Visser, T.J.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals (Article)

2013-03-21T00:00:00Z

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Thyroid hormones and their placental deiodination in normal and pre-eclamptic pregnancy (Article)

2013-03-19T00:00:00Z

Pre-eclampsia is associated with lower serum selenium concentrations and glutathione peroxidase expression/activity; total thyroid hormones are also lower. Objectives, study design and main outcome measures: We hypothesised that the placental selenoprotein deiodinase (D3) will be protected in pre-eclampsia due to the hierarchy of selenoprotein biosynthesis in selenium deficiency. Venous blood and tissue from three standardised placental sites were obtained at delivery from 27 normotensive and 23 pre-eclamptic women. mRNA expression and enzyme activity were assessed for both deiodinases (D2 and D3); protein expression/localisation was also measured for D3. FT4, FT3and TSH concentrations were measured in maternal and umbilical cord blood. Results: No significant differences in D3 mRNA or protein expression between normotensive and pre-eclamptic pregnancies. There was a significant effect of sampling site on placental D3 activity only in pre-eclamptic women (P = 0.034; highest activity nearest the cord). A strong correlation between D3 mRNA expression and enzyme activity existed only in the pre-eclamptic group; further strengthened when controlling for maternal selenium (P < 0.002). No significant differences were observed between groups for any of the maternal thyroid hormones; umbilical TSH concentrations were significantly higher in the pre-eclamptic samples (P < 0.001). Conclusions: D3 mRNA and protein expression appear to be independent of selenium status. Nevertheless, the positive correlation between D3 mRNA expression and activity evident only in pre-eclampsia, suggests that in normotensive controls, where selenium is higher, translation is not affected, but in pre-eclampsia, where selenium is low, enzyme regulation may be altered. The raised umbilical TSH concentrations in pre-eclampsia may be an adaptive fetal response to maximise iodide uptake.

Developmental and cell-specific expression of thyroid hormone transporters in the mouse cochlea (Article)

2011-12-01T00:00:00Z

Thyroid hormone is essential for the development of the cochlea and auditory function. Cochlear response tissues, which express thyroid hormone receptor β (encoded by Thrb), include the greater epithelial ridge and sensory epithelium residing inside the bony labyrinth. However, these response tissues lack direct blood flow, implying that mechanisms exist to shuttle hormone from the circulation to target tissues. Therefore, we investigated expression of candidate thyroid hormone transporters L-type amino acid transporter 1 (Lat1), monocarboxylate transporter (Mct)8, Mct10, and organic anion transporting polypeptide 1c1 (Oatp1c1) in mouse cochlear development by in situ hybridization and immunofluorescence analysis. L-type amino acid transporter 1 localized to cochlear blood vessels and transiently to sensory hair cells. Mct8 localized to the greater epithelial ridge, tympanic border cells underlying the sensory epithelium, spiral ligament fibrocytes, and spiral ganglion neurons, partly overlapping with the Thrb expression pattern. Mct10 was detected in a highly restricted pattern in the outer sulcus epithelium and weakly in tympanic border cells and hair cells. Organic anion transporting polypeptide 1c1 localized primarily to fibrocytes in vascularized tissues of the spiral limbus and spiral ligament and to tympanic border cells. Investigation of hypothyroid Tshr-/-mice showed that transporter expression was delayed consistent with retardation of cochlear tissue maturation but not with compensatory responses to hypothyroidism. The results demonstrate specific expression of thyroid hormone transporters in the cochlea and suggest that a network of thyroid hormone transport underlies cochlear development. Copyright

A child with a deletion in the monocarboxylate transporter 8 gene: 7-year follow-up and effects of thyroid hormone treatment (Article)

2011-11-01T00:00:00Z

Objective: The monocarboxylate transporter 8 (MCT8; SLC16A2) has a pivotal role in neuronal triiodothyronine (T3) uptake. Mutations of this transporter determine a distinct X-linked psychomotor retardation syndrome (Allan-Herndon-Dudley syndrome (AHDS)) that is attributed to disturbed thyroid hormone levels, especially elevated T3levels. We describe the genetic analysis of the MCT8 gene in a patient suspected for AHDS and the clinical and endocrine effects of L-thyroxine (LT4) or liothyronine (LT3) treatment intending to overcome the T3uptake resistance through alternative transporters. Methods: The six exons of the MCT8 gene were amplified individually by PCR. As multiple exons were missing, the length of the X-chromosomal deletion was determined by a dense SNP array, followed by PCR-based fine mapping to define the exact borders of the deleted segment. The clinical and endocrine data of the patient during 6.5 years of LT4treatment and two periods (3 months each) of low- and high-dose LT3were evaluated. Results: A partial deletion of the MCT8 gene (comprising five of six exons) was detected, confirming the suspected AHDS. MCT8 dysfunction was associated with partial resistance to T3at the hypothalamus and pituitary level, with normal responsiveness at the peripheral organs (liver and cardiovascular system). Thyroid hormone administration had no beneficial effect on the neurological status of the patient. Conclusion: We identified a 70 kb deletion encompassing exons 2-6 of the MCT8 gene in our AHDS patient. Both LT4and LT3administration had no therapeutic effect. Alternatively, treatment of AHDS patients with thyroid hormone analogs should be considered.

Isolated GH deficiency: Mutation screening and copy number analysis of HMGA2 and CDK6 genes (Article)

2011-10-01T00:00:00Z

Objective: In most patients, the genetic cause of isolated GH deficiency (IGHD) is unknown. By identifying several genes associated with height variability within the normal population, three separate genome-wide association studies provided new candidate genes for human growth disorders. We selected two of them for genetic screening of our IGHD population. Aim:We aimed to determine whether high-mobility group A2 (HMGA2) and cyclin-dependent protein kinase 6 (CDK6) are involved in the pathogenicity of IGHD. Methods: We directly sequenced coding regions and exon-intron boundaries of the genes HMGA2 and CDK6 in 105 Caucasian IGHD patients from the Dutch HYPOPIT study. In addition, we developed a new probe set of multiplex ligation-dependent probe amplification for both genes in order to detect copy number variations. Results: In one patient with classical IGHD phenotype, we identified a new heterozygous 20 bp deletion in the intronic region of HMGA2 (c.250-29--9del), which was absent in the databases and healthy controls. Together, with recently published data concerning the 12q14 microdeletion syndrome, where patients with an HMGA2 haploinsufficiency had proportionate short stature, this study provides further support of the important role for HMGA2 in growth. In CDK6, we found only known polymorphisms. Conclusions: This study provides the first report of a deletion in the HMGA2 gene that might be related to IGHD. We suggest that this gene is investigated as a second screening in patients with a classical IGHD phenotype in which mutations in classical candidate genes have been excluded.

Sunitinib-induced hypothyroidism is due to induction of type 3 deiodinase activity and thyroidal capillary regression (Article)

2011-10-01T00:00:00Z

Context: Anticancer treatment with the tyrosine kinase inhibitor sunitinib causes thyroid dysfunction. Objective: Our objective was to investigate the time course and underlying mechanisms of sunitinib-induced thyroid dysfunction. Design: Thyroid function tests of 83 patients on sunitinib were collected retrospectively for their total treatment duration between January 2006 and November 2009 and prospectively in 15 patients on sunitinib for 10 wk. Additionally, thyroid function and histology were assessed in rats on sunitinib (8 d; n = 10) and after sunitinib withdrawal (11 d; n = 7) and compared with controls (n = 7). Setting: Patients were seen at a university outpatient oncology clinic. Patients and Animals: Patients with metastatic renal cell carcinoma or gastrointestinal stromal tumors participated in the clinical study and Wistar Kyoto rats were used in the rat study. Intervention: Sunitinib was taken according to a 4 wk "on," 2 wk "off" treatment regimen. Blood samples for measurement of thyroid function were collected at baseline and at wk 4 and 10. In rats, blood, liver, and thyroid were collected to assess thyroid hormones, deiodinase activity, and thyroid histology. Main Outcome Measures: TSH and free T4levels, deiodinase activity, and thyroid histology were assessed. Results: Forty-two percent of patients in the retrospective study developed elevated TSH levels. Prospective analysis showed increased TSH levels within 10 wk of treatment, accompanied by a decreased T3/rT3ratio. In rats, serum T4and T3decreased, hepatic type 3 deiodinase activity increased, andthyroid histology showed marked capillary regression, which all but thyroid hormones reversed after sunitinib withdrawal. Conclusion: Sunitinib induces hypothyroidism due to alterations in T4/T3metabolism as well as thyroid capillary regression. Copyright

The type 2 deiodinase ORFa-Gly3Asp polymorphism (rs12885300) influences the set point of the hypothalamus-pituitary-thyroid axis in patients treated for differentiated thyroid carcinoma (Article)

2011-09-01T00:00:00Z

Context: Iodothyronine deiodinases D1, D2, and D3 play an important role in synthesis and degradation of T3. The relationship between serum TSH and free T4(FT4) levels is determined by an individual set point of the hypothalamus-pituitary-thyroid axis. Objective: Several polymorphisms have been described in D1 and D2 of which some are associated with serum TSH and iodothyronine levels. In this study we investigate whether polymorphisms of D1 and D2 influence the set point of the hypothalamus- pituitary-thyroid axis. Design: We collected 1905 serum FT4and TSH measurements during 11.5 ± 8.8 yr of follow-up in patients treated for differentiated thyroid carcinoma (DTC). We determined these polymorphisms: D1-rs11206244, D1-rs12095080, D2-rs225014, and D2-rs12885300. Effects of these polymorphisms on the set points of the hypothalamus-pituitary-thyroid axis were analyzed with a linear mixed model. Setting: The study was conducted at Leiden University Medical Center, a tertiary referral center for DTC. Patients: One hundred fifty-one consecutive patients were treated and cured for DTC. Main Outcome Measure: Slopes and intercepts of regression equations representing the relationship between InTSH and FT4were measured for all polymorphisms. Results: DTC patients homozygous for the D2-rs12885300 T allele have an altered set point of the hypothalamus-pituitary-thyroid axis. The slope of the regression line (corrected for age, body mass index, and gender) for wild-type patients was -0.32 ± 0.028 (ln[TSH(mU/liter)]/[FT4(pmol/liter)]), the intercept, 4.95. For heterozygous patients, the slope was -0.30 ± 0.028 (ln[TSH(mU/liter)]/[FT4(pmol/liter)]), the intercept, 4.23. The slope of the homozygous patients was -0.35 ± 0.026 (ln[TSH(mU/liter)]/[FT4(pmol/liter)]) and the intercept, 6.07 (P=0.036 compared with wild-type and heterozygous patients). Conclusion: Our data suggest that the negative feedback of FT4on TSH is weaker in patients homozygous for the D2-rs12885300 T allele than in wild-type and heterozygous subjects. Copyright

A nonselenoprotein from amphioxus deiodinates triac but not T3: Is triac the primordial bioactive thyroid hormone? (Article)

2011-08-01T00:00:00Z

Thyroidhormone (TH) is important for metamorphosis inmanyspecies, including the cephalochordate Branchiostoma floridae, a marine invertebrate (amphioxus) living in warmer coastal areas. Branchiostoma expresses a TH receptor, which is activated by 3,3′,5-triiodothyroacetic acid (TA3) but not by T3. The Branchiostoma genome also contains multiple genes coding for proteins homologous to iodothyronine deiodinases in vertebrates, selenoproteins catalyzing the activation or inactivation of TH. Three Branchiostoma deiodinases have been cloned: two have a catalytic Sec, and one, bfDy, has a Cys residue. We have studied the catalytic properties of bfDy in transfected COS1 cells by HPLC analysis of reactions with125I-labeled substrates and dithiothreitol as cofactor.Wecould not detect deiodination of T4, T3, or rT3by bfDy but observed rapid and selective inner ring deiodination (inactivation) of TA3and 3,3′,5,5′- tetraiodothyroacetic acid (TA4). Deiodination of TA3by bfDy was optimal at 25 C and 10 mM dithiothreitol. bfDy was extremely labile at 37 C, showing a half-life of less than 2 min, in contrast with a half-life of more than 60 min at 25 C. Deiodination of labeled TA3was inhibited dose dependently by unlabeled TA3≈TA4>T4≈T3. Michaelis-Menten analysis yielded Michaelis-Menten constant values of 6.8 and 68 nM and maximum velocity values of 1.4 and 5.4 pmol/min·mg protein for TA3and TA4, respectively. bfDy was not inhibited by propylthiouracil and iodoacetate and only weakly by goldthioglucose and iopanoic acid. In conclusion, we demonstrate rapid inactivation of TA3and TA4but not of T3and T4by the first reported natural nonselenodeiodinase. Our findings support the hypothesis that TA3is a primordial bioactive TH.

Expression of thyroid hormone transporters in the human hypothalamus (Article)

2011-06-01T00:00:00Z

Context: Transport of thyroid hormone across the plasma membrane is required for proper thyroid hormone action and metabolism. Several specific thyroid hormone transporters have been identified capable of facilitating uptake and/or efflux of thyroid hormones. Monocarboxylate transporter (MCT)-8, MCT10, and organic anion transporting polypeptide 1C1 (OATP1C1) are the best-characterized specific thyroid hormone transporters to date. Objective: Our earlier studies in the human hypothalamus have shown that MCT8 is present in neurons of the hypothalamic paraventricular nucleus (PVN) and infundibular nucleus (IFN) and in tanycytes. We hypothesized that also MCT10 and OATP1C1 are present in specific areas of the human hypothalamus. Design: We studied postmortem brain samples of patients with known serum thyroid hormone levels using immunocythochemistry to investigate the distribution of MCT10 and OATP1C1 in the hypothalamus. Results: We found strong neuronal MCT10 immunocytochemical staining in a number of hypothalamic nuclei, including the PVN, IFN, and supraoptic nucleus. Intense staining was also observed in neurons of the lateral hypothalamus including the perifornical area. OATP1C1 immunoreactivity was present in glial cells throughout the hypothalamus. In addition, staining was present in capillary walls and in neurons of the PVN, IFN, and supraoptic nucleus. Conclusion: The strong expression of MCT10 and OATP1C1 in the human hypothalamus indicates a possible role in the regulation of the hypothalamus-pituitary-thyroid axis. Copyright

A large-scale association analysis of 68 thyroid hormone pathway genes with serum TSH and FT4 levels (Article)

2011-05-01T00:00:00Z

Objective: Minor variation in serum thyroid hormone (TH) levels can have important effects on various clinical endpoints. Although 45-65% of the inter-individual variation in serum TH levels is due to genetic factors, the causative genes are not well established. We therefore studied the effects of genetic variation in 68 TH pathway genes on serum TSH and free thyroxine (FT4) levels. Design and methods: Sixty-eight genes (1512 polymorphisms) were studied in relation to serum TSH and FT4levels in 1121 Caucasian subjects. Promising hits (P<0.01) were studied in three independent Caucasian populations (2656 subjects) for confirmation. A meta-analysis of all four studies was performed. Results: For TSH, eight PDE8B polymorphisms (P=4×10-17) remained significant in the meta-analysis. For FT4, two DIO1 (P=8×10-12) and one FOXE1 (P=0.0003) polymorphisms remained significant in the meta-analysis. Suggestive associations were detected for one FOXE1 (P=0.0028) and three THRB (P=0.0045) polymorphisms with TSH, and one SLC16A10 polymorphism (P=0.0110) with FT4, but failed to reach the significant multiple-testing corrected P value (P<0.0022 and P<0.0033 respectively). Conclusions: Using a large-scale association analysis, we replicated previously reported associations with genetic variation in PDE8B, THRB, and DIO1. We demonstrate effects of genetic variation in FOXE1 on serum FT4levels, and borderline significant effects on serum TSH levels. A suggestive association of genetic variation in SLC16A10 with serum FT4levels was found. These data provide insight into the molecular basis of inter-individual variation in TH serum levels.

Maternal thyroid function during pregnancy and behavioral problems in the offspring: The generation R study (Article)

2011-05-01T00:00:00Z

Maternal thyroid function during pregnancy is implicated in the neurodevelopment of the offspring, yet little is known about the effect of maternal thyroid parameters on the behavior of children. We investigated the association of maternal thyroid function during the first half of pregnancy with parent-reported problem behavior of the offspring up to age of 3 y. In the Generation R study, a population-based cohort of 3736 children and their mothers, data on maternal thyroid function and child's behavior were examined. The degree of internalizing and externalizing problems in the children were assessed with the Child Behavior Checklist at ages 11/2 and 3 y. Higher levels of maternal TSH during pregnancy predicted a higher externalizing scores in children at 11/2 and 3 y (B = 0.22 per SD of TSH; 95% CI: 0.04, 0.40; B = 0.10 per SD for internalizing scores; 95% CI:-0.01, 0.21). Maternal free thyroxine (T4) and total T4 were not associated with internalizing or externalizing scores of children. The linear relationship with more externalizing scores was across the range of TSH; this implies that subtle impairments of maternal thyroid function may affect the child. The results suggest that thyroid function is crucial for fetal brain development, which determines problem behavior later in life. Copyright

Left-ventricular remodeling after myocardial infarction is associated with a cardiomyocyte-specific hypothyroid condition (Article)

2011-02-01T00:00:00Z

Similarities in cardiac gene expression in hypothyroidism and left ventricular (LV) pathological remodeling after myocardial infarction (MI) suggest a role for impaired cardiac thyroid hormone (TH) signaling in the development of heart failure. Increased ventricular activity of the TH-degrading enzyme type 3 deiodinase (D3) is recognized as a potential cause. In the present study, we investigated the cardiac expression and activity of D3 over an 8-wk period after MI in C57Bl/6J mice. Pathological remodeling of the noninfarcted part of the LV was evident from cardiomyocyte hypertrophy, interstitial fibrosis, and impairment of contractility. These changes were maximal and stable from the first week onward, as was the degree of LV dilation. A strong induction of D3 activity was found, which was similarly stable for the period examined. Plasma T4 levels were transiently decreased at 1 wk after MI, but T3 levels remained normal. The high D3 activity was associated with increased D3 mRNA expression at 1 but not at 4 and 8 wk after MI. Immunohistochemistry localized D3 protein to cardiomyocytes. In vivo measurement of TH-dependent transcription activity in cardiomyocytes using a luciferase reporter assay indicated a 48% decrease in post-MI mice relative to sham-operated animals, and this was associated with a 50% decrease in LVtissue T3 concentration. In conclusion, pathological ventricular remodeling after MI in themouse leads to high and stable induction of D3 activity in cardiomyocytes and a local hypothyroid condition.

Sorafenib induced thyroiditis in two patients with hepatocellular carcinoma (Article)

2011-02-01T00:00:00Z

Background: Sorafenib is a multi-targeted tyrosine kinase inhibitor licensed for the treatment of hepatocellular carcinoma and renal cell carcinoma. Thyroid function test abnormalities have been reported for different tyrosine kinase inhibitors, but only limited data on thyroid function test abnormalities related to sorafenib are available, demonstrating the occurrence of hypothyroidism in patients treated with sorafenib. Summary: We describe two patients who developed temporary hyperthyroidism during the course of sorafenib treatment, which was followed by overt and subclinical hypothyroidism, respectively. Thyroid ultrasonography showed an atrophic thyroid gland in patient 1, and signs of thyroiditis in patient 2. Detailed reassessment of thyroid volumes on routinely performed computerized tomography scans showed a gradual decrease in thyroid volume during sorafenib treatment in one patient, suggesting progressive thyroid destruction. Conclusion: This case report describes in detail and for the first time two cases of sorafenib-induced thyroiditis. We assume that this sorafenib-induced destructive thyroiditis is an important cause of sorafenib-induced hypothyroidism.

Tissue-specific effects of mutations in the thyroid hormone transporter MCT8 (Article)

2011-02-01T00:00:00Z

Effects of methimazole on the elimination of irinotecan (Article)

2011-01-01T00:00:00Z

Purpose: To study the possible pharmacokinetic and pharmacodynamic interactions between irinotecan and methimazole. Methods: A patient treated for colorectal cancer with single agent irinotecan received methimazole co-medication for Graves' disease. Irinotecan pharmacokinetics and side effects were followed during a total of four courses (two courses with and two courses without methimazole). Results: Plasma concentrations of the active irinotecan metabolite SN-38 and its inactive metabolite SN-38-Glucuronide were both higher (a mean increase of 14 and 67%, respectively) with methimazole co-medication, compared to irinotecan monotherapy. As a result, the mean SN-38 glucuronidation rate increased with 47% during concurrent treatment. Other possible confounding factors did not change over time. Specific adverse events due to methimazole co-treatment were not seen. Conclusions: Additional in vitro experiments suggest that these results can be explained by induction of UGT1A1 by methimazole, leading to higher SN-38G concentrations. The prescribed combination of these drugs may lead to highly toxic intestinal SN-38 levels. We therefore advise physicians to be very careful in combining methimazole with regular irinotecan doses, especially in patients who are prone to irinotecan toxicity.

Effects of evening vs morning levothyroxine intake: A randomized double-blind crossover trial (Article)

2010-12-13T00:00:00Z

Background: Levothyroxine sodium is widely prescribed to treat primary hypothyroidism. There is consensus that levothyroxine should be taken in the morning on an empty stomach. A pilot study showed that levothyroxine intake at bedtime significantly decreased thyrotropin levels and increased free thyroxine and total triiodothyronine levels. To date, no large randomized trial investigating the best time of levothyroxine intake, including quality-of-life evaluation, has been performed. Methods: To ascertain if levothyroxine intake at bedtime instead of in the morning improves thyroid hormone levels, a randomized double-blind crossover trial was performed between April 1, 2007, and November 30, 2008, among 105 consecutive patients with primary hypothyroidism at Maasstad Hospital Rotterdam in the Netherlands. Patients were instructed during 6 months to take 1 capsule in the morning and 1 capsule at bedtime (one containing levothyroxine and the other a placebo), with a switch after 3 months. Primary outcome measures were thyroid hormone levels; secondary outcome measures were creatinine and lipid levels, body mass index, heart rate, and quality of life. Results: Ninety patients completed the trial and were available for analysis. Compared with morning intake, direct treatment effects when levothyroxine was taken at bedtime were a decrease in thyrotropin level of 1.25 mIU/L (95% confidence interval [CI], 0.60-1.89 mIU/L; P<.001), an increase in free thyroxine level of 0.07 ng/dL (0.02-0.13 ng/dL; P=.01), and an increase in total triiodothyronine level of 6.5 ng/dL (0.9-12.1 ng/dL; P=.02) (to convert thyrotropin level to micrograms per liter, multiply by 1.0; free thyroxine level to picomoles per liter, multiply by 12.871; and total triiodothyronine level to nanomoles per liter, multiply by 0.0154). Secondary outcomes, including quality-of-life questionnaires (36-Item Short Form Health Survey, Hospital Anxiety and Depression Scale, 20-Item Multidimensional Fatigue Inventory, and a symptoms questionnaire), showed no significant changes between morning vs bedtime intake of levothyroxine. Conclusions: Levothyroxine taken at bedtime significantly improved thyroid hormone levels. Quality-of-life variables and plasma lipid levels showed no significant changes with bedtime vs morning intake. Clinicians should consider prescribing levothyroxine intake at bedtime. Trial Registration: isrctn.org Identifier: ISRCTN17436693 (NTR959).

Impact of monocarboxylate transporter-8 deficiency on the hypothalamus-pituitary-thyroid axis in mice (Article)

2010-10-01T00:00:00Z

In patients, inactivating mutations in the gene encoding the thyroid hormone-transporting monocarboxylate transporter 8 (Mct8) are associated with severe mental and neurological deficits and disturbed thyroid hormone levels. The latter phenotype characterized by high T3and low T4serum concentrations is replicated in Mct8 knockout (ko) mice, indicating that MCT8 deficiency interferes with thyroid hormone production and/or metabolism. Our studies of Mct8 ko mice indeed revealed increased thyroidal T3and T4concentrations without overt signs of a hyperactive thyroid gland. However, upon TSH stimulation Mct8 ko mice showed decreased T4and increased T3secretion compared with wild-type littermates. Moreover, similar changes in the thyroid hormone secretion pattern were observed in Mct8/Trhr1 double-ko mice, which are characterized by normal serum T3levels and normal hepatic and renal D1 expression in the presence of very low T4serum concentrations. These data strongly indicate that absence of Mct8 in the thyroid gland affects thyroid hormone efflux by shifting the ratio of the secreted hormones toward T3. To test this hypothesis, we generated Mct8/Pax8 double-mutant mice, which in addition to Mct8 lack a functional thyroid gland and are therefore completely athyroid. Following the injection of these animals with either T4or T3, serum analysis revealed T3concentrations similar to those observed in Pax8 ko mice under thyroid hormone replacement, indicating that indeed increased thyroidal T3secretion in Mct8 ko mice represents an important pathogenic mechanism leading to the high serum T3levels. Copyright

Maternal thyroid function during early pregnancy and cognitive functioning in early childhood: The generation R study (Article)

2010-09-01T00:00:00Z

Context: Thyroid hormones are essential for neurodevelopment from early pregnancy onward. Yet population-based data on the association between maternal thyroid function in early pregnancy and children's cognitive development are sparse. Objective: Our objective was to study associations of maternal hypothyroxinemia and of early pregnancy maternal TSH and free T4(FT4) levels across the entire range with cognitive functioning in early childhood. Design and Setting: We conducted a population-based cohort in The Netherlands. Participants: Participants included 3659 children and their mothers. Main Measures: In pregnant women with normal TSH levels at 13 wk gestation (SD = 1.7), mild and severe maternal hypothyroxinemia were defined as FT4concentrations below the 10th and 5th percentile, respectively. Children's expressive vocabulary at 18 months was reported by mothers using the MacArthur Communicative Development Inventory. At 30 months, mothers completed the Language Development Survey and the Parent Report of Children's Abilities measuring verbal and nonverbal cognitive functioning. Results: Maternal TSH was not related to the cognitive outcomes. An increase in maternal FT4predicted a lower risk of expressive language delay at 30 months only. However, both mild and severe maternal hypothyroxinemia was associated with a higher risk of expressive language delay across all ages [odds ratio (OR) = 1.44; 95% confidence interval (CI) = 1.09-1.91; P = 0.010 and OR = 1.80; 95% CI = 1.24-2.61; P = 0.002, respectively]. Severe maternal hypothyroxinemia also predicted a higher risk of nonverbal cognitive delay (OR = 2.03; 95% CI = 1.22-3.39; P = 0.007). Conclusions: Maternal hypothyroxinemia is a risk factor for cognitive delay in early childhood. Copyright

Sorafenib-induced hypothyroidism is associated with increased type 3 deiodination (Article)

2010-08-01T00:00:00Z

Background: Therapy with tyrosine kinase inhibitors is associated with thyroid dysfunction. Decreased serum thyroid hormone levels during tyrosine kinase inhibitors are also observed in athyreotic patients with thyroid carcinoma. We therefore hypothesized that tyrosine kinase inhibitors may influence thyroid hormone metabolism. Aim: The aim was to study the effects of sorafenib therapy on serum thyroid hormone concentrations and iodothyronine deiodination in athyreotic patients. Design: The design included a prospective open, single-center, single-arm 26-wk study. Methods: We measured serum thyroxine (T4), free T4, 3,5,3-triiodothyronine (T 3), free T3, reverse T3 (rT3), and TSH concentrations at baseline and after 26 wk in 21 patients with progressive non-medullary thyroid carcinoma treated with sorafenib. Ratios of T 3/T4 and T3/rT3, which are independent of substrate availability and reflect iodothyronine deiodination, were calculated. Results: Serum free T4 and T3 levels, adjusted for levothyroxine dose per kilogram body weight, decreased by 11 and 18%, respectively, whereas TSH levels increased. The serum T3/T 4 and T3/rT3 ratios decreased by 18 and 22%, respectively, which is compatible with increased type 3 deiodination. Conclusions: Sorafenib enhances T4 and T3 metabolism, which is probably caused by increased type 3 deiodination.

Genetics and phenomics of hypothyroidism and goiter due to iodotyrosine deiodinase (DEHAL1) gene mutations (Article)

2010-06-01T00:00:00Z

Iodotyrosine deiodinase is a thyroidal enzyme that deiodinates mono- and di-iodotyrosines (MIT, DIT) and recycles iodine, a scarce element in the environment, for the efficient synthesis of thyroid hormone. Failure of this enzyme leads to hypothyroidism, goiter and mental retardation, a clinical phenotype yet described in the 1950s, whose diagnostic hallmark is the elevation of iodotyrosines in serum and urine.DEHAL1, the gene responsible for this activity, was recently isolated and the molecular basis for the iodotyrosine deiodinase deficiency (ITDD) unraveled. The current clinical picture of mutations in DEHAL1 mostly recapitulates the " classical" phenotype of ITDD, including the psychomotor deficits. This is probably due to the lack of expression of the disease at the beginning of life, which causes ITDD being undetected in current screening programs for congenital hypothyroidism. This worrying feature calls for efforts to improve the preclinical detection of iodotyrosine deiodinase deficiency in the neonatal time.

The type 2 deiodinase Thr92Ala polymorphism is associated with increased bone turnover and decreased femoral neck bone mineral density (Article)

2010-06-01T00:00:00Z

The role of type 2 deiodinase (D2) in the human skeleton remains unclear. The D2 polymorphism Thr92Ala has been associated with lower enzymatic activity, which could result in lower local triiodothyronine (T3) availability in bone. We therefore hypothesized that the D2 Thr92Ala polymorphism may influence bone mineral density (BMD) and bone turnover. We studied 154 patients (29 men, 125 women: 79 estrogen-replete, 46 estrogen-deficient) with cured differentiated thyroid carcinoma. BMD and bone turnover markers [bone-specific alkaline phosphatase (BAP), cross-linking terminal C-telopeptide of type I collagen (CTX), procollagen type 1 aminoterminal propeptide (P1NP), and cross-linked N-telopeptide of type I collagen (NTX)] were measured. Effects of the D2 Thr92Ala polymorphism on BMD and bone turnover markers were assessed by a linear regression model, with age, gender, estrogen state, body mass index (BMI), serum calcium, 25-hydroxyvitamin D, parathyroid hormone (PTH), thyroid-stimulating hormone (TSH), and free triiodothyroxine (T4) as covariables. Sixty patients were wild type (Thr/Thr), 66 were heterozygous (Thr/Ala), and 28 were homozygous (Ala/Ala) for the D2 polymorphism. There were no significant differences in any covariables between the three genotypes. Subjects carrying the D2 Thr92Ala polymorphism had consistently lower femoral neck and total hip densities than wild-type subjects (p=.028), and this was accompanied by significantly higher serum P1NP and CTX and urinary NTX/creatinine levels. We conclude that in patients with cured differentiated thyroid carcinoma, the D2 Thr92Ala polymorphism is associated with a decreased femoral neck BMD and higher bone turnover independent of serum thyroid hormone levels, which points to a potential functional role for D2 in bone.

Pilot study on the assessment of the setpoint of the hypothalamus- pituitary-thyroid axis in healthy volunteers (Article)

2010-02-01T00:00:00Z

Objective: To determine the log-linear relationship between TSH and free thyroxine in healthy subjects, and the variation in baseline TSH/free thyroxine (FT4) combination in each individual. Subjects and methods: Twenty-one healthy volunteers (nine males and 12 females; mean age 60 years, range 51-74) were randomized to receive at 2300 h with 2-week intervals a single dose of placebo, 125 μg T4and 250 mg T4(arm 1, n = 10), or placebo, 25 μg triiodothyronine (T3) and 50 μg T3(arm 2, n = 11). Blood samples were taken in the morning (0800-1100 h) before and following the administration of the drug for the assessment of TSH, FT4and T3. Results: Intra- and inter-individual variation and the individuality index of the four baseline serum samples were respectively 21.6%, 41.9% and 0.52 for TSH; 9.9%, 16.5% and 0.60 for FT4; and 9.3%, 16.0% and 0.58 for T3. Substantial differences existed in the location of individual working points within the reference range. T4administration increased FT4(but not T3) and decreased TSH, resulting in a log-linear relationship (log TSH = 1.50-0.059xFT4, P<0.05) for the whole group. T3administration increased T3and decreased TSH (but not FT4), resulting in a log-linear relationship (log TSH = 0.790-0.245xT3, P<0.001) for the whole group. Log-linear relationships were not always significant when assessed for each subject separately. Conclusion: Individuality indices of TSH, FT4and T3are all ≤0.6, thereby limiting the usefulness of the population-based reference values. Accurate assessment of individual setpoints of the HPT axis was not possible with the applied single doses of T4or T3, and will require either prolonged administration or higher single doses of thyroid hormone.

Consequences of monocarboxylate transporter 8 deficiency for renal transport and metabolism of thyroid hormones in mice (Article)

2010-01-29T00:00:00Z

Patients carrying inactivating mutations in the gene encoding the thyroid hormone transporting monocarboxylate transporter (MCT)-8 suffer from a severe form of psychomotor retardation and exhibit abnormal serum thyroid hormone levels. The thyroidal phenotype characterized by highserum T3and low-serum T4levels is also found in mice mutants deficient in MCT8 although the cause of these abnormalities is still unknown. Here we describe the consequences of MCT8 deficiency for renal thyroid hormone transport, metabolism, and function by studying MCT8 null mice and wild-type littermates. Whereas serum and urinary parameters do not indicate a strongly altered renal function, a pronounced induction of iodothyronine deiodinase type 1 expression together with increased renal T3and T4content point to a general hyperthyroid state of the kidneys in the absence of MCT8. Surprisingly, accumulation of peripherally injected T4and T3into the kidneys was found to be enhanced in the absence of MCT8, indicating that MCT8 deficiency either directly interferes with the renal efflux of thyroid hormones or activates indirectly other renal thyroid hormone transporters that preferentially mediate the renal uptake of thyroid hormones. Our findings indicate that the enhanced uptake and accumulation of T4in the kidneys of MCT8 null mice together with the increased renal conversion of T4into T3by increased renal deiodinase type 1 activities contributes to the generation of the low-serum T4and the increase in circulating T3levels, a hallmark of MCT8 deficiency. Copyright

Molecular aspects of thyroid hormone transporters, including MCT8, MCT10, and OATPs, and the effects of genetic variation in these transporters (Article)

2010-01-01T00:00:00Z

Thyroid hormone is a pleiotropic hormone with widespread biological actions. For instance, adequate levels of thyroid hormone are critical for the development of different tissues such as the central nervous system, but are also essential for the regulation of metabolic processes throughout life. The biological activity of thyroid hormone depends not only on serum thyroid hormone levels, but is also regulated at the tissue level by the expression and activity of deiodinases, which activate thyroid hormone or mediate its degradation. In addition, thyroid hormone transporters are necessary for the uptake of thyroid hormone into target tissues. With the discovery of monocarboxylate transporter 8 (MCT8) as a specific thyroid hormone transporter and the finding that mutations in this transporter lead to a syndrome of severe psychomotor retardation and elevated serum 3,3′,5-tri-iodothyronine levels known as the Allan-Herndon-Dudley syndrome, the interest in this area of research has greatly increased. In this review, we will focus on the molecular aspects of thyroid hormone transporters, including MCT8, MCT10, organic anion transporting polypeptides, and the effects of genetic variation in these transporters.

Expression of thyroid hormone transporters during critical illness (Article)

2009-11-27T00:00:00Z

Objective: Prolonged critically ill patients have low circulating thyroid hormone (TH) levels without a rise in TSH, a condition labeled 'the low tri-iodothyronine (T3) syndrome'. Currently, it is not clear whether this represents an adaptive response. We examined the role of TH transporters monocarboxylate transporter 8 (MCT8, also known as SLC16A2) and MCT10 in the pathogenesis of the low T3 syndrome in prolonged critical illness. Methods: A clinical observational study in critically ill patients and an intervention study in an in vivo animal model of critical illness. Gene expression levels of MCT8 and MCT10 were measured by realtime PCR. Results: In prolonged critically ill patients, we measured increased MCT8 but not MCT10 gene expression levels in liver and skeletal muscle as compared with patients undergoing acute surgical stress. In a rabbit model of prolonged critical illness, gene expression levels of MCT8 in liver and of MCT10 in skeletal muscle were increased as compared with healthy controls. Treatment of prolonged critically ill rabbits with TH (thyroxineCT3) resulted in a downregulation of gene expression levels of MCT8 in liver and of MCT10 in muscle. Transporter expression levels correlated inversely with circulating TH parameters. Conclusions: These data suggest that alterations in the expression of TH transporters do not play a major role in the pathogenesis of the 'low T3 syndrome' but rather reflect a compensatory effort in response to hypothyroidism.

Multiple genomic aberrations in a patient with mental retardation and hypogonadism: 45,X/46,X,psu dic(Y) karyotype, thyroid hormone receptor beta (THRB) mutation and heterozygosity for Wilson disease (Article)

2009-10-01T00:00:00Z

We report on multiple genomic aberrations in a patient with mental retardation. In addition, he had hypogonadism, elevated thyroid hormone levels, hearing loss, delayed speech development and mild dysmorphic features. First, we identified amosaic karyotype, 45,X/46,X,psu dic(Y). The pseudo-dicentric Y chromosome has three short arm segments. Second, we found a germline mutation (Pro453Thr) of the thyroid hormone receptor beta (THRB) which is associated with resistance to thyroid hormone. Third, he was found to be a carrier of a heterozygous ATP7B mutation (c.2575 + 5G>C), the Wilson disease gene. Even though an array-CGH (with a density of ∼1 Mb) did not reveal any further genomic gains or losses, we cannot exclude that all contributing factors have been identified. However, this case report shows that with increasing technological possibilities we can find more than one cause for developmental problems in a single patient. The identification of multiple causes in a single patient may complicate explaining the disorder and genetic counseling.

Changes in the central component of the hypothalamus-pituitary-thyroid axis in a rabbit model of prolonged critical illness (Article)

2009-09-11T00:00:00Z

Introduction: Prolonged critically ill patients reveal low circulating thyroid hormone levels without a rise in thyroid stimulating hormone (TSH). This condition is labeled "low 3,5,3'-tri-iodothyronine (T3) syndrome" or "nonthyroidal illness syndrome (NTI)" or "euthyroid sick syndrome". Despite the low circulating and peripheral tissue thyroid hormone levels, thyrotropin releasing hormone (TRH) expression in the hypothalamus is reduced and it remains unclear which mechanism is responsible. We set out to study whether increased hypothalamic T3availability could reflect local thyrotoxicosis and explain feedback inhibition-induced suppression of the TRH gene in the context of the low T3syndrome in prolonged critical illness.Methods: Healthy rabbits were compared with prolonged critically ill, parenterally fed animals. We visualized TRH mRNA in the hypothalamus by in situ-hybridization and measured mRNA levels for the type II iodothyronine diodinase (D2), the thyroid hormone transporters monocarboxylate transporter (MCT) 8, MCT10 and organic anion co-transporting polypeptide 1C1 (OATP1C1) and the thyroid hormone receptors α (TRα) and β (TRβ) in the hypothalamus. We also measured the activity of the D2 and type III iodothyronine deiodinase (D3) enzymes.Results: In the hypothalamus of prolonged critically ill rabbits with low circulating T3 and TSH, we observed decreased TRH mRNA, increased D2 mRNA and increased MCT10 and OATP1C1 mRNA while MCT8 gene expression was unaltered as compared with healthy controls. This coincided with low hypothalamic thyroxine (T4) and low-normal T3concentrations, without a change at the thyroid hormone receptor level.Conclusions: Although expression of D2 and of the thyroid hormone transporters MCT10 and OATP1C1 were increased in the hypothalamus of prolonged critical ill animals, hypothalamic T4and T3content or thyroid hormone receptor expression were not elevated. Hence, decreased TRH gene expression, and hereby low TSH and T3 during prolonged critical illness, is not exclusively brought about by hypothalamic thyrotoxicosis, and infer other TRH suppressing factors to play a role.

Physiological thyroid hormone levels regulate numerous skeletal muscle transcripts (Article)

2009-09-01T00:00:00Z

Context: Skeletal muscle is an important target tissue for thyroid hormone (TH). It is currently unknown which genes are regulated by physiological TH levels. Objective: We examined the effects of L-thyroxine on human skeletal muscle transcriptome. Design: Microarray analysis of transcript levels was performed using skeletal muscle biopsies from patients under euthyroid and hypothyroid conditions. Setting: The study was conducted in a university hospital laboratory. Patients: We studied skeletal muscle obtained from 10 thyroidectomized patients with differentiated thyroid carcinoma on and after 4 wk off L-thyroxine replacement. Mean Outcome Measures: Gene expression changes were measured using microarrays. Results were analyzed using dedicated statistical methods. Results: We detected 607 differentially expressed genes on L-thyroxine treatment, of which approximately 60% were positively and approximately 40% were negatively regulated. Representative genes were validated by quantitative PCR. Genes involved in energy and fuel metabolism were overrepresented among the up-regulated genes, of which a large number were newly associated with thyroid state. L-thyroxine therapy induced a large down-regulation of the primary transcripts of the noncoding microRNA pair miR-206/miR-133b. Conclusion: We demonstrated that physiological levels of TH regulate a myriad of genes in human skeletal muscle. The identification of novel putatively TH-responsive genes may provide the molecular basis of clinical effects in subjects with different TH status. The observation that TH regulates microRNAs reveals a new layer of complexity by which TH influences cellular processes.

Thr92Ala polymorphism in the type 2 deiodinase is not associated with T4 dose in athyroid patients or patients with Hashimoto thyroiditis (Article)

2009-08-01T00:00:00Z

Objective The type 2 deiodinase (D2)-Thr92Ala polymorphism has been associated with decreased D2 activity in some in vitro experiments but not in others. So far no association between the D2-Thr92Ala polymorphism and serum thyroid hormone levels has been observed in humans, but in a recent study in athyroid patients, it was suggested that patients homozygous for the Ala92allele needed higher T4 doses to achieve TSH suppression. We studied the association between the D2-Thr92Ala polymorphism with thyroid hormone levels and T4 dosage, in patients treated for differentiated thyroid carcinoma (DTC) and in a group of patients treated for Hashimoto thyroiditis. Design Cross-sectional study. Patients We studied 154 patients with DTC treated with TSH suppressive thyroid hormone replacement therapy for longer than 3 years and 141 patients with Hashimoto thyroiditis treated for at least 6 months with T4. Measurements In all patients, serum levels of TSH, free T4, T3 and reverse T3 were measured and genotypes of the D2-Thr92Ala polymorphism were determined by Taqman assay. Univariate regression analysis was performed to determine the relation between T4 dosages and the D2-Thr92Ala polymorphism corrected for age, gender, BMI and serum TSH levels. Results Both in DTC patients and Hashimoto patients, no association was observed between serum thyroid hormone levels or T4 dosages in presence of the D2-Thr92Ala polymorphism. Categorization of DTC patients according to degree of TSH suppression did not change these results. Conclusion The D2-Thr92Ala polymorphism is not associated with thyroid hormone levels or T4 dose in patients treated for DTC or Hashimoto thyroiditis.

Preliminary evidence that a functional polymorphism in type 1 deiodinase is associated with enhanced potentiation of the antidepressant effect of sertraline by triiodothyronine (Article)

2009-07-01T00:00:00Z

Background: Triiodothyronine (T3) is used to potentiate the clinical effect of antidepressant drugs. Inter-individual differences in efficacy may be related to genetically-based variability in thyroid function. Methods: DNA was obtained from 64 patients treated with sertraline plus T3 (SERT-T3, N = 35) or plus placebo (SERT-PLB, N = 29), for 8 weeks. Antidepressant efficacy was rated with the 21 item Hamilton Rating Scale for Depression (HRSD-21). Functional polymorphisms in type 1 (DIO1-C785T, DIO1-A1814G) and type 2 deiodinase (DIO2-Thr92Ala and DIO2-ORFa-Gly3Asp) were genotyped. Results: DIO1-C785T was associated with efficacy of T3 but not placebo supplementation, as indicated by the interaction of treatment, DIO1-C758T genotype and time (p = 0.04) and a stronger effect of SERT-T3 among DIO1-758T allele carriers (p = 0.01). HRSD-21 scores of DIO1-758T allele carriers declined by 68.7 + 26.6% (mean + SD) over 8 weeks compared to 42.9 + 37.8% among non-carriers (p = 0.02). Discussion: DIO1 plays a key-role in T4 to T3 conversion and in clearance of the inactive metabolite, rT3. Previous data associate the DIO1-785T allele with lower DIO1 activity. This is consistent with our observation that responders to T3 supplementation had lower baseline serum T3 levels than non-responders. Depressed patients, who have a genetically determined lower T4 to T3 conversion, may be more likely to benefit from T3 supplementation.

Tissue mRNA expression of the glucocorticoid receptor and its splice variants in fatal critical illness (Article)

2009-07-01T00:00:00Z

Background Critical illness results in activation of the hypothalamic-pituitary-adrenal (HPA) axis, which might be accompanied by a peripheral adaptation in glucocorticoid sensitivity. Tissue sensitivity is determined by the active glucocorticoid receptor GRα, of which two splice variants involving the hormone-binding domain exist, GRβ and GR-P. Objective To study tissue mRNA expression of the GR and its splice variants in fatal critical illness. Design and methods We assessed mRNA expression of the GRα, GRβ and GR-P variants in liver (n = 58) and muscle (n = 65) of patients who had died after intensive care, and had been randomized for insulin treatment. We analysed whether GR mRNA expression was associated with insulin treatment, cortisol levels and glucocorticoid treatment. Results GRα and GR-P mRNA constituted 87 ± 8% and 13 ± 2%, respectively, of total GR mRNA in liver. GRβ mRNA could only be amplified in five liver samples. All variants were present in most muscle samples (α = 96 ± 11%, P = 3·9 ± 0·4%, β = 0·010 ± 0·002%). GR expression was not associated with insulin therapy. A strong positive relationship was observed between the different GR variants in both liver and muscle (P < 0·001 for all). Serum cortisol levels were negatively associated with liver GRα and muscle GR-P expression (P < 0·05). mRNA expression of both liver GRα and GR-P, but not muscle GR, was substantially lower in patients who had received exogenous glucocorticoids (P < 0·01). Conclusion We demonstrate the presence of GRα and GR-P mRNA in liver and of GRα, GRβ and GR-P mRNA in muscle, with no evidence for altered splicing in critical illness. In contrast to muscle GR, liver GR expression was substantially lower in patients receiving exogenous glucocorticoids.

Impact of thyroid function and polymorphisms in the type 2 deiodinase on blood pressure: The Rotterdam Study and the Rotterdam Scan Study (Article)

2009-07-01T00:00:00Z

Introduction Thyroid function and genetic variation in the hypothalamus-pituitary-thyroid axis have been implicated in blood pressure regulation and susceptibility to hypertension. However studies conducted thus far were small with controversial results. Objective To examine whether serum thyroid parameters and polymorphisms in the type 2 deiodinase and the TSH receptor are associated with blood pressure and the presence of hypertension in two large cohorts of elderly subjects. Design and participants We studied a random sample of 1444 subjects of the Rotterdam study, and 997 subjects of the Rotterdam Scan study, two population-based cohort studies among elderly individuals aged 55-90 years. Outcome measurements Data on blood pressure and hypertension were obtained, and serum thyroid parameters, D2-Thr92Ala, D2-ORFa-Gly3Asp and TSHR-Asp727Glu polymorphisms were determined. Results In contrast to previous findings, no consistent and/or significant associations were found between serum TSH and FT4 and blood pressure in both cohorts. In addition, the D2-Thr92Ala, D2-ORFa-Gly3Asp and TSHR-Asp727Glu polymorphisms were not associated with blood pressure or the risk of hypertension. Conclusions In two large populations of elderly subjects, neither serum thyroid parameters nor polymorphisms in the type 2 deiodinase and the TSH receptor, were associated with blood pressure or the presence of hypertension. Our data suggest that thyroid function is not an important determinant of hypertension in elderly Dutch subjects.

The effect of genetic variation in the type 1 deiodinase gene on the interindividual variation in serum thyroid hormone levels: An investigation in healthy Danish twins (Article)

2009-06-01T00:00:00Z

Introduction Genetic factors have a considerable influence on serum thyroid hormone levels. The C785T and A1814G polymorphisms, located in the 3′ untranslated region of the type 1 deiodinase (D1) gene have been associated with serum FT4 and rT3 levels. Objective In healthy Danish twins, we examined the association of these polymorphisms with serum thyroid hormone levels and determined the proportion of genetic influence explained by these variants. We analysed the underlying functional mechanism by performing mRNA stability measurements and analysed the effect of these variants on D1 activity. Methods Serum thyroid measurements and genotypes of the D1-C785T and D1-A1814G polymorphisms were determined in 1192 twins. Structural equation modelling was used to determine heritability estimates. Functional analyses were carried out in D1-transfected JEG3 cells. Results Carriers of the D1-785T allele had 3·8% higher FT4 and 14·3% higher rT3 levels, resulting in a lower T3/T4 and T3/rT3 ratio and a higher rT3/T4 ratio. This polymorphism explained 0·87% and 1·79%, respectively, of the variation in serum FT4 and rT3. The D1-A1814G polymorphism was not associated with serum thyroid hormone levels. No differences in D1 mRNA decay rate or D1 activity were observed between wild-type D1 and the two variants. Conclusion The D1-C785T polymorphism is consistently and significantly associated with serum thyroid hormone levels. However, the proportion of genetic influence explained by this particular polymorphism is small. No effect of the polymorphism on D1 mRNA decay rate or D1 activity was observed. The underlying functional mechanism needs to be elucidated.

Interference of a mutant thyroid hormone receptor α1 with hepatic glucose metabolism (Article)

2009-06-01T00:00:00Z

Mice expressing the mutant thyroid hormone receptor TRα1R384C, which has a 10-fold reduced affinity to the ligand T3, exhibit hypermetabolism due to an overactivation of the sympathetic nervous system. To define the consequences in the liver, we analyzed hepatic metabolism and the regulation of liver genes in the mutant mice. Our results showed that hepatic phosphoenolpyruvate-carboxykinase was up-regulated and pyruvate kinase mRNA down-regulated, contrary to what observed after T3treatment. In contrast, mice expressing a mutant TRα1L400R specifically in the liver did not show a dysregulation of these genes; however, when the TRα1L400R was expressed ubiquitously, the hepatic phenotype differed from TRα1R384C animals, suggesting that the localization of the mutation plays an important role for its consequences on glucose metabolism. Furthermore, we observed that glycogen stores were completely depleted in TRα1R384C animals, despite increased gluconeogenesis and decreased glycolysis. Exposure of the mutant mice to high maternal levels of thyroid hormone during fetal development leads to a normal liver phenotype in the adult. Our results show how genetic and maternal factors interact to determine the metabolic setpoint of the offspring and indicate an important role for maternal thyroid hormone in the susceptibility to metabolic disorders in adulthood. Copyright

Type 2 iodothyronine deiodinase in skeletal muscle: Effects of hypothyroidism and fasting (Article)

2009-06-01T00:00:00Z

Context: The iodothyronine deiodinases D1, D2, and D3 enable tissue-specific adaptation of thyroid hormone levels in response to various conditions, such as hypothyroidism or fasting. The possible expression of D2 mRNA in skeletal muscle is intriguing because this enzyme could play a role in systemic as well as local T3production. Objective: We determined D2 activity and D2 mRNA expression in human skeletal muscle biopsies under control conditions and during hypothyroidism, fasting, and hyperinsulinemia. Design: This was a prospective study. Setting: The study was conducted at a university hospital. Patients: We studied 11 thyroidectomized patients with differentiated thyroid carcinoma (DTC) on and after 4 wk off T4replacement and six healthy lean subjects in the fasting state and during hyperinsulinemia after both 14 and 62 h of fasting. Mean Outcome Measures: D2 activity and D2 mRNA levels were measured in skeletal muscle samples. Results: No differences were observed in muscle D2 mRNA levels in DTC patients on and off T4replacement therapy. In healthy subjects, muscle D2 mRNA levels were lower after 62 h compared to 14 h of fasting. Insulin increased mRNA expression after 62 h, but not after 14 h of fasting. Skeletal muscle D2 activities were very low and not influenced by hypothyroidism and fasting. Conclusion: Human skeletal muscle D2 mRNA expression is modulated by fasting and insulin, but not by hypothyroidism. The lack of a clear effect of D2 mRNA modulation on the observed low D2 activities questions the physiological relevance of D2 activity in human skeletal muscle. Copyright

Transport of thyroxine and 3,3′,5-triiodothyronine in human umbilical vein endothelial cells (Article)

2009-03-01T00:00:00Z

The prerequisite for the uptake of thyroid hormone (TH) in peripheral tissues is the exit of TH from the bloodstream. The first step in this process is transport across the endothelium. Little is known about this important step in TH physiology. Therefore, we aimed to characterize the TH transport processes across the endothelium using human umbilical vein endothelial cells as a model. Transport studies showed rapid uptake of 1 nm [125I]T3 and [125I]T4 in these cells. The apparent Michaelis constant value for [125I]T3 uptake was about 1 μm, and the IC50for T4 inhibition of T3 uptake was about 3 μm. The aromatic amino acids phenylalanine, tyrosine, and tryptophan and the L-type amino acid transporter-specific ligand 2-aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid did not inhibit [125I]T3 or [125I]T4 uptake. Verapamil was capable of reversibly reducing transport of [125I]T3 and [125I]T 4. Human umbilical vein endothelial cells incubated with the affinity label BrAcT3 resulted in a labeling of multiple proteins, which are probably protein disulfide isomerase related. Extrapolating our findings to the endothelial lining of blood vessels suggests that T3 and T 4 uptake is mediated by the same transport system. Because TH transport characteristics do not correspond to known TH transporters, further studies are required to identify the TH transporter protein(s) at the molecular level. Possible candidates may be widely expressed Na+-independent transporter proteins.

Minireview: Pathophysiological importance of thyroid hormone transporters (Article)

2009-03-01T00:00:00Z

Thyroid hormone metabolism and action are largely intracellular events that require transport of iodothyronines across the plasma membrane. It has been assumed for a long time that this occurs by passive diffusion, but it has become increasingly clear that cellular uptake and efflux of thyroid hormone is mediated by transporter proteins. Recently, several active and specific thyroid hormone transporters have been identified, including monocarboxylate transporter 8 (MCT8), MCT10, and organic anion transporting polypeptide 1C1 (OATP1C1). The latter is expressed predominantly in brain capillaries and transports preferentially T4, whereas MCT8 and MCT10 are expressed in multiple tissues and are capable of transporting different iodothyronines. The pathophysiological importance of thyroid hormone transporters has been established by the demonstration of MCT8 mutations in patients with severe psychomotor retardation and elevated serum T3levels. MCT8 appears to play an important role in the transport of thyroid hormone in the brain, which is essential for the crucial action of the hormone during brain development. It is expected that more specific thyroid hormone transporters will be discovered in the near future, which will lead to a better understanding of the tissue-specific regulation of thyroid hormone bioavailability. Copyright

Endocrine effects of hexabromocyclododecane (HBCD) in a one-generation reproduction study in Wistar rats (Article)

2009-02-25T00:00:00Z

The brominated flame retardant (BFR) hexabromocyclododecane was tested in a one-generation reproduction assay in Wistar rats, enhanced for endocrine parameters. A solution of the compound in corn oil was mixed in the feed, targeting at dietary exposure of 0-0.1-0.3-1-3-10-30-100 mg/kg body weight/day (mkd) in parental rats during 10 (males) or 2 (females) weeks premating, during gestation and lactation, and in their F1 offspring from weaning until final necropsy. Effects were assessed in F1 animals. Livers of these animals showed increased HBCD concentrations, in a dose-dependent way. The trabecular bone mineral density of the tibia was dose-dependently decreased in females (BenchMark Dose Lower confidence bound, BMDL = 0.056 mkd). The IgG response after immunization with sheep red blood cells (SRBC) was increased in males (BMDL = 0.46 mkd). Further sensitive effects were decreased weight of the testis (BMDL = 1.5 mkd), increased fraction of neutrophilic granulocytes (BMDL = 7.7 mkd), decreased concentration of apolar retinoids in female livers (BMDL = 1.3 mkd), and decreased plasma alkaline phosphatase in females (BMDL = 8.6 mkd). CYP19/aromatase activity in the ovary was correlated to the concentration of γ-HBCD in the liver. A developmental origin of these effects is considered, and this is also true for sensitive effects observed in neurobehavioural testing in littermates from the same experiment, i.e. in the brainstem auditory evoked potentials and in a catalepsy test [Lilienthal, H., Van der Ven, L.T.M., Piersma, A.H., Vos, J.G. Neurobehavioral effects of the brominated flame retardant hexabromocyclododecane (HBCD) in rats after pre- and postnatal exposure, in press]. The low BMDLs of these effects may raise concern for human health, particularly when based on body burdens of HBCD, which leads to critical margins of exposure particularly for the occupational setting.

Novel pathogenic mechanism suggested by ex vivo analysis of MCT8 (SLC16A2) mutations (Article)

2009-01-01T00:00:00Z

Monocarboxylate transporter 8 (MCT8; approved symbol SLC16A2) facilitates cellular uptake and efflux of 3,3′,5-triiodothyronine (T3). Mutations in MCT8 are associated with severe psychomotor retardation, high serum T3 and low 3,3′,5′-triiodothyronine (rT3) levels. Here we report three novel MCT8 mutations. Two subjects with the F501del mutation have mild psychomotor retardation with slightly elevated T3 and normal rT3 levels. T3 uptake was mildly affected in F501del fibroblasts and strongly decreased in fibroblasts from other MCT8 patients, while T3 efflux was always strongly reduced. Moreover, type 3 deiodinase activity was highly elevated in F501del fibroblasts, whereas it was reduced in fibroblasts from other MCT8 patients, probably reflecting parallel variation in cellular T3 content. Additionally, T3-responsive genes were markedly upregulated by T3 treatment in F501del fibroblasts but not in fibroblasts with other MCT8 mutations. In conclusion, mutations in MCT8 result in a decreased T3 uptake in skin fibroblasts. The much milder clinical phenotype of patients with the F501del mutation may be correlated with the relatively small decrease in T3 uptake combined with an even greater decrease in T3 efflux. If fibroblasts are representative of central neurons, abnormal brain development associated with MCT8 mutations may be the consequence of either decreased or increased intracellular T3 concentrations.

Large induction of type III deiodinase expression after partial hepatectomy in the regenerating mouse and rat liver (Article)

2009-01-01T00:00:00Z

The deiodinase types 1 (D1) and 2 (D2) catalyze the activation of T4to T3, whereas type 3 deiodinase (D3) catalyzes the inactivation of T3and T4. D3 plays a key role in controlling thyroid hormone bioavailability. It is highly expressed during fetal development, but also in other processes with increased cell proliferation, e.g. in vascular tumors. Because tissue regeneration is dependent on cellular proliferation and is associated with activation of fetal genes, we evaluated deiodinase activities and mRNA expression in rat and mouse liver, as well as the local and systemic thyroid hormone status after partial hepatectomy (PH). We observed that in rats, D3 activity was increased 10-fold at 20 h and 3-fold at 48 h after PH; D3 mRNA expression was increased 3-fold at 20 h. The increase in D3 expression was associated with maximum 2-to 3-fold decreases of serum and liver T3and T4levels at 20 to 24 h after PH. In mice, D3 activity was increased 5-fold at 12 h, 8-fold at 24 h, 40-fold at 36 h, 15-fold at 48 h, and 7-fold at 72 h after PH. In correlation with this, D3 mRNA was highest (6-fold increase), and serum T3and T4were lowest at 36 h. Furthermore, as a measure for cell proliferation, 5-bromo-2'- deoxyuridine incorporation peaked at 20-24 h after PH in rats and at 36 h in mice. No significant effect on D1 activity or mRNA expression was found after PH. D2 activity was always undetectable. In conclusion, we found a large induction of hepatic D3 expression after PH that was correlated with an increased cellular proliferation and decreased serum and liver T3and T4levels. Our data suggest that D3 is important in the modulation of thyroid hormone levels in the regenerating liver, in which a decrease in cellular T3permits an increase in proliferation. (Endocrinology 150: 540-545, 2009) Copyright

High-normal thyroid function and risk of atrial fibrillation: The Rotterdam study (Article)

2008-11-10T00:00:00Z

Background: Overt and subclinical hyperthyroidism are both well-known independent risk factors for atrial fibrillation. We aimed to investigate the association of high-normal thyroid function with the development of atrial fibrillation in a prospective population-based study in the elderly. Methods: The association between thyroid-stimulating hormone (TSH) levels and atrial fibrillation was examined in 1426 subjects with TSH levels in the normal range (0.4-4.0 mU/L) and without atrial fibrillation at baseline. In 1177 of the 1426 persons in this group, we also examined the association between free thyroxine levels within the normal range (0.86-1.94 ng/dL [to convert to picomoles per liter, multiply by 12.871]) and atrial fibrillation. During a median follow-up of 8 years, 105 new cases of atrial fibrillation were identified. Hazard ratios (HRs) were calculated with 95% confidence intervals (CIs) using Cox proportional hazards models after adjustment for age, sex, current smoking, former smoking, body mass index, systolic blood pressure, hypertension, history of myocardial infarction, presence of heart failure, left ventricular hypertrophy on the electrocardiogram, diabetes mellitus, total cholesterol level, and time of the drawing of blood samples. Results: The risk of atrial fibrillation was associated with the TSH level. The multivariate adjusted HR was 1.94 (95% CI, 1.13-3.34, lowest vs highest quartile; P for trend, .02). The multivariate adjusted level of free thyroxine showed a graded association with risk of atrial fibrillation (HR, 1.62; 95% CI, 0.84-3.14, highest vs lowest quartile; P for trend, .06). Conclusion: Within the normal range of thyroid parameters, persons with high-normal thyroid function are at an increased risk of atrial fibrillation.

Polymorphisms in the brain-specific thyroid hormone transporter OATP1C1 are associated with fatigue and depression in hypothyroid patients (Article)

2008-11-01T00:00:00Z

Introduction: Some hypothyroid patients continue to have significant impairments in psychological well-being, despite adequate treatment with levothyroxine (LT4). T4 transport across the blood-brain barrier is one of the crucial processes for thyroid hormone action in the brain. OATP1C1, a thyroid hormone transporter expressed at the blood-brain barrier, is considered to play a key role in delivering serum T4 to the brain. Objective: To examine whether polymorphisms in OATP1C1 are determinants of well-being, neurocognitive functioning and preference for replacement therapy with a combination of LT4 and liothyronine (LT3). Design and participants: We studied 141 patients with primary autoimmune hypothyroidism, adequately treated with LT4 monotherapy and participating in a randomized clinical trial comparing LT4 therapy with LT4-LT3 combination therapy. Outcome measurements: Different questionnaires on well-being and neurocognitive tests were performed at baseline. Serum thyroid parameters, OATP1C1-intron3C > T, OATP1C1-Pro143Thr and OATP1C1-C3035T polymorphisms were determined. Results: Allele frequencies of the OATP1C1 polymorphisms in patients with primary hypothyroidism were similar to those of healthy controls. Both the OATP1C1-intron3C > T and the OATP1C1-C3035T polymorphism, but not the OATP1C1-Pro143Thr polymorphism, were associated with symptoms of fatigue and depression. OATP1C1 polymorphisms were not associated with measures of neurocognitive functioning or preference for combined LT4-LT3 therapy. Conclusions: OATP1C1 polymorphisms are associated with fatigue and depression, but do not explain differences in neurocognitive functioning or appreciation of LT4-LT3 combination therapy. Future studies are needed to confirm these findings.

Type 3 deiodinase is highly expressed in infiltrating neutrophilic granulocytes in response to acute bacterial infection (Article)

2008-10-01T00:00:00Z

Background: Macrophages and polymorphonuclear cells (PMNs) play an important role in the first line of defense against bacteria by infiltrating the infected organ in order to clear the harmful pathogen. Our earlier studies showed that granulocytes express type 3 deiodinase (D3) when activated during a turpentine-induced abscess. We hypothesized that D3 expression by granulocytes may also occur during bacterial infection. Methods: In order to test this hypothesis, we used the following experimental infection models: peritonitis induced by Escherichia coli and acute pneumonia induced by Streptococcus pneumoniae. Results: E. coli-induced peritonitis was characterized by infiltration in the liver by inflammatory cells with abundant immunocytochemical D3 expression while no staining was present in hepatocytes of infected or control mice. Acute pneumonia induced by S. pneumoniae resulted in inflamed lungs characterized by numerous infiltrating granulocytes expressing D3 while no D3 staining was present in lung sections without an infiltrate. Serum thyroid hormones were negatively correlated to bacterial outgrowth in both lung and spleen, and thus to the severity of illness. Conclusion: Infiltrating granulocytes during acute bacterial infection express D3. Our work supports the hypothesis that D3 plays an important role during chemical and bacterial inflammation. Whether the resulting decreased local bioavailability of thyroid hormones or rather the increased local availability of iodide is an important element of the innate immune response remains to be studied.

Thyroid hormone signaling in the hypothalamus (Article)

2008-10-01T00:00:00Z

Purpose of review Proper thyroid hormone signaling is essential for brain development and adult brain function. Signaling can be disrupted at many levels due to altered thyroid hormone secretion, conversion or thyroid hormone receptor binding. Recent findings Mutated genes involved in thyroid hormone signaling in patients and animal models have increased the understanding of the (patho-)physiological consequences of altered thyroid hormone signaling. Neuroanatomical studies have provided more insight in the underlying neuroanatomical pathways. Summary A number of thyroid hormone signaling pathways in the hypothalamus have been proposed, which may be involved in the adaptation of the thyroid axis, not only to hypoand hyperthyroidism, but also to inflammation, critical illness and fasting. Studies in knockout and transgenic mouse models have shown that the individual characteristics of mutations in thyroid hormone receptors can cause striking differences in the observed phenotypes.

Identification and consequences of polymorphisms in the thyroid hormone receptor alpha and beta genes (Article)

2008-10-01T00:00:00Z

Objective: Genetic factors exert considerable influence on thyroid function variables. Single nucleotide polymorphisms (SNPs) in thyroid hormone pathway genes have been associated with serum thyroid parameters implying small alterations in the hypothalamus-pituitary-thyroid axis. However, little is known about SNPs in the THRA (17q11.2) and THRB (3p24.2) genes. The aim of this study was to map THRA and THRB for the occurrence and frequencies of SNPs and relate these to thyroid parameters. Design and Methods: SNPs were identified by sequencing all THRA and THRB exons and flanking regions in 52 randomly selected subjects. SNPs were genotyped in 1116 healthy Danish twins by TaqMan assays and related to thyroid parameters. One SNP in THRB was additionally genotyped in the elderly population of the Rotterdam Scan Study (n = 940). Main Outcome: 15 SNPs (7 novel) in THRA and THRB were identified. Two SNPs in the 3′ untranslated region of THRA were genotyped: a novel SNP (2390A/G) and 1895C/A (rs12939700). In THRB, a synonymous (735C/T; rs3752874) and an intronic SNP (in9-G/A; rs13063628) were genotyped. No associations between SNPs and thyroid hormone levels (total and free 3,3′,5-triiodo-L-thyronine [T3] and thyroxine, reverse T3) were found. THRB-in9-G/A was significantly associated with higher serum thyroid stimulating hormone (TSH) (plnTSH = 0.01) in the Danish twins, but not in subjects of the Rotterdam Scan Study, although it showed a similar trend. Conclusions: Analysis of the T3 receptor genes revealed 15 SNPs, including 7 novel. Only THRB-in9-G/A was associated with higher serum TSH in a large population of Danish twins.

Thyroid hormone transport and metabolism by organic anion transporter 1C1 and consequences of genetic variation (Article)

2008-10-01T00:00:00Z

Organic anion transporting polypeptide (OATP) 1C1 has been characterized as a specific thyroid hormone transporter. Based on its expression in capillaries in different brain regions, OATP1C1 is thought to play a key role in transporting thyroid hormone across the blood-brain barrier. For this reason, we studied the specificity of iodothyronine transport by OATP1C1 in detail by analysis of thyroid hormone uptake in OATP1C1-transfected COS1 cells. Furthermore, we examined whether OATP1C1 is rate limiting in subsequent thyroid hormone metabolism in cells cotransfected with deiodinases. We also studied the effect of genetic variation in the OATP1C1 gene: polymorphisms were determined in 155 blood donors and 1192 Danish twins and related to serum thyroid hormone levels. In vitro effects of the polymorphisms were analyzed in cells transfected with the variants. Cells transfected with OATP1C1 showed increased transport of T4 and T4 sulfate (T4S), little transport of rT 3, and no transport of T3 or T3 sulphate, compared with mock transfected cells. Metabolism of T4, T4S, and rT3 by cotransfected deiodinases was greatly augmented in the presence of OATP1C1. The OATP1C1-intron3C>T, Pro143Thr, and C3035T polymorphisms were not consistently associated with thyroid hormone levels, nor did they affect transport function in vitro. In conclusion, OATP1C1 mediates transport of T4, T4S, and rT3 and increases the access of these substrates to the intracellular active sites of the deiodinases. No effect of genetic variation on the function of OATP1C1 was observed.

Organic anion transporter 1B1: An important factor in hepatic thyroid hormone and estrogen transport and metabolism (Article)

2008-09-01T00:00:00Z

Sulfation is an important pathway in the metabolism of thyroid hormone and estrogens. Sulfation of estrogens is reversible by estrogen sulfatase, but sulfation of thyroid hormone accelerates its degradation by the type 1 deiodinase in liver. Organic anion transporters (OATPs) are capable of transporting iodothyronine sulfates such as T4 sulfate (T 4S), T3S, and rT3S or estrogen sulfates like estrone sulfate (E1S), but the major hepatic transporter for these conjugates has not been identified. A possible candidate is OATP1B1 because model substrates for this transporter include the bilirubin mimic bromosulfophthalein (BSP) and E1S, and it is highly and specifically expressed in liver. Therefore, OATP1B1-transfected COS1 cells were studied by analysis of BSP, E1S, and iodothyronine sulfate uptake and metabolism. Two Caucasian populations (155 blood donors and 1012 participants of the Rotterdam Scan Study) were genotyped for the OATP1B1-Val174Ala polymorphism and associated with bilirubin, E1S, and T4S levels. OATP1B1-transfected cells strongly induced uptake of BSP, E1S, T4S, T3S, and rT3S compared with mocktransfected cells. Metabolism of iodothyronine sulfates by cotransfected type 1 deiodinase was greatly augmented in the presence of OATP1B1. OATP1B1-Val174 showed a 40% higher induction of transport and metabolism of these substrates than OATP1B1-Ala174. Carriers of the OATP1B1-Ala174 allele had higher serum bilirubin, E1S, and T 4S levels. In conclusion, OATP1B1 is an important factor in hepatic transport and metabolism of bilirubin, E1S, and iodothyronine sulfates. OATP1B1-Ala174 displays decreased transport activity and thereby gives rise to higher bilirubin, E1S, and T4S levels in carriers of this polymorphism.

A common variation in deiodinase 1 gene DIO1 is associated with the relative levels of free thyroxine and triiodothyronine (Article)

2008-08-01T00:00:00Z

Introduction: Genetic factors influence circulating thyroid hormone levels, but the common gene variants involved have not been conclusively identified. The genes encoding the iodothyronine deiodinases are good candidates because they alter the balance of thyroid hormones. We aimed to thoroughly examine the role of common variation across the three deiodinase genes in relation to thyroid hormones. Methods: We used HapMap data to select single-nucleotide polymorphisms (SNPs) that captured a large proportion of the common genetic variation across the three deiodinase genes. We analyzed these initially in a cohort of 552 people on T4replacement. Suggestive findingsweretakenforwardinto three additional studies in people not on T4(total n = 2513) and metaanalyzed for confirmation. Results: A SNP in the DIO1 gene, rs2235544, was associated with the free T3to free T4ratio with genome-wide levels of significance (P = 3.6 × 10-13). The C-allele of this SNP was associated with increased deiodinase 1 (D1) function with resulting increase in free T3/T4ratio and free T3and decrease in free T4and rT3. There was no effect on serum TSH levels. None of the SNPs in the genes coding for D2 or D3 had any influence on hormone levels. Conclusions: This study provides convincing evidence that common genetic variation in DIO1 alters deiodinase function, resulting in an alteration in the balance of circulating free T3to free T4. This should prove a valuable tool to assess the relative effects of circulating free T3vs. free T4on a wide range of biological parameters. Copyright

Effects of substitution and high-dose thyroid hormone therapy on deiodination, sulfoconjugation, and tissue thyroid hormone levels in prolonged critically ill rabbits (Article)

2008-08-01T00:00:00Z

To delineate the metabolic fate of thyroid hormone in prolonged critically ill rabbits, we investigated the impact of two dose regimes of thyroid hormone on plasma 3,3′-diiodothyronine (T2) and T4S, deiodinase type 1 (D1) and D3 activity, and tissue iodothyronine levels in liver and kidney, as compared with saline and TRH. D2-expressing tissues were ignored. The regimens comprised either substitution dose or a 3- to 5- fold higher dose of T4and T3, either alone or combined, targeted to achieve plasma thyroid hormone levels obtained by TRH. Compared with healthy animals, saline-treated ill rabbits revealed lower plasma T3(P = 0.006), hepatic T3(P = 0.02), and hepatic D1 activity (P = 0.01). Substitution-dosed thyroid hormone therapy did not affect these changes except a further decline in plasma (P = 0.0006) and tissue T4(P = 0.04). High-dosed thyroid hormone therapy elevated plasma and tissue iodothyronine levels and hepatic D1 activity, as did TRH. Changes in iodothyronine tissue levels mimicked changes in plasma. Tissue T3and tissue T3/reverse T3ratio correlated with deiodinase activities. Neither substitution- nor high-dose treatment altered plasma T2. Plasma T4S was increased only by T4in high dose. We conclude that in prolonged critically ill rabbits, low plasma T3levels were associated with low liver and kidney T3levels. Restoration of plasma and liver and kidney tissue iodothyronine levels was not achieved by thyroid hormone in substitution dose but instead required severalfold this dose. This indicates thyroid hormone hypermetabolism, which in this model of critical illness is not entirely explained by deiodination or by sulfoconjugation. Copyright

Thyroid function in short children born small-for-gestational age (SGA) before and during GH treatment (Article)

2008-08-01T00:00:00Z

Context: Disturbances in thyroid function have been described in small-for-gestational age (SGA) children but the influence of prematurity is unclear. In addition, the effect of GH treatment on thyroid function has not been studied in short SGA children. Objectives: To determine whether short SGA children have higher TSH levels compared to age-matched controls and evaluate the influence of gestational age. To investigate whether GH treatment alters thyroid function. Patients: A total of 264 short SGA children (116 preterm), prepubertal and non-GH deficient. Measurements: Serum FT4 and TSH at baseline and after 6, 12 and 24 months of GH treatment. Results: Baseline mean TSH was higher in preterm short SGA children than in age-matched controls (P < 0.05). Mean FT4 was not significantly different between short SGA children and controls. Baseline FT4 or TSH did not correlate with gestational age, or SDS for birth weight, birth length, height, body mass index, IGF-I or IGFBP-3. Mean FT4 decreased significantly during the first 6 months of GH treatment, but remained within the normal range. TSH did not change during treatment. The change in FT4 did not correlate with the change in height SDS during 24 months of GH treatment. Conclusion: Preterm short SGA children have higher, although within the normal range, TSH levels than controls. The level of TSH does not correlate with gestational age, birth weight SDS or birth length SDS. FT4 decreases during GH treatment, but is neither associated with an increase in TSH nor does it affect the response to GH treatment. As these mild alterations in thyroid function do not appear clinically relevant, frequent monitoring of thyroid function during GH therapy is not warranted in short SGA children.

High free thyroxine levels are associated with QTc prolongation in males (Article)

2008-07-01T00:00:00Z

The literature on the effect of excess thyroid hormone on ventricular repolarization is controversial. To study whether free thyroxine (T4) and TSH are associated with QTc prolongation we conducted population-based cohort study. This study was conducted as part of the Rotterdam Study and included 365 men and 574 women aged 55 years and older with an electrocardiogram, who were randomly sampled for the assessment of thyroid status (free T4/TSH) at baseline, after exclusion of participants with hypothyroidism, use of antithyroid drugs, thyroid hormones or digoxin, left ventricular hypertrophy, and left and right bundle branch block. Endpoints were the length of the QTc interval and risk of borderline QTc prolongation. The associations were examined by means of linear and logistic regression analysis, adjusted for age and gender, diabetes mellitus, myocardial infarction, hypertension, and heart failure. Overall, there was no significant association between TSH and QTc interval (0.8 ms (95% confidence interval (Cl) -3.5, 5.2) in the first quintile compared with the fifth quintile). Subjects in the fifth quintile of free T4did not have an increased QTc interval (3.2 ms (95% Cl - 1.1, 7.6)); stratification on gender showed an increment of 10.9 ms (95% Cl 3.4, 18.3) in the fifth quintile in men and 1.1 ms (95% Cl - 4.2, 6.3) in the fifth quintile of free T4in women. When compared with subjects in the first quintile, male subjects in the fifth quintile of free T4had a significantly increased risk of a borderline QTc interval and QTc prolongation (odds ratio 2.40 (95% Cl 1.20, 4.80)). High levels of free T4are associated with substantial QTc prolongation in men of up to 10 ms. The fact that free T4is also associated with a significantly increased risk of borderline and prolonged QTc values with its risk of sudden cardiac death, endorses the clinical importance of our findings.

Thyroid hormone independent associations between serum TSH levels and indicators of bone turnover in cured patients with differentiated thyroid carcinoma (Article)

2008-07-01T00:00:00Z

Objective: It has been proposed that TSH has thyroid hormone-independent effects on bone mineral density (BMD) and bone metabolism. This concept is still controversial and has not been studied in human subjects in detail. We addressed this question by studying relationships between serum TSH concentration and indicators of bone turnover, after controlling for triiodothyronine (T3), free thyroxine (FT4), and non-thyroid factors relevant to BMD and bone metabolism. We also studied the contribution of the TSH receptor (TSHR)-Asp727Glu polymorphism to these relationships. Design: We performed a cross-sectional study with 148 patients, who had been thyroidectomized for differentiated thyroid carcinoma. Methods: We measured BMD of the femoral neck and lumbar spine. FT4, T3, TSH, bone-specific alkaline phosphatase, procollagen type 1 aminoterminal propeptide levels, C-cross-linking terminal telopeptide of type I collagen, and urinary N-telopeptide of collagen cross-links were measured. Genotypes of the TSHR-Asp727Glu polymorphism were determined by Taqman assay. Results: We found a significant, inverse correlation between serum TSH levels and indicators of bone turnover, which was independent of serum FT4and T3levels as well as other parameters influencing bone metabolism. We found that carriers of the TSHR-Asp727Glu polymorphism had an 8.1% higher femoral neck BMD, which was, however, no longer significant after adjusting for body mass index. Conclusion: We conclude that in this group of patients, serum TSH was related to indicators of bone remodeling independently of thyroid hormone levels. This may point to a functional role of the TSHR in bone in humans. Further research into this mechanism needs to be performed.

Identification of DIO2 as a new susceptibility locus for symptomatic osteoarthritis (Article)

2008-06-15T00:00:00Z

Osteoarthritis [MIM 165720] is a common late-onset articular joint disease for which no pharmaceutical intervention is available to attenuate the cartilage degeneration. To identify a new osteoarthritis susceptibility locus, a genome-wide linkage scan and combined linkage association analysis were applied to 179 affected siblings and four trios with generalized osteoarthritis (The GARP study). We tested, for confirmation by association, 1478 subjects who required joint replacement and 734 controls in a UK population. Additional replication was tested in 1582 population-based females from the Rotterdam study that contained 94 cases with defined hip osteoarthritis and in 267 Japanese females with symptomatic hip osteoarthritis and 465 controls. Suggested evidence for linkage in the GARP study was observed on chromosome 14q32.11 (log of odds = 3.03, P = 1.9 × 10-4). Genotyping tagging single-nucleotide polymorphisms covering three important candidate genes revealed a common coding variant (rs225014; Thr92Ala) in the iodothyronine-deiodinase enzyme type 2 (D2) gene (DIO2 [MIM 601413]) which significantly explained the linkage signal (P = 0.006). Confirmation and replication by association in the additional osteoarthritis studies indicated a common DIO2 haplotype, exclusively containing the minor allele of rs225014 and common allele of rs12885300, with a combined recessive odds ratio of 1.79, 95% confidence interval (CI) 1.37-2.34 with P = 2.02 × 10-5in female cases with advanced/symptomatic hip osteoarthritis. The gene product of this DIO2 converts intracellular pro-hormone-3,3′,5,5′-tetraiodothyronine (T4) into the active thyroid hormone 3,3′,5-triiodothyronine (T3) thereby regulating intracellular levels of active T3 in target tissues such as the growth plate. Our results indicate a new susceptibility gene (DIO2) conferring risk to osteoarthritis.

A 28-day oral dose toxicity study in Wistar rats enhanced to detect endocrine effects of decabromodiphenyl ether (decaBDE) (Article)

2008-06-10T00:00:00Z

Decabromodiphenyl ether (decaBDE) is a widely used brominated flame retardant, considered to be of low toxicity. However, previous toxicity studies applied exposure methods with low bioavailability of this compound, and the actual hazard of decaBDE for humans, which are environmentally exposed to decaBDE, may thus be underestimated in current risk assessments. The present 28 days oral toxicity study in Wistar rats was designed to facilitate detection of endocrine and immune modulating effects of decaBDE using an exposure protocol with improved bioavailability. A technical preparation of high purity decaBDE was thus tested by daily exposure through gavage with an emulsion of soy phospholipon/lutrol as a carrier. Most sensitive effect in males were increased weight of seminal vesicle/coagulation gland with BMDL of 0.2 mg/kg bw/day and increased expression of hepatic CYP1A and CYP2B (BMDLs 0.5-0.7 mg/kg bw/day). In females the most sensitive effect was decreased activity of P450c17 (CYP17), which is a key enzyme in the androgen synthesis pathway, in adrenals (BMDL 0.18 mg/kg bw/day). These results suggest that decaBDE may represent an as yet unreported hazard for reproductive health.

Beneficial effects of propylthiouracil plus L-thyroxine treatment in a patient with a mutation in MCT8 (Article)

2008-06-01T00:00:00Z

Context: Mutations of the monocarboxylate transporter 8 (MCT8) gene determine a distinct X-linked phenotype of severe psychomotor retardation and consistently elevated T3levels. Lack of MCT8 transport of T3in neurons could explain the neurological phenotype. Objective: Our objective was to determine whether the high T3levels could also contribute to some critical features observed in these patients. Results: A 16-yr-old boy with severe psychomotor retardation and hypotonia was hospitalized for malnutrition (body weight = 25 kg) and delayed puberty. He had tachycardia (104 beats/min), high SHBG level (261 nmol/liter), and elevated serum free T3(FT3) level (11.3 pmol/liter), without FT4and TSH abnormalities. A missense mutation of the MCT8 gene was present. Oral overfeeding was unsuccessful. The therapeutic effect of propylthiouracil (PTU) and then PTU plus levothyroxine (LT4) wastested. After PTU (200 mg/d), serum FT4was undetectable, FT3was reduced (3.1 pmol/liter) with high TSH levels (50.1 mU/liter). Serum SHBG levels were reduced (72 nmol/liter). While PTU prescription was continued, high LT4doses (100 μg/d) were needed to normalize serum TSH levels (3.18 mU/liter). At that time, serum FT4was normal (16.4 pmol/liter), and FT3was slightly high (6.6 pmol/liter). Tachycardia was abated (84 beats/min), weight gain was 3 kg in 1 yr, and SHBG was 102 nmol/liter. Conclusions: 1) When thyroid hormone production was reduced by PTU, high doses of LT4(3.7 μg/kg·d) were needed to normalize serum TSH, confirming that mutation of MCT8 is a cause of resistance to thyroid hormone. 2) High T3levels might exhibit some deleterious effects on adipose, hepatic, and cardiac levels. 3) PTU plus LT4could be an effective therapy to reduce general adverse features, unfortunately without benefit on the psychomotor retardation. Copyright

Age-related changes in renal and hepatic cellular mechanisms associated with variations in rat serum thyroid hormone levels (Article)

2008-06-01T00:00:00Z

Aging is associated with changes in thyroid gland physiology. Age-related changes in the contribution of peripheral tissues to thyroid hormone serum levels have yet to be systematically assessed. Here, we investigated age-related alterations in the contributions of the liver and kidney to thyroid hormone homeostasis using 6-, 12-, and 24-mo-old male Wistar rats. A significant and progressive decline in plasma thyroxine occurred with age, but triiodothyronine (T3) was decreased only at 24 mo. This was associated with an unchanged protein level of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the kidney and with a decreased MCT8 level in the liver at 24 mo. Hepatic type I deiodinase (D1) protein level and activity declined progressively with age. Renal D1 levels were decreased at both 12 and 24 mo but D1 activity was decreased only at 24 mo. In the liver, no changes occurred in thyroid hormone receptor (TR) TRα1, whereas a progressive increase in TRβ1occurred at both mRNA and total protein levels. In the kidney, both TRα1and TRβ1mRNA and total protein levels were unchanged between 6 and 12 mo but increased at 24 mo. Interestingly, nuclear TRβ1 levels were decreased in both liver and kidney at 12 and 24 mo, whereas nuclear TRα1levels were unchanged. Collectively, our data show differential age-related changes among hepatic and renal MCT8 and D1 and TR expressions, and they suggest that renal D1 activity is maintained with age to compensate for the decrease in hepatic T3production. Copyright

Genotype-phenotype relationship in patients with mutations in thyroid hormone transporter MCT8 (Article)

2008-05-01T00:00:00Z

Loss-of-function mutations in thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to severe X-linked psychomotor retardation and elevated serum T3levels. Most patients, for example those with mutations V235M, S448X, insI189, or delF230, cannot stand, walk, or speak. Patients with mutations L434W, L568P, and S194F, however, walk independently and/or develop some dysarthric speech. To study the relationship between mutation and phenotype, we transfected JEG3 and COS1 cells with wild-type or mutant MCT8. Expression and function of the transporter were studied by analyzing T3and T4uptake, T3metabolism (by cotransfected type 3 deiodinase), Western blotting, affinity labeling with N-bromoacetyl-T3, immunocytochemistry, and quantitative RT-PCR. Wild-type MCT8 increased T3uptake and metabolism about 5-fold compared with empty vector controls. Mutants V235M, S448X, insI189, and delF230 did not significantly increase transport. However, S194F, L568P, and L434W showed about 20, 23, and 37% of wild-type activity.RT-PCR did not show significant differences in mRNA expression between wild-type and mutant MCT8. Immunocytochemistry detected the nonfunctional mutants V235M, insI189, and delF230 mostly in the cytoplasm, whereas mutants with residual function were expressed at the plasma membrane. Mutants S194F and L434W showed high protein expression but low affinity for N-bromoacetyl-T3; L568P was detected in low amounts but showed relatively high affinity. Mutations in MCT8 cause loss of function through reduced protein expression, impaired trafficking to the plasma membrane, or reduced substrate affinity. Mutants L434W, L568P, and S194F showed significant residual transport capacity, which may underlie the more advanced psychomotor development observed in patients with these mutations. Copyright

Identification of molecular mechanisms related to nonthyroidal illness syndrome in skeletal muscle and adipose tissue from patients with septic shock (Article)

2008-05-01T00:00:00Z

Objective: Septic shock is one of various causes of nonthyroidal illness syndrome (NTIS). In humans, the molecular mechanisms involved in NTIS are mostly unknown. The aim of this study was to investigate, in patients with NTIS secondary to septic shock, changes in the expression of genes involved in the actions of thyroid hormones and in the activity of deiodinase enzymes, in two tissues important for protein and energy metabolism, skeletal muscle (SM) and subcutaneous adipose tissue (SAT). Design: Hospitalized patients were divided into a control and a septic shock NTIS group. Measurement: Serum collection for biochemical measurements, and SM and SAT biopsies for mRNA expression analysis of thyroid hormone receptors (THRB1, THRA1), retinoid X receptors (RXRA, RXRB, RXRG), nuclear receptor corepressor (NCOR1), silencing mediator of retinoid and thyroid hormone receptor (SMRT), steroid receptor coactivator (SRC1), type 1 and 2 deiodinases (D1, D2), monocarboxylate transporter 8 (MCT8), SECIS binding protein 2 (SBP2) and uncoupling protein 3 (UCP3) as well as D1, D2 and D3 enzyme activity measurements. Results: The NTIS group had lower serum TSH, and free T3 and higher rT3 than controls. D1 and D3 were detected in SAT, with no differences found between the two groups; SM had very low D2 activity and again no differences were found between groups; D3 activity in SM was higher in NTIS than controls. SM expression of THRB1, RXRG and D2 was lower and RXRA higher in NTIS than controls. SAT from NTIS patients had lower MCT8, THRB1, THRA1, RXRG and SMRT, and higher UCP3 expression than controls. Conclusions: In patients with septic shock NTIS tissue responses are orientated to decrease production and increase degradation (muscle) or decrease uptake (adipose tissue) of T3, as well as to decrease thyroid hormone actions.

Mutations in the iodotyrosine deiodinase gene and hypothyroidism (Article)

2008-04-24T00:00:00Z

DEHAL1 has been identified as the gene encoding iodotyrosine deiodinase in the thyroid, where it controls the reuse of iodide for thyroid hormone synthesis. We screened patients with hypothyroidism who had features suggestive of an iodotyrosine deiodinase defect for mutations in DEHAL1. Two missense mutations and a deletion of three base pairs were identified in four patients from three unrelated families; all the patients had a dramatic reduction of in vitro activity of iodotyrosine deiodinase. Patients had severe goitrous hypothyroidism, which was evident in infancy and childhood. Two patients had cognitive deficits due to late diagnosis and treatment. Thus, mutations in DEHAL1 led to a deficiency in iodotyrosine deiodinase in these patients. Because infants with DEHAL1 defects may have normal thyroid function at birth, they may be missed by neonatal screening programs for congenital hypothyroidism. Copyright

A 28-day oral dose toxicity study enhanced to detect endocrine effects of a purified technical pentabromodiphenyl ether (pentaBDE) mixture in Wistar rats (Article)

2008-03-12T00:00:00Z

A 28-day subacute oral toxicity study was performed in Wistar rats with a purified preparation of the commercial pentabromodiphenyl ether (pentaBDE), DE-71. The applied OECD407 protocol was enhanced for endocrine and immune parameters, and to enable benchmark dose analysis. A vehicle control group and 7 dose groups were included, which received 0.27, 0.82, 2.47, 7.4, 22.2, 66.7 or 200 mg pentaBDE/kg bw/d (mkd). The liver appeared to be a key target organ, showing a marked increase of weight and centrilobular hepatocellular hypertrophy, probably due to the observed induction of P450 enzymes, notably CYP1A and CYP2B. A marked decrease of circulating total thyroxine (TT4) and an increase of plasma cholesterol were probably secondary to the liver effects. Furthermore, dose-dependently decreased weight of epididymis, seminal vesicles, and prostate, as well as sperm head deformities in males, and induction of CYP17 activity in adrenals in females were observed, all possibly related to anti-androgenic activity. Finally, we observed a substantial increase of large unstained cells in the blood and a decrease of apolar retinoids in the liver. All these effects had benchmark doses at the lower confidence bound (BMDL) in the low- or mid-dose range, but particular sensitive, potentially adverse effects were TT4 decrease (BMDLs 1.1 in males and 1.8 mkd in females), and decrease of hepatic apolar retinoids (BMDLs 0.5 mkd in males and 2.3 mkd in females). These results contribute to refinement of the hazard identification of pentaBDE and improved risk assessment of human exposure to this industrial chemical and environmental pollutant.

Endocrine effects of tetrabromobisphenol-A (TBBPA) in Wistar rats as tested in a one-generation reproduction study and a subacute toxicity study (Article)

2008-03-12T00:00:00Z

Endocrine effects of the brominated flame retardant tetrabromobisphenol-A (TBBPA) were studied in a one-generation reproduction assay in Wistar rats via repeated dietary exposure, applying eight dose groups at 0-3-10-30-100-300-1000-3000 mg/kg body weight/day (mkd). This design enables dose-response analysis and calculation of benchmark doses (BMDL). This reproduction study was preceded by a 28-day repeat dose subacute toxicity study, at 0-30-100-300 mkd. Major effects in the reproduction study included decreased circulating thyroxine (T4) with BMDLs of 31 (m) and 16 (f) mkd, and increased weight of testis and male pituitary (BMDLs of 0.5 and 0.6 mkd). The hypothyroxinemia correlated to a cluster of developmental parameters including delayed sexual development in females, decreased pup mortality, and effects on brainstem auditory evoked potentials [Lilienthal, H., Verwer, C.M., Van der Ven, L.T.M., Piersma, A.H., Vos, J.G., 2008. Neurobehavioral effects of tetrabromobisphenol A (TBBPA) in rats after pre- and postnatal exposure. Toxicology]. A second cluster of parameters in F1 animals was correlated to increased testis weight, and included female gonad weight, endometrium height, CYP19/aromatase activity in the ovary, and plasma testosterone levels in males. These two correlation clusters suggest a dual action of TBBPA. The only effects in the subacute study were decreased circulating T4 and increased T3 levels in males (BMDLs 48 and 124 mkd), and non-significant trends for these parameters in females, suggesting that the other effects in the reproduction study were induced during development. Combined with data of human exposure to environmental TBBPA, the margin of exposure for highly exposed populations can be calculated at 2.6, and current use of TBBPA may therefore be a matter of concern for human health.

Hypoxia-inducible factor induces local thyroid hormone inactivation during hypoxic-ischemic disease in rats (Article)

2008-03-01T00:00:00Z

Thyroid hormone is a critical determinant of cellular metabolism and differentiation. Precise tissue-specific regulation of the active ligand 3,5,3′-triiodothyronine (T3) is achieved by the sequential removal of iodine groups from the thyroid hormone molecule, with type 3 deiodinase (D3) comprising the major inactivating pathway that terminates the action of T3 and prevents activation of the prohormone thyroxine. Using cells endogenously expressing D3, we found that hypoxia induced expression of the D3 gene DIO3 by a hypoxiainducible factor-dependent (HIF-dependent) pathway. D3 activity and mRNA were increased both by hypoxia and by hypoxia mimetics that increase HIF-1. Using ChIP, we found that HIF-1α interacted specifically with the DIO3 promoter, indicating that DIO3 may be a direct transcriptional target of HIF-1. Endogenous D3 activity decreased T3-dependent oxygen consumption in both neuronal and hepatocyte cell lines, suggesting that hypoxia-induced D3 may reduce metabolic rate in hypoxic tissues. Using a rat model of cardiac failure due to RV hypertrophy, we found that HIF-1α and D3 proteins were induced specifically in the hypertrophic myocardium of the RV, creating an anatomically specific reduction in local T3 content and action. These results suggest a mechanism of metabolic regulation during hypoxic-ischemic injury in which HIF-1 reduces local thyroid hormone signaling through induction of D3.

Effects of serum TSH and FT4 levels and the TSHR-Asp727Glu polymorphism on bone: The Rotterdam Study (Article)

2008-02-01T00:00:00Z

Background: TSH and thyroid hormone may have independent effects on bone. In this study we investigated the association of TSH and free T4 (FT4) with different bone parameters in human subjects. TSH and FT4 are known to be associated with body mass index (BMI) and a higher BMI gives a higher bone mineral density (BMD). Thus, we aimed to determine whether the effects of TSH and FT4 on bone are mediated by BMI. As TSH exerts its biological effect through the TSH receptor (TSHR), the TSHR gene might be a candidate gene affecting bone mass. The TSHR-Asp727Glu polymorphism is associated with lower TSH levels. We therefore examined the association of this polymorphism with bone parameters. Method: Genotypes were determined by Taqman assay in 4934 elderly Caucasian men and women of the Rotterdam Study, of whom BMD and bone geometry data were available. Serum thyroid parameters were available in a random set of 1327 subjects,. Results: Femoral neck BMD as well as narrow neck BMD and cortical thickness increased with serum TSH. However, FT4 was more strongly and negatively associated with bone parameters. Regression models showed BMI-dependent and -independent effects of both TSH and FT4 on bone. Carriers of the TSHR-Glu727allele had a 2.3% higher femoral neck BMD. Conclusion: In line with the effect of TSH on bone in mice, serum TSH shows a positive trend with BMD in human subjects, a finding that is strengthened by the association between the TSHR-Asp727Glu polymorphism and femoral neck BMD. However, serum FT4 has a much greater influence on bone than TSH.

Biotransformation of brominated flame retardants into potentially endocrine-disrupting metabolites, with special attention to 2,2′,4, 4′-tetrabromodiphenyl ether (BDE-47) (Article)

2008-02-01T00:00:00Z

In this study, the endocrine-disrupting (ED) potency of metabolites from brominated flame retardants (BFRs) was determined. Metabolites were obtained by incubating single-parent compound BFRs with phenobarbital-induced rat liver microsomes. Incubation extracts were tested in seven in vitro bioassays for their potency to compete with thyroxine for binding to transthyretin (TTR), to inhibit estradiol-sulfotransferase (E2SULT), to interact with thyroid hormone-mediated cell proliferation, and to (in-)activate the androgen, progesterone, estrogen, or aryl hydrocarbon receptor. For most BFRs, TTR-binding potencies, and to a lesser extent E2SULT-inhibiting potencies, significantly increased after biotransformation. Microsomal incubation had less pronounced effects on other ED modes of action, due to low biotransformation efficiency and background activities determined in control incubations without BFRs. Moreover, cell-based bioassays suffered from cytotoxicity from metabolites of lower-brominated polybrominated diphenyl ethers. For the environmentally relevant 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), six hydroxylated metabolites were identified. Individual metabolites had TTR-binding and E2SULT-inhibiting potencies 160-1600 and 2.2-220 times higher than BDE-47 itself, whereas their combined potencies in a realistic mixture were well predicted via concentration addition. In combination with other environmentally relevant hydroxylated organohalogens acting on TTR-binding and E2SULT inhibition, internal exposure to BFR metabolites may significantly contribute to the overall risk of endocrine disruption.

The metabolism and dechlorination of chlorotyrosine in vivo (Article)

2007-10-05T00:00:00Z

During inflammation, neutrophil- and monocyte-derived myeloperoxidase catalyzes the formation of hypochlorous acid, which can chlorinate tyrosine residues in proteins to form chlorotyrosine. However, little is known of the metabolism and disposition of chlorotyrosine in vivo. Following infusion of deuterium-labeled [D4]chlorotyrosine into Sprague-Dawley rats, the major urinary metabolites were identified by mass spectrometry. 3-Chloro-4-hydroxyphenylacetic acid was identified as the major chlorinated metabolite of chlorotyrosine and accounted for 3.6 ± 0.3% of infused [D4]chlorotyrosine. The striking observation was that ∼40% (39 ± 1%) of infused [D4]chlorotyrosine was dechlorinated and excreted in the urine as deuterated 4-hydroxyphenylacetic acid, a major metabolite of tyrosine. 1.1 ± 0.1% of infused [D4] chlorotyrosine was excreted as [D4]tyrosine. To determine whether protein-bound chlorotyrosine could undergo dechlorination, chlorinated albumin was incubated with liver homogenate from mutant rats, which did not synthesize albumin. There was ∼20% decrease in the chlorotyrosine content over 1 h. This study is the first to describe the dechlorination of chlorotyrosine as the major metabolic pathway to eliminate this modified amino acid in vivo.

Thyroid hormone levels in children with Prader-Willi syndrome before and during growth hormone treatment (Article)

2007-09-01T00:00:00Z

Background: Prader-Willi syndrome (PWS) is a neurogenetic disorder characterized by muscular hypotonia, psychomotor delay, obesity and short stature. Several endocrine abnormalities have been described, including GH deficiency and hypogonadotrophic hypogonadism. Published data on thyroid hormone levels in PWS children are very limited. Objective: To evaluate thyroid function in children with PWS, before and during GH treatment. Design/patients: At baseline, serum levels of T4, free T4 (fT4), T3, reverse T3 (rT3) and TSH were assessed in 75 PWS children. After 1 year, assessments were repeated in 57 of the them. These children participated in a randomized study with two groups: group A (n = 34) treated with 1 mg GH/m2/day and group B (n = 23) as controls. Results: Median age (interquartile range, IQR) of the total group at baseline was 4.7 (2.7-7.6) years. Median (IQR) TSH level was -0.1 SDS (-0.5 to 0.5), T4 level -0.6 SDS (-1.7 to 0.0) and fT4 level -0.8 SDS (-1.3 to -0.3), the latter two being significantly lower than 0 SDS. T3 level, at 0.3 SDS (-0.3 to 0.9), was significantly higher than 0 SDS. After 1 year of GH treatment, fT4 decreased significantly from -0.8 SDS (-1.5 to -0.2) to -1.4 SDS (-1.6 to -0.7), compared to no change in untreated PWS children. However, T3 did not change, at 0.3 SDS (-0.1 to 0.8). Conclusions: We found normal fT4 levels in most PWS children. During GH treatment, fT4 decreased significantly to low-normal levels. TSH levels remained normal. T3 levels were relatively high or normal, both before and during GH treatment, indicating that PWS children have increased T4 to T3 conversion.

Brief report: The type II iodothyronine deiodinase is up-regulated in skeletal muscle during prolonged critical illness (Article)

2007-08-01T00:00:00Z

Context: Critical illness is associated with the low T3syndrome. It remains unclear whether altered type II deiodinase activity (D2) in skeletal muscle contributes to this syndrome. Objective: Our objective was to study D2 expression and activity in skeletal muscle of acute and prolonged critically ill patients. Design and Setting: We conducted a clinical observational study in acute and prolonged critical illness with comparison with healthy controls at a university hospital surgical intensive care unit. Patients: Subjects included 63 prolonged critically ill patients who died in the intensive care unit, 21 acutely ill patients, and 38 controls matched for age, gender, and body mass index. Results: Elevated expression of the D2 gene and D2 activity in skeletal muscle of prolonged, but not acute, critically ill patients were observed in the face of low circulating thyroid hormone levels. Conclusions: Reduced D2 activity does not appear to play a role in the pathogenesis of the low T3syndrome of critical illness. Copyright

Hypothalamic thyroid hormone catabolism acts as a gatekeeper for the seasonal control of body weight and reproduction (Article)

2007-08-01T00:00:00Z

Seasonal adaptations in physiology exhibited by many animals involve an interface between biological timing and specific neuroendocrine systems, but the molecular basis of this interface is unknown. In this study of Siberian hamsters, we show that the availability of thyroid hormone within the hypothalamus is a key determinant of seasonal transitions. The expression of the gene encoding type III deiodinase (Dio3) and Dio3 activity in vivo (catabolism of T4and T3) is dynamically and temporally regulated by photoperiod, consistent with the loss of hypothalamic T3concentrations under short photoperiods. Chronic replacement of T3in the hypothalamus of male hamsters exposed to short photoperiods, thus bypassing synthetic or catabolic deiodinase enzymes located in cells of the ependyma of the third ventricle, prevented the onset of short-day physiology: hamsters maintained a long-day body weight phenotype and failed to undergo testicular and epididymal regression. However, pelage moult to a winter coat was not affected. Type II deiodinase gene expression was not regulated by photoperiod in these hamsters. Collectively, these data point to a pivotal role for hypothalamic DIO3 and T3catabolism in seasonal cycles of body weight and reproduction in mammals. Copyright

Bexarotene-induced hypothyroidism: Bexarotene stimulates the peripheral metabolism of thyroid hormones (Article)

2007-07-01T00:00:00Z

Objective: Therapy with the retinoid X receptor agonist bexarotene is associated with hypothyroidism caused by decreased pituitary TSH secretion. To evaluate the effects of bexarotene on peripheral thyroid hormone metabolism, we performed a study in athyreotic subjects on a fixed substitution dose with L-T4. Design: The design was an open prospective 6-wk intervention study. Methods: Ten athyreotic patients with pulmonary metastases of differentiated thyroid carcinoma received 6-wk redifferentiation treatment with 300 mg bexarotene/d. L-T4doses were kept stable. Before and in the sixth week of therapy, serum levels of total T4, free T4(FT4), T3, reverse T3(rT3), and TSH were measured. To study nondeiodinase-mediated thyroid hormone degradation, serum levels of T4sulfate (T4S) were measured. Recombinant human TSH was administered before and in the sixth week of bexarotene therapy. Results: Bexarotene induced profound decreases in total T4(56% of baseline), FT4(47%), T3(69%), rT3(51%), and T4S (70%) in all patients, whereas TSH levels were not affected. The T3/rT3ratio increased by 43%, and the T4S/FT4ratio increased by 48%. Serum TSH levels before and after recombinant human TSH were unaffected by bexarotene. Conclusions: In the present study, we demonstrate that increased peripheral degradation of thyroid hormones by a nondeiodinase-mediated pathway contributes to bexarotene induced-hypothyroidism. Copyright

Functional analysis of monocarboxylate transporter 8 mutations identified in patients with X-linked psychomotor retardation and elevated serum triiodothyronine (Article)

2007-06-01T00:00:00Z

Context: T3action in neurons is essential for brain development. Recent evidence indicates that monocarboxylate transporter 8 (MCT8) is important for neuronal T3uptake. Hemizygous mutations have been identified in the X-linked MCT8 gene in boys with severe psychomotor retardation and elevated serum T3levels. Objective: The objective of this study was to determine the functional consequences of MCT8 mutations regarding transport of T3. Design: MCT8 function was studied in wild-type or mutant MCT8-transfected JEG3 cells by analyzing: 1) T3uptake, 2) T3metabolism in cells cotransfected with human type 3 deiodinase, 3) immunoblotting, and 4) immunocytochemistry. Results: The mutations identified in MCT8 comprise four deletions (24.5 kb, 2.4 kb, 14 bp, and 3 bp), three missense mutations (Ala224Val, Arg271His, and Leu471Pro), a nonsense mutation (Arg245stop), and a splice site mutation (94 amino acid deletion). All tested mutants were inactive in uptake and metabolism assays, except MCT8 Arg271His, which showed approximately 20% activity vs. wild-type MCT8. Conclusion: These findings support the hypothesis that the severe psychomotor retardation and elevated serum T3levels in these patients are caused by inactivation of the MCT8 transporter, preventing action and metabolism of T3in central neurons. Copyright

The impact of a TSH receptor gene polymorphism on thyroid-related phenotypes in a healthy Danish twin population (Article)

2007-06-01T00:00:00Z

Objectives: The Asp727Glu polymorphism in the TSH receptor (TSHR) gene is associated with serum TSH levels. However, the proportion of genetic variation accounted for by this polymorphism is unknown. In this study, we (1) examined the association of the Asp727Glu polymorphism with thyroid size, serum levels of TSH, thyroid hormones, and thyroid antibodies in 1241 healthy Danish twin individuals and (2) assessed the contribution of the polymorphism to the trait variation and the genetic variance. Measurements: The effect of the genotype on the traits (mean ± SD) was established; associations between the TSHR-Asp727Glu polymorphism and measures of thyroid homeostasis were assessed and the effect of the polymorphism on the trait's phenotypic variability was quantified by incorporating the genotype information in structural equation modelling. Results: The genotype distribution was Asp/Asp 84.9%; Asp/Glu 14.5% and Glu/Glu 0.6%. Carriers of the TSHR-Glu727 allele had lower TSH levels (noncarriers vs. carriers: 1.78 ± 0.93 vs. 1.60 ± 0.84 mU/l, P = 0.04). Regression analysis showed an association between the TSHR-Asp727Glu polymorphism and serum TSH (P = 0.007). The polymorphism accounted for 0.91% of the total phenotypic variance in serum TSH levels. Including the genotype in quantitative genetic modelling improved the model fit (P = 0.001); however, the genetic influence on serum TSH not attributable to this specific genetic variant was only reduced from 68.2% to 67.8%. The polymorphism was not significantly associated with thyroid size, thyroid hormones or thyroid antibody levels. Conclusions: The TSHR-727Glu allele was associated with decreasing TSH levels; however, the contribution to the genetic variance was very small. No association was found with other thyroid-related measures.

The Asp727Glu polymorphism in the TSH receptor is associated with insulin resistance in healthy elderly men (Article)

2007-06-01T00:00:00Z

Background: Variations in thyroid function within the normal range are associated with differences in metabolism and body composition. For instance, TSH is positively associated with body mass index (BMI). This could be due to alterations in thyroid hormone activity, or to direct effects of TSH, as the TSH receptor (TSHR) is also expressed in adipose tissue. The TSHR-Asp727Glu polymorphism is associated with lower serum TSH levels in vivo. In this study, we analysed whether serum thyroid parameters and the TSHR-Asp727Glu polymorphism were associated with glucose metabolism and insulin resistance. In addition, we analysed the Thr92Ala polymorphism in the type 2 deiodinase (D2), which was recently associated with insulin resistance. Methods: Genotypes were determined in a population of 349 elderly men (age 77.7 ± 3.5 years), for whom serum thyroid parameters and data on insulin resistance, such as fasting blood glucose, serum insulin and homeostasis model assessment (HOMA) values, were available. Results: In nondiabetic, euthyroid subjects, TSH was positively associated with leptin levels, whereas FT4 and rT3 were significantly negatively correlated with insulin and HOMA. Carriers of the TSHR-Glu727allele had a significantly higher glucose (P = 0.01), insulin (P = 0.001), glycated haemoglobin (HbA1c) (P = 0.002), HOMA (P = 0.001) and leptin (P = 0.008). The D2-Ala92allele showed a trend towards higher levels of insulin (P = 0.07) and a higher HOMA (P = 0.09). Conclusion: In this population of nondiabetic elderly men, serum thyroid parameters and the TSHR-Asp727Glu polymorphism were associated with relative insulin resistance. Our study suggests that genetic variation in TSHR plays a role in insulin resistance and thereby influences glucose metabolism.

Genetic variation in thyroid hormone transporters (Article)

2007-06-01T00:00:00Z

Determination of the sequence of the human genome has led to a large expansion of research in this area. Several studies on single nucleotide polymorphisms, i.e. variations in the genome that occur in > 1% of the population, have been published in recent years. In the thyroid field, information about variation in relevant genes is also forthcoming, which is not surprising as these polymorphisms are thought to play a role in determining each individual's thyroid hormone set-point. So far, research has focused mainly on genetic variation in the thyroid-stimulation hormone receptor and the deiodinases, and their association with thyroid parameters and/or clinical endpoints, such as insulin resistance. However, with the characterization of specific thyroid hormone transporters, a new field of research is emerging.

Preface (Article)

2007-06-01T00:00:00Z

Abnormal thyroid hormone metabolism in mice lacking the monocarboxylate transporter 8 (Article)

2007-03-01T00:00:00Z

In humans, inactivating mutations in the gene of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8; SLC16A2) lead to severe forms of psychomotor retardation combined with imbalanced thyroid hormone serum levels. The MCT8-null mice described here, however, developed without overt deficits but also exhibited distorted 3,5,3′-triiodothyronine (T3) and thyroxine (T4) serum levels, resulting in increased hepatic activity of type 1 deiodinase (D1). In the mutants' brains, entry of T4 was not affected, but uptake of T3 was diminished. Moreover, the T4 and T3 content in the brain of MCT8-null mice was decreased, the activity of D2 was increased, and D3 activity was decreased, indicating the hypothyroid state of this tissue. In the CNS, analysis of T3 target genes revealed that in the mutants, the neuronal T3 uptake was impaired in an area-specific manner, with strongly elevated thyrotropin-releasing hormone transcript levels in the hypothalamic paraventricular nucleus and slightly decreased RC3 mRNA expression in striatal neurons; however, cerebellar Purkinje cells appeared unaffected, since they did not exhibit dendritic outgrowth defects and responded normally to T3 treatment in vitro. In conclusion, the circulating thyroid hormone levels of MCT8-null mice closely resemble those of humans with MCT8 mutations, yet in the mice, CNS development is only partially affected.

The association of polymorphisms in the type 1 and 2 deiodinase genes with circulating thyroid hormone parameters and atrophy of the medial temporal lobe (Article)

2007-02-01T00:00:00Z

Context: Thyroid function has been related to Alzheimer disease (AD) and neuroimaging markers thereof. Whether thyroid dysfunction contributes to or results from developing AD remains unclear. Variations in the deiodinase type 1 (DIO1) and type 2 (DIO2) genes that potentially alter thyroid hormone bioactivity may help in elucidating the role of thyroid function in AD. Objective: We investigated the association of recently identified polymorphisms in the DIO1 (D1a-C/T, D1b-A/G) and DIO2 (D2-ORFa-Gly3Asp, D2-Thr92Ala) genes with circulating thyroid parameters and early neuroimaging markers of AD. Design and Participants: The Rotterdam Scan Study is a population-based cohort study among 1,077 elderly individuals aged 60-90 yr. Main Outcome Measures: DIO1 and DIO2 polymorphisms and serum TSH, free T4, T3, and reverse T3(rT3) levels were determined in 995 nondemented elderly, including 473 persons with assessments of hippocampal and amygdalar volume on brain magnetic resonance imaging. Results: Carriers of the D1a-T allele had higher serum free T4and rT3, lower T3, and lower T3/rT3. The D1b-G allele was associated with higher serum T3and T3/rT3. The DIO2 variants were not associated with serum thyroid parameters. No associations were found with hippocampal or amygdalar volume. Conclusion: This is the first study to report an association of D1a-C/T and D1b-A/G polymorphisms with iodothyronine levels in the elderly. Polymorphisms in the DIO1 and DIO2 genes are not associated with early magnetic resonance imaging markers of AD. This suggests that the previously reported association between iodothyronine levels and brain atrophy reflects comorbidity or nonthyroidal illness rather than thyroid hormones being involved in developing AD. Copyright

Differential effects of maternal dexamethasone treatment on circulating thyroid hormone concentrations and tissue deiodinase activity in the pregnant ewe and fetus (Article)

2007-02-01T00:00:00Z

Clinically, treatment of pregnant women at risk of preterm delivery with synthetic glucocorticoids accelerates fetal maturation. This study investigated the effect of maternal dexamethasone treatment, in clinically relevant doses, on plasma thyroid hormone concentrations and tissue deiodinase activities (D1, D2, and D3) in ewes and their fetuses. From 125 d of gestation (term 145 ± 2 d), pregnant ewes were injected twice im with either saline (2 ml of 0.9% NaCl, n = 11) or dexamethasone (2 x 12 mg in 2 ml of saline, n = 10) at 24-h intervals. Maternal dexamethasone treatment increased plasma T3and reverse T3(rT3), but not T4, concentrations in the fetuses. In the dexamethasone-exposed fetuses, hepatic D1 activity was higher, and renal and placental D3 activities were lower, than in the saline-exposed fetuses. In the ewes, plasma concentrations of T3and T4were reduced, and rT3increased, by dexamethasone treatment without any change in tissue deiodinase activity. Therefore, maternal dexamethasone treatment has different effects on the thyroid hormone axis of the pregnant ewe and fetus. In the fetus, the dexamethasone-induced rise in circulating T3may be due to both increased hepatic production of T3from T4, and reduced clearance of T3by the kidney and placenta. Changes in T3bioavailability may mediate some of the maturational effects of antenatal glucocorticoid treatment in the preterm fetus. Copyright

Thyroid hormone receptor isoforni expression in livers of critically ill patients (Article)

2007-02-01T00:00:00Z

Objective: The THRA gene encodes two isoforms of the thyroid hormone receptor (TR), TRα1 and TRα2. The ratio of these splice variants could have a marked influence on T3-regulated gene expression, especially during illness. Design: We studied the expression of the isoforms TRβ1, TRα1, and TRα2 and 5′-deiodinase in postmortem liver biopsies of 58 patients who were critically ill and died in the intensive care unit (ICU). All mRNA levels were determined using real-time PCR. Main outcome: All ratios of the biopsies were higher than those found in three normal liver biopsies due to an increased TRα1 level. The TRα1/TRα2 ratio increased with age and severity of illness following the equation: TRα1/TRα2 ratio = - 1.854 + (0.0323×age) + (0.0431×Therapeutic Intervention Scoring System score) indicating that 28% of the changed TRα1/TRα2 ratio can be predicted by these clinical variables. There was no effect of randomization to intensive insulin therapy or glucocorticoid or thyroid hormone treatment on the TRα1/TRα2 ratio or TRβ1. Furthermore, no relation was seen between the expression levels of the 5′-deiodinase mRNA and TR isoforms or the triiodothyronine (T3) levels. Conclusion: It appears that in critically ill patients the ratio of TRα1/TRα2 expression increases with age and severity of illness, possibly indicating a mechanism to enhance sensitivity to T3in the oldest and sickest patients.

Long-term toxicity of [177Lu-DOTA0,Tyr 3]octreotate in rats (Article)

2007-02-01T00:00:00Z

Purpose and methods: Studies on peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues have shown promising results with regard to tumour control. The efficacy of PRRT is limited by uptake and retention in the proximal tubules of the kidney, which might lead to radiation nephropathy. We investigated the long-term renal toxicity after different doses of [177Lu-DOTA0,Tyr3]octreotate and the effects of dose fractionation and lysine co-injection in two tumour-bearing rat models. Results: Significant renal toxicity was detected beyond 100 days after start of treatment as shown by elevated serum creatinine and proteinuria. Microscopically, tubules were strongly dilated with flat epithelium, containing protein cylinders. Creatinine levels rose significantly after 555 MBq [177Lu-DOTA0,Tyr3]octreotate, but were significantly lower after 278 MBq (single injection) or two weekly doses of 278 MBq. Renal damage scores were maximal after 555 MBq and significantly lower in the 278 and 2x278 MBq groups. Three doses of 185 MBq [177Lu-DOTA0,Tyr3]octreotate with intervals of a day, a week or a month significantly influenced serum creatinine (469±18, 134±70 and 65±15 μmol/l, respectively; p<0.001). Renal histological damage scores were not significantly influenced by dose fractionation. Lysine co-administration with three weekly treatments of 185 MBq significantly lowered serum creatinine and proteinuria. Conclusion: Injection of high doses of [177Lu-DOTA0,Tyr3]octreotate resulted in severe renal damage in rats as indicated by proteinuria, elevated serum creatinine and histological damage. This damage was dose dependent and became overt between 100 and 200 days after treatment. Dose fractionation had significant beneficial effects on kidney function. Also, lysine co-injection successfully prevented functional damage.

Effects of evening vs morning thyroxine ingestion on serum thyroid hormone profiles in hypothyroid patients (Article)

2007-01-01T00:00:00Z

Objective: Standard drug information resources recommend that l-thyroxine be taken half an hour before breakfast on an empty stomach, to prevent interference of its intestinal uptake by food or medication. We observed cases in which TSH levels improved markedly after changing the administration time of l-thyroxine to the late evening. We therefore conducted a pilot-study to investigate whether l-thyroxine administration at bedtime improves TSH and thyroid hormones, and whether the circadian rhythm of TSH remains intact. Design: Patients were studied on two occasions: on a stable regimen of morning thyroxine administration and two months after switching to night-time thyroxine using the same dose. On each occasion patients were admitted for 24 h and serial blood samples were obtained. Patients: We investigated 12 women treated with l-thyroxine because of primary hypothyroidism, who used no medication known to interfere with l-thyroxine uptake. Measurements: Patients were admitted to hospital and blood samples were obtained at hourly intervals for 24 h via an indwelling catheter. Following this first hospital admission, all women were asked to switch the administration time from morning to bedtime or vice versa. After 2 months they were readmitted for a 24-h period of hourly blood sampling. Blood samples were analysed for serum TSH (immunometric assay), FT4 and T3 (competitive immunoassay), T4 and rT3 (radioimmunoassay), serum TBG (immunometric assay) and total protein and albumin (colourimetric methods). Results: A significant difference in TSH and thyroid hormones was found after switching to bedtime administration of l-thyroxine. Twenty-four-hour average serum values amounted to (mean ± SD, morning vs bedtime ingestion): TSH, 5.1 ± 0.9 vs 1.2 ± 0.3 mU/l (P < 0.01); FT4, 16.7 ± 1.0 vs 19.3 ± 0.7 pmol/l (P < 0.01); T3, 1.5 ± 0.05 vs 1.6 ± 0.1 nmol/l (P < 0.01). There was no significant change in T4, rT3, albumin and TBG serum levels, nor in the T3/rT3 ratio. The relative amplitude and time of the nocturnal TSH surge remained intact. Conclusions: l-thyroxine taken at bedtime by patients with primary hypothyroidism is associated with higher thyroid hormone concentrations and lower TSH concentrations compared to the same l-thyroxine dose taken in the morning. At the same time, the circadian TSH rhythm stays intact. Our findings are best explained by a better gastrointestinal uptake of l-thyroxine during the night.

Regulation of type III iodothyronine deiodinase expression in human cell lines. (Article)

2006-12-05T00:00:00Z

Type I iodothyronine deiodinase (D1) and type II iodothyronine deiodinase (D2) catalyze the activation of the prohormone T4 to the active hormone T3; type III iodothyronine deiodinase (D3) catalyzes the inactivation of T4 and T3. D3 is highly expressed in brain, placenta, pregnant uterus, and fetal tissues and plays an important role in regulating thyroid hormone bioavailability during fetal development. We examined the activity of the different deiodinases in human cell lines and investigated the regulation of D3 activity and mRNA expression in these cell lines, as well as its possible coexpression with neighboring genes Dlk1 and Dio3os, which may also be especially important during development. D1 activity and mRNA were only found in HepG2 hepatocarcinoma cells, and D2 activity was observed in none of the cell lines. D3 activity and mRNA was found in ECC-1 endometrium carcinoma cells, MCF-7 mammacarcinoma cells, WRL-68 embryonic liver cells, and SH-SY5Y neuroblastoma cells, but not in the HepG2 hepatocarcinoma cell line or in any choriocarcinoma or astrocytoma cell line. We demonstrated that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate increased D3 activity 2- to 9-fold in ECC-1, MCF-7, WRL-68, and SH-SY5Y cells. Estradiol increased D3 activity 3-fold in ECC-1, but not in any other cells. Dexamethasone decreased D3 activity in WRL-68 cells only in the absence of fetal calf serum. Incubation with retinoids increased D3 activity 2- to 3-fold in ECC-1, WRL-68, and MCF-7 cells but decreased D3 activity in SH-SY5Y cells. D3 expression in the different cells was not affected by cAMP or thyroid hormone. Interestingly, D3 mRNA expression in the different cell lines strongly correlated with Dio3os mRNA expression and in a large set of neuroblastoma cell lines also with Dlk1 expression. In conclusion, we identified different human D3-expressing cell lines, in which the regulation of D3 expression is cell type-specific. Our data suggest that estradiol may be one of the factors contributing to the induction of D3 activity in the pregnant uterus and that in addition to gene-specific regulatory elements, more distant common regulatory elements also may be involved in the regulation of D3 expression.

Developmental control of iodothyronine deiodinases by cortisol in the ovine fetus and placenta near term. (Article)

2006-12-05T00:00:00Z

Preterm infants have low serum T4 and T3 levels, which may partly explain the immaturity of their tissues. Deiodinase enzymes are important in determining the bioavailability of thyroid hormones: deiodinases D1 and D2 convert T4 to T3, whereas deiodinase D3 inactivates T3 and produces rT3 from T4. In human and ovine fetuses, plasma T3 rises near term in association with the prepartum cortisol surge. This study investigated the developmental effects of cortisol and T3 on tissue deiodinases and plasma thyroid hormones in fetal sheep during late gestation. Plasma cortisol and T3 concentrations in utero were manipulated by exogenous hormone infusion and fetal adrenalectomy. Between 130 and 144 d of gestation (term 145+/-2 d), maturational increments in plasma cortisol and T3, and D1 (hepatic, renal, perirenal adipose tissue) and D3 (cerebral), and decrements in renal and placental D3 activities were abolished by fetal adrenalectomy. Between 125 and 130 d, iv cortisol infusion raised hepatic, renal, and perirenal adipose tissue D1 and reduced renal and placental D3 activities. Infusion with T3 alone increased hepatic D1 and decreased renal D3 activities. Therefore, in the sheep fetus, the prepartum cortisol surge induces tissue-specific changes in deiodinase activity that, by promoting production and suppressing clearance of T3, may be responsible for the rise in plasma T3 concentration near term. Some of the maturational effects of cortisol on deiodinase activity may be mediated by T3.

Thyroid hormone transport by the human monocarboxylate transporter 8 and its rate-limiting role in intracellular metabolism. (Article)

2006-11-01T00:00:00Z

Cellular entry of thyroid hormone is mediated by plasma membrane transporters. We have identified rat monocarboxylate transporter 8 (MCT8) as an active and specific thyroid hormone transporter. The MCT8 gene is located on the X-chromosome. The physiological relevance of MCT8 has been demonstrated by the identification of hemizygous mutations in this gene in males with severe psychomotor retardation and elevated serum T(3) levels. We have characterized human (h) MCT8 by analysis of iodothyronine uptake and metabolism in cell lines transiently transfected with hMCT8 cDNA alone or together with cDNA coding for iodothyronine deiodinase D1, D2, or D3. MCT8 mRNA was detected by RT-PCR in a number of human cell lines as well as in COS1 cells but was low to undetectable in other cell lines, including JEG3 cells. MCT8 protein was not detected in nontransfected cell lines tested by immunoblotting using a polyclonal C-terminal hMCT8 antibody but was detectable in transfected cells at the expected size (61 kDa). Transfection of COS1 and JEG3 cells with hMCT8 cDNA resulted in 2- to 3-fold increases in uptake of T(3) and T(4) but little or no increase in rT(3) or 3,3'-diiodothyronine (3,3'-T(2)) uptake. MCT8 expression produced large increases in T(4) metabolism by cotransfected D2 or D3, T(3) metabolism by D3, rT(3) metabolism by D1 or D2, and 3,3'-T(2) metabolism by D3. Affinity labeling of hMCT8 protein was observed after incubation of intact transfected cells with N-bromoacetyl-[(125)I]T(3). hMCT8 also facilitated affinity labeling of cotransfected D1 by bromoacetyl-T(3). Our findings indicate that hMCT8 mediates plasma membrane transport of iodothyronines, thus increasing their intracellular availability.

Euthyroid sick syndrome in meningococcal sepsis: the impact of peripheral thyroid hormone metabolism and binding proteins. (Article)

2005-10-01T00:00:00Z

CONTEXT AND OBJECTIVES: The objective of this study was to elucidate the influence of disease severity, deiodination, sulfation, thyroid hormone binding, and dopamine use on thyroid function in euthyroid sick syndrome. SETTING: The study was performed at a university-affiliated pediatric intensive care unit (PICU). DESIGN: This was an observational cohort study. PATIENTS: Sixty-nine children with meningococcal sepsis were studied. MAIN OUTCOME MEASURES: Differences in thyroid function among nonsurvivors, shock survivors, and sepsis survivors on PICU admission were the main outcome measures. RESULTS: The main study group consisted of 45 non-dopamine-treated children. All children had decreased total T3 (TT3)/rT3 ratios without elevated TSH. T4 sulfate levels were decreased in 88%. Nonsurvivors had paradoxically higher TT3/rT3 ratios than shock survivors (0.71 vs. 0.30); this ratio also correlated with shorter duration of disease (r = -0.43). TT4 and T4-binding globulin (TBG) levels declined with increasing disease severity. TBG levels correlated inversely with elastase levels (r = -0.46). Only TSH levels were significantly lower in 24 dopamine-treated children compared with non-dopamine-treated children (0.65 vs. 0.84), whereas other thyroid hormones did not significantly differ. Both higher TT3/rT3 ratios and lower TT4 levels were predictive for mortality, but this disappeared when IL-6 was entered into the regression model. CONCLUSIONS: All children with meningococcal sepsis showed signs of euthyroid sick syndrome. Alterations in peripheral thyroid hormone metabolism related inversely to the duration of disease and seemed to be enacted by profound induction of type 3 deiodinase rather than by down-regulation of type 1. Lower TT4 levels were related to increased turnover of TBG by elastase. Dopamine was found to suppress only TSH secretion, not other thyroid hormone levels, on PICU admission. Both the TT3/rT3 ratio and TT4 levels were predictive for mortality, but were not superior to IL-6.

Serum 3,3',5'-triiodothyronine (rT3) and 3,5,3'-triiodothyronine/rT3 are prognostic markers in critically ill patients and are associated with postmortem tissue deiodinase activities. (Article)

2005-08-01T00:00:00Z

INTRODUCTION AND METHODS: Critical illness is associated with reduced TSH and thyroid hormone secretion, and with changes in peripheral thyroid hormone metabolism, resulting in low serum T3 and high rT3. In 451 critically ill patients who received intensive care for more than 5 d, serum thyroid parameters were determined on d 1, 5, 15, and last day (LD). All patients had been randomized for intensive or conventional insulin treatment. Seventy-one patients died, and postmortem liver and skeletal muscle biopsies were obtained from 50 of them for analysis of deiodinase (D1-3) activities. RESULTS: Insulin treatment did not affect thyroid parameters. On d 1, rT3 was higher and T3/rT3 was lower in nonsurvivors as compared with survivors (P = 0.001). Odds ratio for survival of the highest vs. the lowest quartile was 0.3 for rT3 and 2.9 for T3/rT3. TSH, T4, and T3 were lower in nonsurvivors from d 5 until LD (P < 0.001). TSH, T4, T3, and T3/rT3 increased over time in survivors, but decreased or remained unaltered in nonsurvivors. Liver D1 activity was positively correlated with LD serum T3/rT3 (R = 0.83, P < 0.001) and negatively correlated with rT3 (R = -0.69, P < 0.001). Both liver and skeletal muscle D3 activity were positively correlated with LD serum rT3 (R = 0.32, P = 0.02 and R = 0.31, P = 0.03). CONCLUSION: In critically ill patients who required more than 5 d of intensive care, rT3 and T3/rT3 were already prognostic for survival on d 1. On d 5, T4, T3, but also TSH levels are higher in patients who will survive. Serum rT3 and T3/rT3 were correlated with postmortem tissue deiodinase activities.

A new polymorphism in the type II deiodinase gene is associated with circulating thyroid hormone parameters. (Article)

2005-07-01T00:00:00Z

Type II deiodinase (D2) is important in the regulation of local thyroid hormone bioactivity in certain tissues. D2 in skeletal muscle may also play a role in serum triiodothyronine (T(3)) production. In this study, we identified a polymorphism in the 5'-UTR of the D2 gene (D2-ORFa-Gly3Asp). We investigated the association of D2-ORFa-Gly3Asp, and of the previously identified D2-Thr92Ala polymorphism, with serum iodothyronine levels. D2-ORFa-Gly3Asp was identified by sequencing the 5'-UTR of 15 randomly selected individuals. Genotypes for D2-ORFa-Gly3Asp were determined in 156 healthy blood donors (age 46.3 +/- 12.2 yr) and 349 ambulant elderly men (age 77.7 +/- 3.5 yr) and related to serum iodothyronine and TSH levels. D2-ORFa-Asp(3) had an allele frequency of 33.9% in blood bank donors and was associated with serum thyroxine (T(4); Gly/Gly vs. Gly/Asp vs. Asp/Asp = 7.06 +/- 0.14 vs. 6.74 +/- 0.15 vs. 6.29 +/- 0.27 microg/dl, P = 0.01), free T(4) (1.22 +/- 0.02 vs. 1.16 +/- 0.02 vs. 1.06 +/- 0.04 ng/dl, P = 0.001), reverse T(3) (P = 0.01), and T(3)/T(4) ratio (P = 0.002) in a dose-dependent manner, but not with serum T(3) (P = 0.59). In elderly men, D2-ORFa-Asp(3) had a similar frequency but was not associated with serum iodothyronine levels. This new polymorphism in the 5'-UTR of D2 is associated with iodothyronine levels in blood donors but not in elderly men. We hypothesize that this might be explained by the decline in skeletal muscle size during aging, resulting in a relative decrease in the contribution of D2 to serum T(3) production.

Expression of glucocorticoid, retinoid, and thyroid hormone receptors during human lung development. (Article)

2005-07-01T00:00:00Z

CONTEXT: Although glucocorticoid hormone, thyroid hormone, and retinoic acid play important roles in fetal development, the expression of their receptors in human lung is still unknown. OBJECTIVE: The aim of this study was to investigate the ontogeny of glucocorticoid receptor (GR)alpha, thyroid hormone receptors (TRs), retinoic acid receptors (RARs), and retinoid X receptors (RXRs) mRNA expression in human lungs. DESIGN: Lungs from human fetuses and neonates (13.5-41 wk gestation; n = 20) as well as adults (n = 5) were analyzed by real-time PCR to monitor the ontogeny of mRNA expression for each receptor. In addition, immunohistochemistry was performed to show the cellular distribution of the different receptors. RESULTS: The expression of GRalpha, TRs, RARs, and RXRs was already detected in the earliest developmental stages analyzed. There was no significant difference in mRNA expression between developmental groups for any of the genes studied. However, for fetal and neonatal samples, there were positive correlations between gestational age and mRNA expression for RARalpha (r = 0.665; P = 0.001), RXRalpha (r = 0.444; P = 0.050), and RXRgamma (r = 0.464; P = 0.039). Immunohistochemical studies showed the presence of GRalpha, TRs, RARs, and RXRs in the nuclei of both epithelial and mesenchymal cells, albeit more pronounced in epithelium of larger airways. CONCLUSIONS: The detection of GRalpha, TRs, RARs, and RXRs expression in human lung as early as 13.5 wk gestation implies an early potential for therapeutic or toxic effects by exogenous analogs or by excess of endogenous ligands.

Expression of recombinant membrane-bound type I iodothyronine deiodinase in yeast. (Article)

2005-06-01T00:00:00Z

The bioactivity of thyroid hormone is determined to a large extent by the monodeiodination of the prohormone thyroxine (T4) by the hepatic selenoenzyme type I iodothyronine deiodinase (D1), i.e. by outer ring deiodination (ORD) to the active hormone triiodothyronine (T3) or by inner ring deiodination (IRD) to the inactive metabolite reverse T3 (rT3). Since D1 is a membrane-bound protein with an N-terminal membrane-spanning domain, the enzyme is very difficult to purify in an active state. This study was undertaken in order to develop a heterologous (over)-expression system that would eventually allow the production of large amounts of purified active D1 protein. We have expressed a mutant rat D1 protein, in which the selenocysteine residue in the core catalytic center was replaced by cysteine (D1 Cys) in yeast cells (Saccharomyces cerevisiae). After yeast cell fractionation, kinetic analysis was performed with dithiothreitol as reducing cofactor. ORD activity was associated with membrane fractions, while no activity could be detected in the cytosolic fraction. The D1 Cys protein displayed a tenfold increase in Km (2 microM) for rT3 as compared with native D1 protein in rat liver microsomes. The D1 protein content is about 65 pmol/mg microsomal protein, as compared with about 3 pmol/mg in rat liver microsomal fraction. SDS-PAGE analysis of N-bromoacetyl-[125I]T3 affinity-labeled D1 protein showed several labeled protein isoforms with apparent molecular masses between 27 and 32 kDa. Immunoblot analysis with a specific D1 antiserum confirmed the observed D1 protein heterogeneity. Site-directed mutagenesis of several potential N-linked glycosylation sites, phosphorylation sites and a unique myristoylation site established that D1 heterogeneity is not caused by N-linked glycosylation, but probably by a combination of O-linked glycosylation and phosphorylation. Deletion of the endoplasmic reticulum (ER)-signal sequence and the membrane-spanning domain (amino acid residue 2-35), did not result in the production of a soluble D1 enzyme. Although this mutated D1 protein is inactive, the fact that it is still membrane bound indicates the existence of additional membrane attachment site(s) or membrane-spanning domains. Overall, our studies indicate that yeast cells provide a useful system for the expression of relatively high levels of D1 protein which could be used for further structure-function analysis.

A polymorphism in type I deiodinase is associated with circulating free insulin-like growth factor I levels and body composition in humans. (Article)

2005-01-01T00:00:00Z

The interaction between the GH-IGF-I axis and thyroid hormone metabolism is complex and not fully understood. T(4) stimulates IGF-I activity in animals in the absence of GH. On the other hand, GH replacement therapy results in an increase in serum T(3) and a decrease in T(4) and rT(3) levels, suggesting a stimulation of type I deiodinase (D1) activity. Recently, we demonstrated the association of two polymorphisms in D1 (D1a-C/T; T = 34%, and D1b-A/G; G = 10%) with serum iodothyronine levels. Haplotype alleles were constructed, suggesting a lower activity of the D1 haplotype 2 allele (aT-bA) and a higher activity of the haplotype allele 3 (aC-bG). In this study, we investigated whether genetic variations in D1 are associated with the IGF-I system.In 156 blood donors and 350 elderly men, the association of the D1 haplotype alleles with circulating IGF-I and free IGF-I levels was studied. In addition, potential associations with muscle strength and body composition were investigated in the elderly population. Finally, the relation between serum iodothyronine levels and IGF-I levels was studied.In blood donors, haplotype allele 2 was associated with higher levels of free IGF-I (302.9 +/- 22.9 vs. 376.3 +/- 19.1 pg/ml, P = 0.02). In elderly men, haplotype allele 2 also showed an allele dose increase in free IGF-I levels (P(trend) = 0.01) and an allele dose decrease in serum T(3) levels (P(trend) = 0.01), independent of age. Carriers of the D1a-T variant also had a higher isometric grip strength (P = 0.047) and maximum leg extensor strength (P = 0.07) as well as a higher lean body mass (P = 0.03).In blood donors, T(4) and free T(4) were negatively correlated with total IGF-I levels (R = -0.18, P = 0.03 and R = -0.24, P = 0.003), whereas T(3) to T(4) and T(3) to reverse T(3) ratios were positively correlated with total IGF-I (R = 0.31, P < 0.001 and R = 0.18, P = 0.03). Free IGF-I showed a negative correlation with T(4) (R = -0.26, P = 0.001) and T(4)-binding globulin (R = -0.31, P < 0.001) and a positive correlation with T(3) to T(4) ratio (R = 0.21, P = 0.01).In conclusion, a polymorphism that results in a decreased D1 activity is associated with an increase in free IGF-I levels. The pathophysiological significance of this association with IGF-I is supported by an increased muscle strength and muscle mass in carriers of the D1 haplotype 2 allele in a population of elderly men. The association of D1 haplotype allele 2 with serum T(3) levels in the elderly population suggests a relative increase in its contribution to circulating T(3) in old age.

Characteristics and thyroid state-dependent regulation of iodothyronine deiodinases in pigs. (Article)

2004-09-01T00:00:00Z

Three iodothyronine deiodinases (D1, D2, and D3) regulate local and systemic availability of thyroid hormone. D1 and D2 activate the prohormone T4 to the thyromimetic T3, and D3 inactivates T4 and T3 to rT3 and 3,3'-diiodothyronine, respectively. The expression of the three deiodinases is tightly regulated with regard to developmental stage and cell type to provide fine tuning of T3 supply to target cells. Most studies regarding distribution and regulation of deiodinases have been carried out in rodents. However, in different respects, rodents do not seem to be the optimal experimental model for human thyroid hormone physiology. For instance, D2 expression has been observed in human thyroid and skeletal muscle but not in these tissues in rodents. In this study, we have explored the pig as an alternative model. Porcine D1, D2, and D3 were cloned by RT-PCR, and their catalytic properties were shown to be virtually identical to those reported for human and rodent deiodinases. The tissue distribution of deiodinases was studied in normal pigs and in pigs made hypothyroid by methimazole treatment or in pigs made hyperthyroid by T4 treatment. D1 activity in liver and kidney was increased in T4-treated pigs. D2 activities in cerebrum and pituitary were decreased after T4 treatment and strongly increased after methimazole treatment. Remarkably, D2 activity in thyroid and skeletal muscle was induced in hypothyroid pigs. Significant expression of D3 was observed in cerebrum and was positively regulated by thyroid state. In conclusion, the pig appears to be a valuable model for human thyroid hormone physiology. The expression of D2 activity in thyroid and skeletal muscle is of particular interest for studies on the importance of this enzyme in (hypothyroid) humans.

Iodothyronine levels in the human developing brain: major regulatory roles of iodothyronine deiodinases in different areas. (Article)

2004-07-01T00:00:00Z

Thyroid hormones are required for human brain development, but data on local regulation are limited. We describe the ontogenic changes in T(4), T(3), and rT(3) and in the activities of the types I, II, and III iodothyronine deiodinases (D1, D2, and D3) in different brain regions in normal fetuses (13-20 wk postmenstrual age) and premature infants (24-42 wk postmenstrual age). D1 activity was undetectable.The developmental changes in the concentrations of the iodothyronines and D2 and D3 activities showed spatial and temporal specificity but with divergence in the cerebral cortex and cerebellum. T(3) increased in the cortex between 13 and 20 wk to levels higher than adults, unexpected given the low circulating T(3). Considerable D2 activity was found in the cortex, which correlated positively with T(4) (r = 0.65). Cortex D3 activity was very low, as was D3 activity in germinal eminence and choroid plexus. In contrast, cerebellar T(3) was very low and increased only after midgestation. Cerebellum D3 activities were the highest (64 fmol/min.mg) of the regions studied, decreasing after midgestation. Other regions with high D3 activities (midbrain, basal ganglia, brain stem, spinal cord, hippocampus) also had low T(3) until D3 started decreasing after midgestation. D3 was correlated with T(3) (r = -0.682) and rT(3)/T(3) (r = 0.812) and rT(3)/T(4) (r = 0.889).Our data support the hypothesis that T(3) is required by the human cerebral cortex before midgestation, when mother is the only source of T(4). D2 and D3 play important roles in the local bioavailability of T(3). T(3) is produced from T(4) by D2, and D3 protects brain regions from excessive T(3) until differentiation is required.

An ascidian homolog of vertebrate iodothyronine deiodinases. (Article)

2004-03-01T00:00:00Z

In all classes of vertebrates, the deiodination of the prohormone T(4) to T(3) represents an essential activation step in thyroid hormone action. The possible presence of iodothyronine deiodinase activity in protochordates has been demonstrated in vivo. Recent molecular cloning of the genomes and transcripts of several ascidian species allows further investigation into thyroid-related processes in ascidians. A cDNA clone from Halocynthia roretzi (hrDx) was found to have significant homology (30% amino acid identity) with the iodothyronine deiodinase gene sequences from vertebrates, including the presence of an in-frame UGA codon that might encode a selenocysteine (SeC) in the active site. Because it was not certain that the 3' untranslated region (UTR) contained a SeC insertion sequence (SECIS) element essential for SeC incorporation, a chimeric expression vector of the hrDx coding sequence and the rat deiodinase SECIS element was produced, as well as an expression vector containing the intact hrDx cDNA. COS, CHO, and HEK cells were transfected with these vectors, and deiodinase activity was measured in cell homogenates. Outer-ring deiodinase activity was detected using both T(4) and reverse T(3) as substrates, and activity was enhanced by the presence of the reductive cofactor dithiothreitol. The enzyme activity was optimal during incubation between 20 and 30 C (pH 6-7) and was strongly inhibited by gold-thioglucose. The Halocynthia deiodinase appears to be a high Michaelis-Menten constant (K(m)) enzyme (K(m) reverse T(3), 2 microM; and K(m) T(4), 4 microM). Deiodinase activity was completely lost upon the substitution of the SeC residue in the putative catalytic center by either cysteine or alanine. Transfection of the full-length hrDx cDNA produced deiodinase activity confirming the presence of a SECIS element in the 3'UTR, as revealed by the SECISearch program. In conclusion, our results show, for the first time, the existence of an ascidian iodothyronine outer-ring deiodinase. This raises the hypothesis that, in protochordates, the prohormone T(4) is activated by enzymatic outer-ring deiodination to T(3).

Molecular basis for the substrate selectivity of cat type I iodothyronine deiodinase. (Article)

2003-12-01T00:00:00Z

The type I iodothyronine deiodinase (D1) catalyzes the activation of T4 to T3 as well as the degradation of T3 (rT3) and sulfated iodothyronines. A comparison of the catalytic activities of D1 in liver microsomal preparations from several species revealed a remarkable difference between cat D1 on one hand and rat/human D1 on the other hand. The Michaelis constant (Km) of cat D1 for rT3 (11 microm) is 30-fold higher than that of rat and human D1 (0.2-0.5 microm). Deiodination of rT3 by cat D1 is facilitated by sulfation [maximal velocity (Vmax)/Km rT3 = 3 and Vmax/Km rT3S = 81]. To understand the molecular basis for the difference in substrate interaction the cat D1 cDNA was cloned, and the deduced amino acid sequence was compared with rat/human D1 protein. In the region between amino acid residues 40 and 70 of cat D1, various differences with rat/human D1 are concentrated. By site-directed mutagenesis of cat D1 it was found that a combination of mutations was necessary to improve the deiodination of rT3 by cat D1 enzyme. For efficient rT3 deiodination, a Phe at position 65 and the insertion of the Thr-Gly-Met-Thr-Arg48-52 sequence as well as the amino acids Gly and Glu at position 45-46 are essential. Either of these changes alone resulted in only a limited improvement of rT3 deiodination. At the same time the combination of the described mutations did not affect the already quite efficient outer ring deiodination of rT3S nor the inner ring deiodination of T3S, whereas each of the described changes alone did affect rT3S deiodination. Our findings suggest great flexibility of the active site in D1 that adapts to its various substrates. The active site of wild-type cat D1 is less flexible than the active site of rat/human D1 and favors sulfated iodothyronines.

Identification of monocarboxylate transporter 8 as a specific thyroid hormone transporter. (Article)

2003-10-10T00:00:00Z

Transport of thyroid hormone across the cell membrane is required for its action and metabolism. Recently, a T-type amino acid transporter was cloned which transports aromatic amino acids but not iodothyronines. This transporter belongs to the monocarboxylate transporter (MCT) family and is most homologous with MCT8 (SLC16A2). Therefore, we cloned rat MCT8 and tested it for thyroid hormone transport in Xenopus laevis oocytes. Oocytes were injected with rat MCT8 cRNA, and after 3 days immunofluorescence microscopy demonstrated expression of the protein at the plasma membrane. MCT8 cRNA induced an approximately 10-fold increase in uptake of 10 nM 125I-labeled thyroxine (T4), 3,3',5-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine. Because of the rapid uptake of the ligands, transport was only linear with time for <4 min. MCT8 did not transport Leu, Phe, Trp, or Tyr. [125I]T4 transport was strongly inhibited by L-T4, D-T4, L-T3, D-T3, 3,3',5-triiodothyroacetic acid, N-bromoacetyl-T3, and bromosulfophthalein. T3 transport was less affected by these inhibitors. Iodothyronine uptake in uninjected oocytes was reduced by albumin, but the stimulation induced by MCT8 was markedly increased. Saturation analysis provided apparent Km values of 2-5 microM for T4, T3, and rT3. Immunohistochemistry showed high expression in liver, kidney, brain, and heart. In conclusion, we have identified MCT8 as a very active and specific thyroid hormone transporter.

Substitution of cysteine for selenocysteine in the catalytic center of type III iodothyronine deiodinase reduces catalytic efficiency and alters substrate preference (Article)

2003-01-01T00:00:00Z

Human type III iodothyronine deiodinase (D3) catalyzes the conversion of T(4) to rT(3) and of T(3) to 3, 3'-diiodothyronine (T2) by inner-ring deiodination. Like types I and II iodothyronine deiodinases, D3 protein contains selenocysteine (SeC) in the highly conserved core catalytic center at amino acid position 144. To evaluate the contribution of SeC144 to the catalytic properties of D3 enzyme, we generated mutants in which cysteine (D3Cys) or alanine (D3Ala) replaces SeC144 (D3wt). COS cells were transfected with expression vectors encoding D3wt, D3Cys, or D3Ala protein. Kinetic analysis was performed on homogenates with dithiothreitol as reducing cofactor. The Michaelis constant of T(3) was 5-fold higher for D3Cys than for D3wt protein. In contrast, the Michaelis constant of T(4) increased 100-fold. The D3Ala protein was enzymatically inactive. Semiquantitative immunoblotting of homogenates with a D3 antiserum revealed that about 50-fold higher amounts of D3Cys and D3Ala protein are expressed relative to D3wt protein. The relative substrate turnover number of D3Cys is 2-fold reduced for T(3) and 6-fold reduced for T(4) deiodination, compared with D3wt enzyme. Studies in intact COS cells expressing D3wt or D3Cys showed that the D3Cys enzyme is also active under in situ conditions. In conclusion, the SeC residue in the catalytic center of D3 is essential for efficient inner-ring deiodination of T(3) and in particular T(4) at physiological substrate concentrations.

Polymorphisms in thyroid hormone pathway genes are associated with plasma TSH and iodothyronine levels in healthy subjects (Article)

2003-01-01T00:00:00Z

Single nucleotide polymorphisms (SNPs) in genes involved in thyroid hormone metabolism may affect thyroid hormone bioactivity. We investigated the occurrence and possible effects of SNPs in the deiodinases (D1-D3), the TSH receptor (TSHR), and the T(3) receptor beta (TR beta) genes. SNPs were identified in public databases or by sequencing of genomic DNA from 15 randomly selected subjects (30 alleles). Genotypes for the identified SNPs were determined in 156 healthy blood donors and related to plasma T(4), free T(4), T(3), rT(3), and TSH levels. Eight SNPs of interest were identified, four of which had not yet been published. Three are located in the 3'-untranslated region: D1a-C/T (allele frequencies, C = 66%, T = 34%), D1b-A/G (A = 89.7%, G = 10.3%), and D3-T/G (T = 85.5%, G = 14.2%). Four are missense SNPs: D2-A/G (Thr92Ala, Thr = 61.2%, Ala = 38.8%), TSHRa-G/C (Asp36His, Asp = 99.4%, His = 0.6%), TSHRb-C/A (Pro52Thr, Pro = 94.2%, Thr = 5.8%), and TSHRc-C/G (Asp727Glu, Asp = 90.7%, Glu = 9.3%). One is a silent SNP: TR beta-T/C (T = 96.8%, C = 3.2%). D1a-T was associated in a dose-dependent manner with a higher plasma rT(3) [CC, 0.29 +/- 0.01; CT, 0.32 +/- 0.01; and TT, 0.34 +/- 0.02 nmol/liter (mean +/- SE); P = 0.017], a higher plasma rT(3)/T(4) (P = 0.01), and a lower T(3)/rT(3) (P = 0.003) ratio. The D1b-G allele was associated with lower plasma rT(3)/T(4) (P = 0.024) and with higher T(3)/rT(3) (P = 0.08) ratios. TSHRc-G was associated with a lower plasma TSH (CC, 1.38 +/- 0.07, vs. GC, 1.06 +/- 0.14 mU/liter; P = 0.04), and with lower plasma TSH/free T(4) (P = 0.06), TSH/T(3) (P = 0.06), and TSH/T(4) (P = 0.08) ratios. No associations with TSH and iodothyronine levels were found for the other SNPs. We have analyzed eight SNPs in five thyroid hormone pathway genes and found significant associations of three SNPs in two genes (D1, TSHR) with plasma TSH or iodothyronine levels in a normal population.

Reduced activation and increased inactivation of thyroid hormone in tissues of critically ill patients (Article)

2003-01-01T00:00:00Z

Critical illness is often associated with reduced TSH and thyroid hormone secretion as well as marked changes in peripheral thyroid hormone metabolism, resulting in low serum T(3) and high rT(3) levels. To study the mechanism(s) of the latter changes, we determined serum thyroid hormone levels and the expression of the type 1, 2, and 3 iodothyronine deiodinases (D1, D2, and D3) in liver and skeletal muscle from deceased intensive care patients. To study mechanisms underlying these changes, 65 blood samples, 65 liver, and 66 skeletal muscle biopsies were obtained within minutes after death from 80 intensive care unit patients randomized for intensive or conventional insulin treatment. Serum thyroid parameters and the expression of tissue D1-D3 were determined. Serum TSH, T(4), T(3), and the T(3)/rT(3) ratio were lower, whereas serum rT(3) was higher than in normal subjects (P < 0.0001). Liver D1 activity was down-regulated and D3 activity was induced in liver and skeletal muscle. Serum T(3)/rT(3) ratio correlated positively with liver D1 activity (P < 0.001) and negatively with liver D3 activity (ns). These parameters were independent of the type of insulin treatment. Liver D1 and serum T(3)/rT(3) were highest in patients who died from severe brain damage, intermediate in those who died from sepsis or excessive inflammation, and lowest in patients who died from cardiovascular collapse (P < 0.01). Liver D3 showed an opposite relationship. Acute renal failure requiring dialysis and need of inotropes were associated with low liver D1 activity (P < 0.01 and P = 0.06) and high liver D3 (P < 0.01) and skeletal muscle D3 (P < 0.05) activity. Liver D1 activity was negatively correlated with plasma urea (P = 0.002), creatinine (P = 0.06), and bilirubin (P < 0.0001). D1 and D3 mRNA levels corresponded with enzyme activities (both P < 0.001), suggesting regulation of the expression of both deiodinases at the pretranslational level. This is the first study relating tissue deiodinase activities with serum thyroid hormone levels and clinical parameters in a large group of critically ill patients. Liver D1 is down-regulated and D3 (which is not present in liver and skeletal muscle of healthy individuals) is induced, particularly in disease states associated with poor tissue perfusion. These observed changes, in correlation with a low T(3)/rT(3) ratio, may represent tissue-specific ways to reduce thyroid hormone bioactivity during cellular hypoxia and contribute to the low T(3) syndrome of severe illness.

Characterization of rat iodothyronine sulfotransferases. (Article)

2003-01-01T00:00:00Z

Sulfation appears to be an important pathway for the reversible inactivation of thyroid hormone during fetal development. The rat is an often used animal model to study the regulation of fetal thyroid hormone status. The present study was done to determine which sulfotransferases (SULTs) are important for iodothyronine sulfation in the rat, using radioactive T4, T3, rT3, and 3,3'-T2 as substrates, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as cofactor, and rat liver, kidney and brain cytosol, and recombinant rat SULT1A1, -1B1, -1C1, -1E1, -2A1, -2A2, and -2A3 as enzymes. Recombinant rat SULT1A1, -1E1, -2A1, -2A2, and -2A3 failed to catalyze iodothyronine sulfation. For all tissue SULTs and for rSULT1B1 and rSULT1C1, 3,3'-T2 was by far the preferred substrate. Apparent Km values for 3,3'-T2 amounted to 1.9 microM in male liver, 4.4 microM in female liver, 0.76 microM in male kidney, 0.23 microM in male brain, 7.7 microM for SULT1B1, and 0.62 microM for SULT1C1, whereas apparent Km values for PAPS showed less variation (2.0-6.9 microM). Sulfation of 3,3'-T2 was inhibited dose dependently by other iodothyronines, with similar structure-activity relationships for most enzymes except for the SULT activity in rat brain. The apparent Km values of 3,3'-T2 in liver cytosol were between those determined for SULT1B1 and -1C1, supporting the importance of these enzymes for the sulfation of iodothyronines in rat liver, with a greater contribution of SULT1C1 in male than in female rat liver. The results further suggest that rSULT1C1 also contributes to iodothyronine sulfation in rat kidney, whereas other, yet-unidentified forms appear more important for the sulfation of thyroid hormone in rat brain.

Effects of thyroid state on the expression of hepatic thyroid hormone transporters in rats. (Article)

2002-01-01T00:00:00Z

Liver uptake of thyroxine (T4) is mediated by transporters and is rate limiting for hepatic 3,3',5-triiodothyronine (T3) production. We investigated whether hepatic mRNA for T4 transporters is regulated by thyroid state using Xenopus laevis oocytes as an expression system. Because X. laevis oocytes show high endogenous uptake of T4, T4 sulfamate (T4NS) was used as an alternative ligand for the hepatic T4 transporters. Oocytes were injected with 23 ng liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats, and after 3-4 days uptake was determined by incubation of injected and uninjected oocytes for 1 h at 25 degrees C or for 4 h at 18 degrees C with 10 nM [125I]T4NS. Expression of type I deiodinase (D1), which is regulated by thyroid state, was studied in the oocytes as an internal control. Uptake of T4NS showed similar approximately fourfold increases after injection of liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats. A similar lack of effect of thyroid state was observed using reverse T3 as ligand. In contrast, D1 activity induced by liver mRNA from hyperthyroid and hypothyroid rats in the oocytes was 2.4-fold higher and 2.7-fold lower, respectively, compared with euthyroid rats. Studies have shown that uptake of iodothyronines in rat liver is mediated in part by several organic anion transporters, such as the Na+/taurocholate-cotransporting polypeptide (rNTCP) and the Na-independent organic anion-transporting polypeptide (rOATP1). Therefore, the effects of thyroid state on rNTCP, rOATP1, and D1 mRNA levels in rat liver were also determined. Northern analysis showed no differences in rNTCP or rOATP1 mRNA levels between hyperthyroid and hypothyroid rats, whereas D1 mRNA levels varied widely as expected. These results suggest little effect of thyroid state on the levels of mRNA coding for T4 transporters in rat liver, including rNTCP and rOATP1. However, they do not exclude regulation of hepatic T4 transporters by thyroid hormone at the translational and posttranslational level.

Characterization of iodothyronine sulfatase activities in human and rat liver and placenta (Article)

2002-01-01T00:00:00Z

In conditions associated with high serum iodothyronine sulfate concentrations, e.g. during fetal development, desulfation of these conjugates may be important in the regulation of thyroid hormone homeostasis. However, little is known about which sulfatases are involved in this process. Therefore, we investigated the hydrolysis of iodothyronine sulfates by homogenates of V79 cells expressing the human arylsulfatases A (ARSA), B (ARSB), or C (ARSC; steroid sulfatase), as well as tissue fractions of human and rat liver and placenta. We found that only the microsomal fraction from liver and placenta hydrolyzed iodothyronine sulfates. Among the recombinant enzymes only the endoplasmic reticulum-associated ARSC showed activity toward iodothyronine sulfates; the soluble lysosomal ARSA and ARSB were inactive. Recombinant ARSC as well as human placenta microsomes hydrolyzed iodothyronine sulfates with a substrate preference for 3,3'-diiodothyronine sulfate (3,3'-T(2)S) approximately T(3) sulfate (T(3)S) >> rT(3)S approximately T(4)S, whereas human and rat liver microsomes showed a preference for 3,3'-T(2)S > T(3)S >> rT(3)S approximately T(4)S. ARSC and the tissue microsomal sulfatases were all characterized by high apparent K(m) values (>50 microM) for 3,3'-T(2)S and T(3)S. Iodothyronine sulfatase activity determined using 3,3'-T(2)S as a substrate was much higher in human liver microsomes than in human placenta microsomes, although ARSC is expressed at higher levels in human placenta than in human liver. The ratio of estrone sulfate to T(2)S hydrolysis in human liver microsomes (0.2) differed largely from that in ARSC homogenate (80) and human placenta microsomes (150). These results suggest that ARSC accounts for the relatively low iodothyronine sulfatase activity of human placenta, and that additional arylsulfatase(s) contributes to the high iodothyronine sulfatase activity in human liver. Further research is needed to identify these iodothyronine sulfatases, and to study the physiological importance of the reversible sulfation of iodothyronines in thyroid hormone metabolism.

Potent inhibition of estrogen sulfotransferase by hydroxylated metabolites of polyhalogenated aromatic hydrocarbons reveals alternative mechanism for estrogenic activity of endocrine disrupters (Article)

2002-01-01T00:00:00Z

Polyhalogenated aromatic hydrocarbons (PHAHs), such as polychlorinated dibenzo-p-dioxins and dibenzofurans, polybrominated diphenylethers, and bisphenol A derivatives are persistent environmental pollutants, which are capable of interfering with reproductive and endocrine function in birds, fish, reptiles, and mammals. PHAHs exert estrogenic effects that may be mediated in part by their hydroxylated metabolites (PHAH-OHs), the mechanisms of which remain to be identified. PHAH-OHs show low affinity for the ER. Alternatively, they may exert their estrogenic effects by inhibiting E2 metabolism. As sulfation of E2 by estrogen sulfotransferase (SULT1E1) is an important pathway for E2 inactivation, inhibition of SULT1E1 may lead to an increased bioavailability of estrogens in tissues expressing this enzyme. Therefore, we studied the possible inhibition of human SULT1E1 by hydroxylated PHAH metabolites and the sulfation of the different compounds by SULT1E1. We found marked inhibition of SULT1E1 by various PHAH-OHs, in particular by compounds with two adjacent halogen substituents around the hydroxyl group that were effective at (sub)nanomolar concentrations. Depending on the structure, the inhibition is primarily competitive or noncompetitive. Most PHAH-OHs are also sulfated by SULT1E1. We also investigated the inhibitory effects of the various PHAH-OHs on E2 sulfation by human liver cytosol and found that the effects were strongly correlated with their inhibitions of recombinant SULT1E1 (r = 0.922). Based on these results, we hypothesize that hydroxylated PHAHs exert their estrogenic effects at least in part by inhibiting SULT1E1-catalyzed E2 sulfation.

Substitution of cysteine for a conserved alanine residue in the catalytic center of type II iodothyronine deiodinase alters interaction with reducing cofactor (Article)

2002-01-01T00:00:00Z

Human type II iodothyronine deiodinase (D2) catalyzes the activation of T(4) to T(3). The D2 enzyme, like the type I (D1) and type III (D3) deiodinases, contains a selenocysteine (SeC) residue (residue 133 in D2) in the highly conserved catalytic center. Remarkably, all of the D2 proteins cloned so far have an alanine two residue-amino terminal to the SeC, whereas all D1 and D3 proteins contain a cysteine at this position. A cysteine residue in the catalytic center could assist in enzymatic action by providing a nucleophilic sulfide or by participating in redox reactions with a cofactor or enzyme residues. We have investigated whether D2 mutants with a cysteine (A131C) or serine (A131S) two-residue amino terminal to the SeC are enzymatically active and have characterized these mutants with regard to substrate affinity, reducing cofactor interaction and inhibitor profile. COS cells were transfected with expression vectors encoding wild-type (wt) D2, D2 A131C, or D2 A131S proteins. Kinetic analysis was performed on homogenates with dithiothreitol (DTT) as reducing cofactor. The D2 A131C and A131S mutants displayed similar Michaelis-Menten constant values for T(4) (5 nM) and reverse T(3) (9 nM) as the wt D2 enzyme. The limiting Michaelis-Menten constant for DTT of the D2 A131C enzyme was 3-fold lower than that of the wt D2 enzyme. The wt and mutant D2 enzymes are essentially insensitive to propylthiouracil [concentration inhibiting 50% of activity (IC(50)) > 2 mM] in the presence of 20 mM DTT, but when tested in the presence of 0.2 mM DTT the IC(50) value for propylthiouracil is reduced to about 0.1 mM. During incubations of intact COS cells expressing wt D2, D2 A131C, or D2 A131S, addition of increasing amounts of unlabeled T(4) resulted in the saturation of [(125)I]T(4) deiodination, as reflected in a decrease of [(125)I]T(3) release into the medium. Saturation first appeared at medium T(4) concentrations between 1 and 10 nM. In conclusion: substitution of cysteine for a conserved alanine residue in the catalytic center of the D2 protein does not inactivate the enzyme in vitro and in situ, but rather improves the interaction with the reducing cofactor DTT in vitro.

Uptake of triiodothyronine and triiodothyroacetic acid in neonatal rat cardiomyocytes: effects of metabolites and analogs (Article)

2002-01-01T00:00:00Z

Cellular and nuclear uptake of [125I]tri-iodothyronine (T3) and [125I]triiodothyroacetic acid (Triac) were compared in cardiomyocytes of 2-3 day old rats, and the effect of thyroid hormone analogs on cellular T(3) uptake was measured. Cells (5-10 x 10(5) per well) were cultured in DMEM-M199 with 5% horse serum and 5% FCS. Incubations were performed for from 15 min to 24 h at 37 degrees C in the same medium, 0.5% BSA and [125I]T3 (100 pM), or [125I]Triac (240 pM). Expressed as % dose, T(3) uptake was five times Triac uptake, but expressed as fmol/pM free hormone, Triac uptake was at least 30% (P<0.001) greater than T3 uptake, whereas the relative nuclear binding of the two tracers was comparable. The 15 min uptake of [125I]T3 was competitively inhibited by 10 microM unlabeled T3 (45-52%; P<0.001) or 3,3'- diiodothyronine (T2) (52%; P<0.001), and to a smaller extent by thyroxine (T(4)) (27%; 0.05

Induction of thyroid hormone-degrading deiodinase in cardiac hypertrophy and failure (Article)

2002-01-01T00:00:00Z

The similarities between the changes in cardiac gene expression in pathological ventricular hypertrophy and hypothyroidism suggest a role of impaired cardiac thyroid hormone (TH) action in the development of contractile dysfunction during chronic cardiac pressure overload. Here we studied the possible involvement of altered cardiac TH metabolism using a rat model of right-ventricular (RV) hypertrophy induced by pressure-overload. Pathological RV hypertrophy was indicated by decreased mRNA levels of sarcoplasmic reticulum(SR) Ca2-ATPase type 2a (SERCA2a) and myosin heavy chain a (MHCalpha), and increased levels of MHCbeta mRNA. Enzyme activity of type HI deiodinase (D3), which converts T4 and T3 to the inactive compounds rT3 and 3,3'-T2, respectively, was identified in ventricular tissue. This activity was stimulated up to five fold in hypertrophic RV, but remained unaltered in the non-hypertrophic left ventricle (LV). A low level of type Ideiodinase activity was also detected, which decreased significantly in both RV and LV. Stimulation of RV D3 activity was significantly higher in those animals in which hypertrophy progressed to heart failure, compared to animals that developed compensatory hypertrophy. The induction of a cardiac TR-degrading deiodinase maybe expected to result in reduced cellular levels of T3 and thereby contribute to a local hypothyroid state in the hypertrophic and, particularly, in the failing ventricle.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability (Article)

2001-01-01T00:00:00Z

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Thyroid hormone transport by the heterodimeric human system L amino acid transporter (Article)

2001-01-01T00:00:00Z

Transport of thyroid hormone across the cell membrane is required for thyroid hormone action and metabolism. We have investigated the possible transport of iodothyronines by the human system L amino acid transporter, a protein consisting of the human 4F2 heavy chain and the human LAT1 light chain. Xenopus oocytes were injected with the cRNAs coding for human 4F2 heavy chain and/or human LAT1 light chain, and after 2 d were incubated at 25 C with 0.01-10 microM [(125)I]T(4), [(125)I]T(3), [(125)I]rT(3), or [(125)I]3,3'-diiodothyronine or with 10-100 microM [(3)H]arginine, [(3)H]leucine, [(3)H]phenylalanine, [(3)H]tyrosine, or [(3)H]tryptophan. Injection of human 4F2 heavy chain cRNA alone stimulated the uptake of leucine and arginine due to dimerization of human 4F2 heavy chain with an endogenous Xenopus light chain, but did not affect the uptake of other ligands. Injection of human LAT1 light chain cRNA alone did not stimulate the uptake of any ligand. Coinjection of cRNAs for human 4F2 heavy chain and human LAT1 light chain stimulated the uptake of phenylalanine > tyrosine > leucine > tryptophan (100 microM) and of 3,3'-diiodothyronine > rT(3) approximately T(3) > T(4) (10 nM), which in all cases was Na(+) independent. Saturation analysis provided apparent Michaelis constant (K(m)) values of 7.9 microM for T(4), 0.8 microM for T(3), 12.5 microM for rT(3), 7.9 microM for 3,3'-diiodothyronine, 46 microM for leucine, and 19 microM for tryptophan. Uptake of leucine, tyrosine, and tryptophan (10 microM) was inhibited by the different iodothyronines (10 microM), in particular T(3). Vice versa, uptake of 0.1 microM T(3) was almost completely blocked by coincubation with 100 microM leucine, tryptophan, tyrosine, or phenylalanine. Our results demonstrate stereospecific Na(+)-independent transport of iodothyronines by the human heterodimeric system L amino acid transporter.

Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes (Article)

2001-01-01T00:00:00Z

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.

Subclinical hypothyroidism is an independent risk factor for atherosclerosis and myocardial infarction in elderly women: the Rotterdam Study (Article)

2000-01-01T00:00:00Z

BACKGROUND: Overt hypothyroidism has been found to be associated with cardiovascular disease. Whether subclinical hypothyroidism and thyroid autoimmunity are also risk factors for cardiovascular disease is controversial. OBJECTIVE: To investigate whether subclinical hypothyroidism and thyroid autoimmunity are associated with aortic atherosclerosis and myocardial infarction in postmenopausal women. DESIGN: Population-based cross-sectional study. SETTING: A district of Rotterdam, The Netherlands. PARTICIPANTS: Random sample of 1149 women (mean age +/- SD, 69.0 +/- 7.5 years) participating in the Rotterdam Study. MEASUREMENTS: Data on thyroid status, aortic atherosclerosis, and history of myocardial infarction were obtained at baseline. Subclinical hypothyroidism was defined as an elevated thyroid-stimulating hormone level (>4.0 mU/L) and a normal serum free thyroxine level (11 to 25 pmol/L [0.9 to 1.9 ng/dL]). In tests for antibodies to thyroid peroxidase, a serum level greater than 10 IU/mL was considered a positive result. RESULTS: Subclinical hypothyroidism was present in 10.8% of participants and was associated with a greater age-adjusted prevalence of aortic atherosclerosis (odds ratio, 1.7 [95% CI, 1.1 to 2.6]) and myocardial infarction (odds ratio, 2.3 [CI, 1.3 to 4.0]). Additional adjustment for body mass index, total and high-density lipoprotein cholesterol level, blood pressure, and smoking status, as well as exclusion of women who took beta-blockers, did not affect these estimates. Associations were slightly stronger in women who had subclinical hypothyroidism and antibodies to thyroid peroxidase (odds ratio for aortic atherosclerosis, 1.9 [CI, 1.1 to 3.6]; odds ratio for myocardial infarction, 3.1 [CI, 1.5 to 6.3]). No association was found between thyroid autoimmunity itself and cardiovascular disease. The population attributable risk percentage for subclinical hypothyroidism associated with myocardial infarction was within the range of that for known major risk factors for cardiovascular disease. CONCLUSION: Subclinical hypothyroidism is a strong indicator of risk for atherosclerosis and myocardial infarction in elderly women.

Potent inhibition of estrogen sulfotransferase by hydroxylated PCB metabolites: a novel pathway explaining the estrogenic activity of PCBs (Article)

2000-01-01T00:00:00Z

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants which exert a variety of toxic effects in animals, including disturbances of sexual development and reproductive function. The estrogenic effects of PCBs may be mediated in part by hydroxylated PCB metabolites (PCB-OHs), but the mechanisms by which they are brought about are not understood. PCBs as well as PCB-Hs show low affinities for both alpha and beta estrogen receptor isoforms. In the present study we demonstrate that various environmentally relevant PCB-OHs are extremely potent inhibitors of human estrogen sulfotransferase, strongly suggesting that they indirectly induce estrogenic activity by increasing estradiol bioavailability in target tissues.

Characterization of human iodothyronine sulfotransferases (Article)

1999-01-01T00:00:00Z

Sulfation is an important pathway of thyroid hormone metabolism that facilitates the degradation of the hormone by the type I iodothyronine deiodinase, but little is known about which human sulfotransferase isoenzymes are involved. We have investigated the sulfation of the prohormone T4, the active hormone T3, and the metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2) by human liver and kidney cytosol as well as by recombinant human SULT1A1 and SULT1A3, previously known as phenol-preferring and monoamine-preferring phenol sulfotransferase, respectively. In all cases, the substrate preference was 3,3'-T2 >> rT3 > T3 > T4. The apparent Km values of 3,3'-T2 and T3 [at 50 micromol/L 3'-phosphoadenosine-5'-phosphosulfate (PAPS)] were 1.02 and 54.9 micromol/L for liver cytosol, 0.64 and 27.8 micromol/L for kidney cytosol, 0.14 and 29.1 micromol/L for SULT1A1, and 33 and 112 micromol/L for SULT1A3, respectively. The apparent Km of PAPS (at 0.1 micromol/L 3,3'-T2) was 6.0 micromol/L for liver cytosol, 9.0 micromol/L for kidney cytosol, 0.65 micromol/L for SULT1A1, and 2.7 micromol/L for SULT1A3. The sulfation of 3,3'-T2 was inhibited by the other iodothyronines in a concentration-dependent manner. The inhibition profiles of the 3,3'-T2 sulfotransferase activities of liver and kidney cytosol obtained by addition of 10 micromol/L of the various analogs were better correlated with the inhibition profile of SULT1A1 than with that of SULT1A3. These results indicate similar substrate specificities for iodothyronine sulfation by native human liver and kidney sulfotransferases and recombinant SULT1A1 and SULT1A3. Of the latter, SULT1A1 clearly shows the highest affinity for both iodothyronines and PAPS, but it remains to be established whether it is the prominent isoenzyme for sulfation of thyroid hormone in human liver and kidney.

Sulfation of thyroid hormone by estrogen sulfotransferase (Article)

1999-01-01T00:00:00Z

Sulfation is one of the pathways by which thyroid hormone is inactivated. Iodothyronine sulfate concentrations are very high in human fetal blood and amniotic fluid, suggesting important production of these conjugates in utero. Human estrogen sulfotransferase (SULT1E1) is expressed among other tissues in the uterus. Here we demonstrate for the first time that SULT1E1 catalyzes the facile sulfation of the prohormone T4, the active hormone T3 and the metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2) with preference for rT3 approximately 3,3'-T2 > T3 approximately T4. Thus, a single enzyme is capable of sulfating two such different hormones as the female sex hormone and thyroid hormone. The potential role of SULT1E1 in fetal thyroid hormone metabolism needs to be considered.

Cloning and characterization of type III iodothyronine deiodinase from the fish Oreochromis niloticus (Article)

1999-01-01T00:00:00Z

Type III iodothyronine deiodinase (D3) catalyzes the inner ring deiodination (IRD) of T4 and T3 to the inactive metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2), respectively. Here we describe the cloning and characterization of complementary DNA (cDNA) coding for D3 in fish (Oreochromis niloticus, tilapia). This cDNA contains 1478 nucleotides and codes for a protein of 267 amino acids, including a putative selenocysteine (Sec) residue, encoded by a TGA triplet, at position 131. The deduced amino acid sequence shows 57-67% identity with frog, chicken, and mammalian D3, 33-39% identity with frog, fish (Fundulus heteroclitus) and mammalian D2, and 30-35% identity with fish (tilapia), chicken, and mammalian D1. The 3' UTR contains a putative Sec insertion sequence (SECIS) element. Recombinant tilapia D3 (tD3) expressed in COS-1 cells and native tD3 in tilapia brain microsomes show identical catalytic activities, with a strong preference for IRD of T3 (Km approximately 20 nM). IRD of [3,5-125I]T3 by native and recombinant tD3 are equally sensitive to inhibition by substrate analogs (T3 > T4 >> rT3) and inhibitors (gold thioglucose >> iodoacetate > propylthiouracil). Northern analysis using a tD3 riboprobe shows high expression of a 1.6-kb messenger RNA in gill and brain, although D3 activity is much higher in brain than in gill. The characterization of tD3 cDNA provides new information about the structure-activity relationship of iodothyronine deiodinases and an important tool to study the regulation of thyroid hormone bioactivity in fish.

Rapid sulfation of 3,3',5'-triiodothyronine in native Xenopus laevis oocytes (Article)

1998-01-01T00:00:00Z

Sulfation is an important metabolic pathway facilitating the degradation of thyroid hormone by the type I iodothyronine deiodinase. Different human and rat tissues contain cytoplasmic sulfotransferases that show a substrate preference for 3,3'-diiodothyronine (3,3'-T2) > T3 > rT3 > T4. During investigation of the expression of plasma membrane transporters for thyroid hormone by injection of rat liver RNA in Xenopus laevis oocytes, we found uptake and metabolism of iodothyronines by native oocytes. Groups of 10 oocytes were incubated for 20 h at 18 C in 0.1 ml medium containing 500,000 cpm (1-5 nM) [125I]T4, [125I]T3, [125I]rT3, or [125I]3,3'-T2. In addition, cytosol prepared from oocytes was tested for iodothyronine sulfotransferase activity by incubation of 1 mg cytosolic protein/ml for 30 min at 21 C with 1 microM [125I]T4, [125I]T3, [125I]rT3, or [125I]3,3'-T2 and 50 microM 3'-phosphoadenosine-5'-phosphosulfate. Incubation media, oocyte extracts, and assay mixtures were analyzed by Sephadex LH-20 chromatography for production of conjugates and iodide. After 20-h incubation, the percentage of added radioactivity present as conjugates in the media and oocytes amounted to 0.9 +/- 0.2 and 1.0 +/- 0.1 for T4, less than 0.1 and less than 0.1 for T3, 32.5 +/- 0.4 and 29.3 +/- 0.2 for rT3, and 3.8 +/- 0.3 and 2.3 +/- 0.2 for 3,3'-T2, respectively (mean +/- SEM; n = 3). The conjugate produced from rT3 was identified as rT3 sulfate, as it was hydrolyzed by acid treatment. After injection of oocytes with copy RNA coding for rat type I iodothyronine deiodinase, we found an increase in iodide production from rT3 from 2.3% (water-injected oocytes) to 46.2% accompanied by a reciprocal decrease in rT3 sulfate accumulation from 53.7% to 7.1%. After 30-min incubation with cytosol and 3'-phosphoadenosine-5'-phosphosulfate, sulfate formation amounted to 1.8% for T4, less than 0.1% for T3, 77.9% for rT3, and 2.9% for 3,3'-T2. These results show that rT3 is rapidly metabolized in native oocytes by sulfation. The substrate preference of the sulfotransferase activity in oocytes is rT3 >> 3,3'-T2 > T4 > T3. The physiological significance of the high activity for rT3 sulfation in X. laevis oocytes remains to be established.

Ontogeny of iodothyronine deiodinases in human liver (Article)

1998-01-01T00:00:00Z

The role of the deiodinases D1, D2, and D3 in the tissue-specific and time-dependent regulation of thyroid hormone bioactivity during fetal development has been investigated in animals but little is known about the ontogeny of these enzymes in humans. We analyzed D1, D2, and D3 activities in liver microsomes from 10 fetuses of 15-20 weeks gestation and from 8 apparently healthy adult tissue transplant donors, and in liver homogenates from 2 fetuses (20 weeks gestation), 5 preterm infants (27-32 weeks gestation), and 13 term infants who survived up to 39 weeks postnatally. D1 activity was determined using 1 microM [3',5'-125I]rT3 as substrate and 10 mM dithiothreitol (DTT) as cofactor, D2 activity using 1 nM [3',5'-125I]T4 and 25 mM DTT in the presence of 1 mM 6-propyl-2-thiouracil (to block D1 activity) and 1 microM T3 (to block D3 activity), and D3 activity using 10 nM [3,5-125I]T3 and 50 mM DTT, by quantitation of the release of 125I. The assays were validated by high performance liquid chromatography of the products, and kinetic analysis [Michaelis-Menten constant (Km) of rT3 for D1: 0.5 microM; Km of T3 for D3: 2 nM]. In liver homogenates, D1 activity was not correlated with age, whereas D3 activity showed a strong negative correlation with age (r -0.84), with high D3 activities in preterm infants and (except in 1 infant of 35 weeks) absent D3 activity in full-term infants. In microsomes, D1 activities amounted to 4.3-60 pmol/min/mg protein in fetal livers and to 170-313 pmol/min/mg protein in adult livers, whereas microsomal D3 activities were 0.15-1.45 pmol/min/mg protein in fetuses and <0.1 pmol/min/mg protein in all but one adult. In the latter sample, D3 activity amounted to 0.36 pmol/min/mg protein. D2 activity was negligible in both fetal and adult livers. These findings indicate high D1 and D3 activities in fetal human liver, and high D1 and mostly absent D3 activities in adult human liver. Therefore, the low serum T3 levels in the human fetus appear to be caused by high hepatic (and placental) D3 activity rather than caused by low hepatic D1 activity. The occasional expression of D3 in adult human liver is intriguing and deserves further investigation.

Expression of rat liver cell membrane transporters for thyroid hormone in Xenopus laevis oocytes (Article)

1997-01-01T00:00:00Z

The present study was conducted to explore the possible use of Xenopus laevis oocytes for the expression cloning of cell membrane transporters for iodothyronines. Injection of stage V-VI X. laevis oocytes with 23 ng Wistar rat liver polyadenylated RNA (mRNA) resulted after 3-4 days in a highly significant increase in [125I]T3 (5 nM) uptake from 6.4 +/- 0.8 fmol/oocyte x h in water-injected oocytes to 9.2 +/- 0.65 fmol/oocyte x h (mean +/- SEM; n = 19). In contrast, [125I]T4 (4 nM) uptake was not significantly stimulated by injection of total liver mRNA. T3 uptake induced by liver mRNA was significantly inhibited by replacement of Na+ in the incubation medium by choline+ or by simultaneous incubation with 1 microM unlabeled T3. In contrast, T3 uptake by water-injected oocytes was not Na+ dependent. Fractionation of liver mRNA on a 6-20% sucrose gradient showed that maximal stimulation of T3 uptake was obtained with mRNA of 0.8-2.1 kilobases (kb). In contrast to unfractionated mRNA, the 0.7- to 2.1-kb fraction also significantly stimulated transport of T4, and it was found to induce uptake of T3 sulfate (T3S). Because T3S is a good substrate for type I deiodinase (D1), 2.3 ng rat D1 complementary RNA (cRNA) were injected either alone or together with 23 ng of the 0.8- to 2.1-kb fraction of rat liver mRNA. Compared with water-injected oocytes, injection of D1 cRNA alone did not stimulate uptake of [125I]T3S (1.25 nM). T3S uptake in liver mRNA and D1 cRNA-injected oocytes was similar to that in oocytes injected with mRNA alone, showing that transport of T3S is independent of the metabolic capacity of the oocyte. Furthermore, coinjection of liver mRNA and D1 cRNA strongly increased the production of 125I-, showing that the T3S taken up by the oocyte is indeed transported to the cell interior. In conclusion, injection of rat liver mRNA into X. laevis oocytes resulted in a stimulation of saturable, Na+-dependent T4, T3 and T3S transport, indicating that rat liver contains mRNA(s) coding for plasma membrane transporters for these iodothyronine derivatives.

Renal clearance of the thyrotropin-releasing hormone-like peptide pyroglutamyl-glutamyl-prolineamide in humans (Article)

1997-01-01T00:00:00Z

TRH-like peptides have been identified that differ from TRH (pGlu-His-ProNH2) in the middle amino acid. We have estimated TRH-like immunoreactivity (TRH-LI) in human serum and urine by RIA with TRH-specific antiserum 8880 or with antiserum 4319, which binds most peptides with the structure pGlu-X-ProNH2. TRH was undetectable in serum (< 25 pg/mL), but TRH-LI was detected with antiserum 4319 in serum of 27 normal subjects, 21 control patients, and 12 patients with carcinoid tumors (range 17-45, 5-79, and 18-16,600 pg/mL, respectively). Because serum was kept for at least 2 h at room temperature, which causes degradation of TRH, pGlu-Phe-ProNH2, and pGlu-Tyr-ProNH2, serum TRH-LI is not caused by these peptides. On high-performance liquid chromatography, serum TRH-LI coeluted with pGlu-Glu-ProNH2 (< EEP-NH2), a peptide produced in, among others, the prostate. Urine of normals and control patients also contained TRH-LI (range 1.14-4.97 and 0.24-5.51 ng/mL, respectively), with similar levels in males and females. TRH represented only 2% of urinary TRH-LI, and anion-exchange chromatography and high-performance liquid chromatography revealed that most TRH-LI in urine was < EEP-NH2. In patients with carcinoid tumors, increased urinary TRH-LI levels were noted (range 1.35-962.4 ng/mL). Urinary TRH-LI correlated positively with urinary creatinine, and the urinary clearance rate of TRH-LI was similar to the glomerular filtration rate. In addition, serum TRH-LI was increased in 17 hemodialysis patients (43-373 pg/mL). This suggests that serum < EEP-NH2 is cleared by glomerular filtration with little tubular resorption. The possible role of the prostate as a source of urinary TRH-LI was evaluated in 11 men with prostate cancer, showing a 25% decrease in urinary TRH-LI excretion after prostatectomy (0.19 +/- 0.02 vs. 0.15 +/- 0.01 ng/mumol creatinine, mean +/- SEM). However, TRH-LI was similar in spontaneously voided urine and in urine obtained through a nephrostomy cannula from 16 patients with unilateral urinary tract obstruction (0.15 +/- 0.01 vs. 0.14 +/- 0.01 ng/mumol creatinine). These data indicate that: 1) TRH-LI in human serum represents largely < EEP-NH2, which is cleared by renal excretion; 2) part of urinary < EEP-NH2 is derived from prostatic secretion into the blood and not directly into urine; and 3) urinary < EEP-NH2 can be used as marker for carcinoid tumors.

Characterization of iodothyronine sulfotransferase activity in rat liver (Article)

1997-01-01T00:00:00Z

Sulfation is an important pathway in the metabolism of thyroid hormone because it strongly facilitates the degradation of the hormone by the type I iodothyronine deiodinase. However, little is known about the properties and possible regulation of the sulfotransferase(s) involved in the sulfation of thyroid hormone. We have developed a convenient method for the analysis of iodothyronine sulfotransferase activity in tissue cytosolic fractions, using radioiodinated 3,3'-diiodothyronine (3,3'-T2) as the preferred substrate, unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor, and Sephadex LH-20 minicolomns for separation of the products. We found that iodothyronine sulfotransferase activity in rat liver cytosol is 1) higher in male than in female rats; 2) optimal at pH 8.0; 3) characterized (at 50 microM PAPS and pH 7.2) by apparent Michaelis-Menton (Km) values for 3,3'-T2 of 1.77 and 4.19 microM, and Vmax values of 1.94 and 1.45 nmol/min per mg protein in male and female rats, respectively; 4) characterized (at 1 microM 3,3'-T2 and pH 7.2) by apparent Km values for PAPS of 4.92 and 3.80 microM and Vmax values of 0.72 and 0.31 nmol/min per mg protein, in males and females, respectively; 5) little affected by hyperthyroidism in both male and female rats, but significantly decreased by hypothyroidism in males but not in females; and 6) not affected by short-term (3 days) fasting in both male and female rats, but significantly decreased by long-term (3 weeks) food restriction to one-third of normal intake in males but not in females. It is suggested that the higher hepatic iodothyronine sulfotransferase activity in male vs. female rats, as well as the decreases induced in males by hypothyroidism and long-term food restriction, represents differences in the expression of the male-dominant isoenzyme rSULT1C1.

Expression of chicken hepatic type I and type III iodothyronine deiodinases during embryonic development (Article)

1997-01-01T00:00:00Z

In embryonic chicken liver (ECL) two types of iodothyronine deiodinases are expressed: D1 and D3. D1 catalyzes the activation as well as the inactivation of thyroid hormone by outer and inner ring deiodination, respectively. D3 only catalyzes inner ring deiodination. D1 and D3 have been cloned from mammals and amphibians and shown to contain a selenocysteine (Sec) residue. We characterized chicken D1 and D3 complementary DNAs (cDNAs) and studied the expression of hepatic D1 and D3 messenger RNAs (mRNAs) during embryonic development. Oligonucleotides based on two amino acid sequences strongly conserved in the different deiodinases (NFGSCTSecP and YIEEAH) were used for reverse transcription-PCR of poly(A+) RNA isolated from embryonic day 17 (E17) chicken liver, resulting in the amplification of two 117-bp DNA fragments. Screening of an E17 chicken liver cDNA library with these probes led to the isolation of two cDNA clones, ECL1711 and ECL1715. The ECL1711 clone was 1360 bp long and lacked a translation start site. Sequence alignment showed that it shared highest sequence identity with D1s from other vertebrates and that the coding sequence probably lacked the first five nucleotides. An ATG start codon was engineered by site-directed mutagenesis, generating a mutant (ECL1711M) with four additional codons (coding for MGTR). The open reading frame of ECL1711M coded for a 249-amino acid protein showing 58-62% identity with mammalian D1s. An in-frame TGA codon was located at position 127, which is translated as Sec in the presence ofa Sec insertion sequence (SECIS) identified in the 3'-untranslated region. Enzyme activity expressed in COS-1 cells by transfection with ECL1711M showed the same catalytic, substrate, and inhibitor specificities as native chicken D1. The ECL1715 clone was 1366 bp long and also lacked a translation start site. Sequence alignment showed that it was most homologous with D3 from other species and that the coding sequence lacked approximately the first 46 nucleotides. The deduced amino acid sequence showed 62-72% identity with the D3 sequences from other species, including a putative Sec residue at a corresponding position. The 3'-untranslated region of ECL1715 also contained a SECIS element. These results indicate that ECL1711 and ECL1715 are near-full-length cDNA clones for chicken D1 and D3 selenoproteins, respectively. The ontogeny of D1 and D3 expression in chicken liver was studied between E14 and 1 day after hatching (C1). D1 activity showed a gradual increase from E14 until C1, whereas D1 mRNA level remained relatively constant. D3 activity and mRNA level were highly significantly correlated, showing an increase from E14 to E17 and a strong decrease thereafter. These results suggest that the regulation of chicken hepatic D3 expression during embryonic development occurs predominantly at the pretranslational level.

Characterization of a propylthiouracil-insensitive type I iodothyronine deiodinase (Article)

1997-01-01T00:00:00Z

Mammalian type I iodothyronine deiodinase (D1) activates and inactivates thyroid hormone by outer ring deiodination (ORD) and inner ring deiodination (IRD), respectively, and is potently inhibited by propylthiouracil (PTU). Here we describe the cloning and characterization of a complementary DNA encoding a PTU-insensitive D1 from teleost fish (Oreochromis niloticus, tilapia). This complementary DNA codes for a protein of 248 amino acids, including a putative selenocysteine (Sec) residue, encoded by a TGA triplet, at position 126. The 3' untranslated region contains two putative Sec insertion sequence (SECIS) elements. Recombinant enzyme expressed in COS-1 cells catalyzes both ORD of T4 and rT3 and IRD of T3 and T3 sulfate with the same substrate specificity as native tilapia D1 (tD1), i.e. rT3 >> T4 > T3 sulfate > T3. Native and recombinant tD1 show equally low sensitivities to inhibition by PTU, iodoacetate, and gold thioglucose compared with the potent inhibitions observed with mammalian D1s. Because the residue 2 positions downstream from Sec is Pro in tD1 and in all (PTU-insensitive) type II and type III iodothyronine deiodinases but Ser in all PTU-sensitive D1s, we prepared the Pro128Ser mutant of tD1. The mutant enzyme showed strongly decreased ORD and somewhat increased IRD activity, but was still insensitive to PTU. These results provide new information about the structure-activity relationship of D1 concerning two characteristic properties, i.e. catalysis of both ORD and IRD, and inhibition by PTU.

Evidence that the TRH-like peptide pyroglutamyl-glutamyl-prolineamide in human serum may not be secreted by the pituitary gland (Article)

1997-01-01T00:00:00Z

Recent studies have revealed that TRH-like immunoreactivity (TRH-LI) in human serum is predominantly pGlu-Glu-ProNH2 (< EEP-NH2), a peptide previously found in, among others tissues, the pituitary gland of various mammalian species. In the rat pituitary, < EEP-NH2 is present in gonadotrophs and its pituitary content is regulated by gonadal steroids and gonadotrophin-releasing hormone (GnRH). Hence, we reasoned that < EEP-NH2 in human serum may also arise, at least in part, from the pituitary, and that its secretion may correlate with that of gonadotrophins. Therefore, blood was simultaneously sampled from both inferior petrosal sinuses, which are major sites of the venous drainage of the pituitary gland, and a peripheral vein from seven patients with suspected adrenocorticotrophin-secreting pituitary tumours. In addition, in six postmenopausal and six cyclic women, peripheral vein blood was collected at 10-min intervals for 6 h, then a standard 100 micrograms GnRH test was performed. In the sera, TRH-LI was estimated by RIA with antiserum 4319, which binds most tripeptides that share the N- and C-terminal amino acids with TRH (pGlu-His-ProNH2). In addition, LH and FSH were measured in these sera by RIA. In the blood samples taken at 10-min intervals, an episodic variation in serum TRH-LI was noted and pulses of TRH-LI were detected at irregular intervals (from one to six pulses per 6 h) in five postmenopausal and six cyclic women. In general, these pulses did not coincide with those of LH and FSH, suggesting that TRH-LI is not co-secreted with gonadotrophins. Moreover, unlike LH and FSH, serum TRH-LI did not increase during the menopause or after exogenous administration of GnRH. Whereas gonadotrophin concentrations were significantly greater in the inferior petrosal sinus than in peripheral serum, there were no differences in TRH-LI concentrations between these serum samples. In conclusion, serum TRH-LI in humans seems not to be regulated by gonadal steroids or GnRH. Moreover, serum derived directly from the pituitary contained no more TRH-LI than did peripheral serum, which suggests that the human pituitary gland does not secrete significant amounts of < EEP-NH2, and therefore does not contribute significantly to serum TRH-LI concentrations. Further research is required to identify the site of origin of < EEP-NH2 in human serum.

Uptake of 3,3',5,5'-tetraiodothyroacetic acid and 3,3',5'-triiodothyronine in cultured rat anterior pituitary cells and their effects on thyrotropin secretion (Article)

1995-01-01T00:00:00Z

We compared the uptake, metabolism, and biological effects of tetraiodothyroacetic acid (Tetrac) and rT3 in anterior pituitary cells with those of T4 and T3. Cells were isolated from adult male Wistar rats and cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C in medium with 0.1% BSA for [125I]Tetrac (200,000 cpm; 240 pM) and [125I]T4 (100,000 cpm; 175 pM) or with 0.5% BSA for [125I]rT3 (100,000 cpm; 250 pM) and [125I]T3 (50,000 cpm; 50 pM). The free fraction of Tetrac was 1% that of T4 (in medium with 0.1 and with 0.5% BSA), and the free fraction of rT3 was half that of T3. Uptake of the four tracers increased sharply up to 1 h of incubation and then leveled off. Expressed as femtomoles per pM free hormone, uptake at equilibrium was 1.16 +/- 0.16 (n = 6) for Tetrac, 0.15 +/- 0.01 (n = 6) for T4, 0.023 +/- 0.003 (n = 6) for rT3, and 0.21 +/- 0.02 (n = 6) for T3. Cell-associated radioactivity after incubation for 24 h with [125I]Tetrac was represented for 15% by [125I]Triac; after incubation with [125I]T4 for 15-20% by [125I]T3, after incubation with [125I]rT3 for 6% by [125I]3,3'-T2, while [125I]T3 was still for 98% [125I]T3. Exposure of cells for 2 h to 100 nM TRH stimulated TSH release by 90-135%. Tetrac was effective in reducing this response at a free concentration of 0.05 pM, but rT3 was effective only at a free concentration of 16 nM. A free Tetrac concentration of 5 pM was equally effective as 50 pM free T4 in reducing the TSH response to TRH. In human serum, Tetrac was exclusively bound to T4-binding prealbumin. The free Tetrac fraction was 0.001% in control subjects and rose 2- to 12-fold in patients with nonthyroidal illness. As uptake of [125I]Tetrac in the pituitary was higher than that of T4 and T3, and it was more potent than T4 in reducing TSH release, Tetrac may be of potential significance for the regulation of TSH secretion in vivo.

Different effects of continuous infusion of interleukin-1 and interleukin-6 on the hypothalamic-hypophysial-thyroid axis (Article)

1994-01-01T00:00:00Z

The cytokines interleukin-1 (IL-1) and IL-6 are thought to be important mediators in the suppression of thyroid function during nonthyroidal illness. In this study we compared the effects of IL-1 and IL-6 infusion on the hypothalamus-pituitary-thyroid axis in rats. Cytokines were administered by continuous ip infusion of 4 micrograms IL-1 alpha/day for 1, 2, or 7 days or of 15 micrograms IL-6/day for 7 days. Body weight and temperature, food and water intake, and plasma TSH, T4, free T4 (FT4), T3, and corticosterone levels were measured daily, and hypothalamic pro-TRH messenger RNA (mRNA) and hypophysial TSH beta mRNA were determined after termination of the experiments. Compared with saline-treated controls, infusion of IL-1, but not of IL-6, produced a transient decrease in food and water intake, a transient increase in body temperature, and a prolonged decrease in body weight. Both cytokines caused transient decreases in plasma TSH and T4, which were greater and more prolonged with IL-1 than with IL-6, whereas they effected similar transient increases in the plasma FT4 fraction. Infusion with IL-1, but not IL-6, also induced transient decreases in plasma FT4 and T3 and a transient increase in plasma corticosterone. Hypothalamic pro-TRH mRNA was significantly decreased (-73%) after 7 days, but not after 1 or 2 days, of IL-1 infusion and was unaffected by IL-6 infusion. Hypophysial TSH beta mRNA was significantly decreased after 2 (-62%) and 7 (-62%) days, but not after 1 day, of IL-1 infusion and was unaffected by IL-6 infusion. These results are in agreement with previous findings that IL-1, more so than IL-6, directly inhibits thyroid hormone production. They also indicate that IL-1 and IL-6 both decrease plasma T4 binding. Furthermore, both cytokines induce an acute and dramatic decrease in plasma TSH before (IL-1) or even without (IL-6) a decrease in hypothalamic pro-TRH mRNA or hypophysial TSH beta mRNA, suggesting that the acute decrease in TSH secretion is not caused by decreased pro-TRH and TSH beta gene expression. The TSH-suppressive effect of IL-6, either administered as such or induced by IL-1 infusion, may be due to a direct effect on the thyrotroph, whereas additional effects of IL-1 may involve changes in the hypothalamic release of somatostatin or TRH.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake of triiodothyroacetic acid and its effect on thyrotropin secretion in cultured anterior pituitary cells (Article)

1994-01-01T00:00:00Z

The uptake of [125I]triiodothyroacetic acid ([125I]Triac) in anterior pituitary cells was investigated and compared with that of [125I]T3. Furthermore, the effects of Triac, T3, and T4 on TSH release were compared. Cells isolated from adult male Wistar rats were cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C with [125I]Triac (100,000 cpm; 120 pM) or [125I]T3 (50,000 cpm; 50 pM) in medium with 0.5% BSA. In this medium, the ratio of the free fractions of Triac, T3, and T4 was 1:8:1. Exposure of cells to 100 nM TRH for 2 h stimulated TSH release by 80-110% (P < 0.001). Comparing total hormone levels (1 nM to 1 microM), Triac and T3 were equally effective in reducing this response, and both were 10-fold more effective than T4. The time course (15 min to 4 h) of [125I]Triac uptake was similar to that of [125I]T3, showing equilibrium after 1 h. Unlabeled Triac (1 microM) reduced the uptake of [125I]Triac and [125I]T3 at all time intervals. Expressed per pM free hormone, the cellular and nuclear uptake of [125I]Triac were twice those of [125I]T3. The 15-min uptake of [125I]Triac was reduced by incubation with 10 nM unlabeled Triac (35%; P < 0.001). Maximum inhibition (56%; P < 0.001) was found with 10 microM Triac. A similar effect was seen with 10 microM T3, T4, or 3,3',5,5'-tetraiodothyroacetic acid. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced the cellular ATP content by 51% (P < 0.001), [125I]T3 uptake by 77% (P < 0.001), and [125I]Triac uptake by only 25% (P < 0.001). The temperature dependence of [125I]Triac and [125I]T3 uptake was the same. Preincubation and incubation with 10 microM monensin (reduces the Na+ gradient) or 10 microM monodansylcadaverine (inhibits receptor-mediated endocytosis) reduced 15-min [125I] Triac uptake by 15% (P < 0.005) and 19% (P < 0.005), respectively. The data show that 1) Triac, on the basis of the free hormone concentration, is more potent than T3 or T4 in suppressing TSH secretion; and 2) the rapid uptake of [125I]Triac by the anterior pituitary occurs by a carrier-mediated mechanism that is only partially dependent on ATP or the Na+ gradient.

Uptake of thyroxine in cultured anterior pituitary cells of euthyroid rats (Article)

1994-01-01T00:00:00Z

The uptake of [125I]T4 was investigated in cultured anterior pituitary cells isolated from adult fed Wistar rats and cultured for 3 days in medium containing 10% fetal calf serum. Experiments were performed with [125I]T4 (10(5) to 2 x 10(6) cpm; 0.35-7 nM) in medium containing 0.5% or 0.1% BSA. The uptake of [125I]T4 increased with time and showed equilibrium after around 1 h of incubation. The presence of 10 microM unlabeled T4 during incubation decreased the uptake of [125I]T4 by 65-70% at all time intervals. After 24 h of incubation, 1.5% iodide and 3.2% conjugates were detected in the medium, whereas around 20% of cellular radioactivity represented [125I]T3. The 15-min uptake of [125I]T4 was significantly reduced by simultaneous incubation with 100 nM T4 (by 24%; P < 0.05), 100 nM T3 (by 38%; P < 0.001), or 10 microM rT3 (by 32%; P < 0.001), whereas 10 microM tetraiodothyroacetic acid (Tetrac) had no effect. Furthermore, preincubation (30 min) and incubation (15 min) with 10 microM monodansylcadaverine, oligomycin, or monensin reduced the uptake of [125I]T4 by 30%, 50%, and 40%, respectively (all P < 0.001). Substitution of Na+ in the buffer by K+ diminished the uptake of [125I]T4 by 39% (P < 0.005); 2 mM phenylalanine, tyrosine, or tryptophan reduced [125I]T4 uptake by 18% (P < 0.05), 18% (P = NS), and 33% (P < 0.005), respectively. Our data suggest that the pituitary contains a specific carrier-mediated energy-requiring mechanism for [125I]T4 uptake that is partly dependent on the Na+ gradient. In addition, part of [125I]T4 uptake in the pituitary might occur through an amino acid transport system. When expressed per pM of free hormone, the 15-min uptake of [125I]T4 was approximately as high as that of [125I]T3. Because the reduction of [125I]T4 uptake by T4, T3, monodansylcadaverine, oligomycin, and monensin was roughly the same as the previously reported reduction of [125I]T3 uptake by the same compounds, it is further suggested that T4 and T3 share a common carrier in cultured anterior pituitary cells.

Thyroid hormone deiodination (Doctoral Thesis)

1980-10-22T00:00:00Z

The enzymatic deiodination of thyroid hormone is an important process since it concerns- among other things- the regulation of thyromimetic activity at the site of the target organ. To understand the mechanism of this regulation it is necessary to have a detailed knowledge of the mode of action of the enzyme(s) involved in the metabolism of thyroid hormone. My investigations of the deiodination of iodothyronines at the subcellular level, forming the basis of this thesis, are described in the appendix papers. It is not intended to deal in extenso with the technical aspects of my studies in the preceeding chapters. Rather it will be attempted to give a general review of the literature including- with some emphasis -my own work. Though not directly related to the subject of this thesis, the biosynthesis of thyroid hormone in the thyroid gland is treated in the first chapter. This is done because of possible similarities between thyroid hormone iodination and deiodination pathways, which are suggested by the finding that some drugs inhibit both processes. In the same chapter the relationship between iodothyronine structure and biological potency is described to illustrate that indeed deiodination has a dramatic effect on the activity of thyroid hormone. Besides deiodination, other pathways of metabolism are also considered. The second chapter concerns the in vivo investigation of thyroid hormone deiodination under physiological and pathological conditions. This includes the effects of internal and external factors which affect deiodination, such as dietary intake, drugs, stress and illness. Since much work has been done to find an explanation for the effect of calorie restriction on deiodination at the molecular level, the role of the diet is emphasized. This appears particularly important since nutritional status must be considered to contribute to the change in thyroid hormone metabolism found in other situations, for example in systemic illness. The in vitro observations of the enzymatic deiodination of thyroid hormone are described in chapter 3. A distinction has been made between (early) reports on the analysis of iodide production using chromatography, and (more recent) studies dealing with the detection of specific metabolites, often by means of radioimmunoassay. My investigations which belong to the latter category are presented in the appendix papers