Vitamin D plays a major role in the regulation of mineral homeostasis and affects bone metabolism. So far, detailed knowledge on the vitamin D endocrine system in human bone cells is limited. Here we investigated the direct effects of 1α,25-(OH)2D3 on osteoblast differentiation and mineralization. Also, we studied the impact of 24-hydroxylation, generally considered as the first step in the degradation pathway of vitamin D, as well as the role of the nuclear and presumed membrane vitamin D receptor (VDR). For this we used a human osteoblast cell line (SV-HFO) that has the potency to differentiate during culture forming a mineralized extracellular matrix in a 3-week period. Transcriptional analyses demonstrated that both 1α,25-(OH)2D3 and the 24-hydroxylated metabolites 24R,25-(OH)2D3 and 1α,24R,25-(OH)3D 3 induced gene transcription. All metabolites dose-dependently increased alkaline phosphatase (ALP) activity and osteocalcin (OC) production (protein and RNA), and directly enhanced mineralization. 1α,24R,25-(OH) 3D3 stimulated ALP activity and OC production most potently, while for mineralization itwasequipotentto 1α,25-(OH) 2D3. The nuclear VDR antagonist ZK159222 almost completely blocked the effects of all metabolites. Interestingly, 1α,25-(OH) 2D3, an inhibitor of membrane effects of 1α,25-(OH)2D3 in the intestine, induced gene transcription and increased ALP activity, OC expression and mineralization. In conclusion, not only 1α,25-(OH)2D3, but also the presumed 24-hydroxylated "degradation" products stimulate differentiation of human osteoblasts. 1α,25-(OH)2D3 as well as the 24-hydroxylated metabolites directly enhance mineralization, with the nuclear VDR playing a central role. The intestinal antagonist 1β,25-(OH)2D3 acts in bone as an agonist and directly stimulates mineralization in a nuclear VDR-dependent way.

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Journal of Cellular Biochemistry
Department of Orthopaedics

van Driel, M, Koedam, M, Buurman, C.J, Roelse, M, Weyts, F.A.A, Chiba, H, … van Leeuwen, J.P.T.M. (2006). Evidence that both 1α,25-dihydroxyvitamin D3 and 24-hydroxylated D3 enhance human osteoblast differentiation and mineralization. Journal of Cellular Biochemistry, 99(3), 922–935. doi:10.1002/jcb.20875