Combined effects of 1,25-dihydroxyvitamin D3 and tamoxifen on the growth of MCF-7 and ZR-75-1 human breast cancer cells

In the present study we assessed the effect of combined treatment with 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and tamoxifen (TAM) on the growth of estrogen-responsive (MCF-7) and estrogen-dependent (ZR-75-1) human breast cancer cells. Both basal and 17β-estradiol (17β-E2)-stimulated growth were studied. 1,25-(OH)2D3 (10-10 - 10-7 M) time- and dose-dependently inhibited basal growth of MCF-7 cells, with growth arrest at 10-7 M. Also, 17β-E2-stimulated growth of MCF-7 and ZR-75-1 cells was inhibited by 1,25-(OH)2D3 in a time- and dose-dependent manner. TAM inhibited 17β-E2-stimulated growth of both cell lines and at high concentration (10-6 M) it also inhibited basal growth of MCF-7 cells. 10-6 M TAM together with 1,25-(OH)2D3 resulted in a further inhibition of basal (MCF-7 cells) as well as 17β-E2-stimulated proliferation (MCF-7 and ZR-75-1 cells) compared to the inhibition by these agents alone. TAM in combination with 10-7 M 1,25-(OH)2D3 resulted in growth arrest of 17β-E2-stimulated growth of MCF-7 cells. The inhibition of basal and 17β-E2-stimulated growth of MCF-7 cells was additive at early time points (4 days), but less than additive at later time points (8-10 days). It was demonstrated that with co-treatment of MCF-7 cells an equipotent inhibition of basal growth could be reached with lower concentrations of 1,25-(OH)2D3, compared to treatment with 1,25-(OH)2D3 alone. Studies with low concentrations (< 10-7 M) of TAM revealed a partial estrogenic effect, i.e. stimulation of MCF-7 proliferation in the absence of 17β-E2. This effect, which may resemble TAM-induced tumor flare, was completely prevented by co-treatment with a low concentration of 1,25-(OH)2D3 (10-9 M). Together, these results demonstrate the potent inhibition of breast cancer cell proliferation by 1,25-(OH)2D3 combined with TAM and indicate a potential benefit of combining these agents for the treatment of breast cancer.

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