Inhibition of Breast Cancer Cell Growth by Combined Treatment with Vitamin D3 Analogues and Tamoxifen

The steroid hormone 1,25-dihydroxyvitamin D3 [l,25-(OH)2D3] has potential to be used as an antitumor agent, but its clinical application is restricted by the strong calcemic activity. Therefore, new vitamin D3 analogues are developed with increased growth inhibitory and reduced calcemic activity. In the present study, we have examined the antiproliferative effects of four novel vitamin D3 analogues (CB966, EB1089, KH1060, and 22-oxa-calcitriol) on breast cancer cells, either alone or in combination with the antiestrogen tamoxifen. The estrogen-dependent ZR-75-1 and estrogen-responsive MCF-7 cell lines were used as a model. It was shown that, with EB1089 and KH1060, the same growth inhibitory effect as l,25-(OH)2D3 could be reached at up to 100-fold lower concentrations, whereas CB966 and 22-oxa-calcitriol were nearly equipotent with 1,25-(OH)2D3. The growth inhibition by the vitamin D3 compounds could be augmented by combined treatment with tamoxifen. At the maximal effective concentrations of the vitamin D3 compounds, the effect of combined treatment was additive (MCF-7 cells) or less than additive (ZR-75-1 cells). Tamoxifen increased the sensitivity of the cells to the vitamin D3 compounds 2- to 4000-fold, which was expressed by a shift to lower median effective concentration values. Thereby, the vitamin D3 compounds may be used at even lower dosages in combination therapy with tamoxifen. A major problem of tamoxifen therapy is the development of tamoxifen resistance. We have observed that tamoxifen-resistant clones of ZR-75-1 cells retain their response to the vitamin D3 compounds. Regulation of the growth-related oncogene c-myc (mRNA level) and the estrogen receptor (protein level) were studied but appeared not to be related to the antiproliferative action of the vitamin D3 compounds. Together, our data point to a potential benefit of combination therapy with 1,25-(OH)2D3 or vitamin D3 analogues and tamoxifen for the treatment of breast cancer.</p

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

Inhibition of Breast Cancer Cell Growth by Combined Treatment with Vitamin D3 Analogues and Tamoxifen

The steroid hormone 1,25-dihydroxyvitamin D3 [l,25-(OH)2D3] has potential to be used as an antitumor agent, but its clinical application is restricted by the strong calcemic activity. Therefore, new vitamin D3 analogues are developed with increased growth inhibitory and reduced calcemic activity. In the present study, we have examined the antiproliferative effects of four novel vitamin D3 analogues (CB966, EB1089, KH1060, and 22-oxa-calcitriol) on breast cancer cells, either alone or in combination with the antiestrogen tamoxifen. The estrogen-dependent ZR-75-1 and estrogen-responsive MCF-7 cell lines were used as a model. It was shown that, with EB1089 and KH1060, the same growth inhibitory effect as l,25-(OH)2D3 could be reached at up to 100-fold lower concentrations, whereas CB966 and 22-oxa-calcitriol were nearly equipotent with 1,25-(OH)2D3. The growth inhibition by the vitamin D3 compounds could be augmented by combined treatment with tamoxifen. At the maximal effective concentrations of the vitamin D3 compounds, the effect of combined treatment was additive (MCF-7 cells) or less than additive (ZR-75-1 cells). Tamoxifen increased the sensitivity of the cells to the vitamin D3 compounds 2- to 4000-fold, which was expressed by a shift to lower median effective concentration values. Thereby, the vitamin D3 compounds may be used at even lower dosages in combination therapy with tamoxifen. A major problem of tamoxifen therapy is the development of tamoxifen resistance. We have observed that tamoxifen-resistant clones of ZR-75-1 cells retain their response to the vitamin D3 compounds. Regulation of the growth-related oncogene c-myc (mRNA level) and the estrogen receptor (protein level) were studied but appeared not to be related to the antiproliferative action of the vitamin D3 compounds. Together, our data point to a potential benefit of combination therapy with 1,25-(OH)2D3 or vitamin D3 analogues and tamoxifen for the treatment of breast cancer