EFFECTS OF A NEW SYNTHETIC VITAMIN-D ANALOG, EB1089, ON THE ESTROGEN-RESPONSIVE GROWTH OF HUMAN BREAST-CANCER CELLS

The anti-proliferative effects of the novel vitamin D analogue, EB1089, were assessed in the hormone-dependent breast cancer cell line, MCF-7, in vitro. In the present study, EB1089 was shown to be at least an order of magnitude more potent at inhibiting MCF-7 cell proliferation than the native hormone, 1 alpha,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3). Treatment of MCF-7 cell cultures with combinations of oestradiol and EB1089 ranging from 5 x 10(-11) M to 5 x 10(-9) M revealed the ability of EB1089 to suppress the mitogenic effects of oestradiol in these cells dose-dependently, as determined by [H-3]thymidine incor poration and cell counts. EB1089 also exhibited a significant time- and dose-dependent decrease in MCF-7 oestrogen receptor (ER) concentration, as assessed by Ligand binding assay. A fourfold reduction of ER levels by 5 x 10(-9) M EB1089 relative to control ER levels was observed, whilst 5 x 10(-9) M 1,25(OH)(2)D-3 produced a significant but less dramatic decrease in ER levels. In addition, reduction of ER protein in EB1089-treated cell cultures was also demonstrated using an oestrogen receptor enzyme immunoassay. The interaction of EB1089 and anti-oestrogens on the oestradiol-stimulated growth of MCF-7 cells was investigated. The treatment of cell cultures with 5 x 10(-10) M EB1089 in combination with the pure anti-oestrogen, ICI 182,780 (5 x 10(-8) M), and in the presence of between 5 x 10(-10) M and 5 x 10(-9) M oestradiol, produced an augmented inhibition of MCF-7 cell proliferation compared with the actions of either compound alone. This study demonstrates that EB1089 is a potent anti-proliferative agent of breast cancer cells in vitro, and that part of its mechanism to inhibit cell growth may involve the modulation of ER expression, such that the responsiveness of cells to the growth-stimulatory effects of oestradiol is diminished. This new vitamin D analogue has potential in the treatment of breast cancer.