1 alpha,25-dihydroxyvitamin D-3 down-regulates estrogen receptor abundanceand suppresses estrogen actions in MCF-7 human breast cancer cells

1 alpha,25-Dihydroxyvitamin D-3 [1,25(OH)(2)D-3], the active metabolite of vitamin D, is a potent inhibitor of breast cancer cell growth. Because the estrogen receptor (ER) plays a key role in breast cancer progression, we ha ve studied the effects of 1,25(OH)(2)D-3 on the regulation of ER in the est rogen-responsive MCF-7 human breast cancer cell line, which is known to pre dominantly express ER alpha. 1,25(OH)(2)D-3 causes significant inhibition o f MCF-7 cell growth, and it also decreases the growth-stimulatory effect of BP-estradiol (E-2), Treatment of MCF-7 cells with 1,25(OH)(2)D-3 reduces E R levels in a dose-dependent manner, as shown by ligand binding assays and Western blot analysis, The 1,25(OH),D, analogues EB-1089, KH-1060, Ro 27-05 74, and Ro 23-7553 are more potent than 1,25(OH)(2)D-3 in both their antipr oliferative actions as well as ER down-regulation. There is a striking corr elation (R-2 = 0.98) between the growth-inhibitory actions of 1,25(OH)(2)D- 3 or analogues and their ability to down-regulate ER levels. Treatment with 1,25(OH)(2)D-3 shows that the reduction in ER is accompanied by a signific ant decrease in the steady-state levels of ER mRNA, The decrease in ER mRNA is not abolished by the protein synthesis inhibitor cycloheximide, Inhibit ion of mRNA synthesis with actinomycin D reveals no significant differences between ER mRNA half-life in control and 1,25(OH)(2)D-3 treated cells. Nuc lear run-on experiments demonstrate significant decreases in ER gene transc ription at the end of 17 h of treatment with 1,25(OH)(2)D-3. These findings indicate that 1,25(OH)(2)D-3 exerts a direct negative effect on ER gene tr anscription. Coincident with the decrease in ER levels there is an attenuat ion of E-2-mediated bioresponses after 1,25(OH)(2)D-3 treatment. Induction of progesterone receptor by E-2 is suppressed by 1,25(OH)(2)D-3, and the E- 2-mediated increase in breast cancer susceptibility gene (BRCA1) protein is reduced by 1,25(OH)(2)D-3 treatment. Overall, these results suggest that t he antiproliferative effects of 1,25(OH)(2)D-3 and its analogues on MCF-7 c ells could partially be mediated through their action to down-regulate ER l evels and thereby attenuate estrogenic bioresponses, including breast cance r cell growth.