1α,25-Dihydroxyvitamin D3 Down-Regulates Estrogen Receptor Abundance and Suppresses Estrogen Actions in MCF-7 Human Breast Cancer Cells

1α,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 have studied the effects of 1,25(OH) 2 D 3 on the regulation of ER in the estrogen-responsive MCF-7 human breast cancer cell line, which is known to predominantly express ERα. 1,25(OH) 2 D 3 causes significant inhibition of MCF-7 cell growth, and it also decreases the growth-stimulatory effect of 17β-estradiol (E 2 ). Treatment of MCF-7 cells with 1,25(OH) 2 D 3 reduces ER levels in a dose-dependent manner, as shown by ligand binding assays and Western blot analysis. The 1,25(OH) 2 D 3 analogues EB-1089, KH-1060, Ro 27-0574, and Ro 23-7553 are more potent than 1,25(OH) 2 D 3 in both their antiproliferative actions as well as ER down-regulation. There is a striking correlation ( 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 significant decrease in the steady-state levels of ER mRNA. The decrease in ER mRNA is not abolished by the protein synthesis inhibitor cycloheximide. Inhibition 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. Nuclear run-on experiments demonstrate significant decreases in ER gene transcription 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 transcription. Coincident with the decrease in ER levels there is an attenuation 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 the antiproliferative effects of 1,25(OH) 2 D 3 and its analogues on MCF-7 cells could partially be mediated through their action to down-regulate ER levels and thereby attenuate estrogenic bioresponses, including breast cancer cell growth.

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