Regulation of estrogen receptor (ER) levels in MCF-7 cells by progesterone metabolites

Estradiol-17β (E2) may participate in carcinoma of mammary cells containing estradiol receptors (ER) at sufficient levels. Hence, the regulation of ER levels may be important for the progression of estrogen-dependent mammary carcinomas. Our previous findings that the progesterone metabolite, 5α-pregnane-3,20-dione (5αP), exhibits marked mitogenic and metastatic properties, whereas the progesterone metabolites, 4-pregnen-3α-ol-20-one (3αHP) and 4-pregnen-20α-ol-3-one (20αHP), oppose these actions, prompted examination of the possible effects of these progesterone metabolites on ER concentration in MCF-7 breast cancer cells. Cells were exposed for 24 h to 0 (control) or 10−10 to 10−6 M E2, 5αP, 3αHP, 20αHP or combinations of these steroids, and ER concentrations were determined for intracellular estrogen receptors by specific binding of [3H]E2. The total ER number (nuclear plus cytosolic) in control samples was 2551 ± 164 per cell. E2 and 5αP resulted in significant dose-dependent increases in total ER numbers (∼1.6-fold and ∼2.2-fold at 10−6 M, respectively). In combination, E2 + 5αP resulted in additive increases in ER numbers. Individually, 3αHP and 20αHP each resulted in dose-dependent decreases (43% and 54% at 10−6 M, respectively) in total ER numbers and inhibited the E2- or 5αP-induced increases in ER levels. In combination, 3αHP + 20αHP resulted in dose-dependent additive suppression of ER levels. Treatment with cycloheximide or actinomycin D indicated that both transcription and translation are involved in 5αP and 3αHP action on ER numbers. Real time RT-PCR showed increases in expression of ERα transcripts due to 5αP and increases in expression of ERβ due to 3αHP; expression levels of either ERα or ERβ were not significantly altered when cells were treated with 5αP + 3αHP. The results are the first to show that the pro- and anti-cancer progesterone metabolites also have marked selective (up or down) regulatory effects on ER levels in MCF-7 breast cancer cells.