Abstract
The regulatory influence of medroxyprogesterone acetate (MPA) on estrogen and androgen receptors of the human breast cancer cell lines MCF-7 and EFM-19 was explored in conjunction with the growth-promoting properties of these steroids. In the absence of steroidal stimulation, up to 1 microM MPA had no effect on the proliferation of the MCF-7 cell strain used and of EFM-19 cells. Under stimulation with 10 nM 17 beta-estradiol or 1 microM dihydrotestosterone, dose-dependent inhibition of the cell proliferation rates by 0.1-1 microM MPA was observed. Binding of MPA to the androgen receptor (Kd = 2.1 nM) but not to the estrogen receptor was demonstrable. During incubation of MCF-7 or EFM-19 cells with 1 microM MPA for 7 days, the estrogen and androgen receptor contents were down-regulated by approximately 50% and 60%, respectively. Likewise, the number of androgen-binding sites was reduced to 35% of the untreated controls after incubation of MCF-7 cells with 1 microM synthetic progestin R5020 for 7 days. The results indicate down-regulation of estrogen and androgen receptors by progestins in the absence of stimulatory effects on the proliferation of mammary carcinoma cells.