Previous studies have shown that the
progesterone metabolite,
5alpha-pregnane-3,20-dione (5alphaP), exhibits mitogenic and metastatic activity in breast cell lines and that specific, high affinity receptors for 5alphaP are located in the plasma membrane fractions of tumorigenic (ER/PR-positive) MCF-7 cells. The aim of this study was to determine the effects of the mitogenic (
estradiol; 5alphaP) and anti-mitogenic (3alpha-hydroxy-4-pregnen-20-one,
3alphaHP; 20alpha-hydroxy-4-pregnen-3-one, 20alphaHP) endogenous
steroid hormones on 5alphaP receptor (5alphaP-R) numbers and on cell proliferation and adhesion of MCF-7 and MCF-10A cells. Exposure of MCF-7 cells for 24h to
estradiol or 5alphaP resulted in significant (p < 0.05-0.001) dose-dependent increases in 5alphaP-R levels. Conversely, treatment with
3alphaHP or 20alphaHP resulted in significant (p < 0.05-0.01) dose-dependent decreases in 5alphaP-R levels. Treatment with one mitogenic and one anti-mitogenic
hormone resulted in inhibition of the
mitogen-induced increases, whereas treatment with two mitogenic or two anti-mitogenic
hormones resulted in additive effects on 5alphaP-R numbers. Treatments with
cycloheximide and
actinomycin D indicate that changes in 5alphaP-R levels depend upon transcription and translation. The non-tumorigenic breast cell line, MCF-10A, was also shown to posses specific, high affinity plasma membrane receptors for 5alphaP that were up-regulated by
estradiol and 5alphaP and down-regulated by
3alphaHP.
Estradiol binding was demonstrated in MCF-10A cell membrane fractions and may explain the
estradiol action in these cells that lack intracellular ER. In both MCF-7 and MCF-10A cells, the increases in 5alphaP-R due to
estradiol or 5alphaP, and decreases due to
3alphaHP or 20alphaHP correlate with respective increases and decreases in cell proliferation as well as detachment. These results show distribution of 5alphaP-R in several cell types and they provide further evidence of the significance of
progesterone metabolites and their novel membrane-associated receptors in
breast cancer stimulation and control. The findings that
3alphaHP and 20alphaHP down-regulate 5alphaP-R and suppress mitogenic and metastatic activity suggest that these endogenous anti-mitogenic
progesterone metabolites deserve considerations in designing new
breast cancer therapeutic agents.