A putative stimulatory role of progesterone acting via progesterone receptors in the steroidogenic cells of the human corpus luteum.

To further explore the proposed auto-regulatory role of progesterone action in the human corpus luteum (CL), the expression and functional roles of progesterone receptor (PR) isoforms A and B during the luteal phase (LP) of the menstrual cycle were investigated. A total of 27 otherwise healthy patients previously scheduled for surgery were recruited after informed consent. An LH rise was detected, and CL were grouped according to age (Days 2-5 post-LH-rise, early LP; Days 6-10, mid LP; Days 11-14, late LP). Using a semiquantitative reverse transcription-polymerase chain reaction assay, the PR-B mRNA levels, which were 100- to 1000-fold lower than PR-A/B mRNA, were 46% lower (P < 0.05, n = 24) in mid LP, compared to early and late LP. CL tissue levels of progesterone and PR-A/B protein levels were inversely correlated to increasing CL age; i.e., significantly reduced levels were observed in the late LP (r(2) = 0.34, P < 0.01, n = 23). Expression of PR-A/B mRNA as well as PR-A/B protein were detected by in situ hybridization and immunohistochemistry, respectively. Both methods revealed a clear and distinct localization to cells in the steroidogenic layer of the CL. Freshly obtained mid-luteal CL cells were cultured in vitro, and media were analyzed for progesterone concentrations after treatment by incremental doses of hCG and the stable PR antagonist mifepristone, alone or in combination. Mifepristone did not per se alter progesterone synthesis, but when it was added in conjunction with hCG, a dose-related inhibitory response was seen, with a maximal 47% reduction in progesterone output at a 10 microM addition (P < 0.05, n = 3). Collectively, these data implicate a stimulatory role of progesterone receptor-mediated action in the steroidogenic cells of the human CL, which may serve as an important pathway for maintaining functional homeostasis during early pregnancy.