Enhancement of cell surface expression and receptor functions of membrane progestin receptor α (mPRα) by progesterone receptor membrane component 1 (PGRMC1): evidence for a role of PGRMC1 as an adaptor protein for steroid receptors

Expression of progesterone receptor membrane component-2 within the immature rat ovary and its role in regulating mitosis and apoptosis of spontaneously immortalized granulosa cells

Progesterone receptor membrane component 2 (Pgrmc2) mRNA was detected in the immature rat ovary. By 48 h after eCG, Pgrmc2 mRNA levels decreased by 40% and were maintained at 48 h post-hCG. Immunohistochemical studies detected PGRMC2 in oocytes and ovarian surface epithelial, interstitial, thecal, granulosa, and luteal cells. PGRMC2 was also present in spontaneously immortalized granulosa cells, localizing to the cytoplasm of interphase cells and apparently to the mitotic spindle of cells in metaphase. Interestingly, PGRMC2 levels appeared to decrease during the G1 stage of the cell cycle. Moreover, overexpression of PGRMC2 suppressed entry into the cell cycle, possibly by binding the p58 form of cyclin dependent kinase 11b. Conversely, Pgrmc2 small interfering RNA (siRNA) treatment increased the percentage of cells in G1 and M stage but did not increase the number of cells, which was likely due to an increase in apoptosis. Depleting PGRMC2 did not inhibit cellular (3)H-progesterone binding, but attenuated the ability of progesterone to suppress mitosis and apoptosis. Taken together these studies suggest that PGRMC2 affects granulosa cell mitosis by acting at two specific stages of the cell cycle. First, PGRMC2 regulates the progression from the G0 into the G1 stage of the cell cycle. Second, PGRMC2 appears to localize to the mitotic spindle, where it likely promotes the final stages of mitosis. Finally, siRNA knockdown studies indicate that PGRMC2 is required for progesterone to slow the rate of granulosa cell mitosis and apoptosis. These findings support a role for PGRMC2 in ovarian follicle development.

Non-canonical progesterone signaling in granulosa cell function

It has been known for over 3 decades that progesterone (P4) suppresses follicle growth. It has been assumed that P4 acts directly on granulosa cells of developing follicles to slow their development, as P4 inhibits both mitosis and apoptosis of cultured granulosa cells. However, granulosa cells of developing follicles of mice, rats, monkeys, and humans do not express the A or B isoform of the classic nuclear receptor for P4 (PGR). By contrast, these granulosa cells express other P4 binding proteins, one of which is referred to as PGR membrane component 1 (PGRMC1). PGRMC1 specifically binds P4 with high affinity and mediates P4's anti-mitotic and anti-apoptotic action as evidenced by the lack of these P4-dependent effects in PGRMC1-depleted cells. In addition, mice in which PGRMC1 is conditionally depleted in granulosa cells show diminished follicle development. While the mechanism through which P4 activation of PGRMC1 affects granulosa cell function is not well defined, it appears that PGRMC1 controls granulosa cell function in part by regulating gene expression in T-cell-specific transcription factor/lymphoid enhancer factor-dependent manner. Clinically, altered PGRMC1 expression has been correlated with premature ovarian failure/insufficiency, polycystic ovarian syndrome, and infertility. These collective studies provide strong evidence that PGRMC1 functions as a receptor for P4 in granulosa cells and that altered expression results in compromised reproductive capacity. Ongoing studies seek to define the components of the signal transduction cascade through which P4 activation of PGRMC1 results in the regulation of granulosa cell function.