Inhibition of extracellular signal-regulated protein kinase-2 phosphorylation by dihydrotestosterone reduces follicle-stimulating hormone-mediated cyclin D2 messenger ribonucleic acid expression in rat granulosa cells.

Granulosa cell mitogenesis is critical for the development of normal ovarian follicles. FSH and other mitogenic stimuli play a crucial role in this process. We have shown that exposing granulosa cells to 5alpha-dihydrotestosterone (DHT) reduces forskolin-stimulated cyclin D2 mRNA expression, which leads to cell cycle arrest resulting in reduced cell proliferation. The present study investigated the signaling molecules upstream of cyclin D2 in FSH-mediated, cAMP-dependent signaling pathway that may be negatively affected by DHT, leading to inhibition of cell cycle progression. Because ERK is an important molecule in mitogenic signaling, the possible effect of DHT on its phosphorylation was examined. Granulosa cells from 3-d estradiol-primed immature rats were treated with DHT (90 ng/ml) for 24 h and subsequently stimulated with forskolin. DHT treatment reduced forskolin stimulation of ERK phosphorylation. Although DHT exposure did not affect cellular cAMP production in response to forskolin, treating the cells with DHT for 24 h significantly reduced protein kinase A activity. DHT also caused a reduction in ERK-2 phosphorylation in response to FSH similar to that seen with forskolin. Furthermore, blocking ERK phosphorylation as well as DHT treatment resulted in a reduction in FSH-stimulated cyclin D2 mRNA expression. From these results, we conclude that DHT treatment reduces the FSH-mediated ERK phosphorylation in granulosa cells, leading to reduced cyclin D2 mRNA expression that culminates in cell cycle arrest.