INHIBITION OF GONADOTROPIN-STIMULATED STEROIDOGENESIS BY THE ERK CASCADE

LH and FSH are two important hormones in the regulation of granulosa cells. Their effects are mediated mainly by cAMP/PKA signaling, bit the activity of the extracellular signal-regulated kinase (ERK) signaling cascade is elevated as well. We studied the involvement of the ERK cascade in LH and FSH-induced steroidogenesis in two granulosa-derived cell lines, rLHR-4 and rFSHR-17, respectively. We found that stimulation of these cells with the appropriate gonadotropin induced ERK activation as well as progesterone production, downstream of PKA. Inhibition of ERK activity enhanced gonadotropin-stimulated progesterone production, which was correlated with increased-expression of the steroidogenic acute regulatory (StAR) protein, a key regulator of progesterone synthesis. Therefore, it is likely that gonadotropin-stimulated progesterone formation is regulated by a pathway that includes PKA and StAR, and this process is downregulated by ERK, due to attenuation of StAR expression. Our results suggest that activation of PKA signaling by gonadotropins not only induces steroidogenesis, but also activates downregulation machinery involving the ERK cascade. The activation of ERK by gonadotropins as well as by other agents, may be a key mechanism for the modulation of gonadotropin-induced steroidogenesis.