Inhibition of gonadotropin-induced granulosa cell differentiation by activation of protein kinase C.

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Abstract
The induction of granulosa cell differentiation by follicle-stimulating hormone (FSH) is characterized by cellular aggregation, expression of luteinizing hormone (LH) receptors, and biosynthesis of steroidogenic enzymes. These actions of FSH are mediated by activation of adenylate cyclase and cAMP-dependent protein kinase and can be mimicked by choleragen, forskolin, and cAMP analogs. Gonadotropin releasing hormone (GnRH) agonists inhibit these maturation responses in a calcium-dependent manner and promote phosphoinositide turnover. The phorbol ester phorbol 12-myristate 13-acetate (PMA) also prevented FSH-induced cell aggregation and suppressed cAMP formation, LH receptor expression, and progesterone production, with an ID50 of 0.2 nM. In FSH-treated cells, PMA did not reduce the initial increase in cAMP formation during the first 24 hr of culture but prevented its secondary increase from 24 to 48 hr. PMA also inhibited LH receptor induction by cholera toxin, forskolin, and 8-bromo-cAMP, but it did not impair cAMP responses to the former two agents, indicating that the site of action of the phorbol ester is distal to adenylate cyclase. The early stimulation of cAMP-dependent protein kinase activity by FSH was also unaffected by PMA, consistent with its lack of effect on the initial cAMP response to FSH. However, PMA caused a marked decrease in cytosolic protein kinase C activity within 1 min of its addition to the cells. The permeant diacylglycerols, 1-oleoyl-2-acetoyl-sn-glycerol and sn-1,2-dioctanoyl glycerol, also inhibited LH receptor formation, while the nonpermeant diacylglycerol, diolein, was inactive. These results indicate that in situ activation of protein kinase C by PMA or permeant diacylglycerols inhibits cAMP-dependent granulosa cell differentiation, and suggest that the inhibitory actions of GnRH agonists on granulosa cell maturation are also mediated by protein kinase C.

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