Activation of FSH-responsive adenylate cyclase by staurosporine: role for protein phosphorylation in gonadotropin receptor desensitization

Abstract

Prolonged stimulation of gonadotropin receptors in granulosa cells leads to desensitization of the cellular response to gonadotropic hormones which is evident by decrease in cAMP formation. In order to explore the mechanism of desensitization and to examine whether protein phosphorylation may play a role in this phenomenon, we have studied the effect of various stimulators and inhibitors of protein phosphorylation on FSH-induced cAMP formation in the FSH-responsive cell line, GFSHR-17, recently established in our laboratory. Both ovine and human FSH activated the hormone sensitive adenylate cyclase in a dose-dependent manner with an ED 50 of 0.5 nM. This stimulation was followed by a sharp decrease in cAMP formation after 30 min incubation of the cell with the hormone. When cells were preincubated for 60 min with staurosporine, cAMP accumulation during 20 min of FSH stimulation was elevated about 500%, compared to cells stimulated by FSH alone. Staurosporine alone showed a negligible effect on cAMP accumulation in these cells. In cells stimulated with forskolin, a non-specific activator of adenylate cyclase, or with cholera toxin (CT), an inhibitor of GTPase activity associated with Gs of adenylate cyclase, preincubation with staurosporine increased cAMP formation in these cells by only 50–70 or 80–120%, respectively. Preincubation of cells with the protein kinase C (PKC) inhibitors chelerythrine and GF109203X increased FSH-stimulated accumulation of cAMP by 50 and 30%, respectively. These drugs exhibit a similar effect on forskolin-stimulated cells. Preincubation of cells for 60 min with a PKC stimulator, TPA, suppressed FSH-mediated cAMP response in these cells by 40%. Tyrosine kinase inhibitors such as AG18, AG33 and genistein exhibit a modest inhibitory effect of up to 20% on FSH-stimulated cAMP accumulation. All the above results were obtained both in the presence and absence of IBMX, a potent inhibitor of the cellular phosphodiesterases. Upon prolonged incubation with FSH (3 h) cells pretreated with staurosporine exhibited a much slower rate of decline in intracellular cAMP levels. Moreover, in desensitized cells, following 1 or 2 h of continuous stimulation with FSH, staurosporine could markedly enhance cAMP formation in the presence of FSH. Our data suggest that staurosporine-sensitive phosphorylation of serine or threonine in the FSH receptor cyclase system may be responsible for desensitization of the FSH coupled activation of cAMP formation, while reactivation of the system can be achieved by protein dephosphorylation at these specific sites. Because specific inhibition of PKC could not mimic the staurosporine effect on FSH-stimulated cAMP formation, nor could activation of kinase C antagonize it, it is suggested that a specific staurosporine-sensitive receptor kinase may be responsible for modulation of the coupling between the gonadotropin receptor and the adenylate cyclase system.