Gonadotropin-Releasing Hormone Couples to 3′,5′-Cyclic Adenosine-5′-Monophosphate Pathway through Novel Protein Kinase Cδ and -ε in LβT2 Gonadotrope Cells

Abstract

GnRH regulates the reproductive system by stimulating synthesis and release of gonadotropins. GnRH acts through a receptor coupled to multiple intracellular events including a rapid phosphoinositide turnover. Although the cAMP pathway is essential for gonadotrope function, the ability of GnRH to induce cAMP, as well as the coupling mechanisms involved, remain controversial. In this study, we established that GnRH increases intracellular cAMP levels in a concentration-dependent manner in LbetaT2 gonadotrope cells (maximal increase, 2.5-fold; EC(50), 0.30 nm), and this was further evidenced by GnRH activation of a cAMP-sensitive reporter gene. The GnRH effect was Ca(2+) independent, mimicked by the phorbol ester phorbol 12-myristate 13-acetate, and blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide, indicating that the GnRH effect was mediated by PKC. Pharmacological inhibition of conventional PKC isoforms with Gö6976 did not prevent GnRH-induced cAMP production, whereas down-regulation of novel PKCdelta, -epsilon, and -theta by a long-term treatment with GnRH markedly reduced it. Expression of dominant-negative (DN) mutants of PKCdelta or -epsilon but not PKCtheta impaired GnRH activation of a cAMP-sensitive promoter, demonstrating that PKCdelta and -epsilon are the two endogenous isoforms mediating GnRH activation of the adenylyl cyclase (AC) pathway in LbetaT2 cells. Accordingly, we identified by RT-PCR and immunocytochemical analysis, two PKC-sensitive AC isoforms, i.e. AC5 and AC7 as potential targets for GnRH. Lastly, we showed that only sustained stimulation of GnRH receptor significantly increased cAMP, suggesting that in vivo, the cAMP signaling pathway may be selectively recruited under intense GnRH release such as the preovulatory GnRH surge.