Differential G protein coupling preference of mammalian and nonmammalian gonadotropin-releasing hormone receptors

Recently, we have identified three distinct types of gonadotropin- releasing hormone receptor (GnRHR) in the bullfrog (designated bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3). In the present study, we compared G protein coupling preference of mammalian and nonmammalian GnRHRs. In a transient expression system, stimulation of either bfGnRHRs or rat GnRHR by GnRH significantly increased both inositol phosphates (IP) and cAMP productions, but ratios of IP to cAMP induction levels were quite different among the receptors, indicating differential G protein coupling preference. Using cAMP-dependent protein kinase A (PKA)-specific (CRE-luc) or protein kinase C (PKC)-specific reporter (c-fos-luc) systems, we further examined G_s and G_q/11 coupling preference of these GnRHRs. Since activities of CRE-luc and c-fos-luc were highly dependent on cell types, GnRH-induced CRE-luc or c-fos-luc activity was normalized by forskolin-induced CRE-luc or 12-O-tetradecanoylphenol-13- acetate (TPA)-induced c-fos-luc activity, respectively. This normalized result indicated that bfGnRHR-2 couples to G_s more actively than G _q/11, while bfGnRHR-1 and -3 couple to G_s and G _q/11 with similar strength. However, the rat GnRHR appeared to couple to G_q/11 more efficiently than G_s. This study was further confirmed by an experiment in which GnRH augmented CRE-driven luciferase activity in αT3-1 cells when CRE-luc was cotransfected with bfGnRHRs but not with vehicle or rat GnRHR. Collectively, these results indicate that mammalian and nonmammalian GnRHRs may induce diverse cellular and physiological responses through differential activation of PKA and PKC signaling pathways.