Gonadotropin-releasing hormone receptor initiates multiple signaling pathways by exclusively coupling to Gq/11 proteins

The agonist-bound gonadotropin-releasing hormone (GnRH) receptor engages several distinct signaling cascades, and it has recently been proposed that coupling of a single type of receptor to multiple G proteins (G(q), G(s), and G(i)) is responsible for this behavior, GnRH-dependent signaling was studied in gonadotropic alpha T3-1 cells endogenously expressing the murine receptor and in CHO-K1 (CHO#3) and COS-7 cells transfected with the human GnRH receptor cDNA In all cell systems studied, GnRH-induced phospholipase C activation and Ca2+ mobilization was pertussis toxin-insensitive, as was GnRH-mediated extracellular signal-regulated kinase activation. Whereas the G(i)-coupled m2 muscarinic receptor interacted with a chimeric G(s) protein (G(s)i5) containing the C-terminal five amino acids of G alpha(12), the human GnRH receptor was unable to activate the G protein chimera. GnRH challenge of alpha T3-1, CHO#3 and of GnRH receptor-expressing COS-7 cells did not result in agonist-dependent cAMP formation. GnRH challenge of CHO#3 cells expressing a cAMP-responsive element-driven firefly luciferase did not result in increased reporter gene expression, However, coexpression of the human GnRH receptor and adenylyl cyclase I in COS-7 cells led to clearly discernible GnRH-dependent cAMP formation subsequent to GnRH-elicited rises in [Ca2+](i), In alpha T3-1 and CHO#3 cell membranes, addition of [alpha-P-32]GTP azidoanilide resulted in GnRH receptor-de pendent labeling of G alpha(q/11) but not of G alpha(i), G alpha(s), or G alpha(12/13) proteins. Thus, the murine and human GnRH receptors exclusively couple to G proteins of the G(q/11) family. Multiple GnRH-dependent signaling pathways are therefore initiated downstream of the receptor/G protein interface and are not indicative of a multiple G protein coupling potential of the GnRH receptor.