Gonadotropin-releasing hormone receptor initiates multiple signaling pathways by exclusively coupling to G(q/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 αT3-1 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 Ga(i2), the human GnRH receptor was unable to activate the G protein chimera. GnRH challenge of αT3-1, 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 αT3-1 and CHO 3 cell membranes, addition of [α-³²P]GTP azidoanilide resulted in GnRH receptor-dependent labeling of Gα(q/11) but not of Gα(i), Gα(s) or Gα(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.