Ligand-Biased Regulation of PtdIns(3,4,5)P3-Dependent Signal Transduction in GPCR Control of Pituitary Hormone Release.

Abstract

Biased signaling describes the selective activation of signal transduction cascades by structurally related ligands downstream of shared G protein-coupled receptors (GPCRs). Although class I phosphoinositide 3-kinases (PI3Ks) are important components of GPCR-controlled transduction networks, little is known regarding the potential for biased regulation of class I PI3K-dependent signaling. The full complement of class I PI3K catalytic subunits (p110α, p110β, p110δ, and p110γ) first appears in bony fishes and, despite being associated with distinct cellular functions, all class I PI3Ks produce the lipid second-messenger phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3]. We have previously shown that 2 endogenous gonadotropin-releasing hormones (GnRH2 and GnRH3), which both signal through shared Gαq/11-coupled receptors, selectively activate different subsets of class I PI3K isoforms in their control of hormone release from goldfish (Carassius auratus) pituitary cells. Here, we tested the hypothesis that the biased activation of class I PI3K isoforms results in the selective recruitment of PtdIns(3,4,5)P3-sensitive effectors downstream of GnRH-stabilized GPCRs using pharmacological mapping. Our results reveal that distinct PtdIns(3,4,5)P3-sensitive effectors are involved in the differential control of GnRH2- and GnRH3-stimulated, as well as basal, hormone release and implicate the participation of noncanonical PtdIns(3,4,5)P3-sensitive transduction elements. Furthermore, observations using a selective inhibitor of the shared Gβγ-effector interaction surface indicate a role for Gβγ-dependent signaling in the integrated control of pituitary hormone exocytosis. These findings add to our understanding of functional selectivity in GPCR signal transduction networks, in general, and reveal the complexity of biased signaling downstream of class I PI3K catalytic activity.