β-Arrestin-dependent signaling in GnRH control of hormone secretion from goldfish gonadotrophs and somatotrophs

In goldfish, two native isoforms of gonadotropin-releasing hormone (GnRH2 and GnRH3) stimulate luteinizing hormone (LH) and growth hormone (GH) release from pituitary cells through activation of cell-surface GnRH-receptors (GnRHRs) on gonadotrophs and somatotrophs. Interestingly, GnRH2 and GnRH3 induce LH and GH release via non-identical post-receptor signal transduction pathways in a ligand- and cell-type-selective manner. In this study, we examined the involvement of beta-arrestins in the control of GnRH-induced LH and GH secretion from dispersed goldfish pituitary cells. Treatment with Barbadin, which interferes with beta-arrestin and beta 2-adaptin subunit interaction, reduced LH responses to GnRH2 and GnRH3, as well as GH responses to GnRH2; but enhanced GnRH3-induced GH secretion. Barbadin also had positive influences on basal hormone release, and basal GH release in particular, as well as basal activity of extracellular signal-regulated kinase (ERIC) and GnRH-induced ERK activation. These findings indicate that beta-arrestins play permissive roles in the control of GnRH-stimulated LH release. However, in somatotrophs, beta-arrestins, perhaps by mediating agonist-selective endosomal trafficking of engaged GnRHRs, participate in GnRH-isoform-specific GH release responses (stimulatory and inhibitory for GnRH2-GnRHR and GnRH3-GnRHR activation, respectively). The correlative stimulatory influences of Barbadin on basal hormone release and ERK activation suggest that beta-arrestins may negatively regulate basal secretion through modulation of basal ERIC activity. These results provide the first direct evidence of a role for beta-arrestins in hormone secretion from an untransformed primary pituitary cell model, and establish these proteins as important receptor-proximal players in mediating functional selectivity downstream of goldfish GnRHRs.