Desensitization and Endocytosis Mechanisms of Ghrelin-Activated Growth Hormone Secretagogue Receptor 1a

In this study, a sequential analysis of pathways involved in the regulation of GH secretagogue receptor subtype 1a (GHSR-1a) signaling has been undertaken to characterize the process of rapid desensitization that is observed after ghrelin binding. This process was evaluated by studying the binding of [125I]ghrelin, measurement of intracellular calcium mobilization, and confocal microscopy. The results indicate that GHSR-1a is mainly localized at the plasma membrane under unstimulated conditions and rapidly desensitizes after stimulation. The agonist-dependent desensitization is not mediated by protein kinase C because phorbol ester, phorbol-12-myristate-13-acetate, failed to block the ghrelin-induced calcium response. The ghrelin/GHSR-1a complex progressively disappears from the plasma membrane after 20 min exposure to ghrelin and accumulates in the perinuclear region after 60 min. Colocalization of the internalized GHSR-1a with the early endosome marker (EEA1) after 20 min exposure to ghrelin suggests th...