Agonist-induced desensitization, internalization, and phosphorylation of the sst2A somatostatin receptor

Cellular responsiveness to the inhibitory peptide somatostatin (SRIF) or its clinically used analogs can desensitize with agonist exposure. While desensitization of other seven-transmembrane domain receptors is mediated by receptor phosphorylation and/or internalization, the mechanisms mediating SRIF receptor (sst) desensitization are unknown. Therefore, we investigated the susceptibility of the sst2A receptor isotype to ligand-induced desensitization, internalization, and phosphorylation in GH-R2 cells, a clone of pituitary tumor cells overexpressing this receptor. A 30-min exposure of cells to either SRIF or the analog SMS 201-995 (SMS) reduced both the potency and efficacy of agonist inhibition of adenylyl cyclase. Internalization of receptor-bound ligand was rapid (t1/2 = 4 min) and temperature-dependent. SRIF and SMS increased the phosphorylation of the 71-kDa sst2A protein 25-fold within 15 min. Receptor phosphorylation was dependent on both the concentration and time of agonist exposure and was not affected by pertussis toxin pretreatment, indicating that receptor occupancy rather than second messenger formation was required. Receptor phosphorylation was also stimulated by phorbol 12-myristate 13-acetate activation of protein kinase C. Both ligand-stimulated and phorbol 12-myristate 13-acetate-stimulated receptor phosphorylation occurred primarily on serine. These studies are the first demonstration of agonist-dependent desensitization, internalization, and phosphorylation of the sst2A receptor and suggest that phosphorylation may mediate the homologous and heterologous regulation of this receptor.