AGONIST-INDUCED DESENSITIZATION, INTERNALIZATION, AND PHOSPHORYLATIONOF THE SST2A SOMATOSTATIN RECEPTOR

Cellular responsiveness to the inhibitory peptide somatostatin (SRIP) 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 mech anisms mediating SRIF receptor (sst) desensitization are unknown. Ther efore, we investigated the susceptibility of the sst2A receptor isotyp e to ligand-induced desensitization, internalization, and phosphorylat ion in GH-R2 cells, a clone of pituitary tumor cells overexpressing th is receptor. A 30-min exposure of cells to either SRIF or the analog S MS 201-995 (SMS) reduced both the potency and efficacy of agonist inhi bition of adenylyl cyclase. Internalization of receptor-bound ligand w as rapid (t(1/2) = 4 min) and temperature-dependent. SRIF and SMS incr eased the phosphorylation of the 71-kDa sst2A protein 25-fold within 1 5 min. Receptor phosphorylation was dependent on both the concentratio n and time of agonist exposure and was not affected by pertussis toxin pretreatment, indicating that receptor occupancy rather than second m essenger formation was required. Receptor phosphorylation was also sti mulated by phorbol 12-myristate 13-acetate activation of protein kinas e C. Both ligand-stimulated and phorbol 12-myristate 13-acetate-stimul ated receptor phosphorylation occurred primarily on serine. These stud ies 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 regu lation of this receptor.