Protein kinase C activation stimulates the phosphorylation and internalization of the sst2A somatostatin receptor

The sst2A receptor is expressed in the endocrine, gastrointestinal, and neu ronal systems as well. as in many hormone-sensitive tumors, This receptor i s rapidly internalized and phosphorylated in growth hormone-R2 pituitary ce lls following somatostatin binding (Hipkin, R. W., Friedman, J., Clark, R, B., Eppler, C. M., and Schonbrunn, A. (1997) J. Biol. Chem. 272, 13869-1387 6). The protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate ( PMA), also stimulates sst2A phosphorylation. Here we examine the mechanisms and consequences of PMA and agonist-induced sst2A phosphorylation. Like so matostatin, both PMA and bombesin increased sst2A receptor phosphorylation within 2 min. The PKC inhibitor GF109203X blocked PMA- and bombesin- stimul ated sst2A phosphorylation, whereas stimulation by the somatostatin analog SMS 201-995 was unaffected. Agonist and PMA each stimulated phosphorylation in two receptor domains, the third intracellular loop and the C-terminal t ail, Functionally, PMA dramatically increased the internalization of the ss t2A receptor-ligand complex, This PMA stimulation was blocked by GF109203X, whereas basal internalization was unaffected. However, neither basal nor P MA-stimulated internalization was altered by pertussis toxin, whereas both were blocked by hypertonic sucrose. Therefore PKC activation and agonist bi nding stimulate sst2A phosphorylation by distinct mechanisms, and PKC poten tiates internalization of the sst2A receptor via clathrin-coated pits. Thus , hormonal stimulation of PKC-coupled receptors may provide a mechanism for regulating the inhibitory actions of somatostatin in target tissue.