Somatostatin (SS) is a neuropeptide which exists in two forms (SS14 and SS28) and acts as a hormone, a neurotransmitter and an autocrine regulator to inhibit secretion and/or proliferation in endocrine, exocrine, neuronal, immune and tumor cells. SS has been implicated in numerous physiological processes including growth, glucose homeostasis and memory. Moreover, SS analogs are used clinically for the diagnosis and treatment of neuroendocrine tumors and are being evaluated for therapy in several diseases including diabetes mellitus. The biological actions of SS are initiated by binding to a family of five plasma membrane receptors (sst1-sst5) which are coupled to effector enzymes and ion channels via G proteins. Each SS receptor subtype shows a specific tissue and cellular distribution pattern in the brain, the pituitary, the endocrine and exocrine pancreas as well as in neuroendocrine tumors. The goal of this proposal is to elucidate the mechanisms involved in homologous and heterologous regulation of SS receptor subtypes. During the previous award period we demonstrated that the sst2A receptor undergoes rapid phosphorylation following hormone binding as well as upon activation of protein kinase C (PKC). Concomitant with receptor phosphorylation, SS stimulates rapid endocytosis of the sst2A receptor-ligand complex and the rate of this endocytosis is dramatically stimulated by PKC activation. In contrast, the sst1 receptor is minimally endocytosed upon agonist binding. However, the sst1 receptor is also rapidly phosphorylated upon SS treatment and both the sst1 and the sst2A receptors undergo rapid homologous desensitization. The investigator proposes to examine the mechanisms involved in sst receptor desensitization, trafficking, and resensitization and to determine the role of homologous and heterologous receptor phosphorylation in sst receptor regulation. Using biochemical analysis of molecular constructs which include site-specific mutants and receptor deletions and chimeras, the investigator will identify the sites of receptor phosphorylation and define key domains and residues for sst receptor function. Sst receptor regulation will be examined in both transfected cell lines and in cells endogenously expressing the native receptor in order to elucidate the role of the cellular environment in sst receptor desensitization and trafficking.
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