Somatostatin is a widely expressed circulating peptide hormone that has been involved in different biological processes including hormone secretion inhibition, neuronal transmission modulation, regulation of cell growth etc., and somatostatin's effects are mediated by a family of GPCRs "somatostatin Receptor-1 to -5" (SSTR1-5) that are differentially expressed in tumours and normal tissues including CNS/PNS, endocrine system and digestive canal. Once these receptors are activated, somatostatin exerts well-characterized direct/indirect antiproliferative effects in a variety of epithelial and endocrine cells. Somatostatin directly antagonizes the mitogenic action of EGF in pancreatic tumor cells and antagonizes additional tyrosine kinases in other cell types. SSTR1 belongs to GPCR1 family and it is a somatostatin receptor with higher affinity for somatostatin-14 than -28. SSTR1 is coupled to phosphotyrosine phosphatase and Na+/H+ exchanger via pertussis toxin insensitive G proteins, and is widely expressed in normal tissues as well as tumors.
