Tissue-specific proteolytic cleavage of prosomatostatin generates two biologically active peptides, somatostatin-14 (SS-14) and somatostatin- 28 (SS-28). These two peptides are thought to activate distinct receptors in the brain, pituitary, pancreas, and GI tract. Because somatostatin regulates the functions of so many target tissues, unraveling the complexities of somatostatinergic signaling is a major problem in neuroendocrinology. We, and others, have recently identified and isolated cDNAs encoding two distinct somatostatin receptors: SSR1 and SSR2. These cDNAs provide the best evidence for the existence of distinct functional subtypes of somatostatin receptors and additionally supply the means to determine the physiological significance of this receptor multiplicity. One of the goals of this proposal is to determine whether these receptor subtypes display different affinities for SS-14 and SS-28. Both somatostatin peptides elicit relatively complex biological responses, by activating or inhibiting multiple signal transduction pathways. For example, alterations in adenylyl cyclase activity, potassium efflux, and calcium influx all contribute to the ability of SS-14 to inhibit prolactin secretion from pituitary cells. A second goal of this proposal is to determine whether these different biological actions of somatostatin are mediated by distinct receptor subtypes and G protein signaling pathways. Perhaps the most unique action of somatostatin is its ability to inhibit tumor cell growth. This antineoplastic action of somatostatin may be mediated through its stimulation of a protein tyrosine phosphatase. We have identified a novel G protein pathway used by somatostatin to stimulate a protein tyrosine phosphatase (PTP) activity in human tumor cells. Our third goal is to characterize the components of the pathway that mediate this effect. In conclusion, somatostatin participates in a unique signaling system capable of activating at least four separate G protein-coupled pathways. Prosomatostatin and the family of somatostatin receptors and effectors can serve as a paradigm for how a single neuropeptide system mediates a wide variety of biological effects. Through the coordinated expression of specific ligands, receptors, G proteins, and effectors somatostatin is able to increase the range and scope of its actions. The specificity of the interactions between ligands, receptors, G proteins and effectors governs the signaling pathway utilized in the cell. We have isolated the necessary reagents and have developed the technical tools to begin to understand the molecular rules that govern the interactions. Understanding these rules will facilitate the development of therapeutic strategies designed to discriminate among the many effects of somatostatin.
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| Florio, T; Rim, C; Hershberger, R E et al. (1994) The somatostatin receptor SSTR1 is coupled to phosphotyrosine phosphatase activity in CHO-K1 cells. Mol Endocrinol 8:1289-97 |
