Neuronal nitric oxide synthase is a SHP‐1 substrate involved in sst2 somatostatin receptor growth inhibitory signaling

Abstract

Somatostatin receptor sst2 is an inhibitory G pr otein-coupled receptor, which inhibits normal and tumor cell growth by a mechanism involving the tyrosine phosphatase SHP-1. We reported previously that SHP-1 associates transiently with and is activated by sst2 and is a critical component for sst2 growth inhibitory signaling. He re, we demonstrate that in Chinese hamster ovary cells expressing sst2, SHP-1 is associated at the basal level with the neuronal nitric oxide synthase (nNOS). Following sst2 activation by the somatostatin analog RC-160, SHP-1 rapidly recruits nNOS tyrosine dephosphorylates and activates it. The resulting NO activates guanylate cyclase and inhibits cell proliferation. Coexpre ssion of a catalytically inactive SHP-1 mutant with sst2 blocks RC-160-induced nNOS dephosphorylation and activation, as well as guanylate cyclase activation. In mouse pancreatic aci ni, RC-160 treatment reduces nNOS tyrosine phosphorylation accompanied by an increase of its activity. By opposition, in acini from viable motheaten (mev/mev) mice, which express a markedly inactive SHP-1, RC-160 has no effect on nNOS activity. Finally, expression of a dominant-negative form of nNOS prevents both RC-160- induced p27 up-regulation and cell proliferation inhibition. We therefore identified nNOS as a novel SHP-1 substrate critical for sst2-induced cell-growth arrest. he ubiquitous regulatory neurohormone somatostatin exerts a wide spectrum of biological functions by its action as inhibitor of cell proliferation and endocrine and exocrine secretory processes, together with its immunomodulatory and vasoconstrictor properties. Cellular mechanisms induced by somatostatin include inhibition of adenylate cyclase, modulation of K+ and Ca 2+ channels, and protein dephosphorylation. The biological effects of somatostatin are mediated through its specific bindi ng to a family of G protein-coupled receptors (sst1-5) (1). The five receptors bind to the natu ral peptides somatostatin 14 and somatostatin 28 with high affinity, whereas stable somatostatin analog used in clinic, including SMS 201-995 (octreotide) or RC-160, selectively interact with sst2, sst5, and sst3 (2). Somatostatin receptors are expressed widely in brain and peripheral organs and in a large variety of human tumors such