GALANIN INHIBITS PROINSULIN GENE-EXPRESSION STIMULATED BY THE INSULINOTROPIC HORMONE GLUCAGON-LIKE PEPTIDE-I(7-37) IN MOUSE INSULINOMA BETA-TC-1 CELLS

The neuropeptide hormone galanin, released by sympathetic stimulation of nerve terminals in the endocrine pancreas, inhibits insulin secretion via a receptor-linked pertussis toxin-sensitive (G(i)) transmembrane signaling pathway. Glucagon-like peptide-I(7-37) [GLP-I(7-37)] is an intestinal hormone shown to have potent insulin-releasing activities in pancreatic B-cells and is believed to serve a physiological role in the augmentation of nutrient-induced insulin release. GLP-1(7-37) binds to specific G8- and adenylate cyclase-coupled receptors on pancreatic B-cells and directly stimulates proinsulin gene transcription, thereby increasing cellular levels of proinsulin messenger RNA (mRNA) and proinsulin biosynthesis. This study examines the effects of galanin on GLP-I(7-37)-stimulated proinsulin gene expression in mouse beta-TC1 cells. The degree of proinsulin gene transcription was assessed by measuring the activity of chloramphenicol acetyl transferase (CAT) expressed from a CAT reporter plasmid linked to the rat insulin-I gene promoter transferred to beta-TC1 cells and by measuring proinsulin mRNA levels by Northern blot analysis. Galanin inhibited both CAT activity and the rise in proinsulin mRNA levels stimulated by either GLP-1(7-37) or forskolin (0.1-mu-m). Notably, galanin was without effect on CAT activity induced by the cAMP analog, 8-bromo-cAMP, the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, or higher concentrations of forskolin. The inhibitory effects of galanin on GLP-1(7-37) and forskolin-induced CAT activity were reversed by the addition of pertussis toxin, a toxin that inactivates inhibitory G-proteins (G(i)). We conclude that galanin inhibits GLP-1(7-37)-stimulated proinsulin gene expression by inhibiting the activation of adenylate cyclase by GLP-I(7-37) and subsequently the production of cAMP in B-cells. Further, our data suggest that these actions of galanin are mediated by a pertussis toxin sensitive pathway involving one or more G(i)s that inhibit adenylate cyclase. Thus, in addition to its well known inhibitory effects on insulin secretion galanin can inhibit proinsulin gene expression stimulated by GLP-1(7-37) activation of the cAMP signaling pathway. These findings may be a unique demonstration of the inhibition of proinsulin gene expression by a substance (galanin) released endogenously within the pancreas.