Glucagon-Like Peptide 1 Stimulates Insulin Gene Promoter Activity by Protein Kinase A–Independent Activation of the Rat Insulin I Gene cAMP Response Element

摘要

Glucagon-like peptide 1 (GLP-1), a hormonal activator of adenyl cyclase, stimulates insulin gene transcription, an effect mediated by the cAMP response element (CRE) of the rat insulin I gene promoter (RIP1). Here we demonstrate that the signaling mechanism underlying stimulatory effects of GLP-1 on insulin gene transcription results from protein kinase A (PKA)-independent activation of the RIP1 CRE. Although GLP-1 stimulates cAMP production in rat INS-1 insulinoma cells, we find accompanying activation of a −410-bp RIP1 luciferase construct (−410RIP1-LUC) to exist independently of this second messenger. GLP-1 produced a dose-dependent stimulation of −410RIP1-LUC (EC50 0.43 nmol/l), an effect reproduced by the GLP-1 receptor agonist exendin-4 and abolished by the antagonist exendin(-). Activation of RIP1 by GLP-1 was not affected by cotransfection with dominant-negative Gs α, was not blocked by cAMP antagonist Rp-cAMPS, and was insensitive to PKA antagonist H-89. Truncation of −410RIP1-LUC to generate −307-, −206-, and −166-bp constructs revealed 2 segments of RIP1 targeted by GLP-1. The first segment, not regulated by forskolin, was located between −410 and −307 bp of the promoter. The second segment, regulated by both GLP-1 and forskolin, included the CRE and was located between −206 and −166 bp. Consistent with these observations, stimulatory effects of GLP-1 at RIP1 were reduced after introduction of Δ-182 and Δ-183/180 inactivating deletions at the CRE. The action of GLP-1 at −410RIP1-LUC was also reduced by cotransfection with A-CREB, a genetically engineered isoform of the CRE binding protein CREB, which dimerizes with and prevents binding of basic-region-leucine-zipper (bZIP) transcription factors to the CRE. In contrast, the action of GLP-1 at the CRE was not blocked by cotransfection with M1-CREB, an isoform that lacks a consensus serine residue serving as substrate for PKA-mediated phosphorylation. On the basis of these studies, it is proposed that PKA-independent stimulatory actions of GLP-1 at RIP1 are mediated by bZIP transcription factors related in structure but not identical to CREB.