Glucagon-like peptide 1 stimulates insulin gene promoter activity by protein kinase A-independent activation of the rat insulin I gene cAMP response element.
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(9-39). Activation of RIP1 by GLP-1 was not affected by cotransfection with dominant-negative Gs alpha, 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 delta-182 and delta-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.
Skoglund
G
G
Laboratory of Physiology, Faculty of Medicine, Pitie Salpetriere, INSERM CJF.
Hussain
M A
MA
Holz
G G
GG
D000818:Animals; D001483:Base Sequence; D000242:Cyclic AMP / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology*; D005934:Glucagon / Q000494:pharmacology*; D052216:Glucagon-Like Peptide 1; D007328:Insulin / Q000235:genetics*; D007340:Insulinoma; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D010190:Pancreatic Neoplasms; D010446:Peptide Fragments / Q000494:pharmacology*; D011401:Promoter Regions, Genetic / Q000187:drug effects; D011498:Protein Precursors / Q000494:pharmacology*; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D015290:Second Messenger Systems / Q000502:physiology*; D017384:Sequence Deletion; D014158:Transcription, Genetic / Q000187:drug effects*; D014407:Tumor Cells, Cultured; D014688:Venoms / Q000494:pharmacology
D007328:Insulin; D010446:Peptide Fragments; D011498:Protein Precursors; D011993:Recombinant Fusion Proteins; D014688:Venoms; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D000242:Cyclic AMP; D008156:Luciferases; D017868:Cyclic AMP-Dependent Protein Kinases