Insulinotropic hormone glucagon-like peptide 1 (GLP-1) activation of insulin gene promoter inhibited by p38 mitogen-activated protein kinase

The insulin gene promoter contains many transcriptional response elements t hat predispose the gene to a wide range of regulatory signals. Glucagon-lik e peptide 1 (GLP-1) stimulates insulin gene transcription by intracellular second messenger cascades leading to direct transcription factor activation or to the up-regulation of insulin promoter specific transcription factors . In these studies, we have identified a novel regulatory signaling mechani sm acting on the rat insulin 1 promoter (rINS1) in the INS-1 beta -cell lin e. In the presence of stimulatory concentrations of GLP-1 (0.1-100 nM) on r INS1 activity, inhibition of p38 mitogen-activated protein kinase (p38 MAPK ) using SE 203580 resulted in a marked increase in promoter activity (maxim um 3-fold) over GLP-1 alone, as determined by rINS1 promoter-luciferase rep orter gene expression. This effect was revealed to be mediated via the cAMP response element (CRE) of rINS 1, because site directed mutagenesis of the CRE motif in rINS1 abolished the in-creased response to SE 203580. Further more, inhibition of p38 MAPK uncovered a similar, more pronounced, response in the expression of a generic CRE promoter driven reporter gene. Time cou rse dose-response studies indicate that the p38 MAPK induced inhibitory res ponse may involve expression of immediate early genes (IEGs); maximum repre ssion of rINS 1 activity occurred after 4 h of treatment, comparable with r egulatory responses by IEGs. In conclusion, these results demonstrate a nov el signaling mechanism whereby p38 MAPK represses rINS1 promoter activity i n response to GLP-1, suggesting the involvement of a robust regulatory cont rol by p38 MAPK in insulin gene expression. The relevance of this mechanism may be most apparent during periods of cellular stress in which p38 MAPK a ctivity is stimulated. In this regard, reduced insulin expression levels ca used by chronic hyperglycemia (glucotoxicity) and/or hyperlipidemia (lipoto xicity) may be a direct consequence of this mechanism.