Identification of a novel human glucagon receptor promoter: regulation by cAMP and PGC-1alpha

Previously we have demonstrated that glucagon receptor mRNA expression in cultured rat hepatocytes and pancreatic islets can be regulated by various factors, including cAMP; however, the regulation of the human glucagon receptor gene has not been well-defined. Here we have characterized the promoter regions of the human glucagon receptor gene. Primer extension studies yielded multiple products in both liver and pancreas, corresponding to transcription start sites situated at -166 and -477 relative to the start of translation, indicating two putative promoters. Both transcription start sites were found to be active, when sequence immediately upstream of the start sites were cloned into luciferase reporter constructs. The transcriptional activity of the proximal promoter, but not the distal promoter, could be inhibited approximately 50% by cAMP, indicating that the previously observed inhibitory effects of cAMP on glucagon receptor mRNA expression is mediated at the level of gene transcription. The cAMP-mediated downregulation of the proximal promoter was examined by deletion analysis in the human hepatoma cell line HepG2 and the cAMP responsiveness was found to be located in a region between 1051 and 1016 base pairs upstream of the transcription start site, which contains several putative cAMP responsive elements. Expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha), known to be upregulated in the liver by fasting, was found to abolish the cAMP-dependent downregulation of glucagon receptor mRNA expression in vitro, whereas overexpression of PGC-1beta had no effect.