Rat glucagon receptor mRNA is directly regulated by glucose through transactivation of the carbohydrate response element binding protein

Abstract

The glucagon receptor (Gcgr) is essential for maintaining glucose homeostasis in the liver and for stimulating insulin secretion in pancreatic β-cells. Glucose induces rat Gcgr mRNA expression; however, the precise mechanism remains unknown. We previously have studied the role of the carbohydrate response element binding protein (ChREBP), a glucose-activated transcription factor, in the regulation of glucose-stimulated gene expression. The G-box has previously been reported to be responsible for glucose regulation of Gcgr mRNA expression. The G-box comprises two E-boxes separated by 3bp, which distinguishes it from the carbohydrate response element (ChoRE), which has 5-bp spacing between the two E-boxes. In the rat Gcgr promoter, a putative ChoRE (−554bp/−538bp) is localized near the G-box (−543bp/−529bp). In rat INS-1E insulinoma cells, deletion studies of the rat Gcgr promoter show that ChoRE is a minimal glucose response element. Moreover, reporter assays using a pGL3 promoter vector, which harbors ChoRE and chromatin immunoprecipitation assays reveal that ChoRE is a functional glucose response element in the rat Gcgr promoter. Furthermore, In contrast, glucagon partly suppresses glucose-induced expression of Gcgr mRNA. Thus, ChREBP directly regulates rat Gcgr expression in INS-1E cells. In addition, negative feedback looping between ChREBP and GCGR may further contribute to the regulation of glucose-induced gene expression.