Regulated expression of adenosine triphosphate-sensitive potassium channel subunits in pancreatic beta-cells.

Abstract

The regulation of glucose-dependent insulin secretion in pancreatic beta-cells is linked to the expression and function of the ATP-sensitive potassium channel (K(ATP)), which is composed of a sulfonylurea receptor (SUR1) and an inwardly rectifying potassium channel (Kir6.2). Previous animal and human genetic studies have demonstrated that disruption or defective expression of K(ATP) subunit genes has a profound impact on the regulation of insulin secretion. Little is known about how SUR1 and Kir6.2 gene expression is regulated. Here we show that high glucose concentrations lead to a marked decrease (approximately 70%) in Kir6.2 messenger RNA (mRNA) levels in isolated rat pancreatic islets as well as in the INS-1 beta-cell line. This effect is reversible, because exposure to low glucose reinduces Kir6.2 transcript levels. The cognate K(ATP) channel subunit SUR1 showed similar down-regulation at high glucose concentration. The K(ATP) channel activity of INS-1 cells cultivated at high glucose was reduced by 33-51%. In contrast, glucagon-like peptide-1 (GLP-1) induced Kir6.2 mRNA steady state levels and was able to prevent glucose-dependent inhibition of Kir6.2 mRNA and K(ATP) channel activity. To provide further insight into the mechanisms by which glucose and GLP-1 regulate beta-cell K(ATP) channel genes, we have cloned and initiated the characterization of the Kir6.2 gene transcriptional regulatory regions contained within the entire 4.5 kb flanked by the SUR1 and Kir6.2 genes. Transient transfection experiments with five deletion constructs in a pancreatic beta-cell line (INS-1) showed that the proximal 988 bp of the Kir6.2 promoter sequence contributes only 25-30% to the total basal promoter activity. The minimal promoter region -67/+140, also encompassing parts of the 5'-untranslated region, confers sensitivity to GLP-1, which stimulates transcriptional activity of the Kir6.2 minigene by about 2-fold. We propose that glucose- and GLP-1-dependent regulation of K(ATP) subunit genes may be important in the adaptation of beta-cells to changes in secretory demands in physiological and diseased states.