Regulated expression of adenosine triphosphate-sensitive potassium channelsubunits in pancreatic beta-cells

The regulation of glucose-dependent insulin secretion in pancreatic beta -c ells is linked to the expression and function of the ATP-sensitive potassiu m channel (K-ATP), which is composed of a sulfonylurea receptor (SUR1) and an inwardly rectifying potassium channel (Kir6.2). Previous animal and huma n 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 reg ulated. Here we show that high glucose concentrations lead to a marked decr ease (similar to 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 lev els. The cognate K-ATP, channel subunit SUR1 showed similar downregulation at high glucose concentration. The K-ATP channel activity of INS-1 cells cu ltivated 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 p revent glucose-dependent inhibition of Kir6.2 mRNA and K-ATP channel activi ty. To provide further insight into the mechanisms by which glucose and GLP -1 regulate beta -cell KATP channel genes, we have cloned and initiated the characterization of the Kir6.2 gene transcriptional regulatory regions con tained within the entire 4,5 kb flanked by the SUR1 and Kir6.2 genes. Trans ient transfection experiments with five deletion constructs in a pancreatic beta -cell line (INS-1) showed that the proximal 988 bp of the Kir6.2 prom oter sequence contributes only 25-30% to the total basal promoter activity. The minimal promoter region -67/+140, also encompassing parts of the 5'-un translated region, confers sensitivity to GLP-1, which stimulates transcrip tional activity of the Kir6.2 minigene by about 2-fold. We propose that glu cose- and GLP-1-dependent regulation of K-ATP subunit genes may be importan t in the adaptation of beta -cells to changes in secretory demands in physi ological and diseased states.