Glucose enhances insulin promoter activity in MIN6 beta-cells independently of changes in intracellular Ca2+ concentration and insulin secretion

Recent studies have suggested that glucose may activate insulin gene transc ription through increases in intracellular Ca2+ concentration, possibly act ing via the release of stored insulin. We have investigated this question b y dynamic photon-counting imaging of insulin- and c-fos-promoter-firefly lu ciferase reporter construct activity. Normalized to constitutive viral prom oter activity, insulin promoter activity in MIN6 beta-cells was increased 1 .6-fold after incubation at 30 mM compared with 3 mM glucose, but was unalt ered at either glucose concentration by the presence of insulin (100 nM) or the Ca2+ channel inhibitor, verapamil (100 mu M). Increases in intracellul ar [Ca2+] achieved by plasma membrane depolarization with KCI failed to enh ance either insulin or c-fos promoter activity in MIN6 beta-cells, but incr eased c-fos promoter activity 5-fold in AtT20 cells. Together, these result s demonstrate that glucose can exert a direct effect on insulin promoter ac tivity in islet beta-cells, via a signalling pathway which does not require increases in intracellular [Ca2+] nor insulin release and insulin receptor activation.