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

Recent studies have suggested that glucose may activate insulin gene transcription through increases in intracellular Ca²⁺ concentration, possibly acting via the release of stored insulin. We have investigated this question by dynamic photon-counting imaging of insulin- and c-fos-promoter-firefly luciferase reporter construct activity. Normalized to constitutive viral promoter activity, insulin promoter activity in MIN6 β-cells was increased 1.6-fold after incubation at 30 mM compared with 3 mM glucose, but was unaltered at either glucose concentration by the presence of insulin (100 nM) or the Ca²⁺ channel inhibitor, verapamil (100 μM). Increases in intracellular [Ca²⁺] achieved by plasma membrane depolarization with KCl failed to enhance either insulin or c-fos promoter activity in MIN6 cells, but increased c-fos promoter activity 5-fold in AtT20 cells. Together, these results demonstrate that glucose can exert a direct effect on insulin promoter activity in islet β-cells, via a signalling pathway which does not require increases in intracellular [Ca²⁺] nor insulin release and insulin receptor activation.