Glucotoxicity inhibits cAMP-protein kinase A-potentiated glucose-stimulated insulin secretion in pancreatic β-cells

Background: The effect of incretin is markedly blunted in patients with type 2 diabetes (T2D), and this reduced effect of incretin is correlated with a diminished insulintropic potency of glucagon-like peptide-1 (GLP-1). We reported recently that GLP-1 potentiates glucose-stimulated insulin secretion (GSIS) mainly via activation of the cAMP-protein kinase A (PKA) signaling pathway in INS-1E cells under hyperglycemic conditions. In the present study, we further explored whether glucotoxicity impairs cAMP-PKA-mediated effects and its relevance to the reduced insulinotropic action of GLP-1 in hyperglycemia. Methods: Mouse islets and INS-1E cells were cultured in 30mmol/L glucose for 72h. The effects of glucotoxicity on cAMP-PKA-linked pathways and its insulinotropic action were then evaluated. Results: Chronic exposure of INS-1E cells and primary mouse islets to 30mmol/L glucose almost abolished GSIS. The cAMP-elevating agent forskolin produced an approximate 1.9-fold increase in GSIS, significantly lower than that observed with 5.5mmol/L glucose (∼3.3-fold). Moreover, 72h culture in the presence of 30mmol/L glucose reduced forskolin-stimulated cAMP accumulation in β-cells. Notably, glucotoxicity reduced the expression and activity of PKA, as well as PKA-mediated effects. In contrast, glucotoxicity had no effect on the expression of Epac2, another cAMP effector. Conclusions: Glucotoxicity-induced reductions in PKA and its signaling account, at least in part, for the decreased incretin effect under conditions of glucotoxicity.