Effects of chronic exposure of clonal beta-cells to elevated glucose and free fatty acids on incretin receptor gene expression and secretory responses to GIP and GLP-1.

AIM: The incretin effect, mediated by glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), is impaired in type 2 diabetes.MATERIALS AND METHODS: The present study examines the effects of prolonged exposure to elevated glucose and free fatty acids in clonal BRIN BD11 cells on GIP and GLP-1 action.RESULTS: Glucotoxic conditions (18 h) had no effect on GIP- or GLP-1-mediated insulinotropic responses. In contrast, 48 h glucotoxic culture impaired (p <0.05 to p <0.001) insulin release in response to GLP-1, and particularly GIP. Culture under lipotoxic conditions (18 h) impaired (p <0.05 to p <0.001) the insulin-releasing effect of GIP, but was without effect on GLP-1. However, 48 h lipotoxic culture compromised both GIP (p <0.05 to p <0.001) and GLP-1 (p <0.05 to p <0.01) insulin-releasing actions. Glucolipotoxic culture (18 h) completely annulled the insulinotropic action of GIP, whereas GLP-1 effects were similar to control. However, when glucolipotoxic culture was extended to 48 h, both GIP- and GLP-1-mediated effects were (p <0.05 to p <0.001) impaired. Assessment of cell viability, number and insulin content revealed detrimental (p <0.05 to p <0.001) effects under all culture conditions, barring 18 h glucotoxic and lipotoxic culture. Finally, GIP-R gene and protein expression was increased (p <0.05 to p <0.01) under glucotoxic culture, with decreased (p <0.05 to p <0.001) expression following glucolipotoxic culture. GLP-1-R gene expression followed a similar trend, but protein levels were generally reduced under all culture conditions.CONCLUSION: The results indicate that impaired insulinotropic response to GIP and GLP-1 under diabetic milieu involves mechanisms beyond simple expression of respective receptors.