Lower expression of PKAα impairs insulin secretion in islets isolated from low-density lipoprotein receptor (LDLR(-/-)) knockout mice

Hypercholesterolemic low-density lipoprotein receptor knockout mice (LDLR(-/-)) show normal whole-body insulin sensitivity, but impaired glucose tolerance due to a reduced insulin secretion in response to glucose. Here, we investigate the possible mechanisms involved in such a defect in isolated LDLR(-/-) mice islets. Low-fat chow-fed female and male mice aged 20 weeks, LDLR(-/-) mice, and wild-type (WT) mice were used in this study. Static insulin secretion, cytoplasmatic Ca(2+) analysis, and protein expression were measured in islets isolated from LDLR(-/-) and WT mice. At basal (2.8 mmol/L) and stimulatory (11.1 mmol/L) glucose concentrations, the insulin secretion rates induced by depolarizing agents such as KCl, L-arginine, and tolbutamide were significantly reduced in LDLR(-/-) when compared with control (WT) islets. In addition, KCl-induced Ca(2+) influx at 2.8 mmol/L glucose was lower in LDLR(-/-) islets, suggesting a defect downstream of the substrate metabolism step of the insulin secretion pathway. Insulin secretion induced by the protein kinase A (PKA) activators forskolin and 3-isobutyl-1-methyl-xanthine, in the presence of 11.1 mmol/L glucose, was lower in LDLR(-/-) islets and was normalized in the presence of the protein kinase C pathway activators carbachol and phorbol 12-myristate 13-acetate. Western blotting analysis showed that phospholipase Cβ(2) expression was increased and PKAα was decreased in LDLR(-/-) compared with WT islets. Results indicate that the lower insulin secretion observed in islets from LDLR(-/-) mice at postprandial levels of glucose can be explained, at least in part, by the reduced expression of PKAα in these islets.