Cholesterol reduction ameliorates glucose-induced calcium handling and insulin secretion in islets from low-density lipoprotein receptor knockout mice

Abstract

Aims/hypothesisChanges in cellular cholesterol level may contribute to beta cell dysfunction. Islets from low density lipoprotein receptor knockout (LDLR−/−) mice have higher cholesterol content and secrete less insulin than wild-type (WT) mice. Here, we investigated the association between cholesterol content, insulin secretion and Ca2+ handling in these islets. MethodsIsolated islets from both LDLR−/− and WT mice were used for measurements of insulin secretion (radioimmunoassay), cholesterol content (fluorimetric assay), cytosolic Ca2+ level (fura-2AM) and SNARE protein expression (VAMP-2, SNAP-25 and syntaxin-1A). Cholesterol was depleted by incubating the islets with increasing concentrations (0–10mmol/l) of methyl-beta-cyclodextrin (MβCD). ResultsThe first and second phases of glucose-stimulated insulin secretion (GSIS) were lower in LDLR−/− than in WT islets, paralleled by an impairment of Ca2+ handling in the former. SNAP-25 and VAMP-2, but not syntaxin-1A, were reduced in LDLR−/− compared with WT islets. Removal of excess cholesterol from LDLR−/− islets normalized glucose- and tolbutamide-induced insulin release. Glucose-stimulated Ca2+ handling was also normalized in cholesterol-depleted LDLR−/− islets. Cholesterol removal from WT islets by 0.1 and 1.0mmol/l MβCD impaired both GSIS and Ca2+ handling. In addition, at 10mmol/l MβCD WT islet showed a loss of membrane integrity and higher DNA fragmentation. ConclusionAbnormally high (LDLR−/− islets) or low cholesterol content (WT islets treated with MβCD) alters both GSIS and Ca2+ handling. Normalization of cholesterol improves Ca2+ handling and insulin secretion in LDLR−/− islets.