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In an effort to better understand the phenomenon of lipotoxicity in human beta-cells, we evaluated the effects of 48-h preculture with 1.0 or 2.0 mmol/l free fatty acid (FFA) (2:1 oleate to palmitate) on the function and survival of isolated human islets and investigated some of the possible mechanisms. Compared with control islets, triglyceride content was significantly increased and insulin content and glucose-stimulated insulin release were significantly reduced in islets precultured with increased FFA concentrations. These changes were accompanied by a significant reduction of glucose utilization and oxidation. By cell death detection techniques, it was observed that exposure to FFAs induced a significant increase of the amount of dead cells. Electron microscopy showed the involvement of beta-cells, with morphological appearance compatible with the presence of apoptotic phenomena. FFA-induced islet cell death was blocked by inhibition of upstream caspases and partially prevented by inhibiton of ceramide synthesis or serine protease activity, whereas inhibition of nitric oxide synthesis had no effect. RT-PCR studies revealed no major change of iNOS and Bax mRNA expression and a marked decrease of Bcl-2 mRNA expression in the islets cultured with FFA. Thus, prolonged exposure to FFAs has cytostatic and pro-apoptotic effects on human pancreatic beta-cells. The cytostatic action is likely to be due to the FFA-induced reduction of intraislet glucose metabolism, and the proapoptotic effects are mostly caspase mediated, partially dependent on ceramide pathway, and possibly Bcl-2 regulated. [ABSTRACT FROM AUTHOR]

Type 2 (non-insulin-dependent) diabetes results from decreased insulin action in peripheral target tissues (insulin resistance) and impaired pancreatic beta-cell function. These defects reflect both genetic components and environmental risk factors. Recently, the common Gly(972)-->Arg amino acid polymorphism of insulin receptor substrate 1 (Arg(972) IRS-1) has been associated with human type 2 diabetes. In this study, we report on some functional and morphological properties of isolated human islets carrying the Arg(972) IRS-1 polymorphism. Insulin content was lower in variant than control islets (94 +/- 47 vs. 133 +/- 56 microU/islet; P < 0.05). Stepwise glucose increase (1.7 to 16.7 mmol/l) significantly potentiated insulin secretion from control islets, but not Arg(972) IRS-1 islets, with the latter also showing a relatively lower response to glyburide and a significantly higher response to arginine. Proinsulin release mirrored insulin secretion, and the insulin-to-proinsulin ratio in response to arginine was significantly lower from Arg(972) IRS-1 islets than from control islets. Glucose utilization and oxidation did not differ in variant and wild-type islets at both low and high glucose levels. Electron microscopy showed that Arg(972) IRS-1 beta-cells had a severalfold greater number of immature secretory granules and a lower number of mature granules than control beta-cells. In conclusion, Arg(972) IRS-1 islets have reduced insulin content, impaired insulin secretion, and a lower amount of mature secretory granules. These alterations may account for the increased predisposition to type 2 diabetes in individuals carrying the Gly(972)-->Arg amino acid polymorphism of IRS-1. [ABSTRACT FROM AUTHOR]