Insulin Secretory Function Is Impaired in Isolated Human Islets Carrying the Gly972→Arg IRS-1 Polymorphism

Abstract

Type 2 (non–insulin-dependent) diabetes results from decreased insulin action in peripheral target tissues (insulin resistance) and impaired pancreatic β-cell function. These defects reflect both genetic components and environmental risk factors. Recently, the common Gly972→Arg amino acid polymorphism of insulin receptor substrate 1 (Arg972 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 Arg972 IRS-1 polymorphism. Insulin content was lower in variant than control islets (94 ± 47 vs. 133 ± 56 μU/islet; P < 0.05). Stepwise glucose increase (1.7 to 16.7 mmol/l) significantly potentiated insulin secretion from control islets, but not Arg972 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 Arg972 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 Arg972 IRS-1 β-cells had a severalfold greater number of immature secretory granules and a lower number of mature granules than control β-cells. In conclusion, Arg972 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 Gly972→Arg amino acid polymorphism of IRS-1.