The Gly(972)-> Arg amino acid polymorphism in IRS-1 impairs insulin secretion in pancreatic beta cells

Recent studies have identified several polymorphisms in the human insulin r eceptor substrate-1 (IRS-1) gene. The most prevalent IRS-1 variant, a Gly-- >Arg change at the codon 972, has been reported to be increased in prevalen ce among patients with type 2 diabetes. Carriers of the Arg(972) substituti on are characterized by lower fasting insulin and C-peptide levels compared with non-carriers, suggesting that the Arg(972) IRS-1 variant may contribu te to impairment of insulin secretion. In this study, we stably overexpress ed both wild-type IRS-1 (RIN-WT) and Arg(972) IRS-1 variant (RIN-Arg(972)) in RIN beta cells to investigate directly whether the polymorphism in codon 972 of IRS-1 impairs insulin secretion. The Arg(972) IRS-1 variant did not affect expression or function of endogenous IRS-2. RIN-WT showed a marked increase in both glucose- and insulin-stimulated tyrosine phosphorylation o f IRS-1 compared with control RIN cells. The Arg(972) IRS-1 variant did not alter the extent of either glucose-or insulin-stimulated tyrosine phosphor ylation of recombinant IRS-1. However, RIN-Arg(972) showed a significant de crease in binding of the p85 subunit of phosphatidylinositol-3-kinase (PI 3 -kinase) with IRS-1, compared with RIN-WT. Compared with control RIN cells, insulin content was reduced to the same extent in RIN-WT or RIN-Arg(972) a t both the protein and mRNA levels. Both glucose- and sulfonylurea-induced insulin secretion was increased in RIN-WT compared with control RIN cells. By contrast, RIN cells expressing Arg(972) IRS-1 exhibited a marked decreas e in both glucose- and sulfonylurea-stimulated insulin secretion compared w ith RIN-WT. These data suggest that the insulin signaling pathway involving the IRS-1/PI 3-kinase may play an important role in the insulin secretory process in pancreatic beta cells. More importantly, the results suggest tha t the common Arg(972) IRS-1 polymorphism may impair glucose-stimulated insu lin secretion, thus contributing to the relative insulin deficiency observe d in carriers of this variant.