The Gly -> Arg(972) amino acid polymorphism in insulin receptor substrate-1 affects glucose metabolism in skeletal muscle cells

Molecular scanning of insulin receptor substrate-1 (IRS-1) revealed several amino acid substitutions. The most common IRS-1 variant, a Gly to Arg(972) change, is more prevalent among type 2 diabetic patients. In this study we overexpressed wild-type and Arg(972)IRS-1 variant in L6 skeletal muscle cells and examined the functional consequences of this polymorphism on insulin metabolic signaling. L6 cells expressing Arg(972)-IRS-1 (L6-Arg(972)) showed a decrease in insulin-stimulated IRS-l-associated phosphatidylinositol 3-binase (PI 3-kinase) activity compared with L6 cells expressing wild-type IRS-1(L6-WT) as a consequence of decreased binding of p85 subunit of PI S-kinase to IRS-1. L6-Ar-972 exhibited a decrease in both basal and insulin-stimulated glucose transport due to a reduction in the amount of both GLUT1 and GLUT4 translocated to the plasma membrane. Both basal and insulin-stimulated Akt phosphorylations were decreased in L6-Arg(972) compared with L6-WT. Basal glycogen synthase kinase-3 (GSK-3) activity was increased in L6-Arg(972) compared with L6-WT, and insulin-induced inactivation of GSK-3 was also reduced in L6-Arg(972). This change was associated with a significant decrease in insulin-stimulated glucose incorporation into glycogen and glycogen synthase activity in L6-Arg(972) compared with L6-WT. These results indicate that the Arg(972)-IRS-1 polymorphism impairs the ability of insulin to stimulate glucose transport, glucose transporter translocation, and glycogen synthesis by affecting the PI 3-kinase/Akt/GSK-3 signaling pathway. The present data indicate that the polymorphism at codon 972 of IRS-1 may contribute to the in vivo insulin resistance observed in carriers of this variant.