Increased Insulin Receptor Substrate 1 Serine Phosphorylation and Stress-Activated Protein Kinase/c-Jun N-Terminal Kinase Activation Associated With Vascular Insulin Resistance in Spontaneously Hypertensive Rats

Abstract

Insulin resistance is associated with cardiovascular disease. Impaired insulin receptor substrate (IRS)–mediated signal transduction is a major contributor to insulin resistance. Recently, IRS-1 phosphorylation at serine 307 by stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) has been highlighted as a molecular event that causes insulin resistance. We investigated IRS-1–mediated insulin signaling, IRS-1 phosphorylation at serine 307, and SAPK/JNK activation status in the aorta of spontaneously hypertensive rats (SHR) by immunoprecipitation and immunoblotting. Insulin-stimulated tyrosine phosphorylation of insulin receptor and IRS-1 in SHR was decreased to 55% ( P <0.01) and 40% ( P <0.01) of the levels in Wistar-Kyoto rats (WKY), respectively. Insulin-stimulated IRS-1–associated phosphatidylinositol 3-kinase activation in SHR was reduced to 28% of the level in WKY ( P <0.0001). Immunoblot analysis revealed that phosphorylated IRS-1 at serine 307 in SHR was increased to 261% ( P <0.001) of the level in WKY. Phosphorylated (activated) SAPK/JNK in SHR was increased to 223% of the level in WKY ( P <0.01). Serine-phosphorylated IRS-1 that was immunoprecipitated from the aorta of SHR was capable of inhibiting in vitro tyrosine phosphorylation by recombinant insulin receptor compared with WKY-derived IRS-1. These findings demonstrate that insulin resistance in the aorta of SHR was associated with elevated IRS-1 phosphorylation at serine 307 and increased SAPK/JNK activation. The present study suggests that increased SAPK/JNK activation may play an important role in the pathogenesis of vascular insulin resistance via inhibitory serine phosphorylation of IRS-1.