Different Subcellular Distribution and Regulation of Expression of Insulin Receptor Substrate (IRS)-3 from Those of IRS-1 and IRS-2

Motonobu Anai,Hiraku Ono,Makoto Funaki,Yasushi Fukushima,Kouichi Inukai,Takehide Ogihara,Hideyuki Sakoda,Yukiko Onishi,Yoshio Yazaki,Masatoshi Kikuchi,Yoshitomo Oka,Tomoichiro Asano

doi:10.1074/jbc.273.45.29686

Abstract

Adipocytes contain three major substrate proteins of the insulin receptor, termed IRS-1, IRS-2, and IRS-3. We demonstrated that IRS-1 and IRS-2 are located mainly in the low density microsome (LDM) fraction and are tyrosine phosphorylated in response to insulin stimulation, leading to phosphatidylinositol (PI) 3-kinase activation. In contrast, IRS-3 is located mainly in the plasma membrane (PM) fraction and contributes to PI 3-kinase activation in the PM fraction. The different cellular localizations of IRS proteins may account for the mechanism of insulin resistance induced by a high fat diet, considering that PI 3-kinase activation in the LDM fraction is reportedly essential for the translocation of GLUT4 in adipocytes. High fat feeding in rats increased both protein and mRNA levels of IRS-3 but decreased those of IRS-1 and IRS-2 in epididymal adipocytes. As a result, selective impairment of insulin-induced PI 3-kinase activation was observed in the LDM fraction, whereas PI 3-kinase activation was conserved in the PM fraction. This is the first report showing that different IRS proteins function in different subcellular compartments, which may contribute to determining the insulin sensitivity in adipocytes.

Highlights

Adipocytes contain three major substrate proteins of the insulin receptor, termed IRS-1, IRS-2, and IRS-3
We demonstrated that IRS-1 and IRS-2 are located mainly in the low density microsome (LDM) fraction and are tyrosine phosphorylated in response to insulin stimulation, leading to phosphatidylinositol (PI) 3-kinase activation
The different cellular localizations of IRS proteins may account for the mechanism of insulin resistance induced by a high fat diet, considering that PI 3-kinase activation in the LDM fraction is reportedly essential for the translocation of GLUT4 in adipocytes

Summary

Introduction

Adipocytes contain three major substrate proteins of the insulin receptor, termed IRS-1, IRS-2, and IRS-3. Selective impairment of insulin-induced PI 3-kinase activation was observed in the LDM fraction, whereas PI 3-kinase activation was conserved in the PM fraction This is the first report showing that different IRS proteins function in different subcellular compartments, which may contribute to determining the insulin sensitivity in adipocytes. The different regulation of IRS-3 from that of IRS-1 and IRS-2 would induce a different degree of PI 3-kinase activation in the different subcellular locations, which may be involved in the pathogenesis of insulin resistance This is the first paper showing the different role of IRS-3 and suggesting the possibility that the difference in regulation of expression of IRS proteins may affect insulin sensitivity in fat cells. The abbreviations used are: PI 3-kinase, phosphatidylinositol 3-kinase; IRS insulin receptor substrate; PH, pleckstrin homology; PTB, phosphotyrosine binding; PM, plasma membrane; HDM, high density microsome; LDM, low density microsome; PVDF, polyvinylidene difluoride; PAGE, polyacrylamide gel electrophoresis

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