DIFFERENT SUBCELLULAR-DISTRIBUTION AND REGULATION OF EXPRESSION OF INSULIN-RECEPTOR SUBSTRATE (IRS)-3 FROM THOSE OF IRS-1 AND IRS-2

Adipocytes contain three major substrate proteins of the insulin recep tor, termed IRS-1, IRS-2, and IRS-3. We demonstrated that IRS-1 and IR S-2 are located mainly in the low density microsome (LDM) fraction and are tyrosine phosphorylated in response to insulin stimulation, leadi ng to phosphatidylinositol (PI) 3-kinase activation. In contrast, IRS- 3 is located mainly in the plasma membrane (PM) fraction and contribut es to PI 3-kinase activation in the PM fraction. The different cellula r localizations of IRS proteins may account for the mechanism of insul in resistance induced by a high fat diet, considering that PI 3-kinase activation in the LDM fraction is reportedly essential for the transl ocation of GLUT4 in adipocytes. High fat feeding in rats increased bot h protein and mRNA levels of IRS-3 but decreased those of IRS-1 and IR S-2 in epididymal adipocytes. As a result, selective impairment of ins ulin-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 di fferent subcellular compartments, which may contribute to determining the insulin sensitivity in adipocytes.