Release of insulin receptor substrate proteins from an intracellular complex coincides with the development of insulin resistance

Insulin receptor substrate (IRS) proteins are major substrates of the insul in receptor (IR), IRS-1 associates with an insoluble multiprotein complex, possibly the cytoskeleton, in adipocytes. This localization may facilitate interaction with the IR at the cell surface. In the present study, we exami ned the hypothesis that the release of IRS proteins from this location may be a mechanism for insulin desensitization. We show that a second IRS prote in, IRS-2, is associated with a multiprotein complex in adipocytes with sim ilar characteristics to the IRS-1 complex. Insulin treatment (15-60 min) ca used the release of IRS-1 and IRS-2 from this complex (high speed pellet; H SP) into the cytosol, whereas the level of tyrpsyl-phosphorylated IRS prote ins remained constant. Chronic insulin treatment resulted in a dramatic red uction in IRS-1 and IRS-2 in the HSP, eventually (>2 h) leading to IRS prot ein degradation and decreased levels of tyrosyl-phosphorylated IRS proteins . Okadaic acid, which rapidly induces insulin resistance in adipocytes inde pendently of TR function, caused an almost quantitative release of IRS-1 in to the cytosol commensurate with a significant reduction in tyrosyl-phospho rylated IRS proteins. Platelet-derived growth factor, a factor known to com promise insulin signaling, caused a more moderate release of IRS proteins f rom the HSP. Collectively, these results suggest that the assembly of IRS-1 /IRS-2 into a multiprotein complex facilitates coupling to the IR and that the regulated release from this location may represent a novel mechanism of insulin resistance.