Persistent activation of phosphatidylinositol 3-kinase causes insulin resistance due to accelerated insulin-induced insulin receptor substrate-1 degradation in 3T3-L1 adipocytes

Recently, we have reported that the overexpression of a membrane-targeted p hosphatidylinositol (PI) 3-kinase (p110CAAX) stimulated p70S6 kinase, Akt, glucose transport, and Ras activation in the absence of insulin but inhibit ed insulin-stimulated glycogen synthase activation and MAP kinase phosphory lation in 3T3-L1 adipocytes. To investigate the mechanism of p110CAAX-induc ed cellular insulin resistance, we have now studied the effect of p110CAAX on insulin receptor substrate (IRS)-1 protein. Overexpression of p110CAAX a lone decreased IRS-1 protein levels to 63 +/- 10% of control values. Insuli n treatment led to an IRS-1 gel mobility shift (most likely caused by serin e/threonine phosphorylation), with subsequent IRS-1 degradation. Moreover, insulin-induced IRS-1 degradation was enhanced by expression of p110CAAX (6 1 +/- 16% vs. 13 +/- 15% at 20 min, and 80 +/- 8% us. 41 +/- 12% at 60 min, after insulin stimulation with or without p110CAAX expression, respectivel y). In accordance with the decreased IRS-1 protein, the insulin-stimulated association between IRS-1 and the p85 subunit of PI 3-kinase was also decre ased in the p110CAAX-expressing cells, and IRS-1-associated PI 3-kinase act ivity was decreased despite the fact that total PI S-kinase activity was in creased. Five hours of wortmannin pretreatment inhibited both serine/threon ine phosphorylation and degradation of IRS-1 protein. These results indicat e that insulin treatment leads to serine/threonine phosphorylation of IRS-1 , with subsequent IRS-1 degradation, through a PI 3-kinase-sensitive mechan ism. Consistent with this, activated PI 3-kinase phosphorylates IRS-1 on se rine/threonine residues, leading to IRS-1 degradation. The similar finding was observed in IRS-2 as well as IRS-1. These results may also explain the cellular insulin-resistant state induced by chronic p110CAAX expression.