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

Endocrinology. 2000 Jun;141(6):1930-5. doi: 10.1210/endo.141.6.7516.


Recently, we have reported that the overexpression of a membrane-targeted phosphatidylinositol (PI) 3-kinase (p110CAAX) stimulated p70S6 kinase, Akt, glucose transport, and Ras activation in the absence of insulin but inhibited insulin-stimulated glycogen synthase activation and MAP kinase phosphorylation in 3T3-L1 adipocytes. To investigate the mechanism of p110CAAX-induced cellular insulin resistance, we have now studied the effect of p110CAAX on insulin receptor substrate (IRS)-1 protein. Overexpression of p110CAAX alone decreased IRS-1 protein levels to 63+/-10% of control values. Insulin treatment led to an IRS-1 gel mobility shift (most likely caused by serine/threonine phosphorylation), with subsequent IRS-1 degradation. Moreover, insulin-induced IRS-1 degradation was enhanced by expression of p110CAAX (61+/-16% vs. 13+/-15% at 20 min, and 80+/-8% vs. 41+/-12% at 60 min, after insulin stimulation with or without p110CAAX expression, respectively). 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 decreased in the p110CAAX-expressing cells, and IRS-1-associated PI 3-kinase activity was decreased despite the fact that total PI 3-kinase activity was increased. Five hours of wortmannin pretreatment inhibited both serine/threonine phosphorylation and degradation of IRS-1 protein. These results indicate that insulin treatment leads to serine/threonine phosphorylation of IRS-1, with subsequent IRS-1 degradation, through a PI 3-kinase-sensitive mechanism. Consistent with this, activated PI 3-kinase phosphorylates IRS-1 on serine/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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • 3T3 Cells
  • Adipocytes / metabolism*
  • Androstadienes / pharmacology
  • Animals
  • Blotting, Northern
  • Blotting, Western
  • Enzyme Activation
  • Gene Expression
  • Insulin / pharmacology*
  • Insulin Receptor Substrate Proteins
  • Insulin Resistance*
  • Mice
  • Phosphatidylinositol 3-Kinases / genetics
  • Phosphatidylinositol 3-Kinases / metabolism*
  • Phosphoproteins / metabolism*
  • Transfection
  • Wortmannin


  • Androstadienes
  • Insulin
  • Insulin Receptor Substrate Proteins
  • Irs1 protein, mouse
  • Phosphoproteins
  • Phosphatidylinositol 3-Kinases
  • Wortmannin