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 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.
Egawa
K
K
Department of Medicine, University of California, San Diego, La Jolla 92093-0673, USA.
Nakashima
N
N
Sharma
P M
PM
Maegawa
H
H
Nagai
Y
Y
Kashiwagi
A
A
Kikkawa
R
R
Olefsky
J M
JM
D016475:3T3 Cells; D017667:Adipocytes / Q000378:metabolism*; D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D004789:Enzyme Activation; D015870:Gene Expression; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance; D051379:Mice; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D014162:Transfection; D000077191:Wortmannin
D000730:Androstadienes; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D010750:Phosphoproteins; D019869:Phosphatidylinositol 3-Kinases; D000077191:Wortmannin