Role of mTOR in the degradation of IRS-1: regulation of PP2A activity.
We have investigated the role of PI 3-kinase and mTOR in the degradation of IRS-1 induced by insulin. Inhibition of mTOR with rapamycin resulted in approximately 50% inhibition of the insulin-induced degradation of IRS-1. In contrast, inhibition of PI-3 kinase, an upstream activator of mTOR, leads to a complete block of the insulin-induced degradation. Inhibition of either PI-3 kinase or mTOR prevented the mobility shift in IRS-1 in response to insulin, a shift that is caused by Ser/Thr phosphorylation. These results indicate that insulin stimulates PI 3-kinase-mediated degradation of IRS-1 via both mTOR-dependent and -independent pathways. Platelet-derived growth factor (PDGF) stimulation leads to a lower level of degradation, but significant phosphorylation of IRS-1. Both the degradation and phosphorylation of IRS-1 in response to PDGF are completely inhibited by rapamycin, suggesting that PDGF stimulates IRS-1 degradation principally via the mTOR-dependent pathway. Inhibition of the serine/threonine phosphatase PP2A with okadaic acid also induced the phosphorylation and degradation of IRS-1. IRS-1 phosphorylation and degradation in response to okadaic acid were not inhibited by rapamycin, suggesting that the action of mTOR in the degradation of IRS-1 results from inhibition of PP2A. Consistent with this, treatment of cells with rapamycin stimulated PP2A activity. While the role of mTOR in the phosphorylation of IRS-1 appears to proceed primarily through the regulation of PP2A, we also provide evidence that the regulation of p70S6 kinase phosphorylation requires the direct activity of mTOR.
Hartley
David
D
Department of Biology, Boston University, Boston, Massachusetts 02215, USA.
Cooper
Geoffrey M
GM
D016475:3T3 Cells / Q000187:drug effects / Q000201:enzymology; D017667:Adipocytes / Q000378:metabolism; D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D000077214:Becaplermin; D024202:Electrophoretic Mobility Shift Assay; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D007004:Hypoglycemic Agents / Q000494:pharmacology; D015151:Immunoblotting; D007328:Insulin / Q000494:pharmacology; D055504:Insulin Receptor Substrate Proteins; D051379:Mice; D019319:Okadaic Acid / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism*; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology; D011494:Protein Kinases / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D020574:Proto-Oncogene Proteins c-sis; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases
D000903:Antibiotics, Antineoplastic; D004791:Enzyme Inhibitors; D007004:Hypoglycemic Agents; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D010750:Phosphoproteins; D010982:Platelet-Derived Growth Factor; D011518:Proto-Oncogene Proteins; D020574:Proto-Oncogene Proteins c-sis; D000077214:Becaplermin; D019319:Okadaic Acid; D011494:Protein Kinases; C546843:mTOR protein, mouse; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D010749:Phosphoprotein Phosphatases; D020123:Sirolimus