title: Mechanisms of high-glucose/insulin-mediated desensitization of acute insulin-stimulated glucose transport and Akt activation.
abstract: High-glucose/low-dose insulin-mediated insulin resistance of glucose transport was studied in 3T3-L1 adipocytes. In this model, proximal insulin signaling, including insulin receptor substrate (IRS)-1-bound phosphatidylinositol 3-kinase (PI 3-kinase) activation, is preserved, but insulin-stimulated protein kinase B (Akt) activation is markedly impaired. To assess a difference in acute insulin-stimulated production of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3], cells were labeled with [32P]orthophosphate, and glycerophosphoinositides were quantified by HPLC. Although basal PtdIns(3,4,5)P3 was similar, insulin stimulated its production 33.6% more in controls (P < 0.03) than in insulin-resistant cells. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) protein, a lipid phosphatase that dephosphorylates PtdIns(3,4,5)P3 in the 3-position, was significantly and specifically increased in insulin-resistant cells. Treatment with rapamycin [a specific inhibitor of mammalian target of rapamycin complex 1 (mTORC1)] inhibited the increased PTEN expression and partially restored insulin-stimulated glucose transport and Akt activation to insulin-resistant cells. Acute insulin markedly stimulated Ser(636/639) phosphorylation of IRS-1; this was rapamycin inhibited but was significantly decreased in cells that had been preexposed to insulin, whereas total IRS-1 was unaffected. These findings were essentially paralleled by changes in the activation of p70 S6 kinase and S6-ribosomal protein. Overexpression of uncoupling protein-1 or manganese superoxide dismutase did not prevent the development of insulin-resistant glucose transport and impaired Akt activation in high-glucose/low-insulin-pretreated cells. The insulin resistance associated with glucotoxicity in our model reflects in part decreased availability of PtdIns(3,4,5)P3, which correlates with increased PTEN protein expression. Chronic activation of mTORC1 plays a role in stimulating PTEN expression and possibly in activation or induction of a phosphoprotein phosphatase. No evidence was found for a role for increased mitochondrial superoxide production in this model.
authors.affiliation: Division of Endocrinology, Department of Medicine, Medical University of South Carolina, Charleston, SC, USA.
authors.forename: Katherine A
authors.initials: KA
authors.lastname: Robinson
authors.forename: Maria G
authors.initials: MG
authors.lastname: Buse
mesh_terms: D016475:3T3 Cells; D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D017667:Adipocytes / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D001711:Biotransformation / Q000187:drug effects; D015153:Blotting, Western; D018274:Electroporation; D005947:Glucose / Q000494:pharmacology*; D051246:Glucose Transport Proteins, Facilitative / Q000378:metabolism*; D049452:Green Fluorescent Proteins; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance / Q000502:physiology; D007473:Ion Channels / Q000378:metabolism; D050356:Lipid Metabolism / Q000187:drug effects; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D024101:Mitochondrial Proteins / Q000378:metabolism; D046912:Multiprotein Complexes; D051058:Oncogene Protein v-akt / Q000378:metabolism*; D051059:PTEN Phosphohydrolase / Q000096:biosynthesis / Q000235:genetics; D010716:Phosphatidylinositols / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D011506:Proteins; D013481:Superoxides / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics; D000071256:Uncoupling Protein 1
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D051246:Glucose Transport Proteins, Facilitative; D007004:Hypoglycemic Agents; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D007473:Ion Channels; C526295:Irs1 protein, mouse; D024101:Mitochondrial Proteins; D046912:Multiprotein Complexes; D010716:Phosphatidylinositols; D011506:Proteins; D014157:Transcription Factors; C000604372:Ucp1 protein, mouse; D000071256:Uncoupling Protein 1; D013481:Superoxides; D049452:Green Fluorescent Proteins; D011494:Protein Kinases; C546843:mTOR protein, mouse; D000076222:Mechanistic Target of Rapamycin Complex 1; D051058:Oncogene Protein v-akt; D058570:TOR Serine-Threonine Kinases; D051059:PTEN Phosphohydrolase; D005947:Glucose