title: Sustained exposure of L6 myotubes to high glucose and insulin decreases insulin-stimulated GLUT4 translocation but upregulates GLUT4 activity.
abstract: Hyperglycemia and hyperinsulinemia are cardinal features of acquired insulin resistance. In adipose cell cultures, high glucose and insulin cause insulin resistance of glucose uptake, but because of altered GLUT4 expression and contribution of GLUT1 to glucose uptake, the basis of insulin resistance could not be ascertained. Here we show that GLUT4 determines glucose uptake in L6 myotubes stably overexpressing myc-tagged GLUT4. Preincubation for 24 h with high glucose and insulin (high Glc/Ins) reduced insulin-stimulated GLUT4 translocation by 50%, without affecting GLUT4 expression. Insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation, phosphatidylinositol 3-kinase activation, and Akt phosphorylation also diminished, as did insulin-mediated glucose uptake. However, basal glucose uptake rose by 40% without any gain in surface GLUT4. High Glc/Ins elevated basal p38 mitogen-activated protein kinase (MAPK) phosphorylation and activity, and a short inhibition of p38 MAPK with SB202190 corrected the rise in basal glucose uptake, suggesting that p38 MAPK activity contributes to this rise. We propose that in a cellular model of skeletal muscle, chronic exposure to high Glc/Ins reduced the acute, insulin-elicited GLUT4 translocation. In addition, basal state GLUT4 activity was augmented to partially compensate for the translocation defect, resulting in a more robust glucose uptake than what would be predicted from the amount of cell surface GLUT4 alone.
authors.affiliation: Programme in Cell Biology, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8.
authors.forename: Carol
authors.initials: C
authors.lastname: Huang
authors.forename: Romel
authors.initials: R
authors.lastname: Somwar
authors.forename: Nish
authors.initials: N
authors.lastname: Patel
authors.forename: Wenyan
authors.initials: W
authors.lastname: Niu
authors.forename: Dóra
authors.initials: D
authors.lastname: Török
authors.forename: Amira
authors.initials: A
authors.lastname: Klip
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects*; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology*; D051275:Glucose Transporter Type 4; D007093:Imidazoles / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology*; D007739:L Cells; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D023421:Models, Animal; D009004:Monosaccharide Transport Proteins / Q000235:genetics* / Q000378:metabolism*; D009124:Muscle Proteins; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D021381:Protein Transport / Q000187:drug effects; D011725:Pyridines / Q000494:pharmacology; D048051:p38 Mitogen-Activated Protein Kinases
chemical_list: D004791:Enzyme Inhibitors; D005947:Glucose; D051275:Glucose Transporter Type 4; D007093:Imidazoles; D007328:Insulin; D020928:Mitogen-Activated Protein Kinases; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D019869:Phosphatidylinositol 3-Kinases; D011725:Pyridines; D048051:p38 Mitogen-Activated Protein Kinases; C495693:Slc2a4 protein, mouse; C090942:4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)imidazole