Tumor necrosis factor alpha-mediated insulin resistance, but not dedifferentiation, is abrogated by MEK1/2 inhibitors in 3T3-L1 adipocytes.
Tumor necrosis factor-alpha (TNFalpha) has been implicated as a contributing mediator of insulin resistance observed in pathophysiological conditions such as obesity, cancer-induced cachexia, and bacterial infections. Previous studies have demonstrated that TNFalpha confers insulin resistance by promoting phosphorylation of serine residues on insulin receptor substrate 1 (IRS-1), thereby diminishing subsequent insulin-induced tyrosine phosphorylation of IRS-1. However, little is known about which signaling molecules are involved in this process in adipocytes and about the temporal sequence of events that ultimately leads to TNFalpha-stimulated IRS-1 serine phosphorylation. In this study, we demonstrate that specific inhibitors of the MAP kinase kinase (MEK)1/2-p42/44 mitogen-activated protein (MAP) kinase pathway restore insulin signaling to normal levels despite the presence of TNFalpha. Additional experiments show that MEK1/2 activity is required for TNFalpha-induced IRS-1 serine phosphorylation, thereby suggesting a mechanism by which these inhibitors restore insulin signaling. We observe that TNFalpha requires 2.5-4 h to markedly reduce insulin-triggered tyrosine phosphorylation of IRS-1 in 3T3-L1 adipocytes. Although TNFalpha activates p42/44 MAP kinase, maximal stimulation is observed within 10-30 min. To our surprise, p42/44 activity returns to basal levels well before IRS-1 serine phosphorylation and insulin resistance are observed. These activation kinetics suggest a mechanism of p42/44 action more complicated than a direct phosphorylation of IRS-1 triggered by the early spike of TNFalpha-induced p42/44 activity. Chronic TNFalpha treatment (>> 72 h) causes adipocyte dedifferentiation, as evidenced by the loss of triglycerides and down-regulation of adipocyte-specific markers. We observe that this longer term TNFalpha-mediated dedifferentiation effect utilizes alternative, p42/44 MAP kinase-independent intracellular pathways. This study suggests that TNFalpha-mediated insulin resistance, but not adipocyte dedifferentiation, is mediated by the MEK1/2-p42/44 MAP kinase pathway.
Engelman
J A
JA
Department of Molecular Pharmacology and Diabetes Research and Training Center, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
Berg
A H
AH
Lewis
R Y
RY
Lisanti
M P
MP
Scherer
P E
PE
D016475:3T3 Cells; D017667:Adipocytes / Q000166:cytology / Q000187:drug effects*; D000818:Animals; D000841:Anisomycin / Q000494:pharmacology; D002454:Cell Differentiation / Q000187:drug effects*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D004815:Epidermal Growth Factor / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance; D007700:Kinetics; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases / Q000037:antagonists & inhibitors*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010768:Phosphoserine / Q000378:metabolism; D019000:Phosphotyrosine / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors; D015398:Signal Transduction; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
D004791:Enzyme Inhibitors; D005419:Flavonoids; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D010750:Phosphoproteins; D014409:Tumor Necrosis Factor-alpha; D010768:Phosphoserine; D019000:Phosphotyrosine; D004815:Epidermal Growth Factor; D000841:Anisomycin; D011505:Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482127:Map2k1 protein, mouse; D020929:Mitogen-Activated Protein Kinase Kinases; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one