title: Ceramide mediates insulin resistance by tumor necrosis factor-alpha in brown adipocytes by maintaining Akt in an inactive dephosphorylated state.
abstract: Tumor necrosis factor (TNF)-alpha causes insulin resistance on glucose uptake in fetal brown adipocytes. We explored the hypothesis that some effects of TNF-alpha could be mediated by the generation of ceramide, given that TNF-alpha treatment induced the production of ceramide in these primary cells. A short-chain ceramide analog, C2-ceramide, completely precluded insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane, as determined by Western blot or immunofluorescent localization of GLUT4. These effects were not produced in the presence of a biologically inactive ceramide analog, C2-dihydroceramide. Analysis of the phosphatidylinositol (PI) 3-kinase signaling pathway indicated that C2-ceramide precluded insulin stimulation of Akt kinase activity, but not of PI-3 kinase or protein kinase C-zeta activity. C2-ceramide completely abolished insulin-stimulated Akt/protein kinase B phosphorylation on regulatory residues Thr 308 and Ser 473, as did TNF-alpha, and inhibited insulin-induced mobility shift in Akt1 and Akt2 separated in PAGE. Moreover, C2-ceramide seemed to activate a protein phosphatase (PP) involved in dephosphorylating Akt because 1) PP2A activity was increased in C2-ceramide- and TNF-alpha-treated cells, 2) treatment with okadaic acid concomitantly with C2-ceramide completely restored Akt phosphorylation by insulin, and 3) transient transfection of a constitutively active form of Akt did not restore Akt activity. Our results indicate that ceramide produced by TNF-alpha induces insulin resistance in brown adipocytes by maintaining Akt in an inactive dephosphorylated state.
authors.affiliation: Departamento de Bioquimica y Biologia Molecular II, Facultad de Farmacia, Universidad Complutense, 28040, Madrid, Spain.
authors.forename: T
authors.initials: T
authors.lastname: Teruel
authors.forename: R
authors.initials: R
authors.lastname: Hernandez
authors.forename: M
authors.initials: M
authors.lastname: Lorenzo
mesh_terms: D017667:Adipocytes / Q000502:physiology*; D002001:Adipose Tissue, Brown / Q000166:cytology / Q000502:physiology*; D000818:Animals; D002518:Ceramides / Q000502:physiology*; D005333:Fetus; D007333:Insulin Resistance / Q000502:physiology*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D017208:Rats, Wistar; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*
chemical_list: D002518:Ceramides; D011518:Proto-Oncogene Proteins; D014409:Tumor Necrosis Factor-alpha; C494920:Akt1 protein, rat; C494923:Akt2 protein, rat; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt