T. Teruel et al., Ceramide mediates insulin resistance by tumor necrosis factor-alpha in brown adipocytes by maintaining Akt in an inactive dephosphorylated state, DIABETES, 50(11), 2001, pp. 2563-2571
Tumor necrosis factor (TNF)-alpha causes insulin resistance on glucose upta
ke in fetal brown adipocytes. We explored the hypothesis that some effects
of TNF-alpha could be mediated by the generation of ceramide, given that TN
F-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 phosphatidyl
inositol (Pl) 3-kinase signaling pathway indicated that C2-ceramide preclud
ed insulin stimulation of Akt kinase activity, but not of PI-3 kinase or pr
otein kinase C-zeta activity. C2-ceramide completely abolished insulin-stim
ulated 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 activa
te 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 res
tored Akt phosphorylation by insulin, and 3) transient transfection of a co
nstitutively active form of Akt did not restore Akt activity. Our results i
ndicate that ceramide produced by TNF-alpha induces insulin resistance in b
rown adipocytes by maintaining Akt in an inactive dephosphorylated state.