Tumor necrosis factor-alpha and ceramides in insulin resistance.
The present studies tested the hypothesis that some effects of tumor necrosis factor-alpha (TNF-alpha) are mediated by activation of sphingomyelinases and the production of ceramides. Differentiated 3T3-L1 adipocytes were incubated with short-chain ceramide analogs, (C2- and C6-ceramides: N-acetyl- and N-hexanoyl-sphingosines, respectively), and this treatment increased 2-deoxyglucose uptake in the absence of insulin progressively from 2-24 h. This effect was inhibited by blocking the activations of mitogen-activated protein kinase, phosphatidylinositol 3-kinase (PI 3-kinase), and ribosomal S6 kinase which mediated an increase in GLUT1 concentrations. Long-term increases in PI 3-kinase activity associated with insulin receptor substrate-1 (IRS-1) increased the proportion of GLUT1 and GLUT4 in plasma membranes. These events explain the increases in noninsulin-dependent glucose uptake and incorporation of this glucose into the fatty acid and glycerol moieties of triacylglycerol. The mechanisms by which TNF-alpha and ceramides increase PI 3-kinase activity were investigated further by using rat2 fibroblasts. Incubation for 20 min with TNF-alpha, bacterial sphingomyelinase, or C2-ceramides increased PI 3-kinase activity by about fivefold, and this effect depended upon a stimulation of tyrosine kinase activity and an increase in Ras-GTP. This demonstrates the existence of a novel signaling pathway for TNF-alpha that could contribute to the effects of this cytokine in stimulating basal glucose uptake. By contrast, treating the 3T3-L1 adipocytes for 2-24 h with C2-ceramide diminished insulin-stimulated glucose uptake by decreasing the insulin-induced translocation of GLUT1 and GLUT4 to plasma membranes. This inhibition was observed when there was no increase in basal glucose uptake, and it occurred downstream of PI 3-kinase. Our work provides further mechanisms whereby TNF-alpha and ceramides produce insulin resistance and decrease the effectiveness of insulin in stimulating glucose disposal from the blood. Conversely, TNF-alpha and ceramides increase the ability of adipocytes to take up glucose and store triacylglycerol in the absence of insulin.
Department of Biochemistry (Signal Transduction Laboratories), University of Alberta, Edmonton, Canada. david.brindley@ualberta.ca
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Brindley
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Wang
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Mei
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Xu
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Hanna
D016475:3T3 Cells; D017667:Adipocytes / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002518:Ceramides / Q000494:pharmacology*; D003847:Deoxyglucose / Q000378:metabolism; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D007328:Insulin / Q000494:pharmacology* / Q000502:physiology; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance; D007700:Kinetics; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D009124:Muscle Proteins; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism*; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000494:pharmacology; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
D002518:Ceramides; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C064769:N-acetylsphingosine; D010750:Phosphoproteins; C495678:Slc2a1 protein, mouse; C495693:Slc2a4 protein, mouse; D014409:Tumor Necrosis Factor-alpha; C101954:N-caproylsphingosine; D003847:Deoxyglucose; D019869:Phosphatidylinositol 3-Kinases; D013110:Sphingosine