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A number of studies have demonstrated that tumor necrosis factor-α (TNF-α) is associated with profound insulin resistance in adipocytes and may also play a critical role in the insulin resistance of obesity and non- insulin-dependent diabetes mellitus. Reports on the mechanism of TNF-α action have been somewhat contradictory. GLUT4 down-regulation has been implicated as a possible cause of insulin resistance as has been the reduced kinase function of the insulin receptor. Here we examine the effects of tumor necrosis factor on the protein components thought to be involved in insulin- stimulated glucose transport in adipocytes, namely the insulin receptor, its major substrate IRS-1, and the insulin responsive glucose transporter GLUT4. Prolonged exposure (72-96 h) of 3T3-L1 adipocytes to TNF-α causes a substantial reduction (> 80%) in IRS-1 and GLUT4 mRNA and protein as well as a lesser reduction (>50%) in the amount of the insulin receptor. Nevertheless, the remaining proteins appear to be biochemically indistinguishable from those in untreated adipocytes. Both the insulin receptor and IRS-1 are tyrosine-phosphorylated to the same extent in response to acute insulin stimulation following cellular TNF-α exposure. Furthermore, the ability of the insulin receptor to phosphorylate exogenous substrate in the test tube is also normal following its isolation from TNF-α-treated cells. These results are confirmed by the reduced but obvious level of insulin-dependent glucose transport and GLUT4 translocation observed in TNF- α-treated adipocytes. We conclude that the insulin resistance of glucose transport in 3T3-L1 adipocytes exposed to TNF-α for 72-96 h results from a reduced amount in requisite proteins involved in insulin action. These results are consistent with earlier studies indicating that TNF-α reduces the transcriptional activity of the GLUT4 gene in murine adipocytes, and reduced mRNA transcription of a number of relevant genes may be the general mechanism by which TNF-α causes insulin resistance in adipocytes.

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Journal of Biological Chemistry

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