Technical Abstract: Insulin resistant states are commonly associated with both increased circulating levels of tumor necrosis factor (TNF-a) and hepatic overproduction of very low density lipoproteins (VLDL). In the present study, we examined the potential mechanistic link between TNF-a and hepatic overproduction of apolipoprotein B (apoB) 100-containing VLDL. In vivo TNF-a infusion for 4 hours in chow-fed hamsters induced whole-body insulin resistance based on euglycemic hyperinsulinemic studies. Immunoprecipitation and immunoblotting analysis of livers from TNF-a-treated hamsters indicated decreased tyrosine phosphorylation of the insulin receptor (IR)-ß, IR substrate (IRS)-1 (Tyr), Akt (ser473), p38, ERK1/2, and JNK, but increased serine phosphorylation of IRS-1 (ser307)and Shc. TNF-a infusion also significantly increased hepatic production of total serum and VLDL-apoB100 in both fasting and postprandial (fat load) states. Ex vivo experiments, using the cultured primary hepatocytes from hamsters, also showed TNF-a-induced VLDL-apoB100 oversecretion, an effect that was blocked by TNF receptor 2 antibody. Moreover, TNF-a decreased the sterol regulatory element binding protein-1c mRNA and increased microsomal triglyceride transfer protein mRNA without altering apoB mRNA levels in primary hepatocytes. In summary, these data provide direct evidence that TNF-a induces whole body insulin resistance and impairs hepatic insulin signaling accompanied by hepatic overproduction of apoB100-containing VLDL particles, an effect likely mediated via TNF receptor 2.