Tumor necrosis factor (TNF)-alpha is one of the candidate mediators of insulin 
resistance associated with obesity, a major risk factor for the development of 
type 2 diabetes. The insulin resistance induced by TNF-alpha is antagonized by 
thiazolidinediones (TZDs), a new class of insulin-sensitizing drugs. The aim of 
the current study was to dissect the mechanism whereby pioglitazone, one of the 
TZDs, ameliorates TNF-alpha-induced insulin resistance in 3T3-L1 adipocytes. 
Pioglitazone restored insulin-stimulated 2-deoxyglucose (DOG) uptake, which was 
reduced by TNF-alpha, with concomitant restorations in tyrosine phosphorylation 
and protein levels of insulin receptor (IR) and insulin receptor substrate 
(IRS)-1, as well as association of the p85 regulatory subunit of 
phosphatidylinositol (PI) 3-kinase with IRS-1 and PI 3-kinase activity. 
Adenovirus-mediated gene transfer of either wild-type human peroxisome 
proliferator-activated receptor (PPAR)-gamma2 or a mutant carrying a replacement 
at the consensus mitogen-activated protein kinase phosphorylation site 
(hPPAR-gamma2-S112A) promoted adipogenesis of 3T3-L1 fibroblasts and restored 
TNF-alpha-induced decrease of triglyceride in adipocytes as effectively as 
pioglitazone. Overexpression of the PPAR-gamma proteins in TNF-alpha-treated 
adipocytes restored protein levels of IR/IRS-1, but did not improve 
insulin-stimulated tyrosine phosphorylation of IR/IRS-1 or insulin-stimulated 
2-DOG uptake. These results indicate that the ability of pioglitazone to restore 
insulin-stimulated tyrosine phosphorylation of IR/IRS-1, which is necessary for 
amelioration of TNF-alpha-induced insulin resistance, may be independent of the 
adipogenic activity of PPAR-gamma that regulates protein levels of IR/IRS-1.