This study was conducted to examine the mechanism by which triglyceride induces insulin resistance and ER stress in HepG2 cells. Using in vitro study models, we show that triglyceride causes insulin resistance through serine phosphorylation of insulin receptor substrate-1 (IRS-1). In addition, triglyceride induces the expression of endogenous endoplasmic reticulum, (ER) stress markers, including GRP 78, IRE-1alpha, XBP-1, p-eIF2alpha, CHOP, and p-JNK. ER stress, in turn, leads to the suppression of insulin receptor signaling through tyrosine dephosphorylation of IRS-1. The results of this study show that triglyceride is a central feature of peripheral insulin resistance, and also suggest that triglyceride-induced ER stress influences insulin resistance. These experiments may be used in the development of an in vitro acute obesity model. (C) 2007 Elsevier Inc. All rights reserved.