Characterization of palmitic acid toxicity induced insulin resistance in HepG2 cells.

An etiology of palmitic acid (PA) induced insulin resistance (IR) is complex for which two mechanisms are proposed namely ROS induced JNK activation and lipid induced protein kinase-C (PKCε) activation. However, whether these mechanisms act alone or in consortium is not clear. In this study, we have characterized PA induced IR in liver cells. These cells were treated with different concentrations of PA for either 8 or 16 h. Insulin responsiveness of cells treated with PA for 8 h was found to be same as that of control. However, cells treated with PA for 16 h, showed increased glucose output both in the presence and in absence of insulin only at higher concentrations, indicating development of IR. In these, both JNK and PKCε were activated in response to increased ROS and lipid accumulation, respectively. Activated JNK and PKCε phosphorylated IRS1 at Ser-307 resulting in inhibition of AKT which in turn inactivated GSK3β, leading to reduced glycogen synthase activity. Inhibition of AKT also reduced insulin suppression of hepatic gluconeogenesis by activating Forkhead box protein O1 (FOXO1) and increased expression of the gluconeogenic enzymes and their transcription factors. Thus, our data clearly demonstrate that both these mechanisms work simultaneously and more importantly, identified a threshold of HepG2 cells, which when crossed led to the pathological state of IR in response to PA.