Inhibition of protein kinase B by palmitate in the insulin signaling of HepG2 cells and the preventive effect of arachidonic acid on insulin resistance.

Elevated plasma levels of free fatty acids (FFAs) may contribute to insulin resistance (IR) that is characteristic of type 2 diabetes mellitus. In this study, we investigated the effects of two fatty acids, palmitate (PA) and arachidonic acid (AA) on glycogenesis under insulin signaling in HepG2cells, a transformed hepatic carcinoma cell line. In the presence of 200 μmol of palmitate, insulin (10(-7) mol/L) stimulation of glycogenesis was inhibited, as evidenced by increased glucose in the medium and decreased intracellular glycogen. Wortmannin (WM), a specific inhibitor of PI3K, dramatically decreased the amount of intracellular glycogen in cells without PA incubation. However, glycogen in PA treated cells was not significantly changed by WM, indicating that PA may also act on PI3K. Interestingly, AA restored the effects of WM inhibition on glycogenesis in PA cells. Western blot analysis demonstrated that PA in the absence of WM increased phosphorylated glycogen synthase (inactive form of GS) and decreased phosphorylated protein kinase B (active form of PKB), causing a reduction of intracellular glycogen. AA, however, reversed the effects of PA on GS and PKB. Furthermore, inhibition of protein kinase C (PKC) by a specific inhibitor chelerythrine chloride (CC) abolished the inhibitory effect of PA on glycogen synthesis by decreasing phosphorylated GS and increasing phosphorylated PKB. However, the effect of CC in the presence of PA disappeared when AA was also present.Our results suggest that there is a disruption of the insulin signaling pathway between PKB and GS when the cells were exposed to PA, contributing to IR. PA may also interrupt the PKC signaling pathway. In contrast, AA could rescue glycogenesis impaired by PA.