Isomers of conjugated linoleic acid induce insulin resistance through a mechanism involving activation of protein kinase Cε in liver cells

Conjugated linoleic acid (CLA) constitutes a group of isomers derived from linoleic acid. Diverse studies have suggested that these unsaturated fatty acids have beneficial effects on human health. However, it has also been reported that their consumption can generate alterations in hepatic tissue. Thus, in the present study, we evaluated the effect of two of the major isomers of CLA, cis-9, trans-11-CLA and trans-10, cis-12-CLA, in the regulation of insulin signaling in a hepatic cell model, clone 9 (C9). We found that the two isomers decrease insulin-stimulated phosphorylation of the main proteins involved in insulin signaling, such as Akt at Seem and Thr(308), the insulin receptor at Tyr(1158), IRS-1 at Tyr(632), and GSK-3 at Ser(9/21). Protein expression, however, was unaffected. Interestingly, both isomers of CLA promoted phosphorylation and activation of PEC epsilon. Inhibition of PKC epsilon activity by a dominant-negative form or knockdown of endogenous PECe prevented the adverse effects of CLA isomers on insulin-induced Akt phosphorylation. Additionally, we also found that both isomers of CLA increase phosphorylation of IRS-1 at Ser(612), a mechanism that probably underlies the inhibition of IRS-1 signaling by PEC epsilon. Using confocal microscopy, we found that both isomers of CLA induced lipid accumulation in C9 cells with the presence of spherical cytosolic vesicles, suggesting their identity as neutral lipid droplets. These findings indicate that cis-9, trans-11-CLA and trans-10, cis-12-CLA isomers could have a significant role in the development of insulin resistance in hepatic C9 cells through IRS-1 serine phosphorylation, PECe activation, and hepatic lipid accumulation.