Background: In metabolic syndrome, down-regulation of the insulin signaling leads to insulin-regulated metabolism and cardiovascular dyfunctions. Free fatty acids (FFAs) in the circulation are increased in this disorder and inhibit insulin signaling. Lipid oversupply contributes to the development of insulin resistance, likely by promoting the accumulation of lipid metabolites capable of inhibiting signal transduction.
Material/methods: This study was designed to examine the effects of FFAs and their metabolites on the insulin signaling pathway that leads to the activation of endothelial nitric oxide synthase (eNOS) and increase in nitric oxide (NO) production in endothelial cells.
Results: Here we demonstrate that exposing human umbilical vein endothelial cells (HUVECs) to palmitate inhibits activation of Akt/eNOS signal pathway by insulin, and subsequently insulin-stimulated NO generation. Palmitate concomitantly induced the accumulation of ceramide, a product of acyl-CoA that has been shown to accumulate in insulin-resistant tissues and to inhibit insulin signaling. Preventing de novo ceramide synthesis abolished the antagonistic effect of palmitate toward the Akt/ eNOS pathway. Moreover, inducing ceramide buildup augmented the inhibitory effect of palmitate.
Conclusions: Taken together, we have demonstrated that palmitic acid induces accumulation of ceramide, which appears to mediate palmitic acid's inhibitory effects on the Akt/eNOS pathway, leading to a significant decrease in NO generation. Therefore, ceramide is a necessary and sufficient intermediate mediating the inhibition of the AKT/eNOS signaling pathway by palmitate in endothelial cells.