A conjugated fatty acid present at high levels in bitter melon seed favorably affects lipid metabolism in hepatocytes by increasing NAD+/NADH ratio and activating PPARα, AMPK and SIRT1 signaling pathway

α-Eleostearic acid (α-ESA), or the cis-9, trans-11, trans-13 isomer of conjugated linolenic acid, is a special fatty acid present at high levels in bitter melon seed oil. The aim of this study was to examine the effect of α-ESA on hepatic lipid metabolism. Using H4IIEC3 hepatoma cell line, we showed that α-ESA significantly lowered intracellular triglyceride accumulation compared to α-linolenic acid (LN), used as a fatty acid control, in a dose- and time-dependent manner. The effects of α-ESA on enzyme activities and mRNA profiles in H4IIEC3 cells suggested that enhanced fatty acid oxidation and lowered lipogenesis were involved in α-ESA-mediated triglyceride lowering effects. In addition, α-ESA triggered AMP-activated protein kinase (AMPK) activation without altering sirtuin 1 (SIRT1) protein levels. When cells were treated with vehicle control (VC), LN alone (LN; 100 μmol/L) or in combination with α-ESA (LN + α-ESA; 75 + 25 μmol/L) for 24 h, acetylation of forkhead box protein O1 was decreased, while the NAD+/NADH ratio, mRNA levels of NAMPT and PTGR1 and enzyme activity of nicotinamide phosphoribosyltransferase were increased by LN + α-ESA treatment compared to treatment with LN alone, suggesting that α-ESA activates SIRT1 by increasing NAD+ synthesis and NAD(P)H consumption. The antisteatosis effect of α-ESA was confirmed in mice treated with a high-sucrose diet supplemented with 1% α-ESA for 5 weeks. We conclude that α-ESA favorably affects hepatic lipid metabolism by increasing cellular NAD+/NADH ratio and activating PPARα, AMPK and SIRT1 signaling pathways.