High doses of medium chain fatty acids do not induce lipotoxicity nor insulin resistance in HEPG2 hepatocytes

• Published in: Applied physiology, nutrition, and metabolism nutrition, and metabolism, 2018-01, Vol.43 (4), p.S36
• Main Authors: Rial, Sabri Ahmed, Ravaut, Gaëtan, Malaret, Tommy Boris, Bergeron, Karl-Frédérik, Mounier, Catherine
• Format: Article
• Language: English
• Subjects: Fatty acids; Insulin resistance; Lipids; Liver cancer; Obesity
• ISSN: 1715-5312

Context: Long chain fatty acids (LCFA) such as palmitate (C16) are potent contributors to obesity, fatty liver disease and insulin resistance (IR). Inversely, growing evidence suggests that medium chain fatty acids (MCFA), namely hexanoate (C6), octanoate (C8) and decanoate (C10), can improve metabolic health. Indeed, we recently reported (APNM, vol.42, 2017) that a high-fat diet loaded with medium chain triglyceride oil (MCT-oil) instead of lard (rich in LCFA) did not induce body-weight gain, liver steatosis, glucose intolerance, IR or global adiposity in mice. Objective: To elucidate the mechanisms by which MCFA avoid hepatic steatosis and IR. Methods: HepG2 cells were exposed 24h to increasing doses of C16, C6, C8 and/or C10 then submitted to an MTT viability test. HepG2 cells exposed 24h to 0.25mM of either C16 or MCFA were submitted to: a) BODIPY fluorophore staining; b) quantification of key anabolic (e.g. SREBP-1) and catabolic (e.g. CPT-1) gene expression, by Western blot and RT-qPCR; c) 3H-deoxyglucose uptake assay following a 1h exposure to 100nM insulin; d) evaluation of the phosphorylation levels of AKT1, mTOR and p70S6K kinases in response to insulin stimulation (100nM, 10min); e) and fatty acid betaoxidation assay based on the conversion rate of 14C-oleate into 14CO2. Results: While C16 significantly decreases HepG2 cell viability in a dose-response fashion, MCFA do not. At 0.25mM, the threshold of C16 toxicity, MCFA do not induce a rise in lipid droplet number and size, while C16 does so substantially. In the same time, MCFA maintain the basal beta-oxidation rate, while C16 inhibits it two-fold. Furthermore, anabolic and catabolic genes are respectively downregulated and upregulated by MCFA, at both mRNA and protein levels, and inversely regulated by C16. Finally, whereas C16 inhibits insulin-induced glucose uptake and the underlying phosphorylation of the AKT1-mTORp70S6K axis, MCFA treatment does not alter them. Conclusion: In hepatocytes, MCFA increase lipid oxidation and impede both lipogenesis and lipid esterification, thus do not cause steatosis or lipotoxicity. Then, insulin signalling pathway remains unaltered, preserving insulin sensitivity. These results demonstrate that MCFA are promising bioactive lipids in the management of metabolic diseases. (Support: NSERC and Aligo Innovation.)