The Inhibition of Metabolic Inflammation by EPA Is Associated with Enhanced Mitochondrial Fusion and Insulin Signaling in Human Primary Myotubes

BACKGROUND: Sustained fuel excess triggers low-grade inflammation that can drive mitochondrial dysfunction, a pivotal defect in the pathogenesis of insulin resistance in skeletal muscle.OBJECTIVES: This study aimed to investigate whether inflammation in skeletal muscle can be prevented by EPA, and if this is associated with an improvement in mitochondrial fusion, membrane potential, and insulin signaling.METHODS: Human primary myotubes were treated for 24h with palmitic acid (PA, 500 muM) under hyperglycemic conditions (13mM glucose), which represents nutrient overload, and in the presence or absence of EPA (100 muM). After the treatments, the expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A) and IL6 was assessed by q-PCR. Western blot was used to measure the abundance of the inhibitor of NF-kappaB (IKBA), mitofusin-2 (MFN2), mitochondrial electron transport chain complex proteins, and insulin-dependent AKT (Ser473) and AKT substrate 160 (AS 160; Thr642) phosphorylation. Mitochondrial dynamics and membrane potential were evaluated using immunocytochemistry and the JC-1 (tetraethylbenzimidazolylcarbocyanine iodide) dye, respectively. Data were analyzed using 1-factor ANOVA followed by Tukey post hoc test.RESULTS: Nutrient excess activated the proinflammatory NFkappaB signaling marked by a decrease in IKBA (40%; P<0.05) and the upregulation of IL6 mRNA (12-fold; P<0.001). It also promoted mitochondrial fragmentation (53%; P<0.001). All these effects were counteracted by EPA. Furthermore, nutrient overload-induced drop in mitochondrial membrane potential (6%; P<0.05) was prevented by EPA. Finally, EPA inhibited fuel surplus-induced impairment in insulin-mediated phosphorylation of AKT (235%; P<0.01) and AS160 (49%; P<0.05).CONCLUSIONS: EPA inhibited NFkappaB signaling, which was associated with an attenuation of the deleterious effects of PA and hyperglycemia on both mitochondrial health and insulin signaling in human primary myotubes. Thus, EPA might preserve skeletal muscle metabolic health during sustained fuel excess but this requires confirmation in human clinical trials.