Eicosapentaenoic acid improves metabolic switching of human myotubes

In skeletal muscle, glucose oxidation dominates in the fed state, while fat oxidation increases both during fasting and during sustained exercise. Metabolic flexibility is the ability to switch from predominantly lipid oxidation during fasting conditions to suppression of lipid oxidation and increased glucose oxidation during feeding conditions. Loss of metabolic flexibillity is related to insulin resistance in skeletal muscle. Skeletal muscle (myotubes) store fat in lipid droplets and is the major tissue for lipid and glucose oxidation in the body. We have tested whether treatment with different fatty acids including eicosapentaenoic acid (EPA), linoleic acid (LA), palmitic acid (PA) and oleic acid (OA), as well as activation of the nuclear receptor liver X receptor, could modify metabolic switching of myotubes and whether these changes is correlated to changes in mitochondria or lipid droplets in the cells. Methods used are radiolabeled substrate oxidation assays and live imaging of lipid droplets and mitochondria. Myotubes were stained with Hoescht, Bodipy 493/503 (lipid droplets) and Mitotracker®Red FM and live imaging was performed using the Scan^R high throughput microscope (Olympus) at the Department of Molecular Biosciences, UiO, NorMIC imaging node. Number, size and size distribution of lipid droplets and mitochondrial content in myotubes dependent on pretreatment was quantified. Results showed that EPA pretreatment of myotubes increased metabolic switching. Furthermore, preatreatment with EPA, as well as with OA and LA and activation of the nuclear receptor liver X receptor increased the number of lipid droplets per nucleus. Lipid droplet size, size distribution and mitochondrial content were independent of pretreatment conditions. This study suggests a possible favorable effect of EPA on skeletal muscle metabolic switching and glucose utilization.