Activation of SIRT1 by l-serine increases fatty acid oxidation and reverses insulin resistance in C2C12 myotubes (l-serine activates SIRT1 in C2C12 myotubes)

Silent information regulator 1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase, and the function is linked to cellular metabolism including mitochondrial biogenesis. Hepatic l-serine concentration is decreased significantly in fatty liver disease. We reported that the supplementation of the amino acid ameliorated the alcoholic fatty liver by enhancing l-serine-dependent homocysteine metabolism. In this study, we hypothesized that the metabolic production of NAD+ from l-serine and thus activation of SIRT1 contribute to the action of l-serine. To this end, we evaluated the effects of l-serine on SIRT1 activity and mitochondria biogenesis in C2C12 myotubes. l-Serine increased intracellular NAD+ content and led to the activation of SIRT1 as determined by p53 luciferase assay and western blot analysis of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) acetylation. l-Serine treatment increased the expression of the genes associated with mitochondrial biogenesis and enhanced mitochondrial mass and function. In addition, l-serine reversed cellular insulin resistance determined by insulin-induced phosphorylation of Akt and GLUT4 expression and membrane translocation. l-Serine-induced mitochondrial gene expression, fatty acid oxidation, and insulin sensitization were mediated by enhanced SIRT1 activity, which was verified by selective SIRT1 inhibitor (Ex-527) and siRNA directed to SIRT1. l-Serine effect on cellular NAD+ level is dependent on the l-serine metabolism to pyruvate that is subsequently converted to lactate by lactate dehydrogenase. In summary, these data suggest that l-serine increases cellular NAD+ level and thus SIRT1 activity in C2C12 myotubes.