Leucine, Palmitate, or Leucine/Palmitate Cotreatment Enhances Myotube Lipid Content and Oxidative Preference

Branched-chain amino acids (BCAA) such as leucine stimulate favorable metabolic processes involved in lean tissue preservation and skeletal muscle metabolism. However, higher levels of circulating BCAA correlate with severity of metabolic disease (including diabetes/insulin resistance), and may result from dysregulated BCAA catabolism. Past observations have demonstrated potential interaction between BCAA and dietary fat; however, much of this relationship remains underexplored. This study investigated the effect of leucine both with and without palmitate on oxidative and glycolytic metabolism, as well as indicators of BCAA catabolism using cultured skeletal muscle cells. Specifically, C2C12 myotubes were treated with or without varying concentrations of leucine both with and without palmitate for 24 h. Leucine treatment significantly elevated mRNA expression of metabolic regulators including peroxisome proliferator-activated receptor-gamma coactivator 1-alpha versus leucine with concurrent palmitate treatment. Interestingly, leucine-only, palmitate-only, and leucine with palmitate all significantly increased cellular lipid content, which translated into significantly increased oxidative capacity under substrate-limited conditions. However, upon the addition of excess substrate and carnitine, discrepancies in peak metabolic capacities between various treatments were no longer observed, suggesting leucine, palmitate, or the combination thereof causes a shift in metabolic preference from glycolytic to oxidative. These data also suggest leucine's effect on mitochondrial metabolism may result in part from increased lipid stores in addition to other previously documented pathways.