Branched‐chain Amino Acid Catabolism is Required for Muscle Cell Differentiation

Abstract

The importance of branched‐chain amino acids (BCAAs) in promoting skeletal muscle anabolism has been well studied. BCAAs isoleucine, leucine, and valine have been shown to have a profound effect on activating anabolic signaling pathways in skeletal muscle. This occurs in part by the upregulation in activity of the mammalian target of rapamycin complex‐1 (mTORC1), resulting in increased protein synthesis. However, the regulation of branched‐chain amino acids during the development of muscle remains yet to be elucidated. Here, we studied BCAA metabolism during a 5‐day differentiation of L6 myoblasts. Although no change in intracellular BCAA concentrations was observed during differentiation, L6 cells cultured in the absence of leucine were severely impaired in their ability to differentiate as expression of myosin heavy chain (MHC) was completed abrogated at day 5. Two enzymes which are critical for BCAA metabolism are the branched‐chain amino transferase‐2 enzyme (BCAT2) and the branched chain α‐keto acid dehydrogenase complex (BCKD). BCAT2 catabolizes BCAAs to their corresponding alpha‐keto acids, which are then irreversibly decarboxylated by the BCKD complex. The abundance of BCAT2 did not change during differentiation, whereas levels of the E1α subunit of BCKD was increased 7x on day 5 compared to day 1 (p<0.05). Finally, when either BCAT2 or BCKDE1α was knocked down in the presence of leucine, myoblast differentiation was abrogated (BCKDE1α RNAi: MHC decreased 4.5x from day 3 to day 5 (p<0.05); BCAT2 RNAi: MHC levels completely abolished). Our findings suggest that BCAA catabolism may be a critical process in facilitating muscle differentiation.