The L-type amino acid transporter inhibitor, BCH, inhibits myotube BCAA uptake and mitochondrial function without altering myotube insulin sensitivity during insulin resistance in C2C12 myotubes.

Introduction: Branched-chain amino acids (BCAA) are essential nutrients involved in protein synthesis. BCAA are absorbed via the L-type amino acid transporter (LAT1) in skeletal muscle where the majority of BCAA are metabolized. Higher circulating BCAA levels have been shown to correlate with insulin resistance. Some speculate that enhanced BCAA metabolism/disposal or reduced BCAA uptake may limit BCAA-mediated anabolic signaling and possibly improve insulin sensitivity.

Aims: This study investigated the effect of 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), a LAT inhibitor, on metabolism and insulin sensitivity in a myotube model of insulin resistance. Because BCAA-mediated anabolic signaling has been linked with insulin resistance, we assessed if reduced BCAA uptake via LAT inhibition would improve insulin sensitivity.

Methods: C2C12 myotubes were cultured in the presence and absence of insulin resistance and treated with and without BCH. Myotube metabolism was assessed via oxygen consumption, and associated gene and protein expression were assessed using qRT-PCR and Western blot, respectively. LC/MS was performed to assess the effect of each condition on extracellular BCAA accumulation.

Results and conclusions: BCH treatment and insulin resistance both increased extracellular BCAA levels which was associated with reduced protein expression/activity of BCAA catabolic enzymes. Additionally, BCH and insulin resistance were both independently associated with reduced mitochondrial function which occurred without significant changes in mitochondrial biogenesis signaling. Importantly, BCH did not alter myotube viability or insulin sensitivity, suggesting reduced metabolism was not a function of reduced viability. These observations demonstrate that reduction of BCAA uptake may not improve insulin resistance and may promote mitochondrial dysfunction.