The Role of Leucine and Its Metabolite (KIC) in Insulin Signalling and Glucose Transport in L6 Myotubes
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Branched-chain Amino Acids (BCAAs) are known to have positive effects in metabolic health through weight management and muscle protein synthesis. However, elevated levels of BCAAs (particularly leucine) and their metabolites have also been implicated in the development of insulin resistance and type 2 diabetes mellitus (T2DM). This study examines the dose-dependent effect of leucine in the presence or absence of other amino acids on glucose transport in L6 rat myotubes. Here we report that leucine significantly suppresses insulin-stimulated glucose uptake in skeletal muscle cells and particularly at 150 M, there is a 75% reduction in insulin-mediated glucose transport (p<0.01). This occurs in parallel with increased activation of proteins involved in the mammalian/mechanistic target of rapamycin complex 1 (mTORC1) pathway (p<0.05), which suggests a link between increased mTORC1 activity and insulin resistance. Interestingly, the suppressive effect of leucine on glucose transport disappears in the presence of other amino acids. We also illustrate that leucines metabolite, -ketoisocaproic acid (KIC) inhibits insulin-stimulated glucose uptake at 200 M by 45% concurrent with increased activation of the mTORC1 pathway (p<0.05). Finally, siRNA knockdown of the branched-chain aminotransferase 2 mitochondrial (BCAT2) enzyme which catalyzes the reversible conversion of leucine to KIC, ameliorated the inhibitory effect of KIC on glucose transport (p<0.05), suggesting that the impairing effects of KIC on glucose transport occur through its conversion back to leucine. Taken together, our results show that in L6 myotubes, leucine and its metabolite significantly suppress insulin-mediated glucose transport. Moreover, modulating the activity of the BCAT2 enzyme could be a new therapeutic approach in patients with high BCAA levels in conditions such obesity and T2DM.