(DCP081) SYSTEMIC INHIBITION OF BRANCHED CHAIN AMINOTRANSFERASE 2 (BCAT2), BUT NOT MUSCLE-SPECIFIC DELETION OF BCAT2, ENHANCES GLUCOSE HOMEOSTASIS IN OBESE AND INSULIN RESISTANT MICE

Background: Branched-chain amino acids (BCAAs) and their subsequent catabolic metabolites, branched-chain alpha-ketoacids (BCKAs), have been identified as potential biomarkers for insulin resistance and type 2 diabetes. However, the specific role and relative contribution of BCAAs versus BCKAs in mediating insulin resistance at the systemic and muscular levels remains unknown. To investigate these questions, we employed a branched-chain aminotransferase 2 (BCAT2)-selective inhibitor to attenuate the global expression of mitochondrial BCAT2, which converts BCAAs to BCKAs, and also generated muscle-specific Bcat2 knockout (Bcat2SkM-/-) mice.

METHODS AND RESULTS: Eight-week-old C57BL/6J mice, or Bcat2SkM-/- mice and their control human α-skeletal actin Cre (HSACre) littermates were fed a high-fat diet for eight weeks to induce obesity and insulin resistance. Subsequently, C57BL/6J obese mice treated with either vehicle control or a BCAT2-selective inhibitor at a dose of 100 mg/kg/day for three weeks, along with Bcat2SkM-/- obese mice and their littermates, underwent assessment of glucose homeostasis via glucose and insulin tolerance tests and whole-body energy metabolism using indirect calorimetry.

Results: Treating C57BL/6J obese mice with a BCAT2-selective inhibitor significantly improved glucose tolerance and insulin sensitivity, without any apparent changes in body weight and energy expenditure. Interestingly, the improvement in glucose homeostasis induced by the BCAT2 inhibitor was accompanied by a systemic decrease in BCKA and an increase in BCAA concentrations, and a phenotype that was reversed upon supplementation with BCKAs. Unlike the whole-body inhibitor, Bcat2SkM-/- obese mice displayed no changes in glucose homeostasis, even though the levels of BCKA in their muscles were significantly reduced and the levels of BCAA were elevated. Moreover, treatment with a BCAT2 inhibitor in Bcat2SkM-/- obese mice resulted in improved glucose homeostasis, indicating that the accumulation of BCKAs, rather than BCAAs, is associated with insulin resistance in tissues beyond muscle. Notably, the deletion of muscle Bcat2 led to increased levels of BCAAs in muscle. This promoted protein synthesis and enhanced exercise performance in mice by activating downstream targets of mTOR, such as 4E-BP1 and p70S6K.

Conclusion: This study establishes a critical role of BCAAs relative to BCKAs in promoting insulin resistance at both the whole-body and muscle levels. Furthermore, our results demonstrate that systemic inhibition of BCAT2 enhances glucose homeostasis, presumably by reducing BCKA levels in organs other than muscle, such as adipose tissue and/or the brain.