Abstract
OBJECTIVES: T cells use the amino acid leucine to respond to their increased biosynthetic demands during activation. However, once inside T cells, leucine is subjected to degradation, which is initiated by the mitochondrial branched-chain aminotransferase (BCATm) that catalyzes the reversible transamination of leucine. We hypothesized that if BCATm is absent from T cells, this would provide more intracellular leucine to stimulate T cell metabolism. METHODS: To explore the dependence of T cells on BCATm function, we isolated CD4(+) T cells from spleens of wild type (WT) and BCATm global knockout (KO) mice, and after cell activation with anti-CD3 and anti-CD28 for 24 h, we measured leucine transamination, glycolysis, mitochondrial respiration and ATP synthesis, the activity of the mammalian target of rapamycin (mTOR) pathway, and the release of IFN-γ. RESULTS: The global deletion of BCATm resulted in a 1.8-fold reduction in leucine transamination and a 1.2-fold increase in the intracellular leucine concentrations in activated CD4(+) T cells from BCATmKO mice. These T cells demonstrated 4.0– and 5.0-fold increases in glycolysis and glycolytic capacity, along with 1.8– and 2-0-fold increases in the maximal respiration and spare respiratory capacity when compared to WT T cells after 24 h of activation. In addition, mTOR signaling was more active in BCATmKO T cells and their IFN-γ release was increased by 2.1-fold relative to WT T cells. CONCLUSIONS: The results suggested that leucine catabolism at the BCATm step negatively affects T cell metabolism by limiting glycolytic intermediates for biosynthetic needs and mitochondrial respiration for energy. Thus, leucine catabolism is regarded as a metabolic checkpoint of T cells that may prove useful for therapeutic purposes. FUNDING SOURCES: Des Moines University, (IOER-112-3705 to EAS), the National Institute of Health (DK 34,738 to SMH).