Evaluating metabolic and non metabolic functions of BCAT1 in T-cell Acute Lymphoblastic Leukaemia

The cytosolic Branched Chain Aminotransferase 1 (BCAT1) catalyses the reversible transamination of Branched Chain Amino Acids generating the respective Branched Chain α-Keto Acids, which can fuel several biosynthetic pathways, and glutamate, which can contribute to glutathione synthesis. Moreover, thanks to its conserved CXXC redox active motif, BCAT1 can be considered an effective intracellular antioxidant system able to prevent irreversible oxidation of target proteins harbouring reactive cysteine residues. Recent findings suggest BCAT1 might contribute to cancer progression: BCAT1 can modulate redox homeostasis and, in a context-dependent manner, can participate in metabolic reprogramming supporting the growth of many types of tumours. Preliminary data from our laboratory have enlightened a strong association between NOTCH1 activation characterising most T-cell acute lymphoblastic leukaemia (T-ALL) cases (60%) and BCAT1 upregulation; however, further studies are necessary. Thus, we aimed at determining which one of the two key activities of BCAT1 are crucial in NOTCH-activated T-ALL. Using the CUTLL1 T-ALL cell line, which does not express BCAT1 due to promoter methylation, the functional consequences of expressing mutants lacking enzymatic transaminase activity (K222A mutation), redox activity (C335S & C338S double mutation) or both (K222A + C335S & C338S mutations) were determined. Under standard culture conditions, the expression of BCAT1 wild type modestly promoted cell survival without significantly affecting proliferation. This finding was associated with lower levels of cytosolic reactive oxygen species (ROS) and higher levels of reduced glutathione. Interestingly, cells expressing BCAT1 transaminase mutant (K222A) showed increased apoptosis compared to control cells, while CUTLL1 cells expressing the redox BCAT1 mutant (C335S & C338S double mutation) tended to have higher levels of cellular ROS and lower levels of reduced GSH. Further, under oxidative stress (H2O2 treatment) BCAT1 expressing CUTLL1 cells showed a significant proliferation and survival advantage compared to control cells, which was partially dependent on its redox activity. Our results suggest that, in CUTLL1 cells grown under standard conditions, wild-type BCAT1 might contribute to buffering cellular ROS levels, with the functional CXXC motif contributing to this activity. The role of this function seems to be particularly important following exposure to an oxidative stress, such as H2O2, where BCAT1 expression significantly contributes to cell viability and cell proliferation. Future studies are required to figure out whether the CXXC motif plays a relevant role in the oncogenic function of BCAT1.