Novel insights into the contribution of human Bcl-2 protein Noxa to the metabolism and differentiation of CD4+ T cells

Abstract

The human Noxa protein was originally identified as a pro-apoptotic member of the large Bcl-2 family of apoptosis regulators but, more recently, rediscovered as a protein that can also promote growth and survival through regulation of glucose and glutamine metabolism in T leukemia cells. Preliminary studies in primary human T cells had also shown that Noxa protein was induced following TCR engagement and was required for their activation-induced switch to glutaminolysis. The research described here focuses on CD4+ T cells and tests the hypothesis that Noxa serves as a critical sensor of glutamine availability and plays a central role in driving T cell differentiation towards glutamine dependent effector phenotypes (Th1/Th17) and away from regulatory and B cell promoting (Treg /Th2) phenotypes. These studies show that Noxa is induced in co-stimulated CD4+ T cells, that the induction is glutamine-dependent and that Noxa, in turn, is required for entry of glutamine carbons into the mitochondrial TCA cycle. Knocking out Noxa does not affect mitochondrial health but increases dependence of activated knockout cells on fatty acid oxidation. Moreover, inhibiting glutaminase activity had little effect on respiration of these cells, confirming the lack of dependence on glutamine in the absence of Noxa in keeping with a Treg or Th2 phenotype. RNA-seq analysis of Noxa KO CD4+ T cells showed decreased expression of genes related to the Th17 subset of helper T cells. Overall, the results show a unique interdependence between Noxa and glutamine and suggest that this interdependence may influence metabolic choices that drive the differentiation of human CD4+T cells subsets. These studies offer a better understanding of the role of Noxa in T cell metabolism and in the generation of CD4+T cell subsets and could eventually lead to the use of Noxa as a therapeutic tool for strategies targeted at metabolic reprogramming in T cells.