Investigating the interdependence of human BH3-only protein Noxa and glutamine in activated CD8+ T cells.

Abstract

Human Noxa, a BH3-only member of the Bcl-2 family of apoptosis regulators, was shown to have both pro-apoptotic and pro-survival functions in human hematological malignancies. Noxa protein is not expressed in unstimulated or naïve human T cells but is induced in CD8+ T cells upon receptor engagement. Activated T cells prepare for rapid proliferation by switching from metabolizing glucose for energy production via OxPhos to diverting the sugar for biomass generation through aerobic glycolysis, utilizing alternative fuels such as glutamine or fatty acids for energy production. Here we have investigated the role of glutamine in regulating Noxa expression and the role of Noxa in the activation-induced metabolic switch in human CD8+ T cells. The absence of glutamine during T cell receptor engagement resulted in delayed activation, poor mitochondrial function, and low proliferation rates in the CD8+ T cells. Our studies showed that Noxa induction in activated CD8+ T cells requires glutamine and that this regulation of Noxa expression by glutamine is post-transcriptional. Glutamine deprivation, or treatment with DON, a glutamine antagonist, prevents Noxa protein induction, but inhibition of glutaminase does not. We show that glutamine promotes the translation of the Noxa protein through the mTOR pathway by facilitating leucine influx. Conversely, the entry of glutamine carbons into the TCA cycle following co-stimulation requires Noxa. Silencing of Noxa through siRNA interferes with glutaminolysis but does not impair mitochondrial respiration, suggesting a switch by the T cells to an alternative fuel source for energy generation. Thus, human BH3-only protein Noxa not only facilitates the entry of glutamine into the TCA cycle for proliferative metabolism but also helps drive CD8+ T cells toward glutamine-dependent effector phenotypes and can potentially be targeted for immunotherapy. Understanding the connection of this Bcl-2 family protein with glutamine could offer insights into metabolic pathways that influence the function of T cells in glutamine-deprived tumor microenvironments.