Alanine – a metabolic liability ‘by choice’
Abstract:
T lymphocytes, the cellular arm of the adaptive immune system, maintain host homeostasis and orchestrate an immune response to pathogens and internal threats like tumors and injuries. Upon encountering their cognate antigen, T cells enter a phase of rapid cell growth expansion. This transition from quiescence to active growth is driven by the reprogramming of cellular metabolism, transitioning from low-rate catabolism to massive biosynthesis. Success in this transformation relies on the microenvironment providing essential fuels such as glucose and amino acids. Our previous work highlighted T cell dependence of extracellular alanine pools for activation, showing that T cells do not express GPT (Glutamate Pyruvate Transaminase), an alanine synthesizing enzyme. As a result, alanine deprivation inhibited protein synthesis and arrested T cell activation. An intriguing question arose: Why do T cells ‘choose’ this metabolic liability?
We hypothesized that T cells purposefully suppress GPT expression to enable pyruvate flux into other metabolic pathways. In this study, we tested this hypothesis by overexpressing GPT in primary T cells and examining its effect on T cell metabolism and function. GPT overexpression rescued T cell growth, activation, and proliferation under conditions of alanine deprivation, albeit with a concomitant decrease in cell viability. Untargeted metabolomics highlighted significant alterations in metabolism driven by GPT overexpression, with lower carbon flux into the mitochondria, reduced cellular pools of aspartate and nucleotides, and accumulation of 2-hydroxyglutarate (2-HG). Intriguingly, these robust changes in metabolism did not affect proliferation but compromised cell viability.
Our findings shed light on T cells fuel portioning and the consequences of its interruption.
