Etomoxir Actions on Regulatory and Memory T Cells Are Independent of Cpt1a-Mediated Fatty Acid Oxidation

Abstract

T cell subsets including effector (T), regulatory (T), and memory (T) cells are characterized by distinct metabolic profiles that influence their differentiation and function. Previous research suggests that engagement of long-chain fatty acid oxidation (LC-FAO) supports Foxp3 T cell and T cell survival. However, evidence for this is mostly based on inhibition of Cpt1a, the rate-limiting enzyme for LC-FAO, with the drug etomoxir. Using genetic models to target Cpt1a specifically in T cells, we dissected the role of LC-FAO in primary, memory, and regulatory T cell responses. Here we show that the ACC2/Cpt1a axis is largely dispensable for T, T, or T cell formation, and that the effects of etomoxir on T cell differentiation and function are independent of Cpt1a expression. Together our data argue that metabolic pathways other than LC-FAO fuel T or T differentiation and suggest alternative mechanisms for the effects of etomoxir that involve mitochondrial respiration.

Fatty acid metabolism in CD8+ T cell memory: Challenging current concepts

“…These data demonstrate that CPT1A-mediated transport of LCFAs into the mitochondria is not required for the development of immunological memory. Furthermore, the differentiation of CD4 + T reg cells, another T cell subset that has been described to depend on LC-FAO (19, 134) was unaffected by the absence of Cpt1a (25). We also evaluated the memory response in mice deficient in other enzymes involved in LC-FAO, namely the long-chain acyl-CoA dehydrogenase (LCAD) and the very-long chain acyl-CoA dehydrogenase (VLCAD), which catalyse the first step in the process of mitochondrial β-oxidation downstream of CPT1A (124, 135, 136).…”

“…Accordingly, complete deletion of ACC2 in mice increases basal oxidation of LCFAs in muscle and reduces accumulation of fat (98, 99, 125), which can be attributed to increased CPT1 activity and transport of LCFAs into the mitochondria for oxidation. It is however not clear if T cells also employ this mechanism to regulate their rates of LC-FAO, given that ACC2 deletion in these cells has only very modest effects on FAO while not influencing their differentiation, effector function or memory development (25, 62, 65, 126). Instead, transcriptional regulation of Cpt1a expression could be more relevant in T cells.…”

“…Instead, transcriptional regulation of Cpt1a expression could be more relevant in T cells. Indeed, it has been reported that PI3K-Akt-mTORC2 signalling initiated by growth factor or TCR/CD28 stimulation induces a downregulation of Cpt1a expression (25, 46, 127, 128), which in activated T cells is accompanied by a reduction of LC-FAO (24). …”

“…To answer the question of whether LC-FAO mediated by CPT1A is critical for the development of CD8 + T mem cells , we generated T-cell specific Cpt1a conditional knockout mice (25). In contrast to the phenotype of TRAF6-deficient mice, deletion of Cpt1a in T cells did not affect the development or activation status of the CD4 + and CD8 + T cell compartment under homeostatic conditions, despite reduced synthesis of long-chain acylcarnitines and rates of LC-FAO.…”

“…Likewise, T-cell specific Cpt1a knockout mice were able to mount a typical primary immune response after infection with LmOVA, with normal numbers of OVA-specific T cells and inflammatory cytokine production. Most importantly, these mice generated a CD8 + T mem population that survived for weeks after the primary response, and robustly proliferated and produced IFN-γ after re-challenge (25). These data demonstrate that CPT1A-mediated transport of LCFAs into the mitochondria is not required for the development of immunological memory.…”