Regulatory T Cell Metabolism in Cancer

Regulatory T cells (Tregs) display metabolic fitness to adopt tumour microenvironment (TME), characterized by hypoxia, acidity and metabolic depletion/competition, in order to impair anti-tumour immunity and allow metastasis. Tregs and other TME immune cells interact metabolically, with glycolysis supporting proliferation of Tregs along with cancer cells and CD8⁺ T cells and a basal oxidative phosphorylation (OXPHOS) promoting Treg and CD8⁺ T cell activity. Lactate is a glycolysis byproduct that its accumulation creates acidosis within TME, and its uptake provides a fuel source for Treg activity and fosters their persistence in the hypoxic TME. Itaconate and hypoxic TME increase succinate accumulation, but they take complex roles on Tregs and T cells. Hypoxia and hypoxia inducible factor-1 (HIF-1) activity induce lactate release and Treg recruitment/accumulation via stimulating glycolysis path and extracellular adenosine aggregation. Knockout of HIF-1α although reduces lactate, it secondarily induces OXPHOS to fulfil Treg immunosuppressive function. FOXP3 is stabilized by mitochondrial transcription factor A (Tfam) and induces Treg CD36 and OXPHOS, which can be disturbed by nucleus accumbens-associated protein 1 (NAC1). Liver kinase B1 (LKB1) and AMP-activated protein kinase (AMPK) although induce FOXP3 stability and OXPHOS in Tregs, their activities downregulate programmed death-1 (PD-1) in such cells. OXPHOS augmentation (by α-ketoglutarate [αKG]) or suppression (by metformin) disrupt Treg metabolism. Finally, indoleamine 2,3-dioxygenase (IDO) seems to affect Tregs and can be a promising target in advanced immunotherapy naïve cancer patients. The focus of this review is to describe Treg metabolic regulators/connectome and opportunities they bring about in cancer therapy.