HCAR1 and neoplasm: Hypoxia enhances the expression of PD-L1 in tumor cells through the hypoxia-inducible factor 1-alpha (HIF-1α)/mTOR pathway, and activates G protein-coupled receptor (GPR81) through lactic acid, inhibiting the antigen-presenting ability of DCs and forming type I TME, namely “cold tumor” type[50].