HCAR2 and cancer: For example, when myeloid cells infiltrated into the HCC microenvironment are defeated by cancer cells in competition for glutamine, their own endoplasmic reticulum homeostasis is disrupted, which leads to up-regulation of IRE1α/XBP1 signaling-induced expression of the G-protein-coupled receptor GPR109A, and myeloid cells thus turn into cancer accomplices with highly immune-suppressing properties (40).