GPR4 and neoplasm: In the intricate landscape of the tumor microenvironment, the signal transduction triggered by lactate stimulation mainly relays on the lactate receptor HCAR1 (GPR81), and proton-sensing G-protein coupled receptors (GPRs), including GPR4, TDAG8 (GPR65), OGR1 (GPR68), and G2A (GPR132) [38, 39].