HCAR1 and neoplasm: Apart from its character as a signaling mediator intracellularly, meanwhile, lactate functions as an extracellular ligand.192–196 G-protein-coupled receptor 81 (GPR81), a G protein-coupled receptor for lactate, exists in colon, breast, lung, hepatocellular, salivary gland, cervical, and pancreatic cancer cell lines.197–200 Lactate supports energy metabolism in tumor cells through binding to GPR81.