hydroxycarboxylic acid receptors, member of the class A family of seven-transmembrane G protein-coupled receptors
Hydroxycarboxylic acid receptor (HCAR) subfamily, a member of the class A G-protein coupled receptors (GPCRs), contains three receptor subtypes: HCAR1, HCAR2, and HCAR3. The endogenous ligand of HCAR1 (also known as lactate receptor 1, GPR104, or GPR81) is L-lactic acid. The endogenous ligands of HCAR2 (also known as niacin receptor 1, GPR109A, or nicotinic acid receptor) and HCAR3 (also known as niacin receptor 2 or GPR109B) are 3-hydroxybutyric acid and 3-hydroxyoctanoic acid, respectively. Because nicotinic acid is capable of stimulating HCAR2 at higher concentrations only (in the range of sub-micromolar concentration), it is unlikely that nicotinic acts as a physiological ligand of HCAR2. All three receptors are expressed in adipocytes and mediate anti-lipolytic effects in fat cells through G(i) type G protein-dependent inhibition of adenylate cyclase.