Functional Studies Of Gpbar2 In Cholesterol Metabolism

G protein coupled receptors(GPCRs)are a family of proteins with seven transmembrane domains.More than 34 percent of prescripted drugs target on GPCR,which makes them the most attractable protein family for drug development.However,the biochemical and biophysical properties of GPCRs make them very difficult for expression,purification,structure and functional study,thus leave a large number of GPCRs with unknown functions.Through genome wide association studies(GWAS),it was found that single nucleotide polymorphisms(SNPs)in the intron of Gpbar2 were strongly associated with abnormal total cholesterol level in the blood.However,its mechanism remains unknown.Gpbar2 is a member of the type A class of GPCR with unknow ligand,so it is also an orphan receptor.We find that Gpbar2 is highly expressed in liver,adipose tissue and lung.In the liver,it mainly expresses in hepatocytes.Liver is the major organ for cholesterol metabolism in vivo.In the liver,cholesterol can be catalyzed by a series of enzymes to produce bile acids,which is the key pathway to metabolise and dispose cholesterol.HepG2 is a well-studied hepatocytes cell line that has been widely used in study of cholesterol metabolism.We find that knocking down Gpbar2 in HepG2 cells significantly up-regulates the expression of cholesterol 7? hydroxylase(CYP7A1).CYP7A1 is the key and rate-limiting enzyme in the synthesis of bile acids.The expression is strictly regulated by two major pathways,the FXR-SHP-LRH pathway and FGF19-FGFR4/?-Klotho pathway.Both of those two pathways response to bile acids and play major roles in bile acids mediated feedback regulation of bile acids synthesis.Meanwhile,other cytokines including hepatocyte growth factor(HGF),tumor necrosis factor ?(7)TNF-?(8)(11)interleukin-1?(IL-1?)and transforming growth factor ?(7)TGF-?)also inhibit the expression of CYP7A1 in the FXR independent pathway.Our current data show that deficiency of Gpbar2 in HepG2 cells prevents the expression of CYP7A1 dose not require any noted pathway.In summary,we find a novel orphan GPCR,Gpbar2 that highly expressed in liver and hepatocytes.It suppresses the expression of CYP7A1 and may play important roles in cholesterol metabolism and bile acids synthesis.We get the tissue-specific Gpbar2 knockout mice and further studies to identify its physiological functions are under way.