Fig. 1

The enterohepatic circulation, homeostasis of bile acids and treatment targets for cholestasis. The grey arrows indicate the route of enterohepatic circulation of bile acids. Bile acids are synthesized from cholesterol in hepatocytes to generate the primary bile acids CA and CDCA. After conjugation with glycine or taurine, bile acids (BAs) are transported from hepatocytes into the bile canaliculi via BSEP. Intestinal microbiota converts primary bile acids into the secondary bile acids DCA and LCA. Most of BAs reabsorbed by the enterocytes through ASBT in the apical membrane and then delivered into the portal circulation system via BA efflux transporter OSTα/β in the basolateral membrane. BAs are re-absorbed into hepatocytes. Hepatocytes secrete these BAs along with the de novo synthesized bile acids enter the next cycle. Bile acids also play roles in signaling to regulate the homeostasis of bile acids. The nuclear receptor FXR is the bile acid receptor to regulate bile acid homeostasis at the synthesis and the elimination levels, acting in the hepatocytes and enterocytes. The figure also shows different therapeutic targets at hepatocellular transport or enterohepatic circulations. 1°BAs, primary bile acids; 2°BAs, secondary bile acids; 4-PB, 4-phenylbutyrate; ASBT, apical sodium dependent bile acid transporter; BAs, bile acids; BSEP, bile salt export pump; CA, cholic acid; CDCD, chenodeoxy cholic acid; DCA, deoxycholic acid; FGFR4, fibroblast growth factor receptor 4; FXR, farnesoid X receptor; G(T)CA, glyco- or tauro-cholic acid; G(T)CDCA, glyco- or tauro-chenodeoxy cholic acid; LCA, lithocholic acid; MRP3, multidrug resistance-associated protein 3; MRP4, multidrug resistance-associated protein 4; NTCP, sodium/taurocholate co-transporting polypeptide; OATP1B1/3, organic-anion-transporting polypeptide 1B1 and 1B3; OSTα/β, organic solute transporter-α/β; RXRα, retinoid X receptor α; SHP, small heterodimer partner; UDCA, ursodeoxycholic acid