Bile acid transporters and regulatory nuclear receptors in the liver and beyond

Abstract

Bile acid (BA) transporters are critical for maintenance of the enterohepatic BA circulation where BAs exert their multiple physiological functions including stimulation of bile flow, intestinal absorption of lipophilic nutrients, solubilization and excretion of cholesterol, as well as antimicrobial and metabolic effects. Tight regulation of BA transporters via nuclear receptors is necessary to maintain proper BA homeostasis. Hereditary and acquired defects of BA transporters are involved in the pathogenesis of several hepatobiliary disorders including cholestasis, gallstones, fatty liver disease and liver cancer, but also play a role in intestinal and metabolic disorders beyond the liver. Thus, pharmacological modification of BA transporters and their regulatory nuclear receptors opens novel treatment strategies for a wide range of disorders.

Fig. 1

Overview of diseases linked to disturbances in enterohepatic bile acid circulation. After their synthesis in hepatocytes, bile acids (BAs) are excreted into the bile and subsequently reabsorbed by enterocytes and, after completing the enterohepatic circulation, by hepatocytes. Efficient reuptake in the ileum preserves 95% of secreted BAs. Disturbances of transport processes within the enterohepatic circulation cause a variety of hepatic and intestinal disorders. Under normal conditions, BAs filtered by the kidney are conserved in the kidney (reabsorption in renal tubules) but can be alternatively excreted when BAs accumulate due to impaired biliary excretion in cholestasis. BAs, bile acids; BRIC, benign recurrent intrahepatic cholestasis; IBD, inflammatory bowel disease; ICP, intrahepatic cholestasis of pregnancy; NAFLD, non-alcoholic fatty liver disease; PBC, primary biliary cholestasis; PFIC; progressive familial intrahepatic cholestasis; PSC, primary sclerosing cholangitis.

Fig. 2

Transcriptional regulation of hepatocellular bile formation. Expression of hepatobiliary transporters in hepatocytes determines hepatic bile acid (BA) flux and hepatocellular concentrations of these potentially toxic metabolites. To ensure the balance between synthesis, uptake and excretion, expression of hepatobiliary transporters is tightly regulated by nuclear receptors (NRs). NRs provide a network of negative feedback and positive feed-forward mechanisms, for the control of intracellular concentration of biliary constituents, which are often also ligands for these NRs. BA-activated FXR is a central player in this network, that represses (via interaction with HNF4 in rats and GR in humans) hepatic BA uptake (NTCP) and (via SHP) synthesis (CYP7A1), promotes bile secretion via induction of canalicular transporters (BSEP, MRP2, ABCG5/8, MDR3) and induces BA elimination via alternative export systems at the hepatic basolateral (sinusoidal) membrane (OSTα/β). Several NR pathways converge at the level of CYP7A1 as the rate limiting enzyme in BA synthesis. CAR and PXR facilitate adaptation to increased intracellular BA concentrations by upregulation of alternative hepatic export routes (MRP3 and MRP4) and induction of detoxification enzymes (not shown). Together with RAR, these xenobiotic receptors also regulate the canalicular expression of MRP2. Cholesterol sensor LXR promotes biliary cholesterol excretion via ABCG5/8. Stimulation of AE2 expression by GR stimulates biliary bicarbonate secretion thus reducing bile toxicity. Green arrows indicate stimulatory and red lines suppressive effects on target genes. In addition to these transcriptional mechanisms, post-transcriptional processes (e.g., vesicular targeting of transporters to the membrane, phosphorylation of transport proteins) and modification of the bile through cholangiocytes (e.g., bicarbonate secretion) also play an important role in bile formation (not shown). BAs, bile acids; Bili-glu, bilirubin glucuronide; BSEP, bile salt export pump; CAR, constitutive androstane receptor; CYP7A1, cholesterol-7α-hydroxylase, FXR, farnesoid X receptor; GR, glucocorticoid receptor; HNF4, hepatocyte nuclear factor 4, LXR, liver X receptor; MDR3, multidrug resistance protein 3, phospholipid flippase; MRP2, multidrug resistance-associated protein 2; MRP3, multidrug resistance-associated protein 3; MRP4, multidrug resistance-associated protein 4; NTCP, sodium taurocholate co-transporting polypeptide; OSTα/β, organic solute transporter alpha and beta, PC, phosphatidylcholine; PXR, pregnane X receptor; PPARα, peroxisome proliferator-activated receptor alpha; RAR, retinoic acid receptor; SHP, small heterodimer partner.