SEARCH

SEARCH BY CITATION

Summary

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Background  Bile salts are produced and secreted by the liver and are required for intestinal absorption of fatty food components and excretion of endobiotics and xenobiotics. They are reabsorbed in the terminal ileum and transported back to the liver via the portal tract. Dedicated bile salt transporters in hepatocytes and enterocytes are responsible for the unidirectional transport of bile salts in the enterohepatic cycle.

Aim  To give an overview of the function and regulations of proteins involved in bile salt synthesis and transport.

Methods  Data presented are obtained from PubMed-accessible literature combined with our own recent research.

Result  Hepatocytes and enterocytes contain unique bile salt importers (sodium-taurocholate cotransporting polypeptide and apical sodium-dependent bile acid transporter, respectively) and exporters (bile salt export pump and organic solute transporter alpha-beta, respectively). Enzymes involved in bile salt biosynthesis reside in different subcellular locations, including the endoplasmic reticulum, mitochondria, cytosol and peroxisomes. Defective expression or function of the transporters or enzymes may lead to cholastasis. The bile salt-activated transcription factor Farnesoid X receptor controls expression of genes involved in bile salt biosynthesis and transport.

Conclusions  Detailed knowledge is available about the enzymes and transporters involved in bile salt homeostasis and how their defective function is associated with cholestasis. In contrast, the process of intracellular bile salt transport is largely unexplored.


Enterohepatic circulation of bile salts

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Bile is a complex fluid containing water, electrolytes and organic molecules including bile salts, cholesterol, phospholipids and bilirubin that flows through the biliary tract into the small intestine. Bile serves two main functions; digestion and absorption of fats and fat-soluble vitamins in the small intestine and elimination of excess cholesterol and many waste product, bilirubin, drugs and toxic compounds. The main driving force of bile flow is the secretion of bile salts. Bile salts are synthesized in the liver from cholesterol and are actively secreted into bile and stored in the gallbladder. Upon food ingestion, the gall bladder contracts and its content is released in the duodenum. In the small intestine it serves its function as ‘detergent’ to keep fat-soluble compounds in solution for uptake (vitamins) or excretion (cholesterol, lipophilic toxins/drugs). In the bile, bile salts form mixed micelles with phospholipids and these are the vehicles that carry cholesterol and other lipophilic compounds through the intestine. In the terminal ileum, 90–95% of bile salts are reabsorbed and returned to the liver. The remainder is lost to the colon, where primary bile salts are transformed by bacterial metabolism into secondary bile salts. Some of the secondary bile salts are also reabsorbed and the rest is excreted with the faeces. Primary and secondary bile salts return to the liver via the portal circulation. In the liver, bile salts are taken up into hepatocytes, thereby completing the enterohepatic cycle. In man, bile acids complete an enterohepatic cycle about 6–10 times per day. Enterohepatic cycling not only serves to reclaim bile acids, but it also enables bile salts to act as messengers that carry signals from intestine to liver. Hereby, they regulate their own synthesis and transport rates.

Bile salt transporters in the liver and intestine

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Hepatocytes and ileal epithelial cells express proteins that efficiently pump bile salts in and out of these cells (Figure 1). Import of bile salts in the hepatocyte is predominantly mediated by the sodium-taurocholate cotransporting polypeptide (NTCP, SLC10A1). NTCP is exclusively expressed in the liver,1, 2 where it is present at the sinusoidal/basolateral membrane of the hepatocyte.3, 4 NTCP preferentially transports conjugated bile salts that are transported in a sodium-dependent manner.5, 6 The Na+/K+-ATPase maintains the Na+ gradient required for this transport. Other transporters are also involved in the bile salt uptake at the basolateral membrane. These transporters include members of the SLCO1 family (organic anion transporting polypeptides; OATPs). These proteins have a broad substrate specificity, not only transporting bile salts, but also hormones, prostanoids, conjugated steroids, cAMP and various xenobiotics (reviewed in Ref.7). In humans, four members of the SLCO1 family are expressed in the hepatocyte that could contribute to sodium-independent bile salt transport, namely the organic anion transporting polypeptide 1B1 (OATP1B1; synonyms SLCO1B1, SLC21A6 LST-1, OATP2, OATP-C), OATP1B3 (synonyms SLCO1B3, SLC21A8, LST3, OATP8, OATP1B3), OATP2B1 (synonyms SLCO2B1, SLC21A9, OATPB, OATP-B) and OATP1A2 (synonyms SLCO1A2, SLC21A3, OATP, OATP-A, OATP1A2).

image

Figure 1.  Enterohepatic circulation of bile salts. Bile salts are maintained in an enterohepatic circulation through the action of selective bile salt transporters in hepatocytes and enterocytes. Import of bile salts in the hepatocyte is predominantly mediated by the sodium-dependent taurocholate-co-transporting polypeptide (SLC10A1). In addition, members of the SLC01 family (organic anion transporting polypeptides) are also involved in basolateral uptake of bile salts. Secretion into bile is mediated by the canalicular bile salt export pump. This member of the ATP-binding cassette transporter family transports bile salts against a steep concentration gradient and constitutes the driving force of the enterohepatic circulation. After passing through the small intestine, where they act as fat solubilizers, bile salts are reabsorbed at the terminal ileum by the apical sodium-dependent bile acid transporter (SLC10A2) in the apical membrane of ileal enterocytes. The last step in the enterohepatic circulation is secretion to the blood by the heterodimeric protein OSTα/OSTβ.

Download figure to PowerPoint

Bile salts are 100- to 1000-fold concentrated in bile compared to plasma, therefore their secretion into the bile canaliculi is against a steep concentration gradient and necessitates active transport. The Bile Salt Export Pump (BSEP, ABCB11) is the hepatocanalicular bile salt transporter responsible for the concentration of bile salts in bile.8 BSEP is crucial for the generation of the bile salt-dependent bile flow. It belongs to the ATP-binding cassette (ABC) transporter superfamily (reviewed in Ref.9). ABC transporters couple the energy generated from ATP to the transport of bile salts across the canalicular membrane against the strong concentration gradient. BSEP is a typical ABC-transporter containing two transmembrane segments and two nucleotide (ATP)-binding domains. BSEP is solely expressed in hepatocytes at the canalicular membrane.8 Its substrate specificity is narrow and is restricted to bile salts (reviewed in Ref.10). Other transporters that may be involved in ATP-dependent bile salt export from hepatocytes are the multidrug resistance-associated proteins 3 and 4 (MRP3/ABCC3 and MRP4/ABCC4) and the organic solute transporter alpha-beta (Ostα/β).11–13 These proteins are located in the basolateral membrane. In normal liver, they are expressed at low levels, but in case of cholestatic liver disease their expression is highly increased inducing a shift towards renal secretion of bile salts.14–16 The proteins are therefore considered escape routes when efficient export to the bile is compromised.

In the terminal ileum, bile salts are reabsorbed to the blood. The first step is uptake from the intestinal lumen by the apical sodium-dependent bile salt transporter (ASBT/SLC10A2) in the apical membrane of ileal enterocytes.17, 18 ASBT is highly homologous to NTCP. As NTCP, it transports bile salts in a sodium-dependent manner. The final step in the enterohepatic circulation of bile salts is the transport of bile salts out of the ileal enterocytes by the recently characterized heterodimeric organic solute transporter Ostα/β.13 OSTα/β transports conjugated bile salts in a sodium-independent manner. Importantly, both subunits are required for bile salt transport and they need to be co-expressed for efficient sorting of Ostα/β to the basolateral membrane. From the ileum, the bile salts are transported to the liver via the portal circulation thereby completing the enterohepatic cycle.

Genetic defects in bile salt transport

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

The importance of effective enterohepatic cycling of bile salts is apparent from the cholestatic syndromes that are caused by mutations in substrate transporters involved in bile formation. These syndromes are collectively called progressive familial intrahepatic cholestasis (PFIC). PFIC is a group of autosomal recessive diseases characterized by cholestasis at infancy. Most patients require liver transplantation to survive.

Mutations in three different genes have been described to cause PFIC. PFIC type 2 is caused by mutations in the ABCB11 gene, encoding BSEP.19, 20 PFIC-2 is characterized by neonatal hepatitis and persistent cholestasis as a result, with malabsorption, stunted growth and haemorrhage. This disease stresses the functional importance of BSEP for normal bile salt homeostasis. Over 40 different mutations in BSEP have been reported to cause PFIC-2. Less severe mutations in BSEP cause a more benign type of relapsing cholestasis, BRIC type 2.21

The ‘solubilization function’ of bile salts for lipophilic compounds is only effective when it is present in mixed micelles with phospholipids. Pure bile salt micelles are extremely cytotoxic because of membranolytic activity. In the canalicular lumen, bile salt micelles interact with the lumen-facing leaflet of the canalicular membranes that is enriched with phosphatidylcholine and cholesterol.22 These are extracted into the bile salt micelle and contribute to the formation of the characteristic bile salt-phospholipid-cholesterol mixed micelle. Phospholipid transfer from the inner to the outer leaflet that faces the canalicular lumen is mediated by multidrug-resistant protein 3, MDR3 (ABCB4; in rodents mdr2, abcb4).23 Genetic deficiency of MDR3 causes PFIC type 3.24, 25 In this disease, phospholipid transfer through the canalicular membrane is abrogated and this results in bile without phospholipids. Bile salt transfer is undisturbed and the bile that is produced under these conditions is extremely cytotoxic. It damages surrounding hepatocytes and bile duct epithelial cells. Hence, liver histology of these patients show portal inflammation, bile duct proliferation and periportal fibrosis.

Progressive familial intrahepatic cholestasis type 1 (or Byler disease) results from mutations in the FIC1 gene affecting the FIC1 (ATP8B1) protein.26, 27 FIC1 is an aminophospholipid translocator, malfunctioning of which affects the distribution of phosphatidylserine (PS) across the two plasma membrane leaflets. Too much PS in the outer membrane leaflet makes the outer membrane leaflet unstable. Fic1−/− mice have excessive outer leaflet-anchored proteins in their bile.28 How cholestasis develops from FIC1 deficiency is not well understood. Benign Recurrent Intrahepatic Cholestasis (BRIC) type 1 is also caused by mutations in the FIC1 gene.27 It is characterized by recurrent episodes of cholestasis, not leading to chronic disease and liver failure. Apparently, the mutant FIC1 proteins in these patients contain residual activity that may respond to environmental factors causing the recurrent episodes of cholestasis.

In contrast to the severe phenotypes observed in the PFIC syndromes, mutations in other bile salt transporting proteins are mild or unknown to date. Mutations that affect the function of ASBT cause bile acid-induced diarrhoea,29 whereas no clinically important genetic defects of NTCP or Ostα/β have been recognized thus far.

Bile salts biosynthesis

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

During the enterohepatic cycling of bile salts, there is continuous loss of a fraction of the bile salt pool that is excreted via the faeces. This is compensated for by de novo synthesis in the liver. The adult human liver converts approximately 500 mg cholesterol to bile acids on a daily basis (reviewed in Ref. 30). This is an exclusively liver-specific process involving at least 13 different enzymes. However, bile salt synthesis may be initiated in extrahepatic tissues also, after which bile salt-intermediates are transported to the liver for further metabolism to the primary bile salts cholic acid (CA) and chenodeoxycholic acid (CDCA). Before entering the enterohepatic circulation, these bile salts are conjugated to either glycine or taurine. In humans, bile salts are predominantly conjugated to glycine. In the intestine, a certain amount of the primary bile salts is de-conjugated and/or converted to the secondary bile salt deoxycholic acid and lithocholic acid by intestinal bacteria.31

Enzymes involved in bile salt biosynthesis

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

For cholesterol to be converted into CA or CDCA, basically three different modifications are required, namely (i) hydroxylation of the steroid nucleus, (ii) shortening of the side chain and (iii) conjugation of glycine or taurine to the side chain. Two main pathways are described that catalyse these reactions; the classic (or neutral) pathway and the alternative (or acidic) pathway (Figure 2). The most important difference between these two pathways is the order of reactions that transforms cholesterol to CA or CDCA and the cellular site where the first reactions take place. The classic pathway starts with 7α-hydroxylation of the steroid nucleus by cholesterol 7α-hydroxylase (CYP7A1), an enzyme that resides in the endoplasmic reticulum and is considered the rate-limiting step in this route.32 In contrast, the alternative pathway starts with the hydroxylation at the cholesterol side chain (at C-position 27) by sterol 27-hydroxylase (CYP27A1), which is a mitochondrial protein.33 The CYP27A1 product, 5-cholesten-3β-27-diol, is not a substrate for CYP7A1, but is hydroxylated at the C-7 position by an alternative cytochrome P450 enzyme, CYP7B1.34 From here on, the neutral and acidic pathway largely overlap (see Figure 2). The next steps of the ring modification are catalysed by 3β-hydroxy C27-steroid dehydrogenase/isomerase (3βHSD/HSD3B7)35 and sterol 12-α-hydroxylase (CYP8B1)36 in the endoplasmic reticulum. The latter adds a hydroxyl group to carbon 12, being the sole determinant of the ratio between double-hydroxylated CDCA and triple-hydroxylated CA. Further modifications of the ring structure are performed in the cytoplasm by two members of the aldo-keto reductase family, Δ4-3-ketosteroid-5-β-reductase (AKR1D1)37 and 3α-hydroxysteroid dehydrogenase AKR1C4.38 Production of the conjugated primary bile salts will now be finalized by several enzymatic steps in yet another organelle, the peroxisome. The first step is catalysed by bile acid coenzyme A ligase, which activates the sterol intermediate by conjugation with coenzyme A.39 Further, 2-methylacyl-coenzyme A racemase (AMACR) transforms the bile salt precursors from the 25(R) isomers to the 25(S) isomers,40 followed by branched chain acyl CoA oxidase (ACOX2) that produces a 24,25-trans-unsaturated derivative.41 The D-bifunctional enzyme then catalyses a combined hydratation and oxidation reaction at the Δ24 bond to form a C24-oxo product.42 Side chain cleavage at the C24-C25 by thiolase 2 (also known as sterol carrier protein 2)43 yields a C24-CoA bile acid intermediate and propionyl-CoA. The final step is the addition of an amino acid, either glycine or taurine, to the carbon 24 by peroxisomal bile acid coenzyme A:amino acid N-acyltransferase (BAAT).44 Conjugation of bile acids increases the amphipathicity and enhances the solubility of the molecules, which makes them impermeable to cell membranes. Glycine- and taurine-conjugated primary bile salts need to be transported out of peroxisomes before they can enter the enterohepatic cycle initiated by BSEP. This activity has recently been experimentally demonstrated, but the identity of the protein remains unknown.45 BAAT activity is also required for the fraction of bile salts that become de-conjugated in the intestine and return to the liver. Recently, we showed that BAAT is strictly located in peroxisomes.46 As BAAT is the only enzyme responsible for glycine and taurine conjugation of bile salts,44, 47 it implies that the de-conjugated bile salts need to be imported into peroxisomes. Also, this bile salt transporter has not been identified yet.

image

Figure 2.  Enzymes and organelles involved in bile salt biosynthesis. Bile salts are synthesized in the hepatocytes in a series of reactions that are spatially separated in different cell compartments. For each enzymatic step the relevant change to the cholesterol backbone is presented in bold. Abbreviation of enzymes: refer to the main text. The rate-limiting enzyme of the neutral (or classic) pathway is CYP7A1, which is located in the endoplasmic reticulum. The first step of the acidic (or alternative) is mitochondrial CYP27A1. Subsequent enzymatic conversions are performed by enzymes residing in the endoplasmic reticulum, cytoplasm and peroxisomes. The final steps leading to the primary bile salts cholic acid or chenodeoxycholic acid that are conjugated to either glycine or taurine are restricted to peroxisomes. After transport to the cytosol, these bile salts are substrates for bile salt export pump (BSEP) for transport to the bile. A fraction of bile salts that return from the intestine to the liver are de-/un-conjugated and require reactivation with CoA by FATP5. Subsequently, CoA-activated bile salts require transport into peroxisomes for glycine or taurine conjugation by bile acid coenzyme A:amino acid N-acyltransferase. As for the de novo synthesized bile salts, the reconjugated bile salts require export out of the peroxisome before they can be transported by BSEP.

Download figure to PowerPoint

At least 95% of the bile acid pool is generated through the combined action of the classic and alternative pathways. Even though the production of bile salts is restricted to the liver, the synthesis may be initiated elsewhere in the body.48 CYP7A1 is solely expressed in the liver, so the classic pathway starts and ends in this organ. CYP27A1 is expressed in the liver but also in peripheral tissues where the first step of the acidic pathway may occur. Besides the classic and acidic route, bile acid synthesis may be initiated in the brain by the action of cholesterol 24-hydroxylase (CYP46) after which the product, 24S-hydroxycholesterol, is transported to the liver via the circulation.49, 50 Humans produce approximately 6–7 mg 24S-hydroxycholesterol per day, so the contribution of this pathway to bile salt synthesis (±500 mg/day) is low (≈1%).51 However, the CYP46 activity is of great importance for cholesterol homeostasis in the brain. In human, it has been estimated that the relative contribution of the classic and acidic pathway to bile salt synthesis is about equal, where around 5–10% is initiated in extrahepatic tissues by CYP27A1 and thus the predominant 90–95% part is synthesized from cholesterol to bile acid in the liver. The exact order of enzymatic steps is not fixed. Many of the biosynthetic intermediates are substrates for more than one enzyme. Extensive intracellular transport of bile acid intermediates occurs between various organelles. Transport in and out these organelles is likely mediated by transport proteins but these have not been characterized yet.

Genetic defects in bile salt biosynthesis

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Bile acid synthesis defects (BASD) are rare genetic disorders that are the underlying cause of approximately 2% of persistent cholestasis in infants. BASDs are recognized by the absence or reduction of normal primary bile salts in serum and/or urine. Instead, non-typical bile acids and sterols are often detected in the body fluids of these patients. These can be identified by fast atom bombardment ionization mass spectrometry (FAB-MS) and gas chromatography mass spectrometry (GC-MS). Disease-causing mutations have been identified in eight of the 16 bile acid biosynthesis enzymes. Cholestasis is a common clinical presentation of these diseases. The associated liver diseases may vary from mild to life-threatening, but in many cases, can be managed by replacement of deficient primary bile salts. This not only leads to restoration of normal bile function, it also induces feedback inhibition on the production of toxic bile acid intermediates.

CYP7A1 deficiency causes a markedly decreased rate of bile acid synthesis and excretion and a significant elevation of total and low density lipoprotein cholesterol levels with substantial accumulation of cholesterol in the liver.52 Remarkably, the loss of Cyp7a1 in mice is not accompanied by elevated cholesterol levels.53 This is probably on account of the different regulation of the gene in the two organisms.

CYP27A1 deficiency is associated with cerebrotendinous xanthomatosis (CTX).54 CTX is a slowly progressive chronic disease characterized by early dementia and xanthomata. Bile acid synthesis is reduced, but the clinical manifestations are caused by the accumulation of cholesterol and cholestenol in the brain. This gradually disrupts the myelin sheets surrounding the neurones. If diagnosed early, CTX can be treated effectively with bile acid therapy.

CYP7B1 deficiency is so far only identified in one patient.55 It is characterized by the accumulation of toxic 27α-hydroxy cholesterol. At present, the only cure seems to be liver transplantation, as this patient did not respond to treatment with bile salts. This suggests that the main cause of disease was the accumulation of toxic oxysterols, rather than the lack of bile salts.

Mutations in the HSD3B7 gene represent the most common disorders of bile acid biosynthesis.56–60 HSD3B7 deficiency causes neonatal jaundice, hepatosplenomegaly, steatorrhoea, pruritis, poor growth and fat-soluble vitamin deficiency.61 Because the symptoms are similar to PFIC, it has been proposed to constitute the fourth type of PFIC-4.35 Urine and plasma bile acid levels are high and consist of abnormal conjugates of the unoxidized precursors di- and tri-hydroxy-Δ-5-cholenic acids. These abnormal bile acids are poorly transported across the canalicular membrane and interfere with the ATP-dependent transport of CA. Bile salts are curative, it is believed, through suppression of CYP7A1 and consequent decrease of toxic intermediates.30

Patients with AKR1D1 deficiency presented with neonatal cholestasis.62, 63 Urine and serum levels of primary bile acids were low, but Δ4-3-oxo bile acids concentrations were elevated. Similar to HSD3B7 deficiency, treatment with primary bile acids is successful, ursodeoxycholic acid is not. Also, in these patients symptoms are related to the accumulation of hepatotoxic Δ4-3-oxo bile acids, the production of which is inhibited by primary bile salts but not by ursodeoxycholic acid.

AMACR deficiency leads to the accumulation of 25(R) isomers of bile salts precursors and of pristanic acid.64 The enzyme racemizes both compounds, which is an indispensable step before β-oxidation can proceed.65 Symptoms in infants are coagulopathy, vitamin D and E deficiencies and mild liver impairment.30 The liver phenotype can be reverted with diet supplementation of bile salts. However, the neuropathy observed in adult patients is caused by the accumulation of pristanic acid.

D-Bifunctional Protein Deficiency shows clinical manifestations that resemble those of adrenoleukodystrophy, only some patients show liver failure.66

Mutations in the BAAT gene cause familial hypercholanaemia (FHC) which is characterized by elevated serum bile acid concentrations in the absence of conjugated bile salts, elevated cholesterol, itching, vitamin K deficiency and fat malabsorption.67

The final enzymatic steps of bile acid biosynthesis take place in peroxisomes. Zellweger Syndrome (ZS) is a genetic disorder that affects the formation of these organelles. Mutations in over a dozen different genes have been shown to be the molecular cause of ZS or the related disorders neonatal adrenoleukodystrophy and Refsum disease.68 These genes encode proteins that are involved in transporting newly synthesized enzymes to peroxisomes or are essential for formation of the peroxisomal membrane. Indirectly, these mutations also affect the enzymes in peroxisomes, including those for bile acid synthesis. Patients present with cerebral neuronal migration disorder, craniofacial dysmorphism, psychomotor retardation and chronic liver disease. ZS is generally fatal in the first 2 years of life. Biochemically, these patients are characterized by increased levels of very long-chain fatty acids, atypical mono-, di- and tri-C27 hydroxy bile acids (such as cholestanoic acid) and hyperpipecolic acidaemia.

Regulation of bile salt homeostasis

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Traditionally, bile salts have been viewed as detergents that simply act to keep fat-soluble compounds in solution for absorption or secretion. This view has drastically changed in the past decade. It has been long known that bile salt synthesis and enterohepatic shuttling are tightly regulated processes. Now we know that bile salts themselves play the leading part in this process. Bile salts are able to activate transcription factors that turn on or off bile salt biosynthesis or transport processes. In particular, this protects the cells that are exposed to the highest intracellular bile salt concentrations, hepatocytes and enterocytes.

Regulation of hepatic and intestinal bile salt transporters

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Exposing hepatocytes or enterocytes to increased bile salt concentrations leads to (i) reduced bile salt synthesis, (ii) decreased bile salt uptake and (iii) increased bile salt efflux. What is the molecular mechanism? The answer to this question came from the detailed analysis of a large family of transcription factors, now knows as nuclear receptors (NRs). These NRs can be activated by ligand binding after which they selectively activate or repress transcription of target genes by binding to specific DNA sequences in their promoter regions. The founder proteins of this family are the retinoic receptors (RAR/NR1B1 and RXR/NR2B1)69, 70 and the peroxisome proliferator-activating receptors (PPARs/NR1Cs).71 These proteins are activated by retinoids and fibrates, respectively. For PPARs, this was followed by the discovery of their physiological ligands, amongst others fatty acids and eicosanoids. The discovery of the bile salt-activated NR followed a similar pattern. Originally identified as a protein similar to RXR and PPARs, it was found that it is activated by farnesol, a key metabolic intermediate of the mevalonate pathway that leads to the synthesis of cholesterol, bile acids, porphyrin, dolichol, ubiquinone, carotenoids, retinoids, vitamin D, steroid hormones and farnesylated proteins.72 Therefore, it was named the Farnesoid X receptor (FXR/NR1H4). The physiological importance became apparent when it was found that bile salts, and in particular CDCA, are the natural high affinity ligands for FXR.73–75 Bile salt-activated FXR regulates genes for bile salt biosynthesis and transport (Figure 3). It does so in a heterodimeric complex with the 9-cis-retinoic acid-activated RXRα.

image

Figure 3.  Bile salts regulate their own synthesis and transport rate. The Farnesoid X receptor (FXR) is the bile salt sensor. It positively regulates Bile Salt Export Pump and Ostα/β expression in hepatocytes and enterocytes, respectively. It does so by binding to a FXR-binding element in the promoter regions of the corresponding genes. FXR inhibits expression of Cyp7A, sodium-dependent taurocholate-co-transporting polypeptide (NTCP) and apical sodium-dependent bile acid transporter (ASBT). This is an indirect effect. FXR induces the expression of the Small Heterodimer Partner that interacts and inhibits the activity of transcription factors that induce expression of Cyp7A, NTCP and ASBT.

Download figure to PowerPoint

In the human liver, FXR positively regulates expression of BSEP. It binds to a DNA sequence in its promoter element characterized as an inverted repeat separated by one nucleotide (IR-1).76, 77 This DNA sequence is also called the FXR responsive element (FXRE). Similar positive regulation has been observed for both subunits of the Ostα/β bile salt transporter in the intestine and the corresponding FXREs have been identified in their promoter elements.78–80 In contrast, expression of both bile salt importers, NTCP and ASBT, as well as the rate-limiting enzyme for bile salt biosynthesis, CYP7A1, are repressed by FXR. Repression of the transcription of these genes is mediated through the FXR-dependent induction of the transcription factor Small Heterodimer Partner (SHP). SHP is also a member of the NR family, but lacks a DNA-binding domain and a ligand has not been identified for this protein. It has been shown to interact with other members of the NR family thereby repressing their transcriptional activity towards their target genes. This way, it interacts with RXR/RAR that are positive regulators of NTCP and ASBT in the liver and intestine, respectively.81–83 Similarly, it inhibits bile salt synthesis by interacting with the liver X receptor (LXR), which is an oxysterol-activated activator of CYP7A1 expression.84

Bile salt-dependent regulation of transcription is not restricted to FXR. Bile salts have also been shown to modulate the activity of the pregnane X receptor (PXR/NR1I2) and the vitamin D receptor (VDR/NR1I1).80, 85, 86 On the other hand, FXR activity is not restricted to bile salt homeostasis but is also involved in regulation of lipoprotein and glucose metabolism, hepatic regeneration, intestinal bacterial growth and the response to hepatotoxins (reviewed in Ref.87). Consequently, the ultimate effect of bile salts and FXR during diseases of the liver and/or intestine will be a complex interplay between different transcription factors and their ligands. These mechanisms act then in concert with post-translational regulation mechanisms that control the amount of bile salt transporters in cellular membranes.13, 88–92

Regulation of bile salt transporters during acquired cholestatic liver disease

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

In the inherited cholestatic syndromes, disease symptoms are the direct result of defective functioning of the mutated bile salt transporter or biosynthetic enzyme. How does this relate to acquired cholestatic liver diseases, for which much more patients require treatment? Indeed, for some forms of acquired cholestasis, there is a direct correlation of the symptoms to the malfunctioning of one bile salt transporter. Drugs like cyclosporin A, rifampicin, glibenclamide and bosentan physically interact with BSEP, thereby inhibiting its bile salt transport activity.93, 94 Also, oestrogen hormone metabolites have been shown to have a similar effect on BSEP.93, 95 The level of transport inhibition may depend on the specific variant of BSEP since recent data show that drug-induced cholestasis is associated with specific single nucleotide polymorphisms (SNPs) in BSEP.96 Genotyping may therefore help to identify individuals who are predisposed to develop drug-induced cholestasis. Proinflammatory cytokines do not directly interact with bile salt transporters, though cholestasis is often associated with inflammation. Transcript (mRNA) levels of BSEP also do not seem to be drastically altered under these conditions. The most dominant effect on BSEP during inflammation appears to be a changed subcellular location of this transporter. It induces retrieval of this transporter from the canalicular membrane to intracellular vesicles, which reduces the bile salt transport capacity of the hepatocyte.97

In most patients with cholestatic liver disease, however, defective transporter function is not the primary cause of cholestasis. In many patients the primary problem exists at the bile duct level. This may be the result of obstruction (gallstones, tumours) or destruction (primary sclerosing cholangitis; PSC and primary biliary cirrhosis; PBC) of bile ducts. In PBC, expression of BSEP is preserved, while bile excretion out of the liver is blocked by the bile duct damage.14 Significant adaptations in transporter expression are, however, observed in the livers of PBC patients. Collectively, these adaptations are probably aimed at protecting the cell against accumulation of toxic bile salts. The bile salt importers, NTCP and OATP2 are strongly downregulated, reducing bile salt import. MRP3, MRP4 and Ostα/β, transporters that are hardly expressed in the healthy liver, are strongly induced. These transporters reside in the basolateral membrane and force export of bile salts back to the portal blood for secretion via the urine. These changes in bile salt transport expression act in concert with reduced bile salt synthesis and phase I and phase II detoxification of bile salts through hydroxylation and sulphation or glucuronidation, respectively. These modifications increase the water solubility of bile salts reducing their toxicity.

Conclusions

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References

Bile salts are synthesized in the liver in a complex pathway with enzymes residing in different cellular locations. Subsequently, they are maintained in an enterohepatic circulation through the action of dedicated bile salt transporters in the plasma membranes of hepatocytes and enterocytes. In contrast to the detailed knowledge about the bile salt transporters in plasma membranes, we know very little about the process of bile salts through organellar membranes. As defective bile salt synthesis and transport is associated with cholestasis, we may expect that such transporters are also important for normal bile salt homeostasis. Our current research is aimed to analyse this process of intracellular transport of bile salts.

Bile salts are important signalling molecules. Through the activation of NRs, they regulate their own synthesis and transport rates. These transcription factors can also be activated by pharmaceutical compounds that may offer new therapeutic options to treat cholestatic liver disease in the future.

References

  1. Top of page
  2. Summary
  3. Enterohepatic circulation of bile salts
  4. Bile salt transporters in the liver and intestine
  5. Genetic defects in bile salt transport
  6. Bile salts biosynthesis
  7. Enzymes involved in bile salt biosynthesis
  8. Genetic defects in bile salt biosynthesis
  9. Regulation of bile salt homeostasis
  10. Regulation of hepatic and intestinal bile salt transporters
  11. Regulation of bile salt transporters during acquired cholestatic liver disease
  12. Conclusions
  13. Acknowledgements
  14. References
  • 1
    Hagenbuch B, Meier PJ. Molecular cloning, chromosomal localization, and functional characterization of a human liver Na+/bile acid cotransporter. J Clin Invest 1994; 93: 132631.
  • 2
    Hagenbuch B, Dawson P. The sodium bile salt cotransport family SLC10. Pflugers Arch-Eur J Physiol 2004; 447: 56670.
  • 3
    Ananthanarayanan M, Ng OC, Boyer JL, Suchy FJ. Characterization of cloned rat liver Na(+)-bile acid cotransporter using peptide and fusion protein antibodies. Am J Physiol 1994; 267: G63743.
  • 4
    Stieger B, Hagenbuch B, Landmann L, Hochli M, Schroeder A, Meier PJ. In-situ localization of the hepatocytic Na+ taurocholate cotransporting polypeptide in rat-liver. Gastroenterology 1994; 107: 17817.
  • 5
    Meier PJ, Eckhardt U, Schroeder A, Hagenbuch B, Stieger B. Substrate specificity of sinusoidal bile acid and organic anion uptake systems in rat and human liver. Hepatology 1997; 26: 166777.
  • 6
    Schroeder A, Eckhardt U, Stieger B, et al. Substrate specificity of the rat liver Na+-bile salt cotransporter in Xenopus laevis oocytes and in CHO cells. Am J Physiol Gastrointest Liver Physiol 1998; 37: G3705.
  • 7
    Mikkaichi T, Suzuki T, Tanemoto M, Ito S, Abe T. The organic anion transporter (OATP) family. Drug Metab Pharmacokinet 2004; 19: 1719.
  • 8
    Gerloff T, Stieger B, Hagenbuch B, et al. The sister of P-glycoprotein represents the canalicular bile salt export pump of mammalian liver. J Biol Chem 1998; 273: 1004650.
  • 9
    Borst P, Elferink RO. Mammalian ABC transporters in health and disease. Annu Rev Biochem 2002; 71: 53792.
  • 10
    Stieger B, Meier Y, Meier PJ. The bile salt export pump. Pflugers Arch-Eur J Physiol 2007; 453: 61120.
  • 11
    Zelcer N, Saeki T, Bot I, Kuil A, Borst P. Transport of bile acids in multidrug-resistance-protein 3-overexpressing cells co-transfected with the ileal Na+-dependent bile-acid transporter. Biochem J 2003; 369: 2330.
  • 12
    Zelcer N, Reid G, Wielinga P, et al. Steroid and bile acid conjugates are substrates of human multidrug-resistance protein (MRP) 4 (ATP-binding cassette C4). Biochem J 2003; 371: 3617.
  • 13
    Dawson PA, Hubbert M, Haywood J, et al. The heteromeric organic solute transporter alpha-beta, ost alpha-ost beta, is an ileal basolateral bile acid transporter. J Biol Chem 2005; 280: 69608.
  • 14
    Zollner G, Fickert P, Silbert D, et al. Adaptive changes in hepatobiliary transporter expression in primary biliary cirrhosis. J Hepatol 2003; 38: 71727.
  • 15
    Keitel V, Burdelski M, Warskulat U, et al. Expression and localization of hepatobiliary transport proteins in progressive familial intrahepatic cholestasis. Hepatology 2005; 41: 116072.
  • 16
    Boyer JL, Trauner M, Mennone A, et al. Upregulation of a basolateral FXR-dependent bile acid efflux transporter OSTalpha-OSTbeta in cholestasis in humans and rodents. Am J Physiol Gastrointest Liver Physiol 2006; 290: G112430.
  • 17
    Shneider BL, Dawson PA, Christie DM, Hardikar W, Wong MH, Suchy FJ. Cloning and molecular characterization of the ontogeny of a rat ileal sodium-dependent bile-acid transporter. J Clin Invest 1995; 95: 74554.
  • 18
    Craddock AL, Love MW, Daniel RW, et al. Expression and transport properties of the human ileal and renal sodium-dependent bile acid transporter. Am J Physiol Gastrointest Liver Physiol 1998; 37: G15769.
  • 19
    Strautnieks SS, Bull LN, Knisely AS, et al. A gene encoding a liver-specific ABC transporter is mutated in progressive familial intrahepatic cholestasis. Nat Genet 1998; 20: 2338.
  • 20
    Jansen PL, Strautnieks SS, Jacquemin E, et al. Hepatocanalicular bile salt export pump deficiency in patients with progressive familial intrahepatic cholestasis. Gastroenterology 1999; 117: 13709.
  • 21
    Van Mil SWC, Van Der Woerd WL, Van Der Brugge G, et al. Benign recurrent intrahepatic cholestasis type 2 is caused by mutations in ABCB11. Gastroenterology 2004; 127: 37984.
  • 22
    Crawford AR, Smith AJ, Hatch VC, Oude Elferink RP, Borst P, Crawford JM. Hepatic secretion of phospholipid vesicles in the mouse critically depends on mdr2 or MDR3 P-glycoprotein expression. Visualization by electron microscopy. J Clin Invest 1997; 100: 25627.
  • 23
    Smit JJ, Schinkel AH, Oude Elferink RP, et al. Homozygous disruption of the murine mdr2 P-glycoprotein gene leads to a complete absence of phospholipid from bile and to liver disease. Cell 1993; 75: 45162.
  • 24
    Deleuze JF, Jacquemin E, Dubuisson C, et al. Defect of multidrug-resistance 3 gene expression in a subtype of progressive familial intrahepatic cholestasis. Hepatology 1996; 23: 9048.
  • 25
    De Vree JM, Jacquemin E, Sturm E, et al. Mutations in the MDR3 gene cause progressive familial intrahepatic cholestasis. Proc Natl Acad Sci U S A 1998; 95: 2827.
  • 26
    Bull LN, Carlton VE, Stricker NL, et al. Genetic and morphological findings in progressive familial intrahepatic cholestasis (Byler disease [PFIC-1] and Byler syndrome): evidence for heterogeneity. Hepatology 1997; 26: 15564.
  • 27
    Bull LN, Van Eijk MJ, Pawlikowska L, et al. A gene encoding a P-type ATPase mutated in two forms of hereditary cholestasis. Nat Genet 1998; 18: 21924.
  • 28
    Paulusma CC, Oude Elferink RP. The type 4 subfamily of P-type ATPases, putative aminophospholipid translocases with a role in human disease. Biochim Biophys Acta 2005; 1741: 1124.
  • 29
    Wong MH, Oelkers P, Dawson PA. Identification of a mutation in the ileal sodium-dependent bile acid transporter gene that abolishes transport activity. J Biol Chem 1995; 270: 2722834.
  • 30
    Russell DW. The enzymes, regulation, and genetics of bile acid synthesis. Annu Rev Biochem 2003; 72: 13774.
  • 31
    Ridlon JM, Kang DJ, Hylemon PB. Bile salt biotransformations by human intestinal bacteria. J Lipid Res 2006; 47: 24159.
  • 32
    Myant NB, Mitropoulos KA. Cholesterol 7alpha-hydroxylase. J Lipid Res 1977; 18: 13553.
  • 33
    Cali JJ, Russell DW. Characterization of human sterol 27-hydroxylase. A mitochondrial cytochrome P-450 that catalyzes multiple oxidation reaction in bile acid biosynthesis. J Biol Chem 1991; 266: 77748.
  • 34
    Toll A, Wikvall K, Sudjana-Sugiaman E, Kondo KH, Bjorkhem I. 7 alpha hydroxylation of 25-hydroxycholesterol in liver microsomes. Evidence that the enzyme involved is different from cholesterol 7 alpha-hydroxylase. Eur J Biochem 1994; 224: 30916.
  • 35
    Schwarz M, Wright AC, Davis DL, Nazer H, Bjorkhem I, Russell DW. The bile acid synthetic gene 3 beta-hydroxy-delta(5)-C-27-steroid oxidoreductase is mutated in progressive intrahepatic cholestasis. J Clin Invest 2000; 106: 117584.
  • 36
    Gafvels M, Olin M, Chowdhary BP, et al. Structure and chromosomal assignment of the sterol 12alpha-hydroxylase gene (CYP8B1) in human and mouse: eukaryotic cytochrome P-450 gene devoid of introns. Genomics 1999; 56: 18496.
  • 37
    Zhu Y, Fillenwarth MJ, Crabb D, Lumeng L, Lin RC. Identification of the 37-kd rat liver protein that forms an acetaldehyde adduct in vivo as delta 4-3-ketosteroid 5 beta-reductase. Hepatology 1996; 23: 11522.
  • 38
    Usui E, Okuda K, Kato Y, Noshiro M. Rat hepatic 3 alpha-hydroxysteroid dehydrogenase: expression of cDNA and physiological function in bile acid biosynthetic pathway. J Biochem (Tokyo) 1994; 115: 2307.
  • 39
    Falany CN, Xie X, Wheeler JB, et al. Molecular cloning and expression of rat liver bile acid CoA ligase. J Lipid Res 2002; 43: 206271.
  • 40
    Cuebas DA, Phillips C, Schmitz W, Conzelmann E, Novikov DK. The role of alpha-methylacyl-CoA racemase in bile acid synthesis. Biochem J 2002; 363: 8017.
  • 41
    Baumgart E, Vanhooren JC, Fransen M, et al. Molecular characterization of the human peroxisomal branched-chain acyl-CoA oxidase: cDNA cloning, chromosomal assignment, tissue distribution, and evidence for the absence of the protein in Zellweger syndrome. Proc Natl Acad Sci U S A 1996; 93: 1374853.
  • 42
    Kurosawa T, Sato M, Yoshimura T, Jiang LL, Hashimoto T, Tohma M. Stereospecific formation of (24R,25R)-3 alpha,7 alpha,12 alpha,24-tetrahydroxy-5 beta-cholestan-26-oic acid catalyzed with a peroxisomal bifunctional D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase. Biol Pharm Bull 1997; 20: 2957.
  • 43
    Seedorf U, Assmann G. Cloning, expression, and nucleotide sequence of rat liver sterol carrier protein 2 cDNAs. J Biol Chem 1991; 266: 6306.
  • 44
    Falany CN, Johnson MR, Barnes S, Diasio RB. Glycine and taurine conjugation of bile-acids by a single enzyme - molecular-cloning and expression of human liver bile-acid Coa-amino acid N-acyltransferase. J Biol Chem 1994; 269: 193759.
  • 45
    Visser WF, Van Roermund CWT, Ijlst L, Waterham HR, Wanders RJA. Demonstration of bile acid transport across the mammalian peroxisomal membrane. Biochem Biophys Res Commun 2007; 357: 33540.
  • 46
    Pellicoro A, Van Den Heuvel FAJ, Geuken M, Moshage H, Jansen PLM, Faber KN. Human and rat bile acid-CoA: amino acid N-acyltransferase are liver-specific peroxisomal enzymes: implications for intracellular bile salt transport. Hepatology 2007; 45: 3408.
  • 47
    Barbanto E, Batta AK, Salen G, et al. High serum and urinary unconjugated bile acid concentrations are associated with homozygous mutation in bile acid coenzyme A:amino acid N-acyltransferase (BAAT). Gastroenterology 2003; 124: A60.
  • 48
    Princen HMG, Post SM, Twisk J. Regulation of bile acid biosynthesis. Curr Pharm Des 1997; 3: 5984.
  • 49
    Bjorkhem I, Lutjohann D, Breuer O, Sakinis A, Wennmalm A. Importance of a novel oxidative mechanism for elimination of brain cholesterol – turnover of cholesterol and 24(S)-hydroxycholesterol in rat brain as measured with O-18(2) techniques in vivo and in vitro. J Biol Chem 1997; 272: 3017884.
  • 50
    Lund EG, Guileyardo JM, Russell DW. cDNA cloning of cholesterol 24-hydroxylase, a mediator of cholesterol homeostasis in the brain. Proc Natl Acad Sci U S A 1999; 96: 723843.
  • 51
    Heverin M, Bogdanovic N, Lutjohann D, et al. Changes in the levels of cerebral and extracerebral sterols in the brain of patients with Alzheimer’s disease. J Lipid Res 2004; 45: 18693.
  • 52
    Pullinger CR, Eng C, Salen G, et al. Human cholesterol 7alpha-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype. J Clin Invest 2002; 110: 10917.
  • 53
    Ishibashi S, Schwarz M, Frykman PK, Herz J, Russell DW. Disruption of cholesterol 7 alpha-hydroxylase gene in mice: 1. Postnatal lethality reversed by bile acid and vitamin supplementation. J Biol Chem 1996; 271: 1801723.
  • 54
    Bjorkhem I, Leitersdorf E. Sterol 27-hydroxylase deficiency: a rare cause of xanthomas in normocholesterolemic humans. Trends Endocrinol Metab 2000; 11: 1803.
  • 55
    Setchell KDR, Schwarz M, O’Connell NC, et al. Identification of a new inborn error in bile acid synthesis: mutation of the oxysterol 7 alpha-hydroxylase gene causes severe neonatal liver disease. J Clin Invest 1998; 102: 1690703.
  • 56
    Clayton PT, Leonard JV, Lawson AM, et al. Familial giant-cell hepatitis associated with synthesis of 3-beta, 7-alpha-dihydroxy-5-cholenoic and 3-beta, 7-alpha, 12-alpha-trihydroxy-5-cholenoic acids. J Clin Invest 1987; 79: 10318.
  • 57
    Ichimiya H, Nazer H, Gunasekaran T, Clayton P, Sjovall J. Treatment of chronic liver-disease caused by 3-beta-hydroxy-delta-5-c27-steroid dehydrogenase-deficiency with chenodeoxycholic acid. Arch Dis Child 1990; 65: 11214.
  • 58
    Witzleben CL, Piccoli DA, Setchell K. A new category of causes of intrahepatic cholestasis. Pediatr Pathol 1992; 12: 26974.
  • 59
    Horslen SP, Lawson AM, Malone M, Clayton PT. 3-Beta-hydroxy-delta-5-C27-steroid dehydrogenase-deficiency – effect of chenodeoxycholic acid therapy on liver histology. J Inherit Metab Dis 1992; 15: 3846.
  • 60
    Jacquemin E, Setchell KDR, Oconnell NC, et al. A new cause of progressive intrahepatic cholestasis – 3-beta-hydroxy-C-27-steroid dehydrogenase/isomerase deficiency. J Pediatr 1994; 125: 37984.
  • 61
    Akobeng AK, Clayton PT, Miller V, Super M, Thomas AG. An inborn error of bile acid synthesis (3 beta-hydroxy-delta(5)-C-27-steroid dehydrogenase deficiency) presenting as malabsorption leading to rickets. Arch Dis Child 1999; 80: 4635.
  • 62
    Setchell KD, Suchy FJ, Welsh MB, Zimmer-Nechemias L, Heubi J, Balistreri WF. Delta 4-3-oxosteroid 5 beta-reductase deficiency described in identical twins with neonatal hepatitis. A new inborn error in bile acid synthesis. J Clin Invest 1988; 82: 214857.
  • 63
    Shneider BL, Setchell KD, Whitington PF, Neilson KA, Suchy FJ. Delta 4-3-oxosteroid 5 beta-reductase deficiency causing neonatal liver failure and hemochromatosis. J Pediatr 1994; 124: 2348.
  • 64
    Ferdinandusse S, Denis S, Clayton PT, et al. Mutations in the gene encoding peroxisomal alpha-methylacyl-CoA racemase cause adult-onset sensory motor neuropathy. Nat Genet 2000; 24: 18891.
  • 65
    Ferdinandusse S, Overmars H, Denis S, Waterham HR, Wanders RJA, Vreken P. Plasma analysis of di- and trihydroxycholestanoic acid diastereoisomers in peroxisomal alpha-methylacyl-CoA racemase deficiency. J Lipid Res 2001; 42: 13741.
  • 66
    Ferdinandusse S, Van Grunsven EG, Oostheim W, et al. Reinvestigation of peroxisomal 3-ketoacyl-CoA thiolase deficiency: identification of the true defect at the level of D-bifunctional protein. Am J Hum Genet 2002; 70: 158993.
  • 67
    Carlton VEH, Harris BZ, Puffenberger EG, et al. Complex inheritance of familial hypercholanemia with associated mutations in TJP2 and BAAT. Nat Genet 2003; 34: 916.
  • 68
    Depreter M, Espeel M, Roels F. Human peroxisomal disorders. Microsc Res Tech 2003; 61: 20323.
  • 69
    Zelent A, Krust A, Petkovich M, Kastner P, Chambon P. Cloning of murine alpha and beta retinoic acid receptors and a novel receptor gamma predominantly expressed in skin. Nature 1989; 339: 7147.
  • 70
    Leid M, Kastner P, Lyons R, et al. Purification, cloning, and RXR identity of the HeLa cell factor with which RAR or TR heterodimerizes to bind target sequences efficiently. Cell 1992; 68: 37795.
  • 71
    Green S, Wahli W. Peroxisome proliferator-activated receptors: finding the orphan a home. Mol Cell Endocrinol 1994; 100: 14953.
  • 72
    Forman BM, Goode E, Chen J, et al. Identification of a nuclear receptor that is activated by farnesol metabolites. Cell 1995; 81: 68793.
  • 73
    Makishima M, Okamoto AY, Repa JJ, et al. Identification of a nuclear receptor for bile acids. Science 1999; 284: 13625.
  • 74
    Parks DJ, Blanchard SG, Bledsoe RK, et al. Bile acids: natural ligands for an orphan nuclear receptor. Science 1999; 284: 13658.
  • 75
    Wang H, Chen J, Hollister K, Sowers LC, Forman BM. Endogenous bile acids are ligands for the nuclear receptor FXR/BAR. Mol Cell 1999; 3: 54353.
  • 76
    Ananthanarayanan M, Balasubramanian N, Makishima M, Mangelsdorf DJ, Suchy FJ. Human bile salt export pump promoter is transactivated by the farnesoid X receptor/bile acid receptor. J Biol Chem 2001; 276: 2885765.
  • 77
    Plass JRM, Mol O, Heegsma J, et al. Farnesoid X receptor and bile salts are involved in transcriptional regulation of the gene encoding the human bile salt export pump. Hepatology 2002; 35: 58996.
  • 78
    Lee H, Zhang YQ, Lee FY, Nelson SF, Gonzalez FJ, Edwards PA. FXR regulates organic solute transporters alpha and beta in the adrenal gland, kidney, and intestine. J Lipid Res 2006; 47: 20114.
  • 79
    Landrier JF, Eloranta JJ, Vavricka SR, Kullak-Ublick GA. The nuclear receptor for bile acids, FXR, transactivates human organic solute transporter-alpha and -beta genes. Am J Physiol Gastrointest Liver Physiol 2006; 290: G47685.
  • 80
    Frankenberg T, Rao A, Chen F, Haywood J, Shneider BL, Dawson PA. Regulation of the mouse organic solute transporter alpha-beta, Ostalpha-Ostbeta, by bile acids. Am J Physiol Gastrointest Liver Physiol 2006; 290: G91222.
  • 81
    Denson LA, Sturm E, Echevarria W, et al. The orphan nuclear receptor, shp, mediates bile acid-induced inhibition of the rat bile acid transporter, ntcp. Gastroenterology 2001; 121: 1407.
  • 82
    Denson LA, Auld KL, Schiek DS, McClure MH, Mangelsdorf DJ, Karpen SJ. The gene promoters for two critical hepatobiliary transporters, NTCP and MRP2, are coordinately induced by retinoids and suppressed by IL-1 beta and bile acids via RXR: RAR response elements. Hepatology 1999; 30: 305A.
  • 83
    Neimark E, Chen F, Li XP, Shneider BL. Bile acid-induced negative feedback regulation of the human lleal bile acid transporter. Hepatology 2004; 40: 14956.
  • 84
    Chiang JYL, Kimmel R, Stroup D. Regulation of cholesterol 7 alpha-hydroxylase gene (CYP7A1) transcription by the liver orphan receptor (LXR alpha). Gene 2001; 262: 25765.
  • 85
    Xie W, Radominska-Pandya A, Shi Y, et al. An essential role for nuclear receptors SXR/PXR in detoxification of cholestatic bile acids. Proc Natl Acad Sci U S A 2001; 98: 337580.
  • 86
    Makishima M, Lu TT, Xie W, et al. Vitamin D receptor as an intestinal bile acid sensor. Science 2002; 296: 13136.
  • 87
    Lee FY, Lee H, Hubbert ML, Edwards PA, Zhang Y. FXR, a multipurpose nuclear receptor. Trends Biochem Sci 2006; 31: 57280.
  • 88
    Rippin SJ, Hagenbuch B, Meier PJ, Stieger B. Cholestatic expression pattern of sinusoidal and canalicular organic anion transport systems in primary cultured rat hepatocytes. Hepatology 2001; 33: 77682.
  • 89
    Dranoff JA, McClure M, Burgstahler AD, et al. Short-term regulation of bile acid uptake by microfilament-dependent translocation of rat ntcp to the plasma membrane. Hepatology 1999; 30: 2239.
  • 90
    Xia XF, Lu XH, Shentu S, Merikhi A, LeSage G. Regulation of apical bile acid transporter (ASBT) activity by intracellular to membrane translocation is dependent on a cAMP- and p38-dependent phosphorylation of ASBT. Gastroenterology 2004; 126: A674.
  • 91
    Suchy FJ, Ananthanarayanan M. Bile salt excretory pump: biology and pathobiology. J Pediatr Gastroenterol Nutr 2006; 43: S106.
  • 92
    Misra S, Varticovski L, Arias IM. Mechanisms by which cAMP increases bile acid secretion in rat liver and canalicular membrane vesicles. Am J Physiol Gastrointest Liver Physiol 2003; 285: G31624.
  • 93
    Stieger B, Fattinger K, Madon J, Kullak-Ublick GA, Meier PJ. Drug- and estrogen-induced cholestasis through inhibition of the hepatocellular bile salt export pump (Bsep) of rat liver. Gastroenterology 2000; 118: 42230.
  • 94
    Fattinger K, Funk C, Pantze M, et al. The endothelin antagonist bosentan inhibits the canalicular bile salt export pump: a potential mechanism for hepatic adverse reactions. Clin Pharmacol Ther 2001; 69: 22331.
  • 95
    Kullak-Ublick GA, Stieger B, Meier PJ. Enterohepatic bile salt transporters in normal physiology and liver disease. Gastroenterology 2004; 126: 32242.
  • 96
    Lang C, Meier Y, Stieger B, et al. Mutations and polymorphisms in the bile salt export pump and the multidrug resistance protein 3 associated with drug-induced liver injury. Pharmacogenet Genomics 2007; 17: 4760.
  • 97
    Elferink MG, Olinga P, Draaisma AL, et al. LPS-induced downregulation of MRP2 and BSEP in human liver is due to a posttranscriptional process. Am J Physiol Gastrointest Liver Physiol 2004; 287: G100816.