Rat liver canalicular membrane vesicles contain an ATP-dependent bile acid transport system

A history of research into the physiology of bile, from Hippocrates to molecular medicine

Content available: Audio Recording.

Bile formation and secretion: An update

Bile formation is a fundamental physiological process that is vital to the survival of all vertebrates. However, little was known about the mechanisms of this secretion until after World War II. Initial studies involved classic physiologic studies in animal models and humans, which progressed to include studies in isolated cells and membrane vesicles. The advent of molecular biology then led to the identification of specific transport systems that are the determinants of this secretion. Progress in this field was reviewed in the American Physiologic Society's series on "Comprehensive Physiology" in 2013. Herein, we provide an in-depth update of progress since that time.

Quantitative Systems Toxicology Analysis of In Vitro Mechanistic Assays Reveals Importance of Bile Acid Accumulation and Mitochondrial Dysfunction in TAK-875-Induced Liver Injury

TAK-875 (fasiglifam), a GPR40 agonist in development for the treatment of type 2 diabetes (T2D), was voluntarily terminated in Phase III trials due to adverse liver effects. The potential mechanisms of TAK-875 toxicity were explored by combining in vitro experiments with quantitative systems toxicology (QST) using DILIsym, a mathematical representation of drug-induced liver injury. In vitro assays revealed that bile acid transporters were inhibited by both TAK-875 and its metabolite, TAK-875-Glu. Experimental data indicated that human bile salt export pump (BSEP) inhibition by TAK-875 was mixed whereas sodium taurocholate co-transporting polypeptide (NTCP) inhibition by TAK-875 was competitive. Furthermore, experimental data demonstrated that both TAK-875 and TAK-875-Glu inhibit mitochondrial electron transport chain (ETC) enzymes. These mechanistic data were combined with a physiologically based pharmacokinetic (PBPK) model constructed within DILIsym to estimate liver exposure of TAK-875 and TAK-875-Glu. In a simulated population (SimPops) constructed to reflect T2D patients, 16/245 (6.5%) simulated individuals developed alanine aminotransferase (ALT) elevations, an incidence similar to that observed with 200 mg daily dosing in clinical trials. Determining the mode of bile acid transporter inhibition (K_i) was critical to accurate predictions. In addition, simulations conducted on a sensitive subset of individuals (SimCohorts) revealed that when either BSEP or ETC inhibition was inactive, ALT elevations were not predicted to occur, suggesting that the two mechanisms operate synergistically to produce the observed clinical response. These results demonstrate how utilizing QST methods to interpret in vitro experimental results can lead to an improved understanding of the clinically relevant mechanisms underlying drug-induced toxicity.

Beyond Competitive Inhibition: Regulation of ABC Transporters by Kinases and Protein-Protein Interactions as Potential Mechanisms of Drug-Drug Interactions

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters mediating the extrusion of an array of substrates ranging from amino acids and lipids to xenobiotics, and many therapeutic compounds, including anticancer drugs. The ABC transporters are also recognized as important contributors to pharmacokinetics, especially in drug-drug interactions and adverse drug effects. Drugs and xenobiotics, as well as pathologic conditions, can influence the transcription of ABC transporters, or modify their activity or intracellular localization. Kinases can affect the aforementioned processes for ABC transporters as do protein interactions. In this review, we focus on the ABC transporters ABCB1, ABCB11, ABCC1, ABCC4, and ABCG2 and illustrate how kinases and protein-protein interactions affect these transporters. The clinical relevance of these factors is currently unknown; however, these examples suggest that our understanding of drug-drug interactions will benefit from further knowledge of how kinases and protein-protein interactions affect ABC transporters.

Nuclear receptor FXR, bile acids and liver damage: Introducing the progressive familial intrahepatic cholestasis with FXR mutations

The nuclear receptor farnesoid X receptor (FXR) is the master regulator of bile acids (BAs) homeostasis since it transcriptionally drives modulation of BA synthesis, influx, efflux, and detoxification along the enterohepatic axis. Due to its crucial role, FXR alterations are involved in the progression of a plethora of BAs associated inflammatory disorders in the liver and in the gut. The involvement of the FXR pathway in cholestasis development and management has been elucidated so far with a direct role of FXR activating therapy in this condition. However, the recent identification of a new type of genetic progressive familial intrahepatic cholestasis (PFIC) linked to FXR mutations has strengthen also the bona fide beneficial effects of target therapies that by-pass FXR activation, directly promoting the action of its target, namely the enterokine FGF19, in the repression of hepatic BAs synthesis with reduction of total BA levels in the liver and serum, accomplishing one of the major goals in cholestasis. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni and Peter Jansen.

Involvement of a cyclic adenosine monophosphate-dependent signal in the diet-induced canalicular trafficking of adenosine triphosphate-binding cassette transporter g5/g8

The adenosine triphosphate-binding cassette (ABC) half-transporters Abcg5 and Abcg8 promote the secretion of neutral sterol into bile. Studies have demonstrated the diet-induced gene expression of these transporters, but the regulation of their trafficking when the nutritional status changes in the liver remains to be elucidated. Here, we generated a novel in vivo kinetic analysis that can monitor the intracellular trafficking of Abcg5/Abcg8 in living mouse liver by in vivo transfection of the genes of fluorescent protein-tagged transporters and investigated how hypernutrition affects the canalicular trafficking of these transporters. The kinetic analysis showed that lithogenic diet consumption accelerated the translocation of newly synthesized fluorescent-tagged transporters to intracellular pools in an endosomal compartment and enhanced the recruitment of these pooled gene products into the bile canalicular membrane in mouse liver. Because some ABC transporters are reported to be recruited from intracellular pools to the bile canaliculi by cyclic adenosine monophosphate (cAMP) signaling, we next evaluated the involvement of this machinery in a diet-induced event. Administration of a protein kinase A inhibitor, N-(2-{[3-(4-bromophenyl)-2-propenyl]amino}ethyl)-5-isoquinolinesulfonamide, decreased the canalicular expression of native Abcg5/Abcg8 in lithogenic diet-fed mice, and injection of a cAMP analog, dibutyryl cAMP, transiently increased their levels in standard diet-fed mice, indicating the involvement of cAMP signaling. Indeed, canalicular trafficking of the fluorescent-tagged Abcg5/Abcg8 was enhanced by dibutyryl cAMP administration.

CONCLUSION

These observations suggest that diet-induced lipid loading into liver accelerates the trafficking of Abcg5/Abcg8 to the bile canalicular membrane through cAMP signaling machinery.

Glyphosate, pathways to modern diseases III: Manganese, neurological diseases, and associated pathologies

Manganese (Mn) is an often overlooked but important nutrient, required in small amounts for multiple essential functions in the body. A recent study on cows fed genetically modified Roundup(®)-Ready feed revealed a severe depletion of serum Mn. Glyphosate, the active ingredient in Roundup(®), has also been shown to severely deplete Mn levels in plants. Here, we investigate the impact of Mn on physiology, and its association with gut dysbiosis as well as neuropathologies such as autism, Alzheimer's disease (AD), depression, anxiety syndrome, Parkinson's disease (PD), and prion diseases. Glutamate overexpression in the brain in association with autism, AD, and other neurological diseases can be explained by Mn deficiency. Mn superoxide dismutase protects mitochondria from oxidative damage, and mitochondrial dysfunction is a key feature of autism and Alzheimer's. Chondroitin sulfate synthesis depends on Mn, and its deficiency leads to osteoporosis and osteomalacia. Lactobacillus, depleted in autism, depend critically on Mn for antioxidant protection. Lactobacillus probiotics can treat anxiety, which is a comorbidity of autism and chronic fatigue syndrome. Reduced gut Lactobacillus leads to overgrowth of the pathogen, Salmonella, which is resistant to glyphosate toxicity, and Mn plays a role here as well. Sperm motility depends on Mn, and this may partially explain increased rates of infertility and birth defects. We further reason that, under conditions of adequate Mn in the diet, glyphosate, through its disruption of bile acid homeostasis, ironically promotes toxic accumulation of Mn in the brainstem, leading to conditions such as PD and prion diseases.

History of hepatic bile formation: old problems, new approaches

Studies of hepatic bile formation reported in 1958 established that it was an osmotically generated water flow. Intravenous infusion of sodium taurocholate established a high correlation between hepatic bile flow and bile acid excretion. Secretin, a hormone that stimulates bicarbonate secretion, was also found to increase hepatic bile flow. The sources of the water entering the biliary system with these two stimuli were differentiated by the use of mannitol. An increase in its excretion parallels the increase in bile flow in response to bile acids but not secretin, which led to a quantitative distinction between canalicular and ductular water flow. The finding of aquaglyceroporin-9 in the basolateral surface of the hepatocyte accounted for the rapid entry of mannitol into hepatocytes and its exclusion from water movement in the ductules where aquaporin-1 is present. Electron microscopy demonstrated that bile acids generate the formation of vesicles that contain lecithin and cholesterol after their receptor-mediated canalicular transport. Biophysical studies established that the osmotic effect of bile acids varies with their concentration and also with the proportion of mono-, di-, and trihydroxy bile acids and provides a basis for understanding their physiological effects. Because of the varying osmotic effect of bile acids, it is difficult to quantify bile acid independent flow generated by other solutes, such as glutathione, which enters the biliary system. Monohydroxy bile acids, by markedly increasing aggregation number, severely reduce water flow. Developing biomarkers for the noninvasive assessment of normal hepatic bile flow remains an elusive goal that merits further study.

Biosynthesis and trafficking of the bile salt export pump, BSEP: therapeutic implications of BSEP mutations

The bile salt export pump (BSEP, ABCB11) is the primary transporter of bile acids from the hepatocyte to the biliary system. This rate-limiting step in bile formation is essential to the formation of bile salt dependent bile flow, the enterohepatic circulation of bile acids, and the digestion of dietary fats. Mutations in BSEP are associated with cholestatic diseases such as progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2), drug-induced cholestasis, and intrahepatic cholestasis of pregnancy. Development of clinical therapies for these conditions necessitates a clear understanding of the cell biology of biosynthesis, trafficking, and transcriptional and translational regulation of BSEP. This chapter will focus on the molecular and cell biological aspects of this critical hepatic membrane transporter.

Clinical application of transcriptional activators of bile salt transporters

Hepatobiliary bile salt (BS) transporters are critical determinants of BS homeostasis controlling intracellular concentrations of BSs and their enterohepatic circulation. Genetic or acquired dysfunction of specific transport systems causes intrahepatic and systemic retention of potentially cytotoxic BSs, which, in high concentrations, may disturb integrity of cell membranes and subcellular organelles resulting in cell death, inflammation and fibrosis. Transcriptional regulation of canalicular BS efflux through bile salt export pump (BSEP), basolateral elimination through organic solute transporters alpha and beta (OSTα/OSTβ) as well as inhibition of hepatocellular BS uptake through basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP) represent critical steps in protection from hepatocellular BS overload and can be targeted therapeutically. In this article, we review the potential clinical implications of the major BS transporters BSEP, OSTα/OSTβ and NTCP in the pathogenesis of hereditary and acquired cholestatic syndromes, provide an overview on transcriptional control of these transporters by the key regulatory nuclear receptors and discuss the potential therapeutic role of novel transcriptional activators of BS transporters in cholestasis.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Hepatic overexpression of Abcb11 in mice promotes the conservation of bile acids within the enterohepatic circulation

The bile salt export pump, encoded by ABCB11, is the predominant canalicular transport protein for biliary bile acid secretion. The level of ABCB11 expression in humans is widely variable yet the impact of this variability on human disease is not well defined. We aim to determine the effect of hepatic Abcb11 overexpression on the enterohepatic circulation (EHC) in mice. We used a stable isotope dilution technique in transgenic mice overexpressing hepatic Abcb11 (TTR-Abcb11) to determine the pool size, fractional turnover rate (FTR), and synthesis rate of the primary bile acid, cholic acid (CA). The gallbladder was cannulated to determine bile flow, bile acid composition, and the biliary secretion rates of CA, total bile acids, phospholipid, and cholesterol. The combined data allowed for estimation of the CA cycling time and the fraction of CA lost per cycle. Hepatic and intestinal gene and protein expression were determined by qPCR and Western blot. Abcb11 overexpression strongly decreased FTR and synthesis rate of CA. Abcb11 overexpression decreased the fraction of CA that was lost per cycle of the EHC. Hepatic expression of Cyp7a1 was suppressed by nearly 50% and ileal expression of FGF15 was increased more than eightfold in TTR-Abcb11 mice. Despite the increased intestinal reabsorption of bile acids, ileal Asbt expression was suppressed. Hepatic Abcb11 overexpression in mice increases the conservation of bile acids within the enterohepatic circulation. These data provide strong evidence for the existence of feed-forward communication between hepatic expression of a bile acid transport protein and the intestine.

A perspective on efflux transport proteins in the liver

Detailed knowledge regarding the influence of hepatic transport proteins on drug disposition has advanced at a rapid pace over the past decade. Efflux transport proteins located in the basolateral and apical (canalicular) membranes of hepatocytes play an important role in the hepatic elimination of many endogenous and exogenous compounds, including drugs and metabolites. This review focuses on the role of these efflux transporters in hepatic drug excretion. The impact of these proteins as underlying factors for disease is highlighted, and the importance of hepatic efflux proteins in the efficacy and toxicity of drugs is discussed. In addition, a brief overview of methodology to evaluate the function of hepatic efflux transport proteins is provided. Current challenges in predicting the impact of altered efflux protein function on systemic, intestinal, and hepatocyte exposure to drugs and metabolites are highlighted.

Hepatic overexpression of abcb11 promotes hypercholesterolemia and obesity in mice

BACKGROUND & AIMS

ABCB11 is a canalicular transport protein that controls the rate-limiting step in hepatic bile acid secretion. Its expression levels vary in humans, and it is not clear how these variations affect lipid metabolism. We investigated whether overexpression of Abcb11 in mice increases lipid absorption in the intestine and affects the development of obesity or hypercholesterolemia.

METHODS

Transgenic mice that overexpress Abcb11 in liver (TTR-Abcb11) and FVB/NJ mice (controls) were fed a high-cholesterol or high-fat diet for 12 weeks. Intestinal lipid absorption was measured by the dual fecal isotope method. Energy expenditure was measured by indirect calorimetry. The bile acid pool was analyzed by high-performance liquid chromatography.

RESULTS

TTR-Abcb11 mice had a nearly 2-fold increase in intestinal cholesterol absorption compared with controls. TTR-Abcb11 mice fed a high-cholesterol diet had greater increases in plasma and hepatic levels of cholesterol and became more obese than controls; they also had increased intestinal absorption of fatty acids and decreased energy expenditure. In the TTR-Abcb11 mice, the sizes of plasma and total bile acid pools were reduced; the bile acid pool contained more species of hydrophobic bile acids compared with controls.

CONCLUSIONS

Hepatic overexpression of Abcb11 in mice promotes diet-induced obesity and hypercholesterolemia; increased intestinal cholesterol absorption by hydrophobic bile acids might cause these features. Increased absorption of fatty acids in the intestine and reduced expenditure of energy could increase weight gain in TTR-Abcb11 mice. In humans, variations in expression of ABCB11 might confer genetic susceptibility to diet-induced hyperlipidemia and obesity.

The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation

Bile formation is an important function of the liver. Bile salts are a major constituent of bile and are secreted by hepatocytes into bile and delivered into the small intestine, where they assist in fat digestion. In the small intestine, bile salts are almost quantitatively reclaimed and transported back via the portal circulation to the liver. In the liver, hepatocytes take up bile salts and secrete them again into bile for ongoing enterohepatic circulation. Uptake of bile salts into hepatocytes occurs largely in a sodium-dependent manner by the sodium taurocholate cotransporting polypeptide NTCP. The transport properties of NTCP have been extensively characterized. It is an electrogenic member of the solute carrier family of transporters (SLC10A1) and transports predominantly bile salts and sulfated compounds, but is also able to mediate transport of additional substrates, such as thyroid hormones, drugs and toxins. It is highly regulated under physiologic and pathophysiologic conditions. Regulation of NTCP copes with changes of bile salt load to hepatocytes and prevents entry of cytotoxic bile salts during liver disease. Canalicular export of bile salts is mediated by the ATP-binding cassette transporter bile salt export pump BSEP (ABCB11). BSEP constitutes the rate limiting step of hepatocellular bile salt transport and drives enterohepatic circulation of bile salts. It is extensively regulated to keep intracellular bile salt levels low under normal and pathophysiologic situations. Mutations in the BSEP gene lead to severe progressive familial intrahepatic cholestasis. The substrates of BSEP are practically restricted to bile salts and their metabolites. It is, however, subject to inhibition by endogenous metabolites or by drugs. A sustained inhibition will lead to acquired cholestasis, which can end in liver injury.

Role of the bile salt export pump, BSEP, in acquired forms of cholestasis

Generation of bile is a key function of the liver. Its impairment leads to accumulation of cytotoxic bile salts in hepatocytes and, consequently, to liver disease. The bile salt export pump, BSEP, is critically involved in the secretion of bile salts into bile. Its function can be disturbed or abolished by inherited mutations. This will lead to progressive intrahepatic cholestais and severe liver disease. In addition to mutations, BSEP can be inhibited by acquired factors, such as xenobiotics or drugs, aberrant bile salt metabolites, or pregnancy. This inhibition will lead to acquired cholestasis. Some drugs are now known to be competitive inhibitors of Bsep. In addition, a polymorphism in the gene coding for BSEP has been identified as a potential susceptibility factor for acquired cholestasis.

The bile salt export pump: clinical and experimental aspects of genetic and acquired cholestatic liver disease

The primary transporter responsible for bile salt secretion is the bile salt export pump (BSEP, ABCB11), a member of the ATP-binding cassette (ABC) superfamily, which is located at the bile canalicular apical domain of hepatocytes. In humans, BSEP deficiency results in several different genetic forms of cholestasis, which include progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2), as well as other acquired forms of cholestasis such as drug-induced cholestasis (DIC) and intrahepatic cholestasis of pregnancy (ICP). Because bile salts play a pivotal role in a wide range of physiologic and pathophysiologic processes, regulation of BSEP expression has been a subject of intense research. The authors briefly describe the molecular characteristics of BSEP and then summarize what is known about its role in the pathogenesis of genetic and acquired cholestatic disorders, emphasizing experimental observations from animal models and cell culture in vitro systems.

Polymorphic variants in the human bile salt export pump (BSEP; ABCB11): functional characterization and interindividual variability

OBJECTIVES

Our aims were to identify and functionally characterize coding region nonsynonymous single nucleotide polymorphisms in the hepatic efflux transporter, bile salt export pump (BSEP; ABCB11), and to assess interindividual variability in BSEP expression.

METHODS

We identified 24 single nucleotide polymorphisms, including nine nonsynonymous variants, in ABCB11 from genomic DNA of approximately 250 ethnically diverse healthy individuals using denaturing high-performance liquid chromatography analysis and DNA sequencing. Wild type and variant BSEP were generated and functionally characterized for taurocholate transport activity in vitro in HeLa cells using a recombinant vaccinia-based method. BSEP expression was assessed by real-time mRNA analysis, western blot analysis, and immunofluorescence confocal microscopy.

RESULTS

For the most part, polymorphisms were rare and ethnic-dependent. In vitro functional studies revealed several rare variants, including 616A>G, 1674G>C, 1772A>G, and 3556G>A, to be associated with significantly impaired taurocholate transport activity while the 890A>G variant trended towards impaired function but was not statistically significant. The 3556G>A variant was associated with reduced cell surface to total protein expression compared with wild-type BSEP. Expression of BSEP by mRNA and protein analysis was determined from a bank of human liver samples. Wide interindividual variability was noted in both mRNA (19-fold) and protein (31-fold) expression levels. The common variant 1331T>C was associated with significantly reduced hepatic BSEP mRNA levels.

CONCLUSION

Accordingly, our study indicates there are functionally relevant polymorphisms in ABCB11 which may be of potential relevance in the predisposition to acquired liver disorders such as drug-induced cholestasis.

[Cholestasis and cholestatic liver diseases]

The main determinant of bile formation is an osmotic filtration process resulting from active transport of bile acids and other osmotic solutes (glutathion). Most of the membrane transporters ensuring bile formation have now been identified. The expression of these membrane transporters is regulated through transcriptional and post-traductional mechanisms. Transcriptional regulation is under the control of nuclear receptors activated by ligands such as bile acids, which act as endogenous steroids synthesized from cholesterol in hepatocytes. Cholestatic liver diseases comprise genetic diseases resulting from the complex interaction between genetic and environmental factors. Monogenic cholestatic diseases recently identified illustrate the key role of membrane transporters in biliary function. Bile acids and inflammatory mediators are potent modulators of transporters and nuclear receptor genes and thus trigger an adaptative response to cholestasis. The extent of this adaptative response could explain the compelling phenotypic variability of cholestatic diseases in childhood and adults. The first-line medical treatment is currently ursodeoxycholic acid and in case of failure of this medical treatment, liver transplantation is required. Recent progress in the molecular pathogenesis of bile formation and cholestatic liver diseases is expected to provide the design of drugs targeted to the molecular abnormalities typical of cholestatic diseases.

Involvement of Mrp2/MRP2 in the species different excretion route of benzylpenicillin between rat and human

1. The purpose of this study was to investigate the involvement of rat Mrp2 and human MRP2 in benzylpenicillin transport using canalicular liver plasma membrane (cLPM) vesicles isolated from Sprague-Dawley or Easai hyperbilirubinemic (EHBR) rats, and MDCKII cells overexpressing MRP2. 2. The adenosine triphosphate (ATP)-dependent uptake of benzylpenicillin and oestradiol-17beta-D-glucuronide (E(2)17betaG), a representative substrate for Mrp2, into EHBR-cLPM vesicles was decreased relative to that seen with control-cLPM vesicles, which may reflect the absence of Mrp2 in the EHBR. The ATP-dependent uptake of taurocholate, which is not a substrate for Mrp2, was similar in both control and EHBR-cLPM vesicles. The concentration dependence of ATP-dependent benzylpenicillin uptake was reflected in a K(m) of 44.0 microM and a V(max) of 508.4 pmol mg(-1) min(-1). Additional inhibition studies using E(2)17betaG and methotrexate as representative substrates for Mrp2/MRP2 demonstrated the involvement of rat Mrp2, but not human MRP2, in benzylpenicillin efflux. Benzylpenicillin appears not to be a substrate for or inhibitor of other human efflux transporters such as MDR1, MRP1, MRP3, or BCRP. 3. In conclusion, rat Mrp2, but not human MRP2, plays an important role in ATP-dependent benzylpenicillin uptake in the bile canalicular membrane, which may explain why biliary excretion of benzylpenicillin is high in the rat but negligible in humans.

Localization of P-gp (Abcb1) and Mrp2 (Abcc2) in freshly isolated rat hepatocytes

Freshly isolated hepatocytes are widely accepted as the "gold standard" for providing reliable data on drug uptake across the sinusoidal (basolateral) membrane. However, the suitability of freshly isolated hepatocytes in suspension to assess efflux by canalicular (apical) proteins or predict biliary excretion in the intact organ is unclear. After collagenase digestion, hepatocytes rapidly lose polarity, but localization of canalicular transport proteins in the first few hours after isolation has not been well characterized. In this study, immunostaining and confocal microscopy have provided, for the first time, a detailed examination of canalicular transport protein localization in freshly isolated rat hepatocytes fixed within 1 h of isolation and in cells cultured for 1 h. Organic anion transporting polypeptide 1a1 (Oatp1a1) was expressed in all hepatocytes and distributed evenly across the basolateral membrane; there was no evidence for colocalization of Oatp1a1 with P-glycoprotein (P-gp) or multidrug resistance-associated protein 2 (Mrp2). In contrast, P-gp and Mrp2 expression was lower than Oatp1a1 and confined to junctions between adjacent cells, intracellular compartments, and "legacy" network structures at or near the cell surface. P-gp and Mrp2 staining was more predominant in regions adjacent to former canalicular spaces, identified by zonula occludens-1 staining. Functional analysis of rat hepatocytes cultured for 1 h demonstrated that the fluorescent anion and Mrp2 substrate, 5-(and-6)-carboxy-2',7'-dichlorofluorescein (CDF), accumulated in cellular compartments; compartmental accumulation of CDF was sensitive to (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl]-[[3-dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK571, Mrp inhibitor) and was not observed in hepatocytes isolated from Mrp2-deficient rats. Drug efflux from freshly isolated hepatocytes as an estimate of apical efflux/biliary excretion would give an inaccurate assessment of true apical elimination and, as such, should not be used to make in vivo extrapolations.

Exploitation of bile acid transport systems in prodrug design

The enterohepatic circulation of bile acids is one of the most efficient recycling routes in the human body. It is a complex process involving numerous transport proteins, which serve to transport bile acids from the small intestine into portal circulation, from the portal circulation into the hepatocyte, from the hepatocyte into the bile, and from the gall bladder to the small intestine. The tremendous transport capacity and organ specificity of enterohepatic circulation combined with versatile derivatization possibilities, rigid steroidal backbone, enantiomeric purity, availability, and low cost have made bile acids attractive tools in designing pharmacological hybrid molecules and prodrugs with the view of improving intestinal absorption, increasing the metabolic stability of pharmaceuticals, specifically targeting drugs to organs involved in enterohepatic circulation, as well as sustaining therapeutically reasonable systemic concentrations of active agents. This article briefly describes bile acid transport proteins involved in enterohepatic circulation, summarizes the key factors affecting on the transport by these proteins, and reviews the use of bile acids and their derivatives in designing prodrugs capable of exploiting the bile acid transport system.

Erythrocytes with T-state-stabilized hemoglobin as a therapeutic tool for postischemic liver dysfunction

This study aimed to examine if T-state stabilization of hemoglobin in erythrocytes could protect against postischemic organ injury. Human erythrocytes containing three different states of Hb allostery were prepared: control Hb (hRBC), CO-Hb that is stabilized under R-state with the 6-coordinated prosthetic heme (CO-hRBC), and alpha-NO-deoxyHb stabilized under T-state (alpha-NO-hRBC). To prepare alpha-NO-RBC, deoxygenated RBC was treated with FK409, a thiol-free NO donor, at its half molar concentration to that of Hb; this procedure resulted in the 5-coordinated NO binding on the alpha-subunit heme, as judged by electron spin resonance spectrometry. Rats were subject to 20 min systemic hemorrhage to maintain mean arterial pressure at 40 mm Hg, and reperfused with one of hRBCs. This protocol for ischemia, followed by 60 min reperfusion with physiological saline, caused modest metabolic acidosis and cholestasis. Administration of hRBC or COhRBC significantly attenuated cholestasis and improved acidosis. Rats treated with alpha-NO-hRBC exhibited greater recovery of metabolic acidosis and bile excretion than those treated with hRBC or CO-hRBC, displaying the best outcome of local oxygen utilization in hepatic lobules. Half-life time of alpha-NO-RBC administered in vivo was approximately 60 min. These results suggest that T-state Hb stabilization by NO serves as a stratagem to treat postischemic organ dysfunction.

Bile salt excretory pump: biology and pathobiology

Bile acids are the major determinant and driving force for the generation of bile flow. Bile acid transport across the canalicular membrane is primarily an ATP-dependent process. The predominant transporter is the bile salt excretory pump (BSEP, ABCB11), a member of the adenosine triphosphate-binding cassette (ABC) family of transporters. Regulatory mechanisms that can coordinate the genes encoding bile acid transport proteins are critically important to avoid hepatocyte damage from intracellar accumulation of bile acids. Bile salts are natural ligands for several nuclear hormone receptors expressed in liver and intestine. Nuclear receptors are transcription factors that bind specific ligands such as bile acids and regulate gene expression according to the metabolic requirements of the cell. In cloning of the BSEP gene, we found a binding site in the promoter for the farnesoid X receptor (FXR), a nuclear receptor for bile acids. FXR activity requires heterodimerization with the 9-cis retinoid receptor (RXR alpha), and when bound by bile acids and retinoic acid, the complex effectively activates the transcription of BSEP. There is a growing body of evidence for the activation of nuclear hormone receptors through the remodeling of chromatin by histone modification involving acetylation, in concert with methylation of H3 and H4 histones. We have recently demonstrated a role for the coactivator-associated arginine methyltransferase 1 (CARM1), as a coactivator of the FXR/RXR receptor and regulator of FXR responsive genes such as BSEP. Chromatin immunoprecipitation showed that the bile acid-dependent activation of the human BSEP is associated with a simultaneous increase of FXR and CARM1 occupation of the BSEP promoter. The increased occupation of the BSEP locus by CARM1 also corresponds with the increased deposition of Arg-17 methylation and Lys-9 acetylation of histone H3 within the FXR DNA-binding element of BSEP. Our work on the role of nuclear receptors in regulation of bile acid homeostasis has led to an increased understanding of the pathogenesis of the disorder, progressive familial intrahepatic cholestasis, type 1 (PFIC1) or Byler disease. The gene mutated in PFIC1 is called FIC1 and codes for a type IV P-type ATPase whose function is unknown. Increased ileal apical sodium-dependent bile acid transporter messenger RNA (mRNA) expression was detected in 3 patients with PFIC1. Ileal FXR and short heterodimer partner (an inhibitory nuclear receptor) messenger RNA levels were reduced in the same 3 patients. In studies of cells after antisense-mediated knock-down of endogenous FIC1, the activity of the ileal apical bile acid transporter promoter was enhanced, whereas the activities of the human FXR and BSEP promoters were reduced. Nuclear but not cytoplasmic localization of FXR is markedly decreased in FIC1-negative cells, indicating that FIC1 is necessary for posttranslational modifications necessary for the nuclear translocation of FXR. This defect leads to enhanced ileal bile salt uptake and impaired canalicular bile salt secretion by BSEP. In PFIC1, an increased load of bile acids is retained in the liver leading to cholestasis and progressive liver injury.

The farnesoid X receptor: a novel drug target?

Bile acids are end products of cholesterol metabolism. They are exclusively synthesised by the liver and subsequently secreted via the bile duct into the intestine to facilitate the absorption of dietary fat and fat-soluble vitamins. Nuclear receptors are ligand-activated transcription factors. The farnesoid X receptor (FXR) has recently been identified as a bile acid-activated nuclear receptor. FXR controls bile-acid synthesis, conjugation and transport, as well as lipid metabolism. Recent advances in FXR biology demonstrate that FXR may represent a valuable target for the identification of novel drugs to treat dyslipidaemia and cholestasis. However, for therapeutic purposes the development of selective FXR modulators, which only activate or inhibit specific FXR target genes and as such induce specific responses, will be required.

The bile salt export pump: molecular properties, function and regulation

Secretion of bile salts from the hepatocyte into bile is the major driving force for the generation of bile flow. Identification of the bile salt export pump (BSEP, ABCB11) as the main adenosine-triphosphate-dependent bile salt transporter in mammalian liver has led to a greater understanding of the biliary bile salt secretory process and its regulation. The biology and pathobiology of BSEP have been the subject of many recent studies. Thus, it has been recognized that while mutations in the gene encoding BSEP are responsible for a subgroup of progressive familial cholestasis (progressive familial intrahepatic cholestasis subtype 2), a pediatric cholestatic disorder that may progress to cirrhosis, defective expression or function of BSEP may underlie some forms of drug-induced cholestasis. The present review summarizes recent data on the molecular properties and regulation of BSEP, as well as the clinical implications of absent or defective function of this hepatic efflux pump.

A progressive familial intrahepatic cholestasis type 2 mutation causes an unstable, temperature-sensitive bile salt export pump

BACKGROUND/AIMS

Progressive familial intrahepatic cholestasis type 2 (PFIC-2) patients have a defect in the hepatocanalicular bile salt secretion. The disease is caused by mutations in the bile salt export pump (BSEP). Ten different missense mutations have been described. In this study, we analysed the effect of the D482G PFIC-2 mutation on BSEP function.

METHODS

Adenosine triphosphatase (ATPase) and taurocholate transport assays were performed with full-length mouse Bsep (mBsep) with and without the D482G mutation. The effect on expression and subcellular sorting was studied in HepG2 cells, stably expressing enhanced green fluorescent protein (EGFP)-tagged mBsep proteins.

RESULTS

The D482G mutation did not significantly affect the taurocholate transport activity of mBsep, even though the bile salt-inducible ATPase activity of the mutant protein was slightly reduced. Protein expression and canalicular sorting were strongly affected by the D482G mutation. Mutant EGFP-mBsep protein was only partly glycosylated and detected in both the canalicular membrane and the cytoplasm. At 30 degrees C, the mutant mRNA and protein levels were strongly increased, and the protein was predominantly glycosylated and efficiently targeted to the canalicular membrane.

CONCLUSIONS

These data suggest that PFIC-2 patients with the D482G mutation express a functional, but highly unstable, temperature-sensitive bile salt export pump.

Mechanisms by which cAMP increases bile acid secretion in rat liver and canalicular membrane vesicles

Bile acid secretion induced by cAMP and taurocholate is associated with recruitment of several ATP binding cassette (ABC) transporters to the canalicular membrane. Taurocholate-mediated bile acid secretion and recruitment of ABC transporters are phosphatidylinositol 3-kinase (PI3K) dependent and require an intact microtubular apparatus. We examined mechanisms involved in cAMP-mediated bile acid secretion. Bile acid secretion induced by perfusion of rat liver with dibutyryl cAMP was blocked by colchicine and wortmannin, a PI3K inhibitor. Canalicular membrane vesicles isolated from cAMP-treated rats manifested increased ATP-dependent transport of taurocholate and PI3K activity that were reduced by prior in vivo administration of colchicine or wortmannin. Addition of a PI3K lipid product, phosphoinositide 3,4-bisphosphate, but not its isomer, phosphoinositide 4,5-bisphosphate, restored ATP-dependent taurocholate in these vesicles. Addition of a decapeptide that activates PI3K to canalicular membrane vesicles increased ATP-dependent transport above baseline activity. In contrast to effects induced by taurocholate, cAMP-stimulated intracellular trafficking of the canalicular ABC transporters was unaffected by wortmannin, and recruitment of multidrug resistance protein 2, but not bile salt excretory protein (bsep), was partially decreased by colchicine. These studies indicate that trafficking of bsep and other canalicular ABC transporters to the canalicular membrane in response to cAMP is independent of PI3K activity. In addition, PI3K lipid products are required for activation of bsep in the canalicular membrane. These observations prompt revision of current concepts regarding the role of cAMP and PI3K in intracellular trafficking, regulation of canalicular bsep, and bile acid secretion.

Prevention by dietary (n-6) polyunsaturated phosphatidylcholines of intrahepatic cholestasis induced by cyclosporine A in animals

Previous findings showed that dietary (n-6) polyunsaturated phosphatidylcholines (vegetable lecithin) could efficiently prevent intrahepatic cholestasis induced by cyclosporine A in rats. Mechanistic studies showed that expressions in rat liver of Na(+), K(+)-ATPase, Ca(2+), Mg(2+)-ATPase and F-actin were both decreased by drug administration and both enhanced by (n-6) lecithin enriched diet. There is a possible direct effect of phosphatidylcholines, vectors of polyunsaturated fatty acids provided by the metabolism of the dietary lecithin, on the aforesaid hepatic parameters. Such modulations by drug and diet result in reversed modifications of membrane composition and fluidity. Final outcome is decreased and enhanced bile lipid secretion by cyclosporine and vegetable lecithin enriched diet respectively. Moreover, we advance the hypothesis of a bypass process including a separate and functional actin-independent way for the non micellar and phospholipid-dependent secretion of bile lipids. The relationships between the ATPases, the microfilament components such as F-actin and the different transporters still remain to be clarified. Furthermore, one can speculate on beneficial effects in humans of diets enriched in vegetable lecithins that might prevent cholestasis induced by cyclosporine A.

Cholestasis

In contrast with urine formation, bile flow is not dependent on hydrostatic forces, but driven by osmotic pressure of solutes secreted across the apical membrane of hepatocytes and bile duct epithelial cells. This secretory process is mediated by a set of primary active transporters that use ATP hydrolysis to pump solutes against the concentration gradient. The most important solutes in bile are bile salts, lipids, electrolytes, and organic anions. The direct consequence of the osmotic mechanism of bile formation is that impaired function of these pumps leads to impaired bile flow-that is, cholestasis. The function of these pumps is highlighted by a number of inherited cholestatic diseases, which are caused by mutations in these genes. Identification of the molecular defect in these diseases was not only important for diagnostic reasons but also emphasised that impaired transporter function has pathological consequences. Indeed, it is now becoming clear that impaired or downregulated transporter function is also involved in the pathogenesis of acquired cholestatic syndromes.

Appearance of atypical 3 alpha,6 beta,7 beta,12 alpha-tetrahydroxy-5 beta-cholan-24-oic acid in spgp knockout mice

Bile formation and its canalicular secretion are essential functions of the mammalian liver. The sister-of-p-glycoprotein (spgp) gene was shown to encode the canalicular bile salt export protein, and mutations in spgp gene were identified as the cause of progressive familial intrahepatic cholestasis type 2. However, target inactivation of spgp gene in mice results in nonprogressive but persistent cholestasis and causes the secretion of unexpectedly large amounts of unknown tetrahydroxylated bile acid in the bile. The present study confirms the identity of this tetrahydroxylated bile acid as 3 alpha,6 beta,7 beta,12 alpha-tetrahydroxy-5 beta-cholan-24-oic acid. The data further show that in serum, liver, and urine of the spgp knockout mice, there is a significant increase in the concentration of total bile salts containing a large amount of tetrahydroxy-5 beta-cholan-24-oic acid. The increase in total bile acids was associated with up-regulation of the mRNA of cholesterol 7 alpha-hydroxylase in male mice only. It is suggested that the lower severity of the cholestasis in the spgp knockout mice may be due to the synthesis of 3 alpha,6 beta,7 beta,12 alpha-tetrahydroxy-5 beta-cholan-24-oic acid, which neutralizes in part the toxic effect of bile acids accumulated in the liver.

Bile acid regulation of hepatic physiology: I. Hepatocyte transport of bile acids

Bile acids are cholesterol derivatives that serve as detergents in bile and the small intestine. Approximately 95% of bile acids secreted by hepatocytes into bile are absorbed from the distal ileum into the portal venous system. Extraction from the portal circulation by the hepatocyte followed by reexcretion into the bile canaliculus completes the enterohepatic circulation of these compounds. Over the past few years, candidate bile acid transport proteins of the sinusoidal and canalicular plasma membranes of the hepatocyte have been identified. The physiology of hepatocyte bile acid transport and its relationship to these transport proteins is the subject of this Themes article.

The human bile salt export pump: characterization of substrate specificity and identification of inhibitors

BACKGROUND & AIMS

The bile salt export pump (BSEP) is the major bile salt transporter in the liver canalicular membrane. Our aim was to determine the affinity of the human BSEP for bile salts and identify inhibitors.

METHODS

Human BSEP was expressed in insect cells. Adenosine triphosphatase (ATPase) assays were performed, and bile salt transport studies were undertaken.

RESULTS

The BSEP gene, ABCB11, was cloned and a recombinant baculovirus was generated. Infected insect cells expressed a 140-kilodalton protein that was absent in uninfected and in mock-infected cells. An ATPase assay showed BSEP to have a high basal ATPase activity. Transport assays were used to determine the Michaelis constant for taurocholate as 4.25 micromol/L, with a maximum velocity of 200 pmol x min(-1) x mg(-1) protein. Inhibition constant values for other bile salts were 11 micromol/L for glycocholate, 7 micromol/L for glycochenodeoxycholate, and 28 micromol/L for taurochenodeoxycholate. Cyclosporin A, rifampicin, and glibenclamide were proved to be competitive inhibitors of BSEP taurocholate transport, with inhibition constant values of 9.5 micromol/L, 31 micromol/L, and 27.5 micromol/L, respectively. Progesterone and tamoxifen did not inhibit BSEP.

CONCLUSIONS

The human BSEP is a high-affinity bile salt transporter. The relative affinities for the major bile salts differ from those seen in rodents and reflect the different bile salt pools. BSEP is competitively inhibited by therapeutic drugs. This is a potentially significant mechanism for drug-induced cholestasis.

Trafficking of canalicular ABC transporters in hepatocytes

ATP-binding cassette (ABC) transporters located in the hepatocyte canalicular membrane of mammalian liver are critical players in bile formation and detoxification. Although ABC transporters have been well characterized functionally, only recently have several canalicular ABC transporters been cloned and their molecular nature revealed. Subsequently, development of specific antibodies has permitted a detailed investigation of ABC transporter intrahepatic distribution under varying physiological conditions. It is now apparent that there is a complex array of ABC transporters in hepatocytes. ABC transporter molecules reside in intrahepatic compartments and are delivered to the canalicular domain following increased physiological demand to secrete bile. Insufficient amounts of ABC transporters in the bile canalicular membrane result in cholestasis (i.e., bile secretory failure). Therefore, elucidation of the intrahepatic pathways and regulation of ABC transporters may help to understand the cause of cholestasis at a molecular level and provide clues for novel therapies.

Bile salt transporters

Bile salts are the major organic solutes in bile and undergo extensive enterohepatic circulation. Hepatocellular bile salt uptake is mediated predominantly by the Na(+)-taurocholate cotransport proteins Ntcp (rodents) and NTCP (humans) and by the Na(+)-independent organic anion-transporting polypeptides Oatp1, Oatp2, and Oatp4 (rodents) and OATP-C (humans). After diffusion (bound by intracellular bile salt-binding proteins) to the canalicular membrane, monoanionic bile salts are secreted into bile canaliculi by the bile salt export pump Bsep (rodents) or BSEP (humans). Both belong to the ATP-binding cassette (ABC) transporter superfamily. Dianionic conjugated bile salts are secreted into bile by the multidrug-resistance-associated proteins Mrp2/MRP2. In bile ductules, a minor portion of protonated bile acids and monomeric bile salts are reabsorbed by non-ionic diffusion and the apical sodium-dependent bile salt transporter Asbt/ASBT, transported back into the periductular capillary plexus by Mrp3/MRP3 [and/or a truncated form of Asbt (tAsbt)], and subjected to cholehepatic shunting. The major portion of biliary bile salts is aggregated into mixed micelles and transported into the intestine, where they are reabsorbed by apical Oatp3, the apical sodium-dependent bile salt transporter (ASBT), cytosolic intestinal bile acid-binding protein (IBABP), and basolateral Mrp3/MRP3 and tAsbt. Transcriptional and posttranscriptional regulation of these enterohepatic bile salt transporters is closely related to the regulation of lipid and cholesterol homeostasis. Furthermore, defective expression and function of bile salt transporters have been recognized as important causes for various cholestatic liver diseases.

Genetic defects in hepatobiliary transport

Bile formation, the exocrine function of the liver, represents a process that is unique to the hepatocyte as a polarized epithelial cell. The generation of bile flow is an osmotic process and largely depends on solute secretion by primary active transporters in the apical membrane of the hepatocyte. In recent years an impressive progress has been made in the discovery of these proteins, most of which belong to the family of ABC transporters. The number of identified ABC transporter genes has been exponentially increasing and the mammalian subfamily now counts at least 52. This development has been of crucial importance for the elucidation of the mechanism of bile formation, and it is therefore not surprising that the development in this field has run in parallel with the discovery of the ABC genes. With the identification of these transporter genes, the background of a number of inherited diseases, which are caused by mutations in these solute pumps, has now been elucidated. We now know that at least six primary active transporters are involved in canalicular secretion of biliary components (MDR1, MDR3, BSEP, MRP2, BCRP and FIC1). Four of these transporter genes are associated with inherited diseases. In this minireview we will shortly describe our present understanding of bile formation and the associated inherited defects.

Human bile salt export pump promoter is transactivated by the farnesoid X receptor/bile acid receptor

The bile salt excretory pump (BSEP, ABCb11) is critical for ATP-dependent transport of bile acids across the hepatocyte canalicular membrane and for generation of bile acid-dependent bile secretion. Recent studies have demonstrated that the expression of this transporter is sensitive to the flux of bile acids through the hepatocyte, possibly at the level of transcription of the BSEP gene. To determine the mechanisms underlying the regulation of BSEP by bile acids, the promoter of the BSEP gene was cloned. The sequence of the promoter contained an inverted repeat (IR)-1 element (5'-GGGACA T TGATCCT-3') at base pairs -63/-50 consisting of two nuclear receptor half-sites organized as an inverted repeat and separated by a single nucleotide. This IR-1 element has been shown in several recent studies to serve as a binding site for the farnesoid X receptor (FXR), a nuclear receptor for bile acids. FXR activity requires heterodimerization with RXR alpha, and when bound by bile acids, the complex effectively regulates the transcription of several genes involved in bile acid homeostasis. Gel mobility shift assays demonstrated specific binding of FXR/RXR alpha heterodimers to the IR-1 element in the BSEP promoter. In HepG2 cells, co-transfection of FXR and RXR alpha is required to attain full transactivation of the BSEP promoter by bile acids. Two FXR transactivation-deficient mutants (an AF-2 deletion and a W469A point mutant) failed to transactivate, indicating that the effect of bile acids is FXR-dependent. Further, mutational analysis confirms that the FXR/RXR alpha heterodimer activates transcription through the IR-1 site in the human BSEP promoter. These results demonstrate a mechanism by which bile acids transcriptionally regulate the activity of the bile salt excretory pump, a critical component involved in the enterohepatic circulation of bile acids.

Transport of bile acids in hepatic and non-hepatic tissues

Bile acids are steroidal amphipathic molecules derived from the catabolism of cholesterol. They modulate bile flow and lipid secretion, are essential for the absorption of dietary fats and vitamins, and have been implicated in the regulation of all the key enzymes involved in cholesterol homeostasis. Bile acids recirculate through the liver, bile ducts, small intestine and portal vein to form an enterohepatic circuit. They exist as anions at physiological pH and, consequently, require a carrier for transport across the membranes of the enterohepatic tissues. Individual bile acid carriers have now been cloned from several species. Na(+)-dependent transporters that mediate uptake into hepatocytes and reabsorption from the intestine and biliary epithelium and an ATP-dependent transporter that pumps bile acids into bile comprise the classes of transporter that are specific for bile acids. In addition, at least four human and five rat genes that code for Na(+)-independent organic anion carriers with broad multi-substrate specificities that include bile acids have been discovered. Studies concerning the regulation of these carriers have permitted identification of molecular signals that dictate eventual changes in the uptake or excretion of bile acids, which in turn have profound physiological implications. This overview summarizes and compares all known bile acid transporters and highlights findings that have identified diseases linked to molecular defects in these carriers. Recent advances that have fostered a more complete appreciation for the elaborate disposition of bile acids in humans are emphasized.

Transporters on demand: intrahepatic pools of canalicular ATP binding cassette transporters in rat liver

ABC transporter trafficking in rat liver induced by cAMP or taurocholate and [(35)S]methionine metabolic labeling followed by subcellular fractionation were used to identify and characterize intrahepatic pools of ABC transporters. ABC transporter trafficking induced by cAMP or taurocholate is a physiologic response to a temporal demand for increased bile secretion. Administration of cAMP or taurocholate to rats increased amounts of SPGP, MDR1, and MDR2 in the bile canalicular membrane by 3-fold; these effects abated after 6 h and were insensitive to prior treatment of rats with cycloheximide. Half-lives of ABC transporters were 5 days, which suggests cycling of ABC transporters between canalicular membrane and intrahepatic sites before degradation. In vivo [(35)S]methionine labeling of rats followed by immunoprecipitation of (sister of P-glycoprotein) (SPGP) from subcellular liver fractions revealed a steady state distribution after 20 h of SPGP between canalicular membrane and a combined endosomal fraction. After mobilization of transporters from intrahepatic sites with cAMP or taurocholate, a significant increase in the amount of ABC transporters in canalicular membrane vesicles was observed, whereas the decrease in the combined endosomal fraction remained below detection limits in Western blots. This observation is in accordance with relatively large intracellular ABC transporter pools compared with the amount present in the bile canalicular membrane. Furthermore, trafficking of newly synthesized SPGP through intrahepatic sites was accelerated by additional administration of cAMP but not by taurocholate, indicating two distinct intrahepatic pools. Our data indicate that ABC transporters cycle between the bile canaliculus and at least two large intrahepatic ABC transporter pools, one of which is mobilized to the canalicular membrane by cAMP and the other, by taurocholate. In parallel to regulation of other membrane transporters, we propose that the "cAMP-pool" in hepatocytes corresponds to a recycling endosome, whereas recruitment from the "taurocholate-pool" involves a hepatocyte-specific mechanism.

Use of ursodeoxycholic acid in liver diseases

Ursodeoxycholic acid is currently the only established drug for the treatment of chronic cholestatic liver diseases. It has cytoprotective, anti-apoptotic, membrane stabilizing, anti-oxidative and immunomodulatory effects. Prolonged administration of ursodeoxycholic acid in patients with primary biliary cirrhosis (PBC) is associated with survival benefit and a delaying of liver transplantation. There is evidence that it might even prevent progression of the histologic stage of PBC. It also has a beneficial effect on primary sclerosing cholangitis, intrahepatic cholestasis of pregnancy, liver disease associated with cystic fibrosis, chronic graft versus host disease, total parenteral nutrition associated cholestasis and various pediatric cholestatic liver diseases. In the present review the current knowledge about the mechanisms of the action and role of ursodeoxycholic acid in the treatment of various liver diseases has been discussed.

ATP-dependent GSH and glutathione S-conjugate transport in skate liver: role of an Mrp functional homologue

Multidrug resistance-associated proteins 1 and 2 (Mrp1 and Mrp2) are thought to mediate low-affinity ATP-dependent transport of reduced glutathione (GSH), but there is as yet no direct evidence for this hypothesis. The present study examined whether livers from the little skate (Raja erinacea) express an Mrp2 homologue and whether skate liver membrane vesicles exhibit ATP-dependent GSH transport activity. Antibodies directed against mammalian Mrp2-specific epitopes labeled a 180-kDa protein band in skate liver plasma membranes and stained canaliculi by immunofluorescence, indicating that skate livers express a homologous protein. Functional assays of Mrp transport activity were carried out using (3)H-labeled S-dinitrophenyl-glutathione (DNP-SG). DNP-SG was accumulated in skate liver membrane vesicles by both ATP-dependent and ATP-independent mechanisms. ATP-dependent DNP-SG uptake was of relatively high affinity [Michaelis-Menten constant (K(m)) = 32 +/- 9 microM] and was cis-inhibited by known substrates of Mrp2 and by GSH. Interestingly, ATP-dependent transport of (3)H-labeled S-ethylglutathione and (3)H-labeled GSH was also detected in the vesicles. ATP-dependent GSH transport was mediated by a low-affinity pathway (K(m) = 12 +/- 2 mM) that was cis-inhibited by substrates of the Mrp2 transporter but was not affected by membrane potential or pH gradient uncouplers. These results provide the first direct evidence for ATP-dependent transport of GSH in liver membrane vesicles and support the hypothesis that GSH efflux from mammalian cells is mediated by members of the Mrp family of proteins.