Hepatobiliary transport: molecular mechanisms of development and cholestasis

Current Models for Predicting Drug-induced Cholestasis: The Role of Hepatobiliary Transport System

Drug-induced cholestasis is the main type of liver disorder accompanied by high morbidity and mortality. Evidence for the role of hepatobiliary pumps in the cholestasis patho-mechanism is constantly increasing. Recognition of the interactions of chemical agents with these transporters at the initial phases of drug discovery can help develop new drug candidates with low cholestasis potential. This review delivers an outline of the role of these transport proteins in bile creation. It addresses the pathophysiological mechanism for drug-induced cholestasis. In-vitro models, including cell-based and membrane-based approaches and In-vivo models such as genetic knockout animals, are considered. The benefits and restrictions of each model are discussed in this review. Current understandings into the cellular and molecular process that control the activity of hepatobiliary pumps have directed to a better understanding of the pathophysiology of drug-induced cholestasis. A combination of in-vitro monitoring for transport interaction, in-silico predicting systems, and consideration of and metabolic and physicochemical properties must cause more effective monitoring of possible liver problems.

Long-Term Outcome after Liver Transplantation for Progressive Familial Intrahepatic Cholestasis

Background and Objectives: Progressive familial intrahepatic cholestasis (PFIC) is a rare autosomal recessive inherited disease divided into five types (PFIC 1-5). Characteristic for all types is early disease onset, which may result clinically in portal hypertension, fibrosis, cirrhosis, hepatocellular carcinoma (HCC), and extrahepatic manifestations. Liver transplantation (LT) is the only successful treatment approach. Our aim is to present the good long-term outcomes after liver transplantation for PFIC1, focusing on liver function as well as the occurrence of extrahepatic manifestation after liver transplantation. Materials and Methods: A total of seven pediatric patients with PFIC1 underwent liver transplantation between January 1999 and September 2019 at the Department of Surgery, Charité Campus Virchow Klinikum and Charité Campus Mitte of Charité-Universitätsmedizin Berlin. Long-term follow-up data were collected on all patients, specifically considering liver function and extrahepatic manifestations. Results: Seven (3.2%) recipients were found from a cohort of 219 pediatric patients. Two of the seven patients had multilocular HCC in cirrhosis. Disease recurrence or graft loss did not occur in any patient. Two patients (male, siblings) had persistently elevated liver parameters but showed excellent liver function. Patient and graft survival during long-term follow-up was 100%, and no severe extrahepatic manifestations requiring hospitalization or surgery occurred. We noted a low complication rate during long-term follow-up and excellent patient outcome. Conclusions: PFIC1 long-term follow-up after LT shows promising results for this rare disease. In particular, the clinical relevance of extrahepatic manifestations seems acceptable, and graft function seems to be barely affected. Further multicenter studies are needed to analyze the clinically inhomogeneous presentation and to better understand the courses after LT.

Pyrrolizidine Alkaloids Disturb Bile Acid Homeostasis in the Human Hepatoma Cell Line HepaRG

1,2-unsaturated pyrrolizidine alkaloids (PAs) belong to a group of secondary plant metabolites. Exposure to PA-contaminated feed and food may cause severe hepatotoxicity. A pathway possibly involved in PA toxicity is the disturbance of bile acid homeostasis. Therefore, in this study, the influence of four structurally different PAs on bile acid homeostasis was investigated after single (24 h) and repeated (14 days) exposure using the human hepatoma cell line HepaRG. PAs induce a downregulation of gene expression of various hepatobiliary transporters, enzymes involved in bile acid synthesis, and conjugation, as well as several transcription regulators in HepaRG cells. This repression may lead to a progressive impairment of bile acid homeostasis, having the potential to accumulate toxic bile acids. However, a significant intracellular and extracellular decrease in bile acids was determined, pointing to an overall inhibition of bile acid synthesis and transport. In summary, our data clearly show that PAs structure-dependently impair bile acid homeostasis and secretion by inhibiting the expression of relevant genes involved in bile acid homeostasis. Furthermore, important biliary efflux mechanisms seem to be disturbed due to PA exposure. These mole-cular mechanisms may play an important role in the development of severe liver damage in PA-intoxicated humans.

Blood-Bile Barrier: Morphology, Regulation, and Pathophysiology

The term blood-bile barrier (BBlB) refers to the physical structure within a hepatic lobule that compartmentalizes and hence segregates sinusoidal blood from canalicular bile. Thus, this barrier provides physiological protection in the liver, shielding the hepatocytes from bile toxicity and restricting the mixing of blood and bile. BBlB is primarily composed of tight junctions; however, adherens junction, desmosomes, gap junctions, and hepatocyte bile transporters also contribute to the barrier function of the BBlB. Recent findings also suggest that disruption of BBlB is associated with major hepatic diseases characterized by cholestasis and aberrations in BBlB thus may be a hallmark of many chronic liver diseases. Several molecular signaling pathways have now been shown to play a role in regulating the structure and function and eventually contribute to regulation of the BBlB function within the liver. In this review, we will discuss the structure and function of the BBlB, summarize the methods to assess the integrity and function of BBlB, discuss the role of BBlB in liver pathophysiology, and finally, discuss the mechanisms of BBlB regulation. Collectively, this review will demonstrate the significance of the BBlB in both liver homeostasis and hepatic dysfunction.

Bile Metabolism and Lithogenesis: An Update

Bile is composed of multiple macromolecules, including bile acids, free cholesterol, phospholipids, bilirubin, and inorganic ions that aid in digestion, nutrient absorption, and disposal of the insoluble products of heme catabolism. The synthesis and release of bile acids is tightly controlled and dependent on feedback mechanisms that regulate enterohepatic circulation. Alterations in bile composition, impaired gallbladder relaxation, and accelerated nucleation are the principal mechanisms leading to biliary stone formation. Various physiologic conditions and disease states alter bile composition and metabolism, thus increasing the risk of developing gallstones.

Dysregulated Bile Transporters and Impaired Tight Junctions During Chronic Liver Injury in Mice

BACKGROUND & AIMS

Liver fibrosis, hepatocellular necrosis, inflammation, and proliferation of liver progenitor cells are features of chronic liver injury. Mouse models have been used to study the end-stage pathophysiology of chronic liver injury. However, little is known about differences in the mechanisms of liver injury among different mouse models because of our inability to visualize the progression of liver injury in vivo in mice. We developed a method to visualize bile transport and blood-bile barrier (BBlB) integrity in live mice.

METHODS

C57BL/6 mice were fed a choline-deficient, ethionine-supplemented (CDE) diet or a diet containing 0.1% 3,5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC) for up to 4 weeks to induce chronic liver injury. We used quantitative liver intravital microscopy (qLIM) for real-time assessment of bile transport and BBlB integrity in the intact livers of the live mice fed the CDE, DDC, or chow (control) diets. Liver tissues were collected from mice and analyzed by histology, immunohistochemistry, real-time polymerase chain reaction, and immunoblots.

RESULTS

Mice with liver injury induced by a CDE or a DDC diet had breaches in the BBlB and impaired bile secretion, observed by qLIM compared with control mice. Impaired bile secretion was associated with reduced expression of several tight-junction proteins (claudins 3, 5, and 7) and bile transporters (NTCP, OATP1, BSEP, ABCG5, and ABCG8). A prolonged (2-week) CDE, but not DDC, diet led to re-expression of tight junction proteins and bile transporters, concomitant with the reestablishment of BBlB integrity and bile secretion.

CONCLUSIONS

We used qLIM to study chronic liver injury, induced by a choline-deficient or DDC diet, in mice. Progression of chronic liver injury was accompanied by loss of bile transporters and tight junction proteins.

Physiological and molecular biochemical mechanisms of bile formation

This review considers the physiological and molecular biochemical mechanisms of bile formation. The composition of bile and structure of a bile canaliculus, biosynthesis and conjugation of bile acids, bile phospholipids, formation of bile micellar structures, and enterohepatic circulation of bile acids are described. In general, the review focuses on the molecular physiology of the transporting systems of the hepatocyte sinusoidal and apical membranes. Knowledge of physiological and biochemical basis of bile formation has implications for understanding the mechanisms of development of pathological processes, associated with diseases of the liver and biliary tract.

P4 ATPases - lipid flippases and their role in disease

P4 ATPases (type 4 P-type ATPases) are multispan transmembrane proteins that have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Studies in Saccharomyces cerevisiae have indicated that P4 ATPases are important in vesicle biogenesis and are required for vesicular trafficking along several intracellular vesicular transport routes. Although little is known about mammalian P4 ATPases, some members of this subfamily appear to be associated with human disease or mouse pathophysiology. ATP8B1, a phosphatidylserine translocase, is the most extensively studied mammalian P4 ATPase. This protein is important for maintaining the detergent resistant properties of the apical membrane of the hepatocyte. Mutations in ATP8B1 give rise to severe liver disease. Furthermore, a role for Atp8b3 in mouse sperm cell capacitation has been suggested, whereas deficiency of Atp10a and Atp10d leads to insulin resistance and obesity in mice. Here we review the present status on the pathophysiological consequences of P4 ATPase deficiency.

Resistance of young rat hepatic mitochondria to bile acid-induced permeability transition: potential role of alpha-tocopherol

Retention of bile acids within the liver is a primary factor in the pathogenesis of cholestatic liver disorders, which are more common in human infants. The objective of this study was to evaluate developmental changes in mitochondrial factors involved in bile acid-induced hepatocyte injury. Hepatic mitochondria from adult rats (aged 9 wk) underwent a mitochondrial permeability transition (MPT) and release of cytochrome c upon exposure to glycochenodeoxycholic acid. In contrast, mitochondria from young rats (age 6-36 d) were resistant to MPT induction and cytochrome c release. Neither mitochondrial levels of MPT-associated proteins (voltage-dependent anion channel, cyclophilin D, or adenine nucleotide translocase), Bcl-2 family proteins, nor antioxidant enzymes explained this resistance. Mitochondria from young rats contained 2- to 3-fold higher alpha-tocopherol (alpha-TH). In vivo alpha-TH enrichment of adult hepatic mitochondria increased their MPT resistance. Tetra-linoleoyl cardiolipin (TL-CL), the primary molecular species of CL, was reduced in mitochondria of the young rat; however, enrichment with CL and TL-CL only modestly increased their MPT susceptibility. In conclusion, we observed an unexpected resistance in young rats to bile acid induction of mitochondrial cell death pathways, which may be related to developmental differences in membrane composition.

Physiology of bile secretion

The formation of bile depends on the structural and functional integrity of the bile-secretory apparatus and its impairment, in different situations, results in the syndrome of cholestasis. The structural bases that permit bile secretion as well as various aspects related with its composition and flow rate in physiological conditions will first be reviewed. Canalicular bile is produced by polarized hepatocytes that hold transporters in their basolateral (sinusoidal) and apical (canalicular) plasma membrane. This review summarizes recent data on the molecular determinants of this primary bile formation. The major function of the biliary tree is modification of canalicular bile by secretory and reabsorptive processes in bile-duct epithelial cells (cholangiocytes) as bile passes through bile ducts. The mechanisms of fluid and solute transport in cholangiocytes will also be discussed. In contrast to hepatocytes where secretion is constant and poorly controlled, cholangiocyte secretion is regulated by hormones and nerves. A short section dedicated to these regulatory mechanisms of bile secretion has been included. The aim of this revision was to set the bases for other reviews in this series that will be devoted to specific issues related with biliary physiology and pathology.

Non invasive in vivo investigation of hepatobiliary structure and function in STII medaka (Oryzias latipes): methodology and applications

Background

A novel transparent stock of medaka (Oryzias latipes; STII), recessive for all pigments found in chromatophores, permits transcutaneous imaging of internal organs and tissues in living individuals. Findings presented describe the development of methodologies for non invasive in vivo investigation in STII medaka, and the successful application of these methodologies to in vivo study of hepatobiliary structure, function, and xenobiotic response, in both 2 and 3 dimensions.

Results

Using brightfield, and widefield and confocal fluorescence microscopy, coupled with the in vivo application of fluorescent probes, structural and functional features of the hepatobiliary system, and xenobiotic induced toxicity, were imaged at the cellular level, with high resolution (< 1 μm), in living individuals. The findings presented demonstrate; (1) phenotypic response to xenobiotic exposure can be investigated/imaged in vivo with high resolution (< 1 μm), (2) hepatobiliary transport of solutes from blood to bile can be qualitatively and quantitatively studied/imaged in vivo, (3) hepatobiliary architecture in this lower vertebrate liver can be studied in 3 dimensions, and (4) non invasive in vivo imaging/description of hepatobiliary development in this model can be investigated.

Conclusion

The non-invasive in vivo methodologies described are a unique means by which to investigate biological structure, function and xenobiotic response with high resolution in STII medaka. In vivo methodologies also provide the future opportunity to integrate molecular mechanisms (e.g., genomic, proteomic) of disease and toxicity with phenotypic changes at the cellular and system levels of biological organization. While our focus has been the hepatobiliary system, other organ systems are equally amenable to in vivo study, and we consider the potential for discovery, within the context of in vivo investigation in STII medaka, as significant.

Molecular pathogenesis of intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy (ICP) occurs mainly in the third trimester and is characterised by pruritus and elevated serum bile acid levels. ICP is associated with an increased perinatal risk and higher rates of foetal morbidity and mortality. Although the pathogenesis of this disease is unknown, a genetic hypersensitivity to female hormones (oestrogen and/or progesterone) or their metabolites is thought to impair bile secretory function. Recent data suggest that mutations or polymorphisms of genes expressing hepatobiliary transport proteins or their nuclear regulators may contribute to the development and/or severity of ICP. Unidentified environmental factors may also influence pathogenesis of the disease. This review summarises current knowledge on the potential mechanisms involved in ICP at the molecular level.

Non invasive high resolution in vivo imaging of alpha-naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity in STII medaka

A novel transparent stock of medaka (Oryzias latipes; STII), homozygous recessive for all four pigments (iridophores, xanthophores, leucophores, melanophores), permits transcutaneous, high resolution (<1 microm) imaging of internal organs and tissues in living individuals. We applied this model to in vivo investigation of alpha -naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity. Distinct phenotypic responses to ANIT involving all aspects of intrahepatic biliary passageways (IHBPs), particularly bile preductular epithelial cells (BPDECs), associated with transitional passageways between canaliculi and bile ductules, were observed. Alterations included: attenuation/dilation of bile canaliculi, bile preductular lesions, hydropic vacuolation of hepatocytes and BPDECs, mild BPDEC hypertrophy, and biliary epithelial cell (BEC) hyperplasia. Ex vivo histological, immunohistochemical, and ultrastructural studies were employed to aid in interpretation of, and verify, in vivo findings. 3D reconstructions from in vivo investigations provided quantitative morphometric and volumetric evaluation of ANIT exposed and untreated livers. The findings presented show for the first time in vivo evaluation of toxicity in the STII medaka hepatobiliary system, and, in conjunction with prior in vivo work characterizing normalcy, advance our comparative understanding of this lower vertebrate hepatobiliary system and its response to toxic insult.

"Let there be bile"--understanding hepatic injury in cholestasis

Cholestatic liver disorders account for a large proportion of chronic liver ailments in adults, children and infants, and are among the leading indications for liver transplantation in all age groups. Recent studies have begun to characterize the cellular and molecular mechanisms of hepatocyte injury caused by the retention of hydrophobic bile acids in cholestasis. Steatocholestasis is the combined presence of hepatic steatosis and cholestasis, common in genetic causes of metabolic liver disease in childhood. Retention of hydrophobic bile acids promotes hepatocellular injury and subsequent portal fibrosis in these conditions. Investigations at the mechanistic level have revealed that activation of hepatocyte death receptors, induction of oxidative stress, mitochondrial perturbations and activation of caspases are intracellular pathways that mediate hepatocyte injury. Several compounds in licorice root have been shown to modulate bile acid-induced apoptosis and necrosis of hepatocytes. Further investigations will be needed to identify novel molecular and cellular targets for which pharmaceuticals might be developed, to reduce liver injury and fibrosis in cholestasis and steatocholestasis.

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.

Hepatobiliary function assessed by 99mTc-mebrofenin cholescintigraphy in the evaluation of severity of steatosis in a rat model

PURPOSE

This study evaluated the utility of non-invasive assessment of hepatobiliary function by 99mTc-mebrofenin cholescintigraphy in a rat model of diet-induced steatosis.

METHODS

Male Wistar rats (250-300 g) were fed a standard methionine- and choline-deficient (MCD) diet for up to 5 weeks, thereby inducing hepatic fat accumulation, progressive inflammation and fibrogenesis corresponding with clinical steatosis. 99mTc-mebrofenin pinhole scintigraphy was used to evaluate the hepatocyte mebrofenin uptake rate, the time of maximum hepatic uptake (T(peak)) and the time required for peak activity to decrease by 50% (T(1/2peak)). Scintigraphic parameters were correlated with biochemical and serological parameters and with liver histopathology.

RESULTS

MCD diet induced mild steatosis after 1 week and severe steatosis with prominent inflammation after 5 weeks. T(peak), T(1/2peak) prolonged and the uptake rate decreased significantly, while the severity of steatosis increased (p<0.05). There was a strong, significant correlation between the severity of steatosis (histopathology, hepatic triglyceride content) and the 99mTc-mebrofenin uptake rate (r2=0.83, p<0.0001 and r2=0.82, p<0.0001, respectively). In addition, the uptake rate correlated significantly with the increased inflammation (plasma and hepatic TNF-alpha, r2=0.72, p<0.0001 and r2=0.52, p=0.001, respectively). The correlation of the uptake rate with hepatocellular damage was weak (AST and ALT, r2=0.29 and 0.32, respectively), but correlation with synthetic function was strong (prothrombin time, r2=0.70, p<0.001).

CONCLUSION

Hepatobiliary function assessed by 99mTc-mebrofenin scintigraphy correlates with the extent and progression of steatosis. These results suggest a potential role for mebrofenin scintigraphy as a non-invasive functional follow-up method for disease progression in steatotic patients.

Multiple mechanisms of ontogenic regulation of nuclear receptors during rat liver development

Nuclear receptors (NRs) play pivotal roles in the regulation of genes contributing to hepatobiliary cholesterol and bile acid homeostasis. We have previously shown that transporters involved in bile formation are developmentally regulated and are poorly developed during the fetal stage, but their expression reached gradual maturity during the postnatal period. To define the molecular mechanisms underlying this regulation and the role that class II NRs and associated members [liver receptor homolog-1 (LRH-1) and short heterodimer partner (SHP)] play, we have analyzed the ontogeny of NR expression during liver development. Real-time PCR analysis of hepatic NR expression from fetal day 17 through adult revealed that steady-state mRNA levels for all NRs were very low during the embryonic period. However, mRNA levels peaked close to that of adult rats (>6 wk-old rats) by 4 wk of age for farnesoid X receptor (FXR), pregnane X receptor (PXR), liver X receptor-alpha (LXRalpha), peroxisome proliferator-activated receptor-alpha (PPARalpha), retinoid acid receptor-alpha (RARalpha), LRH-1, and SHP, whereas RXRalpha mRNA lagged behind. FXR, PXR, LXRalpha, RARalpha, and PPARalpha functional activity in liver nuclear extracts assayed by gel EMSA demonstrated that the activity attained adult levels by 4 wk of age, exhibiting a strict correlation with mRNA levels. Surprisingly, PPARalpha activity was delayed as seen by EMSA assay. Protein levels for NRs also corresponded to the mRNA and functional activity except for RXRalpha. RXRalpha protein levels were higher than message levels, suggesting increased protein stability. We conclude that expression of NRs during rat liver development is primarily regulated by transcriptional mechanisms, which in turn, control the regulation of bile acid and cholesterol metabolic pathways.

Differential expression of bile salt and organic anion transporters in developing rat liver

BACKGROUND/AIMS

Differentiated hepatocytes express distinct transport systems at their basolateral and canalicular membrane domains. Here, we investigated the ontogenesis of the polar expression of hepatocellular organic anion and bile salt transport systems in rat liver.

METHODS

mRNA levels (real time PCR) and protein expression (immunofluorescence microscopy) were investigated for the Na(+)-taurocholate cotransport protein (Ntcp), the organic anion transporting polypeptides (Oatp1a1, Oatp1a4, Oatp1b2), the multidrug resistance associated proteins (Mrp2, Mrp6) and the bile salt export pump (Bsep).

RESULTS

Expression of mRNA and protein was detected first for Oatp1b2, Mrp2 and Mrp6 at embryonic day 16 (E16), followed by Ntcp, Oatp1a1 and Bsep at E20 and by Oatp1a4 at postnatal day 5 (P5). Intracellular localization of Oatps (e.g. Oatp1b2) preceded expression at the plasma membrane. Approximate adult phenotypes of polarized expression were achieved for Ntcp by P5, for Bsep, Mrp2 and Mrp6 by P12 and for Oatp1a1, Oatp1a4 and Oatp1b2 by P29.

CONCLUSIONS

The data demonstrate that full maturation of polarized transporter expression in rat liver requires several weeks. The findings provide a molecular explanation for the previously observed chronology of the functional maturation of bile salt-independent and dependent bile formation and of hepatic detoxification functions in developing rat liver.

Molecular aspects of bile formation and cholestasis

Recent insights into the cellular and molecular mechanisms that control the function and regulation of hepatobiliary transport have led to a greater understanding of the physiological significance of bile secretion. Individual carriers for bile acids and other organic anions in both liver and intestine have now been cloned from several species. In addition, complex networks of signals that regulate key enzymes and membrane transporters located in cells that participate in the metabolism or transport of biliary constituents are being unraveled. This knowledge has major implications for the pathogenesis of cholestatic liver diseases. Here, we review recent information on molecular aspects of hepatobiliary secretory function and its regulation in cholestasis. Potential implications of this knowledge for the design of new therapies of cholestatic disorders are also discussed.

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.

Comparative in Vivo Stability of Copper-64-Labeled Cross-Bridged and Conventional Tetraazamacrocyclic Complexes

The increased use of copper radioisotopes in radiopharmaceutical applications has created a need for bifunctional chelators (BFCs) that form stable radiocopper complexes and allow covalent attachment to biological molecules. The chelators most commonly utilized for labeling copper radionuclides to biomolecules are analogues of 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA); however, recent reports have communicated the instability of the radio-Cu(II)-TETA complexes in vivo. A class of bicyclic tetraazamacrocycles, the ethylene "cross-bridged" cyclam (CB-cyclam) derivatives, form highly kinetically stable complexes with Cu(II) and therefore may be less susceptible to transchelation than their nonbridged analogues in vivo. Herein we report results on the relative biological stabilities and identification of the resulting radiolabeled metabolites of a series of (64)Cu-labeled macrocyclic complexes. Metabolism studies in normal rat liver have revealed that the (64)Cu complex of 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane ((64)Cu-CB-TE2A) resulted in significantly lower values of protein-associated (64)Cu than (64)Cu-TETA [13 +/- 6% vs 75 +/- 9% at 4 h]. A similar trend was observed for the corresponding cyclen derivatives, with the (64)Cu complex of 4,10-bis(carboxymethyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane ((64)Cu-CB-DO2A) undergoing less transchelation than the (64)Cu complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ((64)Cu-DOTA) [61 +/- 14% vs 90.3 +/- 0.5% protein associated (64)Cu at 4 h]. These data indicate that the structurally reinforcing cross-bridge enhances in vivo stability by reducing metal loss to protein in both the cyclam and cyclen cross-bridged (64)Cu complexes and that (64)Cu-CB-TE2A is superior to (64)Cu-CB-DO2A in that regard. These findings further suggest that a bifunctional chelator derivative of CB-TE2A is a highly desirable alternative for labeling copper radionuclides to biological molecules for diagnostic imaging and targeted radiotherapy.

Adaptação do modelo experimental de colestase extra-hepática em ratos jovens

Os autores descrevem a adaptação da técnica de ligadura e ressecção do ducto biliar comum descrita na literatura, enfatizando suas vantagens na recuperação pós-cirúrgica. Este trabalho foi realizado no Laboratório Experimental do Departamento de Pediatria da FMB-UNESP.

Differential developmental regulation of rat liver canalicular membrane transporters Bsep and Mrp2

Bile formation depends on the active secretion of bile salts and other biliary constituents by specific transporters. Recently two major transporters that contribute to bile formation, the bile salt export pump (Bsep) and multidrug resistance protein-2 (Mrp2), have been cloned. The goal of the present study was to define the expression of Bsep and Mrp2 during rat liver development. mRNA expression as assessed by Northern blot and RT-PCR was higher for Mrp2 (40% of adult) at 21 d fetal age relative to Bsep (<20% of adult). The levels of Mrp2 mRNA increased to approximately 50% of adult at 1 d of life and then rapidly increased to adult levels by 1-3 wk. Nuclear run-on assays for Bsep and Mrp2 showed minimal transcription during fetal life with an increase in transcription in the postnatal period. A different pattern of expression was observed for both Mrp2 and Bsep proteins. During fetal life, there was low expression of Mrp2 and Bsep proteins (<20% of adult) with a gradual increase neonatally reaching adult levels at 4 wk. Thus, we noted a temporal delay between the maximal expression of the mRNA (1-3 wk) and protein (4 wk) for Bsep and Mrp2. These results show that 1) expression (of mRNA and protein) of canalicular transporters is developmentally regulated by both transcriptional and posttranscriptional mechanisms and 2) Mrp2 and Bsep gene expression (mRNA) are differentially regulated.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Down-regulation of the Na+/taurocholate cotransporting polypeptide during pregnancy in the rat

BACKGROUND

Experimental studies have shown decreased bile acid (BA) uptake and reduced excretion of cholephilic compounds in pregnant rodents.

AIM

To assess the expression and function of the main BA importer, the Na(+)/taurocholate cotransporting polypeptide (Ntcp) in pregnant rats.

METHODS

BA uptake and Ntcp expression were studied in control and timed-pregnant rats in late gestation. Ntcp protein, messenger RNA (mRNA) expression, and Ntcp tissue localization were determined by Northern blotting, Western analysis, and tissue immunofluorescence. The activity of three transactivators of the Ntcp promoter: hepatocyte nuclear factor 1-alpha (HNF1-alpha), nuclear receptor heterodimer retinoid X receptor:retinoid acid receptor (RXR:RAR) and signal transducer and activator of transcription 5 (Stat5) was assessed using gel electrophoretic mobility shift assays.

RESULTS

A significantly reduced BA uptake and decreased Ntcp mRNA levels (-40%) and protein mass (-60%) was observed in pregnant rats. Nuclear extracts from pregnant rats showed a marked decrease of HNF1-alpha and RXR:RAR binding activities by -80 and -40% of basal activity, respectively. In contrast, binding activity of Stat-5 was increased by 50% in nuclear extracts from pregnant rats.

CONCLUSIONS

Pregnancy is associated with reduced Ntcp expression and function in the rat. Our findings suggest that Ntcp down-regulation during pregnancy occurs primarily at the transcriptional level.

Ceruloplasmin metabolism and function

Ceruloplasmin is a serum ferroxidase that contains greater than 95% of the copper found in plasma. This protein is a member of the multicopper oxidase family, an evolutionarily conserved group of proteins that utilize copper to couple substrate oxidation with the four-electron reduction of oxygen to water. Despite the need for copper in ceruloplasmin function, this protein plays no essential role in the transport or metabolism of this metal. Aceruloplasminemia is a neurodegenerative disease resulting from inherited loss-of-function mutations in the ceruloplasmin gene. Characterization of this disorder revealed a critical physiological role for ceruloplasmin in determining the rate of iron efflux from cells with mobilizable iron stores and has provided new insights into human iron metabolism and nutrition.

Disruption of mitochondrial calcium homeostasis after chronic alpha-naphthylisothiocyanate administration: relevance for cholestasis

BACKGROUND

Hepatocyte dysfunction caused by impaired mitochondrial function has been pointed out as a probable leading cause of cholestatic liver injury. The aim of this study was to evaluate liver mitochondrial bioenergetics that followed repeated in vivo administration of alpha-naphthylisothiocyanate, a known cholestatic agent.

METHODS

Serum markers of liver injury and endogenous adenine nucleotides were measured in alpha-naphthylisothiocyanate-treated rats (intraperitoneally, 100 mg/Kg/wk x 6 wk). Changes in membrane potential, mitochondrial respiration, as well as alterations in mitochondrial calcium homeostasis were monitored.

RESULTS

In rats injected with alpha-naphthylisothiocyanate, liver injury with cholestasis developed within 48 hours, as indicated by both serum enzyme activities and total bilirubin concentration. However, 1 week after the last injection, serum enzyme activity returned to control levels. In addition, after chronic alpha-naphthylisothiocyanate administration, no alterations in mitochondrial respiratory function and membrane potential were observed. Associated with these parameters, mitochondria from treated animals exhibited increased susceptibility to disruption of mitochondrial calcium homeostasis by calcium phosphate and by bile acids, which was probably caused by induction of permeability transition pore.

CONCLUSIONS

Our data suggest that chronic cholestasis in rats leads to impaired mitochondrial function due to the disruption of mitochondrial calcium homeostasis. The initiating event is the induction of a cyclosporine A-sensitive release of calcium. This event may be an important determinant of the progression of cholestatic liver injury and associated liver cirrhosis. In addition, in the present study we observed that impairment of mitochondrial function is potentiated by chenodeoxycholate, a bile acid that is known to be toxic. Ursodeoxycholate (the beta- epimer of chenodeoxycholate) is approved for the treatment of chronic cholestatic liver disease. Interestingly, we show that the susceptibility to the cyclosporine A-sensitive release of calcium was increased by the combination of both bile acids. These results indicate that the reported improvement of biochemical parameters in cholestatic patients treated with ursodeoxycholate would not prevent the associated mitochondrial dysfunction. This may explain the progression of the histological stage and the maintenance of symptoms during cholestasis.

Neonatal cholestasis

Neonatal cholestasis must always be considered in a newborn who is jaundiced for more than 14-21 days and a measurement of the serum total and conjugated bilirubin in these infants is mandatory. Conjugated hyperbilirubinaemia, dark urine and pale stools are pathognomic of the neonatal hepatitis syndrome which should be investigated urgently. The neonatal hepatitis syndrome has many causes and should be investigated using a structured protocol. The most important condition in the differential diagnosis is biliary atresia and affected infants require a Kasai portoenterostomy performed by an experienced surgeon, ideally before the infant is 60 days old. A modified evaluation schedule should be used for preterm infants who have required neonatal intensive care. Genetic causes of the neonatal hepatitis syndrome are increasingly recognized and early diagnosis facilitates genetic counselling and, in some situations, specific treatment. The management of cholestasis is largely supportive, consisting of aggressive nutritional support with particular attention to fat-soluble vitamin status. The use of ursodeoxycholic acid is associated with improvement in biochemical measures of cholestasis and may improve the natural history of cholestasis in some circumstances. Outcome is dependent on aetiology. In idiopathic neonatal hepatitis more than 90% make a complete biochemical and d clinical recovery.

Molecular basis of neonatal cholestasis

At present, specific evidence regarding the molecular mechanisms of neonatal cholestasis is limited. The recent explosion in the understanding of the molecular physiology of bile formation has been fueled by the discovery of several genes that are involved in familial cholestasis. The ever-growing understanding of the functional immaturity of the neonatal liver is sure to be enhanced by the study of the ontogeny of important hepatobiliary transporters as they are discovered. The understanding of the functional differences between the immature and mature liver is key to the understanding of neonatal cholestasis.

A missense mutation in FIC1 is associated with greenland familial cholestasis

Greenland familial cholestasis is a severe form of intrahepatic cholestasis described among indigenous Inuit families in Greenland. Patients present with jaundice, pruritus, bleeding episodes, and steatorrhea, and die in childhood due to end-stage liver disease. We investigated the possibility that Greenland familial cholestasis is caused by a mutation in FIC1, the gene defective in patients with progressive familial intrahepatic cholestasis type 1 and many cases of benign recurrent intrahepatic cholestasis. Using single-strand conformation polymorphism analysis and sequencing of the FIC1 exons, a missense mutation, 1660 G-->A (D554N), was detected and was shown to segregate with the disease in Inuit patients from Greenland and Canada. Examination of liver specimens from 3 Inuit patients homozygous for this mutation revealed bland canalicular cholestasis and, on transmission electron microscopy, coarsely granular Byler bile, as previously described in patients with progressive familial intrahepatic cholestasis type 1. These data establish Greenland familial cholestasis as a form of progressive familial intrahepatic cholestasis type 1 and further underscore the importance of unimpeded FIC1 activity for normal bile formation.

Role of S-adenosylmethionine on the hepatobiliary homeostasis of glutathione during cyclosporine A treatment

The effects of cyclosporine A (CyA) treatment on the hepatic content and biliary output of reduced (GSH) and oxidized (GSSG) glutathione and lipid peroxidation in the liver, and the ability of S-adenosylmethionine (SAMe) to antagonize the CyA-induced alterations were studied in male Wistar rats. To evaluate the efficacy of SAMe, three CyA and SAMe protocols were used: cotreatment with SAMe plus CyA, pretreatment with SAMe before starting cotreatment, and post-treatment with SAMe after beginning treatment with CyA alone. CyA treatment for one and four weeks depleted liver GSH, decreased the GSH/GSSG ratio and significantly reduced GSH and GSSG biliary concentrations and secretion rates. Additionally, long-term treatment enhanced lipid peroxidation. By contrast, when the rats were treated with CyA plus SAMe using any of the administration protocols, SAMe was seen to be efficient in antagonizing the GSH hepatic depletion, the changes in hepatic GSH/GSSG ratio and the increase induced by CyA in lipid peroxidation. Furthermore, SAMe also abolished the effects of CyA on the biliary secretion rates of GSH and GSSG. The efficacy of SAMe was similar, regardless of the administration protocols used. In conclusion, our results clearly demonstrate that SAMe is good for preventing, antagonizing and reversing the CyA-induced alterations in the hepatobiliary homeostasis of glutathione.

Ursodeoxycholic acid treatment in children with Byler disease

BACKGROUND

There have been a few reports of patients with Byler disease and the best medical treatment is not known. The aim of the present study is to show the effect of ursodeoxycholic acid (UDCA) on clinical, laboratory and histologic findings in children with Byler disease.

METHODS

Nine children aged between 1.5 and 9 years with Byler disease were administered UDCA orally at doses of 15-20 mg/kg per day. They were followed for at least 12 months. Clinical, laboratory and histologic outcomes were evaluated after 12 months of treatment.

RESULTS

Seven children presented in the first 6 months of life with itching and/or jaundice. Gamma-glutamyl transpeptidase and cholesterol levels were normal in all patients, despite severe cholestasis. With UDCA therapy, pruritus disappeared/diminished in four (44.4%) patients. The mean serum concentrations of alanine aminotransferase, aspartate aminotransferase (AST), total and conjugated bilirubin decreased, although it was significant only for AST (P = 0.01). Before treatment, all biopsy materials showed cellular/canalicular cholestasis and fibrosis. After UDCA therapy cholestasis was ameliorated. Two patients died during follow up.

CONCLUSIONS

The results suggest that administration of UDCA leads to clinical and biochemical improvement in children with Byler disease. The UDCA ameliorates symptoms partially, improves the life quality of patients and may be given for as long as the disease continues.