Contractility of bile canaliculi: implications for liver function

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

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Bosentan Alters Endo- and Exogenous Bile Salt Disposition in Sandwich-Cultured Human Hepatocytes

Bosentan, a well-known cholestatic agent, was not identified as cholestatic at concentrations up to 200 µM based on the drug-induced cholestasis (DIC) index value, determined in a sandwich-cultured human hepatocyte (SCHH)-based DIC assay. To obtain further quantitative insights into the effects of bosentan on cellular bile salt handling by human hepatocytes, the present study determined the effect of 2.5-25 µM bosentan on endogenous bile salt levels and on the disposition of 10 µM chenodeoxycholic acid (CDCA) added to the medium in SCHHs. Bosentan reduced intracellular as well as extracellular concentrations of both endogenous glycochenodeoxycholic acid (GCDCA) and glycocholic acid in a concentration-dependent manner. When exposed to 10 µM CDCA, bosentan caused a shift from canalicular efflux to sinusoidal efflux of GCDCA. CDCA levels were not affected. Our mechanistic model confirmed the inhibitory effect of bosentan on canalicular GCDCA clearance. Moreover, our results in SCHHs also indicated reduced GCDCA formation. We confirmed the direct inhibitory effect of bosentan on CDCA conjugation with glycine in incubations with liver S9 fraction. SIGNIFICANCE STATEMENT: Bosentan was evaluated at therapeutically relevant concentrations (2.5-25 µM) in sandwich-cultured human hepatocytes. It altered bile salt disposition and inhibited canalicular secretion of glycochenodeoxycholic acid (GCDCA). Within 24 hours, bosentan caused a shift from canalicular to sinusoidal efflux of GCDCA. These results also indicated reduced GCDCA formation. This study confirmed a direct effect of bosentan on chenodeoxycholic acid conjugation with glycine in liver S9 fraction.

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.

Genetic aspects of biliary atresia etiology

Biliary atresia (BA) is a cholestatic disorder of infancy that is fatal if untreated. Despite years of study the etiology of BA remains unknown. Three etiopathogenic mechanisms may be involved, such as immune dysregulation, environmental factors and genetic susceptibility. Genetic predisposition is being actively studied. Candidate genes associated with BA in certain populations, genes affecting the cholangiocyte cilia function, as well as genes involved in stress responses have been identified. However, the long-term follow-up of twins with BA suggests that genotype is not of paramount importance for the disease development. Both epigenetic patterns and postzygotic somatic mutations may contribute to etiology of the disease. Recently, some evidence is being accumulated on the possible genetic predisposition to certain outcome of Kasai portoenterostomy performed in patients with BA. However, the presence of a number of factors contributing to the development of the disease makes it difficult to identify the genetic markers.

Hepatic bile formation: bile acid transport and water flow into the canalicular conduit

Advances in molecular biology identifying the many carrier-mediated organic anion transporters and advances in microscopy that have provided a more detailed anatomy of the canalicular conduit make updating the concept of osmotically determined canalicular flow possible. For the most part water flow is not transmembrane but via specific pore proteins in both the hepatocyte and the tight junction. These pores independently regulate the rate at which water flows in response to an osmotic gradient and therefore are determinants of canalicular bile acid concentration. Review of the literature indicates that the initial effect on hepatic bile flow of cholestatic agents such as Thorazine and estradiol 17β-glucuronide are on water flow and not bile salt export pump-mediated bile acid transport and thus provides new approaches to the pathogenesis of drug-induced liver injury. Attaining a micellar concentration of bile acids in the canaliculus is essential to the formation of cholesterol-lecithin vesicles, which mostly occur in the periportal region of the canalicular conduit. The other regions, midcentral and pericentral, may transport lesser amounts of bile acid but augment water flow. Broadening the concept of how hepatic bile flow is initiated, provides new insights into the pathogenesis of canalicular cholestasis.

Hydrostatic pressure regulates CYP1A2 expression in human hepatocytes via a mechanosensitive aryl hydrocarbon receptor-dependent pathway

Approximately 75% of xenobiotics are primarily eliminated through metabolism; thus the accurate scaling of metabolic clearance is vital to successful drug development. Yet, when data is scaled from in vitro to in vivo, hepatic metabolic clearance, the primary source of metabolism, is still commonly underpredicted. Over the past decades, with biophysics used as a key component to restore aspects of the in vivo environment, several new cell culture settings have been investigated to improve hepatocyte functionalities. Most of these studies have focused on shear stress, i.e., flow mediated by a pressure gradient. One potential conclusion of these studies is that hepatocytes are naturally "mechanosensitive," i.e., they respond to a change in their biophysical environment. We demonstrate that hepatocytes also respond to an increase in hydrostatic pressure that, we suggest, is directly linked to the lobule geometry and vessel density. Furthermore, we demonstrate that hydrostatic pressure improves albumin production and increases cytochrome P-450 (CYP) 1A2 expression levels in an aryl hydrocarbon-dependent manner in human hepatocytes. Increased albumin production and CYP function are commonly attributed to the impacts of shear stress in microfluidic experiments. Therefore, our results highlight evidence of a novel link between hydrostatic pressure and CYP metabolism and demonstrate that the spectrum of hepatocyte mechanosensitivity might be larger than previously thought.

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

Regulation of bile secretion by calcium signaling in health and disease

Calcium (Ca²⁺) signaling controls secretion in many types of cells and tissues. In the liver, Ca²⁺ regulates secretion in both hepatocytes, which are responsible for primary formation of bile, and cholangiocytes, which line the biliary tree and further condition the bile before it is secreted. Cholestatic liver diseases, which are characterized by impaired bile secretion, may result from impaired Ca²⁺ signaling mechanisms in either hepatocytes or cholangiocytes. This review will discuss the Ca²⁺ signaling machinery and mechanisms responsible for regulation of secretion in both hepatocytes and cholangiocytes, and the pathophysiological changes in Ca²⁺ signaling that can occur in each of these cell types to result in cholestasis.

Farnesoid X Receptor Agonists Obeticholic Acid and Chenodeoxycholic Acid Increase Bile Acid Efflux in Sandwich-Cultured Human Hepatocytes: Functional Evidence and Mechanisms

The farnesoid X receptor (FXR) is a nuclear receptor that regulates genes involved in bile acid homeostasis. FXR agonists, obeticholic acid (OCA) and chenodeoxycholic acid (CDCA), increase mRNA expression of efflux transporters in sandwich-cultured human hepatocytes (SCHH). This study evaluated the effects of OCA and CDCA treatment on the uptake, basolateral efflux, and biliary excretion of a model bile acid, taurocholate (TCA), in SCHH. In addition, changes in the protein expression of TCA uptake and efflux transporters were investigated. SCHH were treated with 1 µM OCA, 100 µM CDCA, or vehicle control for 72 hours followed by quantification of deuterated TCA uptake and efflux over time in Ca²⁺-containing and Ca²⁺-free conditions (n = 3 donors). A mechanistic pharmacokinetic model was fit to the TCA mass-time data to obtain estimates for total uptake clearance (CL_Uptake), total intrinsic basolateral efflux clearance (CL_int,BL), and total intrinsic biliary clearance (CL_int,Bile). Modeling results revealed that FXR agonists significantly increased CL_int,BL by >6-fold and significantly increased CL_int,Bile by 2-fold, with minimal effect on CL_Uptake Immunoblotting showed that protein levels of the basolateral transporter subunits organic solute transporter α and β (OSTα and OSTβ) in FXR agonist-treated SCHH were significantly induced by >2.5- and 10-fold, respectively. FXR agonist-mediated changes in the expression of other TCA transporters in SCHH were modest. In conclusion, this is the first report demonstrating that OCA and CDCA increased TCA efflux in SCHH, which contributed to reduced intracellular TCA concentrations. Increased basolateral efflux of TCA was consistent with increased OSTα/β protein expression in OCA- and CDCA-treated SCHH.

A Predictive 3D Multi-Scale Model of Biliary Fluid Dynamics in the Liver Lobule

Bile, the central metabolic product of the liver, is transported by the bile canaliculi network. The impairment of bile flow in cholestatic liver diseases has urged a demand for insights into its regulation. Here, we developed a predictive 3D multi-scale model that simulates fluid dynamic properties successively from the subcellular to the tissue level. The model integrates the structure of the bile canalicular network in the mouse liver lobule, as determined by high-resolution confocal and serial block-face scanning electron microscopy, with measurements of bile transport by intravital microscopy. The combined experiment-theory approach revealed spatial heterogeneities of biliary geometry and hepatocyte transport activity. Based on this, our model predicts gradients of bile velocity and pressure in the liver lobule. Validation of the model predictions by pharmacological inhibition of Rho kinase demonstrated a requirement of canaliculi contractility for bile flow in vivo. Our model can be applied to functionally characterize liver diseases and quantitatively estimate biliary transport upon drug-induced liver injury.

Downregulation of microRNA-145 may contribute to liver fibrosis in biliary atresia by targeting ADD3

BACKGROUND AND OBJECTIVES

Biliary atresia (BA) is a pediatric liver disease characterized by fibro-obliteration and obstruction of the extrahepatic biliary system, that invariably leads to cirrhosis and even death, if left untreated for extended time. However, its pathology and etiology still remained unknown. In this study, we tested the expression of adducin 3 (ADD3), the gene identified as a susceptibility gene in BA by GWAS, and uncovered its upstream regulatory microRNA in the pathogenesis of BA.

METHODS

In this study, 14 infants with BA and 14 infants with choledochal cyst (CC) were enrolled as experimental group and control group, respectively. ADD3 and microRNA-145 (miR-145) expression profiles in liver tissues of BA and CC were determined using qPCR. Luciferase reporter assay was performed to verify the direct interaction between miR-145-5p and ADD3 3' Untranslated Regions (3'UTR). The Lentiviral vectors containing miR-145, miR-145-3p inhibitor, miR-145-5p inhibitor, empty vector were transfected into human hepatic stellate cell line (LX-2) to determine the functional effect of miR-145 on ADD3 expression at both mRNA and protein level.

RESULTS

MiR-145 was shown to be down-regulated in liver tissues of infants with BA compared to CC (p = 0.0267). ADD3, verified as a target of miR-145-5p, was shown to be overexpressed in infants with BA at the mRNA level (p = 0.0118). Transfection of lentiviruses containing miR-145 into LX-2 cells decreased the expression of ADD3 at both mRNA and protein level compared to negative control group, and suppressed the expression of p-Akt at protein level.

CONCLUSIONS

Our study has shown that overexpressed ADD3 and downregulated miR-145 were detected in BA liver tissues. MiR-145-5p was confirmed to target ADD3 by luciferase reporter assay. The downregulation of miR-145 may contribute to liver fibrosis in BA by upregulating the expression of ADD3.

Sandwich-Cultured Hepatocytes as a Tool to Study Drug Disposition and Drug-Induced Liver Injury

Sandwich-cultured hepatocytes (SCH) are metabolically competent and have proper localization of basolateral and canalicular transporters with functional bile networks. Therefore, this cellular model is a unique tool that can be used to estimate biliary excretion of compounds. SCH have been used widely to assess hepatobiliary disposition of endogenous and exogenous compounds and metabolites. Mechanistic modeling based on SCH data enables estimation of metabolic and transporter-mediated clearances, which can be used to construct physiologically based pharmacokinetic models for prediction of drug disposition and drug-drug interactions in humans. In addition to pharmacokinetic studies, SCH also have been used to study cytotoxicity and perturbation of biological processes by drugs and hepatically generated metabolites. Human SCH can provide mechanistic insights underlying clinical drug-induced liver injury (DILI). In addition, data generated in SCH can be integrated into systems pharmacology models to predict potential DILI in humans. In this review, applications of SCH in studying hepatobiliary drug disposition and bile acid-mediated DILI are discussed. An example is presented to show how data generated in the SCH model were used to establish a quantitative relationship between intracellular bile acids and cytotoxicity, and how this information was incorporated into a systems pharmacology model for DILI prediction.

Prediction of Altered Bile Acid Disposition Due to Inhibition of Multiple Transporters: An Integrated Approach Using Sandwich-Cultured Hepatocytes, Mechanistic Modeling, and Simulation

Transporter-mediated alterations in bile acid disposition may have significant toxicological implications. Current methods to predict interactions are limited by the interplay of multiple transporters, absence of protein in the experimental system, and inaccurate estimates of inhibitor concentrations. An integrated approach was developed to predict altered bile acid disposition due to inhibition of multiple transporters using the model bile acid taurocholate (TCA). TCA pharmacokinetic parameters were estimated by mechanistic modeling using sandwich-cultured human hepatocyte data with protein in the medium. Uptake, basolateral efflux, and biliary clearance estimates were 0.63, 0.034, and 0.074 mL/min/g liver, respectively. Cellular total TCA concentrations (Ct,Cells) were selected as the model output based on sensitivity analysis. Monte Carlo simulations of TCA Ct,Cells in the presence of model inhibitors (telmisartan and bosentan) were performed using inhibition constants for TCA transporters and inhibitor concentrations, including cellular total inhibitor concentrations ([I]t,cell) or unbound concentrations, and cytosolic total or unbound concentrations. For telmisartan, the model prediction was accurate with an average fold error (AFE) of 0.99-1.0 when unbound inhibitor concentration ([I]u) was used; accuracy dropped when total inhibitor concentration ([I]t) was used. For bosentan, AFE was 1.2-1.3 using either [I]u or [I]t This difference was evaluated by sensitivity analysis of the cellular unbound fraction of inhibitor (fu,cell,inhibitor), which revealed higher sensitivity of fu,cell,inhibitor for predicting TCA Ct,Cells when inhibitors exhibited larger ([I]t,cell/IC50) values. In conclusion, this study demonstrated the applicability of a framework to predict hepatocellular bile acid concentrations due to drug-mediated inhibition of transporters using mechanistic modeling and cytosolic or cellular unbound concentrations.

Rho-kinase/myosin light chain kinase pathway plays a key role in the impairment of bile canaliculi dynamics induced by cholestatic drugs

Intrahepatic cholestasis represents a frequent manifestation of drug-induced liver injury; however, the mechanisms underlying such injuries are poorly understood. In this study of human HepaRG and primary hepatocytes, we found that bile canaliculi (BC) underwent spontaneous contractions, which are essential for bile acid (BA) efflux and require alternations in myosin light chain (MLC2) phosphorylation/dephosphorylation. Short exposure to 6 cholestatic compounds revealed that BC constriction and dilation were associated with disruptions in the ROCK/MLCK/myosin pathway. At the studied concentrations, cyclosporine A and chlorpromazine induced early ROCK activity, resulting in permanent MLC2 phosphorylation and BC constriction. However, fasudil reduced ROCK activity and caused rapid, substantial and permanent MLC2 dephosphorylation, leading to BC dilation. The remaining compounds (1-naphthyl isothiocyanate, deoxycholic acid and bosentan) caused BC dilation without modulating ROCK activity, although they were associated with a steady decrease in MLC2 phosphorylation via MLCK. These changes were associated with a common loss of BC contractions and failure of BA clearance. These results provide the first demonstration that cholestatic drugs alter BC dynamics by targeting the ROCK/MLCK pathway; in addition, they highlight new insights into the mechanisms underlying bile flow failure and can be used to identify new predictive biomarkers of drug-induced cholestasis.

Species differences in hepatobiliary disposition of taurocholic acid in human and rat sandwich-cultured hepatocytes: implications for drug-induced liver injury

The bile salt export pump (BSEP) plays an important role in bile acid excretion. Impaired BSEP function may result in liver injury. Bile acids also undergo basolateral efflux, but the relative contributions of biliary (CLBile) versus basolateral efflux (CLBL) clearance to hepatocellular bile acid excretion have not been determined. In the present study, taurocholic acid (TCA; a model bile acid) disposition was characterized in human and rat sandwich-cultured hepatocytes (SCH) combined with pharmacokinetic modeling. In human SCH, biliary excretion of TCA predominated (CLBile = 0.14 ± 0.04 ml/min per g liver; CLBL = 0.042 ± 0.019 ml/min per g liver), whereas CLBile and CLBL contributed approximately equally to TCA hepatocellular excretion in rat SCH (CLBile = 0.34 ± 0.07 ml/min per g liver; CLBL = 0.26 ± 0.07 ml/min per g liver). Troglitazone decreased TCA uptake, CLBile, and CLBL; membrane vesicle assays revealed for the first time that the major metabolite, troglitazone sulfate, was a noncompetitive inhibitor of multidrug resistance-associated protein 4, a basolateral bile acid efflux transporter. Simulations revealed that decreased CLBile led to a greater increase in hepatic TCA exposure in human than in rat SCH. A decrease in both excretory pathways (CLBile and CLBL) exponentially increased hepatic TCA in both species, suggesting that 1) drugs that inhibit both pathways may have a greater risk for hepatotoxicity, and 2) impaired function of an alternate excretory pathway may predispose patients to hepatotoxicity when drugs that inhibit one pathway are administered. Simulations confirmed the protective role of uptake inhibition, suggesting that a drug's inhibitory effects on bile acid uptake also should be considered when evaluating hepatotoxic potential. Overall, the current study precisely characterized basolateral efflux of TCA, revealed species differences in hepatocellular TCA efflux pathways, and provided insights about altered hepatic bile acid exposure when multiple transport pathways are impaired.

Species differences in hepatobiliary disposition of taurocholic acid in human and rat sandwich-cultured hepatocytes: implications for drug-induced liver injury

The bile salt export pump (BSEP) plays an important role in bile acid excretion. Impaired BSEP function may result in liver injury. Bile acids also undergo basolateral efflux, but the relative contributions of biliary (CLBile) versus basolateral efflux (CLBL) clearance to hepatocellular bile acid excretion have not been determined. In the present study, taurocholic acid (TCA; a model bile acid) disposition was characterized in human and rat sandwich-cultured hepatocytes (SCH) combined with pharmacokinetic modeling. In human SCH, biliary excretion of TCA predominated (CLBile = 0.14 ± 0.04 ml/min per g liver; CLBL = 0.042 ± 0.019 ml/min per g liver), whereas CLBile and CLBL contributed approximately equally to TCA hepatocellular excretion in rat SCH (CLBile = 0.34 ± 0.07 ml/min per g liver; CLBL = 0.26 ± 0.07 ml/min per g liver). Troglitazone decreased TCA uptake, CLBile, and CLBL; membrane vesicle assays revealed for the first time that the major metabolite, troglitazone sulfate, was a noncompetitive inhibitor of multidrug resistance-associated protein 4, a basolateral bile acid efflux transporter. Simulations revealed that decreased CLBile led to a greater increase in hepatic TCA exposure in human than in rat SCH. A decrease in both excretory pathways (CLBile and CLBL) exponentially increased hepatic TCA in both species, suggesting that 1) drugs that inhibit both pathways may have a greater risk for hepatotoxicity, and 2) impaired function of an alternate excretory pathway may predispose patients to hepatotoxicity when drugs that inhibit one pathway are administered. Simulations confirmed the protective role of uptake inhibition, suggesting that a drug's inhibitory effects on bile acid uptake also should be considered when evaluating hepatotoxic potential. Overall, the current study precisely characterized basolateral efflux of TCA, revealed species differences in hepatocellular TCA efflux pathways, and provided insights about altered hepatic bile acid exposure when multiple transport pathways are impaired.

Association between single nucleotide polymorphisms in the ADD3 gene and susceptibility to biliary atresia

Background and Objectives

Based on the results of previous studies, the ADD3 gene, located in the 10q24.2 region, may be a susceptibility gene of biliary atresia (BA). In this study, two single nucleotide polymorphisms (SNPs) in the ADD3 gene, rs17095355 C/T and rs10509906 G/C, were selected to investigate whether there is an association between these SNPs and susceptibility to BA in a Chinese population.

Methods

A total of 752 Han Chinese (134 BA cases and 618 ethnically matched healthy controls) were included in the present study. The ADD3 gene polymorphisms were genotyped using a TaqMan genotyping assay.

Results

Positive associations were found for the SNP rs17095355 in the codominant model; specifically, the frequencies of the CT and TT genotypes and the T allele were higher in the cases than the controls, demonstrating a significant risk for BA (odds ratio [OR] = 1.62, 95% confidence interval [CI] = 1.02–2.58; OR = 2.89, 95% CI = 1.72–4.86; and OR = 1.75, 95% CI = 1.34–2.29, respectively). Regarding rs10509906, the per-C-allele conferred an OR of 0.70 (95% CI = 0.49–1.00) under the additive model. A greater risk of BA was associated with the T_a-G_b (a for rs17095355 and b for rs10509906) haplotype (OR = 1.82, 95% CI = 1.27–2.61) compared with the C_a-C_b haplotype.

Conclusion

This study suggests that the ADD3 gene plays an important role in BA pathogenesis and reveals a significant association between two SNPs, rs17095355 and rs10509906, and BA.

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.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

Role of hepatic efflux transporters in regulating systemic and hepatocyte exposure to xenobiotics

Hepatic efflux transporters include numerous well-known and emerging proteins localized to the canalicular or basolateral membrane of the hepatocyte that are responsible for the excretion of drugs into the bile or blood, respectively. Altered function of hepatic efflux transporters due to drug-drug interactions, genetic variation, and/or disease states may lead to changes in xenobiotic exposure in the hepatocyte and/or systemic circulation. This review focuses on transport proteins involved in the hepatocellular efflux of drugs and metabolites, discusses mechanisms of altered transporter function as well as the interplay between multiple transport pathways, and highlights the importance of considering intracellular unbound concentrations of transporter substrates and/or inhibitors. Methods to evaluate hepatic efflux transport and predict the effects of impaired transporter function on systemic and hepatocyte exposure are discussed, and the sandwich-cultured hepatocyte model to evaluate comprehensively the role of hepatic efflux in the hepatobiliary disposition of xenobiotics is characterized.

Combination lopinavir and ritonavir alter exogenous and endogenous bile acid disposition in sandwich-cultured rat hepatocytes

Inhibition of the bile salt export pump (BSEP) can cause intracellular accumulation of bile acids and is a risk factor for drug-induced liver injury in humans. Antiretroviral protease inhibitors lopinavir (LPV) and ritonavir (RTV) are reported BSEP inhibitors. However, the consequences of LPV and RTV, alone and combined (LPV/r), on hepatocyte viability, bile acid transport, and endogenous bile acid disposition in rat hepatocytes have not been examined. The effect of LPV, RTV, and LPV/r on cellular viability and the disposition of [(3)H]taurocholic acid (TCA) and [(14)C]chenodeoxycholic acid (CDCA) was determined in sandwich-cultured rat hepatocytes (SCRH) and suspended rat hepatocytes. Lactate dehydrogenase and ATP assays revealed a concentration-dependent effect of LPV and RTV on cellular viability. LPV (5 µM), alone and combined with 5 µM RTV, significantly decreased [(3)H]TCA accumulation in cells + bile of SCRHs compared with control. LPV/r significantly increased [(3)H]TCA cellular accumulation (7.7 ± 0.1 pmol/mg of protein) compared with vehicle and 5 µM LPV alone (5.1 ± 0.7 and 5.0 ± 0.5 pmol/mg of protein). The [(3)H]TCA biliary clearance was reduced significantly by LPV and RTV and further reduced by LPV/r. LPV and RTV did not affect the initial uptake rates of [(3)H]TCA or [(14)C]CDCA in suspended rat hepatocytes. LPV (50 µM), RTV (5 µM), and LPV/r (5 and 50 µM/5 µM) significantly decreased the accumulation of total measured endogenous bile acids (TCA, glycocholic acid, taurochenodeoxycholic acid, glycochenodeoxycholic acid, and α/β-tauromuricholic acid) in SCRH. Quantification of endogenous bile acids in SCRH may reveal important adaptive responses associated with exposure to known BSEP inhibitors.

Genome-wide association study identifies a susceptibility locus for biliary atresia on 10q24.2

Biliary atresia (BA) is characterized by the progressive fibrosclerosing obliteration of the extrahepatic biliary system during the first few weeks of life. Despite early diagnosis and prompt surgical intervention, the disease progresses to cirrhosis in many patients. The current theory for the pathogenesis of BA proposes that during the perinatal period, a still unknown exogenous factor meets the innate immune system of a genetically predisposed individual and induces an uncontrollable and potentially self-limiting immune response, which becomes manifest in liver fibrosis and atresia of the extrahepatic bile ducts. Genetic factors that could account for the disease, let alone for its high incidence in Chinese, are to be investigated. To identify BA susceptibility loci, we carried out a genome-wide association study (GWAS) using the Affymetrix 5.0 and 500 K marker sets. We genotyped nearly 500 000 single-nucleotide polymorphisms (SNPs) in 200 Chinese BA patients and 481 ethnically matched control subjects. The 10 most BA-associated SNPs from the GWAS were genotyped in an independent set of 124 BA and 90 control subjects. The strongest overall association was found for rs17095355 on 10q24, downstream XPNPEP1, a gene involved in the metabolism of inflammatory mediators. Allelic chi-square test P-value for the meta-analysis of the GWAS and replication results was 6.94 x 10(-9). The identification of putative BA susceptibility loci not only opens new fields of investigation into the mechanisms underlying BA but may also provide new clues for the development of preventive and curative strategies.

Hepatobiliary disposition of troglitazone and metabolites in rat and human sandwich-cultured hepatocytes: use of Monte Carlo simulations to assess the impact of changes in biliary excretion on troglitazone sulfate accumulation

This study examined the hepatobiliary disposition of troglitazone (TGZ) and metabolites [TGZ sulfate (TS), TGZ glucuronide (TG), and TGZ quinone (TQ)] over time in rat and human sandwich-cultured hepatocytes (SCH). Cells were incubated with TGZ; samples were analyzed for TGZ and metabolites by liquid chromatography-tandem mass spectrometry. SCH mimicked the disposition of TGZ/metabolites in vivo in rats and humans; TGZ was metabolized primarily to TS and to a lesser extent to TG and TQ. In human SCH, the biliary excretion index (BEI) was negligible for TGZ and TQ, approximately 16% for TS, and approximately 43% for TG over the incubation period; in rat SCH, the BEI for TS and TG was approximately 13 and approximately 41%, respectively. Hepatocyte accumulation of TS was extensive, with intracellular concentrations ranging from 132 to 222 microM in rat SCH; intracellular TGZ concentrations ranged from 7.22 to 47.7 microM. In human SCH, intracellular TS and TGZ concentrations ranged from 136 to 160 microM and from 49.4 to 84.7 microM, respectively. Pharmacokinetic modeling and Monte Carlo simulations were used to evaluate the impact of modulating the biliary excretion rate constant (K(bile)) for TS on TS accumulation in hepatocytes and medium. Simulations demonstrated that intracellular concentrations of TS may increase up to 3.1- and 5.7-fold when biliary excretion of TS was decreased 2- and 10-fold, respectively. It is important to note that altered hepatobiliary transport and the extent of hepatocyte exposure may not always be evident based on medium concentrations (analogous to systemic exposure in vivo). Pharmacokinetic modeling/simulation with data from SCH is a useful approach to examine the impact of altered hepatobiliary transport on hepatocyte accumulation of drug/metabolites.

Ischemic preconditioning and intermittent ischemia preserve bile flow in a rat model of ischemia/reperfusion injury

Ischemia and reperfusion (IR) injury of the liver is associated with impaired bile secretion, but the effects of ischemic preconditioning (IPC) and intermittent ischemia (INT) on bile flow are unknown. A rat model of segmental (60%-70%) hepatic ischemia and reperfusion was employed to test the effects of IPC and INT on bile flow. Continuous clamping for 45 min (CC) substantially reduced bile flow, and this did not recover after 60 min of reperfusion. IPC and INT caused a significant recovery of bile flow. The elevation in plasma liver marker enzymes induced by CC was not reduced by IPC and INT. Light microscopy showed mild hepatocyte damage in all groups. In the CC group, the amount of F-actin localized around the bile canaliculi in the ischemic lobes was less than that in the nonischemic lobes, but this difference was not observed in the IPC and INT groups. It is concluded that IPC and INT substantially alleviate the decrease in bile flow induced by ischemia. Bile flow may be useful in the assessment of IR injury.

Vasopressin-induced morphological changes in polarized rat hepatocyte multiplets: dual calcium-dependent effects

Calcium-mobilizing hormones and neurotransmitters are known to affect cell morphology and function including cell differentiation or division. In this study, we examined vasopressin (AVP)-induced morphological changes in a polarized system of rat hepatocytes. Light and electron microscope observations showed that AVP induced microvilli formation and a remodeling of the isolated hepatocyte F-actin submembrane cytoskeleton, these two events being correlated. We showed that these effects were rapid, reversible, observed at nanomolar AVP concentration and mediated by the V(1a) receptor. On polarized multicellular systems of hepatocytes, we observed a rapid reduction of the bile canaliculi lumen at the apical pole and micovilli formation at the basolateral domain with an enlarged F-actin cytoskeleton. Neither activation of protein kinase C nor A via phorbol ester or dibutyryl cAMP induced such rapid morphological changes, at variance with ionomycin, suggesting that AVP-induced intracellular calcium rise plays a crucial role in those effects. By using spectrofluorimetry and cytochemistry, we showed that calcium release from intracellular stores was involved in bile canaliculus contraction, while calcium entry from the extracellular space controlled microvilli formation. Taken together, AVP and calcium-mobilizing agonists differentially regulate physiological hepatocyte plasma membrane events at the basal and the apical domains via topographically specialized calcium-dependent mechanisms.

Ischemic preconditioning and intermittent ischemia preserve bile flow in a rat model of ischemia reperfusion injury

Ischemia and reperfusion (IR) injury of the liver is associated with impaired bile secretion, but the effects of ischemic preconditioning (IPC) and intermittent ischemia (INT) on bile flow are unknown. A rat model of segmental (60%-70%) hepatic ischemia and reperfusion was employed to test the effects of IPC and INT on bile flow. Continuous clamping for 45 min (CC) substantially reduced bile flow, and this did not recover after 60 min of reperfusion. IPC and INT caused a significant recovery of bile flow. The elevation in plasma liver marker enzymes induced by CC was not reduced by IPC and INT. Light microscopy showed mild hepatocyte damage in all groups. In the CC group, the amount of F-actin localized around the bile canaliculi in the ischemic lobes was less than that in the nonischemic lobes, but this difference was not observed in the IPC and INT groups. It is concluded that IPC and INT substantially alleviate the decrease in bile flow induced by ischemia. Bile flow may be useful in the assessment of IR injury.

Taurolithocholate impairs bile canalicular motility and canalicular bile secretion in isolated rat hepatocyte couplets

AIM

To investigate the effects of taurolithocholate (TLC) on the canalicular motility in isolated rat hepatocyte couplets (IRHC).

METHODS

TLC was added to IRHC at concentrations of 10 and 50 mumol/L, respectively. In each group, five time-lapse movies containing 3 representative bile canaliculi were taken under phase-contrast microscopy for 12 h. The number of bile canalicular contractions and the intervals between consecutive canalicular contractions were calculated. Furthermore, the effects of TLC on IRHC were examined by transmission electron microscopy.

RESULTS

The bile canalicular contractions were spontaneous and forceful in the controls. Active vesicular movement was observed in the pericanalicular region. Immediately after the addition of TLC, the bile canaliculi were deformed, and canalicular bile was incorporated into the vacuoles. The canaliculi were gradually dilated, and canalicular contractions were markedly inhibited by TLC. The vesicular movements became extremely slow in the pericanalicular region. The number of canalicular contractions significantly decreased in the TLC-treated groups, as compared with that in the controls. The time intervals were prolonged, as the TLC dosage increased, indicating that bile secretion into the canaliculi was impaired with TLC. Transmission electron microscopy revealed the lamellar transformation of the canalicular membranes in IRHC treated with TLC.

CONCLUSION

TLC impairs both the bile canalicular contractions and the canalicular bile secretion, possibly by acting directly on the canalicular membranes in TLC-induced cholestasis.

Hepatic ischemia-reperfusion injury: roles of Ca2+ and other intracellular mediators of impaired bile flow and hepatocyte damage

Liver resection and liver transplantation have been successful in the treatment of liver tumors and end-stage liver disease. This success has led to an expansion in the pool of patients potentially treatable by liver surgery and, in the case of transplantation, to a shortage of liver donors. At present, there are significant numbers of potential candidates for liver resection and liver donation who have fatty livers, are aged, or have livers damaged by chemotherapy. All of these are at high risk for ischemic reperfusion (IR) injury. The aims of this review are to assess current knowledge of the clinical effectiveness of ischemic preconditioning and intermittent ischemia in reducing IR damage in liver surgery; to evaluate the use of bile flow as a sensitive indicator of IR liver damage; and to analyze the molecular mechanisms, especially intracellular Ca2+, involved in IR injury and ischemic preconditioning. It is concluded that bile flow is a sensitive indicator of IR injury. Together with reactive oxygen species (ROS) and other extracellular and intracellular signaling molecules, intracellular Ca2+ in hepatocytes plays a key role in the normal regulation of bile flow and in IR-induced injury and cell death. Ischemic preconditioning is an effective strategy to reduce IR injury but there is considerable scope for improvement, especially in patients with fatty and aged livers. The development of effective new strategies to reduce IR injury will depend on improved understanding of the molecular mechanisms involved, especially by gaining a better perspective of the relative importance of the various intrahepatocyte signaling pathways involved.

Cholesterol-regulated translocation of NPC1L1 to the cell surface facilitates free cholesterol uptake

Although NPC1L1 is required for intestinal cholesterol absorption, data demonstrating mechanisms by which this protein facilitates the process are few. In this study, a hepatoma cell line stably expressing human NPC1L1 was established, and cholesterol uptake was studied. A relationship between NPC1L1 intracellular trafficking and cholesterol uptake was apparent. At steady state, NPC1L1 proteins localized predominantly to the transferrin-positive endocytic recycling compartment, where free cholesterol also accumulated as revealed by filipin staining. Interestingly, acute cholesterol depletion induced with methyl-beta-cyclodextrin stimulated relocation of NPC1L1 to the plasma membrane, preferentially to a newly formed "apical-like" subdomain. This translocation was associated with a remarkable increase in cellular cholesterol uptake, which in turn was dose-dependently inhibited by ezetimibe, a novel cholesterol absorption inhibitor that specifically binds to NPC1L1. These findings define a cholesterol-regulated endocytic recycling of NPC1L1 as a novel mechanism regulating cellular cholesterol uptake.

Reconstruction of 3D stacked-up structures by rat small hepatocytes on microporous membranes

The three-dimensional (3D) culture of hepatocytes is essential for the reconstruction of functional hepatic tissues in vitro. In the present experiment, we developed a 3D-culture method in order to reconstruct hepatic cordlike structures by stacking up two-dimensional (2D) tissues composed of rat small hepatocytes (SHs), which are hepatic progenitor cells. Pairs of membranes were prepared and the cells were separately cultured on each membrane. After the SH colonies had developed, one membrane was inverted on top of the other to form an SH bilayer. Thereafter, we investigated whether the stacked cells were organized into differentiated tissues. In the 3D stacked-up structures, bile canaliculi (BC) started to form and gradually developed into anastomosing networks. Transmission electron microscopy revealed that the SHs of the upper and lower layers adhered to one another, and that BC formed between them. Bile canalicular proteins localized on the lumina of the tubular structures. Furthermore, the cells within the structures exhibited mRNA transcription of the hepatic-differentiation markers and maintained a relatively high level of albumin secretion. We conclude that highly differentiated 3D tissues, including functional BC, can be reconstructed by stacking up layers of SHs. This 3D stacked-up culture is useful for the reconstruction of tissue-engineered livers.

Coordinated Movement of Bile Canalicular Networks Reconstructed by Rat Small Hepatocytes

Hepatocytes in vivo have a potential for liver regeneration, but it has been very difficult to reconstruct hepatic organoids in vitro. Recent studies have shown that small hepatocytes (SHs) can reconstruct hepatic organoids including functional bile canaliculi (BC). In the present study we analyzed the movement of BC formed in the hepatic organoids, focusing on the coordination of contraction and dilation among cells and the mechanism producing the coordination. Hepatic cells, including SHs, were isolated from an adult rat liver and cultured. Time-lapse images of BC movements were taken and analyzed in cells treated with or without cytochalasin B (CB). Time-lapse images revealed that all BC, regardless of region contracted in a coordinated manner. Actin filaments were observed along the BC even after the BC networks treated with CB dilated markedly. Microinjection of dye was also carried out to investigate the flow thorough BC. Secreted fluorescein from the injected cell flowed along BC, and gap junctional protein connexin 32 was expressed along BC networks, suggesting cell-to-cell communication. Thus, groups of hepatocytes in the hepatic organoids act in a coordinated manner through intercellular communication.

Senescence marker protein-30 (SMP30) induces formation of microvilli and bile canaliculi in Hep G2 cells

Senescence marker protein-30 (SMP30) is an androgen-independent factor that decreases with aging. To elucidate the physiological functions of SMP30, we transfected human SMP30 cDNA into the human hepatoma cell line, Hep G2. These Hep G2/SMP30 transfectants, which stably expressed large amounts of SMP30, proliferated at a slower rate and synthesized less DNA than mock transfectants (Hep G2/pcDNA3 controls). Thus, enhanced expression of SMP30 retarded the growth of Hep G2/SMP30 cells. Ultrastructural studies by scanning electron microscopy revealed numerous microvilli covering the surfaces of Hep G2/SMP30 cells, whereas few microvilli appeared on control cells. Subsequently, transmission electron microscopy revealed that groups of Hep G2/SMP30 cells exhibited bile canaliculi and possessed specialized adhesion contacts, such as tight junctions and desmosomes, at interplasmic membranes. However, in controls, units of only two cells were seen, and these lacked specialized adhesion junctions. Moesin and ZO-1 are known to be concentrated in microvilli and at tight junctions, respectively. Double-immunostaining was performed to examine whether moesin and ZO-1 were expressed in bile canaliculi with microvilli at the apical regions of Hep G2/SMP30 cells. The intensity of moesin and ZO-1 staining in the contact regions of each cell was markedly higher in Hep G2/SMP30 than in control cells. Moreover, moesin stained more interior areas, which corresponded to the microvilli of bile canaliculi. Clearly, bile canaliculi with microvilli formed at the apical ends of Hep G2/SMP30 cells. These results indicate that SMP30 has an important physiological function as a participant in cell-to-cell interactions and imply that the down-regulation of SMP30 during the aging process contributes to the deterioration of cellular interactivity.

Bile canalicular formation in hepatic organoid reconstructed by rat small hepatocytes and nonparenchymal cells

The morphogenesis and movement of bile canaliculi (BC) are not well understood. This is because culture of hepatocytes that maintain polarity of cell membranes and possess highly differentiated functions has never been successful. We found that small hepatocytes (SHs), which are known to be hepatic progenitor cells, could proliferate and differentiate into mature hepatocytes and that BC-like structures developed between rising/piled-up cells. We investigated how BC-like structures developed with maturation of SHs and whether the structures were functionally active as BC. Hepatic cells, including SHs, were isolated from an adult rat liver and cultured. Immunocytochemistry and immunoblotting for BC proteins, such as ectoATPase, 5'-nucleotidase, dipeptidylpeptidase IV, and multidrug-resistance associated protein 2, were examined and time-lapse microscopy was used for the observation of BC contractions. Secretion of bilirubin into the reconstructed BC was also observed. The results of immunocytochemistry, immunoblots, and immunoelectron micrographs revealed that BC proteins were localized in the intercellular space that coincided with BC-like structures reconstructed between rising/piled-up cells. Tight junction-associated protein ZO-1 was also expressed along the BC-like structures. Bilirubin added to the medium were secreted into BC-like structure and accumulated without leakage. Time-lapse microscopy showed continuous contractions of reconstructed BC. In conclusion, BC-like structures reconstructed by SHs may be functional with membrane polarity, secretory ability, and motility. These results show that this culture system may suitable for investigating the mechanism of the formation of BC and their functions.

Abnormalities in villin gene expression and canalicular microvillus structure in progressive cholestatic liver disease of childhood

BACKGROUND

The molecular basis of clinical cholestasis is a subject of intense investigation. Villin is an actin binding, bundling, and severing protein needed for maintenance of structural integrity of canalicular microvilli, in which membrane transporters required for bile secretion are located. We aimed to investigate the role of canalicular cytoskeletal proteins in three genetically unrelated children with a biliary atresia-like clinical disorder, each of whom developed liver failure requiring liver transplantation.

METHODS

Explanted livers from the three patients were examined by standard pathological methods followed by transmission and cryoimmunoelectron microscopy. With archival tissue samples, a panel of cytoskeletal proteins was investigated by immunohistochemistry and western blotting, with purified canalicular membrane preparations. Villin mRNA analyses were undertaken on liver homogenates, with primers from coding regions of the human villin gene. Classic biliary atresia, other types of cholestasis, and normal livers served as controls.

FINDINGS

In patients, pronounced ultrastructural deformities of canaliculi and especially of their microvilli were noted, which correlated with absence of villin protein by immunostaining of liver tissue sections and by western blot analysis. Additionally, villin mRNA was strikingly reduced or absent. These results differed greatly from those in controls.

INTERPRETATION

These results suggest that the disorder described mimics biliary atresia, but structural and molecular pathological findings differ. We propose that a functional abnormality in villin gene expression is key to the mechanism of cholestasis in patients with progressive cholestasis and hepatic failure.

Organ design for generation and reception of CO: lessons from the liver

Carbon monoxide (CO) is synthesized in vivo by heme oxygenase. Although for many years CO had been regarded as potentially toxic waste, recent studies have indicated that it is a signaling molecule with important physiological functions. Nitric oxide (NO), another diatomic diffusible gas, is regarded as an established signaling molecule. Structural similarities between CO and NO have led many investigators to draw analogies between the two gaseous mediators. Whereas the NO signaling system has been well defined as to its receptor molecule, soluble guanylate cyclase, the CO system has been conceived to require further tuning with respect to identifying its receptor molecules and its downstream effectors. Furthermore, there has been little quantitative information to argue for a physiological role of CO in vivo. This review, therefore, focuses on recent developments on both physiologic and pathophysiologic roles of CO in the model of isolated perfused liver of rats where endogenous production of CO is actually estimated. This model has revealed that CO acts as an endogenous vasorelaxant in the liver and that effects of CO are at least in part cyclic GMP-dependent. It has also provided answers to many questions of hepatobiliary functions that had not been resolved because of the complexity introduced by the interplay between NO and CO.

Distribution of signaling molecules involved in vasopressin-induced Ca2+ mobilization in rat hepatocyte multiplets

In freshly isolated rat hepatocyte multiplets, Ca2+ signals in response to vasopressin are highly organized. In this study we used specific probes to visualize, by fluorescence and confocal microscopy, the main signaling molecules involved in vasopressin-mediated Ca2+ responses. V1a receptors were detected with a novel fluorescent antagonist, Rhm8-PVA. The Galphaq/Galpha11, PLCbeta3, PIP2, and InsP3 receptors were detected with specific antibodies. V1a vasopressin receptors and PIP2 were associated with the basolateral membrane and were not detected in the bile canalicular domain. Galphaq/Galpha11, PLCbeta3, and InsP3 receptors were associated with the basolateral membrane and also with other intracellular structures. We used double labeling, Western blotting, and drugs (cytochalasin D, colchicine) known to disorganize the cytoskeleton to demonstrate the partial co-localization of Galphaq/Galpha11 with F-actin.

Visualization of cell surface vasopressin V1a receptors in rat hepatocytes with a fluorescent linear antagonist

To visualize cell surface V1a vasopressin receptors in rat hepatocytes in the absence of receptor-mediated endocytosis, we used a high-affinity fluorescent linear antagonist, Rhm8-PVA. Epifluorescence microscopy (3CCD camera) and fluorescence spectroscopy were used. Rhm8-PVA alone did not stimulate Ca2+ signals and competitively blocked Ca2+ signals (Kinact of 3.0 nM) evoked by arginine vasopressin (vasopressin). When rat hepatocytes were incubated with 10 nM of Rhm8-PVA for 30 min at 4C, the fluorescent antagonist bound to the surface of cells, presumably the plasma membrane. The V1a receptor specificity of Rhm8-PVA binding was confirmed by its displacement by the nonfluorescent antagonist V4253 and by the natural hormone vasopressin at 4C. Prior vasopressin-mediated endocytosis of V1a receptors at 37C abolished binding of the labeled antagonist, whereas in non-preincubated cells, Rhm8-PVA labeled the cell surface of rat hepatocytes. When cells labeled with Rhm8-PVA at 4C were warmed to 37C to initiate receptor-mediated internalization of the fluorescent complex, Rhm8-PVA remained at the cell surface. Incubation temperature at 4C or 37C had little effect on binding of Rhm8-PVA. We conclude that Rhm8-PVA is unable to evoke receptor-mediated endocytosis and can readily be used to visualize cell surface receptors in living cells.

Mechanisms and functional features of polarized membrane traffic in epithelial and hepatic cells

Epithelial cells express plasma-membrane polarity in order to meet functional requirements that are imposed by their interaction with different extracellular environments. Thus apical and basolateral membrane domains are distinguished that are separated by tight junctions in order to maintain the specific lipid and protein composition of each domain. In hepatic cells, the plasma membrane is also polarized, containing a sinusoidal (basolateral) and a bile canalicular (apical)-membrane domain. Relevant to the biogenesis of these domains are issues concerning sorting, (co-)transport and regulation of transport of domain-specific membrane components. In epithelial cells, specific proteins and lipids, destined for the apical membrane, are sorted in the trans-Golgi network (TGN), which involves their sequestration into cholesterol/sphingolipid 'rafts', followed by 'direct' transport to the apical membrane. In hepatic cells, a direct apical transport pathway also exists, as revealed by transport of sphingolipids from TGN to the apical membrane. This is remarkable, since in these cells numerous apical membrane proteins are 'indirectly' sorted, i.e. they are first transferred to the basolateral membrane prior to their subsequent transcytosis to the apical membrane. This raises intriguing questions as to the existence of specific lipid rafts in hepatocytes. As demonstrated in studies with HepG2 cells, it has become evident that, in hepatic cells, apical transport pathways can be regulated by protein kinase activity, which in turn modulates cell polarity. Finally, an important physiological function of hepatic cells is their involvement in intracellular transport and secretion of bile-specific lipids. Mechanisms of these transport processes, including the role of multidrug-resistant proteins in lipid translocation, will be discussed in the context of intracellular vesicular transport. Taken together, hepatic cell systems provide an important asset to studies aimed at elucidating mechanisms of sorting and trafficking of lipids (and proteins) in polarized cells in general.

An update on high-yield hepatocyte isolation methods and on the potential clinical use of isolated liver cells

Isolated hepatocytes are a suitable system for the study of hepatic physiology and metabolism. They are also used for pharmacological and toxicological studies related to hepatic uptake, metabolism, excretion and toxicity of xenobiotics, as well as morphological and metabolic effects induced in the liver as a result of drug or toxic substance exposure. In this paper, the enzymatic methods for hepatocyte isolation in some mammalian species are reviewed, and methods for evaluating cell purification and assessment of cellular morphology and function are also examined. More recently, interest in hepatocyte transplantation has increased, and the clinical experimentation of hepatocyte-based liver support systems has attracted the attention of scientists and hepatologists. From a clinical perspective, using isolated hepatocytes could be useful both for supporting an acutely devastated liver, a chronically diseased liver, and for correcting genetic disorders resulting in metabolically deficient states. Reports of clinical usage of isolated allogenic hepatocytes in hepatocellular transplantation and of xenogenic liver cells in constructing bio-artificial liver support systems are promising, and are renewing interest in the development of methods for isolation and purification of hepatocytes.

Hepatobiliary effects of tertiary-butylhydroperoxide (tBOOH) in isolated rat hepatocyte couplets

The organic hydroperoxide, tertiary-butylhydroperoxide (tBOOH), causes oxidative damage in a number of cell types. It is used here in an isolated rat hepatocyte couplet preparation to study adverse hepatobiliary effects of peroxidative damage in vitro. At subcytotoxic concentrations (as determined by lactate dehydrogenase release and maintenance of cytoplasmic ATP concentrations) tBOOH caused decreased accumulation of a fluorescent bile acid analogue, cholyl-lysyl-fluorescein (CLF), in the canalicular vacuole of couplets (a hepatobiliary effect; cholestasis). This was dose dependent in the range 100-200 microM. At the same concentrations it brought about release of preaccumulated CLF, suggesting that its effect was more likely to be on sealing properties of the vacuole than processes of uptake, transcytosis, and secretion. Pretreatment of tBOOH-treated couplets with the antioxidants deferoxamine mesylate (iron chelator) and dimethyl sulfoxide (free radical scavenger) resulted in the prevention of both canalicular vacuolar accumulation (cVA, which assesses canalicular function) and canalicular vacuolar retention (cVR, which assesses the retaining ability of couplets) depression at 100 microM tBOOH but not at higher concentrations. This indicates that the cholestatic effect of tBOOH has a preventable and nonpreventable phase and that free radicals are involved in these processes. By selectively generating the two types of tBOOH radical, peroxyl (tBOO.) and alkoxyl (tBO.), using suitable catalysts, we were able to determine that the peroxyl radical was most probably involved in tBOOH-induced cholestasis. This was further supported by experiments employing specific peroxyl and alkoxyl radical scavengers; only the peroxyl scavenger reduced the effect of tBOOH upon canalicular function under the conditions studied.

Carbon monoxide as a regulator of bile canalicular contractility in cultured rat hepatocytes

This study aimed to examine the mechanism(s) by which carbon monoxide (CO), a product of heme oxygenase reaction, controls the contractility of bile canaliculus (BC) in hepatocytes. When BCs associated with the couplet cells in cultured rat hepatocyte suspension were observed using time-lapse video microscopy, they exhibited periodical contractions with a most-probable interval of 6 minutes under our experimental conditions. The addition of 1 micromol/L zinc protoporphyrin IX (ZnPP), a potent inhibitor of heme oxygenase, to the culture medium elicited a 40% shortening of the interval time together with an increase in intracellular calcium concentrations, while the same concentration of iron protoporphyrin IX did not induce such changes. The production of CO, which was 0.5 nmol/h/10(8) cells in the absence of ZnPP, diminished to less than 0.1 nmol/h/10(8) cells upon application of ZnPP. The ZnPP-elicited increases in both contractile frequency and intracellular calcium concentrations were attenuated by the addition of 1 micromol/L CO or 50 micromol/L 1,2-bis(2-aminophenoxy) ethane-tetraacetate, a calcium chelator. Clotrimazole or metyrapone, inhibitors of cytochrome P450-dependent monooxygenase activities, also attenuated the ZnPP-induced elevation of the contractile frequency. On the other hand, intracellular cyclic guanosine monophosphate (cGMP) contents were not altered significantly by the application of ZnPP or by CO. These results indicate that CO generated by heme oxygenase controls the BC function by changing intracellular calcium concentrations presumably through a mechanism involving the cytochrome P450 reaction.

Distribution of heme oxygenase isoforms in rat liver. Topographic basis for carbon monoxide-mediated microvascular relaxation

Carbon monoxide (CO) derived from heme oxygenase has recently been shown to play a role in controlling hepatobiliary function, but intrahepatic distribution of the enzyme is unknown. We examined distribution of two kinds of the heme oxygenase isoforms (HO-1 and HO-2) in rat liver immunohistochemically using monoclonal antibodies. The results showed that distribution of the two isoforms had distinct topographic patterns: HO-1, an inducible isoform, was observed only in Kupffer cells, while HO-2, a constitutive form, distributed to parenchymal cells, but not to Kupffer cells. Both isoforms were undetectable in hepatic stellate cells and sinusoidal endothelial cells. Of the two isoforms, HO-2 in the parenchymal cell rather than HO-1 in the Kupffer cell, appears to play a major role in regulation of microvascular tone. In the perfused liver, administration of HbO2, a CO-trapping reagent that can diffuse across the fenestrated endothelium into the space of Disse, elicited a marked sinusoidal constriction, while administration of a liposome-encapsulated Hb that cannot enter the space had no effect on the microvascular tone. These results suggest that CO evolved by HO-2 in the parenchymal cells, and, released to the extrasinusoidal space, served as the physiological relaxant for hepatic sinusoids.