Abnormal expression of anion exchanger genes in primary biliary cirrhosis

Role of the anion exchanger 2 in the pathogenesis and treatment of primary biliary cirrhosis

The essential anion exchanger (AE) involved in biliary bicarbonate secretion is AE2/SLC4A2, a membrane protein which has also been recognized to be relevant for the regulation of the intracellular pH (pH(i)) in several cell types. Previously, we reported that the expression of AE2 mRNA is diminished in liver biopsies and peripheral blood mononuclear cells from patients with primary biliary cirrhosis (PBC). Immunohistochemical studies indicated that the expression of the AE2 protein is decreased in the bile ducts and hepatocytes in PBC livers. Moreover, we found that bile duct cells isolated from PBC patients and cultured for a few passages exhibit defective Na(+)-independent Cl(-)/HCO(3)(-) exchange. Interestingly, positron emission tomography studies have shown that PBC patients, even at early stages of the disease, fail to secrete bicarbonate to bile in response to secretin, a defect that can be partially reversed after several months of treatment with ursodeoxycholic acid. Altogether, these findings sustain our hypothesis that dysfunctions related to AE2 might have a role in the pathogenesis of PBC. Inadequate AE2 function in lymphocytes may disturb pH(i) regulation in these cells and alter immune homeostasis leading to autoimmunity. On the other hand, reduced AE2 in cholangiocytes could cause cholestasis and oxidative stress of bile duct cells. Cholangiocyte changes, together with altered immune homeostasis, could favor the development of antimitochondrial antibodies and the autoimmune attack on biliary ducts. Our recent findings that Ae2(a,b)-deficient mice indeed display most of these features strongly support the notion that AE2 abnormalities may be involved in the pathogenesis of PBC.

Fibrosis in autoimmune and cholestatic liver disease

Autoimmune and cholestatic liver disease account for a significant part of end-stage liver disease and are leading indications for liver transplantation. Especially cholestatic liver diseases (primary biliary cirrhosis and primary sclerosing cholangitis) appear to be different from other chronic liver diseases with regards to pathogenesis. Portal fibroblasts located in the connective tissue surrounding bile ducts appear to be different from hepatic stellate cells with regards to expression of marker proteins and response the profibrogenic and mitogenic stimuli. In addition there is increasing evidence for a cross talk between activated cholangiocytes and portal myofibroblasts. Several animal models have improved our understanding of the mechanisms underlying these chronic liver diseases. In the present review, we discuss the current concepts and ideas with regards to myofibroblastic cell populations, mechanisms of fibrosis, summarize characteristic histological findings and currently employed animal models of autoimmune and cholestatic liver disease.

Nuclear receptors as new perspective for the management of liver diseases

Nuclear receptors (NRs) are ligand-activated transcription factors that act as sensors for a broad range of natural and synthetic ligands and regulate several key hepatic functions including bile acid homeostasis, bile secretion, lipid and glucose metabolism, as well as drug deposition. Moreover, NRs control hepatic inflammation, regeneration, fibrosis, and tumor formation. Therefore, NRs are key for understanding the pathogenesis and pathophysiology of a wide range of hepatic disorders. Finally, targeting NRs and their alterations offers exciting new perspectives for the treatment of liver diseases.

Chronic fibrosing cholangiopathies: a consequence of a defective HCO₃⁻ "umbrella"?

The pathogenesis of chronic cholangiopathies, in particular primary biliary cirrhosis and primary sclerosing cholangitis, is still obscure. A stimulating hypothesis is proposed by Beuers et al. They reason that, since cholangiocytes are exposed to high concentrations of hydrophobic bile salts that are toxic at μM concentrations, these cells had to develop protective mechanisms. Apart from micelle formation, the authors argue that biliary HCO₃⁻ secretion serves to maintain an alkaline pH near the apical surface of cholangiocytes by forming a HCO₃⁻ "umbrella". In this alkaline environment, glycine conjugated bile salts (which are predominant in man), with a pKa of ~4, remain deprotonated and are unable to permeate the apical membrane and cause cell damage. Functional impairment of biliary HCO₃⁻ secretion leads to enhanced vulnerability of cholangiocytes toward the attack of hydrophobic bile salts, causing cell damage and cholangitis. Such an impairment could be due to genetic factors, like mutations of the anion exchanger 2 (a variant of the Cl⁻/HCO₃⁻ exchanger) in primary biliary cirrhosis or of TGR5 (a bile salt receptor implicated in the regulation of HCO₃⁻ secretion) in primary sclerosing cholangitis. This stimulating hypothesis is amenable to experimental testing and has potential pathophysiological and therapeutic implications.

The biliary HCO(3)(-) umbrella: a unifying hypothesis on pathogenetic and therapeutic aspects of fibrosing cholangiopathies

This review focuses on the hypothesis that biliary HCO(3)(-) secretion in humans serves to maintain an alkaline pH near the apical surface of hepatocytes and cholangiocytes to prevent the uncontrolled membrane permeation of protonated glycine-conjugated bile acids. Functional impairment of this biliary HCO(3)(-) umbrella or its regulation may lead to enhanced vulnerability of cholangiocytes and periportal hepatocytes toward the attack of apolar hydrophobic bile acids. An intact interplay of hepatocellular and cholangiocellular adenosine triphosphate (ATP) secretion, ATP/P2Y- and bile salt/TGR5-mediated Cl(-)/ HCO(3)(-) exchange and HCO(3)(-) secretion, and alkaline phosphatase-mediated ATP breakdown may guarantee a stable biliary HCO(3)(-) umbrella under physiological conditions. Genetic and acquired functional defects leading to destabilization of the biliary HCO(3)(-) umbrella may contribute to development and progression of various forms of fibrosing/sclerosing cholangitis.

Update on the genetics and genomics of PBC

Despite recent progress, the pathogenic mechanisms governing PBC development, treatment response and outcome remain unknown. This deficiency is in large part due to the complex nature of PBC, wherein various environmental factors may be capable of prompting disease, but only in the context of underlying genetic susceptibility. Identification of genomic loci containing these heritable risk factors has been slowed by the rarity and late onset of PBC, which has made difficult the collection of sufficient numbers of patients and family members for meaningful genetic analyses. Advancements in our ability to catalog the genetic variation in large numbers of individuals at a genome-wide scale, coupled with unprecedented efforts to recruit PBC patients for genetic study, positions us to generate data that could fundamentally change our understanding of PBC and lead to clinical innovation. Indeed, the first genome-wide association study for PBC has been published, in which multiple genes involved with IL12 signaling, a pathway that is being targeted in treatment of other inflammatory conditions, were implicated in disease. However, this study was relatively small in the genome-wide milieu and a significantly expanded effort will be necessary to truly elucidate the genetic architecture of PBC. Moving ahead, cooperation between the groups collecting biospecimens and generating genome-wide data from large numbers of patients with PBC will be essential, not only to increase power for fine mapping and future studies of rare variants and epistasis; but to streamline efforts to perform functional validation of novel discoveries. Here we provide a brief update of the current state of genetics in PBC to form a basis for understanding the considerable progress that is likely to be made in the coming years.

Murine models of autoimmune cholangitis

PURPOSE OF REVIEW

Primary biliary cirrhosis (PBC) is a human autoimmune liver disease whose molecular pathogenesis is poorly understood because of the difficulty in accessing human tissue and the absence of appropriate animal models. Recently, several unique murine models of human PBC have been discovered. These models have great potential for illustrating the cause and the cellular events that lead to biliary-specific damage. The purpose of this review is to summarize recent progress in these models.

RECENT FINDINGS

The murine models of autoimmune cholangitis include the transforming growth factor beta receptor II (TGF-betaRII) dominant-negative (dnTGF-betaRII), IL-2 receptor alpha deleted (IL-2Ralpha-/-), scurfy, nonobese diabetic (NOD) c3c4, and Ae2 gene-disrupted (Ae2a,b-/-) mice. Recently, we have also established a successful murine model following the immunization with a chemical mimicry of the lipoyl-lysine residue of the E2 component of PDC-E2.

SUMMARY

These emerging murine models have greatly enabled researchers to address the pathogenesis of human PBC and to elucidate pathogenic factors. These models will ultimately lead to new therapeutic options for human PBC.

Nuclear receptors as drug targets in cholestasis and drug-induced hepatotoxicity

Nuclear receptors are key regulators of various processes including reproduction, development, and metabolism of xeno- and endobiotics such as bile acids and drugs. Research in the last two decades provided researchers and clinicians with a detailed understanding of the regulation of these processes and, most importantly, also prompted the development of novel drugs specifically targeting nuclear receptors for the treatment of a variety of diseases. Some nuclear receptor agonists are already used in daily clinical practice but many more are currently designed or tested for the treatment of diabetes, dyslipidemia, fatty liver disease, cancer, drug hepatotoxicity and cholestasis. The hydrophilic bile acid ursodeoxycholic acid is currently the only available drug to treat cholestasis but its efficacy is limited. Therefore, development of novel treatments represents a major goal for both pharmaceutical industry and academic researchers. Targeting nuclear receptors in cholestasis is an intriguing approach since these receptors are critically involved in regulation of bile acid homeostasis. This review will discuss the general role of nuclear receptors in regulation of transporters and other enzymes maintaining bile acid homeostasis and will review the role of individual receptors as therapeutic targets. In addition, the central role of nuclear receptors and other transcription factors such as the aryl hydrocarbon receptor (AhR) and the nuclear factor-E2-related factor (Nrf2) in mediating drug disposition and their potential therapeutic role in drug-induced liver disease will be covered.

Bicarbonate secretion of mouse cholangiocytes involves Na(+)-HCO(3)(-) cotransport in addition to Na(+)-independent Cl(-)/HCO(3)(-) exchange

Bicarbonate secretion from cholangiocytes is required for appropriate adjustment of primary canalicular bile along the biliary tract. In human and rat cholangiocytes, bicarbonate secretion is mediated by anion exchanger (AE) 2, an electroneutral Na(+)-independent Cl(-)/HCO(3) (-) AE also involved in intracellular pH (pH(i)) regulation. In Ae2(a,b)-deficient mice, pH(i) is increased in lymphocytes and fibroblasts, whereas it is surprisingly normal in cholangiocytes. Here, we analyze the mechanisms for HCO(3) (-) secretion in cultured Ae2(a,b) (+/+) and Ae2(a,b) (-/-) mouse cholangiocytes by microfluorimetric measurement of pH(i) changes upon established perfusion maneuvers. Cl(-) withdrawal by isethionate-based perfusions showed that Ae2(a,b) (+/+) but not Ae2(a,b) (-/-) mouse cholangiocytes can display Cl(-)/HCO(3) (-) exchange, which is therefore entirely mediated by Ae2. Nevertheless, simultaneous withdrawal of Cl(-) and Na(+) revealed that mouse cholangiocytes possess an additional transport activity for HCO(3) (-) secretion not observed in control rat cholangiocytes. Propionate-based maneuvers indicated that this supplemental Na(+)-driven HCO(3) (-)-secreting activity is Cl(-)-independent, consistent with a Na(+)-HCO(3) (-) cotransport (NBC). NBC activity is greater in Ae2(a,b) (-/-) than Ae2(a,b) (+/+) mouse cholangiocytes, and membrane-depolarization experiments showed that it is electrogenic. Consistent with the potential role of Slc4a4/Nbc1 as the involved transporter, Ae2(a,b) (-/-) mouse cholangiocytes exhibit up-regulated expression of this electrogenic NBC carrier. Whereas Ae2-mediated Cl(-)/HCO(3) (-) exchange in Ae2(a,b) (+/+) mouse cholangiocytes is stimulated by cyclic adenosine monophosphate (cAMP) and acetylcholine, the NBC activity is down-regulated by cAMP and adenosine triphosphate (ATP) in Ae2(a,b) (-/-) mouse cholangiocytes. Polarized Ae2(a,b) (-/-) mouse cholangiocytes placed in Ussing chambers show decreased (but not abolished) cAMP-dependent Cl(-) current and increased ATP-dependent/Ca(2+)-activated Cl(-) secretion, which run in parallel with decreased cystic fibrosis transmembrane conductance regulator messenger RNA expression and increased intracellular Ca(2+) levels.

CONCLUSION

Bicarbonate secretion in mouse cholangiocytes involves two differentially regulated activities: Ae2-mediated Cl(-)/HCO(3) (-) exchange and Na(+)-HCO(3) (-) cotransport.

New molecular insights into the mechanisms of cholestasis

Recent progress in basic research has enhanced our understanding of the molecular mechanisms of normal bile secretion and their alterations in cholestasis. Genetic transporter variants contribute to an entire spectrum of cholestatic liver diseases and can cause hereditary cholestatic syndromes or determine susceptibility and disease progression in acquired cholestatic disorders. Cholestasis is associated with complex transcriptional and post-transcriptional alterations of hepatobiliary transporters and enzymes participating in bile formation. Ligand-activated nuclear receptors for bile acids and other biliary compounds play a key role in the regulation of genes required for bile formation. Pharmacological interventions in cholestasis may aim at modulating such novel regulatory pathways. This review will summarize the principles of molecular alterations in cholestasis and will give an overview of potential clinical implications.

Primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is an immune-mediated chronic cholestatic liver disease with a slowly progressive course. Without treatment, most patients eventually develop fibrosis and cirrhosis of the liver and may need liver transplantation in the late stage of disease. PBC primarily affects women (female preponderance 9–10:1) with a prevalence of up to 1 in 1,000 women over 40 years of age. Common symptoms of the disease are fatigue and pruritus, but most patients are asymptomatic at first presentation. The diagnosis is based on sustained elevation of serum markers of cholestasis, i.e., alkaline phosphatase and gamma-glutamyl transferase, and the presence of serum antimitochondrial antibodies directed against the E2 subunit of the pyruvate dehydrogenase complex. Histologically, PBC is characterized by florid bile duct lesions with damage to biliary epithelial cells, an often dense portal inflammatory infiltrate and progressive loss of small intrahepatic bile ducts. Although the insight into pathogenetic aspects of PBC has grown enormously during the recent decade and numerous genetic, environmental, and infectious factors have been disclosed which may contribute to the development of PBC, the precise pathogenesis remains enigmatic. Ursodeoxycholic acid (UDCA) is currently the only FDA-approved medical treatment for PBC. When administered at adequate doses of 13–15 mg/kg/day, up to two out of three patients with PBC may have a normal life expectancy without additional therapeutic measures. The mode of action of UDCA is still under discussion, but stimulation of impaired hepatocellular and cholangiocellular secretion, detoxification of bile, and antiapoptotic effects may represent key mechanisms. One out of three patients does not adequately respond to UDCA therapy and may need additional medical therapy and/or liver transplantation. This review summarizes current knowledge on the clinical, diagnostic, pathogenetic, and therapeutic aspects of PBC.

Common genetic variation and haplotypes of the anion exchanger SLC4A2 in primary biliary cirrhosis

OBJECTIVES

Deficiencies of the anion exchanger SLC4A2 are thought to play a pathogenic role in primary biliary cirrhosis (PBC), as the evidenced by decreased expression and activity in PBC patients and development of disease features in SLC4A2 knockout mice. We hypothesized that genetic variation in SLC4A2 might influence this pathogenic contribution. Thus, we aimed to perform a comprehensive assessment of SLC4A2 genetic variation in PBC using a linkage disequilibrium (LD)-based haplotype-tagging approach.

METHODS

Twelve single nucleotide polymorphisms (SNPs) across SLC4A2 were genotyped in 409 PBC patients and 300 controls and evaluated for association with disease, as well as with prior orthotopic liver transplant and antimitochondrial antibody (AMA) status among the PBC patients, both individually and as inferred haplotypes, using logistic regression.

RESULTS

All SNPs were in Hardy-Weinberg equilibrium. No associations with disease or liver transplantation were detected, but two variants, rs2303929 and rs3793336, were associated with negativity for antimitochondrial antibodies among the PBC patients.

CONCLUSIONS

The common genetic variation of SLC4A2 does not directly affect the risk of PBC or its clinical outcome. Whether the deficiency of SLC4A2 expression and activity observed earlier in PBC patients is an acquired epiphenomenon of underlying disease or is because of heritable factors in unappreciated regulatory regions remains uncertain. Of note, two SLC4A2 variants appear to influence AMA status among PBC patients. The mechanisms behind this finding are unclear.

Nuclear receptors as therapeutic targets in cholestatic liver diseases

Cholestasis results in intrahepatic accumulation of cytotoxic bile acids, which cause liver damage ultimately leading to biliary fibrosis and cirrhosis. Cholestatic liver injury is counteracted by a variety of adaptive hepatoprotective mechanisms including alterations in bile acid transport, synthesis and detoxification. The underlying molecular mechanisms are mediated mainly at a transcriptional level via a complex network involving nuclear receptors including the farnesoid X receptor, pregnane X receptor, vitamin D receptor and constitutive androstane receptor, which target overlapping, although not identical, sets of genes. Because the intrinsic adaptive response to bile acids cannot fully prevent liver injury in cholestasis, therapeutic targeting of these receptors via specific and potent agonists may further enhance the hepatic defence against toxic bile acids. Activation of these receptors results in repression of bile acid synthesis, induction of phases I and II bile acid hydroxylation and conjugation and stimulation of alternative bile acid export while limiting hepatocellular bile acid import. Furthermore, the use of nuclear receptor ligands may not only influence bile acid transport and metabolism but may also directly target hepatic fibrogenesis and inflammation. Many drugs already used to treat cholestasis and its complications such as pruritus (e.g. ursodeoxycholic acid, rifampicin, fibrates) may act via activation of nuclear receptors. More specific and potent nuclear receptor ligands are currently being developed. This article will review the current knowledge on nuclear receptors and their potential role in the treatment of cholestatic liver diseases.

When lightning strikes twice: the plot thickens for a dual role of the anion exchanger 2 (AE2/SLC4A2) in the pathogenesis and treatment of primary biliary cirrhosis

BACKGROUND/AIMS

Cl(-)/HCO(3)(-) anion exchanger 2 (AE2) is involved in intracellular pH (pH(i)) regulation and transepithelial acid-base transport, including secretin-stimulated biliary bicarbonate excretion. AE2 gene expression was found to be reduced in liver biopsy specimens and blood mononuclear cells from patients with primary biliary cirrhosis (PBC), a disease characterized by chronic nonsuppurative cholangitis associated with antimitochondrial antibodies (AMA) and other autoimmune phenomena. In mice with widespread Ae2 gene disruption, we previously reported altered spermiogenesis and reduced gastric acid secretion. We now describe the hepatobiliary and immunologic changes observed in these Ae2(a,b)-deficient mice.

METHODS

In this murine model, splenocyte pH(i) and T-cell populations were studied by flow cytometry. CD3-stimulated cytokine secretion was estimated using cytokine arrays. AMA were evaluated by immunoblotting and proteomics. Hepatobiliary changes were assessed by immunohistopathology, flow cytometry, and serum biochemistry. Cholangiocyte gene expression was analyzed by real-time polymerase chain reaction.

RESULTS

Ae2(a,b)(-/-) mice exhibit splenomegaly, elevated pH(i) in splenocytes, increased production of interleukin-12p70 and interferon gamma, expanded CD8(+) T-cell population, and under represented CD4(+)FoxP3(+)/regulatory T cells. Most Ae2(a,b)(-/-) mice tested positively for AMA, showing increased serum levels of immunoglobulin M and G, and liver-specific alkaline phosphatase. About one third of Ae2(a,b)(-/-) mice had extensive portal inflammation with CD8(+) and CD4(+) T lymphocytes surrounding damaged bile ducts. Cholangiocytes isolated from Ae2(a,b)(-/-) mice showed gene expression changes compatible with oxidative stress and increased antigen presentation.

CONCLUSIONS

Ae2 deficiency alters pH(i) homeostasis in immunocytes and gene expression profile in cholangiocytes, leading to immunologic and hepatobiliary changes that resemble PBC.

Induction of autoimmune cholangitis in non-obese diabetic (NOD).1101 mice following a chemical xenobiotic immunization

Our laboratory has suggested that loss of tolerance to pyruvate dehydrogenase (PDC-E2) leads to an anti-mitochondrial antibody response and autoimmune cholangitis, similar to human primary biliary cirrhosis (PBC). We have suggested that this loss of tolerance can be induced either via chemical xenobiotic immunization or exposure to select bacteria. Our work has also highlighted the importance of genetic susceptibility. Using the non-obese diabetic (NOD) congenic strain 1101 (hereafter referred to as NOD.1101 mice), which has chromosome 3 regions from B6 introgressed onto a NOD background, we exposed animals to 2-octynoic acid (2OA) coupled to bovine serum albumin (BSA). 2OA has been demonstrated previously by a quantitative structural activity relationship to react as well as or better than lipoic acid to anti-mitochondrial antibodies. We demonstrate herein that NOD.1101 mice immunized with 2OA-BSA, but not with BSA alone, develop high titre anti-mitochondrial antibodies and histological features, including portal infiltrates enriched in CD8(+) cells and liver granulomas, similar to human PBC. We believe this model will allow the rigorous dissection of early immunogenetic cause of biliary damage.

Genetic factors of susceptibility and of severity in primary biliary cirrhosis

BACKGROUND/AIMS

In primary biliary cirrhosis (PBC), pathogenesis is influenced by genetic factors that remain poorly elucidated up to now. We investigated the impact of sequence diversity in candidate genes involved in immunity (CTLA-4 and TNFalpha), in bile formation (10 hepatobiliary transporter genes) and in the adaptative response to cholestasis (three nuclear receptor genes) on the susceptibility and severity of PBC.

METHODS

A total of 42 Ht SNPs were identified and compared in 258 PBC patients and two independent groups of 286 and 269 healthy controls. All participants were white continental individuals with French ancestry.

RESULTS

Ht SNPs of CTLA-4 and TNFalpha genes were significantly associated with susceptibility to PBC. The progression rate of liver disease under ursodeoxycholic acid (UDCA) therapy was significantly linked to SNPs of TNFalpha and SLC4A2/anion exchanger 2 (AE2) genes. A multivariate Cox regression analysis including clinical and biochemical parameters showed that SLC4A2/AE2 variant was an independent prognostic factor.

CONCLUSIONS

These data point to a primary role of genes encoding regulators of the immune system in the susceptibility to PBC. They also demonstrate that allelic variations in TNFalpha and SLC4A2/AE2 have a significant impact on the evolutive profile of PBC under UDCA therapy.

Ae2a,b-deficient mice develop antimitochondrial antibodies and other features resembling primary biliary cirrhosis

BACKGROUND & AIMS

Cl(-)/HCO(3)(-) anion exchanger 2 (AE2) is involved in intracellular pH (pH(i)) regulation and transepithelial acid-base transport, including secretin-stimulated biliary bicarbonate excretion. AE2 gene expression was found to be reduced in liver biopsy specimens and blood mononuclear cells from patients with primary biliary cirrhosis (PBC), a disease characterized by chronic nonsuppurative cholangitis associated with antimitochondrial antibodies (AMA) and other autoimmune phenomena. In mice with widespread Ae2 gene disruption, we previously reported altered spermiogenesis and reduced gastric acid secretion. We now describe the hepatobiliary and immunologic changes observed in these Ae2(a.b)-deficient mice.

METHODS

In this murine model, splenocyte pH(i) and T-cell populations were studied by flow cytometry. CD3-stimulated cytokine secretion was estimated using cytokine arrays. AMA were evaluated by immunoblotting and proteomics. Hepatobiliary changes were assessed by immunohistopathology, flow cytometry, and serum biochemistry. Cholangiocyte gene expression was analyzed by real-time polymerase chain reaction.

RESULTS

Ae2(a,b)(-/-) mice exhibit splenomegaly, elevated pH(i) in splenocytes, increased production of interleukin-12p70 and interferon gamma, expanded CD8(+) T-cell population, and under represented CD4(+)FoxP3(+)/regulatory T cells. Most Ae2(a,b)(-/-) mice tested positively for AMA, showing increased serum levels of immunoglobulin M and G, and liver-specific alkaline phosphatase. About one third of Ae2(a,b)(-/-) mice had extensive portal inflammation with CD8(+) and CD4(+) T lymphocytes surrounding damaged bile ducts. Cholangiocytes isolated from Ae2(a,b)(-/-) mice showed gene expression changes compatible with oxidative stress and increased antigen presentation.

CONCLUSIONS

Ae2 deficiency alters pH(i) homeostasis in immunocytes and gene expression profile in cholangiocytes, leading to immunologic and hepatobiliary changes that resemble PBC.

Combination of ursodeoxycholic acid and glucocorticoids upregulates the AE2 alternate promoter in human liver cells

Primary biliary cirrhosis (PBC) is a cholestatic disease associated with autoimmune phenomena and alterations in both biliary bicarbonate excretion and expression of the bicarbonate carrier AE2. The bile acid ursodeoxycholic acid (UCDA) is currently used in treatment of cholestatic liver diseases and is the treatment of choice in PBC; however, a subset of PBC patients respond poorly to UDCA monotherapy. In these patients, a combination of UDCA and glucocorticoid therapy appears to be beneficial. To address the mechanism of this benefit, we analyzed the effects of UDCA and dexamethasone on AE2 gene expression in human liver cells from hepatocyte and cholangiocyte lineages. The combination of UDCA and dexamethasone, but not UDCA or dexamethasone alone, increased the expression of liver-enriched alternative mRNA isoforms AE2b1 and AE2b2 and enhanced AE2 activity. Similar effects were obtained after replacing UDCA with UDCA conjugates. In in vitro and in vivo reporter assays, we found that a UDCA/dexamethasone combination upregulated human AE2 alternate overlapping promoter sequences from which AE2b1 and AE2b2 are expressed. In chromatin immunoprecipitation assays, we demonstrated that combination UCDA/dexamethasone treatment induced p300-related interactions between HNF1 and glucocorticoid receptor on the AE2 alternate promoter. Our data provide a potential molecular explanation for the beneficial effects of the combination of UDCA and glucocorticoids in PBC patients with inadequate response to UDCA monotherapy.

Mechanisms of cholestasis

This article gives an overview of the molecular and cellular mechanisms of cholestasis. Topics reviewed include the pathomechanisms of hereditary cholestasis syndromes, such as progressive familial intrahepatic cholestasis, and hepatocellular transporter defects encountered in various acquired cholestatic disorders, such as intrahepatic cholestasis of pregnancy, drug-induced cholestasis, inflammatory cholestasis, primary sclerosing cholangitis, and primary biliary cirrhosis. In addition, current concepts regarding adaptive hepatocellular mechanisms counteracting cholestatic liver damage are discussed.

Lessons from the toxic bile concept for the pathogenesis and treatment of cholestatic liver diseases

Alterations in bile secretion at the hepatocellular and cholangiocellular levels may cause cholestasis. Formation of 'toxic bile' may be the consequence of abnormal bile composition and can result in hepatocellular and/or bile duct injury. The canalicular phospholipid flippase (Mdr2/MDR3) normally mediates biliary excretion of phospholipids, which normally form mixed micelles with bile acids and cholesterol to protect the bile duct epithelium from the detergent properties of bile acids. Mdr2 knockout mice are not capable of excreting phospholipids into bile and spontaneously develop bile duct injury with macroscopic and microscopic features closely resembling human sclerosing cholangitis. MDR3 mutations have been linked to a broad spectrum of hepatobiliary disorders in humans ranging from progressive familial intrahepatic cholestasis in neonates to intrahepatic cholestasis of pregnancy, drug-induced cholestasis, intrahepatic cholelithiasis, sclerosing cholangitis and biliary cirrhosis in adults. Other examples for bile injury due to the formation of toxic bile include the cholangiopathy seen in cystic fibrosis, after lithocholate feeding (in mice) and vanishing bile duct syndromes induced by drugs and xenobiotics. Therapeutic strategies for cholangiopathies may target bile composition/toxicity and the affected bile duct epithelium itself, and ideally should also have anti-cholestatic, anti-fibrotic and anti-neoplastic properties. Ursodeoxycholic acid (UDCA) shows some of these properties, but is of limited efficacy in the treatment of human cholangiopathies. By contrast to UDCA, its side chain-shortened homologue norUDCA undergoes cholehepatic shunting leading to a bicarbonate-rich hypercholeresis. Moreover, norUDCA has anti-inflammatory, anti-fibrotic and anti-proliferative effects, and stimulates bile acid detoxification. Upcoming clinical trials will have to demonstrate whether norUDCA or other side chain-modified bile acids are also clinically effective in humans. Finally, drugs for the treatment of cholangiopathies may target bile toxicity via nuclear receptors (FXR, PPARalpha) regulating biliary phospholipid and bile acid excretion.

Liver Transplantation in Diego Blood Disparity: A Case Report

The Di antigen in the Diego blood type system is an anthropologic marker of Mongoloids. Here, we report the first case of liver transplantation involving donor/recipient Diego blood type disparity. The recipient was a 58-year-old woman who had developed fulminant hepatic failure, and her 32-year-old daughter was a candidate donor. The recipient and the donor were both ABO blood type O, and were Di (a- b+) and Di (a+ b+), respectively, in the Diego blood system. Living-related liver transplantation was performed, and immediate graft function was obtained. No signs of humoral rejection were observed on postoperative days one to four. Biopsy performed on postoperative days 10, 63, and 87 because of elevation of the serum bilirubin level showed no signs of humoral rejection. In conclusion, liver transplantation can be performed successfully in cases of Diego blood type disparity.

Cytoskeletal and motor proteins facilitate trafficking of AQP1-containing vesicles in cholangiocytes

BACKGROUND INFORMATION

We have previously showed that: (i) cholangiocytes contain AQP1 (aquaporin 1) water channels sequestered in intracellular vesicles; and (ii) upon stimulation with choleretic agonists such as secretin or dibutyryl-cAMP (dbcAMP), the AQP1 vesicles move via microtubules to the apical cholangiocyte membrane to facilitate osmotically driven, passive water movement (i.e. ductal bile secretion). The aim of the present study was to determine which proteins and mechanisms regulate AQP1 trafficking in cholangiocytes.

RESULTS

Using polarized cultured NMCs (normal mouse cholangiocytes) or NRCs (normal rat cholangiocytes) and affinity-purified antibodies, we performed immunofluorescent confocal microscopy on fixed cells or immunoblotting on cell lysates for actin, tubulin, kinesin and dynein, proteins known to regulate intracellular vesicle trafficking. By immunostaining, the appropriate orientation of the actin (i.e. sub-apical) and tubulin (i.e. generalized) cytoskeleton was apparent; kinesin and dynein displayed a homogeneous punctate distribution. Immunoblotting showed kinesin and dynein to be present in both cholangiocyte lysates and in isolated AQP1-containing vesicles. We utilized real-time fluorescence confocal microscopy of NMCs transfected with a GFP (green fluorescent protein)-AQP1 fusion construct in the presence and absence of dbcAMP.

CONCLUSIONS

Our results provide additional insights into the potential molecular mechanisms of ductal bile secretion.

Molecular regulation of hepatobiliary transport systems: clinical implications for understanding and treating cholestasis

Hepatobiliary transport systems are responsible for hepatic uptake and excretion of bile salts and other biliary constituents (eg, bilirubin) into bile. Hereditary transport defects can result in progressive familial and benign recurrent intrahepatic cholestasis. Exposure to acquired cholestatic injury (eg, drugs, hormones, proinflammatory cytokines, biliary obstruction or destruction) also results in altered expression and function of hepatic uptake and excretory systems, changes that may maintain and contribute to cholestasis and jaundice. Recruitment of alternative efflux pumps and induction of phase I and II detoxifying enzymes may limit hepatic accumulation of potentially toxic biliary constituents in cholestasis by providing alternative metabolic and escape routes. These molecular changes are mediated by bile salts, proinflammatory cytokines, drugs, and hormones at a transcriptional and posttranscriptional level. Alterations of hepatobiliary transporters and enzymes are not only relevant for a better understanding of the pathophysiology of cholestatic liver diseases, but may also represent important targets for pharmacotherapy. Drugs (eg, ursodeoxycholic acid, rifampicin) used to treat cholestatic liver diseases and pruritus may counteract cholestasis via stimulation of defective transporter expression and function. In addition, therapeutic strategies may be aimed at supporting and stimulating alternative detoxification pathways and elimination routes for bile salts in cholestasis.

Shared apical sorting of anion exchanger isoforms AE2a, AE2b1, and AE2b2 in primary hepatocytes

AE2 (SLC4A2) is the member of the Na(+)-independent anion exchanger (AE) family putatively involved in the secretion of bicarbonate to bile. In humans, three variants of AE2 mRNA have been described: the full-length transcript AE2a (expressed from the upstream promoter in most tissues), and alternative transcripts AE2b(1) and AE2b(2) (driven from alternate promoter sequences in a tissue-restricted manner, mainly in liver and kidney). These transcripts would result in AE protein isoforms with short N-terminal differences. To ascertain their translation, functionality, and membrane sorting, we constructed expression vectors encoding each AE2 isoform fused to GFP at the C-terminus. Transfected HEK293 cells showed expression of functional GFP-tagged AE2 proteins, all three isoforms displaying comparable AE activities. Primary rat hepatocytes transfected with expression vectors and repolarized in a collagen-sandwich configuration showed a microtubule-dependent apical sorting of each AE2 isoform. This shared apical sorting is liver-cell specific, as sorting of AE2 isoforms was basolateral in control experiments on polarized kidney MDCK cells. Hepatocytic apical targeting of AE2 isoforms suggests that they all may participate in the canalicular secretion of bicarbonate to bile.

Endogenous ursodeoxycholic acid and cholic acid in liver disease due to cystic fibrosis

Focal biliary cirrhosis causes significant morbidity and mortality in cystic fibrosis (CF). Although the mechanisms of pathogenesis remain unclear, bile acids have been proposed as potential mediators of liver injury. This study examined bile acid composition in CF and assessed altered bile acid profiles to determine if they are associated with incidence and progression of liver injury in CF-associated liver disease (CFLD). Bile acid composition was determined by gas-liquid chromatography/mass spectrometry in bile, urine, and serum samples from 30 children with CFLD, 15 children with CF but without liver disease (CFnoLD), and 43 controls. Liver biopsies from 29 CFLD subjects were assessed histologically by grading for fibrosis stage, inflammation, and disruption of the limiting plate. A significantly greater proportion of endogenous biliary ursodeoxycholic acid (UDCA) was demonstrated in CFnoLD subjects vs. both CFLD subjects and controls (2.4- and 2.2-fold, respectively; ANOVA, P =.04), and a 3-4 fold elevation in endogenous serum UDCA concentration was observed in both CFLD subjects and CFnoLD subjects vs. controls (ANOVA, P <.05). In CFLD, there were significant correlations between serum cholic acid and hepatic fibrosis, inflammation, and limiting plate disruption as well as the ratio of serum cholic acid/chenodeoxycholic acid to hepatic fibrosis, inflammation, and limiting plate disruption. In conclusion, elevated endogenous UDCA in CFnoLD suggests a possible protective role against liver injury in these patients. The correlation between both cholic acid and cholic acid/chenodeoxycholic acid levels with histological liver injury and fibrosis progression suggests a potential monitoring role for these bile acids in CFLD.

Increased susceptibility of cholangiocytes to tumor necrosis factor-alpha cytotoxicity after bile duct ligation

Tumor necrosis factor (TNF)-alpha plays a critical role in epithelial cell injury. However, the role of TNF-alpha in mediating cholangiocyte injury under physiological or pathophysiological conditions is unknown. Thus we assessed the effects of TNF-alpha alone or following sensitization by actinomycin D on cell apoptosis, proliferation, and basal and secretin-stimulated ductal secretion in cholangiocytes from normal or bile duct-ligated (BDL) rats. Cholangiocytes from normal or BDL rats were highly resistant to TNF-alpha alone. However, presensitization by actinomycin D increased apoptosis in cholangiocytes following BDL and was associated with an inhibition of proliferation and secretin-stimulated ductal secretion. Thus TNF-alpha mediates cholangiocyte injury and altered ductal secretion following bile duct ligation. These observations suggest that cholestasis may enhance susceptibility to cytokine-mediated cholangiocyte injury.

Adaptive changes in hepatobiliary transporter expression in primary biliary cirrhosis

BACKGROUND/AIMS

Information about alterations of hepatobiliary transporter expression in primary biliary cirrhosis (PBC) could provide important insights into the pathogenesis of cholestasis. This study aimed to determine the expression of hepatobiliary transport systems for bile salts (Na(+)/taurocholate cotransporter, NTCP; bile salt export pump, BSEP), organic anions (organic anion transporting protein, OATP2; canalicular conjugate export pump, MRP2; basolateral MRP homologue, MRP3), organic cations (canalicular multidrug export pump, MDR1), and phospholipids (canalicular phospholipid flippase MDR3) in livers from patients with advanced stages of PBC.

METHODS

Transporter mRNA and protein levels were assessed by reverse transcription polymerase chain reaction and Western blot analysis. Tissue distribution of transporters was investigated by immunohistochemistry and immunofluorescence microscopy. Hepatic bile acids were measured by gas chromatography-mass spectrometry.

RESULTS

Compared to controls, basolateral uptake systems (NTCP, OATP2) were reduced, canalicular export pumps for bile salts and bilirubin (BSEP, MRP2) were preserved, while canalicular MDR P-glycoproteins (MDR1, MDR3) and the basolateral efflux pump MRP3 were increased in PBC. Double immunofluorescence labeling with a canalicular marker (dipeptidyl peptidase IV) demonstrated proper canalicular localization of BSEP and MRP2 in PBC. OATP2 and MRP2 expression correlated inversely with hepatic levels of hydrophobic bile acids, while positively correlating with hepatic enrichment with ursodeoxycholic acid.

CONCLUSIONS

Down-regulation of basolateral uptake systems and maintenance/up-regulation of canalicular and basolateral efflux pumps may represent adaptive mechanisms limiting the accumulation of toxic biliary constituents.

Agonist-induced coordinated trafficking of functionally related transport proteins for water and ions in cholangiocytes

We previously proposed that ductal bile formation is regulated by secretin-responsive relocation of aquaporin 1 (AQP1), a water-selective channel protein, from an intracellular vesicular compartment to the apical membrane of cholangiocytes. In this study, we immunoisolated AQP1-containing vesicles from cholangiocytes prepared from rat liver; quantitative immunoblotting revealed enrichment in these vesicles of not only AQP1 but also cystic fibrosis transmembrane regulator (CFTR) and AE2, a Cl- channel and a Cl-/HCO3- exchanger, respectively. Dual labeled immunogold electron microscopy of cultured polarized mouse cholangiocytes showed significant colocalization of AQP1, CFTR, and AE2 in an intracellular vesicular compartment; exposure of cholangiocytes to dibutyryl-cAMP (100 microm) resulted in co-redistribution of all three proteins to the apical cholangiocyte plasma membrane. After administration of secretin to rats in vivo, bile flow increased, and AQP1, CFTR, and AE2 co-redistributed to the apical cholangiocyte membrane; both events were blocked by pharmacologic disassembly of microtubules. Based on these in vitro and in vivo observations utilizing independent and complementary approaches, we propose that cholangiocytes contain an organelle that sequesters functionally related proteins that can account for ion-driven water transport, that this organelle moves to the apical cholangiocyte membrane in response to secretory agonists, and that these events account for ductal bile secretion at a molecular level.

Ursodeoxycholic acid aggravates bile infarcts in bile duct-ligated and Mdr2 knockout mice via disruption of cholangioles

BACKGROUND & AIMS

The effects of ursodeoxycholic acid (UDCA) in biliary obstruction are unclear. We aimed to determine the effects of UDCA in bile duct-ligated and in Mdr2 knockout (Mdr2(-/-)) mice with biliary strictures.

METHODS

Mice fed UDCA (0.5% wt/wt) or a control diet were subjected to common bile duct ligation (CBDL), selective bile duct ligation (SBDL), or sham operation. UDCA was also fed to 2-month-old Mdr2(-/-) mice. Serum biochemistry, liver histology, and mortality rates were investigated. The biliary tract was studied by plastination, India ink injection, and electron microscopy. The effects of UDCA on biliary pressure were determined by cholangiomanometry.

RESULTS

UDCA feeding in CBDL mice increased biliary pressure, with subsequent rupture of cholangioles and aggravation of hepatocyte necroses, resulting in significantly increased mortality. UDCA feeding in SBDL mice aggravated liver injury exclusively in the ligated lobe. Mdr2(-/-) mice developed liver lesions resembling sclerosing cholangitis characterized by biliary strictures and dilatations. UDCA induced bile infarcts in these animals.

CONCLUSIONS

UDCA aggravates bile infarcts and hepatocyte necroses in mice with biliary obstruction via disruption of cholangioles as a result of increased biliary pressure caused by its choleretic action.

Defective regulation of cholangiocyte Cl-/HCO3(-) and Na+/H+ exchanger activities in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a disorder of unknown origin with autoimmune features. Recently, impaired biliary secretion of bicarbonate has been shown in patients with PBC. Here we have investigated whether bile duct epithelial cells isolated from PBC patients exhibit defects in transepithelial bicarbonate transport by analyzing the activities of 2 ion exchangers, Cl(-)/HCO3(-) anion exchanger 2 (AE2) and Na(+)/H(+) exchanger (NHE) in isolated cholangiocytes. AE2 and NHE activities were studied in basal conditions and after stimulation with cyclic adenosine monophosphate (cAMP) and extracellular adenosine triphosphate (ATP), respectively. Cholangiocytes were grown from needle liver biopsies from 12 PBC patients, 8 normal controls, and 9 patients with other liver diseases. Also, intrahepatic cholangiocytes were cultured after immunomagnetic isolation from normal liver tissue (n = 6), and from recipients undergoing liver transplantation for end-stage PBC (n = 9) and other forms of liver disease (n = 8). In needle-biopsy cholangiocytes, basal AE2 activity was significantly decreased in PBC as compared with normal livers and disease controls. In addition, we observed that though cAMP increased AE2 activity in cholangiocytes from both normal and non-PBC livers, this effect was absent in PBC cholangiocytes. Similarly, though in cholangiocytes from normal and disease control livers extracellular ATP induced a marked enhancement of NHE activity, cholangiocytes from PBC patients failed to respond to purinergic stimulation. In conclusion, our findings provide functional evidence that PBC cholangiocytes exhibit a widespread failure in the regulation of carriers involved in transepithelial H(+)/HCO3(-) transport, thus, providing a molecular basis for the impaired bicarbonate secretion in this cholestatic syndrome.

Regulation of organic anion transporters in a new rat model of acute and chronic cholangitis resembling human primary sclerosing cholangitis

BACKGROUND/AIMS

Primary sclerosing cholangitis (PSC) is a cholestatic liver disease of unknown etiology. Although the primary defect affects cholangiocytes, cholestatic injury of hepatocytes may promote further liver damage. Since down-regulation of hepatocellular organic anion transporters is implicated in the molecular pathogenesis of cholestasis, expression of these transporters was determined in a novel rat model, which closely resembles human PSC.

METHODS

Hepatic protein and mRNA expression of basolateral (Ntcp, Oatp1, 2 and 4) and canalicular (Mrp2, Bsep) organic anion transporters were analyzed 1, 4 and 12 weeks after induction of experimental PSC by 2,4,6-trinitrobenzenesulfonic acid (TNBS).

RESULTS

Specific down-regulation of basolateral and canalicular transport systems except Oatp4 and Bsep proteins occurred during the acute phase of inflammation. In chronic cholangitis 12 weeks after TNBS Mrp2 protein and mRNA remained down-regulated by 40-50% of controls (P<0.05). In addition Oatp1 protein was also reduced by 40+/-13% (P<0.05), whereas all other transporters returned to control values.

CONCLUSIONS

In chronic cholangitis only canalicular Mrp2 expression remained down-regulated. This might represent the first injury to hepatocytes in chronic cholangitis as an extension of liver injury from the level of cholangiocytes to hepatocytes in PSC.

Ursodeoxycholic acid 'mechanisms of action and clinical use in hepatobiliary disorders'

UDCA exerts its beneficial effect in liver diseases through a diverse, probably, complementary array of mechanisms. The clinical use and efficacy of UDCA in PBC have been evident. UDCA may also have a place in the management of PSC, ICP, cystic fibrosis, PFIC and GVHD involving the liver, although, more studies are needed to further determine its therapeutic potential in these diseases and in other hepatobiliary disorders such as liver allograft rejection, drug and TPN-induced cholestasis, NASH, and alcoholic liver disease.

The pathophysiology of cholestasis with special reference to primary biliary cirrhosis

Cholestasis in primary biliary cirrhosis results from impairment of bile flow either by reduced transport at the level of the canaliculi or by disturbed bile flow through damaged intrahepatic bile ductules. Whatever its cause, the expression of hepatic transport proteins will be affected. In cholestatic rats: the expression of the multispecific organic anion transporter mrp2 is decreased; the bile salt export pump bsep and the phospholipid transporter mdr2 are less affected; the carrier protein for hepatic uptake of bile salts ntcp is sharply down-regulated; Mrp3, a basolateral ATP-dependent transporter for glucuronides and bile salts, is upregulated. Thus, bile salts that cannot exit the hepatocyte because of the cholestasis are effectively removed across the basolateral membrane. These may be adaptive responses in defence against overloading of hepatocytes with cytotoxic bile salts. These responses show that the expression of hepatic transporter proteins is highly regulated. This occurs by transcriptional and post-transcriptional mechanisms. Primary biliary cirrhosis starts as a disease of the small intrahepatic bile ducts and therefore the experimental evidence for 'cross-talk' between hepatocytes and cholangiocytes is of great interest for this disease and needs to be further investigated. New insights in bile physiology may enable the development of new therapies for cholestatic liver diseases as primary biliary cirrhosis.

Pathophysiology of the intrahepatic biliary epithelium

The intrahepatic bile duct epithelium modulates the fluidity and alkalinity of the primary hepatocellular bile from which it reabsorbs fluids, amino acids, glucose and bile acids, while secreting water, electrolytes and immunoglobulin A. The transport function of the intrahepatic biliary epithelium is finely regulated by a number of gastrointestinal hormones, neuropeptides and neurotransmitters that promote either secretion or absorption. The intrahepatic biliary epithelium appears to be a primary target in a broad group of chronic cholestatic disorders that represent an important cause of morbidity and mortality. The spectrum of cholangiopathies ranges from conditions in which a normal epithelium is damaged by disordered autoimmunity, infectious agents, toxic compounds or ischaemia, to genetically determined disorders arising from an abnormal bile duct biology, such as cystic fibrosis or biliary atresia. Probably as a result of the known heterogeneity in cholangiocyte function, different portions of the biliary tree appear to be preferentially affected in specific cholangiopathies. From a pathophysiological point of view, cholangiopathies are characterized by the coexistence of cholangiocyte loss (by apoptotic or lytic cell death) with cholangiocyte proliferation and various degrees of portal inflammation, fibrosis and cholestasis. These basic disease mechanisms are discussed in detail. Better understanding of cholangiocyte pathophysiology, in particular the immune regulation of cholangiocyte function, will help in designing newer genetic or pharmacological approaches to treat cholangiopathies.

Tissue-specific N-terminal isoforms from overlapping alternate promoters of the human AE2 anion exchanger gene

Previously, we isolated the human AE2 (SLC4A2) gene, a member of the sodium-independent anion exchanger family. Rat ortholog of this gene was reported to drive alternative transcription yielding N-terminal variants of the AE2a message. We thus analyzed the human AE2 gene in this regard. Using HepG2 cells, two alternative first exons, each splicing to exon 3 in alternative transcripts, were found to be transcribed from overlapping sequences of intron 2. Exon 1b(1) corresponds to the rat variant "b" and encodes three initial residues (MTQ) in AE2b(1) isoform that replace the first 17 amino acids of AE2a protein, while the novel exon 1b(2) encodes eight initial residues (MDFLLRPQ) in AE2b(2) isoform. The relative abundance of AE2b(1) and AE2b(2) mRNAs was about 10% of AE2a mRNA each. Alternate promoter sequences have multiple potential binding motifs for liver-enriched factors, and dual-luciferase assays indicated that they possess the ability for driving transcription in transiently transfected HepG2 cells. Tissue survey showed that expression of human AE2b(1) and AE2b(2) transcripts is restricted to liver and kidney, while AE2a mRNA was encountered in all examined tissues. Our findings reveal a characteristic tissue-specific expression of two N-terminal variants of human AE2 from overlapping sequences within intron 2, one of which is a novel isoform.

Sepsis and cholestasis

Sepsis-associated cholestasis should always be considered as part of the differential diagnosis of jaundice in the hospitalized or critically ill patient. The development of a disproportionate elevation of serum bilirubin in comparison with serum alkaline phosphatase and serum aminotransferases should be considered an early warning sign of an underlying infection, even in the absence of fever, leukocytosis, or other signs or symptoms. Prompt recognition and appropriate medical and surgical intervention may reduce morbidity and mortality.

Review article: mechanisms of action and therapeutic applications of ursodeoxycholic acid in chronic liver diseases

Ursodeoxycholic acid (ursodiol) is a non-toxic, hydrophilic bile acid used to treat predominantly cholestatic liver disorders. Better understanding of the cellular and molecular mechanisms of action of ursodeoxycholic acid has helped to elucidate its cytoprotective, anti-apoptotic, immunomodulatory and choleretic effects. Ursodeoxycholic acid prolongs survival in primary biliary cirrhosis and it improves biochemical parameters of cholestasis in various other cholestatic disorders including primary sclerosing cholangitis, intrahepatic cholestasis of pregnancy, cystic fibrosis and total parenteral nutrition-induced cholestasis. However, a positive effect on survival remains to be established in these diseases. Ursodeoxycholic acid is of unproven efficacy in non-cholestatic disorders such as acute rejection after liver transplantation, non-alcoholic steatohepatitis, alcoholic liver disease and chronic viral hepatitis. This review outlines the present knowledge of the modes of action of ursodeoxycholic acid, and presents data from clinical trials on its use in chronic liver diseases.

Assessment of biliary bicarbonate secretion in humans by positron emission tomography

BACKGROUND & AIMS

Positron emission tomography (PET) allows imaging and quantitative analysis of organ functions in basal and stimulated conditions. We have applied this method to the study of biliary bicarbonate secretion in humans.

METHODS

PET was performed in 5 healthy subjects and 13 patients with hepatobiliary disorders after intravenous injection of NaH11CO3. In each case the study was performed in basal conditions and after secretin stimulation. Positron emission from the hepatic area was scanned, and normalized uptake values for parenchymal and hilar regions were estimated.

RESULTS

In healthy individuals, the injection of NaH11CO3 resulted in a peak uptake of the label in parenchymal and hilar regions 2-3 minutes after the injection. In both normal and cirrhotic subjects, secretin administration increased bicarbonate uptake in the parenchymal region, followed by accumulation of the label in the perihilar area. Normal basal uptake with absent response to secretin was registered in extrahepatic biliary obstruction and in untreated primary biliary cirrhosis (PBC). The secretin response was present in patients with PBC undergoing treatment with ursodeoxycholic acid.

CONCLUSIONS

PET allows investigation of biliary bicarbonate secretion in humans. An impaired response to secretin was observed in cholestatic conditions. Preliminary data suggest that ursodeoxycholic acid might improve the response to secretin in PBC.

A beta3-adrenergic agonist increases muscle GLUT1/GLUT4 ratio, and regulates liver glucose utilization in diabetic rats

AIM

Previous studies have reported that beta3-adrenergic agonists reduce plasma glucose levels in situations of hyperglycaemia and diabetes in rodents. Nevertheless, the mechanisms still remain unclear. In this context Trecadrine, a novel compound with affinity for beta3-adrenergic receptors, has been tested in alloxan-diabetic rats for its potential use as an anti-diabetic drug, but also to elucidate the role of muscle/liver glucose utilization in the process.

METHODS AND RESULTS

Daily oral administration (1 mg/kg) to alloxan-diabetic Wistar rats (n = 10) for 4 days caused a significant reduction in plasma glucose levels (from 15.0 to 8.3 mmol/l) with no apparent effects on insulin secretion. Furthermore, Trecadrine administration tended to normalize glucose storage (estimated by measuring glucokinase activity) and output (by measuring glucose-6-phosphatase activity) in the liver of diabetic animals. On the other hand, Trecadrine administration for 4 days resulted in an increase in GLUT1 gene expression in gastrocnemius muscle as compared to insulin-dependent glucose transporter GLUT4. Furthermore, a significant stimulation of 2-deoxy-D-glucose uptake in extensor digitorum longus muscle and, in a lesser degree, in gastrocnemius, but not in soleus muscle and in white adipose tissue, occurs.

CONCLUSIONS

Trecadrine reduces glucose output from the liver, thus thus contributing to the reduction of plasma glucose levels to achieve the values of control rats. Furthermore, Trecadrine administration stimulates glucose uptake in skeletal muscle, especially in those muscles with predominant glycolytic fast-twitched fibres, apparently by a direct non-insulin-dependent mechanism, involving a relative increase in the content of GLUT1 in the plasma membrane as compared with GLUT4. In conclusion, Trecadrine shows a potent hypoglycaemic effect in the alloxan-induced model of diabetes in rats by decreasing hepatic glucose output and improving muscle glucose uptake.

Intestinal inflammation reduces expression of DRA, a transporter responsible for congenital chloride diarrhea

The pathogenesis of diarrhea in intestinal inflammatory states is a multifactorial process involving the effects of inflammatory mediators on epithelial transport function. The effect of colonic inflammation on the gene expression of DRA (downregulated in adenoma), a chloride-sulfate anion transporter that is mutated in patients with congenital chloridorrhea, was examined in vivo as well as in an intestinal epithelial cell line. DRA mRNA expression was diminished five- to sevenfold in the HLA-B27/beta2m transgenic rat compared with control. In situ hybridization showed that DRA, which is normally expressed in the upper crypt and surface epithelium of the colon, was dramatically reduced in the surface epithelium of the HLA-B27/beta2m transgenic rat, the interleukin-10 (IL-10) knockout mouse with spontaneous colitis, and in patients with ulcerative colitis. Immunohistochemistry demonstrated that mRNA expression of DRA reflected that of protein expression in vivo. IL-1beta reduced DRA mRNA expression in vitro by inhibiting gene transcription. The loss of transport function in the surface epithelium of the colon by attenuation of transporter gene expression, perhaps inhibited at the level of gene transcription by proinflammatory cytokines, may play a role in the pathogenesis of diarrhea in colitis.

Decreased anion exchanger 2 immunoreactivity in the liver of patients with primary biliary cirrhosis

Chloride-bicarbonate anion exchanger 2 (AE2) is expressed in a variety of tissues, including the liver and salivary glands, where it may participate in the generation of hydroionic fluxes into secretions. We have previously reported decreased hepatic levels of AE2 messenger RNA in patients with primary biliary cirrhosis (PBC), a cholestatic condition frequently associated with pluriglandular exocrine failure. Here we investigated the expression of AE2 protein in the liver of PBC patients. Using a monoclonal antibody against an AE2 peptide, immunohistochemistry was performed on liver biopsy specimens from subjects with normal liver (n = 7), patients with PBC (n = 13), and patients with cirrhosis or cholestasis other than PBC (n = 17 and 11, respectively). Immunostaining was graded from 0 to 7, according to its intensity and distribution. AE2 immunoreactivity was observed in normal livers, as previously reported, and in many pathological liver biopsy specimens, being mainly restricted to canaliculi and the luminal membrane of terminal and interlobular bile ducts. Canalicular and ductular scores were significantly reduced in the PBC group compared with each control group (normal liver and cirrhosis or cholestasis other than PBC), whereas no differences in immunoreactivity scores were observed among control groups. When four patients with primary sclerosing cholangitis (PSC) were analyzed, they also differed from those with PBC. These results suggest that PBC is characterized by diminished expression of AE2 in the liver. Reduced levels of this transporter protein might be involved in the pathogenesis of cholestasis in PBC.

Taurocholate-stimulated leukotriene C4 biosynthesis and leukotriene C4-stimulated choleresis in isolated rat liver

BACKGROUND/AIMS

Cysteinyl-containing leukotrienes seem to exert a cholestatic effect. However, leukotriene inhibitors were found to reduce bile salt efflux in isolated rat hepatocytes, suggesting a role for leukotrienes in bile flow formation.

METHODS

In the isolated rat liver, the effects of two different concentrations of leukotriene C4 on bile flow and bile salt excretion are analyzed, as well as the possible effect of taurocholate on the hepatic production of cysteinyl-containing leukotrienes.

RESULTS

Leukotriene C4 (0.25 fmol) increased bile salt excretion (+22.2%; P < 0.05), whereas a much higher dose (0.25 x 10(6) fmol) showed the known cholestatic effect, reducing bile salt excretion (-25.9%; P < 0.01). These dose-dependent biphasic effects were specific because they could be prevented by the simultaneous administration of cysteinyl-containing leukotriene antagonists. On the other hand, taurocholate administration induced a dose-dependent increase in biliary excretion of cysteinyl-containing leukotrienes. Furthermore, taurocholate increased messenger RNA levels of 5-lipoxygenase, a key enzyme in leukotriene biosynthesis. Taurocholate increase of hepatocyte intracellular calcium was not significant, suggesting that taurocholate effects are not mediated by stimulation of calcium metabolism.

CONCLUSIONS

These results constitute evidence for the existence of a positive feedback mechanism by which bile salts stimulate the synthesis of leukotrienes that, in turn, stimulate bile salt excretion.

Immunohistochemical detection of chloride/bicarbonate anion exchangers in human liver

Sodium-independent Cl-/HCO3- exchange activity has been observed in isolated rat hepatocytes and intrahepatic bile duct epithelial cells, where it is involved in intracellular pH regulation and, possibly, biliary bicarbonate secretion. Monoclonal antibodies to the membrane domain of human chloride/bicarbonate anion exchanger proteins, AE1 and AE2, were prepared so that we might determine by immunohistochemical methods the presence and location of these antiporters in the human liver. To obtain the antibody against AE1, we immunized mice with injections of washed human erythrocytes. The selected monoclonal antibody was found to be specific for the 17-kD proteolytic membrane fragment of AE1 protein. The antibody to AE2 was produced with a 14-mer synthetic peptide, whose sequence corresponds specifically to amino acid residues 871 to 884 in the deduced primary structure of human kidney AE2 protein. When the monoclonal antibody to AE2 peptide was employed for the immunohistochemical study of liver specimens (by both immunofluorescence and immunoperoxidase), a clearly defined staining was present at the canalicular membrane of hepatocytes, as well as the luminal side of the membrane of bile duct epithelial cells from small and medium-sized bile ducts. No staining was observed in the liver parenchyma with the monoclonal antibody to AE1, which instead strongly decorated the erythrocytes in liver blood vessels. We conclude that AE2 immunoreactivity is present in human liver, where it localizes very specifically to the membrane regions, which appear most probably involved in the transport of bicarbonate to bile (i.e., the canalicular membrane of hepatocytes and the apical side of epithelial cells of small and medium bile ducts).