Expression of the bile salt export pump is maintained after chronic cholestasis in the rat

Nuclear receptors: mediators and modifiers of inflammation-induced cholestasis

Inflammation-induced cholestasis (IIC) is a frequently occurring phenomenon. A central role in its pathogenesis is played by nuclear receptors (NRs). These ligand-activated transcription factors not only regulate basal expression of hepatobiliary transport systems, but also mediate adaptive responses to inflammation and possess anti-inflammatory characteristics. The latter two functions may be exploited in the search for new treatments for IIC as well as for cholestasis in general. Current knowledge of the pathogenesis of IIC and the dual role NRs in this process are reviewed. Special interest is given to the use of NRs as potential targets for intervention.

Aquaporins: Their role in cholestatic liver disease

This review focuses on current knowledge on hepatocyte aquaporins (AQPs) and their significance in bile formation and cholestasis. Canalicular bile secretion results from a combined interaction of several solute transporters and AQP water channels that facilitate water flow in response to the osmotic gradients created. During choleresis, hepatocytes rapidly increase their canalicular membrane water permeability by modulating the abundance of AQP8. The question was raised as to whether the opposite process, i.e. a decreased canalicular AQP8 expression would contribute to the development of cholestasis. Studies in several experimental models of cholestasis, such as extrahepatic obstructive cholestasis, estrogen-induced cholestasis, and sepsis-induced cholestasis demonstrated that the protein expression of hepatocyte AQP8 was impaired. In addition, biophysical studies in canalicular plasma membranes revealed decreased water permeability associated with AQP8 protein downregulation. The combined alteration in hepatocyte solute transporters and AQP8 would hamper the efficient coupling of osmotic gradients and canalicular water flow. Thus cholestasis may result from a mutual occurrence of impaired solute transport and decreased water permeability.

Dynamic localization of hepatocellular transporters in health and disease

Vesicle-based trafficking of hepatocellular transporters involves delivery of the newly-synthesized carriers from the rough endoplasmic reticulum to either the plasma membrane domain or to an endosomal, submembrane compartment, followed by exocytic targeting to the plasma membrane. Once delivered to the plasma membrane, the transporters usually undergo recycling between the plasma membrane and the endosomal compartment, which usually serves as a reservoir of pre-existing transporters available on demand. The balance between exocytic targeting and endocytic internalization from/to this recycling compartment is therefore a chief determinant of the overall capability of the liver epithelium to secrete bile and to detoxify endo and xenobiotics. Hence, it is a highly regulated process. Impaired regulation of this balance may lead to abnormal localization of these transporters, which results in bile secretory failure due to endocytic internalization of key transporters involved in bile formation. This occurs in several experimental models of hepatocellular cholestasis, and in most human cholestatic liver diseases. This review describes the molecular bases involved in the biology of the dynamic localization of hepatocellular transporters and its regulation, with a focus on the involvement of signaling pathways in this process. Their alterations in different experimental models of cholestasis and in human cholestatic liver disease are reviewed. In addition, the causes explaining the pathological condition (e.g. disorganization of actin or actin-transporter linkers) and the mediators involved (e.g. activation of cholestatic signaling transduction pathways) are also discussed. Finally, several experimental therapeutic approaches based upon the administration of compounds known to stimulate exocytic insertion of canalicular transporters (e.g. cAMP, tauroursodeoxycholate) are described.

Bile acid transporters in health and disease

In recent years the discovery of a number of major transporter proteins expressed in the liver and intestine specifically involved in bile acid transport has led to improved understanding of bile acid homeostasis and the enterohepatic circulation. Sodium (Na(+))-dependent bile acid uptake from portal blood into the liver is mediated primarily by the Na(+) taurocholate co-transporting polypeptide (NTCP), while secretion across the canalicular membrane into the bile is carried out by the bile salt export pump (BSEP). In the ileum, absorption of bile acids from the lumen into epithelial cells is mediated by the apical Na(+) bile salt transporter (ASBT), whereas exit into portal blood across the basolateral membrane is mediated by the organic solute transporter alpha/beta (OSTalpha/beta) heterodimer. Regulation of transporter gene expression and function occurs at several different levels: in the nucleus, members of the nuclear receptor superfamily, regulated by bile acids and other ligands are primarily involved in controlling gene expression, while cell signalling events directly affect transporter function, and subcellular localization. Polymorphisms, dysfunction, and impaired adaptive responses of several of the bile acid transporters, e.g. BSEP and ASBT, results in liver and intestinal disease. Bile acid transporters are now understood to play central roles in driving bile flow, as well as adaptation to various pathological conditions, with complex regulation of activity and function in the nucleus, cytoplasm, and membrane.

Reduced antidiabetic effect of metformin and down-regulation of hepatic Oct1 in rats with ethynylestradiol-induced cholestasis

PURPOSE

To investigate the effect of 17alpha-ethynylestradiol (EE)-induced cholestasis on the expression of organic cation transporters (Octs) in the liver and kidney, as well as the pharmacokinetics and pharmacodynamics of metformin in rats.

METHODS

Octs mRNA and protein expression were determined. The pharmacokinetics and tissue uptake clearance of metformin were determined following iv administration (5 mg/kg). Uptake of metformin, glucagon-mediated glucose production, and AMP-activated protein kinase (AMPK) activation were measured in isolated hepatocytes. The effect of metformin (30 mg/kg) on blood glucose levels was tested using the iv glucose tolerance test (IVGTT).

RESULTS

The mRNAs of hepatic Oct1, renal Oct1, and Oct2 were decreased by 71.1%, 37.6%, and 94.5%, respectively, by EE cholestasis. The hepatic Oct1 and renal Oct2 proteins were decreased by 30.6% and 60.2%, respectively. The systemic and renal clearance of metformin were decreased. The in vitro hepatocyte uptake of metformin was decreased by 86.4% for V (max). Suppression of glucagon-stimulated glucose production and stimulation of AMPK activation in hepatocytes by metformin were diminished. In addition, metformin did not demonstrate a glucose-lowering effect during IVGTT in EE cholestasis.

CONCLUSION

The antidiabetic effect of metformin may be diminished in diabetic patients with EE cholestasis, due to impaired hepatic uptake of the drug via OCT1.

Short-chain ubiquitination is associated with the degradation rate of a cell-surface-resident bile salt export pump (BSEP/ABCB11)

The reduced expression of the bile salt export pump (BSEP/ABCB11) at the canalicular membrane is associated with cholestasis-induced hepatotoxicity due to the accumulation of bile acids in hepatocytes. We demonstrated previously that 4-phenylbutyrate (4PBA) treatment, a U.S. Food and Drug Administration-approved drug for the treatment of urea cycle disorders, induces the cell-surface expression of BSEP by prolonging the degradation rate of cell-surface-resident BSEP. On the other hand, BSEP mutations, E297G and D482G, found in progressive familial intrahepatic cholestasis type 2 (PFIC2), reduced it by shortening the degradation rate of cell-surface-resident BSEP. Therefore, to help the development of the medical treatment of cholestasis, we investigated the underlying mechanism by which 4PBA and PFIC2-type mutations affect the BSEP degradation from cell surface, focusing on short-chain ubiquitination. In Madin-Darby canine kidney II (MDCK II) cells expressing BSEP and rat canalicular membrane vesicles, the molecular mass of the mature form of BSEP/Bsep shifted from 170 to 190 kDa after ubiquitin modification (molecular mass, 8 kDa). Ubiquitination susceptibility of BSEP/Bsep was reduced in vitro and in vivo by 4PBA treatment and, conversely, was enhanced by BSEP mutations E297G and D482G. Moreover, biotin-labeling studies using MDCK II cells demonstrated that the degradation of cell-surface-resident chimeric protein fusing ubiquitin to BSEP was faster than that of BSEP itself. In conclusion, BSEP/Bsep is modified with two to three ubiquitins, and its ubiquitination is modulated by 4PBA treatment and PFIC2-type mutations. Modulation of short-chain ubiquitination can regulate the change in the degradation rate of cell-surface-resident BSEP by 4PBA treatment and PFIC2-type mutations.

Investigating compensation and recovery of fathead minnow (Pimephales promelas) exposed to 17alpha-ethynylestradiol with metabolite profiling

1H NMR spectroscopy was used to profile metabolite changes in the livers of fathead minnows (Pimephales promelas) exposed to the synthetic estrogen 17alpha-ethynylestradiol (EE2) via a continuous flow water exposure. Fish were exposed to either 10 or 100 ng EE2/L for 8 days, followed by an 8 day depuration phase. Livers were collected after days 1, 4, and 8 of the exposure, and at the end of the depuration phase. Analysis of polar extracts of the liver revealed a greater impact of EE2 on males than females, with metabolite profiles of the former assuming similarities with those of the females (i.e., feminization) early in the exposure. Biochemical effects observed in the males included changes in metabolites relating to energetics (e.g., glycogen, glucose, and lactate) and liver toxicity (creatine and bile acids). In addition, amino acids associated with vitellogenin (VTG) synthesis increased in livers of EE2-exposed males, a finding consistent with increased plasma concentrations of the lipoprotein in the fish. Using partial least-squares discriminant analysis (PLS-DA), the response trajectories of the males at both exposure concentrations were compared. This revealed an apparent ability of the fish to compensate for the presence of EE2 later in the exposure, and to partially recover from its effects after the chemical was removed.

Hepatocellular transport in acquired cholestasis: new insights into functional, regulatory and therapeutic aspects

The recent overwhelming advances in molecular and cell biology have added enormously to our understanding of the physiological processes involved in bile formation and, by extension, to our comprehension of the consequences of their alteration in cholestatic hepatopathies. The present review addresses in detail this new information by summarizing a number of recent experimental findings on the structural, functional and regulatory aspects of hepatocellular transporter function in acquired cholestasis. This comprises (i) a short overview of the physiological mechanisms of bile secretion, including the nature of the transporters involved and their role in bile formation; (ii) the changes induced by nuclear receptors and hepatocyte-enriched transcription factors in the constitutive expression of hepatocellular transporters in cholestasis, either explaining the primary biliary failure or resulting from a secondary adaptive response; (iii) the post-transcriptional changes in transporter function and localization in cholestasis, including a description of the subcellular structures putatively engaged in the endocytic internalization of canalicular transporters and the involvement of signalling cascades in this effect; and (iv) a discussion on how this new information has contributed to the understanding of the mechanism by which anticholestatic agents exert their beneficial effects, or the manner in which it has helped the design of new successful therapeutic approaches to cholestatic liver diseases.

P-glycoprotein function and expression during obstructive cholestasis in rats

OBJECTIVES

The present study was aimed at evaluation of in vivo biliary and renal excretion of rhodamine 123 (Rho123), a P-glycoprotein (P-gp) substrate, in rats during either acute or chronic cholestasis induced by bile duct obstruction (BDO).

METHODS

The Rho123 clearance study was performed either one (BDO1) or seven (BDO7) days after BDO. Bile flow was reconstituted, and bile and urine were collected after steady-state plasma concentration of Rho123 was attained. Tissue expression of P-gp was evaluated by quantitative immunohistochemistry, and immunoblotting.

RESULTS

Significant up-regulation of the liver P-gp protein was observed in acute and chronic cholestasis. Primary periportal location of P-gp was enlarged also to pericentral areas. In the kidneys, immunohistochemistry showed pancellular increase in P-gp after 1 day of BDO, which subsided after 7 days of BDO. Nevertheless, biliary and renal clearances (CL(Bile) and CL(R)) of Rho123 did not reflect the induction of P-gp expression. While CL(Bile) was reduced one day after cholestasis and restored on the seventh day, the CL(R) was preserved in BDO1 group and reduced in BDO7 group without change in glomerular filtration rate. In parallel, biliary and renal clearances of conjugated bilirubin were significantly reduced in both cholestatic groups compared with controls.

CONCLUSION

These findings suggest that extrahepatic cholestasis causes time-dependent changes in elimination of Rho123 which do not exactly reflect alteration of P-gp expression in the rat liver and kidney. These data may help to explain impaired elimination of P-gp substrates after short-term cholestasis that may commonly occur in clinical practice.

Dearterialization of the liver causes intrahepatic cholestasis due to reduced bile transporter expression

BACKGROUND

Bile duct injury after hepatic artery thrombosis (HAT) in liver transplantation is believed to be caused by ischemia predominantly. We aimed to define the involvement of bile secretory dysfunction in the pathogenesis of liver injury after HAT.

METHODS

In a murine model, the main hepatic artery, the extrahepatic peribiliary plexus, or both arterial connections to the liver were interrupted (n=5 for each group). After 1, 14, or 28 days, hepatobiliary function was assessed by analysis of bile transporter expression, serum bile acids and bilirubin, and hepatic ATP content. In addition, cellular injury was assessed by light microscopy and biochemical markers.

RESULTS

There were no signs of hepatobiliary dysfunction or injury in sham-operated animals or in mice with interruption of the hepatic artery or the extrahepatic peribiliary plexus alone. However, as early as 24 hr after complete dearterialization, bile transporter expression was significantly reduced and intrahepatic cholestasis started to progress the following weeks. Histologic studies at 28 days after complete dearterialization showed severe hepatobiliary injury.

CONCLUSIONS

This study indicates that arterial blood supply is critical for normal bile secretion. Bile duct injury after complete arterial deprivation is preceded by a loss of bile secretory function and subsequent intrahepatic cholestasis.

Molecular pathogenesis of intrahepatic cholestasis of pregnancy

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

LPS induces the TNF-alpha-mediated downregulation of rat liver aquaporin-8: role in sepsis-associated cholestasis

Although bacterial lipopolysaccharides (LPS) are known to cause cholestasis in sepsis, the molecular mechanisms accounting for this effect are only partially known. Because aquaporin-8 (AQP8) seems to facilitate the canalicular osmotic water movement during hepatocyte bile formation, we studied its gene and functional expression in LPS-induced cholestasis. By subcellular fractionation and immunoblotting analysis, we found that 34-kDa AQP8 was significantly decreased by 70% in plasma (canalicular) and intracellular (vesicular) liver membranes. However, expression and subcellular localization of hepatocyte sinusoidal AQP9 were unaffected. Immunohistochemistry for liver AQPs confirmed these observations. Osmotic water permeability (P(f)) of canalicular membranes, measured by stopped-flow spectrophotometry, was significantly reduced (65 +/- 1 vs. 49 +/- 1 microm/s) by LPS, consistent with defective canalicular AQP8 functional expression. By Northern blot analysis, we found that 1.5-kb AQP8 mRNA expression was increased by 80%, suggesting a posttranscriptional mechanism of protein reduction. The tumor necrosis factor-alpha (TNF-alpha) receptor fusion protein TNFp75:Fc prevented the LPS-induced impairment of AQP8 expression and bile flow, suggesting the cytokine TNF-alpha as a major mediator of LPS effect. Accordingly, studies in hepatocyte primary cultures indicated that recombinant TNF-alpha downregulated AQP8. The effect of TNF-alpha was prevented by the lysosomal protease inhibitors leupeptin or chloroquine or by the proteasome inhibitors MG132 or lactacystin, suggesting a cytokine-induced AQP8 proteolysis. In conclusion, our data suggest that LPS induces the TNF-alpha-mediated posttranscriptional downregulation of AQP8 functional expression in hepatocytes, a mechanism potentially relevant to the molecular pathogenesis of sepsis-associated cholestasis.

Sepsis and cholestasis

Cholestasis is a common complication of bacterial infections and sepsis. This article gives a comprehensive overview of the underlying molecular mechanisms of sepsis-associated cholestasis and jaundice, their clinical presentation, and diagnostic and therapeutic management.

Hepatic synthesis and urinary elimination of acetaminophen glucuronide are exacerbated in bile duct-ligated rats

Renal and intestinal disposition of acetaminophen glucuronide (APAP-GLU), a common substrate for multidrug resistance-associated proteins 2 and 3 (Mrp2 and Mrp3), was assessed in bile duct-ligated rats (BDL) 7 days after surgery using an in vivo perfused jejunum model with simultaneous urine collection. Doses of 150 mg/kg b.w. (i.v.) or 1 g/kg b.w. (i.p.) of acetaminophen (APAP) were administered, and its glucuronide was determined in bile (only Shams), urine, and intestinal perfusate throughout a 150-min period. Intestinal excretion of APAP-GLU was unchanged or decreased (-58%) by BDL for the 150 mg and 1 g/kg b.w. doses of APAP, respectively. In contrast, renal excretion was increased by 200 and 320%, respectively. Western studies revealed decreased levels of apical Mrp2 in liver and jejunum but increased levels in renal cortex from BDL animals, whereas Mrp3 was substantially increased in liver and not affected in kidney or intestine. The global synthesis of APAP-GLU, determined as the sum of cumulative excretions, was higher in BDL rats (+51 and +110%) for these same doses of APAP as a consequence of a significant increase in functional liver mass, with no changes in specific glucuronidating activity. Expression of apical breast cancer resistance protein, which also transports nontoxic metabolites of APAP, was decreased by BDL in liver and renal cortex, suggesting a minor participation of this route. We demonstrate a more efficient hepatic synthesis and basolateral excretion of APAP-GLU followed by its urinary elimination in BDL group, the latter two processes consistent with up-regulation of liver Mrp3 and renal Mrp2.

Obstructive cholestasis induces TNF-alpha- and IL-1 -mediated periportal downregulation of Bsep and zonal regulation of Ntcp, Oatp1a4, and Oatp1b2

Inverse acinar regulation of Mrp2 and 3 represents an adaptive response to hepatocellular cholestatic injury. We studied whether obstructive cholestasis (bile duct ligation) and LPS treatment affect the zonal expression of Bsep (Abcb11), Mrp4 (Abcc4), Ntcp (Slc10a1), and Oatp isoforms (Slco1a1, Slco1a4, and slco1b2) in rat liver, as analyzed by semiquantitative immunofluorescence. Contribution of TNF-alpha and IL-1beta to transporter zonation in obstructive cholestasis was studied by cytokine inactivation. In normal liver Bsep, Mrp4, Ntcp, and Oatp1a1 were homogeneously distributed in the acinus, whereas Oatp1a4 and Oatp1b2 expression increased from zone 1 to 3. Glutamine synthetase-positive pericentral hepatocytes exhibited markedly lower Oatp1a4 expression than the remaining zone 3 hepatocytes. In cholestatic liver Bsep and Ntcp immunofluorescence in periportal hepatocytes significantly decreased to 66 +/- 4% (P < 0.01) and 67 +/- 7% (P < 0.05), whereas it was not altered in pericentral hepatocytes. Oatp1a4 was significantly induced in hepatocytes with a primarily low expression, i.e., in periportal hepatocytes and in glutamine synthetase-positive pericentral hepatocytes. Likewise, Oatp1b2 was upregulated in periportal hepatocytes. Mrp4 zonal induction was homogeneous. Inactivation of TNF-alpha and IL-1beta prevented periportal downregulation of Bsep. Recruitment of neutrophils and polymorphonuclear cells mainly occurred in the periportal zone. Likewise, IL-1beta induction was largely found periportally. No significant transporter zonation was seen following LPS treatment. In conclusion, zonal downregulation of Bsep in obstructive cholestasis is associated with portal inflammation and is mediated by TNF-alpha and IL-1beta. Periportal downregulation of Ntcp and induction of Oatp1a4 and Oatp1b2 may represent adaptive mechanisms to reduce cholestatic injury in hepatocytes with profound downregulation of Bsep and Mrp2.

Extrahepatic cholestasis downregulates Oatp1 by TNF-alpha signalling without affecting Oatp2 and Oatp4 expression and sodium-independent bile salt uptake in rat liver

Hepatic uptake of bile salts is mediated by sodium-dependent and sodium-independent transport systems. During extrahepatic cholestasis, both the function and the expression of the Na(+)/taurocholate cotransporting polypeptide (Ntcp) are downregulated. To test whether sodium-independent organic anion-transporting polypeptides are also affected by extrahepatic cholestasis, the function and expression of all three Oatps have been determined in common bile duct-ligated (CBDL) rats. Oatp1/Oatp1a1 protein mass remained unchanged after CBDL for 1 day, but then declined by 75+/-7% and 90+/-17%, respectively, after 3 and 7 days. In contrast, Oatp2/Oatp1a4 and Oatp4/Oatp1b2 protein expression was not affected by CBDL as compared with controls. After CBDL, Oatp1 mRNA was rapidly downregulated by 68+/-21% of untreated controls (P<0.05) within 24 h, and remained at similar levels at 3 and 7 days. Cytokine-inactivation studies with etanercept pretreatment demonstrated that TNF-alpha-dependent signals mediated the down-regulation of this transporter gene at both protein and mRNA levels during obstructive cholestasis. Sodium-independent uptake of taurocholate and cholate into freshly isolated hepatocyte suspensions showed neither significant differences in K(m) nor V(max) values. These results indicate that sodium-independent transport of bile salts may be mediated by Oatp2 and 4 during biliary obstruction, because its expression remains unaffected and may compensate for loss of Oatp1 expression and function in cholestatic hepatocytes.

Regulation of hepatic bile acid transporters Ntcp and Bsep expression

Sodium-taurocholate cotransporting polypeptide (Ntcp) and bile salt export pump (Bsep) are two key transporters for hepatic bile acid uptake and excretion. Alterations in Ntcp and Bsep expression have been reported in pathophysiological conditions. In the present study, the effects of age, gender, and various chemicals on the regulation of these two transporters were characterized in mice. Ntcp and Bsep mRNA levels in mouse liver were low in the fetus, but increased to its highest expression at parturition. After birth, mouse Ntcp and Bsep mRNA decreased by more than 50%, and then gradually increased to adult levels by day 30. Expression of mouse Ntcp mRNA and protein exhibit higher levels in female than male livers. No gender difference exists in BSEP/Bsep expression in human and mouse livers. Hormone replacements conducted in gonadectomized, hypophysectomized, and lit/lit mice indicate that female-predominant Ntcp expression in mouse liver is due to the inhibitory effect of male-pattern GH secretion, but not sex hormones. Ntcp and Bsep expression are in general resistant to induction by a large battery of microsomal enzyme inducers. Administration of cholestyramine increased Ntcp, whereas chenodeoxycholic acid (CDCA) increased Bsep mRNA expression. In conclusion, mouse Ntcp and Bsep are regulated by age, gender, cholestyramine, and bile acid, but resistant to induction by most microsomal enzyme inducers.

Beneficial Effect of Spironolactone Administration on Ethynylestradiol-Induced Cholestasis in the Rat: Involvement of Up-Regulation of Multidrug Resistance-Associated Protein 2

The effect of spironolactone (SL) administration on 17alpha-ethynylestradiol (EE)-induced cholestasis was studied, with emphasis on expression and activity of Mrps. Adult male Wistar rats were divided into the following groups: EE (5 mg/kg daily for 5 days, s.c.), SL (200 micromol/kg daily for 3 days, i.p.), EE+SL (same doses, SL administered the last 3 days of EE treatment), and controls. SL prevented the decrease in bile salt-independent fraction of bile flow induced by EE, in association with normalization of biliary excretion of glutathione. Western blot studies indicate that EE decreased the expression of multidrug resistance-associated protein 2 (Mrp2) by 41% and increased that of Mrp3 by 200%, whereas SL only affected Mrp2 expression (+60%) with respect to controls. The EE+SL group showed increased levels of Mrp2 and Mrp3 to the same extent as that registered for the individual treatments. Real-time polymerase chain reaction studies indicated that up-regulation of Mrp2 and Mrp3 by SL and EE, respectively, was at the transcriptional level. To estimate Mrp2 and Mrp3 activities, apical and basolateral excretion of acetaminophen glucuronide (APAP-glu), a common substrate for both transporters, was measured in the recirculating isolated perfused liver model. Biliary/perfusate excretion ratio was decreased in EE (-88%) and increased in SL (+36%) with respect to controls. Coadministration of rats with SL partially prevented (-53%) impairment induced by EE in this ratio. In conclusion, SL administration to EE-induced cholestatic rats counteracted the decrease in bile flow and biliary excretion of glutathione and APAP-glu, a model Mrp substrate, findings associated with up-regulation of Mrp2 expression.

Hypoxia-induced changes in the expression of rat hepatobiliary transporter genes

Cholestatic disorders may arise from liver ischemia (e.g., in liver transplantation) through various mechanisms. We have examined the potential of hypoxia to induce changes in the expression of hepatobiliary transporter genes. In a model of arterial liver ischemia subsequent to complete arterial deprivation of the rat liver, the mRNA levels of VEGF, a hypoxia-inducible gene, were increased fivefold after 24 h. The pattern of VEGF-induced expression and ultrastructural changes, including swelling of the endoplasmic reticulum, indicated that hypoxia affected primarily cholangiocytes, but also hepatocytes, predominantly in the periportal area. Serum and bile analyses demonstrated liver dysfunction of cholestatic type with reduced bile acid biliary excretion. Fluorescence-labeled ursodeoxycholic acid used as a tracer displayed no regurgitation, eliminating bile leakage as a significant mechanism of cholestasis in this model. In liver tissue, a marked reduction in the mRNA levels of Na(+)-taurocholate-cotransporting polypeptide (Ntcp), bile salt export protein (Bsep), and multidrug resistance-associated protein 2 (Mrp2) and an increase in those of Cftr were detected before bile duct proliferation occurred. In cultured hepatocytes, a nontoxic hypoxic treatment caused a decrease in the mRNA and protein expression of Ntcp, Bsep, and Mrp2 and in the mRNA levels of nuclear factors involved in the transactivation of these genes, i.e., HNF4alpha, RXRalpha, and FXR. In bile duct preparations, hypoxic treatment elicited an increase in Cftr transcripts, along with a rise in cAMP, a major regulator of Cftr expression and function. In conclusion, hypoxia triggers a downregulation of hepatocellular transporters, which may contribute to cholestasis, whereas Cftr, which drives secretion in cholangiocytes, is upregulated.

Hepatobiliary transporter expression in intercellular adhesion molecule 1 knockout and Fas receptor-deficient mice after common bile duct ligation is independent of the degree of inflammation and oxidative stress

Liver injury in intercellular adhesion molecule 1 knockout (ICAM(-/-)) and Fas receptor-deficient (lpr) mice is markedly reduced after common bile duct ligation (CBDL) due to significantly reduced inflammation and oxidative stress. Liver injury in CBDL rodents is counteracted by adaptive hepatobiliary transporter induction. Since hepatobiliary transporter expression in obstructive cholestasis may be regulated not only by accumulating bile acids but also by inflammatory mediators and oxidative stress, we hypothesized that differences in the inflammatory response may affect hepatobiliary transporter expression in CBDL, which would contribute to reduced liver injury. Therefore, expression of major hepatobiliary transporters (Ntcp, Bsep, Mrp2-4, Ost alpha/beta) was determined by Taqman RT-PCR and Western blotting in sham-operated animals and 3 days after CBDL in wild-type, ICAM(-/-) and lpr mice of the endotoxin-sensitive C57BL/6 and the endotoxin-resistant C3H/HeJ strains. CBDL resulted in a significant decrease of Ntcp in all genotypes. Canalicular transporters Bsep and Mrp2 were repressed only in the endotoxin-sensitive strain regardless of the genotype. Mrp3 was moderately induced in ICAM(-/-), lpr, and endotoxin-resistant mice, whereas Mrp4 was only induced in the endotoxin-resistant strain. Ost beta was massively induced in all CBDL mice, whereas Ost alpha was reduced. In conclusion, markedly reduced inflammation and oxidative stress in CBDL ICAM(-/-) and lpr mice does not profoundly affect hepatobiliary transporter expression. Therefore, transporter expression does not account for reduced liver injury in ICAM(-/-) and lpr mice. Induction of the adaptive transporter response after CBDL is independent of the degree of the inflammatory response. Rather, retention of biliary constituents may determine transporter expression in CBDL.

4-phenylbutyrate enhances the cell surface expression and the transport capacity of wild-type and mutated bile salt export pumps

Progressive familial intrahepatic cholestasis type 2 (PFIC2) is caused by a mutation in the bile salt export pump (BSEP/ABCB11) gene. We previously reported that E297G and D482G BSEP, which are frequently found mutations in European patients, result in impaired membrane trafficking, whereas both mutants retain their transport function. The dysfunctional localization is probably attributable to the retention of BSEP in endoplasmic reticulum (ER) followed by proteasomal degradation. Because sodium 4-phenylbutyrate (4PBA) has been shown to restore the reduced cell surface expression of mutated plasma membrane proteins, in the current study, we investigated the effect of 4PBA treatment on E297G and D482G BSEP. Transcellular transport and cell surface biotinylation studies using Madin-Darby canine kidney (MDCK) II cells demonstrated that 4PBA treatment increased functional cell surface expression of wild-type (WT), E297G, and D482G BSEP. The prolonged half-life of cell surface-resident BSEP with 4PBA treatment was responsible for this result. Moreover, treatment of Sprague-Dawley rats with 4PBA resulted in an increase in BSEP expression at the canalicular membrane, which was accompanied by an increase in the biliary excretion of [(3)H]taurocholic acid (TC).

CONCLUSION

4PBA treatment with a clinically achievable concentration enhances the cell surface expression and the transport capacity of WT, E297G, and D482G BSEP in MDCK II cells, and also induces functional BSEP expression at the canalicular membrane and bile acid transport via canalicular membrane in vivo. 4PBA is a potential pharmacological agent for treating not only PFIC2 patients with E297G and D482G mutations but also other cholestatic patients, in whom the BSEP expression at the canalicular membrane is reduced.

Heat stress modulates hepatocyte membrane proteins during endotoxemia

BACKGROUND

Hepatic failure following sepsis is one of the important features of burns. Studies have shown that in septic rats, heat stress (HS) has a protective effect on bile acid transporters in hepatocyte membranes. This study investigates the influence of HS on hepatocyte membrane proteins during endotoxemia using 2D gel electrophoresis.

METHODS

Endotoxemia in rats was induced by intraperitoneal injection of Escherichia coli lipopolysaccharide (LPS) (n=24), while control rats (n=24) received saline. Twelve rats from each group were exposed to HS 2h prior to LPS or saline injection by external warming to 42 degrees C for 10 min and 12 rats in each group were exposed to ambient temperature 2h prior to LPS or saline injection. Membrane fractions were extracted 12, 24 and 72 h after LPS or saline treatment. Extracted proteins were separated using 2D gel electrophoresis. The most dominant spots were analyzed by MALDI-TOF-MS.

RESULTS

Two-dimensional gel electrophoresis differentially identified expressed proteins in all treatment groups. The majority of the spots developed 24h after injection. Membrane proteins; Wnt 13, ribosomal protein L14, VLCAD, BHMT and HIT-40 were found only in HS-LPS. Protein profiles of the groups returned to normal after 72 h.

CONCLUSION

We propose that HS during endotoxemia changes hepatic membrane proteins expression, which are involved in metabolism.

Defective hepatocyte aquaporin-8 expression and reduced canalicular membrane water permeability in estrogen-induced cholestasis

Our previous work supports a role for aquaporin-8 (AQP8) water channels in rat hepatocyte bile formation mainly by facilitating the osmotically driven canalicular secretion of water. In this study, we tested whether a condition with compromised canalicular bile secretion, i.e., the estrogen-induced intrahepatic cholestasis, displays defective hepatocyte AQP8 functional expression. After 17alpha-ethinylestradiol administration (5 mg x kg body wt(-1).day(-1) for 5 days) to rats, the bile flow was reduced by 58% (P < 0.05). By subcellular fractionation and immunoblotting analysis, we found that 34 kDa AQP8 was significantly decreased by approximately 70% in plasma (canalicular) and intracellular (vesicular) liver membranes. However, 17alpha-ethinylestradiol-induced cholestasis did not significantly affect the protein level or the subcellular localization of sinusoidal AQP9. Immunohistochemistry for liver AQPs confirmed these observations. Osmotic water permeability (P(f)) of canalicular membranes, measured by stopped-flow spectrophotometry, was significantly reduced (73 +/- 1 vs. 57 +/- 2 microm/s) in cholestasis, consistent with defective canalicular AQP8 functional expression. By Northern blotting, we found that AQP8 mRNA expression was increased by 115% in cholestasis, suggesting a posttranscriptional mechanism of protein level reduction. Accordingly, studies in primary cultured rat hepatocytes indicated that the lysosomal protease inhibitor leupeptin prevented the estrogen-induced AQP8 downregulation. In conclusion, hepatocyte AQP8 protein expression is downregulated in estrogen-induced intrahepatic cholestasis, presumably by lysosomal-mediated degradation. Reduced canalicular membrane AQP8 expression is associated with impaired osmotic membrane water permeability. Our data support the novel notion that a defective expression of canalicular AQP8 contributes as a mechanism for bile secretory dysfunction of cholestatic hepatocytes.

Adaptive response to increased bile acids: induction of MDR1 gene expression and P-glycoprotein activity in renal epithelial cells

Cholestatic liver disease and increased serum bile acid concentrations are known to trigger various adaptive responses including the induction of hepatic, intestinal and renal bile acid transport proteins, but renal P-glycoprotein (Pgp, multidrug resistance protein 1, MDR1) remained uninvestigated in this context. We show that treatment of Madin Darby canine kidney (MDCK) cells with pathophysiologically relevant concentrations of chenodeoxycholic acid (CDCA; 100 microM) for 12 h induces MDR1 transcript levels in vitro more than twofold. CDCA and deoxycholic acid pre-treatment for 24-96 h (100 microM) also increased Pgp activity measured as rhodamine efflux, while cholic acid and taurocholic acid were not effective in concentrations up to 600 microM. CDCA pre-treatment (100 microM, 72 h) also resulted in a doubling of rhodamine123 secretion across an epithelium-like monolayer grown on Transwell filters and decreased the sensitivity towards the kidney toxic drugs cyclosporine A and paclitaxel. These findings predict physiologically as well as pharmacologically relevant consequences of liver disease for Pgp substrate transport and toxicity in the kidneys.

Effect of ?-asarone and a derivative on lipids, bile flow and Na+/K+-ATPase in ethinyl estradiol-induced cholestasis in the rat

Administration of ethinyl estradiol (EE), a widely used component of oral contraceptives, has been associated with impairment of bile flow and the capacity to excrete organic anions in man and experimental animals. alpha-Asarone (2,4,5-trimethoxypropenylbenzene) and 2-methoxy-4-(2-propenyl) phenoxyacetic acid (MPPA) have shown hypolipidemic effects. In addition to these effects, we decided to evaluate the properties of these compounds on EE-induced cholestasis. Wistar male rats were injected subcutaneously with 10 mg/kg of EE for 5 days; simultaneously, alpha-asarone or MPPA were also administered and appropriate controls were performed. alpha-asarone and MPPA decreased plasma and bile cholesterol. EE diminished triglycerides total, low-density lipoprotein, high-density lipoprotein and bile cholesterol. MPPA further decreased these lipid parameters. Alkaline phosphatase (an enzyme marker of cholestasis) was increased after administration of EE, but this effect was prevented significantly by alpha-asarone or MPPA administration. Bile flow was importantly decreased by EE and increased by alpha-asarone alone. Furthermore, alpha-asarone or MPPA preserved the normal bile flow in EE-treated rats. EE inhibited the activity of the Na(+)/K(+)-ATPase, while both alpha-asarone and MPPA preserved this enzyme activity. Na(+)/K(+)-ATPase is involved in Na(+)-coupled uptake of bile acids into hepatocytes and, therefore, ultimately is the driving force for the generation of bile flow. Therefore, the anticholestatic effects of alpha-asarone and MPPA, described herein by the first time, may be due to its ability to preserve ATPase activity. This enzyme is negatively regulated by membrane cholesterol, thus the hypolipidemic effects of the compounds tested may be responsible for Na(+)/K(+)-ATPase activity and bile flow maintenance.

Pharmacokinetic Drug Interactions Involving 17??-Ethinylestradiol

17alpha-Ethinylestradiol (EE) is widely used as the estrogenic component of oral contraceptives (OC). In vitro and in vivo metabolism studies indicate that EE is extensively metabolised, primarily via intestinal sulfation and hepatic oxidation, glucuronidation and sulfation. Cytochrome P450 (CYP)3A4-mediated EE 2-hydroxylation is the major pathway of oxidative metabolism of EE. For some time it has been known that inducers of drug-metabolising enzymes (such as the CYP3A4 inducer rifampicin [rifampin]) can lead to breakthrough bleeding and contraceptive failure. Conversely, inhibitors of drug-metabolising enzymes can give rise to elevated EE plasma concentrations and increased risks of vascular disease and hypertension. In vitro studies have also shown that EE inhibits a number of human CYP enzymes, such as CYP2C19, CYP3A4 and CYP2B6. Consequently, there are numerous reports in the literature describing EE-containing OC formulations as perpetrators of pharmacokinetic drug interactions. Because EE may participate in multiple pharmacokinetic drug interactions as either a victim or perpetrator, pharmaceutical companies routinely conduct clinical drug interaction studies with EE-containing OCs when evaluating new chemical entities in development. It is therefore critical to understand the mechanisms underlying these drug interactions. Such an understanding can enable the interpretation of clinical data and lead to a greater appreciation of the profile of the drug by physicians, clinicians and regulators. This article summarises what is known of the drug-metabolising enzymes and transporters governing the metabolism, disposition and excretion of EE. An effort is made to relate this information to known clinical drug-drug interactions. The inhibition and induction of drug-metabolising enzymes by EE is also reviewed.

Bile acid analysis in biliary tract cancer

The etiology of biliary tract cancer is obscure, but there are evidences that bile acid plays a role in carcinogenesis. To find the association between biliary tract cancer and bile acid, this study compared the bile acid concentration and composition among patients with biliary cancer, biliary tract stones, and no biliary disease. Bile was compared among patients with biliary tract cancer (n = 26), biliary tract stones (n = 29), and disease free controls (n = 9). Samples were obtained by percutaneous transhepatic biliary drainage, endoscopic nasobiliary drainage, or gallbladder puncture, and analyzed for cholic, deoxycholic, chenodeoxycholic, lithocholic, and ursodeoxycholic acid composition. Total bile acid concentration was lower in the cancer group than the biliary stone and control groups; the proportions of deoxycholic (2.2% vs. 10.2% and 23.6%, p < 0.001 and p < 0.001, respectively) and lithocholic acid (0.3% vs. 0.6% and 1.0%, p = 0.065 and p < 0.001, respectively) were also lower. This result was similar when disease site was limited to bile duct or gallbladder. Analysis of cases with bilirubin <or= 2.0 mg/dL also showed lower total bile acid concentration and deoxycholic acid composition in the cancer group compared to controls (5.7% vs. 23.6%, p = 0.003). Although the presence of bile duct obstruction explains some of the difference in total concentration and composition of bile acid, there are other contributing mechanisms. We suspect the alteration of bile acid transport might decrease bile acid excretion and cause the accumulation of carcinogenic bile acid in bile duct epithelium.

Role of pituitary hormones on 17alpha-ethinylestradiol-induced cholestasis in rat

Estrogens cause intrahepatic cholestasis in susceptible women during pregnancy, after administration of oral contraceptives, or during postmenopausal hormone replacement therapy. 17alpha-Ethinylestradiol (EE) is a synthetic estrogen widely used to cause experimental cholestasis in rodents with the aim of examining molecular mechanisms involved in this disease. EE actions on the liver are thought to be mediated by estrogen receptor alpha (ERalpha) and pituitary hormones. We tested this hypothesis by analyzing metabolic changes induced by EE in livers from hypophysectomized (HYPOX) and hypothyroid rats. Microarray studies revealed that the number of genes regulated by EE was increased almost 4-fold in HYPOX rat livers compared with intact males. Little overlap was apparent between the effects of EE in intact and HYPOX rats, demonstrating that pituitary hormones play a critical role in the hepatic effects of EE. Consistently, hypophysectomy protects the liver against induction by EE of serum bilirubin and alkaline phosphatase, two markers of cholestasis and hepatotoxicity and modulates the effects of EE on several genes involved in bile acid homeostasis (e.g., FXR, SHP, BSEP, and Cyp8b1). Finally, we demonstrate a novel mechanism of action of EE through binding and negative regulation of glucocorticoid receptor-mediated transcription. In summary, pituitary- and ERalpha-independent mechanisms contribute to development of EE-induced changes in liver transcriptome. Such mechanisms may be relevant when this model of EE-induced cholestasis is evaluated. The observation that the pharmacological effects of estrogen in liver differ in the absence or presence of the pituitary could be clinically relevant, because different drugs that block actions of pituitary hormones are now available.

The bile salt export pump

Canalicular secretion of bile salts mediated by the bile salt export pump Bsep constitutes the major driving force for the generation of bile flow. Bsep is a member of the B-family of the super family of ATP-binding cassette transporters and is classified as ABCB11. Bsep has a narrow substrate specificity, which is largely restricted to bile salts. Bsep is extensively regulated at the transcriptional and posttranscriptional level, which directly modulates canalicular bile formation. Pathophysiological alterations of Bsep by either inherited mutations or acquired processes such as inhibition by drugs or disease-related down regulation may lead to a wide spectrum of mild to severe forms of liver disease. Furthermore, many genetic variants of Bsep are known, some of which potentially render individuals susceptible to acquired forms of liver disease.

Mechanisms of Disease: mechanisms and clinical implications of cholestasis in sepsis

Cholestasis is a common complication in patients with extrahepatic bacterial infection and sepsis. This article gives a comprehensive overview of the molecular and cellular mechanisms of sepsis-associated cholestasis. Recent advances in the understanding of intrahepatic cholestasis have allowed us to delineate the molecular mechanisms that underlie sepsis-associated cholestasis and to describe their potential clinical and therapeutic applications. The mechanisms and clinical presentation of sepsis-associated liver injury vary according to the severity of the bacterial infection. Proinflammatory cytokines and nitric oxide cause cholestasis by impairing hepatocellular and ductal bile formation. Ischemic liver injury and, rarely, progressive sclerosing cholangitis can also be found in patients with septic shock, or major trauma with systemic inflammatory response syndrome. Treatment is mainly focused on eradication of the underlying infection and managing the sepsis. The use of ursodeoxycholic acid or extracorporeal liver support as treatments for sepsis-associated cholestasis is under investigation, but neither can be recommended in routine clinical practice at present. Patients with progressive sclerosing cholangitis should be considered for orthotopic liver transplantation.

Review article: modulation of the brain-gut axis as a therapeutic approach in gastrointestinal disease

BACKGROUND

The importance of bi-directional brain-gut interactions in gastrointestinal illness is increasingly being recognized, most prominently in the area of functional gastrointestinal disorders. Numerous current and emerging therapies aimed at normalizing brain-gut interactions are a focus of interest, particularly for irritable bowel syndrome and functional dyspepsia.

METHODS

A literature search was completed for preclinical and clinical studies related to central modulation of gastrointestinal functions and published in English between 1980 and 2006.

RESULTS

Existing data, while sparse, support the use of different classes of antidepressant drugs, including tricyclics, and selective and non-selective serotonin reuptake inhibitors in irritable bowel syndrome. Serotonin receptor agonists and antagonists with peripheral and possibly central effects are effective in treating specific subtypes of irritable bowel syndrome. Based largely on theoretical and preclinical evidence, several novel compounds that selectively target receptors at multiple levels within the brain-gut axis such as neurokinin, somatostatin and corticotropin-releasing factor receptor antagonists are promising.

CONCLUSIONS

This review discusses the rationale for modulation of the brain-gut axis in the treatment of functional gastrointestinal disorders and highlights the most promising current and future therapeutic strategies.

Principles of hepatic organic anion transporter regulation during cholestasis, inflammation and liver regeneration

Hepatic uptake and biliary excretion of organic anions (e.g., bile acids and bilirubin) is mediated by hepatobiliary transport systems. Defects in transporter expression and function can cause or maintain cholestasis and jaundice. Recruitment of alternative export transporters in coordination with phase I and II detoxifying pathways provides alternative pathways to counteract accumulation of potentially toxic biliary constituents in cholestasis. The genes encoding for organic anion uptake (NTCP, OATPs), canalicular export (BSEP, MRP2) and alternative basolateral export (MRP3, MRP4) in liver are regulated by a complex interacting network of hepatocyte nuclear factors (HNF1, 3, 4) and nuclear (orphan) receptors (e.g., FXR, PXR, CAR, RAR, LRH-1, SHP, GR). Bile acids, proinflammatory cytokines, hormones and drugs mediate causative and adaptive transporter changes at a transcriptional level by interacting with these nuclear factors and receptors. Unraveling the underlying regulatory mechanisms may therefore not only allow a better understanding of the molecular pathophysiology of cholestatic liver diseases but should also identify potential pharmacological strategies targeting these regulatory networks. This review is focused on general principles of transcriptional basolateral and canalicular transporter regulation in inflammation-induced cholestasis, ethinylestradiol- and pregnancy-associated cholestasis, obstructive cholestasis and liver regeneration. Moreover, the potential therapeutic role of nuclear receptor agonists for the management of liver diseases is highlighted.

Heat stress enhances recovery of hepatocyte bile acid and organic anion transporters in endotoxemic rats by multiple mechanisms

Heat stress (HS) reduces the many sequelae of lipopolysaccharide (LPS)-induced endotoxemia. Without HS, endotoxins have been shown to induce a transcriptional down-regulation of hepatocyte transport proteins for bile acids and organic anions. We performed experiments in isolated perfused rat livers at various times after LPS administration with and without HS pretreatment to determine whether HS would correct deficient transport of bromosulfophthalein (BSP). Possible mechanisms involved were investigated in livers from intact animals. In isolated perfused livers, LPS injection reduced BSP excretion to 48% compared with saline-injected controls (P < 0.01). When HS was applied 2 hours prior to LPS, BSP excretion increased to 74% of controls (P < 0.05 vs LPS and controls). Expression of the basolateral (Oatp1a1) and canalicular (Mrp2) organic anion transporter involved in the transport of BSP recovered more rapidly when HS preceded LPS application. Recovery of mRNA levels of these transporters occurred also earlier. Coimmunoprecipitation experiments and immunoelectron microscopy using a double immunogold labeling of heat shock protein 70 (HSP70) and various hepatocyte transporters suggested colocalization with HSP70 for the canalicular bile acid transporter (Bsep) in the subcanalicular space. In contrast, no colocalization was shown for Ntcp and anion transporters. In conclusion, we could show that HS enhances recovery of organic anion transporters and bile acid transporters following endotoxemia. Faster recovery of mRNA seems to be a key mechanism for anion transporters, whereas physical interaction with HSP70 plays a role in preservation of bile acid transporters. This interaction of HSP70 and canalicular transporters occurs only in pericanalicular vesicles but not when the protein is integrated into the plasma membrane.

Mrp4-/- mice have an impaired cytoprotective response in obstructive cholestasis

Mrp4 is a member of the multidrug resistance-associated gene family that is expressed on the basolateral membrane of hepatocytes and undergoes adaptive upregulation in response to cholestatic injury or bile acid feeding. However, the relative importance of Mrp4 in a protective adaptive response to cholestatic injury is not known. To address this issue, common bile duct ligation (CBDL) was performed in wild-type and Mrp4-/- mice and animals followed for 7 days. Histological analysis and serum aminotransferase levels revealed more severe liver injury in the absence of Mrp4 expression. Western analyses revealed that Mrp4, but not Mrp3, was significantly increased after CBDL in wild-type mice. Serum bile acid levels were significantly lower in Mrp4-/- mice than in wild-type CBDL mice, whereas serum bilirubin levels were the same, suggesting that Mrp4 was required to effectively extrude bile acids from the cholestatic liver. Mrp3 and Ostalpha-Ostbeta were upregulated in Mrp4-/- mice but were unable to compensate for the loss of Mrp4. High-performance liquid chromatography analysis on liver extracts revealed that taurine tetrahydroxy bile acid/beta-muricholic acid ratios were increased twofold in Mrp4-/- mice. In conclusion, hepatic Mrp4 plays a unique and essential protective role in the adaptive response to obstructive cholestatic liver injury.

Bile salt toxicity aggravates cold ischemic injury of bile ducts after liver transplantation in Mdr2+/- mice

Intrahepatic bile duct strictures are a serious complication after orthotopic liver transplantation (OLT). We examined the role of endogenous bile salt toxicity in the pathogenesis of bile duct injury after OLT. Livers from wild-type mice and mice heterozygous for disruption of the multidrug resistance 2 Mdr2 gene (Mdr2+/-) were transplanted into wild-type recipient mice. Mdr2+/- mice secrete only 50% of the normal amount of phospholipids into their bile, leading to an abnormally high bile salt/phospholipid ratio. In contrast to homozygous Mdr2-/- mice, the Mdr2+/- mice have normal liver histology and function under normal conditions. Two weeks after OLT, bile duct injury and cholestasis were assessed by light and electron microscopy, as well as through molecular and biochemical markers. There were no signs of bile duct injury or intrahepatic cholestasis in liver grafts from wild-type donors. Liver grafts from Mdr2+/- donors, however, had enlarged portal tracts with cellular damage, ductular proliferation, biliostasis, and a dense inflammatory infiltrate after OLT. Parallel to this observation, recipients of Mdr2+/- livers had significantly higher serum transaminases, alkaline phosphatase, total bilirubin, and bile salt levels, as compared with recipients of wild-type livers. In addition, hepatic bile transporter expression was compatible with the biochemical and histological cholestatic profile found in Mdr2+/- grafts after OLT. In conclusion, toxic bile composition, due to a high biliary bile salt/phospholipid ratio, acted synergistically with cold ischemia in the pathogenesis of bile duct injury after transplantation.

A comparison of gene expression in mouse liver and kidney in obstructive cholestasis utilizing high-density oligonucleotide microarray technology

AIM: To assess the effects of obstructive cholestasis on a wider range of gene expression using microarray technology.

METHODS: Male C57BL/6J mice underwent common bile duct ligation (BDL) and were matched with pair-fed sham-operated controls. After 7 d, the animals were sacrificed and total RNA was isolated from livers and kidneys. Equal amounts of RNA from each tissue were pooled for each group and hybridized to Affymetrix GeneChip^®MG-U74Av2 containing a total of 12 488 probe sets. Data analysis was performed using GeneSpring^®6.0 software. Northern analysis and immunofluorescence were used for validation.

RESULTS: In sham-operated and BDL mice, 44 and 50% of 12 488 genes were expressed in livers, whereas 49 and 51% were expressed in kidneys, respectively. Seven days after BDL, 265 liver and 112 kidney genes with GeneOntology annotation were up-regulated and 113 liver and 36 kidney genes were down-regulated in comparison with sham-operated controls. Many genes were commonly regulated in both tissues and metabolism-related genes represented the largest functional group.

CONCLUSION: Following BDL, microarray analysis reveals a broad range of gene alterations in both liver and kidney.

Estrogen receptor alpha mediates 17alpha-ethynylestradiol causing hepatotoxicity

Estrogens are known to cause hepatotoxicity such as intrahepatic cholestasis in susceptible women during pregnancy, after administration of oral contraceptives, or during postmenopausal replacement therapy. Enterohepatic nuclear receptors including farnesoid X receptor (FXR), pregnane X receptor (PXR), and constitutive active/androstane receptor (CAR) are important in maintaining bile acid homeostasis and protecting the liver from bile acid toxicity. However, no nuclear receptor has been implicated in the mechanism for estrogen-induced hepatotoxicity. Here Era(-/-), Erb(-/-), Fxr(-/-), Pxr(-/-), and Car(-/-) mice were employed to show that Era(-/-) mice were resistant to synthetic estrogen 17alpha-ethynylestradiol (EE2)-induced hepatotoxicity as indicated by the fact that the EE2-treated Era(-/-) mice developed none of the hepatotoxic phenotypes such as hepatomegaly, elevation in serum bile acids, increase of alkaline phosphatase activity, liver degeneration, and inflammation. Upon EE2 treatment, estrogen receptor alpha (ERalpha) repressed the expression of bile acid and cholesterol transporters (bile salt export pump (BSEP), Na(+)/taurocholate cotransporting polypeptide (NTCP), OATP1, OATP2, ABCG5, and ABCG8) in the liver. Consistently, biliary secretions of both bile acids and cholesterol were markedly decreased in EE2-treated wild-type mice but not in the EE2-treated Era(-/-) mice. In addition, ERalpha up-regulated the expression of CYP7B1 and down-regulated the CYP7A1 and CYP8B1, shifting bile acid synthesis toward the acidic pathway to increase the serum level of beta-muricholic acid. ERbeta, FXR, PXR, and CAR were not involved in regulating the expression of bile acid transporter and biosynthesis enzyme genes following EE2 exposure. Taken together, these results suggest that ERalpha-mediated repression of hepatic transporters and alterations of bile acid biosynthesis may contribute to development of the EE2-induced hepatotoxicity.

Galactosamine prevents ethinylestradiol-induced cholestasis

Ethinylestradiol (EE) induces intrahepatic cholestasis in experimental animals being its derivative, ethinylestradiol 17beta-glucuronide, a presumed mediator of this effect. To test whether glucuronidation is a relevant step in the pathogenesis of cholestasis induced by EE (5 mg/kg b.wt. s.c. for 5 consecutive days), the effect of simultaneous administration of galactosamine (200 mg/kg b.wt. i.p.) on biliary secretory function was studied. A single injection of this same dose of galactosamine was able to decrease hepatic UDP-glucuronic acid (UDP-GA) levels by 85% and excretion of EE-17beta-glucuronide after administration of a tracer dose of [3H]EE by 40%. Uridine (0.9 g/kg b.wt. i.p.) coadministration reverted the effect of galactosamine on hepatic UDP-GA levels and restored the excretion of [3H]EE-17beta-glucuronide. When administered for 5 days, galactosamine itself did not alter any of the serum markers of liver injury studied (aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase) or biliary secretory function. When coadministered with EE, galactosamine partially prevented the impairment induced by this estrogen in total bile flow, the bile-salt-independent fraction of bile flow, basal bile salt secretion, and the secretory rate maximum of tauroursodeoxycholate. Uridine coadministration partially prevented galactosamine from exerting its anticholestatic effects. In conclusion, galactosamine administration partially prevented EE-induced cholestasis by a mechanism involving decreased UDP-GA availability for subsequent formation of EE 17beta-glucuronide. The evidence thus supports the hypothesis that EE 17beta-glucuronide is involved in the pathogenesis of EE cholestasis.

Ethynylestradiol increases expression and activity of rat liver MRP3

We evaluated the effect of ethynylestradiol (EE) administration (5 mg/kg b.wt. s.c., for 5 consecutive days) on the expression and activity of multidrug resistance-associated protein 3 (Mrp3) in rats. Western blotting analysis revealed decreased Mrp2 (-41%) and increased Mrp3 (+200%) expression by EE. To determine the functional impact of up-regulation of Mrp3 versus Mrp2, we measured the excretion of acetaminophen glucuronide (APAP-glu), a common substrate for both transporters, into bile and perfusate in the recirculating isolated perfused liver (IPL) model. APAP-glu was generated endogenously from acetaminophen (APAP), which was administered as a tracer dose (2 micromol/ml) into the perfusate. Biliary excretion of APAP-glu after 60 min of perfusion was reduced in EE-treated rats (-80%). In contrast, excretion into the perfusate was increased by EE (+45%). Liver content of APAP-glu at the end of the experiment was reduced by 36% in the EE group. The total amount of glucuronide remained the same in both groups. Taken together, these results indicate that up-regulation of Mrp3 led to an exacerbated basolateral versus canalicular excretion of conjugated APAP in IPL. We conclude that induced expression of basolateral Mrp3 by EE may represent a compensatory mechanism to prevent intracellular accumulation of common Mrp substrates, either endogenous or exogenous, due to reduced expression and activity of apical Mrp2.

Lithocholic acid feeding induces segmental bile duct obstruction and destructive cholangitis in mice

We determined the mechanisms of hepatobiliary injury in the lithocholic acid (LCA)-fed mouse, an increasingly used model of cholestatic liver injury. Swiss albino mice received control diet or 1% (w/w) LCA diet (for 1, 2, and 4 days), followed by assessment of liver morphology and ultrastructure, tight junctions, markers of fibrosis and key proteins of hepatobiliary function, and bile flow and composition. As expected LCA feeding led to bile infarcts, which were followed by a destructive cholangitis with activation and proliferation of periductal myofibroblasts. At the ultrastructural level, small bile ducts were frequently obstructed by crystals. Biliary-excreted fluorescence-labeled ursodeoxycholic acid accumulated in bile infarcts, whereas most infarcts did not stain with India ink injected into the common bile duct; both findings are indicative of partial biliary obstruction. Expression of the main basolateral bile acid uptake proteins (sodium-taurocholate cotransporter and organic anion-transporting polypeptide 1) was reduced, the canalicular transporters bile salt export pump and multidrug-related protein 2 were preserved, and the basolateral transporter multidrug-related protein 3 and the detoxifying enzyme sulfotransferase 2a1 were induced. Thus, we demonstrate that LCA feeding in mice leads to segmental bile duct obstruction, destructive cholangitis, periductal fibrosis, and an adaptive transporter and metabolic enzyme response.

Cytokine-dependent regulation of hepatic organic anion transporter gene transactivators in mouse liver

Proinflammatory cytokines such as TNF-alpha and IL-1beta lead to downregulation of hepatic organic anion transporters in cholestasis. This adapted response is transcriptionally mediated by nuclear hormone receptors and liver-specific transcription factors. Because little is known in vivo about cytokine-dependent regulatory events, mice were treated with either TNF-alpha or IL-1beta for up to 16 h. Transporter mRNA expression was determined by Northern blot analysis, nuclear activity, and protein-expression of transactivators by EMSA and Western blotting. TNF-alpha induces a sustained decrease in Ntcp, Oatp1/Oatp1a1, and Bsep mRNA expression but exerts only transient [multidrug resistance-associated protein 2 (Mrp2)] or no effects (Mrp3) on Mrps. In addition to Ntcp and Oatp1/Oatp1a1, IL-1beta also downregulates Bsep, Mrp2, and Mrp3 mRNAs to some extent. To study transcriptional regulation, Ntcp and Bsep promoters were first cloned from mice revealing a new distal Ntcp hepatocyte nuclear factor 1 (HNF-1) element but otherwise show a conserved localization to known rat regulatory elements. Changes in transporter-expression are preceeded by a reduction in binding activities at IR-1, ER-8, DR-5, and HNF-1alpha sites after 4 h by either cytokine, which remained more sustained by TNF-alpha in the case of nuclear receptors. Nuclear protein levels of retinoid X receptor (RXR)-alpha are significantly decreased by TNF-alpha but only transiently affected by IL-1beta. Minor reductions of retinoic acid receptor, farnesoid X receptor, pregnane X receptor, and constitutive androstane receptor nuclear proteins are restricted to 4 h after cytokine application and paralleled by a decrease in mRNA levels. Basolateral and canalicular transporter systems are downregulated by both cytokines, TNF-alpha and IL-1beta. Activity of HNF-1alpha as regulator of mNtcp is suppressed by both cytokines. Decreased binding activities of nuclear receptor heterodimers may be explained by a reduction of the ubiquitous heterodimerization partner RXR-alpha.

Hepatic pharmacokinetics of taurocholate in the normal and cholestatic rat liver

The disposition kinetics of [3H]taurocholate ([3H]TC) in perfused normal and cholestatic rat livers were studied using the multiple indicator dilution technique and several physiologically based pharmacokinetic models. The serum biochemistry levels, the outflow profiles and biliary recovery of [3H]TC were measured in three experimental groups: (i) control; (ii) 17 alpha-ethynylestradiol (EE)-treated (low dose); and (iii) EE-treated (high dose) rats. EE treatment caused cholestasis in a dose-dependent manner. A hepatobiliary TC transport model, which recognizes capillary mixing, active cellular uptake, and active efflux into bile and plasma described the disposition of [3H]TC in the normal and cholestatic livers better than the other pharmacokinetic models. An estimated five- and 18-fold decrease in biliary elimination rate constant, 1.7- and 2.7-fold increase in hepatocyte to plasma efflux rate constant, and 1.8- and 2.8-fold decrease in [3H]TC biliary recovery ratio was found in moderate and severe cholestasis, respectively, relative to normal. There were good correlations between the predicted and observed pharmacokinetic parameters of [3H]TC based on liver pathophysiology (e.g. serum bilirubin level and biliary excretion of [3H]TC). In conclusion, these results show that altered hepatic TC pharmacokinetics in cholestatic rat livers can be correlated with the relevant changes in liver pathophysiology in cholestasis.

Biliary excretion of taurocholate, organic anions and vinblastine in rats with alpha-naphthylisothiocyanate-induced cholestasis

BACKGROUND AND AIMS

alpha-Naphthylisothiocyanate (ANIT) is known to cause cholestasis due to injury of the bile duct epithelial cells. The aim of the present study was to examine the effect of a single dose of ANIT on the biliary excretion of various cholephilic compounds and on the amount of canalicular transporters.

METHODS

Twenty-four hours after the oral administration of ANIT (100 mg/kg), the biliary excretion of taurocholate, leukotriene C(4), pravastatin and vinblastine was studied. The protein levels of the bile salt export pump and multidrug resistance protein 2 and the immunostaining of multidrug resistance protein 2 in the liver were also examined.

RESULTS

The ANIT treatment markedly decreased the biliary excretion of tracer amounts of taurocholate, leukotriene C(4), pravastatin and vinblastine. The biliary excretory maximum of taurocholate was also markedly decreased after ANIT treatment. The ANIT treatment had no effect on the protein levels of bile salt export pump and multidrug resistance protein 2 and the immunostaining of multidrug resistance protein 2 in the liver.

CONCLUSIONS

These findings support canalicular transporters having little effect on the marked impairment of biliary excretion of cholephilic compounds in ANIT-induced cholestasis.

Impaired expression of hepatic multidrug resistance protein 2 is associated with posthepatectomy hyperbilirubinemia in patients with biliary cancer

BACKGROUND AND AIMS

Hyperbilirubinemia is a critical complication following hepatectomy for biliary cancer. Hepatic multidrug resistance protein 2 (MRP2), a bilirubin transporter, is shown to be down-regulated by acute biliary obstruction in rats. However, little is known about the effect of chronic obstruction by malignancy on the MRP2 expression in patients or the association of MRP2 expression with posthepatectomy hyperbilirubinemia.

MATERIALS AND METHODS

The MRP2 expression before hepatectomy was determined by immunostaining and Western blotting in patients with biliary cancer. To directly determine the effect of chronic bile duct obstruction on the MRP2 expression, the expression levels were compared between the cholestatic and noncholestatic lobes in each of seven patients. In another 39 patients, the correlation of the MRP2 expression of the anticipated remnant liver with the posthepatectomy severe hyperbilirubinemia, defined as a serum total bilirubin concentration>or=200 micromol/l, was evaluated.

RESULTS

The MRP2 staining in the cholestatic lobes was weak and not restricted to the canalicular membrane, unlike the noncholestatic lobes. The expression levels in the cholestatic lobes were 45% of those in the noncholestatic lobes. Postoperative maximum bilirubin levels were significantly correlated with MRP2 expression of the anticipated remnant liver. The MRP2 expression had been already impaired before hepatectomy in all patients who eventually developed severe hyperbilirubinemia.

CONCLUSIONS

Decreased MRP2 expression, caused by biliary obstruction due to cancer, is a possible risk factor for posthepatectomy severe hyperbilirubinemia.

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.