Regulation of hepatic transport systems involved in bile secretion during liver regeneration in rats

Bile acids induce hepatic stellate cell proliferation via activation of the epidermal growth factor receptor

BACKGROUND & AIMS

Hepatic stellate cell (HSC) proliferation is a key event in the development of liver fibrosis. In many liver diseases, HSCs are exposed to inflammatory cytokines, reactive oxygen species, and bile acids. Although inflammatory cytokines and reactive oxygen species are known to promote proliferation of HSCs, nothing is known about the effects of bile acids on HSC proliferation or apoptosis. The aim of this study was to investigate the effects of bile acids on HSC proliferation.

METHODS

HSCs were exposed to bile acids with different hydrophobicity (5-200 micromol/L). HSC proliferation and cell cycle-related events were assessed by bromodeoxyuridine incorporation, cell counting and proliferating cell nuclear antigen and cyclin E expression, apoptosis by caspase-3 activity assay, immunocytochemistry for active caspase-3 and acridine orange staining, and activation of signal transduction pathways by Western blot using phospho-specific antibodies. Uptake of bile acids was investigated using fluorescent bile acids.

RESULTS

All bile acids, at concentrations >25 micromol/L, induce a 2.5- to 3-fold increase in HSC proliferation via activation of the epidermal growth factor receptor. Bile acid-induced proliferation is mediated by activation of a protein kinase C/extracellular signal-regulated kinase/p70S6K-dependent pathway. Bile acids did not induce apoptosis in HSCs. HSCs do not take up fluorescent bile acids and do not express the bile acid importer ntcp.

CONCLUSIONS

Bile acids at levels reached in cholestatic conditions are an independent profibrogenic factor. Bile acids induce HSC proliferation via the activation of the epidermal growth factor receptor, whereas HSCs are protected against bile acid-induced apoptosis by excluding bile acids.

Effects of TVE application during 70% hepatectomy on regeneration capacity of rats1

BACKGROUND

For adequate control of excess bleeding during liver resection, total vascular exclusion (TVE) is preferred by surgeons, especially when the tumor is located in the posterior liver lobes or near the cava. To the authors' knowledge, the effects of TVE technique on the postoperative liver regeneration process have not thus far been evaluated yet in the literature. This study was planned to compare the effects of liver resections performed either with portal pedicle clamping or with TVE on the regeneration process.

MATERIALS AND METHODS

Seventy percent hepatectomy was performed with portal pedicle clamping (n=10, Group A) or with TVE (n=10, Group B) in rats. At 48 h after resection, sampling was performed for the measurement of serum transaminase, alkaline phosphatase (ALP), tissue malondialdehyde (MDA), and glutathione (GSH) levels. Liver regeneration rate, proliferating cell nuclear antigen (PCNA) labeling, and mitotic indices were also evaluated.

RESULTS

Liver injury determinants (serum transaminases, ALP, and tissue MDA levels) were found significantly higher in group B than in group A. Liver regeneration rate, liver GSH levels, PCNA labeling index, and mitotic index were significantly lower in group B than in group A.

CONCLUSIONS

The injury during TVE seems to be greater than during resection with portal pedicle clamping. The negative effect of this oxidative damage may influence the regenerative capacity of the remnant liver tissue.

Transient up-regulation of P-glycoprotein reduces tacrolimus absorption after ischemia–reperfusion injury in rat ileum

Ischemia-reperfusion injury is an unavoidable problem for organ transplantation including small bowel transplantation, and causes a large intra-individual variation of tacrolimus (FK506) pharmacokinetics. Little information is available about the regulation of the intestinal P-glycoprotein expression during tissue regeneration. In the present study, we have examined the molecular and functional variations of ileum P-glycoprotein using rats after ischemia-reperfusion treatment. Morphological study revealed a rapid regeneration of the intestinal wall during 24 h after reperfusion. A reverse transcription-coupled competitive PCR and Western blot analysis revealed that the intestinal expression of P-glycoprotein recovered with time after reperfusion. At 24 h after reperfusion, the ileum P-glycoprotein level was transiently increased to two-fold, and the absorption rate of dihydro-[(3)H]FK506 from in situ ileum loop into portal vein was markedly low in comparison with the control. P-glycoprotein was detected in the crypt area as well as in villous cells at 6 h after reperfusion, and then localized to the apical surface at 24 h consistent with the cell proliferation and differentiation. However, the P-glycoprotein level returned to normal at 48 h. The intra-individual variation in the absorptive rate of tacrolimus was suggested to be regulated by the morphological status of the intestinal epithelium and enterocyte expression level of P-glycoprotein. Therefore, the monitoring of the enterocyte P-glycoprotein level would provide useful information for determining the dosage of tacrolimus immediately after small bowl transplantation.

The influence of brain death on liver function

BACKGROUND

In this study, we investigated the influence of brain death on inflammatory response and the effects of brain death on liver function both directly after explantation and after reoxygenation.

METHODS

The influence of brain death on liver function was studied in rats using a brain death model and the liver slice model to mimic reoxygenation. Liver function was assessed by measuring the ATP content and the ATP-driven urea synthesis. The activation of non-parenchymal cells was studied by measuring mRNA levels of IL-10, cytokine production (IL-10 and IL-1 beta) and inducible nitric oxide synthases (iNOS) upregulation (mRNA) and protein level.

RESULTS

Brain death had no direct influence on the ATP content of the liver. However, it led to induction of several cytokines because of activation of non-parenchymal cells, which led to upregulation of iNOS and to nitric oxide metabolites production. It is known that cytokine production may influence the drug metabolism capacity; however, no influence of brain death on drug metabolism was observed. An explanation may be the relatively short experimental period.

CONCLUSIONS

Kupffer cells seem to be activated during the onset of brain death induction; however, they become quiescent when liver slices of brain dead rats were reoxygenated during incubation. Other non-parenchymal cells, possibly the endothelial cells, remain activated during incubation and reoxygenation in slices from brain death donors but not in slices from control livers. Future experiments in our rat liver transplantation model need to elucidate the implications of these findings.

Involvement of Mrp2 in Hepatic and Intestinal Disposition of Dinitrophenyl-S-glutathione in Partially Hepatectomized Rats

The ability of the liver and small intestine for secretion of dinitrophenyl-S-glutathione (DNP-SG), a substrate for multidrug resistance-associated protein 2 (Mrp2), into bile and lumen, respectively, as well as expression of Mrp2 in both tissues, were assessed in 70-75% hepatectomized rats. An in vivo perfused intestinal model was used. A single i.v. dose of 30 micromol/kg b.w. of 1-chloro-2,4-dinitrobenzene (CDNB) was administered and its glutathione conjugate, DNP-SG, was determined by HPLC in bile and intestinal perfusate. One and seven days after hepatectomy, biliary excretion of DNP-SG was decreased by 90 and 50% with respect to shams, respectively, when expressed per mass unit. In contrast, intestinal excretion was increased by 63% or unchanged one and seven days post-hepatectomy, respectively. Tissue content of DNP-SG 5 min after CDNB administration was substantially decreased in liver and significantly increased in intestine, one day post-hepatectomy. Western and immunofluorescence studies revealed preserved levels and localization of Mrp2 in both tissues from hepatectomized animals, irrespective of the time analyzed. In spite of preserved expression of Mrp2, the higher availability of DNP-SG in intestinal cells, likely as a consequence of increased glutathione-S-transferase-mediated conjugation of CDNB, may explain the in vivo findings. Further experiments in isolated hepatocytes suggested that decreased synthesis of DNP-SG rather than altered canalicular transport is responsible for the substantial impairment in excretion of this compound into bile. Taken together, these results indicate that the intestine may partially compensate for liver DNP-SG disposition, particularly shortly after surgery, while liver capability is recovering.

Diallyl disulfide, a chemopreventive agent in garlic, induces multidrug resistance-associated protein 2 expression

The organosulfur compounds (OSCs), present in garlic, are studied for their protective effect against human cancers. P-glycoprotein (P-gp) and multidrug resistance protein 2 (Mrp2) are two transporters involved in the defense of cells and in the development of multidrug resistance. Whereas OSCs increase glutathione S-transferase activity (GST), Mrp2 plays a role in the transport of glutathione (GSH)-conjugates. In this study, we have investigated the effect of two OSCs, diallyl disulfide (DADS) and S-allyl cysteine (SAC), on P-gp and Mrp2 expression in renal brush-border membranes. By Western blot analysis, our results show that DADS induces Mrp2 expression (by 7-fold), which correlates with the rise of GST activity and GSH levels. Surprisingly, a co-administration of OSC with cisplatin, an anticancer drug, significantly increased Mrp2 gene and protein expression (by 30-fold), suggesting that DADS could potentiate the effects of cisplatin. Interestingly, SAC and cisplatin in co-treatment decreased P-gp protein expression and mdr1b isoform mRNA levels. In addition, modulation of the mdr1b isoform and Mrp2 by cisplatin was completely abolished by a glutathione precursor, N-acetyl cysteine. These results indicate that OSCs present in a garlic-rich diet might alter chemotherapeutic treatments using P-gp or Mrp2 substrates.

Expression of basolateral and canalicular transporters in rat liver and cultures of primary hepatocytes

Basolateral and canalicular proteins are expressed in the liver and besides their role in the transport of bilirubin, glutathione, hormones and various glucuronides, they also function as transporters of a wide range of drugs. Despite their frequent use in drug research, little is known about the expression of genes coding for transporters in cultures of primary sandwiched hepatocytes. The kinetics of gene expression of canalicular and basolateral membrane transporters in cultures of primary rat sandwiched hepatocytes were investigated, and the expression of cMOAT, spgp, mdrla and mdr2 were shown to be comparable with transcript levels observed in vivo. Strikingly, expression of the basolateral membrane transporter ntcp and oatpl and 2 were dramatically reduced, the level being < 10 and < 5%, respectively, of those found in vivo in rat liver tissue. Notably, mRNA expression of the canalicular membrane transporter mdrlb was increased up to 13-fold. The findings point to a dramatic change in the expression of basolateral and canalicular transporters in cultured hepatocytes, and this should be considered when hepatocytes are used for drug profiling studies.

Sustained spatial disturbance of bile canalicular networks during regeneration of the steatotic rat liver

BACKGROUND

Although it is generally considered that livers with moderate steatosis can be safely used in the setting of living-donor liver transplantation, the effect of the regenerative process of such a graft on postoperative liver function is incompletely understood. We assessed the morphologic and functional alterations during the regeneration of fatty liver, with special reference to the biliary system.

METHODS

Wistar rats with normal or fatty livers induced by a choline-deficient diet were subjected to 70% partial hepatectomy (PH). The regenerated liver weight and serum parameters were compared. Furthermore, to assess the spatial alterations of bile canalicular networks, the distribution of AGp110, a fibronectin receptor that localizes on the apical (bile canalicular) membrane of the hepatocytes, was analyzed immunohistochemically.

RESULTS

The serum albumin levels of the fatty-liver rats decreased significantly after 24 hours, and this continued until day 7. The increase in the total bile acid levels of the fatty-liver group was higher and more prolonged compared with that of the normal-liver group. At 24 hours after PH, discontinuity of the AGp110-positive canalicular network was evident in both groups. At 7 days after PH, the typical AGp110-positive canalicular network was almost restored in the normal-liver group. In contrast, the fatty-liver group showed sustained discontinuity of canalicular networks at the same time point.

CONCLUSIONS

The livers with moderate steatosis are associated with prolonged cholestasis after 70% PH, and this was caused, in part, by sustained spatial disturbance of bile canalicular networks during the regenerative process.

Intracellular trafficking during liver regeneration. Alterations in late endocytic and transcytotic pathways

BACKGROUND/AIMS

Liver growth, induced by partial hepatectomy of the organ is a precisely regulated process during which a radical reorganisation of metabolism occurs as the hepatocytes become committed to enter the cell cycle. Recent studies have shown the importance of the endocytic compartment in the control of lipid and protein intracellular trafficking but also in the control of the signal transduction events, which eventually will trigger the initiation of DNA synthesis and the subsequent cell division.

METHODS

We isolated endosomes at different times after partial hepatectomy in male rats and compared with endosomes isolated from sham-operated animals. Also, bile was collected and analysed by 2D-gel electrophoresis.

RESULTS

The amount of late endosomes isolated from regenerating livers decreased, concomitant with decreased cathepsin D specific enzyme activity. Furthermore, secretion of horseradish peroxidase, pIgA and transferrin increased in the pre-replicative phase of liver regeneration.

CONCLUSIONS

At the early stages of liver regeneration, the hepatocellular transport pathway towards degradation (late endosomes and lysosomal pathway) decreases, but the transcytosis and the bile secretion of several major proteins increases.

Acceleration of hepatobiliary dynamics in liver transplant donors

This study compared hepatobiliary scintigraphy findings in livers before and after liver graft donation to examine whether there is a change in hepatobiliary dynamics. Nine donors underwent hepatobiliary scintigraphy with intravenous injection of Tc-99m mebrofenin 1 day before and during the first week after left liver lobectomy. Five donors also underwent additional scintigraphy more than 1 year postsurgery. Images were acquired every second for the first minute, and then every minute for the next 40 minutes. Hepatic arterial perfusion index and portal perfusion index(PPI) were calculated from the images acquired during the first minute. For the function phase the computed parameters included: hepatic extraction efficiency, (HEE), time to appearance of activity in the intrahepatic biliary channels, and in the intestine, time to half maximal activity, and activity retained in the liver parenchyma at 40 minutes. Time to appearance of intrahepatic biliary channels and of intestinal activity was shorter among scintigraphies obtained within 1 week postsurgery compared to the preoperative values. Early after the operation HEE increased and PPI decreased significantly. Visual inspection of the scintigraphy scan obtained in all donors, within the first week postsurgery revealed hypertrophy of the right liver lobe. None of the patients showed progression of right lobe activity to the left side, even among scans obtained more than 1 year after donation. Reduced time to activity in the biliary channels and intestine and increased HEE suggest acceleration of hepatobiliary dynamics.

Disorders of bile formation and biliary transport

A wide range of cholestatic liver diseases result from various primary defects in bile formation. Clinical features include jaundice, pruritus, failure to thrive, fat malabsorption, cholelithiasis, and variably progressive cirrhosis. Accurate diagnosis of these disorders is essential for determination of prognosis and selection of the most appropriate therapies. Severe genetic defects in canalicular bile acid and phospholipid excretion lead to progressive liver disease that often requires liver transplantation. Defects in bile acid biosynthesis and aminophospholipid transport may be responsive to medical or non-transplant surgical approaches.

Growth hormone modulation of the rat hepatic bile transporter system in endotoxin-induced cholestasis

Treatment with high dose human GH, although an effective anabolic agent, has been associated with increased incidence of sepsis, inflammation, multiple organ failure, and death in critically ill patients. We hypothesized that GH might increase mortality by exacerbating cholestasis through modulation of bile acid transporter expression. High dose GH was continuously infused over 4 d into rats, and on the final day lipopolysaccharides were injected. Hepatic bile acid transporter expression was measured by Northern analysis and immunoblotting and compared with serum markers of cholestasis and endotoxinemia. Compared with non-GH-treated controls, GH increased endotoxin-induced markers of cholestasis and liver damage as well as augmented IL-6 induction. In endotoxinemia, GH treatment significantly induced multidrug resistance-associated protein 1 mRNA and protein and suppressed organic anion transporting polypeptides, Oatp1 and Oatp4, mRNA, suggesting impaired uptake of bilirubin and bile acids at the basolateral surface of the hepatocyte, which could contribute to the observed worsening of cholestasis by GH. This study of endotoxinemia may thus provide a mechanistic link between GH treatment and exacerbation of cholestasis through modulation of basolateral bile acid transporter expression in the rat hepatocyte.

Impact of drug transporter studies on drug discovery and development

Drug transporters are expressed in many tissues such as the intestine, liver, kidney, and brain, and play key roles in drug absorption, distribution, and excretion. The information on the functional characteristics of drug transporters provides important information to allow improvements in drug delivery or drug design by targeting specific transporter proteins. In this article we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug discovery and development process. The use of transporter function offers the possibility of delivering a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side effects), controlling the elimination process, and/or improving oral bioavailability. It is useful to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The expression system of transporters is an efficient tool for screening the activity of individual transport processes. The changes in pharmacokinetics due to genetic polymorphisms and drug-drug interactions involving transporters can often have a direct and adverse effect on the therapeutic safety and efficacy of many important drugs. To obtain detailed information about these interindividual differences, the contribution made by transporters to drug absorption, distribution, and excretion needs to be taken into account throughout the drug discovery and development process.

Sestamibi is a substrate for MDR1 and MDR2 P-glycoprotein genes

Technetium-99m sestamibi has attracted interest for assessment of the function of P-glycoproteins, which are well expressed in the liver and have roles in biliary transport and the removal of chemotherapeutic drugs. To further examine the cross-reactivity of (99m)Tc-sestamibi for P-glycoprotein family members, we conducted studies in animals. Hepatobiliary secretion of (99m)Tc-sestamibi was determined in normal FVB/N mice, mutant mice with specific P-glycoprotein deficiencies in the FVB/N background, normal Long-Evans Agouti (LEA) rats, and Long-Evans Cinnamon (LEC) rats with abnormal copper transport and liver disease but intact P-glycoprotein expression. After intrasplenic injection, (99m)Tc-sestamibi was rapidly incorporated in the mouse and rat liver, with maximal accumulation after 102+/-31 and 109+/-16 s, respectively ( P=NS). In normal mice and rats, 55%+/-11% and 55%+/-6%, respectively, of the maximal sestamibi activity was retained in the liver after 1 h ( P=NS). In double knockout mice lacking both mdr1a and mdr1b homologs of the human MDR1 ( ABCB1) gene, 88%+/-11% of maximal sestamibi activity was retained in the liver after 1 h ( P<0.001). In knockout mice deficient in either mdr1a gene or mdr2 ( ABCB4) gene, biliary sestamibi excretion was also impaired, although this impairment was relatively less pronounced in ABCB4-deficient mice than in double knockout mice lacking both ABCB1 gene homologs ( P<0.03). Hepatobiliary sestamibi excretion in LEC rats was not different from that in control normal rats, despite the presence of significant liver disease in the former. Hepatobiliary sestamibi excretion requires P-glycoproteins and is unperturbed in chronic liver disease. Sestamibi appears to be a substrate for both ABCB1 and ABCB4 genes, although the former utilizes it far more efficiently. Assessment of P-glycoprotein activity with sestamibi should consider how regulation of ABCB1 and related family members might modulate sestamibi incorporation.

Decreased expression of P-glycoprotein during differentiation in the human intestinal cell line Caco-2

The expression profile of the multidrug resistance (MDR) 1 gene product P-glycoprotein (Pgp) was examined during culture using Caco-2 cells as an in vitro model. Levels of MDR1 and cyclooxygenase 2 mRNA expression in Caco-2 cells were the highest on day 3 and decreased with days in culture, but the level of cyclooxygenase 1 was stable throughout the culture period. The stability of MDR1 mRNA was 7-fold higher on day 3 than on day 9, and the run-on assay suggested the transcription rate of the MDR1 gene on day 3 tended to be higher than on day 9. In addition, the expression of Pgp was comparable with that of MDR1 mRNA, but was inversely correlated with villin expression. The Pgp-mediated tacrolimus transport was the highest on day 1 and the lowest on day 11. These results suggested that the changeable mRNA stability rather than transcription rate of MDR1 contributed to its up-regulation during cell proliferation and down-regulation after post-confluent differentiation in Caco-2 cells. Therefore, the temporal induction and subsequent down-regulation of the enterocyte Pgp could affect bioavailability of several drugs during the regeneration of the intestinal wall.

Biliary bile acid concentration is a simple and reliable indicator for liver function after hepatobiliary resection for biliary cancer

BACKGROUND

The functional recovery of the remnant liver after an extended hepatectomy is critical for the outcome of the patient. The aim of this prospective study was to examine whether biliary bile acids could be an indicator for postoperative liver function.

METHODS

Externally drained bile samples were obtained from 51 patients with biliary or periampullary carcinomas before and after surgery. Patients were categorized into 3 groups: group A, 29 hepatectomized patients without liver failure; group B, 7 hepatectomized patients with liver failure (maximum serum bilirubin level, >10 mg/dL); and group C, 15 patients who underwent biliopancreatic resection without hepatectomy, with a good postoperative course. Bile samples were withdrawn 1 day before surgery and on postoperative days 1, 2, 3, 4, 6, and 7. Total bile acids were measured with a 3 alpha-hydroxysteroid dehydrogenase method.

RESULTS

Before surgery, the concentration of bile acids was higher in groups A and C than in group B, and correlated significantly with the indocyamine green disappearance rate (KICG) values (R(2) = 0.557; P <.0001). After surgery, bile acid concentrations decreased in all 3 groups until postoperative day 2, which was followed by a gradual increase. The concentration recovered to the preoperative level in groups A and C but remained low in group B. Biliary bile acid concentrations on day 2 correlated significantly with remnant liver KICG values (R(2) = 0.257; P =.0019). Among several parameters studied, including KICG, remnant liver KICG, biliary bile acids, and biliary bilirubin, biliary bile acid concentration had the most predictive power for occurrence of postoperative liver failure.

CONCLUSION

Biliary bile acid concentration could be a simple, real-time, reliable indicator of preoperative and postoperative liver function.

Bile salt transporters: molecular characterization, function, and regulation

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

Cyclosporine a augments P-glycoprotein expression in the regenerating rat liver

In the liver, the multidrug resistance (MDR) protein P-glycoprotein (P-gp) is physiologically expressed at the bile canalicular membrane, where it participates in the biliary excretion of various lipophilic drugs and xenobiotics. Previous studies showed that the immunosuppressive agent cyclosporine A (CsA) modulates P-gp and exerts a hepatotrophic influence in the regenerating liver. Hepatocytes isolated from regenerating rat liver, after 2/3 partial hepatectomy (PH 2/3), were used as an in vivo experimental model of cells with high proliferating activity in order to investigate whether CsA influences cellular levels of P-gp in those cells. Male Wistar rats were treated with CsA (20 mg/kg body weight) for 4 d preoperatively and 1 d postoperatively, and regenerating hepatocytes were isolated by collagenase perfusion 12, 24 and 48 h after PH 2/3. Flow cytometry and Western blotting studies with the monoclonal antibodies C494 and C219 showed that after PH 2/3, cellular levels of P-gp were initially suppressed, 12 h after PH 2/3, by 23%, but were significantly elevated thereafter, 24 and 48 h after PH 2/3 by 28% and 73%, respectively. In CsA pretreated animals, P-gp levels were increased even in normal hepatocytes by 34%, and an additional augmentation was seen in hepatocytes from 24 and 48 h regenerating livers (60% and 56%, respectively). In summary, we demonstrate for the first time that CsA has an additive effect on the expression of P-glycoprotein during liver regeneration in the rat. Therefore, induction of P-gp might also be considered in patients receiving CsA after liver transplantation for hepatocellular carcinoma and chemotherapy as an adjuvant treatment for the prevention of tumor recurrence.

Peroxisome proliferator-activated receptor alpha (PPARalpha)-mediated regulation of multidrug resistance 2 (Mdr2) expression and function in mice

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a nuclear receptor that controls expression of genes involved in lipid metabolism and is activated by fatty acids and hypolipidaemic fibrates. Fibrates induce the hepatic expression of murine multidrug resistance 2 ( Mdr2 ), encoding the canalicular phospholipid translocator. The physiological role of PPARalpha in regulation of Mdr2 and other genes involved in bile formation is unknown. We found no differences in hepatic expression of the ATP binding cassette transporter genes Mdr2, Bsep (bile salt export pump), Mdr1a / 1b, Abca1 and Abcg5 / Abcg8 (implicated in cholesterol transport), the bile salt-uptake systems Ntcp (Na(+)-taurocholate co-transporting polypeptide gene) and Oatp1 (organic anion-transporting polypeptide 1 gene) or in bile formation between wild-type and Ppar alpha((-/-)) mice. Upon treatment of wild-type mice with ciprofibrate (0.05%, w/w, in diet for 2 weeks), the expression of Mdr2 (+3-fold), Mdr1a (+6-fold) and Mdr1b (+11-fold) mRNAs was clearly induced, while that of Oatp1 (-5-fold) was reduced. Mdr2 protein levels were increased, whereas Bsep, Ntcp and Oatp1 were drastically decreased. Exposure of cultured wild-type mouse hepatocytes to PPARalpha agonists specifically induced Mdr2 mRNA levels and did not affect expression of Mdr1a / 1b. Altered transporter expression in fibrate-treated wild-type mice was associated with a approximately 400% increase in bile flow: secretion of phospholipids and cholesterol was increased only during high-bile-salt infusions. No fibrate effects were observed in Ppar alpha((-/-)) mice. In conclusion, our results show that basal bile formation is not affected by PPARalpha deficiency in mice. The induction of Mdr2 mRNA and Mdr2 protein levels by fibrates is mediated by PPARalpha, while the induction of Mdr1a / 1b in vivo probably reflects a secondary phenomenon related to chronic PPARalpha activation.

Drug transport proteins in the liver

Together with drug metabolising enzymes, transmembrane transporters are important determinants of drug metabolism and drug clearance by the liver. Hepatic uptake of organic anions, cations, prostaglandins and bile salts is supported by dedicated transporter proteins in the basolateral (sinusoidal) membrane of hepatocytes: OATPs, OATs, OCTs, PGTs and NTCP, respectively. ATP-binding cassette (ABC) transporter proteins in the canalicular membrane of hepatocytes mediate the hepatic efflux of drugs, bile salts and metabolites against a steep concentration gradient from liver to bile. This transport is driven by ATP hydrolysis. Drugs, endogenous metabolites, bile salts and cytokines affect the expression levels of these transporters. They act through a family of ligand-activated transcription factors, the nuclear hormone receptors. Consequently, the levels of the various transporter proteins are subject to genetic polymorphism in the encoding genes as well as in these transcription factors. Adverse drug reactions may be caused by genetic or disease-induced variations of transporter expression or drug-drug interactions at the level of these transporters.

Single nucleotide polymorphisms in multidrug resistance associated protein 2 (MRP2/ABCC2): its impact on drug disposition

Multidrug resistance associated protein 2 (MRP2/ABCC2), expressed on the bile canalicular membrane, plays an important role in the biliary excretion of various kinds of substrates. In addition, MRP2 is also expressed on the apical membrane of epithelial cells such as enterocytes. It is possible that the inter-individual difference in the function of MRP2 affects the drug disposition. In the present article, we will summarize the physiological and pharmacological role of MRP2, particularly focusing on the factors affecting its transport function such as single nucleotide polymorphisms and/or the induction/down regulation of this transporter. Mutations found in patients suffering from the Dubin-Johnson syndrome, along with the amino acid residues which are involved in supporting the transport activity of MRP2, are also summarized.

Effects of bile salt flux variations on the expression of hepatic bile salt transporters in vivo in mice

BACKGROUND/AIMS

Expression of hepatic bile salt transporters is partly regulated by bile salts via activation of nuclear farnesoid X-activated receptor (Fxr). We investigated the physiological relevance of this regulation by evaluating transporter expression in mice experiencing different transhepatic bile salt fluxes.

METHODS

Bile salt flux was manipulated by dietary supplementation with taurocholate (0.5% w/w) or cholestyramine (2% w/w) or by disruption of the cholesterol 7alpha-hydroxylase-gene (Cyp7A(-/-) mice) leading to reduced bile salt pool size. Expression of hepatic transporters was assessed (polymerase chain reaction (PCR), immunoblotting, and immunohistochemistry).

RESULTS

Biliary bile salt secretion was increased (+350%) or decreased (-50%) after taurocholate or cholestyramine feeding, respectively, but plasma bile salt concentrations and hepatic Fxr expression were not affected. The bile salt uptake system Na(+)-taurocholate co-transporting polypeptide (Ntcp) and organic anion transporting polypeptide-1 (Oatp1) were down-regulated by taurocholate and not affected by cholestyramine feeding. Cyp7A(-/-) mice did not show altered Ntcp or Oatp1 expression. Canalicular bile salt export pump (Bsep) was up-regulated by 65% in taurocholate-fed mice, and slightly down-regulated in Cyp7A(-/-) mice.

CONCLUSIONS

Large variations in hepatic bile salt flux have minor effects on expression of murine Ntcp and Bsep in vivo, suggesting that these transporters are abundantly expressed and able to accommodate a wide range of 'physiological' bile salt fluxes.

Hepatobiliary organic anion transporters are differentially regulated in acute toxic liver injury induced by carbon tetrachloride

BACKGROUND/AIMS

Hepatobiliary transporters are down-regulated in cholestasis, but their expression in acute, non-cholestatic, cytokine-mediated liver injury is unknown. Thus we studied the molecular mechanisms, by which sodium taurocholate cotransporting polypeptide (Ntcp), organic anion transporting polypeptide 1 (Oatp1), Oatp2, Oatp4, multidrug-resistance protein 2 (Mrp2) and bile salt export pump (Bsep) are regulated in liver injury induced by carbon tetrachloride (CCl(4)).

METHODS

mRNA and protein levels were determined in rats 24 and 72h after CCl(4) injection. Transporter gene transcription and binding activities of Ntcp and Mrp2 transactivators were assessed by nuclear runoff and electrophoretic mobility shift assays.

RESULTS

mRNA levels significantly declined to 41+/-44% for Ntcp, 65+/-41% for Oatp1 and 64+/-28% for Oatp2, but remained unchanged for Oatp4, canalicular Mrp2 and Bsep. Protein levels declined only for Oatp4 (-50+/-17%) and Ntcp (-23+/-13%) at 24h. Reduced mRNA levels (Ntcp, Oatp1, Oatp2) were associated with decreased transcriptional activities. Binding activity of Ntcp transactivators (hepatocyte nuclear factor 1 alpha (HNF1alpha) and CAAT enhancer binding protein alpha (C/EBPalpha) were reduced by 24h, whereas retinoid X receptor alpha (RXRalpha):retinoid acid receptor alpha (RARalpha) as transactivator of both Ntcp and Mrp2 remained unaltered. Recovery of acute hepatitis and changes in gene expression occurred after 72h.

CONCLUSIONS

Acute liver injury results in down-regulation of basolateral organic anion transporters similar to liver regeneration after partial hepatectomy, but in contrast to endotoxin-induced cholestasis. Maintained binding activity of RXRalpha:RARalpha may explain differences in Mrp2 expression.

Messenger RNA profiles in liver injury and stress: a comparison of lethal and nonlethal rat models

Liver damage activates processes aimed at repairing damage; simultaneously, liver functions required for survival must be maintained. The expression of genes responsible for both in rat models of lethal (lipopolysaccharide, 90% hepatectomy, and d-galactosamine) and nonlethal (turpentine, 70% hepatectomy, and acetaminophen) liver damage and stress was measured at 3, 6, 12, and 24 h after the intervention and quantitated as the area between the control curves and the test curves (AUC). The expression of genes for cell division and remodeling was upregulated most in the lethal models. The expression of most liver-specific function genes was reduced. Positive AUC was found for ARG, ASL, CPT1, Mdr1b, Mdr2, and PEPCK. It is concluded that a high expression of genes for repair of liver damage is associated with reduced expression of genes for several liver-specific functions, possibly reflecting a limited capacity for transcriptional activity. Maintained or increased expression of selected function genes indicates that the corresponding functions have high priority. The liver sustains metabolic homeostasis ensuring that other organs in the body function normally. Simultaneously, the processes required for the integrity of its own structure and function are maintained as a result of regulated expression of the genes that produce the proteins needed to perform both set of functions.

Bile-salt hydrophobicity is a key factor regulating rat liver plasma-membrane communication: relation to bilayer structure, fluidity and transporter expression and function

Bile-salt hydrophobicity regulates biliary phospholipid secretion and subselection. The aim of this study was to determine whether bile salts can influence liver plasma membrane phospholipids and fluidity in relation to the ATP-dependent transporter. Rats were depleted of bile salts by overnight biliary diversion and then sodium taurocholate was infused intravenously at a constant rate (200 nmol/min per 100 g of body weight), followed by infusion of bile salts with various hydrophobicities (taurochenodeoxycholate, tauroursodeoxycholate, tauro-beta-muricholate, tauro-alpha-muricholate at 200 nmol/min per 100 g of body weight). The hydrophobicity of the infused bile salts correlated with that of biliary phospholipids, but was inversely related to that of the canalicular membrane bilayer. Canalicular membrane fluidity (estimated by 1,6-diphenyl-1,3,5-hexatriene fluorescence depolarization) and expression of multidrug-resistance proteins (Mrp2, Mrp3) and apical Na(+)-dependent bile-salt transporter (ASBT) were increased by hydrophilic bile salts, although there was no marked change in the expression of P-glycoprotein subfamilies (Mdr2). Bile-salt export pump (Bsep) expression was increased along with increasing bile-salt hydrophobicity. Bile salts modulate canalicular membrane phospholipids and membrane fluidity, as well as the ATP-dependent transporter expression and function, and these actions are associated with their hydrophobicity. The cytoprotective effect of hydrophilic bile salts seems to be associated with induction of Mrp2, Mrp3 and ASBT.

Nifedipine modulation of biliary GSH and GSSG/ conjugate efflux in normal and regenerating rat liver

Canalicular glutathione secretion provides the major driving force for bile acid-independent bile flow (BAIF), although the pathways involved are not established. The hypothesis that GSH efflux proceeds by a route functionally distinct from the high-affinity, low-capacity, mrp2-mediated pathway was tested by using perfused rat liver and three choleretic compounds that modify biliary secretion of GSH (the dihydropyridine nifedipine and organic anion probenecid) or GSSG [sodium nitroprusside (SNP)]. Whereas nifedipine (30 microM) stimulated GSH secretion and blocked SNP-stimulated GSSG efflux and choleresis, SNP (1 mM) was ineffective against nifedipine-stimulated GSH efflux or BAIF, suggesting that most GSSG exits through a GSH-inhibitable path independent of high-affinity GSSG/glutathione conjugate transport. Three observations support this proposal. SNP, but not nifedipine, significantly inhibited bromosulfophthalein (BSP, 1 microM) excretion. Probenecid (1 mM) blocked resting or nifedipine-stimulated GSH secretion but only weakly inhibited BSP excretion. Glutathione, but not BSP, efflux capacity was reduced following partial hepatectomy. We suggest GSH efflux is mediated by a high-capacity organic anion pathway capable of GSSG transport when its high-affinity route is saturated.

Isolation of endothelial cells from brain, lung, and kidney: expression of the multidrug resistance P-glycoprotein isoforms

Endothelial cells (EC) were isolated from brain, lung, and renal cortex using magnetic microbeads cross-linked to an antibody directed against the platelet-endothelial cell adhesion molecule-1 (PECAM-1). Levels of endothelial nitric oxide synthase (eNOS) and PECAM-1 were measured by Western blots and both were enriched in the positively selected EC fractions. The multidrug resistance P-glycoprotein (P-gp) was strongly enriched (59-fold) in the EC fraction from brain and was absent in the negative fraction, in which the glial fibrillary acidic protein (GFAP), an astrocyte marker, was present. Lower P-gp levels were detected in EC from renal cortex and lung. Reverse transcription-polymerase chain reaction analysis showed that the mdr1a gene was preferentially expressed in EC fraction from the brain. The mdr1b gene was found in EC from renal cortex whereas both mdr1 genes were detected in EC from lung. Our results indicate that EC can be isolated using microbeads and that the isoform of P-gp found in brain is mostly mdr1a, associated with EC.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

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

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

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.

Cholestatic syndromes

Continued advances in the field of liver cell biology and molecular biology have provided further insights into the normal physiology of bile secretion and the pathogenesis and therapy for cholestatic syndromes. Important new data have also been published about pathogenesis, clinical features, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, drug-induced cholestasis, and cholestatic syndromes caused by viral infections.

Hepatobiliary transport

The alterations of hepatobiliary transport that occur in cholestasis can be divided into primary defects, such as mutations of transporter genes or acquired dysfunctions of transport systems that cause defective canalicular or cholangiocellular secretion, and secondary defects, which result from biliary obstruction. The dysfunction of distinct biliary transport systems as a primary cause of cholestasis is exemplified by the genetic defects in progressive familial intrahepatic cholestasis or by the direct inhibition of transporter gene expression by cytokines. In both, the hepatocellular accumulation of toxic cholephilic compounds causes multiple alterations of hepatocellular transporter expression. In addition, lack of specific components of bile caused by a defective transporter, as in the case of mdr2/MDR3 deficiency, unmasks the toxic potential of other components. The production of bile is critically dependent upon the coordinated regulation and function of sinusoidal and canalicular transporters, for instance of Na+-taurocholate cotransporting polypeptide (NTCP) and bile salt export pump (BSEP). Whereas the downregulation of the unidirectional sinusoidal uptake system NTCP protects the hepatocyte from further intracellular accumulation of bile salts, the relative preservation of canalicular BSEP expression serves to uphold bile salt secretion, even in complete biliary obstruction. Conversely, the strong downregulation of canalicular MRP2 (MRP, multidrug resistance protein) in cholestasis forces the hepatocyte to upregulate basolateral efflux systems such as MRP3 and MRP1, indicating an inverse regulation of basolateral and apical transporters The regulation of hepatocellular transporters in cholestasis adheres to the law of parsimony, since many of the cellular mechanisms are pivotally governed by the effect of bile salts. The discovery that bile salts are the natural ligand of the farnesoid X receptor has shown us how the major bile component is able to regulate its own enterohepatic circulation by affecting transcription of the genes critically involved in transport and metabolism.