Altered localization and activity of canalicular Mrp2 in estradiol-17beta-D-glucuronide-induced cholestasis

Genetics of familial intrahepatic cholestasis syndromes

Bile acids and bile salts have essential functions in the liver and in the small intestine. Their synthesis in the liver provides a metabolic pathway for the catabolism of cholesterol and their detergent properties promote the solubilisation of essential nutrients and vitamins in the small intestine. Inherited conditions that prevent the synthesis of bile acids or their excretion cause cholestasis, or impaired bile flow. These disorders generally lead to severe human liver disease, underscoring the essential role of bile acids in metabolism. Recent advances in the elucidation of gene defects underlying familial cholestasis syndromes has greatly increased knowledge about the process of bile flow. The expression of key proteins involved in bile flow is tightly regulated by transcription factors of the nuclear hormone receptor family, which function as sensors of bile acids and cholesterol. Here we review the genetics of familial cholestasis disorders, the functions of the affected genes in bile flow, and their regulation by bile acids and cholesterol.

Hormonal regulation of hepatic multidrug resistance-associated protein 2 (Abcc2) primarily involves the pattern of growth hormone secretion

Biliary excretion is the rate-limiting step in transfer of bilirubin, other organic anions, and xenobiotics across the liver. Multidrug resistance-associated protein 2 (Mrp2, Abcc2) is the major transporter for conjugated endo- and xenobiotic-conjugated compounds into bile. Hormones regulate bilirubin and xenobiotic secretion into bile, which have dimorphic differences. Therefore, we examined the possible role of sex steroids and growth hormone in the regulation of Mrp2. In approximately 8-wk-old rats, mRNA, transcriptional activity, and hepatic content of Mrp2 were selectively increased fourfold (P < 0.001) in females compared with males. In males, estrogens increased and testosterone decreased Mrp2 mRNA and protein, whereas no significant effect was measured in females, suggesting either a direct effect on the liver or an alteration in growth hormone secretory pattern. After hypophysectomy, Mrp2 mRNA was markedly reduced and the effects of estrogens and testosterone on Mrp2 were prevented, supporting the role of pituitary hormones in controlling Mrp2 expression. Mrp2 increased following growth hormone infusion in males. Mrp2 mRNA was decreased in growth hormone-deficient "Little" mice. Growth hormone infusions in hypophysectomized rats partially restored Mrp2 levels, whereas thyroxine addition returned Mrp2 mRNA and protein to basal levels. Morphology as well as biochemical measurements demonstrated that Mrp2 was localized to the bile canaliculus in equal density in both genders, whereas hormone replacements increased Mrp2 in hypophysectomized animals. In cultured hepatocytes, thyroxine did not have an effect, but growth hormone alone and combined with thyroxine increased Mrp2 mRNA levels. In conclusion, Mrp2 levels are regulated by the combination of thyroxine and different growth hormone secretory patterns.

Human renal organic anion transporters: Characteristics and contributions to drug and drug metabolite excretion

The kidney is a key organ for promoting the excretion of drugs and drug metabolites. One of the mechanisms by which the kidney promotes excretion is via active secretion. Secretion of drugs and their metabolites from blood to luminal fluid in the nephron is a protein-mediated process that normally involves either the direct or indirect expenditure of energy. Renal transporters for organic anions are located in the proximal tubule segment of the nephron. The primary transporters of organic anions on the basolateral membrane (BLM) of proximal tubule cells are members of the organic anion transporter (OAT) family (mainly OAT1 and OAT3). The sulfate-anion antiporter 1 (SAT-1; hsat-1) may also contribute to organic anion transport at the basolateral membrane. On the apical membrane, the multi-drug resistance-associated protein 2 (MRP2) is an important transport protein to complete the secretion process. However, there are several transport proteins on the basolateral and apical membranes of proximal tubule cells in human kidneys that have not been fully characterized and whose role in the secretion of organic anions remains to be determined. This review will primarily focus on the human renal basolateral and apical membrane transporters for organic anions that may play a role in the excretion of drugs and drug metabolites.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

Shift from biliary to urinary elimination of acetaminophen-glucuronide in acetaminophen-pretreated rats

Despite its toxicity, acetaminophen (APAP) is used increasingly as an analgesic, antipyretic, and anti-inflammatory agent. We examined the effect of prior exposure to APAP on its biliary and urinary elimination. The biliary and urinary elimination of a test dose of APAP (150 mg/kg i.v.) was determined in male Wistar rats 24 h after pretreatment with vehicle, a single dose (1.0 g/kg i.p.), or increasing daily doses (0.2, 0.3, 0.6, and 1.0 g/kg/day i.p.) of APAP. Although elimination of the parent APAP was minimally affected, biliary excretion of APAP glucuronide was significantly decreased 70 and 80%, whereas urinary excretion was significantly increased 90 and 100% in the groups pretreated with single and repeated doses of APAP, respectively, relative to vehicle controls. Western analysis and confocal immunofluorescent microscopy indicated a marked increase in hepatic expression of multidrug resistance-associated protein 3 (Mrp3) in both groups pretreated with APAP, relative to expression of Mrp2. ATP-dependent transport of [3H]taurocholate, an Mrp3 substrate, was significantly increased in basolateral liver plasma membrane vesicles from rats pretreated with repeated doses of APAP relative to controls. Enterohepatic recirculation of APAP glucuronide after administration of the same test dose of the drug was significantly decreased in rats pretreated with repeated doses of APAP. These data indicate that APAP pretreatment induced a shift from biliary to urinary elimination of APAP glucuronide, consistent with the increased expression of Mrp3 in the basolateral domain of the hepatocyte. We postulate that decreased enterohepatic recirculation contributes to decreased APAP hepatotoxicity by reducing liver exposure.

Variable expression of MRP2 (ABCC2) in human placenta: influence of gestational age and cellular differentiation

MRP2 (ABCC2) is an ATP-binding cassette (ABC)-type membrane protein involved in transport of conjugates of various drugs and endogenous compounds. MRP2 has been localized to the apical membrane of syncytiotrophoblasts and is assumed to be involved in diaplacental transfer of the above substances. It has been shown that both genetic and environmental factors can influence MRP2 expression. We therefore investigated whether gestational age, cellular differentiation, and genetic polymorphisms influence expression and localization of MRP2 in 58 human placenta samples. We detected a significant increase of transporter-mRNA with gestational age by quantitative real-time polymerase chain reaction (MRP2 mRNA/18S rRNA ratio x 1000 +/- S.D.; 0.43 +/- 0.13 in early preterms versus 1.18 +/- 0.44 in late preterms versus 2.1 +/- 0.63 in terms; p < 0.05). MRP2 protein followed the mRNA amount as shown by Western blotting (mean relative band intensity +/- S.D.; 0.56 +/- 0.1 versus 0.7 +/- 0.18 versus 0.92 +/- 0.19; early preterms versus terms p < 0.05). In cultured cytotrophoblasts, MRP2 expression increased with differentiation to syncytiotrophoblasts, with a peak on day 2 (MRP2 mRNA/18S rRNA ratio x 1000 +/- S.D.; 0.06 +/- 0.01 versus 0.88 +/- 0.27 versus 0.24 +/- 0.02 on days 0, 2, and 4). Moreover, we studied the effect of single nucleotide polymorphisms (C-24T; G1249A, and C3972T) in the MRP2 gene on placental expression. One of these polymorphisms (G1249A) resulted in a significantly reduced expression of MRP2 mRNA in preterms. In summary, the expression of MRP2 in human placenta is influenced by gestational age, cellular differentiation, and genetic factors.

Dapsone impairs the bile salt-independent fraction of bile flow in rats: Possible involvement of its N-hydroxylated metabolite

The effects of dapsone (DDS) treatment (30 mg/kg body wt, twice a day, for 4 days) on biliary secretory function, with special emphasis on bile salt independent bile flow (BSIF), were investigated in male and in female Wistar rats. Because DDS is metabolized to its N-hydroxylated parent compound only in male rats, any gender difference in DDS effect can be causally attributed to this metabolite. The two main driving forces for BSIF, the biliary secretion of HCO(3)(-) and glutathione species, were assessed. BSIF was decreased by about 20% in male but not in female rats after DDS treatment. Basal biliary HCO(3)(-) secretion was decreased also by 20% in males. This was associated with a diminished (-37%) expression of the HCO(3)(-) canalicular transporter, anion exchanger 2 (AE2), detected by western blotting. Biliary output of reduced glutathione (GSH) was not modified by DDS irrespective of gender, whereas excretion of oxidized glutathione (GSSG) was increased by 830% in males. This latter finding confirmed a gender-dependent oxidative stress associated with formation of the N-hydroxylated metabolite of DDS. The expression of multidrug resistance-associated protein 2 (Mrp2), a putative transporter of glutathione species, was decreased by 38% as detected by western blotting, clearly dissociating from preserved or increased biliary excretion of GSH and GSSG. In conclusion, our results show an impairment of BSIF by DDS mainly due to a decreased AE2-mediated biliary excretion of HCO(3)(-), formation of the N-hydroxylated metabolite of DDS being a likely mediator. The clinical relevance of these findings is discussed.

Silibinin prevents cholestasis-associated retrieval of the bile salt export pump, Bsep, in isolated rat hepatocyte couplets: possible involvement of cAMP

Estradiol-17beta-d-glucuronide (E(2)17G) and taurolithocholate (TLC) induce acute cholestasis-associated with retrieval of the bile salt export pump (Bsep), which parallels with alteration in transport activity. cAMP stimulates the apically directed vesicular trafficking of transporters, partially preventing these alterations. The hepatoprotector, silymarin, which inhibits cAMP-phosphodiesterase, prevents the cholestasis induced in vivo by both estrogens and TLC. We aimed to assess the ability of silibinin (Sil), the silymarin active component, to prevent the retrieval of Bsep induced by TLC and E(2)17G, and the associated alteration in its transport function. The possible involvement of cAMP as a second messenger and the intracellular signalling pathways implicated were also evaluated. Functional studies were performed analysing the proportion of isolated rat hepatocyte couplets (IRHC) accumulating the fluorescent bile salt analogue, cholyl-lysylfluorescein (CLF), into their sealed canalicular vacuoles. Cellular localisation of Bsep was assessed by immunofluorescent staining. Intracellular levels of cAMP were measured by ELISA. Sil (2.5microM) elevated by 40+/-3% intracellular cAMP, and mimicked the ability of dibutyryl-cAMP (10microM) to prevent internalisation of Bsep and the TLC (2.5microM)- and E(2)17G (50microM)-induced impairment in the capacity of IRHC to accumulate CLF apically. Preventive effects of Sil and dibutyryl-cAMP were not abolished by the specific protein kinase A inhibitors, KT5720 and H89. Contrarily, the intracellular Ca(2+) chelator, BAPTA/AM, significantly blocked the protective effect of both compounds. We conclude that Sil prevented TLC- and E(2)17G-induced bile salt secretory failure, at least in part, by avoiding redistribution of Bsep, by a mechanism probably involving cAMP-induced cytosolic Ca(2+) elevations.

Role of microtubules in estradiol-17beta-D-glucuronide-induced alteration of canalicular Mrp2 localization and activity

Estradiol-17beta-D-glucuronide (E2-17G) induces a marked but reversible inhibition of bile flow in the rat together with endocytic retrieval of multidrug resistance-associated protein 2 (Mrp2) from the canalicular membrane to intracellular structures. We analyzed the effect of pretreatment (100 min) with the microtubule inhibitor colchicine or lumicholchicine, its inactive isomer (1 micromol/kg iv), on changes in bile flow and localization and function of Mrp2 induced by E2-17G (15 micromol/kg iv). Bile flow and biliary excretion of bilirubin, an endogenous Mrp2 substrate, were measured throughout, whereas Mrp2 localization was examined at 20 and 120 min after E2-17G by confocal immunofluorescence microscopy and Western analysis. Colchicine pretreatment alone did not affect bile flow or Mrp2 localization and activity over the short time scale examined (3-4 h). Administration of E2-17G to colchicine-pretreated rats induced a marked decrease (85%) in bile flow and biliary excretion of bilirubin as well as internalization of Mrp2 at 20 min. These alterations were of a similar magnitude as in rats pretreated with lumicolchicine followed by E2-17G. Bile flow and Mrp2 localization and activity were restored to control levels within 120 min of E2-17G in animals pretreated with lumicolchicine. In contrast, in colchicine-pretreated rats followed by E2-17G, bile flow and Mrp2 activity remained significantly inhibited by 60%, and confocal and Western studies revealed sustained internalization of Mrp2 120 min after E2-17G. We conclude that recovery from E2-17G cholestasis, associated with exocytic insertion of Mrp2 in the canalicular membrane, but not its initial E2-17G-induced endocytosis, is a microtubule-dependent process.

Biliary Transport Systems: Short‐Term Regulation

Bile secretion is conveyed by a large set of transporter proteins. Their activity is controlled on long- and short-term timescales. Short-term regulation of transcellular transport has to guarantee intra- and extracellular molecular homeostasis and has to meet the actual cellular metabolic needs. As transport activity depends not only on transporter expression, measurements of mRNA or protein levels will not fully predict functionality. Transporter activity is also determined by covalent modifications (e.g., phosphorylation), substrate competition, and subcellular transporter localization. The latter is a major target of short-term regulation of bile secretion and involves rapid endo- and exocytosis of transporter-bearing vesicles from and into the respective cell membrane. In liver parenchymal cells, several signaling pathways were identified that govern these processes; however, the underlying molecular mechanisms still need to be characterized. Different techniques have been employed in studies on transporter retrieval and insertion, which are discussed in this chapter.

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.

Estradiol 3-glucuronide is transported by the multidrug resistance-associated protein 2 but does not activate the allosteric site bound by estradiol 17-glucuronide

beta-estradiol 17-(beta-D-glucuronide) (E217G) is a well known cholestatic agent and substrate of multidrug resistance-associated protein 2 (Mrp2), whereas beta-estradiol 3-(beta-D-glucuronide) (E23G) is a noncholestatic regioisomer of E217G with unknown transport properties. The purpose of this study was to compare and contrast the Mrp2-mediated transport of E217G and E23G. The full coding region of rat Mrp2 was cloned into the baculovirus genome, the recombinant baculovirus used to infect Sf9 cells, and ATP-dependent transport of 3H-E23G and 3H-E217G in Sf9 cell membranes was characterized. Mrp2 transported E23G into an osmotically sensitive space, requiring ATP, with S50=55.7 microM, Vmax=326 pmol.mg(-1).min(-1), and a Hill coefficient of 0.88. ATP-dependent Mrp2-mediated E217G transport was markedly stimulated at high E217G concentrations, consistent with positive cooperativity (Hill coefficient 1.5). E217G (5-125 microM) increased S50 but not Vmax for E23G transport, consistent with competitive inhibition. E23G (0.4-400 microM) completely, potently (IC50=14.2 microM), and competitively inhibited E217G transport, but E217G (0.01-250 microM) inhibited only 53% of E23G transport (IC50=33.4 microM). Estriol 16alpha-(beta-D-glucuronide) potently and completely inhibited transport of E23G (IC50=2.23 microM), as did beta-estradiol 3-sulfate 17-(beta-D-glucuronide) (5-50 microM). In summary, E217G binds not only to an Mrp2 transport site, but also to an allosteric site that activates Mrp2 with positive cooperativity, thus activating its own transport and potentially that of other Mrp2 substrates, such as E23G. The noncholestatic E23G is an Mrp2 substrate and competes with E217G for transport, but does not activate the allosteric site.

Effect of acetaminophen on expression and activity of rat liver multidrug resistance-associated protein 2 and P-glycoprotein

We evaluated the effect of acetaminophen (APAP), given as a single, 1g/kg body weight dose, on expression and activity of rat liver multidrug resistance-associated protein 2 (Mrp2) and P-glycoprotein (P-gp), two major canalicular drug transporters. The studies were performed 24h after administration of the drug. APAP induced an increase in plasma membrane content of Mrp2 detected by western blotting, consistent with increased detection of the protein at the canalicular level by immunoflourescence microscopy. In vivo biliary excretion of dinitrophenyl-S-glutathione, a well known Mrp2 substrate, was slightly but significantly increased by APAP, agreeing well with upregulation of the transporter. Basal biliary excretion of oxidized glutathione, an endogenous Mrp2 substrate, was also increased by APAP, likely indicating increased hepatic synthesis as a result of APAP-induced oxidative stress followed by accelerated canalicular secretion mediated by Mrp2. APAP also increased the expression of P-gp detected by western blotting and immunofluorescence microscopy as well as the in vivo biliary secretory rate of digoxin, a model P-gp substrate. Because specific APAP-conjugated metabolites are Mrp2 substrates, we postulate that induction of Mrp2 by APAP may represent an adaptive mechanism to accelerate liver disposition of the drug. In addition, increased Mrp2-mediated elimination of oxidized glutathione may be essential in maintaining the redox equilibrium in the hepatocyte under conditions of APAP-induced oxidative stress.

Bezafibrate stimulates canalicular localization of NBD-labeled PC in HepG2 cells by PPARalpha-mediated redistribution of ABCB4

Fibrates, including bezafibrate (BF), upregulate the expression of ATP binding cassette protein B4 (ABCB4) through gene transcription in mice. To determine the effects of BF on the expression levels of ABCB4 and on the stimulation of biliary phosphatidylcholine (PC) transport in human HepG2 hepatoblastoma cells, mRNA and protein levels as well as subcellular localization were investigated in the cells treated with BF. The canalicular accumulation of a fluorescent PC was assessed by confocal laser scanning microscopy. Treatment with 300 micromol/l BF for 24 h increased levels of ABCB4 mRNA but not protein by up to 151%. BF caused redistribution of ABCB4 into pseudocanaliculi formed between cells. In association with this redistribution, BF accelerated the accumulation of fluorescent PC in bile canaliculi (up to 163% of that in nontreated cells). Suppression of peroxisome proliferator-activated receptor alpha (PPARalpha) expression by either a small interfering RNA duplex or morpholino antisense oligonucleotide attenuated the BF-induced redistribution of ABCB4. These findings suggest that BF may enhance the capacity of human hepatocytes to direct PC into bile canaliculi via PPARalpha-mediated redistribution of ABCB4 to the canalicular membrane. This provides a rationale for the use of BF to improve cholestasis and/or cholangitis that is attributable to hypofunction of ABCB4.

Mrp2 is involved in benzylpenicillin-induced choleresis

Benzylpenicillin (PCG; 180 micromol/kg), a classic beta-lactam antibiotic, was intravenously given to Sprague-Dawley (SD) rats and multidrug resistance-associated protein 2 (Mrp2)-deficient Eisai hyperbilirubinemic rats (EHBR). A percentage of the [(3)H]PCG was excreted into the bile of the rats within 60 min (SD rats: 31.7% and EHBR: 4.3%). Remarkably, a transient increase in the bile flow ( approximately 2-fold) and a slight increase in the total biliary bilirubin excretion were observed in SD rats but not in the EHBR after PCG administration. This suggests that the biliary excretion of PCG and its choleretic effect are Mrp2-dependent. Positive correlations were observed between the biliary excretion rate of PCG and bile flow (r(2) = 0.768) and more remarkably between the biliary excretion rate of GSH and bile flow (r(2) = 0.968). No ATP-dependent uptake of [(3)H]PCG was observed in Mrp2-expressing Sf9 membrane vesicles, whereas other forms of Mrp2-substrate transport were stimulated in the presence of PCG. GSH efflux mediated by human MRP2 expressed in Madin-Darby canine kidney II cells was enhanced in the presence of PCG in a concentration-dependent manner. In conclusion, the choleretic effect of PCG is caused by the stimulation of biliary GSH efflux as well as the concentrative biliary excretion of PCG itself, both of which were Mrp2 dependent.

Enhanced expression of basolateral multidrug resistance protein isoforms Mrp3 and Mrp5 in rat liver by LPS

Lipopolysaccharide (LPS) induces hepatocellular down-regulation and endocytic retrieval of multidrug resistance protein 2 (Mrp2, Abcc2). Basolateral Mrp isoforms may compensate for the intracellular metabolic changes in cholestasis. Therefore, the effect of LPS on the zonal localization of Mrp2 and Mrp3 and the expression of Mrp3, Mrp4, Mrp5, and Mrp6 mRNA were investigated in rat liver. In normal rat liver Mrp3 was found in pericentral hepatocytes also expressing glutamine synthetase. In LPS-treated rat liver the decrease in Mrp2 protein was most pronounced in pericentral hepatocytes, with only minor down-regulation in periportal hepatocytes. Conversely, induction of Mrp3 was found in pericentral hepatocytes with a low expression of Mrp2. Furthermore, we found a strong induction of Mrp5 mRNA. Likewise, Mrp6 mRNA was up-regulated, however Mrp6 protein expression was not significantly altered. It is concluded that Mrp3 is inversely regulated to Mrp2 in a zonal pattern and may compensate for the LPS-induced loss of Mrp2 in the perivenous area. Induction of pericentral Mrp3 and up-regulation of Mrp5 mRNA may play an important role in the hepatocellular clearance of cholephilic substances and cyclic nucleotides accumulating after LPS treatment.

Kupffer cells alter organic anion transport through multidrug resistance protein 2 in the post-cold ischemic rat liver

Although Kupffer cells (KCs) may play a crucial role in post-cold ischemic hepatocellular injury, their role in nonnecrotic graft dysfunction remains unknown. This study examined reveal the role of KC in post-cold ischemic liver grafts. Rat livers treated with or without liposome-encapsulated dichloromethylene diphosphonate, a KC-depleting reagent, were stored in University of Wisconsin (UW) solution at 4 degrees C for 8 to 24 hours and reperfused while monitoring biliary output and constituents. The ability of hepatocytes to excrete bile was assessed through laser-confocal microfluorography in situ. Cold ischemia-reperfused grafts decreased their bile output significantly at 8 hours without any notable cell injury. This event coincided with impaired excretion of glutathione and bilirubin-IXalpha (BR-IXalpha), suggesting delayed transport of these organic anions. We examined whether intracellular relocalization of multidrug resistance protein-2 (Mrp2) occurred. Kinetic analyses for biliary excretion of carboxyfluorescein, a fluoroprobe excreted through this transporter, revealed significant delay of dye excretion from hepatocytes into bile canaliculi. The KC-depleting treatment significantly attenuated this decline in biliary anion transport mediated through Mrp2 in the 8-hour cold ischemic grafts via redistribution of Mrp2 from the cytoplasm to the canalicular membrane. Furthermore, thromboxane A(2) (TXA(2)) synthase in KC appeared involved as blocking this enzyme improved 5-carboxyfluorescein excretion while cytoplasmic internalization of Mrp2 disappeared in the KC-depleting grafts. In conclusion, KC activation is an important determinant of nonnecrotic hepatocellular dysfunction, jeopardizing homeostasis of the detoxification capacity and organic anion metabolism of the post-cold ischemic grafts.

Ca2+-dependent protein kinase C isoforms induce cholestasis in rat liver

Bile secretion is regulated by different signaling transduction pathways including protein kinase C (PKC). However, the role of different PKC isoforms for bile formation is still controversial. This study investigates the effects of PKC isoform selective activators and inhibitors on PKC translocation, bile secretion, bile acid uptake, and subcellular transporter localization in rat liver, isolated rat hepatocytes and in HepG2 cells. In rat liver activation of Ca(2+)-dependent cPKCalpha and Ca(2+)-independent PKCepsilon by phorbol 12-myristate 13-acetate (PMA, 10nmol/liter) is associated with their translocation to the plasma membrane. PMA also induced translocation of the cloned rat PKCepsilon fused to a yellow fluorescent protein (YFP), which was transfected into HepG2 cells. In the perfused liver, PMA induced marked cholestasis. The PKC inhibitors Gö6850 (1 micromol/liter) and Gö6976 (0.2 micromol/liter), a selective inhibitor of Ca(2+)-dependent PKC isoforms, diminished the PMA effect by 50 and 60%, respectively. Thymeleatoxin (Ttx,) a selective activator of Ca(2+)-dependent cPKCs, did not translocate rat PKCepsilon-YFP transfected in HepG2 cells. However, Ttx (0.5-10 nmol/liter) induced cholestasis similar to PMA and led to a retrieval of Bsep from the canalicular membrane in rat liver while taurocholate-uptake in isolated hepatocytes was not affected. Gö6976 completely blocked the cholestatic effect of Ttx but had no effect on tauroursodeoxycholate-induced choleresis. The data identify Ca(2+)-dependent PKC isoforms as inducers of cholestasis. This is mainly due to inhibition of taurocholate excretion involving transporter retrieval from the canalicular membrane.

Changes in the expression and localization of hepatocellular transporters and radixin in primary biliary cirrhosis

BACKGROUND/AIMS

Expression and localization of human hepatocellular transporters and of radixin, cross-linking actin with some membrane transporters, may change in cholestatic liver diseases.

METHODS

We investigated the uptake transporters OATP2 (SLC21A6), OATP8 (SLC21A8), and NTCP (SLC10A1), the export pumps MRP2 (ABCC2), MRP3 (ABCC3), MRP6 (ABCC6), and P-glycoproteins (ABCB1, ABCB4, ABCB11), and radixin, in non-icteric primary biliary cirrhosis (PBC stages I-III) and control human liver needle-biopsies using immunofluorescence microscopy and semi-quantitative RT-PCR.

RESULTS

Expression and localization of all transporters were unchanged in PBC I-II. Immunostaining intensities of uptake transporters decreased in PBC III with a concomitant decrease in mRNA levels. Immunostaining intensities and mRNA levels of export pumps were similar in controls and PBC I-III, however, irregular MRP2 immunostaining suggested redistribution of MRP2 into intracellular structures in PBC III. Areas of irregular MRP2 immunostaining showed largely reduced radixin immunostaining, whereas normal hepatocytes had MRP2 and radixin confined to the canalicular membrane. Disrupted localization of radixin and MRP2 supports the concept that radixin contributes to the canalicular localization of MRP2.

CONCLUSIONS

Down-regulation of uptake transporters may contribute to the impaired hepatobiliary elimination in advanced PBC, and partially altered localization of MRP2 may reflect the onset of changes leading to icteric PBC.

Biliary secretion of glutathione in estradiol 17beta-D-glucuronide-induced cholestasis

Estradiol-17beta-D-glucuronide (E2-17G) induces an acute but reversible inhibition of bile flow after its intravenous administration to rats, due in part to the endocytic retrieval of the canalicular multidrug resistance-associated transporter protein 2 and the bile salt export pump, transporters that contribute to bile flow. Decreased bile salt-independent bile flow (BSIF) is also involved and persists during the phase of recovery from cholestasis. Because glutathione and HCO3- are major contributors to BSIF, we evaluated changes in their biliary excretion and the hepatic content of total glutathione during E2-17G-induced cholestasis. E2-17G acutely decreased bile flow and biliary excretion of total glutathione by about 80%; glutathione excretion was still inhibited at 80 min and 120 min, even though bile flow was partially and totally restored, respectively. Neither liver glutathione content nor the proportions of oxidized glutathione in bile and liver were affected by E2-17G at any time. HCO3- concentrations in bile were unchanged, so that secretion paralleled variations in bile flow. In the isolated perfused liver, addition of E2-17G decreased both bile flow and the biliary concentration of glutathione, whereas addition of its noncholestatic isomer estradiol-3-D-glucuronide (E2-3G) did not inhibit bile flow, but significantly reduced the concentration of glutathione in bile. The bile:liver concentration ratios of glutathione were significantly decreased in vivo by E2-17G and in the perfused liver by E2-17G and E2-3G. These data indicate that E2-17G cis-inhibits the canalicular transport of glutathione and thus contributes to the cholestatic effect by inhibiting BSIF.

Estradiol-17beta-D-glucuronide induces endocytic internalization of Bsep in rats

Endocytic internalization of the multidrug resistance-associated protein 2 (Mrp2) was previously suggested to be involved in estradiol-17beta-D-glucuronide (E217G)-induced cholestasis. Here we evaluated in the rat whether a similar phenomenon occurs with the bile salt export pump (Bsep) and the ability of DBcAMP to prevent it. E217G (15 micromol/kg i.v.) impaired bile salt (BS) output and induced Bsep internalization, as assessed by confocal microscopy and Western blotting. Neither cholestasis nor Bsep internalization occurred in TR- rats lacking Mrp2. DBcAMP (20 micromol/kg i.v.) partially prevented the decrease in bile flow and BS output and substantially prevented E217G-induced Bsep internalization. In hepatocyte couplets, E217G (50 microM) diminished canalicular accumulation of a fluorescent BS and decreased Bsep-associated fluorescence in the canalicular membrane; DBcAMP (10 microM) fully prevented both effects. In conclusion, our results suggest that changes in Bsep localization are involved in E217G-induced impairment of bile flow and BS transport and that DBcAMP prevents this effect by stimulating insertion of canalicular transporter-containing vesicles. Mrp2 is required for E217G to induce its harmful effect.

Drug-induced cholestasis

Drugs may cause several overlapping syndromes of cholestasis, the pathophysiological syndrome resulting from impaired bile flow. These reactions comprise approximately 17% of all hepatic adverse drug reactions (ADRs) and they may be severe. Causes of 'pure' (bland) cholestasis include oestrogens and anabolic steroids; rarer associations are with antimicrobials and NSAIDs. 'Cholestatic hepatitis' is a common drug reaction in which liver injury and inflammation cause significant elevation of serum alanine aminotransferase (ALT) as well as cholestasis. Chlorpromazine and ketoconazole are classic examples, but it is now exemplified by amoxycillin-clavulanate and other oxy-penicillins. Chronic cholestasis results from small bile duct injury leading to the vanishing bile duct syndrome (VBDS), a disorder mimicking primary biliary cirrhosis, or from injury to larger bile ducts causing secondary sclerosing cholangitis. Whilst there is increasing evidence of a genetic predisposition to cholestatic drug reactions, there are currently no pretreatment tests to predict drug safety. Prevention of severe reactions therefore relies on early detection of liver injury and prompt drug withdrawal. Symptomatic management includes relief of pruritus and correction of fat-soluble vitamin deficiency. In small cohort studies, ursodeoxycholic acid (UDCA) arrested progressive cholestasis in two-thirds of cases, but evidence for use of corticosteroids is anecdotal. This review considers diagnosis, pathogenesis, prevention and management of drug-induced cholestasis, with particular reference to frequently- and newly-described causes.

Cholestatic syndromes

Further insights into the cellular and molecular mechanisms underlying hepatobiliary transport function and its regulation now permit a better understanding of the pathogenesis and treatment options of cholestatic liver diseases. Identification of the molecular basis of hereditary cholestatic syndromes will result in an improved diagnosis and management of these conditions. New insights into the pathogenesis of extrahepatic manifestations of cholestasis (eg, pruritus) have facilitated new treatment strategies. Important new studies have been published about the pathogenesis, clinical features, diagnosis, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, total parenteral nutrition-induced cholestasis, drug-induced cholestasis, and viral cholestatic syndromes.

Expression of rat hepatic multidrug resistance-associated proteins and organic anion transporters in pregnancy

The expression of hepatic multidrug resistance-associated protein (Mrp)1, 2, 3, and 6 and organic anion transporting polypeptides (Oatp)1 and 2 were examined in control and 20- to 21-day pregnant rats. Western analysis showed that expression of Oatp2 was decreased 50% in pregnancy, whereas expression of Oatp1 did not change. Expression of Mrp2 protein determined by Western analysis of total liver homogenate decreased to 50% of control levels in pregnant rats, consistent with studies using plasma membranes. Confocal immunohistochemistry showed that Mrp2 expression was confined to the canalicular membrane in both control and pregnant rats and was not detectable in intracellular compartments. In isolated perfused liver, the biliary excretion of 2,4-dintrophenyl-glutathione was significantly decreased in pregnancy, consistent with decreased expression of Mrp2. The expression of the basolateral transporter Mrp1 was not altered in pregnancy, whereas expression of Mrp6 mRNA was decreased by 60%. Expression of Mrp3 was also decreased by 50% in pregnant rat liver, indicating differential regulation of Mrp isoforms in pregnancy. These data also demonstrate that decreased Mrp2 expression is not necessarily accompanied by increased Mrp3 expression.

Regulation of expression of the multidrug resistance-associated protein 2 (MRP2) and its role in drug disposition

The multidrug resistance protein 2 (MRP2; ABCC2) is an ATP-binding cassette transporter accepting a diverse range of substrates, including glutathione, glucuronide, and sulfate conjugates of many endo- and xenobiotics. MRP2 generally performs excretory or protective roles, and it is expressed on the apical domain of hepatocytes, enterocytes of the proximal small intestine, and proximal renal tubular cells, as well as in the brain and the placenta. MRP2 is regulated at several levels, including membrane retrieval and reinsertion, translation, and transcription. In addition to transport of conjugates, MRP2 transports cancer chemotherapeutics, uricosurics, antibiotics, leukotrienes, glutathione, toxins, and heavy metals. Several mutagenesis studies have described critical residues for substrate binding and various naturally occurring mutations that eliminate MRP2 expression or function. MRP2 is important clinically as it modulates the pharmacokinetics of many drugs, and its expression and activity are also altered by certain drugs and disease states.