Cholestasis caused by inhibition of the adenosine triphosphate-dependent bile salt transport in rat liver

Kinetic analysis of P-glycoprotein-mediated transport by using normal human placental brush-border membrane vesicles

PURPOSE

P-Glycoprotein (Pgp) plays an important role in drug disposition and excretion in various tissues such as the brain, intestine, and kidney. Moreover, we have demonstrated that Pgp is expressed on the brush-border membranes of trophoblast cells in the placenta and restricts drug transfer from the maternal circulation to the fetus. However, the transport kinetics of physiologically expressed Pgp has scarcely been investigated.

METHODS

In this study, we assessed the functional kinetics of transport mediated by Pgp that is physiologically expressed in normal tissue by using human placental brush-border membrane vesicles (BBMVs). Digoxin and vinblastine were used as typical substrates of Pgp.

RESULTS

The uptakes of [3H]digoxin and [3H]vinblastine into BBMVs were significantly increased in the presence of an ATP-regenerating system. The ATP-dependent uptakes of [3H]digoxin and [3H]vinblastine into BBMVs exhibited saturable kinetics. The Michaelis constants (Kt values) were 2.65 +/- 1.80 microM and 21.9 +/- 3.37 microM, respectively. In the presence of a Pgp inhibitor such as verapamil, cyclosporine A, or progesterone, the ATP-dependent uptakes of [3H]digoxin and [3H]vinblastine into BBMVs were significantly reduced. Anti-Pgp monoclonal antibody C219 completely inhibited the uptake of [3H]digoxin.

CONCLUSIONS

The transport kinetics of [3H]digoxin and [3H]vinblastine by physiologically expressed Pgp were successfully evaluated by using BBMVs prepared from normal human placenta. The present method enabled us to evaluate the function of physiologically expressed Pgp and is superior to the use of cultured transfectants in terms of the yield of vesicles. The present method may also be applicable to investigating the influence of various factors such as the genotype of the MDR1 gene or various pathophysiologic states of neonates on the function of Pgp.

Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

The characteristics of L-lactic acid transport across the trophoblast basal membrane were investigated and compared with those across the brush-border membrane by using membrane vesicles isolated from human placenta. The uptake of L-[(14)C]lactic acid into basal membrane vesicles was Na(+) independent, and an uphill transport was observed in the presence of a pH gradient ([H(+)](out) > [H(+)](in)). L-[(14)C]lactic acid uptake exhibited saturation kinetics with a K(m) value of 5.89 +/- 0.68 mM in the presence of a pH gradient. p-Chloromercuribenzenesulfonate and alpha-cyano-4-hydroxycinnamate inhibited the initial uptake, whereas phloretin or 4,4'-diisothiocyanostilbene-2,2'-disulfonate did not. Mono- and dicarboxylic acids suppressed the initial uptake. In conclusion, L-lactic acid transport in the basal membrane is H(+) dependent and Na(+) independent, as is also the case for the brush-border membrane transport, and its characteristics resemble those of monocarboxylic acid transporters. However, there were several differences in the effects of inhibitors between basal and brush-border membrane vesicles, suggesting that the transporter(s) involved in L-lactic acid transport in the basal membrane of placental trophoblast may differ from those in the brush-border membrane.

Inhibition of transport across the hepatocyte canalicular membrane by the antibiotic fusidate

Hyperbilirubinemia is a frequent side effect induced by long-term therapy with the antibiotic fusidate. The aim of this study was to elucidate the molecular mechanisms of fusidate-induced hyperbilirubinemia by investigating its influence on hepatic transport systems in the canalicular membrane. Using canalicular membrane vesicles from rat liver, we determined the effect of fusidate on the adenosine 5'-triphosphate (ATP)-dependent transport of substrates of the apical conjugate export pump, multi-drug resistance protein 2 (Mrp2, symbol Abcc2) and the bile salt export pump (Bsep, symbol Abcb11). Fusidate inhibited the ATP-dependent transport of the Mrp2 substrates 17beta-glucuronosyl estradiol and leukotriene C4, and the transport of cholyltaurine by Bsep with Ki values of 2.2+/-0.3, 7.6+/-1.3, and 5.5+/-0.8 microM, respectively. To elucidate the in vivo implication of these findings, the effect of fusidate treatment on the elimination of intravenously administered tracer doses of 17beta-glucuronosyl estradiol and cholyltaurine into bile was studied in rats. Treatment with fusidate (100 micromol/kg body weight) reduced the biliary excretion rate of 17beta-glucuronosyl [3H]estradiol and [3H]cholyltaurine by 75 and 80%, respectively. Extended treatment of rats with fusidate (100 micromol/kg body weight, three times daily i.p. for 3 days) reduced hepatic Mrp2 protein levels by 61% (P<0.001). Our data suggest that there are at least two different mechanisms involved in the impairment of transport processes and hepatobiliary elimination by fusidate, direct inhibition of transport of Mrp2 and Bsep substrates by competitive interaction and impairment by a decreased level of hepatic Mrp2.

Sirolimus/cyclosporine/tacrolimus interactions on bile flow and biliary excretion of immunosuppressants in a subchronic bile fistula rat model

The new immunosuppressive agent sirolimus generally is combined in transplant patients with cyclosporine and tacrolimus which both exhibit cholestatic effects. Nothing is known about possible cholestatic effects of these combinations which might be important for biliary excretion of endogenous compounds as well as of immunosuppressants. Rats were daily treated with sirolimus (1 mg kg(-1) p.o.), cyclosporine (10 mg kg(-1) i.p.), tacrolimus (1 mg kg(-1) i.p.), or a combination of sirolimus with cyclosporine or tacrolimus. After 14 days a bile fistula was installed to investigate the effects of the immunosuppressants and their combinations on bile flow and on biliary excretion of bile salts, cholesterol, and immunosuppressants. Cyclosporine as well as tacrolimus reduced bile flow (-22%; -18%), biliary excretion of bile salts (-15%;-36%) and cholesterol (-15%; -47%). Sirolimus decreased bile flow by 10%, but had no effect on cholesterol or bile salt excretion. Combination of sirolimus/cyclosporine decreased bile flow and biliary bile salt excretion to the same extent as cyclosporine alone, but led to a 2 fold increase of biliary cholesterol excretion. Combination of sirolimus/tacrolimus reduced bile flow only by 7.5% and did not change biliary bile salt and cholesterol excretion. Sirolimus enhanced blood concentrations of cyclosporine (+40%) and tacrolimus (+57%). Sirolimus blood concentration was increased by cyclosporine (+400%), but was not affected by tacrolimus. We conclude that a combination of sirolimus/tacrolimus could be the better alternative to the cotreatment of sirolimus/cyclosporine in cholestatic patients and in those facing difficulties in reaching therapeutic ranges of sirolimus blood concentration.

Bile salt transporters

Bile salts are the major organic solutes in bile and undergo extensive enterohepatic circulation. Hepatocellular bile salt uptake is mediated predominantly by the Na(+)-taurocholate cotransport proteins Ntcp (rodents) and NTCP (humans) and by the Na(+)-independent organic anion-transporting polypeptides Oatp1, Oatp2, and Oatp4 (rodents) and OATP-C (humans). After diffusion (bound by intracellular bile salt-binding proteins) to the canalicular membrane, monoanionic bile salts are secreted into bile canaliculi by the bile salt export pump Bsep (rodents) or BSEP (humans). Both belong to the ATP-binding cassette (ABC) transporter superfamily. Dianionic conjugated bile salts are secreted into bile by the multidrug-resistance-associated proteins Mrp2/MRP2. In bile ductules, a minor portion of protonated bile acids and monomeric bile salts are reabsorbed by non-ionic diffusion and the apical sodium-dependent bile salt transporter Asbt/ASBT, transported back into the periductular capillary plexus by Mrp3/MRP3 [and/or a truncated form of Asbt (tAsbt)], and subjected to cholehepatic shunting. The major portion of biliary bile salts is aggregated into mixed micelles and transported into the intestine, where they are reabsorbed by apical Oatp3, the apical sodium-dependent bile salt transporter (ASBT), cytosolic intestinal bile acid-binding protein (IBABP), and basolateral Mrp3/MRP3 and tAsbt. Transcriptional and posttranscriptional regulation of these enterohepatic bile salt transporters is closely related to the regulation of lipid and cholesterol homeostasis. Furthermore, defective expression and function of bile salt transporters have been recognized as important causes for various cholestatic liver diseases.

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.

Induction of apoptosis in MDR1 expressing cells by daunorubicin with combinations of suboptimal concentrations of P-glycoprotein modulators

The application of most agents with the capacity to reverse multidrug resistance (MDR) via modulation of the multidrug transporter P-glycoprotein (Pgp) was shown to be associated with toxic side-effects. For this reason, we have investigated the effect of combinations of suboptimal concentrations of Pgp blockers on the induction of apoptosis and growth arrest in daunorubicin (D) treated, MDR1 gene transfected cells. We used verapamil, PSC833 and Cremophor EL as Pgp modulators, which affect the function of Pgp by different mechanisms. Treatment of NIH3T3/MDR1 cells with combinations of suboptimal concentrations of Pgp modulators in the presence of D caused apoptosis and G(2) arrest to the same extent as optimal concentrations of singly used blockers. We conclude that combinations of suboptimal concentrations of Pgp modulators may cause effective sensitization of resistant tumor cells, and at the same time, may avoid the frequently observed toxic effects experienced in clinical trials with a single modifier applied at the optimal dose.

Cyclosporin A reduces canalicular membrane fluidity and regulates transporter function in rats

Changes of the biliary canalicular membrane lipid content can affect membrane fluidity and biliary lipid secretion in rats. The immunosuppressant cyclosporin A is known to cause intrahepatic cholestasis. This study investigated whether cyclosporin A influenced canalicular membrane fluidity by altering membrane phospholipids or transporter expression. In male Sprague-Dawley rats, a bile-duct cannula was inserted to collect bile, and sodium taurocholate was infused (100 nmol/min per 100 g) for 60 min. During steady-state taurocholate infusion, cyclosporin A (20 mg/kg) or vehicle was injected intravenously and then bile was collected for 80 min. After killing the rats, canalicular membrane vesicles were prepared. Expression of canalicular membrane transporters was assessed by Western blotting and canalicular membrane vesicle fluidity was estimated by fluorescence polarization. Cyclosporin A reduced biliary lipid secretion along with a disproportionate reduction of lipids relative to bile acids. Cyclosporin A significantly decreased canalicular membrane fluidity along with an increase of the cholesterol/phospholipid molar ratio. Only expression of the transporter P-glycoprotein was increased by cyclosporin A. Because canalicular membrane transporter expression was largely unchanged by cyclosporin A despite a marked decrease of biliary lipid secretion, transporter activity may partly depend upon canalicular membrane fluidity.

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.

Localization and genomic organization of a new hepatocellular organic anion transporting polypeptide

Based on sequence homology to the human organic anion transporting polypeptide 2 (OATP2; SLC21A6), we cloned a new member of the SLC21A superfamily of solute carriers, termed OATP8 (SLC21A8). The protein of 702 amino acids showed an amino acid identity of 80% with human OATP2. Based on Northern blotting, the expression of OATP8 was restricted to human liver. Cosmid clones containing the genes encoding human OATP1 (SLC21A3), OATP2 (SLC21A6), and OATP8 (SLC21A8) served to establish their genomic organization. All three genes contained 14 exons with 13 identical splice sites when transferred to the amino acid sequence. An antibody raised against the carboxyl terminus localized OATP8 to the basolateral membrane of human hepatocytes and the recombinant glycoprotein, expressed in MDCKII cells, to the lateral membrane. Transport properties of OATP8 were studied in stably transfected MDCKII and HEK293 cells. Organic anions transported by human OATP8 included sulfobromophthalein, with a K(m) of 3.3 microm, and 17beta-glucuronosyl estradiol, with a K(m) of 5.4 microm. Several bile salts were not substrates. Thus, human OATP8 is a new uptake transporter in the basolateral hepatocyte membrane with an overlapping but distinct substrate specificity as compared with OATP2, which is localized to the same membrane domain.

Mechanisms of cholestasis

From the multiple mechanisms of cholestasis presented in this article, a unifying hypothesis may be deduced by parsimony. The disturbance of the flow of bile must inevitably lead to the intracellular retention of biliary constituents. Alternatively, the lack of specific components of bile unmasks the toxic potential of other components, as in the case of experimental mdr2 deficiency. In the sequence of events that leads to liver injury, the cytotoxic action of bile salts is pivotal to all forms of cholestasis. The inhibition of the bsep by drugs, sex steroids, or monohydroxy bile salts is an example of direct toxicity to the key mediator in canalicular bile salt excretion. In other syndromes, the dysfunction of distinct hepatocellular transport systems is the primary pathogenetic defect leading to cholestasis. Such dysfunctions include the genetic defects in PFIC and the direct inhibition of gene transcription by cytokines. Perturbations in the short-term regulation of transport protein function are exemplified by the cholestasis of endotoxinemia. The effect of bile salts on signal transduction, gene transcription, and transport processes in hepatocytes and cholangiocytes has become the focus of intense research in recent years. The central role of bile salts in the pathogenesis of cholestasis has, ironically, become all the more evident from the improvement of many cholestatic syndromes with oral bile salt therapy.

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.

Drug- and estrogen-induced cholestasis through inhibition of the hepatocellular bile salt export pump (Bsep) of rat liver

BACKGROUND & AIMS

Drug-induced cholestasis is a frequent form of acquired liver disease. To elucidate the molecular pathogenesis of drug-induced cholestasis, we investigated the effects of prototypic cholestatic drugs on the canalicular bile salt export pump (Bsep) of rat liver.

METHODS

Vesicles were isolated from Bsep-, Mrp2-, and Bsep/Mrp2-expressing Sf9 cells. Canalicular plasma membrane (cLPM) vesicles from rat liver and Sf9 cell vesicles were used to study adenosine triphosphate (ATP)-dependent solute uptake by a rapid filtration technique.

RESULTS

Bsep-expressing Sf9 cell vesicles showed ATP-dependent transport of numerous monoanionic bile salts with similar Michaelis constant values as in cLPM vesicles, whereas several known substrates of the multispecific organic anion transporter Mrp2 were not transported by Bsep. Cyclosporin A, rifamycin SV, rifampicin, and glibenclamide cis-inhibited Bsep-mediated bile salt transport to similar extents as ATP-dependent taurocholate transport in cLPM vesicles. In contrast, the cholestatic estrogen metabolite estradiol-17beta-glucuronide inhibited ATP-dependent taurocholate transport only in normal cLPM and in Bsep/Mrp2-coexpressing Sf9 cell vesicles, but not in Mrp2-deficient cLPM or in selectively Bsep-expressing Sf9 cell vesicles, indicating that it trans-inhibits Bsep only after its secretion into bile canaliculi by Mrp2.

CONCLUSIONS

These results provide a molecular basis for previous in vivo observations and identify Bsep as an important target for induction of drug- and estrogen-induced cholestasis in mammalian liver.

A novel human organic anion transporting polypeptide localized to the basolateral hepatocyte membrane

We cloned and expressed a new organic anion transporting polypeptide (OATP), termed human OATP2, (OATP-C, LST-1; symbol SLC21A6), involved in the uptake of various lipophilic anions into human liver. The cDNA encoding OATP2 comprised 2073 base pairs, corresponding to a protein of 691 amino acids, which were 44% identical to the known human OATP. An antibody directed against the carboxy terminus localized OATP2 to the basolateral membrane of human hepatocytes. Northern blot analysis indicated a strong expression of OATP2 only in human liver. Transport mediated by recombinant OATP2 and its localization were studied in stably transfected Madin-Darby canine kidney strain II (MDCKII) and HEK293 cells. Confocal microscopy localized recombinant OATP2 protein to the lateral membrane of MDCKII cells. Substrates included 17beta-glucuronosyl estradiol, monoglucuronosyl bilirubin, dehydroepiandrosterone sulfate, and cholyltaurine. 17beta-Glucuronosyl estradiol was a preferred substrate, with a Michaelis-Menten constant value of 8.2 microM; its uptake was Na(+) independent and was inhibited by sulfobromophthalein, with a inhibition constant value of 44 nM. Our results indicate that OATP2 is important for the uptake of organic anions, including bilirubin conjugates and sulfobromophthalein, in human liver.

Purification of the human apical conjugate export pump MRP2 reconstitution and functional characterization as substrate-stimulated ATPase

The multidrug resistance protein MRP2 (ABCC2) acts as an ATP-dependent conjugate export pump in apical membranes of polarized cells and confers multidrug resistance. Purified MRP2 is essential for the detailed functional characterization of this member of the family of ATP-binding cassette (ABC) transporter proteins. In human embryonic kidney cells (HEK293), we have permanently expressed MRP2 containing an additional C-terminal (His)6-tag. Immunoblot and immunofluorescence analyses detected the MRP2-(His)6 overexpressing clones. Isolated membrane vesicles from the MRP2-(His)6-expressing cells were active in ATP-dependent transport of the glutathione S-conjugate leukotriene C4 and were photoaffinity-labelled with 8-azido-[alpha-32P]ATP. MRP2-(His)6 was solubilized from membranes of MRP2-(His)6-cells and purified to homogeneity in a three-step procedure using immobilized metal affinity chromatography, desalting, and immunoaffinity chromatography. The identity of the pure MRP2-(His)6 was verified by MS analysis of tryptic peptides. The purified MRP2-(His)6 glycoprotein was reconstituted into proteoliposomes and showed functional activity as ATPase in a protein-dependent manner with a Km for ATP of 2.1 mM and a Vmax of 25 nmol ADP x mg MRP2-1 x min-1. This ATPase activity was substrate-stimulated by oxidized and reduced glutathione and by S-decyl-glutathione. Future studies using pure MRP2 reconstituted in proteoliposomes should allow further insight into the molecular parameters contributing to MRP2 transport function and to define its intracellular partners for transport and multidrug resistance.

Transport of monoglucuronosyl and bisglucuronosyl bilirubin by recombinant human and rat multidrug resistance protein 2

The secretion of bilirubin conjugates from hepatocytes into bile represents a decisive step in the prevention of hyperbilirubinemia. The bilirubin conjugates, monoglucuronosyl bilirubin (MGB) and bisglucuronosyl bilirubin (BGB), were previously suggested to be endogenous substrates for the apical multidrug resistance protein (MRP2), a member of the adenosine triphosphate (ATP)-binding cassette family of transporters (symbol ABCC2), also termed canalicular multispecific organic anion transporter. We have characterized this ATP-dependent transport using membrane vesicles from human embryonic kidney (HEK) cells expressing recombinant rat as well as human MRP2. MGB and BGB, (3)H-labeled in the glucuronosyl moiety, were synthesized enzymatically with recombinant UDP-glucuronosyltransferase 1A1, and stabilized with ascorbate. Rates for ATP-dependent transport of MGB and BGB (0.5 micromol/L each) by human MRP2 were 183 and 104 pmol x mg protein(-1) x min(-1), respectively. K(m) values were 0.7 and 0.9 micromol/L for human MRP2, and 0.8 and 0.5 micromol/L for rat MRP2, with MGB and BGB as substrates, respectively. Leukotriene C(4) and 17beta-glucuronosyl estradiol, which are both known high-affinity substrates for human MRP2, inhibited [(3)H]MGB transport with IC(50) values of 2.3 and 30 micromol/L, respectively. Cyclosporin A competitively inhibited human and rat MRP2-mediated transport of [(3)H]MGB, with K(i) values of 21 and 10 micromol/L, respectively. Our results provide direct evidence that recombinant MRP2, cloned from rat as well as human liver, mediates the primary-active ATP-dependent transport of the bilirubin conjugates MGB and BGB.

Uptake of bromosulfophthalein via SO2-4/OH- exchange increases the K+ conductance of rat hepatocytes

In confluent primary cultures of rat hepatocytes, micromolar concentrations of bromosulfophthalein (BSP) lead to a sizeable hyperpolarization of membrane voltage. The effect is a saturable function of BSP concentration yielding an apparent value of 226 micromol/l and a Vmax of -10.3 mV. The BSP-induced membrane hyperpolarization is inhibited by the K+ channel blocker Ba2+, and in cable-analysis and ion-substitution experiments it becomes evident that the effect is due to a significant increase in cell membrane K+ conductance. Voltage changes were attenuated by the simultaneous administration of SO2-4, succinate, and cholate (cis-inhibition) and increased after preincubation with SO2-4 and succinate (trans-stimulation), suggesting that the effect occurs via BSP uptake through the known SO2-4/OH- exchanger. Microfluorometric measurements reveal that BSP-induced activation of K+ conductance is not mediated by changes in cell pH, cell Ca2+, or cell volume. It is concluded that K+ channel activation by BSP (as well as by DIDS and indocyanine green) may reflect a physiological mechanism linking the sinusoidal uptake of certain anions to their electrogenic canalicular secretion.

Primary active transport of organic anions on bile canalicular membrane in humans

Biliary excretion of several anionic compounds was examined by assessing their ATP-dependent uptake in bile canalicular membrane vesicles (CMV) prepared from six human liver samples. 2, 4-Dinitrophenyl-S-glutathione (DNP-SG), leukotriene C4 (LTC4), sulfobromophthalein glutathione (BSP-SG), E3040 glucuronide (E-glu), beta-estradiol 17-(beta-D-glucuronide) (E2-17G), grepafloxacin glucuronide (GPFXG), pravastatin, BQ-123, and methotrexate, which are known to be substrates for the rat canalicular multispecific organic anion transporter, and taurocholic acid (TCA), a substrate for the bile acid transporter, were used as substrates. ATP-dependent and saturable uptake of TCA, DNP-SG, LTC4, E-glu, E2-17G, and GPFXG was observed in all human CMV preparations examined, suggesting that these compounds are excreted in the bile via a primary active transport system in humans. Primary active transport of the other substrates was also seen in some of CMV preparations but was negligible in the others. The ATP-dependent uptake of all the compounds exhibited a large inter-CMV variation, and there was a significant correlation between the uptake of glutathione conjugates (DNP-SG, LTC4, and BSP-SG) and glucuronides (E-glu, E2-17G, and GPFXG). However, there was no significant correlation between TCA and the other organic anions, implying that the transporters for TCA and for organic anions are different also in humans. When the average value for the ATP-dependent uptake by each preparation of human CMVs was compared with that of rat CMVs, the uptake of glutathione conjugates and nonconjugated anions (pravastatin, BQ-123, and methotrexate) in humans was approximately 3- to 76-fold lower than that in rats, whereas the uptake of glucuronides was similar in the two species. Thus there is a species difference in the primary active transport of organic anions across the bile canalicular membrane that is less marked for glucuronides.

Selective inhibition of MDR1 P-glycoprotein-mediated transport by the acridone carboxamide derivative GG918

The acridone carboxamide derivative GG918 (N-{4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)-ethyl]-pheny l}-9,10dihydro-5-methoxy-9-oxo-4-acridine carboxamide) is a potent inhibitor of MDR1 P-glycoprotein-mediated multidrug resistance. Direct measurements of ATP-dependent MDR1 P-glycoprotein-mediated transport in plasma membrane vesicles from human and rat hepatocyte canalicular membranes indicated 50% inhibition at GG918 concentrations between 8 nM and 80 nM using N-pentyl-[3H]quinidinium, ['4C]doxorubicin and [3H]daunorubicin as substrates. The inhibition constant K for GG918 was 35 nM in rat hepatocyte canalicular membrane vesicles with [3H]daunorubicin as the substrate. Photoaffinity labelling of canalicular and recombinant rat Mdr1b P-glycoprotein by [3H]azidopine was suppressed by 10 muM and 40 muM GG918. The high selectivity of GG918-induced inhibition was demonstrated in canalicular membrane vesicles and by analysis of the hepatobiliary elimination in rats using [3H]daunorubicin, [3H]taurocholate and [3H]cysteinyl leukotrienes as substrates for three distinct ATP-dependent export pumps. Almost complete inhibition of [3H]daunorubicin transport was observed at GG918 concentrations that did not affect the other hepatocyte canalicular export pumps. The high potency and selectivity of GG918 for the inhibition of human MDR1 and rat Mdr1b P-glycoprotein may serve to interfere with this type of multidrug resistance and provides a tool for studies on the function of these ATP-dependent transport proteins.

Changes in the localization of the rat canalicular conjugate export pump Mrp2 in phalloidin-induced cholestasis

Administration of phalloidin, one of the toxic peptides of the mushroom Amanita phalloides, leads to rapid and sustained cholestasis in rats. Although attributed to the interaction of phalloidin with microfilaments, the events leading to cholestasis are incompletely understood. The adenosine triphosphate (ATP)-dependent, apical conjugate export pump, termed multidrug resistance protein 2 (Mrp2) or canalicular multispecific organic anion transporter, is the major driving force for bile salt-independent bile flow. We investigated the role of Mrp2 in phalloidin-induced cholestasis. Bile flow decreased to 53% and 31% of control at 15 and 30 minutes after phalloidin (0.5 mg/kg), respectively. Mrp2-mediated [3H]leukotriene excretion into bile during the initial 45 minutes was reduced to 44% of control when [3H]LTC4 was injected 15 minutes after phalloidin treatment. Mrp2 was progressively lost from the hepatocyte canalicular membrane and detected predominantly on intracellular membrane structures together with other canalicular proteins including P-glycoproteins, ecto-ATPase, and dipeptidyl-peptidase IV. By contrast, structures involved in intercellular adhesion (zonula occludens, zonula adhaerens, and desmosomes) as well as intermediate filaments of the cytokeratin type appeared largely unaffected within 30 minutes after phalloidin. In line with the immunofluorescence analysis, immunoblots indicated a loss of Mrp2 and P-glycoproteins from the canalicular membrane and a 3- and 4.6-fold increase of these transport proteins in the microsomal fraction, respectively. Our results indicate that phalloidin induces marked alterations of the hepatocyte canalicular architecture and a loss of Mrp2 together with other proteins from the canalicular membrane. The resulting cholestasis can therefore be explained in part by the loss of export pumps, including Mrp2, from the canalicular membrane.

Transfected rat cMOAT is functionally expressed on the apical membrane in Madin-Darby canine kidney (MDCK) cells

PURPOSE

The purpose of the present study is to investigate the expression of canalicular multispecific organic anion transporter (cMOAT) by its cDNA transfection in polarized Madin-Darby canine kidney cells (MDCK).

METHODS

MDCK cells were transfected with an expression vector (pCXN2) containing the rat cMOAT cDNA with lipofectamine to obtain the stable transfectant under G418. Cells from a single colony whose cMOAT expression was the highest were seeded to form a tight epithelial monolayer on microporous membrane filters. Export of glutathione S-bimane (GS-B) from monolayers was determined after preloading its precursor, monochloro bimane (MCB).

RESULTS

A comparable amount of GS-B was excreted to the apical and basal compartments in the vector-transfected cells. In contrast, in cMOAT-transfected cells, the amount apically excreted was approximately twice that excreted into the basal compartment. Cyclosporin A (CsA) (30 microM), an inhibitor of cMOAT at higher concentrations, inhibited the preferential apical export of GS-B from cMOAT-transfected cells.

CONCLUSIONS

Rat cMOAT is functionally expressed on the apical membrane of MDCK cells after transfection.

Discontinuous total parenteral nutrition prevents postischemic mitochondrial dysfunction in rat liver

Although discontinuous total parenteral nutrition (d-TPN) has recently been favored for clinical use over continuous total parenteral nutrition (c-TPN) to ameliorate liver dysfunction, mechanisms for the protection against postoperative liver dysfunction remain unknown. This study aimed to examine differences in mitochondrial function in d-TPN- and c-TPN-pretreated livers during ischemia-reperfusion. Rat livers pretreated with d-TPN or c-TPN were perfused with Krebs-Ringer buffer and were exposed to 25% low-flow hypoxia followed by reperfusion. Intrahepatic mitochondrial membrane potential (triangle up) and cell viability were assessed by dual-color digital microfluorography using rhodamine 123 (Rh123) and propidium iodide (PI), respectively. In response to hypoxia, livers pretreated with c-TPN, d-TPN, and an ordinary chow diet exhibited a significant triangle up reduction among the entire lobules. Upon reperfusion, the regional triangle up values further decreased in the c-TPN liver, whereas those in the d-TPN-treated or chow-treated livers displayed a rapid recovery toward the control levels. The severity of cell injury did not differ among the groups, showing that the reperfusion-induced triangle up drop in the c-TPN-pretreated liver is not a consequence of cell injury. Differences in the triangle up drop among the groups appear to occur irrespective of those in the glycogen storage, because the livers undergoing d-TPN display a marked triangle up recovery even when reperfused at the end of a fasted state. These results indicate that c-TPN, but not d-TPN, jeopardizes mitochondrial re-energization and suggest that a circadian pattern of the TPN serves as a potentially beneficial strategy to reduce the risk of postischemic mitochondrial dysfunction in the liver.

Expression of the apical conjugate export pump, Mrp2, in the polarized hepatoma cell line, WIF-B

The polarized rat hepatoma/human fibroblast hybrid cell line, WIF-B, forms apical vacuoles into which cholephilic substances are secreted. We studied expression, localization, and function of the apical conjugate export pump, Mrp2, in WIF-B cells. Mrp2, the apical isoform of the multidrug resistance protein, alternatively termed canalicular Mrp (cMrp) or canalicular multispecific organic anion transporter (cMoat), is a 190-kd membrane glycoprotein mediating adenosine triphosphate (ATP)-dependent transport of glucuronides, glutathione S-conjugates, and other amphiphilic anions across the hepatocyte canalicular membrane into bile. Expression of the rat mrp2 gene in WIF-B cells was shown by reverse-transcription polymerase chain reaction (PCR), followed by sequencing of the amplified 789-bp fragment. Immunoblotting, using antibodies reacting with the amino-terminal or with the carboxyl-terminal sequence of rat Mrp2, detected the 190-kd glycoprotein in WIF-B cell homogenates. Immunofluorescence microscopy localized Mrp2 to the apical membrane domain. Preloading of WIF-B cells with a membrane-permeable ester of the calcium-dependent fluorescent indicator, Fluo-3, was followed by Mrp2-mediated secretion of the amphiphilic anion, Fluo-3, into the apical vacuoles. This transport was potently inhibited by cyclosporin A added to the culture medium. Direct measurements of ATP-dependent transport into Mrp2-containing plasma membrane vesicles in comparison with Mrp2-deficient vesicles established that Fluo-3 is transported by Mrp2 with a Km value of 3.7 micromol/L. Our results indicate that the polarized WIF-B cells express the rat ortholog of the apical conjugate-transporting ATPase, Mrp2. The function of Mrp2 as well as the action of inhibitors can thus be analyzed by use of the fluorescent amphiphilic anion, Fluo-3.

The possible role of endothelin(s) in cyclosporine A preparations--induced contraction of guinea pig isolated gallbladder strips

1. In guinea-pig isolated gallbladder strips, both cyclosporine A (CyA) preparations and their vehicles (10(-7) M-4 x 10(-5) M) caused stable, long-lasting and concentration-dependent contractions. 2. Some gallbladder strips showed spontaneous rhythmic activity. CyA and its vehicles increased this rhythmic activity. Furthermore, they elicited rhythmic activity in the strips that did not show any spontaneous rhythmic activity. 3. Bosentan (10(-5) M) and verapamil (10(-5) M) partly but significantly inhibited the contractions due to CyA preparations and their vehicles except the effect of verapamil on Labrafil-induced contraction. 4. Neither parenteral and oral solutions of CyA nor their vehicles caused any contractile response on the sphincter of Oddi preparations.

Inhibition of biliary glutathione secretion by cyclosporine A in the rat: possible mechanisms and role in the cholestasis induced by the drug

BACKGROUND/AIMS

Biliary glutathione appears to be a major osmotic factor in the generation of bile acid-independent bile flow. This study was designed to investigate its importance in cyclosporine A-induced cholestasis in both acute and short-term-treated rats.

METHODS

Adult male Wistar rats were treated as follows: (i) with a single i.v. dose of cyclosporine or its vehicle (acute assays); (ii) with cyclosporine, its vehicle or physiological saline, i.p., for 7 days once per day (short-term treatment assays). Bile flow and biliary glutathione levels were determined under anesthesia both before and after intrabiliary hydrolysis of the tripeptide had been inhibited.

RESULTS

Acute cyclosporine administration, at a dose of 20 mg/kg, brought about an abrupt and marked fall in bile flow and bile acid secretion simultaneously with a rapid decrease in the biliary concentration and secretion rates of total, reduced and oxidized glutathione. When the rats were treated with cyclosporine A for 1 week, at a dose of 10 mg/kg per day, similar cholestatic and inhibitory effects on the biliary secretion of glutathione were noted both before and after the intrabiliary catabolism of the tripeptide had been inhibited with acivicin; in addition, the hepatic content of glutathione was also reduced. The cholestatic effect of the drug was associated with reductions in the four bile flow fractions evaluated: bile acid- and glutathione-dependent bile flow and bile acid- and glutathione-independent bile flow.

CONCLUSIONS

These findings indicate that cyclosporine-induced cholestasis in the rat is due not only to alterations in the hepatobiliary transport of bile acids but also to an impairment of bile formation dependent on the biliary secretion of glutathione, possibly through inhibition of the canalicular transport of the tripeptide.

Biliary lipid composition after liver transplantation: effect of allograft function and cyclosporine

Biliary lipid composition and bile flow are altered after orthotopic liver transplantation. Cyclosporine may have additional effects on biliary lipid composition and secretion. We studied the effects of liver transplantation, allograft function, and cyclosporine on biliary lipids in humans. Changes in lipid composition and secretion were correlated with serum cyclosporine levels, clinical events, and allograft function. Bile samples were withdrawn via a T-tube at interval time points in 17 patients during the first 3 months posttransplantation. Total and individual bile acid, cholesterol, and phospholipid were determined using high-performance liquid chromatography. Biliary lipid profiles were then correlated with clinical events, serum cyclosporine levels, and other clinical laboratory values. Biliary lipid concentrations decreased in 3 patients during periods of graft dysfunction (acute cellular rejection, drug-induced hepatitis, and inferior vena caval thrombosis) and increased with resolution of the graft injury. Serum cyclosporine levels were positively correlated with total bile acid, cholesterol, and phospholipid concentrations in bile. There was no relationship between the composition of secreted bile acids and serum cyclosporine levels. Bile acid, cholesterol, and phospholipid secretion were not uncoupled in the presence of cyclosporine. We concluded that (1) a decrease in biliary lipid concentrations may be an indicator of worsened graft function in some allografts; (2) biliary lipid concentrations are correlated with increasing cyclosporine levels; and (3) bile acid composition is unchanged, and uncoupling of secretion of other biliary lipids is not observed in the presence of cyclosporine.

Functional analysis of a canalicular multispecific organic anion transporter cloned from rat liver

Transport of many organic anions across the bile canalicular membrane is mediated by the canalicular multispecific organic anion transporter (cMOAT). Previously, we cloned cDNA that may encode cMOAT from Sprague-Dawley rat liver (Ito, K., Suzuki, H., Hirohashi, T., Kume, K., Shimizu, T., and Sugiyama, Y. (1997) Am. J. Physiol. 272, G16-G22). In the present study, the function of this cloned cDNA was investigated by examining the ATP-dependent uptake of S-(2,4-dinitrophenyl)-glutathione (DNP-SG) into membrane vesicles isolated from an NIH/3T3 cell line transfected with an expression vector containing the cloned cDNA. Although the membrane vesicles from the control NIH/3T3 cells exhibited endogenous activity in transporting DNP-SG and leukotriene C4 in an ATP-dependent manner, the transfection of cMOAT cDNA resulted in a significant increase in the transport activity for these ligands. The uptake of DNP-SG into membrane vesicles was osmotically sensitive and was stimulated to some extent by other nucleotide triphosphates (GTP, UTP, and CTP) but not by AMP or ADP. The K(m) and Vmax values for the uptake of DNP-SG by the membrane vesicles were 0.175 +/- 0.031 microM and 11.0 +/- 0.73 pmol/min/mg protein, respectively, for the transfected rat cMOAT and 0.141 +/- 0.036 microM and 3.51 +/- 0.39 pmol/min/mg protein, respectively, for the endogenous transporter expressed on control NIH/3T3 cells. These results suggest that the product of the previously cloned cDNA has cMOAT activity being able to transport organic anions in an ATP-dependent manner. Alternatively, it is possible that the cDNA product encodes an activator of endogenous transporter since the K(m) value for DNP-SG was comparable between the vector- and cMOAT-transfected cells. The transport activity found in the control NIH/3T3 cells may be ascribed to mouse cMOAT since Northern blot analysis indicated the presence of a transcript that hybridyzed to the carboxyl-terminal ATP-binding cassette sequence of the murine protein.

ATP-dependent transport of bilirubin glucuronides by the multidrug resistance protein MRP1 and its hepatocyte canalicular isoform MRP2

Bilirubin is secreted from the liver into bile mainly as monoglucuronosyl and bisglucuronosyl conjugates. We demonstrate for the first time that ATP-dependent transport of both bilirubin glucuronides is mediated by the multidrug resistance protein (MRP1) as well as by the distinct canalicular (apical) isoform MRP2, also termed cMRP or cMOAT (canalicular multispecific organic anion transporter). In membrane vesicles from MRP1-transfected HeLa cells mono[3H]glucuronosylbilirubin and bis[3H]glucuronosylbilirubin (each at 0.5 microM) were transported with rates of 5.3 and 3.1 pmol/min per mg of protein respectively. Rat hepatocyte canalicular membrane vesicles, which contain Mrp2 (the rat equivalent of MRP2), transported mono[3H]glucuronosylbilirubin and bis[3H]glucuronosylbilirubin at rates of 8.9 and 8.5 pmol/min per mg of protein, whereas membrane vesicles from mutant liver lacking Mrp2 showed no transport of the conjugates. In membrane vesicles from human hepatoma Hep G2 cells, which predominantly expressed MRP2, transport rates were 8.3 and 4.4 pmol/min per mg of protein for monoglucuronosylbilirubin and bisglucuronosylbilirubin respectively. ATP-dependent transport of the glutathione S-conjugate -3H-leukotriene C4, an established high-affinity substrate for MRP1 and MRP2, was inhibited by both bilirubin glucuronides with IC50 values between 0.10 and 0.75 microM. The ratios of leukotriene C4 transport and bilirubin glucuronide transport, determined in the same membrane vesicle preparation, indicated substrate specificity differences between MRP1 and MRP2 with a preference of MRP2 for the glucuronides.

Characterization and quantification of rat bile phosphatidylcholine by electrospray-tandem mass spectrometry

Rat bile phosphatidylcholine was structurally characterized and quantified by electrospray mass spectrometry using a triple quadrupole instrument. All results were obtained by direct analysis of an unprocessed total lipid extract from rat bile. Structural characterization of phosphatidylcholine was achieved by collision-induced dissociation of [M + Cl]- ions observed in the negative-ion electrospray mass spectrum. Quantification of phosphatidylcholine was performed in the positive-ion mode using precursor ion scanning of m/z 184 and dimyristoyl-phosphatidylcholine as internal standard. Using this new methodology, the effect of cyclosporin A on biliary phosphatidylcholine excretion in the rat was investigated. After intravenous administration of cyclosporin A (25 mg/kg body wt) the phosphatidylcholine level in bile was reduced to about 30% of the control level. This suggests an inhibition by cyclosporin A of the translocation of phosphatidylcholine across the hepatocyte canalicular membrane which is mediated by the Mdr2 P-glycoprotein.

cDNA cloning of the hepatocyte canalicular isoform of the multidrug resistance protein, cMrp, reveals a novel conjugate export pump deficient in hyperbilirubinemic mutant rats

ATP-dependent transport of glutathione and glucuronate conjugates from hepatocytes into bile is mediated by a distinct member of the ATP-binding cassette superfamily. We have cloned and sequenced the canalicular isoform of the multidrug resistance protein from rat liver, and termed it cMrp. This membrane glycoprotein is composed of 1541 amino acids with an identity of 47.8% with the human multidrug resistance protein (MRP) and of 41.9% with the yeast cadmium factor (YCF1). The carboxyl-terminal 130 amino acids of the human hepatocyte canalicular isoform of MRP (cMRP) were 80.2% identical with rat cMrp. cMrp was not expressed in the liver of two mutant rat strains, the Eisai hyperbilirubinemic rat and the GY/TR- mutant, which are deficient in the ATP-dependent transport of conjugates across the canalicular membrane. Immunoblotting using an antibody raised against the carboxyl terminus of cMrp detected the glycoprotein of about 190 kDa only in the canalicular membrane from normal liver. Double immunofluorescence and confocal laser scanning microscopy localized cMrp exclusively to the canalicular membrane domain of hepatocytes and demonstrated its loss in the hyperbilirubinemic mutant rat. The results identify cMrp as a canalicular transport protein with a novel sequence and with a function similar to the one of the MRP.

Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes

In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7alpha-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the "acidic" and "neutral" pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7alpha-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 microM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 microM), dibutyryl-cAMP (5O microM), nor squalestatin 1 (15O nM-1.0 microM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 microM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-beta-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7alpha-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [14C]cholesterol remained relatively high and consisted of beta-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.

ATP-dependent glutathione disulphide transport mediated by the MRP gene-encoded conjugate export pump

We have previously shown that the multidrug resistance protein (MRP) mediates the ATP-dependent membrane transport of the endogenous glutathione conjugate leukotriene C4 (LTC4) and of structurally related anionic conjugates of lipophilic compounds [Jedlitschky, Leier, Buchholz, Center and Keppler (1994) Cancer Res. 54, 4833-4836; Leier, Jedlitschky, Buchholz, Cole, Deeley and Keppler (1994) J. Biol. Chem. 269, 27807-27810]. We demonstrate in the present study that MRP also mediates the ATP-dependent transport of GSSG, as shown in membrane vesicles from human leukaemia cells overexpressing MRP (HL60/ADR cells) or HeLa cells transfected with an MRP expression vector (HeLa T5 cells) in comparison with the respective parental or control cells. The Km value for ATP-dependent transport of GSSG was 93 +/- 26 microM (mean value +/- S.D., n=5) in membrane vesicles from HeLa T5 cells. GSH, at a concentration of 100 microM, was not a substrate for any significant ATP-dependent MRP-mediated transport. The transport of GSSG was competitively inhibited by LTC4, by the leukotriene D4 receptor antagonist 3-([{3-(2-[7-chloro-2-quinolinyl]ethenyl)phenyl}-{(3-dimethylamino-3- oxopropyl)-thio}-methyl]thio)propanoic acid (MK 571) and by S-decylglutathione, with K1 values of 0.3, 0.6 and 0.7 microM respectively. These studies identify MRP as the membrane glycoprotein which mediates the ATP-dependent export of GSSG from these cells.

Effect of combination of suboptimal concentrations of P-glycoprotein blockers on the proliferation of MDR1 gene expressing cells

Pharmacologically active in vivo doses of P-glycoprotein (Pgp) blockers, specifically verapamil, Cremophor EL and PSC833 cause toxicity in addition to that from the concomitantly used cancer chemotherapeutic drugs. It was shown before that these blockers cause different types of toxicities in vivo. We found that these 3 chemically distinct Pgp blockers exert different biophysical effects on the membranes of L1210 MDR cells. They also affect the general metabolism of these cells differently, but all block affinity labeling of Pgp. We could also show that the combination of suboptimal doses of these blockers can restore the uptake of the Pgp substrate rhodamine 123 into L1210MDR, 3T3MDR and KB-VI cells and can reduce the survival rate of these cells when treated in combination with daunorubicin. Our results suggest that the combination of suboptimal doses of these Pgp blockers may be advantageous in clinical practice.

Bile acids in the assessment of hepatocellular function

Bile acids, which are synthesized in the liver from cholesterol, are important in the production of bile flow, excretion of cholesterol, and intestinal digestion and absorption of fats and fat-soluble vitamins. Increases and/or alterations in concentrations of bile acids in serum are specific and sensitive indicators of hepatobiliary disorders. Synthesis of bile acids in hepatocytes involves steps in endoplasmic reticulum, cytosol, mitochondria, and peroxisomes. Other important hepatocellular processes involving bile acids include active uptake by the basolateral membrane, intracellular transport, P-450-mediated conjugations and hydroxylations, and canalicular secretion. Hydrophobic bile acids produce hepatotoxicity in vivo and in vitro. In experimental and epidemiologic studies, some of these forms have been identified as causative agents in the development of colon and liver (experimental only) cancer. Conversely, several hydrophilic forms, primarily ursodeoxycholic acid, have demonstrated cytoprotective properties in a variety of clinical and experimental hepatobiliary diseases and disorders. Because bile acids can have dramatically different properties and effects, determination of mechanisms of action of these compounds has become an active area of research. Primary isolated hepatocytes provide an opportunity to investigate bile acid-related functions and effects in well-designed, carefully controlled studies. Short-term cultures have been used to study a variety of issues related to bile acids, including cytotoxicity, synthesis, and hepatocellular processing. With these systems, however, many functions of mature hepatocytes, including those pertaining to bile acids, can be lost when cultures are maintained for more than several days. Recent developments in culture techniques permit long-term maintenance of functionally stable, differentiated cells. Pertaining to bile acid research, these systems remain to be fully characterized but, in appropriate situations, they should provide important alternatives to in vivo studies and short-term in vitro assays.

Expression of the MRP gene-encoded conjugate export pump in liver and its selective absence from the canalicular membrane in transport-deficient mutant hepatocytes

We have previously shown that the multi-drug resistance protein (MRP) mediates the ATP-dependent membrane transport of glutathione S-conjugates and additional amphiphilic organic anions. In the present study we demonstrate the expression of MRP in hepatocytes where it functions in hepatobiliary excretion. Analysis by reverse transcription-PCR of human and normal rat liver mRNA resulted in two expected cDNA fragments of MRP. Four different antibodies against MRP reacted on immunoblots with the glycoprotein of about 190 kD from human canalicular as well as basolateral hepatocyte membrane preparations. A polyclonal antibody directed against the carboxy-terminal sequence of MRP detected the rat homolog of MRP in liver. Double immunofluorescence microscopy and confocal laser scanning microscopy showed the presence of human MRP and rat Mrp in the canalicular as well as in the lateral membrane domains of hepatocytes. The transport function of the mrp gene-encoded conjugate export pump was assayed in plasma membrane vesicles with leukotriene C4 as a high-affinity glutathione S-conjugate substrate. The deficient ATP-dependent conjugate transport in canalicular membranes from TR- mutant rat hepatocytes was associated with a lack of amplification of one of the mrp cDNA fragments and with a selective loss of Mrp on immunoblots of canalicular membranes. Double immunofluorescence microscopy of livers from transport-deficient TR- mutant rats localized Mrp only to the lateral but not to the canalicular membrane. Our results indicate that the absence of Mrp or an isoform of Mrp from the canalicular membrane is the basis for the hereditary defect of the hepatobiliary excretion of anionic conjugates by the transport-deficient hepatocyte.

Functional reconstitution of ATP-dependent transporters from the solubilized hepatocyte canalicular membrane

The hepatocyte canalicular membrane contains several primary-active ATP-dependent export carriers including one for bile salts and one for leukotriene C4 and related conjugates. The molecular identity of both transporters has not been fully elucidated. To establish a transport assay that allows the purification and identification of the proteins involved in ATP-dependent bile salt transport and in leukotriene C4 transport, we reconstituted solubilized hepatocyte canalicular membranes into phospholipid bilayers using a rapid dilution method. The proteoliposomes formed exhibited both [3H]taurocholate and [3H]leukotriene C4 uptake, which was much higher in the presence of ATP than in the presence of the non-hydrolyzable ATP-analog AdoPP[CH2]P or in the absence of nucleotides. Nucleotide requirement and osmotic sensitivity of [3H]taurocholate transport indicates true transport into the vesicle lumen. Optimized conditions for reconstitution included the addition of a high concentration of an osmolyte (glycerol) and the presence of exogenous phospholipids (0.3%) during solubilization. Highest transport rates were obtained by reconstitution into acetone/ether-precipitated Escherichia coli phospholipid supplemented with 20% cholesterol and by use of octylglucoside concentrations between 30 mM and 50 mM. Taurocholate transport was non-competitively inhibited by vanadate (Ki = 39 microM). The kinetic parameters of cyclosporin A inhibition (Ki = 2.6 microM for taurocholate and 4.3 microM for leukotriene C4 transport) as well as the affinities of taurocholate (Km = 12 microM) and leukotriene C4 (Km = 0.5 microM) in the proteoliposome system indicate that the reconstitution resulted in functionally active transport systems, which are representative of ATP-dependent transport in the intact plasma membrane.