Isolation and characterization of canalicular and basolateral plasma membrane fractions from human liver

Adenovirus-Mediated ABCC6 Gene Therapy for Heritable Ectopic Mineralization Disorders

Loss-of-function mutations in the ABCC6 gene cause pseudoxanthoma elasticum and type 2 generalized arterial calcification of infancy, heritable ectopic mineralization disorders without effective treatment. ABCC6 encodes the putative efflux transporter ABCC6, which is predominantly expressed in the liver. Although the substrate of ABCC6 remains unknown, recent studies showed that pseudoxanthoma elasticum is a metabolic disorder caused by reduced circulating levels of pyrophosphate, a potent mineralization inhibitor. We hypothesized that reconstitution of ABCC6 might counteract ectopic mineralization in an Abcc6^-/- mouse model of pseudoxanthoma elasticum. Intravenous administration of a recombinant adenovirus expressing wild-type human ABCC6 in Abcc6^-/- mice showed sustained high-level expression of human ABCC6 in the liver for up to 4 weeks, increasing pyrophosphate levels in plasma. In addition, adenovirus injection every 4 weeks restored plasma pyrophosphate levels and, consequently, significantly reduced ectopic mineralization in the skin of young mice. By contrast, the same treatment in old mice with already established mineral deposits failed to reduce mineralization. These results suggest that adenovirus-mediated ABCC6 gene delivery, when initiated early, is a promising prevention therapy for pseudoxanthoma elasticum and generalized arterial calcification of infancy, diseases that currently lack preventive or therapeutic options.

Abcc6 Knockout Rat Model Highlights the Role of Liver in PPi Homeostasis in Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum, a heritable ectopic mineralization disorder, is caused by mutations in the ABCC6 gene primarily expressed in the liver and the kidneys. The fundamental question on pathogenesis of pseudoxanthoma elasticum, whether lack of ABCC6 expression in liver or kidney is the primary site of molecular pathology in peripheral tissues, has not been addressed. We generated a series of Abcc6^-/- rats as models of pseudoxanthoma elasticum depicting ectopic mineralization in the skin, eyes, and the arterial blood vessels. Plasma inorganic pyrophosphate (PPi) level was reduced (<30%) in the Abcc6^-/- rats leading to a lowered PPi/inorganic phosphate plasma ratio. In situ liver and kidney perfusions were performed to determine the relative contribution of these organs to PPi levels in circulation. PPi levels in the perfusates both in the liver and kidney of Abcc6^-/- rats were significantly reduced, but the PPi levels in the liver perfusates of wild-type rats were 10-fold higher than that in the kidney perfusates. These observations suggest a critical role of hepatic ABCC6 in contributing to plasma PPi levels, identifying liver as a target of molecular correction to counteract ectopic mineralization in pseudoxanthoma elasticum.

Interindividual variability of canalicular ATP-binding-cassette (ABC)-transporter expression in human liver

Interindividual variability in hepatic canalicular transporter expression might predispose to the development of hepatic disorders such as acquired forms of intrahepatic cholestasis. We therefore investigated expression patterns of bile salt export pump (BSEP, ABCB11), multidrug resistance protein 3 (MDR3, ABCB4), multidrug resistance associated protein 2 (MRP2, ABCC2) and multidrug resistance protein 1 (MDR1, ABCB1) in healthy liver tissue of a white population. Protein expression levels were correlated with specific single nucleotide polymorphisms (SNPs) in the corresponding transporter genes. Hepatic protein expression levels from 110 individuals undergoing liver resection were assessed by Western blot analysis of liver plasma membranes enriched in canalicular marker enzymes. Each individual was genotyped for the following synonymous (s) and nonsynonymous (ns) SNPs: ABCB11: (ns:1457T>C and 2155A>G), ABCB4: (ns:3826A>G) and ABCC2 (ns:1286G>A,3600T>A and 4581G>A) and ABCB1 (ns:2677G>T/A and s:3435C>T). Transporter expression followed unimodal distribution. However, of all tested individuals 30% exhibited a high expression and 32% a low or very low expression phenotype for at least one of the four investigated transport proteins. Transporter expression levels did not correlate with age, sex, underlying liver disease, or presurgery medication. However, low BSEP expression was associated with the 1457C-allele in ABCB11 (P = .167) and high MRP2 expression was significantly correlated with the 3600A and 4581A ABCC2 variants (P = .006). In conclusion, the results demonstrate a considerable interindividual variability of canalicular transporter expression in normal liver. Furthermore, data suggest a polymorphic transporter expression pattern, which might constitute a risk factor for the development of acquired forms of cholestatic liver diseases.

Use of canalicular membrane vesicles (CMVs) from rats, dogs, monkeys and humans to assess drug transport across the canalicular membrane

INTRODUCTION

A novel application of a Ultrafree filter cartridge/centrifugation method was evaluated to determine uptake in canalicular membrane vesicles (CMVs) from SD rats, beagle dogs, cynomolgus monkeys (common safety species in the pharmaceutical industry) and humans to assess biliary transport.

METHODS

CMVs prepared from fresh livers of rats, dogs, monkeys and humans (four donors) were characterized for enrichment, basolateral and Golgi contamination and orientation. The presence of MRP2 and p-glycoprotein (P-gp) were confirmed by Western blots. Uptake of [3H]-leukotriene C4 (LTC4) and [3H]-estradiol-17beta-d-glucuronide (E2-Gluc) was determined at a low substrate concentration and/or by kinetic measurements (K(m) and V(max)). Correlation of in vitro data with in vivo findings was achieved by determining the biliary clearance of E2-Gluc in rats after a single i.v. dose and with literature in vivo data for LTC4.

RESULTS

CMVs were highly enriched and minimally contaminated based on marker enzyme activities. Uptake clearance among different species varied by approximately ten-fold (rat > dog = human > monkey) for LTC4 and less than two-fold for E2-Gluc. The lower uptake of LTC4 by human than rat CMVs may be attributed to a higher Km value for human than rat CMVs. Uptake of LTC4 or E2-Gluc by human CMVs showed little inter-subject variability (2-5-fold). Differences in in vitro uptake clearance (10-fold) between LTC4 and E2-Gluc in rat CMVs seemed to correlate with differences in their biliary clearance (4-fold) in rats, consistent with LTC4 and E2-Gluc being a high and a low clearance substrate, respectively.

DISCUSSION

A novel application of a Ultrafree filter cartridge/centrifugation method was developed to determine uptake in CMVs from different preclinical animal safety species and humans, and may represent a useful approach to study the mechanism of biliary excretion during drug discovery and development.

Down-regulation of hepatic and intestinal Abcg5 and Abcg8 expression associated with altered sterol fluxes in rats with streptozotocin-induced diabetes

AIM/HYPOTHESIS

Type I diabetes is associated with altered hepatic bile formation and increased intestinal cholesterol absorption. The aim of this study was to evaluate whether altered expression of the ATP-Binding Cassette half-transporters Abcg5 and Abcg8, recently implicated in control of both hepatobiliary cholesterol secretion and intestinal cholesterol absorption, contributes to changed cholesterol metabolism in experimental diabetes.

METHODS

mRNA and protein expression of Abcg5 and Abcg8 were determined in the liver and intestine of rats with streptozotozin-induced diabetes and related to relevant metabolic parameters in plasma, liver and bile.

RESULTS

Hepatic mRNA expression of both Abcg5 (-76%) and Abcg8 (-71%) was reduced in diabetic rats when compared to control rats. In spite of increased HDL cholesterol, considered a major source of biliary cholesterol, secretion of the sterol into bile relative to that of bile salts was reduced by 65% in diabetic animals. Intestinal mRNA expression of Abcg5 (-47%) and Abcg8 (-43%) as well as Abcg5 protein contents were also reduced in insulin-deficient animals. This was accompanied by a three- to four-fold increase in plasma beta-sitosterol and campesterol concentrations and by a doubling of the calculated apparent cholesterol absorption. These effects partially normalized upon insulin supplementation.

CONCLUSION/INTERPRETATION

Our data indicate that effects of insulin-deficiency on bile composition and cholesterol absorption in rats are, at least partly, attributable to changes in hepatic and intestinal Abcg5 and Abcg8 expression.

Cyclosporin a and enterohepatic circulation of bile salts in rats: decreased cholate synthesis but increased intestinal reabsorption

Cyclosporin A (CsA) has been shown to inhibit synthesis and hepatobiliary transport of bile salts. However, effects of CsA on the enterohepatic circulation of bile salts in vivo are largely unknown. We characterized the effects of CsA on the enterohepatic circulation of cholate, with respect to synthesis rate, pool size, cycling time, intestinal absorption, and the expression of relevant transporters in liver and intestine in rats. CsA (1 mg. 100 g(-1). day(-1) s.c.) or its solvent was administered daily to male rats for 10 days. Cholate synthesis rate and pool size were determined by a 2H4-cholate dilution technique. Bile and feces were collected for determination of cholate and total bile salts, respectively. Cycling time and intestinal absorption of cholate were calculated. The mRNA levels and corresponding transporter protein levels in liver and intestine were assessed by real-time polymerase chain reaction and Western analysis, respectively. CsA treatment decreased cholate synthesis rate by 71%, but did not affect pool size or cycling time. CsA reduced the amount of cholate lost per enterohepatic cycle by approximately 70%. Protein levels of the apical sodium-dependent bile salt transporter (Asbt) were 2-fold increased in distal ileum of CsA-treated rats, due to post-transcriptional events. In conclusion, chronic CsA treatment markedly reduces cholate synthesis rate in rats, but does not affect cholate pool size or cycling time. Our results strongly suggest that CsA enhances efficacy of intestinal cholate reabsorption through increased Asbt protein expression in the distal ileum, which contributes to maintenance of cholate pool size in CsA-treated rats.

Differential effects of streptozotocin-induced diabetes on expression of hepatic ABC-transporters in rats

BACKGROUND & AIMS

Diabetes mellitus is associated with changes in bile formation. The aim of our study was to investigate the molecular basis for these changes in rats with experimentally induced diabetes.

METHODS

Expression of bile canalicular transporters was studied by reverse-transcription polymerase chain reaction, immunoblotting, and immunohistochemistry in control, streptozotocin-diabetic, and insulin-treated diabetic rats. Bile formation was studied under basal conditions and during stepwise increasing intravenous infusion of taurocholate to determine bile salt secretory rate maximum (SRm).

RESULTS

In diabetic rats, hepatic gene and protein expression of the multidrug resistance P-glycoprotein type 2 (Mdr2) were increased by 105% and 530%, respectively, associated with increased biliary phospholipid output (+520%) and phospholipid/bile salt ratio (+77%). Protein levels of the canalicular bile salt export pump (Bsep) were unchanged in diabetic rats, but basal biliary bile salt output and the SRm of taurocholate were increased by 260% and 130%, respectively, compared with controls. Alterations in transporter expression and bile formation were partly reversed by insulin administration. The bile salt SRm was strongly correlated with biliary phospholipid concentration (P < 0.001, R = 0.82).

CONCLUSIONS

Induction of Mdr2 expression and biliary phospholipid secretion, rather than Bsep expression, appears to be responsible for the enhanced capacity of biliary bile salt secretion in experimentally induced diabetes.

Differential localization and operation of distinct Mg(2+) transporters in apical and basolateral sides of rat liver plasma membrane

Upon activation of specific cell signaling, hepatocytes rapidly accumulate or release an amount of Mg(2+) equivalent to 10% of their total Mg(2+) content. Although it is widely accepted that Mg(2+) efflux is Na(+)-dependent, little is known about transporter identity and the overall regulation. Even less is known about the mechanism of cellular Mg(2+) uptake. Using sealed and right-sided rat liver plasma membrane vesicles representing either the basolateral (bLPM) or apical (aLPM) domain, it was possible to dissect three different Mg(2+) transport mechanisms based upon specific inhibition, localization within the plasma membrane, and directionality. The bLPM possesses only one Mg(2+) transporter, which is strictly Na(+)-dependent, bi-directional, and not inhibited by amiloride. The aLPM possesses two separate Mg(2+) transporters. One, similar to that in the bLPM because it strictly depends on Na(+) transport, and it can be differentiated from that of the bLPM because it is unidirectional and fully inhibited by amiloride. The second is a novel Ca(2+)/Mg(2+) exchanger that is unidirectional and inhibited by amiloride and imipramine. Hence, the bLPM transporter may be responsible for the exchange of Mg(2+) between hepatocytes and plasma, and vice versa, shown in livers upon specific metabolic stimulation, whereas the aLPM transporters can only extrude Mg(2+) into the biliary tract. The dissection of these three distinct pathways and, therefore, the opportunity to study each individually will greatly facilitate further characterization of these transporters and a better understanding of Mg(2+) homeostasis.

Localization of the Wilson's disease protein in human liver

BACKGROUND & AIMS

Wilson's disease is an autosomal-recessive disorder of copper metabolism that results from the absence or dysfunction of a copper-transporting P-type adenosine triphosphatase that leads to impaired biliary copper excretion and disturbed holoceruloplasmin synthesis. To gain further insight into the role of the Wilson's disease protein in hepatic copper handling, its localization in human liver was investigated.

METHODS

By use of a specific antibody, localization of the Wilson's disease protein was studied in liver membrane fractions and liver sections by immunoblotting, immunohistochemistry, and double-label confocal scanning laser microscopy.

RESULTS

The 165-kilodalton protein, found by immunoblotting, was most abundant mainly in isolated plasma membrane fractions enriched in canalicular domains. Immunohistochemistry revealed intracellular punctuate staining of hepatocytes in certain regions of the liver, whereas a canalicular membrane staining pattern was observed in other regions. Double-labeling studies showed that in the latter regions the transporter is present mainly in vesicular structures just underneath the canalicular membrane that are positive for markers of the trans-Golgi network. A weak staining of the canalicular membrane, identified by staining for P-glycoprotein, was observed.

CONCLUSIONS

These results show that in human liver the Wilson's disease protein is predominantly present in trans-Golgi vesicles in the pericanalicular area, whereas relatively small amounts of the protein appear to localize to the canalicular membrane, consistent with a dual function of the protein in holoceruloplasmin synthesis and biliary copper excretion.

Metabolic fate of glutathione conjugate of benzo[a]pyrene-(7R,8S)-diol (9S,10R)-epoxide in human liver

Benzo[a]pyrene-(7R,8S)-diol (9S,10R)-epoxide [(+)-anti-BPDE] is believed to be the activated form of the widely spread environmental pollutant benzo[a]pyrene. Glutathione (GSH) S-transferase (GST)-catalyzed conjugation of (+)-anti-BPDE with GSH is an important mechanism in its cellular detoxification. Here, we report that the GSH conjugate of (+)-anti-BPDE [(-)-anti-BPD-SG] is a potent inhibitor (K(i) 15 microM) of class Mu human GST isoenzyme, which, among human liver GSTs, is a highly efficient detoxifier of (+)-anti-BPDE. Thus, the inhibition of GST activity by (-)-anti-BPD-SG may hinder GSH conjugation of (+)-anti-BPDE, unless the conjugate is metabolized and/or eliminated. The results of the present study show that gamma-glutamyltranspeptidase (gamma-GT) can metabolize (-)-anti-BPD-SG at a rate of about 0.29 nmol/min/mg protein. Our studies also show that (-)-anti-BPD-SG is transported across the human canalicular liver plasma membrane (cLPM) in an ATP-dependent manner at a rate of about 0.33 nmol/min/mg protein. The ATP-dependent transport of (-)-anti-[(3)H]BPD-SG across human cLPM follows Michaelis-Menten kinetics (K(m) 84 microM; V(max) 0.33 nmol/min/mg). In conclusion, the results of the present study suggest that both gamma-GT-mediated metabolism and ATP-dependent canalicular transport may be important steps in overall detoxification of (+)-anti-BPDE in the human liver.

Biliary fibrosis associated with altered bile composition in a mouse model of erythropoietic protoporphyria

BACKGROUND & AIMS

Reduced activity of ferrochelatase in erythropoietic protoporphyria (EPP) results in protoporphyrin (PP) accumulation in erythrocytes and liver. Liver disease may occur in patients with EPP, some of whom develop progressive liver failure that necessitates transplantation. We investigated the mechanisms underlying EPP-associated liver disease in a mouse model of EPP.

METHODS

Liver histology, indicators of lipid peroxidation, plasma parameters of liver function, and bile composition were studied in mice homozygous (fch/fch) for a point mutation in the ferrochelatase gene and in heterozygous (fch/+) and wild-type (+/+) mice.

RESULTS

Microscopic examination showed bile duct proliferation and biliary fibrosis with portoportal bridging in fch/fch mice. PP content was 130-fold increased, and thiobarbituric acid-reactive substances (+30%) and conjugated dienes (+75%) were slightly higher in fch/fch than in fch/+ and +/+ livers. Levels of hepatic thiols (-12%) and iron (-52%) were reduced in fch/fch livers. Liver enzymes and plasma bilirubin were markedly increased in the homozygotes. Plasma bile salt levels were 80 times higher in fch/fch than in fch/+ and +/+ mice, probably related to the absence of the Na(+)-taurocholate cotransporting protein (Ntcp) in fch/fch liver. Paradoxically, bile flow was not impaired and biliary bile salt secretion was 4 times higher in fch/fch mice than in controls. Up-regulation of the intestinal Na(+)-dependent bile salt transport system in fch/fch mice may enhance efficiency of bile salt reabsorption. The bile salt/lipid ratio and PP content of fch/fch bile were increased 2-fold and 85-fold, respectively, compared with +/+, whereas biliary glutathione was reduced by 90%. Similar effects on bile formation were caused by griseofulvin-induced inhibition of ferrochelatase activity in control mice.

CONCLUSIONS

Bile formation is strongly affected in mice with impaired ferrochelatase activity. Rather than peroxidative processes, formation of cytotoxic bile with high concentrations of bile salts and PP may cause biliary fibrosis in fch/fch mice by damaging bile duct epithelium.

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.

Characterization of two Mg2+ transporters in sealed plasma membrane vesicles from rat liver

The plasma membrane of mammalian cells possesses rapid Mg2+ transport mechanisms. The identity of Mg2+ transporters is unknown, and so are their properties. In this study, Mg2+ transporters were characterized using a biochemically and morphologically standardized preparation of sealed rat liver plasma membranes (LPM) whose intravesicular content could be set and controlled. The system has the advantages that it is not regulated by intracellular signaling machinery and that the intravesicular ion milieu can be designed. The results indicate that 1) LPM retain trapped intravesicular total Mg2+ with negligible leak; 2) the addition of Na+ or Ca2+ induces a concentration- and temperature-dependent efflux corresponding to 30-50% of the intravesicular Mg2+; 3) the rate of flux is very rapid (137.6 and 86.8 nmol total Mg2+ . micrometer -2 . min-1 after Na+ and Ca2+ addition, respectively); 4) coaddition of maximal concentrations of Na+ and Ca2+ induces an additive Mg2+ efflux; 5) both Na+- and Ca2+-stimulated Mg2+ effluxes are inhibited by amiloride, imipramine, or quinidine but not by vanadate or Ca2+ channel blockers; 6) extracellular Na+ or Ca2+ can stimulate Mg2+ efflux in the absence of Mg2+ gradients; and 7) Mg2+ uptake occurs in LPM loaded with Na+ but not with Ca2+, thus indicating that Na+/Mg2+ but not Ca2+/Mg2+ exchange is reversible. These data are consistent with the operation of two distinct Mg2+ transport mechanisms and provide new information on rates of Mg2+ transport, specificity of the cotransported ions, and reversibility of the transport.

Impaired activity of the bile canalicular organic anion transporter (Mrp2/cmoat) is not the main cause of ethinylestradiol-induced cholestasis in the rat

To test the hypothesis that impaired activity of the bile canalicular organic anion transporting system mrp2 (cmoat) is a key event in the etiology of 17alpha-ethinylestradiol (EE)-induced intrahepatic cholestasis in rats, EE (5 mg/kg subcutaneously daily) was administered to male normal Wistar (NW) and mrp2-deficient Groningen Yellow/Transport-deficient Wistar (GY/TR-) rats. Elevated plasma bilirubin levels in GY/TR- rats increased upon EE-treatment from 65 +/- 8.4 micromol/L to 183 +/- 22.7 micromol/L within 3 days, whereas bilirubin levels remained unaffected in NW rats. Biliary bilirubin secretion was 1.5-fold increased in NW rats but remained unaltered in GY/TR- rats. Plasma bile salt concentrations remained unchanged in both strains, although hepatic levels of the sinusoidal Na+-taurocholate cotransporting protein (ntcp) were markedly reduced. Biliary secretion of endogenous bile salt was not affected in either strain. A clear reduction of mrp2 levels in liver plasma membranes of NW rats was found after 3 days of treatment. The bile salt-independent fraction of bile flow (BSIF) was reduced from 2.6 to 2.0 microL/min/100 g body weight in NW rats with a concomitant 62% reduction of biliary glutathione secretion. The absence of mrp2 and biliary glutathione in GY/TR- rats did not prevent induction of EE-cholestasis; a similar absolute reduction of BSIF, i.e., from 1.1 to 0.6 microL/min/100 g bodyweight, was found in these animals. EE treatment caused a reduction of the maximal biliary secretory rate (S(RM)) of the mrp2 substrate, dibromosulphthalein (DBSP), from 1,040 to 695 nmol/min/100 g body weight (-38%) in NW rats and from 615 to 327 nmol/min/100 g body weight (-46%) in GY/TR- rats. These results demonstrate that inhibition of mrp2 activity and/or biliary glutathione secretion is not the main cause of EE-induced cholestasis in rats. The data indicate that alternative pathways exist for the biliary secretion of bilirubin and related organic anions that are also affected by EE.

Hepatic bile salt flux does not modulate level and activity of the sinusoidal Na+-taurocholate cotransporter (ntcp) in rats

BACKGROUND/AIMS

Efficient uptake at the basolateral plasma membrane of hepatocytes is required for maintenance of the enterohepatic circulation of bile salts. Uptake occurs mainly via a Na+-dependent process mediated by ntcp, a recently cloned and characterized 51 kDa glycoprotein. The aim of this study was to evaluate the role of variations in hepatic bile salt flux through the liver in the regulation of ntcp activity and expression under non-cholestatic conditions.

METHODS

We determined the kinetics of Na+-dependent taurocholate transport in isolated basolateral plasma membrane vesicles as well as hepatic ntcp protein and ntcp mRNA levels in long-term (8 days) bile-diverted rats, with a transhepatic bile salt flux of 0, and in streptozotocin-induced diabetic rats with a 2.5-fold increased bile salt flux.

RESULTS

We found no changes in the kinetics of taurocholate transport in the absence of transhepatic bile salt flux due to bile diversion. Ntcp protein and ntcp mRNA levels were also unaffected in bile-diverted rats. Likewise, no changes in taurocholate transport kinetics, ntcp protein or ntcp mRNA levels were detected in streptozotocin-diabetic rats when compared to non-diabetic controls. Thus, variation in hepatic bile salt flux from 0 to 250% of normal values had no effect on hepatic ntcp expression or taurocholate transport activity in basolateral plasma membrane vesicles in rats. In contrast, 4 days of bile duct ligation resulted in a strong decrease in ntcp mRNA and protein levels, as recently also reported by others.

CONCLUSIONS

Our data indicate that ntcp is not regulated by the transhepatic flux of bile acids under non-cholestatic conditions.

Adenosine triphosphate-dependent copper transport in human liver

BACKGROUND/AIM

The recent cloning and sequencing of the Wilson disease gene indicates that hepatic copper (Cu) transport is mediated by a P-type ATPase. The location of this Cu-transporting protein within the hepatocyte is not known; in view of its proposed function and current concepts of hepatic Cu transport, it may reside in intracellular membranes (endoplasmic reticulum (ER), lysosomes) and/or in the bile canalicular membrane. The objective of this study was to establish characteristics and localization of ATP-dependent Cu transport in human liver.

METHODS

We have investigated Cu transport in vesicles of human liver plasma membranes showing a gradual increase in enrichment of canalicular domain markers: i.e. basolateral liver plasma membranes (blLPM), a mixed population of basolateral and canalicular (XLPM) and canalicular liver plasma membranes (cLPM).

RESULTS

In the presence of ATP (4 mM) and an ATP-regenerating system, uptake of radiolabeled Cu (64Cu, 10 microM) into cLPM vesicles and, to a lesser extent, into blLPM and XLPM was clearly stimulated when compared to control AMP values. Initial uptake rates of ATP-dependent Cu transport were 5.6, 7.8 and 13.7 nmol.min-1.mg-1 protein for blLPM, XLPM and cLPM, respectively, and showed no relationship with marker enzyme activity of ER and lysosomes (glucose-6-phosphatase and acid-phosphatase, respectively). Leucine aminopeptidase activity, as a marker for the cLPM, significantly correlated with ATP-dependent uptake rates measured in different membrane preparations: r = 0.70 (n = 9, p < 0.05). Estimated K(m) and Vmax values of ATP-dependent Cu uptake were 49.5 microM and 36.9 nmol.min-1.mg-1 protein, respectively.

CONCLUSION

This study provides biochemical evidence for the presence of an ATP-dependent Cu transport system in human liver (cCOP), mainly localized at the canalicular domain of the hepatocytic plasma membrane.

Current concepts of hepatic uptake, intracellular transport and biliary secretion of bile acids: physiological basis and pathophysiological changes in cholestatic liver dysfunction

Hepatic sinusoidal uptake of bile acids is mediated by defined carrier proteins against unfavourable concentration and electrical gradients. Putative carrier proteins have been identified using bile acid photoaffinity labels and more recently using immunological probes, such as monoclonal antibodies. At the sinusoidal domain, proteins with molecular weights of 49 and 54 kDa have been shown to be carriers for bile acid transport. The 49 kDa protein has been associated with the Na(+)-dependent uptake of conjugated bile acids, while the 54 kDa carrier has been involved in the Na(+)-independent bile acid uptake process. Within the hepatocyte, cytosolic proteins, such as the glutathione S-transferase (also designated the Y protein), the Y binders and the fatty acid binding proteins, are able to bind bile acids and possibly facilitate their movement to the canalicular domain. At the canalicular domain a 100 kDa carrier protein has been isolated and it has been shown by several laboratories that this particular protein is concerned with canalicular bile acid transport. The system is ATP-dependent and follows Michaelis-Menten kinetics. Interference with bile acid transport has been demonstrated by several chemicals. The mechanisms by which these chemicals inhibit bile acid transport may explain the apparent cholestatic properties observed in patients and experimental animals treated with these agents. Several studies have shown that Na+/K(+)-ATPase activity is markedly decreased in cholestasis induced by ethinyloestradiol, taurolithocholate and chlorpromazine. However, other types of interference have been described and the cholestatic effects may be the result of several mechanisms. Cholestasis is associated with several adaptive changes that may be responsible for the accumulation of bile acids and other cholephilic compounds in the blood of these patients. It may be speculated that the nature of these changes is to protect liver parenchymal cells from an accumulation of bile acids to toxic levels. However, more detailed quantitative experiments are necessary to answer questions with regard to the significance of these changes and the effect of various hepatobiliary disorders in modifying these mechanisms. It is expected that the mechanisms by which bile acid transport is regulated and efforts to understand the molecular basis for these processes will be among the areas of future research.

Fluorescent, short-chain C6-NBD-sphingomyelin, but not C6-NBD-glucosylceramide, is subject to extensive degradation in the plasma membrane: implications for signal transduction related to cell differentiation

The involvement of the plasma membrane in the metabolism of the sphingolipids sphingomyelin (SM) and glucosylceramide (GlcCer) was studied, employing fluorescent short-chain analogues of these lipids, 6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl) amino]hexanoylsphingosylphosphorylcholine (C6-NBD-SM), C6-NBD-GlcCer and their common biosynthetic precursor C6-NBD-ceramide (C6-NBD-Cer). Although these fluorescent short-chain analogues are metabolically active, some caution is to be taken in view of potential changes in biophysical/biochemical properties of the lipid compared with its natural counterpart. However, these short-chain analogues offer the advantage of studying the lipid metabolic enzymes in their natural environment, since detergent solubilization is not necessary for measuring their activity. These studies were carried out with several cell types, including two phenotypes (differing in state of differentiation) of HT29 cells. Degradation and biosynthesis of C6-NBD-SM and C6-NBD-GlcCer were determined in intact cells, in their isolated plasma membranes, and in plasma membranes isolated from rat liver tissue. C6-NBD-SM was found to be subject to extensive degradation in the plasma membrane, due to neutral sphingomyelinase (N-SMase) activity. The extent of C6-NBD-SM hydrolysis showed a general cell-type dependence and turned out to be dependent on the state of cell differentiation, as revealed for HT29 cells. In undifferentiated HT29 cells N-SMase activity was at least threefold higher than in its differentiated counterpart. In contrast, in all cell types studied, very little if any biosynthesis of C6-NBD-SM from the precursor C6-NBD-Cer occurred. Moreover, in the case of C6-NBD-GlcCer, neither hydrolytic nor synthetic activity was found to be associated with the plasma membrane. These results are discussed in the context of the involvement of the sphingolipids SM and GlcCer in signal transduction pathways in the plasma membrane.

Adenosine triphosphate-dependent copper transport in isolated rat liver plasma membranes

The process of hepatobiliary copper (Cu) secretion is still poorly understood: Cu secretion as a complex with glutathione and transport via a lysosomal pathway have been proposed. The recent cloning and sequencing of the gene for Wilson disease indicates that Cu transport in liver cells may be mediated by a Cu transporting P-type ATPase. Biochemical evidence for ATP-dependent Cu transport in mammalian systems, however, has not been reported so far. We have investigated Cu transport in rat liver plasma membrane vesicles enriched in canalicular or basolateral membranes in the presence and absence of ATP (4 mM) and an ATP-regenerating system. The presence of ATP clearly stimulated uptake of radiolabeled Cu (64Cu, 10 microM) into canalicular plasma membrane vesicles and, to a lesser extent, also into basolateral plasma membrane vesicles. ATP-dependent Cu transport was dose-dependently inhibited by the P-type ATPase inhibitor vanadate, and showed saturation kinetics with an estimated Km of 8.6 microM and a Vmax of 6.9 nmol/min/mg protein. ATP-stimulated Cu uptake was similar in canalicular membrane vesicles of normal Wistar rats and those of mutant GY rats, expressing a congenital defect in the activity of the ATP-dependent canalicular glutathione-conjugate transporter (cMOAT). These studies demonstrate the presence of an ATP-dependent Cu transporting system in isolated plasma membrane fractions of rat liver distinct from cMOAT.

Characterization of transport in isolated human hepatocytes. A study with the bile acid taurocholic acid, the uncharged ouabain and the organic cations vecuronium and rocuronium

The uptake and efflux of three categories of substrates were measured in isolated human hepatocytes and compared to those in rat hepatocytes. In addition, the extent to which the in vitro experiments quantitatively reflect liver function in vivo in both species was investigated. The anionic bile acid taurocholic acid was taken up by isolated human hepatocytes at a considerably lower rate than observed in isolated rat hepatocytes. Taurocholic acid uptake both in human hepatocytes and in liver plasma membrane vesicles showed sodium dependency. The uptake rate of taurocholic acid in isolated hepatocytes of both species was quantitatively compatible with the reported liver clearance of the bile acid in vivo. Ouabain uptake rate in isolated human hepatocytes was lower than in rat hepatocytes. This species difference was in accordance with pharmacokinetic studies in vivo on hepatic clearance of ouabain in man and rat. Uptake of vecuronium into human hepatocytes was about a factor of 10 lower than that in rat hepatocytes. Uptake into and efflux from human hepatocytes was comparable for the two short acting muscle relaxants vecuronium and rocuronium. Since distribution to the liver is considered to be a major factor in termination of action of vecuronium and rocuronium these observations were in line with the human pharmacokinetic profiles. In conclusion, the uptake rate of the studied model compounds in human hepatocytes appeared to be lower than that in rat hepatocytes. These observed transport rates reflected the relative hepatic transport rates observed in these species in the intact organism, but the absolute values in both species for some substrates may have been somewhat lower than calculated from in vivo data. It is concluded that transport studies in isolated hepatocytes are suitable for comparative drug transport studies, but are less precise in the prediction of quantitative membrane transport.

Ursodeoxycholate conjugates protect against disruption of cholesterol-rich membranes by bile salts

BACKGROUND/AIMS

Ursodeoxycholic acid attenuates hepatocellular injury in cholestatic disorders, possibly by counteracting membrane disruptive effects of endogenous bile salts. The possible physicochemical basis of this protective effect was explored by using model membranes composed of egg phosphatidylcholine and cholesterol.

METHODS

Large unilamellar vesicles containing trapped 3H inulin were prepared by extrusion and gel filtration. Vesicle disruption (release of trapped inulin) was quantified using rapid centrifugal ultrafiltration.

RESULTS

Disruption of membranes increased with bile salt concentration, hydrophobicity, and increasing ionic strength. Disruption decreased with a decreasing bile salt/phospholipid ratio or an increasing cholesterol/phospholipid ratio. Vesicle disruption by taurodeoxycholate (3 alpha, 12 alpha-dihydroxy-5 beta-cholanoyl taurine) was reduced in a concentration-dependent manner by addition of tauroursodeoxycholate (3 alpha,7 beta-dihydroxy-5 beta-cholanoyl taurine) (TUDC) when the cholesterol/phospholipid ratio was > or = 0.5, but TUDC was not protective at a cholesterol/phospholipid ratio < or = 0.2. Glycoursodeoxycholate (3 alpha,7 beta-dihydroxy-5 beta-cholanoyl glycine) was somewhat less protective than TUDC, and unconjugated ursodeoxycholate (3 alpha,7 beta-dihydroxy-5 beta-cholanoate) (UDC) had little effect. Taurine conjugates of several other hydrophilic bile salts were also protective, but protection was not strictly proportional to hydrophilicity.

CONCLUSIONS

Conjugates of UDC and other hydrophilic bile salts can reduce disruption of cholesterol-rich model membranes by more toxic bile salts via a purely physicochemical mechanism. UDC conjugates in vivo may protect the cholestatic liver by preventing bile salt disruption of the cholesterol-rich canalicular membrane.

Inhibition by cyclosporin A of adenosine triphosphate-dependent transport from the hepatocyte into bile

BACKGROUND

Immunosuppressive treatment with cyclosporin A may be associated with impaired hepatobiliary elimination of bile salts and with cholestasis. Inhibition by cyclosporin A of the primary-active adenosine triphosphate (ATP)-dependent transport systems responsible for excretion of bile salts and cysteinyl leukotrienes across the hepatocyte canalicular membrane into bile may explain the cholestatic side effect.

METHODS

ATP-dependent transport of bile salt and of cysteinyl leukotrienes was studied in human liver plasma membrane vesicles and additionally in rat liver plasma membrane vesicles enriched in canalicular membranes.

RESULTS

Inhibition of ATP-dependent taurocholate transport in human liver by 50% was measured at 3 mumol/L cyclosporin A and at 4 mumol/L fujimycin. Kinetic analyses in rat liver indicated non-competitive inhibition by cyclosporin A with respect to ATP and competitive inhibition with respect to taurocholate with inhibition constant (Ki) values of 1.0 and 0.3 mumol/L, respectively.

CONCLUSIONS

The ATP-dependent export carriers for bile salts and cysteinyl leukotrienes in the hepatocyte canalicular membrane are novel targets for inhibitory side effects of cyclosporin A. Inhibition of ATP-dependent bile salt transport may induce cholestasis.

Adenosine triphosphate-dependent taurocholate transport in human liver plasma membranes

Transport systems involved in uptake and biliary secretion of bile salts have been extensively studied in rat liver; however, little is known about these systems in the human liver. In this study, we investigated taurocholate (TC) transport in canalicular and basolateral plasma membrane vesicles isolated from 15 human livers (donor age 6-64 yr). ATP stimulated the uptake of TC into both canalicular and basolateral human liver plasma membrane vesicles (cLPM and blLPM, respectively). Considerable interindividual variations in the transport velocity were observed in the different membrane preparations used: 9.0 +/- 1.3 (mean +/- SEM, n = 17; range 1.6-18.0) and 9.3 +/- 2.0 (range 1.1-29.8) pmol TC.mg protein-1.min-1 at 1.0 microM TC for cLPM and blLPM, respectively. TC transport was temperature sensitive and showed saturation kinetics with a high affinity for TC (Km 4.2 +/- 0.7 microM and 3.7 +/- 0.5 microM for cLPM and blLPM, respectively). Transport was dependent on the ATP concentration and saturable (Km 0.25 +/- 0.03 mM, n = 3). Neither nitrate, which reduces membrane potential, nor the protonophore FCCP strongly inhibited ATP-dependent TC transport, indicating that membrane potential and proton gradient are not involved in this process. TC transport was significantly inhibited by the classical anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonate (250 microM) and the glutathione conjugate S-(2,4-dinitrophenyl)glutathione (100 microM). In conclusion, high affinity ATP-dependent TC transport is present in human liver at both the canalicular and the basolateral sides of the hepatocyte.