Journal of Hepatology
Volume 31, Issue 3 , Pages 563-573, September 1999

Regulation of organic anion and drug transporters of the sinusoidal membrane

  • Gerd A Kullak-Ublick

      Affiliations

    • Corresponding Author InformationGerd A. Kullak-Ublick, Abtlg. Klinische Pharmakologie, Departement Innere Medizin, Universitätsspital, Rämistrasse 100,CH-8091 Zürich, Switzerland. Tel: 41 1 255 4097. Fax: 41 1 255 4411.

Department of Medicine, Division of Clinical Pharmacology and Toxicology, University Hospital, Zurich, Switzerland

Article Outline

 

The liver plays a key role in the clearance and metabolism of endogenous and xenobiotic lipophilic organic substances. Following uptake into the liver cell, these compounds undergo biotransformation and are subsequently eliminated from the hepatocyte via secretion into the bile canaliculus. Bile formation is driven primarily by the vectorial secretion of bile acids from sinusoidal blood into bile. Numerous transport systems have been identified on the functional and the molecular level which are involved in the basolateral uptake and the canalicular secretion of amphipathic organic substrates across the hepatocyte membrane (Fig. 1). The regulative pathways which modulate the expression and function of the multiple transport systems have been partially elucidated, in particular with respect to alterations of transport protein expression in cholestatic liver disease 1., 2., 3.. Several studies have focussed on the effects of cholestasis on hepatic membrane transport processes, both in experimental rat models and in human cholestatic liver disease. The aim of this review is to summarize the most recent developments concerning the function and regulation of basolateral (sinusoidal) transport proteins involved in the uptake of organic anions and cations into the hepatocyte (Table 1).

  • View full-size image.
  • Fig. 1. 

    Transport proteins of the basolateral (sinusoidal) hepatocyte membrane involved in the hepatocellular uptake of organic anions and cations. Four transporter families have been identified: the Na+-taurocholate cotransporting polypeptide (NTCP), the organic anion transporting polypeptides (OATP), the organic cation transporters (OCT) and the organic anion transporters (OAT). The OATP, OCT and OAT transporters function bidirectionally as antiporters, whereas NTCP and the multidrug resistance proteins (mrp (1–6) transport unidirectionally via primary (mrp) or secondary (NTCP) active transport. In cholestasis, rat Ntcp, Oatp1, mrp2 and the canalicular bile salt export pump (Bsep) are downregulated, whereas mrp1 and mrp3 are upregulated. These regulative mechanisms protect the hepatocyte from the intracellular accumulation of potentially toxic cholephils in cholestatic liver disease. OATP, organic anion transporting polypeptide; OCT, organic cation transporter; OAT, organic anion transporter; mdr, multidrug resistance gene product.

TABLE 1. Molecular and functional characteristics of organic anion and cation transporters of the basolateral hepatocyte membrane
TransporterSpeciesAmino acidsTissue distributionSubstratesPromoter binding sitesExpression in cholestatic liver
Ntcprat362LiverBile acids, conjugated sex steroids, T4Stat 5, HNF1, TATA↓↓
Oatp1rat670Liver, Kidney, BrainBromosulphophthalein, bile acids, conjugated sex steroids, ouabain, APD-ajmalinium, temocaprilat, gadoxetate ↓
Oatp2rat661Liver, Kidney, BrainDigoxin, taurocholate, T4, APD-ajmalinium, BQ123 =
Oatp3rat670Kidney, Liver, Cholangiocyte, Ileum, RetinaTaurocholate, T4
Oat-k1rat669Kidney, LiverMethotrexate
OATPhuman670Liver, Kidney, Brain, Lung, TestisBromosulphophthalein, bile acids, conjugated sex steroids, ouabain, CRC 220, Type II cations: APD-ajmalinium, m-quinidine, m-quinineHNF1,3,4,5, AP1,2, C/EBP, GRE= (↑)
Pgtrat643Liver, Kidney, Brain, Lung, Testis, Stomach, Intestine, Eye, Uterus, HeartPGE, PGF, iloprost, arachidonate
rOCT1rat556Liver, Kidney, IntestineType I cations: TEA, MPP, choline
hOCT1human554LiverType I cations: TEA, MPP, choline
ORCTL2*human424Liver, KidneyCations?
OAT2rat535LiverPara-aminohippurate, dicarboxylates, PGE2, salicylate
OAT3rat536Liver, Kidney, BrainPAH, estrone sulfate, ochratoxin A, cimetidine
MRP1human1531Lung, Bladder, Spleen, Thyroid, Testis, Adrenal gland, IntestineGlutathione-S-conjugates, leukotriene C4, steroid conjugates, glucuronidated/sulfated bile saltsp53, Sp1,CRE, ERE,AP1, GRE,AP2↑
MRP3human1526Liver, Intestine, Adrenal gland N-Myc, AP1, AP2, Sp1(↑)?
MRP6human1503Liver, KidneyBQ123

T4, thyroxine; APD-ajmalinium, N-(4,4-azo-n-pentyl)-21-deoxyajmalinium; TEA, tetraethylammonium; MPP, methyl-4-phenylpyridinium; PGE, prostaglandin E; CRC 220, BQ123: synthetic peptides. *organic cation transporter like protein 2 (see Ref. 128).

Back to Article Outline

Sodium-dependent Hepatocellular Bile Acid Uptake: the Na+-taurocholate Cotransporting Polypeptide (Ntcp) 

Bile acids enter the hepatocyte by sodium-dependent and sodium-independent mechanisms (4). The sodium-dependent transport pathway accounts for more than 80% of taurocholate uptake, but for less than 50% of cholate uptake 5., 6., 7., 8., 9., 10.. Sodium-dependent taurocholate uptake is reduced following partial hepatectomy (11) and in primary hepatocyte cultures (12), is absent in various hepatoma cell lines 13., 14., 15. and is rapidly down-regulated in rat models of cholestasis such as bile duct ligation (16) and endotoxinemia 17., 18.. In postpartum rats and in rats supplemented with the hormone prolactin, transport activity is upregulated (19). Prolactin thus represents the only known physiological stimulator of sodium-dependent bile acid uptake.

On the molecular level, the involved carrier system which mediates sodium-dependent uptake of bile acids into the liver has been identified as the so-called Na+-taurocholate cotransporting polypeptide, Ntcp. This protein has been isolated from rat (Ntcp) (20), human (NTCP) (21) and mouse (22) liver and in human liver represents a 349 amino acid protein. A structurally related protein, called the intestinal bile acid transporter (IBAT), mediates sodium-dependent uptake of bile acids in the ileum and has been isolated from hamster (23), rat (24) and human (25) ileum. The sodium-dependent bile acid transporters are unique within the class of sodium cotransporters in that they only possess seven membrane-spanning domains (26). On a functional level, Ntcp mediates the hepatocellular uptake of conjugated bile acids and to a lesser extent of unconjugated bile acids, selected conjugated steroids such as estrone-3-sulfate (27) and dehydroepiandrosterone sulfate (DHEAS) (Kullak-Ublick et al., unpublished data) and even of the organic anion bromosulphophthalein (BSP) in the absence of albumin (27). Na+-dependent hepatocellular cholate uptake may be mediated in part by microsomal epoxide hydrolase (28). Ntcp transport activity is stimulated by pretreatment of hepatocytes with cAMP (29), presumably due to translocation of Ntcp protein from endosomal compartments to the plasma membrane (30).

Based upon previous studies in experimental rats, which showed a reduction of hepatocellular sodiumdependent taurocholate uptake in cholestasis, the expression of the Ntcp transporter was studied in three established rat models of cholestasis. Ntcp was down-regulated in bile duct ligated rat livers 16., 31., in endotoxinemia secondary to lipopolysaccharide treatment 17., 32. and in ethinyl estradiol induced cholestasis (33). Teleologically, the down-regulation of the chief hepatocellular bile acid uptake system in cholestasis probably serves to protect the hepatocyte from further intracellular accumulation of potentially toxic bile salts in situations in which the canalicular efflux systems for bile acids are impaired (34). In choledochocaval fistula (CCF) rats, which have increased concentrations of biliary constituents in systemic blood, Ntcp mRNA but not protein levels transiently decreased, whereas in bile fistula rats, which have decreased concentrations of biliary constituents, Ntcp mRNA and protein levels were unaltered (35). These results suggested a regulatory effect of the increased transhepatic flux of bile acids in the CCF model on Ntcp gene transcription. In maternal obstructive cholestasis, caused by bile duct ligation on day 14 of pregnancy in rats, the fetal and neonatal expression of Ntcp was not different from controls despite a significant elevation of fetal serum bile acid levels (36). Thus the abnormalities in bile secretory function observed in the offspring of cholestatic mothers is not attributable to the defective hepatic uptake of bile acids, but putatively to reduced canalicular bile acid efflux.

In man, cholestatic liver diseases such as primary biliary cirrhosis or primary sclerosing cholangitis evolve chronically as a result of the gradual reduction in the numbers of intrahepatic or extrahepatic bile ducts. The first study analyzing the expression of NTCP in human cholestatic liver disease reported a significant inverse correlation between NTCP mRNA levels in 23 patients with extrahepatic biliary atresia and plasma bilirubin concentrations (37). These results indicated that NTCP is down-regulated in cholestatic liver disease in man, supporting the experimental findings in the rat models.

The molecular basis for down-regulation of Ntcp/NTCP in cholestasis has not been resolved. A first line of evidence has emerged from sequence analysis of the rat Ntcp gene promoter (38), which contains a sequence element that is homologous to the so-called "bile acid response element" originally identified by Chiang et al. in the cholesterol-7α-hydroxylase gene promoter (39). This bile acid response element confers suppression of gene transcription by hydrophobic bile acids such as taurochenodeoxycholate (40). Bile acids bind to a nuclear receptor for bile acids, recently identified as the farnesoid X receptor (FXR) (40a), which subsequently undergoes a conformational change and suppresses gene transcription. Whether Ntcp down-regulation in cholestasis is indeed attributable to the effect of increased intracellular bile acid concentrations on the Ntcp gene promoter is currently under investigation. Two regulatory mechanisms explaining the down-regulation of Ntcp in endotoxin-induced cholestasis have been identified. First, decreased binding activity of the nuclear transcription factor hepatocyte nuclear factor 1 (HNF1), required for basal Ntcp gene transcription (38), occurs in endotoxinemia (41). Second, the human NTCP promoter is dependent upon the CCAAT/enhancer binding protein, the α-form of which is reduced in sepsis 41a., 41b.. In contrast to Ntcp down-regulation observed in cholestasis, the mechanism of up-regulation in postpartum rats which exhibit increased serum prolactin (PRL) concentrations (19) has recently been elucidated (42). Increased serum PRL levels are associated with an increase in nuclear translocation of phosphorylated liver Stat5 (a member of the ignal ransducers and ctivators of ranscription family). Stat5 binds to IFN-γ-activated sequence (GAS)-like elements located in the promoter region and thereby stimulates Ntcp gene expression.

The established notion that hepatoma cell lines such as HepG2, HTC and H4-II-E have a near absent expression of Ntcp 13., 14., 15., 43. has recently been confounded by in vivo findings in human hepatocellular carcinomas (HCC). A comparative analysis of NTCP expression in HCC and in peri-tumor non-malignant liver tissue indicated that the level of NTCP mRNA in HCC amounted to 56% compared to non-malignant liver tissue (44). Immunofluorescence studies confirmed the expression of NTCP on the surface of HCC cells. These data support the feasibility of liver-specific drug targeting strategies for the treatment of HCC which are based upon the covalent coupling of cytostatic agents to bile acids (45).

Back to Article Outline

Multispecific Hepatocellular Bile Acid and Amphipathic Substrate Uptake: the Organic Anion Transporting Polypeptides (Oatp) 

Unlike sodium-dependent bile acid uptake, the sodium-independent uptake of bile acids cannot be attributed to the function of a single transport system. Uptake of unconjugated bile acids is mediated by several transport systems that have been identified on a functional level in basolateral plasma membrane vesicles. These include a sulphate/anion exchanger (46), a dicarboxylate/cholate anion exchanger (47) and arguably an OH−/cholate anion exchanger 10., 48., 49.. An important feature of the sodium-independent uptake of bile acids and other organic anions into the hepatocyte is the multispecificity of this transport pathway. Different experimental models such as the isolated perfused rat liver, primary rat hepatocytes and basolateral plasma membrane vesicles have been used to characterize the sodium-independent uptake of organic anions. The spectrum of substrates which share this transport route includes conjugated and unconjugated bile acids 50., 51., bromosulphophthalein (BSP), DIDS 50., 52., 53., 54., 55., cardiac glycosides and other neutral steroids 56., 57., 58., linear and cyclic peptides 50., 51., 59., selected organic cations 57., 58. and numerous other drugs such as pravastatin (60). In view of this broad spectrum of transported substrates, the involved transport system was termed the "multispecific bile acid transporter" 50., 51..

Expression cloning strategies in Xenopus laevis oocytes subsequently led to the identification of a novel transporter family of "organic anion transporting polypeptides" or Oatps, several of which are also expressed in the hepatocyte (Table 1). In rats, five members of this transporter family have been identified. Oatp1 and Oatp2 show a strong level of expression in the liver and transport bile acids 61., 62.. Both Oatp1 (63) and Oatp2 (64) have been localized to the basolateral hepatocyte membrane. In contrast to Oatp1, which is expressed homogeneously across the liver acinus (65), Oatp2 is expressed predominantly in perivenous hepatocytes (65a). Oatp1 and Oatp2 are also expressed in the choroid plexus epithelium of the brain, where Oatp1 has been localized to the apical plasma membrane (66), whereas Oatp2 has been detected in the basolateral domain (67). The thyroid hormone and bile acid transporter Oatp3 and the methotrexate carrier Oat-k1 are predominantly expressed in the kidney and only have a low level of expression in the hepatocyte 68., 69.. The rat prostaglandin transporter rPgt is a homologue of the Oatp transporter family and exhibits 37% amino acid identity with Oatp1 (70). Pgt occurs ubiquitously in numerous tissues including the liver and Xenopus laevis oocytes injected with rat Pgtor human PGT-cRNA take up PGE, PGF, iloprost and arachidonate sodium independently 70., 71.. Although these functional data suggest that Pgt probably represents a transmembrane transport protein, the membrane localization of Pgt has yet to be confirmed. In man, only one representative of the OATP family has been identified (72). Human OATP is expressed predominantly in the brain, with additional expression in the liver and residual expression in the hepatoblastoma cell line HepG2 (15). In three well-differentiated hepatocellular carcinomas, OATP protein levels were normal compared to non-malignant liver tissue (44). OATP transports organic anions such as bile acids, BSP and the sulfated steroid dehydroepiandrosterone-3-sulfate (73) and also type II cations (74).

The first representative of the Oatp family that was characterized with respect to transport function was Oatp1 (27). The spectrum of substrates transported via Oatp1 ranges from bromosulphophthalein and bile acids 61., 75., 76. to ouabain, estrone-3-sulfate and estradiol-17β-glucuronide 77., 78., the synthetic peptide CRC 220 (79), the mycotoxin ochratoxin A (80), the angiotensin converting enzyme inhibitor temocaprilat (81), and even the organic cation N-(4,4-azo-n-pentyl)-21-deoxyajmalinium or APD-ajmalinium (77). The liver-specific magnetic resonance imaging contrast agent Gd-EOB-DTPA (gadoxetate) appears to be a specific Oatp1 substrate (Km ∼3.3 mmol/l) (82). When expressed in Xenopus laevis oocytes, Oatp1 mediates the uptake of leukotriene C4 and S-dinitrophenyl glutathione and the efflux of reduced glutathione (GSH) (83). Moreover, taurocholate uptake is trans-stimulated by GSH, with a stoichiometry of GSH/taurocholate exchange of 1:1. Oatp1 therefore represents a sinusoidal GSH efflux system.

Oatp2, which is also highly expressed in rat liver, exhibits a partially overlapping substrate specificity, but also transports digoxin (62). Compared to the Oatp1 and Oatp2 of rat liver, the human liver OATP is a less efficient organic anion carrier (27), but appears to be a major transport system for type II organic cations such as APD-ajmalinium, rocuronium, N-methyl-quinine (Km ∼5 μmol/l) and N-methyl-quinidine (Km ∼26 μmol/l) (74). In conjunction with the recently cloned liver-specific organic cation transporter hOCT1 of human liver 84., 85., which mediates the hepatic clearance of small type I cations such as tetraethylammonium (TEA), and the OAT family of transport proteins (83a), the overall transport activity of OATP, hOCT1 and the OATs could account for the bulk of organic cation and drug disposition by human liver (86).

The Oatp transporters appear to function as anion exchangers, mediating not only the hepatocellular uptake but also the efflux of the organic anions BSP and taurocholate across the basolateral membrane into sinusoidal blood 87., 88.. The regulation of Oatp is less well understood than the regulation of the sodium-dependent bile acid transporter Ntcp. Ontogenetically, the Oatp1 mRNA first appears in the liver on day 16 of gestation (65), 4–5 days earlier than Ntcp (43). The Oatp1 mRNA levels remain constant during the perinatal period and increase only after weaning. This delayed expression of the organic anion transporter in the perinatal period could in part explain the immaturity of bile formation and the physiological neonatal cholestasis.

In bile duct ligation, the expression of the Oatp1 mRNA is strongly down-regulated after 1 day but recovers to a certain extent by day 3 (31). However, sodium-independent bile acid uptake into the liver does not seem to be significantly affected by this loss of Oatp1 expression (89), suggesting that other transporters compensate for the down-regulation of Oatp1. In contrast, Oatp2 expression is unaffected by bile duct ligation (90). During liver regeneration following partial hepatectomy, both Oatp1 and Oatp2 are reduced to 50% on the mRNA level after 1 and 4 days, respectively, returning to normal by day 7 and 14, respectively (91). Ntcp mRNA levels also decline to 50% after 1 day. These changes in the expression levels of the basolateral bile acid transport systems may explain the cholestasis that occurs during early liver regeneration. In endotoxin-induced cholestasis, the basolateral transport of the organic anions BSP and sulfolithocholyl-taurine has been shown to be reduced by 40–55% (18) secondary to lipopolysaccharide (LPS) administration. The third model of cholestasis, ethinyl estradiol treatment, decreases hepatic Oatp1 mRNA levels to 15% of controls by day 5 (33). In contrast, testosterone treatment significantly upregulates Oatp1 expression, but only in the kidney (92). In man, hepatic OATP mRNA steady-state levels were moderately increased in four patients with primary sclerosing cholangitis, a chronic cholestatic liver disease, compared to three non-cholestatic controls (93). The discrepancy noted between the down-regulation of Oatp1 and the lack of change of Oatp2 expression in bile duct ligation could be attributable to different regulatory elements in the corresponding gene promoters. Because the Oatps function as bidirectional transporters, the preserved expression of rat Oatp2 and human OATP during cholestasis could serve to enhance the sinusoidal efflux of potentially toxic bile salts in situations in which the canalicular efflux systems for organic anions, such as MRP2, are impaired 94., 95..

On the molecular level, the promoter elements of the human OATP gene have been isolated and partially characterized (93). A silencer element was located in the −662/−440 nucleotide region relative to the transcription initiation site. This silencer element was functional in the liver cell lines HepG2 and Chang liver and in CHO-K1 cells, but not in the kidney cell line MDCK. In HepG2 cells transfected with OATP promoter constructs, promoter activity was stimulated approximately 1.6-fold by 100 μmol/l of the bile salt taurocholate. Since intraparenchymal taurocholate concentrations are increased in cholestatic liver (96), OATP could function as a sinusoidal overflow system in chronic cholestatic conditions. A comparable effect of taurocholate on gene transcription was reported in a recent study on the induction of the ileal bile acid transporter (ibat) mRNA in rat ileum by taurocholate and cholate (97). Although the promoter elements of the ibat gene have not been fully characterized, it seems likely that a specific element which mediates stimulation of gene transcription by taurocholate is present. Whether this element is identical with the putative bile acid responsive OATP promoter element remains to be elucidated.

Because the Oatp family of transporters does not mediate the uptake of para-aminohippurate (PAH) and dicarboxylates (75), a class of organic anions for which carrier-mediated basolateral uptake has been demonstrated in vesicle studies 46., 47., expression cloning in oocytes was again employed to identify the PAH transporter of rat kidney. A 2.227 kb cDNA clone was isolated, coding for ROAT1, a novel renal organic anion transporter (98). ROAT1 mediates basolateral uptake of PAH and 2-oxoglutarate in renal tubule cells. This transport pathway is stimulated by inhibitors of protein kinase C (PKC) in killifish renal proximal tubules and is inhibited by the PKC agonist phorbol 12-myristate 13-acetate (PMA), indicating a negative correlation with PKC activity (99). However, the extracellular signals that initiate the PKC signal cascade are unknown. Although ROAT1 is not expressed in the liver, a protein with 48% similarity and 38% identity called "novel liver-specific transport protein" (NLT) or OAT2 is predominantly expressed in rat liver (100). OAT2 is presumably localized on the basolateral hepatocyte surface and mediates the uptake of the organic anions PAH, dicarboxylates and PGE2 (100). Most recently, a third member of the OAT family, OAT3, was isolated from rat brain (100a). OAT3 is expressed predominantly in the liver, has 42% amino acid identity with OAT2, and transports PAH, estrone-3-sulfate, ochratoxin A and cimetidine. The mechanisms governing the regulation of the OATs have as yet to be defined.

Back to Article Outline

Multidrug Resistance Protein (MRP) Mediated Organic Anion Transport 

Six multidrug resistance-associated proteins (MRP's) have been identified, each with a distinct pattern of tissue-specific expression (101). MRP1 was originally cloned from multidrug-resistant human lung cancer cells and was identified as an integral membrane glycoprotein of 190 kD belonging to the superfamily of ATP-binding cassette (ABC) transporters (102). The physiological function of MRP1 is the ATP-dependent export of glutathione S-conjugates (103), leukotriene C4, steroid conjugates such as 17β-glucuronosyl-estradiol and glucuronidated or sulfated bile salt conjugates (104). The level of MRP1 in normal hepatocytes is very low (3), but expression levels are increased in human hepatoblastoma HepG2 cells and SV40 large T antigen-immortalized human hepatocytes (105). Importantly, the subcellular localization of MRP1 is the lateral hepatocyte membrane (105). The canalicular membrane possesses a homologue of MRP1 with a qualitatively similar substrate specificity, known as MRP2 or the canalicular multispecific organic anion transporter (cMOAT) (106). Human MRP2 is highly and exclusively expressed in the liver and is 46% identical with MRP1. Its selective absence from the canalicular membrane is the pathogenetic basis of the Dubin-Johnson syndrome 107., 108..

In addition to MRP1 and MRP2, four other MRP homologues have so far been identified by screening databases of human expressed sequence tags (101). MRP3 transports sulfated and glucuronidated bile salts, is localized in the basolateral hepatocyte membrane (109) and has a strong level of expression in the liver (101). Moreover, it appears to be upregulated in both the Dubin-Johnson syndrome and in cholestasis and could mediate the efflux of MRP2 substrates into the sinusoidal blood plasma in situations in which MRP2 is absent or downregulated (109). MRP5, also localized in the basolateral hepatocyte membrane, is expressed in the liver and in other tissues 101., 110.. In HepG2 cells, the expression of MRP1, MRP2 and MRP3 is enhanced by phenobarbital in a concentration-dependent manner (111).

Another member of the MRP family which is highly expressed in the liver is MRP6, the rat (112) and human (113) homologues of which have been identified. Rat mrp6 is located in the lateral hepatocyte membrane and mediates the ATP-dependent transport of the endothelin ETA receptor antagonist BQ123 (112). This cyclic peptide is also taken up by mrp2 expressing vesicles isolated from baculovirus infected Sf9 insect cells and by Oatp2-cRNA injected Xenopus laevis oocytes (Km ∼30 μmol/l) (64). No physiological substrate for MRP6 has yet been identified.

The regulatory mechanisms governing the expression of the MRP transporters have been partly characterized. In endotoxin-induced cholestasis, which produces a strong downregulation of canalicular mrp2 and decreased mRNA and protein levels of the canalicular bile salt export pump (Bsep) in rat liver (114), mrp1 mRNA and protein levels are increased (115). Thus both mrp1 and mrp3 appear to compensate for the decreased transport activity of mrp2 in cholestasis by enhancing the efflux of organic anions into the sinusoidal space. Isolation of the promoter region of the MRP1 gene (116) has identified consensus binding sites for numerous transcription factors including the activator proteins AP1 and AP2, Sp1, and glucocorticoid response elements. Transfections using 5′-deleted constructs of the MRP1 promoter fused to a luciferase reporter gene showed that the wild-type tumor suppressor gene p53 suppressed the MRP1 promoter and that the p53-responsive element resided within the −91/+103 nucleotide region (117). Conversely, cotransfection of the MRP1 promoter with an Sp1 expression vector increased promoter activity up to approximately 200-fold, an effect that was attenuated by the additional cotransfection of a wild-type p53 expression plasmid. Thus loss of wild-type p53 function and/or an increase in Sp1 activity could, at least in tumor cells, upregulate the MRP1 gene. The recent cloning of the human MRP3 promoter will reveal whether similar regulatory mechanisms govern the expression of the MRP3 gene (118).

The short-term regulation of MRP1 transport activity is controlled by changes in the degree of phosphorylation of the protein (119). Phosphorylation is mediated by protein kinase C (PKC) isoforms and inhibition of PKC reduces both MRP1 phosphorylation and transport activity in the leukemia cell line HL60/ADR (120). Further studies are required to determine the role of PKC-dependent mechanisms in regulating MRP-mediated organic anion efflux from hepatocytes.

Back to Article Outline

Basolateral Organic Cation Transport 

Organic cation transporters in the liver are critical in the uptake and elimination of many endogenous amines and a wide array of pharmaceuticals. Many therapeutic drugs and environmental toxins are organic cations or bases that are protonated at physiologic pH. Several of the pathways which mediate the sinusoidal uptake of endogenous cations, such as dopamine, choline, noradrenaline or serotonin, are shared by exogenous cations (86).

The first member of a novel organic cation transporter family called SLC22 (121) was isolated from a rat kidney cDNA library by expression cloning in Xenopus laevis oocytes, using uptake of tetraethylammonium (TEA) as a screening assay (122). This transporter, called rOCT1, has been identified as a 67 kD sinusoidal transporter by immunoblotting (123) and mediates polyspecific electrogenic transport of the small type I organic cations TEA, 4-methyl-4-phenylpyridinium (MPP), choline, dopamine and noradrenaline. In contrast, larger type II cations such as vecuronium, quinine and quinidine are high affinity inhibitors of rOCT1 (124). These type II cations are transported by certain members of the Oatp organic anion transporter family, in particular by the human OATP (74). The sinusoidal transport systems for both type I and type II cations have been shown to function as proton-antiport systems (125). The 1.882 kb rOCT1 mRNA is most abundant in kidney, liver, colon and small intestine, whereas its 1.885 kb human homologue, hOCT1, is expressed predominantly in the liver 84., 85..

A protein homologous with the OCT family, called Orct, which exhibits approximately 30% amino acid identity (123), was isolated from murine liver and has recently been shown to transport TEA and choline (126). A 1.8 kb Orct homologue was found to be abundantly expressed in human hepatoblastoma HepG2 cells and presumably represents an uptake system for TEA and choline in this cell line (127). OCT1 also exhibits 36–38% identity with the hepatocellular organic anion transporters OAT2 and OAT3, recently shown to mediate transport of para-aminohippurate in the oocyte expression system 100., 100a..

Back to Article Outline

Conclusions 

Recent years have seen the identification of a rapidly increasing number of distinct transport proteins from a limited number of transporter families. At the level of the sinusoidal hepatocyte membrane these can be divided into the family of Na+-taurocholate cotransporting polypeptides (Ntcp), organic anion transporting polypeptides (Oatp), organic anion transporters (OAT), the organic cation transporter family (OCT/Orct) and the multidrug resistance proteins (mrp). The best-characterized models to study the regulation of hepatocellular transport proteins have been the models of cholestasis. The major uptake system of the hepatocyte for conjugated bile acids, Ntcp, is down-regulated in all rat models of cholestasis as well as in human cholestatic liver disease. This regulatory mechanism is probably attributable to a bile acid responsive sequence element in the Ntcp gene promoter, which confers suppression of gene expression by hydrophobic bile acids. In cholestasis, overall hepatic extraction of bile acids from the blood circulation is therefore reduced to the extent to which canalicular bile acid secretion is impaired. In contrast to the sodium-dependent bile acid uptake pathway, the sodium-independent uptake of bile acids is mediated by a multitude of closely-related transporters, several of which are members of the Oatp transporter family. It seems likely that not all Oatp carriers are regulated in an identical fashion, especially in view of the marked differences in tissue distribution which have been observed. While the Oatp1 of rat liver is down-regulated in the acute models of cholestasis, Oatp2 expression is unaltered. Certain members of the MRP family appear to be upregulated in endotoxin-induced cholestasis in rats and possibly in the Dubin-Johnson syndrome in man. These long-term regulative mechanisms ultimately reflect the hepatocyte's attempt to prevent the intracellular accumulation of bile acids and other potentially toxic cholephils in chronic cholestatic conditions. The implications for the hepatologist are only beginning to emerge and will require further analysis of the transporters acting in human liver and of the regulative adaptations that are operative in liver disease.

Back to Article Outline

Acknowledgements 

The author wishes to thank Peter J. Meier for critical reading of the manuscript. He also thanks his collaborators from the Zurich and Munich transport groups: Peter J. Meier, Gustav Paumgartner, Bruno Stieger, Bruno Hagenbuch, Ulrich Beuers, Monika Oswald, Manfred Ismair, and Thomas Fisch. This work was supported by grant KU 899/5–2 from the Deutsche Forschungsgemeinschaft.

Back to Article Outline

References 

  1. Trauner M, Meier PJ, Boyer JL. Molecular pathogenesis of cholestasis. N Engl J Med. 1998;339:1217–1227
  2. Trauner M, Meier PJ, Boyer JL. Molecular regulation of hepatocellular transport systems in cholestasis. J Hepatol. 1999;31:165–178
  3. Müller M, Jansen PLM. The secretory function of the liver: new aspects of hepatobiliary transport. J Hepatol. 1998;28:344–354
  4. Meier PJ. Molecular mechanisms of hepatic bile salt transport from sinusoidal blood into bile. Am J Physiol. 1995;269:G801–G812
  5. Anwer MS, Hegner D. Effect of Na+ on bile acid uptake by isolated rat hepatocytes: evidence for a heterogeneous system. Hoppe-Seyler's Z Physiol Chem. 1978;359:181–192
  6. Scharschmidt BF, Stephens JE. Transport of sodium, chloride and taurocholate by cultured rat hepatocytes. Proc Natl Acad Sci USA. 1981;78:986–990
  7. Yamazaki M, Suzuki H, Hanano M, Sugiyama Y. Different relationships between cellular ATP and hepatic uptake among taurocholate, cholate, and organic anions. Am J Physiol. 1993;264:G693–G701
  8. Boelsterli UA, Zimmerli B, Meier PJ. Identification and characterization of a basolateral dicarboxylate-cholate antiport system in rat hepatocytes. Am J Physiol. 1995;268:G797–G805
  9. Kouzuki H, Suzuki H, Ito K, Ohashi R, Sugiyama Y. Contribution of sodium taurocholate co-transporting polypeptide to the uptake of its possible substrates into rat hepatocytes. J Pharmacol Exp Ther. 1998;286:1043–1050
  10. Caflisch C, Zimmerli B, Reichen J, Meier PJ. Cholate uptake in basolateral rat liver plasma membrane vesicles and in liposomes. Biochim Biophys Acta. 1990;1021:70–76
  11. Green RM, Gollan JL, Hagenbuch B, Meier PJ, Beier D. Regulation of hepatocyte bile salt transporters during hepatic regeneration. Am J Physiol. 1997;273:G621–G627
  12. Liang D, Hagenbuch B, Stieger B, Meier PJ. Parallel decrease of Na+-taurocholate cotransport and its encoding mRNA in primary cultures of rat hepatocytes. Hepatology. 1993;18:1162–1166
  13. von Dippe P, Levy D. Expression of the bile acid transport protein during liver development and in hepatoma cells. J Biol Chem. 1990;265:5942–5945
  14. Blumrich M, Zeyen-Blumrich U, Pagels P, Petzinger E. Immortalization of rat hepatocytes by fusion with hepatoma cells. Studies on the transport and synthesis of bile acids in hepatocytoma (HPCT) cells. Eur J Cell Biol. 1994;64:339–347
  15. Kullak-Ublick GA, Beuers U, Paumgartner G. Molecular and functional characterization of bile acid transport in human hepatoblastoma HepG2 cells. Hepatology. 1996;23:1053–1060
  16. Gartung C, Ananthanarayanan M, Rahman MA, Schuele S, Nundy S, Soroka CJ, et al.  Down-regulation of expression and function of the rat liver Na+/bile acid cotransporter in extrahepatic cholestasis. Gastroenterology. 1996;110:199–209
  17. Green RM, Beier D, Gollan JL. Regulation of hepatocyte bile salt transporters by endotoxin and inflammatory cytokines in rodents. Gastroenterology. 1996;111:193–198
  18. Bolder U, Ton-Nu H-T, Schteingart CD, Frick E, Hofmann AF. Hepatocyte transport of bile acids and organic anions in endotoxemic rats: impaired uptake and secretion. Gastroenterology. 1997;112:214–225
  19. Ganguly TC, Liu Y, Hyde JF, Hagenbuch B, Meier PJ, Vore M. Prolactin increases hepatic Na+/taurocholate cotransport activity and messenger RNA post partum. Biochem J. 1994;303:33–36
  20. Hagenbuch B, Stieger B, Foguet M, Luebbert H, Meier PJ. Functional expression cloning and characterization of the hepatocyte Na+/bile acid cotransport system. Proc Natl Acad Sci USA. 1991;88:10629–10633
  21. Hagenbuch B, Meier PJ. Molecular cloning, chromosomal localization, and functional characterization of a human liver Na+/bile acid cotransporter. J Clin Invest. 1994;93:1326–1331
  22. Cattori V, Eckhardt U, Hagenbuch B. Molecular cloning and functional characterization of two alternatively spliced Ntcp isoforms from mouse liver. Biochim Biophys Acta. 1999;1445:154–159
  23. Wong MH, Oelkers P, Craddock AL, Dawson PA. Expression cloning and characterization of the hamster ileal sodium-dependent bile acid transporter. J Biol Chem. 1994;269:1340–1347
  24. Shneider BL, Dawson PA, Christie D-M, Hardikar W, Wong MH, Suchy FJ. Cloning and molecular characterization of the ontogeny of a rat ileal sodium-dependent bile acid transporter. J Clin Invest. 1995;95:745–754
  25. Craddock AL, Love MW, Daniel RW, Kirby LC, Walters HC, Wong MH, et al.  Expression and transport properties of the human ileal and renal sodium-dependent bile acid transporter. Am J Physiol. 1998;274:G157–G169
  26. Wright EM, Hager KM, Turk E. Sodium cotransport proteins. Curr Opin Cell Biol. 1992;4:696–702
  27. Meier PJ, Eckhardt U, Schroeder A, Hagenbuch B, Stieger B. Substrate specificity of sinusoidal bile acid and organic anion uptake systems in rat and human liver. Hepatology. 1997;26:1667–1677
  28. von Dippe P, Amoui M, Stellwagen RH, Levy D. The functional expression of sodium-dependent bile acid transport in Madin Darby Canine Kidney Cells transfected with the cDNA for microsomal epoxide hydrolase. J Biol Chem. 1996;271:18176–18180
  29. Grüne S, Engelking LR, Anwer MS. Role of intracellular calcium and protein kinases in the activation of hepatic Na+/taurocholate cotransport by cyclic AMP. J Biol Chem. 1993;268:17734–17741
  30. Mukhopadhayay S, Ananthanarayanan M, Stieger B, Meier PJ, Suchy FJ, Anwer MS. cAMP increases liver Na+-taurocholate cotransport by translocating transporter to plasma membranes. Am J Physiol. 1997;273:G842–G848
  31. Dumont M, Jacquemin E, D'Hont C, Descout C, Cresteil C, Haouzi D, et al.  Expression of the liver Na+-independent organic anion transporting polypeptide (oatp-1) in rats with bile duct ligation. J Hepatol. 1997;27:1051–1056
  32. Moseley RH, Wang W, Takeda H, Lown K, Shick L, Ananthanarayanan M, et al.  Effect of endotoxin on bile acid transport in rat liver: a potential model for sepsis-associated cholestasis. Am J Physiol. 1996;271:G137–G146
  33. Simon FR, Fortune J, Iwahashi M, Gartung C, Wolkoff AW, Sutherland E. Ethinyl estradiol cholestasis involves alterations in expression of liver sinusoidal transporters. Am J Physiol. 1996;271:G1043–G1052
  34. Bossard R, Stieger B, O'Neill B, Fricker G, Meier PJ. Ethinylestradiol treatment induces multiple canalicular membrane transport alterations in rat liver. J Clin Invest. 1993;91:2714–2720
  35. Gartung C, Schuele S, Schlosser SF, Boyer JL. Expression of the rat liver Na+/taurocholate cotransporter is regulated in vivo by retention of biliary constituents but not their depletion. Hepatology. 1997;25:284–290
  36. Arrese M, Trauner M, Ananthanarayanan M, Boyer JL, Suchy FJ. Maternal cholestasis does not affect the ontogenic pattern of expression of the Na+/taurocholate cotransporting polypeptide (ntcp) in the fetal and neonatal rat liver. Hepatology. 1998;28:789–795
  37. Shneider BL, Fox VL, Schwarz KB, Watson CL, Ananthanarayanan M, Thevananther S, et al.  Hepatic basolateral sodiumdependent bile acid transporter expression in two unusual cases of hypercholanemia and in extrahepatic biliary atresia. Hepatology. 1997;25:1176–1183
  38. Karpen SJ, Sun A-Q, Kudish B, Hagenbuch B, Meier PJ, Ananthanarayanan M, et al.  Multiple factors regulate the rat liver basolateral sodium-dependent bile acid cotransporter gene promoter. J Biol Chem. 1996;271:15211–15221
  39. Chiang JYL, Stroup D. Identification and characterization of a putative bile acid-responsive element in cholesterol 7α-hydroxylase gene promoter. J Biol Chem. 1994;269:17502–17507
  40. Stroup D, Crestani M, Chiang JYL. Identification of a bile acid response element in the cholesterol 7α-hydroxylase gene CYP7A. Am J Physiol. 1997;273:G508–G517
  41. Makishima M, Okamoto AY, Repa JJ, Tu H, Learned RM, Luk A, et al.  Identification of a nuclear receptor for bile acids. Science. 1999;284:1362–1365
  42. Trauner M, Arrese M, Lee H, Boyer JL, Karpen SJ. Endotoxin downregulates rat hepatic ntcp gene expression via decreased activity of critical transcription factors. J Clin Invest. 1998;101:2092–2100
  43. McClure MH, Denson LA, Ananthanarayanan M, Suchy FJ, Hardikar W, Karpen SJ. CCAAT/Enhancer Binding Protein α (C/EBPα) and Hepatocyte Nuclear Factor 3β (HNF3β) transactivate the human hepatic Na+/taurocholate cotransporter (NTCP). [abstract] Hepatology. 1998;28:428A
  44. Yin M, Yang SQ, Lin HZ, Lane MD, Chatterjee S, Diehl AM. Tumor necrosis factor α promotes nuclear localization of cytokine-inducible CCAAT/Enhancer Binding Protein isoforms in hepatocytes. J Biol Chem. 1996;271:17974–17978
  45. Ganguly TC, O'Brien ML, Karpen SJ, Hyde JF, Suchy FJ, Vore M. Regulation of the rat liver sodium-dependent bile acid cotransporter gene by prolactin. J Clin Invest. 1997;99:2906–2914
  46. Boyer JL, Hagenbuch B, Ananthanarayanan M, Suchy F, Stieger B, Meier PJ. Phylogenic and ontogenic expression of hepatocellular bile acid transport. Proc Natl Acad Sci USA. 1993;90:435–438
  47. Kullak-Ublick GA, Glasa J, Boeker C, Oswald M, Gruetzner U, Hagenbuch B, et al.  Chlorambucil-taurocholate is transported by bile acid carriers expressed in human hepatocellular carcinomas. Gastroenterology. 1997;113:1295–1305
  48. Kramer W, Wess G, Schubert G, Bickel M, Girbig F, Gutjahr U, et al.  Liver-specific drug targeting by coupling to bile acids. J Biol Chem. 1992;267:18598–18604
  49. Hugentobler G, Meier PJ. Multispecific anion exchange in basolateral (sinusoidal) rat liver plasma membrane vesicles. Am J Physiol. 1986;251:G656–G664
  50. Zimmerli B, O'Neill B, Meier PJ. Identification of sodium-dependent and sodium-independent dicarboxylate transport systems in rat liver basolateral membrane vesicles. Pflügers Arch. 1992;421:329–335
  51. Blitzer BL, Terzakis C, Scott KA. Hydroxyl-bile acid exchange: a new mechanism for the uphill transport of cholate by basolateral liver plasma membrane vesicles. J Biol Chem. 1986;261:12042–12046
  52. Veith CM, Thalhammer T, Felberbauer FX, Graf J. Relationship of hepatic cholate transport to regulation of intracellular pH and potassium. Biochim Biophys Acta. 1992;1103:51–61
  53. Frimmer M, Ziegler K. The transport of bile acids in liver cells. Biochim Biophys Acta. 1988;947:75–99
  54. Petzinger E. Transport of organic anions in the liver. An update on bile acid, fatty acid, monocarboxylate, anionic amino acid, cholephilic organic anion, and anionic drug transport. Rev Physiol Biochem Pharmacol. 1994;123:47–211
  55. Anwer MS, Hegner D. Effect of organic anions on bile acid uptake by isolated rat hepatocytes. Hoppe-Seyler's Z Physiol Chem. 1978;359:1027–1030
  56. Elsner RH, Ziegler K. Radiation inactivation of multispecific transport systems for bile acids and xenobiotics in basolateral rat liver plasma membrane vesicles. J Biol Chem. 1992;267:9788–9793
  57. Wehner F, Rosin-Steiner S, Beetz G, Sauer H. The anion transport inhibitor DIDS increases rat hepatocyte K+ conductance via uptake through the bilirubin pathway. J Physiol Lond. 1993;471:617–635
  58. Ishii K, Wolkoff AW. Inhibition of rat hepatocyte organic anion transport by bile acids. Am J Physiol. 1994;267:G458–G464
  59. Eaton DL, Klaassen CD. Carrier-mediated transport of ouabain in isolated hepatocytes. J Pharmacol Exp Ther. 1978;205:480–488
  60. Okudaira K, Yamazaki M, Sawada Y, Sugiyama Y, Jga T, Hanano M. Correlation between the inhibitory effects of basic drugs on the uptake of cardiac glycosides and taurocholate by isolated rat hepatocytes. Pharm Res. 1992;9:1152–1156
  61. Steen H, Merema M, Meijer DKF. A multispecific uptake system for taurocholate, cardiac glycosides and cationic drugs in the liver. Biochem Pharmacol. 1992;44:2323–2331
  62. Ziegler K, Lins W, Frimmer M. Hepatocellular transport of cyclosomatostatins: evidence for a carrier system related to the multispecific bile acid transporter. Biochim Biophys Acta. 1991;1061:287–296
  63. Ziegler K, Stuenkel W. Tissue-selective action of pravastatin due to hepatocellular uptake via a sodium-independent bile acid transporter. Biochim Biophys Acta. 1992;1139:203–209
  64. Jacquemin E, Hagenbuch B, Stieger B, Wolkoff AW, Meier PJ. Expression cloning of a rat liver Na+-independent organic anion transporter. Proc Natl Acad Sci USA. 1994;91:133–137
  65. Noé B, Hagenbuch B, Stieger B, Meier PJ. Isolation of a multispecific organic anion and cardiac glycoside transporter from rat brain. Proc Natl Acad Sci USA. 1997;94:10346–10350
  66. Bergwerk AJ, Shi X, Ford AC, Kanai N, Jacquemin E, Burk RD, et al.  Immunologic distribution of an organic anion transport protein in rat liver and kidney. Am J Physiol. 1996;271:G231–G238
  67. Reichel C, Gao B, Cattori V, Landmann L, Sugiyama Y, Stieger B, et al.  Heterogenous expression of the polyspecific organic anion transporter Oatp2 in rat liver and its identification as a cyclic peptide transporter. [abstract] Hepatology. 1998;28:425A
  68. Dubuisson C, Cresteil D, Desrochers M, Decimo D, Hadchouel M, Jacquemin E. Ontogenic expression of the Na+-independent organic anion transporting polypeptide (oatp) in rat liver and kidney. J Hepatol. 1996;25:932–940
  69. Kakyo M, Sakagami H, Nishio T, Nakai D, Nakagomi R, Tokui T, et al.  Immunohistochemical distribution and functional characterization of an organic anion transporting polypeptide 2 (oatp2). FEBS Lett. 1999;445:343–346
  70. Angeletti RH, Novikoff PM, Juvvadi SR, Fritschy J-M, Meier PJ, Wolkoff AW. The choroid plexus epithelium is the site of the organic anion transport protein in the brain. Proc Natl Acad Sci USA. 1997;94:283–286
  71. Gao B, Stieger B, Noe B, Fritschy J-M, Meier PJ. Brain capillary endothelium and choroid plexus epithelium express identical multispecific organic anion and drug transporters. J Histochem Cytochem. 1999; in press
  72. Abe T, Kakyo M, Sakagami H, Tokui T, Nishio T, Tanemoto M, et al.  Molecular characterization and tissue distribution of a new organic anion transporter subtype (oatp3) that transports thyroid hormones and taurocholate and comparison with oatp2. J Biol Chem. 1998;273:22395–22401
  73. Saito H, Masuda S, Inui K-I. Cloning and functional characterization of a novel rat organic anion transporter mediating basolateral uptake of methotrexate in the kidney. J Biol Chem. 1996;271:20719–20725
  74. Kanai N, Lu R, Satriano JA, Bao Y, Wolkoff AW, Schuster VL. Identification and characterization of a prostaglandin transporter. Science. 1995;268:866–869
  75. Lu R, Kanai N, Bao Y, Schuster VL. Cloning, in vitro expression, and tissue distribution of a human prostaglandin transporter cDNA (hPGT). J Clin Invest. 1996;98:1142–1149
  76. Kullak-Ublick GA, Hagenbuch B, Stieger B, Schteingart CD, Hofmann AF, Wolkoff AW, et al.  Molecular and functional characterization of an organic anion transporting polypeptide cloned from human liver. Gastroenterology. 1995;109:1274–1282
  77. Kullak-Ublick GA, Fisch T, Oswald M, Hagenbuch B, Meier PJ, Beuers U, et al.  Dehydroepiandrosterone sulfate (DHEAS): identification of a carrier protein in human liver and brain. FEBS Lett. 1998;424:173–176
  78. van Montfoort JE, Hagenbuch B, Fattinger K, Müller M, Groothuis GMM, Meijer DKF, et al.  Polyspecific organic anion transporting polypeptides mediate hepatic uptake of amphipathic type II organic cations. J Pharmacol Exp Ther. 1999; in press
  79. Kullak-Ublick GA, Hagenbuch B, Stieger B, Wolkoff AW, Meier PJ. Functional characterization of the basolateral rat liver organic anion transporting polypeptide. Hepatology. 1994;20:411–416
  80. Eckhardt U, Schroeder A, Stieger B, Höchli M, Landmann L, Tynes R, et al.  Polyspecific substrate uptake by the hepatic organic anion transporter Oatp1 in stably transfected CHO cells. Am J Physiol. 1999;276:G1037–G1042
  81. Bossuyt X, Mueller M, Hagenbuch B, Meier PJ. Polyspecific drug and steroid clearance by an organic anion transporter of mammalian liver. J Pharmacol Exp Ther. 1996;276:891–896
  82. Kanai N, Lu R, Bao Y, Wolkoff AW, Vore M, Schuster VL. Estradiol 17β-D-glucuronide is a high-affinity substrate for oatp organic anion transporter. Am J Physiol. 1996;270:F326–F331
  83. Eckhardt U, Horz JA, Petzinger E, Stueber W, Reers M, Dickneite G, et al.  The peptide-based thrombin inhibitor CRC 220 is a new substrate of the basolateral rat liver organic aniontransporting polypeptide. Hepatology. 1996;24:380–384
  84. Kontaxi M, Eckhardt U, Hagenbuch B, Stieger B, Meier PJ, Petzinger E. Uptake of the mycotoxin Ochratoxin A in liver cells occurs via the cloned organic anion transporting polypeptide. J Pharmacol Exp Ther. 1996;279:1507–1513
  85. Ishizuka H, Konno K, Naganuma H, Nishimura K, Kouzuki H, Suzuki H, et al.  Transport of temocaprilat into rat hepatocytes: role of organic anion transporting polypeptide. J Pharmacol Exp Ther. 1998;287:37–42
  86. van Montfoort JE, Stieger B, Meijer DKF, Weinmann H-J, Meier PJ, Fattinger KE. Hepatic uptake of the MRI-contrast agent gadoxetate by the organic anion transporting polypeptide Oatp1. J Pharmacol Exp Ther. 1999; in press
  87. Li L, Lee TK, Meier PJ, Ballatori N. Identification of glutathione as a driving force and leukotriene C4 as a substrate for Oatp1, the hepatic sinusoidal organic solute transporter. J Biol Chem. 1998;273:16184–16191
  88. Endou H. Recent advances in molecular mechanisms of nephrotoxicity. Toxicol Lett. 1998;102–103:29–33
  89. Gorboulev V, Ulzheimer JC, Akhoundova A, Ulzheimer-Teuber I, Karbach U, Quester S, et al.  Cloning and characterization of two human polyspecific organic cation transporters. DNA Cell Biol. 1997;16:871–881
  90. Zhang L, Dresser MJ, Gray AT, Yost SC, Terashita S, Giacomini KM. Cloning and functional expression of a human liver organic cation transporter. Mol Pharmacol. 1997;51:913–921
  91. Giacomini KM. Membrane transporters in drug disposition. J Pharmacokinet Biopharm. 1997;25:731–741
  92. Shi X, Bai S, Ford AC, Burk RD, Jacquemin E, Hagenbuch B, et al.  Stable inducible expression of a functional rat liver organic anion transport protein in HeLa cells. J Biol Chem. 1995;270:25591–25595
  93. Satlin LM, Amin V, Wolkoff AW. Organic anion transporting polypeptide mediates organic anion/HCO3 exchange. J Biol Chem. 1997;272:26340–26345
  94. Gartung C, Trauner M, Schlosser SF, Hagenbuch B, Meier PJ, Boyer JL. Sodium-independent uptake of bile acids is unaffected by down-regulation of an organic anion transporter (oatp) in rat liver during cholestasis produced by common bile duct ligation (CBDL). [abstract] Hepatology. 1996;24:369A
  95. Gartung C, Gerloff T, Stieger B, Hagenbuch B, Meier PJ, Matern S. Unterschiedliche Expression basolateraler und kanalikulärer organischer Anionentransporter während der Leberregeneration nach partieller Hepatektomie. [abstract] Z Gastroenterol. 1999;37:74
  96. Gerloff T, Gartung C, Stieger B, Hagenbuch B, Meier PJ, Matern S. Differential expression of basolateral and canalicular organic anion transporters during liver regeneration. [abstract] Hepatology. 1998;28:506A
  97. Lu R, Kanai N, Bao Y, Wolkoff AW, Schuster VL. Regulation of renal oatp mRNA expression by testosterone. Am J Physiol. 1996;270:F332–F337
  98. Kullak-Ublick G-A, Beuers U, Fahney C, Hagenbuch B, Meier PJ, Paumgartner G. Indentification and functional characterization of the promoter region of the human organic anion transporting polypeptide gene. Hepatology. 1997;26:991–997
  99. Trauner M, Arrese M, Soroka CJ, Ananthanarayanan M, Koeppel TA, Schlosser SF, et al.  The rat canalicular conjugate export pump (mrp2) is down-regulated in intrahepatic and obstructive cholestasis. Gastroenterology. 1997;113:255–264
  100. Oswald M, Kullak-Ublick GA, Beuers U, Paumgartner G. Expression of the hepatocyte canalicular multidrug resistance associated protein 2 (MRP2) in primary biliary cirrhosis. [abstract] Hepatology. 1998;28:544A
  101. Fischer S, Beuers U, Spengler U, Zwiebel FM, Koebe H-G. Hepatic levels of bile acids in end-stage chronic cholestatic liver disease. Clin Chim Acta. 1996;251:173–186
  102. Stravitz RT, Sanyal AJ, Pandak WM, Vlahcevic ZR, Beets JW, Dawson PA. Induction of sodium-dependent bile acid transporter messenger RNA, protein, and activity in rat ileum by cholic acid. Gastroenterology. 1997;113:1599–1608
  103. Sweet DH, Wolff NA, Pritchard JB. Expression cloning and characterization of ROAT1: the basolateral organic anion transporter in rat kidney. J Biol Chem. 1997;272:30088–30095
  104. Miller DS. Protein kinase C regulation of organic anion transport in renal proximal tubule. Am J Physiol. 1998;274:F156–F164
  105. Sekine T, Cha SH, Tsuda M, Apiwattanakul N, Nakajima N, Kanai Y, et al.  Identification of multispecific organic anion transporter 2 expressed predominantly in the liver. FEBS Lett. 1998;429:179–182
  106. Kusuhara H, Sekine T, Utsunomiya-Tate N, Tsuda M, Kojima R, Cha SH, et al.  Molecular cloning and characterization of a new multispecific organic anion transporter from rat brain. J Biol Chem. 1999;274:13675–13680
  107. Kool M, de Haas M, Scheffer GL, Scheper RJ, van Eijk MJT, Juijn JA, et al.  Analysis of expression of cMOAT (MRP2), MRP3, MRP4, and MRP5, homologues of the multidrug resistance-associated protein gene (MRP1), in human cancer cell lines. Cancer Res. 1997;57:3537–3547
  108. Cole SPC, Bhardwaj G, Gerlach JH, Mackie JE, Grant CE, Almquist KC, et al.  Overexpression of a transporter gene in a multidrug-resistant human lung cancer cell line. Science. 1992;258:1650–1654
  109. Müller M, Mejer C, Zaman GJR, Borst P, Scheper RJ, Mulder NH, et al.  Overexpression of the gene encoding the multidrug resistance-associated protein results in increased ATP-dependent glutathione S-conjugate transport. Proc Natl Acad Sci USA. 1994;91:13033–13037
  110. Jedlitschky G, Leier I, Buchholz U, Barnouin K, Kurz G, Keppler D. Transport of glutathione, glucuronate, and sulfate conjugates by the MRP gene-encoded conjugate export pump. Cancer Res. 1996;56:988–994
  111. Roelofsen H, Vos TA, Schippers IJ, Kuipers F, Koning H, Moshage H, et al.  Increased levels of the multidrug resistance protein in lateral membranes of proliferating hepatocyte-derived cells. Gastroenterology. 1997;112:511–521
  112. Paulusma CC, Bosma PJ, Zaman GJR, Bakker CTM, Otter M, Scheffer GL, et al.  Congenital jaundice in rats with a mutation in a multidrug resistance-associated protein gene. Science. 1996;271:1126–1128
  113. Kartenbeck J, Leuschner U, Mayer R, Keppler D. Absence of the canalicular isoform of the MRP gene-encoded conjugate export pump from the hepatocytes in Dubin-Johnson syndrome. Hepatology. 1996;23:1061–1066
  114. Paulusma CC, Kool M, Bosma PJ, Scheffer GL, Ter Borg F, Scheper RJ, et al.  A mutation in the human canalicular multispecific organic anion transporter gene causes the Dubin-Johnson syndrome. Hepatology. 1997;25:1539–1542
  115. Keppler D, König J, Cui Y, Leier I, Rost D, Nies A. ATP-dependent hepatocellular export pumps of the MRP family. In:  Paumgartner G,  Stiehl A,  Gerok W,  Keppler D,  Leuschner U editor. Bile Acids and Cholestasis. Dordrecht, Boston, London: Kluwer Academic Publishers; 1999; in press
  116. Stieger B, Meier PJ. Bile acid and xenobiotic transporters in liver. Curr Opin Cell Biol. 1998;10:462–467
  117. Kiuchi Y, Suzuki H, Hirohashi T, Tyson CA, Sugiyama Y. cDNA cloning and inducible expression of human multidrug resistance associated protein 3 (MRP3). FEBS Lett. 1998;433:149–152
  118. Madon J, Hagenbuch B, Gerloff T, Landmann L, Meier PJ, Stieger B. Identification of a novel multidrug resistance-associated protein (mrp6) at the lateral plasma membrane of rat hepatocytes. [abstract] Hepatology. 1998;28:400A
  119. Kool M, van der Linden M, de Haas M, Baas F, Borst P. Expression of human MRP6, a homologue of the multidrug resistance protein gene MRP1, in tissues and cancer cells. Cancer Res. 1999;59:175–182
  120. Lee JM, Trauner M, Soroka CJ, Stieger B, Meier PJ, Boyer JL. Molecular expression of the sister of P-glycoprotein in chronic bile duct obstruction in the rat. In:  Paumgartner G,  Stiehl A,  Gerok W,  Keppler D,  Leuschner U editor. Bile Acids and Cholestasis. Dordrecht, Boston, London: Kluwer Academic Publishers; 1999; in press
  121. Vos TA, Hooiveld GJEJ, Koning H, Childs S, Meijer DKF, Moshage H, et al.  Up-regulation of the multidrug resistance genes, mrpl and mdrlb, and down-regulation of the organic anion transporter, mrp2, and the bile salt transporter, spgp, in endotoxemic rat liver. Hepatology. 1998;28:1637–1644
  122. Zhu Q, Center MS. Cloning and sequence analysis of the promoter region of the MRP gene of HL60 cells isolated for resistance to adriamycin. Cancer Res. 1994;54:4488–4492
  123. Wang Q, Beck WT. Transcriptional suppression of multidrug resistance-associated protein (MRP) gene expression by wild-type p53. Cancer Res. 1998;58:5762–5769
  124. Fromm MF, Leake B, Roden DM, Wilkinson GR, Kim RB. Human MRP3 transporter: identification of the 5′-flanking region, genomic organization and alternative splice variants. Biochim Biophys Acta. 1999;1415:369–374
  125. Roelofsen H, Müller M, Jansen PLM. Regulation of organic anion transport in the liver. Yale J Biol Med. 1997;70:435–445
  126. Ma L, Krishnamachary N, Center MS. Phosphorylation of the multidrug resistance associated protein gene encoded protein P190. Biochemistry. 1995;34:3338–3343
  127. Koehler MR, Wissinger B, Gorboulev V, Koepsell H, Schmid M. The two human organic cation transporter genes SLC22A1 and SLC22A2 are located on chromosome 6q26. Cytogenet Cell Genet. 1997;79:198–200
  128. Gründemann D, Gorboulev V, Gambaryan S, Veyhl M, Koepsell H. Drug excretion mediated by a new prototype of polyspecific transporter. Nature. 1994;372:549–552
  129. Meyer-Wentrup F, Karbach U, Gorboulev V, Arndt P, Koepsell H. Membrane localization of the electrogenic cation transporter rOCT1 in rat liver. Biochem Biophys Res Comm. 1998;248:673–678
  130. Nagel G, Volk C, Friedrich T, Ulzheimer JC, Bamberg E, Koepsell H. A reevaluation of substrate specificity of the rat cation transporter rOCT1. J Biol Chem. 1997;272:31953–31956
  131. Koepsell H. Organic cation transporters in intestine, kidney, liver, and brain. Annu Rev Physiol. 1998;60:243–266
  132. Sinclair CJ, Ward KL, Green RM. Stable expression and kinetic characterization of a novel hepatic organic cation transporter (Orct). [abstract] Hepatology. 1998;28:506A
  133. Green RM, Sinclair C, Ward KL. Functional characterization of organic cation transport in HepG2 cells. [abstract] Hepatology. 1998;28:531A
  134. Cooper PR, Smilinich NJ, Day CD, Nowak NJ, Reid LH, Pearsall RS, et al.  Divergently transcribed overlapping genes expressed in liver and kidney and located in the 11p15.5 imprinted domain. Genomics. 1998;49:38–51

PII: S0168-8278(99)80054-3

Journal of Hepatology
Volume 31, Issue 3 , Pages 563-573, September 1999

Article Tools

* Image Usage & Resolution