Mutations in the canilicular multispecific organic anion transporter (cMOAT) gene, a novel ABC transporter, in patients with hyperbilirubinemia II/Dubin-Johnson syndrome

Pseudoxanthoma elasticum: mutations in the MRP6 gene encoding a transmembrane ATP-binding cassette (ABC) transporter

Pseudoxanthoma elasticum (PXE), the prototypic heritable connective tissue disorder affecting the elastic structures in the body, manifests with cutaneous, ophthalmologic, and cardiovascular findings, with considerable morbidity and mortality. The molecular basis of PXE has remained unknown, but the disease locus has recently been mapped to an approximately 500-kb interval on chromosome 16p13.1, without evidence for locus heterogeneity. In this study, we report pathogenetic mutations in MRP6, a member of the ABC transporter gene family, in eight kindreds with PXE. The mutation detection strategy consisted of heteroduplex scanning of coding sequences in the MRP6 gene, which were amplified by PCR by using genomic DNA as template, followed by direct nucleotide sequencing. A total of 13 mutant MRP6 alleles were disclosed in the eight probands with PXE. These genetic lesions consisted of either single base pair substitutions resulting in missense, nonsense, or splice site mutations, or large deletions resulting in allelic loss of the MRP6 locus. Examination of clinically unaffected family members in four multiplex families identified heterozygous carriers, consistent with an autosomal recessive inheritance pattern. Collectively, identification of mutations in the MRP6 gene provides the basis to examine the pathomechanisms of PXE and allows development of DNA-based carrier detection, prenatal testing, and preimplantation genetic diagnosis in families with a history of this disease.

Mechanisms of cholestasis

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

Zonal down-regulation and redistribution of the multidrug resistance protein 2 during bile duct ligation in rat liver

We have studied regulation of the multidrug resistance protein 2 (mrp2) during bile duct ligation (BDL) in the rat. In hepatocytes isolated after 16, 48, and 72 hours of BDL, mrp2-mediated dinitrophenyl-glutathione (DNP-GS) transport was decreased to 65%, 33%, and 33% of control values, respectively. The impaired mrp2-mediated transport coincided with strongly decreased mrp2 protein levels, without any significant changes in mrp2 RNA levels. Restoration of bile flow after a 48-hour BDL period resulted in a slow recovery of mrp2-mediated transport and protein levels. Immunohistochemical detection of the protein in livers of rats undergoing BDL showed strongly reduced mrp2 staining after 48 hours, which was initiated in the periportal areas of the liver lobule and progressed toward the pericentral areas after 96 hours. Immunofluorescent detection of mrp2 in livers of rats undergoing 48 hours of BDL revealed decreased staining accompanied by intracellular localization of the protein in pericanalicular vesicular structures. Within this intracellular compartment, mrp2 colocalized with the bile salt transporter (bsep) and was still active as shown by vesicular accumulation of the fluorescent organic anion glutathione-bimane (GS-B). We conclude that down-regulation of mrp2 during BDL-induced obstructive cholestasis is mainly posttranscriptionally regulated. We propose that this down-regulation is caused by endocytosis of apical transporters followed up by increased breakdown of mrp2, probably in lysosomes. This breakdown of mrp2 is more severe in the periportal areas of the liver lobule.

Conjugate export pumps of the multidrug resistance protein (MRP) family: localization, substrate specificity, and MRP2-mediated drug resistance

The membrane proteins mediating the ATP-dependent transport of lipophilic substances conjugated to glutathione, glucuronate, or sulfate have been identified as members of the multidrug resistance protein (MRP) family. Several isoforms of these conjugate export pumps with different kinetic properties and domain-specific localization in polarized human cells have been cloned and characterized. Orthologs of the human MRP isoforms have been detected in many different organisms. Studies in mutant rats lacking the apical isoform MRP2 (symbol ABCC2) indicate that anionic conjugates of endogenous and exogenous substances cannot exit from cells at a sufficient rate unless an export pump of the MRP family is present in the plasma membrane. Several mutations in the human MRP2 gene have been identified which lead to the absence of the MRP2 protein from the hepatocyte canalicular membrane and to the conjugated hyperbilirubinemia of Dubin-Johnson syndrome. Overexpression of recombinant MRP2 confers resistance to multiple chemotherapeutic agents. Because of its function in the terminal excretion of cytotoxic and carcinogenic substances, MRP2 as well as other members of the MRP family, play an important role in detoxification and chemoprevention.

Inventory and function of yeast ABC proteins: about sex, stress, pleiotropic drug and heavy metal resistance

Saccharomyces cerevisiae was the first eukaryotic organism whose complete genome sequence has been determined, uncovering the existence of numerous genes encoding proteins of the ATP-binding cassette (ABC) family. Fungal ABC proteins are implicated in a variety of cellular functions, ranging from clinical drug resistance development, pheromone secretion, mitochondrial function, peroxisome biogenesis, translation elongation, stress response to cellular detoxification. Moreover, some yeast ABC proteins are orthologues of human disease genes, which makes yeast an excellent model system to study the molecular mechanisms of ABC protein-mediated disease. This review provides a comprehensive discussion and update on the function and transcriptional regulation of all known ABC genes from yeasts, including those discovered in fungal pathogens.

An inventory of the human ABC proteins

Currently 30 human ABC proteins are represented by full sequences in various databases, and this paper provides a brief overview of these proteins. ABC proteins are composed of transmembrane domains (TMDs), and nucleotide binding domains (NBDs, or ATP-binding cassettes, ABSs). The arrangement of these domains, together with available membrane topology models of the family members, are presented. Based on their sequence similarity scores, the members of the human ABC protein family can be grouped into eight subfamilies. At present the MDR/TAP, the ALD, the MRP/CFTR, the ABC1, the White, the RNAseL inhibitor, the ANSA, and the GCN20 subfamilies are identified. Mutations of many human ABC proteins are known to be causative in inherited diseases, and a short description of the molecular pathology of these ABC gene-related genetic diseases is also provided.

The human multidrug resistance protein 2 gene: functional characterization of the 5'-flanking region and expression in hepatic cells

The human multidrug resistance protein 2 (MRP2), also termed as the canalicular multispecific organic anion transporter (cMOAT), is a member of the adenosine triphosphate-binding cassette transporter superfamily. In the liver, MRP2 mediates the multispecific efflux of various types of organic anions, including glucuronate, sulfate, and glutathione conjugates, across the canalicular hepatocyte membrane to the bile. To investigate how the MRP2 gene is expressed in liver cells, the 5'-flanking region of the human MRP2 gene was isolated from a human placental genomic library. Sequence analysis of the MRP2 promoter showed a number of consensus binding sites for both ubiquitous and liver-enriched transcription factors. Transfection of human hepatic HepG2 cells with a series of 5'-deleted promoter luciferase constructs identified a putative silencer element localized in the -1,659/-491 region and a liver-specific positive regulatory element localized in the -491/-258 region. This latter region contained the liver-abundant transcription factor CCAAT-enhancer binding protein beta (C/EBPbeta). The transcriptional activity of the promoter construct containing a mutation in the C/EBPbeta binding site was significantly decreased in HepG2 cells. This study suggests that C/EBPbeta (-356 to -343) may regulate the liver expression of the MRP2 gene.

Effect of organic anions and bile acid conjugates on biliary excretion of taurine-conjugated bile acid sulfates in the rat

Biliary organic anion excretion is mediated by an ATP-dependent primary active transporter, canalicular multispecific organic anion transporter/multidrug resistance protein 2. On the other hand, a multiplicity of canalicular organic anion transporter/multidrug resistance protein 2 has been suggested. Therefore, to examine the effect of hydrophobicity on the substrate specificity of canalicular multispecific organic anion transporter/multidrug resistance protein 2, we examined the effect of organic anions and bile acid conjugates on biliary excretion of three taurine-conjugated bile acid sulfates with different hydrophobicity, taurolithocholate-3-sulfate, taurochenodeoxycholate3-sulfate, and taurocholate-3-sulfate in rats. Biliary excretions of these bile acid conjugates were delayed in Eisai hyperbilirubinemic rats. Biliary excretion of these bile acid conjugates was inhibited by sulfobromophthalein, whereas biliary excretion and taurocholate-3-sulfate was not inhibited by phenolphthalein glucuronide. Taurolithocholate-3-sulfate and ursodeoxycholate-3-glucuronide decreased biliary excretion of taurochenodeoxycholate-3-sulfate and taurocholate-3-sulfate, but ursodeoxycholate-3,7-disulfate did not affect biliary excretion of taurochenodeoxycholate-3-sulfate and taurocholate-3-sulfate. These findings indicate that very hydrophilic organic anions are not good substrates of canalicular multispecific organic anion transporter/multidrug resistance protein 2.

Mechanisms of drug resistance in chemotherapy for urogenital carcinoma

Cancer chemotherapy is the principal approach for urogenital cancers. However, the acquisition of resistance to anticancer agents is a critical factor that limits the successful treatment of malignancies. The multidrug resistant (MDR) phenotype has been widely recognized in cancer chemotherapy in urogenital tumors and the mechanisms underlying MDR have also been extensively studied. One of the principle mechanisms in MDR is caused by the overexpression of P-glycoprotein (P-gp), encoded by the multidrug resistance gene (MDR1). It functions as an ATP-dependent active efflux pump of chemotherapeutic agents in human cancer cells. Recently, other drug resistance proteins, including multidrug resistance-associated protein (MRP1) and cMOAT (or MRP2), were also identified from multidrug resistant cells. A functional analysis of MRP1 has shown that MRP1 may have the potential to act as a transporter of glutathione conjugates, which has been known as a central detoxification pathway in anticancer agents. Furthermore, several other resistance-related proteins (e.g. glutathione S-transferase, metallothionein, thioredoxin, topoisomerase I, II, O6-alkylguanine-DNA methyltransferase, etc.) have been found to be up- or down-regulated in resistant cells and these molecules are believed to contribute to the resistant phenotype as well. Based on the molecular characteristics identified in MDR, several experimental and clinical approaches have been studied to overcome MDR. One of these strategies is to reverse MDR by using such P-gp inhibitors as verapamil and cyclosporine A. In this review, we summarize the recent advances in MDR-related molecules and clinical trials to circumvent MDR in urogenital carcinomas.

Exon-intron organization of the human multidrug-resistance protein 2 (MRP2) gene mutated in Dubin-Johnson syndrome

BACKGROUND & AIMS

The Dubin-Johnson syndrome is characterized by conjugated hyperbilirubinemia and by impaired secretion of anionic conjugates from hepatocytes into bile. Absence of the multidrug-resistance protein 2 (MRP2; symbol ABCC2), an adenosine triphosphate-dependent conjugate export pump, from the hepatocyte canalicular membrane is the molecular basis of this syndrome. The aim of this study was the elucidation of all exon-intron boundaries of the MRP2 gene as a prerequisite for the analysis of mutations in patients with Dubin-Johnson syndrome.

METHODS

Exon-intron boundaries of MRP2 were determined, and the amplified exons were screened for mutations. Immunofluorescence microscopy served to localize the MRP2 protein in human liver.

RESULTS

The human MRP2 gene is approximately 45 kilobases long; it contains 32 exons and a high proportion of class 0 introns. In 2 patients with Dubin-Johnson syndrome, we detected a nonsense mutation at codon 1066 and a 6-nucleotide deletion mutation affecting codons 1392-1394. The MRP2 protein was absent from the canalicular membrane of both patients.

CONCLUSIONS

The mutations detected so far show that various mutations in the MRP2 gene can lead to the Dubin-Johnson syndrome. The exon-intron boundaries established in this article will facilitate the analysis of additional mutations in the MRP2 gene.

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

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

Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1

Tangier disease (TD) was first discovered nearly 40 years ago in two siblings living on Tangier Island. This autosomal co-dominant condition is characterized in the homozygous state by the absence of HDL-cholesterol (HDL-C) from plasma, hepatosplenomegaly, peripheral neuropathy and frequently premature coronary artery disease (CAD). In heterozygotes, HDL-C levels are about one-half those of normal individuals. Impaired cholesterol efflux from macrophages leads to the presence of foam cells throughout the body, which may explain the increased risk of coronary heart disease in some TD families. We report here refining of our previous linkage of the TD gene to a 1-cM region between markers D9S271 and D9S1866 on chromosome 9q31, in which we found the gene encoding human ATP cassette-binding transporter 1 (ABC1). We also found a change in ABC1 expression level on cholesterol loading of phorbol ester-treated THP1 macrophages, substantiating the role of ABC1 in cholesterol efflux. We cloned the full-length cDNA and sequenced the gene in two unrelated families with four TD homozygotes. In the first pedigree, a 1-bp deletion in exon 13, resulting in truncation of the predicted protein to approximately one-fourth of its normal size, co-segregated with the disease phenotype. An in-frame insertion-deletion in exon 12 was found in the second family. Our findings indicate that defects in ABC1, encoding a member of the ABC transporter superfamily, are the cause of TD.

MRP3, a new ATP-binding cassette protein localized to the canalicular domain of the hepatocyte

Bile secretion in liver is driven in large part by ATP-binding cassette (ABC)-type proteins that reside in the canalicular membrane and effect ATP-dependent transport of bile acids, phospholipids, and non-bile acid organic anions. Canalicular ABC-type proteins can be classified into two subfamilies based on membrane topology and sequence identity: MDR1, MDR3, and SPGP resemble the multidrug resistance (MDR) P-glycoprotein, whereas MRP2 is similar in structure and sequence to the multidrug resistance protein MRP1 and transports similar substrates. We now report the isolation of the rMRP3 gene from rat liver, which codes for a protein 1522 amino acids in length that exhibits extensive sequence similarity with MRP1 and MRP2. Northern blot analyses indicate that rMRP3 is expressed in lung and intestine of Sprague-Dawley rats as well as in liver of Eisai hyperbilirubinemic rats and TR- mutant rats, which are deficient in MRP2 expression. rMRP3 expression is also transiently induced in liver shortly after birth and during obstructive cholestasis. Antibodies raised against MRP3 recognize a polypeptide of 190-200 kDa, which is reduced in size to 155-165 kDa after treatment with endoglycosidases. Immunoblot analysis and immunoconfocal microscopy indicate that rMRP3 is present in the canalicular membrane, suggesting that it may play a role in bile formation.

Characterization of the transport properties of cloned rat multidrug resistance-associated protein 3 (MRP3)

We have previously cloned rat MRP3 as an inducible transporter in the liver (Hirohashi, T., Suzuki, H., Ito, K., Ogawa, K., Kume, K., Shimizu, T., and Sugiyama, Y. (1998) Mol. Pharmacol. 53, 1068-1075). In the present study, the function of rat MRP3 was investigated using membrane vesicles isolated from LLC-PK1 and HeLa cell population transfected with corresponding cDNA. The ATP-dependent uptake of both 17beta estradiol 17-beta-D-glucuronide ([3H]E217betaG) and glucuronide of [14C] 6-hydroxy-5, 7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole (E3040), but not that of [3H]leukotriene C4 and [3H]2, 4-dinitrophenyl-S-glutathione, was markedly stimulated by MRP3 transfection in both cell lines. The Km and Vmax values for the uptake of [3H]E217betaG were 67 +/- 14 microM and 415 +/- 73 pmol/min/mg of protein, respectively, for MRP3-expressing membrane vesicles and 3.0 +/- 0.7 microM and 3.4 +/- 0.4 pmol/min/mg of protein, respectively, for the endogenous transporter expressed on HeLa cells. [3H]E217betaG had also a similar Km value for MRP3 when LLC-PK1 cells were used as the host. All glucuronide conjugates examined (E3040 glucuronide, 4-methylumbelliferone glucuronide, and naphthyl glucuronide) and methotrexate inhibited MRP3-mediated [3H]E217betaG transport in LLC-PK1 cells. Moreover, [3H]methotrexate was transported via MRP3. The inhibitory effect of estrone sulfate, [3H]2,4-dinitrophenyl-S-glutathione, and [3H]leukotriene C4 was moderate or minimal, whereas N-acetyl-2,4-dinitrophenylcysteine had no effect on the uptake of [3H]E217betaG. The uptake of [3H]E217betaG was enhanced by E3040 sulfate and 4-methylumbelliferone sulfate. Thus we were able to demonstrate that several kinds of organic anions are transported via MRP3, although the substrate specificity of MRP3 differs from that of MRP1 and cMOAT/MRP2 in that glutathione conjugates are poor substrates for MRP3.

Canalicular multispecific organic anion transporter/multidrug resistance protein 2 mediates low-affinity transport of reduced glutathione

The canalicular multispecific organic anion transporter (cMOAT), a member of the ATP-binding cassette transporter family, mediates the transport of a broad range of non-bile salt organic anions from liver into bile. cMOAT-deficient Wistar rats (TR-) are mutated in the gene encoding cMOAT, leading to defective hepatobiliary transport of a whole range of substrates, including bilirubin glucuronide. These mutants also have impaired hepatobiliary excretion of GSH and, as a result, the bile flow in these animals is reduced. In the present work we demonstrate a role for cMOAT in the excretion of GSH both in vivo and in vitro. Biliary GSH excretion in rats heterozygous for the cMOAT mutation (TR/tr) was decreased to 63% of controls (TR/TR) (114+/-24 versus 181+/-20 nmol/min per kg body weight). Madin-Darby canine kidney (MDCK) II cells stably expressing the human cMOAT protein displayed >10-fold increase in apical GSH excretion compared with wild-type MDCKII cells (141+/-6.1 pmol/min per mg of protein versus 13.2+/-1.3 pmol/min per mg of protein in wild-type MDCKII cells). Similarly, MDCKII cells expressing the human multidrug resistance protein 1 showed a 4-fold increase in GSH excretion across the basolateral membrane. In several independent cMOAT-transfectants, the level of GSH excretion correlated with the expression level of the protein. Furthermore, we have shown, in cMOAT-transfected cells, that GSH is a low-affinity substrate for the transporter and that its excretion is reduced upon ATP depletion. In membrane vesicles isolated from cMOAT-expressing MDCKII cells, ATP-dependent S-(2,4-dinitrophenyl)glutathione uptake is competitively inhibited by high concentrations of GSH (Ki approximately 20 mM). We concluded that cMOAT mediates low-affinity transport of GSH. However, since hepatocellular GSH concentrations are high (5-10 mM), cMOAT might serve an important physiological function in maintenance of bile flow in addition to hepatic GSH turnover.

Molecular aspects of organic compound transport across the plasma membrane of hepatocytes

Many organic compounds are taken up from the blood by membrane transporters, taken across the sinosuidal membrane of hepatocytes and then excreted into bile via the bile canalicular membrane. The hepatic uptake of conjugated bile acids is mediated by the sodium taurocholate cotransporting polypeptide. Many organic anions and bulky organic cations are incorporated into hepatocytes by the organic anion transporting polypeptide, while small organic cations are transported by the organic cation transporter. At the canalicular membrane, organic compounds are excreted into bile by ATP-binding cassette transporters which hydrolyse ATP to ADP. Excretion of monovalent bile acids is mediated by the canalicular bile salt transporter and that of organic anions, including divalent bile acid, conjugates, are mediated by the multi-drug resistance-associated protein 2, also termed canalicular multi-specific organic anion transporter. Organic cations are excreted into bile by the multi-drug resistance gene product (MDR) 1 and phospholipids are excreted by MDR3 (mdr2 in mice and rats). The clinical syndromes associated with alterations of these transporters are also discussed.

Pediatric liver disease

Pediatric hepatology has advanced greatly over the past few years. Many more children with severe liver disease are now surviving into adulthood. There is a number of conditions not seen in adult practice that have been the focus of research efforts, and many of these efforts have borne fruit in the past year. Disorders characterized by intrahepatic cholestasis in particular have now been substantially unravelled, and this work has elucidated a great deal about hepatic physiology. Liver-directed gene therapy is on the threshold of human application. This research has been facilitated by excellent animal models and the advent of isolated hepatocyte transplantation. Relatively common conditions such as biliary atresia, however, remain largely unexplained and the viral hepatitides have no effective form of treatment. It remains a hope that our increase in knowledge in the fields of genetics and immunology will translate into advances in therapy.

Characterization of the human multidrug resistance protein isoform MRP3 localized to the basolateral hepatocyte membrane

Several members of the multidrug resistance protein (MRP) family are expressed in the liver. Adenosine triphosphate (ATP)-dependent transport of glutathione and glucuronoside conjugates across the hepatocyte canalicular membrane is mediated by the apical MRP isoform, MRP2 (APMRP), also known as canalicular multispecific organic anion transporter (cMOAT). We have cloned an additional MRP isoform, MRP3, from human liver and localized it to the basolateral membrane domain of hepatocytes. Basolateral MRP (BLMRP) is composed of 1,527 amino acids and encoded by 4,581 base pairs of complementary DNA. Northern blotting of various human tissues indicated an expression of MRP3 in the liver, colon, pancreas, and, at a lower level, in the kidney. The amino acid identity of MRP3 with MRP1 and MRP2 is 58% and 48%, respectively. These three isoforms, encoded by genes on different chromosomes, have a similar predicted topology of transmembrane segments and ATP-binding domains. Antibodies raised against two peptide sequences of MRP3 that are not shared by other MRP family members detected recombinant MRP3 expressed in polarized MDCK cells. Both antibodies served to localize MRP3 to the basolateral membrane of hepatocytes. Double-label immunofluorescence microscopy confirmed that MRP3 was not detectable in the canalicular membrane domain. A particularly strong expression of the MRP3 protein was observed in the basolateral hepatocyte membrane of two patients with Dubin-Johnson syndrome who are deficient in MRP2. These results indicate that the basolateral MRP isoform, MRP3, may be upregulated when the canalicular secretion of anionic conjugates by MRP2 is impaired.

Genomic structure of the canalicular multispecific organic anion-transporter gene (MRP2/cMOAT) and mutations in the ATP-binding-cassette region in Dubin-Johnson syndrome

Dubin-Johnson syndrome (DJS) is an autosomal recessive disease characterized by conjugated hyperbilirubinemia. Previous studies of the defects in the human canalicular multispecific organic anion transporter gene (MRP2/cMOAT) in patients with DJS have suggested that the gene defects are responsible for DJS. In this study, we determined the exon/intron structure of the human MRP2/cMOAT gene and further characterized mutations in patients with DJS. The human MRP2/cMOAT gene contains 32 exons, and it has a structure that is highly conserved with that of another ATP-binding-cassette gene, that for a multidrug resistance-associated protein. We then identified three mutations, including two novel ones. All mutations identified to date are in the cytoplasmic domain, which includes the two ATP-binding cassettes and the linker region, or adjacent putative transmembrane domain. Our results confirm that MRP2/cMOAT is the gene responsible for DJS. The finding that mutations are concentrated in the first ATP-binding-cassette domain strongly suggests that a disruption of this region is a critical route to loss of function.

A splice mutation in the human canalicular multispecific organic anion transporter gene causes Dubin-Johnson syndrome

The human Dubin Johnson syndrome (DJS) is a rare autosomal recessive disorder characterized by chronic conjugated hyperbilirubinemia and impaired hepatobiliary transport of non-bile salt organic anions. A highly homologous phenotype exists in the transport deficient (TR-) Wistar rat, which has a defective canalicular multispecific organic anion transporter (cMOAT). This protein mediates adenosine triphosphate-dependent transport of a broad range of endogenous and xenobiotic compounds across the (apical) canalicular membrane of the hepatocyte. The cDNA encoding rat cMOAT has recently been cloned, and this mutation in the TR- rat has been identified. Subsequently the human homologue of rat cMOAT localized in the liver was found to be the cause of DJS. In an individual with DJS, we have identified a single novel nucleotide substitution in the exon-intron junction of the cMOAT gene which generates liver cDNA with a 67bp exon deletion.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Isolation of a novel human canalicular multispecific organic anion transporter, cMOAT2/MRP3, and its expression in cisplatin-resistant cancer cells with decreased ATP-dependent drug transport

The human multidrug resistance protein (MRP) gene encodes a membrane protein involved in the ATP-dependent transport of hydrophobic compounds. We previously isolated a canalicular multispecific organic anion transporter, cMOAT1/MRP2, that belongs to the ATP binding cassette (ABC) superfamily, which is specifically expressed in liver, and cMOAT1/MRP2 is responsible for the defects in hyperbilirubinemia II/Dubin-Johnson syndrome. In this study, we isolated a new cDNA of the ABC superfamily designated cMOAT2/MRP3 that is homologous to human MRP1 and cMOAT1/MRP2: cMOAT2/MRP3 is 56% identical to MRP1 and 45% identical to cMOAT1/MRP2, respectively. Fluorescence in situ hybridization demonstrated the chromosomal locus of this gene on chromosome 17q22. The human cMOAT2 cDNA hybridized to a 6.5-kb mRNA that was mainly expressed in liver and to a lesser extent in colon, small intestine, and prostate. The cMOAT2/MRP3 gene was not overexpressed in cisplatin-resistant cell lines with increased ATP-dependent transport of cisplatin over their parental counterparts derived from human head and neck cancer and human prostatic cancer cell lines. The human cMOAT2/MRP3, a novel member of the ABC superfamily, may function as a membrane transporter in liver, colon, and prostate.

A gene encoding a liver-specific ABC transporter is mutated in progressive familial intrahepatic cholestasis

The progressive familial intrahepatic cholestases (PFIC) are a group of inherited disorders with severe cholestatic liver disease from early infancy. A subgroup characterized by normal serum cholesterol and gamma-glutamyltranspeptidase (gammaGT) levels is genetically heterogeneous with loci on chromosomes 2q (PFIC2) and 18q. The phenotype of the PFIC2-linked group is consistent with defective bile acid transport at the hepatocyte canalicular membrane. The PFIC2 gene has now been identified by mutations in a positional candidate, BSEP, which encodes a liver-specific ATP-binding cassette (ABC) transporter, sister of p-glycoprotein (SPGP). The product of the orthologous rat gene has been shown to be an effective bile acid transporter in vitro. These data provide evidence that SPGP is the human bile salt export pump (BSEP).

cDNA cloning and inducible expression of human multidrug resistance associated protein 3 (MRP3)

Previously, we cloned rat MRP3 as a candidate for an inducible transporter for the biliary excretion of organic anions [Hirohashi et al. (1998) Mol. Pharmacol. 53, 1068-10751. In the present study, we cloned human MRP3 (1527 amino acids) from Caco-2 cells. Human MRP3 is predominantly expressed in liver, small intestine and colon; hepatic expression of MRP3 was observed in humans but not in normal rats. In HepG2 cells, the expression of MRP3 was induced by phenobarbital. These results suggest that MRP3 may act as an inducible transporter in the biliary and intestinal excretion of organic anions.

Bile acid and xenobiotic transporters in liver

Identification of transporters involved in bile formation in liver is rapidly progressing. It is now clear that these transporters are also important in drug disposition in the body. Significant recent advances include the cloning of an ATP-dependent bile acid transporter, related to the p-glycoprotein family, in the canalicular plasma membrane of hepatocytes. In addition, liver transporter genes responsible for hereditary forms of cholestatic liver disease have been identified and found to belong to the superfamily of ATP-binding cassette proteins.