Identification of a novel gene family encoding human liver-specific organic anion transporter LST-1

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Cloning, expression, and ontogeny of mouse organic anion-transporting polypeptide-5, a kidney-specific organic anion transporter

The full-length coding sequence of mouse organic anion-transporting polypeptide (designated mouse Oatp-5) has been cloned from mouse kidney cDNA library. Analysis of the 5'-untranslated region (5'-UTR) of Oatp-5 cDNA through capsite cloning reveals two possible transcription start sites that are 4-bp apart. The 3'-untranslated region (3'-UTR) of Oatp-5 cDNA contains an early polyadenylation signal, indicating the possibility that mRNAs with different 3'-UTR lengths may coexist. Deduced amino acid sequence of mouse Oatp-5 protein contains 670 amino acids and has 10 putative transmembrane domains, multiple potential glycosylation and phosphorylation sites. Tissue-specific expression studies indicate that mouse Oatp-5 is expressed only in kidney. Studies on the developmental expression reveal that there is no significant expression of Oatp-5 mRNA in mouse kidney for at least 3 weeks after birth, and adult levels of Oatp-5 mRNA expression are attained more than 6 weeks after birth. Phylogenetic analysis reveals that mouse Oatp-5 is an ortholog of rat Oatp-5.

Blood-brain barrier genomics

The blood-brain barrier (BBB) is formed by the brain microvascular endothelium, and the unique transport properties of the BBB are derived from tissue-specific gene expression within this cell. The current studies developed a gene microarray approach specific for the BBB by purifying the initial mRNA from isolated rat brain capillaries to generate tester cDNA. A polymerase chain reaction-based subtraction cloning method, suppression subtractive hybridization (SSH), was used, and the BBB cDNA was subtracted with driver cDNA produced from mRNA isolated from rat liver and kidney. Screening 5% of the subtracted tester cDNA resulted in identification of 50 gene products and more than 80% of those were selectively expressed at the BBB; these included novel gene sequences not found in existing databases, ESTs, and known genes that were not known to be selectively expressed at the BBB. Genes in the latter category include tissue plasminogen activator, insulin-like growth factor-2, PC-3 gene product, myelin basic protein, regulator of G protein signaling 5, utrophin, IkappaB, connexin-45, the class I major histocompatibility complex, the rat homologue of the transcription factors hbrm or EZH1, and organic anion transporting polypeptide type 2. Knowledge of tissue-specific gene expression at the BBB could lead to new targets for brain drug delivery and could elucidate mechanisms of brain pathology at the microvascular level.

Expression, transport properties, and chromosomal location of organic anion transporter subtype 3

The rat and mouse organic anion-transporting polypeptides (oatp) subtype 3 (oatp3) were cloned to further define components of the intestinal bile acid transport system. In transfected COS cells, oatp3 mediated Na(+)-independent, DIDS-inhibited taurocholate uptake (Michaelis-Menten constant approximately 30 microM). The oatp3-mediated uptake rates and affinities were highest for glycine-conjugated dihydroxy bile acids. In stably transfected, polarized Madin-Darby canine kidney (MDCK) cells, oatp3 mediated only apical uptake of taurocholate. RT-PCR analysis revealed that rat oatp3, but not oatp1 or oatp2, was expressed in small intestine. By RNase protection assay, oatp3 mRNA was readily detected down the length of the small intestine as well as in brain, lung, and retina. An antibody directed to the carboxy terminus localized oatp3 to the apical brush-border membrane of rat jejunal enterocytes. The mouse oatp3 gene was localized to a region of mouse chromosome 6. This region is syntenic with human chromosome 12p12, where the human OATP-A gene was mapped, suggesting that rodent oatp3 is orthologous to the human OATP-A. These transport and expression properties suggest that rat oatp3 mediates the anion exchange-driven absorption of bile acids previously described for the proximal small intestine.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Molecular identification of a rat novel organic anion transporter moat1, which transports prostaglandin D(2), leukotriene C(4), and taurocholate

We have isolated a rat novel multispecific organic anion transporter, moat1. The isolated clones were originated by alternative splicing of the moat1 mRNA. The nucleotide sequences predict a protein of 682 amino acids with moderate sequence similarity to LST-1, the oatp family, and the prostaglandin transporter. Northern blot analysis of rat moat1 identified a predominant transcript of 4.4 kilonucleotides in all tissues. Northern blot and in situ hybridization analyses of rat brain further indicated that moat1 mRNA is widely distributed in neuronal cells of the central nervous system, especially in the hippocampus and cerebellum. moat1 transports prostaglandin D(2) (K(m); 35.5 nM), leukotriene C(4) (K(m); 3.2 microM) and taurocholate (K(m); 17.6 microM) in a sodium-independent manner. moat1 also transports prostaglandin E(1), E(2), thromboxane B(2), and iloprost but not dehydroepiandrosterone sulfate and digoxin, of which the substrate specificity is similar, but definitively different from those of any other organic anion transporters.

The pathophysiology of cholestasis with special reference to primary biliary cirrhosis

Cholestasis in primary biliary cirrhosis results from impairment of bile flow either by reduced transport at the level of the canaliculi or by disturbed bile flow through damaged intrahepatic bile ductules. Whatever its cause, the expression of hepatic transport proteins will be affected. In cholestatic rats: the expression of the multispecific organic anion transporter mrp2 is decreased; the bile salt export pump bsep and the phospholipid transporter mdr2 are less affected; the carrier protein for hepatic uptake of bile salts ntcp is sharply down-regulated; Mrp3, a basolateral ATP-dependent transporter for glucuronides and bile salts, is upregulated. Thus, bile salts that cannot exit the hepatocyte because of the cholestasis are effectively removed across the basolateral membrane. These may be adaptive responses in defence against overloading of hepatocytes with cytotoxic bile salts. These responses show that the expression of hepatic transporter proteins is highly regulated. This occurs by transcriptional and post-transcriptional mechanisms. Primary biliary cirrhosis starts as a disease of the small intrahepatic bile ducts and therefore the experimental evidence for 'cross-talk' between hepatocytes and cholangiocytes is of great interest for this disease and needs to be further investigated. New insights in bile physiology may enable the development of new therapies for cholestatic liver diseases as primary biliary cirrhosis.

Oatp2 mediates bidirectional organic solute transport: a role for intracellular glutathione

One member of the OATP family of transporters, rat Oatp1, functions as an anion exchanger that is driven in part by the glutathione (GSH) electrochemical gradient, indicating that other OATP-related transporters may also be energized by this mechanism. The present study examined whether rat Oatp2 is also an anion exchanger, and, if so, whether it is energized by the GSH electrochemical gradient. As with Oatp1, uptake of 10 microM [(3)H]taurocholate in Oatp2-expressing Xenopus laevis oocytes was trans-stimulated by intracellular 0.2 mM unlabeled taurocholate, indicating bidirectional transport. Interestingly, [(3)H]taurocholate uptake in Oatp2-expressing oocytes was also trans-stimulated when oocytes were preloaded with GSH, S-methylglutathione, S-sulfobromophthalein-glutathione, S-dinitrophenyl glutathione, or ophthalmic acid (a GSH analog) but not by glutarate or N-acetylcysteine, suggesting that GSH derivatives and conjugates may function as intracellular substrates for Oatp2. Support for this hypothesis was provided by the demonstration of enhanced [(3)H]GSH and [(3)H]S-(2,4-dinitrophenyl)-glutathione efflux in Oatp2-expressing oocytes. However, in contrast to Oatp1, extracellular GSH failed to cis-inhibit uptake of [(3)H]taurocholate or [(3)H]digoxin in Oatp2-expressing oocytes, indicating that the stimulatory effect of high intracellular GSH concentrations is not due to a coupled exchange mechanism. Taken together, the results indicate that Oatp2 mediates bidirectional transport of organic anions by a GSH-sensitive facilitative diffusion mechanism and suggest that this transporter may play a role in cellular export of specific organic molecules.

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

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

Cloning of the full-length coding sequence of rat liver-specific organic anion transporter-1 (rlst-1) and a splice variant and partial characterization of the rat lst-1 gene

The full-length coding sequence of rat liver-specific organic anion transporter-1 (lst-1) and its splice variant have been cloned. The full-length rat lst-1 (designated rlst-1a) encodes a protein containing 687 amino acids and has 12-putative transmembrane domains, multiple potential N-glycosylation and phosphorylation sites. Therefore, rat lst-1a has 35 additional amino acid residues compared to the previously reported rat lst-1. A splice variant (designated rlst-1c) reported in this communication encodes a protein containing 654 amino acids and has 10-putative transmembrane domains. PCR analysis suggests that rlst-1a is the most abundant form in liver. Phylogenetic analysis reveals that rat lst-1a is an ortholog of human LST-1 (hLST-1) and mouse lst-1 (mlst-1). The rlst-1 gene is composed of 15 exons and 14 introns. Analysis of exon-intron boundary reveals that the splice variant rlst-1c lacks the entire exon 7, while the previously reported rat lst-1 (designated herein as rlst-1b) lacks approximately half of exon 10, and the splicing has occurred through alternative usage of a splice donor site within exon 10.

Intracellular transport of bile acids

Bile acids originate from the liver and are transported via bile to the intestines where they perform an important role in the absorption of lipids and lipid-soluble nutrients. Most of the bile acids are reclaimed from the terminal ileum and returned to the liver via portal blood for reuse. The transport of bile acids is vectorial in both liver and intestinal cells, originating and terminating at opposite poles. Bile acids enter through the basolateral pole in liver cells, and through the apical pole in intestinal cells. During the past decade, much has been learned about the mechanisms by which bile acids enter and exit liver and intestinal cells. By contrast, the mechanisms by which bile acids are transported across cells remain poorly understood. The current body of evidence suggests that bile acids do not traverse the cell by vesicular transport. Although a carrier-mediated mechanism is a likely alternative, only a handful of intracellular proteins capable of binding bile acids have been described. The significance of these proteins in the intracellular transport of bile acids remains to be tested.

Molecular identification and characterization of novel members of the human organic anion transporter (OATP) family

We identified three novel transporters structurally belonging to the organic anion transporting polypeptide (OATP) family in humans. Since previously known rat oatp1 to 3 do not necessarily correspond to the human OATPs in terms of either tissue distribution or function, here we designate the newly identified human OATPs as OATP-B, -D and -E, and we rename the previously known human OATP as OATP-A. OATP-C proved to be identical with the recently reported LST1/OATP-2. Expression profiles of the five OATPs and the prostaglandin transporter PGT (a member of OATP family) in human tissues showed that OATP-C is exclusively localized in liver, OATP-A and PGT are expressed in restricted ranges of tissues, and OATP-B, -D and -E show broad expression profiles. OATP-B, -C, -D and -E exhibited transport activity for [(3)H]estrone-3-sulfate as a common substrate. OATP-C has a high transport activity with broad substrate specificity.

Full-length cDNA cloning and genomic organization of the mouse liver-specific organic anion transporter-1 (lst-1)

We have cloned a cDNA that codes for mouse liver-specific transporter-1, mouse lst-1. The cDNA is comprised of 3296 base pairs and it contains a coding sequence for a protein of 689 amino acids with 12 putative transmembrane domains. The deduced amino acid sequence of the mouse lst-1 shares 64 and 77% identities with the reported human and rat lsts, respectively. Northern blot analysis demonstrates that mouse lst-1 mRNA is expressed exclusively in liver. We also report here the structural organization of the mouse lst-1 gene as the first evidence for the structure of a gene encoding an lst. The mouse lst-1 gene spans approximately 60 kbp in length and consists of 16 exons, including two noncoding exons. All the introns are flanked by GT-AG consensus splice sequences. 5'-Rapid Amplification of cDNA Ends (RACE) analyses demonstrate three splice variant mRNAs involving the noncoding exon 2 and exon 3. The 5'-flanking region of the gene contains consensus CAAT and TATA boxes and several potential binding sites for transcription factors for CAAT enhancer binding protein (C/EBP) and hepatocyte nuclear factors (HNF-3beta, HFH-1, and HFH-2), transcription factors important for liver-specific gene expression.

Identification of organic anion transporting polypeptide 4 (Oatp4) as a major full-length isoform of the liver-specific transporter-1 (rlst-1) in rat liver

A novel organic anion transporting polypeptide (Oatp)4(1) was isolated from rat liver that is 35 amino acids longer than the reported rat liver specific organic anion transporter (rlst)-1 and exhibits a 64% amino acid sequence identity with the human OATP-C (LST-1/OATP2; gene symbol SLC21A6). When expressed in Xenopus laevis oocytes, Oatp4 (Slc21a10) mediated polyspecific uptake of a variety of organic anions including taurocholate (K(m) approximately 27 microM), bromosulfophthalein (K(m) approximately 1.1 microM) and steroid conjugates. Based on nuclease protection analysis Oatp4 appears to be the predominant transcript in rat liver indicating that rlst-1 plays a minor role in overall hepatic organic anion uptake.

Cholestatic syndromes

Continued advances in the field of liver cell biology and molecular biology have provided further insights into the normal physiology of bile secretion and the pathogenesis and therapy for cholestatic syndromes. Important new data have also been published about pathogenesis, clinical features, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, drug-induced cholestasis, and cholestatic syndromes caused by viral infections.

Mechanisms of cholestasis

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

Hepatobiliary transport

The alterations of hepatobiliary transport that occur in cholestasis can be divided into primary defects, such as mutations of transporter genes or acquired dysfunctions of transport systems that cause defective canalicular or cholangiocellular secretion, and secondary defects, which result from biliary obstruction. The dysfunction of distinct biliary transport systems as a primary cause of cholestasis is exemplified by the genetic defects in progressive familial intrahepatic cholestasis or by the direct inhibition of transporter gene expression by cytokines. In both, the hepatocellular accumulation of toxic cholephilic compounds causes multiple alterations of hepatocellular transporter expression. In addition, lack of specific components of bile caused by a defective transporter, as in the case of mdr2/MDR3 deficiency, unmasks the toxic potential of other components. The production of bile is critically dependent upon the coordinated regulation and function of sinusoidal and canalicular transporters, for instance of Na+-taurocholate cotransporting polypeptide (NTCP) and bile salt export pump (BSEP). Whereas the downregulation of the unidirectional sinusoidal uptake system NTCP protects the hepatocyte from further intracellular accumulation of bile salts, the relative preservation of canalicular BSEP expression serves to uphold bile salt secretion, even in complete biliary obstruction. Conversely, the strong downregulation of canalicular MRP2 (MRP, multidrug resistance protein) in cholestasis forces the hepatocyte to upregulate basolateral efflux systems such as MRP3 and MRP1, indicating an inverse regulation of basolateral and apical transporters The regulation of hepatocellular transporters in cholestasis adheres to the law of parsimony, since many of the cellular mechanisms are pivotally governed by the effect of bile salts. The discovery that bile salts are the natural ligand of the farnesoid X receptor has shown us how the major bile component is able to regulate its own enterohepatic circulation by affecting transcription of the genes critically involved in transport and metabolism.

Molecular cloning and functional characterization of the mouse organic-anion-transporting polypeptide 1 (Oatp1) and mapping of the gene to chromosome X

We have cloned a murine member of the organic-anion-transporting polypeptide (Oatp) family of membrane-transport proteins from mouse liver. The cloned cDNA insert of 2783 bp with an open reading frame of 2011 bp codes for a 12-transmembrane 670-amino-acid protein with highest amino acid identity with the rat Oatp1. When expressed in Xenopus laevis oocytes, the mouse Oatp exhibited the same substrate specificity as the rat Oatp1. Besides the common Oatp substrates bromosulphophthalein, taurocholate, oestrone 3-sulphate and ouabain, the new mouse Oatp also mediates transport of the Oatp1-specific magnetic-resonance-imaging agent gadoxetate. The Oatp2-specific cardiac glycoside digoxin, however, is not transported. Kinetic analyses performed for taurocholate and oestrone 3-sulphate revealed apparent K(m) values of 12 microM and 5 microM respectively. Northern-blot analysis demonstrated a predominant expression in the liver with an additional moderate expression in the kidney. Taken together, the amino acid identity, the functional characteristics and the tissue distribution suggest that we have isolated the murine orthologue of the rat Oatp1, and consequently the identified protein will be called Oatp1. Using fluorescence in situ hybridization, the murine Oatp1 gene was mapped to chromosome XA3-A5.

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

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

A novel human hepatic organic anion transporting polypeptide (OATP2). Identification of a liver-specific human organic anion transporting polypeptide and identification of rat and human hydroxymethylglutaryl-CoA reductase inhibitor transporters

A novel human organic transporter, OATP2, has been identified that transports taurocholic acid, the adrenal androgen dehydroepiandrosterone sulfate, and thyroid hormone, as well as the hydroxymethylglutaryl-CoA reductase inhibitor, pravastatin. OATP2 is expressed exclusively in liver in contrast to all other known transporter subtypes that are found in both hepatic and nonhepatic tissues. OATP2 is considerably diverged from other family members, sharing only 42% sequence identity with the four other subtypes. Furthermore, unlike other subtypes, OATP2 did not transport digoxin or aldosterone. The rat isoform oatp1 was also shown to transport pravastatin, whereas other members of the OATP family, i.e. rat oatp2, human OATP, and the prostaglandin transporter, did not. Cis-inhibition studies indicate that both OATP2 and roatp1 also transport other statins including lovastatin, simvastatin, and atorvastatin. In summary, OATP2 is a novel organic anion transport protein that has overlapping but not identical substrate specificities with each of the other subtypes and, with its liver-specific expression, represents a functionally distinct OATP isoform. Furthermore, the identification of oatp1 and OATP2 as pravastatin transporters suggests that they are responsible for the hepatic uptake of this liver-specific hydroxymethylglutaryl-CoA reductase inhibitor in rat and man.

Molecular characterization and functional regulation of a novel rat liver-specific organic anion transporter rlst-1

BACKGROUND & AIMS

Recently, we isolated a new complementary DNA (cDNA) encoding human liver-specific organic anion transporter (LST-1), representing the multispecificity of human liver. The aim of this study was to isolate a rat counterpart of human LST-1 and examine the expression regulation of its messenger RNA (mRNA) to clarify the molecular basis of cholestasis.

METHODS

A rat liver cDNA library was screened with human LST-1 cDNA as a probe. Xenopus oocyte expression system was used for functional analysis. Northern blot analyses were performed using the isolated cDNA (termed rlst-1). The bile duct ligation model and the cecum ligation and puncture model were used for expression analyses.

RESULTS

rlst-1 encodes 652 amino acids, predicting at least 11 transmembrane regions. The overall homology with human LST-1 was 60.2%, which is the highest among all known organic anion transporters. rlst-1 also belongs to the same new gene family as human LST-1, located between the organic anion transporter family and the prostaglandin transporter. rlst-1 preferably transports taurocholate (K(m), 9.45 micromol/L) in an Na(+)-independent manner. The rlst-1 mRNA is exclusively expressed in the liver. In both the bile duct ligation model and the cecum ligation and puncture model, mRNA expression levels of rlst-1 were down-regulated.

CONCLUSIONS

rlst-1 is a counterpart of human LST-1 and is one of the important transporters in rat liver for the clearance of bile acid. The expression of rlst-1 may be under feedback regulation of cholestasis by biliary obstruction and/or sepsis.