Yunhai Cui

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.

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 aK m of 3.3 μm, and 17β-glucuronosyl estradiol, with a K m of 5.4 μm. 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.

Detection of the human organic anion transporters SLC21A6 (OATP2) and SLC21A8 (OATP8) in liver and hepatocellular carcinoma

Transport proteins mediating the selective uptake of organic anions into human hepatocytes include the organic anion transporters SLC21A6 (also termed OATP2, OATP-C, or LST-1) and SLC21A8 (OATP8). Both transporters are localized to the basolateral membrane of human hepatocytes. Because of the importance of these transporters for hepatobiliary elimination, including the removal of bilirubin and its conjugates from the blood circulation, we have generated monoclonal antibodies for studies on the expression and localization of these transport proteins. We describe two antibodies, designated monoclonal antibody MDQ (mMDQ) and monoclonal antibody ESL (mESL), directed against the amino terminus and the carboxyl terminus of human SLC21A6, respectively. Both antibodies have been characterized by immunoblot analysis, immunoprecipitation, and immunofluorescence microscopy. While mESL reacted specifically with SLC21A6, mMDQ detects both SLC21A6 and SLC21A8. Neither of the two antibodies reacted with other human, or with dog, rat, or mouse liver SLC21A family members. Antibody mMDQ may be used for the simultaneous detection of SLC21A6 and SLC21A8 in immunoblotting because of its immunoreactivity with both molecules and because of the different molecular masses of both glycosylated proteins in human hepatocytes. This is exemplified in hepatocellular carcinomas where SLC21A6 and SLC21A8 were differentially synthesized and showed an irregular staining pattern. Both transport proteins have not been detected in human hepatoma HepG2 cells. In routine paraffin sections, 10 of 12 hepatocellular carcinomas were focally positive with antibody mMDQ. In contrast, cholangiocarcinomas and liver metastases of colorectal and pancreatic adenocarcinoma were negative without exception. This suggests the usefulness of SLC21A6/SLC21A8 within a panel of tumor markers for hepatocellular carcinomas. Moreover, both antibodies should be useful in studies on the expression and localization of two important uptake transporters of human hepatocytes under physiologic and pathophysiologic conditions.

Export pumps for anionic conjugates encoded by MRP genes.

Several members of the multidrug resistance protein (MRP) family mediate the ATP-dependent transport of amphiphilic anions across membranes. The substrate specificity of recombinant human MRP1 has been most extensively defined by use of inside-out membrane vesicles. Substrates include the glutathione S-conjugate leukotriene C4, 17 beta-glucuronosyl estradiol, glucuronosyl bilirubin, glutathione disulfide, in addition to the fluorescent lipophilic anion Fluo-3. These substances are also substrates for the apical isoform MRP2, also termed canalicular multispecific organic anion transporter, cMOAT, which shares only 49% amino acid sequence identity with MRP1. The K(m) of leukotriene C4 for MRP2 is 10-fold higher than for MRP1, and the K(m) of 17 beta-glucuronosyl estradiol is 4.8-fold higher for MRP2 than for recombinant human MRP1. Human as well as rat MRP2 confer multidrug resistance to polarized MDCKII cells permanently expressing the recombinant glycoprotein in their apical plasma membrane. Resistance of cells transfected with human and rat MRP2 to etoposide was enhanced 5-fold and 3.8-fold, and resistance to vincristine was enhanced 2.3-fold and 6.0-fold, respectively. Conjugate-transporting members of the MRP family with a related sequence and a similar function have been detected recently. In addition to several MRP isoforms (MRP1-6) and orthologs in mammals (human, rat, rabbit, mouse), MRP family members have been identified in the nematode Caenorhabditis elegans, in the yeast Saccharomyces cerevisiae, and in the plant Arabidopsis thaliana. These conjugate export pumps of the MRP family play a widespread role in detoxification, drug resistance, and, because of the role in the export of glutathione disulfide by MRP1 and MRP2, in the defense against oxidative stress.

Vectorial Transport of Enalapril by Oatp1a1/Mrp2 and OATP1B1 and OATP1B3/MRP2 in Rat and Human Livers

Although Oatp1a1 (rat organic anion-transporting polypeptide 1a1) was the transporter found responsible for the hepatocellular entry of enalapril (EN) into the rat liver, the canalicular transporter involved for excretion of EN and the metabolite, enalaprilat (ENA), was unknown. The Eisai hyperbilirubinemic rat (EHBR) that lacks Mrp2 (multidrug resistance-associated protein 2) was used to appraise the role of Mrp2 in the excretion of [3H]EN and its metabolite [3H]ENA in single-pass rat liver preparations. Although the total and metabolic clearances and hepatic extraction ratios at steady-state were virtually unaltered for EN in EHBR compared with published values of Sprague-Dawley rats, the biliary clearances of EN and ENA were significantly reduced almost to zero (P < 0.05). Involvement of human OATP1B1, OATP1B3, and MRP2 in EN transport was further assessed in single- or double-transfected mammalian cells. Human embryonic kidney 293 cells that expressed OATP1B1 or OATP1B3 showed that OATP1B3 transport of EN (20-500 μM) was of low affinity, whereas transport of EN by OATP1B1 was associated with the Km of 262 ± 35 μM, a value similar to that for Oatp1a1 (214 μM). The transcellular transport of EN via human OATP1B1 and MRP2, investigated with the double-transfected Madin-Darby canine kidney (MDCK) II cells in the Transwell system, showed that the sinusoidal to canalicular flux of EN in the OATP1B1/MRP2/MDCK cells was significantly higher (P < 0.05) than that of mock/MDCK and OATP1B1/MDCK cells. EN was transported by Oatp1a1 and Mrp2 in rats and OATP1B1/OATP1B3 and MRP2 in humans.

Identification and functional characterization of the natural variant MRP3-Arg1297His of human multidrug resistance protein 3 (MRP3/ABCC3).

The human multidrug resistance protein 3 (MRP3, symbol ABCC3) is an ATP-binding cassette transporter that mediates the efflux of organic anions, including lipophilic substances conjugated with glucuronate, sulphate or glutathione, across the basolateral membrane of polarized cells (e.g. hepatocytes) into blood. Genetic variants of MRP3 may affect the transport of these substances out of cells. The aims of this study were: (i) to identify MRP3 polymorphisms; (ii) to functionally characterize one relatively frequent MRP3 polymorphism; and (iii) to establish whether MRP3 transports bilirubin glucuronosides. Exonic nucleotide variants in the ABCC3 gene were identified by single-strand conformation polymorphism analysis. The 3890G>A mutation, resulting in MRP3-ArgHis, was introduced into the ABCC3 cDNA which was stably transfected into MDCKII cells. For the functional characterization of MRP3-ArgHis in comparison with MRP3, ATP-dependent transport was analysed in isolated membrane vesicles. Two non-synonymous MRP3 variants were identified with an allele frequency of 0.003 for 1643T>A (MRP3-LeuGln) and 0.08 for 3890G>A (MRP3-ArgHis). Because of the high frequency of the 3890G>A mutation, and because of the close proximity of Arg to the second nucleotide-binding domain, we pursued the functional characterization of the MRP3-ArgHis polymorphic variant. MRP3-ArgHis was correctly localized to the basolateral membrane of polarized MDCKII cells. We identified monoglucuronosyl bilirubin, bisglucuronosyl bilirubin and leukotriene C4 as substrates for both MRP3 and MRP3-ArgHis. Dehydroepiandrosterone-3-sulphate and 17beta-glucuronosyl oestradiol were transported with similar kinetics by MRP3 and MRP3-ArgHis. This experimental setup provides a useful tool to analyse the functional consequences of polymorphic variants of MRP3.

CHAPTER 20 – MRP2, THE APICAL EXPORT PUMP FOR ANIONIC CONJUGATES

In the elimination and detoxification of endogenous and xenobiotic substances, the ATP-dependent unidirectional transport of anionic conjugates across the apical membrane domain of polarized cells plays an important role. This transport function was originally described as a glutathione S -conjugate transport system and as a canalicular multispecific organic anion transporter. Subsequent cloning, expression and functional analysis of the recombinant protein has established that the apical conjugate export pump is encoded by the MRP2 ( ABCC2 ) gene. Antibodies raised against various epitopes of MRP2 from several species served to localize the MRP2 glycoprotein to the apical membrane of polarized cells, including hepatocytes, kidney proximal tubules, intestinal epithelia, and lung. The apical localization of MRP2 and its broad substrate specificity for various conjugates qualify this ATP-binding cassette (ABC) transporter as an important terminal component in detoxification, subsequent to the phase I and phase II reactions of xenobiotic metabolism. Mutations and polymorphisms in the human MRP2 gene that affect MRP2 function may be relevant for adverse drug reactions because of an impaired hepatobiliary and renal clearance of anionic drug conjugates. Moreover, such polymorphisms may also affect the oral bioavailability of drugs that are substrates for intestinal MRP2 or become substrates following conjugation inside intestinal epithelia.