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

Recent Trends in Nanocarrier-Based Drug Delivery System for Prostate Cancer

Prostate cancer remains a significant global health concern, requiring innovative approaches for improved therapeutic outcomes. In recent years, nanoparticle-based drug delivery systems have emerged as promising strategies to address the limitations of conventional cancer chemotherapy. The key trends include utilizing nanoparticles for enhancing drug delivery to prostate cancer cells. Nanoparticles have some advantages such as improved drug solubility, prolonged circulation time, and targeted delivery of drugs. Encapsulation of chemotherapeutic agents within nanoparticles allows for controlled release kinetics, reducing systemic toxicity while maintaining therapeutic efficacy. Additionally, site-specific accumulation within the prostate tumor microenvironment is made possible by the functionalization of nanocarrier with targeted ligands, improving therapeutic effectiveness. This article highlights the basics of prostate cancer, statistics of prostate cancer, mechanism of multidrug resistance, targeting approach, and different types of nanocarrier used for the treatment of prostate cancer. It also includes the applications of nanocarriers for the treatment of prostate cancer and clinical trial studies to validate the safety and efficacy of the innovative drug delivery systems. The article focused on developing nanocarrier-based drug delivery systems, with the goal of translating these advancements into clinical applications in the future.

Prevalence of ABCC3-1767G/A polymorphism among patients with antiretroviral-associated hepatotoxicity

Background

Plasma concentrations of antiretrovirals (ARVs) regimens have considerably varied in individuals of human immunodeficiency virus (HIV) because of variations in the expression of drug‐metabolizing and transporter genes. Transporter genes play an important role in the disposition of drugs. Polymorphism in transporter gene (ABCC3) affects the MRP3 expression and varies the treatment outcome.

Method

We examined the polymorphism of ABCC3‐1767G/A gene in a total of 165 HIV patients (out of 165 HIV patients, 34 were with and 131 were without hepatotoxicity) and 156 healthy individuals using the polymerase chain reaction–restriction fragment length polymorphism method.

Results

In univariate analysis, we found a decreased prevalence of ABCC3 1767GA, 1767GA+AA genotypes, and 1767A allele in patients with hepatotoxicity as compared to patients without hepatotoxicity (23.5% vs. 28.2% and 23.5% vs. 30.53%; 11.76% vs. 16.41%), while a higher prevalence of 1767AA genotype was observed in HIV patients in comparison with healthy controls (2.3% vs. 1.3%, odds ratio [OR] = 1.71, 95% confidence interval [CI]: 0.23–15.03, p = .89). The frequency of ABCC3‐1767AA genotype was dispersed higher in individuals with early and advanced HIV disease stage in comparison with healthy controls (5.3% vs. 1.3%, OR = 4.73, p = .70; 8.9% vs. 1.3%, OR = 1.89, p = .91). A higher occurrence of ABCC3‐1767AA genotype was found in tobacco using HIV patients without hepatotoxicity compared with nonusers (4.7% vs. 1.1%, OR = 4.28, p = .52). The distribution of ABCC3‐1767GA genotype was higher in nevirapine receiving HIV patients irrespective of their hepatotoxicity status as compared to nonusers (30.4% vs. 9.1%, OR = 3.34, p = .22; 29.4% vs. 16.7%, OR = 1.69, p = .77). In multivariate analysis, HIV patients receiving nevirapine and with hepatotoxicity was found to have a significant risk for severity of hepatotoxicity (OR = 4.56, 95% CI: 1.60–12.99, p = .004).

Conclusion

ABCC3 1767G/A polymorphism was not significantly associated with susceptibility to ARV‐associated hepatotoxicity, although ABCC3 1767AA genotype designated a risk for acquisition of hepatotoxicity and advancement of the disease. Nevirapine usage emerged as an independent risk factor for hepatotoxicity severity.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

Cisplatin: The first metal based anticancer drug

Cisplatin or (SP-4-2)-diamminedichloridoplatinum(II) is one of the most potential and widely used drugs for the treatment of various solid cancers such as testicular, ovarian, head and neck, bladder, lung, cervical cancer, melanoma, lymphomas and several others. Cisplatin exerts anticancer activity via multiple mechanisms but its most acceptable mechanism involves generation of DNA lesions by interacting with purine bases on DNA followed by activation of several signal transduction pathways which finally lead to apoptosis. However, side effects and drug resistance are the two inherent challenges of cisplatin which limit its application and effectiveness. Reduction of drug accumulation inside cancer cells, inactivation of drug by reacting with glutathione and metallothioneins and faster repairing of DNA lesions are responsible for cisplatin resistance. To minimize cisplatin side effects and resistance, combination therapies are used and have proven more effective to defect cancers. This article highlights a systematic description on cisplatin which includes a brief history, synthesis, action mechanism, resistance, uses, side effects and modulation of side effects. It also briefly describes development of platinum drugs from very small cisplatin complex to very large next generation nanocarriers conjugated platinum complexes.

ABCC3 genetic variants are associated with postoperative morphine-induced respiratory depression and morphine pharmacokinetics in children

Respiratory depression (RD) is a serious side effect of morphine and detrimental to effective analgesia. We reported that variants of the ATP binding cassette gene ABCC3 (facilitates hepatic morphine metabolite efflux) affect morphine metabolite clearance. In this study of 316 children undergoing tonsillectomy, we found significant association between ABCC3 variants and RD leading to prolonged postoperative care unit stay (prolonged RD). Allele A at rs4148412 and allele G at rs729923 caused a 2.36 (95% CI=1.28-4.37, P=0.0061) and 3.7 (95% CI 1.47-9.09, P=0.0050) times increase in odds of prolonged RD, respectively. These clinical associations were supported by increased formation clearance of morphine glucuronides in children with rs4148412 AA and rs4973665 CC genotypes in this cohort, as well as an independent spine surgical cohort of 67 adolescents. This is the first study to report association of ABCC3 variants with opioid-related RD, and morphine metabolite formation (in two independent surgical cohorts).

New insights in the biology of ABC transporters ABCC2 and ABCC3: impact on drug disposition

INTRODUCTION

For the elimination of environmental chemicals and metabolic waste products, the body is equipped with a range of broad specificity transporters that are present in excretory organs as well as in several epithelial blood-tissue barriers.

AREAS COVERED

ABCC2 and ABCC3 (also known as MRP2 and MRP3) mediate the transport of various conjugated organic anions, including many drugs, toxicants and endogenous compounds. This review focuses on the physiology of these transporters, their roles in drug disposition and how they affect drug sensitivity and toxicity. It also examines how ABCC2 and ABCC3 are coordinately regulated at the transcriptional level by members of the nuclear receptor (NR) family of ligand-modulated transcription factors and how this can be therapeutically exploited.

EXPERT OPINION

Mutations in both ABCC2 and ABCC3 have been associated with changes in drug disposition, sensitivity and toxicity. A defect in ABCC2 is associated with Dubin-Johnson syndrome, a recessively inherited disorder characterized by conjugated hyperbilirubinemia. Pharmacological manipulation of the activity of these transporters can potentially improve the pharmacokinetics and thus therapeutic activity of substrate drugs but also affect the physiological function of these transporters and consequently ameliorate associated disease states.

New insights in bilirubin metabolism and their clinical implications

Bilirubin, a major end product of heme breakdown, is an important constituent of bile, responsible for its characteristic colour. Over recent decades, our understanding of bilirubin metabolism has expanded along with the processes of elimination of other endogenous and exogenous anionic substrates, mediated by the action of multiple transport systems at the sinusoidal and canalicular membrane of hepatocytes. Several inherited disorders characterised by impaired bilirubin conjugation (Crigler-Najjar syndrome type I and type II, Gilbert syndrome) or transport (Dubin-Johnson and Rotor syndrome) result in various degrees of hyperbilirubinemia of either the predominantly unconjugated or predominantly conjugated type. Moreover, disrupted regulation of hepatobiliary transport systems can explain jaundice in many acquired liver disorders. In this review, we discuss the recent data on liver bilirubin handling based on the discovery of the molecular basis of Rotor syndrome. The data show that a substantial fraction of bilirubin conjugates is primarily secreted by MRP3 at the sinusoidal membrane into the blood, from where they are subsequently reuptaken by sinusoidal membrane-bound organic anion transporting polypeptides OATP1B1 and OATP1B3. OATP1B proteins are also responsible for liver clearance of bilirubin conjugated in splanchnic organs, such as the intestine and kidney, and for a number of endogenous compounds, xenobiotics and drugs. Absence of one or both OATP1B proteins thus may have serious impact on toxicity of commonly used drugs cleared by this system such as statins, sartans, methotrexate or rifampicin. The liver-blood cycling of conjugated bilirubin is impaired in cholestatic and parenchymal liver diseases and this impairment most likely contributes to jaundice accompanying these disorders.

Basolateral and canalicular transport of xenobiotics in the hepatocyte: A review

The molecular and functional characterization of severalproteins involved in the uptake and excretion of xenobioticsand endogenous compounds in the hepatocyte has been achievedthrough intensive research conducted in the past few years.These studies have lead to the identification of specificmembrane transporters located in the basolateral andcanalicular membrane domains of the hepatocyte. The organicanion-transporting polypeptide (OATP), present in thebasolateral membrane of the hepatocyte, is responsible for thetranslocation of xenobiotics from the sinusoidal space into thehepatocyte. Once inside the cell, unconjugated neutral, anionicand cationic xenobiotics can be secreted into bile by themultidrug-resistance P-glycoprotein 1 (MDR1). Conjugatedxenobiotics (e.g. glucuronides and glutathione conjugates) aresecreted into bile by the canalicular multispecific organicanion transporter (cMOAT). Other transporters play keyphysiological roles, including the basolateral uptake of bilesalts (sodium-taurocholate cotransporter, NTCP) and thesecretion into bile of conjugated and unconjugated bile salts(bile salt export pump, BSEP) and phospholipids (MDR2).Experimental approaches used to investigate the role of thebasolateral and canalicular transporters in the hepatocyte haveincluded both in vivo and in vitro models. Animalmodels lacking canalicular transporters include the;hyperbilirubinemic' rats (Groningen-Yellow (GY), Eisaihyperbilirubinemic (EHB) and TR(-) rats), which aredeficient in the cMOAT protein, and ;knock-out' mice, lackingeither the MDR1 or MDR2 transporter. Although no animal modelsare currently available for the study of basolateraltransporters, their function has been conveniently investigatedthrough heterologous expression in Xenopus laevis oocytesand also with basolateral membrane vesicles isolated fromhepatocytes. The total number of basolateral and canaliculartransport proteins present in the hepatocyte is still unknown,but current knowledge indicates that there are at least fourpresent in the basolateral membrane and five in the canaliculardomain. The present review focuses on the current knowledgeabout the most relevant hepatocyte transporters involved in theuptake of foreign and endogenous compounds from the sinusoidalspace and in their active secretion into bile. The first partof the review deals with the basolateral (sinusoidal) transportof organic anions, and the major basolateral transporters (e.g.NTCP, OATP) are described here, both in terms of their knownbiochemistry and physiology. In the second part of the review,the canalicular (apical) transport of organic anions isdiscussed and the biochemistry and physiological role of MDR1,MDR2, cMOAT and BSEP is described in detail. The concludingremarks point out areas of research that need to be addressedin order to answer important questions that still remainunanswered in this important field of study.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

MRP3: a molecular target for human glioblastoma multiforme immunotherapy

Background

Glioblastoma multiforme (GBM) is refractory to conventional therapies. To overcome the problem of heterogeneity, more brain tumor markers are required for prognosis and targeted therapy. We have identified and validated a promising molecular therapeutic target that is expressed by GBM: human multidrug-resistance protein 3 (MRP3).

Methods

We investigated MRP3 by genetic and immunohistochemical (IHC) analysis of human gliomas to determine the incidence, distribution, and localization of MRP3 antigens in GBM and their potential correlation with survival. To determine MRP3 mRNA transcript and protein expression levels, we performed quantitative RT-PCR, raising MRP3-specific antibodies, and IHC analysis with biopsies of newly diagnosed GBM patients. We used univariate and multivariate analyses to assess the correlation of RNA expression and IHC of MRP3 with patient survival, with and without adjustment for age, extent of resection, and KPS.

Results

Real-time PCR results from 67 GBM biopsies indicated that 59/67 (88%) samples highly expressed MRP3 mRNA transcripts, in contrast with minimal expression in normal brain samples. Rabbit polyvalent and murine monoclonal antibodies generated against an extracellular span of MRP3 protein demonstrated reactivity with defined MRP3-expressing cell lines and GBM patient biopsies by Western blotting and FACS analyses, the latter establishing cell surface MRP3 protein expression. IHC evaluation of 46 GBM biopsy samples with anti-MRP3 IgG revealed MRP3 in a primarily membranous and cytoplasmic pattern in 42 (91%) of the 46 samples. Relative RNA expression was a strong predictor of survival for newly diagnosed GBM patients. Hazard of death for GBM patients with high levels of MRP3 RNA expression was 2.71 (95% CI: 1.54-4.80) times that of patients with low/moderate levels (p = 0.002).

Conclusions

Human GBMs overexpress MRP3 at both mRNA and protein levels, and elevated MRP3 mRNA levels in GBM biopsy samples correlated with a higher risk of death. These data suggest that the tumor-associated antigen MRP3 has potential use for prognosis and as a target for malignant glioma immunotherapy.

Recombinant single-chain variable fragment antibodies against extracellular epitopes of human multidrug resistance protein MRP3 for targeting malignant gliomas

Multidrug resistance protein 3 (MRP3), a multidrug resistance protein identified by serial analysis of gene expression as a glioblastoma multiforme (GBM)-associated molecule, is highly expressed in GBM, but not in normal brain cells. Thus, MRP3 is a candidate for GBM immunotargeting, but to date, no monoclonal antibody has been isolated that can target an extracellular MRP3 epitope. By phage display, we have isolated 3 recombinant, fully human, single-chain Fv (scFv) antibodies, M25, M58 and M89, which specifically react with the extracellular N-terminus of human MRP3. In ELISA, these scFvs reacted only with the peptide used for screening and not with other MRP3-derived peptides. Flow cytometric analysis revealed that these scFv fragments bind specifically to viable human GBM cells displaying different MRP3 expression levels, but not to MRP3-null cells. Furthermore, these scFv antibodies failed to react with tumor cells overexpressing other MRP proteins, including MRP1, MRP2, MRP4 and MRP5. M25 and M58 also bound to viable neurospheres. Iodogen-labeled scFvs demonstrated a yield of 56-76%. The immunoreactive fractions of the radiolabeled M25, M58 and M89 scFvs were 32, 52 and 69%, respectively. M25 exhibited 20% internalization into D2159MG neurospheres, M58, 33% into D54MG cells and M89, 26% into D247MG. Immunohistochemical evaluation of human gliomas to determine the localization of MRP3 antigen using scFvs M25 and M58 showed a dense cytoplasmic and membranous staining pattern. These Fv-based recombinant antibodies, which possess superior tumor penetration capabilities and selectively target tumor cells that express MRP3, may potentially be used in immunotherapy and diagnosis for brain tumors and other cancers.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Update information on drug metabolism systems--2009, part II: summary of information on the effects of diseases and environmental factors on human cytochrome P450 (CYP) enzymes and transporters

The present paper is an update of the data on the effects of diseases and environmental factors on the expression and/or activity of human cytochrome P450 (CYP) enzymes and transporters. The data are presented in tabular form (Tables 1 and 2) and are a continuation of previously published summaries on the effects of drugs and other chemicals on CYP enzymes (Rendic, S.; Di Carlo, F. Drug Metab. Rev., 1997, 29(1-2), 413-580., Rendic, S. Drug Metab. Rev., 2002, 34(1-2), 83-448.). The collected information presented here is as stated by the cited author(s), and in cases when several references are cited the latest published information is included. Inconsistent results and conclusions obtained by different authors are highlighted, followed by discussion of the major findings. The searchable database is available as an Excel file, for information about file availability contact the corresponding author.

Arsenic accumulation decreased in metallothionein null Cisplatin-resistant cell lines

Metallothionein (MT) is known to play an important role in the resistance of tumor cells to cis-diamminedichloroplatinum (II) (cisplatin). To identify non-MT factors of cisplatin resistance, we characterized cisplatin-resistant cell lines derived from MT-null cells. All of the cisplatin-resistant MT- null cell lines, namely MKCr-1, -3, -4, -12, and -13, showed strong cisplatin resistance and decreased platinum accumulation. Some multidrug resistance proteins (MRPs) have been reported to contribute to cisplatin resistance. However, no significant difference of the MRPs was observed in any of these cell lines. The MKCrs showed cross-resistance to other metals such as arsenite, arsenate, cadmium and antimony. The arsenate and arsenite sensitivities were highly correlated with sensitivity to cisplatin. In addition, the degree of arsenite accumulation was correlated with the degree of cisplatin accumulation. These results suggest that the cisplatin resistance was strongly correlated with the arsenite transport mechanism in these cells.

Interaction between CD44 and hyaluronate induces chemoresistance in non-small cell lung cancer cell

CD44s is a principle hyaluronate (HA) receptor and has been reported to play an important role in cancer cell invasion and metastasis. The aim of our study is to determine if the interaction between HA and CD44s influences in vitro chemosensitivity of non-small cell lung cancer (NSCLC). NSCLC cell line, H322 cells, transfected with the CD44s gene (H322/CD44s) cultured on HA coated plates were more resistant to cisplatin (CDDP) than that on bovine serum albumin. Multidrug resistance protein2 (MRP2) expression was induced in H322/CD44s cells cultured on HA. MRP2 inhibitor, MK571, not only suppressed MRP2 expression but also reversed CDDP resistance. These results suggest that the interaction between CD44s and HA play a pivotal role in acquired resistance to CDDP in NSCLC and MRP2 could be involved in this potential mechanism.

Genetic Variations of the ABC Transporter Gene ABCC3 in a Japanese Population

An ATP-binding cassette transporter, multidrug resistance-related protein 3 (MRP3), is encoded by the ABCC3 gene. The MRP3 protein is expressed in several tissues, and functions as an efflux transporter for conjugated as well as unconjugated substrates. In this study, the 31 ABCC3 exons and their flanking introns were comprehensively screened for genetic variations in 89 Japanese subjects. Forty-six genetic variations, including 21 novel ones, were found: 8 were located in the 5'-flanking region, 14 in the coding exons (8 synonymous and 6 nonsynonymous variations), and 24 in the introns. Of these 46 variations, five novel nonsynonymous variations, 2221C>T (Gln741Stop), 2395G>A (Val799Met), 2798_2799delAG (Gln933ArgfsX64), 3657C>A (Ser1219Arg), and 4217C>T (Thr1406Met), were found as heterozygous variations. The allele frequencies were 0.011 for Ser1219Arg and 0.006 for the other four variations. Gln741Stop induces a stop codon at codon 741. Gln933ArgfsX64 causes a frame-shift at codon 933, resulting in early termination at codon 997. Both variations result in loss of 6 transmembrane helices (from the 12th to 17th helices) in the C-terminus and all regions of nucleotide binding domain 2. Thus, both variant proteins are assumed to be inactive. These data provide fundamental and useful information for pharmacogenetic studies on MRP3-transported drugs in Japanese.

Multidrug Resistance-Associated Proteins: Expression and Function in the Central Nervous System

Drug delivery to the brain is highly restricted, since compounds must cross a series of structural and metabolic barriers to reach their final destination, often a cellular compartment such as neurons, microglia, or astrocytes. The primary barriers to the central nervous system are the blood-brain and blood-cerebrospinal fluid barriers. Through structural modifications, including the presence of tight junctions that greatly limit paracellular transport, the cells that make up these barriers restrict diffusion of many pharmaceutically active compounds. In addition, the cells that comprise the blood-brain and blood-cerebrospinal fluid barriers express multiple ATP-dependent, membrane-bound, efflux transporters, such as members of the multidrug resistance-associated protein (MRP) family, which contribute to lowered drug accumulation. A relatively new concept in brain drug distribution just beginning to be explored is the possibility that cellular components of the brain parenchyma could act as a "second" barrier to brain permeation of pharmacological agents via expression of many of the same transporters. Indeed, efflux transporters expressed in brain parenchyma may facilitate the overall export of xenobiotics from the central nervous system, essentially handing them off to the barrier tissues. We propose that these primary and secondary barriers work in tandem to limit overall accumulation and distribution of xenobiotics in the central nervous system. The present review summarizes recent knowledge in this area and emphasizes the clinical significance of MRP transporter expression in a variety of neurological disorders.

High-dose calcitriol and carboplatin in metastatic androgen-independent prostate cancer

Calcitriol acts synergistically with carboplatin in preclinical models of adenocarcinoma of the prostate. The authors sought to test high-dose oral calcitriol in combination with carboplatin in patients with metastatic androgen-independent prostate cancer. Seventeen patients received oral calcitriol (0.5 microg/kg) on day 1 and intravenous carboplatin (AUC 7 or AUC 6 in patients with prior radiation) on day 2, repeated every 4 weeks. PSA response was the primary end point and was defined as a 50% reduction confirmed 4 weeks later. Palliative response (2-point reduction or normalization of pain on the present pain intensity [PPI] scale without increased analgesic consumption) was also examined. One of 17 patients (6%, 95% CI, 0-28) achieved a confirmed PSA response. Four patients (24%, 95% CI, 7-49) had PSA reductions ranging from 24 to 38%. Of the 15 patients with a PPI > or = 1 point on entry, 3 (18%, 95% CI, 4-48) met criteria for palliative response. Treatment-related toxicity was mild and generally similar to that expected with single-agent carboplatin. Despite encouraging preclinical evidence, the addition of oral calcitriol to carboplatin in this study was not associated with an increase in the response rate when compared with the reported activity of carboplatin alone.

Immuno-histochemical detection of MRPs in human lung cells in culture

The transport of molecules across membranes is an essential function of all living organisms and a large number of specific transporters have evolved to carry out this function. The largest transporter gene family is the ATP-binding cassette (ABC) transporter superfamily. The multidrug resistance-associated protein (MRP) family is comprised of nine related ABC transporters. The intra-cellular distribution of the different MRP isoforms in relation to their physiological and non physiological function is still a point of discussion. For this purpose we used normal human lung cells (bronchial epithelial cells, NHBEC, and peripheral lung cells, PLC) as well as tumor cell cultures as test tools to investigate the intracelluar localization of these proteins under classical culture conditions and under air-liquid interface by means of indirect fluorescence microscopy. Characterization of the cultured cells as lung epithelial cells was performed by means of immuno-histochemical analysis. MRP1 and MRP3 were localised to the cellular membrane in all tested lung cell types. In contrast to that MRP2, MRP4 and MRP5 could be described as intracellular proteins in NHBEC and PLC. All MRP1-MRP5 isoforms could be characterized in A549 tumor cell line as membrane proteins. In order to imitate the physiological in vivo circumstances in the lung, we have established a dry/wet method (air-liquid interface) for cell cultivation so that cultured cells have the option to polarize between air and basal membrane and this might influence the distribution pattern of MRP1 and MRP2 in NHBEC. Using confocal laser scanning techniques we could show that in cells kept under dry/wet conditions MRP1 was found to be localised to baso-lateral cell regions while MRP2 was localised to all cell regions. Under classical culture conditions MRP1 was not localized to particular membrane regions and MRP2 was found to be an intracellular protein.

Expression and immunolocalization of the multidrug resistance proteins, MRP1-MRP6 (ABCC1-ABCC6), in human brain

Multidrug resistance proteins (MRPs, symbol ABCC) are membrane glycoproteins that mediate the ATP-dependent export of organic anions, including cytotoxic and antiviral drugs, from cells. To identify MRP family members possibly involved in the intrinsic resistance of human brain to cytotoxic and antiviral drugs, we analyzed the expression and localization of MRP1-MRP6 in rapidly frozen perilesional samples of several regions of adult human brain obtained during neurosurgery. Quantitative polymerase chain reaction analysis showed expression of MRP1, MRP2, MRP3, MRP4, and MRP5 mRNA, whereas MRP6 mRNA was below detectability. However, immunofluorescence microscopy of cryosections from human brain showed no reactivity for the MRP2 or MRP3 proteins. The proteins MRP1, MRP4, and MRP5 were clearly localized by confocal laser scanning microscopy to the luminal side of brain capillary endothelial cells. The MRP4 and MRP5 proteins were also detected in astrocytes of the subcortical white matter. Notably, MRP5 protein was present in pyramidal neurons. MRP proteins may, thus, contribute to the cellular efflux of endogenous anionic glutathione or glucuronate conjugates (substrates for MRP1), cyclic nucleotides (substrates for MRP4 and MRP5), or glutathione (co-substrate for MRP1 and MRP4); in addition, they may play an important role in the resistance of the brain to several cytotoxic and antiviral drugs.

The role of copper transporters in the development of resistance to Pt drugs

Recent studies in yeast, mouse and human cells suggest that the conserved metal binding transporters of the Cu homeostasis pathway can mediate resistance to Pt drugs in cancer cells. This review summarizes the data available from these studies. The observation that cells selected for resistance to Cu or the Pt drugs display bidirectional cross-resistance, parallel defects in the transport of Cu and the Pt drugs and altered expression of Cu transporters is consistent with the concept that the Cu homeostasis proteins regulate sensitivity to the Pt drugs by influencing their uptake, efflux and intracellular distribution. This model is supported by the finding that when mammalian and yeast cells are genetically engineered to express altered levels of the Cu transporters they exhibit altered sensitivity to Pt drugs and are defective in intracellular Pt accumulation due to altered uptake and/or efflux rates. Negative associations between the expression of ATP7A and ATP7B and the outcome of Pt therapy further support the significance of the Cu homeostasis proteins as both markers of and contributors to Pt resistance.

The MRP family of drug efflux pumps

The MRP family is comprised of nine related ABC transporters that are able to transport structurally diverse lipophilic anions and function as drug efflux pumps. Investigations of this family have provided insights not only into cellular resistance mechanisms associated with natural product chemotherapeutic agents, antifolates and nucleotide analogs, but also into factors that influence drug distribution in the body, membrane systems that are involved in the extrusion of reduced folates, cysteinyl leukotrienes and bile acids, and the molecular basis of two hereditary conditions in humans. The review will describe the biochemical properties, drug resistance activities and potential in vivo functions of these unusual pumps.

Substitution of Trp1242 of TM17 alters substrate specificity of human multidrug resistance protein 3

Multidrug resistance protein 3 (MRP3) is an ATP-dependent transporter of 17beta-estradiol 17beta(d-glucuronide) (E(2)17betaG), leukotriene C(4) (LTC(4)), methotrexate, and the bile salts taurocholate and glycocholate. In the present study, the role of a highly conserved Trp residue at position 1242 on MRP3 transport function was examined by expressing wild-type MRP3 and Ala-, Cys-, Phe-, Tyr-, and Pro-substituted mutants in human embryonic kidney 293T cells. Four MRP3-Trp(1242) mutants showed significantly increased E(2)17betaG uptake, whereas transport by the Pro mutant was undetectable. Similarly, the Pro mutant did not transport LTC(4). By comparison, LTC(4) transport by the Ala, Cys, Phe, and Tyr mutants was reduced by approximately 35%. The Ala, Cys, Phe, and Tyr mutants all showed greatly reduced methotrexate and leucovorin transport, except the Tyr mutant, which transported leucovorin at levels comparable with wild-type MRP3. In contrast, the MRP3-Trp(1242) substitutions did not significantly affect taurocholate transport or taurocholate and glycocholate inhibition of E(2)17betaG uptake. Thus Trp(1242) substitutions markedly alter the substrate specificity of MRP3 but leave bile salt binding and transport intact.

Considerations in the design and development of transport inhibitors as adjuncts to drug therapy

With the realization of the importance of drug efflux transporters in disease processes and treatment, development of inhibitors to these transporters has been sought for use as adjuncts to therapy. To date, inhibitors that have been best studied are modulators of P-glycoprotein, a transporter important in the removal of anticancer agents from cells and overexpression of this transporter results in multidrug resistance. There is a delicate balance between efficacy and toxicity. This review summarizes key learning points in the development of P-glycoprotein inhibitors. Topics covered include specificity of the inhibitor for the target transporter, effect on metabolism of coadministered drugs, pharmacokinetic interactions, toxicity and the salient features needed for efficacy. These points will have general application to the development of inhibitors of transporters.

The basic and clinical implications of ABC transporters, Y-box-binding protein-1 (YB-1) and angiogenesis-related factors in human malignancies

In our laboratories, we have been studying molecular targets which might be advantageous for novel cancer therapeutics. In this review, we focus on how ATP-binding cassette (ABC) transporter superfamily genes, Y-box-binding protein-1 (YB-1), and tumor angiogenesis-associated factors could contribute to the development of novel strategies for molecular cancer therapeutics. ABC transporters such as P-glycoprotein/MDR1 and several MRP family proteins function to protect cells from xenobiotics, drugs and poisons, suggesting that ABC transporters are a double-edged sword. In this regard, P-glycoprotein/MDR1 is a representative ABC transporter which plays a critical role in the efflux of a wide range of drugs. We have reported that gene amplification, gene rearrangements, transcription factor YB-1 and CpG methylation on the promoter are involved in MDR1 gene overexpression in cultured cancer cells. Among them, two mechanisms appear to be relevant to the up-regulation of MDR1 gene in human malignancies. We first reported that MDR1 gene promoter is activated in response to environmental stimuli, and is modulated by methylation/demethylation of CpG sites on the MDR1 promoter. We also demonstrated that YB-1 modulates not only transcription of various genes associated with cell growth, drug resistance and DNA synthesis, but also translation, mRNA stabilization and DNA repair/self-defense processes. Angiogenesis is also involved in tumor growth, invasion and metastasis of various malignancies, and so angiogenesis-related molecules also offer novel molecular targets for anticancer therapeutics.

Increased expression of multidrug resistance-associated proteins in bladder cancer during clinical course and drug resistance to doxorubicin

Overexpression of the P-glycoprotein/multidrug resistance 1 (MDR1) and multidrug resistance protein 1 (MRP1) gene is closely associated with the clinical outcome of various malignancies, and it is involved in responses to some anticancer chemotherapeutic agents including doxorubicin. Six human MRP subfamily members (MRP2-7) with structural similarities to MRP1 have been identified. Recently, the relationships between MRP2 and MRP3 expression levels of some cancer cells and drug sensitivity to doxorubicin have been reported, but the relationship between the clinical samples and drug sensitivity remains unclear. We determined the expressions of the MDR1, MRP1, MRP2 and MRP3 gene in bladder cancer during the clinical course and sought to learn whether the expression was correlated with drug responses to doxorubicin. Doxorubicin, used in chemotherapeutic treatment including intravesical and systemic chemotherapy, is an important anticancer agent for the treatment of bladder cancer. We used quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis for our study, and the sensitivity to doxorubicin in bladder cancer was determined using the in vitro succinate dehydrogenase inhibition test. Using 47 clinical samples of bladder cancer, we confirmed the significant correlation of MDR1, MRP1 and MRP3 mRNA levels with resistance to doxorubicin. We showed that the expression of MDR1, MRP1, MRP2 and MRP3 in recurrent tumors and residual tumors after chemotherapeutic treatment was higher than that in untreated primary tumors. In particular, the MDR1 expression in residual tumors was 5.7-fold higher than that in untreated primary tumors.

Tissue distribution and induction of human multidrug resistant protein 3

The multidrug resistance protein (MRP) family consists of several members and, for some of these transporter proteins, distinct roles in multidrug resistance and normal tissue functions have been well established (MRP1 and MRP2) or are still under investigation (MRP3). MRP3 expression studies in human tissues have been largely restricted to the mRNA level. In this report we extended these studies and further explored MRP3 expression at the protein level. Western blot and immunohistochemistry with two MRP3-specific monoclonal antibodies, M(3)II-9 and M(3)II-21, showed MRP3 protein to be present in adrenal gland, and kidney and in tissues of the intestinal tract: colon, pancreas, gallbladder, and liver. In epithelia, MRP3 was found to be located at the basolateral sides of cell membranes. In normal liver, MRP3 was detected at lower levels than anticipated from the mRNA data and was found present mainly in the bile ducts. In livers from patients with various forms of cholestasis, MRP3 levels were frequently increased in the proliferative cholangiocytes, with sometimes additional staining of the basolateral membranes of the hepatocytes. This was especially evident in patients with type 3 progressive familial intrahepatic cholestasis. The present results support the view that MRP3 plays a role in the cholehepatic and enterohepatic circulation of bile and in protection within the biliary tree and tissues along the bile circulation route against toxic bile constituents. The possible functional roles for MRP3 in the adrenal gland and in the kidney remain as yet unknown. In a panel of 34 tumor samples of various histogenetic origins, distinct amounts of MRP3 were detected in a limited number of cases, including lung, ovarian, and pancreatic cancers. These findings may be of potential clinical relevance when considering the drug treatment regimens for these tumor types.

Relevance of multidrug resistance proteins for intestinal drug absorption in vitro and in vivo

Multidrug resistance proteins (p-glycoprotein and mrps) are becoming increasingly important to explain the pharmacokinetics and action of drugs. Located in epithelial and endothelial cells of the gastrointestinal tract, liver, kidney, blood brain barrier, choroid plexus and other organs, they are critical determinants for the movement of a large number of commonly prescribed drugs across cellular barriers. Here we provide a brief overview of the role of multidrug resistance proteins in drug absorption from the gastrointestinal tract. We address the different types of multidrug resistance proteins involved, describe experimental models to study the influence of these proteins on transcellular transport and discuss the impact of multidrug resistance proteins on overall drug bioavailability in vivo.

Enhanced expression of the human multidrug resistance protein 3 by bile salt in human enterocytes. A transcriptional control of a plausible bile acid transporter

The enterohepatic circulation is essential for the maintenance of bile acids and cholesterol homeostasis. The ileal bile acid transporter on the apical membrane of enterocytes mediates the intestinal uptake of bile salts, but little is known about the bile salt secretion from the basolateral membrane of enterocytes into blood. In the basolateral membrane of enterocytes, an ATP-binding cassette transporter, multidrug resistance protein 3 (MRP3), is expressed, which has the ability to transport bile salts. We hypothesized that MRP3 might play a role in the enterohepatic circulation of bile salts by transporting them from enterocytes into circulating blood through the up-regulation of MRP3 expression, so we investigated the transcriptional control of MRP3 in response to bile salts. MRP3 mRNA levels were increased about 3-fold in human colon cells by chenodeoxycholic acid (CDCA), in a dose- and time-dependent manner. In the promoter assay, the promoter activity of MRP3 was increased about 3-fold over the basal promoter activity when treated with CDCA, and the putative bile salt-responsive elements exist in the region -229/-138 including two alpha-1 fetoprotein transcription factor (FTF)-like elements. Constructs with a specific mutation in the consensus sequence of FTF elements showed no increase in basal transcriptional activity following CDCA treatment. In electrophoretic mobility shift assay with nuclear extracts, specific binding of FTF to FTF-like elements was observed when treated with CDCA. The expression of FTF mRNA levels were also markedly enhanced in response to CDCA, and overexpression of FTF specifically activated the MRP3 promoter activity about 4-fold over the basal promoter activity. FTF thus might play a key role not only in the bile salt synthetic pathway in hepatocytes but also in the bile salt excretion pathway in enterocytes through the regulation of MRP3 expression. MRP3 may contribute as a plausible bile salt-exporting transporter to the enterohepatic circulation of bile salts.

Decreased expression of an ATP-binding cassette transporter, MRP2, in human livers with hepatitis C virus infection

BACKGROUND/AIMS

To understand hepatic injury during the process of hepatitis viral infection, determination of liver-specific functions at molecular levels is critical. Because the transport of endogenous/exogenous toxic substances is an intrinsically important hepatic function, we examined whether expression of the ATP-binding cassette (ABC) transporter gene was affected in patients with hepatitis viral infection.

METHODS

To determine which ABC transporter was expressed differently in patients with hepatic viral infection, we assayed the expression of MDR1, MDR3, MRP1, MRP2, and MRP3 in non-cancerous regions in the liver of 42 patients with hepatic tumors using both quantitative RT-PCR and immunological staining analysis, and compared the hepatic expression levels between patients with hepatitis viral infection and non-infected controls.

RESULTS

Of the five ABC transporter genes studied, the mRNAs of MRP2 and MRP3 were highly expressed in the human liver. There was a significant reduction in MRP2 expression to 29% in the virus-infected liver. Treatment of hepatic cells with inflammatory cytokines resulted in decreased mRNA levels of MRP2 and decreased MRP2 promoter activity.

CONCLUSIONS

The down-regulation of MRP2 might induce a failure in the transport of various genotoxic substances in the liver with hepatitis virus infection.

MRP subfamily transporters and resistance to anticancer agents

The MRP subfamily of ABC transporters from mammals consists of at least seven members, six of which have been implicated in the transport of amphipathic anions. MRP1, MRP2, and MRP3 bear a close structural resemblance, confer resistance to a variety of natural products as well as methotrexate, and have the facility for transporting glutathione and glucuronate conjugates. MRP1 is a ubiquitously expressed efflux pump for the products of phase II of xenobiotic detoxification, while MRP2, whose hereditary deficiency results in Dubin-Johnson syndrome, functions to extrude organic anions into the bile. MRP3 is distinguished by its capacity to transport the monoanionic bile constituent glycocholate, and may function as a basolateral back-up system for the detoxification of hepatocytes when the usual canalicular route is impaired by cholestatic conditions. MRP4 and MRP5 resemble each other more closely than they resemble MRPs 1-3 and confer resistance to purine and nucleotide analogs which are either inherently anionic, as in the case of the anti-AIDS drug PMEA, or are phosphorylated and converted to anionic amphiphiles in the cell, as in the case of 6-MP. Given their capacity for transporting cyclic nucleotides, MRP4 and MRP5 have also been implicated in a broad range of cellular signaling processes. The drug resistance activity and physiological substrates of MRP6 are unknown. However, its hereditary deficiency results in pseudoxanthoma elasticum, a multisystem disorder affecting skin, eyes, and blood vessels. It is hoped that elucidation of the resistance profiles and physiological functions of the different members of the MRP subfamily will provide new insights into the molecular basis of clinical drug resistance and spawn new strategies for combating this phenomenon.

Mutation of Trp1254 in the multispecific organic anion transporter, multidrug resistance protein 2 (MRP2) (ABCC2), alters substrate specificity and results in loss of methotrexate transport activity

The ATP-binding cassette (ABC) proteins comprise a large superfamily of transmembrane transporters that utilize the energy of ATP hydrolysis to translocate their substrates across biological membranes. Multidrug resistance protein (MRP) 2 (ABCC2) belongs to subfamily C of the ABC superfamily and, when overexpressed in tumor cells, confers resistance to a wide variety of anticancer chemotherapeutic agents. MRP2 is also an active transporter of organic anions such as methotrexate (MTX), estradiol glucuronide (E217betaG), and leukotriene C4 and is located on the apical membrane of polarized cells including hepatocytes where it acts as a biliary transporter. We recently identified a highly conserved tryptophan residue in the related MRP1 that is critical for the substrate specificity of this protein. In the present study, we have examined the effect of replacing the analogous tryptophan residue at position 1254 of MRP2. We found that only nonconservative substitutions (Ala and Cys) of Trp1254 eliminated [3H]E217betaG transport by MRP2, whereas more conservative substitutions (Phe and Tyr) had no effect. In addition, only the most conservatively substituted mutant (W1254Y) transported [3H]leukotriene C4, whereas all other substitutions eliminated transport of this substrate. On the other hand, all substitutions of Trp1254 eliminated transport of [3H]MTX. Finally, we found that sulfinpyrazone stimulated [3H]E217betaG transport by wild-type MRP2 4-fold, whereas transport by the Trp1254 substituted mutants was enhanced 6-10-fold. In contrast, sulfinpyrazone failed to stimulate [3H]MTX transport by either wild-type MRP2 or the MRP2-Trp1254 mutants. Taken together, our results demonstrate that Trp1254 plays an important role in the ability of MRP2 to transport conjugated organic anions and identify this amino acid in the putative last transmembrane segment (TM17) of this ABC protein as being critical for transport of MTX.

Single amino acid substitution of rat MRP2 results in acquired transport activity for taurocholate

Multidrug resistance-associated protein 3 (MRP3), unlike other MRPs, transports taurocholate (TC). The difference in TC transport activity between rat MRP2 and MRP3 was studied, focusing on the cationic amino acids in the transmembrane domains. For analysis, transport into membrane vesicles from Sf9 cells expressing wild-type and mutated MRP2 was examined. Substitution of Arg at position 586 with Leu and Ile and substitution of Arg at position 1096 with Lys, Leu, and Met resulted in the acquisition of TC transport activity, while retaining transport activity for glutathione and glucuronide conjugates. Substitution of Leu at position 1084 of rat MRP3 (which corresponds to Arg-1096 in rat MRP2) with Lys, but not with Val or Met, resulted in the loss of transport activity for TC and glucuronide conjugates. These results suggest that the presence of the cationic charge at Arg-586 and Arg-1096 in rat MRP2 prevents the transport of TC, whereas the presence of neutral amino acids at the corresponding position of rat MRP3 is required for the transport of substrates.

Elevated expression of vacuolar proton pump genes and cellular PH in cisplatin resistance

V-ATPases are proton-translocating enzymes, which are found not only in numerous intracellular organelles but also in the plasma membranes of many eukaryotic cells. Using differential display, we have identified one of the proton pump subunit genes, ATP6C, as a cisplatin-inducible gene. Northern blot analysis demonstrated that expression of other members of the subunit is inducible by cisplatin treatment. Proton pump gene expression is also upregulated in 3 independent cisplatin-resistant cell lines but not in vincristine- or etoposide-resistant cell lines. Cellular pH was significantly higher in cisplatin-resistant cells than in sensitive parental cells. In vitro DNA-binding activity of cisplatin was markedly increased in acidic conditions, suggesting that the cytotoxicity of cisplatin is modulated by cellular pH. Furthermore, the proton pump inhibitor bafilomycin can synergistically potentiate the cytotoxicity of cisplatin but not of etoposide or camptothecin. These results indicate that cellular pH is one of the critical parameters for effective cancer chemotherapy with cisplatin.

Association between expression of the MRP3 gene and exposure to platinum drugs in lung cancer

To investigate the roles played by the multidrug resistance-associated protein (MRP1) homologues MRP3 and MRP4 in resistance to platinum drugs, we examined steady-state levels of mRNA for both MRP3 and MRP4 in normal lung and lung cancer specimens as well as peripheral mononuclear cells (PMN) after platinum drug exposure. MRP3 and MRP4 gene expression levels were monitored in the PMN of 10 previously untreated lung cancer patients within 24 hr after carboplatin (CBDCA) administration. Expression levels for both genes were also examined in 80 autopsy samples (40 primary tumors and 40 corresponding normal lung tissues) from 40 patients with lung cancer. MRP3 and MRP4 gene expression levels were assessed by quantitative reverse transcription-polymerase chain reaction. MRP3 expression levels in the PMN rose rapidly within 24 hr after administration of CBDCA, whereas MRP4 expression levels remained the same. Furthermore, MRP3 expression levels in normal lung and tumor tissues from autopsy samples that had been exposed to platinum drugs while the patients were alive were significantly higher than those in unexposed tissues, but again MRP4 expression levels remained the same. These results suggest that platinum drugs and/or the physiological stress response to xenobiotics induce expression of the MRP3 gene.

Charged amino acids in the transmembrane domains are involved in the determination of the substrate specificity of rat Mrp2

Multidrug resistance-associated protein 2 (MRP2) transports glutathione conjugates, glucuronide conjugates, and sulfated conjugates of bile acids. In the present study, we examined the role of charged amino acids in the transmembrane domains of rat Mrp2, conserved among MRP families, using the isolated membrane vesicles from Sf9 cells infected with the recombinant baculoviruses. By normalizing the transport activity for compounds by that for estradiol 17beta-D-glucuronide (E(2)17betaG), it was indicated that the site-directed mutagenesis from Lys to Met at 325 (K325M) and from Arg to Leu at 586 (R586L) results in a marked reduction in the transport for glutathione conjugates [2,4-dinitrophenyl-S-glutathione (DNP-SG) and leukotriene (LT) C(4)] without affecting that for 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridymethyl) benzothiazole glucuronide and taurolithocholate sulfate. In contrast to the reduced affinity for DNP-SG, the affinity for E(2)17betaG was increased severalfold in these mutant Mrp2s, suggesting the amino acids at 325 and 586 play an important role in distinguishing between glutathione and glucuronide conjugates. The comparable affinity for LTD(4), LTE(4), and LTF(4) in these mutant Mrp2s with that in wild-type Mrp2 indicates that recognition of LTC(4) metabolites by Mrp2 is different from that of LTC(4). The transport activity for glutathione conjugate was retained on R586K, whereas no such complementary cationic amino acid effect was observed in K325R. In addition, R1206M and E1208Q exhibited the loss of transport activity for the tested compounds. The results of the present study demonstrate that the charged amino acids in the transmembrane domain of rat Mrp2 may play an important role in the recognition and/or transport of its substrates.

Involvement of the multidrug resistance protein 3 in drug sensitivity and its expression in human glioma

The multidrug resistance protein (MRP) family belongs to the ATP-binding cassette superfamily (ABC) of transporters, which are involved in ATP-dependent transport of hydrophobic compounds. One of the MRP family, MRP1, is partially associated with the multidrug resistance phenotype in brain tumors. In this study, we asked whether another MRP family gene, MRP3, could affect drug sensitivity to anticancer agents in human glioma cell lines and clinical glioma specimens. We first produced two antisense transfectants by introduction of antisense MRP3 cDNA into the glioma cell line NHG2, which endogenously expresses MRP3. The two MRP3 antisense transfectants showed 2- to 5-fold increases in drug sensitivity to etoposide and cisplatin compared with NHG2 cells, but their sensitivity to vincristine or nitrosourea was not changed. Two MRP3 cDNA sense transfectants of pig kidney cell lines showed 4- to 6-fold drug resistance to etoposide, but only 1.4- to 1.5-fold to cisplatin. We next compared the mRNA levels of four ABC transporters, multidrug resistance 1 (MDR1), MRP1, MRP2 and MRP3 in clinical samples, including 34 patients with gliomas, by quantitative RT-PCR analysis. In some of the clinical samples, increased expression of MRP1 and MRP3 was apparent in malignant gliomas. In situ hybridization revealed that glioma cells were stained with MRP3 probe. MRP3 may modulate drug sensitivity to certain anticancer agents in human gliomas.

Analysis of the structure and expression pattern of MRP7 (ABCC10), a new member of the MRP subfamily

The MRP subfamily of ABC transporters currently consists of at least six members, several of which have been demonstrated to transport amphipathic anions and to confer in vitro resistance to chemotherapeutic agents. In searching the data bases we identified the product of a cDNA sequencing project that bears significant similarity to MRP subfamily transporters. In this report the predicted coding sequence, protein product and expression pattern of this cDNA, termed MRP7, are analyzed. The MRP7 cDNA sequence encodes a 1492 amino acid ABC transporter whose structural architecture resembles that of MRP1, MRP2, MRP3, and MRP6, in that its transmembrane helices are arranged in three membrane spanning domains. However, in contrast to the latter transporters, a conserved N-linked glycosylation site is not found at the N-terminus of MRP7. Comparisons of the MRP7 amino acid sequence indicated that while it is most closely related to other MRP subfamily members, its degree of relatedness is the lowest of any of the known MRP-related transporters. The integrity of the predicted MRP7 coding sequence was confirmed by the synthesis of an approximately 158 kDa protein in reticulocyte lysates programmed with the MRP7 cDNA. While MRP7 transcript was detected in a variety of tissues by RT/PCR, it was not readily detectable by RNA blot analysis, suggesting that it is expressed at low levels in these tissues. Fluorescence in situ hybridization indicated that MRP7 maps to chromosome 6p12-21, in proximity to several genes associated with glutathione conjugation and synthesis. On the basis of these findings and evolutionary cluster analysis, we conclude that MRP7 is a member of the MRP subfamily of amphipathic anion transporters.

Clinical perspectives on platinum resistance

The platinum compounds cisplatin and carboplatin are widely used in the treatment of a number of solid malignancies. Although some platinum-sensitive tumours may be cured by combination chemotherapy (e.g. testicular cancer), most will relapse and subsequently prove resistant to platinum compounds. The mechanisms of platinum resistance in patients are still poorly understood. Clearly, when a tumour relapses a long time after successful first-line treatment, there is a high chance that it will still be sensitive to platinum compounds. A number of studies have attempted to assess the role of drug transport, the glutathione system, DNA repair and apoptosis genes in the development of resistance in tumours, but no conclusive evidence is available. Approaches to increasing the potency of platinum therapy (to overcome resistance) have been devised and some have proved to be effective; in particular, intraperitoneal administration of cisplatin has shown superiority over intravenous administration in selected patients with ovarian cancer. The development of drugs and techniques to reduce the adverse effects of platinum chemotherapy has greatly improved their administration. Investigations attempting to modulate platinum activity and toxicity have also been performed. Further investigation of in vivo resistance mechanisms should be valuable in allowing prediction of clinical response to chemotherapy and may identify new treatments with the potential to improve outcomes for patients with a variety of platinum-resistant tumour types.

Molecular pharmacology of renal organic anion transporters

Renal organic anion transport systems play an important role in the elimination of drugs, toxic compounds, and their metabolites, many of which are potentially harmful to the body. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine of a wide variety of anionic substrates. Recent studies have shown that organic anion secretion in renal proximal tubule is mediated by distinct sodium-dependent and sodium-independent transport systems. Knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past few years by cloning of various carrier proteins. However, a number of fundamental questions still have to be answered to elucidate the participation of the cloned transporters in the overall tubular secretion of anionic xenobiotics. This review summarizes the latest knowledge on molecular and pharmacological properties of renal organic anion transporters and homologs, with special reference to their nephron and plasma membrane localization, transport characteristics, and substrate and inhibitor specificity. A number of the recently cloned transporters, such as the p-aminohippurate/dicarboxylate exchanger OAT1, the anion/sulfate exchanger SAT1, the peptide transporters PEPT1 and PEPT2, and the nucleoside transporters CNT1 and CNT2, are key proteins in organic anion handling that possess the same characteristics as has been predicted from previous physiological studies. The role of other cloned transporters, such as MRP1, MRP2, OATP1, OAT-K1, and OAT-K2, is still poorly characterized, whereas the only information that is available on the homologs OAT2, OAT3, OATP3, and MRP3-6 is that they are expressed in the kidney, but their localization, not to mention their function, remains to be elucidated.

Comparison of the functional characteristics of the nucleotide binding domains of multidrug resistance protein 1

Multidrug Resistance Protein 1 (MRP1) transports diverse organic anionic conjugates and confers resistance to cytotoxic xenobiotics. The protein contains two nucleotide binding domains (NBDs) with features characteristic of members of the ATP-binding cassette superfamily and exhibits basal ATPase activity that can be stimulated by certain substrates. It is not known whether the two NBDs of MRP1 are functionally equivalent. To investigate this question, we have used a baculovirus dual expression vector encoding both halves of MRP1 to reconstitute an active transporter and have compared the ability of each NBD to be photoaffinity-labeled with 8-azido-[(32)P]ATP and to trap 8-azido-[(32)P]ADP in the presence of orthovanadate. We found that NBD1 was preferentially labeled with 8-azido-[(32)P]ATP, while trapping of 8-azido-[(32)P]ADP occurred predominantly at NBD2. Although trapping at NBD2 was dependent on co-expression of both halves of MRP1, binding of 8-azido-ATP by NBD1 remained detectable when the NH(2)-proximal half of MRP1 was expressed alone and when NBD1 was expressed as a soluble polypeptide. Mutation of the conserved Walker A lysine 684 or creation of an insertion mutation between Walker A and B motifs eliminated binding by NBD1 and all detectable trapping of 8-azido-ADP at NBD2. Both mutations decreased leukotriene C(4) (LTC(4)) transport by approximately 70%. Mutation of the NBD2 Walker A lysine 1333 eliminated trapping of 8-azido-ADP by NBD2 but, in contrast to the mutations in NBD1, essentially eliminated LTC(4) transport activity without affecting labeling of NBD1 with 8-azido-[(32)P]ATP.

Characterization of 5'-flanking region of human MRP3

We previously cloned MRP3, which is responsible for the cellular extrusion of organic anions, as an inducible transporter in the liver under cholestatic conditions. In the present study, we investigated the mechanism for the expression of human MRP3. The cap site hunting method revealed that the transcription starts at -25 and -27 nt upstream of the initiation codon. Luciferase assay with a series of truncated 5'-flanking regions indicated that the region from -127 to -23 nt is important for MRP3 expression. Moreover, carrying out a gel shift assay indicated that Sp1 binds to the sequence between -92 and -58 nt. Collectively, it was demonstrated that human MRP3 is under the control of TATA-less promoter and Sp1 binding sites may be involved in the transcription.

MRP2, a human conjugate export pump, is present and transports fluo 3 into apical vacuoles of Hep G2 cells

The multidrug resistance protein 2 (MRP2, symbol ABCC2) transports anionic conjugates and certain amphiphilic anions across the apical membrane of polarized cells. Human hepatoma Hep G2 cells retain hepatic polarity and form apical vacuoles into which cholephilic substances are secreted. Immunofluorescence microscopy showed that human MRP2 was expressed in the apical vacuole membrane of polarized Hep G2 cells, whereas the isoform MRP3 was localized to the lateral membrane. Expression of both MRP2 and MRP3 was confirmed by immunoblotting and reverse transcription PCR. Fluo 3 secretion into the apical vacuoles was inhibited by cyclosporin A but not by selective inhibitors of multidrug resistance 1 P-glycoprotein. In addition, carboxyfluorescein, rhodamine 123, and the fluorescent bile salt derivatives ursodeoxycholyl-(Nepsilon-nitrobenzoxadiazolyl)-lysine and cholylglycylamido-fluorescein were secreted into the apical vacuoles; the latter two probably via the bile salt export pump. We conclude that MRP2 mediates fluo 3 secretion into the apical vacuoles of polarized Hep G2 cells. Thus the function of human MRP2 and the action of inhibitors can be analyzed by the secretion of fluorescent anions such as fluo 3.

Detailed structural analysis on both human MRP5 and mouse mrp5 transcripts

The multidrug-resistant phenotype in tumor cells is attributed in part to anti-cancer drug efflux transporters such as the MRP family. The amino-terminal structure of MRP5 has not been refined. To determine the amino-terminal structure of a major transcript of the MRP5 gene, we performed primer extension analysis to determine a major transcriptional start site of this gene and compared the structure of human MRP5 and that of mouse mrp5. We successfully determined the structures of human MRP5 and mouse mrp5. Estimated amino acid sequences are 1437 and 1436 amino acids for human MRP5 and mouse mrp5 respectively, and were highly conserved (94.1%). We further showed that our previously identified SMRP mRNA was a splicing variant of the MRP5 gene, which was expressed in various human tissues, suggesting that a short form of MRP5 protein encoded by the SMRP mRNA may have a physiological role.

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.