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

ATP-Binding Cassette and Solute Carrier Transporters: Understanding Their Mechanisms and Drug Modulation Through Structural and Modeling Approaches

The ATP-binding cassette (ABC) and solute carrier (SLC) transporters play pivotal roles in cellular transport mechanisms, influencing a wide range of physiological processes and impacting various medical conditions. Recent advancements in structural biology and computational modeling have provided significant insights into their function and regulation. This review provides an overview of the current knowledge of human ABC and SLC transporters, emphasizing their structural and functional relationships, transport mechanisms, and the contribution of computational approaches to their understanding. Current challenges and promising future research and methodological directions are also discussed.

Blood-brain barrier transporters: An overview of function, dysfunction in Alzheimer's disease and strategies for treatment

The blood-brain-barrier (BBB) has a major function in maintaining brain homeostasis by regulating the entry of molecules from the blood to the brain. Key players in BBB function are BBB transporters which are highly expressed in brain endothelial cells (BECs) and critical in mediating the exchange of nutrients and waste products. BBB transporters can also influence drug delivery into the brain by inhibiting or facilitating the entry of brain targeting therapeutics for the treatment of brain disorders, such as Alzheimer's disease (AD). Recent studies have shown that AD is associated with a disrupted BBB and transporter dysfunction, although their roles in the development in AD are not fully understand. Modulation of BBB transporter activity may pose a novel approach to enhance the delivery of drugs to the brain for enhanced treatment of AD. In this review, we will give an overview of key functions of BBB transporters and known changes in AD. In addition, we will discuss current strategies for transporter modulation for enhanced drug delivery into the brain.

Drug transporters in drug disposition - the year 2022 in review

On behalf of all the authors, I am pleased to share our third annual review on drug transporter science with an emphasis on articles published and deemed influential in signifying drug transporters' role in drug disposition in the year 2022. As the drug transporter field is rapidly evolving several key findings were noted including promising endogenous biomarkers, rhythmic activity, IVIVE approaches in transporter-mediated clearance, new modality interaction, and transporter effect on gut microbiome. As identified previously (Chothe et Cal. 2021, 2022) the goal of this review is to highlight key findings without a comprehensive overview of each article and to this end, each coauthor independently selected 1-3 peer-reviewed articles published or available online in the year 2022 (Table 1). Each article is summarized in synopsis and commentary with unbiased viewpoints by each coauthor. We strongly encourage readers to consult original articles for specifics of the study. Finally, I would like to thank all coauthors for their continued support in writing this annual review on drug transporters and invite anyone interested in contributing to future versions of this review.

Structure and Mechanism of Human ABC Transporters

ABC transporters are essential for cellular physiology. Humans have 48 ABC genes organized into seven distinct families. Of these genes, 44 (in five distinct families) encode for membrane transporters, of which several are involved in drug resistance and disease pathways resulting from transporter dysfunction. Over the last decade, advances in structural biology have vastly expanded our mechanistic understanding of human ABC transporter function, revealing details of their molecular arrangement, regulation, and interactions, facilitated in large part by advances in cryo-EM that have rendered hitherto inaccessible targets amenable to high-resolution structural analysis. As a result, experimentally determined structures of multiple members of each of the five families of ABC transporters in humans are now available. Here we review this recent progress, highlighting the physiological relevance of human ABC transporters and mechanistic insights gleaned from their direct structure determination. We also discuss the impact and limitations of model systems and structure prediction methods in understanding human ABC transporters and discuss current challenges and future research directions.

Neonatal Dubin-Johnson Syndrome and its Differentiation from Biliary Atresia

BACKGROUND AND AIMS

The aim was to determine if liver biochemistry indices can be used as biomarkers to help differentiate patients with neonatal Dubin-Johnson syndrome (nDJS) from those with biliary atresia (BA).

METHODS

Patients with genetically-confirmed nDJS or cholangiographically confirmed BA were retrospectively enrolled and randomly assigned to discovery or verification cohorts. Their liver chemistries, measured during the neonatal period, were compared. Predictive values were calculated by receiver operating characteristic curve analysis.

RESULTS

A cohort of 53 nDJS patients was recruited, of whom 13 presented with acholic stools, and 14 underwent diagnostic cholangiography or needle liver biopsy to differentiate from BA. Thirty-five patients in the cohort, with complete biochemical information measured during the neonatal period, were compared with 133 infants with cholangiographically confirmed BA. Total and direct bilirubin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bile acids, alkaline phosphatase, and gamma-glutamyl transferase were significantly lower in nDJS than in BA. In the discovery cohort, the areas under the curve for ALT and AST were 0.908 and 0.943, respectively. In the validation cohort, 13/15 patients in the nDJS group were classified as nDJS, and 10/53 in the BA control group were positive (p<0.00001) with an ALT biomarker cutoff value of 75 IU/L. Thirteen of 15 patients were classified as nDJS and none were classified positive in the BA group (13/15 vs. 0/53, p<0.00001) with an AST cutoff of 87 IU/L.

CONCLUSIONS

Having assembled and investigated the largest cohort of nDJS patients reported to date, we found that nDJS patients could be distinguished from BA patients using the serum AST level as a biomarker. The finding may be clinically useful to spare cholestatic nDJS patients unnecessary invasive procedures.

The human ATP-binding cassette (ABC) transporter superfamily

The ATP-binding cassette (ABC) transporter superfamily comprises membrane proteins that efflux various substrates across extra- and intracellular membranes. Mutations in ABC genes cause 21 human disorders or phenotypes with Mendelian inheritance, including cystic fibrosis, adrenoleukodystrophy, retinal degeneration, cholesterol, and bile transport defects. To provide tools to study the function of human ABC transporters we compiled data from multiple genomics databases. We analyzed ABC gene conservation within human populations and across vertebrates and surveyed phenotypes of ABC gene mutations in mice. Most mouse ABC gene disruption mutations have a phenotype that mimics human disease, indicating they are applicable models. Interestingly, several ABCA family genes, whose human function is unknown, have cholesterol level phenotypes in the mouse. Genome-wide association studies confirm and extend ABC traits and suggest several new functions to investigate. Whole-exome sequencing of tumors from diverse cancer types demonstrates that mutations in ABC genes are not common in cancer, but specific genes are overexpressed in select tumor types. Finally, an analysis of the frequency of loss-of-function mutations demonstrates that many human ABC genes are essential with a low level of variants, while others have a higher level of genetic diversity.

The Genetics of Inherited Cholestatic Disorders in Neonates and Infants: Evolving Challenges

Many inherited conditions cause cholestasis in the neonate or infant. Next-generation sequencing methods can facilitate a prompt diagnosis in some of these cases; application of these methods in patients with liver diseases of unknown cause has also uncovered novel gene-disease associations and improved our understanding of physiological bile secretion and flow. By helping to define the molecular basis of certain cholestatic disorders, these methods have also identified new targets for therapy as well patient subgroups more likely to benefit from specific therapies. At the same time, sequencing methods have presented new diagnostic challenges, such as the interpretation of single heterozygous genetic variants. This article discusses those challenges in the context of neonatal and infantile cholestasis, focusing on difficulties in predicting variant pathogenicity, the possibility of other causal variants not identified by the genetic screen used, and phenotypic variability among patients with variants in the same genes. A prospective, observational study performed between 2010-2013, which sequenced six important genes (ATP8B1, ABCB11, ABCB4, NPC1, NPC2 and SLC25A13) in an international cohort of 222 patients with infantile liver disease, is given as an example of potential benefits and challenges that clinicians could face having received a complex genetic result. Further studies including large cohorts of patients with paediatric liver disease are needed to clarify the spectrum of phenotypes associated with, as well as appropriate clinical response to, single heterozygous variants in cholestasis-associated genes.

Rat models of human diseases and related phenotypes: a systematic inventory of the causative genes

The laboratory rat has been used for a long time as the model of choice in several biomedical disciplines. Numerous inbred strains have been isolated, displaying a wide range of phenotypes and providing many models of human traits and diseases. Rat genome mapping and genomics was considerably developed in the last decades. The availability of these resources has stimulated numerous studies aimed at discovering causal disease genes by positional identification. Numerous rat genes have now been identified that underlie monogenic or complex diseases and remarkably, these results have been translated to the human in a significant proportion of cases, leading to the identification of novel human disease susceptibility genes, helping in studying the mechanisms underlying the pathological abnormalities and also suggesting new therapeutic approaches. In addition, reverse genetic tools have been developed. Several genome-editing methods were introduced to generate targeted mutations in genes the function of which could be clarified in this manner [generally these are knockout mutations]. Furthermore, even when the human gene causing a disease had been identified without resorting to a rat model, mutated rat strains (in particular KO strains) were created to analyze the gene function and the disease pathogenesis. Today, over 350 rat genes have been identified as underlying diseases or playing a key role in critical biological processes that are altered in diseases, thereby providing a rich resource of disease models. This article is an update of the progress made in this research and provides the reader with an inventory of these disease genes, a significant number of which have similar effects in rat and humans.

Genetic contribution of ABCC2 to Dubin-Johnson syndrome and inherited cholestatic disorders

BACKGROUND AND AIMS

The ABCC2 gene is implicated in Dubin-Johnson syndrome (DJS), a rare autosomal recessive liver disorder. The primary aim of this study was to determine the diagnostic value of ABCC2 genetic testing in the largest cohort of DJS reported to date. The high number of patients with cholestatic manifestations in this series prompted us to evaluate the genetic contribution of rare, potentially pathogenic ABCC2 variants to other inherited cholestatic disorders.

METHODS

The cohort study included 32 patients with clinical DJS diagnosis, and 372 patients referred for the following disorders: low phospholipid-associated cholelithiasis (LPAC) syndrome, intrahepatic cholestasis of pregnancy (ICP) and benign recurrent intrahepatic cholestasis (BRIC). ABCC2 was screened by next-generation sequencing.

RESULTS

Most patients with clinical DJS had positive genetic diagnosis (n = 30; 94%), with a great diversity of point mutations and copy number variations in ABCC2. Strikingly, eight (27%) of these patients showed transient cholestatic features at presentation: four neonatal cholestasis, two ICP, one contraceptive-induced cholestasis and one sporadic cholestasis. Conversely, the frequency of rare, heterozygous, potentially pathogenic ABCC2 variants in patients with LPAC, ICP or BRIC did not differ significantly from that of the general population.

CONCLUSIONS

This large series reveals that DJS is a highly homogeneous Mendelian disorder involving a large spectrum of ABCC2 variants. Genetic testing is crucial to establish early DJS diagnosis in patients with atypical presentations, such as neonatal cholestasis. This study also provides no evidence for the contribution of rare, potentially pathogenic ABCC2 variants to other inherited cholestatic disorders.

Next generation sequencing reveals co-existence of hereditary spherocytosis and Dubin–Johnson syndrome in a Chinese gril: A case report

BACKGROUND

Hereditary spherocytosis (HS) is a hereditary disease of hemolytic anemia that occurs due to the erythrocyte membrane defects. Dubin–Johnson syndrome (DJS), which commonly results in jaundice, is a benign hereditary disorder of bilirubin clearance that occurs only rarely. The co-occurrence of HS and DJS is extremely rare. We recently diagnosed and treated a case of co-occurring HS and DJS.

CASE SUMMARY

A 21-year-old female patient presented to our department because of severe jaundice, severe splenomegaly, and mild anemia since birth. We eventually confirmed the diagnosis of co-occurring DJS and HS by next generation sequencing (NGS). The treatment of ursodeoxycholic acid in combination with phenobarbital successfully increased hemoglobin and reduced total bilirubin and direct bilirubin.

CONCLUSION

The routine application of NGS can efficiently render a definite diagnosis when inherited disorders are suspected.

[Clinical features and ABCC2 genotypic analysis of an infant with Dubin-Johnson syndrome]

Dubin-Johnson syndrome (DJS) is an autosomal recessive disorder resulting from biallelic mutations of ABCC2 gene, with long-term or intermittent conjugated hyperbilirubinemia being the main clinical manifestation. This paper aims to report the clinical features and ABCC2 genotypes of an infant with DJS. A 9.5-month-old male infant was referred to the hospital due to abnormal liver function discovered over 9 months. The major clinical presentation was prolonged jaundice since neonatal period. A series of biochemistry analysis revealed markedly elevated total bilirubin, conjugated bilirubin and total bile acids. The patient had been managed in different hospitals, but the therapeutic effects were unsatisfactory due to undetermined etiology. Physical examination revealed jaundiced skin and sclera, and a palpable liver 3 cm below the right subcostal margin with medium texture. The spleen was not enlarged. Genetic analysis revealed a splice-site variant c.3988-2A>T and a nonsense variant c.3825C>G (p.Y1275X) in the ABCC2 gene of the infant, which were inherited from his mother and father respectively. The former had not been previously reported. Then ursodeoxycholic acid and phenobarbital were given orally. Half a month later, as a result, his jaundice disappeared and the biochemistry indices improved. However, the long-term outcome needs to be observed. Literature review revealed that neonates/infants with DJS presented with cholestatic jaundice soon after birth as the major clinical feature, and the ABCC2 variants exhibited marked heterogeneity.

Clinical, Pathologic, and Genetic Features of Neonatal Dubin-Johnson Syndrome: A Multicenter Study in Japan

OBJECTIVE

To clarify the clinical, pathologic, and genetic features of neonatal Dubin-Johnson syndrome.

STUDY DESIGN

Ten patients with neonatal Dubin-Johnson syndrome were recruited from 6 pediatric centers in Japan between September 2013 and October 2016. Clinical and laboratory course, macroscopic and microscopic liver findings, and molecular genetic findings concerning ATP-binding cassette subfamily C member 2 (ABCC2) were retrospectively and prospectively examined.

RESULTS

All neonates exhibited cholestasis, evident as prolonged jaundice with or without acholic stools and elevations of serum direct bilirubin as well as γ-glutamyltransferase or total bile acids. Only 38% (3 of 8) of patients who underwent liver biopsy showed a grossly black liver or melanin-like pigment deposits in hepatocytes; their biopsies were performed in early infancy. Immunohistochemically, all liver specimens showed no expression of multidrug resistance-associated protein 2 but increased expression of the bile salt export pump protein. Homozygous or compound heterozygous pathogenic variants of ABCC2 were identified in all patients, representing 11 distinct pathogenic variants including 2 not previously reported.

CONCLUSIONS

Immunohistochemical staining of the liver for multidrug resistance-associated protein 2 and molecular genetic analysis of ABCC2 are crucial for accurate diagnosis of neonatal Dubin-Johnson syndrome.

Beyond Competitive Inhibition: Regulation of ABC Transporters by Kinases and Protein-Protein Interactions as Potential Mechanisms of Drug-Drug Interactions

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters mediating the extrusion of an array of substrates ranging from amino acids and lipids to xenobiotics, and many therapeutic compounds, including anticancer drugs. The ABC transporters are also recognized as important contributors to pharmacokinetics, especially in drug-drug interactions and adverse drug effects. Drugs and xenobiotics, as well as pathologic conditions, can influence the transcription of ABC transporters, or modify their activity or intracellular localization. Kinases can affect the aforementioned processes for ABC transporters as do protein interactions. In this review, we focus on the ABC transporters ABCB1, ABCB11, ABCC1, ABCC4, and ABCG2 and illustrate how kinases and protein-protein interactions affect these transporters. The clinical relevance of these factors is currently unknown; however, these examples suggest that our understanding of drug-drug interactions will benefit from further knowledge of how kinases and protein-protein interactions affect ABC transporters.

Mutation responsible for congenital photosensitivity and hyperbilirubinemia in Southdown sheep

OBJECTIVE To identify the genetic cause for congenital photosensitivity and hyperbilirubinemia (CPH) in Southdown sheep. ANIMALS 73 Southdown sheep from a CPH research flock and 48 sheep of various breeds from commercial flocks without CPH. PROCEDURES Whole-genome sequencing was performed for a phenotypically normal Southdown sheep heterozygous for CPH. Heterozygous variants within Slco1b3 coding exons were identified, and exons that contained candidate mutations were amplified by PCR assay methods for Sanger sequencing. Blood samples from the other 72 Southdown sheep of the CPH research flock were used to determine plasma direct and indirect bilirubin concentrations. Southdown sheep with a plasma total bilirubin concentration < 0.3 mg/dL were classified as controls, and those with a total bilirubin concentration ≥ 0.3 mg/dL and signs of photosensitivity were classified as mutants. Sanger sequencing was used to determine the Slco1b3 genotype for all sheep. Genotypes were compared between mutants and controls of the CPH research flock and among all sheep. Protein homology was measured across 8 species to detect evolutionary conservation of Slco1b. RESULTS A nonsynonymous mutation at ovine Chr3:193,691,195, which generated a glycine-to-arginine amino acid change within the predicted Slco1b3 protein, was significantly associated with hyperbilirubinemia and predicted to be deleterious. That amino acid was conserved across 7 other mammalian species. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested a nonsynonymous mutation in Slco1b3 causes CPH in Southdown sheep. This disease appears to be similar to Rotor syndrome in humans. Sheep with CPH might be useful animals for Rotor syndrome research.

The Effect of Albumin on MRP2 and BCRP in the Vesicular Transport Assay

The ABC transporters multidrug resistance associated protein 2 (MRP2) and breast cancer resistance protein (BCRP) are of interest in drug development, since they affect the pharmacokinetics of several drugs. Membrane vesicle transport assays are widely used to study interactions with these proteins. Since albumin has been found to affect the kinetics of metabolic enzymes in similar membrane preparations, we investigated whether albumin affects the kinetic parameters of efflux transport. We found that albumin increased the Vmax of 5(6)-carboxy-2',7'-dichlorofluorescein (CDCF) and estradiol-17-β-D-glucuronide uptake into MRP2 vesicles in the presence of 0.1% bovine serum albumin (BSA) by 2 and 1.5-fold, respectively, while BSA increased Lucifer yellow uptake by 30% in BCRP vesicles. Km values increased slightly, but the change was not statistically significant. The effect of BSA on substrate uptake was dependent on the vesicle amount, while increasing BSA concentration did not significantly improve substrate uptake. These results indicate a minor effect of albumin on MRP2 and BCRP, but it should be considered if albumin is added to transporter assays for example as a solubilizer, since the effect may be substrate or transporter specific.

Genetic and biochemical study of dual hereditary jaundice: Dubin-Johnson and Gilbert's syndromes. Haplotyping and founder effect of deletion in ABCC2

Dual hereditary jaundice, a combination of Dubin-Johnson and Gilbert's syndromes, is a rare clinical entity resulting from the compound defects of bilirubin conjugation and transport. We aimed to study the hereditary jaundice in 56 members from seven seemingly unrelated Roma families, to find the causal genetic defect and to estimate its origin in Roma population. On the basis of biochemical results of total and conjugated serum bilirubin and clinical observations, ABCC2 gene, TATA box and phenobarbital enhancer (PBREM) of UGT1A1 gene were analyzed by sequencing, RFLP and fragment analysis. We found a novel variant c.1013_1014delTG in the eighth exon of ABCC2 gene in 17 individuals in homozygous state. Dual defect NG_011798.1:c.[1013_1014delTG]; NG_002601.2:g.[175492_175493insTA] in homozygous state was found in four subjects. Biochemical analyses of porphyrins and coproporphyrin isomers in urine performed by HPLC showed inverted ratio of excreted coproporphyrin, with the predominance of coproporphyrin I (up to 100%), typical for patients with Dubin-Johnson syndrome. Pursuant cultural and social specifics of the population led us to suspect a founder effect; therefore, we performed a haplotype study using genotyping data from Affymetrix Genome-Wide Human SNP Array 6.0. As a result, we detected a common 86 kbp haplotype encompassing promoter and part of the ABCC2 coding region among all families, and estimated the age of the ancestral variant to 178-185 years. In this study, we found a novel deletion in ABCC2 gene, described genetic and biochemical features of dual hereditary jaundice and confirmed the existence of founder effect and common haplotype among seven Roma families.

Influence of multidrug resistance and drug transport proteins on chemotherapy drug metabolism

INTRODUCTION

Chemotherapy involving the use of anticancer drugs remains an important strategy in the overall management of patients with metastatic cancer. Acquisition of multidrug resistance remains a major impediment to successful chemotherapy. Drug transporters in cell membranes and intracellular drug metabolizing enzymes contribute to the resistance phenotype and determine the pharmacokinetics of anticancer drugs in the body.

AREAS COVERED

ATP-binding cassette (ABC) transporters mediate the transport of endogenous metabolites and xenobiotics including cytotoxic drugs out of cells. Solute carrier (SLC) transporters mediate the influx of cytotoxic drugs into cells. This review focuses on the substrate interaction of these transporters, on their biology and what role they play together with drug metabolizing enzymes in eliminating therapeutic drugs from cells.

EXPERT OPINION

The majority of anticancer drugs are substrates for the ABC transporter and SLC transporter families. Together, these proteins have the ability to control the influx and the efflux of structurally unrelated chemotherapeutic drugs, thereby modulating the intracellular drug concentration. These interactions have important clinical implications for chemotherapy because ultimately they determine therapeutic efficacy, disease progression/relapse and the success or failure of patient treatment.

Report of new haplotype for ABCC2 gene: rs17222723 and rs8187718 in cis

The ATP-binding cassette, subfamily C [CFTR/MRP], member 2 (ABCC2) gene is a member of the ATP-binding cassette transporters and is involved in the transport of molecules across cellular membranes. Substrates transported by ABCC2 include antiepileptics, statins, tenofovir, cisplatin, irinotecan, and carbamazepine. Because of the pharmacogenomics implications, we developed a clinical laboratory-developed assay to test for seven variants in the ABCC2 gene: c.3563T>A (p.V1188E, rs17222723), c.1249G>A (p.V417I, rs2273697), c.3972C>T (p.I1324I, rs3740066), c.2302C>T (p.R768W, rs56199535), c.2366C>T (p.S789F, rs56220353), c.-24C>T (5'UTR, rs717620), and c.4544G>A (p.C1515Y, rs8187710). During the validation process, we noted several DNA samples, obtained from the Coriell Cell Repository, that contained both c.3563T>A, c.4544G>A, and a third variant, suggesting that c.3563T>A and c.4544G>A are in cis on the chromosome in some individuals. We obtained DNA samples from a trio (father, mother, and child), tested their ABCC2 variants, and confirmed that c.3563T>A and c.4544G>A were in cis on the same chromosome. Here, we report a new haplotype in ABCC2.

Transporters at CNS barrier sites: obstacles or opportunities for drug delivery?

The blood-brain barrier (BBB) and blood-cerebrospinal fluid (BCSF) barriers are critical determinants of CNS homeostasis. Additionally, the BBB and BCSF barriers are formidable obstacles to effective CNS drug delivery. These brain barrier sites express putative influx and efflux transporters that precisely control permeation of circulating solutes including drugs. The study of transporters has enabled a shift away from "brute force" approaches to delivering drugs by physically circumventing brain barriers towards chemical approaches that can target specific compounds of the BBB and/or BCSF barrier. However, our understanding of transporters at the BBB and BCSF barriers has primarily focused on understanding efflux transporters that efficiently prevent drugs from attaining therapeutic concentrations in the CNS. Recently, through the characterization of multiple endogenously expressed uptake transporters, this paradigm has shifted to the study of brain transporter targets that can facilitate drug delivery (i.e., influx transporters). Additionally, signaling pathways and trafficking mechanisms have been identified for several endogenous BBB/BCSF transporters, thereby offering even more opportunities to understand how transporters can be exploited for optimization of CNS drug delivery. This review presents an overview of the BBB and BCSF barrier as well as the many families of transporters functionally expressed at these barrier sites. Furthermore, we present an overview of various strategies that have been designed and utilized to deliver therapeutic agents to the brain with a particular emphasis on those approaches that directly target endogenous BBB/BCSF barrier transporters.

A cisplatin-resistant head and neck cancer cell line with cytoplasmic p53(mut) exhibits ATP-binding cassette transporter upregulation and high glutathione levels

PURPOSE

Head and neck squamous cell carcinoma (HNSCC) cell lines with cytoplasmically sequestered mutant p53 (p53(mut_c)) are frequently more resistant to cisplatin (CDDP) than cells with mutant but nuclear p53 (p53(mut_n)). The aim of the study was to identify underlying mechanisms implicated in CDDP resistance of HNSCC cells carrying cytoplasmic p53(mut).

METHODS

Microarray analysis, quantitative reverse transcription polymerase chain reaction, Western blot analysis and immunocytochemistry were used to identify and evaluate candidate genes involved in CDDP resistance of p53(mut_c) cells. RNAi knockdown or treatment with inhibitors together with flow cytometry-based methods was used for functional assessment of the identified candidate genes. Cellular metabolic activity was assessed with the XTT assay, and the redox capacity of cells was evaluated by measuring cellular glutathione (GSH) levels.

RESULTS

Upregulation of ABCC2 and ABCG2 transporters was observed in CDDP-resistant p53(mut_c) HNSCC cells. Furthermore, p53(mut_c) cells exhibited a pronounced side population that could be suppressed by RNAi knockdown of ABCG2 as well as treatment with the ATP-binding-cassette transporter inhibitors imatinib, MK571 and tariquidar. Metabolic activity and cellular GSH levels were higher in CDDP-resistant p53(mut_c) cells, consistent with a higher capacity to fend off cytotoxic oxidative effects such as those caused by CDDP treatment. Finally, ABCC2/G2 inhibition of HNSCC cells with MK571 markedly enhanced CDDP sensitivity of HNSCC cells.

CONCLUSIONS

The observations in this study point to a major role of p53(mut_c) in conferring a stem cell like phenotype to HNSCC cells that is associated with ABCC2/G2 overexpression, high GSH and metabolic activity levels as well as CDDP resistance.

Quantification of biliary excretion and sinusoidal excretion of 5(6)-carboxy-2',7'-dichlorofluorescein (CDF) in cultured hepatocytes isolated from Sprague Dawley, Wistar and Mrp2-deficient Wistar (TR(-)) rats

Hepatic efflux of drug candidates is an important issue in pre-clinical drug development. Here we utilise a method which quantifies and distinguishes efflux of drugs at the canalicular and sinusoidal membranes in rat hepatocyte cultures. Bi-phasic kinetics of transport of 5(6)-carboxydichlorofluorescein (CDF) at the canalicular membrane was demonstrated in Sprague Dawley (SD) and Wistar (W) rat hepatocytes. The high affinity component (Km=3.2±0.8μM (SD), 9.0±3.1μM (W)) was attributed to Mrp2-mediated transport, the low affinity component (Km=192.1±291.5μM (SD), 69.2±36.2μM (W)) may be attributed to transport involving a separate Mrp2 binding site. Data from membranes (Hill coefficient (h)=2.0±0.5) and vesicles (h=1.6±0.2) expressing Mrp2 and from SD (h=1.6±0.4) and Wistar (h=4.0±0.6) hepatocytes suggests transport involves more than one binding site. In TR(-) hepatocytes, CDF efflux was predominantly over the sinusoidal membrane (Km=100.7±36.0μM), consistent with low abcc2 (Mrp2) expression and compensatory increase in abcc3 (Mrp3) expression. This report shows the potential of using this in vitro method to model changes in biliary excretion due to alterations in transporter expression.

The two major isoforms of thyroid hormone receptor, TRα1 and TRβ1, preferentially partner with distinct panels of auxiliary proteins

Thyroid hormone receptors (TRs) are expressed primarily as two major isoforms, TRα1 and TRβ1, which are expressed at different times in development and at different tissue abundances in the adult. The transcription properties and biological properties of TRα1 and TRβ1 can differ. We report here that although overlapping, TRα1 and TRβ1 recruit distinct panels of partner proteins that may account for their divergent biological functions, and which appear to explain their distinct target gene regulatory properties.

Genetic Variations of ABCC2 Gene Associated with Adverse Drug Reactions to Valproic Acid in Korean Epileptic Patients

The multidrug resistance protein 2 (MRP2, ABCC2) gene may determine individual susceptibility to adverse drug reactions (ADRs) in the central nervous system (CNS) by limiting brain access of antiepileptic drugs, especially valproic acid (VPA). Our objective was to investigate the effect of ABCC2 polymorphisms on ADRs caused by VPA in Korean epileptic patients. We examined the association of ABCC2 single-nucleotide polymorphisms and haplotype frequencies with VPA related to adverse reactions. In addition, the association of the polymorphisms with the risk of VPA related to adverse reactions was estimated by logistic regression analysis. A total of 41 (24.4%) patients had shown VPA-related adverse reactions in CNS, and the most frequent symptom was tremor (78.0%). The patients with CNS ADRs were more likely to have the G allele (79.3% vs. 62.7%, p = 0.0057) and the GG genotype (61.0% vs. 39.7%, p = 0.019) at the g.-1774delG locus. The frequency of the haplotype containing g.-1774Gdel was significantly lower in the patients with CNS ADRs than without CNS ADRs (15.8% vs. 32.3%, p = 0.0039). Lastly, in the multivariate logistic regression analysis, the presence of the GG genotype at the g.-1774delG locus was identified as a stronger risk factor for VPA related to ADRs (odds ratio, 8.53; 95% confidence interval, 1.04 to 70.17). We demonstrated that ABCC2 polymorphisms may influence VPA-related ADRs. The results above suggest the possible usefulness of ABCC2 gene polymorphisms as a marker for predicting response to VPA-related ADRs.

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.

CCBE1 mutation in two siblings, one manifesting lymphedema-cholestasis syndrome, and the other, fetal hydrops

Background

Lymphedema-cholestasis syndrome (LCS; Aagenaes syndrome) is a rare autosomal recessive disorder, characterized by 1) neonatal intrahepatic cholestasis, often lessening and becoming intermittent with age, and 2) severe chronic lymphedema, mainly lower limb. LCS was originally described in a Norwegian kindred in which a locus, LCS1, was mapped to a 6.6cM region on chromosome 15. Mutations in CCBE1 on chromosome 18 have been reported in some cases of lymphatic dysplasia, but not in LCS.

Methods

Consanguineous parents of Mexican ancestry had a child with LCS who did not exhibit extended homozygosity in the LCS1 region. A subsequent pregnancy was electively terminated due to fetal hydrops. We performed whole-genome single nucleotide polymorphism genotyping to identify regions of homozygosity in these siblings, and sequenced promising candidate genes.

Results

Both siblings harbored a homozygous mutation in CCBE1, c.398 T>C, predicted to result in the missense change p.L133P. Regions containing known ‘cholestasis genes’ did not demonstrate homozygosity in the LCS patient.

Conclusions

Mutations in CCBE1 may yield a phenotype not only of lymphatic dysplasia, but also of LCS or fetal hydrops; however, the possibility that the sibling with LCS also carries a homozygous mutation in an unidentified gene influencing cholestasis cannot be excluded.

Mutation and functional analysis of ABCC2/multidrug resistance protein 2 in a Japanese patient with Dubin-Johnson syndrome

Dubin-Johnson syndrome (DJS) is a recessive inherited disorder characterized by conjugated hyperbilirubinemia. It is caused by dysfunction of adenosine triphosphate-binding cassette, sub-family C, member 2 (ABCC2/MRP2) on the canalicular membrane of hepatocytes. We performed mutational analysis of the ABCC2/MRP2 gene in a Japanese female with DJS. Furthermore, we investigated the effects of the two identified DJS-associated mutations on MRP2 function. We found a compound heterozygous mutation in the patient: W709R (c.2124T>C), a missense mutation in exon 17, and R1310X (c.3928C>T), a nonsense mutation in exon 28. DJS-associated mutations have been shown to impair the protein maturation and transport activity of ABCC2/MRP2. We established HEK293 cell lines stably expressing one of the two identified DJS-associated mutations. Expressed W709R MRP2 was mainly core-glycosylated, predominantly retained in the endoplasmic reticulum, and exhibited no transport activity, suggesting that this mutation causes deficient maturation and impaired protein sorting. No MRP2 protein was expressed from HEK293 cells transfected with an R1310X-containing construct. This compound heterozygous mutation of the MRP2 gene causes dysfunction of the MRP2 protein and the hyperbilirubinemia seen in DJS.

Stoffwechselerkrankungen

Entsprechend ihrer Wanderung bei isoelektrischer Fokussierung werden die allelen Varianten des α1-AT als Proteinaseinhibitorphänotypen (Pi) klassifiziert. Die dominierende Isoform ist der normale Phänotyp M, daneben gibt es die Mangelvarianten S und Z sowie eine 0-Variante.

MRP2 (ABCC2, cMOAT) expression in nuclear envelope of primary fallopian tube cancer cells is a new unfavorable prognostic factor

Objective

To determine the prognostic value of the immunohistochemical evaluation of the multidrug resistance-associated protein 2 (MRP2) expression, together with its subcellular localization in primary fallopian tube carcinomas (PFTCs).

Methods

The immunohistochemical analysis was performed using samples originating from 70 patients with PFTCs.

Results

(1) We documented that MRP2 can be localized in the plasma membrane (MRP2c), as well as in the nuclear envelope (MRP2n) of the PFTC cells. (2) Patients with more advanced stage, with progression of the disease and patients who died, showed significantly higher expression of the MRP2n. (3) Univariate and multivariate analyses showed that MRP2n is an unfavorable prognostic factor in PFTCs. (4) The analysis of the classic clinicopathological data revealed that only the FIGO stage had prognostic value, both in the univariate, as well as in multivariate analysis.

Conclusions

(1) This study suggests that MRP2n is a new disadvantageous prognostic factor in PFTCs and (2) that expression in nuclear envelope can be associated with lower differentiation of cancer cells and their resistance to the cisplatin. (3) We have also confirmed independent prognostic value of FIGO stage in PFTCs.

ABCC2 is involved in the hepatocyte perinuclear barrier for small organic compounds

Small organic molecules are believed to freely diffuse across nuclear pores but this may not be so if this route is blocked during protein and nucleic acid transfer. Here we have investigated the existence of transport mechanisms across the nuclear envelope (NE) of hepatocytes. Using nuclei isolated from rat liver cells, evidence for the existence of ATP-dependent transporters of organic compounds was found. In rat hepatocyte NE, with negligible contamination by other membranes, the presence of mature and glycosylated ABCC2, but not other ABC export pumps, was detected. ABCC2 was localized in the same membranes as the conjugating enzyme UGT1A1. Human ABCC2 ORF was tagged with V5 and transfected to human hepatoma cells. ABCC2-V5 protein was detected at perinuclear ER vesicles and at the NE. Both compartments expressing ABCC2-V5 were able to exclude calcein. ABCC2 abundance at the NE of rat hepatocytes was modified by treatments able to increase or reduce the expression of canalicular ABCC2. The sensitivity to mitoxantrone was higher for hepatocytes obtained from TR- rats whose NE lacked ABCC2. Incubation with mitoxantrone after depletion of ATP resulted in a marked accumulation of mitoxantrone in the nucleus of wild-type, but not TR-, hepatocytes. In sum, ABCC2 is present at the NE and perinuclear ER where, in combination with the activity of conjugating enzymes, this pump may be involved in the perinuclear barrier for small organic molecules, playing a role in protecting DNA from genotoxic compounds and in the control of intranuclear concentrations of ligands for nuclear receptors.

Pharmacogenetic Screening for Polymorphisms in Drug-Metabolizing Enzymes and Drug Transporters in a Dutch Population

BACKGROUND

A possible explanation for the wide interindividual variability in toxicity and efficacy of drug therapy is variation in genes encoding drug-metabolizing enzymes and drug transporters. The allelic frequency of these genetic variants, linkage disequilibrium (LD), and haplotype of these polymorphisms are important parameters in determining the genetic differences between patients. The aim of this study was to explore the frequencies of polymorphisms in drug-metabolizing enzymes (CYP1A1, CYP2C9, CYP2C19, CYP3A4, CYP2D6, CYP3A5, DPYD, UGT1A1, GSTM1, GSTP1, GSTT1) and drug transporters (ABCB1[MDR1] and ABCC2[MRP2]), and to investigate the LD and perform haplotype analysis of these polymorphisms in a Dutch population.

METHODS

Blood samples were obtained from 100 healthy volunteers and genomic DNA was isolated and amplified by PCR. The amplification products were sequenced and analyzed for the presence of polymorphisms by sequence alignment.

RESULTS

In the study population, we identified 13 new single nucleotide polymorphisms (SNPs) in Caucasians and three new SNPs in non-Caucasians, in addition to previously recognized SNPs. Three of the new SNPs were found within exons, of which two resulted in amino acid changes (A428T in CYP2C9 resulting in the amino acid substitution D143V; and C4461T in ABCC2 in a non-Caucasian producing the amino acid change T1476M). Several LDs and haplotypes were found in the Caucasian individuals.

CONCLUSION

In this Dutch population, the frequencies of 16 new SNPs and those of previously recognized SNPs were determined in genes coding for drug-metabolizing enzymes and drug transporters. Several LDs and haplotypes were also inferred. These data are important for further research to help explain the interindividual pharmacokinetic and pharmacodynamic variability in response to drug therapy.

CM2 antigen, a potential novel molecule participating in glucuronide transport on rat hepatocyte canalicular membrane

The polarized molecules predominately distributing at hepatocyte canalicular surface play a vital role in disclosing the process of bile formation and etiopathogenisis of cholestatic live diseases. Therefore, it is important to find novel polarized molecules on hepatocyte canalicular membrane. In the present study, canalicular membrane vesicles (CMVs) isolated from rat hepatocyte by density gradient centrifugation were used as immunogens to produce hybridoma and 46 strains of monoclonal antibodies (mAb) against CMVs were obtained. With a series of morphological assay methods, including immunohistochemistry, immunofluorescence and immuno-electron microscope, the antigens recognized by canalicular mAb1 (CM1) and canalicular mAb2 (CM2) were confirmed to predominately distribute at hepatocyte canalicular membrane. Transport activity assay revealed that CM2 could inhibit ATP-dependent E217βG uptake of rat hepatocyte CMVs. Meanwhile, Western blotting analysis showed that the molecular mass of CM2 antigen was approximately 110kDa, which was much less than Mr 180kDa of multidrug resistance-associated protein 2 (MRP2) involved in glucuronide transport. These data indicated that CM2 antigen might be a potential novel molecule participating in glucuronide transport on the hepatocyte canalicular membrane.

Gender-specific reduction of hepatic Mrp2 expression by high-fat diet protects female mice from ANIT toxicity

Emerging evidence suggests that feeding a high-fat diet (HFD) to rodents affects the expression of genes involved in drug transport. However, gender-specific effects of HFD on drug transport are not known. The multidrug resistance-associated protein 2 (Mrp2, Abcc2) is a transporter highly expressed in the hepatocyte canalicular membrane and is important for biliary excretion of glutathione-conjugated chemicals. The current study showed that hepatic Mrp2 expression was reduced by HFD feeding only in female, but not male, C57BL/6J mice. In order to determine whether down-regulation of Mrp2 in female mice altered chemical disposition and toxicity, the biliary excretion and hepatotoxicity of the Mrp2 substrate, α-naphthylisothiocyanate (ANIT), were assessed in male and female mice fed control diet or HFD for 4weeks. ANIT-induced biliary injury is a commonly used model of experimental cholestasis and has been shown to be dependent upon Mrp2-mediated efflux of an ANIT glutathione conjugate that selectively injures biliary epithelial cells. Interestingly, HFD feeding significantly reduced early-phase biliary ANIT excretion in female mice and largely protected against ANIT-induced liver injury. In summary, the current study showed that, at least in mice, HFD feeding can differentially regulate Mrp2 expression and function and depending upon the chemical exposure may enhance or reduce susceptibility to toxicity. Taken together, these data provide a novel interaction between diet and gender in regulating hepatobiliary excretion and susceptibility to injury.

ABCC2 (MRP2, cMOAT) localized in the nuclear envelope of breast carcinoma cells correlates with poor clinical outcome

Nuclear expression of ABCC2 can be specific for lower differentiated cells and stem cells. The study aimed at examination of ABCC2 expression in breast cancers. The immunohistochemical analyses were performed on 70 samples of breast cancer. We have also studied prognostic value of the ABCC2 mRNA expression using the KM plotter which assessed the effect of 22,277 genes on survival in 1809 breast cancer patients. Immunohistochemical studies demonstrated that ABCC2 expression may be manifested in nuclear envelope of neoplastic cells (ABCC2n) as well as in their cell membrane and cytoplasm (ABCC2c). The univariate and multivariate analyses showed that higher expression of ABCC2n and ABCC2c was typical for cases of a shorter overall survival time. Higher ABBC2n expression was also typical for cases of a shorter disease-free survival and a shorter progression-free time. The KM plotter analysis of the prognostic value of ABCC2 mRNA expression showed that elevated ABCC2 expression was specific for cases of a shorter relapse-free survival only in the estrogen receptor-negative subgroup. The study demonstrated hat breast cancers manifest ABCC2 expression and that it is linked to a less favourable prognosis. Our results suggested that immunohistochemical tests represent a reliable way to detect prognostic value of ABCC2 expression, allowing to demonstrate differences related to subcellular localization of the protein. Cases with nuclear expression of ABCC2 manifested a more aggressive clinical course, which might reflect a less advanced differentiation of neplastic cells, resistance to the applied cytostatic drugs and tamoxifen.

Multidrug resistance proteins (MRPs/ABCCs) in cancer chemotherapy and genetic diseases

The ATP-binding cassette (ABC) transporters are a superfamily of membrane proteins that are best known for their ability to transport a wide variety of exogenous and endogenous substances across membranes against a concentration gradient via ATP hydrolysis. There are seven subfamilies of human ABC transporters, one of the largest being the 'C' subfamily (gene symbol ABCC). Nine ABCC subfamily members, the so-called multidrug resistance proteins (MRPs) 1-9, have been implicated in mediating multidrug resistance in tumor cells to varying degrees as the efflux extrude chemotherapeutic compounds (or their metabolites) from malignant cells. Some of the MRPs are also known to either influence drug disposition in normal tissues or modulate the elimination of drugs (or their metabolites) via hepatobiliary or renal excretory pathways. In addition, the cellular efflux of physiologically important organic anions such as leukotriene C(4) and cAMP is mediated by one or more of the MRPs. Finally, mutations in several MRPs are associated with human genetic disorders. In this minireview, the current biochemical and physiological knowledge of MRP1-MRP9 in cancer chemotherapy and human genetic disease is summarized. The mutations in MRP2/ABCC2 leading to conjugated hyperbilirubinemia (Dubin-Johnson syndrome) and in MRP6/ABCC6 leading to the connective tissue disorder Pseudoxanthoma elasticum are also discussed.

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.

Resveratrol inhibits genistein-induced multi-drug resistance protein 2 (MRP2) expression in HepG2 cells

The interactions between various dietary cancer chemopreventive phytochemicals in drug transporter functions are not well studied. In this study, the effects of genistein and resveratrol on the multidrug resistance protein 2 (MRP2) expression and the underlying molecular mechanisms were investigated using HepG2-C3 cells that are stably transfected with a construct containing human MRP2 promoter region conjugated with luciferase reporter gene. A 3-fold induction of MRP2 luciferase activity was observed after genistein (50μM) treatment to HepG2-C3 cells, but was diminished by the resveratrol (50μM) cotreatment. This observation was further validated by Western blot analysis and RT-PCR analysis as resveratrol also inhibited genistein-induced MRP2 protein synthesis and mRNA expression. Immunofluorescence study revealed that genistein-induced formation of MRP2 vacuoles was dramatically reduced by resveratrol. The binding affinity between retinoid X receptor alpha (RXRα) and MRP2 promoter was examined by DNA-protein pull-down assay. The results showed that resveratrol inhibited the genistein-induced binding of RXRα to the promoter sequence of MRP2 gene, and this mechanism could potentially contribute to the inhibition of genistein-induced MRP2 expression by resveratrol. Taken together, our present study suggests that naturally occurring phytochemicals can potentially interfere with each other's regulatory function on the cancer chemoprevention-related genes through a competitive mechanism.

Multidrug resistance proteins (MRPs, ABCCs): importance for pathophysiology and drug therapy

The nine multidrug resistance proteins (MRPs) represent the major part of the 12 members of the MRP/CFTR subfamily belonging to the 48 human ATP-binding cassette (ABC) transporters. Cloning, functional characterization, and cellular localization of most MRP subfamily members have identified them as ATP-dependent efflux pumps with a broad substrate specificity for the transport of endogenous and xenobiotic anionic substances localized in cellular plasma membranes. Prototypic substrates include glutathione conjugates such as leukotriene C(4) for MRP1, MRP2, and MRP4, bilirubin glucuronosides for MRP2 and MRP3, and cyclic AMP and cyclic GMP for MRP4, MRP5, and MRP8. Reduced glutathione (GSH), present in living cells at millimolar concentrations, modifies the substrate specificities of several MRPs, as exemplified by the cotransport of vincristine with GSH by MRP1, or by the cotransport of GSH with bile acids or of GSH with leukotriene B(4) by MRP4.The role of MRP subfamily members in pathophysiology may be illustrated by the MRP-mediated release of proinflammatory and immunomodulatory mediators such as leukotrienes and prostanoids. Pathophysiological consequences of many genetic variants leading to a lack of functional MRP protein in the plasma membrane are observed in the hereditary MRP2 deficiency associated with conjugated hyperbilirubinemia in Dubin-Johnson syndrome, in pseudoxanthoma elasticum due to mutations in the MRP6 (ABCC6) gene, or in the type of human earwax and osmidrosis determined by single nucleotide polymorphisms in the MRP8 (ABCC8) gene. The hepatobiliary and renal elimination of many drugs and their metabolites is mediated by MRP2 in the hepatocyte canalicular membrane and by MRP4 as well as MRP2 in the luminal membrane of kidney proximal tubules. Therefore, inhibition of these efflux pumps affects pharmacokinetics, unless compensated by other ATP-dependent efflux pumps with overlapping substrate specificities.

Role of multidrug resistance protein 2 (MRP2) in chemoresistance and clinical outcome in oesophageal squamous cell carcinoma

BACKGROUND

Although multidrug resistance protein 2 (MRP2) confers chemoresistance in some cancer types, its implication on oesophageal squamous cell carcinoma (ESCC) remains unclear.

METHODS

We evaluated MRP2 expression by immunohistochemistry and RT-PCR using 81 resected specimens from ESCC patients who did or did not receive neo-adjuvant chemotherapy (NACT), including 5-fluorouracil, doxorubicin, and cisplatin (CDDP). Correlation between MRP2 expression and response to chemotherapy was also examined in 42 pre-therapeutic biopsy samples and eight ESCC cell lines.

RESULTS

MRP2-positive immunostaining was more frequently observed in ESCCs with NACT than in those without NACT (27.3 vs 5.4%). The MRP2-positive patients showed poorer prognosis than MRP2-negative patients (5-year survival rate, 25.6 vs 55.7%). Concordantly, ESCC with NACT showed 2.1-fold higher mRNA expression of MRP2 than those without NACT (P=0.0350). In pre-therapeutic biopsy samples of patients with NACT, non-responders showed 2.9-fold higher mRNA expression of MRP2 than responders (P=0.0035). Among the panel of ESCC cell lines, TE14 showed the highest MRP2 mRNA expression along with the strongest resistance to CDDP. Inhibition of MRP2 expression by small-interfering RNA reduced chemoresistance to CDDP.

CONCLUSION

Our data suggested that MRP2 is one of molecules, which regulate the sensitivity to chemotherapy including CDDP in advanced ESCC patients.

Structures of the nucleotide-binding domain of the human ABCB6 transporter and its complexes with nucleotides

The human ATP-binding cassette (ABC) transporter ABCB6 is involved in haem-precursor transport across the mitochondrial membrane. The crystal structure of its nucleotide-binding domain (NBD) has been determined in the apo form and in complexes with ADP, with ADP and Mg(2+) and with ATP at high resolution. The overall structure is L-shaped and consists of two lobes, consistent with other reported NBD structures. Nucleotide binding is mediated by the highly conserved Tyr599 and the Walker A motif, and induces notable structural changes. Structural comparison with other structurally characterized NBDs and full-length ABC transporters gives the first insight into the possible catalytic mechanism of ABCB6 and the role of the N-terminal helix alpha(1) in full-length ABCB6.

Comparative and veterinary pharmacogenomics

Pharmacogenomics is the study of the impact of genetic variation on drug effects, with the ultimate goal of achieving "personalised medicine". Since the completion of the Human Genome Project, great strides have been made towards the goal of personalised dosing of drugs in people, as exemplified by the development of gene-guided dosing of the anticoagulant drug, warfarin. Although the pharmacogenomics of domestic animals is still at an early stage of development, there is great potential for advances in the coming years as the direct result of complete genome sequences currently being derived for many of the species of significance to veterinary and comparative medicine. This sequence information is being used to discover sequence variants in candidate genes associated with altered drug response, as well as to develop whole genome high density single nucleotide polymorphism arrays for genotype-phenotype linkage analysis. This review summarises the current state of veterinary pharmacogenomics research, including drug response variability phenotypes with either known genetic aetiology or strong circumstantial evidence for genetic involvement. Polymorphisms and rarer gene variants affecting drug disposition (pharmacokinetics) and drug effect (pharmacodynamics) are discussed. In addition to providing the veterinary clinician with useful information for the practise of therapeutics, it is envisaged that the increasing knowledge base will also provide a resource for individuals involved in veterinary and comparative biomedical research.

Multidrug resistance protein 2-mediated estradiol-17beta-D-glucuronide transport potentiation: in vitro-in vivo correlation and species specificity

Multidrug resistance protein 2 (MRP2) is a multispecific organic anion transporter expressed at important pharmacological barriers, including the canalicular membrane of hepatocytes. At this location it is involved in the elimination of both endogenous and exogenous waste products, mostly as conjugates, to the bile. Estradiol-17beta-d-glucuronide (E(2)17betaG), a widely studied endogenous substrate of MRP2, was shown earlier to recognize two binding sites of the transporter in vesicular transport assays. MRP2 modulators (substrates and nonsubstrates) potentiate the transport of E(2)17betaG by MRP2. We correlated data obtained from studies of different complexities and investigated the species-specific differences between rat and human MRP2-mediated transport. We used vesicular transport assays, sandwich-cultured primary hepatocytes, and in vivo biliary efflux in rats. Our results demonstrate that the rat Mrp2 transporter, unlike the human MRP2, transports E(2)17betaG according to Michaelis-Menten type kinetics. Nevertheless, in the presence of modulator drugs E(2)17betaG transport mediated by the rat transporter also shows cooperative kinetics as potentiation of E(2)17betaG transport was observed in the vesicular transport assay. We also demonstrated that the potentiation exists both in rat and in human hepatocytes and in vivo in rats.