Expression of multidrug transporterMRP4/ABCC4is a marker of poor prognosis in neuroblastoma and confers resistance to irinotecanin vitro

Structural View of Cryo-Electron Microscopy-Determined ATP-Binding Cassette Transporters in Human Multidrug Resistance

ATP-binding cassette (ABC) transporters, acting as cellular "pumps," facilitate solute translocation through membranes via ATP hydrolysis. Their overexpression is closely tied to multidrug resistance (MDR), a major obstacle in chemotherapy and neurological disorder treatment, hampering drug accumulation and delivery. Extensive research has delved into the intricate interplay between ABC transporter structure, function, and potential inhibition for MDR reversal. Cryo-electron microscopy has been instrumental in unveiling structural details of various MDR-causing ABC transporters, encompassing ABCB1, ABCC1, and ABCG2, as well as the recently revealed ABCC3 and ABCC4 structures. The newly obtained structural insight has deepened our understanding of substrate and drug binding, translocation mechanisms, and inhibitor interactions. Given the growing body of structural information available for human MDR transporters and their associated mechanisms, we believe it is timely to compile a comprehensive review of these transporters and compare their functional mechanisms in the context of multidrug resistance. Therefore, this review primarily focuses on the structural aspects of clinically significant human ABC transporters linked to MDR, with the aim of providing valuable insights to enhance the effectiveness of MDR reversal strategies in clinical therapies.

Structural and mechanistic basis of substrate transport by the multidrug transporter MRP4

Multidrug resistance-associated protein 4 (MRP4) is an ATP-binding cassette (ABC) transporter expressed at multiple tissue barriers where it actively extrudes a wide variety of drug compounds. Overexpression of MRP4 provides resistance to clinically used antineoplastic agents, making it a highly attractive therapeutic target for countering multidrug resistance. Here, we report cryo-EM structures of multiple physiologically relevant states of lipid bilayer-embedded human MRP4, including complexes between MRP4 and two widely used chemotherapeutic agents and a complex between MRP4 and its native substrate. The structures display clear similarities and distinct differences in the coordination of these chemically diverse substrates and, in combination with functional and mutational analysis, reveal molecular details of the transport mechanism. Our study provides key insights into the unusually broad substrate specificity of MRP4 and constitutes an important contribution toward a general understanding of multidrug transporters.

Machine learning methods revealed the roles of immune-metabolism related genes in immune infiltration, stemness, and prognosis of neuroblastoma

BACKGROUND

Immunometabolism plays an important role in neuroblastoma (NB). However, the mechanism of immune-metabolism related genes (IMRGs) in NB remains unclear. This study aimed to explore the effects of IMRGs on the prognosis, immune infiltration and stemness of patients with NB using machine learning methods.

METHODS

R software (v4.2.1) was used to identify the differentially expressed IMRGs, and machine learning algorithm was used to screen the prognostic genes from IMRGs. Then we constructed a prognostic model and calculated the risk scores. The NB patients were grouped according to the prognosis scores. In addition, the genes most associated with the immune infiltration and stemness of NB were analyzed by weighted gene co-expression network analysis (WGCNA).

RESULTS

There were 89 differentially expressed IMRGs between the MYCN amplification and the MYCN non-amplification group, among which CNR1, GNAI1, GLDC and ABCC4 were selected by machine learning algorithm to construct the prognosis model due to their better prediction effect. Both the K-M survival curve and the 5-year Receiver operating characteristic (ROC) curve indicated that the prognosis model could predict the prognosis of NB patients, and there was significant difference in immune infiltration between the two groups according to the median of risk score.

CONCLUSIONS

We verified the effects of IMRGs on the prognosis, immune infiltration and stemness of NB. These findings could provide help for predicting prognosis and developing immunotherapy in NB.

TDP1-independent pathways in the process and repair of TOP1-induced DNA damage

Anticancer drugs, such as camptothecin (CPT), trap topoisomerase I (TOP1) on DNA and form TOP1 cleavage complexes (TOP1cc). Alternative repair pathways have been suggested in the repair of TOP1cc. However, how these pathways work with TDP1, a key repair enzyme that specifically hydrolyze the covalent bond between TOP1 catalytic tyrosine and the 3’-end of DNA and contribute to the repair of TOP1cc is poorly understood. Here, using unbiased whole-genome CRISPR screens and generation of co-deficient cells with TDP1 and other genes, we demonstrate that MUS81 is an important factor that mediates the generation of excess double-strand breaks (DSBs) in TDP1 KO cells. APEX1/2 are synthetic lethal with TDP1. However, deficiency of APEX1/2 does not reduce DSB formation in TDP1 KO cells. Together, our data suggest that TOP1cc can be either resolved directly by TDP1 or be converted into DSBs and repaired further by the Homologous Recombination (HR) pathway.

Here the authors find that MUS81 mediates excess DNA double strand break (DSB) generation in TDP1 KO cells after camptothecin treatment. They show that TOP1 cleavage complexes can be either resolved directly by TDP1 or be converted into DSBs and repaired further by the Homologous Recombination pathway.

Specific MRP4 Inhibitor Ceefourin-1 Enhances Apoptosis Induced by 6-Mercaptopurine in Jurkat Leukemic Cells, but Not in Normal Lymphoblast Cell Line CRL-1991

Background and objectives: The multidrug resistance protein 4 (MRP4) is a member of the ABC transporter, which has been extensively related to many types of cancer including leukemia. MRP4 overexpression and activity over the efflux of some chemotherapeutic drugs are the main causes of chemoresistance. 6-mercaptopurine (6-MP) is a chemotherapeutic drug widely used in the consolidation and maintenance phases of leukemia treatment. However, 6-MP is a substrate of MRP4, which decreases its chemotherapeutic efficacy. Current research is focused on the development of MRP4 inhibitors to combat chemoresistance by allowing the accumulation of the drug substrates inside the cells. To date, the only specific MRP4 inhibitor that has been developed is ceefourin-1, which has been reported to inhibit MRP4 in many cancer cells and which makes it an excellent candidate to enhance the activity of 6-MP in a combined treatment in vitro of leukemic cells. Materials and methods: in the present work, we determined the enhancing activity of ceefourin-1 on the antiproliferative and apoptotic effect of 6-MP in leukemic Jurkat cells by trypan blue assay and flow cytometry. Besides, we determined the 6-MP and ceefourin-1 binding sites into MRP4 by molecular docking and molecular dynamics. Results: ceefourin-1 enhanced the apoptotic activity of 6-MP in Jurkat cells, while in CRL-1991 cells both antiproliferative and apoptotic effect were significantly lower. Ceefourin-1 additively cooperates with 6-MP to induce apoptosis in leukemic cells, but normal lymphoblast CRl-1991 showed resistance to both drugs. Conclusion: ceefourin-1 and 6-MP cooperates to trigger apoptosis in leukemic Jurkat cells, but the full mechanism needs to be elucidated in further works. In addition, our perspective is to test the cooperation between ceefourin-1 and 6-MP in samples from patients and healthy donnors.

Excellent Early Outcomes of Combined Chemotherapy With Arsenic Trioxide for Stage 4/M Neuroblastoma in Children: A Multicenter Nonrandomized Controlled Trial

This nonrandomized, multicenter cohort, open-label clinical trial evaluated the efficacy and safety of combined chemotherapy with arsenic trioxide (ATO) in children with stage 4/M neuroblastoma (NB). We enrolled patients who were newly diagnosed with NB and assessed as stage 4/M and received either traditional chemotherapy or ATO combined with chemotherapy according to their own wishes. Twenty-two patients were enrolled in the trial group (ATO combined with chemotherapy), and 13 patients were enrolled in the control group (traditional chemotherapy). Objective response rate (ORR) at 4 weeks after completing induction chemotherapy was defined as the main outcome, and adverse events were monitored and graded in the meantime. Data cutoff date was December 31, 2019. Finally, we found that patients who received ATO combined with chemotherapy had a significantly higher response rate than those who were treated with traditional chemotherapy (ORR: 86.36% vs. 46.16%, p=0.020). Reversible cardiotoxicity was just observed in three patients who were treated with ATO, and no other differential adverse events were observed between the two groups. ATO combined with chemotherapy can significantly improve end-induction response in high-risk NB, and our novel regimen is well tolerated in pediatric patients. These results highlight the superiority of chemotherapy with ATO, which creates new opportunity for prolonging survival. In addition, this treatment protocol minimizes therapeutic costs compared with anti-GD2 therapy, MIBG, and proton therapy and can decrease the burden to families and society. However, we also need to evaluate more cases to consolidate our conclusion.

Multidrug resistance proteins (MRPs): Structure, function and the overcoming of cancer multidrug resistance

ATP-binding cassette (ABC) transporters mediate the ATP-driven translocation of structurally and mechanistically distinct substrates against steep concentration gradients. Among the seven human ABC subfamilies namely ABCA-ABCG, ABCC is the largest subfamily with 13 members. In this respect, 9 of the ABCC members are termed "multidrug resistance proteins" (MRPs1-9) due to their ability to mediate cancer multidrug resistance (MDR) by extruding various chemotherapeutic agents or their metabolites from tumor cells. Furthermore, MRPs are also responsible for the ATP-driven efflux of physiologically important organic anions such as leukotriene C₄, folic acid, bile acids and cAMP. Thus, MRPs are involved in important regulatory pathways. Blocking the anticancer drug efflux function of MRPs has shown promising results in overcoming cancer MDR. As a result, many novel MRP modulators have been developed in the past decade. In the current review, we summarize the structure, tissue distribution, biological and pharmacological functions as well as clinical insights of MRPs. Furthermore, recent updates in MRP modulators and their therapeutic applications in clinical trials are also discussed.

Mouse models of high-risk neuroblastoma

Informative and realistic mouse models of high-risk neuroblastoma are central to understanding mechanisms of tumour initiation, progression, and metastasis. They also play vital roles in validating tumour drivers and drug targets, as platforms for assessment of new therapies and in the generation of drug sensitivity data that can inform treatment decisions for individual patients. This review will describe genetically engineered mouse models of specific subsets of high-risk neuroblastoma, the development of patient-derived xenograft models that more broadly represent the diversity and heterogeneity of the disease, and models of primary and metastatic disease. We discuss the research applications, advantages, and limitations of each model type, the importance of model repositories and data standards for supporting reproducible, high-quality research, and potential future directions for neuroblastoma mouse models.

Effect of ABC transporter expression and mutational status on survival rates of cancer patients

ATP-binding cassette (ABC) transporters mediate multidrug resistance in cancer. In contrast to DNA single nucleotide polymorphisms in normal tissues, the role of mutations in tumors is unknown. Furthermore, the significance of their expression for prediction of chemoresistance and survival prognosis is still under debate. We investigated 18 tumors by RNA-sequencing. The mutation rate varied from 27,507 to 300885. In ABCB1, three hotspots with novel mutations were in transmembrane domains 3, 8, and 9. We also mined the cBioPortal database with 11,814 patients from 23 different tumor entities. We performed Kaplan-Meier survival analyses to investigate the effect of ABC transporter expression on survival rates of cancer patients. Novel mutations were also found in ABCA2, ABCA3, ABCB2, ABCB5, ABCC1-6, and ABCG2. Mining the cBioPortal database with 11,814 patients from 23 different tumor entities validated our results. Missense and in-frame mutations led to altered binding of anticancer drugs in molecular docking approaches. The ABCB1 nonsense mutation Q856* led to a truncated P-glycoprotein, which may sensitize tumors to anticancer drugs. The search for ABC transporter nonsense mutations represents a novel approach for precision medicine.. Low ABCB1 mRNA expression correlated with significantly longer survival in ovarian or kidney cancer and thymoma. In cancers of breast, kidney or lung, ABC transporter expression correlated with different tumor stages and human populations as further parameters to refine strategies for more individualized chemotherapy.

Multidrug transporter MRP4/ABCC4 as a key determinant of pancreatic cancer aggressiveness

Recent findings show that MRP4 is critical for pancreatic ductal adenocarcinoma (PDAC) cell proliferation. Nevertheless, the significance of MRP4 protein levels and function in PDAC progression is still unclear. The aim of this study was to determine the role of MRP4 in PDAC tumor aggressiveness. Bioinformatic studies revealed that PDAC samples show higher MRP4 transcript levels compared to normal adjacent pancreatic tissue and circulating tumor cells express higher levels of MRP4 than primary tumors. Also, high levels of MRP4 are typical of high-grade PDAC cell lines and associate with an epithelial-mesenchymal phenotype. Moreover, PDAC patients with high levels of MRP4 depict dysregulation of pathways associated with migration, chemotaxis and cell adhesion. Silencing MRP4 in PANC1 cells reduced tumorigenicity and tumor growth and impaired cell migration. Transcriptomic analysis revealed that MRP4 silencing alters PANC1 gene expression, mainly dysregulating pathways related to cell-to-cell interactions and focal adhesion. Contrarily, MRP4 overexpression significantly increased BxPC-3 growth rate, produced a switch in the expression of EMT markers, and enhanced experimental metastatic incidence. Altogether, our results indicate that MRP4 is associated with a more aggressive phenotype in PDAC, boosting pancreatic tumorigenesis and metastatic capacity, which could finally determine a fast tumor progression in PDAC patients.

Multidrug resistance protein 4 (MRP4/ABCC4) is overexpressed in clear cell renal cell carcinoma (ccRCC) and is essential to regulate cell proliferation

Kidney cancer accounts for 2.5% of all cancers, with an annual global incidence of almost 300,000 cases leading to 111,000 deaths. Approximately 85% of kidney tumors are renal cell carcinoma (RCC) and their major histologic subtype is clear cell renal cell carcinoma (ccRCC). Although new therapeutic treatments are being designed and applied based on the combination of tyrosine kinase inhibitors and immunotherapy, no major impact on the mortality has been reported so far. MRP4 is a pump efflux that transporters multiple endogenous and exogenous substances. Recently it has been associated with tumoral persistence and cell proliferation in several types of cancer including pancreas, lung, ovary, colon, ostesarcoma, etc. Herein, we demonstrate for the first time, that MRP4 is overexpressed in ccRCC tumors, compared to control renal tissues. In addition, using cell culture models, we observed that MRP4 pharmacological inhibition produces an imbalance in cAMP metabolism, induces cell arrest, changes in lipid composition, increase in cytoplasmic lipid droplets and finally apoptosis. These data provide solid evidence for the future evaluation of MRP4 as a possible new therapeutic target in ccRCC.

ABCC4/MRP4 contributes to the aggressiveness of Myc-associated epithelial ovarian cancer

Epithelial ovarian cancer (EOC) is a complex disease comprising discrete histological and molecular subtypes, for which survival rates remain unacceptably low. Tailored approaches for this deadly heterogeneous disease are urgently needed. Efflux pumps belonging to the ATP-binding cassette (ABC) family of transporters are known for roles in both drug resistance and cancer biology and are also highly targetable. Here we have investigated the association of ABCC4/MRP4 expression to clinical outcome and its biological function in endometrioid and serous tumors, common histological subtypes of EOC. We found high expression of ABCC4/MRP4, previously shown to be directly regulated by c-Myc/N-Myc, was associated with poor prognosis in endometrioid EOC (P = .001) as well as in a subset of serous EOC with a "high-MYCN" profile (C5/proliferative; P = .019). Transient siRNA-mediated suppression of MRP4 in EOC cells led to reduced growth, migration and invasion, with the effects being most pronounced in endometrioid and C5-like serous cells compared to non-C5 serous EOC cells. Sustained knockdown of MRP4 also sensitized endometrioid cells to MRP4 substrate drugs. Furthermore, suppression of MRP4 decreased the growth of patient-derived EOC cells in vivo. Together, our findings provide the first evidence that MRP4 plays an important role in the biology of Myc-associated ovarian tumors and highlight this transporter as a potential therapeutic target for EOC.

CCI52 sensitizes tumors to 6-mercaptopurine and inhibits MYCN-amplified tumor growth

The antimetabolite 6-mercaptopurine (6-MP) is an important component in the treatment of specific cancer subtypes, however, the development of drug resistance and dose-limiting toxicities can limit its effectiveness. The therapeutic activity of 6-MP requires cellular uptake, enzymatic conversion to thio-GMP and incorporation of thio-GTP into RNA and DNA, as well as inhibition of de novo purine synthesis by methyl-thio-IMP. Mechanisms that prevent 6-MP entry into the cell, prevent 6-MP metabolism or deplete thiopurine intermediates, can all lead to 6-MP resistance. We previously conducted a high-throughput screen for inhibitors of the multidrug transporter MRP4 using 6-MP sensitivity as the readout. In addition to MRP4-specific inhibitors, we identified a compound, CCI52, that sensitized cell lines to 6-MP independent of this transporter. CCI52 and its more stable analogue CCI52-14 also function as effective chemosensitizers in vivo, substantially extending survival in a transgenic mouse cancer model treated with 6-MP. Chemosensitization was associated with an increase in thio-IMP, suggesting that CCI52 functions directly on 6-MP uptake or metabolism. In addition to its chemosensitizing effects, CCI52 and CCI52-14 inhibited the growth of MYCN-amplified high-risk neuroblastoma cell lines and delayed tumor progression in a MYCN-driven, transgenic mouse model of neuroblastoma. These multifunctional inhibitors may be useful for the further development of anticancer agents and as tools to better understand 6-MP metabolism.

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.

MRP4/ABCC4 As a New Therapeutic Target: Meta-Analysis to Determine cAMP Binding Sites as a Tool for Drug Design

MRP4 transports multiple endogenous and exogenous substances and is critical not only for detoxification but also in the homeostasis of several signaling molecules. Its dysregulation has been reported in numerous pathological disorders, thus MRP4 appears as an attractive therapeutic target. However, the efficacy of MRP4 inhibitors is still controversial. The design of specific pharmacological agents with the ability to selectively modulate the activity of this transporter or modify its affinity to certain substrates represents a challenge in current medicine and chemical biology. The first step in the long process of drug rational design is to identify the therapeutic target and characterize the mechanism by which it affects the given pathology. In order to develop a pharmacological agent with high specific activity, the second step is to systematically study the structure of the target and identify all the possible binding sites. Using available homology models and mutagenesis assays, in this review we recapitulate the up-to-date knowledge about MRP structure and aligned amino acid sequences to identify the candidate MRP4 residues where cyclic nucleotides bind. We have also listed the most relevant MRP inhibitors studied to date, considering drug safety and specificity for MRP4 in particular. This meta-analysis platform may serve as a basis for the future development of inhibitors of MRP4 cAMP specific transport.

Multidrug resistance-associated protein 4 in pharmacology: Overview of its contribution to pharmacokinetics, pharmacodynamics and pharmacogenetics

MRP4 is an ABC membrane transporter involved in clinical outcomes as it is located in many tissues that manages the transport and the elimination of many drugs. This review explores the implication of MRP4 in clinical pharmacology and the importance of its genetic variability. Although there is no specific recommendation regarding the study of MRP4 in drug development, it should be considered when drugs are eliminated by the kidney or liver or when drug-drug interactions are expected.

A Human ABC Transporter ABCC4 Gene SNP (rs11568658, 559 G > T, G187W) Reduces ABCC4-Dependent Drug Resistance

Broad-spectrum drug resistance is a major obstacle in cancer treatment, which is often caused by overexpression of ABC transporters the levels of which vary between individuals due to single-nucleotide polymorphisms (SNPs) in their genes. In the present study, we focused on the human ABC transporter ABCC4 and one major non-synonymous SNP variant of the ABCC4 gene in the Japanese population (rs11568658, 559 G > T, G187W) whose allele frequency is 12.5%. Cells expressing ABCC4 (G187W) were established using the Flp-In™ system based on Flp recombinase-mediated transfection to quantitatively evaluate the impacts of this non-synonymous SNP on drug resistance profiles of the cells. Cells expressing ABCC4 (WT) or (G187W) showed comparable ABCC4 mRNA levels. 3-(4,5-Dimethyl-2-thiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay indicated that the EC₅₀ value of the anticancer drug, SN-38, against cells expressing ABCC4 (G187W) was 1.84-fold lower than that against cells expressing ABCC4 (WT). Both azathioprine and 6-mercaptopurine showed comparable EC₅₀ values against cells expressing ABCC4 (G187W) and those expressing ABCC4 (WT). These results indicate that the substitution of Gly at position 187 of ABCC4 to Trp resulted in reduced SN-38 resistance.

Drug resistance in topoisomerase-targeting therapy

Drug resistance is a well-known phenomenon that occurs when initially responsive to chemotherapy cancer cells become tolerant and elude further effectiveness of anticancer drugs. Based on their mechanism of action, anticancer drugs can be divided into cytotoxic-based agents and target-based agents. An important role among the therapeutics of the second group is played by drugs targeting topoisomerases, nuclear enzymes critical to DNA function and cell survival. These enzymes are cellular targets of several groups of anticancer agents which generate DNA damage in rapidly proliferating cancer cells. Drugs targeting topoisomerase I are mostly analogs of camtothecin, a natural compound isolated from the bark of a tree growing in China. Drugs targeting topoisomerase II are divided into poisons, such as anthracycline antibiotics, whose action is based on intercalation between DNA bases, and catalytic inhibitors that block topoisomerase II at different stages of the catalytic cycle. Unfortunately, chemotherapy is often limited by the induction of drug resistance. Identifying mechanisms that promote drug resistance is critical for the improvement of patient prognosis. Cancer drug resistance is a complex phenomenon that may be influenced by many factors. Here we discuss various mechanisms by which cancer cells can develop resistance to topoisomerase-directed drugs, which include enhanced drug efflux, mutations in topoisomerase genes, hypophosphorylation of topoisomerase II catalytic domain, activation of NF-κB transcription factor and drug inactivation. All these events may lead to the ineffective induction of cancer cell death. Attempts at circumventing drug resistance through the inhibition of cellular efflux pumps, use of silencing RNAs or inhibition of some important mechanisms, which can allow cancer cells to survive therapy, are also presented.

ABC transporters as cancer drivers: Potential functions in cancer development

BACKGROUND

ABC transporters have attracted considerable attention for their function as drug transporters in a broad range of tumours and are therefore considered as major players in cancer chemoresistance. However, less attention has been focused on their potential role as active players in cancer development and progression.

SCOPE OF REVIEW

This review presents the evidence suggesting that ABC transporters might have a more active role in cancer other than the well known involvement in multidrug resistance and discusses the potential strategies to target each ABC transporter for a specific tumour setting.

MAJOR CONCLUSIONS

Emerging evidence suggests that ABC transporters are able to transport bioactive molecules capable of playing key roles in tumour development. Characterization of the effects of these transporters in specific cancer settings opens the possibility for the development of personalized treatments.

GENERAL SIGNIFICANCE

A more targeted approach of ABC transporters should be implemented that considers which specific transporter is playing a major role in a particular tumour setting in order to achieve a more successful outcome for ABC transporters inhibitors in cancer therapy.

The Effects of Synthetically Modified Natural Compounds on ABC Transporters

Multidrug resistance (MDR) is a major hurdle which must be overcome to effectively treat cancer. ATP-binding cassette transporters (ABC transporters) play pivotal roles in drug absorption and disposition, and overexpression of ABC transporters has been shown to attenuate cellular/tissue drug accumulation and thus increase MDR across a variety of cancers. Overcoming MDR is one desired approach to improving the survival rate of patients. To date, a number of modulators have been identified which block the function and/or decrease the expression of ABC transporters, thereby restoring the efficacy of a range of anticancer drugs. However, clinical MDR reversal agents have thus far proven ineffective and/or toxic. The need for new, effective, well-tolerated and nontoxic compounds has led to the development of natural compounds and their derivatives to ameliorate MDR. This review evaluates whether synthetically modifying natural compounds is a viable strategy to generate potent, nontoxic, ABC transporter inhibitors which may potentially reverse MDR.

MRP4 regulates ENaC-dependent CREB/COX-2/PGE2 signaling during embryo implantation

Multi-drug resistance protein 4 (MRP4), a potential chemotherapeutic target as well as a transporter for endogenous signaling molecules (e.g. prostaglandins), is known to be expressed in the endometrium, although its possible role(s) in the physiology of the endometrium remains unknown. Here, we show that MRP4 is upregulated at implantation window and localized to the basolateral membrane of the endometrial epithelium, the interface between the epithelium and stroma in mice. In human endometrial epithelial cells, MRP4 expression is upregulated by ENaC activation and the inhibition of MRP4 blocks ENaC-dependent PGE₂ release as well as phosphorylation of CREB. Intrauterine injection of MRP4 inhibitor in mice prior to implantation significantly downregulated implantation markers COX-2, Claudin4 and Lif, and reduced implantation rate. These results in together have revealed a previously undefined role of MRP4 in mediating ENaC-dependent CREB/COX-2/PGE₂ signaling essential to embryo implantation with implication in cancer progression as well.

Suppression of the ATP-binding cassette transporter ABCC4 impairs neuroblastoma tumour growth and sensitises to irinotecan in vivo

The ATP-binding cassette transporter ABCC4 (multidrug resistance protein 4, MRP4) mRNA level is a strong predictor of poor clinical outcome in neuroblastoma which may relate to its export of endogenous signalling molecules and chemotherapeutic agents. We sought to determine whether ABCC4 contributes to development, growth and drug response in neuroblastoma in vivo. In neuroblastoma patients, high ABCC4 protein levels were associated with reduced overall survival. Inducible knockdown of ABCC4 strongly inhibited the growth of human neuroblastoma cells in vitro and impaired the growth of neuroblastoma xenografts. Loss of Abcc4 in the Th-MYCN transgenic neuroblastoma mouse model did not impact tumour formation; however, Abcc4-null neuroblastomas were strongly sensitised to the ABCC4 substrate drug irinotecan. Our findings demonstrate a role for ABCC4 in neuroblastoma cell proliferation and chemoresistance and provide rationale for a strategy where inhibition of ABCC4 should both attenuate the growth of neuroblastoma and sensitise tumours to ABCC4 chemotherapeutic substrates.

Quantitative Evaluation of Drug Resistance Profile of Cells Expressing Wild-Type or Genetic Polymorphic Variants of the Human ABC Transporter ABCC4

Broad-spectrum resistance in cancer cells is often caused by the overexpression of ABC transporters; which varies across individuals because of genetic single-nucleotide polymorphisms (SNPs). In the present study; we focused on human ABCC4 and established cells expressing the wild-type (WT) or SNP variants of human ABCC4 using the Flp-In™ system (Invitrogen, Life Technologies Corp, Carlsbad, CA, USA) based on Flp recombinase-mediated transfection to quantitatively evaluate the effects of nonsynonymous SNPs on the drug resistance profiles of cells. The mRNA levels of the cells expressing each ABCC4 variant were comparable. 3-(4,5-Dimethyl-2-thiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay clearly indicated that the EC₅₀ values of azathioprine against cells expressing ABCC4 (WT) were 1.4–1.7-fold higher than those against cells expressing SNP variants of ABCC4 (M184K; N297S; K304N or E757K). EC₅₀ values of 6-mercaptopurine or 7-Ethyl-10-hydroxy-camptothecin (SN-38) against cells expressing ABCC4 (WT) were also 1.4–2.0- or 1.9-fold higher than those against cells expressing the SNP variants of ABCC4 (K304N or E757K) or (K304N; P403L or E757K); respectively. These results indicate that the effects of nonsynonymous SNPs on the drug resistance profiles of cells expressing ABCC4 can be quantitatively evaluated using the Flp-In™ system.

MRP1 expression in CTCs confers resistance to irinotecan-based chemotherapy in metastatic colorectal cancer

Circulating tumor cells are important markers of tumor progression and can reflect tumor behavior in metastatic colorectal cancer (mCRC). Identification of proteins that confer resistance to treatment is an important step to predict response and better selection of treatment for patients. Multidrug resistance-associated protein 1 (MRP1) and Multidrug resistance-associated protein 4 (MRP4) play a role in irinotecan-resistance, and Excision Repair Cross-Complementation group 1 (ERCC1) expression can confer resistance to platinum compounds. Here, we included 34 patients with mCRC and most of them received FOLFIRI or FOLFOX chemotherapy (91.1%). CTCs were isolated by ISET(®) Technology and identified in 30 patients (88.2%), with a median of 2.0 CTCs/mL (0-31.0). We analyzed the immunocytochemical expression of MRP1, MRP4 and ERCC1 only in patients who had previously detectable CTCs, accordingly to treatment received (n = 19, 15 and 13 patients, respectively). Among patients treated with irinotecan-based chemotherapy, 4 out of 19 cases with MRP1 positive CTCs showed a worse progression free survival (PFS) in comparison to those with MRP1 negative CTCs (2.1 months vs. 9.1 months; p = 0.003). None of the other proteins studied in CTCs had significant association with PFS. We analyzed also histological sections of primary tumors and metastases by immunohistochemistry, and found no association with clinicopathological characteristics or with PFS. Our results show MRP1 as a potential biomarker of resistance to treatment with irinotecan when found in CTCs from mCRC patients. This is a small proof-of-principle study and these early findings need to be validated in a larger cohort of patients.

Multidrug resistance protein 4/ ATP binding cassette transporter 4: a new potential therapeutic target for acute myeloid leukemia

Less than a third of adults patients with acute myeloid leukemia (AML) are cured by current treatments, emphasizing the need for new approaches to therapy. We previously demonstrated that besides playing a role in drug-resistant leukemia cell lines, multidrug resistance protein 4 (MRP4/ABCC4) regulates leukemia cell proliferation and differentiation through the endogenous MRP4/ABCC4 substrate, cAMP. Here, we studied the role of MRP4/ABCC4 in tumor progression in a mouse xenograft model and in leukemic stem cells (LSCs) differentiation. We found a decrease in the mitotic index and an increase in the apoptotic index associated with the inhibition of tumor growth when mice were treated with rolipram (PDE4 inhibitor) and/or probenecid (MRPs inhibitor). Genetic silencing and pharmacologic inhibition of MRP4 reduced tumor growth. Furthermore, MRP4 knockdown induced cell cycle arrest and apoptosis in vivo. Interestingly, when LSC population was isolated, we observed that increased cAMP levels and MRP4/ABCC4 blockade resulted in LSCs differentiation. Taken together, our findings show that MRP4/ABCC4 has a relevant role in tumor growth and apoptosis and in the eradication of LSCs, providing the basis for a novel promising target in AML therapy.

Quantitative interactome analysis reveals a chemoresistant edgotype

Chemoresistance is a common mode of therapy failure for many cancers. Tumours develop resistance to chemotherapeutics through a variety of mechanisms, with proteins serving pivotal roles. Changes in protein conformations and interactions affect the cellular response to environmental conditions contributing to the development of new phenotypes. The ability to understand how protein interaction networks adapt to yield new function or alter phenotype is limited by the inability to determine structural and protein interaction changes on a proteomic scale. Here, chemical crosslinking and mass spectrometry were employed to quantify changes in protein structures and interactions in multidrug-resistant human carcinoma cells. Quantitative analysis of the largest crosslinking-derived, protein interaction network comprising 1,391 crosslinked peptides allows for ‘edgotype' analysis in a cell model of chemoresistance. We detect consistent changes to protein interactions and structures, including those involving cytokeratins, topoisomerase-2-alpha, and post-translationally modified histones, which correlate with a chemoresistant phenotype.

Changes in protein–protein interactions result in changes to cellular phenotype. Here the authors use crosslinking mass spectrometry to derive a quantitative protein interaction network in drug-sensitive and -resistant HeLa cells, and uncover a chemoresistant ‘edgotype'.

The Pharmacological and Physiological Role of Multidrug-Resistant Protein 4

Multidrug-resistant protein 4 (MRP4), a member of the C subfamily of ATP-binding cassette transporters, is distributed in a variety of tissues and a number of cancers. As a drug transporter, MRP4 is responsible for the pharmacokinetics and pharmacodynamics of numerous drugs, especially antiviral drugs, antitumor drugs, and diuretics. In this regard, the functional role of MRP4 is affected by a number of factors, such as genetic mutations; tissue-specific transcriptional regulations; post-transcriptional regulations, including miRNAs and membrane internalization; and substrate competition. Unlike other C family members, MRP4 is in a pivotal position to transport cellular signaling molecules, through which it is tightly connected to the living activity and physiologic processes of cells and bodies. In the context of several cancers in which MRP4 is overexpressed, MRP4 inhibition shows striking effects against cancer progression and drug resistance. In this review, we describe the role of MRP4 more specifically in both healthy conditions and disease states, with an emphasis on its potential as a drug target.

Quantitative determination of irinotecan and the metabolite SN-38 by nanoflow liquid chromatography-tandem mass spectrometry in different regions of multicellular tumor spheroids

A new and simple method was developed to evaluate the distribution of therapeutics in three-dimensional multicellular tumor spheroids (MCTS) by combining serial trypsinization and nanoflow liquid chromatography-tandem mass spectrometry (nLC-MS/MS). This methodology was validated with quantitative measurements of irinotecan and its bioactive metabolite, SN-38, in distinct spatial regions of HCT 116 MCTS. Irinotecan showed a time-dependent permeability into MCTS with most of the drug accumulating in the core after 24 h of treatment. The amount of SN-38 detected was 30 times lower than that of the parent drug, and was more abundant in the outer rim and intermediate regions of MCTS where proliferating cells were present. This method can be used to investigate novel and established drugs. It enables investigation of drug penetration properties and identification of metabolites with spatial specificity in MCTS. The new approach has great value in facilitating the drug evaluation process.

Dual role of cAMP in the transcriptional regulation of multidrug resistance-associated protein 4 (MRP4) in pancreatic adenocarcinoma cell lines

Cyclic AMP represents one of the most studied signaling molecules and its role in proliferation and differentiation processes has been well established. Intracellular cAMP levels are tightly regulated where the MRP4 transporter plays a major role. In the present study, we sought to establish whether cAMP modulated MRP4 expression in pancreatic adenocarcinoma cell lines. Quantitative PCR and western blot studies showed that cAMP-increasing agents enhanced MRP4 transcripts and protein levels in PANC-1 cells. Reporter luciferase experiments carried out in pancreatic AR42J cells showed that intracellular cAMP up-regulates MRP4 through an Epac2- and Rap1- mediated mechanism whereas extracellular cAMP reduced MRP4 promoter activity by a MEK/ERK-mediated pathway. Present results show that cAMP regulates MRP4 promoter activity, and further indicate that the balance between intracellular and extracellular cAMP levels determines MRP4 expression.

Silencing of the ABCC4 gene by RNA interference reverses multidrug resistance in human gastric cancer

The identification of genes associated with recurrent drug resistance in gastric cancer and the elucidation of the underlying molecular mechanisms associated with recurrent drug resistance in gastric cancer are important for the effective treatment and prognosis of this cancer. Variations in the expression level of the ATP-binding cassette subfamily C member 4 (ABCC4) gene are correlated with the recurrence, development and chemotherapeutic susceptibility of various types of cancers. In the present study, we demonstrated that the ABCC4 gene was highly expressed in multiple types of gastric cancer cells, and ABCC4 expression was even more prominent in the drug-resistant gastric cancer cells. Conversely, in normal gastric mucosal cells, ABCC4 expression was very low or undetectable. We used RNA interference to decrease the expression of ABCC4 in drug-resistant gastric cancer cells, which resulted in an increase in apoptosis and cell cycle arrest in the G1 phase. In addition, we found that ABCC4 knockdown in 5-fluorouracil (5-FU)-resistant cancer cells restored 5-FU sensitivity, resulting in the inhibition of cell proliferation and tumour growth in nude mice. Our results showed that inhibition of ABCC4 gene expression can inhibit the proliferation of multidrug-resistant gastric cancer cells and can enhance gastric cancer cell sensitivity to chemotherapeutic drugs.

Lipid-coated polymeric nanoparticles for cancer drug delivery

Polymeric nanoparticles and liposomes have been the platform of choice for nanoparticle-based cancer drug delivery applications over the past decade, but extensive research has revealed their limitations as drug delivery carriers. A hybrid class of nanoparticles, aimed at combining the advantages of both polymeric nanoparticles and liposomes, has received attention in recent years. These core/shell type nanoparticles, frequently referred to as lipid-polymer hybrid nanoparticles (LPNs), possess several characteristics that make them highly suitable for drug delivery. This review introduces the formulation methods used to synthesize LPNs and discusses the strategies used to treat cancer, such as by targeting the tumor microenvironment or vasculature. Finally, it discusses the challenges that must be overcome to realize the full potential of LPNs in the clinic.

ABC transporters and neuroblastoma

Neuroblastoma is the most common cancer of infancy and accounts for 15% of all pediatric oncology deaths. Survival rates of high-risk neuroblastoma remain less than 50%, with amplification of the MYCN oncogene the most important aberration associated with poor outcome. Direct transcriptional targets of MYCN include a number of ATP-binding cassette (ABC) transporters, of which ABCC1 (MRP1), ABCC3 (MRP3), and ABCC4 (MRP4) are the best characterized. These three transporter genes have been shown to be strongly prognostic of neuroblastoma outcome in primary untreated neuroblastoma. In addition to their ability to efflux a number of chemotherapeutic drugs, evidence suggests that these transporters also contribute to neuroblastoma outcome independent of any role in cytotoxic drug efflux. Endogenous substrates of ABCC1 and ABCC4 that may be potential candidates affecting neuroblastoma biology include molecules such as prostaglandins and leukotrienes. These bioactive lipid mediators have the ability to influence biological processes contributing to cancer initiation and progression, such as angiogenesis, cell signaling, inflammation, proliferation, and migration and invasion. ABCC1 and ABCC4 are thus potential targets for therapeutic suppression in high-risk neuroblastoma, and recently developed small-molecule inhibitors may be an effective strategy in treating aggressive forms of this cancer, as well as other cancers that express high levels of these transporters.

A Drosophila ABC transporter regulates lifespan

MRP4 (multidrug resistance-associated protein 4) is a member of the MRP/ABCC subfamily of ATP-binding cassette (ABC) transporters that are essential for many cellular processes requiring the transport of substrates across cell membranes. Although MRP4 has been implicated as a detoxification protein by transport of structurally diverse endogenous and xenobiotic compounds, including antivirus and anticancer drugs, that usually induce oxidative stress in cells, its in vivo biological function remains unknown. In this study, we investigate the biological functions of a Drosophila homolog of human MRP4, dMRP4. We show that dMRP4 expression is elevated in response to oxidative stress (paraquat, hydrogen peroxide and hyperoxia) in Drosophila. Flies lacking dMRP4 have a shortened lifespan under both oxidative and normal conditions. Overexpression of dMRP4, on the other hand, is sufficient to increase oxidative stress resistance and extend lifespan. By genetic manipulations, we demonstrate that dMRP4 is required for JNK (c-Jun NH₂-terminal kinase) activation during paraquat challenge and for basal transcription of some JNK target genes under normal condition. We show that impaired JNK signaling is an important cause for major defects associated with dMRP4 mutations, suggesting that dMRP4 regulates lifespan by modulating the expression of a set of genes related to both oxidative resistance and aging, at least in part, through JNK signaling.

The drug transporters are often known for their ability to transport different physiological-related compounds across cell membranes. Although the abnormal up-regulation of some these transporters is believed to be the common cause of the clinic problem called drug resistance, the biological functions of these transporters remain largely unknown. Here we show that a Drosophila homolog of the mammalian drug transporter plays a role in lifespan regulation. Mutations of this gene increase the sensitivity to oxidative stress and reduce lifespan, while overexpression of this gene increases resistance to oxidative stress and extends lifespan. By molecular and genetic analyses, we have linked functions of this gene to a key signaling transduction pathway that has been known to be important in lifespan regulation.

Minireview: SLCO and ABC transporters: a role for steroid transport in prostate cancer progression

Androgens play a critical role in the development and progression of prostate cancer (PCa), and androgen deprivation therapy via surgical or medical castration is front-line therapy for patients with advanced PCa. However, intratumoral testosterone levels are elevated in metastases from patients with castration-resistant disease, and residual intratumoral androgens have been implicated in mediating ligand-dependent mechanisms of androgen receptor activation. The source of residual tissue androgens present despite castration has not been fully elucidated, but proposed mechanisms include uptake and conversion of adrenal androgens, such as dehdroepiandrosterone to testosterone and dihydrotestosterone, or de novo androgen synthesis from cholesterol or progesterone precursors. In this minireview, we discuss the emerging evidence that suggests a role for specific transporters in mediating transport of steroids into or out of prostate cells, thereby influencing intratumoral androgen levels and PCa development and progression. We focus on the solute carrier and ATP binding cassette gene families, which have the most published data for a role in PCa-related steroid transport, and review the potential impact of genetic variation on steroid transport activity and PCa outcomes. Continued assessment of transport activity in PCa models and human tumor tissue is needed to better delineate the different roles these transporters play in physiologic and neoplastic settings, and in order to determine whether targeting the uptake of steroid substrates by specific transporters may be a clinically feasible therapeutic strategy.

Repositioning of Tyrosine Kinase Inhibitors as Antagonists of ATP-Binding Cassette Transporters in Anticancer Drug Resistance

The phenomenon of multidrug resistance (MDR) has attenuated the efficacy of anticancer drugs and the possibility of successful cancer chemotherapy. ATP-binding cassette (ABC) transporters play an essential role in mediating MDR in cancer cells by increasing efflux of drugs from cancer cells, hence reducing the intracellular accumulation of chemotherapeutic drugs. Interestingly, small-molecule tyrosine kinase inhibitors (TKIs), such as AST1306, lapatinib, linsitinib, masitinib, motesanib, nilotinib, telatinib and WHI-P154, have been found to have the capability to overcome anticancer drug resistance by inhibiting ABC transporters in recent years. This review will focus on some of the latest and clinical developments with ABC transporters, TKIs and anticancer drug resistance.

Pharmacogenomic characterization of gemcitabine response--a framework for data integration to enable personalized medicine

Supplemental Digital Content is available in the text.

Objectives

Response to the oncology drug gemcitabine may be variable in part due to genetic differences in the enzymes and transporters responsible for its metabolism and disposition. The aim of our in-silico study was to identify gene variants significantly associated with gemcitabine response that may help to personalize treatment in the clinic.

Methods

We analyzed two independent data sets: (a) genotype data from NCI-60 cell lines using the Affymetrix DMET 1.0 platform combined with gemcitabine cytotoxicity data in those cell lines, and (b) genome-wide association studies (GWAS) data from 351 pancreatic cancer patients treated on an NCI-sponsored phase III clinical trial. We also performed a subset analysis on the GWAS data set for 135 patients who were given gemcitabine+placebo. Statistical and systems biology analyses were performed on each individual data set to identify biomarkers significantly associated with gemcitabine response.

Results

Genetic variants in the ABC transporters (ABCC1, ABCC4) and the CYP4 family members CYP4F8 and CYP4F12, CHST3, and PPARD were found to be significant in both the NCI-60 and GWAS data sets. We report significant association between drug response and variants within members of the chondroitin sulfotransferase family (CHST) whose role in gemcitabine response is yet to be delineated.

Conclusion

Biomarkers identified in this integrative analysis may contribute insights into gemcitabine response variability. As genotype data become more readily available, similar studies can be conducted to gain insights into drug response mechanisms and to facilitate clinical trial design and regulatory reviews.

Targeting inflammation: multiple innovative ways to reduce prostaglandin E₂

The PGE2 pathway is important in inflammation-driven diseases and specific targeting of the inducible mPGES-1 is warranted due to the cardiovascular problems associated with the long-term use of COX-2 inhibitors. This review focuses on patents issued on methods of measuring mPGES-1 activity, on drugs targeting mPGES-1 and on other modulators of free extracellular PGE2 concentration. Perspectives and conclusions regarding the status of these drugs are also presented. Importantly, no selective inhibitors targeting mPGES-1 have been identified and, despite the high number of published patents, none of these drugs have yet made it to clinical trials.

High-throughput screening identifies Ceefourin 1 and Ceefourin 2 as highly selective inhibitors of multidrug resistance protein 4 (MRP4)

Multidrug resistance protein 4 (MRP4/ABCC4), a member of the ATP-binding cassette (ABC) transporter superfamily, is an organic anion transporter capable of effluxing a wide range of physiologically important signalling molecules and drugs. MRP4 has been proposed to contribute to numerous functions in both health and disease; however, in most cases these links remain to be unequivocally established. A major limitation to understanding the physiological and pharmacological roles of MRP4 has been the absence of specific small molecule inhibitors, with the majority of established inhibitors also targeting other ABC transporter family members, or inhibiting the production, function or degradation of important MRP4 substrates. We therefore set out to identify more selective and well tolerated inhibitors of MRP4 that might be used to study the many proposed functions of this transporter. Using high-throughput screening, we identified two chemically distinct small molecules, Ceefourin 1 and Ceefourin 2, that inhibit transport of a broad range of MRP4 substrates, yet are highly selective for MRP4 over other ABC transporters, including P-glycoprotein (P-gp), ABCG2 (Breast Cancer Resistance Protein; BCRP) and MRP1 (multidrug resistance protein 1; ABCC1). Both compounds are more potent MRP4 inhibitors in cellular assays than the most widely used inhibitor, MK-571, requiring lower concentrations to effect a comparable level of inhibition. Furthermore, Ceefourin 1 and Ceefourin 2 have low cellular toxicity, and high microsomal and acid stability. These newly identified inhibitors should be of great value for efforts to better understand the biological roles of MRP4, and may represent classes of compounds with therapeutic application.

Upregulation of COX-2 in the lung cancer promotes overexpression of multidrug resistance protein 4 (MRP4) via PGE2-dependent pathway

It is apparent that lung cancer is associated with inflammation, with accompanying hallmark elevations of cyclooxygenase 2 (COX-2) and prostaglandin E2 (PGE2) levels. However, the effects of these changes on MRP efflux transporters have not been thoroughly investigated before. Here, we report that upregulation of COX-2 can induce overexpression of MRP4 in both A549 non-small-cell lung cancer cell lines and mouse lung cancer models. In A549 cells, phorbol 12-myristate 13-acetate (PMA) treatment induced upregulation of COX-2 and MRP4 together, but not other MRP transporters. Transient overexpression of human COX-2 cDNA also specifically increased COX-2 and MRP4. Moreover, COX inhibitor treatment and COX-2-specific siRNA significantly inhibited the upregulation of MRP4. Additionally, PMA-treatment increased extracellular PGE2 levels, likely due to increased MRP4 function. Likewise, COX-2-specific siRNA reduced extracellular PGE2 levels. Furthermore, COX-2 upregulation resulted in an increase in mPGES-1, an enzyme responsible for PGE2 production. Finally, metastasized lung cancer model mice exhibited increased expression levels of COX-2 and MRP4, as well as mPGES-1. In conclusion, the present study suggests that overexpression of MRP4 in lung cancer may be attributable to COX-2 upregulation via a PGE2-dependent pathway.

Multidrug resistance protein 4 (MRP4/ABCC4) regulates thrombus formation in vitro and in vivo

The multidrug resistance protein 4 (MRP4) is a member of the ABCC subfamily of the adenosine triphosphate-binding cassette transporters that remove cyclic nucleotides from platelets and uptake ADP into dense granule in platelets. However, whether MRP4 directly involves platelet activation remains unclear. Thus, the aim of our study was to determine the detailed mechanisms underlying the regulation of MRP4 in platelet activation. Our results revealed that the MRP4 inhibitor MK571 inhibited collagen-induced platelet aggregation which was partially reversed by the PKA inhibitor H89, but not by the adenylyl cyclase (AC) inhibitor SQ22536 and the guanylyl cyclase (GC) inhibitor ODQ, suggesting that MK571 can prevent collagen-induced aggregation via a route independent of cyclic nucleotide production. In the present study, we found that MK571 inhibited collagen-induced ATP release and calcium mobilization. The phosphorylation of protein kinase C, JNK, and Akt was also inhibited by MK571, and electron spin resonance experiment showed that MK571 significantly reduced hydroxyl radical formation. Moreover, MK571 delayed platelet plug formation in vitro by a PFA-100 device, and delayed thrombus formation in mesenteric venules of mice irradiated by fluorescein sodium. However, previous studies have reported that MK571 also blocks MRP1 and leukotriene D4 (LTD4) receptor. Therefore, whether MK571 inhibits platelet activation through MRP1 or LTD4 receptor needs to be considered and further defined. In conclusion, in addition to blocking the transport of cyclic nucleotides, MRP4 inhibition may prevent thrombus formation in vitro and in vivo. Our findings also support the idea that MRP4 may represent a potential target for the development of novel therapeutic interventions for the treatment of thromboembolic disorders.

ABCC4 is required for cell proliferation and tumorigenesis in non-small cell lung cancer

BACKGROUND

Multidrug resistance protein 4 (MRP4), also known as ATP-cassette binding protein 4 (ABCC4), is a member of the MRP/ABCC subfamily of ATP-binding cassette transporters, which are capable of pumping a wide variety of drugs out of the cell. However, little is known about the function of ABCC4 in the proliferation of lung cancer cells.

METHODS

ABCC4 mRNA and protein levels in lung cancer cell lines were measured by real-time polymerase chain reaction and Western blot, respectively. A lentivirus-mediated RNA interference technique was used to inhibit ABCC4 mRNA expression in A549 and 801D cells. The function of ABCC4 in cell growth was investigated by MTS and colony formation assays. The role of ABCC4 in cell cycle progression was evaluated by flow cytometry and Western blot analysis. ABCC4 mRNA levels in 30 pairs of tumors and corresponding matched adjacent normal tissues from non-small cell lung cancer patients were detected by real-time polymerase chain reaction.

RESULTS

ABCC4 was highly expressed in lung cancer cell lines. ABCC4 expression was markedly downregulated in A549 and 801D cells using the RNA interference technique. Suppression of ABCC4 expression inhibited cell growth. The percentage of cells in G1 phase was increased when ABCC4 expression was suppressed. Phosphorylation of retinoblastoma protein was weakened, originating in the downregulation of ABCC4. ABCC4 mRNA was highly expressed in lung cancer tissue and lung cancer cell lines.

CONCLUSION

ABCC4 may play an important role in the control of A549 and 801D cell growth. ABCC4 is a potential target for lung cancer therapy.

Multidrug resistance and cancer stem cells in neuroblastoma and hepatoblastoma

Chemotherapy is one of the major modalities in treating cancers. However, its effectiveness is limited by the acquisition of multidrug resistance (MDR). Several mechanisms could explain the up-regulation of MDR genes/proteins in cancer after chemotherapy. It is known that cancer stem cells (CSCs) play a role as master regulators. Therefore, understanding the mechanisms that regulate some traits of CSCs may help design efficient strategies to overcome chemoresistance. Different CSC phenotypes have been identified, including those found in some pediatric malignancies. As solid tumors in children significantly differ from those observed in adults, this review aims at providing an overview of the mechanistic relationship between MDR and CSCs in common solid tumors, and, in particular, focuses on clinical as well as experimental evidence of the relations between CSCs and MDR in neuroblastoma and hepatoblastoma. Finally, some novel approaches, such as concomitant targeting of multiple key transcription factors governing the stemness of CSCs, as well as nanoparticle-based approaches will also be briefly addressed.

Interaction of immunosuppressive drugs with human organic anion transporter (OAT) 1 and OAT3, and multidrug resistance-associated protein (MRP) 2 and MRP4

Renal proximal tubule transporters can play a key role in excretion, pharmacokinetic interactions, and toxicity of immunosuppressant drugs. Basolateral organic anion transporters (OATs) and apical multidrug resistance-associated proteins (MRPs) contribute to the active tubular uptake and urinary efflux of these drugs, respectively. We studied the interaction of 12 immunosuppressants with OAT1- and OAT3-mediated [(3)H]-methotrexate (MTX) uptake in cells, and adenosine triphosphate-dependent [(3)H]-MTX transport in membrane vesicles isolated from human embryonic kidney 293 cells overexpressing human MRP2 and MRP4. Our results show that at a clinically relevant concentration of 10 μM, mycophenolic acid inhibited both OAT1- and OAT3-mediated [(3)H]-MTX uptake. Cytarabine, vinblastine, vincristine, hydrocortisone, and mitoxantrone inhibited only OAT1, whereas tacrolimus, azathioprine, dexamethasone, cyclosporine, and 6-mercaptopurine had no effect on both transporters. Cyclophosphamide stimulated OAT1, but did not affect OAT3. With regard to the apical efflux transporters, mycophenolic acid, cyclophosphamide, hydrocortisone, and tacrolimus inhibited MRP2 and MRP4, whereas mitoxantrone and dexamethasone stimulated [(3)H]-MTX transport by both transporters. Cyclosporine, vincristine, and vinblastine inhibited MRP2 only, whereas 6-mercaptopurine inhibited MRP4 transport activity only. Cytarabine and azathioprine had no effect on either transporter. In conclusion, we charted comprehensively the differences in inhibitory action of various immunosuppressive agents against the 4 key renal anion transporters, and we provide evidence that immunosuppressant drugs can modulate OAT1-, OAT3-, MRP2-, and MRP4-mediated transport of MTX to different extents. The data provide a better understanding of renal mechanisms underlying drug-drug interactions and nephrotoxicity concerning combination regimens with these compounds in the clinic.

Clinicopathological impact of ABCC1/MRP1 and ABCC4/MRP4 in epithelial ovarian carcinoma

Ovarian cancer is the main cause of death from gynaecological malignancies. In spite of the efficacy of platinum-paclitaxel treatment in patients with primary epithelial ovarian carcinoma, platinum-based chemotherapy is not curative and resistance remains one of the most important causes of treatment failure. Although ABC transporters have been implicated in cellular resistance to multiple drugs, the clinical relevance of these efflux pumps is still poorly understood. Thus, we examined the prognostic role of transporters of the MRP family (i.e., ABCC1/MRP1, ABCC4/MRP4) to gain insights into their clinical impacts. A case material of 127 patients with ovarian carcinoma at different stages and histotypes was used. The expression of MRP1 and MRP4 was examined by immunohistochemistry using tissue microarrays in tumor specimens collected at the time of initial surgery expression. We found an association between MRP1 expression and grading, and we observed that MRP4 displayed an unfavourable impact on disease relapse in multivariate analysis (HR = 2.05, 95% CI: 1.01–4.11; P = 0.045). These results suggest that in epithelial ovarian cancer, MRP1 may be a marker for aggressiveness because its expression was associated with tumor grade and support that MRP4 may play an unfavourable role in disease outcome.

Downregulation of ATP-binding cassette subfamily C member 4 increases sensitivity to neoadjuvant radiotherapy for locally advanced rectal carcinoma

OBJECTIVE

This study was designed to verify the effect of ATP-binding cassette subfamily C member 4 on radiosensitivity of locally advanced rectal carcinoma.

SETTING

The expression of ATP-binding cassette subfamily C member 4 protein in 121 pretreatment tissue samples from locally advanced rectal carcinoma patients was detected by immunohistochemistry.

DESIGN

Pathological response to radiotherapy was evaluated according to tumor regression grading by postoperative histological examinations after they received long-course preoperative neoadjuvant radiotherapy, and the association between clinicopathological data and tumor regression grading was analyzed retrospectively. For further validation, short hairpin RNA was constructed and transfected into colorectal carcinoma cell line HT29. The knockdown efficiency was confirmed at both RNA and protein levels. The altered radiosensitivity was evaluated by methylthiazolyl tetrazolium assay, colony formation assay, flow cytometry, and Hoechst 33258 staining.

RESULTS

Univariate analysis revealed that ATP-binding cassette subfamily C member 4 expression (p < 0.001), P53 type (p = 0.069), and CEA (p = 0.100) were possibly associated with tumor regression grading, and multivariate analysis demonstrated that ATP-binding cassette subfamily C member 4 expression (p < 0.001) and P53 type (p = 0.039) were positively correlated with response to neoadjuvant radiotherapy in locally advanced rectal carcinoma patients. Lentiviral vector was successfully introduced into HT29 cells and inhibited ATP-binding cassette subfamily C member 4 expression efficiently and persistently. Downregulation of ATP-binding cassette subfamily C member 4 expression significantly enhanced inhibition of cell proliferation, decreased colony formation capacity, and increased cell apoptosis induced by irradiation, as examined by a series of experiments in vitro. In addition, radiobiological parameters calculated according to the single-hit multitarget model were also decreased significantly.

CONCLUSIONS

Our data indicate that ATP-binding cassette subfamily C member 4 may be a useful molecular marker in predicting radiosensitivity, and a potential target in improving the response to neoadjuvant radiotherapy in locally advanced rectal carcinoma patients.

Augmented epithelial multidrug resistance-associated protein 4 expression in peritoneal endometriosis: regulation by lipoxin A(4)

OBJECTIVE

To compare the expression of the prostaglandin (PG) E(2) transporter multidrug resistance-associated protein 4 (MRP4) in eutopic and ectopic endometrial tissue from endometriosis patients with that of control subjects and to examine whether MRP4 is regulated by the antiinflammatory lipid lipoxin A(4) (LXA(4)) in endometriotic epithelial cells.

DESIGN

Molecular analysis in human samples and a cell line.

SETTING

Two university hospitals and a private clinic.

PATIENT(S)

A total of 59 endometriosis patients and 32 age- and body mass index-matched control subjects undergoing laparoscopy or hysterectomy.

INTERVENTION(S)

Normal, eutopic, and ectopic endometrial biopsies as well as peritoneal fluid were obtained during surgery performed during the proliferative phase of the menstrual cycle. 12Z endometriotic epithelial cells were used for in vitro mechanistic studies.

MAIN OUTCOME MEASURE(S)

Tissue MRP4 mRNA levels were quantified by quantitative reverse-transcription polymerase chain reaction (qRT-PCR), and localization was analyzed with the use of immunohistochemistry. Cellular MRP4 mRNA and protein were quantified by qRT-PCR and Western blot, respectively. PGE(2) was measured in peritoneal fluid and cell supernatants using an enzyme immunoassay (EIA).

RESULT(S)

MRP4 was expressed in eutopic and ectopic endometrium, where it was overexpressed in peritoneal lesions and localized in the cytoplasm of glandular epithelial cells. LXA(4) attenuated MRP4 mRNA and protein levels in endometriotic epithelial cells in a dose-dependent manner, while not affecting the expression of enzymes involved in PGE(2) metabolism. Investigations employing receptor antagonists and small interfering RNA revealed that this occurred through estrogen receptor α. Accordingly, LXA(4) treatment inhibited extracellular PGE(2) release.

CONCLUSION(S)

We report for the first time that MRP4 is expressed in human endometrium, elevated in peritoneal endometriosis, and modulated by LXA(4) in endometriotic epithelial cells.