The human multidrug resistance protein MRP5 transports folates and can mediate cellular resistance against antifolates

Inhibition and transport mechanisms of the ABC transporter hMRP5

Human multidrug resistance protein 5 (hMRP5) effluxes anticancer and antivirus drugs, driving multidrug resistance. To uncover the mechanism of hMRP5, we determine six distinct cryo-EM structures, revealing an autoinhibitory N-terminal peptide that must dissociate to permit subsequent substrate recruitment. Guided by these molecular insights, we design an inhibitory peptide that could block substrate entry into the transport pathway. We also identify a regulatory motif, comprising a positively charged cluster and hydrophobic patches, within the first nucleotide-binding domain that modulates hMRP5 localization by engaging with membranes. By integrating our structural, biochemical, computational, and cell biological findings, we propose a model for hMRP5 conformational cycling and localization. Overall, this work provides mechanistic understanding of hMRP5 function, while informing future selective hMRP5 inhibitor development. More broadly, this study advances our understanding of the structural dynamics and inhibition of ABC transporters.

Regulation of the ATP-binding cassette transporters ABCB1, ABCG2 and ABCC5 by nuclear receptors in porcine blood-brain barrier endothelial cells

BACKGROUND AND PURPOSE

Blood-brain barrier (BBB) ABCB1, ABCG2 and ABCC5 transporters influence central therapeutic drug distribution. Transporter expression is regulated by the NR3C1, NR1I3 and NR1I2 nuclear receptors, but their precise roles in brain are poorly understood. We investigated the effects of selective ligand-based activation of NR3C1, NR1I3, NR1I2 and NR2B1 in porcine brain endothelial cells (PBECs).

EXPERIMENTAL APPROACH

Primary cultures of PBECs were exposed to NR3C1, NR1I3 and NR1I2 ligands and ABCB1, ABCG2 and ABCC5 transporter activities determined by measuring intracellular accumulation of fluorescent probes. Western blotting was used to determine the effects of receptor ligands on expression of ABCB1, ABCG2, ABCC5, NR1I2, NR1I3, NR3C1 and NR2B1. Fluorescent immunocytochemistry was employed to assess the effects of receptor ligands on the cellular localisation of NR1I2 and NR1I3.

KEY RESULTS

The NR1I2 agonist rifampicin significantly up-regulated ABCG2 activity, which is counteracted by co-treatment with NR1I2 antagonist l-sulforaphane. The NR1I3 agonist 6-(4-chlorophenyl)-imidazo[2,1-b]thiazole-5-carbaldehyde and inverse agonist meclizine significantly down-regulated ABCB1, ABCG2 and ABCC5 activity. NR3C1 agonist dexamethasone significantly increased ABCB1, ABCG2 and ABCC5 activity and ABCG2 and ABCC5 protein expression, which was counteracted by co-treatment with the NR3C1 antagonist mifepristone. This first study demonstrates that NR1I3 and NR3C1 regulate ABCC5 activity and protein expression in BBB endothelial cells.

CONCLUSIONS AND IMPLICATIONS

In PBECs, expression of key ATP-binding cassette (ABC) transporters and nuclear receptors is differentially regulated by NR1I3, NR1I2, NR3C1 and NR2B1. This will help to better understand the response of the BBB to physiological and pharmacological activation of nuclear receptors.

Notes from the Underground: Heme Homeostasis in C. elegans

Heme is an iron-containing tetrapyrrole that plays a critical role in various biological processes, including oxygen transport, electron transport, signal transduction, and catalysis. However, free heme is hydrophobic and potentially toxic to cells. Organisms have evolved specific pathways to safely transport this essential but toxic macrocycle within and between cells. The bacterivorous soil-dwelling nematode Caenorhabditis elegans is a powerful animal model for studying heme-trafficking pathways, as it lacks the ability to synthesize heme but instead relies on specialized trafficking pathways to acquire, distribute, and utilize heme. Over the past 15 years, studies on this microscopic animal have led to the identification of a number of heme-trafficking proteins, with corresponding functional homologs in vertebrates. In this review, we provide a comprehensive overview of the heme-trafficking proteins identified in C. elegans and their corresponding homologs in related organisms.

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

Plasma Distribution of Methotrexate and Its Polyglutamates in Pediatric Acute Lymphoblastic Leukemia: Preliminary Insights

BACKGROUND AND OBJECTIVE

High-dose methotrexate (HD-MTX) is the mainstream therapy of current acute lymphoblastic leukemia (ALL) regimens, but frequent intra- and interindividual differences in the clinical response to HD-MTX lead to chemotherapeutic interruption or discontinuation. The exact mechanism of transport across the cell membrane and the disposition of active methotrexate metabolites-methotrexate polyglutamates (MTXPGs)-are not well described in the literature. The aim of this study was to gain more insight into the plasma distribution of methotrexate and MTXPGs in pediatric patients with ALL and to clarify the obscure pathways of MTXPGs.

METHODS

We prospectively measured the concentrations of MTXPG_1-7 in plasma samples from three male pediatric patients treated with HD-MTX and leucovorin rescue according to the IC-BFM 2009 protocol using liquid chromatography-mass spectrometry (LC-MS). Blood samples were obtained at 24, 36, 42, and 48 h after the start of HD-MTX treatment.

RESULTS

Noticeable plasma concentrations of MTXPGs with a 2.2-fold interpatient variability were detected. The highest interindividual variability in total plasma MTXPG concentration was observed at 36 h, and ranged from 13.78 to 30.82 μmol/L. Among all patients, the predominant polyglutamate types in relation to the total plasma MTXPG concentration at each time point were MTXPG₃ (16.71-30.02%) and MTXPG₅ (26.23-38.60%), while MTXPG₇ was the least abundant MTXPG (3.22-5.02%).

CONCLUSION

The presence of MTXPGs in plasma of patients with ALL could be related to the action of ABC efflux transporters on blood cells and hepatocytes resulting from the administration of high doses of methotrexate. This study may not draw definitive conclusions, but it does reduce uncertainty about the dynamics of methotrexate and its active metabolites, which may be of vital importance for achieving a clinical response.

The Concept of Folic Acid in Health and Disease

Folates have a pterine core structure and high metabolic activity due to their ability to accept electrons and react with O-, S-, N-, C-bounds. Folates play a role as cofactors in essential one-carbon pathways donating methyl-groups to choline phospholipids, creatine, epinephrine, DNA. Compounds similar to folates are ubiquitous and have been found in different animals, plants, and microorganisms. Folates enter the body from the diet and are also synthesized by intestinal bacteria with consequent adsorption from the colon. Three types of folate and antifolate cellular transporters have been found, differing in tissue localization, substrate affinity, type of transferring, and optimal pH for function. Laboratory criteria of folate deficiency are accepted by WHO. Severe folate deficiencies, manifesting in early life, are seen in hereditary folate malabsorption and cerebral folate deficiency. Acquired folate deficiency is quite common and is associated with poor diet and malabsorption, alcohol consumption, obesity, and kidney failure. Given the observational data that folates have a protective effect against neural tube defects, ischemic events, and cancer, food folic acid fortification was introduced in many countries. However, high physiological folate concentrations and folate overload may increase the risk of impaired brain development in embryogenesis and possess a growth advantage for precancerous altered cells.

Detection of Cross-Resistance Between Methotrexate and Azoles in Candida albicans and Meyerozyma guilliermondii: An In Vitro Study

In recent years, there has been a rapid increase in the incidence of Candida infections. The different species of the genus Candida vary in their virulence abilities and susceptibility to antifungal agents, depending on several external factors. The result of such modifications may be cross-resistance, which is understood as an acquired resistance to a certain antimicrobial agent after exposure to another drug. The aim of this study was to determine the possibility of cross-resistance between fluconazole, voriconazole, itraconazole, and methotrexate in Candida albicans and Meyerozyma guilliermondii (syn. Candida guilliermondii ). Fifteen strains of M. guilliermondii and eight strains of C. albicans , including the standard strains, were tested. For all strains, the minimum inhibitory concentrations (MICs) for fluconazole, voriconazole, and itraconazole were determined before and after stimulation with methotrexate. The median MICs in M. guilliermondii before and after stimulation were 9.333 and 64 mg/L ( p = 0.005) for fluconazole; 0.917 and 1.667 mg/L ( p = 0.001) for itraconazole, respectively. No significant change in MIC was observed for voriconazole. For C. albicans strains, the median MICs before and after stimulation were 0.917 and 64 mg/L ( p = 0.012) for fluconazole; 0.344 and 1.135 mg/L ( p = 0.018) for voriconazole, respectively. There was no significant change in MIC values for itraconazole. Thus, this study demonstrates the presence of cross-resistance between voriconazole, itraconazole, fluconazole, and methotrexate for the selected strains. Methotrexate exposure induces different responses when certain drugs are used for various species. Therefore, if a patient was previously exposed to methotrexate, there may be a higher risk of treatment failure with fluconazole than with other azoles such as voriconazole for fungemia caused by M. guilliermondii or itraconazole for C. albicans infection.

Upregulation of ATP Binding Cassette Subfamily C Member 5 facilitates Prostate Cancer progression and Enzalutamide resistance via the CDK1-mediated AR Ser81 Phosphorylation Pathway

The treatment of advanced prostate cancer, castration-resistant prostate cancer, remains challenging. The mechanisms of action of ATP binding cassette subfamily C member 5 (ABCC5) in prostate cancer and its relationship with drug resistance are still unclear. Expression and prognostic analyses of ABCC5 were performed through bioinformatic methods and immunohistochemistry analyses in multiple public databases as well as in our own prostate cancer cohort. The biological function of ABCC5 in prostate cancer cells was evaluated by in vitro and in vivo cell proliferation and migration and invasion assays. The regulation of CDK1 by ABCC5 was determined via RT-qPCR, western blots, and immunofluorescence. ABCC5 was significantly overexpressed in prostate cancer and positively associated with unfavorable clinicopathological features and prognosis. Upregulation of ABCC5 could enhance the cell proliferation, migration, and invasion of prostate cancer in vitro and in vivo. Mechanistically, ABCC5 exerts a protumor effect by binding to and inhibiting the protein degradation of CDK1, which promotes the phosphorylation of AR at Ser81 by CDK1 and activates the transcriptional activity of AR on target genes. Moreover, the addition of a CDK1 inhibitor or knockdown of CDK1 significantly improved the efficacy of enzalutamide on prostate cancer cells. The ABCC5-CDK1-AR regulatory pathway could be a potential therapeutic target for advanced prostate cancer, especially castration-resistant prostate cancer (CRPC), to enhance the therapeutic effect of enzalutamide.

Role of ATP-Binding Cassette Transporter Proteins in CNS Tumors: Resistance- Based Perspectives and Clinical Updates

Despite gigantic advances in medical research and development, chemotherapeutic resistance remains a major challenge in complete remission of CNS tumors. The failure of complete eradication of CNS tumors has been correlated with the existence of several factors including overexpression of transporter proteins. To date, 49 ABC-transporter proteins (ABC-TPs) have been reported in humans, and the evidence of their strong association with chemotherapeutics' influx, dissemination, and efflux in CNS tumors, is growing. Research studies on CNS tumors are implicating ABC-TPs as diagnostic, prognostic and therapeutic biomarkers that may be utilised in preclinical and clinical studies. With the current advancements in cell biology, molecular analysis of genomic and transcriptomic interplay, and protein homology-based drug-transporters interaction, our research approaches are streamlining the roles of ABC-TPs in cancer and multidrug resistance. Potential inhibitors of ABC-TP for better clinical outcomes in CNS tumors have emerged. Elacridar has shown to enhance the chemo-sensitivity of Dasatanib and Imatinib in various glioma models. Tariquidar has improved the effectiveness of Temozolomide's in CNS tumors. Although these inhibitors have been effective in preclinical settings, their clinical outcomes have not been as significant in clinical trials. Thus, to have a better understanding of the molecular evaluations of ABC-TPs, as well as drug-interactions, further research is being pursued in research labs. Our lab aims to better comprehend the biological mechanisms involved in drug resistance and to explore novel strategies to increase the clinical effectiveness of anticancer chemotherapeutics, which will ultimately improve clinical outcomes.

Updates on Genes and Genetic Mechanisms Implicated in Primary Angle-Closure Glaucoma

Primary angle-closure glaucoma (PACG) is estimated to affect over 30 million people worldwide by 2040 and is highly prevalent in the Asian population. PACG is more severe and carries three times the higher risk of blindness than primary open-angle glaucoma, thus representing a significant public health concern. High heritability and ethnic-specific predisposition to PACG suggest the involvement of genetic factors in disease development. In the recent past, genetic studies have led to the successful identification of several genes and loci associated with PACG across different ethnicities. The precise cellular and molecular roles of these multiple loci in the development and progression of PACG remains to be elucidated. Nonetheless, these studies have significantly increased our understanding of the emerging cellular processes and biological pathways that might provide more significant insights into the disease’s genetic etiology and may be valuable for future clinical applications. This review aims to summarize and update the current knowledge of PACG genetics analysis research.

Human drug efflux transporter ABCC5 confers acquired resistance to pemetrexed in breast cancer

Aim

Pemetrexed, a new generation antifolate drug, has been approved for the treatment of locally advanced or metastatic breast cancer. However, factors affecting its efficacy and resistance have not been fully elucidated yet. ATP-binding cassette (ABC) transporters are predictors of prognosis as well as of adverse effects of several xenobiotics. This study was designed to explore whether ABC transporters affect pemetrexed resistance and can contribute to the optimization of breast cancer treatment regimen.

Methods

First, we measured the expression levels of ABC transporter family members in cell lines. Subsequently, we assessed the potential role of ABC transporters in conferring resistance to pemetrexed in primary breast cancer cells isolated from 34 breast cancer patients and the role of ABCC5 in mediating pemetrexed transport and apoptotic pathways in MCF-7 cells. Finally, the influence of ABCC5 expression on the therapeutic effect of pemetrexed was evaluated in an in vivo xenograft mouse model of breast cancer.

Results

The expression levels of ABCC2, ABCC4, ABCC5, and ABCG2 significantly increased in the pan-resistant cell line, and the ABCC5 level in the MCF-7-ADR cell line was 5.21 times higher than that in the control group. ABCC5 expression was inversely correlated with pemetrexed sensitivity (IC₅₀, r = 0.741; p < 0.001) in breast cancer cells derived from 34 patients. Furthermore, we found that the expression level of ABCC5 influenced the efflux and cytotoxicity of pemetrexed in MCF-7 cells, with IC₅₀ values of 0.06 and 0.20 μg/mL in ABCC5 knockout and over-expression cells, respectively. In the in vivo study, we observed that ABCC5 affected the sensitivity of pemetrexed in breast tumor-bearing mice, and the tumor volume was much larger in the ABCC5-overexpressing group than in the control group when compared with their own initial volumes (2.7-fold vs. 1.3-fold).

Conclusions

Our results indicated that ABCC5 expression was associated with pemetrexed resistance in vitro and in vivo, and it may serve as a target or biomarker for the optimization of pemetrexed regimen in breast cancer treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-01842-x.

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.

Cellular Mechanisms Accounting for the Refractoriness of Colorectal Carcinoma to Pharmacological Treatment

Simple Summary

Colorectal cancer (CRC) causes a high number (more than 800,000) of deaths worldwide each year. Better methods for early diagnosis and the development of strategies to enhance the efficacy of the therapeutic approaches used to complement or substitute surgical removal of the tumor are urgently needed. Currently available pharmacological armamentarium provides very moderate benefits to patients due to the high resistance of tumor cells to respond to anticancer drugs. The present review summarizes and classifies into seven groups the cellular and molecular mechanisms of chemoresistance (MOC) accounting for the failure of CRC response to the pharmacological treatment.

Abstract

The unsatisfactory response of colorectal cancer (CRC) to pharmacological treatment contributes to the substantial global health burden caused by this disease. Over the last few decades, CRC has become the cause of more than 800,000 deaths per year. The reason is a combination of two factors: (i) the late cancer detection, which is being partially solved by the implementation of mass screening of adults over age 50, permitting earlier diagnosis and treatment; (ii) the inadequate response of advanced unresectable tumors (i.e., stages III and IV) to pharmacological therapy. The latter is due to the existence of complex mechanisms of chemoresistance (MOCs) that interact and synergize with each other, rendering CRC cells strongly refractory to the available pharmacological regimens based on conventional chemotherapy, such as pyrimidine analogs (5-fluorouracil, capecitabine, trifluridine, and tipiracil), oxaliplatin, and irinotecan, as well as drugs targeted toward tyrosine kinase receptors (regorafenib, aflibercept, bevacizumab, cetuximab, panitumumab, and ramucirumab), and, more recently, immune checkpoint inhibitors (nivolumab, ipilimumab, and pembrolizumab). In the present review, we have inventoried the genes involved in the lack of CRC response to pharmacological treatment, classifying them into seven groups (from MOC-1 to MOC-7) according to functional criteria to identify cancer cell weaknesses. This classification will be useful to pave the way for developing sensitizing tools consisting of (i) new agents to be co-administered with the active drug; (ii) pharmacological approaches, such as drug encapsulation (e.g., into labeled liposomes or exosomes); (iii) gene therapy interventions aimed at restoring the impaired function of some proteins (e.g., uptake transporters and tumor suppressors) or abolishing that of others (such as export pumps and oncogenes).

Breast cancer: insights in disease and influence of drug methotrexate

According to the World Health Organization, cancer is one of the leading causes of morbidity and mortality worldwide. The previously estimated 14 million new cases in the year of 2012 are expected to rise, yearly, over the following 2 decades. Among women, breast cancer is the most common one. In 2012, almost 1.7 million people were diagnosed worldwide and half a million died from the disease. Despite having several treatments available, from surgery to chemotherapy, most of these treatments have severe adverse effects. Chemotherapy has a narrow therapeutic window and requires high dosage treatment in patients with advanced-stage cancers and further need innovative treatment strategies. Although methotrexate (MTX) is not a first line drug used against breast cancer, however, it might be valuable to fight the disease. MTX is an effective and cheap drug that might impair malignant growth without irreversible damage to normal tissues. Nevertheless, while MTX does present some disadvantages including poor solubility and low permeability, several strategies are being used to discover and provide novel and effective targeted treatment against breast cancer. In this review, we analyze the chemotherapy of breast cancer and its relationship with drug MTX.

Transporters in the Mammary Gland-Contribution to Presence of Nutrients and Drugs into Milk

A large number of nutrients and bioactive ingredients found in milk play an important role in the nourishment of breast-fed infants and dairy consumers. Some of these ingredients include physiologically relevant compounds such as vitamins, peptides, neuroactive compounds and hormones. Conversely, milk may contain substances-drugs, pesticides, carcinogens, environmental pollutants-which have undesirable effects on health. The transfer of these compounds into milk is unavoidably linked to the function of transport proteins. Expression of transporters belonging to the ATP-binding cassette (ABC-) and Solute Carrier (SLC-) superfamilies varies with the lactation stages of the mammary gland. In particular, Organic Anion Transporting Polypeptides 1A2 (OATP1A2) and 2B1 (OATP2B1), Organic Cation Transporter 1 (OCT1), Novel Organic Cation Transporter 1 (OCTN1), Concentrative Nucleoside Transporters 1, 2 and 3 (CNT1, CNT2 and CNT3), Peptide Transporter 2 (PEPT2), Sodium-dependent Vitamin C Transporter 2 (SVCT2), Multidrug Resistance-associated Protein 5 (ABCC5) and Breast Cancer Resistance Protein (ABCG2) are highly induced during lactation. This review will focus on these transporters overexpressed during lactation and their role in the transfer of products into the milk, including both beneficial and harmful compounds. Furthermore, additional factors, such as regulation, polymorphisms or drug-drug interactions will be described.

Abcc5 Knockout Mice Have Lower Fat Mass and Increased Levels of Circulating GLP-1

OBJECTIVE

A previous genome-wide association study linked overexpression of an ATP-binding cassette transporter, ABCC5, in humans with a susceptibility to developing type 2 diabetes with age. Specifically, ABCC5 gene overexpression was shown to be strongly associated with increased visceral fat mass and reduced peripheral insulin sensitivity. Currently, the role of ABCC5 in diabetes and obesity is unknown. This study reports the metabolic phenotyping of a global Abcc5 knockout mouse.

METHODS

A global Abcc5^-/- mouse was generated by CRISPR/Cas9. Fat mass was determined by weekly EchoMRI and fat pads were dissected and weighed at week 18. Glucose homeostasis was ascertained by an oral glucose tolerance test, intraperitoneal glucose tolerance test, and intraperitoneal insulin tolerance test. Energy expenditure and locomotor activity were measured using PhenoMaster cages. Glucagon-like peptide 1 (GLP-1) levels in plasma, primary gut cell cultures, and GLUTag cells were determined by enzyme-linked immunosorbent assay.

RESULTS

Abcc5^-/- mice had decreased fat mass and increased plasma levels of GLP-1, and they were more insulin sensitive and more active. Recombinant overexpression of ABCC5 protein in GLUTag cells decreased GLP-1 release.

CONCLUSIONS

ABCC5 protein expression levels are inversely related to fat mass and appear to play a role in the regulation of GLP-1 secretion from enteroendocrine cells.

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.

What "The Cancer Genome Atlas" database tells us about the role of ATP-binding cassette (ABC) proteins in chemoresistance to anticancer drugs

Introduction: Chemotherapy remains the only option for advanced cancer patients when other alternatives are not feasible. Nevertheless, the success rate of this type of therapy is often low due to intrinsic or acquired mechanisms of chemoresistance. Among them, drug extrusion from cancer cells through ATP-binding cassette (ABC) proteins plays an important role. ABC pumps are primary active transporters involved in the barrier and secretory functions of many healthy cells. Areas covered: In this review, we have used The Cancer Genome Atlas (TCGA) database to explore the relationship between the expression of the major ABC proteins involved in cancer chemoresistance in the most common types of cancer, and the drugs used in the treatment of these tumors that are substrates of these pumps. Expert opinion: From unicellular organisms to humans, several ABC proteins play a major role in detoxification processes. Cancer cells exploit this ability to protect themselves from cytostatic drugs. Among the ABC pumps, MDR1, MRPs and BCRP are able to export many antitumor drugs and are expressed in several types of cancer, and further up-regulated during treatment. This event results in the enhanced ability of tumor cells to reduce intracellular drug concentrations and hence the pharmacological effect of chemotherapy.

Resistin induces multidrug resistance in myeloma by inhibiting cell death and upregulating ABC transporter expression

Despite advances in therapy, multiple myeloma remains incurable, with a high frequency of relapse. This suggests the need to identify additional factors that contribute to drug resistance. Our previous studies revealed that bone marrow adipocytes promote resistance to chemotherapy in myeloma through adipocyte-secreted adipokines, but the mechanism underlying this effect and the specific adipokines involved are not well understood. We proposed to determine the role of resistin, an adipokine that is secreted by adipocytes, in chemotherapy resistance in myeloma. We found that resistin abrogated chemotherapy-induced apoptosis in established myeloma cell lines and primary myeloma samples. Resistin inhibited chemotherapy-induced caspase cleavage through the NF-κB and PI3K/Akt pathways. Resistin also increased the expression and drug efflux function of ATP-binding cassette (ABC) transporters in myeloma cells through decreasing the expression of both DNA methyltransferases DNMT1 and DNMT3a and the methylation levels of ABC gene promoters. In vivo studies further demonstrated the protective effect of resistin in chemotherapy-induced apoptosis. Our study thus reveals a new biological function of resistin in the pathogenesis of myeloma, with the implication that targeting resistin could be a potential strategy to prevent or overcome multidrug resistance in myeloma.

The cardioprotective effects of L-carnitine on rat cardiac injury, apoptosis, and oxidative stress caused by amethopterin

Amethopterin is used as a chemotherapeutic agent, and its antioxidant activity is used to treat many cancer types. This study aimed to study the ameliorating effect of L-carnitine against amethopterin-induced cardiac injury and oxidative stress in male rats. Sixty male albino rats were equally divided into six groups; the first and second groups were the control and L-carnitine groups, respectively, while the third group was treated with amethopterin rat group; the fourth and fifth groups were co-treated and post-treated with amethopterin rat with L-carnitine, respectively, and the sixth group was self-treated with amethopterin rat group. Cholesterol, triglycerides, low-density lipoprotein (LDL), glutathione, and total protein levels in amethopterin group showed a significant decrease when compared with control group, while high-density lipoprotein (HDL), glutamic oxaloacetic transaminase (GOT), malondialdehyde (MDA), catalase, and nitric oxide (NO) levels in amethopterin group showed a significant increase when compared with control group. Cholesterol, triglycerides, LDL, GOT, MDA, and catalase levels in the self-treated group showed a significant increase when compared with amethopterin group, while glutathione, total protein, and NO levels in the self-treated group showed significant decrease when compared with amethopterin group. Many of abnormalities as moderate hydrophobic changes of myofibrillar structure with striations, myocardial atrophy, cytoplasmic vacuoles, edema, and leukocyte infiltration were detected in cardiac tissues in amethopterin rat group. A significant increase of the apoptotic protein p53 and CD68 immunoreactivity, despite a significant decrease in the antiapoptotic Bcl-2 proteins after amethopterin injection when compared with control group, was observed. Treatment (co and post) with L-carnitine improved the biochemical, histopathological, and immunohistochemical alterations in the heart treated with amethopterin.

Genetic Polymorphisms Associated to Folate Transport as Predictors of Increased Risk for Acute Lymphoblastic Leukemia in Mexican Children

Acute lymphoblastic leukemia (ALL) is a frequent neoplasia occurring in children. The most commonly used drug for the treatment of ALL is methotrexate (MTX), an anti-folate agent. Previous studies suggest that folate transporters play a role in ALL prognosis and that genetic polymorphism of genes encoding folate transporters may increase the risk of ALL. Therefore, the main goal of this study was to determine the associations among six genetic polymorphisms in four genes related with the folate transporter pathway to determine a relationship with the occurrence of ALL in Mexican children. A case-control study was performed in 73 ALL children and 133 healthy children from Northern and Northwestern Mexico. COL18A1 (rs2274808), SLC19A1 (rs2838956), ABCB1 (rs1045642 and rs1128503), and ABCC5 (rs9838667 and rs3792585). Polymorphisms were assayed through qPCR. Our results showed an increased ALL risk in children carrying CT genotype (OR = 2.55, CI 95% 1.11–5.83, p = 0.0001) and TT genotype (OR = 21.05, CI 95% 5.62–78.87, p < 0.0001) of COL18A1 rs2274808; in SLC19A1 rs2838956 AG carriers (OR = 44.69, CI 95% 10.42–191.63, p = 0.0001); in ABCB1 rs1045642 TT carriers (OR = 13.76, CI 95% 5.94–31.88, p = 0.0001); in ABCC5 rs9838667 AC carriers (OR = 2.61, CI 95% 1.05–6.48, p < 0.05); and in ABCC5 rs3792585 CC carriers (OR = 9.99, CI 95% 3.19–31.28, p = 0.004). Moreover, several combinations of genetic polymorphisms were found to be significantly associated with a risk for ALL. Finally, two combinations of ABCC5 polymorphisms resulted in protection from this neoplasia. In conclusion, certain genetic polymorphisms related to the folate transport pathway, particularly COL18A1 rs2274808, SLC19A1 rs2838956, ABCB1 rs1045642, and ABCC5 rs3792585, were associated with an increased risk for ALL in Mexican children.

Folylpoly-γ-glutamate synthetase: A key determinant of folate homeostasis and antifolate resistance in cancer

Mammalians are devoid of autonomous biosynthesis of folates and hence must obtain them from the diet. Reduced folate cofactors are B9-vitamins which play a key role as donors of one-carbon units in the biosynthesis of purine nucleotides, thymidylate and amino acids as well as in a multitude of methylation reactions including DNA, RNA, histone and non-histone proteins, phospholipids, as well as intermediate metabolites. The products of these S-adenosylmethionine (SAM)-dependent methylations are involved in the regulation of key biological processes including transcription, translation and intracellular signaling. Folate-dependent one-carbon metabolism occurs in several subcellular compartments including the cytoplasm, mitochondria, and nucleus. Since folates are essential for DNA replication, intracellular folate cofactors play a central role in cancer biology and inflammatory autoimmune disorders. In this respect, various folate-dependent enzymes catalyzing nucleotide biosynthesis have been targeted by specific folate antagonists known as antifolates. Currently, antifolates are used in drug treatment of multiple human cancers, non-malignant chronic inflammatory disorders as well as bacterial and parasitic infections. An obligatory key component of intracellular folate retention and intracellular homeostasis is (anti)folate polyglutamylation, mediated by the unique enzyme folylpoly-γ-glutamate synthetase (FPGS), which resides in both the cytoplasm and mitochondria. Consistently, knockout of the FPGS gene in mice results in embryonic lethality. FPGS catalyzes the addition of a long polyglutamate chain to folates and antifolates, hence rendering them polyanions which are efficiently retained in the cell and are now bound with enhanced affinity by various folate-dependent enzymes. The current review highlights the crucial role that FPGS plays in maintenance of folate homeostasis under physiological conditions and delineates the plethora of the molecular mechanisms underlying loss of FPGS function and consequent antifolate resistance in cancer.

Association of ABCB1, ABCC5 and xanthine oxidase genetic polymorphisms with methotrexate adverse reactions in Mexican pediatric patients with ALL

BACKGROUND

Acute lymphoblastic leukemia (ALL) is one of the most frequent oncological disorders in pediatric populations. To date, the drug of choice for the treatment of ALL is methotrexate, a drug associated with a high risk of adverse reactions (ADRs). The xanthine oxidase (XO) polymorphisms, 1936A>G and 2107A>G, as well as the polymorphic variants derived from ATP-binding cassette transporter gene subfamilies, ABCB1 and ABCC5, of drug resistant codifying genes, are implicated as precursors of drug-related neurologic, hepatic, and renal toxicities. Our aim was to determine whether the mentioned polymorphisms are risk or protective factors for the development of adverse reactions by methotrexate in our pediatric population with ALL.

METHODS

A total of 35 Mexican children from Centro Estatal de Cancerología-Durango, Mexico, with ALL and the previously noted polymorphisms as determined qPCR were studied. At the same time, a 12-month drug monitoring program was conducted in accordance with WHO-PAHO guidelines for pharmacovigilance.

RESULTS

The ABCB11936A>G and 2107A>G and ABCC5 3414+434A>C polymorphisms were not associated with methotrexate ADRs. Single nucleotide polymorphisms (SNPs) of ABCB1 1236C>T (OR 0.19, 95% CI: 0.03-0.9, p<0.05) and ABCC5 3933+313T>C (OR 0.12, 95% CI: 0.027-0.58, p<0.05) were associated with methotrexate ADRs.

CONCLUSIONS

SNPs 1236C>T of ABCB1 and ABCC5 3933+313T>C are not associated with the development of typical ADRs by methotrexate, rather, they showed a protective factor for myelosuppression in the studied sick population.

Heterogeneous response of different tumor cell lines to methotrexate-coupled nanoparticles in presence of hyperthermia

Today, the therapeutic efficacy of cancer is restricted by the heterogeneity of the response of tumor cells to chemotherapeutic drugs. Since those therapies are also associated with severe side effects in nontarget organs, the application of drugs in combination with nanocarriers for targeted therapy has been suggested. Here, we sought to assess whether the coupling of methotrexate (MTX) to magnetic nanoparticles (MNP) could serve as a valuable tool to circumvent the heterogeneity of tumor cell response to MTX by the combined treatment with hyperthermia. To this end, we investigated five breast cancer cell lines of different origin and with different mutational statuses, as well as a bladder cancer cell line in terms of their response to exposure to MTX as a free drug or after its coupling to MNP as well as in presence/absence of hyperthermia. We also assessed whether the effects could be connected to the cell line-specific expression of proteins related to the uptake and efflux of MTX and MNP. Our results revealed a very heterogeneous and cell line-dependent response to an exposure with MTX-coupled MNP (MTX–MNP), which was almost comparable to the efficacy of free MTX in the same cell line. Moreover, a cell line-specific and preferential uptake of MTX–MNP compared with MNP alone was found (probably by receptor-mediated endocytosis), agreeing with the observed cytotoxic effects. Opposed to this, the expression pattern of several cell membrane transport proteins noted for MTX uptake and efflux was only by tendency in agreement with the cellular toxicity of MTX–MNP in different cell lines. Higher cytotoxic effects were achieved by exposing cells to a combination of MTX–MNP and hyperthermal treatment, compared with MTX or thermo-therapy alone. However, the heterogeneity in the response of the tumor cell lines to MTX could not be completely abolished – even after its combination with MNP and/or hyperthermia – and the application of higher thermal dosages might be necessary.

ATP-binding cassette transporters in reproduction: a new frontier

BACKGROUND

The transmembrane ATP-binding cassette (ABC) transporters actively efflux an array of clinically relevant compounds across biological barriers, and modulate biodistribution of many physiological and pharmacological factors. To date, over 48 ABC transporters have been identified and shown to be directly and indirectly involved in peri-implantation events and fetal/placental development. They efflux cholesterol, steroid hormones, vitamins, cytokines, chemokines, prostaglandins, diverse xenobiotics and environmental toxins, playing a critical role in regulating drug disposition, immunological responses and lipid trafficking, as well as preventing fetal accumulation of drugs and environmental toxins.

METHODS

This review examines ABC transporters as important mediators of placental barrier functions and key reproductive processes. Expression, localization and function of all identified ABC transporters were systematically reviewed using PubMed and Google Scholar websites to identify relevant studies examining ABC transporters in reproductive tissues in physiological and pathophysiological states. Only reports written in English were incorporated with no restriction on year of publication. While a major focus has been placed on the human, extensive evidence from animal studies is utilized to describe current understanding of the regulation and function of ABC transporters relevant to human reproduction.

RESULTS

ABC transporters are modulators of steroidogenesis, fertilization, implantation, nutrient transport and immunological responses, and function as 'gatekeepers' at various barrier sites (i.e. blood-testes barrier and placenta) against potentially harmful xenobiotic factors, including drugs and environmental toxins. These roles appear to be species dependent and change as a function of gestation and development. The best-described ABC transporters in reproductive tissues (primarily in the placenta) are the multidrug transporters p-glycoprotein and breast cancer-related protein, the multidrug resistance proteins 1 through 5 and the cholesterol transporters ABCA1 and ABCG1.

CONCLUSIONS

The ABC transporters have various roles across multiple reproductive tissues. Knowledge of efflux direction, tissue distribution, substrate specificity and regulation of the ABC transporters in the placenta and other reproductive tissues is rapidly expanding. This will allow better understanding of the disposition of specific substrates within reproductive tissues, and facilitate development of novel treatments for reproductive disorders as well as improved approaches to protecting the developing fetus.

ATP-binding Cassette Subfamily C Member 5 (ABCC5) Functions as an Efflux Transporter of Glutamate Conjugates and Analogs

The ubiquitous efflux transporter ABCC5 (ATP-binding cassette subfamily C member 5) is present at high levels in the blood-brain barrier, neurons, and glia, but its in vivo substrates and function are not known. Using untargeted metabolomic screens, we show that Abcc5(-/-) mice accumulate endogenous glutamate conjugates in several tissues, but brain in particular. The abundant neurotransmitter N-acetylaspartylglutamate was 2.4-fold higher in Abcc5(-/-) brain. The metabolites that accumulated in Abcc5(-/-) tissues were depleted in cultured cells that overexpressed human ABCC5. In a vesicular membrane transport assay, ABCC5 also transported exogenous glutamate analogs, like the classic excitotoxic neurotoxins kainic acid, domoic acid, and NMDA; the therapeutic glutamate analog ZJ43; and, as previously shown, the anti-cancer drug methotrexate. Glutamate conjugates and analogs are of physiological relevance because they can affect the function of glutamate, the principal excitatory neurotransmitter in the brain. After CO2 asphyxiation, several immediate early genes were expressed at lower levels in Abcc5(-/-) brains than in wild type brains, suggesting altered glutamate signaling. Our results show that ABCC5 is a general glutamate conjugate and analog transporter that affects the disposition of endogenous metabolites, toxins, and drugs.

The role of ABC transporters in ovarian cancer progression and chemoresistance

Over 80% of ovarian cancer patients develop chemoresistance which results in a lethal course of the disease. A well-established cause of chemoresistance involves the family of ATP-binding cassette transporters, or ABC transporters that transport a wide range of substrates including metabolic products, nutrients, lipids, and drugs across extra- and intra-cellular membranes. Expressions of various ABC transporters, shown to reduce the intracellular accumulation of chemotherapy drugs, are increased following chemotherapy and impact on ovarian cancer survival. Although clinical trials to date using ABC transporter inhibitors have been disappointing, ABC transporter inhibition remains an attractive potential adjuvant to chemotherapy. A greater understanding of their physiological functions and role in ovarian cancer chemoresistance will be important for the development of more effective targeted therapies. This article will review the role of the ABC transporter family in ovarian cancer progression and chemoresistance as well as the clinical attempts used to date to reverse chemoresistance.

N-lactoyl-amino acids are ubiquitous metabolites that originate from CNDP2-mediated reverse proteolysis of lactate and amino acids

Despite technological advances in metabolomics, large parts of the human metabolome are still unexplored. In an untargeted metabolomics screen aiming to identify substrates of the orphan transporter ATP-binding cassette subfamily C member 5 (ABCC5), we identified a class of mammalian metabolites, N-lactoyl-amino acids. Using parallel protein fractionation in conjunction with shotgun proteomics on fractions containing N-lactoyl-Phe-forming activity, we unexpectedly found that a protease, cytosolic nonspecific dipeptidase 2 (CNDP2), catalyzes their formation. N-lactoyl-amino acids are ubiquitous pseudodipeptides of lactic acid and amino acids that are rapidly formed by reverse proteolysis, a process previously considered to be negligible in vivo. The plasma levels of these metabolites strongly correlate with plasma levels of lactate and amino acid, as shown by increased levels after physical exercise and in patients with phenylketonuria who suffer from elevated Phe levels. Our approach to identify unknown metabolites and their biosynthesis has general applicability in the further exploration of the human metabolome.

Multidrug Resistance Proteins (MRPs) and Cancer Therapy

The ATP-binding cassette (ABC) transporters are members of a protein superfamily that are known to translocate various substrates across membranes, including metabolic products, lipids and sterols, and xenobiotic drugs. Multidrug resistance proteins (MRPs) belong to the subfamily C in the ABC transporter superfamily. MRPs have been implicated in mediating multidrug resistance by actively extruding chemotherapeutic substrates. Moreover, some MRPs are known to be essential in physiological excretory or regulatory pathways. The importance of MRPs in cancer therapy is also implied by their clinical insights. Modulating the function of MRPs to re-sensitize chemotherapeutic agents in cancer therapy shows great promise in cancer therapy; thus, multiple MRP inhibitors have been developed recently. This review article summarizes the structure, distribution, and physiological as well as pharmacological function of MRP1-MRP9 in cancer chemotherapy. Several novel modulators targeting MRPs in cancer therapy are also discussed.

γ-Glutamyl hydrolase modulation significantly influences global and gene-specific DNA methylation and gene expression in human colon and breast cancer cells

γ-Glutamyl hydrolase (GGH) plays an important role in folate homeostasis by catalyzing hydrolysis of polyglutamylated folate into monoglutamates. Polyglutamylated folates are better substrates for several enzymes involved in the generation of S-adenosylmethionine, the primary methyl group donor, and hence, GGH modulation may affect DNA methylation. DNA methylation is an important epigenetic determinant in gene expression, in the maintenance of DNA integrity and stability, and in chromatin modifications, and aberrant or dysregulation of DNA methylation has been mechanistically linked to the development of human diseases including cancer. Using a recently developed in vitro model of GGH modulation in HCT116 colon and MDA-MB-435 breast cancer cells, we investigated whether GGH modulation would affect global and gene-specific DNA methylation and whether these alterations were associated with significant gene expression changes. In both cell lines, GGH overexpression decreased global DNA methylation and DNA methyltransferase (DNMT) activity, while GGH inhibition increased global DNA methylation and DNMT activity. Epigenomic and gene expression analyses revealed that GGH modulation influenced CpG promoter DNA methylation and gene expression involved in important biological pathways including cell cycle, cellular development, and cellular growth and proliferation. Some of the observed altered gene expression appeared to be regulated by changes in CpG promoter DNA methylation. Our data suggest that the GGH modulation-induced changes in total intracellular folate concentrations and content of long-chain folylpolyglutamates are associated with functionally significant DNA methylation alterations in several important biological pathways.

Overexpression of MRP4 (ABCC4) and MRP5 (ABCC5) confer resistance to the nucleoside analogs cytarabine and troxacitabine, but not gemcitabine

We aimed to determine whether the multidrug-resistance-proteins MRP4 (ABCC4) and MRP5 (ABCC5) confer resistance to the antimetabolites cytarabine (Ara-C), gemcitabine (GEM), and the L-nucleoside analog troxacitabine. For this purpose we used HEK293 and the transfected HEK/MRP4 (59-fold increased MRP4) or HEK/MRP5i (991-fold increased MRP5) as model systems and tested the cells for drug sensitivity using a proliferation test. Drug accumulation was performed by using radioactive Ara-C, and for GEM and troxacitabine with HPLC with tandem-MS or UV detection. At 4-hr exposure HEK/MRP4 cells were 2-4-fold resistant to troxacitabine, ara-C and 9-(2-phosphonylmethoxyethyl)adenine (PMEA), and HEK/MRP5i to ara-C and PMEA, but none to GEM. The inhibitors probenecid and indomethacin reversed resistance. After 4-hr exposure ara-C-nucleotides were 2-3-fold lower in MRP4/5 cells, in which they decreased more rapidly after washing with drug-free medium (DFM). Trocacitabine accumulation was similar in the 3 cell lines, but after the DFM period troxacitabine decreased 2-4-fold faster in MRP4/5 cells. Troxacitabine-nucleotides were about 25% lower in MRP4/5 cells and decreased rapidly in MRP4, but not in MRP5 cells. Accumulation of GEM-nucleotides was higher in the MRP4/5 cells. In conclusion: MRP4 and MRP5 overexpression confer resistance to troxacitabine and ara-C, but not to GEM, which was associated with a rapid decline of the ara-C and troxacitabine-nucleotides in HEK/MRP4-5 cells.

The intestinal absorption of folates

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

P53, Bcl-2 and CD68 expression in response to amethopterin-induced lung injury and ameliorating role of L-carnitine

Amethopterin (methotrexate, MTX) is an antimetabolite and antifolate drug with antiflammatory properities and is used to treat autoimmune diseases, such as psoriasis, rheumatoid arthritis and certain types of cancer, such as breast, lymphoma and lung. The present study aimed to study the changes in P53, Bcl-2 and CD68 expression in response to amethopterin-induced lung injury and ameliorating the role of l-carnitine. A total of 36 male albino rats were equally divided into six groups: the first and second groups were the control and l-carnitine groups respectively while the 3rd group was amethopterin rat group; the 4th and 5th groups were co- and post-treated amethopterin rat with l-carnitine respectively and the 6th group was self treated amethopterin rat group. Our results shows that lung in amethopterin-treated rats showed many of histopathological alterations as severe to strong alveolar damage in the form of collapsed alveoli and strong thickened interalveolar septa with heavy infiltration of inflammatory cells. This damage was increased or remaining in self-amethopterin-treated group. Treatment (co- and post) with l-carnitine were improved in the lung structure that was treated with amethopterin. A significant increase in p53 and CD68 and decrease in Bc1-2 immunoreactivity in the lung in amethopterin group is observed when compared with the control group. However, treatment of rats with l-carnitine decreased the intensity of P53-ir and CD68-ir and increased the intensity of Bcl-2 in lung when compared with amethopterin rat group. Co-treatment with l-carnitine improved lung damage induced with amethopterin.

Molecular diagnosis of ATP-binding cassette transporter-related diseases

ATP-binding cassette (ABC) transporters are involved in a variety of physiologic processes such as xenobiotic defense, lipid metabolism, ion homeostasis and immune functions. A large number of ABC proteins have been causatively linked to rare and common human genetic diseases including familial high-density lipoprotein deficiency, retinopathies, cystic fibrosis, diabetes and cardiomyopathies. Furthermore, genetic variations in ABC transporter genes and dysregulated expression patterns of these molecules significantly contribute to drug resistance in human cancer cells and alter the pharmacokinetic properties of a variety of drugs. In order to analyze DNA sequence alterations or define disease-associated mRNA expression patterns of the complete ABC transporter superfamily, novel high-throughput molecular methods such as quantitative real-time PCR and DNA microarray analysis are emerging. The aim of this review is to provide an overview and to present some examples of human ABC transporters involved in monogenic diseases, cancer and pharmacogenetics. Methodologic aspects of molecular diagnostics applied to analyze genetic variations, mRNA and protein expression levels and functional characteristics of ABC transporters are discussed.

Molecular basis for pharmacokinetics and pharmacodynamics of methotrexate in rheumatoid arthritis therapy

  Methotrexate (MTX) is a derivative of folic acid (folate) and commonly used as an anchor drug for the treatment of rheumatoid arthritis (RA). The pharmacokinetics (PK) and pharmacodynamics (PD) of MTX entirely depends on the function of specific transporters that belong to the two major superfamilies, solute carrier transporters and ATP-binding cassette transporters. Several transporters have been identified as being able to mediate the transport of MTX, and suggested to be involved in the disposition in the body and in the regulation of intracellular metabolism in target cells, together with several enzymes involved in folate metabolism. Thus, drug-drug interactions through the transporters and their genetic polymorphisms may alter the PK and PD of MTX, resulting in an interpatient variability of efficacy. This review summarizes the PK and PD of MTX, particularly in relation to RA therapy and focuses on the roles of transporters involved in PK and PD with the aim of facilitating an understanding of the molecular basis of the mechanism of MTX action to achieve its effective use in RA therapy.

Identification of transporters associated with Etoposide sensitivity of stomach cancer cell lines and methotrexate sensitivity of breast cancer cell lines by quantitative targeted absolute proteomics

Membrane transporter proteins may influence the sensitivity of cancer cells to anticancer drugs that can be recognized as substrates. The purpose of this study was to identify proteins that play a key role in the drug sensitivity of stomach and breast cancer cell lines by measuring the absolute protein expression levels of multiple transporters and other membrane proteins and examining their correlation to drug sensitivity. Absolute protein expression levels of 90 membrane proteins were examined by quantitative targeted absolute proteomics using liquid chromatography-linked tandem mass spectrometry. Among them, 11 and 14 membrane proteins, including transporters, were present in quantifiable amounts in membrane fraction of stomach cancer and breast cancer cell lines, respectively. In stomach cancer cell lines, the protein expression level of multidrug resistance-associated protein 1 (MRP1) was inversely correlated with etoposide sensitivity. MK571, an MRP inhibitor, increased both the cell-to-medium ratio of etoposide and the etoposide sensitivity of MRP1-expressing stomach cancer cell lines. In breast cancer cell lines, the protein expression level of reduced folate carrier 1 (RFC1) was directly correlated with methotrexate (MTX) sensitivity. Initial uptake rate and steady-state cell-to-medium ratio of [(3)H]MTX were correlated with both RFC1 expression level and MTX sensitivity. These results suggest that MRP1 modulates the etoposide sensitivity of stomach cancer cell lines and RFC1 modulates the MTX sensitivity of breast cancer cell lines. Our results indicate that absolute quantification of multiple membrane proteins could be a useful strategy for identification of candidate proteins involved in drug sensitivity.

Involvement of Multiple Transporters-mediated Transports in Mizoribine and Methotrexate Pharmacokinetics

Mizoribine is administered orally and excreted into urine without being metabolized. Many research groups have reported a linear relationship between the dose and peak serum concentration, between the dose and AUC, and between AUC and cumulative urinary excretion of mizoribine. In contrast, a significant interindividual variability, with a small intraindividual variability, in oral bioavailability of mizoribine is also reported. The interindividual variability is mostly considered to be due to the polymophisms of transporter genes. Methotrexate (MTX) is administered orally and/or by parenteral routes, depending on the dose. Metabolic enzymes and multiple transporters are involved in the pharmacokinetics of MTX. The oral bioavailability of MTX exhibits a marked interindividual variability and saturation with increase in the dose of MTX, with a small intraindividual variability, where the contribution of gene polymophisms of transporters and enzymes is suggested. Therapeutic drug monitoring of both mizoribine and MTX is expected to improve their clinical efficacy in the treatment of rheumatoid arthritis.

Effect of 5-Fu on the ratio of SP cells and expression of HIF-2α and ABCG2 in human gastric cancer cell line SGC7901 under hypoxia

AIM: To investigate the mechanism of resistance to 5-fluorouracil (5-Fu) chemotherapy in gastric cancer cells under hypoxia.

METHODS: The proliferative activity of SGC7901 cells was determined by MTT assay, and the half maximal inhibitory concentration (IC₅₀) of 5-Fu under normoxia and hypoxia was calculated. After 5-Fu at a concentration of IC₅₀ was incubated with cells for 24, 48 or 72 h under hypoxia, the percentage of SP cells was detected by Hoechst 33342 staining, the expression of HIF-2α protein was detected by immunocytochemistry, and the expression of ABCG2 was detected by fluorescence immunocytochemistry.

RESULTS: 5-Fu inhibited the proliferation of SGC7901 cells in a dose- and time-dependent manner under normoxia and hypoxia. The IC₅₀ of 5-Fu under normoxia and hypoxia was 100 mg/L and 200 mg/L, respectively. The ratio of SP cells in SGC7901 cells was 1.87% under nomoxia, and significantly increased after induction by hypoxia for 24, 48, and 72 h. Treatment with 5-Fu had no significant impact on the proportion of SP cells under normoxia, but gradually increased SP cell proportion under hypoxia. The expression levels of HIF-2α and ABCG2 proteins in SGC7901 cells were lower under normoxia. Treatment with 5-Fu had no significant impact on the expression of HIF-2α and ABCG2 proteins under normoxia, but gradually increased their expression under hypoxia.

CONCLUSION: The mechanism underlying the resistance of SGC7901 cells to 5-Fu under hypoxia may be related to inducing HIF-2α and ABCG2 expression and promoting stemness.

Intestinal transporters for endogenic and pharmaceutical organic anions: the challenges of deriving in-vitro kinetic parameters for the prediction of clinically relevant drug-drug interactions

OBJECTIVES

This review provides an overview of intestinal human transporters for organic anions and stresses the need for standardization of the various in-vitro methods presently employed in drug-drug interaction (DDI) investigations.

KEY FINDINGS

Current knowledge on the intestinal expression of the apical sodium-dependent bile acid transporter (ASBT), the breast cancer resistance protein (BCRP), the monocarboxylate transporters (MCT) 1, MCT3-5, the multidrug resistance associated proteins (MRP) 1-6, the organic anion transporting polypetides (OATP) 2B1, 1A2, 3A1 and 4A1, and the organic solute transporter α/β (OSTα/β) has been covered along with an overview of their substrates and inhibitors. Furthermore, the many challenges in predicting clinically relevant DDIs from in-vitro studies have been discussed with focus on intestinal transporters and the various methods for deducting in-vitro parameters for transporters (K(m) /K(i) /IC50, efflux ratio). The applicability of using a cut-off value (estimated based on the intestinal drug concentration divided by the K(i) or IC50) has also been considered.

SUMMARY

A re-evaluation of the current approaches for the prediction of DDIs is necessary when considering the involvement of other transporters than P-glycoprotein. Moreover, the interplay between various processes that a drug is subject to in-vivo such as translocation by several transporters and dissolution should be considered.

Mammalian multidrug-resistance proteins (MRPs)

Subfamily C of the human ABC (ATP-binding cassette) superfamily contains nine proteins that are often referred to as the MRPs (multidrug-resistance proteins). The 'short' MRP/ABCC transporters (MRP4, MRP5, MRP8 and ABCC12) have a typical ABC structure with four domains comprising two membrane-spanning domains (MSD1 and MSD2) each followed by a nucleotide-binding domain (NBD1 and NBD2). The 'long' MRP/ABCCs (MRP1, MRP2, MRP3, ABCC6 and MRP7) have five domains with the extra domain, MSD0, at the N-terminus. The proteins encoded by the ABCC6 and ABCC12 genes are not known to transport drugs and are therefore referred to as ABCC6 and ABCC12 (rather than MRP6 and MRP9) respectively. A large number of molecules are transported across the plasma membrane by the MRPs. Many are organic anions derived from exogenous sources such as conjugated drug metabolites. Others are endogenous metabolites such as the cysteinyl leukotrienes and prostaglandins which have important signalling functions in the cell. Some MRPs share a degree of overlap in substrate specificity (at least in vitro), but differences in transport kinetics are often substantial. In some cases, the in vivo substrates for some MRPs have been discovered aided by studies in gene-knockout mice. However, the molecules that are transported in vivo by others, including MRP5, MRP7, ABCC6 and ABCC12, still remain unknown. Important differences in the tissue distribution of the MRPs and their membrane localization (apical in contrast with basolateral) in polarized cells also exist. Together, these differences are responsible for the unique pharmacological and physiological functions of each of the nine ABCC transporters known as the MRPs.

Mechanisms of membrane transport of folates into cells and across epithelia

Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

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.