The MRP family of drug efflux pumps

Genome-wide identification, characterization and phylogenetic analysis of 50 catfish ATP-binding cassette (ABC) transporter genes

Background

Although a large set of full-length transcripts was recently assembled in catfish, annotation of large gene families, especially those with duplications, is still a great challenge. Most often, complexities in annotation cause mis-identification and thereby much confusion in the scientific literature. As such, detailed phylogenetic analysis and/or orthology analysis are required for annotation of genes involved in gene families. The ATP-binding cassette (ABC) transporter gene superfamily is a large gene family that encodes membrane proteins that transport a diverse set of substrates across membranes, playing important roles in protecting organisms from diverse environment.

Methodology/Principal Findings

In this work, we identified a set of 50 ABC transporters in catfish genome. Phylogenetic analysis allowed their identification and annotation into seven subfamilies, including 9 ABCA genes, 12 ABCB genes, 12 ABCC genes, 5 ABCD genes, 2 ABCE genes, 4 ABCF genes and 6 ABCG genes. Most ABC transporters are conserved among vertebrates, though cases of recent gene duplications and gene losses do exist. Gene duplications in catfish were found for ABCA1, ABCB3, ABCB6, ABCC5, ABCD3, ABCE1, ABCF2 and ABCG2.

Conclusion/Significance

The whole set of catfish ABC transporters provide the essential genomic resources for future biochemical, toxicological and physiological studies of ABC drug efflux transporters. The establishment of orthologies should allow functional inferences with the information from model species, though the function of lineage-specific genes can be distinct because of specific living environment with different selection pressure.

PD173074, a selective FGFR inhibitor, reverses ABCB1-mediated drug resistance in cancer cells

PURPOSE

Specific tyrosine kinase inhibitors were recently reported to modulate the activity of ABC transporters, leading to an increase in the intracellular concentration of their substrate drugs. In this study, we determine whether PD173074, a specific fibroblast growth factor receptor (FGFR) inhibitor, could reverse ABC transporter-mediated multidrug resistance.

METHODS

3-(4,5-Dimethylthiazol-yl)-2,5-diphenyllapatinibrazolium bromide assay was used to determine the effect of PD173074 on reversal of ABC transporter-mediated multidrug resistance (MDR). In addition, [³H]-paclitaxel accumulation/efflux assay, western blotting analysis, ATPase, and photoaffinity labeling assays were done to study the interaction of PD173074 on ABC transporters.

RESULTS

PD173074 significantly sensitized both ABCB1-transfected and drug-selected cell lines overexpressing this transporter to substrate anticancer drugs colchicine, paclitaxel, and vincristine. This effect of PD173074 is specific to ABCB1, as no significant interaction was detected with other ABC transporters such as ABCC1 and ABCG2. The observed reversal effect seems to be primarily due to the decreased active efflux of [³H]-paclitaxel in ABCB1 overexpressing cells observed in efflux assay. In addition, no significant change in the ABCB1 expression was observed when ABCB1 overexpressing cells were exposed to 5 μM PD173074 for up to 3 days, thereby further suggesting its role in modulating the function of the transporter. In addition, PD173074 stimulated the ATPase activity of ABCB1 in a concentration-dependent manner, indicating a direct interaction with the transporter. Interestingly, PD173074 did not inhibit photolabeling of ABCB1 with [¹²⁵I]-iodoarylazidoprazosin (IAAP), showing that it binds at a site different from that of IAAP in the drug-binding pocket.

CONCLUSIONS

Here, we report for the first time, PD173074, an inhibitor of the FGFR, to selectively reverse ABCB1 transporter-mediated MDR by directly blocking the efflux function of the transporter.

A burst of ABC genes in the genome of the polyphagous spider mite Tetranychus urticae

Background

The ABC (ATP-binding cassette) gene superfamily is widespread across all living species. The majority of ABC genes encode ABC transporters, which are membrane-spanning proteins capable of transferring substrates across biological membranes by hydrolyzing ATP. Although ABC transporters have often been associated with resistance to drugs and toxic compounds, within the Arthropoda ABC gene families have only been characterized in detail in several insects and a crustacean. In this study, we report a genome-wide survey and expression analysis of the ABC gene superfamily in the spider mite, Tetranychus urticae, a chelicerate ~ 450 million years diverged from other Arthropod lineages. T. urticae is a major agricultural pest, and is among of the most polyphagous arthropod herbivores known. The species resists a staggering array of toxic plant secondary metabolites, and has developed resistance to all major classes of pesticides in use for its control.

Results

We identified 103 ABC genes in the T. urticae genome, the highest number discovered in a metazoan species to date. Within the T. urticae ABC gene set, all members of the eight currently described subfamilies (A to H) were detected. A phylogenetic analysis revealed that the high number of ABC genes in T. urticae is due primarily to lineage-specific expansions of ABC genes within the ABCC, ABCG and ABCH subfamilies. In particular, the ABCC subfamily harbors the highest number of T. urticae ABC genes (39). In a comparative genomic analysis, we found clear orthologous relationships between a subset of T. urticae ABC proteins and ABC proteins in both vertebrates and invertebrates known to be involved in fundamental cellular processes. These included members of the ABCB-half transporters, and the ABCD, ABCE and ABCF families. Furthermore, one-to-one orthologues could be distinguished between T. urticae proteins and human ABCC10, ABCG5 and ABCG8, the Drosophila melanogaster sulfonylurea receptor and ecdysone-regulated transporter E23. Finally, expression profiling revealed that ABC genes in the ABCC, ABCG ABCH subfamilies were differentially expressed in multi-pesticide resistant mite strains and/or in mites transferred to challenging (toxic) host plants.

Conclusions

In this study we present the first comprehensive analysis of ABC genes in a polyphagous arthropod herbivore. We demonstrate that the broad plant host range and high levels of pesticide resistance in T. urticae are associated with lineage-specific expansions of ABC genes, many of which respond transcriptionally to xenobiotic exposure. This ABC catalogue will serve as a basis for future biochemical and toxicological studies. Obtaining functional evidence that these ABC subfamilies contribute to xenobiotic tolerance should be the priority of future research.

Which drug or drug delivery system can change clinical practice for brain tumor therapy?

The prognosis and treatment outcome for primary brain tumors have remained unchanged despite advances in anticancer drug discovery and development. In clinical trials, the majority of promising experimental agents for brain tumors have had limited impact on survival or time to recurrence. These disappointing results are partially explained by the inadequacy of effective drug delivery to the CNS. The impediments posed by the various specialized physiological barriers and active efflux mechanisms lead to drug failure because of inability to reach the desired target at a sufficient concentration. This perspective reviews the leading strategies that aim to improve drug delivery to brain tumors and their likelihood to change clinical practice. The English literature was searched for defined search items. Strategies that use systemic delivery and those that use local delivery are critically reviewed. In addition, challenges posed for drug delivery by combined treatment with anti-angiogenic therapy are outlined. To impact clinical practice and to achieve more than just a limited local control, new drugs and delivery systems must adhere to basic clinical expectations. These include, in addition to an antitumor effect, a verified favorable adverse effects profile, easy introduction into clinical practice, feasibility of repeated or continuous administration, and compatibility of the drug or delivery system with any tumor size and brain location.

Cell-penetrating Peptide-mediated therapeutic molecule delivery into the central nervous system

The blood-brain barrier (BBB), a dynamic and complex barrier formed by endothelial cells, can impede the entry of unwanted substances - pathogens and therapeutic molecules alike - into the central nervous system (CNS) from the blood circulation. Taking into account the fact that CNS-related diseases are the largest and fastest growing unmet medical concern, many potential protein- and nucleic acid-based medicines have been developed for therapeutic purposes. However, due to their poor ability to cross the BBB and the plasma membrane, the above-mentioned bio-macromolecules have limited use in treating neurological diseases. Finding effective, safe, and convenient ways to deliver therapeutic molecules into the CNS is thus urgently required. In recent decades, much effort has been expended in the development of drug delivery technologies, of which cell-penetrating peptides (CPPs) have the most promising potential. The present review covers the latest advances in CPP delivery technology, and provides an update on their use in CNS-targeted drug delivery.

Efflux pump-mediated resistance in chemotherapy

Efflux pump mechanisms perform important physiological functions such as prevention of toxin absorption from the gastrointestinal tract, elimination of bile from the hepatocytes, effective functioning of the blood-brain barrier and placental barrier, and renal excretion of drugs. They exist in all living cells, but those in the bacterial and mammalian cells are more important to the clinician and pharmacologist, as they constitute an important cause of antimicrobial drug resistance, which contributes to treatment failure, high medical bills, and increased mortality / morbidity. This review was aimed at highlighting the role of efflux pump mechanisms in microbial resistance to chemotherapeutic agents. It was also aimed to elucidate their structure and mechanisms of action so as to integrate the efflux pump mechanisms in the design and development of novel antimicrobial agents. Findings from previous studies and research on this subject assessed through Google search, Pubmed, Hinari websites, as well as standard textbooks on chemotherapy, provided the needed information in the process of this review. Efflux pump inhibitors are promising strategies for preventing and reverting efflux-mediated resistance to chemotherapeutic agents. They are usually employed as adjuncts in antimicrobial and cancer chemotherapy. Toxicity, more common with the older-generation inhibitors such as verapamil and reserpine, constitutes the greatest impediment to their clinical applications. No efflux pump inhibitor has been approved for routine clinical use, as a result of doubtful clinical efficacy and unacceptably high incidence of adverse effects, particularly inhibition of the P-450 drug metabolizing enzyme. At present, their applications are mainly restricted to epidemiological studies. Nonetheless, the search for efficacious and tolerable efflux pump inhibitors continues because of the potential benefits. There is a need to consider efflux pump substrate selectivity in the design and development of novel chemotherapeutic agents.

The exposure of cancer cells to hyperthermia, iron oxide nanoparticles, and mitomycin C influences membrane multidrug resistance protein expression levels

Purpose

The presence of multidrug resistance-associated protein (MRP) in cancer cells is known to be responsible for many therapeutic failures in current oncological treatments. Here, we show that the combination of different effectors like hyperthermia, iron oxide nanoparticles, and chemotherapeutics influences expression of MRP 1 and 3 in an adenocarcinoma cell line.

Methods

BT-474 cells were treated with magnetic nanoparticles (MNP; 1.5 to 150 μg Fe/cm²) or mitomycin C (up to 1.5 μg/cm², 24 hours) in the presence or absence of hyperthermia (43°C, 15 to 120 minutes). Moreover, cells were also sequentially exposed to these effectors (MNP, hyperthermia, and mitomycin C). After cell harvesting, mRNA was extracted and analyzed via reverse transcription polymerase chain reaction. Additionally, membrane protein was isolated and analyzed via sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting.

Results

When cells were exposed to the effectors alone or to combinations thereof, no effects on MRP 1 and 3 mRNA expression were observed. In contrast, membrane protein expression was influenced in a selective manner. The effects on MRP 3 expression were less pronounced compared with MRP 1. Treatment with mitomycin C decreased MRP expression at high concentrations and hyperthermia intensified these effects. In contrast, the presence of MNP only increased MRP 1 and 3 expression, and hyperthermia reversed these effects. When combining hyperthermia, magnetic nanoparticles, and mitomycin C, no further suppression of MRP expression was observed in comparison with the respective dual treatment modalities.

Discussion

The different MRP 1 and 3 expression levels are not associated with de novo mRNA expression, but rather with an altered translocation of MRP 1 and 3 to the cell membrane as a result of reactive oxygen species production, and with shifting of intracellular MRP storage pools, changes in membrane fluidity, etc, at the protein level. Our results could be used to develop new treatment strategies by repressing mechanisms that actively export drugs from the target cell, thereby improving the therapeutic outcome in oncology.

Functional regulation of P-glycoprotein at the blood-brain barrier in proton-coupled folate transporter (PCFT) mutant mice

Folate deficiency has been associated with many adverse clinical manifestations. The blood-brain barrier (BBB), formed by brain capillary endothelial cells, protects the brain from exposure to neurotoxicants. The function of BBB is modulated by multiple ABC transporters, particularly P-glycoprotein. A proton-coupled folate transporter (PCFT)-deficient mouse has been previously described as a model for systemic folate deficiency. Herein, we demonstrate that exposing mouse brain capillaries to the antiepileptic drug, valproic acid (VPA; 5 μM), significantly increased P-glycoprotein transport function in the wild-type animals. A ligand to the aryl hydrocarbon receptor, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), produced a similar induction of P-glycoprotein, which tightened the BBB, thereby increasing the neuroprotection. However, VPA- or TCDD-induced P-glycoprotein transport was blocked in the PCFT-nullizygous mice, indicating that multiple neuroprotective mechanisms are compromised under folate-deficient conditions. Brain capillaries from S-folinic acid (SFA; 40 mg/kg)-treated PCFT-nullizygous mice exhibited increased P-glycoprotein transport following VPA exposure. This suggests that SFA supplementation restored the normal BBB function. In addition, we show that tight-junction proteins are disintegrated in the PCFT mutant mice. Taken together, these findings strongly suggest that folate deficiency disrupts the BBB function by targeting the transporter and tight junctions, which may contribute to the development of neurological disorders.

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

Objective

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

Methods

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

Results

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

Conclusions

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

The antifolates

This article focuses on the cellular, biochemical, and molecular pharmacology of antifolates and how a basic understanding of the mechanism of action of methotrexate, its cytotoxic determinants, mechanisms of resistance, and transport into and out of cells has led to the development of a new generation of antifolates, a process that continues in the laboratory and in the clinics. New approaches to folate-based cancer chemotherapy are described based on the targeted delivery of drugs to malignant cells.

Modulation of the ATPase and transport activities of broad-acting multidrug resistance factor ABCC10 (MRP7)

The cell surface molecule ABCC10 is a broad-acting transporter of xenobiotics, including cancer drugs, such as taxanes, epothilone B, and modulators of the estrogen pathway. Abcc10(-/-) mice exhibit increased tissue sensitivity and lethality resulting from paclitaxel exposure compared with wild-type counterparts, arguing ABCC10 functions as a major determinant of taxane sensitivity in mice. To better understand the mechanistic basis of ABCC10 action, we characterized the biochemical and vectorial transport properties of this protein. Using crude membranes in an ABCC10 overexpression system, we found that the ABCC10 transport substrates estrogen estradiol-glucuronide (E(2)17βG) and leukotriene C4 (LTC(4)) significantly stimulated ABCC10 beryllium fluoride (BeFx)-sensitive ATPase activity. We also defined the E(2)17βG antagonist, tamoxifen, as a novel substrate and stimulator of ABCC10. In addition, a number of cytotoxic substrates, including docetaxel, paclitaxel, and Ara-C, increased the ABCC10 basal ATPase activity. We determined that ABCC10 localizes to the basolateral cell surface, using transepithelial well assays to establish that ABCC10-overexpressing LLC-PK1 cells exported [(3)H]-docetaxel from the apical to the basolateral side. Importantly, we found that the clinically valuable multikinase inhibitor sorafenib, and a natural alkaloid, cepharanthine, inhibited ABCC10 docetaxel transport activity. Thus, concomitant use of these agents might restore the intracellular accumulation and potency of ABCC10-exported cytotoxic drugs, such as paclitaxel. Overall, our work could seed future efforts to identify inhibitors and other physiologic substrates of ABCC10.

Nilotinib potentiates anticancer drug sensitivity in murine ABCB1-, ABCG2-, and ABCC10-multidrug resistance xenograft models

A panel of clinically used tyrosine kinase inhibitors were compared and nilotinib was found to most potently sensitize specific anticancer agents by blocking the functions of ABCB1/P-glycoprotein, ABCG2/BCRP and ABCC10/MRP7 transporters involved in multi-drug resistance. Nilotinib appreciably enhanced the antitumor response of (1) paclitaxel in the ABCB1- and novel ABCC10-xenograft models, and (2) doxorubicin in a novel ABCG2-xenograft model. With no apparent toxicity observed in the above models, nilotinib attenuated tumor growth synergistically and increased paclitaxel concentrations in ABCB1-overexpressing tumors. The beneficial actions of nilotinib warrant consideration as viable combinations in the clinic with agents that suffer from MDR-mediated insensitivity.

In vitro and in vivo prostate cancer metastasis and chemoresistance can be modulated by expression of either CD44 or CD147

CD44 and CD147 are associated with cancer metastasis and progression. Our purpose in the study was to investigate the effects of down-regulation of CD44 or CD147 on the metastatic ability of prostate cancer (CaP) cells, their docetaxel (DTX) responsiveness and potential mechanisms involved in vitro and in vivo. CD44 and CD147 were knocked down (KD) in PC-3M-luc CaP cells using short hairpin RNA (shRNA). Expression of CD44, CD147, MRP2 (multi-drug resistance protein-2) and MCT4 (monocarboxylate tranporter-4) was evaluated using immunofluorescence and Western blotting. The DTX dose-response and proliferation was measured by MTT and colony assays, respectively. The invasive potential was assessed using a matrigel chamber assay. Signal transduction proteins in PI3K/Akt and MAPK/Erk pathways were assessed by Western blotting. An in vivo subcutaneous (s.c.) xenograft model was established to assess CaP tumorigenecity, lymph node metastases and DTX response. Our results indicated that KD of CD44 or CD147 decreased MCT4 and MRP2 expression, reduced CaP proliferation and invasive potential and enhanced DTX sensitivity; and KD of CD44 or CD147 down-regulated p-Akt and p-Erk, the main signal modulators associated with cell growth and survival. In vivo, CD44 or CD147-KD PC-3M-luc xenografts displayed suppressed tumor growth with increased DTX responsiveness compared to control xenografts. Both CD44 and CD147 enhance metastatic capacity and chemoresistance of CaP cells, potentially mediated by activation of the PI3K and MAPK pathways. Selective targeting of CD44/CD147 alone or combined with DTX may limit CaP metastasis and increase chemosensitivity, with promise for future CaP treatment.

Deletion of an ATP-binding cassette protein subfamily C transporter in Leishmania donovani results in increased virulence

Genome comparison of Leishmania species sequenced to date has identified several hundred differentially distributed genes which are present as functional genes in some species but as pseudogenes or absent in others. It is not clear whether these differentially distributed genes are important for disease, redundant or even harmful for a particular Leishmania species though a few of these genes have been implicated in Leishmania infection tissue tropism. The L. infantum LinJ.24.1510 gene is an ATP-binding cassette transporter protein subfamily C member which is present in L. donovani complex species and L. mexicana but absent in L. major and L. braziliensis and its substrate is unknown. Experimental deletion of this gene from L. donovani has resulted in an increase in growth as axenic amastigotes and increased virulence in infecting mice. To our knowledge, this is the first gene identified in Leishmania where its deletion results in hypervirulence.

OSI-930 analogues as novel reversal agents for ABCG2-mediated multidrug resistance

OSI-930, a dual c-Kit and KDR tyrosine kinase inhibitor, is reported to have undergone a Phase I dose escalation study in patients with advanced solid tumors. A series of fifteen pyridyl and phenyl analogues of OSI-930 were designed and synthesized. Extensive screening of these compounds led to the discovery that nitropyridyl and ortho-nitrophenyl analogues, VKJP1 and VKJP3, were effective in reversing ABC subfamily G member 2 (ABCG2) transporter-mediated multidrug resistance (MDR). VKJP1 and VKJP3 significantly sensitized ABCG2-expressing cells to established substrates of ABCG2 including mitoxantrone, SN-38, and doxorubicin in a concentration-dependent manner, but not to the non-ABCG2 substrate cisplatin. However, they were unable to reverse ABCB1- or ABCC1-mediated MDR indicating their selectivity for ABCG2. Western blotting analysis was performed to evaluate ABCG2 expression and it was found that neither VKJP1 nor VKJP3 significantly altered ABCG2 protein expression for up to 72 h. [(3)H]-mitoxantrone accumulation study demonstrated that VKJP1 and VKJP3 increased the intracellular accumulation of [(3)H]-mitoxantrone, a substrate of ABCG2. VKJP1 and VKJP3 also remarkably inhibited the transport of [(3)H]-methotrexate by ABCG2 membrane vesicles. Importantly, both VKJP1 and VKJP3 were efficacious in stimulating the activity of ATPase of ABCG2 and inhibited the photoaffinity labeling of this transporter by its substrate [(125)I]-iodoarylazidoprazosin. The results suggested that VKJP1 and VKJP3, specifically inhibit the function of ABCG2 through direct interaction with its substrate binding site(s). Thus VKJP1 and VKJP3 represent a new class of drugs for reducing MDR in ABCG2 over-expressing tumors.

Amonafide: a future in treatment of resistant and secondary acute myeloid leukemia?

Development of the novel topoisomerase II inhibitor, amonafide, began almost 40 years ago. The drug was selected for further investigation owing to evidence of marked antineoplastic efficacy in preclinical models of cancer. When its usefulness in the treatment of various solid malignancies proved limited, focus was shifted to establishing its use as an antileukemic agent, specifically against secondary and treatment-associated acute myeloid leukemia (AML). While Phase I and II studies gave rise to hopes that amonafide might hold the key to treating older patients, including those with multidrug resistant, cytogenetically unfavorable secondary and treatment-associated AML, when used in combination with cytarabine, it failed to demonstrate a survival advantage over standard-of-care therapy in randomized studies. This article will outline the development of amonafide from the laboratory to the bedside and discuss the potential place that this agent has in the current management of AML.

The MUC4 mucin mediates gemcitabine resistance of human pancreatic cancer cells via the Concentrative Nucleoside Transporter family

The fluorinated analog of deoxycytidine, Gemcitabine (Gemzar), is the main chemotherapeutic drug in pancreatic cancer, but survival remains weak mainly because of the high resistance of tumors to the drug. Recent works have shown that the mucin MUC4 may confer an advantage to pancreatic tumor cells by modifying their susceptibility to drugs. However, the cellular mechanism(s) responsible for this MUC4-mediated resistance is unknown. The aim of this work was to identify the cellular mechanisms responsible for gemcitabine resistance linked to MUC4 expression. CAPAN-2 and CAPAN-1 adenocarcinomatous pancreatic cancer (PC) cell lines were used to establish stable MUC4-deficient clones (MUC4-KD) by shRNA interference. Measurement of the IC50 index using tetrazolium salt test indicated that MUC4-deficient cells were more sensitive to gemcitabine. This was correlated with increased Bax/BclXL ratio and apoptotic cell number. Expression of Equilibrative/Concentrative Nucleoside Transporter (hENT1, hCNT1/3), deoxycytidine kinase (dCK), ribonucleotide reductase (RRM1/2) and Multidrug-Resistance Protein (MRP3/4/5) was evaluated by quantitative RT-PCR (qRT-PCR) and western blotting. Alteration of MRP3, MRP4, hCNT1 and hCNT3 expression was observed in MUC4-KD cells, but only hCNT1 alteration was correlated to MUC4 expression and sensitivity to gemcitabine. Decreased activation of MAPK, JNK and NF-κB pathways was observed in MUC4-deficient cells, in which the NF-κB pathway was found to have an important role in both sensitivity to gemcitabine and hCNT1 regulation. Finally, and in accordance with our in vitro data, we found that MUC4 expression was conversely correlated to that of hCNT1 in tissues from patients with pancreatic adenocarcinoma. This work describes a new mechanism of PC cell resistance to gemcitabine, in which the MUC4 mucin negatively regulates the hCNT1 transporter expression via the NF-κB pathway. Altogether, these data point out to MUC4 and hCNT1 as potential targets to ameliorate the response of pancreatic tumors to gemcitabine treatment.

Severe diabetes and leptin resistance cause differential hepatic and renal transporter expression in mice

Background

Type-2 Diabetes is a major health concern in the United States and other Westernized countries, with prevalence increasing yearly. There is a need to better model and predict adverse drug reactions, drug-induced liver injury, and drug efficacy in this population. Because transporters significantly contribute to drug clearance and disposition, it is highly significant to determine whether a severe diabetes phenotype alters drug transporter expression, and whether diabetic mouse models have altered disposition of acetaminophen (APAP) metabolites.

Results

Transporter mRNA and protein expression were quantified in livers and kidneys of adult C57BKS and db/db mice, which have a severe diabetes phenotype due to a lack of a functional leptin receptor. The urinary excretion of acetaminophen-glucuronide, a substrate for multidrug resistance-associated proteins transporters was also determined. The mRNA expression of major uptake transporters, such as organic anion transporting polypeptide Slco1a1 in liver and kidney, 1a4 in liver, and Slc22a7 in kidney was decreased in db/db mice. In contrast, Abcc3 and 4 mRNA and protein expression was more than 2 fold higher in db/db male mouse livers as compared to C57BKS controls. Urine levels of APAP-glucuronide, -sulfate, and N-acetyl cysteine metabolites were higher in db/db mice.

Conclusion

A severe diabetes phenotype/presentation significantly altered drug transporter expression in liver and kidney, which corresponded with urinary APAP metabolite levels.

ABCC1 polymorphisms in anthracycline-induced cardiotoxicity in childhood acute lymphoblastic leukaemia

Anthracyclines are potent cytostatic drugs, the correct dosage being critical to avoid possible cardiac side effects. ABCC1 [ATP-binding cassette, sub-family C, member 1; also denoted as MRP1 (multidrug resistance-associated protein 1)] is expressed in the heart and takes part in the detoxification and protection of cells from the toxic effects of xenobiotics, including anthracyclines. Our objective was to search for associations between LV (left ventricular) function and single-nucleotide polymorphisms of the ABCC1 gene in children receiving anthracycline chemotherapy. Data of 235 paediatric patients with acute lymphoblastic leukaemia was analysed. Patients were followed-up by echocardiography (median follow-up 6.3 years). Nine polymorphisms in the ABCC1 gene were genotyped. The ABCC1 rs3743527TT genotype and rs3743527TT–rs246221TC/TT genotype combination were associated with lower LVFS (left ventricular fractional shortening) after chemotherapy. The results suggest that genetic variants in the ABCC1 gene influence anthracycline-induced LV dysfunction.

Taxane resistance in breast cancer: mechanisms, predictive biomarkers and circumvention strategies

BACKGROUND

Taxanes are established in the treatment of metastatic breast cancer (MBC) and early breast cancer (EBC) as potent chemotherapy agents. However, their therapeutic usefulness is limited by de-novo refractoriness or acquired resistance, which are common drawbacks to most anti-cancer cytotoxics. Considering that the taxanes will remain principle chemotherapeutic agents for the treatment of breast cancer, we reviewed known mechanisms of resistance in with an outlook of optimizing their clinical use.

METHODS

We searched the PubMed and MEDLINE databases for articles (from inception through to 9th January 2012; last search 10/01/2012) and journals known to publish information relevant to taxane chemotherapy. We imposed no language restrictions. Search terms included: cancer, breast cancer, response, resistance, taxane, paclitaxel, docetaxel, taxol. Due to the possibility of alternative mechanisms of resistance all combination chemotherapy treated data sets were removed from our overview.

RESULTS

Over-expression of the MDR-1 gene product Pgp was extensively studied in vitro in association with taxane resistance, but data are conflicting. Similarly, the target components microtubules, which are thought to mediate refractoriness through alterations of the expression pattern of tubulins or microtubule associated proteins and the expression of alternative tubulin isoforms, failed to confirm such associations. Little consensus has been generated for reported associations between taxane-sensitivity and mutated p53, or taxane-resistance and overexpression of Bcl-2, Bcl-xL or NFkB. In contrary sufficient in vitro data support an association of spindle assembly checkpoint (SAC) defects with resistance. Clinical data have been limited and inconsistent, which relate to the variety of methods used, lack of standardization of cut-offs for quantitation, differences in clinical endpoints measured and in methods of tissue collection preparation and storage, and study/patient heterogeneity. The most prominent finding is that pharmaceutical down-regulation of HER-2 appears to reverse the taxane resistance.

CONCLUSIONS

Currently no valid practical biomarkers exist that can predict resistance to the taxanes in breast cancer supporting the principle of individualized cancer therapy. The incorporation of several biomarker analyses into prospectively designed studies in this setting are needed.

ABCC1 polymorphism Arg723Gln (2168G> A) is associated with lung cancer susceptibility in a Chinese population

1. In a previous in vitro study, we showed that the Arg723Gln (2168G > A) polymorphism significantly ABCC1-induced multidrug resistance. The aim of the present study was to further investigate the association of this polymorphism with lung cancer susceptibility and chemotherapy response in a Chinese population. 2. A total of 77 lung cancer patients (54 men, 23 women) and 71 cancer-free controls (49 men, 22 women) were enrolled in the study. Genomic DNA was extracted from peripheral blood and all samples were genotyped using polymerase chain reaction-restriction fragment length polymorphism. 3. Individuals carrying the 723Gln (A) allele have a 3.4-fold increased risk (adjusted odds ratio (OR) 3.42; 95% confidence interval (CI) 1.29-9.06; P = 0.013) of lung cancer compared with wild-type individuals. Further stratified analysis indicated that older individuals (> 50 years) carrying the 723Gln (A) allele have the highest susceptibility to lung cancer (adjusted OR 4.10; 95% CI 1.25-13.48; P = 0.020). However, no substantial association was found between the Arg723Gln (2168G > A) polymorphism and chemotherapy response in Chinese lung cancer patients. 4. In conclusion, the Arg723Gln (2168G > A) polymorphism of ABCC1 appears to be a potential susceptibility marker for lung cancer in the Chinese population, especially in older people.

GW583340 and GW2974, human EGFR and HER-2 inhibitors, reverse ABCG2- and ABCB1-mediated drug resistance

The overexpression of ATP binding cassette (ABC) transporters often leads to the development of multidrug resistance (MDR) and results in a suboptimal response to chemotherapy. Previously, we reported that lapatinib (GW572016), a human epidermal growth factor receptor (EGFR) and HER-2 tyrosine kinase inhibitor (TKI), significantly reverses MDR in cancer cells by blocking the efflux function of ABC subfamily B member 1 (ABCB1) and ABC subfamily G member 2 (ABCG2). In the present study, we conducted in vitro experiments to evaluate if GW583340 and GW2974, structural analogues of lapatinib, could reverse ABCB1- and ABCG2-mediated MDR. Our results showed that GW583340 and GW2974 significantly sensitized ABCB1 and ABCG2 overexpressing MDR cells to their anticancer substrates. GW583340 and GW2974 significantly increased the intracellular accumulation of [(3)H]-paclitaxel in ABCB1 overexpressing cells and [(3)H]-mitoxantrone in ABCG2 overexpressing cells respectively. In addition, GW583340 and GW2974 significantly inhibited ABCG2-mediated transport of methotrexate in ABCG2 overexpressing membrane vesicles. There was no significant change in the expression levels of ABCB1 and ABCG2 in the cell lines exposed to 5μM of either GW583340 or GW2974 for 3 days. In addition, a docking model predicted the binding conformation of GW583340 and GW2974 to be within the transmembrane region of homology modeled human ABCB1 and ABCG2. We conclude that GW583340 and GW2974, at clinically achievable plasma concentrations, reverse ABCB1- and ABCG2-mediated MDR by blocking the drug efflux function of these transporters. These findings may be useful in developing combination therapy for cancer treatment with EGFR TKIs.

Tyrosine kinase inhibitors as modulators of ABC transporter-mediated drug resistance

Tyrosine kinases (TKs) are involved in key signaling events/pathways that regulate cancer cell proliferation, apoptosis, angiogenesis and metastasis. Deregulated activity of TKs has been implicated in several types of cancers. In recent years, tyrosine kinase inhibitors (TKIs) have been developed to inhibit specific kinases whose constitutive activity results in specific cancer types. These TKIs have been found to demonstrate effective anticancer activity and some of them have been approved by the Food and Drug Administration for clinical use or are in clinical trials. However, these targeted therapeutic agents are also transported by ATP-binding cassette (ABC) transporters, resulting in altered pharmacokinetics or development of resistance to these drugs in cancer patients. This review covers the recent findings on the interactions of clinically important TKIs with ABC drug transporters. Future research efforts in the development of novel TKIs with specific targets, seeking improved activity, should consider these underlying causes of resistance to TKIs in cancer cells.

Farnesol-induced apoptosis in Candida albicans is mediated by Cdr1-p extrusion and depletion of intracellular glutathione

Farnesol is a key derivative in the sterol biosynthesis pathway in eukaryotic cells previously identified as a quorum sensing molecule in the human fungal pathogen Candida albicans. Recently, we demonstrated that above threshold concentrations, farnesol is capable of triggering apoptosis in C. albicans. However, the exact mechanism of farnesol cytotoxicity is not fully elucidated. Lipophilic compounds such as farnesol are known to conjugate with glutathione, an antioxidant crucial for cellular detoxification against damaging compounds. Glutathione conjugates act as substrates for ATP-dependent ABC transporters and are extruded from the cell. To that end, this current study was undertaken to validate the hypothesis that farnesol conjugation with intracellular glutathione coupled with Cdr1p-mediated extrusion of glutathione conjugates, results in total glutathione depletion, oxidative stress and ultimately fungal cell death. The combined findings demonstrated a significant decrease in intracellular glutathione levels concomitant with up-regulation of CDR1 and decreased cell viability. However, addition of exogenous reduced glutathione maintained intracellular glutathione levels and enhanced viability. In contrast, farnesol toxicity was decreased in a mutant lacking CDR1, whereas it was increased in a CDR1-overexpressing strain. Further, gene expression studies demonstrated significant up-regulation of the SOD genes, primary enzymes responsible for defense against oxidative stress, with no changes in expression in CDR1. This is the first study describing the involvement of Cdr1p-mediated glutathione efflux as a mechanism preceding the farnesol-induced apoptotic process in C. albicans. Understanding of the mechanisms underlying farnesol-cytotoxicity in C. albicans may lead to the development of this redox-cycling agent as an alternative antifungal agent.

Establishment and characterization of two 5-fluorouracil-resistant hepatocellular carcinoma cell lines

5-Fluorouracil (5-FU) chemotherapy is the first choice treatment for advanced hepatocellular carcinoma (HCC), and resistance is the major obstacle to successful treatment. Recent studies have reported that epithelial-to-mesenchymal transition (EMT) is associated with chemoresistance in cancers. We speculated that EMT and 5-FU metabolism are related to the mechanism of 5-FU resistance. First, two 5-FU-resistant cell lines, HLF-R4 and HLF-R10, were established from the HLF undifferentiated human HCC cell line. Whereas cell growth was similar in the HLF and HLF-R cell lines, HLF-Rs are about 4- and 10-fold more resistant compared with the HLF cells; thus, we named these cell lines HLF-R4 and HLF-R10, respectively. The terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay also showed a dramatically decreased number of apoptotic cells in the HLF-Rs after treatment with 5-FU. We next assessed the characteristics of the HLF, HLF-R4 and HLF-R10 cells. Consistent with our hypothesis, the HLF-Rs had typical morphologic phenotypes of EMT, loss of cell-cell adhesion, spindle-shaped morphology and increased formation of pseudopodia. Real-time quantitative reverse transcriptase polymerase chain reaction data showed downregulated E-cadherin and upregulated Twist-1 and also indicated that EMT changes occurred in the HLF-Rs. We also found decreased ribonucleotide reductase and increased multidrug resistance protein 5 genes in the HLF-R cells. Our results suggested that the metabolism of EMT and 5-FU has important roles in 5-FU chemoresistance in the HLF-R cells, and that the HLF-R cells would be useful in vitro models for understanding the 5-FU-resistant mechanisms in HCC.

Multidrug resistance associated proteins in multidrug resistance

Multidrug resistance proteins (MRPs) are members of the C family of a group of proteins named ATP-binding cassette (ABC) transporters. These ABC transporters together form the largest branch of proteins within the human body. The MRP family comprises of 13 members, of which MRP1 to MRP9 are the major transporters indicated to cause multidrug resistance in tumor cells by extruding anticancer drugs out of the cell. They are mainly lipophilic anionic transporters and are reported to transport free or conjugates of glutathione (GSH), glucuronate, or sulphate. In addition, MRP1 to MRP3 can transport neutral organic drugs in free form in the presence of free GSH. Collectively, MRPs can transport drugs that differ structurally and mechanistically, including natural anticancer drugs, nucleoside analogs, antimetabolites, and tyrosine kinase inhibitors. Many of these MRPs transport physiologically important anions such as leukotriene C4, bilirubin glucuronide, and cyclic nucleotides. This review focuses mainly on the physiological functions, cellular resistance characteristics, and probable in vivo role of MRP1 to MRP9.

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.

ATP-binding cassette transporters as pharmacogenetic biomarkers for kidney transplantation

Immunosuppressive drugs used in organ transplantation are highly effective in preventing acute rejection. However, the clinical use of these drugs is complicated by the fact that they display highly variable pharmacokinetics and pharmacodynamics between individual patients. The influence of genetic variation on the interindividual variability in immunosuppressive drug disposition, efficacy, and toxicity has been explored in recent years. The polymorphically-expressed ATP-binding cassette (ABC) transporter proteins, in particular ABCB1 and ABCC2, have been investigated extensively because they play an important role in the absorption, distribution and elimination of many immunosuppressive drugs in use today. From these studies it can be concluded that polymorphisms in ABCB1 and ABCC2 have no consistent effect on immunosuppressant pharmacokinetics and toxicity although polymorphisms in ABCB1 appear to be related to the risk of developing calcineurin inhibitor-related nephrotoxicity. However, the latter needs to be replicated before an individual's ABCB1 genotype can become a useful marker that is applied in clinical practice. Future studies evaluating the influence of ABC transporter gene polymorphisms should explore the relationship with intracellular rather than systemic drug concentrations further in well-designed clinical studies. Until then, single-nucleotide polymorphisms in ABC transporter genes are not suitable to act as biomarkers for solid organ transplantation.

Contribution of Abcc10 (Mrp7) to in vivo paclitaxel resistance as assessed in Abcc10(-/-) mice

Recently, we reported that the ATP-binding cassette transporter 10 (ABCC10), also known as multidrug resistance protein 7 (MRP7), is able to confer resistance to a variety of anticancer agents, including taxanes. However, the in vivo functions of the pump have not been determined to any extent. In this study, we generated and analyzed Abcc10(-/-) mice to investigate the ability of Abcc10 to function as an endogenous resistance factor. Mouse embryo fibroblasts derived from Abcc10(-/-) mice were hypersensitive to docetaxel, paclitaxel, vincristine, and cytarabine (Ara-C) and exhibited increased cellular drug accumulation, relative to wild-type controls. Abcc10(-/-) null mice treated with paclitaxel exhibited increased lethality associated with neutropenia and marked bone marrow toxicity. In addition, toxicity in spleen and thymus was evident. These findings indicate that Abcc10 is dispensable for health and viability and that it is an endogenous resistance factor for taxanes, other natural product agents, and nucleoside analogues. This is the first demonstration that an ATP-binding cassette transporter other than P-glycoprotein can affect in vivo tissue sensitivity toward taxanes.

Overcoming drug resistance in pancreatic cancer

INTRODUCTION

Pancreatic cancer has the worst survival rate of all cancers. The current standard care for metastatic pancreatic cancer is gemcitabine, however, the success of this treatment is poor and overall survival has not improved for decades. Drug resistance (both intrinsic and acquired) is thought to be a major reason for the limited benefit of most pancreatic cancer therapies.

AREAS COVERED

Previous studies have indicated various mechanisms of drug resistance in pancreatic cancer, including changes in individual genes or signaling pathways, the influence of the tumor microenvironment, and the presence of highly resistant stem cells. This review summarizes recent advances in the mechanisms of drug resistance in pancreatic cancer and potential strategies to overcome this.

EXPERT OPINION

Increasing drug delivery efficiency and decreasing drug resistance is the current aim in pancreatic cancer treatment, and will also benefit the treatment of other cancers. Understanding the molecular and cellular basis of drug resistance in pancreatic cancer will lead to the development of novel therapeutic strategies with the potential to sensitize pancreatic cancer to chemotherapy, and to increase the efficacy of current treatments in a wide variety of human cancers.

COX-2 inhibitors block chemotherapeutic agent-induced apoptosis prior to commitment in hematopoietic cancer cells

Enzymatic inhibitors of pro-inflammatory cyclooxygenase-2 (COX-2) possess multiple anti-cancer effects, including chemosensitization. These effects are not always linked to the inhibition of the COX-2 enzyme. Here we analyze the effects of three COX-2 enzyme inhibitors (nimesulide, NS-398 and celecoxib) on apoptosis in different hematopoietic cancer models. Surprisingly, COX-2 inhibitors strongly prevent apoptosis induced by a panel of chemotherapeutic agents. We selected U937 cells as a model of sensitive cells for further studies. Here, we provide evidence that the protective effect is COX-independent. No suppression of the low basal prostaglandin (PG)E(2) production may be observed upon treatment by COX-2 inhibitors. Besides, the non-active celecoxib analog 2,5-dimethyl-celecoxib is able to protect from apoptosis as well. We demonstrate early prevention of the stress-induced apoptotic signaling, prior to Bax/Bak activation. This preventive effect fits with an impairment of the ability of chemotherapeutic agents to trigger apoptogenic stress. Accordingly, etoposide-induced DNA damage is strongly attenuated in the presence of COX-2 inhibitors. In contrast, COX-2 inhibitors do not exert any anti-apoptotic activity when cells are challenged with physiological stimuli (anti-Fas, TNFα or Trail) or with hydrogen peroxide, which do not require internalization and/or are not targeted by chemoresistance proteins. Altogether, our findings show a differential off-target anti-apoptotic effect of COX-2 inhibitors on intrinsic vs. extrinsic apoptosis at the very early steps of intracellular signaling, prior to commitment. The results imply that an exacerbation of the chemoresistance phenomena may be implicated.

Alteration of hepatic but not renal transporter expression in diet-induced obese mice

Drug pharmacokinetics can be altered in obese and diabetic subjects. In consideration of the prevalence of obesity and diabetes, characterization of transporter expression in mouse models of diabetes and obesity may be a useful tool to aid in prediction of altered drug pharmacokinetics or adverse drug reactions. It has been reported that ob/ob mice, which display a severe obesity and diabetes phenotype, exhibit multiple changes in drug transporter expression in liver and kidney. In the present study, the mRNA and protein expression of major drug transporters was determined in livers and kidneys of diet-induced obese (DIO) C57BL/6J male mice. The mice were fed a high-fat diet (HFD) (60% fat) from 6 weeks of age and display obesity, fatty liver, and mild hyperglycemia. The HFD diet increased expression of multidrug resistance-associated proteins Abcc3 and 4 mRNA and protein in liver by 3.4- and 1.4-fold, respectively, compared with that detected in control mice fed a low-fat diet (LFD). In contrast, Abcc1 mRNA and protein decreased by 50% in livers of DIO mice compared with those in livers to lean mice. The HFD did not alter transporter expression in kidney compared with the LFD. In summary, unlike ob/ob and db/db mice, DIO mice exhibited a selective induction of efflux transporter expression in liver (i.e., Abcc3 and 4). In addition, diet-induced obesity affects transporter expression in liver but not kidney in the C57BL/6J mouse model. These data indicate that hepatic transporter expression is only slightly altered in a model of mild diabetes and nonalcoholic fatty liver disease and obesity.

Up-regulation of P-glycoprotein confers acquired resistance to 6-mercaptopurine in human chronic myeloid leukemia cells

To investigate the mechanisms of cellular resistance to 6-mercaptopurine (6-MP) in chronic myeloid leukemia (CML), a 6-MP resistant cell line (K562-MP5) was established by stepwise selection of the CML cell line (K562). The results of the drug sensitivity analysis of the K562-MP5 cell line revealed the cells to be 339-fold more resistant to 6-MP compared with the parental K562 cells. K562-MP5 cells exhibited decreased accumulation and increased efflux of [(14)C]6-MP and its metabolites. In addition, K562-MP5 cells showed increased [(3)H]MTX transport. K562-MP5 cells over-expressed P-glycoprotein (P-gp) and up-regulated MDR1 mRNA levels. Taken together, these results suggest that the up-regulation of P-gp, which contributes to the decreased accumulation by increasing the efflux of 6-MP and its metabolites, underlies the mechanism of 6-MP resistance in K562 cells.

Discovery of regulatory elements in human ATP-binding cassette transporters through expression quantitative trait mapping

ATP-Binding Cassette (ABC) membrane transporters determine the disposition of many drugs, metabolites and endogenous compounds. Coding region variation in ABC transporters is the cause of many genetic disorders, but much less is known about the genetic basis and functional outcome of ABC transporter expression level variation. We used genotype and mRNA transcript level data from human lymphoblastoid cell lines to assess population and gender differences in ABC transporter expression, and to guide the discovery of genomic regions involved in transcriptional regulation. Nineteen of 49 ABC genes were differentially expressed between individuals of African, Asian and European descent suggesting an important influence of race on expression level of ABC transporters. Twenty-four significant associations were found between transporter transcript levels and proximally located genetic variants. Several of the associations were experimentally validated in reporter assays. Through influencing ABC expression levels, these SNPs may affect disease susceptibility and response to drugs.

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

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

Formaldehyde stimulates Mrp1-mediated glutathione deprivation of cultured astrocytes

Formaldehyde (Fal) is an environmental neurotoxin that is also endogenously produced in brain. Since the tripeptide glutathione (GSH) plays an important role in detoxification processes in brain cells, we have investigated the consequences of a Fal exposure on the GSH metabolism of brain cells, using astrocyte-rich primary cultures as model system. Treatment of these cultures with Fal resulted in a rapid time- and concentration-dependent depletion of cellular GSH and a matching increase in the extracellular GSH content. Exposure of astrocytes to 1mm Fal for 3h did not compromise cell viability but almost completely deprived the cells of GSH. Half-maximal deprivation of cellular GSH was observed after application of 0.3mm Fal. This effect was rather specific for Fal, since methanol, formate or acetaldehyde did not affect cellular GSH levels. The Fal-stimulated GSH loss from viable astrocytes was completely prevented by semicarbazide-mediated chemical removal of Fal or by the application of MK571, an inhibitor of the multidrug resistance protein 1. These data demonstrate that Fal deprives astrocytes of cellular GSH by a multidrug resistance protein 1-mediated process.

Uptake transporters of the human OATP family: molecular characteristics, substrates, their role in drug-drug interactions, and functional consequences of polymorphisms

Organic anion transporting polypeptides (OATPs, gene family: SLC21/SLCO) mediate the uptake of a broad range of substrates including several widely prescribed drugs into cells. Drug substrates for members of the human OATP family include HMG-CoA-reductase inhibitors (statins), antibiotics, anticancer agents, and cardiac glycosides. OATPs are expressed in a variety of different tissues including brain, intestine, liver, and kidney, suggesting that these uptake transporters are important for drug absorption, distribution, and excretion. Because of their wide tissue distribution and broad substrate spectrum, altered transport kinetics, for example, due to drug-drug interactions or due to the functional consequences of genetic variations (polymorphisms), can contribute to the interindividual variability of drug effects. Therefore, the molecular characteristics of human OATP family members, the role of human OATPs in drug-drug interactions, and the in vitro analysis of the functional consequences of genetic variations in SLCO genes encoding OATP proteins are the focus of this chapter.