Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

Aromatic 2-(thio)ureidocarboxylic acids as a new family of modulators of multidrug resistance-associated protein 1: synthesis, biological evaluation, and structure-activity relationships

Four series of aromatic carboxylic acids were prepared with a urea or thiourea moiety at the neighboring position to the carboxyl group and benzene or thiophene as aromatic scaffold. Using a calcein AM assay, these compounds were evaluated as inhibitors of multidrug resistance-associated protein 1 (MRP1) and selected compounds were examined toward P-glycoprotein (P-gp) as well as breast cancer resistance protein (BCRP) to assess selectivity for MRP1. Two 2-thioureidobenzo[b]thiophene-3-carboxylic acids (48, 49) were identified as particularly potent inhibitors of MRP1, with IC50 values of around 1 microM. The structural features of this new family of nontoxic MRP1 inhibitors include a (thio)urea disubstituted with preferentially two alkyl groups at the terminal nitrogen and an additional fused aromatic ring.

Lipid dependence of ABC transporter localization and function

Lipid rafts have been implicated in many cellular functions, including protein and lipid transport and signal transduction. ATP-binding cassette (ABC) transporters have also been localized in these membrane domains. In this review the evidence for this specific localization will be evaluated and discussed in terms of relevance to ABC transporter function. We will focus on three ABC transporters of the A, B and C subfamily, respectively. Two of these transporters are relevant to multidrug resistance in tumor cells (Pgp/ABCB1 and MRP1/ABCC1), while the third (ABCA1) is extensively studied in relation to the reverse cholesterol pathway and cellular cholesterol homeostasis. We will attempt to derive a generalized model of lipid rafts to which they associate based on the use of various different lipid raft isolation procedures. In the context of lipid rafts, modulation of ABC transporter localization and function by two relevant lipid classes, i.e. sphingolipids and cholesterol, will be discussed.

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Tumor cells that are nonsensitive to anticancer drugs frequently have a multidrug resistant (MDR) phenotype. Many studies with cell lines and patient material have been done to investigate the impact of different resistance markers at protein and mRNA level in drug resistance but with contradictory outcome. In the present study, 26 well-characterised patient-derived non-small cell lung cancer xenografts were used. The known chemosensitivity to etoposide, carboplatin, gemcitabine, paclitaxel and erlotinib was compared to the protein and mRNA expression of BCRP, LRP, MDR1, and MRP1. Further, four of these xenografts were short-term treated to analyse possible regulation mechanisms after therapeutic interventions. We found a borderline correlation between the bcrp mRNA expression and the response of xenografts to etoposide. All other constitutive mRNA and protein expression levels were not correlated to any drug response and were not significantly influenced by a short term treatment. The present results indicate that the expression levels of MDR proteins and mRNA investigated do not play an important role in the chemoresistance of NSCLC in the in vivo situation.

The effect of probenecid and MK-571 on the feto-maternal transfer of saquinavir in dually perfused human term placenta

Human placenta, particularly the blood-placenta barrier, with various transporters has crucial role to protect the fetus and, on the other hand, to facilitate movement of compounds towards the fetal circulation. This study aimed to characterize the role of basal transporters of the syncytiotrophoblast, which appear to be yet less studied, in the fetal-to-maternal transfer of saquinavir by use of dually perfused human placentas. A dual perfusion of human placenta was performed to study effect of MK-571 and probenecid, inhibitors of the MRP1 and OATP transporters, expressed in the basal trophoblast membrane, on the transfer of saquinavir. The fetal-to-maternal placental transfer of saquinavir in the control group as measured by TPT(AUC)% (absolute fraction of the dose crossing placenta) was 14.0%, which is 73% less than the transfer of the freely diffusible antipyrine. The two inhibitors, MK-571 and probenecid caused a non-significant (P = 0.34 for ANOVA) reduction of 43% and 24%, respectively, in the mean amount of saquinavir transferred from the fetal to the maternal side. MK-571 also somewhat (by 31%) reduced the TPT(AUC)% of antipyrine, but this finding did not reach statistical significance (P = 0.25). Neither of the employed inhibitors had an effect on the placental transfer index of saquinavir transfer (P = 0.77). The present results indicated lack of significant effect by MK-571 and probenecid on the fetal-to-maternal transfer of saquinavir and suggest that MRP1 and, possibly, OATP2B1 do not play a significant role in the fetal-to-maternal transfer of saquinavir.

Identification of novel specific and general inhibitors of the three major human ATP-binding cassette transporters P-gp, BCRP and MRP2 among registered drugs

PURPOSE

To study the inhibition patterns of the three major human ABC transporters P-gp (ABCB1), BCRP (ABCG2) and MRP2 (ABCC2), using a dataset of 122 structurally diverse drugs.

METHODS

Inhibition was investigated in cellular and vesicular systems over-expressing single transporters. Computational models discriminating either single or general inhibitors from non-inhibitors were developed using multivariate statistics.

RESULTS

Specific (n = 23) and overlapping (n = 19) inhibitors of the three ABC transporters were identified. GF120918 and Ko143 were verified to specifically inhibit P-gp/BCRP and BCRP in defined concentration intervals, whereas the MRP inhibitor MK571 was revealed to inhibit all three transporters within one log unit of concentration. Virtual docking experiments showed that MK571 binds to the ATP catalytic site, which could contribute to its multi-specific inhibition profile. A computational model predicting general ABC inhibition correctly classified 80% of both ABC transporter inhibitors and non-inhibitors in an external test set.

CONCLUSIONS

The inhibitor specificities of P-gp, BCRP and MRP2 were shown to be highly overlapping. General ABC inhibitors were more lipophilic and aromatic than specific inhibitors and non-inhibitors. The identified specific inhibitors can be used to delineate transport processes in complex experimental systems, whereas the multi-specific inhibitors are useful in primary ABC transporter screening in drug discovery settings.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

Vandetanib (Zactima, ZD6474) antagonizes ABCC1- and ABCG2-mediated multidrug resistance by inhibition of their transport function

BACKGROUND

ABCC1 and ABCG2 are ubiquitous ATP-binding cassette transmembrane proteins that play an important role in multidrug resistance (MDR). In this study, we evaluated the possible interaction of vandetanib, an orally administered drug inhibiting multiple receptor tyrosine kinases, with ABCC1 and ABCG2 in vitro.

METHODOLOGY AND PRINCIPAL FINDINGS

MDR cancer cells overexpressing ABCC1 or ABCG2 and their sensitive parental cell lines were used. MTT assay showed that vandetanib had moderate and almost equal-potent anti-proliferative activity in both sensitive parental and MDR cancer cells. Concomitant treatment of MDR cells with vandetanib and specific inhibitors of ABCC1 or ABCG2 did not alter their sensitivity to the former drug. On the other hand, clinically attainable but non-toxic doses of vandetanib were found to significantly enhance the sensitivity of MDR cancer cells to ABCC1 or ABCG2 substrate antitumor drugs. Flow cytometric analysis showed that vandetanib treatment significantly increase the intracellular accumulation of doxorubicin and rhodamine 123, substrates of ABCC1 and ABCG2 respectively, in a dose-dependent manner (P<0.05). However, no significant effect was shown in sensitive parental cell lines. Reverse transcription-PCR and Western blot analysis showed that vandetanib did not change the expression of ABCC1 and ABCG2 at both mRNA and protein levels. Furthermore, total and phosphorylated forms of AKT and ERK1/2 remained unchanged after vandetanib treatment in both sensitive and MDR cancer cells.

CONCLUSIONS

Vandetanib is unlikely to be a substrate of ABCC1 or ABCG2. It overcomes ABCC1- and ABCG2-mediated drug resistance by inhibiting the transporter activity, independent of the blockade of AKT and ERK1/2 signal transduction pathways.

ABCG2: a perspective

ABCG2, or breast cancer resistance protein (BCRP), is an ABC transporter that has been the subject of intense study since its discovery a decade ago. With high normal tissue expression in the brain endothelium, gastrointestinal tract, and placenta, ABCG2 is believed to be important in the protection from xenobiotics, regulating oral bioavailability, forming part of the blood-brain barrier, the blood-testis barrier, and the maternal-fetal barrier. Notably, ABCG2 is often expressed in stem cell populations, where it likely plays a role in xenobiotic protection. However, clues to its epigenetic regulation in various cell populations are only beginning to emerge. While ABCG2 overexpression has been demonstrated in cancer cells after in vitro drug treatment, endogenous ABCG2 expression in certain cancers is likely a reflection of the differentiated phenotype of the cell of origin and likely contributes to intrinsic drug resistance. Notably, research into the transporter's role in cancer drug resistance and its development as a therapeutic target in cancer has lagged. Substrates and inhibitors of the transporter have been described, among them chemotherapy drugs, tyrosine kinase inhibitors, antivirals, HMG-CoA reductase inhibitors, carcinogens, and flavonoids. This broad range of substrates complements the efficiency of ABCG2 as a transporter in laboratory studies and suggests that, while there are redundant mechanisms of xenobiotic protection, the protein is important in normal physiology. Indeed, emerging studies in pharmacology and toxicology assessing polymorphic variants in man, in combination with murine knockout models have confirmed its dynamic role. Work in pharmacology may eventually lead us to a greater understanding of the physiologic role of ABCG2.

Xenobiotic transporters of the human organic anion transporting polypeptides (OATP) family

1. The organic anion transporting polypeptides (humans OATP; other species Oatp) belong to the SLCO gene superfamily of transporters and are twelve transmembrane domain glycoproteins expressed in various epithelial cells. Some OATPs/Oatps are expressed in a single organ, while others are expressed ubiquitously. 2. The functionally characterized members mediate sodium-independent transport of a variety of structurally independent, mainly amphipathic organic compounds, including bile salts, hormones and their conjugates, toxins, and various drugs. 3. This review summarizes the general features and the substrates of the eleven human OATPs. Furthermore, it reviews what is known about the mechanism of their multispecificity, their predicted structure, their role in drug-food interactions, and their role in cancer. 4. Finally, some open questions are raised that need to be addressed to advance OATP research in the near future.

Prediction and identification of drug interactions with the human ATP-binding cassette transporter multidrug-resistance associated protein 2 (MRP2; ABCC2)

The chemical space of registered oral drugs was explored for inhibitors of the human multidrug-resistance associated protein 2 (MRP2; ABCC2), using a data set of 191 structurally diverse drugs and drug-like compounds. The data set included a new reference set of 75 compounds, for studies of hepatic drug interactions with transport proteins, CYP enzymes, and compounds associated with liver toxicity. The inhibition of MRP2-mediated transport of estradiol-17beta-D-glucuronide was studied in inverted membrane vesicles from Sf9 cells overexpressing human MRP2. A total of 27 previously unknown MRP2 inhibitors were identified, and the results indicate an overlapping but narrower inhibitor space for MRP2 compared with the two other major ABC efflux transporters P-gp (ABCB1) and BCRP (ABCG2). In addition, 13 compounds were shown to stimulate the transport of estradiol-17beta-D-glucuronide. The experimental results were used to develop a computational model able to discriminate inhibitors from noninhibitors according to their molecular structure, resulting in a predictive power of 86% for the training set and 72% for the test set. The inhibitors were in general larger and more lipophilic and presented a higher aromaticity than the noninhibitors. The developed computational model is applicable in an early stage of the drug discovery process and is proposed as a tool for prediction of MRP2-mediated hepatic drug interactions and toxicity.

ABC multidrug transporters: structure, function and role in chemoresistance

Three ATP-binding cassette (ABC)-superfamily multidrug efflux pumps are known to be responsible for chemoresistance; P-glycoprotein (ABCB1), MRP1 (ABCC1) and ABCG2 (BCRP). These transporters play an important role in normal physiology by protecting tissues from toxic xenobiotics and endogenous metabolites. Hydrophobic amphipathic compounds, including many clinically used drugs, interact with the substrate-binding pocket of these proteins via flexible hydrophobic and H-bonding interactions. These efflux pumps are expressed in many human tumors, where they likely contribute to resistance to chemotherapy treatment. However, the use of efflux-pump modulators in clinical cancer treatment has proved disappointing. Single nucleotide polymorphisms in ABC drug-efflux pumps may play a role in responses to drug therapy and disease susceptibility. The effect of various genotypes and haplotypes on the expression and function of these proteins is not yet clear, and their true impact remains controversial.

6-(7-Nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol, a specific glutathione S-transferase inhibitor, overcomes the multidrug resistance (MDR)-associated protein 1-mediated MDR in small cell lung cancer

In the present work, we have investigated the antitumor activity of 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol (NBDHEX) on aggressive small cell lung cancer. NBDHEX not only is cytotoxic toward the parental small cell lung cancer H69 cell line (LC(50) of 2.3 +/- 0.6 micromol/L) but also overcomes the multidrug resistance of its variant, H69AR, which overexpresses the ATP-binding cassette transporter multidrug resistance-associated protein 1 (MRP1; LC(50) of 4.5 +/- 0.9 micromol/L). Drug efflux experiments, done in the presence of a specific inhibitor of MRP1, confirmed that NBDHEX is not a substrate for this export pump. Interestingly, NBDHEX triggers two different types of cell death: a caspase-dependent apoptosis in the H69AR cells and a necrotic phenotype in the parental H69 cells. The apoptotic pathway triggered by NBDHEX in H69AR cells is associated with c-Jun NH(2)-terminal kinase and c-Jun activation, whereas glutathione oxidation and activation of p38(MAPK) is observed in the NBDHEX-treated H69 cells. In contrast to the parental cells, the higher propensity to die through apoptosis of the H69AR cell line may be related to the lower expression of the antiapoptotic protein Bcl-2. Therefore, down-regulation of a factor crucial for cell survival makes H69AR cells more sensitive to the cytotoxic action of NBDHEX, which is not a MRP1 substrate. We have previously shown that NBDHEX is cytotoxic toward P-glycoprotein-overexpressing tumor cell lines. Therefore, NBDHEX seems a very promising compound in the search for new molecules able to overcome the ATP-binding cassette family of proteins, one of the major mechanisms of multidrug resistance in cancer cells.

Inhibition of multidrug resistance transporters in the diatom Thalassiosira rotula facilitates dye staining

Cells are protected by multidrug resistance transporters, which remove potentially harmful chemicals entering the cells from the environment or originating endogenously from the cellular metabolism. Multidrug resistance transporters have not been investigated so far in marine eukaryotic algae like diatoms. We investigated the uptake of a calcium-sensitive dye, Fura 2 acetoxymethylester (AM), by the marine diatom Thalassiosira rotula in the presence and absence of substances known to inhibit multidrug resistance transporters (ATP-binding cassette transporters, ABC). Three inhibitors known to block transporters in living organisms were tested in the marine diatom T. rotula. We applied verapamil, which blocks multidrug resistance P-glycoprotein (MDR1), probenecid as an inhibitor of organic anion transport and the specific inhibitor of multidrug resistance-associated protein (MRP), MK571, obtaining positive results with the highly specific MK571. This leads to the assumption that the cells of T. rotula possess MRP transporters. Marine diatom cells can now be loaded by incubation with a calcium-sensitive dye, which facilitates measurements of cellular calcium signals without using methods risking injury of the cell membrane. This opens an avenue for investigation on diatom calcium signalling and perhaps how they process environmental signals.

Expression of thirty-six drug transporter genes in human intestine, liver, kidney, and organotypic cell lines

This study was designed to quantitatively assess the mRNA expression of 36 important drug transporters in human jejunum, colon, liver, and kidney. Expression of these transporters in human organs was compared with expression in commonly used cell lines (Caco-2, HepG2, and Caki-1) originating from these organs to assess their value as in vitro transporter system models, and was also compared with data obtained from the literature on expression in rat tissues to assess species differences. Transporters that were highly expressed in the intestine included HPT1, PEPT1, BCRP, MRP2, and MDR1, whereas, in the liver, OCT1, MRP2, OATP-C, NTCP and BSEP were the main transporters. In the kidney, OAT1 was expressed at the highest levels, followed by OAT3, OAT4, MCT5, MDR1, MRP2, OCT2, and OCTN2. The best agreement between human tissue and the representative cell line was observed for human jejunum and Caco-2 cells. Expression in liver and kidney ortholog cell lines was not correlated with that in the associated tissue. Comparisons with rat transporter gene expression revealed significant species differences. Our results allowed a comprehensive quantitative comparison of drug transporter expression in human intestine, liver, and kidney. We suggest that it would be beneficial for predictive pharmacokinetic research to focus on the most highly expressed transporters. We hope that our comparison of rat and human tissue will help to explain the observed species differences in in vivo models, increase understanding of the impact of active transport processes on pharmacokinetics and distribution, and improve the quality of predictions from animal studies to humans.