Photoaffinity labeling under non-energized conditions of a specific drug-binding site of the ABC multidrug transporter LmrA from Lactococcus lactis

The Lactococcus lactis multidrug resistance ABC transporter protein LmrA has been shown to confer resistance to structurally and functionally diverse antibiotics and anti-cancer drugs. Using a previously characterized photoreactive drug analogue of Rhodamine 123 (iodo-aryl azido-Rhodamine 123 or IAARh123), direct and specific photoaffinity labeling of LmrA in enriched membrane vesicles could be achieved under non-energized conditions. This photoaffinity labeling of LmrA occurs at a physiologically relevant site as it was inhibited by molar excess of ethidium bromide>Rhodamine 6G>vinblastine>doxorubicin>MK571 (a quinoline-based drug) while colchicine had no effect. The MDR-reversing agents PSC 833 and cyclosporin A were similarly effective in inhibiting IAARh123 photolabeling of LmrA and P-glycoprotein. In-gel digestion with Staphyloccocus aureus V8 protease of IAARh123-photolabeled LmrA revealed several IAARh123 labeled polypeptides, in addition to a 6.8kDa polypeptide that comprises the last two transmembrane domains of LmrA.     

Molecular modeling of the human multidrug resistance protein 1 (MRP1/ABCC1)

Multidrug resistance protein 1 (MRP1/ABCC1) is a 190 kDa member of the ATP-binding cassette (ABC) superfamily of transmembrane transporters that is clinically relevant for its ability to confer multidrug resistance by actively effluxing anticancer drugs. Knowledge of the atomic structure of MRP1 is needed to elucidate its transport mechanism, but only low resolution structural data are currently available. Consequently, comparative modeling has been used to generate models of human MRP1 based on the crystal structure of the ABC transporter Sav1866 from Staphylococcus aureus. In these Sav1866-based models, the arrangement of transmembrane helices differs strikingly from earlier models of MRP1 based on the structure of the bacterial lipid transporter MsbA, both with respect to packing of the twelve helices and their interactions with the nucleotide binding domains. The functional importance of Tyr³²⁴ in transmembrane helix 6 predicted to project into the substrate translocation pathway was investigated.     

Disulfiram is a potent modulator of multidrug transporter Cdr1p of Candida albicans

To find novel drugs for effective antifungal therapy in candidiasis, we examined disulfiram, a drug used for the treatment of alcoholism, for its role as a potential modulator of Candida multidrug transporter Cdr1p. We show that disulfiram inhibits the oligomycin-sensitive ATPase activity of Cdr1p and 2.5mM dithiothreitol reverses this inhibition. Disulfiram inhibited the binding of photoaffinity analogs of both ATP ([alpha-(32)P]8-azidoATP; IC(50)=0.76 microM) and drug-substrates ([(3)H]azidopine and [(125)I]iodoarylazidoprazosin; IC(50) approximately 12 microM) to Cdr1p in a concentration-dependent manner, suggesting that it can interact with both ATP and substrate-binding site(s) of Cdr1p. Furthermore, a non-toxic concentration of disulfiram (1 microM) increased the sensitivity of Cdr1p expressing Saccharomyces cerevisiae cells to antifungal agents (fluconazole, miconazole, nystatin, and cycloheximide). Collectively these results demonstrate that disulfiram reverses Cdr1p-mediated drug resistance by interaction with both ATP and substrate-binding sites of the transporter and may be useful for antifungal therapy.     

Cyclohexyl-linked tricyclic isoxazoles are potent and selective modulators of the multidrug resistance protein (MRP1)

Structure–activity relationship (SAR) studies on the tricyclic isoxazole series of MRP1 modulators have resulted in the identification of potent and selective inhibitors containing cyclohexyl-based linkers. These studies ultimately identified compound 21b, which reverses drug resistance to MRP1 substrates, such as doxorubicin, in HeLa-T5 cells (EC₅₀ = 0.093 μM), while showing no inherent cytotoxicity. Additionally, 21b inhibits ATP-dependent, MRP1-mediated LTC₄ uptake into membrane vesicles prepared from the MRP1-overexpressing HeLa-T5 cells (EC₅₀ = 0.064 μM) and shows selectivity (1115-fold) against the related transporter, P-glycoprotein, in HL60/Adr and HL60/Vinc cells. Finally, when dosed in combination with the oncolytic MRP1 substrate vincristine, 21b showed tumor regression and growth delay in MRP1-overexpressing tumors in vivo.     

Interaction of ivermectin with multidrug resistance proteins (MRP1, 2 and 3)

Ivermectin is a potent antiparasitic drug from macrocyclic lactone (ML) family, which interacts with the ABC multidrug transporter P-glycoprotein (Pgp). We studied the interactions of ivermectin with the multidrug resistance proteins (MRPs) by combining cellular and subcellular approaches. The inhibition by ivermectin of substrate transport was measured in A549 cells (calcein or 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, BCECF) and in HL60-MRP1 (calcein). Ivermectin induced calcein and BCECF retention in A549 cells (IC(50) at 1 and 2.5microM, respectively) and inhibited calcein efflux in HL60-MRP1 (IC(50)=3.8microM). The action of ivermectin on the transporters ATPase activity was followed on membranes from Sf9 cells overexpressing human Pgp, MRP1, 2 or 3. Ivermectin inhibited the Pgp, MRP1, 2 and 3 ATPase activities after stimulation by their respective activators. Ivermectin showed a rather good affinity for MRPs, mainly MRP1, in the micromolar range, although it was lower than that for Pgp. The transport of BODIPY-ivermectin was followed in cells overexpressing selectively Pgp or MRP1. In both cell lines, inhibition of the transporter activity induced intracellular retention of BODIPY-ivermectin. Our data revealed the specific interaction of ivermectin with MRP proteins, and its transport by MRP1. Although Pgp has been considered until now as the sole active transporter for this drug, the MRPs should be taken into account for the transport of ivermectin across cell membrane, modulating its disposition in addition to Pgp. This could be of importance for optimizing clinical efficacy of ML-based antiparasitic treatments. This offers fair perspectives for the use of ivermectin or non-toxic derivatives as multidrug resistance-reversing agents.     

A new multidrug resistance protein at the blood-brain barrier

Porcine brain capillary endothelial cells (PBCEC) cultured in serum-free and hydrocortisone supplemented medium are characterised by high transendothelial electrical resistances and low cell monolayer permeabilities for small solutes very similar to the blood-brain barrier (BBB) in vivo. Differential screening of a subtracted cDNA library disclosed a 2.1-kb mRNA that is overexpressed in hydrocortisone treated PBCEC relative to untreated cells. The mRNA encodes a 656-aa member of the ATP-binding cassette (ABC) superfamily of transporters that we named brain multidrug resistance protein (BMDP). Phylogenetic analysis and multiple sequence alignment showed that porcine BMDP is most related to the human and mouse breast cancer resistance protein (BCRP). Northern blot analysis revealed that BMDP is expressed in brain tissue in vivo and was predominantly localised within the endothelial cells isolated from brain capillaries. Thus, we identified a new transport protein at the BBB that might play an important role in the exclusion of xenobiotics from the brain.     

Photoaffinity labeling of the multidrug resistance protein 2 (ABCC2/cMOAT) with a photoreactive analog of LTC4

Several studies have shown that the multidrug resistant protein MRP2 mediates the transport of chemotherapeutic drugs and normal cell metabolites, including Leukotriene C (LTC₄); however direct binding of the LTC₄ to MRP2 has not been demonstrated. In this study, a photoreactive analog of LTC₄ (IAALTC₄) was used to demonstrate its direct binding to MRP2. Our results show specific photoaffinity labeling of MRP2 with IAALTC₄ in plasma membranes from MDCKII^MRP2 cells. The photoaffinity labeling signal of MRP2 with IAALTC₄ was much lower than that of MRP1, consistent with previous studies whereby the measured K_m values of MRP1 and MRP2 for LTC₄ were 1 μM and 0.1 μM LTC₄, respectively. Competition of IAALTC₄ photoaffinity labeling to MRP2 with MK571, a well characterized inhibitor of MRP2 function, showed ~75% reduction in binding in the presence of 50 μM excess MK571. Interestingly, unmodified LTC₄ enhanced the photoaffinity labeling of IAALTC₄ to MRP2, whereas excess GSH and Quercetin had no significant effect. Mild tryptic digestion of photoaffinity labeled MRP2 revealed several photoaffinity labeled peptides that localized the IAALTC₄ binding to a 15 kDa amino acid sequence that contains transmembrane 16 and 17. Together these results provide the first demonstration of direct LTC₄ binding to MRP2.     

Kinetic analysis of rhodamines efflux mediated by the multidrug resistance protein (MRP1)

Characterization of rhodamine 123 as functional assay for MDR has been primarily focused on P-glycoprotein-mediated MDR. Several studies have suggested that Rh123 is also a substrate for MRP1. However, no quantitative studies of the MRP1-mediated efflux of rhodamines have, up to now, been performed. Measurement of the kinetic characteristics of substrate transport is a powerful approach to enhancing our understanding of their function and mechanism. In the present study, we have used a continuous fluorescence assay with four rhodamine dyes (rhodamine 6G, tetramethylrosamine, tetramethylrhodamine ethyl ester, and tetramethylrhodamine methyl ester) to quantify drug transport by MRP1 in living GLC4/ADR cells. The formation of a substrate concentration gradient was observed. MRP1-mediated transport of rhodamine was glutathione-dependent. The kinetics parameter, k(a) = V(M)/k(m), was very similar for the four rhodamine analogs but approximately 10-fold less than the values of the same parameter determined previously for the MRP1-mediated efflux of anthracycline. The findings presented here are the first to show quantitative information about the kinetics parameters for MRP1-mediated efflux of rhodamine dyes.     

Nucleotide triphosphatase activity of the N-terminal nucleotide-binding domains of the multidrug resistance proteins P-glycoprotein and MRP1

The multidrug resistance proteins P-glycoprotein (Pgp) and MRP1 are drug-efflux pumps. In this study, we compared the nucleotide triphosphatase activities of the isolated N-terminal nucleotide binding domains (NBD1) of Pgp and MRP1, and explored the potential role of the phosphorylation target domain of Pgp on the regulation of Pgp NBD1 ATPase activity. We found that: (1) the NBD1s of Pgp and MRP1 have ATPase and GTPase activities, (2) the K(m)s of Pgp NBD1 for ATP and GTP hydrolysis are identical, while the K(m) of MRP1 NBD1 for ATP is lower than that for GTP, and (3) phosphorylation of MLD by PKA or PKC produces a marginal increase of V(max) for ATP hydrolysis, without affecting the affinity for ATP. These results show efficient GTP hydrolysis by the NBD1s of Pgp and MRP1, and a minor role of phosphorylation in the control of Pgp NBD1 ATPase activity.     

Substrate recognition and transport by multidrug resistance protein 1 (ABCC1)

Multidrug resistance protein (MRP) 1 belongs to the 'C' branch of the ABC transporter superfamily. MRP1 is a high-affinity transporter of the cysteinyl leukotriene C(4) and is responsible for the systemic release of this cytokine in response to an inflammatory stimulus. However, the substrate specificity of MRP1 is extremely broad and includes many organic anion conjugates of structurally unrelated endo- and xenobiotics. In addition, MRP1 transports unmodified hydrophobic compounds, such as natural product type chemotherapeutic agents and mutagens, such as aflatoxin B(1). Transport of several of these compounds has been shown to be dependent on the presence of reduced glutathione (GSH). More recently, GSH has also been shown to stimulate the transport of some conjugated compounds, including sulfates and glucuronides. Here, we summarize current knowledge of the substrate specificity and modes of transport of MRP1 and discuss how the protein may recognize its structurally diverse substrates.     

Dibenzocyclooctadiene lignans: a class of novel inhibitors of multidrug resistance-associated protein 1

We recently reported that dibenzocyclooctadiene lignans were a novel class of P-glycoprotein (P-gp) inhibitors. In this study, we demonstrated that the lignans of this class were also effective inhibitors of multidrug resistance-associated protein 1 (MRP1). The activities of 5 dibenzocyclooctadiene lignans (schisandrin A, schisandrin B, schisantherin A, schisandrol A, and schisandrol B) to reverse MRP1-mediated drug resistance were tested using HL60/Adriamycin (ADR) and HL60/Multidrug resistance-associated protein (MRP), two human promyelocytic leukemia cell lines with overexpression of MRP1 but not P-gp. The five lignans could effectively reverse drug resistance of the two cell lines to vincristine, daunorubicin, and VP-16. This study, together with our previous reports, proves that dibenzocyclooctadiene lignans have multiple activities against cancer multidrug resistance, including inhibition of P-gp and MRP1, and enhancement of apoptosis. Considering that cancer multidrug resistance (MDR) is multifactorial, agents with broad activities are preferable to the use of combination of several specific modulators to prevent drug-drug interaction and cumulative toxicity.     

Plasmodium falciparum expresses a multidrug resistance-associated protein

Plasmodium falciparum proteins that efflux toxic metabolic products such as oxidised glutathione (GSSG) are possible targets for anti-malarial drug development. Proteins capable of transporting GSSG and glutathione conjugates include the multidrug resistance-associated transporters (MRPs). A gene, PFA0590w, encoding a MRP homologue, has been identified in P. falciparum. Here we show the presence of full-length mRNA (5.5 kb) of this PfMRP in trophozoites by RT-PCR and Northern blotting. A polyclonal anti-PfMRP antibody generated against two unique, hydrophilic peptides in the predicted sequence produced a strong immunoreactive protein band of 210-215 kDa on Western blots of schizonts of chloroquine-sensitive and chloroquine-resistant strains, confirming expression of PfMRP protein. Using confocal microscopy the protein was seen to be localised at the edge of the schizonts with no obvious staining of the food vacuole. We suggest that PfMRP may act as the GSSG transporter in the parasite plasma membrane.     

The evaluations of 99mTc cyclopentadienyl tricarbonyl triphenyl phosphonium cation for multidrug resistance

A triphenylphosphonium cation, [^99mTc]Technetium cyclopentadienyltricarbonyl-6-hexanoyl-triphenylphosphonium cation ([^99mTc]3) was prepared to target multidrug resistance (MDR). The radiotracer was evaluated in the MDR-negative MCF-7 and MDR-positive MCF-7/ADR cell lines in vitro, as well as animal models in vivo. [^99mTc]3 was proofed to be a substrate of P-glycoprotein and multidrug resistant protein 1, and showed a higher accumulation in the MDR-negative MCF-7 cells compared to ^99mTc-sestamibi in vitro. The MCF-7 tumor-to-MCF-7/ADR tumor ratio of [^99mTc]3 was ～3 at 1 h p.i. in the biodistribution study. These results demonstrated the capability of the radiotracer to detect multidrug resistance in tumor cells.     

Mutation of Glu521 or Glu535 in cytoplasmic loop 5 causes differential misfolding in multiple domains of multidrug and organic anion transporter MRP1 (ABCC1)

The polytopic 5-domain multidrug resistance protein 1 (MRP1/ABCC1) extrudes a variety of drugs and organic anions across the plasma membrane. Four charged residues in the fifth cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 are critical for stable expression of MRP1 at the plasma membrane. Thus Ala substitution of Lys(513), Lys(516), Glu(521), and Glu(535) all cause misfolding of MRP1 and target the protein for proteasome-mediated degradation. Of four chemical chaperones tested, 4-phenylbutyric acid (4-PBA) was the most effective at restoring expression of MRP1 mutants K513A, K516A, E521A, and E535A. However, although 4-PBA treatment of K513A resulted in wild-type protein levels (and activity), the same treatment had little or no effect on the expression of K516A. On the other hand, 4-PBA treatment allowed both E521A and E535A to exit the endoplasmic reticulum and be stably expressed at the plasma membrane. However, the 4-PBA-rescued E535A mutant exhibited decreased transport activity associated with reduced substrate affinity and conformational changes in both halves of the transporter. By contrast, E521A exhibited reduced transport activity associated with alterations in the mutant interactions with ATP as well as a distinct conformational change in the COOH-proximal half of MRP1. These findings illustrate the critical and complex role of CL5 for stable expression of MRP1 at the plasma membrane and more specifically show the differential importance of Glu(521) and Glu(535) in interdomain interactions required for proper folding and assembly of MRP1 into a fully transport competent native structure.     

ABC transporters influence sensitivity of Brugia malayi to moxidectin and have potential roles in drug resistance

Some ABC transporters play a significant role in human health and illness because they confer multidrug resistance (MDR) through their overexpression. Compounds that inhibit the drug efflux mechanism can improve efficacy or reverse resistance. Of the eight described ABC transporter subfamilies, those proteins conferring MDR in humans are in subfamilies A, B, C, and G. In nematodes, transporters in subfamilies B and C are suggested to confer resistance to ivermectin. The Brugia malayi ABC transporter superfamily was examined to assess their potential to influence sensitivity to moxidectin. There was an increase in expression of ABC transporters in subfamilies A, B, C, and G following treatment. Co-administration of moxidectin with inhibitors of ABC transporter function did not enhance sensitivity to moxidectin in males; however, sensitivity was significantly enhanced in females and microfilariae. The work suggests that ABC transporters influence sensitivity to moxidectin and have a potential role in drug resistance.     

Role of the membrane fusion protein in the assembly of resistance-nodulation-cell division multidrug efflux pump in Pseudomonas aeruginosa

The tripartite xenobiotic-antibiotic transporter of Pseudomonas aeruginosa consists of the inner membrane transporter (e.g., MexB, MexY), the periplasmic membrane-fusion-protein (e.g., MexA, MexX), and the outer membrane channel protein (e.g., OprM). These subunits were assumed to assemble into a transporter unit during export of the substrates. However, subunit interaction and their specificity in native form remained to be elucidated. To address these important questions, we analyzed the role of the individual subunits for the assembly of MexAB-OprM by pull-down assay tagging only one of the subunits. We found stable MexA-MexB-OprM complex without chemical cross-linking that withstand all purification procedures. Results of bi-partite interactions analysis showed tight association between MexA and OprM in the absence of MexB, whereas the expression systems lacking MexA failed to co-purify MexB or OprM. None of the heterologous subunit combinations such as MexA+MexY(his)+OprM and MexX+MexB(his)+OprM showed interaction. These results implied that the membrane fusion protein is central to the tripartite xenobiotic transporter assembly.     

The reversion effect of the RNAi-silencing mdr1 gene on multidrug resistance of the leukemia cell HT9

Overexpression of P-glycoprotein (P-gp), the mdr1 gene product, confers multidrug resistance (MDR) to tumor cells and often limits the efficacy of chemotherapy. This study evaluated RNAi for specific silencing of the mdr1 gene and reversion of multidrug resistance. Three different short hairpin RNAs (shRNAs) were designed and constructed in a pSilencer 3.1-H1 neo plasmid. The shRNA recombinant plasmids were transfected into HT9 leukemia cells. The RNAi effect was evaluated by real-time PCR, Western blotting and cell cytotoxicity assay. In the cell, shRNAs can specifically down-regulate the expression of mdr1, mRNA and P-gp. Resistance against harringtonine, doxorubicin and curcumin was decreased. The study indicated that shRNA recombinant plasmids could modulate MDR in vitro.     

Zoledronic acid is synergic with vinblastine to induce apoptosis in a multidrug resistance protein-1 dependent way: an in vitro study

We have explored the action of zoledronic acid, which has an apoptotic effect and is used as an agent for treating skeletal metastases and osteoporosis, in the presence of vinblastine, and whether this effect is associated with MRP-1 (multidrug resistance protein-1) expression. HEK (human embryonic kidney) 293 cells were transfected to form the multidrug resistant cell line designated 293MRP (MRP-1 expressing HEK293 cells). Both lines were treated with varying concentrations of vinblastine and zoledronic acid. Apoptosis was determined by the TUNEL (deoxyuridine triphosphate nick end-labeling) method. The type of treatment, MRP-1 expression status, and the type of treatment with respect to MRP-1 expression status significantly affected (P < 0.001) the degree of apoptosis. The largest increase in cytotoxicity was noted in HEK293 cells, when 100 micromol zoledronic acid was added to 4 microg/ml vinblastine (an increment of 80.3%, P < 0.001). This preliminary work shows that zoledronic acid acts synergistically with vinblastine to induce apoptosis in an MRP-1 dependent way.     

Mutagenesis of the putative nucleotide-binding domains of the multidrug resistance associated protein (MRP). Analysis of the effect of these mutations on MRP mediated drug resistance and transport

Studies were conducted to examine the functional role of the nucleotide-binding domains of MRP in drug resistance and drug transport in isolated membrane vesicles. In vivo studies were conducted by preparing stable transfectants of HeLa cells with wild-type MRP cDNA or MRP cDNAs which had been mutated at certain nucleotide binding domains (NBD). Stable transfectants producing equivalent amounts of the MRP encoded protein P190 were used in this study. The results demonstrated that deletions in the C-motif of NBD1 or the A-motif of NBD2 have a pronounced effect in reducing resistance levels to chemotherapeutic agents. Certain single-site mutations in lysines in these same motifs also reduce IC₅₀ values. It has also been observed that mutation of the MRP NBDs results in an increase in drug accumulation and a reduction in drug efflux. Additional studies have been carried out in which recombinant baculovirus containing either wild-type MRP or MRP containing mutated NBDs was prepared and used to infect SF21 insect cells. Using this system we have analyzed the effects of these mutations on in vitro transport of leukotriene C₄ (LTC₄) 17 β-estradiol 17 (β-D-glucuronide)(E₂17βG) and daunomycin in membrane vesicles prepared from baculovirus infected cells. The results demonstrate that deletions and site-specific mutations in MRP NBDs greatly reduce the ATP dependent transport of all three substrates. The results of these studies conducted both in vivo and in vitro demonstrate that the NBDs of MRP function in a cooperative manner and are critical for the transport activity of the MRP encoded protein P190. These studies also identify specific lysines in NBD1 and NBD2 which are important for optimal MRP activity. This revised version was published online in August 2006 with corrections to the Cover Date.