The First Cytoplasmic Loop in the Core Structure of the ABCC1 (Multidrug Resistance Protein 1; MRP1) Transporter Contains Multiple Amino Acids Essential for Its Expression.

ABCC1 (human multidrug resistance protein 1 (hMRP1)) is an ATP-binding cassette transporter which effluxes xeno- and endobiotic organic anions and confers multidrug resistance through active drug efflux. The 17 transmembrane α-helices of hMRP1 are distributed among three membrane spanning domains (MSD0, 1, 2) with MSD1,2 each followed by a nucleotide binding domain to form the 4-domain core structure. Eight conserved residues in the first cytoplasmic loop (CL4) of MSD1 in the descending α-helix (Gly392, Tyr404, Arg405), the perpendicular coupling helix (Asn412, Arg415, Lys416), and the ascending α-helix (Glu422, Phe434) were targeted for mutagenesis. Mutants with both alanine and same charge substitutions of the coupling helix residues were expressed in HEK cells at wild-type hMRP1 levels and their transport activity was only moderately compromised. In contrast, mutants of the flanking amino acids (G392I, Y404A, R405A/K, E422A/D, and F434Y) were very poorly expressed although Y404F, E422D, and F434A were readily expressed and transport competent. Modeling analyses indicated that Glu422 and Arg615 could form an ion pair that might stabilize transporter expression. However, this was not supported by exchange mutations E422R/R615E which failed to improve hMRP1 levels. Additional structures accompanied by rigorous biochemical validations are needed to better understand the bonding interactions crucial for stable hMRP1 expression.

Mutagenic Analysis of the Putative ABCC6 Substrate-Binding Cavity Using a New Homology Model.

Inactivating mutations in ABCC6 underlie the rare hereditary mineralization disorder pseudoxanthoma elasticum. ABCC6 is an ATP-binding cassette (ABC) integral membrane protein that mediates the release of ATP from hepatocytes into the bloodstream. The released ATP is extracellularly converted into pyrophosphate, a key mineralization inhibitor. Although ABCC6 is firmly linked to cellular ATP release, the molecular details of ABCC6-mediated ATP release remain elusive. Most of the currently available data support the hypothesis that ABCC6 is an ATP-dependent ATP efflux pump, an un-precedented function for an ABC transporter. This hypothesis implies the presence of an ATP-binding site in the substrate-binding cavity of ABCC6. We performed an extensive mutagenesis study using a new homology model based on recently published structures of its close homolog, bovine Abcc1, to characterize the substrate-binding cavity of ABCC6. Leukotriene C4 (LTC4), is a high-affinity substrate of ABCC1. We mutagenized fourteen amino acid residues in the rat ortholog of ABCC6, rAbcc6, that corresponded to the residues in ABCC1 found in the LTC4 binding cavity. Our functional characterization revealed that most of the amino acids in rAbcc6 corresponding to those found in the LTC4 binding pocket in bovine Abcc1 are not critical for ATP efflux. We conclude that the putative ATP binding site in the substrate-binding cavity of ABCC6/rAbcc6 is distinct from the bovine Abcc1 LTC4-binding site.

Structure-guided probing of the leukotriene C4 binding site in human multidrug resistance protein 1 (MRP1; ABCC1).

The human multidrug resistance protein 1 (hMRP1) transporter is implicated in cancer multidrug resistance as well as immune responses involving its physiologic substrate, glutathione (GSH)-conjugated leukotriene C4 (LTC4). LTC4 binds a bipartite site on hMRP1, which a recent cryoelectron microscopy structure of LTC4-bound bovine Mrp1 depicts as composed of a positively charged pocket and a hydrophobic (H) pocket that binds the GSH moiety and surrounds the fatty acid moiety, respectively, of LTC4. Here, we show that single Ala and Leu substitutions of H-pocket hMRP1-Met1093 have no effect on LTC4 binding or transport. Estrone 3-sulfate transport is also unaffected, but both hMRP1-Met1093 mutations eliminate estradiol glucuronide transport, demonstrating that these steroid conjugates have binding sites distinct from each other and from LTC4. To eliminate LTC4 transport by hMRP1, mutation of 3 H-pocket residues was required (W553/M1093/W1246A), indicating that H-pocket amino acids are key to the vastly different affinities of hMRP1 for LTC4vs. GSH alone. Unlike organic anion transport, hMRP1-mediated drug resistance was more diminished by Ala than Leu substitution of Met1093. Although our findings generally support a structure in which H-pocket residues bind the lipid tail of LTC4, their critical and differential role in the transport of conjugated estrogens and anticancer drugs remains unexplained.-Conseil, G., Arama-Chayoth, M., Tsfadia, Y., Cole, S. P. C. Structure-guided probing of the leukotriene C4 binding site in human multidrug resistance protein 1 (MRP1; ABCC1).

An Outward-Facing Aromatic Amino Acid Is Crucial for Signaling between the Membrane-Spanning and Nucleotide-Binding Domains of Multidrug Resistance Protein 1 (MRP1; ABCC1).

The 190-kDa human MRP1 is an ATP-binding cassette multidrug and multiorganic anion efflux transporter. The 17 transmembrane helices of its three membrane-spanning domains, together with its two nucleotide binding domains (NBDs), form a stabilizing network of domain-domain interactions that ensure substrate binding in the cytoplasm is efficiently coupled to ATP binding and hydrolysis to effect solute efflux into the extracellular milieu. Here we show that Ala substitution of Phe583 in an outward-facing loop between the two halves of the transporter essentially eliminates the binding of multiple organic anions by MRP1. Conservative substitutions with Trp and Tyr had little or no effect. The F583A mutation also caused a substantial increase in orthovanadate-induced trapping of azidoADP by the cytoplasmic NBDs of MRP1, although the binding of ATP was unaffected. These observations indicate that the loss of the aromatic side chain at position 583 impairs the release of ADP and thus effectively locks the transporter in a low-affinity solute binding state. Phe583 is the first outward-facing amino acid in MRP1 found to be critical for its transport function. Our data provide evidence for long-range coupling, presumably via allosteric interaction, between this outward-facing region of MRP1 and both the solute binding and nucleotide binding regions of the transporter. Cryoelectron microscopy structural and homology models of MRP1 indicate that the orientation of the Phe583 side chain is altered by ATP binding but are currently unable to provide insights into the molecular mechanism by which this long-range signaling is propagated.

N-linked glycans do not affect plasma membrane localization of multidrug resistance protein 4 (MRP4) but selectively alter its prostaglandin E2 transport activity.

Multidrug resistance protein 4 (MRP4) is a member of subfamily C of the ATP-binding cassette superfamily of membrane transport proteins. MRP4 mediates the ATP-dependent efflux of many endogenous and exogenous solutes across the plasma membrane, and in polarized cells, it localizes to the apical or basolateral plasma membrane depending on the tissue type. MRP4 is a 170 kDa glycoprotein and here we show that MRP4 is simultaneously N-glycosylated at Asn746 and Asn754. Furthermore, confocal immunofluorescence studies showed that N-glycans do not affect MRP4's apical membrane localization in polarized LLC-PK1 cells or basolateral membrane localization in polarized MDCKI cells. However, vesicular transport assays showed that N-glycans differentially affect MRP4's ability to transport prostaglandin E2, but not estradiol glucuronide. Together these data indicate that N-glycosylation at Asn746 and Asn754 is not essential for plasma membrane localization of MRP4 but cause substrate-selective effects on its transport activity.

Cysteinyl Leukotriene Receptor 1/2 Antagonists Nonselectively Modulate Organic Anion Transport by Multidrug Resistance Proteins (MRP1-4).

Active efflux of both drugs and organic anion metabolites is mediated by the multidrug resistance proteins (MRPs). MRP1 (ABCC1), MRP2 (ABCC2), MRP3 (ABCC3), and MRP4 (ABCC4) have partially overlapping substrate specificities and all transport 17ß-estradiol 17-(ß-d-glucuronide) (E217ßG). The cysteinyl leukotriene receptor 1 (CysLT1R) antagonist MK-571 inhibits all four MRP homologs, but little is known about the modulatory effects of newer leukotriene modifiers (LTMs). Here we examined the effects of seven CysLT1R- and CysLT2R-selective LTMs on E217ßG uptake into MRP1-4-enriched inside-out membrane vesicles. Their effects on uptake of an additional physiologic solute were also measured for MRP1 [leukotriene C4 (LTC4)] and MRP4 [prostaglandin E2 (PGE2)]. The two CysLT2R-selective LTMs studied were generally more potent inhibitors than CysLT1R-selective LTMs, but neither class of antagonists showed any MRP selectivity. For E217ßG uptake, LTM IC50s ranged from 1.2 to 26.9 µM and were most comparable for MRP1 and MRP4. The LTM rank order inhibitory potencies for E217ßG versus LTC4 uptake by MRP1, and E217ßG versus PGE2 uptake by MRP4, were also similar. Three of four CysLT1R-selective LTMs also stimulated MRP2 (but not MRP3) transport and thus exerted a concentration-dependent biphasic effect on MRP2. The fourth CysLT1R antagonist, LY171883, only stimulated MRP2 (and MRP3) transport but none of the MRPs were stimulated by either CysLT2R-selective LTM. We conclude that, in contrast to their CysLTR selectivity, CysLTR antagonists show no MRP homolog selectivity, and data should be interpreted cautiously if obtained from LTMs in systems in which more than one MRP is present.

Atypical B cells promote cancer progression and poor response to Bacillus Calmette-Guérin in non-muscle invasive bladder cancer.

Poor response to Bacillus Calmette-Guérin (BCG) immunotherapy remains a major barrier in the management of patients with non-muscle invasive bladder cancer (NMIBC). Multiple factors are associated with poor outcomes, including biological aging and female sex. More recently, it has emerged that a B-cell infiltrated pre-treatment immune microenvironment of NMIBC tumors can influence the response to intra-vesically administered BCG. The mechanisms underlying the roles of B cells in NMIBC are poorly understood. Here, we show that B-cell dominant tertiary lymphoid structures (TLSs), a hallmark feature of the chronic mucosal immune response, are abundant and located close to the epithelial compartment in pre-treatment tumors from BCG non-responders. Digital spatial proteomic profiling of whole tumor sections from male and female patients with NMIBC who underwent treatment with intravesical BCG, revealed higher expression of immune exhaustion-associated proteins within the tumor-adjacent TLSs in both responders and non-responders. Chronic local inflammation, induced by the N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN) carcinogen, led to TLS formation with recruitment and differentiation of the immunosuppressive atypical B-cell (ABC) subset within the bladder microenvironment, predominantly in aging female mice compared to their male counterparts. Depletion of ABCs simultaneous to BCG treatment delayed cancer progression in female mice. Our findings provide evidence indicating a role for ABCs in BCG response and will inform future development of therapies targeting the B cell-exhaustion axis.

Two polymorphic variants of ABCC1 selectively alter drug resistance and inhibitor sensitivity of the multidrug and organic anion transporter multidrug resistance protein 1.

In this study we compared the in silico predictions of the effect of ABCC1 nonsynonymous single nucleotide polymorphisms (nsSNPs) with experimental data on MRP1 transport function and response to chemotherapeutics and multidrug resistance protein 1 (MRP1) inhibitors. Vectors encoding seven ABCC1 nsSNPs were stably expressed in human embryonic kidney (HEK) cells, and levels and localization of the mutant MRP1 proteins were determined by confocal microscopy and immunoblotting. The function of five of the mutant proteins was determined using cell-based drug and inhibitor sensitivity and efflux assays, and membrane-based organic anion transport assays. Predicted consequences of the mutations were determined by multiple bioinformatic methods. Mutants C43S and S92F were correctly routed to the HEK cell plasma membrane, but the levels were too low to permit functional characterization. In contrast, levels and membrane trafficking of R633Q, G671V, R723Q, A989T, and C1047S were similar to wild-type MRP1. In cell-based assays, all five mutants were equally effective at effluxing calcein, but only two exhibited reduced resistance to etoposide (C1047S) and vincristine (A989T; C1047S). The GSH-dependent inhibitor LY465803 (LY465803 [N-[3-(9-chloro-3-methyl-4-oxo-4H-isoxazolo-[4,3-c]quinolin-5-yl)-cyclohexylmethyl]-benzamide)] was less effective at blocking calcein efflux by A989T, but in a membrane-based assay, organic anion transport by A989T and C1047S was inhibited by MRP1 modulators as well as wild-type MRP1. GSH accumulation assays suggest cellular GSH efflux by A989T and C1047S may be impaired. In conclusion, although six in silico analyses consistently predict deleterious consequences of ABCC1 nsSNPs G671V, changes in drug resistance and inhibitor sensitivity were only observed for A989T and C1047S, which may relate to GSH transport differences.

Chalcogenopyrylium dyes as differential modulators of organic anion transport by multidrug resistance protein 1 (MRP1), MRP2, and MRP4.

Multidrug resistance proteins (MRPs) mediate the ATP-dependent efflux of structurally diverse compounds, including anticancer drugs and physiologic organic anions. Five classes of chalcogenopyrylium dyes (CGPs) were examined for their ability to modulate transport of [(3)H]estradiol glucuronide (E(2)17ßG; a prototypical MRP substrate) into MRP-enriched inside-out membrane vesicles. Additionally, some CGPs were tested in intact transfected cells using a calcein efflux assay. Sixteen of 34 CGPs inhibited MRP1-mediated E(2)17ßG uptake by >50% (IC50 values: 0.7-7.6 µM). Of 9 CGPs with IC50 values ≤2 µM, two belonged to class I, two to class III, and five to class V. When tested in the intact cells, only 4 of 16 CGPs (at 10 µM) inhibited MRP1-mediated calcein efflux by >50% (III-1, V-3, V-4, V-6), whereas a fifth (I-5) inhibited efflux by just 23%. These five CGPs also inhibited [(3)H]E(2)17ßG uptake by MRP4. In contrast, their effects on MRP2 varied, with two (V-4, V-6) inhibiting E(2)17ßG transport (IC(50) values: 2.0 and 9.2 µM) and two (V-3, III-1) stimulating transport (>2-fold), whereas CGP I-5 had no effect. Strikingly, although V-3 and V-4 had opposite effects on MRP2 activity, they are structurally identical except for their chalcogen atom (Se versus Te). This study is the first to identify class V CGPs, with their distinctive methine or trimethine linkage between two disubstituted pyrylium moieties, as a particularly potent class of MRP modulators, and to show that, within this core structure, differences in the electronegativity associated with a chalcogen atom can be the sole determinant of whether a compound will stimulate or inhibit MRP2.

Cancer cell genotype associated tumor immune microenvironment exhibits differential response to therapeutic STING pathway activation in high-grade serous ovarian cancer.

BACKGROUND: High-grade serous ovarian carcinoma (HGSC) is the most lethal gynecologic malignancy characterized by resistance to chemotherapy and high rates of recurrence. HGSC tumors display a high prevalence of tumor suppressor gene loss. Given the type 1 interferon regulatory function of BRCA1 and PTENgenes and their associated contrasting T-cell infiltrated and non-infiltrated tumor immune microenvironment (TIME) states, respectively, in this study we investigated the potential of stimulator of interferon genes (STING) pathway activation in improving overall survival via enhancing chemotherapy response, specifically in tumors with PTEN deficiency. METHODS: Expression of PTEN protein was evaluated in tissue microarrays generated using pretreatment tumors collected from a cohort of 110 patients with HGSC. Multiplex immunofluorescence staining was performed to determine spatial profiles and density of selected lymphoid and myeloid cells. In vivo studies using the syngeneic murine HGSC cell lines, ID8-Trp53 -/-; Pten -/- and ID8-Trp53 -/-; Brca1 -/-, were conducted to characterize the TIME and response to carboplatin chemotherapy in combination with exogenous STING activation therapy. RESULTS: Patient tumors with absence of PTEN protein exhibited a significantly decreased disease specific survival and intraepithelial CD68+ macrophage infiltration as compared with intact PTEN expression. In vivo studies demonstrated that Pten-deficient ovarian cancer cells establish an immunosuppressed TIME characterized by increased proportions of M2-like macrophages, GR1+MDSCs in the ascites, and reduced effector CD8+ cytotoxic T-cell function compared with Brca1-deficient cells; further, tumors from mice injected with Pten-deficient ID8 cells exhibited an aggressive behavior due to suppressive macrophage dominance in the malignant ascites. In combination with chemotherapy, exogenous STING activation resulted in longer overall survival in mice injected with Pten-deficient ID8 cells, reprogrammed intraperitoneal M2-like macrophages derived from Pten-deficient ascites to M1-like phenotype and rescued CD8+ cytotoxic T-cell activation. CONCLUSIONS: This study reveals the importance of considering the influence of cancer cell intrinsic genetic alterations on the TIME for therapeutic selection. We establish the rationale for the optimal incorporation of interferon activating therapies as a novel combination strategy in PTEN-deficient HGSC.

Sex differences in the aging murine urinary bladder and influence on the tumor immune microenvironment of a carcinogen-induced model of bladder cancer.

Sex and age associated differences in the tumor immune microenvironment of non-muscle invasive bladder (NMIBC) cancer and associated clinical outcomes are emerging indicators of treatment outcomes. The incidence of urothelial carcinoma of the bladder is four times higher in males than females; however, females tend to present with a more aggressive disease, a poorer response to immunotherapy and suffer worse clinical outcomes. Recent findings have demonstrated sex differences in the tumor immune microenvironment of non-muscle invasive and muscle invasive bladder cancer and associated clinical outcomes. However, a significant gap in knowledge remains with respect to the current pre-clinical modeling approaches to more precisely recapitulate these differences towards improved therapeutic design. Given the similarities in mucosal immune physiology between humans and mice, we evaluated the sex and age-related immune alterations in healthy murine bladders. Bulk-RNA sequencing and multiplex immunofluorescence-based spatial immune profiling of healthy murine bladders from male and female mice of age groups spanning young to old showed a highly altered immune landscape that exhibited sex and age associated differences, particularly in the context of B cell mediated responses. Spatial profiling of healthy bladders, using markers specific to macrophages, T cells, B cells, activated dendritic cells, high endothelial venules, myeloid cells and the PD-L1 immune checkpoint showed sex and age associated differences. Bladders from healthy older female mice also showed a higher presence of tertiary lymphoid structures (TLSs) compared to both young female and male equivalents. Spatial immune profiling of N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN) carcinogen exposed male and female bladders from young and old mice revealed a similar frequency of TLS formation, sex differences in the bladder immune microenvironment and, age associated differences in latency of tumor induction. These findings support the incorporation of sex and age as factors in pre-clinical modeling of bladder cancer and will potentially advance the field of immunotherapeutic drug development to improve clinical outcomes.

GSH facilitates the binding and inhibitory activity of novel multidrug resistance protein 1 (MRP1) modulators.

MRP1 (ABCC1) is a membrane transporter that confers multidrug resistance in cancer cells by exporting chemotherapeutic agents, often in a reduced glutathione (GSH)-dependent manner. This transport activity can be altered by compounds (modulators) that block drug transport while simultaneously stimulating GSH efflux by MRP1. In MRP1-expressing cells, modulator-stimulated GSH efflux can be sufficient to deplete GSH and increase sensitivity to chemotherapy, enhancing cancer cell death. Further development of clinically useful MRP1 modulators requires a better mechanistic understanding of modulator binding and its relationship to GSH binding and transport. Here, we explore the mechanism of two MRP1 small molecule modulators, 5681014 and 7914321, in relation to a bipartite substrate-binding cavity of MRP1. Binding of these modulators to MRP1 was dependent on the presence of GSH but not its reducing capacity. Accordingly, the modulators poorly inhibited organic anion transport by K332L-mutant MRP1, where GSH binding and transport is limited. However, the inhibitory activity of the modulators was also diminished by mutations that limit E2 17ßG but spare GSH-conjugate binding and transport (W553A, M1093A, W1246A), suggesting overlap between the E2 17ßG and modulator binding sites. Immunoblots of limited trypsin digests of MRP1 suggest that binding of GSH, but not the modulators, induces a conformation change in MRP1. Together, these findings support the model, in which GSH binding induces a conformation change that facilitates binding of MRP1 modulators, possibly in a proposed hydrophobic binding pocket of MRP1. This study may facilitate the structure-guided design of more potent and selective MRP1 modulators.

Conserved amino acids in the region connecting membrane spanning domain 1 to nucleotide binding domain 1 are essential for expression of the MRP1 (ABCC1) transporter.

Multidrug resistance protein 1 (MRP1) (gene symbol ABCC1) is an ATP-binding cassette (ABC) transporter which effluxes xeno- and endobiotic organic anions including estradiol glucuronide and the pro-inflammatory leukotriene C4. MRP1 also confers multidrug resistance by reducing intracellular drug accumulation through active efflux. MRP1 has three membrane spanning domains (MSD), and two nucleotide binding domains (NBD). MSD1 and MSD2 are linked to NBD1 and NBD2 by connecting regions (CR) 1 and CR2, respectively. Here we targeted four residues in CR1 (Ser612, Arg615, His622, Glu624) for alanine substitution and unexpectedly, found that cellular levels of three mutants (S612A, R615A, E624A) in transfected HEK cells were substantially lower than wild-type MRP1. Whereas CR1-H622A properly trafficked to the plasma membrane and exhibited organic anion transport activity comparable to wild-type MRP1, the poorly expressing R615A and E624A (and to a lesser extent S612A) mutant proteins were retained intracellularly. Analyses of cryogenic electron microscopic and atomic homology models of MRP1 indicated that Arg615 and Glu624 might participate in bonding interactions with nearby residues to stabilize expression of the transporter. However, this was not supported by double exchange mutations E624K/K406E, R615D/D430R and R615F/F619R which failed to improve MRP1 levels. Nevertheless, these experiments revealed that the highly conserved CR1-Phe619 and distal Lys406 in the first cytoplasmic loop of MSD1 are also essential for expression of MRP1 protein. This study is the first to demonstrate that CR1 contains several highly conserved residues critical for plasma membrane expression of MRP1 but thus far, currently available structures and models do not provide any insights into the underlying mechanism(s). Additional structures with rigorous biochemical validation data are needed to fully understand the bonding interactions critical to stable expression of this clinically important ABC transporter.

Plasmodium falciparum multidrug resistance protein 1 and artemisinin-based combination therapy in Africa.

Plasmodium falciparum response mechanisms to the major artemisinin-based combination therapies (ACTs) are largely unknown. Multidrug-resistance protein (MRP)-like adenosine triphosphate (ATP)-binding cassette transporters are known to be related to multidrug resistance in many organisms. Therefore, we hypothesized that sequence variation in pfmrp1 can contribute to decreased parasite sensitivity to ACT. Through sequencing of the pfmrp1 open reading frame for 103 geographically diverse P. falciparum infections, we identified 27 single-nucleotide polymorphisms (SNPs), of which 21 were nonsynonymous and 6 synonymous. Analyses of clinical efficacy trials with artesunate-amodiaquine and artemether-lumefantrine detected a specific selection of the globally prevalent I876V SNP in recurrent infections after artemether-lumefantrine treatment. Additional in silico studies suggested an influence of variation in amino acid 876 on the ATP hydrolysis cycle of pfMRP1 with potential impact on protein functionality. Our data suggest for the first time, to our knowledge, the involvement of pfMRP1 in P. falciparum in vivo response to ACT.

Multiple roles of charged amino acids in cytoplasmic loop 7 for expression and function of the multidrug and organic anion transporter MRP1 (ABCC1).

Multidrug resistance protein MRP1 mediates the ATP-dependent efflux of many chemotherapeutic agents and organic anions. MRP1 has two nucleotide binding sites (NBSs) and three membrane spanning domains (MSDs) containing 17 transmembrane helices linked by extracellular and cytoplasmic loops (CL). Homology models suggest that CL7 (amino acids 1141-1195) is in a position where it could participate in signaling between the MSDs and NBSs during the transport process. We have individually replaced eight charged residues in CL7 with Ala, and in some cases, an amino acid with the same charge, and then investigated the effects on MRP1 expression, transport activity, and nucleotide and substrate interactions. A triple mutant in which Glu(1169), Glu(1170), and Glu(1172) were all replaced with Ala was also examined. The properties of R1173A and E1184A were comparable with those of wild-type MRP1, whereas the remaining mutants were either poorly expressed (R1166A, D1183A) or exhibited reduced transport of one or more organic anions (E1144A, D1179A, K1181A, (1169)AAQA). Same charge mutant D1183E was also not expressed, whereas expression and activity of R1166K were similar to wild-type MRP1. The moderate substrate-selective changes in transport activity displayed by mutants E1144A, D1179A, K1181A, and (1169)AAQA were accompanied by changes in orthovanadate-induced trapping of [alpha-(32)P]azidoADP by NBS2 indicating changes in ATP hydrolysis or release of ADP. In the case of E1144A, estradiol glucuronide no longer inhibited trapping of azidoADP. Together, our results demonstrate the extreme sensitivity of CL7 to mutation, consistent with its critical and complex dual role in both the proper folding and transport activity of MRP1.

Involvement of NHERF1 in apical membrane localization of MRP4 in polarized kidney cells.

Multidrug resistance protein 4 (MRP4/ABCC4), a member of the ATP-binding cassette protein superfamily, confers resistance to nucleoside and nucleotide analogs as well as camptothecin derivatives. MRP4 also mediates the efflux of certain cyclic nucleotides, eicosanoids, conjugated steroids, and uric acid. Depending on the cell type, MRP4 may localize to either apical or basolateral membranes in polarized cells. The adaptor protein NHERF1 has previously been implicated in MRP4 internalization in non-polarized cells. We have now found that NHERF1 levels are very low in polarized MDCKI cells which express MRP4 on basolateral membranes relative to polarized LLC-PK1 cells which express MRP4 on apical membranes. Furthermore, ectopic expression of FLAG-tagged NHERF1 in MDCKI cells and in MDCKI cells stably expressing eGFP-tagged MRP4 causes endogenous MRP4 and eGFP-MRP4, respectively, to traffic to the apical membranes. These data establish NHERF1 as a major determinant of MRP4 trafficking to apical membranes of mammalian kidney cells.

Molecular basis for reduced estrone sulfate transport and altered modulator sensitivity of transmembrane helix (TM) 6 and TM17 mutants of multidrug resistance protein 1 (ABCC1).

Multidrug resistance protein 1 (MRP1) confers drug resistance and also mediates cellular efflux of many organic anions. MRP1 also transports glutathione (GSH); furthermore, this tripeptide stimulates transport of several substrates, including estrone 3-sulfate. We have previously shown that mutations of Lys(332) in transmembrane helix (TM) 6 and Trp(1246) in TM17 cause different substrate-selective losses in MRP1 transport activity. Here we have extended our characterization of mutants K332L and W1246C to further define the different roles these two residues play in determining the substrate and inhibitor specificity of MRP1. Thus, we have shown that TM17-Trp(1246) is crucial for conferring drug resistance and for binding and transport of methotrexate, estradiol glucuronide, and estrone 3-sulfate, as well as for binding of the tricyclic isoxazole inhibitor N-[3-(9-chloro-3-methyl-4-oxo-4H-isoxazolo-[4,3-c]quinolin-5-yl)-cyclohexylmethyl]-benzamide (LY465803). In contrast, TM6-Lys(332) is important for enabling GSH and GSH-containing compounds to serve as substrates (e.g., leukotriene C(4)) or modulators (e.g., S-decyl-GSH, GSH disulfide) of MRP1 and, further, for enabling GSH (or S-methyl-GSH) to enhance the transport of estrone 3-sulfate and increase the inhibitory potency of LY465803. On the other hand, both mutants are as sensitive as wild-type MRP1 to the non-GSH-containing inhibitors (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK571), 1-[2-hydroxy-3-propyl-4-[4-(1H-tetrazol-5-yl)butoxy]phenyl]-ethanone (LY171883), and highly potent 6-[4'-carboxyphenylthio]-5[S]-hydroxy-7[E], 11[Z]14[Z]-eicosatetrenoic acid (BAY u9773). Finally, the differing abilities of the cysteinyl leukotriene derivatives leukotriene C(4), D(4), and F(4) to inhibit estradiol glucuronide transport by wild-type and K332L mutant MRP1 provide further evidence that TM6-Lys(332) is involved in the recognition of the gamma-Glu portion of substrates and modulators containing GSH or GSH-like moieties.

MRP1 modulators synergize with buthionine sulfoximine to exploit collateral sensitivity and selectively kill MRP1-expressing cancer cells.

Members of the ABC transporter family, particularly P-glycoprotein (P-gp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and multidrug resistance protein 1 (MRP1, ABCC1) are well characterized mediators of multidrug resistance, however their pharmacological inhibition has so far failed as a clinical strategy. Harnessing collateral sensitivity, a form of synthetic lethality where cells with acquired multidrug resistance exhibit hypersensitivity to unrelated agents, may be an alternative approach to targeting multidrug resistant tumour cells. We characterized a novel small molecule modulator that selectively enhanced MRP1-dependent efflux of reduced glutathione (GSH), an endogenous MRP1 substrate. Using cell lines expressing high levels of endogenous MRP1 from three difficult to treat cancer types-lung cancer, ovarian cancer and high-risk neuroblastoma-we showed that the MRP1 modulator substantially lowered intracellular GSH levels as a single agent. The effect was on-target, as MRP1 knockdown abolished GSH depletion. The MRP1 modulator was synergistic with the GSH synthesis inhibitor buthionine sulfoximine (BSO), with the combination exhausting intracellular GSH, increasing intracellular reactive oxygen species (ROS) and abolishing clonogenic capacity. Clonogenicity was rescued by the ROS scavenger N-acetylcysteine, implicating GSH depletion in the effect. The MRP1 modulator in combination with BSO also strongly sensitized cancer cells to MRP1-substrate chemotherapeutic agents, particularly arsenic trioxide, and was more effective than either the MRP1 modulator or BSO alone. GSH-depleting MRP1 modulators may therefore provide an enhanced therapeutic window to treat chemo-resistant MRP1-overexpressing pediatric and adult cancers.

Mechanistic differences between GSH transport by multidrug resistance protein 1 (MRP1/ABCC1) and GSH modulation of MRP1-mediated transport.

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-dependent polytopic membrane protein that transports many anticancer drugs and organic anions. Its transport mechanism is multifaceted, especially with respect to the participation of GSH. For example, vincristine is cotransported with GSH, estrone sulfate transport is stimulated by GSH, or MRP1 can transport GSH alone, and this can be stimulated by compounds such as verapamil or apigenin. Thus, the interactions between GSH and MRP1 are mechanistically complex. To examine the similarities and differences among the various GSH-associated mechanisms of MRP1 transport, we have measured first the effect of GSH and several GSH-associated substrates/modulators on the binding and hydrolysis of ATP by MRP1 using 8-azidoadenosine-5'-[(32)P]-triphosphate ([(32)P]azidoATP) analogs, and second the initial binding of GSH and GSH-associated substrates/modulators to MRP1. We observed that GSH or its nonreducing derivative S-methylGSH (S-mGSH), but none of the GSH-associated substrate/modulators, caused a significant increase in [gamma-(32)P]azidoATP labeling of MRP1. Moreover, GSH and S-mGSH decreased levels of orthovanadate-induced trapping of [alpha-(32)P]azidoADP. [alpha-(32)P]azidoADP.Vi trapping was also decreased by estone sulfate, whereas vincristine, verapamil, and apigenin had no apparent effects on nucleotide interactions with MRP1. Furthermore, estrone sulfate and S-mGSH enhanced the effect of each other 15- and 10-fold, respectively. Second, although GSH binding increased the apparent affinity of MRP1 for all GSH-associated substrates/modulators tested, only estrone sulfate had a reciprocal effect on the apparent affinity of MRP1 for GSH. Overall, these results indicate significant mechanistic differences between MRP1-mediated transport of GSH and the ability of GSH to modulate MRP1 transport.

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

Multidrug resistance protein 1 (MRP1/ABCC1) is a 190kDa 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.