Mutation of proline residues in the NH(2)-terminal region of the multidrug resistance protein, MRP1 (ABCC1): effects on protein expression, membrane localization, and transport function

Emerging roles of ncRNAs regulating ABCC1 on chemotherapy resistance of cancer - a review

In the process of chemotherapy, drug resistance of cancer cells is a common and difficult problem of chemotherapy failure, and it is also the main cause of cancer recurrence and metastasis. Non-coding RNAs (ncRNAs) refer to the RNA that does not encode proteins, including microRNA (miRNA), long non-coding RNA (lncRNA) and circularRNA (circRNA), etc. NcRNAs are involved in a series of important life processes and further regulate the expression of ABCC1 by directly or indirectly up-regulating or down-regulating the expression of targeted mRNAs, making cancer cells more susceptible to drug resistance. A growing number of studies have shown that ncRNAs have effects on cancer cell proliferation, invasion, metastasis, and drug sensitivity, by regulating the expression of ABCC1. In this review, we will discuss the emerging roles of ncRNAs regulating ABCC1 in chemotherapy resistance and mechanisms to reverse drug resistance as well as provide potential targets for future cancer treatment.

LY103, a pomalidomide derivative, alleviates taxol resistance in NSCLC via energy metabolism crosstalk and tumor microenvironment intervention

In this study, we identified HIF 1α as a potential target for reversing taxol resistance in lung cancer by combining bioinformatics analysis with pharmacological analysis. Furthermore, pomalidomide derivative LY103 was also be synthesized by introducing an isatin analogue into the amino terminal ofpomalidomide, and it has a broad antitumor spectrum and showed excellent activity against A549/Taxol cells (IC50 = 6.33 ± 0.51 μM). The results of molecular docking showed that not only LY103 was inclined to bind to HIF 1α stably, it could also form multiple hydrogen bonds with VAL376, ASP256, ILE454, and GLU455 of HIF 1α even was reduced to LY103-NH2 by nitroreductase, which was further stabilized the complex formed by them, thereby inhibiting the activity of HIF 1α. LY103 was able to significantly induce DNA damage and inhibit angiogenesis. Concurrently, LY103 activated the immune response, reduced the expression of cytokines TNF-α, IL-6, and IL-1β, thus might be inhibit the proliferation and metastasis of tumor cells. Pharmacological analysis proved that LY103 led to cell apoptosis through the mitochondrial pathway, and its combination with taxol significantly promoted this process. In general, the consumption of glutathione, the crosstalk of energy metabolism, and the improvement of the tumor microenvironment caused by LY103 eventually led to the decrease of ABCC1 protein expression and the drug resistance was reversed. The rational design of LY103 provided a basis for the application of nitro compounds in the treatment of hypoxic tumors and the reversal of taxol resistance.

Biochemical studies on the structure-function relationship of major drug transporters in the ATP-binding cassette family and solute carrier family

Human drug transporters often play key roles in determining drug accumulation within cells. Their activities are often directly related to therapeutic efficacy, drug toxicity as well as drug-drug interactions. However, the progress for interpretation of their crystal structures is relatively slow. Hence, conventional biochemical studies together with computer modeling became useful manners to reveal essential structures of these membrane proteins. Over the years, quite a few structure-function relationship information had been obtained for members of the two major transporter families: the ATP-binding cassette family and the solute carrier family. Critical structural features of drug transporters include transmembrane domains, post-translational modification sites and domains for cell surface assembly and protein-protein interactions. Alterations at these important sites may affect protein stability, trafficking to the plasma membrane and/or ability of transporters to interact with substrates.

Differential functional rescue of Lys(513) and Lys(516) processing mutants of MRP1 (ABCC1) by chemical chaperones reveals different domain-domain interactions of the transporter

Multidrug resistance protein 1 (MRP1) extrudes drugs as well as pharmacologically and physiologically important organic anions across the plasma membrane in an ATP-dependent manner. We previously showed that Ala substitutions of Lys(513) and Lys(516) in the cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 cause misfolding of MRP1, abrogating its expression at the plasma membrane in transfected human embryonic kidney (HEK) cells. Exposure of HEK cells to the chemical chaperones glycerol, DMSO, polyethylene glycol (PEG) and 4-aminobutyric acid (4-PBA) improved levels of K513A to wild-type MRP1 levels but transport activity was only fully restored by 4-PBA or DMSO treatments. Tryptic fragmentation patterns and conformation-dependent antibody immunoreactivity of the transport-deficient PEG- and glycerol-rescued K513A proteins indicated that the second nucleotide binding domain (NBD2) had adopted a more open conformation than in wild-type MRP1. This structural change was accompanied by differences in ATP binding and hydrolysis but no changes in substrate Km. In contrast to K513A, K516A levels in HEK cells were not significantly enhanced by chemical chaperones. In more permissive insect cells, however, K516A levels were comparable to wild-type MRP1. Nevertheless, organic anion transport by K516A in insect cell membranes was reduced by >80% due to reduced substrate Km. Tryptic fragmentation patterns indicated a more open conformation of the third membrane spanning domain of MRP1. Thus, despite their close proximity to one another in CL5, Lys(513) and Lys(516) participate in different interdomain interactions crucial for the proper folding and assembly of MRP1.

Multidrug resistance: Physiological principles and nanomedical solutions

Multidrug resistance (MDR) is a pathophysiological phenomenon employed by cancer cells which limits the prolonged and effective use of chemotherapeutic agents. MDR is primarily based on the over-expression of drug efflux pumps in the cellular membrane. Prominent examples of such efflux pumps, which belong to the ATP-binding cassette (ABC) superfamily of proteins, are Pgp (P-glycoprotein) and MRP (multidrug resistance-associated protein), nowadays officially known as ABCB1 and ABCC1. Over the years, several strategies have been evaluated to overcome MDR, based not only on the use of low-molecular-weight MDR modulators, but also on the implementation of 1-100(0) nm-sized drug delivery systems. In the present manuscript, after introducing the most important physiological principles of MDR, we summarize prototypic nanomedical strategies to overcome multidrug resistance, including the use of carrier materials with intrinsic anti-MDR properties, the use of nanomedicines to modify the mode of cellular uptake, and the co-formulation of chemotherapeutic drugs together with low- and high-molecular-weight MDR inhibitors within a single drug delivery system. While certain challenges still need to be overcome before such constructs and concepts can be widely applied in the clinic, the insights obtained and the progress made strongly suggest that nanomedicine formulations hold significant potential for improving the treatment of multidrug-resistant malignancies.

Identification of proline residues in or near the transmembrane helices of the human breast cancer resistance protein (BCRP/ABCG2) that are important for transport activity and substrate specificity

The human breast cancer resistance protein (BCRP/ABCG2) confers multidrug resistance and mediates the active efflux of drugs and xenobiotics. BCRP contains one nucleotide-binding domain (NBD) followed by one membrane-spanning domain (MSD). We investigated whether prolines in or near the transmembrane helices are essential for BCRP function. Six proline residues were substituted with alanine individually, and the mutants were stably expressed in Flp-In(TM)-293 cells at levels comparable to that of wild-type BCRP and predominantly localized on the plasma membrane of the cells. While P392A showed a significant reduction (35-50%) in the efflux activity of mitoxantrone, BODIPY-prazosin, and Hoechst 33342, P485A exhibited a significant decrease of approximately 70% in the efflux activity of only BODIPY-prazosin. Other mutants had no significant changes in the efflux activities of these substrates. Drug resistance profiles of the cells expressing the mutants correlated well with the efflux data. ATPase activity was not substantially affected for P392A or P485A compared to that of wild-type BCRP. These results strongly suggest Pro(392) and Pro(485) are important in determining the overall transport activity and substrate selectivity of BCRP, respectively. Prazosin differentially affected the binding of 5D3, a conformation-sensitive antibody, to wild-type BCRP, P392A, or P485A in a concentration-dependent manner. In contrast, mitoxantrone had no significant effect on 5D3 binding. Homology modeling indicates that Pro(392) may play an important role in the communication between the MSD and NBD as it is predicted to be located at the interface between the two functional domains, and Pro(485) induces flexible hinges that may be essential for the broad substrate specificity of BCRP.

Structure of a human multidrug transporter in an inward-facing conformation

Multidrug resistance protein 1 (ABCC1) is a member of the 'C' class of ATP-binding cassette transporters, which can give rise to resistance to chemotherapy via drug export from cells. It also acts as a leukotriene C4 transporter, and hence has a role in adaptive immune response. Most C-class members have an additional NH(2)-terminal transmembrane domain versus other ATP-binding cassette transporters, but little is known about the structure and role of this domain. Using electron cryomicroscopy of 2D crystals, data at 1/6per A(-1) resolution was generated for the full-length ABCC1 protein in the absence of ATP. Analysis using homologous structures from bacteria and mammals allowed the core transmembrane domains to be localised in the map. These display an inward-facing conformation and there is a noteworthy separation of the cytoplasmic nucleotide-binding domains. Examination of non-core features in the map suggests that the additional NH(2)-terminal domain has extensive contacts on one side of both core domains, and mirrors their inward-facing configuration in the absence of nucleotide.

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.

Identification of regions required for apical membrane localization of human multidrug resistance protein 2

Multidrug resistance proteins MRP1 and MRP2 transport a wide range of endo- and xenobiotics. However, with the exception of certain parts of the brain, MRP1 traffics to basolateral membranes of polarized cells, whereas MRP2 is apical in location and thus it is particularly important for systemic elimination of such compounds. Different regions of MRP1 and MRP2 seem to target them to their respective membrane locations. In addition to two "core" membrane spanning domains (MSDs) characteristic of ATP-binding cassette transporters, MRP1 and MRP2 have a third NH2-terminal MSD (MSD0), which is not required for basolateral targeting of MRP1, or for transport of at least some substrates. Here, we demonstrate that all elements necessary for apical targeting of MRP2 reside in MSD0 and the adjacent cytoplasmic loop (CL) 3. Furthermore, we show that this region of MRP2 can target the core of MRP1 to an exclusively apical location. Within MRP2 CL3, we identified a lysine-rich element that is essential for apical targeting. When introduced into MRP1, this element alone is sufficient to result in partial apical localization. However, exclusive targeting to the apical membrane seems to require the integrity of the entire region encompassing MSD0 and CL3 of MRP2. Because CL3 of MRP1 is critical for binding, transport, or both of several compounds, we also examined the function of hybrids containing all, or portions of MRP2 MSD0 and CL3. Our results indicate that CL3 is important for interaction with both the glutathione and glucuronide conjugates tested, but that different regions may be involved.

Regulation of function by dimerization through the amino-terminal membrane-spanning domain of human ABCC1/MRP1

Overexpression of some ATP-binding cassette (ABC) membrane transporters such as ABCB1/P-glycoprotein/MDR1 and ABCC1/MRP1 causes multidrug resistance in cancer chemotherapy. It has been thought that half-ABC transporters with one nucleotide-binding domain and one membrane-spanning domain (MSD) likely work as dimers, whereas full-length transporters with two nucleotide-binding domains and two or three MSDs function as monomers. In this study, we examined the oligomeric status of the human full-length ABC transporter ABCC1/MRP1 using several biochemical approaches. We found 1) that it is a homodimer, 2) that the dimerization domain is located in the amino-terminal MSD0L0 (where L0 is loop 0) region, and 3) that MSD0L0 has a dominant-negative function when coexpressed with wild-type ABCC1/MRP1. These findings suggest that ABCC1/MRP1 may exist and function as a dimer and that MSD0L0 likely plays some structural and regulatory functions. It is also tempting to propose that the MSD0L0-mediated dimerization may be targeted for therapeutic development to sensitize ABCC1/MRP1-mediated drug resistance in cancer chemotherapy.

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

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

Insight in eukaryotic ABC transporter function by mutation analysis

With regard to structure-function relations of ATP-binding cassette (ABC) transporters several intriguing questions are in the spotlight of active research: Why do functional ABC transporters possess two ATP binding and hydrolysis domains together with two ABC signatures and to what extent are the individual nucleotide-binding domains independent or interacting? Where is the substrate-binding site and how is ATP hydrolysis functionally coupled to the transport process itself? Although much progress has been made in the elucidation of the three-dimensional structures of ABC transporters in the last years by several crystallographic studies including novel models for the nucleotide hydrolysis and translocation catalysis, site-directed mutagenesis as well as the identification of natural mutations is still a major tool to evaluate effects of individual amino acids on the overall function of ABC transporters. Apart from alterations in characteristic sequence such as Walker A, Walker B and the ABC signature other parts of ABC proteins were subject to detailed mutagenesis studies including the substrate-binding site or the regulatory domain of CFTR. In this review, we will give a detailed overview of the mutation analysis reported for selected ABC transporters of the ABCB and ABCC subfamilies, namely HsCFTR/ABCC7, HsSUR/ABCC8,9, HsMRP1/ABCC1, HsMRP2/ABCC2, ScYCF1 and P-glycoprotein (Pgp)/MDR1/ABCB1 and their effects on the function of each protein.

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.

Analysis of human multidrug resistance protein 1 (ABCC1) by matrix-assisted laser desorption ionization/time of flight mass spectrometry: toward identification of leukotriene C4 binding sites

Multidrug resistance in tumor cells may be caused by reduced drug accumulation resulting from expression of one or more proteins belonging to the ATP-binding cassette (ABC) transporter superfamily. In addition to their drug efflux properties, certain ABC proteins such as multidrug resistance protein 1 (MRP1) (ABCC1) mediate the ATP-dependent transport of a broad array of organic anions. The intrinsically photoreactive glutathione-conjugated cysteinyl leukotriene C4 (LTC4) is a high-affinity physiological substrate of MRP1 and is widely regarded as a model compound for evaluating the substrate binding and transport properties of wild-type and mutant forms of the transporter. In the present study, we have optimized high-level expression of recombinant human MRP1 in Pichia pastoris and developed a two-step purification scheme that results in purification of the transporter to >90% homogeneity. Peptide mapping by matrix-assisted laser desorption ionization/time of flight mass spectrometry of the peptides generated by in-gel protease digestions of purified underglycosylated MRP1 identified 96.7% of the MRP1 sequence with >98% coverage of its 17 transmembrane helices. Subsequent comparisons with mass spectra of MRP1 photolabeled with LTC4 identified six candidate LTC4-modified peptide fragments that are consistent with the conclusion that the intracellular juxtamembrane positions of transmembrane helices 6, 7, 10, 17, and a COOH-proximal portion of the cytoplasmic loop that links the first and second membrane spanning domains are part of the LTC4 binding site of the transporter. Our studies confirm the usefulness of mass spectrometry for analysis of mammalian polytopic membrane proteins and for identification of substrate binding sites of human MRP1.

Functional importance of three basic residues clustered at the cytosolic interface of transmembrane helix 15 in the multidrug and organic anion transporter MRP1 (ABCC1)

The multidrug resistance protein 1 (MRP1) mediates drug and organic anion efflux across the plasma membrane. The 17 transmembrane (TM) helices of MRP1 are linked by extracellular and cytoplasmic (CL) loops of various lengths and two cytoplasmic nucleotide binding domains. In this study, three basic residues clustered at the predicted TM15/CL7 interface were investigated for their role in MRP1 expression and activity. Thus, Arg1138, Lys1141, and Arg1142 were replaced with residues of the same or opposite charge, expressed in human embryonic kidney cells, and the properties of the mutant proteins were assessed. Neither Glu nor Lys substitutions of Arg1138 and Arg1142 affected MRP1 expression; however, all four mutants showed a decrease in organic anion transport with a relatively greater decrease in leukotriene C4 and glutathione transport. These mutations also modulated MRP1 ATPase activity as reflected by a decreased vanadate-induced trapping of 8-azido-[32P]ADP. Mutation of Lys1141 to either Glu or Arg reduced MRP1 expression, and routing to the plasma membrane was impaired. However, only the Glu-substituted Lys1141 mutant showed a decrease in organic anion transport, and this was associated with decreased substrate binding and vanadate-induced trapping of 8-azido-ADP. These studies identified a cluster of basic amino acids likely at the TM15/CL7 interface as a region important for both MRP1 expression and activity and demonstrated that each of the three residues plays a distinct role in the substrate specificity and catalytic activity of the transporter.

Role of the NH2-terminal membrane spanning domain of multidrug resistance protein 1/ABCC1 in protein processing and trafficking

Multidrug resistance protein (MRP)1/ABCC1 transports organic anionic conjugates and confers resistance to cytotoxic xenobiotics. In addition to two membrane spanning domains (MSDs) typical of most ATP-binding cassette (ABC) transporters, MRP1 has a third MSD (MSD0) of unknown function. Unlike some topologically similar ABCC proteins, removal of MSD0 has minimal effect on function, nor does it prevent MRP1 from trafficking to basolateral membranes in polarized cells. However, we find that independent of cell type, the truncated protein accumulates in early/recycling endosomes. Using a real-time internalization assay, we demonstrate that MSD0 is important for MRP1 retention in, or recycling to, the plasma membrane. We also show that MSD0 traffics independently to the cell surface and promotes membrane localization of the core-region of MRP1 when the two protein fragments are coexpressed. Finally, we demonstrate that MSD0 becomes essential for trafficking of MRP1 when the COOH-terminal region of the protein is mutated. These studies demonstrate that MSD0 and the COOH-terminal region contain redundant trafficking signals, which only become essential when one or the other region is missing or is mutated. These data explain apparent differences in the trafficking requirement for MSD0 and the COOH-terminal region of MRP1 compared with other ABCC proteins.

Identification of proline residues in the core cytoplasmic and transmembrane regions of multidrug resistance protein 1 (MRP1/ABCC1) important for transport function, substrate specificity, and nucleotide interactions

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-binding cassette transporter that confers resistance to drugs and mediates the transport of organic anions. MRP1 has a core structure of two membrane spanning domains (MSDs) each followed by a nucleotide binding domain. This core structure is preceded by a third MSD with five transmembrane (TM) helices, whereas MSD2 and MSD3 each contain six TM helices. We investigated the consequences of Ala substitution of 18 Pro residues in both the non-membrane and TM regions of MSD2 and MSD3 on MRP1 expression and organic anion transport function. All MRP1-Pro mutants except P1113A were expressed in human embryonic kidney cells at levels comparable with wild-type MRP1. In addition, five mutants containing substitutions of Pro residues in or proximal to the TM helices of MSD2 (TM6-Pro(343), TM8-Pro(448), TM10-Pro(557), and TM11-Pro(595)) and MSD3 (TM14-Pro(1088)) exhibited significantly reduced transport of five organic anion substrates. In contrast, mutation of Pro(1150) in the cytoplasmic loop (CL7) linking TM15 to TM16 caused a substantial increase in 17beta-estradiol-17-beta-(D-glucuronide) and methotrexate transport, whereas transport of other organic anions was reduced or unchanged. Significant substrate-specific changes in the ATP dependence of transport and binding by the P1150A mutant were also observed. Our findings demonstrate the importance of TM6, TM8, TM10, TM11, and TM14 in MRP1 transport function and suggest that CL7 may play a differential role in coupling the activity of the nucleotide binding domains to the translocation of different substrates across the membrane.