Molecular cloning and pharmacological characterization of rat multidrug resistance protein 1 (mrp1)

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.

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

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 LTC₄ vs. 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).

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

Multidrug resistance-associated protein 1 decreases the concentrations of antiepileptic drugs in cortical extracellular fluid in amygdale kindling rats

AIM

To investigate whether multidrug resistance-associated protein 1 (MRP1) was responsible for drug resistence in refractory epilepsy in amygdale kindling rats.

METHODS

Rat amygdale kindling was used as a model of refractory epilepsy. The expression of MRP1 mRNA and protein in the brains was examined using RT-PCR and Western blot. MRP1-positive cells in the cortex and hippocampus were studied with immunohistochemical staining. The rats were intraperitoneally injected with phenytoin (50 mg/kg) or carbamazepine (20 mg/kg), and their concentrations in the cortical extracellular fluid were measured using microdialysis and HPLC. Probenecid, a MRP1 inhibitor (40 mmol/L, 50 μL) was administered through an inflow tube into the cortex 30 min before injection of the antiepileptic drugs.

RESULTS

The expression of MRP1 mRNA and protein was significantly up-regulated in the cortex and hippocampus in amygdale kindling rats compared with the control group. Furthermore, the number of MRP1-positive cells in the cortex and hippocampus was also significantly increased in amygdale kindling rats. Microdialysis studies showed that the concentrations of phenytoin and carbamazepine in the cortical extracellular fluid were significantly decreased in amygdale kindling rats. Pre-administration of probenecid could restore the concentrations back to their control levels.

CONCLUSION

Up-regulation of MRP1 is responsible for the resistance of brain cells to antiepileptic drugs in the amygdale kindling rats.

Identification of transporters associated with Etoposide sensitivity of stomach cancer cell lines and methotrexate sensitivity of breast cancer cell lines by quantitative targeted absolute proteomics

Membrane transporter proteins may influence the sensitivity of cancer cells to anticancer drugs that can be recognized as substrates. The purpose of this study was to identify proteins that play a key role in the drug sensitivity of stomach and breast cancer cell lines by measuring the absolute protein expression levels of multiple transporters and other membrane proteins and examining their correlation to drug sensitivity. Absolute protein expression levels of 90 membrane proteins were examined by quantitative targeted absolute proteomics using liquid chromatography-linked tandem mass spectrometry. Among them, 11 and 14 membrane proteins, including transporters, were present in quantifiable amounts in membrane fraction of stomach cancer and breast cancer cell lines, respectively. In stomach cancer cell lines, the protein expression level of multidrug resistance-associated protein 1 (MRP1) was inversely correlated with etoposide sensitivity. MK571, an MRP inhibitor, increased both the cell-to-medium ratio of etoposide and the etoposide sensitivity of MRP1-expressing stomach cancer cell lines. In breast cancer cell lines, the protein expression level of reduced folate carrier 1 (RFC1) was directly correlated with methotrexate (MTX) sensitivity. Initial uptake rate and steady-state cell-to-medium ratio of [(3)H]MTX were correlated with both RFC1 expression level and MTX sensitivity. These results suggest that MRP1 modulates the etoposide sensitivity of stomach cancer cell lines and RFC1 modulates the MTX sensitivity of breast cancer cell lines. Our results indicate that absolute quantification of multiple membrane proteins could be a useful strategy for identification of candidate proteins involved in drug sensitivity.

Implications of ABC transporters on the disposition of typical veterinary medicinal products

The ATP-Binding Cassette (ABC) transporters ABCB1, ABCC2 and ABCG2 are efflux transporters that facilitate the excretion of drugs, contribute to the function of biological barriers and maintain low cytoplasmic substrate concentrations in cells. ABC transporters modulate drug absorption, distribution and elimination according to the level of expression in the intestine, liver, kidney, and at biological barriers such as the blood-brain barrier. Moreover individual transporters are known to convey multi-drug resistance to tumour cells. While these diverse functions have been described in laboratory animal studies and in humans, the available information is very limited in animal species that are typical veterinary patients. This brief review summarizes the available data on organ distribution and expression levels in animals, genetic defects in dogs resulting in a non-functional P-gp expression, and describes examples of kinetic investigations directed to assess the clinical relevance of species differences in ABC-transporter expression.

Increased Estrogen Sulfation of Estradiol 17β-D-Glucuronide in Metastatic Tumor Rat Livers

Changes in the disposition of estradiol 17beta-d-glucuronide (E(2)17G), a substrate of the organic anion-transporting polypeptide family (Oatp) and multidrug resistance-associated protein 2 (Mrp2), were examined in livers of male Wag/Rij rats that were injected with CC531 cells intraportally to induce metastatic tumors (n = 5) or with phosphate-buffered saline for sham-operated controls (n = 4). Multiple indicator dilution, single-pass liver perfusions revealed extremely high influx clearances of [(3)H]E(2)17G (>190 ml/min) in both groups. In recirculating liver perfusions, [(3)H]E(2)17G decayed monoexponentially in the reservoir perfusate, and the total (9.19 +/- 1.33 versus 8.18 +/- 0.94 ml/min) and biliary (4.94 +/- 1.07 versus 4.60 +/- 0.86 ml/min) clearances were similar in both groups (P > 0.05). The metabolic clearance of E(2)17G was higher in the tumor group (4.60 +/- 0.64 versus 3.23 +/- 0.23 ml/min, P < 0.05). E(2)3S17G, the 3-sulfate metabolite, whose identity was confirmed by mass spectrometry, appeared only in bile and not perfusate. Liver microsomal incubations of E(2)3(35)S17G and [(3)H]estrone sulfate revealed similar sulfatase activities between the tumor and sham livers, albeit the activities were much lower for E(2)3(35)S17G. Oatp1a1 and Oatp1b2 protein expression in liver membrane fragments was reduced by 42% and 38%, respectively, whereas that of cytosolic estrogen sulfotransferase (Sult1e1) was significantly increased (41%) with tumor (P < 0.05). All of the observations were captured by modeling. From modeling, we showed that reduction of the high influx clearance (546 to 283 ml/min) failed to lower the total clearance of E(2)17G, whereas up-regulation of Sult1e1 increased the E(2)17G sulfation clearance (2.56 to 3.69 ml/min) in livers with metastatic tumors.

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.

Functional analysis of the extracellular cysteine residues in the human organic anion transporting polypeptide, OATP2B1

Organic anion transporting polypeptide (OATP) superfamily member 2B1 (OATP2B1) mediates the uptake of steroid hormone precursors and selected drugs in the placenta, liver, mammary gland, brain, and intestine. This action is modulated by sulfhydryl reagents. Common to all OATPs is a large extracellular loop between transmembrane domains IX and X with 10 conserved cysteines. To elucidate the structure-function relationship of this cysteine rich ectodomain, a truncated OATP2B1 lacking 10 extracellular cysteines (OATP2B1(Delta489-557)) and 10 OATP2B1 mutants containing individual Cys-to-Ala substitutions were generated and expressed in CHO-K1 cells. The immunolocalization, cell-surface expression, transport activity, and free cysteine labeling with N-biotinoylaminoethylmethane-thiosulfonate of mutant proteins and wild-type OATP2B1 were compared. OATP2B1(Delta489-557) accumulated intracellularly. Nine Cys-to-Ala substitutions, C489A, C495A, C504A, C516A, C520A, C539A, C541A, C553A, and C557A, were misprocessed, appearing predominantly as core-glycosylated, 60-kDa proteins and as 180-kDa complexes. Only C493A was a fully glycosylated 75-kDa protein expressed at the cell surface. Thapsigargin partially corrected the misprocessing of mutants. Compared with OATP2B1, C493A and C557A transported estrone-3-sulfate and dehydroepiandrosterone sulfate less efficiently, whereas all other mutants were functionally impaired. MTSEA labeled free cysteines in all Cys-to-Ala mutants but not in OATP2B1, suggesting that all 10 extracellular cysteines are normally disulfide-bonded. Our findings show that the trafficking and function of OATP2B1 is vulnerable to changes in the cysteine residues of extracellular loop IX-X.

Multidrug Resistance-Associated Proteins: Expression and Function in the Central Nervous System

Drug delivery to the brain is highly restricted, since compounds must cross a series of structural and metabolic barriers to reach their final destination, often a cellular compartment such as neurons, microglia, or astrocytes. The primary barriers to the central nervous system are the blood-brain and blood-cerebrospinal fluid barriers. Through structural modifications, including the presence of tight junctions that greatly limit paracellular transport, the cells that make up these barriers restrict diffusion of many pharmaceutically active compounds. In addition, the cells that comprise the blood-brain and blood-cerebrospinal fluid barriers express multiple ATP-dependent, membrane-bound, efflux transporters, such as members of the multidrug resistance-associated protein (MRP) family, which contribute to lowered drug accumulation. A relatively new concept in brain drug distribution just beginning to be explored is the possibility that cellular components of the brain parenchyma could act as a "second" barrier to brain permeation of pharmacological agents via expression of many of the same transporters. Indeed, efflux transporters expressed in brain parenchyma may facilitate the overall export of xenobiotics from the central nervous system, essentially handing them off to the barrier tissues. We propose that these primary and secondary barriers work in tandem to limit overall accumulation and distribution of xenobiotics in the central nervous system. The present review summarizes recent knowledge in this area and emphasizes the clinical significance of MRP transporter expression in a variety of neurological disorders.

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.

Isoform-specific expression and induction of udp-glucuronosyltransferase in immunoactivated peritoneal macrophages of the rat

Phase I drug-metabolizing enzymes such as cytochrome P450 in immunocytes are known to play a role in metabolic activation of toxic and immunosuppressive compounds such as polycyclic aromatic hydrocarbon (PAH). UDP-glucuronosyltransferase (UGT), a drug-metabolizing phase II enzyme, accelerates elimination of these compounds; however, there is little information on the expression and function of UGT in immunocytes. In this study, we investigated the expressions of UGT isoforms in rat peritoneal macrophages and the role of UGT in macrophage functions. Expressions of UGT1A1, 1A6, and 1A7 were observed in macrophages by immunohistochemical staining and reverse transcriptase-polymerase chain reaction. When macrophage cells cultured in plates were exposed to 1-naphthol and 3-hydroxybenzo-[a]pyrene (3-OH-B[a]P), these glucuronides increased in the medium, indicating that macrophages glucuronidated the chemicals. The production of the glucuronides of 1-naphthol and 3-OH-B[a]P was induced by lipopolysaccharide (LPS) treatment of the cultured macrophage cells. Northern blot analysis revealed that UGT1A7 mRNA was induced by LPS treatment. This result is the first evidence that a drug-metabolizing enzyme is induced by immunoactivation. The results indicated that macrophages can detoxify various toxic and immunosuppressive compounds with UGT, and that ability is enhanced by immunoactivation. We propose that macrophages contribute to protection against not only macromolecules as immunocytes but also small molecules such as the immunosuppressive agents PAHs in peripheral blood and interstitial tissues.

Multidrug resistance proteins: role of P-glycoprotein, MRP1, MRP2, and BCRP (ABCG2) in tissue defense

In tumor cell lines, multidrug resistance is often associated with an ATP-dependent decrease in cellular drug accumulation which is attributed to the overexpression of certain ATP-binding cassette (ABC) transporter proteins. ABC proteins that confer drug resistance include (but are not limited to) P-glycoprotein (gene symbol ABCB1), the multidrug resistance protein 1 (MRP1, gene symbol ABCC1), MRP2 (gene symbol ABCC2), and the breast cancer resistance protein (BCRP, gene symbol ABCG2). In addition to their role in drug resistance, there is substantial evidence that these efflux pumps have overlapping functions in tissue defense. Collectively, these proteins are capable of transporting a vast and chemically diverse array of toxicants including bulky lipophilic cationic, anionic, and neutrally charged drugs and toxins as well as conjugated organic anions that encompass dietary and environmental carcinogens, pesticides, metals, metalloids, and lipid peroxidation products. P-glycoprotein, MRP1, MRP2, and BCRP/ABCG2 are expressed in tissues important for absorption (e.g., lung and gut) and metabolism and elimination (liver and kidney). In addition, these transporters have an important role in maintaining the barrier function of sanctuary site tissues (e.g., blood-brain barrier, blood-cerebral spinal fluid barrier, blood-testis barrier and the maternal-fetal barrier or placenta). Thus, these ABC transporters are increasingly recognized for their ability to modulate the absorption, distribution, metabolism, excretion, and toxicity of xenobiotics. In this review, the role of these four ABC transporter proteins in protecting tissues from a variety of toxicants is discussed. Species variations in substrate specificity and tissue distribution of these transporters are also addressed since these properties have implications for in vivo models of toxicity used for drug discovery and development.

Development and characterization of a recombinant Madin-Darby canine kidney cell line that expresses rat multidrug resistance-associated protein 1 (rMRP1)

Multidrug resistance-associated protein 1 (MRP1) is one of the major proteins shown to mediate efflux transport of a broad range of antitumor drugs, glucuronide conjugates, and glutathione, in addition to endogenous substrates. Significant differences in substrate selectivity were reported for murine and human MRP1. As preclinical drug disposition and pharmacokinetics studies are often conducted in rats, we have recently cloned the rat MRP1 (rMRP1) and demonstrated that rMRP1 expressed in transfected cells effluxes calcein, a commonly used fluorescence substrate for human MRP1. To further characterize the rat ortholog of MRP1, we isolated a cell line stably expressing recombinant rMRP1. These cells were tested for their ability to transport calcein and a range of chemotherapeutic drugs. Our results showed that cells expressing rMRP1 consistently efflux calcein at a rate 5-fold greater than control cells. The rMRP1 transfected cells, like their human ortholog, can confer drug resistance to vinca alkaloid (vinblastine and vincristine) and anthracycline drugs (daunorubcin and doxorubicin), and the resistance conferred by the MRP1 can be partially abolished by the MRP-specific inhibitors. The transepithelial permeability due to rMRP1 expression in differentiated Madin-Darby canine kidney cells (MDCK) cells was also investigated. The MRP1 transport activity is directional, as demonstrated by directional vinblastine transport. Collectively, our results demonstrate that the cellular expression of rMRP1, like its human ortholog, could confer resistance to anticancer drugs.

Evaluation of the binding of the tricyclic isoxazole photoaffinity label LY475776 to multidrug resistance associated protein 1 (MRP1) orthologs and several ATP-binding cassette (ABC) drug transporters

Several of the ATP-binding cassette (ABC) transporters confer resistance to anticancer agents and/or antiviral agents when overexpressed in drug-sensitive cells. Recently a MRP1 (ABCC1) tricyclic isoxazole inhibitor, LY475776 was shown to be a glutathione-dependent photoaffinity label of human MRP1 and showed poor labeling of murine mrp1, an ortholog that does not confer anthracycline resistance. In the present study, the specificity of LY475776 was examined for its ability to modulate or photolabel orthologs of MRP1 and several other drug efflux transporters of the ABC transporter family. LY475776 modulated MRP1 and Pgp-mediated resistance (MDR, ABCB1) in, respectively, HeLa-T5 and CEM/VLB(100) cells to both vincristine and doxorubicin. LY475776 photolabeled 170kDa Pgp and was inhibited by the potent Pgp inhibitor LY335979 (Zosuquidar.3HCl). The labeling of the 190kDa MRP1 protein in membranes of HeLa-T5 cells was inhibited by substrates of MRP1 such as leukotriene C(4), vincrisine, and doxorubicin and by the inhibitor, MK571. LY475776 did not photolabel human MRP2 (ABCC2), MRP3 (ABCC3), MRP5 (ABCC5) or breast cancer resistance protein (ABCG2). Because LY475776 photolabels murine mrp1 less well than human MRP1 and binds to a region believed important for anthracycline binding, studies were conducted with monkey and canine MRP1 which also show a reduced ability to confer resistance to anthracyclines. Unlike murine mrp1, both orthologs were photolabeled well by LY475776. These studies indicate that the specificity of LY475776 is fairly limited to Pgp and MRP1 and further studies will help to define the binding regions.

Selective sterol accumulation in ABCG5/ABCG8-deficient mice

The ATP binding cassette (ABC) transporters ABCG5 and ABCG8 limit intestinal absorption and promote biliary secretion of neutral sterols. Mutations in either gene cause sitosterolemia, a rare recessive disease in which plasma and tissue levels of several neutral sterols are increased to varying degrees. To determine why patients with sitosterolemia preferentially accumulate noncholesterol sterols, levels of cholesterol and the major plant sterols were compared in plasma, liver, bile, and brain of wild-type and ABCG5/ABCG8-deficient (G5G8(-/-)) mice. The total sterol content of liver and plasma was similar in G5G8(-/-) mice and wild-type animals despite an approximately 30-fold increase in noncholesterol sterol levels in the knockout animals. The relative enrichment of each sterol in the plasma and liver of G5G8(-/-) mice (stigmasterol > sitosterol = cholestanol > bassicasterol > campesterol > cholesterol) reflected its relative enrichment in the bile of wild-type mice. These results indicate that 24-alkylated, Delta22, and 5alpha-reduced sterols are preferentially secreted into bile and that preferential biliary secretion of noncholesterol sterols by ABCG5 and ABCG8 prevents the accumulation of these sterols in normal animals. The mRNA levels for 13 enzymes in the cholesterol biosynthetic pathway were reduced in the livers of the G5G8(-/-) mice, despite a 50% reduction in hepatic cholesterol level. Thus, the accumulation of sterols other than cholesterol is sensed by the cholesterol regulatory machinery.

Functional importance of polar and charged amino acid residues in transmembrane helix 14 of multidrug resistance protein 1 (MRP1/ABCC1): identification of an aspartate residue critical for conversion from a high to low affinity substrate binding state

Human multidrug resistance protein 1 (MRP1) confers resistance to many chemotherapeutic agents and transports diverse conjugated organic anions. We previously demonstrated that Glu1089 in transmembrane (TM) 14 is critical for the protein to confer anthracycline resistance. We have now assessed the functional importance of all polar and charged amino acids in this TM helix. Asn1100, Ser1097, and Lys1092, which are all predicted to be on the same face of the helix as to Glu1089, are involved in determining the substrate specificity of the protein. Notably, elimination of the positively charged side chain of Lys1092, increased resistance to the cationic drugs vincristine and doxorubicin, but not the electroneutral drug etoposide (VP-16). In addition, mutations S1097A and N1100A selectively decreased transport of 17beta-estradiol 17-(beta-d-glucuronide) (E217betaG) but not cysteinyl leukotriene 4 (LTC4), demonstrating the importance of multiple residues in this helix in determining substrate specificity. In contrast, mutations of Asp1084 that eliminate the carboxylate side chain markedly decreased resistance to all drugs tested, as well as transport of both E217betaG and LTC4, despite the fact that LTC4 binding was unaffected. We show that these mutations prevent the ATP-dependent transition of the protein from a high to low affinity substrate binding state and drastically diminish ADP trapping at nucleotide binding domain 2. Based on results presented here and crystal structures of prokaryotic ATP binding cassette transporters, Asp1084 may be critical for interaction between the cytoplasmic loop connecting TM13 and TM14 and a region of nucleotide binding domain 2 between the conserved Walker A and ABC signature motifs.