Functional analysis of MRP1 cloned from bovine

A macrocyclic peptide inhibitor traps MRP1 in a catalytically incompetent conformation

Adenosine triphosphate-binding cassette (ABC) transporters, such as multidrug resistance protein 1 (MRP1), protect against cellular toxicity by exporting xenobiotic compounds across the plasma membrane. However, constitutive MRP1 function hinders drug delivery across the blood-brain barrier, and MRP1 overexpression in certain cancers leads to acquired multidrug resistance and chemotherapy failure. Small-molecule inhibitors have the potential to block substrate transport, but few show specificity for MRP1. Here we identify a macrocyclic peptide, named CPI1, which inhibits MRP1 with nanomolar potency but shows minimal inhibition of a related multidrug transporter P-glycoprotein. A cryoelectron microscopy (cryo-EM) structure at 3.27 Å resolution shows that CPI1 binds MRP1 at the same location as the physiological substrate leukotriene C4 (LTC4). Residues that interact with both ligands contain large, flexible sidechains that can form a variety of interactions, revealing how MRP1 recognizes multiple structurally unrelated molecules. CPI1 binding prevents the conformational changes necessary for adenosine triphosphate (ATP) hydrolysis and substrate transport, suggesting it may have potential as a therapeutic candidate.

Insights into Aflatoxin B1 Toxicity in Cattle: An In Vitro Whole-Transcriptomic Approach

Aflatoxins, and particularly aflatoxin B1 (AFB1), are toxic mycotoxins to humans and farm animal species, resulting in acute and chronic toxicities. At present, AFB1 is still considered a global concern with negative impacts on health, the economy, and social life. In farm animals, exposure to AFB1-contaminated feed may cause several untoward effects, liver damage being one of the most devastating ones. In the present study, we assessed in vitro the transcriptional changes caused by AFB1 in a bovine fetal hepatocyte-derived cell line (BFH12). To boost the cellular response to AFB1, cells were pre-treated with the co-planar PCB 3,3′,4,4′,5-pentachlorobiphenyl (PCB126), a known aryl hydrocarbon receptor agonist. Three experimental groups were considered: cells exposed to the vehicle only, to PCB126, and to PCB126 and AFB1. A total of nine RNA-seq libraries (three replicates/group) were constructed and sequenced. The differential expression analysis showed that PCB126 induced only small transcriptional changes. On the contrary, AFB1 deeply affected the cell transcriptome, the majority of significant genes being associated with cancer, cellular damage and apoptosis, inflammation, bioactivation, and detoxification pathways. Investigating mRNA perturbations induced by AFB1 in cattle BFH12 cells will help us to better understand AFB1 toxicodynamics in this susceptible and economically important food-producing species.

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).

Structural Basis of Substrate Recognition by the Multidrug Resistance Protein MRP1

The multidrug resistance protein MRP1 is an ATP-binding cassette (ABC) transporter that confers resistance to many anticancer drugs and plays a role in the disposition and efficacy of several opiates, antidepressants, statins, and antibiotics. In addition, MRP1 regulates redox homeostasis, inflammation, and hormone secretion. Using electron cryomicroscopy, we determined the molecular structures of bovine MRP1 in two conformations: an apo form at 3.5 Å without any added substrate and a complex form at 3.3 Å with one of its physiological substrates, leukotriene C₄. These structures show that by forming a single bipartite binding site, MRP1 can recognize a spectrum of substrates with different chemical structures. We also observed large conformational changes induced by leukotriene C₄, explaining how substrate binding primes the transporter for ATP hydrolysis. Structural comparison of MRP1 and P-glycoprotein advances our understanding of the common and unique properties of these two important molecules in multidrug resistance to chemotherapy.

Tissue distribution and phenobarbital induction of target SLC- and ABC- transporters in cattle

In veterinary pharmaco-toxicological sciences, few data about uptake and efflux drug transporters (DTs) expression and regulation phenomena have been published. In this study, the tissue distribution and transcriptional modulation of solute carrier (SLC) and ATP-binding cassette (ABC) DTs were investigated in cattle orally administered with phenobarbital (PB) by using a quantitative real-time RT-PCR approach. The criterion for target gene selection was the PB-responsiveness in human and rodent model species. All target DTs were expressed in the liver. Only two of the seven PB-responsive target DTs (SLCO1B3 and SLC10A1) were not constitutively expressed in cattle extra-hepatic tissues. The greatest number of DTs (SLCO2B1, ABCB1, ABCC2, ABCG2) were expressed in intestine and testis, followed by, adrenal gland (SLCO2B1, ABCB1, ABCG2), lung (ABCB1, ABCG2), kidney, and skeletal muscle (ABCG2). PB administration never altered DTs mRNA levels, except for an increase in hepatic ABCC2 mRNA and a down-regulation of renal ABCG2. Altogether, these results confirm only to some extent data obtained in humans and laboratory species; clearly, they should be considered a preliminary step for further molecular investigations about species-differences in DT gene expression and regulation as well as in DT expression and function.

Differential expression of ABC transporters and their regulatory genes during lactation and dry period in bovine mammary tissue

ATP-binding cassette (ABC) transporters play a pivotal role in human physiology, and mutations in these genes often result in severe hereditary diseases. ABC transporters are expressed in the bovine mammary gland but their physiological role in this organ remains elusive. Based on findings in the context of human disorders we speculated that candidate ABC transporters are implicated in lipid and cholesterol transport in the mammary gland. Therefore we investigated the expression pattern of selected genes that are associated with sterol transport in lactating and nonlactating mammary glands of dairy cows. mRNA levels from mammary gland biopsies taken during lactation and in the first and second week of the dry period were analysed using quantitative PCR. Five ABC transporter genes, namely ABCA1, ABCA7, ABCG1, ABCG2 and ABCG5, their regulating genes LXRalpha, PPARgamma, SREBP1 and the milk proteins lactoferrin and alpha-lactalbumin were assessed. A significantly enhanced expression in the dry period was observed for ABCA1 while a significant decrease of expression in this period was detected for ABCA7, ABCG2, SREBP1 and alpha-lactalbumin. ABCG1, ABCG5, LXRalpha, PPARgamma and lactoferrin expression was not altered between lactation and dry period. These results indicate that candidate ABC transporters involved in lipid and cholesterol transport show differential mRNA expression between lactation and the dry period. This may be due to physiological changes in the mammary gland such as immigration of macrophages or the accumulation of fat due to the loss of liquid in the involuting mammary gland. The current mRNA expression analysis of transporters in the mammary gland is the prerequisite for elucidating novel molecular mechanisms underlying cholesterol and lipid transfer into milk.

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.

Identification of the bovine cholesterol efflux regulatory protein ABCA1 and its expression in various tissues1

The ATP-binding cassette transporter A1 (ABCA1) is known to play a significant role in cellular export of phospholipids and cholesterol in humans. The ABCA1 transporter might also play a crucial role in cellular cholesterol homeostasis in the cow or in the transfer of cholesterol into the milk, but its presence and tissue distribution in the bovine is unknown. Therefore, we studied the expression and distribution of the bovine ABCA1 transporter using quantitative PCR and sequenced the entire ABCA1 coding region. In addition, the proximal promoter was identified and screened for regulatory elements. Concordant with data from other mammalian species, bovine ABCA1 mRNA was expressed and detected in all tissues tested. The highest expression levels were detected in lung, esophagus, uterus, spleen, and muscle. Sequence analysis revealed that the open reading frame of this gene consists of 6,786 bases and encodes for a protein of 2,261 AA with a predicted molecular weight of 254 kDa. The deduced bovine ABCA1 protein shows the highest AA sequence homology with human (94%), mouse (93%), rat (92%), and chicken (85%). Analysis of the putative ABCA1 promoter region revealed potential transcription factor binding sites associated with ABCA1 transcription and lipid metabolism. This work could open new avenues for elucidating a potential role of ABCA1 in sterol homeostasis in the bovine organism.

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.

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.

Valproic acid uptake by bovine brain microvessel endothelial cells: role of active efflux transport

The basis for low brain permeability of valproic acid (VPA) appears to be the result of efflux transport at the blood-brain barrier (BBB); however, the identity of the putative efflux transporter has not been investigated. The objective of our studies was to determine whether the multidrug resistance-associated protein (MRP) might be involved in efflux transport of VPA. Brain microvessel endothelial cells (BMEC) were isolated from cow brains and grown to confluence. MRP messenger RNA (mRNA) in BMEC were verified by reverse transcriptase-polymerase chain reaction (RT-PCR). Functional activity was demonstrated using the steady-state retention of calcein and MRP inhibitors, indomethacin (IND) and probenecid (PRB). Probenecid (0.50 mM) and indomethacin (10 microM) produced a 26 and 13% ( P<0.05 ) elevation in steady-state cellular VPA uptake following a 30-min-incubation with tracer 3H-VPA and 30 microM cold VPA. In contrast, at higher concentrations of probenecid (2 mM) and indomethacin (500 microM), an 11 and 31% reduction in VPA uptake was observed. The biphasic pattern of VPA uptake suggested concurrent inhibition of uptake and efflux transporters by the inhibitor with differing sensitivities, i.e. the efflux transporter being more susceptible to inhibition than the influx transporter. Similar results were obtained in the MRP overexpressing cell line A549. Overall, the results suggest that MRP(s) is(are) involved in the efflux transport of VPA, but do not preclude the possible contribution(s) of other organic anion transporters. The findings also adds to the growing evidence that up-regulation of active drug efflux transporters at the BBB may contribute to the development of drug resistance to antiepileptic drug therapy.

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.

Molecular identification and characterization of rat Abcc1 cDNA: existence of two splicing variants and species difference in drug-resistance profile

The human ABCC1 gene, a member of the ATP-binding cassette transporter super-family, plays a critical role in conferring cancer cell resistance to chemotherapeutic drugs. In the present study, we have cloned the full-length cDNA of rat Abcc1 and evaluated its significance in drug resistance. Analysis using the currently available genome database revealed that the rat Abcc1 gene is located on rat chromosome 13 and consists of at least 30 exons. The rat Abcc1 cDNA cloned from the spleen was 4981-bp long, within which two additional splicing variants were discovered. The rat Abcc1 gene is expressed in a wide variety of organs, with the highest expression being observed in the spleen. Human embryonic kidney 293 cells were transfected with the rat Abcc1/pcDNA3.1 vector to stably express rat Abcc1. Overexpression of rat Abcc1 elicited high resistance to etoposide. In contrast to the hitherto known drug-resistance profile of human ABCC1, rat Abcc1 did not significantly confer cellular resistance to anthracyclins or Vinca alkaloids. Our results strongly suggest that there is a significant species difference between human ABCC1 and rat Abcc1 in their contribution to the drug-resistance profile.

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

Multidrug resistance protein 1 (MRP1) transports a wide range of structurally diverse conjugated and nonconjugated organic anions and some peptides, including oxidized and reduced glutathione (GSH). The protein confers resistance to certain heavy metal oxyanions and a variety of natural product-type chemotherapeutic agents. Elevated levels of MRP1 have been detected in many human tumors, and the protein is a candidate therapeutic target for drug resistance reversing agents. Previously, we have shown that human MRP1 (hMRP1) and murine MRP1 (mMRP1) differ in their substrate specificity despite a high degree of structural conservation. Since rat models are widely used in the drug discovery and development stage, we have cloned and functionally characterized rat MRP1 (rMRP1). Like mMRP1 and in contrast to hMRP1, rMRP1 confers no, or very low, resistance to anthracyclines and transports the two estrogen conjugates, 17beta-estradiol-17-(beta-d-glucuronide) (E217betaG) and estrone 3-sulfate, relatively poorly. Mutational studies combined with vesicle transport assays identified several amino acids conserved between rat and mouse, but not hMRP1, that make major contributions to these differences in substrate specificity. Despite the fact that the rodent proteins transport E217betaG poorly and the GSH-stimulated transport of estrone 3-sulfate is low compared with hMRP1, site-directed mutagenesis studies indicate that different nonconserved amino acids are involved in the low efficiency with which each of the two estrogen conjugates is transported. Our studies also suggest that although rMRP1 and mMRP1 are 95% identical in primary structure, their substrate specificities may be influenced by amino acids that are not conserved between the two rodent proteins.