Exploring the structure and function of the P-glycoprotein multidrug transporter using fluorescence spectroscopic tools

Exploring the potential of P-glycoprotein inhibitors in the targeted delivery of anti-cancer drugs: A comprehensive review

Due to the high prevalence of cancer, progress in the management of cancer is the need of the hour. Most cancer patients develop chemotherapeutic drug resistance, and many remain insidious due to overexpression of Multidrug Resistance Protein 1 (MDR1), also known as Permeability-glycoprotein (P-gp) or ABCB1 transporter (ATP-binding cassette subfamily B member 1). P-gp, a transmembrane protein that protects vital organs from outside chemicals, expels medications from malignant cells. The blood-brain barrier (BBB), gastrointestinal tract (GIT), kidneys, liver, pancreas, and cancer cells overexpress P-gp on their apical surfaces, making treatment inefficient and resistant. Compounds that compete with anticancer medicines for transportation or directly inhibit P-gp may overcome biological barriers. Developing nanotechnology-based formulations may help overcome P-gp-mediated efflux and improve bioavailability and cell chemotherapeutic agent accumulation. Nanocarriers transport pharmaceuticals via receptor-mediated endocytosis, unlike passive diffusion, which bypasses ABCB1. Anticancer drugs and P-gp inhibitors in nanocarriers may synergistically increase drug accumulation and chemotherapeutic agent toxicity. The projection of desirable binding and effect may be procured initially by molecular docking of the inhibitor with P-gp, enabling the reduction of preliminary trials in formulation development. Here, P-gp-mediated efflux and several possible outcomes to overcome the problems associated with currently prevalent cancer treatments are highlighted.

Non-ionic Surfactants as a P-Glycoprotein(P-gp) Efflux Inhibitor for Optimal Drug Delivery-A Concise Outlook

Significant research efforts have been devoted to unraveling the mystery of P-glycoprotein(P-gp) in drug delivery applications. The efflux membrane transporter P-gp is widely distributed in the body and accountable for restricting drug absorption and bioavailability. For these reasons, it is the primary cause of developing multidrug resistance (MDR) in most drug delivery applications. Therefore, P-gp inhibitors must be explored to address MDR and the low bioavailability of therapeutic substrates. Several experimental models in kinetics and dynamic studies identified the sensitivity of drug molecules and excipients as a P-gp inhibitor. In this review, we aimed to emphasize nonionic surface-active agents for effective reversal of P-gp inhibition. As it is inert, non-toxic, noncharged, and quickly reaching the cytosolic lipid membrane (the point of contact with P-gp efflux protein) enables it to be more efficient as P-gp inhibitors. Moreover, nonionic surfactant improves drug absorption and bioavailability through the various mechanism, involving (i) association of drug with surfactant improves solubilization, facilitating its cell penetration and absorption; (ii) weakening the lateral membrane packing density, facilitating the passive drug influx; and (iii) inhibition of the ATP binding cassette of transporter P-glycoprotein. The application of nonionic surfactant as P-gp inhibitors is well established and supported by various experiments. Altogether, herein, we have primarily focused on various nonionic surfactants and their development strategies to conquer the MDR-causing effects of P-gp efflux protein in drug delivery. Graphical Abstract.

Diminished Expression of P-glycoprotein Using Focused Ultrasound Is Associated With JNK-Dependent Signaling Pathway in Cerebral Blood Vessels

MRI-guided focused ultrasound (MRgFUS) combined with microbubbles (MBs) is a promising technology that can facilitate drug delivery through a temporarily disrupted blood-brain barrier (BBB) and induce the down-regulation of P-glycoprotein (P-gp) expression on the blood vessels. Despite the increasing evidence regarding the down-regulation of P-gp expression after MRgFUS BBB disruption (BBBD), its underlying molecular events remain unclear. The aim of this study was to evaluate the underlying mechanism of FUS BBBD-mediated P-gp down-regulation. While our results showed down-regulation of P-gp at 24 h post-BBBD in transcriptional and translational levels, restoration to the normal expression appeared at different time points for transcriptional (72 h) and translational (120 h) levels. In addition, the signaling molecule, JNK, was significantly activated in the cerebral blood vessels at 24 h post-BBBD. Although P-gp levels were significantly decreased, the expression levels of proteins involved in the integrity of blood vessels, such as Glut1, ZO-1 and occludin, were not decreased at 24 h post-BBBD. Our study suggests that the JNK signaling pathway is involved in the regulation of FUS-induced P-gp expression, without affecting vessel integrity, and a detailed regulatory mechanism can provide the basis for clinical application of FUS to the treatment of neurological disease.

Conformational changes in a multidrug resistance ABC transporter DrrAB: Fluorescence-based approaches to study substrate binding

Bacterial multidrug transporter DrrAB exhibits overlapping substrate specificity with mammalian P-glycoprotein. DrrA hydrolyzes ATP, and the energy is transduced to carrier DrrB resulting in export of drugs. Previous studies suggested that DrrB contains a large and flexible drug-binding pocket made of aromatic residues contributed by several transmembrane helices with different drugs binding to both specific and shared residues in this pocket. However, direct binding of drugs to DrrAB or the mechanism of substrate-induced conformational changes between DrrA and DrrB has so far not been investigated. We used two fluorescence-based approaches to determine substrate binding to purified DrrAB. Our analysis shows that DrrB binds drugs with variable affinities and contains multiple drug binding sites. This work also provides evidence for two asymmetric nucleotide binding sites in DrrA with strikingly different binding affinities. Using targeted fluorescence labeling, we provide clear evidence of long-range conformational changes occurring between DrrA and DrrB. It is proposed that the transduction pathway from the nucleotide-binding DrrA subunit to the substrate binding DrrB subunit includes Q-loop and CREEM motifs in DrrA and EAA-like motif in DrrB. This study lays a solid groundwork for examining roles of various conserved regions of DrrA and DrrB in transduction of conformational changes.

ATP-binding cassette transporters in progression and clinical outcome of pancreatic cancer: What is the way forward?

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive diseases and is characterized by high chemoresistance, leading to the lack of effective therapeutic approaches and grim prognosis. Despite increasing understanding of the mechanisms of chemoresistance in cancer and the role of ATP-binding cassette (ABC) transporters in this resistance, the therapeutic potential of their pharmacological inhibition has not been successfully exploited yet. In spite of the discovery of potent pharmacological modulators of ABC transporters, the results obtained in clinical trials have been so far disappointing, with high toxicity levels impairing their successful administration to the patients. Critically, although ABC transporters have been mostly studied for their involvement in development of multidrug resistance (MDR), in recent years the contribution of ABC transporters to cancer initiation and progression has emerged as an important area of research, the understanding of which could significantly influence the development of more specific and efficient therapies. In this review, we explore the role of ABC transporters in the development and progression of malignancies, with focus on PDAC. Their established involvement in development of MDR will be also presented. Moreover, an emerging role for ABC transporters as prognostic tools for patients' survival will be discussed, demonstrating the therapeutic potential of ABC transporters in cancer therapy.

Therapeutic potential of nanocarrier for overcoming to P-glycoprotein

Enhancement of targeted therapeutic effect in the body and achievement of high bioavailability are major concern for the researchers due to the complex physiology of human body. There are so many barriers that hinder the absorption and permeation of drugs from the body, thus influencing the bioavailability of therapeutics. P-glycoprotein (P-gp) is one of such barrier present on the apical membranes of various organs such as small intestine, brain, kidney and liver. This protein interacts with vast variety of therapeutics and efflux out them preventing their entrance to the desired site, thus modulating their pharmacokinetic properties. To address this, a concerned number of approaches have been used such as the use of chemo sensitizers along with the therapeutics and various novel techniques. In this review, we are going to discuss the basic introduction to this protein and overview of various strategies used earlier to tackle the problem of P-gp efflux as well as the role of nanocarriers in confronting this issue. Nanocarriers have played great role in the enhancement of the bioavailability of many antineoplastic agents as well as other P-gp substrates. Encapsulation of P-gp inhibitors in the nanocarrier system prevents toxicity and gives site-specific action.

Reversible dimers of the atypical antipsychotic quetiapine inhibit p-glycoprotein-mediated efflux in vitro with increased binding affinity and in situ at the blood-brain barrier

The multidrug resistance transporter P-glycoprotein (P-gp) is highly expressed in the capillary endothelial cells of the blood-brain barrier (BBB) where it functions to limit the brain penetration of many drugs, including antipsychotic agents used to treat schizophrenia. Therefore, in an effort to inhibit the transporter, we designed dimers of the antipsychotic drug and P-gp substrate quetiapine (QT), linked by variable length tethers. In P-gp overexpressing cells and in human brain capillary endothelial hCMEC/D3 cells, the dimer with the shortest tether length (QT2C2) (1) was the most potent inhibitor showing >80-fold better inhibition of P-gp-mediated transport than monomeric QT. The dimers, which are linked via ester moieties, are designed to revert to the therapeutic monomer once inside the target cells. We demonstrated that the addition of two sterically blocking methyl groups to the linker (QT2C2Me2, 8) increased the half-life of the molecule in plasma 10-fold as compared to the dimer lacking methyl groups (QT2C2, 1), while retaining inhibitory potency for P-gp transport and sensitivity to cellular esterases. Experiments with purified P-gp demonstrated that QT2C2 (1) and QT2C2Me2 (8) interacted with both the H- and R-binding sites of the transporter with binding affinities 20- to 30-fold higher than that of monomeric QT. Using isolated rat brain capillaries, QT2C2Me2 (8) was a more potent inhibitor of P-gp transport than QT. Lastly, we showed that QT2C2Me2 (8) increased the accumulation of the P-gp substrate verapamil in rat brain in situ three times more than QT. Together, these results indicate that the QT dimer QT2C2Me2 (8) strongly inhibited P-gp transport activity in human brain capillary endothelial cells, in rat brain capillaries, and at the BBB in an animal model.

Complex Interplay between the P-Glycoprotein Multidrug Efflux Pump and the Membrane: Its Role in Modulating Protein Function

Multidrug resistance in cancer is linked to expression of the P-glycoprotein multidrug transporter (Pgp, ABCB1), which exports many structurally diverse compounds from cells. Substrates first partition into the bilayer and then interact with a large flexible binding pocket within the transporter's transmembrane regions. Pgp has been described as a hydrophobic vacuum cleaner or an outwardly directed drug/lipid flippase. Recent X-ray crystal structures have shed some light on the nature of the drug-binding pocket and suggested routes by which substrates can enter it from the membrane. Detergents have profound effects on Pgp function, and several appear to be substrates. Biochemical and biophysical studies in vitro, some using purified reconstituted protein, have explored the effects of the membrane environment. They have demonstrated that Pgp is involved in a complex relationship with its lipid environment, which modulates the behavior of its substrates, as well as various functions of the protein, including ATP hydrolysis, drug binding, and drug transport. Membrane lipid composition and fluidity, phospholipid headgroup and acyl chain length all influence Pgp function. Recent studies focusing on thermodynamics and kinetics have revealed some important principles governing Pgp-lipid and substrate-lipid interactions, and how these affect drug-binding and transport. In some cells, Pgp is associated with cholesterol-rich microdomains, which may modulate its functions. The relationship between Pgp and cholesterol remains an open question; however, it clearly affects several aspects of its function in addition to substrate-membrane partitioning. The action of Pgp modulators appears to depend on their membrane permeability, and membrane fluidizers and surfactants reverse drug resistance, likely via an indirect mechanism. A detailed understanding of how the membrane affects Pgp substrates and Pgp's catalytic cycle may lead to new strategies to combat clinical drug resistance.

The ABC gene family in arthropods: comparative genomics and role in insecticide transport and resistance

About a 100 years ago, the Drosophila white mutant marked the birth of Drosophila genetics. The white gene turned out to encode the first well studied ABC transporter in arthropods. The ABC gene family is now recognized as one of the largest transporter families in all kingdoms of life. The majority of ABC proteins function as primary-active transporters that bind and hydrolyze ATP while transporting a large diversity of substrates across lipid membranes. Although extremely well studied in vertebrates for their role in drug resistance, less is known about the role of this family in the transport of endogenous and exogenous substances in arthropods. The ABC families of five insect species, a crustacean and a chelicerate have been annotated in some detail. We conducted a thorough phylogenetic analysis of the seven arthropod and human ABC protein subfamilies, to infer orthologous relationships that might suggest conserved function. Most orthologous relationships were found in the ABCB half transporter, ABCD, ABCE and ABCF subfamilies, but specific expansions within species and lineages are frequently observed and discussed. We next surveyed the role of ABC transporters in the transport of xenobiotics/plant allelochemicals and their involvement in insecticide resistance. The involvement of ABC transporters in xenobiotic resistance in arthropods is historically not well documented, but an increasing number of studies using unbiased differential gene expression analysis now points to their importance. We give an overview of methods that can be used to link ABC transporters to resistance. ABC proteins have also recently been implicated in the mode of action and resistance to Bt toxins in Lepidoptera. Given the enormous interest in Bt toxicology in transgenic crops, such findings will provide an impetus to further reveal the role of ABC transporters in arthropods.

P-glycoprotein Inhibition for Optimal Drug Delivery

P-glycoprotein (P-gp), an efflux membrane transporter, is widely distributed throughout the body and is responsible for limiting cellular uptake and the distribution of xenobiotics and toxic substances. Hundreds of structurally diverse therapeutic agents are substrates to it and it impedes the absorption, permeability, and retention of the drugs, extruding them out of the cells. It is overexpressed in cancer cells and accountable for obstructing cell internalization of chemotherapeutic agents and for developing transporter mediated resistance by cancer cells during anti-tumor treatments. As it jeopardizes the success of drug delivery and cancer targeting, strategies are being developed to overcome P-gp mediated drug transport. This concise review represents a brief discussion on P-gp mediated drug transport and how it hinders the success of various therapies. Its main focus is on various strategies used to tackle this curb in the field of drug delivery and targeting.

Quantitative fluorescence microscopy provides high resolution imaging of passive diffusion and P-gp mediated efflux at the in vivo blood-brain barrier

Quantitative fluorescent microscopy is an emerging technology that has provided significant insight into cellular dye accumulation, organelle function, and tissue physiology. However, historically dyes have only been used to qualitatively or semi-quantitatively (fold change) determine changes in blood-brain barrier (BBB) integrity. Herein, we present a novel method to calculate the blood to brain transfer rates of the dyes rhodamine 123 and Texas red across the in situ BBB. We observed that rhodamine 123 is subject to p-glycoprotein mediated efflux at the rat BBB and can be increased nearly 20-fold with p-glycoprotein inhibition. However, Texas Red appears to not be subject to MRP2 mediated efflux at the rat BBB, agreeing with literature reports suggesting MRP2 may lack functionality at the normal rat BBB. Lastly, we present data demonstrating that once dyes have crossed the BBB, diffusion of the dye molecule is not as instantaneous as has been previously suggested. We propose that future work can now be completed to (1) match BBB transfer coefficients to interstitial diffusion constants and (2) use dyes with specific affinities to cellular organelles or that have specific properties (e.g., subject to efflux transporters) to more fully understand BBB physiology.

Catalytic and transport cycles of ABC exporters

ABC (ATP-binding cassette) transporters are arguably the most important family of ATP-driven transporters in biology. Despite considerable effort and advances in determining the structures and physiology of these transporters, their fundamental molecular mechanisms remain elusive and highly controversial. How does ATP hydrolysis by ABC transporters drive their transport function? Part of the problem in answering this question appears to be a perceived need to formulate a universal mechanism. Although it has been generally hoped and assumed that the whole superfamily of ABC transporters would exhibit similar conserved mechanisms, this is proving not to be the case. Structural considerations alone suggest that there are three overall types of coupling mechanisms related to ABC exporters, small ABC importers and large ABC importers. Biochemical and biophysical characterization leads us to the conclusion that, even within these three classes, the catalytic and transport mechanisms are not fully conserved, but continue to evolve. ABC transporters also exhibit unusual characteristics not observed in other primary transporters, such as uncoupled basal ATPase activity, that severely complicate mechanistic studies by established methods. In this chapter, I review these issues as related to ABC exporters in particular. A consensus view has emerged that ABC exporters follow alternating-access switch transport mechanisms. However, some biochemical data suggest that alternating catalytic site transport mechanisms are more appropriate for fully symmetrical ABC exporters. Heterodimeric and asymmetrical ABC exporters appear to conform to simple alternating-access-type mechanisms.

Overcoming drug resistance in pancreatic cancer

INTRODUCTION

Pancreatic cancer has the worst survival rate of all cancers. The current standard care for metastatic pancreatic cancer is gemcitabine, however, the success of this treatment is poor and overall survival has not improved for decades. Drug resistance (both intrinsic and acquired) is thought to be a major reason for the limited benefit of most pancreatic cancer therapies.

AREAS COVERED

Previous studies have indicated various mechanisms of drug resistance in pancreatic cancer, including changes in individual genes or signaling pathways, the influence of the tumor microenvironment, and the presence of highly resistant stem cells. This review summarizes recent advances in the mechanisms of drug resistance in pancreatic cancer and potential strategies to overcome this.

EXPERT OPINION

Increasing drug delivery efficiency and decreasing drug resistance is the current aim in pancreatic cancer treatment, and will also benefit the treatment of other cancers. Understanding the molecular and cellular basis of drug resistance in pancreatic cancer will lead to the development of novel therapeutic strategies with the potential to sensitize pancreatic cancer to chemotherapy, and to increase the efficacy of current treatments in a wide variety of human cancers.

Fluorescence studies of drug binding and translocation by membrane transporters

Resistance to multiple drugs is a serious limitation to chemotherapy treatment of human cancers. In addition, many clinically useful drugs show limited uptake in the intestine and cannot gain access to the brain. Three multidrug efflux pumps of the ABC superfamily (P-glycoprotein/ABCB1, MRP1/ABCC1, and BCRP/ABGG2) are responsible for most drug transport out of mammalian cells. P-glycoprotein is the best characterized of the ABC drug transporters. However, the lipophilic nature of its substrates has made it difficult to directly quantitate drug binding to the protein by classical biochemical methods, and the measurement of drug transport rates has also proved challenging. In recent years, fluorescence spectroscopic approaches have proved very useful in overcoming these problems. This chapter focuses on the use of fluorescence tools to quantitate the affinity of binding of various drugs to purified P-glycoprotein and to measure its drug transport activity in reconstituted proteoliposomes in real time. The ability of various drugs to inhibit P-glycoprotein mediated transport can also be assessed using this approach.

P-glycoprotein ATPase from the resistant pest, Helicoverpa armigera: purification, characterization and effect of various insecticides on its transport function

Helicoverpa armigera is a major pest of agricultural crops and has developed resistance to various insecticides. A P-glycoprotein (Pgp) with ATPase activity likely to be involved in insecticide resistance was purified and characterized from insecticide-resistant H. armigera. The purification was 18-fold with 3% yield. The optimum pH and temperature were found to be 7.4 and 30-40 degrees C, respectively. Kinetic studies indicated that this enzyme had a K(m) value of 1.2mM for ATP. Pgp from H. armigera was partially sequenced and found to be homologous to conserved sequences of mammalian Pgps. Pesticides stimulated H. armigera Pgp ATPase activity with a maximum stimulation of up to 40%. Quenching of the intrinsic tryptophan fluorescence of purified Pgp was used to quantitate insecticide binding. Using the high-affinity fluorescent substrate, tetramethylrosamine, transport was monitored in real time in proteoliposomes containing H. armigera Pgp. The presence of Pgp could be one of the reasons for insecticide resistance in this pest.

Role of membrane transporters in the safety profile of drugs

It has increasingly been recognized that few molecules move across the cell membrane without the assistance of transporter proteins. Large superfamilies of transporter proteins have been identified in every living cell, including microorganisms and mitochondria. This report reviews the role of transporters in physiology and pharmacology, and identifies where this may have an impact on drug efficacy and toxicity. This new understanding will require a fresh appreciation of pharmacokinetics and drug effects, as the current paradigms are based largely on the assumption that drug molecules have a reasonable unrestricted permeability across membranes. Rather than just focusing on clearance changes and central compartment pharmacokinetics, it will become increasingly necessary to examine the peripheral tissue distribution of drugs to more accurately predict drug efficacy and toxicity.

Interaction of the P-glycoprotein multidrug efflux pump with cholesterol: effects on ATPase activity, drug binding and transport

Resistance to a broad spectrum of structurally diverse chemotherapeutic drugs (multidrug resistance; MDR) is a major impediment to the treatment of cancer. One cause of MDR is the expression at the tumor cell surface of P-glycoprotein (Pgp), which functions as an ATP-powered multidrug efflux pump. Since Pgp interacts with its substrates after they partition into the lipid bilayer, changes in membrane physicochemical properties may have substantial effects on its functional activity. Various interactions between cholesterol and Pgp have been suggested, including a role for the protein in transbilayer movement of cholesterol. We have characterized several aspects of Pgp-cholesterol interactions, and found that some of the previously reported effects of cholesterol result from inhibition of Pgp ATPase activity by the cholesterol-extracting reagent, methyl-beta-cyclodextrin. The presence of cholesterol in the bilayer modulated the basal and drug-stimulated ATPase activity of reconstituted Pgp in a modest fashion. Both the ability of drugs to bind to the protein and the drug transport and phospholipid flippase functions of Pgp were also affected by cholesterol. The effects of cholesterol on drug binding affinity were unrelated to the size of the compound. Increasing cholesterol content greatly altered the partitioning of hydrophobic drug substrates into the membrane, which may account for some of the observed effects of cholesterol on Pgp-mediated drug transport. Pgp does not appear to mediate the flip-flop of a fluorescent cholesterol analogue across the bilayer. Cholesterol likely modulates Pgp function via effects on drug-membrane partitioning and changes in the local lipid environment of the protein.

Benzodiazepine-mediated structural changes in the multidrug transporter P-glycoprotein: an intrinsic fluorescence quenching analysis

P-glycoprotein expressed in Pichia pastoris was used to study the drug binding sites of different benzodiazepines. The effect of bromazepam, chlordiazepoxide, diazepam and flurazepam on P-glycoprotein structure was investigated by measuring the intrinsic fluorescence of the transporter tryptophan residues. Purified mouse mdr1a transporter in mixed micelles of 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid and 1,2-dimiristoyl-sn-glycerol-3-phosphocholine emitted fluorescence at 340 nm indicative of the fluorophores in a relatively apolar environment. Acrylamide and iodide ion were used as collisional quenchers toward distinct regions of the transporter, the protein and the interface protein-surface, respectively. Binding of ATP induced conformational changes at the protein surface level in accordance with the location of the nucleotide binding sites. Bromazepam interaction with the transporter was located at the protein-surface interface, diazepam at the membrane region and chlordiazepoxide at the protein surface. Only the flurazepam interaction site was not detected by the quenchers used. All benzodiazepines were able to elicit reorientation of the protein fluorophores on the P-glycoprotein-ATP complex.

ABC multidrug transporters: structure, function and role in chemoresistance

Three ATP-binding cassette (ABC)-superfamily multidrug efflux pumps are known to be responsible for chemoresistance; P-glycoprotein (ABCB1), MRP1 (ABCC1) and ABCG2 (BCRP). These transporters play an important role in normal physiology by protecting tissues from toxic xenobiotics and endogenous metabolites. Hydrophobic amphipathic compounds, including many clinically used drugs, interact with the substrate-binding pocket of these proteins via flexible hydrophobic and H-bonding interactions. These efflux pumps are expressed in many human tumors, where they likely contribute to resistance to chemotherapy treatment. However, the use of efflux-pump modulators in clinical cancer treatment has proved disappointing. Single nucleotide polymorphisms in ABC drug-efflux pumps may play a role in responses to drug therapy and disease susceptibility. The effect of various genotypes and haplotypes on the expression and function of these proteins is not yet clear, and their true impact remains controversial.

Interaction of insecticides with mammalian P-glycoprotein and their effect on its transport function

We studied the effects of four commonly used insecticides (methylparathion, endosulfan, cypermethrin and fenvalerate) on P-glycoprotein isolated from multidrug-resistant cells. All the pesticides stimulated P-glycoprotein ATPase activity, with maximum stimulation of up to 213% in a detergent-solubilized preparation, and up to 227% in reconstituted liposomes. The ATPase stimulation profiles were biphasic, displaying lower stimulation, and in the case of methylparathion, inhibition of activity, at higher insecticide concentrations. Quenching of the intrinsic Trp fluorescence of purified P-glycoprotein was used to quantitate insecticide binding; the estimated K(d) values fell in the range 4-6 microM. Transport of the fluorescent substrate tetramethylrosamine (TMR) into proteoliposomes containing P-glycoprotein was monitored in real time. The TMR concentration gradient generated by the transporter was collapsed by the addition of insecticides, and prior addition of these compounds prevented its formation. The rate of TMR transport was inhibited in a saturable fashion by all the compounds, indicating that they compete with the substrate for membrane translocation. Taken together, these data suggest that the insecticides bind to Pgp with high affinity and effectively block drug transport. Inhibition of Pgp by pesticides may compromise its ability to clear xenobiotics from the body, leading to a higher risk of toxicity.

Shedding light on drug transport: structure and function of the P-glycoprotein multidrug transporter (ABCB1)

P-glycoprotein (Pgp; ABCB1), a member of the ATP-binding cassette (ABC) superfamily, exports structurally diverse hydrophobic compounds from the cell, driven by ATP hydrolysis. Pgp expression has been linked to the efflux of chemotherapeutic drugs in human cancers, leading to multidrug resistance (MDR). The protein also plays an important physiological role in limiting drug uptake in the gut and entry into the brain. Substrates partition into the lipid bilayer before interacting with Pgp, which has been proposed to function as a hydrophobic vacuum cleaner. Low- and medium-resolution structural models of Pgp suggest that the 2 nucleotide-binding domains are closely associated to form a nucleotide sandwich dimer. Pgp is an outwardly directed flippase for fluorescent phospholipid and glycosphingolipid derivatives, which suggests that it may also translocate drug molecules from the inner to the outer membrane leaflet. The ATPase catalytic cycle of the protein is thought to proceed via an alternating site mechanism, although the details are not understood. The lipid bilayer plays an important role in Pgp function, and may regulate both the binding and transport of drugs. This review focuses on the structure and function of Pgp, and highlights the importance of fluorescence spectroscopic techniques in exploring the molecular details of this enigmatic transporter.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

Conformational and functional characterization of trapped complexes of the P-glycoprotein multidrug transporter

The Pgp (P-glycoprotein) multidrug transporter couples ATP hydrolysis at two cytoplasmic NBDs (nucleotide-binding domains) to the transport of hydrophobic compounds. Orthovanadate (V(i)) and fluoroaluminate (AlF(x)) trap nucleotide in one NBD by forming stable catalytically inactive complexes (Pgp-M2+-ADP-X), which are proposed to resemble the catalytic transition state, whereas the complex formed by beryllium fluoride (BeF(x)) is proposed to resemble the ground state. We studied the trapped complexes formed via incubation of Pgp with ATP (catalytically forward) or ADP (reverse) and V(i), BeF(x) or AlF(x) using Mg2+ or Co2+ as the bivalent cation. Quenching of intrinsic Pgp tryptophan fluorescence by acrylamide, iodide and caesium indicated that conformational changes took place upon formation of the trapped complexes. Trapping with V(i) and ATP led to a 6-fold increase in the acrylamide quenching constant, K(SV), suggesting that large conformational changes take place in the Pgp transmembrane regions on trapping in the forward direction. Trapping with V(i) and ADP gave only a small change in quenching, indicating that the forward- and reverse-trapped complexes are different. TNP (trinitrophenyl)-ATP/TNP-ADP interacted with all of the trapped complexes, however, the fluorescence enhancement differed for the trapped states, suggesting a change in polarity in the nucleotide-binding sites. The nucleotide-binding site of the BeF(x)-trapped complex was much more polar than that of the V(i) and AlF(x) complexes. Functionally, all the trapped complexes were able to bind drugs and TNP-nucleotides with unchanged affinity compared with native Pgp.

Sensitivity of P-glycoprotein tryptophan residues to benzodiazepines and ATP interaction

Plasma membrane P-glycoprotein is a member of the ATP-binding cassette family of membrane transporters. In the present study tryptophan intrinsic fluorescence was used to understand the P-glycoprotein response to three benzodiazepines (bromazepam, chlordiazepoxide and flurazepam) in the presence and absence of ATP. Fluorescence emission spectra showed a red shift on the maximal emission wavelength upon interaction of P-glycoprotein with all benzodiazepines. Benzodiazepine association with nucleotide-bound P-glycoprotein also showed this trend and the quenching profile was attributed to a sphere-of-action model, for static fluorescence. Furthermore, quenching data of benzodiazepine-bound P-glycoprotein with ATP were concentration dependent and saturable, indicating that nucleotide binds to P-glycoprotein whether drug is present or not. These results seems in agreement with the proposal of the ATP-switch model by Higgins and Linton, where substrate binding to the transporters initiates the transport cycle by increasing the ATP binding affinity.

New Insights into the Drug Binding, Transport and Lipid Flippase Activities of the P-Glycoprotein Multidrug Transporter

The MDR1 P-glycoprotein, an ATP-binding cassette (ABC) superfamily member that functions as an ATP-driven drug efflux pump, has been linked to resistance of human tumors to multiple chemotherapeutic agents. P-glycoprotein binds and actively transports a large variety of hydrophobic drugs and peptides. P-glycoprotein in reconstituted proteoliposomes is also an outwardly directed flippase for membrane phospholipids and simple glycosphinglipids. This review focuses on recent advances in our understanding of P-glycoprotein structure and function, particularly through the use of fluorescence spectroscopic approaches. Progress is being made towards understanding the structure of the transporter, especially the spatial relationship between the two nucleotide-binding domains. Exploration of the P-glycoprotein catalytic cycle using vanadate-trapped complexes has revealed that drug transport likely takes place by concerted conformational changes linked to relaxation of a high energy intermediate. Low resolution mapping of the protein using fluorescence resonance energy transfer showed that both the H and R drug-binding sites are located within the cytoplasmic leaflet. Two drugs can bind to the R-site simultaneously, suggesting that the protein contains a large flexible binding region.

ABC transporters as multidrug resistance mechanisms and the development of chemosensitizers for their reversal

One of the major problems related with anticancer chemotherapy is resistance against anticancer drugs. The ATP-binding cassette (ABC) transporters are a family of transporter proteins that are responsible for drug resistance and a low bioavailability of drugs by pumping a variety of drugs out cells at the expense of ATP hydrolysis. One strategy for reversal of the resistance of tumor cells expressing ABC transporters is combined use of anticancer drugs with chemosensitizers. In this review, the physiological functions and structures of ABC transporters, and the development of chemosensitizers are described focusing on well-known proteins including P-glycoprotein, multidrug resistance associated protein, and breast cancer resistance protein.

Effect of pluronic P85 on ATPase activity of drug efflux transporters

PURPOSE

Pluronic block copolymers are potent sensitizers of multi-drug resistant (MDR) cancer cells. The sensitization effect by Pluronics is a result of two processes acting in concert: i) intracellular ATP depletion, and ii) inhibition of ATPase activity of drug efflux proteins. This work characterizes effects of Pluronic P85 on ATPase activities of Pgp, MRP1, and MRP2 drug efflux transport proteins and interaction of these proteins with their substrates, vinblastine, and leucotriene C4.

METHODS

Using membranes overexpressing Pgp, MRP1, and MRP2, the current study evaluates effects of Pluronic P85 (P85) on the kinetic parameters (Vmax, Km, Vmax/Km) of ATP hydrolysis by these ATPases.

RESULTS

The decreases in the maximal reaction rates (Vmax) and increases in apparent Michaelis constants (Km) for these transporters in the presence of various concentrations of P85 were observed. The mechanism of these effects may involve i) conformational changes of the transporter due to membrane fluidization and/or ii) nonspecific steric hindrance of the drug-binding sites by P85 chains embedded into cellular membranes. The extent of these alterations was increased in the row MRP1 < MRP2 << Pgp.

CONCLUSIONS

These data suggest that there are unifying pathways for the inhibition of Pgp and MRPs by the block copolymer. However, the effect of P85 on Pgp ATPase activity is considerably greater compared with the effects on MRP1 and MRP2 ATPases. This may be a reason for greater inhibitory effects of Pluronic in Pgp- compared with MRP-overexpressing cells.

Isolated Rafts from Adriamycin-Resistant P388 Cells Contain Functional ATPases and Provide an Easy Test System for P-glycoprotein?Related Activities

PURPOSE

P-glycoprotein (P-gp), a membrane ATPase expelling many structurally unrelated compounds out of cells, is one of the major contributors to multidrug resistance. It is enriched in cold TritonX-100 insoluble membrane domains (i.e., rafts). The purpose of this work was to characterize the ATPase activities of raft preparations from P388 cells overexpressing P-gp (P388/ADR) or devoid of P-gp (P388) and to establish a P-gp-enriched screening system for P-gp-interfering compounds.

METHODS

Rafts were extracted with cold TritonX-100. The ATPase activity was characterized in 96-well plates using a fluorescence assay.

RESULTS

The ATPase activity per mg protein was about five times higher in P388/ADR rafts than in crude membranes. The anti-P-gp antibody C219 inhibited 20% of the activity in P388/ADR rafts but only about 10% of the activity in P388/ADR crude membranes and had no effect on the activity of P388 rafts. The known P-gp-activating compounds verapamil, progesterone, and valinomycin revealed the typical bell-shaped activity/concentration profiles in P388/ADR rafts, indicative for activation at low compound concentrations and inhibition at concentrations >10 to 100 microM. The inhibitory effect was also observed in P388 rafts.

CONCLUSIONS

Extracted rafts are rich in functional ATPases. Rafts from P-gp-overexpressing cells display P-gp-typical ATPase activity and provide an easy, P-gp-enriched screening system.

The elementary mass action rate constants of P-gp transport for a confluent monolayer of MDCKII-hMDR1 cells

The human multi-drug resistance membrane transporter, P-glycoprotein, or P-gp, has been extensively studied due to its importance to human health and disease. Thus far, the kinetic analysis of P-gp transport has been limited to steady-state Michaelis-Menten approaches or to compartmental models, neither of which can prove molecular mechanisms. Determination of the elementary kinetic rate constants of transport will be essential to understanding how P-gp works. The experimental system we use is a confluent monolayer of MDCKII-hMDR1 cells that overexpress P-gp. It is a physiologically relevant model system, and transport is measured without biochemical manipulations of P-gp. The Michaelis-Menten mass action reaction is used to model P-gp transport. Without imposing the steady-state assumptions, this reaction depends upon several parameters that must be simultaneously fitted. An exhaustive fitting of transport data to find all possible parameter vectors that best fit the data was accomplished with a reasonable computation time using a hierarchical algorithm. For three P-gp substrates (amprenavir, loperamide, and quinidine), we have successfully fitted the elementary rate constants, i.e., drug association to P-gp from the apical membrane inner monolayer, drug dissociation back into the apical membrane inner monolayer, and drug efflux from P-gp into the apical chamber, as well as the density of efflux active P-gp. All three drugs had overlapping ranges for the efflux active P-gp, which was a benchmark for the validity of the fitting process. One novel finding was that the association to P-gp appears to be rate-limited solely by drug lateral diffusion within the inner monolayer of the plasma membrane for all three drugs. This would be expected if P-gp structure were open to the lipids of the apical membrane inner monolayer, as has been suggested by recent structural studies. The fitted kinetic parameters show how P-gp efflux of a wide range of xenobiotics has been maximized.

Imaging reversal of multidrug resistance in living mice with bioluminescence: MDR1 P-glycoprotein transports coelenterazine

Coelenterazine is widely distributed among marine organisms, producing bioluminescence by calcium-insensitive oxidation mediated by Renilla luciferase (Rluc) and calcium-dependent oxidation mediated by the photoprotein aequorin. Despite its abundance in nature and wide use of both proteins as reporters of gene expression and signal transduction, little is known about mechanisms of coelenterazine transport and cell permeation. Interestingly, coelenterazine analogues share structural and physiochemical properties of compounds transported by the multidrug resistance MDR1 P-glycoprotein (Pgp). Herein, we report that living cells stably transfected with a codon-humanized Rluc show coelenterazine-mediated bioluminescence in a highly MDR1 Pgp-modulated manner. In Pgp-expressing Rluc cells, low baseline bioluminescence could be fully enhanced (reversed) to non-Pgp matched control levels with potent and selective Pgp inhibitors. Therefore, using coelenterazine and noninvasive bioluminescence imaging in vivo, we could directly monitor tumor-specific Pgp transport inhibition in living mice. While enabling molecular imaging and high-throughput screening of drug resistance pathways, these data also raise concern for the indiscriminate use of Rluc and aequorin as reporters in intact cells or transgenic animals, wherein Pgp-mediated alterations in coelenterazine permeability may impact results.

Structure and function of efflux pumps that confer resistance to drugs

Resistance to therapeutic drugs encompasses a diverse range of biological systems, which all have a human impact. From the relative simplicity of bacterial cells, fungi and protozoa to the complexity of human cancer cells, resistance has become problematic. Stated in its simplest terms, drug resistance decreases the chance of providing successful treatment against a plethora of diseases. Worryingly, it is a problem that is increasing, and consequently there is a pressing need to develop new and effective classes of drugs. This has provided a powerful stimulus in promoting research on drug resistance and, ultimately, it is hoped that this research will provide novel approaches that will allow the deliberate circumvention of well understood resistance mechanisms. A major mechanism of resistance in both microbes and cancer cells is the membrane protein-catalysed extrusion of drugs from the cell. Resistant cells exploit proton-driven antiporters and/or ATP-driven ABC (ATP-binding cassette) transporters to extrude cytotoxic drugs that usually enter the cell by passive diffusion. Although some of these drug efflux pumps transport specific substrates, many are transporters of multiple substrates. These multidrug pumps can often transport a variety of structurally unrelated hydrophobic compounds, ranging from dyes to lipids. If we are to nullify the effects of efflux-mediated drug resistance, we must first of all understand how these efflux pumps can accommodate a diverse range of compounds and, secondly, how conformational changes in these proteins are coupled to substrate translocation. These are key questions that must be addressed. In this review we report on the advances that have been made in understanding the structure and function of drug efflux pumps.

Optical single transporter recording: transport kinetics in microarrays of membrane patches

Optical single transporter recording (OSTR) is an emerging technique for the fluorescence microscopic measurement of transport kinetics in membrane patches. Membranes are attached to transparent microarrays of cylindrical test compartments (TCs) approximately 0.1-100 mum in diameter and approximately 10-100 mum in depth. Transport across membrane patches that may contain single transporters or transporter populations is recorded by confocal microscopy. By these means transport of proteins through single nuclear pore complexes has been recorded at rates of <1 translocation/s. In addition to the high sensitivity in terms of measurable transport rates OSTR features unprecedented spatial selectivity and parallel processing. This article reviews the conceptual basis of OSTR and its realization. Applications to nuclear transport are summarized. The further development of OSTR is discussed and its extension to a diversity of transporters, including translocases and ATP-binding cassette (ABC) pumps, projected.

Developmental aspects of blood-brain barrier (BBB) and rat brain endothelial (RBE4) cells as in vitro model for studies on chlorpyrifos transport

The mammalian central nervous system (CNS) is characterized by the blood-brain barrier (BBB), a restrictive barrier endowed with the maintenance of homeostatic control of an optimal milieu within the brain. Whereas in tissues other than the CNS, concentrations of various metabolites (amino acids, K+) can undergo frequent fluctuations, the CNS must keep rigorous control over the extracellular cerebral fluid composition, preventing the mirroring of transient fluctuations in blood, because abrupt changes in these metabolites can translate to aberrant CNS function. The BBB is a specialized structure accomplished by individual endothelial cells that are continuously linked by tight junctions. This brief review will address pertinent issues to development of the BBB. Particular emphasis will be directed at the role of astrocytes in the induction and maintenance of the restrictive properties of this barrier, and the utility of in vitro culture models in surveying transport kinetics, exemplified by recent studies with the pesticide, chlorpyrifos.

Fluorescent substrates of sister-P-glycoprotein (BSEP) evaluated as markers of active transport and inhibition: evidence for contingent unequal binding sites

PURPOSE

Although sister-P-glycoprotein (SPGP, BSEP) is closely related to P-glycoprotein, it is much more selective in distribution and substrate recognition. Moreover, because inhibition or lack of BSEP function has severe consequences including cholestasis, hepatotoxicity, exposure to toxic xenobiotics, and drug interactions, in vitro methods are necessary for quantifying and characterizing specific inhibition of BSEP. Therefore, the objective is to discern a method and quantitatively characterize several example BSEP inhibitors.

METHODS

With fluorescent markers having been used successfully to evaluate and quantify inhibition of P-gp-mediated transport, this study evaluates several compounds for specific cell retention caused by BSEP inhibitors. In addition to the several compounds asserted to be BSEP inhibitors, the compounds suggested to be BSEP substrates might also inhibit BSEP competitively. Retained fluorescence of possible BSEP substrates was measured by a flow cell cytometer using transfected cells presenting the BSEP transporter specifically and abundantly.

RESULTS

Several compounds were shown to inhibit BSEP active transport of the fluorescent substrates dihydrofluorescein and bodipy. The inhibition potency was quantified (i.e., cyclosporin A IC50 approximately 7 microM), revealing incongruent relative sensitivities among the substrate markers, with H2FDA generally the most sensitive of the series of substrate markers evaluated.

CONCLUSIONS

The inconsistent sensitivities of the transport markers (H2FDA and bodipy) were reminiscent of the apparent multiple binding site behaviors observed for P-gp and could indicate opposing and unequal yet interacting binding sites akin to those of P-gp. Nonetheless, notable differences between P-gp and BSEP in marker substrate recognition/transport were apparent despite the observed overlap in xenobiotic recognition and transport. Thus far the most potent inhibitors seem to be cyclosporin, tamoxifen, and valinomycin. There are likely to be much more potent inhibitors, and other substrates also may be more sensitive to inhibition of transport.

The structure and function of drug pumps: an update

Our understanding of the exact mechanisms used by the transmembrane protein pumps that confer cellular resistance to cytotoxic drugs has improved enormously with the recent determination of the structures of three Escherichia coli transporters, two belonging to the ATP-binding cassette (ABC) superfamily and one to the resistance-nodulation-cell division (RND) family. Although these studies do not provide an insight into how drug pumps can recognize several structurally unrelated drugs, important advances have been also made in this area. Information on the molecular basis of multidrug recognition has been provided by determining the structure of transcriptional regulators that can bind, often structurally unrelated, cytotoxic drugs and control the expression of drug pumps.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

NMDA sensitization and stimulation by peroxynitrite, nitric oxide, and organic solvents as the mechanism of chemical sensitivity in multiple chemical sensitivity

Multiple chemical sensitivity (MCS) is a condition where previous exposure to hydrophobic organic solvents or pesticides appears to render people hypersensitive to a wide range of chemicals, including organic solvents. The hypersensitivity is often exquisite, with MCS individuals showing sensitivity that appears to be at least two orders of magnitude greater than that of normal individuals. This paper presents a plausible set of interacting mechanisms to explain such heightened sensitivity. It is based on two earlier theories of MCS: the elevated nitric oxide/peroxynitrite theory and the neural sensitization theory. It is also based on evidence implicating excessive NMDA activity in MCS. Four sensitization mechanisms are proposed to act synergistically, each based on known physiological mechanisms: Nitric oxide-mediated stimulation of neurotransmitter (glutamate) release; peroxynitrite-mediated ATP depletion and consequent hypersensitivity of NMDA receptors; peroxynitrite-mediated increased permeability of the blood-brain barrier, producing increased accessibility of organic chemicals to the central nervous system; and nitric oxide inhibition of cytochrome P450 metabolism. Evidence for each of these mechanisms, which may also be involved in Parkinson's disease, is reviewed. These interacting mechanisms provide explanations for diverse aspects of MCS and a framework for hypothesis-driven MCS research.

Active transport of fluorescent P-glycoprotein substrates: evaluation as markers and interaction with inhibitors

With P-glycoprotein (P-gp) continuing to have prominence among the ABC transporters for its ability to remove various xenobiotics from many cell types, accurate and robust methods for estimating the exposure of drug, carcinogen, toxicant, pesticide, and even some endobiotics to tissues and cells affected by P-gp are valuable. The inhibition of P-gp active transport of molecules, therefore, has often been quantified by concentration dependence of inhibitor effect on fluorescent substrate marker efflux mediated by this enzyme, with much evidence indicating two asymmetric yet interdependent substrate binding sites on P-gp. A uniqueness in the pair of binding sites could result in distinct effects of an inhibitor on the transport of certain substrates, thus leading to differences in fluorescent substrate responsiveness or sensitivity. Seven different fluorescent substrates of P-gp were quantitatively tested for their responsiveness to inhibition by a wide range of P-gp substrates/inhibitors. Interesting differences were observed in the IC(50) values caused by each of the inhibitors employed, in part exemplified by DNR and LDS being generally more sensitive to inhibition effects than any other fluorescent marker. However, no clear trend emerged to designate any fluorochrome marker as the most or least responsive to inhibition. Furthermore, LDS is more sensitive to some P-gp inhibitors than the substrate marker DNR, generally the most responsive. These results support the assertion of two unequal substrate binding sites that are allosterically dependent on each other. Therefore, an inhibitor that favors binding to the site opposite from that favored by a particular marker may have significant transduced effects through the protein between the two binding sites. Nevertheless, although either DNR or LDS is generally the fluorescent substrate most responsive to inhibition, there may be other substrates yet even more sensitive.

Small molecules that dramatically alter multidrug resistance phenotype by modulating the substrate specificity of P-glycoprotein

By screening a chemical library for the compounds protecting cells from adriamycin (Adr), a series of small molecules was isolated that interfered with the accumulation of Adr in mouse fibroblasts by enhancing efflux of the drug. Isolated compounds also stimulated efflux of Rhodamine 123 (Rho-123), another substrate of multidrug transporters. Stimulation of drug efflux was detectable in the cells expressing P-glycoprotein (P-gp), but not in their P-gp-negative variants, and was completely reversible by the P-gp inhibitors. A dramatic stimulation of P-gp activity against Adr and Rho-123 by the identified compounds was accompanied by suppression of P-gp-mediated efflux of other substrates, such as Taxol (paclitaxel) or Hoechst 33342, indicating that they act as modulators of substrate specificity of P-gp. Consistently, P-gp modulators dramatically altered the pattern of cross-resistance of P-gp-expressing cells to different P-gp substrates: an increase in resistance to Adr, daunorubicin, and etoposide was accompanied by cell sensitization to Vinca alkaloids, gramicidin D, and Taxol with no effect on cell sensitivity to colchicine, actinomycin D, puromycin, and colcemid, as well as to several non-P-gp substrates. The relative effect of P-gp modulators against different substrates varied among the isolated compounds that can be used as fine tools for analyzing mechanisms of drug selectivity of P-gp. These results raise the possibility of a rational control over cell sensitivity to drugs and toxins through modulation of P-gp activity by small molecules.

Multidrug resistance protein. Identification of regions required for active transport of leukotriene C4

Multidrug resistance protein (MRP) is a broad specificity, primary active transporter of organic anion conjugates that confers a multidrug resistance phenotype when transfected into drug-sensitive cells. The protein was the first example of a subgroup of the ATP-binding cassette superfamily whose members have three membrane-spanning domains (MSDs) and two nucleotide binding domains. The role(s) of the third MSD of MRP and its related transporters is not known. To begin to address this question, we examined the ability of various MRP fragments, expressed individually and in combination, to transport the MRP substrate, leukotriene C4 (LTC4). We found that elimination of the entire NH2-terminal MSD or just the first putative transmembrane helix, or substitution of the MSD with the comparable region of the functionally and structurally related transporter, the canalicular multispecific organic anion transporter (cMOAT/MRP2), had little effect on protein accumulation in the membrane. However, all three modifications decreased LTC4 transport activity by at least 90%. Transport activity could be reconstituted by co-expression of the NH2-terminal MSD with a fragment corresponding to the remainder of the MRP molecule, but this required both the region encoding the transmembrane helices of the NH2-terminal MSD and the cytoplasmic region linking it to the next MSD. In contrast, a major part of the cytoplasmic region linking the NH2-proximal nucleotide binding domain of the protein to the COOH-proximal MSD was not required for active transport of LTC4.