ATPase activity of partially purified P-glycoprotein from multidrug-resistant Chinese hamster ovary cells

The phenothiazine, trifluoperazine, is selectively lethal to ABCB1-expressing multidrug resistant cells

P-glycoprotein, member of the B-subfamily of the ATP-binding cassette (ABC) superfamily (e.g., ABCB1), has been demonstrated to confer resistance to clinically relevant anticancer drugs. Paradoxically, ABCB1-expressing multidrug resistant (MDR) cells are hypersensitivity or collateral sensitivity to non-toxic drugs. In this report, we demonstrate the capacity of trifluoperazine (TFP), a calmodulin inhibitor, to confer a collateral sensitivity onto ABCB1-overexpressing MDR cells. We show TFP-induced collateral sensitivity to be linked to ABCB1 expression and ATPase activity, as such phenotype is abolished in ABCB1-knockout MDR cells (CHO^RC5^ΔABCB1 clones A1-A3) or with inhibitors of ABCB1 ATPase. TFP-induced collateral sensitivity is mediated by apoptotic cell death, due to enhanced oxidative stress. The findings in this study show for first time the use TFP as a collateral sensitivity drug, at clinically relevant concentrations. Moreover, given the use of trifluoperazine in the treatment for symptoms of schizophrenia and the role of ABCB1 transporter in tissue blood barriers and other physiologic functions, the finding in this study may have implications beyond cancer chemotherapy.

ABCB1/MDR1/P-gp employs an ATP-dependent twist-and-squeeze mechanism to export hydrophobic drugs

ABCB1, also called MDR1 or P-glycoprotein, exports various hydrophobic compounds and plays an essential role as a protective physiological barrier in several organs, including the brain, testis, and placenta. However, little is known about the structural mechanisms that allow ABCB1 to recognize hydrophobic compounds of diverse structures or the coupling of ATP hydrolysis to uphill substrate export. High-resolution X-ray crystal structures of the pre- and post-transport states and FRET analyses in living cells have revealed that an aromatic hydrophobic network at the top of the inner cavity is key for the conformational change in ABCB1 that is triggered by a hydrophobic substrate. ATP binding, but not hydrolysis, induces a progressive network that results in a twisting motion of the whole protein, squeezing out the substrate directly to the extracellular space. This twist-and-squeeze mechanism by which ABCB1 exports hydrophobic substrates is distinct from those of other transporters.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

The effects of anthracycline drugs on the conformational distribution of mouse P-glycoprotein explains their transport rate differences

P-glycoprotein (Pgp) is an ATP-dependent efflux transporter and plays a major role in anti-cancer drug resistance by pumping a chemically diverse range of cytotoxic drugs from cancerous tumors. Despite numerous studies with the transporter, the molecular features that drive anti-cancer drug efflux are not well understood. Even subtle differences in the anti-cancer drug molecular structure can lead to dramatic differences in their transport rates. To unmask these structural differences, this study focused on two closely-related anthracycline drugs, daunorubicin (DNR), and doxorubicin (DOX), with mouse Pgp. While only differing by a single hydroxyl functional group, DNR has a 4 to 5-fold higher transport rate than DOX. They both non-competitively inhibited Pgp-mediated ATP hydrolysis below basal levels. The K_m of Pgp-mediated ATP hydrolysis extracted from the kinetics curves was lower for DOX than DNR. However, the dissociation constants (K_Ds) for these drugs determined by fluorescence quenching were virtually identical. Acrylamide quenching of Pgp tryptophan fluorescence to probe the tertiary structure of Pgp suggested that DNR shifts Pgp to a "closed" conformation, while DOX shifts Pgp to an "intermediate" conformation. The effects of these drugs on the Pgp conformational distributions in a lipid bilayer were also examined by atomic force microscopy (AFM). Analysis of AFM images revealed that DNR and DOX cause distinct and significant shifts in the conformational distribution of Pgp. The results were combined to build a conformational distribution model for anthracycline transport by Pgp.

Global marine pollutants inhibit P-glycoprotein: Environmental levels, inhibitory effects, and cocrystal structure

The world's oceans are a global reservoir of persistent organic pollutants to which humans and other animals are exposed. Although it is well known that these pollutants are potentially hazardous to human and environmental health, their impacts remain incompletely understood. We examined how persistent organic pollutants interact with the drug efflux transporter P-glycoprotein (P-gp), an evolutionarily conserved defense protein that is essential for protection against environmental toxicants. We identified specific congeners of organochlorine pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers that inhibit mouse and human P-gp, and determined their environmental levels in yellowfin tuna from the Gulf of Mexico. In addition, we solved the cocrystal structure of P-gp bound to one of these inhibitory pollutants, PBDE (polybrominated diphenyl ether)-100, providing the first view of pollutant binding to a drug transporter. The results demonstrate the potential for specific binding and inhibition of mammalian P-gp by ubiquitous congeners of persistent organic pollutants present in fish and other foods, and argue for further consideration of transporter inhibition in the assessment of the risk of exposure to these chemicals.

Providing a molecular mechanism for P-glycoprotein; why would I bother?

It is almost 40 years since the drug efflux pump P-glycoprotein (permeability glycoprotein or P-gp) was shown to confer multi-drug resistance in cancer cells. This protein has been one of the most extensively investigated transport proteins due to its intriguing mechanism and its affect in oncology. P-gp is known to interact with over 300 compounds and the ability to achieve this has not yet been revealed. Following the binding of substrate and nucleotide, a complex series of conformational changes in the membrane and cytosolic domains translocates substrate across the membrane. Despite over 30 years of biochemical investigation, the availability of structural data and a plethora of chemical tools to modulate its function, the molecular mechanism remains a mystery. In addition, overcoming its activity in resistant cancer cells has not been achieved in the clinic, thereby garnering some degree of pessimism in the field. This review highlights the progress that has been achieved in understanding this complex protein and the value of undertaking molecular studies.

Current advances on ABC drug transporters in fish

Most members of the large ATP-binding cassette (ABC) gene family are transporters involved in substrate translocation across biological membranes. In eukaryotes, ABC proteins functioning as drug transporters are located in the plasma membrane and mediate the cellular efflux of a wide range of organic chemicals, with some transporters also transporting certain metals. As the enhanced expression of ABC drug transporters can confer multidrug resistance (MDR) to cancers and multixenobiotic resistance (MXR) to organisms from polluted habitats, these ABC family members are also referred to as MDR or MXR proteins. In mammals, ABC drug transporters show predominant expression in tissues involved in excretion or constituting internal or external body boundaries, where they facilitate the excretion of chemicals and their metabolites, and limit chemical uptake and penetration into "sanctuary" sites of the body. Available knowledge about ABC proteins is still limited in teleost fish, a large vertebrate group of high ecological and economic importance. Using transport activity measurements and immunochemical approaches, early studies demonstrated similarities in the tissue distribution of ABC drug transporters between teleosts and mammals, suggesting conserved roles of the transporters in the biochemical defence against toxicants. Recently, the availability of teleost genome assemblies has stimulated studies of the ABC family in this taxon. This review summarises the current knowledge regarding the genetics, functional properties, physiological function, and ecotoxicological relevance of teleostean ABC transporters. The available literature is reviewed with emphasis on recent studies addressing the tissue distribution, substrate spectrum, regulation, physiological function and phylogenetic origin of teleostean ABC transporters.

Kinetic validation of the models for P-glycoprotein ATP hydrolysis and vanadate-induced trapping. Proposal for additional steps

P-Glycoprotein, a member of the ATP-binding cassette (ABC) superfamily, is a multidrug transporter responsible for cellular efflux of hundreds of structurally unrelated compounds, including natural products, many clinically used drugs and anti-cancer agents. Expression of P-glycoprotein has been linked to multidrug resistance in human cancers. ABC transporters are driven by ATP hydrolysis at their two cytoplasmic nucleotide-binding domains, which interact to form a closed ATP-bound sandwich dimer. Intimate knowledge of the catalytic cycle of these proteins is clearly essential for understanding their mechanism of action. P-Glycoprotein has been proposed to hydrolyse ATP by an alternating mechanism, for which there is substantial experimental evidence, including inhibition of catalytic activity by trapping of ortho-vanadate at one nucleotide-binding domain, and the observation of an asymmetric occluded state. Despite many studies of P-glycoprotein ATPase activity over the past 20 years, no comprehensive kinetic analysis has yet been carried out, and some puzzling features of its behaviour remain unexplained. In this work, we have built several progressively more complex kinetic models, and then carried out simulations and detailed analysis, to test the validity of the proposed reaction pathway employed by P-glycoprotein for ATP hydrolysis. To establish kinetic parameters for the catalytic cycle, we made use of the large amount of published data on ATP hydrolysis by hamster P-glycoprotein, both purified and in membrane vesicles. The proposed kinetic scheme(s) include a high affinity priming reaction for binding of the first ATP molecule, and an independent pathway for ADP binding outside the main catalytic cycle. They can reproduce to varying degrees the observed behavior of the protein's ATPase activity and its inhibition by ortho-vanadate. The results provide new insights into the mode of action of P-glycoprotein, and some hypotheses about the nature of the occluded nucleotide-bound state.

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.

Tandutinib (MLN518/CT53518) targeted to stem-like cells by inhibiting the function of ATP-binding cassette subfamily G member 2

Tandutinib is a novel inhibitor of tyrosine kinases FLT3, PDGFR and KIT. Our study was to explore the capability of tandutinib to reverse ABC transporter-mediated multidrug resistance. Tandutinib reversed ABCG2-mediated drug resistance in ABCG2-482-R2, ABCG2-482-G2, ABCG2-482-T7 and S1-M1-80 cells and increased the accumulation of doxorubicin, rhodamine 123 and [H(3)] mitoxantrone in ABCG2-overexpressing cells. Importantly, tandutinib selectively sensitized side population cells to mitoxantrone. Taken together, our results advocate the potency of tandutinib as an ABCG2 modulator and stem-like cells targeted agent to increase efficiency of anticancer drugs.

Molecular analysis of the multidrug transporter, P-glycoprotein

Inherent or acquired resistance of tumor cells to cytotoxic drugs represents a major limitation to the successful chemotherapeutic treatment of cancer. During the past three decades dramatic progress has been made in the understanding of the molecular basis of this phenomenon. Analyses of drug-selected tumor cells which exhibit simultaneous resistance to structurally unrelated anti-cancer drugs have led to the discovery of the human MDR1 gene product, P-glycoprotein, as one of the mechanisms responsible for multidrug resistance. Overexpression of this 170 kDa N-glycosylated plasma membrane protein in mammalian cells has been associated with ATP-dependent reduced drug accumulation, suggesting that P-glycoprotein may act as an energy-dependent drug efflux pump. P-glycoprotein consists of two highly homologous halves each of which contains a transmembrane domain and an ATP binding fold. This overall architecture is characteristic for members of the ATP-binding cassette or ABC superfamily of transporters. Cell biological, molecular genetic and biochemical approaches have been used for structure-function studies of P-glycoprotein and analysis of its mechanism of action. This review summarizes the current status of knowledge on the domain organization, topology and higher order structure of P-glycoprotein, the location of drug- and ATP binding sites within P-glycoprotein, its ATPase and drug transport activities, its possible functions as an ion channel, ATP channel and lipid transporter, its potential role in cholesterol biosynthesis, and the effects of phosphorylation on P-glycoprotein activity.

Lipid bilayer composition affects transmembrane protein orientation and function

Sperm membranes change in structure and composition upon ejaculation to undergo capacitation, a molecular transformation which enables spermatozoa to undergo the acrosome reaction and be capable of fertilization. Changes to the membrane environment including lipid composition, specifically lipid microdomains, may be responsible for enabling capacitation. To study the effect of lipid environment on proteins, liposomes were created using lipids extracted from bull sperm membranes, with or without a protein (Na⁺ K⁺-ATPase or α-amylase). Protein incorporation, function, and orientation were determined. Fluorescence resonance energy transfer (FRET) confirmed protein inclusion in the lipid bilayer, and protein function was confirmed using a colourometric assay of phosphate production from ATP cleavage. In the native lipid liposomes, ATPase was oriented with the β subunit facing the outer leaflet, while changing the lipid composition to 50% native lipids and 50% exogenous lipids significantly altered this orientation of Na⁺ K⁺-ATPase within the membranes.

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.

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

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

Insect cell versus bacterial overexpressed membrane proteins: an example, the human ABCG2 transporter

The multidrug resistance phenotype of cancer cells has been often related to overexpression of plasma membrane ATP-binding cassette transporters, which are able to efflux many types of drug by using the energy of ATP hydrolysis. ABCG2 is a half-transporter recently involved. Its purification would help to understand the mechanism of both transport and its inhibition. Biophysical, structural, and functional studies are consuming great amounts of homogeneous purified proteins and require efficient overexpression systems. Heterologous overexpression of human membrane proteins is actually a challenge because these proteins are toxic for the host, and both translation and chaperone systems of the host are not well adapted to the biosynthesis of human proteins. Overexpression of ABCG2 has been assayed in both bacterial and insect cell/baculovirus systems. Although it was highly overexpressed in bacterial system, neither transport nor ATPase activity was found within inverted membrane vesicles. By contrast, insect cells/baculovirus system produces a low amount of protein, a part of which is active.

Functional role of the linker region in purified human P-glycoprotein

Human P-glycoprotein (P-gp), which conveys multidrug resistance, is an ATP-dependent drug efflux pump that transports a wide variety of structurally unrelated compounds out of cells. P-gp possesses a 'linker region' of approximately 75 amino acids that connects two homologous halves, each of which contain a transmembrane domain followed by a nucleotide-binding domain. To investigate the role of the linker region, purified human P-gp was cleaved by proteases at the linker region and then compared with native P-gp. Based on a verapamil-stimulated ATP hydrolase assay, size-exclusion chromatography analysis and a thermo-stability assay, cleavage of the P-gp linker did not directly affect the preservation of the overall structure or the catalytic process in ATP hydrolysis. However, linker cleavage increased the k(cat) values both with substrate (k(sub)) and without substrate (k(basal)), but decreased the k(sub)/k(basal) values of all 10 tested substrates. The former result indicates that cleaving the linker activates P-gp, while the latter result suggests that the linker region maintains the tightness of coupling between the ATP hydrolase reaction and substrate recognition. Inspection of structures of the P-gp homolog, MsbA, suggests that linker-cleaved P-gp has increased ATP hydrolase activity because the linker interferes with a conformational change that accompanies the ATP hydrolase reaction. Moreover, linker cleavage affected the specificity constants [k(sub)/K(m(D))] for some substrates (i.e. linker cleavage probably shifts the substrate specificity profile of P-gp). Thus, this result also suggests that the linker region regulates the inherent substrate specificity of P-gp.

Characterization of the ATPase activity of a novel chimeric fusion protein consisting of the two nucleotide binding domains of MRP1

Nucleotide Binding Domains (NBDs) are responsible for the ATPase activity of the multidrug resistance protein 1 (MRP1). A series of NBD1-linker-NBD2 chimeric fusion proteins were constructed, expressed and purified, and their ATPase activities were analyzed. We report here that a GST linked NBD1(642-890)-GST-NBD2(1286-1531) was able to hydrolyze ATP at a rate of about 4.6 nmol/mg/min (K(m)=2.17 mM, V(max)=12.36 nmol/mg/min), which was comparable to the purified and reconstituted MRP1. In contrast, neither a mixture of NBD1 and GST-NBD2 nor the NBD1-GST-NBD1 fusion protein showed detectable ATPase activity. Additionally, the E1455Q mutant was found to be nonfunctional. Measurements by both MIANS labeling and circular dichroism spectroscopy revealed significant conformational differences in the NBD1-GST-NBD2 chimeric fusion protein compared to the mixture of NBD1 and GST-NBD2. The results suggest a direct interaction mediated by GST between the two NBDs of MRP1 leading to conformational changes which would enhance its ATPase activity.

A synonymous polymorphism in a common MDR1 (ABCB1) haplotype shapes protein function

The MDR1 (ABCB1) gene encodes a membrane-bound transporter that actively effluxes a wide range of compounds from cells. The overexpression of MDR1 by multidrug-resistant cancer cells is a serious impediment to chemotherapy. MDR1 is expressed in various tissues to protect them from the adverse effect of toxins. The pharmacokinetics of drugs that are also MDR1 substrates also influence disease outcome and treatment efficacy. Although MDR1 is a well-conserved gene, there is increasing evidence that its polymorphisms affect substrate specificity. Three single nucleotide polymorphisms (SNPs) occur frequently and have strong linkage, creating a common haplotype at positions 1236C>T (G412G), 2677G>T (A893S) and 3435C>T (I1145I). The frequency of the synonymous 3435C>T polymorphism has been shown to vary significantly according to ethnicity. Existing literature suggests that the haplotype plays a role in response to drugs and disease susceptibility. This review summarizes recent findings on the 3435C>T polymorphism of MDR1 and the haplotype to which it belongs. A possible molecular mechanism of action by ribosome stalling that can change protein structure and function by altering protein folding is discussed.

Ins and outs of the ABCG2 multidrug transporter: an update on in vitro functional assays

The major aim of this chapter is to provide a critical overview of the in vitro methods available for studying the function of the ABCG2 multidrug transporter protein. When describing the most applicable assay systems, in each case we present a short overview relevant to ABC multidrug transporters in general, and then we concentrate on the tools applicable to analysis of substrate-drug interactions, the effects of potential activators and inhibitors, and the role of polymorphisms of the ABCG2 transporter. Throughout this chapter we focus on recently developed assay systems, which may provide new possibilities for analyzing the pharmacological aspects of this medically important protein.

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.

Utilization of membrane vesicle preparations to study drug-ABC transporter interactions

BACKGROUND

The last 15 years have marked an expansion in our understanding of how ABC transporters modulate the pharmacokinetic properties of drugs. Assays based on different membrane preparations were one of the first methods developed to study ABC transporters. Later, they turned out to be valuable tools to gain insight into the nature of drug-ABC transporter interactions.

OBJECTIVES

Membranes prepared from different sources have been used and characterized; based on the biochemical characteristics of the transport process, a number of different assay types have been developed.

METHODS

This review focuses on the current experiences on how different membrane-based assays can be utilized in pharmaceutical R&D. Sources of membrane preparations, available assay types and correlation studies between different in-vitro and in-vivo methods are discussed.

RESULTS/CONCLUSION

Membrane-based assays are valuable tools in drug discovery to characterize drug-ABC transporter interactions.

P-glycoprotein efflux transporter activity often displays biphasic dose-response relationships

The occurrence of P-glycoprotein efflux transporter systems is recognized as playing critical roles in chemotherapy, drug pharmacokinetics, and the bioavailability of environmental toxins. This article reveals that P-glycoprotein efflux transporter activity commonly displays a biphasic dose response, with low doses being stimulatory and high doses inhibiting. The quantitative features of these biphasic dose responses are consistent with the hormetic dose-response model and are independent of biological model and chemical class. These findings provide further support for the generalizability of the hormetic dose-response model while having important biological and clinical implications, including transport through the blood-brain barrier.

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.

(R)- and (S)-verapamil differentially modulate the multidrug-resistant protein MRP1

The multidrug-resistant protein MRP1 (involved in the cancer cell multidrug resistance phenotype) has been found to be modulated by racemic verapamil (through stimulation of glutathione transport), inducing apoptosis of human MRP1 cDNA-transfected baby hamster kidney 21 (BHK-21) cells and not of control BHK-21 cells. In this study, we show that the two enantiomers of verapamil have different effects on MRP1 activity. Only the S-isomer (not the R-isomer) potently induced the death of MRP1-transfected BHK-21 cells. The decrease in cellular glutathione content induced by the S-isomer, which was not observed with the R-isomer, was stronger than that induced by the racemic mixture, indicating that the R-isomer antagonized the S-isomer effect. Both enantiomers altered leukotriene C(4) and calcein transport by MRP1. Thus, the R-isomer behaved as an inhibitor, which was confirmed by its ability to revert the multidrug resistance phenotype toward vincristine. Molecular studies on purified MRP1 using fluorescence spectroscopy showed that both enantiomers bound to MRP1 with high affinity, with the binding being prevented by glutathione. Furthermore, conformational changes induced by the two enantiomers (monitored by sodium iodide accessibility of MRP1 tryptophan residues) were quite different, correlating with their distinct effects. (S)-Verapamil induces the death of potentially resistant tumor cells, whereas (R)-verapamil sensitizes MRP1-overexpressing cells to chemotherapeutics. These results might be of great potential interest in the design of new compounds able to modulate MRP1 in chemotherapy.

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.

Stimulatory effect of insecticides on partially purified P-glycoprotein ATPase from the resistant pest Helicoverpa armigera

A P-glycoprotein-like protein (Ha-Pgp) was detected in a membrane preparation from the insecticide-resistant pest Helicoverpa armigera (Lepidoptera: Noctüidae) using C219 antibodies that are directed towards an epitope in the nucleotide-binding domains. This protein was partially purified and found to be a glycoprotein displaying ATPase activity. SDS-PAGE confirmed that a high molecular mass glycoprotein (150 kDa) was overexpressed in resistant pests, but was not detected in susceptible pests. The partially purified Ha-Pgp ATPase was reconstituted into proteoliposomes and it was found that some insecticides, namely, monocrotophos, endosulfan, cypermethrin, fenvalerate, and methylparathion, stimulated the ATPase activity. The effect of various inhibitors on partially purified Ha-Pgp showed that orthovanadate is a potent inhibitor of its ATPase activity, inhibiting it by 90% at a concentration of 2 mmol/L. Other inhibitors, such as EDTA, sodium azide, and molybdate resulted in only a 20% decrease in activity. Details of the structure and function of Ha-Pgp will be important in the development of strategies to overcome insecticide resistance in this pest.

Recent progress in understanding the mechanism of P-glycoprotein-mediated drug efflux

P-glycoprotein (P-gp) is an ATP-dependent drug pump that can transport a broad range of hydrophobic compounds out of the cell. The protein is clinically important because of its contribution to the phenomenon of multidrug resistance during AIDS/HIV and cancer chemotherapy. P-gp is a member of the ATP-binding cassette (ABC) family of proteins. It is a single polypeptide that contains two repeats joined by a linker region. Each repeat has a transmembrane domain consisting of six transmembrane segments followed by a hydrophilic domain containing the nucleotide-binding domain. In this mini-review, we discuss recent progress in determining the structure and mechanism of human P-glycoprotein.

ATPase activity of purified plasma membranes and digestive vacuoles from Plasmodium falciparum

The ATPase activity of the human malaria parasite, Plasmodium falciparum was investigated using two experimental systems, (i) digestive vacuoles, and (ii) purified plasma membranes isolated from a chloroquine-sensitive and a chloroquine-resistant strain. No correlation between the level of ATPase activity and chloroquine sensitivity could be detected. In both systems, the ATPase activity of the chloroquine-resistant and -sensitive strain was decreased in the presence of the P-glycoprotein inhibitor vanadate. Susceptibility to inhibition by vanadate together with the lack of effect of ouabain implies a P-type ATPase activity in the plasma membrane. Furthermore, the inhibition of Fac8 ATPase activity by oligomycin both in the digestive vacuoles and the plasma membranes would be consistent with higher levels of Pgh1 in Fac8. Our data are consistent with the presence of a V-type H+-ATPase in the parasite food vacuole. Bafilomycin A1 and N-ethylmaleimide decreased the vacuolar ATPase activity in both chloroquine-resistant and -sensitive strains. Interestingly, a 30% decrease was observed between the ATPase activity of plasma membranes isolated from Fac8 and D10 in the presence of bafilomycin A1, suggesting the presence of a V-type ATPase in D10 plasma membrane that is underexpressed or altered in the plasma membrane of the chloroquine-resistant Fac8. The chemosensitisers tested had no effect on the ATPase activity of chloroquine-resistant P. falciparum in both systems suggesting that their activity is not mediated through an ATP-dependent mechanism. No effect was observed on the vacuolar ATPase activity in the presence of the antimalarials tested indicating that an ATP-dependent transport has not been activated.

Mutational analysis of P-glycoprotein: suppression of caspase activation in the absence of ATP-dependent drug efflux

P-glycoprotein (P-gp) can induce multidrug resistance (MDR) through the ATP-dependent efflux of chemotherapeutic agents. We have previously shown that P-gp can inhibit nondrug apoptotic stimuli by suppressing the activation of caspases. To determine if this additional activity is functionally linked to ATP hydrolysis, we expressed wild-type and ATPase-mutant P-gp and showed that cells expressing mutant P-gp could not efflux chemotherapeutic drugs but remained relatively resistant to apoptosis. CEM lymphoma cells expressing mutant P-gp treated with vincristine showed a decrease in the fraction of cells with apoptotic morphology, cytochrome c release from the mitochondria and suppression of caspase activation, yet still accumulated in mitosis and showed a loss of clonogenic potential. The loss of clonogenicity in vincristine-treated cells expressing mutant P-gp was associated with accumulation of cells in mitosis and the presence of multinucleated cells consistent with mitotic catastrophe. The antiapoptotic effect of mutant P-gp was not affected by antibodies that inhibit the efflux function of the protein. These data are consistent with a dual activity model for P-gp-induced MDR involving both ATPase-dependent drug efflux and ATPase-independent inhibition of apoptosis. The structure-function analyses described herein provide novel insight into the mechanisms of action of P-gp in mediating MDR.

Reconstitution of ATP-dependent cGMP transport into proteoliposomes by membrane proteins from human erythrocytes

The cellular efflux of cGMP from human erythrocytes has previously been characterized in functional studies. The purpose of the present study was to find membrane proteins with the ability to restore ATP-dependent uptake of cGMP into proteoliposomes. Human erythrocyte membranes were solubilized with CHAPS (3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfonate) and gel filtration gave three protein fractions with the ability to restore active transport. Only two of these fractions were retained on a lentil lectin column. By using these two purification steps, active transport was 11 times higher in the first fraction compared to the original material and SDS-PAGE showed the presence of proteins with sizes of 145 kDa and 165 kDa. The second fraction gave 20 times higher active transport after purification and comprised proteins with sizes of 145 kDa and 180 kDa. At present three members of the MRP (multi-resistance associated protein) family have been detected in human erythrocytes: MRPI, MRP4 and MRP5. The last two proteins have been shown to transport cyclic nucleotides. The present findings are compatible with MRP4 as the 145 kDa protein, MRP5 as the 165 kDa protein and MRP1 as the 180 kDa protein. However, the 145 kDa protein could also be SMRP (short multi-resistance protein), the gene splice variant of MRP5. Immunoprecipitation of MRP5 from CHAPS-solubilized extract reduced active transport and specific binding by about 45% and 40%, respectively. This shows that MRP5 is an important cGMP-transporting protein in human erythrocytes.

Methanethiosulfonate Derivatives of Rhodamine and Verapamil Activate Human P-glycoprotein at Different Sites

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) actively extrudes a broad range of potentially cytotoxic compounds out of the cell. Key steps in understanding the transport process are binding of drug substrates in the transmembrane domains, initiation of ATPase activity, and subsequent drug efflux. We used cysteine-scanning mutagenesis of the transmembrane segment residues and reaction with the thiol-reactive drug substrate analog of rhodamine, methane-thiosulfonate-rhodamine (MTS-rhodamine), to test whether P-gp could be trapped in an activated state with high levels of ATPase activity. The presence of such an activated P-gp could be used to further investigate P-gp-drug substrate interactions. Single cysteine mutants (149) were treated with MTS-rhodamine, and ATPase activities were determined after removal of unreacted MTS-rhodamine. One mutant, F343C(TM6), showed a 5.8-fold increase in activity after reaction with MTS-rhodamine. Pre-treatment of mutant F343C with rhodamine B protected it from activation by MTS-rhodamine, indicating that residue Cys-343 contributes to the rhodamine-binding site. The ATPase activity of MTS-rhodamine-treated mutant F343C, however, was not stimulated further by colchicine or calcein-AM. By contrast, verapamil and Hoechst 33342 stimulated and inhibited, respectively, the ATPase activity of the MTS-rhodamine-treated mutant F343C. These results indicate that the MTS-rhodamine binding site overlaps that of colchicine and calcein-AM but not that of verapamil and Hoechst 33342 within the common drug-binding pocket.

Simultaneous binding of two different drugs in the binding pocket of the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) transports a wide variety of structurally diverse compounds out of the cell. The drug-binding pocket of P-gp is located in the transmembrane domains. Although occupation of the drug-binding pocket by one molecule is sufficient to activate the ATPase activity of P-gp, the drug-binding pocket may be large enough to accommodate two different substrates at the same time. In this study, we used cysteine-scanning mutagenesis to test whether P-gp could simultaneously interact with the thiol-reactive drug substrate, Tris-(2-maleimidoethyl)amine (TMEA) and a second drug substrate. TMEA is a cross-linker substrate of P-gp that allowed us to test for stimulation of cross-linking by a second substrate such as calcein-acetoxymethyl ester, colchicine, demecolcine, cyclosporin A, rhodamine B, progesterone, and verapamil. We report that verapamil induced TMEA cross-linking of mutant F343C(TM6)/V982C(TM12). By contrast, no cross-linked product was detected in mutants F343C(TM6), V982C(TM12), or F343C(TM6)/V982C(TM12) in the presence of TMEA alone. The verapamil-stimulated ATPase activity of mutant F343C(TM6)/V982C(TM12) in the presence of TMEA decreased with increased cross-linking of the mutant protein. These results show that binding of verapamil must induce changes in the drug-binding pocket (induced-fit mechanism) resulting in exposure of residues F343C(TM6)/V982C(TM12) to TMEA. The results also indicate that the common drug-binding pocket in P-gp is large enough to accommodate both verapamil and TMEA simultaneously and suggests that the substrates must occupy different regions in the common drug-binding pocket.

Permanent activation of the human P-glycoprotein by covalent modification of a residue in the drug-binding site

The human multidrug resistance P-glycoprotein (ABCB1) transports a broad range of structurally diverse compounds out of the cell. The transport cycle involves coupling of drug binding in the transmembrane domains with ATP hydrolysis. Compounds such as verapamil stimulate ATPase activity. We used cysteine-scanning mutagenesis of the transmembrane segments and reaction with the thiol-reactive substrate analog of verapamil, methanethiosulfonate (MTS)-verapamil, to test whether it caused permanent activation of ATP hydrolysis. Here we report that one mutant, I306C(TM5) showed increased ATPase activity (8-fold higher than untreated) when treated with MTS-verapamil and isolated by nickel-chelate chromatography. Drug substrates that either enhance (calcein acetoxymethyl ester, demecolcine, and vinblastine) or inhibit (cyclosporin A and trans-(E)-flupentixol) ATPase activity of Cys-less or untreated mutant I306C P-glycoprotein did not affect the activity of MTS-verapamil-treated mutant I306C. Addition of dithiothreitol released the covalently attached verapamil, and ATPase activity returned to basal levels. Pretreatment with substrates such as cyclosporin A, demecolcine, verapamil, vinblastine, or colchicine prevented activation of mutant I306C by MTS-verapamil. The results suggest that MTS-verapamil reacts with I306C in a common drug-binding site. Covalent modification of I306C affects the long range linkage between the drug-binding site and the distal ATP-binding sites. This results in the permanent activation of ATP hydrolysis in the absence of transport. Trapping mutant I306C in a permanently activated state indicates that Ile-306 may be part of the signal to switch on ATP hydrolysis when the drug-binding site is occupied.

Substrate-induced conformational changes in the transmembrane segments of human P-glycoprotein. Direct evidence for the substrate-induced fit mechanism for drug binding

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) is quite promiscuous in that it can transport a broad range of structurally diverse compounds out of the cell. We hypothesized that the transmembrane (TM) segments that constitute the drug-binding site are quite mobile such that drug binding occurs through a "substrate-induced fit" mechanism. Here, we used cysteine-scanning mutagenesis and oxidative cross-linking to test for substrate-induced changes in the TM segments. Pairs of cysteines were introduced into a Cys-less P-gp and the mutants treated with oxidant (copper phenanthroline) in the presence or absence of various drug substrates. We show that cyclosporin A promoted cross-linking between residues P350C(TM6)/G939C(TM11), while colchicine and demecolcine promoted cross-linking between residues P350C(TM6)/V991C(TM12). Progesterone promoted cross-linking between residues P350C(TM6)/A935C(TM11), P350C(TM6)/G939C(TM11), as well as between residues P350C(TM6)/V991C(TM12). Other substrates such as vinblastine, verapamil, cis-(Z)-flupenthixol or trans-(E)-flupenthixol did not induce cross-linking at these sites. These results provide direct evidence that the packing of the TM segments in the drug-binding site is changed when P-gp binds to a particular substrate. The induced-fit mechanism explains how P-gp can accommodate a broad range of compounds.

Drug binding in human P-glycoprotein causes conformational changes in both nucleotide-binding domains

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) uses ATP to transport many structurally diverse compounds out of the cell. It is an ABC transporter with two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). Recently, we showed that the "LSGGQ" motif in one NBD ((531)LSGGQ(535) in NBD1; (1176)LSGGQ(1180) in NBD2) is adjacent to the "Walker A" sequence ((1070)GSSGCGKS(1077) in NBD2; (427)GNSGCGKS(434) in NBD1) in the other NBD (Loo, T. W., Bartlett, M. C., and Clarke, D. M. (2002) J. Biol. Chem. 277, 41303-41306). Drug substrates can stimulate or inhibit the ATPase activity of P-gp. Here, we report the effect of drug binding on cross-linking between the LSGGQ signature and Walker A sites (Cys(431)(NBD1)/C1176C(NBD2) and Cys(1074)(NBD2)/L531C(NBD1), respectively). Seven drug substrates (calcein-AM, demecolcine, cis(Z)-flupentixol, verapamil, cyclosporin A, Hoechst 33342, and trans(E)-flupentixol) were tested for their effect on oxidative cross-linking. Substrates that stimulated the ATPase activity of P-gp (calcein-AM, demecolcine, cis(Z)-flupentixol, and verapamil) increased the rate of cross-linking between Cys(431)(NBD1-Walker A)/C1176C(NBD2-LSGGQ) and between Cys(1074)(NBD2-Walker A)/L531C(NBD1-LSGGQ) when compared with cross-linking in the absence of drug substrate. By contrast, substrates that inhibited ATPase activity (cyclosporin A, Hoechst 33342, and trans(E)-flupentixol) decreased the rate of cross-linking. These results indicate that interaction between the LSGGQ motifs and Walker A sites must be essential for coupling drug binding to ATP hydrolysis. Drug binding in the transmembrane domains can induce long range conformational changes in the NBDs, such that compounds that stimulate or inhibit ATPase activity must decrease and increase, respectively, the distance between the Walker A and LSGGQ sequences.

Functional analysis of candidate ABC transporter proteins for sitosterol transport

Two ATP-binding cassette (ABC) proteins, ABCG5 and ABCG8, have recently been associated with the accumulation of dietary cholesterol in the sterol storage disease sitosterolemia. These two 'half-transporters' are assumed to dimerize to form the complete sitosterol transporter which reduces the absorption of sitosterol and related molecules in the intestine by pumping them back into the lumen. Although mutations altering ABCG5 and ABCG8 are found in affected patients, no functional demonstration of sitosterol transport has been achieved. In this study, we investigated whether other ABC transporters implicated in lipid movement and expressed in tissues with a role in sterol synthesis and absorption, might also be involved in sitosterol transport. Transport by the multidrug resistance P-glycoprotein (P-gp; Abcb1), the multidrug resistance-associated protein (Mrp1; Abcc1), the breast cancer resistance protein (Bcrp; Abcg2) and the bile salt export pump (Bsep; Abcb11) was assessed using several assays. Unexpectedly, none of the candidate proteins mediated significant sitosterol transport. This has implications for the pathology of sitosterolemia. In addition, the data suggest that otherwise broad-specific ABC transporters have acquired specificity to exclude sitosterol and related sterols like cholesterol presumably because the abundance of cholesterol in the membrane would interfere with their action; in consequence, specific transporters have evolved to handle these sterols.

Location of the rhodamine-binding site in the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein (P-gp) pumps a wide variety of structurally diverse compounds out of the cell. It is an ATP-binding cassette transporter with two nucleotide-binding domains and two transmembrane (TM) domains. One class of compounds transported by P-gp is the rhodamine dyes. A P-gp deletion mutant (residues 1-379 plus 681-1025) with only the TM domains retained the ability to bind rhodamine. Therefore, to identify the residues involved in rhodamine binding, 252 mutants containing a cysteine in the predicted TM segments were generated and reacted with a thiol-reactive analog of rhodamine, methanethiosulfonate (MTS)-rhodamine. The activities of 28 mutants (in TMs 2-12) were inhibited by at least 50% after reaction with MTS-rhodamine. The activities of five mutants, I340C(TM6), A841C(TM9), L975C(TM12), V981C(TM12), and V982C(TM12), however, were significantly protected from inhibition by MTS-rhodamine by pretreatment with rhodamine B, indicating that residues in TMs 6, 9, and 12 contribute to the binding of rhodamine dyes. These results, together with those from previous labeling studies with other thiol-reactive compounds, dibromobimane, MTS-verapamil, and MTS-cross-linker substrates, indicate that common residues are involved in the binding of structurally different drug substrates and that P-gp has a common drug-binding site. The results support the "substrate-induced fit" hypothesis for drug binding.

Vanadium in cancer treatment

Vanadium compounds exert preventive effects against chemical carcinogenesis on animals, by modifying, mainly, various xenobiotic enzymes, inhibiting, thus, carcinogen-derived active metabolites. Studies on various cell lines reveal that vanadium exerts its antitumor effects through inhibition of cellular tyrosine phosphatases and/or activation of tyrosine phosphorylases. Both effects activate signal transduction pathways leading either to apoptosis and/or to activation of tumor suppressor genes. Furthermore, vanadium compounds may induce cell-cycle arrest and/or cytotoxic effects through DNA cleavage and fragmentation and plasma membrane lipoperoxidation. Reactive oxygen species generated by Fenton-like reactions and/or during the intracellular reduction of V(V) to V(IV) by, mainly, NADPH, participate to the majority of the vanadium-induced intracellular events. Vanadium may also exert inhibitory effects on cancer cell metastatic potential through modulation of cellular adhesive molecules, and reverse antineoplastic drug resistance. It also possesses low toxicity that, in combination with the synthesis of new, more potent and better tolerated complexes, may establish vanadium as an effective non-platinum, metal antitumor agent.

Preparation of pure and intact Plasmodium falciparum plasma membrane vesicles and partial characterisation of the plasma membrane ATPase

BACKGROUND

In host erythrocytes, the malaria parasite must contend with ion and drug transport across three membranes; its own plasma membrane, the parasitophorous membrane and the host plasma membrane. Isolation of pure and intact Plasmodium falciparum plasma membrane would provide a suitable model to elucidate the possible role played by the parasite plasma membrane in ion balance and drug transport.

RESULTS

This study describes a procedure for isolating parasite plasma membrane from P. falciparum-infected erythrocytes. With this method, the trophozoites released by saponin treatment were cleansed of erythrocyte membranes using anti-erythrocyte antibodies fixed to polystyrene beads. These trophozoites were then biotinylated and the parasite plasma membrane was disrupted by nitrogen cavitation. This process allows the membranes to reform into vesicles. The magnetic streptavidin beads bind specifically to the biotinylated parasite plasma membrane vesicles facilitating their recovery with a magnet. These vesicles can then be easily released from the magnetic beads by treatment with dithiotreithol. The parasite plasma membrane showed optimal ATPase activity at 2 mM ATP and 2 mM Mg2+. It was also found that Ca2+ could not substitute for Mg2+ ATPase activity in parasite plasma membranes whereas activity was completely preserved when Mn2+ was used instead of Mg2+. Other nucleoside triphosphates tested were hydrolysed as efficiently as ATP, while the nucleoside monophosphate AMP was not.

CONCLUSIONS

We have described the successful isolation of intact P. falciparum plasma membrane vesicles free of contaminating organelles and determined the experimental conditions for optimum ATPase activity.

Vanadate trapping of nucleotide at the ATP-binding sites of human multidrug resistance P-glycoprotein exposes different residues to the drug-binding site

The human multidrug resistance P-glycoprotein uses ATP to transport a wide variety of structurally unrelated cytotoxic compounds out of the cell. In this study, we used cysteine-scanning mutagenesis and cross-linking studies to identify residues that are exposed to the drug-binding site upon vanadate trapping. In the absence of nucleotides, C222(TM4) was cross-linked to C868(TM10) and C872(TM10); C306(TM5) was cross-linked to C868(TM10), C872(TM10), C945(TM11), C982(TM12), and C984(TM12); and C339(TM6) was cross-linked to C868(TM10), C872(TM10), C942(TM11), C982(TM12), and C985(TM12). These cysteines are in the middle of the predicted transmembrane (TM) segments and form the drug-binding site. Cross-linking between 332C(TM6) and cysteines introduced at the extracellular side of other TM segments was also done. In the absence of nucleotides, residues 332C and 856C on the extracellular side of TMs 6 and 10, respectively, were cross-linked with a 13-A cross-linker (M8M, 3,6-dioxaoctane-1,8-diyl bismethanethiosulfonate). ATP plus vanadate inhibited cross-linking between 332C(TM6) and 856C(TM10) as well as those in the drug-binding site. Instead, vanadate trapping promoted cross-linking between 332C(TM6) and 976C(TM12) with a 10-A cross-linker (M6M, 1,6-hexanediyl bismethanethiosulfonate). When ATP hydrolysis was allowed to proceed, then 332C(TM12) could form a disulfide bond with 975C(TM12). The cross-linking pattern of 332C(TM6) with residues in TM10 and TM12 indicates that the drug-binding site undergoes dynamic and relatively large conformational changes, and that different residues are exposed to the drug-binding site during the resting phase, upon vanadate trapping and at the completion of the catalytic cycle.

MDR1 causes resistance to the antitumour drug miltefosine

Miltefosine (hexadecylphosphocholine) is used for topical treatment of breast cancers. It has been shown previously that a high percentage of breast carcinomas express MDR1 or MRP. We investigated the sensitivity of MDR1 -expressing cells to treatment with miltefosine. We show that cells overexpressing MDR1 (NCI/ADR-RES, KB-8-5, KB-C1, CCRF/VCR1000, CCRF/ADR5000) were less sensitive to miltefosine treatment when compared to the sensitive parental cell lines. HeLa cells transfected with MDR1 exhibited resistance to the compound, indicating that expression of this gene is sufficient to reduce the sensitivity to miltefosine. The resistance of MDR1-expressing cells to miltefosine was less pronounced than that to adriamycin or vinblastine. Expression of MDR2 did not correlate with the resistance to miltefosine. As shown by a fluorescence quenching assay using MIANS-labelled P-glycoprotein (PGP), miltefosine bound to PGP with a K(d)of approximately 7 microM and inhibited PGP-ATPase activity with an IC(50)of approximately 35 microM. Verapamil was not able to reverse the resistance to miltefosine. Concentrations of miltefosine up to approximately 60 microM stimulated, whereas higher concentrations inhibited the transport of [3H]-colchicine with an IC(50)of approximately 297 microM. Binding studies indicated that miltefosine seems to interact with the transmembrane domain and not the cytosolic nucleotide-binding domain of PGP. These data indicate that expression of MDR1 may reduce the response to miltefosine in patients and that this compound interacts with PGP in a manner different from a number of other substrates.

A functional assay for detection of the mitoxantrone resistance protein, MXR (ABCG2)

The fluorescent compounds rhodamine 123, LysoTracker Green DMD-26, mitoxantrone, and BODIPY-prazosin were used with the antagonist fumitremorgin C (FTC) in order to develop functional assays for the half-transporter, MXR/BCRP/ABCP1. A measure of FTC-inhibitable efflux was generated for each compound in a series of MXR-overexpressing drug-selected cell lines and in ten unselected cell lines which were used to determine if the four fluorescent compounds were sensitive enough to detect the low MXR levels found in drug-sensitive cell lines. FTC-inhibitable efflux of mitoxantrone and prazosin was found in four of the ten cell lines, SF295, KM12, NCI-H460, and A549, and low but detectable levels of MXR mRNA were also observed by Northern analysis in these cells. FTC-inhibitable mitoxantrone and prazosin efflux in both selected and unselected cell lines was found to correlate well with MXR levels as determined by Northern blotting, r(2)=0.89 and r(2)=0.70 respectively. In contrast, rhodamine and LysoTracker were not able to reliably detect MXR. Cytotoxicity assays performed on two of the four unselected cell lines confirmed increased sensitivity to mitoxantrone in the presence of FTC. FTC was found to be a specific inhibitor of MXR, with half-maximal inhibition of MXR-associated ATPase activity at 1 microM FTC. Short term selections of the SF295, KM12, NCI-H460 and A549 cell lines in mitoxantrone resulted in a small but measurable increase in MXR by both Northern blot and functional assay. These studies show that flow cytometric measurement of FTC-inhibitable mitoxantrone or prazosin efflux is a sensitive and specific method for measuring the function of the MXR half-transporter in both selected and unselected cell lines.

Inhibition of P-glycoprotein transport function by grapefruit juice psoralen

PURPOSE

The grapefruit juice component bergamottin is known to inactivate cytochrome P450 3A4, with grapefruit juice consumption causing increased absorption and enhanced oral bioavailability of many cytochrome P450 3A4 substrates. Many of these substrates are also recognized by the efflux transporter P-glycoprotein. The gene product of MDR1 (multidrug resistance transporter), P-glycoprotein also confers protection against xenobiotics.

METHODS

Using a whole ceil assay in which the retention of a marker substrate is evaluated and quantified, we studied the ability of grapefruit juice components to inhibit the function of this transporter.

RESULTS

In a cell line presenting an overexpressed amount of the human transporter, the enzyme exhibited a 40 microM IC50 for inhibition by bergamottin. Additionally, using the ATP-hydrolysis assay, we showed that bergamottin increases P-gp-mediated ATP hydrolysis by approximately 2.3 fold with a Km of 8 microM. The concentration for this interaction is similar to that for CYP3A4 inactivation.

CONCLUSIONS

These results suggest that observed grapefruit juice drug pharmacokinetic clinical interactions may be due to P-gp inhibition rather than or in addition to CYP3A4 inhibition. Inhibition of P-gp by citrus psoralens could present ways both to enhance bioavailability of therapies without increasing the dose and to diminish drug resistance in refractory cells.

ATPase activity and transport by a cGMP transporter in human erythrocyte ghosts and proteoliposome-reconstituted membrane extracts

We previously described the [(3)H]cGMP-binding characteristics of a CHAPS-solubilized protein that we proposed to be a cGMP transporter. We now report the ATPase activity of the membrane-bound, solubilized and reconstituted form of a cGMP transporter. The membrane-bound protein of unsealed ghosts had a linear ATPase activity over a 120 min incubation period with optimal activity of about 400 pmol/mg/min. The apparent K(m) and V(max) for ATP were about 0.5 mM and 300 pmol/mg/min, respectively. When solubilized with CHAPS the specific activity of the protein was reduced to about 70 pmol/mg/min. Reconstitution of the CHAPS preparation into phospholipid bilayer using rapid detergent removal by Extracti-gel column resulted in proteoliposomes which had ATPase activity similar to that found in the erythrocyte membranes. The proteoliposomes displayed a linear ATP-dependent uptake of [(3)H]cGMP with an apparent K(m) value of 1. 0 microM. This low K(m)-uptake of [(3)H]cGMP in proteoliposomes was not affected by 10 microM of AMP, cAMP and GMP, but was completely abolished in the presence of the non-hydrolyzable ATP analogue, ATP-gamma-S. Some ATPase activation was also observed in the presence of 2 microM cAMP, but it is unclear whether this activity was coupled to the cGMP transporter. Our results show that the membrane protein responsible for cGMP transport has an ATPase activity and transports the cyclic nucleotide in the presence of ATP.