Purification and reconstitution of functional human P-glycoprotein

Mechanism of 5-fluorouracil induced resistance and role of piperine and curcumin as chemo-sensitizers in colon cancer

Among cancer-related deaths worldwide, colorectal cancer ranks second, accounting for 1.2% of deaths in those under 50 years and 0.6% of deaths in those between 50 and 54 years. The anticancer drug 5-fluorouracil is widely used to treat colorectal cancer. Due to a better understanding of the drug's mechanism of action, its anticancer activity has been increased through a variety of therapeutic alternatives. Clinical use of 5-FU has been severely restricted due to drug resistance. The chemoresistance mechanism of 5-FU is challenging to overcome because of the existence of several drug efflux transporters, DNA repair enzymes, signaling cascades, classical cellular processes, cancer stem cells, metastasis, and angiogenesis. Curcumin, a potent phytocompound derived from Curcuma longa, functions as a nuclear factor (NF)-κB inhibitor and sensitizer to numerous chemotherapeutic drugs. Piperine, an alkaloid found in Piper longum, inhibits cancer cell growth, causing cell cycle arrest and apoptosis. This review explores the mechanism of 5-FU-induced chemoresistance in colon cancer cells and the role of curcumin and piperine in enhancing the sensitivity of 5-FU-based chemotherapy. CLINICAL TRIAL REGISTRATION: Not applicable.

Application and Prospect of CRISPR/Cas9 Technology in Reversing Drug Resistance of Non-Small Cell Lung Cancer

Cancer drug resistance has always been a major factor affecting the treatment of non-small cell lung cancer, which reduces the quality of life of patients. The clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9 (CRISPR/Cas9) technology, as an efficient and convenient new gene-editing technology, has provided a lot of help to the clinic and accelerated the research of cancer and drug resistance. In this review, we introduce the mechanisms of drug resistance in non-small cell lung cancer (NSCLC), discuss how the CRISPR/Cas9 system can reverse multidrug resistance in NSCLC, and focus on drug resistance gene mutations. To improve the prognosis of NSCLC patients and further improve patients' quality of life, it is necessary to utilize the CRISPR/Cas9 system in systematic research on cancer drug resistance.

Large-scale purification of functional human P-glycoprotein (ABCB1)

Human P-glycoprotein (P-gp) is an ATP-binding cassette transporter that has been implicated in altering the pharmacokinetics of anticancer drugs in normal tissues and development of multidrug resistance in tumor cells via drug efflux. There is still no definitive explanation of the mechanism by which P-gp effluxes drugs. One of the challenges of large-scale purification of membrane transporters is the selection of a suitable detergent for its optimal extraction from cell membranes. In addition, further steps of purification can often lead to inactivation and aggregation, decreasing the yield of purified protein. Here we report the large-scale purification of human P-gp expressed in High-Five insect cells using recombinant baculovirus. The purification strategies we present yield homogeneous functionally active wild type P-gp and its E556Q/E1201Q mutant, which is defective in carrying out ATP hydrolysis. Three detergents (1,2-diheptanoyol-sn-glycero-3-phosphocholine, dodecyl maltoside and n-octyl-β-d-glucopyranoside) were used to solubilize and purify P-gp from insect cell membranes. P-gp purification was performed first using immobilized metal affinity chromatography, then followed by a second step of either anion exchange chromatography or size exclusion chromatography to yield protein in concentrations of 2-12 mg/mL. Size exclusion chromatography was the preferred method, as it allows separation of monomeric transporters from aggregates. We show that the purified protein, when reconstituted in proteoliposomes and nanodiscs, exhibits both basal and substrate or inhibitor-modulated ATPase activity. This report thus provides a convenient and robust method to obtain large amounts of active homogeneously purified human P-gp that is suitable for biochemical, biophysical and structural characterization.

Reconstitution and functional studies of hamster P-glycoprotein in giant liposomes

This paper describes the preparation of giant unilamellar vesicles with reconstituted hamster P-glycoprotein (Pgp, ABCB1) for studying the transport activity of this efflux pump in individual liposomes using optical microscopy. Pgp, a member of ABC (ATP-binding cassette) transporter family, is known to contribute to the cellular multidrug resistance (MDR) against variety of drugs. The efficacy of many therapeutics is, thus, hampered by this efflux pump, leading to a high demand for simple and effective strategies to monitor the interactions of candidate drugs with this protein. Here, we applied small Pgp proteoliposomes to prepare giant Pgp-bearing liposomes via modified electroformation techniques. The presence of Pgp in the membrane of giant proteoliposomes was confirmed using immunohistochemistry. Assessment of Pgp ATPase activity suggested that this transporter retained its activity upon reconstitution into giant liposomes, with an ATPase specific activity of 439 ± 103 nmol/mg protein/min. For further confirmation, we assessed the transport activity of Pgp in these proteoliposomes by monitoring the translocation of rhodamine 123 (Rho123) across the membrane using confocal microscopy at various ATP concentrations (0-2 mM) and in the presence of Pgp inhibitors. Rate of change in Rho123 concentration inside the liposomal lumen was used to estimate the Rho123 transport rates (1/s) for various ATP concentrations, which were then applied to retrieve the Michaelis-Menten constant (Km) of ATP in Rho123 transport (0.42 ± 0.75 mM). Similarly, inhibitory effects of verapamil, colchicine, and cyclosporin A on Pgp were studied in this system and the IC50 values for these Pgp inhibitors were found 26.6 ± 6.1 μM, 94.6 ± 47.6 μM, and 0.21 ± 0.07 μM, respectively. We further analyzed the transport data using a kinetic model that enabled dissecting the passive diffusion of Rho123 from its Pgp-mediated transport across the membrane. Based on this model, the permeability coefficient of Rho123 across the liposomal membrane was approximately 1.25×10-7 cm/s. Comparing the membrane permeability in liposomes with and without Pgp revealed that the presence of this protein did not have a significant impact on membrane integrity and permeability. Furthermore, we used this model to obtain transport rate constants for the Pgp-mediated transport of Rho123 (m3/mol/s) at various ATP and inhibitor concentrations, which were then applied to estimate values of 0.53 ± 0.66 mM for Km of ATP and 25.2 ± 5.0 μM for verapamil IC50, 61.8 ± 34.8 μM for colchicine IC50, and 0.23 ± 0.09 μM for cyclosporin A IC50. The kinetic parameters obtained from the two analyses were comparable, suggesting a minimal contribution from the passive Rho123 diffusion across the membrane. This approach may, therefore, be applied for screening the transport activity of Pgp against potential drug candidates.

Human-Mouse Chimeras with Normal Expression and Function Reveal That Major Domain Swapping Is Tolerated by P-Glycoprotein (ABCB1)

The efflux transporter P-glycoprotein (P-gp) plays a vital role in the transport of molecules across cell membranes and has been shown to interact with a panoply of functionally and structurally unrelated compounds. How human P-gp interacts with this large number of drugs has not been well understood, although structural flexibility has been implicated. To gain insight into this transporter's broad substrate specificity and to assess its ability to accommodate a variety of molecular and structural changes, we generated human-mouse P-gp chimeras by the exchange of homologous transmembrane and nucleotide-binding domains. High-level expression of these chimeras by BacMam- and baculovirus-mediated transduction in mammalian (HeLa) and insect cells, respectively, was achieved. There were no detectable differences between wild-type and chimeric P-gp in terms of cell surface expression, ability to efflux the P-gp substrates rhodamine 123, calcein-AM, and JC-1, or to be inhibited by the substrate cyclosporine A and the inhibitors tariquidar and elacridar. Additionally, expression of chimeric P-gp was able to confer a paclitaxel-resistant phenotype to HeLa cells characteristic of P-gp-mediated drug resistance. P-gp ATPase assays and photo-cross-linking with [(125)I]iodoarylazidoprazosin confirmed that transport and biochemical properties of P-gp chimeras were similar to those of wild-type P-gp, although differences in drug binding were detected when human and mouse transmembrane domains were combined. Overall, chimeras with one or two mouse P-gp domains were deemed functionally equivalent to human wild-type P-gp, demonstrating the ability of human P-gp to tolerate major structural changes.

Lipid regulation of the ABCB1 and ABCG2 multidrug transporters

This chapter deals with the interactions of two medically important multidrug ABC transporters (MDR-ABC), ABCB1 and ABCG2, with lipid molecules. Both ABCB1 and ABCG2 are capable of transporting a wide range of hydrophobic drugs and xenobiotics and are involved in cancer chemotherapy resistance. Therefore, the exploration of their mechanism of action has major therapeutic consequences. As discussed here in detail, both ABCB1 and ABCG2 are significantly affected by various lipid compounds especially those residing in their close proximity in the plasma membrane. ABCB1 is capable of transporting lipids and lipid derivatives, and thus may alter the general membrane composition by "flopping" membrane lipid constituents, while there is no such information regarding ABCG2. Still, both ABCB1 and ABCG2 show complex interactions with a variety of lipid molecules, and the transporters are significantly modulated by cholesterol and cholesterol derivatives at the posttranslational level. In this chapter, we explore the molecular details of the direct transporter-lipid interactions, the potential role of lipid-sensor domains within the proteins, as well as the application of experimental site-directed mutagenesis, detailed structural studies, and in silico modeling for examining these interactions. We also discuss the regulation of ABCB1 and ABCG2 expression at the transcriptional level, occurring through nuclear receptors involved in lipid sensing. The better understanding of lipid interactions with these medically important MDR-ABC transporters may significantly improve further drug development and clinical treatment options.

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.

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.

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.

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.

A fluorescence-based technique to construct size distributions from single-object measurements: application to the extrusion of lipid vesicles

We report a novel approach to quantitatively determine complete size distributions of surface-bound objects using fluorescence microscopy. We measure the integrated intensity of single particles and relate it to their size by taking into account the object geometry and the illumination profile of the microscope, here a confocal laser scanning microscope. Polydisperse (as well as monodisperse) size distributions containing objects both below and above the optical resolution of the microscope are recorded and analyzed. The data is collected online within minutes, which allows the user to correlate the size of an object with the response from any given fluorescence-based biochemical assay. We measured the mean diameter of extruded fluorescently labeled lipid vesicles using the proposed method, dynamic light scattering, and cryogenic transmission electron microscopy. The three techniques were in excellent agreement, measuring the same values within 7-9%. Furthermore we demonstrated here, for the first time that we know of, the ability to determine the full size distribution of polydisperse samples of nonextruded lipid vesicles. Knowledge of the vesicle size distribution before and after extrusion allowed us to propose an empirical model to account for the effect of extrusion on the complete size distribution of vesicle samples.

Curcumin as chemosensitizer

This overview presents curcumin as a significant chemosensitizer in cancer chemotherapy. Although the review focuses on curcumin and its analogues on multidrug resistance (MDR) reversal, the relevance of curcumin as a nuclear factor (NF)-KB blocker and sensitizer of many chemoresistant cancer cell lines to chemotherapeutic agents will also be discussed. One of the major mechanisms of MDR is the enhanced ability of tumor cells to actively efflux drugs, leading to a decrease in cellular drug accumulation below toxic levels. Active drug efflux is mediated by several members of the ATP-binding cassette (ABC) superfamily of membrane transporters, which have now been subdivided into seven families designated A through G. Among these ABC families, the classical MDR is attributed to the elevated expression of ABCB1 (Pgp), ABCC1 (MRP1), and ABCG2 (MXR). The clinical importance of Pgp, MRP1, and MXR for MDR and cancer treatment has led to the investigation of the inhibiting properties of several compounds on these transporters. At present, due in part to the disappointing results associated with the many side effects of synthetic modulators that have been used in clinical trials, current research efforts are directed toward the identification of novel compounds, with attention to dietary natural products. The advantage is that they exhibit little or virtually no side effects and do not further increase the patient's medication burden.

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.

Microanalysis for MDR1 ATPase by high-performance liquid chromatography with a titanium dioxide column

MDR1 is clinically important because it is involved in multidrug resistance of cancer cells and affects the pharmacokinetics of various drugs. Because MDR1 harnesses adenosine 5'-triphosphate (ATP) hydrolysis for transporting drugs, examining the effect on ATPase activity is imperative for understanding the interactions between drugs and MDR1. However, conventional assay systems for ATPase activity are not sensitive enough for screening drugs using purified MDR1. Here we report a novel method to measure ATPase activity of MDR1 using high-performance liquid chromatography equipped with a titanium dioxide column. The amount of adenosine 5'-diphosphate (ADP) produced by the ATPase reaction was determined within 2 min with a titanium dioxide column (4.6 mm ID x 100 mm). The relationship between ADP amount and chromatogram peak area was linear from 5 pmol to 10 nmol. This method made it possible to reduce the amount of purified MDR1 required for a reaction to 0.5 ng, about 1/20th of the conventional colorimetric inorganic phosphate detection assay. This method is sensitive enough to detect any subtle changes in ATPase activity of MDR1 induced by drugs and can be applied to measure ATPase activity of any protein.

Targeting the human mdr1 gene by 125I-labeled triplex-forming oligonucleotides

Antigene radiotherapy is our approach to targeting specific sites in the genome by combining the highly localized DNA damage produced by the decay of Auger electron emitters, such as 125I, with the sequence-specific action of triplex-forming oligonucleotides (TFO). As a model, we used the multidrug resistance gene (mdr1) overexpressed and amplified nearly 100 times in the human KB-V1 carcinoma cell line. Phosphodiester pyrrazolopyrimidine dG (PPG)-modified TFO complementary to the polypurine-polypyrimidine region of the mdr1 gene were synthesized and labeled with 125I-dCTP at the C5 position of two cytosines by the primer extension method. 125I-TFO were delivered into KB-V1 cells with several delivery systems. DNA from the 125I-TFO-treated cells was recovered and analyzed for sequence-specific cleavage in the mdr1 target by Southern hybridization. Experiments with plasmid DNA containing the mdr1 polypurine-polypyrimidine region and with purified genomic DNA confirmed the ability of the designed 125I-TFO to bind to and introduce double-strand breaks into the target sequence. We showed that 125I-TFO in nanomolar concentrations can recognize and cleave a target sequence in the mdr1 gene in situ, that is, within isolated nuclei and intact digitonin-permeabilized cells. Our results demonstrate the ability of 125I-TFO to target specific sequences in their natural environment, that is, within the eukaryotic nucleus. The nearly 100-fold amplification of the mdr1 gene in KB-V1 cells affords a very useful cell culture model for evaluation of methods to produce sequence-specific DNA double-strand breaks for gene-specific radiotherapy.

Development of an immobilized P-glycoprotein stationary phase for on-line liquid chromatographic determination of drug-binding affinities

The membrane transporter P-glycoprotein (PGP) has been immobilized on an immobilized artificial membrane (IAM) LC stationary phase. The resulting PGP-IAM phase retained the ability of the native PGP to bind the known PGP-ligand vinblastine. Displacement studies using other known PGP ligands, verapamil and cyclosporin A, demonstrated that there was selective binding between vinblastine and the immobilized PGP transporter. The binding affinity (Kd value) of vinblastine for the PGP-IAM was determined to be 19+/-20 and 71+/-11 nM on two separate columns. These values are consistent with previously reported values of 9+/-2, 8+/-2, and 37+/-10 nM, which were obtained using native membranes. The Kd values obtained on the PGP-IAM for cyclosporin A and verapamil were 492+/-21 and 172+/-29 microM, respectively. These results were higher than the corresponding Kd values obtained using native membranes, but the relative affinities vinblastine > cyclosporin A >> verapamil are consistent in both approaches. During several months of experiments and storage, the PGP-IAM was found to be reproducible and stable. The stationary phase appears to be useful in the on-line screening for PGP ligands.

Simple purification of highly active biotinylated P-glycoprotein: enantiomer-specific modulation of drug-stimulated ATPase activity

A simplified method for the expression and purification of P-glycoprotein (Pgp) is presented. This method is based on the in-frame fusion of both a polyhistidine tail and a 100-amino acid residue biotin acceptor domain of oxaloacetate decarboxylase from Klebsiella pneumoniae at the carboxyl terminus end of Pgp (Pgp-H6BD). The expression/purification protocol for Pgp-H6BD involves high-level expression of the fusion protein in the yeast Pichia pastoris, biotinylation in vitro with biotin ligase, solubilization of crude membrane fractions in detergent, and affinity purification by a combination of nickel and avidin chromatography. Biotinylated Pgp binds to immobilized monomeric avidin and can be eluted with free biotin in a high state of purity. This protocol is rapid and efficient and yields purified Pgp which shows robust ATPase activity, as determined by vanadate-induced trapping of photoactive nucleotides and by direct measurement of ATP hydrolysis by Pgp-H6BD. This method should be useful for structural studies of the protein by spectroscopic or crystallographic approaches. This purified Pgp-H6BD preparation has been used to study the enantiomer-specific effects of inhibitors of Pgp-mediated drug transport on the drug-stimulated ATPase activity of the protein. A series of 1, 4-disubstituted piperazine derivatives with a central chiral carbon and modified at the head and tail groups are shown to stimulate Pgp ATPase activity in a dose-dependent fashion. Some of these compounds are also capable of inhibiting either vinblastine or verapamil stimulation of ATPase activity of Pgp in an enantiomer-specific fashion. The enantiomeric specific inhibitory activity of these compounds suggests complex interactions at a single substrate binding site(s) on Pgp.

The linker peptide of the ArsA ATPase

Plasmid R773 encodes an As(III)/Sb(III)-translocating ATPase that confers resistance to those metalloids in Escherichia coli. The catalytic subunit of the pump, the ArsA ATPase, consists of homologous N- and C-terminal nucleotide-binding domains connected by a 25-residue linker. The role of this linker sequence was examined by deletion of five, 10, 15 or 23 residues or insertion of five glycine residues. Cells expressing arsA with the 5-residue insertion had wild-type arsenite resistance. Resistance of cells expressing modified arsA genes with deletions was dependent on the linker length. Cells with five or 10 deleted residues exhibited slightly reduced resistance. Deletion of 15 or 23 residues resulted in further decreases in resistance. Each altered ArsA was purified. The enzyme with the 5-residue insertion had the same affinity for ATP and Sb(III) as the wild-type enzyme. Enzymes with 5-, 10-, 15- or 23-residue deletions exhibited decreased affinity for both Sb(III) and ATP. The enzyme with a 23-residue deletion exhibited only basal ATPase activity and was unable to be allosterically activated by Sb(III). These results suggest that the linker has evolved to a length optimal for bringing the two halves of the protein into proper contact with each other, facilitating catalysis.

Biochemical, cellular, and pharmacological aspects of the multidrug transporter

Considerable evidence has accumulated indicating that the multidrug transporter or P-glycoprotein plays a role in the development of simultaneous resistance to multiple cytotoxic drugs in cancer cells. In recent years, various approaches such as mutational analyses and biochemical and pharmacological characterization have yielded significant information about the relationship of structure and function of P-glycoprotein. However, there is still considerable controversy about the mechanism of action of this efflux pump and its function in normal cells. This review summarizes current research on the structure-function analysis of P-glycoprotein, its mechanism of action, and facts and speculations about its normal physiological role.

A method for studying plasma membrane transport with intact cells using computerized fluorometry

A new method is presented for measuring rapid efflux of fluorescent compounds from monolayer cells. Cells grown on a glass coverslip were loaded with a fluorescent substrate. Thereafter, the coverslip was installed outside the light path in a stirred and thermostated cuvette of a fluorometer. The efflux was recorded by measuring the changes of fluorescence in the extracellular medium. The method was used to study the kinetics of active and passive plasma membrane transport of the P-glycoprotein substrates rhodamine 123 and daunorubicin. The method has advantages over other methods: (1) no radioactively labeled substrate is needed, (2) fluorescence of the transported substrate is not compromised by the cells, (3) changes in the extracellular concentration of the substrate can be monitored continuously and therefore a substantial improvement of the kinetic resolution is obtained, and (4) the measurement setup is relatively simple and a standard fluorometer can be used. From the efflux data, cellular transport parameters could be calculated, such as passive permeation coefficients and active transport rates.

Purification and reconstitution of human P-glycoprotein

Human Pgp from the vinblastine-resistant cell line, KB-V1, can be purified by sequential conventional chromatography on DEAE-sepharose CL-6B resin followed by a wheat germ agglutinin column. By including glycerol (osmolyte protectant) and lipid during the solubilization and chromatography procedures most of the biological activity of Pgp can be retained. The activity of Pgp in the detergent extract or in the concentrated column fractions is stable for at least 8-10 months when stored at -80 degrees. However, repeated cycles of freezing and thawing of fractions result in considerable loss of activity. We have purified Pgp from KB-C1 (a subclone of KB 3-1 that is resistant to 1 microgram/ml colchicine) by following the same protocol. When this method was used for purification of Pgp from MDR1-transfected NIH 3T3 transfectants (N3-V2400, grown in the presence of 2.4 micrograms/ml vinblastine), the protein was eluted with 0.1 M NaCl from the DEAE-Sepharose CL-6B column as usual. However, during WGA lectin chromatography, the protein was eluted with a lower concentration of sugar (0.1 M instead of 0.25 M NAG). This altered elution pattern appears to be due to a difference in the glycosylation of human Pgp in mouse NIH 3T3 cells. This is consistent with the observation that human Pgp expressed in NIH 3T3 cells migrates faster compared to the protein from KB-V1 cells on 8-10% acrylamide gel. Similarly, other workers have purified Chinese hamster Pgp either by a single-step chromatography on Reactive Red 120 agarose or by a combination of anion exchange and immunoaffinity chromatography (see the article by Senior et al. for the purification and properties of ATPase activity of Chinese hamster Pgp). The high level of drug-stimulated ATP hydrolysis by Pgp (Table I), like other ion-transporting ATPases, indicates that this is a high-capacity pump that can function as an effective multidrug transporter. This is further supported by the qualitative demonstration of ATP-dependent vinblastine transport in proteoliposomes reconstituted with pure Pgp (see Fig. 2). Thus, these experiments provide strong evidence that purified Pgp retains its activity and that it functions as an ATP-dependent drug transporter.

ATPase activity of Chinese hamster P-glycoprotein

We have developed two defined experimental systems for biochemical investigation of P-glycoprotein, namely, plasma membranes highly enriched in Pgp, obtained from the CR1R12 Chinese hamster ovary cell line, and pure, reconstituted Pgp, obtained by solubilization of Pgp from CR1R12 plasma membranes, Reactive Red 120 chromatography, and reconstitution in liposomes. Studies of the ATPase catalytic mechanism by kinetic methods and covalent inactivation have been greatly facilitated by the availability of these experimental systems. The technique of vanadate trapping of nucleotide has been particularly useful. As a result of these studies, we now have explicit, testable, proposals for (1) the normal catalytic pathway of ATP hydrolysis, (2) a postulated alternating catalytic site cycle, and (3) coupling of ATP hydrolysis to drug transport. The experimental methods described here should prove valuable for future studies of Pgp and of ABC transporters in general.

Human P-glycoprotein exhibits reduced affinity for substrates during a catalytic transition state

Human P-glycoprotein (Pgp), a plasma membrane protein that confers multidrug resistance, functions as an ATP-dependent drug efflux pump. Pgp contains two ATP binding/utilization sites and exhibits ATPase activity that is stimulated in the presence of substrates and modulating agents. The mechanism of coupling of ATP hydrolysis to drug transport is not known. To understand the role of ATP hydrolysis in drug binding, it is necessary to develop methods for purifying and reconstituting Pgp that retains properties including stimulation of ATPase activity by known substrates to an extent similar to that in the native membrane. In this study, (His)6-tagged Pgp was expressed in Trichoplusia ni (High Five) cells using the recombinant baculovirus system and purified by metal affinity chromatography. Upon reconstitution into phospholipid vesicles, purified Pgp exhibited specific binding to analogues of substrates and ATP in affinity labeling experiments and displayed a high level of drug-stimulated ATPase activity (specific activity ranging from 4.5 to 6.5 micromol min-1 mg-1). The ATPase activity was inhibited by ADP in a competitive manner, and by vanadate and N-ethylmaleimide at low concentrations. Vanadate which is known to inhibit ATPase activity by trapping MgADP at the catalytic site inhibited photoaffinity labeling of Pgp with substrate analogues, [125I]iodoarylazidoprazosin and [3H]azidopine, only under ATP hydrolysis conditions. Because vanadate-trapped Pgp is known to resemble the ADP and phosphate-bound catalytic transition state, our findings indicate that ATP hydrolysis results in a conformation with reduced affinity for substrates. A catalytic transition conformation with reduced affinity would essentially result in substrate dissociation and supports a model for drug transport in which an ATP hydrolysis-induced conformational change leads to drug release toward the extracellular medium.

A comparison of the phenomenology and genetics of multidrug resistance in cancer cells and quinoline resistance in Plasmodium falciparum

Plasmodium falciparum is the causative agent of the most deadly form of human malaria. Chemotherapy traditionally has been the main line of defense against this parasite, and chloroquine, the drug of choice, has been one of the most successful drugs ever developed. Unfortunately, the evolution and spread of resistance to chloroquine and other quinoline-containing drugs means that these compounds are now virtually useless in many endemic areas. Future prospects for the use of quinoline compounds improved considerably when it was demonstrated that chloroquine resistance could be circumvented in vitro by a number of structurally and functionally unrelated compounds such as verapamil and desipramine. The phenomenon of resistance reversal by compounds such as verapamil is also a key feature of drug resistance in mammalian cells, and this has raised the possibility that the underlying mechanisms of drug resistance of the two cell types could be similar. This hypothesis has prompted a large number of studies into the genetics and biochemistry of resistance to quinoline-containing drugs in P. falciparum. Both the genetic and the biochemical studies have raised issues of controversy and stimulated much debate. These issues are discussed in this review, in the context of a comparison with the genetics and biochemistry of multidrug resistance in mammalian cells.

ATPase activity of purified multidrug resistance-associated protein

Human multidrug resistance protein (MRP) was expressed at high levels in stably transfected baby hamster kidney (BHK-21) cells. These cells exhibited a pattern of cross-resistance to several different drugs typical of an MRP-mediated phenotype despite the addition of 10 histidine residues at the C terminus to facilitate purification. Consistent with this functional evidence of the presence of MRP at the surface of these transfectants, strong signals were detected by immunoblotting and immunofluorescence using a specific monoclonal antibody to MRP. There was intense uniform staining of the cell surface as well as weaker staining of intracellular membranes. MRP-containing membranes were solubilized in 1% N-dodecyl-beta-D-maltoside in the presence of 0.4% sheep brain phospholipids. Two sequential affinity purification steps on Ni-NTA agarose and wheat germ agglutinin agarose provided substantial enrichment, and contaminating bands were not detected. ATPase activity of the purified protein was assayed in the presence of the phospholipids, which had been maintained throughout all purification steps. ATP was hydrolyzed in proportion to the amount of purified protein assayed, and typical Michaelis-Menten behavior was exhibited, yielding estimations of Km of approximately 3.0 mM and Vmax of 0.46 micromol mg-1 min-1. This activity was moderately stimulated by the drugs that others have shown to be transported by MRP-containing membrane vesicles. This stimulation was enhanced by reduced glutathione as is its drug transport, and oxidized glutathione, itself a substrate for transport, caused a strong stimulation. These data describe the first purification of MRP and provide the first direct evidence that the molecule possesses drug-stimulated ATPase activity.

Characterization of a novel bisacridone and comparison with PSC 833 as a potent and poorly reversible modulator of P-glycoprotein

Novel compounds, composed of two acridone moieties connected by a propyl or butyl spacer, were synthesized and tested as potential modulators of P-glycoprotein (P-gp)-mediated multidrug resistance. The propyl derivative 1,3-bis(9-oxoacridin-10-yl)-propane (PBA) was extremely potent and, at a concentration of 1 microM, increased steady state accumulation of vinblastine (VLB) approximately 9-fold in the multidrug-resistant cell line KB8-5. In contrast to the readily reversible effects of VRP and cyclosporin A on VLB uptake and similar to the effects of the cyclosporin analog PSC 833, this modulation by PBA was not fully reversed 6-8 hr after transfer of cells to PBA-free medium. Continuous exposure to 3 microM PBA was nontoxic and could completely reverse VLB resistance in KB8-5 cells. Consistent with its effects on VLB transport, the drug resistance-modulating effect of PSC 833 was significantly more persistent than that of VRP. However, the effect of PBA was, like that of VRP, rapidly reversed once the modulator was removed from the extracellular environment. PBA was able to compete with radiolabeled azidopine for binding to P-gp and to stimulate P-gp ATPase activity. However, both the steady state accumulation of PBA and the rate of efflux of PBA were similar in drug-sensitive KB3-1 and drug-resistant KB8-5 cells, suggesting that this compound is not efficiently transported by P-gp. These results indicate that PBA represents a new class of potent and poorly reversible synthetic modulators of P-gp-mediated VLB transport.

Reconstitution of peptide-binding activity by TAP in proteoliposomes

The purification and functional reconstitution of the transporter associated with antigen processing (TAP) is crucial for a complete molecular understanding of its action. Here, we report the conditions for the successful solubilization of human TAP from cellular membranes while maintaining TAP peptide-binding activity. In addition, solubilized TAP was incorporated into proteoliposomes and shown to possess specific peptide-binding activity. These studies provide the foundation for future attempts to achieve the complete functional reconstitution of TAP, which includes peptide transport.

Phosphorylation site mutations in the human multidrug transporter modulate its drug-stimulated ATPase activity

In the human multidrug transporter (MDR1), three serine residues located in the "linker" region of the protein are targets of in vivo phosphorylation. These three serines, or all eight serines and threonines in the linker, were substituted by alanines (mutants 3A and 8A) or with glutamic acids (mutants 3E and 8E). The wild-type and mutant proteins were expressed in baculovirus-infected Spodoptera frugiperda (Sf9) ovarian insect cells, and the vanadate-sensitive, drug-stimulated ATPase activity was measured in isolated membrane preparations. The maximum drug-stimulated MDR1-ATPase activity was similar for the wild-type and the mutant proteins. However, wild-type MDR1, which is known to be phosphorylated in Sf9 membranes, and the 3E and 8E mutants, which mimic the charge of phosphorylation, achieved half-maximum activation of MDR1-ATPase activity at lower verapamil, vinblastine, or rhodamine 123 concentrations than the nonphosphorylatable 3A and 8A variants. For some other drugs (e.g. valinomycin or calcein acetoxymethylester) activation of the MDR1-ATPase for any of the mutants was indistinguishable from that of the wild-type protein. Kinetic analysis of the data obtained for the 3A and 8A MDR1 variants indicated the presence of more than one drug interaction site, exhibiting an apparent negative cooperativity. This phenomenon was not observed for the wild-type or the 3E and 8E MDR1 proteins. The dependence of the MDR1-ATPase activity on ATP concentration was identical in the wild-type and the mutant proteins, and Hill plots indicated the presence of more than one functional ATP-binding site. These results suggest that phosphorylation of the linker region modulates the interaction of certain drugs with MDR1, especially at low concentrations, although phosphorylation does not alter the maximum level of MDR1-ATPase activity or its dependence on ATP concentration.

Relation between the turnover number for vinblastine transport and for vinblastine-stimulated ATP hydrolysis by human P-glycoprotein

Considerable uncertainty surrounds the stoichiometry of coupling of ATP hydrolysis to drug pumping by P-glycoprotein, the multidrug transporter. To estimate relative turnovers for pumping of the drug vinblastine and ATP hydrolysis, we began by measuring the number of P-glycoprotein molecules on the surface of murine NIH3T3 cells expressing the human MDR1 gene. Fluorescence of cells treated with monoclonal antibody UIC2 was determined as a function of (i) amount of antibody at a fixed number of cells and (ii) increasing cell number at constant antibody. The two together gives 1.95 x 10(6) P-glycoprotein molecules/cell. Initial uptake rates of vinblastine +/- verapamil measure the ability of P-glycoprotein to extract vinblastine from the plasma membrane before it enters the cell. As a function of [vinblastine] at 37 degrees C, they give the maximum rate of this component of outward pumping as 2.1 x 10(6) molecules s-1 cell-1 or a turnover number of 1.1 s-1. Initial rates of one-way efflux as a function of [vinblastine] at 25 degrees C +/- glucose give the maximum rate of this component of pumping as 0.59 x 10(6) molecules s-1 cell-1. The ratio of ATPase activity of P-glycoprotein at 37 and 25 degrees C is 4.6. Appropriating this ratio for pumping, maximum one-way efflux at 37 degrees C is 4.6 x 0.59 = 2.7 x 10(6) molecules s-1 cell-1, a turnover number of 1.4 s-1. The vinblastine-stimulated ATPase activity of P-glycoprotein has a turnover number of 3.5 s-1 at 37 degrees C, giving 2.8 molecules of ATP hydrolyzed for every vinblastine molecule transported in a particular direction. These calculations involve several approximations, but turnover numbers for pumping of vinblastine and for vinblastine-stimulated ATP hydrolysis are comparable. Thus, ATP hydrolysis is probably directly linked to drug transport by P-glycoprotein.

Efficient purification and reconstitution of P-glycoprotein for functional and structural studies

Plasma membrane P-glycoprotein is known as an ATP-dependent drug efflux pump that confers multidrug resistance to tumor cells. None of the reported purification procedures worked properly for our P-glycoprotein-overproducing cell lines, i.e. murine lymphoid leukemia P388/ADR25, rat hepatoma AS30-D/COL10, and human lymphoblastic leukemia CEM/VLB5 cells. We have thus developed a general procedure for efficient purification of P-glycoprotein by combining solubilization with sodium dodecyl sulfate and chromatography on ceramic hydroxyapatite. This procedure was successful for the three cell lines and yielded 70% of the P-glycoprotein present in the starting plasma membranes with more than 99% purity. After exchanging sodium dodecyl sulfate into dodecyl maltoside and reconstitution into liposomes, purified P-glycoprotein exhibited a specific ATPase activity of about 200 nmol/min/mg, which was very similar to that obtained for P-glycoprotein solubilized and purified with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. This ATPase activity was sensitive to orthovanadate inhibition and stimulated by verapamil and other drugs. More importantly, drug transport properties of the reconstituted P-glycoprotein were comparable with those of P-glycoprotein embedded in plasma membranes. Since it is virtually devoid of lipids, this preparation is suitable for both functional and structural investigations.

Functional characterization of a glycine 185-to-valine substitution in human P-glycoprotein by using a vaccinia-based transient expression system

Human P-glycoprotein (Pgp) is a 170-kDa plasma membrane protein that confers multidrug resistance to otherwise sensitive cells. A mutation in Pgp, G185-->V, originally identified as a spontaneous mutation, was shown previously to alter the drug resistance profiles in cell lines that are stably transfected with the mutant MDR1 cDNA and selected with cytotoxic agents. To understand the mechanism by which the V185 mutation leads to an altered drug resistance profile, we used a transient expression system that eliminates the need for drug selection to attain high expression levels and allows for the rapid characterization of many aspects of Pgp function and biosynthesis. The mutant and wild-type proteins were expressed at similar levels after 24-48 h in human osteosarcoma (HOS) cells by infection with a recombinant vaccinia virus encoding T7 RNA polymerase and simultaneous transfection with a plasmid containing MDR1 cDNA controlled by the T7 promoter. For both mutant and wild-type proteins, photolabeling with [3H]azidopine and [125I]iodoarylazidoprazosin, drug-stimulated ATPase activity, efflux of rhodamine 123, and accumulation of radiolabeled vinblastine and colchicine were evaluated. In crude membrane preparations from HOS cells, a higher level of basal Pgp-ATPase activity was observed for the V185 variant than for the wild-type, suggesting partial uncoupling of drug-dependent ATP hydrolysis by the mutant. Several compounds, including verapamil, nicardipine, tetraphenylphosphonium, and prazosin, stimulated ATPase activities of both the wild-type and mutant similarly, whereas cyclosporin A inhibited the ATPase activity of the mutant more efficiently than that of the wild-type. This latter observation explains the enhanced potency of cyclosporin A as an inhibitor of the mutant Pgp. No differences were seen in verapamil-inhibited rhodamine 123 efflux, but the rate of accumulation was slower for colchicine and faster for vinblastine in cells expressing the mutant protein, as compared with those expressing wild-type Pgp. We conclude that the G185-->V mutation confers pleiotropic alterations on Pgp, including an altered basal ATPase activity and altered interaction with substrates and the inhibitor cyclosporin A.

P-glycoprotein and multidrug resistance

Although the phenomenon of simultaneous resistance to multiple cytotoxic drugs (multidrug resistance) in cancer cells has been discussed for more than two decades, and the human and mouse genes encoding an energy-dependent transporter (the multidrug transporter or P-glycoprotein) responsible for multidrug resistance were cloned 10 years ago, there is still considerable controversy about the mechanism of action of this efflux pump and its true biological function. This review summarizes the current research on the mechanism of action of the multidrug transporter, including the hydrophobic cleaner and altered partitioning models, the possible function of P-glycoprotein as a chloride and/or ATP channel, the role of phosphorylation in its function and fact and speculation about its physiological role.