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

Nano-hydroxyapatite improves intestinal absorption of acetazolamide (BCS Class IV drug)–but how?

We earlier reported that coating poorly water-soluble drugs with nano-hydroxyapatite (nano-HAP) improves bioavailability after oral administration. In the present study, we coated BCS Class IV drug acetazolamide (AZ) with nano-HAP (AZ/HAP formulation), and investigated its bioavailability and nano-HAP’s role in promoting it. We tested AZ bioavailability after a single oral dose of the AZ/HAP formulation in rats, followed by a series of in vitro, ex vivo and in vivo testing. The binding state of AZ and nano-HAP was analyzed by gel filtration chromatography. AZ permeability was studied using a Caco-2 cell monolayer assay kit, to test for tight junction penetration, then using an Ussing chamber mounted with intestinal epithelium, both with and without Peyer’s patch tissue, to examine the role of intracellular transport. Fluorescence-labeled nano-HAP particles were administered orally in rats to investigate their localization in the intestinal tract. The area under the blood concentration time-curve in rats was about 4 times higher in the AZ/HAP formulation group than in the untreated AZ group. Gel filtration analysis showed AZ and nano-HAP were not bound. The Caco-2 study showed equivalent AZ permeability for both groups, but without significant change in transepithelial electrical resistance (TEER), indicating that tight junctions were not penetrated. In the Ussing chamber study, no significant difference in AZ permeability between the two groups was observed for epithelium containing Peyer’s patch tissue, but for epithelium without Peyer’s patch tissue, at high concentration, significantly higher permeability in the AZ/HAP formulation group was observed. Fluorescent labeling showed nano-HAP particles were present in both intestinal villi and Peyer’s patch tissue 30 min after oral administration. Our results suggest that nano-HAP’s enhancement of drug permeability from the small intestine occurs not via tight junctions, but intracellularly, via the intestinal villi. Further study to elucidate the mechanism of this permeability enhancement is required.

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.

Membrane protein stability can be compromised by detergent interactions with the extramembranous soluble domains

Detergent interaction with extramembranous soluble domains (ESDs) is not commonly considered an important determinant of integral membrane protein (IMP) behavior during purification and crystallization, even though ESDs contribute to the stability of many IMPs. Here we demonstrate that some generally nondenaturing detergents critically destabilize a model ESD, the first nucleotide-binding domain (NBD1) from the human cystic fibrosis transmembrane conductance regulator (CFTR), a model IMP. Notably, the detergents show equivalent trends in their influence on the stability of isolated NBD1 and full-length CFTR. We used differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy to monitor changes in NBD1 stability and secondary structure, respectively, during titration with a series of detergents. Their effective harshness in these assays mirrors that widely accepted for their interaction with IMPs, i.e., anionic > zwitterionic > nonionic. It is noteworthy that including lipids or nonionic detergents is shown to mitigate detergent harshness, as will limiting contact time. We infer three thermodynamic mechanisms from the observed thermal destabilization by monomer or micelle: (i) binding to the unfolded state with no change in the native structure (all detergent classes); (ii) native state binding that alters thermodynamic properties and perhaps conformation (nonionic detergents); and (iii) detergent binding that directly leads to denaturation of the native state (anionic and zwitterionic). These results demonstrate that the accepted model for the harshness of detergents applies to their interaction with an ESD. It is concluded that destabilization of extramembranous soluble domains by specific detergents will influence the stability of some IMPs during purification.

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.

Cholesterol-mediated activation of P-glycoprotein: distinct effects on basal and drug-induced ATPase activities

Cholesterol promotes basal and verapamil-induced ATPase activity of P-glycoprotein (P-gp). We investigated whether these effects are related to each other and to the impact of the sterol on bilayer fluidity and verapamil membrane affinity. P-gp was reconstituted in egg-phosphatidylcholine (PhC) liposomes with or without cholesterol, 1,2-dipalmitoyl-phosphatidylcholine (DPPC), alpha-tocopherol (alpha-Toc) or 2,2,5,7,8-pentamethyl-6-chromanol (PMC). Basal and verapamil-induced ATPase activities were studied with an enzymatic assay. Membrane fluidity was characterized with diphenyl-hexatriene anisotropy measurements and membrane affinity by equilibrium dialysis. DPPC (70% mol/mol) decreased the fluidity of PhC bilayers to the same level as 20% cholesterol. PMC (20%) and alpha-Toc (20%) decreased the fluidity to lesser extents. alpha-Toc and PMC, but not DPPC increased the verapamil membrane affinity. While 20% cholesterol strikingly enhanced the basal ATPase activity, none of the other constituents had a similar effect. In contrast, verapamil stimulation of P-gp ATPase activity was not only enabled by cholesterol but also by alpha-Toc and DPPC. PMC had no effect. In conclusion, cholesterol exerts distinct effects on basal and verapamil-induced ATPase activity. The influence on basal ATPase activity is sterol-specific while its effect on verapamil-induced ATPase activity is unspecific and not related to its influence on membrane fluidity and on verapamil membrane affinity.

Microtechnologies for membrane protein studies

Despite the rapid and enormous progress in biotechnologies, the biochemical analysis of membrane proteins is still a difficult task. The presence of the large hydrophobic region buried in the lipid bilayer membrane (transmembrane domain) makes it difficult to analyze membrane proteins in standard assays developed for water-soluble proteins. To handle membrane proteins, the lipid bilayer membrane may be used as a platform to sustain their functionalities. Relatively slow progress in developing micro total analysis systems (microTAS) for membrane protein analysis directly reflects the difficulty of handling lipid membranes, which is a common problem in bulk measurement technologies. Nonetheless, researchers are continuing to develop efficient and sensitive analytical microsystems for the study of membrane proteins. Here, we review the latest developments, which enable detection of events caused by membrane proteins, such as ion channel current, membrane transport, and receptor/ligand interaction, by utilizing microfabricated structures. High-throughput and highly sensitive detection systems for membrane proteins are now becoming a realistic goal. Although most of these systems are still in the early stages of development, we believe this field will become one of the most important applications of microTAS for pharmaceutical and clinical screenings as well as for basic biochemical research.

P-glycoprotein in proteoliposomes with low residual detergent: the effects of cholesterol

PURPOSE

There is evidence that cholesterol affects the ATPase and transport functions of P-glycoprotein (P-gp). To study the influence of cholesterol on P-gp in a well defined lipid environment, we reconstituted P-gp in egg phosphatidylcholine (PhC) and PhC/cholesterol proteoliposomes with negligible residual amounts of detergents.

MATERIALS AND METHODS

P-gp proteoliposomes were prepared by continuous dialysis from micelles consisting of P-gp, lipids, sodium dodecyl sulfate and cholate. Basal and modulator-induced ATPase activities were studied in an established enzyme assay. Modulator affinities to P-gp and to the lipid bilayers were determined by equilibrium dialysis.

RESULTS

In the absence of cholesterol the basal ATPase activity was six fold lower than in the presence of 20 or 40% cholesterol, and no P-gp binding and ATPase induction was detected for the tested modulators verapamil and progesterone. In proteoliposomes containing 20 and 40% cholesterol, respectively, the modulators showed significant P-gp binding and ATPase activation. The concentration of the modulators for half maximal activation of the ATPase was higher with 40% than with 20% cholesterol.

CONCLUSIONS

Cholesterol influences P-gp in three ways: (a) it enhances its basal ATPase activity, (b) it renders P-gp sensitive towards the modulators verapamil and progesterone and (c) it affects the modulator concentration at half maximal ATPase activation.

Cholesterol: Coupling between membrane microenvironment and ABC transporter activity

Lipid composition of biological membranes is closely related to the function of the ATP-binding cassette (ABC) transporter P-Glycoprotein (Pgp). Herein, we studied how membrane physico-chemical properties affect Pgp-activity. We effectively modulated the cellular cholesterol content using methyl-beta-cyclodextrin (MbetaCD) and MbetaCD-cholesterol-inclusion complex. Pgp was not liberated from the plasma membrane during cholesterol modulation and functional inhibition of Pgp was related to varying cholesterol levels in the plasma membrane. Our data indicate that membrane fluidity does not solely account for cholesterol dependent modifications of Pgp-activity. Therefore, we isolated lipid rafts and examined distinct membrane microdomains. Both depletion and cholesterol enrichment induces a disassembly of lipid rafts. In cholesterol-depleted cell membranes a shift in the Pgp localisation to detergent soluble fractions was observed. Enrichment of membrane cholesterol changed lipid raft distribution but not the localisation of Pgp. From our data we conclude that Pgp-transport capacity depends on accurate lipid raft properties.

P-glycoprotein effect on the properties of its natural lipid environment probed by Raman spectroscopy and Langmuir-Blodgett technique

Behavior of P-glycoprotein (Pgp) natural lipid environment within the membrane of CEM cells expressing Pgp in the quantities varying from 0% to 32% of the total amount of all membrane proteins is described for the first time. Observed cooperative effect of Pgp-induced increase of membrane stability, decrease of the temperature of gel-to-crystal lipids transition and predominance of the lipid liquid crystalline phase at physiological temperatures should have an impact in development of multidrug resistance phenotype of tumor cells by favoring the Pgp intercellular transfer and Pgp ATPase activity.

Multidrug-resistant cancer cells contain two populations of P-glycoprotein with differently stimulated P-gp ATPase activities: evidence from atomic force microscopy and biochemical analysis

Considerable interest exists about the localization of P-gp (P-glycoprotein) in DRMs (detergent-resistant membranes) of multidrug resistant cancer cells, in particular concerning the potential modulating role of the closely related lipids and proteins on P-gp activity. Our observation of the opposite effect of verapamil on P-gp ATPase activity from DRM and solubilized-membrane fractions of CEM-resistant leukaemia cells, and results from Langmuir experiments on membrane monolayers from resistant CEM cells, strongly suggest that two functional populations of P-gp exist. The first is located in DRM regions: it displays its optimal P-gp ATPase activity, which is almost completely inhibited by orthovanadate and activated by verapamil. The second is located elsewhere in the membrane; it displays a lower P-gp ATPase activity that is less sensitive to orthovanadate and is inhibited by verapamil. A 40% cholesterol depletion of DRM caused the loss of 52% of the P-gp ATPase activity. Cholesterol repletion allowed recovery of the initial P-gp ATPase activity. In contrast, in the solubilized-membrane-containing fractions, cholesterol depletion and repletion had no effect on the P-gp ATPase activity whereas up to 100% saturation with cholesterol induced a 58% increased P-gp ATPase activity, while no significant modification was observed for the DRM-enriched fraction. DRMs were analysed by atomic force microscopy: 40-60% cholesterol depletion was necessary to remove P-gp from DRMs. In conclusion, P-gp in DRMs appears to contain closely surrounding cholesterol that can stimulate P-gp ATPase activity to its optimal value, whereas cholesterol in the second population seems deprived of this function.

Controlling {beta}-amyloid oligomerization by the use of naphthalene sulfonates: trapping low molecular weight oligomeric species

Aggregation of proteins and peptides has been shown to be responsible for several diseases known as amyloidoses, which include Alzheimer disease (AD), prion diseases, among several others. AD is a neurodegenerative disorder caused primarily by the aggregation of beta-amyloid peptide (Abeta). Here we describe the stabilization of small oligomers of Abeta by the use of sulfonated hydrophobic molecules such as AMNS (1-amino-5-naphthalene sulfonate); 1,8-ANS (1-anilinonaphthalene-8-sulfonate) and bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate). The experiments were performed with either Abeta-1-42 or with Abeta-13-23, a shorter version of Abeta that is still able to form amyloid fibrils in vitro and contains amino acid residues 16-20, previously shown to be essential to peptide-peptide interaction and fibril formation. All sulfonated molecules tested were able to prevent Abeta aggregation in a concentration dependent fashion in the following order of efficacy: 1,8-ANS < AMNS < bis-ANS. Size exclusion chromatography revealed that in the presence of bis-ANS, Abeta forms a heterogeneous population of low molecular weight species that proved to be toxic to cell cultures. Since the ANS compounds all have apolar rings and negative charges (sulfonate groups), both hydrophobic and electrostatic interactions may contribute to interpeptide contacts that lead to aggregation. We also performed NMR experiments to investigate the structure of Abeta-13-23 in SDS micelles and found features of an alpha-helix from Lys(16) to Phe(20). 1H TOCSY spectra of Abeta-13-23 in the presence of AMNS displayed a chemical-shift dispersion quite similar to that observed in SDS, which suggests that in the presence of AMNS this peptide might adopt a conformation similar to that reported in the presence of SDS. Taken together, our studies provide evidence for the crucial role of small oligomers and their stabilization by sulfonate hydrophobic compounds.

New invMED1 element cis-activates human multidrug-related MDR1 and MVP genes, involving the LRP130 protein

The MDR1 gene is a key component of the cytotoxic defense network and its overexpression results in the multidrug resistance (MDR) phenotype. However, the molecular mechanisms that regulate the MDR1 gene and coordinate multiple MDR-related genes expression are poorly understood. In a previous study, we identified a new 12 bp cis-activating region in the 5'-flanking region of the human MDR1 gene, which we called inverted MED1. In the present study, we characterized the precise binding element, which we named invMED1, and revealed the presence of the LRP130 protein as the nuclear factor. Its binding intensity increases with the endogenous MDR1 geneexpression and with the MDR level of CEM leukemia cells. Interestingly, the LRP130 level did not vary with the chemoresistance level. We observed the involvement of LRP130 in the transcriptional activity of the MDR1 gene promoter, and moreover, in that of the MDR-related, invMED1-containing, MVP gene promoter. We used siRNAs and transcriptional decoys in two unrelated human cancer cell lines to show the role of the invMED1/LRP130 couple in both MDR1 and MVP endogenous genes activities. We showed that invMED1 was localized in the -105/-100 and -148/-143 regions of the MDR1 and MVP gene promoters, respectively. In addition, since the invMED1 sequence is primarily located in the -160/-100 bp region of mammalian MDR-related genes, our results present the invMED1/LRP130 couple as a potential central regulator of the transcription of these genes.

Optimizing chemotherapy by measuring reversal of P-glycoprotein activity in plasma membrane vesicles

The appearance of multidrug resistance (MDR) of cancer cells is a major obstacle to successful chemotherapy. Several proteins have been identified that pump chemotherapeutic drugs out of cells, thus bringing about MDR. One representative pump is the P-glycoprotein, whose function can be inhibited by blockers (also known as reversers, modulators or chemosensitizers). In clinical application, many of these blockers are often not effective because they become bound to the plasma of the patients. The extent of plasma binding of the blocker varies in different persons and we have developed a 96-well kit to assay such inter-person differences. The assay uses membrane vesicles isolated from a human lymphoblastoid cell line (CEM Col1000). Uptake of rhodamine into the vesicles was measured with different concentrations of the blockers verapamil and XR9576 in presence of human plasma. The reverser XR9576 is nearly 30 times more effective than the classical blocker verapamil, the relevant K(m) values ranging from 2.66 to 45 nM for XR 9576 and 0.7 to 5.5 microM for verapamil. An even greater difference between these two drugs, nearly 1,000-fold, could be shown also in intact cells by calcein AM uptake experiments.

Highly efficient over-production in E. coli of YvcC, a multidrug-like ATP-binding cassette transporter from Bacillus subtilis

ATP-binding cassette (ABC) transporters have often been refractory to over-expression. Using the C41(DE3) E. coli as a host strain, membrane vesicles highly enriched (>50%) in YvcC, a previously uncharacterized ABC transporter from Bacillus subtilis homologous to P-glycoprotein multidrug transporters, were obtained. The functionality of YvcC was assessed by its high vanadate-sensitive ATPase activity and its ability to transport a fluorescent drug, the Hoechst 33342.

Structural and functional asymmetry of the nucleotide-binding domains of P-glycoprotein investigated by attenuated total reflection Fourier transform infrared spectroscopy

The dynamic changes occurring during the catalytic cycle of MDR3 P-glycoprotein (Pgp) and the role of each nucleotide-binding domain (NBD) in the transport process were investigated using attenuated total reflection Fourier transform infrared spectroscopy. For this purpose, wild-type Pgp and two mutations of homologous residues in each NBD were studied. On the one hand, we demonstrate here that, during its catalytic cycle, Pgp does not undergo secondary structure changes, but only modifications in its stability and accessibility to the external environment. On the other hand, amide H/D exchange kinetics demonstrate that homologous mutations in the two NBDs affect, in a different way, the dynamic properties of Pgp and also the dynamic changes occurring during ATP hydrolysis. These observations led to the conclusion that the NBDs have an asymmetric structure and different functions in the catalytic cycle of Pgp. Our data suggest that the release of drug from the membrane into the extracellular environment is due to decreased stability and/or increased accessibility to the external medium of the membrane-embedded drug-binding site(s). NBD1 would play an important role in this first restructuring of the membrane-embedded domains. NBD2 would be directly implicated in the subsequent restructuring of the membrane-embedded binding sites by which they recover their initial stability and accessibility to the membrane. It is proposed that this restructuring step would allow the binding and transport of another molecule of substrate.

P-glycoprotein inserted in planar lipid bilayers formed by liposomes opened on amorphous carbon and Langmuir-Blodgett monolayer

The insertion of proteins into planar lipid layers is of outstanding interest as the resulting films are suitable for the investigation of protein structure and aggregation in a lipid environment and/or the development of biotechnological applications as biosensors. In this study, purified P-glycoprotein (P-gp), a membrane drug pump, was incorporated in model membranes deposited on solid supports according to the method by Puu and Gustafson, Biochim. Biophys. Acta 1327 (1997) 149-161. The models were formed by a double lipid layer obtained by opening P-gp-containing liposomes onto two hydrophobic supports: amorphous carbon films and Langmuir-Blodgett (L-B) lipid monolayers, which were then observed by transmission electron microscopy and atomic force microscopy, respectively. Before the opening of liposomes, the P-gp structure and functionality were verified by circular dichroism spectroscopy and enzymatic assay. Our micrographs showed that liposomes containing P-gp fuse to the substrates more easily than plain liposomes, which keep their rounded shape. This suggests that the protein plays an essential role in the fusion of liposomes. To localize P-gp, the immunogold labeling of two externally exposed protein epitopes was carried out. Both imaging techniques confirmed that P-gp was successfully incorporated in the model membranes and that the two epitopes preserved the reactivity with specific mAbs, after sample preparation. Model membranes obtained on L-B monolayer incorporated few molecules with respect to those incorporated in the model membrane deposited onto amorphous carbon, probably because of the different mechanism of proteoliposome opening. Finally, all particles appeared as isolated units, suggesting that P-gp molecules were present as monomers.

Dissociation and unfolding of GCN4 leucine zipper in the presence of sodium dodecyl sulfate

The dissociation and unfolding behavior of the GCN4 leucine zipper has been studied using SDS titration. Circular dichroism (CD) spectra showed that the alpha-helix content of the leucine zipper (20 microM) decreased during the sodium dodecyl sulfate (SDS) titration. However, the alpha-helix content of the leucine zipper still remained significant in the presence of 1 mM SDS, with little change detected when the SDS concentration further increased to 2 mM. The dimer dissociation of the leucine zipper is also a co-operative process during SDS titration; with no dimer remaining when SDS concentration reached 1 mM, as shown by electrophoresis and the the theta(222)/theta(208) ratio. Our results indicate that SDS efficiently induces leucine zipper dimer dissociation with the monomers still partially folded. The experimental results provide important evidence for the previous model that partial helix formation precedes dimerization in coiled coil folding.

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

P-glycoprotein is an ABC protein that functions as an efflux pump for multiple drugs, natural products and peptides. It is proposed to operate as a hydrophobic vacuum cleaner, expelling non-polar compounds from the membrane bilayer to the exterior, driven by the energy of ATP hydrolysis. The nucleotide-binding domains of P-glycoprotein appear to operate by an alternating sites mechanism to power drug transport. In recent years, purification and functional reconstitution of the protein has allowed the application of fluorescence spectroscopic techniques. This approach has led to insights into the structural architecture of the P-glycoprotein molecule, and a more detailed understanding of the way in which it interacts with nucleotides and drugs.

Immunogold localisation of P-glycoprotein in supported lipid bilayers by transmission electron microscopy and atomic force microscopy

In this study, purified P-glycoprotein molecules, a membrane drug pump responsible for the multidrug resistance phenomenon, were incorporated in model membranes deposited onto solid supports, according to the method described by Puu and Gustafson (1997). The insertion of proteins into planar supported model membranes is of interest, as the films are fundamental in biosensor applications and for the investigation of how proteins conform and aggregate in a lipid environment. In our investigation, two model membranes were prepared by transferring liposomes containing P-glycoprotein to different hydrophobic supports: (a) thin amorphous carbon films; (b) Langmuir-Blodgett lipid monolayers on mica. After the labelling of P-glycoprotein with two well-characterised monoclonal antibodies, MM4.17 and MRK-16, samples (a) were observed by transmission electron microscopy (TEM) and samples (b) by atomic force microscopy (AFM). The comparative analysis performed by TEM and AFM allowed us to demonstrate the successful insertion of P-glycoprotein in the model membranes and their stability under different environmental conditions (vacuum, air and water). P-glycoprotein appeared to maintain, after purification and insertion in lipid bilayers, a good part of its conformational features as shown by the P-glycoprotein segments bearing the specific monoclonal antibody epitopes.

The effect of lipid environment and retinoids on the ATPase activity of ABCR, the photoreceptor ABC transporter responsible for Stargardt macular dystrophy

ABCR is a photoreceptor-specific ATP-binding cassette transporter that has been linked to various retinal diseases, including Stargardt macular dystrophy, and implicated in retinal transport across rod outer segment (ROS) membranes. We have examined the ATPase and GTPase activity of detergent-solubilized and reconstituted ABCR. 3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonic acid-solubilized ABCR had ATPase and GTPase activity (K(m) approximately 75 micrometer V(max) approximately 200 nmol/min/mg) that was stimulated 1.5-2-fold by all-trans-retinal and dependent on phospholipid and dithiothreitol. The K(m) for ATP decreased to approximately 25 micrometer after reconstitution, whereas the V(max) was strongly dependent on the lipid used for reconstitution. ABCR reconstituted in ROS phospholipid had a V(max) for basal and retinal activated ATPase activity that was 4-6 times higher than for ABCR in soybean or brain phospholipid. This enhanced activity was mainly due to the high phosphatidylethanolamine (PE) content of ROS membranes. PE was also required for retinoid-stimulated ATPase activity. ATPase activity of ABCR was stimulated by the addition of N-retinylidene-PE but not the reduced derivative, retinyl-PE. ABCR expressed in COS-1 cells also exhibited retinal-stimulated ATPase activity similar to that of the native protein. These results support the view that ABCR is an active retinoid transporter, the nucleotidase activity of which is strongly influenced by its lipid environment.

Modulation of the typical multidrug resistance phenotype by targeting the MED-1 region of human MDR1 promoter

Multidrug resistance of cancer (MDR) is the major cause of failure of chemotherapy. The typical MDR phenotype is due to the overexpression of membrane proteins among which the main representative is P-glycoprotein (Pgp) encoded by the MDR1 gene. Many attempts to modulate MDR by chemosensitizers have been unsuccessful in human therapy due to their intrinsic toxic effects. In an effort to modulate the MDR phenotype efficiently we designed an antisense and a transcriptional decoy strategy targeting the TATA-less human MDR1 gene promoter. The choice of the start point of transcription in a multiple start site window is related to an upstream MED-1 cis-element, the sequence and configuration of which are specific to human MDR1 gene expressed in Pgp-overproducing cancer cells. A 12mer antisense oligodeoxynucleotide (ODN) and a 12mer double-stranded ODN, both containing the MED-1 sequence, were designed and efficiently vectorized into the nucleus with the chimerical MPG peptide. A synthetic cellular model (NIH-EGFP) and highly resistant human CEM/VLB0.45 leukemia cells, significantly responded to transfection with the ODN/MPG complex. The level of EGFP fluorescence in NIH-EGFP cells decreased, and thus its production, and viability of CEM/VLB0.45 cells decreased by 63% in the presence of vinblastine, revealing that their resistance to the anticancer drug was reversed. These results open new insights into transcriptional decoy and anti-gene therapies of MDR cancers that overproduce Pgp. Gene Therapy (2000) 7, 1224-1233.

Magnetic resonance spectroscopy of cellular lipid extracts from sensitive, resistant and reverting K562 cells and flow cytometry for investigating the P-glycoprotein function in resistance reversion

The proton NMR spectra of K562 cells contain resonances of lipids. When these cells acquire multidrug resistance phenotype, the NMR lipid signals are modified and partially recovered when the resistance is reversed. The goals of the present study are to elucidate the mechanism of the resistance phenotype reversion and to investigate the possible origin of lipid signals detected in whole cells with proton NMR spectroscopy. Therefore, the K562 drug-sensitive cell line, its adriamycin resistant counterpart and two reverting derivates, obtained by verapamil treatment and long term culture in drug-free medium, were used in this study. The P-glycoprotein (P-gp) pump function was measured by flow cytometry and lipids were extracted to be analysed by proton and phosphorus spectroscopy. The phenotype reversion is due to the decrease of the P-gp function and an increased entrance of anthracycline drug when compared with the resistant cells. The spectra obtained on extracts showed no modification of the fatty acid composition and of the ratio of total cholesterol to fatty acid content. A different phospholipid composition in sensitive and resistant cells was found, but the reversion of resistance did not produce a recovery of these lipids. Thus, the lipid NMR spectra of extracts could not explain the spectral modifications observed on whole cells, in relation to acquiring and reverting drug resistance. These results are in favour of a different lipid organization or of localization within the cell.

Insights into the structure and substrate interactions of the P-glycoprotein multidrug transporter from spectroscopic studies

The P-glycoprotein multidrug transporter is a 170-kDa efflux pump which exports a diverse group of natural products, chemotherapeutic drugs, and hydrophobic peptides across the plasma membrane, driven by ATP hydrolysis. The transporter has been proposed to interact with its drug substrates within the membrane environment; however, much remains to be learned about the nature and number of the drug binding site(s). The two nucleotide binding domains are responsible for ATP binding and hydrolysis, which is coupled to drug movement across the membrane. In recent years, P-glycoprotein has been purified and functionally reconstituted in amounts large enough to allow biophysical studies. The use of spectroscopic techniques has led to insights into both its secondary and tertiary structure, and its interaction with nucleotides and drugs. In this review, we will summarise what has been learned by application to purified P-glycoprotein of fluorescence spectroscopy, circular dichroism spectroscopy and infra-red spectroscopy.

Structurally related peptide agonist, partial agonist, and antagonist occupy a similar binding pocket within the cholecystokinin receptor. Rapid analysis using fluorescent photoaffinity labeling probes and capillary electrophoresis

The molecular basis of ligand binding to receptors provides important insights for drug development. Here, we explore domains of the cholecystokinin (CCK) receptor that are critical for ligand binding, using a novel series of fluorescent photolabile probes, receptor proteolysis, and rapid high resolution separation of peptide fragments by capillary electrophoresis. Each probe incorporated the same fluorophore and a photolabile p-benzoylphenylalanine at the amino terminus of the pharmacophoric domain (residue 24 of CCK-33) of CCK analogues representing full agonist, partial agonist, and antagonist of this receptor. Each was used to label the CCK receptor expressed on Chinese hamster ovary-CCKR cells, with the labeled domain then released by cyanogen bromide cleavage. Capillary electrophoresis with laser-induced fluorescence detection achieved an on-capillary mass sensitivity of 1.6 attomoles (10(-18) mol), with an excellent signal-to-noise ratio. Each of the biologically divergent, but structurally similar probes saturably and specifically labeled the same receptor domain, consistent with conservation of "docking" determinants. This had an apparent mass of 2.9 kDa, most consistent with the first extracellular loop domain. An additional probe having its site of covalent attachment in a different region of the probe (residue 29 of CCK-33) labeled a distinct receptor fragment with differential migration on capillary electrophoresis (third extracellular loop). Identification of the specific receptor residue(s) covalently linked to the amino-terminal probes must await further fragmentation and sequence analysis.

ATP- and glutathione-dependent transport of chemotherapeutic drugs by the multidrug resistance protein MRP1

The present study was performed to investigate the ability of the multidrug resistance protein (MRPI) to transport different cationic substrates in comparison with MDR1-P-glycoprotein (MDR1). Transport studies were performed with isolated membrane vesicles from in vitro selected multidrug resistant cell lines overexpressing MDR1 (A2780AD) or MRP1 (GLC4/Adr) and a MRP1-transfected cell line (S1(MRP)). As substrates we used 3H-labelled derivatives of the hydrophilic monoquaternary cation N-(4',4'-azo-in-pentyl)-21-deoxy-ajmalinium (APDA), the basic drug vincristine and the more hydrophobic basic drug daunorubicin. All three are known MDR1-substrates. MRP1 did not mediate transport of these substrates per se. In the presence of reduced glutathione (GSH), there was an ATP-dependent uptake of vincristine and daunorubicin, but not of APDA, into GLC4/Adr and S1(MRP) membrane vesicles which could be inhibited by the MRP1-inhibitor MK571. ATP- and GSH-dependent transport of daunorubicin and vincristine into GLC4/Adr membrane vesicles was inhibited by the MRP1-specific monoclonal antibody QCRL-3. MRP1-mediated daunorubicin transport rates were dependent on the concentration of GSH and were maximal at concentrations > or = 10 mM. The apparent KM value for GSH was 2.7 mM. Transport of daunorubicin in the presence of 10 mM GSH was inhibited by MK571 with an IC50 of 0.4 microM. In conclusion, these results demonstrate that MRP1 transports vincristine and daunorubicin in an ATP- and GSH-dependent manner. APDA is not a substrate for MRP1.

In vitro and in vivo reversal of cancer cell multidrug resistance by the semi-synthetic antibiotic tiamulin

A large number of multidrug resistance (MDR) modulators, termed chemosensitizers, have been identified from a variety of chemicals, but most have been proven to be clinically toxic. Low concentrations of the pleuromutilin-derived semi-synthetic antibiotic tiamulin (0.1 to 10 microM) sensitized the three highly resistant P-glycoprotein (Pgp)-overexpressing tumor cell lines P388 (murine lymphoid leukemia), AS30-D (rat hepatoma), CEM (human lymphoblastic leukemia), and the barely resistant AS30-D/S cell lines to several MDR-related anticancer drugs. Flow cytometric analysis showed that tiamulin significantly increased the intracellular accumulation of daunomycin. When compared to reference modulating agents such as verapamil and cyclosporin A, tiamulin proved to be 1.1 to 8.3 times more efficient in sensitizing the resistant cell lines. Moreover, when given i.p. (1.6 microg/mg body weight), tiamulin increased the survival rate of adriamycin-treated mice bearing the P388/ADR25 tumor line by 29%. In the presence of an anticancer drug, tiamulin inhibited both ATPase and drug transport activities of Pgp in plasma membranes from tumor cells. Tiamulin is thus a potent chemosensitizer that antagonizes the Pgp-mediated chemoresistance in many tumor cell lines expressing the MDR phenotype at different levels and displays no toxic effects on contractile tissues at active doses, therefore providing the promise for potential clinical applications.

Secondary structure of P-glycoprotein investigated by circular dichroism and amino acid sequence analysis

P-glycoprotein (Pgp) is a plasma membrane protein known as an ATP-dependent drug-efflux pump that confers multidrug resistance to tumor cells. Structural analysis of Pgp was investigated by circular dichroism (CD) for the first time and in combination with amino acid sequence analysis. CD of highly purified Pgp from human, rat and murine Pgp-overexpressing drug resistant cells revealed slight variations in the spectral shape when recorded in the presence of dodecyl maltoside (DM). These species-dependent variations in CD shapes resulted from the interaction of the oligosaccharidic part with the protein core since they were abolished either in the presence of sodium dodecyl sulfate (SDS) or after deglycosylation, the latter not altering the Pgp ATP-dependent drug transport activity. Whatever the level of Pgp glycosylation and the detergent used (SDS or DM), the content in secondary structure deduced from deconvolution of CD spectra is almost the same for the three sources of Pgp and estimated to 43% alpha-helix, 16% beta-sheet, 15% beta-turn and 26% of other structures. These data, which constitute the first report of Pgp structure analysis by circular dichroism, are consistent with the 48% alpha-helix and 16% beta-sheets global contents predicted by using recently reported efficient secondary structure prediction methods. This consistency reinforces the reliability of the probable nature and localization of predicted Pgp secondary structure elements. This provides a good framework for precise 3D structure modeling of Pgp by homology with proteins of known 3D structure, as it is illustrated here for the A motifs of the ATP-binding domains of Pgp.

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.

Biochemical, genetic, and metabolic adaptations of tumor cells that express the typical multidrug-resistance phenotype. Reversion by new therapies

Among the genetic and metabolic alterations that cancer cells undergo, several allow their survival under extreme environmental conditions. The resulting aberrant metabolism is compatible with tumor progression at the expenses of high energy needs, especially for maintaining high division rate. When treated with chemotherapeutic drugs many cancer cells take advantage of their ability to develop a resistance phenotype, as part of an adaptative mechanism. Two main actors of this multidrug phenotype (MDR) are represented by the P-glycoprotein and by the more recently discovered multidrug-resistance associated protein (MRP), two membrane proteins of the ABC superfamily of transporters that can extrude chemotherapeutic drugs under an ATP-dependent mechanism. We will briefly review the major metabolic aberrations that several cancers develop, followed by the molecular, genetic, structural, and functional aspects related mainly to P-glycoprotein, with a concern for the regulation of mdr gene expression. We will point out the role that membrane cholesterol may play in the MDR phenotype, relate this phenotype to bioenergetic considerations, and review the ways of modulating it by the use of new therapeutic approaches.

Complete removal and exchange of sodium dodecyl sulfate bound to soluble and membrane proteins and restoration of their activities, using ceramic hydroxyapatite chromatography

Up to now, removal of sodium dodecyl sulfate (SDS) from proteins in terms of restoration of their activity was an unsolved problem. A general procedure using ceramic hydroxyapatite (HAP) chromatography was developed for the complete removal of SDS bound to soluble or membrane proteins. This procedure involves (i) the binding of the SDS-protein complexes onto the ceramic hydroxyapatite column, (ii) extensive washing of bound proteins with phosphate buffer containing a mild detergent to exchange SDS, (iii) elution of the retained protein by increasing the phosphate concentration. Using this approach, complete exchange of [35S]SDS into a nonionic detergent such as dodecyl maltoside was achieved with a 90-100% protein recovery. The efficiency of protein-bound SDS removal is very likely due to the combined effect of phosphate ions and the hydrophobic tail of nonionic detergent: acting together, they are able to displace SDS molecules from their protein-binding sites. The advantages of this HAP-mediated SDS removal method include high efficiency, rapidity, simplicity and general applicability to a wide variety of detergents and soluble or membrane proteins. Of utmost importance, SDS-treated P-glycoprotein, glutamate dehydrogenase, and lysozyme fully recovered their enzymatic activities after HAP chromatography, including lysozyme electroeluted from SDS-polyacrylamide gel electrophoresis. This demonstrates that reactivation of SDS-treated protein can be achieved, provided that SDS is completely removed under mild conditions.