The mechanism of action of multidrug-resistance-linked P-glycoprotein

Mitochondria-targeted drugs synergize with 2-deoxyglucose to trigger breast cancer cell death

Cancer cells are long known to exhibit increased aerobic glycolysis, but glycolytic inhibition has not offered a viable chemotherapeutic strategy in part because of the systemic toxicity of antiglycolytic agents. However, recent studies suggest that a combined inhibition of glycolysis and mitochondrial function may help overcome this issue. In this study, we investigated the chemotherapeutic efficacies of mitochondria-targeted drugs (MTD) in combination with 2-deoxy-d-glucose (2-DG), a compound that inhibits glycolysis. Using the MTDs, termed Mito-CP and Mito-Q, we evaluated relative cytotoxic effects and mitochondrial bioenergetic changes in vitro. Interestingly, both Mito-CP and Mito-Q synergized with 2-DG to decrease ATP levels in two cell lines. However, with time, the cellular bioenergetic function and clonogenic survival were largely restored in some cells. In a xenograft model of human breast cancer, combined treatment of Mito-CP and 2-DG led to significant tumor regression in the absence of significant morphologic changes in kidney, liver, or heart. Collectively, our findings suggest that dual targeting of mitochondrial bioenergetic metabolism with MTDs and glycolytic inhibitors such as 2-DG may offer a promising chemotherapeutic strategy.

Lytic peptides with improved stability and selectivity designed for cancer treatment

Lytic peptides are a group of membrane-acting peptides, which have excellent activity to drug-resistant cells. In this study, the stability and tumor selectivity of newly designed pH-activated lytic peptides were studied. We found that despite varied secondary structures, pH-induced structure changes could not be directly linked to the activity and pH sensitivity of peptides. On the contrary, formation of aggregates had great impacts on peptide binding and insertion into the lipid bilayer of cell membrane. It was found that the pH controlled peptide aggregation and dissolution was responsible for the pH-dependent membrane lysis activity of peptides. One peptide (PTP-7c) formed stable amyloid fibrils, which did not completely dissolve under acidic conditions. As a result, PTP-7c had the lowest membrane lysis and cell killing activities among tested lytic peptides. As solid tumors have consistently low extracellular pHs, peptides with acid-activation features showed improved selectivity to cancer cells. In addition, self-assembled lytic peptides were found to become more stable and showed dramatically increased half lives (up to 11 h) in human plasma. These new lytic peptides with good stability and acid-activated cell lysis activity will have wide biomedical applications especially for the treatment of cancers in which drug resistance has developed.

Improving cancer chemotherapy with modulators of ABC drug transporters

ATP-binding cassette (ABC) transporters, P-glycoprotein (P-gp, ABCB1) and ABCG2, are membrane proteins that couple the energy derived from ATP hydrolysis to efflux many chemically diverse compounds across the plasma membrane, thereby playing a critical and important physiological role in protecting cells from xenobiotics. These transporters are also implicated in the development of multidrug resistance (MDR) in cancer cells that have been treated with chemotherapeutics. One approach to blocking the efflux capability of an ABC transporter in a cell or tissue is inhibiting the activity of the transporters with a modulator. Since ABC transporter modulators can be used in combination with chemotherapeutics to increase the effective intracellular concentration of anticancer drugs, the possible impact of modulators of ABC drug transporters is of great clinical interest. Another possible clinical use of modulators that has recently attracted attention is their ability to increase oral bioavailability or increase tissue penetration of drugs transported by the transporters. Several preclinical and clinical studies have been performed to evaluate the feasibility and the safety of this approach. The primary focus of this review is to discuss progress made in recent years in the identification and applicability of compounds that may serve as ABC transporter modulators and the possible role of these compounds in altering the pharmacokinetics and pharmacodynamics of therapeutic drugs used in the clinic.

Polymorphisms of ERCC1 C118T/C8092A and MDR1 C3435T predict outcome of platinum-based chemotherapies in advanced non-small cell lung cancer: a meta-analysis

BACKGROUND AND AIMS

With great progress made in individualized chemotherapy, pharmacogenetics is gradually put on the agenda. We performed this meta-analysis to compare outcome to platinum-based chemotherapies in advanced non-small cell lung cancer (NSCLC) with different ERCC1 C118T/C8092A and MDR1 C3435T polymorphisms.

METHODS

Relevant studies were identified according to search strategy in this meta-analysis. Inclusion criteria were patients with advanced NSCLC who were receiving platinum-based chemotherapies. We evaluated the relationship between single nucleotide polymorphisms (SNP) and outcome of platinum-based chemotherapies. RevMan and STATA package were used for the comprehensive quantitative analyses.

RESULTS

Twenty studies were included in the meta-analysis. There was no significant association between SNPs and objective response or overall survival of platinum-based chemotherapies with CC vs. CT/TT: ERCC1 C118T (OR 1.21, 95% CI 0.81-1.82 for objective response; HR 1.09, 95% CI 0.79-1.51 for overall survival); ERCC1 C8092A SNP (OR 0.84, 95% CI 0.59-1.18; HR 1.26, 95% CI 0.68-2.36) and MDR1 C3435T SNP (HR 1.11, 95% CI 0.78-1.56). Ethnic stratification provided the same results. We found a significant difference for MDR1 C3435T (OR 2.22, 95% CI 1.46-3.37; OR 2.63, 95% CI 1.56-4.45 for Asians; OR 1.61, 95% CI 0.79-3.28 for Caucasians).

CONCLUSIONS

We found no evidence to support the use of ERCC1 C118T/C8092A polymorphisms as prognostic predictors of platinum-based chemotherapies in NSCLC. For the MDR1 C3435T SNP, a significant association with objective response was detected for CC genotype in overall and Asian populations stratified. Multiple and large-scale studies with ethnic stratification are required for the correlation between biomarkers and tumor prognosis.

An update on ABCB1 pharmacogenetics: insights from a 3D model into the location and evolutionary conservation of residues corresponding to SNPs associated with drug pharmacokinetics

The human ABCB1 protein, (P-glycoprotein or MDR1) is a membrane-bound glycoprotein that harnesses the energy of ATP hydrolysis to drive the unidirectional transport of substrates from the cytoplasm to the extracellular space. As a large range of therapeutic agents are known substrates of ABCB1 protein, its role in the onset of multidrug resistance has been the focus of much research. This role has been of particular interest in the field of pharmacogenomics where genetic variation within the ABCB1 gene, particularly in the form of single nucleotide polymorphisms (SNPs), is believed to contribute to inter-individual variation in ABCB1 function and drug response. In this review we provide an update on the influence of coding region SNPs within the ABCB1 gene on drug pharmacokinetics. By utilizing the crystal structure of the mouse ABCB1 homolog (Abcb1a), which is 87% homologous to the human sequence, we accompany this discussion with a graphical representation of residue location for amino acids corresponding to human ABCB1 coding region SNPs. Also, an assessment of residue conservation, which is calculated following multiple sequence alignment of 11 confirmed sequences of ABCB1 homologs, is presented and discussed. Superimposing a 'heat map' of residue homology to the Abcb1a crystal structure has permitted additional insights into both the conservation of individual residues and the conservation of their immediate surroundings. Such graphical representation of residue location and conservation supplements this update of ABCB1 pharmacogenetics to help clarify the often confounding reports on the influence of ABCB1 polymorphisms on drug pharmacokinetics and response.

Molecular biology of the blood-brain and the blood-cerebrospinal fluid barriers: similarities and differences

Efficient processing of information by the central nervous system (CNS) represents an important evolutionary advantage. Thus, homeostatic mechanisms have developed that provide appropriate circumstances for neuronal signaling, including a highly controlled and stable microenvironment. To provide such a milieu for neurons, extracellular fluids of the CNS are separated from the changeable environment of blood at three major interfaces: at the brain capillaries by the blood-brain barrier (BBB), which is localized at the level of the endothelial cells and separates brain interstitial fluid (ISF) from blood; at the epithelial layer of four choroid plexuses, the blood-cerebrospinal fluid (CSF) barrier (BCSFB), which separates CSF from the CP ISF, and at the arachnoid barrier. The two barriers that represent the largest interface between blood and brain extracellular fluids, the BBB and the BCSFB, prevent the free paracellular diffusion of polar molecules by complex morphological features, including tight junctions (TJs) that interconnect the endothelial and epithelial cells, respectively. The first part of this review focuses on the molecular biology of TJs and adherens junctions in the brain capillary endothelial cells and in the CP epithelial cells. However, normal function of the CNS depends on a constant supply of essential molecules, like glucose and amino acids from the blood, exchange of electrolytes between brain extracellular fluids and blood, as well as on efficient removal of metabolic waste products and excess neurotransmitters from the brain ISF. Therefore, a number of specific transport proteins are expressed in brain capillary endothelial cells and CP epithelial cells that provide transport of nutrients and ions into the CNS and removal of waste products and ions from the CSF. The second part of this review concentrates on the molecular biology of various solute carrier (SLC) transport proteins at those two barriers and underlines differences in their expression between the two barriers. Also, many blood-borne molecules and xenobiotics can diffuse into brain ISF and then into neuronal membranes due to their physicochemical properties. Entry of these compounds could be detrimental for neural transmission and signalling. Thus, BBB and BCSFB express transport proteins that actively restrict entry of lipophilic and amphipathic substances from blood and/or remove those molecules from the brain extracellular fluids. The third part of this review concentrates on the molecular biology of ATP-binding cassette (ABC)-transporters and those SLC transporters that are involved in efflux transport of xenobiotics, their expression at the BBB and BCSFB and differences in expression in the two major blood-brain interfaces. In addition, transport and diffusion of ions by the BBB and CP epithelium are involved in the formation of fluid, the ISF and CSF, respectively, so the last part of this review discusses molecular biology of ion transporters/exchangers and ion channels in the brain endothelial and CP epithelial cells.

Modulation of P-glycoprotein activity by the substituted quinoxalinone compound QA3 in adriamycin-resistant K562/A02 cells

QA3 is a derivative of the substituted 1,3-dimethyl-1H-quinoxalin-2-ones, which are compounds that may selectively antagonize P-glycoprotein (P-gp) in multidrug resistance (MDR) cancer cells. Our previous work identified QA3 as a candidate compound for reversing MDR in cancer cells. In the present study, we found that QA3 significantly decreases the intracellular level of ATP, stimulates ATPase activity in membrane microsomes and decreases protein kinase C (PKC) activity. These results indicated that QA3 inhibits P-gp activity by blocking ATP hydrolysis and ATP regeneration. Furthermore, QA3 triggered and increased adriamycin-induced K562/A02 cell apoptosis as evidenced by Annexin V-FITC plus PI staining.Western blot analysis showed that the levels of cleaved caspase-9 and cleaved caspase-3 proteins increased, and similarly, the levels of procaspase-9 and procaspase-3 decreased after QA3 treatment. Consequently, poly ADP-ribose polymerase (PARP) activity increased as evidenced by the presence of the PARP cleavage product in K562/A02 cells. QA3 also enhanced the potency of adriamycin against K562/A02 cells as demonstrated by increased apoptosis and activation of caspase-9,-3 and PARP. These data support the observation that P-gp activity is inhibited after QA3 treatment. Moreover, these results indicate that QA3 is a novel MDR reversal agent with potent inhibitory action against P-gp MDR cancer cells.

Neurosteroid transport by the organic solute transporter OSTα-OSTβ

A variety of steroids, including pregnenolone sulfate (PREGS) and dehydroepiandrosterone sulfate (DHEAS) are synthesized by specific brain cells, and are then delivered to their target sites, where they exert potent effects on neuronal excitability. The present results demonstrate that [(3)H]DHEAS and [(3)H]PREGS are relatively high affinity substrates for the organic solute transporter, OSTα-OSTβ, and that the two proteins that constitute this transporter are selectively localized to steroidogenic cells in the cerebellum and hippocampus, namely the Purkinje cells and cells in the cornu ammonis region in both mouse and human brain. Analysis of Ostα and Ostβ mRNA levels in mouse Purkinje and hippocampal cells isolated via laser capture microdissection supported these findings. In addition, Ostα-deficient mice exhibited changes in serum DHEA and DHEAS levels, and in tissue distribution of administered [(3)H]DHEAS. OSTα and OSTβ proteins were also localized to the zona reticularis of human adrenal gland, the major region for DHEAS production in the periphery. These results demonstrate that OSTα-OSTβ is localized to steroidogenic cells of the brain and adrenal gland, and that it modulates DHEA/DHEAS homeostasis, suggesting that it may contribute to neurosteroid action.

Chemotherapy resistance research of lung cancer based on micro-fluidic chip system with flow medium

Micro total analysis systems (-TAS) or labs-on-achip, have been spreading rapidly due to their desirable characteristics, including reductions in reagent consumption, space requirements and analysis time. This work aimed at establishing an integrated microfluidic system which can supply the cells with fresh medium of oxygen and nutrition continuously at a control flow rate mimicking the microenvironment in vivo. Human non-small cell lung cancer cell line SPCA1 was seeded in a microchip supplied with fresh medium at a constant rate of 15 mm/24 h controlled by a pump. The expression of P-gp for verapamil-pretreated or non-pretreated cells was assayed with immunofluorescence. Both groups cells were exposed to anticancer drug VP-16 at 30 microM for 6 h before the apoptosis analysis online. The results indicated that the cells could grow and spread well for 4 days in the microfluidic system successively furnished with fresh medium. Immunofluorescence assay showed that the intensity of the fluorescence for the verapamil-pretreated cells was obvious weak compared with that of nonpretreated cells. Apoptosis analysis demonstrated that the percentage of apoptotic cells for verapamil-pretreated group increased around twofold compared with that of nonverapamil pretreated group (26.5+/-2.5% versus 10.9+/- 0.85%, p<0.05), showing a similar results as by flow cytometry analysis. All these indicate that P-gp plays an important role in the resistance to VP-16 in SPCA1, the microfluidic system provides a suitable environment for cells survival and is valuable in long time cell culture and bioassays mimicking the microenvironment in vivo and deserved to be studied further.

Advancement of structure-activity relationship of multidrug resistance-associated protein 2 interactions

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is mainly expressed in the apical phase of barrier membranes. It functions as a critical efflux pump in the biliary excretion of endogenous substances, such as conjugated bilirubin and bile salts, as well as many structurally diverse xenobiotics and their metabolites. Due to its important role in defining ADME/Tox properties, efforts have emerged to build the structure-activity relationship (SAR) for MRP2/ABCC2 at early stages of drug discovery process. MRP2/ABCC2 is a member of the integral membrane protein family whose high-resolution crystal structure has not been described. To overcome the obstacle of lacking detailed structural depiction, various molecular modeling approaches have been applied to derive the structural requirements for binding interactions with MRP2/ABCC2 protein, including two-dimensional (2D) and three-dimensional (3D) quantitative SAR (QSAR) analysis, pharmacophore models, and homology modeling of the transporter. Here we summarize recent progresses in understanding the SAR of MRP2/ABCC2 recognition of substrates and/or inhibitors, and describe some of the useful in vitro tools for characterizing the interactions with the transporter.

Mg2+ -dependent ATP occlusion at the first nucleotide-binding domain (NBD1) of CFTR does not require the second (NBD2)

ATP binding to the first and second NBDs (nucleotide-binding domains) of CFTR (cystic fibrosis transmembrane conductance regulator) are bivalent-cation-independent and -dependent steps respectively [Aleksandrov, Aleksandrov, Chang and Riordan (2002) J. Biol. Chem. 277, 15419-15425]. Subsequent to the initial binding, Mg(2+) drives rapid hydrolysis at the second site, while promoting non-exchangeable trapping of the nucleotide at the first site. This occlusion at the first site of functional wild-type CFTR is somewhat similar to that which occurs when the catalytic glutamate residues in both of the hydrolytic sites of P-glycoprotein are mutated, which has been proposed to be the result of dimerization of the two NBDs and represents a transient intermediate formed during ATP hydrolysis [Tombline and Senior (2005) J. Bioenerg. Biomembr. 37, 497-500]. To test the possible relevance of this interpretation to CFTR, we have now characterized the process by which NBD1 occludes [(32)P]N(3)ATP (8-azido-ATP) and [(32)P]N(3)ADP (8-azido-ADP). Only N(3)ATP, but not N(3)ADP, can be bound initially at NBD1 in the absence of Mg(2+). Despite the lack of a requirement for Mg(2+) for ATP binding, retention of the NTP at 37 degrees C was dependent on the cation. However, at reduced temperature (4 degrees C), N(3)ATP remains locked in the binding pocket with virtually no reduction over a 1 h period, even in the absence of Mg(2+). Occlusion occurred identically in a DeltaNBD2 construct, but not in purified recombinant NBD1, indicating that the process is dependent on the influence of regions of CFTR in addition to NBD1, but not NBD2.

Genomics and the mechanism of P-glycoprotein (ABCB1)

The development of effective clinical interventions against multidrug resistance (MDR) in cancer remains a significant challenge. Single nucleotide polymorphisms (SNPs) contribute to wide variations in how individuals respond to medications and there are several SNPs in human P-glycoprotein (P-gp) that may influence the interactions of drug-substrates with the transporter. Interestingly, even some of the synonymous SNPs have functional consequences for P-gp. It is also becoming increasingly evident that an understanding of the transport pathway of P-gp may be necessary to design effective modulators. In this review we discuss: (1) The potential importance of SNPs (both synonymous and non-synonymous) in MDR and (2) How new concepts that have emerged from structural studies with isolated nucleotide binding domains of bacterial ABC transporters have prompted biochemical studies on P-gp, leading to a better understanding of the mechanism of P-gp mediated transport. Our results suggest that the power-stroke is provided only after formation of the pre-hydrolysis transition-like (E.S) state during ATP hydrolysis.

A mutation of the H-loop selectively affects rhodamine transport by the yeast multidrug ABC transporter Pdr5

The yeast ABC transporter Pdr5 plays a major role in drug resistance against a large number of structurally unrelated compounds. Although Pdr5 has been extensively studied, many important aspects regarding its molecular mechanisms remain unresolved. For example, a striking degeneration of conserved amino acid residues exists in the nucleotide binding domains (NBDs), but their functional relevance is unknown. Here, we performed in vivo and in vitro experiments to address the functional asymmetry of NBDs. It became evident by ATPase activity and drug transport studies that catalysis at only one of the two NBD composite sites is crucial for protein function. Furthermore, mutations of the proposed "catalytic carboxylate" (E1036) and the "catalytic dyad histidine" (H1068) were characterized. Although a mutation of the glutamate abolished ATPase activity and substrate transport, mutation of H1068 had no influence on ATP consumption. However, the H1068A mutation abolished rhodamine transport in vivo and in vitro, while leaving the transport of other substrates unaffected. By contrast to mammalian P-glycoprotein (P-gp), the ATPase activity of yeast Pdr5 is not stimulated by the addition of substrates, indicating that Pdr5 is an uncoupled ABC transporter that constantly hydrolyses ATP to ensure active substrate transport. Taken together, our data provide important insights into the molecular mechanism of Pdr5 and suggest that not solely the transmembrane domains dictate substrate selection.

The Q-loop of DrrA is involved in producing the closed conformation of the nucleotide binding domains and in transduction of conformational changes between DrrA and DrrB

DrrA and DrrB proteins form an ATP-dependent efflux pump for doxorubicin and daunorubicin in Streptomyces peucetius. DrrA, the catalytic subunit, forms a complex with the integral membrane protein DrrB. Previous studies have provided evidence for strong interaction between these two proteins, which was found to be critical for binding of ATP to DrrA and for stability of DrrB. Chemical cross-linking experiments carried out previously showed that in the resting state of the complex DrrA and DrrB are in contact with each other. Use of a cysteine-to-amine cross-linker then allowed identification of the N-terminal cytoplasmic tail of DrrB (residues 1-53) as the primary region of contact with DrrA. In this study, single-cysteine substitutions were introduced into different domains of DrrA in a strain already containing the S23C substitution in the N-terminal tail of DrrB. By using different arm-length disulfide cross-linkers, we found that a cysteine placed in the Q-loop region of DrrA traps DrrA in the dimeric state, thus indicating that in the closed conformation the Q-loops from opposing subunits are in the proximity of each other. Furthermore, the same region of DrrA was also found to interact with the N-terminus of DrrB, although the A-A interaction was much more prominent than the A-B interaction under these conditions. On the basis of additional data shown here, we propose that the interaction of the Q-loop with the N-terminal cytoplasmic tail of DrrB identifies an important step in the communication of conformational changes between DrrA and DrrB. The significance of these findings in the mechanism of the DrrAB complex is discussed, and a model based on analyses of different conformations of DrrA and DrrB is presented.

Catalytic cycle of ATP hydrolysis by P-glycoprotein: evidence for formation of the E.S reaction intermediate with ATP-gamma-S, a nonhydrolyzable analogue of ATP

Structural and biochemical studies of ATP-binding cassette (ABC) transporters suggest that an ATP-driven dimerization of the nucleotide-binding domains (NBDs) is an important reaction intermediate of the transport cycle. Moreover, an asymmetric occlusion of ATP at one of the two ATP sites of P-glycoprotein (Pgp) may follow the formation of the symmetric dimer. It has also been postulated that ADP drives the dissociation of the dimer. In this study, we show that the E.S conformation of Pgp (previously demonstrated in the E556Q/E1201Q mutant Pgp) can be obtained with the wild-type protein by use of the nonhydrolyzable ATP analogue ATP-gamma-S. ATP-gamma-S is occluded into the Pgp NBDs at 34 degrees C but not at 4 degrees C, whereas ATP is not occluded at either temperature. Using purified Pgp incorporated into proteoliposomes and ATP-gamma-35S, we demonstrate that the occlusion of ATP-gamma-35S has an Eact of 60 kJ/mol and the stoichiometry of ATP-gamma-35S:Pgp is 1:1 (mol/mol). Additionally, in the conserved Walker B mutant (E556Q/E1201Q) of Pgp, we find occlusion of the nucleoside triphosphate but not the nucleoside diphosphate. Furthermore, Pgp in the occluded nucleotide conformation has reduced affinity for transport substrates. These data provide evidence for the ATP-driven dimerization and ADP-driven dissociation of the NBDs, and although two ATP molecules may initiate dimerization, only one is driven to an occluded pre-hydrolysis intermediate state. Thus, in a full-length ABC transporter like Pgp, it is unlikely that there is complete association and disassociation of NBDs and the occluded nucleotide conformation at one of the NBDs provides the power-stroke at the transport-substrate site.

Complete inhibition of the Pdr5p multidrug efflux pump ATPase activity by its transport substrate clotrimazole suggests that GTP as well as ATP may be used as an energy source

The yeast Pdr5p transporter is a 160 kDa protein that effluxes a large variety of xenobiotic compounds. In this study, we characterize its ATPase activity and demonstrate that it has biochemical features reminiscent of those of other ATP-binding cassette multidrug transporters: a relatively high Km for ATP (1.9 mM), inhibition by orthovanadate, and the ability to specifically bind an azidoATP analogue at the nucleotide-binding domains. Pdr5p-specific ATPase activity shows complete, concentration-dependent inhibition by clotrimazole, which is also known to be a potent transport substrate. Our results indicate, however, that this inhibition is noncompetitive and caused by the interaction of clotrimazole with the transporter at a site that is distinct from the ATP-binding domains. Curiously, Pdr5p-mediated transport of clotrimazole continues at intracellular concentrations of substrate that should eliminate all ATPase activity. Significantly, however, we observed that the Pdr5p has GTPase and UTPase activities that are relatively resistant to clotrimazole. Furthermore, the Km(GTPase) roughly matches the intracellular concentrations of the nucleotide reported for yeast. Using purified plasma membrane vesicles, we demonstrate that Pdr5p can use GTP to fuel substrate transport. We propose that Pdr5p increases its multidrug transport substrate specificity by using more than one nucleotide as an energy source.

Multidrug resistance in a urothelial cancer cell line after 3, 1-hour exposures to mitomycin C

PURPOSE

The development of multidrug resistance is a problem in chemotherapy for many tumors. In vitro models of multidrug resistance require adapted cell strains that are conventionally produced from parental lines by chronic low dose drug exposure. Because adjunctive intravesical chemotherapy for superficial bladder cancer uses short courses of high dose treatment, we investigated whether such exposure of the RT112 cell line (Catalogue No. ACC 418, Deutsche Sammlung von Mikroorganismen und Zellkulturen, Braunschweig, Germany) to mitomycin C, which is a common intravesical agent, would elicit multidrug resistance.

MATERIALS AND METHODS

Three 1-hour exposures to graded concentrations were done at 3-week intervals. The highest mitomycin C concentrations permitting recovery in cultures and, therefore, available for examination were 3.13 and 1.06 microg/ml. Cross-resistance to epirubicin in surviving cultures was visualized by confocal microscopy and quantified by MTT residual viable biomass assay. Spheroids were made by the agarose technique and exposed to high dose mitomycin C to assess the probability that the relevant concentrations might be found clinically in some cell layers of a superficial lesion.

RESULTS

Resistance was induced by 3 short drug exposures. The evidence for this was functional (MTT assay) and by intracellular localization. Toxicity to an alternative multidrug resistance class drug was lowered in surviving clones and nuclear exclusion of the drug was noted. Spheroid experiments showed sharp gradients of incorporated drug across the outermost layers of cells, suggesting that a proportion of cells in clinical superficial bladder cancer would be exposed to drug at concentrations that generated the resistant clones in these experiments.

CONCLUSIONS

We report multidrug resistance induction using 2 independent methodologies. The results have implications for the development of experimental models and the likelihood of resistance resulting from clinical regimens. Brief exposure can elicit detectable resistance. It is arguable that selective rather than instructive mechanisms are involved, and the levels of drug required are likely to exist in a superficial transitional cell carcinoma frond exposed at its surface to high drug concentrations.

The use of biomarkers in biomonitoring: a 2-tier approach assessing the level of pollutant-induced stress syndrome in sentinel organisms

The paper outlines a 2-tier approach for wide-scale biomonitoring programmes. To obtain a high level of standardization, we suggest the use of caged organisms (mussels or fish). An "early warning", highly sensitive, low-cost biomarker is employed in tier 1 (i.e. lysosomal membrane stability (LMS) and survival rate, a marker for highly polluted sites). Tier 2 is used only for animals sampled at sites in which LMS changes are evident and there is no mortality, with a complete battery of biomarkers assessing the levels of pollutant-induced stress syndrome. Possible approaches for integrating biomarker data in a synthetic index are discussed, along with our proposal to use a recently developed Expert System. The latter system allows a correct selection of biomarkers at different levels of biological organisation (molecular/cellular/tissue/organism) taking into account trends in pollutant-induced biomarker changes (increasing, decreasing, bell-shape). A selection of biomarkers of stress, genotoxicity and exposure usually employed in biomonitoring programmes is presented, together with a brief overview of new biomolecular approaches.

Surfactant-polymer nanoparticles overcome P-glycoprotein-mediated drug efflux

Nanoparticles enhance the therapeutic efficacy of an encapsulated drug by increasing and sustaining the delivery of the drug inside the cell. We have previously demonstrated that Aerosol OT (AOT)-alginate nanoparticles, a novel formulation developed recently in our laboratory, significantly enhance the therapeutic efficacy of encapsulated drugs like doxorubicin in drug-sensitive tumor cells. The purpose of this study is to evaluate the drug delivery potential of AOT-alginate nanoparticles in drug-resistant cells overexpressing the drug efflux transporter, P-glycoprotein (P-gp). AOT-alginate nanoparticles were formulated using an emulsion-cross-linking process. Rhodamine 123 and doxorubicin were used as model P-gp substrates. Cytotoxicity of nanoparticle-encapsulated doxorubicin and kinetics of nanoparticle-mediated cellular drug delivery were evaluated in both drug-sensitive and -resistant cell lines. AOT-alginate nanoparticles enhanced the cytotoxicity of doxorubicin significantly in drug-resistant cells. The enhancement in cytotoxicity with nanoparticles was sustained over a period of 10 days. Uptake studies with rhodamine-loaded nanoparticles indicated that nanoparticles significantly increased the level of drug accumulation in resistant cells at nanoparticle doses higher than 200 microg/mL. Blank nanoparticles also improved rhodamine accumulation in drug-resistant cells in a dose-dependent manner. Nanoparticle-mediated enhancement in rhodamine accumulation was not because of membrane permeabilization. Fluorescence microscopy studies demonstrated that nanoparticle-encapsulated doxorubicin was predominantly localized in the perinuclear vesicles and to a lesser extent in the nucleus, whereas free doxorubicin accumulated mainly in peripheral endocytic vesicles. Inhibition of P-gp-mediated rhodamine efflux with AOT-alginate nanoparticles was confirmed in primary brain microvessel endothelial cells. In conclusion, an AOT-alginate nanoparticle system enhanced the cellular delivery and therapeutic efficacy of P-gp substrates in P-gp-overexpressing cells.

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.

Preliminary examination of short-term cellular toxicological responses of the coral Madracis mirabilis to acute Irgarol 1051 exposure

Irgarol 1051 is an s-triazine herbicide formulated with Cu2O in antifouling paints. Recent studies have shown that Irgarol 1051 inhibits coral photosynthesis at environmentally relevant concentrations, consistent with its mode of action as a photosystem II inhibitor. Related toxicologic effects of this herbicide on coral cellular physiology have not yet been investigated. We used cellular diagnostics to measure changes in 18 toxicologic cellular parameters in endosymbiotic algal (dinoflagellate) and cnidarian (host) fractions of the common branching coral Madracis mirabilis associated with in vivo 8- and 24-hour exposures to a nominal initial Irgarol 1051 concentration of 10 microg L(-1). Responses measured were (1) xenobiotic response, which includes total and dinoflagellate multixenobiotic resistance (MXR), cnidarian cytochrome (CYP) P450-3 and P450-6 classes, cnidarian, and dinoflagellate glutathione-s-transferase (GST); (2) oxidative damage and response, which includes cnidarian and dinoflagellate Cu/Zn and Mn superoxide dismutase (SOD), cnidarian and dinoflagellate glutathione peroxidase (GPx), cnidarian catalase, and total protein carbonyl); (3) metabolic homeostasis, which includes chloroplast and invertebrate small heat-shock proteins (sHsp), cnidarian protoporphyrinogen oxidase IX (PPO), cnidarian ferrochelatase, and cnidarian heme oxygenase; and (4) protein metabolic condition, which includes cnidarian and dinoflagellate heat shock proteins (hsp70 and hsp60), total ubiquitin, and cnidarian ubiquitin ligase. Acute responses to Irgarol 1051 exposure included significant increases in total and dinoflagellate MXR, dinoflagellate Cu/Zn SOD, dinoflagellate chloroplast sHsp, and cnidarian PPO. Irgarol 1051 exposure resulted in decreases in cnidarian GPx, cnidarian ferrochelatase, cnidarian catalase, and cnidarian CYP 450-3 and -6 classes. Related implications of Irgarol 1051 exposure to coral cellular condition are discussed.

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

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

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

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

Differential effects of the immunosuppressive agents cyclosporin A, tacrolimus and sirolimus on drug transport by multidrug resistance proteins

PURPOSE

We sought to determine the effects of the immunosuppressants, cyclosporin A (CsA), tacrolimus and sirolimus, on drug transport by the ATP-binding cassette proteins, P-glycoprotein (Pgp; ABCB1), multidrug resistance protein-1 (MRP-1; ABCC1) and breast cancer resistance protein (BCRP; ABCG2), and the major vault protein lung resistance protein (LRP).

METHODS

Cellular content of mitoxantrone, a Pgp, MRP-1 and BCRP substrate, was measured by flow cytometry in cells overexpressing these proteins following incubation with and without CsA, tacrolimus or sirolimus. Interaction of BCRP with these compounds was studied by photolabeling and ATPase assays. Nuclear-cytoplasmic distribution of doxorubicin was studied by confocal microscopy in cells overexpressing LRP.

RESULTS

CsA increased cellular drug uptake in cells overexpressing Pgp, MRP-1 or BCRP and nuclear drug uptake in cells overexpressing LRP at the clinically achievable concentration of 2.5 microM. Tacrolimus enhanced cellular drug uptake at 1 microM, but not at 0.08 microM, its clinically achievable concentration, and did not enhance nuclear drug uptake. Sirolimus enhanced cellular drug uptake in cells overexpressing Pgp, MRP-1 and BCRP with optimal effects at 2.5 microM, but was effective at its clinically achievable concentration of 0.25 microM if cells were pre-incubated for at least 30 min before drug exposure, and also enhanced nuclear drug uptake at 0.25 microM. BCRP modulation by all three immunosuppressive agents was associated with competitive binding to the drug transport sites.

CONCLUSIONS

CsA, tacrolimus and sirolimus modulate drug transport by Pgp, MRP-1 and BCRP and CsA and sirolimus modulate drug transport by LRP at concentrations that differ from immunosuppressive concentrations and maximum tolerated concentrations.

Neoadjuvant chemotherapy in women with large and locally advanced breast cancer: chemoresistance and prediction of response to drug therapy

Patients with large and locally advanced breast cancer (LLABC) present with a therapeutic challenge and undergo multimodality treatment. Many such patients receive neoadjuvant chemotherapy (NAC) prior to surgery. However, a number of these patients do not respond well to NAC and only a percentage (usually less than 30%) obtains a complete or optimal response. A range of mechanisms are believed to be involved in this chemoresistance, including ATP binding cassette (ABC) transporter overexpression, dysregulation of apoptosis and possibly increased numbers of cancer stem cells. The chemoresistant processes may be due to more than one mechanism. The ability to predict a response to NAC would be beneficial, targeting expensive and toxic drug treatment to those likely to respond and providing a therapeutic strategy for further post-operative chemotherapy. Currently, many biomarkers have been studied with a view to establishing a predictor of response. However, no single biomarker appears to be effective. Genomics is a novel biotechnological process which is being used to predict response to drug therapy; this work is currently at an early stage of development

Modulation of function of three ABC drug transporters, P-glycoprotein (ABCB1), mitoxantrone resistance protein (ABCG2) and multidrug resistance protein 1 (ABCC1) by tetrahydrocurcumin, a major metabolite of curcumin

Many studies have been performed with the aim of developing effective resistance modulators to overcome the multidrug resistance (MDR) of human cancers. Potent MDR modulators are being investigated in clinical trials. Many current studies are focused on dietary herbs due to the fact that these have been used for centuries without producing any harmful side effects. In this study, the effect of tetrahydrocurcumin (THC) on three ABC drug transporter proteins, P-glycoprotein (P-gp or ABCB1), mitoxantrone resistance protein (MXR or ABCG2) and multidrug resistance protein 1 (MRP1 or ABCC1) was investigated, to assess whether an ultimate metabolite form of curcuminoids (THC) is able to modulate MDR in cancer cells. Two different types of cell lines were used for P-gp study, human cervical carcinoma KB-3-1 (wild type) and KB-V-1 and human breast cancer MCF-7 (wild type) and MCF-7 MDR, whereas, pcDNA3.1 and pcDNA3.1-MRP1 transfected HEK 293 and MXR overexpressing MCF7AdrVp3000 or MCF7FL1000 and its parental MCF-7 were used for MRP1 and MXR study, respectively. We report here for the first time that THC is able to inhibit the function of P-gp, MXR and MRP1. The results of flow cytometry assay indicated that THC is able to inhibit the function of P-gp and thereby significantly increase the accumulation of rhodamine and calcein AM in KB-V-1 cells. The result was confirmed by the effect of THC on [(3)H]-vinblastine accumulation and efflux in MCF-7 and MCF-7MDR. THC significantly increased the accumulation and inhibited the efflux of [(3)H]-vinblastine in MCF-7 MDR in a concentration-dependent manner. This effect was not found in wild type MCF-7 cell line. The interaction of THC with the P-gp molecule was clearly indicated by ATPase assay and photoaffinity labeling of P-gp with transport substrate. THC stimulated P-gp ATPase activity and inhibited the incorporation of [(125)I]-iodoarylazidoprazosin (IAAP) into P-gp in a concentration-dependent manner. The binding of [(125)I]-IAAP to MXR was also inhibited by THC suggesting that THC interacted with drug binding site of the transporter. THC dose dependently inhibited the efflux of mitoxantrone and pheophorbide A from MXR expressing cells (MCF7AdrVp3000 and MCF7FL1000). Similarly with MRP1, the efflux of a fluorescent substrate calcein AM was inhibited effectively by THC thereby the accumulation of calcein was increased in MRP1-HEK 293 and not its parental pcDNA3.1-HEK 293 cells. The MDR reversing properties of THC on P-gp, MRP1, and MXR were determined by MTT assay. THC significantly increased the sensitivity of vinblastine, mitoxantrone and etoposide in drug resistance KB-V-1, MCF7AdrVp3000 and MRP1-HEK 293 cells, respectively. This effect was not found in respective drug sensitive parental cell lines. Taken together, this study clearly showed that THC inhibits the efflux function of P-gp, MXR and MRP1 and it is able to extend the MDR reversing activity of curcuminoids in vivo.

Engineering ATPase Activity in the Isolated ABC Cassette of Human TAP1

The human transporter associated with antigen processing (TAP) translocates antigenic peptides from the cytosol into the endoplasmic reticulum lumen. The functional unit of TAP is a heterodimer composed of the TAP1 and TAP2 subunits, both of which are members of the ABC-transporter family. ABC-transporters are ATP-dependent pumps, channels, or receptors that are composed of four modules: two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). Although the TMDs are rather divergent in sequence, the NBDs are conserved with respect to structure and function. Interestingly, the NBD of TAP1 contains mutations at amino acid positions that have been proposed to be essential for catalytic activity. Instead of a glutamate, proposed to act as a general base, TAP1 contains an aspartate and a glutamine instead of the conserved histidine, which has been suggested to act as the linchpin. We used this degeneration to evaluate the individual contribution of these two amino acids to the ATPase activity of the engineered TAP1-NBD mutants. Based on our results a catalytic hierarchy of these two fundamental amino acids in ATP hydrolysis of the mutated TAP1 motor domain was deduced.

Exploiting reaction intermediates of the ATPase reaction to elucidate the mechanism of transport by P-glycoprotein (ABCB1)

The transport cycle of ABC transporters in general and P-glycoprotein in particular has been extensively studied, but the molecular mechanism remains controversial. We identify stable reaction intermediates in the progression of the P-glycoprotein-mediated ATPase reaction equivalent to the enzyme-substrate (E.S, P-glycoprotein.ATP) and enzyme-product (E.P, P-glycoprotein.ADP.P(i)) reaction intermediates. These have been characterized using the photoaffinity analog 8-azido-[alpha-32P]ATP as well as under equilibrium conditions using [alpha-32P]ATP, in which a cross-linking step is not involved. Similar results were obtained when 8-azido-[alpha-32P]ATP or [alpha-32P]ATP was used. The reaction intermediates were characterized based on their kinetic properties and the nature (triphosphate/diphosphate) of the trapped nucleotide. Using this defined framework and the Walker B E556Q/E1201Q mutant that traps nucleotide in the absence of vanadate or beryllium fluoride, the high to low affinity switch in the transport substrate binding site can be attributed to the formation of the E.S reaction intermediate of the ATPase reaction. Importantly, the posthydrolysis E.P state continues to have low affinity for substrate, suggesting that conformational changes that form the E.S complex are coupled to the conformational change at the transport substrate site to do mechanical work. Thus, the formation of E.S reaction intermediate during a single turnover of the catalytic cycle appears to provide the initial power stroke for movement of drug substrate from inner leaflet to outer leaflet of lipid bilayer. This novel approach applies transition state theory to elucidate the mechanism of P-glycoprotein and other ABC transporters and has wider applications in testing cause-effect hypotheses in coupled systems.

An important role of a "probable ATP-binding component of ABC transporter" during the process of Pseudomonas aeruginosa resistance to fluoroquinolone

In order to find new drug target to eliminate the fluoroquinolone resistance, the in vitro progress of Pseudomonas aeruginosa fluoroquinolone resistance was mimicked, and then proteomic analysis was applied to comparing different protein profiles during the resistant process. The results show that the expression of a "probable ATP-binding component of ATP binding cassette (ABC) transporter" existed in ciprofloxacin-intermediate and -resistant strains, but not in sensitive strain. In addition, the ciprofloxacin concentrations in P. aeruginosa strains, which were obtained from the progress of P. aeruginosa fluoroquinolone resistance, were determined by means of HPLC; the results show that the decrease of the intracellular concentration of drug and the expression of this new protein nearly take place simultaneously. The changes of mRNA levels of the probable ATP-binding component of ABC transporter were detected by virtue of RT-PCR and showed that this protein did not express in the sensitive strains but expressed increasingly in the intermediate and resistant strains. In order to determine the relationships between the development of antibiotic resistance and this protein further, a DNAzyme was designed to aim at the mRNA of the probable ATP-binding component of ABC transporter directly; the ciprofloxacin resistance of P. aeruginosa was partially reduced in vivo by inhibiting the expression of this protein. This DNAzyme has no effect on sensitive strain. And the comparison of drug intracellular concentrations between DNAzyme-treated strains and its control strains shows that this protein may be included in the course of active drug efflux.

Molecular dynamics simulations of E. coli MsbA transmembrane domain: formation of a semipore structure

The human P-glycoprotein (MDR1/P-gp) is an ATP-binding cassette (ABC) transporter involved in cellular response to chemical stress and failures of anticancer chemotherapy. In the absence of a high-resolution structure for P-gp, we were interested in the closest P-gp homolog for which a crystal structure is available: the bacterial ABC transporter MsbA. Here we present the molecular dynamics simulations performed on the transmembrane domain of the open-state MsbA in a bilayer composed of palmitoyl oleoyl phosphatidylethanolamine lipids. The system studied contained more than 90,000 atoms and was simulated for 50 ns. This simulation shows that the open-state structure of MsbA can be stable in a membrane environment and provides invaluable insights into the structural relationships between the protein and its surrounding lipids. This study reveals the formation of a semipore-like structure stabilized by two key phospholipids which interact with the hinge region of the protein during the entire simulation. Multiple sequence alignments of ABC transporters reveal that one of the residues involved in the interaction with these two phospholipids are under a strong selection pressure specifically applied on the bacterial homologs of MsbA. Hence, comparison of molecular dynamics simulation and phylogenetic data appears as a powerful approach to investigate the functional relevance of molecular events occurring during simulations.

The power of the pump: mechanisms of action of P-glycoprotein (ABCB1)

Members of the superfamily of ATP-binding cassette (ABC) transporters mediate the movement of a variety of substrates including simple ions, complex lipids and xenobiotics. At least 18 ABC transport proteins are associated with disease conditions. P-glycoprotein (Pgp, ABCB1) is the archetypical mammalian ABC transport protein and its mechanism of action has received considerable attention. There is strong biochemical evidence that Pgp moves molecular cargo against a concentration gradient using the energy of ATP hydrolysis. However, the molecular details of how the energy of ATP hydrolysis is coupled to transport remain in dispute and it has not been possible to reconcile the data from various laboratories into a single model. The functional unit of Pgp consists of two nucleotide binding domains (NBDs) and two trans-membrane domains which are involved in the transport of drug substrates. Considerable progress has been made in recent years in characterizing these functionally and spatially distinct domains of Pgp. In addition, our understanding of the domains has been augmented by the resolution of structures of several non-mammalian ABC proteins. This review considers: (i) the role of specific conserved amino acids in ATP hydrolysis mediated by Pgp; (ii) emerging insights into the dimensions of the drug binding pocket and the interactions between Pgp and the transport substrates and (iii) our current understanding of the mechanisms of coupling between energy derived from ATP binding and/or hydrolysis and efflux of drug substrates.

Functional characterization and ATP-induced dimerization of the isolated ABC-domain of the haemolysin B transporter

Nucleotide-binding domains (NBD) are highly conserved constituents of ATP-binding cassette (ABC) transporters. Members of this family couple ATP hydrolysis to the transfer of various molecules across cell membranes. The NBD of the HlyB transporter, HlyB-NBD, was characterized with respect to its uncoupled ATPase activity, oligomeric state, and stability in solution. Experimental data showed that both the nature and pH of an assay buffer influenced the level of protein activity. Comparative analysis of protein stability and ATPase activity in various buffers suggests an inverse relationship between the two. The highest ATPase activity was detected in HEPES, pH 7.0. A kinetic analysis of the ATPase activity in this buffer revealed an enzyme concentration dependence and ATP-induced protein oligomerization. Assuming that the dimer is the active form of enzyme, at least half of the purified HlyB-NBD was estimated to be a dimer at 1.2 microM under the most optimal conditions for ATP hydrolysis. This is about 2 orders of magnitude lower than reported for other canonical ABC-ATPases. The maximum reaction velocity of 0.6 micromol/mg x min at 22 degrees C and the apparent kinetic constant K(app)(0.5) of 0.26 mM for ATP were determined for the dimerized HlyB-NBD. Gel filtration experiments with the wild-type protein and HlyB-NBD mutated in a key catalytic residue, H662A, provided further evidence for ATP-induced protein dimerization. ATPase activity experiments with protein mixtures composed of wild-type and the ATPase-deficient H662A mutant demonstrated that one intact NBD within a dimer is sufficient for ATP hydrolysis. This single site turnover might suggest a sequential mechanism of ATP hydrolysis in the intact HlyB transporter.

Genomic organization and effects of ivermectin selection on Onchocerca volvulus P-glycoprotein

Ivermectin (IVM) was first developed for use with livestock. It is now the only drug used for mass treatment of onchocerciasis. It is difficult to prove whether reports of sub-optimal responses to IVM in some Onchocerca volvulus infected patients are a result of drug resistance, as procedures typically used to examine IVM efficacy in livestock can not be performed on humans. To determine the effects of IVM on O. volvulus, one approach is to examine allele frequencies before and after treatment. Allele(s) linked to resistance may increase in frequency after repeated treatment. Mass treatment of large human populations to reduce transmission of O. volvulus will impose selection pressure for resistance. P-glycoprotein has been implicated as a candidate IVM resistance gene in nematodes. In this study, the intron-exon structure of O. volvulus P-glycoprotein (OvPGP) has been defined. The gene spans 10.6 kb, is AT-rich, contains 24 exons and a high proportion of class 0 introns. The genetic diversity of 28 loci spanning the entire OvPGP gene was examined in four O. volvulus populations from the Volta Region of Ghana. Worms collected in 1999 and 2002 from IVM treated patients showed reduced genetic polymorphism and an increase in the number of loci not in Hardy-Weinberg equilibrium. Changes in allelic patterns and a reduction in diversity at many loci in P-glycoprotein in the parasites from IVM treated patients in 1999 and 2002 suggest that IVM is imposing selection on this gene, consistent with a possible development of IVM resistance.

Anticancer multidrug resistance mediated by MRP1: recent advances in the discovery of reversal agents

Multidrug resistance protein 1 (MRP1) belongs to the ATP-binding cassette (ABC) transporter family. It is able to transport a broad range of anticancer drugs through cellular membranes, thus limiting their antiproliferative action. Since its discovery in 1992, MRP1 has been the most studied among MRP proteins, which now count nine members. Besides the biological work, which targets structure elucidation, binding sites location, and mode of action, most efforts have been focused on finding molecules which act as MRP1 inhibitors. In this review, we attempt to summarize and highlight studies dealing with modulators of MRP1-mediated multidrug resistance (MDR), which have been accomplished in the last 5 years. The reported MRP1 inhibitors are discussed according to their chemical class. Finally, we try to bring information on structure-activity relationship (SAR) aspects and how modulators might interact with MRP1. This study may facilitate the rational design of future modulators of MDR.

Changes in P-glycoprotein activity are mediated by the growth of a tumour cell line as multicellular spheroids

BACKGROUND

Expression of P-glycoprotein (P-gp), the multidrug resistance (MDR) 1 gene product, can lead to multidrug resistance in tumours. However, the physiological role of P-gp in tumours growing as multicellular spheroids is not well understood. Recent evidence suggests that P-gp activity may be modulated by cellular components such as membrane proteins, membrane-anchoring proteins or membrane-lipid composition. Since, multicellular spheroids studies have evidenced alterations in numerous cellular components, including those related to the plasma membrane function, result plausible that some of these changes might modulate P-gp function and be responsible for the acquisition of multicellular drug resistance. In the present study, we asked if a human lung cancer cell line (INER-51) grown as multicellular spheroids can modify the P-gp activity to decrease the levels of doxorubicin (DXR) retained and increase their drug resistance.

RESULTS

Our results showed that INER-51 spheroids retain 3-folds lower doxorubicin than the same cells as monolayers however; differences in retention were not observed when the P-gp substrate Rho-123 was used. Interestingly, neither the use of the P-gp-modulating agent cyclosporin-A (Cs-A) nor a decrease in ATP-pools were able to increase DXR retention in the multicellular spheroids. Only the lack of P-gp expression throughout the pharmacological selection of a P-gp negative (P-gpneg) mutant clone (PSC-1) derived from INER-51 cells, allow increase of DXR retention in spheroids.

CONCLUSION

Thus, multicellular arrangement appears to alter the P-gp activity to maintain lower levels of DXR. However, the non expression of P-gp by cells forming multicellular spheroids has only a minor impact in the resistance to chemotherapeutic agents.

Interaction of LDS-751 with P-glycoprotein and mapping of the location of the R drug binding site

One cause of multidrug resistance is the overexpression of P-glycoprotein, a 170 kDa plasma membrane ABC transporter, which functions as an ATP-driven efflux pump with broad specificity for hydrophobic drugs, peptides, and natural products. The protein appears to interact with its substrates within the membrane environment. Previous reports suggested the existence of at least two binding sites, possibly overlapping and displaying positively cooperative interactions, termed the H and R sites for their preference for Hoechst 33342 and rhodamine 123, respectively. In this work, we have used several fluorescence approaches to characterize the molecular interaction of purified P-glycoprotein (Pgp) with the dye LDS-751, which is proposed to bind to the R site. A 50-fold enhancement of LDS-751 fluorescence indicated that the protein binding site was located in a hydrophobic environment, with a polarity lower than that of chloroform. LDS-751 bound with sub-micromolar affinity (K(d) = 0.75 microM) and quenched P-glycoprotein intrinsic Trp fluorescence by 40%, suggesting that Trp emitters are probably located close to the drub-binding regions of the transporter and may interact directly with the dye. Using a FRET approach, we mapped the possible locations of the LDS-751 binding site relative to the NB domain active sites. The R site appeared to be positioned close to the membrane boundary of the cytoplasmic leaflet. The location of both H and R drug binding sites is in agreement with the idea that Pgp may operate as a drug flippase, moving substrates from the inner leaflet to the outer leaflet of the plasma membrane.

Biochemical mechanism of modulation of human P-glycoprotein (ABCB1) by curcumin I, II, and III purified from Turmeric powder

P-glycoprotein (Pgp, ABCB1) is an ATP-dependent drug efflux pump linked to development of multidrug resistance (MDR) in cancer cells. Previously [Biochem Pharmacol 2002;64:573-82], we reported that a curcumin mixture could modulate both function and expression of Pgp. This study focuses on the effect of three major curcuminoids--curcumin I, II and III purified from a curcumin mixture--on modulation of Pgp function in a multidrug resistant human cervical carcinoma cell line (KB-V1). The similar IC(50) values for cytotoxicity of curcuminoids of KB-V1, and KB-3-1 (parental drug sensitive cell line) suggest that these curcuminoids may not be substrates for Pgp. Treating the cells with non-toxic doses of curcuminoids increased their sensitivity to vinblastine only in the Pgp expressing drug resistant cell line, KB-V1, and curcumin I retained the drug in KB-V1 cells more effectively than curcumin II and III, respectively. Effects of each curcuminoid on rhodamine123, calcein-AM, and bodipy-FL-vinblastine accumulation confirmed these findings. Curcumin I, II and III increased the accumulation of fluorescent substrates in a dose-dependent manner, and at 15 microM, curcumin I was the most effective. The inhibitory effect in a concentration-dependent manner of curcuminoids on verapamil-stimulated ATPase activity and photoaffinity labeling of Pgp with the [(125)I]-iodoarylazidoprazosin offered additional support; curcumin I was the most potent modulator. Taken together, these results indicate that curcumin I is the most effective MDR modulator among curcuminoids, and may be used in combination with conventional chemotherapeutic drugs to reverse MDR in cancer cells.

Nonequivalence of the nucleotide binding domains of the ArsA ATPase

The arsRDABC operon of Escherichia coli plasmid R773 encodes the ArsAB pump that catalyzes extrusion of the metalloids As(III) and Sb(III), conferring metalloid resistance. The catalytic subunit, ArsA, is an ATPase with two homologous halves, A1 and A2, connected by a short linker. Each half contains a nucleotide binding domain. The overall rate of ATP hydrolysis is slow in the absence of metalloid and is accelerated by metalloid binding. The results of photolabeling of ArsA with the ATP analogue 8-azidoadenosine 5'-[alpha-(32)P]-triphosphate at 4 degrees C indicate that metalloid stimulation correlates with a >10-fold increase in affinity for nucleotide. To investigate the relative contributions of the two nucleotide binding domains to catalysis, a thrombin site was introduced in the linker. This allowed discrimination between incorporation of labeled nucleotides into the two halves of ArsA. The results indicate that both the A1 and A2 nucleotide binding domains bind and hydrolyze trinucleotide, even in the absence of metalloid. Sb(III) increases the affinity of the A1 nucleotide binding domain to a greater extent than the A2 nucleotide binding domain. The ATP analogue labeled with (32)P at the gamma position was used to measure hydrolysis of trinucleotide at 37 degrees C. Under these catalytic conditions, both nucleotide binding domains hydrolyze ATP, but hydrolysis in A1 is stimulated to a greater degree by Sb(III) than A2. These results suggest that the two homologous halves of the ArsA may be functionally nonequivalent.

Cellular diagnostics and coral health: declining coral health in the Florida Keys

Coral reefs within the Florida Keys are disappearing at an alarming rate. Coral cover in the Florida Keys National Marine Sanctuary declined by 38% from 1996 to 2000. In 2000, populations of Montastraea annularis at four sites near Molasses Reef within the Florida Keys National Marine Sanctuary and one reef within Biscayne National Park were sampled on a quarterly basis. Anecdotal observations showed corals at Alina's Reef in Biscayne National Park appeared healthy in March, but experienced an acute loss of coral cover by August. Cellular Diagnostic analysis indicated that Alina's Reef corals were in distress: they had been afflicted with a severe oxidative damaging and protein-denaturing stress that affected both the corals and their symbiotic zooxanthellae. This condition was associated with a significant xenobiotic detoxification response in both species, reflecting probable chemical contaminant exposure. These results demonstrate that applying a Cellular Diagnostic approach can be effective in helping to identify stress and its underlying causes, providing diagnostic and prognostic biomarkers of coral health.

Multidrug resistance protein 4 (ABCC4)-mediated ATP hydrolysis: effect of transport substrates and characterization of the post-hydrolysis transition state

Multidrug resistance protein 4 (MRP4/ABCC4), transports cyclic nucleoside monophosphates, nucleoside analog drugs, chemotherapeutic agents, and prostaglandins. In this study we characterize ATP hydrolysis by human MRP4 expressed in insect cells. MRP4 hydrolyzes ATP (Km, 0.62 mm), which is inhibited by orthovanadate and beryllium fluoride. However, unlike ATPase activity of P-glycoprotein, which is equally sensitive to both inhibitors, MRP4-ATPase is more sensitive to beryllium fluoride than to orthovanadate. 8-Azido[alpha-32P]ATP binds to MRP4 (concentration for half-maximal binding approximately 3 microm) and is displaced by ATP or by its non-hydrolyzable analog AMPPNP (concentrations for half-maximal inhibition of 13.3 and 308 microm). MRP4 substrates, the prostaglandins E1 and E2, stimulate ATP hydrolysis 2- to 3-fold but do not affect the Km for ATP. Several other substrates, azidothymidine, 9-(2-phosphonylmethoxyethyl)adenine, and methotrexate do not stimulate ATP hydrolysis but inhibit prostaglandin E2-stimulated ATP hydrolysis. Although both post-hydrolysis transition states MRP4.8-azido[alpha-32P]ADP.Vi and MRP4.8-azido[alpha-32P]ADP.beryllium fluoride can be generated, nucleotide trapping is approximately 4-fold higher with beryllium fluoride. The divalent cations Mg2+ and Mn2+ support comparable levels of nucleotide binding, hydrolysis, and trapping. However, Co2+ increases 8-azido[alpha-32P]ATP binding and beryllium fluoride-induced 8-azido[alpha-32P]ADP trapping but does not support steady-state ATP hydrolysis. ADP inhibits basal and prostaglandin E2-stimulated ATP hydrolysis (concentrations for half-maximal inhibition 0.19 and 0.25 mm, respectively) and beryllium fluoride-induced 8-azido[alpha-32P]ADP trapping, whereas Pi has no effect up to 20 mm. In aggregate, our results demonstrate that MRP4 exhibits substrate-stimulated ATP hydrolysis, and we propose a kinetic scheme suggesting that ADP release from the post-hydrolysis transition state may be the rate-limiting step during the catalytic cycle.

Characterization of YvcC (BmrA), a multidrug ABC transporter constitutively expressed in Bacillus subtilis

The involvement of transporters in multidrug resistance of bacteria is an increasingly challenging problem, and most of the pumps identified so far use the protonmotive gradient as the energy source. A new member of the ATP-binding cassette (ABC) family, known in Bacillus subtilis as YvcC and homologous to each half of mammalian P-glycoprotein and to LmrA of Lactococcus lactis, has been studied here. The yvcC gene was constitutively expressed in B. subtilis throughout its growth, and a knockout mutant showed a lower rate of ethidium efflux than the wild-type strain. Overexpression of yvcC in Escherichia coli allowed the preparation of highly enriched inverted-membrane vesicles that exhibited high transport activities of three fluorescent drugs, namely, Hoechst 33342, doxorubicin, and 7-aminoactinomycin D. After solubilization with n-dodecyl beta-D-maltoside, the hexahistidine-tagged YvcC was purified by a one-step affinity chromatography, and its ability to bind many P-glycoprotein effectors was evidenced by fluorescence spectroscopy experiments. Collectively, these results showed that YvcC is a multidrug ABC transporter functionally active in wild-type B. subtilis, and YvcC was therefore renamed BmrA for Bacillus multidrug resistance ATP. Besides, reconstitution of YvcC into liposomes led to the highest, vanadate-sensitive, ATPase activity reported so far for an ABC transporter. Interestingly, such a high ATP hydrolysis proceeds with a positive cooperativity mechanism, a property only found so far with ABC importers.