P-glycoprotein substrate transport assessed by comparing cellular and vesicular ATPase activity

Trends in Molecular Properties, Bioavailability, and Permeability across the Bayer Compound Collection

For oral drugs, medicinal chemists aim to design compounds with high oral bioavailability, of which permeability is a key determinant. Taking advantage of >2000 compounds tested in rat bioavailability studies and >20,000 compounds tested in Caco2 assays at Bayer, we have examined the molecular properties governing bioavailability and permeability. In addition to classical parameters such as logD and molecular weight, we also investigated the relationship between calculated pK_a and permeability. We find that neutral compounds retain permeability up to a molecular weight limit of 700, while stronger acids and bases are restricted to weights of 400-500. We also investigate trends for common properties such as hydrogen bond donors and acceptors, polar surface area, aromatic ring count, and rotatable bonds, including compounds which exceed Lipinski's rule of five (Ro5). These property-structure relationships are combined to provide design guidelines for bioavailable drugs in both traditional and "beyond rule of 5" (bRo5) chemical space.

Selection of an Optimal In Vitro Model to Assess P-gp Inhibition: Comparison of Vesicular and Bidirectional Transcellular Transport Inhibition Assays

The multidrug resistance protein 1 (MDR1) P-glycoprotein (P-gp) is a clinically important transporter. In vitro P-gp inhibition assays have been routinely conducted to predict the potential for clinical drug-drug interactions (DDIs) mediated by P-gp. However, high interlaboratory and intersystem variability of P-gp IC50 data limits accurate prediction of DDIs using static models and decision criteria recommended by regulatory agencies. In this study, we calibrated two in vitro P-gp inhibition models: vesicular uptake of N-methyl-quinidine (NMQ) in MDR1 vesicles and bidirectional transport (BDT) of digoxin in Lilly Laboratories Cell Porcine Kidney 1 cells overexpressing MDR1 (LLC-MDR1) using a total of 48 P-gp inhibitor and noninhibitor drugs and digoxin DDI data from 70 clinical studies. Refined thresholds were derived using receiver operating characteristic analysis, and their predictive performance was compared with the decision frameworks proposed by regulatory agencies and selected reference. Furthermore, the impact of various IC50 calculation methods and nonspecific binding of drugs on DDI prediction was evaluated. Our studies suggest that the concentration of inhibitor based on highest approved dose dissolved in 250 ml divided by IC50(I2/IC50) is sufficient to predict P-gp related intestinal DDIs. IC50 obtained from vesicular inhibition assay with a refined threshold of I2/IC50 ≥ 25.9 provides comparable predictive power over those measured by net secretory flux and efflux ratio in LLC-MDR1 cells. We therefore recommend vesicular P-gp inhibition as our preferred method given its simplicity, lower variability, higher assay throughput, and more direct estimation of in vitro kinetic parameters, rather than BDT assay. SIGNIFICANCE STATEMENT: This study has conducted comprehensive calibration of two in vitro P-gp inhibition models: uptake in MDR1 vesicles and bidirectional transport in LLC-MDR1 cell monolayers to predict DDIs. This study suggests that IC50s obtained from vesicular inhibition with a refined threshold of I2/IC50 ≥ 25.9 provide comparable predictive power over those in LLC-MDR1 cells. Therefore, vesicular P-gp inhibition is recommended as the preferred method given its simplicity, lower variability, higher assay throughput, and more direct estimation of in vitro kinetic parameters.

Nitrogen-containing naringenin derivatives for reversing multidrug resistance in cancer

Naringenin (1), isolated from Euphorbia pedroi, was previously derivatized yielding compounds 2-13. In this study, aiming at expanding the pool of analogues of the flavanone core towards better multidrug resistance (MDR) reversal agents, alkylation reactions and chemical modification of the carbonyl moiety was performed (15-39). Compounds structures were assigned mainly by 1D and 2D NMR experiments. Compounds 1-39 were assessed as MDR reversers, in human ABCB1-transfected mouse T-lymphoma cells, overexpressing P-glycoprotein (P-gp). The results revealed that O-methylation at C-7, together with the introduction of nitrogen atoms and aromatic moieties at C-4 or C-4', significantly improved the activity, being compounds 27 and 37 the strongest P-gp modulators and much more active than verapamil. In combination assays, synergistic interactions of selected compounds with doxorubicin substantiated the results. While molecular docking suggested that flavanone derivatives act as competitive modulators, molecular dynamics showed that dimethylation promotes binding to a modulator-binding site. Moreover, flavanones may also interact with a vicinal ATP-binding site in both nucleotide-binding domains, hypothesizing an allosteric mode of action.

New uracil analogs as downregulators of ABC transporters in 5-fluorouracil-resistant human leukemia HL-60 cell line

Overexpression of ATP-binding cassette (ABC) transporters causing multidrug resistance (MDR) in cancer cells is one of the major obstacles in cancer chemotherapy. The 5-FU resistant subclone (HL-60/5FU) of the human HL-60 promyelocytic leukemia cell line was selected by the conventional method of continuous exposure of the cells to the drug up to 0.08 mmol/L concentration. HL-60/5FU cells exhibited six-fold enhanced resistance to 5-FU than HL-60 cells. RT-PCR and ELISA assay showed significant overexpression of MDR-related ABC transporters, ABCB1, ABCG2 but especially ABCC1 in the HL-60/5FU as compared with the parental cell line. Three novel synthetic 5-methylidenedihydrouracil analogs, U-236, U-332 and U-359, selected as highly cytotoxic for HL-60 cells in MTT test, showed similar cytotoxicity in the resistant cell line. When co-incubated with 5-FU, these analogs were found to down-regulate the expression of all three transporters. However, the most pronounced effect was caused by U-332 which almost completely abolished ABCC1 expression in the resistant HL-60/5FU cells. Additionally, U-332 inhibited the activity of ATPase, an enzyme which catalyzes hydrolysis of ATP, providing energy to efflux drugs from the cells through the cellular membranes. Taken together, the obtained data suggest that acquired 5-FU resistance in HL-60/5FU cells results from overexpression of ABCC1 and that targeting ABCC1 expression could be a potential approach to re-sensitize resistant leukemia cells to 5-FU. The synthetic uracil analog U-332, which can potently down-regulate ABC transporter expression and therefore disturb drug efflux, can be considered an efficient ABCC1 regulator in cancer cells.

Gut Microbiota-Mediated Bile Acid Transformations Alter the Cellular Response to Multidrug Resistant Transporter Substrates in Vitro: Focus on P-glycoprotein

Pharmacokinetic research at the host-microbe interface has been primarily directed toward effects on drug metabolism, with fewer investigations considering the absorption process. We previously demonstrated that the transcriptional expression of genes encoding intestinal transporters involved in lipid translocation are altered in germ-free and conventionalized mice possessing distinct bile acid signatures. It was consequently hypothesized that microbial bile acid metabolism, which is the deconjugation and dehydroxylation of the bile acid steroid nucleus by gut bacteria, may impact upon drug transporter expression and/or activity and potentially alter drug disposition. Using a panel of three human intestinal cell lines (Caco-2, T84, and HT-29) that differ in basal transporter expression level, bile acid conjugation-, and hydroxylation-status was shown to influence the transcription of genes encoding several major influx and efflux transporter proteins. We further investigated if these effects on transporter mRNA would translate to altered drug disposition and activity. The results demonstrated that the conjugation and hydroxylation status of the bile acid steroid nucleus can influence the cellular response to multidrug resistance (MDR) substrates, a finding that did not directly correlate with directionality of gene or protein expression. In particular, we noted that the cytotoxicity of cyclosporine A was significantly augmented in the presence of the unconjugated bile acids deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) in P-gp positive cell lines, as compared to their taurine/glycine-conjugated counterparts, implicating P-gp in the molecular response. Overall this work identifies a novel mechanism by which gut microbial metabolites may influence drug accumulation and suggests a potential role for the microbial bile acid-deconjugating enzyme bile salt hydrolase (BSH) in ameliorating multidrug resistance through the generation of bile acid species with the capacity to access and inhibit P-gp ATPase. The physicochemical property of nonionization is suggested to underpin the preferential ability of unconjugated bile acids to attenuate the efflux of P-gp substrates and to sensitize tumorigenic cells to cytotoxic therapeutics in vitro. This work provides new impetus to investigate whether perturbation of the gut microbiota, and thereby the bile acid component of the intestinal metabolome, could alter drug pharmacokinetics in vivo. These findings may additionally contribute to the development of less toxic P-gp modulators, which could overcome MDR.

Unidirectional Transport Mechanism in an ATP Dependent Exporter

ATP-binding cassette (ABC) transporters use the energy of ATP binding and hydrolysis to move a large variety of compounds across biological membranes. P-glycoprotein, involved in multidrug resistance, is the most investigated eukaryotic family member. Although a large number of biochemical and structural approaches have provided important information, the conformational dynamics underlying the coupling between ATP binding/hydrolysis and allocrite transport remains elusive. To tackle this issue, we performed molecular dynamic simulations for different nucleotide occupancy states of Sav1866, a prokaryotic P-glycoprotein homologue. The simulations reveal an outward-closed conformation of the transmembrane domain that is stabilized by the binding of two ATP molecules. The hydrolysis of a single ATP leads the X-loop, a key motif of the ATP binding cassette, to interfere with the transmembrane domain and favor its outward-open conformation. Our findings provide a structural basis for the unidirectionality of transport in ABC exporters and suggest a ratio of one ATP hydrolyzed per transport cycle.

Korean red ginseng extract enhances paclitaxel distribution to mammary tumors and its oral bioavailability by P-glycoprotein inhibition

1. Drug efflux by P-glycoprotein (P-gp) is a common resistance mechanism of breast cancer cells to paclitaxel, the primary chemotherapy in breast cancer. As a means of overcoming the drug resistance-mediated failure of paclitaxel chemotherapy, the potential of Korean red ginseng extract (KRG) as an adjuvant chemotherapy has been reported only in in vitro. Therefore, we assessed whether KRG alters P-gp mediated paclitaxel efflux, and therefore paclitaxel efficacy in in vitro and vivo models. 2. KRG inhibited P-gp protein expression and transcellular efflux of paclitaxel in MDCK-mdr1 cells, but KRG was not a substrate of P-gp ATPase. In female rats with mammary tumor, the combination of paclitaxel with KRG showed the greater reduction of tumor volumes, lower P-gp protein expression and higher paclitaxel distribution in tumors, and greater oral bioavailability of paclitaxel than paclitaxel alone. 3. From these results, KRG increased systemic circulation of oral paclitaxel and its distribution to tumors via P-gp inhibition in rats and under the current study conditions.

Allocrite Sensing and Binding by the Breast Cancer Resistance Protein (ABCG2) and P-Glycoprotein (ABCB1)

The ATP binding cassette (ABC) transporters ABCG2 and ABCB1 perform ATP hydrolysis-dependent efflux of structurally highly diverse compounds, collectively called allocrites. Whereas much is known about allocrite-ABCB1 interactions, the chemical nature and strength of ABCG2-allocrite interactions have not yet been assessed. We quantified and characterized interactions of allocrite with ABCG2 and ABCB1 using a set of 39 diverse compounds. We also investigated potential allocrite binding sites based on available transporter structures and structural models. We demonstrate that ABCG2 binds its allocrites from the lipid membrane, despite their hydrophilicity. Hence, binding of allocrite to both transporters is a two-step process, starting with a lipid-water partitioning step, driven mainly by hydrophobic interactions, followed by a transporter binding step in the lipid membrane. We show that binding of allocrite to both transporters increases with the number of hydrogen bond acceptors in allocrites. Scrutinizing the transporter translocation pathways revealed ample hydrogen bond donors for allocrite binding. Importantly, the hydrogen bond donor strength is, on average, higher in ABCG2 than in ABCB1, which explains the higher measured affinity of allocrite for ABCG2. π-π stacking and π-cation interactions play additional roles in binding of allocrite to ABCG2 and ABCB1. With this analysis, we demonstrate that these membrane-mediated weak electrostatic interactions between transporters and allocrites allow for transporter promiscuity toward allocrites. The different sensitivities of the transporters to allocrites' charge and amphiphilicity provide transporter specificity. In addition, we show that the different hydrogen bond donor strengths in the two transporters allow for affinity tuning.

P-glycoprotein and membrane roles in multidrug resistance

Multidrug-resistance (MDR) phenomena are a worldwide health concern. ATP-binding cassette efflux pumps as P-glycoprotein have been thoroughly studied in a frantic run to develop new efflux modulators capable to reverse MDR phenotypes. The study of efflux pumps has provided some key aspects on drug extrusion, however the answers could not be found solely on ATP-binding cassette transporters. Its counterpart – the plasma membrane – is now emerging as a critical structure able to modify drug behavior and efflux pump activity. Alterations in the membrane surrounding P-glycoprotein are now known to modulate drug efflux, with membrane-related biophysical, biochemical and mechanical aspects further increasing the complexity of an already multifaceted phenomena. This review summarizes the main knowledge comprising the plasma membrane role in MDR.

The Q loops of the human multidrug resistance transporter ABCB1 are necessary to couple drug binding to the ATP catalytic cycle

For a primary active pump, such as the human ATP-binding-cassette (ABC) transporter ABCB1, coupling of drug-binding by the two transmembrane domains (TMDs) to the ATP catalytic cycle of the two nucleotide-binding domains (NBDs) is fundamental to the transport mechanism, but is poorly understood at the biochemical level. Structure data suggest that signals are transduced through intracellular loops of the TMDs that slot into grooves on the NBDs. At the base of these grooves is the Q loop. We therefore mutated the eponymous glutamine in one or both NBD Q loops and measured the effect on conformation and function by using a conformation-sensitive antibody (UIC2) and a fluorescent drug (Bodipy-verapamil), respectively. We showed that the double mutant is trapped in the inward-open state, which binds the drug, but cannot couple to the ATPase cycle. Our data also describe marked redundancy within the transport mechanism, because single-Q-loop mutants are functional for Bodipy-verapamil transport. This result allowed us to elucidate transduction pathways from twin drug-binding cavities to the Q loops using point mutations to favor one cavity over the other. Together, the data show that the Q loop is the central flexion point where the aspect of the drug-binding cavities is coupled to the ATP catalytic cycle.

Restricting CNS penetration of drugs to minimise adverse events: role of drug transporters

Some drug discovery approaches can benefit from restricting the access of compounds to the central nervous system (CNS) to minimise the risk of side-effects. Designing compounds that act as substrates for efflux transporters in the blood–brain barrier can achieve CNS restriction without significantly impairing absorption in the intestine. In vitro assays can be deployed to optimise a balance between passive permeability and active efflux via the ABC family transporters P-glycoprotein (P-gp, ABCB1) and Breast Cancer Resistance Protein (BCRP, ABCG2) whilst in vivo estimates of distribution of unbound concentrations of drug are needed to understand pharmacologically relevant exposure in peripheral and central compartments. This strategy can deliver significant CNS restriction whilst retaining good oral bioavailability, cell penetration and pharmacological activity. The possible risks of targeting P-gp and BCRP in orally delivered drugs are discussed.

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.

Sav1866 from Staphylococcus aureus and P-glycoprotein: similarities and differences in ATPase activity assessed with detergents as allocrites

The ATP-binding cassette exporters Sav1866 from Staphylococcus aureus and P-glycoprotein are known to share a certain sequence similarity and disposition for cationic allocrites. Conversely, the two ATPases react very differently to neutral detergents that have previously been shown to be inhibitory allocrites for P-glycoprotein. To gain insight into the functional differences of the two proteins, we compared their basal and detergent-stimulated ATPase activity. P-Glycoprotein was investigated in NIH-MDR1-G185 plasma membrane vesicles and Sav1866 in lipid vesicles exhibiting a membrane packing density and a surface potential similar to those of the plasma membrane vesicles. Under basal conditions, Sav1866 revealed a lower catalytic efficiency and concomitantly a more pronounced sodium chloride and pH dependence than P-glycoprotein. As expected, the cationic allocrites (alkyltrimethylammonium chlorides) induced similar bell-shaped activity curves as a function of concentration for both exporters, suggesting stimulation upon binding of the first and inhibition upon binding of the second allocrite molecule. However, the neutral allocrites (n-alkyl-β-d-maltosides and n-ethylene glycol monododecyl ethers) reduced P-glycoprotein's ATPase activity at concentrations well below their critical micelle concentration (CMC) but strongly enhanced Sav1866's ATPase activity even at concentrations above their CMC. The lack of ATPase inhibition at high concentrations of neutral of detergents could be explained by their comparatively low binding affinity for the transmembrane domains of Sav1866, which seems to prevent binding of a second inhibitory molecule. The high ATPase activity in the presence of hydrophobic, long chain detergents moreover revealed that Sav1866, despite its lower basal catalytic efficiency, is a more efficient floppase for lipidlike amphiphiles than P-glycoprotein.

P-glycoprotein-ATPase modulation: the molecular mechanisms

P-glycoprotein-ATPase is an efflux transporter of broad specificity that counteracts passive allocrit influx. Understanding the rate of allocrit transport therefore matters. Generally, the rates of allocrit transport and ATP hydrolysis decrease exponentially with increasing allocrit affinity to the transporter. Here we report unexpectedly strong down-modulation of the P-glycoprotein-ATPase by certain detergents. To elucidate the underlying mechanism, we chose 34 electrically neutral and cationic detergents with different hydrophobic and hydrophilic characteristics. Measurement of the P-glycoprotein-ATPase activity as a function of concentration showed that seven detergents activated the ATPase as expected, whereas 27 closely related detergents reduced it significantly. Assessment of the free energy of detergent partitioning into the lipid membrane and the free energy of detergent binding from the membrane to the transporter revealed that the ratio, q, of the two free energies of binding determined the rate of ATP hydrolysis. Neutral (cationic) detergents with a ratio of q = 2.7 ± 0.2 (q > 3) followed the aforementioned exponential dependence. Small deviations from the optimal ratio strongly reduced the rates of ATP hydrolysis and flopping, respectively, whereas larger deviations led to an absence of interaction with the transporter. P-glycoprotein-ATPase inhibition due to membrane disordering by detergents could be fully excluded using (2)H-NMR-spectroscopy. Similar principles apply to modulating drugs.

Fitting the elementary rate constants of the P-gp transporter network in the hMDR1-MDCK confluent cell monolayer using a particle swarm algorithm

P-glycoprotein, a human multidrug resistance transporter, has been extensively studied due to its importance to human health and disease. In order to understand transport kinetics via P-gp, confluent cell monolayers overexpressing P-gp are widely used. The purpose of this study is to obtain the mass action elementary rate constants for P-gp's transport and to functionally characterize members of P-gp's network, i.e., other transporters that transport P-gp substrates in hMDR1-MDCKII confluent cell monolayers and are essential to the net substrate flux. Transport of a range of concentrations of amprenavir, loperamide, quinidine and digoxin across the confluent monolayer of cells was measured in both directions, apical to basolateral and basolateral to apical. We developed a global optimization algorithm using the Particle Swarm method that can simultaneously fit all datasets to yield accurate and exhaustive fits of these elementary rate constants. The statistical sensitivity of the fitted values was determined by using 24 identical replicate fits, yielding simple averages and standard deviations for all of the kinetic parameters, including the efflux active P-gp surface density. Digoxin required additional basolateral and apical transporters, while loperamide required just a basolateral tranporter. The data were better fit by assuming bidirectional transporters, rather than active importers, suggesting that they are not MRP or active OATP transporters. The P-gp efflux rate constants for quinidine and digoxin were about 3-fold smaller than reported ATP hydrolysis rate constants from P-gp proteoliposomes. This suggests a roughly 3∶1 stoichiometry between ATP hydrolysis and P-gp transport for these two drugs. The fitted values of the elementary rate constants for these P-gp substrates support the hypotheses that the selective pressures on P-gp are to maintain a broad substrate range and to keep xenobiotics out of the cytosol, but not out of the apical membrane.

Euphorbia factor L1 reverses ABCB1-mediated multidrug resistance involving interaction with ABCB1 independent of ABCB1 downregualtion

Euphorbia factor L1 (EFL1) belongs to diterpenoids of genus Euphorbia. In this article, its reversal activity against ABCB1-mediated MDR in KBv200 and MCF-7/adr cells was reported. However, EFL1 did not alter the sensitivity of KB and MCF-7 cells to chemotherapeutic agents. Meanwhile, EFL1 significantly increased accumulation of doxorubicin and rhodamine 123 in KBv200 and MCF-7/adr cells, showing no significant influence on that of KB and MCF-7 cells. Furthermore, EFL1 could enhance the ATP hydrolysis activity of ABCB1 stimulated by verapamil. At the same time, EFL1 inhibited the efflux of ABCB1 in KBv200 and MCF-7/adr cells. In addition, EFL1 did not downregulate expression of ABCB1 in KBv200 and MCF-7/adr cells either in mRNA or protein level.

First characterization of fish P-glycoprotein (abcb1) substrate specificity using determinations of its ATPase activity and calcein-AM assay with PLHC-1/dox cell line

P-glycoprotein (P-gp; abcb1) is one of the major ABC transport proteins that mediates multixenobiotic resistance (MXR) defense in fish. In order to offer a sound evaluation of its ecotoxicological relevance it is critical to characterize substrate specificity of fish P-gp. Measurement of the ATPase activity is a reliable approach often used to discern type of interaction of various drugs with mammalian P-gp. A similar assay has never been used for characterization of P-gp in aquatic organisms and the main goal of this study was to develop a specific ATPase assay for characterization of fish P-gp. For this purpose we have used P-gp enriched membrane vesicles isolated from fish hepatoma PLHC-1/dox cells characterized by high overexpression of P-gp. As additional demonstration of a P-gp specific phenotype, we have quantified transcript expression of a series of eight ABC efflux transporter genes constitutively expressed in PLHC-1 wild type and PLHC-1/dox cells. Transcript expression analysis confirmed high and specific P-gp transcript overexpression in PLHC-1/dox cells. Provided that the transcript abundance is translated to protein, the development of ATPase assay is enabled. Using this model we determined Km(ATP) of 0.4mM, baseline ATPase activity from 35-50nmol/mg(PROT)/min, and maximal activation of ATPase activity obtained for fish P-gp in our system was 1.8-2.5-fold over baseline. All these values were in good agreement with data previously reported for mammalian P-gp. In order to perform a more detailed characterization of fish P-gp substrate specificity, in the next step of our study we used the developed ATPase assay to test 50 different compounds for their interaction with fish P-gp. The same set of compounds was also tested with calcein-AM (Ca-AM) transport activity assay both using PLHC-1/dox cells and NIH 3T3/MDR1 fibroblast cells overexpressing human P-gp. Our results showed that there is a clear difference for some substances-five compounds specifically interacted only with fish P-gp, while seven compounds exhibited interaction with human P-gp only. Most of the compounds tested in this study showed similar behavior in respect to fish or human P-gp and relatively high correlation in the interaction potency was found between fish and human P-gp. In summary, the described results represent the first in depth insight into substrate specificity of an important xenobiotic efflux transporter in fish. In addition, our study showed that combination of Ca-AM assay and the developed ATPase assay using inside/out vesicles isolated from PLHC-1/dox cells, offers a high-throughput and reliable approach for identification of environmentally relevant pollutants that interact with fish P-gp.

Exploring the P-glycoprotein binding cavity with polyoxyethylene alkyl ethers

P-glycoprotein (ABCB1) moves allocrits from the cytosolic to the extracellular membrane leaflet, preventing their intrusion into the cytosol. It is generally accepted that allocrit binding from water to the cavity lined by the transmembrane domains occurs in two steps, a lipid-water partitioning step, and a cavity-binding step in the lipid membrane, whereby hydrogen-bond (i.e., weak electrostatic) interactions play a crucial role. The remaining key question was whether hydrophobic interactions also play a role for allocrit binding to the cavity. To answer this question, we chose polyoxyethylene alkyl ethers, C(m)EO(n), varying in the number of methylene and ethoxyl residues as model allocrits. Using isothermal titration calorimetry, we showed that the lipid-water partitioning step was purely hydrophobic, increasing linearly with the number of methylene, and decreasing with the number of ethoxyl residues, respectively. Using, in addition, ATPase activity measurements, we demonstrated that allocrit binding to the cavity required minimally two ethoxyl residues and increased linearly with the number of ethoxyl residues. The analysis provides the first direct evidence, to our knowledge, that allocrit binding to the cavity is purely electrostatic, apparently without any hydrophobic contribution. While the polar part of allocrits forms weak electrostatic interactions with the cavity, the hydrophobic part seems to remain associated with the lipid membrane. The interplay between the two types of interactions is most likely essential for allocrit flipping.

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

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