Modulation of ATP and drug binding by monoclonal antibodies against P-glycoprotein

Reversing ABCB1-mediated multi-drug resistance from within cells using translocating immune conjugates

Multi-drug resistance (MDR) is still a major cause of the eventual failure of chemotherapy in cancer treatment. Different approaches have been taken to render these cells drug sensitive. Here, we attempted sensitizing drug-resistant cells from within, using a translocating immune conjugate approach. To that effect, a monoclonal antibody, C219, directed against the intracellular ATP-binding site of the membrane-anchored MDR transporter ABCB1 [P-glycoprotein (P-gp), MDR1], was conjugated to human immunodeficiency virus [HIV(37-72)Tat] translocator peptide through a disulfide bridge. Fluorescence-labelled IgG-Tat conjugates accumulated in drug resistant Chinese hamster ovary (CHO) cells within less than 20 min. Preincubation with C219-S-S-(37-72)Tat conjugate augmented calcein accumulation in drug-resistant CHO and mouse lymphoma cells, indicating reduction in ABCB1 transporter activity. A thioether conjugate C219-S-(37-72)Tat was ineffective, as were disulfide and thioether conjugates of an irrelevant antibody. Furthermore, in the presence of C219-S-S-(37-72)Tat, drug resistant cells were sensitized to colchicine and doxorubicin. Taken together, these findings demonstrate, as proof of principle, a novel approach for the reversal of MDR from within cells, by delivery of translocating immune conjugates as sensitizing agents towards chemotherapy.

Thyroxine (T4) transfer from CSF to choroid plexus and ventricular brain regions in rabbit: contributory role of P-glycoprotein and organic anion transporting polypeptides

This study investigated the transfer of T4 from cerebrospinal fluid (CSF) into the choroid plexuses (CP) and ventricular brain regions, and the role of P-glycoprotein (P-gp), multidrug resistance protein 1 (mrp1) and organic anion transporting polypeptides (oatps). During in vivo ventriculo-cisternal (V-C) perfusion in the anesthetized rabbit (meditomidine hydrochloride 0.5 mg kg(-1), pentobarbitone 10 mg kg(-1) i.v.), 125I-T4 was perfused continuously into ventricular CSF with reference molecules 14C-mannitol and blue dextran. Over 2 h, 36.9+/-4.6% 125I-T4 was recovered in cisternal CSF. Addition of P-gp substrate verapamil increased CSF 125I-T4 recovery to 51.4+/-2.8%, although mrp1 and oatp substrates had no significant effect. In brain, 125I-T4 showed greatest accumulation in the CP (1.52+/-0.31 ml g(-1)), followed by ventricular regions (caudate putamen, ependyma, hippocampus, 0.05-0.14 ml g(-1)). At the CP, verapamil and probenecid (but not indomethacin) significantly increased 125I-T4 accumulation, implicating a role for P-gp and oatps. Of other brain regions, all three drugs increased accumulation in caudate putamen 3-5 times, and indomethacin and probenecid increased accumulation in ependyma 4-5 times. The role of P-gp was investigated further in isolated incubated CPs using 5 microg/ml C219 anti-P-gp antibody. Both 125I-T4 and 3H-cyclosporin accumulation increased by 80%, suggesting that P-gp is functional in the CP and has a role in removal of T4. Combined with the in vivo results, these studies suggest that P-gp provides a local homeostatic mechanism, maintaining CSF T4 levels. We conclude that P-gp and oatps contribute to the transfer of 125I-T4 between the CSF, CP and brain, hence regulating 125I-T4 availability in CSF.

Modulation of GSH levels in ABCC1 expressing tumor cells triggers apoptosis through oxidative stress

The over-expression of ABCC1 transmembrane protein has been shown to cause multidrug resistance in tumor cell lines. ABCC1 is a member of the ABC transmembrane proteins that function as efflux pumps with diverse substrate specificity. Several endogenous cell metabolites, including the leukotriene C4 (LTC(4)) and glutathione (GSH) are substrates for ABCC1 protein. ABCC1 expression in certain tumor cells was demonstrated to confer hypersensitivity to glutathione modulating agents. In this report we have investigated the mechanism of collateral sensitivity seen in tumor cells over-expressing ABCC1 protein. The results of this study show that ABCC1 expression in tumor cells correlates with their hypersensitivity to various glutathione modulating agents, as demonstrated in H69AR-drug selected and HeLa/ABCC1-transfectant cells. This effect was triggered either through inhibition of GSH synthesis with BSO or by increasing ABCC1-mediated GSH transport with verapamil or apigenin. In addition, our results show that the hypersensitivity of ABCC1-expressing cells to BSO, verapamil or apigenin was preceded by an increase in reactive oxygen species (or ROS). A decrease in GSH level is also observed prior the increase in ROS. In addition, we show that hypersensitivity to the BSO, verapamil or apigenin leads to tumor cell death by apoptosis. Together, the results of this study demonstrate that ABCC1 potentiates oxidative stress in tumor cells through reductions in cellular GSH levels.

Allosteric modulation of the human P-glycoprotein involves conformational changes mimicking catalytic transition intermediates

The drug transport function of human P-glycoprotein (Pgp, ABCB1) can be inhibited by a number of pharmacological agents collectively referred to as modulators or reversing agents. In this study, we demonstrate that certain thioxanthene-based Pgp modulators with an allosteric mode of action induce a distinct conformational change in the cytosolic domain of Pgp, which alters susceptibility to proteolytic digestion. Both cis and trans-isomers of the Pgp modulator flupentixol confer considerable protection of an 80 kDa Pgp fragment against trypsin digestion, that is recognized by a polyclonal antibody specific for the NH(2)-terminal half to Pgp. The protection by flupentixol is abolished in the Pgp F983A mutant that is impaired in modulation by flupentixols, indicating involvement of the allosteric site in generating the conformational change. A similar protection to an 80 kDa fragment is conferred by ATP, its nonhydrolyzable analog ATPgammaS, and by trapping of ADP-vanadate at the catalytic domain, but not by transport substrate vinblastine or by the competitive modulator cyclosporin A, suggesting different outcomes from modulator interaction at the allosteric site and at the substrate site. In summary, we demonstrate that allosteric interaction of flupentixols with Pgp generates conformational changes that mimic catalytic transition intermediates induced by nucleotide binding and hydrolysis, which may play a crucial role in allosteric inhibition of Pgp-mediated drug transport.

Regulation of volume-sensitive Cl- channels in multi-drug resistant MCF7 cells

The P-glycoprotein (P-gp) is thought to be involved in the regulation of volume-sensitive chloride channels. In this study, the possible coupling between P-gp and swelling-activated chloride channels has been examined in MCF7 cells with sensitive (MDR-), resistant (MDR+), and reversed resistant (MDR(REV)) phenotypes. Western blot analysis showed that incubation of cells with doxorubicin induced P-gp expression in a reversible manner. Exposure of MDR+ cells to hypotonicity resulted in an inhibition of P-gp activity while hypotonic challenges induced swelling-activated chloride currents (I(Cl-swell)) in MDR-, MDR+, and MDR(REV) MCF7 cells. While verapamil inhibited I(Cl-swell) in all cell types, doxorubicin and vincristine rapidly and reversibly inhibited I(Cl-swell) uniquely in MDR+. Intracellular dialysis of MDR+ cells with C219 anti-P-gp antibody abolished the sensitivity of I(Cl-swell) to doxorubicin and led to a response pattern very close to that of MDR- cells. Taken together, these results strongly suggest that the P-glycoprotein regulates I(Cl-swell) in resistant MCF7.

Isolated Rafts from Adriamycin-Resistant P388 Cells Contain Functional ATPases and Provide an Easy Test System for P-glycoprotein?Related Activities

PURPOSE

P-glycoprotein (P-gp), a membrane ATPase expelling many structurally unrelated compounds out of cells, is one of the major contributors to multidrug resistance. It is enriched in cold TritonX-100 insoluble membrane domains (i.e., rafts). The purpose of this work was to characterize the ATPase activities of raft preparations from P388 cells overexpressing P-gp (P388/ADR) or devoid of P-gp (P388) and to establish a P-gp-enriched screening system for P-gp-interfering compounds.

METHODS

Rafts were extracted with cold TritonX-100. The ATPase activity was characterized in 96-well plates using a fluorescence assay.

RESULTS

The ATPase activity per mg protein was about five times higher in P388/ADR rafts than in crude membranes. The anti-P-gp antibody C219 inhibited 20% of the activity in P388/ADR rafts but only about 10% of the activity in P388/ADR crude membranes and had no effect on the activity of P388 rafts. The known P-gp-activating compounds verapamil, progesterone, and valinomycin revealed the typical bell-shaped activity/concentration profiles in P388/ADR rafts, indicative for activation at low compound concentrations and inhibition at concentrations >10 to 100 microM. The inhibitory effect was also observed in P388 rafts.

CONCLUSIONS

Extracted rafts are rich in functional ATPases. Rafts from P-gp-overexpressing cells display P-gp-typical ATPase activity and provide an easy, P-gp-enriched screening system.

Arginine482 to threonine mutation in the breast cancer resistance protein ABCG2 inhibits rhodamine 123 transport while increasing binding

ABCG2 [also known as BCRP (breast cancer resistance protein) or MXR] is an ABC (ATP-binding cassette) protein shown to confer multidrug resistance. ABCG2 was initially identified in resistant breast carcinoma cells (MCF-7/AdrVp1000) selected with doxorubicin and verapamil. Later studies demonstrated the presence of a point mutation (Arg482 to Thr) in ABCG2 in MCF-7/AdrVp1000 cells. This mutation was shown to modulate the transport of Rh123 (rhodamine 123). In the present study, we have used a previously characterized photoreactive drug analogue of Rh123, IAARh123 (iodoaryl-azido-Rh123), to examine the effects of the Arg482Thr mutation on Rh123 binding and transport by ABCG2. Our results show that both wild-type (ABCG2R482) and mutant (ABCG2T482) ABCG2 bound directly to IAARh123. Surprisingly, however, wild-type ABCG2R482, which does not transport Rh123, was more intensely photolabelled than mutant ABCG2T482. In addition, inhibition of IAARh123 photolabelling using various drug substrates of ABCG2 revealed some differences between wild-type and mutant ABCG2. For example, a molar excess of mitoxantrone was more effective at inhibiting IAARh123 labelling of wild-type than of mutant ABCG2, while excess cisplatin, taxol and methotrexate showed significant inhibition of IAARh123 binding to both wild-type and mutant ABCG2. Taken together, the results of this study provide the first demonstration of the direct binding of drugs to ABCG2.

Transport of somatostatin and substance P by human P-glycoprotein

P-glycoprotein is an efflux pump for a broad spectrum of hydrophobic agents. We found that bioactive peptides including somatostatin and substance P inhibit ATP-dependent vincristine binding to P-glycoprotein-overexpressing K562/ADM membrane vesicles. Some of these bioactive peptides including somatostatin stimulate basal ATPase activity of P-glycoprotein; in contrast, other peptides including substance P inhibit it. The K562/ADM membrane vesicles showed an ATP-dependent, osmotically sensitive uptake of somatostatin and substance P, which was inhibited by valspodar, an inhibitor of P-glycoprotein. These findings suggested that certain bioactive peptides such as somatostatin and substance P directly interact with human P-glycoprotein as endogenous substrates for P-glycoprotein-mediated transport.

Photoaffinity labeling under non-energized conditions of a specific drug-binding site of the ABC multidrug transporter LmrA from Lactococcus lactis

The Lactococcus lactis multidrug resistance ABC transporter protein LmrA has been shown to confer resistance to structurally and functionally diverse antibiotics and anti-cancer drugs. Using a previously characterized photoreactive drug analogue of Rhodamine 123 (iodo-aryl azido-Rhodamine 123 or IAARh123), direct and specific photoaffinity labeling of LmrA in enriched membrane vesicles could be achieved under non-energized conditions. This photoaffinity labeling of LmrA occurs at a physiologically relevant site as it was inhibited by molar excess of ethidium bromide>Rhodamine 6G>vinblastine>doxorubicin>MK571 (a quinoline-based drug) while colchicine had no effect. The MDR-reversing agents PSC 833 and cyclosporin A were similarly effective in inhibiting IAARh123 photolabeling of LmrA and P-glycoprotein. In-gel digestion with Staphyloccocus aureus V8 protease of IAARh123-photolabeled LmrA revealed several IAARh123 labeled polypeptides, in addition to a 6.8kDa polypeptide that comprises the last two transmembrane domains of LmrA.

Binding of a photoaffinity analogue of glutathione to MRP1 (ABCC1) within two cytoplasmic regions (L0 and L1) as well as transmembrane domains 10-11 and 16-17

MRP1 (or ABCC1) is an ABC membrane protein that transports a wide range of natural products as well as glutathione (GSH)-, glucuronate-, and sulfate-conjugated metabolites. In addition, free GSH is required for MRP1 to transport several chemotherapeutic drugs. However, the mechanisms regulating the influence of GSH on MRP1 is poorly understood, and the location of GSH binding site(s) within MRP1 have yet to be determined. To address these issues, we have synthesized a [(125)I] labeled azido-derivative of GSH (IAAGSH) to photoaffinity label MRP1. Our results revealed that IAAGSH labeled MRP1 with high specificity, and binding was inhibited by MRP1 substrates leukotriene C(4) and MK571. Interestingly, verapamil and vincristine enhanced IAAGSH photolabeling of MRP1, in agreement with observations that both drugs enhance GSH transport. We observed GSH to be the best inhibitor of photoaffinity labeling, as compared to oxidized glutathione (GSSG) and four different GSH alkyl derivatives. These observations indicate that IAAGSH interacted with MRP1 in a similar manner as unmodified GSH. Moreover, using eight MRP1-HA variants, each containing hemagglutinin A (HA) epitopes inserted at different sites in MRP1, we mapped the GSH binding sites in MRP1. Our GSH analogue photoaffinity labeled four MRP1 polypeptides that were located within two cytoplasmic domains in linker sequences (L0 and L1) as well as transmembrane domains 10-11 and 16-17. The photoaffinity labeling of polypeptides within L0 and L1 domains is further confirmed using two MRP1-specific monoclonal antibodies (MRPr1 and QCRL1) with epitopes within the linker domains. Taken together, this study provides the most precise information to date on the location of GSH binding sites in MRP1.

The involvement of P-glycoprotein in berberine absorption

Berberine is an important ingredient in a number of traditional Chinese medicines but has been shown to have poor bioavailability in the dog. The aim of this study was to use the P-glycoprotein (P-glycoprotein) inhibitors cyclosporin A, verapamil and the monoclonal antibody C219 in in vivo and in vitro models of intestinal absorption to determine the role of P-glycoprotein in berberine absorption. In the rat recirculating perfusion model, berberine absorption was improved 6-times by P-glycoprotein inhibitors. In the rat everted intestinal sac model, berberine serosal-to-mucosal transport was significantly decreased by cyclosporin A. In Ussing-type chambers, the rate of serosal-to-mucosal transport across rat ileum was 3-times greater than in the reverse direction and was significantly decreased by cyclosporin A. In Caco-2 cells, berberine uptake was significantly increased by P-glycoprotein inhibitors and by monoclonal antibody C219. P-glycoprotein appears to contribute to the poor intestinal absorption of berberine which suggests P-glycoprotein inhibitors could be of therapeutic value by improving its bioavailability.

Proteolytic Cleavage of the Linker Region of the Human P-glycoprotein Modulates Its ATPase Function

P-glycoprotein (Pgp), an anticancer drug-translocating ATPase, is responsible for multidrug resistance in cancer. We have previously shown (Nuti, S. L., Mehdi, A., and Rao, U. S. (2000) Biochemistry 39, 3424-3432) that tryptic cleavage of Pgp results in the activation of basal and drug-stimulated ATPase functions of Pgp. To understand this phenomenon, we determined the sites cleaved by trypsin and further examined whether the modulation of Pgp function is trypsin-specific or the result of proteolysis in general. The effects of chymotrypsin and proteinase K on Pgp ATPase function were studied. The results show that proteolysis of Pgp irrespective of the protease employed resulted in the activation of basal ATPase activity. However, drug-stimulated ATPase activities were differentially modulated. Immunoblot analysis of proteolytic digests indicated that, irrespective of the protease employed, Pgp was predominantly cleaved in the middle of the molecule. N-terminal amino acid sequencing of Pgp tryptic and chymotryptic peptides indicated Arg(680) and Leu(682) as the sites of cleavage, respectively. These two cleavage sites are part of the predicted linker region that joins the two halves of Pgp. Together, these results suggest that the linker region in Pgp is primarily accessible to protease action and that cleavage of this region modulates Pgp ATPase function.

Functional expression of multidrug resistance protein 1 in Pichia pastoris

Overexpression of the multidrug resistance-associated protein (MRP1) causes multidrug resistance in cultured cells. MRP1 transports a large number of glutathione, glucuronide, and sulfate-conjugated organic anions by an ATP-dependent efflux mechanism. Six other MRP proteins exist (MRP2-7), and mutations in some of these genes cause major pathological conditions in humans. A detailed characterization of the structure and mechanism of action of these proteins requires an efficient expression system from which large amounts of active protein can be obtained. We report the expression of a recombinant MRP1 in the methylotrophic yeast Pichia pastoris. The protein is expressed in the membrane fraction of these cells, as a stable and underglycosylated 165 kDa peptide. Expression levels are very high, and 30 times superior to those seen in multidrug-resistant HeLa/MRP1 transfectants. MRP1 expressed in P. pastoris binds 8-azido[alpha-(32)P]ATP in a Mg(2+)-dependent and EDTA-sensitive fashion, which can be competed by a molar excess of ADP and ATP. Under hydrolysis conditions (at 37 degrees C), orthovanadate induces trapping of the 8-azido[alpha-(32)P]nucleotide in MRP1, which can be further modulated by known MRP1 ligands. MRP1 is also labeled by a photoactive analogue of rhodamine 123 (IAARh123) in P. pastoris/MRP1 membranes, and this can be competed by known MRP1 ligands. Finally, MRP1-positive membrane vesicles show ATP-dependent uptake of LTC(4). Thus, MRP1 expressed in P. pastoris is active and shows characteristics of MRP1 expressed in mammalian cells, including drug binding, ligand-modulated formation of the MRP1-MgADP-P(i) intermediate (ATPase activity), and ATP-dependent substrate transport. The successful expression of catalytically active and transport-competent MRP1 in P. pastoris should greatly facilitate the efficient production and isolation of the wild type or inactive mutants of MRP1, or of other MRP proteins for structural and functional characterization.

Role of glycine-534 and glycine-1179 of human multidrug resistance protein (MDR1) in drug-mediated control of ATP hydrolysis

The human multidrug resistance protein (MDR1) (P-glycoprotein), a member of the ATP-binding cassette (ABC) family, causes multidrug resistance by an active transport mechanism, which keeps the intracellular level of hydrophobic compounds below a cell-killing threshold. Human MDR1 variants with mutations affecting a conserved glycine residue within the ABC signature of either or both ABC units (G534D, G534V, G1179D and G534D/G1179D) were expressed and characterized in Spodoptera frugiperda (Sf9) cell membranes. These mutations caused a loss of measurable ATPase activity but still allowed ATP binding and the formation of a transition-state intermediate (nucleotide trapping). In contrast with the wild-type protein, in which substrate drugs accelerate nucleotide trapping, in the ABC signature mutants nucleotide trapping was inhibited by MDR1-substrate drugs, suggesting a miscommunication between the drug-binding site(s) and the catalytic domains. Equivalent mutations of the two catalytic sites resulted in a similar effect, indicating the functional equivalence of the two sites. On the basis of these results and recent structural information on an ABC-ABC dimer [Hopfner, Karcher, Shin, Craig, Arthur, Carney and Tainer (2000) Cell 101, 789-800], we propose a key role of these glycine residues in the interdomain communication regulating drug-induced ATP hydrolysis.

Rhodamine 123 binds to multiple sites in the multidrug resistance protein (MRP1)

The mechanisms of MRP1-drug binding and transport are not clear. In this study, we have characterized the interaction between MRP1 and rhodamine 123 (Rh123) using the photoreactive-iodinated analogue, [(125)I]iodoaryl azido-rhodamine 123 (or IAARh123). Photoaffinity labeling of plasma membranes from HeLa cells transfected with MRP1 cDNA (HeLa-MRP1) with IAARh123 shows the photolabeling of a 190 kDa polypeptide not labeled in HeLa cells transfected with the vector alone. Immunoprecipitation of a 190 kDa photolabeled protein with MRP1-sepcific monoclonal antibodies (QCRL-1, MRPr1, and MRPm6) confirmed the identity of this protein as MRP1. Analysis of MRP1-IAARh123 interactions showed that photolabeling of membranes from HeLa-MRP1 with increasing concentrations of IAARh123 was saturable, and was inhibited with excess of IAARh123. Furthermore, the photoaffinity labeling of MRP1 with IAARh123 was greatly reduced in the presence of excess Leukotreine C(4) or MK571, but to a lesser extent with excess doxorubicin, colchicine or chloroquine. Cell growth assays showed 5-fold and 14-fold increase in the IC(50) of HeLa-MRP1 to Rh123 and the Etoposide VP16 relative to HeLa cells, respectively. Analysis of Rh123 fluorescence in HeLa and HeLa-MRP1 cells with or without ATP suggests that cross-resistance to Rh123 is in part due to reduced drug accumulation in the cytosol of HeLa-MRP1 cells. Mild digestion of purified IAARh123-photolabeled MRP1 with trypsin showed two large polypeptides (approximately 111 and approximately 85 kDa) resulting from cleavage in the linker domain (L1) connecting the multiple-spanning domains MSD0 and MSD1 to MSD2. Exhaustive proteolysis of purified IAARh123-labeled 85 and 111 kDa polypeptides revealed one (6 kDa) and two (approximately 6 plus 4 kDa) photolabeled peptides, respectively. Resolution of total tryptic digest of IAARh123-labeled MRP1 by HPLC showed three radiolabeled peaks consistent with the three Staphylococcus aureus V8 cleaved peptides from the Cleveland maps. Together, the results of this study show direct binding of IAARh123 to three sites that localize to the N- and C-domains of MRP1. Moreover, IAARh123 provides a sensitive and specific probe to study MRP1-drug interactions.

Transition-state formation in ATPase-negative mutants of human MDR1 protein

In this work we have studied the partial catalytic reactions in MDR1 variants carrying mutations in the conserved Walker A region (K433M and K1076M) of either the N-terminal or C-terminal ABC domain. Both mutations have been demonstrated to cause a loss of drug transport, drug-stimulated ATPase, and vanadate-dependent nucleotide trapping activity. Here we show that these mutants still allow transition state formation (nucleotide trapping) when fluoro-aluminate or beryllium fluoride is used as a complex-stabilizing anion. Drug stimulation of nucleotide trapping was found to be preserved in both mutants. Limited trypsin digestion revealed that whenever MDR1-nucleotide trapping occurred, both ABC domains were involved in the formation of the catalytic intermediates. Our results show that details of the MDR1-ATPase cycle can be studied even in ATPase-negative mutants. These data also demonstrate that the conformational alteration caused by a mutation in one of the ABC domains is propagated to the other, nonmutated domain, indicating a tight coupling between the functioning of the two ABC domains.

Investigation of the role of glutamine-471 and glutamine-1114 in the two catalytic sites of P-glycoprotein

P-glycoprotein, also known as multidrug resistance protein, pumps drugs out of cells using ATP hydrolysis as the energy source. Glutamine-471 and the corresponding glutamine-1114 in the two catalytic sites of P-glycoprotein are conserved in ABC transporters. X-ray structures show that they lie close to the bound nucleotide. Proposed functional roles are (1) activation of the attacking water for ATP hydrolysis, (2) coordination of the essential Mg(2+) cofactor in Mg nucleotide, and (3) signal communication between catalytic site reaction chemistry and drug-binding sites. We made mutations Q471A, Q471E, Q1114A, and Q1114E in mouse MDR3 P-glycoprotein. Pure mutant and wild-type proteins were prepared and subjected to enzymatic and biochemical characterization. We conclude from the results that the primary role of this glutamine residue is in interdomain signal communication. Coordination of the Mg(2+) cofactor is not a critical functional role, neither is activation of the attacking water molecule, although an auxiliary role in positioning the water cannot be ruled out. We found that equivalent mutations (Ala or Glu) in either of the two P-glycoprotein catalytic sites produced the same effects, implying functional symmetry of the two sites.

Molecular analysis of multidrug resistance in feline lymphoma cells

OBJECTIVE

To evaluate the mechanism of multidrug resistance in feline lymphoma cell lines.

SAMPLE POPULATION

A feline lymphoma cell line (FT-1) and its adriamycin (ADM)-resistant subline (FT-1/ADM).

PROCEDURES

The FT-1 cell line was cultivated in the presence of a gradually increasing concentration of ADM to generate its ADM-resistant subline (FT-1/ADM). Susceptibility of cells from the parental FT-1 cell line and the FT-1/ADM subline to antineoplastic drugs was determined. From the complementary DNA (cDNA) template of FT-1/ADM cells, feline MDR1 cDNA was amplified by use of polymerase chain reaction (PCR) and sequenced. Reverse transcription (RT)-PCR and Western blot analyses were performed to assess expression of the MDR1 gene and P-glycoprotein (P-gp) in FT-1/ADM cells, compared with that in FT-1 cells.

RESULTS

A drug sensitivity assay revealed that FT-1/ADM cells were much more resistant to ADM and vincristine than the parental FT-1 cells. The feline MDR7 cDNA amplified by use of PCR was 3,489 base pairs long, corresponding to approximately 90% of the whole open reading frame of human MDR1 cDNA; its amino acid sequence was 91.5, 87.0, and 79.4% identical to that of human MDR1, mouse mdr1a, and mdr1b cDNA, respectively. By RT-PCR analysis, expression of MDR1 messenger RNA was clearly detected in FT-1/ADM cells but not in the parental FT-1 cells. Western blot analysis also revealed the expression of P-gp encoded by the MDR1 gene in FT-1/ADM cells but not in FT-1 cells.

CONCLUSIONS

The basic structure of the feline MDR1 gene was essentially the same as that of multidrug-resistance genes of other species. Expression of P-gp appeared to be one of the mechanisms responsible for the development of multidrug resistance in feline lymphoma cell lines in vitro.

Conserved walker A Ser residues in the catalytic sites of P-glycoprotein are critical for catalysis and involved primarily at the transition state step

P-glycoprotein mutants S430A/T and S1073A/T, affecting conserved Walker A Ser residues, were characterized to elucidate molecular roles of the Ser and functioning of the two P-glycoprotein catalytic sites. Results showed the Ser-OH is critical for MgATPase activity and formation of the normal transition state, although not for initial MgATP binding. Mutation to Ala in either catalytic site abolished MgATPase and transition state formation in both sites, whereas Thr mutants had similar MgATPase to wild-type. Trapping of 1 mol of MgADP/mol of P-glycoprotein by vanadate, shown here with pure protein, yielded full inhibition of ATPase. Thus, congruent with previous work, both sites must be intact and must interact for catalysis. Equivalent mutations (Ala or Thr) in the two catalytic sites had identical effects on a wide range of activities, emphasizing that the two catalytic sites function symmetrically. The role of the Ser-OH is to coordinate Mg(2+) in MgATP, but only at the stage of the transition state are its effects tangible. Initial substrate binding is apparently to an "open" catalytic site conformation, where the Ser-OH is dispensable. This changes to a "closed" conformation required to attain the transition state, in which the Ser-OH is a critical ligand. Formation of the latter conformation requires both sites; both sites may provide direct ligands to the transition state.

Multidrug-resistance transporters

P-glycoprotein was initially isolated due to its role in multidrug resistance to cancer chemotherapeutics. Recent work, however, makes it increasingly apparent that this transporter is also involved in the pharmacokinetics of many drugs. P-gp is strategically expressed in the luminal epithelial cells of organs often associated with drug absorption and disposition, for example, hepatocyte canalicular membrane, renal proximal tubules, and the intestinal mucosa. P-gp is also expressed in the endothelial cells comprising the blood-brain barrier. This localization clearly suggests the potential for this protein to serve as a protective mechanism against entry of toxic xenobiotics and also suggests that P-gp is well situated to participate in the removal of therapeutic agents. Numerous investigations with drugs such as digoxin, etoposide, cyclosporine, vinblastine, Taxol, loperamide, dom-peridone, and ondansteron demonstrate that P-gp has an important role in determining the pharmacokinetics of substrate drugs. Pharmacological modulation of P-gp function to increase drug bioavailability, both on a organismal and a cellular level, is one approach currently being explored to enhance therapeutic effectiveness. This approach is not without potential collateral consequences given the wide tissue distribution of P-gp. While animals deficient in P-gp are viable and without obvious abnormalities, the pharmacokinetics and toxic consequences of several compounds are significantly altered in these animals. Thus blockade of the protective P-gp barrier in humans may have adverse effects on substrate drugs. In particular, this situation may arise when several compounds which may be substrates compete for P-gp-mediated transport. Additional multidrug transporters, notably MRP and family members, have been identified and may also determine the fate of pharmaceuticals. Further understanding the physiological role of each of the multidrug transporters is critical for determining their role in pharmacokinetics and for evaluating the consequences of modification of their activities. Such information is also important in the development of novel drugs which may be substrates for these transporters.

The multidrug resistance protein is photoaffinity labeled by a quinoline-based drug at multiple sites

Tumor cells overcome cytotoxic drug pressure by the overexpression of either or both transmembrane proteins, the P-glycoprotein (P-gp) and the multidrug resistance protein (MRP). The MRP has been shown to mediate the transport of cytotoxic natural products, in addition to glutathione-, glucuronidate-, and sulfate-conjugated cell metabolites. However, the mechanism of MRP drug binding and transport is at present not clear. In this study, we have used a photoreactive quinoline-based drug, N-(hydrocinchonidin-8'-yl)-4-azido-2-hydroxybenzamide (IACI), to show the photoaffinity labeling of the 190 kDa protein in membranes from the drug resistant SCLC H69/AR cells. The photoaffinity labeling of the 190 kDa protein by IACI was saturable and specific. The identity of the IACI-photolabeled protein as the MRP was confirmed by immunoprecipitation with the monoclonal antibody QCRL-1. Furthermore, a molar excess of leukotriene C(4), doxorubicin, colchicine, and other quinoline-based drugs, including MK571, inhibited the photoaffinity labeling of the MRP. Drug transport studies showed lower IACI accumulation in MRP-expressing cells which was reversed by depleting ATP levels in H69/AR cells. Mild digestion of the purified IACI-photolabeled MRP with trypsin showed two large polypeptides ( approximately 111 and approximately 85 kDa). The 85 kDa polypeptide which contains the QCRL-1 and MRPm6 monoclonal antibody epitopes corresponds to the C-terminal half of the MRP (amino acids approximately 900-1531) containing the third multiple spanning domain (MSD3) and the second nucleotide binding site. The 111 kDa polypeptide which contains the epitope sequence of the MRPr1 monoclonal antibody encodes the remainder of the MRP sequence (amino acids 1-900) containing the MSD1 and MSD2 plus the first nucleotide binding domain. Cleveland maps of purified IACI-labeled 85 and 111 kDa polypeptides revealed 6 kDa and approximately 6 plus 4 kDa photolabeled peptides, respectively. In addition, resolution of the exhaustively digested IACI-photolabeled MRP by HPLC showed two major and one minor radiolabeled peaks that eluted late in the gradient (60 to 72% acetonitrile). Taken together, the results of this study show direct binding of IACI to the MRP at physiologically relevant sites. Moreover, IACI photolabels three small peptides which localize to the N- and C-halves of the MRP. Finally, IACI provides a sensitive and specific probe for studying MRP-drug interactions.

Nucleotide-induced conformational changes in P-glycoprotein and in nucleotide binding site mutants monitored by trypsin sensitivity

Limited trypsin digestion was used to monitor nucleotide-induced conformational changes in wild-type P-glycoprotein (Pgp) as well as in nucleotide binding domain (NBD) Pgp mutants. Purified and reconstituted wild-type or mutant mouse Mdr3 Pgps were preincubated with different hydrolyzable or nonhydrolyzable nucleotides, followed by limited proteolytic cleavage at different trypsin:protein ratios. The Pgp tryptic digestion products were separated by SDS-PAGE followed by immunodetection with the mouse monoclonal anti-Pgp antibody C219, which recognizes a conserved epitope (VVQE/AALD) in each half of the protein. Different trypsin digestion patterns were observed for wild-type Pgp incubated with MgCl(2) alone, MgADP, MgAMP.PNP, MgATP, and MgATP + vanadate. A unique trypsin digestion profile suggestive of enhanced resistance to trypsin was observed under conditions of vanadate-induced trapping of nucleotides (MgATP + vanadate). The trypsin sensitivity profiles of Pgp mutants bearing either single or double mutations in Walker A (K429R, K1072R) and Walker B (D551N, D1196N) sequence signatures of NBD1 and NBD2 were analyzed under conditions of vanadate-induced trapping of nucleotides. The proteolytic cleavage pattern observed for the double mutants K429R/K1072R and D551N/D1196N, and for the single mutants K429R, K1072R, and D1196N were similar and clearly distinct from wild-type Pgp under the same conditions. This is consistent with the absence of ATP hydrolysis and of vanadate-induced trapping of 8-azido-ADP previously reported for these mutants [Urbatsch et al. (1998) Biochemistry 37, 4592-4602]. Interestingly, the trypsin digestion profiles observed under vanadate-induced trapping for the D551N and D1196N mutants were quite different, with the D551N mutant showing a profile resembling that seen for wild-type Pgp. The different sensitivity profiles of Pgp mutants bearing mutations at the homologous residue in NBD1 (D551N) and NBD2 (D1196N) suggest possible structural and functional differences between the two sites.

Activation of the human P-glycoprotein ATPase by trypsin

The human MDR1 gene product, P-glycoprotein (Pgp), a tandemly duplicated molecule containing two putative ATP- and perhaps two drug-binding sites, is responsible for multidrug resistance in tumors. In this report, we characterized the effects of trypsinization of Pgp on its ATPase function. Incubation of Pgp-containing membranes with trypsin at a ratio of 1000:1 (w/w) resulted in a gradual increase in the basal- and the drug-stimulated ATPase activities of Pgp in a time-dependent manner. The maximal basal-, verapamil-, and vinblastine-stimulated ATPase activities of the trypsinized Pgp were approximately 1.8-, 1.5-, and 1.75-fold higher than the activities of the native Pgp, respectively. Increased basal- and drug-stimulated ATPase activities of the Pgp were also observed when the ratio of membrane protein to trypsin in the incubation mixtures was raised to 10:1 (w/w). Immunoblotting analysis of Pgp tryptic digests using Pgp-specific NH(2)11, C219, and C494 antibodies together revealed the degradation of full-length Pgp and formation of at least eight peptides migrating in the 36-60 kDa range. Immunoprecipitation reactions using NH(2)11 and C494 antibodies have suggested that the peptides originating from the NH(2) half of Pgp are in strong association with the COOH half of the peptide. These findings suggest that while Pgp fragments together exhibit the ATPase functional characteristics, Pgp possesses a cleavage activation site or region, and its cleavage leads to the activation of basal ATPase function of Pgp.

ATP binding properties of the nucleotide-binding folds of SUR1

Pancreatic beta cell ATP-sensitive potassium (K(ATP)) channels regulate glucose-induced insulin secretion. The activity of the K(ATP) channel, composed of SUR1 and Kir6.2 subunits, is regulated by intracellular ATP and ADP, but the molecular mechanism is not clear. To distinguish the ATP binding properties of the two nucleotide-binding folds (NBFs) of SUR1, we prepared antibodies against NBF1 and NBF2, and the tryptic fragment of SUR1 was immunoprecipitated after photoaffinity labeling with 8-azido-[(32)P]ATP. The 35-kDa fragment was strongly labeled with 5 microM 8-azido-[(32)P]ATP even in the absence of Mg(2+) and was immunoprecipitated with the antibody against NBF1. The 65-kDa fragment labeled with 100 microM 8-azido-[alpha-(32)P]ATP in the presence of Mg(2+) was immunoprecipitated with anti-NBF2 and anti-C terminus antibodies. These results indicate that NBF1 of SUR1 binds 8-azido-ATP strongly in a magnesium-independent manner and that NBF2 binds 8-azido-ATP weakly in a magnesium-dependent manner. Furthermore, the 65-kDa tryptic fragment was not photoaffinity-labeled with 8-azido-[gamma-(32)P]ATP at 37 degrees C, whereas the 35-kDa tryptic fragment was, suggesting that NBF2 of SUR1 may have ATPase activity and that NBF1 has none or little.

Antibody C219 recognizes an alpha-helical epitope on P-glycoprotein

The ABC transporter, P-glycoprotein, is an integral membrane protein that mediates the ATP-driven efflux of drugs from multidrug-resistant cancer and HIV-infected cells. Anti-P-glycoprotein antibody C219 binds to both of the ATP-binding regions of P-glycoprotein and has been shown to inhibit its ATPase activity and drug binding capacity. C219 has been widely used in a clinical setting as a tumor marker, but recent observations of cross-reactivity with other proteins, including the c-erbB2 protein in breast cancer cells, impose potential limitations in detecting P-glycoprotein. We have determined the crystal structure at a resolution of 2.4 A of the variable fragment of C219 in complex with an epitope peptide derived from the nucleotide binding domain of P-glycoprotein. The 14-residue peptide adopts an amphipathic alpha-helical conformation, a secondary structure not previously observed in structures of antibody-peptide complexes. Together with available biochemical data, the crystal structure of the C219-peptide complex indicates the molecular basis of the cross-reactivity of C219 with non-multidrug resistance-associated proteins. Alignment of the C219 epitope with the recent crystal structure of the ATP-binding subunit of histidine permease suggests a structural basis for the inhibition of the ATP and drug binding capacity of P-glycoprotein by C219. The results provide a rationale for the development of C219 mutants with improved specificity and affinity that could be useful in antibody-based P-glycoprotein detection and therapy in multidrug resistant cancers.

Cloning and partial characterization of the proteasome S4 ATPase from Plasmodium falciparum

Certad, G., Abrahem, A., and Georges, E. 1999. Cloning and Partial characterization of the proteasome S4 ATPase from Plasmodium falciparum. Experimental Parasitology 93, 123-131. The ATP-ubiquitin-proteasome pathway mediates the nonlysosomal degradation of cytosolic proteins in eukaryotic cells. The activities of this pathway have been shown to regulate cell growth and differentiation through modulation of regulatory proteins. The proteasome is a large complex consisting of two multisubunit structures, the 20S and 19S(PA700) or P28 complexes, that combine to form the 26S particles. In this study, we describe the cloning of a cDNA encoding the proteasome subunit 4 ATPase homologue from Plasmodium falciparum (PFS4). Analysis of the PFS4 cDNA sequence shows an open reading frame encoding a deduced protein of 455 amino acids. Moreover, comparison of PFS4 cDNA sequence to that of genomic fragments encoding PFS4 showed identical sequences with no detectable introns. Database searches revealed a high sequence identity to those of rice, yeast, mouse, Drosophila, and human S4 ATPases. However, PFS4 contains two unique inserts of nine and seven amino acid residues in the N-terminal domain. Interestingly, only the rice S4 contains the latter (seven amino acids) insert with four identical amino acids. In vitro expression of the full-length cDNA encoding the PFS4, using a transcription-translation-coupled reticulocyte lysate, shows a 50-kDa [(35)S]methionine-labeled protein which was immunoprecipitated with PFS4 anti-peptide antiserum. Southern blot analysis of genomic DNA digests shows a single gene copy of PFS4 in P. falciparum. Of interest was the effect of the proteasome-specific natural product, lactacystin, on the growth of the parasite, with IC(50) values of 0.6-0.92 microM. The latter IC(50) values of lactacystin for different clones of P. falciparum are comparable to those obtained for mammalian cell lines (0.65 microM), suggesting the presence of a conserved proteasome complex. Moreover, lactacystin was equally toxic to drug-sensitive and resistant parasites.

Monoclonal antibodies that inhibit the transport function of the 190-kDa multidrug resistance protein, MRP. Localization of their epitopes to the nucleotide-binding domains of the protein

Multidrug resistance in tumor cells is often accompanied by overexpression of multidrug resistance protein (MRP), a 190-kDa transmembrane protein that belongs to the ATP-binding cassette superfamily of transport proteins. MRP mediates ATP-dependent transport of a variety of conjugated organic anions and can also transport several unmodified xenobiotics in a glutathione-dependent manner. To facilitate structure-function studies of MRP, we have generated a panel of MRP-specific monoclonal antibodies (mAbs). Four of these mAbs, QCRL-2, -3, -4, and -6, bind intracellular conformation-dependent epitopes, and we have shown that they can inhibit the transport of several MRP substrates. Binding competition and immunoprecipitation assays indicated that mAbs QCRL-4 and -6 probably recognize the same detergent-sensitive epitope in MRP, whereas mAbs QCRL-2, -3, and -4 each bind distinct, non-overlapping epitopes. Fab fragments inhibit transport as effectively as the intact mAbs, suggesting that inhibition results from direct interactions of the mAbs with MRP. Immunodot blot and immunoprecipitation analyses revealed that the minimal regions of MRP sufficient for full reactivity of mAbs QCRL-2 and -3 are amino acids 617-858 and 617-932, respectively, which encompass the NH2-proximal nucleotide-binding domain (NBD). In contrast, the epitope bound by mAb QCRL-4 localized to amino acids 1294-1531, a region that contains the COOH-proximal NBD. However, none of the mAbs inhibited photolabeling of intact MRP with 8-azido-[alpha-32P]ATP. This suggests that rather than preventing nucleotide binding, the mAbs inhibit transport by interfering with substrate binding or by trapping MRP in a conformation that does not allow transport to occur. Our results also demonstrate for the first time that the NBDs of MRP can be expressed as soluble polypeptides that retain a native conformation.

ATPase activity of Chinese hamster P-glycoprotein

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

P-glycoprotein shows strong catalytic cooperativity between the two nucleotide sites

P-Glycoprotein (Pgp) (also known as multidrug-resistance protein) contains two nucleotide binding sites, both of which are catalytic ATPase sites. The covalent reagent 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) reacts in catalytic sites, and full inactivation of ATPase activity occurs at a reaction stoichiometry of 1 mol of NBD-Cl/mol of Pgp. We show that, at reaction stoichiometry of < or = 1 mol/mol, both nucleotide sites become labeled in relatively nonselective fashion. There is therefore strong interaction between the two nucleotide sites because (a) reaction of one site with NBD-Cl severely impedes reaction of reagent with the other site, and (b) reaction of one site inhibits steady-state ATPase, i.e. both sites are inhibited. Vanadate-trapping experiments revealed that when one nucleotide site was reacted with NBD-Cl, not even a single ATPase turnover event could occur in the other, intact, nucleotide site. The data demonstrate therefore that catalytic cooperativity between the two nucleotide sites in Pgp is extremely strong and mandatory for catalysis.

Conformational changes of P-glycoprotein by nucleotide binding

P-glycoprotein (Pgp) is a membrane protein that transports chemotherapeutic drugs, causing multidrug resistance in human cancer cells. Pgp is a member of the ATP-binding cassette superfamily and functions as a transport ATPase. It has been suggested that the conformation of Pgp changes in the catalytic cycle. In this study, we tested this hypothesis by using limited proteolysis as a tool to detect different conformational states trapped by binding of nucleotide ligands and inhibitors. Pgp has high basal ATPase activity; that is, ATP hydrolysis by Pgp is not rigidly associated with drug transport. This activity provides a convenient method for studying the conformational change of Pgp induced by nucleotide ligands, in the absence of drug substrates which may generate complications due to their own binding. Inside-out membrane vesicles containing human Pgp were isolated from multidrug-resistant SKOV/VLB cells and treated with trypsin in the absence or presence of MgATP, Mg-adenosine 5'-[beta,gamma-imido]triphosphate (Mg-p[NH]ppA) and MgADP. Changes in the proteolysis profile of Pgp owing to binding of nucleotides were used to indicate the conformational changes in Pgp. We found that generation of tryptic fragments, including the loop linking transmembrane (TM) regions TM8 and TM9 of Pgp, were stimulated by the binding of Mg-p[NH]ppA, MgATP and MgADP, indicating that the Pgp conformation was changed by the binding of these nucleotides. The effects of nucleotides on Pgp conformation are directly associated with the binding and/or hydrolysis of these ligands. Four conformational states of Pgp were stabilized under different conditions with various ligands and inhibitors. We propose that cycling through these four states couples the Pgp-mediated MgATP hydrolysis to drug transport.

P-glycoprotein function involves conformational transitions detectable by differential immunoreactivity

The MDR1 P-glycoprotein (Pgp), a member of the ATP-binding cassette family of transporters, is a transmembrane ATPase efflux pump for various lipophilic compounds, including many anti-cancer drugs. mAb UIC2, reactive with the extracellular moiety of Pgp, inhibits Pgp-mediated efflux. UIC2 reactivity with Pgp was increased by the addition of several Pgp-transported compounds or ATP-depleting agents, and by mutational inactivation of both nucleotide-binding domains (NBDs) of Pgp. UIC2 binding to Pgp mutated in both NBDs was unaffected in the presence of Pgp transport substrates or in ATP-depleted cells, whereas the reactivities of the wild-type Pgp and Pgps mutated in a single NBD were increased by these treatments to the level of the double mutant. These results indicate the existence of different Pgp conformations associated with different stages of transport-associated ATP hydrolysis and suggest trapping in a transient conformation as a mechanism for antibody-mediated inhibition of Pgp.

Photoaffinity labelling of P-glycoprotein catalytic sites

Photoaffinity labelling of hamster P-glycoprotein was carried out after trapping of radioactive Mg-8-azido-ADP in the catalytic sites by vanadate or beryllium fluoride. With either trapping agent the same labelled peptide was obtained in homogeneous form, with the sequence -FNEVVFNxPTRPDI-, corresponding to residues 1034-1037 in the C-terminal nucleotide binding site. The missing residue 'x' corresponds to Tyr-1041, which is therefore a primary reaction target of 8-azido-ADP. This tyrosine is conserved in all hamster, mouse and human P-glycoproteins. A second major labelled peptide fraction was also identified. The major sequence in this fraction was -NIHFSxPSR-, corresponding to residues 393-401 of hamster P-glycoprotein, where 'x' corresponds to Tyr-398 in the N-terminal nucleotide binding site. Therefore Tyr-398, which is also conserved in other P-glycoproteins, is also a reaction target for 8-azido-ADP. In sequence alignment of the two nucleotide binding sites, Tyr-398 exactly corresponds to Tyr-1041. The data indicate that these two tyrosines lie close to the adenine ring of bound substrate MgATP in the respective catalytic sites of P-glycoprotein.

Evidence for two nonidentical drug-interaction sites in the human P-glycoprotein

Human P-glycoprotein (Pgp) confers multidrug resistance to cancer cells by ATP-dependent extrusion of a great many structurally dissimilar hydrophobic compounds. The manner in which Pgp recognizes these different substrates is unknown. The protein shows internal homology between its N- and C-terminal halves, each comprised of six putative transmembrane helices and a consensus ATP binding/utilization site. Photoactive derivatives of certain Pgp substrates specifically label two regions, one on each half of the protein. In this study, using [125I]iodoarylazidoprazosin ([125I]IAAP), a photoactive analog of prazosin, we have demonstrated the presence of two nonidentical drug-interaction sites within Pgp. Taking advantage of a highly susceptible trypsin cleavage site in the linker region of Pgp, we characterized the [125I]IAAP binding to the N- and C-terminal halves. cis(Z)-Flupentixol, a modulator of Pgp function, preferentially increased the affinity of [125I]IAAP for the C-terminal half of the protein (C-site) by reducing the Kd from 20 to 6 nM without changing the labeling or affinity (Kd = 42-46 nM) of the N-terminal half (N-site). Also, the concentration of vinblastine (Pgp substrate) and cyclosporin A (Pgp modulator) required for 50% inhibition of [125I]IAAP binding to the C-site was increased 5- to 6-fold by cis(Z)-flupentixol without any effect on the N-site. In addition, [125I]IAAP binding to the N-site was less susceptible than to C-site to inhibition by vanadate which blocks ATP hydrolysis and drug transport. These data demonstrate the presence of at least two nonidentical substrate interaction sites in Pgp.

Interaction of cyclosporin derivatives with the ATPase activity of human P-glycoprotein

1. P-glycoprotein, a 170-180 kDa membrane glycoprotein that mediates multidrug resistance, hydrolyses ATP to efflux a broad spectrum of hydrophobic agents. In this study, we analysed the effects of three MDR reversing agents, verapamil, cyclosporin A and [3'-keto-Bmt1]-[Val2]-cyclosporin (PSC 833), on the adenosine triphosphatase (ATPase) activity of human P-glycoprotein. 2. P-glycoprotein was immunoprecipitated with a monoclonal antibody (MRK-16) and the P-glycoprotein-MRK-16-Protein A-Sepharose complexes obtained were subjected to a coupled enzyme ATPase assay. 3. While verapamil activated the ATPase, the cyclosporin derivatives inhibited both the substrate-stimulated and the basal P-glycoprotein ATPase. No significant difference was observed between PSC 833 and cyclosporin A on the inhibition of basal P-glycoprotein ATPase activity. PSC 833 was more potent than cyclosporin A for the substrate-stimulated activity. 4. Kinetic analysis indicated a competitive inhibition of verapamil-stimulated ATPase by PSC 833. 5. The binding of 8-azido-[alpha-32P]-ATP to P-glycoprotein was not altered by the cyclosporin derivatives, verapamil, vinblastine and doxorubicin, suggesting that the modulation by these agents of P-glycoprotein ATPase cannot be attributed to an effect on ATP binding to P-glycoprotein. 6. The interaction of the cyclosporin derivatives with ATPase of P-glycoprotein might present an alternative and/or additional mechanism of action for the modulation of P-glycoprotein function.

Interactions of forskolin and ATP with the cytosolic domains of mammalian adenylyl cyclase

Fragments of the two cytoplasmic domains of mammalian adenylyl cyclases can be synthesized independently (and abundantly) as soluble proteins; Gsalpha- and forskolin-stimulated enzymatic activity is restored upon their mixture. We have utilized this system to characterize the interactions of adenylyl cyclase with forskolin and its substrate, ATP. In the presence of Gsalpha, adenylyl cyclase is activated in response to occupation of only one forskolin-binding site. A single binding site for forskolin was identified by equilibrium dialysis; its Kd (0.1 microM) corresponds to the EC50 for enzyme activation. The affinity of forskolin for adenylyl cyclase is greatly reduced in the absence of Gsalpha ( approximately 40 microM). Binding of forskolin to the individual cytoplasmic domains of the enzyme was not detected. A single binding site for the ATP analog, alpha,beta-methylene ATP (Ap(CH2)pp), was also detected by equilibrium dialysis. Such binding was not observed with the individual domains. Binding of Ap(CH2)pp was unaffected by P-site inhibitors of adenylyl cyclase. A modified P-loop sequence located near the carboxyl terminus of adenylyl cyclase has been implicated in ATP binding. Mutation of the conserved, non-glycine residues within this region caused no significant changes in the Km for ATP or the Ki for Ap(CH2)pp. It thus seems unlikely that this region is part of the active site. However, a mutation in the C1 domain (E518A) causes a 10-fold decrease in the binding affinity for Ap(CH2)pp. This residue and the active site of the enzyme may lie at the interface between the two cytosolic domains.

Localization of the iodomycin binding site in hamster P-glycoprotein

P-glycoprotein, the overexpression of which is a major cause for the failure of cancer chemotherapy in man, recognizes and transports a broad range of structurally unrelated amphiphilic compounds. This study reports on the localization of the binding site of P-glycoprotein for iodomycin, the Bolton-Hunter derivative of the anthracycline daunomycin. Plasma membrane vesicles isolated from multidrug-resistant Chinese hamster ovary B30 cells were photolabeled with [125I]iodomycin. After chemical cleavage behind the tryptophan residues, 125I-labeled peptides were separated by electrophoresis and high performance liquid chromatography. Edman sequencing revealed that [125I]iodomycin had been predominantly incorporated into the fragment 230-312 of isoform I of hamster P-glycoprotein. According to models based on hydropathy plots, the amino acid sequence 230-312 forms the distal part of transmembrane segment 4, the second cytoplasmic loop, and the proximal part of transmembrane segment 5 in the N-terminal half of P-glycoprotein. The binding site for iodomycin is recognized with high affinity by vinblastine and cyclosporin A.

Overexpression of a 40-kDa protein in human multidrug resistant cells

The use of anticancer drugs in the chemotherapeutic treatment of cancer patients frequently results in the emergence of drug resistant tumors. Selection of tumor cell lines in vitro has led to the identification of several proteins that mediate drug resistance to anticancer drugs. In this study, an immuno-dot blot method was used to isolate a monoclonal antibody (IPM96) which recognized a 40 kDa protein (or P-40) co-expressed with P-glycoprotein and MRP in several multidrug resistant cell lines (MCF-7/Adr, SKOV/VLB1.0, H69/Adr, and HL60/AR). Furthermore, P-40 levels dropped significantly in one revertant cell line (H69/PR) derived from H69/AR cells. Interestingly, the expression of P-40 was also higher in two tumor cell lines (SKTax6a and A2780CP) that were selected with paclitaxel or cisplatin but do not express P-gp or MRP. Immuno-fluorescence staining of cells with IPM96 showed both membrane and cytoplasmic staining. These results were confirmed by Western blot analysis of different subcellular fractions from MCF-7/Adr cells. The membrane bound P-40 was resistant to extraction with high salt, chelating agents, and denaturing agents, but was solubilized with 10 mM CHAPS. Taken together, the overexpression of P-40 in multidrug resistant cells has not been previously determined and therefore could be important in the expression of the drug resistance phenotype.

Inhibition of P-glycoprotein ATPase activity by beryllium fluoride

ATPase activity of P-glycoprotein (multidrug-resistance protein) was found to be potently inhibited by beryllium fluoride (BeFx) in combination with MgATP, MgADP, or corresponding Mg-8-azido-nucleotides. Inhibition was due to trapping of nucleoside diphosphate at catalytic sites. Full inhibition was achieved on trapping of 1 mol of nucleotide per mol of Pgp. Reactivation was slow (t(1/2) = 32 min at 37 degrees C), and release of trapped nucleotide correlated with recovery of ATPase. Trapping of 8-azido-ADP followed by UV irradiation yielded permanent inactivation and specific labeling of Pgp in plasma membranes. Both N- and C-terminal nucleotide binding sites were labeled. These findings give strong confirmation of the concepts that in intact Pgp both nucleotide sites are active in MgATP hydrolysis, and that they interact strongly. The characteristics of inhibition by BeFx were similar in general to those seen with vanadate. However, PPi gave strong protection against BeFx inhibition, and in this respect, inhibition by BeFx was clearly different from vanadate inhibition.

Inhibition of P-glycoprotein ATPase activity by procedures involving trapping of nucleotide in catalytic sites

Fluoroaluminate in combination with nucleotide inhibited ATPase activity of P-glycoprotein (Pgp) in plasma membranes and in pure reconstituted form. Low nucleotide concentrations were effective, e.g., half-maximal inhibition was obtained with 10 microM MgATP. With MgATP or MgADP, reactivation occurred with t1/2 = 7 min at 37 degrees C. With 8-azido-ATP, UV irradiation of inhibited Pgp gave specific photolabeling of both nucleotide sites. Fluoroaluminate therefore provides a valuable tool for functional and structural characterization of P-glycoprotein and probably of other ABC transporters. 2-Azido-ATP, in combination with vanadate, fluoroaluminate, or beryllium fluoride, inhibited Pgp ATPase activity. Low concentrations of 2-azido-ATP were effective. However, after UV irradiation of the inhibited Pgp species, in no case was there evidence of covalent labeling of nucleotide sites. Therefore in the Pgp catalytic sites, under conditions of nucleotide trapping, there is no suitable amino acid side chain adjacent to the photoactivated 2-position of bound 2-azido-nucleotide, and 8-azido-ATP is the preferred photolabeling analog.

The 220-kDa rim protein of retinal rod outer segments is a member of the ABC transporter superfamily

Outer segments of mammalian rod photoreceptor cells contain an abundantly expressed membrane protein that migrates with an apparent molecular mass of 220 kDa by SDS-gel electrophoresis. We have purified the bovine protein by immunoaffinity chromatography, determined its primary structure by cDNA cloning and direct peptide sequence analysis, and mapped its distribution in photoreceptors by immunocytochemical and biochemical methods. The full-length cDNA encodes a 2280-amino acid protein (calculated molecular mass of 257 kDa) consisting of two structurally related, tandem arranged halves. Each half consists of a hydrophobic domain containing six putative transmembrane segments followed by an ATP-binding cassette. A data base homology search showed that the rod outer segment 220-kDa protein is 40-50% identical in amino acid sequence to the ABC1 and ABC2 proteins cloned from a mouse macrophage cell line. Photoaffinity labeling with 8-azido-ATP and nucleotide inhibition studies confirmed that both ATP and GTP bind to this protein with similar affinities. Concanavalin A labeling and endoglycosidase H digestion indicated that the rod outer segment protein contains at least one carbohydrate chain. Immunocytochemical and biochemical studies have revealed that the 220-kDa glycoprotein is distributed along the rim region and incisures of rod outer segment disc membranes. From these studies we conclude that the 220-kDa glycoprotein of bovine rod outer segment disc membranes or Rim ABC protein is a new member of the superfamily of ABC transporters and is the mammalian homolog of the frog photoreceptor rim protein.

Effect of quercetin on Hoechst 33342 transport by purified and reconstituted P-glycoprotein

Multidrug resistance due to P-glycoprotein is a serious impediment to successful chemotherapy of cancer. Numerous compounds are known that inhibit the drug-exporting function of P-glycoprotein. Understanding the mechanisms of action of these chemosensitizers is made difficult by the complexity of the in vivo cell systems usually employed. To examine the direct effects of chemosensitizers, we have developed a system in which purified and reconstituted P-glycoprotein transports. Hoechst 33342 from the lipid membrane to the aqueous interior of proteoliposomes, requiring ATP hydrolysis (Shapiro AB and Ling V, J Biol Chem 270: 16167-16175, 1995). Here, we use this system to understand the effect on P-glycoprotein of quercetin, one of three flavonoids that have been reported to have the unique property of stimulating drug transport by P-glycoprotein in vivo (Phang et al., Cancer Res 53: 5977-5981, 1993). Since flavonoids are abundant in food, it is important to understand their effects on the function of P-glycoprotein because of the implications for cancer chemotherapy. In our hands, quercetin inhibited P-glycoprotein-mediated Hoechst 33342 efflux and enhanced accumulation, as measured by flow cytometry, by multidrug-resistant CHRC5 cells. In the purified system, quercetin strongly inhibited Hoechst 33342 transport by P-glycoprotein, at least in part by inhibiting the ATPase activity of P-glycoprotein required for transport. We conclude that the previously reported stimulatory effect of quercetin on drug efflux from multidrug-resistant cells is not a direct effect on P-glycoprotein. The ATPase domain of P-glycoprotein may be an attractive target for new chemosensitizing agents.

Functional modulation of ATPase of P-glycoprotein by C219, a monoclonal antibody against P-glycoprotein

P-glycoprotein functions as an ATP-driven efflux pump for antitumor agents. C219 is a monoclonal antibody which recognizes regions near both ATP binding domains in each half of P-glycoprotein. In this study, we have demonstrated that C219 inhibits the ATPase activity of P-glycoprotein based on the following findings: 1) the inhibition of total ATPase activity by C219 was selective to P-glycoprotein-positive membranes; 2) the C219-sensitive fraction of ATPase correlated the expression of P-glycoprotein; and 3) modulators of P-glycoprotein ATPase, verapamil and cyclosporin A, affected the C219-sensitive fraction of ATPase. The photolabeling of P-glycoprotein with 8-azido-[alpha-32P]ATP was inhibited by C219, suggesting that the inhibition of ATP binding by C219 reduced the activity. Since C219 interacts with P-glycoprotein ATPase, C219 might become a useful tool for studying the role of P-glycoprotein ATPase.

Topological folding and proteolysis profile of P-glycoprotein in membranes of multidrug-resistant cells: implications for the drug-transport mechanism

P-glycoprotein (Pgp)1 is a polytopic membrane protein and functions as an energy-dependent drug efflux pump. It is responsible for multidrug resistance (MDR) in cancer cell lines. Recently, the topological structure of Pgp has been investigated. However, the results are in dispute. A major question concerning the Pgp topology is the membrane orientation of the loop linking TM4 and TM5 (loop 4) and the loop linking TM8 and TM9 (loop 8). In this study, we generated polyclonal antibodies specific to these two loops. In combination with a panel of other well-characterized site-specific polyclonal- and monoclonal antibodies of Pgp, we tested the membrane orientation of these two loops of Pgp in multidrug-resistant cells using immunocytochemistry and proteolysis/membrane protection assay. Our results showed that (1) both loops 4 and 8 are located extracellularly whereas other domains, such as the ATP-binding sites, are in the cytoplasm and (2) proteolysis of Pgp is not a random event and the trypsin-sensitive sites are cleaved in orders. Since the Pgp was not genetically manipulated in this study, in contrast to previous studies, we believe that naturally expressed Pgp molecules have an unconventional topology. We speculate that this alternate topology of Pgp may represent a different functional state of the protein. Further detailed analysis of Pgp topology will help to understand the fundamental mechanism of drug transport by Pgp.

Characterization and epitope mapping of several new anti-P-glycoprotein monoclonal antibodies

Monoclonal antibodies (MAbs) were raised against partially purified Class I P-glycoprotein from multidrug-resistant Chinese hamster ovary CHRB30 cells. Fifteen stable monoclonal hybridoma cell lines were established, and the secreted antibodies were classified into 8 groups on the basis of banding pattern on immunoblots of P-glycoprotein digested with cyanogen bromide or partially digested with proteases. One representative of each group was tested further for several activities. Six of the 8 recognized human P-glycoprotein in the multidrug-resistant SKVLBI cell line. None of the antibodies recognized P-glycoprotein in unfixed cells, suggesting that all recognize cytoplasmic epitopes or extracellular epitopes not accessible in native P-glycoprotein. All 8 antibodies were able to immunoprecipitate P-glycoprotein from non-denaturing detergent solution. The linear epitopes of the antibodies were mapped to 11-27 amino acids. Two of the antibodies had epitopes in the linker region, 3 in the N-terminal nucleotide binding domain, 2 in the C-terminal nucleotide binding domain and 1 in the predicted cytoplasmic loop between predicted transmembrane helices 8 and 9. These antibodies, with known epitopes, could have uses for P-glycoprotein detection, structure/function studies, purification and quantitation.

The catalytic cycle of P-glycoprotein

P-glycoprotein is a plasma-membrane glycoprotein which confers multidrug-resistance on cells and displays ATP-driven drug-pumping in vitro. It contains two nucleotide-binding domains, and its structure places it in the 'ABC transporter' family. We review recent evidence that both nucleotide-sites bind and hydrolyse Mg-ATP. The two catalytic sites interact strongly. A minimal scheme for the MgATP hydrolysis reaction is presented. An alternating catalytic sites scheme is proposed, in which drug transport is coupled to relaxation of a high-energy catalytic site conformation generated by the hydrolysis step. Other ABC transporters may show similar catalytic features.

Volume-sensitive chloride channel activity does not depend on endogenous P-glycoprotein

To determine whether endogenous P-glycoprotein, the MDR1 gene product that functions as a drug transport pump, is a volume-sensitive Cl- channel molecule or a protein kinase C-mediated regulator of the Cl- channel, whole-cell patch-clamp and molecular biological experiments were carried out in a human small intestinal epithelial cell line. Endogenous expression of P-glycoprotein was confirmed by Northern blot analysis, reverse transcription-polymerase chain reaction, Western blot analysis, and immunostaining. The P-glycoprotein expression was abolished by the antisense (but not sense) oligonucleotide for the MDR1 gene, whereas the magnitude of the Cl- current activated by osmotic swelling was not distinguishable between both antisense- and sense-treated cells. The volume-sensitive Cl- currents were not specifically affected by the anti-P-glycoprotein monoclonal antibodies, MRK16, C219, and UIC2. An inhibitor of P-glycoprotein-mediated pump activity, verapamil, was found to never affect the Cl- current. A substrate for the P-glycoprotein-mediated drug pump, vincristine or daunomycin, did not prevent swelling-induced activation of the Cl- current. Furthermore, the Cl- current was not affected by an activator of protein kinase C (12-O-tetradecanoylphorbol-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol). Thus, it is concluded that the endogenous P-glycoprotein molecule is not itself a volume-sensitive Cl- channel nor a protein kinase C-mediated regulator of the channel in the human epithelial cells.

Both P-glycoprotein nucleotide-binding sites are catalytically active

The technique of vanadate trapping of nucleotide was used to study catalytic sites of P-glycoprotein (Pgp) in plasma membranes from multidrug-resistant Chinese hamster ovary cells. Vanadate trapping of Mg- or Co-8-azido-nucleotide (1 mol/mol of Pgp) caused complete inhibition of Pgp ATPase activity, with reactivation rates at 37 degrees C of 1.4 x 10(-3) s-1 (t1/2 = 8 min) or 3.3 x 10(-4) s-1 (t1/2 = 35 min), respectively. UV irradiation of the inhibited Pgp yielded permanent inactivation of ATPase activity and specific photolabeling of Pgp. Mild trypsin digestion showed that the two nucleotide sites were labeled in equal proportion. The results show that both nucleotide sites in Pgp are capable of nucleotide hydrolysis, that vanadate trapping of nucleotide at either site completely prevents hydrolysis at both sites, and that vanadate trapping of nucleotide in the N- or C-terminal nucleotide sites occurs non-selectively. A minimal scheme is presented to explain inhibition by vanadate trapping of nucleotide and to describe the normal catalytic pathway. The inhibited Pgp-Mg-nucleotide.vanadate complex is probably an analog of the catalytic transition state, implying that when one nucleotide site assumes the catalytic transition state conformation the other site cannot do so and suggesting that the two sites may alternate in catalysis.