Structure-activity studies of verapamil analogs that modulate transport of leukotriene C(4) and reduced glutathione by multidrug resistance protein MRP1.

The 190-kDa multidrug resistance protein MRP1 is an ATP-binding cassette protein that confers resistance to multiple antineoplastic agents and actively transports conjugated organic anions. We have previously shown that MRP1-mediated GSH transport is stimulated by verapamil but transport of verapamil in the presence or absence of GSH is not observed. We have now examined 20 sulfur-containing verapamil analogs for their ability to inhibit MRP1-mediated leukotriene C(4) (LTC(4)) transport and stimulate GSH uptake into inside-out membrane vesicles. All of the derivatives were poor inhibitors of LTC(4) uptake. However, the inhibitory potency of the more lipophilic dithiane compounds could be enhanced by coincubation with GSH whereas this was not the case for the more hydrophilic dithiane tetraoxides. The dithiane derivatives stimulated GSH transport whereas, with one exception, the dithiane tetraoxides did not. One pair of dithiane stereoisomers differed significantly in their ability to stimulate GSH transport although their ability to inhibit LTC(4) uptake in the presence of GSH was comparable. Our findings indicate that the GSH transport activity of MRP1 can be dissociated from its conjugated organic anion transport activity.

Comparison of the functional characteristics of the nucleotide binding domains of multidrug resistance protein 1.

Multidrug Resistance Protein 1 (MRP1) transports diverse organic anionic conjugates and confers resistance to cytotoxic xenobiotics. The protein contains two nucleotide binding domains (NBDs) with features characteristic of members of the ATP-binding cassette superfamily and exhibits basal ATPase activity that can be stimulated by certain substrates. It is not known whether the two NBDs of MRP1 are functionally equivalent. To investigate this question, we have used a baculovirus dual expression vector encoding both halves of MRP1 to reconstitute an active transporter and have compared the ability of each NBD to be photoaffinity-labeled with 8-azido-[(32)P]ATP and to trap 8-azido-[(32)P]ADP in the presence of orthovanadate. We found that NBD1 was preferentially labeled with 8-azido-[(32)P]ATP, while trapping of 8-azido-[(32)P]ADP occurred predominantly at NBD2. Although trapping at NBD2 was dependent on co-expression of both halves of MRP1, binding of 8-azido-ATP by NBD1 remained detectable when the NH(2)-proximal half of MRP1 was expressed alone and when NBD1 was expressed as a soluble polypeptide. Mutation of the conserved Walker A lysine 684 or creation of an insertion mutation between Walker A and B motifs eliminated binding by NBD1 and all detectable trapping of 8-azido-ADP at NBD2. Both mutations decreased leukotriene C(4) (LTC(4)) transport by approximately 70%. Mutation of the NBD2 Walker A lysine 1333 eliminated trapping of 8-azido-ADP by NBD2 but, in contrast to the mutations in NBD1, essentially eliminated LTC(4) transport activity without affecting labeling of NBD1 with 8-azido-[(32)P]ATP.

Verapamil stimulates glutathione transport by the 190-kDa multidrug resistance protein 1 (MRP1).

Multidrug resistance in tumor cells is often associated with reduced drug accumulation resulting from increased expression of the 190-kDa multidrug resistance protein 1 (MRP1) or the 170-kDa P-glycoprotein. However, unlike P-glycoprotein, MRP1 is a primary active transporter of many conjugated organic anions, including the cysteinyl leukotriene LTC(4). Moreover, agents such as verapamil that reverse P-glycoprotein-mediated resistance are often poorly, or not at all, effective in MRP1-overexpressing cells. In the present study, we investigated the effects of verapamil on MRP1-mediated transport processes. We found that verapamil inhibited LTC(4) transport into inside-out membrane vesicles prepared from MRP1-transfected cells in a competitive manner, but only in the presence of reduced glutathione (GSH) or its nonreducing S-methyl derivative. In the presence of 1 mM GSH, the apparent K(i) for verapamil was 1.2 microM, and in the presence of 100 microM verapamil, the apparent K(i) for GSH was 77 microM. Verapamil itself was not transported by MRP1 in either intact cells or membrane vesicles. However, verapamil strongly stimulated MRP1-mediated GSH uptake by membrane vesicles in a concentration-dependent and osmotically sensitive manner that was inhibitable by MRP1-specific monoclonal antibodies. In the presence of 100 microM verapamil, the apparent K(m) and V(max) for GSH uptake were 83 microM and 55 pmol mg(-1) min(-1), respectively. It is proposed that the variable ability of verapamil to modulate MRP1-mediated resistance in different cell lines may be more closely linked to its effect on the GSH status of the cells than on its ability to inhibit the MRP1 transporter itself.

Characterization of vincristine transport by the M(r) 190,000 multidrug resistance protein (MRP): evidence for cotransport with reduced glutathione.

The M(r) 190,000 multidrug resistance protein (MRP) confers resistance to a broad spectrum of natural product drugs. However, it has not been possible to demonstrate that MRP can actively transport unmodified forms of these compounds, although the protein has been shown to transport structurally diverse glutathione (GSH)- and glucuronide-conjugated molecules. Previously, we showed that ATP-dependent uptake of vincristine by MRP-enriched, inside-out membrane vesicles could be stimulated by physiological concentrations of GSH (Loe et al., J. Biol. Chem., 271: 9675-9682, 1996). We have now established that the ATP/GSH dependent vincristine uptake is both proportional to the level of MRP in the membrane vesicles and can be inhibited by monoclonal antibodies shown previously to inhibit transport of established MRP substrates, such as leukotriene C4. We also show that short-chain alkyl derivatives of GSH can stimulate drug uptake, which suggests that vincristine transport does not necessarily involve a change in redox state or glutathionylation of the protein. Furthermore, vincristine uptake is accompanied by ATP- and drug-dependent accumulation of GSH, which can also be stimulated to a lesser extent by vinblastine but not daunorubicin or doxorubicin. Although GSH or vincristine alone are very poor inhibitors of MRP-mediated transport of leukotriene C4, together they act as relatively potent competitive inhibitors. Overall, our data demonstrate that MRP can actively cotransport GSH and unmodified vincristine and that these compounds probably interact, either with the leukotriene C4 binding site(s) on the protein or with a mutually exclusive site.

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.

Pharmacological characterization of the murine and human orthologs of multidrug-resistance protein in transfected human embryonic kidney cells.

Overexpression of the human multidrug-resistance protein (MRP) causes a form of multidrug resistance similar to that conferred by P-glycoprotein, although the two proteins are only distantly related. In contrast to P-glycoprotein, human MRP has also been shown to be a primary active transporter of a structurally diverse range of organic anionic conjugates, some of which may be physiological substrates. At present, the mechanism by which MRP transports these compounds and mediates multidrug resistance is not understood. With the objective of developing an animal model for studies on the normal functions of MRP and its ability to confer multidrug resistance in vivo, we recently cloned the murine ortholog of MRP (mrp). To assess the degree of functional conservation between mrp and MRP, we directly compared the drug cross-resistance profiles they confer when transfected into human embryonic kidney cells, as well as their ability to actively transport leukotriene C4, 17beta-Estradiol 17beta-(D-glucuronide), and vincristine; mrp and MRP conferred similar drug resistance profiles, with the exception that only MRP conferred resistance to the anthracyclines tested. Consistent with these findings, accumulation of [3H]vincristine and [3H]VP-16 was decreased, and efflux of [3H]vincristine was increased in both murine and human MRP-transfected cell populations, whereas only human MRP-transfected cells displayed decreased accumulation and increased efflux of [3H]daunorubicin. Membrane vesicles derived from both transfected cell populations transported leukotriene C4 in an ATP-dependent manner with comparable efficiency, although the efficiency of 17beta-estradiol 17beta-(D-glucuronide) transport was somewhat higher with MRP transfectants. ATP-dependent transport of vincristine was also observed with vesicles from mrp and MRP transfectants but only in the presence of glutathione. These studies reveal intrinsic differences between the murine and human MRP orthologs with respect to their ability to confer resistance to a major class of chemotherapeutic drugs.

ATP-dependent transport of aflatoxin B1 and its glutathione conjugates by the product of the multidrug resistance protein (MRP) gene.

Glutathione-S-transferase-catalyzed conjugation of glutathione (GSH) to aflatoxin B1-8,9-epoxide plays an important role in preventing binding of this ultimate carcinogen to target macromolecules. Once formed, the aflatoxin B1-epoxide-GSH conjugates are actively extruded from the cell by an unidentified ATP-dependent export pump or pumps. Two possible candidates for this GSH conjugate pump are the 190-kDa multidrug resistance protein (MRP) and the 170-kDa P-glycoprotein. Both proteins belong to the ATP-binding cassette superfamily of transmembrane transport proteins and confer resistance to a similar spectrum of natural-product drugs. Using membrane vesicles from MRP-transfected cells, we found that MRP transports GSH conjugates of both the endo-isomers and exo-isomers of aflatoxin B1-8,9-epoxide in an ATP-dependent, osmotically sensitive manner (V(max) = 180 pmol/mg/min, K(m) = 189 nM). Membrane vesicles from P-glycoprotein-overexpressing cells showed very low levels of transport. MRP-mediated transport was inhibited by an MRP-specific monoclonal antibody and by a variety of GSH derivatives and cholestatic steroid glucuronides. ATP-dependent transport of unmodified aflatoxin B1 by MRP-enriched membrane vesicles was low but markedly enhanced in the presence of 5 mM GSH, even though GSH conjugates of aflatoxin B1 were not formed by the vesicles. These data demonstrate that MRP is capable of energy-dependent transport of aflatoxin B1 and its GSH conjugates and suggest a potential protective role for MRP in mammalian chemical carcinogenesis.

Reconstitution of ATP-dependent leukotriene C4 transport by Co-expression of both half-molecules of human multidrug resistance protein in insect cells.

Multidrug resistance protein (MRP) confers a multidrug resistance phenotype similar to that associated with overexpression of P-glycoprotein. Unlike P-glycoprotein, MRP has also been shown to be a primary active ATP-dependent transporter of conjugated organic anions. The mechanism(s) by which MRP transports these compounds and increases resistance to natural product drugs is unknown. To facilitate studies on the structure and function of MRP, we have determined whether a baculovirus expression system can be used to produce active protein. Full-length MRP as well as molecules corresponding to either the NH2- or COOH-proximal halves of the protein were expressed individually and in combination in Spodoptera frugiperda Sf21 cells. High levels of intact and half-length proteins were detected in membrane vesicles from infected cells. Although underglycosylated, the full-length protein transported leukotriene C4 (LTC4) with kinetic parameters very similar to those of MRP produced in transfected HeLa cells. Neither half-molecule was able to transport LTC4. However, a functional transporter with characteristics similar to those of intact protein could be reconstituted when both half-molecules were co-expressed. Transport of LTC4 by Sf21 membrane vesicles containing either intact or reconstituted MRP was competitively inhibited by both S-decylglutathione and 17beta-estradiol 17-(beta-D-glucuronide), with Ki values similar to those reported previously for MRP expressed in HeLa cells (Loe, D. W., Almquist, K. C., Deeley, R. G., and Cole, S. P. C. (1996) J. Biol. Chem. 271, 9675-9682; Loe, D. W., Almquist, K. C., Cole, S. P. C., and Deeley, R. G. (1996) J. Biol. Chem. 271, 9683-9689). These studies demonstrate that human MRP produced in insect cells can function as an active transporter of LTC4 and that the NH2- and COOH-proximal halves of the protein can assemble efficiently to form a transporter with functional characteristics similar to those of the intact protein.

Multidrug resistance protein (MRP)-mediated transport of leukotriene C4 and chemotherapeutic agents in membrane vesicles. Demonstration of glutathione-dependent vincristine transport.

The 190-kDa multidrug resistance protein (MRP) has recently been associated with the transport of cysteinyl leukotrienes and several glutathione (GSH) S-conjugates. In the present study, we have examined the transport of leukotriene C4 (LTC4) in membrane vesicles from MRP-transfected HeLa cells (T14), as well as drug-selected H69AR lung cancer cells which express high levels of MRP. V(max) and K(m) values for LTC4 transport by membrane vesicles from T14 cells were 529 +/- 176 pmol mg(-1) min(-1) and 105 +/- 31 nM, respectively. At 50 nM LTC4, the K(m) (ATP) was 70 micron. Transport in T14 vesicles was osmotically-sensitive and was supported by various nucleoside triphosphates but not by non- or slowly-hydrolyzable ATP analogs. LTC4 transport rates in membrane vesicles derived from H69AR cells and their parental and revertant variants were consistent with their relative levels of MRP expression. A 190-kDa protein in T14 membrane vesicles was photolabeled by [3H]LTC4 and immunoprecipitation with MRP-specific monoclonal antibodies (mAbs) confirmed that this protein was MRP. LTC4 transport was inhibited by an MRP-specific mAb (QCRL-3) directed against an intracellular conformational epitope of MRP, but not by a mAb (QCRL-1) which recognizes a linear epitope. Photolabeling with [3H]LTC4 was also inhibitable by mAb QCRL-3 but not mAb QCRL-1. GSH did not inhibit LTC4 transport. However, the ability of alkylated GSH derivatives to inhibit transport increased markedly with the length of the alkyl group. S-Decylglutathione was a potent competitive inhibitor of [3H]LTC4 transport (K(i(app)) 116 nM), suggesting that the two compounds bind to the same, or closely related, site(s) on MRP. Chemotherapeutic agents including colchicine, doxorubicin, and daunorubicin were poor inhibitors of [3H]LTC4 transport. Taxol, VP-16, vincristine, and vinblastine were also poor inhibitors of LTC4 transport but inhibition by these compounds was enhanced by GSH. Uptake of [3H]vincristine into T14 membrane vesicles in the absence of GSH was low and not dependent on ATP. However, in the presence of GSH, ATP-dependent vincristine transport was observed. Levels of transport increased with concentrations of GSH up to 5 mM. The identification of an MRP-specific mAb that inhibits LTC4 transport and prevents photolabeling of MRP by LTC4, provides conclusive evidence of the ability of MRP to transport cysteinyl leukotrienes. Our studies also demonstrate that MRP is capable of mediating ATP-dependent transport of vincristine and that transport is GSH-dependent.

ATP-dependent 17 beta-estradiol 17-(beta-D-glucuronide) transport by multidrug resistance protein (MRP). Inhibition by cholestatic steroids.

In addition to its ability to confer resistance to a range of natural product type chemotherapeutic agents, multidrug resistance protein (MRP) has been shown to transport the cysteinyl leukotriene, LTC4, and several other glutathione (GSH) S-conjugates. We now demonstrate that its range of potential physiological substrates also includes cholestatic glucuronidated steroids. ATP dependent, osmotically sensitive transport of the naturally occurring conjugated estrogen, 17 beta-estradiol 17-(beta-D-glucuronide) (E(2)17 beta G), was readily demonstrable in plasma membrane vesicles from populations of MRP-transfected HeLa cells (Vmax 1.4 nmol mg-1 min-1, K(m) 2.5 micron). The involvement of MRP was confirmed by demonstrating that transport was completely inhibited by a monoclonal antibody specific for an intracellular conformational epitope of the protein. MRP-mediated transport of LTC4, was competitively inhibited by E(2)17 beta G (K(i(app)) 22 micron), despite the lack of structural similarity between these two substrates. Competitive inhibition of [3H]E(2)17 beta G transport was also observed with a number of other cholestatic conjugated steroids. All of these compounds prevented photolabeling of MRP with [3H]LTC4, demonstrating that the cholestatic steroid and leukotriene conjugates compete either for the same or possibly overlapping sites on the protein. Consistent with the presence of overlapping but non-identical sites, studies using chemotherapeutic drugs to inhibit MRP-mediated E(2)17 beta G transport indicated that daunorubicin had the highest relative potency of the drugs tested, whereas it was the least potent inhibitor of LTC4 transport. Non-cholestatic steroids glucuronidated at the 3 position of the steroid nucleus, such as 17 beta-estradiol 3-(beta-D-glucuronide), did not compete for transport of E(2)17 beta G by MRP, nor did they inhibit photolabeling of the protein with [3H]LTC4. These data identify MRP as a potential transporter of cholestatic conjugated estrogens and demonstrate site-specific requirements for glucuronidation of the steroid nucleus.

Characterization of the M(r) 190,000 multidrug resistance protein (MRP) in drug-selected and transfected human tumor cell.

Overexpression of multidrug resistance-associated protein (MRP) has been detected in resistant cell lines derived from a variety of tumor types. The deduced amino acid sequence of MRP suggests that it is a member of the ATP-binding cassette transmembrane transporter superfamily that may be glycosylated and/or phosphorylated [S. P. C. Cole et al., Science Washington, DC), 258: 1650-1654, 1992]. Recently, transfection of HeLa cells with MRP expression vectors has demonstrated that the protein is capable of increasing resistance to natural product drugs such as anthracyclines, Vinca alkaloids, and epipodophyllotoxins (C. E. Grant et al., Cancer Res., 54: 357-361, 1994). Although the resistance phenotype of the transfectants is similar to that of the human small cell lung cancer cell line, H69AR, from which MRP was originally cloned, the transfectants differ in their drug accumulation characteristics, relative resistance to certain drugs, and MRP mRNA:protein ratio. Such differences have also been observed among drug-selected cell lines that overexpress MRP, and the underlying causes of these variable phenotypes are presently not known. We have utilized polyclonal anti-MRP-peptide antibodies to compare MRP post-translational modification, stability, processing, and subcellular distribution in the HeLa transfectants and in the drug-selected H69AR cells. These studies establish that MRP in both the transfected and selected cells is an ATP-binding, integral membrane glycophosphoprotein with an apparent molecular weight of 190,000. No obvious differences were detected in the extent or type of glycosylation or the kinetics of processing and turnover of the protein that might contribute to the different characteristics of the transfected and drug-selected cells. Analyses of the subcellular distribution of MRP by isopyknic density gradient centrifugation revealed that approximately 80% of MRP in the HeLa transfectants was associated with a low density plasma membrane fraction while the comparable fraction in the drug-selected H69AR cells contained only approximately 50% of the protein. The remaining MRP and plasma membrane markers were codistributed in higher density fractions consistent with the presence of MRP in endocytotic vesicles. The relatively high proportion of MRP associated with these fractions in H69AR cells may contribute to the lack of an observable accumulation defect in these cells when compared with the transfectants.

Pharmacological characterization of multidrug resistant MRP-transfected human tumor cells.

We have previously identified and characterized a novel member of the ATP-binding cassette superfamily of transport proteins, multidrug resistance protein (MRP), and subsequently demonstrated that its overexpression is sufficient to confer multidrug resistance on previously sensitive cells (Cole et al., Science (Washington DC), 258: 1650-1654, 1992; Grant et al., Cancer Res. 54: 357-361, 1994). In the present study, we have transfected two different eukaryotic expression vectors containing MRP complementary DNA into HeLa cells to study the pharmacological phenotype produced exclusively by overexpression of human MRP. The drug resistance patterns of the two MRP-transfected cell populations were similar. They were characterized by a moderate (5- to 15-fold) level of resistance to doxorubicin, daunorubicin, epirubicin, vincristine, and etoposide, and a low (< or = 3-fold) level of resistance to taxol, vinblastine, and colchicine. The transfectants were not resistant to 9-alkyl anthracyclines, mitoxantrone, or cisplatin. The MRP-transfected cells were also resistant to some heavy metal anions including arsenite, arsenate, and trivalent and pentavalent antimonials but were not resistant to cadmium chloride. Accumulation of radiolabeled vincristine was reduced by 45% in the MRP-transfected cells and could be restored to the levels found in sensitive cells by depletion of ATP. Rates of vincristine efflux did not differ greatly in the sensitive and resistant cells. The cytotoxic effects of vincristine and doxorubicin could be enhanced in a dose-dependent fashion by coadministration of verapamil. Cyclosporin A also increased vincristine toxicity but had less effect on doxorubicin toxicity. The degree of chemosensitization by verapamil and cyclosporin A was similar in MRP-transfected cells and in cells transfected with the vector alone, suggesting that sensitization involved mechanisms independent of MRP expression. Verapamil and cyclosporin A caused a modest increase in vincristine accumulation in the resistant cells but did not restore levels to those of the sensitive cells. Taken together, these data indicate that drug-resistant cell lines generated by transfection with MRP complementary DNA display some but not all of the characteristics of MRP-overexpressing cell lines produced by drug selection in vitro. They further demonstrate that the multidrug resistance phenotype conferred by MRP is similar but not identical to that conferred by P-glycoprotein and includes resistance to arsenical and antimonial oxyanions.

ATP is not required for anion current activated by cell swelling in multidrug-resistant lung cancer cells.

During whole cell recording with 4 mM ATP and 0.1 mM GTP in the pipette, outwardly rectifying Cl- currents (155 +/- 20.5 pA/pF) were repetitively activated on reduction of bath solution osmolarity from 290 mosM (control) to 210 mosM. These currents were sensitive to 0.1-1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Omission of ATP from the pipette solution reduced the current magnitude to 42.7 +/- 9.5 pA/pF and prevented repetitive activation. More hyposmotic solutions (160 mosM) usually elicited current repetitively despite an ATP-free pipette solution. In cells depleted of ATP (to < 5% of control) by preincubation with 2-deoxyglucose (10 mM) and rotenone (100 nM), hyposmotic solutions failed to activate significant current. Cell volume increased to 230 +/- 18% of control (19.1 +/- 1.2 microns) in 210 mosM bath (normal cells) but only to 114 +/- 13% of control in ATP-depleted cells exposed to 160 mosM solution. This failure of ATP-depleted cells to swell in hypotonic external solutions was reversed by overnight pretreatment with cytochalasin D (2 micrograms/ml; n = 6) but not by colchicine (250 microM; n = 8). In outside-out patches of membrane dialyzed with zero ATP and excised from swollen cells, we observed sustained activation of a 53-pS outwardly rectifying channel (chord conductance, +100 mV; open probability approximately 1.0). In cell-attached patches from normal and ATP-depleted cells, we activated similar channels by suction. ATP does not appear to be an absolute requirement for the activation of this Cl- channel in H69AR cells but may be essential for the normal volume response and channel activation mediated through cytoskeletal elements within cells.

Interaction of multidrug-resistant Chinese hamster ovary cells with the peptide ionophore gramicidin D.

A major form of multidrug resistance results from the overexpression of P-glycoprotein, a 170 kDa membrane protein. Multidrug resistant (MDR) Chinese hamster ovary (CHO) cells and mdrl transfectants displayed cross-resistance to the channel-forming peptide ionophore gramicidin D, which was reversed by various chemosensitizers, thus directly implicating P-glycoprotein as the mediator of resistance. However, gramicidin D was not able to inhibit [3H]azidopine photolabelling of P-glycoprotein. MDR cells were not resistant to other pore-forming ionophores, but showed a modest level of cross-resistance to the mobile ionophore valinomycin. There was no difference in 125I-gramicidin D uptake by resistant and sensitive cells. Resistant cells showed lower 86Rb+ uptake, relative to the drug-sensitive parent. Addition of GmD increased both the rate and the level of 86Rb+ uptake in sensitive cells, but had no effect on MDR cells. MDR cells also showed much lower rates of gramicidin D-dependent 86Rb+ efflux than sensitive cells, and this was greatly increased by verapamil. These results suggest that P-glycoprotein interferes with the formation of ion-conducting gramicidin D channels. In contrast, valinomycin had the same effect on gramicidin D-dependent cation efflux in MDR and sensitive cells. Gramicidin D is thus unique among the ionophores is being a substrate for P-glycoprotein, which appears to greatly reduce the formation of active dimeric channels in the plasma membrane of MDR cells.

Interaction of multidrug-resistant Chinese hamster ovary cells with amphiphiles.

The interaction of membrane-active amphiphiles with a series of MDR Chinese hamster ovary (CHO) cell lines was investigated. Cross-resistance to cationic amphiphiles was observed, which was effectively sensitised by verapamil. MDR cells showed collateral sensitivity to polyoxyethylene amphiphiles (Triton X-100/Nonidet P-40), which reached a maximum at 9-10 ethylene oxide units. Resistant lines were also highly collaterally sensitive (17-fold) to dibutylphthalate. mdrl transfectants showed cross-resistance to cationic amphiphiles, but no collateral sensitivity to nonionic species. Triton X-100/Nonidet P-40 inhibited 3H-azidopine photoaffinity labelling at low concentrations, perhaps reflecting a specific interaction with P-glycoprotein. Further investigation of the molecular basis of collateral sensitivity revealed that association of 3H-Triton X-100 with MDR cells reached steady state levels rapidly, and occurred by a non-mediated mechanism. The equilibrium level of X-100 uptake was inversely related to drug resistance. Collateral sensitivity is thus not a result of decreased Triton X-100 association with the cell. The fluorescent probe merocyanine 540 was used to examine the MDR plasma membrane microenvironment for physicochemical changes. Increasing levels of drug resistance correlated with a progressive shift in the mean cell fluorescence to lower levels, which suggests that the packing density in the outer leaflet of MDR cells is increased relative to that of the drug-sensitive parent.

Solubilization, characterization, and detergent interactions of lymphocyte 5'-nucleotidase.

5'-Nucleotidase is a member of a recently identified class of membrane proteins that is anchored via a phosphatidylinositol-containing glycolipid. The enzyme was readily solubilized with full retention of catalytic activity by nonionic and anionic detergents such as alkylthioglucosides, deoxycholate, and 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane-sulfonate (CHAPS), while the cationic detergent dodecyltrimethylammonium bromide (DTAB) caused loss of activity. 5'-Nucleotidase was released only at high detergent concentrations, suggesting that it is tightly associated with the membrane. DTAB and deoxycholate caused a loss of heat stability, while alkylthioglucosides had no effect. CHAPS produced a remarkable increase in the heat stability of the partially purified (glycoprotein fraction) and purified enzyme. Arrhenius plots of solubilized 5'-nucleotidase showed "break points" for all detergents in the temperature range 30-37 degrees C. SDS-PAGE of pure 5'-nucleotidase showed a single subunit of molecular mass 70 kilodaltons (kDa), while sucrose density gradient sedimentation gave a peak of activity corresponding to 132 kDa, indicating that the enzyme exists as a dimer. Gel filtration of the solubilized enzyme in several detergents showed apparent molecular masses between 200-630 kDa, suggesting that lymphocyte 5'-nucleotidase may be present in high molecular mass aggregates in its native state.