ATPase activity of purified multidrug resistance-associated protein

Magnolol derivatives as specific and noncytotoxic inhibitors of breast cancer resistance protein (BCRP/ABCG2)

The breast cancer resistance protein (BCRP/ABCG2) transporter mediates the efflux of numerous antineoplastic drugs, playing a central role in multidrug resistance related to cancer. The absence of successful clinical trials using specific ABCG2 inhibitors reveals the urge to identify new compounds to attend this critical demand. In this work, a series of 13 magnolol derivatives was tested as ABCG2 inhibitors. Only two compounds, derivatives 10 and 11, showed partial and complete ABCG2 inhibitory effect, respectively. This inhibition was selective toward ABCG2, since none of the 13 compounds inhibited neither P-glycoprotein nor MRP1. Both inhibitors (10 and 11) were not transported by ABCG2 and demonstrated a low cytotoxic profile even at high concentrations (up to 100 µM). 11 emerged as the most promising compound of the series, considering the ratio between cytotoxicity (IG50) and ABCG2 inhibition potency (IC50), showing a therapeutic ratio (TR) higher than observed for 10 (10.5 versus 1.6, respectively). This derivative showed a substrate-independent and a mixed type of inhibition. The effect of compound 11 on the ABCG2 ATPase activity and thermostability revealed allosteric protein changes. This compound did not affect the expression levels of ABCG2 and increased the binding of the conformational-sensitive antibody 5D3. A docking study showed that 11 did not share the same binding site with ABCG2 substrate mitoxantrone. Finally, 11 could revert the chemoresistance to SN-38 mediated by ABCG2.

Cell type-specific regulation of CFTR trafficking-on the verge of progress

Trafficking of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein is a complex process that starts with its biosynthesis and folding in the endoplasmic reticulum. Exit from the endoplasmic reticulum (ER) is coupled with the acquisition of a compact structure that can be processed and traffic through the secretory pathway. Once reaching its final destination-the plasma membrane, CFTR stability is regulated through interaction with multiple protein partners that are involved in its post-translation modification, connecting the channel to several signaling pathways. The complexity of the process is further boosted when analyzed in the context of the airway epithelium. Recent advances have characterized in detail the different cell types that compose the surface epithelium and shifted the paradigm on which cells express CFTR and on their individual and combined contribution to the total expression (and function) of this chloride/bicarbonate channel. Here we review CFTR trafficking and its relationship with the knowledge on the different cell types of the airway epithelia. We explore the crosstalk between these two areas and discuss what is still to be clarified and how this can be used to develop more targeted therapies for CF.

Targeting BPTF Sensitizes Pancreatic Ductal Adenocarcinoma to Chemotherapy by Repressing ABC-Transporters and Impairing Multidrug Resistance (MDR)

Simple Summary

Pancreatic ductal adenocarcinoma is a devastating disease and an extremely chemoresistant tumour. In the present manuscript, we described the role of BPTF during tumour pancreatic ductal adenocarcinoma progression and in response to gemcitabine treatment, a gold standard treatment in this tumour type. Through different genetic approaches, we reduced BPTF levels in a panel of pancreatic ductal adenocarcinoma cell lines. We validated its therapeutic effect in cell cultures and in mouse models of pancreatic cancer. A reduction in BPTF levels impaired cell proliferation and sensitized pancreatic tumour cells to gemcitabine. We demonstrated that BPTF-silencing reduced the expression of several ABC-transporters, which are involved in gemcitabine resistance, and enhanced its accumulation in the tumour cell, improving its therapeutic effect.

Abstract

Pancreatic ductal adenocarcinoma (PDA) is characterized by an extremely poor prognosis due to its late diagnosis and strong chemoresistance to the current treatments. Therefore, finding new therapeutic targets is an urgent need nowadays. In this study, we report the role of the chromatin remodeler BPTF (Bromodomain PHD Finger Transcription Factor) as a therapeutic target in PDA. BPTF-silencing dramatically reduced cell proliferation and migration in vitro and in vivo in human and mouse PDA cell lines. Moreover, BPTF-silencing reduces the IC50 of gemcitabine in vitro and enhanced its therapeutic effect in vivo. Mechanistically, BPTF is required for c-MYC recruitment to the promoter of ABC-transporters and its downregulation facilitates gemcitabine accumulation in tumour cells, increases DNA damage, and a generates a strong synergistic effect in vivo. We show that BPTF is a therapeutic target in pancreatic ductal adenocarcinoma due to its strong effect on proliferation and in response to gemcitabine.

Mechanistic basis of breast cancer resistance protein inhibition by new indeno[1,2-b]indoles

The ATP-binding cassette transporter ABCG2 mediates the efflux of several chemotherapeutic drugs, contributing to the development of multidrug resistance (MDR) in many cancers. The most promising strategy to overcome ABCG2-mediated MDR is the use of specific inhibitors. Despite many efforts, the identification of new potent and specific ABCG2 inhibitors remains urgent. In this study, a structural optimization of indeno[1,2-b]indole was performed and a new generation of 18 compounds was synthesized and tested as ABCG2 inhibitors. Most compounds showed ABCG2 inhibition with IC₅₀ values below 0.5 µM. The ratio between cytotoxicity (IG₅₀) and ABCG2 inhibition potency (IC₅₀) was used to identify the best inhibitors. In addition, it was observed that some indeno[1,2-b]indole derivatives produced complete inhibition, while others only partially inhibited the transport function of ABCG2. All indeno[1,2-b]indole derivatives are not transported by ABCG2, and even the partial inhibitors are able to fully chemosensitize cancer cells overexpressing ABCG2. The high affinity of these indeno[1,2-b]indole derivatives was confirmed by the strong stimulatory effect on ABCG2 ATPase activity. These compounds did not affect the binding of conformation-sensitive antibody 5D3 binding, but stabilized the protein structure, as revealed by the thermostabilization assay. Finally, a docking study showed the indeno[1,2-b]indole derivatives share the same binding site as the substrate estrone-3-sulfate.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

Optimizing the flavanone core toward new selective nitrogen-containing modulators of ABC transporters

Aim: Naringenin (1), isolated in large amount from the aerial parts of Euphorbia pedroi, was chemically derivatized to yield 18 imine derivatives (2–19) and three alkylated derivatives through a Mannich-type reaction (20–22) that were tested as multidrug resistance (MDR) reversers in cancer cells. Results/methodology: While hydrazone (2–4) and azine (5–13) derivatives showed an improvement in their MDR reversal activities against the breast cancer resistance protein, carbohydrazides 14–19 revealed an enhancement in MDR reversal activity toward the multidrug resistance protein 1. Conclusion: The observed activities, together with pharmacophoric analysis and molecular docking studies, identified the spatial orientation of the substituents as a key structural feature toward a possible mechanism by which naringenin derivatives may reverse MDR in cancer.

CRISPR/Cas9-Mediated Three Nucleotide Insertion Corrects a Deletion Mutation in MRP1/ABCC1 and Restores Its Proper Folding and Function

A three-nucleotide deletion in cystic fibrosis transmembrane conductance regulator/ATP-binding cassette transporter C7 (CFTR/ABCC7) resulting in the absence of phenylalanine at 508 leads to mis-fold of the mutated protein and causes cystic fibrosis. We have used a comparable three-nucleotide deletion mutant in another ABCC family member, multidrug resistance-associated protein (MRP1)/ABCC1, to determine whether CRISPR-Cas9-mediated recombination can safely and efficiently knock in three-nucleotide to correct the mutation. We have found that the rate of homology-directed recombination mediated by guideRNA (gRNA) complementary to the deletion mutant is significantly higher than the one mediated by gRNA complementary to the wild-type (WT) donor. In addition, the rate of homology-directed recombination mediated by gRNA complementary to the WT donor is significantly higher than that of gRNAs complementary to the 5' or 3' side of the deletion mutant. Interestingly, the frequency of mutations introduced by gRNA complementary to the deletion mutant is significantly higher than with gRNA complementary to WT donor. However, combination of gRNAs complementary to both WT donor and deletion mutant decreased the rate of homology-directed recombination, but did not significantly decrease the mutation rate introduced by this system. Thus, the data presented here provide guidance for designing of gRNA and donor DNA to do genome editing, especially to correct the mutations with three mismatched nucleotides, such as three-nucleotide deletion or insertion.

Ferrocene-embedded flavonoids targeting the Achilles heel of multidrug-resistant cancer cells through collateral sensitivity

With the aim to develop anticancer agents acting selectively against resistant tumor cells, we investigated ferrocene embedded into chalcone, aurone and flavone skeletons. These compounds were conceived and then investigated based on the concept of collateral sensitivity, where the target is the Achilles Heel of cancer cells overexpressing the multidrug ABC transporter MRP1. The 14 synthesized compounds were evaluated for their ability to induce efflux of glutathione (GSH) from tumor cells overexpressing MRP1. When tested at 5 and 20 μM, at least one compound from each series was found to be a highly inducer of GSH efflux. The different compounds inducing a high efflux of GSH were evaluated on both sensitive and resistant cell lines, and two of them, belonging to the flavones class were found to be more cytotoxic on resistant cancer cells, with the best selectivity ratio >9.1. Our results bring chemical and biological bases for further optimization.

Specific stabilization of CFTR by phosphatidylserine

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR, ABCC7) is a plasma membrane chloride ion channel in the ABC transporter superfamily. CFTR is a key target for cystic fibrosis drug development, and its structural elucidation would advance those efforts. However, the limited in vivo and in vitro stability of the protein, particularly its nucleotide binding domains, has made structural studies challenging. Here we demonstrate that phosphatidylserine uniquely stimulates and thermally stabilizes the ATP hydrolysis function of purified human CFTR. Among several lipids tested, the greatest stabilization was observed with brain phosphatidylserine, which shifted the Tm for ATPase activity from 22.7±0.8°C to 35.0±0.2°C in wild-type CFTR, and from 26.6±0.7°C to 42.1±0.2°C in a more stable mutant CFTR having deleted regulatory insertion and S492P/A534P/I539T mutations. When ATPase activity was measured at 37°C in the presence of brain phosphatidylserine, Vmax for wild-type CFTR was 240±60nmol/min/mg, a rate higher than previously reported and consistent with rates for other purified ABC transporters. The significant thermal stabilization of CFTR by phosphatidylserine may be advantageous in future structural and biophysical studies of CFTR.

Flavonoid dimers are highly potent killers of multidrug resistant cancer cells overexpressing MRP1

MRP1 overexpression in multidrug-resistant cancer cells has been shown to be responsible for collateral sensitivity to some flavonoids that stimulate a huge MRP1-mediated GSH efflux. This massive GSH depletion triggers the death of these cancer cells. We describe here that bivalent flavonoid dimers strikingly stimulate such MRP1-mediated GSH efflux and trigger a 50-100 fold more potent cell death than their corresponding monomers. This selective and massive cell death of MRP1-overexpressing cells (both transfected and drug-selected cell lines) is no longer observed either upon catalytic inactivation of MRP1 or its knockdown by siRNA. The best flavonoid dimer, 4e, kills MRP1-overexpressing cells with a selective ratio higher than 1000 compared to control cells and an EC₅₀ value of 0.1 μM, so far unequaled as a collateral sensitivity agent targeting ABC transporters. This result portends the flavonoid dimer 4e as a very promising compound to appraise in vivo the therapeutic potential of collateral sensitivity for eradication of MRP1-overexpressing chemoresistant cancer cells in tumors.

Expression of the cereblon binding protein argonaute 2 plays an important role for multiple myeloma cell growth and survival

BACKGROUND

Immunomodulatory drugs (IMiDs), such as lenalidomide, are therapeutically active compounds that bind and modulate the E3 ubiquitin ligase substrate recruiter cereblon, thereby affect steady-state levels of cereblon and cereblon binding partners, such as ikaros and aiolos, and induce many cellular responses, including cytotoxicity to multiple myeloma (MM) cells. Nevertheless, it takes many days for MM cells to die after IMiD induced depletion of ikaros and aiolos and thus we searched for other cereblon binding partners that participate in IMiD cytotoxicity.

METHODS

Cereblon binding partners were identified from a MM cell line expressing histidine-tagged cereblon by pulling down cereblon and its binding partners and verified by co-immunoprecipitation. IMiD effects were determined by western blot analysis, cell viability assay, microRNA array and apoptosis analysis.

RESULTS

We identified argonaute 2 (AGO2) as a cereblon binding partner and found that the steady-state levels of AGO2 were regulated by cereblon. Upon treatment of IMiD-sensitive MM cells with lenalidomide, the steady-state levels of cereblon were significantly increased, whereas levels of AGO2 were significantly decreased. It has been reported that AGO2 plays a pivotal role in microRNA maturation and function. Interestingly, upon treatment of MM cells with lenalidomide, the steady-state levels of microRNAs were significantly altered. In addition, silencing of AGO2 in MM cells, regardless of sensitivity to IMiDs, significantly decreased the levels of AGO2 and microRNAs and massively induced cell death.

CONCLUSION

These results support the notion that the cereblon binding partner AGO2 plays an important role in regulating MM cell growth and survival and AGO2 could be considered as a novel drug target for overcoming IMiD resistance in MM cells.

Functional and pharmacological induced structural changes of the cystic fibrosis transmembrane conductance regulator in the membrane solved using SAXS

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is a membrane-integral protein that belongs to the ATP-binding cassette superfamily. Mutations in the CFTR gene cause cystic fibrosis in which salt, water, and protein transports are defective in various tissues. To investigate the conformation of the CFTR in the membrane, we applied the small-angle x-ray scattering (SAXS) technique on microsomal membranes extracted from NIH/3T3 cells permanentely transfected with wild-type (WT) CFTR and with CFTR carrying the ΔF508 mutation. The electronic density profile of the membranes was calculated from the SAXS data, assuming the lipid bilayer electronic density to be composed by a series of Gaussian shells. The data indicate that membranes in the microsome vesicles, that contain mostly endoplasmic reticulum membranes, are oriented in the outside-out conformation. Phosphorylation does not change significantly the electronic density profile, while dephosphorylation produces a significant modification in the inner side of the profile. Thus, we conclude that the CFTR and its associated protein complex in microsomes are mostly phosphorylated. The electronic density profile of the ΔF508-CFTR microsomes is completely different from WT, suggesting a different assemblage of the proteins in the membranes. Low-temperature treatment of cells rescues the ΔF508-CFTR protein, resulting in a conformation that resembles the WT. Differently, treatment with the corrector VX-809 modifies the electronic profile of ΔF508-CFTR membrane, but does not recover completely the WT conformation. To our knowledge, this is the first report of a direct physical measurement of the structure of membranes containing CFTR in its native environment and in different functional and pharmacological conditions.

Structure and function of the bacterial heterodimeric ABC transporter CydDC: stimulation of ATPase activity by thiol and heme compounds

In Escherichia coli, the biogenesis of both cytochrome bd-type quinol oxidases and periplasmic cytochromes requires the ATP-binding cassette-type cysteine/GSH transporter, CydDC. Recombinant CydDC was purified as a heterodimer and found to be an active ATPase both in soluble form with detergent and when reconstituted into a lipid environment. Two-dimensional crystals of CydDC were analyzed by electron cryomicroscopy, and the protein was shown to be made up of two non-identical domains corresponding to the putative CydD and CydC subunits, with dimensions characteristic of other ATP-binding cassette transporters. CydDC binds heme b. Detergent-solubilized CydDC appears to adopt at least two structural states, each associated with a characteristic level of bound heme. The purified protein in detergent showed a weak basal ATPase activity (approximately 100 nmol P_i/min/mg) that was stimulated ∼3-fold by various thiol compounds, suggesting that CydDC could act as a thiol transporter. The presence of heme (either intrinsic or added in the form of hemin) led to a further enhancement of thiol-stimulated ATPase activity, although a large excess of heme inhibited activity. Similar responses of the ATPase activity were observed with CydDC reconstituted into E. coli lipids. These results suggest that heme may have a regulatory role in CydDC-mediated transmembrane thiol transport.

Identification of cereblon-binding proteins and relationship with response and survival after IMiDs in multiple myeloma

Cereblon (CRBN) mediates immunomodulatory drug (IMiD) action in multiple myeloma (MM). Using 2 different methodologies, we identified 244 CRBN binding proteins and established relevance to MM biology by changes in their abundance after exposure to lenalidomide. Proteins most reproducibly binding CRBN (>fourfold vs controls) included DDB1, CUL4A, IKZF1, KPNA2, LTF, PFKL, PRKAR2A, RANGAP1, and SHMT2. After lenalidomide treatment, the abundance of 46 CRBN binding proteins decreased. We focused attention on 2 of these-IKZF1 and IKZF3. IZKF expression is similar across all MM stages or subtypes; however, IKZF1 is substantially lower in 3 of 5 IMiD-resistant MM cell lines. The cell line (FR4) with the lowest IKZF1 levels also harbors a damaging mutation and a translocation that upregulates IRF4, an IKZF target. Clinical relevance of CRBN-binding proteins was demonstrated in 44 refractory MM patients treated with pomalidomide and dexamethasone therapy in whom low IKZF1 gene expression predicted lack of response (0/11 responses in the lowest expression quartile). CRBN, IKZF1, and KPNA2 levels also correlate with significant differences in overall survival. Our study identifies CRBN-binding proteins and demonstrates that in addition to CRBN, IKZF1, and KPNA2, expression can predict survival outcomes.

Efficient purification and reconstitution of ATP binding cassette transporter B6 (ABCB6) for functional and structural studies

The mitochondrial ATP binding cassette transporter ABCB6 has been associated with a broad range of physiological functions, including growth and development, therapy-related drug resistance, and the new blood group system Langereis. ABCB6 has been proposed to regulate heme synthesis by shuttling coproporphyrinogen III from the cytoplasm into the mitochondria. However, direct functional information of the transport complex is not known. To understand the role of ABCB6 in mitochondrial transport, we developed an in vitro system with pure and active protein. ABCB6 overexpressed in HEK293 cells was solubilized from mitochondrial membranes and purified to homogeneity. Purified ABCB6 showed a high binding affinity for MgATP (Kd = 0.18 μM) and an ATPase activity with a Km of 0.99 mM. Reconstitution of ABCB6 into liposomes allowed biochemical characterization of the ATPase including (i) substrate-stimulated ATPase activity, (ii) transport kinetics of its proposed endogenous substrate coproporphyrinogen III, and (iii) transport kinetics of substrates identified using a high throughput screening assay. Mutagenesis of the conserved lysine to alanine (K629A) in the Walker A motif abolished ATP hydrolysis and substrate transport. These results suggest a direct interaction between mitochondrial ABCB6 and its transport substrates that is critical for the activity of the transporter. Furthermore, the simple immunoaffinity purification of ABCB6 to near homogeneity and efficient reconstitution of ABCB6 into liposomes might provide the basis for future studies on the structure/function of ABCB6.

Mechanisms of verapamil-enhanced chemosensitivity of gallbladder cancer cells to platinum drugs: glutathione reduction and MRP1 downregulation

Gallbladder cancer (GBC) is highly malignant with a low response rate after chemotherapy and platinum drugs are currently prominent in the treatment of biliary tract cancers. Therefore, the development of novel strategies to enhance the sensitivity of GBC to platinum drugs is required. In the present study, we examined the effects of verapamil, a classic chemosensitizer whose reported mechanisms of action include inhibiting the transport function of P-glycoprotein (MDR1) or stimulating glutathione (GSH) transport by multidrug resistance-related protein 1 (MRP1), in combination with cisplatin (CDDP), carboplatin (CBP) or oxaliplatin on the GBC cell lines, SGC996 and GBC-SD. Our results demonstrated that the co-treatment with verapamil markedly enhanced the chemosensitivity of GBC cells in comparison with platinum drug treatment alone. The mechanisms involved included GSH reduction and MRP1 downregulation. Verapamil/CDDP co-treatment inhibited tumor xenograft growth via the downregulation of MRP1 expression. MRP1 was highly expressed in human GBC tissue compared to non-tumorous gallbladder tissue. Our data demonstrate that verapamil may be used as a safe chemosensitizer for platinum drugs in the treatment of GBC. It functions by ROS and ATP-binding cassette transporter-related mechanisms.

Long term myriocin treatment increases MRP1 transport activity

We investigated the effect of myriocin treatment, which extensively depletes sphingolipids from cells, on multidrug resistance-related protein 1 (MRP1) efflux activity in MRP1 expressing cells and isolated plasma membrane vesicles. Our data reveal that both short term (3 days) and long term (7 days) treatment effectively reduce the cellular sphingolipid content to the same level. Intriguingly, a two-fold increase in MRP1-mediated efflux activity was observed following long term treatment, while short term treatment had no impact. Very similar data were obtained with plasma membrane vesicles isolated from myriocin-treated cells. Exploiting the cell-free vesicle system, Michaelis-Menten analysis revealed that the intrinsic MRP1 activity remained unaltered; however, the fraction of active transporter molecules increased. We demonstrate that the latter effect is due to an enhanced recruitment of MRP1 into lipid raft fractions, thereby promoting MRP1 activity.

Diallyl sulfide, diallyl disulfide and diallyl trisulfide affect drug resistant gene expression in colo 205 human colon cancer cells in vitro and in vivo

To elevate chemo-resistance of human cancer cells is a major obstacle in the treatment and management of malignant cancers. Diallyl sulfide (DAS), diallyl disulfide (DADS) and diallyl trisulfide (DATS) are presented in the Alliaceae family particularly in garlic. Although DAS, DADS and DATS have been shown to exhibit anticancer activities, there is little information on effects of these compounds on drug resistant genes in human colon cancer cells in vitro and in vivo. Herein, we are the first to show that DAS, DADS and DATS at 25 μM for 24-h and 48-h incubations promoted expression of drug resistant genes in colo 205 human colon cancer cells. In vitro experiments indicated that DATS promoted gene expression of multidrug resistant 1 (Mdr1) (p<0.05), and DAS and DADS promoted MRP3 gene expression and DATS alone stimulated gene expression of multidrug resistance-associated protein-1 (MRP1) (p<0.05) in colo 205 cells. In vivo studies demonstrated that DADS and DATS induced Mdr1 and MRP1 gene expression (p<0.05). DADS promoted MRP3 gene expression (p<0.05) as well as DADS and DATS increased MRP4 and MRP6 gene expression (p<0.05) in the colo 205 xenograft mice. Based on our in vitro and in vivo results, diallyl polysulfides (DAS, DADS and DATS) affected the gene expression of the multidrug resistance in colo 205 human colon cancer cells in vitro and in vivo.

Function of MRP1/ABCC1 is not dependent on cholesterol or cholesterol-stabilized lipid rafts

MRP1 (multidrug-resistance-related protein 1)/ABCC1 (ATP-binding cassette transporter C1) has been localized in cholesterol-enriched lipid rafts, which suggests a role for these lipid rafts and/or cholesterol in MRP1 function. In the present study, we have shown for the first time that nearly complete oxidation of free cholesterol in the plasma membrane of BHK-MRP1 (MRP1-expressing baby hamster kidney) cells did not affect MRP1 localization in lipid rafts or its efflux function, using 5-carboxyfluorescein diacetate as a substrate. Inhibition of cholesterol biosynthesis, using lovastatin in combination with RO 48-8071, an inhibitor of oxidosqualene cyclase, resulted in a shift of MRP1 out of lipid raft fractions, but did not affect MRP1-mediated efflux in Neuro-2a (neuroblastoma) cells. Short-term methyl-β-cyclodextrin treatment was equally effective in removing free cholesterol from Neuro-2a and BHK-MRP1 cells, but affected MRP1 function only in the latter. The kinetics of loss of both MRP1 efflux function and lipid raft association during long-term methyl-β-cyclodextrin treatment did not match the kinetics of free cholesterol removal in both cell lines. Moreover, MRP1 activity was measured in vesicles consisting of membranes isolated from BHK-MRP1 cells using the substrate cysteinyl leukotriene C4 and was not changed when the free cholesterol level of these membranes was either decreased or increased. In conclusion, MRP1 activity is not correlated with the level of free cholesterol or with localization in cholesterol-dependent lipid rafts.

Multidrug resistance-related protein 1 (MRP1) function and localization depend on cortical actin

MRP1 (ABCC1) is known to be localized in lipid rafts. Here we show in two different cell lines that localization of Mrp1/MRP1 (Abcc1/ABCC1) in lipid rafts and its function as an efflux pump are dependent on cortical actin. Latrunculin B disrupts both cortical actin and actin stress fibers. This results in partial loss of actin and Mrp1/MRP1 (Abcc1/ABCC1) from detergent-free lipid raft fractions, partial internalization of Mrp1/MRP1 (Abcc1/ABCC1), and reduction of Mrp1/MRP1 (Abcc1/ABCC1)-mediated efflux. Pretreatment with nocodazole prevents latrunculin B-induced loss of cortical actin and all effects of latrunculin B on Mrp1 (Abcc1) localization and activity. However, pretreatment with tyrphostin A23 does not prevent latrunculin B-induced loss of cortical actin, lipid raft association, and efflux activity, but it does prevent latrunculin B-induced internalization of Mrp1 (Abcc1). Cytochalasin D disrupts actin stress fibers but not cortical actin and this inhibitor much less affects Mrp1/MRP1 (Abcc1/ABCC1) localization in lipid rafts, internalization, and efflux activity. In conclusion, cortical actin disruption results in reduced Mrp1/MRP1 (Abcc1/ABCC1) activity concomitant with a partial shift of Mrp1/MRP1 (Abcc1/ABCC1) out of lipid raft fractions and partial internalization of the ABC transporter. The results suggest that reduced Mrp1 (Abcc1) function is correlated to the loss of lipid raft association but not internalization of Mrp1 (Abcc1).

Extensive sphingolipid depletion does not affect lipid raft integrity or lipid raft localization and efflux function of the ABC transporter MRP1

We show that highly efficient depletion of sphingolipids in two different cell lines does not abrogate the ability to isolate Lubrol-based DRMs (detergent-resistant membranes) or detergent-free lipid rafts from these cells. Compared with control, DRM/detergent-free lipid raft fractions contain equal amounts of protein, cholesterol and phospholipid, whereas the classical DRM/lipid raft markers Src, caveolin-1 and flotillin display the same gradient distribution. DRMs/detergent-free lipid rafts themselves are severely depleted of sphingolipids. The fatty acid profile of the remaining sphingolipids as well as that of the glycerophospholipids shows several differences compared with control, most prominently an increase in highly saturated C(16) species. The glycerophospholipid headgroup composition is unchanged in sphingolipid-depleted cells and cell-derived detergent-free lipid rafts. Sphingolipid depletion does not alter the localization of MRP1 (multidrug-resistance-related protein 1) in DRMs/detergent-free lipid rafts or MRP1-mediated efflux of carboxyfluorescein. We conclude that extensive sphingolipid depletion does not affect lipid raft integrity in two cell lines and does not affect the function of the lipid-raft-associated protein MRP1.

A distinct mechanism for the ABC transporter BtuCD-BtuF revealed by the dynamics of complex formation

ATP-binding cassette (ABC) transporters are integral membrane proteins that translocate a diverse array of substrates across cell membranes. We present here the dynamics of complex formation of three structurally characterized ABC transporters-the BtuCD vitamin B(12) importer and MetNI d/l-methionine importer from Escherichia coli and the Hi1470/1 metal-chelate importer from Haemophilus influenzae-in complex with their cognate binding proteins. Similarly to other ABC importers, MetNI interacts with its binding protein with low affinity (K(d) approximately 10(-4) M). In contrast, BtuCD-BtuF and Hi1470/1-Hi1472 form stable, high-affinity complexes (K(d) approximately 10(-13) and 10(-9) M, respectively). In BtuCD-BtuF, vitamin B(12) accelerates the complex dissociation rate approximately 10(7)-fold, with ATP having an additional destabilizing effect. The findings presented here highlight substantial mechanistic differences between BtuCD-BtuF, and likely Hi1470/1-Hi1472, and the better-characterized maltose and related ABC transport systems, indicating that there is considerable mechanistic diversity within this large protein super-family.

Molecular mechanism of ATP-dependent solute transport by multidrug resistance-associated protein 1

Millions of new cancer patients are diagnosed each year and over half of these patients die from this devastating disease. Thus, cancer causes a major public health problem worldwide. Chemotherapy remains the principal mode to treat many metastatic cancers. However, occurrence of cellular multidrug resistance (MDR) prevents efficient killing of cancer cells, leading to chemotherapeutic treatment failure. Over-expression of ATP-binding cassette transporters, such as P-glycoprotein, breast cancer resistance protein and/or multidrug resistance-associated protein 1 (MRP1), confers an acquired MDR due to their capabilities of transporting a broad range of chemically diverse anticancer drugs across the cell membrane barrier. In this review, the molecular mechanism of ATP-dependent solute transport by MRP1 will be addressed.

Effects of putative catalytic base mutation E211Q on ABCG2-mediated methotrexate transport

ABCG2 is a half-ATP binding cassette (ABC) drug transporter that consists of a nucleotide binding domain (NBD) followed by a transmembrane domain. This half-ABC transporter is thought to form a homodimer in the plasma membrane where it transports anticancer drugs across the biological membranes in an ATP-dependent manner. Substitution of the putative catalytic residue E211 with a nonacidic amino acid glutamine (E211Q) completely abolished its ATPase activity and ATP-dependent methotrexate transport, suggesting that ATP hydrolysis is required for the ATP-dependent solute transport. However, whether one ATP hydrolysis or two ATP hydrolyses in the homodimer of ABCG2 with the NBD.ATP.ATP.NBD sandwich structure is/are required for the ATP-dependent solute transport is not known yet. To address this question, we have made an YFP/ABCG2 fusion protein and expressed this 99 kDa fusion protein alone or along with the 70 kDa E211Q-mutated ABCG2 in BHK cells. Although membrane vesicles prepared from BHK cells expressing YFP/ABCG2 exert higher ATPase activity than that of wt ABCG2, the dATP-dependent methotrexate transport activities of these two proteins are the same. Interestingly, membrane vesicles prepared from BHK cells expressing both YFP/ABCG2 and E211Q-mutated ABCG2 (with a ratio of 1:1) form homodimers and heterodimer and exert 55% of wt ABCG2 ATPase activity that can be further enhanced by anticancer drugs, suggesting that the wt NBD in the heterodimer of YFP/ABCG2 and E211Q may be able to hydrolyze ATP. Furthermore, the membrane vesicles containing both YFP/ABCG2 and E211Q exert approximately 79% of wt ABCG2-mediated methotrexate transport activity, implying that the heterodimer harboring YFP/ABCG2 and E211Q may be able to transport the anticancer drug methotrexate across the biological membranes.

Characterization of the ATPase activity of a novel chimeric fusion protein consisting of the two nucleotide binding domains of MRP1

Nucleotide Binding Domains (NBDs) are responsible for the ATPase activity of the multidrug resistance protein 1 (MRP1). A series of NBD1-linker-NBD2 chimeric fusion proteins were constructed, expressed and purified, and their ATPase activities were analyzed. We report here that a GST linked NBD1(642-890)-GST-NBD2(1286-1531) was able to hydrolyze ATP at a rate of about 4.6 nmol/mg/min (K(m)=2.17 mM, V(max)=12.36 nmol/mg/min), which was comparable to the purified and reconstituted MRP1. In contrast, neither a mixture of NBD1 and GST-NBD2 nor the NBD1-GST-NBD1 fusion protein showed detectable ATPase activity. Additionally, the E1455Q mutant was found to be nonfunctional. Measurements by both MIANS labeling and circular dichroism spectroscopy revealed significant conformational differences in the NBD1-GST-NBD2 chimeric fusion protein compared to the mixture of NBD1 and GST-NBD2. The results suggest a direct interaction mediated by GST between the two NBDs of MRP1 leading to conformational changes which would enhance its ATPase activity.

Purification and structural analyses of ABCG2

ABCG2 is best known as a multidrug transporter capable of conferring resistance to cancer cells. However, the protein is also inherently expressed in numerous barrier tissues and intriguingly within hematopoietic stem cells. Unlike its partners ABCB1 and ABCC1, there is considerably less information available on the molecular mechanism of ABCG2. The transporter has a distinct topology and is presumed to function as a homodimer. However, a number of biochemical studies have presented data to suggest that the protein adopts higher order oligomers. This review focuses on this controversial issue with particular reference to findings from low resolution structural data. In addition, a number of molecular models of ABCG2 based on high resolution structures of bacterial ABC transporters have recently become available and are critically assessed. ABCG2 is a structurally distinct member of the triumvirate of human multidrug transporters and continues to evade description of a unifying molecular mechanism.

Perturbation of the lipid phase of a membrane is not involved in the modulation of MRP1 transport activity by flavonoids

The expression of transmembrane transporter multidrug resistance-associated protein 1 (MRP1) confers the multidrug-resistant phenotype (MDR) on cancer cells. Since the activity of the other MDR transporter, P-glycoprotein, is sensitive to membrane perturbation, we aimed to check whether the changes in lipid bilayer properties induced by flavones (apigenin, acacetin) and flavonols (morin, myricetin) were related to their MRP1 inhibitory activity. All the flavonoids inhibited the efflux of MRP1 fluorescent substrate from human erythrocytes and breast cancer cells. Morin was also found to stimulate the ATPase activity of erythrocyte ghosts. All flavonoids intercalated into phosphatidylcholine bilayers as judged by differential scanning calorimetry and fluorescence spectroscopy with the use of two carbocyanine dyes. The model of an intramembrane localization for flavones and flavonols was proposed. No clear relationship was found between the membrane-perturbing activity of flavonoids and their potency to inhibit MRP1. We concluded that mechanisms other than perturbation of the lipid phase of membranes were responsible for inhibition of MRP1 by the flavonoids.

Mechanistic differences between GSH transport by multidrug resistance protein 1 (MRP1/ABCC1) and GSH modulation of MRP1-mediated transport

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-dependent polytopic membrane protein that transports many anticancer drugs and organic anions. Its transport mechanism is multifaceted, especially with respect to the participation of GSH. For example, vincristine is cotransported with GSH, estrone sulfate transport is stimulated by GSH, or MRP1 can transport GSH alone, and this can be stimulated by compounds such as verapamil or apigenin. Thus, the interactions between GSH and MRP1 are mechanistically complex. To examine the similarities and differences among the various GSH-associated mechanisms of MRP1 transport, we have measured first the effect of GSH and several GSH-associated substrates/modulators on the binding and hydrolysis of ATP by MRP1 using 8-azidoadenosine-5'-[(32)P]-triphosphate ([(32)P]azidoATP) analogs, and second the initial binding of GSH and GSH-associated substrates/modulators to MRP1. We observed that GSH or its nonreducing derivative S-methylGSH (S-mGSH), but none of the GSH-associated substrate/modulators, caused a significant increase in [gamma-(32)P]azidoATP labeling of MRP1. Moreover, GSH and S-mGSH decreased levels of orthovanadate-induced trapping of [alpha-(32)P]azidoADP. [alpha-(32)P]azidoADP.Vi trapping was also decreased by estone sulfate, whereas vincristine, verapamil, and apigenin had no apparent effects on nucleotide interactions with MRP1. Furthermore, estrone sulfate and S-mGSH enhanced the effect of each other 15- and 10-fold, respectively. Second, although GSH binding increased the apparent affinity of MRP1 for all GSH-associated substrates/modulators tested, only estrone sulfate had a reciprocal effect on the apparent affinity of MRP1 for GSH. Overall, these results indicate significant mechanistic differences between MRP1-mediated transport of GSH and the ability of GSH to modulate MRP1 transport.

Interaction between the bound Mg.ATP and the Walker A serine residue in NBD2 of multidrug resistance-associated protein MRP1 plays a crucial role for the ATP-dependent leukotriene C4 transport

Structural analysis of human MRP1-NBD1 revealed that the Walker A S685 forms a hydrogen bond with the Walker B D792 and interacts with the Mg (2+) cofactor and the beta-phosphate of the bound Mg.ATP. We have found that substitution of the S685 with an amino acid that potentially prevents the formation of the hydrogen bond resulted in misfolding of the protein and significantly affect the ATP-dependent leukotriene C4 (LTC4) transport. In this report we tested whether the corresponding substitution in NBD2 would also result in misfolding of the protein. In contrast to the NBD1 mutations, none of the mutations in NBD2, including S1334A, S1334C, S1334D, S1334H, S1334N, and S1334T, caused misfolding of the protein. However, elimination of the hydroxyl group at S1334 in mutations including S1334A, S1334C, S1334D, S1334H, and S1334N drastically reduced the ATP binding and the ATP-enhanced ADP trapping at the mutated NBD2. Due to this low efficient ATP binding at the mutated NBD2, the inhibitory effect of ATP on the LTC4 binding is significantly decreased. Furthermore, ATP bound to the mutated NBD2 cannot be efficiently hydrolyzed, leading to almost completely abolishing the ATP-dependent LTC4 transport. In contrast, S1334T mutation, which retained the hydroxyl group at this position, exerts higher LTC4 transport activity than the wild-type MRP1, indicating that the hydroxyl group at this position plays a crucial role for ATP binding/hydrolysis and ATP-dependent solute transport.

Glutathione-dependent interaction of heavy metal compounds with multidrug resistance proteins MRP1 and MRP2

The interactions of three heavy metal-containing compounds, cisplatin (CDDP), arsenic trioxide (As(2)O(3)), and mercury dichloride (HgCl(2)), with the multidrug resistance transporters MRP1 and MRP2 and the involvement of glutathione (GSH)-related processes herein were investigated. In Madin-Darby canine kidney cells stably expressing MRP1 or MRP2, viability, GSH content, calcein efflux and polarized GSH efflux were measured as a function of exposure to CDDP, As(2)O(3) and HgCl(2). In isolated Sf9-MRP1 and Sf9-MRP2 membrane vesicles, the interaction with MRP-associated ATPase activity was measured. In the latter model system adduct formation with GSH is not an issue. The data show that (1) CDDP interacts with both MRP1 and MRP2, and GSH appears to play no major role in this process, (2) As(2)O(3) interacts with MRP1 and MRP2 in which process GSH seems to be essential, and (3) HgCl(2) interacts with MRP1 and MRP2, either alone and/or as a metal-GSH complex.

Role of proline 1150 in functional interactions between the membrane spanning domains and nucleotide binding domains of the MRP1 (ABCC1) transporter

The ATP-binding cassette multidrug resistance protein 1 (MRP1) mediates ATP-dependent cellular efflux of drugs and organic anions. We previously described a mutant, MRP1-Pro1150Ala, which exhibits selectively increased estradiol glucuronide (E217betaG) and methotrexate transport as well as altered interactions with ATP. We have now further explored the functional importance of MRP1-Pro1150 at the interface of transmembrane helix 15 and cytoplasmic loop 7 (CL7) by replacing it with Gly, Ile, Leu and Val. All four mutants exhibited a phenotype similar to MRP1-Pro1150Ala with respect to organic anion transport and [gamma32P]8N3ATP photolabeling. They also displayed very low levels of substrate-independent vanadate-induced trapping of [alpha32P]8N3ADP. To better understand the relationship between the altered nucleotide interactions and transport activity of these mutants, [alpha32P]8N3ADP trapping experiments were performed under different conditions. Unlike leukotriene C4, E217betaG decreased [alpha32P]8N3ADP trapping by both wild-type and mutant MRP1. [alpha32P]8N3ADP trapping by MRP1-Pro1150Ala could be increased by using Ni2+ instead of Mg2+, and by decreasing temperature; however, the transport properties of the mutant remained unchanged. We conclude that the reduced [alpha32P]8N3ADP trapping associated with loss of Pro1150, or the presence of E217betaG, is due to enhanced ADP release following ATP hydrolysis rather than a reduction in ATP hydrolysis itself. We hypothesize that loss of Pro1150 alters the role of CL7 as a coupling helix that mediates signaling between the nucleotide binding domains and some substrate binding sites in the membrane spanning domains of MRP1.

The hydroxyl group of S685 in Walker A motif and the carboxyl group of D792 in Walker B motif of NBD1 play a crucial role for multidrug resistance protein folding and function

Structural analysis of MRP1-NBD1 revealed that the Walker A S685 forms hydrogen-bond with the Walker B D792 and interacts with magnesium and the beta-phosphate of the bound ATP. We have found that substitution of the D792 with leucine resulted in misfolding of the protein. In this report we tested whether substitution of the S685 with residues that prevent formation of this hydrogen-bond would also cause misfolding. Indeed, substitution of the S685 with residues potentially preventing formation of this hydrogen-bond resulted in misfolding of the protein. In addition, some substitutions that might form hydrogen-bond with D792 also yielded immature protein. All these mutants are temperature-sensitive variants. However, these complex-glycosylated mature mutants prepared from the cells grown at 27 degrees C still significantly affect ATP binding and ATP-dependent solute transport. In contrast, substitution of the S685 with threonine yielded complex-glycosylated mature protein that is more active than the wild-type MRP1, indicating that the interaction between the hydroxyl group of 685 residue and the carboxyl group of D792 plays a crucial role for the protein folding and the interactions of the hydroxyl group at 685 with magnesium and the beta-phosphate of the bound ATP play an important role for ATP-binding and ATP-dependent solute transport.

(R)- and (S)-verapamil differentially modulate the multidrug-resistant protein MRP1

The multidrug-resistant protein MRP1 (involved in the cancer cell multidrug resistance phenotype) has been found to be modulated by racemic verapamil (through stimulation of glutathione transport), inducing apoptosis of human MRP1 cDNA-transfected baby hamster kidney 21 (BHK-21) cells and not of control BHK-21 cells. In this study, we show that the two enantiomers of verapamil have different effects on MRP1 activity. Only the S-isomer (not the R-isomer) potently induced the death of MRP1-transfected BHK-21 cells. The decrease in cellular glutathione content induced by the S-isomer, which was not observed with the R-isomer, was stronger than that induced by the racemic mixture, indicating that the R-isomer antagonized the S-isomer effect. Both enantiomers altered leukotriene C(4) and calcein transport by MRP1. Thus, the R-isomer behaved as an inhibitor, which was confirmed by its ability to revert the multidrug resistance phenotype toward vincristine. Molecular studies on purified MRP1 using fluorescence spectroscopy showed that both enantiomers bound to MRP1 with high affinity, with the binding being prevented by glutathione. Furthermore, conformational changes induced by the two enantiomers (monitored by sodium iodide accessibility of MRP1 tryptophan residues) were quite different, correlating with their distinct effects. (S)-Verapamil induces the death of potentially resistant tumor cells, whereas (R)-verapamil sensitizes MRP1-overexpressing cells to chemotherapeutics. These results might be of great potential interest in the design of new compounds able to modulate MRP1 in chemotherapy.

A molecular understanding of ATP-dependent solute transport by multidrug resistance-associated protein MRP1

Over a million new cases of cancers are diagnosed each year in the United States and over half of these patients die from these devastating diseases. Thus, cancers cause a major public health problem in the United States and worldwide. Chemotherapy remains the principal mode to treat many metastatic cancers. However, occurrence of cellular multidrug resistance (MDR) prevents efficient killing of cancer cells, leading to chemotherapeutic treatment failure. Numerous mechanisms of MDR exist in cancer cells, such as intrinsic or acquired MDR. Overexpression of ATP-binding cassette (ABC) drug transporters, such as P-glycoprotein (P-gp or ABCB1), breast cancer resistance protein (BCRP or ABCG2) and/or multidrug resistance-associated protein (MRP1 or ABCC1), confers an acquired MDR due to their capabilities of transporting a broad range of chemically diverse anticancer drugs. In addition to their roles in MDR, there is substantial evidence suggesting that these drug transporters have functions in tissue defense. Basically, these drug transporters are expressed in tissues important for absorption, such as in lung and gut, and for metabolism and elimination, such as in liver and kidney. In addition, these drug transporters play an important role in maintaining the barrier function of many tissues including blood-brain barrier, blood-cerebral spinal fluid barrier, blood-testis barrier and the maternal-fetal barrier. Thus, these ATP-dependent drug transporters play an important role in the absorption, disposition and elimination of the structurally diverse array of the endobiotics and xenobiotics. In this review, the molecular mechanism of ATP-dependent solute transport by MRP1 will be addressed.

Purification and 3D structural analysis of oligomeric human multidrug transporter ABCG2

ABCG2 is a multidrug efflux pump associated with resistance of cancer cells to a plethora of unrelated drugs. ABCG2 is a "half-transporter," and previous studies have indicated that it forms homodimers and higher oligomeric species. In this manuscript, electron microscopic structural analysis directly addressed this issue. An N-terminal hexahistidine-tagged ABCG2(R482G) isoform was expressed to high levels in insect cells. An extensive detergent screen was employed to effect extraction of ABCG2(R482G) from membranes and identified only the fos-choline detergents as efficient. Soluble protein was purified to >95% homogeneity by a three-step procedure while retaining the ability to bind substrates. Cryonegative stain electron microscopy of purified ABCG2(R482G) provided 3D structural data at a resolution of approximately 18 A. Single-particle analysis revealed that the complex forms a tetrameric complex ( approximately 180 A in diameter x approximately 140 A high) with an aqueous central region. We interpret the tetrameric structure as comprising four homodimeric ABCG2(R482G) complexes.

Misassembled mutant DeltaF508 CFTR in the distal secretory pathway alters cellular lipid trafficking

Most patients with cystic fibrosis (CF) have a single codon deletion (DeltaF508) in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) that impairs assembly of the multidomain glycoprotein. The mutant protein escapes endoplasmic reticulum (ER) quality control at low temperature, but is rapidly cleared from the distal secretory pathway and degraded in lysosomes. CF cells accumulate free cholesterol similar to Niemann-Pick disease type C cells. We show that this lipid alteration is caused by the presence of misassembled mutant CFTR proteins, including DeltaF508, in the distal secretory pathway rather than the absence of functional CFTR. By contrast, cholesterol distribution is not changed by either D572N CFTR, which does not mature even at low temperature, or G551D, which is processed normally but is inactive. On expression of the DeltaF508 mutant, cholesterol and glycosphingolipids accumulate in punctate endosomal structures and cholesterol esters are reduced, indicating a block in the translocation of cholesterol to the ER for esterification. This is overcome by Rab9 overexpression, resulting in clearance of accumulating intracellular cholesterol. Similar but less pronounced alterations in intracellular cholesterol distribution are observed on expression of a temperature-rescued mutant variant of the related ATP-binding cassette (ABC) protein multidrug resistance-associated protein 1 (MRP1). Thus, on escape from ER quality control, misassembled mutants of CFTR and MRP1 impair lipid homeostasis in endocytic compartments.

Structural and functional fingerprint of the mitochondrial ATP-binding cassette transporter Mdl1 from Saccharomyces cerevisiae

The ATP-binding cassette half-transporter Mdl1 from Saccharomyces cerevisiae has been proposed to be involved in the quality control of misassembled respiratory chain complexes by exporting degradation products generated by the m-AAA proteases from the matrix. Direct functional or structural data of the transport complex are, however, not known so far. After screening expression in various hosts, Mdl1 was overexpressed 100-fold to 1% of total mitochondrial membrane protein in S. cerevisiae. Based on detergent screens, Mdl1 was solubilized and purified to homogeneity. Mdl1 showed a high binding affinity for MgATP (Kd = 0.26 microm) and an ATPase activity with a Km of 0.86 mm (Hill coefficient of 0.98) and a turnover rate of 2.6 ATP/s. Mutagenesis of the conserved glutamate downstream of the Walker B motif (E599Q) or the conserved histidine of the H-loop (H631A) abolished ATP hydrolysis, whereas ATP binding was not affected. Mdl1 reconstituted into liposomes showed an ATPase activity similar to the solubilized complex. By single particle electron microscopy, a first three-dimensional structure of the mitochondrial ATP-binding cassette transporter was derived at 2.3-nm resolution, revealing a homodimeric complex in an open conformation.

Hydrogen-bond formation of the residue in H-loop of the nucleotide binding domain 2 with the ATP in this site and/or other residues of multidrug resistance protein MRP1 plays a crucial role during ATP-dependent solute transport

MRP1 couples ATP binding/hydrolysis to solute transport. We have shown that ATP binding to nucleotide-binding-domain 1 (NBD1) plays a regulatory role whereas ATP hydrolysis at NBD2 plays a crucial role in ATP-dependent solute transport. However, how ATP is hydrolyzed at NBD2 is not well elucidated. To partially address this question, we have mutated the histidine residue in H-loop of MRP1 to either a residue that prevents the formation of hydrogen-bonds with ATP and other residues in MRP1 or a residue that may potentially form these hydrogen-bonds. Interestingly, substitution of H827 in NBD1 with residues that prevented formation of these hydrogen-bonds had no effect on the ATP-dependent solute transport whereas corresponding mutations in NBD2 almost abolished the ATP-dependent solute transport completely. In contrast, substitutions of H1486 in H-loop of NBD2 with residues that might potentially form these hydrogen-bonds exerted either full function or partial function, implying that hydrogen-bond formation between the residue at 1486 and the gamma-phosphate of the bound ATP and/or other residues, such as putative catalytic base E1455, together with S769, G771, T1329 and K1333, etc., holds all the components necessary for ATP binding/hydrolysis firmly so that the activated water molecule can efficiently hydrolyze the bound ATP at NBD2.

ATP-binding and -hydrolysis activities of ALDP (ABCD1) and ALDRP (ABCD2), human peroxisomal ABC proteins, overexpressed in Sf21 cells

The peroxisomal ATP-binding cassette (ABC) proteins, adrenoleukodystrophy protein (ALDP, ABCD1) and ALD-related protein (ALDRP, ABCD2), were expressed in Spodoptera frugiperda 21 (Sf21) insect cells using a baculovirus-mediated expression system. Immunoelectron microscopy and subcellular fractionation revealed that the overexpressed ALDP was distributed in various subcellular organelles including mitochondria, nucleus and peroxisomes. The ALDP was not extractable with Na(2)CO(3) treatment, suggesting that it integrated into membranes. ATPase activity was detected in the membrane fraction expressing ALDP. The nucleotide-binding capacities of the expressed ALDP were estimated by the binding to ATP- or ADP-agarose. ALDP exhibited an affinity to both ADP and ATP. In contrast, ALDRP exhibited an affinity to ADP but scarcely to ATP. The ALDP in the Sf21 membrane fraction was extracted with n-dodecyl-beta-maltoside and successively purified with a chelate column. The nucleotide-binding and ATPase activities of the purified ALDP were, however, not detected. It may be that certain membranous components are required for the activity. We demonstrate for the first time that the peroxisomal ABC proteins can be expressed in Sf21 membranes maintaining their nucleotide-binding abilities and ATPase activities, and the expressed proteins will be of use for further characterization.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

Replacement of the positively charged Walker A lysine residue with a hydrophobic leucine residue and conformational alterations caused by this mutation in MRP1 impair ATP binding and hydrolysis

MRP1 (multidrug resistance protein 1) couples ATP binding/hydrolysis at its two non-equivalent NBDs (nucleotide-binding domains) with solute transport. Some of the NBD1 mutants, such as W653C, decreased affinity for ATP at the mutated site, but increased the rate of ATP-dependent solute transport. In contrast, other NBD1 mutants, such as K684L, had decreased ATP binding and rate of solute transport. We now report that mutations of the Walker A lysine residue, K684L and K1333L, significantly alter the tertiary structure of the protein. Due to elimination of the positively charged group and conformational alterations, the K684L mutation greatly decreases the affinity for ATP at the mutated NBD1 and affects ATP binding at the unmutated NBD2. Although K684L-mutated NBD1 can bind ATP at higher concentrations, the bound nucleotide at that site is not efficiently hydrolysed. All these alterations result in decreased ATP-dependent solute transport to approx. 40% of the wild-type. In contrast, the K1333L mutation affects ATP binding and hydrolysis at the mutated NBD2 only, leading to decreased ATP-dependent solute transport to approx. 11% of the wild-type. Consistent with their relative transport activities, the amount of vincristine accumulated in cells is in the order of K1333L> or =CFTR (cystic fibrosis transmembrane conductance regulator)>K684L>>>wild-type MRP1. Although these mutants retain partial solute transport activities, the cells expressing them are not multidrug-resistant owing to inefficient export of the anticancer drugs by these mutants. This indicates that even partial inhibition of transport activity of MRP1 can reverse the multidrug resistance caused by this drug transporter.

The ABC transporter BmrA from Bacillus subtilis is a functional dimer when in a detergent-solubilized state

BmrA from Bacillus subtilis is a half-size ABC (ATP-binding cassette) transporter involved in multidrug resistance. Although its supramolecular organization has been investigated after reconstitution in a lipid bilayer environment, and shows a dimeric and possibly a tetrameric form, the precise quaternary structure in a detergent-solubilized state has never been addressed. In the present study, BmrA was purified from Escherichia coli membranes using an optimized purification protocol and different detergents. Furthermore, the ATPase activity of BmrA and the quantity of bound lipids and detergent were determined, and the oligomeric state was analysed using SEC (size-exclusion chromatography) and analytical ultracentrifugation. The activity and the quaternary structure of BmrA appeared to be strongly influenced by the type and concentration of the detergent used. SEC data showed that BmrA could be purified in a functional form in 0.05 and 0.01% DDM (n-dodecyl-beta-D-maltoside) and was homogeneous and monodisperse with an R(s) (Stokes radius) of 5.6 nm that is compatible with a dimer structure. Sedimentation-velocity and equilibrium experiments unequivocally supported that BmrA purified in DDM is a dimer and excluded the presence of other oligomeric states. These observations, which are discussed in relation to results obtained in proteoliposomes, also constitute an important first step towards crystallographic studies of BmrA structure.

Drug resistance-associated changes in sphingolipids and ABC transporters occur in different regions of membrane domains

We have recently shown that two ATP binding cassette (ABC) transporters are enriched in Lubrol-resistant noncaveolar membrane domains in multidrug-resistant human cancer cells [Hinrichs, J. W. J., K. Klappe, I. Hummel, and J. W. Kok. 2004. ATP-binding cassette transporters are enriched in non-caveolar detergent-insoluble glycosphingolipid-enriched membrane domains (DIGs) in human multidrug-resistant cancer cells. J. Biol. Chem. 279: 5734-5738]. Here, we show that aminophospholipids are relatively enriched in Lubrol-resistant membrane domains compared with Triton X-100-resistant membrane domains, whereas sphingolipids are relatively enriched in the latter. Moreover, Lubrol-resistant membrane domains contain more protein and lipid mass. Based on these results, we postulate a model for detergent-insoluble glycosphingolipid-enriched membrane domains consisting of a Lubrol-insoluble/Triton X-100-insoluble region and a Lubrol-insoluble/Triton X-100-soluble region. The latter region contains most of the ABC transporters as well as lipids known to be necessary for their efflux activity. Compared with drug-sensitive cells, the detergent-insoluble glycosphingolipid-enriched membrane domains (DIGs) in drug-resistant cells differ specifically in sphingolipid content and not in protein, phospholipid, or cholesterol content. In drug-resistant cells, sphingolipids with specific fatty acids (especially C24:1) are enriched in these membrane domains. Together, these data show that multidrug resistance-associated changes in both sphingolipids and ABC transporters occur in DIGs, but in different regions of these domains.

Interactions of mefloquine with ABC proteins, MRP1 (ABCC1) and MRP4 (ABCC4) that are present in human red cell membranes

Human erythrocyte membranes express the multidrug resistance-associated proteins, MRP1, MRP4 and 5, that collectively can efflux oxidised glutathione, glutathione conjugates and cyclic nucleotides. It is already known that the quinoline derivative, MK-571, is a potent inhibitor of MRP-mediated transport. We here examine whether the quinoline-based antimalarial drugs, amodiaquine, chloroquine, mefloquine, primaquine, quinidine and quinine, also interact with erythrocyte MRPs with consequences for their access to the intracellular parasites or for efflux of oxidised glutathione from infected cells. Using inside-out vesicles prepared from human erythrocytes we have shown that mefloquine and MK-571 inhibit transport of 3 microM [(3)H]DNP-SG known to be mediated by MRP1 (IC(50) 127 and 1.1 microM, respectively) and of 3.3 microM [(3)H]cGMP thought but not proven to be mediated primarily by MRP4 (IC(50) 21 and 0.41 microM). They also inhibited transport in membrane vesicles prepared from tumour cells expressing MRP1 or MRP4 and blocked calcein efflux from MRP1-overexpressing cells and BCECF efflux from MRP4-overexpressing cells. Both stimulated ATPase activity in membranes prepared from MRP1 and MRP4-overexpressing cells and inhibited activity stimulated by quercetin or PGE(1), respectively. Neither inhibited [alpha-(32)P]8-azidoATP binding confirming that the interactions are not at the ATP binding site. These results demonstrate that mefloquine and MK-571 both inhibit transport of other substrates and stimulate ATPase activity and thus may themselves be substrates for transport. But at concentrations achieved clinically mefloquine is unlikely to affect the MRP1-mediated transport of GSSG across the erythrocyte membrane.