Molecular analysis of the massive GSH transport mechanism mediated by the human Multidrug Resistant Protein 1/ABCC1.

The transporter Multidrug Resistance Protein 1 (MRP1, ABCC1) is implicated in multidrug resistant (MDR) phenotype of cancer cells. Glutathione (GSH) plays a key role in MRP1 transport activities. In addition, a ligand-stimulated GSH transport which triggers the death of cells overexpressing MRP1, by collateral sensitivity (CS), has been described. This CS could be a way to overcome the poor prognosis for patients suffering from a chemoresistant cancer. The molecular mechanism of such massive GSH transport and its connection to the other transport activities of MRP1 are unknown. In this context, we generated MRP1/MRP2 chimeras covering different regions, MRP2 being a close homolog that does not trigger CS. The one encompassing helices 16 and 17 led to the loss of CS and MDR phenotype without altering basal GSH transport. Within this region, the sole restoration of the original G1228 (D1236 in MRP2) close to the extracellular loop between the two helices fully rescued the CS (massive GSH efflux and cell death) but not the MDR phenotype. The flexibility of that loop and the binding of a CS agent like verapamil could favor a particular conformation for the massive transport of GSH, not related to other transport activities of MRP1.

5-Oxo-hexahydroquinoline derivatives as modulators of P-gp, MRP1 and BCRP transporters to overcome multidrug resistance in cancer cells.

Multidrug resistance (MDR) in cancer cells is often associated with overexpression of ATP-binding cassette (ABC) transporters, including P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1) and breast cancer resistance protein (BCRP/ABCG2). Modulators of these transporters might be helpful in overcoming MDR. Moreover, exploiting collateral sensitivity (CS) could be another approach for efficient treatment of cancer. Twelve novel 5-oxo-hexahydroquinoline derivatives bearing different aromatic substitutions at C4, while having 2-pyridyl alkyl carboxylate substituents at the C3 were synthesized and evaluated for MDR reversal activity by flow cytometric determination of rhodamine 123, calcein and mitoxantrone accumulations in P-gp, MRP1 and BCRP-overexpressing cell lines, respectively. Furthermore, to confirm the P-gp inhibitory activity, the effect of compounds on the reduction of doxorubicin's IC50 of drug-resistant human uterine sarcoma cell line, MES-SA/DX5, was evaluated. Compounds D6, D5 and D3 (bearing 3-chlorophenyl, 2,3-dichlorophenyl and 4-chlorophenyl substituents at C4 position of 5-oxo-hexahydroquinoline core) were the most potent P-gp, MRP1 and BCRP inhibitors, respectively, causing significant MDR reversal at concentrations of 1-10 µM. Additionally, D4 (containing 3-flourophenyl) was the most effective MRP1-dependent CS inducing agent. Overall, chlorine containing compounds D6, C4 and D3 were capable of significant inhibition of all 3 important efflux pumps in cancer cells. Moreover, D6 also induced CS triggered by reducing glutathione efflux. In conclusion, some of the 5-oxo-hexahydroquinoline derivatives are effective efflux pump inhibitors capable of simultaneously blocking 3 important ABC transporters involved in MDR, and represent promising agents to overcome MDR in cancer cells.

Methyl-cyclopentadienyl Ruthenium Compounds with 2,2'-Bipyridine Derivatives Display Strong Anticancer Activity and Multidrug Resistance Potential.

New ruthenium methyl-cyclopentadienyl compounds bearing bipyridine derivatives with the general formula [Ru(η5-MeCp)(PPh3)(4,4'-R-2,2'-bpy)]+ (Ru1, R = H; Ru2, R = CH3; and Ru3, R = CH2OH) have been synthesized and characterized by spectroscopic and analytical techniques. Ru1 crystallized in the monoclinic P21/ c, Ru2 in the triclinic P1̅, and Ru3 in the monoclinic P21/ n space group. In all molecular structures, the ruthenium center adopts a "piano stool" distribution. Density functional theory calculations were performed for all complexes, and the results support spectroscopic data. Ru1 and Ru3 were poor substrates of the main multidrug resistance human pumps, ABCB1, ABCG2, ABCC1, and ABCC2, while Ru2 displayed inhibitory properties of ABCC1 and ABCC2 pumps. Importantly, all compounds displayed a very high cytotoxic profile for ovarian cancer cells (sensitive and resistant) that was much more pronounced than that observed with cisplatin, making them very promising anticancer agents.

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.

Cytotoxicity of η6-areneruthenium-based molecules to glioblastoma cells and their recognition by multidrug ABC transporters.

A new series of amphiphilic η6-areneruthenium(II) compounds containing phenylazo ligands (group I: compounds 1a, 1b, 2a and 2b) and phenyloxadiazole ligands (group II: compounds 3a, 3b, 4a and 4b) were synthesized and characterized for their anti-glioblastoma activity. The effects of the amphiphilic η6-areneruthenium(II) complexes on the viability of three human glioblastoma cell lines, U251, U87MG and T98G, were evaluated. The azo-derivative ruthenium complexes (group I) showed high cytotoxicity to all cell lines, whilst most oxadiazole-derivative complexes (group II) were less cytotoxic, except for compound 4a. The cationic complexes 2a, 2b and 4b were more cytotoxic than the neutral complexes. Compounds 2a and 2b caused a significant reduction in the percentage of cells in the G0/G1 phase, with concomitant increases in the G2/M phase and fragmented DNA in the T98G cell line. The η6-areneruthenium(II) compounds were also tested in cell lines that overexpress the multidrug ABC transporters P-gp, MRP1 and ABCG2. Compounds 2b and 4a were substrates for the P-gp protein, with resistance indexes of 8.6 and 1.9, respectively. Compound 2b was also a substrate for ABCG2 and MRP1 proteins, with lower resistance indexes (1.8 and 1.6, respectively). The contribution of multidrug ABC transporters to the cytotoxicity of compound 2b in T98G cells was evidenced, since verapamil (a characteristic inhibitor of MRP1) increased the cytotoxicity of compound 2b at concentrations up to 20 µmol L-1, whilst GF120918 and Ko143 (specific inhibitors of P-gp and ABCG2, respectively) had no significant effect. In addition, we showed that compound 2b interacts with glutathione (GSH), which could explain its cellular efflux by MRP1. Our results showed that the amphiphilic η6-areneruthenium(II) complexes are promising anti-glioblastoma compounds, especially compound 2b, which was cytotoxic for all three cell lines, although it is transported by the three main multidrug ABC transporters.

Monoterpene indole alkaloid azine derivatives as MDR reversal agents.

Aiming at generating a library of bioactive indole alkaloid derivatives as multidrug resistance (MDR) reversers, two epimeric indole alkaloids (1 and 2) were submitted to chemical transformations, giving rise to twenty-four derivatives (5-28), bearing new aromatic or aliphatic azine moieties. The structure of the compounds was established by 1D and 2D NMR (COSY, HMBC, HMQC and NOESY) experiments. Two different strategies were employed for assessing their anti-MDR potential, namely through the evaluation of their activity as inhibitors of typical MDR ABC transporters overexpressed by cell transfection, such as ABCB1 (P-gp), ABCC1 (MRP1), and ABCG2 (BCRP), or by evaluating their ability as collateral sensitivity (CS) agents in cells overexpressing MRP1. A considerable MDR reversing activity was observed for compounds bearing the aromatic azine moiety. The strongest and most selective P-gp inhibition was found for the epimeric azines 5 and 6, bearing a para-methylbenzylidene moiety. Instead, compounds 17 and 18 that possess a di-substituted benzylidene portion with methoxy and hydroxyl groups, selectively inhibited MRP1 drug-efflux. None of these compounds inhibited BCRP. Compounds 5, 6 and 18 were further investigated in drug combination experiments, which corroborated their anti-MDR potential. Moreover, it was observed that compound 12, with an aromatic azine moiety, and compounds 23-26, sharing a new aliphatic substituent, displayed a CS activity, selectively killing MRP1-overexpressing cells. Among these last compounds, it could be established that addition of 19, 23 and 25 to MRP1-overexpressing cells led to glutathione depletion triggering cell death through apoptosis.

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.

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.

MRP1-dependent Collateral Sensitivity of Multidrug-resistant Cancer Cells: Identifying Selective Modulators Inducing Cellular Glutathione Depletion.

Cancer cells are permanently being selected for survival and proliferation. During this process, tumor cells often co-opt basic physiological mechanisms to protect themselves from toxic chemotherapy. One of these mechanisms is the overexpression of ATP-binding cassette (ABC) drug efflux pumps leading to multidrug resistance (MDR) of cancer cells through an increase of drug efflux. In the past 20 years, many efforts were done to circumvent MDR through the inhibition of ABC transporters. A number of inhibitors of these transporters were found but are rarely specific or rationally developed. Beside this approach, a new therapeutic strategy towards eradicating drug resistant tumor cells has recently emerged from the observation that cancer cells expressing a high level of these pumps show an unexpected hypersensitivity, called collateral sensitivity (CS) to a selected subset of chemical compounds. In this review, we target the multidrug resistance protein 1 (MRP1) and after a non-exhaustively highlighting of some of the most exemplary inhibitors of MRP1 and modulators of its expression, we focus on CS agents specifically targeting MRP1 which becomes, when overexpressed, the so called "Achilles' heel" of multidrug resistant cancer cells. We discuss the link between the prominent role of glutathione translocation and related redox balance of the cell and the CS induced by certain types of compounds. The latter are discussed according to their chemical class, and perspectives in their development for successful eradication of resistant cancer are proposed.

2-Indolylmethylenebenzofuranones as first effective inhibitors of ABCC2.

ABC-transporters play a vital role in drugs bioavailability. They prevent intracellular accumulation of toxic compounds, rendering them a major defense mechanism against harmful substances. In this large family, ABCC2 is an apical efflux pump representing about 10% of all membrane proteins in liver and small intestine, and up to 25% in colon. In these tissues, ABCC2 plays a major role in the pharmacokinetics and pharmacodynamics of endo- and xenobiotics. To gain insight in the function of this crucial protein, we have investigated and developed the first effective inhibitors of this pump. Firstly, we set up a cellular flow cytometry assay for monitoring the drug efflux carried out by ABCC2, and used it for the screening of chemical libraries derived from several chemical classes. We found that 2-indolylmethylenebenzofuranone derivatives as promising candidates. Optimization of the hits provided new compounds that inhibit ABCC2 in the micromolar range, making them the first potent ABCC2 inhibitors reported so far. Such compounds would constitute valuable tools to further investigate the role of ABCC2 in the pharmacokinetics and pharmacodynamics of drugs.

Mitochondrial metabolomics using high-resolution Fourier-transform mass spectrometry.

High-resolution Fourier-transform mass spectrometry (FTMS) provides important advantages in studies of metabolism because more than half of common intermediary metabolites can be measured in 10 min with minimal pre-detector separation and without ion dissociation. This capability allows unprecedented opportunity to study complex metabolic systems, such as mitochondria. Analysis of mouse liver mitochondria using FTMS with liquid chromatography shows that sex and genotypic differences in mitochondrial metabolism can be readily distinguished. Additionally, differences in mitochondrial function are readily measured, and many of the mitochondria-related metabolites are also measurable in plasma. Thus, application of high-resolution mass spectrometry provides an approach for integrated studies of complex metabolic processes of mitochondrial function and dysfunction in disease.

Collateral sensitivity of resistant MRP1-overexpressing cells to flavonoids and derivatives through GSH efflux.

The multidrug resistance protein 1 (MRP1) is involved in multidrug resistance of cancer cells by mediating drug efflux out of cells, often in co-transport with glutathione (GSH). GSH efflux mediated by MRP1 can be stimulated by verapamil. In cells overexpressing MRP1, we have previously shown that verapamil induced a huge intracellular GSH depletion which triggered apoptosis of the cells. That phenomenon takes place in the more global anticancer strategy called "collateral sensitivity" and could be exploited to eradicate some chemoresistant cancer cells. Seeking alternative compounds to verapamil, we screened a library of natural flavonoids and synthetic derivatives. A large number of these compounds stimulate MRP1-mediated GSH efflux and the most active ones have been evaluated for their cytotoxic effect on MRP1-overexpressing cells versus parental cells. Interestingly, some are highly and selectively cytotoxic for MRP1-cells, leading them to apoptosis. However, some others do not exhibit any cytotoxicity while promoting a strong GSH efflux, indicating that GSH efflux is necessary but not sufficient for MRP1-cells apoptosis. In support to this hypothesis, structure activity relationships show that the absence of a hydroxyl group at position 3 of the flavonoid C ring is an absolute requirement for induction of MRP1-cells death, but is not for GSH efflux stimulation. Chrysin (compound 8) and its derivatives, compounds 11 and 22, exhibit a high selectivity toward MRP1-cells with a IC50 value of 4.1 µM for compound 11 and 4.9 µM for chrysin and compound 22, making them among the best described selective killer compounds of multidrug ABC transporter-overexpressing cells.

New structure-activity relationships of chalcone inhibitors of breast cancer resistance protein: polyspecificity toward inhibition and critical substitutions against cytotoxicity.

Adenosine triphosphate-binding cassette subfamily G member 2 (ABCG2) plays a major role in cancer cell multidrug resistance, which contributes to low efficacy of chemotherapy. Chalcones were recently found to be potent and specific inhibitors, but unfortunately display a significant cytotoxicity. A cellular screening against ABCG2-mediated mitoxantrone efflux was performed here by flow cytometry on 54 chalcone derivatives from three different series with a wide panel of substituents. The identified leads, with submicromolar IC50 (half maximal inhibitory concentration) values, showed that the previously identified 2'-OH-4',6'-dimethoxyphenyl, as A-ring, could be efficiently replaced by a 2'-naphthyl group, or a 3',4'-methylenedioxyphenyl with lower affinity. Such a structural variability indicates 3polyspecificity of the multidrug transporter for inhibitors. At least two methoxyl groups were necessary on B-ring for optimal inhibition, but substitution at positions 3, 4, and 5 induced cytotoxicity. The presence of a large O-benzyl substituent at position 4 and a 2'-naphthyl as A-ring markedly decreased the cytotoxicity, giving a high therapeutic ratio, which constitutes a critical requirement for future in-vivo assays in animal models.

Identification of xanthones as selective killers of cancer cells overexpressing the ABC transporter MRP1.

Multidrug-resistance protein 1 (MRP1) belongs to the ATP-binding cassette (ABC) transporter family. MRP1 mediates MDR (multidrug resistance) by causing drug efflux either by conjugation to glutathione (GSH) or by co-transport with free GSH (without covalent bonding between the drug and GSH). We recently reported that the calcium channel blocker verapamil can activate massive GSH efflux in MRP1-overexpressing cells, leading to cell death through apoptosis. However, clinical use of verapamil is hampered by its cardiotoxicity. Then, in the search for compounds that act similarly to verapamil, but without major side effects, we investigated xanthones. Herein we show that xanthones induce apoptosis among resistant cells overexpressing MRP1 similarly to the verapamil effect. Among the xanthones studied, 1,3-dihydroxy-6-methoxyxanthone was identified as the most active derivative, able to specifically kill cells transfected with human MRP1 with even greater potency than verapamil. Under the same conditions, the active xanthones have no toxic effect on control (sensitive) cells. Xanthones could therefore be considered as new potential anticancer agents for the selective treatment of MRP1-positive tumors.

Iodination of verapamil for a stronger induction of death, through GSH efflux, of cancer cells overexpressing MRP1.

The multidrug resistance protein 1 (MRP1), involved in multidrug resistance (MDR) of cancer cells, was found to be modulated by verapamil, through stimulation of GSH transport, leading to apoptosis of MRP1-overexpressing cells. In this study, various iodinated derivatives of verapamil were synthesized, including iodination on the B ring, known to be involved in verapamil cardiotoxicity, and assayed for the stimulation of GSH efflux by MRP1. The iodination, for nearly all compounds, led to a higher stimulation of GSH efflux. However, determination of concomitant cytotoxicity is also important for selecting the best compound, which was found to be 10-fold more potent than verapamil. This will then allow us to design original anti-cancer compounds which could specifically kill the resistant cancer cells.

(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.

Influence of tyrosine-kinase Wzc activity on colanic acid production in Escherichia coli K12 cells.

Bacterial tyrosine-kinases have been demonstrated to participate in the regulation of capsule polysaccharides (CPS) and exopolysaccharides (EPS) production and export. However, discrepant data have been reported on the molecular mechanism responsible for this regulation depending on the bacterial species analyzed. Special attention was previously paid to the tyrosine-kinase Wzc(ca) of Escherichia coli K-12, which is involved in the production of the exopolysaccharide, colanic acid, and autophosphorylates by using a cooperative two-step process. In this work, we took advantage of these observations to investigate in further detail the effect of Wzc(ca) phosphorylation on the colanic acid production. First, it is shown that expression of the phosphorylated form of Wzc prevents production of colanic acid whereas expression of the non-phosphorylated form allows biosynthesis of this exopolysaccharide. However, we provide evidence that, in the latter case, the size distribution of the colanic acid polymer is less scattered than in the case of the wild-type strain expressing both phosphorylated and non-phosphorylated forms of Wzc. It is then demonstrated that colanic acid production is not merely regulated by an on/off mechanism and that, instead, both phosphorylated and non-phosphorylated forms of Wzc are required to promote colanic acid synthesis. Moreover, a series of data suggests that besides the involvement of phosphorylated and non-phosphorylated forms of Wzc in the production of colanic acid, two particular regions of this kinase play as such an important role in the synthesis of this exopolysaccharide: a proline-rich domain located in the N-terminal part of Wzc(ca), and a tyrosine cluster present in the C-terminal portion of the enzyme. Furthermore, considering that polysaccharides are known to facilitate bacterial resistance to certain environmental stresses, it is shown that the resistance of E. coli to desiccation is directly connected with the phosphorylation state of Wzc(ca).

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

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

Verapamil and its derivative trigger apoptosis through glutathione extrusion by multidrug resistance protein MRP1.

This study demonstrates that verapamil and a newly synthesized verapamil derivative, NMeOHI(2), behave as apoptogens in multidrug resistance protein 1 (MRP1)-expressing cells. When treated with either verapamil or NMeOHI(2), surprisingly, baby hamster kidney-21 (BHK) cells transfected with human MRP1 were killed. Because parental BHK cells were not, as well as cells expressing an inactive (K1333L) MRP1 mutant, this indicated that cell death involved functional MRP1 transporter. Cell death was identified as apoptosis by using annexin V-fluorescein labeling and was no longer observed in the presence of the caspase inhibitor Z-Val-Ala-Asp(OMe)-CH(2)F (Z-VAD-FMK). In vitro, both verapamil and its derivative inhibited leukotriene C4 transport by MRP1-enriched membrane vesicles in a competitive manner, with a K(i) of 48.6 microm for verapamil and 5.5 microm for NMeOHI(2,) and stimulated reduced glutathione (GSH) transport 3-fold and 9-fold, respectively. Treatment of MRP1-expressing cells with either verapamil or the derivative quickly depleted intracellular GSH content with a strong decrease occurring in the first hour of treatment, which preceded cell death beginning at 8-16 h. Furthermore, addition of GSH to the media efficiently prevented cell death. Therefore, verapamil and its derivative trigger apoptosis through stimulation of GSH extrusion mediated by MRP1. This new information on the mechanism of induced apoptosis of MDR cells may represent a novel approach in the selective treatment of MRP1-positive tumors.