Multidrug resistance in cancer: role of ATP-dependent transporters

Strategies for inhibition of MDR1 gene expression

Several distinct strategies have been used to modulate the expression of cancer-associated genes, including antisense oligonucleotides, small interfering RNAs (siRNAs), and artificial transcriptional factors. One major cause for chemotherapeutic treatment failure in cancer is the overexpression of P-glycoprotein, the product of the multidrug resistance gene MDR1. In this study, we tested the ability of siRNAs to inhibit MDR1 gene expression. We evaluated the efficiency of chemically synthesized dsRNAs as well as vector-based hairpin siRNAs and investigated the behavior of clones of multidrug-resistant NCI/ADR-RES breast carcinoma cells stably transfected with hairpin siRNA vectors. The effects of siRNA on the MDR phenotype were compared with those elicited by antisense oligonucleotides or by designed transcription factors targeting the MDR1 promoter. These studies suggest that there are several comparably effective strategies for inhibiting MDR1 expression.

Predicting drug sensitivity and resistance: profiling ABC transporter genes in cancer cells

For analysis of multidrug resistance, a major barrier to effective cancer chemotherapy, we profiled mRNA expression of the 48 known human ABC transporters in 60 diverse cancer cell lines (the NCI-60) used by the National Cancer Institute to screen for anticancer activity. The use of real-time RT-PCR avoided artifacts commonly encountered with microarray technologies. By correlating the results with the growth inhibitory profiles of 1,429 candidate anticancer drugs tested against the cells, we identified which transporters are more likely than others to confer resistance to which agents. Unexpectedly, we also found and validated compounds whose activity is potentiated, rather than antagonized, by the MDR1 multidrug transporter. Such compounds may serve as leads for development.

Expression of lung resistance-related protein in transitional cell carcinoma of bladder

OBJECTIVES

To determine the role of lung resistance-related protein (LRP) in intrinsic multidrug-resistance (MDR) of bladder cancer.

METHODS

The study group consisted of 66 patients with newly diagnosed primary bladder cancer. No patient had been treated preoperatively with either radiotherapy or chemotherapy. Reverse transcriptase-polymerase chain reaction was performed to measure LRP, multidrug resistance-associated protein 1 (MDR1), and MRP1 mRNA expression. The expression of LRP, p53 proteins, and p63 proteins was examined by immunohistochemistry. We analyzed the correlation of LRP with the above indexes and the clinical pathologic parameters.

RESULTS

The expression rate of LRP mRNA (63.6%) was the greatest among the three MDR markers in primary bladder cancer without exposure to chemotherapy. The LRP mRNA level was significantly greater in normal bladder tissue than in transitional cell carcinoma bladder tissue (P <0.01) and in superficial cancer than in invasive cancer (P = 0.013). LRP mRNA expression showed no association with either MDR1 or MRP1, but close correlation with the LRP level (P = 0.001). LRP was associated with low-grade (P <0.01) and low-stage (P <0.05) cancer but had no association with tumor suppressor p53 or p63.

CONCLUSIONS

The grade and stage-related expression pattern of LRP indicates that it may be a predictive index for intrinsic MDR in early bladder cancer. Anticancer drugs out of the MDR spectrum of LRP may be more effective for patients with early bladder cancer.

Reduced expression of small GTPases and hypermethylation of the folate binding protein gene in cisplatin-resistant cells

Reduced accumulation of cisplatin is the most consistent feature seen in cisplatin-resistant (CP-r) cells that are cross-resistant to other cytotoxic compounds, such as methotrexate. In this report, defective uptake of a broad range of compounds, including [(14)C]-carboplatin, [(3)H]MTX, [(3)H]folic acid (FA), [(125)I]epidermal growth factor, (59)Fe, [(3)H]glucose, and [(3)H]proline, as well as (73)As(5+) and (73)As(3+), was detected in CP-r human hepatoma and epidermal carcinoma cells that we have previously shown are defective in fluid-phase endocytosis. Downregulation of several small GTPases, such as rab5, rac1, and rhoA, which regulate endocytosis, was found in CP-r cells. However, expression of an early endosomal protein and clathrin heavy chain was not changed, suggesting that the defective endocytic pathway is clathrin independent. Reduced expression of the cell surface protein, folate-binding protein (FBP), which is a carrier for the uptake of MTX, was also observed in the CP-r cells by confocal immunofluorescence microscopy and Real-Time PCR. Reactivation of the silenced FBP gene in the CP-r cells by a DNA demethylation agent, 2-deoxy-5-aza-cytidine (DAC) demonstrates that hypermethylation occurred in the CP-r cells. The uptake of [(14)C]carboplatin, [(3)H]FA, and [(3)H]MTX increased in an early stage CP-r cell line (KB-CP1) after treatment with DAC. Both a defective endocytic pathway and DNA hypermethylation resulting in the downregulation of small regulatory GTPases and cell surface receptors contribute to the reduced accumulation of a broad range of compounds in CP-r cells.

Reduced expression of small GTPases and hypermethylation of the folate binding protein gene in cisplatin-resistant cells

Reduced accumulation of cisplatin is the most consistent feature seen in cisplatin-resistant (CP-r) cells that are cross-resistant to other cytotoxic compounds, such as methotrexate. In this report, defective uptake of a broad range of compounds, including [(14)C]-carboplatin, [(3)H]MTX, [(3)H]folic acid (FA), [(125)I]epidermal growth factor, (59)Fe, [(3)H]glucose, and [(3)H]proline, as well as (73)As(5+) and (73)As(3+), was detected in CP-r human hepatoma and epidermal carcinoma cells that we have previously shown are defective in fluid-phase endocytosis. Downregulation of several small GTPases, such as rab5, rac1, and rhoA, which regulate endocytosis, was found in CP-r cells. However, expression of an early endosomal protein and clathrin heavy chain was not changed, suggesting that the defective endocytic pathway is clathrin independent. Reduced expression of the cell surface protein, folate-binding protein (FBP), which is a carrier for the uptake of MTX, was also observed in the CP-r cells by confocal immunofluorescence microscopy and Real-Time PCR. Reactivation of the silenced FBP gene in the CP-r cells by a DNA demethylation agent, 2-deoxy-5-aza-cytidine (DAC) demonstrates that hypermethylation occurred in the CP-r cells. The uptake of [(14)C]carboplatin, [(3)H]FA, and [(3)H]MTX increased in an early stage CP-r cell line (KB-CP1) after treatment with DAC. Both a defective endocytic pathway and DNA hypermethylation resulting in the downregulation of small regulatory GTPases and cell surface receptors contribute to the reduced accumulation of a broad range of compounds in CP-r cells.

Effect of valspodar on the pharmacokinetics of unbound paclitaxel

The aim of this multicenter study was to determine whether valspodar (Amdray; code designation, SDZ PSC 833), a potent P-glycoprotein (P-gp) inhibitor, affects the pharmacokinetics of unbound paclitaxel (Cu). Data were obtained from 31 patients with advanced breast cancer. Thirteen patients were treated with paclitaxel alone (3-h infusion at 175 mg/m2) and another 18 received paclitaxel (3-h infusion at 70 mg/m2) in combination with a 21-day cycle of oral valspodar (5 mg/kg given four times a day) starting 1 day before administration of paclitaxel. Serial blood samples were taken in the first course and Cu in plasma determined using equilibrium dialysis with a [G-3H]paclitaxel tracer. The apparent clearance of Cu was not significantly different between the two groups, with mean +/- standard deviation (+/- SD) values of 230 +/- 56.0 and 202 +/- 49.9 L/h/m2 in the absence and presence of valspodar, respectively (P = 0.17). The volume of Cu distribution was slightly larger in the presence of valspodar (1160 +/- 474 vs. 1620 +/- 552 L/m2; P = 0.025), which contributed to a minor difference in the terminal disposition half-life (6.12 +/- 3.42 vs. 8.50 +/- 2.06 h; P = 0.028). These data indicate that (i) valspodar lacks the significant interaction with paclitaxel observed previously with other P-gp modulators, (ii) the majority of the increased toxicity of the combination does not appear to be attributable to increased levels of Cu, and (iii) provide further evidence of the conjecture that the plasma concentration of paclitaxel may not be an appropriate measure to monitor the impact of P-gp inhibition.

Classification analysis of P-glycoprotein substrate specificity

Prediction of P-glycoprotein substrate specificity (S(PGP)) can be viewed as a constituent part of a compound's "pharmaceutical profiling" in drug design. This task is difficult to achieve due to several factors that raised many contradictory opinions: (i) the disparity between the S(PGP) values obtained in different assays, (ii) the confusion between Pgp substrates and inhibitors, (iii) the confusion between lipophilicity and amphiphilicity of Pgp substrates, and (iv) the dilemma of describing class-specific relationships when Pgp has no binding sites of high ligand specificity. In this work, we compiled S(PGP) data for 1000 compounds. All data were represented in a binary format, assigning S(PGP) = 1 for substrates and S(PGP) = 0 for non-substrates. Each value was ranked according to the reliability of experimental assay. Two data sets were considered. Set 1 included 220 compounds with S(PGP) from polarized transport across MDR1 transfected cell monolayers. Set 2 included the entire list of 1000 compounds, with S(PGP) values of generally lower reliability. Both sets were analysed using a stepwise classification structure-activity relationship (C-SAR) method, leading to derivation of simple rules for crude estimation of S(PGP) values. The obtained rules are based on the following factors: (i) compound's size expressed through molar weight or volume, (ii) H-accepting given by the Abraham's beta (that can be crudely approximated by the sum of O and N atoms), and (iii) ionization given by the acid and base pKa values. Very roughly, S(PGP) can be estimated by the "rule of fours". Compounds with (N + O) > or = 8, MW > 400 and acid pKa > 4 are likely to be Pgp substrates, whereas compounds with (N + O) < or = 4, MW < 400 and base pKa < 8 are likely to be non-substrates. The obtained results support the view that Pgp functioning can be compared to a complex "mini-pharmacokinetic" system with fuzzy specificity. This system can be described by a probabilistic version of Abraham's solvation equation, suggesting a certain similarity between Pgp transport and chromatographic retention. The chromatographic model does not work in the case of "marginal" compounds with properties close to the "global" physicochemical cut-offs. In the latter case various class-specific rules must be considered. These can be associated with the "amphiphilicity" and "biological similarity" of compounds. The definition of class-specific effects entails construction of the knowledge base that can be very useful in ADME profiling of new drugs.

Mutational Analysis of ABCG2: Role of the GXXXG Motif

ABCG2 (BCRP/MXR/ABCP) is a half-transporter associated with multidrug resistance that presumably homodimerizes for function. It has a conserved GXXXG motif in its first transmembrane segment, a motif that has been linked with dimerization in other proteins, e.g., glycophorin A. We substituted either or both glycines of this GXXXG motif with leucines to evaluate the impact on drug transport, ATP hydrolysis, cross-linking, and susceptibility to degradation. All mutants also carried the R482G gain-of-function mutation, and all migrated to the cell surface. The mutations resulted in lost transport for rhodamine 123 and impaired mitoxantrone, pheophorbide a, and BODIPY-prazosin transport, particularly in the double leucine mutant (G406L/G410L). Basal ATPase activity of the G406L/G410L mutant was comparable to the empty vector transfected cells with no substrate induction. Despite impaired function, the mutants retained susceptibility to cross-linking using either disuccinimidyl suberate (DSS) or the reducible dithiobis(succinimidyl propionate) (DSP) and demonstrated a high molecular weight complex under nonreducing conditions. Mutations to alanine at the same positions yielded fully functional transporters. Finally, we exposed cells to mitoxantrone to promote folding and processing of the mutant proteins, which in the leucine mutants resulted in increased amounts detected on immunoblot and by immunofluorescence. These studies support a hypothesis that the GXXXG motif promotes proper packing of the transmembrane segments in the functional ABCG2 homodimer, although it does not solely arbitrate dimerization.

Systemic treatment and liver transplantation for hepatocellular carcinoma: two ends of the therapeutic spectrum

Hepatocellular carcinoma is the fifth most common malignant disorder and causes nearly 1 million deaths a year worldwide. A background of cirrhosis is the major risk factor, and in Asia and subSaharan Africa, cirrhosis is attributable mainly to endemic hepatitis B infection. In Europe and the USA the incidence of hepatocellular carcinoma is increasing as a result of the high prevalence of hepatitis C. The only curative treatments are surgical resection or liver transplantation, but only a few patients are eligible for these procedures. Local ablative treatments such as ethanol injection can lengthen survival in selected patients, and radiofrequency ablation also shows promise. Unfortunately, most patients are suitable only for palliative treatment because of the extent of their tumour or background liver disease or both. For these patients, a wide range of therapeutic interventions have been assessed, including transarterial embolisation (with or without chemotherapy), hormone therapy with antioestrogens and androgens, octreotide, interferon, and both arterial and systemic chemotherapy, of which only chemoembolisation improves survival over symptomatic care. Tamoxifen is ineffective, and there are insufficient randomised data to show the benefit of any other intervention. In this review, we focus on two ends of the therapeutic spectrum--transplantation, which is highly effective but applicable to only a few patients, and systemic chemotherapy, which is of uncertain benefit but widely applicable.

Biochemical basis of polyvalency as a strategy for enhancing the efficacy of P-glycoprotein (ABCB1) modulators: stipiamide homodimers separated with defined-length spacers reverse drug efflux with greater efficacy

Human P-glycoprotein (Pgp) is as an ATP-dependent efflux pump for a variety of chemotherapeutic drugs. The aim of this study is to evaluate whether Pgp modulators can be engineered to exhibit high-affinity binding using polyvalency. Five bivalent homodimeric polyenes based on stipiamide linked with polyethylene glycol ethers in the range of 3-50 A were synthesized and quantitatively characterized for their effect on Pgp function. The stipiamide homodimers displaced [(125)I]iodoarylazidoprazoin (IAAP), an analogue of the Pgp substrate prazosin. A minimal spacer of 11 A is necessary for inhibition of IAAP labeling, beyond which there is an inverse correlation between the length of the spacer and the IC(50) for the displacement of IAAP. ATP hydrolysis by Pgp on the other hand is stimulated by the dimers with spacers of up to 22 A, whereas dimers with longer spacers inhibit ATP hydrolysis. Finally, the homodimers reverse Pgp-mediated drug efflux in intact cells overexpressing Pgp, and 11 A is a threshold beyond which the effectiveness of the homodimers increases exponentially and levels off at 33 A. We demonstrate that dimerization and identification of an optimal spacer length increase by 11-fold the affinity of stipiamide, and this is reflected in the efficacy with which Pgp-mediated drug efflux is reversed. These results suggest that polyvalency could be a useful strategy for the development of more potent Pgp modulators.

Identifying candidate causal variants responsible for altered activity of the ABCB1 multidrug resistance gene

The difficulty of fine localizing the polymorphisms responsible for genotype-phenotype correlations is emerging as an important constraint in the implementation and interpretation of genetic association studies, and calls for the definition of protocols for the follow-up of associated variants. One recent example is the 3435C>T polymorphism in the multidrug transporter gene ABCB1, associated with protein expression and activity, and with several clinical conditions. Available data suggest that 3435C>T may not directly cause altered transport activity, but may be associated with one or more causal variants in the poorly characterized stretch of linkage disequilibrium (LD) surrounding it. Here we describe a strategy for the follow-up of reported associations, including a Bayesian formalization of the associated interval concept previously described by Goldstein. We focus on the region of high LD around 3435C>T to compile an exhaustive list of variants by (1) using a relatively coarse set of marker typings to assess the pattern of LD, and (2) resequencing derived and ancestral chromosomes at 3435C>T through the associated interval. We identified three intronic sites that are strongly associated with the 3435C>T polymorphism. One of them is associated with multidrug resistance in patients with epilepsy (chi2 = 3.78, P = 0.052), and sits within a stretch of significant evolutionary conservation. We argue that these variants represent additional candidates for influencing multidrug resistance due to P-glycoprotein activity, with the IVS 26+80 T>C being the best candidate among the three intronic sites. Finally, we describe a set of six haplotype tagging single-nucleotide polymorphisms that represent common ABCB1 variation surrounding 3435C>T in Europeans.

ydaG and ydbA of Lactococcus lactis encode a heterodimeric ATP-binding cassette-type multidrug transporter

Multidrug resistance (MDR)-type transporters mediate the active extrusion of structurally and functionally dissimilar compounds from the cells, thereby rendering cells resistant to a range of drugs. The ydaG and ydbA genes of Lactococcus lactis encode two ATP-binding cassette half-transporters, which both share homology with MDR proteins such as LmrA from L. lactis or the mammalian P-glycoprotein. The ydaG/ydbA genes were cloned and expressed separately and jointly in L. lactis using the nisin-inducible system. When both proteins are co-expressed, several structurally dissimilar drugs such as ethidium, daunomycin, and BCECF-AM are extruded from the cell. YdaG and YdbA could be co-purified as a stable heterodimer. ATPase activity was found to be associated with the YdaG/YdbA heterodimer only and not with the individual subunits. Both the ydaG and ydbA genes are up-regulated in multidrug-resistant L. lactis strains selected for growth in the presence of a variety of toxic compounds. This is the first demonstration of a functional heterodimeric ATP-binding cassette-type MDR transporter.

Phase II trial of carboplatin and infusional cyclosporine in platinum-resistant recurrent ovarian cancer

PURPOSE

To determine the response rate to 26-h continuous infusion cyclosporine A (CSA) combined with a fixed dose level of carboplatin (CBDCA) in patients with recurrent ovarian cancer, and to determine the effect of CSA on the pharmacokinetics of CBDCA.

EXPERIMENTAL DESIGN

To examine the effect of duration of CSA exposure on reversal of CBDCA resistance, clonogenic assays were performed in vitro in platinum-resistant A2780 cells. CBDCA (AUC 4) with CSA repeated every 3 weeks was then administered to patients on this phase II study. Pharmacokinetics of CSA and CBDCA were determined in a subset of patients.

RESULTS

Preincubation of platinum-resistant A2780 cells with CSA reversed CBDCA resistance in a concentration-dependent and time-dependent manner. A group of 23 patients received 58 courses of CBDCA/CSA therapy. One partial response was observed. Eight patients achieved disease stabilization. Toxicity was similar to that observed in our previous phase I study and consisted of myelosuppression, nausea, vomiting, and headache. The mean +/- SD end-of-infusion CSA level (HPLC assay) was 1253 +/- 400 microg/ml. The pharmacokinetic studies suggest that CSA does not increase CBDCA AUC.

CONCLUSIONS

Steady-state levels of >1 microg/ml CSA (HPLC assay) are achievable in vivo. Modest partial reversal of platinum resistance (in one patient with an objective response and in eight patients with stable disease noted) is achievable in vivo in patients pretreated with CSA. This phenomenon is not explained by alterations in CBDCA pharmacokinetics.

High-affinity interaction of tyrosine kinase inhibitors with the ABCG2 multidrug transporter

Tyrosine kinase inhibitors (TKIs) are promising new agents for specific inhibition of malignant cell growth and metastasis formation. Because most of the TKIs have to reach an intracellular target, specific membrane transporters may significantly modulate their effectiveness. In addition, the hydrophobic TKIs may interact with so-called multidrug transporters and thus alter the cellular distribution of unrelated pharmacological agents. In the present work, we show that certain TKIs, already in the clinical phase of drug development, directly interact with the ABCG2 multidrug transporter protein with a high affinity. We found that in several in vitro assay systems, STI-571 (Gleevec; imatinib mesylate), ZD1839 (Iressa; gefitinib), and N-[4-[(3-bromophenyl)amino]-6-quinazolinyl]-2-butynamide (EKI-785) interacted with ABCG2 at submicromolar concentrations, whereas other multidrug transporters, human multidrug resistance protein (P-glycoprotein, ABCB1) and human multidrug resistance protein 1 (ABCC1), showed much lower reactivity toward these agents. Low concentrations of the TKIs examined selectively modulated ABCG2-ATPase activity, inhibited ABCG2-dependent active drug extrusion, and significantly affected drug resistance patterns in cells expressing ABCG2. Our results indicate that multidrug resistance protein modulation by TKIs may be an important factor in the clinical treatment of cancer patients. These data also raise the possibility that an extrusion of TKIs by multidrug transporters, e.g., ABCG2, may be involved in tumor cell TKI resistance.

Application of expression genomics in drug development and genomic medicine

Expressed sequence tags and genome sequencing have virtually uncovered all the genes and transcripts in the human genome. Functional genomic analysis of these genes will undoubtedly reveal their physiological roles in cells in the near future. Coupled with the advent of DNA microarray, cellular response to perturbation can now be examined at genome‐wide levels in a single analysis, yielding clues to the network of pathways and interacting pathways that underlie a comprehensive systems response to exposure to small molecule perturbants. This post‐genomic information will be the foundation for knowledge‐based in silico systems biology. It is envisioned that querying such relational databases will generate testable hypotheses as well as revealing information on networks of regulatory genes and pathways that could further fuel and shape molecular and drug target discovery, and ushering in a new era in genomic medicine. In this review, we will first discuss examples of laboratory results generated from high‐throughput microarray analyses that illuminated previously unrecognized networks of regulatory genes and pathways that point to the mechanisms of action for the tumor promoter 12‐O‐tetradecanoyl‐phorbol‐13‐acetate, and the green tea polyphenol, epi‐gallocatechin gallate. We will then focus on the application of gene expression profiles in genomic medicine for predicting treatment response in cancer, using clinical specimens obtained from patients. These studies and others point to an increasing trend in modern biology in which high‐throughput genome scale comprehensive biological information is incorporated into databases for constructing and reconstructing pathways and networks of interacting pathways that constitute a physiological response to environmental perturbation at the organism or systems levels. Drug Dev. Res. 62:124–133, 2004. © 2004 Wiley‐Liss, Inc.

Highly Altered Protein Expression Profile in the Adriamycin Resistant MCF-7 Cell Line

The protein expression pattern in the cytosol fraction of the adriamycin resistant MCF-7 cell line (MCF-7/ADR) was compared to that of the parental MCF-7 cell line using two-dimensional gel electrophoresis and mass spectrometry. Twenty proteins with altered abundances were identified and studied in MCF-7/ADR. Both up regulation and down regulation are characterized. The most striking differences were found for proteins that were uniquely expressed in this cell line and not detectable in the parental MCF-7 cell line. These proteins include annexin I, the neuronal ubiquitin carboxyl hydrolase isoenzyme L-1 (also known as PGP9.5), glutathione-S-transferase pi class, nicotinamide N-methyltransferase, and interleukin-18 precursor. On the other hand, catechol-O-methyltransferase was expressed in the parental cell line, but was not detected in the adriamycin resistant cell line. This protein expression pattern was unique to MCF-7/ADR and not observed in MCF-7 cell lines selected for resistant to etoposide, mitoxantrone or melphalan.

Expression and cellular distribution of multidrug resistance-related proteins in the hippocampus of patients with mesial temporal lobe epilepsy

PURPOSE

This study investigated the cellular distribution of different multidrug resistance (MDR)-related proteins such as P-glycoprotein (P-gp), the multidrug resistance-associated proteins (MRP) 1 and 2, and the major vault protein (MVP) in normal and sclerotic hippocampus of patients with medically refractory mesial temporal lobe epilepsy (MTLE).

METHODS

Single- and double-label immunocytochemistry was used on brain sections of control hippocampus and of hippocampus of refractory MTLE patients.

RESULTS

In TLE cases with hippocampal sclerosis (HS), all four MDR proteins examined that had low or no expression in control tissue were upregulated, albeit with different cellular distribution patterns. P-gp immunoreactivity (IR) was observed in astrocytes in regions with diffuse reactive gliosis. In 75% of HS cases, strong P-gp IR was detected in blood vessels, with prominent endothelial labeling. Reactive astrocytes displayed low MRP1 IR. However, glial MRP1 expression was noted in glial endfoot processes around blood vessels. Neuronal MRP1 expression was observed in hypertrophic hilar neurons and in a few residual neurons of the CA1 region. Hippocampal MRP2 expression was observed in the large majority of HS cases in blood vessels. Hypertrophic hilar neurons and blood vessels within the sclerotic hippocampus expressed major vault protein (MVP).

CONCLUSIONS

These findings indicate that MDR proteins are upregulated in concert in the hippocampus of patients with refractory MTLE, supporting their role in the mechanisms underlying drug resistance. The specific cell-distribution patterns within the sclerotic hippocampus suggest different cellular functions, not necessarily linked only to clinical drug resistance.

Role of a conserved membrane-embedded acidic residue in the multidrug transporter MdfA

According to the current topology model of the Escherichia coli multidrug transporter MdfA, it contains a membrane-embedded negatively charged residue, Glu26, which was shown to play an important role in substrate recognition. To further elucidate the role of this substrate recognition determinant, various Glu26 replacements were characterized. Surprisingly, studies with neutral MdfA substrates showed that, unlike many enzymatic systems where the size and chemical properties of binding site residues are relatively defined, MdfA tolerates a variety of changes at position 26, including size, hydrophobicity, and charge. Moreover, although efficient transport of positively charged substrates requires a negative charge at position 26 (Glu or Asp), neutralization of this charge does not always abrogate the interaction of MdfA with cationic drugs, thus demonstrating that the negative charge does not play an essential role in the multidrug transport mechanism. Collectively, these results suggest a link between the broad substrate specificity profile of multidrug transporters and the structural and chemical promiscuity at their substrate recognition pockets.

Combined mutation of catalytic glutamate residues in the two nucleotide binding domains of P-glycoprotein generates a conformation that binds ATP and ADP tightly

Combined mutation of "catalytic carboxylates" in both nucleotide binding domains (NBDs) of P-glycoprotein generates a conformation capable of tight binding of 8-azido-ADP (Sauna, Z. E., Müller, M., Peng, X. H., and Ambudkar, S. V. (2002) Biochemistry 41, 13989-14000). Here we characterized this conformation using pure mouse MDR3 P-glycoprotein and natural MgATP and MgADP. Mutants E552A/E1197A, E552Q/E1197Q, E552D/E1197D, and E552K/E1197K had low but real ATPase activity in the order Ala > Gln > Asp > Lys, emphasizing the requirement for Glu stereochemistry. Mutant E552A/E1197A bound MgATP and MgADP (1 mol/mol) with K(d) 9.2 and 92 microm, showed strong temperature sensitivity of MgATP binding and equal dissociation rates for MgATP and MgADP. With MgATP as the added ligand, 80% of bound nucleotide was in the form of ATP. None of these parameters was vanadate-sensitive. The other mutants showed lower stoichiometry of MgATP and MgADP binding, in the order Ala > Gln > Asp > Lys. We conclude that the E552A/E1197A mutation arrests the enzyme in a conformation, likely a stabilized NBD dimer, which occludes nucleotide, shows preferential binding of ATP, does not progress to a normal vanadate-sensitive transition state, but hydrolyzes ATP and releases ADP slowly. Impairment of turnover is primarily due to inability to form the normal transition state rather than to slow ADP release. The Gln, Asp, and Lys mutants are less effective at stabilizing the occluded nucleotide, putative dimeric NBD, conformation. We envisage that in wild-type the occluded nucleotide conformation occurs transiently after MgATP binds to both NBDs with associated dimerization, and before progression to the transition state.

Therapeutic implications of cancer stem cells

Most cancers comprise a heterogenous population of cells with marked differences in their proliferative potential as well as the ability to reconstitute the tumor upon transplantation. Cancer stem cells are a minor population of tumor cells that possess the stem cell property of self-renewal. In addition, dysregulation of stem cell self-renewal is a likely requirement for the development of cancer. This new model for cancer will have significant ramifications for the way we study and treat cancer. In addition, through targeting the cancer stem cell and its dysregulated self-renewal, our therapies for treating cancer are likely to improve.

ABCG2 overexpression in colon cancer cells resistant to SN38 and in irinotecan-treated metastases

Overcoming drug resistance has become an important issue in cancer chemotherapy. Among all known mechanisms that confer resistance, active efflux of chemotherapeutic agents by proteins from the ATP-binding cassette family has been extensively reported. The aim of the present study was to determine the involvement of ABCG2 in resistance to SN38 (the active metabolite of irinotecan) in colorectal cancer. By progressive exposure to increasing concentrations of SN38, we isolated 2 resistant clones from the human colon carcinoma cell line HCT116. These clones were 6- and 53-fold more resistant to SN38 than the HCT116-derived sensitive clone. Topoisomerase I expression was unchanged in our resistant variants. The highest resistance level correlated with an ABCG2 amplification. This overexpression was associated with a marked decrease in the intracellular accumulation of SN38. The inhibition of ABCG2 function by Ko143 demonstrated that enhanced drug efflux from resistant cells was mediated by the activity of ABCG2 protein and confirmed that ABCG2 is directly involved in acquired resistance to SN38. Furthermore, we show, for the first time in clinical samples, that the ABCG2 mRNA content in hepatic metastases is higher after an irinotecan-based chemotherapy than in irinotecan-naive metastases. In conclusion, this study supports the potential involvement of ABCG2 in the development of irinotecan resistance in vivo.

Down-regulation and altered localization of gamma-catenin in cisplatin-resistant adenocarcinoma cells

Resistance to cisplatin, one of the most widely used anticancer chemotherapeutic agents, is a major clinical problem. There is no effective way to predict development of cisplatin resistance in cancers. As determined by reverse transcription-polymerase chain reaction and Western blotting, the expression of gamma-catenin, an adherens junction protein, was decreased in KB-CP20 and 7404-CP20 cells compared with parental-sensitive cells. Short-term treatment with cisplatin of the parental cells resulted in proteolysis of gamma-catenin as evaluated in membrane pellet preparations, and the extent of cleavage increased as cisplatin concentration was raised from 1 to 5 microg/ml during 1 h of treatment. Uncleaved cytoplasmic gamma-catenin increased under the same conditions. These biochemical results were supported by confocal microscopy, which showed a loss of gamma-catenin from adherens plaques after cisplatin treatment. Cleavage of gamma-catenin was specific to cisplatin treatment in that cleavage did not occur after treatment with doxorubicin and cytosine arabinoside. Pretreatment of KB and 7404 cells with cisplatin for 1 h resulted in reduced uptake of [(14)C]carboplatin, suggesting that the biochemical changes induced by cisplatin treatment, including cleavage of gamma-catenin, could affect the ability of cells to internalize platinum compounds. Cells transfected with the gamma-catenin gene are sensitive to cisplatin compared with cells transfected with a control vector. Our data suggest that proteolysis and altered localization of gamma-catenin are early markers for the response of cells to cisplatin, and reduced levels of gamma-catenin in resistant cells may indicate an important role for gamma-catenin in mediating or modulating the toxicity of cisplatin in cancer cells.

Overexpression of glucosylceramide synthase and P-glycoprotein in cancer cells selected for resistance to natural product chemotherapy

Resistance to natural product chemotherapy drugs is a major obstacle to successful cancer treatment. This type of resistance is often acquired in response to drug exposure; however, the mechanisms of this adverse reaction are complex and elusive. Here, we have studied acquired resistance to Adriamycin, Vinca alkaloids, and etoposide in MCF-7 breast cancer cells, KB-3-1 epidermoid carcinoma cells, and other cancer cell lines to determine if there is an association between expression of glucosylceramide synthase, the enzyme catalyzing ceramide glycosylation to glucosylceramide, and the multidrug-resistant (MDR) phenotype. This work shows that glucosylceramide levels increase concomitantly with increased drug resistance in the KB-3-1 vinblastine-resistant sublines KB-V.01, KB-V.1, and KB-V1 (listed in order of increasing MDR). The levels of glucosylceramide synthase mRNA, glucosylceramide synthase protein, and P-glycoprotein (P-gp) also increased in parallel. Increased glucosylceramide levels were also present in Adriamycin-resistant KB-3-1 sublines KB-A.05 and KB-A1. In breast cancer, detailed analysis of MCF-7 wild-type and MCF-7-AdrR cells (Adriamycin-resistant) demonstrated enhanced glucosylceramide synthase message and protein, P-gp message and protein, and high levels of glucosylceramide in resistant cells. Similar results were seen in vincristine-resistant leukemia, etoposide-resistant melanoma, and Adriamycin-resistant colon cancer cell lines. Cell-free glucosylceramide synthase activity was higher in lysates obtained from drug-resistant cells. Lastly, glucosylceramide synthase promoter activity was 15-fold higher in MCF-7-AdrR compared with MCF-7 cells. We conclude that selection pressure for resistance to natural product chemotherapy drugs selects for enhanced ceramide metabolism through glucosylceramide synthase in addition to enhanced P-gp expression. A possible connection between glucosylceramide synthase and P-gp in drug resistance biology is suggested.

Pheophorbide a is a specific probe for ABCG2 function and inhibition

Pheophorbide a (PhA), a chlorophyll catabolite, was shown to be an ABCG2 substrate based on Abcg2(-/-) knockout mouse studies (J. W. Jonker et al., Proc. Natl. Acad. Sci. USA, 99: 15649-15654, 2002). We developed a functional assay for ABCG2 using PhA and the ABCG2 inhibitor fumitremorgin C. In selected cell lines expressing high levels of P-glycoprotein, multidrug resistance-associated protein 1, or ABCG2, PhA transport was observed only in cells expressing ABCG2. Fumitremorgin C-inhibitable PhA transport was found to correlate with cell surface ABCG2 expression as measured by the anti-ABCG2 antibody 5D3. We found that 100 micro M of the cyclin-dependent kinase inhibitor UCN-01 or 1 micro M of the P-glycoprotein inhibitor tariquidar inhibited ABCG2-mediated PhA transport. In 4-day cytotoxicity assays, ABCG2-mediated resistance to SN-38 and topotecan was abrogated in ABCG2-transfected HEK-293 cells treated with 1 micro M tariquidar, and ABCG2-transfected cells were 6-7-fold resistant to UCN-01. PhA is an ABCG2-specific substrate with potential value in measuring ABCG2 function and expression in clinical samples.

Carvedilol: a new candidate for reversal of MDR1/P-glycoprotein-mediated multidrug resistance

In 1983, carvedilol [1-[carbazolyl-(4)-oxy]-3-[(2-methoxyphenoxyethyl)amino]-2-propanol] was designed and developed as a beta-adrenoceptor antagonist with vasodilating activity for efficacious and safe treatment of hypertension and coronary artery disease. Carvedilol belongs to the 'third generation' of beta-adrenoceptor antagonists and shows selectivity for the beta1- rather than beta2-adrenoceptor. Carvedilol is also an alpha1-blocking agents, with around 2- to 3-fold more selectivity for beta1- than alpha1-adrenoceptors. This degree of alpha1-blockade is responsible for the moderate vasodilator properties of carvedilol, being different from other beta-adrenoceptor antagonists. In addition, carvedilol is a potent antioxidant, with a 10-fold greater activity than vitamin E. Some carvedilol metabolites found in human plasma also exhibit antioxidative activity approximately 50- to 100-fold greater than carvedilol and other antioxidants. These unique properties of carvedilol, i.e. adrenergic (beta1, beta2 and alpha1) blockade and antioxidative activity, may be important in preventing progressive deterioration of left ventricular dysfunction and chronic heart failure. Recently, carvedilol has been demonstrated to reverse multidrug resistance (MDR) to anticancer drugs in tumor cells in vitro and its reversal effects were comparable with verapamil, which has been used in the first clinical trial for the reversal of MDR. This review introduces the reversal activity and usefulness against MDR, as well as an overview of the pharmacological and pharmacokinetic properties, of carvedilol.

Functional Imaging of Multidrug Resistant Phenotype by 99mTc-MIBI Scan in Patients with Multiple Myeloma

Overexpression of P-glycoprotein (Pgp) is one of the primary mechanisms of multidrug resistance (MDR) in several diseases, including multiple myeloma. The aim of this study was to investigate whether the washout of 99mTc-MIBI, a transport substrate of Pgp, is enhanced in the bone marrow of patients with multiple myeloma overexpressing Pgp. Seventeen (17) patients were i.v. injected with 555 MBq of 99mTc-MIBI, and whole-body scans were performed at 10 and 60 minutes. A region of interest (ROI) was drawn over the thoracic spine of each scan, and the washout of 99mTc-MIBI was calculated, after decay correction, as: (10-minute counts/pixel minus 60-minute counts/pixel) divided by 10-minute counts/pixel. Pgp expression was determined in 17 bone marrow samples obtained from the same patients immediately before the 99mTc-MIBI scan. Following centrifugation over the Ficoll-Hypaque gradient, cytospins were obtained and immunostained with C219 monoclonal antibody. The immunostaining of Pgp was graded as 1, 2, or 3 when a faint, moderate, or intense reaction, respectively, was observed in infiltrating plasma cells. Washout of 99mTc-MIBI ranged between 5% and 26%. A statistically significant direct correlation was found between the washout of the tracer and Pgp expression (Spearman rank correlation coefficient r = 0.74, p < 0.001). A partial overlap of washout values was observed in different classes of Pgp expression, thus preventing the discrimination of individual patients. Washout of 99mTc-MIBI, expressed as the percentage of radioactivity cleared from the bone marrow over a 1-hour period, may be used as a noninvasive tool for in vivo whole-body imaging of Pgp expression and function in multiple myeloma patients.

Cholesterol modulates P-glycoprotein activity in human peripheral blood mononuclear cells

P-glycoprotein (P-gp) is expressed in a wide range of cell types including peripheral blood mononuclear cells (PBMCs) where it may restrict intracellular accumulation of substrates like antineoplastic agents, HIV protease inhibitors, or rhodamine123. P-gp is known to be located in membrane microdomains, whose structure and function are susceptible to cholesterol alterations. This study evaluated the effect of cholesterol alteration in human PBMCs on P-gp activity. Whereas cholesterol depletion had no effect, cholesterol repletion of depleted cells significantly decreased intracellular rhodamine123 concentrations in lymphocytes to 32.2%+/-2.7 (p<0.001) and to 41.9%+/-3.5 (p<0.001) in monocytes. After cholesterol saturation of native cells intracellular rhodamine123 fluorescence decreased to 12.4%+/-1.6 (p<0.001) in lymphocytes and 12.9%+/-3.5 (p<0.001) in monocytes. These data demonstrate that elevated cellular cholesterol levels can markedly increase P-gp activity in human PBMCs.

Microtubule inhibitory effects of various SJ compounds on tissue culture cells

SJ compounds (SJ8002 and related compounds) are a group of novel anticancer agents (Cho, Chung, Lee, Kwon, Kang, Joo, and Oh. PCT/KR02/00392). To explore the anticancer mechanism of these compounds, we examined the effect of SJ8002 on microtubules of six human cell lines. At a high concentration (2 microg/mL), SJ8002 effectively disrupted microtubules of the six cell lines within 1 h. At lower concentrations (0.05 to approximately 1.0 microg/mL), the antimicrotubule activity of SJ8002 varied defending on cell lines. The inhibition of in vitro polymerization of pure tubulin by SJ8002 suggested that SJ8002 acts on free tubulin, inhibits the polymerization of tubulin dimer into microtubules, and hence induces the depolymerization of microtubules.

Glutathione S-transferase M1 and multidrug resistance protein 1 act in synergy to protect melanoma cells from vincristine effects

Previous studies have shown that glutathione S-transferases (GSTs) can operate in synergy with efflux transporters, multi-drug resistance proteins (MRPs), to confer resistance to several carcinogens, mutagens and anticancer drugs. To address the poorly documented role of the GSTM1 in cancer chemoresistance, we used CAL1 human melanoma cells expressing no endogenous GSTM1 and a high level of MRP1. Cells were transfected with an expression vector containing the GSTM1 cDNA, and different clones were selected expressing different levels of GSTM1 (RT-PCR, Western blot, and enzyme activity). Cells overexpressing GSTM1 displayed a 3- to 4-fold increase in resistance to anticancer drugs vincristine (VCR) and chlorambucil (CHB) in proliferation, cytotoxic, and clonogenic survival assays. Inhibitors of MRP1 (sulfinpyrazone, verapamil) and GST (dicumarol, curcumin) completely reversed the GSTM1-associated resistance to VCR, indicating that a MRP efflux function is necessary to potentiate GSTM1-mediated resistance to VCR. Conversely, MRP1 inhibitors had no effect on the sensitivity to CHB. Using immunofluorescence assay, GSTM1 was also shown to protect microtubule network integrity from VCR-induced inhibition of microtubule polymerization. In conclusion, these results show that GSTM1 alone is involved in melanoma resistance to CHB, whereas it can act in synergy with MRP1 to protect cells from toxic effects of VCR.

P-glycoprotein-mediated drug efflux is a resistance mechanism of chronic myelogenous leukemia cells to treatment with imatinib mesylate

Imatinib (Glivec), STI571) is an intracellular acting drug that demonstrates high activity against BCR-ABL-positive chronic myelogenous leukemia (CML) or acute lymphoblastic leukemia (ALL). However, many patients, especially with advanced disease, develop drug resistance. Here, we show by a novel high-performance liquid chromatography-based method that intracellular levels of imatinib decrease in P-glycoprotein (Pgp)-positive leukemic cells. In a model of K562 cells with gradually increasing Pgp expression, a Pgp-dependent decline of intracellular imatinib levels was observed. Decreased imatinib levels were associated with a retained phosphorylation pattern of the Bcr-Abl target Crkl and loss of effect of imatinib on cellular proliferation and apoptosis. The modulation of Pgp by cyclosporin A (CSA) readily restored imatinib cytotoxicity in these cells. Finally, we provide first data showing a biological effect of Pgp modulation in the imatinib treatment of a patient with BCR-ABL-positive ALL. MDR1 overexpression must therefore be considered as an important clinical mechanism in the diversity of resistance development to imatinib treatment.

Prodrug chemotherapeutics bypass p-glycoprotein resistance and kill tumors in vivo with high efficacy and target-dependent selectivity

Doxorubicin intercalates into DNA, causes double-strand breaks, and leads to apoptotic death. Limitations to the efficacy and therapeutic index of doxorubicin include poor tumor selectivity, high systemic toxicity, and the development of resistance, especially p-glycoprotein (p-gp)-mediated. We chemically coupled doxorubicin to a monoclonal antibody directed to the insulin-like growth factor-1 receptor, a receptor highly overexpressed in most tumors and validated as a tumor target. The prodrug conjugate bounded to tumor cells selectively, and accumulated efficiently and only in receptor-expressing cells. The conjugate was processed to release free doxorubicin inside target cells leading to selective toxicity, had >200-fold improved therapeutic index, and in vivo reduced tumor load with no systemic toxicity. Importantly, the prodrug conjugate is not subject to p-gp efflux and can bypass resistance in vivo. Our studies define a strategy to develop improved and more selective anticancer agents.

Dynamic and intracellular trafficking of P-glycoprotein-EGFP fusion protein: Implications in multidrug resistance in cancer

In our present study, a P-glycoprotein-EGFP (P-gp-EGFP) fusion plasmid was constructed and functionally expressed in HeLa cells to investigate the intracellular localization and trafficking of P-glycoprotein (P-gp). Using immunocytochemistry and fluorescent confocal microscopy techniques, colocalization studies showed that after transfection, P-gp-EGFP was progressively transported from the endoplasmic reticulum (ER) to the Golgi and finally to the plasma membrane within 12-48 hr. The degree of intracellular accumulation of daunorubicin was related to the particular localization of P-gp-EGFP. Significant daunorubicin accumulation occurred in transfected cells when P-gp-EGFP was localized predominantly within the ER, and accumulation remained high when P-gp-EGFP was mainly localized in the Golgi. However, there was little or no intracellular accumulation of daunorubicin when P-gp-EGFP was localized predominantly on the plasma membrane. Blocking the intracellular trafficking of P-gp-EGFP with brefeldin A (BFA) and monensin resulted in inhibition of traffic of P-gp-EGFP and retention of P-gp-EGFP intracellularly. Intracellular accumulation of daunorubicin also increased in the presence of BFA or monensin. Our study shows that P-gp-EGFP can be used to define the dynamics of P-gp traffic in a transient expression system, and demonstrates that localization of P-gp on the plasma membrane is associated with the highest level of resistance to daunorubicin accumulation in cells. Modulation of intracellular localization of P-gp with agents designed to selectively modify its traffic may provide a new strategy for overcoming multidrug resistance in cancer cells.

Single nucleotide polymorphisms result in impaired membrane localization and reduced atpase activity in multidrug transporter ABCG2

ABCG2/MXR/ABCP1/BCRP is a member of the ATP-binding cassette membrane transporter, which consists of six transmembrane regions and one ATP-binding cassette. The transporter is known to be involved in the efflux of various anticancer compounds such as mitoxantrone, doxorubicin and topoisomerase I inhibitor. In this study, we analyzed the effects of polymorphisms in ABCG2, V12M and Q141K on transporter function. When polarized LLC-PK1 cells were transfected with variant ABCG2, drug-resistance to topoisomerase I inhibitor of cells expressing V12M or Q141K was less than 1/10 that of wild-type ABCG2 transfected cells, and was accompanied by increased drug accumulation and decreased drug efflux in the variant ABCG2-expressing cells. We further elucidated the molecular mechanisms of the transport dysfunction by investigating membrane localization and ATPase activity. Confocal microscopic analysis revealed that apical plasma membrane localization of V12M was disturbed, while the localization of wild-type transporters occurred specifically in the apical plasma membrane of polarized LLC-PK1 cells. Also, ATPase activities measured in the membrane of SF9 cells infected with variant ABCG2 showed that Q141K decreased activity by 1.3 below that of wild-type ABCG2. In addition, kinetic analysis of ATPase activity showed that the K(m) value in Q141K was 1.4-fold higher than that of wild-type ABCG2. These results indicated that naturally occurring SNPs alter transport functions of ABCG2 transporter and analysis of SNPs in ABCG2 may hold great importance in understanding the response/metabolism of chemotherapy compounds that act as substrates for ABCG2.

Characterization of oligomeric human half-ABC transporter ATP-binding cassette G2

Human ATP-binding cassette G2 (ABCG2, also known as mitoxantrone resistance protein, breast cancer-resistance protein, ABC placenta) is a member of the superfamily of ATP-binding cassette (ABC) transporters that have a wide variety of substrates. Overexpression of human ABCG2 in model cancer cell lines causes multidrug resistance by actively effluxing anticancer drugs. Unlike most of the other ABC transporters which usually have two nucleotide-binding domains and two transmembrane domains, ABCG2 consists of only one nucleotide-binding domain followed by one transmembrane domain. Thus, ABCG2 has been thought to be a half-transporter that may function as a homodimer. In this study, we characterized the oligomeric feature of human ABCG2 using non-denaturing detergent perfluoro-octanoic acid and Triton X-100 in combination with gel filtration, sucrose density gradient sedimentation, and gel electrophoresis. Unexpectedly, we found that human ABCG2 exists mainly as a tetramer, with a possibility of a higher form of oligomerization. Monomeric and dimeric ABCG2 did not appear to be the major form of the protein. Further immunoprecipitation analysis showed that the oligomeric ABCG2 did not contain any other proteins. Taken together, we conclude that human ABCG2 likely exists and functions as a homotetramer.

MYCN-mediated regulation of the MRP1 promoter in human neuroblastoma

In the childhood cancer neuroblastoma (NB), the level of expression of the multidrug resistance-associated protein (MRP1) gene is strongly correlated with expression of the MYCN oncogene in primary NB tumors, suggesting that MRP1 may be a target for MYCN-mediated gene regulation. In this study, we show that MYCN induction in human NB cells results in increased MRP1 mRNA and protein levels, which in turn is accompanied by increased drug resistance and enhanced MRP1-mediated drug efflux. Furthermore, luciferase activity from MRP1 promoter/luciferase gene reporter constructs was significantly increased in NB cells with exogenous overexpression of MYCN, whereas activity was decreased in NB cells stably transfected with MYCN-antisense vectors. Decreased luciferase activity was observed with promoter constructs that lacked one or two E-box sequences or had E-box double point mutations, while a truncated MRP1 promoter lacking all three E-boxes exhibited only basal levels of activity. Specific electrophoretic mobility shifts of MRP1 E-box sequences were detected with nuclear extracts from NB cells with MYCN overexpression, and complex formation was inhibited with the addition of antibodies directed against MYCN or MYC. These findings indicate that by interacting with E-box elements within the promoter, MYCN can upregulate MRP1 expression and modulate drug resistance in NB.

Glypican-3 is involved in cellular protection against mitoxantrone in gastric carcinoma cells

Elevated expression of the heparan sulphate proteoglycan glypican-3 (GPC3) was found on mRNA and protein levels in the atypical multidrug-resistant gastric carcinoma cell line EPG85-257RNOV, which was established by in vitro selection against mitoxantrone. In order to elucidate a putative role of GPC3 in the drug-resistant phenotype, the mitoxantrone-resistant cell line EPG85-257RNOV was transfected with an expression vector construct carrying an anti-GPC3 hammerhead ribozyme. It could be demonstrated that in anti-GPC3 ribozyme-transfected cell clones, the GPC3-specific mRNA and corresponding protein expression levels were decreased to levels that are similar to those observed in nonresistant, parental cells. The anti-GPC3 ribozyme-containing clones reduced the mitoxantrone resistance level up to 21% of the original resistance and the crossresistance against etoposide to 33% of the original value. This reversal of drug resistance was accompanied by an increased cellular mitoxantrone accumulation in the anti-GPC3 ribozyme-expressing cells. In conclusion, it was verified that GPC3 is involved in the cellular protection against mitoxantrone in the atypical multidrug-resistant gastric carcinoma cell line EPG85-257RNOV.

Inhibition of P-glycoprotein function by XR9576 in a solid tumour model can restore anticancer drug efficacy

Resistance to cancer chemotherapy involves both altered drug activity at the designated target and modified intra-tumour pharmacokinetic properties (e.g. uptake, metabolism). The membrane transporter P-glycoprotein (P-gp) plays a major role in pharmacokinetic resistance by preventing sufficient intracellular accumulation of several anticancer agents. Whilst inhibiting P-gp has great potential to restore chemotherapeutic effectiveness in blood-borne cancers, the situation in solid tumours is less clear. Therefore, the degree of resistance tumours pose to the cytotoxicity of vinblastine and doxorubicin was characterised using the multicellular tumour spheroid model. Tumour spheroids were generated from either drug-sensitive MCF7(WT) breast cancer cells or a resistant P-gp-expressing variant (NCI/ADR(Res)). Drug-induced cytotoxicity in tumour spheroids was measured using an outgrowth assay and compared with that observed in monolayer cultures. As anticipated, the 3-D organisation of MCF7(WT) in tumour spheroids was associated with a reduction in the potency of doxorubicin and vinblastine-i.e. the inherent multicellular resistance phenomenon. In contrast, tumour spheroids from NCI/ADR(Res) cells did not display multicellular resistance. However their constitutive expression of P-gp reduced the potency of both anticancer drugs. Moreover, the highly potent P-gp inhibitor, the anthranilic acid derivative, XR9576, was able to restore the cytotoxic efficacy of both drugs in tumour spheroids comprising NCI/ADR(Res) cells. The results suggest that inhibition of P-gp in solid tumours is achievable and that generation of potent inhibitors will provide a significant benefit towards restoration of chemotherapy in solid tissues.

The molecular basis of the action of disulfiram as a modulator of the multidrug resistance-linked ATP binding cassette transporters MDR1 (ABCB1) and MRP1 (ABCC1)

The overexpression of multidrug resistance protein 1 (MDR1) and multidrug resistance protein 1 (MRP1) gene products is a major cause of multidrug resistance in cancer cells. A recent study suggested that disulfiram, a drug used to treat alcoholism, might act as a modulator of P-glycoprotein. In this study, we investigated the molecular and chemical basis of disulfiram as a multidrug resistance modulator. We demonstrate that in intact cells, disulfiram reverses either MDR1- or MRP1-mediated efflux of fluorescent drug substrates. Disulfiram inhibits ATP hydrolysis and the binding of [alpha-32P]8-azidoATP to P-glycoprotein and MRP1, with inhibition curves comparable with those of N-ethylmaleimide, a cysteine-modifying agent. However, if the ATP sites are protected with excess ATP, disulfiram stimulates ATP hydrolysis by both transporters in a concentration-dependent manner. Thus, in addition to modifying cysteines at the ATP sites, disulfiram may interact with the drug-substrate binding site. We demonstrate that disulfiram, but not N-ethylmaleimide, inhibits in a concentration-dependent manner the photoaffinity labeling of the multidrug transporter with 125I-iodoarylazidoprazosin and [3H]azidopine. This suggests that the interaction of disulfiram with the drug-binding site is independent of its role as a cysteine-modifying agent. Finally, we have exploited MRP4 (ABCC4) to demonstrate that disulfiram can inhibit ATP binding by forming disulfide bonds between cysteines located in the vicinity of, although not in, the active site. Taken together, our results suggest that disulfiram has unique molecular interactions with both the ATP and/or drug-substrate binding sites of multiple ATP binding cassette transporters, which are associated with drug resistance, and it is potentially an attractive agent to combat multidrug resistance.

Mitochondria in tumor cells studied by laser scanning confocal microscopy

We present here a confocal fluorescence microscopy study of mitochondria in sensitive and resistant carcinoma cells by using two potentiometric probes of mitochondria, rhodamine 123 (R123) and dimethylaminostyryl-methylpyridiniumiodine. We have found that active mitochondria in sensitive MCF-7 and multidrug resistant MCF-7/DX carcinoma cells are very different in localization and morphology. In sensitive cells active mitochondria are found in the perinuclear region, whereas in the multidrug resistance (MDR) subline they are confined to the cell periphery. Interestingly, the MDR revertant verapamil has been found to restore in MCF-7/DX cells the same pattern of active mitochondria seen in sensitive cells. We have also studied R123 in human lung carcinoma A549 cells, which display a low responsivity to doxorubicin, and overexpress the lung resistance-related protein. In addition to perinuclear mitochondria, peripheral mitochondria with weaker fluorescence can be seen in this cell line. Interestingly, in the two examined carcinoma lines we have been able to recognize by image analysis a common new star-lobed morphology. Our results indicate that in resistant carcinoma cells two populations of mitochondria coexist with different localization, morphology, and activity.

Reversal of breast cancer resistance protein-mediated drug resistance by tryprostatin A

MDR in human cancers is one of the major causes of failure of chemotherapy. A member of the superfamily of ABC transporters, BCRP, was demonstrated to confer an atypical MDR phenotype to tumor cells. To overcome the BCRP-mediated drug resistance, the fungal secondary metabolite TPS-A, a diketopiperazine, was analyzed with regard to its potency to reverse the BCRP-mediated drug-resistant phenotype. At concentrations of 10-50 microM, TPS-A reversed a mitoxantrone-resistant phenotype and inhibited the cellular BCRP-dependent mitoxantrone accumulation in the human gastric carcinoma cell line EPG85-257RNOV, the human breast cancer cell line MCF7/AdrVp (both exhibiting acquired BCRP-mediated MDR) and the BCRP cDNA-transfected breast cancer cell line MCF-7/BCRP clone 8. No cytotoxicity was seen at effective concentrations. These data indicate that TPS-A is a novel BCRP inhibitor.

Acquired resistance of human T cells to sulfasalazine: stability of the resistant phenotype and sensitivity to non-related DMARDs

BACKGROUND

A recent study from our laboratory showed that induction of the multidrug resistance related drug efflux pump ABCG2 contributed to acquired resistance of human T cells to the disease modifying antirheumatic drug (DMARD) sulfasalazine (SSZ).

OBJECTIVES

To investigate the duration of SSZ resistance and ABCG2 expression after withdrawal of SSZ and rechallenging with SSZ, and to assess the impact of SSZ resistance on responsiveness to other DMARDs.

METHODS

Human CEM cells (T cell origin) with acquired resistance to SSZ (CEM/SSZ) were characterised for (a) SSZ sensitivity and ABCG2 expression during withdrawal and rechallenge of SSZ, and (b) antiproliferative efficacy of other DMARDs.

RESULTS

ABCG2 protein expression was stable for at least 4 weeks when CEM/SSZ cells were grown in the absence of SSZ, but gradually declined, along with SSZ resistance levels, to non-detectable levels after withdrawal of SSZ for 6 months. Rechallenging with SSZ led to a rapid (<2.5 weeks) resumption of SSZ resistance and ABCG2 expression as in the original CEM/SSZ cells. CEM/SSZ cells displayed diminished sensitivity to the DMARDs leflunomide (5.1-fold) and methotrexate (1.8-fold), were moderately more sensitive (1.6-2.0 fold) to cyclosporin A and chloroquine, and markedly more sensitive (13-fold) to the glucocorticoid dexamethasone as compared with parental CEM cells.

CONCLUSION

The drug efflux pump ABCG2 has a major role in conferring resistance to SSZ. The collateral sensitivity of SSZ resistant cells for some other (non-related) DMARDs may provide a further rationale for sequential mono- or combination therapies with distinct DMARDs upon decreased efficacy of SSZ.

Development of sulfasalazine resistance in human T cells induces expression of the multidrug resistance transporter ABCG2 (BCRP) and augmented production of TNFalpha

OBJECTIVE

To determine whether overexpression of cell membrane associated drug efflux pumps belonging to the family of ATP binding cassette (ABC) proteins contributes to a diminished efficacy of sulfasalazine (SSZ) after prolonged cellular exposure to this disease modifying antirheumatic drug (DMARD).

METHODS

A model system of human T cells (CEM) was used to expose cells in vitro to increasing concentrations of SSZ for a period of six months. Cells were then characterised for the expression of drug efflux pumps: P-glycoprotein (Pgp, ABCB1), multidrug resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2).

RESULTS

Prolonged exposure of CEM cells to SSZ provoked resistance to SSZ as manifested by a 6.4-fold diminished antiproliferative effect of SSZ compared with parental CEM cells. CEM cells resistant to SSZ (CEM/SSZ) showed a marked induction of ABCG2/BCRP, Pgp expression was not detectable, while MRP1 expression was even down regulated. A functional role of ABCG2 in SSZ resistance was demonstrated by 60% reversal of SSZ resistance by the ABCG2 blocker Ko143. Release of the proinflammatory cytokine tumour necrosis factor alpha (TNFalpha) was threefold higher in CEM/SSZ cells than in CEM cells. Moreover, twofold higher concentrations of SSZ were required to inhibit TNFalpha release from CEM/SSZ cells compared with CEM cells.

CONCLUSION

Collectively, ABCG2 induction, augmented TNFalpha release, and less efficient inhibition of TNFalpha production by SSZ may contribute to diminished efficacy after prolonged exposure to SSZ. These results warrant further clinical studies to verify whether drug efflux pumps, originally identified for their roles in cytostatic drug resistance, can also be induced by SSZ or other DMARDs.

Prognosis in childhood and adult acute lymphoblastic leukaemia: a question of maturation?

Acute lymphoblastic leukaemia (ALL) is a disease diagnosed in children as well as adults. Progress in the treatment of ALL has led to better survival rates, however, children have benefited more from improved treatment modalities than adults. Recent evidence has underscored that the difference in characteristics and biology of adult versus childhood ALL might be the result of a different origin. According to the two-hit paradigm of Knudson, to develop cancer two genetic events are necessary. It has been suggested, that in childhood ALL the first genetic event happens in the more mature lymphoid committed progenitor cells, whereas in adult ALL the first hit occurs in multipotent stem cells. This review compares patient characteristics, the extent of the disease, leukaemic cell characteristics and treatment between childhood and adult ALL. This is discussed in relation to the hypothesis that the maturation stage of the cells, from which the leukaemia arises, is responsible for the differential behaviour of adult and childhood ALL.

Imaging reversal of multidrug resistance in living mice with bioluminescence: MDR1 P-glycoprotein transports coelenterazine

Coelenterazine is widely distributed among marine organisms, producing bioluminescence by calcium-insensitive oxidation mediated by Renilla luciferase (Rluc) and calcium-dependent oxidation mediated by the photoprotein aequorin. Despite its abundance in nature and wide use of both proteins as reporters of gene expression and signal transduction, little is known about mechanisms of coelenterazine transport and cell permeation. Interestingly, coelenterazine analogues share structural and physiochemical properties of compounds transported by the multidrug resistance MDR1 P-glycoprotein (Pgp). Herein, we report that living cells stably transfected with a codon-humanized Rluc show coelenterazine-mediated bioluminescence in a highly MDR1 Pgp-modulated manner. In Pgp-expressing Rluc cells, low baseline bioluminescence could be fully enhanced (reversed) to non-Pgp matched control levels with potent and selective Pgp inhibitors. Therefore, using coelenterazine and noninvasive bioluminescence imaging in vivo, we could directly monitor tumor-specific Pgp transport inhibition in living mice. While enabling molecular imaging and high-throughput screening of drug resistance pathways, these data also raise concern for the indiscriminate use of Rluc and aequorin as reporters in intact cells or transgenic animals, wherein Pgp-mediated alterations in coelenterazine permeability may impact results.

Analysis of catalytic carboxylate mutants E552Q and E1197Q suggests asymmetric ATP hydrolysis by the two nucleotide-binding domains of P-glycoprotein

In the nucleotide-binding domains (NBDs) of ABC transporters, such as mouse Mdr3 P-glycoprotein (P-gp), an invariant carboxylate residue (E552 in NBD1; E1197 in NBD2) immediately follows the Walker B motif (hyd(4)DE/D). Removal of the negative charge in mutants E552Q and E1197Q abolishes drug-stimulated ATPase activity measured by P(i) release. Surprisingly, drug-stimulated trapping of 8-azido-[alpha-(32)P]ATP is still observed in the mutants in both the presence and absence of the transition-state analogue vanadate (V(i)), and ADP can be recovered from the trapped enzymes. The E552Q and E1197Q mutants show characteristics similar to those of the wild-type (WT) enzyme with respect to 8-azido-[alpha-(32)P]ATP binding and 8-azido-[alpha-(32)P]nucleotide trapping, with the latter being both Mg(2+) and temperature dependent. Importantly, drug-stimulated nucleotide trapping in E552Q is stimulated by V(i) and resembles the WT enzyme, while it is almost completely V(i) insensitive in E1197Q. Similar nucleotide trapping properties are observed when aluminum fluoride or beryllium fluoride is used as an alternate transition-state analogue. Partial proteolytic cleavage of photolabeled enzymes indicates that, in the absence of V(i), nucleotide trapping occurs exclusively at the mutant NBD, whereas in the presence of V(i), nucleotide trapping occurs at both NBDs. Together, these results suggest that there is single-site turnover occurring in the E552Q and E1197Q mutants and that ADP release from the mutant site, or another catalytic step, is impaired in these mutants. Furthermore, our results support a model in which the two NBDs of P-gp are not functionally equivalent.

Val133 and Cys137 in transmembrane segment 2 are close to Arg935 and Gly939 in transmembrane segment 11 of human P-glycoprotein

P-glycoprotein (P-gp; ABCB1) transports a wide variety of structurally diverse compounds out of the cell. The protein has two homologous halves joined by a linker region. Each half consists of a transmembrane (TM) domain with six TM segments and a nucleotide-binding domain. The drug substrate-binding pocket is at the interface between the TM segments in each half of the protein. Preliminary studies suggested that the arrangement of the two halves of P-gp shows rotational symmetry (i.e. "head-to-tail" arrangement). Here, we tested this model by determining whether the cytoplasmic ends of TM2 and TM3 in the N-terminal half are in close contact with TM11 in the C-terminal half. Mutants containing a pair of cysteines in TM2/TM11 or TM3/TM11 were subjected to oxidative cross-linking with copper phenanthroline. Two of the 110 TM2/TM11 mutants, V133C(TM2)/G939C(TM11) and C137C(TM2)/A935C (TM11), were cross-linked at 4 degrees C, when thermal motion is reduced. Cross-linking was specific since no cross-linked product was detected in the 100 double Cys TM3/TM11 mutants. Vanadate trapping of nucleotide or the presence of some drug substrates inhibited cross-linking of mutants V133C(TM2)/G939C(TM11) and C137C(TM2)/A935C(TM11). Cross-linking of TM2 and TM11 also blocked drug-stimulated ATPase activity. The close proximity of TM2/TM11 and TM5/TM8 (Loo, T. W., Bartlett, M. C., and Clarke, D. M. (2004) J. Biol. Chem. 279, 7692-7697) indicates that these regions between the two halves must enclose the drug-binding pocket at the cytoplasmic side of P-gp. They may form the "hinges" required for conformational changes during the transport cycle.

Broad-spectrum modulation of ATP-binding cassette transport proteins by the taxane derivatives ortataxel (IDN-5109, BAY 59-8862) and tRA96023

PURPOSE

The taxanes paclitaxel and docetaxel are substrates for P-glycoprotein (Pgp), an ATP-binding cassette (ABC) transport protein associated with multidrug resistance (MDR). In contrast, the synthetic taxane ortataxel (BAY 59-8862, IDN-5109) is effective against Pgp-expressing cells by virtue of modulation of Pgp-mediated transport. The synthetic taxane tRA96023 also modulates Pgp and is noncytotoxic due to removal of the tubulin-binding side chain at the C-13 position of the taxane backbone. We studied the effects of ortataxel and tRA96023 on the other MDR-associated ABC transport proteins, multidrug resistance protein (MRP-1) and breast cancer resistance protein (BCRP, MXR, ABCG2).

METHODS

Modulation of mitoxantrone, daunorubicin and doxorubicin retention and cytotoxicity by ortataxel and tRA96023 was studied in established cell lines overexpressing Pgp, MRP-1 and wild type (BCRP(R482)) and mutant (BCRP(R482T)) BCRP, and was compared with modulation by the established Pgp-, MRP-1- and BCRP-specific modulators PSC-833, probenecid and fumitremorgin C, respectively.

RESULTS

Ortataxel effectively modulated drug retention and cytotoxicity in cell lines overexpressing MRP-1 and BCRP(R482), in addition to Pgp. tRA96023 modulated drug retention and cytotoxicity in cell lines overexpressing BCRP(R482) and Pgp, but not those overexpressing MRP-1. Neither ortataxel nor tRA96023 modulated BCRP(R482T).

CONCLUSIONS

The synthetic taxane derivatives ortataxel and tRA96023 are broad-spectrum ABC protein modulators. Further studies will seek to identify a noncytotoxic synthetic taxane that modulates Pgp, MRP-1 and BCRP.

Persistence of a small subpopulation of cancer stem-like cells in the C6 glioma cell line

Both stem cells and cancer cells are thought to be capable of unlimited proliferation. Paradoxically, however, some cancers seem to contain stem-like cells (cancer stem cells). To help resolve this paradox, we investigated whether established malignant cell lines, which have been maintained for years in culture, contain a subpopulation of stem cells. In this article, we show that many cancer cell lines contain a small side population (SP), which, in many normal tissues, is thought to contain the stem cells of the tissue. We demonstrate that in the absence of serum the combination of basic fibroblast growth factor and platelet-derived growth factor maintains SP cells in the C6 glioma cell line. Moreover, we show that C6 SP cells, but not non-SP cells, can generate both SP and non-SP cells in culture and are largely responsible for the in vivo malignancy of this cell line. Finally, we provide evidence that C6 SP cells can produce both neurons and glial cells in vitro and in vivo. We propose that many cancer cell lines contain a minor subpopulation of stem cells that is enriched in an SP, can be maintained indefinitely in culture, and is crucial for their malignancy.