Multidrug resistance in cancer: role of ATP-dependent transporters

Drug resistance reversal--are we getting closer?

Clinical drug resistance is a major barrier to overcome before chemotherapy can become curative for most patients presenting with metastatic cancer. Rational attempts to tackle clinical drug resistance need to be based on an understanding of the mechanisms involved; these are likely to be complex and multifactorial, and may be due to inadequate drug exposure or alterations in the cancer cell itself. This article reviews a number of strategies used to tackle drug resistance, focussing on work in our institution related to the treatment of ovarian cancer and resistance to platinum and taxane-based chemotherapy. Further progress towards drug resistance reversal will require a three-pronged approach, namely: the development of novel cytotoxics which exploit selectively expressed targets; modulation of resistance to conventional agents and, most importantly, a serious attempt to understand resistance mechanisms in tumour samples taken both pre- and post-chemotherapy.

Exploring the quantitative relationship between the level of MDR1 transcript, protein and function using digoxin as a marker of MDR1-dependent drug efflux activity

A limited number of gene expression studies have investigated the quantitative relationships between the amount of transcript, level of protein or activity/function, with disparate conclusions regarding these relationships. Collectively these studies indicate that the relevance of quantitative transcript analysis as a predictor of phenotype has to be evaluated on a gene-by-gene or even a case-by-case basis. The purpose of this study was to define a suitable marker for MDR1-dependent drug efflux, and to quantitatively investigate the relationships between the amount of transcript, protein and drug efflux in the frequently used Caco-2 cell model. The substrate specificity of digoxin, a commonly used marker for MDR1, was investigated using transgenic MDCK II or LLC-PK1 cell lines expressing the efflux proteins MDR1, BCRP and MRP2, since these proteins are localised to the apical part of the enterocyte plasma membrane and exhibit comparatively high transcript levels in the human small intestine. Relationships between levels of transcript, protein and function were investigated quantitatively using real-time RT-PCR, ECL western blot analysis and basolateral-to-apical and apical-to-basolateral efflux ratios. Our results indicate that digoxin is a specific marker for MDR1-dependent drug efflux in the Caco-2 cell drug absorption model and that MDR1 transcript abundance is at least as valid as MDR1 protein abundance as a predictor of MDR1 efflux activity.

Co-administration of simvastatin and cytotoxic drugs is advantageous in myeloma cell lines

We have evaluated the potential application of simvastatin (Sim) combined with conventional cytotoxic drugs for the treatment of multiple myeloma. RPMI 8226 and U266 myeloma cells seeded in culture plates were treated with Sim (5 and 10 microM, respectively) combined with melphalan (Mel; 25 and 20 microM, respectively) or dexamethasone (Dex; 1 microM). We assessed cell cycle (propidium iodide staining and flow cytometric analysis), cell morphology, viability (WST1), total cell count and cell death (Trypan blue exclusion). Sim significantly enhanced the anti-myeloma activity of cytotoxic agents in vitro (p<0.05). Incubation of U266 and RPMI 8226 with Sim prior to Mel increased the cytotoxicity in an additive manner, whereas the exposure of U266 to combined Sim and Dex resulted in a synergistic amplification of the individual effects. Combined application of Dex and Sim to RPMI 8226 cells resulted in antagonistic activity. The possible roles of Ras and phosphoinositol 3-kinase are discussed.

Characterization of salvicine-resistant lung adenocarcinoma A549/SAL cell line

Salvicine is a diterpenoid quinone derived from a traditional Chinese medication that has been shown to possess potent in vitro and in vivo antitumor effects. This compound, which inhibits the activity of Topoisomerase II, was found to equipotently kill various multidrug-resistant tumor cells and their corresponding parental counterparts in vitro and to inhibit mdr1/P-gp expression in multidrug-resistant K562/A02 cells. To examine the features of tumor resistance to salvicine, we established a salvicine-resistant tumor cell subline of A549 lung adenocarcinoma cells. Compared with parental cells, A549/SAL cells displayed 8.91-fold resistance to salvicine and an average of 6.70-fold resistance to the antimetabolites. A549/SAL cells, however, were not resistant to alkylating agents, platinum compounds and other naturally-derived antineoplastics. RT-PCR analysis showed that the expression of mRNAs from the mdr-1, MRP, PCNA, topoisomerase II alpha and beta, GSTpi, p21 and GADD45 genes was not altered in the salvicine-resistant subline. In contrast, expression of p53 and bax mRNA was significantly lower, and expression of mdm2 mRNA was significantly higher, in A549/SAL cells compared to A549 cells. A549/SAL cells grew more slowly, and in a more scattered pattern, than A549 cells. In addition, the A549/SAL cells showed enhanced ability to migrate and invade in comparison to the parental cells. These results indicate that exposure to salvicine does not induce a tumor multidrug-resistant phenotype.

Clinical molecular imaging

This review summarizes the rapidly growing field of molecular imaging, the spatially localized and/or temporally resolved sensing of molecular and cellular processes in vivo. Molecular imaging is used to map the anatomic locations of specific molecules of interest within living tissue and has enormous potential as a powerful means to diagnose and monitor disease. Molecular imaging agents comprise a targeting component that confers localization and a component that enables external detectability with an imaging modality, such as PET, SPECT, MRI, optical, and ultrasound. The advantages and disadvantages of each of these modalities are discussed in regard to spatial resolution, temporal resolution, sensitivity, and cost. Molecular imaging agents can be divided into three categories, Type A, which bind directly to a target molecule, Type B, which are accumulated by molecular or cellular activity by the target, and Type C, which are undetectable when injected but can be imaged after they are activated by the target. The current status of clinical molecular imaging agents is presented as well as examples of some preclinical applications. The value of molecular imaging is illustrated by some examples for diseases such as cancer, neurological and psychiatric disorders, cardiovascular disease, infection and inflammation, and the monitoring of gene therapy and stem cell therapy.

Intestinal membrane transport of drugs and nutrients: genomics of membrane transporters using expression microarrays

Carrier-mediated transport across membranes plays an important role in drug and nutrient absorption. However, relevant transporters remain largely unknown for most substrates. Their identification requires global analysis of expressed mRNAs in intestinal tissues. Microarray technologies capable of measuring mRNA profiles have proven useful in detecting the expression of genes encoding transporters and ion channels in intestines and Caco-2 cells. This colon carcinoma cell line with characteristics of absorptive enterocytes serves as a common model for drug absorption studies. Gene expression patterns of membrane transporters and channels define the cell's overall transport capacity. Moreover, transporter mRNA profiles provide a basis for assessing drug-drug and drug-food interactions in intestinal absorption. To determine relevant transporters for any given substrate, chemogenomic methods have emerged correlating mRNA expression in multiple tissues to drug transport or response. The resultant drug-transporter databases permit the search for transporter-drug relationships at a genomic scale.

'Side Population' cells in adult mouse testis express Bcrp1 gene and are enriched in spermatogonia and germinal stem cells

Stem cells in various somatic tissues (bone marrow, skeletal muscle) can be identified by the 'Side Population' marker based on Hoechst 33342 efflux. We show that mouse testicular cells also display a 'Side Population' that express Bcrp1 mRNA, the ABC transporter responsible for Hoechst efflux in hematopoietic cells. Inhibition of Hoechst efflux by specific BCRP1 inhibitor Ko143 show that germinal 'Side Population' phenotype is dependent on BCRP1 activity. Analysis of two well-defined models of altered spermatogenesis (W/Wv mutants and cryptorchid male mice) and RNA expression studies of differentiation markers demonstrate that germinal 'Side Population' contains spermatogonial cells. In addition, alpha 6-integrin and Stra8 germinal stem cell markers, are expressed in the 'Side Population'. In vivo repopulation assay clearly establishes that testis 'Side Population' in adult mice is highly enriched in male germ stem cells.

Bcl-2 overexpression prevents 99mTc-MIBI uptake in breast cancer cell lines

We have previously shown a correlation between the absence of technetium-99m methoxyisobutylisonitrile ((99m)Tc-MIBI) uptake and overexpression of the anti-apoptotic protein Bcl-2 in human breast carcinoma. To establish a direct cause-effect relationship between Bcl-2 overexpression and reduced (99m)Tc-MIBI uptake, MCF-7 and T47D breast cancer cell lines were stably transfected with the human Bcl-2 gene to increase intracellular protein levels and tested for (99m)Tc-MIBI uptake. All clones overexpressing Bcl-2 showed a dramatic reduction of (99m)Tc-MIBI uptake as compared with mock transfected control cells. Tracer uptake was promptly and partially restored by induction of apoptosis with staurosporine treatment. After 4.5 h of staurosporine treatment, a tenfold increase in (99m)Tc-MIBI uptake was observed in treated as compared with untreated Bcl-2 overexpressing cells. Our findings provide a rational basis for the development of an in vivo test to detect Bcl-2 overexpression in human tumours.

Disulfide cross-linking analysis shows that transmembrane segments 5 and 8 of human P-glycoprotein are close together on the cytoplasmic side of the membrane

Human P-glycoprotein (P-gp) transports a wide variety of structurally diverse compounds out of the cell. Knowledge about the packing of the transmembrane (TM) segments is essential for understanding the mechanism of drug recognition and transport. We used cysteine-scanning mutagenesis and disulfide cross-linking analysis to determine which TM segment in the COOH half of P-gp was close to TMs 5 and 6 since these segments in the NH(2) half are important for drug binding. An active Cys-less P-gp mutant cDNA was used to generate 240 double cysteine mutants that contained 1 cysteine in TMs 5 or 6 and another in TMs 7 or 8. The mutants were subjected to oxidative cross-linking analysis. No disulfide cross-linking was observed in the 140 TM6/TM7 or TM6/TM8 mutants. By contrast, cross-linking was detected in several P-gp TM5/TM8 mutants. At 4 degrees C, when thermal motion is low, P-gp mutants N296C(TM5)/G774C(TM8), I299C(TM5)/F770C(TM8), I299C(TM5)/G774C(TM8), and G300C(TM5)/F770C(TM8) showed extensive cross-linking with oxidant. These mutants retained drug-stimulated ATPase activity, but their activities were inhibited after treatment with oxidant. Similarly, disulfide cross-linking was inhibited by vanadate trapping of nucleotide. These results indicate that significant conformational changes must occur between TMs 5 and 8 during ATP hydrolysis. We revised the rotational symmetry model for TM packing based on our results and by comparison to the crystal structure of MsbA (Chang, G. (2003) J. Mol. Biol. 330, 419-430) such that TM5 is adjacent to TM8, TM2 is adjacent to TM11, and TMs 1 and 7 are next to TMs 6 and 12, respectively.

Ecteinascidin-743 drug resistance in sarcoma cells: transcriptional and cellular alterations

A human chondrosarcoma cell line, CS-1, was treated successively with increasing concentrations of the marine chemotherapeutic Ecteinascidin-743 (ET-743), yielding a variant cell line displaying a significant degree of resistance to the cytotoxic action of this drug. Various experiments were performed to discern molecular aberrations between the parent and resistant cell line, and also identify potential molecular markers indicative of drug resistance. Although no significant differences in the levels of membrane transporters such as P-glycoprotein or multidrug resistance protein 1 (MRP1) were detected, the cell migratory ability of the ET-743-resistant cell variant was reduced, as was its attachment capability to gelatin-coated cell culture dishes. Staining of the actin-containing cytoskeleton with fluorescent-labeled phalloidin revealed marked differences in the cytoskeleton architecture between the parent and ET-743-resistant CS-1 cell lines. Comparison of serum-free conditioned medium from both cell lines showed conspicuous differences in the levels of several proteins, including a quartet of high molecular weight proteins (> or =140 kDa). The protein sequences of two of these high molecular weight proteins, present at significantly higher concentrations in conditioned medium obtained from the parent cell line, corresponded to subunits of types I and IV collagen. Analysis of type I collagen alpha1 chain mRNA revealed a significantly lower level in the ET-743-resistant CS-1 cell line. Thus, prolonged exposure to ET-743 may cause changes in cell function through cytoskeleton rearrangement and/or modulation of collagen levels.

Pharmacogenetics goes genomic

Most people in the developed world will sooner or later be given prescription drugs to treat common diseases or to reduce the risk of getting them. Almost everyone who takes medicines will, at some stage, encounter those that do not work as well as they do in other people or even that cause an adverse reaction. Pharmacogenetics seeks to reduce the variation in how people respond to medicines by tailoring therapy to individual genetic make-up. It seems increasingly likely that investment in this field might be the most effective strategy for rapidly delivering the public health benefits that are promised by the Human Genome Project and related endeavours.

The expression of P-glycoprotein does influence the distribution of novel fluorescent compounds in solid tumour models

Solid tumours display a complex drug resistance phenotype that involves inherent and acquired mechanisms. Multicellular resistance is an inherent feature of solid tumours and is known to present significant barriers to drug permeation in tumours. Given this barrier, do acquired resistance mechanisms such as P-glycoprotein (P-gp) contribute significantly to resistance? To address this question, the multicellular tumour spheroid (MCTS) model was used to examine the influence of P-gp on drug distribution in solid tissue. Tumour spheroids (TS) were generated from either drug-sensitive MCF7(WT) cells or a drug-resistant, P-gp-expressing derivative MCF7(Adr). Confocal microscopy was used to measure time courses and distribution patterns of three fluorescent compounds; calcein-AM, rhodamine123 and BODIPY-taxol. These compounds were chosen because they are all substrates for P-gp-mediated transport, exhibit high fluorescence and are chemically dissimilar. For example, BODIPY-taxol and rhodamine 123 showed high accumulation and distributed extensively throughout the TS(WT), whereas calcein-AM accumulation was restricted to the outermost layers. The presence of P-gp in TS(Adr) resulted in negligible accumulation, regardless of the compound. Moreover, the inhibition of P-gp by nicardipine restored intracellular accumulation and distribution patterns to levels observed in TS(WT). The results demonstrate the effectiveness of P-gp in modulating drug distribution in solid tumour models. However, the penetration of agents throughout the tissue is strongly determined by the physico-chemical properties of the individual compounds.

Synergy between conserved ABC signature Ser residues in P-glycoprotein catalysis

Functional roles of the two ABC signature sequences ("LSGGQ") in the N- and C-terminal nucleotide binding domains of P-glycoprotein were studied by mutating the conserved Ser residues to Ala. The two single mutants (S528A; S1173A) each impaired ATPase activity mildly, and showed generally symmetrical effects on function, consistent with equivalent mechanistic roles of the two nucleotide sites. Synergy between the two mutations when combined was remarkable and resulted in strong catalytic impairment. The Ser residues are not involved significantly in MgATP- or MgADP-binding or in interdomain communication between catalytic sites and drug binding sites. Retention of product MgADP is not the cause of reduced turnover. Mutation of Ser to Ala reduced the strength of interaction with the chemical transition state specifically, as shown by vanadate-ADP and beryllium fluoride-ADP trapping experiments. Therefore, the two conserved ABC signature motif Ser residues of P-glycoprotein cooperatively accelerate ATP hydrolysis via chemical transition state interaction. Because the transition state complex is currently believed to form in the dimerized state of the nucleotide binding domains, one may also conclude that both Ser-OH are necessary for correct formation of the dimer state.

Role of the tumor microenvironment in mediating de novo resistance to drugs and physiological mediators of cell death

The emergence of clinical drug resistance continues to be an obstacle for the successful treatment of cancer. Our current understanding of mechanisms associated with drug resistance has been ascertained by investigating drug-resistant models created by exposing a parental population to increasing concentrations of a cytotoxic. These unicellular drug-resistant models have been critical in elucidating drug-resistant mechanism and in some cases have aided in the identification of drug targets. However, these models do not address resistance mechanisms that contribute to de novo drug resistance. We propose that specific niches within the tumor microenvironment may provide a sanctuary for subpopulations of tumors cells that affords a survival advantage following initial drug exposure and may facilitate the acquisition of acquired drug resistance. More specifically, we propose that the bone marrow microenvironment is a sanctuary for hema-topoietic cancers. This review will focus on the bone marrow microenvironment and its role in conferring resistance to cytotoxics and physiological mediators of cell death.

P-glycoprotein: from genomics to mechanism

Resistance to chemically different natural product anti-cancer drugs (multidrug resistance, or MDR) results from decreased drug accumulation, resulting from expression of one or more ATP-dependent efflux pumps. The first of these to be identified was P-glycoprotein (P-gp), the product of the human MDR1 gene, localized to chromosome 7q21. P-gp is a member of the large ATP-binding cassette (ABC) family of proteins. Although its crystallographic 3-D structure is yet to be determined, sequence analysis and comparison to other ABC family members suggest a structure consisting of two transmembrane (TM) domains, each with six TM segments, and two nucleotide-binding domains. In the epithelial cells of the gastrointestinal tract, liver, and kidney, and capillaries of the brain, testes, and ovaries, P-gp acts as a barrier to the uptake of xenobiotics, and promotes their excretion in the bile and urine. Polymorphisms in the MDR1 gene may affect the pharmacokinetics of many commonly used drugs, including anticancer drugs. Substrate recognition of many different drugs occurs within the TM domains in multiple-overlapping binding sites. We have proposed a model for how ATP energizes transfer of substrates from these binding sites on P-gp to the outside of the cell, which accounts for the apparent stoichiometry of two ATPs hydrolysed per molecule of drug transported. Understanding of the biology, genetics, and biochemistry of P-gp promises to improve the treatment of cancer and explain the pharmacokinetics of many commonly used drugs.

Importance of P-glycoprotein for drug disposition in humans

The ATP-binding cassette transporter P-glycoprotein is now recognized as an important determinant for disposition of multiple drugs. The use of P-glycoprotein-expressing cell lines, the generation of P-glycoprotein knockout mice as well as studies in animals and humans contributed to a better understanding on the role of active transport processes for drug disposition. P-glycoprotein is located in tissues with excretory function such as intestine, liver and kidney. Moreover, due to its expression in important blood-tissue barriers (blood-brain and blood-testis barriers), in lymphocytes and in placenta it limits tissue penetration of its substrates. Induction and inhibition of P-glycoprotein have now been identified as important underlying mechanisms of drug interactions in humans. Using selected examples, this review summarizes currently available data on the impact of P-glycoprotein for bioavailability of drugs, drug interactions and drug effects.

Pharmacogenomics in the treatment of epilepsy

Genetic variability has recently been implicated in the development of familial epilepsy syndromes and in heterogeneous responses of epilepsy patients to drug treatment. Mutations in distinct proteins have been shown to underlie the development of epilepsy, increase propensity for drug resistance, and alter drug metabolism. Improved understanding of how individual genetic variability may alter the efficacy of pharmacological therapeutic interventions is an important and timely goal. The investigation of relationships between genotype and patient responses to drug treatment is termed pharmacogenomics.

Side Population Keratinocytes Resembling Bone Marrow Side Population Stem Cells Are Distinct From Label-Retaining Keratinocyte Stem Cells

Very primitive hematopoietic stem cells have been identified as side population cells based on their ability to efflux a fluorescent vital dye, Hoechst 33342. In this study we show that keratinocytes with the same side population phenotype are also present in the human epidermis. Although side population keratinocytes have the same dye-effluxing phenotype as bone marrow side population cells and can be blocked by verapamil, they do not express increased levels of the ABCG2 transporter that is believed to be responsible for the bone marrow side population phenotype. Because bone marrow side population cells have stem cell characteristics, we sought to determine if side population keratinocytes represent a keratinocyte stem cell population by comparing side population keratinocytes with a traditional keratinocyte stem cell candidate, label-retaining keratinocytes. Flow cytometric analyses demonstrated that side population keratinocytes have a different cell surface phenotype (low beta1 integrin and low alpha6 integrin expression) than label-retaining keratinocytes and represent a unique population of keratinocytes distinctly different from the traditional keratinocyte stem cell candidate. Future in vivo studies will be required to analyze the function of side population keratinocytes in epidermal homeostasis and to determine if side population keratinocytes have characteristics of keratinocyte stem cells.

Potent Killing of Paclitaxel- and Doxorubicin-resistant Breast Cancer Cells by Calphostin C Accompanied by Cytoplasmic Vacuolization

Drug resistance is a major impediment to the successful treatment of breast cancer using chemotherapy. The photoactivatable drug calphostin C has shown promise in killing select drug-resistant tumor cells lines in vitro. To assess the effectiveness of this agent in killing doxorubicin- or paclitaxel-resistant breast tumor cells and to explore its mode of action, MCF-7 cells were exposed to increasing concentrations of either doxorubicin or paclitaxel until maximum resistance was obtained. This resulted in the creation of isogenic drug-resistant MCF-7TAX and MCF-7DOX cell lines, which were approximately 50- and 65-fold resistant to paclitaxel and doxorubicin, respectively. Interestingly, calphostin C was able to kill MCF-7TAX cells as efficiently as wildtype MCF-7 cells (IC50s were 9.2 and 13.2 nM, respectively), while MCF-7DOX cells required a 5-fold higher concentration of calphostin C to achieve the same killing (IC50 = 64.2 nM). Consistent with their known mechanisms of action, paclitaxel killed tumor cells by inducing mitotic arrest and cell multinucleation, while doxorubicin induced plasma membrane blebbing and decreased nuclear staining with propidium iodide. In contrast, cytoplasmic vacuolization accompanied cell killing by calphostin C in these cell lines, without the induction of caspase-8 or PARP cleavage or the release of cytochrome c from mitochondria. Calphostin C had little effect on the uptake of either paclitaxel or doxorubicin by the cells. Taken together, the above data suggests that calphostin C is able to potently kill drug-resistant breast tumor cells through a mechanism that may involve the induction of cytoplasmic vacuolization, without activation of typical apoptotic pathways. Consequently, calphostin C may prove useful clinically to combat tumor growth in breast cancer patients whose tumors have become unresponsive to anthracyclines or taxanes, particularly in association with photodynamic therapy.

Molecular modeling correctly predicts the functional importance of Phe594 in transmembrane helix 11 of the multidrug resistance protein, MRP1 (ABCC1)

The human ATP-binding cassette (ABC) transporter, multidrug resistance protein 1 (MRP1/ABCC1), confers resistance to a broad range of anti-cancer agents and transports a variety of organic anions. At present, essentially no structural data exists for MRP1 that might be used to elucidate its mechanism of transport. Consequently, we have applied a modeling strategy incorporating crystal and indirect structural data from other ABC transporters to construct a model of the transmembrane domains of the core region of MRP1 that includes the amino acid side chains. Three conserved Trp residues and one non-conserved Tyr residue, shown previously to be of functional importance (Koike, K., Oleschuk, C. J., Haimeur, A., Olsen, S. L., Deeley, R. G., and Cole, S. P. C. (2002) J. Biol. Chem. 277, 49495-49503), were found to line the "pore" in our model proximal to the membrane cytosol interface. A fifth aromatic residue (Phe594) was identified that, with the Trp and Tyr residues, completed a ring or "basket" of aromatic amino acids and, accordingly, we postulated that it would also be of functional importance. To test this idea, MRP1-Phe594 mutants were expressed in human embryonic kidney cells, and their properties were examined using membrane vesicles. Substitution of Phe594 with Ala substantially reduced or eliminated the transport of five organic anion substrates by MRP1 and abrogated the binding of leukotriene C4. On the other hand, the conservatively substituted F594W and F594Y mutants remained transport competent, although significant substrate- and substitution-specific changes were observed. These studies provide some structural insight into a possible substrate binding/transport site of MRP1 at the beginning of a putative substrate translocation pathway and demonstrate the usefulness of modeling for directing structure-function analyses of this transporter.

Simultaneous binding of two different drugs in the binding pocket of the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) transports a wide variety of structurally diverse compounds out of the cell. The drug-binding pocket of P-gp is located in the transmembrane domains. Although occupation of the drug-binding pocket by one molecule is sufficient to activate the ATPase activity of P-gp, the drug-binding pocket may be large enough to accommodate two different substrates at the same time. In this study, we used cysteine-scanning mutagenesis to test whether P-gp could simultaneously interact with the thiol-reactive drug substrate, Tris-(2-maleimidoethyl)amine (TMEA) and a second drug substrate. TMEA is a cross-linker substrate of P-gp that allowed us to test for stimulation of cross-linking by a second substrate such as calcein-acetoxymethyl ester, colchicine, demecolcine, cyclosporin A, rhodamine B, progesterone, and verapamil. We report that verapamil induced TMEA cross-linking of mutant F343C(TM6)/V982C(TM12). By contrast, no cross-linked product was detected in mutants F343C(TM6), V982C(TM12), or F343C(TM6)/V982C(TM12) in the presence of TMEA alone. The verapamil-stimulated ATPase activity of mutant F343C(TM6)/V982C(TM12) in the presence of TMEA decreased with increased cross-linking of the mutant protein. These results show that binding of verapamil must induce changes in the drug-binding pocket (induced-fit mechanism) resulting in exposure of residues F343C(TM6)/V982C(TM12) to TMEA. The results also indicate that the common drug-binding pocket in P-gp is large enough to accommodate both verapamil and TMEA simultaneously and suggests that the substrates must occupy different regions in the common drug-binding pocket.

Chemoembolization in the Management of Liver Tumors

The dual vascular supply of the liver affords a unique opportunity to explore intraarterial therapies for hepatic malignancies. Chemoembolization is a well-established technique combining intra-arterial chemotherapy with delivery of embolic agents in order to achieve an antitumor effect due to a high local concentration and prolonged dwell time of the drug, along with select ischemia. Many tumors, such as hepatocellular carcinoma, colorectal cancer, and neuroendocrine tumors, cause symptoms and death by local growth and destruction of the liver. While there are other methods capable of controlling small or isolated hepatic neoplasms, none are suitable for the majority of these patients. Chemoembolization can produce significant results in terms of tumor shrinkage in many of these patients, and there are studies to suggest a survival advantage in hepatocellular carcinoma. Toxicity, however, may be substantial, and patient selection is crucial in order to achieve the optimal benefit of this powerful technique for individual populations.

Saquinavir induces stable and functional expression of the multidrug transporter P-glycoprotein in human CD4 T-lymphoblastoid CEMrev cells

BACKGROUND

The multidrug transporter P-glycoprotein (P-gp) is expressed in HIV-1 target cells, in a range of pharmacological barriers and in AIDS-associated tumours. P-gp substrates include HIV-1 protease inhibitors (PIs) and anticancer drugs, which are efficiently effluxed from multidrug-resistant (MDR) cells.

OBJECTIVES

The aim of this study was to investigate the effect on human CD4 T-lymphoblastoid CEMrev cells of saquinavir and other PIs in terms of P-gp expression and to characterize the functional and biochemical patterns of PI-induced P-gp molecules.

METHODS

CEMrev cells no longer expressing detectable amounts of P-gp were cultured for a prolonged period in the presence of 10 microg/mL saquinavir (CEMsaq10) and tested for P-gp expression and function. Subsequently, CEMsaq10 cells were transferred into medium containing 15 microg/mL saquinavir (CEMsaq15) and cultured for several months. These cell lines were continuously monitored for P-gp expression, function and immunochemical patterns. A similar strategy was adopted to determine whether other PIs, such as ritonavir and indinavir, were able to induce P-gp expression in CEMrev cells.

RESULTS

Compared with the drug-diluent control, the exposure of CEMrev cells to 10 microg/mL saquinavir induced, in a consistent fraction of cells (45-50%), de novo expression of functioning P-gp molecules. The transfer of CEMsaq10 cells to 15 microg/mL saquinavir was associated with a dramatic increase in P-gp expression and function (85-90% of CEMsaq15 cells expressed P-gp and effluxed P-gp dye substrates). These saquinavir-induced P-gp molecules included 75-kDa molecules as well as the classical 170-kDa form of P-gp, suggesting induction of a particular isoform of P-gp termed mini-P-glycoprotein. Conversely, ritonavir and indinavir induced transient P-gp expression in a small percentage of the CEMrev cells.

CONCLUSIONS

Treatment of human CD4 T-lymphoblastoid CEMrev cells with saquinavir caused over-expression of functioning P-gp molecules. This de novo acquired MDR phenotype, which differed from that induced by other PIs, was stable, as expression and activity of P-gp were observed in CEMsaq10 and CEMsaq15 cells during prolonged in vitro culturing, even in drug-free conditions.

Screening for effectors that modify multidrug resistance in yeast

The yeast transcription factors Pdr1p and Pdr3p regulate the expression of several genes that encode energy-dependent efflux pumps involved in multidrug resistance. They recognize specific pleiotropic drug resistance elements in the promoters of the target gene such as PDR5 coding for a major multidrug transporter. Gain-of-function mutations in Pdr1p/Pdr3p result in over-expression of transporter genes and establishment of multidrug resistance. We developed a novel yeast-based screening procedure designed to detect compounds that specifically modify multidrug resistance due to an interference with the expression of drug efflux transporter genes. The screening is based on the ability to abrogate the growth defect of cells suffering from the galactose induced Pdr3p driven over-expression of a dominant-lethal allele of the PMA1 gene placed under the control of the PDR5 promoter. Validation of the assay was achieved by showing that growth inhibition was relieved by mutant Pdr3p devoid of activation domain. This screening system may also be used to select the loss-of-function pdr3 (or pdr1) mutants and to identify specific gene(s) whose over-expression or deletion will suppress the expression of multidrug transporters and increase the susceptibility of yeast cells to antifungals.

Characterization of the role of polar amino acid residues within predicted transmembrane helix 17 in determining the substrate specificity of multidrug resistance protein 3

Human multidrug resistance protein (MRP) 3 is the most closely related homologue of MRP1. Like MRP1, MRP3 confers resistance to etoposide (VP-16) and actively transports 17 beta-estradiol 17-(beta-D-glucuronide) (E(2)17 beta G), cysteinyl leukotriene 4 (LTC(4)), and methotrexate, although with generally lower affinity. Unlike MRP1, MRP3 also transports monovalent bile salts. We have previously demonstrated that hydrogen-bonding residues predicted to be in the inner-leaflet spanning segment of transmembrane (TM) 17 of MRP1 are important for drug resistance and E(2)17 beta G transport. We have now examined the importance of the hydrogen-bonding potential of residues in TM17 of MRP3 on both substrate specificity and overall activity. Mutation S1229A reduced only methotrexate transport. Mutations S1231A and N1241A decreased resistance to VP-16 and transport of E(2)17 beta G and methotrexate but not taurocholate. Mutation Q1235A also reduced resistance to VP-16 and transport of E(2)17beta G but increased taurocholate transport without affecting transport of methotrexate. Mutations Y1232F and S1233A reduced resistance to VP-16 and the transport of all three substrates tested. In contrast, mutation T1237A markedly increased VP-16 resistance and transport of all substrates. On the basis of the substrates analyzed, residues Ser(1229), Ser(1231), Gln(1235), and Asn(1241) play an important role in determining the specificity of MRP3, while mutation of Tyr(1232), Ser(1233), and Thr(1237) affects overall activity. Unlike MRP1, the involvement of polar residues in determining substrate specificity extends throughout the TM helix. Furthermore, elimination of the hydrogen-bonding potential of a single amino acid, Thr(1237), markedly enhanced the ability of the protein to confer drug resistance and to transport all substrates examined.

Quinine as a multidrug resistance inhibitor: a phase 3 multicentric randomized study in adult de novo acute myelogenous leukemia

Based on our previous demonstration that quinine could be used clinically to reverse P-glycoprotein-mediated resistance, we designed a multicenter, randomized trial aiming to determine whether quinine would improve the survival of adult patients (15-60 years old) with de novo acute myelogenous leukemia (AML). These patients randomly received (n = 213) or did not receive (n = 212) a 30 mg/kg/day continuous intravenous infusion of quinine in combination with induction chemotherapy combining idarubicine and cytarabine and, depending on bone marrow examination at day 20, an additional course of cytarabine and mitoxantrone. The mean steady-state quinine concentration was 7.8 mg/L and the mean multidrug resistance reversing activity of serum was 1.96. Complete remission (CR) was obtained in 344 patients (80.9%) without significant influence of quinine. Of the patients in complete remission, 82 were assigned to receive HLA-matched bone marrow transplants, whereas 262 were assigned to 2 courses of intensive consolidation chemotherapy, with or without quinine, depending on initial randomization. The 4-year actuarial overall survival (OS) of the 425 eligible patients was 42.0% +/- 2.5%, without significant influence of quinine. Of 160 patients who could be studied, 54 demonstrated rhodamine 123 efflux. In these patients, quinine significantly improved the CR rate from 12 of 25 (48.0%) to 24 of 29 (82.8%) (P =.01). However, there was no significant difference in OS. Neither mdr1 gene nor P-glycoprotein expression influenced the outcome. We conclude that quinine does not improve the survival of adult patients with de novo AML, even though it improves CR rate in a small subgroup of patients defined by rhodamine 123 efflux.

Clinical Relevance of P-Glycoprotein in Drug Therapy

The drug efflux transporter P-glycoprotein (P-gp) is known to confer multidrug resistance in cancer chemotherapy. The P-gp is highly expressed in many types of tumor cells, as well as many normal tissues, including the apical surface of intestinal epithelial cells, and the luminal surface of capillary endothelial cells in the brain. Because of its expression and localization, it has been suggested that P-gp plays an important role in cancer chemotherapy, intestinal absorption, and brain uptake. This review addresses the significance of the role of P-gp in cancer chemotherapy, drug absorption, and brain uptake. Based on the clinical and animal studies with P-gp modulators, it has become apparent that the role of P-gp in multidrug resistance is far less important compared to other biological factors. Although P-gp is highly expressed in both intestinal epithelial cells and endothelial cells of brain capillaries and functions as an efflux transporter in both organs, the magnitude of P-gp's impact on intestinal absorption and brain uptake of drugs is quantitatively very different. From animal and clinical studies, it is evident that P-gp plays a very important role in CNS penetration of drugs, whereas the effect of P-gp on drug absorption is not as important as generally believed.

Cytotoxicity, apoptosis induction and downregulation of MDR-1 expression by the anti-topoisomerase II agent, salvicine, in multidrug-resistant tumor cells

Salvicine, a novel topoisomerase II inhibitor and a diterpenoid quinone compound, exerts potent in vitro and in vivo antitumor effects. In our study, we show that salvicine effectively kills multidrug-resistant (MDR) sublines, such as K562/A02, KB/VCR and MCF-7/ADR, and parental K562, KB and MCF-7 cell lines to an equivalent degree. These cytotoxic activities of salvicine were much more potent than those of several classical anticancer drugs (average resistance factor: 1.42 for salvicine vs. 344.35, 233.19 and 71.22 for vincristine, doxorubicin and etoposide, respectively). Flow cytometry and DNA agarose gel electrophoresis demonstrated that salvicine induced similar levels of apoptosis in MDR K562/A02 and parental cells. The compound activated caspase-1 and -3 (but not caspase-8) and increased the ratio of bax to bcl-2 mRNA via reduction of bcl-2 mRNA expression in the same cells. Furthermore, salvicine induced the downregulation of mdr-1 gene and P-gp expression but had no effect on MRP and LRP gene expression in MDR K562/A02 cells. These results suggest that the reduction of mdr-1 and bcl-2 expression by salvicine possibly contributes to its cytotoxicity and apoptotic induction in this system. The effectiveness, broad-spectrum activity and possibly novel mechanism of killing MDR tumor cells in vitro of salvicine signify promising in vivo and clinical activity. The novel chemical structure of this compound further implies a role for salvicine in future MDR tumor therapy.

Quantitative real-time reverse transcription-polymerase chain reaction analysis of drug metabolizing and cytoprotective genes in psoriasis and regulation by ultraviolet radiation

There are unpredictable inter-individual differences in response to ultraviolet radiation, used in the treatment of psoriasis and other common skin diseases. It is therefore essential that we attempt to identify phenotypic markers that correlate with individual treatment outcomes. Exposure of human skin to ultraviolet radiation results in the generation of reactive intermediates and oxidative stress. Hepatic drug metabolizing and cytoprotective genes are induced as an adaptive response to xenobiotics and reactive intermediates; as several of these genes are present in skin, we hypothesized that their cutaneous expression and regulation may be implicated in responses to ultraviolet radiation. We used quantitative real-time reverse transcription-polymerase chain reaction to investigate interindividual differences in the cutaneous expression of a variety of drug metabolizing and cytoprotective genes, including cytochrome P450s, glutathione S-transferases and drug transporters, and investigated the regulation of gene expression by ultraviolet radiation and in lesional psoriatic skin. We confirmed significant induction of cyclooxygenase 2 (mean 3.63-fold, range 0.14-22.6, p<0.0001) by ultraviolet radiation and showed more modest (approximately 2-fold) inductions of glutathione peroxidase, and novel inductions of glutathione S-transferase P1 and the drug transporter multidrug resistance associated protein-1. Glutathione S-transferase P1 (3.74-fold, 1.3-33.1, p<0.0001) and multidrug resistance associated protein-1 (4.06-fold, 1.3-24.8, p<0.0001) were also significantly increased in psoriatic plaque, as were P450 CYP2E1 (3.64-fold, 1-28.9 p<0.0001) and heme oxygenase-1 (10.19-fold, 2.9-49.7, p<0.0001), implying a differential adaptive response to oxidant exposure in lesional psoriatic skin. We found considerable interindividual variation in constitutive gene expression and inducibility, indicating that these genes may be associated with individuality in response to ultraviolet radiation.

Reversal of multidrug resistance in cancer cells by pyranocoumarins isolated from Radix Peucedani

The pyranocoumarins, (+/-)-3'-angeloyl-4'-acetoxy-cis-khellactone, were isolated from Radix Peucedani, the dry root of Peucedanum praeruptorum Dunn, through bioassay-guided fractionation. The chemical structure of pyranocoumarins was determined by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. X-ray crystallography showed that there are eight molecules (i.e. two each of four conformers) in each unit cell with their optical activities equally cancelled out. The four conformers are 3'(R)-angeloyl-4'(R)-acetoxy-khellactone in two conformational forms, and 3'(S)-angeloyl-4'(S)-acetoxy-khellactone in two conformational forms. Pyranocoumarins caused apoptotic cell death with IC50 of 41.9+/-2.8 and 17.3+/-8.2 microM for drug-sensitive KB-3-1 and multidrug resistant (MDR) KB-V1, respectively. The two- to threefold sensitivity difference between the two cell lines is interesting considering that the same ratio for doxorubicin is 50-300. Strong synergistic interactions were demonstrated when pyranocoumarins were combined with common anti-tumor drugs including doxorubicin, paclitaxel, puromycin or vincristine in MDR KB-V1 cell line, but not in drug-sensitive KB-3-1 cells. Pyranocoumarins increased doxorubicin accumulation in KB-V1 cells by about 25% after 6 h of incubation. Pyranocoumarins treatment for 24 h down-regulated the expression of P-glycoprotein in KB-V1 cells at both protein and mRNA levels. Pyranocoumarins also transiently reduced the cellular ATP contents in KB-V1 cells in a dose-dependent manner. Our results suggest that pyranocoumarins could be a potential MDR reversing agent.

Regulation of multidrug resistance in cancer cells by hyaluronan

Multidrug resistance in cancer cells is often due to ATP-dependent efflux pumps, but is also linked to alterations in cell survival and apoptotic signaling pathways. We have found previously that perturbation of hyaluronan-tumor cell interaction by treatment with hyaluronan oligosaccharides suppresses the phosphoinositide 3-kinase/Akt cell survival signaling pathway in cancer cells and reduces tumor growth in vivo. Here we find that these oligomers suppress both the MAP kinase and phosphoinositide 3-kinase pathways in multidrug resistant tumor cells and sensitize these cells to a variety of chemotherapeutic drugs. On the other hand, increased hyaluronan production induces resistance in drug-sensitive tumor cells. Likewise, increased expression of emmprin, which is a glycoprotein that is present on the surface of most malignant cancer cells and that stimulates hyaluronan production, also induces increased resistance. Thus, perturbation of hyaluronan signaling may provide a dual therapeutic role, since it has intrinsic suppressive effects on tumor growth as well as sensitizing cancer cells to chemotherapeutic agents.

Crystal structures of the ATPase subunit of the glucose ABC transporter from Sulfolobus solfataricus: nucleotide-free and nucleotide-bound conformations

The ABC-ATPase GlcV energizes a binding protein-dependent ABC transporter that mediates glucose uptake in Sulfolobus solfataricus. Here, we report high-resolution crystal structures of GlcV in different states along its catalytic cycle: distinct monomeric nucleotide-free states and monomeric complexes with ADP-Mg(2+) as a product-bound state, and with AMPPNP-Mg(2+) as an ATP-like bound state. The structure of GlcV consists of a typical ABC-ATPase domain, comprising two subdomains, connected by a linker region to a C-terminal domain of unknown function. Comparisons of the nucleotide-free and nucleotide-bound structures of GlcV reveal re-orientations of the ABCalpha subdomain and the C-terminal domain relative to the ABCalpha/beta subdomain, and switch-like rearrangements in the P-loop and Q-loop regions. Additionally, large conformational differences are observed between the GlcV structures and those of other ABC-ATPases, further emphasizing the inherent flexibility of these proteins. Notably, a comparison of the monomeric AMPPNP-Mg(2+)-bound GlcV structure with that of the dimeric ATP-Na(+)-bound LolD-E171Q mutant reveals a +/-20 degrees rigid body re-orientation of the ABCalpha subdomain relative to the ABCalpha/beta subdomain, accompanied by a local conformational difference in the Q-loop. We propose that these differences represent conformational changes that may have a role in the mechanism of energy-transduction and/or allosteric control of the ABC-ATPase activity in bacterial importers.

Sestamibi is a substrate for MDR1 and MDR2 P-glycoprotein genes

Technetium-99m sestamibi has attracted interest for assessment of the function of P-glycoproteins, which are well expressed in the liver and have roles in biliary transport and the removal of chemotherapeutic drugs. To further examine the cross-reactivity of (99m)Tc-sestamibi for P-glycoprotein family members, we conducted studies in animals. Hepatobiliary secretion of (99m)Tc-sestamibi was determined in normal FVB/N mice, mutant mice with specific P-glycoprotein deficiencies in the FVB/N background, normal Long-Evans Agouti (LEA) rats, and Long-Evans Cinnamon (LEC) rats with abnormal copper transport and liver disease but intact P-glycoprotein expression. After intrasplenic injection, (99m)Tc-sestamibi was rapidly incorporated in the mouse and rat liver, with maximal accumulation after 102+/-31 and 109+/-16 s, respectively ( P=NS). In normal mice and rats, 55%+/-11% and 55%+/-6%, respectively, of the maximal sestamibi activity was retained in the liver after 1 h ( P=NS). In double knockout mice lacking both mdr1a and mdr1b homologs of the human MDR1 ( ABCB1) gene, 88%+/-11% of maximal sestamibi activity was retained in the liver after 1 h ( P<0.001). In knockout mice deficient in either mdr1a gene or mdr2 ( ABCB4) gene, biliary sestamibi excretion was also impaired, although this impairment was relatively less pronounced in ABCB4-deficient mice than in double knockout mice lacking both ABCB1 gene homologs ( P<0.03). Hepatobiliary sestamibi excretion in LEC rats was not different from that in control normal rats, despite the presence of significant liver disease in the former. Hepatobiliary sestamibi excretion requires P-glycoproteins and is unperturbed in chronic liver disease. Sestamibi appears to be a substrate for both ABCB1 and ABCB4 genes, although the former utilizes it far more efficiently. Assessment of P-glycoprotein activity with sestamibi should consider how regulation of ABCB1 and related family members might modulate sestamibi incorporation.

Targeting cancer cells by exploiting their resistance

Cancer cells are intrinsically resistant to growth arrest and can further acquire multidrug resistance. Current approaches to this problem are intended to reverse, overcome or prevent the drug resistance. However, the resistance of cancer cells can be exploited to kill resistant cells selectively, while sparing sensitive normal cells. As the simplest example, multidrug-resistant cells pump out protectors, such as pharmacological inhibitors of apoptosis. A sequence of at least two agents must include an exclusive protector, which is ineffective in resistant cancer cells, and an inclusive cytotoxic drug, which kills unprotected cells. By abolishing several dose-limiting side effects of chemotherapy, this strategy provides a means to treat selectively most deranged, aggressive and resistant cancers.

The distribution of Abcc6 in normal mouse tissues suggests multiple functions for this ABC transporter

We have studied the tissue distribution of Abcc6, a member of the ABC transmembrane transporter subfamily C, in normal C57BL/6 mice. RNase protection assays revealed that although almost all tissues studied contained detectable levels of the mRNA encoding Abcc6, the highest levels of Abcc6 mRNA were found in the liver. In situ hybridization (ISH) demonstrated abundant Abcc6 mRNA in epithelial cells from a variety of tissues, including hepatic parenchymal cells, bile duct epithelia, kidney proximal tubules, mucosa and gland cells of the stomach, intestine, and colon, squamous epithelium of the tongue, corneal epithelium of the eye, keratinocytes of the skin, and tracheal and bronchial epithelium. Furthermore, we detected Abcc6 mRNA in arterial endothelial cells, smooth muscle cells of the aorta and myocardium, in circulating leukocytes, lymphocytes in the thymus and lymph nodes, and in neurons of the brain, spinal cord, and the specialized neurons of the retina. Immunohistochemical analysis using a polyclonal Abcc6 rabbit antibody confirmed the tissue distribution of Abcc6 suggested by our ISH studies and revealed the cellular localization of Abcc6 in the basolateral plasma membrane in the epithelial cells of proximal convoluted tubules in the kidney. Although the function of Abcc6 is unknown, mutations in the human ABCC6 gene result in a heritable disorder of connective tissue called pseudoxanthoma elasticum (PXE). Our results demonstrating the presence of Abcc6 in epithelial and endothelial cells in a variety of tissues, including those tissues affected in PXE patients, suggest a possible role for Abcc6 in the normal assembly of extracellular matrix components. However, the presence of Abcc6 in neurons and leukocytes, two cell populations not associated with connective tissue, also suggests a more complex multifunctional role for Abcc6.

Communication between the nucleotide binding domains of P-glycoprotein occurs via conformational changes that involve residue 508

Our aim is to provide molecular understanding of the mechanisms underlying the (i) interaction between the two nucleotide binding domains (NBDs) and (ii) coupling between NBDs and transmembrane domains within P-glycoprotein (Pgp) during a transport cycle. To facilitate this, we have introduced a number of unique cysteine residues at surface exposed positions (E393C, S452C, I500C, N508C, and K578C) in the N-terminal NBD of Pgp, which had previously been engineered to remove endogenous cysteines. Positions of the mutations were designed using a model based on crystallographic features of prokaryotic NBDs. The single cysteine mutants were expressed in insect cells using recombinant baculovirus and the proteins purified by metal affinity chromatography by virtue of a polyhistidine tag. None of the introduced cysteine residues perturbed the function of Pgp as judged by the characteristics of drug stimulated ATP hydrolysis. The role of residues at each of the introduced sites in the catalytic cycle of Pgp was investigated by the effect of covalent conjugation with N-ethyl-maleimide (NEM). All but one mutation (K578C) was accessible to labeling with [(3)H]-NEM. However, perturbation of ATPase activity was only observed for the derivitized N508C isoform. The principle functional manifestation was a marked inhibition of the "basal" rate of ATP hydrolysis. Neither the extent nor potency to which a range of drugs could affect the ATPase activity were altered in the NEM conjugated N508C isoform. The results imply that the accessibility of residue 508, located in the alpha-helical subdomain of NBD1 in Pgp, is altered by the conformational changes that occur during ATP hydrolysis.

Cancer gene therapy: an awkward adolescence

At the Eleventh International Conference on Gene Therapy of Cancer (December 12-14, 2002, San Diego, CA) progress on using gene transfer technology to treat cancer was presented. Although there is as yet no cancer gene therapy being marketed, considerable progress has been made in defining likely strategies and likely targets for gene therapy of cancer. These strategies, including viral and non-viral delivery systems, and potential targets in cancer cells linked to our developing knowledge of cancer cell biology, are reviewed in this paper. Use of gene therapy to sensitize tumors to radiation and chemotherapy is one promising area of investigation. Some of the ancillary benefits of research on cancer gene therapy, including the development of public-private partnerships, recruitment of laboratory scientists into clinical research, and credentialing of potential cancer cell targets for therapies other than gene therapy, are noted.

Modulation of the classical multidrug resistance (MDR) phenotype by RNA interference (RNAi)

For reversal of MDR1 gene-dependent multidrug resistance (MDR), two small interfering RNA (siRNA) constructs were designed to inhibit MDR1 expression by RNA interference. SiRNA duplexes were used to treat human pancreatic carcinoma (EPP85-181RDB) and gastric carcinoma (EPG85-257RDB) cells. In both cellular systems, siRNAs could specifically inhibit MDR1 expression up to 91% at the mRNA and protein levels. Resistance against daunorubicin was decreased to 89% (EPP85-181RDB) or 58% (EPG85-257RDB). The data indicate that this approach may be applicable to cancer patients as a specific means to reverse tumors with a P-glycoprotein-dependent MDR phenotype back to a drug-sensitive one.

Differential response of human acute myeloid leukemia cells to gemtuzumab ozogamicin in vitro: role of Chk1 and Chk2 phosphorylation and caspase 3

Gemtuzumab ozogamicin (GO) is a humanized anti-CD33 antibody conjugated to the anticancer agent calicheamicin, approved for the treatment of CD33+-relapsed acute myeloid leukemia. We have investigated the effects of GO on 4 human myeloid leukemia lines of different French-American-British (FAB) types (KG-1, THP-1, HL-60, and NB-4), observing 3 different types of response. Exposure to GO (10-1000 ng/mL) induced G2 arrest (up to 80% of the cells) followed by apoptosis (45% of the cells) in HL-60 and NB-4 cells. By contrast, in THP-1 cells we observed a strong G2 arrest (up to 75% of the cells) with little apoptosis. Finally, the KG-1 line was completely resistant to the same concentrations of GO. These different responses did not correlate with the levels of expression of either CD33 or multiple-drug resistance proteins, although the higher cyclosporin A (CsA)-inhibitable efflux activity of KG-1 cells may play a role in the resistance of this line to the drug. We could show that Chk1 and Chk2 phosphorylation, but not p53 or p21 expression, correlated with G2 arrest, implicating the ataxia-telangiectasia mutated/ataxia-telangiectasia related (ATM/ATR)-Chk1/Chk2 pathway in the cell cycle response to GO. However, apoptosis was associated with caspase 3 activation. Freshly isolated acute myeloid leukemia (AML) cells showed patterns of response to GO in vitro similar to those observed with the cell lines, including phosphorylation of Chk2 and caspase 3 activation. Our results suggest that the different molecular pathways induced by the drug in vitro may reflect, at least in part, the variable response to GO obtained in vivo.

Inhibition of early 99mTc-MIBI uptake by Bcl-2 anti-apoptotic protein overexpression in untreated breast carcinoma

Lack of technetium-99m methoxyisobutylisonitrile ((99m)Tc-MIBI) uptake is consistently reported to predict poor response to subsequent chemotherapy in a variety of human malignant tumours. Since (99m)Tc-MIBI accumulates within mitochondria, which also play a central role in apoptosis through the integration of death signals by Bcl-2 family members, we tested whether early (99m)Tc-MIBI uptake is affected by alterations of the apoptotic pathway. Forty-two breast cancer patients were intravenously injected with 740 MBq of (99m)Tc-MIBI and planar images were obtained 10 min post injection with the patients in the prone lateral position. Ten carcinomas failed to accumulate (99m)Tc-MIBI and could not be visualised on scintigraphic images despite being larger than 1.8 cm (MIBI negative). Thirty-two of the 42 breast carcinomas showed focal uptake of (99m)Tc-MIBI (MIBI positive), and 10 min tumour-to-background ratios (T/B) varied between 1.14 and 6.93. The apoptotic index, the rate of proliferation, and the expression of the anti-apoptotic Bcl-2 protein and pro-apoptotic Bax protein were assessed in surgically excised tumours. All MIBI-negative carcinomas showed a dramatic and statistically significant reduction in the apoptotic index as compared with MIBI-positive lesions (mean+/-SD, 0.14+/-0.15 vs 1.28+/-0.83, P<0.0001) independently of rate of proliferation, tumour size and P-glycoprotein expression. Significantly higher levels of Bcl-2 were also found in MIBI-negative as compared with MIBI-positive carcinomas. In MIBI-positive lesions, an inverse significant correlation was found between T/B ratios and Bcl-2 levels ( r=-0.50, P<0.01). Our findings indicate that early uptake of (99m)Tc-MIBI in breast carcinomas is affected by alterations of apoptotic pathway. High levels of Bcl-2, despite the stabilisation of mitochondrial membrane potentials, prevent accumulation of (99m)Tc-MIBI in tumour cells. In conclusion, absent or reduced early (99m)Tc-MIBI uptake in large tumours may indicate a Bcl-2-mediated resistance to chemo- and radiotherapy.

Multidrug resistance-1 (MDR-1) in autoimmune disorders III: increased P-glycoprotein activity in lymphocytes from immune thrombocytopenic purpura patients

OBJECTIVE

P-glycoprotein (P-gp) expression has been widely observed in normal and neoplastic cells. The physiologic role of P-gp involves hormone and metabolite secretion, bacterial product detoxification, and transport of several drugs to the extracellular space. Multidrug resistance-1 is characterized by drug extrusion through P-gp, reducing the intracellular levels of drugs and diminishing their pharmacological effects. Treatment of immune thrombocytopenic purpura (ITP) includes agents that are substrates of P-gp; hence, the objective of this study was to analyze the functional activity of P-gp in lymphocytes from patients with ITP.

PATIENTS AND METHODS

30 ITP patients (9 refractory, 5 dependent, 14 responders to treatment, and 2 with stable disease) and 25 healthy controls were studied. Peripheral blood mononuclear cells were isolated by gradient centrifugation and incubated with daunorubicin (a fluorescent drug extruded by P-gp). Functional activity of P-gp was analyzed by flow cytometry. Results were expressed as the percentage of lymphocytes able to extrude daunorubicin.

RESULTS

ITP patients showed an increased number of lymphocytes with P-gp activity (mean=12.3%+/-16%) when compared to controls (mean=0.87%+/-0.72%) (p<0.05). P-gp function was higher in the refractory group (median=9.4%) than in the treatment-dependent (median=5.4%), responder (median=6.4%), and stable disease (median=5.2%) groups, although no statistical differences were found among them.

CONCLUSION

Enhanced P-gp activity in ITP may be related to an unfavorable clinical outcome and poor response to treatment. Furthermore, P-gp function might affect therapeutic requirements for disease control.

Structure of the substrate binding pocket of the multidrug transporter EmrE: site-directed spin labeling of transmembrane segment 1

Site-directed spin labeling (SDSL) was used to explore the structural framework responsible for the obligatory drug-proton exchange in the Escherichia coli multidrug transporter, EmrE. For this purpose, a nitroxide scan was carried out along a stretch of 26 residues that include transmembrane segment 1 (TMS1). This segment has been implicated in the catalytic mechanism of EmrE due to the presence of the highly conserved glutamate 14, a residue absolutely required for ligand binding. Sequence-specific variation in the accessibilities of the introduced nitroxides to molecular oxygen reveals a transmembrane helical conformation along TMS1. One face of the helix is in contact with the hydrocarbon interior of the detergent micelle while the other face appears to be solvated by an aqueous environment, resulting in significant exposure of the nitroxides along this face to NiEDDA. TMS1 from two different subunits are in close proximity near a 2-fold axis of symmetry as revealed by the analysis of spin-spin interactions at sites 14 and 18. The limited extent of spin-spin interactions is consistent with a scissor-like packing of the two TMS1. This results in a V-shaped chamber which is in contact with the aqueous phase near the N-terminus. The spatial organization of TMS1, particularly the close proximity of E14, is consistent with a proposed mechanistic model of EmrE [Yerushalmi, H., and Schuldiner, S. (2000) Biochemistry 39, 14711-14719] where substrate extrusion is coupled to proton influx through electrostatic interactions and shifts of the glutamate 14 pK(a) during the cycle.

Mechanisms of anticancer drug resistance

Chemotherapy agents are extremely important in the treatment of liquid malignancies, such as lymphoma, myeloma, and chronic lymphocytic leukemia. In addition, chemotherapy agents have proven effective in the adjuvant treatment of solid tumors, such as osteosarcoma, hemangiosarcoma, transitional cell carcinoma, and others. Unfortunately, chemotherapy resistance in these situations is the most significant cause of treatment failure. Therefore, the ability to predict, treat, or circumvent resistance is extremely likely to improve clinical outcomes. This article has reviewed the most widely investigated forms of chemotherapy resistance, such as reduced drug accumulation, increased DNA damage repair, decreased apoptosis, and others; however, new mechanisms are being found at an alarming pace. In addition, investigations to date have routinely centered on single-cell mechanisms of drug resistance, and cancer is truly a three dimensional disease. The elucidation of mechanisms surrounding (1) how tumors interact with their normal microenvironment, (2) how tumors interact in a three-dimensional environment, and (3) a better understanding of basic tumor physiology and biology may supersede in importance those previously elucidated single-cell mechanisms of chemoresistance.

DJ-927, a novel oral taxane, overcomes P-glycoprotein-mediated multidrug resistance in vitro and in vivo

DJ-927 is a novel taxane, which was selected for high solubility, non-neurotoxicity, oral bioavailability, and potent antitumor activity. In this study, we compared the in vitro and in vivo efficacy of DJ-927 with those of paclitaxel and docetaxel. DJ-927 exhibited stronger cytotoxicity than paclitaxel and docetaxel in various tumor cell lines, especially against P-glycoprotein (P-gp)-expressing cells. The cytotoxicity of DJ-927, unlike those of other taxanes, was not affected by the P-gp expression level in tumor cells, or by the co-presence of a P-gp modulator. When intracellular accumulation of the three compounds was compared, intracellular amounts of DJ-927 were much higher than those of paclitaxel or docetaxel, particularly in P-gp-positive cells. In vivo, DJ-927 showed potent antitumor effects against two human solid tumors in male BALB/c-nu/nu mice, and yielded significant life-prolongation in a murine liver metastasis model with male C57BL/6 mice, in which neither paclitaxel nor docetaxel was effective. The results demonstrate the superior efficacy of orally administered DJ-927 over intravenously administered paclitaxel or docetaxel against P-gp-expressing tumors, probably due to higher intracellular accumulation. A phase I clinical trials of DJ-927 is currently ongoing in the US.

Multidrug resistance modulators PSC 833 and CsA show differential capacity to induce apoptosis in lymphoid leukemia cell lines independently of their MDR phenotype

Among the mechanisms that induce multidrug resistance (MDR), one of those most frequent is over-expression of a phosphoglycoprotein (Pgp) encoded in the mouse by the mdr-1 and mdr-3 genes. We have demonstrated that cyclosporin-A (CsA) as well as its analogue PSC 833 were able to revert the MDR phenotype in murine cell lines resistant to vincristine (LBR-V160) or doxorubicin (LBR-D160). The aim of this work was to evaluate the ability of PSC 833 and CsA to modulate mdr-1, mdr-3 and mrp-1 genes as well as to induce apoptosis analyzing the mechanism involved in the above tumor cell lines. By semi-quantitative RT-PCR, we demonstrated that mdr-3 was over-expressed in both resistant lines while mdr-1 was over-expressed only in LBR-V160; in contrast, mrp-1 expression was not evidenced in any of the cell lines. After treatment with 0.1 microg ml(-1) of either PSC 833 or CsA, LBR-V160 showed no changes in mdr-1 but decreased mdr-3 expression, while LBR-D160 failed to display any modification in the expression of these genes. Apoptosis was evidenced by fluorescence microscopy, S minuscule accumulation and agarose gel electrophoresis. Our results demonstrated that CsA (1 microg ml(-1)) was able to induce apoptosis in all cell lines: 18.31% (+/-4.46) for LBR-, 25.96% (+/-5.24) for LBR-V160 and 27.36% (+/-4.12) for LBR-D160, while PSC 833 (1 microg ml(-1)) only induced apoptosis 21.51% (+/-5.73) in LBR-V160 cell line. The expression of Bcl-2 family proteins (Bcl-2, Bax and Bcl-x(L)) was analyzed by flow cytometry showing high expression of the three proteins which was not significantly modified after treatment with either PSC 833 or CsA on the sensitive as well as on the resistant cell lines. Single stranded conformation polymorphisms analysis of p53 (Trp53) gene in the cell lines showed no mutation in exons 5-8 of the tumor suppressor gene. We conclude that depending on the concentration used, PSC 833 and CsA may act either by modulating the mdr-3 gene (0.1 microg ml(-1)) or by direct impact on the cells through induction of apoptosis (1 microg ml(-1)), in the latter case through a mechanism that might act independent of the Bcl-2 family proteins.

Xenobiotic transporter expression and function in the human mammary gland

Xenobiotic transport in the mammary gland has tremendous clinical, toxicological and nutritional implications. Mechanisms such as passive diffusion, carrier-mediated transport, and transcytosis mediate xenobiotic transfer into milk. In vivo animal and human studies suggest the functional expression of both xenobiotic and nutrient transporters in the lactating mammary gland and the potential involvement of such systems in the significant accumulation of certain compounds in milk. In vitro cell culture systems provide further evidence for carrier-mediated transport across the lactating mammary epithelium. Additionally, molecular characterization studies indicate the expression of various members of the organic cation transporter, organic anion transporter, organic anion polypeptide transporter, oligopeptide transporter, nucleoside and nucleobase transporter, multidrug resistant transporter, and multidrug resistant-like protein transporter families at the lactating mammary epithelium. The in vivo relevance of the expression of such xenobiotic and nutrient transporters and their involvement in drug disposition at the mammary gland requires investigation.

Drug sensitivity and resistance genes in cancer chemotherapy: a chemogenomics approach

Resistance to antineoplastic drugs represents a serious obstacle to successful cancer treatment. Genome-wide studies correlating drug response phenotypes with large DNA/tissue microarray and proteomic datasets have been performed to identify the genes and proteins involved in chemosensitivity or drug resistance. The goal is to identify a set of chemosensitivity and/or resistance genes for each drug that are predictive of treatment response. Therefore, validated pharmacogenomic biomarkers offer the potential for the selection of optimal treatment regimens for individual patients and for identifying novel therapeutic targets to overcome drug resistance.

Association of multidrug resistance in epilepsy with a polymorphism in the drug-transporter gene ABCB1

BACKGROUND

One third of patients with epilepsy have drug-resistant epilepsy, which is associated with an increased risk of death and debilitating psychosocial consequences. Because this form is resistant to multiple antiepileptic drugs, the mode of resistance must be nonspecific, involving drug-efflux transporters such as ATP-binding cassette sub-family B member 1 (ABCB1, also known as MDR1 and P-glycoprotein 170). We hypothesized that the CC genotype at the ABCB1 C3435T polymorphism, which is associated with increased expression of the protein, influences the response to antiepileptic-drug treatment.

METHODS

ABCB1 3435 was genotyped in 315 patients with epilepsy, classified as drug-resistant in 200 and drug-responsive in 115, and 200 control subjects without epilepsy. Recently devised methods were used to control for population stratification, and linkage disequilibrium was calculated across the gene.

RESULTS

As compared with patients with drug-responsive epilepsy, patients with drug-resistant epilepsy were more likely to have the CC genotype at ABCB1 3435 than the TT genotype (odds ratio, 2.66; 95 percent confidence interval, 1.32 to 5.38; P=0.006). There was no genetic stratification between the two groups of patients. The polymorphism fell within an extensive block of linkage disequilibrium spanning much or all of the gene, implying that the polymorphism may not itself be causal but rather may be linked with the causal variant.

CONCLUSIONS

These pharmacogenomic results identify a genetic factor associated with resistance to antiepileptic drugs.