Selection and maintenance of multidrug-resistant cells

Multidrug Resistance Protein-4 Influences Aspirin Toxicity in Human Cell Line

Overexpression of efflux transporters, in human cells, is a mechanism of resistance to drug and also to chemotherapy. We found that multidrug resistance protein-4 (MRP4) overexpression has a role in reducing aspirin action in patients after bypass surgery and, very recently, we found that aspirin enhances platelet MRP4 levels through peroxisome proliferator activated receptor-α (PPARα). In the present paper, we verified whether exposure of human embryonic kidney-293 cells (Hek-293) to aspirin modifies MRP4 gene expression and its correlation with drug elimination and cell toxicity. We first investigated the effect of high-dose aspirin in Hek-293 and we showed that aspirin is able to increase cell toxicity dose-dependently. Furthermore, aspirin effects, induced at low dose, already enhance MRP4 gene expression. Based on these findings, we compared cell viability in Hek-293, after high-dose aspirin treatment, in MRP4 overexpressing cells, either after aspirin pretreatment or in MRP4 transfected cells; in both cases, a decrease of selective aspirin cell growth inhibition was observed, in comparison with the control cultures. Altogether, these data suggest that exposing cells to low nontoxic aspirin dosages can induce gene expression alterations that may lead to the efflux transporter protein overexpression, thus increasing cellular detoxification of aspirin.

Characterization of Abcc4 gene amplification in stepwise-selected mouse J774 macrophages resistant to the topoisomerase II inhibitor ciprofloxacin

Exposure of J774 mouse macrophages to stepwise increasing concentrations of ciprofloxacin, an antibiotic inhibiting bacterial topoisomerases, selects for resistant cells that overexpress the efflux transporter Abcc4 (Marquez et al. [2009] Antimicrob. Agents Chemother. 53: 2410-2416), encoded by the Abcc4 gene located on Chromosome 14qE4. In this study, we report the genomic alterations occurring along the selection process. Abcc4 expression progressively increased upon selection rounds, with exponential changes observed between cells exposed to 150 and 200 µM of ciprofloxacin, accompanied by a commensurate decrease in ciprofloxacin accumulation. Molecular cytogenetics experiments showed that this overexpression is linked to Abcc4 gene overrepresentation, grading from a partial trisomy of Chr 14 at the first step of selection (cells exposed to 100 µM ciprofloxacin), to low-level amplifications (around three copies) of Abcc4 locus on 1 or 2 Chr 14 (cells exposed to 150 µM ciprofloxacin), followed by high-level amplification of Abcc4 as homogeneous staining region (hsr), inserted on 3 different derivative Chromosomes (cells exposed to 200 µM ciprofloxacin). In revertant cells obtained after more than 60 passages of culture without drug, the Abcc4 hsr amplification was lost in approx. 70% of the population. These data suggest that exposing cells to sufficient concentrations of an antibiotic with low affinity for eukaryotic topoisomerases can cause major genomic alterations that may lead to the overexpression of the transporter responsible for its efflux. Gene amplification appears therefore as a mechanism of resistance that can be triggered by non-anticancer agents but contribute to cross-resistance, and is partially and slowly reversible.

Reduction of doxorubicin resistance in P-glycoprotein overexpressing cells by hybrid cell-penetrating and drug-binding peptide

Drug efflux by the membrane transporter P-glycoprotein (P-gp) plays a key role in multidrug resistance (MDR). In order to bypass P-gp, thus overcoming MDR, a hybrid peptide comprising a cell penetrating peptide (Tat) and a drug binding motif (DBM) has been developed to noncovalently bind and deliver doxorubicin (Dox) into MDR cells. The uptake of Dox into the leukemia cell line K562 and its P-gp overexpressing subline KD30 increased in the presence of DBM-Tat peptide. Confocal microscopy indicated that DBM-Tat associated Dox was directed to a perinuclear area of KD30 cells, while this was not observed in parent K562 cells. When KD30 cells were pretreated with the endosomotropic agent chloroquine (CLQ), peptide associated Dox redistributed into the cytosol, indicating that endocytosis was the predominant uptake route. Altered drug uptake kinetics observed by cellular accumulation assay also supported an endocytic uptake. In the presence of CLQ, DBM-Tat was able to enhance the cytotoxicity of Dox by 68.4% at 5 microM peptide concentration in KD30 cells but there were only minor effects on Dox cytotoxicity in K562 cells even in the presence of CLQ. Thus, combining Dox with DBM-Tat reduces P-gp mediated drug efflux, without a requirement for drug modification or inhibiting P-gp function.

Molecular mechanisms of drug resistance in single-step and multi-step drug-selected cancer cells

Multidrug resistance (MDR) remains one of the key determinants in chemotherapeutic success of cancer patients. Often, acquired resistance is mediated by the overexpression of ATP-binding cassette (ABC) drug transporters. To study the mechanisms involved in the MDR phenotype, investigators have generated a variety of in vitro cell culture models using both multi-step and single-step drug selections. Sublines produced from multi-step selections have led to the discovery of several crucial drug transporters including ABCB1, ABCC1, and ABCG2. Additionally, a number of mechanisms causing gene overexpression have been elucidated. To more closely mimic in vivo conditions, investigators have also established MDR sublines with single-step drug selections. Here, we examine some of the multi-step and single-step selected cell lines generated to elucidate the mechanisms involved in the development of MDR in cancer cells.

Identification of the efflux transporter of the fluoroquinolone antibiotic ciprofloxacin in murine macrophages: studies with ciprofloxacin-resistant cells

Ciprofloxacin, the most widely used totally synthetic antibiotic, is subject to active efflux mediated by a MRP-like transporter in wild-type murine J774 macrophages. To identify the transporter among the seven potential Mrps, we used cells made resistant to ciprofloxacin obtained by long-term exposure to increasing drug concentrations (these cells show less ciprofloxacin accumulation and provide a protected niche for ciprofloxacin-sensitive intracellular Listeria monocytogenes). In the present paper, we first show that ciprofloxacin-resistant cells display a faster efflux of ciprofloxacin which is inhibited by gemfibrozil (an unspecific MRP inhibitor). Elacridar, at a concentration known to inhibit P-glycoprotein and breast cancer resistance protein (BCRP), only slightly increased ciprofloxacin accumulation, with no difference between resistant and wild-type cells. Analysis at the mRNA (real-time PCR) and protein (Western blotting) levels revealed an overexpression of Mrp2 and Mrp4. Mrp4 transcripts, however, were overwhelmingly predominant (45% [wild-type cells] to 95% [ciprofloxacin-resistant cells] of all Mrp transcripts tested [Mrp1 to Mrp7]). Silencing of Mrp2 and Mrp4 with specific small interfering RNAs showed that only Mrp4 is involved in ciprofloxacin transport in both ciprofloxacin-resistant and wild-type cells. The study therefore identifies Mrp4 as the most likely transporter of ciprofloxacin in murine macrophages but leaves open a possible common upregulation mechanism for both Mrp4 and Mrp2 upon chronic exposure of eukaryotic cells to this widely used antibiotic.

Combination of tenofovir and emtricitabine plus efavirenz: in vitro modulation of ABC transporter and intracellular drug accumulation

Efflux proteins have been shown to greatly affect the uptake of antiretroviral drugs by cells and to hamper their access to the human immunodeficiency virus type 1 replication site. This study evaluated the factors that may lead to drug-drug interactions between emtricitabine (FTC), tenofovir (TFV), and efavirenz (EFV), including the modulation of efflux transporter expression and function. Peripheral blood mononuclear cells from healthy volunteers were used to determine whether or not an interaction between antiretroviral drugs and target cells occurred in any combination of FTC, TFV, EFV, FTC-TFV, TFV-EFV, or FTC-TFV-EFV. Following 20 h of treatment, intracellular drug concentrations were measured by liquid chromatography-tandem mass spectrometry. Efflux transporter functionality and inhibitor drug properties were assessed by measuring fluorescent dye efflux. ABCB1 (P-glycoprotein), ABCC 1 to 6 (multidrug resistance-associated protein), and OAT (organic anion transporter) expression in response to the treatments was quantified by semiquantitative real-time PCR. Cells treated with a double combination (FTC-TFV or TFV-EFV) or the triple combination (FTC-TFV-EFV) produced higher FTC and TFV intracellular concentrations than cells treated with FTC or TFV alone. However, no change in the EFV intracellular concentration was observed. FTC tended to induce abcc5 mRNA expression and EFV tended to induce abcc1 and abcc6 mRNA expression, whereas TFV tended to reduce mdr1, abcc1, abcc5, and abcc6 mRNA expression. Under these conditions, a decrease in the functionality of ABCC was observed, and this decrease was associated with the direct inhibitory actions of these drugs. This in vitro study reveals a benefit of the combination FTC-TFV-EFV in terms of the intracellular FTC and TFV concentrations and highlights the pharmacological mechanisms that lead to this effect.

Cellular accumulation and activity of quinolones in ciprofloxacin-resistant J774 macrophages

Ciprofloxacin is the substrate for a multidrug resistance-related protein (MRP)-like multidrug transporter in J774 mouse macrophages, which also modestly affects levofloxacin but only marginally affects garenoxacin and moxifloxacin (J.-M. Michot et al., Antimicrob. Agents Chemother. 49:2429-2437, 2005). Two clones of ciprofloxacin-resistant cells were obtained by a stepwise increase in drug concentration (from 34 to 51 to 68 mg/liter) in the culture fluid. Compared to wild-type cells, ciprofloxacin-resistant cells showed (i) a markedly reduced ciprofloxacin accumulation (12% of control) and (ii) a two- to threefold lower sensitivity to the enhancing effect exerted by MRP-inhibitors (probenecid and MK571) on ciprofloxacin accumulation or by ciprofloxacin itself. ATP-depletion brought ciprofloxacin accumulation to similarly high levels in both wild-type and ciprofloxacin-resistant cells. Garenoxacin and moxifloxacin accumulation remained unaffected, and levofloxacin showed an intermediate behavior. DNA and protein synthesis were not impaired in ciprofloxacin-resistant cells for ciprofloxacin concentrations up to 100 mg/liter (approximately 85 and 55% inhibition, respectively, in wild-type cells). In Listeria monocytogenes-infected ciprofloxacin-resistant cells, 12-fold higher extracellular concentrations of ciprofloxacin were needed to show a bacteriostatic effect in comparison with wild-type cells. The data suggest that the resistance mechanism is mediated by an overexpression and/or increased activity of the MRP-like ciprofloxacin transporter expressed at a basal level in wild-type J774 macrophages, which modulates both the intracellular pharmacokinetics and activity of ciprofloxacin.

Cells resistant to HTI-286 do not overexpress P-glycoprotein but have reduced drug accumulation and a point mutation in α-tubulin

HTI-286, a synthetic analogue of hemiasterlin, depolymerizes microtubules and is proposed to bind at the Vinca peptide site in tubulin. It has excellent in vivo antitumor activity in human xenograft models, including tumors that express P-glycoprotein, and is in phase II clinical evaluation. To identify potential mechanisms of resistance induced by HTI-286, KB-3-1 epidermoid carcinoma cells were exposed to increasing drug concentrations. When maintained in 4.0 nmol/L HTI-286, cells had 12-fold resistance to HTI-286. Cross-resistance was observed to other Vinca peptide-binding agents, including hemiasterlin A, dolastatin-10, and vinblastine (7- to 28-fold), and DNA-damaging drugs, including Adriamycin and mitoxantrone (16- to 57-fold), but minimal resistance was seen to taxanes, epothilones, or colchicine (1- to 4-fold). Resistance to HTI-286 was retained when KB-HTI-resistant cells were grown in athymic mice. Accumulation of [3H]HTI-286 was lower in cells selected in intermediate (2.5 nmol/L) and high (4.0 nmol/L) concentrations of HTI-286 compared with parental cells, whereas accumulation of [14C]paclitaxel was unchanged. Sodium azide treatment partially reversed low HTI-286 accumulation, suggesting involvement of an ATP-dependent drug pump. KB-HTI-resistant cells did not overexpress P-glycoprotein, breast cancer resistance protein (BCRP/ABCG2/MXR), MRP1, or MRP3. No mutations were found in the major β-tubulin isoform. However, 4.0 nmol/L HTI-286-selected cells had a point mutation in α-tubulin that substitutes Ser for Ala12 near the nonexchangeable GTP-binding site of α-tubulin. KB-HTI-resistant cells removed from drug became less resistant to HTI-286, no longer had low HTI-286 accumulation, and retained the Ala12 mutation. These data suggest that HTI-286 resistance may be partially mediated by mutation of α-tubulin and by an ATP-binding cassette drug pump distinct from P-glycoprotein, ABCG2, MRP1, or MRP3.

Active efflux of ciprofloxacin from J774 macrophages through an MRP-like transporter

The accumulation and efflux kinetics of ciprofloxacin have been examined by using murine J774 macrophages. Accumulation (at equilibrium) was increased (three- to fourfold) (i) when cells were incubated with high extracellular drug concentrations (typically 200 mg/liter) as opposed to clinically meaningful concentrations (10 mg/liter or lower), (ii) during ATP- depletion and at acid pH, and (iii) during coincubation with probenecid, gemfibrozil and the preferential multidrug resistance-related protein (MRP) inhibitor MK571. All these conditions were also associated with a marked decrease in ciprofloxacin efflux (half-lives increased from <2 min in controls to up to 10 min). Monensin (a proton ionophore), verapamil, and the preferential P-glycoprotein (P-gp) inhibitor GF120918 had no or only minimal effect, while cyclosporin A, which is not specific for P-gp but also acts on MRP, had an intermediate effect. Short-term uptake studies showed that the influence of the modulators on the apparent drug influx was almost immediate (delay of < or =1 min). Cells made resistant to probenecid and showing a marked overexpression of MRP1 (by Western blot analysis and confocal microscopy) accumulated ciprofloxacin to almost the same extent as did control cells, but efflux was inhibited less by probenecid, gemfibrozil, and MK571. We conclude that ciprofloxacin is subject to constitutive efflux in J774 macrophages through the activity of an MRP-related transporter which is probably distinct from MRP1. We also suggest that the cellular accumulation of ciprofloxacin in wild-type cells is constitutively impaired at therapeutically meaningful concentrations.

Isolation and characterization of Caco-2 subclones expressing high levels of multidrug resistance protein efflux transporter

PURPOSE

The purpose of this study was to isolate Caco-2 subclones that express high levels of multidrug resistance protein (MDR1) and to characterize their kinetics and affinity parameters for MDR1 substrate/inhibitors.

METHODS

The subclones were selected by a dilution cloning technique. The polarized efflux of [3H]-vinblastine across subclone cell monolayers was quantified by measuring the apparent permeability coefficients (Papp) of [3H]-vinblastine in the basolateral (BL)-to-apical (AP) direction and in the AP-to-BL direction (Papp BL-to-AP/Papp AP-to-BL) across the cell monolayers. The expression of MDR1 in the Caco-2 subclones compared with the parental Caco-2 cells was confirmed by Western blotting analysis. The kinetics parameters (Km, Vmax) of [3H]-vinblastine and the inhibitory constants (KI) of several known MDR1 substrates/inhibitors on the transport of [3H]-digoxin determined in the parental Caco-2 cells and Caco-2 subclones were also compared.

RESULTS

Three subclones (#1, #20, #21) were selected based on their polarized efflux of [3H]-vinblastine. The Papp BL-to-AP/Papp AP-to-BL ratios for #1, #20, and #21 were 110, 140, and 112, respectively, and were about 6-fold higher than the ratio observed for the parental Caco-2 cells. In the presence of GF-120918 (2 microM), a known MDR1-specific inhibitor, the Papp BL-to-AP/Papp AP-to-BL ratios were significantly decreased, suggesting that these cells were overexpressing MDRI. The Km values observed for vinblastine in the Caco-2 subclones were nearly identical to the value observed in the parental Caco-2 cells. In contrast, the Vmax values observed in the subclones were approximate 26-69% higher. The KI values observed for various known MDR1 substrates/inhibitors on [3H]-digoxin transport were nearly identical to those in the parental Caco-2 cells and Caco-2 subclones. The high functional efflux activities of these subclones were stable up to 6 months.

CONCLUSIONS

Subclones #1, #20, #21 express high levels of MDR1. These Caco-2 subclones may be useful models for profiling drugs for their MDR1 substrate activity and for establishing structure-transport relationships for this efflux transporter.