Kinetics of daunorubicin transport in Ehrlich ascites tumor cells with different expression of P-glycoprotein

Thiosemicarbazones Can Act Synergistically with Anthracyclines to Downregulate CHEK1 Expression and Induce DNA Damage in Cell Lines Derived from Pediatric Solid Tumors

Anticancer therapy by anthracyclines often leads to the development of multidrug resistance (MDR), with subsequent treatment failure. Thiosemicarbazones have been previously suggested as suitable anthracycline partners due to their ability to overcome drug resistance through dual Pgp-dependent cytotoxicity-inducing effects. Here, we focused on combining anthracyclines (doxorubicin, daunorubicin, and mitoxantrone) and two thiosemicarbazones (DpC and Dp44mT) for treating cell types derived from the most frequent pediatric solid tumors. Our results showed synergistic effects for all combinations of treatments in all tested cell types. Nevertheless, further experiments revealed that this synergism was independent of Pgp expression but rather resulted from impaired DNA repair control leading to cell death via mitotic catastrophe. The downregulation of checkpoint kinase 1 (CHEK1) expression by thiosemicarbazones and the ability of both types of agents to induce double-strand breaks in DNA may explain the Pgp-independent synergism between anthracyclines and thiosemicarbazones. Moreover, the concomitant application of these agents was found to be the most efficient approach, achieving the strongest synergistic effect with lower concentrations of these drugs. Overall, our study identified a new mechanism that offers an avenue for combining thiosemicarbazones with anthracyclines to treat tumors regardless the Pgp status.

Modulation of Doxorubicin Intracellular Accumulation and Anticancer Activity by Triterpene Glycoside Cucumarioside A2-2

The effect of treatment of Ehrlich ascites carcinoma (EAC) cells with multidrug resistance by holothurian triterpene glycoside, cucumarioside A₂-2 (CA₂-2) was evaluated. Calcein-AM efflux assay and doxorubicin (DOX) uptake and retention measurement in cancer cells, as well as determination of DOX cytotoxic and anticancer effects were applied. Treatment of EAC cells with CA₂-2 (0.01–0.1 μM) blocked Calcein-AM and DOX efflux from cancer cells and increased the accumulation and cytotoxicity of DOX in EAC cells. Moreover, pre-treatment of mice with EAC by CA₂-2 (10 μg/kg/5 days, intraperitoneal injection (i.p.)), then transplantation of tumor cells into fresh animals and subsequent treatment of these mice with DOX (2 mg/kg/3 days i.p.) significantly increased average life span (ALS) of mice bearing a tumor and therefore boosted the antitumor effect of doxorubicin in vivo.

Metabolic and non-metabolic pathways that control cancer resistance to anthracyclines

Anthracyclines Doxorubicin, Epirubicin, Daunorubicin and Idarubicin are used to treat a variety of tumor types in the clinics, either alone or, most often, in combination therapies. While their cardiotoxicity is well known, the emergence of chemoresistance is also a major issue accounting for treatment discontinuation. Resistance to anthracyclines is associated to the acquisition of multidrug resistance conferred by overexpression of permeability glycoprotein-1 or other efflux pumps, by altered DNA repair, changes in topoisomerase II activity, cancer stemness and metabolic adaptations. This review further details the metabolic aspects of resistance to anthracyclines, emphasizing the contributions of glycolysis, the pentose phosphate pathway and nucleotide biosynthesis, glutathione, lipid metabolism and autophagy to the chemoresistant phenotype.

MicroRNA expression profiles associated with development of drug resistance in Ehrlich ascites tumor cells

Multidrug resistance (MDR) poses a major obstacle to successful chemotherapeutic treatment of cancer, and often involves multiple genes, which may be regulated post-transcriptionally by microRNAs (miRNAs). The purpose of the present study was therefore to identify any resistance-associated changes in miRNA expression in a sensitive and five increasingly drug-resistant Ehrlich ascites tumor (EAT) cell lines, representing different steps in the development of resistance. We used an LNA-enhanced microarray platform to study the global miRNA expression profiles in the six murine EAT cell lines, and identified growth-, hypoxia-, and resistance-specific miRNA patterns. Among the differentially expressed miRNAs, we found the two clusters miR-183∼miR-96∼miR-182 and miR-200b∼miR-200a∼miR-429 as well as miR-141 to be consistently upregulated in the MDR cell lines, while miR-125b-5p and the two clusters miR-30d∼miR-30b and miR-23b∼miR-27b∼miR-24-1 were downregulated in most of the resistant EAT cells. Several of the target genes for these miRNAs-including Zeb1/Zeb2 and members of the Fox gene family-could contribute to the drug-resistant phenotype, although we did not find that the degree of resistance was directly correlated to any specific changes in miRNA expression. Probably, the observed miRNA expression patterns reflect the underlying genomic instability of the tumor cells, and further studies are needed to explore how the highly complex regulatory miRNA networks contribute to the development of MDR.

Pumping of drugs by P-glycoprotein: a two-step process?

The apparent inhibition constant, Kapp, for the blockade of P-glycoprotein (P-gp) by four drugs, verapamil, cyclosporin A, XR9576 (tariquidar), and vinblastine, was measured by studying their ability to inhibit daunorubicin and calcein-AM efflux from four strains of Ehrlich cells with different levels of drug resistance and P-gp content. For daunorubicin as a transport substrate, Kapp was independent of [P-gp] for verapamil but increased strictly linearly with [P-gp] for vinblastine, cyclosporin A, and XR9576. A theoretical analysis of the kinetics of drug pumping and its reversal shows that Kapp for inhibition should increase linearly with the amount of pumps present in the membrane for a reverser that inhibits pumping from the cytoplasmic face. In contrast, if the reverser acts by blocking transport from the outer face, i.e., preemptively, Kapp should be independent of the number of pumps present. The experimental data suggest that verapamil blocks pumping at the extracellular face of the membrane, whereas the other three blockers act on pumping from the cytoplasmic phase. The maximum degree of inhibition was the same for all four blockers; thus, they do not act in parallel but rather, in serial, i.e., a drug that is pumped from the cytoplasmic phase has to pass the preemptive route upon leaving the cell. Our results are consistent with the Sauna-Ambudkar two-step model for pumping by P-gp. We suggest that the vinblastine/cyclosporin A/XR9576-binding site accepts daunorubicin at the cytoplasmic face and transfers it to the verapamil-binding site, from where daunorubicin is emptied at the extracellular surface.

Characterisation of non-P-glycoprotein multidrug-resistant Ehrlich ascites tumour cells selected for resistance to mitoxantrone

An Ehrlich ascites tumour cell line (EHR2) was selected in vivo for resistance to mitoxantrone (MITOX). The resistant cell line (EHR2/MITOX) was 6123-, 33-, and 30-fold-resistant to mitoxantrone, daunorubicin, and etoposide, respectively, but retained sensitivity to vincristine. The resistant cells showed moderate sensitisation to mitoxantrone on treatment with verapamil or cyclosporin A. Compared with EHR2, the multidrug resistance-associated protein mRNA was increased 13-fold in EHR2/MITOX. Western blot analysis showed an unchanged, weak expression of P-glycoprotein. Topoisomerase IIalpha was reduced to one-third in EHR2/MITOX relative to EHR2 cells, whereas topoisomerase IIbeta was present in EHR2 but could not be detected in EHR2/MITOX. In the resistant subline, net accumulation of MITOX (120 min) and daunorubicin (60 min) was reduced by 43% and 27%, respectively, as compared with EHR2. The efflux of daunorubicin from preloaded EHR2/MITOX cells was significantly increased. EHR2/MITOX microsomes had a significant basal unstimulated ATPase activity. The apparent K(i) value for vanadate inhibition of the ATPase activity in EHR2/MITOX microsomes was not significantly different from the K(i) value for P-glycoprotein-positive cells. However, whereas verapamil (50 microM) inhibited the ATPase activity of EHR2/MITOX microsomes, it stimulated the ATPase activity of microsomes derived from P-glycoprotein-positive cells. In conclusion, the resistance in EHR2/MITOX was multifactorial and appeared to be associated with: 1) a quantitative reduction in topoisomerase IIalpha and beta protein; 2) reduced drug accumulation, probably as a result of increased expression of a novel transport protein with ATPase activity; and 3) increased expression of MRP mRNA.

Characterisation of multidrug-resistant Ehrlich ascites tumour cells selected in vivo for resistance to etoposide

An Ehrlich ascites tumour cell line (EHR2) was selected for resistance to etoposide (VP16) by in vivo exposure to this agent. The resulting cell line (EHR2/VP16) was 114.3-, 5.7-, and 4.0-fold resistant to VP16, daunorubicin, and vincristine, respectively. The amount of salt-extractable immunoreactive topoisomerase IIalpha and beta in EHR2/VP16 was reduced by 30-40% relative to that in EHR2. The multidrug resistance-associated protein (MRP) mRNA was increased 20-fold in EHR2/VP16 as compared with EHR2, whereas the expression of P-glycoprotein was unchanged. In EHR2/VP16, the steady-state accumulation of [(3)H]VP16 and daunorubicin was reduced by 64% and 17%, respectively, as compared with EHR2. Deprivation of energy by addition of sodium azide increased the accumulation of both drugs to the level of sensitive cells. When glycolysis was restored by the addition of glucose to EHR2/VP16 cells loaded with drug in the presence of sodium azide, extrusion of [(3)H]VP16 and daunorubicin was induced. Addition of verapamil (25 microM) decreased the efflux of daunorubicin to the level of sensitive cells, but had only a moderate effect on the efflux of [(3)H]VP16. The resistant cells showed moderate sensitisation to VP16 on treatment with verapamil, whereas cyclosporin A had no effect. Compared with that of sensitive cells, the ATPase activity of plasma membrane vesicles prepared from EHR2/VP16 cells was very low. Vanadate inhibited the ATPase activity of EHR2/VP16 microsomes with a K(i) value of 30 microM. ATPase activity was slightly stimulated by daunorubicin, whereas vinblastine, verapamil, and cyclosporin A had no effect. In conclusion, development of resistance to VP16 in EHR2 is accompanied by a significant reduction in topoisomerase II (alpha and beta) and by increased expression of MRP mRNA (20-fold). MRP displays several points of resemblance to P-glycoprotein in its mode of action: 1) like P-glycoprotein, MRP causes resistance to a range of hydrophobic drugs; 2) MRP decreases drug accumulation in the cells and this decrease is abolished by omission of energy; and 3) MRP increases efflux of drug from cells. However, compared with that of P-glycoprotein-positive cells, the ATPase activity of MRP-positive cells is found to be low and not able to be stimulated by verapamil.

Pleiotropic resistance to diverse antimalarials in actinomycin D-resistant Plasmodium falciparum

The development and spread of multidrug-resistant Plasmodium falciparum are major health concerns. The molecular mechanisms of multidrug resistance, including resistance to many quinoline-based antimalarials, are largely unknown. In this study, we report on the isolation and partial characterization of actinomycin D (actD)-resistant P. falciparum (3D7(R)/actD2.3) from a chloroquine-susceptible strain, 3D7. The stepwise selection of an actD-resistant clone (3D7(R)/actD2.3) led to the isolation and cloning of P. falciparum that grew in the presence of 2 ng/mL of actD. The parental isolate (3D7) did not grow in the presence of a 10-fold lower drug concentration (0.2 ng/mL). The latter estimate of parasite growth was determined by direct counting of parasites in infected red blood cells. Estimates of drug resistance levels to actD, using a [(3)H]hypoxanthine uptake and incorporation method, showed a 3-fold difference in the IC(50) between 3D7 and 3D7(R)/actD2.3. Interestingly, 3D7(R)/actD2.3 P. falciparum parasites were less sensitive to several antimalarials (chloroquine, mefloquine, quinidine, and artemisinin) and to the mitochondrial specific dye Rhodamine 123. Drug transport studies using [(3)H]actD showed that 3D7(R)/actD2.3 accumulated less drug than 3D7. Moreover, the accumulation of [(3)H]actD was energy dependent. To determine if Pfmdr1 expression, previously implicated in drug resistance to certain antimalarials, mediated the resistance phenotype of 3D7(R)/actD2.3, Pfmdr1 levels in 3D7 and 3D7(R)/actD2.3 were compared by Southern and northern blot analyses. Our results revealed no differences in Pfmdr1 copy number or mRNA levels between 3D7 and 3D7(R)/actD2.3. Furthermore, comparison of Pfmdr1 sequences between 3D7 and 3D7(R)/actD2.3 showed no differences. In addition, verapamil, which reverses P-glycoprotein-mediated drug resistance in mammalian cells, did not reverse the resistance of 3D7(R)/actD2.3 to actD or chloroquine. Taken together, the findings of this study demonstrated that in vitro selection of P. falciparum for resistance to actD leads to decreased sensitivity to diverse drugs and that this pleiotropic drug resistance is associated with reduced drug accumulation not mediated by Pfmdr1.

Mechanism of action of P-glycoprotein in relation to passive membrane permeation

This review presents a survey of studies of the movement of chemotherapeutic drugs into cells, their extrusion from multidrug-resistant (MDR) cells overexpressing P-glycoprotein (Pgp), and the mode of sensitization of MDR cells to anticancer drugs by Pgp modulators. The consistent features of the kinetics from studies of the operation of Pgp in cells were combined in a computer model that enables the simulation of experimental scenarios. MDR-type drugs are hydrophobic and positively charged and as such bind readily to negatively charged phospholipid head groups of the membrane. Transmembrane movement of MDR-type drugs, such as doxorubicin, occurs by a flip-flop mechanism with a lifetime of about 1 min rather than by diffusion down a gradient present in the lipid core. A long residence time of a drug in the membrane leaflet increases the probability that P-glycoprotein will remove it from the cell. In a manner similar to ion-transporting ATPases, such as Na+,K(+)-ATPase, Pgp transports close to one drug molecule per ATP molecule hydrolyzed. Computer simulation of cellular pharmacokinetics, based on partial reactions measured in vitro, show that the efficiency of Pgp, in conferring MDR on cells, depends on the pumping capacity of Pgp and its affinity toward the specific drug, the transmembrane movement rate of the drug, the affinity of the drug toward its pharmacological cellular target, and the affinity of the drug toward intracellular trapping sites. Pgp activities present in MDR cells allow for the efficient removal of drugs, whether directly from the cytoplasm or from the inner leaflet of the plasma membrane. A prerequisite for a successful modulator, capable of overcoming cellular Pgp, is the rapid passive transbilayer movement, allowing it to reenter the cell immediately and thus successfully occupy the Pgp active site(s).

Direct binding of chloroquine to the multidrug resistance protein (MRP): possible role for MRP in chloroquine drug transport and resistance in tumor cells

Multidrug resistance protein (MRP) transports a range of compounds that include glutathione S-conjugates, amphiphilic anionic drugs, and natural-product toxins. However, the mechanism of MRP drug binding and transport is presently unclear. We recently demonstrated the direct binding of a quinoline-based photoactive drug, N-[4-[1-hydroxy-2-(dibutylamino)ethyl]quinolin-8-yl]-4-az idosalicylamide (IAAQ), to MRP at a biologically relevant site [Vezmar et al., Biochem Biophys Res Commun 241: 104-111, 1997]. In the present report, we demonstrated that the lysosomotropic or antimalarial drug chloroquine is a substrate for MRP. Specifically, our results showed that chloroquine, similar to leukotriene C4 (LTC4) and 3-(3-(2-(7-chloro-2-quinolinyl)ethenyl-phenyl)((3-(dimethyl amino-3-oxo propyl)thio)methyl)thio) propanoic acid (MK 571), inhibits the photoaffinity labeling of MRP by IAAQ. Furthermore, cell growth assays showed MRP-expressing multidrug-resistant cells (H69/AR and HL60/AR) to be more resistant to chloroquine than their parental cells (i.e., IC50 of 121 microM versus 28 microM chloroquine for H69/AR and H69, respectively). Moreover, MK 571, an LTD4 receptor antagonist, reversed the resistance of H69/AR cells to chloroquine. Drug transport studies using [14C]chloroquine demonstrated that MRP-expressing cells accumulate less drug than the parental drug-sensitive cells. The reduced accumulation of [14C]chloroquine in resistant cells was ATP dependent and was due to enhanced drug efflux. Taken together, the results of this study show that MRP modulates the transport of chloroquine by direct binding.

Relation between the turnover number for vinblastine transport and for vinblastine-stimulated ATP hydrolysis by human P-glycoprotein

Considerable uncertainty surrounds the stoichiometry of coupling of ATP hydrolysis to drug pumping by P-glycoprotein, the multidrug transporter. To estimate relative turnovers for pumping of the drug vinblastine and ATP hydrolysis, we began by measuring the number of P-glycoprotein molecules on the surface of murine NIH3T3 cells expressing the human MDR1 gene. Fluorescence of cells treated with monoclonal antibody UIC2 was determined as a function of (i) amount of antibody at a fixed number of cells and (ii) increasing cell number at constant antibody. The two together gives 1.95 x 10(6) P-glycoprotein molecules/cell. Initial uptake rates of vinblastine +/- verapamil measure the ability of P-glycoprotein to extract vinblastine from the plasma membrane before it enters the cell. As a function of [vinblastine] at 37 degrees C, they give the maximum rate of this component of outward pumping as 2.1 x 10(6) molecules s-1 cell-1 or a turnover number of 1.1 s-1. Initial rates of one-way efflux as a function of [vinblastine] at 25 degrees C +/- glucose give the maximum rate of this component of pumping as 0.59 x 10(6) molecules s-1 cell-1. The ratio of ATPase activity of P-glycoprotein at 37 and 25 degrees C is 4.6. Appropriating this ratio for pumping, maximum one-way efflux at 37 degrees C is 4.6 x 0.59 = 2.7 x 10(6) molecules s-1 cell-1, a turnover number of 1.4 s-1. The vinblastine-stimulated ATPase activity of P-glycoprotein has a turnover number of 3.5 s-1 at 37 degrees C, giving 2.8 molecules of ATP hydrolyzed for every vinblastine molecule transported in a particular direction. These calculations involve several approximations, but turnover numbers for pumping of vinblastine and for vinblastine-stimulated ATP hydrolysis are comparable. Thus, ATP hydrolysis is probably directly linked to drug transport by P-glycoprotein.

Relationship between expression level of P-glycoprotein and daunorubicin transport in LLC-PK1 cells transfected with human MDR1 gene

P-Glycoprotein-mediated transcellular transport and intracellular accumulation of [3H]daunorubicin were examined in cell monolayers with different levels of P-glycoprotein. The porcine kidney epithelial cell line LLC-PK1 was transfected with MDR1 cDNA, and four sublines, LLC-GA5, LLC-GA5-VLB4, LLC-GA5-COL10, and LLC-GA5-COL150, were obtained by culturing the cells in the absence or in the presence of 4 ng/mL vinblastine, 10 ng/mL colchicine, and 150 ng/mL colchicine, respectively. Western blot analysis showed a large difference in P-glycoprotein expression within these sublines. The degree of drug resistance was dependent on the expression level of P-glycoprotein. The amount of the unidirectional transport of [3H]daunorubicin by P-glycoprotein corresponded to the expression level of P-glycoprotein, which was followed by the decrease in intracellular accumulation of the agent. The concentration of cyclosporin A required for the inhibition of P-glycoprotein-mediated transport of [3H]daunorubicin was higher in cells with a high expression of P-glycoprotein. These findings suggest that the transport of daunorubicin by P-glycoprotein and its inhibition by cyclosporin A correspond to the expression level of P-glycoprotein.

Cellular resistance to anthracyclines

The antracyclines induce multiple intracellular effects; however, inhibition of the nuclear enzyme topoisomerase II (TOPO II) is the main mechanism of action. Resistance to anthracyclines in tumor cells is multifactorial. The main mechanisms are: (1) the classic multidrug resistance (MDR) phenotype, which is due to the presence of P-glycoprotein (PGP) in plasma membrane, that is, a "pump" that can extrude a wide range of anticancer drugs. Membrane-active drugs (e.g., verapamil) have been found in vitro to reverse this phenotype. Most clinical studies including chemosensitizers have, however, been disappointing. (2) Non-PGP-mediated MDR: this phenotype is characterized by expression of other proteins in the plasma membrane which are also able to extrude anticancer drugs. (3) Changes in the intracellular distribution of drug: this mechanism has been demonstrated in several cell lines, most often in combination with PGP or non-PGP-mediated resistance. (4) Glutathione transferases (GST) and detoxification mechanisms: these represent a multigene family of enzymes that conjugate glutathione to chemically reactive groups. Direct evidence for a causative role of GST in anthracycline resistance is missing. (5) Alterations in TOPO II (at-MDR): DNA topoisomerases are involved in several aspects of DNA metabolism, in particular genetic recombination, DNA transcription, and chromosome segregation. Low levels of expression or alterations in TOPO II are associated in vitro with resistance. (6) Increased DNA repair: in several cell lines, an increase in the efficacy of DNA repair has been associated with resistance to doxorubicin (DOX). So far, only classic MDR has been shown to contribute to resistance in clinical conditions, whereas evidence for the other mechanisms of resistance is still missing.

Influx of daunorubicin in multidrug resistant Ehrlich ascites tumour cells: correlation to expression of P-glycoprotein and efflux. Influence of verapamil

Classic multidrug resistance is characterized by a decrease in the intracellular concentration of drugs in resistant cells as compared to sensitive cells. This is correlated with the presence of P-glycoprotein in the membrane. P-glycoprotein is responsible for an active efflux of drug. In this study we investigated the correlation between P-glycoprotein and influx of daunorubicin. Four Ehrlich ascites tumour cell lines selected in vivo for resistance to daunorubicin were investigated. The sublines EHR2/0.1, EHR2/0.2, passage no. 12 of EHR2/0.8, EHR2/0.4, and passage no. 72 of EHR2/0.8 were 6-, 6-, 5-, 33-, and 35-fold resistant to daunorubicin, respectively. All sublines overexpressed P-glycoprotein as determined with Western blot. Influx was measured over 40 sec. In glucose-enriched medium influx was significantly decreased in all but one of the resistant sublines. A correlation between P-glycoprotein, degrees of resistance, and influx was demonstrated in four sublines. Comparing influx experiments with efflux experiments (Nielsen et al., Biochem Pharmacol 1994, 47, 2125-2135) we found a linear relationship between influx and efflux in the resistant sublines (r = 0.97). Verapamil (5.5 microM, 11.0 microM) increased influx significantly in all resistant sublines, whereas the drug had no effect on sensitive cells. Verapamil (3.3 microM) increased influx in the EHR2/0.8 (passage no. 72) subline to the level of sensitive cells. Comparing this result with efflux experiments, verapamil was found to increase influx preferentially. Depletion of energy (medium without glucose including Na(+)-azide) increased influx in all resistant sublines. In EHR2/0.4 and EHR2/0.8 (passage no. 72) the influx, however, was still significantly decreased after depletion of energy. In these cells further addition of verapamil increased influx to the level of EHR2. These data were consistent with the hypothesis that P-glycoprotein effluxes drug directly from the plasma membrane.