Cellular uptake, cytotoxicity, and transport kinetics of anthracyclines in human sensitive and multidrug-resistant K562 cells

Characterization of K562 cells: uncovering novel chromosomes, assessing transferrin receptor expression, and probing pharmacological therapies

Human erythroleukemic K562 cells represent the prototypical cell culture model of chronic myeloid leukemia (CML). The cells are pseudo-triploid and positive for the Philadelphia chromosome. Therefore, K562 cells have been widely used for investigating the BCR/ABL1 oncogene and the tyrosine kinase inhibitor, imatinib-mesylate. Further, K562 cells overexpress transferrin receptors (TfR) and have been used as a model for targeting cytotoxic therapies, via receptor-mediated endocytosis. Here, we have characterized K562 cells focusing on the karyotype of cells in prolonged culture, regulation of expression of TfR in wildtype (WT) and doxorubicin-resistant cells, and responses to histone deacetylase inhibition (HDACi). Karyotype analysis indicates novel chromosomes and gene expression analysis suggests a shift of cultured K562 cells away from patient-derived leukemic cells. We confirm the high expression of TfR on K562 cells using immunofluorescence and cell-surface receptor binding radioassays. Importantly, high TfR expression is observed in patient-derived cells, and we highlight the persistent expression of TfR following doxorubicin acquired resistance. Epigenetic analysis indicates that permissive histone acetylation and methylation at the promoter region regulates the transcription of TfR in K562 cells. Finally, we show relatively high expression of HDAC enzymes in K562 cells and demonstrate the chemotoxic effects of HDACi, using the FDA-approved hydroxamic acid, vorinostat. Together with a description of morphology, infrared spectral analysis, and examination of metabolic properties, we provide a comprehensive characterization of K562 cells. Overall, K562 cell culture systems remain widely used for the investigation of novel therapeutics for CML, which is particularly important in cases of imatinib-mesylate resistance.

The effects of anthracycline drugs on the conformational distribution of mouse P-glycoprotein explains their transport rate differences

P-glycoprotein (Pgp) is an ATP-dependent efflux transporter and plays a major role in anti-cancer drug resistance by pumping a chemically diverse range of cytotoxic drugs from cancerous tumors. Despite numerous studies with the transporter, the molecular features that drive anti-cancer drug efflux are not well understood. Even subtle differences in the anti-cancer drug molecular structure can lead to dramatic differences in their transport rates. To unmask these structural differences, this study focused on two closely-related anthracycline drugs, daunorubicin (DNR), and doxorubicin (DOX), with mouse Pgp. While only differing by a single hydroxyl functional group, DNR has a 4 to 5-fold higher transport rate than DOX. They both non-competitively inhibited Pgp-mediated ATP hydrolysis below basal levels. The K_m of Pgp-mediated ATP hydrolysis extracted from the kinetics curves was lower for DOX than DNR. However, the dissociation constants (K_Ds) for these drugs determined by fluorescence quenching were virtually identical. Acrylamide quenching of Pgp tryptophan fluorescence to probe the tertiary structure of Pgp suggested that DNR shifts Pgp to a "closed" conformation, while DOX shifts Pgp to an "intermediate" conformation. The effects of these drugs on the Pgp conformational distributions in a lipid bilayer were also examined by atomic force microscopy (AFM). Analysis of AFM images revealed that DNR and DOX cause distinct and significant shifts in the conformational distribution of Pgp. The results were combined to build a conformational distribution model for anthracycline transport by Pgp.

A Flow Cytometric Clonogenic Assay Reveals the Single-Cell Potency of Doxorubicin

Standard cell proliferation assays use bulk media drug concentration to ascertain the potency of chemotherapeutic drugs; however, the relevant quantity is clearly the amount of drug actually taken up by the cell. To address this discrepancy, we have developed a flow cytometric clonogenic assay to correlate the amount of drug in a single cell with the cell's ability to proliferate using a cell tracing dye and doxorubicin, a naturally fluorescent chemotherapeutic drug. By varying doxorubicin concentration in the media, length of treatment time, and treatment with verapamil, an efflux pump inhibitor, we introduced 10(5) -10(10) doxorubicin molecules per cell; then used a dye-dilution assay to simultaneously assess the number of cell divisions. We find that a cell's ability to proliferate is a surprisingly conserved function of the number of intracellular doxorubicin molecules, resulting in single-cell IC50 values of 4-12 million intracellular doxorubicin molecules. The developed assay is a straightforward method for understanding a drug's single-cell potency and can be used for any fluorescent or fluorescently labeled drug, including nanoparticles or antibody-drug conjugates.

Expression of P-gp in acute myeloid leukemia and the reversal function of As2O3 on drug resistance

To study the expression of P-glycoprotein (P-gp) and the reversal function of As₂O₃, the active ingredient of arsenic, on drug resistance in acute myeloid leukemia (AML) patients, P-gp and cluster of differentiation 34 (CD34) were examined in primary mononuclear and resistant cells, with or without As₂O₃. In addition, multidrug resistance gene 1 (MDR1) mRNA expression was investigated in K562/D cells and AML patients. In total, 28.6% of newly-treated (NT) patients and 59.1% of relapsed/refractory (RR) patients were P-gp_function⁺, and 31.7% of NT patients and 59.1% of RR patients were CD34⁺. The positivity rate of P-gp_function and CD34⁺ expression in the RR group were significantly higher compared with that in the NT group (P<0.05). In addition, higher CD34⁺, P-gp_expression⁺ and P-gp_function⁺ values were observed in older patients compared with younger patients. MDR1 expression was downregulated in certain patients following treatment with AS₂O₃. In the present study, the overexpression of P-gp was the primary cause of drug resistance in the AML patients, and MDR1 expression was downregulated by As₂O₃ in primary leukemia and drug-resistant cells.

A fluorescence-based assay for monitoring clinical drug resistance

Background and aims

Multidrug resistance (MDR) limits effectiveness in treating malignancy by modifying internalisation and/or externalisation of drugs through cancer cell membranes. In this study we describe an assay to monitor patients’ responses to chemotherapy.

Methods

The assay is based on the fluorescent properties of doxorubicin alone as well as in combination with methotrexate, vinblastine, doxorubicin and cisplatin (MVAC). The slide-based cell imaging technique was first optimised using a panel of breast and urothelial cancer cell lines and then extended to fine needle breast aspiration biopsy and urine cytology.

Results

The drug fluorescence behaviour observed on smears of clinical specimens is identical to that obtained using fixed cultured cells. The fluorescence of sensitive cells to chemotherapy is mainly localised in the nucleus, whereas resistant cells show a weak fluorescence signal localised in the cytoplasm. The difference in terms of fluorescence intensity is also highlighted through fluorescence spectra.

Conclusions

The results suggest that the assay provides clinically valuable information in predicting responses to doxorubicin and/or MVAC therapy. Originally set up on a confocal microscope, the assay was also effective using a standard epifluorescence microscope; as such it is technically simple, reliable and inexpensive.

2-Pyrrolinodoxorubicin and its peptide-vectorized form bypass multidrug resistance

A well-known mechanism leading to the emergence of multidrug-resistant tumor cells is the overexpression of P-glycoprotein, which is capable of lowering intracellular drug concentrations. In the present study, we tested the capability of 2-pyrrolinodoxorubicin (p-DOX), a highly potent derivative of DOX, to bypass multidrug resistance. The accumulation, intracellular distribution and cytotoxicity of p-DOX were tested in two cell lines (K562 and A2780) and their DOX-resistant counterparts (K562/ADR and A2780/ADR). Cellular accumulation and cytotoxicity were dramatically lowered for DOX in resistant cell lines, in comparison with non-resistant cells. In contrast, cellular accumulation, intracellular distribution and cytotoxicity of p-DOX were independent of the nature of the cell lines. The p-DOX showed potent dose-dependent inhibition of cell growth against resistant cells as compared with DOX. After treatment of resistant cells with verapamil, the intracellular levels of DOX were markedly increased and consequent cytotoxicity improved. In contrast, treatment of resistant cells with verapamil did not cause any further enhancement of cell uptake or an increase in the cytotoxic effect of the derivative p-DOX, indicating that the compound bypasses the P-glycoprotein. Finally, we show that vectorization of p-DOX by a peptide vector (SynB3) which has been shown to enhance the brain uptake of DOX and to decrease its heart accumulation does not affect this property. These results indicate that p-DOX and its vectorized form are potent and effective in overcoming multidrug resistance.

Interactions between doxorubicin and the human iron regulatory system

Anthracyclines are included in clinical treatments against various malignancies, but severe cardiotoxic side-effects and the development of resistance mechanisms limit their usefulness. Many aspects of the cellular response to anthracyclines remain debated. The status of the main regulator of iron homeostasis, namely the RNA-binding activity of iron regulatory proteins (IRPs), has been assessed herein for two types of human tumor cells and their derived doxorubicin-resistant sublines. IRPs were always fully activated in the latter, whereas only partial activation occurred in the former. Doxorubicin exposure reversibly inactivated IRP1 in small cell lung carcinoma (GLC(4)) and myelogenous leukemia (K562) cell lines, but was without effect in their derived doxorubicin-resistant sublines. In contrast, adding doxorubicin to cytosolic fractions of untreated cells or to purified IRPs led to the irreversible alteration of the RNA-binding activity of IRP1. In these different conditions, interaction between doxorubicin and the iron regulatory system disturbs iron metabolism, and cells having developed a resistance mechanism are tuned to maximize the iron supply. The results reported herein may lead the path toward a better therapeutic management of cancer patients receiving doxorubicin by discriminating between the antiproliferative and cardiotoxic properties of this anthracycline.

Infrared spectroscopy as a tool for discrimination between sensitive and multiresistant K562 cells

Fourier transform infrared spectroscopy was performed on human leukemic daunorubicin-sensitive K562 cells and their multiresistant counterpart derived by selection. Statistical analysis, including variable reduction and linear discriminant analysis was performed on sensitive and multiresistant cells spectra in order to establish a diagnostic tool for multiresistant pattern. For each of the two methods of data reduction tested [genetic algorithm or principal component analysis (PCA)] discrimination between the two cell lines was found to be possible. The best results, obtained with PCA-reduction, showed an accuracy of 93% on a distinct test set of spectra. These results demonstrate the efficiency of Fourier transform infrared spectroscopy for classification. Further analysis of the spectral differences indicated that discrimination between resistant and sensitive cells was based on variations in all cellular contents. Lipid and nucleic acid decreased, relatively, while the protein content increased.

Partial circumvention of P-glycoprotein-mediated multidrug resistance by doxorubicin-14-O-hemiadipate

Previously, we have reported partial circumvention of P-glycoprotein (Pgp)-associated resistance to doxorubicin (Dox) in MCF7/R human breast carcinoma and P388/R murine leukemia cell lines by doxorubicin-14-O-hemiadipate (H-Dox) [Povarov L.S. et al. (1995) Russian J. Bioorganic Chemistry 21: 797-803]. We felt that these changes were due to alterations in the cellular pharmacokinetics of the analog in multidrug (MDR) resistant cells, as compared to that of Dox. To address this hypothesis, we performed comparative studies of the accumulation, retention and intracellular localization of H-Dox and Dox in Dox-sensitive murine leukemia cell line P388/S and its Dox-selected. Pgp-positive drug resistant P388/R subline. These studies were performed in the presence or absence of cyclosporin A (CsA), a competitive inhibitor of Pgp. Flow cytometric analysis revealed significant differences in Dox and H-Dox accumulation in P388/R cells when compared to P388/S cells. In P388/R versus P388/S cells, there was a 38-fold decrease in Dox accumulation, but only a 5-fold decrease in H-Dox accumulation, indicating over a 7-fold increase in H-Dox buildup in resistant cells. CsA did not affect uptake or retention of either drug by sensitive cells. However, coincubation with CsA resulted in a 54-fold increase in Dox accumulation and only a 5-fold increase in H-Dox uptake in P388/R cells, restoring anthracycline levels in P388/R to 100% of that found in P388/S cells. Once internalized by the resistant cells, H-Dox was retained better than Dox regardless of presence or absence of CsA. Confocal microscopic analysis revealed the presence of H-Dox but no Dox in cellular nuclei of P388/R cells. Thus, increased activity of H-Dox toward P388/R cells was correlated with its enhanced ability to enter and be retained in these cells, and also with redistribution of H-Dox into the nuclei of the resistant cells as compared to Dox. Overall, our findings support our initial hypothesis and provide evidence that H-Dox, a 14-O-hemiadipate of doxorubicin, is affected by Pgp-mediated MDR to a lesser extent than parental Dox due to changes iin the cellular pharmacokinetics of the analog.

Nucleotide-induced conformational changes in the human multidrug resistance protein MRP1 are related to the capacity of chemotherapeutic drugs to accumulate or not in resistant cells

Intracellular accumulation of anthracycline derivatives was measured in a human embryonic kidney cell line (HEK) and a resistant subline (HEK/multidrug resistance protein (MRP1)) overexpressing MRP1 at the plasma membrane surface. Two compounds (daunorubicin and doxorubicin) were rejected outside the multidrug-resistant cells. On the contrary, three compounds (4'-deoxy-4'-iodo-doxorubicin, 4-demethoxy-daunorubicin and 3'-(3-methoxymorpholino)doxorubicin) accumulated equally within sensitive HEK cells and resistant HEK/MRP1 cells. Our main objective here was to characterize the MRP1 conformational changes mediated by the binding of these anthracycline derivatives and to determine whether these conformational changes are related to MRP1-mediated drug transport. MRP1 was reconstituted in lipid vesicles as previously described [Manciu, L., Chang, X.B., Riordan, J.R. and Ruysschaert, J.-M. (2000) Biochemistry 39, 13026-13033]. The reconstituted protein was shown to conserve its ATPase and drug transport activity. Acrylamide quenching of Trp fluorescence was used to monitor drug-dependent conformational changes. Binding of drugs (4-demethoxy-daunorubicin and 3'-(3-methoxymorpholino)doxorubicin) which accumulate in resistant cells immobilizes MRP1 in a conformational state that is insensitive to ATP binding whereas drugs rejected outside the resistant cells (daunorubicin, doxorubicin) favor a conformational change which may be a required step in the transport process.

Doxorubicin-peptide conjugates overcome multidrug resistance

A well-known mechanism leading to the emergence of multidrug-resistant tumor cells is the overexpression of P-glycoprotein (P-gp), which is capable of lowering intracellular drug concentrations. To overcome this problem, we tested the capability of two peptide vectors that are able to cross cellular membranes to deliver doxorubicin in P-gp-expressing cells. The antitumor effect of peptide-conjugated doxorubicin was tested in human erythroleukemic (K562/ ADR) resistant cells. The conjugate showed potent dose-dependent inhibition of cell growth against K562/ADR cells as compared with doxorubicin alone. Doxorubicin exhibited IC50 concentrations of 65 microM in the resistant cells, whereas vectorized doxorubicin was more effective with IC50 concentrations of 3 microM. After treatment of the resistant cells with verapamil, the intracellular levels of doxorubicin were markedly increased and consequent cytotoxicity was improved. In contrast, treatment of resistant cells with verapamil did not cause any further enhancement in the cell uptake nor in the cytotoxic effect of the conjugated doxorubicin, indicating that the conjugate bypasses the P-gp. Finally, we show by the in situ brain perfusion method in P-gp-deficient and competent mice that vectorized doxorubicin bypasses the P-gp present at the luminal site of the blood-brain barrier. These results indicate that vectorization of doxorubicin with peptide vectors is effective in overcoming multidrug resistance.

Modulation of anthracycline activity in canine mammary tumour cells in vitro by medroxyprogesterone acetate

Failure of chemotherapy with anthracyclines as a result of drug resistance and toxicity is a major problem in the clinical management of neoplasia. The aim of the present study was to evaluate the activity of medroxyprogesterone acetate (MPA) as a chemosensitiser on anthracycline cytotoxicity. The study investigated whether such an effect could be related to an increase in lipid peroxidation, nitric oxide production, membrane fluidity and intracellular anthracycline concentration. The results showed that anthracyclines decreased nitric oxide production but increased membrane viscosity (polarisation constant) and lipid hydroperoxide formation in canine mammary tumour cells. Moreover, it was found that both drug-induced cytotoxicity and membrane viscosity increased in the presence of MPA. Conversely, lipid hydroperoxides decreased in MPA-supplemented cells. Medroxyprogesterone acetate did not show any effect on nitric oxide production. The two anthracyclines used (doxorubicin and idarubicin) showed differential intranuclear accumulation in canine mammary tumour cells, and MPA significantly modified intracellular concentration of anthracyclines.

Study of apoptosis-related responses of leukemic blast cells to in vitro anthracycline treatment

Anthracyclines trigger an apoptotic cell death but their molecular targets are not totally explored. We investigated the apoptotic response of blast cells and lymphocytes from medullary samples of 31 de novo acute leukemia. Mononuclear cells were treated in vitro by therapeutic concentrations of either daunorubicin (DNR) or idarubicin (IDA) for 1 h, washed and cultured for 18 h. A multivariate analysis using flow cytometry and a CD45 gating on lymphocytes and blast cells was performed. DNR and IDA induced a Fas enhancement on both leukemic and normal cells. In blast cells the DEVDases were activated and the caspase 3 was cleaved in relation to phosphatidyl serine exposure, showing a caspase-dependent pathway in anthracycline-induced apoptosis. Apoptotic percentages were always higher for blast cells than for lymphocytes, confirming that anthracycline toxicity mainly affected tumor cells. Moreover, drug-induced apoptosis was not related to spontaneous apoptosis, suggesting that variations in response intensities were due to individual variations of sensitivity rather than to programmed life span time. The apoptotic response of P-glycoprotein-expressing blast cells was not significant, giving biological argument for the poor prognosis of multidrug resistance leukemia. Finally, Fas induction and anthracycline-induced apoptosis on blast cells were significantly higher when a complete remission was achieved, thus shedding light on potential new prognostic factors in acute leukemia.

The relative importance of passive and P-glycoprotein mediated anthracycline efflux from multidrug-resistant cells

For the four anthracyclines idarubicin, daunorubicin, epirubicin and doxorubicin the passive and active efflux rates in intact multidrug resistant cells were compared. Although highly similar structurally, these anti-tumor agents differ in lipophilicity and membrane permeability (k). The method we used was based on the continuous measurement of the cellular efflux and determination of the ratio (RVp) of transport rates just before and just after inhibition of the active transport with verapamil (Vp). Hence, RVp - 1 should reflect the active transport rate relative to the passive transport rate. If cells were single, well-stirred compartments, RVp - 1 should equal Vmax/(k.Km), where Vmax is the maximal pumping rate and Km is the Michaelis constant. However, using the plasma membrane permeabilizing agent digitonin, we found an effective intracellular anthracycline store. Particularly, when the efflux was fast, e.g. with idarubicin or in intensively pumping cells, the intracellular transport began to control the cellular efflux. Under these conditions, k underestimated the true plasma membrane permeability (k0) and RVp - 1 underestimated Vmax/(k.Km). Based on the effects of digitonin on the efflux rates in pumping and nonpumping cells, we developed an index (RVp,corrected - 1) which should equal Vmax/(k0. Km). The term Vmax/(k0.Km) varied substantially between the drugs. It appears that differences in lipophilicity between the drugs do not affect passive efflux and pumping equally. This demonstrates that passive permeation plays a substantial and independent role in determining the drug resistance for these anthracyclines. The methods developed here enable dissection of this role from that of drug pumping and intracellular subcompartmentation.

Ligand-mediated tertiary structure changes of reconstituted P-glycoprotein. A tryptophan fluorescence quenching analysis

Ligand-dependent changes in accessibility of purified P-glycoprotein, functionally reconstituted in liposomes, were investigated by fluorescence measurements. Trp quenching experiments provided evidence that P-glycoprotein adopts different tertiary structures upon binding of drug substrates in the absence and presence of MgATP and its nonhydrolyzable analog, MgATPgammaS. Five anthracycline derivatives were tested as drug substrates: daunorubicin, 4'-epi-doxorubicin, iododoxorubicin, 4-demethoxy-daunorubicin, and methoxy-morpholino-doxorubicin. Among them, daunorubicin and 4'-epi-doxorubicin have been shown to be rejected outside the multidrug-resistant cells, whereas the three others have been shown to accumulate in multidrug-resistant cells overexpressing P-glycoprotein and therefore retain their cytotoxic activity. A small conformational change was associated with nucleotide binding and amplified after nucleotide hydrolysis. Different conformational states were adopted by P-glycoprotein upon the addition of the anthracycline derivatives in the absence and presence of MgATP or MgATPgammaS. These conformational changes are shown to be related to the nature of the antitumor agents and more precisely to their capacity to accumulate in resistant cells. These data also suggest that the cytotoxicity of iododoxorubicin and 4-demethoxy-daunorubicin is related to the fact they are not transported by P-glycoprotein. On the contrary, methoxy-morpholino-doxorubicin cytotoxicity may be explained in terms of its rapid reincorporation into the plasma membrane after being transported by P-glycoprotein.