Functional study of multidrug resistance with fluorescent dyes. Limits of the assay for low levels of resistance and application in clinical samples

Proteomic analysis of imatinib-resistant CML-T1 cells reveals calcium homeostasis as a potential therapeutic target

Chronic myeloid leukemia (CML) therapy has markedly improved patient prognosis after introduction of imatinib mesylate for clinical use. However, a subset of patients develops resistance to imatinib and other tyrosine kinase inhibitors (TKIs), mainly due to point mutations in the region encoding the kinase domain of the fused BCR-ABL oncogene. To identify potential therapeutic targets in imatinib-resistant CML cells, we derived imatinib-resistant CML-T1 human cell line clone (CML-T1/IR) by prolonged exposure to imatinib in growth media. Mutational analysis revealed that the Y235H mutation in BCR-ABL is probably the main cause of CML-T1/IR resistance to imatinib. To identify alternative therapeutic targets for selective elimination of imatinib-resistant cells, we compared the proteome profiles of CML-T1 and CML-T1/IR cells using 2-DE-MS. We identified eight differentially expressed proteins, with strongly upregulated Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) in the resistant cells, suggesting that this protein may influence cytosolic pH, Ca²⁺ concentration or signaling pathways such as Wnt in CML-T1/IR cells. We tested several compounds including drugs in clinical use that interfere with the aforementioned processes and tested their relative toxicity to CML-T1 and CML-T1/IR cells. Calcium channel blockers, calcium signaling antagonists and modulators of calcium homeostasis, namely thapsigargin, ionomycin, verapamil, carboxyamidotriazole and immunosuppressive drugs cyclosporine A and tacrolimus (FK-506) were selectively toxic to CML-T1/IR cells. The putative cellular targets of these compounds in CML-T1/IR cells are postulated in this study. We propose that Ca²⁺ homeostasis can be a potential therapeutic target in CML cells resistant to TKIs. We demonstrate that a proteomic approach may be used to characterize a TKI-resistant population of CML cells enabling future individualized treatment options for patients.

Paired image- and FACS-based toxicity assays for high content screening of spheroid-type tumor cell cultures

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Design of a high throughput toxicity assay for spheroid-type tumor cell cultures.

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Cell viability, rather than a proxy thereof, is measured.

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Cell viability is measured on a per-cell, rather than per-well, basis.

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The method can be implemented on standard automated microscopes.

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It provides a low-cost approach for accurate and reproducible toxicity screens.

Novel spheroid-type tumor cell cultures directly isolated from patients’ tumors preserve tumor characteristics better than traditionally grown cell lines. However, such cultures are not generally used for high-throughput toxicity drug screens. In addition, the assays that are commonly used to assess drug-induced toxicity in such screens usually measure a proxy for cell viability such as mitochondrial activity or ATP-content per culture well, rather than actual cell death. This generates considerable assay-dependent differences in the measured toxicity values. To address this problem we developed a robust method that documents drug-induced toxicity on a per-cell, rather than on a per-well basis. The method involves automated drug dispensing followed by paired image- and FACS-based analysis of cell death and cell cycle changes. We show that the two methods generate toxicity data in 96-well format which are highly concordant. By contrast, the concordance of these methods with frequently used well-based assays was generally poor. The reported method can be implemented on standard automated microscopes and provides a low-cost approach for accurate and reproducible high-throughput toxicity screens in spheroid type cell cultures. Furthermore, the high versatility of both the imaging and FACS platforms allows straightforward adaptation of the high-throughput experimental setup to include fluorescence-based measurement of additional cell biological parameters.

Microparticle drug sequestration provides a parallel pathway in the acquisition of cancer drug resistance

Expanding on our previous findings demonstrating that microparticles (MPs) spread cancer multidrug resistance, we now show that MPs sequester drugs, reducing the free drug concentration available to cells. MPs were isolated from drug-sensitive and drug-resistant sub-clones of a human breast adenocarcinoma cell line and from human acute lymphoblastic leukemia cells. MPs were assessed for size, mitochondria, RNA and phospholipid content, P-glycoprotein (P-gp) expression and orientation and ATPase activity relative to drug sequestration capacity. Of the drug classes examined, MPs sequestered the anthracycline class to a significant degree. The degree of sequestration was likely due to the size of MPs and thus the amount of cargo they contain, to which the anthracyclines bind. Moreover, a proportion of the P-gp present on MPs was inside-out in orientation, enabling it to influx drugs rather than its typical efflux function. This was confirmed by surface immunofluorescence and by assessment of drug-stimulated ATPase activity following MP permeabilization. Thus we determined that breast cancer MPs carried a proportion of their P-gp oriented inside-out, providing active sequestration within the microvesicular compartment. These results demonstrate a capacity for MPs to sequester chemotherapeutic drugs, which has a predominantly active sequestration component for MPs derived from drug-resistant cells and a predominantly passive component for MPs derived from drug-sensitive cells. This reduction in available drug concentration has potential to contribute to a parallel pathway and complements that of the intercellular transfer of P-gp. These findings lend further support to the role of MPs in limiting the successful management of cancer.

DNA labeling in living cells

BACKGROUND

Live cell fluorescence microscopy experiments often require visualization of the nucleus and the chromatin to determine the nuclear morphology or the localization of nuclear compartments.

METHODS

We compared five different DNA dyes, TOPRO-3, TOTO-3, propidium iodide, Hoechst 33258, and DRAQ5, to test their usefulness in live cell experiments with continuous imaging and photobleaching in widefield epifluorescence and confocal laser scanning microscopy. In addition, we compared the DNA stainings with fluorescent histones as an independent fluorescent label to mark chromatin.

RESULTS

From the dyes tested, only Hoechst and DRAQ5 could be used to stain DNA in living cells. However, DRAQ5 had several advantages, namely low photobleaching, labeling of the chromatin compartments comparable to that of H2B-GFP fusion proteins, and deep red excitation/emission compatible with available genetically encoded fluorescent proteins such as C/G/YFP or mRFP.

CONCLUSIONS

The DNA dye DRAQ5 is well suited for chromatin visualization in living cells and can easily be combined with other fluorophores with blue to orange emission.

In vitro low-level resistance to azoles in Candida albicans is associated with changes in membrane lipid fluidity and asymmetry

The present study tracks the development of low-level azole resistance in in vitro fluconazole-adapted strains of Candida albicans, which were obtained by serially passaging a fluconazole-susceptible dose-dependent strain, YO1-16 (fluconazole MIC, 16 microg ml(-1)) in increasing concentrations of fluconazole, resulting in strains YO1-32 (fluconazole MIC, 32 microg ml(-1)) and YO1-64 (MIC, 64 microg ml(-1)). We show that acquired resistance to fluconazole in this series of isolates is not a random process but is a gradually evolved complex phenomenon that involves multiple changes, which included the overexpression of ABC transporter genes, e.g., CDR1 and CDR2, and the azole target enzyme, ERG11. The sequential rise in fluconazole MICs in these isolates was also accompanied by cross-resistance to other azoles and terbinafine. Interestingly, fluorescent polarization measurements performed by using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene revealed that there was a gradual increase in membrane fluidity of adapted strains. The increase in fluidity was reflected by observed change in membrane order, which was considerably decreased (decrease in fluorescence polarization values, P value) in the adapted strain (P value of 0.1 in YO1-64, compared to 0.19 in the YO1-16 strain). The phospholipid composition of the adapted strain was not significantly altered; however, ergosterol content was reduced in YO1-64 from that in the YO1-16 strain. The asymmetrical distribution of phosphatidylethanolamine (PE) between two monolayers of plasma membrane was also changed, with PE becoming more exposed to the outer monolayer in the YO1-64 strain. The results of the present study suggest for the first time that changes in the status of membrane lipid phase and asymmetry could contribute to azole resistance in C. albicans.