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Liposome-mediated modulation of multidrug resistance in human HL-60 leukemia cells
A Rahman1, S R Husain, J Siddiqui
1Department of Medicine, Vincent T. Lombardi Cancer Research Center, Georgetown University Medical Center, Washington, D.C.
Background:
Multidrug resistance (MDR) is a major obstacle in cancer treatment. Resistance of cultured tumor cells to major classes of cytotoxic drugs is frequently due to expression of a plasma membrane P-glycoprotein encoded by MDR genes. We have demonstrated that liposome-encapsulated doxorubicin is more toxic than the free drug and that it modulates MDR in Chinese hamster LZ cells and human colon cancer cells.
Purpose:
To investigate further the association between expression of P-glycoprotein and modulation of MDR by liposome-encapsulated doxorubicin, we studied vincristine-resistant HL-60/VCR leukemia cells, which express P-glycoprotein, and doxorubicin-resistant HL-60/ADR leukemia cells, which do not.
Methods:
Cells were exposed to various concentrations of free doxorubicin and liposome-encapsulated doxorubicin. The cellular content of doxorubicin was determined by fluorescence analysis, and cytotoxicity was determined by cell growth inhibition. Photoaffinity-labeling studies of P-glycoprotein binding were performed on HL-60/VCR and HL-60/ADR cells and KB-GSV2 cells transfected with the MDR1 gene (also known as PGY1).
Results:
The concentrations that caused 50% inhibition of growth (IC50) for free doxorubicin in HL-60, HL-60/ADR, and HL-60/VCR cells were 30 nM, 9 microM, and 0.9 microM, respectively. The values for liposome-encapsulated doxorubicin in parental HL-60 cells and HL-60/ADR cells were 20 nM and 9 microM, respectively, indicating little or no sensitization. In contrast, HL-60/VCR cells were fivefold more sensitive to liposome-encapsulated doxorubicin than to free doxorubicin, and IC50 was reduced to 0.17 microM. In HL-60 cells exposed to liposome-encapsulated doxorubicin, intracellular doxorubicin accumulation was less than that seen with free drug. In contrast, in HL-60/VCR cells, accumulation was twofold to threefold higher than that with free doxorubicin. Liposome-encapsulated doxorubicin completely inhibited the photoaffinity labeling of P-glycoprotein by azidopine in membrane vesicles of HL-60/VCR cells, with a potency comparable to that of azidopine, suggesting that circumvention of MDR by liposomes is related to their specific interaction with P-glycoprotein. The studies with KB-GSV2 cells indicated that blank liposomes can directly inhibit photoaffinity labeling of P-glycoprotein.
Conclusions:
These results demonstrate the effectiveness of liposome-encapsulated doxorubicin in overcoming resistance in the multidrug-resistant phenotype of HL-60/VCR cells by direct interaction with P-glycoprotein. Furthermore, they indicate that liposome-encapsulated doxorubicin may be an effective treatment for human cancers.
Insights
Liposome-encapsulated doxorubicin overcomes multidrug resistance (MDR) in cancer cells expressing P-glycoprotein. This liposomal formulation shows enhanced efficacy by interacting directly with P-glycoprotein, suggesting potential for improved cancer treatments.
Area of Science:
- Oncology
- Pharmacology
- Biotechnology
Background:
- Multidrug resistance (MDR) is a significant challenge in cancer therapy, often mediated by P-glycoprotein.
- Liposome-encapsulated doxorubicin has shown increased toxicity and MDR modulation in preclinical models.
Purpose of the Study:
- To investigate the role of P-glycoprotein in MDR modulation by liposomal doxorubicin.
- To compare the effects of liposomal doxorubicin on vincristine-resistant (HL-60/VCR) and doxorubicin-resistant (HL-60/ADR) leukemia cells.
Main Methods:
- Cells were treated with free and liposome-encapsulated doxorubicin.
- Cytotoxicity was assessed by cell growth inhibition (IC50).
- Doxorubicin accumulation and P-glycoprotein binding were analyzed using fluorescence and photoaffinity labeling.
Main Results:
- Liposomal doxorubicin demonstrated significantly higher sensitivity in HL-60/VCR cells (5-fold increase) compared to free doxorubicin.
- Intracellular doxorubicin accumulation increased in HL-60/VCR cells treated with liposomal doxorubicin.
- Liposomes directly inhibited P-glycoprotein binding, suggesting circumvention of MDR.
Conclusions:
- Liposome-encapsulated doxorubicin effectively overcomes MDR in P-glycoprotein-expressing cells through direct interaction.
- This liposomal formulation holds promise as an effective therapeutic strategy for human cancers exhibiting MDR.