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We established a daunorubicin (DNR)-resistant cell line derived from human leukemia cell line K562, (K562/D1-9), which also shows multidrug resistance (MDR). K562 cells were cultured with serially increasing concentrations up to 1.0 microM of DNR and then cloned by the limiting dilution method. K562/D1-9 cells were found to be 28 times more resistant to DNR than their its parent cells. Intracellular accumulation of DNR in K562/D1-9 was less than in the wild type, and P-glycoprotein (PGP) was overexpressed. Both DNR resistance and its intracellular accumulation were partially reversed by addition of verapamil to K562/D1-9 cells, but not to K562 cells. Topoisomerase II (Topo II) activity was decreased in K562/D1-9 cells. In contrast to other drugs, such as doxorubicin and vincristine, verapamil could not reverse drug resistance to VP-16 in the K562/D1-9 cell line, suggesting the importance of Topo II as the target of MDR. Protein kinase C (PKC) level was higher in K562/D1-9 than in K562. These findings suggested that the mechanism of MDR in this cell line might be multifactorial, including PGP, topo II and PKC. The K562/D1-9 cell line may be a good model for studying drug resistance in leukemia chemotherapy.
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