Drug-resistant sublines of the human U-937 myeloid leukemia cell line were selected in doxorubicin concentrations of 10, 40, and 200 ng/mL (designated U-A10, U-A40, and U-A200, respectively). Northern blot analysis showed overexpression of the multidrug resistance-associated protein (MRP) gene, but not MDR1, in U-A10 cells as compared with parental U-937 cells. Prolonged passage of U-A10 cells in 10 ng/mL of doxorubicin had little effect on MRP RNA levels, but increased MDR1 expression. The U-A40 and U-A200 cells, derived by selection of U-A10 cells, showed high levels of both MRP and MDR1 expression. None of the drug-resistant cell lines showed MRP or MDR1 gene amplification as judged by Southern blot analysis. U-A10 cells exhibited minimal decreased net accumulation of anthracycline, whereas U-A40 and U-A200 cells showed more significantly decreased drug accumulation as compared with U-937 cells. Subcellular anthracycline accumulation in U-937 cells as determined by fluorescence microscopy showed daunorubicin fluorescence predominately in the nucleus. However, the drug-resistant cell lines showed minimal nuclear drug accumulation with marked redistribution of drug into a vesicular compartment. Treatment with sodium azide/2-deoxyglucose, 2,4-dinitrophenol, or monensin, but not verapamil, abolished the vesicular accumulation. These studies in doxorubicin-selected U-937 cells indicate that induction of MRP overexpression occurs before that for the MDR1 gene. In addition, the drug-resistant cells possess an energy-dependent redistribution of anthracyclines into a nonnuclear vesicular compartment.

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