Partial characterization of human leukemia U-937 cell sublines resistant to 9-nitrocamptothecin

Human leukemia U-937 cell sublines exhibiting various levels of resistance to 9-nitrocamptothecin (9NC) were developed after exposure to progressively increased 9NC concentrations. Increases in 9NC resistance of the cells were accompanied by decreases in proliferation rate; appearance of morphological and functional features that correlate with granulocytic maturation; decreased synthesis of topoisomerase I; increased synthesis of topoisomerase II; and inability or decreased ability to induce tumors when xenografted in nude mice. 9NC-resistant cells, transferred and propagated in 9NC-free media for 6 months, continue to exhibit resistance and other features similar to cells propagated in continual presence of 9NC. Finally, 9NC-resistant U-937 cells respond to physiological and non-physiological agents of cell differentiation, indicating that alternative treatments can be successfully used to inhibit growth of 9NC-resistant U-937 cells and tumors.