Abstract

Human leukemic HL60 cells were selected for resistance to alkylating agents by stepwise exposure to increasing concentrations ofl-phenylalanine mustard (melphalan). The resulting resistant cell line (R-HL60) was 4-fold resistant (melphalan IC₅₀ value, 27.84 ± 4.2 μM) to melphalan compared with parental HL60 cells (melphalan IC₅₀ value, 6.9 ± 1.78 μM). Nuclear extracts from R-HL60 cells possess a ∼4-fold increase in DNA topoisomerase II activity compared with parental HL60 cells. As determined using Western blot analysis, the level of topoisomerase IIα protein expressed inR-HL60 cells was approximately 3-fold that of parental HL60 cells. However, there were no differences observed in the level of topoisomerase IIβ protein, in the topoisomerase I activity, or in the level of topoisomerase I protein expression comparing the two cell lines. R-HL60 cells were 5-fold more sensitive than parental HL60 cells to the cytotoxic effect of the topoisomerase II inhibitor doxorubicin. The sensitivity to the cytotoxic effects of the topoisomerase I inhibitor camptothecin did not differ inR-HL60 and parental HL60 cell lines. Preincubation with doxorubicin significantly increased melphalan-induced interstrand DNA cross-link formation and cytotoxicity in R-HL60 cells compared with the parental HL60 cells. The affinity of topoisomerase II for UV-irradiated cross-linked HL60 DNA was increased by ∼2.5-fold compared with that of HL60 native DNA. The affinity of topoisomerase II for both UV-irradiated (cross-linked) and native DNA was significantly decreased after doxorubicin pretreatment. Elevated topoisomerase II activity and the increased affinity of topoisomerase II for cross-linked DNA in melphalan-resistant cells seems to contribute to alkylator resistance by changing DNA topology, thereby facilitating DNA repair.