ALTERED STABILITY OF ETOPOSIDE-INDUCED TOPOISOMERASE-II-DNA COMPLEXESIN RESISTANT HUMAN LEUKEMIA K562 CELLS

K562 leukaemia cells were selected for resistance using 0.5 muM etopos ide (VP-16). Cloned K/VP. 5 cells were 30-fold resistant to growth inh ibition by VP-16 and 5- to 13-fold resistant to m-AMSA, adriamycin and mitoxantrone. K/VP.5 cells did not overexpress P-glycoprotein; VP-16 accumulation was similar to that in K562 cells. VP-16-induced DNA dama ge was reduced in cells and nuclei from K/VP.5 cells compared with K56 2 cells. Topoisomerase II protein was reduced 3- to 7-fold and topoiso merase IIalpha and topoisomerase IIbeta mRNAs were each reduced 3-fold in resistant cells. After drug removal, VP-16-induced DNA damage disa ppeared 1.7 times more rapidly and VP-16-induced DNA-topoisomerase II adducts dissociated 1.5 times more rapidly in K/VP.5 cells than in K56 2 cells. ATP (1 mM) was more effective in enhancing VP-16-induced DNA damage in nuclei isolated from sensitive cells than in nuclei from res istant cells. In addition, ATP (0.3-5 mM) stimulated VP-16-induced DNA -topoisomerase II adducts to a greater extent in K562 nuclei than in K /VP.5 nuclei. Taken together, these results indicate that resistance t o VP-16 in a K562 subline is associated with a quantitative reduction in topoisomerase II protein and, in addition, a distinct qualitative a lteration in topoisomerase II affecting the stability of drug-induced DNA-topoisomerase II complexes.