Altered DNA-cleavage activity of topoisomerase II from WEHI-3B leukemia cells with specific resistance to ciprofloxacin

In order to investigate the mechanisms of drug resistance arising in tumor cells, we investigated the capacity of fluoroquinolones to inhibit the in v itro growth of WEHI-3B monomyelocytic leukemia cells and then we establishe d a variant of this line (currently maintained in the absence of drug). The line, named WEHI-3B/CPX, expresses a specific resistance to ciprofloxacin (CPX; resistance index=17.3+/-2.2), and does not show cross-resistance with other fluoroquinolones, camptothecin and topoisomerase II inhibitors such as doxorubicin, etoposide and teniposide. Although a little decrease in int racellular accumulation of CPX is observed in WEHI-3B/CPX cells, these cell s do not express MDR or LRP markers, and the resistance is not circumvented by verapamil, Purified nuclear extracts from WEHI-3B and WEHI-3B/CPX cells were tested for topoisomerase I catalytic activity and checking in vitro t opoisomerase I sensitivity to CPX and camptothecin inhibition, but no diffe rence was observed. As the treatment with CPX showed that the resistant cel l line suffers a significantly lower number of breaks in the DNA molecule w e also addressed our investigations to the topoisomerase Ii-dependent DNA c leavage that, in the resistant clone, was found dramatically less susceptib le to be enhanced by CPX both in pre-strand and post-strand DNA passage con ditions. WEHI-3B/CPX cells do not express any character of multidrug resist ance and represent a rare case of specific drug resistance to CPX, The spec ific resistance to CPX observed in these cells is related to a functional d ecrease of topoisomerase II cleavage activity. It could be consequent to a decreased binding affinity of CPX for the topoisomerase II-DNA complex or t o a decreased affinity or specificity of topoisomerase II for its DNA cleav age sites. [(C) 2001 Lippincott Williams & Wilkins,].