Altered DNA-cleavage activity of topoisomerase II from WEHI-3B leukemia cells with specific resistance to ciprofloxacin |
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Authors: | Pessina A Raimondi A Croera C Acchini M Mineo E Foti P Neri M G |
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Affiliation: | Institute of Microbiology, Faculty of Medicine, University of Milan, 20133 Milan, Italy. pessinaa@mailserver.unimi.it |
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Abstract: | In order to investigate the mechanisms of drug resistance arising in tumor cells, we investigated the capacity of fluoroquinolones to inhibit the in vitro growth of WEHI-3B monomyelocytic leukemia cells and then we established 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 intracellular accumulation of CPX is observed in WEHI-3B/CPX cells, these cells 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 topoisomerase I sensitivity to CPX and camptothecin inhibition, but no difference was observed. As the treatment with CPX showed that the resistant cell line suffers a significantly lower number of breaks in the DNA molecule we also addressed our investigations to the topoisomerase II-dependent DNA cleavage that, in the resistant clone, was found dramatically less susceptible to be enhanced by CPX both in pre-strand and post-strand DNA passage conditions. WEHI-3B/CPX cells do not express any character of multidrug resistance and represent a rare case of specific drug resistance to CPX. The specific resistance to CPX observed in these cells is related to a functional decrease of topoisomerase II cleavage activity. It could be consequent to a decreased binding affinity of CPX for the topoisomerase II--DNA complex or to a decreased affinity or specificity of topoisomerase II for its DNA cleavage sites. |
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