A new multiple drug-resistant Chinese hamster ovary cell line, CHO-SMR5, has been isolated which demonstrates a direct correlation between reduced cellular topoisomerase II activity (5-fold reduction) and a low level of resistance (3- to 7-fold) to topoisomerase II inhibitors. This cell line, initially selected for resistance to 9-(4,6-O-ethylidene-β-d-glucopyranosyl)-4′-demethylepipodophyllotoxin, exhibits cross-resistance to other topoisomerase II inhibitors including 4′-(9-acridinylamino)methanesulfon-m-anisidide, doxorubicin, and mitoxantrone. The resistant cells show a 4,5-fold decrease in topoisomerase II by immunoblotting when compared to wild-type cells. Drug uptake studies reveal equivalent equilibrium intracellular concentrations of [3H]9-(4,6-O-ethylidene-β-d-glucopyranosyl)-4′-demethylepipodophyllotoxin in the resistant and parental cells. The catalytic activity of topoisomerase II (decatenation of kinetoplast DNA) is 5-fold less in the drug-resistant cell line relative to wild-type Chinese hamster ovary cells. Drug-induced DNA damage, measured as either formation of DNA double-strand breaks or covalent DNA-enzyme complexes, is 4-fold less in the resistant cell line. Finally, Northern blot analysis demonstrates a 5-fold reduction in topoisomerase II mRNA isolated from log phase CHO-SMR5 cells. These findings suggest that a reduced level of topoisomerase II is likely to be the sole mechanism of drug resistance in this novel cell line.


This work was supported by USPHS Grant CA01124 awarded by the National Cancer Institute and by Brown Cancer Center Funds.

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