"Atypical" multidrug resistance in human ovarian cancer cell line A2780 selected for resistance to doxorubicin (A2780 DX3).

Human ovarian cancer cells A2780, selected for resistance to doxorubicin (A2780-DX3), are cross-resistant to various other topoisomerase-II-targeted drugs but not to vinblastine. The parental cell line was very sensitive to doxorubicin-, mitoxantrone- or etoposide(VP16)-induced DNA single-strand breaks, under deproteinizing conditions. In contrast, little or no DNA strand breakage was seen in resistant A2780-DX3 cells, even at very high concentrations, indicating a good correlation, with cytotoxicity. No significant alterations in cellular drug uptake were observed in DX3 cells. Further studies showed that the nuclei isolated from resistant cells were also resistant to mitoxantrone- or VP16-induced single-strand breaks, indicating that nuclear modifications in resistant cells are responsible for this resistance. Catalytic activity in crude nuclear extracts from wild-type and DX3 cells was almost equal. However, an assay that specifically measures generation of 5'-protein-linked breaks in 32P-labeled 3 DNA revealed that, DNA cleavage activity in nuclear extract from the DX3 cell line is profoundly resistant to a stimulation by VP16. These data indicate that stimulation of topoisomerase-II-mediated DNA cleavage is responsible for topoisomerase-II-targeted drug-cytotoxicity rather than loss of normal topoisomerase catalytic function. These data support the hypothesis that A2780-DX3 cells display an "atypical" multidrug resistance.