HT1080/DR4 - A P-GLYCOPROTEIN-NEGATIVE HUMAN FIBROSARCOMA CELL-LINE EXHIBITING RESISTANCE TO TOPOISOMERASE-II-REACTIVE DRUGS DESPITE THE PRESENCE OF A DRUG-SENSITIVE TOPOISOMERASE-II

HT1080/DR4 (DR4) is a doxorubicin-resistant human fibrosarcma line that exhibits 150-fold cross-resistance to etoposide but does not overexpress P-glycoprotein (one mechanism of multiple drug resistance). We examined anothe possible of multiple drug resistance). We examined another possible mechanism that could explain resistance to bothe doxorubicin and etoposide: a quantitative or qualitative alteration in topoisomerase II, the putative nuclear target of these agents. The amount of immunoreactive topoisomerase II present in whole-cell Iysates and nuclear extracts was three- to 10-fold lower in DRA than in HT1080 cells. However the topoisomerase II in nuclear extracts from both lines was sensitive to the effects of amascrine (AMSA) and etoposide. Following treatment with AMSA, etoposide, and 5-iminodaunorubicin, topoisomerase II-mediated DNA cleavage in DRA cells and nuclei was reduced compared with cleavage in HT1080 parent cells and nuclei. The difference between the Ht1080 and DR4 lines in AMSA-and 5-iminodaunorubicin-induced cleavage was similar in cells and nuclei and could be due to the lower amount of DR4 topoisomerase II. By contrast, the difference between the HT1080 and DR4 lines in etoposideinduced DNA cleavage was much greater in cells than in nuclei. This finding suggested that cytosolic factors, removed from isolated nuclei, could influence the susceptibility of intact cells to the cyotoxic and DNA-cleaving actions of etoposide. The specific activities of several against free-radical damage that may be produced by doxorubicin or etoposide, were significantly differences may constitute an additional mechanism of resistance. Regardless, the magnitude of the resistance of DR4 to doxorubicin and etoposide cannot be explained solely on the basis of a topoisomerase II-related mechanism.[J Natl Cancer Inst 82:1553-1561, 1990]. © 1990 Oxford University Press.