Reduced DNA topoisomerase II in VP-16 resistant mouse breast cancer cell line

Abstract VP-16 resistant cells, Fvp r 50 (50B-3), were isolated from mouse breast cancer cell line FM3A. 50B-3 cells showed 84-fold higher resistance than their parent cells. Reduced drug uptake was not found in resistant cells. Quantitative analysis of drug-stimulated DNA cleavage activity using 3' 32 P end-labeled pBR322 restriction fragments showed that VP-16 stimulated DNA-topoisomerase II cleavable complex forming activity in crude nuclear extract from 50B-3 cells was approximately one-fifth as compared with that of FM3A wild-type cells. Dot-blot analysis of RNA extracted from the two cell lines showed that mRNA levels of topoisomerase II in 50B-3 cells drastically decreased and catalytic activity was also 1 2 – 1 3 as compared with that of parent cells. 50B-3 cells showed cross resistance to VM-26, m -AMSA, adriamycin. Thes findings suggest that reduced topoisomerase II activity (cellular levels) and cleavable complex forming activity may be significant factors in the marked drug resistance. DNA topoisomerase II / VP-16 / drag resistance.