Correlation between DNA topoisomerase II activity and cytotoxicity in pMC540 and merodantoin sensitive and resistant human breast cancer cells.

We have shown previously that preactivated merocyanine 540 (pMC540) and merodantoin appear to mediate their cytotoxic effects via interaction with Topo II. Now, we demonstrate a correlation between DNA Topo II activity and drug-sensitive (MCF-7) and -insensitive (MDA-MB-231) breast cancer cell lines. Further studies indicate that MDA-MB-231 cells are insensitive to the cytotoxic and DNA cleavage effects of pMC540 and merodantoin. This loss of sensitivity is not associated with M(r) 170,000 P-glycoprotein over expression. However, in drug insensitive cells, the Topo II catalytic activity in crude nuclear extract was reduced two- to-three-fold and in cellular extracts was virtually absent as determined by decatenation of kDNA. Topoisomerase I activities appeared similar in extracts from MCF-7 and MDA-MB-231 cell lines. Drug-induced DNA cleavage was reduced two-to-threefold in nuclear extracts from MDA-MB-231. m-AMSA was more effective in inhibiting the decatenation activity in the nuclear extracts from MDA-MB-231 as compared to MCF-7 cells. Western blot analysis of whole-cell lysates revealed undetectable immunoreactivity of Topo II in the drug-insensitive cells. These data indicate that insensitivity of MDA-MB-231 to pMC540 and merodantoin is in part due to the reduced drug-induced formation of the cleavage complex and Topo II (170 kD) enzyme content.