RT Journal Article SR Electronic T1 Maleimide Is a Potent Inhibitor of Topoisomerase II in Vitro and in Vivo: A New Mode of Catalytic Inhibition JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 1235 OP 1243 DO 10.1124/mol.61.5.1235 VO 61 IS 5 A1 Lars H. Jensen A1 Axelle Renodon-Corniere A1 Irene Wessel A1 Seppo W. Langer A1 Birgitte Søkilde A1 Elisabeth V. Carstensen A1 Maxwell Sehested A1 Peter B. Jensen YR 2002 UL http://molpharm.aspetjournals.org/content/61/5/1235.abstract AB Maleimide, N-ethyl-maleimide (NEM), andN-methyl-maleimide (NMM) were identified as potent catalytic inhibitors of purified human topoisomerase IIα, whereas the ring-saturated analog succinimide was completely inactive. Catalytic inhibition was not abrogated by topoisomerase II mutations that totally abolish the effect of bisdioxopiperazine compounds on catalytic inhibition, suggesting a different mode of action by these maleimides. Furthermore, in DNA cleavage assay maleimide and NEM could antagonize etoposide-induced DNA double-strand breaks. Consistently, maleimide could antagonize the effect of topoisomerase II poisons in three different in vivo assays: 1) In an alkaline elution assay maleimide protected against etoposide-induced DNA damage. 2) In a band depletion assay maleimide reduced etoposide-induced trapping of topoisomerase IIα and β on DNA. 3) In a clonogenic assay maleimide antagonized the cytotoxicity of etoposide and daunorubicin on four different cell lines of human and murine origin. at-MDR cell lines with reduced nuclear topoisomerase IIα content are fully sensitive to maleimide, indicating that it is not a topoisomerase II poison in vivo. Our finding that topoisomerase II is sensitive to maleimide, NMM, and NEM but insensitive to succinimide demonstrates a strict requirement for the unsaturated ring bond for activity. We suggest that the observed antagonism in vitro and in vivo is caused by covalent modification of topoisomerase II cysteine residues reducing the amount of catalytically active enzyme sensitive to the action of topoisomerase II poisons.