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Vol. 61, Issue 5, 1235-1243, May 2002
Laboratory for Experimental Medical Oncology, Finsen Center,
Copenhagen, Denmark (L.H.J., S.W.L., P.B.J.); Department of Pathology,
Laboratory Center, Copenhagen, Denmark (L.H.J., A.-R.C., I.W., M.S.);
and Topo Target A/S, Copenhagen, Denmark (L.H.J., I.W., S.W.L., B.S.,
E.V.C., M.S., P.B.J.)
Maleimide, N-ethyl-maleimide (NEM), and
N-methyl-maleimide (NMM) were identified as potent
catalytic inhibitors of purified human topoisomerase II
, 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
II
and
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 II
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.
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