Etoposide is a topoisomerase II poison
that is utilized to treat
a broad spectrum of human cancers. Despite its wide clinical use,
2–3% of patients treated with etoposide eventually develop
treatment-related acute myeloid leukemias (t-AMLs) characterized by
rearrangements of the MLL gene. The molecular basis
underlying the development of these t-AMLs is not well understood;
however, previous studies have implicated etoposide metabolites (i.e.,
etoposide quinone) and topoisomerase IIβ in the leukemogenic
process. Although interactions between etoposide quinone and topoisomerase
IIα have been characterized, the effects of the drug metabolite
on the activity of human topoisomerase IIβ have not been reported.
Thus, we examined the ability of etoposide quinone to poison human
topoisomerase IIβ. The quinone induced ∼4 times more
enzyme-mediated DNA cleavage than did the parent drug. Furthermore,
the potency of etoposide quinone was ∼2 times greater against
topoisomerase IIβ than it was against topoisomerase IIα,
and the drug reacted ∼2–4 times faster with the β
isoform. Etoposide quinone induced a higher ratio of double- to single-stranded
breaks than etoposide, and its activity was less dependent on ATP.
Whereas etoposide acts as an interfacial topoisomerase II poison,
etoposide quinone displayed all of the hallmarks of a covalent poison:
the activity of the metabolite was abolished by reducing agents, and
the compound inactivated topoisomerase IIβ when it was incubated
with the enzyme prior to the addition of DNA. These results are consistent
with the hypothesis that etoposide quinone contributes to etoposide-related
leukemogenesis through an interaction with topoisomerase IIβ.