Topoisomerases are essential
enzymes that are involved in
DNA metabolism.
Topoisomerase II generates transient
DNA strand breaks that are stabilized by anticancer drugs, such as
doxorubicin, causing an accumulation of DNA damage. However,
doxorubicin causes
cardiac toxicity and, like
etoposide and other
topoisomerase II-targeted agents, can induce DNA damage, resulting in secondary
cancers. The
cannabinoid quinone HU-331 has been identified as a potential anticancer
drug that demonstrates more potency in
cancer cells with less off-target toxicity than that of
doxorubicin. Reports indicate that
HU-331 does not promote cell death via apoptosis, cell cycle arrest,
caspase activation, or
DNA strand breaks. However, the precise mechanism of action is poorly understood. We employed biochemical assays to study the mechanism of action of
HU-331 against purified topoisomerase IIα. These assays examined
DNA binding, cleavage,
ligation, relaxation, and
ATPase activities of topoisomerase IIα. Our results demonstrate that
HU-331 inhibits topoisomerase IIα-mediated
DNA relaxation at micromolar levels. We find that
HU-331 does not induce
DNA strand breaks in vitro. When added prior to the
DNA substrate,
HU-331 blocks DNA cleavage and relaxation activities of topoisomerase IIα in a redox-sensitive manner. The action of
HU-331 can be blocked, but not reversed, by the presence of
dithiothreitol. Our results also show that
HU-331 inhibits the
ATPase activity of topoisomerase IIα using a noncompetitive mechanism. Preliminary binding studies also indicate that
HU-331 decreases the ability of topoisomerase IIα to bind
DNA. In summary,
HU-331 inhibits relaxation activity without
poisoning DNA cleavage. This action is sensitive to
reducing agents and appears to involve noncompetitive inhibition of the
ATPase activity and possibly inhibition of
DNA binding. These studies provide a promising foundation for the exploration of
HU-331 as a catalytic inhibitor of topoisomerase IIα.