Topoisomerase II inhibition by the bioactivated benzene metabolite hydroquinone involves multiple mechanisms.

While benzene is widely recognized as a human and animal carcinogen, the key mechanisms underlying its carcinogenic effects remain unknown. Inhibition of topoisomerase II (topoII) by benzene and its metabolites represents a potential mechanism by which benzene could induce its chromosome-altering and leukemogenic effects. Previous work from our laboratory and others has demonstrated that bioactive benzene metabolites are capable of inhibiting topoII in isolated enzyme and cell culture systems. Similarly, a decrease in topoII activity has been seen in the bone marrow of mice administered benzene in vivo. The objective of these studies was to further investigate the mechanisms by which the bioactivated benzene metabolite, hydroquinone (BAHQ), inhibits topoII in vitro, and to identify the point(s) in the enzyme's catalytic cycle where inhibition occurs. Our experiments indicate that BAHQ inhibits topoII at the DNA binding stage as well as in the closed clamp stage in the catalytic cycle, thereby interfering with either the binding to, or the release of, DNA from the enzyme. While increases in the cleavable complex were also seen with BAHQ treatment, our results suggest that this is related to a shift in equilibrium due to an accumulation of the topoII enzyme at the closed clamp stage rather than a major inhibitory effect on the religation step. An increase in cleavable complex formation as well as the inhibition of enzymatic activity at the closed clamp and other stages of the catalytic cycle in bone marrow cells would likely result in DNA breakage, the formation of chromosomal aberrations, and could potentially result in leukemia-associated chromosomal translocations, similar to those seen in leukemias induced by the bisdioxopiperazine type of catalytic topoII inhibitors.

Topoisomerase II inhibition by myeloperoxidase-activated hydroquinone: a potential mechanism underlying the genotoxic and carcinogenic effects of benzene.

Benzene is an established human and animal carcinogen. While many of the key mechanisms underlying its carcinogenic effects remain unknown, there is increasing evidence that chromosomal alterations play an important role in the development of the induced leukemias. Inhibition of enzymes involved in DNA replication and maintenance such as topoisomerases by benzene metabolites represents a potential mechanism by which benzene may induce its chromosome-altering effects. Previous work from our laboratory and others has demonstrated that bioactivated benzene metabolites are capable of inhibiting topoisomerase II (topo II) in isolated enzyme and cell systems as well as in mice administered benzene in vivo. The current studies were designed to build upon this hypothesis, and show that in the presence of human myeloperoxidase and H2O2, hydroquinone can be activated to a potent topo II inhibitor. In the absence of dithiothreitol, partial inhibition can be seen at hydroquinone concentrations as low as 50 nM. The potential role of topo II inhibition in the development of benzene-induced leukemia is also discussed in the context of other known leukemia-inducing agents. Current evidence indicates that multiple mechanisms are likely to contribute to benzene-induced leukemias, and that inhibition of topo II could represent an important step in the development of certain leukemia subtypes.