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Evidence against the nuclear in situ binding of arsenicals-oxidative stress theory of arsenic carcinogenesis
Citation:
KITCHIN, K. T. AND K. WALLACE. Evidence against the nuclear in situ binding of arsenicals-oxidative stress theory of arsenic carcinogenesis. TOXICOLOGY AND APPLIED PHARMACOLOGY. Academic Press Incorporated, Orlando, FL, 2(232):252-257, (2008).
Impact/Purpose:
A large amount of evidence suggests that arsenicals act via oxidative stress in causing cancer in humans and experimental animals. This study was done to see if there was any high affinity, specific binding of arsenite to several constituents of the cells nucleus particularly DNA and histones.
Description:
A large amount of evidence suggests that arsenicals act via oxidative stress in causing cancer in humans and experimental animals. It is possible that arsenicals could bind in situ close to nuclear DNA followed by Haber-Weiss type oxidative DNA damage. Therefore, we tested this hypothesis by using radioactive 73As labeled arsenite and vacuum filtration methodology to determine the binding affinity and capacity of 73As arsenite to calf thymus DNA and Type 2A unfractionated histones, histone H3, H4 and horse spleen ferritin. Arsenicals are known to release redox active Fe from ferritin. At concentrations up to about 1mM, neither DNA nor any of the three proteins studied, Type II-A histones, histone H3, H4 or ferritin, bound radioactive arsenite in a specific manner. Therefore, it appears highly unlikely that initial in situ binding of trivalent arsenicals, followed by in situ oxidative DNA damage, can account for arsenic’s carcinogenicity. This experimental evidence (lack of arsenite binding to DNA, histone Type II-A and histone H3, H4) does not rule out other possible oxidative stress modes of action for arsenic such as (a) diffusion of longer lived oxidative stress molecules, such as H2O2 into the nucleus and ensuing oxidative damage, (b) redox chemistry by unbound arsenicals in the nucleus, or (c) arsenical-induced perturbations in Fe, Cu or other metals which are already known to oxidize DNA in vitro and in vivo.
What is the study?
We were interested in the scientific hypothesis that trivalent arsenicals bind to components of the nucleus such as DNA, run in situ Fenton type oxidation of DNA bases and this resulting DNA oxidation causes arsenic-induced carcinogenesis. We tested this hypothesis by using radioactive 73As labeled arsenite and vacuum filtration methodology to determine the binding affinity and capacity of 73As arsenite to calf thymus DNA and Type 2A unfractionated histones, histone H3, H4 and horse spleen ferritin.
Why it was done?
A large amount of evidence suggests that arsenicals act via oxidative stress in causing cancer in humans and experimental animals. This study was done to see if there was any high affinity, specific binding of arsenite to several constituents of the cells nucleus particularly DNA and histones. Any oxidation of DNA bases could be occurring from in situ oxidations via arsenicals inside of the nucleus, free unbound arsenicals in the nucleus or chemical species that are formed farther away from the DNA.
What is the impact on the field and the Agency?
From this study we learned that arsenite does not specifically bind to either DNA, histones or ferritin. Therefore the hypothesis that arsenite binds specifically to nuclear components and produces in situ Fenton-style oxidation of DNA does not seem to be a viable hypothesis of arsenic¬ induced carcinogenesis. This experimental evidence of this study (lack of arsenite binding to DNA and histones) does not rule out (a) diffusion of longer lived oxidative stress molecules, such as H2O2 into the nucleus and ensuing oxidative DNA damage, (b) redox chemistry by free (unbound) arsenicals in the nucleus or (c) arsenical-induced perturbations in Fe, Cu or other redox active metals which are already the known to oxidize DNA in vitro and in vivo. It is always difficult to rule out Fe- and Cu-mediated pathways in mammalian oxidative stress and carcinogenesis.