Reply to Kojo: Mechanisms of antioxidant-induced DNA damage

There are several different mechanisms by which antioxidants induce DNA damage in cultured cells. In a Letter to the Editor, Kojo (1) suggests that one of these mechanisms, the generation of hydrogen peroxide (H₂O₂) resulting from the exposure of antioxidants to high levels of oxygen in cell culture media, could be primarily responsible for the DNA damage and cell death observed in in vitro studies using resveratrol, genistein, and baicalein (2). Thus, the author questions whether the three antioxidants could act as genotoxins (and by extension, chemotherapeutic agents) in vivo where oxygen concentrations are low. Although this is a valid concern, there are two key pieces of evidence that argue against it. First, recognizing that resveratrol, genistein, and baicalein could be acting as prooxidants in our studies, we had measured the production of reactive oxygen species in cultured cells following treatment with these compounds at concentrations of 92 μM using the general oxidative stress indicator CM-H2DCF-DA (Invitrogen) and found no difference compared with treatment with DMSO alone. Second, other groups have shown that resveratrol, genistein, and baicalein can successfully reduce tumor growth in mice (3–5). Taken together, these data suggest that the production of H₂O₂ does not make a significant contribution to the phenotypes observed following resveratrol, genistein, and baicalein treatments. Rather, these compounds act through other mechanisms, which may involve the intercalation of DNA and/or the inhibition of topoisomerase, DNA polymerase, and ribonucleotide reductase.