Hydrogen peroxide lethality is associated with a decreased ability to maintain positive DNA supercoiling

Potentiation of hydrogen peroxide toxicity: From catalase inhibition to stable DNA-iron complexes

Hydrogen peroxide (H₂O₂) is unique among general toxins, because it is stable in abiotic environments at ambient temperature and neutral pH, yet rapidly kills any type of cells by producing highly-reactive hydroxyl radicals. This life-specific reactivity follows the distribution of soluble iron, Fe(II) (which combines with H₂O₂ to form the famous Fenton's reagent),Fe(II) is concentrated inside cells, but is virtually absent outside them. Because of the immediate danger of H₂O₂, all cells have powerful H₂O₂ scavengers, the equally famous catalases, which enable cells to survive thousand-fold higher concentrations of H₂O₂ and, in combination with adequate movement of H₂O₂ across membranes, make the killing H₂O₂ concentrations virtually impractical to generate in vivo. And yet, low concentrations of H₂O₂ are somehow used as an efficient biological weapon. Here we review several examples of how cells potentiate H₂O₂ toxicity with other chemicals. At first, these potentiators were thought to simply inhibit catalases, but recent findings with cyanide suggest that potentiators mostly promote the other side of Fenton's reaction, recruiting iron from cell depots into stable DNA-iron complexes that, in the presence of elevated H₂O₂, efficiently break duplex DNA, pulverizing the chromosome. This multifaceted potentiation of H₂O₂ toxicity results in robust and efficient killing.

The role of nonhomologous DNA end joining, conservative homologous recombination, and single-strand annealing in the cell cycle-dependent repair of DNA double-strand breaks induced by H(2)O(2) in mammalian cells

The purpose of this study was to investigate the cell cycle-dependent role of nonhomologous DNA end joining (NHEJ), conservative homologous recombination (HR), and single-strand annealing (SSA) for the repair of simple DNA double-strand breaks (DSBs) induced by H(2)O(2)-mediated OH radicals in CHO cells. Cells of the cell lines V3 (NHEJ-deficient), irs1SF (HR-deficient) and UV41 (SSA-deficient) and their parental cell line AA8 were exposed to various concentrations of H(2)O(2) in G(1) or S phase of the cell cycle and their colony-forming ability was assayed. In G(1) phase, NHEJ was the most important-if not the only-mechanism to repair H(2)O(2)-mediated DSBs; this was similar to results obtained in a parallel study of more complex DSBs induced by sparsely or densely ionizing radiation. Unlike HR (irs1SF)- and SSA (UV41)-deficient cells, the sensitivity of NHEJ-deficient V3 cells to H(2)O(2) relative to parental AA8 cells in G(1) phase is about 50 times higher compared to 200 kV X rays. This points to a specific role of the catalytic subunit of DNA-PK for efficient NHEJ of H(2)O(2)-mediated DSBs that are located at sites critical for the maintenance of the higher-order structure of cellular DNA, whereas X-ray-induced DSBs are distributed stochastically. Surprisingly, SSA-deficient cells in G(1) phase showed an increased sensitivity to high concentrations of H(2)O(2) relative to the parental wild-type cells and to HR-deficient cells, which may be interpreted in terms of a specific type of H(2)O(2)-induced damage requiring SSA for repair after its transfer into S phase. In S phase, HR is the most important mechanism to repair H(2)O(2)-mediated DSBs, followed by NHEJ. In contrast, the action of error-prone SSA may not be beneficial, since SSA-deficient cells are three times more resistant to H(2)O(2) than wild-type AA8 cells. This is likely due to channeling of DSBs into the error-free HR repair pathway or into the potentially error-prone NHEJ pathway. Cells with or without a defect in DSB repair are considerably more sensitive to H(2)O(2) in S phase compared to G(1) phase. This effect is likely due to the fact that topoisomerase II, which is expressed only in proliferating cells, is a target of H(2)O(2), resulting in enhanced accumulation of DSBs and killing of cells treated in S phase with H(2)O(2). The relative sensitivities to H(2)O(2) differ by orders of magnitude for the four cell lines. This seems to be caused mainly by H(2)O(2)-mediated poisoning of topoisomerase IIalpha rather than by a defect in DSB repair.

Enhanced sensitivity and long-term G2 arrest in hydrogen peroxide-treated Ku80-null cells are unrelated to DNA repair defects

While the Ku complex, comprised of Ku70 and Ku80, is primarily involved in the repair of DNA double-strand breaks, it is also believed to participate in additional cellular processes. Here, treatment of embryo fibroblasts (MEFs) derived from either wild-type or Ku80-null (Ku80(-/-)) mice with various stress agents revealed that hydrogen peroxide (H(2)O(2)) was markedly more cytotoxic for Ku80(-/-) MEFs and led to their long-term accumulation in the G2 phase. This differential response was not due to differences in DNA repair, since H(2)O(2)-triggered DNA damage was repaired with comparable efficiency in both Wt and Ku80(-/-) MEFs, but was associated with differences in the expression of important cell cycle regulatory genes. Our results support the notion that Ku80-mediated cytoprotection and G2-progression are not only dependent on the cell's DNA repair but also may reflect Ku80's influence on additional cellular processes such as gene expression.

Glutathione peroxidase and catalase modulate the genotoxicity of arsenite

The X-ray hypersensitive Chinese hamster ovary (CHO) cells, xrs-5, are also more sensitive to sodium arsenite in terms of cell growth and micronucleus induction than CHO-K1 cells. Since reactive oxygen species are suggested to be involved in arsenic toxicity, we have measured antioxidant mechanisms in xrs-5 as well as CHO-K1 cells. There were no apparent differences in the activities of superoxide dismutase, glutathione S-transferase, glutathione reductase, and the levels of glutathione between xrs-5 and CHO-K1 cells. However, the activities of glutathione peroxidase and catalase were 5.4- and 5.8-fold lower, respectively, in xrs-5 cells. The addition of catalase or glutathione peroxidase to cultures reduced the arsenite-induced micronuclei in xrs-5 cells. Whereas, simultaneous treatment with mercaptosuccinate, an inhibitor of glutathione peroxidase, and 3-aminotriazole, an inhibitor of catalase, synergistically increased the arsenite-induced micronuclei. These results suggest that both catalase and glutathione peroxidase are involved in defense against arsenite genotoxicity. The xrs-6 cells, another line of x-ray hypersensitive CHO cells, which had 1.6-fold higher catalase activity and 2.5-fold higher glutathione peroxidase activity than xrs-5 cells, were also more sensitive than CHO-K1 cells but were less sensitive than xrs-5 cells to cell growth inhibition of arsenite. Moreover, a 1.6-fold increase of glutathione peroxidase activity by selenite adaptation effectively removed the arsenite-induced micronuclei in CHO-K1 cells. These results suggest that glutathione peroxidase is more important than catalase in defending against arsenite toxicity. Our results also suggest that increasing the intracellular antioxidant level may have preventive or therapeutic effects in arsenic poisoning.

Mutagenic activity and specificity of hydrogen peroxide in the ad-3 forward-mutation test in two-component heterokaryons of Neurospora crassa

In the ad-3 forward-mutation test, hydrogen peroxide was at best a weak mutagen in nongrowing conidia from a DNA repair-proficient heterokaryon (H-12, uvs-2+/uvs-2+) but was a moderate mutagen in nongrowing conidia from a DNA-repair-deficient heterokaryon (H-59, uvs-2/uvs-2) over a narrow range of high concentrations. H-59 also was more sensitive than H-12 to the killing activity of hydrogen peroxide at high concentrations. Thus, a DNA-repair pathway, of which the gene product of the uvs-2+ allele is a part, appears to be involved in the repair of hydrogen peroxide-induced DNA lesions at low survival in these strains. There was slightly, but significantly, more killing by hydrogen peroxide of nongrowing conidia from H-12 and H-59 in the presence of O2 than in the absence of O2 (presence of N2). Thus, the killing activity of hydrogen peroxide was enhanced by O2. The Mutational Spectra of hydrogen peroxide-induced ad-3 mutants shows that hydrogen peroxide induced mainly gene/point mutations but also some multilocus deletion mutations in H-12 and H-59. Multiple-locus mutations occurred only in H-59, but the frequency was very low. The frequencies of the 3 kinds of intracistronic complementation pattern among ad-3BR mutants (gene/point mutations) suggest that hydrogen peroxide induced both base-pair substitutions and frameshift mutations in both strains.

Content of iron and copper in the nuclei and induction of pH 9-labile lesions in L5178Y sublines inversely cross-sensitive to H2O2 and x-rays

Cells from the L5178Y murine lymphoma subline LY-R are twice as resistant to killing by ionizing radiation than the subline LY-S. In contrast, LY-R cells are more sensitive to killing by H2O2, the effect being more pronounced at 37 degrees C than 0 degree C. Initial DNA damage after H2O2 treatment (both temperatures, 5 min) has been estimated by the 'comet' assay (single-cell gel electrophoresis) and fluorescent halo technique. According to both methods, the initial damage is significantly higher in LY-R cells, particularly that inflicted at 0 degree C. Differences between DNA unwinding and rewinding abilities at pH 9 and 6.9 (estimated by the fluorescent halo technique) point to a considerable difference in pH-9-labile damage between the sublines, as observed previously for x-irradiated cells (Kapiszewska et al. 1992). In contrast to findings with x-irradiated cells, however, after H2O2 treatment this damage is more extensive in LY-R cells than in LY-S cells. Thus, the initial pH-9-labile damage corresponds to the pattern of sensitivity to H2O2 and x-rays. We suggest that this is caused by different proportions of cuprous and ferric ions found in the nuclei of LY sublines and by the different ability of these ions to react with H2O2 and water radiolysis products. The copper/iron ratio in the nucleus is 1.31 in LY-R cells and 4.84 in LY-S cells.

Comparison of effects of iron and calcium chelators on the response of L5178Y sublines to X-rays and H2O2

The L5178Y murine lymphoma subline LY-R is twofold more resistant to killing by ionizing radiation than the subline LY-S. In contrast, LY-R cells are more sensitive to killing by hydrogen peroxide: at 37 degrees C LY-R cells are 1.4 times more sensitive to the killing effect of H2O2 than LY-S cells. Pretreatment with the iron chelator desferroxamine followed by hydrogen peroxide treatment at 37 degrees C gives a considerable sparing effect, which is substantially greater for the LY-R subline than for the LY-S subline. This is reflected in the initial DNA damage (estimated by single cell gel electrophoresis), survival and mutation frequency in the HGPRT locus. Similar results have been obtained with calcium chelators, which, according to recent findings (Sandström and Granström 1993), also are efficient iron chelators. In contrast, the response to X-rays is not modified by the above chelators, with the exception of mutation frequencies: lower mutant numbers are found in desferroxamine pretreated LY-R cells.

DNA strand breakage, cytotoxicity and mutagenicity of hydrogen peroxide treatment at 4 degrees C and 37 degrees C in L5178Y sublines

Cells from the L5178Y murine lymphoma subline LY-R are twofold more resistant to killing by ionizing radiation than the subline LY-S. In contrast, LY-R cells are more sensitive to killing by hydrogen peroxide. Cells of the two sublines in logarithmic growth phase were treated with hydrogen peroxide in phosphate-buffered saline for 1 h at 4 degrees C or 37 degrees C. From the comparison of D(o) values it followed that at 37 degrees C LY-R were 3.6 times more sensitive to the killing effect of H2O2 than LY-S cells; at 4 degrees C they were 11 times more sensitive. Treatment with hydrogen peroxide at 4 degrees C gave a considerable sparing effect, which was substantially greater for the LY-S subline; for LY-S cells D(o) was 5.7 times lower at 37 degrees C than at 4 degrees C, for LY-R cells only 1.9 times. The mutation frequency (HGPRT) in LY-R cells was increased in proportion to H2O2 concentration and was the same at both treatment temperatures. In contrast, mutation frequencies initially increased, then decreased with increasing H2O2 concentration in LY-S cells treated at 4 or 37 degrees C. The concentration at which the decline was initiated was higher at 4 than at 37 degrees C. DNA damage after H2O2 treatment (both temperatures, 5 min) was estimated from the 'comet' assay (single-cell gel electrophoresis). The initial damage, but not the residual damage, differed significantly in LY sublines. A period of slower repair (between 3 and 10 min) was found in LY-R cells.