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Nickel is a metal carcinogen and chronic exposure to humans occurs via electroplating, nickel refining and welding. Nickel is found to cause various pathological disorders like lung fibrosis and respiratory cancers. Studies have shown that Nickel can cause DNA hypermethylation and histone hypoacetylation. These results in chromatin structure modifications, which lead to silencing of some tumor suppressor genes. Although there are reports about exposure to Nickel generates DNA-damaging reactive oxygen species and inhibits DNA repair, the exact molecular mechanisms which lead to Nickel-induced damages are yet not clear. In our study we focused on known carcinogenic- water insoluble Nickel compounds, Nickel Oxide (black), Nickel Oxide (green) and Nickel Subsulfide (Ni3S2), as a source of ionic Nickel (II). Here we report that cells exposed to Nickel Subsulfide, but not to Nickel Oxide (black), Nickel Oxide (green), show comet tail and γ-H2AX foci formation, indicative of DNA double strand breaks. Counter-staining the cells with an anti-phospho-Histone H3 antibody and propidium iodide, we find that Nickel Subsulfide increases Histone H3 phosphorylation, suggesting that exposure to Nickel cause mitotic arrest. Measuring 3H-thymidine incorporation, we also find that Nickel Subsulfide induces a replication blockage that can be abrogated by caffeine. Furthermore, we find that cells with defective ATM have normal response to Nickel exposure. However, the cells are still sensitive to caffeine exposure, indicating another protein that can be inhibited by caffeine might be involved in regulation of Nickel response. In summary, our findings suggest that Nickel Subsulfide is the most potent compound to induce DNA damage and cause mitotic arrest and replication blockage among the class of Nickel compounds tested in this study.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]