Genotoxicity Assessment of Co(II) and Ni(II) in HepG2 Cells: Insights into Combined Metal Exposure

The usage of cobalt (Co) and nickel (Ni) in numerous commercial, industrial, and military applications causes widespread exposure nowadays, and concerns are rising about adverse impacts on human health. Emphasis is on the respiratory system, with both metals classified as (possibly) carcinogenic upon inhalation by the International Agency for Research on Cancer (IARC), but limited data are available upon oral exposure. Therefore, this study aims to evaluate the in vitro genotoxicity of Co(II) and Ni(II) and their combination in HepG2 cells, since exposure of those environmental pollutants occurs realistically in concert. Here, Co(II) exposure led to the induction of single-strand breaks and oxidative DNA damage detected by the Comet assay as FPG-sensitive sites, while Ni(II) increased the abundance of γ-H2AX, an indicator for double-strand breaks. Notably, combined exposure to Co(II) and Ni(II) resulted in enhanced DNA damage, especially at the chromosomal level, with increased formation of micronuclei as well as polynucleated cells, indicating a stronger effect compared to single exposure. Furthermore, both metals induced the DNA damage response pathway PARylation. As this process involves the consumption of large amounts of cellular NAD⁺ after DNA damage, the energy state was assessed upon exposure with Co(II) and Ni(II). Current data indicate that especially Co(II) altered the cellular energy state. This study reveals distinct mechanisms of DNA damage exhibited by Co(II) and Ni(II), which were enhanced after a combined treatment. This highlights the need for further research to estimate the genotoxic potential of targeting cells upon oral intake with increasing environmental entry.