DNA-protein cross-linking by chromium salts

Area of Science:

• Toxicology
• Molecular Biology
• Cell Biology

Background:

• Chromate salts are known environmental and occupational toxicants.
• Human bronchial epithelial cells are the suspected origin of malignancies in chromate-exposed workers.
• The genotoxic mechanisms of chromate exposure require further elucidation.

Purpose of the Study:

• To investigate the induction and persistence of DNA-protein cross-links by chromate salts in mammalian cells.
• To identify the specific chromium species responsible for DNA-protein cross-linking.
• To assess the potential for DNA repair of chromate-induced damage.

Main Methods:

• Exposure of mammalian cell cultures (including human bronchial epithelial cells) to various concentrations of chromate salts.
• Detection and quantification of DNA-protein cross-links and DNA single-strand breaks (SSB).
• Incubation of cells and isolated nuclei with different chromium valence states (chromate vs. chromic salts) to determine the active form.

Main Results:

• Chromate salts induced dose-dependent DNA-protein cross-links in cultured mammalian cells.
• These cross-links were persistent, with no significant removal observed after 12 hours of repair incubation.
• Chromic (trivalent) salts, but not chromate (hexavalent) salts, induced DNA-protein cross-links in isolated nuclei and in solution, suggesting reduction is key.
• A low level of DNA single-strand breaks (SSB) was observed but were rejoined within 4 hours.

Conclusions:

• Chromate exposure leads to the formation of stable DNA-protein cross-links in mammalian cells.
• The genotoxic effect is mediated by the trivalent chromium ion, likely after cellular uptake and reduction of extracellular chromate.
• The persistent nature of these DNA-protein cross-links may contribute to the carcinogenicity of chromate, particularly in lung tissues.