Mechanisms of Hexavalent Chromium-Induced Lung Carcinogenesis: DNA Double-Strand Breaks and DNA Repair Inhibition in a Guinea Pig Model.

Hexavalent chromium [Cr(VI)] is an environmental and occupational pollutant and a known lung carcinogen. Despite the exposure risks, it is unclear how Cr(VI) causes lung cancer. Prolonged Cr(VI) exposure induces DNA double-strand breaks and inhibits their repair in exposed cells, leading to chromosome instability, a hallmark of lung cancer. Specifically, Cr(VI) inhibits RAD51, a key effector protein in the homologous recombination repair pathway. The goal of this study was to translate this molecular mechanism of Cr(VI)-induced lung carcinogenesis to a guinea pig model. Hartley guinea pigs 10 to 11-week-old were exposed to zinc chromate via oropharyngeal aspiration for acute (24 h) or subchronic (90-day) exposure. Lungs were dissected and formalin-fixed paraffin-embedded. DNA double-strand breaks were measured using gamma-H2AX foci, and homologous recombination repair was assessed using RAD51 foci. Acute Cr(VI) exposure induced gamma-H2AX foci and RAD51 foci, whereas subchronic exposure also caused gamma-H2AX foci but inhibited RAD51 foci. These effects of Cr(VI)-induced DNA damage and repair inhibition were largely observed in bronchiolar regions but not in alveolar regions. This study establishes a new human-relevant model to study Cr(VI)-induced carcinogenesis and translates the mechanism of Cr(VI)-induced DNA double-strand break formation and repair inhibition to guinea pig lungs.