Elucidating a Mechanism of Particulate Hexavalent Chromium-Induced Carcinogenesis

Hexavalent chromium (Cr(VI)) compounds are well established human lung carcinogens. Solubility plays a key role in the carcinogenicity of Cr(VI), with the most potent carcinogens being the particulate Cr(VI) compounds; however, their carcinogenic mechanism remains poorly understood. The best mechanistic model for particulate Cr(VI) appears to be one that involves genomic instability, but little is known about the ability of particulate Cr(VI) to induce numerical chromosome instability. We found that chronic exposure to particulate Cr(VI) induced concentration- and time-dependent increases in aneuploidy in the form of hypodiploidy, hyperdiploidy and tetraploidy, and these aneuploid cells were able to survive and emerge with a transformed phenotype. Centrosome amplification and spindle assembly checkpoint defects are two mechanisms for chromosome instability. Consistent with these mechanisms, chronic exposure to particulate Cr(VI) induced concentration- and time-dependent increases in spindle assembly checkpoint bypass and centrosome amplification that correlated with aneuploidy. Centrosome amplification persisted in cells transformed with particulate Cr(VI) and in Cr-induced tumors suggesting centrosome amplification is an early and persistent event in particulate Cr(VI)-induced carcinogenesis. The mechanisms of particulate Cr(VI)-induced chromosome instability include a physico-chemical and a biological component. For the physico-chemical mechanism, we found that chromate anions are responsible for the particulate Cr(VI)-induced aneuploidy, centrosome amplification, spindle assembly checkpoint bypass and DNA damage and the particle and lead cation appear to have no effect. In addition, zinc chromate is more potent than lead chromate, suggesting that unlike lead, zinc may be involved in zinc chromate-induced chromosome instability. We focused on the biological mechanism of particulate Cr(VI)-induced centrosome amplification and found that multiple mechanisms were involved. Particulate Cr(VI) induced numerous centriolar defects indicating that centriole splitting, acentriolar centrosome formation and centrosome overduplication were involved in Cr(VI)-induced centrosome amplification. We delved further into the mechanism of centrosome overduplication and found that a DNA-damage induced prolonged G2 arrest involving dysregulation of Nek2 was most likely involved. Altogether, these data indicate that Cr(VI) induces cancer by disrupting genomic stability. We propose chronic DNA damage induces a prolonged G2 arrest facilitating centrosome amplification and spindle assembly checkpoint bypass, which induce numerical chromosome instability, ultimately leading to neoplastic transformation and cancer