Long-term exposure to submicromolar arsenite induces bypass of the spindle assembly checkpoint in mammalian cells

Mitosis is regulated by checkpoints that delay mitotic progression when chromosome segregation errors occur. Inaccuracy in checkpoint processes can lead to chromosome instability both in number and structure (CIN). Arsenic is reported to induce CIN by perturbing mitotic spindles and checkpoints, however, its carcinogenic mechanisms are poorly understood. We previously studied the long-term progression of chromosomal instability in V79 cells treated acutely with arsenite (10 M, 24 hr) followed by growth in arsenic-free medium for 120 cell generations, and found time-dependent increase of aneuploid cells. Here, we treated V79-derived G12 cells with sub-lethal doses (0.1-1.0 µM) of arsenite for 10 days followed by growth in arsenite-free medium for 40 cell generations. Cytogenetic analysis at the end of treatment showed concentration-dependent increases in the frequency of aneuploid cells that was even higher after 40 cell generations. Furthermore, the mitotic index (MI) of arsenite-exposed cells was higher than untreated cells at the end of the 10-day treatment or after 40 cell generations without arsenite. Surprisingly, we found that submicromolar 10-day arsenic exposure induced a large amount of cells in anaphase with premature centromere division. These aberrant mitotic figures dramatically increased after 40 cell generations. Taken together, these results raise the possibility that chronic exposure to sub-lethal arsenic doses induces bypass of the spindle assembly checkpoints and may be mechanistically involved in the induction of cell transformation.