As cancer cells are actively dividing, mitosis is one of the most common targets for anticancer chemotherapy. Mitotic cell death can be induced by spindle poisons, which prevent the proper attachment of spindle microtubules to the kinetochores. This activates the spindle-assembly checkpoint, leading to cell cycle arrest in mitosis, and subsequently triggers profound cell death after prolonged exposure. The precise mechanism of mitotic cell death induced by spindle poisons is still unclear. I have examined the role of mitotic slippage, B-type cyclins, and the spindle-assembly checkpoint in nocodazole-induced cell death. I found that the onset of slippage and cell death were independent events. Downregulation of cyclin B1, but not cyclin B2, by RNA interference reduced nocodazole-induced...[ Read more ]

As cancer cells are actively dividing, mitosis is one of the most common targets for anticancer chemotherapy. Mitotic cell death can be induced by spindle poisons, which prevent the proper attachment of spindle microtubules to the kinetochores. This activates the spindle-assembly checkpoint, leading to cell cycle arrest in mitosis, and subsequently triggers profound cell death after prolonged exposure. The precise mechanism of mitotic cell death induced by spindle poisons is still unclear. I have examined the role of mitotic slippage, B-type cyclins, and the spindle-assembly checkpoint in nocodazole-induced cell death. I found that the onset of slippage and cell death were independent events. Downregulation of cyclin B1, but not cyclin B2, by RNA interference reduced nocodazole-induced cell death. Silencing of both cyclin B1 and cyclin B2 prevented cell death completely. Moreover, cyclin B1-CDC2 could drive apoptosis even in the absence of the spindle-assembly checkpoint. These data underscore the importance of cyclin B1-CDC2, and not the activation or collapse of the checkpoint, as the basis of spindle poisons-mediated apoptosis.

After prolonged activation of spindle-assembly, some cells can exit mitosis aberrantly by mitotic slippage. These cells are arrested in the subsequent tetraploid G₁ phase by a p53-dependent mechanism. Failure of this G₁ checkpoint leads to DNA re-replication and polyploidization. I found that disruption of either p53 or the spindle-assembly checkpoint led to a failure of the postmitotic G₁ arrest. However, disruption of the spindle-assembly checkpoint accelerated mitotic exit and cyclin E accumulation, which led to S phase entry before sufficient p53 and p21 were induced.

Besides spindle disruption, another way to induce mitotic cell death is by the uncoupling of the G₂ DNA damage checkpoint. This forces cells to enter mitosis prematurely with damaged DNA, with the consequence of cell death termed “mitotic catastrophe”. Cyclin B1 was found to be cleaved during mitotic catastrophe by a caspases-dependent mechanism after Asp123. Expression of the cleavage product induced mitotic cell death, suggesting that the cleavage of cyclin B1 may contribute to mitotic or apoptotic functions during mitotic catastrophe.

Together, these results underscore complex interplay between the activities of the checkpoints and cyclin B1-CDC2 during normal mitosis and mitotic cell death.