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UBC Theses and Dissertations

Small molecules as modulators of mitotic arrest and senescence in cancer Riffell, Jenna Louise


Manipulation of the cell cycle is an extensively used and promising strategy for cancer therapy. To identify novel cell cycle modulators, automated fluorescence microscopy assays were designed and used to screen chemical libraries for modulators of mitotic arrest and senescence. 8-azaguanine, IC 261, erysolin and SKF 96365 were identified as chemicals that stimulate senescence, a state of prolonged growth arrest, in a p53-mutated growth arrest-deficient cell line. Microtubule-targeting cancer therapies such as paclitaxel block cell cycle progression at mitosis by prolonged activation of the mitotic checkpoint. Cells arrested in mitosis may remain arrested for extended periods of time, or undergo mitotic slippage through degradation of cyclin B1 in the presence of an active mitotic checkpoint and enter interphase without having separated their chromosomes. Regulation of extended mitotic arrest and mitotic slippage and their contribution to subsequent cell death or survival is incompletely understood. Chlorpromazine and triflupromazine were identified as drugs that inhibit mitotic exit through mitotic slippage. Using these drugs to extend mitotic arrest imposed by low concentrations of paclitaxel led to increased cell survival and proliferation after drug removal. SU6656 and geraldol were identified as chemicals that induce mitotic slippage. Cells arrested at mitosis with paclitaxel or vinblastine and induced by these compounds to undergo mitotic slippage underwent several rounds of DNA replication without cell division and exhibited signs of senescence but eventually all died. These results show that reinforcing mitotic arrest with drugs that inhibit mitotic slippage can lead to increased cell survival and proliferation, while inducing mitotic slippage in cells treated with microtubule-targeting drugs seems to invariably lead to protracted cell death. Mitotic slippage induced by SU6656 or geraldol involved proteasome-dependent degradation of cyclin B1, but also required proteasome- and caspase-3-dependent inactivation of the mitotic checkpoint through degradation of the mitotic checkpoint protein BubR1. Caspase-3 and p53, both apoptotic effectors, did not affect cell death after exposure to paclitaxel, with or without mitotic slippage induction. The requirement for caspase-3 for chemically induced mitotic slippage reveals a new mechanism for mitotic exit and a link between mitosis and apoptosis that has implications for the outcome of cancer chemotherapy.

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