Different Susceptibilities of Postmitotic Checkpoint‐proficient and ‐deficient Balb/3T3 Cells to ICRF‐193, a Catalytic Inhibitor of DNA Topoisomerase II

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Abstract
Two distinct types of Balb/3T3 cells were isolated which exhibit either 4 N DNA or both 4 N and 8 N DNA after exposure to colcemid for 48 h. They were found to differ with respect to the postmi‐totic checkpoint, but not the mitotic checkpoint. Firstly, the checkpoint‐proficient and ‐deficient cells exhibited the same accumulation and subsequent decrease in the number of mitotic cells following exposure to microtubule inhibitors. Secondly, after exit from abnormal mitosis in the presence of ICRF (Imperial Cancer Research Fund)‐193, the checkpoint‐proficient cells were arrested in the next cycle Gl, while the checkpoint‐deficient cells progressed into S and G2 phase. When either mitotic or asynchronous cells were exposed to ICRF‐193, the checkpoint‐proficient cells proved more sensitive to the cytotoxic effect of this agent than the checkpoint‐deficient cells. The different susceptibilities of the two types of cells to ICRF‐193 were not caused by variation in topoisomerase (topo) II function since both the biochemical activity of this enzyme and chromosome segregation were inhibited by similar concentrations of ICRF‐193 in both checkpoint‐proficient and ‐deficient cells. We propose that the inhibition of chromosome segregation by ICRF‐193 is monitored by the next Gl checkpoint, resulting in an irreversible Gl block in the case of postmi‐totic checkpoint‐proficient cells. As the checkpoint‐deficient cells can escape this Gl block, these cells have an increased survival capacity. In summary, ICRF‐193 may prove to be a very useful drug for examination of the postmitotic checkpoint.