Modulation of cell cycle checkpoints by anti-cancer agents

Progression through the cell cycle is a tightly-regulated process. Such regulation is necessary to prevent propagation of genomic errors. Despite these regulatory controls, some errors may elude detection, and cancerous cells often are generated as a result. The major mechanisms that function to monitor critical cellular processes and prevent defective cell cycle progression are referred to as checkpoints. Experimental chemotherapeutic agents that modulate cell cycle checkpoints were examined for their effects on cultured cancer cells. First, using the microtubule depolymerizing agent CI-980, we identified a lung carcinoma cell line designated A427 that does not arrest in mitosis following disruption of microtubules, indicating a defective spindle assembly checkpoint response. In contrast, the A549 lung carcinoma cell line exhibited a functional spindle assembly checkpoint in response to CI-980 by arresting cell cycle progression during mitosis. Thus, the response of A427 cells to microtubule depolymerization suggests that the deregulation of a key spindle assembly checkpoint component may contribute to the generation of a cancerous state. Second, by experimentally activating the DNA replication checkpoint in HeLa cells, we have demonstrated that the kinase inhibitor UCN-01 abrogated this checkpoint. The DNA replication checkpoint normally suppresses the onset of mitosis in the absence of completely-replicated DNA. Abrogation of the checkpoint results in progression into mitosis prior to full replication of the genome. Therefore, elimination of a checkpoint by chemotherapeutic agents may also result in the death of cancer cells. Treatment of HeLa cells with the nuclear export inhibitor leptomycin B resulted in the nuclear localization of cyclin B1, which normally accumulates in the cytoplasm. The leptomycin B-induced redistribution of proteins increased the sensitivity of cells to UCN-01. Furthermore, by examining the effect of the nuclear export inhibitor leptomycin B on cells undergoing the mitosis-to-G1 transition, we have obtained evidence for a checkpoint that monitors the establishment of proper nucleocytoplasmic compartmentalization necessary to establish the interphase state following mitosis. HeLa cells exiting mitosis in the presence of leptomycin B triggered an apoptotic response both more quickly and extensively than control populations. Hence, chemotherapeutic agents that eliminate cell cycle checkpoints may demonstrate efficacy toward treatment of certain cancers.