CELL CYCLE CHECKPOINTS AND CHEMOSENSITIVITY OF HUMAN CANCER CELLS

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The major focus of this research group is to study the role of cell cycle checkpoints in the chemosensitivity of human cancer cells. Such studies are aimed at addressing 4 fundamental questions: (1) Do cell cycle checkpoint differences exist in human cancer cells; (2) Do checkpoint differences, when observed, impart altered sensitivity to currently available anticancer agents; (3) Can we dissect out the molecular components of these checkpoints and determine how these components mediate cell cycle arrest, DNA repair and apoptosis, and (4) Can we discover new chemotherapeutic agents and/or stratagems that exploit checkpoint differences as a basis for the preferential destruction of cancer cells. We are focusing on the checkpoints that respond to DNA damage to arrest cell cycle progression in G1 and/or G2 phase. The G1 DNA damage checkpoint is commonly disrupted in cancer cells and this results from defects in the p53 pathway. We have found that a number of cancer cell lines also exhibit an attenuated G2 checkpoint response to DNA damage. Although this attenuation cannot be fully explained on the basis of p53 status, we have found that checkpoint modulators such as pentoxifylline and UCN-01, preferentially abrogate the G2 checkpoint in p53 defective cells. We have exploited this vulnerability in drug combinations to preferential kill p53 defective cancer cell lines in vitro. We aim to disclose the molecular basis of this vulnerability and will focus on UCN-01, which has just entered clinical trials. We have found that p53 disruption imparts altered sensitivity to a number of commonly used anticancer agents and these effects are dependent upon both the cell type and class of agents used to treat the cells. Most recently we have found that disruption of the p53-regulated gene CIP1/WAF1 preferentially sensitizes cells to DNA crosslinking agents. In collaboration with Dr. Fornace's laboratory, we have found this sensitization correlates with reduced DNA repair activity in the CIP1/WAF1 disrupted cells, implicating Cip1/Waf1 in DNA repair. We and others have now characterized the integrity of the p53 pathway in the NCI cell screen. This analysis has allowed the discovery of agents with potentially greater activity in cells with mutant p53. We are preparing several isogenic cell systems for further investigation of these agents. We are continuing to investigate the mechanisms of G1 and G2 arrest. For G2 arrest, we are focusing on the Wee1 and PLK kinases and for G1 arrest we are investigating the role of a novel p53-regulated phosphatase (Wip1), that was recently cloned as part of a collaboration with Dr. Appella's group at the NCI.