| The tumor suppressor p53 is a crucial mediator of the cellular response to stress, including the DNA damage response. Its activation leads to the transcription of many target genes involved in growth arrest, apoptosis, and DNA repair. The role of p53 in the G1 cell cycle checkpoint has been extensively studied, but its participation in the regulation of the G2/M checkpoint remains largely unknown. Following genotoxic stress, p53 has been shown to induce the expression of several mediators of cell cycle arrest in G2 as well as to repress other genes required for cell cycle progression from G2 to M. However, many of the specific roles of p53 in G2 cell cycle arrest remain uncharacterized, and its participation in the maintenance of such an arrest have been questioned. Dysregulation of the p53 pathway is seen in the majority of human malignancies. Therefore, a complete understanding of the function of p53 in regulating cellular checkpoint function is essential for the development and advancement of cancer therapies.; Presented is a comprehensive examination of the role of p53 in the DNA damage response. A special focus is placed on the involvement of the cyclin-dependent kinase inhibitor p21CIP1, a p53 target gene. Using two independent isogenic systems in U2OS cells, the regulation of the response to genotoxic stress by both p53 and p21 are examined. Data presented herein indicate that cells lacking either protein are highly sensitive to DNA damaging agents and undergo apoptosis following prolonged exposure to genotoxic stress. The presence of p53 and p21 enables cells to stably arrest in response to DNA damage and is essential for the restriction of entry into mitosis. Furthermore, it is demonstrated that cells with p53, but not p53-ablated cells, are able to recover from short exposures to DNA damaging agents. Additionally, the characterization of a novel p53 response element in the human PIG3 promoter is reported and the transactivation function of two human tumor-derived p53 mutants on various pro-apoptotic promoters is characterized. |
