Mediators ofp53-dependent DNA damage induced responses in vivo

p53, a tumor suppressor and transcription factor, is a critical mediator of the cellular response to DNA damage or mitogenic stimulation. Although apoptosis is essential for p53-dependent tumor suppression, little is known about which p53-target genes are required for p53-dependent apoptosis. To elucidate the p53-dependent cell death pathway, we examined the p53 regulation of the pro-apoptotic TRAIL receptor, KILLER/DR5. KILLER/DR5 was induced after exposure to DNA damaging agents in a p53-dependent apoptosis-specific manner. Since KILLER/DR5 was a potent inducer of cell death, we examined the cellular mechanisms of resistance that allow tumor cells to inhibit this pathway through a genetic approach. Interestingly, both TRAIL receptor- and p53-mediated apoptosis was abrogated by cellular inhibitors from the extrinsic and intrinsic apoptotic pathways. These studies suggested that these pathways were critical for p53-dependent death. To identify the critical p53 target genes that mediate apoptosis, we examined p53-dependent apoptosis in vivo where these apoptotic pathways are intact. p53 wild-type and null animals were irradiated and global p53 gene expression patterns in response to ionizing radiation were examined in several tissues undergoing p53-dependent apoptosis. These studies revealed striking tissue specific expression of p53 target genes. In addition, several novel p53 target genes were identified, which suggested novel p53 functions that may contribute to p53-dependent tumor suppression. The p53-dependent regulation of Snk/Plk2, a Polo-like kinase family member, suggested that p53 might regulate a mitotic checkpoint in response to anti-microtubule agents. Since loss of p53 or Snk/Plk2 sensitize cells to anti-microtubule agents, these studies have identified a possible novel therapeutic intervention in tumor cells. Our examination of the tissue specific targets critical for p53-dependent apoptosis in normal tissues, suggests novel pathways to target therapeutically to prevent the dose-limiting side effects of chemotherapeutic agents in normal cells. Conversely, activation of these apoptotic pathways, in tumor cells, may also prove to be an effective therapeutic strategy. In summary, our studies provide novel insights into apoptosis and other checkpoint mechanisms whose loss in cancer may contribute to tumor development, progression and/or therapeutic resistance.