Chapter 080. Cancer Cell Biology and Angiogenesis (Part 3) Figure 80-1 Induction of p53 by the DNA damage and oncogene checkpoints. In response to noxious stimuli, p53 and mdm2 are phosphorylated by the ataxia telangiectasia mutated (ATM) and related ATR serine/threonine kinases, as well as the immediated downstream checkpoint kinases, Chk1 and Chk2. This causes dissociation of p53 from mdm2, leading to increased p53 protein levels and transcription of genes leading to cell cycle arrest (p21 Cip1/Waf1 ) or apoptosis (e.g., the proapoptotic Bcl-2 family members Noxa and Puma). Inducers of p53 include hypoxia, DNA damage (caused by ultraviolet radiation, gamma irradiation, or chemotherapy), ribonucleotide depletion, and telomere shortening. A second mechanism of p53 induction is activated by oncogenes such as Myc, which promote aberrant G 1 /S transition. This pathway is regulated by a second product of the Ink4a locus, p19 ARF , which is encoded by an alternative reading frame of the same stretch of DNA that codes for p16 Ink4a . Levels of ARF are upregulated by Myc and E2F, and ARF binds to mdm2 and rescues p53 from its inhibitory effect. This oncogene checkpoint leads to the death or senescence (an irreversible arrest in G1 of the cell cycle) of renegade cells that attempt to enter S phase without appropriate physiologic signals. Senescent cells have been identified in patients whose premalignant lesions harbor activated oncogenes, for instance, dysplastic nevi that encode an activated form of BRAF (see below), demonstrating that induction of senescence is a protective mechanism that operates in humans to prevent the outgrowth of neoplastic cells. Acquired mutation in p53 is the most common genetic alteration found in human cancer (>50%); germline mutation in p53 is the causative genetic lesion of the Li-Fraumeni familial cancer syndrome. In many tumors, one p53 allele on chromosome 17p is deleted and the other is mutated. The mutations often abrogate the DNA binding function of p53 that is required for its transcription factor activity and tumor-suppressor functions, and also result in high intracellular levels of p53 protein. Inactivation of the p53 pathway compromises cell cycle arrest, attenuates apoptosis induced by DNA damage or other stimuli, and predisposes cells to chromosome instability. This genomic instability greatly increases the probability that p53 null cells will acquire additional mutations and become malignant. In summary, it is likely that all human cancers have genetic alterations that inactivate the Rb and p53 tumor-suppressor pathways. Tumors expressing mutant p53 are more resistant to radiation therapy and chemotherapy than tumors with wild-type p53. If the transcriptional functions of the mutant p53 could be reestablished in tumor cells, massive apoptosis might ensue, whereas normal cells would be protected because they express very low levels of wild-type p53. Investigators have screened chemical libraries for compounds that inhibit tumor cell growth in a mutant p53-dependent manner. One compound entered cells and induced mutant p53 to adopt an active conformation such that p53-dependent transcriptional activation was restored and apoptosis was selectively induced. This compound also had anti-tumor activity in murine xenograft models. Other investigators have identified a low-molecular-weight, cell-permeable compound that inhibits the apoptotic functions of wild-type p53 found in normal host cells. This compound protected mice from the toxic effects of radiation therapy and chemotherapy, including bone marrow suppression, gastrointestinal dysfunction, and hair loss. Taken together, these approaches provide proof of principle for the pharmacologic manipulation of p53 function (mutant or wild-type) that could greatly enhance therapeutic efficacy while decreasing toxicity. Knowledge of the molecular events governing cell cycle regulation has led to the development of viruses that replicate selectively in tumor cells with defined genetic lesions. Such "oncolytic" viruses include adenoviruses designed to replicate in tumor cells that lack functional p53 or have defects in the pRB pathway. The former group includes an adenovirus mutant in which the viral p55 protein (which binds and inhibits p53) was deleted; this virus selectively replicates in tumor cells lacking p53 function. This virus has shown efficacy in phase II clinical trials of head and neck tumors, especially when combined with 5-fluorouracil and cisplatin (50% partial or complete response). The complexities of virus-host interactions (i.e., immune response against replicating virus) will require further refinements of this novel technology before the clinical utility of this approach can be fully realized. . Chapter 080. Cancer Cell Biology and Angiogenesis (Part 3) Figure 80-1 Induction of p53 by the DNA damage and oncogene checkpoints. In response to noxious stimuli, p53 and mdm2. upregulated by Myc and E2F, and ARF binds to mdm2 and rescues p53 from its inhibitory effect. This oncogene checkpoint leads to the death or senescence (an irreversible arrest in G1 of the cell cycle). transcription factor activity and tumor-suppressor functions, and also result in high intracellular levels of p53 protein. Inactivation of the p53 pathway compromises cell cycle arrest, attenuates