Abstract SY02-02: Tumor suppression in the absence of p53-mediated cell cycle arrest, apoptosis, and senescence.
Authors: Li Tongyuan , Kon Ning , Jiang Le , Zhao Ying , Baer Richard , Gu Wei 
Published: August 2014

Cell-cycle arrest, apoptosis, and senescence are widely accepted as the major mechanisms by which p53 suppresses tumor formation. Nevertheless, it remains unclear how these p53 responses are differentially controlled in vivo and whether they are absolutely required for tumor suppression. p53 is regulated by an exquisite network of fine-tuning mechanisms that ensure proper responses to the various stress signals encountered by cells. In broad terms, p53 activities are controlled via protein levels, coactivator/corepressor recruitment, and a diverse array of post-translational modifications that includes acetylation, ubiquitination, phosphorylation, methylation, sumoylation, and neddylation. p53 was the first non-histone protein shown to be regulated functionally by acetylation and deacetylation and subsequent work has established that acetylation plays a major role in controlling promoter-specific activation of p53 targets during stress responses. We and others recently showed that p53 is acetylated at K120 by Tip60/MOF, while K164 is acetylated by CBP and p300, but not Tip60/MOF. Both sites are mutated in human tumors and well conserved in all species known to encode p53. Interestingly, K120 acetylation is crucial for p53-mediated apoptosis but has no obvious effect on cell cycle arrest, while simultaneous loss of acetylation at all major sites abolishes the ability of p53 to induce both cell cycle arrest and apoptosis, suggesting that acetylation is essential for both p53-mediated processes. Nevertheless, the role of acetylation in the most critical aspect of p53 function, its capacity to suppress tumor formation, has not been addressed. To investigate whether p53 acetylation is important for tumor suppression, we generated p53-mutant mice (p53K117R/K117R) in which K117 (K120 in human) is replaced by arginine. In these animals, p53-mediated apoptosis is abrogated but p53-dependent cell cycle arrest and senescence remain intact. We also established mice (p533KR/3KR) in which the three acetylation sites of the DNA-binding domain (K117, K161, K162) were simultaneously replaced by arginine. Significantly, loss of acetylation at these three sites completely abolished the ability of p53 to mediate cell cycle arrest, apoptosis, and senescence in vivo. To evaluate whether these p53-dependent processes are required for tumor suppression, we monitored tumor formation in cohorts of p53 acetylation-deficient mice. Although p53-null mice rapidly develop spontaneous thymic lymphomas, neither p53K117R/K117R nor p533KR/3KR mice are prone to early-onset tumorigenesis. Since tumor suppression can be mediated by a p53 polypeptide (e.g., p533KR) that lacks the ability to induce p53-dependent cell cycle arrest, apoptosis, and senescence, these results indicate that other aspects of p53 function are sufficient to suppress tumor formation. Notably, p533KR retains the ability to modulate energy metabolism and reactive oxygen species (ROS) production by regulating metabolic target expression. These findings underscore the crucial role of acetylation in differentially modulating p53 responses and suggest that unconventional activities of p53, such as metabolic regulation and antioxidant function, are critical for suppression of early-onset spontaneous tumorigenesis. Our study thus reveals that current views regarding the mechanism of p53-mediated tumor suppression should be reconsidered.

Citation Format: Tongyuan Li, Ning Kon, Le Jiang, Ying Zhao, Richard Baer, Wei Gu. Tumor suppression in the absence of p53-mediated cell cycle arrest, apoptosis, and senescence. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr SY02-02. doi:10.1158/1538-7445.AM2013-SY02-02