276

The p53 protein plays a central role as a tumor suppressor. Although p53 is clearly a transcriptional activator capable of transactivating target genes, a number of other activities have been ascribed to p53 using in vitro assays. To define the contribution of transactivation to p53 function in vivo, we generated a knock-in mouse strain in which the wild-type p53 locus is substituted with a mutant significantly compromised for transactivation, p53QS. Using mouse embryo fibroblasts (MEFs), we confirmed that that p53QS is greatly impaired in activation of most p53 target genes examined. To assess p53QS activity in p53 biological function, we assayed apoptotic responses of E1A-expressing MEFs to different cellular stresses. Whereas wild-type p53 can induce apoptosis in response to diverse stresses, p53QS showed selectivity in its action. Specifically, p53QS was completely defective in inducing apoptosis in response to DNA damage, and very effective at inducing apoptosis in response to hypoxia. These findings suggest that p53 acts via distinct mechanisms to induce cell death in different contexts, and that hypoxia-induced apoptosis does not require p53 transactivation function. Analysis of the p53QS mutant provides a unique means to assess the relative contribution of DNA damage-induced and hypoxia-induced, p53-dependent apoptosis for tumor suppression. Toward this end, we have transformed MEFs with E1A and ras and compared p53QSΠexpressing cells with p53-expressing cells. We have determined that p53QS, despite being transactivation defective, has tumor suppressor activity. Together, these results suggest the importance of transactivation-independent activities of p53 for its function in vivo. Additional investigation of the p53QS mutant mice will provide a powerful means for further dissecting the functions of p53 critical for apoptosis and tumor suppression in different settings.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]