Drosophila p53 preserves genomic stability by regulating cell death

Abstract

When animal cells are exposed to stressful conditions, the tumor suppressor protein p53 restrains growth by promoting an arrested cell cycle or initiating a cell death program. How these distinct fates are specified through the action of a single protein is not known. To study its functions in vivo we produced a targeted mutation at the Drosophila p53 (Dmp53) locus. We show that Dmp53 is required for damage-induced apoptosis but not for cell-cycle arrest. Dmp53 function is also required for damage-induced transcription of two tightly linked cell death activators, reaper and sickle. When challenged by ionizing radiation, Dmp53 mutants exhibit radiosensitivity and genomic instability. Hence, elevated mutant loads were not caused by defective checkpoint functions but instead correlated with failures in p53-associated cell death. Our studies support the notion that core ancestral functions of the p53 gene family are intimately coupled to cell death as an adaptive response to maintain genomic stability.T he p53 tumor suppressor limits oncogenesis through activities that govern adaptive responses to stress. As such, p53 is thought to function as a ''guardian of the genome'' that becomes mutated or altered in most human cancers (1, 2). When cells are stressed by exposure to genotoxic agents, radiation, hypoxia, or inappropriate oncogene activation (1-3), p53 restrains growth through activities that reversibly arrest the cell cycle or promote apoptosis (4, 5), but how these alternative fates become specified through the action of this tumor suppressor is not understood. The primary mechanism of p53 action clearly involves transcriptional regulation (6-8), although other activities might contribute to accessory functions (9). The protein requires at least three functional domains to regulate downstream target genes but many mutations commonly found in human cancers map to the DNA-binding domain (10-13). A major effort in cancer biology is focused toward understanding downstream effectors of p53 function. The p53 target gene, p21, acts directly as a cyclindependent kinase inhibitor (14-16) and appears largely responsible for p53-dependent G 1 ͞S arrest. The in vivo downstream targets responsible for p53-mediated apoptosis, however, are less well defined. Important candidates for in vivo death effectors include members of the bcl2 family, bax (17), noxa (18), puma (19,20), and the death receptor fas (21,22). In many cases, regulated expression of these genes is context-specific and whether there exists a generic p53 apoptosis program for all cells is not known. Genome-wide searches for p53-responsive genes have also been reported, but it is not yet clear whether these loci represent direct or indirect targets (23,24).A Drosophila homolog of p53, Dmp53, was recently identified (25-27). Like mammalian counterparts (12, 13, 28-31), Dmp53 has a well conserved DNA-binding domain with a transcriptional activation domain at its N terminus, and an oligomerization domain at its C terminus. In previous studies, forced expression of ...

FRA16D common chromosomal fragile site oxido-reductase (FOR/WWOX) protects against the effects of ionizing radiation in Drosophila

“…A significant amount of variation in the percentage survival was observed between experiments as evident by the large standard deviation at the different dosages. A similar level of variation in survival has previously been reported for both wild-type and Dmp53 mutants following irradiation (Sogame et al, 2003).…”

“…Transgenic lines of UAS-DmWWOX and UAShWWOX were generated by standard transformation procedures (Chang et al, 2001), although the significance of this interaction has been challenged (Aqeilan et al, 2004c;Ludes-Meyers et al, 2004). Mutations generated in the Drosophila orthologue of p53, Dmp53, also resulted in viable adult flies with no obvious phenotype (Lee et al, 2003;Sogame et al, 2003). However, both reports of Dmp53 mutant animals showed an increased sensitivity to ionizing radiation.…”

Stress-induced corneal epithelial apoptosis mediated by K+ channel activation

“…Although the mechanisms underlying p53-dependent apoptotic responses remain incompletely characterized, p53 is known to be involved in both the extrinsic and the intrinsic pathways of apoptosis by initiating apoptosis through mitochondrial depolarization and sensitizing cells to inducers of apoptosis (Wang et al, 1998(Wang et al, ,1999aHaupt et al, 2003;Tang et al, 2003). The phosphorylation status of p53 determines the stability of the protein and controls cell cycle progression, which serves as a master-switch for promoting apoptosis (Ginsberg et al, 1991;Ashcroft and Vousden, 1999;Colman et al, 2000;Asher et al, 2001;Sogame et al, 2003). Alterations of p53 protein, such as missense mutations and loss of its expression caused by nonsense or frame-shift mutations, can result in carcinogenesis (Hussain and Harris, 1998;Medina et al, 2002;Shirai et al, 2002;Nishikawa et al, 2003;Hofseth et al, 2004).…”

Lessons from p53 in non-mammalian models

“…3,5 These findings were further supported by genetic lossof-function studies, which established that dmp53 is required for transcriptional induction of rpr proteins (rpr, hid, skl) and for radiation-induced apoptosis. 3 General development was not affected in the dmp53 mutants, [19][20][21] but mild defects in longevity and fertility were found. Where studied, dmp53 does not engage damage-induced cell cycle checkpoints and, consistent with this, the Drosophila ortholog of p21 is unresponsive to irradiation.…”