Transcriptional Regulation by p 53 and p 73

than 50% of human cancer, is stabilized and activated in cells in response to genotoxic stresses or oncogenic stimuli and then initiates different cellular outcomes, including cell cycle arrest, apoptosis, or senescence (for review, see Ko and Prives 1996; Levine 1997; Bates and Vousden 1999; Prives and Hall 1999). The biological functions of p53 involve its activities as a sequence-specific transcription factor, and cellular outcomes following stress are determined through the transactivation of a wide variety of p53 target genes such as p21WAF1, MDM2, PIG3, BAX, PUMA, p53AIP1, and numerous others. Several models have been proposed to explain how the activity of p53 is regulated and how the promoter discrimination of p53 is achieved on the molecular level, leading, in some cases, to a decision between cell cycle arrest and apoptosis in response to distinct stimuli. It has been proposed that the choice in response depends on the amount of p53 present in the cell as well as the modification state of p53 (Wang and Prives 1995; Gu and Roeder 1997; Sakaguchi et al. 1998; Oda et al. 2000; Appella and Anderson 2001). The stabilization of p53 after DNA damage is thought to occur largely through phosphorylation events that disrupt its interaction with its negative regulator, MDM2, although the literature reveals considerable complexity in that regard (Shieh et al. 1997; Prives and Hall 1999; Moll and Petrenko 2003; Poyurovsky and Prives 2006). Furthermore, some modifications, such as phosphorylation of Ser-46, may increase the affinity of p53 for a specific promoter independently of p53 expression level (Oda et al. 2000). Analysis of target gene mRNA induction and chromatin immunoprecipitation (ChIP) assays has shown a difference in binding affinity and kinetics of p53’s association with and transactivation of its target gene promoters (Szak et al. 2001; Kaeser and Iggo 2002). Although there are notable exceptions (e.g., the PUMA gene), low-affinity sites are frequently found in pro-apoptotic genes, suggesting that the level of p53 expression can determine the threshold for promoter activation and cellular outcome in some experimental conditions (Chen et al. 1996; Ludwig et al. 1996). In situ, this might permit p53, when present at low levels, to induce cell cycle arrest, allowing time to repair the DNA damage, and at higher levels (perhaps following more extensive damage), to activate the apoptotic program. It has also been shown that some proteins, for example ASPP1/2 (Samuels-Lev et al. 2001), augment the p53-dependent transactivation of only the pro-apoptotic target genes. This suggests an initial weaker binding to some pro-apoptotic promoters that, in the case of severe enough DNA damage, is augmented by p53 modification and cofactors. The p53 family member p73 was first identified in 1997 (Jost et al. 1997; Kaghad et al. 1997). Since then, much has been discovered about this protein, but many questions remain unanswered (Irwin and Kaelin 2001a,b; Yang et al. 2002; Moll and Slade 2004). The p73 gene contains 14 exons, which, through splicing, can produce seven TA full-length isoforms that differ in their carboxyl terminus (α–η) and the corresponding ΔN isoforms. The ΔN isoforms result from transcription from an alternative promoter and have a transactivation domain different from that of the full-length isoforms. In vivo, p73α and β are the most commonly found forms, in both the TA and ΔN varieties. The p73 protein has approximately 65% homology with p53 in its DNA-binding domain, and some homology in the transactivation and oligomerization domains (20–30% and 35–45%, respectively.) Like p53, p73 functions as a transcription factor and has been shown to transactivate a number of p53 target genes, although each protein also transactivates its own unique class of downstream targets (for review, see Harms et al. 2004). Also like p53, p73 is up-regulated in the cell following DNA damage (Agami et al. 1999; Yuan et al. 1999), although Transcriptional Regulation by p53 and p73