Are Interactions with P63 and P73 Involved in Mutant P53 Gain of Oncogenic Function?
Total Page：16
File Type：pdf, Size：1020Kb
Oncogene (2007) 26, 2220–2225 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc REVIEW Are interactions with p63 and p73 involved in mutant p53 gain of oncogenic function? Y Li and C Prives Department of Biological Sciences, Columbia University, New York, NY, USA Although still controversial, the presence of mutant p53 in deletion or frameshift mutations resulting in their lack cancer cells mayresult in more aggressive tumors and of expression, however, many tumor-derived p53 muta- correspondinglyworse outcomes. The means bywhich tions are of the missense variety. These mutations are mutant p53 exerts such pro-oncogenic activityare usually located within the central conserved region of currentlyunder extensive investigation and different p53 that binds to DNA in a sequence-specific manner models have been proposed. We focus here on a proposed and result in either direct loss of DNA contact or gross mechanism bywhich a subset of tumor-derived p53 mut- change of the conformation that no longer permits p53 ants physically interact with p53 family members, p63 and to recognize its wild-type DNA targets (reviewed by p73, and negativelyregulate their proapoptotic function. Vousden and Lu, 2002). Approximately 30% of these Both cell-based assays and knock-in mice expressing tumor-derived p53 mutations affect only six ‘hot-spot’ mutant forms of p53 support this model. As more than half residues (175, 245, 248, 249, 273 and 282). In many cases of human tumors harbor mutant forms of p53 protein, tumor-derived p53 mutants are expressed at far higher approaches aimed at disrupting the pathological interac- levels than wild-type p53 protein due in part to their tions among p53 familymembers might be of clinical failure to be degraded by Mdm2. Related to this, many value. experimental and clinical lines of evidence suggest that Oncogene (2007) 26, 2220–2225. doi:10.1038/sj.onc.1210311 tumor-derived p53 mutants have acquired novel func- tions that promote cancer cell growth. The underlying Keywords: mutant p53; p63; p73; protein–protein inter- mechanisms by which these gain-of-function p53 mu- action tants act, a subject of intense study, can be at multiple levels. For example such mutants might upregulate a specific set of genes promoting cell growth and/or inactivate distinct sets of genes that have tumor suppressor activity. Indeed, according to one model Introduction different p53 mutants might each display unique properties in regulating gene expression (Resnick and The p53 tumor suppressor plays a central role in Inga, 2003). We discuss in this review a hypothesis to regulating cell cycle and apoptosis. Under a variety of explain how mutant p53 promotes pro-oncogenic stress conditions, wild-type p53 protein quickly accu- activities which posits that some mutant p53 proteins mulates in the nucleus and can then transcriptionally can interact with and downregulate the transcriptionally regulate a number of downstream genes that control cell active forms of the p53 homologues, p63 and p73, cycle arrest or apoptosis (reviewed by Vogelstein et al., leading to reduced apoptotic response and radioresis- 2000; Laptenko and Prives, 2006). In addition, wild-type tance in tumor cells. Evidence to support this hypothesis p53 can contribute to apoptosis through transcription- is described and discussed below. independent pathway(s) in the cytosol (reviewed by The p53 gene family consists of three members p53, Chipuk and Green, 2006). Loss of p53 function p63 and p73 that are homologous in three domains: inevitably promotes tumorigenesis and also renders their N-terminal transcriptional activation domain tumor cells resistant to chemo- and radiotherapy. (TAD), central core sequence-specific DNA-binding There are several lines of evidence that p53 functions domain (DBD) and the C-terminally located oligomer- as a canonical tumor suppressor. Notably p53-null mice ization domain (OD). P63 and p73, which generally acquire tumors early with 100% penetrance, highly share more homology with each other than with p53, cancer-prone Li–Fraumeni families carry germline can be expressed as two N-terminal isoforms that either mutant forms of p53, and p53 is among the most contain (TA) or lack (DN) a full TAD through alternate commonly mutated genes in all human cancers. In promoter usage and these two isoforms in turn can each contrast to other tumor suppressors that undergo large have multiple C-termini generated as a result of alternative splicing. In general, the full-length TA Correspondence: Dr C Prives, Department of Biological Science, versions of p63/p73 can exert p53-like activities, whereas Columbia University, New York, NY 10027, USA. the DN versions have the opposite effect (reviewed by E-mail: [email protected] Yang et al., 2002). More recently it was reported that The physical and functional interaction between mutant p53 and p63/p73 Y Li and C Prives 2221 p53 can also be expressed as multiple isoforms differing sufficient to bind p63 or p73 both in vivo and in vitro in N and C-termini (reviewed by Murray-Zmijewski (Strano et al., 2000, 2002; Gaiddon et al., 2001). et al., 2006). The different p53 C-termini, however, are Moreover, either a construct expressing the p73 core quite distinct from those of p63 and p73. DNA binding and OD, or one expressing just the DBD It is still unclear whether and to what extent p63 and of p63, can associate with mutant p53 (Strano et al., p73 play roles in tumor suppression. p63 and p73 2000, 2002). As p63 and p73 are most homologous to knockout mice display defects in development rather each other in their core domains and they were shown to than increased tumorigenesis. Furthermore, p63/p73 bind to the same subset of p53 mutants, it is likely that mutations are not commonly observed in human their core domains are the critical if not sole binding tumors. Nevertheless, several lines of evidence suggest surface for mutant p53. that p63 and p73 can also mediate cellular responses to How might such interactions occur? A number of DNA damage agents and have potential tumor suppres- results collectively suggest that conformational changes sion activity in vivo. First, when overexpressed, TAp63/ in the p53 core domain are responsible for its interaction p73 can activate a number of common p53 responsive with p63/p73. First, most p53 hot-spot mutants undergo genes involved in cell cycle arrest and apoptosis structural changes in the DBD as determined by nuclear (Kaghad et al., 1997; Osada et al., 1998; Yang et al., magnetic resonance spectroscopy (Wong et al., 1999). 1998; Wu et al., 2003; Lokshin et al., 2005). Second, There is also a strong correlation between the ability of endogenous TAp73 can be stabilized by DNA-damaging mutant p53 to interact with p63/p73 and their reactivity agents and its ablation can result in chemoresistance to the conformation sensitive monoclonal antibody (Bergamaschi et al., 2003; Irwin et al., 2003; Urist et al., PAb240 (Gaiddon et al., 2001). This antibody recog- 2004). Although less explored, endogenous TAp63 can nizes a cryptic epitope within the core domain when p53 also determine chemotherapeutic efficiency (Gressner is unfolded and therefore reflects an altered conforma- et al., 2005). Third, Flores et al. (2002) showed that p63 tion of the protein (discussed by Cho et al., 1994). and p73 are required by p53 to activate a few Moreover, wild-type Xenopus laevis p53 protein that was proapoptotic genes in mouse embryo fibroblasts (MEFs) shown to be intrinsically temperature sensitive at 371C, expressing adenoviral E1A. Finally, in one genetic binds to both human p73 and the heat-shock protein background mice heterozygous for p63 and p73 can hsp70 in a temperature-dependent manner (Bensaad developtumors (Flores et al., 2005). Nevertheless, in et al., 2003). In fact, some p53 mutants with conforma- contrast to p63 or p73, the central role of wild-type p53 tional distortion can associate with hsp70 protein in protection from cancer is unequivocal. (Gannon et al., 1990) and some of these can also interact with p73 (Gaiddon et al., 2001). Furthermore, immunopurified wild-type p53 protein contains a frac- tion that is conformationally changed during the A subset of p53 mutants can interact with p63 and p73 procedure and is both recognized by PAb 240 and can interact with p73 (Gaiddon et al., 2001). In addition, Biochemical studies have revealed that the tetrameriza- wild-type p53 prepared under harsh conditions or upon tion domains of p63 and p73 can hetero-oligomerize repeated freeze and thaw cycles that partially denature with each other but neither can form heterotetramers proteins can associate with p73 (Bensaad et al., 2003). with p53 (Davison et al., 1999). Despite this it has been Despite indications that the main determinant for shown that a number of p53 mutants can associate with interaction with p63 and p73 is an unfolded p53 core either p73 or p63 as evidenced by co-immunoprecipita- domain there are many observations suggesting that this tion assays when they are either ectopically or endogen- conclusion may be an oversimplification. For example, ously expressed (Di Como et al., 1999; Marin et al., unlike full-length p53, several versions of truncated 2000; Strano et al., 2000, 2002; Gaiddon et al., 2001; wild-type p53 can interact with p73 when they are Bergamaschi et al., 2003; Irwin et al., 2003). Both coexpressed in insect cells (Gaiddon et al., 2001). These ‘conformation’ mutants (e.g. R175 and G245) and include p53 variants with a wild-type core domain but ‘contact’ tumor derived p53 mutants (e.g. R248, R273, lacking either the OD, the N-terminal 96 amino acids or R283) have been reported to associate with p63/p73. the C-terminal 30 amino acids, suggesting that such Furthermore, purified mutant p53 and p63/p73 can bind deletions could impose long range influence on the to each other in vitro, strongly indicating that such conformation of the core domain.