Living with p53, Dying of p53
2007; Cell Press; Volume: 130; Issue: 4 Linguagem: Inglês
10.1016/j.cell.2007.08.005
ISSN1097-4172
Autores Tópico(s)Ubiquitin and proteasome pathways
ResumoThe p53 tumor suppressor protein acts as a major defense against cancer. Among its most distinctive features is the ability to elicit both apoptotic death and cell cycle arrest. In this issue of Cell, Das et al., 2007Das S. Raj L. Zhao B. Kimura Y. Bernstein A. Aaronson S.A. Lee S.W. Cell. 2007; (this issue)Google Scholar and Tanaka et al., 2007Tanaka T. Ohkubo S. Tatsuno I. Prives C. Cell. 2007; (this issue)Google Scholar provide new insights into the mechanisms that dictate the life and death decisions of p53. The p53 tumor suppressor protein acts as a major defense against cancer. Among its most distinctive features is the ability to elicit both apoptotic death and cell cycle arrest. In this issue of Cell, Das et al., 2007Das S. Raj L. Zhao B. Kimura Y. Bernstein A. Aaronson S.A. Lee S.W. Cell. 2007; (this issue)Google Scholar and Tanaka et al., 2007Tanaka T. Ohkubo S. Tatsuno I. Prives C. Cell. 2007; (this issue)Google Scholar provide new insights into the mechanisms that dictate the life and death decisions of p53. Cells are incessantly bombarded by an assortment of environmental and intrinsic factors that cause cellular damage. Although mild damage is often reparable, extensive damage poses a potential oncogenic danger. In the latter case, the benefit of the organism calls for the eradication of the potentially life-threatening cells, which often is achieved through activation of an apoptotic cell death program. Thus, the cell is continually faced with an agonizing choice: repair and live, or die. Defects in this decision process can lead to cancer, and insights into the mechanisms of dysregulation can improve strategies for designing more effective therapies. This cell fate choice often depends on the tumor suppressor protein p53. Sitting at the junction of an extremely complex network of cellular signaling, p53 assimilates disparate input signals such as oncogene activation, DNA damage, mitotic impairment or oxidative stress to initiate appropriate outputs—DNA repair, cell cycle arrest, senescence, or apoptosis (Harris and Levine, 2005Harris S.L. Levine A.J. Oncogene. 2005; 24: 2899-2908Crossref PubMed Scopus (1391) Google Scholar). This begs the question of how one molecule is able to mediate such a wide spectrum of responses. Even though p53 can also function in a transcription-independent manner (Fuster et al., 2007Fuster J.J. Sanz-Gonzalez S.M. Moll U.M. Andres V. Trends Mol. Med. 2007; 13: 192-199Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar), the best understood functions of p53 have been attributed to its transcriptional activity. In fact, approximately half of all cancers bear p53 gene mutations, the vast majority of which impair the ability of p53 to act as a sequence-specific transcriptional activator. This underscores the importance of p53-regulated genes for p53's tumor suppressor activity. How p53 “knows” which genes to turn on or off in order to achieve the desirable outcome has been a focus of intensive research (Harris and Levine, 2005Harris S.L. Levine A.J. Oncogene. 2005; 24: 2899-2908Crossref PubMed Scopus (1391) Google Scholar, Laptenko and Prives, 2006Laptenko O. Prives C. Cell Death Differ. 2006; 13: 951-961Crossref PubMed Scopus (370) Google Scholar). Particular effort has been devoted to understanding how p53 is instructed to favor activation of growth-inhibitory genes in response to limited damage that calls for a transient cell cycle arrest in conjunction with repair, and activation of proapoptotic genes in response to extensive irreparable damage. Two exciting reports (Das et al., 2007Das S. Raj L. Zhao B. Kimura Y. Bernstein A. Aaronson S.A. Lee S.W. Cell. 2007; (this issue)Google Scholar, Tanaka et al., 2007Tanaka T. Ohkubo S. Tatsuno I. Prives C. Cell. 2007; (this issue)Google Scholar) in this issue describe two new partners for p53—the zinc finger protein Hzf and the chromatin-associated protein CAS/CSE1L—that are involved in opposite arms of the p53 response. Sequence-specific DNA binding of p53 is a prerequisite for the transactivation of target genes. Typically, p53 response elements (p53REs) are located within a few thousand nucleotides upstream or downstream from the transcription start site. Frequently, p53 targets contain at least two widely spaced p53REs. However, not all target genes are equally responsive to p53, suggesting additional layers of regulation. DNA topology of p53REs may serve as one structural determinant influencing promoter discrimination. The fact that regions proximal to some p53REs (for example, those of the GADD45 and Mdm2 genes) exist constitutively in open, non-nucleosome occupied states whereas others do not, might contribute to differential activation of p53 target genes (Braastad et al., 2003Braastad C.D. Han Z. Hendrickson E.A. J. Biol. Chem. 2003; 278: 8261-8268Crossref PubMed Scopus (20) Google Scholar). Furthermore, conformation of the DNA may be important. The recognition of elements in the Mdm2 and p21 promoters is dependent on their differential propensities to adopt non-B-DNA conformations (Kim and Deppert, 2003Kim E. Deppert W. Biochem. Cell Biol. 2003; 81: 141-150Crossref PubMed Scopus (55) Google Scholar). More generally, the binding affinities of p53 for specific p53REs differ widely; high affinity sites tend to associate with growth arrest-related genes, whereas low affinity sites are more frequent in proapoptotic genes (Inga et al., 2002Inga A. Storici F. Darden T.A. Resnick M.A. Mol. Cell. Biol. 2002; 22: 8612-8625Crossref PubMed Scopus (151) Google Scholar). The different affinities of p53 toward different p53REs suggest that levels of p53 protein may profoundly affect promoter choice and cell fate. Indeed, low p53 levels tend to favor growth arrest, whereas higher levels override this default pathway and trigger apoptosis (Laptenko and Prives, 2006Laptenko O. Prives C. Cell Death Differ. 2006; 13: 951-961Crossref PubMed Scopus (370) Google Scholar). This might explain, at least in part, why binding to proapoptotic promoters, such as PIG3, is markedly delayed relative to binding to cell cycle arrest promoters such as p21 (Szak et al., 2001Szak S.T. Mays D. Pietenpol J.A. Mol. Cell. Biol. 2001; 21: 3375-3386Crossref PubMed Scopus (157) Google Scholar). Not surprisingly, the interaction between p53 and its DNA target sequences is highly influenced by the cellular context. A plethora of partner proteins have been implicated in modulating the selection of p53 targets (Figure 1). Some of those proteins are transcription factors themselves, which presumably bind to promoter sites adjacent to p53REs to selectively induce specific response genes. Others influence the ability of p53 itself to bind preferentially to particular DNA target sequences and not to others. The cellular environment as well as the relative abundance of these potential partners under different conditions could obviously tip the life-or-death balance of p53 activity. ASPP family proteins comprise three members: ASPP1, ASPP2 and inhibitory ASPP (iASPP) (Sullivan and Lu, 2007Sullivan A. Lu X. Br. J. Cancer. 2007; 96: 196-200Crossref PubMed Scopus (149) Google Scholar). Upon DNA damage, ASPP1 and 2 are activated and then interact with the DNA binding domain (DBD) of p53, enhancing its tumor suppressor activity. Specifically, they enhance p53's apoptotic capabilities by guiding p53 to the promoters of proapoptotic genes, such as Bax and PIG3, but not to the promoters of proarrest genes such as p21 or regulatory genes such as Mdm2. In accordance with their potential tumor suppressor activity, ASPP1 and 2 are frequently downregulated in human tumors (Sullivan and Lu, 2007Sullivan A. Lu X. Br. J. Cancer. 2007; 96: 196-200Crossref PubMed Scopus (149) Google Scholar). In contrast iASPP, which counters the effects of ASPP1 and 2 and interferes with activation of proapoptotic genes, is often overexpressed in human tumors. The Brn3 family of POU domain transcription factors also modulates p53 target selectivity. Whereas Brn3b augments the activation of proapoptotic genes, such as Bax (Budhram-Mahadeo et al., 2006Budhram-Mahadeo V.S. Bowen S. Lee S. Perez-Sanchez C. Ensor E. Morris P.J. Latchman D.S. Nucleic Acids Res. 2006; 34: 6640-6652Crossref PubMed Scopus (29) Google Scholar), Brn3a has the opposite effect. Brn3a and p53 directly interact during neuronal differentiation to mutually modulate their respective transcriptional outputs. In that role, Brn3a diminishes the ability of p53 to transactivate the Bax promoter, while stimulating transcription of p21, resulting in a net outcome of cell cycle arrest rather than apoptosis. YB1 is a DNA binding protein overexpressed in many tumor types. Stress signals trigger proteolytic cleavage of YB1 and p53-dependent nuclear import at the G1/S stage of the cell cycle. Once in the nucleus, the N-terminal fragment of YB1 directly binds to p53 to prevent transactivation of proapoptotic genes (Homer et al., 2005Homer C. Knight D.A. Hananeia L. Sheard P. Risk J. Lasham A. Royds J.A. Braithwaite A.W. Oncogene. 2005; 24: 8314-8325Crossref PubMed Scopus (69) Google Scholar). The transcription factor NF-κB is also an important modulator of p53 transcriptional activity. While often antagonizing p53 function, NF-κB can also sometimes cooperate with p53. NF-κB negatively affects p53 protein levels via positive regulation of Mdm2 expression. Competition for coactivators, such as p300 and CBP, might provide additional means for mutual negative regulation. Recent data indicate that phosphorylation of CBP by IKKα, which occurs excessively in certain human cancers, can direct CBP to bind preferentially to NF-κB and not p53, thereby favoring proliferation and survival over p53-dependent apoptosis (Huang et al., 2007Huang W.C. Ju T.K. Hung M.C. Chen C.C. Mol. Cell. 2007; 26: 75-87Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar). There is also crosstalk between p53 and the NF-κB subunit p52. Under some conditions, p52 can be recruited directly to p53 target promoters, leading to repression of p21 and activation of proapoptotic DR5 and PUMA (Schumm et al., 2006Schumm K. Rocha S. Caamano J. Perkins N.D. EMBO J. 2006; 25: 4820-4832Crossref PubMed Scopus (105) Google Scholar), thus tipping the balance toward apoptosis and away from cell cycle arrest. A number of additional p53-interacting proteins, including the p63 and p73 members of the p53 family, can also modulate promoter choice by p53, even though the exact underlying mechanisms remain to be elucidated (Laptenko and Prives, 2006Laptenko O. Prives C. Cell Death Differ. 2006; 13: 951-961Crossref PubMed Scopus (370) Google Scholar). In their new work, Das et al., 2007Das S. Raj L. Zhao B. Kimura Y. Bernstein A. Aaronson S.A. Lee S.W. Cell. 2007; (this issue)Google Scholar report that the zinc finger protein Hzf, encoded by a gene previously shown to be a p53 target (Sugimoto et al., 2006Sugimoto M. Gromley A. Sherr C.J. Mol. Cell. Biol. 2006; 26: 502-512Crossref PubMed Scopus (27) Google Scholar), directly interacts with the p53 DBD, inducing preferential expression of p53 target genes that block the cell cycle. Thus, Hzf favors the transactivation of p21 and 14-3-3σ genes while simultaneously attenuating transcription of proapoptotic genes such as Bax, Perp, Puma and Noxa (Das et al., 2007Das S. Raj L. Zhao B. Kimura Y. Bernstein A. Aaronson S.A. Lee S.W. Cell. 2007; (this issue)Google Scholar). The Mdm2 gene, which belongs to neither group and encodes a regulator of p53, is not affected either way. Following short-term etoposide treatment to induce mild DNA damage, Hzf-bound p53 engages proarrest but not proapoptotic targets. However, in similarly treated Hzf-deficient cells p53 is detected primarily on proapoptotic targets. Remarkably, after prolonged etoposide treatment, which inflicts extensive DNA damage, Hzf undergoes proteasomal degradation. The resulting reduction in Hzf levels now enables activation of proapoptotic genes, providing an appealing explanation for the observation that extended damage triggers a switch from a growth inhibitory transcriptional program to a proapoptotic one (Figure 1). As anticipated from its remarkable ability to instruct p53 to distinguish between the two classes of target genes, Hzf has a profound impact on cell fate decisions downstream of p53 activation: in its absence, even a mild genotoxic insult is sufficient to trigger apoptosis. It will be of particular interest to find out to whether alterations in Hzf expression or activity are involved in human tumors, particularly those that retain a wild-type p53 gene. Covalent modifications of p53 may also change target gene preferences, possibly by imposing conformational changes in p53 that encourage selective recognition of different p53REs. It has been suggested that p53 mutants that are able to activate only a subset of targets may be “locked” into a particular conformation that only recognizes particular types of promoters. Wild-type p53, however, is conceivably flexible enough to go between different conformations, thereby allowing diverse promoter recognition (Kim and Deppert, 2003Kim E. Deppert W. Biochem. Cell Biol. 2003; 81: 141-150Crossref PubMed Scopus (55) Google Scholar). The list of reported post-translational modifications on p53 is long and continuously growing, and includes phosphorylation of multiple serine (Ser) and threonine (Thr) residues, acetylation, mono- and polyubiquitylation, sumoylation, neddylation and more. Much recent attention has focused on p53 phosphorylation on Ser46, which specifically favors transactivation of proapoptotic genes (Shmueli and Oren, 2007Shmueli A. Oren M. Mol. Cell. 2007; 25: 794-796Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). Indeed, mutation of Ser46 to Ala reduces the ability of p53 to transactivate proapoptotic genes such as p53AIP1, Noxa, Dr5, Pidd, Perp and PUMA and to trigger apoptosis, but not cell cycle arrest, in transfected cells as well as cells derived from Ala46 knock-in mice (Feng et al., 2006Feng L. Hollstein M. Xu Y. Cell Cycle. 2006; 5: 2812-2819Crossref PubMed Scopus (83) Google Scholar, Oda et al., 2000Oda K. Arakawa H. Tanaka T. Matsuda K. Tanikawa C. Mori T. Nishimori H. Tamai K. Tokino T. Nakamura Y. et al.Cell. 2000; 102: 849-862Abstract Full Text Full Text PDF PubMed Scopus (1003) Google Scholar). Interestingly, a naturally occurring p53-46F mutant mimics Ser46 phosphorylation and specifically induces p53 proapoptotic target genes, including Noxa, p53AIP1 and p53RFP (Nakamura et al., 2006Nakamura Y. Futamura M. Kamino H. Yoshida K. Nakamura Y. Arakawa H. Cancer Sci. 2006; 97: 633-641Crossref PubMed Scopus (23) Google Scholar), in keeping with the notion that phosphorylation of Ser46 is key to p53 cell fate choice. Ser46 is the target of several kinases, including HIPK2, DYRK2, protein kinase Cδ and p38 (Shmueli and Oren, 2007Shmueli A. Oren M. Mol. Cell. 2007; 25: 794-796Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). These kinases preferentially phosphorylate Ser46 in response to extensive DNA damage, thereby contributing to the increased likelihood of cell death under such conditions. Although sharing a common target on p53, the mechanisms that direct the individual kinases to p53 upon severe genotoxic damage vary greatly. For instance, whereas such damage drives DYRK2 from the cytoplasm into the nucleus, granting it access to its p53 substrate, HIPK2 benefits from a more intricate process, wherein its levels are increased owing to its release from Mdm2-mediated proteasomal degradation (Shmueli and Oren, 2007Shmueli A. Oren M. Mol. Cell. 2007; 25: 794-796Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). How is target gene choice dictated by phosphorylation of Ser46 or of other residues lying outside the p53 DBD? One possibility is that such modifications change the overall conformation of p53, thereby also affecting the DBD. Alternatively, by modulating coactivator binding, they may indirectly affect chromatin states in the vicinity of p53REs, favoring the activation of particular genes over others. Acetylation also plays a role in dictating the target preferences of p53. Lysine 120 (K120) of p53 is sometimes mutated in human cancers. Remarkably, tumor-derived K120R mutations abrogate p53-mediated apoptosis, but not cell cycle arrest. In response to severe DNA damage, K120 is acetylated by the MYST family of acetyl transferases, MOF and TIP60 (Sykes et al., 2006Sykes S.M. Mellert H.S. Holbert M.A. Li K. Marmorstein R. Lane W.S. McMahon S.B. Mol. Cell. 2006; 24: 841-851Abstract Full Text Full Text PDF PubMed Scopus (530) Google Scholar, Tang et al., 2006Tang Y. Luo J. Zhang W. Gu W. Mol. Cell. 2006; 24: 827-839Abstract Full Text Full Text PDF PubMed Scopus (514) Google Scholar). This acetylated form accumulates preferentially on proapoptotic promoters, such as Bax and PUMA, and presumably serves to recruit other p53 cofactors necessary to override the transcriptional barriers in proapoptotic genes. Recently, it has been reported that Lysine 320 (K320) of p53 is also important for the life-death decision. Competition between acytelation and ubiquitylation at this site directs cell fate toward apoptosis or growth arrest, respectively (Le Cam et al., 2006Le Cam L. Linares L.K. Paul C. Julien E. Lacroix M. Hatchi E. Triboulet R. Bossis G. Shmueli A. Rodriguez M.S. et al.Cell. 2006; 127: 775-788Abstract Full Text Full Text PDF PubMed Scopus (167) Google Scholar). Although ubiquitylation is often used to mark proteins for proteasomal degradation, modification of K320 by the E3 ubiquitin ligase E4F1, which promotes K48-type ubiquitylation on chromatin-bound p53, competes with PCAF-mediated acetylation and thereby causes activation of proarrest genes such as p21 and cyclin G1 (Figure 1). Competition between acetylation and ubiquitylation also occurs on numerous additional lysines, located within the C-terminal part of p53. Depending on the extent of ubiquitylation, ubiquitylation may either promote p53 degradation or export into the cytoplasm, in both cases reducing the amount of nuclear p53 available for DNA binding. Conversely, acetylation on these lysines can stabilize p53 and augment its interaction with DNA within the nucleus. At first approximation, acetylation will thus benefit selectively those target genes whose activation requires higher levels of p53. The dynamic nature of p53 acetylation, involving a multitude of histone acetyltransferases (HATs) and histone deactylases (HDACs), endows it with enhanced potential to modulate p53 target gene choice. Binding of p53 to promoter regions presumably recruits factors that act locally on chromatin to “open” the gene for transcription. Thus, the p300/CBP HATs have been implicated as physiological regulators of p53-mediated transcription. In addition to targeting chromatin components, such as histones, their HAT activity also targets p53 itself. Either of these functions might modulate p53 DNA binding activity and promoter choice. JMY is a p300 cofactor that binds p53. Together, p300/JMY are recruited to p53 in response to genotoxic stress, and significantly augment p53-dependent transcription of Bax but not p21 (Coutts and La Thangue, 2006Coutts A.S. La Thangue N. Biochem. Soc. Symp. 2006; 73: 181-189PubMed Google Scholar). This enhances the apoptotic response and decreases the likelihood of cell cycle arrest. Conversely, components of the chromatin remodeling SWI/SNF complex, SNF5 and BRG1, bind to p53 and augment transcription from the p21 promoter, enhancing p53-mediated cell growth arrest (Lee et al., 2002Lee D. Kim J.W. Seo T. Hwang S.G. Choi E.J. Choe J. J. Biol. Chem. 2002; 277: 22330-22337Crossref PubMed Scopus (181) Google Scholar). The exact factors that dictate which chromatin-modifying complex will associate with p53 under a given set of circumstances remain largely to be determined. In a new study, Tanaka et al., 2007Tanaka T. Ohkubo S. Tatsuno I. Prives C. Cell. 2007; (this issue)Google Scholar now describe the role of a chromatin-modifying complex in dictating p53 target gene preferences. The authors discovered that CAS/CSE1L, previously described as a nuclear transport factor, associates selectively in vivo with a subset of p53 target promoters, including PIG3 and p53AIP1, but not p21. CAS co-sedimented with p53 in a high molecular weight complex, suggesting that many additional proteins accompany p53 and CAS. Somewhat surprisingly, the binding of CAS to the specific promoters was independent of p53, implying that it accesses the chromatin of those promoters via other recognition motifs. Consistent with a role in modulating the transcriptional activity of p53, depletion of cellular CAS quenched the p53-dependent expression of genes with whose promoters it tends to associate. Mechanistically, CAS was shown to relieve histone H3 lysine 27 (H3K27) methylation on the chromatin associated with the transcribed region of the genes to which it binds. Given that H3K27 methylation blocks transcription, reversal of this modification should enable efficient transcription, which likely accounts for the stimulatory effect of CAS on p53-mediated gene expression. Unlike Hzf, CAS does not appear to possess a global discrimination between proapoptotic and growth inhibitory genes. Thus, it does not associate with the chromatin of the PUMA gene and does not affect its expression. Yet, the presence of CAS and its recruitment to a subset of proapoptotic p53 target genes is sufficient for enhanced apoptosis. It still remains to be determined whether CAS itself, or its association with p53 and with other partners in the high molecular weight complex, is differentially affected by mild versus severe DNA damage. Nevertheless, the findings of Tanaka et al. imply that much of the target gene selectivity of p53 may be orchestrated not only at the level of binding of p53 to its response elements within the DNA, but also at the level of gene-specific chromatin modifications dictated by the particular preferences of proteins such as CAS. A future challenge is to understand the crosstalk between these diverse regulators of p53 target gene choice. Other factors, including cell type, absence or presence of other transcription factors, and cellular micro- and macroenvironments, are also likely to provide additional layers of regulation. Further understanding of the rules that govern p53's choice may be critical in our battle against cancer, particularly if these efforts lead to therapeutic approaches that tip the balance toward the apoptosis of cancer cells. hCAS/CSE1L Associates with Chromatin and Regulates Expression of Select p53 Target GenesTanaka et al.CellAugust 24, 2007In BriefThe p53 tumor suppressor protein regulates many genes that can determine different cellular outcomes such as growth arrest or cell death. Promoter-selective transactivation by p53, although critical for the different cellular outcomes, is not well understood. We report here that the human cellular apoptosis susceptibility protein (hCAS/CSE1L) associates with a subset of p53 target promoters, including PIG3, in a p53-autonomous manner. Downregulation of hCAS/CSE1L decreases transcription from those p53 target promoters to which it preferentially binds and reduces apoptosis. Full-Text PDF Open ArchiveHzf Determines Cell Survival upon Genotoxic Stress by Modulating p53 TransactivationDas et al.CellAugust 24, 2007In BriefA critical unresolved issue about the genotoxic stress response is how the resulting activation of the p53 tumor suppressor can lead either to cell-cycle arrest and DNA repair or to apoptosis. We show here that hematopoietic zinc finger (Hzf), a zinc-finger-containing p53 target gene, modulates p53 transactivation functions in an autoregulatory feedback loop. Hzf is induced by p53 and binds to its DNA-binding domain, resulting in preferential transactivation of proarrest p53 target genes over its proapoptotic target genes. Full-Text PDF Open Archive
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