Stem Cells, Vol. 16, No. 4, 248-260,
July 1998
© 1998 AlphaMed Press
The Challenge of p53: Linking Biochemistry, Biology, and Patient Management
Susan E. Bray,
Christoph Schorl,
Peter A. Hall
Department of Cellular and Molecular Pathology, University of Dundee, Ninewells Hospital and Medical School, Dundee, Scotland, UK
Key Words. p53 • Stress responses • Physiology • Biochemistry • Therapy
Correspondence:
Dr. Peter A. Hall, Department of Cellular and Molecular Pathology, University of Dundee, Ninewells Hospital and Medical School, Dundee, DD1 9SY, Scotland, UK.
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Abstract
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Abnormalities of the p53 tumor suppressor gene are the single most common molecular abnormality seen in human cancer. Considerable evidence indicates that the product of this gene has critical roles in coordinating the response of cells to a diverse range of environmental stresses. At present, there is a gamut of biochemical properties and interactions ascribed to p53, but the in vivo physiological relevance of many of these remains uncertain. The development of clinical applications and novel therapeutic strategies utilizing our knowledge of p53 is contingent upon bridging the gap between rigorous biochemistry and holistic in vivo studies.
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p53: An Overview
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A complex array of homeostatic mechanisms have evolved that allow cellular adaptation to a diverse range of physiological and pathological stresses. Such responses must be carefully coordinated, but the mechanisms by which eukaryotes respond to harmful exogenous insults by the cessation of cell cycle progression or by the regulated loss of damaged or unwanted cells by apoptosis remain poorly understood. Originally described as a cellular protein that interacted with a critical transforming antigen in the SV40 tumor virus, p53 has emerged as an important, and perhaps pivotal, player in the coordination of adaptive responses to diverse cellular stresses [1-6]. Much attention has focused upon p53 with the recognition that abnormalities of the p53 gene are the most common molecular change seen in human and rodent neoplasia [7, 8], with missense point mutations in the sequence-specific DNA-binding domain and allelic loss at the p53 locus on chromosome 17. A large body of data points to p53 as a transcription factor, activated by diverse stresses, that can regulate the expression of an as yet poorly defined constellation of genes with diverse roles. Most notably, the activation of endogenous p53 or overexpression of exogenous wild-type p53 in many cell types results in cell cycle arrest. Other cell types, however, undergo rapid apoptotic death following wild-type p53 expression, and there is currently intense interest in how cells choose between these two fates. This is not simply an in vitro artifact, since in whole animals the induction of p53 protein expression and its downstream consequences are tightly regulated in a distinct and tissue-specific manner. Indeed the analysis of the in vivo relevance of the biochemically defined properties of p53 represents a great challenge to the field. The purpose of this concise review is to draw attention to many of the questions that exist in the p53 field, highlighting the areas of uncertainty that exist. Of particular importance is the perspective that the enthusiasms for potential therapeutic opportunities should be tempered by a clear recognition of the complexity of the p53 pathway. Furthermore, the unquestioning perspective that p53 is simply a tumor suppressor should be revisited; the evolutionary drive to the emergence of the p53 pathway is not entirely clear, and tumor suppression may not be the only factor [9].
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Structure-Function Relationships of the p53 Molecule
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Understanding the properties and activities of p53 requires familiarity with the relationship between p53 protein structure and function. Such an understanding also provides a framework for understanding the consequences of mutation in the p53 gene and explains why clinically relevant inactivating mutations are concentrated in certain regions of the gene. Human p53 protein ( Fig. 1) consists of 393 amino acids and can be divided into five functional domains which are well conserved in vertebrates [10, 11]. The N-terminus contains a transactivation domain which is able to interact with the basal transcription machinery to regulate the expression of p53 target genes as a consequence of the binding of the central DNA-binding domain to DNA in a sequence-specific manner. The ability of p53 to interact with the basal transcriptional apparatus can itself be modified by binding of proteins, such as mdm2, to specific residues at the N-terminus of p53, particularly residues F19, W23, and L26 [12]. The region between the N-terminal activation domain and the central DNA-binding domain contains five repeats of the motif proline-X-X-proline and is capable of binding to SH3 domains. Deletion of this domain reduces p53-mediated cell arrest or apoptosis but permits p53 transcriptional activation [13]. Perhaps interaction between SH3-containing proteins and p53 is one way in which p53 can "tune into" signal transduction pathways, including those involving proteins such as c-abl, sensing alterations in cellular homeostasis.
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Figure 1. A representation of the structure of mammalian p53. In man, p53 is composed of 393 residues. There are five highly conserved "boxes" and five identifiable regions subserving different functions. However, it should be recognized that the functions are interdependent, and regulation in one "domain" can profoundly influence other domains. Interactions with other macro-molecules are of great significance (see text).
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The central DNA-binding domain of p53 (residues 102-292) confers the ability of p53 to act as a transcription factor, and it is within this region where the majority of point mutations are found in tumors [8]. There are a number of critical residues that contact DNA and others essential for the stability of the amino acid backbone that orientate those contact residues. Specifically, structural studies have revealed that residues K120, S241, R273, A276, and R283 make contacts with the phosphate backbone in the major groove of the DNA duplex, while K120, C277, and R248 interact via hydrogen bonds to the DNA bases [14]. These contact residues are the mutational hot spots in the p53 gene, and more than 90% of missense mutations in p53 are found in this domain; our understanding of this was dramatically enhanced by knowledge of the crystal structure of the p53 molecule. p53 functions as a tetramer as a consequence of homotypic binding through the oligomerization domain between residues 324 and 355 [15]. Occasional mutations in this domain can inactivate p53 function [16]. As a tetramer, p53 binds specifically to a sequence consisting of two copies of the 10 bp motif 5'- PuPuPuC(A/T)(T/A)GPyPyPy-3' which can be separated by 0-13 bp [17, 18]. The C-terminus of p53 appears to have important regulatory functions. It was also established that the carboxy terminus of p53 functions to allosterically regulate the conversion of p53 between forms that are active and inactive for DNA binding [19]. Small peptides were also capable of converting the latent, inactive form of p53 into the active molecule, an important aspect for therapy [20].
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The Induction of the p53 Response
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The induction of p53 accumulation following exposure to UV light was first reported by Maltzman and Cyzyzk [21] in 1984, but the pivotal significance of this observation could not be appreciated at the time. In the early 1990s, a series of observations led to a model of p53 function in which genotoxic damage induced p53 protein accumulation/activation, which in turn was able to trigger specific cell cycle arrest or apoptosis [22-26]. The availability of p53 knock-out mice allowed this model to be critically evaluated and established the absolute dependence on p53 function for some (but not all) of the growth arrest and apoptotic sequelae following genotoxic insult in vivo and in vitro [27-30]. A critical issue that emerged from these and related studies was the role of p53 in inducing apoptosis in the protection of organisms from neoplastic transformation. Furthermore, the contribution of mutational inactivation of p53 to tumor progression in vivo is strikingly highlighted by the studies of van Dyke and colleagues [31]. In these studies, transgenic mice were engineered such that the inactivation of critical genes such as p53 or Rb could be controlled. At least in this model, it was shown that the loss of p53 function was associated with loss of apoptosis and progression from benign to malignant tumors. In addition to a role in control of apoptosis, loss of p53 function also appears to contribute to neoplasia by allowing the development of genetic instability [3, 4, 32, 33].
p53 is well known as a "mediator" or "sensor" of DNA damage and has been christened "guardian of the genome" [33]. Diverse types of genotoxic stress such as 
-irradiation, UV light, and drugs such as etoposide and methyl methanesulfate (MMS) invoke a p53 response, causing the protein to stabilize and increase in level [22, 23, 25]. This is achieved by increasing the molecule's short half-life from approximately 20 min to several hours and presumably by switching the latent form to an active DNA-binding form. It has been postulated that proteins that recognize DNA damage relay this information to p53 by an as yet poorly defined series of signal transduction pathways. For example, patients with the radiosensitive, cancer-prone disease ataxia-telangiectasia lack the ionizing radiation-induced p53 response seen in healthy individuals [22-34]. However, it is possible that p53 itself senses DNA damage as the molecule binds to single-stranded DNA, possibly at excision-repair damage sites [35], and can localize with sites of damage [36]. What is not clear are the biochemical mechanisms that regulate basal levels of p53 protein and signal transduction pathways that are activated by incoming signals and finally culminate in the activation of p53.
An important issue was whether DNA damage was the only insult that could stimulate the pathway or whether other signals were capable of inducing the p53 response. Because the level of DNA damage to induce the p53 response was very low (perhaps just a single strand break [37]), this question has proved to be difficult to answer. However, there is clear evidence that signals not normally thought to be genotoxic are indeed efficient inducers of the p53 pathway ( Fig. 2). Examples include hypoxia [38], change in cell attachment [39], oxidative stress [40], change in cellular growth factor/cytokine milieu [41], and metabolic depletion of ribonucleotide precursors [42]. Indeed, it may be that many other environmental stress signals can induce p53 protein. For example, variable responses to heat shock have been seen, and this may be very cell-type-dependent. Since these stresses induce many other specific transcriptional responses, there is almost certainly an overlap and integration of the p53 response and other stress pathways. This is hardly surprising when viewed from the teleological perspective of the cell. It seems likely that there will be considerable redundancy in these pathways with scope for a range of different pathways acting on p53, presumably by post-translational modification of p53 leading to altered activity of this critical molecule [43, 44]. There is burgeoning evidence that phosphorylation events can critically alter the properties of p53, and in particular, the interaction with other cellular proteins. A central element in this control is the regulation of the p53 level within a cell.
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Figure 2. The p53 pathway is not a simple linear system. Rather, there are many "inputs" and many "outputs." It is quite wrong to think only of DNA damage as the initiating stimulus to the pathway, and it is similarly incorrect to view the resultant consequences as restricted to growth arrest and apoptosis. Furthermore, there are many levels of control with inputs at all points from initiating insult to resultant adaptive response.
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p53 Protein Levels: Regulation and Consequences
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In in vitro studies, p53 induction has been measured by accumulation of the protein, activation of downstream responsive genes, and induction of resultant biological effects such as cell cycle arrest and apoptosis. As these various measures have been compared, an increasing degree of complexity in the p53 response has been revealed ( Fig. 3). Certain elements of this complexity are now being clarified; for example, it has been shown that the absolute level of p53 protein in a cell can modulate the biological effects, with low levels being antiapoptotic, modest levels inducing growth arrest, and higher levels causing apoptosis [45, 46]. Superimposed upon this is clearly a diverse range of other regulators of the cell death versus arrest pathway such as RB, p21waf1, bax, and bcl2 levels. However, levels of p53 are not the only mechanism of regulation, and a series of experiments suggests that in response to specific signals, the specific activity of p53 can be regulated. For example, high levels of wild-type p53 protein are found in teratocarcinoma cells, but these molecules are inactive, at least as transcription factors [47], and the specific activity of p53 can be regulated post-translationally as established by in vitro studies of regulation of the DNA-binding function of p53 protein [20]. Modification of p53 by covalent linkage to RNA and O-glycosylation has been reported, but the physiological significance remains unclear at present.
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Figure 3. There are diverse levels of control of p53. p53 protein is tightly regulated by control of production of protein (mRNA production, stability, and efficiency of translation) and of its degradation by ubiquitin-mediated proteolysis. Additional levels of control include diverse protein-protein interactions, post-translational modifications (including phosphorylation, RNA binding and glycosylation), and also by regulation of subcellular localization. By this means, p53 protein can be switched from inactive to active forms, and as a result, lead to transcriptional activation (or repression) of downstream target genes. Activities which do not relate to transcriptional control have also been suggested.
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While protein levels are not the sole regulatory control on p53, it is clear that this is important. Furthermore, we now have some mechanistic insights into how this is regulated, for example, by p53 binding proteins. mdm2 binds to the transcriptional activation domain of p53, and by so doing, blocks the ability of p53 to act as a transcription factor [48, 49]. That this interaction is of profound biological significance is evidenced by the dramatic embryonic lethal phenotype of mdm2 null mice. However, if mice are engineered which lack both p53 and mdm2, the offspring are fully viable [50]. The precise interaction of mdm2 with the transcriptional activation domain of p53 has been mapped at the molecular level, and armed with this knowledge, the protein-protein interaction can be manipulated with extreme precision [51, 52]. Assaying mdm2 in clinical situations is, however, complex [53].
As well as controlling the ability of p53 to function as a transcription factor, the interaction of p53 and mdm2 has a second and complementary activity. Elegant studies by Haupt et al. [54] and Kubbutat et al. [55] provide clear evidence that the level of p53 is controlled to a large extent by the targeting of p53 for ubiquitin-mediated degradation by the interaction of mdm2 with p53 [56]. Furthermore, the model of p53 level control by mdm2 that results from this observation provides a clear explanation for the widely recognized phenotype of elevated (but nonfunctional) p53 levels seen in human (and animal) tumors. This phenotype has long been recognized to be closely associated with neoplasia and reasonably closely with mutation of the p53 gene [57, 58]. For some time, the p53 overexpression phenotype was felt to be related to an intrinsic alteration in p53 protein stability, despite the fact that mutations could be widely scattered in the p53 gene. A new model now pertains in which the inability of mutant p53 to activate mdm2 gene expression means that p53 levels are no longer downregulated by mdm2-mediated ubiquitin targeting. Direct experimental support for that model comes from Midgley and Lane [59], who showed that in cells with high levels of mutant p53, the introduction of mdm2 would lead to a rapid decline in p53 as a consequence of enhanced degradation. However, while this is certainly true in model systems, it is probably not the complete explanation for p53 stabilization since (A) elevated p53 levels can occur without p53 mutation [58] and (B) in clinical samples, there is a wide range of phenotypes of p53 and mdm2 levels [53].
Another method to control p53 level which is increasingly recognized as being of biological relevance is the notion that its subcellular localization can be regulated. For example, the seminal observations of Moll [60, 61] suggest that the activity of p53 can depend upon regulation of its nuclear or cytoplasmic localization. Furthermore, it is now evident that mdm2 is an RNA-binding protein [62] that can shuttle between the nucleus and cytoplasm via a rev-dependent pathway [63]. This may reflect a further means of regulating p53 activity by the control of subcellular localization of either p53, mdm2, or of both. Whether further compartmentalization within nuclear domains is relevant will require further investigation. Of course, in mechanistic terms, it may well be that the critical issue in p53 function is not overall p53 level but the amount that is able to bind to and activate specific target gene promoters, and a gamut of mechanisms may regulate this "concentration at a point."
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Events Downstream of p53
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p53 exerts most of its effects within a cell by acting as a transcriptional regulator of diverse cellular genes. While transactivation of target genes appears to be the normal consequence, the repression of some cellular genes has been described, although it is less well established in physiological terms. The range of p53 target genes is growing rapidly, and certain key points should be emphasized. For example, we do not know the physiological significance of many of the target genes. Furthermore, it is perhaps incorrect to assume that all p53 target genes are necessarily activated in the same way within a cell under different circumstances, and there may be marked differences in the way different cell types activate target genes, perhaps depending upon cofactors that enhance (or repress) the DNA-binding or transactivation (transrepression) functions of p53. Evidence for these concepts comes from recent studies of p53-interacting proteins that modify either p53 or target DNA [64-66]. In addition, the nature and context of p53 response elements in different genes may have dramatically different abilities to respond to particular p53 levels and may be differentially affected by associated partner proteins or by post-translational modifications. Certainly it is clear that different p53 responsive promoters are differentially activated by different p53 mutants [67]. Indeed, this is perhaps an explanation for the association of different p53 mutations and variable clinical consequences and disease courses.
The list of p53-responsive genes is ever-increasing, with a whole series of novel p53-inducible genes being identified using the differential display [68-71], SAGE [72, 73], and other methods [18]. The physiological functions of the currently known p53-regulated genes are very diverse, and the biological relevance of many remains poorly defined at present, as does the significance of p53 regulation of many. At present, few conclusions can be drawn. However, there are two well-known consequences of p53 activation which, at least in part, depend upon transcriptional regulation of downstream genes: cell cycle arrest and apoptosis. Of the downstream genes that mediate cell cycle arrest, the most well studied is p21WAF1/CIP-1 [74, 75]. p21 is a cyclin-dependent kinase enzyme inhibitor that also blocks DNA replication by binding to proliferation cell nuclear antigen [76] but does not inhibit the ability of PCNA to mediate DNA repair processes. GADD45, another p53 downstream target gene, also interacts with p21 [77] and PCNA [78]. Although the exact function of GADD45 remains elusive, it has been speculated that its interaction with p21 may be to act as a molecular chaperone, guiding p21 to PCNA. Other genes that contain the p53-dependent DNA-binding element are IGF-BP3, insulin-like growth factor binding protein-3, which blocks signaling of a mitogenic growth factor and Bax, a member of the bcl-2 family that promotes apoptosis [3, 4]. p53 can also act to repress transcription of a number of cellular genes (PCNA, c-fos, c-jun, interleukin 6, Rb, and bcl-2); indeed, repression of gene expression may be of considerable biological importance. It is also important to realize that functions of p53 which are independent of transcriptional regulation have been reported [79], although their physiological significance remains uncertain.
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In Vivo Studies: the Biology of p53
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Even from the brief overview described above, it is clear that the burgeoning biochemical database relating to p53 presents a complex and intimidating edifice. However, a critical issue is the in vivo physiological relevance of the disparate and often confusing biochemical data. Of course, it is well accepted that direct extrapolation between in vitro and cell culture assays and in vivo physiology is often difficult. Moreover, the diversity of in vitro and cell culture assays employed is such that it can be very difficult to reconcile apparently contradictory data in the literature. Additional layers of complexity become apparent when studies are performed in model animal systems [6]. For example, the treatment of tissues with UV light [25] or ionizing radiation [29, 80, 81] indicates that there are profound differences in the response of different cells within mammalian tissues to DNA damage—a point not readily apparent from cell culture experiments. These differences are very extreme and are not easily explained at present in mechanistic terms. For example, using 5 Gy of gamma irradiation, conventional radiation target theory suggests that every cell in a mouse should be traversed by many tracks of ionizing radiation, and this should cause considerable DNA damage in all cells in terms of double and single strand breaks as well as multiple ionization events. It is thus curious that not only are there differences in the effect on p53 accumulation between liver (unresponsive) and heart (responsive), but that there are differences within tissues, and indeed, between adjacent cells in a tissue. One plausible interpretation is that this reflects proliferative activity (or potential), but this is hard to reconcile with the observation that myocardium potently induces p53 protein while skeletal muscle does not. If we are to understand the complex biochemistry of p53, then these in vivo issues must be addressed mechanistically.
Following on from these profound differences in in vivo responses in adult tissues, the question of how this might arise in development can be posed [9, 81]. In the 1980s, it was assumed that p53 may have a role in development from observations of developmentally regulated p53 expression. For example, when mRNA levels in normal adult and embryonic murine tissues were measured, it was found that substantial amounts of p53 are found in fetuses up to day 11, dropping rapidly at later times [82]. However the observation that the p53 knock-out mice develop almost normally [83] casts doubt on this perspective and suggests that normal p53 function is dispensable for embryonic development. More recently, it has become clear that although p53 null mice develop normally, some p53–/– mice exhibit a range of developmental abnormalities, and in particular, are associated with defects in neural tube closure [84, 85]. It is noteworthy that this is the only site of p53 expression in the unstressed, developing mouse [81]. Expression of p53 in developing neurons and the effect of p53 suppression on neuronal differentiation show that p53 is expressed only in proliferative neuroblasts and is downregulated upon normal differentiation. Suppression of p53 resulted in accelerated differentiation of neurons, suggesting that p53 maintains the active proliferation of neuroblasts by fine-tuning neuronal differentiation [86]. Apart from the loss of p53 impairing normal developmental and differentiation processes, its loss may also act in an indirect way by preventing the elimination of abnormal cells from the developing embryo. Teratogenic agents administered to pregnant, p53–/– mice resulted in increased embryotoxicity and teratogenicity (compared with wild-type mice), suggesting that p53 may play an important role as a teratological suppressor gene [87, 88]. These data indeed raise the possibility that a major role for p53 is as a stress response gene in the critical developmental stage of higher eukaryotes [9]. Given the data presented above, perhaps the greatest surprise and a major impediment to p53 being regarded primarily as a teratogenic suppressor, is the modest phenotype of the p53 null mouse. One possible insight into this comes from the dramatic recent description of p53 homologs.
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Homologs of p53
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As has been discussed, it is apparent that p53 is a critical, adaptive regulator of diverse stress responses [2, 9]. However, its 18-year reign as the main attraction in a one-man show is clearly over with the discovery of putative p53 homologs: p73 [89, 90], KET [91], and possibly p53CP [92], although this remains as yet incompletely characterized. The search for other members of a p53 family remained fruitless for so long that many in the p53 field became convinced that p53 ruled supreme and alone. Indeed, this view was generally held despite the fact that the p53 null mice developed mostly normally, even when lack of an expected phenotype following inactivation of a gene is explained by functional redundancy where related gene products fulfill the role of the missing gene. This is indeed the case for the retinoblastoma protein, where Rb-related gene proteins p107 and p130 compensate for loss of Rb function [93].
Ironically, the two p53 homologs so far discovered, p73 and KET, were identified serendipitously by groups not working in the p53 field. Kaghad et al. [89] cloned p73 in a screen for an unrelated protein [94, 95]. p73 protein shows significant sequence identity to p53 with the homology extensive in the most conserved p53 domains ( Fig. 4) including the transcriptional activation domain, the DNA-binding domain, and the oligomerization domain. The p73 gene encodes two polypeptides, the products of an alternatively spliced mRNA transcript. The longer of the two, the p73
, consists of 636 residues, while the shorter isoform, p73ß, contains 499 residues, differing from p73 by only five residues at the carboxy terminus. Interestingly, the longer p73 amino acid sequence shows significant homology to a p53-like protein from squid ( Fig. 4), suggesting that p53 may have evolved from a more phylogenetically ancient p73 protein. Of particular interest is the fact that the p73 gene maps to the chromosome region 1p36, a region frequently deleted in neuroblastoma and other human cancers. Neuroblastoma, unlike the majority of tumors, does not carry p53 mutations. Therefore, it seems reasonable to speculate that it is p73 rather than p53 that is performing a crucial tumor-suppressor role in this type of tumorigenesis.
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Figure 4. p53 homologs: similarities and differences. The recognition of possible homologs of p53 in the mammalian genome and the identification of a potential mollusc (invertebrate: Mollusc, squid, Loligo) homolog raise the notion of a family of p53 proteins. All have similarities in the conserved boxes, including conservation of critical contact residues in the N-terminal transactivation domain and in the DNA-binding domain. Similarities in the oligomerization domain exist with the figures representing percentage identity. In the long C-terminal extension of p73, KET, and squid sequence there is a region of homology (thick bar) denoted a SAM domain (see text). This is absent from the splice variant of p73, p73ß.
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Kaghad et al. [89] have reported using the yeast two-hybrid system in which p73ß can form homotypic interactions and, perhaps of greater significance, heterotypic interactions with p53, albeit weakly. The physiological significance of these interactions remains elusive, but it is tempting to speculate that p73 may be capable of regulating p53 function. p73 protein is capable of p21 induction and growth suppression but is not induced in response to DNA-damaging events. Jost et al. [90] showed that transient expression of p73 in SAOS-2 cells (p53 null cell line) and baby-hamster kidney cells promotes apoptosis, and they speculated that p73 when over-produced is capable of activating the programmed cell death response; however, whether this is via a p53-independent or a novel p73-dependent pathway remains to be determined, as well as whether this phenomenon occurs under physiological conditions. At the same time as the p73 discovery, a group searching for signal transduction components that are expressed specifically in rat taste receptor cells amplified another p53 homolog termed KET, as it is expressed in keratinocytes [91]. KET is expressed during embryonic development and in certain adult tissues. Like p73, KET shows homology close to squid p53 ( Fig. 4), again suggesting that KET and p73 may represent primordial p53 ancestral genes. The restricted expression pattern of KET, especially during embryogenesis, suggests that it is involved in tissue-specific differentiation processes, which is probably a function that cannot be fulfilled by p53.
It is noteworthy that both p73 (alpha and beta) and KET have remarkable conservation of critical contact residues for mdm2 (equivalent to residues F19, D23, and L26 in p53) and the DNA-binding domain (equivalent to residues R175, G245, R248, R249, R273, and R282 in p53) as well as homology in the oligomerization domain. It might then be predicted that these proteins will all interact with mdm2, have similar DNA-binding properties, and potentially homo- and hetero-oligomerize. The long C-terminal extensions of p73, KET, and the squid homolog have a motif called the SAM domain which is seen in diverse proteins throughout eukaryotic phyla [96, 97]. Interestingly, this potential protein-protein interaction domain is specifically excluded from p73ß ( Fig. 4). The biological significance of this domain and the homologs will await the generation of suitable reagents for their biochemical analysis and a definition of expression profiles.
The discovery of p53 homologs means that p53 may have to share center stage with its new cousins. While the new homologs answer some old questions, such as the almost-normal development of p53 null mice, they pose many more questions than they answer. Do these homologs activate the same, an overlapping, or a different set of downstream genes? Do p73 and p53 interact to yield novel interactions not displayed by either molecule alone? Do the proteins serve distinct functions in a cell? What signals influence them? p73 mRNA is not induced by DNA damage, but protein levels have not yet been examined. What is the functional significance of the carboxy terminal extensions of p73 and KET? Presumably, p73 and KET will be regulated in a different manner from p53. Perhaps p73 and KET substitute for developmental functions of p53. p73 and KET knockout mice are required to answer this theory. Are there more homologs to be identified? The multifaceted p53 protein provided us with a complex enough story in its own right. Now with the discovery of its new family members, a reevaluation of p53 function is necessary. There is much work to be undertaken in elucidating the roles of the new homologs. For instance, antibodies are urgently required that will allow the homologs to be assayed in vitro and in vivo.
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The Future: p53 in the Clinic
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What of the clinical implications of our burgeoning knowledge of p53? Certainly, there is no lack of information about the prevalence of p53 abnormalities in human disease, including recently in nonneoplastic disorders [98, 99]. Indeed, while the field has focused on p53 and neoplasia, roles in other "stress-related" situations should not be neglected. Witness the possible role of p53 in development (discussed above) and roles in cardiovascular and neurological disease and in the pathogenesis of viral infections [100-107]. Nevertheless, it is in neoplasia where the most attention has focused. Unfortunately, in many situations, the available information is descriptive and sometimes poorly controlled and analyzed. However, several points are clear. For example, there is a remarkable correlation between the phenotype of p53 overexpression and neoplasia, borne out by very many retrospective studies as well as a large prospective analysis [57]. Indeed, while it must be recognized that there are physiological reasons for the accumulation of p53 protein in cells, the widespread availability of robust anti-p53 reagents means that p53 "staining" can be operationally employed as an indication of neoplasia. Of course, caveats to this exist, not least of which is the important point that if extremely sensitive detection systems are employed, p53 immunoreactivity can be seen in a great many normal cells and tissues. A detailed commentary on this general area was published several years ago, and the general points remain valid today [58].
A second area that has received considerable interest and has generated a huge literature is the relationship of p53 and clinical outcome; indeed, of the 11,091 references containing the term "p53" accessed on NCBI PubMed on May 6th 1998, 1,383 contained the word "prognosis." Much "hype" has been used in this area, with a number of rather exaggerated claims being made, ranging from p53 overexpression or mutation being associated with adverse or even better prognosis. A more objective analysis suggests that while there are good theoretical reasons for associating p53 abnormalities with adverse clinical course, in most clinical situations the effect of p53 status on outcome is rather modest [108]. Notwithstanding this rather cynical perspective, other studies have suggested that there is, at least in some tumor types, clinical benefit in assaying p53 status—in breast cancer, for example [109]. However, the true utility of assaying p53 in clinical practice appears to be best defined by the Scottish legal verdict of ... case ... "not proven." One perspective on this is that while we have considerable knowledge of the biochemistry of p53 and it is clear that abnormalities of p53 contribute extensively to the genesis and progression of human tumors, there are many other genes involved, and it is the analysis of the p53 pathway rather than simply p53 per se that is important. Could it be that if we were to analyze the pathway then we would obtain a more effective diagnostic tool which might also provide more robust prognostic information? Of course, to do this we must have a better understanding of the whole pathway; something that is still not in our grasp. Moreover, it is not simply that we still do not understand the pathway in biochemical terms but that we do not understand how this biochemistry functions in a physiological setting in vivo. The implication of the existence of p53 homologs that might be regulated in tissue-specific manners is thus clear. When we have a better basic knowledge of these issues, the possibilities raised by the seminal observations of Lowe [110, 111], positing that analysis of p53 might allow prediction of therapeutic response, may be realized.
Many experiments have demonstrated that the introduction of functional p53 protein into cells can induce growth arrest and/or cell death. Indeed, this provides good evidence for the tumor suppressor activities of p53, although the studies usually employ considerable overexpression of p53, not the modest physiologically relevant levels normally seen in vivo. Consequently, the final area of clinical relevance is the possibility that interfering with p53 might allow novel therapeutic interventions. A number of approaches are being explored. The simplest approach (in concept) is the introduction of wild-type p53 into tumor cells using retroviral or adenoviral delivery systems, which has been mooted as one approach to tumor gene therapy [112]. Indeed, exciting preliminary results have been reported [113], despite all sorts of theoretical reasons why such an approach might not work. For example, the targeting of delivery systems to affect all tumor cells is fraught with difficulty, although it may be that "bystander" effects will eliminate the necessity for this. Another difficulty is that the existence of mutant p53 protein in a cell may inactivate wild-type protein by a "dominant negative" effect. Certainly, this phenomenon can be seen in experimental systems and may compromise the clinical utility of this approach.
A further strategy that has attracted considerable interest is the idea that the inactive mutant p53 protein present in many tumor cells might be reactivated such that it can exert at least some of its wild-type characteristics. Again, this is an approach which has theoretical difficulties, but recent biochemical data do strongly suggest that it is of possible value. For example, the introduction of certain second mutations into p53 that already has an inactivating mutation can restore some normal activity, raising the hope that small molecules might be designed that have similar function-restoring effects [114]. Furthermore, the ability of various peptides and antibodies to affect the C-terminus of mutant p53 protein in such a way as to restore its ability to function in some assays does lend further support to the notion that a suitable small molecule might be designed (or at least identified in a natural product screen) which has therapeutic potential [19, 20, 115-117]. Another strategy is to manipulate the critical regulatory interactions that control p53 function and activity. For example, it might be that the manipulation of the phosphorylation status of p53 with suitable kinase or phosphatase inhibitors could be of utility. Alternatively, the manipulation of critical protein-protein interactions that control p53 function could be targeted. For example, the recognition that mdm2 plays a critical role in p53 homeostasis and our detailed biochemical and structural understanding of this interaction make it a particularly attractive target, at least from the point of view of "proof of principle" [51].
One approach that has provided some exciting insights as to what may be possible is the use of a mutant adenovirus to kill cells that have a defective p53 pathway [118, 119]. The approach is as follows. An adenovirus encodes a protein (E1b) whose function is to inactivate p53; without this functionality, it cannot replicate in mammalian cells. An adenovirus which cannot produce functional E1b protein will not replicate (a process that will eventually kill the infected cell). By engineering an adenovirus with exactly this defect, it is found that this virus will not grow in any normal cell but only in those in which p53 is inactivated by some other route—for example, by missense mutation or by allelic loss—events typical of many human tumors. Thus, the introduction of the virus into normal cells has no effect, but tumor cells with mutant p53 are killed by the productive lytic infection. This is, of course, very useful, since the infected cells are not only killed, but they also then die and release more virus then capable of infecting additional tumor cells and killing them, a system with intrinsic amplification. Both experimental and phase I clinical studies of this approach are very encouraging. An alternate strategy to the utilization of our knowledge of p53 for therapeutic benefit may be to target the normal cells in the body which limit the effectiveness of radio- and chemotherapy (bone marrow, gut, mucosa, etc.) and to inhibit the p53-dependent apoptotic response here, hence enhancing the potential therapeutic ration of current regimes. A limitation on this may be the existence of apoptotic pathways independent of p53. For example, even in p53 null mice where there is a clear relationship between apoptosis and p53 function [29], apoptosis is seen in the gut in the absence of p53 function at later time points [120].
Clearly, then, manipulating p53 will be of clinical benefit if technical issues (delivery, viral safety, suitable drug discovery) are overcome—in themselves major tasks. Or are there reasons to be a little more skeptical? Despite the importance of p53 in human neoplasia and the evidence that overexpression of p53 in cell lines in vitro can reverse the transformed phenotype, it may be quite inappropriate to conclude that restoration of p53 activity will necessarily be beneficial. A particularly striking example of this comes from the elegant studies of Ewald et al. [121]. Using a tissue-specific regulatable promoter, p53 function was inactivated in vivo by the expression of the p53 binding domain of SV40 large T antigen in salivary gland epithelium. When T antigen is turned on (and hence p53 inactivated), there are progressive cellular hyperplasia and atypia. Turning T antigen off (and hence restoring p53 function) leads to regression of these morphological lesions. In separate experiments, if T antigen is left on longer, there are more severe changes which appear to be overtly neoplastic but benign. Again, these will regress if p53 function is restored by turning T antigen off. However, if T antigen is left on longer, then overtly malignant tumors arise which persist even in the absence of T-antigen expression; presumably, other molecular events have occurred, leading to the tumors persisting irrespective of p53 function. Remembering that the clinical phase of tumors is short compared with their life spans, any manipulation of p53 will be late and presumably in the milieu of multiple genetic changes and heterogeneity derived from the existence of multiple clonal sublines within the tumor.
In conclusion, this review has developed the idea that the considerable complexity of the p53 pathway (which inevitably, we have only touched upon) must be interpreted in the light of its physiological relevance in in vivo systems. Without doubt, p53 has clear relevance to the biology of neoplasia, but this should not obscure other possible biologically important and potentially clinically relevant areas (such as development, cardiovascular, and neurological disease). While at present the direct clinical relevance of assaying the p53 pathway and its abnormalities is unclear (witness the conflicting data on p53 and prognosis), the potential therapeutic opportunities are of great interest and may prove to be immensely beneficial. Perhaps, however, it might be prudent to view some of the enthusiasm with a degree of healthy skepticism. One fact is certain—the identification of the new homologs complicates an already complex field and potentially has important implications for our understanding, not only of the causes of certain cancers, but also for the evolution and emergence of the p53 gene. Finally, it is apparent to us, and we hope to the readers, that the challenge for the future will be the linking of the detailed and rigorous biochemistry to the complex and difficult-to-study physiological systems of whole organisms. We know a lot about p53 in cells in culture, but very little (in truth) about p53 in complex tissues in vivo. Bridging this gap will be the key to utilizing our knowledge of p53 to benefit the patient. This is a tall order, and there may yet be more surprises for us.
Useful Web Sites
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Acknowledgments
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Work in the Hall laboratory is supported by the Cancer Research Campaign, the AICR, the European Union, the Department of Health and the Pathological Society of Great Britain and Ireland. We are indebted to the other members of my laboratory (see http://www.dundee.ac.uk/pathology/pah.htm) and also to David Meek, David Lane and Eric Wright for many invaluable discussions. We apologize to the many authors whose work we have not cited because of space constraints.
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References
|
|---|
-
Harris CC. p53 tumor suppressor gene: from the basic research laboratory to the clinic—an abridged historical perspective. Carcinogenesis 1996;17:1187-1198.[Free Full Text]
-
Hall PA, Meek D, Lane DP. p53—integrating the complexity. J Pathol 1996;180:1-5.[Medline]
-
Ko JL, Prives C. p53: puzzle and paradigm. Genes Dev 1996;10:1054-1072.[Free Full Text]
-
Levine AJ. p53, the cellular gatekeeper for growth and division. Cell 1997;88:323-331.[Medline]
-
Agarwal ML, Taylor WR, Chernov MV et al. The p53 network. J Biol Chem 1998;273:1-4.[Free Full Text]
-
Campbell SJ, O'Neill M, Hall PA. The p53 gene family—an holistic perspective. In: Dive C, Hickman J, eds. Cell Death and Chemotherapy. Totowa, NJ: Humana Press (in press).
-
Greenblatt MS, Bennett WP, Hollstein M et al. Mutations in the p53 tumor suppressor gene: clues to cancer etiology and molecular pathogenesis. Cancer Res 1994;54:4855-4878.[Free Full Text]
-
Hollstein M, Rice K, Greenblatt MS et al. Database of p53 gene somatic mutations in human tumours and cell lines. Nucleic Acids Res 1994;22:3551-3555.
-
Hall PA, Lane DP. p53 - a developing role? Curr Biol 1997;7:144-147.[Medline]
-
Soussi T, May P. Structural aspects of the p53 protein in relation to gene evolution: a second look. J Mol Biol 1996;260:623-637.[Medline]
-
Arrowsmith CH, Morin P. New insights into p53 function from structural studies. Oncogene 1996;12:1379-1385.[Medline]
-
Kussie PH, Gorina S, Marechal V et al. Structure of the MDM2 oncoprotein bound to the p53 tumor suppressor transactivation domain. Science 1996;274:948-953.[Abstract/Free Full Text]
-
Walker KK, Levine AJ. Identification of a novel p53 functional domain that is necessary for efficient growth suppression. Proc Natl Acad Sci USA 1996;93:15335-15340.[Abstract/Free Full Text]
-
Cho Y, Gorina S, Jeffrey PD et al. Crystal structure of p53 tumour suppressor-DNA complex: understanding tumourigenic mutations. Science 1994;265:1353-1356.
-
Jeffrey PD, Gorina S, Pavletich NP. Crystal structure of the tetramerization domain of the p53 tumor suppressor at 1.7 angstroms. Science 1995;267:1498-1502.[Abstract/Free Full Text]
-
Lomax ME, Barnes DM, Gilchrist R et al. Two functional assays employed to detect an unusual mutation in the oligomerisation domain of p53 in a Li-Fraumeni like family. Oncogene 1997;14:1869-1874.[Medline]
-
El-Deiry WS, Kern SE, Pietenpol JA et al. Definition of a consensus binding site for p53. Nat Genet 1992;1:45-49.[Medline]
-
Bourdon JC, Deguin-Chambon V, Lelong JC et al. Further characterisation of the p53 responsive element—identification of new candidate genes for trans-activation by p53. Oncogene 1997;14:85-94.[Medline]
-
Hupp TR, Meek DW, Midgley CA et al. Regulation of the specific DNA binding function of p53. Cell 1992;71:875-886.[Medline]
-
Hupp TR, Sparks A, Lane DP. Small peptides activate the latent sequence-specific DNA binding function of p53. Cell 1995;83:237-245.[Medline]
-
Maltzman W, Czyzyk L. UV irradiation stimulates levels of p53 cellular tumor antigen in nontransformed mouse cells. Mol Cell Biol 1984;4:1689-1694.[Abstract/Free Full Text]
-
Kastan MB, Onyekwere O, Sidransky D et al. Participation of p53 protein in the cellular response to DNA damage. Cancer Res 1991;51:6304-6311.[Abstract/Free Full Text]
-
Kastan MB, Zhan Q, el-Deiry WS et al. A mammalian cell cycle checkpoint pathway utilizing p53 and GADD45 is defective in ataxia-telangiectasia. Cell 1992;71:587-597.[Medline]
-
Lu X, Lane DP. Differential induction of transcriptionally active p53 following UV or ionizing radiation: defects in chromosome instability syndromes? Cell 1993;75:765-778.[Medline]
-
Hall PA, Mckee PH, Menage HP et al. High levels of p53 protein in UV-irradiated normal human skin. Oncogene 1993;8:203-207.[Medline]
-
Yonish-Rouach E, Resnitzky D, Lotem J et al. Wild-type p53 induces apoptosis of myeloid leukaemic cells that is inhibited by interleukin-6. Nature 1991;352:345-347.[Medline]
-
Clarke AR, Purdie CA, Harrison DJ et al. Thymocyte apoptosis induced by p53-dependent and independent pathways. Nature 1993;362:849-852.[Medline]
-
Lowe SW, Schmitt EM, Smith SW et al. p53 is required for radiation-induced apoptosis in mouse thymocytes. Nature 1993;362:847-849.[Medline]
-
Merritt A, Potten CS, Kemp CJ et al. The role of p53 in spontaneous and radiation induced apoptosis in the gastrointestinal tract of normal and p53 deficient mice. Cancer Res 1994;54:614-617.[Abstract/Free Full Text]
-
Clarke AR, Gledhill S, Hooper ML et al. p53 dependence of early apoptotic and proliferative responses within the mouse intestinal epithelium following gamma-irradiation. Oncogene 1994;9:1767-1773.[Medline]
-
Symonds H, Krall L, Remington L et al. p53-dependent apoptosis suppresses tumour growth and progression in vivo. Cell 1994;78:703-711.[Medline]
-
Griffiths SD, Clarke AR, Healy LE et al. Absence of p53 permits propagation of mutant cells following genotoxic damage. Oncogene 1997;14:523-531.[Medline]
-
Lane DP. Cancer. p53, guardian of the genome. Nature 1992;358:15-16.[Medline]
-
Zakian VA. ATM-related genes: what do they tell us about functions of the human gene? Cell 1995;82:685-687.[Medline]
-
Bakalin G, Yakovleva T, Selivanova G et al. p53 binds single-stranded DNA ends and catalyses DNA renaturation and strand transfer. Proc Natl Acad Sci USA 1994;91:413-417.[Abstract/Free Full Text]
-
Coates PJ, Save V, Ansari B et al. Demonstration of DNA damage/repair in individual cells using in situ end labelling: association of p53 with sites of DNA damage. J Pathol 1995;176:19-26.[Medline]
-
Huang LC, Clarkin KC, Wahl GM. Sensitivity and selectivity of the DNA damage sensor responsible for activating p53-dependent G1 arrest. Proc Natl Acad Sci USA 1996;93:4827-4832.[Abstract/Free Full Text]
-
Graeber TG, Osmanian C, Jacks T et al. Hypoxia-mediated selection of cells with diminished apoptotic potential in solid tumors.Nature 1996;379:8891-8894.
-
Nikiforov MA, Hagen K, Ossovskaya VS et al. p53 modulation of anchorage independent growth and experimental metastasis. Oncogene 1996;13:1709-1719.[Medline]
-
Renzing J, Hansen S, Lane DP. Oxidative stress is involved in the UV activation of p53. J Cell Sci 1996;109:1105-1112.[Abstract]
-
Canman CE, Gilmer TM, Coutts SB et al. Growth factor modulation of p53-mediated growth arrest versus apoptosis. Genes Dev 1995;9:600-611.[Abstract/Free Full Text]
-
Linke SP, Clarkin KC, Di Leonardo A et al. A reversible, p53-dependent G0/G1 cell cycle arrest induced by ribonucleotide depletion in the absence of detectable DNA damage. Genes Dev 1996;10:934-947.[Abstract/Free Full Text]
-
Meek DW. Post-translational modification of p53 and the integration of stress signals. Path Biol 1997;45:804-814.
-
Meek DW. Multisite phosphorylation and the integration of stress signals at p53. Cell Signal 1998;10:159-166.[Medline]
-
Chen X, Ko LJ, Jayaraman L et al. p53 levels, functional domains, and DNA damage determine the extent of the apoptotic response of tumor cells. Genes Dev 1996;10:2438-2451.[Abstract/Free Full Text]
-
Lassus P, Ferlin M, Piette J et al. Anti-apoptotic activity of low levels of wild-type p53. EMBO J 1996;15:4566-4573.[Medline]
-
Lutzker SG, Levine AJ. A functionally inactive p53 protein in teratocarcinoma cells is activated by either DNA damage or cellular differentiation. Nat Med 1996;2:804-810.[Medline]
-
Chen J, Marechal V, Levine AJ. Mapping of the p53 and mdm-2 interaction domains. Mol Cell Biol 1993;13:4107-4114.[Abstract/Free Full Text]
-
Momand J, Zambetti GP, Olson DC et al. The mdm-2 oncogene product forms a complex with the p53 protein and inhibits p53-mediated transactivation. Cell 1992;69:1237-1245.[Medline]
-
Montes de Oca Luna R, Wagner DS, Lozano G. Rescue of early embryonic lethality in mdm2-deficient mice by deletion of p53. Nature 1995;378:203-206.[Medline]
-
Bottger A, Bottger V, Sparks A et al. Design of a synthetic Mdm2-binding mini protein that activates the p53 response in vivo. Curr Biol 1997;7:860-869.[Medline]
-
Bottger A, Bottger V, Garcia-Echeverria C et al. Molecular characterization of the hdm2-p53 interaction. J Mol Biol 1997;269:744-756.[Medline]
-
O'Neill M, Campbell SJ, Save V et al. An immunochemical analysis of mdm2 expression in human breast cancer and the identification of a growth regulated cross reacting species, p170. Journal of Pathology (in press).
-
Haupt Y, Maya R, Kazaz A et al. Mdm2 promotes the rapid degradation of p53. Nature 1997;387:296-299.[Medline]
-
Kubbutat MH, Jones SN, Vousden KH. Regulation of p53 stability by Mdm2. Nature 1997;387:299-303.[Medline]
-
Lane DP, Hall PA. MDM2: arbiter of p53's destruction. Trends Biochem Sci 1997;22:372-374.[Medline]
-
Dowell SP, Derias N, Wilson POG et al. The clinical utility of the immunocytochemical detection of p53 protein in cytological specimens. Cancer Res 1994;54:1914-1918.
-
Hall PA, Lane DP. p53 in tumour pathology: can we trust immunohistochemistry ... revisited! J Pathol 1994;172:1-4.[Medline]
-
Midgley CA, Lane DP. p53 protein stability in tumour cells is not determined by mutation but is dependent on Mdm2 binding. Oncogene 1997;15:1179-1189.[Medline]
-
Moll UM, Ostermeyer AG, Haladay R et al. Cytoplasmic sequestration of wild-type p53 protein impairs the G1 checkpoint after DNA damage. Mol Cell Biol 1996;16:1126-1137.[Abstract]
-
Ostermeyer AG, Runko E, Winkfield B et al. Cytoplasmically sequestered wild-type p53 protein in neuroblastoma is relocated to the nucleus by a C-terminal peptide. Proc Natl Acad Sci USA 1996;93:15190-15194.[Abstract/Free Full Text]
-
Elenbaas B, Dobbelstein M, Roth J et al. The MDM2 oncoprotein binds specifically to RNA through its RING finger domain. Mol Med 1996;2:439-451.[Medline]
-
Roth J, Dobbelstein M, Freedman DA et al. Nucleo-cytoplasmic shuttling of the hdm2 oncoprotein regulates the levels of the p53 protein via a pathway used by the human immunodeficiency virus rev protein. EMBO J 1998;17:554-564.[Medline]
-
Jayaraman L, Moorthy NC, Murthy KGK et al. High mobility group protein-1 (HMG-1) is a unique activator of p53. Genes Dev 1998;12:462-472.[Abstract/Free Full Text]
-
Jayaraman L, Murthy KG, Zhu C et al. Identification of redox/repair protein Ref-1 as a potent activator of p53. Genes Dev 1997;11:558-570.[Abstract/Free Full Text]
-
Avantaggiati ML, Ogryzko V, Gardner K et al. Recruitment of p300/CBP in p53-dependent signal pathways. Cell 1997;89:1175-1184.[Medline]
-
Friedlander P, Haupt Y, Prives C et al. mutant p53 that discriminates between p53-responsive genes cannot induce apoptosis. Mol Cell Biol 1996;16:4961-4971.[Abstract]
-
Lehar SM, Nacht M, Jacks T et al. Identification and cloning of E124, a gene induced by p53 in etoposide-treated cells. Oncogene 1996;12:1181-1187.[Medline]
-
Amson AB, Nemani M, Roperch JP et al. Isolation of 10 differentially expressed cDNAs in p53-induced apoptosis: activation of the vertebrate homolog of the Drosophila seven in absentia gene. Proc Natl Acad Sci USA 1996;93:3953-3957.[Abstract/Free Full Text]
-
Morita N, Kiryu S, Kiyama KH. p53-independent cyclin G expression in a group of mature neurons and its enhanced expression during nerve regeneration. J Neurosci 1996;16:5961-5966.[Abstract/Free Full Text]
-
Israeli D, Tessler E, Haupt Y et al. A novel p53-inducible gene, PAG608, encodes a nuclear zinc finger protein whose overexpression promotes apoptosis. EMBO J 1997;16:4384-4392.[Medline]
-
Polyak K, Xia Y, Zweier JL et al. A model for p53-induced apoptosis. Nature 1997;389:300-305.[Medline]
-
Madden SL, Galella EA, Zhu J et al. Transcript profiles for p53-dependent growth regulation. Oncogene 1997;15:1079-1085.[Medline]
-
El-Deiry WS, Tokino T, Velculescu VE et al. WAF1, a potential mediator of p53 tumor suppression. Cell 1993;75:817-825.[Medline]
-
Harper JW, Adami GR, Wei N et al. The p21 interacting protein Cip 1 is a potent inhibitor of G1 cyclin-dependent kinases. Cell 1993;75:805-816.[Medline]
-
Warbrick E, Lane DP, Glover DP et al. A small peptide inhibitor of DNA replication defines the site of interaction between the cyclin-dependent kinase inhibitor p21/WAF1 and proliferating cell nuclear antigen. Curr Biol 1995;5:275-282.[Medline]
-
Kearsey JM, Coates PJ, Prescott AR et al. Gadd 45 is a nuclear cell cycle regulated protein which interacts with p21/Cip1. Oncogene 1995;11:1675-1683.[Medline]
-
Hall PA, Kearsey JM, Coates PJ et al. Characterisation of the interaction between PCNA and Gadd45. Oncogene 1995;10:2427-2433.[Medline]
-
Caelles C, Helmberg A, Karin M. p53-dependent apoptosis in the absence of transcriptional activation of p53-target genes. Nature 1994;370:220-223.[Medline]
-
Midgley CA, Owens B, Briscoe CV et al. Coupling between gamma irradiation, p53 induction and the apoptotic response depends upon cell type in vivo. J Cell Sci 1995;108:1843-1848.[Abstract]
-
MacCallum DE, Hupp TR, Midgley CA et al. The p53 response to ionising radiation in adult and developing murine tissues. Oncogene 1996;13:2575-2587.[Medline]
-
Rogel A, Popliker M, Webb CG et al. p53 cellular tumour antigen: analysis of mRNA levels in normal adult tissues, embryos and tumours. Mol Cell Biol 1995;5:2851-2855.
-
Donehower LA, Harver M, Slagle BL et al. Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours. Nature 1992;356:215-221.[Medline]
-
Armstrong JF, Kaufman MH, Harrison DJ et al. High-frequency developmental abnormalities in p53-deficient mice. Curr Biol 1995;5:931-936.[Medline]
-
Sah VP, Attardi LD, Mulligan GJ et al. A subset of p53-deficient embryos exhibit exencephaly. Nat Genet 1995;10:175-180.[Medline]
-
Ferreira A, Kosik KS. Accelerated neuronal differentiation induced by p53 suppression. J Cell Sci 1996;109:1509-1516.[Abstract]
-
Nicol CJ, Harrison ML, Laposa RR et al. A teratologic suppressor role for p53 in benzo[a]pyrene-treated transgenic p53-deficient mice. Nat Genet 1995;10:181-187.[Medline]
-
Norimura T, Nomoto S, Katsuki M et al. p53-dependent apoptosis suppresses radiation-induced teratogenesis. Nat Med 1996;2:577-578.[Medline]
-
Kaghad M, Bonnet H, Yang A et al. Monoallelically expressed gene related to p53 at 1p36, a region frequently deleted in neuroblastoma and other human cancers. Cell 1997;90:809-819.[Medline]
-
Jost CA, Marin MC, Kaelin WG Jr. p73 is a human p53-related protein that can induce apoptosis. Nature 1997;389:191-194.[Medline]
-
Schmale S, Bamberger C. A novel protein with strong homology to the tumour suppressor p53. Oncogene 1997;15:1363-1367.[Medline]
-
Bian J, Sun Y. p53CP, a putative p53 competing protein that specifically binds to the consensus p53 binding sites: a third member of the p53 family? Proc Natl Acad Sci USA 1997;94:14753-14758.[Abstract/Free Full Text]
-
Herwig S, Strauss M. The retinoblastoma protein: a master regulator of cell cycle, differentiation and apoptosis. Eur J Biochem 1997;246:581-601.[Medline]
-
Clurman B, Groudine M. Tumour-suppressor genes. Killer in search of a motive? Nature 1997;389:122-123.[Medline]
-
Oren M. Lonely no more: p53 finds its kin in a tumour suppressor haven. Cell 1997;90:829-832.[Medline]
-
Bork P, Koonin EV. Predicting functions from protein sequences—where are the bottlenecks? Nat Genet 1998;18:313-318.[Medline]
-
Schultz J, Ponting CP, Hofmann K. SAM as a protein interaction domain involved in developmental regulation. Protein Sci 1997;6:249-253.[Medline]
-
Firestein GS, Echeverri F, Yeo M. Somatic mutations in the p53 tumor suppressor gene in rheumatoid arthritis synovium. Proc Natl Acad Sci USA 1997;94:10895-10900.[Abstract/Free Full Text]
-
Aupperle KR, Boyle DL, Hendrix M et al. Regulation of synoviocyte proliferation, apoptosis, and invasion by the p53 tumor suppressor gene. Am J Pathol 1998;152:1091-1098.[Abstract]
-
Bennett MR, Littlewood TD, Schwartz SM et al. Increased sensitivity of human vascular smooth muscle cells from atherosclerotic plaques to p53-mediated apoptosis. Circ Res 1997;81:591-599.[Abstract/Free Full Text]
-
Kim KK, Soonpaa MH, Daud AI et al. Tumor suppressor gene expression during normal and pathologic myocardial growth. J Biol Chem 1994;269:22607-22613.[Abstract/Free Full Text]
-
Li Y, Chopp M, Zhang ZG et al. p53-immunoreactive protein and p53 mRNA expression after transient middle cerebral artery occlusion in rats. Stroke 1994;25:849-855.[Abstract]
-
Crumrine RC, Thomas AL, Morgan PF. Attenuation of p53 expression protects against focal ischemic damage in transgenic mice. J Cereb Blood Flow Metab 1994;14:887-891.[Medline]
-
Mattila KJ, Valtonen VV, Nieminen MS et al. Role of infection as a risk factor for atherosclerosis, myocardial infarction, and stroke. Clin Infect Dis 1998;26:719-734.[Medline]
-
Marx J. CMV-p53 interaction may help explain clogged arteries. Science 1994;265:320.[Free Full Text]
-
Baas IO, Offerhaus JA, El-Deiry WS et al. The WAF1-mediated p53 growth-suppressor pathway is intact in the coronary arteries of heart transplant recipients. Hum Pathol 1996;27:324-329.[Medline]
-
Garcia JF, Piris MA, Lloret E et al. p53 expression in CMV-infected cells: association with the alternative expression of the p53 transactivated genes p21/WAF1 and MDM2. Histopathol 1997;30:120-125.[Medline]
-
Dowell SP, Hall PA. The p53 tumour suppressor gene and tumour prognosis: is there a relationship? J Pathol 1995;177:221-224.[Medline]
-
Sjogren S, Inganas M, Norberg T et al. The p53 gene in breast cancer: prognostic value of complementary DNA sequencing versus immunohistochemistry. J Natl Cancer Inst 1996;88:173-182.
-
Lowe SW, Ruley HE, Jacks T et al. p53-dependent apoptosis modulates the cytotoxicity of anticancer agents. Cell 1993;74:957-967.[Medline]
-
Lowe SW, Bodis S, McClatchey A et al. p53 status and the efficacy of cancer therapy in vivo. Science 1994;266:807-810.[Abstract/Free Full Text]
-
Roth JA. Gene replacement strategies for lung cancer. Curr Opin Oncol 1998;10:127-132.[Medline]
-
Roth JA, Nguyen D, Lawrence DD et al. Retrovirus-mediated wild-type p53 gene transfer to tumors of patients with lung cancer. Nat Med 1996;2:985-991.[Medline]
-
Brachmann RK, Yu K, Eby Y et al. Genetic selection of intragenic suppressor mutations that reverse the effect of common p53 cancer mutations. EMBO J 1998;17:1847-1859.[Medline]
-
Abarzua P, Losardo JE, Gubler ML et al. Microinjection of monoclonal antibody PAb421 into human SW480 colorectal carcinoma cells restores the transcription activation function to mutant p53. Cancer Res 1995;55:3490-3494.[Abstract/Free Full Text]
-
Abarzua P, Losardo JE, Gubler ML et al. Restoration of the transcription activation function to mutant p53 in human cancer cells. Oncogene 1996;13:2477-2482.[Medline]
-
Hansen S, Hupp TR, Lane DP. Allosteric regulation of the thermostability and DNA binding activity of human p53 by specific interacting proteins. J Biol Chem 1996;271:3917-3924.[Abstract/Free Full Text]
-
Bischoff JR, Kirn DH, Williams A et al. An adenovirus mutant that replicates electively in p53-deficient human tumor cells. Science 1996;274:373-376.[Abstract/Free Full Text]
-
Heise C, Sampson-Johannes A, Williams A et al. ONYX-015, an E1B gene-attenuated adenovirus, causes tumor-specific cytolysis and antitumoral efficacy that can be augmented by standard chemotherapeutic agents. Nat Med 1997;3:639-645.[Medline]
-
Merritt AJ, Allen TD, Potten CS et al. Apoptosis in small intestinal epithelial from p53-null mice: evidence for a delayed, p53-independent G2/M-associated cell death after gamma-irradiation. Oncogene 1997;14:2759-2766.[Medline]
-
Ewald D, Li M, Efrat S et al. Time-sensitive reversal of hyperplasia in transgenic mice expressing SV40 T antigen. Science 1996;273:1384-1386.[Abstract]
accepted for publication on May 19, 1998.
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Abstract
p53: An Overview
