Abstract
p53 is a protein with many talents. One of the most fundamental is the ability to act as essential growth checkpoint that protects cells against cellular transformation. p53 does so through the induction of genes leading to growth arrest or apoptosis. Most of the studies focusing on the mechanisms of p53 activity have been performed in cultured cells upon treatment with well-established p53-activating inputs, such as high doses of radiations, DNA-damaging drugs and activated oncogenes. However, how the tumor suppressive functions of p53 become concerted with the extracellular cues arriving at the cell surface during tissue homeostasis, remains largely unknown. Intriguingly, two recent papers have shed new light into this unexplored field, indicating that p53 plays a key role in TGF-beta-induced growth arrest and, unexpectedly, in the developmental effects of TGF-beta in early embryos. Here we review and comment on these findings and on their implications for cancer biology.

Keywords: p53, TGF-beta, Growth control, Cancer, Embryonic development

1. Introduction
Members of the TGF-beta growth factor family are fundamental signals regulating cell behaviour in a variety of cellular contexts [1-4]. In adult tissues, TGF-beta conveys crucial cytostatic signals to epithelial, immune and other cells types; the loss of this response is the hallmark of many cancers [5]. Research over more than a decade has brought us a fairly satisfactory view of how the TGF-beta signal is transduced from the cell surface to the nucleus. TGF-beta ligands bind to cognate serine/threonine kinase receptors leading, intracellularly, to phosphorylation and activation of the Receptor-Smads signal transducers (R-Smad), such as Smad2 and Smad3. Once activated, the R-Smads translocate into the nucleus to control gene expression in concert with Smad4. The Smads bind to DNA with low affinity and, unless their consensus is reiterated, they require transcription factors as partners for specific interaction with target promoters [6]. The archetypal Smad partner is FAST-1 (forkhead activin signal transducer), which lacks intrinsic transcriptional activity, but is essential for targeting the Smads to some developmentally relevant promoters, such as Mix.2 [7-10].
One may expect that such remarkably simple transduction machinery may leave little room for flexibility and lead to a rather stardardized gene expression profile. In contrast, TGF-beta can control hundreds of target genes in different cells [11-13]. Although the mechanisms by which such transcriptional plasticity is attained are unknown, they must entail the interaction of the Smads with other "informational nodes" of the cell. These nodes should be able to integrate multiple inputs from inside and outside the cell - in one word - setting the "context" in which the TGF-beta signal must act.

2. TGF-beta and growth control
In epithelial cells, TGF-beta controls cell growth by the up-regulation of the cyclin-dependent-kinase inhibitors p15INK4b and p21WAF1 [14, 15] and the concomitant repression of the growth promoting transcription factors c-MYC, Id1, Id2 and Id3 [16-18]. In normal cells, the anti-mitogenic activities of TGF-beta prevail over inputs favoring proliferation but, in cancer cells, the TGF-beta cytostatic program fails to be activated. This resistance can be acquired in some cancers through inactivation of the genes encoding for TGF-beta receptors and Smads [19]. However, the vast majority of cancers lose selectively the growth-inhibitory response to TGF-beta in the presence of intact TGF-beta receptors and Smads, leaving other responses fully operative. Indeed, advanced carcinomas typically express high levels of TGF-beta to foster their own invasive/pro-metastatic abilities [20, 21]. Therefore, as loss of TGF-beta responsiveness is rare in cancer, it is of great interest to determine how cancer cells acquire selective resistance to TGF-beta in growth-inhibition. Conceivably, loss of TGF-beta tumor suppressing effects should occur by genetic modifications in unknown Smad effectors or downstream molecules. The identification of these modulators is therefore essential for understanding tumor incidence and progression.

3. Identification of p53 as Smad partner
p53 was isolated independently in this laboratory and by Atsushi Suzuki' s group in unbiased screens for biologically active molecules expressed during embryonic development [22, 23]. The two groups searched for genes endowed with biological activity within small pools of cDNAs (n<100) from embryonic libraries using Xenopus assays. Curiously, the same result was obtained by quite distinct screening strategies. One group set to identify mouse genes whose expression promoted the activation of mesoderm and endoderm markers in embryonic cells [22], which is a typical TGF-beta-related effect in frog embryos. The second group screened for molecules able to transform the most anterior neural tissue into posterior brain or spinal cord [23]. During Xenopus development, this "posteriorizing" activity emanates from the mesoderm cells underlying the developing CNS (such as those of the notocord and somites) [24]. In both screens, sib-selection revealed that p53 was, unexpectedly, the biologically active cDNA [22, 23].
Injection of titrated doses of p53 mRNA in Xenopus cells causes them to adopt mesodermal and endodermal fates, in a dose-dependent fashion. Molecularly, these inductions are direct (that is, cycloheximide insensitive) and require p53-dependent transcription [22, 23].
Importantly, p53 requires Smad activity to engage its TGF-beta-like effects [22, 23]. This suggests that overexpression of p53 can make cells responsive to otherwise sub-threshold amounts of endogenous TGF-beta ligands. Along the same line, it will be interesting to check whether TGF-beta and unbalanced levels of p53 may cooperate during mammalian development; indeed, mice mutant for the p53-inhibitor mdm-2 [25, 26] die very early during embryonic development (around E6.5), leaving unaddressed the possibility that the expression of early mesoderm/endoderm markers (whose development is controlled by nodal, a TGF-beta-related ligand) may be perturbed in this genetic background as an effect of increased p53 levels.
Of note, a closer comparison of the immediate gene responses triggered by Smad2 and different doses of p53 mRNA in Xenopus cells reveals a large, but not complete overlap of activity, being p53 unable to activate some TGF-beta targets (such as Goosecoid, Wnt8 and Xnr-1) [22, 23]. This finding is revealing how p53 is not a general enhancer of TGF-beta responses, but rather acts selectively on a subset of TGF-beta target promoters. Interestingly, biochemical experiments unveiled that the mechanism of this selectivity is dual: p53 associates with Smad2 and Smad3 in vivo in a TGF-beta-dependent manner but, at the same time, p53 must contact its own cognate DNA site on a promoter to facilitate a robust TGF-beta induced transcription [22]. This model is supported by two observations. Firstly, the Mix.2 and PAI-1 promoters, two paradigms of TGF-beta-induced transcription, contain a p53-binding element in proximity of the TGF-beta enhancer. Mutation of these p53-elements - or depletion of endogenous p53 by siRNA - quantitatively inhibits the inducibility of these reporters by TGF-beta. Conversely, overexpression of p53 leads to a synergistic increase in TGF-beta-induced transcription of the same reporters. Secondly, point-mutations disrupting p53 DNA binding activity, do block its biological activity in Xenopus assays [22]. In other words, p53 is unable to enter into a Smad-activated complex unless it can bind to neighboring cognate DNA sequences. Perhaps, the affinity of the interaction between p53 and Smad is low and can occur efficiently at physiological concentrations only when the two proteins are brought close on promoter DNA. Alternatively, only "activated"/DNA-bound p53 may be competent to interact with the Smads. Whatever the mechanisms, it is the concomitant presence of a Smad-regulated enhancer and of a p53 binding site that ultimately defines a gene under joined-control of p53 and TGF-beta.

4. A new twist for the p53 C-terminus?
p53 binds the N-terminal MH1 domain of Smad2, that includes the Smads' DNA-binding site and also contains the binding domain for other transcription factors, including JunD, LEF-1 and FoxO [15, 22, 27, 28]. This raises the intriguing possibility that these other proteins may modulate the interaction between p53 and Smads. In the p53/Smad2 complex, the C-terminal MH2 domain of Smad2 is free to interact with FAST-1 and Smad4. This suggests that Smad2 may bridge the DNA-bound p53 and FAST-1 leading to the assembly of a more stable and specific DNA multifactorial complex [29] (Fig. 1).

Fig. 1. A model depicting the convergence of p53 and Smads in TGF-beta-mediated gene transcription. Upon TGF-beta stimulation, Smad2 moves to the nucleus where it associates with Smad4 and specific DNA binding co-factors, such as FAST1, and the resulting complex binds to a specific promoter sequence. p53 and the activated Smad complex bind to each other and to their cognate sites on DNA, leading to a synergistic transcriptional activation. Other components of the TGF-beta signaling cascade, transcriptional cofactors and the basal transcriptional machinery were omitted for simplicity.

5. A developmental role for p53
p53 is not only instrumental for fostering TGF-beta gene responses but it is also required for correct TGF-beta responsiveness. Indeed, Xenopus cells depleted of p53 are less responsive to Activin, and p53 knock-down in frog embryos inhibits mesoderm formation and patterning [22]. These studies suggest that the endogenous TGF-beta signals are by themselves insufficient but require the concomitant presence of p53 for turning-on effectively several of their targets. Finding a developmental role for p53 is obviously surprising, given that p53 knock-out mice develop normally [36]. One possible explanation is offered by the finding that different p53 family members are expressed - and remarkably abundant - in early mouse embryos and may compensate for loss of p53 [22]. This is not the case in lower vertebrates where p53 is the only member of its family expressed during early embryogenesis [37-40].
Of note, p63 and p73 are critical for embryonic development [41]. Recent work demonstrated that p53-related proteins may converge to regulate common biological processes and, in so doing, one may receive assistance from its siblings [42, 43]. One shared biological properties may be the ability to bind and to cooperate with Smads. Indeed, p73 can also bind the Smads and different isoforms of p63/p73 can synergize with Smads in promoting a very robust increase in TGF-beta induced transcription of cloned reporter constructs [22].
In addition to redundancy, it has been proposed that the different developmental phenotypes caused by lack of p53 in frog and mammals may also reflect distinct developmental strategies among vertebrate embryos [29]. Indeed, amphibian embryos develop rapidly, whereas viviparity imposes that gastrulation is a late event in mammals, taking place days after fertilization. Thus, mammals might have evolved systems that bypass of the requirement of p53 function, although this would contrast not only with the high level of expression of all p53 family members at early stages, but also with the phylogenetic conservation of the p53 binding elements in frog, human and mouse Mix.2-related genes at the 3' of their Smad enhancer (Cordenonsi and Piccolo, unpublished).
A particularly intriguing issue is whether a cooperation between p53 family members and TGF-beta signals may underlie some of the developmental roles of p63 and p73. p63 is essential for the maintenance of stem cells in several epithelial tissues, such as skin, prostate and breast and for the stratification of the skin [44, 45] whereas p73 is essential for neurogenesis, pheromone signaling and inflammation [46]. Clearly, genetic interaction studies and biochemical experiments will be required to understand precisely the developmental links between p63 and p73 with TGF-beta/Smad signaling.

6. Interplay of p53 and TGF-beta in the control of cell growth
Irrespective of its conservation, one attractive hypothesis is that the link between p53 and TGF-beta in frog development may have served as a prelude for its tumor suppressing roles in longer-living vertebrates [29]. Indeed, it has been shown that p53 can be crucial for TGF-beta-induced growth arrest in mammalian cells. Lack of p53, either transiently (upon siRNA transfection) or genetic (using p53-/- MEFs) involves a defective cytostatic response to TGF-beta and the concomitant incapacity to turn-on efficiently the expression of the CDK inhibitor p21WAF1 in some cellular contexts [22]. Interestingly, this may have consequences for some slowly-proliferating adult stem cells, as shown for mouse hematopoietic progenitors, whose expansion in vitro is antagonized by TGF-beta in a p53-dependent manner [22].
The finding of an involvement of p53 in TGF-beta induced growth arrest is intriguing but it is apparently at odd with the observation that HaCaT keratinocytes, carrying inactivated p53 alleles, do respond to TGF-beta-promoted cytostasis [47]; yet, siRNA against p63 in HaCaT keratinocytes leads to an attenuated induction of p21 by TGF-beta [22]. This finding suggests that p53 may be the prominent regulator of TGF-beta induced growth inhibition only in some cells, such as those expressing mainly p53, or comparable amounts of the three family members. In contrast, it has been calculated that p53 is a fraction of p63/p73 in several epithelial cells, suggesting that p63/p73 isoforms may easily take-over in case of p53 loss [48]. In this context, it would be important to systematically correlate TGF-beta responsiveness with the expression levels and genetic status of p53 family members in primary cancers and cell lines. Finally, it must be recalled that the cell's genetic make-up has a pivotal role in regulating TGF-beta-gene responses [6] and therefore mechanisms other than redundancy might be taken into account to compensate for p53 deficiencies.

7. How many jointly-controlled genes?
The link between p53 and TGF-beta in development and somatic cell-growth is intriguing but opens many new issues that need to be addressed. A crucial one is what are the genes - and the corresponding biological processes - that are under joint control of p53 and TGF-beta and, oppositely, what are those in which they act independently. As shown in Fig. 1, the cooperating effects of p53 toward TGF-beta responses are delivered through a p53-binding element in the vicinity of a Smad enhancer on a target promoter. Unfortunately, a TGF-beta responsive element has been characterized only in a very restricted number of promoters. p53 contributes to TGF-beta-mediated activation of the PAI-1 and Mix.2 genes, that bear a functional p53-binding element. In contrast, we could not identify a p53-binding element in the known regulatory regions of p53-independent TGF-beta targets, such as goosecoid and TIEG [22].
Despite the lack of systematic characterization of Smad enhancers, gene expression profiles of immediate TGF-beta target genes have been carried out for various cells by several laboratories [11-13]. Interestingly, in a bioinformatic cross-comparison between the regulatory sequences of these 800 TGF-beta targets and a genomic database of p53-targets [49], we identified about 200 genes putatively co-regulated. Strikingly, clustering this p53-positive genes suggests that only two cellular responses might be predicted to be under a global control of p53 family members and TGF-beta, namely, growth inhibition and, surprisingly, extracellular matrix remodelling and attachment (Cordenonsi and Piccolo, unpublished). It is temptive to speculate that these genes may be part of the pro-invasive phenotypes triggered by TGF-beta in advanced carcinomas, a context in which TGF-beta fosters migration, epithelial-mesenchymal transdifferentiation and induction of tissue specific metastasis [2, 19, 50, 51]. Interestingly, p53 family members have been shown to be instrumental for cell movements and migration through a 3D collagen gel [52]. Although p53 itself is unlikely to be involved in these processes, several reports have indicated Delta-Np63 as a marker of malignant esophageal, breast and lung carcinomas [53-56]. DeltaNp63 is an isoform of p63 that lacks the canonical N-terminal transcriptional activation domain, but, nevertheless, may unveil transcriptional activation ability in some assays [57, 58]. DeltaNp63 can be oncogenic, as it antagonizes p53 activity, but it is still able to cooperate with the Smads [22, 59]. Thus, it will be interesting to test whether overexpression of DeltaNp63 may constitute a step toward the gain of a pro-invasive TGF-beta response in some tumors. In contrast to cytostasis and control of extracellular environment, processes such as TGF-beta-controlled apoptosis, metabolism and cytoskeletal remodeling are unlikely to be under global joined-control of p53 family members and TGF-beta (Cordenonsi and Piccolo, unpublished).

8. p53, TGF-beta and cancer
The p53/TGF-beta connection suggests that, during tumorigenesis, genetic or epigenetic modifications in p53 may contribute to the gain of resistance to TGF-beta induced growth inhibition in cancer cells. The role of p53 and TGF-beta has been studied only independently in the mouse skin model of chemical carcinogenesis, the best understood system for epithelial tumor progression [60]. In this system, rather than influencing the incidence of benign tumors, p53 acts to prevent transition from papilloma to carcinoma, such that in p53-null mice the frequency of malignant conversion is dramatically augmented [61]. A similar phenotype is observed in mice overexpressing a dominant negative form of type II TGF-beta receptor in the basal/follicular cells of the skin [62].
These results raise the tantalizing possibility that the selective loss of the cytostatic response to TGF-beta may coincide with loss of p53. Indeed, p53 mutations in chemically induced mouse skin appear to occur at the benign-malignant transition [63, 64].
An aspect related to cancer that merits discussion is the possible role of mutant-p53. p53 mutations possess transforming activity and are found with very high frequency in common human malignancies [65]. Given the role of wild-type p53 in TGF-beta gene responses, one might predict that blockade of endogenous p53 functions with a mutant p53 allele would interfere with TGF-beta sensitivity. Indeed, this hypothesis is supported by experiments performed in the early '90s, where introduction of a p53^G132F allele in nonneoplastic BALB/MK keratinocytes triggered an attenuated response to TGF-beta [66]. However, the biological functions and the operating principles of mutant p53 remain poorly understood. Indeed, during cancer progression, there is a strong selective pressure for maintaining a mutant-p53 at high levels and losing the second p53 wild-type allele [67]. This suggests that mutant-p53 is endowed with dominant gain-of function properties going above and beyond its antagonist activity over the wild-type protein. For instance, it will be very important to establish whether mutant p53 does inhibit Smad activity directly or, oppositely, by acting downstream/in parallel to TGF-beta induced cytostasis.
As originally shown by Vize and co-workers, the developing frog embryo may provide useful insights into the study of mutant p53, as localized expression of p53^R175H mRNA leads to an impaired terminal differentiation and the growth of "tumor-like" masses that fail to be incorporated in normal tissues [68]. Once again, this result parallels with the dramatic effects of p53-deficiency on cancer cell differentiation [61, 69]. Why the role of p53 in differentiation becomes apparent only in the context of developing tumors - as well as the involvement of TGF-beta in these processes - remains a mystery.

9. Regulation of p53 activity and TGF-beta gene responses: hints and challenges toward and from embryonic development
Distinct intracellular and extracellular stimuli, such as several types of stress, mitogens, and activation of oncogenes, converge on p53 to promote its transcriptional activity [65]. p53 is therefore an important element of the cellular context, able to rapidly integrate distinct inputs. Precedents for this type of combinatorial control have been described in Drosophila embryos. During specification of mesoderm precursors of the fly, transcription of even-skipped requires local activators to support the effect of dpp/TGF-beta/MAD signaling, being the latter insufficient by itself [70]. Therefore, in light of our results, integration of context-dependent activators and TGF-beta signals may be a general and conserved strategy to restrict the expression of key downstream effector of TGF-beta only to specific-competent cells.
There is no direct study on the spatial/temporal distribution and activity of p53 during embryogenesis. One possibility is that p53 may be equally active in all the cells of the early embryo. In this case, p53 may contribute to the TGF-beta response by broadening the effectiveness of the endogenous Nodal signals. p53 might be envisioned also as a competence factor, because its absence would render the Smad complex insufficient to reach the minimal threshold for promoter activation in vivo.
Alternatively, p53 protein/mRNA stability may be patterned during embryogenesis; this is attractive because it may provide a mechanism to lock the activation of certain genes only to restricted zones of competence. Consistent with this notion is the intriguing finding that p53-dependent transcription from a cloned p53-reporter is higher in the more vegetal cells, i.e. those devoted to form endoderm or mesoderm in response to TGF-beta, than in animal cells, the prospective ectoderm [23].
A prominent modulator of p53 is the p53-E3-ubiquitin ligase mdm2, which promotes p53 degradation and cytoplasmic retention [71]. In the frog, mdm2 mRNA is expressed early during embryogenesis and its RNA decreases at the time of mesoderm specification, suggesting that it may play a role in regulating the timing of p53 activity in the embryo [72].
An additional way to regulate p53 may occur via E2F proteins, in keeping with the notion that loss of Rb or overexpression of E2F1 result in increased levels of p53 [73-75] (Fig. 2). It is unclear at present whether this pathway exists in early embryo to regulate p53 activity, as p19/14ARF was found only in mammals. However, a recent report has challenged this notion, suggesting that ARF may have been present in the common ancestors of mammals and birds [76]. Interestingly, E2F has been isolated in an expression cloning screen for genes able to activate posterior development, an approach very similar to that one used to isolate p53, and p53 and E2F do share the ability to activate posterior homeobox genes [23, 77].

Fig. 2. Simplified model illustrating the regulatory interactions between molecules involved in the G1/S-phase transition.

Aknowledgements
Work in SP lab is supported by AIRC, Telethon, ISS and MIUR. SD is recipient of a post-doctoral fellowship from University of Padua.

References
[1] Y. Shi, J. Massague, Mechanisms of TGF-beta signaling from cell membrane to the nucleus, Cell 113 (2003) 685-700.

[2] A.B. Roberts, L.M. Wakefield, The two faces of transforming growth factor beta in carcinogenesis, Proc Natl Acad Sci U S A 100 (2003) 8621-3.

[3] R. Derynck, Y.E. Zhang, Smad-dependent and Smad-independent pathways in TGF-beta family signalling, Nature 425 (2003) 577-84.

[4] P. Dijke Pt, C.S. Hill, New insights into TGF-beta-Smad signalling, Trends Biochem Sci 29 (2004) 265-73.

[5] P.M. Siegel, J. Massague, Cytostatic and apoptotic actions of TGF-beta in homeostasis and cancer, Nat Rev Cancer 3 (2003) 807-20.

[6] J. Massague, How cells read TGF-beta signals, Nat Rev Mol Cell Biol 1 (2000) 169-78.

[7] X. Chen, M.J. Rubock, M. Whitman, A transcriptional partner for MAD proteins in TGF-beta signalling, Nature 383 (1996) 691-6.

[8] X. Chen, E. Weisberg, V. Fridmacher, M. Watanabe, G. Naco, M. Whitman, Smad4 and FAST-1 in the assembly of activin-responsive factor, Nature 389 (1997) 85-9.

[9] C.Y. Yeo, X. Chen, M. Whitman, The role of FAST-1 and Smads in transcriptional regulation by activin during early Xenopus embryogenesis, J Biol Chem 274 (1999) 26584-90.

[10] M. Howell, G.J. Inman, C.S. Hill, A novel Xenopus Smad-interacting forkhead transcription factor (XFast-3) cooperates with XFast-1 in regulating gastrulation movements, Development 129 (2002) 2823-34.

[11] C.R. Chen, Y. Kang, J. Massague, Defective repression of c-myc in breast cancer cells: A loss at the core of the transforming growth factor beta growth arrest program, Proc Natl Acad Sci U S A 98 (2001) 992-9.

[12] F. Verrecchia, M.L. Chu, A. Mauviel, Identification of novel TGF-beta /Smad gene targets in dermal fibroblasts using a combined cDNA microarray/promoter transactivation approach, J Biol Chem 276 (2001) 17058-62.

[13] J. Zavadil, M. Bitzer, D. Liang, Y.C. Yang, A. Massimi, S. Kneitz, E. Piek, E.P. Bottinger, Genetic programs of epithelial cell plasticity directed by transforming growth factor-beta, Proc Natl Acad Sci U S A 98 (2001) 6686-91.

[14] J. Seoane, C. Pouponnot, P. Staller, M. Schader, M. Eilers, J. Massague, TGFbeta influences Myc, Miz-1 and Smad to control the CDK inhibitor p15INK4b, Nat Cell Biol 3 (2001) 400-8.

[15] J. Seoane, H.V. Le, L. Shen, S.A. Anderson, J. Massague, Integration of Smad and forkhead pathways in the control of neuroepithelial and glioblastoma cell proliferation, Cell 117 (2004) 211-23.

[16] Y. Kang, C.R. Chen, J. Massague, A self-enabling TGFbeta response coupled to stress signaling: Smad engages stress response factor ATF3 for Id1 repression in epithelial cells, Mol Cell 11 (2003) 915-26.

[17] C.R. Chen, Y. Kang, P.M. Siegel, J. Massague, E2F4/5 and p107 as Smad cofactors linking the TGFbeta receptor to c-myc repression, Cell 110 (2002) 19-32.

[18] M. Kowanetz, U. Valcourt, R. Bergstrom, C.H. Heldin, A. Moustakas, Id2 and Id3 define the potency of cell proliferation and differentiation responses to transforming growth factor beta and bone morphogenetic protein, Mol Cell Biol 24 (2004) 4241-54.

[19] R. Derynck, R.J. Akhurst, A. Balmain, TGF-beta signaling in tumor suppression and cancer progression, Nat Genet 29 (2001) 117-29.

[20] Y.A. Yang, O. Dukhanina, B. Tang, M. Mamura, J.J. Letterio, J. MacGregor, S.C. Patel, S. Khozin, Z.Y. Liu, J. Green, M.R. Anver, G. Merlino, L.M. Wakefield, Lifetime exposure to a soluble TGF-beta antagonist protects mice against metastasis without adverse side effects, J Clin Invest 109 (2002) 1607-15.

[21] R.S. Muraoka, N. Dumont, C.A. Ritter, T.C. Dugger, D.M. Brantley, J. Chen, E. Easterly, L.R. Roebuck, S. Ryan, P.J. Gotwals, V. Koteliansky, C.L. Arteaga, Blockade of TGF-beta inhibits mammary tumor cell viability, migration, and metastases, J Clin Invest 109 (2002) 1551-9.

[22] M. Cordenonsi, S. Dupont, S. Maretto, A. Insinga, C. Imbriano, S. Piccolo, Links between tumor suppressors: p53 is required for TGF-beta gene responses by cooperating with Smads, Cell 113 (2003) 301-14.

[23] K. Takebayashi-Suzuki, J. Funami, D. Tokumori, A. Saito, T. Watabe, K. Miyazono, A. Kanda, A. Suzuki, Interplay between the tumor suppressor p53 and TGF beta signaling shapes embryonic body axes in Xenopus, Development 130 (2003) 3929-39.

[24] R. Harland, J. Gerhart, Formation and function of Spemann's organizer, Annu Rev Cell Dev Biol 13 (1997) 611-67.

[25] R. Montes de Oca Luna, D.S. Wagner, G. Lozano, Rescue of early embryonic lethality in mdm2-deficient mice by deletion of p53, Nature 378 (1995) 203-6.

[26] S.N. Jones, A.E. Roe, L.A. Donehower, A. Bradley, Rescue of embryonic lethality in Mdm2-deficient mice by absence of p53, Nature 378 (1995) 206-8.

[27] N.T. Liberati, M.B. Datto, J.P. Frederick, X. Shen, C. Wong, E.M. Rougier-Chapman, X.F. Wang, Smads bind directly to the Jun family of AP-1 transcription factors, Proc Natl Acad Sci U S A 96 (1999) 4844-9.

[28] E. Labbe, A. Letamendia, L. Attisano, Association of Smads with lymphoid enhancer binding factor 1/T cell- specific factor mediates cooperative signaling by the transforming growth factor-beta and wnt pathways, Proc Natl Acad Sci U S A 97 (2000) 8358-63.

[29] M. Whitman, F. McKeon, p53 and TGF-beta in development: prelude to tumor suppression?, Cell 113 (2003) 275-6.

[30] J. Ahn, C. Prives, The C-terminus of p53: the more you learn the less you know, Nat Struct Biol 8 (2001) 730-2.

[31] T.R. Hupp, A. Sparks, D.P. Lane, Small peptides activate the latent sequence-specific DNA binding function of p53, Cell 83 (1995) 237-45.

[32] J.M. Espinosa, B.M. Emerson, Transcriptional regulation by p53 through intrinsic DNA/chromatin binding and site-directed cofactor recruitment, Mol Cell 8 (2001) 57-69.

[33] Y. Wu, H. Huang, Z. Miner, M. Kulesz-Martin, Activities and response to DNA damage of latent and active sequence-specific DNA binding forms of mouse p53, Proc Natl Acad Sci U S A 94 (1997) 8982-7.

[34] X. Chen, L.J. Ko, L. Jayaraman, C. Prives, p53 levels, functional domains, and DNA damage determine the extent of the apoptotic response of tumor cells, Genes Dev 10 (1996) 2438-51.

[35] N. Almog, N. Goldfinger, V. Rotter, p53-dependent apoptosis is regulated by a C-terminally alternatively spliced form of murine p53, Oncogene 19 (2000) 3395-403.

[36] L.A. Donehower, M. Harvey, B.L. Slagle, M.J. McArthur, C.A. Montgomery, Jr., J.S. Butel, A. Bradley, Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours, Nature 356 (1992) 215-21.

[37] P. Lu, M. Barad, P.D. Vize, Xenopus p63 expression in early ectoderm and neurectoderm, Mech Dev 102 (2001) 275-8.

[38] F. Rentzsch, C. Kramer, M. Hammerschmidt, Specific and conserved roles of TAp73 during zebrafish development, Gene 323 (2003) 19-30.

[39] H. Lee, D. Kimelman, A dominant-negative form of p63 is required for epidermal proliferation in zebrafish, Dev Cell 2 (2002) 607-16.

[40] J. Bakkers, M. Hild, C. Kramer, M. Furutani-Seiki, M. Hammerschmidt, Zebrafish DeltaNp63 is a direct target of Bmp signaling and encodes a transcriptional repressor blocking neural specification in the ventral ectoderm, Dev Cell 2 (2002) 617-27.

[41] A. Yang, M. Kaghad, D. Caput, F. McKeon, On the shoulders of giants: p63, p73 and the rise of p53, Trends Genet 18 (2002) 90-5.

[42] E.R. Flores, K.Y. Tsai, D. Crowley, S. Sengupta, A. Yang, F. McKeon, T. Jacks, p63 and p73 are required for p53-dependent apoptosis in response to DNA damage, Nature 416 (2002) 560-4.

[43] M. Urist, C. Prives, p53 leans on its siblings, Cancer Cell 1 (2002) 311-3.

[44] A.A. Mills, B. Zheng, X.J. Wang, H. Vogel, D.R. Roop, A. Bradley, p63 is a p53 homologue required for limb and epidermal morphogenesis, Nature 398 (1999) 708-13.

[45] A. Yang, R. Schweitzer, D. Sun, M. Kaghad, N. Walker, R.T. Bronson, C. Tabin, A. Sharpe, D. Caput, C. Crum, F. McKeon, p63 is essential for regenerative proliferation in limb, craniofacial and epithelial development, Nature 398 (1999) 714-8.

[46] A. Yang, N. Walker, R. Bronson, M. Kaghad, M. Oosterwegel, J. Bonnin, C. Vagner, H. Bonnet, P. Dikkes, A. Sharpe, F. McKeon, D. Caput, p73-deficient mice have neurological, pheromonal and inflammatory defects but lack spontaneous tumours, Nature 404 (2000) 99-103.

[47] M.B. Datto, Y. Li, J.F. Panus, D.J. Howe, Y. Xiong, X.F. Wang, Transforming growth factor beta induces the cyclin-dependent kinase inhibitor p21 through a p53-independent mechanism, Proc Natl Acad Sci U S A 92 (1995) 5545-9.

[48] M. Levrero, V. De Laurenzi, A. Costanzo, J. Gong, J.Y. Wang, G. Melino, The p53/p63/p73 family of transcription factors: overlapping and distinct functions, J Cell Sci 113 (2000) 1661-70.

[49] J. Hoh, S. Jin, T. Parrado, J. Edington, A.J. Levine, J. Ott, The p53MH algorithm and its application in detecting p53-responsive genes, Proc Natl Acad Sci U S A 99 (2002) 8467-72.

[50] S. Grunert, M. Jechlinger, H. Beug, Diverse cellular and molecular mechanisms contribute to epithelial plasticity and metastasis, Nat Rev Mol Cell Biol 4 (2003) 657-65.

[51] Y. Kang, P.M. Siegel, W. Shu, M. Drobnjak, S.M. Kakonen, C. Cordon-Cardo, T.A. Guise, J. Massague, A multigenic program mediating breast cancer metastasis to bone, Cancer Cell 3 (2003) 537-49.

[52] A.A. Sablina, P.M. Chumakov, B.P. Kopnin, Tumor suppressor p53 and its homologue p73alpha affect cell migration, J Biol Chem 278 (2003) 27362-71.

[53] X. Wang, I. Mori, W. Tang, M. Nakamura, Y. Nakamura, M. Sato, T. Sakurai, K. Kakudo, p63 expression in normal, hyperplastic and malignant breast tissues, Breast Cancer 9 (2002) 216-9.

[54] C.J. Di Como, M.J. Urist, I. Babayan, M. Drobnjak, C.V. Hedvat, J. Teruya-Feldstein, K. Pohar, A. Hoos, C. Cordon-Cardo, p63 expression profiles in human normal and tumor tissues, Clin Cancer Res 8 (2002) 494-501.

[55] H. Hu, S.H. Xia, A.D. Li, X. Xu, Y. Cai, Y.L. Han, F. Wei, B.S. Chen, X.P. Huang, Y.S. Han, J.W. Zhang, X. Zhang, M. Wu, M.R. Wang, Elevated expression of p63 protein in human esophageal squamous cell carcinomas, Int J Cancer 102 (2002) 580-3.

[56] P.P. Massion, P.M. Taflan, S.M. Jamshedur Rahman, P. Yildiz, Y. Shyr, M.E. Edgerton, M.D. Westfall, J.R. Roberts, J.A. Pietenpol, D.P. Carbone, A.L. Gonzalez, Significance of p63 amplification and overexpression in lung cancer development and prognosis, Cancer Res 63 (2003) 7113-21.

[57] M. Dohn, S. Zhang, X. Chen, p63alpha and DeltaNp63alpha can induce cell cycle arrest and apoptosis and differentially regulate p53 target genes, Oncogene 20 (2001) 3193-205.

[58] P. Ghioni, F. Bolognese, P.H. Duijf, H. Van Bokhoven, R. Mantovani, L. Guerrini, Complex transcriptional effects of p63 isoforms: identification of novel activation and repression domains, Mol Cell Biol 22 (2002) 8659-68.

[59] K. Hibi, B. Trink, M. Patturajan, W.H. Westra, O.L. Caballero, D.E. Hill, E.A. Ratovitski, J. Jen, D. Sidransky, AIS is an oncogene amplified in squamous cell carcinoma, Proc Natl Acad Sci U S A 97 (2000) 5462-7.

[60] R.J. Akhurst, A. Balmain, Genetic events and the role of TGF beta in epithelial tumour progression, J Pathol 187 (1999) 82-90.

[61] C.J. Kemp, L.A. Donehower, A. Bradley, A. Balmain, Reduction of p53 gene dosage does not increase initiation or promotion but enhances malignant progression of chemically induced skin tumors, Cell 74 (1993) 813-22.

[62] C. Amendt, P. Schirmacher, H. Weber, M. Blessing, Expression of a dominant negative type II TGF-beta receptor in mouse skin results in an increase in carcinoma incidence and an acceleration of carcinoma development, Oncogene 17 (1998) 25-34.

[63] P.A. Burns, C.J. Kemp, J.V. Gannon, D.P. Lane, R. Bremner, A. Balmain, Loss of heterozygosity and mutational alterations of the p53 gene in skin tumours of interspecific hybrid mice, Oncogene 6 (1991) 2363-9.

[64] B. Ruggeri, J. Caamano, T. Goodrow, M. DiRado, A. Bianchi, D. Trono, C.J. Conti, A.J. Klein-Szanto, Alterations of the p53 tumor suppressor gene during mouse skin tumor progression, Cancer Res 51 (1991) 6615-21.

[65] B. Vogelstein, D. Lane, A.J. Levine, Surfing the p53 network, Nature 408 (2000) 307-10.

[66] M. Reiss, V.F. Vellucci, Z.L. Zhou, Mutant p53 tumor suppressor gene causes resistance to transforming growth factor beta 1 in murine keratinocytes, Cancer Res 53 (1993) 899-904.

[67] A.N. Bullock, A.R. Fersht, Rescuing the function of mutant p53, Nat Rev Cancer 1 (2001) 68-76.

[68] J.B. Wallingford, D.W. Seufert, V.C. Virta, P.D. Vize, p53 activity is essential for normal development in Xenopus, Curr Biol 7 (1997) 747-57.

[69] C.J. Sherr, Principles of tumor suppression, Cell 116 (2004) 235-46.

[70] M.S. Halfon, A. Carmena, S. Gisselbrecht, C.M. Sackerson, F. Jimenez, M.K. Baylies, A.M. Michelson, Ras pathway specificity is determined by the integration of multiple signal-activated and tissue-restricted transcription factors, Cell 103 (2000) 63-74.

[71] Y. Yang, C.C. Li, A.M. Weissman, Regulating the p53 system through ubiquitination, Oncogene 23 (2004) 2096-106.

[72] V. Marechal, B. Elenbaas, L. Taneyhill, J. Piette, M. Mechali, J.C. Nicolas, A.J. Levine, J. Moreau, Conservation of structural domains and biochemical activities of the MDM2 protein from Xenopus laevis, Oncogene 14 (1997) 1427-33.

[73] K.D. Robertson, P.A. Jones, The human ARF cell cycle regulatory gene promoter is a CpG island which can be silenced by DNA methylation and down-regulated by wild-type p53, Mol Cell Biol 18 (1998) 6457-73.

[74] M. Choi, H. Lee, H.M. Rho, E2F1 activates the human p53 promoter and overcomes the repressive effect of hepatitis B viral X protein (Hbx) on the p53 promoter, IUBMB Life 53 (2002) 309-17.

[75] K.F. Macleod, Y. Hu, T. Jacks, Loss of Rb activates both p53-dependent and independent cell death pathways in the developing mouse nervous system, Embo J 15 (1996) 6178-88.

[76] S.H. Kim, M. Mitchell, H. Fujii, S. Llanos, G. Peters, Absence of p16INK4a and truncation of ARF tumor suppressors in chickens, Proc Natl Acad Sci U S A 100 (2003) 211-6.

[77] A. Suzuki, A. Hemmati-Brivanlou, Xenopus embryonic E2F is required for the formation of ventral and posterior cell fates during early embryogenesis, Mol Cell 5 (2000) 217-29.

[78] E. Appella, C.W. Anderson, Post-translational modifications and activation of p53 by genotoxic stresses, Eur J Biochem 268 (2001) 2764-72.

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