Carcinogenesis vol.26 no.8 pp.1317–1322, 2005
doi:10.1093/carcin/bgi122
Advance Access publication May 11, 2005
REVIEW
Complicating the complexity of p53
Karen S.Yee and Karen H.Vousden
The Beatson Institute for Cancer Research, Garscube Estate, Switchback
Road, Bearsden, Glasgow G61 1BD, UK
To whom correspondence should be addressed.
Tel: þ44 (0)141 330 2424; Fax: þ44 (0)141 943 0372;
Email: [email protected]
Recent studies have suggested that the straightforward role
of p53 as a transcription factor that functions by inducing
apoptotic target genes to eliminate developing tumor cells
is only part of a much more complicated story. There is
now a firm body of evidence supporting a transcriptionally
independent activity of p53 as a functional, if not structural, homologue of the BH3-only proteins. Although this
information adds another nuance to the mechanism by
which p53 can induce apoptosis, further studies indicate
that the apoptotic function of p53 represents only a part of
its tumor suppressive activity. Although complicating our
understanding of p53, these new insights may also provide
some exciting new targets for the design of therapeutics
that can reactivate p53 in cancers.
The p53 tumor suppressor gene encodes one of the most
intensively studied proteins. This interest in p53 is engendered
not by an essential role for p53 during normal growth and
development, but by the contribution of p53 to tumor
suppression. Loss or alterations in the function of p53 have
been found in most human cancers, including the major epithelial malignancies that may soon be responsible for most
deaths in the Western world (1,2). Understanding and treating
these cancers has become a matter of some urgency, and the
p53 pathway is an attractive candidate for the development of
targeted cancer therapies. But can such therapies be successfully developed? Compared with the kinases, the most popular
and successful cancer drug targets to date, p53 remains a more
elusive quarry. Many functions have been ascribed to p53
that relate to activities as diverse as transcriptional activation,
mitochondrial membrane permeabilization, exonuclease activity, DNA repair and regulation of angiogenesis (3–6). The
ability of p53 to signal a variety of growth inhibitory responses, including the induction of cell-cycle arrest, senescence,
differentiation and apoptosis (7,8), establish a central role
for p53 as a master regulator of tumor suppression (9).
However, despite the efforts of many researchers over
many years, the relative contributions of these activities to
tumor suppression remain hotly disputed. Over the past
few years even the most unassailable ‘truths’ about p53 have
been called into question, leaving us with the uncomfortable
feeling that the more we learn, the less we know. Two
examples of reasonably well-established facts about p53 that
have recently become more complicated are the importance of
p53 as a transcription factor and the role of apoptosis in tumor
suppression, each of which we will discuss briefly in this
review.
Carcinogenesis vol.26 no.8 # Oxford University Press 2005; all rights reserved.
Of all the activities of p53, none is more firmly accepted
than the function of p53 as a transcription factor (10–12).
Assembling into a tetramer, p53 shows sequence specific
DNA binding activity through its central domain (Figure 1),
and activates the expression of genes that contain p53 binding
sites in their promoters by virtue of interactions of the Nterminal domain of p53 with the transcriptional machinery.
Almost all tumor derived p53 mutants contain a point mutation
within the DNA binding domain that prevents, or in some
cases alters, the recognition of p53 binding sites. In general,
p53 mutants expressed in tumors lose the ability to activate
expression of the target genes that are responsive to wild-type
p53—although they may acquire unique transcriptional activities not shared by the wild-type protein—and this by itself
suggests that the transcriptional function of p53 is likely to be
important for tumor suppression. Identification of target genes
that are activated by p53 has further supported this concept; of
the hundreds of p53-inducible target genes that have been
identified so far there is compelling evidence that several of
these play a role in mediating the various downstream
responses to p53. The p21Waf1/Cip1 cyclin-dependent kinase
inhibitor was amongst the first of the p53 targets to be identified, and deletion of p21Waf1/Cip1 in a number of cell systems
strongly abrogates the ability of p53 to induce a G1 cell-cycle
arrest (13,14). More recently, the products of other transcriptional targets of p53 have been shown to play a role in mediating various p53 responses, including a large group of genes
encoding proteins with apoptotic activity (3). Analysis of a
number of these genes indicated that knocking out any one
individually failed to protect the cells completely from
p53-induced death, suggesting that the response reflects the
combined effect of a number of targets. However, more
recently, the BH3 domain protein PUMA has emerged as a
promising candidate for a key p53-inducible apoptotic target
(15,16). PUMA was shown to be critical for p53-mediated cell
death in a number of cells and tissues, with knock-out or
knock-down of PUMA resulting in a severely impeded to
apoptosis (17,18) and in some models, enhancing tumor development (19). However, given the all-or-nothing threshold
nature of the apoptotic response, it seems unwise to conclude
that PUMA is the be-all and end-all to p53-induced apoptosis.
Indeed, it is clear that in some cell types inactivation of other
p53 target genes, like NOXA, has an equally profound effect
on the apoptotic response (18,20). Taken together, it seems
most likely that the coordinated activity of many of the
p53-inducible apoptotic target genes will play a key role in
governing the life and death decision in response to stresses
that activate p53.
Despite the strong evidence supporting the role of p53 as a
transcription factor, there has been consistent debate over
whether p53 may also have transcriptionally independent
activities. Certainly, p53 also shows transcriptional repressor
functions that are strongly correlated with apoptosis (21–23).
However, a number of early studies suggested that p53 may
1317
K.S.Yee and K.H.Vousden
have an apoptotic function that is completely separate from
the regulation of gene expression (24–27). An explanation for
this activity has been provided very recently with the observation that following stress, a proportion of p53 appears to
function outside the nucleus, in the cytoplasm or at the mitochondria, and that p53 can be found in association with several
members of the Bcl2 family of proteins (28). The Bcl2 related
proteins are the key components of the intrinsic apoptotic
pathway and fall into three main groups; those that can
function to perturb mitochondrial membrane potential directly
(Bax and Bak), the BH3-only proteins that directly or
indirectly drive the activation of Bax and Bak, and the
anti-apoptotic proteins that sequester pro-apoptotic family
members and hold them inactive (such as Bcl2, BclxL and
Mcl1). The BH3-only proteins are further divided into two
groups; those that bind Bax and Bak directly to activate them
(the ‘activators’, such as Bim and Bid) and those that bind the
anti-apoptotic family members to release the activators (the
‘enablers’ like Bad and Bik) (29,30) (Figure 2). Unexpectedly,
it would seem that p53 can function in a manner analogous to
the BH3-only proteins, with evidence to support two broad, but
not mutually exclusive, models. In the first, p53 functions like
Fig. 1. Diagram showing the domain structure of the p53 protein. The p53 protein is a transcription factor that contains several well-defined domains. At the
N-terminus are the transactivation domain and a proline-rich region, which is required for apoptotic function. Within the N-terminus are the interaction sites of p53
with components of the transcriptional machinery as well as ubiquitin ligase Mdm2. The central domain harbors the sequence specific DNA binding region, where
most of the tumor associated mutations occur. This central region also contains binding sites for interaction with members of the Bcl2 protein family. The
C-terminal region contains the oligomerization domain as well as nuclear localisation and export signals. Several sites within the N-terminal region have been
shown to be phosphorylated and the C-terminal region contains numerous sites of modification which influence stability, localization and activity of p53.
Fig. 2. Models of p53 function as a BH3-only protein. On the left, two possible roles for p53 as an ‘enabler’ type BH3-only protein. In these models p53 is able to
disrupt the interaction between anti-apoptotic proteins (such as BclxL, Bcl2 and Mcl1) and pro-apoptotic proteins. This could directly relieve the inhibition of Bax
and Bak (A), or free ‘activator’ type BH3 only proteins (Bid and Bim), which can then activate Bax and Bak (B). On the right, models in which p53 functions as an
‘activator’ type BH3-only protein. Activation of Bax and Bak might involve binding and releasing them from interaction with the anti-apoptotic proteins (C).
Alternatively, p53 itself may be held inactive by interaction with the anti-apoptotic proteins. In this case it is possible that other ‘enabler’ BH3-only proteins like
PUMA might be able to displace p53 from the anti-apoptotic proteins, thus allowing it to activate Bax/Bak (D). Note that although there is evidence to support each
of the models, not all the interactions indicated in the Figure have been confirmed.
1318
Complicating the complexity of p53
an ‘enabler’ BH3 domain protein, interacting with the antiapoptotic proteins and presumably releasing pro-apoptotic
BH3 domain proteins to drive apoptosis (Figure 2A and B)
(31). The second model is based on the recent observation that
p53 can function in a manner more analogous to the ‘activator’
BH3-only proteins, by directly activating the apoptotic function of Bax or Bak (Figure 2C and D) (32,33). In either case,
the remarkable parallels in the function between p53 and the
BH3-only proteins is not reflected by any obvious amino acid
sequence similarity, although it is possible that parts of p53
adopt a similar structure to the BH3 domain. Interestingly,
however, mutations in the DNA binding domain of p53 that
are frequently found in tumors also prevent the interaction of
p53 with BclxL (31), indicating that the loss of apoptotic
activity of these mutants may derive from the concomitant
failure to induce transcription and loss of this mitochondrial
activity.
Although in experimental systems the mitochondrial activity
of p53 alone can be sufficient for cell death, it seems likely that
under physiological conditions this function will cooperate
with the ability of p53 to activate transcription of genes like
PUMA to drive a full apoptotic response. Although it is possible that PUMA and mitochondrial p53 activate independent
apoptotic signals that combine to push the cell over an apoptotic threshold, it is interesting to consider a closer relationship
between these two p53 activities. One possibility is that the
accumulation of mitochondrial p53 is an immediate response
to stress, sensitizing the cells to further apoptotic signals like
the expression of PUMA. Recent studies have shown that in
mice subjected to DNA damage treatment, a rapid p53 mitochondrial translocation (which precedes p53 target gene
activation) triggers an early wave of apoptosis, which is followed later by a second wave that is transcription dependent
(34). In this study, mitochondrial p53 was found preferentially
in radiosensitive organs or in cultured cells that respond to p53
by undergoing apoptosis, rather than a cell-cycle arrest. These
results suggest that the ability to accumulate mitochondrial
p53 may be a distinguishing feature between radiosensitive
and radioresistant organs and a determinant of whether or not a
cell will die in response to p53. However, another model has
recently been proposed which intertwines the activity of p53 at
the mitochondria and the function of PUMA even more closely
(35). As mentioned earlier, p53 can interact with the prosurvival BH3 proteins like BclxL, and it is possible that this
interaction serves to inhibit the ‘activator’ function of p53.
PUMA has a very high affinity for the pro-survival proteins
and appears to function like an ‘enabler’ BH3-only protein
(36,37). Therefore, when expressed at sufficiently high levels,
PUMA may be able to dissociate p53 from the pro-survival
proteins and thereby drive the activation of apoptosis
(Figure 2D). Clearly, this cannot be the only route through
which PUMA functions, since PUMA induces apoptosis in
p53-null cells too (15). However, it is likely that the principal
role for PUMA is to release the ‘enabler’ proteins (which
would now encompass Bid, Bim and p53) from the antiapoptotic proteins to drive the activation of Bax and Bak.
This model nicely ties together the transcriptionally independent activity of p53 with the transcriptionally dependent activation of proteins like PUMA and suggests how the final
apoptotic response may require both of these events.
The ability of p53 to function at the mitochondria has also
led to a reassessment of the consequences of how nuclear/
cytoplasmic shuttling of p53 might be regulated. Rather
intriguingly, this brings us back to considering one of the
main negative regulators of p53 function, Mdm2. The role of
Mdm2 in controlling p53 function is clear from numerous
studies in cells and in mice, and stress-induced inhibition of
Mdm2 function is key to the activation of p53 (38). Mdm2
binds to the N-terminal region of p53 that also contains the
transcriptional activation domain (Figure 1), and this interaction of Mdm2 with p53 can block the binding of other components of the transcriptional machinery and inhibit the ability
of p53 to activate transcription. Mdm2 can also directly repress
transcription, in part through its ability to function as a ubiquitin ligase (E3) and ubiquitinate histones (39). This E3
activity of Mdm2 is also critically important for the ubiquitination of p53, an activity that leads to the degradation and
maintenance of low levels of p53 in unstressed cells. The
importance of the negative regulation of p53 by Mdm2 has
been demonstrated in many systems, and it is generally accepted that inhibition of Mdm2 will result in the activation of p53.
It is, therefore, somewhat heretical to suggest that Mdm2 may
also be enabling some p53 functions, but the observation that
Mdm2 is required for p53 to export from the nucleus (40,41)
suggests that Mdm2 activity might contribute to the mitochondrial or cytoplasmic activities of p53. The mechanism through
which Mdm2 drives nuclear export of p53 is not completely
clear, although it is associated with ubiquitination of p53
(42,43) and may involve the unmasking of the nuclear export
sequences that are present in the C-terminal region of p53
(Figure 1). Ubiquitination of p53 by Mdm2, therefore, appears
to have two roles, targeting of p53 to the proteasome for
degradation and nuclear export. Differentiation between these
two responses depends on the extent of ubiquitination—
whereas mono-ubiquitination of p53 is sufficient for nuclear
export, polyubiquitination is necessary for degradation (43).
Interestingly, in support of this model the p53 found at the
mitochondria has been shown to be ubiquitin modified (44).
The idea that Mdm2 may actually contribute to an apoptotic
activity of p53 provides some rationale for the observed differences in apoptotic activity of the two most common polymorphic forms of p53, carrying either arginine or proline at
amino acid residue 72. The Arg72 variant shows an enhanced
ability to interact with Mdm2 and Crm1 and is more efficiently
exported from the nucleus and localized to the mitochondria
than the Pro72 form (44). Consistent with the importance for
this localization of p53, the Arg72 variant shows significantly
higher apoptotic activity. Interestingly, the apoptotic defect of
p53 proteins mutated in the N-terminus that had been ascribed
to loss of transcriptional function (45–47) may also reflect a
failure to bind Mdm2 and so relocalize to the mitochondria.
Taken together, it seems that p53-induced apoptosis represents the culmination of many activities, including the activation of expression of a number of target genes, repression of
gene expression and an ability to activate Bax or Bak in a
transcriptionally independent manner. Dissecting the relative
importance of each of these functions to the overall apoptotic
activity of p53 will be complex, and the identification of a
mutant that could separate these functions would certainly be
extremely helpful.
The clarity of our vision of how p53 functions is becoming
even more clouded with a growing appreciation of the importance of p53-induced responses other than apoptosis
in preventing tumor development. Certainly, the ability to
induce cell death has been strongly linked to the function
of p53 as a tumor suppressor (48,49). Compelling studies
1319
K.S.Yee and K.H.Vousden
showing that transformed cells are more sensitive to
p53-mediated apoptosis, and that under conditions of Myc or
E1A driven tumorigenesis in mice, blocking apoptosis by
overexpression of Bcl2 (50) or loss of Apaf1 (51) could substitute for loss of p53, strongly suggest that the ability to
induce cell death might be key to the success of p53 as a
tumor suppressor. In vivo model systems examining Mycinduced lymphomas also provide elegant support for a
tumor suppressive role of PUMA—in these studies loss of
PUMA was as effective at accelerating tumor development
as loss of p53 (19). However, there is now accumulating
evidence that simply preventing the apoptotic response to
p53 is not the same as losing p53 function completely. For
example, the PUMA knock-out mice do not resemble p53-null
animals with respect to tumor development, despite showing
profound defects in their apoptotic response in many tissues
(17,18). These observations therefore suggest that other activities of p53, such as the induction of cell-cycle arrest or senescence and the contribution to the maintenance of genomic
stability, also play an important role in tumor suppression. As
with apoptosis, pinning down the contribution of the cell-cycle
arrest response to p53 tumor suppression has been complicated. Although the deletion of p21WAF1/CIP1, one of the key
mediators of the proliferative block induced by p53, can
enhance susceptibility to cancer development in some models
(52), loss of p21WAF1/CIP1 is not equivalent to loss of p53.
Indeed in some systems, loss of p21WAF1/CIP1 may even
impede tumorigenesis, possibly as a reflection of tissue and
system-dependent pro-apoptotic and anti-apoptotic activities
of p21WAF1/CIP1 (53). However, notwithstanding this
complexity in the contribution of p21WAF1/CIP1, it seems
clear that the proliferative block induced by p53 in the shape
of cell-cycle arrest or senescence may be as important in
preventing cancer development as the induction of cell death.
Some of this evidence has come from a reexamination of
specific tumor derived p53 mutants that dissociate p53’s
apoptotic and cell-cycle responses. Initial tissue culture studies
indicated that while these mutants remain competent in the
induction of cell-cycle arrest, they fail to induce apoptosis
(54). Interestingly, these p53 mutants function to enhance the
transformation of cells in culture (55), adding further weight to
the suggestion that apoptosis is the key tumor suppressor
function of p53. More recently, however, a knock-in mouse
expressing one such mutant p53 was created and interestingly,
despite showing a complete loss of p53-mediated apoptosis
and a reduced p53-induced cell-cycle arrest, these mice
retained some ability to resist tumor development (56).
Although clearly more susceptible to cancer than their wildtype littermates (suggesting apoptosis does indeed play some
role), the incidence of tumor development was far less than
that seen in p53-null mice, strongly indicating that the ability
to induce a proliferative block, which was matched by a
maintenance of genomic stability, is an important weapon in
the tumor suppressive armoury of p53. These model systems
will encourage us to think again about which p53 activities we
should be trying to restore for therapy—should we concentrate
on apoptosis, cell cycle progression or even senescence (57)?
Even if the ability of p53 to induce a proliferative block can
inhibit tumor development, will it also be true that reinstating
such a p53-dependent cell-cycle arrest can help to
cure a cancer? More importantly, will attempts to modulate
the proliferative block in tumor cells have effects on the
apoptotic response? For example, the identification of
1320
p21WAF1/CIP1 as a survival factor that might ideally be inhibited as a part of tumor therapy (58) suggests that reactivating
this p53 response may have unanticipated and undesirable
consequences for tumor cell survival.
Clearly, there are many questions that remain to be
answered, but despite the obvious gaps in our understanding
of exactly how p53 works, the role of p53 in tumor suppression
is indisputable. This has encouraged the development of
numerous approaches to harnessing the power of p53 for
tumor therapy, including gene therapy to directly express
wild-type p53 (59) as well as the development of small molecules that can restore function to mutant p53 (60) or reactivate
wild-type p53 (61). The latter approach depends on the observation that many tumors that retain a wild-type p53 gene show
defects in the pathways that allow the activation of p53 in
response to stress, in most cases resulting in an inability to
turn off Mdm2. In these tumors, targeting Mdm2 would be
expected to stabilize and activate p53. One extremely promising approach has been to identify small molecule inhibitors of
the Mdm2–p53 interaction (62). Although inhibition of protein–
protein interactions is not usually a favourite target for drug
development, in this case, the success of the Nutlin compounds
may reflect the extremely tightly defined interaction of a small
domain of p53 into a deep pocket in Mdm2 (63). Other
approaches include the use of antisense oligonucleotides to
inihibit Mdm2 expression (64) and the identification of small
molecules that inhibit the E3 activity of Mdm2 (65). Interestingly, although the inhibition of Mdm2 activity should also
hinder the mitochondrial function of p53 by preventing nuclear
export, the results indicate that p53 activity can be restored
with sufficient efficiency to induce an apoptotic response.
Although promising, these approaches have by no means
exhausted the potential of p53 as a therapeutic target. As we
discover more about p53, the list of possible drug targets that
might allow for the reactivation of at least some facet of p53
tumor suppression also increases. We look forward to a continuing boom in our understanding of the basic biology of p53
advancing, hand-in-hand, with an increased ability to exploit
this knowledge for cancer therapy.
Acknowledgements
We are grateful to Kevin Ryan for his helpful comments. We would also like to
thank Cancer Research UK and the West of Scotland Women’s Bowling
Association for providing generous support to KSY.
Conflict of Interest Statement: None declared.
References
1. Hollstein,M., Rice,K., Greenblatt,M.S., Soussi,T., Fuchs,R., Sørlie,T.,
Hovig,E., Smith-Sørensen,B., Montesano,R. and Harris,C.C. (1994)
Database of p53 gene somatic mutations in human tumors and cell lines.
Nucleic Acids Res., 22, 3551–3555.
2. Olivier,M., Hussain,S.P., de Fromentel,C., Hainaut,P. and Harris,C.C.
(2004) TP53 mutation spectra and load: a tool for generating hypotheses
on the etiology of cancer. IARC Sci. Publ., 157, 247–270.
3. Vousden,K.H. and Lu,X. (2002) Live or let die: the cell’s response to p53.
Nat. Rev. Cancer, 2, 594–604.
4. Meek,D.W. (2004) The p53 response to DNA damage. DNA Repair, 3,
1049–1056.
5. Manfredi,J.J. (2003) p53 and apoptosis: it’s not just in the nucleus
anymore. Mol. Cell, 11, 552–554.
6. Sengupta,S. and Harris,C.C. (2005) p53: traffic cop at the crossroads of
DNA repair and recombination. Nat. Rev. Mol. Cell. Biol., 6, 44–55.
Complicating the complexity of p53
7. Vogelstein,B., Lane,D. and Levine,A.J. (2000) Surfing the p53 network.
Nature, 408, 307–310.
8. Hofseth,L.J., Hussain,S.P. and Harris,C.C. (2004) p53: 25 years after its
discovery. Trends Pharmacol. Sci., 25, 177–181.
9. Lowe,S.W., Cepero,E. and Evan,G. (2004) Intrinsic tumour suppression.
Nature, 432, 307–315.
10. Polyak,K., Xia,Y., Zweier,J.L., Kinzler,K.W. and Vogelstein,B. (1997) A
model for p53-induced apoptosis. Nature, 389, 300–305.
11. Yu,J., Zhang,L., Hwang,P.M., Rago,C., Kinzler,K. and Vogelstein,B.
(1999) Identification and classification of p53-regulated genes. Proc. Natl
Acad. Sci. USA, 96, 14517–14522.
12. Zhao,R., Gish,K., Murphy,M., Yin,Y., Notterman,D., Hoffman,W.H.,
Tom,E., Mack,D.H. and Levine,A.J. (2000) Analysis of p53-regulated
gene expression patterns using oligonucleotide arrays. Genes Dev., 14,
981–993.
13. Brugarolas,J., Chandrasekaran,C., Gordon,J.I., Beach,D., Jacks,T. and
Hannon,G.J. (1995) Radiation-induced cell cycle arrest compromised by
p21 deficiency. Nature, 377, 552–556.
14. Deng,C., Zhang,P., Harper,J.W., Elledge,S.J. and Leder,P. (1995) Mice
lacking p21CIP1/WAF1 undergo normal development, but are defective in
G1 checkpoint control. Cell, 82, 675–684.
15. Nakano,K. and Vousden,K.H. (2001) PUMA, a novel pro-apopototic gene,
is induced by p53. Mol. Cell, 7, 683–694.
16. Yu,J., Zhang,L., Hwang,P.M., Kinzler,K.W. and Vogelstein,B. (2001)
PUMA induces the rapid apoptosis of colorectal cancer cells. Mol. Cell, 7,
673–682.
17. Jeffers,J.R., Parganas,E., Lee,Y. et al. (2003) Puma is an essential
mediator of p53-dependent and -independent apoptotic pathways. Cancer
Cell, 4, 321–3238.
18. Villunger,A., Michalak,E.M., Coultas,L., Mullauer,F., Bock,G.,
Ausserlechner,M.J., Adams,J.M. and Strasser,A. (2003) p53- and
drug-induced apoptotic responses mediated by BH3-only proteins puma
and noxa. Science, 302, 1036–1038.
19. Hemann,M.T., Zilfou,J.T., Zhao,Z., Burgess,D.J., Hannon,G.L. and
Lowe,S.W. (2004) Suppression of tumorigenesis by the p53 target
PUMA. Proc. Natl Acad. Sci. USA, 101, 9333–9338.
20. Shibue,T., Takeda,K., Oda,E., Tanaka,H., Murasawa,H., Takaoka,A.,
Morishita,Y., Akira,S., Taniguchi,T. and Tanaka,N. (2003) Integral
role of Noxa in p53-mediated apoptotic response. Genes Dev., 17,
2233–2238.
21. Ryan,K.M. and Vousden,K.H. (1998) Characterization of structural p53
mutants which show selective defects in apoptosis, but not cell cycle
arrest. Mol. Cell. Biol., 18, 3692–3698.
22. Murphy,M., Ahn,J., Walker,K.K., Hoffman,W.H., Evans,R.M.,
Levine,A.J. and George,D.L. (1999) Transcriptional repression by wildtype p53 utilizes histone deacetylases, mediated by interaction with
mSin3a. Genes Dev., 13, 2490–2501.
23. Kho,P.S., Wang,Z., Zhuang,L., Li,Y., Chew,J.L., Ng,H.H., Liu,E.T.
and Yu,Q. (2004) p53-regulated transcriptional program associated
with genotoxic stress-induced apoptosis. J. Biol. Chem., 279,
21183–21192.
24. Caelles,C., Helmberg,A. and Karin,M. (1994) p53-dependent apoptosis in
the absence of transcriptional activation of p53-target genes. Nature, 370,
220–223.
25. Wagner,A.J., Kokontis,J.M. and Hay,N. (1994) Myc-mediated apoptosis
requires wild-type p53 in a manner independent of cell cycle arrest and the
ability of p53 to induce p21waf1/cip1. Genes Dev., 8, 2817–2830.
26. Haupt,Y., Rowan,S., Shaulian,E., Vousden,K.H. and Oren,M. (1995)
Induction of apoptosis in HeLa cells by trans-activation deficient p53.
Genes Dev., 9, 2170–2183.
27. Chen,X., Ko,L.J., Jayaraman,L. and Prives,C. (1996) p53 levels,
functional domains, and DNA damage determine the extent of the
apoptotic response of tumor cells. Genes Dev., 10, 2438–2451.
28. Cory,S. and Adams,J.M. (2002) The Bcl2 family: regulators of the cellular
life-or-death switch. Nat. Rev. Cancer, 2, 647–656.
29. Chittenden,T. (2002) BH3 domains: intracellular death-ligands critical for
initiating apoptosis. Cancer Cell, 2, 165–166.
30. Letai,K., Bassik,M.C., Walensky,L.D., Sorcinelli,M.D., Weiler,S. and
Korsmeyer,S.J. (2002) Distinctive BH3 domains either sensitize or
activate mitochondrial apoptosis, serving as prototype cancer therapeutics.
Cancer Cell, 2, 183–192.
31. Mihara,M., Erster,S., Zaika,A., Petrenko,O., Chittenden,T., Pancoska,P.
and Moll,U.M. (2003) p53 has a direct apoptogenic role at the
mitochondria. Mol. Cell, 11, 577–590.
32. Chipuk,J.E., Kuwana,T., Bouchier-Hayes,L., Droin,N.M., Newmeyer,D.D.,
Schuler,M. and Green,D.R. (2004) Direct activation of Bax by p53
mediates mitochondrial membrane permeabilization an apoptosis. Science,
303, 1010–1014.
33. Leu,J.I., Dumont,P., Hafey,M., Murphy,M.P. and George,D.L. (2004)
Mitochondrial p53 activates Bak and causes disruption of a Bak-Mcl1
complex. Nature Cell Biol., 6, 443–450.
34. Erster,S., Mihara,M., Kim,R.H., Petrenko,O. and Moll,U.M. (2004) In vivo
mitochrondrial p53 translocation triggers a rapid first wave of cell death
in response to DNA damage that can precede p53 target gene activation.
Mol. Cell Biol., 24, 6728–6741.
35. Schuler,M. and Green,D.R. (2005) Transcription, apoptosis and p53:
catch-22. Trends Genet., 21, 182–187.
36. Chen,L., Willis,S.N., Wei,A., Smith,B.J., Fletcher,J.I., Hinds,M.G.,
Colman,P.M., Day,C.L., Adams,J.M. and Huang,D.C. (2005)
Differential targeting of prosurvival Bcl-2 proteins by their BH3-only
ligands allows complementary apoptotic function. Mol. Cell, 17, 393–403.
37. Kuwana,T., Bouchier-Hayes,L., Chipuk,J.E., Bonzon,C., Sullivan,B.A.,
Green,D.R. and Newmeyer,D.D. (2005) BH3 domains of BH3-only
proteins differentially regulate Bax-mediated mitochondrial membrane
permeabilization both directly and indirectly. Mol. Cell, 17, 525–535.
38. Bond,G.L., Hu,W. and Levine,A.J. (2005) MDM2 is a central node in
the p53 pathway: 12 years and counting. Curr. Cancer Drug Targets, 5,
3–8.
39. Minsky,N. and Oren,M. (2004) The RING domain of Mdm2 mediates
histone ubiquitylation and transcriptional repression. Mol. Cell, 16,
631–639.
40. Boyd,S.D., Tsai,K.Y. and Jacks,T. (2000) An intact HDM2 RING-finger
domain is required for nuclear exclusion of p53. Nat. Cell Biol., 2,
563–568.
41. Geyer,R.K., Yu,Z.K. and Maki,C.G. (2000) The MDM2 RING-finger
domain is required to promote p53 nuclear export. Nat. Cell Biol., 2,
569–573.
42. Lohrum,M.A.E., Woods,D.B., Ludwig,R.L., Bálint,E. and Vousden,K.H.
(2001) C-terminal ubiquitination of p53 contributes to nuclear export. Mol.
Cell Biol., 21, 8521–8532.
43. Li,M., Brooks,C.L., Wu-Baer,F., Chen,D., Baer,R. and Gu,W. (2003)
Mono-versus polyubiquitination: differential control of p53 fate by Mdm2.
Science, 302, 1972–1975.
44. Dumont,P., Leu,J.I., Della Pietra,A.C., George,D.L. and Murphy,M.P.
(2003) The codon 72 polymorphic variants of p53 have markedly different
apoptotic potential. Nat. Genet., 33, 357–365.
45. Chao,C., Saito,S., Kang,J., Anderson,C.W., Appella,E. and Xu,Y. (2000)
p53 transcriptional activity is essential for p53-dependent apoptosis
following DNA damage. EMBO J., 19, 4967–4975.
46. Jimenez,G.S., Nister,M., Stommel,J.M., Beeche,M., Barcarse,E.A.,
Zhang,X.-Q., O’Gorman,S. and Wahl,G. (2000) A transactivationdeficient mouse model provides insights into Trp53 regulation and
function. Nat. Genet., 26, 37–41.
47. Nister,M., Tang,M., Zhang,X.Q., Yin,C., Beeche,M., Hu,X., Enblad,G.,
Van Dyke,T. and Wahl,G.M. (2005) p53 must be competent for
transcriptional regulation to suppress tumor formation. Oncogene, 24,
3563–3573.
48. Wang,X.W. (1999) Role of p53 and apoptosis in carcinogenesis.
Anticancer Res., 19, 4759–4771.
49. Fridman,S.J. and Lowe,S.W. (2003) Control of apoptosis by p53.
Oncogene, 22, 9030–9040.
50. Schmitt,C.A., Fridman,S.J., Yang,M., Baranov,E., Hoffman,R.M. and
Lowe,S.W. (2002) Dissecting p53 tumor suppressor functions in vivo.
Cancer Cell, 1, 289–298.
51. Soengas,M.S., Alarcon,R.M., Yoshida,H., Giaccia,A.J., Hakem,R.,
Mak,T.W. and Lowe,S.W. (1999) Apaf-1 and caspase-9 in p53dependent apoptosis and tumor inhibition. Science, 284, 156–9.
52. Martin-Caballero,J., Flores,J.M., Garcia-Palencia,P. and Serrano,M.
(2001) Tumor susceptibility of p21 (Waf1/Cip1)-deficicent mice. Cancer
Res., 61, 6234–6238.
53. Liu,S., Bishop,W.R. and Liu,M. (2003) Differential effects of cell cycle
regulatory protien p21 (WAF1/Cip1) on apoptosis and sensitivity to cancer
chemotherapy. Drug Resist. Updat., 6, 183–195.
54. Rowan,S., Ludwig,R.L., Haupt,Y., Bates,S., Lu,X., Oren,M. and
Vousden,K.H. (1996) Specific loss of apoptotic but not cell cycle
arrest function in a human tumour derived p53 mutant. EMBO J., 15,
827–838.
55. Crook,T., Marston,N.J., Sara,E.A. and Vousden,K.H. (1994)
Transcriptional activation by p53 correlates with suppression of growth
but not transformation. Cell, 79, 817–827.
56. Liu,G., Parant,J.M., Lang,G., Chau,P., Chavez-Reyes,A., El-Naggar,A.K.,
Multani,A., Chang,S. and Lozano,G. (2004) Chromosomal stability, in the
1321
K.S.Yee and K.H.Vousden
absence of apoptosis, is critical for suppression of tumorigenesis in Trp53
mutant mice. Nat. Genet., 36, 63–68.
57. Campisi,J. (2005) Senescent cells, tumor suppression, and organismal
aging: good citizens, bad neighbors. Cell, 120, 513–522.
58. Weiss,R.H. (2003) p21Waf1/Cip1 as a therapeutic target in breast and
other cancers. Cancer Cell, 4, 425–429.
59. Edelman,J., Edelman,J. and Nemunaitis,J. (2003) Adenoviral p53 gene
therapy in squamous cell cancer of the head and neck region. Curr. Opin.
Mol. Ther., 5, 611–617.
60. Wang,W., Rastinejad,F. and El-Deiry,W.S. (2003) Restoring
p53-dependent tumor suppression. Cancer Biol. Ther., 2, S55–63.
61. Bykov,V.J. and Wiman,K.G. (2003) Novel cancer therapy by reactivation
of the p53 apoptosis pathway. Ann. Med., 35, 458–465.
62. Klein,C. and Vassilev,L.T. (2004) Targeting the p53–MDM2 interaction
to treat cancer. Br. J. Cancer, 91, 1415–1419.
1322
63. Vassilev,L.T., Vu,B.T., Graves,B., Carvajal,D., Podlaski,F., Filipovic,Z.,
King,N., Kammlott,U., Lukacs,C., Klein,C., Fotouhi,N. and Liu,E.A.
(2004) In vivo activation of the p53 pathway by small-molecular
antagonists of MDM2. Science, 303, 844–848.
64. Zhang,R., Wang,H. and Agrawal,S. (2005) Novel antisense antiMDM2 mixed-backbone oligonuceotides: proof of principal, in vitro
and in vivo activities, and mechanisms. Curr. Cancer Drug Targets, 5,
43–49.
65. Yang,Y., Ludwig,R.L., Jensens,J.P., Pierre,S., Medaglia,M.V., Davydov,I.,
Safiran,Y.J., Oberoi,P., Kenten,J., Phillips,A.C., Weissman,A.M.
and Vousden,K.H. (2005) Small molecule inhibitors of HDM2
ubiquitin ligase activity stabilize and activate p53 in cells. Cancer Cell,
7, 547–559.
Received April 30, 2005; accepted May 3, 2005