REVIEW
The p53 family and programmed cell death
EC Pietsch1, SM Sykes2, SB McMahon3 and ME Murphy1
1Division of Medical Sciences, Fox Chase Cancer Center, Philadelphia, PA, USA; 2Department of Medicine, Brigham and Women’s
Hospital, Boston, MA, USA and 3Department of Cancer Biology, Kimmel Cancer Center, Thomas Jefferson Medical College,
Philadelphia, PA, USA
The p53 tumor suppressor continues to hold distinction as
the most frequently mutated gene in human cancer. The
ability of p53 to induce programmed cell death, or
apoptosis, of cells exposed to environmental or oncogenic
stress constitutes a major pathway whereby p53 exerts its
tumor suppressor function. In the past decade, we have
discovered that p53 is not alone in its mission to destroy
damaged or aberrantly proliferating cells: it has two
homologs, p63 and p73, that in various cellular contexts
and stresses contribute to this process. In this review, the
mechanisms whereby p53, and in some cases p63 and p73,
induce apoptosis are discussed. Other reviews have
focused more extensively on the contribution of individual
p53-regulated genes to apoptosis induction by this protein,
whereas in this review, we focus more on those factors that
mediate the decision between growth arrest and apoptosis
by p53, p63 and p73, and on the post-translational
modifications and protein–protein interactions that influ-
ence this decision.
Oncogene (2008) 27, 6507–6521; doi:10.1038/onc.2008.315
Keywords: p53; p63; p73; apoptosis; transcription;
mitochondria
The p53 family
The tumor suppressor p53 is vital in maintaining cellular
genomic integrity and controlled cell growth (Levine,
1997; Bargonetti and Manfredi, 2002; Fridman and
Lowe, 2003). Loss or gain of p53 function results in the
aberrant growth of cells. Hence, both the cellular
expression and the activity of p53 are tightly regulated.
p53 protein has a very short half-life and thus is usually
present at extremely low levels within cells. In response
to stress, DNA-damaging agents and chronic mitogenic
stimulation, p53 is transiently stabilized and activated.
Depending on cell type, cell environment and oncogenic
alterations, p53 activation leads to inhibition of cell
cycle progression, induction of senescence, differentia-
tion or apoptosis (Vousden and Lu, 2002). Over a
decade after the identification of the tumor suppressor
p53, two p53-related genes, p63 and p73, were identified
(Kaghad et al., 1997; Schmale and Bamberger, 1997;
Osada et al., 1998; Trink et al., 1998; Yang et al., 1998;
Zeng et al., 2001). Like p53, both p63 and p73 possess
an N-terminal transactivation domain (TAD), a DNA-
binding domain (DBD), and a C-terminal oligomeriza-
tion domain (OD) (Murray-Zmijewski et al., 2006).
Although p63 and p73 demonstrate relatively little
homology with p53 in their TAD and OD, both share
approximately 60% similarity with the p53 DBD,
including conservation of essential DNA contact re-
sidues (Deyoung and Ellisen, 2007) (see Figure 1). This
similarity allows p63 and p73 to regulate p53 target
genes and, similar to p53, to induce cell cycle arrest and
apoptosis. However, studies of knockout mice have
demonstrated that, even though these proteins clearly
share some activities with p53, each of these proteins
also has functions that are very distinct. TP53 null mice
are viable and develop normally (Donehower et al.,
1992). In contrast, p63 knockout mice show severe
developmental defects, including failure to develop
limbs, skin and other epithelial tissue; these mice do
not survive beyond a few days after birth (Mills et al.,
1999). p73 knockout mice exhibit neurodevelopmental
(hippocampal dysgenesis and hydrocephalus) and in-
flammatory (chronic infections and excessive inflamma-
tion) defects (Yang et al., 2000).
Isoforms of p53 family members
The p53, p63 and p73 genes are located on chromosomes
17 (17p13.1), 3 (3q27–29) and 1 (1q36), respectively, and
all three genes are now known to express many
differentially spliced isoforms (Murray-Zmijewski
et al., 2006; Mu¨ller et al., 2006). All three genes encode
two primary transcripts that are controlled by separate
promoters (P1 and P2) (Figure 1). The P1 promoter of
each gene is embedded in a non-coding region of exon 1.
The P2 promoter of p63 and p73 is located in intron 3,
whereas the P2 promoter of p53 is located in intron 4.
Transcripts generated from the P1 promoter produce
proteins that contain the TAD, the DBD and the OD
(TAp53, TAp63 and TAp73). In contrast, in transcripts
generated from the P2 promoter, the N-terminal TAD is
Correspondence: Dr ME Murphy, Division of Medical Sciences, Fox
Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA 19111,
USA.
E-mail: Maureen.Murphy@FCCC.edu or Dr SB McMahon, Kimmel
Cancer Center, Thomas Jefferson Medical College, 233 S. 10th
St. Philadelphia, PA 19107, USA.
E-mail: steven.mcmahon@mail.jci.tju.edu
Oncogene (2008) 27, 6507–6521
& 2008 Macmillan Publishers Limited All rights reserved 0950-9232/08 $32.00
www.nature.com/onc
found to be absent (D133p53, DNp63 and DNp73).
These DN-terminal variants are generally regarded as
dominant-negative versions of p53 family members, as
these can occupy promoter-binding sites but fail to
transactivate gene expression. For both p53 and p73,
additional DN variants can be generated by alternative
splicing of the N terminus or alternative initiation of
translation (D40p53, Dex2p73 and Dex2/3p73) (Murray-
Zmijewski et al., 2006; Mu¨ller et al., 2006). Additional
complexity to p53 family member isoforms is added by
virtue of the fact that P1 and P2 transcripts can be
spliced at the C terminus into several splice variants
(a, b, g, and so on). For p53, transcription from the P1
and P2 promoters and alternative splicing of intron 9
can generate at least six p53 isoforms, including three
TA variants (p53, p53b and p53bg) and three DN
variants (D133p53, D133p53b and D133p53g). For p63,
three TA variants (TAp63a, TAp63b and TAp63g) and
three DN variants (DNp63a, DNp63b and DNp63g) have
been identified. Finally, for p73, eight different isoforms
from the p73 P1 promoter and P2 promoter elements
have been described (TA and DN p73a, b, g, d, e, z, Z
and y) (Figure 1).
Tumor suppression by p53 family members
p53 can be found biallelically mutated or deleted in
more than 50% of human tumors, and there are p53
mutation databases containing a wealth of information
in this regard (Olivier et al., 2002). Germline mutations
in p53 are found in the Li–Fraumeni syndrome, a
tumor-prone disorder predisposing affected individuals
to tumors of the brain, breast, bone and adrenal cortex
(Malkin et al., 1992). Finally, the p53 knockout mouse is
predisposed to early onset tumors, chiefly thymic
lymphoma, sarcoma and testicular tumors (Donehower
et al., 1992).
p63 and p73 map to two regions within the human
genome that are often altered and deleted in cancers
(Kaelin, 1999). This observation, combined with the
observation that p63 and p73 mimic p53 action in tissue
culture when overexpressed, suggests that p63 and p73
have tumor-suppressive properties analogous to p53.
However, characterization of these proteins as bona fide
tumor suppressors has not been straightforward. Studies
aimed at identifying mutations within the p63 and p73
genes in human cancers have demonstrated that p53 is
mutated in 50% of all human cancers, whereas p63 and
p73 mutations occur rarely (Irwin and Kaelin, 2001;
Melino et al., 2003; Deyoung and Ellisen, 2007). Rather,
altered expression of p63 and p73 isoforms is more
commonly observed (Mu¨ller et al., 2006). For example,
p63 is upregulated in 80% of head and neck squamous
cell carcinomas, with DNp63 being the predominant
isoform overexpressed (Weber et al., 2002; Sniezek et al.,
2004; Rocco et al., 2006; DeYoung et al., 2006). DNp63
overexpression is observed in more than 60% of bladder
Figure 1 p53 family member isoforms. Schematic presentation of p53, p63 and p73 isoforms. Approximate location of the
transactivation (TA) domain, proline-rich domain (PR), DNA-binding domain (DBD), oligomerization domain (OD) and the sterile
alpha motif (SAM) are indicated. The amino acid identity between the TA, DBD and OD of p53 and p63/p73 as well as between p63 and
p73 is denoted. Full-length (FL) p53 or TAp63 and TAp73 protein are transcribed from a promoter located in the non-coding region of
exon 1 (P1 promoter) of the p53, p63 and p73 gene. D133p53, DNp63 and DNp73 isoforms are generated by transcription from a
promoter (P2 promoter) located in intron 3 of the p63 and p73 gene or intron 4 of the p53 gene. Furthermore, DN variants are generated
by alternative splicing of the N terminus (D40p53, Dex2p73 and Dex2/3p73). Alternative splicing of the C-terminal region yields additional
variants for p53 (FL, b, and g), p63 (a, b, and g) and p73 (a, b, g, d, e, z, Z, and y). p53 b and g lack the oligomerization domain.
p53 family and apoptosis
EC Pietsch et al
6508
Oncogene
carcinomas (Park et al., 2000), whereas loss of TAp63
expression occurs in bladder cancer and is associated
with metastasis and poor prognosis (Urist et al., 2002;
Koga et al., 2003). Similar to p63, altered expression of
p73 occurs in a multitude of different cancers (Mu¨ller
et al., 2006). Recent studies analysing p73 isoform
expression indicate that both TAp73 and DNp73
isoforms are upregulated in ovarian cancer and rhabdo-
myosarcomas (Concin et al., 2004; Cam et al., 2006),
whereas exclusive upregulation of DNp73 isoforms can
be observed in gliomas as well as in carcinomas of the
breast and the colon (Domı´nguez et al., 2006; Wager
et al., 2006). Downregulation of TAp73 is found in
lymphoblastic leukemias and Burkitt’s lymphoma as a
result of p73 P1 promoter methylation (Corn et al.,
1999; Kawano et al., 1999). The general, but by no
means definitive, consensus appears to be that either
methylation-mediated silencing of the entire gene or
upregulation of the dominant-negative DN variant
predominates in human tumors for p53 family members.
How upregulation of specific DN variants occurs in
human tumors is not presently understood.
Research on the predisposition of p63 and p73
knockout mice to tumor development has been conflict-
ing and has added to the complexity of defining p63 and
p73 as tumor suppressor proteins. p73 knockout mice
do not develop spontaneous tumors (Yang et al., 2000)
and heterozygous p63 mice are neither tumor prone nor
do they develop tumors at an accelerated rate on
exposure to chemical carcinogens (Keyes et al., 2006). In
fact, p63 deficiency was found to induce a cellular
senescence program, increased aging and shortened life
span (Keyes et al., 2008). In contrast to these observa-
tions is a study examining tumor development in aging
heterozygous p63 and p73 knockout mice. This study
demonstrated that p63þ /� and p73þ /� mice develop
spontaneous tumors, similar to p53þ /� mice, with
a median survival time of 10 months for p53þ /�,
15 months for p63þ /� and 14 months for p73þ /�
mice (Flores et al., 2005). These latter data present the
firmest evidence that p63 and p73 have tumor-suppres-
sive properties analogous to p53.
Further evidence that p63 and p73 have tumor
suppressor function comes from four independent
observations, they are: (1) siRNAs specific for p63 and
p73 enhance the transformation potential of p53�/�
mouse embryo fibroblasts (Lang et al., 2004); (2) both
p63 and p73 can mediate chemosensitivity independent
of p53 status by the induction of apoptosis (Irwin et al.,
2000, 2003; Bergamaschi et al., 2003a; Lang et al., 2004;
Gressner et al., 2005); (3) within tumor cells, mutant p53
directly binds to both p63 and p73 via the DBD,
rendering p63 and p73 impaired in their ability to induce
growth suppression and apoptosis (Di Como et al.,
1999; Marin et al., 2000; Gaiddon et al., 2001; Strano
et al., 2002); and (4) silencing mutant p53 expression by
siRNA-mediated knockdown sensitizes cells to antic-
ancer agents (Bergamaschi et al., 2003a). These obser-
vations indicate that the inactivation of p63 and p73
confers a growth advantage to tumor cells that have
mutant forms of p53. Binding of mutant p53 to p73 is
influenced by a polymorphism at codon 72, which
encodes either arginine or proline. p53 mutants asso-
ciated with the arginine polymorphic variant bind and
inactivate p73 most efficiently (Marin et al., 2000;
Bergamaschi et al., 2003a). Consistent with this obser-
vation, these p53 mutants demonstrate a less favorable
response to chemotherapy (Bergamaschi et al., 2003a).
Likewise, in osteosarcoma cells from p53R172H
(equivalent to human hot spot mutant R175H) knockin
mice, mutant p53 co-immunoprecipitates and function-
ally inactivates p63 as well as p73, resulting in enhanced
cellular transformation (Lang et al., 2004). Collectively,
these observations suggest that p63 and p73 may
complement, and in some situations may substitute
for, the actions of p53. However, which effects p63 and
p73 exert on cell proliferation and death may ultimately
depend on the cellular context and genetic background,
as p63 and p73 may not be required for the development
and apoptosis of all cell types (Senoo et al., 2004).
Apoptosis by p53
Transcription-dependent apoptosis
The best understood activity of p53 is as a transcription
factor that binds to the promoters and introns of target
genes, and that recruits the basal transcriptional
machinery to activate expression of that gene. In 1992,
a paper that searched for the consensus binding site for
p53 using an unbiased affinity-binding approach re-
vealed that p53 bound best to two tandem repeats of the
consensus 50-Pu-Pu-Pu-C-(A/T)-(T/A)-G-Pyr-Pyr-Pyr-30,
where Pu is purine and Pyr is pyrimidine, separated by
0–13 bases (el-Deiry et al., 1992). Since that time,
literally hundreds of p53 target genes have been
identified, and each contain one or more consensus sites
in their 50 promoter regions, introns and, in some cases,
exons. These include p53AIP1, Apaf-1, BAX, Caspase-1,
Caspase-6, cathepsin D, DINP1, DR4, DR5, Fas, IGF-
BP3, NOXA, p85, PERP, PIDD, PIG3, PTEN, PUMA,
Scotin, EI24/PIG8 as well as others (for review see Riley
et al., 2008). Once bound to the promoter regions of
these genes, p53 recruits general transcription factors as
well as histone acetyltransferases (HATs), such as
CREB-binding protein (CBP), p300 and PCAF. These
HATs acetylate lysine residues of histones, thereby
increasing the accessibility of chromatin to the trans-
criptional apparatus.
Two p53 target genes deserve specific mention as
being particularly important in the apoptotic armamen-
tarium of p53: these are PUMA and NOXA, BH-3-only
members of the BCL-2 family. Mice lacking either
PUMA or NOXA, or mice doubly knocked out for both
genes (PUMA/NOXA double knockout) develop nor-
mally and are not tumor-prone (Jeffers et al., 2003;
Villunger et al., 2003; Michalak et al., 2008). The
importance of PUMA to p53-dependent apoptosis is
underscored by the fact that thymocytes and cells from
the developing nervous system from PUMA knockout
mice are almost completely impaired for p53-dependent
apoptosis to levels equivalent to the p53 knock-out
p53 family and apoptosis
EC Pietsch et al
6509
Oncogene
mouse (Jeffers et al., 2003). In contrast, apoptosis in
other cell types of the PUMA knockout mouse is only
partially impaired, and in these cells NOXA appears to
contribute. For example, in mouse embryo fibroblasts,
both PUMA and NOXA proteins have critical func-
tions, as mouse embryonic fibroblasts (MEFs) from the
PUMA/NOXA double knockout are most protected
from cell death by etoposide to levels similar to p53
knockout MEFs (Michalak et al., 2008). Finally, there
are other cell types in the mouse where even the PUMA/
NOXA double-knockout mouse is not protected from
apoptosis as well as the p53 knockout mouse; this
includes mature T and B cells, which are partially
but not completely protected by PUMA/NOXA loss
(Michalak et al., 2008). At least in these cell types, these
data not only firmly implicate PUMA and NOXA
proteins as critical players in the p53 apoptotic program
but also suggest that the overall importance of
individual p53 target genes in apoptosis is likely to be
cell context and/or stress-specific.
Transcription-independent apoptosis by p53
Several studies, which have examined p53-dependent
apoptosis in the presence of inhibitors of RNA and
protein synthesis, indicate that p53 induces apoptosis
not only by transcription-dependent but also by
transcription-independent mechanisms (Caelles et al.,
1994; Wagner et al., 1994; Yan et al., 1997; Gao and
Tsuchida, 1999). Moreover, deletion or mutation of the
p53 TAD, which is critical to the induction of p53
apoptotic target genes, does not eliminate the ability of
p53 to induce apoptosis (Haupt et al., 1995). Likewise, a
p53 deletion mutant containing only the first 214 N-
terminal amino acid residues induces apoptosis and
suppresses the transformation of rat embryo fibroblasts
by several oncogenes, but it does not transactivate p53
target genes (Chen et al., 1996; Haupt et al., 1997).
Compelling evidence has accumulated over the last 10
years, indicating that p53 can directly activate compo-
nents of the apoptotic machinery and that this involves
translocation of p53 to the mitochondria. More
specifically, the addition of recombinant p53 to nucle-
ar-free cytosolic extract of irradiated cells leads to
cytochrome C release, caspase activation and apoptosis
induction (Ding et al., 1998, 2000; Schuler et al., 2000).
Therefore, the activation of p53 in enucleated cytoplasts
is sufficient to induce hallmarks of apoptosis. Immuno-
fluorescence, electron microscopy and cell fractionation
studies have demonstrated that in response to genotoxic
stress, including DNA damage, hypoxia and activated
oncogenes, p53 rapidly translocates to the mitochondria
(Marchenko et al., 2000; Sansome et al., 2001; Mihara
et al., 2003; Nemajerova et al., 2005). Mitochondrial
translocation of p53 precedes loss of mitochondrial
membrane potential and caspase activation, suggesting
that p53 may trigger mechanisms at the mitochondria
that ultimately lead to the induction of apoptosis
(Marchenko et al., 2000). Indeed, mitochondrial trans-
location of p53 in irradiated mouse tissues triggers an
early wave of caspase activation, which occurs long
before the transcriptional program of p53 is activated
(Erster et al., 2004). Notably, specific targeting of p53 to
the mitochondria by fusion of the p53 N terminus to the
mitochondrial import leader peptide from ornithine
transcarbamylase is sufficient to induce apoptosis,
suppress colony formation and to allow p53 to function
effectively as a tumor suppressor in vivo (Marchenko
et al., 2000; Dumont et al., 2003; Talos et al., 2005;
Palacios and Moll, 2006).
The importance of p53 mitochondrial translocation in
apoptosis is underscored by several observations. First,
in cells or tissues that undergo cell cycle arrest (and not
apoptosis), p53 mitochondrial translocation does not
occur after exposure to genotoxic stress (Marchenko
et al., 2000; Erster et al., 2004). Second, studies on the
apoptotic potential of the proline and arginine p53
codon 72 polymorphic variants revealed that the proline
codon 72 polymorphic variant, which is markedly
impaired for apoptosis induction as compared with the
arginine codon 72 variant, does not translocate to the
mitochondria as efficiently as the arginine variant
(Dumont et al., 2003). In a mouse model for B-cell
lymphoma, p53 targeted exclusively to the mitochondria
is efficiently tumor suppressive in vivo (Talos et al.,
2005). In contrast, an artificial p53 chimeric mutant that
is capable of robustly transactivating pro-apoptotic p53
target genes is capable of inducing senescence, but
incapable of inducing apoptosis in knockin mouse
embryo fibroblasts (Johnson et al., 2008). Moreover,
the compound pifithrin m, which selectively blocks p53’s
mitochondrial function but leaves intac
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