
[Cancer Research 60, 1217-1220, March 1, 2000]
© 2000 American Association for Cancer Research
CD95 (Fas/APO-1) and p53 Signal Apoptosis Independently in Diverse Cell Types1
Liam OConnor,
Alan W. Harris and
Andreas Strasser2
The Walter and Eliza Hall Institute of Medical Research, Melbourne, Victoria 3050, Australia
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ABSTRACT
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The tumor suppressor p53 exerts its antioncogenic effects in cells
chiefly by regulating their progression through the cell cycle and by
inducing cell death. It has been claimed that p53-transduced apoptosis
involves the death receptor CD95 (Fas/APO-1). We report that thymocytes
from mice lacking functional Fas ligand (gld) show
normal sensitivity to apoptosis transduced by p53, and that hepatocytes
from p53-/- mice have normal sensitivity to
apoptosis triggered through ligation of CD95. p53 and CD95, therefore,
function in independent pathways to cell death in these diverse cell
types.
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Introduction
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The tumor suppressor p53 exerts its antioncogenic potential in
cells chiefly by managing their responses to stress (1
, 2) . When cells are exposed to DNA-damaging agents, p53 levels
rise, attributable at least in part to increased stability of the
protein (3, 4, 5, 6)
, and posttranslational changes cause an
increase in p53 transcription factor activity (7)
. This
increase in p53 activity leads to induction of genes that control cell
cycle arrest and apoptosis. For instance, the p53 gene is
essential for DNA damage-induced cell cycle arrest in fibroblasts and
apoptosis in lymphoid cells (8, 9, 10)
.
Exactly how p53 triggers apoptosis is unclear, but it has been reported
to involve both transcription-dependent and -independent mechanisms.
One study reported that increased levels of p53 induced apoptosis
through transcriptional activation of proapoptotic genes such as
bax (11)
. Induction of bax, however,
is not sufficient for p53-transduced apoptosis, because expression of a
bax transgene does not restore DNA damage-induced apoptosis
in p53-/- thymocytes (12)
.
p53-induced apoptosis has been reported to be dependent on
transcription (13)
, but there have also been reports that
p53 can induce apoptosis without new RNA or protein synthesis (1
, 2) . A possible mechanism for this has been suggested by recent
reports that p53 can trigger apoptosis through CD95, a member of the
tumor necrosis factor superfamily of cell surface receptors
(14)
. Overexpression of p53 in transformed cell lines has
been reported to result in increased cell surface expression of CD95
and subsequent apoptosis (15
, 16)
. Other studies, however,
have suggested there is no link between p53 and CD95 in signaling
apoptosis. Cell lines with inactive p53 have been shown to have
equivalent sensitivity to CD95-triggered apoptosis to cells with
wild-type p53 (17)
.
We believe any physiological significance of cross-talk between CD95
and p53 in signaling apoptosis is best established by comparing animals
that express these molecules at normal levels and mutant animals that
lack them altogether. We report that cells from mice lacking functional
Fas ligand (gld) show normal sensitivity to apoptosis caused
by DNA-damaging agents, and that cells from
p53-/- mice have normal sensitivity to
apoptosis triggered by ligation of CD95. p53 and CD95, therefore,
function in independent pathways to cell death in the cell types
studied.
 |
Materials and Methods
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Induction of Apoptosis in Tissue Culture.
The derivation of p53-/-
(18)
, gld, and lpr (19)
mice have been described. These strains had been backcrossed with
normal C57BL/6J mice for six or more generations. The procedures for
culture of primary cells are described elsewhere (20)
.
Thymocytes were cultured in the presence of a range of concentrations
of dexamethasone (0.011.0 µM) or etoposide
(0.110 µg/ml) or subjected to 2.510 Gy
-irradiation from a
60Co source. Samples were taken at various times
after this treatment, and cell viability was determined by flow
cytometric analysis of propidium iodide (2 µg/ml)-stained cells using
a FACScan (Becton Dickinson).
Immunofluorescence Staining for Detection of CD95 Expression.
Liver samples were fixed in 2% paraformaldehyde and then sections were
cut and embedded in wax. After blocking of endogenous Fc receptors with
normal mouse serum, the sections were incubated serially with
monoclonal hamster anti-Fas (Jo2) antibody (2.5 µg/ml), followed by
FITC-conjugated mouse antihamster IgG antibody (2.5 µg/ml;
PharMingen). CD95 expression on thymocytes was detected using a similar
staining protocol.
Induction of Apoptosis through CD95 in Vivo.
A range of doses of Jo2 anti-CD95 monoclonal antibody was injected in
100 µl of saline into the tail vein of mice 814 weeks of age. The
mice were killed after 2 h, and then liver and blood samples were
taken for analysis.
Detection of Apoptotic Cells by
TUNEL.3
Liver sections were treated with 20 µg/ml proteinase K for 15 min at
22°C, and then DNA free ends were labeled with dUTP-biotin (Roche
Diagnostics) using terminal deoxynucleotidyl transferase (Promega
Corp.; Ref. 21
) and revealed with avidin-biotinylated
horseradish peroxidase and diaminobenzidine. TUNEL-labeled nuclei were
counted in 15 or more 0.56-mm2 fields.
Liver Enzyme Assays.
Blood samples were taken 2 h after injection of Jo2 anti-Fas
antibody. ALP, ALT, and AST activity in serum were measured on a CX3
automatic analyzer (Beckman).
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Results
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Functional FasL Expression Is Not Needed for DNA Damage-induced
Apoptosis.
We isolated thymocytes from gld and control mice and
examined their cell surface expression of CD95 and sensitivity to
various apoptotic stimuli. Thymocytes from gld mice had
normal cell surface levels of CD95 (Fig. 1A)
and showed normal responses to a range of doses of the
glucocorticoid dexamethasone, the topoisomerase inhibitor etoposide,
and
-irradiation (Fig. 1B
, upper panels). gld
thymocytes also died at equivalent rates to control cells when exposed
to these apoptotic stimuli over 48 h (Fig. 1B
, lower
panels). These results show that thymocytes from mice lacking
functional FasL (gld) are unaffected in their response to
p53-transduced apoptotic stimuli, such as those elicited by genotoxic
stress, suggesting that FasL plays no role in p53-transduced apoptosis.

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Fig. 1. Apoptosis caused by DNA damage is normal in thymocytes
from gld mice. A, CD95 expression on
gld and control thymocytes was determined by
immunofluorescence staining and flow cytometric analysis.
B, apoptosis in gld and control thymocyte
cultures exposed to various cytotoxic treatments. Upper
panels, dose response at 24 h. Lower
panels, time course with dexamethasone (0.1 µM),
etoposide (10 µg/ml), and -irradiation (5 Gy).
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CD95-induced Apoptosis Proceeds Independently of p53 Expression.
We isolated thymocytes from p53-/- and
control mice and examined their CD95 expression and sensitivity to
CD95-induced apoptosis. Thymocytes isolated from
p53-/- mice showed equivalent cell
surface expression of CD95 to control thymocytes (Fig. 2A)
and equivalent sensitivity to a range of doses of
monoclonal Jo2 anti-CD95 antibody and at various time points up to
48 h after administration (Fig. 2B
and not shown).

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Fig. 2. Apoptosis triggered through CD95 is normal in
p53-/- thymocytes. A, CD95
expression on p53-/- and control
thymocytes was determined by immunofluorescence staining and flow
cytometric analysis. B, apoptosis in
p53-/- and control thymocyte cultures
treated with a range of doses of Jo2 anti-CD95 antibody.
Bars, SD.
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We also decided to test the role of p53 in CD95-induced apoptosis in
whole animals. We injected groups of
p53-/- and control mice with a range of
doses of Jo2 anti-CD95 antibody and examined their liver for CD95
expression and apoptosis. Immunostaining of liver sections from both
groups of mice showed equivalent levels of CD95 expression (Fig. 3A)
. We then examined apoptosis in liver sections by TUNEL
staining (Fig. 3B)
. Quantitation of TUNEL-positive cells in
liver sections from mice treated with varying doses of Jo2 anti-CD95
antibody showed no difference between
p53-/- and control mice (Fig. 3C)
. We also measured the serum levels of enzymes known to
be released as a result of hepatocyte destruction (22)
.
p53-/- and control mice had equivalent
levels of ALP, ALT, and AST in their serum after
administration of Jo2 anti-CD95 antibody (Fig. 4)
. Cell surface CD95 levels, CD95-induced apoptosis, and liver damage
enzymes induced by Jo2 anti-CD95 antibody administration were all
unaffected in mice lacking p53, suggesting that p53 does not play a
role in CD95-induced apoptosis.

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Fig. 3. Liver apoptosis caused by injection of anti-CD95 antibody
is normal in p53-/- mice.
A, CD95 expression in liver sections from
lpr, control, and p53-/-
mice was determined by immunofluorescence staining. B,
TUNEL staining of p53-/- and control liver
sections from mice injected with 100 µg of Jo2 anti-CD95 antibody.
C, apoptosis in liver sections from
p53-/- and control mice injected with a
range of doses of Jo2 anti-CD95 antibody. Bars,
SD.
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Fig. 4. Acute liver damage caused by injection of Jo2 anti-CD95
antibody in normal and in p53-/-
mice. ALP, ALT, and AST levels in serum from
p53-/- and control mice after injection
with Jo2 anti-CD95 antibody. Bars, SD.
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Discussion
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We have investigated whether apoptosis transduced by p53 and
apoptosis triggered through the death receptor CD95 are linked in
vivo. A mechanistic link between p53 and CD95 in signaling
apoptosis can exist as one of two possibilities: either p53 activation
results in CD95-transduced apoptosis, i.e., p53 is upstream
of CD95, or intracellular events after cross-linking of CD95 on cell
surfaces are somehow transduced by activated p53, i.e., CD95
is upstream of p53.
If p53 is upstream of CD95, the mechanism does not appear to be a
simple effect of p53 on basal CD95 expression, because CD95 levels were
normal in animals lacking p53 (Figs. 2A
and 3A)
.
CD95 may be involved in p53-transduced apoptosis through, for instance,
increased expression of CD95 after activation of p53. If this is the
case, animals in which the CD95/CD95L system is disrupted should show
an impaired response to p53-dependent apoptotic stimuli. It has already
been established that mice that lack CD95 (lpr) have
unimpaired p53-transduced apoptosis (20
, 23)
, and the
results presented here show that thymocytes from mice lacking
functional CD95L are similarly unaffected (Fig. 1)
. It is also unlikely
that other members of the tumor necrosis factor receptor family of
death receptors are involved in signaling p53-transduced apoptosis.
Cells expressing a dominant-negative form of the death adapter molecule
FADD (also called MORT-1) or the caspase inhibitor crmA, which block
all death receptor-induced apoptosis, have unimpaired responses to
p53-transduced apoptosis (24
, 25)
.
The second possibility is that CD95 is upstream of p53. Apoptosis
pathways triggered through CD95 and related death receptors have been
characterized extensively (14
, 26)
. After cross-linking of
CD95 on the cell surface, the intracellular region of CD95 recruits the
adapter molecule FADD/MORT-1, which then recruits procaspase-8,
resulting in its autocatalytic activation. This starts the proteolytic
avalanche that leads to apoptosis. This model leaves no obvious place
for p53 to exert an effect once apoptosis has been triggered through
CD95. Nevertheless, if there were some unknown mechanism, animals
lacking p53 should have impaired CD95-transduced apoptosis. When mice
lacking p53 were injected with Jo2 anti-CD95 antibody, the ensuing
hepatocyte apoptosis was indistinguishable from that found in control
mice (Figs. 2
3
4)
; therefore, p53 cannot play a role in CD95-transduced
apoptosis in hepatocytes. This finding is consistent with results from
a study of transformed cell lines, in which CD95-transduced apoptosis
was unaffected by the status of their p53 genes
(17)
, and with a study showing that thymocytes from
p53-/- mice were normally sensitive to
anti-CD95 antibody (23)
. Studies in cells from mutant mice
have shown that absence of either Apaf-1 or caspase-9 substitutes for
p53 loss in promoting oncogenesis (27)
, which suggests
that Apaf-1 and caspase-9 are in the apoptotic pathway triggered by
p53. CD95-triggered apoptosis is unaffected in cells from either Apaf-1
or caspase-9 null mice, however (28, 29, 30, 31)
. These results
provide further evidence that CD95 and p53 signal apoptosis
independently.
Why have other studies shown a link between p53 and CD95 in signaling
apoptosis when our data suggest there is none? One explanation is that
previous studies were performed in cell lines in which p53 was
overexpressed at levels far in excess of those found in normal cells,
even after genotoxic stress. Moreover, such lines are likely to already
have aberrant apoptotic responses as a result of the transformation
process, and many will also have lost endogenous p53 function. We
believe studies based on extreme overexpression should always be viewed
in the light of experiments performed under physiological conditions.
There also exists the possibility that we have been selective in only
studying thymocytes and hepatocytes, and that a link between p53 and
CD95 in apoptosis signaling may exist in other cell types. However, the
well-documented role of p53 and CD95 in lymphocyte apoptosis and the
observation that animals injected with anti-CD95 antibody die from
acute liver failure (22
, 26)
mean that the cell types
studied are among the most physiologically important.
The p53 gene is deleted or inactivated in about 50% of
human tumors (1)
. Understanding how p53 triggers apoptosis
is therefore central to understanding how these tumors develop. CD95
has been suggested as a possible player in p53-transduced apoptosis
(15
, 16) . It has even been proposed that loss of p53 in
tumors may allow increased survival because of loss of CD95-induced
apoptosis, and CD95 is therefore a potential target for therapy of such
tumors (32)
. Our data, from non-transformed cells in whole
animals and in tissue culture, strongly suggest that pathways to
apoptosis that are activated through p53 are distinct from those
triggered by CD95.
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ACKNOWLEDGMENTS
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We thank D. Huang and K. Newton for comments on the manuscript,
S. Nagata for kindly providing Jo2 monoclonal antibody, D. Deam for
serum enzyme assays, W. Heath for providing gld mice, J.
Merryful for expert animal husbandry, L. Cullen for mouse genotyping,
and S. Mihajlovic for preparing histological sections.
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FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This work was supported by the Josef Steiner
Cancer Foundation, the National Health and Medical Research Council,
Canberra, and the Anti-Cancer Council of Victoria. L. O. was supported
by an Edith Moffatt Postgraduate Scholarship and an Anti-Cancer Council
of Victoria Postdoctoral Fellowship. 
2 To whom requests for reprints should be
addressed, at The Walter and Eliza Hall Institute of Medical Research,
Post Office Royal Melbourne Hospital, Melbourne, Victoria 3050,
Australia. Phone: 61-3-9345-2624; Fax: 61-3-9347-0852; E-mail: strasser{at}wehi.edu.au 
3 The abbreviations used are: TUNEL, terminal
deoxynucleotidyl transferase-mediated nick end labeling; ALP, alkaline
phosphatase; ALT, alanine aminotransferase; AST, aspartate
aminotransferase; FasL, Fas ligand. 
Received 10/ 7/99.
Accepted 1/18/00.
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