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J Biol Chem, Vol. 273, Issue 49, 33091-33098, December 4, 1998
From the In this study we show that TRAIL (tumor necrosis
factor-related apoptosis-inducing ligand), also called Apo2L, activates
the c-Jun N-terminal kinase (JNK). Interestingly, TRAIL-induced JNK activation occurs in a cell type-specific manner. In HeLa cells, TRAIL-induced JNK activation can be completely blocked with the cysteine protease inhibitor zVAD-fmk, whereas the same inhibitor has
no, or even a stimulatory, effect on JNK activation in Kym-1 cells.
Hence, TRAIL can engage at least two independent pathways leading to
JNK activation, one that is cysteine protease-dependent and
one that is cysteine protease-independent. To investigate whether the
cysteine protease-dependent signaling of TRAIL leading to
JNK activation is related to the apoptotic pathway engaged by this
ligand, we investigated HeLa cells stably overexpressing a dominant
negative mutant of FADD (Fas-associating protein with death domain)
(GFP(green fluorescent protein) Members of the tumor necrosis factor
(TNF)1 receptor superfamily
of proteins are critically involved in inflammatory, immune regulatory,
and pathophysiological reactions, including the induction of apoptosis
(1, 2). The receptors of this superfamily are characterized by two to
six extracellular copies of a canonical motif of cysteine-rich
pseudorepeats, each comprising six conserved cysteines in a stretch of
about 40 amino acids (1, 2). The ligands of these receptors belong to a
complementary family of structurally related molecules, the TNF ligand
family. Most of these ligands are primarily expressed as biologically
active type II membrane proteins, from which soluble forms are produced
by proteolytical cleavage or alternative splicing (1, 2). A subgroup of
the TNF receptor superfamily, comprising TNF-R1 (3), Apo1/Fas (4),
Wsl/DR3/Apo-3/TRAMP/LARD (5-9), CAR1 (10), DR4/TRAIL-R1 (11), and
DR5/TRAIL-R2/TRICK2/KILLER (12-19), can be defined by their capability
to induce cell death in various cell lines. These receptors share a
common intracellular 80 amino acid domain, called the death domain,
that is indispensable for initiation of the intracellular signaling
cascade leading to cell death (3-18). The death domain motif is also
found in the cytoplasmic adaptor proteins TNF-R1-associated death
domain protein (TRADD; Ref. 20), Fas-associating protein with death
domain (FADD; Refs. 21 and 22), receptor-interacting protein (RIP; Ref.
23), mitogen-activated protein kinase activator with death domain
(MADD; Ref. 24), myeloid differentiation marker 88 (MyD88; Ref. 25), and RIP-associated ICH1/CED-3 homologous protein with death domain (RAIDD; Ref. 26). Most of these molecules are involved in the mediation
of cell death by the beforementioned death domain-containing receptors.
All of these adaptor molecules are of at least bipartite structure.
Aside from the death domain mediating binding to other death
domain-containing proteins, these adaptor molecules have additional
domains enabling the interaction with, e.g. the prodomains of apoptotic caspases (26-28) or members of the TNF
receptor-associated factor (TRAF) family (29, 30), molecules involved
in activation of NF- Reagents and Cell Lines--
The HeLa cells stably transfected
with the green fluorescent protein (GFP) and a GFP-tagged dominant
negative mutant of FADD, respectively, have been described elsewhere
(53). For analysis of GFP a FACStar+ (Becton and Dickinsion, San Jose,
Ca) has been used. Purified recombinant FLAG-tagged human TRAIL was
used as complex with anti-FLAG M2 antibody (Kodak International
Biotechnologies) as described (53). HeLa and Kym-1 cells were grown in
Clicks RPMI 1640 medium supplemented with 5% (HeLa) and 10% (Kym-1)
heat-inactivated fetal calf serum, 2 mM
L-glutamine, 50 units/ml penicillin, and 50 µg/ml streptomycin.
RNase Protection Assay and RT-PCR--
Total RNA was isolated
from HeLa and Kym-1 cells (10 × 106) with the RNA
INSTAPURE kit (Eurogentech, Seraing, Belgium) according to the
manufacturer's recommendations. The presence of transcripts of
caspase-8, FasL, Fas, FADD, DR3, TRAIL-R1, TRAIL-R2, TRAIL-R3, TRAIL
TNF-R1, TRADD, and RIP as well as the internal controls L32 and
glyceraldehyde-3-phosphate dehydrogenase were analyzed using the
hApo-3c Multi-Probe template set (PharMingen, Hamburg, Germany). Probe
synthesis, hybridization, and RNase treatment were performed with the
RiboQuant Multi-Probe RNase Protection Assay System (PharMingen,
Hamburg, Germany) according to the manufacturer's recommendations.
Finally, samples were resolved by electrophoresis on denaturing
polyacrylamide gels (5%) and analyzed by phosphorimaging. First-strand
cDNA synthesis was performed with the First Strand Synthesis Kit
(Amersham Pharmacia Biotech, Freiburg, Germany) and oligo(dT) as a
primer according to the manufacturer's protocol. Aliquots of 1 µl of
cDNA were used as template in a final volume of 50 µl in a
standard High Fidelity PCR (Boehringer Mannheim, Mannheim, Germany)
reaction. Samples were overlaid with 50 µl of mineral oil and
amplified as followed: 1) station: 94 °C, 2 min; 1 cycle; 2)
station: 94 °C, 15 s; 50 °C, 30 s; 72 °C, 90 s;
10 cycles; 3) station: 94 °C, 15 s; 50 °C, 30 s;
72 °C, 90 s; time increase 20 s per cycle; 10-20 cycles
(as indicated). For the PCR reaction 0.25 µg of each of the following
primers comprising the intracellular domains of TRAIL-R1, TRAIL-R2, and TRAIL-R4 as well as the extracellular domain of TRAIL-R3, respectively, was used: TRAIL-R1: F-NcoI, 5'-CAG CAC CCA TGG GTT GTG GAG
GGG ACC CCA AGT GCA TGG AC-3'; R-NotI, 5'-GTG CTG GCG GCC
GCT CAC TCC AAG GAC ACG GCA GAG CCT GT-3'; TRAIL-R2: F-NcoI,
5'-CAG CAC CCA TGG TTG TTT GCA AGT CTT TAC TGT GGA AG-3';
R-NotI, 5'-GTG CTG GCG GCC GCT GAA GAG AAT CAC ACT TAG GAC
ATG GC-3'; TRAIL-R3: F, 5'-GAG ATG CAA GGG GTG AAG GAG CGC TTC-3'; R,
5'-CCA CAG TGC AGT CTT TCA AAC AAA CAC-3'; TRAIL-R4:
F-BspHI, 5'-CAG CAC TCA TGA GTC GGA AGA AAT TCA TTT CTT ACC
TCA AA-3'; R-NotI, 5'-GTG CTG GCG GCC GCT TTC CTG AAG AGA
TTC TTT CAC AGG CA-3'. Please obey that the primers for TRAIL-R1,
TRAIL-R2, and TRAIL-R4 contain restriction sites for NotI,
BspHI, and NcoI for cloning purposes not relevant
for this study. Control reactions without or with 1 µg of genomic DNA
as template was performed in each experiment.
Cell Death Assays--
HeLa cells (1.5 × 104/well) were cultivated in 96-well microtiter plates
overnight. Next day TRAIL-M2 complex was titrated and 2.5 µg/ml
cycloheximide were added. After 18 h culture supernatants were
discarded, and the cells were washed once with phosphate-buffered saline followed by crystal violet staining (20% methanol, 0.5% crystal violet) for 15 min. The wells were washed with H2O
and air-dried. The dye was resolved with methanol for 15 min, and optical density at 550 nm was determined with a R5000 enzyme-linked immunosorbent assay plate reader (Dynatech, Guernsey, Great Britain). Kym-1 cells (1 × 104/well) were cultivated in 96-well
microtiter plates overnight. Next day TRAIL-M2 complex was titrated and
after additional 18 h of culture metabolic activity was measured
by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide method.
Immune Complex JNK Assay--
Following stimulation, cells
(2-3 × 106) were lysed in 1 ml of kinase lysis
buffer (200 mM Tris, pH 7.4, 5 mM
MgCl2, 1% Triton X-100, 150 mM NaCl, 0.1 mM phenylmethylsulfonyl fluoride, 1 µg/ml leupeptin, 2 mM Na3VO4, and 10 mM
NaF) for 20 min on ice. Cell debris was removed by centrifugation at
10,000 × g for 10 min at 4 °C. JNK was
immunoprecipitated with 0.5 µg of a rabbit polyclonal anti-JNK
antiserum (Santa Cruz Biotechnology) and 20 µl of protein A-Sepharose
beads for 2 h. Beads were washed three times with kinase lysis
buffer and twice in assay buffer (20 mM MOPS, pH 7.2, 10 mM EGTA, 10 mM MgCl2, 0.1% Triton
X-100, and 1 mM dithiothreitol). After the last wash, beads
were left in a 1:1 suspension, and kinase reactions were carried out at
room temperature for 20 min after addition of 0.5 µg GST-Jun(1-79)
and ATP (100 µM ATP and 5 µCi of
[ TRAIL/Apo2L Activates JNK--
It has been shown that JNK activity
is induced upon cross-linking of non-death domain-containing receptors
of the TNF receptor superfamily, e.g. CD40 (54), as well as
by stimulation of the death domain-containing receptors TNF-R1 and
Apo1/Fas (55-58). Accordingly, we were interested to know whether
TRAIL/Apo2L is also able to activate this pathway. For this purpose we
analyzed HeLa and Kym-1 cells, as both cell lines are susceptible
toward the cytotoxic action of TRAIL and should therefore express at least one of the two death domain-containing receptors of TRAIL. To get
first insights in the expression status of the various TRAIL receptors
in these cells, we analyzed the transcription level by RNase protection
assays and RT-PCR. Using a template set including TRAIL-R1, TRAIL-R2,
and TRAIL-R3-specific probes we found significant level of TRAIL-R1 and
TRAIL-R2 mRNA but no detectable amounts of TRAIL-R3 mRNA (Fig.
1A). Nevertheless, with RT-PCR
transcripts of all four TRAIL-R were detectable within a reasonable
number of cycles in RNA from both cell lines (Fig. 1B). For
analysis of JNK activation HeLa and Kym-1 cells were cultured for 0-5
h in the presence of an agonistic TRAIL-FLAG-M2 complex or TNF, and JNK
activity was determined by an immunocomplex kinase assay using an
amino-terminal (residues 1-79) c-Jun-GST fusion protein as a substrate
(Fig. 2A). A low basal level
of JNK activity was found in unstimulated cells, which was
significantly increased by treatment with TRAIL-FLAG-M2 complex as well
as TNF. However, whereas TNF induced the activation of JNK immediately and transiently for 10-30 min, TRAIL-FLAG-M2 complex-induced
activation of JNK could be discerned only after 2 h and reached a
plateau after around 3-4 h of stimulation (Fig. 2A). Next,
Apo1/Fas-mediated JNK activation was investigated in HeLa cells.
Because HeLa cells express only low levels of Apo1/Fas (data not
shown), we analyzed a HeLa transfectant stably overexpressing this
receptor. In these cells, cross-linking of Apo1/Fas using the agonistic
antibody anti-Apo1 resulted in a significant activation of JNK with
kinetics resembling that of TRAIL/Apo2L-induced JNK
activation (Fig. 2A). Interestingly, TRAIL-FLAG-M2 complex
induced apoptosis in Kym-1 but not in HeLa cells without further
treatment. Nevertheless, HeLa cells became sensitive for
TRAIL/Apo2L-induced cell death after addition of
cycloheximide (Fig. 2B). In contrast, TRAIL-mediated JNK
activation occurs in the absence of cycloheximide or other metabolic
inhibitors in both cell lines. In the presence of cycloheximide TRAIL-mediated JNK activation remained unaffected, whereby the basal
level of JNK activity was somewhat increased (data not shown). This
suggests that, at least in HeLa cells, activation of JNK by TRAIL/Apo2L
is not sufficient for induction of cell death. As expected, in both
cell lines TRAIL/Apo2L-induced cell death could be
inhibited by the addition of the caspase inhibitor z-VAD-fmk (Fig.
2B).
TRAIL/Apo2L-induced Activation of JNK Can Occur via a
Caspase-dependent and via a Caspase-independent
Pathway--
Induction of apoptosis by death domain-containing
receptors critically depends on the proteolytic activation of members
of the caspase family, as indicated by the ability of caspase
inhibitors, e.g. the cowpox viral protein cytokine response
modifier A (CrmA) or the inhibitory peptide z-VAD-fmk, to prevent cell
death. In addition, it has been demonstrated that a dominant negative
mutant of the FADD-associated caspase-8 also counteracts the apoptotic signaling of the death domain receptors (13, 14, 16, 29, 35). Moreover,
for Apo1/Fas, the involvement of caspases in the activation of JNK and
the p38 mitogen-activated protein kinase has been clearly demonstrated
(56, 57, 59). To unravel a possible regulatory role of caspases in
activation of JNK by other members of the TNF receptor superfamily, we
stimulated HeLa and Kym-1 cells with TRAIL-FLAG-M2 complex, anti-Apo1,
or TNF in the absence or presence of z-VAD-fmk. Interestingly, aside
from its inhibitory activity on the induction of apoptosis in both cell lines (Fig. 2B), z-VAD-fmk was able to completely block the
activation of JNK in HeLa cells induced by TRAIL/Apo2L and anti-Apo1,
but not TNF (Fig. 3, A and
C). However, in Kym-1 cells, no inhibitory effect of
z-VAD-fmk on TRAIL/Apo2L-mediated JNK activation was observed (Fig. 3, B and C). These data clearly
indicate that: (i) in HeLa cells, TRAIL/Apo2L and FasL on the one side
and TNF on the other side utilize different pathways for activation of JNK. (ii) TRAIL/Apo2L-induced JNK activation occurs in a
cell type-specific manner via two distinct pathways, of which one is cysteine protease-dependent and one cysteine
protease-independent. The dose dependence of the inhibitory effect of
z-VAD-fmk on JNK activation and induction of cell death by TRAIL is
nearly identical (data not shown). As the latter is mediated by the
caspase subgroup of the cysteine protease family, this argues also for
an involvement of caspases in TRAIL-mediated JNK activation in HeLa
cells.
TRAIL/Apo2L- and Apo1/Fas-mediated Cell Death Is Prevented by
Overexpression of GFP Stimulation of the TNF receptor superfamily members TNF-R1 (55),
CD40 (54), HVEM/ATAR (60), Apo1/Fas (56-58), the TRANCE receptor RANK
(61), as well as triggering of the interleukin-1R leads to activation
of the stress activated kinase JNK. Transient expression experiments
with members of the TRAF family, and dominant negative mutants derived
thereof, as well as studies with transgenic mice (62, 63), have
suggested that TRAF molecules are indispensable for JNK activation by
these receptors. Interestingly, the TNF receptor superfamily member
Apo1/Fas, representing the prototype of a death-inducing receptor, does
not associate with TRAFs, but is nevertheless able to induce JNK
activation (56-58, 64-66). Both receptors, TNF-R1, which uses TRAF2
for signaling, and Apo1/Fas, which signals TRAF independently, belong
to a death-inducing subgroup of the TNF receptor superfamily that is
defined by a common intracellular domain, called the death domain. It
was therefore of interest to define whether TRAIL/Apo2L, which binds to
two additional members of this receptor subgroup, is also able to
activate JNK and, if yes, whether TRAIL/Apo2L-induced JNK
activation resembles the TNF-R1 or the Apo1/Fas pathway. HeLa and Kym-1
cells are highly susceptible toward the cytotoxic action of a
TRAIL-FLAG-M2 complex (Fig. 2B) and express all four TRAIL
receptors, in particular the two recently cloned death
domain-containing receptors for TRAIL/Apo2L (Fig. 1). In addition,
cross-linking of the TRAIL receptor(s) in these cell lines resulted in
a stimulation of JNK (Fig. 2A). TRAIL-induced JNK activation
could be detected 1-2 h after stimulation, reaching a plateau after
3-4 h. Hence, the time course of JNK activation was clearly different
from the rapid and transient JNK stimulation by TNF (Fig.
2A), but resembled the sustained JNK activation found after
stimulation of Apo1/Fas (Fig. 2A). It has been reported that
both the overexpression of the serpin family protease inhibitor CrmA
and treatment with the cysteine protease inhibitor z-VAD-fmk can
interfere with Apo1/Fas-mediated p38 and JNK activation (56, 57, 59,
66). We therefore analyzed the effect of z-VAD-fmk on
TRAIL/Apo2L-induced activation of JNK.
TRAIL/Apo2L- and anti-Apo1-, but not TNF-induced, JNK activation was blocked by treatment with z-VAD-fmk (Fig. 3,
A and C) in HeLa cells. This suggests a
caspase-dependent pathway of TRAIL- and anti-Apo1-induced
JNK activation. In contrast, in Kym-1 cells, treatment with z-VAD-fmk
had a rather stimulatory effect on JNK activation (Fig. 3C).
Although at present we cannot rule out that the apparent stimulatory
effect of caspase inhibitors on TRAIL-induced JNK-activation in Kym-1
cells is just a consequence of protection from the apoptotic effect of
TRAIL, it is evident that JNK activation in Kym-1 cells does not
required caspases or other cysteine proteases.
Our data, showing a cell type-specific mode of JNK activation by TRAIL,
are in agreement with results described by different groups for
Apo1/Fas. While Cahill et al. (56) and Lenczowski et
al. (57) demonstrated caspase-dependent activation of
JNK by Apo1/Fas-mediated in Jurkat cells, Yang et al. (58)
found no caspase dependence of Fas/Apo1-induced JNK activation using transient transfection assays in 293 cells. In the case of TNF, the
induction of cell death via the caspase cascade and the
TRAF2-dependent activation of JNK by TNF-R1 bifurcate at
the level of the receptor-associated protein TRADD (43). Hence, it is
apparent that JNK activation is dispensable for TNF-induced cell death.
However, several reports point to a critical role of JNK activation for
induction of apoptosis by other stimuli (e.g. Refs. 67-69).
The question therefore arose whether JNK activation has an essential
function for TRAIL/Apo2L-induced cell death in the cellular
system studied here. Two recent studies have described the generation
of FADD We thank Ingolf Berberich, University of
Würzburg, Germany, for GST-Jun and Marcus Peter, Heidelberg,
Deutsches Krebsforchungszentrum, Germany, for anti-Apo1 antibody.
*
This work was supported by Deutsche Forschungsgemeinschaft
Grant Wa 1025/3-1.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
¶
To whom correspondence should be addressed. Tel.:
49-711-685-7446; Fax: 49-711-685-7484; E-mail:
harald.wajant{at}po.uni-stuttgart.de.
The abbreviations used are:
JNK, c-Jun
amino-terminal kinase; GFP, green fluorescent protein; TNF, tumor
necrosis factor; TRAIL, TNF-related apoptosis-inducing ligand; TRAIL-R, TRAIL receptor; TRADD, TNF-R1-associated death domain protein; FADD, Fas-associating protein with death domain; RIP, receptor-interacting
protein; TRAF, TNF receptor-associated factor; RT-PCR, reverse
transcription-polymerase chain reaction; MOPS, 4-morpholinepropanesulfonic acid; GST, glutathione
S-transferase; FACS, fluorescence-activated cell sorter.
TRAIL/Apo2L Activates c-Jun NH2-terminal Kinase (JNK)
via Caspase-dependent and Caspase-independent
Pathways*
,
,
,
,
,
, and
¶
Institute of Cell Biology and Immunology,
University of Stuttgart, Allmandring 31, 70569 Stuttgart, Germany and
the § Immunex Research and Development Corporation,
Seattle, Washington 98101
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ABSTRACT
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Abstract
Introduction
Procedures
Results
Discussion
References
FADD). In these cells, TRAIL-induced
cell death and activation of the apoptosis executioner caspase-8
(FLICE/MACH) and caspase-3 (YAMA, CPP-32, Apopain), that belong to
caspase subfamily of cysteine proteases, were abrogated, whereas JNK
activation remained unaffected and was still sensitive toward
z-VAD-fmk. Similar data were found in HeLa cells overexpressing
Apo1/Fas and GFP
FADD upon stimulation with agonistic antibodies.
These data suggest that cross-linking of the TRAIL receptors and
Apo1/Fas, respectively, engages a FADD-dependent pathway
leading to the activation of apoptotic caspases and, in parallel, a
FADD-independent pathway leading to the stimulation of one or more
cysteine proteases capable to activate JNK but not sufficient for the
induction of cell death.
![]()
INTRODUCTION
Top
Abstract
Introduction
Procedures
Results
Discussion
References
B and JNK (31, 32). In particular, it has been
shown that the lack of FADD in FADD
/
mice (33, 34) or
overexpression of a FADD molecule lacking the amino-terminal death
effector domain, which mediates association with the prodomain of
caspase-8, interferes with TNF-, Apo1/Fas-, and Wsl/DR3-mediated
apoptosis (6, 29, 35). This suggests that activation of the apoptotic
caspase cascade by these death domain-containing receptors requires
recruitment of the adaptor molecule FADD into the respective receptor
signaling complexes. As shown in detail for Apo1/Fas, FADD in turn
recruits caspase-8 to the death-inducing receptor signaling complex
leading to proteolytic activation of this proximal caspase and to
initiation of the apoptotic program (27, 28, 36, 37). Interestingly, all death domain-containing receptors are in principle able to activate
the transcription factor NF-
B (5, 7, 8, 13, 14, 38-40), which is
frequently associated with protection from apoptosis (41-45). In the
case of TNF-R1, the activation of NF-
B occurs via recruitment of RIP
and TRAF2 into the receptor signaling complex through association with
TRADD and subsequent activation of the TRAF-associated kinase,
NF-
B-inducing kinase. Subsequently, NF-
B-inducing kinase
phosphorylates the I
B kinases
and
(IKK
and IKK
),
leading to activation of NF-
B (46, 47). In contrast, the molecular
mechanisms involved in activation of NF-
B by other death
domain-containing receptors remain to be elucidated. The most recently
identified death domain-containing receptors DR4/TRAIL-R1 and
DR5/TRAIL-R2/TRICK/KILLER are receptors for the cytotoxic ligand
TRAIL/Apo2L (48, 49). Remarkably, TRAIL/Apo2L can also interact with
two additional nonapoptotic members of the TNF receptor superfamily,
the glycophospholipid-anchored cell surface protein DcRI/TRID/TRAIL-R3
(12, 15, 18, 50) and DcRII/TRAIL-R4 (51-52), a receptor containing a
truncated death domain that is still able to activate NF-
B (52).
Overexpression of these nonapoptotic TRAIL receptors protects mammalian
cells from TRAIL/Apo2L-induced cell death (12, 15, 18,
50-52), thereby defining a novel cell death control mechanism within
the TNF receptor superfamily. In this report, we demonstrate the
capability of TRAIL/Apo2L to induce JNK by a cysteine
protease-dependent and by a cysteine protease-independent
pathway. Moreover, using a dominant negative mutant of FADD as well as
the cysteine protease inhibitor z-VAD-fmk, we are able to show that, in
contrast to induction of apoptosis, the cysteine
protease-dependent mode of JNK activation is
FADD-independent. Hence, TRAIL/Apo2L has the capability to engage at
least three different pathways: two independent pathways with distinct
cysteine protease requirements leading to cell death or JNK activation and, in addition, a cysteine protease-independent pathway also linked
to JNK activation.
![]()
EXPERIMENTAL PROCEDURES
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Abstract
Introduction
Procedures
Results
Discussion
References
-32P]ATP. Reactions were stopped by adding 25 µl of
6-fold concentrated Laemmli buffer and boiling for 5 min. Samples were
resolved on SDS-polyacrylamide gel, transferred to nitrocellulose, and
analyzed using a PhosphoImager.
![]()
RESULTS
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Abstract
Introduction
Procedures
Results
Discussion
References

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Fig. 1.
Detection of TRAIL receptor
transcripts. A, RNase protection assay analysis of
steady state levels of TRAIL receptors in HeLa and Kym-1 cells. Whole
RNAs were isolated and 10 µg of each RNA was analyzed with the
hApo-3c Multi-Probe template set to detect caspase-8, FasL, Fas, FADD,
DR3, TRAIL-R1, TRAIL-R2, TRAIL-R3, TRAIL TNF-R1, TRADD, and RIP as well
as the internal controls L32 and glyceraldehyde-3-phosphate
dehydrogenase. Samples were resolved by electrophoresis on denaturing
polyacrylamide gels (5%) and analyzed by phosphorimaging.
B, detection of TRAIL receptor mRNA by RT-PCR. 1 µl of
oligo(dT)-primed cDNA was amplified with the indicated number of
cycles and analyzed by agarose gel electrophoresis. As a control for
cross-contaminations a reaction without template was performed. In
addition, control reactions with 1 µg of genomic DNA indicate that
residual genomic DNA does not interfere with the detection of specific
TRAIL receptor transcripts.

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Fig. 2.
TRAIL/Apo-2L induces the JNK pathway and
apoptosis. A, time course of JNK activation by members
of the TNF receptor superfamily. Cell lysates were prepared from HeLa,
HeLa-Fas, and Kym-1 cells, stimulated for the indicated times with TNF
(5 ng/ml), TRAIL-FLAG-M2 complex (200 ng/ml), and anti-Apo1 antibody
(50 ng/ml), respectively. JNK activity was measured by immunocomplex
kinase assay with GST-c-Jun(1-79) as substrate. B,
TRAIL/Apo-2L induces apoptosis. HeLa and Kym-1 cells were plated in
triplicates overnight at 37 °C in 96-well microtiter plates (HeLa,
1.5 × 104 cells/well; Kym-1, 1 × 104 cells/well). Next day the cells were treated for
additional 16 h with various concentrations of TRAIL-FLAG-M2
complex in the presence or absence of 20 µM z-VAD-fmk. In
addition, HeLa cells were treated with 2.5 µg/ml cycloheximide.
Viable HeLa cells were quantified by staining with crystal violet, and
viable Kym-1 cells were quantitated using the
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide
assay.

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Fig. 3.
TRAIL/Apo-2L-mediated JNK activation can
occur both in a caspase-dependent and a caspase-independent
manner. A and B, HeLa (A) and
Kym-1 (B) cells were treated with the indicated
concentrations of TNF, TRAIL-FLAG-M2 complex, and anti-Apo1 alone or in
combination with z-VAD-fmk (20 µM) for 20 min
(TNF) or 4 h (TRAIL-M2 complex,
anti-Apo1). JNK activity was measured using an immunocomplex
kinase assay with GST-c-Jun(1-79) as substrate. C, after
preincubation with the indicated concentrations of z-VAD-fmk for 3 h, HeLa cells were treated with 200 ng/ml TRAIL-FLAG-M2 complex
for an additional 4 h. JNK activity was then determined as
described in the legend to Fig. 1.
FADD, but JNK Activation Still Occurs in a
Caspase-dependent Manner--
From the data described so
far it was unclear whether TRAIL-R- and Apo1/Fas-mediated JNK
activation in HeLa cells occurs via the apoptotic caspase cascade or
via a distinct pathway with other cysteine protease requirements. As
mentioned above, several recent reports have demonstrated that
overexpression of a dominant negative mutant of FADD, which lacks the
amino-terminal death effector domain and is thus unable to recruit
caspase-8 to receptor signaling complexes, efficiently abrogates
apoptotic signaling induced by death domain-containing receptors (13,
14, 16, 29, 35, 53). We therefore analyzed JNK activation in HeLa cells
stably transfected with a GFP-tagged version of the dominant negative mutant of FADD, GFP
FADD (Fig.
4A). These cells are
completely resistant against TRAIL/Apo2L-induced cell death
(Ref. 53; Fig. 4B). However, as shown in Fig. 4C,
in these cells the TRAIL-FLAG-M2 complex still induced JNK activation
in a cysteine protease-dependent manner. As GFP
FADD
prevented activation of the apoptotic caspases, caspase-3 and caspase-8
(Fig. 4D), these data strongly argue for a TRAIL-initiated
pathway that leads to activation of FADD-independent caspases, which in
turn initiate JNK activation. Moreover, as shown in Fig.
5 for HeLa cells stably transfected with
Apo1/Fas and GFP
FADD (Fig. 5A), the concept of different
caspase(s) requirements of JNK activation and apoptosis also holds true
for Apo1/Fas (Fig. 5, B and C).

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Fig. 4.
HeLa cells stably overexpressing GFP
FADD
are protected against TRAIL/Apo2L-mediated cell death but are still
able to activate JNK via a caspase-dependent pathway.
A, GFP and GFP
FADD expression was analyzed in stable HeLa
transfectants by FACS analysis and immunoblotting. FACS analysis was
performed with living cells without further treatment. For
immunoblotting GFP and GFP
FADD were immunoprecipitated from
cytosolic extracts of 1.5 × 106 cells using a rabbit
polyclonal IgG fraction against GFP, transferred to nitrocellulose, and
detected with a GFP-specific monoclonal antibody and an alkaline
phosphatase-conjugated secondary antibody. B,
HeLa-GFP
FADD cells are resistant toward
TRAIL/Apo2L-induced cell death. 1.5 × 104
cells/well were cultured in a 96-well microtiter plate overnight. The
cells were then incubated for additional 16 h with the indicated
concentrations of TRAIL-FLAG-M2 complex in the presence of 2.5 µg/ml
cycloheximide. Viable cells were quantified after staining with crystal
violet. C, HeLa-GFP
FADD cells were treated with the
indicated concentrations of TRAIL-FLAG-M2 complex alone or in
combination with z-VAD-fmk (20 µM) for 4 h. JNK
activity was measured in an immunocomplex kinase assay using
GST-c-Jun(1-79) as a substrate. D, GFP
FADD prevents
activation of caspase-3 and caspase-8. 2 × 106 HeLa,
HeLa-GFP, and HeLa-GFP
FADD cells were treated with TRAIL-FLAG-M2
complex (200 ng/ml) for 5 h in the presence or absence of
cycloheximide (1 µg/ml). Protein lysates and immunoblotting of
proteins were performed as described under "Experimental
Procedures." Blots were probed with a polyclonal IgG fraction
directed against caspase-3 or a monoclonal caspase-8-specific
antibody.

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Fig. 5.
HeLa-Fas cells stably overexpressing
GFP
FADD are protected against anti-Apo1-induced cell death but are
still able to activate JNK by a caspase-dependent
pathway. A, GFP
FADD expression was analyzed in
stable HeLa-Apo1 transfectants by FACS analysis and immunoblotting as
described previously in the figure legend to Fig. 3. B,
HeLa-Fas-GFP
FADD cells are resistant toward anti-Apo1-induced cell
death. 1.5 × 104 cells/well were cultured overnight
in a 96-well microtiter plate. The cells were then incubated for
additional 16 h with 100 ng/ml anti-Apo1 in the presence of 2.5 µg/ml cycloheximide. Viable cells were quantified after staining with
crystal violet. C, HeLa-Fas and HeLa-Fas-GFP
FADD cells
were treated with the indicated concentrations of anti-Apo1 monoclonal
antibody alone or in combination with z-VAD-fmk (20 µM)
for 4 h. JNK activity was measured in an immunocomplex kinase
assay using GST-c-Jun(1-79) as a substrate.
![]()
DISCUSSION
Top
Abstract
Introduction
Procedures
Results
Discussion
References
/
mice (33-34). In one of these studies it has been shown
in embryonic fibroblasts that the lack of FADD interferes with TNF-R1,
Apo1/Fas, and DR3, but not with TRAIL-R1-induced apoptosis (33). This
argues for a FADD-independent mechanism of TRAIL-R1-induced apoptosis;
however, it is still unclear whether FADD is involved in
TRAIL-R2-induced apoptosis as suggested by some studies based on
transient transfection assays. We have shown recently that HeLa cells
expressing a dominant negative GFP
FADD fusion protein are completely
resistant toward induction of apoptosis by TNF, agonistic
Apo1/Fas-specific antibodies, and TRAIL (53). This indicates that FADD
itself or a FADD-related factor is involved in TRAIL-induced apoptosis
in these cells. More important for this study, TRAIL-mediated
processing of the apoptotic caspase-8 and caspase-3 (Fig.
4D) is blocked by overexpression of GFP
FADD. As JNK
activation by TRAIL/Apo2L is not impaired in the GFP
FADD-expressing
cells, but is still z-VAD-fmk-sensitive (Fig. 4C), the
cysteine protease(s) involved in this process must be distinct from
caspase-3 and caspase-8. Moreover, in HeLa cells, TRAIL-induced
processing of caspase-3 and caspase-8, and therefore induction of cell
death, only occur in the presence of cycloheximide, whereas JNK
activation takes place without further treatment. Together, these data
suggest (i) that a FADD-dependent as well as a
FADD-independent pathway of cysteine protease activation exist and (ii)
that both pathways activate different subsets of cysteine protease, one
not sufficient to induce apoptosis but essential for JNK activation and
one dispensable for JNK activation but involved in apoptosis (Fig.
6). Transcripts of all known TRAIL receptors containing an intracellular domain (TRAIL-R1/DR4,
TRAIL-R2/DR5, TRAIL-R4) have been easily detected in HeLa and Kym-1
cells using RNase protection assays and/or RT-PCR (Fig. 1). It is
therefore rather unlikely that the cysteine
protease-dependent and cysteine protease-independent modes
of JNK activation in HeLa and Kym-1 cells reflect the utilization of
distinct TRAIL receptors. Moreover, the similarities to the Apo1L/Apo1
system rather point to distinct cellular pathways involved in JNK
activation, because triggering of Apo1/Fas, the only known receptor for
Apo1L, also results in cell type-specific,
caspase-dependent and caspase-independent activation of
JNK. At least in HeLa cells, TRAIL-R- and Apo1/Fas-mediated JNK
activation is independent of FADD. How this pathway is molecularly linked to the respective receptors remains to be elucidated. Two possible candidates are the recently cloned molecules RAIDD and DAXX,
proteins that are involved in TNF-R1 and Apo1/Fas signaling. However,
overexpression of dominant negative forms of these molecules in HeLa
cells exerted no effect on apoptosis or JNK activation initiated by
TRAIL/Apo2L (data not shown).

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Fig. 6.
Model of TRAIL/Apo2L-induced pathways leading
to JNK activation and apoptosis.
![]()
ACKNOWLEDGEMENTS
![]()
FOOTNOTES
![]()
REFERENCES
Top
Abstract
Introduction
Procedures
Results
Discussion
References
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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