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INTRODUCTION |
The endoplastic reticulum
(ER)1 is sensitive to
alterations in homeostasis from a variety of different stimuli, such as
glucose deprivation, perturbation of calcium homeostasis, and exposure to free radicals. Under such conditions, perturbation of protein folding and the accumulation of malfolded proteins in the ER induce ER
stress (1).
ER stress elicits two major cellular-protecting responses. One is the
attenuation of protein synthesis, and the other is the up-regulation of
genes encoding chaperones that facilitate the protein folding process
in the ER known as the unfolded protein response (UPR). Both responses
reduce the accumulation and aggregation of malfolded proteins in the
compartments of the cells (1).
IRE1
(2) and IRE1
(3) are believed to be ER stress sensor
proteins and play important roles in transducing the stress signals
initiated by the accumulation of malfolded proteins from the ER to the
cytoplasm and nucleus. IRE1s are known to participate in the UPR and
control the expression of ER molecular chaperones. When cells are
exposed to excess levels of stimuli causing ER stress, apoptotic
signals are transduced from the ER and promote apoptotic cell death. It
is reported that c-Jun N-terminal kinases (JNKs) are activated by the
accumulation of malfolded proteins in the ER (4). JNKs constitute a
family of signal transducers that are activated by a variety of
exogenous stimuli, such as growth factor deprivation, Fas or
Tumor necrosis factor
(TNF
) treatments (5, 6), anticancer drug
treatments, and UV light irradiation. JNKs regulate gene expression
through the phosphorylation and activation of transcription factors
such as cJUN or the activator protein-1 family (7). The
activation of JNK requires TNF receptor-associated factor 2 (TRAF2), a
member of the TRAF family of proteins, which transduce signals from
IREs. In addition, dominant-negative TRAF2, which is truncated in the
N-terminal RING effector domain of TRAF2, inhibits the activation of
JNK by signals from IRE1s (4).
During apoptosis induced by ER stress, caspase-12 is localized to the
ER and is activated (8). The activation of caspase-12 is not mediated
by other stimuli. Furthermore, it has also been reported that
caspase-12-deficient mice are resistant to ER stress-induced apoptosis,
but their cells are led to apoptosis in response to other stimuli (8).
Although it has been shown that caspase-12 is activated during ER
stress-induced apoptosis, the mechanisms of its activation by ER
apoptotic signals are still unknown, and even less is known about how
TRAF2 can transduce ER stress signals from IRE1s to its downstream
signaling events. To address these issues in this study, we describe
the identification and the characterization of interactions among the
ER stress-associated molecules, and we present a picture of how they
are coupled to the activation of this apoptotic signal cascade.
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EXPERIMENTAL PROCEDURES |
Cell Lines, Reagents, and Antibodies--
Human embryonic kidney
293T (HEK293T) cells were grown in Dulbecco's modified Eagle's medium
(Life Technologies, Inc.) supplemented with 10% fetal calf serum, 100 units/ml of penicillin, and 100 mg/ml streptomycin at 37 °C in a 5%
CO2 incubator and were used in all experiments. cDNA
plasmids encoding human IRE1
and human IRE1
were kindly provided
by Dr. R. J. Kaufman and Dr. D. Ron, respectively. Human JNK
inhibitory kinase (JIK), human TRAF2, and mouse procaspase-12
expression vector were obtained by reverse transcriptional polymerase
chain reaction in our laboratory. The procaspase-12 expression
vector-fused HA or FLAG tag in its C-terminal were also engineered.
Rabbit polyclonal anti-caspase-12 antibody was originally raised
against recombinant polypeptide. The other antibodies were purchased
from manufacturers, such as anti-FLAG antibody (Sigma), anti-HA epitope
antibody, phosphospecific anti-JNK antibody, anti-JNK1 antibody, and
anti-TRAF2 antibody (Santa Cruz Biotechnology, Santa Cruz, CA). In all
of the experiments, typical results from at least three repeated
experiments are shown.
Yeast Two-Hybrid System--
To analyze the function of the IRE1
cytosolic domain in the apoptotic pathway, we employed a yeast
two-hybrid system to search for proteins that interact with IRE1. Yeast
cells, strain Y190, were transformed with an expression vector encoding
the GAL4 DNA-binding domain combined with the IRE1 kinase domain. Then
the yeast-expressing bait protein was transformed with an expression
vector containing the cDNA library fused to its DNA activation
domain. We used Matchmaker cDNA library (mRNA sources: normal
whole brain pooled from nine spontaneously aborted male/female
Caucasian fetuses aged 20-25 weeks, 3.5 × 106
independent transformants, CLONTECH). Screenings
were performed according to the manufacturer protocol. As positive and
negative controls, we used packaged transformants supplied with a
screening system.
In Vitro Binding Assays--
HEK293T cells were transfected with
plasmids using LipofectAMINE 2000 transfection reagents (Life
Technologies, Inc.) according to the manufacturer protocols. The total
amount of transfected plasmid DNA was adjusted to the same levels
within each individual experiment. Cells were harvested at 24 h
after transfection and lysed with 0.2% Nonidet P-40 in Dulbecco's
modified Eagle's medium-phosphate-buffered saline. Subsequently, 1 mg
of soluble protein was incubated with 1 µg/ml antibody for 2 h
at 4 °C, and proteins captured with this antibody were
coprecipitated with G protein agarose (Life Technologies, Inc.).
Immunoprecipitates or cell lysates were loaded onto appropriate SDS-polyacrylamide gels, electrophoresed, and immunoblotted with antibodies for detections.
JNK/Stress-activated Protein Kinase Assay--
The activations
of JNK in HEK293T cells transfected with mock vector or various
expression vectors were examined at 3 h after treatment with or
without stimulations. These experiments were performed using a
stress-activated protein kinase/JNK assay kit (New England Biolabs).
The detections of phosphorylated JNK under various conditions were also
examined using cells transfected with JNK and various expression
vectors. The immunoprecipitation was performed with a phosphospecific
anti-JNK antibody followed by Western blotting with an anti-JNK1
antibody. Transfection, immunoprecipitation, and Western blotting were
performed essentially as described above.
Metabolic-labeling Experiments--
Metabolic labeling with
32Pi was performed as described in our previous
paper (9). HEK293T cells in 6-well plates were incubated at
37 °C for 3 h with or without ER stress after the addition of
32Pi. Radiolabeled lysates from each sample
were immunoprecipitated with anti-FLAG antibody or anti-TRAF2 antibody.
The immunoprecipitates were separated by 12% SDS-polyacrylamide gel
electrophoresis and subsequently electrotransferred onto polyvinylidene
difluoride filters (Millipore). The filters were then exposed to x-ray
film for the detection of 32P. To control for loading in
this procedure, the filters were stained with the specific antibody
after autoradiography.
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RESULTS |
Interactions among IRE1
, TRAF2, and JIK--
It is believed
that IRE1s sense ER stress through their lumenal domains (10, 11). The
cytosolic parts of IRE1s, the kinase domains and RNaseL domains, are
reported to transduce ER stress signals to the downstream events to
promote the transcription of ER molecular chaperones. These transduced
signals are known to mediate the increase of gene expressions, such as
ER molecular chaperones, prolyl peptidyl isomerases, and disulfide
exchange proteins (12, 13). In contrast, excess levels of stress in the
ER result in apoptosis (14, 15). We hypothesized that the cytosolic
portion of ER stress sensor molecules can activate not only the UPR
pathway but also the apoptotic-signaling pathway. To examine this
hypothesis, the yeast two-hybrid systems were employed to search for
genes that interact with the cytosolic kinase domain of IRE1
. As a
result of screening the human fetus brain cDNA library, JIK was
identified as a possible binding partner of IRE1
(data not shown).
It has been reported that JIK is a human STE20-related
serine/threonine kinase and that JIK activity is decreased upon
epidermal growth factor receptor activation, but it is not modulated by
other exogenous stimuli, such as UV irradiation, TNF-
, NaCl,
H2O2, and anisomycin treatments (16). However,
there is no information about JIK in the ER stress signaling. Because
JIK is implicated in the JNK signaling, we suspected that JIK also
interacts with TRAF2 and forms complexes with both IRE1s and TRAF2.
To demonstrate the interactions of these molecules in mammalian cells,
we performed coimmunoprecipitation experiments in HEK293T cells
cotransfected with various combinations of expression plasmids for
IRE1
-FLAG, JIK-HA, TRAF2, and mock control. Immunoprecipitations of
full-length JIK-HA with the anti-HA antibody and Western blot analyses
with anti-FLAG or anti-TRAF2 antibodies revealed that JIK could be
coimmunoprecipitated with both IRE1
-FLAG and TRAF2 (Fig.
1, A and B). The
results suggested that IRE1
, TRAF2, and JIK form complexes and might
influence the functions of one another. The same results were obtained
when the order of precipitation was reversed (Fig. 1, A and
B). During ER stress condition, it was reported that
activated IRE1s could recruit TRAF2 to the ER (4). We also
characterized the complex both in normal and the UPR-induced states.
Treatments of 1.0 µg/ml and 2.5 µg/ml tunicamycin for 3 h
resulted in no significant alteration of the bindings between JIK and
IRE1
compared with that of no treated control. On the other hand,
the complex formation between JIK and TRAF2 was facilitated by the same
stimulations as described above (data not shown). These observations
mean that TRAF2 is recruited to the JIK-IRE1
complex in response to
the ER stress.

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Fig. 1.
Association of JIK with
IRE1 and TRAF2. A,
interactions between IRE1 and JIK. Coimmunoprecipitation analyses
were performed as described under "Experimental Procedures."
Specific bindings between JIK and IRE1 are shown in the lanes from
coexpressed cell lysates. Molecular mass markers are indicated
on the left. B, interaction between JIK and
TRAF2. Coimmunoprecipitation experiments revealed interactions between
JIK and TRAF2.
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JIK Functions as a Regulator of the JNK-signaling Pathway during ER
Stress--
It is well known that ER stress activates JNKs (4). We
speculated that JIK might play regulatory roles in JNK activation under
ER stress conditions. Therefore, we measured the relative levels of JNK
activity in cells that were treated with various manipulations by using
an immune complex kinase activity assay. After treatments with 2.5 µg/ml tunicamycin for 3 h, JNK was activated in 293T cells
transfected with mock vectors. Overexpression of JIK resulted in the
acceleration of JNK activation induced by treatment with tunicamycin.
Alternatively, transfection of catalytically inactive mutant
JIK(A181F183) (16) inhibited the activation of JNK by the same stress
(Fig. 2(a)). The cell lysates
were also immunoprecipitated with phosphospecific anti-JNK antibody.
Immunoprecipitated proteins were detected by immunoblotting analysis
using anti-JNK1 antibody. Transfection of wild JIK caused increases in
the amounts of phospho-JNK, which is known as an active form of JNK
during ER stress, but mutant JIK did not increase the amounts compared with the controls (Fig. 2(b)). The results were consistent
with those of the JNK kinase activity assay as described above.

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Fig. 2.
Modulations of JNK activation in HEK293T
cells in response to ER stress. 2 × 105 HEK293T
cells transfected with 0.5 µg expression vectors were treated with or
without 2.5 µg/ml tunicamycin (Tm) for 3 h.
Phosphorylation of c-Jun at Ser-63 was measured by Western blotting
using the phosphospecific anti-c-Jun antibody. The cell lysates were
also immunoprecipitated with anti-c-Jun antibody (a).
Activations of JNK were also detected by immunoblotting analysis using
the phosphospecific anti-JNK antibody. To control for loading, Western
blotting analyses of lysates were performed with anti-JNK1 antibody
(b). JIK promoted the activation of both JNK and c-Jun in
response to ER stress, and the overexpression of mutant JIK suppressed
these activations as much as did dominant-negative TRAF2. The relative
intensities of protein bands were determined using the NIH Image
software.
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It has been reported that the cytoplasmic portion of IRE1s binds to
TRAF2 that couples the ER stress sensor to JNK activation, and
dominant-negative TRAF2 inhibits ER stress-induced c-Jun/JNK activation
(4). As described above, we revealed that JIK activates the JNK pathway
under the ER stress conditions and interacts with IRE1
. Therefore,
it is not inconceivable that JIK affects the function of TRAF2 through
phosphorylation to regulate the signal transduction from IRE1s to JNKs
during ER stress. To investigate whether JIK influences TRAF2, we
examined the phosphorylation levels of TRAF2 with or without JIK
coexpression by the metabolic-labeling method using
32Pi. As a result, transfection of wild JIK did
not alter the expression levels of TRAF2, but the amounts of
phosphorylated TRAF2 were significantly increased (Fig.
3A). At the same time in
another coimmunoprecipitation experiment, we found that the
overexpression of JIK also promoted interactions between TRAF2 and
IRE1
(Fig. 3B). Taken together, these results suggested
that JIK alters the status of TRAF2 phosphorylation and that the
resultant-altered TRAF2 phosphorylation may change the interactions
between IRE1
and TRAF2 themselves that might regulate the activity
of JNK signaling under ER stress conditions.

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Fig. 3.
Implications of JIK in the ER
stress-signaling pathway. A, phosphorylation of TRAF2
increased by JIK coexpression. HEK293T cells were cotransfected with
various plasmids as indicated, and metabolic-labeling experiments were
performed as described under "Experimental Procedures" using
anti-TRAF2 antibody. Increased phosphorylation of TRAF2 could be
detected in the JIK-expressed cells compared with that in the untreated
cells. The cell lysates were Western blotted with anti-TRAF2 antibody
to verify similar levels of expression in all samples. B,
the enhancement of the interactions between TRAF2 and IRE1 by JIK
expression. HEK293T cells were transiently cotransfected with the
indicated amounts (in µg). We used antibodies in the indicated
combinations for coprecipitation and detection. The expression of JIK
strengthened the interactions between IRE1 and TRAF2.
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Interactions between TRAF2 and Procaspase-12--
It is still
unknown whether the activation of JNK signaling by TRAF2 during ER
stress is directly implicated in apoptosis. On the other hand, another
group reported that ER resident caspase-12, which is one of the
cystein-protease family, plays an essential role in ER stress-induced
cell death (8). Procaspase-12 contains the caspase recruitment domain
in its N-terminal region, which is known as a predicted prodomain (8).
Because caspase recruitment domain is known as the domain that
interacts with apoptosis-associated proteins, such as TRAF2, Apaf1, and
other caspases (17), we tried to examine whether procaspase-12 is able
to interact with TRAF2.
Total cell lysates from HEK293T cells transfected with procaspase-12
and/or TRAF2 expression vectors were immunoprecipitated using
anti-TRAF2 antibody and were immunoblotted by anti-caspase-12 antibody.
As shown in Fig. 4, procaspase-12
immunoreactive 60-kDa protein was detected only in TRAF2 and
procaspase-12-cotransfected cells (Fig. 4, lane 3). No bands
were detected in the cells transfected with either vector alone (Fig.
4, lanes 1 and 2). These results indicate that
TRAF2 directly associates with procaspase-12 under normal conditions.
On the other hand, treatments of TRAF2 and procaspase-12-cotransfected
cells with 2.5 µg/ml tunicamycin for 3 h inhibited the
interactions between these molecules (Fig. 4, lanes 4 and
5). The same findings, such as the reduction of the coprecipitated procaspase-12 by TRAF2 during ER stress, were observed when IRE1
was overexpressed (Fig. 4, lanes 6 and
7). Reverse experiments also showed similar consistent
results. We ascertained that the results documented above were not
attributed to the reduction of protein expression by using direct
Western blotting (Fig. 4, each lower panel).

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Fig. 4.
Altered interactions between TRAF2 and
procaspase-12 under ER stress conditions. HEK293T cells were
transfected with the indicated expression vectors and treated with or
without tunicamycin (2.5 µg/ml) for 3 h. Cell lysates were
immunoprecipitated with anti-FLAG or anti-TRAF2 antibodies. These
coprecipitated complexes were subjected to Western blotting using
anti-TRAF2 or anti-caspase-12 antibodies. ER stress signals reduced the
interactions between TRAF2 and procaspase-12. Each lower
panel shows Western blot to indicate the relative amount of
protein expressions in each sample.
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It is known that procaspase-12 is cleaved in response to ER stress (8).
We examined whether TRAF2 overexpression affected the cleavage of
procaspase-12. Our results showed that the coexpression of TRAF2 with
procaspase-12, in fact, emphasized the cleavage of procaspase-12 (Fig.
4(a)). This acceleration of the cleavage seems to be based
on the activations of procaspase-12 as initiated by TRAF2.
Caspase-12 Homodimerization--
Overexpression of procaspase-12
in cells encouraged the cleavage of caspase-12 itself (data not shown).
We hypothesized that after procaspase-12 was recruited to the ER stress
signal transducer in response to ER stress, this protease was activated
through homodimerization and cleavage by some protein such as calpain (18). Initially, we tried to demonstrate procaspase-12 dimerization through coimmunoprecipitation experiments. Procaspase-12-tagged HA
epitope at its C-terminal was coexpressed in 293T cells with procaspase-12-tagged FLAG at its C-terminal. After immunoprecipitation of procaspase-12 with a FLAG antibody, coprecipitating HA-tagged procaspase-12 was detected by Western blotting with an anti-HA antibody. As we expected, procaspase-12-tagged HA was coprecipitated only from the lysates expressing both the HA-tagged procaspase-12 and
the FLAG-tagged procaspase-12 (Fig. 5,
lane 3). The same results were obtained when the order of
precipitation was reversed (data not shown). This procaspase-12
homodimerization was increased by both tunicamycin treatment and
IRE1
expression (Fig. 5, lanes 4-7). Moreover, the
overexpressions of TRAF2 also reinforced this complex formation (Fig.
5, lanes 8 and 9). In addition, we found that
TRAF2 increased the level of phosphorylated procaspase-12 in
metabolic-labeling experiments (data not shown). These results revealed
that TRAF2 plays a significant function in the oligomerization and
phosphorylation of procaspase-12.

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Fig. 5.
Dimerization of procaspase-12 and its
cleavage. Homophilic associations of procaspase-12 were examined
by the coprecipitation experiments. These oligomerizations or
dimerizations of procaspase-12 were increased by various manipulations,
such as treatments with tunicamycin for 3 h, IRE1 , TRAF2, or
dominant-negative TRAF2 coexpressions. To verify similar levels of
procaspase-12 expression in all samples, the amounts of expression in
the cell lysates were analyzed by immunoblotting with anti FLAG
antibody (lower panel).
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Associations of Mutant TRAF2 with Procaspase-12--
TRAF2 can be
divided into four subdomains. The N-terminal RING finger and the
adjacent zinc finger motifs of TRAF2 are known to be required
for nuclear factor
B and JNK activation (19). The TRAFN, which forms
-helical coiled-coil structures, and the TRAFC subdomains
independently interact with TRAF2 and TRAF2-associated proteins such as
TNF-R2 and TRADD (20, 21). It is reported that the binding domain of
TRAF2 with IRE1s is the TRAFC domain (4). Thus, the distinct domains of
TRAF2 are involved in the recruitment of signaling molecules and in the
activation of downstream effectors.
The dominant-negative TRAF2 (RING finger deletion mutant, TRAF2-dR) was
reported to be able to inhibit the activation of JNK signaling in
response to ER stress (4). Consistent with this finding, the nuclear
factor
B activation ability of TRAF2 was also found to reside within
its N-terminal half (19). In contrast, the self-association of
procaspase-12 and its cleavage during ER stress was increased by
TRAF2-dR in comparison with the mock control but not as much as by the
full-length TRAF2 (Fig. 5). We also found that TRAF2 binds with
procaspase-12dependent TRAFN domain by deletion mutant analysis
(Fig. 6). The results suggested that the
structure of TRAFN domain plays an important part for the ER stress
signal transduction to caspase-12 activation. They also revealed that
the ER stress signal transductions to activate the caspase-12 and to
activate JNK pathway were mediated by distinct TRAF2 subdomains.
However, against our expectations, none of the TRAF2 deletion mutants
could get dominant-negative effects on the procaspase-12 activation
(data not shown).

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Fig. 6.
Mapping of TRAF2 domains required for
interaction with procaspase-12. A, expression vectors
for various deletion mutants of TRAF2. These mutants were tagged with
FLAG epitope at its N-terminal (TRAF2-dRZ, TRAF2-dRZC, and
TRAF2-dRZN) or its C-terminal (TRAF2-dNC and
TRAF2-dR). B, interactions between procaspase-12
and TRAF2 deletion mutants. These vectors were transiently
cotransfected with the procaspase-12 expression plasmid.
Coprecipitating FLAG-tagged mutant TRAF2s or procaspase-12 was detected
by immunoblot analyses. Mutant TRAF2s containing the TRAFN
domain bound with procaspase-12, but those lacking the TRAFN domain did
not interact with procaspase-12 (panels (a) and
(c)). Protein expressions in the cells were confirmed with
Western blotting analyses (panels (b) and
(d)).
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DISCUSSION |
IRE1s are known as ER stress transducers transmitting ER stress
signals from the ER to cytosol and to nuclei to fold malfolded proteins. In addition, IRE1s are also reported to activate the JNK-signaling pathway mediated by TRAF2 functions in response to the
perturbation of protein folding in the ER (4). Also, JNK has been
extensively demonstrated to be associated with apoptosis (22).
Therefore, different types of signals are thought to be transduced from
IRE1s, one a cell survival signal and another a death signal mediated
by TRAF2. In this study, we demonstrated that TRAF2 plays crucial roles
not only in the signaling of the JNK pathway, which is controlled by
JIK, but also in the activation of caspase-12 to transduce signals from
IRE1s under ER stress conditions.
We identified JIK as a binding partner of IRE1
using a two-hybrid
system. In mammalian cells, JIK had the potential to bind to both IRE1
and TRAF2, and the expression of JIK caused increases in the binding
amounts of IRE1 and TRAF2 in a dose-dependent manner. These
results suggested that JIK is implicated in the modification of
IRE1s-TRAF2 complex formation to regulate the transduction of the
signals that are initiated by perturbation of protein folding in the
ER. This hypothesis is supported by the findings that overexpression of
JIK increased JNK phosphorylation and JNK activities during ER stress
and that the catalytically overexpressed inactive mutated JIK inhibited
the activation of JNK in response to ER stress as much as the
dominant-negative form of TRAF2. As phosphorylated TRAF2 was
up-regulated in synchrony with the activation of JNK when JIK was
overexpressed, this JIK action on JNK signaling was possibly because of
increased phosphorylated forms of TRAF2. However, it is not known
whether TRAF2 is phosphorylated directly or indirectly by JIK.
Previously, JIK was reported to inhibit the JNK activation induced by
epidermal growth factor receptor (16). This finding is not consistent
with our present data that JIK promoted the activation of the
JNK-signaling pathway in response to ER stress. According to Tassi
et al. (16), any stimuli except treatment with epidermal
growth factor did not affect the JNK pathway, indicating that JIK may
play diverse roles in modulating the signaling upstream of JNK pathway
in response to various stimuli. However, at present the reasons for the
discrepancy in the function of JIK are unclear. To clarify the
mechanisms of the activation of JNK through JIK under ER stress
conditions, further analyses are needed including identification of
specific substrate(s) for JIK.
Caspase-12 is known to be essential for cell death induced by ER
stress. Indeed, procaspase-12 is cleaved, and the activated forms are
accumulated under ER stress conditions (8). However to date, the
mechanisms of the activation of caspase-12 in response to ER stress
have not been demonstrated. This study showed that TRAF2 plays an
essential role in the activation of caspase-12. In unstressed cells,
TRAF2 formed a stable complex with procaspase-12. The stimuli that
induce ER stress led to the dissociation of procaspase-12 from TRAF2,
and simultaneously dimerization (or oligomerization) of procaspase-12
was promoted. These findings raise the possible mechanisms that the
dissociation of TRAF2 from caspase-12 is a trigger for the activation
of caspase-12 during ER stress and that the resultant-free
procaspase-12 is clustered to the ER.
Several recent studies demonstrate that procaspases, such as caspase-2,
-8, and -10, can be activated through dimerization/oligomerization mediated through their prodomains. Specific adapter molecules are
reported to be able to interact with these procaspases. For example,
the prodomains of caspase-8 and -10 interact with the adapter molecule
Fas-associated death domain protein (23-25). In a similar
manner, caspase-2 is thought to bind the death receptor through the
adapter RAIDD (24). It is suggested that the primary role of these
adapter molecules may be to bring procaspase molecules into close
proximity with each other to enable dimerization. The mechanism
regarding the activation of procaspase-12 might be similar to these
recent reports. Although we could not show the recruitment of
procaspase-12 to IRE1s during ER stress in the present study, we cannot
negate the possibility that procaspase-12 is recruited to IRE1s before
oligomerization and that TRAF2 plays a role as an adapter molecule that
recruit procaspase-12. Further studies are needed to elucidate the more
detailed mechanisms responsible for the activation of caspase-12
focusing on the recruitment of procaspase-12 to IRE1s under ER stress conditions.
In conclusion, in this study we provide a missing link in the ER
stress-induced apoptosis-signaling pathway, which connects between the
stress sensor molecule IRE1 and the caspase-12. We demonstrate that
TRAF2 is a key mediator that transduces the signals from the ER to
cytosol during ER stress. Therefore, TRAF2 might become a target
molecule with which we can try to control ER stress-induced apoptosis.