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* From Tularik Inc, South San Francisco, CA 94080.
Correspondence to: David V. Goeddel, PhD, Tularik Inc, Two Corporate Dr, South San Francisco, CA 94080; e-mail: goeddel{at}tularik.com
| Introduction |
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The availability of pure recombinant TNF permitted many studies of its biological properties in the late 1980s. TNF is now known to be a pleiotropic cytokine with many beneficial and harmful actions. In addition to its activities described above, it is a major mediator of immune regulation and inflammatory responses. It has antiviral, mitogenic, and numerous other activities.5 6 TNF entered clinical trials in the late 1980s as an anticancer agent, but its systemic toxicity has limited its anticancer efficacy to date. Recently, efforts have been directed at blocking the actions of TNF since inappropriate production of TNF is clearly associated with a number of disease states.6 7
| Two TNF Receptors |
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TNF-R1 and TNF-R2 are coexpressed on most cell types. Transfection
experiments, receptor-specific antibodies, and experiments with gene
knock-out mice have shown that TNF-R1 is the dominant signaling
receptor.13
14
TNF-R1 can signal apoptosis and activate
the transcription factor nuclear factor-kappa B (NF-
B). TNF-R2 can
directly signal certain activities in lymphocytes such as NF-
B
activation, and also plays an auxiliary role by helping deliver TNF to
TNF-R1.16
The intracellular domains (ICDs) of the TNF receptor family do not specify enzymatic activity and are largely unrelated to each other, with the primary amino acid sequences of the ICDs of the two TNF receptors being completely unrelated. However, the identification of several TNF-receptor-associated proteins and studies of their assembly into signaling complexes have advanced our knowledge in this area. Common themes for signaling by the entire superfamily have emerged from studies of the two distinct TNF receptor complexes.
| TNF Receptor 2 Signaling Complex |
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B activation in these
cells. These same activities are seen in response to human TNF in
stably transfected CT6 cells that express human TNF-R2. The study of
deletion mutants of TNF-R2 in CT6 cells resulted in the identification
of a 78 amino acid region in the ICD that was required for
signaling.17
Proteins of 45, 56, 66, and 68 kd were found
to bind to this region of TNF-R2 and were purified by affinity
chromatography.17
18 The 45- and 56-kd proteins were called TRAF1 and TRAF2, respectively.17 Protein sequencing and complementary DNA cloning experiments revealed that TRAF1 and TRAF2 were novel proteins that contained a conserved C-terminal "TRAF" domain of about 230 amino acids. The TRAF domain can be further divided into two subdomains, TRAF-N and TRAF-C, which share approximately 33% and 63% sequence identity, respectively. TRAF1 and TRAF2 can form both homodimers and heterodimers through their TRAF domains, but only TRAF2 interacts directly with TNF-R2.17 This interaction occurs via the TRAF-C subdomain.19 20 Thus, our initial purification of TRAF1 was a consequence of the TRAF2-TRAF1 heterodimer associating with TNF-R2. Northern blots demonstrate that TRAF2 expression is ubiquitous, whereas TRAF1 is expressed in a tissue-specific fashion. Therefore, depending on cell type, either a TRAF2-TRAF2 or a TRAF1-TRAF2 complex can be found in association with TNF-R2.
The 68- and 66-kd proteins that were affinity purified were given the names cellular inhibitor of apoptosis protein 1 and 2, or c-IAP1 and c-IAP2, respectively.18 These proteins are structurally similar to baculoviral inhibitors of apoptosis proteins (IAPs), which are only about 30 kd in size, but like the mammalian c-IAPs, contain a C-terminal RING finger and an N-terminal baculovirus IAP repeat (BIR) motif. Since baculovirus IAPs prevent apoptosis of virally infected insect cells,21 it is likely that the mammalian c-IAPs are also involved in providing protection against cell death. c-IAP1 messenger RNA is expressed in all tissues examined, whereas the expression of c-IAP2 is expressed preferentially in cells of the immune system.18 Neither c-IAP1 nor c-IAP2 interacts with TNF-R2 directly. The BIR motifs of the cIAPs interact with the TRAF-N domains of both TRAF1 and TRAF2. The TRAF1-TRAF2 heterodimer recruits both c-IAP1 and c-IAP2 to the TNF-R2 complex.18
TNF-R2 is capable of signaling NF-
B activation in some cell
types. Therefore, we overexpressed the four individual proteins in
human 293 cells in an attempt to mimic TNF-induced receptor
aggregation. Only TRAF2 was found to activate NF-
B when
overexpressed. A TRAF2 mutant lacking its N-terminal RING finger
behaved as a dominant negative mutant and acted as an inhibitor of
TNF-R2 mediated NF-
B activation.19
TRAF2 can also
activate the signaling pathway leading to activation of the Jun
N-terminal kinase (JNK).22
TNF-R2, TRAF1, TRAF2, and the c-IAPs form a TNF-R2 complex when overexpressed in a process that is TNF independent.18 To determine whether the TRAFs and c-IAPs are associated with TNF-R2 under physiologic conditions, endogenous TNF-R2 was examined by immunoprecipitation in nontransfected CT6 cells before and after TNF treatment. The TNF-R2 immunocomplex contained TRAF2 only after TNF treatment (unpublished results). Since TRAF2 is required for other components of the TNF-R2 complex to interact with the receptor, the entire signaling complex (Fig 1) must be assembled in a ligand-dependent process.
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| TNF Receptor 1 Signaling Complex |
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B
activation.23
24
A search for proteins that interact with
the ICD of TNF-R1 resulted in the identification of TNF
receptor-associated death domain (TRADD), a 34-kd cytoplasmic protein
that contains a C-terminal death domain with 30% sequence identity to
the death domain of TNF-R1.24
TNF-R1 and TRADD interact
via their respective death domains, which are also capable of
self-association. Overexpression of TRADD induces apoptosis and
activates NF-
B. TRADD and TNF-R1 are not preassociated, but interact
following TNF-mediated receptor aggregation.25
Additional components of the TNF-R1 apoptosis and NF-
B activation
pathways were identified by two-hybrid screens utilizing TRADD as
"bait." The proteins TRAF2, Fas-associated death domain (FADD), and
receptor interacting protein (RIP) were obtained. Each of these
proteins appears to play a role in TNF-R1 signaling.25
26
In addition, c-IAP1 is also a component of the TNF-R1 complex as it is
present in immunoprecipitates of TNF-R1.27
A proposed role
for each of these proteins in TNF-R1 signaling is discussed below.
TRAF2
The N-terminal domain of TRADD interacts with the TRAF-C
domain of TRAF2, and the death domain of TRADD interacts with the death
domain of TNF-R1.25
These results suggested that TRADD
might function as an adapter protein that could simultaneously bind
TRAF2 and TNF-R1. This prediction has been confirmed in both
overexpression25
and endogenous systems.27
The presence of TRAF2 in the TNF-R1 complex suggested that it may play
a similar role in TNF-R1 and TNF-R2 signaling. Overexpression of an
N-terminal deletion mutant of TRAF2 inhibited NF-
B activation by
TNF-R1, but had no effect on apoptosis.25
These results
suggested that TRAF2 plays a role in NF-
B activation by both TNF
receptors, but it is probably not involved in the signaling of
apoptosis.
Gene targeting was performed to generate mice lacking
TRAF2.28
These mice appeared normal at birth, but became
runted and died prematurely. Thymocytes and fetal liver hematopoietic
progenitors were sensitive to TNF-induced cell death. TRAF2-deficient
embryonic fibroblasts had defective JNK activation, but near normal
NF-
B activation in response to TNF treatment. These results
demonstrated that a TRAF2-independent pathway exists for TNF-induced
NF-
B activation.
FADD
FADD is a cytoplasmic death domain-containing protein that
interacts with the death domain of Fas, a member of the TNF receptor
superfamily.29
30
FADD also has an N-terminal death
effector domain. The death effector domain of FADD recruits the
cysteine protease caspase-8 to the Fas signaling complex where
caspase-8 is activated and initiates Fas-mediated
apoptosis.31
32
33
The death domains of TRADD and FADD interact efficiently. FADD does not
interact directly with TNF-R1, but is recruited by TRADD to the TNF-R1
complex when all three proteins are overexpressed.25
This
suggests that death domains can form complexes larger than dimers, and
that FADD may be involved in TNF-R1 mediated signaling. A deletion
mutant of FADD lacking its N-terminal 79 amino acids is a potent
inhibitor of TNF-R1-mediated cell death, but has no effect on
TNF-R1-mediated NF-
B activation, indicating that FADD is not a part
of the latter pathway.25
Despite considerable effort, we
were unable to demonstrate that FADD is a part of the TNF-R1 complex in
nontransfected cells (unpublished results). To address this question,
we generated FADD-deficient mice. These mice died before 11.5 days of
gestation.34
However, FADD null mutant embryonic
fibroblasts were examined. TNF-R1 activation did not induce apoptosis
in these cells, demonstrating that FADD is required for TNF-mediated
apoptosis.
RIP
RIP is a cytoplasmic serine/threonine protein kinase that contains
a C-terminal death domain. It was originally described as a
Fas-interacting protein.35
Our characterization of RIP
suggests that it does not interact with Fas in mammalian
cells.26
Rather, TRADD interacts strongly with RIP and
recruits RIP to the TNF-R1 complex. Following TNF treatment, RIP is
recruited to the TNF-R1 complex in nontransfected cells.26
RIP activates NF-
B26
and JNK22
when
overexpressed. Both activities seem to reside in the kinase and
intermediate domains.22
26
The C-terminal death domain of
RIP is a very effective dominant negative inhibitor of TNF-induced
NF-
B and JNK activation. These data suggested a model in which RIP
is required for TNF-mediated activation of NF-
B and JNK. However,
gene targeting experiments demonstrated that RIP deficiency results in
defective TNF-induced NF-
B activation but does not affect
TNF-induced JNK activation.36
c-IAP1
Following TNF treatment, the endogenous TNF-R1 complex from HeLa
and U937 cells contains c-IAP1.27
Prior to TNF-treatment,
c-IAP1 is found complexed with TRAF2, suggesting that TRAF2 recruits
c-IAP1 to TNF-R1. Reconstitution experiments confirm that c-IAP1
recruitment to TNF-R1 requires both TRADD and TRAF2.27
The
role of c-IAP1 in TNF-R1 signaling remains uncertain at this time.
However, overexpression of c-IAP1 and c-IAP2 together blocks
TRADD-mediated apoptosis (unpublished results).
SODD
Since the death domain of TNF-R1 can self-associate, a mechanism
must exist that prevents unregulated receptor aggregation and
signaling. Yeast two-hybrid interaction cloning experiments identified
silencer of death domains (SODD) as a 60-kd protein that performs this
function.37
SODD directly interacts with the death domain
of TNF-R1 under normal physiologic conditions. When TNF-R1 is bound to
SODD, it cannot simultaneously interact with TRADD. A direct signaling
role of SODD has not yet been demonstrated.
Assembly of the TNF-R1 Signaling Complex
In our model for the TNF-R1 signaling complex, TNF-R1 is
associated with SODD prior to its interaction with TNF. Following TNF
treatment, TNF-R1 trimerizes and SODD is released from the
receptor.37
The death domains of TNF-R1 then associate and
stabilize the aggregated receptor. TRADD, which normally exists as
either a monomer or dimer, is thus able to associate with the death
domain of TNF-R1. This interaction allows TRADD to become an adapter
protein that binds and recruits other signaling proteins. The
N-terminus of TRADD recruits TRAF2 and the death domain of TRADD
recruits RIP and FADD to the TNF-R1 complex. Each of these three
recruited proteins activates a distinct signaling pathway. A model of
the assembly process for the TNF-R1 complex is shown in Figure 2
.
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| Conclusions |
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| References |
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B by TNF receptor 2 and CD40. Science 269,1424-1427
B activation prevents cell death. Cell 87,565-576[CrossRef][ISI][Medline]
B Activation. Cell 81,495-504[CrossRef][ISI][Medline]
B activation, and increased sensitivity to TNF-induced cell death in TRAF2-deficient mice. Immunity 7,715-725[CrossRef][ISI][Medline]
B signal. Immunity 8,297-303[CrossRef][ISI][Medline]
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