Biomedical Engineering Reference
In-Depth Information
experiments are consistent with a role for TRAF6, together with TRAF2, in TCR
activation of IKK in Jurkat T cells, there are currently no genetic data to confirm a
role for TRAFs in TCR signaling in primary T cells.
4.7
UBIQUITIN-MEDIATED MECHANISMS FOR
TERMINATING NF-
κ
B ACTIVITY
To limit the potentially damaging effects of excessive production of proinflammatory
mediators, cells have evolved mechanisms to ensure that the duration of NF-
κ
B
activation is tightly controlled. The best characteristic of these is the NF-
κ
B-depen-
dent transcriptional induction of I
κ
B
α
[143,144,145], which enters the nucleus free
of associated NF-
κ
B [146]. I
κ
B
α
then dissociates NF-
κ
B from DNA and exports it
back to the cytoplasm, thereby turning off NF-
B-dependent transcription [147,148].
Recent research has demonstrated that several other important ubiquitin-dependent
cellular control mechanisms to terminate NF-
κ
κ
B activation exist, which are described
in Section 4.7 (see
Figure 4.2
).
4.7.1
D
E
-U
BIQUITINATION
OF
C
YTOPLASMIC
R
EGULATORY
P
ROTEINS
As discussed above,
in vitro
reconstitution experiments have identified a nondegra-
dative role for ubiquitination in the activation of the IKK complex [30]. This regu-
latory ubiquitination involves the attachment of K63-linked ubiquitin to target pro-
teins, including TRAFs (TRAF2 and TRAF6) [129] and NEMO [130], which then
trigger IKK activation. The physiological significance of this model of IKK activa-
tion was initially unclear due to its reliance on
in vitro
assays and the inhibitory
effects on NF-
B activation of overexpressed, catalytically inactive Ubc13. However,
strong genetic support for the hypothesis that K63-linked ubiquitination is important
in regulating NF-
κ
B activation was provided by the demonstration that cylindroma-
tosis (CYLD), a tumor suppressor protein linked to predisposition to cylindroma
benign skin tumors [149], downregulates NF-
κ
κ
B activation [150,151,152].
B activation was suggested when CYLD was identified
as NEMO-interacting protein and also in an RNA interference screen for deubiquit-
ination enzymes that could block NF-
A role for CYLD in NF-
κ
B activation [150,151,152]. CYLD contains
two sequence motifs similar to the cysteine and histidine boxes found in the UBP
ubiquitin-specific protease subfamily of deubiquitination enzymes [149]. Cell trans-
fection experiments indicate that CYLD facilitates the disassembly of the K63-linked
polyubiquitin chains on TRAF2, TRAF6, and NEMO but does affect K48-linked
ubiquitination of I
κ
-catenin [150,151,152]. As a consequence of this activity,
overexpressed CYLD impairs NF-
κ
B
α
or
β
κ
B activation by multiple agonists, including
TNF
and LPS [150,151,152]. Furthermore, CYLD knockdown by RNA
interference enhances NF-
α
, IL-1
β,
κ
B activation by IL-1
β
and LPS [153]. CYLD depletion
has been reported to have no effect on TNF
α
activation of NF-
κ
B. However,
JNK
is hyperactivated in CYLD-deficient cells following TNF
α
stimulation [153]. Con-
sistently, TNF
-induced polyubiquitination of TRAF2 has been shown to couple to
JNK, rather than NF-
α
κ
B activation [154].
