Biomedical Engineering Reference
In-Depth Information
an N-terminal coiled-coil (CC) region and the TRAF-C domain, the latter conferring
binding to upstream molecules [43].
In some cases, TRAFs bind directly to the intracellular domain of the respective
TNFR, e.g., CD40, while in others additional adaptors are required, such as TRADD
in case of TNFRI [44]. Tumor necrosis factor receptor associated death domain
(TRADD) not only recruits TRAFs, specifically TRAF2 and TRAF5, but also another
molecule named RIP1 (receptor interacting protein) [45]. RIP1 contains different
structural motifs including a death domain, controlling homotypic interaction with
TRADD, a so-called RHIM (RIP homotypic interaction motif) and a kinase domain.
Based on sequence similarities in the kinase domain, three other RIP family members
have been cloned and three more were found by database homology searches [46].
However, thus far, only RIP1 is known to be required for TNFR-dependent NF-
B
activation [47,48]. When overexpressed, all these molecules, TRADD, RIP1, and
TRAF2/5 activate NF-
κ
B activation was confirmed genetically for
some of these molecules. RIP1 knockout cells fail to activate IKK (and JNK1/2) in
response to TNF
κ
B. A role in NF-
κ
and TRAF2-deficient cells show reduced levels of IKK and almost
no jun N-terminal kinase (JNK)1/2-activation [48,49]. Residual NF-
α
B activation in
TRAF2 knockout cells might be due to compensation through other TRAF members,
as TRAF2/TRAF5 double knockout cells show negligible TNF
κ
-dependent IKK
activity [50]. Notably, the kinase activity of RIP1 is dispensable for NF-
α
B (and
JNK1/2) activation, as reconstitution of RIP1-deficient cells with a kinase dead RIP1
mutant confers full responsiveness [47]. Therefore, to some extent all of the above
mentioned proteins only postpone the problem of the lack of enzymatic activity of
the TNFR family members, when trying to understand how IKK is activated. Fur-
thermore, the mechanisms by which these molecules act are only partially defined.
TRAF2 has been demonstrated to recruit the IKK complex to the activated TNFRI
via interaction with IKK
κ
leucine zipper motifs [51]. This initial recruitment of
IKK seems not to depend on RIP1. However, it triggers RIP1-dependent binding to
NEMO and activation of the IKK complex [52]. As RIP1 is known to be ubiquitinated
during TNFR1 activation and TRAF2 has been found to be critically involved in this
process [53], it is possible that TRAF2-dependent ubiquitination of RIP1 is involved
in formation of a stable supramolecular complex between TRAF2, RIP1, and IKK.
Although induced proximity of IKK catalytic subunits within this signaling
complex might explain IKK activation, there are two other molecules, mitogen
activated protein (MAP) extracellular signal regulated protein kinase (ERK) 3
(MEKK3) and transforming growth factor-
α/β
)-activated kinase 1 (TAK1),
that have been demonstrated to play a role in IKK activation.
When overexpressed, MEKK3 and TAK1 induce NF-
β
(TGF-
β
B activation, the response
to TAK1 being dependent on coexpression of its associated adaptor proteins, that
is, TAB1, TAB2, or TAB3 [54-57]. Data from gene deficient cells have been pub-
lished for MEKK3 and for TAK1 [58-61]. Mice deficient in either gene die during
embryogenesis, MEKK3
-/-
embryos die around embryonic (E) day 11, exhibiting
defects in blood vessel development [62], while TAK1
-/-
embryos die around E10,
exhibiting developmental defects of head fold and neural tube [60]. Unfortunately,
these distinct phenotypes do not contribute to our understanding of the roles of the
two kinases in NF-
κ
κ
B signaling, but suggest additional functions in other pathways.
