Biomedical Engineering Reference
In-Depth Information
is also the reason why it is still not clear whether, under physiological circumstances,
it is this form of IKK
α
that phosphorylates p100. Nevertheless, it is clear that the
alternative IKK
α
-dependent pathway is fully operational in the absence NEMO or
IKK
[37], demonstrating that formation of the classical trimolecular IKK complex
is not essential for p100 processing.
Notably, however, there are at least two examples, where catalytically active
IKK
β
as part of the classical NEMO-containing complex, seems to be required for
inducible I
α,
phosphorylation and degradation. The first one is IKK activation in
mammary gland epithelial cells, which proliferate in response to receptor activator
of NF-
κ
B
α
κ
B (RANK) ligand (RANK-L) in a NF-
κ
B-, cyclin D1-dependent manner
[38]. In wild-type (wt) cells, but not in IKK
-AA expressing cells, RANK-L induces
IKK activity, which can be isolated with NEMO-specific antibodies. This IKK
α
α
-
dependent activity regulates a classic NF-
B activation pathway that drives cyclin
D1 expression. This pathway is inhibited by expression of a nonphosphorylatable
form of I
κ
does not seem to be
required for proliferation of mammary epithelial cells (unpublished observation).
These data indicate that IKK
κ
B
α
(so-called superrepressor) [38]. Curiously, IKK
β
α
, in a complex with NEMO can regulate classic “I
κ
B-
sensitive” NF-
κ
B activity in a cell type specific manner. The second example of
IKK
B pathway is signaling by the TNF receptor
family members cluster of differentiation 27 (CD27) and CD40 [39]. Based on
experiments with a human B cell line, both receptors used the upstream kinase
NF-
α
being required for the classic NF-
κ
B pathways
[39]. RNAi-based knock-down experiments indicated that both of these pathways
are also IKK
κ
B inducing kinase (NIK) to activate the classic and alternative NF-
κ
kinase. These data,
however, need to be confirmed in primary, gene deficient B cells. Nevertheless, it
appears that at least for some pathways leading to IKK activation, the IKK-activating
signal is “channeled” through NIK and IKK
α
-dependent, consistent with NIK being an IKK
α
α
into the IKK complex to regulate
“classic” NF-
κ
B activity. Other stimuli channel their activity into this complex via
IKK
β.
It is currently unknown, what dictates the subunit specificity of such stimuli.
3.4
SIGNALING PATHWAYS LEADING TO
IKK ACTIVATION
As discussed above, it is likely that activation of IKK at some point involves trans-
autophosphorylation, which is needed for amplification of kinase activity and efficient
substrate phosphorylation (
Figure 3.1b
).
Different molecular mechanisms were pro-
posed as the initiating event that triggers this autophosphorylation. Notably, for none
of the stimuli that lead to IKK activation is the molecular mechanism of kinase
activation unequivocally clear. Generally, three mechanisms can be envisioned, which
are not mutually exclusive: (i) Direct phosphorylation of one of the IKK subunits,
thereby inducing a conformational change in the kinase domains of the catalytic
subunits that triggers their activity. In this scenario, akin to other kinase activation
pathways, the most likely, but not exclusive phosphorylation site would be the activa-
tion loop of the catalytic subunits IKK
.
While this may depend on an
upstream kinase (IKK-K), it should be noted that phosphorylation of the activation
loop also occurs during autophosphorylation of IKK
α
or IKK
β
α/β
themselves and that so far
