Biomedical Engineering Reference
In-Depth Information
3.4.1.2
Activation of the Alternative NF-
κ
B Pathway
As mentioned above, the alternative pathway is activated by a rather limited number
of TNFR family members, including BAFF-R and CD40 (on B cells) and LT
β
R (on
stromal cells) [37,76-78]. The hallmark of the alternative NF-
κ
B pathway is inducible
NF-
B2/p100 processing, leading to liberation of the mature transcription factor p52.
Like phosphorylated I
κ
,
phosphorylated p100 is recognized by the E3 ligase
SCF
βTrCP
and is targeted for proteasome-dependent proteolysis [3]. The exact reason
why the proteolysis of p100, in contrast to I
κ
B
α
B proteolysis, is partial, is not fully clear,
but may have to do with the presence of stop signals that impede proteasome action
[79]. The alternative NF-
κ
B pathway was first discovered and described based on the
observation that B cells from IKK
κ
-
AA mutant mice have a defect
in the constitutive processing of p100
in vivo
, suggesting that B cells are exposed to
a physiological ligand that induces p100 processing in an IKK
α
-
deficient or IKK
α
-
dependent pathway
[36]. This ligand and receptor pair was found to be BAFF and BAFF-R [76]. Two
kinases are critically involved in activation of the alternative NF-
α
κ
B pathway: NIK
and IKK
[36,80]. Accordingly, mice carrying an inactivating mutation in the NIK
gene, the so-called alymphoplasia (aly) mice or NIK knockout mice have defects in
BAFF- and LT
α
β
R-induced processing of p100 that are similar to those of IKK
α
-
AA
and NEMO are dispensable for this pathway
[37]. There is ample evidence that catalytic activity of both kinases, NIK and IKK
mice [37,77]. As mentioned, both IKK
β
α
is required for inducible p100 processing. Overexpression of NIK or a constitutive
active form of IKK
induces processing of p100 via specific phosphorylation of p100
C-terminal serine residues [36,80]. The response to NIK depends on the presence of
catalytically active IKK
α
α
and
in vitro,
NIK is a potent IKK
α
-
activating kinase [81].
The IKK
-
AA mutant, containing alanine substitutions in its activation loop, is no
longer phosphorylated by NIK and fails to induce LT
α
β
R- or BAFF-R-dependent
processing of p100 [37,82]. Moreover, IKK
has been cloned as a NIK-binding
protein, demonstrating direct interaction between the two proteins [10].
Thus, the alternative pathway seems to function as a typical phosphorylation
α
dependent kinase cascade (
Figure 3.2b
);
however, there remain issues in this pathway
as well. One is the remarkably slow kinetics of p100 processing, requiring several
hours in contrast to the classic NF-
degradation occurs
within minutes. Also, it is not clear why it is so difficult to demonstrate inducible
IKK
κ
B pathway, in which I
κ
B
α
R engagement
(unpublished observation). It is possible that these problems are related and reflect
a low affinity of IKK
α
kinase activity in response either to BAFF-R, CD40 or LT
β
α
for p100. In this context it is important to note that by itself
IKK
binds only weakly to p100 and the interaction between the two is significantly
enhanced in the presence of NIK, implicating NIK as an adaptor for IKK
α
docking
to its substrate [83,84]. It is well established that protein kinases need to physically
dock onto their substrates [84] and in the case of the large IKK complex, such a
function may be mediated by the ELKS subunit [85]. Interestingly, regulation of
NIK activity has been demonstrated to depend on TRAF proteins, particularly on
TRAF3, and suggested to involve ubiquitination [86]. NIK seems to be constitutively
bound by TRAF3, resulting in constitutive ubiquitination and degradation of NIK
[86]. Receptor-induced degradation of TRAF3 precedes p100 processing,
α
