Integrating cell-signalling pathways with NF- κB and IKK function
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REVIEWS
Integrating cell-signalling pathways
with NF-κB and IKK function
Neil D. Perkins
Abstract | Nuclear factor (NF)-κB and inhibitor of NF-κB kinase (IKK) proteins regulate many
physiological processes, including the innate- and adaptive-immune responses, cell death
and inflammation. Disruption of NF-κB or IKK function contributes to many human diseases,
including cancer. However, the NF-κB and IKK pathways do not exist in isolation and there
are many mechanisms that integrate their activity with other cell-signalling networks.
This crosstalk constitutes a decision-making process that determines the consequences
of NF-κB and IKK activation and, ultimately, cell fate.
Nuclear factor (NF)-κB transcription factors can both inflammatory diseases and cancers. The development of
induce and repress gene expression by binding to NF-κB and IKK inhibitors for the treatment of inflamma-
discrete DNA sequences, known as κB elements, in tory diseases and cancers, together with the observation
promoters and enhancers1. In mammalian cells, there that many drugs currently in clinical use have significant
are five NF-κB family members, RelA (p65), RelB, c-Rel, effects on NF-κB activity, means that understanding the
p50/p105 (NF-κB1) and p52/p100 (NF-κB2) (FIG. 1), and pathways that regulate NF-κB function will have many
different NF-κB complexes are formed from their homo- diagnostic and prognostic applications6–9.
and heterodimers. In most cell types, NF-κB complexes
are retained in the cytoplasm by a family of inhibitory NF-κ κB complexes
proteins known as inhibitors of NF-κB (IκBs) (FIG. 1). All NF-κB family members contain an N-terminal
Activation of NF-κB typically involves the phosphory- domain of approximately 300 amino acids called the
lation of IκB by the IκB kinase (IKK) complex, which Rel-homology domain (RHD), which mediates DNA
results in IκB degradation. This releases NF-κB and binding and dimerization. Furthermore, Rel subfamily
allows it to translocate freely to the nucleus1 (BOX 1). members, RelA, RelB and c-Rel, all contain unrelated
The genes regulated by NF-κB include those con- C-terminal transcriptional activation domains (FIG. 1).
trolling programmed cell death (apoptosis), cell adhe- Despite obvious structural similarities and their ability to
sion, proliferation, the innate- and adaptive-immune bind related DNA sequences, genetic studies have shown
responses, inflammation, the cellular-stress response that all NF-κB subunits have distinct and non-overlap-
and tissue remodelling1–5. However, the expression of ping functions3,10. In most unstimulated, non-diseased
these genes is tightly coordinated with the activity of mammalian cells, Rel subunits are found predominantly
many other signalling and transcription-factor pathways. in the cytoplasm bound to a member of the IκB family
Therefore, the outcome of NF-κB activation depends on of inhibitory proteins1. In mammalian cells, there are
the nature and the cellular context of its induction. For three principal IκBs, IκBα, IκBβ and IκBε, which func-
example, it has become apparent that NF-κB activity can tion in part by masking a conserved nuclear localization
be regulated by both oncogenes and tumour suppressors, sequence (NLS) that is found in the RHD of the NF-κB
resulting in either stimulation or inhibition of apoptosis subunits. For IκBα, this NLS masking is only partially
and proliferation6. Alternatively, NF-κB regulation of the effective and NF-κB–IκBα complexes shuttle into the
College of Life Sciences,
Jun N-terminal kinase (JNK)-signalling pathway can have nucleus even in the absence of cellular stimulation1.
Division of Gene Regulation
and Expression, James Black opposing results that depend on the cell type (discussed However, IκBα also contains a nuclear export sequence
Centre, Dow Street, below). This review discusses the mechanisms that link (NES), which causes the rapid export of such complexes
University of Dundee, the NF-κB pathway to other cell-signalling pathways back to the cytoplasm1.
Dundee, DD1 5EH, and asks why integration of NF-κB function is required Both p50 and p52 are synthesized as longer precursor
Scotland, UK.
e-mail:
for the specific regulation of its target genes. Defects proteins, p105 and p100, that contain ankyrin-repeat
n.d.perkins@dundee.ac.uk in these mechanisms can lead to inappropriate NF-κB motifs in their C termini, similar to those found in IκB
doi:10.1038/nrm2083 or IKK activity and can contribute to the pathology of proteins (FIG. 1). p100 and p105 also function as IκB-like
NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 8 | JANUARY 2007 | 49
© 2007 Nature Publishing GroupREVIEWS
a The NF-κB family and p52 can function as nuclear transcription factors.
Both p50 and p52 homodimers can interact with
RelA 1 RHD TA2 TA1 551
(p65)
another IκB family member, BCL-3, which, unlike the
19 301 428 521 other IκBs, is nuclear and functions as a transcriptional
co-activator1.
RelB 1 LZ RHD TAD 557
Homodimers composed of the p50 and p52 NF-κB
410
subunits evade regulation by IκBs and, consequently, are
c-Rel 1 RHD TA1 TA2 587 often found constitutively in the nucleus. However, when
8 290 422 497 p50 and p52 form heterodimers with the Rel subunits,
518 these subunits are also subject to cytoplasmic retention
p105 1 RHD ANK DD 969
(p50) by IκBs1. The exception to this is the p52–RelB complex,
43 363 543 751 818 893 which has a low affinity for IκBα and can therefore evade
p100 1 RHD ANK DD 900 this mechanism of regulation11.
(p52) 38 338 487 699 851
There are several distinct NF-κB-activation path-
776 ways. The most frequently observed is the canonical,
b The IκB family or classical, pathway (BOX 1) , which is induced in
ANK response to various inflammatory stimuli, including the
IκBα 1 PEST 317 pro-inflammatory cytokines tumour necrosis factor-α
73 256 (TNFα) and interleukin-1 (IL-1), engagement of the
264
IκBβ 1 PEST 361
T-cell receptor (TCR) or exposure to bacterial products
such as lipopolysaccharide (LPS)1. This pathway is typi-
57 298
307 fied by the rapid phosphorylation of IκBα at Ser32 and
IκBε 1 500 Ser36 and subsequent ubiquitin-induced degradation by
258 473 the 26S proteasome1. In many cell types, IκBβ and IκBε
are also subject to phosphorylation and degradation, but
BCL-3 1 446 with slower kinetics1,12.
126 351 In the canonical pathway, IκB phosphorylation is due
to IKK-complex activation1. The IKK complex consists
c The IKK family of three core subunits, the catalytic subunits IKKα and
IKKβ (also known as IKK1 and IKK2) and several copies
NEMO 1 CC1 CC2 LZ ZF 419
(IKKγ) of a regulatory subunit called the NF-κB essential modi-
63 193 258 319 397 fier (NEMO, also known as IKKγ)1. Genetic experiments
296 346 417
have shown that IKKβ is the predominant IκB kinase in
IKKα 1 Kinase domain LZ HLH NBD 745 the canonical pathway2,4.
15 301 455 483 599 638 738 743 A subset of NF-κB-induction stimuli, such as
stimulation of the CD40 and lymphotoxin-β receptors,
IKKβ 1 Kinase domain LZ HLH NBD 756 B-cell-activating factor of the TNF family (BAFF), LPS
15 300 458 486 603 642 737 742 and latent membrane protein-1 (LMP1) of Epstein–
Figure 1 | The mammalian members of the NF-κB, IκB and IKK families. Barr virus, activate the non-canonical, or alternative,
a | In mammalian cells, there are five nuclear factor (NF)-κB family members, RelA (p65), pathway2,13. Here, activation of IKKα by the NF-κB-
RelB, c-Rel, p50/p105 (NF-κB1) and p52/p100 (NF-κB2). p50 and p52 (not shown) are inducing kinase (NIK) results in the formation of
derived from the longer precursor proteins p105 and p100, respectively. All NF-κB family p52 from p100, as a consequence of phosphorylation-
members contain an N-terminal Rel-homology domain (RHD) that mediates DNA
induced, ubiquitin-dependent processing of p100 by the
binding and dimerization and contains the nuclear-localization domain. The Rel
subfamily, RelA, RelB and c-Rel, contain unrelated C-terminal transcriptional activation 26S proteasome2,12. p52–RelB heterodimers, which are
domains (TADs). TA1 and TA2 are subdomains of the RelA transactivation domain. frequently activated as a consequence of non-canonical
b | The inhibitor of NF-κB (IκB) family consists of IκBα, IκBβ, IκBε and BCL-3. Like p105 pathway activation, have a higher affinity for distinct κB
and p100, the IκB proteins contain ankyrin-repeat motifs (ANK) in their C termini. c | The elements and might therefore regulate a distinct subset
three core subunits of the IκB kinase (IKK) complex are shown: the catalytic subunits of NF-κB target genes (BOX 1). It has been suggested
IKKα and IKKβ and the regulatory subunit called the NF-κB essential modifier (NEMO, that the non-canonical pathway is regulated by IKKα
also known as IKKγ). The principal structural motifs of each protein are shown, together homodimers that function independently of the larger
with amino-acid numbers corresponding to the human proteins, although some IKK complex2. The non-canonical pathway does not
definitions of where a domain begins and ends might differ between publications. lead to the formation of p50 from p105 and, in many
CC, coiled-coil; DD, region with homology to a death domain; HLH, helix–loop–helix;
cells, high levels of p50 are created in the absence of
LZ, RelB-transactivation-domain containing a putative leucine-zipper-like motif;
NBD, NEMO-binding domain; PEST, domain rich in proline (P), glutamate (E), serine (S) cell stimulation by a co-translational mechanism. In this
and threonine (T); ZF, zinc-finger domain. co-translational mechanism, p50 is generated during
mRNA translation as a consequence of proteolysis of
the nascent p105 polypeptide by the 26S proteasome14,
proteins and retain their NF-κB-subunit dimeric although other mechanisms of p50 generation have
partners in the cytoplasm, inhibiting their activity1. also been proposed15,16. However, IKKβ-inducible and
Processing of p100 and p105 can occur through several ubiquitin-dependent total degradation of p105, without
mechanisms (discussed below) and is required before p50 p50 generation, can also occur15.
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Box 1 | Pathways leading to the activation of NF-κB
Atypical pathway Non-canonical pathway
(IKK independent) (LPS, CD40 and lymphotoxin
Hypoxia, H2O2 UV, HER2/Neu receptors, LMP1)
Tyr kinase CK2 NIK
TA elA
lA
P
D
Re
R
(Tyr42)
Canonical IKKα IKKβ
p50 RelA
pathway RelA
(TNFα, IL-1, LPS) RHD IκBα RHD p50
IκBα
NEMO
Re
P P
lB
IKKα IKKβ Degradation or Degradation P
dissociation of IκB of IκB
p1 NK
P
00
A
(Ser32) p100 (Ser866)
P RHD RelB (Ser870)
lA
Activation of P
Re
IKK complex (Ser36)
RelA Ubiquitylation
p50 and proteasomal
Ub IκBα
processing of p100
Atypical pathway NEMO Ubiquitylation
(genotoxic stress, and proteasomal
Re AD
ATM
lA
lB
T
degradation of IκB
Re
IKK dependent)
NEMO
p50 RelA p52 RelB
Cytoplasm RHD
Nucleus Kinases,
acetylases,
SUMO Ub phosphatases
NEMO NEMO
ATM ATM
Zn-finger TFs
bZIP
bZIP
HMG-I
Co-activator Co-repressor
P
P P P
P P
Re
lA
lA
lA
lB
Re
Re
Re
Ac
p50 RelA p50 RelA p50 RelA p52 RelB
Ac TF
Transcriptional Transcriptional Promoter targeting Distinct κB elements
activation repression and selectivity
The canonical pathway is induced by tumour necrosis factor-α (TNFα), interleukin-1 (IL-1) and many other stimuli, and is
dependent on activation of IKKβ. This activation results in the phosphorylation (P) of IκBα at Ser32 and Ser36, leading to its
ubiquitylation (Ub) and subsequent degradation by the 26S proteasome. Release of the NF-κB complex allows it to relocate to
the nucleus. Under some circumstances, the NF-κB–IκBα complex shuttles between the cytoplasm and the nucleus (not
shown). IKK-dependent activation of NF-κB can occur following genotoxic stress. Here, NF-κB essential modifier (NEMO)
localizes to the nucleus, where it is sumoylated and then ubiquitylated, in a process that is dependent on the ataxia
Co-activators and telangiectasia mutated (ATM) checkpoint kinase. NEMO relocates back to the cytoplasm together with ATM, where activation
co-repressors of IKKβ occurs. IKK-independent atypical pathways of NF-κB activation have also been described, which include casein
Transcriptional regulatory kinase-II (CK2) and tyrosine-kinase-dependent pathways. The non-canonical pathway results in the activation of IKKα by the
proteins that are typically NF-κB-inducing kinase (NIK), followed by phosphorylation of the p100 NF-κB subunit by IKKα. This results in proteasome-
recruited to promoters and dependent processing of p100 to p52, which can lead to the activation of p52–RelB heterodimers that target distinct
enhancers by interacting with κB elements. Phosphorylation of NF-κB subunits by nuclear kinases, and modification of these subunits by acetylases and
DNA-bound transcription phosphatases, can result in transcriptional activation and repression as well as promoter-specific effects. Moreover,
factors. Co-activators stimulate
cooperative interactions with heterologous transcription factors can target NF-κB complexes to specific promoters, resulting
transcription whereas
co-repressors do the opposite.
in the selective activation of gene expression following cellular exposure to distinct stimuli. Ac, acetylation; bZIP, leucine-
Co-activators are frequently zipper-containing transcription factor; HMG-I, high-mobility-group protein-I; IκB, inhibitor of κB; IKK, IκB kinase;
histone acetyltransferases LMP1, latent membrane protein-1; LPS, lipopolysaccharide; NF-κB, nuclear factor-κB; RHD, Rel-homology domain;
whereas co-repressors are TAD, transcriptional activation domain; TF, transcription factor; UV, ultraviolet; Zn-finger TF, zinc-finger-containing
often histone deacetylases. transcription factor. This figure is modified with permission from REF. 6 © (2006) Macmillan Publishers Ltd.
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IKK-complex activation IKK-independent pathways have the potential to
Numerous NF-κB-activating stimuli, which frequently induce differentially modified forms of NF-κB subunits
engage distinct cell-surface receptors and/or cytoplasmic with distinct functions.
signalling pathways, converge on the IKK complex
and induce the canonical pathway1,12,17–19. The key to NF-κκ B-independent effects of the IKKs
activation of the IKK complex is the regulatory subunit IKK activation does more than merely induce IκB deg-
NEMO. Activation of the canonical pathway results radation and the release of NF-κB. IKK subunits also
in complex, Lys63-linked ubiquitylation of signal- programme the cellular response to the initial activating
ling molecules, ultimately resulting in Lys63-linked stimulus (FIGS 2,3). In part, this can occur through post-
ubiquitylation of NEMO (discussed in more detail in translational modification of NF-κB subunits, such as
REFS 12,18–20). In contrast to Lys48-linked ubiquityl- RelA, which helps to define which genes are induced
ation, which results in protein degradation by the 26S or repressed by the NF-κB complexes6,12. However,
proteasome, Lys63-linked ubiquitylation has a role in several NF-κB- and IκB-independent targets of IKKα
signalling by facilitating interactions with proteins that and IKKβ are now being identified, which reveals that
contain ubiquitin-binding domains12,18–20. Ubiquitylation activation of the IKKs could result in more widespread
of NEMO allows the recruitment of kinases — such effects on cell signalling than previously realized.
as transforming growth factor-β (TGFβ)-activated Many of these NF-κB-independent functions of IKKs
kinase-1 (TAK1), which phosphorylates IKKβ in its have only been recently discovered, and these putative
activation loop at Ser177 and Ser181 — to the IKK roles for IKKs therefore require further confirmation.
complex 12,18–20. However, a mouse knockout of the Nonetheless, there is a clear trend towards recognizing
E2 ubiquitin-conjugating enzyme UBC13, which selec- a more promiscuous role for IKKs, and it is likely that
tively stimulates Lys63-linked ubiquitylation, has cast many more substrates have yet to be discovered.
doubt on whether ubiquitylation of NEMO is required
for NF-κB activation under all circumstances. Instead, The consequences of IKKβ activation. Activation of
this study indicated that NEMO ubiquitylation might be IKKβ stimulates anti-apoptotic, pro-inflammatory
required for the activation of mitogen-activated protein and proliferative pathways 24,25 (FIG. 2) , although
(MAP)-kinase signalling21 (see below). pro-apoptotic functions have also been described26,27.
Other mechanisms of canonical pathway activation Consistent with its anti-apoptotic role, IKKβ has
— for example, by genotoxic stimuli, such as ionizing been shown to phosphorylate and inhibit FOXO3a, a
radiation, or by some chemotherapeutic drugs — also transcription factor and tumour suppressor that can
result in NEMO-dependent IKKβ activation22. Here, in induce either apoptosis or cell-cycle arrest28. However,
some contexts, NEMO first translocates to the nucleus, such effects might be context dependent as, conversely,
where it is sumoylated and then phosphorylated by IKKβ has also been found to activate FOXO3a in
the checkpoint kinase ataxia telangiectasia mutated regressing mammary glands, leading to upregulation
(ATM)23 (BOX 1). NEMO sumoylation is then replaced of the death-receptor ligand TWEAK and induction of
by mono-ubiquitylation, leading to the nuclear export apoptosis27. Whether these latter effects involve direct
Checkpoint kinases of NEMO as a complex with ATM. The NEMO–ATM phosphorylation of FOXO3a was not investigated.
The checkpoint kinases ATM complex subsequently activates the IKK complex in a IKKβ can also phosphorylate 14-3-3β when this
and ATR are differentially process that requires the IKK-associated protein ELKS23 protein is complexed with tristetraprolin (TPP), an
activated in response to (a protein that is rich in glutamate (E), leucine (L), lysine AU-rich element (ARE) binding protein that regulates
distinct forms of genotoxic
stress. Generally, ATR and ATM
(K) and serine (S)). Whether this pathway is activated mRNA stability 29. IKKβ phosphorylation inhibits
activate the downstream in response to all genotoxic stresses, in all cell types, is TPP–14-3-3β ARE binding and might therefore pro-
checkpoint kinases CHK1 and currently unclear. Nonetheless, this serves as an illustra- mote the stability of cytokine, chemokine and growth
CHK2, respectively. Together tion of how IKK activity and, consequently, induction factor mRNAs, which all contain the ARE motif 29.
these kinases help orchestrate
of NF-κB DNA binding, can be integrated with parallel Pro-inflammatory cytokines, such as TNFα and IL-1,
the cellular response to DNA
damage. signalling pathways. can lead to insulin resistance and the development of
Atypical, IKK-independent pathways of NF-κB type-2 diabetes. This effect can, in part, be attributed
14-3-3 proteins induction also provide mechanisms to integrate par- to inhibition of insulin signalling that results from
A large eukaryotic class of allel signalling pathways with NF-κB activity (BOX 1). IKKβ phosphorylation of insulin-receptor-substrate-1
proteins that are involved in
cell division, apoptosis, signal
For example, some stimuli, such as hypoxia and (IRS1)30–32.
transduction, transmitter reoxygenation, hydrogen-peroxide stimulation and IKKβ can also participate in a negative feedback
release, receptor function, treatment of cells with nerve growth factor (NGF) loop that downregulates the signalling pathways that
gene expression and enzyme or the tyrosine-phosphatase inhibitor pervanadate lead to its activation. The TCR induces IKKβ through
activation. They function by
result in the phosphorylation of IκBα at Tyr42, which activation of the Carma1–BCL10–MALT1 (CBM) com-
binding to various different,
specific target proteins, usually results in either its degradation or dissociation from plex. IKKβ is required for the induction of the CBM
in response to phosphorylation NF-κB6,12. Treatment with ultraviolet (UV) light, or complex through an ill-defined mechanism. However,
of these targets. expression of the HER2 (also known as erbB-2 or Neu) it is also necessary for the attenuation of CBM-complex
oncogene in breast cancer cells, can result in IκBα signalling. This attenuation effect results from IKKβ
Type-2 diabetes
Diabetes that results from
phosphorylation by casein kinase-II (CK2) at sites directly phosphorylating BCL10, which disrupts its
insulin resistance or from in its C-terminal PEST domain12. As activated IKKα association with MALT1, thereby inactivating the CBM
reduced production of insulin. and IKKβ also phosphorylate RelA (see below), these complex33.
52 | JANUARY 2007 | VOLUME 8 www.nature.com/reviews/molcellbio
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Cell proliferation
Disruption of BCL10–MALT1
interaction, inhibition of MAP kinase activation
T-cell signalling and impaired
activation of NF-κB
TPL2/
COT
P (Ser927)
Carma1 P
BCL10 P (Ser932)
p1 NK
MALT1
05
A
p50 p105
RHD RHD
TPL2/
COT
Inhibition of P (Ser536)
insulin Nuclear
IRS1 IKKβ P localization
signalling
(Ser32) P of RelA,
stimulation of
TA elA
(Ser36) P
D
R
transactivation
functions
RelA
p50 RHD
P IκBα
DOK1
P P Canonical pathway
Downregulation of MAP 14-3-3β FOXO3a activation
kinase signalling TPP
Inhibition of mRNA binding, Inactivation of FOXO3a, Innate immune response,
possible stabilization of cytokine, inhibition of apoptosis inflammation, cell survival
chemokine and growth factor and cell adhesion
mRNAs
Cell proliferation
Cell proliferation
Figure 2 | The consequences of IKKβ activation. Activation of IKKβ stimulates anti-apoptotic, pro-inflammatory and
proliferative pathways. As well as activating the canonical NF-κB-signalling pathway through phosphorylation (P) of the
IκB proteins, IKKβ phosphorylates several other substrates, including NF-κB subunits. For example, phosphorylation of
RelA at Ser536 can result in nuclear localization and stimulation of its transactivation functions by promoting interactions
with co-activator proteins. In some unactivated cells, the p105 NF-κB subunit is found in a complex with the mitogen-
activated protein (MAP)/extracellular signal-regulated kinase (ERK) (MEK) kinase TPL2 (also known as COT). IKKβ
phosphorylates p105 resulting in its degradation, which releases TPL2 resulting in activation of the pro-proliferative MAP
kinase-signalling pathway. Another way in which IKKβ might exert its anti-apoptotic effects is through phosphorylation
and inhibition of the tumour suppressor FOXO3a. By contrast, phosphorylation of the adaptor protein DOK1 is thought to
contribute to its ability to inhibit MAP kinase signalling and cell proliferation. IKKβ phosphorylates and inhibits the 14-3-3β
protein when complexed with tristetraprolin (TPP), an AU-rich element (ARE) binding protein that regulates mRNA
stability. IKKβ phosphorylation inhibits TPP–14-3-3β ARE binding and might therefore stabilize cytokine, chemokine and
growth factor transcripts. IKKβ can also inhibit insulin signalling by targeting insulin-receptor substrate-1 (IRS1).
Phosphorylation of BCL10 as part of the T-cell receptor (TCR) Carma1–BCL10–MALT1 complex disrupts the BCL10–MALT1
interaction, therefore attenuating T-cell signalling. As TCR signalling activates the NF-κB pathway, BCL10 phosphorylation
by IKKβ serves as a negative feedback loop. ANK, ankyrin-repeat motif; IκB, inhibitor of κB; IKK, IκB kinase; NF-κB, nuclear
factor-κB; RHD, Rel-homology domain; TAD, transcriptional activation domain.
There are also links between IKKβ activity and the the cellular FLICE-like inhibitory protein (c-FLIP), the
pro-proliferative MAP kinase pathway. In unstimulated coatomer-β subunit protein COPB2, JNK-interacting
cells, such as macrophages and fibroblasts, the MAP/ leucine-zipper protein (JLP) and A20-binding inhibitor
extracellular signal-regulated kinase (ERK) (MEK) of NF-κB (ABIN)36–39. So, IKKβ-induced p105 proteolysis
kinase TPL2 (also known as COT) is found in a complex might conceivably activate various NF-κB-independent
with a subset of the p105 NF-κB subunit. IKKβ activity pathways. IKKβ can also phosphorylate DOK1, an adap-
can induce proteolysis of p105, which in turn releases tor protein that downregulates MAP kinase activation,
bHLH transcription factor the bound TPL2, resulting in MAP kinase-pathway acti- providing another possible link for the regulation of this
A transcription factor vation34,35. Whether this pathway provides the basis for pathway 40.
containing a DNA-binding and activation of MAP kinase signalling by Lys63 modified A clear implication of these results is that the pro-
dimerization domain
characterized by a region of
NEMO21 is not currently clear. p105 has been reported to inflammatory, anti-apoptotic, pro-proliferative and
basic amino acids followed by associate with several other non-NF-κB proteins, such as tumour-promoting characteristics that have been
a helix–loop–helix motif. the basic helix–loop–helix (bHLH) transcription factor LYL1, ascribed to IKKβ2,4,8 might in part result from these
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Degradation of
P RelA
Regulates chromatin (Ser536)
TA elA
remodelling, activates
D
R
transcription Resolution of
inflammation
P (H3 Ser10) p50 RelA
RHD RHD
Re
Proteolytic
lB
CBP (Ser866)
Induction of P processing
P IKKα
p1 NK
INFα P of p100 to p52
00
A
expression IRF7 p100 (Ser870)
RHD
RelB
IKKα
Non-canonical pathway
activation
P P
Adaptive immune response,
Cyclin D1 cell proliferation
SMRT
Degradation of cyclin D1 P P Derepression of NF-κB-
P ERα β-catenin dependent transactivation
SRC3
Induction of cyclin D1 expression
Cell proliferation
Figure 3 | The consequences of IKKα activation. In addition to activation of the non-canonical signalling pathway
through phosphorylation of the p100 NF-κB subunit, IKKα is now thought to phosphorylate a number of other substrates.
For example, phosphorylation of RelA at Ser536 in macrophages can stimulate its degradation and thus promote
resolution of inflammation. IKKα also phosphorylates proteins that are not components of the NF-κB and IKK pathway.
Through associating with cyclic-AMP responsive element binding (CREB)-binding protein (CBP), IKKα functions as a
histone H3 serine 10 (H3 Ser10) kinase and thereby regulates chromatin remodelling. IKKα also regulates cyclin D1
expression independently of NF-κB by phosphorylating oestrogen receptor-α (ERα), its co-activator SRC3 as well as
β-catenin and cyclin D1 itself. IKKα also affects transcription through phosphorylation of the SMRT co-repressor, which
Cyclin D1 leads to derepression of NF-κB transactivation, and interferon regulatory factor-7 (IRF7), which can induce the expression
A cyclin that forms
of interferon-α (IFNα).Not shown are the kinase-independent effects of IKKα on keratinocyte differentiation and tooth
heterodimers with the cyclin-
dependent kinases CDK4 or
development. ANK, ankyrin-repeat motif; IKK, inhibitor of NF-κB kinase; NF-κB, nuclear factor-κB; RHD, Rel homology
CDK6, thereby regulating domain; TAD, transcriptional activation domain.
transition through the
G1 phase. Cyclin D1 also has
CDK-independent functions
and can function as a regulator NF-κB-independent effects. Furthermore, it should factor (TCF) family of transcription factors, which bind
of transcription. be appreciated that although the development of IKKβ directly to the cyclin D1 promoter45–47. Interestingly,
inhibitors has numerous potential therapeutic application β-catenin has also been described as an inhibitor of
β-catenin
A dual-function protein that for both inflammatory diseases and cancer7–9, their NF-κB transcriptional activity48,49. Furthermore, it has
has a role in the cytoplasm effects will not be totally mediated through the inhibition been reported that IKKα can directly phosphorylate the
regulating cell adhesion and a of NF-κB function. cyclin D1 protein at Thr286 which, in direct contrast to
role in the nucleus as a its more established effects on the cyclin D1 promoter,
transcriptional co-activator
that mediates the Wnt-signal-
The consequences of IKKα activation. IKKα can have a seems to induce cyclin D1 degradation50.
transduction pathway. pro-proliferative function by inducing p52 activity and IKKα has also been found to regulate oestrogen-
Frequently mutated in cancer, by consequently inducing the cyclin D1 promoter41,42. induced cell-cycle progression by indirectly inducing
β-catenin can function as an Indeed, IKKα also seems to be able to regulate cyclin D1 the expression, acetylation and target-gene expression
oncogene.
expression through numerous NF-κB-independent of the E2F1 transcription factor, indicating a potentially
E2F1 mechanisms (FIG. 3). These include the phosphorylation significant role for IKKα in breast cancer51. Other
A member of the E2F family of and activation of the oestrogen receptor-α (ERα) trans- NF-κB-independent transcriptional effects of IKKα
transcription factors, which, cription factor, together with its co-activator protein include the phosphorylation of the SMRT co-repressor
together with the SRC3, in breast cancer cells43,44. Moreover, IKKα can while it is bound to NF-κB complexes on the promoter52.
retinoblastoma tumour
suppressor, regulate genes that
stimulate the cyclin D1 promoter through the phospho- This does not seem to disrupt the SMRT–NF-κB inter-
control cell-cycle progression rylation and stabilization of the β-catenin oncoprotein, a action, but instead results in release of chromatin bound
and DNA synthesis. transcriptional co-activator for members of the T-cell histone deacetylase-3 (HDAC3), leading to derepression
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© 2007 Nature Publishing GroupREVIEWS
of NF-κB-dependent transcription52. IKKα can also although a more dynamic role as a regulator of protein
phosphorylate and activate interferon regulatory function is also possible. For example, a nuclear role for
factor-7 (IRF7), which is required for interferon-α IκBα as a promoter-bound transcriptional repressor of
(IFNα) production53. the Notch-target gene hes1, which encodes a transcrip-
In addition, IKKα regulates chromatin structure and tional repressor and regulator of cell differentiation, has
facilitates gene expression by functioning as a histone been described68. One implication of these findings is
H3 serine 10 (H3 Ser10) kinase54,55. In part, this function that, similar to the phenotypes of IKK-knockout mice,
is facilitated by IKKα interacting with the transcrip- the phenotypes of model systems that rely solely on the
tional co-activator cyclic-AMP responsive element expression of a non-degradable IκBα ‘super-repressor’
binding (CREB)-binding protein55 (CBP). Under some protein might not result exclusively from inhibition of
circumstances, the predominant H3 Ser10 kinases are NF-κB function.
mitogen- and stress-activated protein kinase-1 (MSK1)
and MSK2, which are induced by p38 MAP kinase Modulation of NF-κ κB subunits
activation56,57. H3 Ser10 phosphorylation is crucial for NF-κB subunits are subject to many modifications and
the activation of a subset of NF-κB-dependent genes58. regulatory interactions that define their transcriptional
As p38 MAP kinase is induced by many of the same activity and target-gene specificity. These interactions
inflammatory stimuli and cell stresses that also induce form a crucial interface with other signalling pathways
NF-κB, this provides a mechanism through which this in the cell; modifications of NF-κB subunits by IKK-
signalling pathway can determine the consequences of independent routes can determine their ability to interact
NF-κB activation. with co-activators or co-repressors, whereas cooperative
IKKα also has kinase-independent roles in the DNA binding with heterologous transcription factors
regulation of keratinocyte differentiation59 and tooth can determine their ability to selectively target promo-
development60. Taken together, it can be concluded that ters and enhancers (BOX 1). Indeed, these regulatory
IKKα regulation of these diverse pathways will result mechanisms provide explanations for cell-type- and
in profound effects on the expression of many genes, stimulus-specific effects of NF-κB, such as the ability to
independently of any direct regulation of NF-κB. either inhibit or facilitate apoptosis6,12,25. These pathways
also provide important routes through which oncogenes
IKK-independent functions of NEMO. There is also and tumour suppressors can modulate NF-κB activity 6.
evidence that NEMO has IKK-independent functions.
For example, the ability of NEMO to translocate to the Post-translational modifications. The true extent of
nucleus and to associate with ATM following DNA NF-κB-subunit modifications is currently unknown,
damage23 clearly implies that NEMO can function inde- with only the RelA protein having been studied in any
pendently of IKKα or IKKβ. Indeed, nuclear-localized detail12. However, from these studies, it is clear that there
NEMO has been shown to compete with both RelA and are many subunit modifications with diverse functional
IKKα for binding to the N-terminal domain of CBP61. consequences12. Both IKKα and IKKβ are RelA kinases.
This interaction has the potential to regulate many other However, many other signalling pathways also use
transcription factors that use CBP. For example, NEMO modification of RelA and the other NF-κB subunits to
stimulates the interaction of CBP with hypoxia-inducible control NF-κB-pathway function. For example, RelA is
factor-2α (HIF2α), independently of the IKK subunits, phosphorylated at: Ser276 by the catalytic subunit of
and also directly interacts with HIF2α, thereby enhancing protein kinase A (PKAc), MSK1 and MSK2; at Ser311
its transcriptional activity62. by the atypical PKCζ; at Ser468 by IKKβ, IKKε and
glycogen-synthase kinase-3β (GSK3β); at Ser529 by CK2;
κB proteins
Iκ and at Ser536 by IKKβ, IKKα, IKKε, NF-κB activating
Whether the IκB proteins themselves have NF-κB- kinase (NAK, also known as TANK-binding kinase-1
independent functions is less clear, although there have (TBK1)) and RSK1 (also known as p90 ribosomal
been interesting, albeit largely unconfirmed, reports protein S6 kinase (p90S6K))12. These can all generally be
indicating that this might be the case. For example, pro- described as stimulatory modifications that enhance the
teomic analysis of IκB-associated proteins indicated that, transcriptional activity of RelA and its ability to interact
in addition to interactions with NF-κB subunits, IKKs with co-activators, such as p300 and CBP69,70. RelA is
and components of the ubiquitin-proteasome machin- also acetylated at several sites by p300 and CBP12,69–71.
ery, IκBα also associates with the replication proteins By contrast, phosphorylation at Thr505 by the CHK1
minichromosome maintenance-5 (MCM5) and MCM7 checkpoint kinase, which can be activated by ATM- and
(REF. 36). IκBβ and IκBε were found to associate with Rad3-related (ATR) kinase in response to ARF tumour
several uncharacterized proteins in this study. IκBα was suppressor induction or DNA damage following cisplatin
also reported to interact with cyclin-dependent kinase-4 treatment, inhibits RelA transactivation and results in
CBP and p300 (CDK4)63 together with the p53 tumour suppressor64–66, its increased association with HDAC1 (REFS 72–74).
CBP and p300 are highly and can inhibit HIV-1 Rev protein function independ- This results in the repression of BCL-xL expression, an
homologous transcriptional ently of NF-κB67. The significance of these reports is not anti-apoptotic gene that is typically induced by NF-κB
co-activator proteins with
histone acetyltransferase
currently clear. It is possible that IκB degradation might in response to inflammatory stimuli, and sensitization to
domains. Both interact with release sequestered proteins in a manner that is ana- apoptosis. RelA-dependent repression of anti-apoptotic
various transcription factors. logous to p105-mediated activation of TPL2 (see above), gene expression is also sometimes observed following
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NF-κB activation by UV-light exposure and some chemo- for the exquisite virus-specific expression of IFNβ: other
therapeutic drugs, although the mechanisms involved inducers of NF-κB fail to activate the full complement
are probably diverse and do not all involve Thr505 of IFNβ enhancer-binding proteins and so fail to induce
phosphorylation74–77. The effects of the many subunit its formation. The principles of enhanceosome forma-
modifications are also probably promoter specific78 and tion, including the integration of NF-κB activity with
allow heterologous signalling pathways to specifically other signalling pathways and the specificity this brings,
modulate the NF-κB pathway. are found at many other promoters, although the
RelA Ser536 phosphorylation by IKKβ provides a components might differ88.
mechanism to integrate NF-κB with the phosphoinositide NF-κB can interact with many types of transcription fac-
3-kinase (PI3K) and PKB/AKT-signalling pathway, which tor, including bZIP transcription factors such as C/EBPβ,
is induced by growth factors, cytokines and oncogenes members of the Jun, ATF, CREB and Fos family and zinc-
such as Ras79. Similar to IKKβ activation, this pathway finger-containing proteins such as specificity protein-1
promotes cell survival, and several reports indicate that (Sp1) and early growth response-1 (EGR1)86,87,91,92 (BOX 1).
IKKβ-dependent RelA Ser536 phosphorylation requires Also, recruitment of NF-κB to promoters might not
PI3K/AKT activity80–82. Interestingly, in vitro, AKT is always require a κB element. Recruitment of the RelA–
required for the IKKβ-mediated phosphorylation of HDAC4 complex to the Kruppel-like factor-2 (KLF2)
RelA, but not of IκBα81. Underlining its potential role promoter by the MADS box factor myocyte-enhancer
as an integration point, IKKβ-dependent phosphoryla- factor-2 (MEF2), independently of RelA DNA bind-
tion of RelA at Ser536 that follows TCR stimulation is ing, has recently been proposed93. Furthermore, the
PI3K/AKT-independent but requires TPL2, PKCθ and p52 NF-κB subunit can be recruited by p53 to multiple
NIK kinases83. The complexity of these signalling path- p53-regulated promoters, independently of the DNA-
ways is further illustrated by the diverse consequences of binding activity of p52 (REF. 94). There might well be many
Ser536 phosphorylation which, in addition to enhancing other mechanisms of crosstalk between transcription
RelA transactivation, can also induce its translocation factors. For example, the non-canonical NF-κB pathway
to the nucleus, independently of IκB degradation, and can be induced by the signal transducer and activator of
enhance its proteolytic degradation84,85. transcription-3 (STAT3), which binds DNA in a complex
with p52 (REF. 95). By contrast, active STAT1, induced by
Crosstalk with heterologous transcription factors. IFNα expression, has been reported to interact with RelA
Cooperative interactions with DNA-bound transcrip- and inhibit its DNA binding and nuclear localization96.
tion factors provide another important mechanism to Recent data indicate that cooperative DNA binding
integrate NF-κB function with other signalling pathways. might not always be the mechanism of the synergistic
bZIP-motif-containing
The position and orientation of many (if not most) NF-κB transcriptional effects that are observed between NF-κB
transcription factor binding sites in promoters and enhancers is not random, and heterologous transcription factors87,97. Rather, these
A transcription factor that and both the context and the sequence of the κB element might derive from cooperative recruitment of co-activa-
contains a DNA-binding and can determine the function of NF-κB subunits. Promoter tor proteins, as also occurs at the IFNβ enhanceosome98,
dimerization domain
structure can therefore facilitate the cooperative bind- or from the ability of different transcription factors to
characterized by a region of
basic amino acids followed by ing of NF-κB subunits with heterologous transcription affect different steps in the transcriptional activation
a leucine-zipper motif. factors, thereby enhancing the recruitment of one or both process. For example, synergism between RelA and
to weak or nucleosome-associated binding sites86,87. the constitutive and widely expressed DNA-binding
Jun, ATF, CREB and Fos One of the best characterized examples of such effects protein Sp1 might derive in part from the ability of Sp1
family
A transcription factor family,
is provided by the IFNβ enhancer. IFNβ expression is to stimulate the initiation of transcription, whereas RelA
members of which all contain induced upon viral infection and is NF-κB depend- enhances subsequent re-initiation steps99. Similarly, trans-
related bZIP domains and form ent88. However, other non-viral inducers of NF-κB fail criptional synergy between the Drosophila melanogaster
homo- and heterodimers that to induce IFNβ expression. This is due to the specific NF-κB homologue Dorsal and the bHLH transcription
bind related DNA sequences.
architecture of the IFNβ enhancer, in which the κB factor Twist, which is essential for the expression of Snail,
Collectively, dimers of Jun and
Fos family members are often motif is part of a composite element that contains a protein required for the specification of mesoderm, is
referred to as the AP1 DNA-binding sites for a heterodimer of the bZIP-motif- thought to occur primarily through their ability to make
transcription factor. containing transcription factors Jun and ATF2, members separate contacts with distinct, rate-limiting compo-
of the IRF family and the high-mobility-group protein nents of the RNA polymerase II (Pol II) transcriptional
Kruppel-like factor
A protein that is homologous
HMG-I89. Mutation or altering the spacing or orienta- machinery100. Whatever the mechanism, the many inter-
to the Kruppel transcription tion of the transcription-factor binding sites in the actions between NF-κB and other transcription factors
factor, originally characterized enhancer prevents cooperative DNA binding and the provide a key mechanism to both integrate NF-κB sig-
in Drosophila melanogaster. formation of a higher-order transcription-factor–DNA nalling with other cellular processes and define the exact
Kruppel-like factors possess
complex, known as the enhanceosome90. Furthermore, nature of the NF-κB response to specific cell stimuli.
zinc-finger-containing DNA-
binding domains. the IFNβ κB element differs from those in many other
promoters, as it contains a central core with the sequence Feedback loops. Activation of NF-κB results in repro-
MADS box AAATT that allows binding of HMG-I to the minor gramming of the cell’s gene-expression patterns to
The MADS box is a conserved groove89. HMG-I also binds other sites in the enhancer respond to a specific stimulus or changed environment.
DNA-binding motif originally
identified in the MCM1, AG,
and facilitates cooperative DNA binding of other tran- Many NF-κB target genes generate feedback to influ-
DEFA and SRF proteins, from scription factors as well as enhanceosome formation89. ence the function of NF-κB and also to profoundly affect
which it derives its name. Formation of the enhanceosome structure is required other signalling pathways.
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© 2007 Nature Publishing GroupREVIEWS
TNFα B cells107. These, in turn, through cooperative effects
NEMO/IKKβ
similar to those described above, are then required for
MnSOD
the induction of a second wave of NF-κB target genes.
The duration of the NF-κB response is prolonged
ROS
FHC following LPS stimulation relative to that caused by other
NF-κB stimuli, such as TNFα. This is a result of IRF3-dependent
GADD45β UV light TNFα synthesis leading to a positive feedback loop that
MKK7 restimulates IKK activity108,109. Interestingly, RelA has
XIAP been shown to function as a transcriptional co-activator
for IRF3 following treatment with LPS, but not other
JNK PKCδ
stimuli110.
Together, mechanisms such as these can control
Proliferation the timing and functional consequences of NF-κB
(epidermal cells) activation, allowing its cell-type and stimulus-specific
functions to be revealed.
Integration with other signalling pathways
The integration of the NF-κB pathway with some important
Apoptosis cell-signalling pathways has been analysed in detail. The
Figure 4 | Crosstalk between the NF-κB and JNK- complex relationships that exist provide examples of the
signalling pathways. Exposure of cells to tumour intricate crosstalk that can occur between fundamental
necrosis factor-α (TNFα) can result in the generation of
cell-signalling pathways. Importantly, these studies also
reactive oxygen species (ROS), leading to activation of
mitogen-activated-protein kinase kinase-7 (MKK7) and show that feedback can occur in both directions and that
Jun N-terminal kinase (JNK). If left unopposed, JNK the functional consequences of this can vary, depending
activation will lead to apoptosis, except in some cell types, on the context.
such as epidermal cells, in which it can stimulate cell
proliferation. However, TNFα signalling also activates IKKβ Crosstalk with the JNK-signalling pathway. Many cell
and the canonical NF-κB pathway. NF-κB induces the stresses and stimuli that induce the NF-κB pathway,
expression of antioxidizing enzymes such as manga- such as TNFα, can also activate JNK signalling111,112,
nese-superoxide dismutase (MnSOD) and ferritin heavy which has many consequences, including pro-apoptotic
chain (FHC), which counteract ROS generation. Also, effects 111,112. However, NF-κB activation by TNFα
other proteins activated by NF-κB, such as GADD45β and
prevents apoptosis through the induction of several
XIAP, can oppose JNK activation by inhibiting other
components of the JNK-signalling pathway. By contrast, anti-apoptotic genes25 and by the suppression of the
following ultraviolet (UV)-light exposure, NF-κB induces JNK pathway111,112 (FIG. 4). Activation of JNK by TNFα
the expression of protein kinase Cδ (PKCδ), which requires the generation of reactive oxygen species (ROS),
activates JNK. The point at which XIAP affects the JNK a process that can be counteracted by NF-κB through
pathway is not currently known. IKK, inhibitor of NF-κB the induction of genes that encode antioxidizing
kinase; NEMO, NF-κB essential modifier; NF-κB, nuclear enzymes such as manganese-superoxide dismutase
factor-κB. (MnSOD) and ferritin heavy chain (FHC)111–113. In addi-
tion, the products of other activated NF-κB target genes,
such as GADD45β (also known as MYD118), which is an
With the exception of RelA, all the NF-κB subunits inhibitor of the JNK upstream kinase mitogen-activated-
and IκBα contain κB sites in their promoters1,5. The protein kinase kinase-7 (MKK7, also known as JNKK2)
positive regulation of NF-κB subunits can result in the and the anti-apoptotic protein XIAP, also suppress
IRF3
composition of the NF-κB complex changing over time, JNK activation (the effect of XIAP occurs through an
A transcription factor that
regulates genes that contain whereas induction of IκBα provides a crucial negative undefined mechanism)111,112. In many cell types, sup-
the interferon-sensitive feedback loop that helps terminate the NF-κB response. pression of JNK-induced apoptosis can contribute to the
response element (ISRE) Other NF-κB target genes, such as the CYLD tumour tumour-promoting activities of NF-κB111–113. However,
sequence. IRF3 activity can be suppressor and A20, both of which encode deubiquityl- in epidermal cells, JNK stimulates cell proliferation and
induced through binding of LPS
to Toll-like receptor-4, which
ating enzymes, can also inhibit and limit the NF-κB therefore, in these cells, the inhibition of JNK by NF-κB
also activates NF-κB. response by targeting signalling molecules, such as has a tumour-suppressing function6,114. By contrast, after
TRAF proteins, which are required for IKK-complex UV stimulation, RelA directly induces the expression of
E3 ubiquitin ligase activation101–106. Interestingly, CYLD was also recently PKCδ, which in turn activates JNK77.
A protein that directly interacts
shown to deubiquitylate the p50 and p52 co-activator
with target proteins and
catalyses their ubiquitylation. BCL-3, leading to inhibition of both cyclin D1 expression Crosstalk with p53. The p53 tumour suppressor and
Lys48-linked ubiquitylation and proliferation in keratinocytes106. transcription factor is one of the first lines of defence
can lead to proteasomal Activation of other genes also determines NF-κB-target- against the effects of genotoxic damage or oncogene
degradation whereas Lys63- gene specificity. For example, members of the activator activation, and it typically induces apoptosis or cell-
linked ubiquitylation can
facilitate protein interactions
protein-1 (AP1) and ATF families of transcription factors, cycle arrest in response to these stimuli115. The amount
and activation of signalling such as JunB, JunD, B-ATF and ATF4, are rapidly induced of p53 protein in cells is regulated by HDM2 (known as
pathways. by NF-κB following LPS stimulation of precursor MDM2 in mice) which functions as an E3 ubiquitin ligase
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ATR CHK1
p300/CBP
Competition for
binding
Upregulation of
HDM2 promoter RSK1
Ser536 P P Thr505
ARF HDM2 p53 NF-κB
Induction of p53
expression
BCL-3
lA
Re
p50 RelA
p53
Genotoxic Inflammatory stimuli,
stress, ARF induction genotoxic stress
Induction of Repression of Induction of Induction of
pro-apoptotic genes: anti-apoptotic genes: pro-apoptotic anti-apoptotic genes:
Bax, PUMA, DR5 BCL-xL, XIAP, A20 genes: BCL-xL, XIAP, IAP1, IAP2,
Fas, FasL, DR5 A20
Apoptosis
Figure 5 | Integration of the p53 pathway with the function of the RelA (p65) NF-κB subunit. Crosstalk between
the tumour suppressor p53 and NF-κB pathways regulates apoptosis. In response to inflammatory stimuli, NF-κB
induces anti-apoptotic genes that antagonize the pro-apoptotic function of p53. Moreover, NF-κB, or the IκB family
member BCL-3, can induce the expression of HDM2 (known as MDM2 in mice) and reduce p53 protein levels.
Additional antagonism comes from competition between p53 and RelA for binding to shared co-activator proteins
such as p300 and cyclic-AMP responsive element binding (CREB)-binding protein (CBP). By contrast, there are also
cooperative pathways between p53 and NF-κB. Some inducers of NF-κB result in repression of anti-apoptotic genes
and induction of pro-apoptotic genes. Furthermore, induction of the ARF tumour suppressor can activate the CHK1
checkpoint kinase, which phosphorylates (P) RelA at Thr505, leading to repressed expression of the anti-apoptotic
NF-κB gene target BCL-xL. p53 can induce RelA nuclear localization and DNA binding through activation of RSK1
(also known as p90 ribosomal protein S6 kinase (p90S6K)). NF-κB has also been reported to induce the expression of
p53. Which of these pathways dominates will probably depend on the cell type and the nature of the inducing stimulus
for both p53 and NF-κB. Not shown are the regulatory mechanisms linking p53 to the p52 NF-κB subunit. ATR, ataxia
telangiectasia mutated (ATM)- and Rad3-related; IκB, inhibitor of κB; NF-κB, nuclear factor-κB.
and induces p53 proteolysis115. Expression of HDM2 In contrast to these antagonistic effects, there is also
and the consequent downregulation of p53 can be significant cooperative crosstalk between the NF-κB
induced by IKK and by NF-κB activation116, or by the and p53 pathways (FIG. 5). For example, p53 can induce
IκB-like BCL-3 co-activator117 (FIG. 5). The RelA NF-κB RSK1 activity, which phosphorylates RelA at Ser536,
subunit has also been shown to antagonize p53 trans- resulting in the nuclear localization of NF-κB 84.
activation through sequestration of the p300 and CBP Moreover, in some circumstances RelA and p53
co-activators118. These effects on p53, together with the cooperatively induce apoptosis119–121. This can occur
other functions of NF-κB, such as induction of anti- through the coordinated induction of pro-apoptotic
apoptotic genes, all contribute to the tumour-promoting target genes such as death receptor-5 (DR5), which
effects of NF-κB. contains both p53 and NF-κB response elements122.
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© 2007 Nature Publishing GroupREVIEWS
a Induction of IκBα expression b Co-activator competition There is also crosstalk between p53 and the p52
Ligand NF-κB subunit. Activation of p53 can inhibit cyclin D1
IκBα expression by inducing the association of the p52 NF-κB
p50 RelA p300/CBP subunit with co-repressor complexes42. Moreover, in some
cell lines, p52 is directly recruited to p53-regulated pro-
moters, where it regulates co-activator and co-repressor
NF-κB recruitment, leading to either repression or activation
of p53 target genes such as p21WAF1, DR5 and PUMA94.
Iκ Bα κB element Through this pathway, p52 has the potential to regulate
Nucleus Cytoplasm
p53-dependent cell-cycle arrest and apoptosis.
c Sequestration of NF-κB d Disruption of co-activator-
complex formation Crosstalk with nuclear receptors. Nuclear receptors (NRs)
are transcription factors that are typically regulated by
IRF3
the binding of specific ligands, such as glucocorticoids,
oestrogen, thyroid hormone and retinoic acid124–126.
Many NRs possess anti-inflammatory functions that are
mediated largely through the inhibition of NF-κB and
AP1 transcriptional activity124. What is revealing about
studies of NRs, and what is especially relevant when
IRF3 IRF3 considering the other pathways engaged in crosstalk by
NF-κB, is the diversity of mechanisms through which
IRSE κB element
NRs can inhibit NF-κB function124 (FIG. 6).
e Inhibition of RNA polymerase II f Recruitment of HDACs Most mechanisms of NF-κB repression seem to
hyperphosphorylation involve direct interactions between NF-κB and NRs, and
P-TEFb these are referred to as transrepression124–126. However,
HDAC2 there are also indirect mechanisms, including induction
P of IκBα expression127,128 and competition for co-activators
such as p300 and CBP129.
Pol II Direct interaction with NRs can result in seques-
tration of NF-κB, which inhibits IRF3-dependent
promoters at which RelA functions as a transcriptional
κB element κB element co-activator110,130. Conversely, IRF3 can also function
as a co-activator for some NF-κB-regulated genes and
Figure 6 | Crosstalk between the NF-κB and nuclear-receptor pathways. Multiple here, binding of RelA to the glucocorticoid receptor
mechanisms contribute to the ability of nuclear receptors (NRs) to repress the NF-κB
prevents IRF3 recruitment, leading to transcriptional
pathway and thereby function in an anti-inflammatory manner. Different mechanisms
seem to selectively regulate distinct subsets of NF-κB-regulated genes. Some repression130. Interestingly, other NRs do not disrupt the
pathways of NR-mediated repression are indirect and involve: (a) induction of IκBα interaction between IRF3 and RelA, but rather repress
expression; or (b) competition for co-activator proteins such as cyclic-AMP responsive distinct subsets of NF-κB-regulated genes, indicating
element binding (CREB)-binding protein (CBP) and p300. However, most mechanisms that there are other mechanisms of repression124,130.
of NF-κB repression involve direct interactions with NRs and are referred to as Direct recruitment of NRs by RelA to NF-κB-regulated
transrepression. (c ) Direct interaction with NRs can result in sequestration of NF-κB, promoters can have various other effects that can lead
which inhibits interferon regulatory factor-3 (IRF3)-dependent promoters, for which to repression of transcription. These include the recruit-
RelA functions as a transcriptional co-activator. (d) Conversely, interactions of NRs ment of HDACs (or a failure to clear co-repressors from
with RelA can prevent IRF3 from functioning as a co-activator at some NF-κB- the repressed basal state of the gene) and the inhibition
regulated promoters. RelA-dependent recruitment of NRs to promoters can lead to
of Pol II phosphorylation131,132. These diverse mecha-
repression of transcription by other mechanisms, including: (e) inhibition of
RNA polymerase II (Pol II) phosphorylation (P) by P-TEFb; (f) recruitment of histone nisms seem to affect specific subsets of NF-κB-regulated
deacetylases (HDACs); or a failure to clear co-repressors from the repressed basal state genes, providing opportunities for selective and context-
of the gene (not shown). IκB, inhibitor of κB; IRSE, interferon-sensitive response dependent regulation of NF-κB function.
element; NF-κB, nuclear factor-κB; P-TEFb, positive transcription elongation factor b.
This figure is modified with permission from REF. 124 © (2006) Elsevier. Conclusions
These examples illustrate the complexity of the NF-κB
pathway, but only scratch the surface of this complicated
Alternatively, in contrast to the results described area of biology. The different modifications of the NF-κB
above, NF-κB activity can also increase p53 expres- subunits, and how they control the selectivity of NF-κB-
sion and stability, which results at least in part from target-gene regulation, has only begun to be investigated.
the direct regulation of the p53 gene by NF-κB120,121. Furthermore, in many contexts, the contribution of
In addition, the ARF tumour suppressor, which binds NF-κB interactions with other transcriptional regulators
to and inhibits HDM2, thereby activating p53, can to NF-κB subunit function and target-gene specificity has
induce ATR and CHK1 checkpoint kinase activity73,123 not been investigated in depth, and the significance of
leading to RelA Thr505 phosphorylation, BCL-xL IKK-independent mechanisms of NF-κB activation has
repression and sensitization to apoptosis73. not yet been clearly defined. Therefore, although much
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