|NF-KappaB Family Pathway
NF-KappaB (Nuclear Factor-KappaB) is a
heterodimeric protein composed of different combinations of members of the Rel family of
transcription factors. The Rel/ NF-KappaB family of transcription factors are involved
mainly in stress-induced, immune, and inflammatory responses. In addition, these molecules
play important roles during the development of certain hemopoietic cells, keratinocytes,
and lymphoid organ structures. More recently, NF-KappaB family members have been
implicated in neoplastic progression and the formation of neuronal synapses. NF-KappaB is
also an important regulator in cell fate decisions, such as programmed cell death and
proliferation control, and is critical in tumorigenesis (Ref.1).
NF-KappaB is composed of homo- and
heterodimers of five members of the Rel family including NF-KappaB1(p50), NF-KappaB2
(p52), RelA (p65), RelB, and c-Rel (Rel). Hetero and Homo-dimerization of NF-KappaB
proteins which exhibit differential binding specificities includes p50/RelA, p50/c-Rel,
p52/c-Rel, p65/c-Rel, RelA/RelA, p50/p50, p52/p52, RelB/p50 and RelB/p52. All the Rel
proteins contain a conserved N-terminal region, called the RHD (Rel Homology Domain). The
N-terminal part of the RHD contains the DNA-binding domain, whereas the dimerization
domain is located in the C-terminal region of the RHD. Close to the C-terminal end of the
RHD lies the NLS (Nuclear Localization Signal), which is essential for the transport of
active NF-KappaB complexes into the nucleus. NF-KappaB1 and RelA were the first NF-KappaB
proteins to be identified. Their N-terminal 300 AA revealed high similarity to the
oncoprotein v-Rel, its cellular homologue c-Rel and the Drosophila protein Dorsal what
resulted in the terms Rel proteins and RHD. The Rel/ NF-KappaB proteins can be divided
into two groups: Only RelA (p65), RelB and c-Rel (and Dorsal and Dif in Drosophila)
contain potent TDs (Transactivation Domains) within sequences C-terminal to the RHD. The
TDs consist of abundant serine, acidic and hydrophobic aminoacids that are essential for
transactivation activity. In contrast, p50 and p52 do not possess TDs, and therefore
cannot act as transcriptional activators by themselves. NF-KappaB1and NF-KappaB2 are
produced as p105 and p100 precursors, respectively. The NF-KappaB1 p105 precursor appears
to undergo constitutive processing by the cellular proteasome that removes the C-terminal
I-KappaB-like portion to generate p50. NF-KappaB2 p100 precursor can be processed to
remove the I-KappaB-like C-terminus, allowing the active p52 N-terminal half to function
in transcriptional regulation. Homo- or heterodimers of p50 and p52 were even reported to
repress KappaB site-dependent transcription, possibly by competing with other
transcriptionally active dimers (e.g. p50/RelA) for DNA binding (Ref.2).
NF-KappaB dimers are sequestered in the
cytosol of unstimulated cells via non-covalent interactions with a class of inhibitor
proteins, called I-KappaBs. To date seven I-KappaBs have been identified: I-KappaB-alpha,
I-KappaB-beta, I-KappaB-gamma, I-KappaB-epsilon, BCL3, p100 and p105. All known I-KappaBs
contain multiple copies of a 30-33 aa sequence, called ankyrin repeats which mediate the
association between I-KappaB and NF-KappaB dimers. The ankyrin repeats interact with a
region in the RHD of the NF-KappaB proteins and by this mask their NLS and prevent nuclear
translocation. Signals that induce NF-KappaB activity cause the phosphorylation of
I-KappaBs, their dissociation and subsequent degradation, thereby allowing activation of
the NF-KappaB complex. Activated NF-KappaB complex translocates into the nucleus and binds
DNA at Kappa-B-binding motifs such as 5-prime GGGRNNYYCC 3-prime or 5-prime HGGARNYYCC
3-prime (where H is A, C, or T; R is an A or G purine; and Y is a C or T pyrimidine) and
induce gene expression. The degradation of I-KappaB proteins that permits NF-KappaB
molecules to move into the nucleus is also carried out by the proteasome but only after
prior phosphorylation of I-KappaB by the IKK (I-KappaB Kinase Complex). The IKK is
composed of three subunits: two, IKK-alpha (IKK1) and IKK-beta (IKK2), are bonafide
kinases, while the third, IKK-gamma (NEMO), has no catalytic activity but plays a critical
regulatory role. IKK-alpha is the predominant I-KappaB kinase. Phosphorylated I-KappaB is
recognized by Beta-TrCP, a component of the SCF (skp-1/ Cul/F box) ubiquitin ligase
complex that mediates poly-ubiquitination of I-KappaB and its subsequent proteasomal
degradation. In contrast, IKK-alpha mediates the phosphorylation-dependent processing of
p100, resulting in the generation of p52 (Ref.3).
NF-KappaB can be activated by exposure of
cells to LPS (Lipopolysaccharides) or inflammatory cytokines such as TNF (Tumour Necrosis
Factor) or IL-1 (Interleukin-1), growth factors, lymphokines, oxidant-free radicals,
inhaled particles, viral infection or expression of certain viral or bacterial gene
products, UV irradiation, B or T-Cell activation, and by other physiological and non
physiological stimuli. The most potent NF-KappaB activators are the proinflammatory
cytokines IL-1 and TNF, which cause rapid phosphorylation of KappaBs at two sites within
their N-terminal regulatory domain. TNF, which is the best-studied activator, binds to its
receptor and recruits a protein called TRADD (TNF-Associated Receptor Death Domain). TRADD
binds to the TRAF2 (TNF Receptor-Associated Factor-2) that recruits NIK
(NF-KappaB-Inducible Kinase). Both IKK1 and IKK2 have canonical sequences that can be
phosphorylated by the MAP (Mitogen Activated Protein) kinase NIK/MEKK1 and both kinases
can independently phosphorylate I-KappaB-alpha or I-KappaB-beta. TRAF2 also interacts with
A20, a zinc finger protein whose expression is induced by agents that activate NF-KappaB.
A20 functions to block TRAF2-mediated NF-KappaB activation. A20 also inhibits TNF and IL-1
induced activation of NF-KappaB suggesting that it may act as a general inhibitor of
NF-KappaB activation. CD40, another member of the TNF receptor family, can signal the
induced processing of p100 to p52. The ligand for CD40, CD40L (CD154), is expressed on
activated CD4-T cells, and when it engages CD40 in a T:B interaction, can induce B-Cell
proliferation and differentiation. CD40 signaling induces p100 processing through NIK in
the non-canonical pathway. LT-BetaR (LymphotoxinBeta Receptor), which is critically
important for the development and organization of lymphoid tissue also gives way to two
separate pathways, one that activates the canonical NF-KappaB pathway and depends upon
IKK-beta and IKK-gamma/NEMO and another that induces p100 processing dependent on NIK and
The recognition of bacterial and viral
products by Toll-like receptors on cells of the innate immune system also results in
NF-KappaB induction, leading to the production of proinflammatory cytokines and the
activation of Antigen Presenting Cell for T-Cell costimulation in the adaptive immune
response. Viral infection leads to the increased expression and secretion of the cytokine
interferon gamma (IFN-gamma) from host cells. IFN-gamma activates the double-stranded RNA
(dsRNA)-dependent serine-threonine protein kinase R (PKR). dsRNA produced during viral
replication induces PKR dimerization, autophosphorylation, and activation of the
eIF-2alpha kinase activity. When eIF-2alpha is phosphorylated, cellular and viral protein
translation cannot efficiently occur. Alternatively, bacterial products or cellular stress
can also activate PKR by an endogenous gene product called PACT. The binding of PACT to
PKR promotes conformational changes that allow PKR to activate the downstream signaling
pathways leading to the activation of NF-KappaB. Several survival and stimulatory factors
that is important in the development and BAFF (B-Cell activating factor) coordinated
response of B and T lymphocytes also employ NF-KappaB to carry out their instructive
functions. BAFF induces B-Cell survival and development and requires the specific BAFF
receptor, BAFFR (BR3), as well as NF-KappaB2, and the NIK kinase. In both immature and
mature B cells, BAFF induced processing of p100 to p52, dependent on BAFF-R and NIK, and
in addition this process is independent of the canonical IKK complex (Ref.5).
The interaction of the Antigen Presenting
Cell and T-Cell also causes NF-KappaB activation in both cell types NF-KappaB activation
is triggered in T-Cells by the engagement of the T cell receptor and the CD28 receptor
with their ligands, MHC class II, and the costimulatory molecules CD80 and CD86 presented
by Antigen Presenting Cells. The T-Cell receptor and CD28 synergize in induction of the
NF-KappaB -dependent genes required for T-Cell activation and proliferation, such as IL-2,
IL-2 receptor (IL-2R), and IFN (Interferons). Activated T-Cells, in turn, elicit NF-KappaB
activation in Antigen Presenting Cells (Ref.6).
Exposure of cells to different forms of
radiation and other genotoxic stresses stimulates signaling pathways that activate
transcription factor NF-KappaB, which elicit various biological responses through
induction of target genes. UV-C or UV-B also induces NF-KappaB activity. In addition to
short wavelength UV radiation, NF-KappaB activity is also induced by exposure to even
shorter wavelength photons, gamma rays or IR (Ionizing Radiation). Although both
radiations induce I-KappaB degradation they operate through two distinct mechanisms.
Whereas IR exposure results in IKK activation and IR-induced I-KappaB degradation is
phosphorylation dependent, exposure to UV-C does not result in IKK activation, and
UV-induced I-KappaB degradation occurs independently of its N-terminal serine
Several Growth factors also activates
NF-KappaB. HGF (Hepatocyte Growth Factor) stimulation enhances both NF-KappaB DNA binding
and NF-KappaB-dependent transcriptional activity. The signaling mechanisms mediating these
effects include the classical I-KappaB-alpha phosphorylation-degradation cycle, as well as
the ERK1/2 (Extracellular Signal-Regulated Kinases-1 and -2) and p38 MAPK. NF-KappaB
activation contributes to HGF-mediated proliferation and tubulogenesis. Another pathway,
which has been implicated in the enhancement of NF-KappaB-dependent transcription, is
PI3K/Akt. An essential role for PI3K/Akt in enhancing the transcriptional activity of the
NF-KappaB p65 subunit has been described downstream of TNF-Alpha or IL-1. However, this
pathway does not seem to be required in all cell types or for all stimuli (Ref.8).
NF-KappaB was also found to stimulate
transcription of Cyclin-D1, a key regulator of G1 checkpoint control. Two NF-KappaB
binding sites in the human Cyclin-D1 promoter conferred activation by NF-KappaB as well as
by growth factors. Both levels and kinetics of Cyclin-D1 expression during G1 phase were
controlled by NF-KappaB. Moreover, inhibition of NF-KappaB caused a pronounced reduction
of serum-induced Cyclin-D1 associated kinase activity and resulted in delayed
phosphorylation of the retinoblastoma protein. Inappropriate activation of NF-KappaB has
been linked to inflammatory events associated with autoimmune arthritis, asthma, septic
shock, lung fibrosis, glomerulonephritis, atherosclerosis, and AIDS. In contrast, complete
and persistent inhibition of NF-KappaB has been linked directly to apoptosis,
inappropriate immune cell development, and delayed cell growth (Ref.9).
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- Baeuerle, P. A.
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- Joel L. Pomerantz and David Baltimore.
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- Fulvio D'Acquisto, and Sankar Ghosh. PACT and PKR: Turning on NF- B in the Absence of
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NF-KappaB, Nuclear Factor-KappaB,
NF-KappaB1, p50, NF-KappaB2, p52, RelA, p65, RelB, c-Rel, Rel, RHD, Rel Homology Domain,
NLS, Nuclear Localization Signal, TDs, Transactivation Domains, IKK-alpha, IKK1, IKK-beta,
IKK2, LPS, Lipopolysaccharides, TNF, Tumour Necrosis Factor, TNFR, TNF Receptor, IL-1,
IL-1R, Interleukin-1, GF, Growth Factors, Lymphokines, Oxidant-Free Radicals, Inhaled
Particles, Viral Infection, Certain Viral or Bacterial Gene Products, UV Irradiation, B or
T Cell Activation, TRADD, TNF-Associated Receptor Death Domain, TRAF2, TNF
Receptor-Associated Factor-2, NIK, NF-KappaB Inducible Kinase, MAP, Mitogen Activated
Protein, PKR, Protein Kinase-R, ERK1/2, Extracellular Signal-Regulated Kinases-1/2 p38
MAPK, FADD, Fas Associated Death Domain, IRAK, IL-1 Receptor-Associated Kinases, RIP,
Receptor-Interacting Protein, TRAFs, TNF Receptor-Associated Factors, MyD88, Myeloid
Differentiation Primary Response Gene-88, TollIP, Toll-Interacting Protein, TAK1,
TGF-Beta-Activating Kinase-1, Fyn, Lyn, PKC, Protein Kinase-C, IKK Complex, IKK-Alpha,
IKK-Beta, IKK-Gamma, BAFF, B-cell Activation Factor, HGF, Hepatocyte Growth Factor, APC,
Antigen Presenting Cell, CBP, CREB Binding Protein, p300.
MALP, Mycoplasmal macrophage-Activating
Lipopeptide, PGN, Peptidoglycans, PI3K, Phosphoinositide-3-Kinase, Akt, Cyclin-D1, Vav,
ZAP70, Zeta-Chain-Associated Protein Kinase 70, BIMP, BCL10, MALT, Mucosa Associated
Lymphoid Tissue Lymphoma Translocation Gene, BLK, B Lymphoid Tyrosine Kinase.
Transcription Regulation, Cell Signaling.