Effect of activation NF-κB is crucial in regulating cellular responses because it belongs to the category of "rapid-acting" primary transcription factors, i.e., transcription factors that are present in cells in an inactive state and do not require new protein synthesis in order to become activated (other members of this family include transcription factors such as
c-Jun,
STATs, and
nuclear hormone receptors). This allows NF-κB to be a first responder to harmful cellular stimuli. Known inducers of NF-κB activity are highly variable and include reactive oxygen species (
ROS), tumor necrosis factor alpha (
TNFα), interleukin 1-beta (
IL-1β), bacterial lipopolysaccharides (
LPS),
isoproterenol,
cocaine,
endothelin-1 and
ionizing radiation. NF-κB suppression of
tumor necrosis factor cytotoxicity (apoptosis) is due to induction of
antioxidant enzymes and sustained suppression of
c-Jun N-terminal kinases (JNKs). Receptor activator of NF-κB (
RANK), which is a type of
TNFR, is a central activator of NF-κB.
Osteoprotegerin (OPG), which is a
decoy receptor homolog for RANK ligand (
RANKL), inhibits RANK by binding to RANKL, and, thus, osteoprotegerin is tightly involved in regulating NF-κB activation. Many bacterial products and stimulation of a wide variety of cell-surface
receptors lead to NF-κB activation and fairly rapid changes in gene expression. TLRs are key regulators of both innate and adaptive immune responses. Unlike RelA, RelB, and c-Rel, the p50 and p52 NF-κB subunits do not contain
transactivation domains in their C terminal halves. Nevertheless, the p50 and p52 NF-κB members play critical roles in modulating the specificity of NF-κB function. Although homodimers of p50 and p52 are, in general, repressors of κB site transcription, both p50 and p52 participate in target gene transactivation by forming
heterodimers with RelA, RelB, or c-Rel. In addition, p50 and p52 homodimers also bind to the nuclear protein
Bcl-3, and such complexes can function as transcriptional activators.
Inhibition In unstimulated cells, the NF-κB dimers are sequestered in the
cytoplasm by a family of inhibitors, called IκBs (Inhibitor of κB), which are proteins that contain multiple copies of a sequence called
ankyrin repeats. By virtue of their ankyrin repeat domains, the IκB proteins mask the
nuclear localization signals (NLS) of NF-κB proteins and keep them sequestered in an inactive state in the cytoplasm.
IκBs are a family of related proteins that have an N-terminal regulatory domain, followed by six or more ankyrin repeats and a
PEST domain near their C terminus. Although the IκB family consists of
IκBα,
IκBβ,
IκBε, and
Bcl-3, the best-studied and major IκB protein is IκBα. Due to the presence of ankyrin repeats in their C-terminal halves, p105 and p100 also function as IκB proteins. The c-terminal half of p100, that is often referred to as IκBδ, also functions as an inhibitor. IκBδ degradation in response to developmental stimuli, such as those transduced through
LTβR, potentiate NF-κB dimer activation in a NIK dependent non-canonical pathway.
Activation process (canonical/classical) Activation of the NF-κB is initiated by the signal-induced degradation of IκB proteins. This occurs primarily via activation of a kinase called the
IκB kinase (IKK). IKK is composed of a heterodimer of the catalytic IKKα and IKKβ subunits and a "master" regulatory protein termed
NEMO (NF-κB essential modulator) or IKKγ. When activated by signals, usually coming from the outside of the cell, the IκB kinase phosphorylates two serine residues located in an IκB regulatory domain. When phosphorylated on these serines (e.g., serines 32 and 36 in human IκBα), the IκB proteins are modified by a process called
ubiquitination, which then leads them to be degraded by a cell structure called the
proteasome. With the degradation of IκB, the NF-κB complex is then freed to enter the nucleus where it can 'turn on' the expression of specific genes that have DNA-binding sites for NF-κB nearby. The activation of these genes by NF-κB then leads to the given physiological response, for example, an inflammatory or immune response, a cell survival response, or cellular proliferation. Translocation of NF-κB to nucleus can be detected immunocytochemically and measured by laser scanning cytometry. NF-κB turns on expression of its own repressor, IκBα. The newly synthesized IκBα then re-inhibits NF-κB and, thus, forms an auto feedback loop, which results in oscillating levels of NF-κB activity. In addition, several viruses, including the AIDS virus HIV, have binding sites for NF-κB that controls the expression of viral genes, which in turn contribute to viral replication or viral pathogenicity. In the case of HIV-1, activation of NF-κB may, at least in part, be involved in activation of the virus from a latent, inactive state. YopP is a factor secreted by
Yersinia pestis, the causative agent of plague, that prevents the ubiquitination of IκB. This causes this pathogen to effectively inhibit the NF-κB pathway and thus block the immune response of a human infected with Yersinia.
Inhibitors of NF-κB activity Concerning known protein inhibitors of NF-κB activity, one of them is
IFRD1, which represses the activity of NF-κB p65 by enhancing the
HDAC-mediated deacetylation of the p65 subunit at lysine 310, by favoring the recruitment of HDAC3 to p65. In fact IFRD1 forms trimolecular complexes with p65 and HDAC3. The NAD-dependent protein deacetylase and longevity factor
SIRT1 inhibits NF-κB gene expression by deacetylating the RelA/p65 subunit of NF-κB at lysine 310.
Non-canonical/alternate pathway A select set of cell-differentiating or developmental stimuli, such as
lymphotoxin β-receptor (LTβR),
BAFF or
RANKL, activate the non-canonical NF-κB pathway to induce NF-κB/RelB:p52 dimer in the nucleus. In this pathway, activation of the
NF-κB inducing kinase (NIK) upon receptor ligation led to the phosphorylation and subsequent proteasomal processing of the NF-κB2 precursor protein p100 into mature p52 subunit in an IKK1/IKKa dependent manner. Then p52 dimerizes with RelB to appear as a nuclear RelB:p52 DNA binding activity. RelB:p52 regulates the expression of homeostatic lymphokines, which instructs lymphoid organogenesis and lymphocyte trafficking in the secondary lymphoid organs. In contrast to the canonical signaling that relies on NEMO-IKK2 mediated degradation of IκBα, -β, -ε, non-canonical signaling depends on NIK mediated processing of p100 into p52. Given their distinct regulations, these two pathways were thought to be independent of each other. However, it was found that syntheses of the constituents of the non-canonical pathway, viz RelB and p52, are controlled by canonical IKK2-IκB-RelA:p50 signaling. Moreover, generation of the canonical and non-canonical dimers, viz RelA:p50 and RelB:p52, within the cellular milieu are mechanistically interlinked. Mechanistically, TNF inactivated NIK in LTβR‐stimulated cells and induced the synthesis of
Nfkb2 mRNA encoding p100; these together potently accumulated unprocessed p100, which attenuated the RelB activity. A role of p100/
Nfkb2 in dictating lymphocyte ingress in the inflamed lymphoid tissue may have broad physiological implications. In addition to its traditional role in lymphoid organogenesis, the non-canonical NF-κB pathway also directly reinforces inflammatory immune responses to microbial pathogens by modulating canonical NF-κB signalling. It was shown that p100/
Nfkb2 mediates stimulus-selective and cell-type-specific crosstalk between the two NF-κB pathways and that
Nfkb2-mediated crosstalk protects mice from gut pathogens. On the other hand, a lack of p100-mediated regulations repositions RelB under the control of TNF-induced canonical signalling. In fact, mutational inactivation of p100/
Nfkb2 in multiple myeloma enabled TNF to induce a long-lasting RelB activity, which imparted resistance in myeloma cells to chemotherapeutic drug.
In immunity NF-κB is a major transcription factor that regulates genes responsible for both the
innate and
adaptive immune response. Upon activation of either the
T- or
B-cell receptor, NF-κB becomes activated through distinct signaling components. Upon ligation of the T-cell receptor, protein kinase
Lck is recruited and phosphorylates the
ITAMs of the
CD3 cytoplasmic tail.
ZAP70 is then recruited to the phosphorylated ITAMs and helps recruit
LAT and
PLC-γ, which causes activation of
PKC. Through a cascade of phosphorylation events, the kinase complex is activated and NF-κB is able to enter the nucleus to upregulate genes involved in T-cell development, maturation, and proliferation.
In the nervous system In addition to roles in mediating cell survival, studies by
Mark Mattson and others have shown that NF-κB has diverse functions in the
nervous system including roles in
plasticity, learning, and memory. In addition to stimuli that activate NF-κB in other tissues, NF-κB in the nervous system can be activated by Growth Factors (
BDNF,
NGF) and synaptic transmission such as
glutamate. and mice. as well as by regulating the growth of
dendrites and
dendritic spines. cytokines (
TNF-alpha,
TNFR) and kinases (
PKAc). Despite the functional evidence for a role for Rel-family transcription factors in the nervous system, it is still not clear that the neurological effects of NF-κB reflect transcriptional activation in neurons. Most manipulations and assays are performed in the mixed-cell environments found in vivo, in "neuronal" cell cultures that contain significant numbers of glia, or in tumor-derived "neuronal" cell lines. When transfections or other manipulations have been targeted specifically at neurons, the endpoints measured are typically electrophysiology or other parameters far removed from gene transcription. Careful tests of NF-κB-dependent transcription in highly purified cultures of neurons generally show little to no NF-κB activity. Some of the reports of NF-κB in neurons appear to have been an artifact of antibody nonspecificity. Of course, artifacts of cell culture—e.g., removal of neurons from the influence of glia—could create spurious results as well. But this has been addressed in at least two co-culture approaches. Moerman et al. used a coculture format whereby neurons and glia could be separated after treatment for
EMSA analysis, and they found that the NF-κB induced by glutamatergic stimuli was restricted to glia (and, intriguingly, only glia that had been in the presence of neurons for 48 hours). The same investigators explored the issue in another approach, utilizing neurons from an NF-κB reporter transgenic mouse cultured with wild-type glia; glutamatergic stimuli again failed to activate in neurons. Some of the DNA-binding activity noted under certain conditions (particularly that reported as constitutive) appears to result from Sp3 and Sp4 binding to a subset of κB enhancer sequences in neurons. This activity is actually inhibited by glutamate and other conditions that elevate intraneuronal calcium. In the final analysis, the role of NF-κB in neurons remains opaque due to the difficulty of measuring transcription in cells that are simultaneously identified for type. Certainly, learning and memory could be influenced by transcriptional changes in astrocytes and other glial elements. And it should be considered that there could be mechanistic effects of NF-κB aside from direct transactivation of genes. == Clinical significance ==