NF-κB family members share structural homology with the retroviral oncoprotein v-Rel, resulting in their classification as NF-κB/Rel proteins.[1]
There are five proteins in the mammalian NF-κB family:[18]
| Class | Protein | Aliases | Gene |
|---|---|---|---|
| I | NF-κB1 | p105 → p50 | NFKB1 |
| NF-κB2 | p100 → p52 | NFKB2 | |
| II | RelA | p65 | RELA |
| RelB | RELB | ||
| c-Rel | REL |
An oncogene is a gene that has the potential to cause cancer.[1] In tumor cells, these genes are often mutated, or expressed at high levels.[2]
Most normal cells will undergo programmed form of rapid cell death (apoptosis) when critical functions are altered and malfunctioning. Activated oncogenes can cause those cells designated for apoptosis to survive and proliferate instead.[3] Most oncogenes began as proto-oncogenes: normal genes involved in cell growth and proliferation or inhibition of apoptosis. If, through mutation, normal genes promoting cellular growth are up-regulated (gain-of-function mutation), they will predispose the cell to cancer; thus, they are termed "oncogenes". Usually multiple oncogenes, along with mutated apoptotic or tumor suppressor genes will all act in concert to cause cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. Many cancer drugs target the proteins encoded by oncogenes.[2][4][5][6]
https://en.wikipedia.org/wiki/Oncogene
The ankyrin repeat is a 33-residue motif in proteins consisting of two alpha helices separated by loops, first discovered in signaling proteins in yeast Cdc10 and Drosophila Notch. Domains consisting of ankyrin tandem repeats mediate protein–protein interactions and are among the most common structural motifs in known proteins. They appear in bacterial, archaeal, and eukaryotic proteins, but are far more common in eukaryotes. Ankyrin repeat proteins, though absent in most viruses, are common among poxviruses. Most proteins that contain the motif have four to six repeats, although its namesake ankyrin contains 24, and the largest known number of repeats is 34, predicted in a protein expressed by Giardia lamblia.[2]
Ankyrin repeats typically fold together to form a single, linear solenoid structure called ankyrin repeat domains. These domains are one of the most common protein–protein interaction platforms in nature. They occur in a large number of functionally diverse proteins, mainly from eukaryotes. The few known examples from prokaryotes and viruses may be the result of horizontal gene transfers.[3] The repeat has been found in proteins of diverse function such as transcriptional initiators, cell cycle regulators, cytoskeletal, ion transporters, and signal transducers. The ankyrin fold appears to be defined by its structure rather than its function, since there is no specific sequence or structure that is universally recognised by it.
Considering the atomic structures of individual ankyrin repeats, the loop is often a type 1 beta bulge loop, while both alpha-helices commonly have a Schellman loop at their N-terminus.
https://en.wikipedia.org/wiki/Ankyrin_repeat
In addition to mammals, NF-κB is found in a number of simple animals as well.[19] These include cnidarians (such as sea anemones, coral and hydra), porifera (sponges), single-celled eukaryotes including Capsaspora owczarzaki and choanoflagellates, and insects (such as moths, mosquitoes and fruitflies). The sequencing of the genomes of the mosquitoes A. aegypti and A. gambiae, and the fruitfly D. melanogaster has allowed comparative genetic and evolutionary studies on NF-κB. In those insect species, activation of NF-κB is triggered by the Toll pathway (which evolved independently in insects and mammals) and by the Imd (immune deficiency) pathway.[20]
NF-κB (green) heterodimerizes with RelB (cyan) to form a ternary complex with DNA (orange) that promotes gene transcription.[21]
NF-κB is important 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.[22]
NF-κB suppression of tumor necrosis factor cytotoxicity (apoptosis) is due to induction of antioxidant enzymesand sustained suppression of c-Jun N-terminal kinases (JNKs).[23]
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.[24]
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.[1] The identification of Toll-like receptors (TLRs) as specific pattern recognition molecules and the finding that stimulation of TLRs leads to activation of NF-κB improved our understanding of how different pathogens activate NF-κB. For example, studies have identified TLR4 as the receptor for the LPS component of Gram-negative bacteria.[25] TLRs are key regulators of both innate and adaptive immune responses.[26]
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.[27] In addition, p50 and p52 homodimers also bind to the nuclear protein Bcl-3, and such complexes can function as transcriptional activators.[28][29][30]
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.[31]
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.[32][33] 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.[32][34]
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.[35] 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.[36] 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.[37] 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.[38]
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.[39][40]
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.[41]
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.[42] 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.[43] Moreover, generation of the canonical and non-canonical dimers, viz RelA:p50 and RelB:p52, within the cellular milieu are mechanistically interlinked.[43] These analyses suggest that an integrated NF-κB system network underlies activation of both RelA and RelB containing dimer and that a malfunctioning canonical pathway will lead to an aberrant cellular response also through the non-canonical pathway. Most intriguingly, a recent study identified that TNF-induced canonical signalling subverts non-canonical RelB:p52 activity in the inflamed lymphoid tissues limiting lymphocyte ingress.[44] 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.[45][46] 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.[47]
NF-κB is a major transcription factor that regulates genes responsible for both the innate and adaptive immune response.[48] 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.[49]
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 systemincluding roles in plasticity, learning, and memory.[50] 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.[8] These activators of NF-κB in the nervous system all converge upon the IKK complex and the canonical pathway.
Recently there has been a great deal of interest in the role of NF-κB in the nervous system. Current studies suggest that NF-κB is important for learning and memory in multiple organisms including crabs,[10][11] fruit flies,[51] and mice.[8][9] NF-κB may regulate learning and memory in part by modulating synaptic plasticity,[7][52] synapse function,[51][53][54] as well as by regulating the growth of dendrites[55] and dendritic spines.[54]
Genes that have NF-κB binding sites are shown to have increased expression following learning,[9] suggesting that the transcriptional targets of NF-κB in the nervous system are important for plasticity. Many NF-κB target genes that may be important for plasticity and learning include growth factors (BDNF, NGF)[56] cytokines (TNF-alpha, TNFR)[57] and kinases (PKAc).[52]
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.[58][59]
Some of the reports of NF-κB in neurons appear to have been an artifact of antibody nonspecificity.[60] 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 coculture approaches. Moerman et al.[61] 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.[62] 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.[63] 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.
As such, many different types of human tumors have misregulated NF-κB: that is, NF-κB is constitutively active. Active NF-κB turns on the expression of genes that keep the cell proliferating and protect the cell from conditions that would otherwise cause it to die via apoptosis. In cancer, proteins that control NF-κB signaling are mutated or aberrantly expressed, leading to defective coordination between the malignant cell and the rest of the organism. This is evident both in metastasis, as well as in the inefficient eradication of the tumor by the immune system.[64]
Normal cells can die when removed from the tissue they belong to, or when their genome cannot operate in harmony with tissue function: these events depend on feedback regulation of NF-κB, and fail in cancer.[65]
Defects in NF-κB results in increased susceptibility to apoptosis leading to increased cell death. This is because NF-κB regulates anti-apoptotic genes especially the TRAF1 and TRAF2 and therefore abrogates the activities of the caspase family of enzymes, which are central to most apoptotic processes.[66]
However, even though convincing experimental data have identified NF-κB as a critical promoter of tumorigenesis, which creates a solid rationale for the development of antitumor therapy that is based upon suppression of NF-κB activity, caution should be exercised when considering anti-NF-κB activity as a broad therapeutic strategy in cancer treatment as data has also shown that NF-κB activity enhances tumor cell sensitivity to apoptosis and senescence.
In addition, it has been shown that canonical NF-κB is a Fas transcription activator and the alternative NF-κB is a Fas transcription repressor.[73] Therefore, NF-κB promotes Fas-mediated apoptosis in cancer cells, and thus inhibition of NF-κB may suppress Fas-mediated apoptosis to impair host immune cell-mediated tumor suppression.
https://en.wikipedia.org/wiki/NF-κB
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