NF-kB was originally named for the context in which it was discovered, a Nuclear Factor binding near the kappa light chain locus in B cells4. NF-kB is not a single protein but a group of homo- and hetero-dimers of a family of five individual subunits: RelA (p65), RelB, c-Rel, NFKB1 (p105), and NFKB2 (p100)4. Dimers of NF-kB bind to a consensus DNA sequence known as kB. Only RelA (p65), RelB, and c-Rel contain transcriptional transactivation domains1. The most iconic of the NF-kB dimers are p50:p65 (RelA) and p52:RelB4.
This small set of proteins plays a critical role in the regulation of a multitude of physiological processes including cell survival, proliferation, and differentiation in the immune system, bone development, and the nervous and cardiovascular systems2. Disruption of NF-kB pathways by genetic mutation results in a wide range of inflammatory and inherited diseases, as well as cancer.
In quiescent cells NF-kB proteins are sequestered in the cytoplasm by a family of IkB (Inhibitors of kB) proteins1,2,4. Upon receptor-mediated stimulation, IkB proteins are degraded, releasing the NF-kB proteins and allowing them to translocate to the nucleus where they can activate gene expression. Coincident with the degradation of IkB is the cleavage of NFKB1 or NFKB2 to yield p50 or p52 respectively. The p50:RelA and p52:RelB homodimers are the active nuclear forms of NF-kB1,2,4.
Thus the key trigger for NF-kB activation is the dissociation and degradation of IkB. IkB stability is regulated by a combination of phosphorylation and ubiquitination events, leading to degradation in the proteasome1,2,4.
The effect of NF-kB activation, positive or negative, survival or death, and the set of target genes to be acted on is determined by the cell type, the surface receptor(s) engaged, and the physiological context of the cells.
There are two pathways leading from receptor stimulation to NF-kB activation: the Canonical Pathway (CP) and the Non-canonical (Alternative) Pathway (NP) 1-4. The pathways differ in the receptor proximal signaling molecules and the NF-kB and IKK components involved. The common feature of both pathways is the release of NF-kB from IkB followed by the translocation of NF-kB to the nucleus1-4.
Cell surface receptors associated with the CP include the TNFR, TLRs, TCR, BCR, and IL-1R2. Receptor stimulation activates the IkB kinase (IKK) complex which consists of IKKa, IKKb, and NEMO (NF-kB Essential Modifier)2,4. The IKK complex phosphorylates IkB proteins leading to their ubiquitination and degradation. Once released from IkB, the subunits of NF-kB are free to move into the nucleus and activate gene expression. Regulation of the CP involves a number of other proteins and complexes including TRAFs, TAK, Caspase 8, Casein Kinase 1, the CBM complex (in lymphocytes), and a multi-component ubiquitinating complex known as the LUBAC (Linear Ubiquitin Assembly Complex)3,4.
The NP is activated by engagement of an alternative set of receptors including CD40L, LTbR, BAFFR2,3. Stimulation of these receptors activates the serine/threonine kinase NIK (NF-kB inducing kinase), which in turn phosphorylates the IKKa homodimer. The IKKa homodimer phosphorylates p100 (NFKB2) resulting in its processing to p52. p52 combines with RelB to produce an activated form of NF-kB which translocates to the nucleus2,3. As with the CP, there are a number of intermediate proteins that impact the activity of the NP3.
Though NF-kB is an attractive target for a number of autoimmune and inflammatory diseases, as well as cancer therapy, its pleiotropic effects on so many physiological systems make it difficult to access and manipulate therapeutically. Potential sites for intervention are (1) the activity of the IKK complex, and (2) the degradation of IkB, either of which could affect the stability and localization of active NF-kB2. A less direct approach is illustrated by the use of proteasome inhibitors such as bortezomib4.
- Hayden MS, Ghosh S. 2012. NF-κB, the first quarter-century: remarkable progress and outstanding questions. Genes Dev. 26(3): 203-34. PMID: 22302935.
- Pires BRB, Silva RCMC, Ferreira GM, Abdelhay E. 2018. NF-kappaB: Two Sides of the Same Coin. Genes 9(1): 24. PMID: 29315242.
- Sun SC. 2017. The non-canonical NF-κB pathway in immunity and inflammation. Nat Rev Immunol. 17(9): 545-558. PMID: 28580957.
- Zhang Q, Lenardo MJ, Baltimore D. 2017. 30 Years of NF-κB: A Blossoming of Relevance to Human Pathobiology. Cell. 168(1-2): 37-57. PMID: 28086098.