P65 and the NF-κB inflammatory pathway

The NF-κB family of transcription factors comprises structurally related proteins that play pivotal roles in immune regulation and cellular stress responses. The remaining members include RelB, c-Rel, and cleaved proteins NF-κB1 (p50) and  NF-κB2(p52). p50 and p65 form the most common heterodimer in the NF-κB signaling pathway, present in most cell types. In contrast, other members, for example, c-Rel, are predominantly found only in hematopoietic cells^1,2.

p65, RelB, and c-Rel are characterized by a Rel homology domain located in the N-terminal, which is combined with a transactivating domain in the C-terminal. In contrast, NF-κB1 and NF-κB2 are processed from large precursors (p105 and p100, respectively) into p50 and p52 subunits. This is a ubiquitin-mediated process, involving selective degradation from the C-terminal to remove ankyrin repeats^1,2.

As part of the NF-κB signaling pathway, p65 is typically involved in the body’s inflammatory response. This pathway can be induced by stressful stimuli, including free radicals, ultraviolet irradiation (UV), tumor necrosis factor α (TNFα), interleukin 1-beta (IL-1β), pathogen-associated molecular patterns (PAMPs), or bacterial lipopolysaccharides (LPS). NF-κB has been implicated in memory and synaptic plasticity, while aberrant protein levels have been linked to cancer^3,4.

NF-κB pathway following inflammatory signaling

In unstimulated cells, NF-κB subunits are restricted to the cytoplasm due to the inhibitory effects of the inhibitor of κB (IκB) family. IκBα or IκBβ selectively binds to the p50/p65 heterodimer and masks their nuclear localization signal (NLS), preventing their nuclear translocation.

Stressful stimuli induce NF-κB signaling through several cell membrane-bound receptors that converge on the activation of the IκB kinase (IKK) complex. The activated IKK complex phosphorylates IκBα, priming it for subsequent ubiquitylation and proteasome-mediated degradation. Disassociation of IκBα induces NF-κB heterodimers to relocate to the nucleus and bind to specific gene promoters to modulate the expression of pro- and anti-inflammatory proteins. As a negative feedback loop, the expression of IκBα and IκBβ is up-regulated by the NF-κB heterodimers in order to terminate the signaling pathway.

NF-κB subunits have been reported to be extensively modified, including O-linked N-acetyl glycosylation, ubiquitylation, nitrosylation, acetylation, prolyl isomerization, methylation as well as phosphorylation. The best-studied family member is p65^1,5.

NF-κB signaling plays a crucial role to fight infections, however, some pathogens have developed ways to alter NF-κB activity to their advantage. For example, the Human immunodeficiency virus (HIV) contains binding sites within its genome for NF-κB subunits, which drive viral gene expression as well as boosting its replication⁶.

References

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2. Courtois, G. & Gilmore, T. D. Mutations in the NF-κB signaling pathway: implications for human disease.  Oncogene   25, 6831–6843 (2006).
3. Gilmore, T. D. Introduction to NF-κB: players, pathways, perspectives.  Oncogene   25, 6680–6684 (2006).
4. Gutierrez, H. & Davies, A. M. Regulation of neural process growth, elaboration and structural plasticity by NF-κB.  Trends Neurosci.   34, 316–325 (2011).
5. Hiscott, J., Kwon, H. & Génin, P. Hostile takeovers: viral appropriation of the NF-κB pathway.  J. Clin. Invest.   107, 143–151 (2001).
6. Napetschnig, J. & Wu, H. Molecular basis of NF-κB signaling.  Annu. Rev. Biophys.   42, 443–468 (2013).
7. Surjit, M., Varshney, B. & Lal, S. The ORF2 glycoprotein of hepatitis E virus inhibits cellular NF-κB activity by blocking ubiquitination-mediated proteasomal degradation of IκBα in human hepatoma cells.  BMC Biochem.   13, 7 (2012).