NF-kappaB Signaling Pathway

NF-kappaB is a family of transcription factors that sits at the centre of inflammatory, immune, and stress responses. In resting cells it is held inactive in the cytoplasm by inhibitor (IkappaB) proteins; signals that trigger IkappaB degradation release NF-kappaB to enter the nucleus and switch on genes for inflammation, cell survival, and immune defence.

Definition

NF-kappaB signalling is a pathway in which a stimulus activates the IkappaB kinase complex, leading to phosphorylation and proteasomal degradation of IkappaB inhibitors; the freed NF-kappaB dimers translocate to the nucleus and bind kappaB DNA elements to regulate transcription of inflammatory and survival genes.

Scope

The entry covers the canonical and non-canonical NF-kappaB activation routes, the inhibitor-degradation mechanism that controls them, the gene programmes they induce, and the pathway's links to chronic inflammation and cancer. It is reference material on mechanism, not clinical advice.

Core questions

• How is NF-kappaB kept inactive until a signal arrives?
• What distinguishes the canonical from the non-canonical activation route?
• How does one pathway produce diverse, stimulus-specific gene programmes?
• Why does chronic NF-kappaB activation link inflammation to cancer?

Key concepts

• Inhibitor (IkappaB) sequestration
• IkappaB kinase (IKK) complex
• Regulated proteasomal degradation
• Canonical and non-canonical pathways
• kappaB response elements
• Inflammatory and pro-survival gene induction
• Negative feedback through IkappaBalpha resynthesis

Mechanisms

In unstimulated cells, NF-kappaB dimers are bound by IkappaB proteins that mask their nuclear-localisation signal. A wide range of stimuli, including inflammatory cytokines and microbial products, activate the IkappaB kinase (IKK) complex, which phosphorylates IkappaB and marks it for ubiquitination and proteasomal destruction. Freed NF-kappaB then moves to the nucleus and binds kappaB elements to induce target genes (Hayden & Ghosh, 2008). A canonical route depends on IKK-beta and rapid IkappaB degradation, while a non-canonical route processes a precursor protein to release distinct dimers (Oeckinghaus & Ghosh, 2009). The response is self-limiting, in part because NF-kappaB induces resynthesis of its own inhibitor.

Clinical relevance

Because NF-kappaB drives expression of inflammatory and anti-apoptotic genes, sustained activity is implicated in chronic inflammation and in the connection between inflammation and tumour development (Coussens & Werb, 2002). This entry presents those associations as background knowledge and is not a guide to diagnosis or treatment.

Evidence & guidelines

The pathway is defined by extensive biochemical, genetic, and structural studies summarised in authoritative reviews; it is reference science rather than the subject of clinical guidelines. The cited reviews represent the consensus view of its mechanism and disease links.

History

NF-kappaB was first identified in the mid-1980s as a nuclear factor binding the immunoglobulin kappa light-chain enhancer in B cells, then recognised as an inducible regulator present in many cell types. Subsequent work defined the IkappaB inhibitors and the IKK complex, establishing the inhibitor-degradation paradigm that now organises understanding of the pathway.

Key figures

• David Baltimore
• Sankar Ghosh
• Matthew S. Hayden
• Lisa M. Coussens

Seminal works

• hayden-2008
• oeckinghaus-2009

Frequently asked questions

How is NF-kappaB activated so quickly after a signal?

It does not need to be newly made; it is already present in the cytoplasm held by an inhibitor, so degrading that inhibitor releases pre-existing NF-kappaB to act within minutes.

What kinds of genes does NF-kappaB turn on?

Mainly genes involved in inflammation, immune defence, and cell survival, which is why it is central to host defence and, when chronically active, to inflammatory disease.