NF-κB Signaling Pathway: The "Master Switch" of Inflammation

Introduction: Why is NF-κB Central to Inflammation?

NF-κB (Nuclear Factor-kappa B) is a transcription factor family governing immune and inflammatory responses, often called the "Master Switch of Inflammation." In response to stimuli such as infection, tissue injury, and stress, it controls the expression of hundreds of genes, including pro-inflammatory cytokines like TNF-α, IL-1β, and IL-6, as well as adhesion molecules and chemokines (Nature Reviews Immunology). Excessive activation of NF-κB is a major driving force for chronic inflammatory diseases (Rheumatoid Arthritis, IBD, COPD) and fibrosis, and has long been a focus as a therapeutic target.

1. NF-κB Family and Dimer Formation

NF-κB is a transcription factor family composed of five subunits (RelA/p65, RelB, c-Rel, p50, p52). These form dimers and bind to sequences called κB sites on DNA to control gene expression.

Major Dimers

• p65/p50: The most common combination in the canonical pathway. Has strong transcriptional activation ability.
• p52/RelB: The major product of the non-canonical pathway. Involved in lymphoid tissue development and B cell maturation.
• p50/p50: Transcriptional repressor type. Contributes to the resolution of inflammation.

2. Canonical Pathway: Rapid Inflammatory Response

Stimuli and Receptors

The canonical pathway is activated by stimuli such as:

• Pro-inflammatory Cytokines: TNF-α (via TNFR), IL-1β (via IL-1R)
• Microbial Molecules: LPS (via TLR4), Bacterial DNA (via TLR9)
• Stress: Oxidative stress, DNA damage

Activation Mechanism

1. Activation of IKK Complex

   • Receptor signals activate the IKK (IκB kinase) complex.
   • The IKK complex consists of IKKα, IKKβ (major catalytic subunit), and NEMO (regulatory subunit).
   • E3 ubiquitin ligases (cIAP1/2) add K63 ubiquitin chains to RIP1, forming a scaffold for IKK recruitment.

2. Phosphorylation and Degradation of IκB

   • Activated IKKβ phosphorylates Ser32/36 of the inhibitory protein IκBα.
   • Phosphorylated IκBα is recognized by the E3 ubiquitin ligase β-TrCP and ubiquitinated.
   • It is degraded by the 26S proteasome.

3. Nuclear Translocation and Gene Expression of NF-κB

   • Degradation of IκBα releases the p65/p50 dimer that was sequestered in the cytoplasm.
   • It translocates into the nucleus and binds to κB sites in the promoter regions of target genes.
   • Induces pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), adhesion molecules (ICAM-1, VCAM-1), chemokines (MCP-1, IL-8), etc.

3. Non-Canonical Pathway: Lymphoid Tissue Development and Adaptive Immunity

Stimuli and Receptors

The non-canonical pathway is activated by more limited stimuli:

• TNF Superfamily Receptors: BAFFR (B cell survival), CD40 (B cell activation), LTβR, RANK (Osteoclast differentiation)

Activation Mechanism

1. Stabilization of NIK

   • Normally, NIK (NF-κB-inducing kinase) is continuously degraded by the TRAF2/TRAF3/cIAP complex.
   • Receptor activation leads to TRAF3 degradation and NIK accumulation.

2. Activation of IKKα

   • Accumulated NIK phosphorylates and activates the IKKα homodimer.
   • NEMO is not required in the non-canonical pathway.

3. Processing of p100 and Generation of p52

   • Activated IKKα phosphorylates the C-terminus of p100 (NF-κB2 precursor).
   • Phosphorylated p100 is partially degraded by the proteasome and converted to the mature p52 form.

4. Nuclear Translocation of p52/RelB Dimer

   • The p52/RelB dimer translocates into the nucleus and induces genes involved in lymphoid tissue development and immune response.

4. Negative Feedback: Self-Limitation of NF-κB Signaling

Resynthesis of IκBα

• NF-κB induces transcription of the IκBα gene, its own inhibitor (negative feedback).
• Newly synthesized IκBα binds to NF-κB in the nucleus and pulls it back to the cytoplasm, terminating the signal.

Deubiquitinating Enzymes (DUBs)

• A20, CYLD: Remove ubiquitin chains from upstream signaling molecules (RIP1, TRAF, etc.), suppressing IKK activation.
• USP11, USP15: Remove ubiquitin directly from IκBα, preventing degradation.

5. NF-κB Pathway as a Therapeutic Target

Strategies for NF-κB Inhibition

• IKKβ Inhibitors: Directly inhibit the activity of the IKK complex (e.g., BMS-345541).
• Proteasome Inhibitors: Prevent degradation of IκBα (e.g., Bortezomib, approved for cancer treatment).
• Steroids (Glucocorticoids): Indirectly suppress NF-κB activity. Standard anti-inflammatory drugs.
• Biologics: Antibody drugs neutralizing upstream cytokines (TNF-α, IL-1β) (e.g., Infliximab, Etanercept).

Challenges

Since NF-κB is also essential for immune response and cell survival, systemic inhibition carries risks of severe side effects (susceptibility to infection, immunodeficiency, hepatotoxicity). Disease-specific or tissue-specific inhibition strategies are required.

Conclusion

The NF-κB signaling pathway is a central mechanism controlling the "On/Off" of inflammation. While it works defensively in acute inflammation, chronic activation leads to tissue destruction and fibrosis. It interacts with other pathways like TGF-β, Wnt, and YAP/TAZ to drive the transition from inflammation to fibrosis. Our inflammation and fibrosis models serve as a platform to evaluate the efficacy of therapeutics targeting the NF-κB pathway, from the molecular level to the individual level.

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References

1. Hayden MS, Ghosh S. Shared principles in NF-kappaB signaling. Cell. 2008;132(3):344-362.
2. Liu T, et al. NF-κB signaling in inflammation. Signal Transduct Target Ther. 2017;2:17023.
3. Sun SC. The non-canonical NF-κB pathway in immunity and inflammation. Nat Rev Immunol. 2017;17(9):545-558.