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

"Just inhibit NF-κB and problem solved"—why that's dangerously simplistic.

NF-κB is indispensable for infection defense and immune response. Broadly suppress it, and opportunistic infections spike. Yet chronically activated NF-κB drives fibrosis. This article explores how to wield this "double-edged sword" therapeutically—via selective IKK inhibitors and tissue-specific approaches.

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.