Introduction
Nuclear Factor kappa B (NF-κB) is a ubiquitous transcription factor present in the cytoplasm of all cells. Upon activation by stress, cytokines, pathogens, free radicals, carcinogens, tumor promoters, or endotoxins, NF-κB translocates to the nucleus and regulates over 400 genes, including those encoding enzymes (COX-2, iNOS), cytokines (TNF, IL-1, IL-6, IL-8), chemokines, adhesion molecules, viral proteins, angiogenic factors, and cell cycle regulators. NF-κB is implicated in numerous diseases, including asthma, atherosclerosis, diabetes, Alzheimer’s disease, AIDS, and cancer. It orchestrates both innate and adaptive immune responses and plays a pivotal role in inflammation, apoptosis, and tumorigenesis.
1. Overview of NF-κB
NF-κB was first identified by David Baltimore and colleagues in 1986 as a nuclear factor in B cells that binds the kappa light chain enhancer of immunoglobulins. It exists in all animal cells in an inactive cytoplasmic state and is conserved from Drosophila to humans. NF-κB regulates over 300 genes involved in immune responses, cell growth, and inflammation.
NF-κB Family
The NF-κB family consists of five mammalian members:
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NF-κB1 (p50/p105)
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NF-κB2 (p52/p100)
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RelA (p65)
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RelB
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c-Rel
These proteins contain a Rel Homology Domain (RHD) for DNA binding, dimerization, and interaction with inhibitors (IκBs). NF-κB forms homo- or heterodimers, with the p50/p65 heterodimer being the most common inducible form. Some dimers act as transcriptional repressors unless associated with co-activators such as Bcl-3.
2. NF-κB Activation and Signaling
2.1 Inactive State
In resting cells, NF-κB is sequestered in the cytoplasm by inhibitors (IκBα, IκBβ, IκBγ, p105, p100). IκBα is the most abundant and blocks nuclear translocation.
2.2 Activation Mechanism
Upon stimulation:
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IκB proteins are phosphorylated at conserved serine residues by the IκB kinase (IKK) complex.
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Phosphorylated IκBs undergo polyubiquitination via the SCF-β-TrCP complex.
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IκBs are degraded by the 26S proteasome, freeing NF-κB to enter the nucleus.
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NF-κB binds kB elements in target gene promoters and initiates transcription.
2.3 IKK Complex
The IKK complex consists of:
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Catalytic subunits IKKα and IKKβ
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Regulatory subunit IKKγ (NEMO)
IKKβ is the primary kinase for IκB phosphorylation. Activation occurs via microbial products, cytokines, and other stress signals.
3. NF-κB Signaling Pathways
3.1 Canonical (Classical) Pathway
Triggered by TNF-α, IL-1, or TLR ligands, this pathway activates p50/RelA dimers. IKK-mediated IκB degradation allows nuclear translocation, crucial for innate immunity and inflammation.
3.2 Non-Canonical (Alternative) Pathway
Activated by BAFF and lymphoid organ development signals, it involves p100/RelB processing via NF-κB-inducing kinase (NIK) and IKKα, generating p52/RelB heterodimers. This pathway is essential for B-cell maturation and adaptive immunity.
4. Molecular Functions of NF-κB
4.1 Immune and Inflammatory Response
NF-κB activates genes for cytokines, chemokines, and adhesion molecules in response to infection. It regulates dendritic cells, macrophages, and T/B lymphocytes, maintaining immune homeostasis and orchestrating adaptive responses.
4.2 
Apoptosis and Anti-Apoptotic Genes
NF-κB promotes cell survival by up-regulating anti-apoptotic proteins such as Bcl-xL, c-IAP1/2, TRAF1/2, and c-FLIP. It interferes with caspase activation and mitochondrial apoptotic pathways. In certain contexts, NF-κB can enhance apoptosis via Fas/FasL induction.
4.3 Cell Cycle Regulation
NF-κB regulates cyclin D1 for G1/S transition and modulates cyclin-A and cyclin E/CDK2 complexes, influencing cell proliferation.
4.4 Negative Feedback
NF-κB activation is self-limiting via induction of IκBα, A20, and CYLD, which terminate signaling and stabilize homeostasis.
5. NF-κB in Human Diseases
5.1 Autoimmune and Chronic Inflammatory Diseases
Aberrant NF-κB signaling contributes to:
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Rheumatoid arthritis (RA) : Constitutive p50/p65 activation in synovium promotes TNFα and IL-1 expression.
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Multiple sclerosis (MS) :p65 upregulation in oligodendrocytes and macrophages amplifies CNS inflammation.
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Type-1 diabetes : NF-κB activation in pancreatic β-cells mediates inflammatory destruction.
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Thyroid autoimmunity :NF-κB modulates IL-6 expression and B-cell activation.
5.2 Asthma
NF-κB regulates cytokines, chemokines (RANTES, eotaxin), enzymes, and adhesion molecules, perpetuating airway inflammation. Corticosteroids inhibit NF-κB, highlighting its therapeutic relevance.
5.3 Cardiovascular Diseases
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Atherosclerosis : NF-κB in endothelial cells (ECs), smooth muscle cells (SMCs), and macrophages promotes inflammation and plaque formation.
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Myocardial infarction (MI) and reperfusion injury : Hypoxia and ROS activate NF-κB, influencing apoptosis and tissue repair.
5.4 Cancer
NF-κB is constitutively active in many tumors, including lymphomas, breast, prostate, lung, colon, head and neck cancers. It drives proliferation, angiogenesis, metastasis, chemoresistance, and survival via VEGF, cyclin D1, COX-2, and adhesion molecules.
6. Therapeutic Implications
NF-κB is a central regulator of inflammation, immunity, apoptosis, and cell proliferation. Targeting NF-κB signaling presents opportunities for therapy in autoimmune diseases, chronic inflammation, and cancer, though careful modulation is required due to its widespread role in homeostasis.