NFkB Flashcards
NFkB relevance
The role of the NF-κB signaling pathway in the development of hepatocellular carcinoma, for example, continues to remain controversial. Elimination of NF-κB activity in hepatocytes surprisingly resulted in elevated inflammatory cytokine expression and spontaneous carcinogenesis in every animal within a year, suggesting a tumor suppressor role for NF-κB.
IKK relevance
They showed that the conditional knockout of TAK1, the kinase that phosphorylates IKK, gives a similar phenotype.
Recent experimental data from several laboratories have revealed that IKKα also functions as a tumor suppressor in human squamous cell carcinomas of the skin, lungs, and head and neck
Chemical carcinogenesis studies in mice have shown that reduction in IKKα expression increased the number and size of Ras initiated skin tumors and promoted their progression, indicating that reduced IKKα expression provides a selective growth advantage that cooperates with Ras activity to promote skin carcinogenesis
Why NF-κB is a good target for drug development?
Activation of NF-κB blocks apoptosis and mediates tumor cell proliferation.
Tumor cells frequently express constitutively activated form of NFκB.
Tumor microenvironment can induce NF-κB activation.
NF-κB activation induces resistance to chemotherapeutic agents and radiation.
Several genes involved in tumor initiation, promotion, and metastasis are regulated by NF-kB.
IKK
- An activation loop similar to the one found in the MAP-kinase kinase(MEK) family of proteins has been identified between the kinase subdomains VII and VIII.
- Studies by in vitro or ex vivo approaches indicate that both IKKα and IKKα are interchangeable in phosphorylating S32/S36 of IκBα, and S19/S23 of IκBβ.
IKKb vs IKKa
- First, IKKβ is far more potent than IKKα in IκBα phosphorylation in response to proinflammatory stimuli, such as the signals induced by TNFa, IL-1 and LPS .
- Second, whereas IKKα seems to be more responsible for NF- kB-inducing kinase (NIK) signals, IKKβ appears more important in mediating TAK1/MEKK1 reactions.
- Third, IKKα, but not IKKβ, is physiologically involved in NIK-mediated carboxyl terminal phosphorylation and subsequent process of NF-κB2 (p100) precursor.
- Fourth, IKKα controls keratinocyte differentiation by a kinase-independent mechanism that affects the production of keratinocyte differentiation-inducing factor (kDIF). IKKβ, in contrast, is not necessary for this function.
- Finally, although both IKKα and IKKβ can phosphorylate multiple regions of β-catenin, an opposite effect of IKKα and IKKβ on the transcriptional activity and intracellular localization of β-catenin has been observed. IKKα increased the nuclear localization and transcriptional activity of β-catenin , whereas IKKβ decreased the nuclear localization and transcriptional activity of β-catenin.
IKK disease
Incontinentia Pigmenti(IKKy absent)
* Rare X-linked dominant condition.
* Characteristics include
– Skin lesions
– Hair, eye, teeth, and nail abnormalities
– Osteosclerosis
– Immune system disorders (immunodeficiency resulting in recurrent infections)
– Some males do survive for several years (usually have a milder genetic abnormality)
NIK
- NF-κB-Inducing Kinase (NIK) NIK, a member of the MAPKKK family, was originally identified as a tumor necrosis factor (TNFα) receptor associating factor 2 (TRAF2)- interacting kinase whose overexpression results in potent NF-kB activation without any considerable effect on MAPKs .
- Transient transfection of NIK into human embryonic kidney 293 cells indicated that IKKα was more responsive to NIK, whereas IKKβ was slightly more responsive to MEKK1.
- When the abilities of MEKK1 and NIK to activate total IKK kinase activity are compared, most of studies show that NIK is a much stronger activator of the NF-κB transcriptional reporter than MEKK1. NIK could preferentially phosphorylate IKKα on Sl76 in the activation loop, leading to the activation of IKKα kinase activity.
- In contrast, MEKK1 was found to preferentially phosphorylate the corresponding serine residue, Sl77, in the activation loop of IKKβ. However, studies using cells derived from NIK-deficient mice have indicated that NIK appears to be dispensable in IKK activation induced by TNFα or IL-1 .
TAK1
- TGFβ-Activated Kinase 1 (TAK1) is a member of the MAPKKK (MAP3K) family, which was originally identified as a kinase mediating the signaling pathway of TGFβ superfamily members.
- Overexpression of TAK1 together with its activator protein, TAK1 binding protein 1 (TAB1), induced the nuclear translocation of NF-kB in a NIK-independent manner.
- Recruitment of TAB1 and/or TAB2 to TAK1 activates the kinase activity of TAK1, resulting in phosphorylation of the serine residues in the activation loop of IKK and subsequent dissociation of TAK1 from IKK complex.
TNFa
Tumor Necrosis Factor α (TNF-α)
Pro-inflammatory cytokine produced by activated macrophages and tumor cells.
Induce DNA damage and inhibit DNA repair, growth factor for cancer cells.
Anti-apoptotic (inducer of NF-κB)
Promote angiogenesis (inducer of angiogenesis factors, thymidine phosphorylase and MMPs)
Regulate a network of chemokines that promote cancer (upregulate CXCR4 in ovarian cancer)
Pro-cancer
Blocking : Promote gastric lesion and inflamed colonic mucosa
Anti-cancer (high concentration): destruct tumor vasculature and kill tumor cell
Overexpression : Resistance to skin carcinogenesis
TNF blockers adverse effects: risks of tuberculosis reactivation and invasive fungal infections
TNF inhibitors have a number of known side effects, mainly related to their immunosuppressant activity.
Since TNF is a important cytokine when fighting against tuberculosis, these drugs can reactivate a latent tuberculosis infection.
Natural IKK inhibitors
- ATP analogs include natural products such as β-carboline and its derivatives, and several synthetic compounds developed by pharmaceutical companies,
including SC-839 which has an approximately 200-fold preference for IKKβ as compared to IKKα.
Synthetic IKK inhibitors
- On the other hand, the synthetic IKKβ inhibitor BMS-345541 binds to an allosteric site on the kinase. Several thiol-reactive compounds, such as parthenolide, certain epoxyquinoids, and arsenite, have been shown to block IKKβ activity through Cys-179 probably through direct conjugation to the thiol group of this cysteine. Although not formally proven, it is likely that interaction of such compounds with Cys-179 interferes with phosphorylation-induced activation of IKKβ, in that Cys-179 is located between Ser-177 and Ser-181 which are part of the kinase activation loop and must be phosphorylated for IKKβ to be activated in response to many upstream signals, such as LPS and TNF.
NEMO inhibitors
- NEMO can also serve as a target for IKK complex inhibition. In particular, introduction of a cell-permeable peptide corresponding to the NEMO-binding domain (NBD) of IKKβ can block both the binding of NEMO to IKK and induction of the classical NF-κB pathway by TNF. Moreover, the NBD peptide has shown efficacy in mouse models of inflammation by both topical and systemic administration. Similarly, peptides corresponding to the region of NEMO required for oligomerization can also block NF-κB activation. Chemical inhibitors of NEMO have not yet been identified or developed.
Proteosome Inhibitors
- There are three general classes of proteasome inhibitors. One class consists of peptide aldehydes, which can inhibit the chymotrypsin-like activity of the proteasome complex; these include MG101, MG132, and MG115. A second class includes microbial products lactacystin and its precursor β-lactone, which block proteasome activity by acylating an active site threonine residue in the b subunits of the 20S proteasome. A third class is comprised of peptide boronic acids (or dipeptidyl boronates), such as PS-262, PS-273, PS-341, and PS-402. These molecules block the chymotrypsin-like activity in the 20S subunit core. Most prominent among these is PS-341 (now generally called bortezomib), which has significant efficacy against multiple myeloma (MM), as well as several other hematological and solid tumors.
Bortezomib
- Multiple myeloma is a malignancy of mature plasma B cells and is heavily dependent on growth factors secreted by its stromal cell environment. Many primary MMs and MM cell lines have mutations in genes encoding components of both classical (p105, TRAF2/3, CYLD) and alternative (p100, NIK, CD40, LTβR) NF-κB signaling pathways. MM cell growth is extremely sensitive to proteasome inhibitors (e.g., bortezomib), which are highly effective against MM in the clinic.
- Bortezomib is an efficient blocker of NF-κB in MM cells in vitro, but it is not clear that its anti-MM effects are mediated entirely through inhibition of NF-κB. Namely, bortezomib has non-NF-κB effects on cancer cell growth and additional cancer-related protein targets are affected by its proteasome inhibitory activity
Bortezomib side effects
- In addition, the major side effect associated with bortezomib is that of peripheral neuropathy. There were several mechanisms proposed, one of the mechanisms was that as Bortezomib inhibits NF-KB activity, this results in the blocking of the transcription of nerve growth factor-mediated neuron survival. Another mechanism proposed was the accumulation of Bortezomib in the dorsal root ganglia cells, leading to mitochondrial damage and subsequent dysregulation of Ca2+ homeostasis.
