Figure 1. TNF-α signaling pathway.
Introduction of TNF-α signaling pathway
Tumor necrosis factor (TNF) is a kind of cytokine with many biological effects. It promotes cell growth, differentiation, apoptosis and inflammation by binding to specific receptors on the cell membrane. TNF-α belongs to the TNF family and can activate ERK (extracellular signal 2 regulated protein kinase), Caspase protease, and JNK. It also has independent pathways to achieve its biological functions such as cytotoxicity, antiviral, immune regulation and apoptosis. Since TNF-α is directly related to cell homeostasis and many human diseases, such as tumors, research on TNF-α signaling pathway has become a hot topic in biomedical research in the past decade.
The family of TNF-α
TNF-α is mainly secreted by activated mononuclear macrophages and belongs to type II membrane proteins, which act in the form of trimers. There are two forms of TNF-α, 17 kD of secreted soluble sTNF-α and 26 kD of membrane-associated mTNF-α. mTNF-α is generally considered to be a precursor of sTNF-α, which is cleaved from the membrane by membrane metalloproteinase and sheds into STNF-α. TNF-α achieves its biological function by binding to the receptor TNF-R on the cell surface. There are two types of TNR, TNF-Rl (55 kD) and TNF-R2 (75 kD), both of which belong to type I membrane proteins, consisting of four parts, signal peptide, extracellular domain, transmembrane domain and intracellular domain. The intracellular domain of the two has low homology. TNF-R1 contains a death domain (DD), and TNF-R2 does not contain DD, suggesting that the receptors of the two types activate different signaling pathways and mediate different biological effects. Early studies have found that TNF-R2 is more susceptible to mTNF activation than sTNF, and it is speculated that in sTNF-mediated cellular responses, mTNF plays a major role in ligand delivery, ie, TNF-R2 binds to TNF first, then TNF-R2 delivers TNF to TNF-R1 and increases the affinity of TNF-R1 and TNF. This explains to some extent how low concentrations of TNF trigger biological reactions.
TNF-α signaling pathway
The cascade of TNF-α signaling pathway
It is now recognized that TNF-α mediates two signaling pathways in vivo, and one signaling pathway for apoptosis; after TRADD(TNF-R1 associated death domain) recruitment of TNF-R1, TNF-R1 binds to the death domain of TRADD, and recruits FADD to TNF-R1 complex physically. The N-terminus of FADD contains a novel domain, called the MORT domain or the death effector domain (DED), through which Caspase 8 can be activated, which belongs to the ICE/CED3 cysteine protease family, and activated Caspase-8 then activates other ICE/CED3 proteases. Thus, apoptosis is induced by a cascade amplification reaction of caspasse-8 protease. JNK signaling pathway: TNF-α stimulates cell to form TNF-R1 complex, and the complex activates through a series of protein phosphorylation, then MAPKKK activates MAPKK, and MAPKK activates MAPK, which activates JNK signaling pathway. Both JNK and P38 belong to the MAPK family. The study found that the upstream molecules that activate JNK are mainly members of the MAPKK family, MKK4 and MKK7. Among them, MK is more prone to phosphorylate JNK Tyrl85, and MKK7 tends to phosphorylate Thrl83, although activation of JNK requires phosphorylation at both sites. P38 is mainly composed of MKK3.6 and MKK, which are also members of the MAPKK family. The study found that the activation of JNK was also inhibited after knocking out the makk1 gene in mice, indicating that MEKK1 is indispensable in the JNK signaling pathway. A further study revealed that MEKK1 was activated after stimulation of TNF, and combined with TRAF2, phosphorylated MKK4, and finally activated JNK. However, it is not yet certain whether there are other kinases between MEKK1 and TRAF2.
NF-KB antagonizes TNF-induced apoptosis, and the main cellular effects of NF-KB activation are to promote the expression of inflammatory proteins and antagonize apoptosis. Its anti-apoptotic effect is mainly achieved by inducing the expression of apoptotic inhibitors. Apoptotic inhibitors include c-IAP, c-FLIP, TRAF2. C-IAP can directly bind caspase-3 or caspase-7 to inhibit its activity and can also inhibit the activation of caspase-6 or caspase-9 precursors, and ultimately inhibit TNF-α-induced apoptosis. Negative regulation of A20 on NF- signaling pathway: mice with a20 gene knockout were unable to reduce the regulation of TNF-α, leading to chronic inflammation and cell death, indicating that A20 was an inhibitor of NF-α. A20 is a deubiquitination enzyme, whose N-terminal domain can remove lys63-linked ubiquitin chain in RIP molecule, and C-terminal domain contains seven C2/C2 zinc finger domains, which have the activity of ubiquitin ligase and can connect the ubiquitin chain at lys48 of RIP, and finally enable RIP to enter the protease complex degradation pathway.
Relationship with diseases
Because TNF-α is directly related to tumor, TNF-α has always been a hot topic in the field of cancer research. Now, through in-depth study of TNF-α, some efficient and low-toxic TNF-α allosterants have been developed. Reducing side effects is a new research idea.
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Zhao G, Yu Y M, Kaneki M, et al. Simvastatin Reduces Burn Injury-induced Splenic Apoptosis via Down-regulation of the TNF-α/ NF-κB Pathway. Annals of Surgery. 2015, 261(5):1006-12.
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