Tumor necrosis factor (TNF, cachexin, or cachectin; formerly known as tumor necrosis factor alphaorTNF-α[5][6]) is a cytokine and member of the TNF superfamily, which consists of various transmembrane proteins with a homologous TNF domain. It is the first cytokine to be described as an adipokine as secreted by adipose tissue.[7]
TNF signaling occurs through two receptors: TNFR1 and TNFR2.[8][9] TNFR1 is constitutively expressed on most cell types, whereas TNFR2 is restricted primarily to endothelial, epithelial, and subsets of immune cells.[8][9] TNFR1 signaling tends to be pro-inflammatory and apoptotic, whereas TNFR2 signaling is anti-inflammatory and promotes cell proliferation.[8][9] Suppression of TNFR1 signaling has been important for treatment of autoimmune diseases,[10] whereas TNFR2 signaling promotes wound healing.[9]
TNF-α exists as a transmembrane form (mTNF-α) and as a soluble form (sTNF-α). sTNF-α results from enzymatic cleavage of mTNF-α,[11] by a process called substrate presentation. mTNF-α is mainly found on monocytes/macrophages where it interacts with tissue receptors by cell-to-cell contact.[11] sTNF-α selectively binds to TNFR1, whereas mTNF-α binds to both TNFR1 and TNFR2.[12] TNF-α binding to TNFR1 is irreversible, whereas binding to TNFR2 is reversible.[13]
The cDNAs encoding LT and TNF were cloned in 1984[26] and were revealed to be similar. The binding of TNF to its receptor and its displacement by LT confirmed the functional homology between the two factors. The sequential and functional homology of TNF and LT led to the renaming of TNF as TNFα and LT as TNFβ. In 1985, Bruce A. Beutler and Anthony Cerami discovered that cachectin (a hormone which induces cachexia) was actually TNF.[27] They then identified TNF as a mediator of lethal endotoxin poisoning.[28]Kevin J. Tracey and Cerami discovered the key mediator role of TNF in lethal septic shock, and identified the therapeutic effects of monoclonal anti-TNF antibodies.[29][30]
Research in the laboratory led by Mark Mattson has shown that TNF can prevent the death/apoptosis of neurons by a mechanism involving activation of the transcription factor NF-κB which induces the expression of antioxidant enzymes and Bcl-2.[31][32]
TNF is primarily produced as a 233-amino acid-long type II transmembrane protein arranged in stable homotrimers.[35][36] From this membrane-integrated form the soluble homotrimeric cytokine (sTNF) is released via proteolytic cleavage by the metalloprotease TNF alpha converting enzyme (TACE, also called ADAM17).[37] The soluble 51 kDa trimeric sTNF tends to dissociate at concentrations below the nanomolar range, thereby losing its bioactivity. The secreted form of human TNF takes on a triangular pyramid shape, and weighs around 17-kDa. Both the secreted and the membrane bound forms are biologically active, although the specific functions of each is controversial. But, both forms do have overlapping and distinct biological activities.[38]
TNF can bind two receptors, TNFR1 (TNF receptor type 1; CD120a; p55/60) and TNFR2 (TNF receptor type 2; CD120b; p75/80). TNFR1 is 55-kDa and TNFR2 is 75-kDa.[40] TNFR1 is expressed in most tissues, and can be fully activated by both the membrane-bound and soluble trimeric forms of TNF, whereas TNFR2 is found typically in cells of the immune system, and responds to the membrane-bound form of the TNF homotrimer. As most information regarding TNF signaling is derived from TNFR1, the role of TNFR2 is likely underestimated. At least partly because TNFR2 has no intracellular death domain, it shows neuroprotective properties.[32]
Upon contact with their ligand, TNF receptors also form trimers, their tips fitting into the grooves formed between TNF monomers. This binding causes a conformational change to occur in the receptor, leading to the dissociation of the inhibitory protein SODD from the intracellular death domain. This dissociation enables the adaptor proteinTRADD to bind to the death domain, serving as a platform for subsequent protein binding. Following TRADD binding, three pathways can be initiated.[41][42]
Activation of NF-κB: TRADD recruits TRAF2 and RIP. TRAF2 in turn recruits the multicomponent protein kinaseIKK, enabling the serine-threonine kinase RIP to activate it. An inhibitory protein, IκBα, that normally binds to NF-κB and inhibits its translocation, is phosphorylated by IKK and subsequently degraded, releasing NF-κB. NF-κB is a heterodimeric transcription factor that translocates to the nucleus and mediates the transcription of a vast array of proteins involved in cell survival and proliferation, inflammatory response, and anti-apoptotic factors.
Activation of the MAPK pathways: Of the three major MAPK cascades, TNF induces a strong activation of the stress-related JNK group, evokes moderate response of the p38-MAPK, and is responsible for minimal activation of the classical ERKs. TRAF2/Rac activates the JNK-inducing upstream kinasesofMLK2/MLK3,[43]TAK1, MEKK1 and ASK1 (either directly or through GCKs and Trx, respectively). SRC- Vav- Rac axis activates MLK2/MLK3 and these kinasesphosphorylateMKK7, which then activates JNK. JNK translocates to the nucleus and activates transcription factors such as c-Jun and ATF2. The JNK pathway is involved in cell differentiation, proliferation, and is generally pro-apoptotic.
Induction of death signaling: Like all death-domain-containing members of the TNFR superfamily, TNFR1 is involved in death signaling.[44] However, TNF-induced cell death plays only a minor role compared to its overwhelming functions in the inflammatory process. Its death-inducing capability is weak compared to other family members (such as Fas), and often masked by the anti-apoptotic effects of NF-κB. Nevertheless, TRADD binds FADD, which then recruits the cysteine proteasecaspase-8. A high concentration of caspase-8 induces its autoproteolytic activation and subsequent cleaving of effector caspases, leading to cell apoptosis.
The myriad and often-conflicting effects mediated by the above pathways indicate the existence of extensive cross-talk. For instance, NF-κB enhances the transcription of C-FLIP, Bcl-2, and cIAP1 / cIAP2, inhibitory proteins that interfere with death signaling. On the other hand, activated caspases cleave several components of the NF-κB pathway, including RIP, IKK, and the subunits of NF-κB itself. Other factors, such as cell type, concurrent stimulation of other cytokines, or the amount of reactive oxygen species (ROS) can shift the balance in favor of one pathway or another.[citation needed] Such complicated signaling ensures that, whenever TNF is released, various cells with vastly diverse functions and conditions can all respond appropriately to inflammation.[citation needed] Both protein molecules tumor necrosis factor alpha and keratin 17 appear to
be related in case of oral submucous fibrosis[45]
There is also evidence that TNF-α signaling triggers downstream epigenetic modifications that result in lasting enhancement of pro-inflammatory responses in cells.[47][48][49][50]
On macrophages: stimulates phagocytosis, and production of IL-1 oxidants and the inflammatory lipid prostaglandin E2 (PGE2)
On other tissues: increasing insulin resistance. TNF phosphorylates insulin receptor serine residues, blocking signal transduction.
On metabolism and food intake: regulates bitter taste perception.[55]
A local increase in concentration of TNF will cause the cardinal signs of Inflammation to occur: heat, swelling, redness, pain and loss of function.
Whereas high concentrations of TNF induce shock-like symptoms, the prolonged exposure to low concentrations of TNF can result in cachexia, a wasting syndrome. This can be found, for example, in cancer patients.
Said et al. showed that TNF causes an IL-10-dependent inhibition of CD4 T-cell expansion and function by up-regulating PD-1 levels on monocytes which leads to IL-10 production by monocytes after binding of PD-1 by PD-L.[56]
The research of Pedersen et al. indicates that TNF increase in response to sepsis is inhibited by the exercise-induced production of myokines. To study whether acute exercise induces a true anti-inflammatory response, a model of 'low grade inflammation' was established in which a low dose of E. coli endotoxin was administered to healthy volunteers, who had been randomised to either rest or exercise prior to endotoxin administration. In resting subjects, endotoxin induced a 2- to 3-fold increase in circulating levels of TNF. In contrast, when the subjects performed 3 hours of ergometer cycling and received the endotoxin bolus at 2.5 h, the TNF response was totally blunted.[57] This study provides some evidence that acute exercise may inhibit TNF production.[58]
In the brain TNF can protect against excitotoxicity.[32] TNF strengthens synapses.[8] TNF in neurons promotes their survival, whereas TNF in macrophages and microglia results in neurotoxins that induce apoptosis.[32]
TNF-α and IL-6 concentrations are elevated in obesity.[59][60][61] Use of monoclonal antibodies against TNF-α is associated with increases rather than decreases in obesity, indicating that inflammation is the result, rather than the cause, of obesity.[61] TNF and IL-6 are the most prominent cytokines predicting COVID-19 severity and death.[7]
TNFα in Liver Fibrosis
TNFα mediates the inflammation that activates resident Hepatic Stellate Cells (HSCs) into the fibrogenic myofibroblasts that are largely responsible for liver fibrosis. However, whereas TNF receptor 1 knock-out mice demonstrate reduced fibrosis, TNFα can also suppress collagen α1(I) gene expression in fibroblasts in vitro, raising questions in regard to the complexity of its role in liver fibrosis.[62]
While TNFα treatment suppresses collagen α1 gene expression, apoptosis, and proliferation in activated HSCs in vitro, an activity that should ameliorate fibrosis, it has also been shown to inhibit apoptosis in activated HSCs, an activity which should, in principle, induce fibrosis.[63] Specifically, TNFα produced by hepatic macrophages is known to support the survival of HSCs, the source of hepatic myofibroblasts.[64] TNFα is therefore believed to promote liver fibrosis through its pro-survival effect, despite its pleiotropic effects on HSCs.[65]
Yet another way TNFα contributes to the worsening of liver fibrosis is by stimulating the production of TGF-β by hepatocytes and TIMP1byhepatocytes and HSCs.[66]
It should furthermore be noted that CCR9+ macrophages, which play an essential role in the pathogenesis of liver fibrosis, are TNFα-dependent. When TNFα is attenuated using an anti-TNFα antibody, hepatic HSCs are not activated by CCR9+ macrophages.[67]
TNFα in NAFLD
TNFα has a dual role in the development of NAFLD. Firstly, it is released among other pro-inflammatory cytokines, such as IL-6 and IL-1β, in response to the increased signaling from NF-κB during steatosis. TNFα then participates in the recruitment of Kuppfer cells, which increase inflammation and lead to the development of NASH.[68]
Secondly, the binding of TNFα to TNFα receptor 1 (TNFR1) facilitates insulin resistance, a known contributor to NAFLD progression, by suppressing insulin signaling.[69] Following the binding of TNF-α to TNFR1, intracellular c-JUN N terminal kinase (JNK) and IkB kinase (IKK) signals are activated, and the phosphorylation of JNK (p-JNK) and IKK1/1KK2 further attenuate insulin receptor substrate 1 (IRS-1). The phosphorylation of IRS-1 leads to the suppression of insulin signaling and, subsequently, to insulin resistance.[70] Indeed, a study has demonstrated that blocking TNFR1 protected Wistar rats from diet-induced obesity and insulin resistance.[71]
TNFR1 inhibition has been suggested as a possible therapy for NAFLD. A high-fat diet (HFD) mouse model of NAFLD has been used to demonstrate that the use of an anti-TNFR1-antibody can reduce liver steatosis and triglyceride content, as well as the activation of downstream target genes of lipogenesis. Insulin resistance likewise improved in these mice as a result of the reduced activation of MAP kinase MKK7 and its downstream target JNK.[72]
It is additionally thought that TNFα increases the production of MCP-1 (monocyte chemoattractant protein-1). MCP-1 is known to be overexpressed in obesity and is believed to be responsible for the recruitment of macrophages into adipose tissue and contribute to insulin resistance.[73] The production of MCP-1 increases in primary hepatocytes exposed to TNFα; TNF-α stimulates Mcp1 gene transcription by activating the Akt/PKB pathway.[74]
The dual role of TNFα in the development of NAFLD is counteracted by the anti-inflammatory action of adiponectin, whose production is impaired in metabolic syndrome.[75]
TNFα is, therefore, thought to play a deleterious role in the progression of NAFLDtoNASH and cirrhosis.
On the other hand, some patients treated with TNF inhibitors develop an aggravation of their disease or new onset of autoimmunity. TNF seems to have an immunosuppressive facet as well. One explanation for a possible mechanism is this observation that TNF has a positive effect on regulatory T cells (Tregs), due to its binding to the tumor necrosis factor receptor 2 (TNFR2).[77]
Anti-TNF therapy has shown only modest effects in cancer therapy. Treatment of renal cell carcinoma with infliximab resulted in prolonged disease stabilization in certain patients. Etanercept was tested for treating patients with breast cancer and ovarian cancer showing prolonged disease stabilization in certain patients via downregulation of IL-6 and CCL2. On the other hand, adding infliximab or etanercept to gemcitabine for treating patients with advanced pancreatic cancer was not associated with differences in efficacy when compared with placebo.[78]
Because LTα is no longer referred to as TNFβ,[81] TNFα, as the previous gene symbol, is now simply called TNF, as shown in HGNC (HUGO Gene Nomenclature Committee) database.
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