TNF Alpha Overview
Tumour necrosis factor alpha (TNF alpha) is a 26 kDa transmembrane protein and a member of the TNF superfamily, with its 17 kDa secreted form mediating intracellular signalling. Originally identified in 1975 as an endotoxin-induced glycoprotein in macrophages, and later determined to be ubiquitously expressed in multiple cell types (Carswell et al., 1975), TNF alpha stimulates several critical inflammatory and apoptotic pathways by binding to one of its two membrane bound receptors, TNFR1 and TNFR2 (Aggarwall, 2003). TNF alpha levels are not detectable under healthy conditions in humans, however elevated levels of TNF alpha in tissues and serum are found in inflammatory conditions (Robak et al, 1998). TNF alpha was first cloned in 1984, and in the following two decades it has become the focus of both in vivo and in vitro experimentation, with a critical focus on its role in inducing tumourigenesis (Burugu et al, 2017; Wang et al, 2016; Pennica et al, 2005).
TNF alpha Receptors and signalling cascades
TNF alpha has a similar affinity to both of its receptors; however, TNFR1 is found on all cell types, whereas TNFR2 is specifically expressed primarily on cells of haematopoietic lineage, including specific T cell subsets, and also on endothelial cells involved in immune signalling, and mesenchymal stem cells (Naudé et al, 2007; Aggarwal et al, 2003). An enzyme known as TNF-converting enzyme, or TACE, regulates TNF secretion and processing of TNF receptors thus mediating both ligand and receptor and holding a cell specific pro- or anti- inflammatory role (Black et al, 1997). The structure of TNFR1 embodies a death domain on the cytoplasmic portion, initiating downstream signalling through the caspase cascade, whereas TNFR2 lacks a death domain and binds directly to TNF-receptor associated factor 1 (TRAF1) or TRAF2 at the membrane (Nagata et al, 1995). Both TNFR1 and TNFR2 facilitate inflammatory pathways by inducing NF-Kappa Beta and mitogen-associated protein kinase (MAPK) activation (Ledgerwood et al, 1999).
TNF alpha signalling pathway ELISA kits
TNF Alpha and Apoptosis
Ligand binding to TNFR1 induces apoptosis via the caspase cascade. Specifically, the cytoplasmic death domain of TNFR1 recruits TNFR-associated death domain protein (TRADD) which forms a complex with Fas-associated death domain (FADD) and caspase 8, preceding either extrinsic or intrinsic apoptosis (mitochondrial-mediated). Extrinsic apoptosis involves the cleavage and subsequent activation of caspase 3 by active caspase 8. Intrinsic apoptosis is facilitated via the truncated form of the pro-apoptotic Bcl-2 protein Bid, leading to the translocation of Bax and Bak to the mitochondrial membrane which induces Cytochrome-c release and the formation of the apoptosome. Both extrinsic and intrinsic apoptotic pathways culminate in caspase-3 activation (Kantari and Walczak, 2011; Wang et al, 1996). As TNFR2 is death domain free, it does not induce apoptosis alone, however there is some evidence for crosstalk of both TNFR1 and TNFR2 (Naudé et al, 2007).
TNF Alpha and Inflammatory pathways
Ligation of TNFR1 and -2 also induce inflammatory pathway activation. TRADD association with TRAF2 and Receptor-interacting protein (RIP1) induce downstream IKK-complex activation, IKappa Beta alpha phosphorylation and degradation and the release of NF-Kappa Beta subunits inducing nuclear translocation and NF-Kappa Beta activation. Similarly, TRADD mediated recruitment of TRAF2 can activate MAPK pathways, including c-Jun N-terminal kinase (JNK) and p38, which result in activation of the transcription factor AP-1 (Blonska et al, 2005; Devin et al 2001). Additionally, there is crosstalk between inflammatory and apoptotic pathways, evident in TNF alpha-induced NF-Kappa Beta activation, which can regulate apoptosis by upregulating cFLIP levels, which in turn inhibit caspase 8 activation (Micheau et al, 2001).
TNF Alpha in Inflammatory Disease and Cancer
Importantly, acute inflammation is a therapeutic and preventative natural defense; however, chronic inflammation is detrimental and drives a plethora of pathological conditions. TNF alpha has been identified as the critical mediator of the endotoxin-induced response, specifically facilitating that of Escherichia coli and tnfr1-/-mice are resistant to lipopolysaccharides and Staphylococcus aureus (Pfeffer et al, 1993; Beutler et al, 1985). The inflammatory effects of TNF alpha are primarily mediated via NF-Kappa Beta. Chronic inflammation is now classified as a precursor and leading risk factor to tumorigenesis, with inflammatory signalling in the tumour microenvironment promoting tumour progression and resistance (Balkwill et al, 2005; Karin et al, 2005). Moreover, TNF alpha has been identified to play a crucial role in tumour initiation and development (Aggarwal et al, 2002; Sungamuma et al, 1999; Deigel et al, 1989). One of the mechanisms employed by TNF alpha which promotes tumour development is the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which in turn lead to DNA damage (Woo et al, 2000). Additionally, TNF alpha induces angiogenesis of tumour cells by upregulating several angiogenic factors such as vascular endothelial growth factor (VEGF) (Jing et al, 2011). Furthermore, TNF alpha promotes motility of epithelial tumour cells (Rosen et al, 1991). However, the role of TNF alpha in carcinogenesis is complex due to the range of signalling responses described above, which effectively lead to activation of 4 distinct pathways: a death domain induced pro-apoptotic pathway; an anti-apoptotic pathway, facilitated by TRAF2 associations with the E3 ubiquitin ligase cellular inhibitor of apoptosis-1 (cIAP1); MAPK-mediated AP-1 activation, and RIP1 induced NF-Kappa Beta (Chen and Goeddel, 2002). The current consensus eludes to the TNF alpha tumour microenvironment concentration and cell-specific site of elevated expression (Ohri et al, 2010).
Therapeutic potential of targeting TNF Alpha
TNF alpha offers a prime therapeutic target for cancer. To date, several drugs have been developed which target and inhibit TNF alpha, including inhibitors of TNF alpha expression (Majumdar et al, 2002); TNF alpha antibodies which bind and neutralize TNF alpha, for example infliximab, although several of these have been shown to have adverse effects (Keystone et al, 2004); and inhibitors of TNF alpha signalling primarily targeting NF-Kappa Beta (Aggarwal et al, 2006). Several methods of tumour modulation targeting TNF alpha, among other cytokines, include adoptive T cell therapy, where patients’ dendritic cells are stimulated with TNF alpha in order to induce antigen presentation (Yee et al, 2002). A similar principal has been applied for the development of dendritic cell targeted cancer vaccines which have proven to have some success in clinical trials (Ridgway, 2003). Overall, targeting cytokines, particularly TNF alpha which has the opposing roles of apoptosis and cell survival signalling, is a field of huge interest and potential with relation to cancer therapeutics and treatments for inflammatory disorders.
Figure 1: TNFa singalling pathways. TNFa binds to the membrane-bound receptors TNFR1 or TNFR2, leading to downstream apoptosis and inflammatory signalling. Activation of TNFR1 induces the formation of a death inducing signalling complex, containing TRADD, FADD and caspase-8. Formation of this complex leads to the induction of apoptosis by either the intrinsic or extrinsic pathways, both culminating in the cleavage of caspase-3. Pro-inflammatory singalling pathways can also be induced by either TNFR1 or TNFR2 activation, which signal through the adaptor protein TRAF2 to activate RIP1, NIK or MEKs, resulting in the activation of MAPK and NF-kB activation.
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