TNF (Tumor Necrosis Factor) is a multifunctional proinflammatory cytokine, with effects on lipid metabolism, coagulation, insulin resistance, and endothelial function. TNF has been considered as an anti-cancer agent since its discovery two decades ago. Members of the TNFR (TNF Receptor) superfamily can send both survival and death signals to cells (Ref.1). TNF family members play important roles in various physiological and pathological processes, including cell proliferation, differentiation, apoptosis, modulation of immune responses and induction of inflammation. TNF acts through two receptors, TNFR1 (TNF Receptor-1) and TNFR2 (TNF Receptor-2). TNFR1 is expressed by all human tissues and is the major signaling receptor for TNF-α. TNFR2 is mostly expressed in immune cells and mediates limited biological responses. TNFR2 binds both TNF-α and TNF-β.
TNF-α, a pro-inflammatory cytokine, is produced by many cell types, including macrophages, lymphocytes, fibroblasts and keratinocytes, in response to inflammation, infection, and other environmental stresses. TNF-α induces a heterogeneous array of biological effects according to cell type. TNF-α acts by binding to its receptors, TNFR1 (p55) and TNFR2 (p75), on the cell surface. Most cells express TNFR1, which is believed to be the major mediator of the cytotoxicity of TNF-α (Ref.2). Binding of TNF-α to its two receptors, TNFR1 and TNFR2, results in recruitment of signal transducers that activate at least three distinct effectors. Through complex signaling cascades and networks, these effectors lead to the activation of Caspases and two transcription factors, Activation Protein-1 and NF-κB (Nuclear Factor-κB) (Ref.3). Initially, TRADD (TNFR-Associated Death Domain) protein, binds to TNFR1. Then, TRADD recruits FADD (Fas-Associated Death Domain), RAIDD (RIP-Associated ICH-1/CED-3-homologous protein with a Death Domain), MADD (MAPK Activating Death Domain) and RIP (Receptor-Interacting Protein). Binding of TRADD and FADD to TNFR1 leads to the recruitment, oligomerization, and activation of Caspase8. Activated Caspase8 subsequently initiates a proteolytic cascade that includes other Caspases (Caspases3,6,7) and ultimately induces apoptosis. Caspase8 also cleaves BID (BH3 Interacting Death Domain). tBID (Truncated BID) disrupts the outer mitochondrial membrane to cause release of the pro-apoptotic factors CytoC (Cytochrome-C). CytoC that is released from the intermembrane space binds to APAF1 (Apoptotic Protease Activating Factor-1), which recruits Caspase9 and in turn can proteolytically activate Caspase3. A novel 61-kDa protein kinase RIP2 is related to RIP that is a component of TNFR1 which mediates the recruitment of Caspase death proteases. Overexpression of RIP2 signaled both NF-κB activation and cell death.
TRAF2 (TNF Receptor-Associated Factor-2) has been implicated in the activation of two distinct pathways that leads to the activation of Activation Protein-1 via the JNK (Jun NH2-terminal Kinase), MEKK (MEK Kinase), p38 and, together with RIP, NF-κB activation, via the NIK (NF-κB-Inducing Kinase). TNF-α has been shown to activate MAPKs (Mitogen Activated Protein Kinases): ERK1 and ERK2 (Extracellular signal-Regulated Kinases). Overexpression of SODD (Silencer Of Death Domains), a 60 kDa protein, associated with the DD of TNFRI suppresses TNF-induced cell death and NF-κB activation demonstrating its role as a negative regulatory protein for these signaling pathways.
TNFR2 is the receptor for TNF-β. TNF-β is produced by activated lymphocytes and can be cytotoxic to many tumor and other cells. In neutrophils, endothelial cells and osteoclasts TNF-β can lead to activation while in many other cell types it can lead to increased expression of MHC and adhesion molecules (Ref.4). TNF signaling has been implicated in many other diseases including: multiple sclerosis, Alzheimer’s disease, and TRAPS (TNF-Receptor-Associated Periodic Syndrome). A better understanding of TNF and its relatives should eventually result in the development of small molecules that can successfully inhibit and modulate the biological activity of these cytokines and thereby provide new avenues for therapeutic intervention.
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