Jessica Lazarus' Individual Project
TUMOR NECROSIS FACTOR ALPHA
Tumor Necrosis Factor Alpha (TNF-α) belongs to the Tumor Necrosis Factor superfamily (Member 2). It was first discovered by Carswell et al. in 1975 in mouse serum after injection with Mycobacterium bovis strain bacillus Calmette-Guerin (BCG) and endotoxin. TNF-α is now often recognized under various other titles including simply Tumor Necrosis Factor (TNF), Cachectin, Monocyte-derived TNF, Macrophage-derived TNF and is now known to be one of the most clinically significant members of its family.
TNF-α is a serum glycoprotein and a potent cytokine responsible for initiating a broad spectrum of processes that can have effects at both the cellular and systemic levels. Activation of these processes occurs upon binding of the protein to its various cell-surface receptors. TNF-α is predominantly produced and secreted (in soluble form) by activated monocytes/macrophages during the inflammatory response (acute and chronic) however, TNF-α is also produced by other mononuclear leukocytes. Since TNF-α is a potent inflammatory glycoprotein, its production is typically stimulated by both exogenous and endogenous factors including bacteria, viruses, and other cytokines. Bacteria cell walls contain lipopolysaccharides which are a particularly potent stimulus for TNF-α synthesis.
TNF-α is a multifunctional ligand that binds to its receptors on target cells to produce a diverse range of specific responses. With a wide spectrum of regulative effects on biologic processes, TNF-α has been implicated in lipid metabolism, coagulation, cell proliferation, differentiation, apoptosis, necrosis and endothelial function, amongst various other processes. Previous studies have also revealed that TNF-α has been implicated in the induction of a variety of pathologic conditions including autoimmune diseases, insulin resistance, and cancer where much of these studies involved experimentation using knockout mice models to demonstrate and better understand the role of TNF-α in the human body.
Although TNF-α is a multifunctional ligand it is most recognised for its primary roles during the inflammatory immune response and cell death, including both apoptosis and necrosis. During an inflammatory immune response, soluble TNF-α synthesis by macrophages and mast cells is significantly increased. This in turn promotes the production of other key cytokines; particularly Interleukin-1 (IL-1) and stimulates T cells, B cells and other inflammatory cells in response to antigenic invasion. Increased expression of MHC Class I and II molecules is also strongly associated with TNF-α which plays a chief role in antigen presentation.
One of the most clinically significant roles of TNF-α is its ability to necrotise cells with damaged DNA and hence some tumor cell lines. This accounts for TNF-α’s large immunotherapeutic potential. TNF-α has been found to increase the sensitivity of both normal and neoplastic thyrocytes to FasL and TRAIL which are anti-cancerous agents in thyroid cancer.
Structure and Gene
TNF-α was initially identified in its soluble, cleaved form of approximately 17-kD, however with further research it was later identified in its insoluble, non-cleaved form of approximately 26-kD, existing as a trans-membrane protein. Both TNF-α and its receptors are currently described as trimeric proteins where the ligand is encoded within the Major Histocompatibility Complex (MHC) and has been mapped to chromosome 6p21.3. The following is a link to an interactive murine model of TNF-α. Model of Murine TNF-Alpha at 1.4 A resolution from NCBI Structure Databank - Link to figure of TNF-α: http://www.bio.davidson.edu/COURSES/Immunology/Students/spring2000/wolf/tnfalpha.html
Crystal Structure of TNF-α from Protein Data Bank: http://www.rcsb.org/pdb/explore.do?structureId=1TNF
TNF-α Receptors & Signaling
There is a variety of Tumor Necrosis Factor Receptors that exist as trans-membrane proteins. Each receptor that belongs to the TNF family consists of three primary parts: an extracellular ligand-binding domain, a trans-membrane part and an intracellular death domain; however the defining trait of all Tumor Necrosis Factor Receptors is the extracellular domain comprised of two to six repeats of cysteine rich motifs. The key receptors for TNF-α include Tumor Necrosis Factor Receptor-1 (TNFR1) and Tumor Necrosis Factor Receptor-2 (TNFR2).
Once the ligand, that is TNF-α, binds to the extracellular domain of the receptor, certain pathways are activated that produce intracellular effects. A prominent example is the Caspase Cascade pathway which can be activated by TNF-α during apoptosis. In this example, TNF-α binds to its receptor: TNFR1, which in turn allows receptor trimerisation and clustering of intracellular death domains of the TNF receptors. TNFR-associated death domain (TRADD) is then able to bind to the intracellular death domain of TNFR1 for the recruitment of FAD; an intracellular protein which can then cleave caspase molecules, which subsequently cleave further caspases. This initiates the Caspase Cascade, eventually resulting in apoptotic death of the cell. The following is a link to a video that further explains this process together with a clear illustration of the structure of TNFR1: http://www.sgul.ac.uk/depts/immunology/~dash/apoptosis/receptors.htm
TNF-A has proven to be a vital component of normal physiological function in the human body. On the contrary, it has also been implicated in a wide variety of human diseases such as cachexia, sepsis, diabetes, cancer, osteoporosis and autoimmune diseases including multiple sclerosis and rheumatoid arthritis. Knockout mice models have demonstrated that the mutated TNF-α gene can cause an array of pathologies including cancer formation rather than destruction. However, TNF-α also has enormous potential for cancer treatment. Currently, the focal use of TNF-α in cancer therapy is in the localised treatment of advanced soft tissue sarcomas and metastatic melanomas and other irresectable tumors together with cytostatic drugs. Further to this TNF-α targets the tumor-associated vasculature (TAV) by greatly increasing permeability of intratumoral vasculature and destroying vascular epithelia. This significantly helps to starve the tumor of essential nutrients leading to the death of the cancer cells. These important findings construct a solid platform upon which pharmaceutical companies are able develop more efficacious and targeted anti-tumor drugs.
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