Tumour necrosis factors TNFs

Tumour necrosis factor (TNF) was originally described as a factor produced following exposure of Bacille-Calmette-Guerin-treated animals to bacterial endotoxin. It was so named because it possessed the ability to necrotise tumours. This factor is now named TNF-a to distinguish it from another, related cytokine lymphotoxin, which is sometimes referred to as TNF-/J Alternative names for TNF-a include cachectin and cytotoxin. Its primary cellular source in the body is the activated macrophage, but some other cell types (e.g. NK cells, astrocytes, some lymphocytes, fibroblasts, many tumour cells, endothelial cells and neutrophils) have also been shown to synthesise this cytokine. 

TNF- a was first purified from conditioned medium from HL-60 cells. It has a relative molecular mass of 17 kDa when analysed by SDS-PAGE, but 45 kDa when analysed by gel filtration. Thus, the molecule exists as a non-glycosylated trimer with a pi of 5.3. Each monomer comprises 157 amino acids and contains two cysteine residues that form a disulphide bridge. Trimer formation appears to be due to noncovalent interactions between the monomers. Human TNF-a is synthesised as a 233-amino-acid protein that is proteolytically cleaved during processing. Whilst the 17-kDa form is readily secreted (and hence can function as an extracellular mediator), a 26-kDa transmembrane form has also been identified. This form of TNF-a may thus function in cytotoxicity resulting from cell-cell contact. 

TNF-a elicits a wide range of responses in cells and tissues. Apart from causing the lysis of certain tumours (by mechanisms probably related to the ability of the target to induce the synthesis of the mitochondrial manganese-dependent superoxide dismutase, Mn-SOD), it can also kill normal cells. These effects on normal cells are more apparent when biosynthesis is blocked - for example, by the addition of inhibitors of macromolecular bio- 

TNF-a is chemotactic for monocytes and neutrophils. Its effects on neutrophils are numerous for example, it can prime degranulation and reactive oxidant production, enhance phagocytosis and ADCC and up-regulate the expression of some surface receptors, such as CR3. Whilst low concentrations of TNF-a are required to prime the cells subsequent to stimulation by other agonists, such as fMet-Leu-Phe, higher concentrations of TNF-a alone can activate low levels of oxidant production. This activity is even more pronounced if the neutrophils are adhered to surfaces. 

Two types of receptors for TNF-a have been identified. The type A receptor has a relative molecular mass of 75 kDa and binds the ligand with high affinity. The type B receptor is 55 kDa and has an affinity for TNF-a that is between five- and sevenfold lower than that of the type A receptor. The level of expression of the type A receptor can be regulated independently of changes in expression of the type B receptor. 

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Hyperphosphorylation of ERAK-1 by itself and ERAK-4 causes ERAK-1 to dissociate from the membrane-bound complex. Tumour necrosis factor (TNF) receptor-associated factor-6 ( TRAF-6), a cytoplasmic protein, is activated by ERAK-1 and with TAB-2, another cytoplasmic protein, activates transforming growth factor-P (TFG-P)-activating kinase (TAK-1). During this process both TRAF-6 and TAK-1 become ubiquitinated. TAK-1 then promotes activation of the IkB kinases, or the IKK family, EKKa and EKK 3 (found in a complex with NFicB-essential modulator [NEMO]), which phosphorylate the IkB family, notably IkB-u. IkB-u is an inhibitor of NFkB as it sequesters NFkB in an... 

During ischaemia, NOS is activated by calcium influx or by cytokines like tumour necrosis factor (TNF) or by lipopolysaccharide (LPS) and NO is produced in excess. It has been proposed that the excitotoxic effect of glutamate, which contributes to ischaemia-induced neuronal damage, is mediated by increased production of NO via a chain of events that includes increases in intracellular calcium (via glutamate activation of NMDA receptors), calcium activation of NOS, production of NO and peroxynitrite, and induction of lipid peroxidation. In fact, N-nitro-L-atginine, a selective inhibitor of NOS, has been shown to prevent glutamate-induced neurotoxicity in cortical cell cultures (Dawson rf /., 1991). 

VORDERMEIER S, SlNGH S, BiGGERSTAFF J, Harrison P, Grech H, Pearson TC, Dumonde DC, Brown KA. Red blood cells from patients with sickle cell disease exhibit an increased adherence to cultured endothelium pretreated with tumour necrosis factor (TNF). Br J Haematol 1992 81 591-597. 

Additional products Tumour necrosis factor (TNF), therapeutic enzymes 18 Various chapters... 

In addition to directly eliciting cell chemotaxis and free-radical production, PAF can also induce the release of various inflammatory cytokines, amongst which tumour necrosis factor (TNF) is of particular importance [ 312 ]. We have recently shown that PAF stimulates TNF production from peripheral blood derived monocytes and at picomolar concentrations amplifies lipopoly-saccharide (LPS)-induced TNF production, effects inhibited by various PAF antagonists [313]. PAF also acts synergistically with interferon-y (IFN-y) to increase the monocyte cytotoxicity. Furthermore, PAF can modulate the production of both interleukin 1 and interleukin 2 (IL-1, IL-2) from rat monocytes and lymphocytes, respectively [222, 223], cytokines which in turn elicit the release of other mediators and growth factors. 

IL-3 also has effects on mature cells. Those reported include the enhancement of the cytotoxicity of macrophages, stimulation of the proliferation of tissue-derived mast cells and (in the presence of endotoxin) the stimulation of the tumouricidal activity of monocytes, possibly via enhanced production of tumour necrosis factor (TNF). However, there are no reported effects of IL-3 on mature neutrophil function. 

Almost 20 years ago the ammonium tellnrolate AS 101 was demonstrated to possess immnnomodnlating properties and to mediate anti-tumour effects in rats. The same com-ponnd stimnlates hnman lymphoid cells to proliferate and produce lymphokines, tumour necrosis factor (TNF) and other cytokines in vitro ... 

A third biological activity of IL-2 pertinent to immunostimulation is its ability to promote the growth of natural killer (NK) cells. It also promotes further dilferentiation of NK cells, forming lymphokine-activated killer cells (LAKs), which exhibit an enhanced ability to directly kill tumour cells or virally infected cells. NK cells express the /I and y IL-2 receptor subunits only, thus their stimulation by IL-2 requires elevated concentrations of this cytokine. NK cells are also activated by a variety of additional cytokines, including all IFNs as well as tumour necrosis factor (TNF). 

The growth of eukaryotic cells is modulated by various influences, of which growth factors are amongst the most important for many cell types. A wide range of polypeptide growth factors have been identified (Table 7.1) and more undoubtedly remain to be characterized. Factors that inhibit cell growth also exist, e.g. interferons (IFNs) and tumour necrosis factor (TNF) inhibit proliferation of various cell types. 

Cytokines, including tumour necrosis factor (TNF) and interferon-y, favour the secretion of numerous chemokines and the expression of adhesion molecules by endothelial cells. The mechanisms of action of the principle drugs used in MS, and in priority beta interferons, are the following (1) inhibition of the expression of major histocompatibility complex class II molecules, (2) inhibition of metal-loproteases, (3) induction of immunosuppressor cytokines. 

Tumour necrosis factors (TNF)—cytokines that are immunotoxic to tumors and bacteria... 

Figure 14.9. Summary of the role of glutamate, p-amyloid and pro-inflammatory cytokines such as tumour necrosis factor (TNF) in the production of reactive oxygen species leading to neuronal death.

F. Interleukins — peptides which transfer signals between white blood cells. In this they are helped by interferons (which are growth inhibitors), tumour necrosis factor (TNF), the CSFs and TGF-/3. 

Molybdenum carbonyl complexes useful for inhibiting tumour necrosis factor (TNF) production and for treating inflammatory diseases [242]... 

Signalling for apoptosis can involve a plasma Fas ligand which binds to the PM Fas receptor with resultant activation of an associated cytosol-side Fas death domain of Fas and activation of caspase 8. Caspase 8 is a thiol protease and once activated initiates a so-called caspase cascade leading to activation of further caspases (with consequent proteolysis) and activation of a DNase (leading to DNA destruction with formation of a characteristic DNA fragment ladder ). Caspase 8 acts on mitochondria with resultant release of cytochrome c, which promotes caspase 3 activation by caspase 8 and hence the caspase cascade . Another signalling pathway for apoptosis involves tumour necrosis factor (TNF) binding to the TNF receptor with consequent activation of a cytosolic-side TNF receptor-associated death domain (TRADD) and resultant activation of the caspase cascade and cell death. 

Caspases are involved in intracellular proteolytic protease activation cascades leading to apoptosis that are initiated by ligands such as tumour necrosis factor (TNF) and Fas ligand. These proteins bind to PM receptors with cytosolic death domains that activate the cas-pase cascades leading to cell death. Caspases are cysteine proteases that cleave peptide bonds on the carboxyl side of aspartate (hence c-asp-ases). 

THY-R, thyroid hormone receptor TIMP, tissue inhibitor of metalloprotease TK, tyrosine kinase TLC, thin layer chromatography TLRs, Toll-like receptors TMAOX, trimethylamine oxidase TMY tobacco mosaic virus TNF, tumour necrosis factor TNF-a, tumour necrosis factor-a TNF-a-RTK, tumour necrosis factor-a receptor tyrosine kinase TOPI, DNA topoisomerase I TOPII, DNA topoisomerase II t-PA, tissue plasminogen activator TPA, 12-Tetradecanoylphorbol 13-acetate... 

Osteoclasts are multinucleated cells found on the endosteal surface of bone, in Haversian systems and periosteal surfaces. PTH activates osteoclasts (indirectly via osteoblasts that possess PTH receptors). Calcitonin is a potent inhibitor of osteoclast activity. Local cytokine factors, including interleukin-1 (IL-1), tumour-necrosis factor (TNF), TGF- 0 and interferon-y (INF-y), are important regulators. Osteoclast resorption of bone releases collagen peptides, pyridinoline cross-links and calcium from the bone matrix, through the action of lysosomal enzymes (collagenases and cathepsins). The collagen breakdown products in serum and urine (e.g. hydroxyproline) can be used as biochemical markers. 

The MHC region is divided into three snbgronps, class I, class II and class IE. Class El has a function very different from that of class I and class II (Table 15.4), but since it has a locus between the other two (on chromosome 6 in humans), the three classes are frequently discussed together MHC class III encodes for immune components such as complement (C2, C4, factor B), as well as cytokines (e.g. tumour necrosis factor (TNF)). 

The X-ray crystal structure of the complex of the extracellular domain of the human 55 kDa tumour necrosis factor (TNF) receptor with human TNF-P has been determined at 2.85 A resolution. It provides a model for TNF receptor activation. The complex contains three receptor molecules bound symmetrically to one TNF-P trimer. The TNF-P subunits form a groove into which the receptor is inserted. The structure of the receptor-ligand complex also determines its orientation with respect to the cell membrane. The TNF-receptor structure is likely to be representative of the TNF-receptor family as a whole, including the NGF receptor (see Chapter 1). 

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