Tumor necrosis factor alpha

Tumor necrosis factor (TNF, tumor necrosis factor alpha, TNFα, cachexin, or cachectin) is a cell signaling protein (is_associated_with::cytokine) involved in systemic is_associated_with::inflammation and is one of the cytokines that make up the is_associated_with::acute phase reaction. It is produced chiefly by activated is_associated_with::macrophages, although it can be produced by many other cell types such as is_associated_with::CD4+ lymphocytes, is_associated_with::NK cells, is_associated_with::neutrophils, is_associated_with::mast cells, is_associated_with::eosinophils, and is_associated_with::neurons.

The primary role of TNF is in the regulation of is_associated_with::immune cells. TNF, being an endogenous pyrogen, is able to induce fever, apoptotic cell death, is_associated_with::cachexia, inflammation and to inhibit is_associated_with::tumorigenesis and is_associated_with::viral replication and respond to is_associated_with::sepsis via IL1 & IL6 producing cells. Dysregulation of TNF production has been implicated in a variety of human is_associated_with::diseases including is_associated_with::Alzheimer's disease, is_associated_with::cancer, is_associated_with::major depression and is_associated_with::inflammatory bowel disease (IBD). While still controversial, studies of depression and IBD are currently being linked to TNF levels. Recombinant TNF is used as an is_associated_with::immunostimulant under the INN tasonermin. TNF can be produced ectopically in the setting of malignancy and parallels parathyroid hormone both in causing secondary hypercalcemia and in the cancers with which excessive production is associated.

Discovery
The theory of an is_associated_with::anti-tumoral response of the is_associated_with::immune system is_associated_with::in vivo was recognized by the physician is_associated_with::William B. Coley. In 1968, Dr. Gale A Granger from the is_associated_with::University of California, Irvine, reported a cytotoxic factor produced by is_associated_with::lymphocytes and named it is_associated_with::lymphotoxin (LT). Credit for this discovery is shared by Dr. Nancy H. Ruddle from is_associated_with::Yale University, who reported the same activity in a series of back-to-back articles published in the same month. Subsequently in 1975 Dr. is_associated_with::Lloyd J. Old from is_associated_with::Memorial Sloan-Kettering Cancer Center, New York, reported another cytotoxic factor produced by is_associated_with::macrophages and named it tumor necrosis factor (TNF). Both factors were described based on their ability to kill mouse is_associated_with::fibrosarcoma L-929 cells. These concepts were extended to systemic disease in 1981, when is_associated_with::Ian A. Clark, from the is_associated_with::Australian National University, in collaboration with is_associated_with::Elizabeth Carswell in Dr Old's group, working with pre-sequencing era data, reasoned that excessive production of TNF causes malaria disease and endotoxin poisoning.

The is_associated_with::cDNAs encoding LT and TNF were cloned in 1984 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 (this article) and LT as TNFβ. In 1985, is_associated_with::Bruce A. Beutler and is_associated_with::Anthony Cerami discovered that cachectin (a hormone which induces is_associated_with::cachexia) was actually TNF. They then identified TNF as a mediator of lethal is_associated_with::endotoxin poisoning. is_associated_with::Kevin J. Tracey and Cerami discovered the key mediator role of TNF in lethal is_associated_with::septic shock, and identified the therapeutic effects of monoclonal anti-TNF antibodies. More recently, research in the Laboratory of is_associated_with::Mark Mattson has shown that TNF can prevent the death/is_associated_with::apoptosis of neurons by a mechanism involving activation of the transcription factor is_associated_with::NF-kappaB which induces the expression of is_associated_with::Mn-SOD and is_associated_with::Bcl-2.

Gene
The human TNF is_associated_with::gene (TNFA) was cloned in 1985. It maps to is_associated_with::chromosome 6p21.3, spans about 3 kilobases and contains 4 is_associated_with::exons. The last exon codes for more than 80% of the secreted protein. The 3' UTR of TNFα contains an is_associated_with::AU-rich element (ARE).

Structure
TNF is primarily produced as a 212-is_associated_with::amino acid-long type II transmembrane protein arranged in stable homotrimers. 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 is_associated_with::ADAM17). The soluble 51 kDa trimeric sTNF tends to dissociate at concentrations below the nanomolar range, thereby losing its bioactivity. The secreted form of human TNFα takes on a triangular pyramid shape, and weighs around 17-kD. 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 biology activities.

The common house mouse TNFα and human TNF are structurally different. The 17-is_associated_with::kilodalton (kDa) TNF protomers (185-amino acid-long) are composed of two antiparallel &beta;-pleated sheets with antiparallel &beta;-strands, forming a 'jelly roll' β-structure, typical for the TNF family, but also found in viral capsid proteins.

Cell signaling
TNF can bind two receptors, is_associated_with::TNFR1 (is_associated_with::TNF receptor type 1; CD120a; p55/60) and is_associated_with::TNFR2 (TNF receptor type 2; CD120b; p75/80). TNFR1 is 55-kDa and TNFR2 is 75-kDa. 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 only in cells of the is_associated_with::immune system, and respond 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.



Upon contact with their is_associated_with::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 is_associated_with::adaptor protein is_associated_with::TRADD to bind to the death domain, serving as a platform for subsequent protein binding. Following TRADD binding, three pathways can be initiated.


 * Activation of is_associated_with::NF-κB: TRADD recruits is_associated_with::TRAF2 and RIP. is_associated_with::TRAF2 in turn recruits the multicomponent protein is_associated_with::kinase IKK, enabling the serine-threonine is_associated_with::kinase RIP to activate it. An inhibitory protein, is_associated_with::IκBα, that normally binds to NF-κB and inhibits its translocation, is is_associated_with::phosphorylated by IKK and subsequently degraded, releasing NF-κB. NF-κB is a heterodimeric is_associated_with::transcription factor that translocates to the nucleus and mediates the transcription of a vast array of proteins involved in cell survival and proliferation, is_associated_with::inflammatory response, and anti-is_associated_with::apoptotic factors.


 * Activation of the is_associated_with::MAPK pathways: Of the three major is_associated_with::MAPK cascades, TNF induces a strong activation of the stress-related is_associated_with::JNK group, evokes moderate response of the p38-MAPK, and is responsible for minimal activation of the classical ERKs. TRAF2/Rac activates the is_associated_with::JNK-inducing upstream is_associated_with::kinases of MLK2/MLK3, TAK1, is_associated_with::MEKK1 and is_associated_with::ASK1 (either directly or through GCKs and Trx, respectively). SRC- Vav- Rac axis activates MLK2/MLK3  and these is_associated_with::kinases is_associated_with::phosphorylate MKK7, which then activates is_associated_with::JNK. is_associated_with::JNK translocates to the nucleus and activates is_associated_with::transcription factors such as is_associated_with::c-Jun and is_associated_with::ATF2. The is_associated_with::JNK pathway is involved in is_associated_with::cell differentiation, proliferation, and is generally pro-is_associated_with::apoptotic.


 * Induction of death signaling: Like all death-domain-containing members of the TNFR superfamily, TNFR1 is involved in death signaling. 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-is_associated_with::apoptotic effects of NF-κB. Nevertheless, TRADD binds is_associated_with::FADD, which then recruits the is_associated_with::cysteine protease is_associated_with::caspase-8. A high concentration of is_associated_with::caspase-8 induces its autoproteolytic activation and subsequent cleaving of effector is_associated_with::caspases, leading to cell is_associated_with::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 is_associated_with::C-FLIP, is_associated_with::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 is_associated_with::cytokines, or the amount of is_associated_with::reactive oxygen species (ROS) can shift the balance in favor of one pathway or another. Such complicated signaling ensures that, whenever TNF is released, various cells with vastly diverse functions and conditions can all respond appropriately to is_associated_with::inflammation.

Enzyme regulation
This protein may use the is_associated_with::morpheein model of is_associated_with::allosteric regulation.

Physiology
TNF was thought to be produced primarily by is_associated_with::macrophages, but it is produced also by a broad variety of cell types including is_associated_with::lymphoid cells, is_associated_with::mast cells, is_associated_with::endothelial cells, is_associated_with::cardiac myocytes, is_associated_with::adipose tissue, is_associated_with::fibroblasts, and is_associated_with::neurons. Large amounts of TNF are released in response to is_associated_with::lipopolysaccharide, other is_associated_with::bacterial products, and is_associated_with::Interleukin-1 (IL-1). In the skin, mast cells appear to be the predominant source of pre-formed TNF, which can be released upon inflammatory stimulus (e.g., LPS).

It has a number of actions on various organ systems, generally together with IL-1 and is_associated_with::Interleukin-6 (IL-6):
 * On the is_associated_with::hypothalamus:
 * Stimulation of the is_associated_with::hypothalamic-pituitary-adrenal axis by stimulating the release of is_associated_with::corticotropin releasing hormone (CRH)
 * Suppressing is_associated_with::appetite
 * is_associated_with::Fever
 * On the is_associated_with::liver: stimulating the is_associated_with::acute phase response, leading to an increase in is_associated_with::C-reactive protein and a number of other mediators. It also induces is_associated_with::insulin resistance by promoting serine-phosphorylation of is_associated_with::insulin receptor substrate-1 (IRS-1), which impairs insulin signaling
 * It is a potent chemoattractant for is_associated_with::neutrophils, and promotes the expression of adhesion molecules on is_associated_with::endothelial cells, helping is_associated_with::neutrophils migrate.
 * On macrophages: stimulates is_associated_with::phagocytosis, and production of IL-1 oxidants and the inflammatory lipid is_associated_with::Prostaglandin E2 (PGE2)
 * On other tissues: increasing is_associated_with::insulin resistance. This mechanism occurs as TNF phosphorylates serine residues on the insulin receptor causing signal to stop at the cell surface.

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 is_associated_with::cachexia, a wasting syndrome. This can be found, for example, in is_associated_with::cancer patients.

Said et al. showed that TNFα 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.

Recent research by Pedersen et al. indicates that TNFα increase in response to sepsis is inhibited by the exercise-induced production of is_associated_with::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 TNFα. In contrast, when the subjects performed 3 hours of ergometer cycling and received the endotoxin bolus at 2.5 h, the TNFα response was totally blunted. This study provides some evidence that acute exercise may inhibit TNF production.

Pharmacology
TNF promotes the inflammatory response, which, in turn, causes many of the clinical problems associated with autoimmune disorders such as is_associated_with::rheumatoid arthritis, is_associated_with::ankylosing spondylitis, is_associated_with::inflammatory bowel disease, is_associated_with::psoriasis, is_associated_with::hidradenitis suppurativa and refractory is_associated_with::asthma. These disorders are sometimes treated by using a is_associated_with::TNF inhibitor. This inhibition can be achieved with a is_associated_with::monoclonal antibody such as is_associated_with::infliximab (Remicade), is_associated_with::adalimumab (Humira) or is_associated_with::certolizumab pegol (Cimzia), or with a circulating receptor is_associated_with::fusion protein such as is_associated_with::etanercept (Enbrel).

Interactions
TNFα has been shown to interact with is_associated_with::TNFRSF1A.

Nomenclature
Some recent papers have argued that TNFα should simply be called TNF, as LTα is no longer referred to as TNFβ.