The tumor necrosis factor (TNF) superfamily is a protein superfamily of type II transmembrane proteins containing TNF homology domain and forming trimers. Members of this superfamily can be released from the cell membrane by extracellular proteolytic cleavage and function as a cytokine. These proteins are expressed predominantly by immune cells and regulate diverse cell functions, including regulation of immune response and inflammation, and proliferation, differentiation, apoptosis and embryogenesis. The superfamily contains 19 members that bind to 29 members of TNF receptor superfamily. An occurrence of orthologs in invertebrates hints at ancient origin of this superfamily in evolution.
The PROSITE pattern of this superfamily is located in a beta sheet in the central section of the protein that is conserved across all members.
Members of TNF Superfamily
Table 1. TNF superfamily related products
|TNF Superfamily Ligands||CD154 / CD40L||CD70 / CD27L||EDA|
|Fas ligand / FASL||Lymphotoxin Alpha / LT-α / TNF-beta||LT-beta / LTB|
|TNFAIP8L3||TNFSF4 / OX40L||TNFSF8|
|TNFSF9 / 4-1BB||TNFSF10 / TRAIL||TNFSF11 / RANKL|
|TNFSF12 / TWEAK||TNFSF13 / APRIL||TNFSF13B / BAFF|
|TNFSF14 / LIGHT||TNFSF15 / VEGI||TNFSF18|
|TNF Superfamily Receptors||TNFRSF1A||TNFRSF1B||TNFRSF4|
|TNF Superfamily Regulators||TRAF1||TRAF2||TRAF3|
Lymphotoxin-alpha (LT-α) or tumor necrosis factor-beta (TNF-β) is a protein that in humans is encoded by the LTA gene. Belonging to the hematopoietic cell line, LT-α exhibits anti-proliferative activity and causes the cellular destruction of tumor cell lines. As a cytotoxic protein, LT-α performs a variety of important roles in immune regulation depending on the form that it is secreted as. In other words, LT-α has both a membrane-bound and soluble form that perform distinct roles in immune regulation. The human gene encoding for LT-α was cloned in 1985. The gene of LT-α is located on chromosome 6 and is in close proximity of the gene encoding major histocompatibility complex. LT-α is translated as a 25 kDa glycosylated polypeptide with 171 amino acid residues. Furthermore, human LT-α is 72% identical to mouse LT-α at the protein's primary sequence.
Tumor necrosis factor (TNF, tumor necrosis factor alpha, TNFα, cachexin, or cachectin) is a cell signaling protein (cytokine) involved in systemic inflammation and is one of the cytokines that make up the acute phase reaction. It is produced chiefly by activated macrophages, although it can be produced by many other cell types such as CD4+ lymphocytes, NK cells, neutrophils, mast cells, eosinophils, and neurons. TNF is primarily produced as a 233-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 ADAM17).
Figure 1. TNF protein
Lymphotoxin-beta (LT-beta) also known as tumor necrosis factor C (TNF-C) is a protein that in humans is encoded by the LTB gene. Lymphotoxin beta is a type II membrane protein of the TNF family. It anchors lymphotoxin-alpha to the cell surface through heterotrimer formation. The predominant form on the lymphocyte surface is the lymphotoxin-alpha 1/beta 2 complex (e.g. 1 molecule alpha/2 molecules beta) and this complex is the primary ligand for the lymphotoxin-beta receptor. The minor complex is lymphotoxin-alpha 2/beta 1. LTB is an inducer of the inflammatory response system and involved in normal development of lymphoid tissue. Lymphotoxin-beta isoform b is unable to complex with lymphotoxin-alpha suggesting a function for lymphotoxin-beta which is independent of lymphotoxin-alpha. Alternative splicing results in multiple transcript variants encoding different isoforms.
OX40L is the ligand for CD134 and is expressed on such cells as DC2s (a subtype of dendritic cells) enabling amplification of Th2 cell differentiation. OX40L has also been designated CD252 (cluster of differentiation 252).
CD154, also called CD40 ligand or CD40L, is a protein that is primarily expressed on activated T cells and is a member of the TNF superfamily of molecules. It binds to CD40 on antigen-presenting cells (APC), which leads to many effects depending on the target cell type. In total CD40L has three binding partners: CD40, α5β1 integrin and αIIbβ3. CD154 acts as a costimulatory molecule and is particularly important on a subset of T cells called T follicular helper cells (TFH cells). On TFH cells, CD154 promotes B cell maturation and function by engaging CD40 on the B cell surface and therefore facilitating cell-cell communication.
Fas ligand (FasL or CD95L) is a type-II transmembrane protein that belongs to the tumor necrosis factor (TNF) family. Its binding with its receptor induces apoptosis. Fas ligand/receptor interactions play an important role in the regulation of the immune system and the progression of cancer. Soluble Fas ligand is generated by cleaving membrane-bound FasL at a conserved cleavage site by the external matrix metalloproteinase MMP-7.
CD70 (Cluster of Differentiation 70) is a ligand for CD27. The CD70 protein is expressed on highly activated lymphocytes (like in T- and B-cell lymphomas). It is therefore suggested that anti-CD70 antibodies might be a possible treatment for CD70 positive lymphomas as normal lymphocytes have low CD70 expression.
4-1BB is a type 2 transmembrane glycoprotein receptor belonging to the TNF superfamily, expressed on activated T Lymphocytes. 4-1BBL (4-1BB ligand) is found on APCs (antigen presenting cells) and binds to 4-1BB.
In the field of cell biology, TNF-related apoptosis-inducing ligand (TRAIL), is a protein functioning as a ligand that induces the process of cell death called apoptosis. TRAIL is a cytokine that is produced and secreted by most normal tissue cells. It causes apoptosis primarily in tumor cells, by binding to certain death receptors. TRAIL shows homology to other members of the tumor necrosis factor superfamily. It is composed of 281 amino acids and has characteristics of a type II transmembrane protein (i.e. no leader sequence and an internal transmembrane domain). The N-terminal cytoplasmic domain is not conserved across family members, however, the C-terminal extracellular domain is conserved and can be proteolytically cleaved from the cell surface. TRAIL forms a homotrimer that binds three receptor molecules.
Receptor activator of nuclear factor kappa-Β ligand (RANKL), also known as tumor necrosis factor ligand superfamily member 11 (TNFSF11), TNF-related activation-induced cytokine (TRANCE), osteoprotegerin ligand (OPGL), and osteoclast differentiation factor (ODF), is a protein that in humans is encoded by the TNFSF11 gene. RANKL is known as a type II membrane protein and is a member of the tumor necrosis factor (TNF) superfamily. RANKL has been identified to affect the immune system and control bone regeneration and remodeling. RANKL is an apoptosis regulator gene, a binding partner of osteoprotegerin (OPG), a ligand for the receptor RANK and controls cell proliferation by modifying protein levels of Id4, Id2 and cyclin D1.
Figure 2. RANKL protein
Tumor necrosis factor ligand superfamily member 12 also known as TNF-related weak inducer of apoptosis (TWEAK) is a protein that in humans is encoded by the TNFSF12 gene. This protein is a ligand for the FN14/TWEAKR receptor. This cytokine has overlapping signaling functions with TNF, but displays a much wider tissue distribution.
A proliferation-inducing ligand (APRIL), also known as tumor necrosis factor ligand superfamily member 13 (TNFSF13), is a protein of the TNF superfamily recognized by the cell surface receptor TACI. This protein is a ligand for TNFRSF17/BCMA, a member of the TNF receptor family. This protein and its receptor are both found to be important for B cell development.
B-cell activating factor (BAFF) also known as tumor necrosis factor ligand superfamily member 13B is a protein that in humans is encoded by the TNFSF13B gene. BAFF is also known as B Lymphocyte Stimulator (BLyS) and TNF- and APOL-related leukocyte expressed ligand (TALL-1) and the dendritic cell-derived TNF-like molecule (CD257 antigen; cluster of differentiation 257). BAFF is a 285-amino acid long peptide glycoprotein which undergoes glycosylation at residue 124. It is expressed as a membrane-bound type II transmembrane protein on various cell types including monocytes, dendritic cells and bone marrow stromal cells.
Figure 3. BAFF protein
LIGHT, also known as tumor necrosis factor superfamily member 14 (TNFSF14), is a secreted protein of the TNF superfamily. It is recognized by herpesvirus entry mediator (HVEM), as well as decoy receptor 3. LIGHT stands for "homologous to lymphotoxin, exhibits inducible expression and competes with HSV glycoprotein D for binding to herpesvirus entry mediator, a receptor expressed on T lymphocytes". In the cluster of differentiation terminology it is classified as CD258.
Vascular endothelial growth inhibitor (VEGI), also known as TNF-like ligand 1A (TL1A) and TNF superfamily member 15 (TNFSF15), is protein that in humans is encoded by the TNFSF15 gene. VEGI is an anti-angiogenic protein. It belongs to tumor necrosis factor (ligand) superfamily, where it is member 15. It is the sole known ligand for death receptor 3, and it can also be recognized by decoy receptor 3.
Tumor necrosis factor ligand superfamily member 18 is a protein that in humans is encoded by the TNFSF18 gene. This cytokine is a ligand for receptor TNFRSF18/AITR/GITR. It has been shown to modulate T lymphocyte survival in peripheral tissues. This cytokine is also found to be expressed in endothelial cells, and is thought to be important for interaction between T lymphocytes and endothelial cells.
Ectodysplasin A (EDA) is a protein that in humans is encoded by the EDA gene. Ectodysplasin A is a transmembrane protein of the TNF family which plays an important role in the development of ectodermal tissues such as skin in humans. It is recognized by the ectodysplasin A receptor. The protein encoded by this gene is a type II membrane protein that can be cleaved by furin to produce a secreted form. The encoded protein, which belongs to the tumor necrosis factor family, acts as a homotrimer and may be involved in cell-cell signaling during the development of ectodermal organs. Along with c-Met, it has been shown to be involved in the differentiation of anatomical placodes, precursors of scales, feathers and hair follicles in vertebrates. Defects in this gene are a cause of ectodermal dysplasia, anhidrotic, which is also known as X-linked hypohidrotic ectodermal dysplasia.
As a signaling molecule, LT-α is involved in the regulation of cell survival, proliferation, differentiation, and apoptosis. LT-α plays an important role in innate immune regulation and its presence has been shown to prevent tumor growth and destroy cancerous cell lines. In contrast, unregulated expression of LT-α can result in a constantly active signaling pathway, thus leading to uncontrolled cellular growth and creation of tumors. Hence depending on the context, LT-α may function to prevent growth of cancer cells or facilitate the development of tumors. Furthermore, LT-α effects depend on the type of organ it acts upon, type of cancer cells, cellular environment, gender, and time of effect during an immune response.
The primary role of TNF is in the regulation of immune cells. TNF, being an endogenous pyrogen, is able to induce fever, apoptotic cell death, cachexia, inflammation and to inhibit tumorigenesis and viral replication and respond to sepsis via IL1 & IL6 producing cells.
CD154 acts as a costimulatory molecule and is particularly important on a subset of T cells called T follicular helper cells (TFH cells). On TFH cells, CD154 promotes B cell maturation and function by engaging CD40 on the B cell surface and therefore facilitating cell-cell communication.
Fas ligand or FasL is a homotrimeric type II transmembrane protein expressed on cytotoxic T lymphocytes. It signals through trimerization of FasR, which spans the membrane of the "target" cell. This trimerization usually leads to apoptosis, or cell death.
RANKL binds to RANK on cells of the myeloid lineage and functions as a key factor for osteoclast differentiation and activation. RANKL may also bind to osteoprotegerin, a protein secreted mainly by cells of the osteoblast lineage which is a potent inhibitor of osteoclast formation by preventing binding of RANKL to RANK. RANKL also has a function in the immune system, where it is expressed by T helper cells and is thought to be involved in dendritic cell maturation. This protein was shown to be a dendritic cell survival factor and is involved in the regulation of T cell-dependent immune response. T cell activation was reported to induce expression of this gene and lead to an increase of osteoclastogenesis and bone loss. This protein was shown to activate antiapoptotic kinase AKT/PKB through a signaling complex involving SRC kinase and tumor necrosis factor receptor-associated factor 6 (TRAF6), which indicated this protein may have a role in the regulation of cell apoptosis.
TWEAK can induce apoptosis via multiple pathways of cell death in a cell type-specific manner. This cytokine is also found to promote proliferation and migration of endothelial cells, and thus acts as a regulator of angiogenesis.
In vivo experiments suggest an important role for APRIL in the long-term survival of plasma cells in the bone marrow. Mice deficient in APRIL have normal immune system development. However, APRIL-deficient mice have also been reported to possess a reduced ability to support plasma cell survival. In vitro experiments suggested that this protein may be able to induce apoptosis through its interaction with other TNF receptor family proteins such as TNFRSF6/FAS and TNFRSF14/HVEM.
BAFF is a ligand for receptors TNFRSF13B/TACI, TNFRSF17/BCMA, and TNFRSF13C/BAFF-R. This cytokine is expressed in B cell lineage cells, and acts as a potent B cell activator. It has been also shown to play an important role in the proliferation and differentiation of B cells.
LIGHT may function as a costimulatory factor for the activation of lymphoid cells and as a deterrent to infection by herpesvirus. This protein has been shown to stimulate the proliferation of T cells, and trigger apoptosis of various tumor cells. This protein is also reported to prevent tumor necrosis factor alpha mediated apoptosis in primary hepatocyte. Two alternatively spliced transcript variant encoding distinct isoforms have been reported.
VEGI is a ligand for receptor TNFRSF25 (death receptor 3) and TNFRSF6B (decoy receptor 3). It can activate both the NF-κB and MAPK signaling pathways, and acts as an autocrine factor to induce apoptosis in endothelial cells. This cytokine is also found to inhibit endothelial cell proliferation, and thus may function as an angiogenesis inhibitor. An additional isoform encoded by an alternatively spliced transcript variant has been reported but the sequence of this transcript has not been determined.
Role in disease
LT-α mediates a large variety of inflammatory, immunostimulatory, and antiviral responses. LT-α is also involved in the formation of secondary lymphoid organs during development and plays a role in apoptosis. As a cytotoxic protein, LT-α causes the destruction of cancerous cell lines, activates signaling pathways, and effectively kills transformed tumor cells.
Dysregulation of TNF production has been implicated in a variety of human diseases including Alzheimer's disease, cancer, major depression, psoriasis and inflammatory bowel disease (IBD). Though controversial, studies of depression and IBD are currently being linked to TNF levels. Recombinant TNF is used as an 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.
Various single nucleotide polymorphisms (SNPs) of the OX40L gene have been identified. For some of them association with systemic lupus erythematosus has been reported: No association with occurrence of atherosclerosis has been found.
A defect in this gene results in an inability to undergo immunoglobulin class switching and is associated with hyper IgM syndrome. Absence of CD154 also stops the formation of germinal centers and therefore prohibiting antibody affinity maturation, an important process in the adaptive immune system.
Defective Fas-mediated apoptosis may lead to oncogenesis as well as drug resistance in existing tumors. Germline mutation of Fas is associated with autoimmune lymphoproliferative syndrome (ALPS), a childhood disorder of apoptosis.
TRAIL and its receptors have been used as the targets of several anti-cancer therapeutics since the mid-1990s, such as mapatumumab.
RANKL, through its ability to stimulate osteoclast formation and activity, is a critical mediator of bone resorption and overall bone density. Osteoclastic activity is triggered via the osteoblasts' surface-bound RANKL activating the osteoclasts' surface-bound receptor activator of nuclear factor kappa-B (RANK). Recent studies suggest that in postnatal bones, the osteocyte is the major source of RANKL regulating bone remodeling. RANKL derived from other cell types contributes to bone loss in conditions involving inflammation such as rheumatoid arthritis, and in lytic lesions caused by cancer, such as in multiple myeloma.
APRIL is being explored as a target for autoimmune diseases and B cell malignancies. At least one anti-APRIL monoclonal antibody has been announced to enter phase I clinical trials for multiple myeloma.
As an immunostimulant, BAFF (BLyS, TALL-1) is necessary for maintaining normal immunity. Excessive level of BAFF causes abnormally high antibody production, results in systemic lupus erythematosus, rheumatoid arthritis, and many other autoimmune diseases. Overexpression of BAFF also correlates with enhanced humoral immunity against malaria infection. Blisibimod, a fusion protein inhibitor of BAFF, is in development by Anthera Pharmaceuticals, also primarily for the treatment of systemic lupus erythematosus.
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