The Fas receptor, also known as Fas, FasR, apoptosis antigen 1 (APO-1 or APT), cluster of differentiation 95 (CD95) or tumor necrosis factor receptor superfamily member 6 (TNFRSF6), is a protein that in humans is encoded by the FAS gene. [5] [6] Fas was first identified using a monoclonal antibody generated by immunizing mice with the FS-7 cell line. Thus, the name Fas is derived from FS-7-associated surface antigen. [7]
The Fas receptor is a death receptor on the surface of cells that leads to programmed cell death (apoptosis) if it binds its ligand, Fas ligand (FasL). It is one of two apoptosis pathways, the other being the mitochondrial pathway. [8]
FAS receptor gene is located on the long arm of chromosome 10 (10q24.1) in humans and on chromosome 19 in mice. The gene lies on the plus (Watson strand) and is 25,255 bases in length organized into nine protein encoding exons. Similar sequences related by evolution (orthologs) [9] are found in most mammals.
Previous reports have identified as many as eight splice variants, which are translated into seven isoforms of the protein. Apoptosis-inducing Fas receptor is dubbed isoform 1 and is a type 1 transmembrane protein. Many of the other isoforms are rare haplotypes that are usually associated with a state of disease. However, two isoforms, the apoptosis-inducing membrane-bound form and the soluble form, are normal products whose production via alternative splicing is regulated by the cytotoxic RNA binding protein TIA1. [10]
The mature Fas protein has 319 amino acids, has a predicted molecular weight of 48 kilodaltons and is divided into three domains: an extracellular domain, a transmembrane domain, and a cytoplasmic domain. The extracellular domain has 157 amino acids and is rich in cysteine residues. The transmembrane and cytoplasmic domains have 17 and 145 amino acids respectively. Exons 1 through 5 encode the extracellular region. Exon 6 encodes the transmembrane region. Exons 7-9 encode the intracellular region. [ citation needed ]
Fas forms the death-inducing signaling complex (DISC) upon ligand binding. Membrane-anchored Fas ligand trimer on the surface of an adjacent cell causes oligomerization of Fas. Recent studies which suggested the trimerization of Fas could not be validated. Other models suggested the oligomerization up to 5–7 Fas molecules in the DISC. [11] This event is also mimicked by binding of an agonistic Fas antibody, though some evidence suggests that the apoptotic signal induced by the antibody is unreliable in the study of Fas signaling. To this end, several clever ways of trimerizing the antibody for in vitro research have been employed. [ citation needed ]
Upon ensuing death domain (DD) aggregation, the receptor complex is internalized via the cellular endosomal machinery. This allows the adaptor molecule FADD to bind the death domain of Fas through its own death domain. [12]
FADD also contains a death effector domain (DED) near its amino terminus, [13] which facilitates binding to the DED of FADD-like interleukin-1 beta-converting enzyme (FLICE), more commonly referred to as caspase-8. FLICE can then self-activate through proteolytic cleavage into p10 and p18 subunits, two each of which form the active heterotetramer enzyme. Active caspase-8 is then released from the DISC into the cytosol, where it cleaves other effector caspases, eventually leading to DNA degradation, membrane blebbing, and other hallmarks of apoptosis. [ citation needed ]
Recently, Fas has also been shown to promote tumor growth, since during tumor progression, it is frequently downregulated or cells are rendered apoptosis resistant. Cancer cells in general, regardless of their Fas apoptosis sensitivity, depend on constitutive activity of Fas. This is stimulated by cancer-produced Fas ligand for optimal growth. [14]
Although Fas has been shown to promote tumor growth in the above mouse models, analysis of the human cancer genomics database revealed that FAS is not significantly focally amplified across a dataset of 3131 tumors (FAS is not an oncogene), but is significantly focally deleted across the entire dataset of these 3131 tumors, [15] suggesting that FAS functions as a tumor suppressor in humans.
In cultured cells, FasL induces various types of cancer cell apoptosis through the Fas receptor. In AOM-DSS-induced colon carcinoma and MCA-induced sarcoma mouse models, it has been shown that Fas acts as a tumor suppressor. [16] Furthermore, the Fas receptor also mediates tumor-specific cytotoxic T lymphocyte (CTL) anti-tumor cytotoxicity. [17] In addition to the well-described on-target CTL anti-tumor cytotoxicity, Fas has been ascribed with a distinct function – the induction of bystander tumor cell death even amongst cognate antigen non-expressing (bystander) cells. CTL-mediated bystander killing was described by the Fleischer Lab in 1986 [18] and later attributed to fas-mediated lysis in vitro by the Austin Research Institute, Cellular Cytotoxicity Laboratory. [19] More recently, fas-mediated bystander tumor cell killing was demonstrated in vivo by the Lymphoma Immunotherapy Program at Mount Sinai School of Medicine using T cells and CAR-T cells, [20] similar to additional in vitro work using bispecific antibodies performed at Amgen. [21]
Some reports have suggested that the extrinsic Fas pathway is sufficient to induce complete apoptosis in certain cell types through DISC assembly and subsequent caspase-8 activation. These cells are dubbed Type 1 cells and are characterized by the inability of anti-apoptotic members of the Bcl-2 family (namely Bcl-2 and Bcl-xL) to protect from Fas-mediated apoptosis. Characterized Type 1 cells include H9, CH1, SKW6.4 and SW480, all of which are lymphocyte lineages except the latter, which is a colon adenocarcinoma lineage. However, evidence for crosstalk between the extrinsic and intrinsic pathways exists in the Fas signal cascade. [ citation needed ]
In most cell types, caspase-8 catalyzes the cleavage of the pro-apoptotic BH3-only protein Bid into its truncated form, tBid. BH-3 only members of the Bcl-2 family exclusively engage anti-apoptotic members of the family (Bcl-2, Bcl-xL), allowing Bak and Bax to translocate to the outer mitochondrial membrane, thus permeabilizing it and facilitating release of pro-apoptotic proteins such as cytochrome c and Smac/DIABLO, an antagonist of inhibitors of apoptosis proteins (IAPs). [ citation needed ]
Fas receptor has been shown to interact with:
Apoptosis is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The average adult human loses 50 to 70 billion cells each day due to apoptosis. For the average human child between 8 and 14 years old, each day the approximate loss is 20 to 30 billion cells.
Caspases are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cysteine protease activity – a cysteine in its active site nucleophilically attacks and cleaves a target protein only after an aspartic acid residue. As of 2009, there are 12 confirmed caspases in humans and 10 in mice, carrying out a variety of cellular functions.
Fas ligand is a type-II transmembrane protein expressed on various types of cells, including cytotoxic T lymphocytes, monocytes, neutrophils, breast epithelial cells, vascular endothelial cells and natural killer (NK) cells. It binds with its receptor, called FAS receptor and plays a crucial role in the regulation of the immune system and in induction of apoptosis, a programmed cell death.
The death-effector domain (DED) is a protein interaction domain found only in eukaryotes that regulates a variety of cellular signalling pathways. The DED domain is found in inactive procaspases and proteins that regulate caspase activation in the apoptosis cascade such as FAS-associating death domain-containing protein (FADD). FADD recruits procaspase 8 and procaspase 10 into a death induced signaling complex (DISC). This recruitment is mediated by a homotypic interaction between the procaspase DED and a second DED that is death effector domain in an adaptor protein that is directly associated with activated TNF receptors. Complex formation allows proteolytic activation of procaspase into the active caspase form which results in the initiation of apoptosis. Structurally the DED domain are a subclass of protein motif known as the death fold and contains 6 alpha helices, that closely resemble the structure of the Death domain (DD).
Betulinic acid is a naturally occurring pentacyclic triterpenoid which has antiretroviral, antimalarial, and anti-inflammatory properties, as well as a more recently discovered potential as an anticancer agent, by inhibition of topoisomerase. It is found in the bark of several species of plants, principally the white birch from which it gets its name, same as the bracket fungus Fomitopsis betulina, but also the ber tree, selfheal, the tropical carnivorous plants Triphyophyllum peltatum and Ancistrocladus heyneanus, Diospyros leucomelas, a member of the persimmon family, Tetracera boiviniana, the jambul, flowering quince, rosemary, and Pulsatilla chinensis.
The death-inducing signaling complex (DISC) is a multi-protein complex formed by members of the death receptor family of apoptosis-inducing cellular receptors. A typical example is FasR, which forms the DISC upon trimerization as a result of its ligand (FasL) binding. The DISC is composed of the death receptor, FADD, and caspase 8. It transduces a downstream signal cascade resulting in apoptosis.
FAS-associated death domain protein, also called MORT1, is encoded by the FADD gene on the 11q13.3 region of chromosome 11 in humans.
The BH3 interacting-domain death agonist, or BID, gene is a pro-apoptotic member of the Bcl-2 protein family. Bcl-2 family members share one or more of the four characteristic domains of homology entitled the Bcl-2 homology (BH) domains, and can form hetero- or homodimers. Bcl-2 proteins act as anti- or pro-apoptotic regulators that are involved in a wide variety of cellular activities.
Caspase-8 is a caspase protein, encoded by the CASP8 gene. It most likely acts upon caspase-3. CASP8 orthologs have been identified in numerous mammals for which complete genome data are available. These unique orthologs are also present in birds.
Caspase-3 is a caspase protein that interacts with caspase-8 and caspase-9. It is encoded by the CASP3 gene. CASP3 orthologs have been identified in numerous mammals for which complete genome data are available. Unique orthologs are also present in birds, lizards, lissamphibians, and teleosts.
Tumor necrosis factor receptor type 1-associated DEATH domain protein is a protein that in humans is encoded by the TRADD gene.
Cancer immunology (immuno-oncology) is an interdisciplinary branch of biology and a sub-discipline of immunology that is concerned with understanding the role of the immune system in the progression and development of cancer; the most well known application is cancer immunotherapy, which utilises the immune system as a treatment for cancer. Cancer immunosurveillance and immunoediting are based on protection against development of tumors in animal systems and (ii) identification of targets for immune recognition of human cancer.
Tumor necrosis factor receptor 1 (TNFR1), also known as tumor necrosis factor receptor superfamily member 1A (TNFRSF1A) and CD120a, is a ubiquitous membrane receptor that binds tumor necrosis factor-alpha (TNFα).
Death receptor 4 (DR4), also known as TRAIL receptor 1 (TRAILR1) and tumor necrosis factor receptor superfamily member 10A (TNFRSF10A), is a cell surface receptor of the TNF-receptor superfamily that binds TRAIL and mediates apoptosis.
Caspase-10 is an enzyme that, in humans, is encoded by the CASP10 gene.
Death receptor 5 (DR5), also known as TRAIL receptor 2 (TRAILR2) and tumor necrosis factor receptor superfamily member 10B (TNFRSF10B), is a cell surface receptor of the TNF-receptor superfamily that binds TRAIL and mediates apoptosis.
Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) functions in a variety of cellular pathways related to both cell survival and death. In terms of cell death, RIPK1 plays a role in apoptosis, necroptosis, and PANoptosis Some of the cell survival pathways RIPK1 participates in include NF-κB, Akt, and JNK.
Death receptor 3 (DR3), also known as tumor necrosis factor receptor superfamily member 25 (TNFRSF25), is a cell surface receptor of the tumor necrosis factor receptor superfamily which mediates apoptotic signalling and differentiation. Its only known TNFSF ligand is TNF-like protein 1A (TL1A).
The death domain (DD) is a protein interaction module composed of a bundle of six alpha-helices. DD is a subclass of protein motif known as the death fold and is related in sequence and structure to the death effector domain (DED) and the caspase recruitment domain (CARD), which work in similar pathways and show similar interaction properties. DD bind each other forming oligomers. Mammals have numerous and diverse DD-containing proteins. Within these proteins, the DD domains can be found in combination with other domains, including: CARDs, DEDs, ankyrin repeats, caspase-like folds, kinase domains, leucine zippers, leucine-rich repeats (LRR), TIR domains, and ZU5 domains.
AICD is programmed cell death caused by the interaction of Fas receptors and Fas ligands. AICD is a negative regulator of activated T lymphocytes that results from repeated stimulation of their T-cell receptors (TCR) and helps to maintain peripheral immune tolerance. Alteration of the process may lead to autoimmune diseases.