Fas ligand

FASLG
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 4MSV
Identifiers
Aliases	FASLG, ALPS1B, APT1LG1, APTL, CD178, CD95-L, CD95L, FASL, TNFSF6, TNLG1A, Fas ligand
External IDs	OMIM: 134638; MGI: 99255; HomoloGene: 533; GeneCards: FASLG; OMA:FASLG - orthologs
Gene location (Human) Chr. Chromosome 1 (human)[1] Band 1q24.3 Start 172,659,103 bp[1] End 172,666,876 bp[1]
Gene location (Mouse) Chr. Chromosome 1 (mouse)[2] Band 1 H2.1|1 69.95 cM Start 161,608,258 bp[2] End 161,616,064 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in granulocyte blood lymph node spleen jejunal mucosa mucosa of ileum gallbladder duodenum rectum bone marrow Top expressed in blastocyst lumbar subsegment of spinal cord embryo blood embryo spermatid placenta spleen decidua duodenum More reference expression data BioGPS More reference expression data
Gene ontology Molecular function cytokine activity protein binding tumor necrosis factor receptor binding signaling receptor binding death receptor binding Cellular component integral component of membrane membrane cytoplasmic vesicle lumen plasma membrane integral component of plasma membrane cell surface lysosomal lumen caveola perinuclear region of cytoplasm membrane raft lysosome extracellular exosome cytoplasmic vesicle nucleus external side of plasma membrane extracellular region extracellular space Biological process T cell apoptotic process regulation of transcription, DNA-templated cell-cell signaling endosomal lumen acidification negative regulation of transcription by RNA polymerase II retinal cell programmed cell death transcription, DNA-templated response to lipopolysaccharide positive regulation of neuron apoptotic process positive regulation of endothelial cell apoptotic process necroptotic signaling pathway immune response positive regulation of cell population proliferation positive regulation of apoptotic process positive regulation of I-kappaB kinase/NF-kappaB signaling negative regulation of angiogenesis regulation of extrinsic apoptotic signaling pathway via death domain receptors cellular chloride ion homeostasis response to growth factor inflammatory cell apoptotic process positive regulation of epidermal growth factor receptor signaling pathway extrinsic apoptotic signaling pathway via death domain receptors signal transduction extrinsic apoptotic signaling pathway activation of cysteine-type endopeptidase activity involved in apoptotic process necroptosis apoptotic signaling pathway negative regulation of extrinsic apoptotic signaling pathway via death domain receptors apoptotic process release of sequestered calcium ion into cytosol by endoplasmic reticulum regulation of signaling receptor activity cytokine-mediated signaling pathway Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 356 14103 Ensembl ENSG00000117560 ENSMUSG00000000817 UniProt P48023 P41047 RefSeq (mRNA) NM_001302746 NM_000639 NM_001205243 NM_010177 RefSeq (protein) NP_000630 NP_001289675 NP_001192172 NP_034307 Location (UCSC) Chr 1: 172.66 – 172.67 Mb Chr 1: 161.61 – 161.62 Mb PubMed search [3] [4]
Wikidata
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Fas ligand (FASL or CD95L) 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 (also called CD95) and plays a crucial role in the regulation of the immune system and in induction of apoptosis, a programmed cell death.^[5]

Fas ligand or FasL is a type II transmembrane protein belonging to the tumor necrosis factor superfamily (TNFSF). It is homotrimeric, which means it consists of three identical polypeptides. It has a long cytoplasmic domain, a stalk region, a transmembrane domain (TM), a TNF homology domain (THD) responsible for the homotrimerization. Including a C-terminal region involved in binding to CD95, also known as the fas receptor. ^[6][7]

FasL binds to fas, leading to the formation of fas:FasL assemble. This interaction initiates the formation of the death-inducing signaling complex, resulting in apoptosis.^[6]

FasL is expressed on various cell types, including T cells, natural killer cells, monocytes, neutrophils, and vascular endothelial cells. FasL exists in both membrane-anchored and soluble forms.^[5]

• FasR: The Fas receptor (FasR), or CD95, is the most intensely studied member of the death receptor family. The gene is situated on chromosome 10 in humans and 19 in mice. Previous reports have identified as many as eight splice variants, which are translated into seven isoforms of the protein. Many of these isoforms are associated with rare haplotypes that are usually associated with a state of disease. Apoptosis-inducing Fas receptor is dubbed isoform 1 and is a type 1 transmembrane protein. It consists of three cysteine-rich pseudorepeats, a transmembrane domain, and an intracellular death domain.^[8]
• DcR3: Decoy receptor 3 (DcR3) is a recently discovered decoy receptor of the tumor necrosis factor superfamily that binds to FasL, LIGHT, and TL1A. DcR3 is a soluble receptor that has no signal transduction capabilities (hence a "decoy") and functions to prevent FasR-FasL interactions by competitively binding to membrane-bound Fas ligand and rendering them inactive.^[9]

Fas signaling pathway involves activating apoptosis (programmed cell death). This happens through the interaction of Fas receptor and Fas ligand. As mentioned, Fas ligand/FasL is a type II transmembrane protein that can exist in both membrane-anchored and soluble forms. The interaction between FasR on an adjacent cell and membrane anchored FasL leads to the trimerization, forming the death-inducing signaling complex (DISC). ^[10]

Upon ensuing death domain (DD) aggregation, the receptor complex is internalized via the cellular endosomal machinery. This allows the adaptor molecule Fas-associated death domain (FADD) to bind the death domain (DD) of Fas through its own death domain (DD). FADD also contains a death effector domain (DED) near its amino terminus, which facilitates binding to the DED of FADD-like ICE (FLICE), more commonly referred to as caspase-8. FLICE can then self-activate through proteolytic cleavage into p10 and p18 subunits, of which two 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.^[11][10]

Some reports have suggested that the extrinsic Fas pathway is sufficient to induce complete apoptosis in certain cell types through death-inducing signaling complex (DISC) assembly and subsequent caspase-8 activation. ^[10] 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 for SW480, which is of the colon adenocarcinoma lineage.^[10]

Moreover, the pathways in the Fas signal cascade exhibit evidence for crosstalk. 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 engage exclusively 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). ^[10]

Additionally, the c-FLIP protein, structurally resembling caspase-8 but lacking enzymatic activity, plays a dual role in Fas-induced apoptosis. At low concentrations, c-FLIP is believed to promote caspase-8 activation. There is a possibility it is because caspase-8 binds to c-FLIP with higher affinity than to itself (caspase-8 homo-dimerization). However, at high concentrations, c-FLIP reduces the proteolytic activity of caspase-8, potentially by competing for binding to FADD. This dual role underscores the complexity of Fas signaling and its regulation by c-FLIP at different concentrations.^[10]

Apoptosis triggered by FasR-Fas ligand binding plays a fundamental role in the regulation of the immune system. Its functions include:

• T-cell homeostasis: the activation of T-cells leads to their expression of the Fas ligand. T cells are initially resistant to Fas-mediated apoptosis during clonal expansion, but become progressively more sensitive the longer they are activated, ultimately resulting in activation-induced cell death (AICD). This process is needed to prevent an excessive immune response and eliminate autoreactive T-cells. Humans and mice with deleterious mutations of Fas or Fas ligand develop an accumulation of aberrant T-cells, leading to lymphadenopathy, splenomegaly, and lupus erythematosus. ^[12]
• Cytotoxic T-cell activity: Fas-induced apoptosis and the perforin pathway are the two main mechanisms by which cytotoxic T lymphocytes induce cell death in cells expressing foreign antigens.^[13]
• Immune privilege: Cells in immune privileged areas such as the cornea or testes express Fas ligand and induce the apoptosis of infiltrating lymphocytes. It is one of many mechanisms the body employs in the establishment and maintenance of immune privilege.^[14]
• Maternal tolerance: Fas ligand may be instrumental in the prevention of leukocyte trafficking between the mother and the fetus, although no pregnancy defects have yet been attributed to a faulty Fas-Fas ligand system.^[14]
• Tumor counterattack: Tumors may over-express Fas ligand and induce the apoptosis of infiltrating lymphocytes, allowing the tumor to escape the effects of an immune response.^[15] The up-regulation of Fas ligand often occurs following chemotherapy, from which the tumor cells have attained apoptosis resistance.^[16]

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.^[17]

Increases in Fas-mediated signaling have been implicated in the pathology of low-risk myelodysplastic syndromes (MDS)^[18] and glioblastoma.^[19]

More recently, FasL-mediated apoptosis of T cells has also been suggested as an immune-evasive mechanism by which tumors can suppress T cell infiltration similar to inhibitory immune checkpoints such as PD-1 and CTLA-4.^[20][21][22]

Fas ligand has been shown to interact with:

• FAF1

• GeneReviews/NCBI/NIH/UW entry on Autoimmune Lymphoproliferative Syndrome
• Online Mendelian Inheritance in Man (OMIM): 601859
• Fas+Ligand+Protein at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• Overview of all the structural information available in the PDB for UniProt: P48023 (Tumor necrosis factor ligand superfamily member 6) at the PDBe-KB.