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]
Structural features
editFas 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]
Receptors
edit- 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]
Cell signaling and mechanism
editFas 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]
Function of apoptosis in the immune system
editApoptosis 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]
Role in disease
editDefective 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]
Interactions
editFas ligand has been shown to interact with:
See also
editReferences
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- ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000000817 – Ensembl, May 2017
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- ^ Yolcu ES, Shirwan H, Askenasy N (2017-03-27). "Fas/Fas-Ligand Interaction As a Mechanism of Immune Homeostasis and β-Cell Cytotoxicity: Enforcement Rather Than Neutralization for Treatment of Type 1 Diabetes". Frontiers in Immunology. 8: 342. doi:10.3389/fimmu.2017.00342. PMC 5366321. PMID 28396667.
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- ^ Igney FH, Krammer PH (November 2005). "Tumor counterattack: fact or fiction?". Cancer Immunology, Immunotherapy. 54 (11): 1127–1136. doi:10.1007/s00262-005-0680-7. PMC 11034178. PMID 15889255. S2CID 19331352.
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- ^ Parlato S, Giammarioli AM, Logozzi M, Lozupone F, Matarrese P, Luciani F, et al. (October 2000). "CD95 (APO-1/Fas) linkage to the actin cytoskeleton through ezrin in human T lymphocytes: a novel regulatory mechanism of the CD95 apoptotic pathway". The EMBO Journal. 19 (19): 5123–5134. doi:10.1093/emboj/19.19.5123. PMC 302100. PMID 11013215.
- ^ a b c Ghadimi MP, Sanzenbacher R, Thiede B, Wenzel J, Jing Q, Plomann M, et al. (May 2002). "Identification of interaction partners of the cytosolic polyproline region of CD95 ligand (CD178)". FEBS Letters. 519 (1–3): 50–58. doi:10.1016/s0014-5793(02)02709-6. PMID 12023017. S2CID 26765451.
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- ^ Hane M, Lowin B, Peitsch M, Becker K, Tschopp J (October 1995). "Interaction of peptides derived from the Fas ligand with the Fyn-SH3 domain". FEBS Letters. 373 (3): 265–268. doi:10.1016/0014-5793(95)01051-f. PMID 7589480. S2CID 24130275.
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- ^ Yu KY, Kwon B, Ni J, Zhai Y, Ebner R, Kwon BS (May 1999). "A newly identified member of tumor necrosis factor receptor superfamily (TR6) suppresses LIGHT-mediated apoptosis". The Journal of Biological Chemistry. 274 (20): 13733–13736. doi:10.1074/jbc.274.20.13733. PMID 10318773.
- ^ Hsu TL, Chang YC, Chen SJ, Liu YJ, Chiu AW, Chio CC, et al. (May 2002). "Modulation of dendritic cell differentiation and maturation by decoy receptor 3". Journal of Immunology. 168 (10): 4846–4853. doi:10.4049/jimmunol.168.10.4846. PMID 11994433.
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Further reading
edit- Choi C, Benveniste EN (January 2004). "Fas ligand/Fas system in the brain: regulator of immune and apoptotic responses". Brain Research. Brain Research Reviews. 44 (1): 65–81. doi:10.1016/j.brainresrev.2003.08.007. PMID 14739003. S2CID 46587211.
- Tolstrup M, Ostergaard L, Laursen AL, Pedersen SF, Duch M (April 2004). "HIV/SIV escape from immune surveillance: focus on Nef". Current HIV Research. 2 (2): 141–151. doi:10.2174/1570162043484924. PMID 15078178.
External links
edit- 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.