Fas ligand explained

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.^[1]

Structural features

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. ^{[2] [3]}

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.

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.

Receptors

• 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.^[4]
• 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.^[5]

Cell signaling and mechanism

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). ^[6]

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.^[7]

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. 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.

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).

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.

Function of apoptosis in the immune system

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. ^[8]
• 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.^[9]
• 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.^[10]

• 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.
• 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.^[11] The up-regulation of Fas ligand often occurs following chemotherapy, from which the tumor cells have attained apoptosis resistance.^[12]

Role in disease

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.^[13]

Increases in Fas-mediated signaling have been implicated in the pathology of low-risk myelodysplastic syndromes (MDS)^[14] and glioblastoma.^[15]

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.^{[16] [17] [18]}

Interactions

Fas ligand has been shown to interact with:

• CASP8,
• EZR,^[19]
• FADD,^{[20] [21]}
• FNBP1,
• FYN,^[22]
• FAS,^{[20] [21] [23] [24]}
• Grb2,^{[25] [26]}
• PACSIN2,^[25] and
• TNFRSF6B.^{[27] [28] [29]}

See also

• FAF1

Further reading

• Choi C, Benveniste EN . Fas ligand/Fas system in the brain: regulator of immune and apoptotic responses . Brain Research. Brain Research Reviews . 44 . 1 . 65–81 . January 2004 . 14739003 . 10.1016/j.brainresrev.2003.08.007 . 46587211 .
• Tolstrup M, Ostergaard L, Laursen AL, Pedersen SF, Duch M . HIV/SIV escape from immune surveillance: focus on Nef . Current HIV Research . 2 . 2 . 141–151 . April 2004 . 15078178 . 10.2174/1570162043484924 .

External links

• GeneReviews/NCBI/NIH/UW entry on Autoimmune Lymphoproliferative Syndrome

Notes and References

1. Book: Krippner-Heidenreich A, Scheurich P . 2006 . FasL and Fas. Typical Members of the TNF Ligand and Receptor Family . Fas Signaling. Medical Intelligence Unit. . Springer . Boston, MA . 10.1007/0-387-34573-6_1 . 0-387-34573-6 .
2. Levoin N, Jean M, Legembre P . CD95 Structure, Aggregation and Cell Signaling . Frontiers in Cell and Developmental Biology . 8 . 314 . 2020 . 32432115 . 7214685 . 10.3389/fcell.2020.00314 . free .
3. Orlinick JR, Vaishnaw AK, Elkon KB . Structure and function of Fas/Fas ligand . International Reviews of Immunology . 18 . 4 . 293–308 . 1999 . 10626245 . 10.3109/08830189909088485 .
4. Liu W, Ramagopal U, Cheng H, Bonanno JB, Toro R, Bhosle R, Zhan C, Almo SC . Crystal Structure of the Complex of Human FasL and Its Decoy Receptor DcR3 . Structure . 24 . 11 . 2016–2023 . November 2016 . 27806260 . 10.1016/j.str.2016.09.009 . free .
5. Sheikh MS, Fornace AJ . Death and decoy receptors and p53-mediated apoptosis . Leukemia . 14 . 8 . 1509–1513 . August 2000 . 10942251 . 10.1038/sj.leu.2401865 . 12572810 .
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7. Yolcu ES, Shirwan H, Askenasy N . 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 . 2017-03-27 . 28396667 . 5366321 . 10.3389/fimmu.2017.00342 . free .
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15. Tachibana O, Nakazawa H, Lampe J, Watanabe K, Kleihues P, Ohgaki H . Expression of Fas/APO-1 during the progression of astrocytomas . Cancer Research . 55 . 23 . 5528–5530 . December 1995 . 7585627 .
16. Motz GT, Santoro SP, Wang LP, Garrabrant T, Lastra RR, Hagemann IS, Lal P, Feldman MD, Benencia F, Coukos G . Tumor endothelium FasL establishes a selective immune barrier promoting tolerance in tumors . Nature Medicine . 20 . 6 . 607–615 . June 2014 . 24793239 . 4060245 . 10.1038/nm.3541 .
17. Zhu J, Powis de Tenbossche CG, Cané S, Colau D, van Baren N, Lurquin C, Schmitt-Verhulst AM, Liljeström P, Uyttenhove C, Van den Eynde BJ . Resistance to cancer immunotherapy mediated by apoptosis of tumor-infiltrating lymphocytes . Nature Communications . 8 . 1 . 1404 . November 2017 . 29123081 . 5680273 . 10.1038/s41467-017-00784-1 . 2017NatCo...8.1404Z .
18. Lakins MA, Ghorani E, Munir H, Martins CP, Shields JD . Cancer-associated fibroblasts induce antigen-specific deletion of CD8 ⁺ T Cells to protect tumour cells . Nature Communications . 9 . 1 . 948 . March 2018 . 29507342 . 5838096 . 10.1038/s41467-018-03347-0 . 2018NatCo...9..948L .
19. Parlato S, Giammarioli AM, Logozzi M, Lozupone F, Matarrese P, Luciani F, Falchi M, Malorni W, Fais S . 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 . October 2000 . 11013215 . 302100 . 10.1093/emboj/19.19.5123 .
20. Gajate C, Mollinedo F . Cytoskeleton-mediated death receptor and ligand concentration in lipid rafts forms apoptosis-promoting clusters in cancer chemotherapy . The Journal of Biological Chemistry . 280 . 12 . 11641–11647 . March 2005 . 15659383 . 10.1074/jbc.M411781200 . free .
21. Micheau O, Tschopp J . Induction of TNF receptor I-mediated apoptosis via two sequential signaling complexes . Cell . 114 . 2 . 181–190 . July 2003 . 12887920 . 10.1016/s0092-8674(03)00521-x . 17145731 .
22. Hane M, Lowin B, Peitsch M, Becker K, Tschopp J . Interaction of peptides derived from the Fas ligand with the Fyn-SH3 domain . FEBS Letters . 373 . 3 . 265–268 . October 1995 . 7589480 . 10.1016/0014-5793(95)01051-f . 24130275 . free .
23. Starling GC, Bajorath J, Emswiler J, Ledbetter JA, Aruffo A, Kiener PA . Identification of amino acid residues important for ligand binding to Fas . The Journal of Experimental Medicine . 185 . 8 . 1487–1492 . April 1997 . 9126929 . 2196280 . 10.1084/jem.185.8.1487 .
24. Schneider P, Bodmer JL, Holler N, Mattmann C, Scuderi P, Terskikh A, Peitsch MC, Tschopp J . Characterization of Fas (Apo-1, CD95)-Fas ligand interaction . The Journal of Biological Chemistry . 272 . 30 . 18827–18833 . July 1997 . 9228058 . 10.1074/jbc.272.30.18827 . free .
25. Ghadimi MP, Sanzenbacher R, Thiede B, Wenzel J, Jing Q, Plomann M, Borkhardt A, Kabelitz D, Janssen O . Identification of interaction partners of the cytosolic polyproline region of CD95 ligand (CD178) . FEBS Letters . 519 . 1-3 . 50–58 . May 2002 . 12023017 . 10.1016/s0014-5793(02)02709-6 . 26765451 .
26. Wenzel J, Sanzenbacher R, Ghadimi M, Lewitzky M, Zhou Q, Kaplan DR, Kabelitz D, Feller SM, Janssen O . Multiple interactions of the cytosolic polyproline region of the CD95 ligand: hints for the reverse signal transduction capacity of a death factor . FEBS Letters . 509 . 2 . 255–262 . December 2001 . 11741599 . 10.1016/s0014-5793(01)03174-x . 33084576 .
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