DAXX, FLASH, Caspase-3, Caspase-9, FasR(CD95), MEKK1(MAP3K1), Cytochrome c, BMF, tBid, NUMA1, BRE, Lamin A/C, DFF40 (CAD), c-FLIP (S), Caspase-8, DcR3(TNFRSF6B), Apaf-1, Caspase-7, FADD, Bcl-2, XIAP, RIPK1, Bim, FasR(CD95), SUMO-1, Lamin B, Caspase-10, c-IAP1, ICAD, Bax, MEK4 (MAP2K4), PARP-1, Caspase-6, JNK (MAPK8-10), RAIDD, ASK1 (MAP3K5), FasL(TNFSF6), PAK2, HtrA2, Caspase-2, Bid = tBid, Bid, Smac/Diablo, HSP27
FAS signaling cascade
Death receptors such as FasR belong to a Tumour Necrosis Factor (TNF) superfamily of receptors involved in proliferation, differentiation and apoptosis. FasR is ubiquitously expressed in various tissues, but its ligand FasL is expressed mainly in activated T lymphocytes and natural killer cells. The binding of ligands to receptor induces receptor trimerisation. Clustering on the plasma membrane is required to initiate apoptosis in cells.
FasR have some splice variants and isoforms. Isoforms which are missing the transmembrane domain (soluble form), or the intracellular domain, ( sFasR), can sequester FasL and inhibit apoptosis. In addition to death receptors, there are decoy receptors (DcR). DcR3 is a soluble receptor secreted by cells and binds with Fas ligand ( FasL). Decoy receptors possess functional extracellular ligand binding domains but do not contain intracellular death domains and cannot recruit adaptor proteins required for apoptosis. The principle function of decoy receptors is modulating the sensitivity to death-receptor-mediated apoptosis in vivo. DcR3 sequesters and inactivates the membrane-bound Fas ligand on adjacent cells and prevents activation of Fas receptor ( FasR ).
Activation of FasR lead to stimulation of several signal cascades: activation of caspase cascade, activation of intrinsic apoptotic pathway mediated by mitochondria, and activation of JNK-cascade .
Upon binding FasL to FasR, the receptor recruits a cytosolic adapter protein FADD (Fas-associated death domain), FLASH (CASP8 associated protein 2), and RAIDD ( CASP2 and RIPK1 domain containing adaptor with death domain) via RIPK1 (receptor (TNFRSF)-interacting serine-threonine kinase 1). Adaptor proteins transmit activating signal from the activated receptor FasR to initiator caspases caspase-2, -8, and -10. Recruitment of caspases by adaptors to the plasma membrane increases local concentration of these proteases and induces autocleavage and activation of caspases. The complex formed by FasR, FADD, Caspase-8, and possibly other proteins is known as DISC (death-inducing signaling complex). CASP8 and FADD-like apoptosis regulator ( c-FLIP ) share sequence homology with Caspase-8 and can bind to the FADD in competition with Caspase-8. c-FLIP inhibits Fas-mediated apoptosis when overexpressed in cells .
Activated initiator caspases cleave and activate effector caspases-3, -6 and -7. Once activated, the effector caspases are responsible for the proteolytic cleavage of a broad spectrum of cellular targets, which ultimately lead to cell death .
The extrinsic apoptotic pathway, induced by FasR can crosstalk to the intrinsic pathway through the caspase-8-mediated cleavage of BID (a BH3-ONLY member of the BCL2 family of proteins), which result to produce the pro-apoptotic tBID fragment . In mitochondria, the tBID activates BCL2-associated X protein ( BAX ) and thereby triggers the release of mitochondrial proteins Cytochrome C, diablo homolog ( Smac/DIABLO), and HtrA-like serine protease (HtrA2/OMI) .
These proteins reinforce the caspase cascade. Cytochrome C induces oligomerization of Apaf-1 into complexes which recruit and activate Caspases-9. Smac/DIABLO and HtrA2/OMI antagonize inhibitor of apoptosis proteins ( XIAP, IAP-1 ), a family of cellular caspase inhibitors .
FasR also stimulates the JNK signaling cascade. FasR recruit mitogen-activated protein kinase kinase kinase 5 ( ASK1 ) via adapter death-associated protein 6 ( DAXX ). Activation of ASK1 occur following recruitment to the DISC and subsequent JNK activation is believed to promote apoptosis in cells . JNK inhibit of action of B-cell lymphoma protein 2 ( Bcl-2 ), which blocks the release of mitochondrial proteins.
PAK2 is cleaved into two defined fragments during Fas-induced apoptosis and can activate JNK cascade .