Fas receptor

Overview
The Fas receptor (FasR) is the most intensely studied death receptor. Its aliases include CD95, Apo-1, and tumor necrosis factor receptor superfamily, member 6 (TNFRSf6). The gene is situated on chromosome 10 in humans and 19 in mice.

Isoforms
Previous reports have identified as many as eight splice variants, which are translated into seven isoforms of the protein.

Many of these isoforms are 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.

Function
Fas forms the death inducing signalling complex (DISC) upon ligand binding. Membrane-anchored Fas ligand trimer on the surface of an adjacent cell causes trimerization of Fas receptor. 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.

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.

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, 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.

Role in apoptosis
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.

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).