Molecular oncology. Causes of cancer and targets for treatment. Cambridge University Press (2014)
Ligand binding to death receptors leads to recruitment of DDcontaining adaptor proteins that bind to the DD domains of death receptors (15; Figure 30.2). The DD-containing adaptor proteins include FADD (FAS-associated death-domain protein; 16) and TRADD (TNF-R1-associated death domain protein; 17). FADD contains an N-terminal DD and a C-terminal death-effector domain (DED). The DED of FADD binds and recruits the N-terminal DED of initiator caspases. In this manner, the death receptors Fas, TRAIL-R1, and TRAIL-R2 assemble FADD and pro-caspase-8/-10 to form a death-inducing signaling complex known as the DISC (4). For death receptors TNF-R1 and TRAMP, TRADD is recruited as an intermediary adaptor to further recruit FADD (4). Besides recruiting procaspase-8/-10 to induce apoptosis, TRADD is also a platform to recruit TRAF2 (TNF receptor-associated factor 2), RIP1 (RIPK1, receptor-interacting protein kinase 1), cIAP (cellular inhibitor of apoptosis), and other factors, to activate both cell-survival and cell-death pathways (4; see Figure 30.3).
Given the requirement for FADD to signal cell death, it was initially surprising that FADD-deficient mice and caspase-8-deficient mice are embryonically lethal and exhibit defects in T-cell proliferation (18–21). New findings help resolve this paradox, as FADD-deficient and caspase-8-deficient mice can be rescued by simultaneously deleting either RIP1 or RIP3 (22–24). The reason is that FADD-dependent caspase-8 activity is required to inhibit RIP-dependent necroptosis, a form of programmed necrotic cell death, although the details are unresolved. In this manner, low caspase-8 activity may prevent the slower necrotic death, thereby preserving cell survival and proliferation in cells that are also poised to undergo apoptosis on demand by amplifying caspase-8 activation.
Although the components of the DISC are well established, the molecular details of DISC assembly and activation remain elusive. Current models indicate that instead of triggering trimerization of death receptors, ligand-binding causes clustering of death-receptor trimers (25). Several lines of evidence have suggested that clustering of death receptors is essential for DISC formation (26–27). A hexamer formed by physically linking two soluble ligand trimers is highly competent to induce cell death, while the soluble trimer cannot, suggesting that the minimum signaling ligand-bound form of receptors is hexameric (28).
Figure 30.2. Formation of the death-inducing signaling complex (DISC). The death domain (DD) of the death receptors Fas, TRAIL-R1, and TRAIL-R2 binds to the DD of adaptor protein FADD. FADD further recruits caspase-8 through DED–DED interactions for DISC formation. TRADD further recruits components to TNF-R1 and TRAMP.
Figure 30.3. Death receptors induce non-apoptotic pathways. TNF can trigger apoptosis, necrosis, or cell-survival pathways through distinct signaling complexes. Cell-death complexes are also regulated by cFLIP and cIAPs (not shown).
Figure 30.4. Sequential activation steps for pro-caspase-8. Dimerization and two proteolytic processing steps activate initiator caspase-8. Initial cleavage at Asp374 separates large and small subunits after proximity-driven dimerization and activation of pro-caspase-8. Subsequent cleavage at Asp216 and Asp384 produces active enzyme subunits p18 and p10 of the heterotetrameric enzyme (p18/p10)2.
Death receptors were previously thought to interact with adaptor proteins in the DISC as a Fas-DD–FADD-DD complex with 4:4 stoichiometry, based on crystal structures (29). This would have required significant conformational changes of monomeric Fas-DD domains to adopt a more “open” form to bind the DD of FADD (29–30). It was also troubling that mutations in Fas known to disrupt DISC formation were not found at positions predicted to interface with FADD. Two independent research groups resolved this dilemma by determining the structures of Fas-DD–FADD-DD complexes at near physiological conditions (low salt and more neutral pH; 31–32). These structures reveal that the Fas–FADD complex forms a two-layered structure with five Fas DDs in the upper layer and five FADD DDs in the lower layer. These new structures are consistent with mutation sites known to disrupt DISC formation at Fas–FADD interfaces, and are similar to other DD–DD structures, such as the RAIDD–PIDD complex (33–34). This highly oligomeric structure also predicts a higher order of caspase-8 aggregation and activation in apoptotic cells (35).