Apoptosis is specifically induced via signaling through a family of receptors known collectively as 'death receptors' including Fas, TNFR, DR3, -4 and -5. Death receptor ligands characteristically initiate signaling via receptor oligomerization, recruitment of specialized adaptor proteins and activation of caspase cascades. Apo3L recruits initiator caspase 8 via the adapter protein FADD. Caspase 8 then oligomerizes and is activated via autocatalysis. Activated caspase 8 stimulates apoptosis via two parallel cascades: it directly cleaves and activates caspase-3, and it cleaves Bid (a Bcl-2 family protein). Truncated Bid (tBid) translocates to mitochondria, inducing cytochrome C release, which sequentially activates caspases 9 and 3. DR-3L can deliver pro- or anti-apoptotic signals. DR-3 promote apoptosis via the adaptor proteins TRADD/FADD and the activation of caspase 8. Alternatively, apoptosis inhibited via an adaptor protein complex including RIP which activates NF-kB and induces survival genes including IAP. Induction of apoptosis via Apo2L requires caspase activity, but the adaptor requirement is unclear.
