**3. TRAIL-induced apoptosis signaling**

Soluble TRAIL forms homotrimers that bind three receptor molecules, each at the interface between two of its subunits. A Zn atom bound to cysteine residues in the trimeric ligand is essential for trimer stability and optimal biologic activity. Binding of TRAIL to the extracellular domain of agonistic receptors results in trimerization of the receptors and clustering of the intracellular DDs, which leads to the recruitment of the adaptor molecule Fas-associated protein with death domain (FADD). Subsequently, FADD recruits initiator caspase-8 and -10, leading to the formation of the death-inducing signaling complex (DISC), where initiator caspases are autoactivated by proteolysis. Once they become enzymatically active, caspase-8 and/or -10 are released from the DISC and signal through two different proteolytic pathways that converge on caspase-3 and lead to cellular disassembly (Kaufmann & Steensma, 2005). In type I cells, activation of initiator caspases upon death receptors ligation is sufficient to directly activate downstream effector caspases, such as caspase-3 and/or -7 (Scaffidi et al., 1998). This extrinsic pathway is independent of the mitochondria and is not blocked by overexpression of Bcl-2. In type II cells, the commitment from death receptor ligation to apoptosis is less direct (Scaffidi et al., 1998). The amount of initially cleaved caspase-8 and/or -10 is not enough to directly trigger effector caspases activation. Consequently, apoptotic signaling requires an amplification loop by mithocondrial pathway engagement through caspase 8-mediated cleavage of Bid (BH3 interacting death domain agonist), which, in turn, induces the cytosolic Bcl-2 family member Bax (Bcl-2-associated X protein) and/or the loosely bound mitochondrial homolog Bak (Bcl-2 antagonist/killer) to insert into the mitochondrial membrane, where they contribute to the mitochondrial release of cytochrome c (Lucken-Ardjomande & Martinou, 2005). In the cytosol, cytochrome c binds the adaptor protein Apaf-1 (apoptotic protease activating factor 1) to form an apoptosome with recruitment and activation of the apoptosis-initiating caspase-9, which proteolytically activates additional caspase-3. These events are further amplified by apoptogenic factors released from the mitochondrial space, including Smac/DIABLO (second mitochondrial activator of caspases/direct IAP-binding protein with low pI) (Verhagen & Vaux, 2002).
