Biochemical analysis of the native TRAIL death-inducing signaling complex

The extrinsic apoptosis pathway is activated when certain members of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF) are oligomerized by their cognate ligands that are members of the TNF superfamily (TNFSF). The apoptosis-inducing capacity of a member of the TNFRSF relies on the presence of a death domain (DD) in the intracellular portion of the receptor protein. Such receptors are also referred to as death receptors. Binding of a TNFSF ligand to a TNFRSF receptor that is expressed on the surface of a cell results in the formation of a receptor proximal protein complex. This protein complex is the platform for further signaling events within the cell. In case of death receptors like TNF-related apoptosis-inducing ligand receptor 1 (TRAIL-R1/DR4), TRAIL-R2 (KILLER/APO-2/DR5/TRICK), CD95 (Fas, APO-1), or TNF receptor 1 (TNF-R1), this complex is termed death-inducing signaling complex (DISC). The compositions of the various DISCs have been intensively studied in the last 12 years. For the CD95 and the TRAIL-R1/R2 DISCs, it is now clear that the adaptor protein Fas-associated DD protein (FADD) forms part of these complexes and is necessary for recruitment of the proapoptotic signaling molecules caspase-8 and caspase-10. Recruitment of these proteases allows for their activation at the DISC and subsequent induction of apoptosis. The caspase-8 homologous cellular FLICE-like inhibitory protein (cFLIP) can also be recruited to the DISC. cFLIP acts as an anti-apoptotic regulator by interfering with activation of caspases 8 and 10 at the DISC. Interestingly, treatment of TRAIL-resistant tumor cells with conventional chemotherapeutic drugs or with proteasome inhibitors renders these cells sensitive for TRAIL-induced apoptosis. By applying the methodology of the biochemical analysis of the TRAIL DISC described here, we were able to show that this sensitization is mainly due to changes in the biochemical composition of the DISC as the apoptosis-initiating protein complex of the extrinsic pathway.