| Description |
Apoptosis, or programmed cell death (PCD), is a cell-intrinsic, genetically directed system of cell termination that is important in a wide range of physiological and pathological processes. Apoptotic signals conducted through death receptors are important in inflammation, autoimmunity, and organ development and degeneration. Adapter proteins with death domains, such as FADD and TRADD, play a critical role in pathways activated by Fas and TNF-R1 through the recruitment and activation of caspases. The basic model describing Fas and TNF-receptor mediated cell death consists of linear pathways from ligand to caspases. Studies have continued to show that these pathways are the principal mechanism by which death receptors initiate the apoptotic process. However, a number of other signals that are initiated by death receptors may play important roles within different cellular contexts. In addition, caspase inhibition fails to stop death initiated by death receptors, suggesting that both caspase-dependent and caspase-independent pathways are activated by the receptor. These pathways likely involve death domain proteins, including TRADD and FADD. This treatise describes novel behaviors of the isolated death domains of FADD and TRADD. The death domain of FADD (FADD-DN) usually acts as to block apoptosis. However, in primary prostate epithelial cells, it initiates cell death. This effect appears to be both tissue- and cell-type-specific in that prostate tumor cell lines, primary prostate tumor cells, and normal primary prostate stromal cells fail to die similarly. A new assay for caspase activation in living, single cells, which utilizes fluorescence resonance energy transfer (FRET), is described and is used to show that caspase activation occurs in these cells in response to FADD-DN. The isolated death domain of TRADD also has a novel behavior in that it localizes in "dot-like" structures in the nucleus. In addition, the mechanism of death induced by nuclear TRADD appears to be distinct from its function at the cell membrane in that death is not blocked with FADD-DN, but is inhibited by Bcl-xL. In the novel behaviors described herein, both FADD and TRADD appear to activate caspase-dependent and independent processes distinct from ones previously described. |
