Improved Artificial Death Switches Based on Caspases and FADD

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Abstract
A number of "suicide genes" have been developed as safety switches for gene therapy vectors or as potential inducible cytotoxic agents for hyperproliferative disorders, such as cancer or restenosis. However, most of these approaches have relied on foreign proteins, such as HSV thymidine kinase, that primarily target rapidly dividing cells. In contrast, novel artificial death switches based on chemical inducers of dimerization (CIDs) and endogenous proapoptotic molecules function efficiently in both dividing and nondividing cells. In this approach, lipid-permeable, nontoxic CIDs are used to conditionally cross-link target proteins that are fused to CID-binding domains (CBDs), thus activating signaling cascades leading to apoptosis. In previous reports, CID-regulated Fas and caspases 1, 3, 8, and 9 were described. Since the maximum efficacy of these artificial death switches requires low basal and high specific activity, we have optimized these death switches for three parameters: (1) extent of oligomerization, (2) spacing between CBDs and target proteins, and (3) intracellular localization. We describe improved conditional Fas and caspase 1, 3, 8, and 9 alleles that function at subnanomolar levels of the CID AP1903 to trigger apoptosis. Further, we demonstrate for the first time that oligomerization of the death effector domain of the Fas-associated protein, FADD, is sufficient to trigger apoptosis, suggesting that the primary function of FADD, like that of Apaf-1, is oligomerization of associated caspases. Finally, we demonstrate that nuclear-targeted caspases 1, 3, and 8 can trigger apoptosis efficiently, implying that the cleavage of nuclear targets is sufficient for apoptosis.