NEMO/IKKgamma regulates an early NF-kappaB-independent cell-death checkpoint during TNF signaling

TNF receptor 1 (TNFR1) ligation can result in cell survival or cell death. What determines which of the two opposing responses is triggered is not fully understood. The current model suggests that it is the activation of the NF-kappaB pathway and its induction of prosurvival genes, or the lack thereof, which determines the outcome. NF-kappaB essential modifier (NEMO)/IkappaB kinase-gamma (IKKgamma)-deficient cells are highly sensitive to apoptosis, and as NEMO is essential for NF-kappaB activation, it has been assumed that this is due to the lack of NF-kappaB. This study demonstrates that this assumption was incorrect and that NEMO has another antiapoptotic function that is independent of its role in the NF-kappaB pathway. NEMO prevents receptor interacting protein-1 (RIP1) from engaging CASPASE-8 before NF-kappaB-mediated induction of antiapoptotic genes. Without NEMO, RIP1 associates with CASPASE-8 resulting in rapid tumor necrosis factor (TNF)-induced apoptosis. These results suggest that there are two cell-death checkpoints following TNF stimulation: an early transcription-independent checkpoint whereby NEMO restrains RIP1 from activating the caspase cascade, followed by a later checkpoint dependent on NF-kappaB-mediated transcription of prosurvival genes.