A Non-canonical PDK1-RSK Signal Diminishes Pro-caspase-8-Mediated Necroptosis Blockade

Necroptosis induction in vitro often requires caspase-8 (Casp8) inhibition by zVAD because pro-Casp8 cleaves RIP1 to disintegrate the necrosome. It has been unclear how the Casp8 blockade of necroptosis is eliminated naturally. Here, we show that pro-Casp8 within the necrosome can be inactivated by phosphorylation at Thr265 (pC8(T265)), pC8(T265) occurs in vitro in various necroptotic cells and in the cecum of TNF-treated mice. p90 RSK is the kinase of pro-Casp8. It is activated by a mechanism that does not need ERK but PDK1, which is recruited to the RIP1-RIP3-MLKL-containing necrosome. Phosphorylation of proCasp8 at Thr265 can substitute for zVAD to permit necroptosis in vitro, pC8(T265 )mimic T265E knockin mice are embryonic lethal due to unconstrained necroptosis, and the pharmaceutical inhibition of RSK-mediated pC8(T265) diminishes TNF-induced cecum damage and lethality in mice by halting necroptosis. Thus, phosphorylation of pro-Casp8 at Thr265 by RSK is an intrinsic mechanism for passing the Casp8 checkpoint of necroptosis.