Caspase-8 Restricted By RSK-dependent Phosphorylation On T265 In Necrosome Ensures Necroptosis

Cell death plays an important role in the development and pathogenesis.Apoptosis and necroptosis are the most thorough and in-depth studied types of programmed cell death.Caspase-8,a key protein of apoptosis,inhibits the occurrence of necroptosis by cleavage of its the core protein kinases RIPK1 and RIPK3 and the deubiquitinase CYLD.Caspase-8 is essential for the survival of mouse embryos,because knockout of caspase-8 causes embryonic death.Necrosome is a core functional complex of necroptosis,mainly composed of RIPK3,RIPK1,FADD,MLKL and Caspase-8,and how to regulate the activity of Caspase-8 in necrosome is a question need to be answered.90kDa ribosomal S6 protein kinase 1(RSK1)is a Ser/Thr protein kinase that is specifically recruited by RIPK3 and MLKL to necrome in tumor necrosis factor(TNF)induced necroptosis.RSK1 specifical phosphorylate the T265 site of Caspase-8 in necrosome.This phosphorylation does not cause Caspase-8 to be degraded by the proteasome,but the ability of Caspase-8 to bind its substrates is possible weakened so that the substrates cannot be cleaved,thereby stabilizing the necrosome.Caspase8T265E/T265E mice have the similar embryonic lethal phenotype as Caspase-8-/-mice,but RIPK3-/-or MLKL-/-cannot completely inhibit the postnatal death of Caspase8T265E/T265E mice While,Caspase-8T265A/T265A mice showed high sensitivity to TNF.BI-D1870 is a pan-inhibitor of the RSK family.In TNF-stimulated cells,BI-D1870 significantly inhibited the activation levels of p-C8(T265)and p-RIPK3,releasing the enzymatic activity of Caspase-8.BI-D1870 can well alleviate TNF-induced mouse death and reduce the activation levels of p-MLKL and p-C8(T265)in mouse cecal tissue.In this thesis,by using biochemical,cellular and animal experiments,we have demonstrated that necrosome can phosphorylate the T265 site of Caspase-8 via RSK1,is an intrinsic mechanism for passing the caspase-8-checkpoint of necroptosis.This conclusion helps us to get a better understanding of how Caspase-8 is regulated in necrosome,and also provide us with new sight for treating diseases caused by necroptosis.