Caspase-8, receptor-interacting protein kinase 1 (RIPK1), and RIPK3 regulate retinoic acid-induced cell differentiation and necroptosis

Among caspase family members, Caspase-8 is unique, with associated critical activities to induce and suppress death receptor-mediated apoptosis and necroptosis, respectively. Caspase-8 inhibits necroptosis by suppressing the function of receptor-interacting protein kinase 1 (RIPK1 or RIP1) and RIPK3 to activate mixed lineage kinase domain-like (MLKL). Disruption ofCaspase-8expression causes embryonic lethality in mice, which is rescued by depletion of eitherRipk3orMlkl, indicating that the embryonic lethality is caused by activation of necroptosis. Here, we show that knockdown ofCaspase-8expression in embryoid bodies derived from ES cells markedly enhances retinoic acid (RA)-induced cell differentiation and necroptosis, both of which are dependent onRipk1andRipk3; however, the enhancement of RA-induced cell differentiation is independent ofMlkland necrosome formation. RA treatment obviously enhanced the expression of RA-specific target genes having the retinoic acid response element (RARE) in their promoter regions to induce cell differentiation, and induced marked expression of RIPK1, RIPK3, and MLKL to stimulate necroptosis.Caspase-8knockdown induced RIPK1 and RIPK3 to translocate into the nucleus and to form a complex with RA receptor (RAR), and RAR interacting with RIPK1 and RIPK3 showed much stronger binding activity toRAREthan RAR without RIPK1 or RIPK3. InCaspase-8-deficient as well asCaspase-8- andMlkl-deficient mouse embryos, the expression of RA-specific target genes was obviously enhanced. Thus, Caspase-8, RIPK1, and RIPK3 regulate RA-induced cell differentiation and necroptosis both in vitro and in vivo.