Abstract
Receptor-interacting protein kinase (RIPK) 1 and RIPK3 have emerged as essential kinases mediating a regulated form of necrosis, known as necroptosis, that can be induced by tumor necrosis factor (TNF) signaling. As a consequence, inhibiting RIPK1 kinase activity and repressing RIPK3 expression levels have become commonly used approaches to estimate the contribution of necroptosis to specific phenotypes. Here, we report that RIPK1 kinase activity and RIPK3 also contribute to TNF-induced apoptosis in conditions of cellular inhibitor of apoptosis 1 and 2 (cIAP1/2) depletion or TGF-β-activated kinase 1 (TAK1) kinase inhibition, implying that inhibition of RIPK1 kinase activity or depletion of RIPK3 under cell death conditions is not always a prerequisite to conclude on the involvement of necroptosis. Moreover, we found that, contrary to cIAP1/2 depletion, TAK1 kinase inhibition induces assembly of the cytosolic RIPK1/Fas-associated protein with death domain/caspase-8 apoptotic TNF receptor 1 (TNFR1) complex IIb without affecting the RIPK1 ubiquitylation status at the level of TNFR1 complex I. These results indicate that the recruitment of TAK1 to the ubiquitin (Ub) chains, and not the Ub chains per se, regulates the contribution of RIPK1 to the apoptotic death trigger. In line with this, we found that cylindromatosis repression only provided protection to TNF-mediated RIPK1-dependent apoptosis in condition of reduced RIPK1 ubiquitylation obtained by cIAP1/2 depletion but not upon TAK1 kinase inhibition, again arguing for a role of TAK1 in preventing RIPK1-dependent apoptosis downstream of RIPK1 ubiquitylation. Importantly, we found that this function of TAK1 was independent of its known role in canonical nuclear factor-κB (NF-κB) activation. Our study therefore reports a new function of TAK1 in regulating an early NF-κB-independent cell death checkpoint in the TNFR1 apoptotic pathway. In both TNF-induced RIPK1 kinase-dependent apoptotic models, we found that RIPK3 contributes to full caspase-8 activation independently of its kinase activity or intact RHIM domain. In contrast, RIPK3 participates in caspase-8 activation by acting downstream of the cytosolic death complex assembly, possibly via reactive oxygen species generation.