Activation pathways of cell death. (A) Binding of TNF to TNFR1 causes a conformational change and the intracellular assembly of TNFR complex I. TNFR complex I includes TRADD, RIP1, cellular inhibitor of apoptosis proteins (cIAPs), TRAF2, and TRAF5. Ubiquitylation of RIP1 results in recruitment of transforming growth factor-β–activated kinase 1 (TAK1), TAK1-binding protein 2 (TAB2), and TAB3, which initiate the nuclear factor-κB (NF-κB) activation pathway. Riboflavin kinase (RFK) links the TNFR1 death domain to p22phox, a subunit of NADPH oxidase 1 (NOX1), which contributes to TNFα-induced necroptosis by generating ROS. Deubiquitylation of RIP1 results in 2 distinct types of cell death. (B) The internalization of TNFR1 and the cytosolic assembly of TNFR complex II, which often contain TRADD, FADD, caspase 8, RIP1, and RIP3. Caspase 8 triggers apoptosis by activating the classical caspase cascade, and cleaves and inactivates RIP1 and RIP3. (C) When caspase 8 is inhibited, RIP1 and RIP3 become phosphorylated, triggering necroptosis. (D) ROS and lipid hydroperoxides are normally kept in check by Gpx4 but in oxidative conditions may increase and inhibit caspase 8. Loss of caspase 8 activity activates RIP1 and RIP3, resulting in a caspase-independent, TNFR-independent form of necroptosis. CYLD, cylindromatosis (turban tumor syndrome); FAD, flavin adenine nucleotide; FMN, flavin mononucleotide; NADPH, nicotinamide adenine dinucleotide phosphate; P, phosphate; RF, riboflavin; U, ubiquitin; USP, ubiquitin-specific peptidase; X, unidentified kinase. Adapted from Figure 1 of Vandenabeele et al³  with permission. Professional illustration by Patrick Lane, ScEYEnce Studios.