TRAF2

The caspase-8 zymogens are present in the cells as inactive monomers, which are recruited to the death-inducing signaling complex (DISC) by homophilic interactions with the DED domain of FADD. The monomeric zymogens undergo dimerization and the subsequent conformational changes at the receptor complex, which results in the formation of catalytically active form of procaspase-8.[Boatright KM et al 2003; Donepudi M et al 2003; Keller N et al 2010; Oberst A et al 2010]

All c-FLIP proteins carry two DEDs at their N termini, which can bind FADD and procaspase-8. In addition to two DEDs, FLIP(L) contains a large (p20) and a small (p12) caspase-like domain without catalytic activity. FLIP(S) and FLIP(R) consist of two DEDs and a small C terminus. Depending on its level of expression FLIP(L) may function as an anti-apoptotic or pro-apoptotic factor, while FLIP(S) and FLIP(R) protect cells from apoptosis by blocking the processing of caspase-8 at the receptor level (Scaffidi C et al. 1999; Golks A et al. 2005; Toivonen HT et al. 2011; Fricker N et al. 2010)

The Reactome module describes MLKL-mediated necroptotic events on the plasma membrane

The binding of TNF-alpha to TNFR1 leads to recruitment of the adapter protein TNFR1-associated death domain (TRADD) and of receptor‑interacting protein 1 (RIPK1). TRADD subsequently recruits also TNF receptor-associated factor 2 (TRAF2). RIPK1 is promptly K63-polyubiquitinated which results in the recruitment of the TAB2:TAK1 complex and the IkB kinase (IKK) complex to TNFR1. The activated IKK complex mediates phosphorylation of the inhibitor of NFkappaB (IkB), which targets IkB for ubiquitination and subsequent degradation. Released NFkappaB induces the expression of a variety of genes including inflammation-related genes and anti-apoptotic genes encoding proteins such as inhibitor of apoptosis proteins cIAP1/2, Bcl-2, Bcl-xL or cellular FLICE-like inhibitory protein (FLIP) (Blonska M et al. 2005; Ea CK et al. 2006; Wu CJ et al. 2006; Chen C et al. 2000; Manna SK et al. 2000; Kreuz S et al. 2001; Micheau O et al. 2001). NFkB-mediated inhibition of cell death also involves attenuating TNF-induced activation of c-Jun activating kinase (JNK). Whereas transient activation of JNK upon TNF treatment is associated with cellular survival, prolonged JNK activation contributes to cell death. However, as caspases activate JNK quite efficiently, JNKs are also regularly stimulated in course of apoptosis without being essential for cell death (Wicovsky A et al. 2007). AP1-mediated gene induction results from activation of JNK via TRAF2 (not shown here) (Tsou HK et al. 2012). While pro-survival signaling is initiated and regulated via the activated TNFR1 receptor complex at the cell membrane, cell death signals are induced by internalization-associated fashion upon the release of RIPK1 from the membrane complex (Micheau O and Tschopp J 2003; Schneider-Brachert W et al. 2004; Tchikov V et al. 2011).

TNFR1-mediated transcriptional activity of NFkB is both antiapoptotic and highly proinflammatory and thus must be tightly regulated to prevent constitutive activation that leads to persistent inflammation and cancer (Ward C et al. 1999; Fujihara S et al. 2002; Pekalski J et al. 2013; Kankaanranta H et al. 2014; Shukla S and Gupta S 2004; Jackson-Bernitsas DG et al. 2007; Zhang JY et al. 2007). Multiple mechanisms normally ensure the proper control of NFkappaB activation including two negative feedback loops mediated by NFkappaB inducible inhibitors, IkB-alpha (NFKBIA) and ubiquitin-editing protein A20 (He KL & Ting AT 2002; Wertz IE et al. 2004; Vereecke L et al. 2009; Pekalski J et al. 2013)

TNF binding to TNFR1 results initially in the formation of Complex I that consists of TNFR1, TRADD (TNFR1-associated death domain), TRAF2 (TNF receptor associated factor-2), RIPK1 (receptor interacting protein kinase 1), and BIRC1 and BIRC3 (cIAP1/2,cellular inhibitor of apoptosis) (Micheau O and Tschopp J 2003). This complex by default activates NFkappaB promoting cell survival (induction of anti-apoptotic proteins such as BIRC, cFLIP) and secretion of pro-inflammatory cytokines (TNF and IL-6). When the survival pathway is inhibited, TNF-induced signaling leads to the formation of Complex II that is made up of TRADD, FADD (Fas-associated death domain-containing protein, RIPK1,and procaspase-8 leading to the activation of caspase-8 and apoptotic cell death. When caspase activity is inhibited under certain pathophysiological conditions (e.g. caspase-8 inhibitory proteins such as CrmA and vICA after infection with cowpox virus or CMV) or by pharmacological agents, deubiquitinated RIPK1 is physically and functionally engaged by its homolog RIPK3 leading to formation of the necrosome, a necroptosis-inducing complex consisting of RIPK1 and RIPK3 (Tewari M & Dixit VM 1995; Fliss PM & Brune W 2012; Sawai H 2013; Moquin DM et al. 2013; Kalai M et al. 2002; Cho YS et al. 2009, He S et al. 2009, Zhang DW et al., 2009).TNF-alpha can also activate sphingomyelinase (SMASE, such as SMPD2,3) proteins to catalyze hydrolysis of sphingomyeline into ceramide (Adam D et al.1996; Adam-Klages S et al. 1998; Ségui B et al. 2001). Activation of neutral SMPD2,3 leads to an accumulation of ceramide at the cell surface and has proinflammatory effects. However, TNF can also activate the pro-apoptotic acidic SMASE via caspase-8 mediated activation of caspase-7 which in turn proteolytically cleaves and activates the 72kDa pro-A-SMase form (Edelmann B et al. 2011). Ceramide induces anti-proliferative and pro-apoptotic responses. Further, ceramide can be converted by ceramidase into sphingosine, which in turn is phosphorylated by sphingosine kinase into sphingosine-1-phosphate (S1P). S1P exerts the opposite biological effects to ceramide by activating cytoprotective signaling to promote cell growth counteracting the apoptotic stimuli (Cuvillier O et al. 1996). Thus, TNF-alpha-induced TNFR1 activation leads to divergent intracellular signaling networks with extensive cross-talk between the pro-apoptotic pathway, and the other NFkappaB, and JNK pathways providing highly specific cell responses initiated by various types of stimuli