The inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) is expressed in immune and nonimmune cell types including macrophages, T cells, mast cells, granulocytes, natural killer (NK) cells, fibroblasts, neurons, keratinocytes and smooth muscle cells as a response to tissue injury or upon immune responses to pathogenic stimuli (Köck A. et al. 1990; Dubravec DB et al. 1990; Walsh LJ et al. 1991; te Velde AA et al. 1990; Imaizumi T et al. 2000). TNF-alpha interacts with two receptors, namely TNF receptor 1 (TNFR1) and TNF receptor 2 (TNFR2). Activation of TNFR1 can trigger multiple signal transduction pathways inducing inflammation, proliferation, survival or cell death (Ward C et al. 1999; Micheau O and Tschopp J 2003; Widera D et al. 2006). Whether a TNF-alpha-stimulated cell will survive or die is dependent on autocrine/paracrine signals, and on the cellular context.
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 serin/threonine protein kinase 1), and E3 ubiquitin ligases BIRC2,BIRC3 (cIAP1/2,cellular inhibitor of apoptosis) and LUBAC (Micheau O and Tschopp J 2003). The conjugation of ubiquitin chains by BIRC2/3 and LUBAC (composed of HOIP, HOIL-1 and SHARPIN ) to RIPK1 allows further recruitment and activation of the TAK1 (also known as mitogen-activated protein kinase kinase kinase 7 (MAP3K7)) complex and IκB kinase (IKK) complex. TAK1 and IKK phosphorylate RIPK1 to limit its cytotoxic activity and activate both nuclear factor kappa‐light‐chain‐enhancer of activated B cells (NFkappaB) and mitogen‐activated protein (MAP) kinase signaling pathways promoting cell survival by induction of anti-apoptotic proteins such as BIRC, cellular FLICE (FADD-like IL-1β-converting enzyme)-like inhibitory protein (cFLIP) and secretion of pro-inflammatory cytokines (TNF and IL-6). When the survival pathway is inhibited, the TRADD:TRAF2:RIPK1 detaches from the membrane-bound TNFR1 signaling complex and recruits Fas-associated death domain-containing protein (FADD) and procaspase-8 (also known as complex II). Once recruited to FADD, multiple procaspase-8 molecules interact via their tandem death-effector domains( DED), thereby facilitating both proximity-induced dimerization and proteolytic cleavage of procaspase-8, which are required for initiation of apoptotic cell death (Hughes MA et al. 2009; Oberst A et al. 2010). 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). Within the complex II procaspase-8 can also form heterodimers with cFLIP isoforms, FLIP long (L) and FLIP short (S), which are encoded by the NFkappaB target gene CFLAR (Irmler M et al. 1997; Boatright KM et al. 2004; Yu JW et al. 2009; Pop C et al. 2011). FLIP(S) appears to act purely as an antagonist of caspase-8 activity blocking apoptotic but promoting necroptotic cell death (Feoktistova et al. 2011). The regulatory function of FLIP(L) has been found to differ depending on its expression levels. FLIP(L) was shown to inhibit death receptor (DR)-mediated apoptosis only when expressed at high levels, while low cell levels of FLIP(L) enhanced DR signaling to apoptosis (Boatright KM et al. 2004; Okano H et al. 2003; Yerbes R et al. 2011; Yu JW et al. 2009; Hughes MA et al. 2016). In addition, caspase-8:FLIP(L) heterodimer activity within the TRADD:TRAF2:RIPK1:FADD:CASP8:FLIP(L) complex allowed cleavage of RIPK1 to cause the dissociation of the TRADD:TRAF2:RIP1:FADD:CASP8, thereby inhibiting RIPK1-mediated necroptosis (Feoktistova et al. 2011, 2012). 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/necroptotic pathway, and the other NFkappaB, and MAPK pathways providing highly specific cell responses initiated by various types of stimuli.