The balance between caspase-dependent apoptosis and RIPK-dependent necroptosis was found to depend on the levels of caspase-8 (CASP8) and cellular FADD-like interleukin-1 beta converting enzyme (FLICE)-inhibitory protein (cFLIP, encoded by the CFLAR gene) (Feoktistova M et al. 2011; Hughes MA et al. 2016; reviewed in Tummers B & Green DR 2017). cFLIP exists in two main isoforms: long FLIP(L) and short FLIP(S) forms. Both FLIP(L) and FLIP(S) form heterodimers with procaspase-8, however they differentially regulate CASP8 activation (Feoktistova M et al. 2011; Dillon CP et al. 2012). The pseudoprotease FLIP(L) interacts with procaspase-8 through both death effector domains (DED) and caspase-like domain (CLD) that lacks catalytic activity due to absence of a cysteine residue in FLIP(L). The procaspase-8 catalytic domain prefers heterodimerization with the CLD of FLIP(L) over homodimerization with catalytic domains of other procaspase-8 molecules (Boatright KM et al. 2004; Yu JW et al. 2009). Heterodimerization to FLIP(L) rearranges the catalytic site of procaspase-8, producing a conformation that renders the heterodimer highly active even in the absence of proteolytic processing of either caspase-8 or cFLIPL (Micheau O et al. 2002; Yu JW et al. 2009; reviewed in Tummers B & Green DR 2017). 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; Hughes MA et al. 2016). When FLIP(L) is expressed at high levels, the enzymatic activity of the FLIP(L):CASP8 heterodimer with procaspase-8 being an active unit is insufficient to generate active CASP8 heterotetramers for the apoptosis induction in mammalian cells. In contrary, the residual catalytic activity of FLIP(L):CASP8 is sufficient for RIPK1/RIPK3 cleavage, which inhibited the necroptotic cell death mode (Feoktistova M et al. 2011; Dillon CP et al. 2012; Oberst A et al. 2011).
Van Raam, BJ,