SummaryConcomitant hepatocyte apoptosis and regeneration is a hallmark of chronic liver diseases (CLDs) predisposing to hepatocellular carcinoma (HCC). Here, we mechanistically link caspase-8-dependent apoptosis to HCC development via proliferation- and replication-associated DNA damage. Proliferation-associated replication stress, DNA damage, and genetic instability are detectable in CLDs before any neoplastic changes occur. Accumulated levels of hepatocyte apoptosis determine and predict subsequent hepatocarcinogenesis. Proliferation-associated DNA damage is sensed by a complex comprising caspase-8, FADD, c-FLIP, and a kinase-dependent function of RIPK1. This platform requires a non-apoptotic function of caspase-8, but no caspase-3 or caspase-8 cleavage. It may represent a DNA damage-sensing mechanism in hepatocytes that can act via JNK and subsequent phosphorylation of the histone variant H2AX.

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Apoptosis is a form of programmed cell death, deregulation of which occurs in multiple disorders, including neurodegenerative and autoimmune diseases as well as cancer. The formation of a death-inducing signaling complex (DISC) and death effector domain (DED) filaments are critical for initiation of the extrinsic apoptotic pathway. Post-translational modifications (PTMs) of DED-containing DISC components such as FADD, procaspase-8, and c-FLIP comprise an additional level of apoptosis regulation, which is necessary to overcome the threshold for apoptosis induction. In this review we discuss the influence of PTMs of FADD, procaspase-8, and c-FLIP on DED filament assembly and cell death induction, with a focus on the 3D organization of the DED filament. Death Effector Domain (DED) Proteins in Extrinsic Apoptosis Signaling In multicellular organisms, tissue homeostasis is maintained through a fine-tuned balance between cell proliferation and cell death [1-5]. Several physiological and pathological stimuli have been reported to trigger programmed cell death [6]. Apoptosis can be induced via two pathways: the extrinsic and the intrinsic or mitochondrial pathway [2] (Figure 1). The extrinsic pathway is triggered upon binding of death ligands, including CD95 ligand (CD95L) and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) to death receptors (DRs) such as Fas/CD95 and TRAIL receptor-1/2 (TRAILR1/2), respectively [7]. This association leads to the recruitment of proteins with death domains (DD) such as Fas-associated protein with death domain (FADD) to the DD of CD95 or TRAIL-R1/2, resulting in the formation of the death-inducing signaling complex (DISC) [8,9], which in turn directs cleavage and activation of caspases for inducing apoptosis. DISC is comprised of procaspase-8/10 and cellular FLICE-like inhibitory protein (c-FLIP). Upon induction of apoptosis, the DED of FADD interacts with DEDs of procaspase-8, procaspase-10, and c-FLIP, resulting in the formation of DED-filaments, which serve as a platform for procaspase-8 dimerization and subsequent activation [10-12] (Figure 1). DED belongs to the DD superfamily [3,13]. DED comprises six α-helixes arranged in a Greek key structural motif. FADD contains one DD and one DED, whereas both caspase-8 and c-FLIP contain two DEDs at their N terminus. DED filaments are formed through so-called type I, II, and III interactions between DEDs (Box 1) [12]. Proper architecture of DED filaments at DR complexes provide an extra layer of regulatory control of cell death. Recent findings are revealing a role for post-translational modifications (PTMs) such as phosphorylation, ubiquitylation, SUMOylation, and nitrosylation in regulating these interactions [6,14-17]. These modifications assist in maintaining proper conformations of DEDs in DED filaments, which is required for efficient caspase-8 activation, and in overcoming a threshold for apoptosis induction [18,19]. In this review, we consider the role of PTMs in controlling the mechanisms of DISC and DED filament...

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Mutations of fms-like tyrosine kinase 3 (FLT3) are the most frequent mutations in acute myeloid leukemia (AML). Furthermore, the internal tandem duplication (ITD) represents the most common mutation of FLT3 in AML. To explore therapeutic strategies for AML patients carrying FLT3-ITD, we analyzed death receptor (DR) signaling networks in AML cells comprising FLT3-ITD. We have started with murine myeloid progenitor 32D cells that ectopically express human FLT3-ITD (32D-FLT3-ITD) and found that RIPK1 is strongly upregulated in these cells. Subsequently, we have shown that combinatorial treatment of 32D-FLT3-ITD cells with the SMAC mimetic BV6 and CD95L sensitizes these cells toward apoptosis and necroptosis. Moreover, combinatorial treatment with death ligands (DLs), for example, CD95L or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and BV6 enhanced cell death in primary AML blasts from patients carrying FLT3-ITD mutation. Finally, pharmacological and genetic targeting of RIPK1 inhibited DL/BV6-mediated cell death in cells with FLT3-ITD mutations. Taken together, our study suggests a promising therapeutic opportunity for AML cancer cells harboring FLT3-ITD mutation via targeting RIPK1 pathways.