Activation of caspases-8 and -10 by FLIP(L)

USP8 suppresses death receptor-mediated apoptosis by enhancing FLIPL stability

FLICE-like inhibitory protein (FLIP) is a critical regulator of death receptor-mediated apoptosis. Here, we found ubiquitin-specific peptidase 8 (USP8) to be a novel deubiquitylase of the long isoform of FLIP (FLIP_L). USP8 directly deubiquitylates and stabilizes FLIP_L, but not the short isoform. USP8 depletion induces FLIP_L destabilization, promoting anti-Fas-, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)- and tumor necrosis factor alpha-induced extrinsic apoptosis by facilitating death-inducing signaling complex or TNFR1 complex II formation, which results in the activation of caspase-8 and caspase-3. USP8 mRNA levels are elevated in melanoma and cervical cancers, and the protein levels of USP8 and FLIP_L are positively correlated in these cancer cell lines. Xenograft analyses using ME-180 cervical cancer cells showed that USP8 depletion attenuated tumor growth upon TRAIL injection. Taken together, our data indicate that USP8 functions as a novel deubiquitylase of FLIP_L and inhibits extrinsic apoptosis by stabilizing FLIP_L.

Onto better TRAILs for cancer treatment

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), also known as Apo-2 ligand (Apo2L), is a member of the TNF cytokine superfamily. By cross-linking TRAIL-Receptor (TRAIL-R) 1 or TRAIL-R2, also known as death receptors 4 and 5 (DR4 and DR5), TRAIL has the capability to induce apoptosis in a wide variety of tumor cells while sparing vital normal cells. The discovery of this unique property among TNF superfamily members laid the foundation for testing the clinical potential of TRAIL-R-targeting therapies in the cancer clinic. To date, two of these therapeutic strategies have been tested clinically: (i) recombinant human TRAIL and (ii) antibodies directed against TRAIL-R1 or TRAIL-R2. Unfortunately, however, these TRAIL-R agonists have basically failed as most human tumors are resistant to apoptosis induction by them. It recently emerged that this is largely due to the poor agonistic activity of these agents. Consequently, novel TRAIL-R-targeting agents with increased bioactivity are currently being developed with the aim of rendering TRAIL-based therapies more active. This review summarizes these second-generation novel formulations of TRAIL and other TRAIL-R agonists, which exhibit enhanced cytotoxic capacity toward cancer cells, thereby providing the potential of being more effective when applied clinically than first-generation TRAIL-R agonists.

Major apoptotic mechanisms and genes involved in apoptosis

As much as the cellular viability is important for the living organisms, the elimination of unnecessary or damaged cells has the opposite necessity for the maintenance of homeostasis in tissues, organs and the whole organism. Apoptosis, a type of cell death mechanism, is controlled by the interactions between several molecules and responsible for the elimination of unwanted cells from the body. Apoptosis can be triggered by intrinsically or extrinsically through death signals from the outside of the cell. Any abnormality in apoptosis process can cause various types of diseases from cancer to auto-immune diseases. Different gene families such as caspases, inhibitor of apoptosis proteins, B cell lymphoma (Bcl)-2 family of genes, tumor necrosis factor (TNF) receptor gene superfamily, or p53 gene are involved and/or collaborate in the process of apoptosis. In this review, we discuss the basic features of apoptosis and have focused on the gene families playing critical roles, activation/inactivation mechanisms, upstream/downstream effectors, and signaling pathways in apoptosis on the basis of cancer studies. In addition, novel apoptotic players such as miRNAs and sphingolipid family members in various kind of cancer are discussed.

Balance between short and long isoforms of cFLIP regulates Fas-mediated apoptosis in vivo

cFLIP, an inhibitor of apoptosis, is a crucial regulator of cellular death by apoptosis and necroptosis; its importance in development is exemplified by the embryonic lethality in cFLIP-deficient animals. A homolog of caspase 8 (CASP8), cFLIP exists in two main isoforms: cFLIPL (long) and cFLIPR (short). Although both splice variants regulate death receptor (DR)-induced apoptosis by CASP8, the specific role of each isoform is poorly understood. Here, we report a previously unidentified model of resistance to Fas receptor-mediated liver failure in the wild-derived MSM strain, compared with susceptibility in C57BL/6 (B6) mice. Linkage analysis in F2 intercross (B6 x MSM) progeny identified several MSM loci controlling resistance to Fas-mediated death, including the caspase 8- and FADD-like apoptosis regulator (Cflar) locus encoding cFLIP. Furthermore, we identified a 21-bp insertion in the 3' UTR of the fifth exon of Cflar in MSM that influences differential splicing of cFLIP mRNA. Intriguingly, we observed that MSM liver cells predominantly express the FLIPL variant, in contrast to B6 liver cells, which have higher levels of cFLIPR. In keeping with this finding, genome-wide RNA sequencing revealed a relative abundance of FLIPL transcripts in MSM hepatocytes whereas B6 liver cells had significantly more FLIPR mRNA. Importantly, we show that, in the MSM liver, CASP8 is present exclusively as its cleaved p43 product, bound to cFLIPL. Because of partial enzymatic activity of the heterodimer, it might prevent necroptosis. On the other hand, it prevents cleavage of CASP8 to p10/20 necessary for cleavage of caspase 3 and, thus, apoptosis induction. Therefore, MSM hepatocytes are predisposed for protection from DR-mediated cell death.

Research Advances on Pathways of Nickel-Induced Apoptosis

High concentrations of nickel (Ni) are harmful to humans and animals. Ni targets a number of organs and produces multiple toxic effects. Apoptosis is important in Ni-induced toxicity of the kidneys, liver, nerves, and immune system. Apoptotic pathways mediated by reactive oxygen species (ROS), mitochondria, endoplasmic reticulum (ER), Fas, and c-Myc participate in Ni-induced cell apoptosis. However, the exact mechanism of apoptosis caused by Ni is still unclear. Understanding the mechanism of Ni-induced apoptosis may help in designing measures to prevent Ni toxicity.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

cFLIP (cellular FLICE-like inhibitory protein) is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIP_L, cFLIP_S, and cFLIP_R). cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

Molecular basis of death effector domain chain assembly and its role in caspase-8 activation

Assembly of a death-inducing signaling complex is a key event in the extrinsic apoptotic pathway, enabling activation of the caspase cascade and subsequent cell death. However, the molecular events governing DISC assembly have remained largely elusive because of the lack of information on mechanism and specificity regulating the death effector domain (DED)-DED interaction network. Using molecular modeling, mutagenesis, and biochemical and ex vivo experiments, we identified the precise binding interface and hot spots crucial for intermolecular DED chain assembly. Mutation of key interface residues (Leu42/Phe45) in procaspase-8 DED-A completely abrogated DED chain formation in HEK293 cells and prevented its association with FADD. A significant 2.6-3.6-fold reduction in procaspase-8 activation was observed in functional cell-death assays after substitution of the interfacial residues. Based on our results we propose a new model for DISC formation that refines the current understanding of the activation mechanism. Upon stimulation, FADD self-associates weakly via reciprocal interaction between helices α1/α4 and α2/α3 of the DED to form an oligomeric signaling platform that provides a stage for the initial recruitment of procaspase-8 through direct interaction with α1/α4 of DED-A, followed by sequential interaction mediated by helices α2/α5 of DED-B, to form the procaspase-8 DED chain that is crucial for its activation and subsequent cell death.

DED or alive: assembly and regulation of the death effector domain complexes

Death effector domains (DEDs) are protein–protein interaction domains initially identified in proteins such as FADD, FLIP and caspase-8 involved in regulating apoptosis. Subsequently, these proteins have been shown to have important roles in regulating other forms of cell death, including necroptosis, and in regulating other important cellular processes, including autophagy and inflammation. Moreover, these proteins also have prominent roles in innate and adaptive immunity and during embryonic development. In this article, we review the various roles of DED-containing proteins and discuss recent developments in our understanding of DED complex formation and regulation. We also briefly discuss opportunities to therapeutically target DED complex formation in diseases such as cancer.

Pseudoproteases: mechanisms and function

Catalytically inactive enzymes (also known as pseudoproteases, protease homologues or paralogues, non-peptidase homologues, non-enzymes and pseudoenzymes) have traditionally been hypothesized to act as regulators of their active homologues. However, those that have been characterized demonstrate that inactive enzymes have an extensive and expanding role in biological processes, including regulation, inhibition and immune modulation. With the emergence of each new genome, more inactive enzymes are being identified, and their abundance and potential as therapeutic targets has been realized. In the light of the growing interest in this emerging field the present review focuses on the classification, structure, function and mechanism of inactive enzymes. Examples of how inactivity is defined, how this is reflected in the structure, functions of inactive enzymes in biological processes and their mode of action are discussed.

A novel caspase 8 selective small molecule potentiates TRAIL-induced cell death

Recombinant soluble TRAIL and agonistic antibodies against TRAIL receptors (DR4 and DR5) are currently being created for clinical cancer therapy, due to their selective killing of cancer cells and high safety characteristics. However, resistance to TRAIL and other targeted therapies is an important issue facing current cancer research field. An attractive strategy to sensitize resistant malignancies to TRAIL-induced cell death is the design of small molecules that target and promote caspase 8 activation. For the first time, we describe the discovery and characterization of a small molecule that directly binds caspase 8 and enhances its activation when combined with TRAIL, but not alone. The molecule was identified through an in silico chemical screen for compounds with affinity for the caspase 8 homodimer's interface. The compound was experimentally validated to directly bind caspase 8, and to promote caspase 8 activation and cell death in single living cells or population of cells, upon TRAIL stimulation. Our approach is a proof-of-concept strategy leading to the discovery of a novel small molecule that not only stimulates TRAIL-induced apoptosis in cancer cells, but may also provide insights into the structure-function relationship of caspase 8 homodimers as putative targets in cancer.

Dysfunctional HIV-specific CD8+ T cell proliferation is associated with increased caspase-8 activity and mediated by necroptosis

Decreased HIV-specific CD8(+) T cell proliferation is a hallmark of chronic infection, but the mechanisms of decline are unclear. We analyzed gene expression profiles from antigen-stimulated HIV-specific CD8(+) T cells from patients with controlled and uncontrolled infection and identified caspase-8 as a correlate of dysfunctional CD8(+) T cell proliferation. Caspase-8 activity was upregulated in HIV-specific CD8(+) T cells from progressors and correlated positively with disease progression and programmed cell death-1 (PD-1) expression, but negatively with proliferation. In addition, progressor cells displayed a decreased ability to upregulate membrane-associated caspase-8 activity and increased necrotic cell death following antigenic stimulation, implicating the programmed cell death pathway necroptosis. In vitro necroptosis blockade rescued HIV-specific CD8(+) T cell proliferation in progressors, as did silencing of necroptosis mediator RIPK3. Thus, chronic stimulation leading to upregulated caspase-8 activity contributes to dysfunctional HIV-specific CD8(+) T cell proliferation through activation of necroptosis and increased cell death.

Regulated cell death: signaling and mechanisms

Cell turnover is a fundamental feature in metazoans. Cells can die passively, as a consequence of severe damage to their structural integrity, or actively, owing to a more confined biological disruption such as DNA damage. Passive cell death is uncontrolled and often harmful to the organism. In contrast, active cell death is tightly regulated and serves to support the organism's life. Apoptosis-the primary form of regulated cell death-is relatively well defined. Necroptosis-an alternative, distinct kind of regulated cell death discovered more recently-is less well understood. Apoptosis and necroptosis can be triggered either from within the cell or by extracellular stimuli. Certain signaling components, including several death ligands and receptors, can regulate both processes. Whereas apoptosis is triggered and executed via intracellular proteases called caspases, necroptosis is suppressed by caspase activity. Here we highlight current understanding of the key signaling mechanisms that control regulated cell death.

Functions of caspase 8: the identified and the mysterious

Initially discovered as an initiator protease in apoptosis mediated by death receptors, caspase-8 is now known to have an apparently confounding opposing effect in securing cell survival. It is required to allow mouse embryo survival, and the survival of hematopoietic cells during their development and activation. Classic models in which caspase-8 is depleted or inhibited frequently result in inhibition of apoptosis, and conversion to death through a necrotic pathway. This bewildering switch is now known to be driven by activation of a pathway dependent on protein kinases of the RIP family, which engage a pathway known as necroptosis. If caspase-8 does not control this pathway, necrotic death results. The pro-apoptotic and pro-survival functions of caspase-8 are regulated by a specific interaction with the pseudo-caspase cFLIP, and it is thought that the heterocomplex between these two partners alters the substrate specificity of caspase-8 in favor of inactivating components of the RIP kinase pathway. The description of how caspase-8 and cFLIP coordinate the switch between apoptosis and survival is just beginning. The mechanism is not known, the differential targets are not known, and the reason of why an apoptotic initiator has been co-opted as a critical survival factor is only guessed at. Elucidating these unknowns will be important in understanding mechanisms and possible therapeutic targets in autoimmune, inflammatory, and metastatic diseases.

Unnatural amino acids increase sensitivity and provide for the design of highly selective caspase substrates

Traditional combinatorial peptidyl substrate library approaches generally utilize natural amino acids, limiting the usefulness of this tool in generating selective substrates for proteases that share similar substrate specificity profiles. To address this limitation, we synthesized a Hybrid Combinatorial Substrate Library (HyCoSuL) with the general formula of Ac-P4-P3-P2-Asp-ACC, testing the approach on a family of closely related proteases - the human caspases. The power of this library for caspase discrimination extends far beyond traditional PS-SCL approach, as in addition to 19 natural amino acids we also used 110 diverse unnatural amino acids that can more extensively explore the chemical space represented by caspase-active sites. Using this approach we identified and employed peptide-based substrates that provided excellent discrimination between individual caspases, allowing us to simultaneously resolve the individual contribution of the apical caspase-9 and the executioner caspase-3 and caspase-7 in the development of cytochrome-c-dependent apoptosis for the first time.

Intra- and interdimeric caspase-8 self-cleavage controls strength and timing of CD95-induced apoptosis

Apoptosis in response to the ligand CD95L (also known as Fas ligand) is initiated by caspase-8, which is activated by dimerization and self-cleavage at death-inducing signaling complexes (DISCs). Previous work indicated that the degree of substrate cleavage by caspase-8 determines whether a cell dies or survives in response to a death stimulus. To determine how a death ligand stimulus is effectively translated into caspase-8 activity, we assessed this activity over time in single cells with compartmentalized probes that are cleaved by caspase-8 and used multiscale modeling to simultaneously describe single-cell and population data with an ensemble of single-cell models. We derived and experimentally validated a minimal model in which cleavage of caspase-8 in the enzymatic domain occurs in an interdimeric manner through interaction between DISCs, whereas prodomain cleavage sites are cleaved in an intradimeric manner within DISCs. Modeling indicated that sustained membrane-bound caspase-8 activity is followed by transient cytosolic activity, which can be interpreted as a molecular timer mechanism reflected by a limited lifetime of active caspase-8. The activation of caspase-8 by combined intra- and interdimeric cleavage ensures weak signaling at low concentrations of CD95L and strongly accelerated activation at higher ligand concentrations, thereby contributing to precise control of apoptosis.

Redesigning the procaspase-8 dimer interface for improved dimerization

Caspase-8 is a cysteine directed aspartate-specific protease that is activated at the cytosolic face of the cell membrane upon receptor ligation. A key step in the activation of caspase-8 depends on adaptor-induced dimerization of procaspase-8 monomers. Dimerization is followed by limited autoproteolysis within the intersubunit linker (IL), which separates the large and small subunits of the catalytic domain. Although cleavage of the IL stabilizes the dimer, the uncleaved procaspase-8 dimer is sufficiently active to initiate apoptosis, so dimerization of the zymogen is an important mechanism to control apoptosis. In contrast, the effector caspase-3 is a stable dimer under physiological conditions but exhibits little enzymatic activity. The catalytic domains of caspases are structurally similar, but it is not known why procaspase-8 is a monomer while procaspase-3 is a dimer. To define the role of the dimer interface in assembly and activation of procaspase-8, we generated mutants that mimic the dimer interface of effector caspases. We show that procaspase-8 with a mutated dimer interface more readily forms dimers. Time course studies of refolding also show that the mutations accelerate dimerization. Transfection of HEK293A cells with the procaspase-8 variants, however, did not result in a significant increase in apoptosis, indicating that other factors are required in vivo. Overall, we show that redesigning the interface of procaspase-8 to remove negative design elements results in increased dimerization and activity in vitro, but increased dimerization, by itself, is not sufficient for robust activation of apoptosis.

Involvement of caspase 8 and c-FLIPL in the proangiogenic effects of the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), a cytokine of the tumour necrosis factor superfamily, is a potent cell-apoptosis inducer, although its effects vary as a function of concentration. In fact, low concentrations of TRAIL are associated with non-apoptotic effects, such as cell proliferation. Here, the effects of TRAIL at different concentrations have been evaluated on mitogenesis and migration on human umbilical vein endothelial cells (HUVEC) in vitro, as well as in the chick embryo chorioallantoic membrane (CAM) angiogenesis model in vivo. At low concentrations, TRAIL promoted either mitogenesis or migration of HUVEC, evaluated using the wound healing method. Cleavage of caspase 8 was evaluated along with expression of the caspase 8-like molecule, cellular FLICE-inhibitory protein (long form) (c-FLIPL ). Low concentrations of TRAIL failed to induce caspase 8 processing, whereas high concentrations induced apoptosis of HUVEC and activation of caspase 8. Moreover, TRAIL induced a significant angiogenic response in the CAM assay in vivo, comparable with that of vascular endothelial growth factor. These data suggest that the non-apoptotic effects of TRAIL include mitogenesis and increased mobility of endothelial cells, and eventually angiogenesis. In addition, the results demonstrate that the c-FLIPL level is also modulated by differences in TRAIL concentration, suggesting its involvement in the divergent effects of TRAIL. In conclusion, this study envisions a proangiogenic role of TRAIL, suggesting that TRAIL may represent a target for pharmacological manipulation.

The C-terminal domain of the long form of cellular FLICE-inhibitory protein (c-FLIPL) inhibits the interaction of the caspase 8 prodomain with the receptor-interacting protein 1 (RIP1) death domain and regulates caspase 8-dependent nuclear factor κB (NF-κB) activation

Caspase 8 plays an essential role in the regulation of apoptotic and non-apoptotic signaling pathways. The long form of cellular FLICE-inhibitory protein (c-FLIPL) has been shown previously to regulate caspase 8-dependent nuclear factor κB (NF-κB) activation by receptor-interacting protein 1 (RIP1) and TNF receptor-associated factor 2 (TRAF2). In this study, the molecular mechanism by which c-FLIPL regulates caspase 8-dependent NF-κB activation was further explored in the human embryonic kidney cell line HEK 293 and variant cells barely expressing caspase 8. The caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone greatly diminished caspase 8-dependent NF-κB activation induced by Fas ligand (FasL) when c-FLIPL, but not its N-terminal fragment c-FLIP(p43), was expressed. The prodomain of caspase 8 was found to interact with the RIP1 death domain and to be sufficient to mediate NF-κB activation induced by FasL or c-FLIP(p43). The interaction of the RIP1 death domain with caspase 8 was inhibited by c-FLIPL but not c-FLIP(p43). Thus, these results reveal that the C-terminal domain of c-FLIPL specifically inhibits the interaction of the caspase 8 prodomain with the RIP1 death domain and, thereby, regulates caspase 8-dependent NF-κB activation.

The c-FLIPL cleavage product p43FLIP promotes activation of extracellular signal-regulated kinase (ERK), nuclear factor κB (NF-κB), and caspase-8 and T cell survival

Caspase-8 is now appreciated to govern both apoptosis following death receptor ligation and cell survival and growth via inhibition of the Ripoptosome. Cells must therefore carefully regulate the high level of caspase-8 activity during apoptosis versus the modest levels observed during cell growth. The caspase-8 paralogue c-FLIP is a good candidate for a molecular rheostat of caspase-8 activity. c-FLIP can inhibit death receptor-mediated apoptosis by competing with caspase-8 for recruitment to FADD. However, full-length c-FLIPL can also heterodimerize with caspase-8 independent of death receptor ligation and activate caspase-8 via an activation loop in the C terminus of c-FLIPL. This triggers cleavage of c-FLIPL at Asp-376 by caspase-8 to produce p43FLIP. The continued function of p43FLIP has, however, not been determined. We demonstrate that acute deletion of endogenous c-FLIP in murine effector T cells results in loss of caspase-8 activity and cell death. The lethality and caspase-8 activity can both be rescued by the transgenic expression of p43FLIP. Furthermore, p43FLIP associates with Raf1, TRAF2, and RIPK1, which augments ERK and NF-κB activation, IL-2 production, and T cell proliferation. Thus, not only is c-FLIP the initiator of caspase-8 activity during T cell activation, it is also an initial caspase-8 substrate, with cleaved p43FLIP serving to both stabilize caspase-8 activity and promote activation of pathways involved with T cell growth.

Apoptosis and necrosis in the liver

Because of its unique function and anatomical location, the liver is exposed to a multitude of toxins and xenobiotics, including medications and alcohol, as well as to infection by hepatotropic viruses, and therefore, is highly susceptible to tissue injury. Cell death in the liver occurs mainly by apoptosis or necrosis, with apoptosis also being the physiologic route to eliminate damaged or infected cells and to maintain tissue homeostasis. Liver cells, especially hepatocytes and cholangiocytes, are particularly susceptible to death receptor-mediated apoptosis, given the ubiquitous expression of the death receptors in the organ. In a quite unique way, death receptor-induced apoptosis in these cells is mediated by both mitochondrial and lysosomal permeabilization. Signaling between the endoplasmic reticulum and the mitochondria promotes hepatocyte apoptosis in response to excessive free fatty acid generation during the metabolic syndrome. These cell death pathways are partially regulated by microRNAs. Necrosis in the liver is generally associated with acute injury (i.e., ischemia/reperfusion injury) and has been long considered an unregulated process. Recently, a new form of "programmed" necrosis (named necroptosis) has been described: the role of necroptosis in the liver has yet to be explored. However, the minimal expression of a key player in this process in the liver suggests this form of cell death may be uncommon in liver diseases. Because apoptosis is a key feature of so many diseases of the liver, therapeutic modulation of liver cell death holds promise. An updated overview of these concepts is given in this article.

Targeting the Anti-Apoptotic Protein c-FLIP for Cancer Therapy

Cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) is a major resistance factor and critical anti-apoptotic regulator that inhibits tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis as well as chemotherapy-triggered apoptosis in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 in a ligand-dependent and-independent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. Moreover, c-FLIP(L) and c-FLIP(S) are known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective signaling molecules. Upregulation of c-FLIP has been found in various tumor types, and its downregulation has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and efforts are underway to develop other c-FLIP-targeted cancer therapies. This review focuses on (1) the functional role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and drug resistance; (2) the molecular mechanisms that regulate c-FLIP expression; and (3) strategies to inhibit c-FLIP expression and function.

Caspase-cleaved arrestin-2 and BID cooperatively facilitate cytochrome C release and cell death

Apoptosis is programmed cell death triggered by activation of death receptors or cellular stress. Activation of caspases is the hallmark of apoptosis. Arrestins are best known for their role in homologous desensitization of G protein-coupled receptors (GPCRs). Arrestins quench G protein activation by binding to activated phosphorylated GPCRs. Recently, arrestins have been shown to regulate multiple signalling pathways in G protein-independent manner via scaffolding signalling proteins. Here we demonstrate that arrestin-2 isoform is cleaved by caspases during apoptosis induced via death receptor activation or by DNA damage at evolutionarily conserved sites in the C-terminus. Caspase-generated arrestin-2-(1-380) fragment translocates to mitochondria increasing cytochrome C release, which is the key checkpoint in cell death. Cells lacking arrestin-2 are significantly more resistant to apoptosis. The expression of wild-type arrestin-2 or its cleavage product arrestin-2-(1-380), but not of its caspase-resistant mutant, restores cell sensitivity to apoptotic stimuli. Arrestin-2-(1-380) action depends on tBID: at physiological concentrations, arrestin-2-(1-380) directly binds tBID and doubles tBID-induced cytochrome C release from isolated mitochondria. Arrestin-2-(1-380) does not facilitate apoptosis in BID knockout cells, whereas its ability to increase caspase-3 activity and facilitate cytochrome C release is rescued when BID expression is restored. Thus, arrestin-2-(1-380) cooperates with another product of caspase activity, tBID, and their concerted action significantly contributes to cell death.

Cellular FLICE-like inhibitory protein long form (c-FLIPL) overexpression is related to cervical cancer progression

Cervical cancer is a leading cause of cancer deaths in women worldwide and infection by high-risk human papillomavirus types is a precursor event. The cellular FLICE-like inhibitory protein (c-FLIP) has been found to be overexpressed in several types of cancers and could be associated with cervical cancer progression because of its ability to inhibit the apoptotic process. To detect c-FLIP expression in cervical cancer, an immunohistochemical staining was performed, using tissue microarrays, on a series of 536 archival biopsy samples, including normal cervical tissues, low-grade and high-grade squamous intraepithelial lesions, and squamous cervical carcinomas. The epithelium in the normal cervix and low-grade squamous intraepithelial lesions mainly stained negatively for c-FLIP, whereas high-grade intraepithelial lesions and cancer samples showed an elevated expression of c-FLIP. A direct association was observed between the increasing grade of the lesion and the intensity of c-FLIP staining, in which the frequency of intense c-FLIP expression increased from 12.5% in the normal tissue to 82.1% in the cervical cancer tissue. An increased expression of c-FLIP may be an important factor in the progression of cervical cancer. This finding could aid in identifying patients with preneoplastic lesions at greater risk of developing cervical cancer. c-FLIP expression in cervical tissue may be a potential cervical cancer progression marker.

Apoptosis, necrosis and necroptosis: cell death regulation in the intestinal epithelium

Intestinal epithelial cells (IEC) are organised as a single cell layer which covers the intestine. Their primary task is to absorb nutrients present in the intestinal lumen. However, IEC also play an important role in the immune defence of our body by building a barrier that separates the bowel wall from potentially hazardous bacteria present in the gut lumen. The life cycle of IEC is determined by the time span in which cells migrate from their place of origin at the crypt base to the villus tip, from where they are shed into the lumen. Cell death in the intestinal epithelium has to be tightly regulated and irregularities might cause pathologies. Excessive cell death has been associated with chronic inflammation as seen in patients with Crohn's disease and ulcerative colitis. While until recently apoptosis was discussed as being essential for epithelial turnover and tissue homeostasis in the intestinal epithelium, recent data using gene deficient mice have challenged this concept. Moreover, an apoptosis-independent mode of programmed cell death, termed necroptosis, has been identified and described in the intestinal epithelium. The following article reviews previous studies on cell death regulation in IEC and a potential role of necroptosis for gut homeostasis.

Cellular mechanisms controlling caspase activation and function

Caspases are the primary drivers of apoptotic cell death, cleaving cellular proteins that are critical for dismantling the dying cell. Initially translated as inactive zymogenic precursors, caspases are activated in response to a variety of cell death stimuli. In addition to factors required for their direct activation (e.g., dimerizing adaptor proteins in the case of initiator caspases that lie at the apex of apoptotic signaling cascades), caspases are regulated by a variety of cellular factors in a myriad of physiological and pathological settings. For example, caspases may be modified posttranslationally (e.g., by phosphorylation or ubiquitylation) or through interaction of modulatory factors with either the zymogenic or active form of a caspase, altering its activation and/or activity. These regulatory events may inhibit or enhance enzymatic activity or may affect activity toward particular cellular substrates. Finally, there is emerging literature to suggest that caspases can participate in a variety of cellular processes unrelated to apoptotic cell death. In these settings, it is particularly important that caspases are maintained under stringent control to avoid inadvertent cell death. It is likely that continued examination of these processes will reveal new mechanisms of caspase regulation with implications well beyond control of apoptotic cell death.

Caspase functions in cell death and disease

Caspases are a family of endoproteases that provide critical links in cell regulatory networks controlling inflammation and cell death. The activation of these enzymes is tightly controlled by their production as inactive zymogens that gain catalytic activity following signaling events promoting their aggregation into dimers or macromolecular complexes. Activation of apoptotic caspases results in inactivation or activation of substrates, and the generation of a cascade of signaling events permitting the controlled demolition of cellular components. Activation of inflammatory caspases results in the production of active proinflammatory cytokines and the promotion of innate immune responses to various internal and external insults. Dysregulation of caspases underlies human diseases including cancer and inflammatory disorders, and major efforts to design better therapies for these diseases seek to understand how these enzymes work and how they can be controlled.

The roles of FADD in extrinsic apoptosis and necroptosis

Fas-associated protein with death domain (FADD), an adaptor that bridges death receptor signaling to the caspase cascade, is indispensible for the induction of extrinsic apoptotic cell death. Interest in the non-apoptotic function of FADD has greatly increased due to evidence that FADD-deficient mice or dominant-negative FADD transgenic mice result in embryonic lethality and an immune defect without showing apoptotic features. Numerous studies have suggested that FADD regulates cell cycle progression, proliferation, and autophagy, affecting these phenomena. Recently, programmed necrosis, also called necroptosis, was shown to be a key mechanism that induces embryonic lethality and an immune defect. Supporting these findings, FADD was shown to be involved in various necroptosis models. In this review, we summarize the mechanism of extrinsic apoptosis and necroptosis, and discuss the in vivo and in vitro roles of FADD in necroptosis induced by various stimuli.

Tumor Necrosis Factor-α: Life and Death of Hepatocytes During Liver Ischemia/Reperfusion Injury

BACKGROUND

Tumor necrosis factor-α (TNF-α) is a potent proinflammatory cytokine involved in a variety of disease pathologies, including ischemia/reperfusion (I/R) injuries in transplantation. The interaction of TNF-α with its cognate receptor TNF receptor I (TNFRI) results in the activation of signal transduction pathways that regulate either cell survival or cell death. Hepatocytes express TNFRI and respond to TNF-α released by resident Kupffer cells as well as leukocytes that migrate to the liver during I/R injury. Upon binding TNF-α, the hepatocyte proliferates or undergoes apoptosis or necroptosis. The decision by the cell to commit to one path or the other is not understood. The damaged tissue exhibits cell death and hemorrhaging from the influx of immune mediators. TNF-α inhibitors ameliorate the injury in animal models, suggesting that lowering (but not eliminating) TNF-α levels shifts the balance of TNF-α toward its beneficial functions.

METHODS

We review TNF-α signal transduction pathways and the role of TNF-α in liver I/R injury.

CONCLUSIONS

Because TNF-α plays an important role in hepatocyte proliferation, complete inhibition of TNF-α is not desirable in treating liver I/R injury. The strategy for developing pharmacological therapies may be the identification of specific intermediates in the TNF-α/TNFR1 signal transduction pathway and directed targeting of proapoptotic and pronecroptotic events.

Altered of apoptotic markers of both extrinsic and intrinsic pathways induced by hepatitis C virus infection in peripheral blood mononuclear cells

Background

Chronic hepatitis C (CHC) has emerged as a leading cause of cirrhosis in the U.S. and across the world. To understand the role of apoptotic pathways in hepatitis C virus (HCV) infection, we studied the mRNA and protein expression patterns of apoptosis-related genes in peripheral blood mononuclear cells (PBMC) obtained from patients with HCV infection.

Methods

The present study included 50 subjects which plasma samples were positive for HCV, but negative for human immunodeficiency virus (HIV) or hepatitis B virus (HBV). These cases were divided into four groups according to METAVIR, a score-based analysis which helps to interpret a liver biopsy according to the degree of inflammation and fibrosis. mRNA expression of the studied genes were analyzed by reverse transcription of quantitative polymerase chain reaction (RT-qPCR) and protein levels, analyzed by ELISA, was also conducted. HCV genotyping was also determined.

Results

HCV infection increased mRNA expression and protein synthesis of caspase 8 in group 1 by 3 fold and 4 fold, respectively (p < 0.05). In group 4 HCV infection increased mRNA expression and protein synthesis of caspase 9 by 2 fold and 1,5 fold, respectively (p < 0.05). Also, caspase 3 mRNA expression and protein synthesis had level augumented by HCV infection in group 1 by 4 fold and 5 fold, respectively, and in group 4 by 6 fold and 7 fold, respectively (p < 0.05).

Conclusions

HCV induces alteration at both genomic and protein levels of apoptosis markers involved with extrinsic and intrinsic pathways.

c-FLIP, a master anti-apoptotic regulator

Cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) is a master anti-apoptotic regulator and resistance factor that suppresses tumor necrosis factor-α (TNF-α), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, as well as apoptosis triggered by chemotherapy agents in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 and TRAIL receptor 5 (DR5) in a ligand-dependent and -independent fashion and forms an apoptosis inhibitory complex (AIC). This interaction in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. c-FLIP(L) and c-FLIP(S) are also known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective and pro-survival signaling proteins including Akt, ERK, and NF-kB. Upregulation of c-FLIP has been found in various tumor types, and its silencing has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and much effort is focused on developing other c-FLIP-targeted cancer therapies. This review focuses on (1) the anti-apoptotic role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and chemotherapy drug resistance, (2) the molecular mechanisms and factors that regulate c-FLIP expression, and (3) modulation of c-FLIP expression and function to eliminate cancer cells or increase the efficacy of anticancer agents. This article is part of a Special Issue entitled "Apoptosis: Four Decades Later".

Intrinsic cleavage of receptor-interacting protein kinase-1 by caspase-6

Necroptosis is a form of programmed cell death that occurs in the absence of caspase activation and depends on the activity of the receptor-interacting protein kinases. Inactivation of these kinases by caspase-mediated cleavage has been shown to be essential for successful embryonic development, survival and activation of certain cell types. The initiator of extrinsic apoptosis, caspase-8, which has a pro-death as well as a pro-life function, has been assigned this role. In the present study we demonstrate that caspase-6, an executioner caspase, performs this role during apoptosis induced through the intrinsic pathway. In addition, we demonstrate that in the absence of caspase activity, intrinsic triggers of apoptosis induce the receptor-interacting-kinase-1-dependent production of pro-inflammatory cytokines. We show that ubiquitously expressed caspase-6 has a supporting role in apoptosis by cleaving this kinase, thus preventing production of inflammatory cytokines as well as inhibiting the necroptotic pathway. These findings shed new light on the regulation of necroptosis as well as cell death in an inflammatory environment wherein cells receive both intrinsic and extrinsic death signals.

Cathepsin D primes caspase-8 activation by multiple intra-chain proteolysis

During the resolution of inflammatory responses, neutrophils rapidly undergo apoptosis. A direct and fast activation of caspase-8 by cathepsin D was shown to be crucial in the initial steps of neutrophil apoptosis. Nevertheless, the activation mechanism of caspase-8 remains unclear. Here, by using site-specific mutants of caspase-8, we show that both cathepsin D-mediated proteolysis and homodimerization of caspase-8 are necessary to generate an active caspase-8. At acidic pH, cathepsin D specifically cleaved caspase-8 but not the initiator caspase-9 or -10 and significantly increased caspase-8 activity in dimerizing conditions. These events were completely abolished by pepstatin A, a pharmacological inhibitor of cathepsin D. The cathepsin D intra-chain proteolysis greatly stabilized the active site of caspase-8. Moreover, the main caspase-8 fragment generated by cathepsin D cleavage could be affinity-labeled with the active site probe biotin-VAD-fluoromethyl ketone, suggesting that this fragment is enzymatically active. Importantly, in an in vitro cell-free assay, the addition of recombinant human caspase-8 protein, pre-cleaved by cathepsin D, was followed by caspase-3 activation. Our data therefore indicate that cathepsin D is able to initiate the caspase cascade by direct activation of caspase-8. As cathepsin D is ubiquitously expressed, this may represent a general mechanism to induce apoptosis in a variety of immune and nonimmune cells.

Mechanism and specificity of the human paracaspase MALT1

The paracaspase domain of MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1) is a component of a gene translocation fused to the N-terminal domains of the cellular inhibitor of apoptosis protein 2. The paracaspase itself, commonly known as MALT1, participates in the NF-κB (nuclear factor κB) pathway, probably by driving survival signals downstream of the B-cell antigen receptor through MALT1 proteolytic activity. We have developed methods for the expression and purification of recombinant full-length MALT1 and its constituent catalytic domain alone. Both are activated by dimerization without cleavage, with a similar dimerization barrier to the distantly related cousins, the apical caspases. By using positional-scanning peptidyl substrate libraries we demonstrate that the activity and specificity of full-length MALT1 is recapitulated by the catalytic domain alone, showing a stringent requirement for cleaving after arginine, and with striking peptide length constraints for efficient hydrolysis. Rates of cleavage (k_cat/K_m values) of optimal peptidyl substrates are in the same order (10³–10⁴ M⁻¹·s⁻¹) as for a putative target protein CYLD. Thus MALT1 has many similarities to caspase 8, even cleaving the putative target protein CYLD with comparable efficiencies, but with diametrically opposite primary substrate specificity.

Viral infection and the evolution of caspase 8-regulated apoptotic and necrotic death pathways

Pathogens specifically target both the caspase 8-dependent apoptotic cell death pathway and the necrotic cell death pathway that is dependent on receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3). The fundamental co-regulation of these two cell death pathways emerged when the midgestational death of mice deficient in FAS-associated death domain protein (FADD) or caspase 8 was reversed by elimination of RIP1 or RIP3, indicating a far more entwined relationship than previously appreciated. Thus, mammals require caspase 8 activity during embryogenesis to suppress the kinases RIP1 and RIP3 as part of the dialogue between two distinct cell death processes that together fulfil reinforcing roles in the host defence against intracellular pathogens such as herpesviruses.

RIPK-dependent necrosis and its regulation by caspases: a mystery in five acts

Caspase-8, FADD, and FLIP orchestrate apoptosis in response to death receptor ligation. Mysteriously however, these proteins are also required for normal embryonic development and immune cell proliferation, an observation that has led to their implication in several nonapoptotic processes. While many scenarios have been proposed, recent genetic and biochemical evidence points to unregulated signaling by the receptor-interacting protein kinases-1 (RIPK1) and RIPK3 as the lethal defect in caspase-8-, FADD-, and FLIP-deficient animals and tissues. The RIPKs are known killers, being responsible for a nonapoptotic form of cell death with features similar to necrosis. However, the mechanism by which caspase-8, FADD, and FLIP prevent runaway RIPK activation is unknown, and the signals that trigger these events during development and immune cell activation remain at large. In this review, we will lay out the evidence as it now stands, reinterpreting earlier observations in light of new clues and considering where the investigation might lead.

IAPs: guardians of RIPK1

Deregulation of innate immune signalling and cell death form the basis of most human disease pathogenesis. Inhibitor of APoptosis (IAP) protein-family members are frequently overexpressed in cancer and contribute to tumour cell survival, chemo-resistance, disease progression and poor prognosis. Although best known for their ability to regulate caspases, IAPs also influence ubiquitin-dependent pathways that modulate innate immune signalling by activation of NF-κB. Recent advances in our understanding of the molecular mechanisms through which IAPs influence cell death and innate immune responses have provided new insights into novel strategies for treatment of cancer. In this review we discuss our current understanding of IAP-mediated NF-κB signalling, as well as elaborate on unexpected insights into the involvement of IAPs in regulating the 'Ripoptosome', a novel intrinsic cell death-inducing platform. We propose an evolutionarily conserved concept whereby IAPs function as guardians of killer platforms such as the apoptosome in Drosophila and the Ripoptosome in mammals.

It cuts both ways: reconciling the dual roles of caspase 8 in cell death and survival

Caspase 8 can initiate apoptosis, but it also has non-apoptotic roles; for example, it is required for embryonic development and immune cell proliferation. Recent work has indicated that the requirement for caspase 8 in development and immune cell proliferation is defined by suppression of receptor-interacting protein kinase 3 (RIPK3), a kinase that triggers an alternative form of cell death called programmed necrosis. Interestingly, these recent findings can be reconciled with earlier work on the non-apoptotic roles of caspase 8.

Fasudil hydrochloride hydrate, a Rho-kinase inhibitor, suppresses isoproterenol-induced heart failure in rats via JNK and ERK1/2 pathways

The Rho-kinase (ROCK) plays an important role in the pathogenesis of heart injury. Recent cellular and molecular biology studies indicated a pivotal role of the RhoA/ROCK cascade in many aspects of cardiovascular function such as heart failure, cardiac hypertrophy, and ventricular remodeling following myocardial infarction. However, the signal transduction of RhoA/ROCK and its down-stream signaling pathways remains elusive, and the mechanism of ROCK-mediated isoproterenol (ISO)-induced heart failure is still not thoroughly understood. In the present study, we investigated the effect of the ROCK inhibitor, fasudil hydrochloride hydrate, on ISO-induced heart failure and the potential relationship of RhoA/ROCK to the extracellular signal-regulated kinases (ERK) and the c-jun NH 2-terminal kinase (JNK) pathways. Male Sprague-Dawley (SD) rats, maintained on a normal diet, were randomly divided into four groups given control, ISO alone, ISO with low-dose fasudil, or ISO with high-dose fasudil treatments. Fasudil effectively inhibited ISO-induced heart failure, as evaluated by biometric, hemodynamic, and histological examinations. Consistently, ISO-induced ROCK-1 mRNA expression and myosin phosphatase target subunit-1 (MYPT-1) phosphorylation were markedly suppressed by fasudil. In addition, fasudil significantly decreased ISO-induced JNK activation, ERK translocation to the nucleus and subsequent c-fos, c-jun expression and upregulated c-FLIP(L) expression. Taken together, these results indicate that the RhoA/ROCK pathway is essential for ISO induced heart failure, which can be effectively suppressed by fasudil.

Ripped to death

An old puzzle in the field of cell death was solved recently: the mysterious embryonic lethality of animals deficient in caspase-8 or Fas-associated death domain (FADD), proteins involved in a pathway of apoptosis. This lethality is caused by a failure to develop the yolk sac vasculature rather than a lack of apoptosis. Remarkably, development is rescued by ablation of either of two receptor interacting serine-threonine kinases (RIPKs). Despite being well known cell killers, caspase-8 and FADD act together to block RIPK-mediated necrosis. To manifest this newly elucidated pro-survival function, FADD and caspase-8 depend on FLIP(Long), a catalytically inactive caspase-8 homolog. In this review, the mechanism by which RIPK necrotic death is inhibited by this trio is discussed, as well as how RIPKs might themselves mediate cell death.

Apoptosis and autophagy in the regulation of T lymphocyte function

During the development and normal function of T lymphocytes, the cells are subject to several checkpoints at which they must "decide" to live or die. At these critical times and during homeostasis, the molecules that regulate the classical apoptotic pathways and survival pathways such as autophagy have critical roles in controlling this decision. Our laboratory has focused on the roles of apoptotic and autophagic proteins in T lymphocyte development and function. Using genetic models in mice and in vitro analyses of T cell functions, we have outlined critical roles for the Bcl-2 family (regulators of the intrinsic pathway of apoptosis), c-FLIP (an anti-apoptotic protein in the extrinsic pathway of apoptosis), and autophagy in T lymphocytes.

cIAPs block Ripoptosome formation, a RIP1/caspase-8 containing intracellular cell death complex differentially regulated by cFLIP isoforms

The intracellular regulation of cell death pathways by cIAPs has been enigmatic. Here we show that loss of cIAPs promotes the spontaneous formation of an intracellular platform that activates either apoptosis or necroptosis. This 2 MDa intracellular complex that we designate “Ripoptosome” is necessary but not sufficient for cell death. It contains RIP1, FADD, caspase-8, caspase-10, and caspase inhibitor cFLIP isoforms. cFLIP_L prevents Ripoptosome formation, whereas, intriguingly, cFLIP_S promotes Ripoptosome assembly. When cIAPs are absent, caspase activity is the “rheostat” that is controlled by cFLIP isoforms in the Ripoptosome and decides if cell death occurs by RIP3-dependent necroptosis or caspase-dependent apoptosis. RIP1 is the core component of the complex. As exemplified by our studies for TLR3 activation, our data argue that the Ripoptosome critically influences the outcome of membrane-bound receptor triggering. The differential quality of cell death mediated by the Ripoptosome may cause important pathophysiological consequences during inflammatory responses.

The Ripoptosome, a signaling platform that assembles in response to genotoxic stress and loss of IAPs

A better understanding of the mechanisms through which anticancer drugs exert their effects is essential to improve combination therapies. While studying how genotoxic stress kills cancer cells, we discovered a large ∼2MDa cell death-inducing platform, referred to as "Ripoptosome." It contains the core components RIP1, FADD, and caspase-8, and assembles in response to genotoxic stress-induced depletion of XIAP, cIAP1 and cIAP2. Importantly, it forms independently of TNF, CD95L/FASL, TRAIL, death-receptors, and mitochondrial pathways. It also forms upon Smac-mimetic (SM) treatment without involvement of autocrine TNF. Ripoptosome assembly requires RIP1's kinase activity and can stimulate caspase-8-mediated apoptosis as well as caspase-independent necrosis. It is negatively regulated by FLIP, cIAP1, cIAP2, and XIAP. Mechanistically, IAPs target components of this complex for ubiquitylation and inactivation. Moreover, we find that etoposide-stimulated Ripoptosome formation converts proinflammatory cytokines into prodeath signals. Together, our observations shed new light on fundamental mechanisms by which chemotherapeutics may kill cancer cells.

Drug-induced caspase 8 upregulation sensitises cisplatin-resistant ovarian carcinoma cells to rhTRAIL-induced apoptosis

Background:

Drug resistance is a major problem in ovarian cancer. Triggering apoptosis using death ligands such as tumour necrosis factor-related apoptosis inducing ligand (TRAIL) might overcome chemoresistance.

Methods:

We investigated whether acquired cisplatin resistance affects sensitivity to recombinant human (rh) TRAIL alone or in combination with cisplatin in an ovarian cancer cell line model consisting of A2780 and its cisplatin-resistant subline CP70.

Results:

Combining cisplatin and rhTRAIL strongly enhanced apoptosis in both cell lines. CP70 expressed less caspase 8 protein, whereas mRNA levels were similar compared with A2780. Pre-exposure of particularly CP70 to cisplatin resulted in strongly elevated caspase 8 protein and mRNA levels. Caspase 8 mRNA turnover and protein stability in the presence or absence of cisplatin did not differ between both cell lines. Cisplatin-induced caspase 8 protein levels were essential for the rhTRAIL-sensitising effect as demonstrated using caspase 8 small-interfering RNA (siRNA) and caspase-8 overexpressing constructs. Cellular FLICE-inhibitory protein (c-FLIP) and p53 siRNA experiments showed that neither an altered caspase 8/c-FLIP ratio nor a p53-dependent increase in DR5 membrane expression following cisplatin were involved in rhTRAIL sensitisation.

Conclusion:

Cisplatin enhances rhTRAIL-induced apoptosis in cisplatin-resistant ovarian cancer cells, and induction of caspase 8 protein expression is the key factor of rhTRAIL sensitisation.

Proliferative versus apoptotic functions of caspase-8 Hetero or homo: the caspase-8 dimer controls cell fate

Caspase-8, the initiator of extrinsically-triggered apoptosis, also has important functions in cellular activation and differentiation downstream of a variety of cell surface receptors. It has become increasingly clear that the heterodimer of caspase-8 with the long isoform of cellular FLIP (FLIP(L)) fulfills these pro-survival functions of caspase-8. FLIP(L), a catalytically defective caspase-8 paralog, can interact with caspase-8 to activate its catalytic function. The caspase-8/FLIP(L) heterodimer has a restricted substrate repertoire and does not induce apoptosis. In essence, caspase-8 heterodimerized with FLIP(L) prevents the receptor interacting kinases RIPK1 and -3 from executing the form of cell death known as necroptosis. This review discusses the latest insights in caspase-8 homo- versus heterodimerization and the implication this has for cellular death or survival. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Modeling reveals that dynamic regulation of c-FLIP levels determines cell-to-cell distribution of CD95-mediated apoptosis

The expression levels of caspase-8 inhibitory c-FLIP proteins play an important role in regulating death receptor-mediated apoptosis, as their concentration at the moment when the death-inducing signaling complex (DISC) is formed determines the outcome of the DISC signal. Experimental studies have shown that c-FLIP proteins are subject to dynamic turnover and that their stability and expression levels can be rapidly altered. Even though the influence of c-FLIP on the apoptotic behavior of a single cell has been captured in mathematical simulation studies, the effect of c-FLIP turnover and stability has not been investigated. In this study, a mathematical model of apoptosis was developed to analyze how the dynamic turnover and stability of the c-FLIP isoforms regulate apoptotic signaling for both individual cells and cell populations. Intercellular parameter and concentration distributions were used to describe the behavior of cell populations. Monte-Carlo simulations of cell populations showed that c-FLIP turnover is a key determinant of death receptor responses. The fact that the developed model simulates the state of whole cell populations makes it possible to validate it by comparison with empirical data. The proposed modeling approach can be used to further determine limiting factors in the DISC signaling process.