zVAD-induced necroptosis in L929 cells depends on autocrine production of TNFα mediated by the PKC-MAPKs-AP-1 pathway

The effect and underlying mechanism of Timosaponin B-II on RGC-5 necroptosis induced by hydrogen peroxide

BACKGROUND

Necroptosis is an important mode of cell death, which is due to oxidant stress accumulation. Our previous study indicated that oxidant stresses could be reduced by Timosaponin B-II (TBII), a kind of Chinese herb RhizomaAnemarrhenae monomer extraction. We wonder the possible effect of Timosaponin B-II, whether it can protect cells from necroptosis via reducing the oxidant stress, in RGC-5 following hydrogen peroxide (H2O2) insult.

METHODS

RGC-5 cells were grown in DMEM, the model group was exposed in H2O2 with the concentration of 300 μM, and the experimental group was pre-treated with Timosaponin B-II at different concentrations (1 μM, 10 μM, 100 μM and 1000 μM) for 24 hrs. MTT assay was carried out to measure the cytotoxicity of H2O2, MDA concentration assay was executed to evaluate the degree of oxidative stress, TNF-α ELISA Assay was used to measure the concentration of TNF-α, finally, the degree of necrosis were analyzed using flow cytometry.

RESULTS

We first constructed the cell injury model of necroptosis in RGC-5 upon H2O2 exposure. Morphological observation and MTT assay were used to evaluate the degree of RGC-5 death. MDA assay were carried out to describe the degree of oxidant stress. Annexin V/PI staining was used to detect necroptotic cells pre-treated with or without Timosaponin B-II following H2O2 injury. TNF-α ELISA was carried out to detect the TNF-α accumulation in RGC-5. Upon using Timosaponin B-II with concentration of 100 μM, the percentage of cell viability was increased from 50% to 75%, and the necrosis of cells was reduced from 35% to 20% comparing with H2O2 injury group. Oxidant stress and TNF-α was reduced upon injury which decreased the ratio of RGC-5 necroptosis.

CONCLUSION

Our study found out that Timosaponin B-II might reduce necroptosis via inhibition of ROS and TNF-α accumulation in RGC-5 following H2O2 injury.

Programmed necrosis and necroptosis signalling

In recent years, the paradigm of cell death regulation has changed. Nowadays, not only apoptosis but also several forms of necrosis (e.g. necroptosis) are considered to be regulated. The central roles of receptor-interacting serine/threonine protein kinase1 (RIPK1), RIPK3, and mixed-lineage kinase domain-like protein, and the molecular signalling platforms in which these molecules participate, are being intensively studied. In particular, the role of RIPK1, being both a kinase and a scaffold molecule, in different cell death regulatory complexes is of great relevance for the field. This minireview aims to introduce the emerging and dynamic field of necroptosis to the reader, with a specific focus on intracellular signalling pathways involved in this process.

Anti-necroptosis chemical necrostatin-1 can also suppress apoptotic and autophagic pathway to exert neuroprotective effect in mice intracerebral hemorrhage model

Necroptosis was recently discovered as one form of programmed cell death (PCD) and could be specifically inhibited by necrostatin-1. The aim of this study was to examine the effect of necrostatin-1 on brain injury and investigate the role of necrostatin-1 on the other two types PCD (apoptosis and autophagic cell death) in a mouse intracerebral hemorrhage (ICH) model. Male ICR mice received an infusion of type IV collagenase to induce ICH or saline as control into the left striatum. In the presence of vehicle, 3-MA, zVAD, and necrostatin-1 were pretreated with a single intracerebroventricular (i.c.v.) injection in the ipsilateral ventricle 15 min before ICH, respectively. Compared with vehicle groups, necrostatin-1 treatment significantly reduced injury volume and propidium iodide-positive cells at 24 and 72 h after ICH. Immunoblotting analysis showed that necrostatin-1 treatment suppressed autophagic-associated proteins (LC3-II, Beclin-1) and maintained p62 at normal level at 24 and 72 h after ICH. In addition, necrostatin-1 treatment enhanced the protein level of Bcl-2 and decreased the protein level of cleaved caspase-3 and the Beclin-1/Bcl-2 ratio at 24 and 72 h after ICH. Moreover, both 3-MA and necrostatin-1 treatment could suppress cleaved caspase-3 and LC3-II production, whereas zVAD treatment could inhibit caspase-3 cleavage but increased LC3-II protein levels at 72 h after ICH. Taken together, the data demonstrated for the first time that the specific inhibitor necrostatin-1 suppressed apoptosis and autophagy to exert these neuroprotective effects after ICH and that there existed a cross-talk among necroptosis, apoptosis, and autophagy after ICH.

Docosahexanoic acid antagonizes TNF-α-induced necroptosis by attenuating oxidative stress, ceramide production, lysosomal dysfunction, and autophagic features

OBJECTIVE

It was previously reported that docosahexanoic acid (DHA) reduces TNF-α-induced necrosis in L929 cells. However, the mechanisms underlying this reduction have not been investigated. The present study was designed to investigate cellular and biochemical mechanisms underlying the attenuation of TNF-α-induced necroptosis by DHA in L929 cells.

METHODS

L929 cells were pre-treated with DHA prior to exposure to TNF-α, zVAD, or Necrostatin-1 (Nec-1). Cell death and survival were assessed by MTT and caspase activity assays, and microscopic visualization. Reactive oxygen species (ROS) were measured by flow cytometry. C16- and C18-ceramides were measured by mass spectrometry. Lysosomal membrane permeabilization (LMP) was evaluated by fluorescence microscopy and flow cytometry using Acridine Orange. Cathepsin L activation was evaluated by immunoblotting and fluorescence microscopy. Autophagy was assessed by immunoblotting of LC3-II and Beclin.

RESULTS

Exposure of L929 cells to TNF-α alone for 24 h induced necroptosis, as evidenced by the inhibition of cell death by Nec-1, absence of caspase-3 activity and Lamin B cleavage, and morphological analysis. DHA attenuated multiple biochemical events associated with TNF-α-induced necroptosis, including ROS generation, ceramide production, lysosomal dysfunction, cathepsin L activation, and autophagic features. DHA also attenuated zVAD-induced necroptosis but did not attenuate the enhanced apoptosis and necrosis induced by the combination of TNF-α with Actinomycin D or zVAD, respectively, suggesting that its protective effects might be limited by the strength of the cell death insult induced by TNF-α.

CONCLUSIONS

DHA effectively attenuates TNF-α-induced necroptosis and autophagy, most likely via its ability to inhibit TNF-α-induced sphingolipid metabolism and oxidative stress. These results highlight the role of this Omega-3 fatty acid in antagonizing inflammatory cell death.

Harnessing of Programmed Necrosis for Fighting against Cancers

Chemotherapy has long been considered as one of useful strategies for cancer treatment. It is primarily based on the apoptosis that can selectively kill cancer cells. However, cancer cells can progressively develop an acquired resistance to apoptotic cell death, rendering refractory to chemo- and radiotherapies. Although the mechanism by which cells attained resistance to drug remains to be clarified, it might be caused by either pumping out of them or interfering with apoptotic signal cascades in response to cancer drugs. In case that cancer cells are defective in some part of apoptotic machinery by repeated exposure to anticancer drugs, alternative cell death mechanistically distinct from apoptosis could be adopted to remove cancer cells refractory to apoptosis-inducing agents. This review will mainly deal with harnessing of necrotic cell death, specifically, programmed necrosis and practical uses. Here, we begin with various defects of apoptotic death machinery in cancer cells, and then provide new perspective on programmed necrosis as an alternative anticancer approach.

Receptor interactive protein kinase 3 promotes Cisplatin-triggered necrosis in apoptosis-resistant esophageal squamous cell carcinoma cells

Cisplatin-based chemotherapy is currently the standard treatment for locally advanced esophageal cancer. Cisplatin has been shown to induce both apoptosis and necrosis in cancer cells, but the mechanism by which programmed necrosis is induced remains unknown. In this study, we provide evidence that cisplatin induces necrotic cell death in apoptosis-resistant esophageal cancer cells. This cell death is dependent on RIPK3 and on necrosome formation via autocrine production of TNFα. More importantly, we demonstrate that RIPK3 is necessary for cisplatin-induced killing of esophageal cancer cells because inhibition of RIPK1 activity by necrostatin or knockdown of RIPK3 significantly attenuates necrosis and leads to cisplatin resistance. Moreover, microarray analysis confirmed an anti-apoptotic molecular expression pattern in esophageal cancer cells in response to cisplatin. Taken together, our data indicate that RIPK3 and autocrine production of TNFα contribute to cisplatin sensitivity by initiating necrosis when the apoptotic pathway is suppressed or absent in esophageal cancer cells. These data provide new insight into the molecular mechanisms underlying cisplatin-induced necrosis and suggest that RIPK3 is a potential marker for predicting cisplatin sensitivity in apoptosis-resistant and advanced esophageal cancer.

Distinct roles of RIP1-RIP3 hetero- and RIP3-RIP3 homo-interaction in mediating necroptosis

Necroptosis is mediated by a signaling complex called necrosome, containing receptor-interacting protein (RIP)1, RIP3, and mixed-lineage kinase domain-like (MLKL). It is known that RIP1 and RIP3 form heterodimeric filamentous scaffold in necrosomes through their RIP homotypic interaction motif (RHIM) domain-mediated oligomerization, but the signaling events based on this scaffold has not been fully addressed. By using inducible dimer systems we found that RIP1-RIP1 interaction is dispensable for necroptosis; RIP1-RIP3 interaction is required for necroptosis signaling, but there is no necroptosis if no additional RIP3 protein is recruited to the RIP1-RIP3 heterodimer, and the interaction with RIP1 promotes the RIP3 to recruit other RIP3; RIP3-RIP3 interaction is required for necroptosis and RIP3-RIP3 dimerization is sufficient to induce necroptosis; and RIP3 dimer-induced necroptosis requires MLKL. We further show that RIP3 oligomer is not more potent than RIP3 dimer in triggering necroptosis, suggesting that RIP3 homo-interaction in the complex, rather than whether RIP3 has formed homo polymer, is important for necroptosis. RIP3 dimerization leads to RIP3 intramolecule autophosphorylation, which is required for the recruitment of MLKL. Interestingly, phosphorylation of one of RIP3 in the dimer is sufficient to induce necroptosis. As RIP1-RIP3 heterodimer itself cannot induce necroptosis, the RIP1-RIP3 heterodimeric amyloid fibril is unlikely to directly propagate necroptosis. We propose that the signaling events after the RIP1-RIP3 amyloid complex assembly are the recruitment of free RIP3 by the RIP3 in the amyloid scaffold followed by autophosphorylation of RIP3 and subsequent recruitment of MLKL by RIP3 to execute necroptosis.

Cathepsins limit macrophage necroptosis through cleavage of Rip1 kinase

It has recently been shown that programmed necrosis, necroptosis, may play a key role in the development of inflammation. Deciphering the regulation of this pathway within immune cells may therefore have implications in pathology associated with inflammatory diseases. We show that treatment of macrophages with the pan caspase inhibitor (zVAD-FMK) results in both increased phosphorylation and decreased cleavage of receptor interacting protein kinase-1 (Rip1), leading to necroptosis that is dependent on autocrine TNF signaling. Stimulation of cells with TLR agonists such as LPS in the presence of zVAD-FMK also induced Rip1-phosphorylation via a TNFR-independent mechanism. Further examination of Rip1 expression under these stimulatory conditions revealed a regulatory cleavage of Rip1 in macrophages that is not apparently attributable to caspase-8. Instead, we provide novel evidence that cysteine family cathepsins, which are highly abundant in myeloid cells, can also cleave Rip1 kinase. Using small interfering RNA knockdown, specific cathepsin inhibitors, and cell-free cleavage assays, we demonstrate that cysteine cathepsins B and S can directly cleave Rip1. Finally, we demonstrate that only through combined inhibition of cathepsins and caspase-8 could a potent induction of macrophage necroptosis be achieved. These data reveal a novel mechanism of regulation of necroptosis by cathepsins within macrophage cells.

Regulator of G-protein signaling 19 (RGS19) and its partner Gα-inhibiting activity polypeptide 3 (GNAI3) are required for zVAD-induced autophagy and cell death in L929 cells

Autophagy has diverse biological functions and is involved in many biological processes. The L929 cell death induced by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-(OMe)-fluoromethyl ketone (zVAD) was shown to be an autophagy-mediated death for which RIP1 and RIP3 were both required. It was also reported that zVAD can induce a small amount of TNF production, which was shown to be required for zVAD-induced L929 cell death, arguing for the contribution of autophagy in the zVAD-induced L929 cell death. In an effort to study RIP3 mediated cell death, we identified regulator of G-protein signaling 19 (RGS19) as a RIP3 interacting protein. We showed that RGS19 and its partner Gα-inhibiting activity polypeptide 3 (GNAI3) are involved in zVAD-, but not TNF-, induced cell death. The role of RGS19 and GNAI3 in zVAD-induced cell death is that they are involved in zVAD-induced autophagy. By the use of small hairpin RNAs and chemical inhibitors, we further demonstrated that zVAD-induced autophagy requires not only RIP1, RIP3, PI3KC3 and Beclin-1, but also RGS19 and GNAI3, and this autophagy is required for zVAD-induced TNF production. Collectively, our data suggest that zVAD-induced L929 cell death is a synergistic result of autophagy, caspase inhibition and autocrine effect of TNF.

An extracellular bacterial pathogen modulates host metabolism to regulate its own sensing and proliferation

Successful infection depends on the ability of the pathogen to gain nutrients from the host. The extracellular pathogenic bacterium group A Streptococcus (GAS) causes a vast array of human diseases. By using the quorum-sensing sil system as a reporter, we found that, during adherence to host cells, GAS delivers streptolysin toxins, creating endoplasmic reticulum stress. This, in turn, increases asparagine (ASN) synthetase expression and the production of ASN. The released ASN is sensed by the bacteria, altering the expression of ∼17% of GAS genes of which about one-third are dependent on the two-component system TrxSR. The expression of the streptolysin toxins is strongly upregulated, whereas genes linked to proliferation are downregulated in ASN absence. Asparaginase, a widely used chemotherapeutic agent, arrests GAS growth in human blood and blocks GAS proliferation in a mouse model of human bacteremia. These results delineate a pathogenic pathway and propose a therapeutic strategy against GAS infections.

Development of a novel method for quantification of autophagic protein degradation by AHA labeling

Autophagy is a catabolic process during which cellular components including protein aggregates and organelles are degraded via a lysosome-dependent process to sustain metabolic homeostasis during nutrient or energy deprivation. Measuring the rate of proteolysis of long-lived proteins is a classical assay for measurement of autophagic flux. However, traditional methods, such as a radioisotope labeling assay, are technically tedious and have low sensitivity. Here, we report a novel method for quantification of long-lived protein degradation based on L-azidohomoalanine (AHA) labeling in mouse embryonic fibroblasts (MEFs) and in human cancer cells. AHA is a surrogate for L-methionine, containing a bio-orthogonalazide moiety. When added to cultured cells, AHA is incorporated into proteins during active protein synthesis. After a click reaction between an azide and an alkyne, the azide-containing proteins can be detected with an alkyne-tagged fluorescent dye, coupled with flow cytometry. Induction of autophagy by starvation or mechanistic target of rapamycin (MTOR) inhibitors was able to induce a significant reduction of the fluorescence intensity, consistent with other autophagic markers. Coincidently, inhibition of autophagy by pharmacological agents or by Atg gene deletion abolished the reduction of the fluorescence intensity. Compared with the classical radioisotope pulse-labeling method, we think that our method is sensitive, quantitative, nonradioactive, and easy to perform, and can be applied to both human and animal cell culture systems.

The role of necroptosis, an alternative form of cell death, in cancer therapy

Programmed cell death plays an important role in animal development, tissue homeostasis and eliminating harmful or virally infected cells. Necroptosis, a novel form of programmed cell death, is caspase independent but RIPK and RIPK3 dependent. Moreover, it is suggested that necroptosis can be specifically inhibited by small molecular inhibitors such as necrostatin-1. Its signaling pathways have something in common with apoptosis, although the molecular mechanisms of necroptosis need to be further elucidated. Previous evidences suggest that necroptosis has significant effects in regulating various physiological processes and disease, such as ischemic brain injury, immune system disorders and cancer. In this review, the molecular mechanism of necroptosis is described and how it could be manipulated in the treatment of cancer is summarized.

The role of apoptosis in megakaryocytes and platelets

The role of apoptotic pathways in the development and function of the megakaryocyte lineage has generated renewed interest in recent years. This has been driven by the advent of BH3 mimetic drugs that target BCL2 family proteins to induce apoptosis in tumour cells: agents such as ABT-263 (navitoclax, which targets BCL2, BCL-XL [BCL2L1] and BCL2L2) and ABT-199 (a BCL2-specific agent) are showing great promise in early stage clinical trials. However, the major dose-limiting toxicity of navitoclax has proven to be thrombocytopenia, an on-target effect of inhibiting BCL-XL . It transpires that the anucleate platelet contains a classical intrinsic apoptosis pathway, which at steady state regulates its life span in the circulation. BCL-XL is the critical pro-survival protein that restrains apoptosis and maintains platelet viability. These findings have paved the way to a deeper understanding of apoptotic pathways and processes in platelets, and their precursor cell, the megakaryocyte.

Necrosis-like death can engage multiple pro-apoptotic Bcl-2 protein family members

Necroptosis is a physiologically relevant mode of cell death with some well-described initiating events, but largely unknown executioners. Here we investigated necrostatin-1 (Nec-1) sensitive death elicited by different necroptosis stimuli in L929 mouse fibrosarcoma cells, mouse embryonic fibroblasts (MEF) and bone marrow-derived macrophages. We found that TNFα- or zVAD-induced necroptosis occurs independently of the recently implicated executioners Bmf or PARP-2, but can involve the Bcl-2 family proteins Bid and Bak. Furthermore, this type of necroptosis is associated with mitochondrial cytochrome c release and partly sensitive to cyclosporine A inhibition, suggesting a cross talk with the mitochondrial permeability transition pore. Necroptosis triggered by cadmium (Cd) exposure caused fully Nec-1-sensitive and caspase-independent death in L929 cells that was associated with autocrine TNFα-mediated feed-forward signalling. In MEF Cd-exposure elicited a mixed mode of cell death that was to some extent Nec-1-sensitive but also displayed features of apoptosis. It was partly dependent on Bmf and Bax/Bak, proteins typically considered to act pro-apoptotic, but ultimately insensitive to caspase inhibition. Overall, our study indicates that inducers of "extrinsic" and "intrinsic" necroptosis can both trigger TNF-receptor signalling. Further, necroptosis may depend on mitochondrial changes engaging proteins considered critical for MOMP during apoptosis that ultimately contribute to caspase-independent necrotic cell death.

Autophagy in tumour cell death

In every moment of a cell's existence one key question is always asked, "To be or not to be"? Cells constantly weigh up signals from their environment against their own integrity and metabolic status and decide whether to live or die. Such cell death decisions are central to the progression and treatment of cancer. The term autophagy describes three processes that deliver cytoplasmic macromolecules and organelles to lysosomes for degradation, the difference between each form being the method of delivery. The most extensively studied form is macroautophagy (hereafter referred to as autophagy) where cytosolic components are engulfed by double membraned autophagosomes. Autophagosomes fuse with lysosomes to form structures called autolysosomes, within which organelles, proteins and other macromolecules are degraded by catabolic enzymes in the acidic lysosome environment. Autophagy, which normally occurs at low levels in unstressed cells, is widely regarded as having a positive effect on cell health as potentially harmful protein aggregates and damaged organelles can be recycled. During periods of nutrient shortage autophagy is enhanced to provide, albeit temporarily, an internal energy source. Autophagy is also enhanced by other stresses encountered by tumour cells and this may protect the cell or aid its demise. In this review we examine the effect of autophagy on cell death decisions in tumour cells.

Metabolic profiling of human CD4+ cells following treatment with methotrexate and anti-TNF-α infliximab

The autoimmune process in rheumatoid arthritis depends on activation of immune cells, which utilize intracellular kinases to respond to external stimuli such as cytokines, immune complexes, and antigens. CD4+ T cells comprise a large proportion of the inflammatory cells that invade the synovial tissue and may therefore be a cell type of pathogenic importance. Both methotrexate and infliximab are effective in the treatment of inflammatory arthritis; however, the biological effects triggered by these treatments and the biochemical mechanisms underlining the cell response are still not fully understood. Thus, in this study the global metabolic changes associated with methotrexate or infliximab treatment of isolated human CD4+ T cells were examined using gas chromatography/mass spectrometry or liquid chromatography/mass spectrometry. In total 148 metabolites involved in selective pathways were found to be significantly altered. Overall, the changes observed are likely to reflect the effort of CD4+ cells to increase the production of cellular reducing power to offset the cellular stress exerted by treatment. Importantly, analysis of the global metabolic changes associated with MTX or infliximab treatment of isolated human CD4+ T cells suggested that the toxicity associated with these agents is minimal when used at clinically relevant concentrations.

Mechanism of Siglec-8-mediated cell death in IL-5-activated eosinophils: role for reactive oxygen species-enhanced MEK/ERK activation

BACKGROUND

Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is expressed on human eosinophils, where its ligation induces cell death. Paradoxically, Siglec-8-mediated cell death is markedly enhanced by the presence of the activation and survival factor IL-5 and becomes independent of caspase activity.

OBJECTIVE

In this report we investigate the mechanism of Siglec-8-mediated cell death in activated eosinophils.

METHODS

Human peripheral blood eosinophils were treated with agonistic anti-Siglec-8 antibody and IL-5, and cell death was determined by using flow cytometry and morphology. Phosphorylation of mitogen-activated protein kinase (MAPK) was determined by using phosphoLuminex, flow cytometry, and Western blotting. Reactive oxygen species (ROS) accumulation was determined by using dihydrorhodamine fluorescence.

RESULTS

Costimulation with anti-Siglec-8 and IL-5 significantly increased the rate and proportion of cell death by means of necrosis accompanied by granule release compared with that seen after stimulation with anti-Siglec-8 alone, in which apoptosis predominated. Together with the caspase-independent mode of cell death in costimulated cells, these findings suggest the activation of a specific and distinct biochemical pathway of cell death during anti-Siglec-8/IL-5 costimulation. Phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and MAPK-ERK kinase (MEK) 1 was significantly enhanced and sustained in costimulated cells compared with that seen in cells stimulated with IL-5 alone; anti-Siglec-8 alone did not cause ERK1/2 phosphorylation. MEK1 inhibitors blocked anti-Siglec-8/IL-5-induced cell death. ROS accumulation was induced by Siglec-8 ligation in a MEK-independent manner. In contrast, an ROS inhibitor prevented the anti-Siglec-8/IL-5-induced enhancement of ERK phosphorylation and cell death. Exogenous ROS mimicked stimulation by anti-Siglec-8 and was sufficient to induce enhanced cell death in IL-5-treated cells. Collectively, these data suggest that the enhancement of ERK phosphorylation is downstream of ROS generation.

CONCLUSIONS

In activated eosinophils ligation of Siglec-8 leads to ROS-dependent enhancement of IL-5-induced ERK phosphorylation, which results in a novel mode of biochemically regulated eosinophil cell death.

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.

Crosstalk between apoptosis, necrosis and autophagy

Apoptosis and necrosis are the two major modes of cell death, the molecular mechanisms of which have been extensively studied. Although initially thought to constitute mutually exclusive cellular states, recent findings reveal cellular contexts that require a balanced interplay between these two modes of cellular demise. Several death initiator and effector molecules, signaling pathways and subcellular sites have been identified as key mediators in both processes, either by constituting common modules or alternatively by functioning as a switch allowing cells to decide which route to take, depending on the specific situation. Importantly, autophagy, which is a predominantly cytoprotective process, has been linked to both types of cell death, serving either a pro-survival or pro-death function. Here we review the recent literature that highlights the intricate interplay between apoptosis, necrosis and autophagy, focusing on the relevance and impact of this crosstalk in normal development and in pathology. This article is part of a Special Section entitled: Cell Death Pathways.

Axin expression enhances herpes simplex virus type 1 replication by inhibiting virus-mediated cell death in L929 cells

Herpes simplex virus type 1 (HSV-1) replicates in various cell types and induces early cell death, which limits viral replication in certain cell types. Axin is a scaffolding protein that regulates Wnt signalling and participates in various cellular events, including cellular proliferation and cell death. The effects of axin expression on HSV-1 infection were investigated based on our initial observation that Wnt3a treatment or axin knockdown reduced HSV-1 replication. L929 cells expressed the axin protein in a doxycycline-inducible manner (L-axin) and enhanced HSV-1 replication in comparison to control cells (L-EV). HSV-1 infection induced cell death as early as 6 h after infection through the necrotic pathway and required de novo protein synthesis in L929 cells. Subsequent analysis of viral protein expression suggested that axin expression led to suppression of HSV-1-induced premature cell death, resulting in increased late gene expression. In analysis of axin deletion mutants, the regulators of the G-protein signalling (RGS) domain were involved in the axin-mediated enhancement of viral replication and reduction in cell death. These results suggest that viral replication enhancement might be mediated by the axin RGS domain.

Akt Regulates TNFα synthesis downstream of RIP1 kinase activation during necroptosis

Necroptosis is a regulated form of necrotic cell death that has been implicated in the pathogenesis of various diseases including intestinal inflammation and systemic inflammatory response syndrome (SIRS). In this work, we investigated the signaling mechanisms controlled by the necroptosis mediator receptor interacting protein-1 (RIP1) kinase. We show that Akt kinase activity is critical for necroptosis in L929 cells and plays a key role in TNFα production. During necroptosis, Akt is activated in a RIP1 dependent fashion through its phosphorylation on Thr308. In L929 cells, this activation requires independent signaling inputs from both growth factors and RIP1. Akt controls necroptosis through downstream targeting of mammalian Target of Rapamycin complex 1 (mTORC1). Akt activity, mediated in part through mTORC1, links RIP1 to JNK activation and autocrine production of TNFα. In other cell types, such as mouse lung fibroblasts and macrophages, Akt exhibited control over necroptosis-associated TNFα production without contributing to cell death. Overall, our results provide new insights into the mechanism of necroptosis and the role of Akt kinase in both cell death and inflammatory regulation.

GX15-070 (obatoclax) induces apoptosis and inhibits cathepsin D- and L-mediated autophagosomal lysis in antiestrogen-resistant breast cancer cells

In estrogen receptor-positive (ER+) breast cancer cells, BCL2 overexpression contributes to antiestrogen resistance. Direct targeting of the antiapoptotic BCL2 members with GX15-070 (obatoclax), a BH3-mimetic currently in clinical development, is an attractive strategy to overcome antiestrogen resistance in some breast cancers. Recently, GX15-070 has been shown to induce both apoptosis and autophagy, yet the underlying cell death mechanisms have yet to be elucidated. Here, we show that GX15-070 is more effective in reducing the cell density of antiestrogen-resistant breast cancer cells versus sensitive cells and that this increased sensitivity of resistant cells to GX15-070 correlates with an accumulation of autophagic vacuoles. Formation of autophagosomes in GX15-070-treated cells was verified by changes in expression of the lipidation of microtubule-associated protein-1 light chain-3 and both confocal and transmission electron microscopy. While GX15-070 treatment promotes autophagic vacuole and autolysosome formation, p62/SQSTM1, a marker for autophagic degradation, levels accumulate. Moreover, GX15-070 exposure leads to a reduction in cathepsin D (CTSD) and L (CTSL1) protein expression that would otherwise digest autolysosome cargo. Thus, GX15-070 has dual roles in promoting cell death: (i) directly inhibiting antiapoptotic BCL2 family members, thereby inducing apoptosis; and (ii) inhibiting downstream CTSD and CTSL1 protein expression to limit the ability of cells to use degraded material to fuel cellular metabolism and restore homeostasis. Our data highlight a new mechanism of GX15-070-induced cell death that could be used to design novel therapeutic interventions for antiestrogen resistant breast cancer.

Time-lapse imaging of necrosis

The processes of dying are as tightly regulated as those of growth and proliferation. Recent work into the molecular pathways that regulate and execute cell death have uncovered a plethora of signalling cascades that lead to distinct modes of cell death, including "apoptosis," "necrosis," "autophagic cell death," and "mitotic catastrophe." Given that cells can readily switch from one form of death to another, it is vital to carefully monitor the form of death under investigation. Particularly, end-point techniques are intrinsically unsuitable for assessing apoptosis versus necrosis, as they cannot reconstruct the sequence of events that have led to cell death. Since apoptotic cells frequently undergo secondary necrosis under in vitro culture conditions, novel methods relying on high-throughput time-lapse fluorescence video microscopy have been developed. Here we describe the use of this technique to reliably distinguish necrosis from apoptosis and secondary necrosis.

New frontiers in promoting tumour cell death: targeting apoptosis, necroptosis and autophagy

Cancer is a multifaceted disease comprising a combination of genetic, metabolic and signalling aberrations, which severely disrupt the normal homeostasis of cell growth and death. Many oncogenic events while promoting tumour development also increase the sensitivity of cells to cell death stimuli including chemotherapeutic drugs. As a result, tumour cells often acquire the ability to evade death by inactivating cell death pathways that normally function to eliminate damaged and harmful cells. The impairment of cell death function is also often the reason for the development of chemotherapeutic resistance encountered during treatment. It is therefore necessary to achieve a comprehensive understanding of existing cell death pathways and the relevant regulatory components involved, with the intention of identifying new strategies to kill cancer cells. This review provides an insightful overview of the common forms of cell death signalling pathways, the interactions between these pathways and the ways in which these pathways are deregulated in cancer. We also discuss the emerging therapies targeted at activating or restoring cell death pathways to induce tumour cell death, which are currently being tested in clinical trials.

A role for c-FLIP(L) in the regulation of apoptosis, autophagy, and necroptosis in T lymphocytes

Caspase 8 plays a dual role in the survival of T lymphocytes. Although active caspase 8 mediates apoptosis upon death receptor signaling, the loss of caspase 8 activity leads to receptor-interacting protein (RIP)-1/RIP-3-dependent necrotic cell death (necroptosis) upon TCR activation. The anti-apoptotic protein c-FLIP (cellular caspase 8 (FLICE)-like inhibitory protein) suppresses death receptor-induced caspase 8 activation. Moreover, recent findings suggest that c-FLIP is also involved in inhibiting necroptosis and autophagy. It remains unclear whether c-FLIP protects primary T lymphocytes from necroptosis or regulates the threshold at which autophagy occurs. Here, we used a c-FLIP isoform-specific conditional deletion model to show that c-FLIP(L)-deficient T cells underwent RIP-1-dependent necroptosis upon TCR stimulation. Interestingly, although previous studies have only described necroptosis in the absence of caspase 8 activity, we found that pro-apoptotic caspase 8 activity and apoptosis were also enhanced in c-FLIP(L)-deficient T lymphocytes. Furthermore, c-FLIP(L)-deficient T cells exhibited enhanced autophagy, which served a cytoprotective function. Together, these findings indicate that c-FLIP(L) plays an important antinecroptotic role and is a key regulator of apoptosis, autophagy, and necroptosis in T lymphocytes.

TAK1 is essential for osteoclast differentiation and is an important modulator of cell death by apoptosis and necroptosis

Transforming growth factor β (TGF-β)-activated kinase 1 (TAK1), a mitogen-activated protein 3 (MAP3) kinase, plays an essential role in inflammation by activating the IκB kinase (IKK)/nuclear factor κB (NF-κB) and stress kinase (p38 and c-Jun N-terminal kinase [JNK]) pathways in response to many stimuli. The tumor necrosis factor (TNF) superfamily member receptor activator of NF-κB ligand (RANKL) regulates osteoclastogenesis through its receptor, RANK, and the signaling adaptor TRAF6. Because TAK1 activation is mediated through TRAF6 in the interleukin 1 receptor (IL-1R) and toll-like receptor (TLR) pathways, we sought to investigate the consequence of TAK1 deletion in RANKL-mediated osteoclastogenesis. We generated macrophage colony-stimulating factor (M-CSF)-derived monocytes from the bone marrow of mice with TAK1 deletion in the myeloid lineage. Unexpectedly, TAK1-deficient monocytes in culture died rapidly but could be rescued by retroviral expression of TAK1, inhibition of receptor-interacting protein 1 (RIP1) kinase activity with necrostatin-1, or simultaneous genetic deletion of TNF receptor 1 (TNFR1). Further investigation using TAK1-deficient mouse embryonic fibroblasts revealed that TNF-α-induced cell death was abrogated by the simultaneous inhibition of caspases and knockdown of RIP3, suggesting that TAK1 is an important modulator of both apoptosis and necroptosis. Moreover, TAK1-deficient monocytes rescued from programmed cell death did not form mature osteoclasts in response to RANKL, indicating that TAK1 is indispensable to RANKL-induced osteoclastogenesis. To our knowledge, we are the first to report that mice in which TAK1 has been conditionally deleted in osteoclasts develop osteopetrosis.

Ubiquitination and degradation of the FADD adaptor protein regulate death receptor-mediated apoptosis and necroptosis

Fas-associated protein with death domain (FADD) is a pivotal component of death receptor-mediated extrinsic apoptosis and necroptosis. Here we show that FADD is regulated by Makorin Ring Finger Protein 1 (MKRN1) E3 ligase-mediated ubiquitination and proteasomal degradation. MKRN1 knockdown results in FADD protein stabilization and formation of the rapid death-inducing signalling complex, which causes hypersensitivity to extrinsic apoptosis by facilitating caspase-8 and caspase-3 cleavage in response to death signals. We also show that MKRN1 and FADD are involved in the regulation of necrosome formation and necroptosis upon caspase inhibition. Downregulation of MKRN1 results in severe defects of tumour growth upon tumour necrosis factor-related apoptosis-inducing ligand treatment in a xenograft model using MDA-MB-231 breast cancer cells. Suppression of tumour growth by MKRN1 depletion is relieved by simultaneous FADD knockdown. Our data reveal a novel mechanism by which fas-associated protein with death domain is regulated via an ubiquitination-induced degradation pathway.

Caloric restriction: is mammalian life extension linked to p53?

Caloric restriction, that is limiting food intake, is recognized in mammals as the best characterized and most reproducible strategy for extending lifespan, retarding physiological aging and delaying the onset of age-associated diseases. The aim of this mini review is to argue that p53 is the connection in the abilities of both the Sirt-1 pathway and the TOR pathway to impact on longevity of cells and organisms. This novel, lifespan regulating function of p53 may be evolutionarily more ancient than its relatively recent role in apoptosis and tumour suppression, and is likely to provide many new insights into lifespan modulation.

Combined treatment of human colorectal tumor cell lines with chemotherapeutic agents and ionizing irradiation can in vitro induce tumor cell death forms with immunogenic potential

Chemotherapeutic agents (CT) and ionizing radiation (X-ray) induce DNA damage and primarily aim to stop the proliferation of tumor cells. However, multimodal anti-cancer therapies should finally result in tumor cell death and, best, in the induction of systemic anti-tumor immunity. Since distinct therapy-induced tumor cell death forms may create an immune activating tumor microenvironment, this study examined whether sole treatment with CT that are used in the therapy for colorectal cancer or in combination with X-ray result in colorectal tumor cell death with immunogenic potential. 5-Fluorouracil (5-FU), Oxaliplatin (Oxp), or Irinotecan (Irino) in combination with X-ray were all potent inhibitors of colorectal tumor cell colony formation. This study then examined the forms of cell death with AnnexinA5-FITC/Propidium Iodide staining. Necrosis was the prominent form of cell death induced by CT and/or X-ray. While only a combination of Irino with X-ray leads to death induction already 1 day after treatment, also the combinations of Oxp or 5-FU with X-ray and X-ray alone resulted in high necrosis rates at later time points after treatment. Inhibition of apoptosis increased the amount of necrotic tumor cells, suggesting that a programmed form of necrosis can be induced by CT + X-ray. 5-FU and Oxp alone or in combination with X-ray and Irino plus X-ray were most effective in increasing the expression of RIP, IRF-5, and p53, proteins involved in necrotic and apoptotic cell death pathways. All treatments further resulted in the release of the immune activating danger signals high-mobility group box 1 (HMGB1) and heat shock protein 70 (HSP70). The supernatants of the treated tumor cells induced maturation of dendritic cells. It is, therefore, concluded that combination of CT with X-ray is capable of inducing in vitro cell death forms of colorectal tumors with immunogenic potential.

A novel role for RIP1 kinase in mediating TNFα production

Receptor-interacting protein 1 (RIP1) is a Ser/Thr kinase with both kinase-dependent and kinase-independent roles in death receptor signaling. The kinase activity of RIP1 is required for necroptosis, a caspase-independent pathway of programmed cell death. In some cell types, the inhibition of caspases leads to autocrine production of TNFα, which then activates necroptosis. Here, we describe a novel role for RIP1 kinase in regulating TNFα production after caspase inhibition. Caspase inhibitors activate RIP1 kinase and another protein, EDD, to mediate JNK signaling, which stimulates Sp1-dependent transcription of TNFα. This pathway is independent of nuclear factor κB and also occurs after Smac mimetic/IAP antagonist treatment or the loss of TNF receptor-associated factor 2 (Traf2). These findings implicate cIAP1/2 and Traf2 as negative regulators of this RIP1 kinase-dependent TNFα production pathway and suggest a novel role for RIP1 kinase in mediating TNFα production under certain conditions.

Dichotomy between RIP1- and RIP3-mediated necroptosis in tumor necrosis factor-α-induced shock

Tumor necrosis factor receptor (TNFR) signaling may result in survival, apoptosis or programmed necrosis. The latter is called necroptosis if the receptor-interacting protein 1 (RIP1) inhibitor necrostatin-1 (Nec-1) or genetic knockout of RIP3 prevents it. In the lethal mouse model of TNFα-mediated shock, addition of the pan-caspase inhibitor zVAD-fmk (zVAD) accelerates time to death. Here, we demonstrate that RIP3-deficient mice are protected markedly from TNFα-mediated shock in the presence and absence of caspase inhibition. We further show that the fusion protein TAT-crmA, previously demonstrated to inhibit apoptosis, also prevents necroptosis in L929, HT29 and FADD-deficient Jurkat cells. In contrast to RIP3-deficient mice, blocking necroptosis by Nec-1 or TAT-crmA did not protect from TNFα/zVAD-mediated shock, but further accelerated time to death. Even in the absence of caspase inhibition, Nec-1 application led to similar kinetics. Depletion of macrophages, natural killer (NK) cells, granulocytes or genetic deficiency for T lymphocytes did not influence this model. Because RIP3-deficient mice are known to be protected from cerulein-induced pancreatitis (CIP), we applied Nec-1 and TAT-crmA in this model and demonstrated the deterioration of pancreatic damage upon addition of these substances. These data highlight the importance of separating genetic RIP3 deficiency from RIP1 inhibition by Nec-1 application in vivo and challenge the current definition of necroptosis.

Structure-activity relationships of α-, β(1)-, γ-, and δ-tomatine and tomatidine against human breast (MDA-MB-231), gastric (KATO-III), and prostate (PC3) cancer cells

Partial acid hydrolysis of the tetrasaccharide (lycotetraose) side chain of the tomato glycoalkaloid α-tomatine resulted in the formation of four products with three, two, one, and zero carbohydrate side chains, which were separated by high-performance liquid chromatography (HPLC) and identified by thin-layer chromatography (TLC) and liquid chromatography ion-trap time-of-flight mass spectrometry (LCMS-IT-TOF). The inhibitory activities in terms of IC(50) values (concentration that inhibits 50% of the cells under the test conditions) of the parent compound and the hydrolysates, isolated by preparative HPLC, against normal human liver and lung cells and human breast, gastric, and prostate cancer cells indicate that (a) the removal of sugars significantly reduced the concentration-dependent cell-inhibiting effects of the test compounds, (b) PC3 prostate cancer cells were about 10 times more susceptible to inhibition by α-tomatine than the breast and gastric cancer cells or the normal cells, (c) the activity of α-tomatine against the prostate cancer cells was 200 times greater than that of the aglycone tomatidine, and (d) the activity increased as the number of sugars on the aglycone increased, but this was only statistically significant at p < 0.05 for the normal lung Hel299 cell line. The effect of the alkaloids on tumor necrosis factor α (TNF-α) was measured in RAW264.7 macrophage cells. There was a statistically significant negative correlation between the dosage of γ- and α-tomatine and the level of TNF-α. α-Tomatine was the most effective compound at reducing TNF-α. The dietary significance of the results and future research needs are discussed.

Macroautophagy in T lymphocyte development and function

Macroautophagy (referred to as autophagy) is a fundamental intracellular process characterized by the sequestration of cytoplasmic compartments through double-membrane vesicles, termed autophagosomes. Recent studies have established important roles of autophagy in regulating T lymphocyte development and function. Resting T lymphocytes have basal levels of autophagy that is upregulated by T cell receptor stimulation. Several specific knockout or transgenic models have been developed during the past few years, and it has been revealed that autophagy plays an essential role in regulating thymocyte selection, peripheral T cell survival, and proliferation. The regulation of T cell development and function by autophagy is mediated through its role in regulating self-antigen presentation, intracellular organelle homeostasis, and energy production. Here we will review the current findings concerning how autophagy regulates T cell function, as well as compare different models in studying autophagy in T lymphocytes.

Autophagy is a survival force via suppression of necrotic cell death

Macroautophagy or autophagy is a self-digesting mechanism that the cellular contents are engulfed by autophagosomes and delivered to lysosomes for degradation. Although it has been well established that autophagy is an important protective mechanism for cells under stress such as starvation via provision of nutrients and removal of protein aggregates and damaged mitochondria, there is a very complex relation between autophagy and cell death. At present, the molecular cross-talk between autophagy and apoptosis has been well discussed, while the relationship between autophagy and programmed necrotic cell death is less understood. In this review we focus on the role of autophagy in necrotic cell death by detailed discussion on two important forms of necrotic cell death: (i) necroptosis and (ii) poly-(ADP-ribose) polymerase (PARP)-mediated cell death. It is believed that one important aspect of the pro-survival function of autophagy is achieved via its ability to block various forms of necrotic cell death.

Necroptosis: an emerging form of programmed cell death

Necrosis plays an important role in multiple physiological and pathological processes. Recently, a relatively new form of necrosis has been characterized as "necroptosis". Morphologically, necroptosis exhibits the features of necrosis; however, necroptosis exhibits a unique signaling pathway that requires the involvement of receptor interaction protein kinase 1 and 3 (RIP1 and RIP3) and can be specifically inhibited by necrostatins. Necroptosis has been found to contribute to the regulation of immune system, cancer development as well as cellular responses to multiple stresses. In this review, we will summarize the signaling pathway, biological effects and pathological significance of this specific form of programmed cell death.

Oncogenic stress induced by acute hyper-activation of Bcr-Abl leads to cell death upon induction of excessive aerobic glycolysis

In response to deregulated oncogene activation, mammalian cells activate disposal programs such as programmed cell death. To investigate the mechanisms behind this oncogenic stress response we used Bcr-Abl over-expressing cells cultivated in presence of imatinib. Imatinib deprivation led to rapid induction of Bcr-Abl activity and over-stimulation of PI3K/Akt-, Ras/MAPK-, and JAK/STAT pathways. This resulted in a delayed necrosis-like cell death starting not before 48 hours after imatinib withdrawal. Cell death was preceded by enhanced glycolysis, glutaminolysis, and amino acid metabolism leading to elevated ATP and protein levels. This enhanced metabolism could be linked to induction of cell death as inhibition of glycolysis or glutaminolysis was sufficient to sustain cell viability. Therefore, these data provide first evidence that metabolic changes induced by Bcr-Abl hyper-activation are important mediators of oncogenic stress-induced cell death.

During the first 30 hours after imatinib deprivation, Bcr-Abl hyper-activation did not affect proliferation but resulted in cellular swelling, vacuolization, and induction of eIF2α phosphorylation, CHOP expression, as well as alternative splicing of XPB, indicating endoplasmic reticulum stress response. Cell death was dependent on p38 and RIP1 signaling, whereas classical death effectors of ER stress, namely CHOP-BIM were antagonized by concomitant up-regulation of Bcl-xL.

Screening of 1,120 compounds for their potential effects on oncogenic stress-induced cell death uncovered that corticosteroids antagonize cell death upon Bcr-Abl hyper-activation by normalizing cellular metabolism. This protective effect is further demonstrated by the finding that corticosteroids rendered lymphocytes permissive to the transforming activity of Bcr-Abl. As corticosteroids are used together with imatinib for treatment of Bcr-Abl positive acute lymphoblastic leukemia these data could have important implications for the design of combination therapy protocols.

In conclusion, excessive induction of Warburg type metabolic alterations can cause cell death. Our data indicate that these metabolic changes are major mediators of oncogenic stress induced by Bcr-Abl.

Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets

Deletion of Bak and Bax, the effectors of mitochondrial apoptosis, does not affect platelet production, however, loss of prosurvival Bcl-x_L results in megakaryocyte apoptosis and failure of platelet shedding.

It is believed that megakaryocytes undergo a specialized form of apoptosis to shed platelets. Conversely, a range of pathophysiological insults, including chemotherapy, are thought to cause thrombocytopenia by inducing the apoptotic death of megakaryocytes and their progenitors. To resolve this paradox, we generated mice with hematopoietic- or megakaryocyte-specific deletions of the essential mediators of apoptosis, Bak and Bax. We found that platelet production was unperturbed. In stark contrast, deletion of the prosurvival protein Bcl-x_L resulted in megakaryocyte apoptosis and a failure of platelet shedding. This could be rescued by deletion of Bak and Bax. We examined the effect on megakaryocytes of three agents that activate the intrinsic apoptosis pathway in other cell types: etoposide, staurosporine, and the BH3 mimetic ABT-737. All three triggered mitochondrial damage, caspase activation, and cell death. Deletion of Bak and Bax rendered megakaryocytes resistant to etoposide and ABT-737. In vivo, mice with a Bak^−/− Bax^−/− hematopoietic system were protected against thrombocytopenia induced by the chemotherapeutic agent carboplatin. Thus, megakaryocytes do not activate the intrinsic pathway to generate platelets; rather, the opposite is true: they must restrain it to survive and progress safely through proplatelet formation and platelet shedding.

RIP1-dependent and independent effects of necrostatin-1 in necrosis and T cell activation

BACKGROUND

Programmed necrosis/necroptosis is an emerging form of cell death that plays important roles in mammalian development and the immune system. The pro-necrotic kinases in the receptor interacting protein (RIP) family are crucial mediators of programmed necrosis. Recent advances in necrosis research have been greatly aided by the identification of chemical inhibitors that block programmed necrosis. Necrostatin-1 (Nec-1) and its derivatives were previously shown to target the pro-necrotic kinase RIP1/RIPK1. The protective effect conferred by Nec-1 and its derivatives in many experimental model systems was often attributed to the inhibition of RIP1 function.

METHODOLOGY/PRINCIPAL FINDINGS

We compared the effect of Nec-1 and siRNA-mediated silencing of RIP1 in the murine fibrosarcoma cell line L929. Treatment of L929 cells with the pan-caspase inhibitor zVAD-fmk or exogenous TNF induces necrosis. Strikingly, we found that siRNA-mediated silencing of RIP1 inhibited zVAD-fmk induced necrosis, but not TNF-induced necrosis. TNF-induced cell death in RIP1 knocked down L929 cells was inhibited by Nec-1, but not the caspase inhibitor zVAD-fmk. We found that PKA-C§ expression, but not Jnk or Erk activation, was moderately inhibited by Nec-1. Moreover, we found that Nec-1 inhibits proximal T cell receptor signaling independent of RIP1, leading to inhibition of T cell proliferation.

CONCLUSIONS/SIGNIFICANCE

Our results reveal that besides RIP1, Nec-1 also targets other factors crucial for necrosis induction in L929 cells. In addition, high doses of Nec-1 inhibit other signal transduction pathways such as that for T cell receptor activation. These results highlight the importance to independently validate results obtained using Nec-1 with other approaches such as siRNA-mediated gene silencing. We propose that some of the previous published results obtained using Nec-1 should be re-evaluated in light of our findings.

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.

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.

Cellular FLICE-inhibitory protein (cFLIP) isoforms block CD95- and TRAIL death receptor-induced gene induction irrespective of processing of caspase-8 or cFLIP in the death-inducing signaling complex

Death receptors (DRs) induce apoptosis but also stimulate proinflammatory "non-apoptotic" signaling (e.g. NF-κB and mitogen-activated protein kinase (MAPK) activation) and inhibit distinct steps of DR-activated maturation of procaspase-8. To examine whether isoforms of cellular FLIP (cFLIP) or its cleavage products differentially regulate DR signaling, we established HaCaT cells expressing cFLIP(S), cFLIP(L), or mutants of cFLIP(L) (cFLIP(D376N) and cFLIP(p43)). cFLIP variants blocked TRAIL- and CD95L-induced apoptosis, but the cleavage pattern of caspase-8 in the death inducing signaling complex was different: cFLIP(L) induced processing of caspase-8 to the p43/41 fragments irrespective of cFLIP cleavage. cFLIP(S) or cFLIP(p43) blocked procaspase-8 cleavage. Analyzing non-apoptotic signaling pathways, we found that TRAIL and CD95L activate JNK and p38 within 15 min. cFLIP variants and different caspase inhibitors blocked late death ligand-induced JNK or p38 MAPK activation suggesting that these responses are secondary to cell death. cFLIP isoforms/mutants also blocked death ligand-mediated gene induction of CXCL-8 (IL-8). Knockdown of caspase-8 fully suppressed apoptotic and non-apoptotic signaling. Knockdown of cFLIP isoforms in primary human keratinocytes enhanced CD95L- and TRAIL-induced NF-κB activation, and JNK and p38 activation, underscoring the regulatory role of cFLIP for these DR-mediated signals. Whereas the presence of caspase-8 is critical for apoptotic and non-apoptotic signaling, cFLIP isoforms are potent inhibitors of TRAIL- and CD95L-induced apoptosis, NF-κB activation, and the late JNK and p38 MAPK activation. cFLIP-mediated inhibition of CD95 and TRAIL DR could be of crucial importance during keratinocyte skin carcinogenesis and for the activation of innate and/or adaptive immune responses triggered by DR activation in the skin.

zVAD-induced autophagic cell death requires c-Src-dependent ERK and JNK activation and reactive oxygen species generation

The treatment of L929 fibrosarcoma cells with zVAD has been shown to induce necroptosis. However, whether autophagy is involved or not in this event remains controversial. In this study, we re-examined the role of autophagy in zVAD-induced cell death in L929 cells and further elucidated the signaling pathways triggered by caspase inhibition and contributing to autophagic death. First, we found that zVAD can stimulate LC3-II formation, autophagosome and autolysosome formation, and ROS accumulation. Antioxidants, beclin 1 or Atg5 silencing, and class III PtdIns3K inhibitors all effectively blocked ROS production and cell death, suggesting ROS accumulation downstream of autophagy contributes to cell necrosis. zVAD also stimulated PARP activation, and the PARP inhibitor DPQ can reduce zVAD-induced cell death, but did not affect ROS production, suggesting the increased ROS leads to PARP activation and cell death. Notably, our data also indicated the involvement of Src-dependent JNK and ERK in zVAD-induced ROS production and autophagic death. We found caspase 8 is associated with c-Src at the resting state, and upon zVAD treatment this association was decreased and accompanied by c-Src activation. In conclusion, we confirm the autophagic death in zVAD-treated L929 cells, and define a new molecular pathway in which Src-dependent ERK and JNK activation can link a signal from caspase inhibition to autophagy, which in turn induce ROS production and PARP activation, eventually leading to necroptosis. Thus, in addition to initiating proteolytic activity for cell apoptosis, inactivated caspase 8 also functions as a signaling molecule for autophagic death.

Programmed necrosis from molecules to health and disease

During the past decade, cell death researchers have witnessed a gradual but deep conceptual revolution: it has been unequivocally shown that necrosis, which for long had been considered as a purely accidental cell death mode, can also be induced by finely regulated signal transduction pathways. In particular, when caspases are inhibited by pharmacological or genetic means, the ligation of death receptors such as the tumor necrosis factor receptor 1 (TNFR1) can lead to the assembly of a supramolecular complex containing the receptor-interacting protein kinases 1 and 3 (RIP1 and RIP3) that delivers a pronecrotic signal. Such complex has recently been dubbed necrosome and mediates the execution of a specific instance of regulated necrosis, necroptosis. Soon, it turned out that programmed necrosis occurs in nonmammalian model organisms and that it is implicated in human diseases including ischemia and viral infection. In this review, we first describe the historical evolution of the concept of programmed necrosis and the molecular mechanisms that underlie necroptosis initiation and execution. We then provide evidence suggesting that necroptosis represents an ancient and evolutionarily conserved cell death modality that may be targeted for drug development.