Insights into the regulatory mechanism for caspase-8 activation

Molecular signatures in prion disease: altered death receptor pathways in a mouse model

BACKGROUND

Prion diseases are transmissible and fatal neurodegenerative diseases characterized by accumulation of misfolded prion protein isoform (PrPSc), astrocytosis, microgliosis, spongiosis, and neurodegeneration. Elevated levels of cell membrane associated PrPSc protein and inflammatory cytokines hint towards the activation of death receptor (DR) pathway/s in prion diseases. Activation of DRs regulate, either cell survival or apoptosis, autophagy and necroptosis based on the adaptors they interact. Very little is known about the DR pathways activation in prion disease. DR3 and DR5 that are expressed in normal mouse brain were never studied in prion disease, so also their ligands and any DR adaptors. This research gap is notable and investigated in the present study.

METHODS

C57BL/6J mice were infected with Rocky Mountain Laboratory scrapie mouse prion strain. The progression of prion disease was examined by observing morphological and behavioural abnormalities. The levels of PrP isoforms and GFAP were measured as the marker of PrPSc accumulation and astrocytosis respectively using antibody-based techniques that detect proteins on blot and brain section. The levels of DRs, their glycosylation and ectodomain shedding, and associated factors warrant their examination at protein level, hence western blot analysis was employed in this study.

RESULTS

Prion-infected mice developed motor deficits and neuropathology like PrPSc accumulation and astrocytosis similar to other prion diseases. Results from this research show higher expression of all DR ligands, TNFR1, Fas and p75NTR but decreased levels DR3 and DR5. The levels of DR adaptor proteins like TRADD and TRAF2 (primarily regulate pro-survival pathways) are reduced. FADD, which primarily regulate cell death, its level remains unchanged. RIPK1, which regulate pro-survival, apoptosis and necroptosis, its expression and proteolysis (inhibits necroptosis but activates apoptosis) are increased.

CONCLUSIONS

The findings from the present study provide evidence towards the involvement of DR3, DR5, DR6, TL1A, TRAIL, TRADD, TRAF2, FADD and RIPK1 for the first time in prion diseases. The knowledge obtained from this research discuss the possible impacts of these 16 differentially expressed DR factors on our understanding towards the multifaceted neuropathology of prion diseases and towards future explorations into potential targeted therapeutic interventions for prion disease specific neuropathology.

Evolution of Caspases and the Invention of Pyroptosis

The protein scaffold that includes the caspases is ancient and found in all domains of life. However, the stringent specificity that defines the caspase biologic function is relatively recent and found only in multicellular animals. During the radiation of the Chordata, members of the caspase family adopted roles in immunity, events coinciding with the development of substrates that define the modern innate immune response. This review focuses on the switch from the non-inflammatory cellular demise of apoptosis to the highly inflammatory innate response driven by distinct members of the caspase family, and the interplay between these two regulated cell death pathways.

Death and survival from executioner caspase activation

Executioner caspases are evolutionarily conserved regulators of cell death under apoptotic stress. Activated executioner caspases drive apoptotic cell death through cleavage of diverse protein substrates or pyroptotic cell death in the presence of gasdermin E. On the other hand, activation of executioner caspases can also trigger pro-survival and pro-proliferation signals. In recent years, a growing body of studies have demonstrated that cells can survive from executioner caspase activation in response to stress and that the survivors undergo molecular and phenotypic alterations. This review focuses on death and survival from executioner caspase activation, summarizing the role of executioner caspases in apoptotic and pyroptotic cell death and discussing the potential mechanism and consequences of survival from stress-induced executioner caspase activation.

A biochemical necroptosis model explains cell-type-specific responses to cell death cues

Necroptosis is a form of regulated cell death associated with degenerative disorders, autoimmune and inflammatory diseases, and cancer. To better understand the biochemical mechanisms regulating necroptosis, we constructed a detailed computational model of tumor necrosis factor-induced necroptosis based on known molecular interactions from the literature. Intracellular protein levels, used as model inputs, were quantified using label-free mass spectrometry, and the model was calibrated using Bayesian parameter inference to experimental protein time course data from a well-established necroptosis-executing cell line. The calibrated model reproduced the dynamics of phosphorylated mixed lineage kinase domain-like protein, an established necroptosis reporter. A subsequent dynamical systems analysis identified four distinct modes of necroptosis signal execution, distinguished by rate constant values and the roles of the RIP1 deubiquitinating enzymes A20 and CYLD. In one case, A20 and CYLD both contribute to RIP1 deubiquitination, in another RIP1 deubiquitination is driven exclusively by CYLD, and in two modes either A20 or CYLD acts as the driver with the other enzyme, counterintuitively, inhibiting necroptosis. We also performed sensitivity analyses of initial protein concentrations and rate constants to identify potential targets for modulating necroptosis sensitivity within each mode. We conclude by associating numerous contrasting and, in some cases, counterintuitive experimental results reported in the literature with one or more of the model-predicted modes of necroptosis execution. In all, we demonstrate that a consensus pathway model of tumor necrosis factor-induced necroptosis can provide insights into unresolved controversies regarding the molecular mechanisms driving necroptosis execution in numerous cell types under different experimental conditions.

The protective effects of hyperoside on Ang II-mediated apoptosis of bEnd.3 cells and injury of blood-brain barrier model in vitro

Background

Hypertension and its associated dysfunction of the blood-brain barrier (BBB) are considered to contribute to cerebral small vessel disease (cSVD). Angiotensin II (Ang II), as an important vasoactive peptide of the renin-angiotensin system (RAS), is not only a pivotal molecular signal in hypertension, but also causes BBB leakage, cSVD and its related cognitive impair. Hyperoside (Hyp), a flavone glycoside, has antioxidant, antiphlogistic and anti-apoptosis effects. In this study, we investigate the protection of Hyp on apoptosis of bEnd.3 cells and BBB disruption in vitro induced by Ang II.

Methods

We used bEnd.3 cells to imitate a BBB monolayer model and explored the protection of Hyp on Ang II-induced BBB leakage. The apoptotic activity was assessed by TUNEL staining and flow cytometry. The expression of apoptosis pathway related proteins, tight junction proteins and transcytosis related proteins were detected by western blot assay. The BBB model permeability was detected through measuring the flux of sodium fluorescein (Na-F).

Results

We found that Hyp can not only effectively inhibit the apoptosis of bEnd.3 induced by Ang II, but also protect the structural soundness and functional integrity of BBB model by affecting the expression levels of junctional adhesion molecule A (JAM-A), Claudin-5, zonula occludens-1 (ZO-1), Caveolin-1 (Cav-1) and major facilitator superfamily domain-containing protein 2a (Mfsd2a).

Conclusion

Hyp might be a potent compound for preventing Ang II-induced BBB disruption.

Caspase mechanisms in the regulation of inflammation

Regulated cell death is defined as genetically encoded pathways that lead towards the demise of cells. In mammals, cell demise can be either inflammatory or non-inflammatory, depending on whether the mechanism of death results in cell rupture or not. Inflammatory cell death can lead towards acute and chronic disease. Therefore, it becomes important to distinguish the mechanisms that result in these different inflammatory cell death outcomes. Apoptosis is a non-inflammatory form of cell death where cells resist rupture. In contrast, pyroptosis and necroptosis are inflammatory forms of cell death principally because of release of pro-inflammatory mediators from cells undergoing lysis. This review focusses on the mechanisms of these different cell death outcomes with specific emphasis on the caspase family of proteolytic enzymes.

A High Throughput Approach for Designing Polymers That Mimic the TRAIL Protein

We have leveraged a high throughput approach to design a fully synthetic polymer mimic of the chemotherapeutic protein "TRAIL". Our design enables the synthesis of libraries of star-shaped polymers presenting exactly one receptor binding peptide at the end of each arm with no purification steps. Clear structure-activity relationships in screening for receptor binding and the apoptotic activity on colon cancer lines (COLO205) led us to identify trivalent structures, ∼1.5 nm in hydrodynamic radius as the best mimics. These showed IC₅₀ values ∼2 μM and resulted in the elevated levels of caspase-8 expected from this mechanism of cell death. Our results demonstrate the potential for HTP screening methods to be used in the design of polymers that can mimic a whole range of complex therapeutic proteins.

Inflammatory Caspases: Toward a Unified Model for Caspase Activation by Inflammasomes

Inflammasomes are inflammatory signaling complexes that provide molecular platforms to activate the protease function of inflammatory caspases. Caspases-1, -4, -5, and -11 are inflammatory caspases activated by inflammasomes to drive lytic cell death and inflammatory mediator production, thereby activating host-protective and pathological immune responses. Here, we comprehensively review the mechanisms that govern the activity of inflammatory caspases. We discuss inflammatory caspase activation and deactivation mechanisms, alongside the physiological importance of caspase activity kinetics. We also examine mechanisms of caspase substrate selection and how inflammasome and cell identities influence caspase activity and resultant inflammatory and pyroptotic cellular programs. Understanding how inflammatory caspases are regulated may offer new strategies for treating infection and inflammasome-driven disease. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Caspase-8 auto-cleavage regulates programmed cell death and collaborates with RIPK3/MLKL to prevent lymphopenia

Caspase-8 is an initiator of death receptor-induced apoptosis and an inhibitor of RIPK3-MLKL-dependent necroptosis. In addition, caspase-8 has been implicated in diseases such as lymphoproliferation, immunodeficiency, and autoimmunity in humans. Although auto-cleavage is indispensable for caspase-8 activation, its physiological functions remain poorly understood. Here, we generated a caspase-8 mutant lacking E385 in auto-cleavage site knock-in mouse (Casp8^{ΔE385/ΔE385}). Casp8^{ΔE385/ΔE385} cells were expectedly resistant to Fas-induced apoptosis, however, Casp8^{ΔE385/ΔE385} cells could switch TNF-α-induced apoptosis to necroptosis by attenuating RIPK1 cleavage. More importantly, CASP8(ΔE385) sensitized cells to RIPK3-MLKL-dependent necroptosis through promoting complex II formation and RIPK1-RIPK3 activation. Notably, Casp8^{ΔE385/ΔE385}Ripk3^-/- mice partially rescued the perinatal death of Ripk1^-/- mice by blocking apoptosis and necroptosis. In contrast to the Casp8^-/-Ripk3^-/- and Casp8^-/-Mlkl^-/- mice appearing autoimmune lymphoproliferative syndrome (ALPS), both Casp8^{ΔE385/ΔE385}Ripk3^-/- and Casp8^{ΔE385/ΔE385}Mlkl^-/- mice developed transplantable lymphopenia that could be significantly reversed by RIPK1 heterozygosity, but not by RIPK1 kinase dead mutation. Collectively, these results demonstrate previously unappreciated roles for caspase-8 auto-cleavage in regulating necroptosis and maintaining lymphocytes homeostasis.

Exogenous Introduction of Initiator and Executioner Caspases Results in Different Apoptotic Outcomes

The balance of pro-apoptotic and pro-survival proteins defines a cell's fate. These processes are controlled through an interdependent and finely tuned protein network that enables survival or leads to apoptotic cell death. The caspase family of proteases is central to this apoptotic network, with initiator and executioner caspases, and their interaction partners, regulating and executing apoptosis. In this work, we interrogate and modulate this network by exogenously introducing specific initiator or executioner caspase proteins. Each caspase is exogenously introduced using redox-responsive polymeric nanogels. Although caspase-3 might be expected to be the most effective due to the centrality of its role in apoptosis and its heightened catalytic efficiency relative to other family members, we observed that caspase-7 and caspase-9 are the most effective at inducing apoptotic cell death. By critically analyzing the introduced activity of the delivered caspase, the pattern of substrate cleavage, as well as the ability to activate endogenous caspases, we conclude that the efficacy of each caspase correlated with the levels of pro-survival factors that both directly and indirectly impact the introduced caspase. These findings lay the groundwork for developing methods for exogenous introduction of caspases as a therapeutic option that can be tuned to the apoptotic balance in a proliferating cell.

Clustering of Death Receptor for Apoptosis Using Nanoscale Patterns of Peptides

The nanoscale spatial organization of transmembrane tumor necrosis factor (TNF) receptors has been implicated in the regulation of cellular fate. Accordingly, molecular tools that can induce specific arrangements of these receptors on cell surfaces would give us an opportunity to study these effects in detail. To achieve this, we introduce DNA origami nanostructures that precisely scaffold the patterning of TNF-related apoptosis-inducing ligand-mimicking peptides at nanoscale level. Stimulating human breast cancer cells with these patterns, we find that around 5 nm is the critical interligand distance of hexagonally patterned peptides to induce death receptor clustering and a resulting apoptosis. We thus offer a strategy to reverse the non-efficacy of current ligand- and antibody-based methods for TNF superfamily activation.

Synthesis and antitumor potential of new arylidene ursolic acid derivatives via caspase-8 activation

Continuing our studies on NO-donating ursolic acid-benzylidene derivatives as potential antitumor agents, we designed and synthesized a series of new arylidene derivatives containing NO-donating ursolic acid and aromatic heterocyclic units. Compounds 5c and 6c showed a significant broad-spectrum antitumor activity. Compound 5c exhibited nearly three- to nine-fold higher cytotoxicity as compared with the parent drug in A549, MCF-7, HepG-2, HT-29, and HeLa cells, and it was also found to be the most potent apoptosis inducer of MCF-7 cells. More importantly, compound 5c arrested the MCF-7 cell cycle in the G1 phase, which was associated with caspase activation and a decrease of the Bcl-2/Bax ratio. Meanwhile, compound 5c caused changes in morphological features, dissipation of the mitochondrial membrane potential, and accumulation of reactive oxygen species. A docking study revealed that the nitroxyethyl moiety of compound 5c may form hydrogen bonds with caspase-8 amino acid residues (SER256 and HIS255). Together, these data suggest that NO-donating ursolic acid-arylidene derivatives are potent apoptosis inducers in tumor cells.

Identification and functional characterization of SlDronc in Spodoptera littoralis

Background

Apoptosis is responsible for eliminating damaged and virus-infected cells, regulating normal cell turnover, and maintaining the immune system’s development and function. Caspases play a vital role in both mammal and invertebrate apoptosis. Spodoptera littoralis is a generalist insect herbivore that is one of the most destructive pests in tropical and subtropical areas and attacks a wide range of commercially important crops. Although S. littoralis is a model organism in the study of baculovirus infection, its apoptotic pathway has not been explored.

Methods

We cloned a new caspase gene named sldronc in S. littoralis using Rapid Amplification of cDNA Ends (RACE). We then measured caspase activity on synthetic caspase substrates and S. littoralis’ effector caspase. SlDronc’s function in the apoptotic pathway and its interaction with caspase inhibitors were also tested in SL2 cells.

Results

We found that the initiator caspase SlDronc cleaved and activated effector caspase in S. littoralis. SlDronc overexpression induced apoptosis in SL2 cells, and Sldronc knockdown decreased apoptosis induced by UV irradiation in SL2 cells. Our results indicate that SlDronc acts as an apoptotic initiator caspase in S. littoralis. Additionally, we found that processed forms of SlDronc increased in the presence of N-terminally truncated S. littoralis inhibitors of apoptosis (SlIAP) and that SlDronc was inhibited by P49. This study contributes to the further understanding of S. littoralis’ apoptotic pathway and may facilitate future studies on baculovirus infection-induced apoptosis.

Identification and functional characterization of three caspases in Takifugu obscurus in response to bacterial infection

Caspases are evolutionarily conserved proteases, which are inextricably linked with the apoptosis and immune system in mammals. However, the expression pattern and function of some caspases remain largely unknown in pufferfish. In this study, three different pufferfish caspases (caspase-2 (Pfcasp-2), caspase-3 (Pfcasp-3), and caspase-8 (Pfcasp-8)) were characterized, and their expression patterns and functions were determined following Aeromonas hydrophila infection. The open reading frames of Pfcasp-2, -3, and -8 are 1,320, 846, and 1455 bp, respectively. Analyses of sequence alignment and phylogenetic tree showed that casp-2, -3, and -8 share 52%-65%, 33%-40%, 63%-78% overall sequence identities with those of other vertebrates, respectively. 3D structures of Pfcasp-2, -3, and -8 enjoy conservation in core area together, while each owns a distinctive profile. Comparisons of deduced amino acid sequences indicated that Pfcaspases possessed the caspase domain and conserved active sites like 'HG' and 'QACXG' (X for R or G). qRT-PCR results revealed that Pfcasp-2, -3, and -8 were expressed constitutively in a wide range of organs, especially in immune-related organs including whole blood and kidney. In vitro, the expressions of the three caspases (Pfcasp-2, 3, and -8) and immune-related genes (IgM and IL-8) were significantly up-regulated in kidney leukocytes after A. Hydrophila challenge and inhibitors treatment. The expressions of Pfcasp-2 and Pfcasp-3 were successfully inhibited in the kidney leukocytes by Ac-DEVD-CHO (an inhibitor to caspase-3), but the expression of Pfcasp-8 was not affected. Cellular localization analysis showed that the distribution of Pfcasp-2, -3, and -8 was in cytoplasm. Further, overexpression of Pfcasp-2, -3, or -8 was found to cause DNA damage and apoptosis, suggesting that three caspases may be related to apoptosis and mediate different apoptosis pathways in pufferfish. Moreover, the expressions of these caspases were also up-regulated in whole blood and kidney after A. hydrophila challenge, indicating their possible involvement in the immune response against A. hydrophia stimulation. Taken together, the results of this study suggest that the caspase-2,-3, and -8 may play an important role in the apoptosis and immune response in pufferfish.

Nicotine induces apoptosis through exacerbation of blocked alveolar macrophage autophagic degradation in silicosis

Silica dust mainly attacks alveolar macrophages (AMs). The apoptosis of AMs is correlated with the progress of silicosis. Our previous study showed that autophagic degradation was blocked in AMs from silicosis patients. However, the effects of nicotine on AM autophagy and apoptosis in silicosis are unknown. In this study, we collected AMs from twenty male workers exposed to silica and divided them into observer and silicosis patient groups, according to the tuberous pathological changes observed by X-ray. The AMs from both groups were exposed to nicotine. We found increased levels of LC3, p62, and cleaved caspase-3, decreased levels of LAMP2, and damaged lysosomes after nicotine stimulation of the AMs from both groups. We also found that the autophagy inhibitor 3-methyladenine (3MA) inhibited nicotine-induced apoptosis in the AMs. Furthermore, 3MA reversed both the nicotine-induced decrease in Bcl-2 and the increase in Bax in both groups. These results suggest that nicotine may induce apoptosis by blocking AM autophagic degradation in human silicosis.

Trace of survivin in cancer

Survivin is one of the most cancer-specific proteins overexpressed in almost all malignancies, but is nearly undetectable in most normal tissues in adults. Functionally, as a member of the inhibitor of apoptosis family, survivin has been shown to inhibit apoptosis and increase proliferation. The antiapoptotic function of survivin seems to be related to its ability to inhibit caspases directly or indirectly. Furthermore, the role of survivin in cell cycle division control is related to its role in the chromosomal passenger complex. Consistent with its determining role in these processes, survivin plays a crucial role in cancer progression and cancer cell resistance to anticancer drugs and ionizing radiation. On the basis of these findings, recently survivin has been investigated intensively as an ideal tumor biomarker. Thus, multiple molecular approaches such as use of the RNA interfering technique, antisense oligonucleotides, ribozyme, and small molecule inhibitors have been used to downregulate survivin regulation and inhibit its biological function consequently. In this review, all these approaches are explained and other compounds that induced apoptosis in different cell lines through survivin inhibition are also reported.

Caspase-8: The double-edged sword

Caspase-8 is a cysteine - aspartate specific protease that classically triggers the extrinsic apoptotic pathway, in response to the activation of cell surface Death Receptors (DRs) like FAS, TRAIL-R and TNF-R. Besides it's roles in triggering death receptor-mediated apoptosis, Caspase-8 has also been implicated in the onsets of anoikis, autophagy and pyroptosis. Furthermore, Caspase-8 also plays a crucial pro-survival function by inhibiting an alternative form of programmed cell death called necroptosis. Low expression levels of pro-Caspase-8 is therefore associated with the malignant transformation of cancers. However, the long-held notion that pro-Caspase-8 expression/activity is generally lost in most cancers, thereby contributing to apoptotic escape and enhanced resistance to anti-cancer therapeutics, has been found to be true for only a minority of cancers types. In the majority of cases, pro-Caspase-8 expression is maintained and sometimes elevated, while it's apoptotic activity is regulated through different mechanisms. This supports the notion that the non-apoptotic functions of Caspase-8 offer growth advantage in these cancer types and have, therefore, gained renewed interest in the recent years. In light of these reasons, a number of therapeutic approaches have been employed, with the intent of targeting pro-Caspase-8 in cancer cells. In this review, we would attempt to discuss - the classic roles of Caspase-8 in initiating apoptosis; it's non-apoptotic functions; it's the clinical significance in different cancer types; and the therapeutic applications exploiting the ability of pro-Caspase-8 to regulate various cellular functions.

Effects of selenium on the proliferation and apoptosis of sheep spermatogonial stem cells in vitro

The objective of this study was to investigate effects of selenium (Se) on proliferation and apoptosis of sheep spermatogonial stem cells (SSC) in vitro. The SSC were assigned to five treatment groups (0, 2.0, 4.0, 8.0 and 16.0 μmol/L Se). After treatment with Se for 96 h, cell proliferation and apoptosis were evaluated. The relative abundance of P53 mRNA transcript and protein, cell cycle and apoptosis-related genes were detected using real-time PCR and Western blot quantifications, respectively. The results indicate there were the least cell proliferation rates in the Se_16.0 group. Treatments with relatively greater Se concentrations (8.0 and 16.0 μmol/L) resulted in a greater percentage of apoptotic cells, which was consistent with the relative abundances of P53, P21, P27 and pro-apoptosis mRNA transcripts. There were relatively greater ROS concentrations in the control, Se_8.0 and Se_16.0 groups. Compared with the control group, treatment with the Se concentration of 16.0 μmol/L resulted in an increased abundance of P53, P21, P27 and BAX proteins. Treatment with Pifithrin-α suppressed the increase in abundance of P53 and P21 proteins induced by the relatively greater concentration of Se (16.0 μmol/L), however, did not result in a change in abundances of P27 and BAX proteins. These results indicate the regulatory functions of Se on proliferation and apoptosis of sheep SSC is associated with the P21-mediated P53 signalling pathway. The P27 and BAX proteins have limited functions during the apoptotic process of SSC induced by the relatively greater concentrations of Se.

High-Concentrate Feeding to Dairy Cows Induces Apoptosis via the NOD1/Caspase-8 Pathway in Mammary Epithelial Cells

(1) Background: The effects of a high-concentrate (HC) diet in inducing mammary epithelial cell apoptosis in dairy cows via the NOD1/Caspase-8 pathway have never been investigated before the current study. (2) Methods: Twelve Holstein Frisian cows at mid-lactation were selected to conduct this research. The animals were randomly allocated to two groups (n = 6), and both groups received one of two diets: a low-concentrate (LC) (forage: concentrate 6:4) or a high-concentrate (HC) (forage: concentrate 4:6) diet. Furthermore, an enzyme activity assay, tunnel cell assay, RT-qPCR, western blotting, and an immunofluorescence antibody (IFA) assay were performed to elucidate the effect of an HC diet in the mammary gland of dairy cows. (3) Results: The tunnel cell assay revealed a significant number of apoptotic cells in HC group, and the concentration of Caspase-3, and Caspase-8 was higher in the HC group than in the LC group. NOD1, Rip-2, Caspase-3, Caspase-8, Caspase-9, and Bax mRNA expressions, and NOD1, Caspase-3, Caspase-8, and Bax protein expressions, in the HC group were markedly higher than those in the LC group. Furthermore, Bcl-2 mRNA and protein expressions were markedly decreased in the HC compared to those in the LC group. (4) Conclusions: A HC diet fed to dairy cows incites subacute ruminal acidosis (SARA), which increases the iE-DAP concentration and induces apoptosis in the mammary gland via the NOD1/Caspase-8 pathway.

Heterogeneous responses to low level death receptor activation are explained by random molecular assembly of the Caspase-8 activation platform

Ligand binding to death receptors activates apoptosis in cancer cells. Stimulation of death receptors results in the formation of intracellular multiprotein platforms that either activate the apoptotic initiator Caspase-8 to trigger cell death, or signal through kinases to initiate inflammatory and cell survival signalling. Two of these platforms, the Death-Inducing Signalling Complex (DISC) and the RIPoptosome, also initiate necroptosis by building filamentous scaffolds that lead to the activation of mixed lineage kinase domain-like pseudokinase. To explain cell decision making downstream of death receptor activation, we developed a semi-stochastic model of DISC/RIPoptosome formation. The model is a hybrid of a direct Gillespie stochastic simulation algorithm for slow assembly of the RIPoptosome and a deterministic model of downstream caspase activation. The model explains how alterations in the level of death receptor-ligand complexes, their clustering properties and intrinsic molecular fluctuations in RIPoptosome assembly drive heterogeneous dynamics of Caspase-8 activation. The model highlights how kinetic proofreading leads to heterogeneous cell responses and results in fractional cell killing at low levels of receptor stimulation. It reveals that the noise in Caspase-8 activation-exclusively caused by the stochastic molecular assembly of the DISC/RIPoptosome platform-has a key function in extrinsic apoptotic stimuli recognition.

c-FLIP and CD95 signaling are essential for survival of renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC) is the most-prominent tumor type of kidney cancers. Resistance of renal cell carcinoma (RCC) against tumor therapy is often owing to apoptosis resistance, e.g., by overexpression of anti-apoptotic proteins. However, little is known about the role of the apoptosis inhibitor c-FLIP and its potential impact on death receptor-induced apoptosis in ccRCC cells. In this study, we demonstrate that c-FLIP is crucial for resistance against CD95L-induced apoptosis in four ccRCC cell lines. Strikingly, downregulation of c-FLIP expression by short hairpin RNA (shRNA)interference led to spontaneous caspase activation and apoptotic cell death. Of note, knockdown of all c-FLIP splice variants was required to induce apoptosis. Stimulation of ccRCC cells with CD95L induced NF-κB and MAP kinase survival pathways as revealed by phosphorylation of RelA/p65 and Erk1/2. Interestingly, CD95L surface expression was high in all cell lines analyzed, and CD95 but not TNF-R1 clustered at cell contact sites. Downstream of CD95, inhibition of the NF-κB pathway led to spontaneous cell death. Surprisingly, knockdown experiments revealed that c-FLIP inhibits NF-κB activation in the context of CD95 signaling. Thus, c-FLIP inhibits apoptosis and dampens NF-κB downstream of CD95 but allows NF-κB activation to a level sufficient for ccRCC cell survival. In summary, we demonstrate a complex CD95-FLIP-NF-κB-signaling circuit, in which CD95-CD95L interactions mediate a paracrine survival signal in ccRCC cells with c-FLIP and NF-κB both being required for inhibiting cell death and ensuring survival. Our findings might lead to novel therapeutic approaches of RCC by circumventing apoptosis resistance.

Cucurbit[8]uril Reactivation of an Inactivated Caspase-8 Mutant Reveals Differentiated Enzymatic Substrate Processing

Caspase-8 constructs featuring an N-terminal FGG sequence allow for selective twofold recognition by cucurbit[8]uril, which leads to an increase of the enzymatic activity in a cucurbit[8]uril dose-dependent manner. This supramolecular switching has enabled for the first time the study of the same caspase-8 in its two extreme states; as full monomer and as cucurbit[8]uril induced dimer. A mutated, fully monomeric caspase-8 (D384A), which is enzymatically inactive towards its natural substrate caspase-3, could be fully reactivated upon addition of cucurbit[8]uril. In its monomeric state caspase-8 (D384A) still processes a small synthetic substrate, but not the natural caspase-3 substrate, highlighting the close interplay between protein dimerization and active site rearrangement for substrate selectivity. The ability to switch the caspase-8 activity by a supramolecular system thus provides a flexible approach to studying the activity of a protein at different oligomerization states.

C-27-carboxylated oleanane triterpenoids up-regulate TRAIL DISC assembly via p38 MAPK and CHOP-mediated DR5 expression in human glioblastoma cells

Despite recent tremendous progress, targeting of TNF-related apoptosis-inducing ligand (TRAIL) as a cancer therapy has limited success in many clinical trials, in part due to inactivation of death inducing signaling complex (DISC)-mediated caspase-8 signaling cascade in highly malignant tumors such as glioblastoma. In this study, screening of constituents derived from Astilbe rivularis for TRAIL-sensitizing activity identified C-27-carboxylated oleanolic acid derivatives (C27OAs) including 3β-hydroxyolean-12-en-27-oic acid (C27OA-1), 3β,6β,7α-trihydroxyolean-12-en-27-oic acid (C27OA-2), and 3β-trans-p-coumaroyloxy-olean-12-en-27-oic acid (C27OA-3) as novel TRAIL sensitizers. Interestingly, these C27OAs did not affect apoptotic cell death induced by either ligation of other death receptor (DR) types, such as TNF and Fas or DNA damaging agents, which suggests that C27OAs effectively and selectively sensitize TRAIL-mediated caspase-8 activation. Mechanistically, C27OAs upregulate the expression of cell surface DR5 and DISC formation without affecting downstream intracellular apoptosis-related proteins. The upregulation of DR5 expression by C27OAs strictly depends on transactivation of C/EBP homology protein, which is regulated through the p38 MAPK pathway, rather than p53 and intracellular reactive oxygen species status. Taken together, our results identify the novel C27OAs as TRAIL sensitizers targeting the upstream DISC assembly of DR5, and provide a rationale for further development of C27OAs for facilitating TRAIL-based chemotherapy in glioblastoma patients.

Elevated A20 promotes TNF-induced and RIPK1-dependent intestinal epithelial cell death

Intestinal epithelial cell (IEC) death is a common feature of inflammatory bowel disease (IBD) that triggers inflammation by compromising barrier integrity. In many patients with IBD, epithelial damage and inflammation are TNF-dependent. Elevated TNF production in IBD is accompanied by increased expression of the TNFAIP3 gene, which encodes A20, a negative feedback regulator of NF-κB. A20 in intestinal epithelium from patients with IBD coincided with the presence of cleaved caspase-3, and A20 transgenic (Tg) mice, in which A20 is expressed from an IEC-specific promoter, were highly susceptible to TNF-induced IEC death, intestinal damage, and shock. A20-expressing intestinal organoids were also susceptible to TNF-induced death, demonstrating that enhanced TNF-induced apoptosis was a cell-autonomous property of A20. This effect was dependent on Receptor Interacting Protein Kinase 1 (RIPK1) activity, and A20 was found to associate with the Ripoptosome complex, potentiating its ability to activate caspase-8. A20-potentiated RIPK1-dependent apoptosis did not require the A20 deubiquitinase (DUB) domain and zinc finger 4 (ZnF4), which mediate NF-κB inhibition in fibroblasts, but was strictly dependent on ZnF7 and A20 dimerization. We suggest that A20 dimers bind linear ubiquitin to stabilize the Ripoptosome and potentiate its apoptosis-inducing activity.

Molecular basis of dimerization of initiator caspase was revealed by crystal structure of caspase-8 pro-domain

The assembly of death-inducing signaling complex (DISC) for activation of initiator caspase is a key step for the receptor-mediated apoptosis signaling. Many death effector domain (DED)-containing proteins are involved in DISC assembly and controlling. One of the main DISC component, caspase-8, contains DED and DED-mediated dimerization and oligomerization in the DISC is critical for the activation of this initiator caspase. There have been intensive studies to understand DED-mediated dimerization and oligomerization for the DISC assembly but no clear answer has been provided and there are many controversial arguments. Here, we suggested novel dimerization process of tandem DED of caspase-8 with crystallographic study.

Multiple Mechanisms of Zinc-Mediated Inhibition for the Apoptotic Caspases-3, -6, -7, and -8

Zinc is emerging as a widely used and important biological regulatory signal. Cellular zinc levels are tightly regulated by a complex array of zinc importers and exporters to control processes such as apoptotic cell death. While caspase inhibition by zinc has been reported previously, the reported inhibition constants were too weak to suggest a critical biological role for zinc-mediated inhibition. In this work, we have adopted a method of assessing available zinc. This allowed assessment of accurate inhibition constants for apoptotic caspases, caspase-3, -6, -7, and -8. Each of these caspases are inhibited by zinc at intracellular levels but with widely differing inhibition constants and different zinc binding stoichiometries. Caspase-3, -6, and -8 appear to be constitutively inhibited by typical zinc levels, and this inhibition must be lifted to allow activation. The inhibition constant for caspase-7 (76 nM) is much weaker than for the other apoptotic caspases (2.6-6.9 nM) suggesting that caspase-7 is not inactivated by normal zinc concentrations but can be inhibited under conditions of zinc stress. Caspase-3, -7, and -8 were found to bind three, one, and two zincs, respectively. In each of these caspases, zinc was present in the active site, in contrast to caspase-6, which binds one zinc allosterically. The most notable new mechanism to emerge from this work is for zinc-mediated inhibition of caspase-8. Zinc binds caspase-8 directly at the active site and at a second site. Zinc binding inhibits formation of the caspase-8 dimer, the activated form of the enzyme. Together these findings suggest that zinc plays a critical role in regulation of apoptosis by direct inactivation of caspases, in a manner that is unique for each caspase.

Mitochondrial pathways are involved in Eimeria tenella-induced apoptosis of chick embryo cecal epithelial cells

Accumulating evidence suggests that Eimeria tenella severely damages the intestinal mucosa in infected poultry, resulting in deadly haemorrhagic typhlocolitis and major economic losses. Damage to host tissue is believed to arise mainly from apoptosis, which is, in general, intimately related to mitochondrial function. However, it is unclear whether mitochondria-dependent apoptotic pathways are specifically involved in parasite-induced apoptosis of chick embryo cecal epithelial cells. Because the mitochondrial permeability transition pore (MPTP) and caspase-9 are important elements in these pathways, we studied the effects of their respective inhibitors (i.e., cyclosporine A [CsA] and Z-LEHD-FMK, respectively) in primary cultures of chicken embryonic cecum epithelial cells using histopathological techniques, terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assays, flow cytometry (FCM) and ELISA. Results indicated that the inhibitors significantly decreased (p < 0.01) DNA injury, apoptosis and caspase-9 and caspase-3 activity of chick embryo cecal epithelial cells at 24, 48, 72, 96 and 120 h after E. tenella infection. Thus, our data supported that mitochondria-dependent apoptotic pathways were involved in apoptosis of parasitised chick embryo cecal epithelial cells.

Discovery of oral-available resveratrol-caffeic acid based hybrids inhibiting acetylated and phosphorylated STAT3 protein

Constitutive activation of STAT3 has been found in a wide variety of cancers and demonstrated as a very attractive therapeutic target. Disrupting both acetylation and phosphorylation of STAT3 protein was hypothesized to greatly deactivate STAT3, therefore, treating cancers. To demonstrate the hypothesis, two series of novel resveratrol-caffeic acid hybrids were designed aiming to regulate both acetylation and phosphorylation of STAT3 protein, which is also the first report of the synthetic inhibitors simultaneously regulating two biological reactions of STAT3 to our knowledge. Most of these compounds were demonstrated with preferential antitumor activity with low IC₅₀ values against two cancer cell lines. Particularly, compound 7d was found as an excellent STAT3 inhibitor with over 50-fold better potency than resveratrol and caffeic acid. Meanwhile, the novel derivatives significantly inhibited the proliferation and induced the apoptosis of tumor cells. Molecular docking further disclosed the binding modes of STAT3 with the inhibitors. In addition, compound 7d orally and significantly suppressed breast cancer 4T1 xenograft tumor growth in vivo, indicating its great potential as an efficacious drug candidate for human cancer therapy.

Activity of Uncleaved Caspase-8 Controls Anti-bacterial Immune Defense and TLR-Induced Cytokine Production Independent of Cell Death

Caspases regulate cell death programs in response to environmental stresses, including infection and inflammation, and are therefore critical for the proper operation of the mammalian immune system. Caspase-8 is necessary for optimal production of inflammatory cytokines and host defense against infection by multiple pathogens including Yersinia, but whether this is due to death of infected cells or an intrinsic role of caspase-8 in TLR-induced gene expression is unknown. Caspase-8 activation at death signaling complexes results in its autoprocessing and subsequent cleavage and activation of its downstream apoptotic targets. Whether caspase-8 activity is also important for inflammatory gene expression during bacterial infection has not been investigated. Here, we report that caspase-8 plays an essential cell-intrinsic role in innate inflammatory cytokine production in vivo during Yersinia infection. Unexpectedly, we found that caspase-8 enzymatic activity regulates gene expression in response to bacterial infection as well as TLR signaling independently of apoptosis. Using newly-generated mice in which caspase-8 autoprocessing is ablated (Casp8^DA/DA), we now demonstrate that caspase-8 enzymatic activity, but not autoprocessing, mediates induction of inflammatory cytokines by bacterial infection and a wide variety of TLR stimuli. Because unprocessed caspase-8 functions in an enzymatic complex with its homolog cFLIP, our findings implicate the caspase-8/cFLIP heterodimer in control of inflammatory cytokines during microbial infection, and provide new insight into regulation of antibacterial immune defense.

TLR signaling induces expression of key inflammatory cytokines and pro-survival factors that facilitate control of microbial infection. TLR signaling can also engage cell death pathways through activation of enzymes known as caspases. Caspase-8 activates apoptosis in response to infection by pathogens that interfere with NF-κB signaling, including Yersinia, but has also recently been linked to control of inflammatory gene expression. Pathogenic Yersinia can cause severe disease ranging from gastroenteritis to plague. While caspase-8 mediates cell death in response to Yersinia infection as well as other signals, its precise role in gene expression and host defense during in vivo infection is unknown. Here, we show that caspase-8 activity promotes cell-intrinsic cytokine expression, independent of its role in cell death in response to Yersinia infection. Our studies further demonstrate that caspase-8 enzymatic activity plays a previously undescribed role in ensuring optimal TLR-induced gene expression by innate cells during bacterial infection. This work sheds new light on mechanisms that regulate essential innate anti-bacterial immune defense.

Death Receptor 5 Networks Require Membrane Cholesterol for Proper Structure and Function

Death receptor 5 (DR5) is an apoptosis-inducing member of the tumor necrosis factor receptor superfamily, whose activity has been linked to membrane cholesterol content. Upon ligand binding, DR5 forms large clusters within the plasma membrane that have often been assumed to be manifestations of receptor co-localization in cholesterol-rich membrane domains. However, we have recently shown that DR5 clusters are more than just randomly aggregated receptors. Instead, these are highly structured networks held together by receptor dimers. These dimers are stabilized by specific transmembrane helix-helix interactions, including a disulfide bond in the long isoform of the receptor. The complex relationships among DR5 network formation, transmembrane helix dimerization, membrane cholesterol, and receptor activity has not been established. It is unknown whether the membrane itself plays an active role in driving DR5 transmembrane helix interactions or in the formation of the networks. We show that cholesterol depletion in cells does not inhibit the formation of DR5 networks. However, the networks that form in cholesterol-depleted cells fail to induce caspase cleavage. These results suggest a potential structural difference between active and inactive networks. As evidence, we show that cholesterol is necessary for the covalent dimerization of DR5 transmembrane domains. Molecular simulations and experiments in synthetic vesicles on the DR5 transmembrane dimer suggest that dimerization is facilitated by increased helicity in a thicker bilayer.

Fas-Fas Ligand: Checkpoint of T Cell Functions in Multiple Sclerosis

Fas and Fas Ligand (FasL) are two molecules involved in the regulation of cell death. Their interaction leads to apoptosis of thymocytes that fail to rearrange correctly their T cell receptor (TCR) genes and of those that recognize self-antigens, a process called negative selection; moreover, Fas–FasL interaction leads to activation-induced cell death, a form of apoptosis induced by repeated TCR stimulation, responsible for the peripheral deletion of activated T cells. Both control mechanisms are particularly relevant in the context of autoimmune diseases, such as multiple sclerosis (MS), where T cells exert an immune response against self-antigens. This concept is well demonstrated by the development of autoimmune diseases in mice and humans with defects in Fas or FasL. In recent years, several new aspects of T cell functions in MS have been elucidated, such as the pathogenic role of T helper (Th) 17 cells and the protective role of T regulatory (Treg) cells. Thus, in this review, we summarize the role of the Fas–FasL pathway, with particular focus on its involvement in MS. We then discuss recent advances concerning the role of Fas–FasL in regulating Th17 and Treg cells’ functions, in the context of MS.

Sestrin2 facilitates death receptor-induced apoptosis in lung adenocarcinoma cells through regulation of XIAP degradation

Apoptosis plays a critical physiological role in controlling cell number and eliminating damaged, non-functional and transformed cells. Cancerous cells as well as some types of normal cells are often resistant to cell death induced by pro-inflammatory cytokines through death receptors. This potentially allows cancer cells to evade the control from the immune system and to proceed toward a more malignant stage, although the mechanisms of this evasion are not well established. We have recently identified the stress-responsive Sestrin2 protein as a critical regulator of cell viability under stress conditions. Sestrin2 is a member of a small family of antioxidant proteins and inhibitors of mechanistic Target of Rapamycin Complex 1 (mTORC1) kinase. Down-regulation of Sestrin1/2 leads to genetic instability and accelerates the growth of lung adenocarcinoma xenografts. Here we addressed the potential role of Sestrin2 in regulation of cell death induced by TNFR1 and related Fas and TRAIL receptors in lung adenocarcinoma cells. We found that Sestrin2 silencing strongly inhibits cytokine-induced cell death through a mechanism independent of ROS and mTORC1 regulation. We determined that the X-linked inhibitor of apoptosis protein (XIAP) plays a critical role in the control of cytokine-induced cell death by Sestrin2. Thus our study defines a new, previously unrecognized role of Sestrin2 in the regulation of apoptosis.

Tyrosine Phosphorylation of Caspase-8 Abrogates Its Apoptotic Activity and Promotes Activation of c-Src

Src family tyrosine kinases (SFKs) phosphorylate caspase-8A at tyrosine (Y) 397 resulting in suppression of apoptosis. In addition, the phosphorylation of caspase-8A at other sites including Y465 has been implicated in the regulation of caspase-8 activity. However, the functional consequences of these modifications on caspase-8 processing/activity have not been elucidated. Moreover, various Src substrates are known to act as potent Src regulators, but no such role has been explored for caspase-8. We asked whether the newly identified caspase-8 phosphorylation sites might regulate caspase-8 activation and conversely, whether caspase-8 phosphorylation might affect Src activity. Here we show that Src phosphorylates caspase-8A at multiple tyrosine sites; of these, we have focused on Y397 within the linker region and Y465 within the p12 subunit of caspase-8A. We show that phosphomimetic mutation of caspase-8A at Y465 prevents its cleavage and the subsequent activation of caspase-3 and suppresses apoptosis. Furthermore, simultaneous phosphomimetic mutation of caspase-8A at Y397 and Y465 promotes the phosphorylation of c-Src at Y416 and increases c-Src activity. Finally, we demonstrate that caspase-8 activity prevents its own tyrosine phosphorylation by Src. Together these data reveal that dual phosphorylation converts caspase-8 from a pro-apoptotic to a pro-survival mediator. Specifically, tyrosine phosphorylation by Src renders caspase-8 uncleavable and thereby inactive, and at the same time converts it to a Src activator. This novel dynamic interplay between Src and caspase-8 likely acts as a potent signal-integrating switch directing the cell towards apoptosis or survival.

Structural Insight for Roles of DR5 Death Domain Mutations on Oligomerization of DR5 Death Domain-FADD Complex in the Death-Inducing Signaling Complex Formation: A Computational Study

Death receptor 5 (DR5)-induced apoptosis that prioritizes the death of tumor cells has been proposed as one of the promising cancer therapies. In this process, oligomerized DR5 death domain (DD) binding to Fas-associated death domain (FADD) leads to FADD activating caspase-8, which marks the formation of the death-inducing signaling complex (DISC) that initiates apoptosis. DR5 DD mutations found in cancer cells have been suggested to play an important pathological role, the mechanism through which those mutants prevent the DR5-activated DISC formation is not clear yet. This study sought to provide structural and molecular insight for the roles of four selected DR5 DD mutations (E355K, E367K, K415N, and L363F) in the oligomerization of DR5 DD-FADD complex during the DISC formation. Results from the molecular dynamics simulations show that the simulated mutants induce conformational, dynamical motions and interactions changes in the DR5 DD-FADD tetramer complex, including changes in a protein's backbone flexibility, less exposure of FADD DED's caspase-8 binding site, reduced H-bonding and hydrophobic contacts at the DR5 DD-FADD DD binding, altered distribution of the electrostatic potentials and correlated motions of residues, and reduced binding affinity of DR5 DD binding to FADD. This study provides structural and molecular insight for the influence of DR5 DD mutations on oligomerization of DR5 DD-FADD complex, which is expected to foster understanding of the DR5 DD mutants' resistance mechanism against DR5-activated DISC formation.

Multifunctional murrel caspase 1, 2, 3, 8 and 9: Conservation, uniqueness and their pathogen-induced expression pattern

Caspases are evolutionarily conserved proteases which play fundamental role in apoptosis. Invasion of pathogen triggers the activation of caspases-mediated pro-inflammatory and pro-apoptotic pathways, where multifunctional caspases are involved. In striped murrel Channa striatus, epizootic ulcerative syndrome (EUS) causes endemics resulting in huge economic loss. Aphanomyces invadans, an oomycete is the primary causative agent of EUS which further induces secondary bacterial infections especially Aeromonas hydrophila. In order to get insights into the caspase gene family in C. striatus during EUS infection, we performed various physicochemical and structural analyses on the cDNA and protein sequences of five different murrel caspases namely CsCasp 1, 2, 3, 8 and 9. Sequence analysis of murrel caspase proteins showed that in spite of the conserved CASC domain, each caspase embraces some unique features which made them functionally different. Tissue distribution analysis showed that all the murrel caspases are highly expressed in one of the immune organs such as liver, kidney, spleen and blood cells. Further, to understand the role of caspase during EUS infection, modulation in expression of each caspase gene was analysed after inducing fungal and bacterial infection in C. striatus. Pathogen-induced gene expression pattern revealed an interesting fact that the expression of all the caspase genes reached a maximum level at 24 h post-infection (p.i) in case of bacteria, whereas it was 48 h in fungus. However, the initiation of elevated expression differed between each caspase based on their role such as pro-inflammatory, initiator and executioner caspase. Overall, the results suggested that the caspases in murrel are diverse in their structure and function. Here, we discuss the similarities and differences of five different murrel caspases.

Surviving apoptosis: life-death signaling in single cells

Tissue development and homeostasis are regulated by opposing pro-survival and pro-death signals. An interesting feature of the Tumor Necrosis Factor (TNF) family of ligands is that they simultaneously activate opposing signals within a single cell via the same ligand-receptor complex. The magnitude of pro-death events such as caspase activation and pro-survival events such as Nuclear Factor (NF)-κB activation vary not only from one cell type to the next but also among individual cells of the same type due to intrinsic and extrinsic noise. The molecules involved in these pro-survival and/or pro-death pathways, and the different phenotypes that result from their activities, have been recently reviewed. Here we focus on the impact of cell-to-cell variability in the strength of these opposing signals on shaping cell fate decisions.

Effects of dietary selenium on apoptosis of germ cells in the testis during spermatogenesis in roosters

The aim of this study was to determine the effects of dietary selenium (Se) supplementation on apoptosis of germ cells in the testis during spermatogenesis in roosters. Eighty 12-week-old Hy-Line Variety white roosters with an averaged body weight of 1.38 ± 0.2 kg were selected and randomly divided into four experimental groups. They were fed the basal diet (0.044 mg/kg Se dry matter) supplemented with 0 (control), 0.5, 1.0, or 2.0 mg/kg of Se dry matter (from sodium selenite). After the 45-day feeding experiment, testis samples were collected from the roosters of each treatment group to detect the population of apoptotic germ cells using the terminal deoxynucleotidy1 transferase dUTP nick end labeling assay. The protein expression of cell cycle-related genes and the messenger RNA (mRNA) expression of apoptosis and cell cycle-related genes had also been detected. The results show that the population of apoptotic germ cells in the control and 2.0 mg/kg groups was increased (P < 0.05) compared with that in the 0.5 mg/kg and 1.0 mg/kg groups. Expressions of CDC2 and CCNB1 protein in the control and 2.0 mg/kg groups were lower (P < 0.05) than those in the 0.5 mg/kg and 1.0 mg/kg groups. The mRNA level of CDC2 in the 0.5 mg/kg group was higher (P < 0.05) than that in other groups. The lowest (P < 0.05) mRNA expressions of apoptosis-related genes (BCL-2, CASPASE 3, CASPASE 8) were also obtained in the 0.5 mg/kg group. These results show that dietary Se of roosters can affect apoptosis of germ cells by regulating the mRNA expressions of apoptosis- and cell cycle-related genes in the testis during spermatogenesis.

Structure and mechanism of an aspartimide-dependent peptide ligase in human legumain

Peptide ligases expand the repertoire of genetically encoded protein architectures by synthesizing new peptide bonds, energetically driven by ATP or NTPs. Here, we report the discovery of a genuine ligase activity in human legumain (AEP) which has important roles in immunity and tumor progression that were believed to be due to its established cysteine protease activity. Defying dogma, the ligase reaction is independent of the catalytic cysteine but exploits an endogenous energy reservoir that results from the conversion of a conserved aspartate to a metastable aspartimide. Legumain's dual protease-ligase activities are pH- and thus localization controlled, dominating at acidic and neutral pH, respectively. Their relevance includes reversible on-off switching of cystatin inhibitors and enzyme (in)activation, and may affect the generation of three-dimensional MHC epitopes. The aspartate-aspartimide (succinimide) pair represents a new paradigm of coupling endergonic reactions in ATP-scarce environments.

Baicalein induces apoptosis of human cervical cancer HeLa cells in vitro

A number of studies have shown that baicalein shows high antitumor activity in vitro and in vivo. In this study, the inhibitory effect of baicalein on human cervical cancer HeLa cells was studied in vitro. HeLa cells were treated with high (100 µg/ml) and low (50 µg/ml) doses of baicalein, and cell growth inhibition rates were examined by the MTT assay. The morphological changes of apoptotic cells were observed under the light and electron microscope, while the rate of cell apoptosis was examined by flow cytometry. The expression of apoptosis-related proteins was analyzed by western blot, and caspase-3 activation was examined by a caspase-3 activity assay and spectrophotometry. The results demonstrated that baicalein inhibits the proliferation of HeLa cells and induces apoptosis in a caspase-3-dependent pathway, through downregulation of the B-cell lymphoma 2 (Bcl-2) protein and upregulation of the Bcl-2-associated X protein (Bax), Fas, Fas ligand (FasL) and caspase-8. Thus, we conclude that baicalein induces apoptosis of HeLa cells via the mitochondrial and the death receptor pathways. Cell apoptosis in HeLa cells was most likely promoted by the activation of the proteolytic enzyme caspase-3 in both pathways.

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.

Necroptosis in health and diseases

Necroptosis is a form of regulated necrosis that can be activated by ligands of death receptors and stimuli that induce the expression of death receptor ligands under apoptotic deficient conditions. Activation of necroptosis by ligands of death receptors requires the kinase activity of RIP1, which mediates the activation of RIP3 and MLKL, two critical downstream mediators of necroptosis. Blocking the kinase activity of RIP1, a key druggable target in the necroptosis pathway, by necrostatins inhibits the activation of necroptosis and allows cell survival and proliferation in the presence of death receptor ligands. The activation of necroptosis is modulated by different forms of ubiquitination, including K63, linear and K48 ubiquitination, as well as phosphorylation of RIP1, RIP3 and MLKL. Necroptosis is suppressed by caspase-8/FADD-mediated apoptosis. Deficiency in caspase-8 and FADD leads to embryonic lethality, tissue degeneration and inflammation which can be suppressed by inhibition of RIP1 kinase and RIP3. On the other hand, the lack of RIP3 kinase activity leads to early embryonic lethality which can be suppressed by the loss of caspase-8, suggesting that although the kinase activity of RIP3 is involved in mediating necroptosis, the basal activity of RIP3 kinase may be required for suppressing caspase-8 mediated apoptosis. Necroptosis as well as RIP1- and RIP3-mediated inflammatory response have been implicated in mediating multiple human diseases including TNF-mediated hypothermia and systemic inflammation, ischemic reperfusion injury, neurodegeneration, Gaucher's disease, progressive atherosclerotic lesions, etc. Targeting RIP1 kinase may provide therapeutic benefits for the treatment of human diseases characterized by necrosis and inflammation.

Necroptotic signaling in adaptive and innate immunity

The vertebrate immune system is highly dependent on cell death for efficient responsiveness to microbial pathogens and oncogenically transformed cells. Cell death pathways are vital to the function of many immune cell types during innate, humoral and cellular immune responses. In addition, cell death regulation is imperative for proper adaptive immune self-tolerance and homeostasis. While apoptosis has been found to be involved in several of these roles in immunity, recent data demonstrate that alternative cell death pathways are required. Here, we describe the involvement of a programmed form of cellular necrosis called "necroptosis" in immunity. We consider the signaling pathways that promote necroptosis downstream of death receptors, type I transmembrane proteins of the tumor necrosis factor (TNF) receptor family. The involvement of necroptotic signaling through a "RIPoptosome" assembled in response to innate immune stimuli or genotoxic stress is described. We also characterize the induction of necroptosis following antigenic stimulation in T cells lacking caspase-8 or FADD function. While necroptotic signaling remains poorly understood, it is clear that this pathway is an essential component to effective vertebrate immunity.

Supplementation with conjugated linoeic acid decreases pig back fat deposition by inducing adipocyte apoptosis

Background

Conjugated linoleic acid (CLA), a C18 fatty acid with conjugated double bonds, has been shown to serve as a powerful anti-obesity agent by several research groups, although the precise mechanism remains elusive. Previous studies showed that CLA induced apoptosis in 3T3-L1 cells and in mice. The aim of this research was to clarify the role of CLA in adipocyte apoptosis in pigs, a relevant model for obesity research.

Results

Our results clearly show that back fat deposition of CLA-fed pigs was significantly lower than that of pigs in the control group. Moreover, some typical apoptotic cells were observed among the adipocytes of CLA-fed pigs. Furthermore, the CLA-fed pigs had reduced expression of the anti-apoptosis factor Bcl-2 and increased expression of the pro-apoptosis factors Bax and P53. Subsequently, increased cytochrome C was released from the mitochondria to the endochylema, and the caspase cascade was activated, resulting in cellular apoptosis. These results are consistent with the effects of Bcl-2 and Bax in regulating CLA-induced adipocyte apoptosis via the mitochondrial signaling pathway. However, the increased expression of tumor necrosis factor (TNF)-α and its receptor TNFR indicate that the effect of CLA might partly be through the death receptor signaling pathway in adipose cells.

Conclusions

Our study has demonstrated that CLA reduces pig body fat deposition, an outcome that is partly meditated by apoptosis of adipose cells, and that both the mitochondrial pathway and the death receptor pathway are involved in this effect.

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.

Constitutive expression of murine c-FLIPR causes autoimmunity in aged mice

Death receptor-mediated apoptosis is a key mechanism for the control of immune responses and dysregulation of this pathway may lead to autoimmunity. Cellular FLICE-inhibitory proteins (c-FLIPs) are known as inhibitors of death receptor-mediated apoptosis. The only short murine c-FLIP splice variant is c-FLIP_Raji (c-FLIP_R). To investigate the functional role of c-FLIP_R in the immune system, we used the vavFLIP_R mouse model constitutively expressing murine c-FLIP_R in all hematopoietic compartments. Lymphocytes from these mice are protected against CD95-mediated apoptosis and activation-induced cell death. Young vavFLIP_R mice display normal lymphocyte compartments, but the lymphocyte populations alter with age. We identified reduced levels of T cells and slightly higher levels of B cells in 1-year-old vavFLIP_R mice compared with wild-type (WT) littermates. Moreover, both B and T cells from aged vavFLIP_R animals show activated phenotypes. Sera from 1-year-old WT and transgenic animals were analysed for anti-nuclear antibodies. Notably, elevated titres of these autoantibodies were detected in vavFLIP_R sera. Furthermore, tissue damage in kidneys and lungs from aged vavFLIP_R animals was observed, indicating that vavFLIP_R mice develop a systemic lupus erythematosus-like phenotype with age. Taken together, these data suggest that c-FLIP_R is an important modulator of apoptosis and enforced expression leads to autoimmunity.

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.