In vitro activation of CPP32 and Mch3 by Mch4, a novel human apoptotic cysteine protease containing two FADD-like domains

Role of T cell death in maintaining immune tolerance during persistent viral hepatitis

Virus-specific T cells play an important role in the resolution of hepatic infection. However, during chronic hepatitis infection these cells lack their effector functions and fail to control the virus. Hepatitis B virus and hepatitis C virus have developed several mechanisms to generate immune tolerance. One of these strategies is the depletion of virus-specific T cells by apoptosis. The immunotolerogenic liver has unique property to retain and activate naïve T cell to avoid the over reactivation of immune response against antigens which is exploited by hepatotropic viruses to persist. The deletion of the virus-specific T cells occurs by intrinsic (passive) apoptotic mechanism. The pro-apoptotic molecule Bcl-2 interacting mediator (Bim) has attracted increasing attention as a pivotal involvement in apoptosis, as a regulator of tissue homeostasis and an enhancer for the viral persistence. Here, we reviewed our current knowledge on the evidence showing critical role of Bim in viral-specific T cell death by apoptotic pathways and helps in the immune tolerance.

Caspases playing in the field of neuroinflammation: old and new players

Neuroinflammation is a complex immune response against the harmful effects of diverse stimuli within the central nervous system. Caspases are a family of intracellular cysteine proteases that mediate proteolytic events indispensable for transduction of signaling pathway-controlling biological phenomena such as apoptosis and inflammation. To date, 14 players have been identified in mammals. For many years, caspases were simply divided into 'apoptotic' and 'proinflammatory' caspases and this classification remains useful to some extent. However, increasing evidence indicates that many of these so-called apoptotic caspases also exert nonapoptotic functions. In addition, the role of certain members of the supposed inflammatory caspases in the inflammatory process per se has also been discussed. In this review, we highlight the role for 'apoptotic' and 'proinflammatory' caspases in the regulation of the inflammation response with a special focus on the central nervous system.

Induction of selective cytotoxicity and apoptosis in human T4-lymphoblastoid cell line (CEMss) by boesenbergin a isolated from boesenbergia rotunda rhizomes involves mitochondrial pathway, activation of caspase 3 and G2/M phase cell cycle arrest

Background

Boesenbergia rotunda (Roxb.) Schlecht (family zingiberaceae) is a rhizomatous herb that is distributed from north-eastern India to south-east Asia, especially in Indonesia, Thailand and Malaysia. Previous research has shown that the crude extract of this plant has cytotoxic properties. The current study examines the cytotoxic properties of boesenbergin A isolated from Boesenbergia rotunda.

Methods

MTT assay was used to check the cytotoxicity of boesenbergin A. The morphological assessment of apoptosis was monitored using normal and fluorescence microscopy. The early and late phase of apoptosis was investigated using annexin V and DNA laddering assays, respectively. The mitochondrial membrane potential (MMP) was assessed by fluorescence microscopy. Human apoptosis proteome profiler assays were performed to investigate the mechanism of cell death. In addition, the protein levels of Bax, Bcl2 and HSP 70 were also analyzed using western blot. Assays of caspase =-3/7, -8 and =-9 were carried out in order to test for induction during treatment. Lastly, cell cycle progression was analyzed using flow cytometry.

Results

Boesenbergin A was found to have the highest toxicity towards CEMss cancer cells (IC₅₀ = 8 μg/ml). The morphology of CEMss cells after treatment showed evidence of apoptosis that included blebbing and chromatin condensation. The annexin V assay revealed that early apoptosis is induced after treatment. The DNA laddering assay confirmed that DNA fragmentation had occurred during late apoptosis. The cell cycle analysis indicated that boesenbergin A was able to induce G2/M phase arrest in CEMss cells. The activity of caspases -3/7, -8 and -9 was increased after treatment which indicates both intrinsic and extrinsic pathways are induced during apoptosis. The involvement of mitochondria was established by increased mitochondrial membrane potential and up and down regulation of Bcl2 and Bax proteins as well as HSP70.

Conclusion

In conclusion, the results demonstrated that boesenbergin A induced apoptosis of CEMss cells through Bcl2/Bax signaling pathways with the involvement of caspases and G2/M phase cell cycle arrest. The current findings warrant further research on boesenbergin A as a novel chemotherapeutic agent for leukemia intervention including studies in animal models.

Parkin induces apoptotic cell death in TNF-α-treated cervical cancer cells

Many malignant tumors become resistant to tumor necrosis factor-alpha (TNF-α)-induced cell death during carcinogenesis. In the present study, we examined whether parkin acts as a tumor suppressor in HeLa cells, a human cervical cancer cell line resistant to TNF-α-induced cell death. TNF-α-treatment alone did not affect HeLa cell viability. However, expression of parkin restored TNF-α-induced apoptosis in HeLa cells. Increased cell death was due to the activation of the apoptotic pathway. Expression of parkin in TNF-α-treated HeLa cells stimulated cleavage of the pro-apoptotic proteins caspase-8, -9, -3, -7 and poly ADP ribose polymerase (PARP). In addition, parkin expression resulted in decreased expression of the caspase inhibitory protein, survivin. These results suggest that parkin acts as a tumor suppressor in human cervical cancer cells by modulating survivin expression and caspase activity. We propose that this pathway is a novel molecular mechanism by which parkin functions as a tumor suppressor.

An upstream initiator caspase 10 of snakehead murrel Channa striatus, containing DED, p20 and p10 subunits: molecular cloning, gene expression and proteolytic activity

Caspase 10 (CsCasp10) was identified from a constructed cDNA library of freshwater murrel (otherwise called snakehead) Channa striatus. The CsCasp10 is 1838 base pairs (bp) in length and it is encoding 549 amino acid (aa) residues. CsCasp10 amino acid contains two death effector domains (DED) in the N-terminal at 2-77 and 87-154 and it contains caspase family p20 domain (large subunit) and caspase family p10 domain (small subunit) in the C-terminal at 299-425 and 449-536 respectively. Pairwise analysis of CsCasp10 showed the highest sequence similarity (79%) with caspase 10 of Paralichthys olivaceus. Moreover, the phylogenetic analysis showed that CsCasp10 is clustered together with other fish caspase 10, formed a sister group with caspase 10 from other lower vertebrates including amphibian, reptile and birds and finally clustered together with higher vertebrates such as mammals. Significantly (P < 0.05) highest CsCasp10 gene expression was noticed in gills and lowest in intestine. Furthermore, the CsCasp10 gene expression in C. striatus was up-regulated in gills by fungus Aphanomyces invadans and bacteria Aeromonas hydrophila induction. The proteolytic activity was analyzed using the purified recombinant CsCasp10 protein. The results showed the proteolytic activity of CsCasp10 for caspase 10 substrate was 2.5 units per μg protein. Moreover, the proteolytic activities of CsCasp10 in kidney and spleen induced by A. invadans and A. hydrophila stimulation were analyzed by caspase 10 activity assay kit. All these results showed that CsCasp10 are participated in immunity of C. striatus against A. invadans and A. hydrophila infection.

Cytotoxicity of (-)-vitisin B in human leukemia cells

Vitis thunbergii var. taiwaniana (VTT) is an indigenous Taiwanese wild grape and is used as a folk medicine in Taiwan. VTT is rich in polyphenols, especially quercetin and resveratrol derivatives, which were demonstrated to exhibit inhibitory activities against carcinogenesis and prevent some neurodegenerative diseases. (-)-Vitisin B is one of the resveratrol tetramers extracted from VTT. In this study, we investigated the mechanisms of (-)-vitisin B on the induction of apoptosis in human HL-60 promyelocytic leukemia cells. First, (-)-vitisin B significantly inhibited cell proliferation through inducing cell apoptosis. This effect appeared to occur in a time- and dose-dependent manner. Cell-cycle distribution was also examined, and we found that (-)-vitisin B significantly induced a sub-G1 population in a dose-dependent manner. In addition, (-)-vitisin B exhibited stronger inhibitory effects on cell proliferation than resveratrol. Second, (-)-vitisin B dose dependently induced apoptosis-related protein expressions, such as the cleavage form of caspase-3, caspase-8, caspase-9, poly(ADP ribose) polymerase, and the proapoptotic Bax protein. Third, (-)-vitisin B treatment also resulted in increases in c-Jun N-terminal kinase (JNK) phosphorylation and Fas ligand (FasL) expression. Moreover, the (-)-vitisin B-induced FasL expression and caspase-3 activation could be reversed by a JNK inhibitor. These results suggest that (-)-vitisin B-induced apoptosis of leukemia cells might be mediated through activation of JNK and Fas death-signal transduction.

Regulation of proteases after spinal cord injury

Spinal cord injury is a major medical problem worldwide. Unfortunately, we still do not have suitable therapeutic agents for the treatment of spinal cord injury and prevention of its devastating consequences. Scientists and physicians are baffled by the challenges of controlling progressive neurodegeneration in spinal cord injury, which has not been healed with any currently-available treatments. Although extensive work has been carried out to better understand the pathophysiology of spinal cord injury, our current understanding of the repair mechanisms of secondary injury processes is still meager. Several investigators reported the crucial role played by various proteases after spinal cord injury. Understanding the beneficial and harmful roles these proteases play after spinal cord injury will allow scientists to plan and design appropriate treatment strategies to improve functional recovery after spinal cord injury. This review will focus on various proteases such as matrix metalloproteinases, cysteine proteases, and serine proteases and their inhibitors in the context of spinal cord injury.

Human caspases in vitro: expression, purification and kinetic characterization

A number of strategies and protocols for the expression, purification and kinetic characterization of human caspases are described in the literature. We have systematically revised these protocols and present comprehensive optimized expression and purification protocols for caspase-1 to -9 as well as improved assay conditions for their reproducible kinetic characterization. Our studies on active site titration revealed that the reproducibility is strongly affected by the presence of DTT in the assay buffer. Furthermore, we observed that not all caspases show a linear relationship between enzymatic activity and protein concentration, which explains the discrepancy between published values of specific activities from different laboratories. Our broad kinetic analysis allows the conclusion that the dependency of caspase activities on protein concentration is an effect of concentration-dependent dimerization, which can also be influenced by kosmotropic salts. The protocol recommendations as an outcome of this work will yield higher reproducibility regarding expression and purification of human caspases and contribute to standardization of enzyme kinetic data.

Sorafenib-induced mitochondrial complex I inactivation and cell death in human neuroblastoma cells

Sorafenib is a multikinase inhibitor that is approved for use against renal cell and hepatocellular carcinoma. We found that sorafenib potently induced cell death in human neuroblastoma cells. To understand the molecular basis of sorafenib-mediated cell death in human SH-SY5Y cells, we performed a temporal quantitative proteome analysis. The results showed significant quantitative changes of 193 unique proteins. Bioinformatics-assisted pathway analysis of the regulated proteins revealed that mitochondrial proteins, especially components of the electron transport chain and the mitochondrial ribosomes, were significantly affected upon exposure to sorafenib. The observed down-regulation of the respiratory chain complex I (NADH dehydrogenase) was accompanied with loss of mitochondrial transmembrane potential (Δψm) and complete impairment of complex I enzyme activity. The destabilization of complex I subunits was consistent, rapid, and independent of caspase activation as well as Bcl-2 overexpression. This study provides an overview of the molecular machinery driving sorafenib-mediated cell death in neuroblastoma cells and suggests that sorafenib could be a potential therapeutic drug for the treatment of neuroblastoma.

Ordering of ceramide formation and caspase-9 activation in CD95L-induced Jurkat leukemia T cell apoptosis

Ceramide, a biologically active sphingolipid in cell death signaling, accumulates upon CD95L treatment, concomitantly to apoptosis induction in Jurkat leukemia T cells. Herein, we show that ceramide did not increase in caspase-8 and -10-doubly deficient Jurkat cells in response to CD95L, indicating that apical caspases are essential for CD95L-triggered ceramide formation. Jurkat cells are typically defined as type 2 cells, which require the activation of the mitochondrial pathway for efficient apoptosis induction in response to CD95L. Caspase-9-deficient Jurkat cells significantly resisted CD95L-induced apoptosis, despite ceramide accumulation. Knock-down of sphingomyelin synthase 1, which metabolizes ceramide to sphingomyelin, enhanced (i) CD95L-triggered ceramide production, (ii) cytochrome c release from the mitochondria and (iii) caspase-9 activation. Exogenous ceramide-induced caspase-3 activation and apoptosis were impaired in caspase-9-deficient Jurkat cells. Conversely, caspase-9 re-expression in caspase-9-deficient Jurkat cells restored caspase-3 activation and apoptosis upon exogenous ceramide treatment. Collectively, our data provide genetic evidence that CD95L-triggered endogenous ceramide increase in Jurkat leukemia T cells (i) is not a mere consequence of cell death and occurs mainly in a caspase-9-independent manner, (ii) is likely involved in the pro-apoptotic mitochondrial pathway leading to caspase-9 activation.

Effect of infectious hypodermal and haematopoietic necrosis virus (IHHNV) infection on caspase 3c expression and activity in freshwater prawn Macrobrachium rosenbergii

Caspase 3c (MrCasp3c) was sequenced from the freshwater giant prawn Macrobrachium rosenbergii using Illumina Solexa Genome Analyzer Technique. MrCasp3c consisted of 2080 bp nucleotide encoded 521 polypeptide with an estimated molecular mass of 59 kDa. MrCasp3c sequence contains caspase family p20 domain profile and caspase family p10 domain profile at 236-367 and 378-468 respectively. The quantitative real time PCR analysis revealed a broad expression of MrCasp3c with the highest expression in haemocyte and the lowest in stomach. The expression of MrCasp3c after challenge with the infectious hypodermal and haematopoietic necrosis virus (IHHNV) was tested in haemocyte. In addition, MrCasp3c was expressed in Escherichia coli by prokaryotic expression plasmid pMAL-c2x. The enzyme activity of MrCasp3c was also found to be up-regulated by IHHNV in haemocyte and hepatopancreas tissues. This study suggested that MrCasp3c may be an effector caspase associated with the induction of apoptosis which is potentially involved in the immune defence of M. rosenbergii.

Downregulation of caspase-10 predicting poor survival after resection of stage II colorectal cancer

PURPOSE

The aim of this study was to evaluate the prevalence and clinical significance of caspase-10 mRNA expression in stage II colorectal cancer.

METHODS

Quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) was used to analyze caspase-10 expression in cancer tissue and corresponding normal mucosa from 120 patients with stage II colorectal cancer. Variables were analyzed by Chi-square test or Fisher's exact test. Survival was evaluated with method of Kaplan-Meier. Multivariate analysis was performed with Cox's proportional hazards model.

RESULTS

The expression of caspase-10 mRNA was found to be downregulated in cancer tissue compared to normal mucosa (P = 0.001). Poorly differentiated cancer showed lower mRNA expression than cancer with greater differentiation (P = 0.031). Univariate survival curves, estimated using the method of Kaplan-Meier, defined a significant association between caspase-10 expression and both overall survival (P = 0.012) and disease-free survival (P = 0.021). A multivariate analysis, performed by Cox's proportional hazards regression model, confirmed that a low caspase-10 expression was the only significant factor to predict poor prognosis in patients with stage II colorectal cancer.

CONCLUSION

Our data indicate that caspase-10 expression, measured by quantitative real-time RT-PCR, is a possible prognostic factor in patients with stage II colorectal cancer.

Curcumin induces Apaf-1-dependent, p21-mediated caspase activation and apoptosis

Previous studies have demonstrated that curcumin induces mitochondria-mediated apoptosis. However, understanding of the molecular mechanisms underlying curcumin-induced cell death remains limited. In this study, we demonstrate that curcumin treatment of cancer cells caused dose- and time-dependent caspase-3 activation, which is required for apoptosis as confirmed using the pan caspase inhibitor, z-VAD. Knockdown experiments and knockout cells excluded a role of caspase-8 in curcumin-induced caspase-3 activation. In contrast, Apaf-1 deficiency or silencing inhibited the activity of caspase-3, pointing to a requisite role of Apaf-1 in curcumin-induced apoptotic cell death. Curcumin treatment led to Apaf-1 upregulation both at the protein and mRNA levels. Cytochrome c release from mitochondria to the cytosol in curcumin-treated cells was associated with upregulation of proapoptotic proteins such as Bax, Bak, Bid, and Bim. Crosslinking experiments demonstrated Bax oligomerization during curcumin-induced apoptosis, suggesting that induced expression of Bax, Bid, and Bim causes Bax-channel formation on the mitochondrial membrane. The release of cytochrome c was unaltered in p53-deficient cells, whereas absence of p21 blocked cytochrome c release, caspase activation, and apoptosis. Importantly, p21-deficiency resulted in reduced expression of Apaf-1 during curcumin treatment, indicating a requirement of p21 in Apaf-1 dependent caspase activation and apoptosis. Together, our findings demonstrate that Apaf-1, Bax, and p21 as novel potential targets for curcumin or curcumin-based anticancer agents.

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

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

Proteasome inhibition can induce an autophagy-dependent apical activation of caspase-8

Antiapoptotic Bcl-2 family proteins are often highly expressed in chemotherapy-resistant cancers and impair mitochondrial outer membrane permeabilisation (MOMP), an important requirement for caspase activation via the intrinsic apoptosis pathway. Interestingly, although Bcl-2 overexpression in HeLa cervical cancer cells abrogated caspase processing in response to intrinsic apoptosis induction by staurosporine, tunicamycin or etoposide, residual caspase processing was observed following proteasome inhibition by bortezomib ([(1R)-3-methyl-1-({(2S)-3-phenyl-2-[(pyrazin-2-ylcarbonyl)amino]propanoyl}amino)butyl]boronic acid), epoxomicin (N-acetyl-N-methyl-lisoleucyl-L-isoleucyl-N-[(1S)-3-methyl-1-[[(2R)-2-methyloxiranyl]carbonyl]butyl]-L-threoninamide) or MG-132 (N-(benzyloxycarbonyl)leucinylleucinylleucinal). Similar responses were found in Bcl-2-overexpressing H460 NSCLC cells and Bax/Bak-deficient mouse embyronic fibroblasts. Mild caspase processing resulted in low DEVDase activities, which were MOMP independent and persisted for long periods without evoking immediate cell death. Surprisingly, depletion of caspase-3 and experiments in caspase-7-depleted MCF-7-Bcl-2 cells indicated that the DEVDase activity did not originate from effector caspases. Instead, Fas-associated death domain (FADD)-dependent caspase-8 activation was the major contributor to the slow, incomplete substrate cleavage. Caspase-8 activation was independent of death ligands, but required the induction of autophagy and the presence of Atg5. Depletion of XIAP or addition of XIAP-antagonising peptides resulted in a switch towards efficient apoptosis execution, suggesting that the requirement for MOMP was bypassed by activating the caspase-8/caspase-3 axis. Combination treatments of proteasome inhibitors and XIAP antagonists therefore represent a promising strategy to eliminate highly resistant cancer cells, which overexpress antiapoptotic Bcl-2 family members.

Fas death receptor signalling: roles of Bid and XIAP

Fas (also called CD95 or APO-1), a member of a subgroup of the tumour necrosis factor receptor superfamily that contain an intracellular death domain, can initiate apoptosis signalling and has a critical role in the regulation of the immune system. Fas-induced apoptosis requires recruitment and activation of the initiator caspase, caspase-8 (in humans also caspase-10), within the death-inducing signalling complex. In so-called type 1 cells, proteolytic activation of effector caspases (-3 and -7) by caspase-8 suffices for efficient apoptosis induction. In so-called type 2 cells, however, killing requires amplification of the caspase cascade. This can be achieved through caspase-8-mediated proteolytic activation of the pro-apoptotic Bcl-2 homology domain (BH)3-only protein BH3-interacting domain death agonist (Bid), which then causes mitochondrial outer membrane permeabilisation. This in turn leads to mitochondrial release of apoptogenic proteins, such as cytochrome c and, pertinent for Fas death receptor (DR)-induced apoptosis, Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP binding protein with low Pi), an antagonist of X-linked inhibitor of apoptosis (XIAP), which imposes a brake on effector caspases. In this review, written in honour of Juerg Tschopp who contributed so much to research on cell death and immunology, we discuss the functions of Bid and XIAP in the control of Fas DR-induced apoptosis signalling, and we speculate on how this knowledge could be exploited to develop novel regimes for treatment of cancer.

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.

Inhibition of SREBP1 sensitizes cells to death ligands

Evasion of death receptor ligand-induced apoptosis contributs to cancer development and progression. To better understand mechanisms conferring resistance to death ligands, we screened an siRNA library to identify sequences that sensitize resistant cells to fas activating antibody (CH-11). From this screen, we identified the Sterol-Regulatory Element-Binding Protein 1 (SREBP1), a transcription factor, which regulates genes involved in cholesterol and fatty acid synthesis including fatty acid synthase. Inhibition of SREBP1 sensitized PPC-1 and HeLa to the death receptor ligands CH-11 and TRAIL. In contrast, DU145 prostate cancer cells that are resistant to death ligands despite expressing the receptors on their cell surface remained resistant to CH-11 and TRAIL after knockdown of SREBP1. Consistent with the effects on cell viability, the addition of CH-11 activated caspases 3 and 8 in HeLa but not DU145 cells with silenced SREBP1. We demonstrated that knockdown of SREBP1 produced a marked decrease in fatty acid synthase expression. Furthermore, genetic or chemical inhibition of fatty acid synthase with shRNA or orlistat, respectively, recapitulated the effects of SREBP1 inhibition and sensitized HeLa but not DU145 cells to CH-11 and TRAIL. Sensitization to death receptor ligands by inhibition of fatty acid synthase was associated with activation of caspase 8 prior to caspase 9. Neither silencing of SREBP1 or fatty acid synthase changed basal expression of the core death receptor components Fas, caspase 8, FADD, caspase 3 or FLIP. Thus, inhibition of SREBP1 or its downstream target fatty acid synthase sensitizes resistant cells to death ligands.

The ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 protect cells against TNFα- and FasL-induced apoptosis by interacting with caspase-8

We previously reported that HSV-2 R1, the R1 subunit (ICP10; UL39) of herpes simplex virus type-2 ribonucleotide reductase, protects cells against apoptosis induced by the death receptor (DR) ligands tumor necrosis factor-alpha- (TNFα) and Fas ligand (FasL) by interrupting DR-mediated signaling at, or upstream of, caspase-8 activation. Further investigation of the molecular mechanism underlying HSV-2 R1 protection showed that extracellular-regulated kinase 1/2 (ERK1/2), phosphatidylinositol 3-kinase (PI3-K)/Akt, NF-κB and JNK survival pathways do not play a major role in this antiapoptotic function. Interaction studies revealed that HSV-2 R1 interacted constitutively with caspase-8. The HSV-2 R1 deletion mutant R1(1-834)-GFP and Epstein-Barr virus (EBV) R1, which did not protect against apoptosis induced by DR ligands, did not interact with caspase-8, indicating that interaction is required for protection. HSV-2 R1 impaired caspase-8 activation induced by caspase-8 over-expression, suggesting that interaction between the two proteins prevents caspase-8 dimerization/activation. HSV-2 R1 bound to caspase-8 directly through its prodomain but did not interact with either its caspase domain or Fas-associated death domain protein (FADD). Interaction between HSV-2 R1 and caspase-8 disrupted FADD-caspase-8 binding. We further demonstrated that individually expressed HSV-1 R1 (ICP6) shares, with HSV-2 R1, the ability to bind caspase-8 and to protect cells against DR-induced apoptosis. Finally, as the long-lived Fas protein remained stable during the early period of infection, experiments with the HSV-1 UL39 deletion mutant ICP6∆ showed that HSV-1 R1 could be essential for the protection of HSV-1-infected cells against FasL.

Destabilization of CARP mRNAs by aloe-emodin contributes to caspase-8-mediated p53-independent apoptosis of human carcinoma cells

Using short hairpin RNA against p53, transient ectopic expression of wild-type p53 or mutant p53 (R248W or R175H), and a p53- and p21-dependent luciferase reporter assay, we demonstrated that growth arrest and apoptosis of FaDu (human pharyngeal squamous cell carcinoma), Hep3B (hepatoma), and MG-63 (osteosarcoma) cells induced by aloe-emodin (AE) are p53-independent. Co-immunoprecipitation and small interfering RNA (siRNA) studies demonstrated that AE caused S-phase cell cycle arrest by inducing the formation of cyclin A-Cdk2-p21 complexes through extracellular signal-regulated kinase (ERK) activation. Ectopic expression of Bcl-X(L) and siRNA-mediated Bax attenuation significantly inhibited apoptosis induced by AE. Cyclosporin A or the caspase-8 inhibitor Z-IETD-FMK blocked AE-induced loss of mitochondrial membrane potential and prevented increases in reactive oxygen species and Ca(++). Z-IETD-FMK inhibited AE-induced apoptosis, Bax expression, Bid cleavage, translocation of tBid to mitochondria, ERK phosphorylation, caspase-9 activation, and the release of cytochrome c, apoptosis-inducing factor (AIF), and endonuclease G from mitochondria. The stability of the mRNAs encoding caspase-8 and -10-associated RING proteins (CARPs) 1 and 2 was affected by AE, whereas CARP1 or 2 overexpression inhibited caspase-8 activation and apoptosis induced by AE. Collectively, our data indicate AE induces caspase-8-mediated activation of mitochondrial death pathways by decreasing the stability of CARP mRNAs in a p53-independent manner.

Simple and tunable Förster resonance energy transfer-based bioprobes for high-throughput monitoring of caspase-3 activation in living cells by using flow cytometry

Sensing systems based on Förster resonance energy transfer (FRET) can be used to monitor enzymatic reactions, protein-protein interactions, changes in conformation, and Ca2+ oscillations in studies on cellular dynamics. We developed a series of FRET-based chimeric bioprobes, each consisting of fluorescent protein attached to a fluorescent dye. Green and red fluorescent proteins were used as donors and a series of Alexa Fluor dyes was used as acceptors. The basic fluorescent proteins were substituted with appropriate amino acids for recognition of the target (caspase-3) and subjected to site-directed modification with a fluorescent dye. Variants that retained similar emission profiles to the parent proteins were readily derived for use as FRET-based bioprobes with various fluorescent patterns by incorporating various fluorescent proteins and dyes, the nature of which could be adjusted to experimental requirements. All the constructs prepared functioned as bioprobes for quantitative measurement of caspase-3 activity in vitro. Introduction of the bioprobes into cells was so simple and efficient that activation of caspase-3 upon apoptosis could be monitored by means of cytometric analysis. FRET-based bioprobes are valuable tool for high-throughput flow-cytometric analysis of many cellular events when used in conjunction with other fluorescent labels or markers. Statistical dynamic studies on living cells could provide indications of paracrine signaling.

The protein Dredd is an essential component of the c-Jun N-terminal kinase pathway in the Drosophila immune response

The Drosophila immune deficiency (IMD) pathway mobilizes c-Jun N-terminal kinase (JNK), caspase, and nuclear factor-κB (NF-κB) modules to counter infection with gram-negative bacteria. Dredd is an essential caspase in the IMD pathway, and it is widely established that NF-κB activation depends on Dredd. More recent cell culture studies suggested a role for Dredd in the activation of dJNK (Drosophila JNK). However, there are no epistatic or mechanistic data on the involvement of Dredd in dJNK activation. More importantly, there is no in vivo evidence to demonstrate a physiological requirement for Dredd in the IMD/dJNK pathway. We performed a comprehensive analysis of the role of Dredd in the IMD/dJNK pathway, and we demonstrated that Dredd is essential for the activation of IMD/dJNK in cell culture. We positioned Dredd activity at an early point of the IMD/dJNK pathway and uncovered a series of interactions between Dredd and additional proximal IMD pathway molecules. Mechanistically, we showed that the caspase activity inhibitor p35 blocked dJNK activation and the induction of dJNK-dependent genes in cell culture and in vivo. Most importantly, we demonstrated that dredd mutant flies are completely inhibited in their ability to activate dJNK or express dJNK-responsive target genes after bacterial infection in vivo. In conclusion, we established Dredd as an essential component of the IMD pathway required for the full activation of IMD/dJNK in cell culture and in vivo. Our data enhance our appreciation of Dredd-dependent IMD signal transduction events.

Resveratrol induces p53-independent, X-linked inhibitor of apoptosis protein (XIAP)-mediated Bax protein oligomerization on mitochondria to initiate cytochrome c release and caspase activation

Resveratrol, a naturally occurring phytoalexin, is known to induce apoptosis in multiple cancer cell types, but the underlying molecular mechanisms remain unclear. Here, we show that resveratrol induced p53-independent, X-linked inhibitor of apoptosis protein (XIAP)-mediated translocation of Bax to mitochondria where it underwent oligomerization to initiate apoptosis. Resveratrol treatment promoted interaction between Bax and XIAP in the cytosol and on mitochondria, suggesting that XIAP plays a critical role in the activation and translocation of Bax to mitochondria. This process did not involve p53 but required accumulation of Bim and t-Bid on mitochondria. Bax primarily underwent homo-oligomerization on mitochondria and played a major role in release of cytochrome c to the cytosol. Bak, another key protein that regulates the mitochondrial membrane permeabilization, did not interact with p53 but continued to associate with Bcl-xL. Thus, the proapoptotic function of Bak remained suppressed during resveratrol-induced apoptosis. Caspase-9 silencing inhibited resveratrol-induced caspase activation, whereas caspase-8 knockdown did not affect caspase activity, suggesting that resveratrol induces caspase-9-dependent apoptosis. Together, our findings characterize the molecular mechanisms of resveratrol-induced caspase activation and subsequent apoptosis in cancer cells.

Caspase signalling controls microglia activation and neurotoxicity

Activation of microglia and inflammation-mediated neurotoxicity are suggested to play a decisive role in the pathogenesis of several neurodegenerative disorders. Activated microglia release pro-inflammatory factors that may be neurotoxic. Here we show that the orderly activation of caspase-8 and caspase-3/7, known executioners of apoptotic cell death, regulate microglia activation through a protein kinase C (PKC)-δ-dependent pathway. We find that stimulation of microglia with various inflammogens activates caspase-8 and caspase-3/7 in microglia without triggering cell death in vitro and in vivo. Knockdown or chemical inhibition of each of these caspases hindered microglia activation and consequently reduced neurotoxicity. We observe that these caspases are activated in microglia in the ventral mesencephalon of Parkinson's disease (PD) and the frontal cortex of individuals with Alzheimer's disease (AD). Taken together, we show that caspase-8 and caspase-3/7 are involved in regulating microglia activation. We conclude that inhibition of these caspases could be neuroprotective by targeting the microglia rather than the neurons themselves.

Individual caspase-10 isoforms play distinct and opposing roles in the initiation of death receptor-mediated tumour cell apoptosis

The cysteine protease caspase-8 is an essential executioner of the death receptor (DR) apoptotic pathway. The physiological function of its homologue caspase-10 remains poorly understood, and the ability of caspase-10 to substitute for caspase-8 in the DR apoptotic pathway is still controversial. Here, we analysed the particular contribution of caspase-10 isoforms to DR-mediated apoptosis in neuroblastoma (NB) cells characterised by their resistance to DR signalling. Silencing of caspase-8 in tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-sensitive NB cells resulted in complete resistance to TRAIL, which could be reverted by overexpression of caspase-10A or -10D. Overexpression experiments in various caspase-8-expressing tumour cells also demonstrated that caspase-10A and -10D isoforms strongly increased TRAIL and FasL sensitivity, whereas caspase-10B or -10G had no effect or were weakly anti-apoptotic. Further investigations revealed that the unique C-terminal end of caspase-10B was responsible for its degradation by the ubiquitin-proteasome pathway and for its lack of pro-apoptotic activity compared with caspase-10A and -10D. These data highlight in several tumour cell types, a differential pro- or anti-apoptotic role for the distinct caspase-10 isoforms in DR signalling, which may be relevant for fine tuning of apoptosis initiation.

TIMP-3 promotes apoptosis in nonadherent small cell lung carcinoma cells lacking functional death receptor pathway

Tissue inhibitor of metalloproteinases-3 (TIMP-3) has previously been identified as a tumor suppressor for adherent malignant and normal cells. TIMP-3 inhibits adhesion of cells to extracellular matrix and promotes apoptosis through death receptor-activated, caspase-8-mediated pathway. Here, we have studied the effect of adenovirally mediated overexpression of TIMP-3 on small cell lung cancer (SCLC) cell lines SW2 and N417, which grow in suspension and lack functional caspase-8. The results show that adenoviral delivery of TIMP-3 promotes apoptotic cell death in SCLC cells in the absence of caspase-8 activation. These results suggest TIMP-3 as a promising therapeutic anticancer protein also in nonadherent malignant cells lacking functional death receptor signaling.

Silencing of PTK7 in colon cancer cells: caspase-10-dependent apoptosis via mitochondrial pathway

Protein tyrosine kinase-7 (PTK7) is a catalytically inactive receptor tyrosine kinase (RTK). PTK7 is upregulated in many common human cancers, including colon cancer, lung cancer, gastric cancer and acute myeloid leukemia. The reason for this up-regulation is not yet known. To explore the functional role of PTK7, the expression of PTK7 in HCT 116 cells was examined using small interference (siRNA)-mediated gene silencing. Following transfection, the siRNA successfully suppressed PTK7 mRNA and protein expression. Knocking down of PTK7 in HCT 116 cells inhibited cell proliferation compared to control groups and induced apoptosis. Furthermore, this apoptosis was characterized by decreased mitochondrial membrane potential and activation of caspase-9 and -10. Addition of a caspase-10 inhibitor totally blocked this apoptosis, suggesting that caspase-10 may play a critical role in PTK7-knockdown-induced apoptosis, downstream of mitochondria. These observations may indicate a role for PTK7 in cell proliferation and cell apoptosis and may provide a potential therapeutic pathway for the treatment of a variety of cancers.

Caspase-10-dependent cell death in Fas/CD95 signalling is not abrogated by caspase inhibitor zVAD-fmk

BACKGROUND

Upon CD95/Fas ligation, the initiator caspase-8 is known to activate effector caspases leading to apoptosis. In the presence of zVAD-fmk, a broad-spectrum caspase inhibitor, Fas engagement can also trigger an alternative, non-apoptotic caspase-independent form of cell death, which is initiated by RIP1. Controversy exists as to the ability of caspase-10 to mediate cell death in response to FasL (CD95L or CD178). Herein, the role of caspase-10 in FasL-induced cell death has been re-evaluated.

METHODOLOGY AND PRINCIPAL FINDINGS

The present study shows that FasL-induced cell death was completely impaired in caspase-8- and caspase-10-doubly deficient (I9-2e) Jurkat leukaemia T-cell lines. Over-expressing of either caspase-8 or caspase-10 in I9-2e cells triggered cell death and restored sensitivity to FasL, further arguing for a role of both initiator caspases in Fas apoptotic signalling. In the presence of zVAD-fmk, FasL triggered an alternative form of cell death similarly in wild-type (A3) and in caspase-8-deficient Jurkat cells expressing endogenous caspase-10 (clone I9-2d). Cell death initiated by Fas stimulation in the presence of zVAD-fmk was abrogated in I9-2e cells as well as in HeLa cells, which did not express endogenous caspase-10, indicating that caspase-10 somewhat participates in this alternative form of cell death. Noteworthy, ectopic expression of caspase-10 in I9-2e and HeLa cells restored the ability of FasL to trigger cell death in the presence of zVAD-fmk. As a matter of fact, FasL-triggered caspase-10 processing still occurred in the presence of zVAD-fmk.

CONCLUSIONS AND SIGNIFICANCE

Altogether, these data provide genetic evidence for the involvement of initiator caspase-10 in FasL-induced cell death and indicate that zVAD-fmk does not abrogate caspase-10 processing and cytotoxicity in Fas signalling. Our study also questions the existence of an alternative caspase-independent cell death pathway in Fas signalling.

Activation and specificity of human caspase-10

Two apical caspases, caspase-8 and -10, are involved in the extrinsic death receptor pathway in humans, but it is mainly caspase-8 in its apoptotic and nonapoptotic functions that has been an intense research focus. In this study we concentrate on caspase-10, its mechanism of activation, and the role of the intersubunit cleavage. Our data obtained through in vitro dimerization assays strongly suggest that caspase-10 follows the proximity-induced dimerization model for apical caspases. Furthermore, we compare the specificity and activity of the wild-type protease with a mutant incapable of autoprocessing by using positional scanning substrate analysis and cleavage of natural protein substrates. These experiments reveal a striking difference between the wild type and the mutant, leading us to hypothesize that the single chain enzyme has restricted activity on most proteins but high activity on the proapoptotic protein Bid, potentially supporting a prodeath role for both cleaved and uncleaved caspase-10.

Clustering of the neural cell adhesion molecule (NCAM) at the neuronal cell surface induces caspase-8- and -3-dependent changes of the spectrin meshwork required for NCAM-mediated neurite outgrowth

Changes in neuronal morphology underlying neuronal differentiation depend on rapid and sustained cytoskeleton rearrangements in the growing neurites. Whereas cell adhesion molecules are well established as regulators of neuronal differentiation, less is known about the signaling mechanisms by which they influence the cytoskeleton. Here we show that the neural cell adhesion molecule (NCAM) associates with the active form of caspase-8 and that clustering of NCAM at the neuronal cell surface leads to activation of caspase-8 and -3 followed by the cleavage of the sub-membranous brain spectrin meshwork, but not of the actin or tubulin cytoskeleton. Inhibitors of caspase-8 and -3 specifically block the NCAM-dependent spectrin cleavage and abolish NCAM-dependent neurite outgrowth. NCAM-dependent rearrangements of the membrane associated spectrin meshwork via caspase-8 dependent caspase-3 activation are thus indispensable for NCAM-mediated neurite outgrowth.

Fundamentals of neuronal apoptosis relevant to pediatric anesthesia

The programmed cell death or apoptosis is a complex biochemical process that has risen to prominence in pediatric anesthesia. Preclinical studies report a dose-dependent neuronal apoptosis during synaptogenesis following exposure to intravenous and volatile anesthetic agents. Although emerging clinical data do not universally indicate an increased neurodegenerative risk of general anesthesia in early human life, a great deal of uncertainty was created within the pediatric anesthesia community. This was at least partially caused by the demand of understanding of basic science concepts and knowledge of apoptosis frequently out of reach to the clinician. It is, however, important for the pediatric anesthesiologist to be familiar with the basic science concepts of neuronal apoptosis to be able to critically evaluate current and future preclinical data in this area and future clinical studies. This current review describes the extrinsic and intrinsic pathways involved in the cell death process and discusses techniques commonly employed to determine apoptosis. In addition, potential mechanisms of anesthesia-induced neuronal apoptosis are illustrated in this review.

Screening of cell death genes with a mammalian genome-wide RNAi library

We report the construction and application of a mammalian genome-wide RNAi library. The oligodeoxynucleotides encoding approximately 200,000 shRNA sequences that targeted 47,400 human transcripts were inserted into a lentivirus vector pFIV-H1-puro, and a pool of pseudovirus particles with a complexity of approximately 200,000 were used to infect target cells. From the cells surviving apoptogenic Fas stimulation, four candidate shRNA sequences were obtained that provided resistance to Fas-induced cell death, including two shRNAs for caspase-8, an shRNA for Bid, and an shRNA for Fas. The reconstructed shRNAs with these sequences were shown to reduce expression of the respective gene products and increase survival after Fas stimulation. When similar selection was performed for tunicamycin-induced apoptosis, no shRNA strongly inhibiting tunicamycin-induced cell death was isolated, although a few reconstructed shRNAs led to a slight increase of survival. Thus, this genome-wide shRNA library proved useful for selection of genes that are involved in cell death, but some limitation was also revealed.

First molecular cloning of a molluscan caspase from variously colored abalone (Haliotis diversicolor) and gene expression analysis with bacterial challenge

Mammal caspases have been demonstrated to possess important functions in apoptosis and immune signaling, but there is less knowledge available on abalone caspases. In the present study, a molluscan caspase gene, abCaspase, was cloned for the first time from the variously colored abalone (Haliotis diversicolor) and its full-length cDNA sequence was 2427 bp, with a 1008 bp of open reading frame encoding a protein of 336 aa. The molecular mass of the deduced protein was approximately 36.97 kDa with an estimated pI of 5.28. The predicted amino acid sequence of abCaspase contained two domains of p20 and p10 which were conserved in the caspase family, including the cysteine active site pentapeptide "QSCRG" and the histidine active site signature "HTVYDCVVVIFLTHG". Homology analysis showed that abCaspase shared high similarity with apoptotic caspases and it was grouped together with vertebrate caspase-8s and caspase-10s using phylogenetic analysis, suggesting that abCaspase belonged to a typical apoptotic caspase and might possess the characteristic of human caspase-8 and -10. The mRNA transcripts of abCaspase were widely distributed in various tissues of H. diversicolor. Expression of the abCaspase gene was significantly induced in the tissues tested, especially in the hemocytes, gill and mantle with bacterial challenge. This study suggested that abCaspase may be an initiator caspase associated with the induction of apoptosis which is potentially involved in the immune defense of H. diversicolor.

A new caspase-8 isoform caspase-8s increased sensitivity to apoptosis in Jurkat cells

Caspase-8 is a key initiator of death receptor-induced apoptosis. Here we report a novel short isoform of caspase-8 (caspase-8s), which encodes the first (Death Effector Domain) DED and part of the second DED, missing the C-terminal caspase domain. In vivo binding assays showed that transfected caspase-8s bound to (Fas-associated death domain protein) FADD, the adaptor protein in (death-induced signal complex) DISC. To investigate the potential effects of caspase-8s on cell apoptosis, Jurkat cells were stably transfected with caspase-8s. Overexpression of caspase-8s increased sensitivity to the apoptotic stimuli, Fas-agonistic antibody CH11. These results suggest that caspase-8s may act as a promoter of apoptosis through binding to FADD and is involved in the regulation of apoptosis. In addition, the results also indicate that the first DED was an important structure mediating combination between caspase-8 and FADD.

Small-molecule activators of a proenzyme

Virtually all of the 560 human proteases are stored as inactive proenyzmes and are strictly regulated. We report the identification and characterization of the first small molecules that directly activate proenzymes, the apoptotic procaspases-3 and -6. It is surprising that these compounds induce autoproteolytic activation by stabilizing a conformation that is both more active and more susceptible to intermolecular proteolysis. These procaspase activators bypass the normal upstream proapoptotic signaling cascades and induce rapid apoptosis in a variety of cell lines. Systematic biochemical and biophysical analyses identified a cluster of mutations in procaspase-3 that resist small-molecule activation both in vitro and in cells. Compounds that induce gain of function are rare, and the activators reported here will enable direct control of the executioner caspases in apoptosis and in cellular differentiation. More generally, these studies presage the discovery of other proenzyme activators to explore fundamental processes of proenzyme activation and their fate-determining roles in biology.

Tubular cell HIV-1 gp120 expression induces caspase 8 activation and apoptosis

Renal biopsy data indicate that tubular epithelial cells serve as a reservoir for HIV-1 infection. We studied the effect of HIV-1 gp120 envelope gene expression on tubular cell apoptosis. HIV-1 gp120 was expressed in a lentiviral vector pHR-CMV-IRES2-EGFP-DeltaB. This plasmid construct was used to produce pseudotyped virus using VSV-G envelope to enhance the tropism for efficient viral transduction. Human proximal tubular (HK-2) cells were transduced and assayed for cellular injury by trypan blue exclusion, Hoechst and PI staining, TUNEL, and cell cycle staging. HIV-1 gp120-transduced HK-2 cells showed cellular injury in a time-dependent manner. Gp120-transduced cells showed 2.5-fold greater apoptosis when compared with vector-transduced cells. Cell cycle analysis did not reveal any alteration between gp120-transduced cells and vector-transduced cells. Gp120-transduced cells showed higher expression of both Fas and FasL, whereas pretreatment with anti-FasL antibody partially inhibited gp120-induced tubular cell apoptosis. Similarly, pretreatment with caspase-8 inhibitor attenuated gp120-induced HK2 cell apoptosis. Moreover, gp120-transduced cells showed activation of caspase 8. These results suggest that HIV-1 gp120 expression induces tubular cell apoptosis through the extrinsic pathway by enhancing Fas and FasL expression and activation of caspase-8.

PAC-1 activates procaspase-3 in vitro through relief of zinc-mediated inhibition

The direct induction of apoptosis has emerged as a powerful anticancer strategy, and small molecules that either inhibit or activate certain proteins in the apoptotic pathway have great potential as novel chemotherapeutic agents. Central to apoptosis is the activation of the zymogen procaspase-3 to caspase-3. Caspase-3 is the key "executioner" caspase, catalyzing the hydrolysis of a multitude of protein substrates within the cell. Interestingly, procaspase-3 levels are often elevated in cancer cells, suggesting a compound that directly stimulates the activation of procaspase-3 to caspase-3 could selectively induce apoptosis in cancer cells. We recently reported the discovery of a compound, PAC-1, which enhances procaspase-3 activity in vitro and induces apoptotic death in cancer cells in culture and in mouse xenograft models. Described herein is the mechanism by which PAC-1 activates procaspase-3 in vitro. We show that zinc inhibits the enzymatic activity of procaspase-3 and that PAC-1 strongly activates procaspase-3 in buffers that contain zinc. PAC-1 and zinc form a tight complex with one another, with a dissociation constant of approximately 42 nM. The combined data indicate that PAC-1 activates procaspase-3 in vitro by sequestering inhibitory zinc ions, thus allowing procaspase-3 to autoactivate itself to caspase-3. The small-molecule-mediated activation of procaspases has great therapeutic potential and thus this discovery of the in vitro mechanism of action of PAC-1 is critical to the development and optimization of other procaspase-activating compounds.

Genetic deletion of faim reveals its role in modulating c-FLIP expression during CD95-mediated apoptosis of lymphocytes and hepatocytes

Fas-apoptosis inhibitory molecule (FAIM) is inducibly expressed in B lymphocytes and had been shown to antagonize Fas-mediated killing of B-cell lines in vitro. However, its mechanism and role in vivo are unknown. We have generated faim(-/-) mice and found these mutants to be viable. In contrast to fas(-/-) mice, faim(-/-) mice have normal B- and T-cell populations. However, faim(-/-) B cells and thymocytes show increased sensitivity to Fas-triggered apoptosis in vitro, and faim(-/-) mice suffer greater mortality and exhibit exacerbated liver damage in response to Fas (CD95) engagement in vivo. The lack of FAIM results in greater activation of caspase-8 and -3 in Fas-stimulated thymocytes. Detailed biochemical analyses further reveal the decreased expression of c-FLIP(L) and c-FLIP(R) in faim(-/-) thymocytes and increased association of caspase-8 with Fas in Fas-activated mutant cells. Decreased levels of c-FLIP(L) and c-FLIP(R) are also evident in faim(-/-) liver. Thus, FAIM plays a novel role in modulating Fas-mediated apoptosis and acts through influencing the expression of c-FLIP and regulating the physical binding of caspase-8 to Fas.