Fas- and tumor necrosis factor-induced apoptosis is inhibited by the poxvirus crmA gene product

Granzymes in health and diseases: the good, the bad and the ugly

Granzymes are a family of serine proteases, composed of five human members: GA, B, H, M and K. They were first discovered in the 1980s within cytotoxic granules released during NK cell- and T cell-mediated killing. Through their various proteolytic activities, granzymes can trigger different pathways within cells, all of which ultimately lead to the same result, cell death. Over the years, the initial consideration of granzymes as mere cytotoxic mediators has changed due to surprising findings demonstrating their expression in cells other than immune effectors as well as new intracellular and extracellular activities. Additional roles have been identified in the extracellular milieu, following granzyme escape from the immunological synapse or their release by specific cell types. Outside the cell, granzyme activities mediate extracellular matrix alteration via the degradation of matrix proteins or surface receptors. In certain contexts, these processes are essential for tissue homeostasis; in others, excessive matrix degradation and extensive cell death contribute to the onset of chronic diseases, inflammation, and autoimmunity. Here, we provide an overview of both the physiological and pathological roles of granzymes, highlighting their utility while also recognizing how their unregulated presence can trigger the development and/or worsening of diseases.

The road to death: Caspases, cleavage, and pores

Vishva Dixit recounts his favorite discoveries after 30-plus years studying the proteins that allow infected, damaged, or obsolete cells to die.

Dying in self-defence: a comparative overview of immunogenic cell death signalling in animals and plants

Host organisms utilise a range of genetically encoded cell death programmes in response to pathogen challenge. Host cell death can restrict pathogen proliferation by depleting their replicative niche and at the same time dying cells can alert neighbouring cells to prepare environmental conditions favouring future pathogen attacks. As expected, many pathogenic microbes have strategies to subvert host cell death to promote their virulence. The structural and lifestyle differences between animals and plants have been anticipated to shape very different host defence mechanisms. However, an emerging body of evidence indicates that several components of the host-pathogen interaction machinery are shared between the two major branches of eukaryotic life. Many proteins involved in cell death execution or cell death-associated immunity in plants and animals exert direct effects on endomembrane and loss of membrane integrity has been proposed to explain the potential immunogenicity of dying cells. In this review we aim to provide a comparative view on how cell death processes are linked to anti-microbial defence mechanisms in plants and animals and how pathogens interfere with these cell death programmes. In comparison to the several well-defined cell death programmes in animals, immunogenic cell death in plant defence is broadly defined as the hypersensitive response. Our comparative overview may help discerning whether specific types of immunogenic cell death exist in plants, and correspondingly, it may provide new hints for previously undiscovered cell death mechanism in animals.

The various regulatory functions of long noncoding RNAs in apoptosis, cell cycle, and cellular senescence

Long noncoding RNAs (lncRNAs) are a group of noncoding cellular RNAs involved in significant biological phenomena such as differentiation, cell development, genomic imprinting, adjusting the enzymatic activity, regulating chromosome conformation, apoptosis, cell cycle, and cellular senescence. The misregulation of lncRNAs interrupting normal biological processes has been implicated in tumor formation and metastasis, resulting in cancer. Apoptosis and cell cycle, two main biological phenomena, are highly conserved and intimately coupled mechanisms. Hence, some cell cycle regulators can influence both programmed cell death and cell division. Apoptosis eliminates defective and unwanted cells, and the cell cycle enables cells to replicate themselves. The improper regulation of apoptosis and cell cycle contributes to numerous disorders such as neurodegenerative and autoimmune diseases, viral infection, anemia, and mainly cancer. Cellular senescence is a tumor-suppressing response initiated by environmental and internal stress factors. This phenomenon has recently attained more attention due to its therapeutic implications in the field of senotherapy. In this review, the regulatory roles of lncRNAs on apoptosis, cell cycle, and senescence will be discussed. First, the role of lncRNAs in mitochondrial dynamics and apoptosis is addressed. Next, the interaction between lncRNAs and caspases, pro/antiapoptotic proteins, and also EGFR/PI3K/PTEN/AKT/mTORC1 signaling pathway will be investigated. Furthermore, the effect of lncRNAs in the cell cycle is surveyed through interaction with cyclins, cdks, p21, and wnt/β-catenin/c-myc pathway. Finally, the function of essential lncRNAs in cellular senescence is mentioned.

Suppression of colorectal carcinogenesis by naringin

BACKGROUND

Colorectal cancer is the third most malignant cancer worldwide. Despite novel treatment options, the incidence and mortality rates of colon cancer continue to increase in most countries, especially in US, European and Asian countries. Colorectal carcinogenesis is multifactorial, including dietary and genetic factors, as well as lacking physical activity. Vegetables and fruits contain high amounts of secondary metabolites, which might reduce the risk for colorectal carcinogenesis. Flavonoids are important bioactive polyphenolic compounds. There are more than 4,000 different flavonoids, including flavanones, flavonoids, isoflavonoids, flavones, and catechins in a large variety of plant.

HYPOTHESIS

Among various other flavonoids, naringin in Citrus fruits has been a subject of intense scrutiny for its activity against many types of cancer, including colorectal cancer. We hypothesize that naringin is capable to inhibit the growth of transformed colonocytes and to induce programmed cell death in colon cancer cells.

RESULTS

We comprehensively review the inhibitory effects of naringin on colorectal cancers and address the underlying mechanistic pathways such as NF-κB/IL-6/STAT3, PI3K/AKT/mTOR, apoptosis, NF-κB-COX-2-iNOS, and β-catenin pathways.

CONCLUSION

Naringin suppresses colorectal inflammation and carcinogenesis by various signaling pathways. Randomized clinical trials are needed to determine their effectiveness in combating colorectal cancer.

Signatures of host-pathogen evolutionary conflict reveal MISTR-A conserved MItochondrial STress Response network

Host-pathogen conflicts leave genetic signatures in genes that are critical for host defense functions. Using these "molecular scars" as a guide to discover gene functions, we discovered a vertebrate-specific MItochondrial STress Response (MISTR) circuit. MISTR proteins are associated with electron transport chain (ETC) factors and activated by stress signals such as interferon gamma (IFNγ) and hypoxia. Upon stress, ultraconserved microRNAs (miRNAs) down-regulate MISTR1(NDUFA4) followed by replacement with paralogs MItochondrial STress Response AntiViral (MISTRAV) and/or MItochondrial STress Response Hypoxia (MISTRH). While cells lacking MISTR1(NDUFA4) are more sensitive to chemical and viral apoptotic triggers, cells lacking MISTRAV or expressing the squirrelpox virus-encoded vMISTRAV exhibit resistance to the same insults. Rapid evolution signatures across primate genomes for MISTR1(NDUFA4) and MISTRAV indicate recent and ongoing conflicts with pathogens. MISTR homologs are also found in plants, yeasts, a fish virus, and an algal virus indicating ancient origins and suggesting diverse means of altering mitochondrial function under stress. The discovery of MISTR circuitry highlights the use of evolution-guided studies to reveal fundamental biological processes.

Viral MLKL Homologs Subvert Necroptotic Cell Death by Sequestering Cellular RIPK3

Necroptotic cell death has been implicated in many human pathologies and is thought to have evolved as an innate immunity mechanism. The pathway relies on two key effectors: the kinase receptor-interacting protein kinase 3 (RIPK3) and the terminal effector, the pseudokinase mixed-lineage kinase-domain-like (MLKL). We identify proteins with high sequence similarity to the pseudokinase domain of MLKL in poxvirus genomes. Expression of these proteins from the BeAn 58058 and Cotia poxviruses, but not swinepox, in human and mouse cells blocks cellular MLKL activation and necroptotic cell death. We show that viral MLKL-like proteins function as dominant-negative mimics of host MLKL, which inhibit necroptosis by sequestering RIPK3 via its kinase domain to thwart MLKL engagement and phosphorylation. These data support an ancestral role for necroptosis in defense against pathogens. Furthermore, mimicry of a cellular pseudokinase by a pathogen adds to the growing repertoire of functions performed by pseudokinases in signal transduction.

Pupae protein extracts exert anticancer effects by downregulating the expression of IL-6, IL-1β and TNF-α through biomolecular changes in human breast cancer cells

The Pupae of Bombyx mori and Samia ricini are a source of high-quality proteins and essential nutrient elements for human. Recent studies revealed that protein extracted from pupae possessed therapeutic benefits for the treatment of many diseases. However, the anticancer activity of protein extracts from the pupae of B. mori and S. ricini has been rarely reported. Our objective was to study the effect of protein extracts from the pupae of B. mori and S. ricini on cytotoxicity and expression of pro-inflammatory cytokines; IL-6, IL-1β and TNF-α, in breast cancer cells (MCF-7). Additionally, anticancer action of protein extracted from the pupae was further investigated through biomolecular changes in MCF-7 cells using Fourier transform infrared (FTIR) spectroscopy. Pupae protein extracts of B. mori exhibited cytotoxic effects with an IC₅₀ value of 15.23 + 0.4 μg/mL with higher selectivity than doxorubicin on MCF-7 cells. Fourier transform infrared (FTIR) spectroscopy revealed that lipid contents in MCF-7 cells treated with pupae protein extracts of B. mori were higher than untreated cells. Treatment with protein extracts from pupae of B. mori or S. ricini caused significantly reduced protein and nucleic acid contents of MCF-7 cells. The expression of IL-6, IL-1β and TNF-α in MCF-7 treated cells was investigated using RT-qPCR and ELISA. Our results revealed that protein extracts from the pupae of B. mori or S. ricini significantly decreased IL-6, IL-1β and TNF-α in MCF-7 cells both at mRNA and protein levels. Expression of IL-6 and IL-1β in MCF-7 treated cells, especially IL-6, was strongly reduced compared to untreated cells, while TNF-α expression was slightly decreased. These findings suggest that pupae protein extracted from B. mori or S. ricini may play a role in breast cancer through a down-regulatory action on the expression of IL-6, IL-1β and TNF-α, and may also exert anticancer effects by causing biochemical changes of lipids, proteins and nucleic acids. These findings indicate that pupae protein extracted from B. mori or S. ricini may provide a potential novel therapeutic target for breast cancer.

Evaluation of Ultra-Microscopic Changes and Proliferation of Apoptotic Glioblastoma Multiforme Cells Induced by Velogenic Strain of Newcastle Disease Virus AF2240

Aim:

Newcastle disease virus (NDV) is a member of genus Avulavirus within the family Paramyxoviridae. Interest of using NDV as an anticancer agent has arisen from its ability to kill tumor cells with limited toxicity to normal cells.

Methods:

In this investigation, the proliferation of brain tumor cell line, glioblastoma multiform (DBTRG.05MG) induced by NDV strain AF2240 was evaluated in-vitro, by using MTT proliferation assay. Furthermore, Cytological observations were studied using fluorescence microscopy and transmission electron microscopy, DNA laddering in agarose gel electrophoresis assay used to detect the mode of cell death and analysis of the cellular DNA content by flowcytometery.

Results:

MTT proliferation assay, Cytological observations using fluorescence microscopy and transmission electron microscopy show the anti-proliferation effect and apoptogenic features of NDV on DBTRG.05MG. Furthermore, analysis of the cellular DNA content showed that there was a loss of treated cells in all cell cycle phases (G1, S and G2/M) accompanied with increasing in sub-G1 region (apoptosis peak).

Conclusion:

It could be concluded that NDV strain AF2240 is a potent antitumor agent that induce apoptosis and its cytotoxicity increasing while increasing of time and virus titer.

DNA-stimulated cell death: implications for host defence, inflammatory diseases and cancer

The immune system detects disturbances in homeostasis that occur during infection, sterile tissue damage and cancer. This initiates immune responses that seek to eliminate the trigger of immune activation and to re-establish homeostasis. At the same time, these mechanisms can also play a crucial role in the progression of disease. The occurrence of DNA in the cytosol constitutes a potent trigger for the innate immune system, governing the production of key inflammatory cytokines such as type I interferons and IL-1β. More recently, it has become clear that cytosolic DNA also triggers other biological responses, including various forms of programmed cell death. In this article, we review the emerging literature on the pathways governing DNA-stimulated cell death and the current knowledge on how these processes shape immune responses to exogenous and endogenous challenges.

The SIAH1-HIPK2-p53ser46 Damage Response Pathway is Involved in Temozolomide-Induced Glioblastoma Cell Death

Patients suffering from glioblastoma have a dismal prognosis, indicating the need for new therapeutic targets. Here we provide evidence that the DNA damage kinase HIPK2 and its negative regulatory E3-ubiquitin ligase SIAH1 are critical factors controlling temozolomide-induced cell death. We show that HIPK2 downregulation (HIPK2kd) significantly reduces the level of apoptosis. This was not the case in glioblastoma cells expressing the repair protein MGMT, suggesting that the primary DNA lesion responsible for triggering HIPK2-mediated apoptosis is O⁶ -methylguanine. Upon temozolomide treatment, p53 becomes phosphorylated whereby HIPK2kd had impact exclusively on ser46, but not ser15. Searching for the transcriptional target of p-p53ser46, we identified the death receptor FAS (CD95, APO-1) being involved. Thus, the expression of FAS was attenuated following HIPK2kd, supporting the conclusion that HIPK2 regulates temozolomide-induced apoptosis via p-p53ser46-driven FAS expression. This was substantiated in chromatin-immunoprecipitation experiments, in which p-p53ser46 binding to the Fas promotor was regulated by HIPK2. Other pro-apoptotic proteins such as PUMA, NOXA, BAX, and PTEN were not affected in HIPK2kd, and also double-strand breaks following temozolomide remained unaffected. We further show that downregulation of the HIPK2 inactivator SIAH1 significantly ameliorates temozolomide-induced apoptosis, suggesting that the ATM/ATR target SIAH1 together with HIPK2 plays a proapoptotic role in glioma cells exhibiting p53wt status. A database analysis revealed that SIAH1, but not SIAH2, is significantly overexpressed in glioblastomas. IMPLICATIONS: The identification of a novel apoptotic pathway triggered by the temozolomide-induced DNA damage O⁶ -methylguanine supports the role of p53 in the decision between survival and death and suggests SIAH1 and HIPK2 as new therapeutic targets.

Interview: a conversation with Vishva M Dixit on his journey from remote African village to apoptosis, necroptosis and the inflammasome

Vishva M. Dixit, M.D., Vice President of Physiological Chemistry at Genentech, Inc. has made many contributions to biomedicine, and his early work on apoptosis is prominent in introductory textbooks of biology and medicine.

He is a member of the National Academy of Sciences, the National Academy of Medicine, the American Academy of Arts and Sciences, and a Foreign Member, European Molecular Biology Organization.

Additionally, he serves on the Boards of the Gates Foundation, Howard Hughes Medical Institute, and Keystone Symposia.

The Structural Basis of Necroptotic Cell Death Signaling

The recent implication of the cell death pathway, necroptosis, in innate immunity and a range of human pathologies has led to intense interest in the underlying molecular mechanism. Unlike the better-understood apoptosis pathway, necroptosis is a caspase-independent pathway that leads to cell lysis and release of immunogens downstream of death receptor and Toll-like receptor (TLR) ligation. Here we review the role of recent structural studies of the core machinery of the pathway, the protein kinases receptor-interacting protein kinase (RIPK)1 and RIPK3, and the terminal effector, the pseudokinase mixed lineage kinase domain-like protein (MLKL), in shaping our mechanistic understanding of necroptotic signaling. Structural studies have played a key role in establishing models that describe MLKL's transition from a dormant monomer to a killer oligomer and revealing important interspecies differences.

Vaccinia virus evasion of regulated cell death

Regulated cell death is a powerful anti-viral mechanism capable of aborting the virus replicative cycle and alerting neighbouring cells to the threat of infection. The biological importance of regulated cell death is illustrated by the rich repertoire of host signalling cascades causing cell death and by the multiple strategies exhibited by viruses to block death signal transduction and preserve cell viability. Vaccinia virus (VACV), a poxvirus and the vaccine used to eradicate smallpox, encodes multiple proteins that interfere with apoptotic, necroptotic and pyroptotic signalling. Here the current knowledge on cell death pathways and how VACV proteins interact with them is reviewed. Studying the mechanisms evolved by VACV to counteract host programmed cell death has implications for its successful use as a vector for vaccination and as an oncolytic agent against cancer.

Serro 2 Virus Highlights the Fundamental Genomic and Biological Features of a Natural Vaccinia Virus Infecting Humans

Vaccinia virus (VACV) has been implicated in infections of dairy cattle and humans, and outbreaks have substantially impacted local economies and public health in Brazil. During a 2005 outbreak, a VACV strain designated Serro 2 virus (S2V) was collected from a 30-year old male milker. Our aim was to phenotypically and genetically characterize this VACV Brazilian isolate. S2V produced small round plaques without associated comets when grown in BSC40 cells. Furthermore, S2V was less virulent than the prototype strain VACV-Western Reserve (WR) in a murine model of intradermal infection, producing a tiny lesion with virtually no surrounding inflammation. The genome of S2V was sequenced by primer walking. The coding region spans 184,572 bp and contains 211 predicted genes. Mutations in envelope genes specifically associated with small plaque phenotypes were not found in S2V; however, other alterations in amino acid sequences within these genes were identified. In addition, some immunomodulatory genes were truncated in S2V. Phylogenetic analysis using immune regulatory-related genes, besides the hemagglutinin gene, segregated the Brazilian viruses into two clusters, grouping the S2V into Brazilian VACV group 1. S2V is the first naturally-circulating human-associated VACV, with a low passage history, to be extensively genetically and phenotypically characterized.

A tale of two domains - a structural perspective of the pseudokinase, MLKL

Recently, the programmed necrosis or 'necroptosis' cell death pathway has attracted much interest because of its implication in multiple pathologies, including inflammatory diseases and the cell death arising from ischaemia reperfusion injuries. Pharmacologically, necroptosis is an attractive target because, unlike the counterpart pathway, apoptosis, it is dispensable for mammalian development. In particular, the most terminal-known obligate effector in the necroptosis pathway, the pseudokinase MLKL (mixed lineage kinase domain-like), holds particular appeal because, thus far, its only known function is as a mediator of necroptotic cell death. We review the current understanding and gaps in knowledge relating to how MLKL can be activated by receptor interacting protein kinase (RIPK)3 downstream of tumour necrosis factor receptor 1:RIPK1, Toll like receptor-3:TRIF and viral DNA: DAI (DNA-dependent activator of interferon regulatory factors)/ZBF1. We also discuss the potential mechanism(s) by which MLKL induces necroptotic cell death, with particular emphasis on insights arising from structural studies of mouse and human MLKL.

Analysis of the anti-apoptotic activity of four vaccinia virus proteins demonstrates that B13 is the most potent inhibitor in isolation and during viral infection

Vaccinia virus (VACV) is a large dsDNA virus encoding ~200 proteins, several of which inhibit apoptosis. Here, a comparative study of anti-apoptotic proteins N1, F1, B13 and Golgi anti-apoptotic protein (GAAP) in isolation and during viral infection is presented. VACVs strains engineered to lack each gene separately still blocked apoptosis to some degree because of functional redundancy provided by the other anti-apoptotic proteins. To overcome this redundancy, we inserted each gene separately into a VACV strain (vv811) that lacked all these anti-apoptotic proteins and that induced apoptosis efficiently during infection. Each protein was also expressed in cells using lentivirus vectors. In isolation, each VACV protein showed anti-apoptotic activity in response to specific stimuli, as measured by immunoblotting for cleaved poly(ADP ribose) polymerase-1 and caspase-3 activation. Of the proteins tested, B13 was the most potent inhibitor, blocking both intrinsic and extrinsic stimuli, whilst the activity of the other proteins was largely restricted to inhibition of intrinsic stimuli. In addition, B13 and F1 were effective blockers of apoptosis induced by vv811 infection. Finally, whilst differences in induction of apoptosis were barely detectable during infection with VACV strain Western Reserve compared with derivative viruses lacking individual anti-apoptotic genes, several of these proteins reduced activation of caspase-3 during infection by vv811 strains expressing these proteins. These results illustrated that vv811 was a useful tool to determine the role of VACV proteins during infection and that whilst all of these proteins have some anti-apoptotic activity, B13 was the most potent.

TNF and TNF-receptors: From mediators of cell death and inflammation to therapeutic giants - past, present and future

Tumor Necrosis Factor (TNF), initially known for its tumor cytotoxicity, is a potent mediator of inflammation, as well as many normal physiological functions in homeostasis and health, and anti-microbial immunity. It also appears to have a central role in neurobiology, although this area of TNF biology is only recently emerging. Here, we review the basic biology of TNF and its normal effector functions, and discuss the advantages and disadvantages of therapeutic neutralization of TNF - now a commonplace practice in the treatment of a wide range of human inflammatory diseases. With over ten years of experience, and an emerging range of anti-TNF biologics now available, we also review their modes of action, which appear to be far more complex than had originally been anticipated. Finally, we highlight the current challenges for therapeutic intervention of TNF: (i) to discover and produce orally delivered small molecule TNF-inhibitors, (ii) to specifically target selected TNF producing cells or individual (diseased) tissue targets, and (iii) to pre-identify anti-TNF treatment responders. Although the future looks bright, the therapeutic modulation of TNF now moves into the era of personalized medicine with society's challenging expectations of durable treatment success and of achieving long-term disease remission.

Induction of serpinb1a by PACAP or NGF is required for PC12 cells survival after serum withdrawal

PC12 cells are used to study the signaling mechanisms underlying the neurotrophic and neuroprotective activities of pituitary adenylate cyclase-activating polypeptide (PACAP) and nerve growth factor (NGF). Previous microarray experiments indicated that serpinb1a was the most induced gene after 6 h of treatment with PACAP or NGF. This study confirmed that serpinb1a is strongly activated by PACAP and NGF in a time-dependent manner with a maximum induction (~ 50-fold over control) observed after 6 h of treatment. Co-incubation with PACAP and NGF resulted in a synergistic up-regulation of serpinb1a expression (200-fold over control), suggesting that PACAP and NGF act through complementary mechanisms. Consistently, PACAP-induced serpinb1a expression was not blocked by TrkA receptor inhibition. Nevertheless, the stimulation of serpinb1a expression by PACAP and NGF was significantly reduced in the presence of extracellular signal-regulated kinase, calcineurin, protein kinase A, p38, and PI3K inhibitors, indicating that the two trophic factors share some common pathways in the regulation of serpinb1a. Finally, functional investigations conducted with siRNA revealed that serpinb1a is not involved in the effects of PACAP and NGF on PC12 cell neuritogenesis, proliferation or body cell volume but mediates their ability to block caspases 3/7 activity and to promote PC12 cell survival.

Proteases in cardiometabolic diseases: Pathophysiology, molecular mechanisms and clinical applications

Cardiovascular disease is the leading cause of death in the U.S. and other developed countries. Metabolic syndrome, including obesity, diabetes/insulin resistance, hypertension and dyslipidemia is a major threat for public health in the modern society. It is well established that metabolic syndrome contributes to the development of cardiovascular disease collective called as cardiometabolic disease. Despite documented studies in the research field of cardiometabolic disease, the underlying mechanisms are far from clear. Proteases are enzymes that break down proteins, many of which have been implicated in various diseases including cardiac disease. Matrix metalloproteinase (MMP), calpain, cathepsin and caspase are among the major proteases involved in cardiac remodeling. Recent studies have also implicated proteases in the pathogenesis of cardiometabolic disease. Elevated expression and activities of proteases in atherosclerosis, coronary heart disease, obesity/insulin-associated heart disease as well as hypertensive heart disease have been documented. Furthermore, transgenic animals that are deficient in or over-express proteases allow scientists to understand the causal relationship between proteases and cardiometabolic disease. Mechanistically, MMPs and cathepsins exert their effect on cardiometabolic diseases mainly through modifying the extracellular matrix. However, MMP and cathepsin are also reported to affect intracellular proteins, by which they contribute to the development of cardiometabolic diseases. On the other hand, activation of calpain and caspases has been shown to influence intracellular signaling cascade including the NF-κB and apoptosis pathways. Clinically, proteases are reported to function as biomarkers of cardiometabolic diseases. More importantly, the inhibitors of proteases are credited with beneficial cardiometabolic profile, although the exact molecular mechanisms underlying these salutary effects are still under investigation. A better understanding of the role of MMPs, cathepsins, calpains and caspases in cardiometabolic diseases process may yield novel therapeutic targets for treating or controlling these diseases. This article is part of a Special Issue entitled: Autophagy and protein quality control in cardiometabolic diseases.

Host defense mechanism-based rational design of live vaccine

Live attenuated vaccine (LAV), mimicking natural infection, provides an excellent protection against microbial infection. The development of LAV, however, still remains highly empirical and the rational design of clinically useful LAV is scarcely available. Apoptosis and caspase activation are general host antiviral responses in virus-infected cells. Utilizing these tightly regulated host defense mechanisms, we present a novel apoptosis-triggered attenuation of viral virulence as a rational design of live attenuated vaccine with desired levels of safety, efficacy, and productivity. Mutant influenza viruses carrying caspase recognition motifs in viral NP and the interferon-antagonist NS1 proteins were highly attenuated both in vitro and in vivo by caspase-mediated cleavage of those proteins in infected cells. Both viral replication and interferon-resistance were substantially reduced, resulting in a marked attenuation of virulence of the virus. Despite pronounced attenuation, the viruses demonstrated high growth phenotype in embryonated eggs at lower temperature, ensuring its productivity. A single dose vaccination with the mutant virus elicited high levels of systemic and mucosal antibody responses and provided complete protection against both homologous and heterologous lethal challenges in mouse model. While providing a practical means to generate seasonal or pandemic influenza live vaccines, the sensitization of viral proteins to pathogen-triggered apoptotic signals presents a potentially universal, mechanism-based rational design of live vaccines against many viral infections.

An emerging role for Serine Protease Inhibitors in T lymphocyte immunity and beyond

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmo-regulation, tissue re-modeling and immunity to infection. T lymphocytes are required for adaptive cell mediated immunity and serine proteases are not only important for effector function but also homeostatic regulation of cell numbers. Serine Protease Inhibitors (Serpins) are the physiological regulators of serine proteases activity. In this review, I will discuss the role of serpins in controlling the recognition of antigen, effector function and homeostatic control of T lymphocytes through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This will be discussed in the context of the T lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. The potent anti-apoptotic properties of serpins can also work against adaptive cell immunity by protecting viruses and tumors from eradication by cytotoxic T cells (CTL). Recent insights from knock-out mouse models demonstrate that these serpins also are required for hematological progenitor cells and so are critical for the development of lineages other than T lymphocytes. Given the emerging role of serpins in multiple aspects of lymphocyte immunity and blood development I will review the progress to date in developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Apoptosis resistance in tumor cells

Various antitumor agents induce apoptotic cell death in tumor cells. Since the apoptosis program in tumor cells plays a critical role in the chemotherapy-induced tumor cell killing, it is suggested that the defect in the signaling pathway of apoptosis could cause a new form of multidrug resistance in tumor cells. This article describes the recent findings concerning the mechanisms of chemotherapy-induced apoptosis and discusses the implication of apoptosis resistance in cancer chemotherapy.

Effects of newcastle disease virus strains AF2240 and V4-UPM on cytolysis and apoptosis of leukemia cell lines

Newcastle disease virus (NDV) is used as an antineoplastic agent in clinical tumor therapy. It has prompted much interest as an anticancer agent because it can replicate up to 10,000 times better in human cancer cells than in most normal cells. This study was carried out to determine the oncolytic potential of NDV strain AF2240 and V4-UPM on WEHI-3B leukemia cell line. Results from MTT cytotoxicity assay showed that the CD₅₀ values for both strains were 2 and 8 HAU for AF2240 and V4-UPM, respectively. In addition, bromodeoxyuridine (BrdU) and trypan blue dye exclusion assays showed inhibition in cell proliferation after different periods. Increase in the cellular level of caspase-3 and detection of DNA laddering using agarose gel electrophoresis on treated cells with NDV confirmed that the mode of cell death was apoptosis. In addition, flow-cytometry analysis of cellular DNA content showed that the virus caused an increase in the sub-G1 region (apoptosis peaks). In conclusion, NDV strains AF2240 and V4-UPM caused cytolytic effects against WEHI-3B leukemic cell line.

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.

Cytolytic effects and apoptosis induction of Newcastle disease virus strain AF2240 on anaplastic astrocytoma brain tumor cell line

Newcastle disease virus (NDV) is a member of genus Avulavirus within the family Paramyxoviridae. Interest of using NDV as an anticancer agent has arisen from its ability to kill tumor cells with limited toxicity to normal cells. In this investigation, the cytotolytic properties of NDV strain AF2240 were evaluated on brain tumor cell line, anaplastic astrocytoma (U-87MG), by using MTT assay. Cytological observations were studied using fluorescence microscopy and transmission electron microscopy to show the apoptogenic features of NDV on U-87MG. DNA laddering in agarose gel electrophoresis and terminal deoxyribonucleotide transferase-mediated dUTP-X nick end-labeling staining assay confirmed that the mode of cell death was by apoptosis. However, analysis of the cellular DNA content by flowcytometery showed that there was a loss of treated U-87MG cells in all cell cycle phases (G1, S and G2/M) accompanied with increasing in sub-G1 region (apoptosis peak). Early apoptosis was observed 6 h post-inoculation by annexin-V flow-cytometry method. It could be concluded that NDV strain AF2240 is a potent antitumor agent that induce apoptosis and its cytotoxicity increasing while increasing of time and virus titer.

Interictal spikes in developing rats cause long-standing cognitive deficits

Frequent interictal spikes are a common finding in the electroencephalograms of children with epileptic encephalopathies. While it is well recognized that interictal spikes are a biological marker of seizures and can lead to transitory cognitive impairment, whether interictal spikes can result in long-standing adverse effects on learning and memory in children is not known. Here we investigated the consequences of interictal spikes in rat pups without seizures on long-term learning and memory. Rat pups were given a low dose of flurothyl for 4h for 10 days during continuous electroencephalographic monitoring. Rats developed interictal spikes without seizures while age-matched controls under similar testing conditions had few interictal spikes. When rats were tested as adults, there was impairment in reference memory in the probe test of the Morris water maze, reference memory impairment in the four-trial radial-arm water maze and impaired long-term potentiation. Early-life interictal spikes resulted in impaired new cell formation and decreased cell counts in the hippocampus but did not cause an increase in apoptosis. This study, for the first time demonstrates that interictal spikes in rat pups without seizures can result in long-standing spatial cognitive impairment. Our findings suggest that suppressing IIS may be as important as treating seizures during brain development.

Serine protease inhibitors and cytotoxic T lymphocytes

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmoregulation, tissue remodeling, and immunity to infection. Cytotoxic T lymphocytes (CTLs) are required for adaptive cell-mediated immunity to intracellular pathogens by killing infected cells and through the development of memory T cells. Serine proteases not only allow a CTL to kill but also impose homeostatic control on CTL number. Serine protease inhibitors (serpins) are the physiological regulators of serine proteases' activity. In this review, I discuss the role of serpins in controlling the recognition of antigen, effector function, and homeostatic control of CTLs through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This view is discussed in the context of the T-lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. Given the important role serpins play in CTL immunity, I discuss the potential for developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Effective blockage of both the extrinsic and intrinsic pathways of apoptosis in mice by TAT-crmA

Evidence accumulates that in clinically relevant cell death, both the intrinsic and extrinsic apoptotic pathway synergistically contribute to organ failure. In search for an inhibitor of apoptosis that provides effective blockage of these pathways, we analyzed viral proteins that evolved to protect the infected host cells. In particular, the cowpox virus protein crmA has been demonstrated to be capable of blocking key caspases of both pro-apoptotic pathways. To deliver crmA into eukaryotic cells, we fused the TAT protein transduction domain of HIV to the N terminus of crmA. In vitro, the TAT-crmA fusion protein was efficiently translocated into target cells and inhibited apoptosis mediated through caspase-8, caspase-9, and caspase-3 after stimulation with alpha-Fas, etoposide, doxorubicin, or staurosporine. The extrinsic apoptotic pathway was investigated following alpha-Fas stimulation. In vivo 90% of TAT-crmA-treated animals survived an otherwise lethal dose of alpha-Fas and showed protection from Fas-induced organ failure. To examine the intrinsic apoptotic pathway, we investigated the survival of mice treated with an otherwise lethal dose of doxorubicin. Whereas all control mice died within 31 days, 40% of mice that concomitantly received intraperitoneal injections of TAT-crmA survived. To test the ability to comprehensively block both the intrinsic and extrinsic apoptotic pathway in a clinically relevant setting, we employed a murine cardiac ischemia-reperfusion model. TAT-crmA reduced infarction size by 40% and preserved left ventricular function. In summary, these results provide a proof of principle for the inhibition of apoptosis with TAT-crmA, which might provide a new treatment option for ischemia-reperfusion injuries.

Metaxin deficiency alters mitochondrial membrane permeability and leads to resistance to TNF-induced cell killing

Metaxin, a mitochondrial outer membrane protein, is critical for TNF-induced cell death in L929 cells. Its deficiency, caused by retroviral insertion-mediated mutagenesis, renders L929 cells resistance to TNF killing. In this study, we further characterized metaxin deficiency-caused TNF resistance in parallel with Bcl-X(L) overexpression-mediated death resistance. We did not find obvious change in mitochondria membrane potential in metaxin-deficient (Met(mut)) and Bcl-X(L)-overexpressing cells, but we did find an increase in the release rate of the mitochondrial membrane potential probe rhodamine 123 (Rh123) that was preloaded into mitochondria. In addition, overexpression of a function-interfering mutant of metaxin (MetaΔTM/C) or Bcl-X(L) in MCF-7.3.28 cells also resulted in an acquired resistance to TNF killing and a faster rate of Rh123 release, indicating a close correlation between TNF resistance and higher rates of the dye release from the mitochondria. The release of Rh123 can be controlled by the mitochondrial membrane permeability transition (PT) pore, as targeting an inner membrane component of the PT pore by cyclosporin A (CsA) inhibited Rh123 release. However, metaxin deficiency and Bcl-X(L) overexpression apparently affect Rh123 release from a site(s) different from that of CsA, as CsA can overcome their effect. Though both metaxin and Bcl-X(L) appear to function on the outer mitochondrial membrane, they do not interact with each other. They may use different mechanisms to increase the permeability of Rh123, since previous studies have suggested that metaxin may influence certain outer membrane porins while Bcl-X(L) may form pores on the outer membrane. The alteration of the mitochondrial outer membrane properties by metaxin deficiency and Bcl-X(L) overexpression, as indicated by a quicker Rh123 release, may be helpful in maintaining mitochondrial integrity.

p53 mutant human glioma cells are sensitive to UV-C-induced apoptosis due to impaired cyclobutane pyrimidine dimer removal

The p53 protein is a key regulator of cell responses to DNA damage, and it has been shown that it sensitizes glioma cells to the alkylating agent temozolomide by up-regulating the extrinsic apoptotic pathway, whereas it increases the resistance to chloroethylating agents, such as ACNU and BCNU, probably by enhancing the efficiency of DNA repair. However, because these agents induce a wide variety of distinct DNA lesions, the direct importance of DNA repair is hard to access. Here, it is shown that the induction of photoproducts by UV light (UV-C) significantly induces apoptosis in a p53-mutated glioma background. This is caused by a reduced level of photoproduct repair, resulting in the persistence of DNA lesions in p53-mutated glioma cells. UV-C-induced apoptosis in p53 mutant glioma cells is preceded by strong transcription and replication inhibition due to blockage by unrepaired photolesions. Moreover, the results indicate that UV-C-induced apoptosis of p53 mutant glioma cells is executed through the intrinsic apoptotic pathway, with Bcl-2 degradation and sustained Bax and Bak up-regulation. Collectively, the data indicate that unrepaired DNA lesions induce apoptosis in p53 mutant gliomas despite the resistance of these gliomas to temozolomide, suggesting that efficiency of treatment of p53 mutant gliomas might be higher with agents that induce the formation of DNA lesions whose global genomic repair is dependent on p53.

Roles of proinflammatory cytokines and the Fas/Fas ligand interaction in the pathogenesis of inflammatory myopathies

Within the lesions of inflammatory myopathies, muscle fibres and invading mononuclear cells express Fas and Fas ligand (FasL), respectively. However, the roles of the Fas/FasL interaction in the pathogenesis of inflammatory myopathies are not fully understood. In the present study, we investigated the roles of proinflammatory cytokines and the Fas/FasL system in the pathogenesis of inflammatory myopathies. In vitro culturing of muscle cells with the proinflammatory cytokines interferon-gamma, tumour necrosis factor-alpha, and interleukin (IL)-1beta synergistically increased Fas expression, susceptibility to Fas-mediated apoptosis, and the expression of cytoplasmic caspases 8 and 3. In addition, culturing of muscle cells with activated CD4(+) T cells induced muscle cell apoptosis, which was partially inhibited by anti-FasL antibody. We also tested the possibility that T helper (Th) 17, which is an IL-17-producing helper T-cell subset that plays crucial roles in autoimmune and inflammatory responses, participates in the pathogenesis of inflammatory myopathies. Interestingly, in vitro culturing of dendritic cells with anti-Fas immunoglobulin M (IgM) or activated CD4(+) T cells induced the expression of mRNA for IL-23p19, but not for IL-12p35, in addition to proinflammatory cytokines. Furthermore, IL-23p19 and IL-17 mRNAs were detected in the majority of biopsy samples from patients with inflammatory myopathies. Taken together, these results suggest that proinflammatory cytokines enhance Fas-mediated apoptosis of muscle cells, and that the Fas/FasL interaction between invading dendritic cells and CD4(+) T cells induces local production of IL-23 and proinflammatory cytokines, which can promote the proliferation of Th17 cells and enhance Fas-mediated apoptosis of muscle cells, respectively.

Regulation of caspase activation in apoptosis: implications for transformation and drug resistance

Recent developments in the apoptosis field have uncovered a family of cysteine proteases, the Caspases, that act as signalling components as well as effectors of the cell death machinery. Caspases are constitutively present as inactive precursors within most cells and undergo proteolytic processing in response to diverse death-inducing stimuli to initiate the death programme. Active caspases can process other caspases of the same type as well as process caspases further downstream in the pathway that ultimately leads to collapse of the cell. This cellular collapse is thought to occur as a consequence of caspase-mediated cleavage of a diverse array of cellular substrates. Regulation of entry into the death programme is controlled at a number of levels by members of the Bcl-2 family, as well as by other cell death regulatory proteins. Recent data has shed light upon the mechanism of action of these regulatory molecules and suggests that the point of caspase activation is a major checkpoint in the cell death programme. Because many transformed cell populations possess derangements in cell death-regulatory genes, such as bcl-2, such cells frequently exhibit elevated resistance to cytotoxic chemotherapy. Thus, a deeper understanding of how apoptosis is normally regulated has therapeutic implications for disease states where the normal controls on the cell death machinery have been subverted.

Inhibition of proinflammatory and innate immune signaling pathways by a cytomegalovirus RIP1-interacting protein

TNFalpha is an important cytokine in antimicrobial immunity and inflammation. The receptor-interacting protein RIP1 is an essential component of the TNF receptor 1 signaling pathway that mediates the activation of NF-kappaB, MAPKs, and programmed cell death. It also transduces signals derived from Toll-like receptors and intracellular sensors of DNA damage and double-stranded RNA. Here, we show that the murine CMV M45 protein binds to RIP1 and inhibits TNFalpha-induced activation of NF-kappaB, p38 MAPK, and caspase-independent cell death. M45 also inhibited NF-kappaB activation upon stimulation of Toll-like receptor 3 and ubiquitination of RIP1, which is required for NF-kappaB activation. Hence, M45 functions as a viral inhibitor of RIP1-mediated signaling. The results presented here reveal a mechanism of viral immune subversion and demonstrate how a viral protein can simultaneously block proinflammatory and innate immune signaling pathways by interacting with a central mediator molecule.

Tumor necrosis factor-alpha-induced caspase-1 gene expression. Role of p73

Tumour necrosis factor-alpha (TNF-alpha) is a cytokine that is involved in many functions, including the inflammatory response, immunity and apoptosis. Some of the responses of TNF-alpha are mediated by caspase-1, which is involved in the production of the pro-inflammatory cytokines interleukin-1beta, interleukin-18 and interleukin-33. The molecular mechanisms involved in TNF-alpha-induced caspase-1 gene expression remain poorly defined, despite the fact that signaling by TNF-alpha has been well studied. The present study was undertaken to investigate the mechanisms involved in the induction of caspase-1 gene expression by TNF-alpha. Treatment of A549 cells with TNF-alpha resulted in an increase in caspase-1 mRNA and protein expression, which was preceded by an increase in interferon regulatory factor-1 and p73 protein levels. Caspase-1 promoter reporter was activated by the treatment of cells with TNF-alpha. Mutation of the interferon regulatory factor-1 binding site resulted in the almost complete loss of basal as well as of TNF-alpha-induced caspase-1 promoter activity. Mutation of the p53/p73 responsive site resulted in reduced TNF-alpha-induced promoter activity. Blocking of p73 function by a dominant negative mutant or by a p73-directed small hairpin RNA reduced basal as well as TNF-alpha-induced caspase-1 promoter activity. TNF-alpha-induced caspase-1 mRNA and protein levels were reduced when p73 mRNA was down-regulated by small hairpin RNA. Caspase-5 gene expression was induced by TNF-alpha, which was inhibited by the small hairpin RNA-mediated down-regulation of p73. Our results show that TNF-alpha induces p73 gene expression, which, together with interferon regulatory factor-1, plays an important role in mediating caspase-1 promoter activation by TNF-alpha.

Ectopic expression of the serine protease inhibitor PI9 modulates death receptor-mediated apoptosis

Apoptosis is a highly controlled process, whose triggering is associated with the activation of caspases. Apoptosis can be induced via a subgroup of the tumor necrosis factor (TNF) receptor superfamily, which recruit and activate pro-caspase-8 and -10. Regulation of apoptosis is achieved by several inhibitors, including c-FLICE-inhibitory protein, which prevents apoptosis by inhibiting the pro-apoptotic activation of upstream caspases. Here we show that the human intracellular serine protease inhibitor (serpin), protease inhibitor 9 (PI9), inhibits TNF-, TNF-related apoptosis-inducing ligand- and Fas ligand-mediated apoptosis in certain TNF-sensitive cell lines. The reactive center P1 residue of PI9 was required for this inhibition since PI9 harboring a Glu --> Ala mutation in its reactive center failed to impair death receptor-induced cell death. This suggests a classical serpin-protease interaction. Indeed, PI9 inhibited apoptotic death by directly interacting with the intermediate active forms of caspase-8 and -10. This indicates that PI9 can regulate pro-apoptotic apical caspases.

The large and small isoforms of human papillomavirus type 16 E6 bind to and differentially affect procaspase 8 stability and activity

Human papillomavirus type 16 (HPV-16) has developed numerous ways to modulate host-initiated immune mechanisms. The HPV-16 E6 oncoprotein, for example, can modulate the cellular level, and consequently the activity, of procaspase 8, thus modifying the cellular response to cytokines of the tumor necrosis factor family. E6 from HPV-16, but not E6 from the low-risk types 6b and 11, alters the cellular level of procaspase 8 in a dose-dependent manner. Both the large and small (E6*) isoforms of E6, which originate by way of alternate splicing, can modulate procaspase 8 stability. Intriguingly, although both isoforms bind to procaspase 8, the large isoform accelerates the degradation of procaspase 8 while the small isoform stabilizes it. Binding leads to a change in the ability of procaspase 8 to bind either to itself or to FADD (Fas-associated death domain), with the large version of E6 able to inhibit this binding while the small isoform does not. Consistent with this model, knockdown of the large version of E6 by small interfering RNA leads to increases in the levels of procaspase 8 and its binding to both itself and FADD. Thus, these alternatively spliced isoforms can modulate both the level and the activity of procaspase 8 in opposite directions.

The effects of nicotinamide on apoptosis and blood-brain barrier breakdown following traumatic brain injury

Nicotinamide has been shown to protect against many of the pathophysiological factors associated with both ischemic and traumatic brain injuries. The present study evaluated the neuroprotective effect of nicotinamide on the breakdown of the blood-brain barrier (BBB) and apoptosis expression following traumatic brain injury (TBI). Animals were prepared with a unilateral cortical contusion injury (CCI). Fifteen minutes following injury the animals received either nicotinamide (500 mg/kg, ip) or 0.9% saline. The animals were perfused at 5, 24, and 72 h post-injury. BBB integrity was assessed by endogenous rat IgG immunoreactivity. Recent studies have shown that IgG immunoreactivity is a reliable measure of BBB integrity. The results indicated that IgG immunoreactivity was greatest at 5 h and declined at 24 h after injury. Nicotinamide significantly reduced IgG expression at every time point following injury. Apoptosis was examined using the TUNEL method. The results indicated that TUNEL immunoreactivity peaked at 24 h. TUNEL(+) cells were classified morphologically as nonapoptotic (Type I) or apoptotic (Type II) to verify that the neuroprotective effects of nicotinamide occur by inhibiting apoptosis or necrosis. Administration of nicotinamide significantly reduced the expression of all TUNEL(+) cells in the tissue surrounding the lesion cavity. Specifically there was a significant reduction in the number of Type I, Type II, and Total TUNEL(+) cells in the nicotinamide-treated animals. In addition, nicotinamide reduced lesion cavity expansion 72 h following CCI. These findings suggest that nicotinamide reduces BBB breach and neuronal cell loss acutely following injury and that these reductions may account for the beneficial behavioral effects seen in previous studies.

Caspase inhibitors: viral, cellular and chemical

Caspases, key mediators of apoptosis, are a structurally related family of cysteine proteases that cleave their substrates at aspartic acid residues either to cause cell death or to activate cytokines as part of an immune response. They can be controlled upstream by the regulation of signals that lead to zymogen activation, or downstream by inhibitors that prevent them from reaching their substrates. This review specifically looks at caspase inhibitors as distinct from caspase regulators: those produced by the cell itself; those whose genes are carried by viruses; and artificial caspase inhibitors used for research and potentially as therapeutics.

Apoptosis in malignant glioma cells triggered by the temozolomide-induced DNA lesion O6-methylguanine

Methylating drugs such as temozolomide (TMZ) are widely used in the treatment of brain tumours (malignant gliomas). The mechanism of TMZ-induced glioma cell death is unknown. Here, we show that malignant glioma cells undergo apoptosis following treatment with the methylating agents N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and TMZ. Cell death determined by colony formation and apoptosis following methylation is greatly stimulated by p53. Transfection experiments with O(6)-methylguanine-DNA methyltransferase (MGMT) and depletion of MGMT by O(6)-benzylguanine showed that, in gliomas, the apoptotic signal originates from O(6)-methylguanine (O(6)MeG) and that repair of O(6)MeG by MGMT prevents apoptosis. We further demonstrate that O(6)MeG-triggered apoptosis requires Fas/CD95/Apo-1 receptor activation in p53 non-mutated glioma cells, whereas in p53 mutated gliomas the same DNA lesion triggers the mitochondrial apoptotic pathway. This occurs less effectively via Bcl-2 degradation and caspase-9, -2, -7 and -3 activation. O(6)MeG-triggered apoptosis in gliomas is a late response (occurring >120 h after treatment) that requires extensive cell proliferation. Stimulation of cell cycle progression by the Pasteurella multocida toxin promoted apoptosis whereas serum starvation attenuated it. O(6)MeG-induced apoptosis in glioma cells was preceded by the formation of DNA double-strand breaks (DSBs), as measured by gammaH2AX formation. Glioma cells mutated in DNA-PK(cs), which is involved in non-homologous end-joining, were more sensitive to TMZ-induced apoptosis, supporting the involvement of DSBs as a downstream apoptosis triggering lesion. Overall, the data demonstrate that cell death induced by TMZ in gliomas is due to apoptosis and that determinants of sensitivity of gliomas to TMZ are MGMT, p53, proliferation rate and DSB repair.

Induction of apoptosis limits cytomegalovirus cross-species infection

Cross-species infections are responsible for the majority of emerging and re-emerging viral diseases. However, little is known about the mechanisms that restrict viruses to a certain host species, and the factors viruses need to cross the species barrier and replicate in a different host. Cytomegaloviruses (CMVs) are representatives of the beta-herpesviruses that are highly species specific. They replicate only in cells of their own or a closely related species. In this study, the molecular mechanism underlying the cytomegalovirus species specificity was investigated. We show that infection of human cells with the murine cytomegalovirus (MCMV) triggers the intrinsic apoptosis pathway involving caspase-9 activation. MCMV can break the species barrier and replicate in human cells if apoptosis is blocked by Bcl-2 or a functionally analogous protein. A single gene of the human cytomegalovirus encoding a mitochondrial inhibitor of apoptosis is sufficient to allow MCMV replication in human cells. Moreover, the same principle facilitates replication of the rat cytomegalovirus in human cells. Thus, induction of apoptosis serves as an innate immune defense to inhibit cross-species infections of rodent CMVs.

Dual role of TMS1/ASC in death receptor signaling

Aberrant DNA methylation of promoter region CpG islands is associated with gene silencing and serves as an alternative to mutations in the inactivation of tumor suppressor genes in human cancers. We identified a gene TMS1 (for Target of Methylation-mediated Silencing) that is subject to such epigenetic silencing in a significant proportion of human breast and other cancers. Also known as ASC and PYCARD, TMS1 encodes a bipartite intracellular signaling molecule with proposed roles in apoptosis and inflammation. However, the precise role of this protein in the pathogenesis of breast and other cancers has not been clearly defined. In this study, we examined the role of TMS1/ASC in death receptor signaling. We found that TMS1/ASC is upregulated in response to treatment with TNF-related apoptosis-inducing ligand (TRAIL) and tumor necrosis factor-alpha (TNFalpha) in breast epithelial cells, but not in human fibroblasts. This upregulation was not dependent on the synthesis of a TNFalpha-regulated intermediate or alterations in mRNA stability, suggesting a direct effect on TMS1/ASC transcription. Induction of TMS1/ASC by TNFalpha was blocked by co-expression of a dominant negative IkappaBalpha, small interfering RNA-mediated knockdown of RelA/p65, or concurrent treatment with SP600125, indicating a requirement for the nuclear factor-kappaB (NF-kappaB) and jun kinase signaling pathways. Although previous work has suggested that TMS1/ASC may be directly regulated by p53, we found that whereas treatment of breast epithelial cells or normal diploid fibroblasts with DNA damaging agents resulted in the stabilization of endogenous p53 and a concomitant increase in p21, it had little impact on the expression of TMS1/ASC mRNA or protein. We further show that whereas TMS1/ASC is not required for TNFalpha or TRAIL-induced activation of NF-kappaB or caspase-8, it can promote caspase-8 activation independently of death receptor-ligand interactions. Taken together, these data suggest that upregulation of TMS1/ASC by TNFalpha and subsequent activation of caspase-8 could function to amplify the apoptotic signal induced by death receptors in some cell types, including breast epithelial cells.

Src kinase phosphorylates Caspase-8 on Tyr380: a novel mechanism of apoptosis suppression

We identified Caspase-8 as a new substrate for Src kinase. Phosphorylation occurs on Tyr380, situated in the linker region between the large and the small subunits of human Procaspase-8, and results in downregulation of Caspase-8 proapoptotic function. Src activation triggers Caspase-8 phosphorylation on Tyr380 and impairs Fas-induced apoptosis. Accordingly, Src failed to protect Caspase-8-defective human cells in which a Caspase-8-Y380F mutant is expressed from Fas-induced cell death. Remarkably, Src activation upon EGF-receptor stimulation triggers endogenous Caspase-8 phosphorylation and prevents Fas-induced apoptosis. Tyr380 is phosphorylated also in human colon cancers where Src is aberrantly activated. These data provide the first evidence for a direct role of tyrosine phosphorylation in the control of caspases and reveal a new mechanism through which tyrosine kinases inhibit apoptosis and participate in tumor progression.

Near death experiences: poxvirus regulation of apoptotic death

Apoptosis, or programmed cell death, plays a critical role in the elimination of virus-infected cells. As a result, a growing number of viruses encode numerous potent anti-apoptotic proteins to counteract apoptosis in an effort to prolong their own survival. This review describes the numerous mechanisms by which poxviruses inhibit apoptosis thereby modulating life and death of the cell.

Modulation of tumor necrosis factor by microbial pathogens

In response to invasion by microbial pathogens, host defense mechanisms get activated by both the innate and adaptive arms of the immune responses. TNF (tumor necrosis factor) is a potent proinflammatory cytokine expressed by activated macrophages and lymphocytes that induces diverse cellular responses that can vary from apoptosis to the expression of genes involved in both early inflammatory and acquired immune responses. A wide spectrum of microbes has acquired elegant mechanisms to overcome or deflect the host responses mediated by TNF. For example, modulatory proteins encoded by multiple families of viruses can block TNF and TNF-mediated responses at multiple levels, such as the inhibition of the TNF ligand or its receptors, or by modulating key transduction molecules of the TNF signaling pathway. Bacteria, on the other hand, tend to modify TNF-mediated responses specifically by regulating components of the TNF signaling pathway. Investigation of these diverse strategies employed by viral and bacterial pathogens has significantly advanced our understanding of both host TNF responses and microbial pathogenesis. This review summarizes the diverse microbial strategies to regulate TNF and how such insights into TNF modulation could benefit the treatment of inflammatory or autoimmune diseases.

Granule-mediated killing by granzyme B and perforin requires a mannose 6-phosphate receptor and is augmented by cell surface heparan sulfate

During granule-mediated killing by cytotoxic T lymphocytes or natural killer cells, the serine protease granzyme B enters the target cell by endocytosis and induces apoptosis. Previous studies suggested a role for the mannose 6-phosphate receptor, but further experiments with purified granzyme B indicated this was not essential. Additionally, it is now clear that grB is exocytosed from killer cells in a high-molecular-weight complex with the proteoglycan serglycin. Here granzyme B was delivered as a purified monomer, or in complex with either glycosaminoglycans or serglycin, and killing was evaluated. When granzyme B was a monomer, soluble mannose 6-phosphate had a limited impact, whereas apoptosis induced by the complexed grB was effectively inhibited by mannose 6-phosphate. Most importantly, when granzyme B and perforin were delivered together from granules, inhibition by mannose 6-phosphate was also observed. In pulldown assays mediated by the cation-independent mannose 6-phosphate receptor, granzyme B bound to the receptor more intensely in the presence of immobilized heparan sulfate. We therefore propose the model that under physiological conditions serglycin-bound granzyme B is critically endocytosed by a mannose 6-phosphate receptor, and receptor binding is enhanced by cell surface heparan sulfate.