Oligomerization is a general mechanism for the activation of apoptosis initiator and inflammatory procaspases

Selective induction of programmed cell death using synthetic biology tools

Regulated cell death (RCD) controls the removal of dispensable, infected or malignant cells, and is thus essential for development, homeostasis and immunity of multicellular organisms. Over the last years different forms of RCD have been described (among them apoptosis, necroptosis, pyroptosis and ferroptosis), and the cellular signaling pathways that control their induction and execution have been characterized at the molecular level. It has also become apparent that different forms of RCD differ in their capacity to elicit inflammation or an immune response, and that RCD pathways show a remarkable plasticity. Biochemical and genetic studies revealed that inhibition of a given pathway often results in the activation of back-up cell death mechanisms, highlighting close interconnectivity based on shared signaling components and the assembly of multivalent signaling platforms that can initiate different forms of RCD. Due to this interconnectivity and the pleiotropic effects of 'classical' cell death inducers, it is challenging to study RCD pathways in isolation. This has led to the development of tools based on synthetic biology that allow the targeted induction of RCD using chemogenetic or optogenetic methods. Here we discuss recent advances in the development of such toolset, highlighting their advantages and limitations, and their application for the study of RCD in cells and animals.

CARD-only proteins regulate in vivo inflammasome responses and ameliorate gout

Inflammatory responses are crucial for controlling infections and initiating tissue repair. However, excessive and uncontrolled inflammation causes inflammatory disease. Processing and release of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18 depend on caspase-1 activation within inflammasomes. Assembly of inflammasomes is initiated upon activation of cytosolic pattern recognition receptors (PRRs), followed by sequential polymerization of pyrin domain (PYD)-containing and caspase recruitment domain (CARD)-containing proteins mediated by homotypic PYD and CARD interactions. Small PYD- or CARD-only proteins (POPs and COPs, respectively) evolved in higher primates to target these crucial interactions to limit inflammation. Here, we show the ability of COPs to regulate inflammasome activation by modulating homotypic CARD-CARD interactions in vitro and in vivo. CARD16, CARD17, and CARD18 displace crucial CARD interactions between caspase-1 proteins through competitive binding and ameliorate uric acid crystal-mediated NLRP3 inflammasome activation and inflammatory disease. COPs therefore represent an important family of inflammasome regulators and ameliorate inflammatory disease.

Streptococcus pneumoniae and Its Virulence Factors H2O2 and Pneumolysin Are Potent Mediators of the Acute Chest Syndrome in Sickle Cell Disease

Sickle cell disease (SCD) is one of the most common autosomal recessive disorders in the world. Due to functional asplenia, a dysfunctional antibody response, antibiotic drug resistance and poor response to immunization, SCD patients have impaired immunity. A leading cause of hospitalization and death in SCD patients is the acute chest syndrome (ACS). This complication is especially manifested upon infection of SCD patients with Streptococcus pneumoniae (Spn)—a facultative anaerobic Gram-positive bacterium that causes lower respiratory tract infections. Spn has developed increased rates of antibiotics resistance and is particularly virulent in SCD patients. The primary defense against Spn is the generation of reactive oxygen species (ROS) during the oxidative burst of neutrophils and macrophages. Paradoxically, Spn itself produces high levels of the ROS hydrogen peroxide (H₂O₂) as a virulence strategy. Apart from H₂O₂, Spn also secretes another virulence factor, i.e., the pore-forming exotoxin pneumolysin (PLY), a potent mediator of lung injury in patients with pneumonia in general and particularly in those with SCD. PLY is released early on in infection either by autolysis or bacterial lysis following the treatment with antibiotics and has a broad range of biological activities. This review will discuss recent findings on the role of pneumococci in ACS pathogenesis and on strategies to counteract the devastating effects of its virulence factors on the lungs in SCD patients.

An overview of the non-canonical inflammasome

Inflammasomes are large cytosolic multiprotein complexes assembled in response to infection and cellular stress, and are crucial for the activation of inflammatory caspases and the subsequent processing and release of pro-inflammatory mediators. While caspase-1 is activated within the canonical inflammasome, the related caspase-4 (also known as caspase-11 in mice) and caspase-5 are activated within the non-canonical inflammasome upon sensing of cytosolic lipopolysaccharide (LPS) from Gram-negative bacteria. However, the consequences of canonical and non-canonical inflammasome activation are similar. Caspase-1 promotes the processing and release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and the release of danger signals, as well as a lytic form of cell death called pyroptosis, whereas caspase-4, caspase-5 and caspase-11 directly promote pyroptosis through cleavage of the pore-forming protein gasdermin D (GSDMD), and trigger a secondary activation of the canonical NLRP3 inflammasome for cytokine release. Since the presence of the non-canonical inflammasome activator LPS leads to endotoxemia and sepsis, non-canonical inflammasome activation and regulation has important clinical ramifications. Here we discuss the mechanism of non-canonical inflammasome activation, mechanisms regulating its activity and its contribution to health and disease.

Zingerone protects liver and kidney tissues by preventing oxidative stress, inflammation, and apoptosis in methotrexate-treated rats

The clinical use of drugs used in the treatment of diseases is limited due to the toxic side effects, and many studies have been conducted to benefit from herbal adjuvant therapies recently to eliminate these effects. In this study, the protective effect of zingerone against liver and kidney damage generated in rats through methotrexate (MTX). Histopathological investigations were performed to determine tissue damage caused by MTX and the healing effect of zingone and liver function markers such as serum alanine transaminase (ALT), aspartate transaminase (AST), alkaline phosphatase (ALP), and renal function markers such as urea, creatine, and aquaporin-1 (AQP-1) were measured. The effects of MTX and protective properties of zingerone on oxidative stress were investigated through the measurement of malondialdehyde and reduced glutathione (GSH) levels, catalase (CAT), and glutathione peroxidase (GPx) enzyme activities. The anti-inflammatory effect of zingerone was determined by measuring the cytokine levels causing inflammation such as nuclear factor-kappa B (NF-κB), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β), and its effects on apoptosis were determined by immunohistochemical analysis of caspase-3 and B-cell lymphoma-2 (Bcl-2) expression levels. According to the results obtained within the scope of the study, it was determined that zingerone treatment prevented the increase in MTX-induced liver and kidney function markers, showed healing effects on antioxidant parameters degraded in both tissues, and decreased the inflammation parameters. It was determined that it also prevented apoptosis and possessed a protective effect on disrupted tissue architecture by decreasing the increased caspase-3 expression and increasing the decreased Bcl-2 level.

Relevance and Regulation of Cell Density

Cell density shows very little variation within a given cell type. For example, in humans variability in cell density among cells of a given cell type is 100 times smaller than variation in cell mass. This tight control indicates that maintenance of a cell type-specific cell density is important for cell function. Indeed, pathological conditions such as cellular senescence are accompanied by changes in cell density. Despite the apparent importance of cell-type-specific density, we know little about how cell density affects cell function, how it is controlled, and how it sometimes changes as part of a developmental process or in response to changes in the environment. The recent development of new technologies to accurately measure the cell density of single cells in suspension and in tissues is likely to provide answers to these important questions.

Caspase-10 inhibits ATP-citrate lyase-mediated metabolic and epigenetic reprogramming to suppress tumorigenesis

Caspase-10 belongs to the class of initiator caspases and is a close homolog of caspase-8. However, the lack of caspase-10 in mice and limited substrate repertoire restricts the understanding of its physiological functions. Here, we report that ATP-citrate lyase (ACLY) is a caspase-10 substrate. Caspase-10 cleaves ACLY at the conserved Asp1026 site under conditions of altered metabolic homeostasis. Cleavage of ACLY abrogates its enzymatic activity and suppresses the generation of acetyl-CoA, which is critical for lipogenesis and histone acetylation. Thus, caspase-10-mediated ACLY cleavage results in reduced intracellular lipid levels and represses GCN5-mediated histone H3 and H4 acetylation. Furthermore, decline in GCN5 activity alters the epigenetic profile, resulting in downregulation of proliferative and metastatic genes. Thus caspase-10 suppresses ACLY-promoted malignant phenotype. These findings expand the substrate repertoire of caspase-10 and highlight its pivotal role in inhibiting tumorigenesis through metabolic and epigenetic mechanisms.

Caspases are most closely associated with cell death, but many have other cellular functions. Here, Das et al. find that upon metabolic stress, caspase-10 cleaves ACLY to regulate metabolic homeostasis and epigenetic reprogramming by altering Acetyl-CoA levels.

A soluble tau fragment generated by caspase-2 is associated with dementia in Lewy body disease

Lewy body diseases are neurodegenerative disorders characterized by Lewy bodies in the brain. Lewy body dementia (LBD) refers to two forms of Lewy body disease: Parkinson’s disease with dementia (PDD) and dementia with Lewy bodies (DLB). Tau is a cytoskeletal protein found in neurofibrillary tangles, but not Lewy bodies. The gene encoding tau, MAPT, is a well-established genetic risk factor for LBD; odds ratios of the H1:H2 MAPT haplotypes have been reported in the range of 2 to 4. Despite this genetic association, the mechanism by which tau contributes to dementia is unclear. Recently, a soluble form of tau, Δtau314, which is generated when caspase-2 (Casp2) cleaves tau at Asp314, was reported to be associated with impaired cognition in mice modeling frontotemporal dementia, and with mild cognitive impairment and Alzheimer’s disease (AD) in humans. To investigate whether Δtau314 is associated with dementia in Lewy body disease, we compared Δtau314 levels in aqueous extracts from the superior temporal gyrus of pathologically confirmed LBD (n = 21) and non-dementia Parkinson’s disease (PD) (n = 12). We excluded subjects with AD or microvascular pathology, which could mask potential associations of Δtau314 with LBD.

Using a Δtau314-specific ELISA, we found ~ 2-fold higher levels of Δtau314 in LBD relative to PD (p = 0.009). Additionally, we found ~40% lower levels of soluble total tau and the neuronal marker β-III-tubulin in LBD. These results suggest that in LBD, there is substantial neuron loss or axonal degeneration in the neocortex but disproportionately high levels of Δtau314 in the surviving neurons.

Our results indicate an association between Δtau314 and dementia in Lewy body disease. Cleavage of tau by Casp2 promotes the mislocalization of tau to dendritic spines leading to a reduction in postsynaptic AMPA receptors and excitatory neurotransmission, which suggests a mechanism of the synaptic dysfunction underlying cognitive impairment in LBD. These findings support the potential of Casp2 as a novel drug target for treating LBD.

Therapeutic potential of Vanillylacetone against CCl4 induced hepatotoxicity by suppressing the serum marker, oxidative stress, inflammatory cytokines and apoptosis in Swiss albino mice

The aim of this research was to investigate the therapeutic potential of Vanillylacetone against carbon tetrachloride (CCl₄) induced hepatotoxicity in mice through understanding the serum marker, oxidative stress mechanism and cytokine networks. Carbon tetrachloride is highly hepatotoxic used as research based on animal model. The mice were classified into five groups and each had eight mice. Group-I was controlled and the vehicle was given orally. Group-II was toxic and carbon tetrachloride (1.5 ml/kg) twice a week for 15 days was administered by intra-peritoneal injections. Group- III and IV were pre-treated with Vanillylacetone 50 & 100 mg kg^-1 body weight given every day p.o. while, Group-V received only Vanillylacetone (100 mg kg^-1 body weight) for 15 days orally. The finding indicates that the administration of CCl₄ causes significant elevation of enzyme markers, oxidative stress, inflammatory cytokine and apoptotic markers in Group-II as compared to Group-I. The administration of Vanillylacetone (50 and100 mg kg^-1) significantly suppresses the elevated serum enzymes, oxidative stress (TBARS), an inflammatory cytokine (IL2 and TNFα) and apoptotic markers (Caspase-3 and 9) in Group-III and IV as compared to Group-II. It was also noticed that the higher dose of Vanillylacetone (100 mg) is more effective than lower dose of Vanillylacetone (50 mg). There were no significant changes observed with higher dose of Vanillylacetone (100 mg kg^-1) in Group-V as compared to Group-I. Histopathological analysis also supported the above findings. Overall, this results shows that Vanillylacetone has a good antioxidant and therapeutic properties which can help in preventing the chemically (CCl₄) induced hepatotoxicity.

Apoptosis in Primary Hyperparathyroidism

Primary hyperparathyroidism (PHPT) is defined by inappropriate elevation of parathormone, caused by parathyroid hyperplasia, also known as multi-gland disease (MGD), parathyroid adenoma (PA), or parathyroid carcinoma (PC). Although several studies have already been conducted, there is a lack of a definite diagnostic marker, which could unambiguously distinguish MGD from PA or PC. The accurate and prompt diagnosis has the key meaning for effective treatment and follow-up. This review paper presents the role of apoptosis in PHPT. The comparison of the expression of Fas, TRAIL, BCL-2 family members, p53 in MGD, PA, and PC, among others, was described. The expression of described factors varies among proliferative lesions of parathyroid gland; therefore, these could serve as additional markers to assist in the diagnosis.

Insights into the mechanism of human papillomavirus E2-induced procaspase-8 activation and cell death

High-risk human papillomavirus (HR-HPV) E2 protein, the master regulator of viral life cycle, induces apoptosis of host cell that is independent of its virus-associated regulatory functions. E2 protein of HR-HPV18 has been found to be involved in novel FADD-independent activation of caspase-8, however, the molecular basis of this unique non-death-fold E2-mediated apoptosis is poorly understood. Here, with an interdisciplinary approach that involves in silico, mutational, biochemical and biophysical probes, we dissected and characterized the E2-procasapse-8 binding interface. Our data demonstrate direct non-homotypic interaction of HPV18 E2 transactivation domain (TAD) with α2/α5 helices of procaspase-8 death effector domain-B (DED-B). The observed interaction mimics the homotypic DED-DED complexes, wherein the conserved hydrophobic motif of procaspase-8 DED-B (F122/L123) occupies a groove between α2/α3 helices of E2 TAD. This interaction possibly drives DED oligomerization leading to caspase-8 activation and subsequent cell death. Furthermore, our data establish a model for E2-induced apoptosis in HR-HPV types and provide important clues for designing E2 analogs that might modulate procaspase-8 activation and hence apoptosis.

An updated view on the structure and function of PYRIN domains

The PYRIN domain (PYD) is a protein-protein interaction domain, which belongs to the death domain fold (DDF) superfamily. It is best known for its signaling function in innate immune responses and particularly in the assembly of inflammasomes, which are large protein complexes that allow the induced proximity-mediated activation of caspase-1 and subsequently the release of pro-inflammatory cytokines. The molecular mechanism of inflammasome assembly was only recently elucidated and specifically requires PYD oligomerization. Here we discuss the recent advances in our understanding of PYD signaling and its regulation by PYD-only proteins.

Revisiting caspase-11 function in host defense

Proinflammatory caspases play important roles in innate immunity. Much attention has focused on caspase-1, which acts to eliminate pathogens by obliterating their replicative niches as well as alerting the host to their presence. Now, emerging data have shed light on the lesser-studied proinflammatory caspase-11 in the combat between host and pathogens. Using the new tools available, researchers are further elucidating the mechanisms by which caspase-11 contributes to host defense. Here, we review the emerging understanding of caspase-11 functions and the mechanisms of activation and discuss the implications for human disease.

Tetramers are the activation-competent species of the HOCl-specific transcription factor HypT

Hypochlorous acid (HOCl) is an important component of the immune system and is produced by neutrophils to kill invading microorganisms. The transcription factor HypT is specifically activated by HOCl by methionine oxidation and protects Escherichia coli cells from the detrimental effects of HOCl. HypT forms dodecameric ring-like oligomers. Binding of HypT to DNA induces dissociation of the dodecamers into dimers and tetramers, thus forming the DNA-binding species. To dissect HypT dissociation, binding to DNA, and activation, we aimed to dissociate the dodecamers independently of DNA and to analyze HOCl-dependent activation in vitro. We found that HypT dodecamers dissociated into tetramers in the presence of l-arginine and NaCl, which was reversible upon dilution of the additive. Making use of the reversible dissociation, we generated mixed assemblies consisting of wild-type and mutant HypT subunits and determined that mutant subunits with reduced thermal stability were stabilized by wild-type HypT in the mixed assembly. HypT tetramers, as present at high NaCl concentrations, were stabilized against thermal unfolding and aggregation triggered by high HOCl concentrations. Importantly, in vitro activation by HOCl of HypT tetramers was completed within 1 min, whereas activation of dodecamers required 1 h for completion. Furthermore, activation of HypT tetramers required stoichiometric amounts of HOCl instead of an excess of HOCl, as observed for dodecamers. This supports the idea that small HypT oligomers are the activation-competent species, whereas the dodecamers are a storage form. Our study reveals the importance of the dynamic oligomeric state for HypT activation by HOCl.

Regulation of cell death by transfer RNA

SIGNIFICANCE

Both transfer RNA (tRNA) and cytochrome c are essential molecules for the survival of cells. tRNA decodes mRNA codons into amino-acid-building blocks in protein in all organisms, whereas cytochrome c functions in the electron transport chain that powers ATP synthesis in mitochondrion-containing eukaryotes. Additionally, in vertebrates, cytochrome c that is released from mitochondria is a potent inducer of apoptosis, activating apoptotic proteins (caspases) in the cytoplasm to dismantle cells. A better understanding of both tRNA and cytochrome c is essential for an insight into the regulation of cell life and death.

RECENT ADVANCES

A recent study showed that the mitochondrion-released cytochrome c can be removed from the cell-death pathway by tRNA molecules. The direct binding of cytochrome c by tRNA provides a mechanism for tRNA to regulate cell death, beyond its role in gene expression.

CRITICAL ISSUES

The nature of the tRNA-cytochrome c binding interaction remains unknown. The questions of how this interaction affects tRNA function, cellular metabolism, and apoptotic sensitivity are unanswered.

FUTURE DIRECTIONS

Investigations into the critical issues raised above will improve the understanding of tRNA in the fundamental processes of cell death and metabolism. Such knowledge will inform therapies in cell death-related diseases.

Engineering a BCR-ABL-activated caspase for the selective elimination of leukemic cells

Increased understanding of the precise molecular mechanisms involved in cell survival and cell death signaling pathways offers the promise of harnessing these molecules to eliminate cancer cells without damaging normal cells. Tyrosine kinase oncoproteins promote the genesis of leukemias through both increased cell proliferation and inhibition of apoptotic cell death. Although tyrosine kinase inhibitors, such as the BCR-ABL inhibitor imatinib, have demonstrated remarkable efficacy in the clinic, drug-resistant leukemias emerge in some patients because of either the acquisition of point mutations or amplification of the tyrosine kinase, resulting in a poor long-term prognosis. Here, we exploit the molecular mechanisms of caspase activation and tyrosine kinase/adaptor protein signaling to forge a unique approach for selectively killing leukemic cells through the forcible induction of apoptosis. We have engineered caspase variants that can directly be activated in response to BCR-ABL. Because we harness, rather than inhibit, the activity of leukemogenic kinases to kill transformed cells, this approach selectively eliminates leukemic cells regardless of drug-resistant mutations.

Apoptosis in health and disease and modulation of apoptosis for therapy: An overview

Apoptosis a physiological mechanism that eliminates excessive, damaged or unwanted cells, is a highly regulated pathway important for maintaining homeostasis in multicellular organisms. It can be initiated through various signals via the extrinsic pathway which involves death receptors, or via the intrinsic pathway which is initiated by intracellular damage and involves the mitochondria and release of cytochrome c from it to further activate caspases. The Bcl-2 family of proteins is situated upstream to the irreversible damage of cellular constituents and is an important checkpoint in the fate of a cell. The pro-apoptotic members, BH3 only members include BID, BAD and BIM. They directly or indirectly activate multidomain BAX/BAK that constitute the requisite gateway to the intrinsic pathway which operates at the mitochondrial surface and endoplasmic reticulum. In contrast, antiapoptotic members such as Bcl-2, Bcl-XL bind and sequester activation. Downstream of mitochondria, the apoptosome involvement is seen to generate caspase activity. Post mitochondria regulation involves IAPs, and their inhibitors. The pathogenesis of several diseases such as cancer, neurodegenerative disorders, autoimmune disorders, heart disease, infectious diseases including AIDS is closely related to aberrant apoptosis. Consequently interest has emerged in employing various the rapeutic approaches such as gene therapy, antisense therapy, recombinant biologicals, organic and combinatorial chemistry, to specifically target apoptosis signaling pathways such as death receptors FAS/TRAIL, Bcl-2, p53, IAPs, SMAC and caspases, etc. and are now advancing from preclinical to clinical phase.

["The inflammasomes"]

Inflammation is a host adaptational response to cell injury caused by various exogenous and endogenous stimuli. IL-1β, which is an important proinflammatory cytokine secreted at the site of cellular injury, plays an important role in inflammation. Inflammasome is an intracellular multi-protein complex that mediates caspase-1-dependent processing of IL-1β. In this review, inflammasome function and its dysregulation are discussed in relation to autoinflammatory diseases.

Squamous cell carcinoma antigen 1 promotes caspase-8-mediated apoptosis in response to endoplasmic reticulum stress while inhibiting necrosis induced by lysosomal injury

Squamous cell carcinoma antigen 1 (SCCA1) is a member of the serine protease inhibitor (serpin) family of proteins, whose target proteases include the cathepsins. Initially identified as a serological marker for advanced squamous cell carcinomas of the cervix, SCCA1 has also been found to be associated with other cancer types of epithelial or endodermal origins such as lung cancer, head and neck cancer, melanoma, and hepatocellular carcinoma. While the biological function of SCCA1 remains largely unclear, it is believed to limit cellular damage resulting from lysosomal cathepsin release. Here, we show that SCCA1 acts as a molecular switch that inhibits cell death induced by lysosomal injury resulting from DNA alkylating agents and hypotonic shock, whereas it promotes a caspase-8-mediated apoptosis in response to endoplasmic reticulum (ER) stress. In response to ER stress, SCCA1 blocks both lysosomal and proteasomal protein degradation pathways and enhances the interaction between sequestosome 1/p62 and caspase-8, which leads to the aggregation of intracellular caspase-8 and its subsequent cleavage and activation. Hence, on one hand, SCCA1 inhibits cell death induced by lysosomal injury while, on the other hand, it sensitizes cells to ER stress by activating caspase-8 independently of the death receptor apoptotic pathway.

Integrity of ATP binding site is essential for effective inhibition of the intrinsic apoptosis pathway by NAIP

The importance of the ATP binding site of human Neuronal Apoptosis Inhibitory Protein (NAIP) on its ability in prevention of intrinsic apoptotic pathway was investigated. Thus, ATP binding lysine 476 of NAIP, which is located at the Nucleotide Binding Oligomerization Domain (NOD) was mutated to threonine and the effect of this mutation on autoproteolysis of procaspase-9 and the cleavage of procaspase-3 by apoptosome was investigated. Formation of apoptosome was induced by the addition of cytochrome c and dATP to lysates of HeLa cells transfected with pcDNA-NAIP or pcDNA-NAIP (K476T). Full length wild type NAIP prevented the cleavage of both procaspase-9 to caspase-9 and procaspase-3 to caspase-3. However, K476T variant of NAIP did not block autocleavage of procaspase-9 efficiently. Furthermore, cleavage pattern of procaspase-9 was altered in the presence of mutant NAIP. Interestingly no effect on the procaspase-3 cleavage by apoptosome was observed. The presence of NOD domain by itself had no effect on autocleavage of procaspase-9 yet slightly reduced the cleavage of procaspase-3 by apoptosome. Pull down experiment showed direct interaction of the NOD domain of NAIP with the CARD-NOD domain of Apoptotic Protease Activating Factor 1 (APAF-1). The physical association of these domains was confirmed by pull-down assays. These observations taken with previous findings indicate that the integrity of the NOD domain is essential for effective inhibition of procaspase-9 and procaspase-3 cleavage by the NAIP protein.

Raf-1 activation prevents caspase 9 processing downstream of apoptosome formation

In many cell types, growth factor removal induces the release of cytochrome-c from mitochondria that leads to activation of caspase-9 in the apoptosome complex. Here, we show that sustained stimulation of the Raf-1/MAPK1,3 pathway prevents caspase-9 activation induced by serum depletion in CCL39/ΔRaf-1:ER fibroblasts. The protective effect mediated by Raf-1 is sensitive to MEK inhibition that is sufficient to induce caspase-9 cleavage in exponentially growing cells. Raf-1 activation does not inhibit the release of cytochrome-c from mitochondria while preventing caspase-9 activation. Gel filtration chromatography analysis of apoptosome formation in cells shows that Raf-1/MAPK1,3 activation does not interfere with APAF-1 oligomerization and recruitment of caspase 9. Raf-1-mediated caspase-9 inhibition is sensitive to emetine, indicating that the protective mechanism requires protein synthesis. However, the Raf/MAPK1,3 pathway does not regulate XIAP. Taken together, these results indicate that the Raf-1/MAPK1,3 pathway controls an apoptosis regulator that prevents caspase-9 activation in the apoptosome complex.

tRNA binds to cytochrome c and inhibits caspase activation

The specific molecular events that characterize the intrinsic apoptosis pathway have been the subject of intense research due to the pathway's fundamental role in development, homeostasis, and cancer. This pathway is defined by the release of cytochrome c from mitochondria into the cytosol and subsequent binding of cytochrome c to the caspase activator Apaf-1. Here, we report that both mitochondrial and cytosolic transfer RNA (tRNA) bind to cytochrome c. This binding prevents cytochrome c interaction with Apaf-1, blocking Apaf-1 oligomerization and caspase activation. tRNA hydrolysis in living cells and cell lysates enhances apoptosis and caspase activation, whereas microinjection of tRNA into living cells blocks apoptosis. These findings suggest that tRNA, in addition to its well-established role in gene expression, may determine cellular responsiveness to apoptotic stimuli.

Direct activation of caspase 8 by the proapoptotic E2 protein of HPV18 independent of adaptor proteins

The self-activation of initiator caspases is dependent on their oligomerization driven by interaction with the death fold domains (DFD) of adaptor proteins. Here, we show that the E2 protein of human papillomavirus type 18 triggers apoptosis by assembling cytoplasmic filaments together with caspase 8, in which its efficient self-activation occurs. The E2 protein binds directly to the death effector domains (DED) of caspase 8 through non-DFD interaction. This interaction is independent of FADD, but it can cooperate with FADD homotypic binding to caspase 8 to induce its oligomerization; hence cell death, while it is antagonized by competitive binding of MC159 FLICE inhibitory protein. The amino-terminal domain of E2 contains a 27 amino-acid alpha-helix, which is necessary and sufficient to induce caspase oligomerization and cell death. Our results provide evidence for adaptor-independent oligomerization of caspase 8, mediated by non-DFD direct interactions with the HPV18 E2 protein, thus deciphering a new pathway for caspase 8 activation.

IGF1 promotes resistance to apoptosis in melanoma cells through an increased expression of BCL2, BCL-X(L), and survivin

IGF1 plays a key role in the development and growth of multiple tumors and in the prevention of apoptosis. In melanoma cells, IGF1 has been shown to mediate resistance to anoikis-induced apoptosis. However, the effect of IGF1 on other proapoptotic stimuli has never been reported. Further, the molecular mechanisms by which IGF1 mediates its prosurvival properties in melanoma cells remain unknown. Here, we demonstrate that IGF1 impairs the onset of tumor necrosis factor-related apoptosis-inducing ligand and staurosporine-induced apoptosis in melanoma cells expressing either wild-type or oncogenic B-Raf. Further, we show that IGF1 inhibits mitochondrial damage that occurs during apoptosis, thereby indicating that IGF1 acts at the level of mitochondria to mediate its antiapoptotic stimuli. Accordingly, IGF1 increases the mRNA levels and protein expression of antiapoptotic members of the BCL2 family--BCL2 and BCL-X(L)--and that of the inhibitor of apoptosis protein, survivin. Further, their specific silencing by small interfering RNA prevents the protective effect of IGF1. These findings therefore delineate the molecular mechanisms by which IGF1 mediates its prosurvival properties and provide a basis for clinical strategies designed to neutralize IGF1 or its target genes.

Functional protein expression from a DNA based wheat germ cell-free system

Wheat germ based eukaryotic cell-free systems have been shown to be applicable for both functional and structural analyses of proteins. However, the existing methods might require specialized instrumentation and/or a separate mRNA synthesis step. We have developed a DNA based, highly productive, coupled transcription/translation wheat germ cell-free system that incorporates the normally separate mRNA synthesis step and does not require specialized instrumentation. Using a small-volume batch reaction with fluorescence labeling, DNA templates predicted to encode proteins could be quickly screened for their ability to direct the expression of proteins of the appropriate size. Protein yield can be increased as much as 2 to 4-fold in this system using a dialysis reaction, reaching approximately 200-440 microg/ml in 10-20 h. Furthermore, enzyme activities can be assayed directly in the extract without further purification. Simple purification with affinity tags can be achieved in one-step and with minor modifications, efficient SeMet and [U-15N] labeling of >95% can be accomplished in this system. Thus, this efficient cell-free expression system can facilitate both functional and structural proteomics.

Neuropilin-1 modulates p53/caspases axis to promote endothelial cell survival

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF), one of the crucial pro-angiogenic factors, functions as a potent inhibitor of endothelial cell (EC) apoptosis. Previous progress has been made towards delineating the VPF/VEGF survival signaling downstream of the activation of VEGFR-2. Here, we seek to define the function of NRP-1 in VPF/VEGF-induced survival signaling in EC and to elucidate the concomitant molecular signaling events that are pivotal for our understanding of the signaling of VPF/VEGF. Utilizing two different in vitro cell culture systems and an in vivo zebrafish model, we demonstrate that NRP-1 mediates VPF/VEGF-induced EC survival independent of VEGFR-2. Furthermore, we show here a novel mechanism for NRP-1-specific control of the anti-apoptotic pathway in EC through involvement of the NRP-1-interacting protein (NIP/GIPC) in the activation of PI-3K/Akt and subsequent inactivation of p53 pathways and FoxOs, as well as activation of p21. This study, by elucidating the mechanisms that govern VPF/VEGF-induced EC survival signaling via NRP-1, contributes to a better understanding of molecular mechanisms of cardiovascular development and disease and widens the possibilities for better therapeutic targets.

Caspase-15 is autoprocessed at two sites that contain an aspartate residue in the P1' position

Our recent characterization of porcine caspase-15 suggested that the amino terminus of the small catalytic subunit is formed by proteolytic processing between the consecutive aspartate residues D277 and D278. Since a charged residue (D278) is highly unusual in the P1' position of a caspase cleavage site, we further characterized the mechanism of caspase-15 autoproteolysis. Amino acid sequence alignments showed that D277 and D278 as well as another pair of aspartates, D270 and D271, were evolutionarily conserved among species of the mammalian clade Laurasiatheria. Site-directed mutations of these four residues and analysis of recombinant proteins showed that D270 was crucial for autoproteolysis whereas the three other aspartates were dispensable for separation of the catalytic subunits. Mutation of D270 prevented catalytic activation and abolished subsequent processing at D277. Together with previous reports, our results show that caspase-15, unlike all other caspases, efficiently cleaves sites with an aspartate in the P1' position.

Activation pathways and signal-mediated upregulation of the insect Spodoptera frugiperda caspase-1

Sf-caspase-1 is the most studied effector caspase of Lepidoptera. Its activation is believed to follow a two-step mechanism: The first step requires cleavage by an initiator caspase at D195 (between the large and small subunits) releasing the C-terminal small subunit. This is blocked by the baculovirus caspase inhibitor P49. The second step removes the N-terminal prodomain by cleavage at D28 to generate the large subunit that is blocked by the baculovirus caspase inhibitor P35. In this study, we identified an alternative mechanism of Sf-caspase-1 activation. This additional two-step mechanism involves first cleavage of pro-Sf-caspase-1 at D28 to remove the N-terminal prodomain and subsequently cleavage at D195 to generate the large and small subunits. Both mechanisms are triggered by apoptotic stimuli following a distinct pattern. We also showed that expression of Sf-caspase-1 was upregulated upon reception of apoptotic stimuli. Different from all published data, this upregulation occurred as a post-transcriptional event. Moreover, we proved that the stronger the stimuli, the higher the upregulation. And we demonstrated that P49 and P35 inhibited the cleavage at D28 and D195 respectively, independently of wether the first cleavage was at D195 or at D28.

Kinetic comparison of procaspase-3 and caspase-3

Caspases, the key enzymes in apoptosis, are synthesized as proenzymes and converted into active form by proteolytic cleavage. The residues on active site reorganize during the activation process as shown in the comparative studies of crystallographic structures of procaspase-7 and its mature form. On the other hand, the proenzyme itself has some activity. Aiming to characterize the activation process, the comparative kinetic study for the pro- and mature caspase-3 was performed. In 1/K(M) versus pH study, a residue with pKa of 6.89+/-0.13 was detected only in caspase-3. While Vmax versus pH kinetic results were consistent with the existence of a residue with pKa of 6.21+/-0.06 in procaspase-3 mutant (D9A/D28A/D175A) but not in caspase-3. In the inactivation assays with diethylpyrocarbonate, a residue (pKa, 6.61+/-0.05) could be determined only for caspase-3 whereas with iodoacetamide a residue with pKa value (6.01+/-0.05) could be assigned only for procaspase-3. Considering that those residues could be protected by caspase-3-specific inhibitor from the inactivation, the modifiers are histidine- and cysteine-specific, respectively, and the involvement of these residues in the characteristic catalytic dyad of caspases, the results indicate that the pKa values of the catalytic histidine and cysteine residues are changed during the activation process.

Tumor necrosis factor-related apoptosis-inducing ligand alters mitochondrial membrane lipids

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to have selective antitumor activity. TRAIL induces ubiquitous pathways of cell death in which caspase activation is mediated either directly or via the release of apoptogenic factors from mitochondria; however, the precise components of the mitochondrial signaling pathway have not been well defined. Notably, mitochondria constitute an important target in overcoming resistance to TRAIL in many types of tumors. Bid is considered to be fundamental in engaging mitochondria during death receptor-mediated apoptosis, but this action is dependent on mitochondrial lipids. Here, we report that TRAIL signaling induces an alteration in mitochondrial membrane lipids, particularly cardiolipin. This occurs independently of caspase activation and primes mitochondrial membranes to the proapoptotic action of Bid. We unveil a link between TRAIL signaling and alteration of membrane lipid homeostasis that occurs in parallel to apical caspase activation but does not take over the mode of cell death because of the concurrent activation of caspase-8. In particular, TRAIL-induced alteration of mitochondrial lipids follows an imbalance in the cellular homeostasis of phosphatidylcholine, which results in an elevation in diacylglycerol (DAG). Elevated DAG in turn activates the delta isoform of phospholipid-dependent serine/threonine protein kinase C, which then accelerates the cleavage of caspase-8. We also show that preservation of phosphatidylcholine homeostasis by inhibition of lipid-degrading enzymes almost completely impedes the activation of pro-caspase-9 while scarcely changing the activation of caspase-8.

Expression of apoptosome pathway-related transcripts in non-small cell lung cancer

PURPOSE

Tumour cells killing by cytotoxic therapies largely depends on triggering the intrinsic apoptosome-mediated caspase activation pathway but it had never been evaluated whether the expression of transcripts encoding the core components of apoptosome pathway is altered in non-small cell lung carcinoma (NSCLC).

METHODS

We investigated the expression status of several apoptosome pathway-related transcripts including Apaf-1, procaspase-9, -3, -6, -7 and Smac in tumour and lung tissue samples from 65 surgically treated NSCLC patients and in 10 NSCLC cell lines with using real time RT-PCR.

RESULTS

NSCLC tissues and cell lines showed significantly increased expression of procaspase-9, -3, -6 and Smac mRNAs as compared to the lungs and expression of these transcripts was simultaneously upregulated in a subset of NSCLCs belonging to different histopathological type, grade and stage categories. The expression of procaspase-7 mRNA in NSCLC tissues and cell lines and lungs was not significantly different. By contrast, the expression of Apaf-1 mRNA was frequently downregulated in the tumours as compared to matched lungs. Nevertheless, the examined NSCLC cell lines showed significantly higher expression of Apaf-1 mRNA than the lungs. The expression of Apaf-1, procaspase-9 and -6 mRNAs was higher in lung adenocarcinomas as compared to squamous cell lung carcinomas but the expression levels of the studied apoptosome pathway-related transcripts in the tumours were independent of tumour's grade and stage.

CONCLUSIONS

The results of the present study suggest that there is a subgroup of NSCLCs, which may be intrinsically primed for apoptosis through upregulated expression of transcripts encoding the apoptosome pathway components.

A role for transferrin receptor in triggering apoptosis when targeted with gambogic acid

Transferrin receptor (TfR) has been shown to be significantly overexpressed in different types of cancers. We discovered TfR as a target for gambogic acid (GA), used in traditional Chinese medicine and a previously undiscovered link between TfR and the rapid activation of apoptosis. The binding site of GA on TfR is independent of the transferrin binding site, and it appears that GA potentially inhibits TfR internalization. Down-regulation of TfR by RNA interference decreases sensitivity to GA-induced apoptosis, further supporting TfR as the primary GA receptor. In summary, GA binding to TfR induces a unique signal leading to rapid apoptosis of tumor cells. These results suggest that GA may provide an additional approach for targeting the TfR and its use in cancer therapy.

Caspases-2, -3, and -7 are involved in thapsigargin-induced apoptosis of SH-SY5Y neuroblastoma cells

Caspase-2 has been reported to play a role in the cell death observed under a number of different conditions; however, it is unclear whether caspase-2 plays a role in cell death triggered by endoplasmic reticulum (ER) stress. The purpose of this study was to determine whether caspase-2 is involved in SH-SY5Y neuroblastoma cell death caused by thapsigargin-induced ER stress. Thapsigargin treatment (1 microM, 16 hr) stimulated the proteolytic processing of caspases-2, -3, and -7, suggesting that these caspases are activated by ER stress. The role of these caspases in thapsigargin-induced cell death was examined by using cell-permeable caspase inhibitors. In the absence of pretreatment with caspase inhibitors, thapsigargin (0.1 microM, 20 hr) reduced the number of viable cells to 53.9% +/- 3.3% of starting-time control. Pretreatment for 90 min with either the pan-caspase inhibitor Z-VAD-FMK or the caspase-2-selective inhibitor Z-VDVAD-FMK inhibited thapsigargin-stimulated cell death, resulting in the number of viable cells being 115.6% +/- 5.3% (P < 0.001) and 69.3% +/- 2.9% (P < 0.01), respectively, of starting-time control. Neither the caspase-3- and -7-selective inhibitor Z-DEVD-FMK nor the caspase-9-selective inhibitor Z-LEHD-FMK significantly affected thapsigargin-stimulated cell death. An anticaspase-12-reactive protein was also identified in SH-SY5Y cells, but thapsigargin had no effect on proteolysis of this protein. These data demonstrate that caspases-2, -3, and -7 are involved in ER stress-mediated death of SH-SY5Y cells.

Caspase-2 is resistant to inhibition by inhibitor of apoptosis proteins (IAPs) and can activate caspase-7

Caspases are a family of cysteine proteases with roles in cytokine maturation or apoptosis. Caspase-2 was the first pro-apoptotic caspase identified, but its functions in apoptotic signal transduction are still being elucidated. This study examined the regulation of the activity of caspase-2 using recombinant proteins and a yeast-based system. Our data suggest that for human caspase-2 to be active its large and small subunits must be separated. For maximal activity its prodomain must also be removed. Consistent with its proposed identity as an upstream caspase, caspase-2 could provoke the activation of caspase-7. Caspase-2 was not subject to inhibition by members of the IAP family of apoptosis inhibitors.

Folding pathways for initiator and effector procaspases from computer simulations

The folding pathways of procaspases 3, 7, and 8 have been studied using a Go-like Hamiltonian and molecular dynamics simulations coupled with a parallel tempering scheme. The folding pathways and the overall structures of procaspases 3 and 7 are similar, and are characterized by monomeric as well as dimeric folding intermediates in agreement with the available structural and thermochemical data. The folding pathway of procaspase 8, on the other hand, is characterized by a larger population of monomers and partially folded dimer intermediates, and only a relatively small population of folded dimer species. The most stable structure predicted for procaspase 8 is a dimer, in which the position of the linker is remarkably different from the one observed in procaspases 3 and 7, leading to the fact that all the contacts that stabilize the active site are essentially formed. This novel and unexpected structure provides a rationale for the observed activity of the procaspase 8 dimer, and thus could be highly relevant for the initiation of FAS-mediated apoptosis.

The biochemical mechanism of caspase-2 activation

A unified model for initiator caspase activation has previously been proposed based on the biochemical analysis of caspase-8 and -9. Caspase-2 is structurally related to caspase-9, but its mechanism of activation is not known. Using an uncleavable mutant of caspase-2, we show that dimerization (and not processing) is the key event that drives initial procaspase-2 activation. Following dimerization, caspase-2 undergoes autocatalytic cleavage that promotes its stable dimerization and further enhances the catalytic activity of caspase-2. Although the caspase-2 zymogen does not require cleavage for the initial acquisition of activity, intersubunit cleavage is required to generate levels of activity required to induce cell death by overexpression. We also provide evidence that the reported disulfide bond linkage between two caspase-2 monomers is dispensable for caspase-2 dimerization. As caspase-2 does not require cleavage for its initial activation, our findings confirm caspase-2 to be a bona fide initiator caspase.

Translocation of Fas by LPA prevents ovarian cancer cells from anti-Fas-induced apoptosis

OBJECTIVES

Alterations in the expression of Fas have been demonstrated in various cancers as a mechanism for tumor cells to escape from immune surveillance. In this study, we observed the effect of lysophosphatidic acid (LPA) on Fas expression and function in ovarian cancer cells.

METHODS

Ovarian cancer cell lines were incubated with or without LPA and Fas cell surface presentations were detected by flow cytometry. Anti-Fas IgM was added for induction and analysis of apoptosis by flow cytometry. Cell lysis and subcellular fractions were probed for protein expression by Western blot. Cells were also stained with human anti-Fas Ab, followed with Rhodamine red-X-conjugated goat anti-mouse IgG, and immunofluorescence images were acquired on a Nikon digital camera.

RESULTS

Following treatment with LPA, ovarian cancer cells showed significant rapid reduction of Fas presentation on the cell surface. LPA protected ovarian cancer cells from anti-Fas-induced apoptosis. Cell lysis and subcellular fractionations proved that LPA treatment induced a translocation of Fas receptors, along with phosphorylated ezrin, from the membrane anchored to the actin cytoskeleton, to the cytosol. Translocation of the Fas receptor reduced Fas concentration in the membrane and may inhibit its clustering and internalization during early apoptosis induced by anti-Fas. DISC staining proved that LPA inhibited Fas receptor aggregation and caspase-8 activation at the membrane, which further inhibited caspase-3 and 7 activation in the cytosol.

CONCLUSIONS

Our studies suggest that LPA induces translocation of Fas from the cell membrane to the cytosol, which may provide a mechanism by which ovarian cancer cells evade FasL-bearing immune cells.

Caspases: potential targets for regulating cell death

While in multicellular organisms all cells inexorably die, there are several different ways provided for the realization of cell death. One of them, apoptosis, represents a universal energy-dependent and tightly regulated physiologic process of cell death in both normal and pathologic tissues. The execution of apoptosis appears to be uniformly mediated through consecutive activation of the members of a caspase family. This review briefly summarizes current knowledge on the molecular mechanisms of caspase activation and the inhibitory components of caspase cascades. The suitability of caspases as a new potential therapeutic target is discussed next. Particular attention is focused on two broad categories of caspase-directed compounds: highly specific caspase inhibitors that distinctly block the progress of apoptosis and caspase activators that selectively induce cell death in a variety of in vitro and in vivo systems. These agents promise to be useful clinically, either alone or in combination with more conventional therapeutics.

Interdimer processing and linearity of procaspase-3 activation. A unifying mechanism for the activation of initiator and effector caspases

Caspase activation during apoptosis occurs in a cascade from the initiator caspase(s) (e.g. caspase-8) to the effector caspases (e.g. caspase-3), which ensures the generation of large amounts of active caspases to dismantle cells. However, the mechanism that safeguards against inadvertent caspase activation is not well understood. Previous studies have suggested that the activation of procaspase-8 is mediated by cross-cleavage of precursor dimers, formed upon apoptosis induction, which are not only enzymatically competent but also highly susceptible to cleavage, and that procaspase-8 activation is a linear process without self-amplification. Effector procaspases constitutively exist as dimers and their activation is started by trans-cleavage by an initiator caspase followed by autocleavage of effector caspases. Here we show that the dimerization of caspase-3 molecules through their protease domains is required for their processing by initiator caspases. The subsequent autoprocessing takes place through cleavage between the dimeric intermediates. Moreover, mature caspase-3 fails to process its own precursor. Thus, despite a marked difference in the generation of active intermediates, the activation of initiator and effector caspases shares the features of interdimer cleavage and lack of self-amplification. These features may be important in preventing accidental cell death.

Caspase-8 can be activated by interchain proteolysis without receptor-triggered dimerization during drug-induced apoptosis

Proteases of the caspase family are thought to be activated by proteolytic processing of their inactive zymogens. However, although proteolytic cleavage is sufficient for executioner caspases, a different mechanism has been recently proposed for initiator caspases, such as caspase-8, which are believed to be activated by proximity-induced dimerization. According to this model, dimerization rather than proteolytic processing is considered as the critical event for caspase-8 activation. Such a mechanism would suggest that in the absence of a dimerization platform such as the death-inducing signaling complex, caspase-8 proteolytic cleavage would result in an inactive enzyme. As several studies have described caspase-8 cleavage during mitochondrial apoptosis, we now investigated whether caspase-8 becomes indeed catalytically active in this pathway. Using an in vivo affinity labeling approach, we demonstrate that caspase-8 is activated in etoposide-treated cells in vivo in the absence of the receptor-induced death-inducing signaling complex formation. Furthermore, we show that both caspase-3 and -6 are required for the efficient activation of caspase-8. Our data therefore indicate that interchain cleavage of caspase-8 in the mitochondrial pathway is sufficient to produce an active enzyme even in the absence of receptor-driven procaspase-8 dimerization.

The protein structures that shape caspase activity, specificity, activation and inhibition

The death morphology commonly known as apoptosis results from a post-translational pathway driven largely by specific limited proteolysis. In the last decade the structural basis for apoptosis regulation has moved from nothing to 'quite good', and we now know the fundamental structures of examples from the initiator phase, the pre-mitochondrial regulator phase, the executioner phase, inhibitors and their antagonists, and even the structures of some substrates. The field is as well advanced as the best known of proteolytic pathways, the coagulation cascade. Fundamentally new mechanisms in protease regulation have been disclosed. Structural evidence suggests that caspases have an unusual catalytic mechanism, and that they are activated by apparently unrelated events, depending on which position in the apoptotic pathway they occupy. Some naturally occurring caspase inhibitors have adopted classic inhibition strategies, but other have revealed completely novel mechanisms. All of the structural and mechanistic information can, and is, being applied to drive therapeutic strategies to combat overactivation of apoptosis in degenerative disease, and underactivation in neoplasia. We present a comprehensive review of the caspases, their regulators and inhibitors from a structural and mechanistic point of view, and with an aim to consolidate the many threads that define the rapid growth of this field.

Taxol Induces Caspase-10-dependent Apoptosis

Taxol (paclitaxel) is known to inhibit cell growth and trigger significant apoptosis in various cancer cells. Although taxol induces apoptosis of cancer cells, its exact mechanism of action is not yet known. In this study we investigated death receptors, FAS-associated death domain protein (FADD), the activation of caspases-10 and -8 as well as the downstream caspases, and reactive oxygen species (ROS) in taxol-induced apoptosis in the CCRF-HSB-2 human lymphoblastic leukemia cell line. Pretreating the cells with neutralizing antibodies to Fas, tumor necrosis factor (TNF)-alpha receptor 1, or TNF-related apoptosis-inducing ligand receptors (DR4 and DR5) did not affect taxol-induced apoptosis, but transfection of the cells with a dominant negative FADD plasmid resulted in inhibition of taxol-induced apoptosis, revealing that taxol induces apoptosis independently of these death receptors but dependently on FADD. Furthermore, the drug induced activation of caspases-10, -8, -6, and -3, cleaved Bcl-2, Bid, poly(ADP-ribose) polymerase, and lamin B, and down-regulated cellular levels of FLICE-like inhibitory protein (FLIP) and X-chromosome-linked inhibitor of apoptosis protein (XIAP). However, despite the release of cytochrome c from the mitochondria in taxol-treated cells, caspase-9 was not activated. Inhibitors of caspases-8, -6, or -3 partially inhibited taxol-induced apoptosis, whereas the caspase-10 inhibitor totally abrogated this process. Taxol-induced apoptosis was also associated with decreased mitochondrial membrane potential (Deltapsim) and a significant increase in ROS generation. However, increased ROS production was not directly involved in taxol-triggered apoptosis. Therefore, these results demonstrate for the first time that taxol induces FADD-dependent apoptosis primarily through activation of caspase-10 but independently of death receptors.

Caspase-dependent cytochrome c release and cell death in chick cardiomyocytes after simulated ischemia-reperfusion

We recently demonstrated that reperfusion rapidly induces the mitochondrial pathway of apoptosis in chick cardiomyocytes after 1 h of simulated ischemia. Here we tested whether ischemia-reperfusion (I/R)-induced apoptosis could be initiated by caspase-dependent cytochrome c release in this model of cardiomyocyte injury. Fluorometric assays of caspase activity showed little, if any, activation of caspases above baseline levels induced by 1 h of ischemia alone. However, these assays revealed rapid activation of caspase-2, yielding a 2.95 ± 0.52-fold increase (over ischemia only) within the 1st h of reperfusion, whereas activities of caspases-3, -8, and -9 increased only slightly from their baseline levels. The rapid and prominent activation of caspase-2 suggested that it could be an important initiator caspase in this model, and using specific caspase inhibitors given only at the point of reperfusion, we tested this hypothesis. The caspase-2 inhibitor benzyloxycarbonyl-Val-Asp(Ome)-Val-Ala-Asp(Ome)-CH2F was the only caspase inhibitor that significantly inhibited cytochrome c release from mitochondria. This inhibitor also completely blocked activation of caspases-3, -8, and -9. The caspase-3/7 inhibitor transiently and only partially blocked caspase-2 activity and was less effective in blocking the activities of caspases-8 and -9. The caspase-8 inhibitor failed to significantly block caspase-2 or -3, and the caspase-9 inhibitor blocked only caspase-9. Furthermore, the caspase-2 inhibitor protected against I/R-induced cell death, but the caspase-8 inhibitor failed to do so. These data suggest that active caspase-2 initiates cytochrome c release after reperfusion and that it is critical for the I/R-induced apoptosis in this model.

Increased expression of Apaf-1 and procaspase-3 and the functionality of intrinsic apoptosis apparatus in non-small cell lung carcinoma

The intrinsic apoptosis apparatus plays a significant role in generating and amplifying cell death signals. In this study we examined whether there are differences in the expression of its components and in its functioning in non-small cell lung carcinoma (NSCLC) and the lung. We show that NSCLC cell lines express Apaf-1 and procaspase-9 and -3 proteins and that the expression of Apaf-1 and procaspase-3, but not of procaspase-9 and -7, is frequently up-regulated in NSCLC tissues as compared to the lung. NSCLC tissues and lungs and some NSCLC cell lines expressed also caspase-9S(b) and displayed a high caspase-9S(b)/procaspase-9 expression ratio. Procaspase-3 from NSCLCs and lungs was readily processed to caspase-3 by granzyme B or caspase-8, and the granzyme B-generated caspase-3-like activity was significantly higher in tumor tissues and cells than in lungs. By contrast, cytochrome c plus dATP could induce a significant increase of caspase-3-like activity in cytosol only in some NSCLC cell lines and in subsets of studied NSCLC tissues and lungs, while procaspase-3 and -7 were detectably processed only in NSCLC tissues which showed a high (cytochrome c+dATP)-induced caspase-3-like activity. Taken together, the present study provides evidence that the expression of Apaf-1 and procaspase-3 is up-regulated in NSCLCs and indicates that the tumors have a capability to suppress the apoptosome-driven caspase activation in their cytosol.

The NRIF3 family of transcriptional coregulators induces rapid and profound apoptosis in breast cancer cells

Many anticancer drugs kill cancer cells by inducing apoptosis. Despite the progress in understanding apoptosis, how to harness the cellular death machinery to selectively deliver tumor-specific cytotoxicity (while minimizing damage to other cells) remains an important challenge. We report here that expression of the NRIF3 family of transcriptional coregulators in a variety of breast cancer cell lines induces rapid and profound apoptosis (nearly 100% cell death within 24 h). A novel death domain (DD1) was mapped to a short 30-amino-acid region common to all members of the NRIF3 family. Mechanistic studies showed that DD1-induced apoptosis occurs through a novel caspase 2-mediated pathway that involves mitochondrial membrane permeabilization but does not require other caspases. Interestingly, the cytotoxicity of NRIF3 and DD1 appears to be cell type specific, as they selectively kill breast cancer or related cells but not other examined cells of different origins. Our study demonstrates the feasibility of selectively inducing cytotoxicity in a specific cancer and suggests that breast cancer cells contain a novel "death switch" that can be specifically triggered by NRIF3 or DD1. Strategies utilizing NRIF3 and/or DD1 and/or targeting this death switch may lead to the development of novel and more selective therapeutics against breast cancer.

The interplay between the Bcl-2 family and death receptor-mediated apoptosis

Two principal pathways for apoptosis initiation exist. One pathway, which is also termed the 'extrinsic' pathway, is mediated by death receptors, a subgroup of the TNF receptor superfamily. The second pathway, which is also referred to as the 'intrinsic' pathway is controlled by members of the Bcl-2 family. A long standing discussion revolves around the question of how these two pathways influence each other in regulating the decision about life or death of a cell. Here, we review our current knowledge about the interactions between these two pathways and discuss current models which could help to resolve previous apparently contradictory results.