The adapter protein apoptotic protease-activating factor-1 (Apaf-1) is proteolytically processed during apoptosis

Induction of apoptosis in A431 cells via ROS generation and p53-mediated pathway by chloroform fraction of Argemone mexicana (Pepaveraceae)

Argemone mexicana(Pepaveraceae) is an important medicinal plant commonly known as 'maxican prickly poppy' and is traditionally used to treat skin diseases. In the present study, the extract/fractions of aerial parts of A. mexicana after carrying out the organoleptic characteristics were sequentially extracted with the solvents of increasing polarities. Total fractions were examined for their radical scavenging activities in DPPH and DNA nicking assays. Among all, maximum antioxidant activity was shown by chloroform fraction (AmC) in DPPH assay with IC50 of 26.12 μg/ml, and DNA nicking assay showed 80.91% protective potential. The AmC fraction was analyzed for its antibacterial, cytotoxic potential, cell cycle analysis, mitochondrial membrane potential (MMP) and accumulation of reactive oxygen species (ROS) using A431 cell line. The AmC fraction exhibited remarkable antibacterial activity against bacterial strains in the order Klebsiella pneumoniae> Bacillussubtilis> Salmonella typhi> Staphylococcus epidermidis. The cytotoxic potential of the AmC fraction was analyzed in skin epidermoid carcinoma (A431) cells, osteosarcoma (MG-63) and cervical (HeLa) cell lines with a GI50 value of 47.04 μg/ml, 91.46 μg/ml and 102.90 μg/ml, respectively. The AmC fraction was extended further to explore its role in cell death using A431 cell line. Phase contrast and scanning electron microscopic studies on A431 cells exhibited all the characteristics indicative of apoptosis, viz., viability loss, cell shrinkage, cell rounding-off, DNA fragmentation and formation of apoptotic bodies. Flow cytometric analysis revealed enhanced ROS level, decreased MMP and arrest cell cycle at the G0/G1 phase further strengthened cell death by apoptosis. Increased expressions of apoptotic markers (p53, PUMA, cyt c, Fas and Apaf-1) were confirmed by RT-qPCR analysis. Furthermore, the AmC fraction was subjected to ultra-high-performance liquid chromatography, which revealed the presence of different polyphenols in the order: caffeic acid> epicatechin> kaempferol> chlorogenic acid> gallic acid> catechin> ellagic acid >umbeliferone> quercetin> coumaric acid. A critical analysis of results revealed that the AmC fraction induced cell death in epidermoid carcinoma cells via ROS and p53-mediated apoptotic pathway which may be ascribed to the presence of polyphenols in it.

Contribution of Apaf-1 to the pathogenesis of cancer and neurodegenerative diseases

Deregulation of apoptosis is associated with various pathologies, such as neurodegenerative disorders at one end of the spectrum and cancer at the other end. Generally speaking, differentiated cells like cardiomyocytes, skeletal myocytes and neurons exhibit low levels of Apaf-1 (Apoptotic protease activating factor 1) protein suggesting that down-regulation of Apaf-1 is an important event contributing to the resistance of these cells to apoptosis. Nonetheless, upregulation of Apaf-1 has not emerged as a common phenomenon in pathologies associated with enhanced neuronal cell death, i.e., neurodegenerative diseases. In cancer, on the other hand, Apaf-1 downregulation is a common phenomenon, which occurs through various mechanisms including mRNA hyper-methylation, gene methylation, Apaf-1 localization in lipid rafts, inhibition by microRNAs, phosphorylation, and interaction with specific inhibitors. Due to the diversity of these mechanisms and involvement of other factors, defining the exact contribution of Apaf-1 to the development of cancer in general and neurodegenerative disorders, in particular, is complicated. The current review is an attempt to provide a comprehensive image of Apaf-1's contribution to the pathologies observed in cancer and neurodegenerative diseases with the emphasis on the therapeutic aspects of Apaf-1 as an important target in these pathologies.

Coenzyme Q10 inhibits RANKL-induced osteoclastogenesis by regulation of mitochondrial apoptosis and oxidative stress in RAW264.7 cells

Coenzyme Q10 (CoQ10) has been reported to improve bone density and the number of trabeculae in postmenopausal osteoporosis, but the mechanism remains to be elucidated. We aimed to investigate the effects of CoQ10 on receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis and the underlying molecular mechanisms. RAW264.7 cells were treated with different concentrations of RANKL to differentiate into osteoclasts, and then these cells were treated with different concentrations of CoQ10 with or without H₂ O₂ . Tartrate-resistant acid phosphatase staining was performed to detect osteoclasts. Cell viability was tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, cell apoptosis was examined by flow cytometry, and the effects of CoQ10 on protein and messenger RNA expression of mitochondrial apoptosis-associated proteins and osteoclast marker proteins were measured by quantitative reverse transcription polymerase chain reaction and western blot, respectively. Furthermore, enzyme-linked immunosorbent assay was conducted to analyze the activities of malondialdehyde (MDA), superoxide dismutase (SOD), and catalase (CAT). RANKL significantly induced osteoclastogenesis in RAW264.7 cells, with the greatest efficiency at 50 ng/ml. CoQ10 had no significant effects on cell viability but it significantly increased the percentages of cell apoptosis. Mechanically, CoQ10 statistically decreased the levels of Bcl-2 and cytochrome C in mitochondria and upregulated the levels of Bax, cleaved caspase 3, and cytochrome C in the cytoplasm. Moreover, CoQ10 significantly decreased RANKL-induced osteoclastogenesis regardless of exposure to H₂ O₂ . In addition, CoQ10 statistically reduced MDA activity and elevated the activities of SOD and CAT, as well as the expression of oxidative stress-related proteins. CoQ10 may inhibit RANKL-induced osteoclastogenesis by regulation of mitochondrial apoptosis and oxidative stress in RAW264.7 cells.

Functions of reactive oxygen species in apoptosis and ganoderic acid biosynthesis in Ganoderma lucidum

Ganoderma lucidum is a medicinal fungus that is widely used in traditional medicine. Fungal PacC is recognized as an important transcription factor that functions during adaptation to environmental pH, fungal development and secondary metabolism. Previous studies have revealed that GlPacC plays important roles in mycelial growth, fruiting body development and ganoderic acid (GA) biosynthesis. In this study, using a terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay, we found that the apoptosis level was increased when PacC was silenced. The transcript and activity levels of caspase-like proteins were significantly increased in the PacC-silenced (PacCi) strains compared with the control strains. Silencing PacC also resulted in an increased reactive oxygen species (ROS) levels (∼2-fold) and decreased activity levels of enzymes involved in the antioxidant system. Further, we found that the intracellular ROS levels contributed to apoptosis and GA biosynthesis. Adding N-acetyl-cysteine and vitamin C decreased intracellular ROS and resulted in the inhibition of apoptosis in the PacCi strains. Additionally, the GA biosynthesis was different between the control strains and the PacCi strains after intracellular ROS was eliminated. Taken together, the findings showed that silencing PacC can result in an intracellular ROS burst, which increases cell apoptosis and GA biosynthesis levels. Our study provides novel insight into the functions of PacC in filamentous fungi.

A novel HSP90 inhibitor with reduced hepatotoxicity synergizes with radiotherapy to induce apoptosis, abrogate clonogenic survival, and improve tumor control in models of colorectal cancer

The chaperone heat shock protein 90 (HSP90) crucially supports the maturation, folding, and stability of a variety of client proteins which are of pivotal importance for the survival and proliferation of cancer cells. Consequently, targeting of HSP90 has emerged as an attractive strategy of anti-cancer therapy, and it appears to be particularly effective in the context of molecular sensitization towards radiotherapy as has been proven in preclinical models of different cancer entities. However, so far the clinical translation has largely been hampered by suboptimal pharmacological properties and serious hepatotoxicity of first- and second-generation HSP90 inhibitors. Here, we report on NW457, a novel radicicol-derived member of the pochoxime family with reduced hepatotoxicity, how it inhibits the DNA damage response and how it synergizes with ionizing irradiation to induce apoptosis, abrogate clonogenic survival, and improve tumor control in models of colorectal cancer in vitro and in vivo.

Highly pathogenic avian influenza A virus H5N1 non-structural protein 1 is associated with apoptotic activation of the intrinsic mitochondrial pathway

Outbreaks of avian influenza A (H5N1) virus infection have significant health and economic consequences. Non-structural protein 1 (NS1) is an essential virulence factor of the highly pathogenic H5N1 avian influenza virus and of the apoptosis associated with the pathogenesis of H5N1. Previous studies have revealed that the NS1 protein is able to induce apoptosis via an extrinsic pathway. However, it remains unclear whether the intrinsic pathway is also associated with this apoptosis. The present study used a clone of the NS1 gene from avian influenza A/Jiangsu/1/2007 and observed the localization of the NS1 protein and cytochrome c release from mitochondria and the change of mitochondrial membrane potential (MMP) in lung cancer cells. Cytotoxicity was detected using an MTT assay and the number of apoptotic cells was counted using a flow cytometer. Following the isolation of mitochondria, western blotting was performed to compare cytochrome c release from the mitochondria in cells before and after apoptosis. The change of MMP was detected using JC-1 staining. Furthermore, the results reveal that the majority of the NS1 protein was localized in the cell nucleus, and that it may induce apoptosis of human lung epithelial cells. The apoptosis occurred with marked cytochrome c release from mitochondria and a change of the MMP. This indicated that the NS1 protein may be associated with apoptosis induced by an intrinsic mitochondrial pathway.

A synthetic lethal screen identifies ATR-inhibition as a novel therapeutic approach for POLD1-deficient cancers

The phosphoinositide 3-kinase-related kinase ATR represents a central checkpoint regulator and mediator of DNA-repair. Its inhibition selectively eliminates certain subsets of cancer cells in various tumor types, but the underlying genetic determinants remain enigmatic. Here, we applied a synthetic lethal screen directed against 288 DNA-repair genes using the well-defined ATR knock-in model of DLD1 colorectal cancer cells to identify potential DNA-repair defects mediating these effects. We identified a set of DNA-repair proteins, whose knockdown selectively killed ATR-deficient cancer cells. From this set, we further investigated the profound synthetic lethal interaction between ATR and POLD1. ATR-dependent POLD1 knockdown-induced cell killing was reproducible pharmacologically in POLD1-depleted DLD1 cells and a panel of other colorectal cancer cell lines by using chemical inhibitors of ATR or its major effector kinase CHK1. Mechanistically, POLD1 depletion in ATR-deficient cells caused caspase-dependent apoptosis without preceding cell cycle arrest and increased DNA-damage along with impaired DNA-repair. Our data could have clinical implications regarding tumor genotype-based cancer therapy, as inactivating POLD1 mutations have recently been identified in small subsets of colorectal and endometrial cancers. POLD1 deficiency might thus represent a predictive marker for treatment response towards ATR- or CHK1-inhibitors that are currently tested in clinical trials.

MicroRNA-17-mediated down-regulation of apoptotic protease activating factor 1 attenuates apoptosome formation and subsequent apoptosis of cardiomyocytes

Heart diseases such as myocardial infarction (MI) can damage individual cardiomyocytes, leading to the activation of cell death programs. The most scrutinized type of cell death in the heart is apoptosis, and one of the key events during the propagation of apoptotic signaling is the formation of apoptosomes, which relay apoptotic signals by activating caspase-9. As one of the major components of apoptosomes, apoptotic protease activating factor 1 (Apaf-1) facilitates the formation of apoptosomes containing cytochrome c (Cyto-c) and deoxyadenosine triphosphate (dATP). Thus, it may be possible to suppress the activation of the apoptotic program by down-regulating the expression of Apaf-1 using miRNAs. To validate this hypothesis, we selected a number of candidate miRNAs that were expected to target Apaf-1 based on miRNA target prediction databases. Among these candidate miRNAs, we empirically identified miR-17 as a novel Apaf-1-targeting miRNA. The delivery of exogenous miR-17 suppressed Apaf-1 expression and consequently attenuated formation of the apoptosome complex containing caspase-9, as demonstrated by co-immunoprecipitation and immunocytochemistry. Furthermore, miR-17 suppressed the cleavage of procaspase-9 and the subsequent activation of caspase-3, which is downstream of activated caspase-9. Cell viability tests also indicated that miR-17 pretreatment significantly prevented the norepinephrine-induced apoptosis of cardiomyocytes, suggesting that down-regulation of apoptosome formation may be an effective strategy to prevent cellular apoptosis. These results demonstrate the potential of miR-17 as an effective anti-apoptotic agent.

Serum-derived plasminogen is activated by apoptotic cells and promotes their phagocytic clearance

The elimination of apoptotic cells, called efferocytosis, is fundamentally important for tissue homeostasis and prevents the onset of inflammation and autoimmunity. Serum proteins are known to assist in this complex process. In the current study, we performed a multistep chromatographic fractionation of human serum and identified plasminogen, a protein involved in fibrinolysis, wound healing, and tissue remodeling, as a novel serum-derived factor promoting apoptotic cell removal. Even at levels significantly lower than its serum concentration, purified plasminogen strongly enhanced apoptotic prey cell internalization by macrophages. Plasminogen acted mainly on prey cells, whereas on macrophages no enhancement of the engulfment process was observed. We further demonstrate that the efferocytosis-promoting activity essentially required the proteolytic activation of plasminogen and was completely abrogated by the urokinase plasminogen activator inhibitor-1 and serine protease inhibitor aprotinin. Thus, our study assigns a new function to plasminogen and plasmin in apoptotic cell clearance.

Involvement of GDH3-encoded NADP+-dependent glutamate dehydrogenase in yeast cell resistance to stress-induced apoptosis in stationary phase cells

Glutamate metabolism is linked to a number of fundamental metabolic pathways such as amino acid metabolism, the TCA cycle, and glutathione (GSH) synthesis. In the yeast Saccharomyces cerevisiae, glutamate is synthesized from α-ketoglutarate by two NADP(+)-dependent glutamate dehydrogenases (NADP-GDH) encoded by GDH1 and GDH3. Here, we report the relationship between the function of the NADP-GDH and stress-induced apoptosis. Gdh3-null cells showed accelerated chronological aging and hypersusceptibility to thermal and oxidative stress during stationary phase. Upon exposure to oxidative stress, Gdh3-null strains displayed a rapid loss in viability associated with typical apoptotic hallmarks, i.e. reactive oxygen species accumulation, nuclear fragmentation, DNA breakage, and phosphatidylserine translocation. In addition, Gdh3-null cells, but not Gdh1-null cells, had a higher tendency toward GSH depletion and subsequent reactive oxygen species accumulation than did WT cells. GSH depletion was rescued by exogenous GSH or glutamate. The hypersusceptibility of stationary phase Gdh3-null cells to stress-induced apoptosis was suppressed by deletion of GDH2. Promoter swapping and site-directed mutagenesis of GDH1 and GDH3 indicated that the necessity of GDH3 for the resistance to stress-induced apoptosis and chronological aging is due to the stationary phase-specific expression of GDH3 and concurrent degradation of Gdh1 in which the Lys-426 residue plays an essential role.

The central role of initiator caspase-9 in apoptosis signal transduction and the regulation of its activation and activity on the apoptosome

Key structural and catalytic features are conserved across the entire family of cysteine-dependent aspartate-specific proteases (caspases). Of the caspases involved in apoptosis signal transduction, the initiator caspases-2, -8 and -9 are activated at multi-protein activation platforms, and activation is thought to involve homo-dimerisation of the monomeric zymogens. Caspase-9, the essential initiator caspase required for apoptosis signalling through the mitochondrial pathway, is activated on the apoptosome complex, and failure to activate caspase-9 has profound pathophysiological consequences. Here, we review the pertinent literature on which the currently prevalent understanding of caspase-9 activation is based, extend this view by insight obtained from recent structural and kinetic studies on caspase-9 signalling, and describe an emerging model for the regulation of caspase-9 activation and activity that arise from the complexity of multi-protein interactions at the apoptosome. This integrated view allows us to postulate and to discuss functional consequences for caspase-9 activation and apoptosis execution that may take centre stage in future experimental cell research on apoptosis signalling.

Regulation of apoptotic pathways by Stylophora pistillata (Anthozoa, Pocilloporidae) to survive thermal stress and bleaching

Elevated seawater temperatures are associated with coral bleaching events and related mortality. Nevertheless, some coral species are able to survive bleaching and recover. The apoptotic responses associated to this ability were studied over 3 years in the coral Stylophora pistillata from the Gulf of Eilat subjected to long term thermal stress. These include caspase activity and the expression profiles of the S. pistillata caspase and Bcl-2 genes (StyCasp and StyBcl-2-like) cloned in this study. In corals exposed to thermal stress (32 or 34°C), caspase activity and the expression levels of the StyBcl-2-like gene increased over time (6–48 h) and declined to basal levels within 72 h of thermal stress. Distinct transcript levels were obtained for the StyCasp gene, with stimulated expression from 6 to 48 h of 34°C thermal stress, coinciding with the onset of bleaching. Increased cell death was detected in situ only between 6 to 48 h of stress and was limited to the gastroderm. The bleached corals survived up to one month at 32°C, and recovered back symbionts when placed at 24°C. These results point to a two-stage response in corals that withstand thermal stress: (i) the onset of apoptosis, accompanied by rapid activation of anti-oxidant/anti-apoptotic mediators that block the progression of apoptosis to other cells and (ii) acclimatization of the coral to the chronic thermal stress alongside the completion of symbiosis breakdown. Accordingly, the coral's ability to rapidly curb apoptosis appears to be the most important trait affecting the coral's thermotolerance and survival.

Cleavage of annexin A1 by ADAM10 during secondary necrosis generates a monocytic "find-me" signal

Annexin A1 is an intracellular calcium/phospholipid-binding protein that is involved in membrane organization and the regulation of the immune system. It has been attributed an anti-inflammatory role at various control levels, and recently we could show that annexin A1 externalization during secondary necrosis provides an important fail-safe mechanism counteracting inflammatory responses when the timely clearance of apoptotic cells has failed. As such, annexin A1 promotes the engulfment of dying cells and dampens the postphagocytic production of proinflammatory cytokines. In our current follow-up study, we report that exposure of annexin A1 during secondary necrosis coincided with proteolytic processing within its unique N-terminal domain by ADAM10. Most importantly, we demonstrate that the released peptide and culture supernatants of secondary necrotic, annexin A1-externalizing cells induced chemoattraction of monocytes, which was clearly reduced in annexin A1- or ADAM10-knockdown cells. Thus, altogether our findings indicate that annexin A1 externalization and its proteolytic processing into a chemotactic peptide represent final events during apoptosis, which after the transition to secondary necrosis contribute to the recruitment of monocytes and the prevention of inflammation.

Cell apoptosis induced by zinc deficiency in osteoblastic MC3T3-E1 cells via a mitochondrial-mediated pathway

Deficiency of zinc plays an important role in the pathogenesis of osteoporosis; however, the underlying mechanism is not well understood. Apoptosis of osteoblast causing the loss of bone mass is an important event in the osteoporosis. In this article, we investigated whether zinc deficiency would induce cell apoptosis in MC3T3-E1 cells and ask if it is involved in mitochondrial-mediated pathway. Significant increased apoptosis were observed in zinc deficiency group (ZnD: 5 μM TPEN and 1 μM zinc) compared with untreated control or zinc adequacy group (ZnA: 5 μM TPEN and 15 μM zinc). The mitochondrial membrane potential was strikingly reduced in ZnD group. Furthermore, we observed that the levels of Bax in mitochondria fraction and cyto c, AIF, and cleaved caspase-3/-9 in cytosol fraction were increased in ZnD group. We proposed that zinc deficiency would induce the translocation of Bax into mitochondria, which could lead to the reduction in mitochondrial membrane potential as well as the increase in mitochondrial membrane permeability. In addition, cyto c and AIF were released from mitochondria into the cytosol, which finally activated caspase-dependent and caspase-independent apoptosis processes in MC3T3-E1 cells. Our findings suggested that zinc deficiency is capable of inducing apoptosis through a mitochondria-mediated pathway in osteoblastic cells.

Triggering of a novel intrinsic apoptosis pathway by the kinase inhibitor staurosporine: activation of caspase-9 in the absence of Apaf-1

The protein kinase inhibitor staurosporine is one of the most potent and frequently used proapoptotic stimuli, although its mechanism of action is poorly understood. Here, we show that staurosporine as well as its analog 7-hydroxystaurosporine (UCN-01) not only trigger the classical mitochondrial apoptosis pathway but, moreover, activate an additional novel intrinsic apoptosis pathway. Unlike conventional anticancer drugs, staurosporine and UCN-01 induced apoptosis in a variety of tumor cells overexpressing the apoptosis inhibitors Bcl-2 and Bcl-x(L). Furthermore, activation of this novel intrinsic apoptosis pathway by staurosporine did not rely on Apaf-1 and apoptosome formation, an essential requirement for the mitochondrial pathway. Nevertheless, as demonstrated in caspase-9-deficient murine embryonic fibroblasts, human lymphoma cells, and chicken DT40 cells, staurosporine-induced apoptosis was essentially mediated by caspase-9. Our results therefore suggest that, in addition to the classical cytochrome c/Apaf-1-dependent pathway of caspase-9 activation, staurosporine can induce caspase-9 activation and apoptosis independently of the apoptosome. Since staurosporine derivatives have proven efficacy in clinical trials, activation of this novel pathway might represent a powerful target to induce apoptosis in multidrug-resistant tumor cells.

Apoptosis and the selective survival of host animals following thermal bleaching in zooxanthellate corals

During the past several decades, numerous reports from disparate geographical areas have documented an increased frequency of "bleaching" in reef-forming corals. The phenomenon, triggered by increased sea surface temperatures, occurs when the cnidarian hosts digest and/or expel their intracellular, photosynthetic dinoflagellate symbionts ("zooxanthellae" in the genus Symbiodinium). Although coral bleaching is often followed by the death of the animal hosts, in some cases, the animal survives and can be repopulated with viable zooxanthellae. The physiological factors determining the ability of the coral to survive bleaching events are poorly understood. In this study, we experimentally established that bleaching and death of the host animal involve a caspase-mediated apoptotic cascade induced by reactive oxygen species produced primarily by the algal symbionts. In addition, we demonstrate that, although some corals naturally suppress caspase activity and significantly reduce caspase concentration under high temperatures as a mechanism to prevent colony death from apoptosis, even sensitive corals can be prevented from dying by application of exogenous inhibitors of caspases. Our results indicate that variability in response to thermal stress in corals is determined by a four-element, combinatorial genetic matrix intrinsic to the specific symbiotic association. Based on our experimental data, we present a working model in which the phenotypic expression of this symbiont/host relationship places a selective pressure on the symbiotic association. The model predicts the survival of the host animals in which the caspase-mediated apoptotic cascade is down-regulated.

Cell surface externalization of annexin A1 as a failsafe mechanism preventing inflammatory responses during secondary necrosis

The engulfment of apoptotic cells is of crucial importance for tissue homeostasis in multicellular organisms. A failure of this process results in secondary necrosis triggering proinflammatory cytokine production and autoimmune disease. In the present study, we investigated the role of annexin A1, an intracellular protein that has been implicated in the efficient removal of apoptotic cells. Consistent with its function as bridging protein in the phagocyte synapse, opsonization of apoptotic cells with purified annexin A1 strongly enhanced their phagocytic uptake. A detailed analysis, however, surprisingly revealed that annexin A1 was hardly exposed to the cell surface of primary apoptotic cells, but was strongly externalized only on secondary necrotic cells. Interestingly, while the exposure of annexin A1 failed to promote the uptake of these late secondary necrotic cells, it efficiently prevented induction of cytokine production in macrophages during engulfment of secondary necrotic cells. Our results therefore suggest that annexin A1 exposure during secondary necrosis provides an important failsafe mechanism counteracting inflammatory responses, even when the timely clearance of apoptotic cells has failed.

Hepatitis C virus core protein and cellular protein HAX-1 promote 5-fluorouracil-mediated hepatocyte growth inhibition

Hepatitis C virus (HCV) often causes chronic infection and may lead to hepatocellular carcinoma (HCC). We have shown previously that HCV core protein has pleiotropic functions, including transcriptional regulation of a number of cellular genes, although the mechanism for gene regulation remains unclear. In this study, a mammalian two-hybrid screen identified a novel binding partner, HS1-associated protein X-1 (HAX-1), for HCV core protein from a human liver cDNA library. An association between HAX-1 and HCV core protein was further verified by confocal microscopy and coimmunoprecipitation in HepG2 cells expressing HCV core or full-length (FL) gene. Both HCV core protein and a chemotherapeutic agent for HCC, 5-flouorouracil (5-FU), are known to modulate p53. We examined here whether an association between core and HAX-1 has any functional relevance to p53 modulation in 5-FU-treated cells. For this, the role of HAX-1 on 5-FU treatment was examined in HepG2 cells expressing HCV core or FL gene using cell proliferation, p53 expression, and caspase activation analysis. Cells expressing HCV-core or FL gene were more susceptible to 5-FU-induced growth inhibition than control cells, whereas cell survival was enhanced after suppression of HAX-1 by small interfering RNA. Further, 5-FU-mediated p53 expression was reduced with concurrent HAX-1 suppression in core- or polyprotein-expressing cells compared to control HepG2 cells, and caspase-2 and -7 activities were diminished. On the other hand, HCV core protein did not play a detectable role in 5-FU-mediated caspase-7 activation in the absence of functional p53 in Hep3B or Huh-7 cells. These observations underscore an association between HCV core and HAX-1, which promotes 5-FU mediated p53-dependent caspase-7 activation and hepatocyte growth inhibition.

The anti-apoptotic activity of BAG3 is restricted by caspases and the proteasome

Background

Caspase-mediated cleavage and proteasomal degradation of ubiquitinated proteins are two independent mechanisms for the regulation of protein stability and cellular function. We previously reported BAG3 overexpression protected ubiquitinated clients, such as AKT, from proteasomal degradation and conferred cytoprotection against heat shock. We hypothesized that the BAG3 protein is regulated by proteolysis.

Methodology/Principal Findings

Staurosporine (STS) was used as a tool to test for caspase involvement in BAG3 degradation. MDA435 and HeLa human cancer cell lines exposed to STS underwent apoptosis with a concomitant time and dose-dependent loss of BAG3, suggesting the survival role of BAG3 was subject to STS regulation. zVAD-fmk or caspase 3 and 9 inhibitors provided a strong but incomplete protection of both cells and BAG3 protein. Two putative caspase cleavage sites were tested: KEVD (BAG3^E345A/D347A) within the proline-rich center of BAG3 (PXXP) and the C-terminal LEAD site (BAG3^E516A/D518A). PXXP deletion mutant and BAG3^E345A/D347A, or BAG3^E516A/D518A respectively slowed or stalled STS-mediated BAG3 loss. BAG3, ubiquitinated under basal growth conditions, underwent augmented ubiquitination upon STS treatment, while there was no increase in ubiquitination of the BAG3^E516A/D518A caspase-resistant mutant. Caspase and proteasome inhibition resulted in partial and independent protection of BAG3 whereas inhibitors of both blocked BAG3 degradation. STS-induced apoptosis was increased when BAG3 was silenced, and retention of BAG3 was associated with cytoprotection.

Conclusions/Significance

BAG3 is tightly controlled by selective degradation during STS exposure. Loss of BAG3 under STS injury required sequential caspase cleavage followed by polyubiquitination and proteasomal degradation. The need for dual regulation of BAG3 in apoptosis suggests a key role for BAG3 in cancer cell resistance to apoptosis.

Vitamins C and E inhibit apoptosis of cultured human term placenta trophoblast

Preeclampsia can be lethal to both mother and baby. The prominent symptoms of this syndrome are hypertension, proteinuria and oedema, resulting from an exaggerated aseptic systemic inflammatory response, triggered by placental factors shed into the maternal circulation. Syncytiotrophoblast microparticles (STBM) are one possible factor, shed when the placenta is exposed to stressors such as hypoxia/reperfusion. These can disrupt mitochondria, triggering apoptosis and necrosis, placental pathologies which are increased in preeclampsia. We tested the effects of antioxidant vitamins C (50 microM) and E (50 microM) on trophoblast in culture, using term villous cytotrophoblast preparations. Following Percoll gradient centrifugation and MHC class I expressing cell depletion of placenta digests, syncytial fragments were removed using anti-placental alkaline phosphatase antibody. This yielded cytotrophoblasts of consistently high purity. EGF (10 ng/ml) stimulated syncytialisation and hCG and progesterone production. However, mitochondrial induced apoptosis (MIA) was evident 96h post-isolation, as mitochondrial membrane potential loss and caspase 9 and caspase 3 activation. ROCK-1 cleavage and syncytiotrophoblast particle shedding increased concurrently with apoptosis induction. Vitamins blocked MIA and syncytiotrophoblast particle shedding and significantly increased hCG (p<0.005) and progesterone (p<0.02) concentrations in culture supernatants, reflecting the increased survival rates. Although more cells survived in culture, syncytialisation rate (%) was significantly reduced (p<0.005). We conclude that vitamins C and E can significantly reduce mitochondrial damage generated following syncytialisation in vitro. However, further work is required to determine whether antioxidant vitamins interfere with normal fusion processes.

Apoptosis-associated antigens recognized by autoantibodies in patients with the autoimmune liver disease primary biliary cirrhosis

There is growing evidence that the onset of autoimmune disorders can be linked to the inefficient removal of apoptotic cells. Since defects in the elimination of apoptotic cells lead to secondary necrosis and subsequent release of intracellular components, this might explain the generation of autoantibodies against intracellular antigens. Accordingly, we wanted to investigate, whether antibodies from patients with the autoimmune liver disease primary biliary cirrhosis (PBC) recognize self-proteins generated and released during apoptosis. Using Western blot analyses we could detect intracellular antigens with serum IgG from PBC patients but not with serum IgG from healthy donors in lysates of Jurkat T-leukemia, HepG2 hepatoma, and HT-29 colon-carcinoma cells. Interestingly, PBC serum IgG also recognized caspase substrates in cells undergoing apoptosis induced by staurosporine or TRAIL (TNF-related apoptosis inducing ligand). In addition to intracellular antigens, serum IgG from PBC patients detected caspase-dependent antigens in the supernatants of apoptotic (secondary necrotic) cells and antigens on the surface of apoptotic Jurkat cells. Among the caspase substrates recognized by PBC serum IgG we could identify the components PDC-E2 and -E1beta of the known autoantigen PDC (pyruvate dehydrogenase complex). Thus, caspase-mediated processing of intracellular proteins might generate de novo autoantigens that upon release contribute to the generation of autoantibodies and autoimmune diseases as PBC.

Caspase-8 is activated by cathepsin D initiating neutrophil apoptosis during the resolution of inflammation

In the resolution of inflammatory responses, neutrophils rapidly undergo apoptosis. We describe a new proapoptotic pathway in which cathepsin D directly activates caspase-8. Cathepsin D is released from azurophilic granules in neutrophils in a caspase-independent but reactive oxygen species–dependent manner. Under inflammatory conditions, the translocation of cathepsin D in the cytosol is blocked. Pharmacological or genetic inhibition of cathepsin D resulted in delayed caspase activation and reduced neutrophil apoptosis. Cathepsin D deficiency or lack of its translocation in the cytosol prolongs innate immune responses in experimental bacterial infection and in septic shock. Thus, we identified a new function of azurophilic granules that is in addition to their role in bacterial defense mechanisms: to regulate the life span of neutrophils and, therefore, the duration of innate immune responses through the release of cathepsin D.

Big wheel keeps on turning: apoptosome regulation and its role in chemoresistance

Apoptosis, a form of programmed cell death, enables organisms to maintain tissue homeostasis through deletion of extraneous cells and also serves as a means to eliminate potentially harmful cells. Numerous stress signals have been shown to engage the intrinsic pathway of apoptosis, with the release from mitochondria of proapoptotic factors such as cytochrome c and the subsequent formation of a cytosolic complex between apoptotic protease-activating factor-1 (Apaf-1) and procaspase-9, known as the apoptosome. Recent studies have led to the identification of an array of factors that control the formation and activation of the apoptosome under physiological conditions. Moreover, deregulation of apoptosome function has been documented in various forms of human cancer, and may play a role in both carcinogenesis and chemoresistance. We discuss how the apoptosome is regulated in normal and disease states, and how targeting of apoptosome-dependent, post-mitochondrial stages of apoptosis may serve as a rational approach to cancer treatment.

Potentiation of chemotherapeutics by the Hsp90 antagonist geldanamycin requires a steady serum condition

Inhibition of Hsp90 potentiates diverse chemotherapeutics, but it is not clear if this applies only to specific agents, tumor types or conditions. The aim of this report is to determine the effect of serum starvation (SS) on potentiation. SUIT2 cells were cultured with and without the presence of serum and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assays were carried out at time intervals. Cytotoxic agents were added individually or in combination. Immunohistochemistry of tumor samples and immunofluorescence of cultured cells were used to examine Hsp90 localization. In the presence of serum an at least additive effect of combining the Hsp90 inhibitor geldanamycin (GA) with 5-fluorouracil (5FU) was demonstrated. Following pretreatment with GA, 5FU and GA were synergistic. However, during SS GA was protective against 5FU. Geldanamycin also protected cells from 12-O-tetradecanoylphorbol-13-acetate (TPA) during SS. Protection of cells is transitory, as after 24 h of SS GA again has an at least additive negative effect on vitality with 5FU or TPA. Serum starvation of pancreatic cancer cell lines causes normally largely cytoplasmic Hsp90 to become predominantly nuclear localized. Hsp90 nuclear localization was observed in pancreatic and prostate tumors. Hsp90 binding to a pro-apoptotic client could explain the transitory protection of cells by Hsp90 inhibition during SS. Although potentiation of chemotherapeutics by Hsp90 inhibition is probably a general phenomenon, design of clinical trials should take into account that continuous co-administration may be ineffective because of a balance of synergy of the drugs in some cells and mutual inhibition of the two drug activities in other cells.

Mitochondria are not required for death receptor-mediated cytosolic acidification during apoptosis

In addition to cell shrinkage, membrane blebbing, DNA fragmentation and phosphatidylserine exposure, intracellular acidification represents a hallmark of apoptosis. Although the mechanisms underlying cytosolic acidification during apoptosis remained largely elusive, a pivotal role of mitochondria has been proposed. In order to investigate the involvement of mitochondria in cytosolic acidification during apoptosis, we blocked the mitochondrial death pathway by overexpression of Bcl-2 and subsequently activated the death receptor pathway by anti-CD95 or TRAIL or the mitochondrial pathway by staurosporine. We show that Bcl-2 but not caspase inhibition prevented staurosporine-induced intracellular acidification. Thus, intracellular acidification in mitochondrial apoptosis is a Bcl-2-inhibitable, but caspase-independent process. In contrast, Bcl-2 only slightly delayed, but did not prevent intracellular acidification upon triggering of death receptors. The Na(+)/H(+) exchanger NHE1 was partially degraded during apoptosis but only to a small extent and and at a delayed time point when cytosolic acidification was almost completed. We therefore conclude that cytosolic acidification is mitochondrially controlled in response to mitochondria-dependent death stimuli, but requires additional caspase-dependent mechanisms during death receptor-mediated apoptosis.

Leishmania major metacaspase can replace yeast metacaspase in programmed cell death and has arginine-specific cysteine peptidase activity

The human protozoan parasite Leishmania major has been shown to exhibit several morphological and biochemical features characteristic of a cell death program when differentiating into infectious stages and under a variety of stress conditions. Although some caspase-like peptidase activity has been reported in dying parasites, no caspase gene is present in the genome. However, a single metacaspase gene is present in L. major whose encoded protein harbors the predicted secondary structure and the catalytic dyad histidine/cysteine described for caspases and other metacaspases identified in plants and yeast. The Saccharomyces cerevisiae metacaspase YCA1 has been implicated in the death of aging cells, cells defective in some biological functions, and cells exposed to different environmental stresses. In this study, we describe the functional heterologous complementation of a S. cerevisiae yca1 null mutant with the L. major metacaspase (LmjMCA) in cell death induced by oxidative stress. We show that LmjMCA is involved in yeast cell death, similar to YCA1, and that this function depends on its catalytic activity. LmjMCA was found to be auto-processed as occurs for caspases, however LmjMCA did not exhibit any activity with caspase substrates. In contrast and similarly to Arabidopsis thaliana metacaspases, LmjMCA was active towards substrates with arginine in the P1 position, with the activity being abolished following H147A and C202A catalytic site mutations. These results suggest that metacaspases are members of a family of peptidases with a role in cell death conserved in evolution notwithstanding possible differences in their catalytic activity.

Differential regulation of the intrinsic pathway of apoptosis in brain and liver during ageing

The intrinsic (mitochondria-dependent) pathway of apoptosis is one of the major pathways leading to cell death. We evaluated cytochrome c/apoptotic protease activation factor-1 (Apaf-1)-dependent activation of caspase-3 in brain and liver of different strains of rodents at different stages of development. In cell-free extracts from brain and liver of Sprague-Dawley rats, caspase was activated by cytochrome c/2'-deoxyadenosine 5'-triphosphate at both neonatal and adult stages. In adult brain extracts from Wistar rats, no activation of caspase was observed while extracts from neonatal brain and liver and from adult liver were activated. In CD-1 mouse, only neonatal extracts were activated. Alteration in levels of endogenous inhibitors of apoptosis were not responsible for the lack of activation observed. Instead, decrease in the content of Apaf-1 and caspase-3 and some degradation of caspase-9 during brain ageing were observed. These results suggest that a decrease in apoptosis activation during ageing is not tissue-specific, but rather displays a complex dependence on species and strains of animals.

Unique and overlapping substrate specificities of caspase-8 and caspase-10

Although caspase-8 has an established role as an initiator of death receptor-mediated apoptosis, the function of its closest homolog, caspase-10, is almost completely unknown. To gain a closer insight into the physiological function of caspase-10, we compared the cleavage of known caspase-8 substrates by both initiator caspases. We demonstrate that caspase-10 and -8 have overlapping cleavage preferences for several substrates such as the kinases RIP and PAK2. Interestingly, in other substrates, such as the Bcl-2 protein Bid, we found additional and distinct cleavage sites for both caspases, which might have important consequences for mitochondrial targeting and propagation of the death signal. Caspase-8 and -10 also caused different interchain cleavage patterns of their enzyme precursors. Together, these results suggest that caspase-8 and -10, despite having overlapping functions, also have selective substrate cleavage specificities and might thereby exert nonredundant roles in apoptosis signaling.

Role of Apaf-1, a key regulator of apoptosis, in melanoma progression and chemoresistance

Apoptosis protease-activating factor-1 (Apaf-1) is a key regulator of the mitochondrial apoptotic pathway, being the central element of the multimeric apoptosome formed by procaspase 9, cytochrome c, and Apaf-1 itself. In this review, the principal aspects about Apaf-1 gene structure and function, and its role in the apoptotic machinery, are described. Moreover, the most recent findings about the involvement of this molecule in melanoma progression and chemoresistance, as well as the clinico-pathological relevance of these findings in the treatment of this deadly disease, are reported.

E1A sensitizes cancer cells to TRAIL-induced apoptosis through enhancement of caspase activation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce apoptosis of cancer cells. Sensitization of cancer cells to TRAIL, particularly TRAIL-resistant cancer cells, could improve the effectiveness of TRAIL as an anticancer agent. The adenovirus type 5 E1A that associates with anticancer activities including sensitization to apoptosis induced by tumor necrosis factor is currently being tested in clinical trials. In this study, we investigated the sensitivity to TRAIL in the E1A transfectants ip1-E1A2 and 231-E1A cells and the parental TRAIL-resistant human ovarian cancer SKOV3.ip1 and TRAIL-sensitive human breast cancer MDA-MB-231 cells. The results indicated that the percentage of TRAIL-induced apoptotic cells was significantly higher in the E1A transfectants of both cell lines than it was in the parental cell lines. To further investigate the cellular mechanism of this effect, we found that E1A enhances TRAIL-induced activation of caspase-8, caspase-9, and caspase-3. Inhibition of caspase-3 activity by a specific inhibitor, Z-DEVD-fmk, abolished TRAIL-induced apoptosis. In addition, E1A enhanced TRAIL expression in ip1-E1A2 cells, but not in 231-E1A cells, and the anti-TRAIL neutralizing antibody N2B2 blocked the E1A-mediated bystander effect in vitro. Taken together, these results suggest that E1A sensitizes both TRAIL-sensitive and TRAIL-resistant cancer cells to TRAIL-induced apoptosis, which occurs through the enhancement of caspase activation; activation of caspase-3 is required for TRAIL-induced apoptosis; and E1A-induced TRAIL expression is involved in the E1A-mediated bystander effect. Combination of E1A and TRAIL could be an effective treatment for cancer.

Inhibition of hepatitis C virus core protein expression in immortalized human hepatocytes induces cytochrome c-independent increase in Apaf-1 and caspase-9 activation for cell death

Hepatitis C virus (HCV) core protein has multifunctional activities. We have previously reported that the core protein of HCV immortalizes primary human hepatocytes, which may relate to multistage hepatocarcinogenic events. These immortalized human hepatocytes (IHH) served as a model to study the mechanism of HCV core protein-mediated cell growth regulation. Inhibition of core protein expression in earlier stages after hepatocyte immortalization leads to the induction of apoptosis. Here, we have observed that introduction of antisense core (AS-Core) sequences for inhibition of core protein expression enhanced the expression of E2F1 and p53 levels in early passage IHH. Inhibition of core protein expression also altered the expression level of Bcl-2 family proteins, displaying an increase of the proapoptotic Bax and a decrease in the level of the anti-apoptotic Bcl-xL proteins. These alterations, however, did not result in the release of cytochrome c from the mitochondria. Apaf-1 is frequently deregulated under various pathologic conditions, and examination of AS-Core-expressing apoptotic cells indicated a significant increase in the level of Apaf-1, which coincided with caspase-9 activation. Knockdown of Apaf-1 or the transcriptional regulatory proteins, E2F1 or p53, by small interfering RNA (siRNA) duplexes inhibited the activation of caspase-9 and enhanced cell viability in AS-Core-expressing cells. These findings may contribute to the understanding of the pathophysiology of HCV core protein-mediated hepatocyte growth regulation and disease progression.

Amyloid β peptide binds a novel death‐inducing protein, AB‐DIP

Amyloid beta-peptide (Abeta) plays a central role in the pathogenesis of Alzheimer's disease (AD). It is toxic to neurons, but the mechanism for its action remains largely unknown. Here, we have identified a novel death-inducing protein, Abeta-related DIP (AB-DIP), by two-hybrid screening of the human brain cDNA library and confirmed the binding of Abeta with AB-DIP by coimmunoprecipitation. Overexpression of AB-DIP-induced cell death and coexpression of Abeta enhanced the cell death. During apoptosis, the 97-kDa AB-DIP was cleaved to a 62-kDa protein (AB-DIP p62) at the caspase cleavage site, LEKD. It is more important that cotransfection of Abeta with AB-DIP produced the AB-DIP p62 fragment. Small interfering RNA-mediated knockdown of AB-DIP protein expression significantly protected neuroblastoma cells from Abeta-induced neurotoxicity. AB-DIP may mediate the neurotoxicity of Abeta, and therefore, AB-DIP may be a potential, therapeutic target for AD.

Plasma membrane sequestration of apoptotic protease-activating factor-1 in human B-lymphoma cells: a novel mechanism of chemoresistance

Burkitt lymphoma (BL) is a highly aggressive B-cell neoplasm harboring chromosomal rearrangements of the c-myc oncogene. BL cells frequently resist apoptosis induction by chemotherapeutic agents; however, the mechanism of unresponsiveness has not been elucidated. Here, we show that cytochrome c fails to stimulate apoptosome formation and caspase activation in cytosolic extracts of human BL-derived cell lines, due to insufficient levels of apoptotic protease-activating factor-1 (Apaf-1). Enforced expression of Apaf-1 increased its concentration in the cytosolic compartment, restored cytochrome c-dependent caspase activation, and rendered the prototypic Raji BL cell line sensitive to etoposide- and staurosporine-induced apoptosis. Surprisingly, in nontransfected BL cells, the bulk of Apaf-1 was found to associate with discrete domains in the plasma membrane. Disruption of lipid raft domains or the actin cytoskeleton of Raji cells liberated Apaf-1 and restored sensitivity to cytochrome c–dependent apoptosis, indicating that constitutive Apaf-1 retained its ability to promote caspase activation. Moreover, disruption of lipid rafts sensitized BL cells to apoptosis induced by etoposide. Together, our findings suggest that ectopic (noncytosolic) localization of Apaf-1 may constitute a novel mechanism of chemoresistance in B lymphoma.

Upon drug-induced apoptosis expression of prostate-apoptosis-response-gene-4 promotes cleavage of caspase-8, bid and mitochondrial release of cytochrome c

Par-4 functions as a tumor suppressor antagonizing the transforming capacity and the resistance of malignant cells towards apoptotic stimuli. After demonstrating that par-4 promotes apoptosis by activating signaling of the intrinsic pathway of apoptosis, we hypothesized that par-4 also impacts on key molecules of the extrinsic pathway without the requirement of a receptor/ligand interaction. Here, we provide first evidence, that expression of par-4 increases cleavage of caspase-8, truncation of Bid and its translocation to the mitochondria, resulting in an augmentation of cytochrome c and AIF efflux into the cytosol, effects par-4-positive cells are able to retain to a higher extent than par-4-negative cells upon inhibition of caspase-3 activation.

The kinder side of killer proteases: caspase activation contributes to neuroprotection and CNS remodeling

Caspases are a family of cysteine proteases that are expressed as inactive zymogens and undergo proteolytic maturation in a sequential manner in which initiator caspases cleave and activate the effector caspases 3, 6 and 7. Effector caspases cleave structural proteins, signaling molecules, DNA repair enzymes and proteins which inhibit apoptosis. Activation of effector, or executioner, caspases has historically been viewed as a terminal event in the process of programmed cell death. Emerging evidence now suggests a broader role for activated caspases in cellular maturation, differentiation and other non-lethal events. The importance of activated caspases in normal cell development and signaling has recently been extended to the CNS where these proteases have been shown to contribute to axon guidance, synaptic plasticity and neuroprotection. This review will focus on the adaptive roles activated caspases in maintaining viability, the mechanisms by which caspases are held in check so as not produce apoptotic cell death and the ramifications of these observations in the treatment of neurological disorders.

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.

Intracellular signaling mechanisms in photodynamic therapy

In photodynamic therapy (PDT) a sensitizer, light and oxygen are used to induce death of tumor cells and in the treatment of certain noncancerous conditions. Cell death in PDT may occur by apoptosis or by necrosis, depending on the sensitizer, on the PDT dose and on the cell genotype. Some sensitizers that have been used in PDT are accumulated in the mitochondria, and this may explain their efficiency in inducing apoptotic cell death, both in vitro and in vivo. In this review we will focus on the events that characterize apoptotic death in PDT and on the intracellular signaling events that are set in motion in photosensitized cells. Activation of phospholipases, changes in ceramide metabolism, a rise in the cytosolic free Ca2+ concentration, stimulation of nitric oxide synthase (NOS), changes in protein phosphorylation and alterations in the activity of transcription factors and on gene expression have all been observed in PDT-treated cells. Although many of these metabolic reactions contribute to the demise process, some of them may antagonize cell death. Understanding the signaling mechanisms in PDT may provide means to modulate the PDT effects at the molecular level and potentiate its antitumor effectiveness.

Apoptosis Protease Activator Protein-1 Expression Is Dispensable for Response of Human Melanoma Cells to Distinct Proapoptotic Agents

Loss of expression of the apoptosis protease activator protein-1 (APAF-1) in human melanoma is thought to promote resistance to programmed cell death by preventing caspase-9 activation. However, the role of the APAF-1-dependent pathway in apoptosis activated by cellular stress and/or DNA damage has been recently questioned. We investigated APAF-1 expression in a large panel of human melanomas and assessed cellular response to several proapoptotic agents in tumors expressing or lacking APAF-1 protein. In two melanomas with wild-type p53 but with differential expression of APAF-1, treatment with camptothecin, celecoxib, or an nitric oxide synthase inhibitor (1400W) significantly modulated expression of 36 of 96 genes in an apoptosis-specific cDNA macroarray, but APAF-1 mRNA levels were not induced (in APAF-1(-) cells) nor up-regulated (in APAF-1(+) cells), a finding confirmed at the protein level. Treatment with cisplatin, camptothecin, etoposide, betulinic acid, celecoxib, 1400W, and staurosporine promoted enzymatic activity not only of caspases -2, -8, and -3 but also of caspase-9 in both APAF-1(+) and APAF-1(-) tumor cells. Moreover, drug-induced caspase-9 enzymatic activity could be not only partially but significantly reduced by caspase-2, -3, and -8 -specific inhibitors in both APAF-1(+) and APAF-1(-) tumor cells. In response to 1 to 100 micromol/L of cisplatin, camptothecin, or celecoxib, APAF-1(+) melanomas (n = 12) did not show significantly increased levels of apoptosis compared with APAF-1(-) tumors (n = 7), with the exception of enhanced apoptosis in response to a very high dose (100 micromol/L) of etoposide. These results suggest that the response of human melanoma cells to different proapoptotic agents may be independent of their APAF-1 phenotype.

Bartonella henselae inhibits apoptosis in Mono Mac 6 cells

Bartonella henselae causes the vasculoproliferative disorders bacillary angiomatosis and peliosis probably resulting from the release of vasculoendothelial growth factor (VEGF) from infected epithelial or monocytic host cells. Here we demonstrate that B. henselae in addition to VEGF induction was also capable of inhibiting the endogenous sucide programme of monocytic host cells. Our results show that B. henselae inhibits pyrrolidine dithiocarbamate (PDTC)-induced apoptosis in Mono Mac 6 cells. B. henselae was observed to be present in a vacuolic compartment of Mono Mac 6 cells. Direct contact of B. henselae with Mono Mac 6 cells was crucial for inhibition of apoptosis as shown by the use of a two-chamber model. Inhibition of apoptosis was paralleled by diminished caspase-3 activity which was significantly reduced in PDTC-stimulated and B. henselae-infected cells. The anti-apoptotic effect of B. henselae was accompanied by (i) the activation of the transcription factor NF-kappaB and (ii) the induction of cellular inhibitor of apoptosis proteins-1 and -2 (cIAP-1, -2). Our results suggest a new synergistic mechanism in B. henselae pathogenicity by (i) inhibition of host cell apoptosis via activation of NF-kappaB and (ii) induction of host cell VEGF secretion.

Chemical-induced apoptosis: formation of the Apaf-1 apoptosome

Many environmental and therapeutic agents initiate apoptotic cell death by inducing the release of cytochrome c from the mitochondria, which activates Apaf-1 (apoptotic protease-activating factor-1). This large (approximately 130kD) protein is a mammalian homologue of CED-4, an essential protein involved in programmed cell death in the nematode C. elegans. Cytochrome c activates Apaf-1, which oligomerizes to form an approximately 700-1400-kDa caspase-activating complex known as the Apaf-1 apoptosome. Caspase-9, an initiator caspase, is then recruited to the complex by binding to Apaf-1 through CARD-CARD (caspase recruitment domain) interactions to form a holoenzyme complex. Subsequently, the Apaf-1/caspase-9 holoenzyme complex recruits the effector caspase-3 via an interaction between the active site cysteine in caspase-9 and the critical aspartate, which is the cleavage site for generating the large and small subunits of caspase-3 that constitute the activated form of caspase-3. This initiates the caspase cascade that is responsible for the execution phase of apoptosis. Intracellular levels of K+, XIAP an inhibitor of apoptosis protein, and at least two mitochondrial released proteins, Smac/DIABLO and Omi/Htra 2 a serine protease, tightly regulate formation and function of the apoptosome. Thus, a number of physiological mechanisms ensure that the apoptosome complex is only fully assembled and functional when the cell is irrevocably committed to die. It is interesting that more recent studies show that a variety of small molecules can directly activate or inhibit caspase activation by interfering with the formation and function of the apoptosome complex. The cytotoxicity of many conventional chemotherapeutic drugs rests on their ability to induce apoptosome formation and apoptosis. Defects in this pathway can result in drug resistance, and the discovery that small molecules can directly activate or inhibit the apoptosome may provide new alternative treatments for cancer.

Role of caspases-3 and -7 in Apaf-1 proteolytic cleavage and degradation events during cisplatin-induced apoptosis in melanoma cells

Apoptosis protease-activating factor-1 (Apaf-1), the central element in the mitochondrial pathway of apoptosis, is frequently absent or poorly expressed in metastatic melanomas, a tumor type showing a low degree of spontaneous apoptosis and a poor response to conventional therapies. In the present study, we used the Apaf-1-positive Me665/2/21 melanoma cell line to investigate the fate of Apaf-1 during cisplatin-induced apoptosis. As novel findings described for the first time in melanoma cells, we observed that Apaf-1 was markedly decreased during apoptosis, already at early stages of cell damage; concurrently, an immunoreactive N-terminal fragment of congruent with 26 kDa was evident. In spite of the remarkable decrease of Apaf-1 in apoptotic cells, caspase-9 was found to be processed and enzymatically active. Both Apaf-1 depletion and its proteolytic cleavage were markedly prevented in presence of the caspase-3/-7 inhibitor ac-DEVD-CHO. In presence of ac-DEVD-CHO, caspase-9 activity was also inhibited, along with a partially different pattern of caspase-9 processing forms. Unexpectedly, the inhibition afforded by ac-DEVD-CHO on several components, that is, caspase-3/-7 and caspase-9 activities, and Apaf-1 proteolytic degradation, did not abrogate the apoptotic morphology and cell detachment, nor the proteolytic degradation of crucial targets, such as poly(ADP-ribose) polymerase (PARP) and lamin B. Together, our results suggest that caspase-3 and -7, proved to be dispensable for the above apoptosis-associated events, play a role on Apaf-1 handling and possibly on apoptosome function.

Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand Cooperates with Anticancer Drugs to Overcome Chemoresistance in Antiapoptotic Bcl-2 Family Members Expressing Jurkat Cells

PURPOSE

Overexpression of antiapoptotic Bcl-2 family members has recently been related to resistance to chemo/radiotherapy in several human malignancies, particularly lymphomas. Hence, innovative approaches bypassing this resistance mechanism are required in the therapeutic approach. This study evaluated whether chemoresistance associated with Bcl-2 and Bcl-x(L) overexpression would be overcome by activating the death receptor pathway by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in the Jurkat cell model

EXPERIMENTAL DESIGN

We made use of genetically modified Jurkat cells to evaluate the effect of Bcl-2 or Bcl-x(L) overexpression on the cytotoxic effect produced by the anticancer drugs doxorubicin, etoposide, and oxaliplatin and TRAIL. Caspase activation was detected by cleavage of caspase-8 and -3. The mitochondrial transmambrane potential was assessed by staining with DiOC(6) and flow cytometry. Caspase activity was blocked by the broad-spectrum caspase inhibitor zVAD-fmk.

RESULTS

Bcl-2 and Bcl-x(L) overexpression but not lack of caspase-8 protects the Jurkat cells from the anticancer drug-induced cytolysis. However, Bcl-2/Bcl-x(L) Jurkat cells retained some susceptibility to TRAIL-induced cytolysis. A highly synergistic cytotoxic effect of the combination of TRAIL with any of the antiblastic used in this study was detected in the chemoresistant cells. This effect was associated with mitochondrial disassemblage and dependent on caspase activation

CONCLUSIONS

The combination of TRAIL with conventional anticancer drugs may prove to be useful in the treatment of antiapoptotic Bcl-2 family proteins-expressing malignancies.

Cyclopentenone prostaglandins induce lymphocyte apoptosis by activating the mitochondrial apoptosis pathway independent of external death receptor signaling

15-Deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) is a naturally occurring cyclopentenone metabolite of PGD(2) that possesses both peroxisome proliferator-activated receptor gamma (PPAR-gamma)-dependent and PPAR-gamma-independent anti-inflammatory properties. Recent studies suggest that cyclopentenone PGs may play a role in the down-regulation of inflammation-induced immune responses. In this study, we report that 15d-PGJ(2) as well as synthetic PPAR-gamma agonists inhibit lymphocyte proliferation. However, only 15d-PGJ(2), but not the specific PPAR-gamma activators, induce lymphocyte apoptosis. We found that blocking of the death receptor pathway in Fas-associated death domain(-/-) or caspase-8(-/-) Jurkat T cells has no effect on apoptosis induction by 15d-PGJ(2). Conversely, overexpression of Bcl-2 or Bcl-x(L) completely inhibits the initiation of apoptosis, indicating that 15d-PGJ(2)-mediated apoptosis involves activation of the mitochondrial pathway. In line with these results, 15d-PGJ(2) induces mitochondria disassemblage as demonstrated by dissipation of mitochondrial transmembrane potential (Deltapsi(m)) and cytochrome c release. Both of these events are partially inhibited by the broad spectrum caspase inhibitor benzyloxycarbonil-Val-Ala-Asp-fluoromethylketone, suggesting that caspase activation may amplify the mitochondrial alterations initiated by 15d-PGJ(2). We also demonstrate that 15d-PGJ(2) potently stimulates reactive oxygen species production in Jurkat T cells, and Deltapsi(m) loss induced by 15d-PGJ(2) is prevented by the reactive oxygen species scavenger N-acetyl-L-cysteine. In conclusion, our data indicate that cyclopentenone PGs like 15d-PGJ(2) may modulate immune responses even independent of PPAR-gamma by activating the mitochondrial apoptosis pathway in lymphocytes in the absence of external death receptor signaling.

CD26/dipeptidyl peptidase IV enhances expression of topoisomerase II alpha and sensitivity to apoptosis induced by topoisomerase II inhibitors

CD26/dipeptidyl peptidase IV (DPPIV) is a cell surface-bound ectopeptidase with important roles in T-cell activation and tumour biology. We now report that CD26/DPPIV enhances sensitivity to apoptosis induced by the antineoplastic agents doxorubicin and etoposide. In particular, CD26/DPPIV presence is associated with increased susceptibility to the mitochondrial pathway of apoptosis, documented by enhanced cleavage of poly (ADP ribose) polymerase (PARP), caspase-3 and caspase-9, Bcl-xl, and Apaf-1, as well as increased expression of death receptor 5 (DR5). We also show that the caspase-9-specific inhibitor z-LEHD-fmk inhibits drug-mediated apoptosis, leading to decreased PARP and caspase-3 cleavage, and reduced DR5 expression. Importantly, through detailed studies that demonstrate the association between topoisomerase II alpha expression and DPPIV activity, our data provide further evidence of the key role played by CD26 in biological processes.

15-deoxy-delta-12-14-PGJ2 regulates apoptosis induction and nuclear factor-kappaB activation via a peroxisome proliferator-activated receptor-gamma-independent mechanism in hepatocellular carcinoma

The peroxisome proliferator-activated receptor-gamma (PPARgamma) high-affinity ligand, 15-deoxy-Delta-12,14-PGJ(2) (15d-PGJ(2)), is toxic to malignant cells through cell cycle arrest and apoptosis induction. In this study, we investigated the effects of 15d-PGJ(2) on apoptosis induction and expression of apoptosis-related proteins in hepatocellular carcinoma (HCC) cells. 15d-PGJ(2) induced apoptosis in SK-Hep1 and HepG2 cells at a 50 micro M concentration. Pretreatment with the pan-caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethyl ketone (2-VAD-fmk), only partially blocked apoptosis induced by 40 micro M 15d-PGJ(2). This indicated that 15d-PGJ(2) induction of apoptosis was associated with a caspase-3-independent pathway. 15d-PGJ(2) also induced down-regulation of the X chromosome-linked inhibitor of apoptosis (XIAP), Bclx, and apoptotic protease-activating factor-1 in SK-Hep1 cells but not in HepG2 cells. However, 15d-PGJ(2) sensitized both HCC cell lines to TNF-related apoptosis-induced ligand-induced apoptosis. In SK-Hep1 cells, cell toxicity, nuclear factor-kappaB (NF-kappaB) suppression, and XIAP down-regulation were induced by 15d-PGJ(2) treatment under conditions in which PPARgamma was down-regulated. These results suggest that the effect of 15d-PGJ(2) was through a PPARgamma-independent mechanism. Although cell toxicity was induced when PPARgamma was down-regulated in HepG2 cells, NF-kappaB suppression and XIAP down-regulation were not induced. In conclusion, 15d-PGJ(2) induces apoptosis of HCC cell lines via caspase-dependent and -independent pathways. In SK-Hep1 cells, the ability of 15d-PGJ(2) to induce cell toxicity, NF-kappaB suppression, or XIAP down-regulation seemed to occur via a PPARgamma-independent mechanism, but in HepG2 cells, NF-kappaB suppression by 15d-PGJ(2) was dependent on PPARgamma.