Processed caspase-2 can induce mitochondria-mediated apoptosis independently of its enzymatic activity

Neat1 decreases neuronal apoptosis after oxygen and glucose deprivation

Studies have shown that downregulation of nuclear-enriched autosomal transcript 1 (Neat1) may adversely affect the recovery of nerve function and the increased loss of hippocampal neurons in mice. Whether Neat1 has protective or inhibitory effects on neuronal cell apoptosis after secondary brain injury remains unclear. Therefore, the effects of Neat1 on neuronal apoptosis were observed. C57BL/6 primary neurons were obtained from the cortices of newborn mice and cultured in vitro, and an oxygen and glucose deprivation cell model was established to simulate the secondary brain injury that occurs after traumatic brain injury in vitro. The level of Neat1 expression in neuronal cells was regulated by constructing a recombinant adenovirus to infect neurons, and the effects of Neat1 expression on neuronal apoptosis after oxygen and glucose deprivation were observed. The experiment was divided into four groups: the control group, without any treatment, received normal culture; the oxygen and glucose deprivation group were subjected to the oxygen and glucose deprivation model protocol; the Neat1 overexpression and Neat1 downregulation groups were treated with Neat1 expression intervention techniques and were subjected to the in oxygen and glucose deprivation protocol. The protein expression levels of neurons p53-induced death domain protein 1 (PIDD1, a pro-apoptotic protein), caspase-2 (an apoptotic priming protein), cytochrome C (a pro-apoptotic protein), and cleaved caspase-3 (an apoptotic executive protein) were measured in each group using the western blot assay. To observe changes in the intracellular distribution of cytochrome C, the expression levels of cytochrome C in the cytoplasm and mitochondria of neurons from each group were detected by western blot assay. Differences in the cell viability and apoptosis rate between groups were detected by cell-counting kit 8 assay and terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, respectively. The results showed that the apoptosis rate, PIDD1, caspase-2, and cleaved caspase-3 expression levels significantly decreased, and cell viability significantly improved in the Neat1 overexpression group compared with the oxygen and glucose deprivation group; however, Neat1 downregulation reversed these changes. Compared with the Neat1 downregulation group, the cytosolic cytochrome C level in the Neat1 overexpression group significantly decreased, and the mitochondrial cytochrome C level significantly increased. These data indicate that Neat1 upregulation can reduce the release of cytochrome C from the mitochondria to the cytoplasm by inhibiting the PIDD1-caspase-2 pathway, reducing the activation of caspase-3, and preventing neuronal apoptosis after oxygen and glucose deprivation, which might reduce secondary brain injury after traumatic brain injury. All experiments were approved by the Animal Ethics Committee of the First Affiliated Hospital of Chongqing Medical University, China, on December 19, 2020 (approval No. 2020-895).

Caspase-2 Substrates: To Apoptosis, Cell Cycle Control, and Beyond

Caspase-2 belongs to the caspase family of proteins responsible for essential cellular functions including apoptosis and inflammation. Uniquely, caspase-2 has been identified as a tumor suppressor, but how it regulates this function is still unknown. For many years, caspase-2 has been considered an “orphan” caspase because, although it is able to induce apoptosis, there is an abundance of conflicting evidence that questions its necessity for apoptosis. Recent evidence supports that caspase-2 has non-apoptotic functions in the cell cycle and protection from genomic instability. It is unclear how caspase-2 regulates these opposing functions, which has made the mechanism of tumor suppression by caspase-2 difficult to determine. As a protease, caspase-2 likely exerts its functions by proteolytic cleavage of cellular substrates. This review highlights the known substrates of caspase-2 with a special focus on their functional relevance to caspase-2’s role as a tumor suppressor.

Mechanisms of synergy of autoantibodies to M3 muscarinic acetylcholine receptor and secretory pathway Ca2+/Mn2+-ATPase isoform 1 in patients with non-desmoglein pemphigus vulgaris

Pemphigus vulgaris (PV) is a potentially lethal mucocutaneous blistering disease characterized by IgG autoantibodies (AuAbs) binding to epidermal keratinocytes and inducing a devastating blistering disease affecting oral and/or esophageal surfaces and, sometimes, also the skin. Anti-keratinocyte AuAbs developed by the desmoglein (Dsg) 1/3 AuAb-negative acute PV patients are pathogenic, as they induced acantholysis and epidermal split in the experimental models of PV in vitro and in vivo. These PV patients have various combinations of AuAbs to keratinocyte muscarinic acetylcholine receptor subtype M3 (M3AR), the secretory pathway Ca²⁺/Mn²⁺-ATPase isoform 1 (SPCA1), and desmocollin 3 whose relative concentrations correlate with the disease activity. In this study, we identified new molecular mechanisms of the synergistic cooperation of AuAbs to M3AR and SPCA1 in inducing acantholysis in the anti-Dsg 1/3 AuAb-negative PV patients. Anti-M3AR AuAb was found to play an important role in determining the level of intraepidermal split just above the basal cells, caspase to mediate early pro-apoptotic events triggered by anti-SPCA1 AuAb, and the neonatal Fc receptor (FcRn) to contribute to the pathobiological actions of both anti-M3AR and anti-SPCA1 AuAbs. Altogether, these novel results support our original hypothesis that pemphigus acantholysis is a complex disease process (also known as apoptolysis) initiated by AuAbs directed against different keratinocyte proteins that play important roles in supporting cell viability and regulating vital cell functions.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

Effects of major ozonated autoheamotherapy on functional recovery, ischemic brain tissue apoptosis and oxygen free radical damage in the rat model of cerebral ischemia

Stroke is the second leading cause of death and disability in the world, with a heavy burden on patients, their families, and society. At present, a major focus of cerebrovascular disease research is to find a safe and effective new method to promote early functional recovery in the acute phase of cerebral infarction. Major ozonated autohemotherapy (MOAH) can maintain ATP and energy metabolism in cerebral ischemia and hypoxia, and reduce cell apoptosis. In the current study, the model of middle cerebral artery occlusion in the Sprague Dawley rat was established and evaluated by the clinical functional score, Hoechst staining, immunohistochemistry, Western blot analysis, and biochemical detection. Then, the effects of MOAH on neurological function, apoptosis, and oxygen free radical damage after acute ischemia in middle cerebral artery were evaluated. Moreover, the potential two mechanisms have been illustrated for MOAH effects. This study would lay a theoretical foundation for the application of MOAH and find an effective and early treatment method for the cerebral infarction.

PIDD-dependent activation of caspase-2-mediated mitochondrial injury in E1A-induced cellular sensitivity to macrophage nitric oxide-induced apoptosis

Expression of the adenovirus E1A oncogene sensitizes tumor cells to innate immune rejection by apoptosis induced by macrophage-produced tumor necrosis factor (TNF)-α and nitric oxide (NO). E1A sensitizes cells to TNF-α and NO through two distinct mechanisms, by repressing NF-κB-dependent antiapoptotic responses and enhancing caspase-2 activation and mitochondrial injury, respectively. The mechanisms through which E1A enhances caspase-2 activation in response to NO were unknown. Here, we report that E1A-induced sensitization to NO-induced apoptosis is dependent on expression of PIDD (p53-inducible protein with a death domain) and enhancement of primary immunodeficiency diseases (PIDD) processing for formation of the PIDDosome, the core component of the caspase-2 activation complex. NO-induced apoptosis in E1A-expressing cells did not require expression Bak or Bax, indicating that NO-induced caspase-2-mediated mitochondrial injury does not proceed through the activities of typical, proapoptotic Bcl-2 family members that induce mitochondrial cytochrome C release. These results define a PIDD-dependent pathway, through which E1A enhances casapse-2-mediated mitochondrial injury, resulting in increased sensitivity of mammalian cells to macrophage-induced, NO-mediated apoptosis.

Role of Cell Death in Toxicology: Does It Matter How Cells Die?

My research activity started with studies on drug metabolism in rat liver microsomes in the early 1960s. The CO-binding pigment (cytochrome P450) had been discovered a few years earlier and was subsequently found to be involved in steroid hydroxylation in adrenal cortex microsomes. Our early studies suggested that it also participated in the oxidative demethylation of drugs catalyzed by liver microsomes, and that prior treatment of the animals with phenobarbital caused increased levels of the hemoprotein in the liver, and similarly enhanced rates of drug metabolism. Subsequent studies of cytochrome P450-mediated metabolism of toxic drugs in freshly isolated rat hepatocytes characterized critical cellular defense systems and identified mechanisms by which accumulating toxic metabolites could damage and kill the cells. These studies revealed that multiple types of cell death could result from the toxic injury, and that it is important to know which type of cell death results from the toxic injury.

Cannabinoid derivatives exert a potent anti-myeloma activity both in vitro and in vivo

Although hematopoietic and immune system show high levels of the cannabinoid receptor CB2, the potential effect of cannabinoids on hematologic malignancies has been poorly determined. Here we have investigated their anti-tumor effect in multiple myeloma (MM). We demonstrate that cannabinoids induce a selective apoptosis in MM cell lines and in primary plasma cells of MM patients, while sparing normal cells from healthy donors, including hematopoietic stem cells. This effect was mediated by caspase activation, mainly caspase-2, and was partially prevented by a pan-caspase inhibitor. Their pro-apoptotic effect was correlated with an increased expression of Bax and Bak, a decrease of Bcl-xL and Mcl-1, a biphasic response of Akt/PKB and an increase in the levels of ceramide in MM cells. Inhibition of ceramide synthesis partially prevented apoptosis, indicating that these sphingolipids play a key role in the pro-apoptotic effect of cannabinoids in MM cells. Remarkably, blockage of the CB2 receptor also inhibited cannabinoid-induced apoptosis. Cannabinoid derivative WIN-55 enhanced the anti-myeloma activity of dexamethasone and melphalan overcoming resistance to melphalan in vitro. Finally, administration of cannabinoid WIN-55 to plasmacytoma-bearing mice significantly suppressed tumor growth in vivo. Together, our data suggest that cannabinoids may be considered as potential therapeutic agents in the treatment of MM.

Mitochondria in traumatic brain injury and mitochondrial-targeted multipotential therapeutic strategies

Traumatic brain injury (TBI) is a major health and socioeconomic problem throughout the world. It is a complicated pathological process that consists of primary insults and a secondary insult characterized by a set of biochemical cascades. The imbalance between a higher energy demand for repair of cell damage and decreased energy production led by mitochondrial dysfunction aggravates cell damage. At the cellular level, the main cause of the secondary deleterious cascades is cell damage that is centred in the mitochondria. Excitotoxicity, Ca(2+) overload, reactive oxygen species (ROS), Bcl-2 family, caspases and apoptosis inducing factor (AIF) are the main participants in mitochondria-centred cell damage following TBI. Some preclinical and clinical results of mitochondria-targeted therapy show promise. Mitochondria- targeted multipotential therapeutic strategies offer new hope for the successful treatment of TBI and other acute brain injuries.

Photodynamic therapeutic activity of indocyanine green entrapped in polymeric nanoparticles

BACKGROUND

Photodynamic therapy (PDT) is a therapeutic modality involving the use of a photosensitizer agent activated by light of appropriate wavelength to selectively destroy tumor cells. Indocyanine green (ICG) is a promising photosensitive agent for PDT of tumor cells. The main disadvantage of using ICG in PDT is the instability of ICG in aqueous solutions. Encapsulating ICG dye in a biocompatible matrix based on PEBBLE technology showed an improvement of aqueous stability comparing with free ICG dye. The main objective of this study is to investigate the photodynamic effect of ICG-ormosil PEBBLEs on two different cell lines: human breast adenocarcinoma cells (MCF-7) and hepatocellular carcinoma cells (HepG2).

METHODS

ICG-embedded ormosil PEBBLEs were synthesized based on a sol-gel process, and characterized by transmission electron microscopy and other fluorescence tests. The cell viability was evaluated by MTT and trypan blue assays. Apoptosis, necrosis, and DNA damage (comet assay), were evaluated by fluorescence microscopic tests.

RESULTS

The results declared that ICG-ormosil PEBBLEs and free ICG both have the same cytotoxic and phototoxic effect on MCF-7 and HepG2 cell lines, where the apoptotic mode of cell death is preferentially occurred in case of PDT using ICG-ormosil PEBBLEs. Both ICG and ICG-ormosil PEBBLEs induced DNA damage after laser exposure. These results would suggest that entrapping ICG in Polymeric nanoparticles forming ICG-ormosil PEBBLEs improve the aqueous stability of the photosensitizer and in the same time retain its photodynamic activity, suggesting that it is preferred to use ICG-ormosil PEBBLEs instead of free ICG dye.

Terfenadine induces apoptosis and autophagy in melanoma cells through ROS-dependent and -independent mechanisms

Previously we found that terfenadine, an H1 histamine receptor antagonist, acts as a potent apoptosis inducer in melanoma cells through modulation of Ca²⁺ homeostasis. In this report, focusing our attention on the apoptotic mechanisms activated by terfenadine, we show that this drug can potentially activate distinct intrinsic signaling pathways depending on culture conditions. Serum-deprived conditions enhance the cytotoxic effect of terfenadine and caspase-4 and -2 are activated upstream of caspase-9. Moreover, although we found an increase in ROS levels, the apoptosis was ROS independent. Conversely, terfenadine treatment in complete medium induced ROS-dependent apoptosis. Caspase-4, -2, and -9 were simultaneously activated and p73 and Noxa induction were involved. ROS inhibition prevented p73 and Noxa expression but not p53 and p21 expression, suggesting a role for Noxa in p53-independent apoptosis in melanoma cells. Finally, we found that terfenadine induced autophagy, that can promote apoptosis. These findings demonstrate the great potential of terfenadine to kill melanoma cells through different cellular signaling pathways and could contribute to define new therapeutic strategies in melanoma.

DNA damage-induced cell death: from specific DNA lesions to the DNA damage response and apoptosis

DNA damaging agents are potent inducers of cell death triggered by apoptosis. Since these agents induce a plethora of different DNA lesions, it is firstly important to identify the specific lesions responsible for initiating apoptosis before the apoptotic executing pathways can be elucidated. Here, we describe specific DNA lesions that have been identified as apoptosis triggers, their repair and the signaling provoked by them. We discuss methylating agents such as temozolomide, ionizing radiation and cisplatin, all of them are important in cancer therapy. We show that the potentially lethal events for the cell are O(6)-methylguanine adducts that are converted by mismatch repair into DNA double-strand breaks (DSBs), non-repaired N-methylpurines and abasic sites as well as bulky adducts that block DNA replication leading to DSBs that are also directly induced following ionizing radiation. Transcriptional inhibition may also contribute to apoptosis. Cells are equipped with sensors that detect DNA damage and relay the signal via kinases to executors, who on their turn evoke a process that inhibits cell cycle progression and provokes DNA repair or, if this fails, activate the receptor and/or mitochondrial apoptotic cascade. The main DNA damage recognition factors MRN and the PI3 kinases ATM, ATR and DNA-PK, which phosphorylate a multitude of proteins and thus induce the DNA damage response (DDR), will be discussed as well as the downstream players p53, NF-κB, Akt and survivin. We review data and models describing the signaling from DNA damage to the apoptosis executing machinery and discuss the complex interplay between cell survival and death.

The PIDDosome, DNA-damage-induced apoptosis and beyond

P53-induced protein with a death domain (PIDD) was cloned as a death domain (DD)-containing protein whose expression is induced by p53. It was later described as the core of a molecular platform-activating caspase-2, named the PIDDosome. These first results pointed towards a role for PIDD in apoptosis, in response to DNA damage. Identification of new PIDDosome complexes involved in DNA repair and nuclear factor-κB signaling challenged this early concept. PIDD functions are growing as new complexes and new interaction partners are being discovered, and as additional functions are being revealed. A fascinating feature of PIDD lies within its complex and tight regulation mechanisms, which allow the molecule to fine-tune its different functions: from transcriptional regulation to the expression of different isoforms, and from the interaction with regulatory proteins to an ingenious post-translational cleavage mechanism generating various active fragments with specific functions. Further studies still need to be carried out to provide answers to many unresolved issues and to reconcile conflicting results. This review aims at providing an overview of the current PIDD knowledge status.

Wogonin and related natural flavones overcome tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) protein resistance of tumors by down-regulation of c-FLIP protein and up-regulation of TRAIL receptor 2 expression

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent that kills various tumor cells without damaging normal tissues. However, many cancers remain resistant to TRAIL. To overcome TRAIL resistance, combination therapies using sensitizers of the TRAIL pathway would be an efficacious approach. To investigate potential sensitizers of TRAIL-induced apoptosis, we used TRAIL-resistant human T cell leukemia virus type 1 (HTLV-1)-associated adult T cell leukemia/lymphoma (ATL) cells as a model system. So far, HTLV-1-associated ATL is incurable by presently known therapies. Here, we show that wogonin and the structurally related natural flavones apigenin and chrysin break TRAIL resistance in HTLV-1-associated ATL by transcriptional down-regulation of c-FLIP, a key inhibitor of death receptor signaling, and by up-regulation of TRAIL receptor 2 (TRAIL-R2). This effect is mediated through transcriptional inhibition of the p53 antagonist murine double minute 2 (Mdm2), leading to an increase in p53 levels and, consequently, to up-regulation of the p53 target gene TRAIL-R2. We also show that these flavones can sensitize to TNFα- and CD95-mediated cell death. Furthermore, we show that wogonin, apigenin, and chrysin also enhance TRAIL-mediated apoptosis in other human cancer cell lines including breast cancer cell line MDA-MB-231, colon cancer cell line HT-29, hepatocellular carcinoma cell line HepG2, melanoma cell line SK-MEL-37, and pancreatic carcinoma cell line Capan-1 by the same mechanism. Thus, our study suggests the potential use of these flavones as an adjuvant for TRAIL-mediated anticancer therapy.

A novel transcription complex that selectively modulates apoptosis of breast cancer cells through regulation of FASTKD2

We previously reported that expression of NRIF3 (nuclear receptor interacting factor-3) rapidly and selectively leads to apoptosis of breast cancer cells. DIF-1 (also known as interferon regulatory factor-2 binding protein 2 [IRF-2BP2]), the cellular target of NRIF3, was identified as a transcriptional repressor, and DIF-1 knockdown leads to apoptosis of breast cancer cells but not other cell types. Here, we identify IRF-2BP1 and EAP1 (enhanced at puberty 1) as important components of the DIF-1 complex mediating both complex stability and transcriptional repression. This interaction of DIF-1, IRF-2BP1, and EAP1 occurs through the conserved C4 zinc fingers of these proteins. Microarray studies were carried out in breast cancer cell lines engineered to conditionally and rapidly increase the levels of the death domain (DD1) region of NRIF3. The DIF-1 complex was found to repress FASTKD2, a putative proapoptotic gene, in breast cancer cells and to bind to the FASTKD2 gene by chromatin immunoprecipitation. FASTKD2 knockdown prevents apoptosis of breast cancer cells from NRIF3 expression or DIF-1 knockdown, while expression of FASTKD2 leads to apoptosis of both breast and nonbreast cancer cells. Thus, regulation of FASTKD2 by NRIF3 and the DIF-1 complex acts as a novel death switch that selectively modulates apoptosis in breast cancer.

Silencing receptor EphA2 induces apoptosis and attenuates tumor growth in malignant mesothelioma

Receptor EphA2 over-expression is associated with the aggressive nature of growth in malignant mesothelioma (MM) and silencing EphA2 with interference RNA suppressed MM proliferation. The mechanisms associated with targeting the EphA2 gene in MM were not clear. We sought to determine whether silencing EphA2 induces apoptosis in MM cells by either extrinsic or intrinsic pathways. The receptor EphA2 signaling pathway may provide attractive therapeutic strategy for MM. Apoptosis was determined by Cell Death ELISA in MM Cells transfected with siRNA-EphA2 and control siRNA. The gene expression profile of apoptosis pathways were analyzed by GEArray. Selected genes were further studied by quantitative PCR, Western analysis, and immunofluorescence. Caspases activities were measured by fluorescence spectrometer. Silencing EphA2 expression induced apoptosis in MMC. Apoptosis was characterized by FADD expression, activated caspase-8, caspase-3 and induction of Bax, Bak, and Bid as revealed by GEArray and protein fractionation assays. The expression of FADD, Bid, caspase-8, cytochrome-c and apaf-1 were significantly higher in the cytosolic fractions of EphA2-siRNA transfected cells. Furthermore, blocking the expression of caspase-8 by an inhibitor blunted FADD expression, indicating that caspase-8 is implicated in EphA2-siRNA induced apoptosis in MMC. Our data indicates that targeting the EphA2 gene by siRNA induced both extrinsic and intrinsic apoptotic pathways in MM Cells. Receptor EphA2 inhibition may be an effective approach for inhibiting MM growth and a promising direction for MM therapy.

LBH589, a deacetylase inhibitor, induces apoptosis in adult T-cell leukemia/lymphoma cells via activation of a novel RAIDD-caspase-2 pathway

Adult T-cell leukemia/lymphoma (ATLL), an aggressive neoplasm etiologically associated with human T-lymphotropic virus type-1 (HTLV-1), is resistant to treatment. In this study, we examined the effects of a new inhibitor of deacetylase enzymes, LBH589, on ATLL cells. LBH589 effectively induced apoptosis in ATLL-related cell lines and primary ATLL cells and reduced the size of tumors inoculated in SCID mice. Analyses, including with a DNA microarray, revealed that neither death receptors nor p53 pathways contributed to the apoptosis. Instead, LBH589 activated an intrinsic pathway through the activation of caspase-2. Furthermore, small interfering RNA experiments targeting caspase-2, caspase-9, RAIDD, p53-induced protein with a death domain (PIDD) and RIPK1 (RIP) indicated that activation of RAIDD is crucial and an event initiating this pathway. In addition, LBH589 caused a marked decrease in levels of factors involved in ATLL cell proliferation and invasion such as CCR4, IL-2R and HTLV-1 HBZ-SI, a spliced form of the HTLV-1 basic zipper factor HBZ. In conclusion, we showed that LBH589 is a strong inducer of apoptosis in ATLL cells and uncovered a novel apoptotic pathway initiated by activation of RAIDD.

Rosmanol potently induces apoptosis through both the mitochondrial apoptotic pathway and death receptor pathway in human colon adenocarcinoma COLO 205 cells

Rosemary (Rosmarinus officinalis), a culinary spice and medicinal herb, has been widely used in European folk medicine to treat numerous ailments. Many studies have shown that rosemary extracts play important roles in anti-inflammation, anti-tumor, and anti-proliferation in various in vitro and in vivo settings. The roles of tumor suppression of rosemary have been attributed to the major components, including carnosic acid, carnosol, and rosmarinic acid, rosmanol, and ursolic acid. This study was to explore the effect of rosmanol on the growth of COLO 205 human colorectal adenocarcinoma cells and to delineate the underlying mechanisms. When treated with 50 μM of rosmanol for 24h, COLO 205 cells displayed a strong apoptosis-inducing response with a 51% apoptotic ratio (IC(50) ∼42 μM). Rosmanol increased the expression of Fas and FasL, led to the cleavage and activation of pro-caspase-8 and Bid, and mobilized Bax from cytosol into mitochondria. The mutual activation between tBid and Bad decreased the mitochondrial membrane potential and released cytochrome c and apoptosis-inducing factor (AIF) to cytosol. In turn, cytochrome c induced the processing of pro-caspase-9 and pro-caspase-3, followed by the cleavage of poly-(ADP-ribose) polymerase (PARP) and DNA fragmentation factor (DFF-45). These results demonstrate that the rosmanol-induced apoptosis in COLO 205 cells is involvement of caspase activation and involving complicated regulation of both the mitochondrial apoptotic pathway and death receptor pathway.

Fundamentals of neuronal apoptosis relevant to pediatric anesthesia

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

Chemopreventive effects of Frondanol A5, a Cucumaria frondosa extract, against rat colon carcinogenesis and inhibition of human colon cancer cell growth

Sea cucumber extracts have been widely used to treat individuals with inflammatory conditions in East Asia. The present study has been designed to test potential colon cancer-preventive properties of Frondanol A5, a glycolipid extract from the sea cucumber, Cucumaria frondosa, using in vivo and in vitro models of colon cancer. Chemopreventive efficacy of Frondanol A5 was evaluated on azoxymethane-induced rat colon carcinogenesis using colonic aberrant crypt foci (ACF) as efficacy marker. At 7 weeks of age, groups of rats (12 per group) were fed the AIN-76A diet, and ACFs were induced by azoxymethane (15 mg/kg body weight). Three days after azoxymethane treatment, rats were fed with the diets containing 0, 150, and 450 ppm of Frondanol A5 and continued on the diets for 8 weeks, at which time ACFs were evaluated. Expression levels of proliferating cell nuclear antigen and p21(WAF1/CIP1) were determined in ACFs. Further, Frondanol A5 (10-120 microg/mL) was studied for its growth-inhibitory and apoptotic effects in the HCT-116 cell line. Dietary administration of 150 and 450 ppm of Frondanol A5 significantly suppressed azoxymethane-induced total colonic ACF formation, approximately 34% to 55% (P < 0.01 to P < 0.0001), and multicrypt aberrant foci (48-68.5%, P < 0.0001) in a dose-dependent manner. ACFs in rats treated with Frondanol A5 showed significant upregulation of p21(WAF1/CIP1) and downregulation of proliferating cell nuclear antigen compared with control group. Frondanol A5 showed growth inhibition at S and G(2)-M phase with a decrease in Cdc25c and an increase in p21(WAF1/CIP) with significant apoptosis associated with H2AX phosphorylation and caspase-2 cleavage in HCT116 cells. Overall, Frondanol A5 exhibits potential chemopreventive properties for colon carcinogenesis, which suggests further development of this sea cucumber extract.

Goniothalamin-induced oxidative stress, DNA damage and apoptosis via caspase-2 independent and Bcl-2 independent pathways in Jurkat T-cells

Goniothalamin (GTN) isolated from Goniothalamus sp. has been demonstrated to induce apoptosis in a variety of cancer cell lines including Jurkat T leukemia cells. However, the mechanism of GTN-induced apoptosis upstream of mitochondria is still poorly defined. In this study, GTN caused a decrease in GSH with an elevation of reactive oxygen species as early as 30 min and DNA damage as assessed by Comet assay. Analysis using topoisomerase II processing of supercoiled pBR 322 DNA showed that GTN caused DNA damage via a topoisomerase II-independent pathway suggesting that cellular oxidative stress may contribute to genotoxicity. A 12-fold increase of caspase-2 activity was observed in GTN-treated Jurkat cells after 4 h treatment and this was confirmed using Western blotting. Although the caspase-2 inhibitor Z-VDVAD-FMK inhibited the proteolytic activity of caspase-2, apoptosis ensued confirming that caspase-2 activity was not crucial for GTN-induced apoptosis. However, GTN-induced apoptosis was completely abrogated by N-acetylcysteine further confirming the role of oxidative stress. Since cytochrome c release was observed as early as 1 h without any appreciable change in Bcl-2 protein expression, we further investigated whether overexpression of Bcl-2 confers resistance in GTN-induced cytotoxicity. Using a panel of Jurkat Bcl-2 transfectants, GTN cytotoxicity was not abrogated in these cells. In conclusion, GTN induces DNA damage and oxidative stress resulting in apoptosis which is independent of both caspase-2 and Bcl-2.

Hexokinase II detachment from the mitochondria potentiates cisplatin induced cytotoxicity through a caspase-2 dependent mechanism

Cancer cells are frequently glycolytic and overexpress hexokinase II (HXK II). In cancer cells, the majority of hexokinase II is localized to the mitochondria through interaction with the voltage dependent anion channel (VDAC). Disruption in the binding of hexokinase II to the mitochondria has been shown to promote mitochondrial injury provoked by pro-apoptotic proteins. The present study demonstrates that cisplatin induces the PIDD (p53 induced protein with a death domain) dependent activation of caspase-2. In turn, caspase-2 cleaves and activates Bid, resulting in the oligomerization of Bak and the release of cytochrome c. Notably, the detachment of hexokinase II from the mitochondria markedly potentiates the onset of caspase-2 induced mitochondrial damage, thus resulting in a synergistic induction of cisplatin induced cytotoxicity.

Unilateral blood flow decrease induces bilateral and symmetric responses in the immature brain

The effects of hemodynamic changes in the developing brain have yet to be fully understood. The aim of this study was to explore the relationship between perturbations of the cerebral blood flow in the developing brain via unilateral hypoperfusion in P7 rats. As expected, nuclear caspase-3 (casp3) cleavage and DNA fragmentation were detected at 48 hours after stroke in the injured cortex. Surprisingly, casp3 was also cleaved in the contralateral cortex, although without cell death markers. Delayed (48 hours) casp3 cleavage without DNA fragmentation was also identified after unilateral common carotid artery occlusion, both in the hypoperfused cortex and the unaffected cortex, producing mirror images. Upstream calpain activation, caspase-2 cleavage, and mitochondrial cytochrome c release initiated casp3 cleavage, but did not produce preconditioning. The neuronal marker NeuN co-localized with cleaved casp3 in cortical layers II-III and VI and with gaba-amino butyric acid in layer III. Indeed, collateral supply was provided from the opposite side during carotid artery occlusion but not after reperfusion, and the number of cleaved casp3-positive cells significantly negatively correlated with the common carotid artery immediate reperfusion percentage. In summary, unilateral hypoperfusion, while insufficient to induce cell death, may active bilateral and symmetric casp3 in the P7 rat brain. Additionally, the opposite healthy hemisphere is altered due to the injury and thus should not be used as an internal control.

Caspase-2: controversial killer or checkpoint controller?

The caspases are an evolutionarily conserved family of cysteine proteases, with essential roles in apoptosis or inflammation. Caspase-2 was the second caspase to be cloned and it resembles the prototypical nematode caspase CED-3 more closely than any other mammalian protein. An absence of caspase-2-specific reagents and the subtle phenotype of caspase-2-deficient mice have hampered definition of the physiological role of caspase-2 and identification of factors regulating its activity. Although some data implicate caspase-2 in apoptotic pathways, a link with apoptosis has been less firmly established for caspase-2 than for some other caspases. Emerging evidence suggests that caspase-2 regulates the cell cycle and may act as a tumour suppressor. This article critically reviews the current state of knowledge regarding the biochemistry and biology of this controversial caspase.

Mitochondrial and apoptotic neuronal death signaling pathways in cerebral ischemia

Mitochondria play important roles as the powerhouse of the cell. After cerebral ischemia, mitochondria overproduce reactive oxygen species (ROS), which have been thoroughly studied with the use of superoxide dismutase transgenic or knockout animals. ROS directly damage lipids, proteins, and nucleic acids in the cell. Moreover, ROS activate various molecular signaling pathways. Apoptosis-related signals return to mitochondria, then mitochondria induce cell death through the release of pro-apoptotic proteins such as cytochrome c or apoptosis-inducing factor. Although the mechanisms of cell death after cerebral ischemia remain unclear, mitochondria obviously play a role by activating signaling pathways through ROS production and by regulating mitochondria-dependent apoptosis pathways.

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.

Caspase-2 activation in the absence of PIDDosome formation

PIDD (p53-induced protein with a death domain [DD]), together with the bipartite adapter protein RAIDD (receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a DD), is implicated in the activation of pro–caspase-2 in a high molecular weight complex called the PIDDosome during apoptosis induction after DNA damage. To investigate the role of PIDD in cell death initiation, we generated PIDD-deficient mice. Processing of caspase-2 is readily detected in the absence of PIDDosome formation in primary lymphocytes. Although caspase-2 processing is delayed in simian virus 40–immortalized pidd^−/− mouse embryonic fibroblasts, it still depends on loss of mitochondrial integrity and effector caspase activation. Consistently, apoptosis occurs normally in all cell types analyzed, suggesting alternative biological roles for caspase-2 after DNA damage. Because loss of either PIDD or its adapter molecule RAIDD did not affect subcellular localization, nuclear translocation, or caspase-2 activation in high molecular weight complexes, we suggest that at least one alternative PIDDosome-independent mechanism of caspase-2 activation exists in mammals in response to DNA damage.

Mechanisms of prostate cancer cell survival after inhibition of AR expression

Recent reports have shown that the AR is the key determinant of the molecular changes required for driving prostate cancer cells from an androgen-dependent to an androgen-independent or androgen depletion-independent (ADI) state. Several recent publications suggest that down-regulation of AR expression should therefore be considered the principal strategy for the treatment of ADI prostate cancer. However, no valid data is available about how androgen-dependent prostate cancer cells respond to apoptosis-inducing drugs after knocking down AR expression and whether prostate cancer cells escape apoptosis after inhibition of AR expression. This review will focus on mechanisms of prostate cancer cell survival after inhibition of AR activity mediated either by androgen depletion or by targeting the expression of AR by siRNA. We have shown that knocking down AR expression by siRNA induced PI3K-independent activation of Akt, which was mediated by calcium/calmodulin-dependent kinase II (CaMKII). We also showed that the expression of CaMKII genes is under AR control: active AR in the presence of androgens inhibits CaMKII gene expression whereas inhibition of AR activity results in an elevated level of kinase activity and in enhanced expression of CaMKII genes. This in turn activates the anti-apoptotic PI3K/Akt pathways. CaMKII also express anti-apoptotic activity that is independent from the Akt pathway. This may therefore be an important mechanism by which prostate cancer cells escape apoptosis after androgen depletion or knocking down AR expression. In addition, we have found that there is another way to escape cell death after AR inhibition: DNA damaging agents cannot fully activate p53 in the absence of AR and as a result p53 down stream targets, for example, microRNA-34, cannot be activated and induce apoptosis. This implies that there may be a need for re-evaluation of the therapeutic approaches to human prostate cancer.

Formosanin C-induced apoptosis requires activation of caspase-2 and change of mitochondrial membrane potential

Formosanin C is a pure compound isolated from Paris formosana Hayata (Liliaceae). The antitumor efficacy of formosanin C has been observed in cultured cells and animal systems. However, the molecular mechanisms of formosanin C remain unknown. The results of the present study indicate that formosanin C induced apoptosis of HT-29 cells characterized by exposure of phosphatidylserine, accumulation of cells at the sub-G(1) phase, fragmentation of DNA, and change of nuclear morphology in a time- and dose-related manner. The apoptotic signaling cascades may proceed via proteolytic activation of caspase-2, change of mitochondrial membrane potential (Deltapsi(m)), release of cytochrome c and second mitochondria-derived activator of caspase/direct IAP binding protein with low pI (Smac/DIABLO), activation of caspase-9 and -3, and cleavage of poly(ADP-ribose) polymerase (PARP). Increase in apoptosis-inducing factor and endonuclease G expressions in nuclei, and increase in Bax and Bak expressions and decrease in Bcl-X(L) expression on mitochondria were also observed in formosanin C-treated HT-29 cells. Attenuation of formosanin C-induced change of Deltapsi(m) by caspase-2 inhibitor (Z-VDVAC) implies that caspase-2 acts upstream of the mitochondria. Blockage of formosanin C-induced apoptotic process by using either permeability transition pore inhibitor (cyclosporine A) or caspase-9 inhibitor (Z-LEHD) demonstrates the necessity of mitochondria and caspase-9 in formosanin C-induced apoptosis of HT-29 cells. Taken together, the apoptotic mechanism of formosanin C in human colorectal cancer HT-29 cells involves activation of caspase-2 and the dysfunction of mitochondria.

The enigma of caspase-2: the laymen's view

Proteolysis of cellular substrates by caspases (cysteine-dependent aspartate-specific proteases) is one of the hallmarks of apoptotic cell death. Although the activation of apoptotic caspases is considered a 'late-stage' event in apoptosis signaling, past the commitment stage, one caspase family member, caspase-2, splits the cell death community into half - those searching for evidence of an apical initiator function of this molecule and those considering it as an amplifier of the apoptotic caspase cascade, at best, if relevant for apoptosis at all. This review screens past and present biochemical as well as genetic evidence for caspase-2 function in cell death signaling and beyond.

The PIDDosome mediates delayed death of hippocampal CA1 neurons after transient global cerebral ischemia in rats

A brief period of global brain ischemia, such as that induced by cardiac arrest or cardiopulmonary bypass surgery, causes cell death in vulnerable hippocampal CA1 pyramidal neurons days after reperfusion. Although numerous factors have been suggested to account for this phenomenon, the mechanisms underlying it are poorly understood. We describe a cell death signal called the PIDDosome, a protein complex of p53-induced protein with a death domain (PIDD), receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD), and procaspase-2. We induced 5 min of transient global cerebral ischemia (tGCI) using bilateral common carotid artery occlusion with hypotension. Western blot analysis showed that expression of twice-cleaved fragment of PIDD (PIDD-CC) increased in the cytosolic fraction of the hippocampal CA1 subregion and preceded procaspase-2 activation after tGCI. Caspase-2 cleaved Bid in brain homogenates. Co-immunoprecipitation and immunofluorescent studies demonstrated that PIDD-CC, RAIDD, and procaspase-2 were co-localized and bound directly, which indicates the formation of the PIDD death domain complex. Furthermore, we tested inhibition of PIDD expression by using small interfering RNA (siRNA) treatment that was initiated 48 h before tGCI. Administration of siRNA against PIDD decreased not only expression of PIDD-CC, but also activation of procaspase-2 and Bid, resulting in a decrease in histological neuronal damage and DNA fragmentation in the hippocampal CA1 subregion after tGCI. These results imply that PIDD plays an important role in procaspase-2 activation and delayed CA1 neuronal death after tGCI. We propose that PIDD is a hypothetical molecular target for therapy against neuronal death after tGCI.

Death receptors and caspases but not mitochondria are activated in the GDNF- or BDNF-deprived dopaminergic neurons

Neurotrophic factors, including glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF), promote survival of midbrain dopaminergic neurons, but the death pathways activated in the dopaminergic neurons by deprivation of these factors are poorly studied. We show here that deprivation of GDNF or BDNF triggers a novel mitochondria-independent death pathway in the cultured embryonic dopaminergic neurons: cytochrome c was not released from the mitochondria to cytosol, proapoptotic protein Bax was not activated, and overexpressed Bcl-xL did not block the death. Caspases were critically required, because the death was completely blocked by caspase inhibitor BAF [boc-aspartyl(OMe)-fluoromethylketone] and overexpression of dominant-negative mutants of caspase-9, -3, and -7 significantly blocked the death. Also, the death receptor pathway was involved, because blockage of caspase-8 or FADD (Fas-associated protein with death domain), an adapter required for caspase-8 activation, inhibited death induced by GDNF or BDNF deprivation. Ligation of Fas by agonistic anti-Fas antibody induced apoptosis in the GDNF- or BDNF-maintained neurons, and inhibition of Fas by Fas-Fc chimera blocked the death of GDNF- or BDNF-deprived neurons, whereas FAIM(L) (long isoform of Fas apoptosis inhibitory molecule) could control the activity of Fas in the dopaminergic neurons.

Cytochrome c: functions beyond respiration

Cytochrome c is primarily known for its function in the mitochondria as a key participant in the life-supporting function of ATP synthesis. However, when a cell receives an apoptotic stimulus, cytochrome c is released into the cytosol and triggers programmed cell death through apoptosis. The release of cytochrome c and cytochrome-c-mediated apoptosis are controlled by multiple layers of regulation, the most prominent players being members of the B-cell lymphoma protein-2 (BCL2) family. As well as its role in canonical intrinsic apoptosis, cytochrome c amplifies signals that are generated by other apoptotic pathways and participates in certain non-apoptotic functions.

Down-regulation of caspase-2 by rottlerin via protein kinase C-delta-independent pathway

Protein kinase C-delta (PKC delta) plays an important role in DNA damage-induced apoptosis. We have previously shown that the PKC delta inhibitor rottlerin protects against cisplatin-induced apoptosis acting upstream of caspase-9. In the present study, we have investigated if rottlerin regulates caspase-2 activation. Knockdown of caspase-2 by siRNA inhibited processing of apical caspase-9 and caspase-8, whereas depletion of caspase-9 had little effect on caspase-2 processing. Rottlerin inhibited activation and processing of caspase-9 and caspase-8 and cleavage of poly(ADP)ribose polymerase. We made a novel observation that rottlerin induced down-regulation of caspase-2 but not of caspase-3, caspase-7, caspase-8, or caspase-9. Pharmacologic inhibitors of PKC, such as Gö 6983 and bisindolylmaleimide, or depletion of PKC delta by siRNA had no effect on the down-regulation of caspase-2 by rottlerin. The proteasome inhibitor MG132 reversed caspase-2 down-regulation by rottlerin, whereas calpain inhibitor had no effect. These results suggest that rottlerin induces down-regulation of caspase-2 via PKC delta-independent but ubiquitin proteasome-mediated pathway. Furthermore, down-regulation of caspase-2 by rottlerin can explain its antiapoptotic function during DNA damage-induced apoptosis.

Calpain-mediated pathway dominates cisplatin-induced apoptosis in human lung adenocarcinoma cells as determined by real-time single cell analysis

Cisplatin is an efficient anticancer agent. Cisplatin-based chemotherapy is believed to involve different signal transduction pathways, among which calpain activation has been proposed as an important factor in the induced apoptosis. In our study, based on real-time single cell analysis, we investigated the molecular involvement of calpain in cisplatin-induced apoptosis in living human lung adenocarcinoma cells. After cisplatin treatment, calpain was activated, resulting in Bid cleavage at 4-5 hr, followed by Bid translocation and cytochrome c release, leading to cell death. Calpeptin and PD150606, specific inhibitors of calpain, blocked Bid activation completely; however, cytochrome c release was delayed by more than 2 hr, which was associated with the delay of caspase-3 activation and cell death. Remarkably, calpain-mediated release of cytochrome c and cell death was significantly compromised in the Bid knockdown cells. Z-IETD-fmk and Z-VDVAD-fmk were used to block the activation of caspase-8 and caspase-2, respectively; however, the progression of apoptosis were not affected, suggesting that caspase-8 and caspase-2 were not involved in this experimental model. Taken together, the data demonstrate that calpain mediated cisplatin-induced apoptosis in human lung adenocarcinoma cells through activating Bid, which then regulated the mitochondrial apoptotic pathway. The delays of cytochrome c release, caspase-3 activation and subsequent cell death by inactivating calpain or silencing Bid exclude other earlier or parallel pathways, strongly suggesting that the calpain-mediated pathway is the kinetically earliest one, which dominates the cisplatin-induced apoptosis.

Cell death by SecTRAPs: thioredoxin reductase as a prooxidant killer of cells

Background

SecTRAPs (selenium compromised thioredoxin reductase-derived apoptotic proteins) can be formed from the selenoprotein thioredoxin reductase (TrxR) by targeting of its selenocysteine (Sec) residue with electrophiles, or by its removal through C-terminal truncation. SecTRAPs are devoid of thioredoxin reductase activity but can induce rapid cell death in cultured cancer cell lines by a gain of function.

Principal Findings

Both human and rat SecTRAPs killed human A549 and HeLa cells. The cell death displayed both apoptotic and necrotic features. It did not require novel protein synthesis nor did it show extensive nuclear fragmentation, but it was attenuated by use of caspase inhibitors. The redox active disulfide/dithiol motif in the N-terminal domain of TrxR had to be maintained for manifestation of SecTRAP cytotoxicity. Stopped-flow kinetics showed that NADPH can reduce the FAD moiety in SecTRAPs at similar rates as in native TrxR and purified SecTRAPs could maintain NADPH oxidase activity, which was accelerated by low molecular weight substrates such as juglone. In a cellular context, SecTRAPs triggered extensive formation of reactive oxygen species (ROS) and consequently antioxidants could protect against the cell killing by SecTRAPs.

Conclusions

We conclude that formation of SecTRAPs could contribute to the cytotoxicity seen upon exposure of cells to electrophilic agents targeting TrxR. SecTRAPs are prooxidant killers of cells, triggering mechanisms beyond those of a mere loss of thioredoxin reductase activity.

A role for caspase 2 and PIDD in the process of p53-mediated apoptosis

When treated with some DNA-damaging agents, human tumor-derived H1299 cells expressing inducible versions of wild-type or mutant p53 with inactive transactivation domain I (p53(Q22/S23)) undergo apoptosis as evidenced by cytochrome c release, nuclear fragmentation, and sub-G1 DNA content. Apoptosis induced by p53(Q22/S23) is relatively slow, however, and key downstream effector caspases are not activated. Nevertheless, with either version of p53, caspase 2 activation is required for release of cytochrome c and cell death. Remarkably, although p53(Q22/S23) is known to be defective in transcriptional activation of numerous p53 target genes, it can induce expression of proapoptotic targets including PIDD and AIP1 at least to the same extent as wild-type p53. Furthermore, RNAi silencing of PIDD, previously shown to be required for caspase 2 activation, suppresses apoptosis by both wild-type p53 and p53(Q22/S23). Thus, the initial stage of DNA damage-facilitated, p53-mediated apoptosis occurs by a PIDD- and caspase 2-dependent mechanism, and p53's full transcriptional regulatory functions may be required only for events that are downstream of cytochrome c release.

The mitochondrial permeability transition pore and its involvement in cell death and in disease pathogenesis

Current research on the mitochondrial permeability transition pore (PTP) and its role in cell death faces a paradox. Initially considered as an in vitro artifact of little pathophysiological relevance, in recent years the PTP has received considerable attention as a potential mechanism for the execution of cell death. The recent successful use of PTP desensitizers in several disease paradigms leaves little doubt about its relevance in pathophysiology; and emerging findings that link the PTP to key cellular signalling pathways are increasing the interest on the pore as a pharmacological target. Yet, recent genetic data have challenged popular views on the molecular nature of the PTP, and called into question many early conclusions about its structure. Here we review basic concepts about PTP structure, function and regulation within the framework of intracellular death signalling, and its role in disease pathogenesis.

Mitochondrial targeting of human peroxiredoxin V protein and regulation of PRDX5 gene expression by nuclear transcription factors controlling biogenesis of mitochondria

Peroxiredoxin V (PRDX5) is a member of the family of mammalian proteins that neutralize reactive oxygen species. The PRDX5 gene is constitutively expressed at a high level in many human tissues, but functional elements of its promoter responsible for a high basal activity in the absence of oxidative stress have still not been identified. Among predicted binding sites for transcription factors in the human PRDX5 promoter are binding sites for nuclear respiratory factor 1 (NFR-1) and nuclear respiratory factor 2 (also called GABPA), which regulate the biogenesis of mitochondria. We constructed luciferase reporter gene plasmids containing stepwise deletions of the PRDX5 promoter and examined their activities in transient transfections. Our results suggest that basal PRDX5 promoter activity mostly depends on NFR-1 and GABPA sites. The latter, in the PRDX5 promoter, were conserved in the six mammalian genomes analyzed (human, chimpanzee, cow, mouse, rat and dog) and a fraction of human PRDX5 associates with the mitochondrial matrix. We also found that the N-terminal 50 amino acids of the full-length human PRDX5 (24 kDa) translated from its first AUG codon targets this protein exclusively to mitochondria. However, the short form of PRDX5 (17 kDa), translated from its second AUG codon, has cytoplasmic and nuclear localization, which is also typical for endogenously expressed protein. Together, our results indicate that high basal expression of the PRDX5 gene is coordinated with the expression of nuclear genes encoding mitochondrial proteins and that the PRDX5 protein might play a major role in permanent defense against reactive oxygen species produced by mitochondria.

Caspase-2 cleaves DNA fragmentation factor (DFF45)/inhibitor of caspase-activated DNase (ICAD)

To investigate the signal transduction pathway of caspase-2, cell permeable Tat-reverse-caspase-2 was constructed, characterized and utilized for biochemical and cellular studies. It could induce the cell death as early as 2h, and caspase-2-specific VDVADase activity but not other caspase activities including DEVDase and IETDase. Interestingly, nuclear DNA fragmentation occurred and consistently DNA fragmentation factor (DFF45)/Inhibitor of caspase-activated DNase (ICAD) was cleaved inside the cell as well as in vitro, suggesting a role of caspase-2 in nuclear DNA fragmentation.

PKB and the mitochondria: AKTing on apoptosis

Cellular homeostasis depends upon the strict regulation of responses to external stimuli, such as signalling cascades triggered by nutrients and growth factors, and upon cellular metabolism. One of the major molecules coordinating complex signalling pathways is protein kinase B (PKB), a serine/threonine kinase also known as Akt. The number of substrates known to be phosphorylated by PKB and its interacting partners, as well as our broad understanding of how PKB is implicated in responses to growth factors, metabolic pathways, proliferation, and cell death via apoptosis is constantly increasing. Activated by the insulin/growth factor-phosphatidylinositol 3-kinase (PI3K) cascade, PKB triggers events that promote cell survival and prevent apoptosis. It is also now widely accepted that mitochondria are not just suppliers of ATP, but that they participate in regulatory and signalling events, responding to multiple physiological inputs and genetic stresses, and regulate both cell proliferation and death. Thus, mitochondria are recognized as important players in apoptotic events and it is logical to predict some form of interplay with PKB. In this review, we will summarize mechanisms by which PKB mediates its anti-apoptotic activities in cells and survey recent developments in understanding mitochondrial dynamics and their role during apoptosis.