Mechanisms of cytochrome c release from mitochondria

Cardiolipin: setting the beat of apoptosis

Cardiolipin (CL) is a mitochondria-specific phospholipid which is known to be intimately linked with the mitochondrial bioenergetic machinery. Accumulating evidence now suggests that this unique lipid also has active roles in several of the mitochondria-dependent steps of apoptosis. CL is closely associated with cytochrome c at the outer leaflet of the mitochondrial inner membrane. This interaction makes the process of cytochrome c release from mitochondria more complex than previously assumed, requiring more than pore formation in the mitochondrial outer membrane. While CL peroxidation could be crucial for enabling cytochrome c dissociation from the mitochondrial inner membrane, cytochrome c itself catalyzes CL peroxidation. Moreover, peroxy-CL directly activates the release of cytochrome c and other apoptogenic factors from the mitochondria. CL is also directly involved in mitochondrial outer membrane permeabilization by enabling docking and activation of pro-apoptotic Bcl-2 proteins. It appears therefore that CL has multiple roles in apoptosis and that CL metabolism contributes to the complexity of the apoptotic process.

Methods to dissect mitochondrial membrane permeabilization in the course of apoptosis

In several paradigms of cell death, mitochondrial membrane permeabilization (MMP) delimits the frontier between life and death. Mitochondria control the intrinsic pathway of apoptosis and participate in the extrinsic pathway. Moreover, they have been implicated in nonapoptotic cell death modalities. Irrespective of its initiation at the inner or the outer mitochondrial membrane (IM and OM, respectively), MMP culminates in the functional (dissipation of the mitochondrial transmembrane potential, shutdown of ATP synthesis, redox imbalance) and structural (reorganization of cristae, release of toxic intermembrane space proteins into the cytosol) collapse of mitochondria. This has a profound impact on cellular metabolism, activates caspase-dependent and -independent executioner mechanisms, and finally results in the demise of the cell. However, the partial and/or temporary permeabilization of one or both mitochondrial membranes is not always a prelude to cell death. This chapter proposes a method and several guidelines to discriminate between IM and OM permeabilization and to identify MMP that does indeed precede cell death. This approach relies on the integration of currently available techniques and may be easily introduced in the laboratory routine for a more precise detection of cell death.

Paraquat-induced apoptosis in human neuroblastoma SH-SY5Y cells: involvement of p53 and mitochondria

The herbicide paraquat is a suspected etiologic factor in the development of Parkinson's disease (PD). Paraquat was therefore used to reproduce Parkinsonian syndromes in lab animals, in which it produces dopaminergic pathogenesis. However, the factors or mechanisms by which paraquat kills dopaminergic neurons are not fully understood. Based on reported evidence that paraquat increases p53 protein levels and inhibits mitochondrial function, it was hypothesized that paraquat induces cell death in dopaminergic neurons through a mechanism in which p53 and mitochondrial apoptotic pathway are linked. To explore this possibility, dopaminergic SY5Y cells were treated with paraquat for 48 h and p53 responses were investigated, as well as biomarkers of the mitochondrial intrinsic pathway of apoptosis. Paraquat significantly increased protein levels of p53 and one of its target genes, Bax. By 24 h, paraquat decreased mitochondrial complex I activity and mitochondrial transmembrane potential and induced the release of cytochrome c from mitochondria. In addition, paraquat increased the activities of caspases 9 and 3. Finally, nuclear condensation and DNA fragmentation occurred 48 h after treatment. The decrease of mitochondrial functions, the release of cytochrome c, the increase of caspase 9 and 3 activities, and DNA damage that were produced by paraquat were inhibited by a specific p53 inhibitor, pifithrin-alpha. These findings support the conclusion that paraquat produced apoptosis in SY5Y cells through the mitochondrial intrinsic pathway associated with p53.

Molecular links between autophagy and apoptosis

Macroautophagy (herein referred to as autophagy) contributes to the control of life and death throughout the animal and plant kingdoms. Bilateral links have been found between apoptosis and autophagy where inducers of apoptosis also induce autophagy and vice versa. In some cases, autophagy delays the onset of apoptosis and thus prolongs life although it may also promote apoptosis and other forms of cell death. It is thus of great biological and medical interest to understand the molecular connections between these two pathways, and try to utilize-or block-them selectively to aid induction of cell death (e.g., cancer cells) or inhibit death (e.g., in degenerative disorders). This chapter describes methods for studying apoptotic induction of autophagy and its effects on cell function. We also discuss potential pitfalls. Although cell lines are used as model systems, the substances and methods described here can be applied to primary cells and tissues.

Human 8-oxoguanine DNA glycosylase suppresses the oxidative stress induced apoptosis through a p53-mediated signaling pathway in human fibroblasts

Human 8-oxoguanine DNA glycosylase (hOGG1) is the main defense enzyme against mutagenic effects of cellular 7,8-dihydro-8-oxoguanine. In this study, we investigated the biological role of hOGG1 in DNA damage-related apoptosis induced by hydrogen peroxide (H(2)O(2))-derived oxidative stress. The down-regulated expression of hOGG1 by its small interfering RNA prominently triggers the H(2)O(2)-induced apoptosis in human fibroblasts GM00637 and human lung carcinoma H1299 cells via the p53-mediated apoptotic pathway. However, the apoptotic responses were specifically inhibited by hOGG1 overexpression. The p53-small interfering RNA transfection into the hOGG1-deficient GM00637 markedly inhibited the H(2)O(2)-induced activation of p53-downstream target proteins such as p21, Noxa, and caspase-3/7, which eventually resulted in the increased cell viability. Although the cell viability of hOGG1-knockdown H1299 p53 null cells was similar to that of the hOGG1 wild-type H1299, after the overexpression of p53 the hOGG1-knockdown H1299 showed the significantly decreased cell viability compared with that of the hOGG1 wild-type H1299 at the same experimental condition. Moreover, the array comparative genome hybridization analyses revealed that the hOGG1-deficient GM00637 showed more significant changes in the copy number of large regions of their chromosomes in response to H(2)O(2) treatment. Therefore, we suggest that although p53 is a major modulator of apoptosis, hOGG1 also plays a pivotal role in protecting cells against the H(2)O(2)-induced apoptosis at the upstream of the p53-dependent pathway to confer a survival advantage to human fibroblasts and human lung carcinomas through maintaining their genomic stability.

A comparison of three flow cytometry methods for evaluating mitochondrial damage during staurosporine-induced apoptosis in Jurkat cells

Measuring cytochrome c release during apoptosis provides valuable information about the nature and extent of apoptosis. Several years ago a flow cytometric method (based on selective permeabilization of the plasma membrane with digitonin) was developed that has advantages over other techniques. These experiments describe a comprehensive evaluation of that method. Apoptosis was triggered in Jurkat cells with staurosporine and then flow cytometry was used to measure three aspects of mitochondrial damage: (1) cytochrome c release (with the digitonin assay and a commercially available kit based on the same principle), using a DNA-binding dye to define cell cycle stage; (2) loss of mitochondrial cardiolipin, assessed by a decrease in 10 N-nonyl acridine orange (NAO) binding; and (3) loss of mitochondrial membrane potential, assessed by a decrease in tetramethylrhodamineethylester (TMRE) binding. The results from these three assays were compared with an antibody-based assay for cleaved caspase 3. The digitonin assay and the commercially available kit gave comparable results, showing that staurosporine caused cytochrome c release in all phases of the cell cycle and clearly defining those cells that had lost DNA due to internucleosomal DNA fragmentation. The pattern of fluorescence demonstrated that the mitochondrial apoptotic pathway was either the sole or the predominant pathway to be activated and that cytochrome c release in an individual cell was all-or-nothing. However, comparison with the other assays showed that the cytochrome c release assay underestimated the true extent of apoptosis. This was caused by the selective loss of some digitonin-treated apoptotic cells. The flow cytometry assay for cytochrome c release provides valuable information but it underestimates the percentage of apoptotic cells.

The apoptotic response to strenuous exercise of the gastrocnemius and solues muscle fibers in rats

The purposes of this study were to investigate the effects of strenuous exercise on apoptosis of the gastrocnemius and soleus muscle fibers and clarify the role of oxidative metabolism in the strenuous exercise-induced apoptosis. The experiment was designed with 49 (n = 49) male, 24-week-old, L. Wistar albino rats. Strenuous exercise model was applied to 42 (n = 42) rats and seven (n = 7) rats served as rested controls. All rats were randomly assigned to one of the following groups (n = 7): rested control (C), immediately after exercise (0 h) and 3, 6, 12, 24, and 48 h after exercise. Apoptotic nuclei were shown by single stranded DNA (ssDNA) determination. Oxidative damage in mitochondrial fractions of the muscle tissues was evaluated by malondialdehyde (MDA) levels and reduced/oxidized glutathione (GSH/GSSG) ratios. Caspase-9, -8 and -3 activities and the level of cytochrome c (Cyt c) were measured in the cytosolic fractions of muscle tissues to follow mitochondrial-dependent (intrinsic) or ligand-mediated death receptor (extrinsic) pathways of apoptosis. Plasma interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) levels were also determined. Based on our results, apoptosis is significantly triggered in muscle fibers by strenuous exercise (P < 0.05). Apoptosis in the soleus muscle tissues mostly depends on the intrinsic pathway and may be triggered by increased oxidative stress. In contrast, extrinsic pathway of apoptosis was predominant in the gastrocnemius muscle and increases of TNF-alpha and IL-6 may play a significant role.

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.

Nuclear-mitochondrial cross-talk during heat shock in Arabidopsis cell culture

Apart from energy generation, mitochondria perform a signalling function determining the life and death of a cell under stress exposure. In the present study we have explored patterns of heat-induced synthesis of Hsp101, Hsp70, Hsp17.6 (class I), Hsp17.6 (class II) and Hsp60, and the development of induced thermotolerance in Arabidopsis thaliana cell culture under conditions of mitochondrial dysfunction. It was shown that treatment by mitochondrial inhibitors and uncouplers at the time of mild heat shock downregulates HSP synthesis, which is important for induced thermotolerance in plants. The exposure to elevated temperature induced an increase in cell oxygen consumption and hyperpolarization of the inner mitochondrial membrane. Taken together, these facts suggest that mitochondrial functions are essential for heat-induced HSP synthesis and development of induced thermotolerance in A. thaliana cell culture, suggesting that mitochondrial-nuclear cross-talk is activated under stress conditions. Treatment of Arabidopsis cell culture at 50 degrees C initiates a programmed cell death determined by the time course of viability decrease, DNA fragmentation and cytochrome c release from mitochondria. As treatment at 37 degrees C protected Arabidopsis cells from heat-induced cell death, it may be suggested that Hsp101, Hsp70 and small heat-shock proteins, the synthesis of which is induced under these conditions, are playing an anti-apoptotic role in the plant cell. On the other hand, drastic heat shock upregulated mitochondrial Hsp60 synthesis and induced its release from mitochondria to the cytosol, indicating a pro-apoptotic role of plant Hsp60.

Aloe-emodin-induced apoptosis in human gastric carcinoma cells

The purpose of this study was to investigate the anticancer effect of aloe-emodin, an anthraquinone compound present in the leaves of Aloe vera, on two distinct human gastric carcinoma cell lines, AGS and NCI-N87. We demonstrate that aloe-emodin induced cell death in a dose- and time-dependent manner. Noteworthy is that the AGS cells were generally more sensitive than the NCI-N87 cells. Aloe-emodin caused the release of apoptosis-inducing factor and cytochrome c from mitochondria, followed by the activation of caspase-3, leading to nuclear shrinkage and apoptosis. In addition, exposure to aloe-emodin suppressed the casein kinase II activity in a time-dependent manner and was accompanied by a reduced phosphorylation of Bid, a downstream substrate of casein kinase II and a pro-apoptotic molecule. These preclinical studies suggest that aloe-emodin represents a suitable and novel chemotherapeutic drug candidate for the treatment of human gastric carcinoma.

PI3K/AKT inhibition induces caspase-dependent apoptosis in HTLV-1-transformed cells

The phosphatidylinositol-3-kinase (PI3K) and AKT (protein kinase B) signaling pathways play an important role in regulating cell cycle progression and cell survival. In previous studies, we demonstrated that AKT is activated in HTLV-1-transformed cells and that Tax activation of AKT is linked to p53 inhibition and cell survival. In the present study, we extend these observations to identify regulatory pathways affected by AKT in HTLV-1-transformed cells. We demonstrate that inhibition of AKT reduces the level of phosphorylated Bad, an important member of the pro-apoptotic family of proteins. Consistent with the decrease of phosphorylated Bad, cytochrome c is released from the mitochondria and caspase-9 is activated. Pretreatment of the cells with caspase-9 specific inhibitor z-LEHD-FMK or pan caspase inhibitor Ac-DEVD-CHO prevented LY294002-induced apoptosis. Of interest, p53 siRNA prevents LY294002-induced apoptosis in HTLV-1-transformed cells, suggesting that p53 reactivation is linked to apoptosis. In conclusion, the AKT pathway is involved in targeting multiple proteins which regulate caspase- and p53-dependent apoptosis in HTLV-1-transformed cells. Since AKT inhibitors simultaneously inhibit NF-kappaB and activate p53, these drugs should be promising candidates for HTLV-1-associated cancer therapy.

Genetic ablation of calcium-independent phospholipase A2gamma leads to alterations in mitochondrial lipid metabolism and function resulting in a deficient mitochondrial bioenergetic phenotype

Previously, we identified a novel calcium-independent phospholipase, designated calcium-independent phospholipase A(2) gamma (iPLA(2)gamma), which possesses dual mitochondrial and peroxisomal subcellular localization signals. To identify the roles of iPLA(2)gamma in cellular bioenergetics, we generated mice null for the iPLA(2)gamma gene by eliminating the active site of the enzyme through homologous recombination. Mice null for iPLA(2)gamma display multiple bioenergetic dysfunctional phenotypes, including 1) growth retardation, 2) cold intolerance, 3) reduced exercise endurance, 4) greatly increased mortality from cardiac stress after transverse aortic constriction, 5) abnormal mitochondrial function with a 65% decrease in ascorbate-induced Complex IV-mediated oxygen consumption, and 6) a reduction in myocardial cardiolipin content accompanied by an altered cardiolipin molecular species composition. We conclude that iPLA(2)gamma is essential for maintaining efficient bioenergetic mitochondrial function through tailoring mitochondrial membrane lipid metabolism and composition.

Changes in mitochondrial dynamics during ceramide-induced cardiomyocyte early apoptosis

AIMS

In cells, mitochondria are organized as a network of interconnected organelles that fluctuate between fission and fusion events (mitochondrial dynamics). This process is associated with cell death. We investigated whether activation of apoptosis with ceramides affects mitochondrial dynamics and promotes mitochondrial fission in cardiomyocytes.

METHODS AND RESULTS

Neonatal rat cardiomyocytes were incubated with C(2)-ceramide or the inactive analog dihydro-C(2)-ceramide for up to 6 h. Three-dimensional images of cells loaded with mitotracker green were obtained by confocal microscopy. Dynamin-related protein-1 (Drp-1) and mitochondrial fission protein 1 (Fis1) distribution and levels were studied by immunofluorescence and western blot. Mitochondrial membrane potential (DeltaPsi(m)) and cytochrome c (cyt c) distribution were used as indexes of early activation of apoptosis. Cell viability and DNA fragmentation were determined by propidium iodide staining/flow cytometry, whereas cytotoxicity was evaluated by lactic dehydrogenase activity. To decrease the levels of the mitochondrial fusion protein mitofusin 2, we used an antisense adenovirus (AsMfn2). C(2)-ceramide, but not dihydro-C(2)-ceramide, promoted rapid fragmentation of the mitochondrial network in a concentration- and time-dependent manner. C(2)-ceramide also increased mitochondrial Drp-1 and Fis1 content, Drp-1 colocalization with Fis1, and caused early activation of apoptosis. AsMfn2 accentuated the decrease in DeltaPsi(m) and cyt c redistribution induced by C(2)-ceramide. Doxorubicin, which induces cardiomyopathy and apoptosis through ceramide generation, also stimulated mitochondrial fragmentation.

CONCLUSION

Ceramides stimulate mitochondrial fission and this event is associated with early activation of cardiomyocyte apoptosis.

Another piece of the puzzle of apoptotic cytochrome c release

Involvement of the mitochondrial permeability transition pore (PTP) in apoptosis and PTP structure are highly controversial. In this issue of Molecular Microbiology, experiments based on yeast genetics analyse the roles of the three proteins commonly considered to form the PTP, i.e. porin, ADP/ATP carrier (ACC) and mitochondrial cyclophilin, on apoptosis-like cell death. Whereas knocking out cyclophilin had no effect, the porin-1 knockout yeast showed enhanced apoptosis, suggesting that porin-1 has an antiapoptotic role. Loss of the ACC proteins afforded protection against some causes of death, but enhanced death induced by H(2)O(2), suggesting a more complex role for the ACC proteins in regulating apoptosis-like death in yeast.

ADP/ATP carrier is required for mitochondrial outer membrane permeabilization and cytochrome c release in yeast apoptosis

Adenine nucleotide translocator (ANT) is a mitochondrial inner membrane protein involved in the ADP/ATP exchange and is a component of the mitochondrial permeability transition pore (PTP). In mammalian apoptosis, the PTP can mediate mitochondrial outer membrane permeabilization (MOMP), which is suspected to be responsible for the release of apoptogenic factors, including cytochrome c. Although release of cytochrome c in yeast apoptosis has previously been reported, it is not known how it occurs. Herein we used yeast genetics to investigate whether depletion of proteins putatively involved in MOMP and cytochrome c release affects these processes in yeast. While deletion of POR1 (yeast voltage-dependent anion channel) enhances apoptosis triggered by acetic acid, H(2)O(2) and diamide, CPR3 (mitochondrial cyclophilin) deletion had no effect. Absence of ADP/ATP carrier (AAC) proteins, yeast orthologues of ANT, protects cells exposed to acetic acid and diamide but not to H(2)O(2). Expression of a mutated form of Aac2p (op1) exhibiting very low ADP/ATP translocase activity indicates that AAC's pro-death role does not require translocase activity. Absence of AAC proteins impairs MOMP and release of cytochrome c, which, together with other mitochondrial inner membrane proteins, is degraded. Our findings point to a crucial role of AAC in yeast apoptosis.

Apoptosis: a review of programmed cell death

The process of programmed cell death, or apoptosis, is generally characterized by distinct morphological characteristics and energy-dependent biochemical mechanisms. Apoptosis is considered a vital component of various processes including normal cell turnover, proper development and functioning of the immune system, hormone-dependent atrophy, embryonic development and chemical-induced cell death. Inappropriate apoptosis (either too little or too much) is a factor in many human conditions including neurodegenerative diseases, ischemic damage, autoimmune disorders and many types of cancer. The ability to modulate the life or death of a cell is recognized for its immense therapeutic potential. Therefore, research continues to focus on the elucidation and analysis of the cell cycle machinery and signaling pathways that control cell cycle arrest and apoptosis. To that end, the field of apoptosis research has been moving forward at an alarmingly rapid rate. Although many of the key apoptotic proteins have been identified, the molecular mechanisms of action or inaction of these proteins remain to be elucidated. The goal of this review is to provide a general overview of current knowledge on the process of apoptosis including morphology, biochemistry, the role of apoptosis in health and disease, detection methods, as well as a discussion of potential alternative forms of apoptosis.

Anti-tumor potential of 15,16-dihydrotanshinone I against breast adenocarcinoma through inducing G1 arrest and apoptosis

Chemotherapeutic drugs are usually designed to induce cancer cell death via cell cycle arrest and/or apoptosis pathways. In this study, we used the chemical drug 15,16-dihydrotanshinone I (DHTS) to inhibit breast cancer cell proliferation and tumor growth, and investigate the underlying molecular mechanisms. Human breast cancer cell lines MCF-7 and MDA-MB-231 were both used in this study, and DHTS was found to significantly decrease cell proliferation by a dose-dependent manner in both cells. Flow cytometry indicated that DHTS induced G1 phase arrest in synchronous MCF-7 and MDA-MB-231 cells. When analyzing the expression of cell cycle-related proteins, we found that DHTS reduced cyclin D1, cyclin D3, cyclin E, and CDK4 expression, and increased CDK inhibitor p27 expression in a dose-dependent manner. In addition, DHTS inhibited the kinase activities of CDK2 and CDK4 by an immunocomplex kinase assay. In addition, DHTS also induced apoptosis in both cells through mainly mitochondrial apoptosis pathways. We found that DHTS decreased the anti-apoptotic protein Bcl-xL level and increased the loss of mitochondria membrane potential and the amount of cytochrome c released. Moreover, DHTS activated caspase-9, caspase-3, and caspase-7 and caused cell apoptosis. The fact that DHTS-induced apoptosis could be blocked by pretreating cells with pan-caspase inhibitor confirmed that it is mediated through activation of the caspase-3-dependent pathway. In a nude mice xenograft experiment, DHTS significantly inhibited the tumor growth of MDA-MB-231 cells. Taken together, these results suggest that DHTS can inhibit human breast cancer cell proliferation and tumor growth, and might have potential chemotherapeutic applications.

Fowlpox virus encodes a Bcl-2 homologue that protects cells from apoptotic death through interaction with the proapoptotic protein Bak

Poxviruses are renowned for encoding numerous immunomodulatory proteins capable of undermining potent immune defenses. One effective barrier against infection is apoptosis, a process controlled at the mitochondria by pro- and antiapoptotic members of the highly conserved Bcl-2 family of proteins. Although poxviruses are known to encode an array of effective inhibitors of apoptosis, members of the Avipoxvirus genus, which includes fowlpox virus, encode proteins with Bcl-2 homology. Here, we show that FPV039, a fowlpox virus protein with limited Bcl-2 homology, inhibited apoptosis in response to a variety of cytotoxic stimuli, including virus infection itself. Similar to other antiapoptotic Bcl-2 proteins, FPV039 localized predominantly to the mitochondria in both human and chicken cells and protected human cells from tumor necrosis factor alpha-induced loss of the mitochondrial membrane potential. In addition, coimmunoprecipitation revealed that FPV039 interacted constitutively with the proapoptotic Bcl-2 protein, Bak, in both human and chicken cells. Concordantly, FPV039 also inhibited apoptosis induced by the transient overexpression of Bak. To confirm these results in the context of virus infection, we generated a recombinant vaccinia virus lacking F1L, the endogenous apoptotic inhibitor in vaccinia virus, and expressing FPV039. In the context of vaccinia virus infection, FPV039 retained the ability to localize to the mitochondria and interacted with Bak. Moreover, FPV039 prevented the activation of Bak and protected infected cells from apoptosis induced by staurosporine and virus infection. Together, our data indicate that FPV039 is a functional Bcl-2 homologue that inhibits apoptosis by neutralizing the proapoptotic Bcl-2 family member Bak.

Role of Nrf2 in protection against intracerebral hemorrhage injury in mice

Nrf2 is a key transcriptional factor for antioxidant response element (ARE)-regulated genes. While its beneficial role has been described for stroke, its contribution to intracerebral hemorrhage (ICH)-induced early brain injury remains to be determined. Using wild-type (WT) and Nrf2 knockout (Nrf2(-/-)) mice, the role of Nrf2 in ICH induced by intracerebral injection of collagenase was investigated. The results showed that injury volume was significantly larger in Nrf2(-/-) mice than in WT controls 24 h after induction of ICH (P<0.05), an outcome that correlated with neurological deficits. This exacerbation of brain injury in Nrf2(-/-) mice was also associated with an increase in leukocyte infiltration, production of reactive oxygen species, DNA damage, and cytochrome c release during the critical early phase of the post-ICH period. In combination, these results suggest that Nrf2 reduces ICH-induced early brain injury, possibly by providing protection against leukocyte-mediated free radical oxidative damage.

Propofol attenuates oxidative stress-induced PC12 cell injury via p38 MAP kinase dependent pathway

AIM

To investigate the neuroprotective effect of propofol and its intracellular mechanism on neurons in vitro.

METHODS

Cell viability was determined with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide reduction. Apoptotic cell death was determined by Hoechst 33258 staining and a fluorescence-activated cell sorter. The caspase-3 activity was measured by fluorometric assay. Mitogen-activated protein (MAP) kinase phosphorylation was detected with Western blotting.

RESULTS

The pretreatment of rat pheochromocytoma cell line PC12 with propofol (1-10 micromol/L) resulted in a significant recovery from hydrogen peroxide (H2O2)-induced cell death and the inhibition of H2O2 induced caspase-3 activation and PC12 cell apoptosis. Propofol inhibited the H2O2-induced p38 MAP kinase, but not c-Jun N-terminal kinase or extracellular signal-regulated kinase 1 and 2 activations.

CONCLUSION

Propofol might attenuate H2O2-induced PC12 cell death through the inhibition of signaling pathways mediated by the p38 MAP kinase.

Calcium-dependent proteolytic system and muscle dysfunctions: a possible role of calpains in sarcopenia

The calcium-dependent proteolytic system is composed of cysteine proteases named calpains. They are ubiquitous or tissue-specific enzymes. The two best characterised isoforms are the ubiquitously expressed mu- and m-calpains. Besides its regulation by calcium, calpain activity is tightly controlled by calpastatin, the specific endogenous inhibitor, binding to phospholipids, autoproteolysis and phosphorylation. Calpains are responsible for limited proteolytic events. Among the multitude of substrates identified so far are cytoskeletal and membrane proteins, enzymes and transcription factors. Calpain activity is involved in a large number of physiological and pathological processes. In this review, we will particularly focus on the implication of the calcium-dependent proteolytic system in relation to muscle physiology. Because of their ability to remodel cytoskeletal anchorage complexes, calpains play a major role in the regulation of cell adhesion, migration and fusion, three key steps of myogenesis. Calcium-dependent proteolysis is also involved in the control of cell cycle. In muscle tissue, in particular, calpains intervene in the regeneration process. Another important class of calpain substrates belongs to apoptosis regulating factors. The proteases may thus play a role in muscle cell death, and as a consequence in muscle atrophy. The relationships between calcium-dependent proteolysis and muscle dysfunctions are being further developed in this review with a particular emphasis on sarcopenia.

A high-throughput screening for mammalian cell death effectors identifies the mitochondrial phosphate carrier as a regulator of cytochrome c release

Functional annotation of complex genomes requires the development of novel experimental platforms with increased capacity. Here, we describe a high-throughput system designed to identify cDNAs whose overexpression induces morphologically distinct cell death modalities. The methodology incorporates two robotized steps, and relies on coexpression of library clones with GFP to reveal the morphological features presented by the dying cells. By using this system we screened 135 000 cDNA clones and obtained 90 independent molecules. Interestingly, three death categories were identified, namely; apoptotic, vacuolated and autophagic. Among the pro-apoptotic clones, we found four members of the mitochondrial carrier family: the phosphate and adenine nucleotide (type 3) transporters, and the mitochondrial carrier homologs (MTCHs) 1 and 2. Expression of these molecules induced cytochrome c release and caspase-9-dependent death. One of them, the phosphate carrier, was able to interact with members of the permeability transition pore complex ANT1 and VDAC1, and its binding to ANT1 was stabilized in the presence of apoptotic activators. Depletion of this carrier by siRNA delayed cytochrome c mobilization and apoptosis. These results attribute a previously undescribed apoptotic function to the phosphate carrier and, more generally, suggest that a common property of various mitochondrial transporters was exploited during evolution to regulate apoptosis.

A Novel Epidermal Growth Factor Receptor Inhibitor Promotes Apoptosis in Non–Small Cell Lung Cancer Cells Resistant to Erlotinib

Non-small cell lung cancer (NSCLC) with activating mutations in the epidermal growth factor receptor (EGFR) responds to EGFR tyrosine kinase inhibitors such as erlotinib. However, secondary somatic EGFR mutations (e.g., T790M) confer resistance to erlotinib. BMS-690514, a novel panHER/vascular endothelial growth factor receptor (VEGFR) inhibitor described here, exerted antiproliferative and proapoptotic effects on NSCLC cell lines, with prominent efficacy on H1975 cells expressing the T790M mutation. In this model, BMS-690514 induced a G(1) cell cycle arrest, as well as ultrastructural hallmarks of apoptosis, mitochondrial release of cytochrome c, and activation of caspases involved in the intrinsic (e.g., caspase-2, caspase-3, caspase-7, and caspase-9), but not in the extrinsic (e.g., caspase-8), pathway. Caspase inhibition conferred partial protection against BMS-690514 cytotoxicity, pointing to the involvement of both caspase-dependent and caspase-independent effector mechanisms. Transcriptome analyses revealed the up-regulation of proapoptotic (e.g., Bim, Puma) and cell cycle inhibitory (e.g., p27(Kip1), p57(Kip2)) factors, as well as the down-regulation of antiapoptotic (e.g., Mcl1), heat shock (e.g., HSP40, HSP70, HSP90), and cell cycle promoting [e.g., cyclins B1, D1, and D3; cyclin-dependent kinase 1 (CDK1); MCM family proteins; proliferating cell nuclear antigen (PCNA)] proteins. BMS-690514-induced death of H1975 cells was modified in a unique fashion by a panel of small interfering RNAs targeting apoptosis modulators. Down-regulation of components of the nuclear factor-kappaB survival pathway (e.g., p65, Nemo/IKK gamma, TAB2) sensitized cells to BMS-690514, whereas knockdown of proapoptotic factors (e.g., Puma, Bax, Bak, caspase-2, etc.) and DNA damage-related proteins (e.g., ERCC1, hTERT) exerted cytoprotective effects. BMS-690514 is a new pan-HER/VEGFR inhibitor that may become an alternative to erlotinib for the treatment of NSCLC.

LY303511 amplifies TRAIL-induced apoptosis in tumor cells by enhancing DR5 oligomerization, DISC assembly, and mitochondrial permeabilization

Certain classes of tumor cells respond favorably to TRAIL due to the presence of cell surface death receptors DR4 and DR5. Despite this preferential sensitivity, resistance to TRAIL remains a clinical problem and therefore the heightened interest in identifying compounds to revert tumor sensitivity to TRAIL. We recently demonstrated that the phosphatidylinositide-3-kinase (PI3K) inhibitor, LY294002, and its inactive analog LY303511, sensitized tumor cells to vincristine-induced apoptosis, independent of PI3K/Akt pathway. Intrigued by these findings, we investigated the effect of LY303511 on TRAIL-induced apoptosis in HeLa cells. Preincubation of cells with LY30 significantly amplified TRAIL signaling as evidenced by enhanced DNA fragmentation, caspases 2, 3, 8, and 9 activation, and reduction in the tumor colony formation. This increase in TRAIL sensitivity involved mitochondrial membrane permeabilization resulting in the egress of cytochrome c and second mitochondrial activator of caspase/direct IAP-binding protein with low PI, cleavage of X-linked inhibitor of apoptosis protein, and activation of caspase 9. We link this execution signal to the ability of LY30 to downregulate cFLIP(S) and oligomerize DR5, thus facilitating the signaling of the death initiating signaling complex. The subsequent exposure to TRAIL resulted in processing/activation of caspase 8 and cleavage of its substrate, the BH3 protein Bid. These data provide a novel mechanism of action of this small molecule with the potential for use in TRAIL-resistant tumors.

A novel Bcl-2-like inhibitor of apoptosis is encoded by the parapoxvirus ORF virus

Apoptotic cell death forms part of the host defense against virus infection. We tested orf virus, a member of the poxvirus family, for the ability to inhibit apoptosis and found that orf virus-infected cells were fully resistant to UV-induced changes in cell morphology, caspase activation, and DNA fragmentation. By using a library of vaccinia virus-orf virus recombinants, we identified an orf virus gene (ORFV125) whose presence was linked with the inhibition of apoptosis. The 173-amino-acid predicted protein had no clear homologs in public databases other than those encoded by other parapoxviruses. However, ORFV125 possessed a distinctive C-terminal domain which was necessary and sufficient to direct the protein to the mitochondria. We determined that ORFV125 alone could fully inhibit UV-induced DNA fragmentation, caspase activation, and cytochrome c release and that its mitochondrial localization was required for its antiapoptotic function. In contrast, ORFV125 did not prevent UV-induced activation of c-Jun NH2-terminal kinase, an event occurring upstream of the mitochondria. These features are comparable to the antiapoptotic properties of the mitochondrial regulator Bcl-2. Furthermore, bioinformatic analyses revealed sequence and secondary-structure similarities to Bcl-2 family members, including characteristic residues of all four Bcl-2 homology domains. Consistent with this, the viral protein inhibited the UV-induced activation of the proapoptotic Bcl-2 family members Bax and Bak. ORFV125 is the first parapoxvirus apoptosis inhibitor to be identified, and we propose that it is a new antiapoptotic member of the Bcl-2 family.

Hyperglycemia: Cell death in a cave

Mitochondria play a central role in mediating high glucose-induced apoptosis. A recent study has shown that increases in glucose levels induce significant alterations in caveolae components, suggesting that high glucose may affect apoptotic signaling initiated in caveolae.

Mitochondrial control of cell death induced by hyperosmotic stress

HeLa and HCT116 cells respond differentially to sorbitol, an osmolyte able to induce hypertonic stress. In these models, sorbitol promoted the phenotypic manifestations of early apoptosis followed by complete loss of viability in a time-, dose-, and cell type-specific fashion, by eliciting distinct yet partially overlapping molecular pathways. In HCT116 but not in HeLa cells, sorbitol caused the mitochondrial release of the caspase-independent death effector AIF, whereas in both cell lines cytochrome c was retained in mitochondria. Despite cytochrome c retention, HeLa cells exhibited the progressive activation of caspase-3, presumably due to the prior activation of caspase-8. Accordingly, caspase inhibition prevented sorbitol-induced killing in HeLa, but only partially in HCT116 cells. Both the knock-out of Bax in HCT116 cells and the knock-down of Bax in A549 cells by RNA interference reduced the AIF release and/or the mitochondrial alterations. While the knock-down of Bcl-2/Bcl-X(L) sensitized to sorbitol-induced killing, overexpression of a Bcl-2 variant that specifically localizes to mitochondria (but not of the wild-type nor of a endoplasmic reticulum-targeted form) strongly inhibited sorbitol effects. Thus, hyperosmotic stress kills cells by triggering different molecular pathways, which converge at mitochondria where pro- and anti-apoptotic members of the Bcl-2 family exert their control.

High Bcl-2/Bax ratio in Walker tumor cells protects mitochondria but does not prevent H2O2-induced apoptosis via calcineurin pathways

It has been previously shown that Walker 256 tumor cells express a high content of the anti-apoptotic protein Bcl-2 which protects mitochondria against the damaging effects of Ca(2+). In the present study, we analyze H(2)O(2)-induced apoptotic death in two different types of tumor cells: Walker 256 and SCC-25. Treatment with H(2)O(2) (4mM) increased reactive oxygen species generation and the concentration of cytosolic free Ca(2+). These alterations preceded apoptosis in both cell lines. In Walker cells, which show a high Bcl-2/Bax ratio, apoptosis was dependent on calcineurin activation and independent of changes in mitochondrial membrane potential (DeltaPsi(m)), as well as cytochrome c release. In contrast, in SCC-25 cells, which show a lower Bcl-2/Bax ratio, apoptosis was preceded by a decrease in DeltaPsi(m), mitochondrial permeability transition, and cytochrome c release. Caspase-3 activation occurred in both cell lines. The data suggest that although the high Bcl-2/Bax ratio protected the mitochondria of Walker cells from oxidative stress, it was not sufficient to prevent apoptosis through calcineurin pathways.

Intersectin-1s regulates the mitochondrial apoptotic pathway in endothelial cells

Intersectins (ITSNs) are multidomain adaptor proteins implicated in endocytosis, regulation of actin polymerization, and Ras/MAPK signaling. We have previously shown that ITSN-1s is required for caveolae fission and internalization in endothelial cells (ECs). In the present study, using small interfering RNA to knock down ITSN-1s protein expression, we demonstrate a novel role of ITSN-1s as a key antiapoptotic protein. Knockdown of ITSN-1s in ECs activated the mitochondrial pathway of apoptosis as determined by genomic DNA fragmentation, extensive mitochondrial fission, activation of the proapoptotic proteins BAK and BAX, and cytochrome c efflux from mitochondria. ITSN-1 knockdown acts as a proapoptotic signal that causes mitochondrial outer membrane permeabilization, dissipation of the mitochondrial membrane potential, and generation of reactive oxygen species. These effects were secondary to decreased activation of Erk1/2 and its direct activator MEK. Bcl-X(L) overexpression prevented BAX activation and the apoptotic ECs death induced by suppression of ITSN-1s. Our findings demonstrate a novel role of ITSN-1s as a negative regulator of the mitochondrial pathway-dependent apoptosis secondary to activation of the Erk1/2 survival signaling pathway.

Increased mitochondrial mass characterizes the survival defect of HIV-specific CD8(+) T cells

What governs the increased apoptosis sensitivity of HIV-specific CD8(+) T cells is poorly understood. Here, we examined the involvement of mitochondria in this apoptosis. Remarkably higher mitochondrial mass (MM) was found in HIV-specific compared with CMV-specific CD8(+) T cells from HIV(+) patients and this could not be attributed to their different differentiation status. MM(High) phenotype characterized those CD8(+) T cells from HIV(+) patients that are sensitive to spontaneous and CD95/Fas-induced apoptosis. CD38 expression did not correlate with high MM, whereas Bcl-2 levels were significantly reduced in both CD38(+) and CD38(-) HIV-specific CD8(+) T cells. Although CD38(+) HIV-specific CD8(+) T cells were more susceptible to apoptosis, CD38 expression does not explain on its own the selective apoptosis sensitivity of HIV-specific CD8(+) T cells, as CD38(-) HIV-specific CD8(+) T cells were more apoptotic than CD38(+) CMV-specific ones. Proapoptotic HIV-specific CD8(+) T cells were CD38(+)Bcl-2(Low)MM(High). Copolarization of mitochondria with CD95/Fas capping, very early in CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells, suggests that mitochondria act as an amplification step for this apoptosis. Thus, an extensive mitochondrial network contributes to apoptosis sensitivity of CD8(+) T cells and, when this occurs together with reduced levels of Bcl-2 and chronic activation, determines the proapoptotic state of HIV-specific CD8(+) T cells.

Interaction of a cyclin E fragment with Ku70 regulates Bax-mediated apoptosis

The cyclin E/Cdk2 complex plays an essential role in the G(1)/S cell cycle transition and DNA replication. Earlier we showed that in hematopoietic tumor cells, caspase-mediated cleavage of cyclin E generates p18-cyclin E, which is unable to interact with Cdk2 and therefore plays a role independent of the cell cycle. The expression of a cleavage-resistant cyclin E mutant greatly diminishes apoptosis, indicating the critical role of cyclin E cleavage. p18-cyclin E expression can induce apoptosis or sensitization to apoptotic stimuli in many cell types. Here we identify Ku70 as a specific p18-cyclin E-interacting partner. In hematopoietic tumor cell lines, the association of p18-cyclin E with Ku70 induces the dissociation of Bax from Ku70, followed by Bax activation. This mechanism of Bax activation leads to the amplification of the apoptosis signal in all tumor cell lines examined. N-terminal Ku70 deletion mutants are unable to bind to p18-cyclin E to regulate its apoptotic effect. p18-cyclin E-mediated amplification of apoptosis is dependent on Bax and Ku70 being greatly diminished in Ku70(-/-) and Bax(-/-) mouse embryo fibroblasts and in hematopoietic cells where Bax knockdown was achieved by short interfering RNA. The p18-cyclin E/Ku70 and Bax/Ku70 interactions provide a balance between apoptosis and the survival of cells exposed to genotoxic stress.

Cardiolipin switch in mitochondria: shutting off the reduction of cytochrome c and turning on the peroxidase activity

Upon interaction with anionic phospholipids, particularly mitochondria-specific cardiolipin (CL), cytochrome c (cyt c) loses its tertiary structure and its peroxidase activity dramatically increases. CL-induced peroxidase activity of cyt c has been found to be important for selective CL oxidation in cells undergoing programmed death. During apoptosis, the peroxidase activity and the fraction of CL-bound cyt c markedly increase, suggesting that CL may act as a switch to regulate cyt c's mitochondrial functions. Using cyclic voltammetry and equilibrium redox titrations, we show that the redox potential of cyt c shifts negatively by 350-400 mV upon binding to CL-containing membranes. Consequently, functions of cyt c as an electron transporter and cyt c reduction by Complex III are strongly inhibited. Further, CL/cyt c complexes are not effective in scavenging superoxide anions and are not effectively reduced by ascorbate. Thus, both redox properties and functions of cyt c change upon interaction with CL in the mitochondrial membrane, diminishing cyt c's electron donor/acceptor role and stimulating its peroxidase activity.

Reactive Oxygen Species: A Breath of Life or Death?: Fig. 1

New insights into cancer cell-specific biological pathways are urgently needed to promote development of rationally targeted therapeutics. Reactive oxygen species (ROS) and their role in cancer cell response to growth factor signaling and hypoxia are emerging as verdant areas of exploration on the road to discovering cancer's Achilles heel. One of the distinguishing and near-universal hallmarks of cancer growth is hypoxia. Unregulated cellular proliferation leads to formation of cellular masses that extend beyond the resting vasculature, resulting in oxygen and nutrient deprivation. The resulting hypoxia triggers a number of critical adaptations that enable cancer cell survival, including apoptosis suppression, altered glucose metabolism, and an angiogenic phenotype. Ironically, recent investigations suggest that oxygen depletion stimulates mitochondria to elaborate increased ROS, with subsequent activation of signaling pathways, such as hypoxia inducible factor 1alpha, that promote cancer cell survival and tumor growth. Because mitochondria are key organelles involved in chemotherapy-induced apoptosis induction, the relationship between mitochondria, ROS signaling, and activation of survival pathways under hypoxic conditions has been the subject of increased study. Insights into mechanisms involved in ROS signaling may offer novel avenues to facilitate discovery of cancer-specific therapies. Preclinical and clinical evaluation of agents that modify ROS signaling in cancer offers a novel avenue for intervention. This review will cover recent work in ROS-mediated signaling in cancer cells and its potential as a target for developmental therapeutics.

Oxidative stress and disuse muscle atrophy

Skeletal muscle inactivity is associated with a loss of muscle protein and reduced force-generating capacity. This disuse-induced muscle atrophy results from both increased proteolysis and decreased protein synthesis. Investigations of the cell signaling pathways that regulate disuse muscle atrophy have increased our understanding of this complex process. Emerging evidence implicates oxidative stress as a key regulator of cell signaling pathways, leading to increased proteolysis and muscle atrophy during periods of prolonged disuse. This review will discuss the role of reactive oxygen species in the regulation of inactivity-induced skeletal muscle atrophy. The specific objectives of this article are to provide an overview of muscle proteases, outline intracellular sources of reactive oxygen species, and summarize the evidence that connects oxidative stress to signaling pathways contributing to disuse muscle atrophy. Moreover, this review will also discuss the specific role that oxidative stress plays in signaling pathways responsible for muscle proteolysis and myonuclear apoptosis and highlight gaps in our knowledge of disuse muscle atrophy. By presenting unresolved issues and suggesting topics for future research, it is hoped that this review will serve as a stimulus for the expansion of knowledge in this exciting field.

Methods for the assessment of mitochondrial membrane permeabilization in apoptosis

Mitochondrial membrane permeabilization (MMP) is considered as the "point-of-no-return" in numerous models of programmed cell death. Indeed, mitochondria determine the intrinsic pathway of apoptosis, and play a major role in the extrinsic route as well. MMP affects the inner and outer mitochondrial membranes (IM and OM, respectively) to a variable degree. OM permeabilization culminates in the release of proteins that normally are confined in the mitochondrial intermembrane space (IMS), including caspase activators (e.g. cytochrome c) and caspase-independent death effectors (e.g. apoptosis-inducing factor). Partial IM permeabilization disrupts mitochondrial ion and volume homeostasis and dissipates the mitochondrial transmembrane potential (DeltaPsi(m)). The assessment of early mitochondrial alterations allows for the identification of cells that are committed to die but have not displayed yet the apoptotic phenotype. Several techniques to measure MMP by cytofluorometry and fluorescence microscopy have been developed. Here, we summarize the currently available methods for the detection of MMP, and provide a comparative analysis of these techniques.

Apoptosis versus cell differentiation: role of heat shock proteins HSP90, HSP70 and HSP27

Heat shock proteins HSP27, HSP70 and HSP90 are molecular chaperones whose expression is increased after many different types of stress. They have a protective function helping the cell to cope with lethal conditions. The cytoprotective function of HSPs is largely explained by their anti-apoptotic function. HSPs have been shown to interact with different key apoptotic proteins. As a result, HSPs can block essentially all apoptotic pathways, most of them involving the activation of cystein proteases called caspases. Apoptosis and differentiation are physiological processes that share many common features, for instance, chromatin condensation and the activation of caspases are frequently observed. It is, therefore, not surprising that many recent reports imply HSPs in the differentiation process. This review will comment on the role of HSP90, HSP70 and HSP27 in apoptosis and cell differentiation. HSPs may determine de fate of the cells by orchestrating the decision of apoptosis versus differentiation.

The catalytically inactive precursor of cathepsin D induces apoptosis in human fibroblasts and HeLa cells

In several reports cathepsin D has been implicated in apoptosis. In some systems the effects of agents considered to be mediated by cathepsin D were inhibited in the presence of pepstatin A, an inhibitor of the enzyme. In other studies the effect of a mutant cathepsin D deprived of activity was indistinguishable from that of the normal enzyme. Here we show that in human fibroblasts and in HeLa cells apoptosis can be induced by microinjecting into cytosol either mature cathepsin D or its inactive precursor procathepsin D. The microinjected precursor remains in the uncleaved form. These results confirm that the proapoptotic effect of cathepsin D in the cytosol is independent of its catalytic activity and suggest that the interaction of cathepsin D with the downstream effector does not involve the active site of the enzyme, since in the proenzyme the active site is masked by the prosequence.

Mitochondrial perturbation, oxidative stress and lysosomal destabilization are involved in 7β-hydroxysitosterol and 7β-hydroxycholesterol triggered apoptosis in human colon cancer cells

We reported previously that 7beta-hydroxysitosterol and 7beta-hydroxycholesterol induced apoptosis in Caco-2 cells. Apoptosis caused by 7beta-hydroxysitosterol but not by 7beta-hydroxycholesterol was related to a caspase-dependent process. In the present report, we compared the effects of both compounds on mitochondria integrity and on various modulators of apoptosis. When Caco-2 cells were exposed to both hydroxysterols, no changes in Bcl-2 and Bax expressions were detected indicating a Bcl-2/Bax-independent cell death pathway, whereas loss of mitochondrial membrane potential and cytochrome c release were observed. Endonuclease G expression and enhanced production of reactive oxygen species were detected in 7beta-hydroxycholesterol treated cells, but not with 7beta-hydroxysitosterol. Loss of mitochondrial membrane potential and cell death produced by both hydroxysterols were prevented by vitamin C. Lysosomal membrane integrity was altered with both hydroxysterols, but 7beta-hydroxysitosterol was significantly more active on than 7beta-hydroxycholesterol. Both hydroxysterols induced apoptosis by mitochondrial membrane permeabilization. However, 7beta-hydroxycholesterol exhibited a specific enhancement of oxidative stress and of endonuclease G expression despite its closely related chemical structure with 7beta-hydroxysitosterol. The two hydroxysterols exhibit different lipophilic properties which may explain their different biological effects.

Cardiolipin clusters and membrane domain formation induced by mitochondrial proteins

We show in this study that mitochondrial creatine kinase promotes segregation and clustering of cardiolipin in mixed membranes, a phenomenon that has been proposed to occur at contact sites in the mitochondria. This property of mitochondrial creatine kinase is dependent on the native octameric structure of the protein and does not occur after heat-denaturation or with the native dimeric form of the protein. Cardiolipin segregation was demonstrated by differential scanning calorimetry using membranes containing cardiolipin and either dipalmitoylphosphatidylethanolamine or 1-palmitoyl-2-oleoylphosphatidylethanolamine. Addition of the ubiquitous form of mitochondrial creatine kinase leads to the formation of a phosphatidylethanolamine-rich domain as a result of the protein binding preferentially to the cardiolipin. Such phase separation does not occur if cardiolipin is replaced with dioleoyl phosphatidylglycerol. Lipid phase separation is observed with other cardiolipin-binding proteins, including cytochrome c and, to a very small extent, with truncated Bid (t-Bid), as well as with the cationic polypeptide poly-L-lysine, but among these proteins the octameric form of mitochondrial creatine kinase is by far the most effective in causing segregation and clustering of cardiolipin. The proteins included in this study are found at mitochondrial contact sites where they are known to associate with cardiolipin. Domains in mitochondria enriched in cardiolipin play an important role in apoptosis and in energy flux processes.

Mathematical modeling identifies inhibitors of apoptosis as mediators of positive feedback and bistability

The intrinsic, or mitochondrial, pathway of caspase activation is essential for apoptosis induction by various stimuli including cytotoxic stress. It depends on the cellular context, whether cytochrome c released from mitochondria induces caspase activation gradually or in an all-or-none fashion, and whether caspase activation irreversibly commits cells to apoptosis. By analyzing a quantitative kinetic model, we show that inhibition of caspase-3 (Casp3) and Casp9 by inhibitors of apoptosis (IAPs) results in an implicit positive feedback, since cleaved Casp3 augments its own activation by sequestering IAPs away from Casp9. We demonstrate that this positive feedback brings about bistability (i.e., all-or-none behaviour), and that it cooperates with Casp3-mediated feedback cleavage of Casp9 to generate irreversibility in caspase activation. Our calculations also unravel how cell-specific protein expression brings about the observed qualitative differences in caspase activation (gradual versus all-or-none and reversible versus irreversible). Finally, known regulators of the pathway are shown to efficiently shift the apoptotic threshold stimulus, suggesting that the bistable caspase cascade computes multiple inputs into an all-or-none caspase output. As cellular inhibitory proteins (e.g., IAPs) frequently inhibit consecutive intermediates in cellular signaling cascades (e.g., Casp3 and Casp9), the feedback mechanism described in this paper is likely to be a widespread principle on how cells achieve ultrasensitivity, bistability, and irreversibility.

Erythropoietin administration protects against functional impairment and cell death after ischaemic renal injury in pigs

OBJECTIVE

To determine whether the administration of erythropoietin at the time of ischaemic renal injury (IRI) inhibits apoptosis, enhances tubular epithelial regeneration and promotes renal functional recovery, as it does in rodent models, in a higher mammalian model.

MATERIALS AND METHODS

The model of IRI involved unilateral nephrectomy in pigs, followed a week later by renal artery occlusion for 1 h, followed by reperfusion for 5 days. Pigs were randomized to receive erythropoietin 5000 units/kg intravenously at the time of ischaemia, followed by 1000 units/kg subcutaneously daily, or no treatment (six pigs each). Renal function and structure were analysed; blood and urine were collected daily to determine serum creatinine level, blood urea nitrogen, and creatinine clearance. Animals were killed after 5 days to obtain the injured kidneys. The kidneys were examined histologically for evidence of cellular mitosis, apoptosis and necrosis.

RESULTS

Erythropoietin significantly abrogated renal dysfunction after IRI compared with controls at 12 h after injury; the mean (sem) creatinine clearance (as a percentage of baseline) for IRI was 68.2 (6)% vs erythropoietin-IRI 94.9 (8.9)% (P = 0.027), although by 36 h this was no longer significant, with values of 73.8 (12.7)% vs 95.9 (12)%, respectively (P = 0.23). Erythropoietin also significantly reduced the amount of cell death on histological analysis after 5 days of reperfusion, with a median (range) for IRI of 5.5 (1-45) vs erythropoietin-IRI of 1.5 (0-4) (P = 0.043).

CONCLUSION

This study confirms the potential clinical applications of erythropoietin as a novel therapeutic agent in patients at risk of IRI.

Chronic restraint stress modulates expression of genes in murine spleen

Psychological and physical stress can alter the immune system in both humans and animals. We have reported that mice subjected to chronic 12-h daily physical restraint for 2 days showed dramatic apoptosis in splenocytes. To identify genes that contribute to the splenocyte apoptosis, we compare gene expression in the spleens of restrained and unstressed mice using oligo microarrays consisting of 226 genes. We report here that mice subjected to chronic 12-h daily physical restraint for 2 days exhibited significantly altered expression of 50 of 226 genes. These genes included pro-apoptotic genes. We selected 5 genes of interest and confirmed the microarray results by real-time PCR. In this study, we identify a potentially important component of pro-apoptotic activity in restraint stress and suggest a possible target for anti-apoptotic therapy to protect splenocytes against stress-induced apoptosis.