Necrosis, a well-orchestrated form of cell demise: signalling cascades, important mediators and concomitant immune response

Chitosan-induced programmed cell death in plants

Chitosan, CN(-), or H(2)O(2) caused the death of epidermal cells (EC) in the epidermis of pea leaves that was detected by monitoring the destruction of cell nuclei; chitosan induced chromatin condensation and marginalization followed by the destruction of EC nuclei and subsequent internucleosomal DNA fragmentation. Chitosan did not affect stoma guard cells (GC). Anaerobic conditions prevented the chitosan-induced destruction of EC nuclei. The antioxidants nitroblue tetrazolium or mannitol suppressed the effects of chitosan, H(2)O(2), or chitosan + H(2)O(2) on EC. H(2)O(2) formation in EC and GC mitochondria that was determined from 2',7'-dichlorofluorescein fluorescence was inhibited by CN(-) and the protonophoric uncoupler carbonyl cyanide m-chlorophenylhydrazone but was stimulated by these agents in GC chloroplasts. The alternative oxidase inhibitors propyl gallate and salicylhydroxamate prevented chitosan- but not CN(-)-induced destruction of EC nuclei; the plasma membrane NADPH oxidase inhibitors diphenylene iodonium and quinacrine abolished chitosan- but not CN(-)-induced destruction of EC nuclei. The mitochondrial protein synthesis inhibitor lincomycin removed the destructive effect of chitosan or H(2)O(2) on EC nuclei. The effect of cycloheximide, an inhibitor of protein synthesis in the cytoplasm, was insignificant; however, it was enhanced if cycloheximide was added in combination with lincomycin. The autophagy inhibitor 3-methyladenine removed the chitosan effect but exerted no influence on the effect of H(2)O(2) as an inducer of EC death. The internucleosome DNA fragmentation in conjunction with the data on the 3-methyladenine effect provides evidence that chitosan induces programmed cell death that follows a combined scenario including apoptosis and autophagy. Based on the results of an inhibitor assay, chitosan-induced EC death involves reactive oxygen species generated by the NADPH oxidase of the plasma membrane.

Adhesion-mediated apoptosis resistance in cancer

Adhesion-mediated apoptosis resistance (AMAR) is an emerging concept that may explain the observed differences in survival between cells within the three-dimensional structure of a tumor and the standard monolayer culture conditions in the laboratory. Not only the cancer cells' motility and invasiveness are different in a three-dimensional tumor, but - crucially - the cells' sensitivity towards apoptosis, a form of programmed cell death, varies widely between the in vivo and in vitro situation. Tumor cells interacting either with a specific extracellular matrix protein substrate or with each other or with non-transformed cells, such as fibroblasts, exhibit increased resistance towards a wide variety of therapeutic approaches. In this review we discuss the molecular basis of these interactions and the main downstream effectors that are involved in the enhancement of the tumor cells' survival. In particular, we show that the pathways activated by adhesion are not unique, but involve the MAPK/ERK and PI3K/Akt pathways, which are reused between different forms of AMAR and are also found in adhesion-independent modes of resistance. Thus, the tools to overcome AMAR are already at our disposal and using them in this novel context of AMAR should lead to significant therapeutic benefit.

Ineffective GSH regeneration enhances G6PD-knockdown Hep G2 cell sensitivity to diamide-induced oxidative damage

Glucose-6-phosphate dehydrogenase (G6PD) has been recently found to play growth-regulatory roles in nucleated cells. To identify any other physiologic roles of G6PD, we generated G6PD-knockdown Hep G2 cells and investigated their susceptibility to oxidants. Hep G2 cells expressing shRNA against G6PD (Gi) were more susceptible to diamide-induced cytotoxicity than control cells expressing scrambled control shRNA (Sc). The level of reactive oxygen species in the Gi cells substantially exceeded that in Sc cells. This was accompanied by increased membrane peroxidation and the appearance of high-molecular-weight aggregates of membrane-associated cytoskeletal proteins in Gi cells. G6PD knockdown was associated with an impaired ability to regenerate glutathione. Diamide caused a considerable decrease in cellular glutathione level and a concomitant increase in glutathione disulfide in Gi cells. Consistent with this finding, N-acetylcysteine mitigated diamide-induced oxidative stress and cell death. Our findings suggest that G6PD confers protection against oxidant-induced cytotoxicity through effective glutathione regeneration.

HYD1-induced increase in reactive oxygen species leads to autophagy and necrotic cell death in multiple myeloma cells

HYD1 is a D-amino acid peptide that was previously shown to inhibit adhesion of prostate cancer cells to the extracellular matrix. In this study, we show that in addition to inhibiting adhesion of multiple myeloma (MM) cells to fibronectin, HYD1 induces cell death in MM cells as a single agent. HYD1-induced cell death was necrotic in nature as shown by: (a) decrease in mitochondrial membrane potential (Deltapsi(m)), (b) loss of total cellular ATP, and (c) increase in reactive oxygen species (ROS) production. Moreover, HYD1 treatment does not result in apoptotic cell death because it did not trigger the activation of caspases or the release of apoptosis-inducing factor and endonuclease G from the mitochondria, nor did it induce double-stranded DNA breaks. HYD1 did initiate autophagy in cells; however, autophagy was found to be an adaptive response contributing to cell survival rather than the cause of cell death. We were further able to show that N-acetyl-L-cysteine, a thiol-containing free radical scavenger, partially protects MM cells from HYD1-induced death. Additionally, N-acetyl-L-cysteine blocked HYD1-induced as well as basal levels of autophagy, suggesting that ROS can potentially trigger both cell death and cell survival pathways. Taken together, our data describe an important role of ROS in HYD1-induced necrotic cell death in MM cells.

Programmed cellular necrosis mediated by the pore-forming alpha-toxin from Clostridium septicum

Programmed necrosis is a mechanism of cell death that has been described for neuronal excitotoxicity and ischemia/reperfusion injury, but has not been extensively studied in the context of exposure to bacterial exotoxins. The α-toxin of Clostridium septicum is a β-barrel pore-forming toxin and a potent cytotoxin; however, the mechanism by which it induces cell death has not been elucidated in detail. We report that α-toxin formed Ca²⁺-permeable pores in murine myoblast cells, leading to an increase in intracellular Ca²⁺ levels. This Ca²⁺ influx did not induce apoptosis, as has been described for other small pore-forming toxins, but a cascade of events consistent with programmed necrosis. Ca²⁺ influx was associated with calpain activation and release of cathepsins from lysosomes. We also observed deregulation of mitochondrial activity, leading to increased ROS levels, and dramatically reduced levels of ATP. Finally, the immunostimulatory histone binding protein HMGB1 was found to be released from the nuclei of α-toxin-treated cells. Collectively, these data show that α-toxin initiates a multifaceted necrotic cell death response that is consistent with its essential role in C. septicum-mediated myonecrosis and sepsis. We postulate that cellular intoxication with pore-forming toxins may be a major mechanism by which programmed necrosis is induced.

Clostridium septicum is a highly virulent pathogen that causes spontaneous gas gangrene or clostridial myonecrosis. The essential virulence factor of C. septicum is a β-barrel toxin, α-toxin, that forms small pores in host cell membranes. This toxin is frequently described as a hemolysin, because the formation of these pores causes lysis of red blood cell cells due to membrane disruption. However, this description does not recognize additional effects that may be observed in nucleated host cells, which are more sensitive to α-toxin. We investigated how nucleated cells responded to α-toxin by treating a physiologically relevant muscle cell line with purified toxin and monitoring the response using various assays. We observed α-toxin-mediated programmed cellular necrosis that culminated in the release of the immunostimulatory molecule, HMGB1. This mechanism of cell death induction is consistent with the extensive necrosis that is evident in C. septicum-mediated myonecrosis and with the overwhelming sepsis that frequently contributes to the high mortality rate. These results represent an important advance in the understanding of the toxicity of β-barrel pore-forming toxins and how they may contribute to necrotic and systemic disease pathology.

Activity-dependent survival of neurons in culture: a model of slow neurodegeneration

Central neurons express persistent spontaneous electrical network activity both in the developing brain in vivo as well as in dissociated cultures. This electrical activity is important for the formation of connections among neurons, and for their survival. Prolonged suppression of the spontaneous activity using the sodium channel blocker tetrodotoxin (TTX) causes the death of the cultured neurons. In the present study, we investigated molecular mechanisms that may underlie the activity-suppressed slow degeneration of cortical neurons in culture. Already after 6-7 days of exposure to TTX, neurons begin to express apoptotic vacuoles and shrunken dendrites. Eventually, neurons activate p53, caspase-3 and BAX, hallmarks of neuronal apoptosis, before they die. This death is restricted to neurons, and no parallel process is seen in glial cells that co-exist in the culture. These experiments may lead to a better understanding of slow neuronal death, akin to that found in neurodegenerative diseases of the brain.

The cell cycle and acute kidney injury

Acute kidney injury (AKI) activates pathways of cell death and cell proliferation. Although seemingly discrete and unrelated mechanisms, these pathways can now be shown to be connected and even to be controlled by similar pathways. The dependence of the severity of renal-cell injury on cell cycle pathways can be used to control and perhaps to prevent acute kidney injury. This review is written to address the correlation between cellular life and death in kidney tubules, especially in acute kidney injury.

RIP3, an energy metabolism regulator that switches TNF-induced cell death from apoptosis to necrosis

Necrosis can be induced by stimulating death receptors with tumor necrosis factor (TNF) or other agonists; however, the underlying mechanism differentiating necrosis from apoptosis is largely unknown. We identified the protein kinase receptor-interacting protein 3 (RIP3) as a molecular switch between TNF-induced apoptosis and necrosis in NIH 3T3 cells and found that RIP3 was required for necrosis in other cells. RIP3 did not affect RIP1-mediated apoptosis but was required for RIP1-mediated necrosis and the enhancement of necrosis by the caspase inhibitor zVAD. By activating key enzymes of metabolic pathways, RIP3 regulates TNF-induced reactive oxygen species production, which partially accounts for RIP3's ability to promote necrosis. Our data suggest that modulation of energy metabolism in response to death stimuli has an important role in the choice between apoptosis and necrosis.

Immunogenic and tolerogenic cell death

The immune system is routinely exposed to dead cells during normal cell turnover, injury and infection. Mechanisms must exist to discriminate between different forms of cell death to correctly eliminate pathogens and promote healing while avoiding responses to self, which can result in autoimmunity. However, an effective immune response against host tissue is often needed to eliminate tumours following treatment with chemotherapeutic agents that trigger tumour cell death. Consequently, a central problem in immunology is to understand how the immune system determines whether cell death is immunogenic, tolerogenic or 'silent'.

Overexpression of CHMP6 induces cellular oncosis and apoptosis in HeLa cells

Cell death can proceed via at least two distinct pathways, apoptosis and oncosis. Apoptosis is an energy-dependent process characterized morphologically by cell shrinkage, whereas oncosis is defined as a prelethal pathway leading to cell death associated with cellular swelling, organelle swelling, and increased membrane permeability. In this study, we found that overexpression of chromatin modifying protein 6 (CHMP6) induced cell death by a series of experiments, including morphological observation, intracellular ATP determination, caspase-3 activity, and flow cytometry. Typical morphological characteristics consistent with oncosis were observed by transmission electron microscopy. Simultaneously, we obtained some results that indicated apoptosis, but the anti-apoptotic gene Bcl-xL and caspase family inhibitor Z-VAD-FMK had little effect on CHMP6-induced cell death. These results suggest that CHMP6 overexpression can cause cell death, predominantly via oncosis and to a certain extent via apoptosis, and that CHMP6 might be a novel regulator involved in both oncosis and apoptosis.

Shift from apoptotic to necrotic cell death during human papillomavirus-induced transformation of keratinocytes

Oncogenic transformation is a complex, multistep process, which goes through several stages before complete malignant transformation occurs. To identify early processes in carcinogenesis, we used an in vitro model, based on the initiating event in cervical cancer, papillomavirus transformation of keratinocytes. We compared gene expression in primary keratinocytes (K) and papillomavirus-transformed keratinocytes from early (E) and late (L) passages and from benzo[a]pyrene-treated L cells (BP). The transformed cells exhibit similar transcriptional changes to clinical cervical carcinoma. The number of transcripts expressed progressively decreased during the evolution from K to BP cells. Bioinformatic analysis, validated by detailed biochemical analysis, revealed substantial contraction of both pro- and antiapoptotic networks during transformation. Nonetheless, L and BP cells were not resistant to apoptotic stimuli. At doses of cisplatin that led to 30-60% apoptosis of K and E cells, transformed L and BP cells underwent 80% necrotic cell death, which became the default response to genotoxic stress. Moreover, appreciable necrotic fractions were observed in the cervical carcinoma cell line, HeLa, in response to comparable doses of cisplatin. The shrinkage of biochemical networks, including the apoptotic network, may allow a cancer cell to economize on energy usage to facilitate enhanced proliferation but leaves it vulnerable to stress. This study supports the hypothesis that the process of cancer transformation may be accompanied by a shift from apoptosis to necrosis.

Connexin-related signaling in cell death: to live or let die?

Evidence is accumulating that some forms of cell death, like apoptosis, are not only governed by the complex interplay between extracellular and intracellular signals but are also strongly influenced by intercellular communicative networks. The latter is provided by arrays of channels consisting of connexin proteins, with gap junctions directly connecting the cytoplasm of neighboring cells and hemichannels positioned as pores that link the cytoplasm to the extracellular environment. The role of gap junctions in cell death communication has received considerable interest and recently hemichannels have joined in as potentially toxic pores adding their part to the cell death process. However, despite a large body of existing evidence, especially for gap junctions, the exact contribution of the connexin channel family still remains controversial, as both gap junctions and hemichannels may furnish cell death as well as cell survival signals. An additional layer of complexity is formed by the fact that connexin proteins as such, beyond their channel function, may influence the cell death process. We here review the current knowledge on connexins and their channels in cell death and specifically address the molecular mechanisms that underlie connexin-related signaling. We also briefly focus on pannexins, a novel set of connexin-like proteins that have been implicated in cellular responses to pathological insults.

Epinecidin-1, an antimicrobial peptide from fish (Epinephelus coioides) which has an antitumor effect like lytic peptides in human fibrosarcoma cells

Epinecidin-1, a synthetic 21-mer antimicrobial peptide originally identified from grouper (Epinephelus coioides), specifically exhibited high antimicrobial activities against both Gram-negative and Gram-positive bacteria. In the current study we report on the in vitro cytotoxicity of the peptide, an important factor before it can be considered for further applications in cancer therapy. The cytotoxicity of epinecidin-1 was investigated against several cancer cells (A549, HA59T/VGH, HeLa, HepG2, HT1080, RAW264.7, and U937) and normal cells (AML-12, NIH3T3, and WS-1) with the MTT assay, and the inhibition of cancer cell growth was confirmed by a soft agar assay and scanning electron microscopy. However, cell variations were detected with AO/EtBr staining, while apoptosis and necrosis gene expressions in HT1080 cells after treatment with the epinecidin-1 peptide and Nec-1 showed that epinecidin-1 had an anti-necrosis function in HT1080 cells. The data presented here indicate that epinecidin-1 has in vitro antitumor activity against the HT1080 cell line, and functions like lytic peptides. In addition, our results suggest that epinecidin-1 may prove to be an effective chemotherapeutic agent for human fibrosarcoma cells in the future.

The p53-cathepsin axis cooperates with ROS to activate programmed necrotic death upon DNA damage

Three forms of cell death have been described: apoptosis, autophagic cell death, and necrosis. Although genetic and biochemical studies have formulated a detailed blueprint concerning the apoptotic network, necrosis is generally perceived as a passive cellular demise resulted from unmanageable physical damages. Here, we conclude an active de novo genetic program underlying DNA damage-induced necrosis, thus assigning necrotic cell death as a form of "programmed cell death." Cells deficient of the essential mitochondrial apoptotic effectors, BAX and BAK, ultimately succumbed to DNA damage, exhibiting signature necrotic characteristics. Importantly, this genotoxic stress-triggered necrosis was abrogated when either transcription or translation was inhibited. We pinpointed the p53-cathepsin axis as the quintessential framework underlying necrotic cell death. p53 induces cathepsin Q that cooperates with reactive oxygen species (ROS) to execute necrosis. Moreover, we presented the in vivo evidence of p53-activated necrosis in tumor allografts. Current study lays the foundation for future experimental and therapeutic discoveries aimed at "programmed necrotic death."

Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

BACKGROUND

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

REVIEW

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

CONCLUSION

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

Autophagic or necrotic cell death triggered by distinct motifs of the differentiation factor DIF-1

Autophagic or necrotic cell death (ACD and NCD, respectively), studied in the model organism Dictyostelium which offers unique advantages, require triggering by the same differentiation-inducing factor DIF-1. To initiate these two types of cell death, does DIF-1 act through only one or through two distinct recognition structures? Such distinct structures may recognize distinct motifs of DIF-1. To test this albeit indirectly, DIF-1 was modified at one or two of several positions, and the corresponding derivatives were tested for their abilities to induce ACD or NCD. The results strongly indicated that distinct biochemical motifs of DIF-1 were required to trigger ACD or NCD, and that these motifs were separately recognized at the onset of ACD or NCD. In addition, both ACD and NCD were induced more efficiently by DIF-1 than by either its precursors or its immediate catabolite. These results showed an unexpected relation between a differentiation factor, the cellular structures that recognize it, the cell death types it can trigger and the metabolic state of the cell. The latter seems to guide the choice of the signaling pathway to cell death, which in turn imposes the cell death type and the recognition pattern of the differentiation factor.

Activation and clinical significance of p38 MAPK signaling pathway in patients with severe trauma

BACKGROUND

Organ dysfunction or multiple organ dysfunction syndrome caused by developing immunological dysfunction and subsequent sepsis or the systemic inflammatory response syndrome after trauma is the leading cause of death in trauma patient. It is believed that mitogen-activated protein kinase) (p38MAPK) is one of the most important kinases in inflammatory signaling. In this study, the change of p38 MAPK signaling pathway in trauma patient with different severity and its clinical significance in trauma inflammation were investigated.

METHODS

One hundred fifty major trauma patients were included in the study and divided into three groups according to injury severity score (ISS). All data required to calculate ISS and determine organ function were registered on admission and during the ICU-stay. Peripheral blood samples were collected from trauma patients 6 h, 1 d, 3 d, 5 d, and 7 d after injury. RQ-PCR and Western blot was used to examine the changes in gene expression, protein expression, and activation level of leukocyte p38 MAPK. Plasma IL-6 and TNFalpha were assayed by ELISA.

RESULTS

Organ dysfunction in 33 trauma patients developed and eight deaths occurred after 24 h in ICU. The causes of death included severe ARDS, MODS, and irreversible brain injury. Incidence of organ dysfunction was related to the increase of injury severity (P < 0.01). Compared with healthy control, the gene expression of p38 MAPK in trauma patients increased significantly 6 h after injury (P < 0.05), and reached a maximum in 1 d (P < 0.01). The expression maintained a high level for 7 d (P < 0.05). One day after injury, significant elevation was observed in protein expression and activation level of p38 MAPK (P < 0.05), as well as the plasma TNFalpha and IL-6 level (P < 0.01). Further investigation found that the gene expression, protein expression, and activation levels of p38 MAPK increased with higher ISS (P < 0.05), and the elevation of plasma TNFalpha and IL-6 level was associated with the increase of activated p38 MAPK and ISS (P < 0.05).

CONCLUSION

p38 MAPK signal pathway was activated in trauma patients. The severity of trauma had highly positive correlation with the expression and activation of p38 MAPK, as well as the elevation of plasma TNFalpha and IL-6 expression. These findings indicate that p38 MAPK signaling pathway plays an important role in the pathological mechanism of trauma.

Autophagic cell death: the story of a misnomer

Dying cells often display a large-scale accumulation of autophagosomes and hence adopt a morphology called autophagic cell death. In many cases, it is agreed that this autophagic cell death is cell death with autophagy rather than cell death by autophagy. Here, we evaluate the accumulating body of literature that argues that cell death occurs by autophagy. We also list the caveats that must be considered when deciding whether or not autophagy is an important effector mechanism of cell death.

Oxidative stress and left ventricular remodelling after myocardial infarction

In acute myocardial infarction (MI), reactive oxygen species (ROS) are generated in the ischaemic myocardium especially after reperfusion. ROS directly injure the cell membrane and cause cell death. However, ROS also stimulate signal transduction to elaborate inflammatory cytokines, e.g. tumour necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta and -6, in the ischaemic region and surrounding myocardium as a host reaction. Inflammatory cytokines also regulate cell survival and cell death in the chain reaction with ROS. Both ROS and inflammatory cytokines are cardiodepressant mainly due to impairment of intracellular Ca(2+) homeostasis. Inflammatory cytokines stimulate apoptosis through a TNF-alpha receptor/caspase pathway, whereas Ca(2+) overload induced by extensive ROS generation causes necrosis through enhanced permeability of the mitochondrial membrane (mitochondrial permeability transition). Apoptosis signal-regulating kinase-1 (ASK1) is an ROS-sensitive, mitogen-activated protein kinase kinase kinase that is activated by many stress signals and can activate nuclear factor kappaB and other transcription factors. ASK1-deficient mice demonstrate that the ROS/ASK1 pathway is involved in necrotic as well as apoptotic cell death, indicating that ASK1 may be a therapeutic target to reduce left ventricular (LV) remodelling after MI. ROS and inflammatory cytokines activate matrix metalloproteinases which degrade extracellular matrix, causing a slippage of myofibrils and hence LV dilatation. Consequently, collagen deposition is increased and tissue repair is enhanced with myocardial fibrosis and angiogenesis. Since the extent of LV remodelling is a major predictor of prognosis of the patients with MI, the therapeutic approach to attenuating LV remodelling is critically important.

Lysosomal membrane permeabilization in cell death

Mitochondrial outer membrane permeabilization (MOMP) constitutes one of the major checkpoint(s) of apoptotic and necrotic cell death. Recently, the permeabilization of yet another organelle, the lysosome, has been shown to initiate a cell death pathway, in specific circumstances. Lysosomal membrane permeabilization (LMP) causes the release of cathepsins and other hydrolases from the lysosomal lumen to the cytosol. LMP is induced by a plethora of distinct stimuli including reactive oxygen species, lysosomotropic compounds with detergent activity, as well as some endogenous cell death effectors such as Bax. LMP is a potentially lethal event because the ectopic presence of lysosomal proteases in the cytosol causes digestion of vital proteins and the activation of additional hydrolases including caspases. This latter process is usually mediated indirectly, through a cascade in which LMP causes the proteolytic activation of Bid (which is cleaved by the two lysosomal cathepsins B and D), which then induces MOMP, resulting in cytochrome c release and apoptosome-dependent caspase activation. However, massive LMP often results in cell death without caspase activation; this cell death may adopt a subapoptotic or necrotic appearance. The regulation of LMP is perturbed in cancer cells, suggesting that specific strategies for LMP induction might lead to novel therapeutic avenues.

Real-time detection of the early event of cytotoxicity of herbal ingredients on single leukemia cells studied in a microfluidic biochip

A microfluidic approach has been developed for the real-time detection of drug effects, based on the quantitative measurement of calibrated cytosolic calcium ([Ca(2+)](i)) on single cancer cells. This microfluidic method is rapid by detecting the early event of cytotoxicity of drug candidates on cancer cells, without waiting for a couple of days needed for cell seeding and drug treatment by conventional assays. The miniaturized biochip consists of a V-shaped structure for the single-cell selection and retention. Various test reagents such as the chemotherapy drug (daunorubicin), an ionophore (ionomycin), and herbal ingredients from licorice (isoliquiritigenin or IQ) were investigated for their abilities to stimulate sustained cellular [Ca(2+)](i) elevations. The microfluidic results obtained in hours have been confirmed by conventional cytotoxicity assays which take days to complete. Moreover, any color or chemical interference problems found in the conventional assays of herbal compounds could be resolved. Using the microfluidic approach, IQ (50 microM) has been found to cause a sustained [Ca(2+)](i) elevation and cytotoxic effects on leukemia cells. The microfluidic single-cell analysis not only reduces reagent cost, and demands less cells, but also reveals some phenomena due to cellular heterogeneity that cannot be observed in bulk analysis.

Antigen-mediated T cell expansion regulated by parallel pathways of death

T cells enigmatically require caspase-8, an inducer of apoptosis, for antigen-driven expansion and effective antiviral responses, and yet the pathways responsible for this effect have been elusive. A defect in caspase-8 expression does not affect progression through the cell cycle but causes an abnormally high rate of cell death that is distinct from apoptosis and does not involve a loss of NFkappaB activation. Instead, antigen or mitogen activated Casp8-deficient T cells exhibit an alternative type of cell death similar to programmed necrosis that depends on receptor interacting protein (Ripk1). The selective genetic ablation of caspase-8, NFkappaB, and Ripk1, reveals two forms of cell death that can regulate virus-specific T cell expansion.

Bz-423 superoxide signals apoptosis via selective activation of JNK, Bak, and Bax

Bz-423 is a proapoptotic 1,4-benzodiazepine with potent therapeutic properties in murine models of lupus and psoriasis. Bz-423 modulates the F(1)F(0)-ATPase, inducing the formation of superoxide within the mitochondrial respiratory chain, which then functions as a second messenger initiating apoptosis. Herein, we report the signaling pathway activated by Bz-423 in mouse embryonic fibroblasts containing knockouts of key apoptotic proteins. Bz-423-induced superoxide activates cytosolic ASK1 and its release from thioredoxin. A mitogen-activated protein kinase cascade follows, leading to the specific phosphorylation of JNK. JNK signals activation of Bax and Bak which then induces mitochondrial outer membrane permeabilization to cause the release of cytochrome c and a commitment to apoptosis. The response of these cells to Bz-423 is critically dependent on both superoxide and JNK activation as antioxidants and the JNK inhibitor SP600125 prevents Bax translocation, cytochrome c release, and cell death. These results demonstrate that superoxide generated from the mitochondrial respiratory chain as a consequence of a respiratory transition can signal a sequential and specific apoptotic response. Collectively, these data suggest that the selectivity of Bz-423 observed in vivo results from cell-type specific differences in redox balance and signaling by ASK1 and Bcl-2 proteins.

Autophagic cell death: the story of a misnomer

Dying cells often display a large-scale accumulation of autophagosomes and hence adopt a morphology called autophagic cell death. In many cases, it is agreed that this autophagic cell death is cell death with autophagy rather than cell death by autophagy. Here, we evaluate the accumulating body of literature that argues that cell death occurs by autophagy. We also list the caveats that must be considered when deciding whether or not autophagy is an important effector mechanism of cell death.

Redox mechanisms in hepatic chronic wound healing and fibrogenesis

Reactive oxygen species (ROS) generated within cells or, more generally, in a tissue environment, may easily turn into a source of cell and tissue injury. Aerobic organisms have developed evolutionarily conserved mechanisms and strategies to carefully control the generation of ROS and other oxidative stress-related radical or non-radical reactive intermediates (that is, to maintain redox homeostasis), as well as to 'make use' of these molecules under physiological conditions as tools to modulate signal transduction, gene expression and cellular functional responses (that is, redox signalling). However, a derangement in redox homeostasis, resulting in sustained levels of oxidative stress and related mediators, can play a significant role in the pathogenesis of major human diseases characterized by chronic inflammation, chronic activation of wound healing and tissue fibrogenesis. This review has been designed to first offer a critical introduction to current knowledge in the field of redox research in order to introduce readers to the complexity of redox signalling and redox homeostasis. This will include ready-to-use key information and concepts on ROS, free radicals and oxidative stress-related reactive intermediates and reactions, sources of ROS in mammalian cells and tissues, antioxidant defences, redox sensors and, more generally, the major principles of redox signalling and redox-dependent transcriptional regulation of mammalian cells. This information will serve as a basis of knowledge to introduce the role of ROS and other oxidative stress-related intermediates in contributing to essential events, such as the induction of cell death, the perpetuation of chronic inflammatory responses, fibrogenesis and much more, with a major focus on hepatic chronic wound healing and liver fibrogenesis.

Necrotic cell death: From reversible mitochondrial uncoupling to irreversible lysosomal permeabilization

Dictyostelium atg1- mutant cells provide an experimentally and genetically favorable model to study necrotic cell death (NCD) with no interference from apoptosis or autophagy. In such cells subjected to starvation and cAMP, induction by the differentiation-inducing factor DIF or by classical uncouplers led within minutes to mitochondrial uncoupling, which causally initiated NCD. We now report that (1) in this model, NCD included a mitochondrial-lysosomal cascade of events, (2) mitochondrial uncoupling and therefore initial stages of death showed reversibility for a surprisingly long time, (3) subsequent lysosomal permeabilization could be demonstrated using Lysosensor blue, acridin orange, Texas red-dextran and cathepsin B substrate, (4) this lysosomal permeabilization was irreversible, and (5) the presence of the uncoupler was required to maintain mitochondrial lesions but also to induce lysosomal lesions, suggesting that signaling from mitochondria to lysosomes must be sustained by the continuous presence of the uncoupler. These results further characterized the NCD pathway in this priviledged model, contributed to a definition of NCD at the lysosomal level, and suggested that in mammalian NCD even late reversibility attempts by removal of the inducer may be of therapeutic interest.

Classification of cell death: recommendations of the Nomenclature Committee on Cell Death 2009

Different types of cell death are often defined by morphological criteria, without a clear reference to precise biochemical mechanisms. The Nomenclature Committee on Cell Death (NCCD) proposes unified criteria for the definition of cell death and of its different morphologies, while formulating several caveats against the misuse of words and concepts that slow down progress in the area of cell death research. Authors, reviewers and editors of scientific periodicals are invited to abandon expressions like 'percentage apoptosis' and to replace them with more accurate descriptions of the biochemical and cellular parameters that are actually measured. Moreover, at the present stage, it should be accepted that caspase-independent mechanisms can cooperate with (or substitute for) caspases in the execution of lethal signaling pathways and that 'autophagic cell death' is a type of cell death occurring together with (but not necessarily by) autophagic vacuolization. This study details the 2009 recommendations of the NCCD on the use of cell death-related terminology including 'entosis', 'mitotic catastrophe', 'necrosis', 'necroptosis' and 'pyroptosis'.

Cell death in heart failure

Heart failure (HF) has become the dominant cardiovascular disorder in the Western world and Japan, so there is an urgent need to clarify the mechanisms governing pathological remodeling mediated through cell death, and to identify ways of preventing and treating HF. Historically, there are 3 types of cell death: apoptosis, autophagy and necrosis. Apoptosis, a form of programmed cell death, has been well characterized and the molecular events involved in apoptotic death are well understood. Necrosis is often defined in a negative manner: death lacking the characteristics of programmed cell death and thus accidental and uncontrolled. However, recent studies indicate that necrosis is tightly regulated. Autophagy is a cell survival mechanism that involves degradation and recycling of cytoplasmic components. In contrast to the other 2 mechanisms, autophagy may mediate cell death under specific circumstances. In fact, damaged cardiomyocytes that show characteristics of autophagy have been observed during HF. However, a recent study indicated that upregulation of autophagy in the failing heart is an adaptive response. This review summarizes recent findings regarding the molecular mechanisms of cardiomyocyte cell death in HF.

Vibrio parahaemolyticus orchestrates a multifaceted host cell infection by induction of autophagy, cell rounding, and then cell lysis

The bacterial pathogen Vibrio parahaemolyticus utilizes a type III secretion system to cause death of host cells within hours of infection. We report that cell death is completely independent of apoptosis and occurs by a mechanism in which injection of multiple type III effectors causes induction of autophagy, cell rounding, and the subsequent release of cellular contents. Autophagy is detected by the appearance of lipidated light chain 3 (LC3) and by increases in punctae and vacuole formation. Electron microscopy reveals the production of early autophagic vesicles during infection. Consistent with phosphoinositide 3 (PI3) kinase playing a role in autophagy, treatment of infected cells with a PI3 kinase inhibitor attenuates autophagy in infected cells. Because many effectors are injected during a V. parahaemolyticus infection, it is not surprising that the presence of a sole PI3 kinase inhibitor does not prevent inevitable host-cell death. Our studies reveal an infection paradigm whereby an extracellular pathogen uses its type III secretion system to cause at least three parallel events that eventually result in the proinflammatory death of an infected host cell.

Molecular regulation of inflammation and cell death

Cell death and innate immunity are ancient evolutionary conserved processes that utilize a dazzling number of related molecular effectors and parallel signal transduction mechanisms. The investigation of the molecular mechanisms linking the sensing of a danger signal (pathogens or tissue damage) to the induction of an inflammatory response has witnessed a renaissance in the last few years. This was initiated by the identification of pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and more recently cytosolic Nod-like receptors (NLRs), that brought innate immunity to center stage and opened the field to the study of signal transduction pathways, adaptors and central effectors linked to PRRs. This led to the characterization of the inflammasome, a macromolecular complex, scaffolded by NLRs, that recruits and activates inflammatory caspases, which are essential effectors in inflammation and cell death responses. In this review, we describe the molecular pathways of cell death and innate immunity with a focus on recent advancements in both fields and an emphasis on the striking analogies between NLR innate immunity and mitochondrial apoptosis pathways.

Necrotic cell death and 'necrostatins': now we can control cellular explosion

The receptor-interacting protein 1 (RIP1) kinase activity is necessary for death-receptor-induced necrotic cell death. Recently, it has been demonstrated that 'necrostatins' efficiently block tumor necrosis factor-induced necrotic cell death through the inhibition of RIP1 kinase activity. This discovery supports the concept that receptor-induced necrosis, just like apoptosis, is a controlled cellular process. In addition, necrostatins are becoming important tools for evaluating the contribution of necrotic cell death in experimental disease models.

TNFalpha and reactive oxygen species in necrotic cell death

Death receptors, including the TNF receptor-1 (TNF-RI), have been shown to be able to initiate caspase-independent cell death. This form of "necrotic cell death" appears to be dependent on the generation of reactive oxygen species. Recent data have indicated that superoxide generation is dependent on the activation of NADPH oxidases, which form a complex with the adaptor molecules RIP1 and TRADD. The mechanism of superoxide generation further establishes RIP1 as the central molecule in ROS production and cell death initiated by TNFalpha and other death receptors. A role for the sustained JNK activation in necrotic cell death is also suggested. The sensitization of virus-infected cells to TNFalpha indicates that necrotic cell death may represent an alternative cell death pathway for clearance of infected cells.

ATP depletion alters the mode of cell death induced by benzyl isothiocyanate

Pro-inflammatory death is presumably an undesirable event in cancer prevention process, thus biochemical comprehension and molecular definition of this process could have important clinical implications. In the present study, we examined the cytophysiological conversion of cell death mode by benzyl isothiocyanate (BITC) in human cervical cancer HeLa cells. The detailed studies using flow cytometric and morphological analyses demonstrated that the cells treated with appropriate concentration (25 microM) of BITC showed apoptotic feature, such as chromatin condensation, DNA fragmentation, and preserved plasma membrane integrity, whereas these features were disappeared by treatment with higher concentration (100 microM). The treatment with 2-deoxyglucose, an inhibitor of ATP synthesis, drastically increased in the ratio of necrotic dead cells, while it influences little that of apoptotic cells. Moreover, an analysis using the mitochondrial DNA-deficient HeLa cells demonstrated that the rho degrees cells were more susceptible to the BITC-induced necrosis-like cell death compared to the wild-type (rho(+)) cells, whereas the ROS production was significantly inhibited in the rho degrees cells. It is likely that the BITC-induced ROS is derived from mitochondrial respiratory chain and ruled out the contribution to the mechanism of cell death mode switching. In addition, the BITC treatment resulted in a more rapid depletion of ATP in the rho degrees cells than in the rho(+) cells. Furthermore, a caspase inhibitor, Z-VAD-fmk counteracted not only apoptosis, but also necrosis-like cell death induced by BITC, suggesting that increment in this cell death pattern might be due to the interruption of events downstream of a caspase-dependent pathway. The obtained data suggested that the decline in the intracellular ATP level plays an important role in tuning the mode of cell death by BITC.

Preferential cytolysis of peripheral memory CD4+ T cells by in vitro X4-tropic human immunodeficiency virus type 1 infection before the completion of reverse transcription

CD4+ T-cell depletion is the hallmark of AIDS pathogenesis. Multiple mechanisms may contribute to the death of productively infected CD4+ T cells and innocent-bystander cells. In this study, we characterize a novel mechanism in which human immunodeficiency virus type 1 (HIV-1) infection preferentially depletes peripheral memory CD4+ T cells before the completion of reverse transcription. Using a recombinant HIV-1 carrying the green fluorescent protein reporter gene, we demonstrate that memory CD4+ T cells were susceptible to infection-induced cell death at a low multiplicity of infection. Infected memory CD4+ T cells underwent rapid necrotic cell death. Killing of host cells was dependent on X4 envelope-mediated viral fusion, but not on virion-associated Vpr or Nef. In contrast to peripheral resting CD4+ T cells, CD4+ T cells stimulated by mitogen or certain cytokines were resistant to HIV-1-induced early cell death. These results demonstrate that early steps in HIV-1 infection have a detrimental effect on certain subsets of CD4+ T cells. The early cell death may serve as a selective disadvantage for X4-tropic HIV-1 in acute infection but may play a role in accelerated disease progression, which is associated with the emergence of X4-tropic HIV-1 in the late stage of AIDS.

Treatment of PC-3 and DU145 prostate cancer cells by prenylflavonoids from hop (Humulus lupulus L.) induces a caspase-independent form of cell death

Xanthohumol (X), isoxanthohumol (IX), 8-prenylnaringenin (8PN) and 6-prenylnaringenin (6PN), prenylflavonoids from hop (Humulus lupulus L.), were investigated for their cytotoxicity and the mechanism by which they exert cell death when incubated with prostate cancer cell lines PC-3 and DU145. All compounds induced cell death in the absence of caspase-3 activation and typical apoptotic morphological features. The general pan-caspase inhibitor zVAD-fmk could not protect this form of cell death. In addition, the formation of vacuoles was observed in PC-3 cells treated with IX and 6PN, and in DU145 treated with IX, 8PN and 6PN, which could suggest the induction of autophagy and consequent cell death. The results indicate that hop-derived prenylflavanones (IX, 8PN, 6PN), but not prenylchalcones (X) induce a caspase-independent form of cell death, suggested to be autophagy. Therefore, IX, 8PN and 6PN appear to be promising candidates for further investigation in prostate anticancer therapy.

Extracellular signal-regulated kinase activation mediates mitochondrial dysfunction and necrosis induced by hydrogen peroxide in renal proximal tubular cells

Although tubular necrosis in acute renal failure is associated with excessive production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), the mechanism of ROS-induced cell necrosis remains poorly understood. In this study, we examined the role of the extracellular signaling-regulated kinase (ERK) pathway in H2O2-induced necrosis of renal proximal tubular cells (RPTC) in primary culture. Exposure of 60 to 70% confluent RPTC to 1 mM H2O2 for 3 h resulted in 44% necrotic cell death, as measured by trypan blue uptake, and inactivation of mitogen-activated protein kinase kinase (MEK), the upstream activator of ERK, by either 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (U0126) or 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059) or overexpression of dominant-negative mutant of MEK1, inhibited cell death. In contrast, overexpression of active MEK1 enhanced H2O2-induced cell death. H2O2 treatment led to the loss of mitochondrial membrane potential (MMP) in RPTC, which was decreased by U0126 and PD98059. Furthermore, inhibition of the MEK/ERK pathway decreased oxidant-mediated ERK1/2 activation and mitochondrial swelling in isolated renal cortex mitochondria. However, treatment with cyclosporin A (CsA), a mitochondrial permeability transition blocker, did not suppress RPTC necrotic cell death, loss of MMP, and mitochondrial swelling. We suggest that ERK is a critical mediator of mitochondrial dysfunction and necrotic cell death of renal epithelial cells following oxidant injury. Oxidant-induced necrotic cell death was mediated by a CsA-insensitive loss of MMP that is regulated by the ERK pathway.

NOX enzymes as novel targets for drug development

The members of the NOX/DUOX family of NADPH oxidases mediate such physiologic functions as host defense, cell signaling, and thyroid hormone biosynthesis through the generation of reactive oxygen species (ROS), including superoxide anion and hydrogen peroxide. Moreover, ROS are involved in a broad range of fundamental biochemical and cellular processes, and data accumulated in recent years indicate that the NOX enzymes comprise one of the most important biological sources of ROS. Given the high biochemical reactivity of ROS, it is not surprising that they have been implicated in a wide variety of pathologies and diseases. Prominent among the settings that feature ROS-mediated tissue injury are disorders associated with inflammation, aging, and progressive degenerative changes in cells and organ systems, and it appears that essentially no organ system is exempt. Among the disorders currently believed to be mediated at least in part by NOX-derived ROS are hypertension, aortic aneurysm, myocardial infarction (and other ischemia-reperfusion disorders), pulmonary fibrosis and hypertension, amyotropic lateral sclerosis, Alzheimer's disease, Parkinson's disease, ischemic stroke, diabetic nephropathy, and renal cell carcinoma. Several small-molecule and peptide inhibitors of the NOX enzymes have been useful in experimental studies, but issues of specificity, potency, and toxicity militate against any of the existing published compounds as candidates for drug development. Given the broad array of disease targets documented in recent work, the time is here for vigorous efforts to develop clinically useful inhibitors of the NOX enzymes. As most (though not all) NOX-related diseases appear to be mediated by a single member of the NOX family, agents with isoform specificity will be preferred, although broadly active NOX inhibitors may prove to be useful in some settings.

Regulation of H(2)O(2)-induced necrosis by PKC and AMP-activated kinase signaling in primary cultured hepatocytes

Recent studies have suggested that, in certain cases, necrosis, like apoptosis, may be programmed, involving the activation and inhibition of many signaling pathways. In this study, we examined whether necrosis induced by H(2)O(2) is regulated by signaling pathways in primary hepatocytes. A detailed time course revealed that H(2)O(2) treated to hepatocytes is consumed within minutes, but hepatocytes undergo necrosis several hours later. Thus, H(2)O(2) treatment induces a "lag phase" where signaling changes occur, including PKC activation, Akt (PKB) downregulation, activation of JNK, and downregulation of AMP-activated kinase (AMPK). Investigation of various inhibitors demonstrated that PKC inhibitors were effective in reducing necrosis caused by H(2)O(2) (~80%). PKC inhibitor treatment decreased PKC activity but, surprisingly, also upregulated Akt and AMPK, suggesting that various PKC isoforms negatively regulate Akt and AMPK. Akt did not appear to play a significant role in H(2)O(2)-induced necrosis, since PKC inhibitor treatment protected hepatocytes from H(2)O(2) even when Akt was inhibited. On the other hand, compound C, a selective AMPK inhibitor, abrogated the protective effect of PKC inhibitors against necrosis induced by H(2)O(2). Furthermore, AMPK activators protected against H(2)O(2)-induced necrosis, suggesting that much of the protective effect of PKC inhibition was mediated through the upregulation of AMPK. Work with PKC inhibitors suggested that atypical PKC downregulates AMPK in response to H(2)O(2). Knockdown of PKC-alpha using antisense oligonucleotides also slightly protected (~22%) against H(2)O(2). Taken together, our data demonstrate that the modulation of signaling pathways involving PKC and AMPK can alter H(2)O(2)-induced necrosis, suggesting that a signaling "program" is important in mediating H(2)O(2)-induced necrosis in primary hepatocytes.

Expansion and evolution of cell death programmes

Cell death has historically been subdivided into regulated and unregulated mechanisms. Apoptosis, a form of regulated cell death, reflects a cell's decision to die in response to cues and is executed by intrinsic cellular machinery. Unregulated cell death (often called necrosis) is caused by overwhelming stress that is incompatible with cell survival. Emerging evidence, however, suggests that these two processes do not adequately explain the various cell death mechanisms. Recent data point to the existence of multiple non-apoptotic, regulated cell death mechanisms, some of which overlap or are mutually exclusive with apoptosis. Here we examine how and why these different cell death programmes have evolved, with an eye towards new cytoprotective therapeutic opportunities.