Molecular definitions of cell death subroutines: recommendations of the Nomenclature Committee on Cell Death 2012

Structural basis of signal transduction in the TNF receptor superfamily

Members of the tumor necrosis factor receptor superfamily play key roles in innate and adaptive immunity. Here, we review recent structural studies in the intracellular signal transduction of these receptors. A central theme revealed from these structural studies is that upon ligand binding, multiple intracellular proteins form higher-order signaling machines to transduce and amplify receptor activation information to different cellular fates, including NF-κB activation, apoptosis, and programmed necrosis. These studies open a new vista for understanding the biophysical principles in these signaling cascades.

Roles of poly(ADP-ribose) glycohydrolase in DNA damage and apoptosis

Poly(ADP-ribose) glycohydrolase (PARG) is the primary enzyme that catalyzes the hydrolysis of poly(ADP-ribose) (PAR), an essential biopolymer that is synthesized by poly(ADP-ribose) polymerases (PARPs) in the cell. By regulating the hydrolytic arm of poly(ADP-ribosyl)ation, PARG participates in a number of biological processes, including the repair of DNA damage, chromatin dynamics, transcriptional regulation, and cell death. Collectively, the research investigating the roles of PARG in the cell has identified the importance of PARG and its value as a therapeutic target. However, the biological role of PARG remains less understood than the role of PAR synthesis by the PARPs. Further complicating the study of PARG is the existence of multiple PARG isoforms in the cell, the lack of optimal PARG inhibitors, and the lack of viable PARG-null animals. This review will present our current knowledge of PARG, with a focus on its roles in DNA-damage repair and cell death.

Immunogenic cell death: can it be exploited in PhotoDynamic Therapy for cancer?

Immunogenic Cell Death (ICD) could represent the keystone in cancer management since tumor cell death induction is crucial as well as the control of cancer cells revival after neoplastic treatment. In this context, the immune system plays a fundamental role. The concept of Damage-Associated Molecular Patterns (DAMPs) has been proposed to explain the immunogenic potential of stressed or dying/dead cells. ICD relies on DAMPs released by or exposed on dying cells. Once released, DAMPs are sensed by immune cells, in particular Dendritic Cells (DCs), acting as activators of Antigen-Presenting Cells (APCs), that in turn stimulate both innate and adaptive immunity. On the other hand, by exposing DAMPs, dying cancer cells change their surface composition, recently indicated as vital for the stimulation of the host immune system and the control of residual ill cells. It is well established that PhotoDynamic Therapy (PDT) for cancer treatment ignites the immune system to elicit a specific antitumor immunity, probably linked to its ability in inducing exposure/release of certain DAMPs, as recently suggested. In the present paper, we discuss the DAMPs associated with PDT and their role in the crossroad between cancer cell death and immunogenicity in PDT.

TPEN induces apoptosis independently of zinc chelator activity in a model of acute lymphoblastic leukemia and ex vivo acute leukemia cells through oxidative stress and mitochondria caspase-3- and AIF-dependent pathways

Acute lymphoblastic leukemia is still an incurable disease with resistance to therapy developing in the majority of patients. We investigated the effect of TPEN, an intracellular zinc chelator, in Jurkat and in ex vivo acute lymphoblastic leukemia (ALL) cells resistant to chemotherapy. Changes of nuclei morphology, reactive oxygen species generation, presence of hypodiploid cells, phosphatidylserine translocation, mitochondrial membrane depolarization, immunohistochemical identification of cell death signalling molecules, and pharmacological inhibition were assayed to detect the apoptotic cell death pathways. We found that TPEN induces apoptosis in both types of cells by a molecular oxidative stress pathway involving O(2)(•-) > H(2)O(2) >> NF-κB (JNK/c-Jun) >p53> loss ΔΨ(m)> caspase-3, AIF > chromatin condensation/DNA fragmentation. Interestingly, TPEN induced apoptosis independently of glucose; leukemic cells are therefore devoid of survival capacity by metabolic resistance to treatment. Most importantly, TPEN cytotoxic effect can eventually be regulated by the antioxidant N-acetyl-cysteine and zinc ions. Our data suggest that TPEN can be used as a potential therapeutic prooxidant agent against refractory leukemia. These data contribute to understanding the importance of oxidative stress in the treatment of ALL.

Targeting c-FLICE-like inhibitory protein (CFLAR) in cancer

INTRODUCTION

Evasion of apoptosis (programmed cell death) is one of the characteristic hallmarks of human cancers and may be caused by aberrant expression of antiapoptotic proteins. Among those is c-FLICE-like inhibitory protein (c-FLIP), a protein that not only blocks apoptosis signaling but also regulates additional cell death pathways.

AREAS COVERED

Because c-FLIP is regulated both at the transcriptional and posttranscriptional level by various mechanisms and is a short-lived protein with a rapid turnover, the regulation of c-FLIP expression represents a versatile tool to modulate cell death signaling pathways. Because c-FLIP is aberrantly expressed in various cancers, it represents a promising target for therapeutic intervention.

EXPERT OPINION

Therefore, insights into the molecular events that regulate c-FLIP expression and activity in human cancers will provide the basis for the development of new strategies to target c-FLIP expression in human cancers.

Transgenerational cell fate profiling: a method for the graphical presentation of complex cell cycle alterations

The illicit generation of tetraploid cells constitutes a prominent driver of oncogenesis, as it often precedes the development of aneuploidy and genomic instability. In addition, tetraploid (pre-)malignant cells display an elevated resistance against radio- and chemotherapy. Here, we report a strategy to preferentially kill tetraploid tumor cells based on the broad-spectrum kinase inhibitor SP600125. Live videomicroscopy revealed that SP600125 affects the execution of mitosis, impedes proper cell division and/or activates apoptosis in near-to-tetraploid, though less so in parental, cancer cells. We propose a novel graphical model to quantify the differential response of diploid and tetraploid cells to mitotic perturbators, including SP600125, which we baptized "transgenerational cell fate profiling." We speculate that this representation constitutes a valid alternative to classical "single-cell fate" and "genealogical" profiling and, hence, may facilitate the analysis of cell fate within a heterogeneous population as well as the visual examination of cell cycle alterations.

TRPM7, the cytoskeleton and neuronal death

Ischemic stroke is one of the leading causes of disability and death in the world. Elucidation of the underlying mechanisms associated with neuronal death during this detrimental process has been of significant interest in the field of research. One principle component vital to the maintenance of cellular integrity is the cytoskeleton. Studies suggest that abnormalities at the level of this fundamental structure are directly linked to adverse effects on cellular well-being, including cell death. In recent years, evidence has also emerged regarding an imperative role for the transient receptor potential (TRP) family member TRPM7 in the mediation of excitotoxic-independent neuronal demise. In this review, we will elaborate on the current knowledge and unique properties associated with the functioning of this structure. In addition, we will deliberate the involvement of distinct mechanistic pathways during TRPM7-dependent cell death, including modifications at the level of the cytoskeleton.

Glutathione efflux and cell death

SIGNIFICANCE

Glutathione (GSH) depletion is a central signaling event that regulates the activation of cell death pathways. GSH depletion is often taken as a marker of oxidative stress and thus, as a consequence of its antioxidant properties scavenging reactive species of both oxygen and nitrogen (ROS/RNS).

RECENT ADVANCES

There is increasing evidence demonstrating that GSH loss is an active phenomenon regulating the redox signaling events modulating cell death activation and progression.

CRITICAL ISSUES

In this work, we review the role of GSH depletion by its efflux, as an important event regulating alterations in the cellular redox balance during cell death independent from oxidative stress and ROS/RNS formation. We discuss the mechanisms involved in GSH efflux during cell death progression and the redox signaling events by which GSH depletion regulates the activation of the cell death machinery.

FUTURE DIRECTIONS

The evidence summarized here clearly places GSH transport as a central mechanism mediating redox signaling during cell death progression. Future studies should be directed toward identifying the molecular identity of GSH transporters mediating GSH extrusion during cell death, and addressing the lack of sensitive approaches to quantify GSH efflux.

TLR activation regulates damage-associated molecular pattern isoforms released during pyroptosis

Infection of macrophages by bacterial pathogens can trigger Toll-like receptor (TLR) activation as well as Nod-like receptors (NLRs) leading to inflammasome formation and cell death dependent on caspase-1 (pyroptosis). Complicating the study of inflammasome activation is priming. Here, we develop a priming-free NLRC4 inflammasome activation system to address the necessity and role of priming in pyroptotic cell death and damage-associated molecular pattern (DAMP) release. We find pyroptosis is not dependent on priming and when priming is re-introduced pyroptosis is unaffected. Cells undergoing unprimed pyroptosis appear to be independent of mitochondrial involvement and do not produce inflammatory cytokines, nitrous oxide (NO), or reactive oxygen species (ROS). Nevertheless, they undergo an explosive cell death releasing a chemotactic isoform of the DAMP high mobility group protein box 1 (HMGB1). Importantly, priming through surface TLRs but not endosomal TLRs during pyroptosis leads to the release of a new TLR4-agonist cysteine redox isoform of HMGB1. These results show that pyroptosis is dominant to priming signals and indicates that metabolic changes triggered by priming can affect how cell death is perceived by the immune system.

Oxidative stress and pulmonary fibrosis

Oxidative stress is implicated as an important molecular mechanism underlying fibrosis in a variety of organs, including the lungs. However, the causal role of reactive oxygen species (ROS) released from environmental exposures and inflammatory/interstitial cells in mediating fibrosis as well as how best to target an imbalance in ROS production in patients with fibrosis is not firmly established. We focus on the role of ROS in pulmonary fibrosis and, where possible, highlight overlapping molecular pathways in other organs. The key origins of oxidative stress in pulmonary fibrosis (e.g. environmental toxins, mitochondria/NADPH oxidase of inflammatory and lung target cells, and depletion of antioxidant defenses) are reviewed. The role of alveolar epithelial cell (AEC) apoptosis by mitochondria- and p53-regulated death pathways is examined. We emphasize an emerging role for the endoplasmic reticulum (ER) in pulmonary fibrosis. After briefly summarizing how ROS trigger a DNA damage response, we concentrate on recent studies implicating a role for mitochondrial DNA (mtDNA) damage and repair mechanisms focusing on 8-oxoguanine DNA glycosylase (Ogg1) as well as crosstalk between ROS production, mtDNA damage, p53, Ogg1, and mitochondrial aconitase (ACO2). Finally, the association between ROS and TGF-β1-induced fibrosis is discussed. Novel insights into the molecular basis of ROS-induced pulmonary diseases and, in particular, lung epithelial cell death may promote the development of unique therapeutic targets for managing pulmonary fibrosis as well as fibrosis in other organs and tumors, and in aging; diseases for which effective management is lacking. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Nanomaterials toxicity and cell death modalities

In the last decade, the nanotechnology advancement has developed a plethora of novel and intriguing nanomaterial application in many sectors, including research and medicine. However, many risks have been highlighted in their use, particularly related to their unexpected toxicity in vitro and in vivo experimental models. This paper proposes an overview concerning the cell death modalities induced by the major nanomaterials.

Copper-dependent cytotoxicity of 8-hydroxyquinoline derivatives correlates with their hydrophobicity and does not require caspase activation

This study reports the structure-activity relationship of a series of 8-hydroxoquinoline derivatives (8-HQs) and focuses on the cytotoxic activity of 5-Cl-7-I-8-HQ (clioquinol, CQ) copper complex (Cu(CQ)). 8-HQs alone cause a dose-dependent loss of viability of the human tumor HeLa and PC3 cells, but the coadministration of copper increases the ligands effects, with extensive cell death occurring in both cell lines. Cytotoxic doses of Cu(CQ) promote intracellular copper accumulation and massive endoplasmic reticulum vacuolization that precede a nonapoptotic (paraptotic) cell death. The cytotoxic effect of Cu(CQ) is reproduced in normal human endothelial cells (HUVEC) at concentrations double those effective in tumor cells, pointing to a potential therapeutic window for Cu(CQ). Finally, the results show that the paraptotic cell death induced by Cu(CQ) does not require nor involve caspases, giving an indication for the current clinical assessment of clioquinol as an antineoplastic agent.

Neutrophils come of age in chronic inflammation

Neutrophils have long been known to participate in acute inflammation, but a role in chronic inflammatory and autoimmune diseases is now emerging. These cells are key players in the recognition and elimination of pathogens, but they also sense self components, including nucleic acids and products of sterile tissue damage. While this normally contributes to tissue repair, it can also lead to the release of highly immunogenic products that can trigger and/or amplify autoimmune pathogenic loops. Understanding the mechanisms that underlie neutrophil activation, migration, survival and their various forms of death in health and disease might provide us with new approaches to treat chronic inflammatory conditions.

Therapeutic approaches to preventing cell death in Huntington disease

Neurodegenerative diseases affect the lives of millions of patients and their families. Due to the complexity of these diseases and our limited understanding of their pathogenesis, the design of therapeutic agents that can effectively treat these diseases has been challenging. Huntington disease (HD) is one of several neurological disorders with few therapeutic options. HD, like numerous other neurodegenerative diseases, involves extensive neuronal cell loss. One potential strategy to combat HD and other neurodegenerative disorders is to intervene in the execution of neuronal cell death. Inhibiting neuronal cell death pathways may slow the development of neurodegeneration. However, discovering small molecule inhibitors of neuronal cell death remains a significant challenge. Here, we review candidate therapeutic targets controlling cell death mechanisms that have been the focus of research in HD, as well as an emerging strategy that has been applied to developing small molecule inhibitors-fragment-based drug discovery (FBDD). FBDD has been successfully used in both industry and academia to identify selective and potent small molecule inhibitors, with a focus on challenging proteins that are not amenable to traditional high-throughput screening approaches. FBDD has been used to generate potent leads, pre-clinical candidates, and has led to the development of an FDA approved drug. This approach can be valuable for identifying modulators of cell-death-regulating proteins; such compounds may prove to be the key to halting the progression of HD and other neurodegenerative disorders.

AMPK mediates a pro-survival autophagy downstream of PARP-1 activation in response to DNA alkylating agents

In this study we aim to elucidate the signaling pathway and biological function of autophagy induced by MNNG, a commonly used DNA alkylating agent. We first observed that MNNG is able to induce necrotic cell death and autophagy in Bax-/- Bak-/- double knockout MEFs. We analyzed the critical role of PARP-1 activation and ATP depletion in MNNG-mediated cell death and autophagy via AMPK activation and mTOR suppression. We provide evidence that suppression of AMPK blocks MNNG-induced autophagy and enhances cell death, suggesting the pro-survival function of autophagy in MNNG-treated cells. Taken together, data from this study reveal a novel mechanism in controlling MNNG-mediated autophagy via AMPK activation downstream of PARP-1 activation and ATP depletion.

Transitory phases of autophagic death and programmed necrosis during superoxide-induced neuronal cell death

Neurons can undergo a diverse range of death responses under oxidative stress, encompassing apoptosis (caspase-dependent, programmed cell death) to various forms of caspase-independent death, including necrosis. We recently showed that primary murine cortical neurons exposed acutely to hydrogen peroxide undergo caspase-independent death, both autophagic cell death and programmed necrosis. To determine how oxidative stress induced by superoxide affects the route to cellular demise, we exposed primary cortical neurons to extended superoxide insult (provided by exogenous xanthine and xanthine oxidase in the presence of catalase). Under these conditions, over 24h, the nitroblue tetrazolium-reducing activity (indicative of superoxide) rose significantly during the first 4 to 8h and then declined to background levels. As with hydrogen peroxide, this superoxide insult failed to activate downstream caspases (-3, -7, and -9). Substantial depolarization of mitochondria occurred after 1h, and nuclear morphology changes characteristic of oxidative stress became maximal after 2h. However, death indicated by plasma membrane permeabilization (cellular uptake of propidium iodide) approached maximal levels only after 4h, at which time substantial redistribution to the cytosol of death-associated mitochondrial intermembrane space proteins, notably endonuclease G, had occurred. Applying established criteria for autophagic death (knockdown of Atg7) or programmed necrosis (knockdown of endonuclease G), cells treated with the relevant siRNA showed significant blockade of each type of cell death, 4h after onset of the superoxide flux. Yet at later times, siRNA-mediated knockdown failed to prevent death, monitored by cellular uptake of propidium iodide. We conclude that superoxide initially invokes a diverse programmed caspase-independent death response, involving transient manifestation in parallel of autophagic death and programmed necrosis. Ultimately most neurons become overwhelmed by the consequences of severe oxidative stress and die. This study reveals the multiple phases of neuronal cell death modalities under extended oxidative stress.

Design and synthesis of a luminescent cyclometalated iridium(III) complex having N,N-diethylamino group that stains acidic intracellular organelles and induces cell death by photoirradiation

Cyclometalated iridium(III) complexes have received considerable attention and are important candidates for use as luminescent probes for cellular imaging because of their potential photophysical properties. We previously reported that fac-Ir(atpy)(3)4 (atpy = 2-(5'-amino-4'-tolyl)pyridine) containing three amino groups at the 5'-position of the atpy ligand shows a maximum red emission (at around 600 nm) under neutral and basic conditions and a green emission (at 531 nm) at acidic pH (pH 3-4). In this Article, we report on the design and synthesis of a new pH-sensitive cyclometalated Ir(III) complex containing a 2-(5'-N,N-diethylamino-4'-tolyl)pyridine (deatpy) ligand, fac-Ir(deatpy)(3)5. The complex exhibits a considerable change in emission intensity between neutral and slightly acidic pH (pH 6.5-7.4). Luminescence microscopic studies using HeLa-S3 cells indicate that 5 can be used to selectively stain lysosome, an acidic organelle in cells. Moreover, complex 5 is capable of generating singlet oxygen in a pH-dependent manner and inducing the death of HeLa-S3 cells upon photoirradiation at 377 or 470 nm.

In vitro approach to predict post-translational phosphorylation response to mixtures

While exposure to chemical mixtures is an everyday reality, an understanding of their combined effects, and any potential prediction thereof, is extremely limited. Realistic exposures potentially consist of hundreds to thousands of chemicals per day, but even relatively simple binary mixture interactions can be inherently difficult to predict based upon the lack of temporal and spatial mechanisms for the individual constituents. To this end, we explore the concept of capitalizing on downstream convergence of intracellular signal transduction to experimentally simplify the means of determining xenobiotics that, when combined, could result in increased or unexpected toxicity. In a proof of principle study, we exposed HepG2 cells to deguelin, a natural isoflavonoid, alone and in combination with KCN, and determined the relative post-translational phosphorylation responses to several key proteins related to mitochondrial outer membrane permeabilization. Dose-dependent phosphorylation activity provides a clear identification of threshold response to low-level exposures, and crosstalk amongst selected proteins correctly forecasts mixtures interactions that may lead to increased toxicity. We then used Bliss Independence to determine if the experimentally measured mixture phosphorylation responses could be predicted with individual responses. Independence accurately predicted mixture interactions for deguelin and KCN (87.5%). To more fully exhaust independence as a model for determining potential pharmacodynamic interactions, we exposed HepG2 cells to deguelin and staurosporine, a broad kinase inhibitor; independence accurately predicted these mixture responses (77.5%). In this study, we demonstrate the potential of a new in vitro approach for the prediction of toxic mixtures interactions that is fundamentally driven by the interdependence of signal transduction and apoptosis.

Membrane trafficking in neuronal maintenance and degeneration

Defects in membrane trafficking and degradation are hallmarks of most, and maybe all, neurodegenerative disorders. Such defects typically result in the accumulation of undegraded proteins due to aberrant endosomal sorting, lysosomal degradation, or autophagy. The genetic or environmental cause of a specific disease may directly affect these membrane trafficking processes. Alternatively, changes in intracellular sorting and degradation can occur as cellular responses of degenerating neurons to unrelated primary defects such as insoluble protein aggregates or other neurotoxic insults. Importantly, altered membrane trafficking may contribute to the pathogenesis or indeed protect the neuron. The observation of dramatic changes to membrane trafficking thus comes with the challenging need to distinguish pathological from protective alterations. Here, we will review our current knowledge about the protective and destructive roles of membrane trafficking in neuronal maintenance and degeneration. In particular, we will first focus on the question of what type of membrane trafficking keeps healthy neurons alive in the first place. Next, we will discuss what alterations of membrane trafficking are known to occur in Alzheimer's disease and other tauopathies, Parkinson's disease, polyQ diseases, peripheral neuropathies, and lysosomal storage disorders. Combining the maintenance and degeneration viewpoints may yield insight into how to distinguish when membrane trafficking functions protectively or contributes to degeneration.

Role of apoptosis in disease

Since the initial description of apoptosis, a number of different forms of cell death have been described. In this review we will focus on classic caspase-dependent apoptosis and its variations that contribute to diseases. Over fifty years of research have clarified molecular mechanisms involved in apoptotic signaling as well and shown that alterations of these pathways lead to human diseases. Indeed both reduced and increased apoptosis can result in pathology. More recently these findings have led to the development of therapeutic approaches based on regulation of apoptosis, some of which are in clinical trials or have entered medical practice.

New components of the necroptotic pathway

Programmed necrosis, also known as necroptosis, has recently drawn great attention. As an important cellular regulation mechanism, knowledge of its signaling components is expanding. Necroptosisis demonstrated to be regulated by the RIP1 and RIP3 kinases, and its pathophysiological importance has been confirmed in a number of disease models. Here we review the new members of this necroptosis pathway, MLKL, PGAM5, Drp1 and DAI, and discuss some of their possible applications according to recent findings.

CHK1 cleavage in programmed cell death is intricately regulated by both caspase and non-caspase family proteases

BACKGROUND

CHK1 is an important effector kinase that regulates the cell cycle checkpoint. Previously, we showed that CHK1 is cleaved in a caspase (CASP)-dependent manner during DNA damage-induced programmed cell death (PCD) and have examined its physiological roles.

METHODS AND RESULTS

In this study, we investigated the behavior of CHK1 in PCD. Firstly, we found that CHK1 is cleaved at three sites in PCD, and all cleavages were inhibited by the co-treatment of a pan-CASP inhibitor or serine protease inhibitors. We also showed that CHK1 is cleaved by CASP3 and/or CASP7 recognizing at (296)SNLD(299) and (348)TCPD(351), and that the cleavage results in the enhancement of CHK1 kinase activity. Furthermore, as a result of the characterization of cleavage sites by site-directed mutagenesis and an analysis performed using deletion mutants, we identified (320)EPRT(323) as an additional cleavage recognition sequence. Considering the consensus sequence cleaved by CASP, it is likely that CHK1 is cleaved by non-CASP family protease(s) recognizing at (320)EPRT(323). Additionally, the cleavage catalyzed by the (320)EPRT(323) protease(s) markedly and specifically increased when U2OS cells synchronized into G1 phase were induced to PCD by cisplatin treatment.

CONCLUSION

CHK1 cleavage is directly and indirectly regulated by CASP and non-CASP family proteases including serine protease(s) and the "(320)EPRT(323) protease(s)." Furthermore, (320)EPRT(323) cleavage of CHK1 occurs efficiently in PCD which is induced at the G1 phase by DNA damage.

GENERAL SIGNIFICANCE

CASP and non-CASP family proteases intricately regulate cleavage for up-regulation of CHK1 kinase activity during PCD.

Chronic myeloid leukemic cells trigger poly(ADP-ribose) polymerase-dependent inactivation and cell death in lymphocytes

NK cells and T cells are commonly dysfunctional in CML, and their status may determine the course of disease. We aimed to define the molecular mechanisms of leukemia-induced immunosuppression with focus on the role of ROS and the PARP-1 pathway of cell death. Malignant granulocytes from patients with BCR-ABL-positive CML expressed the oxygen radical-producing enzyme NOX, produced large amounts of ROS, and triggered extensive cell death in NK cells. Inhibition of PARP-1 maintained NK cell viability in cocultures with suppressive leukemic cells. Under conditions of oxidative stress, PARP-1 inhibition upheld the capacity of NK cells to kill myeloid leukemic cells, in addition to restoring the proliferation and cytokine production of NK cells and cytotoxic T cells. Our findings are suggestive of a novel pathway of relevance to immunosuppression in CML.

Hydrogen peroxide-induced poly(ADP-ribosyl)ation regulates osteogenic differentiation-associated cell death

We set out to investigate the role of poly(ADP-ribosylation), the attachment of NAD(+)-derived (ADP-ribose)(n) polymers to proteins, in the regulation of osteogenic differentiation of SAOS-2 cells and mesenchymal stem cells. In osteogenic differentiation medium, SAOS-2 cells showed mineralization and expressed alkaline phosphatase and osteoblastic marker genes such as Runx2, osterix, BMP2, and osteopontin. The cells also released hydrogen peroxide, displayed poly(ADP-ribose) polymerase (PARP) activation, and showed commitment to cell death (apoptosis and necrosis). Scavenging reactive oxygen species by glutathione or decomposing hydrogen peroxide by the addition of catalase reduced differentiation, PARP activation, and cell death. We silenced the expression of the main PAR-synthesizing enzyme PARP-1 and the PAR-degrading enzyme poly(ADP-ribose) glycohydrolase (PARG) in SAOS-2 osteosarcoma cells (shPARP-1 and shPARG, respectively). Both shPARP-1- and shPARG-silenced cells exhibited altered differentiation, with the most notable change being increased osteopontin expression but decreased alkaline phosphatase activity. PARP-1 silencing suppressed both apoptotic and necrotic cell death, but the PARP inhibitor PJ34 sensitized cells to cell death, indicating that the effects of PARP-1 silencing are not related to the activity of the enzyme. PARG silencing resulted in more apoptosis and, in the last days of differentiation, a shift from apoptosis toward necrosis. In conclusion our data prove that hydrogen peroxide-induced poly(ADP-ribose) signaling regulates cell death and osteodifferentiation.

Cytopathic effects: virus-modulated manifestations of innate immunity?

The capacity to injure infected cells is a widespread property of viruses. Usually, this cytopathic effect (CPE) is ascribed to viral hijacking of cellular resources to fulfill viral needs. However, evidence is accumulating that CPE is not necessarily directly coupled to viral reproduction but may largely be due to host defensive and viral antidefensive activities. A major part in this virus–cell interaction appears to be played by a putative host-encoded program with multiple competing branches, leading to necrotic, apoptotic, and, possibly, other types of cell suicide. Manifestations of this program are controlled and modulated by host, viral, and environmental factors.

Transient receptor potential melastatin 4 and cell death

Cell death proceeds by way of a variety of “cell death subroutines,” including several types of “apoptosis,” “regulated necrosis,” and others. “Accidental necrosis” due to profound adenosine triphosphate (ATP) depletion or oxidative stress is distinguished from regulated necrosis by the absence of death receptor signaling. However, both accidental and regulated necrosis have in common the process of “oncosis,” a physiological process characterized by Na⁺ influx and cell volume increase that, in necrotic cell death, is required to produce the characteristic features of membrane blebbing and membrane rupture. Here, we review emerging evidence that the monovalent cation channel, transient receptor potential melastatin 4 (TRPM4), is involved in the cell death process of oncosis. Potential involvement of TRPM4 in oncosis is suggested by the fact that the two principal regulators of TRPM4, intracellular ATP and Ca²⁺, are both altered during necrosis in the direction that causes TRPM4 channel opening. Under physiological conditions, activation of TRPM4 promotes Na⁺ influx and cell depolarization. Under pathological conditions, unchecked activation of TRPM4 leads to Na⁺ overload, cell volume increase, blebbing and cell membrane rupture, the latter constituting the irreversible end stage of necrosis. Emerging data indicate that TRPM4 plays a crucial role as end executioner in the accidental necrotic death of ATP-depleted or redox-challenged endothelial and epithelial cells, both in vitro and in vivo. Future studies will be needed to determine whether TRPM4 also plays a role in regulated necrosis and apoptosis.

Mitochondrial Control of Cellular Life, Stress, and Death

Since the discovery that mitochondrial membrane permeabilization represents a critical step in the regulation of intrinsic apoptosis, mitochondria have been viewed as pluripotent organelles, controlling cell death as well as several aspects of cell survival. Mitochondria constitute the most prominent source of ATP and are implicated in multiple anabolic and catabolic circuitries. In addition, mitochondria coordinate cell-wide stress responses, such as autophagy, and control nonapoptotic cell death routines, such as regulated necrosis. Thus, mitochondria seem to regulate a continuum of cellular functions, spanning from physiological metabolism to stress responses and death. The involvement of mitochondria in both vital and lethal processes is crucial for both embryonic and postembryonic development, as well as for the maintenance of adult tissue homeostasis. In line with this notion, primary mitochondrial defects or alterations in the signaling pathways that converge on or emanate from mitochondria underpin a large number of human diseases, including premature aging, neurodegenerative disorders, cardiovascular disorders, and cancer. Here, we provide an overview of the molecular mechanisms that enable mitochondria to sustain cell survival, coordinate stress responses, and mediate cell death, linking these pathways—whenever relevant—to cardiovascular health and disease.

Violacein induces death of resistant leukaemia cells via kinome reprogramming, endoplasmic reticulum stress and Golgi apparatus collapse

It is now generally recognised that different modes of programmed cell death (PCD) are intimately linked to the cancerous process. However, the mechanism of PCD involved in cancer chemoprevention is much less clear and may be different between types of chemopreventive agents and tumour cell types involved. Therefore, from a pharmacological view, it is crucial during the earlier steps of drug development to define the cellular specificity of the candidate as well as its capacity to bypass dysfunctional tumoral signalling pathways providing insensitivity to death stimuli. Studying the cytotoxic effects of violacein, an antibiotic dihydro-indolone synthesised by an Amazon river Chromobacterium, we observed that death induced in CD34(+)/c-Kit(+)/P-glycoprotein(+)/MRP1(+) TF1 leukaemia progenitor cells is not mediated by apoptosis and/or autophagy, since biomarkers of both types of cell death were not significantly affected by this compound. To clarify the working mechanism of violacein, we performed kinome profiling using peptide arrays to yield comprehensive descriptions of cellular kinase activities. Pro-death activity of violacein is actually carried out by inhibition of calpain and DAPK1 and activation of PKA, AKT and PDK, followed by structural changes caused by endoplasmic reticulum stress and Golgi apparatus collapse, leading to cellular demise. Our results demonstrate that violacein induces kinome reprogramming, overcoming death signaling dysfunctions of intrinsically resistant human leukaemia cells.

Selected anti-tumor vaccines merit a place in multimodal tumor therapies

Multimodal approaches are nowadays successfully applied in cancer therapy. Primary locally acting therapies such as radiotherapy (RT) and surgery are combined with systemic administration of chemotherapeutics. Nevertheless, the therapy of cancer is still a big challenge in medicine. The treatments often fail to induce long-lasting anti-tumor responses. Tumor recurrences and metastases result. Immunotherapies are therefore ideal adjuncts to standard tumor therapies since they aim to activate the patient's immune system against malignant cells even outside the primary treatment areas (abscopal effects). Especially cancer vaccines may have the potential both to train the immune system against cancer cells and to generate an immunological memory, resulting in long-lasting anti-tumor effects. However, despite promising results in phase I and II studies, most of the concepts finally failed. There are some critical aspects in development and application of cancer vaccines that may decide on their efficiency. The time point and frequency of medication, usage of an adequate immune adjuvant, the vaccine's immunogenic potential, and the tumor burden of the patient are crucial. Whole tumor cell vaccines have advantages compared to peptide-based ones since a variety of tumor antigens (TAs) are present. The master requirements of cell-based, therapeutic tumor vaccines are the complete inactivation of the tumor cells and the increase of their immunogenicity. Since the latter is highly connected with the cell death modality, the inactivation procedure of the tumor cell material may significantly influence the vaccine's efficiency. We therefore also introduce high hydrostatic pressure (HHP) as an innovative inactivation technology for tumor cell-based vaccines and outline that HHP efficiently inactivates tumor cells by enhancing their immunogenicity. Finally studies are presented proving that anti-tumor immune responses can be triggered by combining RT with selected immune therapies.

Prevention of cellular suicide by cytomegaloviruses

As intracellular parasites, viruses rely on many host cell functions to ensure their replication. The early induction of programmed cell death (PCD) in infected cells constitutes an effective antiviral host mechanism to restrict viral spread within an organism. As a countermeasure, viruses have evolved numerous strategies to interfere with the induction or execution of PCD. Slowly replicating viruses such as the cytomegaloviruses (CMVs) are particularly dependent on sustained cell viability. To preserve viability, the CMVs encode several viral cell death inhibitors that target different key regulators of the extrinsic and intrinsic apoptosis pathways. The best-characterized CMV-encoded inhibitors are the viral inhibitor of caspase-8-induced apoptosis (vICA), viral mitochondrial inhibitor of apoptosis (vMIA), and viral inhibitor of Bak oligomerization (vIBO). Moreover, a viral inhibitor of RIP-mediated signaling (vIRS) that blocks programmed necrosis has been identified in the genome of murine CMV (MCMV), indicating that this cell death mode is a particularly important part of the antiviral host response. This review provides an overview of the known cell death suppressors encoded by CMVs and their mechanisms of action.

Death receptors and mitochondria: two prime triggers of neural apoptosis and differentiation

BACKGROUND

Stem cell therapy is a strategy far from being satisfactory and applied in the clinic. Poor survival and differentiation levels of stem cells after transplantation or neural injury have been major problems. Recently, it has been recognized that cell death-relevant proteins, notably those that operate in the core of the executioner apoptosis machinery are functionally involved in differentiation of a wide range of cell types, including neural cells.

SCOPE OF REVIEW

This article will review recent studies on the mechanisms underlying the non-apoptotic function of mitochondrial and death receptor signaling pathways during neural differentiation. In addition, we will discuss how these major apoptosis-regulatory pathways control the decision between differentiation, self-renewal and cell death in neural stem cells and how levels of activity are restrained to prevent cell loss as final outcome.

MAJOR CONCLUSIONS

Emerging evidence suggests that, much like p53, caspases and Bcl-2 family members, the two prime triggers of cell death pathways, death receptors and mitochondria, may influence proliferation and differentiation potential of stem cells, neuronal plasticity, and astrocytic versus neuronal stem cell fate decision.

GENERAL SIGNIFICANCE

A better understanding of the molecular mechanisms underlying key checkpoints responsible for neural differentiation as an alternative to cell death will surely contribute to improve neuro-replacement strategies.

Recent advances in the molecular imaging of programmed cell death: part I--pathophysiology and radiotracers

In humans, apoptosis (programmed cell death) is the most common form of cell death after necrosis. Apoptosis is a series of genetically preprogrammed biochemical and morphologic energy-requiring events that, after a specific external or internal stimulus, results in the physiologic disappearance of a cell via its self-disintegration and packaging of its contents into membrane vesicles called apoptotic bodies. Apoptotic bodies can readily be ingested, with their nutrients and even organelles recycled by neighboring cells or phagocytes without local inflammation. In contrast, necrosis is characterized by the primary loss of plasma membrane integrity and the uncontrolled release of a cell's contents, often causing local inflammation, tissue damage, and scarring. Alternate forms of cell death also exist, associated with specific molecular mechanisms involving enzymes, organelles, genes, external stimuli, or blockade of normal cell proliferation. In this review we will briefly outline the molecular mechanisms of apoptosis that can be imaged with radiotracers now under development.

Cytometry of apoptosis. Historical perspective and new advances

Characteristic changes in cell morphology paralleled by the appearance of a multitude of molecular and biochemical markers occur during apoptosis. These changes vary depending on the cell type, mechanism of induction of apoptosis, and the time-window at which the process of apoptosis is analyzed. By virtue of the capability of rapid measurement of individual cells the flow- and imaging-cytometry become preferred technologies to detect, identify and record incidence of apoptosis in large cell populations. It also provided a valuable tool to investigate molecular mechanisms in field of necrobiology. This review outlines the progress in development of the most commonly used cytometric methods probing cells death based on analysis of fragmentation of DNA, activation of caspases, analysis of mitochondrial potential, alterations in plasma membrane structure and other features that characterize programmed cell death. This article is part of a Special Issue entitled "Apoptosis: Four Decades Later"

Apoptotic-like activity of staurosporine in axenic cultures of Trypanosoma evansi

Trypanosoma evansi is a blood protozoan parasite of the genus Trypanosoma which is responsible for surra (Trypanosomosis) in domestic and wild animals. This study addressed apoptotic-like features in Trypanosoma evansi in vitro. The mechanism of parasite death was investigated using staurosporine as an inducing agent. We evaluated its effects through several cytoplasmic features of apoptosis, including cell shrinkage, phosphatidylserine exposure, maintenance of plasma membrane integrity, and mitochondrial trans-membrane potential. For access to these features we have used the flow cytometry and fluorescence microscopy with cultures in the stationary phase and adjusted to a density of 10(6) cells/mL. The apoptotic effect of staurosporine in T. evansi was evaluated at 20 nM final concentration. There was an increase of phosphatidylserine exposure, whereas mitochondrial potential was decreased. Moreover, no evidence of cell permeability increasing with staurosporine was observed in this study, suggesting the absence of a necrotic process. Additional studies are needed to elucidate the possible pathways associated with this form of cell death in this hemoparasite.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

The pantheon of the fallen: why are there so many forms of cell death?

Cells die by various mechanisms, only some of which have been elucidated in detail. Numerous 'active' forms of cell death exist in which the cell participates in its own death, including apoptosis, programmed necrosis, mitotic catastrophe, and the recently described ferroptosis. Here, we attempt to explain why there are so many different forms of cell death, and propose a distinction between active death that constitutes 'suicide' versus 'sabotage'.

Cancer cell growth and survival as a system-level property sustained by enhanced glycolysis and mitochondrial metabolic remodeling

Systems Biology holds that complex cellular functions are generated as system-level properties endowed with robustness, each involving large networks of molecular determinants, generally identified by “omics” analyses. In this paper we describe four basic cancer cell properties that can easily be investigated in vitro: enhanced proliferation, evasion from apoptosis, genomic instability, and inability to undergo oncogene-induced senescence. Focusing our analysis on a K-ras dependent transformation system, we show that enhanced proliferation and evasion from apoptosis are closely linked, and present findings that indicate how a large metabolic remodeling sustains the enhanced growth ability. Network analysis of transcriptional profiling gives the first indication on this remodeling, further supported by biochemical investigations and metabolic flux analysis (MFA). Enhanced glycolysis, down-regulation of TCA cycle, decoupling of glucose and glutamine utilization, with increased reductive carboxylation of glutamine, so to yield a sustained production of growth building blocks and glutathione, are the hallmarks of enhanced proliferation. Low glucose availability specifically induces cell death in K-ras transformed cells, while PKA activation reverts this effect, possibly through at least two mitochondrial targets. The central role of mitochondria in determining the two investigated cancer cell properties is finally discussed. Taken together the findings reported herein indicate that a system-level property is sustained by a cascade of interconnected biochemical pathways that behave differently in normal and in transformed cells.

Live or let die: manipulation of cellular suicide programs by murine cytomegalovirus

Cytomegaloviruses (CMVs) are large double-stranded DNA viruses that replicate slowly and cause life-long persisting infections in their hosts. To achieve this, the CMVs had to evolve numerous countermeasures against innate and adaptive immune responses. Induction of programmed cell death is one important host defense mechanism against intracellular pathogens such as viruses. For a multicellular organism, it is advantageous to let infected cells die in order to thwart viral replication and dissemination. For a virus, by contrast, it is better to inhibit cell death and keep infected cells alive until the viral replication cycle has been completed. As a matter of fact, the CMVs encode a number of proteins devoted to interfering with different forms of programmed cell death: apoptosis and necroptosis. In this review, we summarize the known functions of the four best characterized cell death inhibitors of murine cytomegalovirus (MCMV), which are encoded by open reading frames, M36, m38.5, m41.1, and M45. The viral proteins interact with key molecules within different cell death pathways, namely caspase-8, Bax, Bak, and RIP1/RIP3. In addition, we discuss which events during MCMV infection might trigger apoptosis or necrosis and how MCMV's countermeasures compare to those of other herpesviruses. Since both, MCMV and its natural host, are amenable to genetic manipulation, the mouse model for CMV infection provides a particularly suitable system to study mechanisms of cell death induction and inhibition.

Dying cell clearance and its impact on the outcome of tumor radiotherapy

The induction of tumor cell death is one of the major goals of radiotherapy and has been considered to be the central determinant of its therapeutic outcome for a long time. However, accumulating evidence suggests that the success of radiotherapy does not only derive from direct cytotoxic effects on the tumor cells alone, but instead might also depend – at least in part – on innate as well as adaptive immune responses, which can particularly target tumor cells that survive local irradiation. The clearance of dying tumor cells by phagocytic cells of the innate immune system represents a crucial step in this scenario. Dendritic cells and macrophages, which engulf, process and present dying tumor cell material to adaptive immune cells, can trigger, skew, or inhibit adaptive immune responses, respectively. In this review we summarize the current knowledge of different forms of cell death induced by ionizing radiation, the multi-step process of dying cell clearance, and its immunological consequences with special regard toward the potential exploitation of these mechanisms for the improvement of tumor radiotherapy.

Protective effect of NecroX, a novel necroptosis inhibitor, on gentamicin-induced ototoxicity

INTRODUCTION

NecroX is a novel necrosis and necroptosis inhibitor that shows scavenger activity against mitochondrial reactive oxygen species (ROS) and cytoprotective activity against various insults. These findings raise the possibility of its protective effect in ototoxicity. This study was performed to investigate the protective effect of NecroX on gentamicin (GM)-induced hair cell loss in neonatal mouse cochlea cultures.

MATERIALS AND METHODS

The protective effects of NecroX were measured by phalloidin staining of cultures from postnatal day 2-3 mice with GM-induced hair cell loss. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was used to detect apoptosis. The radical-scavenging activity of NecroX was assessed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay.

RESULTS

NecroX showed a significant and concentration-dependent protective effect against GM-induced hair cell loss, and hair cells retained their stereocilia well. NecroX decreased GM-induced apoptosis of hair cells as assessed by TUNEL staining. Additionally, NecroX showed direct radical scavenging activity in the DPPH assay.

CONCLUSIONS

In this study, we demonstrated the protective effect of NecroX on GM-induced hair cell loss in neonatal cochlea cultures, and suggest that it may be of therapeutic use in the treatment of drug-induced ototoxicity.

Doxorubicin and NRG-1/erbB4-Deficiency Affect Gene Expression Profile: Involving Protein Homeostasis in Mouse

The accumulating evidence demonstrates the essential role of neuregulin-1 signaling in the adult heart, and, moreover, indicates that an impaired neuregulin signaling exacerbates the doxorubicin-mediated cardiac toxicity. Despite this strong data, the specific cardiomyocyte targets of the active erbB2/erbB4 heterodimer remain unknown. In this paper, we examined pathways involved in cardiomyocyte damage as a result of the cardiac sensitization to anthracycline toxicity in the ventricular muscle-specific erbB4 knockout mouse. We performed morphological analyses to evaluate the ventricular remodeling and employed a cDNA microarray to assess the characteristic gene expression profile, verified data by real-time RT-PCR, and then grouped into functional categories and pathways. We confirm the upregulation of genes related to the classical signature of a hypertrophic response, implicating an erbB2-dependent mechanism in doxorubicin-treated erbB4-KO hearts. Our results indicate the remarkable downregulation of IGF-I/PI-3′ kinase pathway and extends our current knowledge by uncovering an altered ubiquitin-proteasome system leading to cardiomyocyte autophagic vacuolization.