Salmonella-induced cell death: apoptosis, necrosis or programmed cell death?

Over the past several years, it has become apparent that enteropathogens activate cell death programs. For Salmonella and Shigella species, the induction of cell death is required for pathogenesis, and the mechanisms by which these bacteria induce cell death is an area of intense investigation. Although initial studies suggested that Salmonella induce cell death through an apoptotic pathway, recent studies demonstrate that cell death occurs through a unique caspase 1-dependent mechanism.     

Cell death in Leishmania induced by stress and differentiation: programmed cell death or necrosis?

Unicellular organisms, such as the protozoan parasite Leishmania, can be stimulated to show some morphological and biochemical features characteristic of mammalian apoptosis. This study demonstrates that under a variety of stress conditions such as serum deprivation, heat shock and nitric oxide, cell death can be induced leading to genomic DNA fragmentation into oligonucleosomes. DNA fragmentation was observed, without induction, in the infectious stages of the parasite, and correlated with the presence of internucleosomal nuclease activity, visualisation of 45 to 59 kDa nucleases and detection of TUNEL-positive nuclei. DNA fragmentation was not dependent on active effector downstream caspases nor on the lysosomal cathepsin L-like enzymes CPA and CPB. These data are consistent with the presence of a caspase-independent cell death mechanism in Leishmania, induced by stress and differentiation that differs significantly from metazoa.     

Nitric oxide: promoter or suppressor of programmed cell death?

Nitric oxide (NO) is a short-lived gaseous free radical that predominantly functions as a messenger and effector molecule. It affects a variety of physiological processes, including programmed cell death (PCD) through cyclic guanosine monophosphate (cGMP)-dependent and-independent pathways. In this field, dominant discoveries are the diverse apoptosis networks in mammalian cells, which involve signals primarily via death receptors (extrinsic pathway) or the mitochondria (intrinsic pathway) that recruit caspases as effector molecules. In plants, PCD shares some similarities with animal cells, but NO is involved in PCD induction via interacting with pathways of phytohormones. NO has both promoting and suppressing effects on cell death, depending on a variety of factors, such as cell type, cellular redox status, and the flux and dose of local NO. In this article, we focus on how NO regulates the apoptotic signal cascade through protein S-nitrosylation and review the recent progress on mechanisms of PCD in both mammalian and plant cells.     

Apoptosis or programmed cell death?

Although there are different ways in which cells may die, it is now thought that in a developmental context cells are induced to positively commit suicide whilst in a homeostatic context the absence of certain survival factors may provide the impetus for suicide. There appears to be some variation in the morphology and indeed the biochemistry of these suicide pathways; some treading the path of "apoptosis", others following a more generalized pathway to deletion, but both usually being genetically and synthetically motivated. There is some evidence that certain symptoms of "apoptosis" such as endonuclease activation can be spuriously induced without engaging a genetic cascade, however, presumably true apoptosis and programmed cell death must be genetically mediated. It is also becoming clear that mitosis and apoptosis are toggled or linked in some way and that the balance achieved depends on signals received from appropriate growth or survival factors.     

The role of VDAC in cell death: Friend or foe?

As the voltage-dependent anion channel (VDAC) forms the interface between mitochondria and the cytosol, its importance in metabolism is well understood. However, research on VDAC's role in cell death is a rapidly growing field, unfortunately with much confusing and contradictory results. The fact that VDAC plays a role in outer mitochondrial membrane permeabilization is undeniable, however, the mechanisms behind this remain very poorly understood. In this review, we will summarize the studies that show evidence of VDAC playing a role in cell death. To begin, we will discuss the evidence for and against VDAC's involvement in mitochondrial permeability transition (MPT) and attempt to clarify that VDAC is not an essential component of the MPT pore (MPTP). Next, we will evaluate the remaining literature on VDAC in cell death which can be divided into three models: proapoptotic agents escaping through VDAC, VDAC homo- or hetero-oligomerization, or VDAC closure resulting in outer mitochondrial membrane permeabilization through an unknown pathway. We will then discuss the growing list of modulators of VDAC activity that have been associated with induction/protection against cell death. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.     

Virus-induced dysregulation of programmed immunocompetent cell death: pathology or adaptation?

The review summarizes information from recent literature and results of the authors' own investigations concerning dysbalance of programmed cell death in establishment of a long-term virus persistense. The article discusses molecular mechanisms of apoptosis modulation of immune cellls by persistent viruses.     

Giant cell formation: the way to cell death or cell survival?

The study of giant cells in populations of different tumor cells and evaluation of their role in cancer development is an expanding field. The formation of giant cells has been shown to be followed by mitotic catastrophe, apoptosis, necrosis, and other types of cell elimination. Reports also demonstrate that giant cells can escape cell death and give rise to new cancer cells. However, it is not known if the programmed cell death is involved in this type of cell cycle disorders. Here we describe principal events that are observed during giant cell formation. We also consider the role of giant cells in cancer development, taking into account both published work and our own recent data in this field.     

Senescence and programmed cell death: substance or semantics?

The terms senescence and programmed cell death (PCD) have led to some confusion. Senescence as visibly observed in, for example, leaf yellowing and petal wilting, has often been taken to be synonymous with the programmed death of the constituent cells. PCD also obviously refers to cells, which show a programme leading to their death. Some scientists noted that leaf yellowing, if it has not gone too far, can be reversed. They suggested calling leaf yellowing, before the point of no return, 'senescence' and the process after it 'PCD'. However, this runs into several problems. It is counter to the historical definitions of senescence, both in animal and plant science, which stipulate that senescence is programmed and directly ends in death. It would also mean that only leaves and shoots show senescence, whereas several other plant parts, where reversal has not (yet) been shown, have no senescence, but only PCD. This conflicts with ordinary usage (as in root and flower senescence). Moreover, a programme can be reversible and therefore it is not counter to logic to regard the cell death programme as potentially reversible. In green leaf cells a decision to die, in a programmed way, has been taken, in principle, before the cells start to remobilize their contents (that is, before visible yellowing) and only rarely is this decision reversed. According to the arguments developed here there are no good reasons to separate a senescence phase and a subsequent PCD phase. Rather, it is asserted, senescence in cells is the same as PCD and the two are fully synchronous.     

Salt-induced oxidative stress in rosemary plants: Damage or protection?

Mechanisms of photoprotection and antioxidant protection, including changes in chlorophylls, xanthophyll cycle components and levels of low-molecular-weight chloroplastic antioxidants (lutein, β-carotene and α-tocopherol) were studied together with levels of malondialdehyde, a product of lipid peroxidation, in the response of rosemary (Rosmarinus officinalis L.) plants to salt stress. Plants were exposed to increasing NaCl concentrations (50, 100 and 150 mM) for 6 weeks, and two concentrations of the following chloride salts: KCl, CaCl₂, MgCl₂ and FeCl₃, were used together with 100 mM NaCl to explore the extent to which these salts can alter the mechanisms of photoprotection, antioxidant protection and malondialdehyde accumulation in leaves. Increasing concentrations of NaCl decreased leaf water contents and photosynthetic pigment levels, while the contents of α-tocopherol and malondialdehyde increased, but with completely different kinetics. α-Tocopherol levels increased in a dose-dependent manner as stress progressed, while malondialdehyde levels increased at the highest dose (150 mM NaCl) but only during early phases of stress. Furthermore, although the addition of chloride salts to NaCl-treated plants apparently improved leaf physiological status, in terms of water and chlorophyll contents, plants showed an increased photoprotective demand and increased oxidative stress, particularly in FeCl₃-treated plants. It is concluded that (i) rosemary plants can withstand moderate doses of NaCl in the medium (at least 150 mM NaCl for 6 weeks), (ii) oxidative stress may be a mechanism for protecting plants from moderate doses of salt stress rather than causing damage to plants, and (iii) the addition of chloride salts to NaCl-treated plants may dramatically increase the photoprotective demand and oxidative stress of leaves, while plant growth is not negatively affected.     

β-cell death and proliferation after intermittent hypoxia: Role of oxidative stress

Intermittent hypoxia (IH), such as occurs in sleep apnea, induces increased oxidative stress and is associated with altered glucose homeostasis. Because pancreatic β cells are very sensitive to oxidative stress we tested whether they could be affected by IH. The effects of IH exposure (24 h/day, 5.7 and 21% O₂ alternation) in mice on β-cell proliferation and β-cell death were tested using Ki67 staining and TUNEL staining, respectively. To assess the role of oxidative stress in these processes, transgenic mice with β-cell-specific overexpression of the antioxidant protein MnSOD were exposed to IH. After 4 days of IH exposure, β-cell proliferation was increased almost fourfold. Coinciding with the increase in proliferation, the subcellular localization of the cell cycle regulator cyclin D2 was increased in the nucleus. In addition, β-cell death was increased approximately fourfold. MnSOD transgene did not alter the effects of IH on β-cell proliferation, but completely abrogated the IH effects on cell death. Thus, IH exposure that mimics sleep apnea can lead to increased β-cell proliferation and cell death. Furthermore, the cell death response seems to be due to oxidative stress.     

Calcium signalling and pancreatic cell death: apoptosis or necrosis?

Secretagogues, such as cholecystokinin and acetylcholine, utilise a variety of second messengers (inositol trisphosphate, cADPR and nicotinic acid adenine dinucleotide phosphate) to induce specific oscillatory patterns of calcium (Ca(2+)) signals in pancreatic acinar cells. These are tightly controlled in a spatiotemporal manner, and are coupled to mitochondrial metabolism necessary to fuel secretion. When Ca(2+) homeostasis is disrupted by known precipitants of acute pancreatitis, for example, hyperstimulation or non-oxidative ethanol metabolites, Ca(2+) stores (endoplasmic reticulum and acidic pool) become depleted and sustained cytosolic [Ca(2+)] elevations replace transient signals, leading to severe consequences. Sustained mitochondrial depolarisation, possibly via opening of the mitochondrial permeability transition pore (MPTP), elicits cellular ATP depletion that paralyses energy-dependent Ca(2+) pumps causing cytosolic Ca(2+) overload, while digestive enzymes are activated prematurely within the cell; Ca(2+)-dependent cellular necrosis ensues. However, when stress to the acinar cell is milder, for example, by application of the oxidant menadione, release of Ca(2+) from stores leads to oscillatory global waves, associated with partial mitochondrial depolarisation and transient MPTP opening; apoptotic cell death is promoted via the intrinsic pathway, when associated with generation of reactive oxygen species. Apoptosis, induced by menadione or bile acids, is potentiated by inhibition of an endogenous detoxifying enzyme NAD(P)H:quinone oxidoreductase 1 (NQO1), suggesting its importance as a defence mechanism that may influence cell fate.     

Autophagic cell death: Loch Ness monster or endangered species?

The concept of autophagic cell death was first established based on observations of increased autophagic markers in dying cells. The major limitation of such a morphology-based definition of autophagic cell death is that it fails to establish the functional role of autophagy in the cell death process, and thus contributes to the confusion in the literature regarding the role of autophagy in cell death and cell survival. Here we propose to define autophagic cell death as a modality of non-apoptotic or necrotic programmed cell death in which autophagy serves as a cell death mechanism, upon meeting the following set of criteria: (i) cell death occurs without the involvement of apoptosis; (ii) there is an increase of autophagic flux, and not just an increase of the autophagic markers, in the dying cells; and (iii) suppression of autophagy via both pharmacological inhibitors and genetic approaches is able to rescue or prevent cell death. In light of this new definition, we will discuss some of the common problems and difficulties in the study of autophagic cell death and also revisit some well-reported cases of autophagic cell death, aiming to achieve a better understanding of whether autophagy is a real killer, an accomplice or just an innocent bystander in the course of cell death. At present, the physiological relevance of autophagic cell death is mainly observed in lower eukaryotes and invertebrates such as Dictyostelium discoideum and Drosophila melanogaster. We believe that such a clear definition of autophagic cell death will help us study and understand the physiological or pathological relevance of autophagic cell death in mammals.     

Autophagic cell death: Loch Ness monster or endangered species?

The concept of autophagic cell death was first established based on observations of increased autophagic markers in dying cells. The major limitation of such a morphology-based definition of autophagic cell death is that it fails to establish the functional role of autophagy in the cell death process, and thus contributes to the confusion in the literature regarding the role of autophagy in cell death and cell survival. Here we propose to define autophagic cell death as a modality of non-apoptotic or necrotic programmed cell death in which autophagy serves as a cell death mechanism, upon meeting the following set of criteria: (i) cell death occurs without the involvement of apoptosis; (ii) there is an increase of autophagic flux, and not just an increase of the autophagic markers, in the dying cells; and (iii) suppression of autophagy via both pharmacological inhibitors and genetic approaches is able to rescue or prevent cell death. In light of this new definition, we will discuss some of the common problems and difficulties in the study of autophagic cell death and also revisit some well-reported cases of autophagic cell death, aiming to achieve a better understanding of whether autophagy is a real killer, an accomplice or just an innocent bystander in the course of cell death. At present, the physiological relevance of autophagic cell death is mainly observed in lower eukaryotes and invertebrates

such as Dictyostelium discoideum and Drosophila melanogaster. We believe that such a clear definition of autophagic cell death will help us study and understand the physiological or pathological relevance of autophagic cell death in mammals.     

Glutathione half-cell reduction potential: A universal stress marker and modulator of programmed cell death?

The elucidation of factors that contribute to cell viability loss is presently compromised by the lack of a universal measure that quantifies “stress.” We have investigated mechanisms of viability loss in plant seeds to find a reliable marker of stress response. Oxidative damage has previously been correlated with degenerative processes and death, but how exactly this contributes to viability loss is unknown. We show in four species subjected to ageing or desiccation that seed viability decreased by 50% when the half-cell reduction potential of glutathione (E_GSSG/2GSH), a major cellular antioxidant and redox buffer, increased to −180 to −160 mV. We then conducted a metaanalysis of data representative of 13 plant and fungal orders to show that plant stress generally becomes lethal when E_GSSG/2GSH exceeds −160 mV. We put forward that this change in E_GSSG/2GSH is part of the signaling cascade that initiates programmed cell death (PCD), finally causing internucleosomal DNA fragmentation in the final, or execution phase, of PCD. E_GSSG/2GSH is therefore a universal marker of plant cell viability and allows us to predict whether a seed will live, germinate, and produce a new plant, or if it will die.     

The end of autophagic cell death?

In the mammalian system, cell death is often preceded or accompanied by autophagic vacuolization, a finding that initially led to the widespread belief that so-called "autophagic cell death" would be mediated by autophagy. Thanks to the availability of genetic tools to disable the autophagic machinery, it has become clear over recent years that autophagy usually constitutes a futile attempt of dying cells to adapt to lethal stress rather than a mechanism to execute a cell death program. Recently, we systematically addressed the question as to whether established or prospective anticancer agents may induce "autophagic cell death". Although a considerable portion among the 1,400 compounds that we evaluated induced autophagic puncta and actually increased autophagic flux, not a single one turned out to kill tumor cells through the induction of autophagy. Thus, knockdown of essential autophagy genes (such as ATG5 and ATG7) failed to prevent and rather accelerated chemotherapy-induced cell death, in spite of the fact that this manipulation efficiently inhibits autophagosome formation. Herein, we review these finding and--polemically--raise doubts as to the very existence of "autophagic cell death".