植物细胞活性氧种类、代谢及其信号转导

越来越明显的证据表明,植物体十分活跃的产生着活性氧并将之作为信号分子、进而控制着诸如细胞程序性死亡、非生物胁迫响应、病原体防御和系统信号等生命过程,而不仅是传统意义上的活性氧是有氧代谢的附产物。日益增多的证据显示,由脱落酸、水杨酸、茉莉酸与乙烯以及活性氧所调节的激素信号途径,在生物和非生物胁迫信号的“交谈”中起重要作用。活性氧最初被认为是动物吞噬细胞在宿主防御反应时所释放的附产物,现在的研究清楚的表明,活性氧在动物和植物细胞信号途径中均起作用。活性氧可以诱导细胞程序性死亡或坏死、可以诱导或抑制许多基因的表达,也可以激活上述级联信号。近来生物化学与遗传学研究证实过氧化氢是介导植物生物胁迫与非生物胁迫的信号分子,过氧化氢的合成与作用似乎与一氧化氮有关系。过氧化氢所调节的下游信号包括钙“动员”、蛋白磷酸化和基因表达等。     

Mitochondrial and apoptotic neuronal death signaling pathways in cerebral ischemia

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

Differential effects of Bcl-2 and caspases on mitochondrial permeabilization during endogenous or exogenous reactive oxygen species-induced cell death

In this study, we have compared several features of cell death triggered by classical inducers of apoptotic pathways (etoposide and tumour necrosis factor (TNF)-α) versus exogenous reactive oxygen species (ROS; hydrogen peroxide (H?O?), tert-butyl hydroperoxide (t-BHP)) or a ROS generator (paraquat). Our aim was to characterize relationships that exist between ROS, mitochondrial perturbations, Bcl-2 and caspases, depending on source and identity of ROS. First, we have found that these five inducers trigger oxidative stress, mitochondrial membrane permeabilization (MMP), cytochrome c (cyt c) release from mitochondria and cell death. In each case, cell death could be inhibited by several antioxidants, showing that it is primarily ROS dependent. Second, we have highlighted that during etoposide or TNF-α treatments, intracellular ROS level, MMP and cell death are all regulated by caspases and Bcl-2, with caspases acting early in the process. Third, we have demonstrated that H?O?-induced cell death shares many of these characteristics with etoposide and TNF-α, whereas t-BHP induces both caspase-dependent and caspase-independent cell death. Surprisingly, paraquat-induced cell death, which harbours some characteristics of apoptosis such as cyt c release and caspase-3 activation, is not modulated by Bcl-2 and caspase inhibitors, suggesting that paraquat also triggers non-apoptotic cell death signals. On the one hand, these results show that endogenous or exogenous ROS can trigger multiple cell death pathways with Bcl-2 and caspases acting differentially. On the other hand, they suggest that H?O? could be an important mediator of etoposide and TNF-α-dependent cell death since these inducers trigger similar phenotypes.     

The role of phytohormone signaling in ozone-induced cell death in plants

Ozone is the main photochemical oxidant that causes leaf damage in many plant species, and can thereby significantly decrease the productivity of crops and forests. When ozone is incorporated into plants, it produces reactive oxygen species (ROS), such as superoxide radicals and hydrogen peroxide. These ROS induce the synthesis of several plant hormones, such as ethylene, salicylic acid, and jasmonic acid. These phytohormones are required for plant growth, development, and defense responses, and regulate the extent of leaf injury in ozone-fumigated plants. Recently, responses to ozone have been studied using genetically modified plants and mutants with altered hormone levels or signaling pathways. These researches have clarified the roles of phytohormones and the complexity of their signaling pathways. The present paper reviews the biosynthesis of the phytohormones ethylene, salicylic acid, and jasmonic acid, their roles in plant responses to ozone, and multiple interactions between these phytohormones in ozone-exposed plants.Key words: cross-talk, ethylene, jasmonic acid, ozone, phytohormones, programmed cell death, salicylic acid, signaling pathways     

Signalling and cell death in ozone-exposed plants

Experiments with Arabidopsis mutants and sensitive and tolerant pairs in several other species have elucidated the molecular basis of plant ozone sensitivity and ozone lesion development. They have indicated an important role for hormonal signalling in determining the outcome of ozone challenge at the cellular level. The reactive oxygen species (ROS) from ozone degradation can cause either direct necrotic damage or induce the process of programmed cell death. Perception of ozone or ROS from its degradation in the apoplast activates several signal transduction pathways that regulate the responses of the cells to the increased oxidative load. Plant hormones salicylic acid, jasmonic acid, ethylene and abscisic acid are involved in determining the duration and extent of ozone-induced cell death and its propagation. Salicylic acid is required for the programmed cell death, ethylene promotes endogenous ROS formation and lesion propagation, and jasmonic acid is involved in limiting the lesion spreading. Abscisic acid is most likely involved through the regulation of stomata and thus is expected to affect lesion initiation. The roles and interactions of perception of ozone, the immediate downstream responses, hormone biosynthesis and signalling during ozone lesion initiation and formation are reviewed.     

Direct oxidative modifications of signalling proteins in mammalian cells and their effects on apoptosis

The production of ROS is an inevitable consequence of metabolism. However, high levels of ROS within a cell can be lethal and so the cell has a number of defences against oxidative cell stress. Occasionally the cell's antioxidant mechanisms fail and oxidative stress occurs. High levels of ROS within a cell have a number of direct and indirect consequences on cell signalling pathways and may result in apoptosis or necrosis. Although some of the indirect effects of ROS are well known, limitations in technology mean that the direct effects of the cell's redox environment upon proteins are less understood. Recent work by a number of groups has demonstrated that ROS can directly modify signalling proteins through different modifications, for example by nitrosylation, carbonylation, di-sulphide bond formation and glutathionylation. These modifications modulate a protein's activity and several recent papers have demonstrated their importance in cell signalling events, especially those involved in cell death/survival. Redox modification of proteins allows for further regulation of cell signalling pathways in response to the cellular environment. Understanding them may be critical for us to modulate cell pathways for our own means, such as in cytotoxic drug treatments of cancer cells. Protein modifications mediated by oxidative stress can modulate apoptosis, either through specific protein modifications resulting in regulation of signalling pathways, or through a general increase in oxidised proteins resulting in reduced cellular function. This review discusses direct oxidative protein modifications and their effects on apoptosis.     

Reactive oxygen species and seed germination

Reactive oxygen species (ROS) are continuously produced by the metabolically active cells of seeds, and apparently play important roles in biological processes such as germination and dormancy. Germination and ROS accumulation appear to be linked, and seed germination success may be closely associated with internal ROS contents and the activities of ROS-scavenging systems. Although ROS were long considered hazardous molecules, their functions as cell signaling compounds are now well established and widely studied in plants. In seeds, ROS have important roles in endosperm weakening, the mobilization of seed reserves, protection against pathogens, and programmed cell death. ROS may also function as messengers or transmitters of environmental cues during seed germination. Little is currently known, however, about ROS biochemistry or their functions or the signaling pathways during these processes, which are to be considered in the present review.     

Reactive oxygen species,cellular redox systems,and apoptosis

Reactive oxygen species (ROS) are products of normal metabolism and xenobiotic exposure, and depending on their concentration, ROS can be beneficial or harmful to cells and tissues. At physiological low levels, ROS function as “redox messengers” in intracellular signaling and regulation, whereas excess ROS induce oxidative modification of cellular macromolecules, inhibit protein function, and promote cell death. Additionally, various redox systems, such as the glutathione, thioredoxin, and pyridine nucleotide redox couples, participate in cell signaling and modulation of cell function, including apoptotic cell death. Cell apoptosis is initiated by extracellular and intracellular signals via two main pathways, the death receptor- and the mitochondria-mediated pathways. Various pathologies can result from oxidative stress-induced apoptotic signaling that is consequent to ROS increases and/or antioxidant decreases, disruption of intracellular redox homeostasis, and irreversible oxidative modifications of lipid, protein, or DNA. In this review, we focus on several key aspects of ROS and redox mechanisms in apoptotic signaling and highlight the gaps in knowledge and potential avenues for further investigation. A full understanding of the redox control of apoptotic initiation and execution could underpin the development of therapeutic interventions targeted at oxidative stress-associated disorders.     

Direct oxidative modifications of signalling proteins in mammalian cells and their effects on apoptosis

Abstract

The production of ROS is an inevitable consequence of metabolism. However, high levels of ROS within a cell can be lethal and so the cell has a number of defences against oxidative cell stress. Occasionally the cell's antioxidant mechanisms fail and oxidative stress occurs. High levels of ROS within a cell have a number of direct and indirect consequences on cell signalling pathways and may result in apoptosis or necrosis. Although some of the indirect effects of ROS are well known, limitations in technology mean that the direct effects of the cell's redox environment upon proteins are less understood. Recent work by a number of groups has demonstrated that ROS can directly modify signalling proteins through different modifications, for example by nitrosylation, carbonylation, di-sulphide bond formation and glutathionylation. These modifications modulate a protein's activity and several recent papers have demonstrated their importance in cell signalling events, especially those involved in cell death/survival. Redox modification of proteins allows for further regulation of cell signalling pathways in response to the cellular environment. Understanding them may be critical for us to modulate cell pathways for our own means, such as in cytotoxic drug treatments of cancer cells. Protein modifications mediated by oxidative stress can modulate apoptosis, either through specific protein modifications resulting in regulation of signalling pathways, or through a general increase in oxidised proteins resulting in reduced cellular function. This review discusses direct oxidative protein modifications and their effects on apoptosis.     

Iron homeostasis and iron-regulated ROS in cell death,senescence and human diseases

BackgroundIron is essential for many types of biological processes. However, excessive iron can be cytotoxic and can lead to many diseases. Since ferroptosis, which is an iron-dependent regulated form of necrosis, was recently discovered, iron and iron-catalysed oxidative stress have attracted much interest because of their sophisticated mechanism of cellular signalling leading to cell death and associated with various diseases.Scope of reviewIn this review, we first focus on how iron catalyses reactive oxygen species (ROS). Next, we discuss the roles of iron in cell death and senescence and, in particular, the downstream signalling pathways of ROS. Finally, we discuss the potential regulation mechanism of iron as a therapeutic target for various iron-related diseases.Major conclusionsBoth labile iron released from organelles upon various stresses and iron incorporated in enzymes produce ROS, including lipid ROS. ROS produced by iron activates various signalling pathways, including mitogen-activated protein kinase (MAPK) signalling pathways such as the apoptosis signal-regulating kinase 1 (ASK1)-p38/JNK pathway. These ROS-activated signalling pathways regulate senescence or cell death and are linked to cancer, ischaemia-reperfusion injury during transplantation and ageing-related neurodegenerative diseases.General significanceIron overload damages cells and causes harmful effects on the body through oxidative stress. Thus, understanding the spatiotemporal availability of iron and the role of iron in generating ROS will provide clues for the suppression of ROS and cytotoxic redox-active iron. Moreover, elucidating the molecular mechanisms and signalling pathways of iron-dependent cytotoxicity will enable us to find novel therapeutic targets for various diseases.     

Ethylene signaling in salt stress- and salicylic acid-induced programmed cell death in tomato suspension cells

Salt stress- and salicylic acid (SA)-induced cell death can be activated by various signaling pathways including ethylene (ET) signaling in intact tomato plants. In tomato suspension cultures, a treatment with 250 mM NaCl increased the production of reactive oxygen species (ROS), nitric oxide (NO), and ET. The 10^?3 M SA-induced cell death was also accompanied by ROS and NO production, but ET emanation, the most characteristic difference between the two cell death programs, did not change. ET synthesis was enhanced by addition of ET precursor 1-aminocyclopropane-1-carboxylic acid, which, after 2 h, increased the ROS production in the case of both stressors and accelerated cell death under salt stress. However, it did not change the viability and NO levels in SA-treated samples. The effect of ET induced by salt stress could be blocked with silver thiosulfate (STS), an inhibitor of ET action. STS reduced the death of cells which is in accordance with the decrease in ROS production of cells exposed to high salinity. Unexpectedly, application of STS together with SA resulted in increasing ROS and reduced NO accumulation which led to a faster cell death. NaCl- and SA-induced cell death was blocked by Ca²⁺ chelator EGTA and calmodulin inhibitor W-7, or with the inhibitors of ROS. The inhibitor of MAPKs, PD98059, and the cysteine protease inhibitor E-64 reduced cell death in both cases. These results show that NaCl induces cell death mainly by ET-induced ROS production, but ROS generated by SA was not controlled by ET in tomato cell suspension.     

Adenoviral SERCA1 overexpression triggers an apoptotic response in cultured neonatal but not in adult rat cardiomyocytes

Although apoptosis and necrosis have been considered different pathways to cell death, only one compound induces both types of cell death. Diethyldithiocarbamate (DDC) has been shown to have antioxidant or prooxidant effects in several different systems. We observed in our present study that DDC induced not only apoptosis but also necrosis depending on its dosage in HL60 premyelocytic leukemia cells. Moreover, in hypoxia cell culture conditions, DDC-induced necrotic cells decreased but DDC-induced apoptosis continued. We investigated the DDC-induced different cell death mechanisms as they are correlated with reactive oxygen species (ROS). High-dose DDC-induced necrotic cell death is thought to depend on the increase of intracellular ROS, while low-dose DDC-induced apoptosis is thought to depend on changes of the intracellular redox state by the transporting of external metal ions. There was no sequential or quantitative change of Bcl-2 family proteins in DDC-induced apoptotic or necrotic pathways. However, the mitochondrial transmembrane potential was remarkably decreased in the DDC-induced necrosis. Finally, duration of c-Jun N-terminal kinase (JNK) activation resulted in different types of cell death.     

Cellular response to oxidative stress: signaling for suicide and survival

Reactive oxygen species (ROS), whether produced endogenously as a consequence of normal cell functions or derived from external sources, pose a constant threat to cells living in an aerobic environment as they can result in severe damage to DNA, protein, and lipids. The importance of oxidative damage to the pathogenesis of many diseases as well as to degenerative processes of aging has becoming increasingly apparent over the past few years. Cells contain a number of antioxidant defenses to minimize fluctuations in ROS, but ROS generation often exceeds the cell's antioxidant capacity, resulting in a condition termed oxidative stress. Host survival depends upon the ability of cells and tissues to adapt to or resist the stress, and repair or remove damaged molecules or cells. Numerous stress response mechanisms have evolved for these purposes, and they are rapidly activated in response to oxidative insults. Some of the pathways are preferentially linked to enhanced survival, while others are more frequently associated with cell death. Still others have been implicated in both extremes depending on the particular circumstances. In this review, we discuss the various signaling pathways known to be activated in response to oxidative stress in mammalian cells, the mechanisms leading to their activation, and their roles in influencing cell survival. These pathways constitute important avenues for therapeutic interventions aimed at limiting oxidative damage or attenuating its sequelae.     

Role of oxidative stress and caspase 3 in CD47-mediated neuronal cell death

CD47 or integrin-associated protein promotes cell death in blood and tumor cells. Recently, CD47 signaling has been identified in neurons as well. In this study, we investigated the role of CD47 in neuronal cell death. Exposure of primary mouse cortical neurons to the CD47 ligand thrombospondin-1 or the specific CD47-activating peptide 4N1K induced cell death. Activation of CD47 elevated levels of active caspase 3 and increased the generation of reactive oxygen species (ROS) in a time-dependent manner. Both ROS scavengers and caspase inhibitors attenuated cell death. But ROS scavenging did not reduce the activation of caspase 3, and combination treatments with a caspase inhibitor plus free radical scavenger did not yield additive protection. Taken together, these data suggest that parallel and redundant pathways of oxidative stress and caspase-mediated cell death are involved. We conclude that CD47 mediates neuronal cell death through caspase-dependent and caspase-independent pathways.     

The actin cytoskeleton: a key regulator of apoptosis and ageing?

Evidence from many organisms has shown that the accumulation of reactive oxygen species (ROS) has a detrimental effect on cell well-being. High levels of ROS have been linked to programmed cell death pathways and to ageing. Recent reports have implicated changes to the dynamics of the actin cytoskeleton in the release of ROS from mitochondria and subsequent cell death.     

Inhibition of Oxidative Phosphorylation Induces a Rapid Death of GA-Pretreated Aleurone Cells,But Not of ABA-Pretreated Aleurone Cells

Reactive oxygen species (ROS) mediate programmed cell death in aleurone cells, which is promoted by gibberellic acid (GA) and prevented by abscisic acid (ABA). Plant mitochondria contain two distinct respiratory pathways: respiration through cytochrome c oxidase increases ROS production, whereas respiration through the alternative oxidase pathway lowers it. While studying the effects of GA and ABA on partitioning of respiration between those two pathways during the germinating process, we discovered that oxidative phosphorylation inhibitors like sodium azide and 2, 4-dinitrophenol induce rapid death of GA-pretreated aleurone cells but not of ABA-pretreated cells. Functional aerobic respiration was required for GA signaling, and 6 to 12 hours of GA signaling altered the cellular state of aleurone cells to be extremely susceptible to inhibition of oxidative phosphorylation. Anaerobic conditions were also able to mimic the effects of respiratory inhibitors in specifically inducing cell death in GA-treated cells, but cell death was provoked much more slowly. Cotreatment with various antioxidants did not prevent this process at all, suggesting that no ROS are responsible for this respiratory inhibitor-induced cell death. Our observation implicates that GA may partition all the electrons produced during mitochondrial respiration only to the cytochrome oxidase pathway, which would at least partly contribute to cellular accumulation of ROS.     

Molecular effectors of multiple cell death pathways initiated by photodynamic therapy

Photodynamic therapy (PDT) is a recently developed anticancer modality utilizing the generation of singlet oxygen and other reactive oxygen species, through visible light irradiation of a photosensitive dye accumulated in the cancerous tissue. Multiple signaling cascades are concomitantly activated in cancer cells exposed to the photodynamic stress and depending on the subcellular localization of the damaging ROS, these signals are transduced into adaptive or cell death responses. Recent evidence indicates that PDT can kill cancer cells directly by the efficient induction of apoptotic as well as non-apoptotic cell death pathways. The identification of the molecular effectors regulating the cross-talk between apoptosis and other major cell death subroutines (e.g. necrosis, autophagic cell death) is an area of intense research in cancer therapy. Signaling molecules modulating the induction of different cell death pathways can become useful targets to induce or increase photokilling in cancer cells harboring defects in apoptotic pathways, which is a crucial step in carcinogenesis and therapy resistance. This review highlights recent developments aimed at deciphering the molecular interplay between cell death pathways as well as their possible therapeutic exploitation in photosensitized cells.     

Critical roles of reactive oxygen species in mitochondrial permeability transition in mediating evodiamine-induced human melanoma A375-S2 cell apoptosis

Previous studies have shown that evodiamine could trigger apoptosis in human malignant melanoma A375-S2 cells within 24 h. To further investigate the biochemical basis of this activity, the roles of reactive oxygen species (ROS) and mitochondrial permeability transition (MPT) were evaluated. Exposure to evodiamine led to a rapid increase in intracellular ROS followed by an onset of mitochondrial depolarization. ROS scavenger rescued the ΔΨm dissipation and cell death induced by evodiamine, whilst MPT inhibitor blocked the second-time ROS formation as well as cell death. Expressions of key proteins in Fas- and mitochondria-mediated pathways were furthermore examined. Both pathways were activated and regulated by ROS and MPT and were converged to a final common pathway involving the activation of caspase-3. These data suggested that a phenomenon termed ROS-induced ROS release (RIRR) was involved in evodiamine-treated A375-S2 cells and greatly contributed to the apoptotic process through both extrinsic and intrinsic pathways.     

Critical roles of reactive oxygen species in mitochondrial permeability transition in mediating evodiamine-induced human melanoma A375-S2 cell apoptosis

Previous studies have shown that evodiamine could trigger apoptosis in human malignant melanoma A375-S2 cells within 24 h. To further investigate the biochemical basis of this activity, the roles of reactive oxygen species (ROS) and mitochondrial permeability transition (MPT) were evaluated. Exposure to evodiamine led to a rapid increase in intracellular ROS followed by an onset of mitochondrial depolarization. ROS scavenger rescued the ΔΨm dissipation and cell death induced by evodiamine, whilst MPT inhibitor blocked the second-time ROS formation as well as cell death. Expressions of key proteins in Fas- and mitochondria-mediated pathways were furthermore examined. Both pathways were activated and regulated by ROS and MPT and were converged to a final common pathway involving the activation of caspase-3. These data suggested that a phenomenon termed ROS-induced ROS release (RIRR) was involved in evodiamine-treated A375-S2 cells and greatly contributed to the apoptotic process through both extrinsic and intrinsic pathways.