S-nitrosylated GAPDH initiates apoptotic cell death by nuclear translocation following Siah1 binding

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) influences cytotoxicity, translocating to the nucleus during apoptosis. Here we report a signalling pathway in which nitric oxide (NO) generation that follows apoptotic stimulation elicits S-nitrosylation of GAPDH, which triggers binding to Siah1 (an E3 ubiquitin ligase), nuclear translocation and apoptosis. S-nitrosylation of GAPDH augments its binding to Siah1, whose nuclear localization signal mediates translocation of GAPDH. GAPDH stabilizes Siah1, facilitating its degradation of nuclear proteins. Activation of macrophages by endotoxin and of neurons by glutamate elicits GAPDH-Siah1 binding, nuclear translocation and apoptosis, which are prevented by NO deletion. The NO-S-nitrosylation-GAPDH-Siah1 cascade may represent an important molecular mechanism of cytotoxicity.     

Say NO to neurodegeneration: role of S-nitrosylation in neurodegenerative disorders

Nitric oxide (NO) is an important signaling molecule that controls a wide range of biological processes. One of the signaling mechanisms of NO is through the S-nitrosylation of cysteine residues on proteins. S-nitrosylation is now regarded as an important redox signaling mechanism in the regulation of different cellular and physiological functions. However, deregulation of S-nitrosylation has also been linked to various human diseases such as neurodegenerative disorders. Nitrosative stress has long been considered as a major mediator in the development of neurodegeneration, but the molecular mechanism of how NO can contribute to neurodegeneration is not completely clear. Early studies suggested that nitration of proteins, which can induce protein aggregation might contribute to the neurodegenerative process. However, several recent studies suggest that S-nitrosylation of proteins that are important for neuronal survival contributes substantially in the development of various neurodegenerative disorders. Thus, in-depth understanding of the mechanism of neurodegeneration in relation to S-nitrosylation will be critical for the development of therapeutic treatment against these neurodegenerative diseases.     

S-nitrosylated GAPDH mediates neuronal apoptosis induced by amyotrophic lateral sclerosis-associated mutant SOD1G93A

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with the selective loss of motor neurons in the brain, brain stem, and spinal cord. A number of the mutants of the human gene for superoxide dismutase 1 (SOD1) have been shown to cause familial ALS as a result of gain-of-function toxicity by an unknown mechanism. In this study, we show that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) functions as a critical mediator of the apoptotic cell death signaling cascade induced by the ALS-associated G93A mutant of human SOD1 [SOD1(G93A)]. We observed that SOD1(G93A) induces S-nitrosylation of GAPDH and the subsequent binding of GAPDH and Siah1 in NSC34 motor neuron-like cells. Furthermore, SOD1(G93A) promoted nuclear translocation of S-nitrosylated GAPDH in the cells. In addition, SOD1(G93A)-induced apoptotic cell death was inhibited by deprenyl, a chemical inhibitor of GAPDH S-nitrosylation, in NSC34 cells. Taken together, our findings suggest that S-nitrosylation of GAPDH plays a critical role in SOD1(G93A)-induced neuronal apoptosis.     

Regulation of apoptosis by protein S-nitrosylation

Summary. S-nitrosylation/denitrosylation of critical cysteine residues on proteins serves as a redox switch that regulates the function of a wide array of proteins. A key signaling pathway that is regulated by S-nitrosylation is apoptotic cell death. Here we will review the proteins in apoptotic pathways that are known to be S-nitrosylated by endogenous NO production. The targets and functional consequences of S-nitrosylation during apoptosis are multifaceted, allowing cells to fine tune their response to apoptotic signals.     

S-nitrosylation of Bcl-2 inhibits its ubiquitin-proteasomal degradation. A novel antiapoptotic mechanism that suppresses apoptosis

Bcl-2 is a key apoptosis regulatory protein of the mitochondrial death pathway whose function is dependent on its expression levels. Although Bcl-2 expression is controlled by various mechanisms, post-translational modifications, such as ubiquitination and proteasomal degradation, have emerged as important regulators of Bcl-2 function. However, the underlying mechanisms of this regulation are unclear. We report here that Bcl-2 undergoes S-nitrosylation by endogenous nitric oxide (NO) in response to multiple apoptotic mediators and that this modification inhibits ubiquitin-proteasomal degradation of Bcl-2. Inhibition of NO production by the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and by NO synthase inhibitor aminoguanidine effectively inhibited S-nitrosylation of Bcl-2, increased its ubiquitination, and promoted apoptotic cell death induced by chromium (VI). In contrast, the NO donors dipropylenetriamine NONOate and sodium nitroprusside showed opposite effects. The effect of NO on Bcl-2 stability was shown to be independent of its dephosphorylation. Mutational analysis of Bcl-2 further showed that the two cysteine residues of Bcl-2 (Cys158 and Cys229) are important in the S-nitrosylation process and that mutations of these cysteines completely inhibited Bcl-2 S-nitrosylation. Treatment of the cells with other stress inducers, including Fas ligand and buthionine sulfoxide, also induced Bcl-2 S-nitrosylation, suggesting that this is a general phenomenon that regulates Bcl-2 stability and function under various stress conditions. These findings indicate a novel function of NO and its regulation of Bcl-2, which provides a key mechanism for the control of apoptotic cell death and cancer development.     

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.     

Glyceraldehyde-3-phosphate, a glycolytic intermediate, prevents cells from apoptosis by lowering S-nitrosylation of glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate (G-3-P), the substrate of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is a key intermediate in several metabolic pathways. Recently, we reported that G-3-P directly inhibits caspase-3 activity in a reversible noncompetitive mode, suggesting the intracellular G-3-P level as a cell fate decision factor. It has been known that apoptotic stimuli induce the generation of NO, and NO S-nitrosylates GAPDH at the catalytic cysteine residue, which confers GAPDH the ability to bind to Siah-1, an E3 ubiquitin ligase. The GAPDH-Siah-1 complex is translocated into the nucleus and subsequently triggers the apoptotic process. Here, we clearly showed that intracellular G-3-P protects GAPDH from S-nitrosylation at above a certain level, and consequently maintains the cell survival. In case G-3-P drops below a certain level as a result of exposure to specific stimuli, G-3-P cannot inhibit S-nitrosylation of GAPDH anymore, and consequently GAPDH translocates with Siah-1 into the nucleus. Based on these results, we suggest that G-3-P functions as a molecule switch between cell survival and apoptosis by regulating S-nitrosylation of GAPDH.     

Selective role of intracellular chloride in the regulation of the intrinsic but not extrinsic pathway of apoptosis in Jurkat T-cells

Apoptosis is a genetic program for the removal of unwanted cells from an organism, which is distinct from necrosis by its characteristic volume loss or apoptotic volume decrease. This cell shrinkage is the result of ion redistribution that is crucial for both the activation and execution of apoptosis. Here we report that UV-C but not Fas ligand treatment results in a significant decrease in intracellular chloride that can be abolished by modulation of chloride flux using either the chloride channel inhibitor SITS or medium with a reduced chloride concentration. Accordingly, downstream events are diminished during UV-C-induced apoptosis following chloride flux modulation, whereas Fas ligand-induced apoptotic characteristics are not affected. Moreover, the activation of the mitogen-activated protein kinase signal transduction pathway early in the apoptotic signaling cascade was affected by chloride flux in Jurkat T-cells. Thus, an alteration of intracellular chloride plays an important role in the activation of signaling molecules upstream of the mitochondria, specifically impairing the intrinsic but not extrinsic apoptotic pathway.     

Molecular mechanisms of hepatic apoptosis

Apoptosis is a prominent feature of liver diseases. Causative factors such as alcohol, viruses, toxic bile acids, fatty acids, drugs, and immune response, can induce apoptotic cell death via membrane receptors and intracellular stress. Apoptotic signaling network, including membrane death receptor-mediated cascade, reactive oxygen species (ROS) generation, endoplasmic reticulum (ER) stress, lysosomal permeabilization, and mitochondrial dysfunction, is intermixed each other, but one mechanism may dominate at a particular stage. Mechanisms of hepatic apoptosis are complicated by multiple signaling pathways. The progression of liver disease is affected by the balance between apoptotic and antiapoptotic capabilities. Therapeutic options of liver injury are impacted by the clear understanding toward mechanisms of hepatic apoptosis.     

S-Nitrosylation of mitochondrial caspases

Caspase-3 is a cysteine protease located in both the cytoplasm and mitochondrial intermembrane space that is a central effector of many apoptotic pathways. In resting cells, a subset of caspase-3 zymogens is S-nitrosylated at the active site cysteine, inhibiting enzyme activity. During Fas-induced apoptosis, caspases are denitrosylated, allowing the catalytic site to function. In the current studies, we sought to identify the subpopulation of caspases that is regulated by S-nitrosylation. We report that the majority of mitochondrial, but not cytoplasmic, caspase-3 zymogens contain this inhibitory modification. In addition, the majority of mitochondrial caspase-9 is S-nitrosylated. These studies suggest that S-nitrosylation plays an important role in regulating mitochondrial caspase function and that the S-nitrosylation state of a given protein depends on its subcellular localization.     

Nucleocytoplasmic transport in apoptosis

The apoptotic demolition of the nucleus is accomplished by diverse proapoptotic factors, most of which are activated in the cytoplasm and gain access to the nucleoplasm during the cell death process. The nucleus is also the main target for genotoxic insult, a potent apoptotic trigger. Signals generated in the nucleus by DNA damage have to propagate to all cellular compartments to ensure the coordinated execution of cell demise. The nucleocytoplasmic shuttling of signalling and execution factors is thus an integral part of the apoptotic programme. Several proteins implicated in apoptotic cell death have been shown to migrate in and out of the nucleus following apoptosis induction. This review summarises the current knowledge on nucleocytoplasmic trafficking of apoptosis-relevant proteins. The effects of apoptosis induction on the nucleocytoplasmic transport machinery are also discussed. Finally, a potential role of nuclear transport as a critical control point of the apoptotic signal cascade is proposed.     

BOK and NOXA are essential mediators of p53-dependent apoptosis

Cellular stress leads to DNA damage and activation of the intrinsic apoptotic pathway in which translocation of mitochondrial cytochrome c to the cytosol plays a critical role. Previous studies have suggested alternative mechanisms responsible for this process. We examined initiation mechanisms of the intrinsic apoptotic pathway using human neuroblastoma and breast cancer cells. Results indicated that translocation of cytochrome c does not require prior activation of caspases but rather depends on activation of specific BCL-2 family members, depending upon the type of death signal. Thus, DNA damage-induced apoptosis requires new protein synthesis, accumulation of p53 tumor suppressor protein, and p53-dependent induction of BOK and NOXA genes, while a role for BAX in this pathway is not essential. In contrast, apoptosis induced by staurosporine does not require protein synthesis but is characterized by translocation of BAX. Based on these findings, we propose a model of the intrinsic apoptotic cascade induced by DNA damage where proapoptotic BOK substitutes for a function of BAX.     

Nitrosative stress inhibits the aminophospholipid translocase resulting in phosphatidylserine externalization and macrophage engulfment: implications for the resolution of inflammation

Macrophage recognition of apoptotic cells depends on externalization of phosphatidylserine (PS), which is normally maintained within the cytosolic leaflet of the plasma membrane by aminophospholipid translocase (APLT). APLT is sensitive to redox modifications of its -SH groups. Because activated macrophages produce reactive oxygen and nitrogen species, we hypothesized that macrophages can directly participate in apoptotic cell clearance by S-nitrosylation/oxidation and inhibition of APLT causing PS externalization. Here we report that exposure of target HL-60 cells to nitrosative stress inhibited APLT, induced PS externalization, and enhanced recognition and elimination of "nitrosatively" modified cells by RAW 264.7 macrophages. Using S-nitroso-L-cysteine-ethyl ester (SNCEE) and S-nitrosoglutathione (GSNO) that cause intracellular and extracellular trans-nitrosylation of proteins, respectively, we found that SNCEE (but not GSNO) caused significant S-nitrosylation/oxidation of thiols in HL-60 cells. SNCEE also strongly inhibited APLT, activated scramblase, and caused PS externalization. However, SNCEE did not induce caspase activation or nuclear condensation/fragmentation suggesting that PS externalization was dissociated from the common apoptotic pathway. Dithiothreitol reversed SNCEE-induced S-nitrosylation, APLT inhibition, and PS externalization. SNCEE but not GSNO stimulated phagocytosis of HL-60 cells. Moreover, phagocytosis of target cells by lipopolysaccharide-stimulated macrophages was significantly suppressed by an NO. scavenger, DAF-2. Thus, macrophage-induced nitrosylation/oxidation plays an important role in cell clearance, and hence in the resolution of inflammation.     

Clofibrate-induced apoptosis is mediated by Ca2+-dependent caspase-12 activation

The mechanism of fibrate-induced myopathy was investigated in this report. When clofibrate (30 to 300 microM) was applied to L6 rat skeletal myoblasts, dose-dependently apoptosis was observed within 24 h. In the apoptotic myoblasts, a caspase-12 cleavage was observed at 2 h and with following caspases-9 and -3-related cascade activation. In contrast, the neutral protease calpain, that is a key enzyme in ER stress-related apoptosis via caspase-12 activation, was significantly decreased during apoptosis. Next, the authors evaluated a role of calcium-dependent signal(s). When clofibrate was added into medium, cytosolic calcium concentration was rapidly and persistently increased. On the other hand, an addition of 10 mM EGTA depressed sustained calcium phase, and concurrent myoblasts apoptosis was completely inhibited. Taken together, our findings indicate that the clofibrate-induced myopathy is triggered by Ca2+ influx, then activated cytosolic caspase-12 through calpain-independent cascade, and consequently caused apoptotic DNA fragmentation.     

Nitric oxide-GAPDH-Siah: a novel cell death cascade

1. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is an extremely abundant glycolytic enzyme, and exemplifies the class of proteins with multiple, seemingly unrelated functions. Recent studies indicate that it is a major intracellular messenger mediating apoptotic cell death. This paper reviews the GAPDH cell death cascade and discusses its clinical relevance. 2. A wide range of apoptotic stimuli activate NO formation, which S-nitrosylates GAPDH. The S-nitrosylation abolishes catalytic activity and confers upon GAPDH the ability to bind to Siah, an E3-ubiquitin-ligase, which translocates GAPDH to the nucleus. In the nucleus, GAPDH stabilizes the rapidly turning over Siah, enabling it to degrade selected target proteins and affect apoptosis. 3. The cytotoxicity of mutant Huntingtin (mHtt) requires nuclear translocation which appears to be mediated via a ternary complex of GAPDH-Siah-mHtt. The neuroprotective actions of the monoamine oxidase inhibitor R-(-)-deprenyl (deprenyl) reflect blockade of GAPDH-Siah binding. Thus, novel cytoprotective therapies may emerge from agents that prevent GAPDH-Siah binding.     

Detection by the comet assay of apoptosis induced in lymphoid cell lines after growth factor deprivation

Dysregulation of apoptosis contributes to various diseases such as neurodegenerative or aging disorders, autoimmune syndromes or cancers. Numerous experimental paradigms have been explored to characterize molecular and cellular modulators of apoptosis. Similarly, numerous techniques have been described for detecting and/or quantifying accurately cells committed to apoptosis. Besides the conventional techniques, we describe in this report that the comet assay, which detects DNA single- and double-strand breaks in situ, at the cellular level, is relevant for the characterization of apoptotic cells. The comet assay is very sensitive and detects DNA fragmentation occurring in the apoptotic process as early as exposure of phosphatidylserine residues on the outer leaflet. Thus the comet assay can be used for the recognition of apoptosis that follows the death signal caused, for example, by genotoxic stress as well as lack of survival signal as in growth factor deprivation.