Activation of src tyrosine kinases by peroxynitrite

Unveiling the human nitroproteome: Protein tyrosine nitration in cell signaling and cancer

Covalent amino acid modification significantly expands protein functional capability in regulating biological processes. Tyrosine residues can undergo phosphorylation, sulfation, adenylation, halogenation, and nitration. These posttranslational modifications (PTMs) result from the actions of specific enzymes: tyrosine kinases, tyrosyl-protein sulfotransferase(s), adenylate transferase(s), oxidoreductases, peroxidases, and metal-heme containing proteins. Whereas phosphorylation, sulfation, and adenylation modify the hydroxyl group of tyrosine, tyrosine halogenation and nitration target the adjacent carbon residues. Because aberrant tyrosine nitration has been associated with human disorders and with animal models of disease, we have created an updated and curated database of 908 human nitrated proteins. We have also analyzed this new resource to provide insight into the role of tyrosine nitration in cancer biology, an area that has not previously been considered in detail. Unexpectedly, we have found that 879 of the 1971 known sites of tyrosine nitration are also sites of phosphorylation suggesting an extensive role for nitration in cell signaling. Overall, the review offers several forward-looking opportunities for future research and new perspectives for understanding the role of tyrosine nitration in cancer biology.

c-Src Promotes Tumorigenesis and Tumor Progression by Activating PFKFB3

Reprogramming of glucose metabolism is a key event in tumorigenesis and progression. Here, we show that active c-Src stimulates glycolysis by phosphorylating (Tyr194) and activating PFKFB3, a key enzyme that boosts glycolysis by producing fructose-2,6-bisphosphate and activating PFK1. Increased glycolysis intermediates replenish non-oxidative pentose phosphate pathway (PPP) and serine pathway for biosynthesis of cancer cells. PFKFB3 knockout (KO) cells and their counterpart reconstituted with PFKFB3-Y194F show comparably impaired abilities for proliferation, migration, and xenograft formation. Furthermore, PFKFB3-Y194F knockin mice show impaired glycolysis and, mating of these mice with APC^min/+ mice attenuates spontaneous colon cancer formation in APC^min/+ mice. In summary, we identify a specific mechanism by which c-Src mediates glucose metabolism to meet cancer cells' requirements for maximal biosynthesis and proliferation. The PFKFB3-Tyr194 phosphorylation level highly correlates with c-Src activity in clinical tumor samples, indicating its potential as an evaluation for tumor prognosis.

Oxidation Impacts the Intracellular Signaling Machinery in Hematological Disorders

The dynamic coordination between kinases and phosphatases is crucial for cell homeostasis, in response to different stresses. The functional connection between oxidation and the intracellular signaling machinery still remains to be investigated. In the last decade, several studies have highlighted the role of reactive oxygen species (ROS) as modulators directly targeting kinases, phosphatases, and downstream modulators, or indirectly acting on cysteine residues on kinases/phosphatases resulting in protein conformational changes with modulation of intracellular signaling pathway(s). Translational studies have revealed the important link between oxidation and signal transduction pathways in hematological disorders. The intricate nature of intracellular signal transduction mechanisms, based on the generation of complex networks of different types of signaling proteins, revealed the novel and important role of phosphatases together with kinases in disease mechanisms. Thus, therapeutic approaches to abnormal signal transduction pathways should consider either inhibition of overactivated/accumulated kinases or homeostatic signaling resetting through the activation of phosphatases. This review discusses the progress in the knowledge of the interplay between oxidation and cell signaling, involving phosphatase/kinase systems in models of globally distributed hematological disorders.

Activation of Phosphotyrosine-Mediated Signaling Pathways in the Cortex and Spinal Cord of SOD1G93A, a Mouse Model of Familial Amyotrophic Lateral Sclerosis

Degeneration of cortical and spinal motor neurons is the typical feature of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease for which a pathogenetic role for the Cu/Zn superoxide dismutase (SOD1) has been demonstrated. Mice overexpressing a mutated form of the SOD1 gene (SOD1^G93A) develop a syndrome that closely resembles the human disease. The SOD1 mutations confer to this enzyme a “gain-of-function,” leading to increased production of reactive oxygen species. Several oxidants induce tyrosine phosphorylation through direct stimulation of kinases and/or phosphatases. In this study, we analyzed the activities of src and fyn tyrosine kinases and of protein tyrosine phosphatases in synaptosomal fractions prepared from the motor cortex and spinal cord of transgenic mice expressing SOD1^G93A. We found that (i) protein phosphotyrosine level is increased, (ii) src and fyn activities are upregulated, and (iii) the activity of tyrosine phosphatases, including the striatal-enriched tyrosine phosphatase (STEP), is significantly decreased. Moreover, the NMDA receptor (NMDAR) subunit GluN2B tyrosine phosphorylation was upregulated in SOD1^G93A. Tyrosine phosphorylation of GluN2B subunits regulates the NMDAR function and the recruitment of downstream signaling molecules. Indeed, we found that proline-rich tyrosine kinase 2 (Pyk2) and ERK1/2 kinase are upregulated in SOD1^G93A mice. These results point out an involvement of tyrosine kinases and phosphatases in the pathogenesis of ALS.

Mechanical perturbations trigger endothelial nitric oxide synthase activity in human red blood cells

Nitric oxide (NO), a vascular signaling molecule, is primarily produced by endothelial NO synthase. Recently, a functional endothelial NO synthase (eNOS) was described in red blood cells (RBC). The RBC-eNOS contributes to the intravascular NO pool and regulates physiological functions. However the regulatory mechanisms and clinical implications of RBC-eNOS are unknown. The present study investigated regulation and functions of RBC-eNOS under mechanical stimulation. This study shows that mechanical stimuli perturb RBC membrane, which triggers a signaling cascade to activate the eNOS. Extracellular NO level, estimated by the 4-Amino-5-Methylamino-2', 7'-Difluorofluorescein Diacetate probe, was significantly increased under mechanical stimuli. Immunostaining and western blot studies confirmed that the mechanical stimuli phosphorylate the serine 1177 moiety of RBC-eNOS, and activates the enzyme. The NO produced by activation of RBC-eNOS in vortexed RBCs promoted important endothelial functions such as migration and vascular sprouting. We also show that mechanical perturbation facilitates nitrosylation of RBC proteins via eNOS activation. The results of the study confirm that mechanical perturbations sensitize RBC-eNOS to produce NO, which ultimately defines physiological boundaries of RBC structure and functions. Therefore, we propose that mild physical perturbations before, after, or during storage can improve viability of RBCs in blood banks.

Effect of Cigarette Smoking on Epithelial to Mesenchymal Transition (EMT) in Lung Cancer

Epithelial to mesenchymal transition (EMT) is a process that allows an epithelial cell to acquire a mesenchymal phenotype through multiple biochemical changes resulting in an increased migratory capacity. During cancer progression, EMT is found to be associated with an invasive or metastatic phenotype. In this review, we focus on the discussion of recent studies about the regulation of EMT by cigarette smoking. Various groups of active compounds found in cigarette smoke such as polycyclic aromatic hydrocarbons (PAH), nicotine-derived nitrosamine ketone (NNK), and reactive oxygen specicies (ROS) can induce EMT through different signaling pathways. The links between EMT and biological responses to cigarette smoke, such as hypoxia, inflammation, and oxidative damages, are also discussed. The effect of cigarette smoke on EMT is not only limited to cancer types directly related to smoking, such as lung cancer, but has also been found in other types of cancer. Altogether, this review emphasizes the importance of understanding molecular mechanisms of the induction of EMT by cigarette smoking and will help in identifying novel small molecules for targeting EMT induced by smoking.

4-Hydroxynonenal activates Src through a non-canonical pathway that involves EGFR/PTP1B

Src, a non-receptor protein tyrosine kinase involved in many biological processes, can be activated through both redox-dependent and independent mechanisms. 4-Hydroxy-2-nonenal (HNE) is a lipid peroxidation product that is increased in pathophysiological conditions associated with Src activation. This study examined how HNE activates human c-Src. In the canonical pathway Src activation is initiated by dephosphorylation of pTyr530 followed by conformational change that causes Src auto-phosphorylation at Tyr419 and its activation. HNE increased Src activation in both dose- and time-dependent manner, while it also increased Src phosphorylation at Tyr530 (pTyr530 Src), suggesting that HNE activated Src via a non-canonical mechanism. Protein tyrosine phosphatase 1B inhibitor (539741), at concentrations that increased basal pTyr530 Src, also increased basal Src activity and significantly reduced HNE-mediated Src activation. The EGFR inhibitor, AG1478, and EGFR silencing, abrogated HNE-mediated EGFR activation and inhibited basal and HNE-induced Src activity. In addition, AG1478 also eliminated the increase of basal Src activation by a PTP1B inhibitor. Taken together these data suggest that HNE can activate Src partly through a non-canonical pathway involving activation of EGFR and inhibition of PTP1B.

New Insight into the Role of Reactive Oxygen Species (ROS) in Cellular Signal-Transduction Processes

Reactive oxygen species (ROS) were once considered to be deleterious agents, contributing to a vast range of pathologies. But, now their protective effects are being appreciated. Both their damaging and beneficial effects are initiated when they target distinct molecules and consequently begin functioning as part of complex signal-transduction pathways. The recognition of ROS as signaling mediators has driven a wealth of research into their roles in both normal and pathophysiological states. The present review assesses the relevant recent literature to outline the current perspectives on redox-signaling mechanisms, physiological implications, and therapeutic strategies. This study highlights that a more fundamental knowledge about many aspects of redox signaling will allow better targeting of ROS, which would in turn improve prophylactic and pharmacotherapy for redox-associated diseases.

TGFβ1 rapidly activates Src through a non-canonical redox signaling mechanism

Transforming growth factor-β1 (TGF-β) is involved in multiple cellular processes through Src activation. In the canonical pathway, Src activation is initiated by pTyr530 dephosphorylation followed by a conformational change allowing Tyr419 auto-phosphorylation. A non-canonical pathway in which oxidation of cysteine allows bypassing of pTyr530 dephosphorylation has been reported. Here, we examined how TGF-β activates Src in H358 cells, a small cell lung carcinoma cell line. TGF-β increased Src Tyr419 phosphorylation, but surprisingly, Tyr530 phosphorylation was increased rather than decreased. Vanadate, a protein tyrosine phosphatase inhibitor, stimulated Src activation itself, but rather than inhibiting Src activation by TGF-β, activation by vanadate was additive with TGF-β showing that pTyr530 dephosphorylation was not required. Thus, the involvement of the non-canonical oxidative activation was suspected. TGF-β increased extracellular H2O2 transiently while GSH-ester and catalase abrogated Src activation by TGF-β. Apocynin, a NADPH oxidase inhibitor, inhibited TGF-β-stimulated H2O2 production. Furthermore, mutation of cysteines to alanine, 248C/A, 277C/A, or 501C/A abrogated, while 490C/A significantly reduced, TGF-β-mediated Src activation. Taken together, the results indicate that TGF-β-mediated Src activation operates largely through a redox dependent mechanism, resulting from enhanced H2O2 production through an NADPH oxidase and that cysteines 248, 277, 490, and 501 are critical for this activation.

Oxidative and Nitrosative Stress and Immune-Inflammatory Pathways in Patients with Myalgic Encephalomyelitis (ME)/Chronic Fatigue Syndrome (CFS)

Myalgic Encephalomyelitis (ME) / Chronic Fatigue Syndrome (CFS) has been classified as a disease of the central nervous system by the WHO since 1969. Many patients carrying this diagnosis do demonstrate an almost bewildering array of biological abnormalities particularly the presence of oxidative and nitrosative stress (O&NS) and a chronically activated innate immune system. The proposal made herein is that once generated chronically activated O&NS and immune-inflammatory pathways conspire to generate a multitude of self-sustaining and self-amplifying pathological processes which are associated with the onset of ME/CFS. Sources of continuous activation of O&NS and immune-inflammatory pathways in ME/CFS are chronic, intermittent and opportunistic infections, bacterial translocation, autoimmune responses, mitochondrial dysfunctions, activation of the Toll-Like Receptor Radical Cycle, and decreased antioxidant levels. Consequences of chronically activated O&NS and immune-inflammatory pathways in ME/CFS are brain disorders, including neuroinflammation and brain hypometabolism / hypoperfusion, toxic effects of nitric oxide and peroxynitrite, lipid peroxidation and oxidative damage to DNA, secondary autoimmune responses directed against disrupted lipid membrane components and proteins, mitochondrial dysfunctions with a disruption of energy metabolism (e.g. compromised ATP production) and dysfunctional intracellular signaling pathways. The interplay between all of these factors leads to self-amplifying feed forward loops causing a chronic state of activated O&NS, immune-inflammatory and autoimmune pathways which may sustain the disease.

Acute lung injury and pulmonary vascular permeability: use of transgenic models

Acute lung injury is a general term that describes injurious conditions that can range from mild interstitial edema to massive inflammatory tissue destruction. This review will cover theoretical considerations and quantitative and semi-quantitative methods for assessing edema formation and increased vascular permeability during lung injury. Pulmonary edema can be quantitated directly using gravimetric methods, or indirectly by descriptive microscopy, quantitative morphometric microscopy, altered lung mechanics, high-resolution computed tomography, magnetic resonance imaging, positron emission tomography, or x-ray films. Lung vascular permeability to fluid can be evaluated by measuring the filtration coefficient (Kf) and permeability to solutes evaluated from their blood to lung clearances. Albumin clearances can then be used to calculate specific permeability-surface area products (PS) and reflection coefficients (σ). These methods as applied to a wide variety of transgenic mice subjected to acute lung injury by hyperoxic exposure, sepsis, ischemia-reperfusion, acid aspiration, oleic acid infusion, repeated lung lavage, and bleomycin are reviewed. These commonly used animal models simulate features of the acute respiratory distress syndrome, and the preparation of genetically modified mice and their use for defining specific pathways in these disease models are outlined. Although the initiating events differ widely, many of the subsequent inflammatory processes causing lung injury and increased vascular permeability are surprisingly similar for many etiologies.

Posttranslational nitration of tyrosine residues modulates glutamate transmission and contributes to N-methyl-D-aspartate-mediated thermal hyperalgesia

Activation of the N-methyl-D-aspartate receptor (NMDAR) is fundamental in the development of hyperalgesia. Overactivation of this receptor releases superoxide and nitric oxide that, in turn, forms peroxynitrite (PN). All of these events have been linked to neurotoxicity. The receptors and enzymes involved in the handling of glutamate pathway—specifically NMDARs, glutamate transporter, and glutamine synthase (GS)—have key tyrosine residues which are targets of the nitration process causing subsequent function modification. Our results demonstrate that the thermal hyperalgesia induced by intrathecal administration of NMDA is associated with spinal nitration of GluN1 and GluN2B receptor subunits, GS, that normally convert glutamate into nontoxic glutamine, and glutamate transporter GLT1. Intrathecal injection of PN decomposition catalyst FeTM-4-PyP⁵⁺ prevents nitration and overall inhibits NMDA-mediated thermal hyperalgesia. Our study supports the hypothesis that nitration of key proteins involved in the regulation of glutamate transmission is a crucial pathway used by PN to mediate the development and maintenance of NMDA-mediated thermal hyperalgesia. The broader implication of our findings reinforces the notion that free radicals may contribute to various forms of pain events and the importance of the development of new pharmacological tool that can modulate the glutamate transmission without blocking its actions directly.

The good and bad effects of cysteine S-nitrosylation and tyrosine nitration upon insulin exocytosis: a balancing act

As understanding of the mechanisms driving and regulating insulin secretion from pancreatic beta cells grows, there is increasing and compelling evidence that nitric oxide (•NO) and other closely-related reactive nitrogen species (RNS) play important roles in this exocytic process. •NO and associated RNS, in particular peroxynitrite, possess the capability to effect signals across both intracellular and extracellular compartments in rapid fashion, affording extraordinary signaling potential. It is well established that nitric oxide signals through activation of guanylate cyclase-mediated production of cyclic GMP. The intricate intracellular redox environment, however, lends credence to the possibility that •NO and peroxynitrite could interact with a wider variety of biological targets, with two leading mechanisms involving 1) Snitrosylation of cysteine, and 2) nitration of tyrosine residues comprised within a variety of proteins. Efforts aimed at delineating the specific roles of •NO and peroxynitrite in regulated insulin secretion indicate that a highly-complex and nuanced system exists, with evidence that •NO and peroxynitrite can contribute in both positive and negative regulatory ways in beta cells. Furthermore, the ultimate biochemical outcome within beta cells, whether to compensate and recover from a given stress, or not, is likely a summation of contributory signals and redox status. Such seeming regulatory dichotomy provides ample opportunity for these mechanisms to serve both physiological and pathophysiologic roles in onset and progression of diabetes. This review focuses attention upon recent accumulating evidence pointing to roles for nitric oxide induced post-translational modifications in the normal regulation as well as the dysfunction of beta cell insulin exocytosis.

Cigarette smoke extract stimulates epithelial-mesenchymal transition through Src activation

Epithelial-mesenchymal transition (EMT) is implicated in the pathogenesis of lung fibrosis and cancer metastasis, two conditions associated with cigarette smoke (CS). CS has been reported to promote the EMT process. CS is the major cause of lung cancer and nearly half of lung cancer patients are active smokers. Nonetheless, the mechanism whereby CS induces EMT remains largely unknown. In this study we investigated the induction of EMT by CS and explored the underlying mechanisms in the human non-small-cell lung carcinoma (H358) cell line. We demonstrate that exposure to an extract of CS (CSE) decreases E-cadherin and increases N-cadherin and vimentin, markers of EMT, in H358 cells cultured in RPMI 1640 medium with 1% fetal bovine serum. Pretreatment with N-acetylcysteine (NAC), a potent antioxidant and precursor of glutathione, abrogated changes in these EMT markers. In addition, CSE activated Src kinase (shown as increased phosphorylation of Src at Tyr418), and the Src kinase inhibitor PP2 inhibited CS-stimulated EMT changes, suggesting that Src is critical in CSE-stimulated EMT induction. Furthermore, NAC treatment abrogated CSE-stimulated Src activation. However, co-incubation with catalase had no effect on CSE-mediated Src activation. Finally, acrolein, an unsaturated aldehyde present in CSE, caused Src activation. Taken together, these data suggest that CSE initiates EMT through Src, which is activated by CS through redox modification.

Stored red blood cells: a changing universe waiting for its map(s)

The availability of stored red blood cells (RBCs) for transfusion remains an important aspect of the treatment of polytrauma, acute anemia or major bleedings. RBCs are prepared by blood banks from whole blood donations and stored in the cold in additive solutions for typically six weeks. These far from physiological storage conditions result in the so-called red cell storage lesion that is of importance both to blood bankers and to clinical practitioners. Here we review the current state of knowledge about the red cell storage lesion from a proteomic perspective. In particular, we describe the current models accounting for RBC aging and response to lethal stresses, review the published proteomic studies carried out to uncover the molecular basis of the RBC storage lesion, and conclude by suggesting a few possible proteomic studies that would provide further knowledge of the molecular alterations carried by RBCs stored in the cold for six weeks.

Peroxynitrite induces tyrosine residue modifications in synaptophysin C-terminal domain, affecting its interaction with src

Peroxynitrite is a potent oxidant that contributes to tissue damage in neurodegenerative disorders. We have previously reported that treatment of rat brain synaptosomes with peroxynitrite induced post-translational modifications in pre- and post-synaptic proteins and stimulated soluble N-ethylmaleimide sensitive fusion proteins attachment receptor complex formation and endogenous glutamate release. In this study we show that, following peroxynitrite treatment, the synaptic vesicle protein synaptophysin (SYP) can be both phosphorylated and nitrated in a dose-dependent manner. We found that tyrosine-phosphorylated, but not tyrosine-nitrated, SYP bound to the src tyrosine kinase and enhanced its catalytic activity. These effects were mediated by direct and specific binding of the SYP cytoplasmic C-terminal tail with the src homology 2 domain. Using mass spectrometry analysis, we mapped the SYP C-terminal tail tyrosine residues modified by peroxynitrite and found one nitration site at Tyr250 and two phosphorylation sites at Tyr263 and Tyr273. We suggest that peroxynitrite-mediated modifications of SYP may be relevant in modulating src signalling of synaptic terminal in pathophysiological conditions.

Effect of nitric oxide donor and gamma irradiation on modifications of ERK and JNK in murine peritoneal macrophages

Mitogen activated protein kinases (MAPKs) play an important role in activation, differentiation and proliferation of macrophages. Macrophages, upon activation, produce large amounts of nitric oxide that inhibit the growth of variety of microorganisms and tumor cells. This nitric oxide which is known to interfere with tyrosine phosphorylation may result in changes in the pattern of activation of MAPKs. In a previous study we have found that tyrosine phosphorylation of MAPKs was completely abolished in the presence of nitric oxide donor and radiation but this did not affect the function of macrophages. In this study the other post translational modifications namely nitration and ubiquitination of JNK and ERK have been looked at. Both ERK and JNK were found to be nitrated. However, there was no increase in ubiquitination of ERK and JNK, indicating that ubiquitination, in this case was not a natural consequence of nitration and may serve in signaling. Additionally, when the nitration was extensive, phosphorylation was also inhibited. The activation of substrates of ERK and JNK were looked at to determine the consequences of such modifications. Inhibition of phosphorylation and extensive nitration of JNK did not prevent activation of its substrate, c-jun. This study indicates that ERK and JNK may be under regulation by different type of modifications in macrophages.

Effect of nitric oxide donor and gamma irradiation on MAPK signaling in murine peritoneal macrophages

Irradiation (IR) of cells is known to activate enzymes of mitogen activated protein kinase (MAPK) family. These are known to be involved in cellular response to stress and are determinants of cell death or survival. When radiotherapy is delivered to malignant cells, macrophages, being radioresistant, survive, get activated, and produce large amounts of nitric oxide. As a result of activation they recognize and phagocytose tumor and normal cell apoptotic bodies leading to tumor regression. In this study, the MAPK signaling in peritoneal macrophages was investigated which plays an important role in its various functions, in an environment which is predominantly nitric oxide, as is after IR. The behavior of macrophages in such an environment was also looked at. The three MAPK (ERK1/2, p38, and JNK) respond differently to Sodium nitroprusside (SNP) alone or IR alone. All the three were activated following IR but only JNK was activated following SNP treatment. Surprisingly, when both the stresses were given simultaneously or one after the other, this differential response was lost and there was a complete inhibition of phosphorylation of all the three MAPKs, irrespective of the order of the two insults (IR and SNP). The noteworthy observation was that despite the complete inhibition of MAPK signaling there was no effect on either the viability or the phagocytic efficiency of peritoneal macrophages.

Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling

Reversible phosphorylation of protein tyrosine residues by polypeptide growth factor-receptor protein tyrosine kinases is implicated in the control of fundamental cellular processes including the cell cycle, cell adhesion, and cell survival, as well as cell proliferation and differentiation. During the last decade, it has become apparent that receptor protein tyrosine kinases and the signaling pathways they activate belong to a large signaling network. Such a network can be regulated by various extracellular cues, which include cell adhesion, agonists of G protein-coupled receptors, and oxidants. It is well documented that signaling initiated by receptor protein tyrosine kinases is directly dependent on the intracellular production of oxidants, including reactive oxygen and nitrogen species. Accumulated evidence indicates that the intracellular redox environment plays a major role in the mechanisms underlying the actions of growth factors. Oxidation of cysteine thiols and nitration of tyrosine residues on signaling proteins are described as posttranslational modifications that regulate, positively or negatively, protein tyrosine phosphorylation (PTP). Early observations described the inhibition of PTP activities by oxidants, resulting in increased levels of proteins phosphorylated on tyrosine. Therefore, a redox circuitry involving the increasing production of intracellular oxidants associated with growth-factor stimulation/cell adhesion, oxidative reversible inhibition of protein tyrosine phosphatases, and the activation of protein tyrosine kinases can be delineated.

Cellular prion protein electron microscopy: attempts/limits and clues to a synaptic trait. Implications in neurodegeneration process

Prion diseases are caused by an infectious agent constituted by a rogue protein called prion (PrP Sc) of neuronal origin (PrP c) and are exemplified by Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle. Considerable efforts have been made to understand the cerebral damage caused by these diseases but a clear comprehensive view cannot be achieved without defining the neurophysiological function of PrP c. This lack of information is in part attributable to our ignorance of the precise localization of PrP c in the brain neuronal cell. One relevant option to explore this aspect is to undertake PrP immunohistochemistry at the electron-microscopy level, knowing that this challenge raises major technical constraints. In describing the attempts and restrictions of the various approaches used, we review here the efforts that have been invested in this particular field of prionology. The common result emerging from these contributions is that the synapse could be the site at which PrP c exerts its critical activity. This location suggests, in the perspective of synaptic regulation, that PrP c can be assigned multiple biological functions and supports the novel concept that prion-like changes are involved in long-term memory formation. The synaptic trait of PrP c and PrP Sc suggests that synapse loss is the key event in neuronal death. Interestingly, synaptic alterations are also considered to be predominant in the pathophysiological mechanism in Alzheimer, Parkinson and Huntington diseases. All these brain disorders, characterized by the formation of a specific amyloid protein of synaptic origin, can be classified under the heading of amyloidogenic synaptopathies.

Exercise and immobilization in aging animals: the involvement of oxidative stress and NF-kappaB activation

In the early 1980s, the concept of threshold of age in exercise and aging was proposed. In several studies it was shown that subjecting young animals to short periods of moderate to intense exercise improved the biochemical and morphological status of their skeletal muscles. This was not the case for old animals subjected to the same exercise regimens. Thus, by measuring several muscle energy-providing enzymes as well as antioxidant enzymes it was demonstrated that their levels and activities increased in young animals postexercise, while in old animals reduced activity of these enzymes was found on completion of the training. However, old animals that started training in young and middle age were still capable of improving their muscle condition as a result of exercise, as long as the onset of training was below a specific age threshold. In the following years, it was shown that intense physical exercise in young humans and animals is accompanied by elevation of oxidative stress parameters in muscles and other organs. Specifically, strenuous training of animals led to increased protein oxidation as measured by protein carbonyl accumulation in muscles, which could be attenuated by the administration of vitamin E. Nuclear factor kappaB (NF-kappaB) is a redox-sensitive transcription factor responsive to closely related reactive oxygen species (ROS) and reactive nitrogen species (RNS) redox cascades. Its involvement in exercise and immobilization has been demonstrated in several studies, indicating that these conditions may lead to inflammatory responses and to oxidative damage to tissues. Indeed, recent studies have revealed that NF-kappaB is involved in inflammatory responses that may result in muscle protein degradation. Additional studies have also demonstrated that the pattern and type of the NF-kappaB activation pathway vary between muscles of young and old animals subjected to limb immobilization for several weeks. This indicates that NF-kappaB may play a crucial role in the regulation of both inflammatory processes and protein turnover and degradation in muscles of old animals. Thus, the modulation of NF-kappaB activity in muscles of old animals by specific inhibitors may provide a means to retard muscle damage and protein degradation under conditions of immobilization.

Red blood cells as a model to differentiate between direct and indirect oxidation pathways of peroxynitrite

Red blood cells are the major physiological scavengers of reactive nitrogen species and have been proposed as real-time biomarkers of some vascular-related diseases. This chapter proposes that the erythrocyte is a suitable cell model for studying the modifications induced by peroxynitrite. Peroxynitrite decays both extra- and intracellularly as a function of cell density and CO(2) concentration, inducing the appearance of distinct cellular biomarkers, as well as the modulation of signaling and metabolism. Intracellular oxidations are due mostly to direct reactions of peroxynitrite with hemoglobin but also lead to the appearance of apoptotic biomarkers. Surface/membrane oxidations are due principally to indirect radical reactions generated by CO(2)-catalyzed peroxynitrite homolysis.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Proteomic analysis of phosphorylation, oxidation and nitrosylation in signal transduction

Signal transduction pathways control cell fate, survival and function. They are organized as intricate biochemical networks which enable biochemical protein activities, crosstalk and subcellular localization to be integrated and tuned to produce highly specific biological responses in a robust and reproducible manner. Post translational Modifications (PTMs) play major roles in regulating these processes through a wide variety of mechanisms that include changes in protein activities, interactions, and subcellular localizations. Determining and analyzing PTMs poses enormous challenges. Recent progress in mass spectrometry (MS) based proteomics have enhanced our capability to map and identify many PTMs. Here we review the current state of proteomic PTM analysis relevant for signal transduction research, focusing on two areas: phosphorylation, which is well established as a widespread key regulator of signal transduction; and oxidative modifications, which from being primarily viewed as protein damage now start to emerge as important regulatory mechanisms.

Quinolinic acid modulates the activity of src family kinases in rat striatum: in vivo and in vitro studies

Quinolinic acid (QA) has been shown to evoke neurotoxic events via NMDA receptor (NMDAR) overactivation and oxidative stress. NMDARs are particularly vulnerable to free radicals, which can modulate protein tyrosine kinase (PTK) and phosphotyrosine phosphatase (PTP) activities. The src family of tyrosine kinases are associated with the NMDAR complex and regulate NMDA channel function. Because QA is an NMDAR agonist as well as a pro-oxidant agent, we investigated whether it may affect the activity of PTKs and PTPs in vivo and in vitro. In synaptosomes prepared from striata dissected 15 min, 30 min or 15 days after bilateral injection of QA we observed modulation of the phosphotyrosine pattern; a significant decrease in PTP activity; and a sustained increase in c-src and lyn activity at 15 and 30 min after treatment with QA, followed by a decrease 2 weeks later. Striatal synaptosomes treated in vitro with QA showed time- and dose-dependent modulation of c-src and lyn kinase activities. Moreover, the nitric oxide synthase inhibitor NG-nitro-L-arginine-methyl ester, the NMDAR antagonist d-2-amino-5-phosphonovaleric acid and pyruvate suppressed the QA-induced modulation of c-src activity. These findings suggest a novel feature of QA in regulating src kinase activity through the formation of reactive radical species and/or NMDAR overactivation.

Survival pathways triggered by peroxynitrite in cells belonging to the monocyte/macrophage lineage

Peroxynitrite, a highly reactive nitrogen species, promotes in U937 cells (a promonocytic cell line) a mitochondrial permeability transition (MPT)-dependent necrosis. An initial event triggered by peroxynitrite (i.e., inhibition of complex III of the mitochondrial respiratory chain) is responsible for the time-dependent formation of H(2)O(2), essential for the occurrence of cell death. Otherwise non-toxic concentrations of peroxynitrite nevertheless commit cells to MPT-dependent necrosis, which is however prevented by a cytoprotective signaling driven by arachidonic acid (AA) released by the cytosolic PLA(2) isoform. Interestingly, the mechanism whereby delayed formation of H(2)O(2) promotes toxicity in cells exposed to intrinsically toxic concentrations of peroxynitrite is independent of the accumulation of additional damage. Cell death is in fact mediated by inhibition of the AA-dependent cytoprotective signaling. Exogenous AA, however, prevented toxicity also under these conditions. An additional point to be made is that the major findings obtained using U937 cells were reproduced in different cell types belonging to the monocyte/macrophage lineage. Hence, within the context of the inflammatory response, monocytes and macrophages may cope with peroxynitrite by using AA, a signaling molecule largely available at the inflammatory sites.

Protein phosphatase 1alpha is tyrosine-phosphorylated and inactivated by peroxynitrite in erythrocytes through the src family kinase fgr

Protein serine/threonine phosphorylation is a significant component of the intracellular signal that together with tyrosine phosphorylation regulates several processes, including cell-cycle progression, muscle contraction, transcription, and neuronal signaling. Cross-talk between phosphoserine/threonine- and phosphotyrosine-mediated pathways is not yet well understood. In this study we found that peroxynitrite, a physiological oxidant formed by the fast radical-radical reaction between the nitric oxide and the superoxide anion, induced tyrosine phosphorylation of the serine/threonine protein phosphatase 1alpha (PP1alpha) in human erythrocytes through activation of src family kinases. We have previously shown in mouse red cells that upregulation of the src kinase fgr phosphorylates PP1alpha, acting as an upstream negative regulator of PP1alpha, and downregulates K-Cl cotransport. Here we found that PP1alpha is a selective substrate of peroxynitrite-activated fgr and that tyrosine phosphorylation of PP1alpha corresponds to an inhibition of its enzymatic activity. Despite fgr activation and PP1alpha downregulation, peroxynitrite stimulated in a dose-dependent fashion the function of the K-Cl cotransporter. In an attempt to understand the mechanism of K-Cl cotransport activation, we found that the effect of peroxynitrite is completely reversed by dithriothreitol, suggesting that peroxynitrite acts as an oxidizing agent by an SH-dependent and PP1alpha-independent mechanism. These findings highlight a novel function of peroxynitrite in regulating the intracellular signal transduction pathways involving serine/threonine phosphorylation and the functional role of proteins that are targets of these phosphatases.

Effector-induced Syk-mediated phosphorylation in human erythrocytes

Band 3 (AE1), the most prominent polypeptide of the human erythrocyte membrane, becomes heavily tyrosine phosphorylated following treatment of intact cells with protein tyrosine phosphatase inhibitors such as diamide, pervanadate, vanadate, or N-ethylmaleimide (NEM). The mechanism underlying this tyrosine phosphorylation is thought to involve the sequential action of two protein tyrosine kinases, Syk (p72syk) and Lyn (p53/56lyn). While Lyn catalysed phosphorylation appears to be strictly dependent on prior phosphorylation of Tyr8 and 21 of band 3 by Syk, little is known about the mechanism of induction of Syk phosphorylation. Data presented here show that both the fraction of Syk that associates with the membrane and the extent of phosphorylation of band 3 differ in response to the above inhibitors. While diamide and NEM stimulate syk translocation to the membrane during their induction of band 3 tyrosine phosphorylation, pervanadate and vanadate induce no change in kinase distribution. Moreover, diamide and NEM-induced Syk recruitment to the membrane are phosphotyrosine independent and involve their preferential association with Triton X-100-insoluble membrane skeletons. Together these data reveal a complex process controlling the association and catalytic activity of protein tyrosine kinases syk and lyn with the human erythrocyte membrane.

Peroxynitrite-Dependent Upregulation of Src Kinases in Red Blood Cells: Strategies to Study the Activation Mechanisms

Several studies have demonstrated that treatment of cells with oxidants, and in particular with peroxynitrite, may cause the upregulation of tyrosine phosphorylation signaling. In erythrocytes, peroxynitrite induces tyrosine phosphorylation of the major intrinsic membrane protein, band 3. A closer look at the enzymes involved revealed that the effect of peroxynitrite was due to the inhibition of phosphotyrosine phosphatases and/or to the activation of src kinases. The activity of src kinases is modulated not only by phosphatases and other kinases but also through redox modification of cysteine residues: Peroxynitrite can, thus, affect src kinase activity by means of direct and indirect mechanisms. In this chapter, we describe the different pathways leading to src kinase activation and the experimental procedures that can be performed to reveal the activation mechanism. The aim is to provide a more general strategy adaptable to different cell types and different oxidants.

Epidermal Growth Factor Receptor-Dependent and -Independent Pathways in Hydrogen Peroxide-Induced Mitogen-Activated Protein Kinase Activation in Cardiomyocytes and Heart Fibroblasts

Mild doses of oxidative stress in the heart correlate with the induction of apoptosis or hypertrophy in cardiomyocytes (CMCs) and fibrosis or proliferation of fibroblasts. Three branches of mitogen-activated protein kinases (MAPKs), i.e., c-Jun N-terminal kinases (JNKs), extracellular signal-related kinases 1 and 2 (ERK1/2), and p38, are activated by oxidants in a variety of cell types, including CMCs. However, the initiation process of these signaling pathways remains unsolved. We explored the role of the epidermal growth factor (EGF) receptor in H(2)O(2)-induced MAPK activation using two different cell types from the same organ: CMCs and heart fibroblasts (HFs). Pretreatment of each cell type with EGF revealed differences in how CMCs and HFs responded to subsequent treatment with H(2)O(2): in CMCs, the second treatment resulted in little further activation of JNKs and ERK1/2, whereas HFs retained the full response of JNKs and ERK1/2 activation by H(2)O(2) regardless of EGF pretreatment. AG-1478 [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline], a pharmacologic inhibitor of the EGF receptor tyrosine kinase, inhibited JNK and ERK1/2 activations but not p38 in both cell types. The data using the Src inhibitor PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] resemble those found when using AG-1478 in either cell type. Pharmacologic inhibitors of matrix metalloproteinases (MMPs) further illustrated the difference between the two cell types. In HFs, MMP inhibitors GM6001 [N-[(2R)-2-(hydroxamidocarbonylmethyl)-4-methylpentanoyl]-l-tryptophan methylamide] and BB2516 [[2S-[N4(R(*)),2R(*),3S(*)]]-N4-[2,2-dimethyl-1-[(methylamino)carbonyl]propyl]-N1,2-dihydroxy-3-(2-methylpropyl)butanediamide, marimastat] inhibited JNKs and ERK1/2 activation without affecting p38 activation by H(2)O(2) inhibitors. In contrast, these MMP failed to significantly inhibit the activation of JNKs, ERKs, or p38 in CMCs. These data suggest the complexity of the cell type-dependent signaling web initiated by oxidants in the heart.

Nitric oxide synthase activity is required for postsynaptic differentiation of the embryonic neuromuscular junction

Agrin, a synapse-organizing protein externalized by motor axons at the neuromuscular junction (NMJ), initiates a signaling cascade in muscle cells leading to aggregation of postsynaptic proteins, including acetylcholine receptors (AChRs). We examined whether nitric oxide synthase (NOS) activity is required for agrin-induced aggregation of postsynaptic AChRs at the embryonic NMJ in vivo and in cultured muscle cells. Inhibition of NOS reduced AChR aggregation at embryonic Xenopus NMJs by 50-90%, whereas overexpression of NOS increased AChR aggregate area 2- to 3-fold at these synapses. NOS inhibitors completely blocked agrin-induced AChR aggregation in cultured embryonic muscle cells. Application of NO donors to muscle cells induced AChR clustering in the absence of agrin. Our results indicate that NOS activity is necessary for postsynaptic differentiation of embryonic NMJs and that NOS is a likely participant in the agrin-MuSK signaling pathway of skeletal muscle cells.

Peroxynitrite inhibits enterocyte proliferation and modulates Src kinase activity in vitro

Overproduction of nitric oxide (NO) or its toxic metabolite, peroxynitrite (ONOO-), after endotoxemia promotes gut barrier failure, in part, by inducing enterocyte apoptosis. We hypothesized that ONOO- may also inhibit enterocyte proliferation by disrupting the Src tyrosine kinase signaling pathway, thereby blunting repair of the damaged mucosa. We examined the effect of ONOO- on enterocyte proliferation and Src kinase activity. Sprague-Dawley rats were challenged with LPS or saline, whereas intestinal epithelial cell line cells were treated with ONOO- or decomposed ONOO- in vitro. Enterocyte proliferation in vivo and in vitro was measured by 5-bromo-2'-deoxyuridine (BrdU) or [3H]thymidine incorporation. Src kinase activity in cell lysates was determined at various times. LPS challenge in vivo and ONOO- treatment in vitro inhibited enterocyte proliferation. ONOO- treatment blunted the activity of Src and its downstream target, focal adhesion kinase, in a time-dependent manner. ONOO- blocked mitogen (FBS, EGF)-induced enterocyte proliferation and Src phosphorylation while increasing Src nitration. Thus ONOO- may promote gut barrier failure not only by inducing enterocyte apoptosis but also by disrupting signaling pathways involved in enterocyte proliferation.

Gab1 is an integrator of cell death versus cell survival signals in oxidative stress

Upon the addition of different growth factors and cytokines, the Gab1 docking protein is tyrosine phosphorylated and in turn activates different signaling pathways. On the basis of the large body of evidence concerning cross talk between the signaling pathways activated by growth factors and oxidative stress, we decided to investigate the role of Gab1 in oxidative injury. We stimulated wild-type mouse embryo fibroblasts (MEF) or MEF with a homozygous deletion of the Gab1 gene (-/- MEF) with H(2)O(2). Our results show that Gab1 is phosphorylated in a dose- and time-dependent manner after H(2)O(2) triggering. Gab1 then recruits molecules such as SHP2, phosphatidylinositol 3-kinase (PI3K), and Shc. Gab1 phosphorylation is sensitive to the Src family kinase inhibitor PP2. Furthermore, we demonstrate that Gab1 is required for H(2)O(2)-induced c-Jun N-terminal kinase (JNK) activation but not for ERK2 or p38 activation. Reconstitution of Gab1 in -/- MEF rescues JNK activation, and we find that this is dependent on the SHP2 binding site in Gab1. Cell viability assays reveal that Gab1 has a dual role in cell survival: a positive one through its interaction with PI3K and a negative one through its interaction with SHP2. This is the first report identifying Gab1 as a component in oxidative stress signaling and one that is required for JNK activation.

Differential effects of quercetin and resveratrol on Band 3 tyrosine phosphorylation signalling of red blood cells

The protective effects of eating fruits and vegetables in the prevention of several degenerative pathologies have been attributed at least in part to the antioxidant and anti-inflammatory properties of polyphenols. In this study, we investigated the effects of two polyphenols, quercetin and resveratrol, on red blood cell Band 3 tyrosine phosphorylation signalling activated by peroxynitrite. Peroxynitrite is a physiological oxidant scavenged largely by the erythrocyte and formed by the reaction between nitrogen monoxide and superoxide anion. Quercetin and its structurally analogous (+)-catechin inhibited the peroxynitrite-dependent upregulation of Band 3 tyrosine phosphorylation. Quercetin was found to downregulate the activity of syk, which is upstream in the Band 3 tyrosine phosphorylation cascade, and partially prevented peroxynitrite-mediated phosphotyrosine phosphatase inhibition. Resveratrol and hydroxytyrosol, unexpectedly, amplified peroxynitrite-dependent upregulation of Band 3 tyrosine phosphorylation through the activation of lyn, a kinase of the src family. The present results clearly indicate that polyphenols may activate cell transduction pathways in different and sometimes opposite ways.

Mitogen-activated protein kinases mediate peroxynitrite-induced cell death in human bronchial epithelial cells

Peroxynitrite, formed by the reaction of nitric oxide (NO. ) with superoxide anions (O(2)(-).), may play a role in the pathophysiology of inflammation. The effects of 3-morpholinosydnonimine (SIN-1), a peroxynitrite generator, on the human bronchial epithelial cell line BEAS-2B, were examined. SIN-1 exposure resulted in cell death in a time- and dose-dependent manner. Depletion of intracellular glutathione increased the vulnerability of the cells. Pretreatment with Mn(III)tetrakis(N-methyl-4'-pyridyl)porphyrin (MnTMPyP) or hydroxocobalamin (HC), O(2)(-). and NO. scavengers, respectively, reduced significantly SIN-1-induced cell death (18.66 +/- 3.57 vs. 77.01 +/- 14.07 or 82.20 +/- 9.64, % cell viability SIN-1 vs. MnTMPyP or HC). Moreover, the mitogen-activated protein kinases (MAPK) p44/42 (ERK), p38, and p54/46 (JNK) were also activated in a time- and concentration-dependent manner. PD-98059 and SB-239063, specific inhibitors of ERK and p38 MAPK pathways, failed to protect cells against 1 mM SIN-1. However, PD-98059 partially inhibited (60% cell survival) SIN-1 effects at < or =0.25 mM, and this was increased with the inclusion of SB-239063. Therefore, MAPKs may mediate signal transduction pathways induced by peroxynitrite in lung epithelial cells leading to cell death.

Defenses against peroxynitrite: selenocompounds and flavonoids

The inflammatory mediator peroxynitrite, when generated in excess, may damage cells by oxidizing and nitrating cellular components. Defense against this reactive species may be at the level of prevention of the formation of peroxynitrite, at the level of interception, or at the level of repair of damage caused by peroxynitrite. Several selenocompounds serve this purpose and include selenoproteins such as glutathione peroxidase (GPx), selenoprotein P and thioredoxin reductase, or low-molecular-weight substances such as ebselen. Further, flavonoids, such as (-)-epicatechin, which occurs in green tea or cocoa as monomer or in the form of oligomers, can contribute to cellular defense against peroxynitrite.

Involvement of tyrosine kinase in peroxynitrite-induced preconditioning in rat isolated heart

We have investigated the role of tyrosine kinase in the antiarrhythmic effects of peroxynitrite preconditioning in rat isolated heart by using a tyrosine phosphatase inhibitor, sodium orthovanadate, and tyrosine kinase inhibitors, genistein and tyrphostin. Rat hearts were preconditioned by peroxynitrite administration at 1 microM for 3 min, which was followed by 10-min washout and 30 min of ischemia. None of the hearts had ventricular fibrillation in the peroxynitrite preconditioning group (from 64%, n=11, to 0%, n=11). Neither sodium orthovanadate (10 microM) nor genistein (50 microM) or tyrphostin (100 microM) alone showed any effects on arrhythmias. Peroxynitrite preserved its beneficial effects on arrhythmias (to 0% ventricular fibrillation, n=7) during sodium orthovanadate infusion (for 23 min) prior to 30 min of an ischemic period. On the other hand, genistein or tyrphostin treatment significantly reversed the protective effects of the peroxynitrite preconditioning (to 71% ventricular fibrillation, n=14, genistein and, to 75% ventricular fibrillation, n=8, tyrphostin). These results suggest that the tyrosine kinase pathway plays a significant role in peroxynitrite-induced preconditioning in rat isolated heart.

Alternatively spliced FGFR-1 isoform signaling differentially modulates endothelial cell responses to peroxynitrite

Mounting experimental evidence has suggested that the trophic environment of cells in culture is an important determinant of their vulnerability to the cytotoxic effects of reactive oxidants such as peroxynitrite (ONOO(-)). However, acidic fibroblast growth factor (FGF-1)-induced signaling renders some cells more sensitive and others resistant to the cytotoxic effects of ONOO(-). To determine whether alternatively spliced fibroblast growth factor receptor (FGFR-1) isoforms are responsible for this differential response, we have stably transfected FGFR-negative rat brain-derived resistant vessel endothelial cells (RVEC) with human cDNA sequences encoding either FGFR-1 alpha or FGFR-1 beta. FGF-1 treatment of RVEC(R-1 alpha) transfectants enhanced ONOO(-)-mediated cell death in a manner dependent upon FGFR-1 tyrosine kinase, MEK/Erk 1/2 kinase, and p38 MAP kinase activities and independent of Src-family kinase (SFK) activity. FGF-1 treatment of RVEC(R-1 beta) transfectants inhibited the cytotoxic effects of ONOO(-) in a manner dependent upon FGFR-1 tyrosine kinase, MEK/Erk 1/2 kinase, and SFK activities and independent of p38 MAP kinase activity. FGF-1-induced preactivation of both FGFR-1 tyrosine and Erk 1/2 kinases was detected in both RVEC(R-1 alpha) and RVEC(R-1 beta) transfectants. FGF-1-induced preactivation of p38 MAPK was restricted to RVEC(R-1 alpha) transfectants, whereas, ligand-induced preactivation of SFK was limited to RVEC(R-1 beta) transfectants. Collectively, these results both reemphasize the role of extracellular trophic factors and their receptor-mediated signaling pathways during cellular responses to oxidant stress and provide a first indication that the alternatively spliced FGFR-1 isoforms induce differential signal transduction pathways.

Interaction between nitric oxide, reactive oxygen intermediates, and peroxynitrite in the regulation of 5-lipoxygenase metabolism

We have shown that overnight lipopolysaccharide (LPS) suppresses alveolar macrophage (AM) leukotriene (LT) synthesis mediated in part by induction of inducible nitric oxide synthase (iNOS) and NO production. Here we examined the possibility that reactive oxygen intermediates (ROI) generated by LPS pretreatment contribute to the suppression of 5-lipoxygenase (5-LO) metabolism. Pretreatment of AM with xanthine/xanthine oxidase, which generates high concentrations of ROI, resulted in suppression of LT synthetic capacity. Since NO and ROI reactive species are known to react and form peroxynitrite (ONOO(-)), we examined the effect of ONOO(-) on 5-LO metabolism. Exogenous ONOO(-) caused a dose-dependent suppression of recombinant 5-LO cell-free activity. ONOO(-) also suppressed LT synthesis in intact AM, which was reversed by the ONOO(-) scavenger tetrakis(4-benzoic acid)porphyrin. ONOO(-) treatment also resulted in dose-dependent nitrotyrosination and S-nitrosylation of the recombinant 5-LO enzyme. Since the direct 5-LO inhibitor zileuton prevents the LPS-induced suppression of LT synthesis, we examined if 5-LO itself was the source of ROI. Zileuton reduced ROI generation in LPS-treated cells. These studies identify an important role for ROI and ONOO(-) in the suppression of 5-LO metabolism by LPS.

Peroxynitrite activates kinases of the src family and upregulates tyrosine phosphorylation signaling

The hypothesis that peroxynitrite may act as a signaling molecule able to upregulate protein tyrosine phosphorylation is discussed. This article focuses on the mechanisms for activating kinases of the src family, an important class of nonreceptor tyrosine kinases implicated in the regulation of cell communication, proliferation, migration, differentiation, and survival. Recent in vitro findings show that in erythrocytes, synaptosomes, and cerebellar primary culture cells peroxynitrite is able to inhibit phosphatases and to activate different members of the src kinase family through different mechanisms involving cysteine-dependent and -independent processes. The ability of nitrotyrosine-containing peptides with SH2 binding affinity to activate src kinases is also discussed.

Peroxynitrite signaling: receptor tyrosine kinases and activation of stress-responsive pathways

Peroxynitrite, generated for example in inflammatory processes, is capable of nitrating and oxidizing biomolecules, implying a considerable impact on the integrity of cellular structures. Cells respond to stressful conditions by the activation of signaling pathways, including receptor tyrosine kinase-dependent pathways such as mitogen-activated protein kinases and the phosphoinositide-3-kinase/Akt pathway. Peroxynitrite affects signaling pathways by nitration as well as by oxidation: while nitration of tyrosine residues by peroxynitrite modulates signaling processes relying on tyrosine phosphorylation and dephosphorylation, oxidation of phosphotyrosine phosphatases may lead to an alteration in the tyrosine phosphorylation/dephosphorylation balance. The flavanol (-)-epicatechin is a potent inhibitor of tyrosine nitration and may be employed as a tool to distinguish signaling effects due to tyrosine nitration from those that are due to oxidation reactions.

Signal transduction by protein tyrosine nitration: competition or cooperation with tyrosine phosphorylation-dependent signaling events?

This review article is an attempt to stimulate a discussion on the significance of protein tyrosine nitration to cellular signaling and its relationships with protein tyrosine phosphorylation. Initially, it provides basic information on growth factor and oxidants as modulators/mediators of tyrosine phosphorylation-dependent signal transduction pathways. The effects of exogenous and endogenous tyrosine nitration on such pathways were examined by reviewing published and unpublished observations. From an initial perspective that tyrosine nitration was a toxic manifestation of nitric oxide, the concept evolved to a protein modification that could also function in cellular signaling events, possibly cooperating with tyrosine phosphorylation.

Peroxynitrite enhances astrocytic volume-sensitive excitatory amino acid release via a src tyrosine kinase-dependent mechanism

Volume-regulated anion channels (VRACs) are critically important for cell volume homeostasis, and under pathological conditions contribute to neuronal damage via excitatory amino (EAA) release. The precise mechanisms by which brain VRACs are activated and/or modulated remain elusive. In the present work we explored the possible involvement of nitric oxide (NO) and NO-related reactive species in the regulation of VRAC activity and EAA release, using primary astrocyte cultures. The NO donors sodium nitroprusside and spermine NONOate did not affect volume-activated d-[3H]aspartate release. In contrast, the peroxynitrite (ONOO-) donor 3-morpholinosydnomine hydrochloride (SIN-1) increased volume-dependent EAA release by approx. 80-110% under identical conditions. Inhibition of ONOO- formation with superoxide dismutase completely abolished the effects of SIN-1. Both the volume- and SIN-1-induced EAA release were sensitive to the VRAC blockers NPPB and ATP. Further pharmacological analysis ruled out the involvement of cGMP-dependent reactions and modification of sulfhydryl groups in the SIN-1-inducedmodulation of EAA release. The src family tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d]pyrimidine (PP2), but not its inactive analog PP3, abolished the effects of SIN-1. A broader spectrum tyrosine kinase inhibitor tyrphostin A51, also completely eliminated the SIN-1-induced EAA release. Our data suggest that ONOO- up-regulates VRAC activity via a src tyrosine kinase-dependent mechanism. This modulation may contribute to EAA-mediated neuronal damage in ischemia and other pathological conditions favoring cell swelling and ONOO- production.