Phosphatidylinositol 3-kinase is a target for protein tyrosine nitration

Tyrosine nitration as a key event of signal transduction that regulates functional state of the cell

This review is dedicated to the role of nitration of proteins by tyrosine residues in physiological and pathological conditions. First of all, we analyze the biochemical evidence of peroxynitrite formation and reactions that lead to its formation, types of posttranslational modifications (PTMs) induced by reactive nitrogen species, as well as three biological pathways of tyrosine nitration. Then, we describe two possible mechanisms of protein nitration that are involved in intracellular signal transduction, as well as its interconnection with phosphorylation/dephosphorylation of tyrosine. Next part of the review is dedicated to the role of proteins nitration in different pathological conditions. In this section, special attention is devoted to the role of nitration in changes of functional properties of actin-protein that undergoes PTMs both in normal and pathological conditions. Overall, this review is devoted to the main features of protein nitration by tyrosine residue and the role of this process in intracellular signal transduction in basal and pathological conditions.

High Glucose-Mediated Tyrosine Nitration of PI3-Kinase: A Molecular Switch of Survival and Apoptosis in Endothelial Cells

Diabetes and hyperglycemia are associated with increased retinal oxidative and nitrative stress and vascular cell death. Paradoxically, high glucose stimulates expression of survival and angiogenic growth factors. Therefore, we examined the hypothesis that high glucose-mediated tyrosine nitration causes inhibition of the survival protein PI3-kinase, and in particular, its regulatory p85 subunit in retinal endothelial cell (EC) cultures. Retinal EC were cultured in high glucose (HG, 25 mM) for 3 days or peroxynitrite (PN, 100 µM) overnight in the presence or absence of a peroxynitrite decomposition catalyst (FeTPPs, 2.5 µM), or the selective nitration inhibitor epicatechin (100 µM). Apoptosis of ECs was assessed using TUNEL assay and caspase-3 activity. Immunoprecipitation and Western blot were used to assess protein expression and tyrosine nitration of p85 subunit and its interaction with the p110 subunit. HG or PN accelerated apoptosis of retinal ECs compared to normal glucose (NG, 5 mM) controls. HG- or PN-treated cells also showed significant increases in tyrosine nitration on the p85 subunit of PI3-kinase that inhibited its association with the catalytic p110 subunit and impaired PI3-kinase/Akt kinase activity. Decomposing peroxynitrite or blocking tyrosine nitration of p85 restored the activity of PI3-kinase, and prevented apoptosis and activation of p38 MAPK. Inhibiting p38 MAPK or overexpression of the constitutively activated Myr-Akt construct prevented HG- or peroxynitrite-mediated apoptosis. In conclusion, HG impairs pro-survival signals and causes accelerated EC apoptosis, at least in part via tyrosine nitration and inhibition of PI3-kinase. Inhibitors of nitration can be used in adjuvant therapy to delay diabetic retinopathy and microvascular complication.

Molecular Basis of Nitrative Stress in the Pathogenesis of Pulmonary Hypertension

Pulmonary hypertension (PH) is a lung vascular disease with marked increases in pulmonary vascular resistance and pulmonary artery pressure (>25 mmHg at rest). In PH patients, increases in pulmonary vascular resistance lead to impaired cardiac output and reduced exercise tolerance. If untreated, PH progresses to right heart failure and premature lethality. The mechanisms that control the pathogenesis of PH are incompletely understood, but evidence from human and animal studies implicate nitrative stress in the development of PH. Increased levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) result in nitrative stress, which in turn induces posttranslational modification of key proteins important for maintaining pulmonary vascular homeostasis. This affects their functions and thereby contributes to the pathogenesis of PH. In this chapter, molecular mechanisms underlying nitrative stress-induced PH are reviewed, molecular sources of ROS and RNS are delineated, and evidence of nitrative stress in PH patients is described. A better understanding of such mechanisms could lead to the development of novel treatments for PH.

Metformin treatment in the period after stroke prevents nitrative stress and restores angiogenic signaling in the brain in diabetes

Diabetes impedes vascular repair and causes vasoregression in the brain after stroke, but mechanisms underlying this response are still unclear. We hypothesized that excess peroxynitrite formation in diabetic ischemia/reperfusion (I/R) injury inactivates the p85 subunit of phosphoinositide 3-kinase (PI3K) by nitration and diverts the PI3K-Akt survival signal to the p38-mitogen-activated protein kinase apoptosis pathway. Nitrotyrosine (NY), Akt and p38 activity, p85 nitration, and caspase-3 cleavage were measured in brains from control, diabetic (GK), or metformin-treated GK rats subjected to sham or stroke surgery and in brain microvascular endothelial cells (BMVECs) from Wistar and GK rats subjected to hypoxia/reoxygenation injury. GK rat brains showed increased NY, caspase-3 cleavage, and p38 activation and decreased Akt activation. Metformin attenuated stroke-induced nitrative signaling in GK rats. GK rat BMVECs showed increased basal nitrative stress compared with controls. A second hit by hypoxia/reoxygenation injury dramatically increased the nitration of p85 and activation of p38 but decreased Akt. These effects were associated with impairment of angiogenic response and were restored by treatment with the peroxynitrite scavenger 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron III chloride or the nitration inhibitor epicatechin. Our results provide evidence that I/R-induced peroxynitrite inhibits survival, induces apoptosis, and promotes peroxynitrite as a novel therapeutic target for the improvement of reparative angiogenesis after stroke in diabetes.

Dynamic conformations of nucleophosmin (NPM1) at a key monomer-monomer interface affect oligomer stability and interactions with granzyme B

Nucleophosmin (NPM1) is an abundant, nucleolar tumor antigen with important roles in cell proliferation and putative contributions to oncogenesis. Wild-type NPM1 forms pentameric oligomers through interactions at the amino-terminal core domain. A truncated form of NPM1 found in some hepatocellular carcinoma tissue formed an unusually stable oligomer and showed increased susceptibility to cleavage by granzyme B. Initiation of translation at the seventh methionine generated a protein (M7-NPM) that shared all these properties. We used deuterium exchange mass spectrometry (DXMS) to perform a detailed structural analysis of wild-type NPM1 and M7-NPM, and found dynamic conformational shifts or local “unfolding” at a specific monomer-monomer interface which included the β-hairpin “latch.” We tested the importance of interactions at the β-hairpin “latch” by replacing a conserved tyrosine in the middle of the β-hairpin loop with glutamic acid, generating Y67E-NPM. Y67E-NPM did not form stable oligomers and further, prevented wild-type NPM1 oligomerization in a dominant-negative fashion, supporting the critical role of the β-hairpin “latch” in monomer-monomer interactions. Also, we show preferential cleavage by granzyme B at one of two available aspartates (either D161 or D122) in M7-NPM and Y67E-NPM, whereas wild-type NPM1 was cleaved at both sites. Thus, we observed a correlation between the propensity to form oligomers and granzyme B cleavage site selection in nucleophosmin proteins, suggesting that a small change at an important monomer-monomer interface can affect conformational shifts and impact protein-protein interactions.

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.

3-Nitrotyrosine: A biomarker of nitrogen free radical species modified proteins in systemic autoimmunogenic conditions

The free radical-mediated damage to proteins results in the modification of amino acid residues, cross-linking of side chains and fragmentation. l-Tyrosine and protein bound tyrosine are prone to attack by various mediators and reactive nitrogen intermediates to form 3-nitrotyrosine (3-NT). Activated macrophages produce superoxide (O2(·-)) and NO, which are converted to peroxynitrite ONO2(-). 3-NT formation is also catalyzed by a class of peroxidases utilizing nitrite and hydrogen peroxide as substrates. Evidence supports the formation of 3-NT in vivo in diverse pathologic conditions and 3-NT is thought to be a relatively specific marker of oxidative damage mediated by peroxynitrite. Free/protein-bound tyrosines are attacked by various RNS, including peroxynitrite, to form free/protein-bound 3-NT, which may provide insight into the etiopathogenesis of autoimmune conditions. The formation of nitrotyrosine represents a specific peroxynitrite-mediated protein modification; thus, detection of nitrotyrosine in proteins is considered as a biomarker for endogenous peroxynitrite activity. The peroxynitrite-driven oxidation and nitration of biomolecules may lead to autoimmune diseases such as systemic lupus. The subsequent release of altered proteins may enable them to act as antigen-inducing antibodies against self-proteins. Hence, tyrosine nitrated proteins can act as neoantigens and lead to the generation of autoantibodies against self proteins in various autoimmune disorders.

Delayed preconditioning prevents ischemia/reperfusion-induced endothelial injury in rats: role of ROS and eNOS

Ischemic preconditioning (IPC) strongly protects against ischemia/reperfusion (I/R) injury; however, the molecular mechanism involved in delayed preconditioning-induced endothelial protection in peripheral arteries is unknown. Therefore, we examined using functional, morphologic and molecular biologic studies whether delayed IPC decreases formation of reactive oxygen species and upregulates endothelial nitric oxide synthase (eNOS) that in turn contributes to vascular endothelial protection. Adult male Sprague-Dawley rats were subjected to 30-min ischemia induced by mesenteric artery occlusion followed by 60-min reperfusion 24 h after sham surgery or preconditioning (three cycles of 5-min ischemia/5-min reperfusion). Delayed preconditioning prevented the I/R-induced impairment of endothelium-dependent relaxations to acetylcholine (maximal relaxation: sham 91.4±2.2%; I/R 54.0±4.0%; IPC 80.2±6.3%). This protective effect was abolished by NOS inhibitor N(G)-nitro-L-arginine methyl ester and not changed by ascorbic acid. Electron microscopy showed marked endothelial damage after I/R and the ultrastructural changes were prevented by delayed preconditioning. Following I/R, the impairment of eNOS phosphorylation and expression was observed in mesenteric vessels. Furthermore, phosphatidylinositol 3-kinase (PI3K) and Akt phosphorylation were reduced, although total PI3K and Akt remained unchanged. IPC restored I/R-induced impairment of eNOS expression and activity. This was possibly the result of the recovery of PI3K/Akt phosphorylation. Furthermore, I/R increased serum level of malondialdehyde, intravascular superoxide and nitrotyrosine generation, which were abrogated by IPC. These results suggest that delayed preconditioning prevented I/R-induced endothelial injury in peripheral resistance vasculature, both in terms of functional and structural changes. Endothelial protection afforded by delayed IPC is associated with inhibition of oxidative stress and upregulation of PI3K/Akt/eNOS pathway.

Remote ischemic perconditioning is effective alone and in combination with intravenous tissue-type plasminogen activator in murine model of embolic stroke

BACKGROUND AND PURPOSE

Remote ischemic conditioning is cardioprotective in myocardial infarction and neuroprotective in mechanical occlusion models of stroke. However, there is no report on its therapeutic potential in a physiologically relevant embolic stroke model (embolic middle cerebral artery occlusion) in combination with intravenous tissue-type plasminogen activator (tPA).

METHODS

We tested remote ischemic perconditioning therapy (RIPerC) at 2 hours after embolic middle cerebral artery occlusion in the mouse with and without intravenous tPA at 4 hours. We assessed cerebral blood flow up to 6 hours, neurological deficits, injury size, and phosphorylation of Akt (Serine(473)) as a prosurvival signal in the ischemic hemisphere at 48 hours poststroke.

RESULTS

RIPerC therapy alone improved the cerebral blood flow and neurological outcomes. tPA alone at 4 hours did not significantly improve the neurological outcome even after successful thrombolysis. Individual treatments with RIPerC and intravenous tPA reduced the infarct size (25.7% and 23.8%, respectively). Combination therapy of RIPerC and tPA resulted in additive effects in further improving the neurological outcome and reducing the infarct size (50%). All the therapeutic treatments upregulated phosphorylation of Akt in the ischemic hemisphere.

CONCLUSIONS

RIPerC is effective alone after embolic middle cerebral artery occlusion and has additive effects in combination with intravenous tPA. RIPerC may be a simple, safe, and inexpensive combination therapy with intravenous tPA.

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.

Sepsis, leukocytes, and nitric oxide (NO): an intricate affair

Sepsis is exceedingly burdensome for hospital intensive care unit caregivers, and its incidence, as well as sepsis-related deaths, is increasing steadily. Sepsis is characterized by a robust increase in NO production throughout the organism that is driven by iNOS. Moreover, NO is an important factor in the development of septic shock and is synthesized by NOS, an enzyme expressed by a variety of cells, including vascular endothelium, macrophages, and neutrophils. However, the effects of NO on leukocyte functions, and the underlying mechanisms, are relatively unknown. Thus, the present review focuses on the effects of NO and its derivatives on cells of the immune system. Experimental evidences discussed herein show that NO induces posttranslational modifications of key proteins in targeted processes with the potential of deterring cellular physiology. Consequently, the manipulation of NO distribution in septic patients, used in conjunction with conventional treatments aimed at restoring normal immune functions, may represent a valuable therapeutic strategy.

Oxidized low-density lipoprotein induces apoptosis in endothelial progenitor cells by inactivating the phosphoinositide 3-kinase/Akt pathway

We tested the hypothesis that oxidized low-density lipoprotein (oxLDL)-induced inactivation of Akt within endothelial progenitor cells (EPCs) is mediated at the level of phosphoinositide 3-kinase (PI3K), specifically by nitrosylation of the p85 subunit of PI3K, and that this action is critical in provoking oxLDL-induced EPC apoptosis. Hypercholesterolemic ApoE null mice had a significant reduction of the phosphorylated Akt (p-Akt)/Akt ratio in EPCs, as well as a greater percentage of apoptosis in these cells than EPCs isolated from wild-type (WT) C57Bl/6 mice. EPCs were isolated from WT spleen and exposed to oxLDL in vitro. oxLDL increased O₂⁻ and H₂O₂ in these cells and induced a dose- and time-dependent reduction in the p-Akt/Akt ratio and increase in EPC apoptosis. These effects were significantly reduced by the antioxidants superoxide dismutase, L-NAME, epicatechin and FeTPPs. oxLDL also induced nitrosylation of the p85 subunit of PI3K and subsequent dissociation of the p85 and p110 subunits, an effect significantly reduced by all the antioxidant agents tested. EPC transfection with a constitutively active Akt isoform (Ad-myrAkt) significantly reduced oxLDL-induced apoptosis of WT EPCs. The present findings indicate that oxLDL disrupts the PI3K/Akt signaling pathway at the level of p85 in EPCs. This dysfunction can be reversed by ex vivo antioxidant therapy.

Early intervention of tyrosine nitration prevents vaso-obliteration and neovascularization in ischemic retinopathy

Diabetic retinopathy and retinopathy of prematurity are blinding disorders that follow a pathological pattern of ischemic retinopathy and affect premature infants and working-age adults. Yet, the treatment options are limited to laser photocoagulation. The goal of this study is to elucidate the molecular mechanism and examine the therapeutic effects of inhibiting tyrosine nitration on protecting early retinal vascular cell death and late neovascularization in the ischemic retinopathy model. Ischemic retinopathy was developed by exposing neonatal mice to 75% oxygen [postnatal day (p) 7-p12] followed by normoxia (21% oxygen) (p12-p17). Peroxynitrite decomposition catalyst 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron III chloride (FeTPPS) (1 mg/kg), the nitration inhibitor epicatechin (10 mg/kg) or the thiol donor N-acetylcysteine (NAC, 150 mg/kg) were administered (p7-p12) or (p7-p17). Vascular endothelial cells were incubated at hyperoxia (40% oxygen) or normoxia (21% oxygen) for 48 h. Vascular density was determined in retinal flat mounts labeled with isolectin B4. Expression of vascular endothelial growth factor, caspase-3, and poly(ADP ribose) polymerase (PARP), activation of Akt and p38 mitogen-activated protein kinase (MAPK), and tyrosine nitration of the phosphatidylinositol (PI) 3-kinase p85 subunit were analyzed by Western blot. Hyperoxia-induced peroxynitrite caused endothelial cell apoptosis as indicated by expression of cleaved caspase-3 and PARP leading to vaso-obliteration. These effects were associated with significant tyrosine nitration of the p85 subunit of PI 3-kinase, decreased Akt activation, and enhanced p38 MAPK activation. Blocking tyrosine nitration of PI 3-kinase with epicatechin or NAC restored Akt phosphorylation, and inhibited vaso-obliteration at p12 and neovascularization at p17 comparable with FeTPPS. Early inhibition of tyrosine nitration with use of epicatechin or NAC can represent safe and effective vascular-protective agents in ischemic retinopathy.

Reactive nitrogen species: molecular mechanisms and potential significance in health and disease

Reactive nitrogen species (RNS) are various nitric oxide-derived compounds, including nitroxyl anion, nitrosonium cation, higher oxides of nitrogen, S-nitrosothiols, and dinitrosyl iron complexes. RNS have been recognized as playing a crucial role in the physiologic regulation of many, if not all, living cells, such as smooth muscle cells, cardiomyocytes, platelets, and nervous and juxtaglomerular cells. They possess pleiotropic properties on cellular targets after both posttranslational modifications and interactions with reactive oxygen species. Elevated levels of RNS have been implicated in cell injury and death by inducing nitrosative stress. The aim of this comprehensive review is to address the mechanisms of formation and removal of RNS, highlighting their potential cellular targets: lipids, DNA, and proteins. The specific importance of RNS and their paradoxic effects, depending on their local concentration under physiologic conditions, is underscored. An increasing number of compounds that modulate RNS processing or targets are being identified. Such compounds are now undergoing preclinical and clinical evaluations in the treatment of pathologies associated with RNS-induced cellular damage. Future research should help to elucidate the involvement of RNS in the therapeutic effect of drugs used to treat neurodegenerative, cardiovascular, metabolic, and inflammatory diseases and cancer.

Prostaglandin E2 signals monocyte/macrophage survival to peroxynitrite via protein kinase A converging in bad phosphorylation with the protein kinase C alpha-dependent pathway driven by 5-hydroxyeicosatetraenoic acid

Monocytes/macrophages committed to death by peroxynitrite nevertheless survive with a signaling response promoting Bad phosphorylation, as well as its cytosolic localization, via upstream activation of cytosolic phospholipase A(2), 5-lipoxygenase, and protein kinase C alpha. We now report evidence for an alternative mechanism converging in Bad phosphorylation when the expression/activity of the above enzymes are suppressed. Under these conditions, also associated with peroxynitrite-dependent severe inhibition of Akt, an additional Bad kinase, Bad dephosphorylation promoted its accumulation in the mitochondria and a prompt lethal response. PGE(2) prevented toxicity via EP(2) receptor-mediated protein kinase A-dependent Bad phosphorylation. This notion was established in U937 cells by the following criteria: 1) there was a strong correlation between survival and cAMP accumulation, both in the absence and presence of phosphodiesterase inhibitors; 2) direct activation of adenylyl cyclase afforded cytoprotection; and 3) PGE(2) promoted loss of mitochondrial Bad and cytoprotection, mimicked by EP(2) receptor agonists, and prevented by EP(2) receptor antagonists or protein kinase A inhibitors. Finally, selected experiments performed in human monocytes/macrophages and in rat peritoneal macrophages indicated that the above cytoprotective pathway is a general response of cells belonging to the monocyte/macrophage lineage to both exogenous and endogenous peroxynitrite. The notion that two different pathways mediated by downstream products of arachidonic acid metabolism converge in Bad phosphorylation emphasizes the relevance of this strategy for the regulation of macrophage survival to peroxynitrite at the inflammatory sites.

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.

Vascular endothelial growth factor in eye disease

Collectively, angiogenic ocular conditions represent the leading cause of irreversible vision loss in developed countries. In the US, for example, retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration are the principal causes of blindness in the infant, working age and elderly populations, respectively. Evidence suggests that vascular endothelial growth factor (VEGF), a 40kDa dimeric glycoprotein, promotes angiogenesis in each of these conditions, making it a highly significant therapeutic target. However, VEGF is pleiotropic, affecting a broad spectrum of endothelial, neuronal and glial behaviors, and confounding the validity of anti-VEGF strategies, particularly under chronic disease conditions. In fact, among other functions VEGF can influence cell proliferation, cell migration, proteolysis, cell survival and vessel permeability in a wide variety of biological contexts. This article will describe the roles played by VEGF in the pathogenesis of retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration. The potential disadvantages of inhibiting VEGF will be discussed, as will the rationales for targeting other VEGF-related modulators of angiogenesis.

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.

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.

Melatonin arrests peroxynitrite-induced tau hyperphosphorylation and the overactivation of protein kinases in rat brain

The purpose of this study was to examine the in vivo effect of melatonin (MEL) on peroxynitrite-induced tau hyperphosphorylation and the involvement of glycogen synthase kinase-3beta (GSK-3beta) and mitogen-activated protein kinase (MAPK) families. Melatonin was injected into the right cerebroventricle of the rats 1 hr before the bilateral hippocampal injection of 3-morpholino-sydnonimine chloride (SIN-1), the recognized donor of peroxynitrite. Thereafter, the phosphorylation level of tau and the activity of the kinases were analyzed. The injection of SIN-1 induced hyperphosphorylation of tau at pS396 epitope with a concomitant activation of GSK-3beta and selective MAPK isoforms including p38alpha, p38beta, and p38delta but not p38gamma. The effect of peroxynitrite was confirmed using uric acid, a recognized scavenger of peroxynitrite. Preinjection of MEL significantly arrested the peroxynitrite-induced hyperphosphorylation of tau and the activation of GSK-3beta and MAPKs. Melatonin also ameliorated peroxynitrite-induced oxidative stress. We conclude that MEL can efficiently arrest peroxynitrite-induced tau hyperphosphorylation, and the underlying mechanism may involve scavenging the reactive species and suppressing the activated GSK-3beta and p38 MAPK family.

Peroxynitrite induces Alzheimer-like tau modifications and accumulation in rat brain and its underlying mechanisms

To investigate the upstream effector that led to tau hyperphosphorylation, nitration, and accumulation as seen in Alzheimer's disease brain, and the underlying mechanisms, we bilaterally injected SIN-1, a recognized peroxynitrite donor, into the hippocampus of rat brain. We observed that the level of nitrated and hyperphosphorylated tau was markedly increased in rat hippocampus 24 h after drug administration, and these alterations were prevented by preinjection of uric acid, a natural scavenger of peroxynitrite. Concomitantly, we detected a significant activation in glycogen synthase kinase-3beta (GSK-3beta) and p38 MAPKs, including p38alpha, p38beta, and p38delta, but no obvious change was measured in the activity of p38gamma, ERK, and c-Jun amino-terminal kinase (JNK). Both nitrated tau and hyperphosphorylated tau were aggregated in the hippocampus, in which the activity of 20S proteasome was significantly arrested in SIN-1-injected rats. Further studies demonstrated that the hyperphosphorylated tau was degraded as efficiently as normal tau by 20S proteasome, but the nitrated tau with an unorderly secondary structure became more resistant to the proteolysis. These results provide the first in vivo evidence showing that peroxynitrite simultaneously induces tau hyperphosphorylation, nitration, and accumulation, and that activation of GSK-3beta, p38alpha, p38beta, p38delta isoforms and the inhibition of proteasome activity are respectively responsible for the peroxynitrite-induced tau hyperphosphorylation and accumulation. Our findings reveal a common upstream stimulator and a potential therapeutic target for Alzheimer-like neurodegeneration.

Nitric oxide induces tau hyperphosphorylation via glycogen synthase kinase-3beta activation

Nitric oxide is associated with neurofibrillary tangle, which is composed mainly of hyperphosphorylated tau in the brain of Alzheimer's disease (AD). However, the role of nitric oxide in tau hyperphosphorylation is unclear. Here we show that nitric oxide produced by sodium nitroprusside (SNP), a recognized donor of nitric oxide, induces tau hyperphosphorylation at Ser396/404 and Ser262 in HEK293/tau441 cells with a simultaneous activation of glycogen synthase kinase-3beta (GSK-3beta). Pretreatment of the cells with 10 mM lithium chloride (LiCl), an inhibitor of GSK-3, 1 h before SNP administration inhibits GSK-3beta activation and prevents tau from hyperphosphorylation. This is the first direct evidence demonstrating that nitric oxide induces AD-like tau hyperphosphorylation in vitro, and GSK-3beta activation is partially responsible for the nitric oxide-induced tau hyperphosphorylation. It is suggested that nitric oxide may be an upstream element of tau abnormal hyperphosphorylation in AD.

Oxidative stress inactivates VEGF survival signaling in retinal endothelial cells via PI 3-kinase tyrosine nitration

In diabetic retinopathy, endothelial cell apoptosis is paradoxically increased despite upregulation of the potent pro-survival factor VEGF. We tested the hypothesis that elevated glucose levels disrupt VEGF's pro-survival function via peroxynitrite-mediated alteration of the Akt-1/p38 MAP kinase signaling pathway by studies of retinal endothelial cells in vitro. High glucose or exogenous peroxynitrite caused significant increases in apoptosis in the presence or absence of VEGF. Treatment with a peroxynitrite decomposition catalyst blocked these effects, implying a causal role of peroxynitrite. Peroxynitrite or high glucose treatment also increased phosphorylation of p38 MAP kinase, whereas phosphorylation of Akt-1 was significantly decreased in basal or VEGF-stimulated conditions. High glucose- or peroxynitrite-treated cells also showed significant increases in tyrosine nitration on the p85 subunit of PI 3-kinase that blocked PI 3-kinase and Akt-1 kinase activity. Inhibiting peroxynitrite formation or blocking tyrosine nitration of p85 restored the activity of PI 3-kinase and Akt-1 kinase, blocked phosphorylation of p38 MAP kinase and normalized pro-survival function. Transfecting the cells with constitutively active Akt-1 or inhibiting activity of p38 MAP kinase completely masked the pro-apoptotic effects of high glucose and exogenous peroxynitrite, suggesting an interaction between the Akt-1 and p38 MAP kinase pathways. In conclusion, high glucose treatment blocks the pro-survival effect of VEGF and causes accelerated endothelial cell apoptosis via the action of peroxynitrite in causing tyrosine nitration of PI 3-kinase, inhibiting activity of Akt-1 kinase and increasing the activity of p38 MAP kinase.

Immunogenicity of autologous IgG bearing the inflammation-associated marker 3-nitrotyrosine

To explore the link between inflammation and autoimmunity, we analyzed the immunogenicity of 3-nitrotyrosine (NT)-bearing self-proteins, an inflammation-associated marker that is formed by nitration of protein tyrosine residues with peroxynitrite generated during inflammation. An interesting feature of NT is its structural similarity to a synthetic hapten, 4-hydroxy-3-nitrophenylacetyl (NP), with which some anti-DNA antibodies (Abs) have been reported to show cross-reactivity. We confirmed that some of anti-DNA monoclonal Abs (mAbs) obtained from MRL/lpr mice also bound NT as well as NP. Based on these findings, we examined whether NT-bearing autologous IgG (NT-IgG) as a model of NT-self proteins is immunogenic to induce a DNA-cross-reactive anti-NT Ab response in autologous normal mice. Anti-NT IgM and IgG Ab responses were elicited after the third immunization with NT-IgG, concomitant with an increase in anti-single stranded (ss)DNA titer. Interestingly, a part of anti-NT mAbs thus induced showed cross-reactivity with ssDNA, some of which used VH sequences that were highly homologous to those reported in anti-DNA Abs from NZB/WF1 mice. Splenic T cells primed with NT-IgG, but not with unmodified IgG, showed a proliferative response to the inducing antigen. Collectively, NT-IgG is immunogenic in autologous hosts, and can induce anti-NT Abs that are cross-reactive with ssDNA.

Mechanisms regulating phosphoinositide 3-kinase signalling in insulin-sensitive tissues

A great deal of evidence has accumulated indicating that the activity of PI 3-kinase is necessary, and in some cases sufficient, for a wide range of insulin's actions in the cell. Most biochemical, genetic and pharmacological studies have focused on identifying potential roles for the class-Ia PI 3-kinases which are rapidly activated following insulin stimulation. However, recent evidence indicates the alpha isoform of class-II PI 3-kinase (PI3K-C2alpha) may also play a role as insulin causes a very rapid activation of this as well. The basic mechanisms by which insulin activates the various members of the PI 3-kinase family are increasingly well understood and these studies reveal multiple mechanisms for modulating the activity and functionality of PI 3-kinase and for down regulating the signals they generate. These include inhibitory phosphorylation events, lipid phosphatases such as PTEN and SHIP2 and inhibitor proteins of the suppressors of cytokine signalling (SOCS) family. The current review will focus on these mechanisms and how defects in these might contribute to the development of insulin resistance.

Reduction of insulin-stimulated glucose uptake by peroxynitrite is concurrent with tyrosine nitration of insulin receptor substrate-1

Inducible nitric oxide synthetase plays an essential role in insulin resistance induced by a high-fat diet. The reaction of nitric oxide with superoxide leads to the formation of peroxynitrite (ONOO-), which can modify several proteins. In this study, we investigated whether peroxynitrite impairs insulin-signalling pathway. Our experiments showed that 3-(4-morpholinyl)sydnonimine hydrochloride (SIN-1), a constitutive producer of peroxynitrite, dose-dependently inhibited insulin-stimulated glucose uptake. While SIN-1 did not affect the insulin receptor protein level and tyrosine phosphorylation, it reduced the insulin receptor substrate-1 (IRS-1) protein level, and IRS-1 associated phosphatidylinositol-3 kinase (PI-3 kinase) activity. Although SIN-1 did not induce Ser307 phosphorylation of IRS-1, tyrosine nitration of IRS-1 was detected in SIN-1-treated-Rat1 fibroblasts expressing human insulin receptors. Mass spectrometry showed that peroxynitrite induced at least four nitrated tyrosine residues in rat IRS-1, including Tyr939, which is critical for association of IRS-1 with the p85 subunit of PI-3 kinase. Our results suggest that peroxynitrite reduces the IRS-1 protein level and decreases phosphorylation of IRS-1 concurrent with nitration of its tyrosine residues.

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.

Vascular endothelial growth factor and diabetic retinopathy: pathophysiological mechanisms and treatment perspectives

Retinal neovascularization and macular edema are central features of diabetic retinopathy, the major cause of blindness in the developed world. Current treatments are limited in their efficacy and are associated with significant adverse effects. Characterization of the molecular and cellular processes involved in vascular growth and permeability has led to the recognition that the angiogenic growth factor and vascular permeability factor vascular endothelial growth factor (VEGF) plays a pivotal role in the retinal microvascular complications of diabetes. Therefore, VEGF represents an exciting target for therapeutic intervention in diabetic retinopathy. This review highlights the current understanding of the mechanisms that regulate VEGF gene expression and mediate its biological effects and how these processes may become altered during diabetes. The cellular and molecular alterations that characterize experimental models of diabetes are considered in relation to the influence of high glucose-mediated oxidative stress on VEGF expression and on the mechanisms of VEGF's actions under hyperglycemic induction. Finally, potential therapeutic strategies for preventing VEGF overexpression or blocking its pathological effects in the diabetic retina are considered.

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

Presentation of nitric oxide regulates monocyte survival through effects on caspase-9 and caspase-3 activation

In the absence of survival factors, blood monocytes undergo spontaneous apoptosis, which involves the activation of caspase-3. Although nitric oxide can block caspase-3 activation and promote cell survival, it can also induce apoptosis. We hypothesized that nitrosothiols that promote protein S-nitrosylation would reduce caspase-3 activation and cell survival, whereas nitric oxide donors (such as 1-propamine 3-(2-hydroxy-2-nitroso-1-propylhydrazine (PAPA) NONOate and diethylamine (DEA) NONOate) that do not target thiol residues would not. Using human monocytes as a model, we observed that nitrosothiol donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine suppressed caspase-9 and caspase-3 activity and DNA fragmentation. In contrast, PAPA or DEA NONOate did not promote monocyte survival events and appeared to inhibit monocyte survival induced by macrophage colony-stimulating factor. The caspase-3-selective inhibitor DEVD-fluoromethyl ketone reversed DNA fragmentation events, and the caspase-9 inhibitor LEHD-fluoromethyl ketone reversed caspase-3 activity in monocytes treated with PAPA or DEA NONOate in the presence of macrophage colony-stimulating factor. These results were not caused by differences in glutathione levels or the kinetics of nitric oxide release. Moreover, S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine directly blocked the activity of recombinant caspase-3, which was reversed by the reducing agent dithiothreitol, whereas PAPA or DEA NONOate did not block the enzymatic activity of caspase-3. These data support the hypothesis that nitrosylation of protein thiol residues by nitric oxide is critical for promoting the survival of human monocytes.

Early decrease of survival factors and DNA repair enzyme in spinal motor neurons of presymptomatic transgenic mice that express a mutant SOD1 gene

The primary pathogenetic mechanisms of amyotrophic lateral sclerosis (ALS) have been elusive. Some of the mechanisms would be implicated in an imbalance between death and survival factors, and impairment of DNA repair possibly caused by oxidative stress. Phosphatidylinositol 3-kinase (PI3-K) and its downstream effector, Akt/protein kinase B (PKB), have been shown to play a pivotal role in neuronal survival against apoptosis supported by neurotrophic factors. To elucidate the mechanisms of motor neuron death in ALS, we examined the expression of PI3-K, Akt, and the DNA repair enzyme redox factor-1 (Ref-1) protein in the spinal cord of transgenic mice with an ALS-linked mutant Cu/Zn superoxide dismutase (SOD1) gene, a valuable model for human ALS. Immunoblotting and immunocytochemical analyses showed that most spinal motor neurons lost immunoreactivity for PI3-K, Akt, and Ref-1 in the presymptomatic stage that preceded a significant loss of neurons. These results suggest that an early decrease of survival signal proteins and a DNA repair enzyme in the spinal motor neurons may account for the mutant SOD1-mediated motor neuron death in this animal model of ALS.

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.

Nitric oxide (NO) induces nitration of protein kinase Cepsilon (PKCepsilon ), facilitating PKCepsilon translocation via enhanced PKCepsilon -RACK2 interactions: a novel mechanism of no-triggered activation of PKCepsilon

Activation of protein kinase C (PKC) epsilon by nitric oxide (NO) has been implicated in the development of cardioprotection. However, the cellular mechanisms underlying the activation of PKCepsilon by NO remain largely unknown. Nitration of protein tyrosine residues has been shown to alter functions of a variety of proteins, and NO-derived peroxynitrite is known as a strong nitrating agent. In this investigation, we demonstrate that NO donors promote translocation and activation of PKCepsilon in an NO- and peroxynitrite-dependent fashion. NO induces peroxynitrite-mediated tyrosine nitration of PKCepsilon in rabbit cardiomyocytes in vitro, and nitrotyrosine residues were also detected on PKCepsilon in vivo in the rabbit myocardium preconditioned with NO donors. Furthermore, coimmunoprecipitation of PKCepsilon and its receptor for activated C kinase, RACK2, illustrated a peroxynitrite-dependent increase in PKCepsilon-RACK2 interactions in NO donor-treated cardiomyocytes. Moreover, using an enzyme-linked immunosorbent assay-based protein-protein interaction assay, PKCepsilon proteins treated with the peroxynitrite donor SIN-1 exhibited enhanced binding to RACK2 in an acellular environment. Our data demonstrate that post-translational modification of PKCepsilon by NO donors, namely nitration of PKCepsilon, facilitates its interaction with RACK2 and promotes translocation and activation of PKCepsilon. These findings offer a plausible novel mechanism by which NO activates the PKC signaling pathway.

Selective nitration of histone tyrosine residues in vivo in mutatect tumors

Nitric oxide-derived reactive species have been implicated in many disorders. Protein nitrotyrosine is often used as a stable marker of these reactive species. Using immunohistochemistry, we have previously detected nitrotyrosine in murine Mutatect tumors, where neutrophils are the principal source of nitric oxide. We now report on the identification of several prominent nitrotyrosine-containing proteins. Using Western blot analysis, nitrotyrosine in higher molecular mass proteins (>20 kDa) was detected in tumors containing a high number of neutrophils but not in tumors with fewer neutrophils. Staining for nitrotyrosine was consistently seen in low molecular mass proteins (< or =15 kDa), regardless of the level of neutrophils. Protein nitrotyrosine was not seen in Mutatect cells growing in vitro. Treatment with nitric oxide donors produced nitration of < or =15-kDa proteins, but only after extended periods. These small proteins, both from tumors and cultured cells, were identified by mass spectrometry to be histones. Only a subset of tyrosine residues was nitrated. Selective nitration may reflect differential accessibility of different tyrosine residues and the influence of neighboring residues within the nucleosome. The prominence of histone nitration may reflect its relative stability, making this post-translational modification a potentially useful marker of extended exposure of cells or tissues to nitric oxide-derived reactive species.

Epicatechin selectively prevents nitration but not oxidation reactions of peroxynitrite

The flavanol (-)-epicatechin has been found to protect against damage inflicted by peroxynitrite, an inflammatory intermediate. Here, epicatechin was tested in systems of increasing complexity. The compound efficiently protected against nitration of protein tyrosine residues by peroxynitrite (IC(50) approximately 0.02 mol epicatechin/mol peroxynitrite). However, at epicatechin concentrations completely preventing nitration of tyrosine by peroxynitrite, protection against the oxidative inactivation of glyceraldehyde-3-phosphate dehydrogenase or soybean lipoxygenase-1 was marginal (IC(50) > 1 mol epicatechin/mol peroxynitrite), approximately two orders of magnitude less. Likewise, epicatechin was relatively ineffective against oxidation of thiols in cell lysates, and against the oxidation of 2',7'-dichlorodihydrofluorescein in cultured cells. The activation of the kinases Akt/protein kinase B, ERK1/2 and p38-MAPK by peroxynitrite in murine aorta endothelial cells was not altered by epicatechin, suggesting that activation of these kinases is due to processes other than tyrosine nitration.

Tyrosine nitration: localisation, quantification, consequences for protein function and signal transduction

The nitration of free tyrosine or protein tyrosine residues generates 3-nitrotyrosine the detection of which has been utilised as a footprint for the in vivo formation of peroxynitrite and other reactive nitrogen species. The detection of 3-nitrotyrosine by analytical and immunological techniques has established that tyrosine nitration occurs under physiological conditions and levels increase in most disease states. This review provides an updated, comprehensive and detailed summary of the tissue, cellular and specific protein localisation of 3-nitrotyrosine and its quantification. The potential consequences of nitration to protein function and the pathogenesis of disease are also examined together with the possible effects of protein nitration on signal transduction pathways and on the metabolism of proteins.

Interactions between phosphatidylinositol 3-kinase and nitric oxide: explaining the paradox

Nitric oxide (NO) and the many derivatives and reactive oxygen intermediates thereof are all molecules that are utilised by mammalian cells in the war against microbial pathogens and tumours. They are potentially toxic molecules and, with damage control being crucial, the production and metabolism of nitric oxide is a tightly regulated process. The duality of NO is well documented. On the one hand, beneficial effects include normal healing in the skin and intestinal mucosa, killing of certain bacteria, regulating T cell proliferation and differentiation (Th1 vs Th2), and regulating leukocyte recruitment, by affecting adhesion molecule expression. On the other hand, persistent high levels of NO can lead to the production of toxic metabolites (peroxynitrite and hydroxyls), which can have detrimental effects, such as increased microvascular and epithelial permeability, increased oxidative stress (which can damage DNA), and damage to iron-sulphur proteins in mitochondria. NO has been reported to modulate its own production and the mechanisms involved in this self-regulation are being hotly pursued. The purpose of this review is to update recent intriguing advances in our understanding of the interaction of the phosphatidylinositol (PI) 3-kinase-dependent signal transduction pathway in regulating the activity of the enzymes that generate NO, namely, the nitric oxide synthases.

Reactive oxygen species and reactive nitrogen species: relevance to cyto(neuro)toxic events and neurologic disorders. An overview

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are formed under physiological conditions in the human body and are removed by cellular antioxidant defense system. During oxidative stress their increased formation leads to tissue damage and cell death. This process may be especially important in the central nervous system (CNS) which is vulnerable to ROS and RNS damage as the result of the brain high O(2) consumption, high lipid content and the relatively low antioxidant defenses in brain, compared with other tissues. Recently there has been an increased number of reports suggesting the involvement of free radicals and their non-radical derivatives in a variety of pathological events and multistage disorders including neurotoxicity, apoptotic death of neurons and neural disorders: Alzheimer's (AD), Parkinson's disease (PD) and schizophrenia. Taking into consideration the basic molecular chemistry of ROS and RNS, their overall generation and location, in order to control or suppress their action it is essential to understand the fundamental aspects of this problem. In this presentation we review and summarize the basics of all the recently known and important properties, mechanisms, molecular targets, possible involvement in cellular (neural) degeneration and apoptotic death and in pathogenesis of AD, PD and schizophrenia. The aim of this article is to provide an overview of our current knowledge of this problem and to inspire experimental strategies for the evaluation of optimum innovative therapeutic trials. Another purpose of this work is to shed some light on one of the most exciting recent advances in our understanding of the CNS: the realisation that RNS pathway is highly relevant to normal brain metabolism and to neurologic disorders as well. The interactions of RNS and ROS, their interconversions and the ratio of RNS/ROS could be an important neural tissue injury mechanism(s) involved into etiology and pathogenesis of AD, PD and schizophrenia. It might be possible to direct therapeutic efforts at oxidative events in the pathway of neuron degeneration and apoptotic death. From reviewed data, no single substance can be recommended for use in human studies. Some of the recent therapeutic strategies and neuroprotective trials need further development particularly those of antioxidants enhancement. Such an approach should also consider using combinations of radical(s) scavengers rather than a single substance.

Transplant atherosclerosis: role of phenotypic modulation of vascular smooth muscle by nitric oxide

Occlusive accelerated atherosclerosis of coronary grafts is the predominant factor that limits longevity of heart transplant recipients. This form of vascular disease affects both the large epicardial and the smaller intramyocardial vessels, leading to characteristic clinical presentation that necessitates the use of sophisticated techniques for their accurate detection. Accelerated atherosclerosis after transplantation is a multifactorial disease with many events contributing to its progression. The initial vascular injury associated with ischemia-reperfusion appears to aggravate preexisting conditions in the donor vasculature in addition to activation of new immunological and nonimmunological mechanisms. Throughout these events, the endothelium remains a primary target of cell- and humoral-mediated injury. Changes in the vascular intima leads to alterations in vascular smooth muscle cell (VSMC) physiology, resulting in VSMC phenotypic modulation with the orchestration of a broad spectrum of growth and inflammatory reactions, which might be a healing response to vascular injury. Endogenous nitric oxide (NO) pathways regulate a multiplicity of cellular mechanisms that play a major role in determining the structure and function of the vessel wall during normal conditions and during remodeling associated with accelerated atherosclerosis. Recently identified signaling pathways, including mitogen-activated protein kinase, cGMP-dependent protein kinase, phosphatidylinositol 3-kinase, and transcriptional events in which nuclear factor kappa B and activator protein 1 take part, can be associated with NO modulation of cell cycle perturbations and phenotypic alteration of VSMC during accelerated atherosclerosis. This article reviews recent progress covering the aforementioned matters. We start by summarizing the clincal aspects and pathogenesis of accelerated atherosclerosis associated with transplantation, including clinical presentation and detection. This summary is followed by a discussion of the multiple factors of the disease process, including immunological and nonimmunolgical contributions. The next section focuses on cellular responses of the VSMCs relevant to lesion formation, with special emphasis on classical and recent paradigms of phenotypic modulation of these cells. To examine the influence of NO on VSMC phenotypic modulation and consequent lesion development, we briefly overview characteristics of NO production in the normal coronary vascular bed and the changes in endogenous NO release and activity during atherosclerosis. This overview is followed by a section covering molecular mechanisms whereby NO regulates a range of signaling pathways, transcriptional events underlying cell cycle perturbation, and phenotypic alteration of VSMC in accelerated atherosclerosis.

Tyrosine nitration in blood vessels occurs with increasing nitric oxide concentration

1. Experiments were designed to explore the effects of nitric oxide (NO) donors on generation of superoxide (O2.-) and peroxynitrite (ONOO-) in rabbit aortic rings. 2. Following inhibition of endogenous superoxide dismutase (SOD), significant basal release of O2.- was revealed (0.9 +/- 0.01 x 10(-12) mol min-1 mg-1 tissue). Generation of O2.- increased in a concentration-dependent manner in response to NADH or NADPH (EC50 = 2.34 +/- 1.18 x 10(-4) and 6.21 +/- 1.79 x 10(-3) M respectively, n = 4). NADH-stimulated O2.- chemiluminescence was reduced by approximately 85% in the presence of exogenous SOD (15 x 10(3) U ml-1). 3. Incubation of aortic rings with S-nitrosoglutathione (GSNO; 1 x 10(-5)-3 x 10(-3) M) or sodium nitroprusside (SNP; 1 x 10(-8)-1 x 10(-3) M), resulted in a concentration-dependent quenching of O2.- chemiluminescence which was proportional to NO release. 4. ONOO- formation was assessed indirectly by determining protein tyrosine nitration in rabbit aorta using a specific antibody against nitrotyrosine. Basally and in the presence of NADH, a single band was detected. Incubation of aortic rings with either GSNO (1 x 10(-3) M) alone or GSNO with NADH resulted in the appearance of additional nitrotyrosine bands. Incubation of serum albumin with GSNO alone did not cause nitrotyrosine formation. In contrast, incubation with 3-morpholinosydonomine (SIN-1; 1 x 10(-3) M, 10 min), resulted in marked nitration of albumin which was reduced by oxyhaemoglobin or SOD. Incubation of albumin with GSNO and pyrogallol, a O2.- generator, also resulted in protein nitration. 5. Addition of exogenous NO results in nitrotyrosine formation in rabbit aortic rings. Nitrotyrosine formation is likely to result from the reaction of exogenous NO and basal endogenous O2.- resulting in the formation of ONOO-. Formation of ONOO- and nitration of tyrosine residues potentially could lead to vascular damage and might represent unexpected adverse effects of long-term nitrate therapy.