Redox-dependent effects of nitric oxide on microvascular integrity in oxygen-induced retinopathy

Retinopathy of prematurity: A review of pathophysiology and signaling pathways

Retinopathy of prematurity (ROP) is a vasoproliferative disorder of the retina and a leading cause of visual impairment and childhood blindness worldwide. The disease is characterized by an early stage of retinal microvascular degeneration, followed by neovascularization that can lead to subsequent retinal detachment and permanent visual loss. Several factors play a key role during the different pathological stages of the disease. Oxidative and nitrosative stress and inflammatory processes are important contributors to the early stage of ROP. Nitric oxide synthase and arginase play important roles in ischemia/reperfusion-induced neurovascular degeneration. Destructive neovascularization is driven by mediators of the hypoxia-inducible factor pathway, such as vascular endothelial growth factor and metabolic factors (succinate). The extracellular matrix is involved in hypoxia-induced retinal neovascularization. Vasorepulsive molecules (semaphorin 3A) intervene preventing the revascularization of the avascular zone. This review focuses on current concepts about signaling pathways and their mediators, involved in the pathogenesis of ROP, highlighting new potentially preventive and therapeutic modalities. A better understanding of the intricate molecular mechanisms underlying the pathogenesis of ROP should allow the development of more effective and targeted therapeutic agents to reduce aberrant vasoproliferation and facilitate physiological retinal vascular development.

Oxidative Stress Implication in Retinal Diseases-A Review

Oxidative stress (OS) refers to an imbalance between free radicals (FRs), namely highly reactive molecules normally generated in our body by several pathways, and intrinsic antioxidant capacity. When FR levels overwhelm intrinsic antioxidant defenses, OS occurs, inducing a series of downstream chemical reactions. Both reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced by numerous chemical reactions that take place in tissues and organs and are then eliminated by antioxidant molecules. In particular, the scientific literature focuses more on ROS participation in the pathogenesis of diseases than on the role played by RNS. By its very nature, the eye is highly exposed to ultraviolet radiation (UVR), which is directly responsible for increased OS. In this review, we aimed to focus on the retinal damage caused by ROS/RNS and the related retinal pathologies. A deeper understanding of the role of oxidative and nitrosative stress in retinal damage is needed in order to develop targeted therapeutic interventions to slow these pathologies.

3β-Hydroxy-5β-hydroxy-B-norcholestane-6β-carboxaldehyde (SEC-B) Induces Proinflammatory Activation of Human Endothelial Cells Associated with Nitric Oxide Production and Endothelial Nitric Oxide Synthase/Caveolin-1 Dysregulation

Oxysterols are a family of 27-carbon cholesterol oxidation derivatives found in low-density lipoproteins (LDLs) and atherosclerotic plaques where they trigger several biological responses involved in the initiation and progression of atherosclerosis. Several pieces of evidence suggest that oxysterols contribute to endothelial dysfunction (ED) due to their ability to alter membrane fluidity and cell permeability leading to inflammation, oxidative stress and apoptosis. The present study aimed to investigate the molecular events occurring in human microvascular endothelial cells (HMEC-1) in response to autoxidation-generated 3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxaldehyde (SEC-B) exposure. Our results highlight that SEC-B rapidly activates HMEC-1 by inducing oxidative stress, nitric oxide (NO) production and pro-inflammatory cytokine release. Exposure to SEC-B up to 24 h results in persistent accumulation of the vasodilator NO paralleled by an upregulation of the endothelial nitric oxide synthase (eNOS) enzyme and downregulation of Caveolin-1 (Cav-1) protein levels. Moreover, reduced expression and extracellular release of the vasoconstrictor factor endothelin-1 (ET-1) are observed. Furthermore, SEC-B stimulates the expression of the cytokines interleukin-6 (IL-6) and tumor necrosis factor-like weak inducer of apoptosis (TWEAK). This proinflammatory state leads to increased monocyte recruitment on activated HMEC-1 cells. Our findings add new knowledge on the role of SEC-B in ED and further support its potential implication in atherosclerosis.

[Retinopathy of prematurity: from prevention to treatment]

Retinopathy of prematurity (ROP) is one of the leading cause of preventable blindness in children. Its incidence increases with increasing survival of extremely preterm babies. ROP results from a multifactorial impairment of retinal development, the retinal vascular network, involving both oxygen-dependent and nutritional factors. The numerous factors involved in ROP development suggest that preventive strategies should be synergistic and complementary, including tight control of oxygen therapy, optimized nutritional intakes and postnatal growth, breastfeeding, adequate ω-3 PUFAs supply and control of hyperglycemic episodes associated with prematurity. ROP requires a multidisciplinary management, which includes systematic screening, appropriate treatment and long-term follow-up. Current screening modalities are based on wide-field digital retinal imaging systems, which also allow screening by telemedicine. The gold-standard treatment for ROP remains laser photocoagulation. It may be combined with intravitreal anti-VEGF administration, which is currently being evaluated, or surgery for advanced stages.

Oxidative Stress Markers and the Retinopathy of Prematurity

Retinopathy of prematurity (ROP) is a leading cause of potentially preventable blindness in low birth weight preterm infants. Several perinatal and postnatal factors contribute to the incomplete maturation of retinal vascularization, leading to oxidative stress damage. Literature data suggest that the lack of equilibrium between pro-oxidants and anti-oxidants plays a key role. In the last decade, there has been an increasing interest in identifying the antecedents of ROP and the relevant pathogenic mechanisms involved. In this context, a panel of biomarkers was investigated in order to achieve early detection of oxidative stress occurrence and to prevent retinal damage. Several nutritional elements have been found to play a relevant role in ROP prevention. At this stage, no conclusive data have been shown to support the usefulness of one biomarker over another. Recently, the Food and Drugs Administration, the European Medicine Agency, and the National Institute of Health proposed a series of criteria in order to promote the inclusion of new biomarkers in perinatal clinical guidelines and daily practice. The aim of the present review is to offer an update on a panel of biomarkers, currently investigated as potential predictors of ROP, highlighting their strengths and weaknesses.

Redox Regulation of the Microcirculation

The microcirculation maintains tissue homeostasis through local regulation of blood flow and oxygen delivery. Perturbations in microvascular function are characteristic of several diseases and may be early indicators of pathological changes in the cardiovascular system and in parenchymal tissue function. These changes are often mediated by various reactive oxygen species and linked to disruptions in pathways such as vasodilation or angiogenesis. This overview compiles recent advances relating to redox regulation of the microcirculation by adopting both cellular and functional perspectives. Findings from a variety of vascular beds and models are integrated to describe common effects of different reactive species on microvascular function. Gaps in understanding and areas for further research are outlined. © 2020 American Physiological Society. Compr Physiol 10:229-260, 2020.

Vascular and Neuronal Protection in the Developing Retina: Potential Therapeutic Targets for Retinopathy of Prematurity

Retinopathy of prematurity (ROP) is a common retinal disease in preterm babies. To prolong the lives of preterm babies, high oxygen is provided to mimic the oxygen level in the intrauterine environment for postnatal organ development. However, hyperoxia-hypoxia induced pathological events occur when babies return to room air, leading to ROP with neuronal degeneration and vascular abnormality that affects retinal functions. With advances in neonatal intensive care, it is no longer uncommon for increased survival of very-low-birth-weight preterm infants, which, therefore, increased the incidence of ROP. ROP is now a major cause of preventable childhood blindness worldwide. Current proven treatment for ROP is limited to invasive retinal ablation, inherently destructive to the retina. The lack of pharmacological treatment for ROP creates a great need for effective and safe therapies in these developing infants. Therefore, it is essential to identify potential therapeutic agents that may have positive ROP outcomes, especially in preserving retinal functions. This review gives an overview of various agents in their efficacy in reducing retinal damages in cell culture tests, animal experiments and clinical studies. New perspectives along the neuroprotective pathways in the developing retina are also reviewed.

Oxygen-Induced Retinopathy and Choroidopathy: In Vivo Longitudinal Observation of Vascular Changes Using OCTA

Purpose

The purpose of this study was to assess the retinal and choroidal vasculatures of an oxygen-induced retinopathy (OIR) rat model using optical coherence tomography angiography (OCTA) as well as to verify the performance of OCTA for visualizing in vivo vascular alterations, longitudinally and quantitatively.

Methods

To induce OIR, Sprague Dawley rat pups were incubated in an 80% oxygen chamber from postnatal day 1 (P1) to P11 and returned to room air. OCTA imaging was performed in six eyes at P15, P18, P21, and P24. All eyes were imaged with ex vivo retinal flat mount immunofluorescence microscopy for comparison with OCTA. The areas of the neovascular tufts, retinal vessel tortuosities and diameters, and vessel densities of different retinal and choroidal layers were quantified.

Results

The neovascular tufts were observed in two OIR eyes. The tuft areas decreased spontaneously from P18 to P24. The increase in arterial tortuosity and venous dilation were observed in the OIR eyes at P15 and P18. The retardation of vascular developments was observed in the deep vascular plexus and the choroidal layer in the OIR group while the superficial vascular plexus did not show developmental delay.

Conclusions

This study demonstrates an application of OCTA for quantitative and longitudinal studies on in vivo vascular alterations, including neovascular tufts, increase in arterial tortuosity, venous dilation, and developmental delay in the OIR rat model.

Propranolol 0.2% Eye Micro-Drops for Retinopathy of Prematurity: A Prospective Phase IIB Study

Background: Oral propranolol reduces retinopathy of prematurity (ROP) progression, although not safely. Propranolol 0.1% eye micro-drops administered to newborns with stage 2 ROP are well-tolerated, but not sufficiently effective. Methods: A multi-center open-label trial was conducted to assess the safety and efficacy of propranolol 0.2% eye micro-drops in newborns with stage 1 ROP. The progression of the disease was evaluated with serial ophthalmologic examinations. Hemodynamic, respiratory, biochemical parameters, and propranolol plasma levels were monitored. Demographic and perinatal characteristics, co-morbidities and co-intervention incidences, together with ROP progression, were compared with a historical control group in the same centers participating in the trial. Results: Ninety-eight newborns were enrolled and compared with the historical control group. Populations were not perfectly homogeneous (as demonstrated by the differences in the Apgar score and the different incidence rate in surfactant administration and oxygen exposure). The progression to ROP stage 2 or 3 plus was significantly lower than the incidence expected on the basis of historical data (Risk Ratio 0.521, 95% CI 0.297- 0.916). No adverse effects related to propranolol were observed and the mean propranolol plasma level was significantly lower than the safety cut-off of 20 ng/mL. Unexpectedly, three newborns treated with oral propranolol before the appearance of ROP, showed a ROP that was unresponsive to propranolol eye micro-drops and required laser photocoagulation treatment. Conclusion: Propranolol 0.2% eye micro-drops were well-tolerated and appeared to reduce the ROP progression expected on the basis of a comparison with a historical control group. Propranolol administered too early appears to favor a more aggressive ROP, suggesting that a β-adrenoreceptor blockade is only useful during the proliferative phase. Further randomized placebo-controlled trials are required to confirm the current results. Clinical Trial Registration  The trial was registered at ClinicalTrials.gov with Identifier NCT02504944 and with EudraCT Number 2014-005472-29.

Ischemic Retinopathies: Oxidative Stress and Inflammation

Ischemic retinopathies (IRs), such as retinopathy of prematurity (ROP), diabetic retinopathy (DR), and (in many cases) age-related macular degeneration (AMD), are ocular disorders characterized by an initial phase of microvascular changes that results in ischemia, followed by a second phase of abnormal neovascularization that may culminate into retinal detachment and blindness. IRs are complex retinal conditions in which several factors play a key role during the development of the different pathological stages of the disease. Increasing evidence reveals that oxidative stress and inflammatory processes are important contributors to the pathogenesis of IRs. Despite the beneficial effects of the photocoagulation and anti-VEGF therapy during neovascularization phase, the need to identify novel targets to prevent initial phases of these ocular pathologies is still needed. In this review, we provide an update on the involvement of oxidative stress and inflammation in the progression of IRs and address some therapeutic interventions by using antioxidants and anti-inflammatory agents.

Retinopathy of prematurity: inflammation, choroidal degeneration, and novel promising therapeutic strategies

Retinopathy of prematurity (ROP) is an important cause of childhood blindness globally, and the incidence is rising. The disease is characterized by initial arrested retinal vascularization followed by neovascularization and ensuing retinal detachment causing permanent visual loss. Although neovascularization can be effectively treated via retinal laser ablation, it is unknown which children are at risk of entering this vision-threatening phase of the disease. Laser ablation may itself induce visual field deficits, and there is therefore a need to identify targets for novel and less destructive treatments of ROP. Inflammation is considered a key contributor to the pathogenesis of ROP. A large proportion of preterm infants with ROP will have residual visual loss linked to loss of photoreceptor (PR) and the integrity of the retinal pigment epithelium (RPE) in the macular region. Recent studies using animal models of ROP suggest that choroidal degeneration may be associated with a loss of integrity of the outer retina, a phenomenon so far largely undescribed in ROP pathogenesis. In this review, we highlight inflammatory and neuron-derived factors related to ROP progression, as well, potential targets for new treatment strategies. We also introduce choroidal degeneration as a significant cause of residual visual loss following ROP. We propose that ROP should no longer be considered an inner retinal vasculopathy only, but also a disease of choroidal degeneration affecting both retinal pigment epithelium and photoreceptor integrity.

Accelerated oxygen-induced retinopathy is a reliable model of ischemia-induced retinal neovascularization

Retinal ischemia and pathological angiogenesis cause severe impairment of sight. Oxygen-induced retinopathy (OIR) in young mice is widely used as a model to investigate the underlying pathological mechanisms and develop therapeutic interventions. We compared directly the conventional OIR model (exposure to 75% O2 from postnatal day (P) 7 to P12) with an alternative, accelerated version (85% O2 from P8 to P11). We found that accelerated OIR induces similar pre-retinal neovascularization but greater retinal vascular regression that recovers more rapidly. The extent of retinal gliosis is similar but neuroretinal function, as measured by electroretinography, is better maintained in the accelerated model. We found no systemic or maternal morbidity in either model. Accelerated OIR offers a safe, reliable and more rapid alternative model in which pre-retinal neovascularization is similar but retinal vascular regression is greater.

Review of the mechanisms and therapeutic avenues for retinal and choroidal vascular dysfunctions in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a multifactorial disease and the main cause of visual impairment and blindness in premature neonates. The inner retina has been considered the primary region affected in ROP, but choroidal vascular degeneration and progressive outer retinal dysfunctions have also been observed. This review focuses on observations regarding neurovascular dysfunctions in both the inner and outer immature retina, the mechanisms and the neuronal-derived factors implicated in the development of ROP, as well potential therapeutic avenues for this disorder.

CONCLUSION

Alterations in the neurovascular integrity of the inner and outer retina contribute to the development of ROP.

Deletion of thioredoxin interacting protein (TXNIP) augments hyperoxia-induced vaso-obliteration in a mouse model of oxygen induced-retinopathy

We have recently shown that thioredoxin interacting protein (TXNIP) is required for VEGF-mediated VEGFR2 receptor activation and angiogenic signal. Retinas from TXNIP knockout mice (TKO) exhibited higher cellular antioxidant defense compared to wild type (WT). This study aimed to examine the impact of TXNIP deletion on hyperoxia-induced vaso-obliteration in ischemic retinopathy. TKO and WT pups were subjected to oxygen-induced retinopathy model. Retinal central capillary dropout was measured at p12. Retinal redox and nitrative state were assessed by reduced-glutathione (GSH), thioredoxin reductase activity and nitrotyrosine formation. Western blot and QT-PCR were used to assess VEGF, VEGFR-2, Akt, iNOS and eNOS, thioredoxin expression, ASK-1 activation and downstream cleaved caspase-3 and PARP in retinal lysates. Retinas from TKO mice exposed to hyperoxia showed significant increases (1.5-fold) in vaso-obliteration as indicated by central capillary drop out area compared to WT. Retinas from TKO showed minimal nitrotyrosine levels (10% of WT) with no change in eNOS or iNOS mRNA expression. There was no change in levels of VEGF or activation of VEGFR2 and its downstream Akt in retinas from TKO and WT. In comparison to WT, retinas from TKO showed significantly higher level of GSH and thioredoxin reductase activity in normoxia but comparable levels under hyperoxia. Exposure of TKO to hyperoxia significantly decreased the anti-apoptotic thioredoxin protein (∼50%) level compared with WT. This effect was associated with a significant increase in activation of the apoptotic ASK-1, PARP and caspase-3 pathway. Our results showed that despite comparable VEGF level and signal in TKO, exposure to hyperoxia significantly decreased Trx expression compared to WT. This effect resulted in liberation and activation of the apoptotic ASK-1 signal. These findings suggest that TXNIP is required for endothelial cell survival and homeostasis especially under stress conditions including hyperoxia.

Novel peptide for attenuation of hyperoxia-induced disruption of lung endothelial barrier and pulmonary edema via modulating peroxynitrite formation

Pulmonary damages of oxygen toxicity include vascular leakage and pulmonary edema. We have previously reported that hyperoxia increases the formation of NO and peroxynitrite in lung endothelial cells via increased interaction of endothelial nitric oxide (eNOS) with β-actin. A peptide (P326TAT) with amino acid sequence corresponding to the actin binding region of eNOS residues 326-333 has been shown to reduce the hyperoxia-induced formation of NO and peroxynitrite in lung endothelial cells. In the present study, we found that exposure of pulmonary artery endothelial cells to hyperoxia (95% oxygen and 5% CO2) for 48 h resulted in disruption of monolayer barrier integrity in two phases, and apoptosis occurred in the second phase. NOS inhibitor N(G)-nitro-L-arginine methyl ester attenuated the endothelial barrier disruption in both phases. Peroxynitrite scavenger uric acid did not affect the first phase but ameliorated the second phase of endothelial barrier disruption and apoptosis. P326TAT inhibited hyperoxia-induced disruption of monolayer barrier integrity in two phases and apoptosis in the second phase. More importantly, injection of P326TAT attenuated vascular leakage, pulmonary edema, and endothelial apoptosis in the lungs of mice exposed to hyperoxia. P326TAT also significantly reduced the increase in eNOS-β-actin association and protein tyrosine nitration. Together, these results indicate that peptide P326TAT ameliorates barrier dysfunction of hyperoxic lung endothelial monolayer and attenuates eNOS-β-actin association, peroxynitrite formation, endothelial apoptosis, and pulmonary edema in lungs of hyperoxic mice. P326TAT can be a novel therapeutic agent to treat or prevent acute lung injury in oxygen toxicity.

Thioredoxin-interacting protein expression is required for VEGF-mediated angiogenic signal in endothelial cells

AIMS

Thioredoxin-interacting protein (TXNIP) contributes to cellular redox-state homeostasis via binding and inhibiting thioredoxin (TRX). Increasing evidence suggests that cellular redox homeostasis regulates vascular endothelial growth factor (VEGF)-mediated signaling. This study aims to examine the redox-dependant role of TXNIP in regulating VEGF-mediated S-glutathionylation and angiogenic signaling. TXNIP-knockout mice (TKO) or wild-type (WT) treated with the reduced glutathione (GSH)-precursor, N-acetyl cysteine (WT-NAC, 500 mg/kg) were compared to WT using hypoxia-induced neovascularization model.

RESULTS

In response to hypoxia, retinas from TKO and WT-NAC mice showed significant decreases in reparative revascularization and pathological neovascularization with similar VEGF expression compared with WT. VEGF failed to stimulate vascular sprouting from aortic rings of TKO compared to WT mice. TKO mice or WT+NAC experienced reductive stress as indicated by twofold increase in TRX reductase activity and fourfold increase in reduced-GSH levels compared with WT. In human microvascular endothelial (HME) cells, VEGF stimulated co-precipitation between vascular endothelial growth factor receptor 2 (VEGFR2) with low molecular weight protein tyrosine phosphatase (LMW-PTP). Silencing TXNIP expression blunted VEGF-induced oxidation of GSH and S-glutathionylation of the LMW-PTP in HME cells. These effects were associated with impaired VEGFR2 phosphorylation that culminated in inhibiting cell migration and tube formation. Overexpression of TXNIP restored VEGFR2 phosphorylation and cell migration in TKO-endothelial cells.

INNOVATION

TXNIP expression is required for VEGF-mediated VEGFR2 activation and angiogenic response in vivo and in vitro. TXNIP expression regulates VEGFR-2 phosphorylation via S-glutathionylation of LMW-PTP in endothelial cells.

CONCLUSION

Our results provide novel mechanistic insight into modulating TXNIP expression as a potential therapeutic target in diseases characterized by aberrant angiogenesis.

Microglia and interleukin-1β in ischemic retinopathy elicit microvascular degeneration through neuronal semaphorin-3A

OBJECTIVE

Proinflammatory cytokines contribute to the development of retinal vasculopathies. However, the role of these factors and the mechanisms by which they elicit their effects in retina are not known. We investigated whether activated microglia during early stages of ischemic retinopathy produces excessive interleukin-1β (IL-1β), which elicits retinal microvascular degeneration not directly but rather by triggering the release of the proapoptotic/repulsive factor semaphorin-3A (Sema3A) from neurons.

APPROACH AND RESULTS

Sprague Dawley rats subjected to retinopathy induced by hyperoxia (80% O2; O2-induced retinopathy) exhibited retinal vaso-obliteration associated with microglial activation, NLRP3 upregulation, and IL-1β and Sema3A release; IL-1β was mostly generated by microglia. Intraperitoneal administration of IL-1 receptor antagonists (Kineret, or rytvela [101.10]) decreased these effects and enhanced retinal revascularization; knockdown of Sema3A resulted in microvessel preservation and, conversely, administration of IL-1β caused vaso-obliteration. In vitro, IL-1β derived from activated primary microglial cells, cultured under hyperoxia, stimulated the release of Sema3A in retinal ganglion cells-5, which in turn induced apoptosis of microvascular endothelium; antagonism of IL-1 receptor decreased microglial activation and on retinal ganglion cells-5 abolished the release of Sema3A inhibiting ensuing endothelial cell apoptosis. IL-1β was not directly cytotoxic to endothelial cells.

CONCLUSIONS

Our findings suggest that in the early stages of O2-induced retinopathy, retinal microglia are activated to produce IL-1β, which sustains the activation of microglia and induces microvascular injury through the release of Sema3A from adjacent neurons. Interference with IL-1 receptor or Sema3A actions preserves the microvascular bed in ischemic retinopathies and, consequently, decreases ensued pathological preretinal neovascularization.

Reactive oxygen species, Nox and angiotensin II in angiogenesis: implications for retinopathy

Pathological angiogenesis is a key feature of many diseases including retinopathies such as ROP (retinopathy of prematurity) and DR (diabetic retinopathy). There is considerable evidence that increased production of ROS (reactive oxygen species) in the retina participates in retinal angiogenesis, although the mechanisms by which this occurs are not fully understood. ROS is produced by a number of pathways, including the mitochondrial electron transport chain, cytochrome P450, xanthine oxidase and uncoupled nitric oxide synthase. The family of NADPH oxidase (Nox) enzymes are likely to be important given that their primary function is to produce ROS. Seven isoforms of Nox have been identified named Nox1-5, Duox (dual oxidase) 1 and Duox2. Nox1, Nox2 and Nox4 have been most extensively studied and are implicated in the development of conditions such as hypertension, cardiovascular disease and diabetic nephropathy. In recent years, evidence has accumulated to suggest that Nox1, Nox2 and Nox4 participate in pathological angiogenesis; however, there is no clear consensus about which Nox isoform is primarily responsible. In terms of retinopathy, there is growing evidence that Nox contribute to vascular injury. The RAAS (renin-angiotensin-aldosterone system), and particularly AngII (angiotensin II), is a key stimulator of Nox. It is known that a local RAAS exists in the retina and that blockade of AngII and aldosterone attenuate pathological angiogenesis in the retina. Whether the RAAS influences the production of ROS derived from Nox in retinopathy is yet to be fully determined. These topics will be reviewed with a particular emphasis on ROP and DR.

Targets of vascular protection in acute ischemic stroke differ in type 2 diabetes

Hemorrhagic transformation is an important complication of acute ischemic stroke, particularly in diabetic patients receiving thrombolytic treatment with tissue plasminogen activator, the only approved drug for the treatment of acute ischemic stroke. The objective of the present study was to determine the effects of acute manipulation of potential targets for vascular protection [i.e., NF-κB, peroxynitrite, and matrix metalloproteinases (MMPs)] on vascular injury and functional outcome in a diabetic model of cerebral ischemia. Ischemia was induced by middle cerebral artery occlusion in control and type 2 diabetic Goto-Kakizaki rats. Treatment groups received a single dose of the peroxynitrite decomposition catalyst 5,10,15,20-tetrakis(4-sulfonatophenyl)prophyrinato iron (III), the nonspecific NF-κB inhibitor curcumin, or the broad-spectrum MMP inhibitor minocycline at reperfusion. Poststroke infarct volume, edema, hemorrhage, neurological deficits, and MMP-9 activity were evaluated. All acute treatments reduced MMP-9 and hemorrhagic transformation in diabetic groups. In addition, acute curcumin and minocycline therapy reduced edema in these animals. Improved neurological function was observed in varying degrees with treatment, as indicated by beam-walk performance, modified Bederson scores, and grip strength; however, infarct size was similar to untreated diabetic animals. In control animals, all treatments reduced MMP-9 activity, yet bleeding was not improved. Neuroprotection was only conferred by curcumin and minocycline. Uncovering the underlying mechanisms contributing to the success of acute therapy in diabetes will advance tailored stroke therapies.

Signaling pathways triggered by oxidative stress that mediate features of severe retinopathy of prematurity

Oxidative stress has been implicated in the pathogenesis of retinopathy of prematurity for decades. It is becoming increasingly understood that reactive oxygen species can trigger signaling pathways that have beneficial or pathologic outcomes. Broad inhibition of reactive oxygen species in the preterm infant may lead to unwanted consequences, as has been experienced with vitamin E studies in the past. In this study, we provide a current understanding of the role of oxidative stress in activating signaling pathways that cause pathologic features in severe retinopathy of prematurity as it manifests in the era of oxygen regulation.

MEF2C ablation in endothelial cells reduces retinal vessel loss and suppresses pathologic retinal neovascularization in oxygen-induced retinopathy

Ischemic retinopathies, including retinopathy of prematurity and diabetic retinopathy, are major causes of blindness. Both have two phases, vessel loss and consequent hypoxia-driven pathologic retinal neovascularization, yet relatively little is known about the transcription factors regulating these processes. Myocyte enhancer factor 2 (MEF2) C, a member of the MEF2 family of transcription factors that plays an important role in multiple developmental programs, including the cardiovascular system, seems to have a significant functional role in the vasculature. We, therefore, generated endothelial cell (EC)-specific MEF2C-deficient mice and explored the role of MEF2C in retinal vascularization during normal development and in a mouse model of oxygen-induced retinopathy. Ablation of MEF2C did not cause appreciable defects in normal retinal vascular development. However, MEF2C ablation in ECs suppressed vessel loss in oxygen-induced retinopathy and strongly promoted vascular regrowth, consequently reducing retinal avascularity. This finding was associated with suppression of pathologic retinal angiogenesis and blood-retinal barrier dysfunction. MEF2C knockdown in cultured retinal ECs using small-interfering RNAs rescued ECs from death and stimulated tube formation under stress conditions, confirming the endothelial-autonomous and antiangiogenic roles of MEF2C. HO-1 was induced by MEF2C knockdown in vitro and may play a role in the proangiogenic effect of MEF2C knockdown on retinal EC tube formation. Thus, MEF2C may play an antiangiogenic role in retinal ECs under stress conditions, and modulation of MEF2C may prevent pathologic retinal neovascularization.

Overexpression of extracellular superoxide dismutase has a protective role against hyperoxia-induced brain injury in neonatal mice

There is increasing evidence that hyperoxia, particularly at the time of birth, may result in neurological injury, in particular to the susceptible vasculature of these tissues. This study was aimed at determining whether overexpression of extracellular superoxide dismutase (EC-SOD) is protective against brain injury induced by hyperoxia. Transgenic (TG) mice (with an extra copy of the human extracellular superoxide dismutase gene) and wild-type (WT) neonate mice were exposed to hyperoxia (95% of F(i) o(2) ) for 7 days after birth versus the control group in room air. Brain positron emission tomography (PET) scanning with fludeoxyglucose (FDG) isotope uptake was performed after exposure. To assess apoptosis induced by hyperoxia exposure, caspase 3 ELISA and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining were performed. Quantitative western blot for the following inflammatory markers was performed: glial fibrillary acidic protein, ionized calcium-binding adaptor molecule 1, macrophage-inhibiting factor, and phospho-AMP-activated protein kinase. PET scanning with FDG isotope uptake showed significantly higher uptake in the WT hyperoxia neonate brain group (0.14 ± 0.03) than in both the TG group (0.09 ± 0.01) and the control group (0.08 ± 0.02) (P< 0.05). Histopathological investigation showed more apoptosis and dead neurons in hippocampus and cerebellum brain sections of WT neonate mice after exposure to hyperoxia than in TG mice; this finding was also confirmed by TUNEL staining. The caspase 3 assay confirmed the finding of more apoptosis in WT hyperoxia neonates (0.814 ± 0.112) than in the TG hyperoxic group (0.579 ± 0.144) (P < 0.05); this finding was also confirmed by TUNEL staining. Quantitative western blotting for the inflammatory and metabolic markers showed significantly higher expression in the WT group than in the TG and control groups. Thus, overexpression of EC-SOD in the neonate brain offers significant protection against hyperoxia-induced brain damage.

Early transient hypoglycemia is associated with increased albumin nitration in the preterm infant

BACKGROUND

The clinical significance of early transient hypoglycemia (ETH), a frequent event in preterm newborns, is a highly controversial issue. In experimental models, hypoglycemia has been reported to cause oxidative stress. Among the reactive species, early generated peroxynitrite is responsible for protein nitration and lipid peroxidation, a process referred to as nitrative stress.

OBJECTIVES

The aim of the present study is to investigate whether ETH is associated with protein nitration in the preterm newborn.

METHODS

Using a novel highly sensitive ELISA, we quantified plasma nitroalbumin (PNA) as a marker of peroxynitrite generation in 72 preterm newborns (28-36 weeks), among which 25 had a glycemia level of <2.5 mmol/l during the first hour of life (H1).

RESULTS

PNA was significantly higher in ETH than in normoglycemic infants at H1 [median = 6.3 (3.8-8.8) vs. 3.4 ng/ml (2.1-5.1), p = 0.027] and at day 1 [median = 6.6 (5.6-15.3) vs. 3.9 ng/ml (2.3-4.6), p = 0.014]. PNA was inversely correlated with glycemia at H1 (r = -0.30, p = 0.01) and at day 1 (r = -0.63, p = 0.001). In ETH infants, lactatemia was inversely correlated with PNA. At day 1, PNA was higher in ETH infants treated by gavage than in those treated with intravenous dextrose [median = 8.9 ng/ml (7.1-10.4) vs. 4.4 ng/ml (2.6-5.7), p = 0.008].

CONCLUSIONS

These results indicate that ETH is associated with increased peroxynitrite generation resulting in systemic protein nitration in premature newborns. Treatment of ETH with intravenous dextrose is associated with lower PNA levels than gavage.

Understanding retinopathy of prematurity: update on pathogenesis

Retinopathy of prematurity (ROP), an ocular disease characterized by the onset of vascular abnormalities in the developing retina, is the major cause of visual impairment and blindness in premature neonates. ROP is a complex condition in which various factors participate at different stages of the disease leading to microvascular degeneration followed by neovascularization, which in turn predisposes to retinal detachment. Current ablative therapies (cryotherapy and laser photocoagulation) used in the clinic for the treatment of ROP have limitations and patients can still have long-term effects even after successful treatment. New treatment modalities are still emerging. The most promising are the therapies directed against VEGF; more recently the use of preventive dietary supplementation with ω-3 polyunsaturated fatty acid may also be promising. Other than pharmacologic and nutritional approaches, cell-based strategies for vascular repair are likely to arise from advances in regenerative medicine using stem cells. In addition to all of these, a greater understanding of other factors involved in regulating pathologic retinal angiogenesis continues to emerge, suggesting potential targets for therapeutic approaches. This review summarizes an update on the current state of knowledge on ROP from our and other laboratories, with particular focus on the role of nitro-oxidative stress and notably trans-arachidonic acids in microvascular degeneration, semaphorin 3 operating as vasorepulsive molecules in the avascular hypoxic retina and in turn impairing revascularization, succinate and its receptor GPR91 in neuron-mediated retinal neovascularization, and ω-3 lipids as modulators of preretinal neovascularization.

Retinopathy of prematurity: understanding ischemic retinal vasculopathies at an extreme of life

Retinopathy of prematurity (ROP) is a major complication of preterm birth. It encompasses a spectrum of pathologies that affect vision, from mild disease that resolves spontaneously to severe disease that causes retinal detachment and subsequent blindness. The pathologies are characterized by an arrest in normal retinal vascular development associated with microvascular degeneration. The resulting ischemia and retinal hypoxia lead to excessive abnormal compensatory blood vessel growth. However, this neovascularization can lead to fibrous scar formation and culminate in retinal detachment. Present therapeutic modalities to limit the adverse consequences of aberrant neovascularization are invasive and/or tissue-destructive. In this Review, we discuss current concepts on retinal microvascular degeneration, neovascularization, and available treatments, as well as present future perspectives toward more profound elucidation of the pathogenesis of ROP.

The mouse retina as an angiogenesis model

The mouse retina has been used extensively over the past decades to study both physiologic and pathologic angiogenesis. Over time, various mouse retina models have evolved into well-characterized and robust tools for in vivo angiogenesis research. This article is a review of the angiogenic development of the mouse retina and a discussion of some of the most widely used vascular disease models. From the multitude of studies performed in the mouse retina, a selection of representative works is discussed in more detail regarding their role in advancing the understanding of both the ocular and general mechanisms of angiogenesis.

Sustained hypercapnia induces cerebral microvascular degeneration in the immature brain through induction of nitrative stress

Hypercapnia is regularly observed in chronic lung disease, such as bronchopulmonary dysplasia in preterm infants. Hypercapnia results in increased nitric oxide synthase activity and in vitro formation of nitrates. Neural vasculature of the immature subject is particularly sensitive to nitrative stress. We investigated whether exposure to clinically relevant sustained high CO2 causes microvascular degeneration in the newborn brain by inducing nitrative stress, and whether this microvascular degeneration has an impact on brain growth. Newborn rat pups were exposed to 10% CO2 as inspired gas (PaCO2 = 60–70 mmHg) starting within 24 h of birth until postnatal day 7 (P7). Brains were notably collected at different time points to measure vascular density, determine brain cortical nitrite/nitrate, and trans-arachidonic acids (TAAs; products of nitration) levels as effectors of vessel damage. Chronic exposure of rat pups to high CO2 (PaCO2 ≈ 65 mmHg) induced a 20% loss in cerebrovascular density at P3 and a 15% decrease in brain mass at P7; at P30, brain mass remained lower in CO2-exposed animals. Within 24 h of exposure to CO2, brain eNOS expression and production of nitrite/nitrate doubled, lipid nitration products (TAAs) increased, and protein nitration (3-nitrotyrosine immunoreactivity) was also coincidently augmented on brain microvessels (lectin positive). Intracerebroventricular injection of TAAs (10 μM) replicated cerebrovascular degeneration. Treatment of rat pups with NOS inhibitor (l-Nω-nitroarginine methyl ester) or a peroxynitrite decomposition catalyst (FeTPPS) prevented hypercapnia-induced microvascular degeneration and preserved brain mass. Cytotoxic effects of high CO2 were reproduced in vitro /ex vivo on cultured endothelial cells and sprouting microvessels. In summary, hypercapnia at values frequently observed in preterm infants with chronic lung disease results in increased nitrative stress, which leads to cerebral cortical microvascular degeneration and curtails brain growth.

CYP1B1 and endothelial nitric oxide synthase combine to sustain proangiogenic functions of endothelial cells under hyperoxic stress

We have recently shown that deletion of constitutively expressed CYP1B1 is associated with attenuation of retinal endothelial cell (EC) capillary morphogenesis (CM) in vitro and angiogenesis in vivo. This was largely caused by increased intracellular oxidative stress and increased production of thrombospondin-2, an endogenous inhibitor of angiogenesis. Here, we demonstrate that endothelium nitric oxide synthase (eNOS) expression is dramatically decreased in the ECs prepared from retina, lung, heart, and aorta of CYP1B1-deficient (CYP1B1(-/-)) mice compared with wild-type (CYP1B1(+/+)) mice. The eNOS expression was also decreased in retinal vasculature of CYP1B1(-/-) mice. Inhibition of eNOS activity in cultured CYP1B1(+/+) retinal ECs blocked CM and was concomitant with increased oxidative stress, like in CYP1B1(-/-) retinal ECs. In addition, expression of eNOS in CYP1B1(-/-) retinal ECs or their incubation with a nitric oxide (NO) donor enhanced NO levels, lowered oxidative stress, and improved cell migration and CM. Inhibition of CYP1B1 activity in the CYP1B1(+/+) retinal ECs resulted in reduced NO levels and attenuation of CM. In contrast, expression of CYP1B1 increased NO levels and enhanced CM of CYP1B1(-/-) retinal ECs. Furthermore, attenuation of CYP1B1 expression with small interfering RNA proportionally lowered eNOS expression and NO levels in wild-type cells. Together, our results link CYP1B1 metabolism in retinal ECs with sustained eNOS activity and NO synthesis and/or bioavailability and low oxidative stress and thrombospondin-2 expression. Thus CYP1B1 and eNOS cooperate in different ways to lower oxidative stress and thereby to promote CM in vitro and angiogenesis in vivo.

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.

Stimulation of peroxynitrite catalysis improves insulin sensitivity in high fat diet-fed mice

Peroxynitrite synthesis is increased in insulin resistant animals and humans. Peroxynitirite-induced nitration of insulin signalling proteins impairs insulin action in vitro, but the role of peroxynitrite in the pathogenesis of insulin resistance in vivo is not known. We therefore assessed the effects of a 1-week treatment with the peroxynitrite decomposition catalyst FeTPPS on insulin sensitivity in insulin resistant high fat diet-fed (HFD) and control mice. FeTPPS normalized the fasting plasma glucose and insulin levels (P < 0.01), attenuated the hyperglycaemic response to an intraperitoneal glucose challenge by roughly 50% (P < 0.05), and more than doubled the insulin-induced decrease in plasma glucose levels in HFD-fed mice (P < 0.001). Moreover, FeTPPS restored insulin-stimulated Akt phosphorylation and insulin-stimulated glucose uptake in isolated skeletal muscle in vitro. Stimulation of peroxynitrite catalysis attenuates HFD-induced insulin resistance in mice by restoring insulin signalling and insulin-stimulated glucose uptake in skeletal muscle tissue.

Trans-arachidonic acids: new mediators of nitro-oxidative stress

A reaction of arachidonic acid with the nitrogen dioxide radical (*NO2) or its precursors (peroxynitrite, nitrous acid, nitrogen trioxide) generates a group of nitro lipids named nitroeicosanoids. A distinct feature of this reaction is abundant formation of four trans isomers of arachidonic acid (TAA) via reversible addition of the NO2 radical to the arachidonic acid cis double bonds. This cis-trans isomerization is biologically relevant because many pathologies that involve NO formation such as inflammation, hyperoxia, hypercapnia or exposure to cigarette smoke increase the TAA levels in cells, tissues and in the systemic circulation. Inflammatory conditions have been known to stimulate formation of a variety of oxidized lipids from unsaturated fatty acid precursors via lipid peroxidation mechanisms; however, nitration-dependent cis-trans-isomerization of arachidonic acid is a characteristic process for *NO2. TAA are likely to function as specific and selective biomarkers of the pathologic conditions that define nitro-oxidative stress. Diet independent biosynthesis of trans fatty acids as a result of disease is our new observation. In the past, experimental feeding and clinical studies have supported the concerns that dietary trans fatty acids are cardiovascular risk factors, however, clinical consequences of the endogenous formation of trans fatty acids are not known but potentially important given available studies on TAA. This review aims to summarize the emerging role of TAA as a unique group of biomarkers that target microcirculation and other systems. A biological mechanism that generates endogenous trans fatty acids poses new challenges for pharmacologic intervention and we suggest approaches that may limit TAA effects.

Neutralizing VEGF decreases tortuosity and alters endothelial cell division orientation in arterioles and veins in a rat model of ROP: relevance to plus disease

PURPOSE

To study the effects of vascular endothelial growth factor (VEGF) on endothelial nitric oxide synthetase (eNOS) and retinal vascular tortuosity and cleavage planes in a rat model of retinopathy of prematurity (ROP).

METHODS

Within 4 hours of birth, pups and mothers were cycled between 50% and 10% oxygen daily. At postnatal day (p)12, pups received either intravitreous anti-rat neutralizing antibody to VEGF or control nonimmune rat IgG in one eye and returned to oxygen cycling until p14 when they were placed in room air (RA) for 4 days (50/10 oxygen-induced retinopathy [50/10 OIR]). Tortuosity indices and endothelial cleavage plane angles relative to the long axes of the major retinal vessels during anaphase were calculated from phosphohistone- and Alexa-isolectin-stained retinal flatmounts. Some retinas were processed for eNOS protein or phosphorylated/total eNOS.

RESULTS

Retinas from 50/10 OIR had increased tortuosity over time with peaks at p12 and p14 (P < 0.001 vs. RA) before the development of intravitreous neovascularization, which peaked at p18. Compared with RA, eNOS/actin in 50/10 OIR retinas was increased at p12 (P = 0.0003) and p14 (P = 0.047). Inhibition of VEGF with a neutralizing antibody decreased tortuosity and caused endothelial mitosis cleavage planes to orient in favor of vessel elongation but did not affect eNOS protein or activation.

CONCLUSIONS

In the 50/10 OIR model, a model with relevance to ROP, arteriolar tortuosity, and venous dilation are increased through VEGF, which influences the orientation of endothelial cell cleavage in major arterioles and veins, independent of eNOS.

[Hypercapnia- and trans-arachidonic acid-induced retinal microvascular degeneration: implications in the genesis of retinopathy of prematurity]

High oxygen tension is a major factor in the genesis of retinopathy of prematurity (ROP). However, clinical and experimental evidence also suggest a significant role for high levels of carbon dioxide (CO(2)). Hypercapnia is a facilitator of nitration in vitro, and nitrative stress is known to have an important role in microvascular degeneration leading to ischemia in conditions such as ROP. We hereby present evidence that prolonged exposure to CO(2) impairs developmental retinal neovascularisation through a mechanism involving increased endothelial nitric oxide synthase and induction of a nitrative stress; effects of hypercapnia are independent of its hyperaemic effects. Moreover, in a model of oxygen-induced retinopathy, we demonstrate that an in vivo nitrative stress associated with retinal vasoobliteration results in nitration of cis-arachidonic acids into trans-arachidonic acids (TAAs). TAAs act in turn as mediators of nitrative stress by causing microvascular degeneration by inducing expression of the anti-angiogenic factor thrombospondin-1. These recent findings establish a previously unexplored means by which hypercapnia hinders efficient neovascularisation and provide new insight into the molecular mechanisms of nitrative stress on microvascular injury involving TAA, therefore opening new therapeutic avenues in the management of nitrative stress disorders such as in ischemic retinopathies (of prematurity and of diabetes) and encephalopathies.

Peroxynitrite: biochemistry, pathophysiology and development of therapeutics

Peroxynitrite--the product of the diffusion-controlled reaction of nitric oxide with superoxide radical--is a short-lived oxidant species that is a potent inducer of cell death. Conditions in which the reaction products of peroxynitrite have been detected and in which pharmacological inhibition of its formation or its decomposition have been shown to be of benefit include vascular diseases, ischaemia-reperfusion injury, circulatory shock, inflammation, pain and neurodegeneration. In this Review, we first discuss the biochemistry and pathophysiology of peroxynitrite and then focus on pharmacological strategies to attenuate the toxic effects of peroxynitrite. These include its catalytic reduction to nitrite and its isomerization to nitrate by metalloporphyrins, which have led to potential candidates for drug development for cardiovascular, inflammatory and neurodegenerative diseases.

Renal and cardiac oxidative/nitrosative stress in salt-loaded pregnant rat

Sodium supplementation given for 1 wk to nonpregnant rats induces changes that are adequate to maintain renal and circulatory homeostasis as well as arterial blood pressure. However, in pregnant rats, proteinuria, fetal growth restriction, and placental oxidative stress are observed. Moreover, the decrease in blood pressure and expansion of circulatory volume, normally associated with pregnancy, are prevented by high-sodium intake. We hypothesized that, in these pregnant rats, a loss of the balance between prooxidation and antioxidation, particularly in kidneys and heart, disturbs the normal course of pregnancy and leads to manifestations such as gestational hypertension. We thus investigated the presence of oxidative/nitrosative stress in heart and kidneys following high-sodium intake in pregnant rats. Markers of this stress [8-isoprostaglandin F(2alpha) (8-iso-PGF(2alpha)) and nitrotyrosine], producer of nitric oxide [nitric oxide synthases (NOSs)], and antioxidants [superoxide dismutase (SOD) and catalase] were measured. Then, molecules (Na(+)-K(+)-ATPase and aconitase) or process [apoptosis (Bax and Bcl-2), inflammation (monocyte chemoattractant protein-1, connective tissue growth factor, and TNF-alpha)] susceptible to free radicals was determined. In kidneys from pregnant rats on 1.8% NaCl-water, NOSs, apoptotic index, and nitrotyrosine expression were increased, whereas Na(+)-K(+)-ATPase mRNA and activity were decreased. In the left cardiac ventricle of these rats, heightened nitrotyrosine, 8-iso-PGF(2alpha), and catalase activity together with reduced endothelial NOS protein expression and SOD and aconitase activities were observed. These findings suggest that oxidative/nitrosative stress in kidney and left cardiac ventricle destabilizes the normal course of pregnancy and could lead to gestational hypertension.

Peroxynitrite mediates VEGF's angiogenic signal and functionviaa nitration‐independent mechanism in endothelial cells

The modulation of angiogenic signaling by reactive oxygen species (ROS) is an emerging area of interest in cellular and vascular biology research. We provide evidence here that peroxynitrite, the powerful oxidizing and nitrating free radical, is critically involved in transduction of the VEGF signal. We tested the hypothesis that VEGF induces peroxynitrite formation, which causes tyrosine phosphorylation and mediates endothelial cell migration and tube formation, by studies of vascular endothelial cells in vitro and in a model of hypoxia-induced neovascularization in vivo. The specific peroxynitrite decomposition catalyst FeTPPs blocked VEGF-induced phosphorylation of VEGFR2 and c-Src and inhibited endothelial cell migration and tube formation. Furthermore, exogenous peroxynitrite mimicked VEGF activity in causing phosphorylation of VEGFR2 and stimulating endothelial cell growth and tube formation in vitro and new blood vessel growth in vivo. The selective nitration inhibitor epicatechin enhanced VEGF's angiogenic function in activating VEGFR2, c-Src, and promoting endothelial cell growth, migration, and tube formation in vitro and retinal neovascularization in vivo. Decomposing peroxynitrite with FeTPPs or blocking oxidation using the thiol donor NAC blocked VEGF's angiogenic functions in vitro and in vivo. In conclusion, peroxynitrite is critically involved in transducing VEGF's angiogenic signal via nitration-independent and oxidation-mediated tyrosine phosphorylation.

Lysophosphatidic acid induces endothelial cell death by modulating the redox environment

Oxidant stress plays a significant role in hypoxic-ischemic injury to the susceptible microvascular endothelial cells. During oxidant stress, lysophosphatidic acid (LPA) concentrations increase. We explored whether LPA caused cytotoxicity to neuromicrovascular cells and the potential mechanisms thereof. LPA caused a dose-dependent death of porcine cerebral microvascular as well as human umbilical vein endothelial cells; cell death appeared oncotic rather than apoptotic. LPA-induced cell death was mediated via LPA(1) receptor, because the specific LPA(1) receptor antagonist THG1603 fully abrogated LPA's effects. LPA decreased intracellular GSH levels and induced a p38 MAPK/JNK-dependent inducible nitric oxide synthase (NOS) expression. Pretreatment with the antioxidant GSH precursor N-acetyl-cysteine (NAC), as well as with inhibitors of NOS [N(omega)-nitro-l-arginine (l-NNA); 1400W], significantly prevented LPA-induced endothelial cell death (in vitro) to comparable extents; as expected, p38 MAPK (SB203580) and JNK (SP-600125) inhibitors also diminished cell death. LPA did not increase indexes of oxidation (isoprostanes, hydroperoxides, and protein nitration) but did augment protein nitrosylation. Endothelial cytotoxicity by LPA in vitro was reproduced ex vivo in brain and in vivo in retina; THG1603, NAC, l-NNA, and combined SB-203580 and SP600125 prevented the microvascular rarefaction. Data implicate novel properties for LPA as a modulator of the cell redox environment, which partakes in endothelial cell death and ensued neuromicrovascular rarefaction.

Hyperoxic exposure leads to nitrative stress and ensuing microvascular degeneration and diminished brain mass and function in the immature subject

BACKGROUND AND PURPOSE

Neonates that survive very preterm birth have a high prevalence of cognitive impairment in later life. A common factor detected in premature infants is their postnatal exposure to high oxygen tension relative to that in utero. Hyperoxia is known to elicit injury to premature lung and retina. Because data on the exposure of the brain to hyperoxia are limited, we studied the effects of high oxygen on this tissue.

METHODS

Rat pups were exposed from birth until day 6 to 21% or 80% O(2). Cerebral vascular density was quantified by lectin immunohistochemistry. Immunoblots for several proteins were performed on brain extracts. We assessed cerebral functional deficits by visual evoked potentials.

RESULTS

Exposure of pups to hyperoxia leads to cerebral microvascular degeneration, diminished brain mass, and cerebral functional deficits. These effects are preceded by an upregulation of endothelial nitric oxide synthase (eNOS) in cerebral capillaries and a downregulation of Cu/Zn superoxide dismutase (SOD). The imbalance in nitric oxide (NO) production and antioxidant defenses favors the formation of nitrating agents in the microvessels revealed by increased nitrotyrosine (3-nt) immunoreactivity and decreased expression of NF-kappaB and the dependent vascular endothelial growth factor receptor 2. NOS inhibitors and eNOS deletion as well as an SOD mimetic (CuDIPS) restore vascular endothelial growth factor receptor-2 levels and nearly abolish the vasoobliteration. NOS inhibitors and SOD mimetic also prevent O(2)-induced diminished brain mass and functional deficit.

CONCLUSIONS

Data identify NO and nitrating agents as major mediators of cerebral microvascular damage, ensuing impaired brain development and function in immature subjects exposed to hyperoxia.

Hypercapnia- and trans-arachidonic acid-induced retinal microvascular degeneration: implications in the genesis of retinopathy of prematurity

High oxygen tension is a major factor in the genesis of retinopathy of prematurity (ROP). However, clinical and experimental evidence suggests a significant role for high carbon dioxide (CO(2)) tension as well. Along these lines, although ischemia is often considered to be synonymous with an oxygen deficit, it is also associated with a concomitant local elevation of CO(2) that can lead to impaired developmental and ischemic neovascularization. The mechanisms by which hypercapnia induces retinal microvascular degeneration, a critical step which precedes the subsequent proliferative preretinal neovascularization, are not known. Nitrative stress has an important role in microvascular degeneration leading to ischemia in conditions such as ROP. Hypercapnia is a facilitator of nitration in vitro. We hereby present evidence that prolonged exposure to CO(2) impairs developmental retinal neovascularization through a mechanism involving increased endothelial nitric oxide synthase and induction of a nitrative stress; effects of hypercapnia are independent of its hyperaemic effects. Moreover, we demonstrate that an in vivo nitrative stress associated with retinal vasoobliteration results in nitration of arachidonic acids into trans-arachidonic acids (TAAs), which can act as mediators of nitrative stress by causing microvascular degeneration by inducing expression of the antiangiogenic factor thrombospondin-1. These recent findings establish a previously unexplored means by which hypercapnia hinders efficient neovascularization and provide new insight into the molecular mechanisms of nitrative stress on microvascular injury involving TAA, and suggest new therapeutic avenues in the management of nitrative stress disorders such as in ischemic retinopathies (of prematurity and of diabetes) and encephalopathies.

Proangiogenic Effects of Protease-Activated Receptor 2 Are Tumor Necrosis Factor-α and Consecutively Tie2 Dependent

Objective— Angiogenesis is essential physiologically in growth and pathologically in tumor development, chronic inflammatory disorders, and proliferative retinopathies. Activation of protease-activated receptor 2 (PAR2) leads to a proangiogenic response, but its mechanisms have yet to be specifically described. Here, we investigated the mode of action of PAR2 in retinal angiogenesis.

Methods and Results— PAR2-activating peptide, SLIGRL, increased retinal angiogenesis associated with an induction of vascular endothelial growth factor and angiopoetin-2 and most notably tie2 in the retina in vivo as well as in cultured neuroretinal endothelial cells. SLIGRL also induced release of the proinflammatory and angiogenic mediator tumor necrosis factor-α (TNF-α) via the MEK/extracellular signal-regulated kinase (ERK) (MEK/ERK) pathway in these endothelial cells. TNF-α, in turn, elicited tie2 expression by activating the MEK/ERK pathway. PAR2-evoked tie2 expression, endothelium proliferation (in vitro), and retinal neovascularization (in vivo) were abrogated by selective TNF-α blockers (neutralizing antibody infliximab and soluble TNF-α receptor-Fc fusion protein etanercept) as well as the MEK inhibitor PD98059.

Conclusion— The proangiogenic properties of PAR2 are intertwined with its proinflammatory effects, such that in retinal vasculature, they depend on TNF-α and subsequent induction of tie2 via the MEK/ERK pathway.

Nitric oxide signaling via nuclearized endothelial nitric-oxide synthase modulates expression of the immediate early genes iNOS and mPGES-1

Stimulation of freshly isolated rat hepatocytes with lysophosphatidic acid (LPA) resulted in LPA1 receptor-mediated and nitricoxide-dependent up-regulation of the immediate early genes iNOS (inducible nitric-oxide synthase (NOS)) and mPGES-1 (microsomal prostaglandin E synthase-1). Because LPA is a ligand for both cell surface and intracellular receptor sites and a potent endothelial NOS (eNOS) activator, we hypothesized that NO derived from activated nuclearized eNOS might participate in gene regulation. Herein we show, by confocal microscopy performed on porcine cerebral endothelial cells expressing native LPA1-receptor and eNOS and on HTC4 rat hepatoma cells co-transfected with recombinant human LPA1-receptor and fused eNOS-GFP cDNA, a dynamic eNOS translocation from peripheral to nuclear regions upon stimulation with LPA. Nuclear localization of eNOS and its downstream effector, soluble guanylate cyclase, were demonstrated in situ in rat liver specimens by immunogold labeling using specific antibodies. Stimulation of this nuclear fraction with LPA and the NO donor sodium nitroprusside resulted, respectively, in increased production of nitrite (and eNOS phosphorylation) and cGMP; these separate responses were also correspondingly blocked by NOS inhibitor L-NAME and soluble guanylate cyclase inhibitor ODQ. In addition, sodium nitroprusside evoked a sequential increase in nuclear Ca2+ transients, activation of p42 MAPK, NF-kappaB binding to DNA consensus sequence, and dependent iNOS RNA. This study describes a hitherto unrecognized molecular mechanism by which nuclear eNOS through ensuing NO modulates nuclear calcium homeostasis involved in gene transcription-associated events. Moreover, our findings strongly support the concept of the nucleus as an autonomous signaling compartment.

Hypercapnia prevents neovascularization via nitrative stress

Neovascularization after an ischemic insult is a beneficial attempt to salvage the injured tissue. Yet, despite the production of angiogenic factors within ischemic tissues, compensatory growth of new vessels fails to provide adequate vascularization. Thus, we hypothesized that local factors counter efficient revascularization. Whereas ischemia is often considered to be synonymous with an oxygen deficit, it is also associated with a concomitant local elevation of carbon dioxide (CO2). Although studies suggest that hypercapnia impacts tissue neovascularization, its significance relative to the abundantly described effects of hypoxia and its underlying mechanisms have yet to be elucidated. Therefore, we investigated the effects of hypercapnia on blood vessel growth in models of developmental and ischemic neovascularization. Acute and prolonged CO2 exposure inhibited developmental neovascularization of the rodent retina, as well as revascularization of the ischemic retina. Hypercapnia induced early increases in endothelial nitric oxide synthase and nitrative stress, associated with astrocyte impairment and endothelial cell death, as well as downregulation of the proangiogenic prostaglandin E2 receptor EP3. These results establish a previously unexplored means by which hypercapnia hinders efficient neovascularization, a mechanism that may contribute to ischemic tissue injury.

Trans-arachidonic acids generated during nitrative stress induce a thrombospondin-1-dependent microvascular degeneration

Nitrative stress has an important role in microvascular degeneration leading to ischemia in conditions such as diabetic retinopathy and retinopathy of prematurity. Thus far, mediators of nitrative stress have been poorly characterized. We recently described that trans-arachidonic acids are major products of NO(2)(*)-mediated isomerization of arachidonic acid within the cell membrane, but their biological relevance is unknown. Here we show that trans-arachidonic acids are generated in a model of retinal microangiopathy in vivo in a NO(*)-dependent manner. They induce a selective time- and concentration-dependent apoptosis of microvascular endothelial cells in vitro, and result in retinal microvascular degeneration ex vivo and in vivo. These effects are mediated by an upregulation of the antiangiogenic factor thrombospondin-1, independently of classical arachidonic acid metabolism. Our findings provide new insight into the molecular mechanisms of nitrative stress in microvascular injury and suggest new therapeutic avenues in the management of disorders involving nitrative stress, such as ischemic retinopathies and encephalopathies.