Oxidative burst and NO generation as initial response to ischemia in flow-adapted endothelial cells

The effect of shear stress reduction on endothelial cells: A microfluidic study of the actin cytoskeleton

Reduced blood flow, as occurring in ischemia or resulting from exposure to microgravity such as encountered in space flights, induces a decrease in the level of shear stress sensed by endothelial cells forming the inner part of blood vessels. In the present study, we use a microvasculature-on-a-chip device in order to investigate in vitro the effect of such a reduction in shear stress on shear-adapted endothelial cells. We find that, within 1 h of exposition to reduced wall shear stress, human umbilical vein endothelial cells undergo reorganization of their actin skeleton with a decrease in the number of stress fibers and actin being recruited into the cells' peripheral band, indicating a fairly fast change in the cells' phenotype due to altered flow.

VascuTrainer: A Mobile and Disposable Bioreactor System for the Conditioning of Tissue-Engineered Vascular Grafts

In vitro tissue engineering of vascular grafts requires dynamic conditioning in a bioreactor system for in vitro tissue maturation and remodeling to receive a mechanically adequate and hemocompatible implant. The goal of the current work was to develop a bioreactor system for the conditioning of vascular grafts which is (i) able to create a wide range of flow, pressure and frequency conditions, including physiological ones; (ii) compact and easy to assemble; (iii) transportable; (iv) disposable. The system is driven by a small centrifugal pump controlled via a custom-made control unit, which can also be operated on batteries to allow for autonomous transportation. To show the potential of the newly developed bioreactor system small-caliber vascular composite grafts (n = 5, internal diameter = 3 mm, length = 12.5 cm) were fabricated using a fibrin scaffold embedding human umbilical artery smooth muscle cells and a polyvinylidene fluoride warp-knitted macroporous mesh. Subsequently, the vascular grafts were endothelialized and mounted in the bioreactor system for conditioning. The conditioning parameters remained within the predefined range over the complete conditioning period and during operation on batteries as tested for up to 25 h. Fabrication and pre-conditioning under arterial pressure and shear stress conditions resulted in robust and hemocompatible tissue-engineered vascular grafts. Analysis of immunohistochemical stainings against extracellular matrix and cell-specific proteins revealed collagen I and collagen III deposition. The luminal surface was confluently covered with endothelial cells. The developed bioreactor system showed cytocompatibility and pH, pO₂, pCO₂, glucose and lactate stayed constant. Sterility was maintained during the complete fabrication process of the vascular grafts. The potential of a versatile and mobile system and its functionality by conditioning tissue-engineered vascular grafts under physiological pressure and flow conditions could be demonstrated.

Primary Graft Dysfunction After Lung Transplantation

Primary graft dysfunction is a form of acute injury after lung transplantation that is associated with significant short- and long-term morbidity and mortality. Multiple mechanisms contribute to the pathogenesis of primary graft dysfunction, including ischemia reperfusion injury, epithelial cell death, endothelial cell dysfunction, innate immune activation, oxidative stress, and release of inflammatory cytokines and chemokines. This article reviews the epidemiology, pathogenesis, risk factors, prevention, and treatment of primary graft dysfunction.

Salvianolic acid A inhibits calpain activation and eNOS uncoupling during focal cerebral ischemia in mice

BACKGROUND

Salvianolic acid A (SAA) is obtained from Chinese herb Salviae Miltiorrhizae Bunge (Labiatae), has been reported to have the protective effects against cardiovascular and neurovascular diseases.

HYPOTHESIS

The aim of present study was to investigate the relationship between the effectiveness of SAA against neurovascular injury and its effects on calpain activation and endothelial nitric oxide synthase (eNOS) uncoupling.

STUDY DESIGN

SAA or vehicle was given to C57BL/6 male mice for seven days before the occlusion of middle cerebral artery (MCAO) for 60min.

METHODS

High-resolution positron emission tomography scanner (micro-PET) was used for small animal imaging to examine glucose metabolism. Rota-rod time and neurological deficit scores were calculated after 24h of reperfusion. The volume of infarction was determined by Nissl-staining. The calpain proteolytic activity and eNOS uncoupling were determined by western blot analysis.

RESULTS

SAA administration increased glucose metabolism and ameliorated neuronal damage after brain ischemia, paralleled with decreased neurological deficit and volume of infarction. In addition, SAA pretreatment inhibited eNOS uncoupling and calpain proteolytic activity. Furthermore, SAA inhibited peroxynitrite (ONOO^-) generation and upregulates AKT, FKHR and ERK phosphorylation.

CONCLUSION

These findings strongly suggest that SAA elicits a neurovascular protective role through the inhibition of eNOS uncoupling and ONOO^- formation. Moreover, SAA attenuates spectrin and calcineurin breakdown and therefore protects the brain against ischemic/reperfusion injury.

Onset of Inflammation With Ischemia: Implications for Donor Lung Preservation and Transplant Survival

Lungs stored ahead of transplant surgery experience ischemia. Pulmonary ischemia differs from ischemia in the systemic organs in that stop of blood flow in the lung leads to loss of shear alone because the lung parenchyma does not rely on blood flow for its cellular oxygen requirements. Our earlier studies on the ischemia-induced mechanosignaling cascade showed that the pulmonary endothelium responds to stop of flow by production of reactive oxygen species (ROS). We hypothesized that ROS produced in this way led to induction of proinflammatory mediators. In this study, we used lungs or cells subjected to various periods of storage and evaluated the induction of several proinflammatory mediators. Isolated murine, porcine and human lungs in situ showed increased expression of cellular adhesion molecules; the damage-associated molecular pattern protein high-mobility group box 1 and the corresponding pattern recognition receptor, called the receptor for advanced glycation end products; and induction stabilization and translocation of hypoxia-inducible factor 1α and its downstream effector VEGFA, all of which are participants in inflammation. We concluded that signaling with lung preservation drives expression of inflammatory mediators that potentially predispose the donor lung to an inflammatory response after transplant.

Hemodynamic Flow-Induced Mechanotransduction Signaling Influences the Radiation Response of the Vascular Endothelium

Hemodynamic shear stress is defined as the physical force exerted by the continuous flow of blood in the vascular system. Endothelial cells, which line the inner layer of blood vessels, sense this physiological force through mechanotransduction signaling and adapt to maintain structural and functional homeostasis. Hemodynamic flow, shear stress and mechanotransduction signaling are, therefore, an integral part of endothelial pathophysiology. Although this is a well-established concept in the cardiovascular field, it is largely dismissed in studies aimed at understanding radiation injury to the endothelium and subsequent cardiovascular complications. We and others have reported on the differential response of the endothelium when the cells are under hemodynamic flow shear compared with static culture. Further, we have demonstrated significant differences in the gene expression of static versus shear-stressed irradiated cells in four key pathways, reinforcing the importance of shear stress in understanding radiation injury of the endothelium. This article further emphasizes the influence of hemodynamic shear stress and the associated mechanotransduction signaling on physiological functioning of the vascular endothelium and underscores its significance in understanding radiation injury to the vasculature and associated cardiac complications. Studies of radiation effect on endothelial biology and its implication on cardiotoxicity and vascular complications thus far have failed to highlight the significance of these factors. Factoring in these integral parts of the endothelium will enhance our understanding of the contribution of the endothelium to radiation biology. Without such information, the current approaches to studying radiation-induced injury to the endothelium and its consequences in health and disease are limited.

Redox homeostasis: The Golden Mean of healthy living

The notion that electrophiles serve as messengers in cell signaling is now widely accepted. Nonetheless, major issues restrain acceptance of redox homeostasis and redox signaling as components of maintenance of a normal physiological steady state. The first is that redox signaling requires sudden switching on of oxidant production and bypassing of antioxidant mechanisms rather than a continuous process that, like other signaling mechanisms, can be smoothly turned up or down. The second is the misperception that reactions in redox signaling involve “reactive oxygen species” rather than reaction of specific electrophiles with specific protein thiolates. The third is that hormesis provides protection against oxidants by increasing cellular defense or repair mechanisms rather than by specifically addressing the offset of redox homeostasis. Instead, we propose that both oxidant and antioxidant signaling are main features of redox homeostasis. As the redox shift is rapidly reversed by feedback reactions, homeostasis is maintained by continuous signaling for production and elimination of electrophiles and nucleophiles. Redox homeostasis, which is the maintenance of nucleophilic tone, accounts for a healthy physiological steady state. Electrophiles and nucleophiles are not intrinsically harmful or protective, and redox homeostasis is an essential feature of both the response to challenges and subsequent feedback. While the balance between oxidants and nucleophiles is preserved in redox homeostasis, oxidative stress provokes the establishment of a new radically altered redox steady state. The popular belief that scavenging free radicals by antioxidants has a beneficial effect is wishful thinking. We propose, instead, that continuous feedback preserves nucleophilic tone and that this is supported by redox active nutritional phytochemicals. These nonessential compounds, by activating Nrf2, mimic the effect of endogenously produced electrophiles (parahormesis). In summary, while hormesis, although globally protective, results in setting up of a new phenotype, parahormesis contributes to health by favoring maintenance of homeostasis.

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Redox homeostasis is the continuously challenged oxidative/nucleophilic balance.

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Rheostatic redox signaling enzymes maintain oxidative/nucleophilic homeostasis.

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Phytochemicals assist redox homeostasis through oxidative feedback (parahormesis).

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Adaptation and hormesis while protective establish a new phenotype and set point.

Primary graft dysfunction: lessons learned about the first 72 h after lung transplantation

PURPOSE OF REVIEW

In 2005, the International Society for Heart and Lung Transplantation published a standardized definition of primary graft dysfunction (PGD), facilitating new knowledge on this form of acute lung injury that occurs within 72 h of lung transplantation. PGD continues to be associated with significant morbidity and mortality. This article will summarize the current literature on the epidemiology of PGD, pathogenesis, risk factors, and preventive and treatment strategies.

RECENT FINDINGS

Since 2011, several manuscripts have been published that provide insight into the clinical risk factors and pathogenesis of PGD. In addition, several transplant centers have explored preventive and treatment strategies for PGD, including the use of extracorporeal strategies. More recently, results from several trials assessing the role of extracorporeal lung perfusion may allow for much-needed expansion of the donor pool, without raising PGD rates.

SUMMARY

This article will highlight the current state of the science regarding PGD, focusing on recent advances, and set a framework for future preventive and treatment strategies.

Shear stress-related mechanosignaling with lung ischemia: lessons from basic research can inform lung transplantation

Cessation of blood flow represents a physical event that is sensed by the pulmonary endothelium leading to a signaling cascade that has been termed "mechanotransduction." This paradigm has clinical relevance for conditions such as pulmonary embolism, lung bypass surgery, and organ procurement and storage during lung transplantation. On the basis of our findings with stop of flow, we postulate that normal blood flow is "sensed" by the endothelium by virtue of its location at the interface of the blood and vessel wall and that this signal is necessary to maintain the endothelial cell membrane potential. Stop of flow is sensed by a "mechanosome" consisting of PECAM-VEGF receptor-VE cadherin that is located in the endothelial cell caveolae. Activation of the mechanosome results in endothelial cell membrane depolarization that in turn leads to activation of NADPH oxidase (NOX2) to generate reactive oxygen species (ROS). Endothelial depolarization additionally results in opening of T-type voltage-gated Ca(2+) channels, increased intracellular Ca(2+), and activation of nitric oxide (NO) synthase with resultant generation of NO. Increased NO causes vasodilatation whereas ROS provide a signal for neovascularization; however, with lung transplantation overproduction of ROS and NO can cause oxidative injury and/or activation of proteins that drive inflammation and cell death. Understanding the key events in the mechanosignaling cascade has important lessons for the design of strategies or interventions that may reduce injury during storage of donor lungs with the goal to increase the availability of lungs suitable for donation and thus improving access to lung transplantation.

Mechanotransduction in the endothelium: role of membrane proteins and reactive oxygen species in sensing, transduction, and transmission of the signal with altered blood flow

SIGNIFICANCE

Changes in shear stress associated with alterations in blood flow initiate a signaling cascade that modulates the vascular phenotype. Shear stress is "sensed" by the endothelium via a mechanosensitive complex on the endothelial cell (EC) membrane that has been characterized as a "mechanosome" consisting of caveolae, platelet endothelial cell adhesion molecule (PECAM), vascular endothelial growth factor receptor 2 (VEGFR2), vascular endothelial (VE)-cadherin, and possibly other elements. This shear signal is transduced by cell membrane ion channels and various kinases and results in the activation of NADPH oxidase (type 2) with the production of reactive oxygen species (ROS).

RECENT ADVANCES

The signaling cascade associated with stop of shear, as would occur in vivo with various obstructive pathologies, leads to cell proliferation and eventual revascularization.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Although several elements of mechanosensing such as the sensing event, the transduction, transmission, and reception of the mechanosignal are now reasonably well understood, the links among these discrete steps in the pathway are not clear. Thus, identifying the mechanisms for the interaction of the K(ATP) channel, the kinases, and ROS to drive long-term adaptive responses in ECs is necessary. A critical re-examination of the signaling events associated with complex flow patterns (turbulent, oscillatory) under physiological conditions is also essential for the progress in the field. Since these complex shear patterns may be associated with an atherosclerosis susceptible phenotype, a specific challenge will be the pharmacological modulation of the responses to altered signaling events that occur at specific sites of disturbed or obstructed flow.

Mechanotransduction drives post ischemic revascularization through K(ATP) channel closure and production of reactive oxygen species

AIMS

We reported earlier that ischemia results in the generation of reactive oxygen species (ROS) via the closure of a K(ATP) channel which causes membrane depolarization and NADPH oxidase 2 (NOX2) activation. This study was undertaken to understand the role of ischemia-mediated ROS in signaling.

RESULTS

Angiogenic potential of pulmonary microvascular endothelial cells (PMVEC) was studied in vitro and in the hind limb in vivo. Flow adapted PMVEC injected into a Matrigel matrix showed significantly higher tube formation than cells grown under static conditions or cells from mice with knockout of K(ATP) channels or the NOX2. Blocking of hypoxia inducible factor-1 alpha (HIF-1α) accumulation completely abrogated the tube formation in wild-type (WT) PMVEC. With ischemia in vivo (femoral artery ligation), revascularization was high in WT mice and was significantly decreased in mice with knockout of K(ATP) channel and in mice orally fed with a K(ATP) channel agonist. In transgenic mice with endothelial-specific NOX2 expression, the revascularization observed was intermediate between that of WT and knockout of K(ATP) channel or NOX2. Increased HIF-1α activation and vascular endothelial growth factor (VEGF) expression was observed in ischemic tissue of WT mice but not in K(ATP) channel and NOX2 null mice. Revascularization could be partially rescued in K(ATP) channel null mice by delivering VEGF into the hind limb.

INNOVATION

This is the first report of a mechanosensitive ion channel (K(ATP) channel) initiating endothelial signaling that drives revascularization.

CONCLUSION

The K(ATP) channel responds to the stop of flow and activates signals for revascularization to restore the impeded blood flow.

Sensing hypoxia by mitochondria: a unifying hypothesis involving S-nitrosation

SIGNIFICANCE

Sudden hypoxia requires a rapid response in tissues with high energy demand. Mitochondria are rapid sensors for a lack of oxygen, but no consistent mechanism for the sensing process and the subsequent counter-regulation has been described.

RECENT ADVANCES

In the present hypothesis review, we suggest an oxygen-sensing mechanism by mitochondria that is initiated at low oxygen tension by electrons from the respiratory chain, leading to the reduction of intracellular nitrite to nitric oxide ((•)NO) that would subsequently compete with oxygen for binding to cytochrome c oxidase. This allows superoxide ((•)O2(-)) formation in hypoxic areas, leading to S-nitrosation and the inhibition of mitochondrial Krebs cycle enzymes. With more formation of (•)O2(-), peroxynitrite is generated and known to damage the connection between the mitochondrial matrix and the outer membrane.

CRITICAL ISSUES

A fundamental question on a regulatory mechanism is its reversibility. Readmission of oxygen and opening of the mitochondrial KATP-channel would allow electrons from glycerol-3-phosphate to selectively reduce the ubiquinone pool to generate (•)O2(-) at both sides of the inner mitochondrial membrane. On the cytosolic side, superoxide is dismutated and will support H2O2/Fe(2+)-dependent transcription processes and on the mitochondrial matrix side, it could lead to the one-electron reduction and reactivation of S-nitrosated proteins.

FUTURE DIRECTIONS

It remains to be elucidated up to which stage the herein proposed silencing of mitochondria remains reversible and when irreversible changes that ultimately lead to classical reperfusion injury are initiated.

PECAM-1 and caveolae form the mechanosensing complex necessary for NOX2 activation and angiogenic signaling with stopped flow in pulmonary endothelium

We showed that stop of flow triggers a mechanosignaling cascade that leads to the generation of reactive oxygen species (ROS); however, a mechanosensor coupled to the cytoskeleton that could potentially transduce flow stimulus has not been identified. We showed a role for KATP channel, caveolae (caveolin-1), and NADPH oxidase 2 (NOX2) in ROS production with stop of flow. Based on reports of a mechanosensory complex that includes platelet endothelial cell adhesion molecule-1 (PECAM-1) and initiates signaling with mechanical force, we hypothesized that PECAM-1 could serve as a mechanosensor in sensing disruption of flow. Using lungs in situ, we observed that ROS production with stop of flow was significantly reduced in PECAM-1(-/-) lungs compared with lungs from wild-type (WT) mice. Lack of PECAM-1 did not affect NOX2 activation machinery or the caveolin-1 expression or caveolae number in the pulmonary endothelium. Stop of flow in vitro triggered an increase in angiogenic potential of WT pulmonary microvascular endothelial cells (PMVEC) but not of PECAM-1(-/-) PMVEC. Obstruction of flow in lungs in vivo showed that the neutrophil infiltration as observed in WT mice was significantly lowered in PECAM-1(-/-) mice. With stop of flow, WT lungs showed higher expression of the angiogenic marker VEGF compared with untreated (sham) and PECAM-1(-/-) lungs. Thus PECAM-1 (and caveolae) are parts of the mechanosensing machinery that generates superoxide with loss of shear; the resultant ROS potentially drives neutrophil influx and acts as an angiogenic signal.

Stop the flow: a paradigm for cell signaling mediated by reactive oxygen species in the pulmonary endothelium

The lung endothelium is exposed to mechanical stimuli through shear stress arising from blood flow and responds to altered shear by activation of NADPH (NOX2) to generate reactive oxygen species (ROS). This review describes the pathway for NOX2 activation and the downstream ROS-mediated signaling events on the basis of studies of isolated lungs and flow-adapted endothelial cells in vitro that are subjected to acute flow cessation (ischemia). Altered mechanical stress is detected by a cell-associated complex involving caveolae and other membrane proteins that results in endothelial cell membrane depolarization and then the activation of specific kinases that lead to the assembly of NOX2 components. ROS generated by this enzyme amplify the mechanosignal within the endothelial cell to regulate activation and/or synthesis of proteins that participate in cell growth, proliferation, differentiation, apoptosis, and vascular remodeling. These responses indicate an important role for NOX2-derived ROS associated with mechanotransduction in promoting vascular homeostasis.

Membrane depolarization is the trigger for PI3K/Akt activation and leads to the generation of ROS

Loss of fluid shear stress (ischemia) to the lung endothelium causes endothelial plasma membrane depolarization via ATP-sensitive K(+) (K(ATP)) channel closure, initiating a signaling cascade that leads to NADPH oxidase (NOX2) activation and ROS production. Since wortmannin treatment significantly reduces ROS production with ischemia, we investigated the role of phosphoinositide 3-kinase (PI3K) in shear-associated signaling. Pulmonary microvascular endothelial cells in perfused lungs subjected to abrupt stop of flow showed membrane depolarization and ROS generation. Stop of flow in flow-adapted mouse pulmonary microvascular endothelial cells in vitro resulted in the activation of PI3K and Akt as well as ROS generation. ROS generation in the lungs in situ was almost abolished by the PI3K inhibitor wortmannin and the PKC inhibitor H7. The combination of the two (wortmannin and H7) did not have a greater effect. Activation of NOX2 was greatly diminished by wortmannin, knockout of Akt1, or dominant negative PI3K, whereas membrane depolarization was unaffected. Ischemia-induced Akt activation (phosphorylation) was not observed with K(ATP) channel-null cells, which showed minimal changes in membrane potential with ischemia. Activation of Akt was similar to wild-type cells in NOX2-null cells, which do not generate ROS with ischemia. Cromakalim, a K(ATP) channel agonist, prevented both membrane depolarization and Akt phosphorylation with ischemia. Thus, Akt1 phosphorylation follows cell membrane depolarization and precedes the activation of NOX2. These results indicate that PI3K/Akt and PKC serve as mediators between endothelial cell membrane depolarization and NOX2 assembly.

Regulation of the ischemia-induced autophagy-lysosome processes by nitrosative stress in endothelial cells

The cellular mechanisms that underlie the diverse nitrosative stress-mediated cellular events associated with ischemic complications in endothelial cells are not yet clear. To characterize whether autophagic elements are associated with the nitrosative stress that causes endothelial damage after ischemia injury, an in vitro sustained oxygen-glucose deprivation (OGD) and an in vivo microsphere embolism model were used in the present study. Consistent with OGD-induced peroxynitrite formation, a rapid induction of microtubule-associated protein 1 light chain 3 (LC3)-I/II conversion and green fluorescent protein-LC3 puncta accumulation were observed in endothelial cells. The Western blot analyses indicated that OGD induced elevations in lysosome-associated membrane protein 2 and cathepsin B protein levels. Similar results were observed in the microvessel insult model, following occlusion of the microvessels using microsphere injections in rats. Furthermore, cultured endothelial cells treated with peroxynitrite (1-50 μm) exhibited a concentration-dependent change in the pattern of autophagy-lysosome signaling. Intriguingly, OGD-induced autophagy-lysosome processes were attenuated by PEP-19 overexpression and by a small-interfering RNA (siRNA)-mediated knockdown of eNOS. The importance of nitrosative stress in ischemia-induced autophagy-lysosome cascades is further supported by our finding that pharmacological inhibition of nitrosative stress by melatonin partially inhibits the ischemia-induced autophagy-lysosome cascade and the degradation of the tight junction proteins. Taken together, the present results demonstrate that peroxynitrite-mediated nitrosative stress at least partially potentiates autophagy-lysosome signaling during sustained ischemic insult-induced endothelial cell damage.

A nonaggregating surfactant protein C mutant is misdirected to early endosomes and disrupts phospholipid recycling

Interstitial lung disease in both children and adults has been linked to mutations in the lung-specific surfactant protein C (SFTPC) gene. Among these, the missense mutation [isoleucine to threonine at codon 73 = human surfactant protein C (hSP-C(I73T) )] accounts for ∼30% of all described SFTPC mutations. We reported previously that unlike the BRICHOS misfolding SFTPC mutants, expression of hSP-C(I73T) induces lung remodeling and alveolar lipoproteinosis without a substantial Endoplasmic Reticulum (ER) stress response or ER-mediated intrinsic apoptosis. We show here that, in contrast to its wild-type counterpart that is directly routed to lysosomal-like organelles for processing, SP-C(I73T) is misdirected to the plasma membrane and subsequently internalized to the endocytic pathway via early endosomes, leading to the accumulation of abnormally processed proSP-C isoforms. Functionally, cells expressing hSP-C(I73T) demonstrated both impaired uptake and degradation of surfactant phospholipid, thus providing a molecular mechanism for the observed lipid accumulation in patients expressing hSP-C(I73T) through the disruption of normal phospholipid recycling. Our data provide evidence for a novel cellular mechanism for conformational protein-associated diseases and suggest a paradigm for mistargeted proteins involved in the disruption of the endosomal/lysosomal sorting machinery.

Production of reactive oxygen species in endothelial cells under different pulsatile shear stresses and glucose concentrations

A hemodynamic Lab-on-a-chip system was developed in this study. This system has two unique features: (1) it consists of a microfluidic network with an array of endothelial cell seeding sites for testing them under multiple conditions, and (2) the flow rate and the frequency of the culture medium in the microchannel are controlled by a pulsation free pump to mimic the flow profile of the blood in the blood vessel under different physiological conditions. The investigated physiological conditions were: (1) the resting condition in a normal shear stress of 15 dyne cm(-2) with a normal heart rate of 70 bpm, (2) an exhaustive exercise condition with a high shear stress of 30 dyne cm(-2) and a fast heart rate of 140 bpm, and (3) a constant high shear stress of 30 dyne cm(-2). Two chemical conditions were investigated (10 mM and 20 mM glucose) to mimic hyperglycemic conditions in diabetes patients. The effects of various shear stresses either alone or in combination with different glucose concentrations on endothelial cells were examined using the developed hemodynamic Lab-on-a-chip system by assessing two parameters. One is the intracellular level of reactive oxygen species (ROS) determined by a fluorescent probe, H(2)DCFDA. Another is the mitochondrial morphology revealed with a fluorescent dye, MitoTracker Green FM. The results showed that ROS level was elevated nearly 4-fold after 60 min of exhaustive exercise. We found that the pulsatile nature of the fluid was the determination factor for causing ROS generation in the cells as almost no increase of ROS was detected in the constant shear stress condition. Similarly, much higher level of ROS was detected when 10 mM glucose was applied to the cells under normal or high pulsatile shear stresses compared with under a static condition. These results suggest that it is necessary to use pulsatile shear stress to represent the physiological conditions of the blood flow, and demonstrate the advantage of utilizing this newly developed hemodynamic Lab-on-a-chip system over the conventional non-pulsatile system in the future shear stress related studies.

Melatonin ameliorates ischemic-like injury-evoked nitrosative stress: Involvement of HtrA2/PED pathways in endothelial cells

Peroxynitrite contributes to diverse cellular stresses in the pathogenesis of ischemic complications. Here, we investigate the downstream effector signaling elements of nitrosative stress which regulate ischemia-like cell death in endothelial cells and protective effect of melatonin. When the mitochondrial membrane potential (ΔΨm) of oxygen-glucose deprivation (OGD)-treated cells was assessed using the fluorescent probe 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazol -carbocyanine iodide, we observed spontaneous changes in peroxynitrite formation. Concomitantly, western blot and confocal microscopy analyses indicated that prolonged OGD exposure initiates the release of mitochondrial HtrA2 and dramatically decreases phosphoprotein enriched in astrocytes (PED or PEA-15) protein levels. Consistently, cultured endothelial cells treated with peroxynitrite (1-50 μm) exhibited a concentration-dependent release of mitochondrial HtrA2 and concomitant PED degradation in vitro. Notably, HtrA2 activation coincided with increased nitrotyrosine immunoreactivity in microvessels of rats following microsphere embolism. Additionally, the protective effect of PED overexpression in OGD-induced apoptosis was abolished by transfection with the PED(S104A/S116A) mutant. Furthermore, the effect of melatonin, an potential antioxidant, on endothelial apoptotic cascade was examined in OGD-evoked nitrosative stress. Our data showed that the application of melatonin provided significant protection against OGD-induced peroxynitrite formation and mitochondrial HtrA2 release, accompanied with a decrease in degradation PED and x-linked inhibitor of apoptosis protein, which is associated with activation of the caspase cascade. Taken together, the protective effect of melatonin is likely mediated, in part, by inhibition of peroxynitrate-mediated nitrosative stress, which in turn relieves imbalance of mitochondrial HtrA2-PED signaling and endothelial cell death.

Reactive oxygen species cause endothelial dysfunction in chronic flow overload

Although elevation of shear stress increases production of vascular reactive oxygen species (ROS), the role of ROS in chronic flow overload (CFO) has not been well investigated. We hypothesize that CFO increases ROS production mediated in part by NADPH oxidase, which leads to endothelial dysfunction. In six swine, CFO in carotid arteries was induced by contralateral ligation for 1 wk. In an additional group, six swine received apocynin (NADPH oxidase blocker and anti-oxidant) treatment in conjunction with CFO for 1 wk. The blood flow in carotid arteries increased from 189.2 ± 25.3 ml/min (control) to 369.6 ± 61.9 ml/min (CFO), and the arterial diameter increased by 8.6%. The expressions of endothelial nitric oxide synthase (eNOS), p22/p47(phox), and NOX2/NOX4 were upregulated. ROS production increased threefold in response to CFO. The endothelium-dependent vasorelaxation was compromised in the CFO group. Treatment with apocynin significantly reduced ROS production in the vessel wall, preserved endothelial function, and inhibited expressions of p22/p47phox and NOX2/NOX4. Although the process of CFO remodeling to restore the wall shear stress has been thought of as a physiological response, the present data implicate NADPH oxidase-produced ROS and eNOS uncoupling in endothelial dysfunction at 1 wk of CFO.

Diazeniumdiolate mediated nitrosative stress alters nitric oxide homeostasis through intracellular calcium and S-glutathionylation of nitric oxide synthetase

BACKGROUND

PABA/NO is a diazeniumdiolate that acts as a direct nitrogen monoxide (NO) donor and is in development as an anticancer drug. Its mechanism of action and effect on cells is not yet fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

We used HPLC and mass spectrometry to identify a primary nitroaromatic glutathione metabolite of PABA/NO and used fluorescent assays to characterize drug effects on calcium and NO homeostasis, relating these to endothelial nitric oxide synthase (eNOS) activity. Unexpectedly, the glutathione conjugate was found to be a competitive inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) presumably at the same site as thapsigargin, increasing intracellular Ca2+ release and causing auto-regulation of eNOS through S-glutathionylation.

CONCLUSIONS/SIGNIFICANCE

The initial direct release of NO after PABA/NO was followed by an eNOS-mediated generation of NO as a consequence of drug-induced increase in Ca2+ flux and calmodulin (CaM) activation. PABA/NO has a unique dual mechanism of action with direct intracellular NO generation combined with metabolite driven regulation of eNOS activation.

The induction of reactive oxygen species and loss of mitochondrial Omi/HtrA2 is associated with S-nitrosoglutathione-induced apoptosis in human endothelial cells

The pathophysiological relevance of S-nitrosoglutathione (GSNO)-induced endothelial cell injury remains unclear. The main objective of this study was to elucidate the molecular mechanisms of GSNO-induced oxidative stress in endothelial cells. Morphological evaluation through DAPI staining and propidium iodide (PI) flow cytometry was used to detect apoptosis. In cultured EA.hy926 endothelial cells, exposure to GSNO led to a time- and dose-dependent apoptotic cascade. When intracellular reactive oxygen species (ROS) production was measured in GSNO-treated cells with the fluorescent probes 5-(and-6)-carboxy-2',7'-dichlorofluorescein diacetate, we observed elevated ROS levels and a concomitant loss in mitochondrial membrane potential, indicating that GSNO-induced death signaling is mediated through a ROS-mitochondrial pathway. Importantly, we found that peroxynitrite formation and Omi/HtrA2 release from mitochondria were involved in this phenomenon, whereas changes of death-receptor dependent signaling were not detected in the same context. The inhibition of NADPH oxidase activation and Omi/HtrA2 by a pharmacological approach provided significant protection against caspase-3 activation and GSNO-induced cell death, confirming that GSNO triggers the death cascade in endothelial cells in a mitochondria-dependent manner. Taken together, our results indicate that ROS overproduction and loss of mitochondrial Omi/HtrA2 play a pivotal role in reactive nitrogen species-induced cell death, and the modulation of these pathways can be of significant therapeutic benefit.

Redox signaling across cell membranes

Generation of reactive oxygen species (ROS) by plasma membrane-localized NADPH oxidase (Nox 2) is a major mechanism of cell signaling associated with activation of the enzyme by a variety of agonists. With activation, the integral membrane flavocytochrome of Nox 2 transfers an electron from intracellular NADPH to extracellular O(2), generating superoxide anion (O(2)(*-)). The latter dismutes to H(2)O(2) which can diffuse through aquaporin channels in the plasma membrane to elicit an intracellular signaling response. O(2)(*-) also can initiate intracellular signaling by penetration of the cell membrane through anion channels (Cl(-) channel-3, ClC-3). Endosomes containing Nox2 and ClC-3 (called signaling endosomes) are composed of internalized plasma membrane and generate O(2)(*-) in the endosomal lumen to initiate signaling at intracellular sites. Thus, cellular signaling by Nox2 is dependent on the transmembrane flux of ROS. The role of this pathway has only recently been described and will require additional investigation to appreciate its physiological significance fully.

Selective blockade of PGE2 EP1 receptor protects brain against experimental ischemia and excitotoxicity, and hippocampal slice cultures against oxygen-glucose deprivation

Cyclooxygenase-2 (COX-2) enzyme increases abnormally during excitotoxicity and cerebral ischemia and promotes neurotoxicity. Although COX-2 inhibitors could be beneficial, they have significant side effects. We and others have shown that the EP1 receptor is important in mediating PGE2 toxicity. Here, we tested the hypothesis that pretreatment with a highly selectiveEP1 receptor antagonist, ONO-8713, would improve stroke outcome and that post-treatment would attenuate NMDA-induced acute excitotoxicity and protect organotypic brain slices from oxygen-glucose deprivation (OGD)-induced toxicity. Male C57BL/6 mice were injected intracerebroventricularly with ONO-8713 before being subjected to 90-min middle cerebral artery occlusion (MCAO) and 96-h reperfusion.Significant reduction in infarct size was observed in groups given 0.1 (25.9 +/- 4.7%) and 1.0 nmol(27.7 +/- 2.8%) ONO-8713 as compared with the vehicle-treated control group. To determine the effects of ONO-8713 post-treatment on NMDA induced excitotoxicity, mice were given a unilateral intrastriatal NMDA injection followed by one intraperitoneal injection of 10 microg/kg ONO-8713, 1 and 6 h later. Significant attenuation of brain damage (26.6 +/-4.9%) was observed at 48 hin the ONO-8713-treated group. Finally, brain slice cultures were protected (25.5 +/- 2.9%) by the addition of ONO-8713 to the medium after OGD.These findings support the notion that the EP1receptor propagates neurotoxicity and that selective blockade could be considered as a potential preventive and/or therapeutic tool against ischemic/hypoxic neurological conditions.

Exercise affects tissue lymphocyte apoptosis via redox-sensitive and Fas-dependent signaling pathways

Intensive and exhaustive exercise induces an activation of blood T-lymphocytes, which seems to be terminated by apoptotic processes in the postexercise period. Here, we report that exercise-induced T-lymphocyte apoptosis is a systemic phenomenon occurring in various lymphoid and nonlymphoid tissues. The apoptosis rate could be related to exercise intensity and type. Although in some tissues, such as the spleen and Peyer's patches, an early start of apoptosis (1-3 h postexercise) could be detected, a delayed apoptosis (24 h postexercise) was observed in lung, bone marrow, and lymph nodes. Further analysis showed a similar apoptosis distribution among lymphocyte subpopulations. We tested whether components of the extrinsic or the intrinsic apoptotic pathways or both were involved in these processes. Elevated levels of lipid peroxidation-product malondialdehyde (MDA), indicating an increased production of reactive oxygen species (ROS), were found after exercise in Peyer's patches, lung, and spleen, but not in lymph nodes. Application of N-acetyl-cysteine (NAC) prevented exercise-induced T-cell apoptosis completely in spleen and bone marrow, partially in lung and Peyer's patches, while it was ineffective in lymph nodes. Additionally, exercise addressed the Fas-mediated apoptosis. The percentage of Fas-receptor (Fas+) and Fas-ligand positive (FasL+) lymphocytes was enhanced in Peyer's patches after exercise. Moreover, FasL+ T cells were increased in the lung, while in lymph nodes Fas+ cells were increased. The critical role of Fas signaling in exercise-induced apoptosis was supported by using Fas-deficient MRL/lpr-mice. In Fas-deficient mice, exercise-induced T-lymphocyte apoptosis was prevented in spleen, lung, bone marrow, and lymph nodes, but not in Peyer's patches. These data demonstrate that exercise-induced lymphocyte apoptosis is a transient systemic process with tissue-type specific apoptosis-inducing mechanisms, whose relevance for the adaptive immune competence remains to be shown.

Salvage of nonischemic control lung from injury by unilateral ischemic lung with apocynin, a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, in isolated perfused rat lung

Ischemia reperfusion (I/R) injury of the lung affects the function of the nonischemic lung. Our objective is to determine how apocynin, which is a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, protects the nonischemic control right lung (RL) from injury by the unilateral ischemic left lung (LL). In isolated ventilated (by air containing 5% CO(2)) rat lungs, in which differential perfusion of the RL or LL was feasible, the LL was selectively made ischemic (60 min) and reperfused (30 min) in a nonrecirculating or recirculating manner with buffer (Krebs-Henseleit) solution, or in a recirculating manner with buffer that contained apocynin (10 mmol/L) or apocynin + TACEI (tumor necrosis factor)-alpha converting enzyme inhibitor; 10 microg/mL) (each group: n = 12) or with buffer that contained SOD (superoxide dismutase, 3000 U before ischemia and at reperfusion) or SOD + TACEI (each group: n = 5). The permeability of pulmonary endothelium/epithelium (wet/dry ratio and protein content of bronchoalveolar lavage fluid of each lung), perfusion pressure, and cytokine messenger RNA (mRNA) expression was increased not only in the LL (compared with nonischemic control RL, P < 0.01 with paired-samples T) but also in the RL in recirculating groups (compared with RL in the nonrecirculating group). Apocynin + TACEI as well as SOD + TACEI prevented those permeability increases in the RL by the ischemic LL. However, apocynin with or without TACEI as well as SOD with or without TACEI could only partially ameliorate I/R injury in the LL (P < 0.01 by 1-way analysis of variance (ANOVA)). TNF-alpha and possibly reactive oxygen species produced and released from the ischemic lung may synergistically induce control RL (remote organ) damage.

Lung ischemia: a model for endothelial mechanotransduction

Endothelial cells in vivo are constantly exposed to shear associated with blood flow and altered shear stress elicits cellular responses (mechanotransduction). This review describes the role of shear sensors and signal transducers in these events. The major focus is the response to removal of shear as occurs when blood flow is compromised (i.e., ischemia). Pulmonary ischemia studied with the isolated murine lung or flow adapted pulmonary microvascular endothelial cells in vitro results in endothelial generation of reactive oxygen species (ROS) and NO. The response requires caveolae and is initiated by endothelial cell depolarization via K(ATP) channel closure followed by activation of NADPH oxidase (NOX2) and NO synthase (eNOS), signaling through MAP kinases, and endothelial cell proliferation. These physiological mediators can promote vasodilation and angiogenesis as compensation for decreased tissue perfusion.

Caveolae are an essential component of the pathway for endothelial cell signaling associated with abrupt reduction of shear stress

Abrupt cessation of flow representing the acute loss of shear stress (simulated ischemia) to flow-adapted pulmonary microvascular endothelial cells (PMVEC) leads to reactive oxygen species (ROS) generation that signals for EC proliferation. We evaluated the role of caveolin-1 on this cellular response with mouse PMVEC that were preconditioned for 72 h to laminar flow at 5 dyn/cm(2) followed by stop of flow ("ischemia"). Preconditioning resulted in a 2.7-fold increase in cellular expression of K(ATP) (K(IR) 6.2) channels but no change in expression level of caveolin-1, gp91(phox), or MAP kinases. The initial response to ischemia in wild type cells was cell membrane depolarization that was abolished by gene targeting of K(IR) 6.2. The subsequent response was increased ROS production associated with activation of NADPH oxidase (NOX2) and then phosphorylation of MAP kinases (Erk, JNK). After 24 h of ischemia in wild type cells, the cell proliferation index increased 2.5 fold and the % of cells in S+G(2)/M phases increased 6-fold. This signaling cascade (cell membrane depolarization, ROS production, MAP kinase activation and cell proliferation) was abrogated in caveolin-1 null PMVEC or by treatment of wild type cells with filipin. These studies indicate that caveolin-1 functions as a shear sensor in flow-adapted EC resulting in ROS-mediated cell signaling and endothelial cell proliferation following the abrupt reduction in flow.

Rac and PI3 kinase mediate endothelial cell-reactive oxygen species generation during normoxic lung ischemia

Abrupt reduction of flow (ischemia) leads to endothelial cell membrane depolarization, NADPH oxidase activation, and reactive oxygen species (ROS) generation in isolated rat and mouse lungs and in flow-adapted endothelial cells in vitro. Here we evaluated the role of PI-3-kinase and rac in activation of endothelial NADPH oxidase. Endothelium of isolated perfused mouse lungs labeled with 2',7'-dichlorodihydrofluorescein (H(2)DCF) or hydroethidine (HE) showed increased ROS generation with ischemia; these results were supported by TBARS measurement in whole-lung homogenate and by in vitro studies using flow-adapted mouse pulmonary microvascular endothelial cells. Ischemia-induced ROS generation in intact lung or isolated cells was blocked by pretreatment with Clostridium difficile toxin B, a rac inhibitor, and by wortmannin or LY294002, PI3 kinase inhibitors. In cells, immunofluorescence and immunoblot after subcellular fractionation showed ischemia-induced translocation of rac, p47(phox), and p67(phox) to the plasma membrane. Increased extracellular K(+) also resulted in rac translocation, providing evidence that this pathway is sensitive to alterations of endothelial cell membrane potential. These results indicate that PI-3-kinase and the small G protein rac are involved in the activation of endothelial cell NADPH oxidase that is associated with the acute loss of shear stress.

Differential regulation of pulmonary endothelial monolayer integrity by varying degrees of cyclic stretch

Ventilator-induced lung injury is a life-threatening complication of mechanical ventilation at high-tidal volumes. Besides activation of proinflammatory cytokine production, excessive lung distension directly affects blood-gas barrier and lung vascular permeability. To investigate whether restoration of pulmonary endothelial cell (EC) monolayer integrity after agonist challenge is dependent on the magnitude of applied cyclic stretch (CS) and how these effects are linked to differential activation of small GTPases Rac and Rho, pulmonary ECs were subjected to physiologically (5% elongation) or pathologically (18% elongation) relevant levels of CS. Pathological CS enhanced thrombin-induced gap formation and delayed monolayer recovery, whereas physiological CS induced nearly complete EC recovery accompanied by peripheral redistribution of focal adhesions and cortactin after 50 minutes of thrombin. Consistent with differential effects on monolayer integrity, 18% CS enhanced thrombin-induced Rho activation, whereas 5% CS promoted Rac activation during the EC recovery phase. Rac inhibition dramatically attenuated restoration of monolayer integrity after thrombin challenge. Physiological CS preconditioning (5% CS, 24 hours) enhanced EC paracellular gap resolution after step-wise increase to 18% CS (30 minutes) and thrombin challenge. These results suggest a critical role for the CS amplitude and the balance between Rac and Rho in mechanochemical regulation of lung EC barrier.

PREVENTION OF ISCHEMIA REPERFUSION INJURY BY POSITIVE PULMONARY VENOUS PRESSURE IN ISOLATED RAT LUNG

Pulmonary ischemia-reperfusion (I/R) without tissue hypoxia induces inflammatory cytokine mRNA expression in the lung under the condition of 0 mm Hg pulmonary venous pressure (0PVP), which might be a cause of I/R injury. Our aim is to determine whether the pulmonary vascular endothelium expresses cytokine mRNAs and their corresponding proteins or develops I/R injury when positive PVP is maintained during ischemia to provide a positive stretch to the endothelium throughout the ischemic period. In isolated, perfused, and ventilated rat lungs, the right and left pulmonary arteries were isolated, and the left lung was selectively occluded for 60 min and then reperfused for 30 min. During ischemia, the left atrial pressure was maintained at 5 mm Hg (5PVP) or 0PVP. TNF-alpha, IL-1beta, IL-6, and IL-10 mRNA expression in the lungs was evaluated by RT-PCR and in situ hybridization, and the production and localization of corresponding proteins were determined by staining with fluorescence-labeled antibodies against the cytokines and an antibody against CD34. Pulmonary vascular/epithelial permeability was evaluated by measuring albumin content in bronchoalveolar lavage (BAL) fluid and wet/dry ratio. At 5PVP, there were no increases in the left lung perfusion pressure, albumin content in BAL fluid, wet/dry ratio, or expression of cytokine mRNAs and their corresponding proteins on the vascular endothelium by I/R. In contrast, at 0PVP, the increased expression of cytokine mRNAs and their corresponding proteins on the vascular endothelium by I/R was verified. The finding that the application of 5PVP during ischemia abolished the expression of cytokine mRNAs and their corresponding proteins as well as the I/R injury gives us new insights in the study of lung preservation for transplantation.

Activation of endothelial NADPH oxidase during normoxic lung ischemia is KATP channel dependent

Previous studies have shown endothelial cell membrane depolarization and generation of reactive oxygen species (ROS) in endothelial cells with abrupt reduction in shear stress (ischemia). This study evaluated the role of ATP-sensitive potassium (K(ATP)) channels and NADPH oxidase in the ischemic response by using Kir6.2-/- and gp91(phox)-/- mice. To evaluate ROS generation, we subjected isolated perfused mouse lungs labeled with 2',7'-dichlorodihydrofluorescein (DCF), hydroethidine (HE), or diphenyl-1-pyrenylphosphine (DPPP) to control perfusion followed by global ischemia. In wild-type C57BL/6J mice, imaging of subpleural endothelial cells showed a time-dependent increase in intensity for all three fluorescence probes with ischemia, which was blocked by preperfusion with cromakalim (a K(ATP) channel agonist) or diphenyleneiodonium (DPI, a flavoprotein inhibitor). Endothelial cell fluorescence with bis-oxonol, a membrane potential probe, increased during lung ischemia indicating cell membrane depolarization. The change in membrane potential with ischemia in lungs of gp91(phox)-/- mice was similar to wild type, but ROS generation did not occur. Lungs from Kir6.2-/- showed marked attenuation of the change in both membrane potential and ROS production. Thus membrane depolarization during lung ischemia requires the presence of a K(ATP) channel and is required for activation of NADPH oxidase and endothelial ROS generation.

Biomedical aspects of targeted delivery of drugs to pulmonary endothelium

Drug targeting to selected subcellular compartments of the pulmonary endothelium may optimise treatment of many diseases. This paper describes endothelial determinants that are potentially useful for such targeting, including endothelial ectopeptidases, cell adhesion molecules and novel candidates identified by high-throughput methods, as well as the means to achieve optimal subcellular targeting of drugs in the endothelium that have been explored in cell culture and animal studies. Criteria for determining the applicability for targeting include accessibility, specificity, safety and subcellular precision. The effects of endothelial delivery of therapeutic agents, including the effects mediated by the intervention in the function of the target determinants, must be characterised in the context of given pathological conditions.

Reactive oxygen species as signaling molecules in cardiovascular differentiation of embryonic stem cells and tumor-induced angiogenesis

Besides the well known pathophysiological impact of oxidative stress in cardiovascular disease, reactive oxygen species (ROS) generated at low concentrations exert a role as signaling molecules that are involved in signal transduction cascades of numerous growth factor-, cytokine-, and hormone-mediated pathways, and regulate biological effects such as apoptosis, cell proliferation, and differentiation. Embryonic stem cells have the capacity to differentiate into the cardiovascular cell lineage. Furthermore, upon confrontation culture with tumor tissue, they form blood vessel-like structures that induce tumor-induced angiogenesis within tumor tissues. The role of ROS in cardiovascular differentiation of embryonic stem cells appears to be antagonistic. Whereas continuous exposure to ROS results in inhibition of cardiomyogenesis and vasculogenesis, pulse chase exposure to low-level ROS enhances differentiation toward the cardiomyogenic as well as vascular cell lineage. This review summarizes the current knowledge of ROS-induced cardiovascular differentiation of embryonic stem cells as well as the role of ROS in tumor-induced angiogenesis.

Lung endothelial cell proliferation with decreased shear stress is mediated by reactive oxygen species

Acute cessation of flow (ischemia) leads to depolarization of the endothelial cell (EC) membrane mediated by K(ATP) channels and followed by production of reactive oxygen species (ROS) from NADPH oxidase. We postulated that ROS are a signal for initiating EC proliferation associated with the loss of shear stress. Flow cytometry was used to identify proliferating CD31-positive pulmonary microvascular endothelial cells (mPMVECs) from wild-type, Kir6.2-/-, and gp91phox-/- mice. mPMVECs were labeled with PKH26 and cultured in artificial capillaries for 72 h at 5 dyn/cm2 (flow adaptation), followed by 24 h of stop flow or continued flow. ROS production during the first hour of ischemia was markedly diminished compared with wild-type mice in both types of gene-targeted mPMVECs. Cell proliferation was defined as the proliferation index (PI). After 72 h of flow, >98% of PKH26-labeled wild-type mPMVECs were at a single peak (PI 1.0) and the proportion of cells in the S+G2/M phases were at 5.8% on the basis of cell cycle analysis. With ischemia (24 h), PI increased to 2.5 and the ratio of cells in S+G2/M phases were at 35%. Catalase, diphenyleneiodonium, and cromakalim markedly inhibited ROS production and cell proliferation in flow-adapted wild-type mPMVECs. Significant effects of ischemia were not observed in Kir6.2-/- and gp91phox-/- cells. ANG II activation of NADPH oxidase was unaffected by KATP gene deletion. Thus loss of shear stress in flow-adapted mPMVECs results in cell division associated with ROS generated by NADPH oxidase. This effect requires a functioning cell membrane KATP channel.

Endothelial cell superoxide generation: regulation and relevance for cardiovascular pathophysiology

The endothelial generation of reactive oxygen species (ROS) is important both physiologically and in the pathogenesis of many cardiovascular disorders. ROS generated by endothelial cells include superoxide (O2-*), hydrogen peroxide (H2O2), peroxynitrite (ONOO-*), nitric oxide (NO), and hydroxyl (*OH) radicals. The O2-* radical, the focus of the current review, may have several effects either directly or through the generation of other radicals, e.g., H2O2 and ONOO-*. These effects include 1) rapid inactivation of the potent signaling molecule and endothelium-derived relaxing factor NO, leading to endothelial dysfunction; 2) the mediation of signal transduction leading to altered gene transcription and protein and enzyme activities ("redox signaling"); and 3) oxidative damage. Multiple enzymes can generate O2-*, notably xanthine oxidase, uncoupled NO synthase, and mitochondria. Recent studies indicate that a major source of endothelial O2-* involved in redox signaling is a multicomponent phagocyte-type NADPH oxidase that is subject to specific regulation by stimuli such as oscillatory shear stress, hypoxia, angiotensin II, growth factors, cytokines, and hyperlipidemia. Depending on the level of oxidants generated and the relative balance between pro- and antioxidant pathways, ROS may be involved in cell growth, hypertrophy, apoptosis, endothelial activation, and adhesivity, for example, in diabetes, hypertension, atherosclerosis, heart failure, and ischemia-reperfusion. This article reviews our current knowledge regarding the sources of endothelial ROS generation, their regulation, their involvement in redox signaling, and the relevance of enhanced ROS generation and redox signaling to the pathophysiology of cardiovascular disorders where endothelial activation and dysfunction are implicated.

Membrane depolarization and NADPH oxidase activation in aortic endothelium during ischemia reflect altered mechanotransduction

We previously showed that "ischemia" (abrupt cessation of flow) leads to rapid membrane depolarization and increased generation of reactive oxygen species (ROS) in lung microvascular endothelial cells. This response is not associated with anoxia but, rather, reflects loss of normal shear stress. This study evaluated whether a similar response occurs in aortic endothelium. Plasma membrane potential and production of ROS were determined by fluorescence microscopy and cytochrome c reduction in flow-adapted rat or mouse aorta or monolayer cultures of rat aortic endothelial cells. Within 30 s after flow cessation, endothelial cells that had been flow adapted showed plasma membrane depolarization that was inhibited by pretreatment with cromakalim, an ATP-sensitive K(+) (K(ATP)) channel agonist. Flow cessation also led to ROS generation, which was inhibited by cromakalim and the flavoprotein inhibitor diphenyleneiodonium. Aortic endothelium from mice with "knockout" of the K(ATP) channel (K(IR)6.2) showed a markedly attenuated change in membrane potential and ROS generation with flow cessation. In aortic endothelium from mice with knockout of NADPH oxidase (gp91(phox)), membrane depolarization was similar to that in wild-type mice but ROS generation was absent. Thus rat and mouse aortic endothelial cells respond to abrupt flow cessation by K(ATP) channel-mediated membrane depolarization followed by NADPH oxidase-mediated ROS generation, possibly representing a cell-signaling response to altered mechanotransduction.

Stable compounds of cigarette smoke induce endothelial superoxide anion production via NADPH oxidase activation

OBJECTIVE

Endothelial dysfunction is an early manifestation of cigarette smoke (CS) toxicity. We have previously demonstrated that CS impairs nitric oxide (NO)-mediated endothelial function via increased generation of superoxide anion (O2*). In these studies, we investigated whether stable compounds present in CS activate specific pathways responsible for the increased endothelial O2* production.

METHODS AND RESULTS

Short exposure of bovine pulmonary artery endothelial cells (BPAECs), human pulmonary artery endothelial cells, and rat pulmonary arteries to CS extracts (CSEs) resulted in a large increase in O2* production (20-fold, 3-fold, and 2-fold increase, respectively; P<0.05 versus control), which was inhibited by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors diphenyleneiodinium, apocynin, and gp91 docking sequence-tat peptide but not by oxypurinol, the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester, or the mitochondrial respiration inhibitor rotenone. Exposure of BPAECs to acrolein, a stable thiol-reactive agent found in CS, increased O2* production 5-fold, which was prevented by prior inhibition of NADPH oxidase.

CONCLUSIONS

These studies demonstrate that thiol-reactive stable compounds in CS can activate NADPH oxidase and increase endothelial O2* production, thereby reducing NO bioactivity and resulting in endothelial dysfunction. Clinically, these studies may contribute to the development of agents able to mitigate CS-mediated vascular toxicity.

Endothelial cell proliferation associated with abrupt reduction in shear stress is dependent on reactive oxygen species

We have shown previously that flow-adapted endothelial cells respond to cessation of flow with cell membrane depolarization and increased production of reactive oxygen species, resulting in activation of transcription factors and increased DNA synthesis. This study utilized flow cytometry to evaluate cellular proliferation with ischemia and to determine the role of reactive oxygen species and apoptosis. PKH26-labeled rat pulmonary microvascular endothelial cells were seeded in an artificial capillary system and subjected to flow at 5 dynes/cm(2) for 96 h or for 72 h followed by 24 h of simulated "ischemia." Ischemia resulted in a 2.5-fold increase in the cellular proliferation index. Cell-cycle analysis showed G0/G1 arrest and decreased S plus G2/M during flow adaptation, whereas ischemia resulted in a three-fold increase of cells in S plus G2/M phases. Apoptotic cells as detected by TUNEL and annexin V binding assays were ~5% of total cells with no differences between static, flow-adapted, and "ischemic" groups. Reactive oxygen species production during a 1-h period following onset of ischemia was confirmed by oxidation of the fluorophore, dichlorofluorescein, and was inhibited by cromakalim, a K(ATP) channel agonist, or diphenyleneiodonium, a flavoprotein inhibitor. Cromakalim and diphenyleneiodonium also markedly inhibited cell proliferation in the flow-adapted ischemic cells, but had no effect on subconfluent cells cultured under static conditions. These results indicate reactive oxygen species-dependent endothelial cell proliferation in flow-adapted microvascular endothelial cells as a response to ischemia and indicate that this response is not a consequence of apoptosis.

Metabolic control of muscle blood flow during exercise in humans

During muscle contraction, several mechanisms regulate blood flow to ensure a close coupling between muscle oxygen delivery and metabolic demand. No single factor has been identified to constitute the primary metabolic regulator, yet there are signal transduction pathways between skeletal muscle and the vasculature that induce vasodilation. A link between muscle metabolic events and microvascular control of blood flow is illustrated by local dilation of terminal arterioles during contraction of muscle fibers and conduction of vasodilation upstream. Endothelial-derived vasodilator mechanisms are known to exert control of muscle vasodilation. Adenosine, nitric oxide (NO), prostacyclin (PGI2), and endothelial-derived hyperpolarization factor (EDHF) are possible mediators of muscle vasodilation during exercise. In humans, adenosine has been shown to contribute to functional hyperemia as blood flow is reduced under nonselective adenosine-receptor blockade. No clear role has been demonstrated for either NO or PGI2(2), based on studies employing selective inhibition of these substances individually, suggesting a redundancy of vasodilator mechanisms. This is supported by recent work demonstrating that combined blockade of NOS and PGI2, and NOS and cytochrome P450, both attenuate exercise-induced hyperemia in humans. Combined vasodilator blockade studies offer the potential to uncover important interactions and compensatory vasodilator responses. The signaling pathways that link metabolic events evoked by muscle contraction to vasodilatory signals in the local vascular bed remains an important area of study.

Cytokine mRNA expression in unilateral ischemic-reperfused rat lung with salt solution supplemented with low-endotoxin or standard bovine serum albumin

Our aim was to determine whether cytokine mRNA expression is induced by experimental manipulation including artificial perfusate or ischemia-reperfusion (I/R) in an isolated, perfused rat lung model. Constant pulmonary flow [Krebs-Henseleit solution supplemented with lowendotoxin (LE) or standard (ST) bovine serum albumin 4%, 0.04 ml/g body wt] and ventilation were maintained throughout. Right and left pulmonary arteries were isolated, and the left pulmonary artery was occluded for 60 min and then reperfused for 30 min. Analysis of tumor necrosis factor-α, IL-1β, IL-6, IL-10, and IFN-γ mRNA expression by RT-PCR and evaluation of vascular permeability by bronchoalveolar lavage (BAL) fluid albumin content were conducted separately in right and left lung. Both LE and ST groups (each 12 rats) showed increases in vascular permeability by I/R (BAL fluid albumin content: 5.53 ± 1.55 vs. 15.63 ± 8.87 and 4.76 ± 2.71 vs. 16.72 ± 4.85 mg·ml BAL fluid-1·g lung dry wt-1, mean ± SD; right vs. left lung in LE and ST groups, P < 0.05 between right and left). Cytokine mRNA expression was significantly higher in the I/R lung than in the control lung in the LE group, whereas it was higher in the control lung in the ST group ( P < 0.05). mRNAs of not only proinflammatory but also anti-inflammatory cytokines were expressed in I/R lung, which are expected to aggravate I/R injury. The reversed pattern of cytokine mRNA expression in the ST group was possibly due to the longer perfusion of control lung with perfusate containing endotoxin, which caused no lung damage without I/R.

Shear stress increases expression of a KATP channel in rat and bovine pulmonary vascular endothelial cells

We have shown previously that acute ischemia leads to depolarization of pulmonary microvascular endothelial cells that is prevented with cromakalim, suggesting the presence of ATP-sensitive K+ (KATP) channels in these cells. Thus KATP channel expression and activity were evaluated in rat pulmonary microvascular endothelial cells (RPMVEC) by whole cell current measurements, dot blot (mRNA), and immunoblot (protein) for the inwardly rectifying K+ channel (KIR) 6.2 subunit and fluorescent ligand binding for the sulfonylurea receptor (SUR). Low-level expression of a KATP channel was detected in endothelial cells in routine (static) culture and led us to examine whether its expression is inducible when endothelial cells are adapted to flow. Channel expression (mRNA and both KIR6.2 and SUR proteins) and inwardly rectified membrane current by patch clamp increased significantly when RPMVEC were adapted to flow at 10 dyn/cm2 for 24 h in either a parallel plate flow chamber or an artificial capillary system. Induction of the KATP channel with flow adaptation was also observed in bovine pulmonary artery endothelial cells. Flow-adapted but not static RPMVEC showed cellular plasma membrane depolarization upon stop of flow that was inhibited by a KATP channel opener and prevented by addition of cycloheximide to the medium during the flow adaptation period. These studies indicate the induction of KATP channels by flow adaptation in pulmonary endothelium and that the expression and activity of this channel are essential for the endothelial cell membrane depolarization response with acute decrease in shear stress.

Potential Interactions Among Vascular and Muscular Functional Compartments During Active Hyperemia

The increase in blood flow that accompanies the start of contractions (active hyperemia) is a complex phenomenon involving a fast phase in which blood flow increases quickly and then slows or decreases (seek phase) before stabilizing at a flow corresponding to the metabolic rate (matched phase). This pattern of blood flow change involves contributions from a flow-induced increase in flow, a response to short periods of occlusion or partial occlusion due to force generated by the muscle contraction, and metabolism. Even denervated, the vascular bed, which consists of endothelial cells, vascular smooth muscle cells, and an adventitial layer that has significant secretory potential, is able to coordinate the response pattern. Within the vascular wall, communication is possible bidirectionally across the wall and also along the wall in a retrograde or upstream direction. The signals involved, which range from endothelial cell products such as nitric oxide and endothelin to adenosine, a skeletal muscle metabolite, appear to be situation- and time-dependent. In addition to the communication potential within and along the vascular wall, signals from the vascular system are able to exert inotropic effects on mammalian skeletal muscle. Key words: bidirectional signaling, postcontraction hyperemia, flow-induced flow changes, signal plasticity

Lung ischemia: endothelial cell signaling by reactive oxygen species. A progress report

These studies using both intact lung and reconstituted cell systems have shown that pulmonary endothelial cells respond rapidly (within several seconds) to the acute cessation of perfusate flow, i.e., ischemia. These effects represent a response to the loss of shear stress and are unrelated to changes in cellular oxygenation. The immediate response is partial depolarization of the endothelial cell membrane followed by activation of endothelial NADPH oxidase and the extracellular generation of superoxide anion. Dismutation of superoxide to H2O2 generates a cell signaling molecule that results in the activation of protein kinases and transcription factors which in turn lead to NO generation and activation of endothelial cell division. The presumed physiological role of this signal cascade is the generation of a vasodilator (NO) and the formation of new capillaries in the effort to restore blood flow.

Reduced perivascular PO2 increases nitric oxide release from endothelial cells

Many studies have suggested that endothelial cells can act as "oxygen sensors" to large reductions in oxygen availability by increasing nitric oxide (NO) production. This study determined whether small reductions in oxygen availability enhanced NO production from in vivo intestinal arterioles, venules, and parenchymal cells. In vivo measurements of perivascular NO concentration ([NO]) were made with NO-sensitive microelectrodes during normoxic and reduced oxygen availability. During normoxia, intestinal first-order arteriolar [NO] was 397 +/- 26 nM (n = 5), paired venular [NO] was 298 +/- 34 nM (n = 5), and parenchymal cell [NO] was 138 +/- 36 nM (n = 3). During reduced oxygen availability, arteriolar and venular [NO] significantly increased to 695 +/- 79 nM (n = 5) and 534 +/- 66 nM (n = 5), respectively, whereas parenchymal [NO] remained unchanged at 144 +/- 34 nM (n = 4). During reduced oxygenation, arteriolar and venular diameters increased by 15 +/- 3% and 14 +/- 5%, respectively: NG-nitro-L-arginine methyl ester strongly suppressed the dilation to lower periarteriolar Po2. Micropipette injection of a CO2 embolus into arterioles significantly attenuated arteriolar dilation and suppressed NO release in response to reduced oxygen availability. These results indicated that in rat intestine, reduced oxygen availability increased both arteriolar and venular NO and that the main site of NO release under these conditions was from endothelial cells.

In vitro intermittent hypoxia: challenges for creating hypoxia in cell culture

Intermittent hypoxia has been implicated in morbidities associated with sleep apnea, and may be a novel cellular signal for inflammation [J. Appl. Physiol. 90 (2001) 1986]. Standard cell culture has two major limitations for studying the effects of steady-state P(O(2)) and intermittent hypoxia. First, convective mixing in the culture media can be variable, making precise control of cellular P(O(2)) difficult. Second, diffusion of oxygen through the culture media slows changes in cellular P(O(2)) after rapid changes in the gas phase P(O(2)). Our estimates of diffusional transients for standard cell culture suggest significant restrictions in the ability to cycle P(O(2)) at frequencies relevant to intermittent hypoxia. We present a novel system for forced convection cell culture with adherent cells inside capillary tubing. Steady state cellular P(O(2)) is regulated to an accuracy of approximately 1 Torr. The response time for cycling of P(O(2)) is less than 1.6 sec. This system is ideally suited for studies of intermittent hypoxia in adherent cells.

Altered production of nitric oxide and reactive oxygen species in rat nodose ganglion neurons during acute hypoxia

Nitric oxide (NO) production in the sensory neurons of the rad nodose ganglion was studied by examining the distribuiotn of NO synthase (NOS) by use of NADPH diaphorase (NADPHD) histochemistry and immunohistochemistry ofr the presence of isoformes of NOS: neuronal (nNOS), endothelial (eNOS) and the inducible isoform (iNOS). Distribution and changes in NO production during acute hypoxia were studied in vital vibratome sections with the fluorescent marker for NO, diaminotriazolofluorescein (DAF-2T). Furthermore, changes in reactive oxygen species (ROS) in vibratome slices were examined utilizing 2',7'-dichlorofluorescein (DCF). By use of these histochemical methods, a positive NADPH reaction and positive immunoreactivity for eNOS were noted in all neurons observed. While for nNOS immunoreactivity, both strongly positive cells but also many negative cells are seen., no iNOS immunoreactive cells were observed. In vital vibratome slices, a dot-like distribution of fluorescence for DAF-2T, indicating production of NO, was observed in the nodose ganglion cells. Neurons exposed to hypoxia showed stronger DAF-2T fluorescence than cells exposed to normoxia, indicating an increased production of NO during hypoxia. When Ca(2+) was removed from the incubation buffer, the intensity of fluorescence for DAF-2T decreased but did not disappear completely. Using a photoconversion technique, DAF-2T was localized in the inner membrane of mitochondria in the ganglion cells by electron microscopy. The level of DCF signals for detection of ROS was higher in neurons incubated in the normoxic medium than those incubated under conditions of hypoxia. Nerve cells exposed to hypoxia followed by reoxygenation (3 min in normoxic conditions) showed higher fluorescence for DCF than those exposed to normoxia. The results of the present study demonstrate clearly that the basal production of NO in viscerosensory neurons is increased during hypoxia and is due to the isoform eNOS rather than nNOS, moreover, that ROS is augmented by reoxygenation but not during hypoxia.

Shear stress and endothelial cell activation

We have shown previously that ischemia in an isolated rat lung that is normally oxygenated by continued ventilation results in lipid and protein oxidation, indicating the generation of reactive oxygen species. By using a variety of biochemical and imaging techniques, we determined that the initial response, which occurs within the first second of ischemia, is partial depolarization of the endothelial cell plasma membrane. This event is followed within several seconds by activation of endothelial NADPH oxidase and generation of superoxide anion at the extracellular surface of the cell membrane where it is dismutated to freely diffusible H2O2. Approximately 15 secs after the onset of ischemia, we detected an elevation of intracellular Ca2+ caused by its release from intracellular stores, followed by Ca2+ influx, possibly through T-type voltage-dependent Ca2+ channels. Increased nitric oxide generation through activation of endothelial nitric oxide synthase is detected after about 45 secs of ischemia. These changes (endothelial membrane depolarization, reactive oxygen species production, elevation of intracellular Ca2+ levels, and nitric oxide generation) were confirmed in isolated endothelial cells that had been adapted to shear stress in vitro. The in vitro model also demonstrates reactive oxygen species-dependent activation of nuclear factor-kappaB and activator protein-1 and that 24 hrs of ischemia results in increased cell division. These results indicate a novel cell-signaling pathway in response to loss of shear stress. The basis for these changes in endothelial function is related to mechanotransduction, i.e., altered shear stress rather than a metabolic response to ischemia. The biological function for the response may be an attempt to restore blood flow through vasodilatation and new capillary formation.

Signaling pathway for nitric oxide generation with simulated ischemia in flow-adapted endothelial cells

Ischemia in the intact ventilated lung (oxygenated ischemia) leads to endothelial generation of reactive oxygen species (ROS) and nitric oxide (NO). This study investigated the signaling pathway for NO generation with oxygenated ischemia in bovine pulmonary artery endothelial cells (BPAEC) that were flow adapted in vitro. BPAECs were cultured in an artificial capillary system and subjected to abrupt cessation of flow (ischemia) under conditions where cellular oxygenation was maintained. Immunoblotting and dichlorofluorescein/triazolofluorescein fluorescence were used to assess extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation and ROS/NO generation, respectively. ERK1/2 phosphorylation significantly increased during ischemia, whereas total ERK1/2 did not change. ERK1/2 phosphorylation was suppressed by an inhibitor of tyrosine phosphorylation (genestein), cholesterol-binding reagents (filipin or cyclodextrin), or inhibitors of ROS (diphenyleneiodonium, N-acetylcysteine, or catalase), suggesting a role for both membrane cholesterol and ROS in ERK1/2 activation. Ischemia resulted in a 1.8-fold increase in NO generation that was suppressed by inhibitors of ERK1/2 activation (PD-98059 or U-0126). A calmodulin inhibitor (calmidizolium) or removal of Ca2+ from the medium also blocked NO generation, indicating that endothelial NO synthase (eNOS) is the activated isoform. These results indicate ischemia induces NO generation (possibly through a membrane cholesterol-sensitive flow sensor), the ERK1/2 cascade mediates signaling from the sensor to eNOS, and ROS are required for ERK activation.