Pulmonary microvascular endothelial cells constitutively release xanthine oxidase

Hyperuricemia-induced endothelial insulin resistance: the nitric oxide connection

Hyperuricemia, defined as elevated serum concentrations of uric acid (UA) above 416 µmol L^-1, is related to the development of cardiometabolic disorders, probably via induction of endothelial dysfunction. Hyperuricemia causes endothelial dysfunction via induction of cell apoptosis, oxidative stress, and inflammation; however, it's interfering with insulin signaling and decreased endothelial nitric oxide (NO) availability, resulting in the development of endothelial insulin resistance, which seems to be a major underlying mechanism for hyperuricemia-induced endothelial dysfunction. Here, we elaborate on how hyperuricemia induces endothelial insulin resistance through the disruption of insulin-stimulated endothelial NO synthesis. High UA concentrations decrease insulin-induced NO synthesis within the endothelial cells by interfering with insulin signaling at either the receptor or post-receptor levels (i.e., proximal and distal steps). At the proximal post-receptor level, UA impairs the function of the insulin receptor substrate (IRS) and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) in the insulin signaling pathway. At the distal level, high UA concentrations impair endothelial NO synthase (eNOS)-NO system by decreasing eNOS expression and activity as well as by direct inactivation of NO. Clinically, UA-induced endothelial insulin resistance is translated into impaired endothelial function, impaired NO-dependent vasodilation, and the development of systemic insulin resistance. UA-lowering drugs may improve endothelial function in subjects with hyperuricemia.

Redox Regulation of Ion Channels and Receptors in Pulmonary Hypertension

Significance: Pulmonary hypertension (PH) is characterized by elevated vascular resistance due to vasoconstriction and remodeling of the normally low-pressure pulmonary vasculature. Redox stress contributes to the pathophysiology of this disease by altering the regulation and activity of membrane receptors, K+ channels, and intracellular Ca2+ homeostasis. Recent Advances: Antioxidant therapies have had limited success in treating PH, leading to a growing appreciation that reductive stress, in addition to oxidative stress, plays a role in metabolic and cell signaling dysfunction in pulmonary vascular cells. Reactive oxygen species generation from mitochondria and NADPH oxidases has substantial effects on K+ conductance and membrane potential, and both receptor-operated and store-operated Ca2+ entry. Critical Issues: Some specific redox changes resulting from oxidation, S-nitrosylation, and S-glutathionylation are known to modulate membrane receptor and ion channel activity in PH. However, many sites of regulation that have been elucidated in nonpulmonary cell types have not been tested in the pulmonary vasculature, and context-specific molecular mechanisms are lacking. Future Directions: Here, we review what is known about redox regulation of membrane receptors and ion channels in PH. Further investigation of the mechanisms involved is needed to better understand the etiology of PH and develop better targeted treatment strategies.

Xanthine Oxidoreductase: A Novel Therapeutic Target for the Treatment of Chronic Wounds?

Significance: Chronic wounds are a major burden to patients and to healthcare systems worldwide. These wounds are difficult to heal and treatment is often lengthy and expensive. This has led to research efforts focussed on the wound environment attempting to understand the underlying pathological mechanisms of impaired wound healing. While some of this research has translated to advancements in wound therapies and implementation of new treatment options, chronic wounds remain a significant challenge to treat. Thus, identification of effective, low-cost, advanced wound therapies that enhance healing rates of these problematic wounds is still essential. Recent Advances and Critical Issues: Xanthine oxidoreductase (XOR), a molybdoflavin enzyme, is emerging as an important source of reactive oxygen species (ROS) in various pathologies, including diabetes and chronic wounds. XOR has recently been shown to be upregulated in chronic wounds, stimulating the overproduction of ROS during dysfunctional wound healing. XOR-induced ROS can amplify and potentiate inflammation in the wound environment further delaying wound closure. Future Directions: The detrimental role of XOR in impaired healing indicates it may be a therapeutic target. Targeted inhibition of XOR has been shown to reduce the expression and activity of this enzyme in diabetic wound models. In turn, this resulted in a significant decrease in ROS levels in the wound environment and improved wound healing. Therefore, repurposing existing XOR inhibitors that are approved for human use may be able to restore homeostasis at the wound site and enable damaged tissue to return to normal healing.

Dispelling dogma and misconceptions regarding the most pharmacologically targetable source of reactive species in inflammatory disease, xanthine oxidoreductase

Xanthine oxidoreductase (XOR), the molybdoflavin enzyme responsible for the terminal steps of purine degradation in humans, is also recognized as a significant source of reactive species contributory to inflammatory disease. In animal models and clinical studies, inhibition of XOR has resulted in diminution of symptoms and enhancement of function in a number of pathologies including heart failure, diabetes, sickle cell anemia, hypertension and ischemia-reperfusion injury. For decades, XOR involvement in pathologic processes has been established by salutary outcomes attained from treatment with the XOR inhibitor allopurinol. This has served to frame a working dogma that elevation of XOR-specific activity is associated with enhanced rates of reactive species generation that mediate negative outcomes. While adherence to this narrowly focused practice of designating elevated XOR activity to be "bad" has produced some benefit, it has also led to significant underdevelopment of the processes mediating XOR regulation, identification of alternative reactants and products as well as micro-environmental factors that alter enzymatic activity. This is exemplified by recent reports: (1) identifying XOR as a nitrite reductase and thus a source of beneficial nitric oxide ((•)NO) under in vivo conditions similar to those where XOR inhibition has been assumed an optimal treatment choice, (2) describing XOR-derived uric acid (UA) as a critical pro-inflammatory mediator in vascular and metabolic disease and (3) ascribing an antioxidant/protective role for XOR-derived UA. When taken together, these proposed and countervailing functions of XOR affirm the need for a more comprehensive evaluation of product formation as well as the factors that govern product identity. As such, this review will critically evaluate XOR-catalyzed oxidant, (•)NO and UA formation as well as identify factors that mediate their production, inhibition and the resultant impact on inflammatory disease.

Hydrogen peroxide generated by xanthine/xanthine oxidase system represses the proliferation of colorectal cancer cell line Caco-2

The twin character of reactive oxygen species is substantiated by a growing body of evidence that reactive oxygen species within cells act as inducers and accelerators of the oncogenic phenotype of cancer cells, while reactive oxygen species can also induce cancer cell death and can therefore function as anti-tumorigenic species. The aim of this study was to assess a possible influence of xanthine/xanthine oxidase on the proliferation of colorectal cancer cell line Caco-2. xanthine/xanthine oxidase (2.5 µM/0.25 mU/ml–25 µM/2.5 mU/ml) dose-dependently inhibited the proliferation of Caco-2 cells. Experiments utilizing reactive oxygen species scavengers (superoxide dismutase, catalase and mannitol) and exogenous hydrogen peroxide revealed a major role of hydrogen peroxide in the xanthine/xanthine oxidase effect. Investigations utilizing annexin V-fluorescein/PI assay using flow cytometry, and the lactate dehydrogenase extracellular release assay indicated that hydrogen peroxide induced necrosis, but not apoptosis, in Caco-2 cells. These results suggest that hydrogen peroxide generated by xanthine/xanthine oxidase has the potential to suppress colorectal cancer cell proliferation.

Pulmonary arterial hypertension: the clinical syndrome

Pulmonary arterial hypertension is a progressive disorder in which endothelial dysfunction and vascular remodeling obstruct small pulmonary arteries, resulting in increased pulmonary vascular resistance and pulmonary pressures. This leads to reduced cardiac output, right heart failure, and ultimately death. In this review, we attempt to answer some important questions commonly asked by patients diagnosed with pulmonary arterial hypertension pertaining to the disease, and aim to provide an explanation in terms of classification, diagnosis, pathophysiology, genetic causes, demographics, and prognostic factors. Furthermore, important molecular pathways that are central to the pathogenesis of pulmonary arterial hypertension are reviewed, including nitric oxide, prostacyclin, endothelin-1, reactive oxygen species, and endothelial and smooth muscle proliferation.

Reactive oxygen species as therapeutic targets in pulmonary hypertension

Pulmonary hypertension (PH) is characterized by a progressive elevation of pulmonary arterial pressure due to alterations of both pulmonary vascular structure and function. This disease is rare but life-threatening, leading to the development of right heart failure. Current PH treatments, designed to target altered pulmonary vascular reactivity, include vasodilating prostanoids, phosphodiesterase-5 inhibitors and endothelin-1 receptor antagonists. Although managing to slow the progression of the disease, these molecules still do not cure PH. More effective treatments need to be developed, and novel therapeutic strategies, targeting in particular vascular remodelling, are currently under investigation. Reactive oxygen species (ROS) are important physiological messengers in vascular cells. In addition to atherosclerosis and other systemic vascular diseases, emerging evidence also support a role of ROS in PH pathogenesis. ROS production is increased in animal models of PH, associated with NADPH oxidases increased expression, in particular of several Nox enzymes thought to be the major source of ROS in the pulmonary vasculature. These increases have also been observed in vitro and in vivo in humans. Moreover, several studies have shown either the deleterious effect of agents promoting ROS generation on pulmonary vasculature or, conversely, the beneficial effect of antioxidant agents in animal models of PH. In these studies, ROS production has been directly linked to pulmonary vascular remodelling, endothelial dysfunction, altered vasoconstrictive responses, inflammation and modifications of the extracellular matrix, all important features of PH pathophysiology. Altogether, these findings indicate that ROS are interesting therapeutic targets in PH. Blockade of ROS-dependent signalling pathways, or disruption of sources of ROS in the pulmonary vasculature, targeting in particular Nox enzymes, represent promising new therapeutic strategies in this disease.

Lung oxidative damage by hypoxia

One of the most important functions of lungs is to maintain an adequate oxygenation in the organism. This organ can be affected by hypoxia facing both physiological and pathological situations. Exposure to this condition favors the increase of reactive oxygen species from mitochondria, as from NADPH oxidase, xanthine oxidase/reductase, and nitric oxide synthase enzymes, as well as establishing an inflammatory process. In lungs, hypoxia also modifies the levels of antioxidant substances causing pulmonary oxidative damage. Imbalance of redox state in lungs induced by hypoxia has been suggested as a participant in the changes observed in lung function in the hypoxic context, such as hypoxic vasoconstriction and pulmonary edema, in addition to vascular remodeling and chronic pulmonary hypertension. In this work, experimental evidence that shows the implied mechanisms in pulmonary redox state by hypoxia is reviewed. Herein, studies of cultures of different lung cells and complete isolated lung and tests conducted in vivo in the different forms of hypoxia, conducted in both animal models and humans, are described.

Redox regulation of endothelial canonical transient receptor potential channels

The endothelium is a highly dynamic structure lining the inside of blood vessels that exhibits physical and chemical properties that are critical determinants of overall vascular function. Physically, the endothelium constitutes a semipermeable barrier. Chemically, the endothelium synthesizes numerous factors such as reactive oxygen species (ROS) that can act as autocrine and paracrine signaling molecules. Oxidative stress results when ROS levels increase to levels that cause cellular injury, and, in the endothelium oxidative stress leads to barrier disruption. Endothelial barrier disruption also results from increased cytosolic calcium through store-operated calcium (SOC) entry channels. Although it is known that ROS can interact with and regulate some ion channels, relatively little is known about the interaction of these species with components of endothelial SOC entry channels, the canonical transient receptor potential (TRPC) proteins. Here we review our current understanding of ROS-mediated TRPC channel function and how it affects SOC entry and endothelial barrier disruption.

Reactive oxygen species signaling in pulmonary vascular smooth muscle

In recent years, it has become evident that reactive oxygen species (ROS) play a critical role in the regulation of several physiological and pathophysiological processes. Herein we review the main sources, targets and pathophysiological roles of ROS in pulmonary vascular smooth muscle. Mitochondria and NADPH oxidases represent the major sources of ROS in vascular cells. In addition, ROS can be produced by different pathways of arachidonic acid metabolism, endothelial NO synthase (eNOS) and xantine oxidase. There is increasing evidence for the role of ROS, specially hydrogen peroxide, as signaling moieties to induce increase in intracellular calcium concentration ([Ca2+]i) and contraction in pulmonary artery smooth muscle cells (PASMC) through the modulation of a variety of targets, such as Rho kinases (ROCK), protein kinase C (PKC), voltage-gated potassium K+ (Kv) channels and ryanodine receptors (RyR). Thus, an increase in ROS has been reported to contribute to the responses induced by different vasoconstrictor stimuli, including hypoxia. Finally, results from recent studies highlighting the involvement of ROS in the development of pulmonary hypertension are discussed in the present paper.

Moderate hypoxia induces xanthine oxidoreductase activity in arterial endothelial cells

Xanthine oxidoreductase (XOR) activity has been previously noted to be responsive to changes in O2 tension. While prior studies have focused on the extremes (0-3% and 95-100%) of O2 tensions, we report the influence of 10% O2 on endothelial cell XOR, a concentration resembling modest arterial hypoxia commonly found in patients with chronic cardiopulmonary diseases. Exposure of bovine aortic endothelial cells to 10% O2 increased XOR mRNA and protein abundance by 50%. Concomitantly, there was a 3-fold increase in XOR activity, XOR-dependent reactive oxygen species production, and cellular export of active enzyme. Although increases in mRNA and immunoreactive protein levels were observed, inhibition of transcription, translation, or protein degradation did not significantly alter cellular XOR specific activity, suggesting only modest contributions to 10% O2-induced effects. Exposure to 10% O2 did not increase cellular HIF-1alpha protein levels and hypoxia mimics did not alter XOR activity. Treatment of control cells with adenosine resulted in increased XOR activity similar to hypoxia. Exposure to the adenosine receptor agonist NECA increased enzymatic activity 4-fold while 8SPT, an adenosine receptor antagonist, reduced hypoxic induction of XOR activity approximately 50%. Combined, these data reveal that moderate hypoxia significantly enhances endothelial XOR specific activity, release, and XOR-derived reactive oxygen species generation. These effects appear to be mediated in part via adenosine-dependent processes.

Reoxygenation results in cell death of human alveolar epithelial cells

BACKGROUND

The functional response of isolated alveolar epithelial cells (AECs) to ischemia/reperfusion injury (I/R) is incompletely understood. Using a cell culture model, we investigated the tolerance of human type II alveolar cells (ATII) to hypoxia and subsequent reoxygenation.

METHODS

Cell cultures of A549 cells (human lung adenocarcinoma) and primary ATII were incubated in 95% N(2)/5% CO(2) saturated medium at 37 degrees C for 48 hours or 72 hours. The hypoxic medium was subsequently exchanged to normoxic medium at 37 degrees C. Lactate dehydrogenase (LDH) release and mitochondrial viability, as assessed by WST-1 metabolism, were determined during both hypoxia and reoxygenation. A549 cells and ATII maintained under normoxic conditions served as controls.

RESULTS

Before reoxygenation, after 48 or 72 hours of hypoxia, WST-1 metabolism in A549 cells was significantly reduced (p < 0.05), but LDH release remained low in both cell types. Reoxygenation after 48 h of hypoxia was associated with recovery of WST-1 metabolism and an only minimal increase in LDH release. Reoxygenation after 72 hours of hypoxia, in contrast, induced marked injury in both A549 cells and primary ATII as indicated by significantly reduced WST-1 metabolism and a dramatic increase of LDH release compared with normoxic controls (p < 0.05).

CONCLUSIONS

Viability of alveolar cell lines and primary ATII is maintained during hypoxia for up to 72 hours. Reoxygenation after 72 hours of hypoxia results in rapid development of injury and cell death in both cell types.

Vascular physiology and pathology of circulating xanthine oxidoreductase: from nucleotide sequence to functional enzyme

The evolutionarily conserved, cofactor-dependent, enzyme xanthine oxidoreductase exists in both cell-associated and circulatory forms. The exact role of the circulating form is not known; however, several putative physiological and pathological functions have been suggested that range from purine catabolism to a mediator of acute respiratory distress syndrome. Regulation of gene expression, cofactor synthesis and insertion, post-translational conversion, entry into the circulation, and putative physiological and pathological roles for human circulating xanthine oxidoreductase are discussed.

Oxidative stress in the placenta

Pregnancy is a state of oxidative stress arising from increased placental mitochondrial activity and production of reactive oxygen species (ROS), mainly superoxide anion. The placenta also produces other ROS including nitric oxide, carbon monoxide, and peroxynitrite which have pronounced effects on placental function including trophoblast proliferation and differentiation and vascular reactivity. Excessive production of ROS may occur at certain windows in placental development and in pathologic pregnancies, such as those complicated by preeclampsia and/or IUGR, overpowering antioxidant defenses with deleterious outcome. In the first trimester, establishment of blood flow into the intervillous space is associated with a burst of oxidative stress. The inability to mount an effective antioxidant defense against this results in early pregnancy loss. In late gestation increased oxidative stress is seen in pregnancies complicated by diabetes, IUGR, and preeclampsia in association with increased trophoblast apoptosis and deportation and altered placental vascular reactivity. Evidence for this oxidative stress includes increased lipid peroxides and isoprostanes and decreased expression and activity of antioxidants. The interaction of nitric oxide and superoxide produces peroxynitrite, a powerful prooxidant with diverse deleterious effects including nitration of tyrosine residues on proteins thus altering function. Nitrative stress, subsequent to oxidative stress is seen in the placenta in preeclampsia and diabetes in association with altered placental function.

Molecular and cellular basis of pulmonary vascular remodeling in pulmonary hypertension

Clinical pulmonary hypertension is characterized by a sustained elevation in pulmonary arterial pressure. Pulmonary vascular remodeling involves structural changes in the normal architecture of the walls of pulmonary arteries. The process of vascular remodeling can occur as a primary response to injury, or stimulus such as hypoxia, within the resistance vessels of the lung. Alternatively, the changes seen in more proximal vessels may arise secondary to a sustained increase in intravascular pressure. To withstand the chronic increase in intraluminal pressure, the vessel wall becomes thickened and stronger. This "armouring" of the vessel wall with extra-smooth muscle and extracellular matrix leads to a decrease in lumen diameter and reduced capacity for vasodilatation. This maladaptive response results in increased pulmonary vascular resistance and consequently, sustained pulmonary hypertension. The process of pulmonary vascular remodeling involves all layers of the vessel wall and is complicated by the finding that cellular heterogeneity exists within the traditional compartments of the vascular wall: intima, media, and adventitia. In addition, the developmental stage of the organism greatly modifies the response of the pulmonary circulation to injury. This review focuses on the latest advances in our knowledge of these processes as they relate to specific forms of pulmonary hypertension and particularly in the light of recent genetic studies that have identified specific pathways involved in the pathogenesis of severe pulmonary hypertension.

Serum xanthine oxidase: origin, regulation, and contribution to control of trypanosome parasitemia

African trypanosomiasis is caused by Salivarian trypanosomes, tsetse fly-transmitted protozoa that inhabit the blood plasma, lymph and interstitial fluids, and, in the case of Trypanosoma brucei species, also the cerebrospinal fluid of mammal hosts. Trypanosomiasis in people and domestic animals manifests as recurring waves of parasites in the blood and is typically fatal. In contrast, trypanosomiasis in Cape buffaloes, which are naturally selected to resist the disease, is characterized by the development of only one or a few waves of parasitemia, after which the infection becomes cryptic, being maintained by the presence of 1-20 mammal-infective organisms/ml of blood. The control of the acute phase of parasitemia in Cape buffaloes correlates with a decline in blood catalase activity and the generation of trypanocidal H(2)O(2) in serum during the catabolism of endogenous purine by xanthine oxidase. Here we review features of this response, and of trypanosome metabolism, that facilitate H(2)O(2)-mediated killing of the parasites with minimal damage to the host. We also discuss the origin and regulation of serum xanthine oxidase and catalase, and show how recovery of serum catalase in infected Cape buffaloes precludes a role for H(2)O(2) in the long-term, stable suppression of trypanosome parasitemia.

Generation of oxidative stress contributes to the development of pulmonary hypertension induced by hypoxia

Chronic hypoxia causes pulmonary hypertension and right ventricular hypertrophy associated with pulmonary vascular remodeling. Because hypoxia might promote generation of oxidative stress in vivo, we hypothesized that oxidative stress may play a role in the hypoxia-induced cardiopulmonary changes and examined the effect of treatment with the antioxidant N-acetylcysteine (NAC) in rats. NAC reduced hypoxia-induced cardiopulmonary alterations at 3 wk of hypoxia. Lung phosphatidylcholine hydroperoxide (PCOOH) increased at days 1 and 7 of the hypoxic exposure, and NAC attenuated the increase in lung PCOOH. Lung xanthine oxidase (XO) activity was elevated from day 1 through day 21, especially during the initial 3 days of the hypoxic exposure. The XO inhibitor allopurinol significantly inhibited the hypoxia-induced increase in lung PCOOH and pulmonary hypertension, and allopurinol treatment only for the initial 3 days also reduced the hypoxia-induced right ventricular hypertrophy and pulmonary vascular thickening. These results suggest that oxidative stress produced by activated XO in the induction phase of hypoxic exposure contributes to the development of chronic hypoxic pulmonary hypertension.

Binding of xanthine oxidase to vascular endothelium. Kinetic characterization and oxidative impairment of nitric oxide-dependent signaling

Concentrations of up to 1.5 milliunits/ml xanthine oxidase (XO) (1.1 micrograms/ml) are found circulating in plasma during diverse inflammatory events. The saturable, high affinity binding of extracellular XO to vascular endothelium and the effects of cell binding on both XO catalytic activity and differentiated vascular cell function are reported herein. Xanthine oxidase purified from bovine cream bound specifically and with high affinity (Kd = 6 nM) at 4 degreesC to bovine aortic endothelial cells, increasing cell XO specific activity up to 10-fold. Xanthine oxidase-cell binding was not inhibited by serum or albumin and was partially inhibited by the addition of heparin. Pretreatment of endothelial cells with chondroitinase, but not heparinase or heparitinase, diminished endothelial binding by approximately 50%, suggesting association with chondroitin sulfate proteoglycans. Analysis of rates of superoxide production by soluble and cell-bound XO revealed that endothelial binding did not alter the percentage of univalent reduction of oxygen to superoxide. Comparison of the extent of CuZn-SOD inhibition of native and succinoylated cytochrome c reduction by cell-bound XO indicated that XO-dependent superoxide production was occurring in a cell compartment inaccessible to CuZn-SOD. This was further supported by the observation of a shift of exogenously added XO from extracellular binding sites to intracellular compartments, as indicated by both protease-reversible cell binding and immunocytochemical localization studies. Endothelium-bound XO also inhibited nitric oxide-dependent cGMP production by smooth muscle cell co-cultures in an SOD-resistant manner. This data supports the concept that circulating XO can bind to vascular cells, impairing cell function via oxidative mechanisms, and explains how vascular XO activity diminishes vasodilatory responses to acetylcholine in hypercholesterolemic rabbits and atherosclerotic humans. The ubiquity of cell-XO binding and endocytosis as a fundamental mechanism of oxidative tissue injury is also affirmed by the significant extent of XO binding to human vascular endothelial cells, rat lung type 2 alveolar epthelial cells, and fibroblasts.

Upregulation of xanthine oxidase by lipopolysaccharide, interleukin-1, and hypoxia. Role in acute lung injury

LPS and selected cytokines upregulate xanthine dehydrogenase/xanthine oxidase (XDH/XO) in cellular systems. However, the effect of these factors on in vivo XDH/XO expression, and their contribution to lung injury, are poorly understood. Rats were exposed to normoxia or hypoxia for 24 h after treatment with LPS (1 mg/kg) and IL-1beta (100 microg/kg) or sterile saline. Lungs were then harvested for measurement of XDH/XO enzymatic activity and gene expression, and pulmonary edema was assessed by measurement of the wet/dry lung weight ratio (W/D). Although treatment with LPS + IL-1beta or hypoxia independently produced a 2-fold elevation (p < 0. 05 versus exposure to normoxia and treatment with saline) in lung XDH/XO activity and mRNA, the combination of LPS + IL-1beta and hypoxia caused a 4- and 3.5-fold increase in these values, respectively. XDH/XO protein expression was increased 2-fold by hypoxia alone and 1.3-fold by treatment with LPS + IL-1beta alone or combination treatment. Compared with normoxic lungs, W/D was significantly increased by exposure to hypoxia, LPS + IL-1beta, or combination treatment. This increase was prevented by treatment of the animals with tungsten, which abrogated lung XDH/XO activity. In conclusion, LPS, IL-1beta, and hypoxia significantly upregulate lung XDH/XO expression in vivo. The present data support a role for this enzyme in the pathogenesis of acute lung injury.

Endothelial cell monolayers as a tool for studying microvascular pathophysiology

Endothelial cells contribute to a variety of biological responses that facilitate organ function. This critical role of the endothelial cell has resulted in the development of different in vitro models that utilize monolayers of cultured cells to simulate conditions that exist in the intact animal. This review focuses on endothelial cell monolayers as a model system for research on certain pathophysiological conditions affecting the gastrointestinal tract. The advantages and limitations of endothelial cell monolayers are addressed, along with evolving technologies and strategies that hold promise for extending the utility of this in vitro model for studies of gastrointestinal function and disease.

NADH oxidase activity of human xanthine oxidoreductase--generation of superoxide anion

Human xanthine oxidase was purified from breast milk. The dehydrogenase form of the enzyme, which predominates in most mammalian tissues, catalyses the oxidation of NADH by oxygen, generating superoxide anion significantly faster than does the oxidase form. The corresponding forms of bovine enzyme behave very similarly. The steady-state kinetics of NADH oxidation and superoxide production, including inhibition by NAD, by the dehydrogenase forms of both enzymes, are analysed in terms of a model involving two-stage recycling of oxidised enzyme. Established inhibitors of xanthine oxidoreductases (allopurinol oxypurinol, amflutizole and BOF 4272), which block all other reducing substrates, were ineffective in the case of NADH. Diphenyleneiodonium, on the other hand, was a powerful inhibitor of NADH oxidation. The potential involvement of reactive oxygen species arising from NADH oxidation by xanthine oxidoreductase in ischaemia-reperfusion injury and other disease states, as well as in normal signal transduction, is discusssed.

Reperfusion injury as the mechanism of brain damage after perinatal asphyxia

Upon reperfusion of ischemic tissues, reactive oxygen metabolites are generated and are responsible for much of the organ damage. Experimental studies have revealed two main sources of these metabolites: 1) the oxidation of hypoxanthine to xanthine and on to uric acid by the oxidase form of xanthine oxidoreductase and 2) neutrophils accumulating in ischemic and reperfused tissue. Blocking either source will reduce reperfusion damage in a number of experimental situations. Although xanthine oxidoreductase activity may be unmeasurably low in organs other than liver and intestine, it may be involved in reperfusion injury elsewhere because of its localization in capillary endothelial cells. Time course considerations suggest that substrate accumulation and NADH inhibition of dehydrogenase activity may be more important in the pathogenesis than conversion of xanthine dehydrogenase into the oxidase form. Neutrophil accumulation may be partly due to oxidants in the first place, suggesting a link between the two sources of reactive oxygen metabolites. In the clinical context, many of the sequelae of perinatal asphyxia may be accounted for by reperfusion damage to organs such as brain, kidney, heart, liver, and lungs. During asphyxia, substrates of xanthine oxidase accumulate, upon resuscitation the cosubstrate oxygen is introduced, and evidence for oxidant production and effects has been obtained. In the pathogenesis of brain damage after asphyxia, both microvascular injury and parenchymal cell damage are important. Oxygen metabolites are involved in the former, but in the latter process their role is less clear because ischemia-reperfusion triggers not only oxidant production but many other phenomena, including gene activation, ATP depletion, glutamate accumulation, and increase of intracellular calcium. A severe insult results in cell necrosis, but more moderate asphyxia may cause delayed neuronal death through apoptosis. The time course of the changes in high energy phosphates as well as of selective neuronal death suggest that in the first hours of life there is a "therapeutic window," with future possibilities for prevention of permanent damage.

Alterations of nitric oxide synthase and xanthine oxidase activities of human keratinocytes by ultraviolet B radiation. Potential role for peroxynitrite in skin inflammation

In the present study, we demonstrated that NO synthase (cNOS) and xanthine oxidase (XO) of human keratinocytes can be activated to release NO, superoxide (O2-) and peroxynitrite (ONOO-) following exposure to ultraviolet B (UVB) radiation. We defined that this photo induced response may be involved in the pathogenesis of sunburn erythema and inflammation. Treatment of human keratinocytes with UVB (290-320 nm) radiation (up to 200 mJ/cm2) resulted in a dose-dependent increase in NO and ONOO- release that was inhibited by N-monomethyl-L-arginine (L-NMMA). NO and ONOO- release from keratinocytes was accompanied by an increase in intracellular cGMP levels. Treatment of human keratinocyte cytosol with various doses of UVB (up to 100 mJ/cm2) resulted in an increase in XO activity that was inhibited by oxypurinol. UVB radiation (up to 100 mJ/cm2) of keratinocytes resulted in a 15-fold increase in S-nitrosothiol formation, which directly increased purified soluble guanylate cyclase (sGC) activity by a mechanism characteristic of release of NO from a carrier molecule. In reconstitution experiments, when UVB-irradiated (20 mJ/cm2) purified cNOS isolated from keratinocyte cytosol was combined with UVB-irradiated (20 mJ/cm2) purified XO, a 4-fold increase in ONOO- production, as compared to nonirradiated enzymes, was observed. ONOO- synthesized by NO and O2- following UVB radiation of cNOS and XO was inhibited by oxypurinol (100 microM). UVB radiation of keratinocyte cytosol resulted in an increase in oxygen free radical production, consistent with the increased production of ONOO- by UVB-irradiated keratinocyte cytosol. In in vivo experiments, when experimental animals were subjected to UVB radiation, a protection factor (PF) of 6.5 +/- 1.8 was calculated when an emulsified cream formulation containing nitro-L-arginine (L-NA) (2%) and L-NMMA (2%) was applied to their skin. The present study indicates that UVB radiation acts as a potent stimulator of cNOS and XO activities in human keratinocytes. NO and ONOO- may exert cytotoxic effects in keratinocytes themselves, as well as in their neighboring endothelial and smooth muscle cells. This may be a major part of the integrated response leading to erythema production and the inflammation process.

Xanthine oxidase activity in the circulation of rats following hemorrhagic shock

Reactive oxygen metabolites generated from xanthine oxidase play an important role in the pathogenesis of ischemia-induced tissue injury. In a hemorrhagic shock model of ischemia-reperfusion, the intracellular enzyme xanthine oxidase was released into the vasculature. This intravascular source of superoxide (O2.-) and hydrogen peroxide (H2O2) interacted reversibly with glycosaminoglycans of vascular endothelium and markedly concentrated xanthine oxidase at cell surfaces, enhancing its ability to produce extensive damage to remote tissues. Rats were made hypotensive by hemorrhage, maintained for 2h, and reinfused with shed blood. Blood samples were obtained prior to hemorrhage and 15, 30, 60, and 90 min after reperfusion for determination of xanthine oxidase (XO), lactate dehydrogenase (LDH), and alanine transaminase (AST). These enzymes were not significantly elevated in control animals. Reperfusion after hemorrhage-induced ischemia resulted in significantly elevated AST and LDH in both low heparin (100 U/h) and high heparin (1000 U/h) groups. Xanthine oxidase was detected in the circulation only after 90 min reperfusion in the low heparin group and was elevated during the entire reperfusion period in the high heparin group. Studies with cultured vascular endothelium showed significant heparin-reversible binding of XO to cellular glycosaminoglycans. These results suggest that XO can gain access to the circulation following ischemia, where it then binds to the vascular endothelial cells to produce site-specific oxidant injury to organs remote from the site of XO release.