Induction of glutathione synthesis by oxidized low-density lipoprotein and 1-palmitoyl-2-arachidonyl phosphatidylcholine: protection against quinone-mediated oxidative stress

The Effect of High-Salt Diet on Oxidative Stress Production and Vascular Function in Tff3-/-/C57BL/6N Knockout and Wild Type (C57BL/6N) Mice

INTRODUCTION

It is well documented that high-salt (HS) diet increases systemic and vascular oxidative stress in various animal models and in humans, leading to impairment of vascular reactivity. The present study examined the interaction of genotype and HS diet intake and the potential effects of oxidative stress - antioxidative system balance on the flow-induced dilation (FID) in pressurized carotid arteries of normotensive Tff3-/-/C57BL/6N knockout mice and their wild-type (WT) controls.

METHODS

Male, ten-week-old transgenic Tff3-/-/C57BL/6N (Tff3-/-) knockout mice and WT/C57BL/6N (WT) (parental strain) healthy mice were divided in LS (0.4% NaCl in rodent chow) and HS (4% NaCl in rodent chow fed for 1 week) groups. Additionally, LS and HS groups were treated with 1 mmol/L 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) dissolved in the drinking water. After anesthesia with ketamine chloride (100 mg/kg) and midazolam (5 mg/kg), blood pressure was measured, carotid arteries and aortas were isolated, and blood samples were collected.

RESULTS

FID was decreased in WT_HS mice and restored by superoxide scavenger TEMPOL in vivo. On the other hand, attenuated FID of Tff3-/- mice was not further affected by HS diet or TEMPOL in vivo treatment. Vascular superoxide/reactive oxygen species levels were increased with HS diet in both strains and restored by TEMPOL. HS upregulated glutathione peroxidase 1 (GPx1) gene expression in WT_HS and Tff3-/-_HS mice, while GPx activity was significantly decreased only in WT_HS group. Systemic (serum) markers of oxidative stress (oxLDL and AOPP) and arterial blood pressure were similar among groups.

CONCLUSION

HS diet increases vascular oxidative stress and impairs vasodilation in WT mice. Tff3 gene deficiency attenuates vasodilation per se, without further effects of HS intake. This can be attributed to vascular upregulation of antioxidative enzyme GPx1 in Tff3-/-/C57BL/6N mice conferring protection from oxidative stress.

Redox status in mammalian cells and stem cells during culture in vitro: critical roles of Nrf2 and cystine transporter activity in the maintenance of redox balance

Culturing cells and tissues in vitro has provided valuable insights into the molecular mechanisms regulating redox signaling in cells with implications for medicine. However, standard culture techniques maintain mammalian cells in vitro under an artificial physicochemical environment such as ambient air and 5% CO2. Oxidative stress is caused by the rapid oxidation of cysteine to cystine in culture media catalyzed by transition metals, leading to diminished intracellular cysteine and glutathione (GSH) pools. Some cells, such as fibroblasts and macrophages, express cystine transport activity, designated as system [Formula: see text], which enables cells to maintain these pools to counteract oxidative stress. Additionally, many cells have the ability to activate the redox sensitive transcription factor Nrf2, a master regulator of cellular defenses against oxidative stress, and to upregulate xCT, the subunit of the [Formula: see text] transport system leading to increases in cellular GSH. In contrast, some cells, including lymphoid cells, embryonic stem cells and iPS cells, express relatively low levels of xCT and cannot maintain cellular cysteine and GSH pools. Thus, fibroblasts have been used as feeder cells for the latter cell types based on their ability to supply cysteine. Other key Nrf2 regulated gene products include heme oxygenase 1, peroxiredoxin 1 and sequestosome1. In macrophages, oxidized LDL activates Nrf2 and upregulates the scavenger receptor CD36 forming a positive feedback loop to facilitate removal of the oxidant from the vascular microenvironment. This review describes cell type specific responses to oxygen derived stress, and the key roles that activation of Nrf2 and membrane transport of cystine and cysteine play in the maintenance and proliferation of mammalian cells in culture.

Rat liver mitochondrial damage under acute or chronic carbon tetrachloride-induced intoxication: protection by melatonin and cranberry flavonoids

In current societies, the risk of toxic liver damage has markedly increased. The aim of the present work was to carry out further research into the mechanism(s) of liver mitochondrial damage induced by acute (0.8 g/kg body weight, single injection) or chronic (1.6g/ kg body weight, 30 days, biweekly injections) carbon tetrachloride - induced intoxication and to evaluate the hepatoprotective potential of the antioxidant, melatonin, as well as succinate and cranberry flavonoids in rats. Acute intoxication resulted in considerable impairment of mitochondrial respiratory parameters in the liver. The activity of mitochondrial succinate dehydrogenase (complex II) decreased (by 25%, p<0.05). Short-term melatonin treatment (10 mg/kg, three times) of rats did not reduce the degree of toxic mitochondrial dysfunction but decreased the enhanced NO production. After 30-day chronic intoxication, no significant change in the respiratory activity of liver mitochondria was observed, despite marked changes in the redox-balance of mitochondria. The activities of the mitochondrial enzymes, succinate dehydrogenase and glutathione peroxidase, as well as that of cytoplasmic catalase in liver cells were inhibited significantly. Mitochondria isolated from the livers of the rats chronically treated with CCl₄ displayed obvious irreversible impairments. Long-term melatonin administration (10 mg/kg, 30 days, daily) to chronically intoxicated rats diminished the toxic effects of CCl₄, reducing elevated plasma activities of alanine aminotransferase and aspartate aminotransferase and bilirubin concentration, prevented accumulation of membrane lipid peroxidation products in rat liver and resulted in apparent preservation of the mitochondrial ultrastructure. The treatment of the animals by the complex of melatonin (10 mg/kg) plus succinate (50 mg/kg) plus cranberry flavonoids (7 mg/kg) was even more effective in prevention of toxic liver injury and liver mitochondria damage.

Physiological effects of oxidized phospholipids and their cellular signaling mechanisms in inflammation

Oxidized phospholipids, such as the products of the oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine by nonenzymatic radical attack, are known to be formed in a number of inflammatory diseases. Interest in the bioactivity and signaling functions of these compounds has increased enormously, with many studies using cultured immortalized and primary cells, tissues, and animals to understand their roles in disease pathology. Initially, oxidized phospholipids were viewed largely as culprits, in line with observations that they have proinflammatory effects, enhancing inflammatory cytokine production, cell adhesion and migration, proliferation, apoptosis, and necrosis, especially in vascular endothelial cells, macrophages, and smooth muscle cells. However, evidence has emerged that these compounds also have protective effects in some situations and cell types; a notable example is their ability to interfere with signaling by certain Toll-like receptors (TLRs) induced by microbial products that normally leads to inflammation. They also have protective effects via the stimulation of small GTPases and induce up-regulation of antioxidant enzymes and cytoskeletal rearrangements that improve endothelial barrier function. Oxidized phospholipids interact with several cellular receptors, including scavenger receptors, platelet-activating factor receptors, peroxisome proliferator-activated receptors, and TLRs. The various and sometimes contradictory effects that have been observed for oxidized phospholipids depend on their concentration, their specific structure, and the cell type investigated. Nevertheless, the underlying molecular mechanisms by which oxidized phospholipids exert their effects in various pathologies are similar. Although our understanding of the actions and mechanisms of these mediators has advanced substantially, many questions do remain about their precise interactions with components of cell signaling pathways.

The association between adiponectin, HDL-cholesterol and α1-antitrypsin-LDL in female subjects without metabolic syndrome

Background

Oxidized low-density lipoprotein (LDL) may act as an atheroprotective (anti-atherosclerotic) agent under some conditions. While the α1-antitrypsin (AT)-LDL complex is considered a type of oxidized LDL, its clinical relevance remains unknown. The aim of the present study was to investigate the association between AT-LDL and anti-atherosclerotic variables such as HDL-cholesterol and adiponectin in subjects with and without metabolic syndrome (MetS).

Methods

In asymptomatic females (n = 194; mean age, 54 years) who were divided into non-MetS (n = 108) and MetS groups (n = 86), the fasting levels of serum AT-LDL, adiponectin and glucose/lipid panels were measured, in addition to body mass index (BMI) and blood pressure.

Results

The MetS group showed significantly higher BMI, blood pressure, glucose and triglyceride levels as well as significantly lower levels of HDL-cholesterol and adiponectin than the non-MetS group. A multivariate-adjusted analysis revealed that in the non-MetS group, AT-LDL was significantly, independently and positively correlated with adiponectin (β = 0.297, P < 0.05), along with HDL-cholesterol (β = 0.217, P < 0.05). In the MetS group, AT-LDL was significantly, independently and positively correlated with LDL-cholesterol only (β = 0.342, P < 0.05).

Conclusions

These data suggest that AT-LDL may exert anti-atherosclerotic effects in female subjects without MetS. More studies are required to clarify the clinical roles of AT-LDL in relation to the pathophysiology of MetS.

Mitochondrial targeting of the electrophilic lipid 15-deoxy-Delta12,14-prostaglandin J2 increases apoptotic efficacy via redox cell signalling mechanisms

Prototypical electrophiles such as the lipid 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) are well recognized for their therapeutic potential. Electrophiles modify signalling proteins in both the cytosol and mitochondrion, which results in diverse cellular responses, including cytoprotective effects and, at high doses, cell death. These findings led us to the hypothesis that targeting electrophiles to specific compartments in the cell could fine-tune their biological effects. To examine this, we synthesized a novel mitochondrially targeted analogue of 15d-PGJ2 (mito-15d-PGJ2) and tested its effects on redox cell signalling. Mito-15d-PGJ2 caused profound defects in mitochondrial bioenergetics and mitochondrial membrane depolarization when compared with 15d-PGJ2. We also found that mito-15d-PGJ2 modified different members of the electrophile-responsive proteome, was more potent at initiating intrinsic apoptotic cell death and was less effective than 15d-PGJ2 at up-regulating the expression of HO-1 (haem oxygenase-1) and glutathione. These results demonstrate the feasibility of modulating the biological effects of electrophiles by targeting the pharmacophore to mitochondria.

Dinitrosyliron complexes and the mechanism(s) of cellular protein nitrosothiol formation from nitric oxide

Nitrosothiols (RSNO), formed from thiols and metabolites of nitric oxide (*NO), have been implicated in a diverse set of physiological and pathophysiological processes, although the exact mechanisms by which they are formed biologically are unknown. Several candidate nitrosative pathways involve the reaction of *NO with O(2), reactive oxygen species (ROS), and transition metals. We developed a strategy using extracellular ferrocyanide to determine that under our conditions intracellular protein RSNO formation occurs from reaction of *NO inside the cell, as opposed to cellular entry of nitrosative reactants from the extracellular compartment. Using this method we found that in RAW 264.7 cells RSNO formation occurs only at very low (<8 microM) O(2) concentrations and exhibits zero-order dependence on *NO concentration. Indeed, RSNO formation is not inhibited even at O(2) levels <1 microM. Additionally, chelation of intracellular chelatable iron pool (CIP) reduces RSNO formation by >50%. One possible metal-dependent, O(2)-independent nitrosative pathway is the reaction of thiols with dinitrosyliron complexes (DNIC), which are formed in cells from the reaction of *NO with the CIP. Under our conditions, DNIC formation, like RSNO formation, is inhibited by approximately 50% after chelation of labile iron. Both DNIC and RSNO are also increased during overproduction of ROS by the redox cycler 5,8-dimethoxy-1,4-naphthoquinone. Taken together, these data strongly suggest that cellular RSNO are formed from free *NO via transnitrosation from DNIC derived from the CIP. We have examined in detail the kinetics and mechanism of RSNO formation inside cells.

Aberrant insulin receptor signaling and amino acid homeostasis as a major cause of oxidative stress in aging

The mechanisms leading to the increase in free radical-derived oxidative stress in "normal aging" remains obscure. Here we present our perspective on studies from different fields that reveal a previously unnoticed vicious cycle of oxidative stress. The plasma cysteine concentrations during starvation in the night and early morning hours (the postabsorptive state) decreases with age. This decrease is associated with a decrease in tissue concentrations of the cysteine derivative and quantitatively important antioxidant glutathione. The decrease in cysteine reflects changes in the autophagic protein catabolism that normally ensures free amino acid homeostasis during starvation. Autophagy is negatively regulated by the insulin receptor signaling cascade that is enhanced by oxidative stress in the absence of insulin. This synopsis of seemingly unrelated processes reveals a novel mechanism of progressive oxidative stress in which decreasing antioxidant concentrations and increasing basal (postabsorptive) insulin receptor signaling activity compromise not only the autophagic protein catabolism but also the activity of FOXO transcription factors (i.e., two functions that were found to have an impact on lifespan in several animal models of aging). In addition, the aging-related decrease in glutathione levels is likely to facilitate certain "secondary" disease-related mechanisms of oxidative stress. Studies on cysteine supplementation show therapeutic promise.

Lipid peroxidation and decomposition--conflicting roles in plaque vulnerability and stability

The low density lipoprotein (LDL) oxidation hypothesis has generated considerable interest in oxidative stress and how it might affect atherosclerosis. However, the failure of antioxidants, particularly vitamin E, to affect the progression of the disease in humans has convinced even staunch supporters of the hypothesis to take a step backwards and reconsider alternatives. Preponderant evidence for the hypothesis came from animal antioxidant intervention studies. In this review we point out basic differences between animal and human atherosclerosis development and suggest that human disease starts where animal studies end. While initial oxidative steps in the generation of early fatty streak lesions might be common, the differences might be in the steps involved in the decomposition of peroxidized lipids into aldehydes and their further oxidation into carboxylic acids. We suggest that these steps may not be amenable to attenuation by antioxidants and antioxidants might actually counter the stabilization of plaque by preventing the formation of carboxylic acids which are anti-inflammatory in nature. The formation of such dicarboxylic acids may also be conducive to plaque stabilization by trapping calcium. We suggest that agents that would prevent the decomposition of lipid peroxides and promote the formation and removal of lipid hydroxides, such as paraoxonase (PON 1) or apo A1/high density lipoprotein (HDL) might be more conducive to plaque regression.

Dual effects of oxidized low-density lipoprotein on LXR-ABCA1-apoA-I pathway in 3T3-L1 cells

BACKGROUND

The adipocyte has been proven to recognize and degrade oxidized low-density lipoprotein (oxLDL), while cholesterol efflux from adipocytes to clear excess cholesterol loaded by oxLDL is essential to maintain its normal function. Thus, it is intriguing to explore the effects of oxLDL on cholesterol efflux in adipocytes.

METHODS

Fully differentiated 3T3-L1 cells were incubated in the medium containing various concentrations of oxLDL (0 to 50 microg/mL) for 8 or 24 h. 10 micromol/L 22(R)-hydroxycholesterol was exposed to preconditioned adipocytes with 25 microg/mL oxLDL for 24 h. Reverse transcription polymerase chain reaction (RT-PCR) was used to evaluate adipocytes mRNA expression. Cholesterol efflux rate was determined through measuring release of radioactivity from (3)H-cholesterol prelabeled cells into medium containing apolipoprotein A-I (apoA-I).

RESULTS

Low concentrations of oxLDL caused a significant increase in apoA-I-mediated cholesterol efflux via enhancement of ATP binding cassette transporter A1 (ABCA1) pathway, whereas higher concentrations were incapable. In adipocytes preincubated with 25 microg/mL oxLDL for 24 h, 22(R)-hydroxycholesterol could increase ABCA1 and LXR* mRNA levels and apoA-I-mediated cholesterol efflux.

CONCLUSION

OxLDL has dual effects on ABCA1 pathway in adipocytes. It depends on the concentration and exposure time. The new action of low levels of oxLDL may provide further understanding to its atheroprotective effects.

Oxidized phospholipids: From molecular properties to disease

Oxidized lipids are generated from (poly)unsaturated diacyl- and alk(en)ylacyl glycerophospholipids under conditions of oxidative stress. The great variety of reaction products is defined by the degree of modification, hydrophobicity, chemical reactivity, physical properties and biological activity. The biological activities of these compounds may depend on both, the recognition of the particular molecular structures by specific receptors and on the unspecific physical and chemical effects on their target systems (membranes, proteins). In this review, we aim at highlighting the molecular features that are essential for the understanding of the biological actions of pure oxidized phospholipids. Firstly, their chemical structures are described as a basis for an understanding of their physical and (bio)chemical properties in membrane- and protein-bound form. Secondly, the biological activities of oxidized phospholipids are discussed in terms of their unspecific effects on the membrane level as well as their potential interactions with specific targets (receptors) affecting a large set of (signaling) molecules. Finally, the role of oxidized phospholipids as important mediators in pathophysiology is discussed with emphasis on atherosclerosis.

Glutathione regulation of redox-sensitive signals in tumor necrosis factor-α-induced vascular endothelial dysfunction

We investigated the regulatory role of glutathione in tumor necrosis factor-alpha (TNF-alpha)-induced vascular endothelial dysfunction as evaluated by using vascular endothelial adhesion molecule expression and monocyte-endothelial monolayer binding. Since TNF-alpha induces various biological effects on vascular cells, TNF-alpha dosage could be a determinant factor directing vascular cells into different biological fates. Based on the adhesion molecule expression patterns responding to different TNF-alpha concentrations, we adopted the lower TNF-alpha (0.2 ng/ml) to rule out the possible involvement of other TNF-alpha-induced biological effects. Inhibition of glutathione synthesis by l-buthionine-(S,R)-sulfoximine (BSO) resulted in down-regulations of the TNF-alpha-induced adhesion molecule expression and monocyte-endothelial monolayer binding. BSO attenuated the TNF-alpha-induced nuclear factor-kappaB (NF-kappaB) activation, however, with no detectable effect on AP-1 and its related mitogen-activated protein kinases (MAPKs). Deletion of an AP-1 binding site in intercellular adhesion molecule-1 (ICAM-1) promoter totally abolished its constitutive promoter activity and its responsiveness to TNF-alpha. Inhibition of ERK, JNK, or NF-kappaB attenuates TNF-alpha-induced ICAM-1 promoter activation and monocyte-endothelial monolayer binding. Our study indicates that TNF-alpha induces adhesion molecule expression and monocyte-endothelial monolayer binding mainly via activation of NF-kappaB in a glutathione-sensitive manner. We also demonstrated that intracellular glutathione does not modulate the activation of MAPKs and/or their downstream AP-1 induced by lower TNF-alpha. Although AP-1 activation by the lower TNF-alpha was not detected in our systems, we could not rule out the possible involvement of transiently activated MAPKs/AP-1 in the regulation of TNF-alpha-induced adhesion molecule expression.

Oxidized low-density lipoprotein: A double-edged sword on atherosclerosis

It is well known that atherosclerosis is closely related to lipoprotein metabolism, particularly to the low density lipoprotein (LDL). LDL becomes atherogenic after undergoing oxidation by vascular cells, that transform them into highly bioreactive oxidized LDL (oxLDL). oxLDL is generally though to be involved in foam cell formation, and trigger an array of proatherogenic events. However, there are accumulating evidences that low levels of oxLDL can be atheroprotective through its cytoprotection, modulation of immunity and activation of reverse cholesterol transport. Thus, oxLDL may exert biphasic effects on atherosclerosis, just like a doubled-edged sword.

ICAM-1 cross-linking stimulates endothelial glutathione synthesis

What mechanisms regulate endothelial glutathione (GSH) during inflammation? Addressing this question is critical in understanding mechanisms leading to endothelial dysfunction and cardiovascular disease. Herein, the authors show data that support the hypothesis that the intercellular cell adhesion molecule-1 (ICAM-1) regulates GSH. Ligating either constitutive or induced ICAM-1 on the endothelial surface, or exposing endothelial cells to soluble ICAM-1, increases GSH concentrations. ICAM-1 is important in mediating leukocyte adhesion and modulates endothelial signaling pathways important in controlling transmigration. The present data underscore a novel function for ICAM-1 in modulating GSH metabolism and raise the hypothesis that this adhesion molecule controls endothelial redox status under basal and inflammatory conditions.

Effects of venous needle turbulence during ex vivo hemodialysis on endothelial morphology and nitric oxide formation

Arteriovenous grafts used for hemodialysis frequently develop intimal hyperplasia (IH), which ultimately leads to graft failure. Although the turbulent jet from the dialysis needle may contribute to vessel wall injury, its role in the pathogenesis of IH is relatively unexplored. In the current study, using bovine aortic endothelial cells (BAEC) cultured on the inner surface of a compliant tube, we evaluated the effects of simulated hemodialysis conditions on morphology and nitric oxide (NO) production. The flows via the graft and needle were 500 ml/min (Reynolds number=819) and 100ml/min (Reynolds number=954), respectively. In the presence of the needle jet for 6h, 19.3% (+/-1.53%) of BAEC were sheared off, whereas no loss of BAEC was observed in the presence of graft flow alone (P<0.05). In the presence of graft flow alone, assessment of cell orientation by the Saltykov method revealed that BAEC were oriented along the flow direction. This alignment, however, was lost in the presence of needle flow. Finally, NO production was also significantly decreased in the presence of the needle flow compared to the presence of graft flow alone (16+/-3.1 vs 34.7+/-1.9 nmol/10(6)cells/h, P<0.05). NO is a key player in vascular homeostasis mechanisms modulating vasomotor tone, inhibiting inflammation and smooth muscle cell proliferation. Thus, the loss of NO signaling and the loss of endothelial integrity caused by needle jet turbulence may contribute to the cascade of events leading to IH formation during hemodialysis.

ATP activates a reactive oxygen species-dependent oxidative stress response and secretion of proinflammatory cytokines in macrophages

Secretion of the proinflammatory cytokines, interleukin (IL)-1beta and IL-18, usually requires two signals. The first, due to microbial products such as lipopolysaccharide, initiates transcription of the cytokine genes and accumulation of the precursor proteins. Cleavage and secretion of the cytokines is mediated by caspase-1, in association with an inflammasome containing Nalp3, which can be activated by binding of extracellular ATP to purinergic receptors. We show that treatment of macrophages with ATP results in production of reactive oxygen species (ROS), which stimulate the phosphatidylinositol 3-kinase (PI3K) pathway and subsequent Akt and ERK1/2 activation. ROS exerts its effect through glutathionylation of PTEN (phosphatase and tensin homologue deleted from chromosome 10), whose inactivation would shift the equilibrium in favor of PI3K. ATP-dependent ROS production and PI3K activation also stimulate transcription of genes required for an oxidative stress response. In parallel, ATP-mediated ROS-dependent PI3K is required for activation of caspase-1 and secretion of IL-1beta and IL-18. Thus, an increase in ROS levels in ATP-treated macrophages results in activation of a single pathway that promotes both adaptation to subsequent exposure to oxidants or inflammation, and processing and secretion of proinflammatory cytokines.

Antioxidant and cytoprotective properties of high-density lipoproteins in vascular cells

Beside their key role in the regulation of cholesterol homeostasis, HDL exhibit antioxidant and anti-inflammatory properties that participate to their general antiatherogenic effect. The purpose of this review is to summarize the recent findings on antioxidant activity and cytoprotective cell signalling elicited by HDL against oxidized LDL and proatherogenic agents in vascular cells. HDL exhibit an antioxidant activity efficient to prevent LDL oxidation, or to inactivate newly formed lipid oxidation products. The antioxidant ability of HDL is due to the apoprotein moiety and to the presence of associated enzymes, paraoxonase and PAF-Acetyl Hydrolase. HDL prevent the intracellular oxidative stress and the inflammatory response elicited by oxidized LDL (ox-LDL), by inhibiting the NFkappaB signalling pathway, and the subsequent inflammatory events (expression of adhesion molecules, recruitment and proliferation of mononuclear cells within the vascular wall). HDL prevent ox-LDL-mediated cell activation and proliferation, this being also attributed to the presence in HDL of sphingosine-1 phosphate which modulates the migration and survival of vascular cells. Lastly, HDL inhibit apoptosis elicited by ox-LDL in vascular cells. Recent evidences indicate that, beside their strong antiatherogenic properties, HDL could exert their protective effect in diseases generally associated to inflammatory events.

Activation of mitogen-activated protein kinases by lysophosphatidylcholine-induced mitochondrial reactive oxygen species generation in endothelial cells

Lysophosphatidylcholine (lysoPC) evokes diverse biological responses in vascular cells including Ca(2+) mobilization, production of reactive oxygen species, and activation of the mitogen-activated protein kinases, but the mechanisms linking these events remain unclear. Here, we provide evidence that the response of mitochondria to the lysoPC-dependent increase in cytosolic Ca(2+) leads to activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase through a redox signaling mechanism in human umbilical vein endothelial cells. ERK activation was attenuated by inhibitors of the electron transport chain proton pumps (rotenone and antimycin A) and an uncoupler (carbonyl cyanide p-trifluoromethoxyphenylhydrazone), suggesting that mitochondrial inner membrane potential plays a key role in the signaling pathway. ERK activation was also selectively attenuated by chain-breaking antioxidants and by vitamin E targeted to mitochondria, suggesting that transduction of the mitochondrial hydrogen peroxide signal is mediated by a lipid peroxidation product. Inhibition of ERK activation with MEK inhibitors (PD98059 or U0126) diminished induction of the antioxidant enzyme heme oxygenase-1. Taken together, these data suggest a role for mitochondrially generated reactive oxygen species and Ca(2+) in the redox cell signaling path-ways, leading to ERK activation and adaptation of the pathological stress mediated by oxidized lipids such as lysoPC.

The effects of stress and aging on glutathione metabolism

Glutathione plays a critical role in many biological processes both directly as a co-factor in enzymatic reactions and indirectly as the major thiol-disulfide redox buffer in mammalian cells. Glutathione also provides a critical defense system for the protection of cells from many forms of stress. However, mild stress generally increases glutathione levels, often but not exclusively through effects on glutamate cysteine ligase, the rate-limiting enzyme for glutathione biosynthesis. This upregulation in glutathione provides protection from more severe stress and may be a critical feature of preconditioning and tolerance. In contrast, during aging, glutathione levels appear to decline in a number of tissues, thereby putting cells at increased risk of succumbing to stress. The evidence for such a decline is strongest in the brain where glutathione loss is implicated in both Parkinson's disease and in neuronal injury following stroke.

Changes in tetrahydrobiopterin levels in endothelial cells and adult cardiomyocytes induced by LPS and hydrogen peroxide--a role for GFRP?

Alterations in tetrahydrobiopterin (BH4) levels have significant consequences in vascular pathophysiology. However, the mechanisms regulating BH4 remain poorly understood. The activity of GTP cyclohydrolase I (GTPCH-I), the first enzyme in BH4 biosynthesis, is controlled by protein levels, posttranslational modifications and interaction with GTPCH-I feedback regulatory protein (GFRP). This work examined the correlation between GTPCH-I protein levels and activity and changes in BH4 in human endothelial cells (HAECs) and adult rat cardiomyocytes (ARCM). Changes in BH4 were stimulated with LPS in HAECs and ARCM, and with hydrogen peroxide in HAECs only. Biopterin production by HAECs and ARCM were attained with concentrations of LPS >>1 microg/ml and responses were nonlinear with respect to LPS concentrations. Western blot analysis demonstrated that induction of biopterin synthesis in HAECs and ARCM by LPS does not entail augmentation of constitutive GTPCH-I protein levels. However, LPS diminished GFRP mRNA, suggesting that disruption of GTPCH-I:GFRP complex enhances de novo biopterin synthesis. Conversely, treatment with hydrogen peroxide increased GTPCH-I and GFRP mRNA levels in HAECs while depleting BH4 and GSH, which was counteracted by catalase. This indicates that GFRP may override increases in GTPCH-I protein inhibiting enzyme activity. This conclusion is further supported by depletion of biopterin in cells transiently transfected with GFRP. Thus, allosteric regulation of GTPCH-I activity in the cardiovascular system maybe an important mechanism regulating BH4 levels through GFRP signaling.

Cellular mechanisms of redox cell signalling: role of cysteine modification in controlling antioxidant defences in response to electrophilic lipid oxidation products

The molecular mechanisms through which oxidized lipids and their electrophilic decomposition products mediate redox cell signalling is not well understood and may involve direct modification of signal-transduction proteins or the secondary production of reactive oxygen or nitrogen species in the cell. Critical in the adaptation of cells to oxidative stress, including exposure to subtoxic concentrations of oxidized lipids, is the transcriptional regulation of antioxidant enzymes, many of which are controlled by antioxidant-responsive elements (AREs), also known as electrophile-responsive elements. The central regulator of the ARE response is the transcription factor Nrf2 (NF-E2-related factor 2), which on stimulation dissociates from its cytoplasmic inhibitor Keap1, translocates to the nucleus and transactivates ARE-dependent genes. We hypothesized that electrophilic lipids are capable of activating ARE through thiol modification of Keap1 and we have tested this concept in an intact cell system using induction of glutathione synthesis by the cyclopentenone prostaglandin, 15-deoxy-Delta12,14-prostaglandin J2. On exposure to 15-deoxy-Delta12,14-prostaglandin J2, the dissociation of Nrf2 from Keap1 occurred and this was dependent on the modification of thiols in Keap1. This mechanism appears to encompass other electrophilic lipids, since 15-A(2t)-isoprostane and the lipid aldehyde 4-hydroxynonenal were also shown to modify Keap1 and activate ARE. We propose that activation of ARE through this mechanism will have a major impact on inflammatory situations such as atherosclerosis, in which both enzymic as well as non-enzymic formation of electrophilic lipid oxidation products are increased.

Apoptotic cells as sources for biologically active oxidized phospholipids

Acute inflammation is characterized by an accumulation of polymorphonuclear cells (PMNs), generation of reactive oxygen species, subsequent apoptosis of PMNs, and finally phagocytosis of apoptotic cells by macrophages. Recently, it has been demonstrated that during apoptosis oxidation of membrane phospholipids, especially phosphatidylserine, occurs. Moreover, we have shown that membrane vesicles released from apoptotic cells contain biologically active oxidized phospholipids. The involvement of oxidized phospholipids in the development of atherosclerosis, which is described as a chronic inflammatory disease, is increasingly recognized. These oxidized phospholipids were shown to induce several proinflammatory genes, such as monocyte chemoattractant protein 1 or interleukin-8, and it is hypothesized that lipid oxidation products also play a role in other chronic inflammatory disorders. On the other hand, oxidized phospholipids were shown to exert antiendotoxin effects by inhibiting lipopolysaccharide-induced signaling, representing a possible feedback loop during gram-negative infection. Additionally, it has been described that oxidized phospholipids are capable of inducing genes such as heme oxygenase-1 that are important for the resolution of acute inflammation. Moreover, oxidized phospholipids serve as recognition signals on apoptotic cells facilitating phagocytosis. In this review, we discuss the hypothesis that oxidized phospholipids generated in apoptotic cells (a) propagate chronic inflammation and (b) contribute to the resolution of acute inflammation.

Induced expression of manganese superoxide dismutase by non-toxic concentrations of oxidized low-density lipoprotein (oxLDL) protects against oxLDL-mediated cytotoxicity

Oxidized low-density lipoprotein (oxLDL) affects macrophages and plays a critical role in the development of atherosclerosis. In the present paper, we demonstrate that high concentrations of oxLDL provoked apoptosis of human Mono-Mac-6 cells, which was blocked by diphenylene-iodonium (DPI), an inhibitor of flavin-containing enzymes, such as NADPH oxidase, suggesting the involvement of reactive oxygen species (ROS). Importantly, pre-treatment of cells with low concentrations of oxLDL prevented apoptosis in response to high concentrations of oxLDL by up-regulating manganese superoxide dismutase (MnSOD). DPI prevented expression of MnSOD by oxLDL, whereas inhibitors of cytochrome P450 (methoxalen) or xanthine oxidase (allopurinol) did not, thus pointing to a role of NADPH-oxidase-derived ROS in oxLDL-induced MnSOD expression. Transfection of cells with MnSOD antisense, but not scrambled antisense, oligonucleotides significantly attenuated oxLDL-mediated MnSOD expression and hindered cytoprotective effects of non-toxic oxLDL concentrations. Our findings suggest that up-regulation of MnSOD by low concentrations of oxLDL is critical for protection towards oxLDL-mediated cytotoxicity.

Cytoprotection against oxidative stress and the regulation of glutathione synthesis

Adaptation to oxidative and nitrosative stress occurs in cells first exposed to a nontoxic stress, resulting in the ability to tolerate a toxic challenge of the same or a related oxidant. Adaptation is observed in a wide variety of cells including endothelial cells on exposure to nitric oxide or oxidized lipids, and lung epithelial cells exposed to air-borne pollutants and toxicants. This acquired characteristic has been related to the regulation of a family of stress responding proteins including those that control the synthesis of the intracellular antioxidant glutathione. The focus of this article, which includes a review of recent results along with new data, is the regulation and signaling of glutathione biosynthesis, especially those relating to adaptive mechanisms. These concepts are illustrated with examples using nitric oxide and oxidized low density lipoprotein mediated adaptation to oxidative stress. These data are discussed in the context of other adaptive mechanisms relating to glutathione synthesis including those from dietary constituents such as curcumin.

Selenium supplementation acting through the induction of thioredoxin reductase and glutathione peroxidase protects the human endothelial cell line EAhy926 from damage by lipid hydroperoxides

The human endothelial cell line EAhy926 was used to determine the importance of selenium in preventing oxidative damage induced by tert-butyl hydroperoxide (tert-BuOOH) or oxidised low density lipoprotein (LDLox). In cells grown in a low selenium medium, tert-BuOOH and LDLox killed cells in a dose-dependent manner. At 555 mg/l LDLox or 300 microM tert-BuOOH, >80% of cells were killed after 20 h. No significant cell kill was achieved by these agents if cells were pre-incubated for 48 h with 40 nM sodium selenite, a concentration that maximally induced the activities of cytoplasmic glutathione peroxidase (cyGPX; 5.1-fold), phospholipid hydroperoxide glutathione peroxidase (PHGPX;1.9-fold) and thioredoxin reductase (TR; 3.1-fold). Selenium-deficient cells pre-treated with 1 microM gold thioglucose (GTG) (a concentration that inhibited 25% of TR activity but had no inhibitory effect on cyGPX or PHGPX activity) were significantly (P<0.05) more susceptible to tert-BuOOH toxicity (LC(50) 110 microM) than selenium-deficient cells (LC(50) 175 microM). This was also the case for LDLox. In contrast, cells pre-treated with 40 nM selenite prior to exposure to GTG were significantly more resistant to damage from tert-BuOOH and LDLox than Se-deficient cells. Treatment with GTG or selenite had no significant effect on intracellular total glutathione concentrations. These results suggest that selenium supplementation, acting through induction of TR and GPX, has the potential to protect the human endothelium from oxidative damage.

Novel insights into the molecular mechanisms of the antiatherosclerotic properties of antioxidants: the alternatives to radical scavenging

Since the oxidation hypothesis of atherogenesis was first proposed, mechanisms of low density lipoprotein (LDL) oxidation and the biological properties of oxidized LDL have been investigated in depth. The major mechanism for the antiatherogenic effects of antioxidants, especially radical scavenging antioxidants, has been thought to be direct inhibition of LDL oxidation. The recently developed genomic technology has allowed this hypothesis to be addressed more rigorously than relying on the simple chemical properties of these therapeutic agents. Oxidized LDL, which is known to be proatherogenic, induces many categories of genes that have a potential involvement in the development of atherosclerotic lesions. The genes involved in cell growth, survival, adhesion, and inflammatory responses were upregulated through some nuclear receptor-depending pathways in cells exposed to stimulants such as shear stress, TNF-alpha, and oxidized LDL. On the other hand, these transcriptome analyses have shown a novel mechanism underlying phenolic antioxidants contribute to antiatherogenicity by regulating the expression of genes involved in protein degradation and transcriptional pathways. These studies reveal the often-suspected complexity of the atherogenic process and have the potential for novel therapeutic intervention.

Obesity, aerobic exercise, and vascular disease: the role of oxidant stress

Oxidant formation in the vasculature contributes to vascular disease and dysfunction associated with obesity. In contrast, exercise-dependent production of oxidants may stimulate adaptive responses that protect against the development of such diseases. In this review, we discuss current concepts in the biology of reactive oxygen and nitrogen species and how their function is modulated in the context of vascular disease, obesity, and aerobic exercise.

Cellular glutathione and thiols metabolism

Low molecular weight thiol-containing compounds have an essential role in many biochemical and pharmacological reactions due to the ease with each they are oxidized, and the rapidity with which they can be regenerated. Thioredoxin and glutathione (GSH) are two of the major small molecular weight thiol-containing compounds synthesized de novo in mammalian cells that participate in those functions. Understanding the mechanisms of thiol metabolism has special relevance to understanding the cell's defense against toxicant exposure and as the focal point in redox signaling. This commentary will, however, focus on GSH consumption and synthesis, and the role of thiols in signaling. The chemical reactions of GSH, including conjugation reactions mediated by glutathione S-transferases (GST) and oxidation reactions mediated by glutathione peroxidases will be described. The regulation of GSH synthesis will be illustrated from a compilation of studies designed to understand the various levels at which enzymatic GSH biosynthesis is controlled, and the signaling pathways that mediate them. The response of the cell to 4-hydroxynonenal (4HNE), a reactive aldehyde produced physiologically in response to inflammation and various air pollutants, will be explored in detail. Finally, the direct role of thiols as signaling molecules will be addressed, with particular attention given to "redox state." It is our aim that this commentary will lead the reader to appreciate that studies investigating the signaling for and regulation of thiol metabolism must never be generalized, and that perturbations in any of step of thiol metabolism may have etiological roles in genetically, virally, and environmentally borne pathologies.