Differential regulation of metabolism by nitric oxide and S-nitrosothiols in endothelial cells

β-Cell-selective regulation of gene expression by nitric oxide

Nitric oxide is produced at low micromolar levels following the induction of inducible nitric oxide synthase (iNOS) and is responsible for mediating the inhibitory actions of cytokines on glucose-stimulated insulin secretion by islets of Langerhans. It is through the inhibition of mitochondrial oxidative metabolism, specifically aconitase and complex 4 of the electron transport chain, that nitric oxide inhibits insulin secretion. Nitric oxide also attenuates protein synthesis, induces DNA damage, activates DNA repair pathways, and stimulates stress responses (unfolded protein and heat shock) in β-cells. In this report, the time- and concentration-dependent effects of nitric oxide on the expression of six genes known to participate in the response of β-cells to this free radical were examined. The genes included Gadd45α (DNA repair), Puma (apoptosis), Hmox1 (antioxidant defense), Hsp70 (heat shock), Chop (UPR), and Ppargc1α (mitochondrial biogenesis). We show that nitric oxide stimulates β-cell gene expression in a narrow concentration range of ∼0.5-1 µM or levels corresponding to iNOS-derived nitric oxide. At concentrations greater than 1 µM, nitric oxide fails to stimulate gene expression in β-cells, and this is associated with the inhibition of mitochondrial oxidative metabolism. This narrow concentration range of responses is β-cell selective, as the actions of nitric oxide in non-β-cells (α-cells, mouse embryonic fibroblasts, and macrophages) are concentration dependent. Our findings suggest that β-cells respond to a narrow concentration range of nitric oxide that is consistent with the levels produced following iNOS induction, and that these concentration-dependent actions are selective for insulin-containing cells.

Endothelial cell and T-cell crosstalk: Targeting metabolism as a therapeutic approach in chronic inflammation

The role of metabolic reprogramming in the coordination of the immune response has gained increasing consideration in recent years. Indeed, it has become clear that changes in the metabolic status of immune cells can alter their functional properties. During inflammation, T cells need to generate sufficient energy and biomolecules to support growth, proliferation, and effector functions. Therefore, T cells need to rearrange their metabolism to meet these demands. A similar metabolic reprogramming has been described in endothelial cells, which have the ability to interact with and modulate the function of immune cells. In this overview, we will discuss recent insights in the complex crosstalk between endothelial cells and T cells as well as their metabolic reprogramming following activation. We highlight key components of this metabolic switch that can lead to the development of new therapeutics against chronic inflammatory disorders. LINKED ARTICLES: This article is part of a themed issue on Cellular metabolism and diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.10/issuetoc.

Inhibition of oxidative metabolism by nitric oxide restricts EMCV replication selectively in pancreatic beta-cells

Environmental factors, such as viral infection, are proposed to play a role in the initiation of autoimmune diabetes. In response to encephalomyocarditis virus (EMCV) infection, resident islet macrophages release the pro-inflammatory cytokine IL-1β, to levels that are sufficient to stimulate inducible nitric oxide synthase (iNOS) expression and production of micromolar levels of the free radical nitric oxide in neighboring β-cells. We have recently shown that nitric oxide inhibits EMCV replication and EMCV-mediated β-cell lysis and that this protection is associated with an inhibition of mitochondrial oxidative metabolism. Here we show that the protective actions of nitric oxide against EMCV infection are selective for β-cells and associated with the metabolic coupling of glycolysis and mitochondrial oxidation that is necessary for insulin secretion. Inhibitors of mitochondrial respiration attenuate EMCV replication in β-cells, and this inhibition is associated with a decrease in ATP levels. In mouse embryonic fibroblasts (MEFs), inhibition of mitochondrial metabolism does not modify EMCV replication or decrease ATP levels. Like most cell types, MEFs have the capacity to uncouple the glycolytic utilization of glucose from mitochondrial respiration, allowing for the maintenance of ATP levels under conditions of impaired mitochondrial respiration. It is only when MEFs are forced to use mitochondrial oxidative metabolism for ATP generation that mitochondrial inhibitors attenuate viral replication. In a β-cell selective manner, these findings indicate that nitric oxide targets the same metabolic pathways necessary for glucose stimulated insulin secretion for protection from viral lysis.

Nitric Oxide in Macrophage Immunometabolism: Hiding in Plain Sight

Nitric Oxide (NO) is a soluble endogenous gas with various biological functions like signaling, and working as an effector molecule or metabolic regulator. In response to inflammatory signals, immune myeloid cells, like macrophages, increase production of cytokines and NO, which is important for pathogen killing. Under these proinflammatory circumstances, called “M1”, macrophages undergo a series of metabolic changes including rewiring of their tricarboxylic acid (TCA) cycle. Here, we review findings indicating that NO, through its interaction with heme and non-heme metal containing proteins, together with components of the electron transport chain, functions not only as a regulator of cell respiration, but also a modulator of intracellular cell metabolism. Moreover, diverse effects of NO and NO-derived reactive nitrogen species (RNS) involve precise interactions with different targets depending on concentration, temporal, and spatial restrictions. Although the role of NO in macrophage reprogramming has been in evidence for some time, current models have largely minimized its importance. It has, therefore, been hiding in plain sight. A review of the chemical properties of NO, past biochemical studies, and recent publications, necessitates that mechanisms of macrophage TCA reprogramming during stimulation must be re-imagined and re-interpreted as mechanistic results of NO exposure. The revised model of metabolic rewiring we describe here incorporates many early findings regarding NO biochemistry and brings NO out of hiding and to the forefront of macrophages immunometabolism.

iNOS as a metabolic enzyme under stress conditions

Nitric oxide (NO) is a free radical acting as a cellular signaling molecule in many different biochemical processes. NO is synthesized from l-arginine through the action of the nitric oxide synthase (NOS) family of enzymes, which includes three isoforms: endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS). iNOS-derived NO has been associated with the pathogenesis and progression of several diseases, including liver diseases, insulin resistance, obesity and diseases of the cardiovascular system. However, transient NO production can modulate metabolism to survive and cope with stress conditions. Accumulating evidence strongly imply that iNOS-derived NO plays a central role in the regulation of several biochemical pathways and energy metabolism including glucose and lipid metabolism during inflammatory conditions. This review summarizes current evidence for the regulation of glucose and lipid metabolism by iNOS during inflammation, and argues for the role of iNOS as a metabolic enzyme in immune and non-immune cells.

Don't just say no: Differential pathways and pharmacological responses to diverse nitric oxide donors

Nitric oxide (NO) is a gaseous free radical molecule with a short half-life (∼1 s), which can gain or lose an electron into three interchangeable redox-dependent forms, the radical (NO), the nitrosonium cation (NO⁺), and nitroxyl anion (HNO). NO acts as an intra and extracellular signaling molecule regulating a wide range of functions in the cardiovascular, immune, and nervous system. NO donors are collectively known by their ability to release NOin vitro and in vivo, being proposed as therapeutic pharmacological tools for the treatment of several pathologies, such as cardiovascular disease. The highly reactive NO molecule is easily oxidized under physiological conditions to N-oxides, nitrate/nitrite and nitrogen dioxide. Different cellular responses are triggered depending on: 1) NO concentration [e.g., nanomolar for heme coordination in the allosteric site of guanylate cyclase (sGC) enzyme]; 2) the type of chemical bound to the nitrosated group (i.e., bound to nitrogen, N-nitro, or bound to sulphur atom, S-nitro) determining post-translational cysteine nitrosation; 3) the time-dependent availability of molecular targets. Classic NO donors are: organic nitrates (e.g., nitroglycerin, or glyceryl trinitrate, GTN; isosorbide mononitrate, ISMN), diazeniumdiolates having a diolate group [or NONOates, e.g., 2-(N,N-diethylamino)-diazenolate-2-oxide], S-nitrosothiols (e.g., S-nitroso glutathione, GSNO; S-nitroso-N-acetylpenicillamine, SNAP) or the organic salt sodium nitroprusside (SNP). In addition, nitroxyl (HNO) donors such as Piloty's acid and Angeli's salt can also be considered. The specific NO form released, as well as its differential reactivity to thiols, could act on different molecular targets and should be discussed in the context of: a) the type and amount of NO species determining the sensitivity of molecular targets (e.g., heme coordination, or S-nitrosation); b) the cellular redox state that could gate different effects. Experimental designs should take special care when choosing which NO donors to use, since different outcomes are to be expected. This article will comment recent findings regarding physiological responses involving NO species and their pharmacological modulation with donor drugs, especially in the context of the photic transduction pathways at the hypothalamic circadian clock.

'SNO'-Storms Compromise Protein Activity and Mitochondrial Metabolism in Neurodegenerative Disorders

The prevalence of neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), is currently a major public health concern due to the lack of efficient disease-modifying therapeutic options. Recent evidence suggests that mitochondrial dysfunction and nitrosative/oxidative stress are key common mediators of pathogenesis. In this review, we highlight molecular mechanisms linking NO-dependent post-translational modifications, such as cysteine S-nitrosylation and tyrosine nitration, to abnormal mitochondrial metabolism. We further discuss the hypothesis that pathological levels of NO compromise brain energy metabolism via aberrant S-nitrosylation of key enzymes in the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, contributing to neurodegenerative conditions. A better understanding of these pathophysiological events may provide a potential pathway for designing novel therapeutics to ameliorate neurodegenerative disorders.

Similarities in the Metabolic Reprogramming of Immune System and Endothelium

Cellular metabolism has been known for its role in bioenergetics. In recent years, much light has been shed on the reprogrammable cellular metabolism underlying many vital cellular processes, such as cell activation, proliferation, and differentiation. Metabolic reprogramming in immune and endothelial cells (ECs) is being studied extensively. These cell compartments are implicated in inflammation and pathogenesis of many diseases but their similarities in metabolic reprogramming have not been analyzed in detail. One of the most notable metabolic reprogramming is the Warburg-like effect, famously described as one of the hallmarks of cancer cells. Immune cells and ECs can display this phenotype that is characterized by a metabolic switch favoring glycolysis over oxidative phosphorylation (OXPHOS) in aerobic conditions. Though energy-inefficient, aerobic glycolysis confers many benefits to the respiring cells ranging from higher rate of adenosine triphosphate production to maintaining redox homeostasis. Chemical and biological regulators either promote or perturb this effect. In this review, nitric oxide, hypoxia-inducible factor, and adenosine monophosphate-activated protein kinase have been discussed for their common involvement in metabolic reprogramming of both systems. From in vitro and animal studies, various discrepancies exist regarding the effects of those regulators on metabolic switch. However, it is generally accepted that glycolysis favors inflammatory reactions while OXPHOS favors anti-inflammatory processes. The reasons for such observation are currently subject of intense studies and not completely understood. Finally, metabolic reprogramming in immune cells and ECs does not limit to the physiological state in health but can also be observed in pathological states, such as atherosclerosis and cancer. These new insights provide us with a better understanding of the similarities in metabolic reprogramming across a number of cell types, which could pave the way for future research and possible metabolic-based therapeutics.

Nitric Oxide Synthase-2-Derived Nitric Oxide Drives Multiple Pathways of Breast Cancer Progression

SIGNIFICANCE

Breast cancer is the second leading cause of cancer-related deaths among women in the United States. Development and progression of malignancy are associated with diverse cell signaling pathways that control cell proliferation, survival, motility, invasion, and metastasis. Recent Advances: An increasing number of clinical studies have implicated a strong relationship between elevated tumor nitric oxide synthase-2 (NOS2) expression and poor patient survival.

CRITICAL ISSUES

Herein, we review what we believe to be key mechanisms in the role(s) of NOS2-derived nitric oxide (NO) as a driver of breast cancer disease progression. High NO increases cyclooxygenase-2 activity, hypoxia inducible factor-1 alpha protein stabilization, and activation of important cell signaling pathways, including phosphoinositide 3-kinase/protein kinase B, mitogen-activated protein kinase, epidermal growth factor receptor, and Ras, through post-translational protein modifications. Moreover, dysregulated NO flux within the tumor microenvironment has other important roles, including the promotion of angiogenesis and modulation of matrix metalloproteinase/tissue inhibitor matrix metalloproteinase associated with tumor progression.

FUTURE DIRECTIONS

The elucidation of these and other NO-driven pathways implicates NOS2 as a key driver of breast cancer disease progression and provides a new perspective in the identification of novel targets that may be therapeutically beneficial in the treatment of estrogen receptor-negative disease. Antioxid. Redox Signal. 26, 1044-1058.

Effect of Dietary Bioactive Compounds on Mitochondrial and Metabolic Flexibility

Metabolic flexibility is the capacity of an organism to adequately respond to changes in the environment, such as nutritional input, energetic demand, etc. An important player in the capacity of adaptation through different stages of metabolic demands is the mitochondrion. In this context, mitochondrial dysfunction has been attributed to be the onset and center of many chronic diseases, which are denoted by an inability to adapt fuel preferences and induce mitochondrial morphological changes to respond to metabolic demands, such as mitochondrial number, structure and function. Several nutritional interventions have shown the capacity to induce changes in mitochondrial biogenesis/degradation, oxidative phosphorylation efficiency, mitochondrial membrane composition, electron transfer chain capacity, etc., in metabolic inflexibility states that may open new target options and mechanisms of action of bioactive compounds for the treatment of metabolic diseases. This review is focused in three well-recognized food bioactive compounds that modulate insulin sensitivity, polyphenols, ω-3 fatty acids and dietary fiber, by several mechanism of action, like caloric restriction properties and inflammatory environment modulation, both closely related to mitochondrial function and dynamics.

Maternal high-fat diet impairs cardiac function in offspring of diabetic pregnancy through metabolic stress and mitochondrial dysfunction

Offspring of diabetic pregnancies are at risk of cardiovascular disease at birth and throughout life, purportedly through fuel-mediated influences on the developing heart. Preventative measures focus on glycemic control, but the contribution of additional offenders, including lipids, is not understood. Cellular bioenergetics can be influenced by both diabetes and hyperlipidemia and play a pivotal role in the pathophysiology of adult cardiovascular disease. This study investigated whether a maternal high-fat diet, independently or additively with diabetes, could impair fuel metabolism, mitochondrial function, and cardiac physiology in the developing offspring's heart. Sprague-Dawley rats fed a control or high-fat diet were administered placebo or streptozotocin to induce diabetes during pregnancy and then delivered offspring from four groups: control, diabetes exposed, diet exposed, and combination exposed. Cardiac function, cellular bioenergetics (mitochondrial stress test, glycolytic stress test, and palmitate oxidation assay), lipid peroxidation, mitochondrial histology, and copy number were determined. Diabetes-exposed offspring had impaired glycolytic and respiratory capacity and a reduced proton leak. High-fat diet-exposed offspring had increased mitochondrial copy number, increased lipid peroxidation, and evidence of mitochondrial dysfunction. Combination-exposed pups were most severely affected and demonstrated cardiac lipid droplet accumulation and diastolic/systolic cardiac dysfunction that mimics that of adult diabetic cardiomyopathy. This study is the first to demonstrate that a maternal high-fat diet impairs cardiac function in offspring of diabetic pregnancies through metabolic stress and serves as a critical step in understanding the role of cellular bioenergetics in developmentally programmed cardiac disease.

Signaling and stress: The redox landscape in NOS2 biology

Nitric oxide (NO) has a highly diverse range of biological functions from physiological signaling and maintenance of homeostasis to serving as an effector molecule in the immune system. However, deleterious as well as beneficial roles of NO have been reported. Many of the dichotomous effects of NO and derivative reactive nitrogen species (RNS) can be explained by invoking precise interactions with different targets as a result of concentration and temporal constraints. Endogenous concentrations of NO span five orders of magnitude, with levels near the high picomolar range typically occurring in short bursts as compared to sustained production of low micromolar levels of NO during immune response. This article provides an overview of the redox landscape as it relates to increasing NO concentrations, which incrementally govern physiological signaling, nitrosative signaling and nitrosative stress-related signaling. Physiological signaling by NO primarily occurs upon interaction with the heme protein soluble guanylyl cyclase. As NO concentrations rise, interactions with nonheme iron complexes as well as indirect modification of thiols can stimulate additional signaling processes. At the highest levels of NO, production of a broader range of RNS, which subsequently interact with more diverse targets, can lead to chemical stress. However, even under such conditions, there is evidence that stress-related signaling mechanisms are triggered to protect cells or even resolve the stress. This review therefore also addresses the fundamental reactions and kinetics that initiate signaling through NO-dependent pathways, including processes that lead to interconversion of RNS and interactions with molecular targets.

Alpha-lipoic acid supplementation protects enzymes from damage by nitrosative and oxidative stress

BACKGROUND

S-nitrosylation of mitochondrial enzymes involved in energy transfer under nitrosative stress may result in ATP deficiency. We investigated whether α-lipoic acid, a powerful antioxidant, could alleviate nitrosative stress by regulating S-nitrosylation, which could result in retaining the mitochondrial enzyme activity.

METHODS

In this study, we have identified the S-nitrosylated forms of subunit 1 of dihydrolipoyllysine succinyltransferase (complex III), and subunit 2 of the α-ketoglutarate dehydrogenase complex by implementing a fluorescence-based differential quantitative proteomics method.

RESULTS

We found that the activities of these two mitochondrial enzymes were partially but reversibly inhibited by S-nitrosylation in cultured endothelial cells, and that their activities were partially restored by supplementation of α-lipoic acid. We show that protein S-nitrosylation affects the activity of mitochondrial enzymes that are central to energy supply, and that α-lipoic acid protects mitochondrial enzymes by altering S-nitrosylation levels.

CONCLUSIONS

Inhibiting protein S-nitrosylation with α-lipoic acid seems to be a protective mechanism against nitrosative stress.

GENERAL SIGNIFICANCE

Identification and characterization of these new protein targets should contribute to expanding the therapeutic power of α-lipoic acid and to a better understanding of the underlying antioxidant mechanisms.

Respiratory substrates regulate S-nitrosylation of mitochondrial proteins through a thiol-dependent pathway

S-Nitrosylation is a reversible post-translational modification on cysteinyl thiols that can modulate the function of redox-sensitive proteins. The S-nitrosylation of mitochondrial proteins has been shown to regulate various mitochondrial activities involved in energy-transducing systems and mitochondrion-driven apoptosis. In isolated rat brain mitochondria, we demonstrate that mitochondrial protein S-nitrosylation is regulated by respiratory substrates (glutamate/malate) through a thiol-dependent pathway. Mitochondrial proteins become susceptible to S-nitrosoglutathione (GSNO)-induced S-nitrosylation in mitochondria with an oxidized environment (low glutathione (GSH), NADH, and NADPH, and high GSSG, NAD⁺, and NADP⁺) caused by isolation of mitochondria using a discontinuous Percoll gradient. Activation of mitochondrial respiration by respiratory substrates leads to increased NAD(P)H and GSH levels, which in turn reduces mitochondrial S-nitrosylated proteins. 1-Chloro-2,4-dinitrobenzene (CDNB), which depletes mitochondrial GSH and inhibits the thioredoxin–thioredoxin reductase system, prevented the denitrosylation of mitochondrial proteins caused by respiratory substrate treatment. Using biotin-switch coupled with LC-MS/MS, several mitochondrial proteins were identified as targets of S-nitrosylation including adenine nucleotide translocase (ANT) and voltage-dependent anion channel (VDAC), important components of the mitochondria permeability transition pore (MPTP), as well as ATP synthase. The S-nitrosylation of ATP synthase by GSNO was found to inhibit its activity. These findings emphasize the importance of respiratory substrates in regulating S-nitrosylation through a thiol-dependent (GSH and/or thioredoxin) pathway, with implications for mitochondrial bioenergetics and mitochondrion-driven apoptosis.

S-Nitrosation of monocarboxylate transporter 1: inhibition of pyruvate-fueled respiration and proliferation of breast cancer cells

Energy substrates metabolized through mitochondria (e.g., pyruvate, glutamine) are required for biosynthesis of macromolecules in proliferating cells. Because several mitochondrial proteins are known to be targets of S-nitrosation, we determined whether bioenergetics are modulated by S-nitrosation and defined the subsequent effects on proliferation. The nitrosating agent S-nitroso-L-cysteine (L-CysNO) was used to initiate intracellular S-nitrosation, and treatment decreased mitochondrial function and inhibited proliferation of MCF7 mammary adenocarcinoma cells. Surprisingly, the d-isomer of CysNO (D-CysNO), which is not transported into cells, also caused mitochondrial dysfunction and limited proliferation. Both L- and D-CysNO also inhibited cellular pyruvate uptake and caused S-nitrosation of thiol groups on monocarboxylate transporter 1, a proton-linked pyruvate transporter. These data demonstrate the importance of mitochondrial metabolism in proliferative responses in breast cancer and highlight a novel role for inhibition of metabolic substrate uptake through S-nitrosation of exofacial protein thiols in cellular responses to nitrosative stress.

Low-density lipoprotein-cholesterol-induced endothelial dysfunction and oxidative stress: the role of statins

SIGNIFICANCE

Cardiovascular diseases (CVD) represent a major public health burden. High low-density lipoprotein (LDL)-cholesterol is a recognized pathogenic factor for atherosclerosis, and its complications and statins represent the most potent and widely used therapeutic approach to prevent and control these disorders.

RECENT ADVANCES

A number of clinical and experimental studies concur to identify endothelial dysfunction as a primary step in the development of atherosclerosis, as well as a risk factor for subsequent clinical events. Oxidant stress resulting from chronic elevation of plasma LDL-cholesterol (LDL-chol) is a major contributor to both endothelial dysfunction and its complications, for example, through alterations of endothelial nitric oxide signaling.

CRITICAL ISSUES

Statin treatment reduces morbidity and mortality of CVD, but increasing evidence questions that this is exclusively through reduction of plasma LDL-chol. The identification of ancillary effects on (cardio)vascular biology, for example, through their modulation of oxidative stress, will not only increase our understanding of their mechanisms of action, with a potential broadening of their indication(s), but also lead to the identification of new molecular targets for future therapeutic developments in CVD.

FUTURE DIRECTIONS

Further characterization of molecular pathways targeted by statins, for example, not directly mediated by changes in plasma lipid concentrations, should enable a more comprehensive approach to the pathogenesis of (cardio)vascular disease, including, for example, epigenetic regulation and fine tuning of cell metabolism.

Inhibition of autophagy and glycolysis by nitric oxide during hypoxia-reoxygenation impairs cellular bioenergetics and promotes cell death in primary neurons

Excessive nitric oxide (NO) production is known to damage mitochondrial proteins and the autophagy repair pathway and so can potentially contribute to neurotoxicity. Accordingly, we hypothesized that protection against protein damage from reactive oxygen and nitrogen species under conditions of low oxygen by the autophagy pathway in neurons would be impaired by NO and enhance bioenergetic dysfunction. Rat primary cortical neurons had the same basal cellular respiration in hypoxia as in normoxia, whereas NO-exposed cells exhibited a gradual decrease in mitochondrial respiration in hypoxia. Upon reoxygenation, the respiration in NO-treated cells did not recover to prehypoxic levels. Hypoxia-reoxygenation in the presence of NO was associated with inhibition of autophagy, and the inability to recover during reoxygenation was exacerbated by an inhibitor of autophagy, 3-methyladenine. The effects of hypoxia could be recapitulated by inhibiting glycolytic flux under normoxic conditions. Under both normoxic and hypoxic conditions NO exposure induced immediate stimulation of glycolysis, but prolonged NO exposure, associated with irreversible inhibition of mitochondrial respiration in hypoxia, inhibited glycolysis. Importantly, we found that NO inhibited basal respiration under normoxic conditions only when glucose was absent from the medium or glycolysis was inhibited by 2-deoxy-d-glucose, revealing a novel NO-dependent mechanism for the inhibition of mitochondrial respiration that is modulated by glycolysis. Taken together these data suggest an oxygen-dependent interaction between mitochondrial respiration, glycolysis, and autophagy in protecting neuronal cells exposed to NO. Importantly, they indicate that mitochondrial dysfunction is intimately linked to a failure of glycolytic flux induced by exposure to NO. In addition, these studies provide new insights into the understanding of how autophagy and NO may play interactive roles in neuroinflammation-induced cellular damage, which is pertinent to our understanding of the pathology of neurodegenerative diseases in which excessive NO is generated.

Effect of nitric oxide on naphthoquinone toxicity in endothelial cells: role of bioenergetic dysfunction and poly (ADP-ribose) polymerase activation

When produced at physiological levels, reactive oxygen species (ROS) can act as signaling molecules to regulate normal vascular function. Produced under pathological conditions, ROS can contribute to the oxidative damage of cellular components (e.g., DNA and proteins) and trigger cell death. Moreover, the reaction of superoxide with nitric oxide (NO) produces the strong oxidant peroxynitrite and decreases NO bioavailability, both of which may contribute to activation of cell death pathways. The effects of ROS generated from the 1,4-naphthoquinones alone and in combination with NO on the activation status of poly(ADP-ribose) polymerase (PARP) and cell viability were examined. Treatment with redox cycling quinones activates PARP, and this stimulatory effect is attenuated in the presence of NO. Mitochondria play a central role in cell death signaling pathways and are a target of oxidants. We show that simultaneous exposure of endothelial cells to NO and ROS results in mitochondrial dysfunction, ATP and NAD(+) depletion, and cell death. Alone, NO and ROS have only minor effects on cellular bioenergetics. Further, PARP inhibition does not attenuate reduced cell viability or mitochondrial dysfunction. These results show that concomitant exposure to NO and ROS impairs energy metabolism and triggers PARP-independent cell death. While superoxide-mediated PARP activation is attenuated in the presence of NO, PARP inhibition does not modify the loss of mitochondrial function or adenine and pyridine nucleotide pools and subsequent bioenergetic dysfunction. These findings suggest that the mechanisms by which ROS and NO induce endothelial cell death are closely linked to the maintenance of mitochondrial function and not overactivation of PARP.

Nitrosative stress and redox-cycling agents synergize to cause mitochondrial dysfunction and cell death in endothelial cells

Nitric oxide production by the endothelium is required for normal vascular homeostasis; however, in conditions of oxidative stress, interactions of nitric oxide with reactive oxygen species (ROS) are thought to underlie endothelial dysfunction. Beyond canonical nitric oxide signaling pathways, nitric oxide production results in the post-translational modification of protein thiols, termed S-nitrosation. The potential interplay between S-nitrosation and ROS remains poorly understood and is the focus of the current study. The effects of the S-nitrosating agent S-nitrosocysteine (CysNO) in combination with redox-cycling agents was examined in bovine aortic endothelial cells (BAEC). CysNO significantly impairs mitochondrial function and depletes the NADH/NAD⁺ pool; however, these changes do not result in cell death. When faced with the additional stressor of a redox-cycling agent used to generate ROS, further loss of NAD⁺ occurs, and cellular ATP pools are depleted. Cellular S-nitrosothiols also accumulate, and cell death is triggered. These data demonstrate that CysNO sensitizes endothelial cells to redox-cycling agent-dependent mitochondrial dysfunction and cell death and identify attenuated degradation of S-nitrosothiols as one potential mechanism for the enhanced cytotoxicity.

Is There a Link between Mitochondrial Reserve Respiratory Capacity and Aging?

Oxidative phosphorylation is an indispensable resource of ATP in tissues with high requirement of energy. If the ATP demand is not met, studies suggest that this will lead to senescence and cell death in the affected tissue. The term reserve respiratory capacity or spare respiratory capacity is used to describe the amount of extra ATP that can be produced by oxidative phosphorylation in case of a sudden increase in energy demand. Depletion of the reserve respiratory capacity has been related to a range of pathologies affecting high energy requiring tissues. During aging of an organism, and as a result of mitochondrial dysfunctions, the efficiency of oxidative phosphorylation declines. Based on examples from the energy requiring tissues such as brain, heart, and skeletal muscle, we propose that the age-related decline of oxidative phosphorylation decreases the reserve respiratory capacity of the affected tissue, sensitizes the cells to surges in ATP demand, and increases the risk of resulting pathologies.

Cytochrome c-mediated formation of S-nitrosothiol in cells

S-nitrosothiols are products of nitric oxide (NO) metabolism that have been implicated in a plethora of signalling processes. However, mechanisms of S-nitrosothiol formation in biological systems are uncertain, and no efficient protein-mediated process has been identified. Recently, we observed that ferric cytochrome c can promote S-nitrosoglutathione formation from NO and glutathione by acting as an electron acceptor under anaerobic conditions. In the present study, we show that this mechanism is also robust under oxygenated conditions, that cytochrome c can promote protein S-nitrosation via a transnitrosation reaction and that cell lysate depleted of cytochrome c exhibits a lower capacity to synthesize S-nitrosothiols. Importantly, we also demonstrate that this mechanism is functional in living cells. Lower S-nitrosothiol synthesis activity, from donor and nitric oxide synthase-generated NO, was found in cytochrome c-deficient mouse embryonic cells as compared with wild-type controls. Taken together, these data point to cytochrome c as a biological mediator of protein S-nitrosation in cells. This is the most efficient and concerted mechanism of S-nitrosothiol formation reported so far.

Assessing bioenergetic function in response to oxidative stress by metabolic profiling

It is now clear that mitochondria are an important target for oxidative stress in a broad range of pathologies, including cardiovascular disease, diabetes, neurodegeneration, and cancer. Methods for assessing the impact of reactive species on isolated mitochondria are well established but constrained by the need for large amounts of material to prepare intact mitochondria for polarographic measurements. With the availability of high-resolution polarography and fluorescence techniques for the measurement of oxygen concentration in solution, measurements of mitochondrial function in intact cells can be made. Recently, the development of extracellular flux methods to monitor changes in oxygen concentration and pH in cultures of adherent cells in multiple-sample wells simultaneously has greatly enhanced the ability to measure bioenergetic function in response to oxidative stress. Here we describe these methods in detail using representative cell types from renal, cardiovascular, nervous, and tumorigenic model systems while illustrating the application of three protocols to analyze the bioenergetic response of cells to oxidative stress.