Metabolic Bypass Rescues Aberrant S-nitrosylation-Induced TCA Cycle Inhibition and Synapse Loss in Alzheimer's Disease Human Neurons

In Alzheimer's disease (AD), dysfunctional mitochondrial metabolism is associated with synaptic loss, the major pathological correlate of cognitive decline. Mechanistic insight for this relationship, however, is still lacking. Here, comparing isogenic wild-type and AD mutant human induced pluripotent stem cell (hiPSC)-derived cerebrocortical neurons (hiN), evidence is found for compromised mitochondrial energy in AD using the Seahorse platform to analyze glycolysis and oxidative phosphorylation (OXPHOS). Isotope-labeled metabolic flux experiments revealed a major block in activity in the tricarboxylic acid (TCA) cycle at the α-ketoglutarate dehydrogenase (αKGDH)/succinyl coenzyme-A synthetase step, metabolizing α-ketoglutarate to succinate. Associated with this block, aberrant protein S-nitrosylation of αKGDH subunits inhibited their enzyme function. This aberrant S-nitrosylation is documented not only in AD-hiN but also in postmortem human AD brains versus controls, as assessed by two separate unbiased mass spectrometry platforms using both SNOTRAP identification of S-nitrosothiols and chemoselective-enrichment of S-nitrosoproteins. Treatment with dimethyl succinate, a cell-permeable derivative of a TCA substrate downstream to the block, resulted in partial rescue of mitochondrial bioenergetic function as well as reversal of synapse loss in AD-hiN. These findings have therapeutic implications that rescue of mitochondrial energy metabolism can ameliorate synaptic loss in hiPSC-based models of AD.

S-Nitrosylation-mediated dysfunction of TCA cycle enzymes in synucleinopathy studied in postmortem human brains and hiPSC-derived neurons

A causal relationship between mitochondrial metabolic dysfunction and neurodegeneration has been implicated in synucleinopathies, including Parkinson disease (PD) and Lewy body dementia (LBD), but underlying mechanisms are not fully understood. Here, using human induced pluripotent stem cell (hiPSC)-derived neurons with mutation in the gene encoding α-synuclein (αSyn), we report the presence of aberrantly S-nitrosylated proteins, including tricarboxylic acid (TCA) cycle enzymes, resulting in activity inhibition assessed by carbon-labeled metabolic flux experiments. This inhibition principally affects α-ketoglutarate dehydrogenase/succinyl coenzyme-A synthetase, metabolizing α-ketoglutarate to succinate. Notably, human LBD brain manifests a similar pattern of aberrantly S-nitrosylated TCA enzymes, indicating the pathophysiological relevance of these results. Inhibition of mitochondrial energy metabolism in neurons is known to compromise dendritic length and synaptic integrity, eventually leading to neuronal cell death. Our evidence indicates that aberrant S-nitrosylation of TCA cycle enzymes contributes to this bioenergetic failure.

ASS1 and ASL suppress growth in clear cell renal cell carcinoma via altered nitrogen metabolism

BACKGROUND

Kidney cancer is a common adult malignancy in the USA. Clear cell renal cell carcinoma (ccRCC), the predominant subtype of kidney cancer, is characterized by widespread metabolic changes. Urea metabolism is one such altered pathway in ccRCC. The aim of this study was to elucidate the contributions of urea cycle enzymes, argininosuccinate synthase 1 (ASS1), and argininosuccinate lyase (ASL) towards ccRCC progression.

METHODS

We employed a combination of computational, genetic, and metabolomic tools along with in vivo animal models to establish a tumor-suppressive role for ASS1 and ASL in ccRCC.

RESULTS

We show that the mRNA and protein expression of urea cycle enzymes ASS1 and ASL are reduced in ccRCC tumors when compared to the normal kidney. Furthermore, the loss of ASL in HK-2 cells (immortalized renal epithelial cells) promotes growth in 2D and 3D growth assays, while combined re-expression of ASS1 and ASL in ccRCC cell lines suppresses growth in 2D, 3D, and in vivo xenograft models. We establish that this suppression is dependent on their enzymatic activity. Finally, we demonstrate that conservation of cellular aspartate, regulation of nitric oxide synthesis, and pyrimidine production play pivotal roles in ASS1+ASL-mediated growth suppression in ccRCC.

CONCLUSIONS

ccRCC tumors downregulate the components of the urea cycle including the enzymes argininosuccinate synthase 1 (ASS1) and argininosuccinate lyase (ASL). These cytosolic enzymes lie at a critical metabolic hub in the cell and are involved in aspartate catabolism and arginine and nitric oxide biosynthesis. Loss of ASS1 and ASL helps cells redirect aspartate towards pyrimidine synthesis and support enhanced proliferation. Additionally, reduced levels of ASS1 and ASL might help regulate nitric oxide (NO) generation and mitigate its cytotoxic effects. Overall, our work adds to the understanding of urea cycle enzymes in a context-independent of ureagenesis, their role in ccRCC progression, and uncovers novel potential metabolic vulnerabilities in ccRCC.

Organic mercury solid phase chemoselective capture for proteomic identification of S-nitrosated proteins and peptides

Cysteine S-nitrosation mediates NO signaling and protein function under pathophysiological conditions. Herein, we provide a detailed protocol regarding the organic mercury chemoselective enrichment of S-nitrosated proteins and peptides. We discuss key aspects of the enrichment strategy and provide technical tips for the best performance of the experimental protocol.

TCA cycle metabolic compromise due to an aberrant S-nitrosoproteome in HIV-associated neurocognitive disorder with methamphetamine use

In the brain, both HIV-1 and methamphetamine (meth) use result in increases in oxidative and nitrosative stress. This redox stress is thought to contribute to the pathogenesis of HIV-associated neurocognitive disorder (HAND) and further worsening cognitive activity in the setting of drug abuse. One consequence of such redox stress is aberrant protein S-nitrosylation, derived from nitric oxide, which may disrupt normal protein activity. Here, we report an improved, mass spectrometry-based technique to assess S-nitrosylated protein in human postmortem brains using selective enrichment of S-nitrosocysteine residues with an organomercury resin. The data show increasing S-nitrosylation of tricarboxylic acid (TCA) enzymes in the setting of HAND and HAND/meth use compared with HIV+ control brains without CNS pathology. The consequence is systematic inhibition of multiple TCA cycle enzymes, resulting in energy collapse that can contribute to the neuronal and synaptic damage observed in HAND and meth use.

Multimodality assessment of heart failure with preserved ejection fraction skeletal muscle reveals differences in the machinery of energy fuel metabolism

AIMS

Skeletal muscle (SkM) abnormalities may impact exercise capacity in patients with heart failure with preserved ejection fraction (HFpEF). We sought to quantify differences in SkM oxidative phosphorylation capacity (OxPhos), fibre composition, and the SkM proteome between HFpEF, hypertensive (HTN), and healthy participants.

METHODS AND RESULTS

Fifty-nine subjects (20 healthy, 19 HTN, and 20 HFpEF) performed a maximal-effort cardiopulmonary exercise test to define peak oxygen consumption (VO_{2, peak} ), ventilatory threshold (VT), and VO₂ efficiency (ratio of total work performed to O₂ consumed). SkM OxPhos was assessed using Creatine Chemical-Exchange Saturation Transfer (CrCEST, n = 51), which quantifies unphosphorylated Cr, before and after plantar flexion exercise. The half-time of Cr recovery (t_{1/2, Cr} ) was taken as a metric of in vivo SkM OxPhos. In a subset of subjects (healthy = 13, HTN = 9, and HFpEF = 12), percutaneous biopsy of the vastus lateralis was performed for myofibre typing, mitochondrial morphology, and proteomic and phosphoproteomic analysis. HFpEF subjects demonstrated lower VO_2,peak , VT, and VO₂ efficiency than either control group (all P < 0.05). The t_{1/2, Cr} was significantly longer in HFpEF (P = 0.005), indicative of impaired SkM OxPhos, and correlated with cycle ergometry exercise parameters. HFpEF SkM contained fewer Type I myofibres (P = 0.003). Proteomic analyses demonstrated (a) reduced levels of proteins related to OxPhos that correlated with exercise capacity and (b) reduced ERK signalling in HFpEF.

CONCLUSIONS

Heart failure with preserved ejection fraction patients demonstrate impaired functional capacity and SkM OxPhos. Reductions in the proportions of Type I myofibres, proteins required for OxPhos, and altered phosphorylation signalling in the SkM may contribute to exercise intolerance in HFpEF.

The effect of dietary nitrate on exercise capacity in chronic kidney disease: a randomized controlled pilot study

BACKGROUND

Chronic Kidney Disease (CKD) patients exhibit a reduced exercise capacity that impacts quality of life. Dietary nitrate supplementation has been shown to have favorable effects on exercise capacity in disease populations by reducing the oxygen cost of exercise. This study investigated whether dietary nitrates would acutely improve exercise capacity in CKD patients.

METHODS AND RESULTS

In this randomized, double-blinded crossover study, 12 Stage 3-4 CKD patients (Mean ± SEM: Age, 60 ± 5yrs; eGFR, 50.3 ± 4.6 ml/min/1.73 m²) received an acute dose of 12.6 mmol of dietary nitrate in the form of concentrated beetroot juice (BRJ) and a nitrate depleted placebo (PLA). Skeletal muscle mitochondrial oxidative function was assessed using near-infrared spectroscopy. Cardiopulmonary exercise testing was performed on a cycle ergometer, with intensity increased by 25 W every 3 min until volitional fatigue. Plasma nitric oxide (NO) metabolites (NOm; nitrate, nitrite, low molecular weight S-nitrosothiols, and metal bound NO) were determined by gas-phase chemiluminescence. Plasma NOm values were significantly increased following BRJ (BRJ vs. PLA: 1074.4 ± 120.4 μM vs. 28.4 ± 6.6 μM, p < 0.001). Total work performed (44.4 ± 10.6 vs 39.6 ± 9.9 kJ, p = 0.03) and total exercise time (674 ± 85 vs 627 ± 86s, p = 0.04) were significantly greater following BRJ. Oxygen consumption at the ventilatory threshold was also improved by BRJ (0.90 ± 0.08 vs. 0.74 ± 0.06 L/min, p = 0.04). These changes occurred in the absence of improved skeletal muscle mitochondrial oxidative capacity (p = 0.52) and VO_2peak (p = 0.35).

CONCLUSIONS

Our findings demonstrate that inorganic nitrate can acutely improve exercise capacity in CKD patients. The effects of chronic nitrate supplementation on CKD related exercise intolerance should be investigated in future studies.

AMPA Receptor Surface Expression Is Regulated by S-Nitrosylation of Thorase and Transnitrosylation of NSF

The regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking affects multiple brain functions, such as learning and memory. We have previously shown that Thorase plays an important role in the internalization of AMPARs from the synaptic membrane. Here, we show that N-methyl-d-aspartate receptor (NMDAR) activation leads to increased S-nitrosylation of Thorase and N-ethylmaleimide-sensitive factor (NSF). S-nitrosylation of Thorase stabilizes Thorase-AMPAR complexes and enhances the internalization of AMPAR and interaction with protein-interacting C kinase 1 (PICK1). S-nitrosylated NSF is dependent on the S-nitrosylation of Thorase via trans-nitrosylation, which modulates the surface insertion of AMPARs. In the presence of the S-nitrosylation-deficient C137L Thorase mutant, AMPAR trafficking, long-term potentiation, and long-term depression are impaired. Overall, our data suggest that both S-nitrosylation and interactions of Thorase and NSF/PICK1 are required to modulate AMPAR-mediated synaptic plasticity. This study provides critical information that elucidates the mechanism underlying Thorase and NSF-mediated trafficking of AMPAR complexes.

Endothelial nitric oxide synthase-derived nitric oxide in the regulation of metabolism

Nitric oxide is an endogenously formed gas that acts as a signaling molecule in the human body. The signaling functions of nitric oxide are accomplished through two primer mechanisms: cGMP-mediated phosphorylation and the formation of S-nitrosocysteine on proteins. This review presents and discusses previous and more recent findings documenting that nitric oxide signaling regulates metabolic activity. These discussions primarily focus on endothelial nitric oxide synthase (eNOS) as the source of nitric oxide.

The Metabolomic Signature of the Placenta in Spontaneous Preterm Birth

The placenta is metabolically active and supports the growth of the fetus. We hypothesize that deficits in the capacity of the placenta to maintain bioenergetic and metabolic stability during pregnancy may result in spontaneous preterm birth (SPTB). To explore this hypothesis, we performed a nested cased control study of metabolomic signatures in placentas from women with SPTB (<36 weeks gestation) compared to normal pregnancies (≥38 weeks gestation). To control for the effects of gestational age on placenta metabolism, we also studied a subset of metabolites in non-laboring preterm and term Rhesus monkeys. Comprehensive quantification of metabolites demonstrated a significant elevation in the levels of amino acids, prostaglandins, sphingolipids, lysolipids, and acylcarnitines in SPTB placenta compared to term placenta. Additional quantification of placental acylcarnitines by tandem mass spectrometry confirmed the significant elevation in SPTB human, with no significant differences between midgestation and term placenta in Rhesus macaque. Fatty acid oxidation as measured by the flux of ³H-palmitate in SPTB placenta was lower than term. Collectively, significant and biologically relevant alterations in the placenta metabolome were identified in SPTB placenta. Altered acylcarnitine levels and fatty acid oxidation suggest that disruption in normal substrate metabolism is associated with SPTB.

Analysis of Cysteine Post Translational Modifications Using Organic Mercury Resin

The wide reactivity of the thiol group enables the formation of a variety of reversible, covalent modifications on cysteine residues. S-nitrosylation, like many other post-translational modifications, is site selective, reversible, and necessary for a wide variety of fundamental cellular processes. The overall abundance of S-nitrosylated proteins and reactivity of the nitrosyl group necessitates an enrichment strategy for accurate detection with adequate depth. Herein, a method is presented for the enrichment and detection of endogenous protein S-nitrosylation from complex mixtures of cell or tissue lysate utilizing organomercury resin. Minimal adaptations to the method also support the detection of either S-glutathionylation or S-acylation using the same enrichment platform. When coupled with high accuracy mass spectrometry, these methods enable a site-specific level of analysis, facilitating the curation comparable datasets of three separate cysteine post-translational modifications. © 2018 by John Wiley & Sons, Inc.

Oral nitrite restores age-dependent phenotypes in eNOS-null mice

Alterations in the synthesis and bioavailability of NO are central to the pathogenesis of cardiovascular and metabolic disorders. Although endothelial NO synthase-derived (eNOS-derived) NO affects mitochondrial long-chain fatty acid β-oxidation, the pathophysiological significance of this regulation remains unclear. Accordingly, we determined the contributions of eNOS/NO signaling in the adaptive metabolic responses to fasting and in age-induced metabolic dysfunction. Four-month-old eNOS-/- mice are glucose intolerant and exhibit serum dyslipidemia and decreased capacity to oxidize fatty acids. However, during fasting, eNOS-/- mice redirect acetyl-CoA to ketogenesis to elevate circulating levels of β-hydroxybutyrate similar to wild-type mice. Treatment of 4-month-old eNOS-/- mice with nitrite for 10 days corrected the hypertension and serum hyperlipidemia and normalized the rate of fatty acid oxidation. Fourteen-month-old eNOS-/- mice exhibited metabolic derangements, resulting in reduced utilization of fat to generate energy, lower resting metabolic activity, and diminished physical activity. Seven-month administration of nitrite to eNOS-/- mice reversed the age-dependent metabolic derangements and restored physical activity. While the eNOS/NO signaling is not essential for the metabolic adaptation to fasting, it is critical for regulating systemic metabolic homeostasis in aging. The development of age-dependent metabolic disorder is prevented by low-dose replenishment of bioactive NO.

Diet-Induced Circadian Enhancer Remodeling Synchronizes Opposing Hepatic Lipid Metabolic Processes

Overnutrition disrupts circadian metabolic rhythms by mechanisms that are not well understood. Here, we show that diet-induced obesity (DIO) causes massive remodeling of circadian enhancer activity in mouse liver, triggering synchronous high-amplitude circadian rhythms of both fatty acid (FA) synthesis and oxidation. SREBP expression was rhythmically induced by DIO, leading to circadian FA synthesis and, surprisingly, FA oxidation (FAO). DIO similarly caused a high-amplitude circadian rhythm of PPARα, which was also required for FAO. Provision of a pharmacological activator of PPARα abrogated the requirement of SREBP for FAO (but not FA synthesis), suggesting that SREBP indirectly controls FAO via production of endogenous PPARα ligands. The high-amplitude rhythm of PPARα imparted time-of-day-dependent responsiveness to lipid-lowering drugs. Thus, acquisition of rhythmicity for non-core clock components PPARα and SREBP1 remodels metabolic gene transcription in response to overnutrition and enables a chronopharmacological approach to metabolic disorders.

Hepatic metal ion transporter ZIP8 regulates manganese homeostasis and manganese-dependent enzyme activity

Genetic variants at the solute carrier family 39 member 8 (SLC39A8) gene locus are associated with the regulation of whole-blood manganese (Mn) and multiple physiological traits. SLC39A8 encodes ZIP8, a divalent metal ion transporter best known for zinc transport. Here, we hypothesized that ZIP8 regulates Mn homeostasis and Mn-dependent enzymes to influence metabolism. We generated Slc39a8-inducible global-knockout (ZIP8-iKO) and liver-specific-knockout (ZIP8-LSKO) mice and observed markedly decreased Mn levels in multiple organs and whole blood of both mouse models. By contrast, liver-specific overexpression of human ZIP8 (adeno-associated virus-ZIP8 [AAV-ZIP8]) resulted in increased tissue and whole blood Mn levels. ZIP8 expression was localized to the hepatocyte canalicular membrane, and bile Mn levels were increased in ZIP8-LSKO and decreased in AAV-ZIP8 mice. ZIP8-LSKO mice also displayed decreased liver and kidney activity of the Mn-dependent enzyme arginase. Both ZIP8-iKO and ZIP8-LSKO mice had defective protein N-glycosylation, and humans homozygous for the minor allele at the lead SLC39A8 variant showed hypogalactosylation, consistent with decreased activity of another Mn-dependent enzyme, β-1,4-galactosyltransferase. In summary, hepatic ZIP8 reclaims Mn from bile and regulates whole-body Mn homeostasis, thereby modulating the activity of Mn-dependent enzymes. This work provides a mechanistic basis for the association of SLC39A8 with whole-blood Mn, potentially linking SLC39A8 variants with other physiological traits.

Pharmacokinetics and Pharmacodynamics of Inorganic Nitrate in Heart Failure With Preserved Ejection Fraction

RATIONALE

Nitrate-rich beetroot juice has been shown to improve exercise capacity in heart failure with preserved ejection fraction, but studies using pharmacological preparations of inorganic nitrate are lacking.

OBJECTIVES

To determine (1) the dose-response effect of potassium nitrate (KNO3) on exercise capacity; (2) the population-specific pharmacokinetic and safety profile of KNO3 in heart failure with preserved ejection fraction.

METHODS AND RESULTS

We randomized 12 subjects with heart failure with preserved ejection fraction to oral KNO3 (n=9) or potassium chloride (n=3). Subjects received 6 mmol twice daily during week 1, followed by 6 mmol thrice daily during week 2. Supine cycle ergometry was performed at baseline (visit 1) and after each week (visits 2 and 3). Quality of life was assessed with the Kansas City Cardiomyopathy Questionnaire. The primary efficacy outcome, peak O2-uptake, did not significantly improve (P=0.13). Exploratory outcomes included exercise duration and quality of life. Exercise duration increased significantly with KNO3 (visit 1: 9.87, 95% confidence interval [CI] 9.31-10.43 minutes; visit 2: 10.73, 95% CI 10.13-11.33 minute; visit 3: 11.61, 95% CI 11.05-12.17 minutes; P=0.002). Improvements in the Kansas City Cardiomyopathy Questionnaire total symptom (visit 1: 58.0, 95% CI 52.5-63.5; visit 2: 66.8, 95% CI 61.3-72.3; visit 3: 70.8, 95% CI 65.3-76.3; P=0.016) and functional status scores (visit 1: 62.2, 95% CI 58.5-66.0; visit 2: 68.6, 95% CI 64.9-72.3; visit 3: 71.1, 95% CI 67.3-74.8; P=0.01) were seen after KNO3. Pronounced elevations in trough levels of nitric oxide metabolites occurred with KNO3 (visit 2: 199.5, 95% CI 98.7-300.2 μmol/L; visit 3: 471.8, 95% CI 377.8-565.8 μmol/L) versus baseline (visit 1: 38.0, 95% CI 0.00-132.0 μmol/L; P<0.001). KNO3 did not lead to clinically significant hypotension or methemoglobinemia. After 6 mmol of KNO3, systolic blood pressure was reduced by a maximum of 17.9 (95% CI -28.3 to -7.6) mm Hg 3.75 hours later. Peak nitric oxide metabolites concentrations were 259.3 (95% CI 176.2-342.4) μmol/L 3.5 hours after ingestion, and the median half-life was 73.0 (interquartile range 33.4-232.0) minutes.

CONCLUSIONS

KNO3 is potentially well tolerated and improves exercise duration and quality of life in heart failure with preserved ejection fraction. This study reinforces the efficacy of KNO3 and suggests that larger randomized trials are warranted.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT02256345.

Heart Failure, Left Ventricular Remodeling, and Circulating Nitric Oxide Metabolites

Background

Stable plasma nitric oxide (NO) metabolites (NO_M), composed predominantly of nitrate and nitrite, are attractive biomarkers of NO bioavailability. NO_M levels integrate the influence of NO‐synthase‐derived NO production/metabolism, dietary intake of inorganic nitrate/nitrite, and clearance of NO_M. Furthermore, nitrate and nitrite, the most abundant NO_M, can be reduced to NO via the nitrate‐nitrite‐NO pathway.

Methods and Results

We compared serum NO_M among subjects without heart failure (n=126), subjects with heart failure and preserved ejection fraction (HFpEF; n=43), and subjects with heart failure and reduced ejection fraction (HFrEF; n=32). LV mass and extracellular volume fraction were measured with cardiac MRI. Plasma NO_M levels were measured after reduction to NO via reaction with vanadium (III)/hydrochloric acid. Subjects with HFpEF demonstrated significantly lower unadjusted levels of NO_M (8.0 μmol/L; 95% CI 6.2–10.4 μmol/L; ANOVA P=0.013) than subjects without HF (12.0 μmol/L; 95% CI 10.4–13.9 μmol/L) or those with HFrEF (13.5 μmol/L; 95% CI 9.7–18.9 μmol/L). There were no significant differences in NO_M between subjects with HFrEF and subjects without HF. In a multivariable model that adjusted for age, sex, race, diabetes mellitus, body mass index, current smoking, systolic blood pressure, and glomerular filtration rate, HFpEF remained a predictor of lower NO_M (β=−0.43; P=0.013). NO_M did not correlate with LV mass, or LV diffuse fibrosis.

Conclusions

HFpEF, but not HFrEF, is associated with reduced plasma NO_M, suggesting greater endothelial dysfunction, enhanced clearance, or deficient dietary ingestion of inorganic nitrate. Our findings may underlie the salutary effects of inorganic nitrate supplementation demonstrated in recent clinical trials in HFpEF.

S-Nitrosylation of Calcium-Handling Proteins in Cardiac Adrenergic Signaling and Hypertrophy

RATIONALE

The regulation of calcium (Ca(2+)) homeostasis by β-adrenergic receptor (βAR) activation provides the essential underpinnings of sympathetic regulation of myocardial function, as well as a basis for understanding molecular events that result in hypertrophic signaling and heart failure. Sympathetic stimulation of the βAR not only induces protein phosphorylation but also activates nitric oxide-dependent signaling, which modulates cardiac contractility. Nonetheless, the role of nitric oxide in βAR-dependent regulation of Ca(2+) handling has not yet been explicated fully.

OBJECTIVE

To elucidate the role of protein S-nitrosylation, a major transducer of nitric oxide bioactivity, on βAR-dependent alterations in cardiomyocyte Ca(2+) handling and hypertrophy.

METHODS AND RESULTS

Using transgenic mice to titrate the levels of protein S-nitrosylation, we uncovered major roles for protein S-nitrosylation, in general, and for phospholamban and cardiac troponin C S-nitrosylation, in particular, in βAR-dependent regulation of Ca(2+) homeostasis. Notably, S-nitrosylation of phospholamban consequent upon βAR stimulation is necessary for the inhibitory pentamerization of phospholamban, which activates sarcoplasmic reticulum Ca(2+)-ATPase and increases cytosolic Ca(2+) transients. Coincident S-nitrosylation of cardiac troponin C decreases myocardial sensitivity to Ca(2+). During chronic adrenergic stimulation, global reductions in cellular S-nitrosylation mitigate hypertrophic signaling resulting from Ca(2+) overload.

CONCLUSIONS

S-Nitrosylation operates in concert with phosphorylation to regulate many cardiac Ca(2+)-handling proteins, including phospholamban and cardiac troponin C, thereby playing an essential and previously unrecognized role in cardiac Ca(2+) homeostasis. Manipulation of the S-nitrosylation level may prove therapeutic in heart failure.

Regulation of brain glutamate metabolism by nitric oxide and S-nitrosylation

Nitric oxide (NO) is a signaling intermediate during glutamatergic neurotransmission in the central nervous system (CNS). NO signaling is in part accomplished through cysteine S-nitrosylation, a posttranslational modification by which NO regulates protein function and signaling. In our investigation of the protein targets and functional impact of S-nitrosylation in the CNS under physiological conditions, we identified 269 S-nitrosocysteine residues in 136 proteins in the wild-type mouse brain. The number of sites was significantly reduced in the brains of mice lacking endothelial nitric oxide synthase (eNOS(-/-)) or neuronal nitric oxide synthase (nNOS(-/-)). In particular, nNOS(-/-) animals showed decreased S-nitrosylation of proteins that participate in the glutamate/glutamine cycle, a metabolic process by which synaptic glutamate is recycled or oxidized to provide energy. (15)N-glutamine-based metabolomic profiling and enzymatic activity assays indicated that brain extracts from nNOS(-/-) mice converted less glutamate to glutamine and oxidized more glutamate than those from mice of the other genotypes. GLT1 [also known as EAAT2 (excitatory amino acid transporter 2)], a glutamate transporter in astrocytes, was S-nitrosylated at Cys(373) and Cys(561) in wild-type and eNOS(-/-) mice, but not in nNOS(-/-) mice. A form of rat GLT1 that could not be S-nitrosylated at the equivalent sites had increased glutamate uptake compared to wild-type GLT1 in cells exposed to an S-nitrosylating agent. Thus, NO modulates glutamatergic neurotransmission through the selective, nNOS-dependent S-nitrosylation of proteins that govern glutamate transport and metabolism.

Strategies for correcting very long chain acyl-CoA dehydrogenase deficiency

Very long acyl-CoA dehydrogenase (VLCAD) deficiency is a genetic pediatric disorder presenting with a spectrum of phenotypes that remains for the most part untreatable. Here, we present a novel strategy for the correction of VLCAD deficiency by increasing mutant VLCAD enzymatic activity. Treatment of VLCAD-deficient fibroblasts, which express distinct mutant VLCAD protein and exhibit deficient fatty acid β-oxidation, with S-nitroso-N-acetylcysteine induced site-specific S-nitrosylation of VLCAD mutants at cysteine residue 237. Cysteine 237 S-nitrosylation was associated with an 8-17-fold increase in VLCAD-specific activity and concomitant correction of acylcarnitine profile and β-oxidation capacity, two hallmarks of the disorder. Overall, this study provides biochemical evidence for a potential therapeutic modality to correct β-oxidation deficiencies.

Effect of inorganic nitrate on exercise capacity in heart failure with preserved ejection fraction

BACKGROUND

Inorganic nitrate (NO3(-)), abundant in certain vegetables, is converted to nitrite by bacteria in the oral cavity. Nitrite can be converted to nitric oxide in the setting of hypoxia. We tested the hypothesis that NO3(-) supplementation improves exercise capacity in heart failure with preserved ejection fraction via specific adaptations to exercise.

METHODS AND RESULTS

Seventeen subjects participated in this randomized, double-blind, crossover study comparing a single dose of NO3-rich beetroot juice (NO3(-), 12.9 mmol) with an identical nitrate-depleted placebo. Subjects performed supine-cycle maximal-effort cardiopulmonary exercise tests, with measurements of cardiac output and skeletal muscle oxygenation. We also assessed skeletal muscle oxidative function. Study end points included exercise efficiency (total work/total oxygen consumed), peak VO2, total work performed, vasodilatory reserve, forearm mitochondrial oxidative function, and augmentation index (a marker of arterial wave reflections, measured via radial arterial tonometry). Supplementation increased plasma nitric oxide metabolites (median, 326 versus 10 μmol/L; P=0.0003), peak VO2 (12.6±3.7 versus 11.6±3.1 mL O2·min(-1)·kg(-1); P=0.005), and total work performed (55.6±35.3 versus 49.2±28.9 kJ; P=0.04). However, efficiency was unchanged. NO3(-) led to greater reductions in systemic vascular resistance (-42.4±16.6% versus -31.8±20.3%; P=0.03) and increases in cardiac output (121.2±59.9% versus 88.7±53.3%; P=0.006) with exercise. NO3(-) reduced aortic augmentation index (132.2±16.7% versus 141.4±21.9%; P=0.03) and tended to improve mitochondrial oxidative function.

CONCLUSIONS

NO3(-) increased exercise capacity in heart failure with preserved ejection fraction by targeting peripheral abnormalities. Efficiency did not change as a result of parallel increases in total work and VO2. NO3(-) increased exercise vasodilatory and cardiac output reserves. NO3(-) also reduced arterial wave reflections, which are linked to left ventricular diastolic dysfunction and remodeling.

CLINICAL TRIAL REGISTRATION URL

www.clinicaltrials.gov. Unique identifier: NCT01919177.

Neutralizing Th2 inflammation in neonatal islets prevents β-cell failure in adult IUGR rats

Intrauterine growth restriction (IUGR) leads to development of type 2 diabetes (T2D) in adulthood. The mechanisms underlying this phenomenon have not been fully elucidated. Inflammation is associated with T2D; however, it is unknown whether inflammation is causal or secondary to the altered metabolic state. Here we show that the mechanism by which IUGR leads to the development of T2D in adulthood is via transient recruitment of T-helper 2 (Th) lymphocytes and macrophages in fetal islets resulting in localized inflammation. Although this immune response is short-lived, it results in a permanent reduction in islet vascularity and impaired insulin secretion. Neutralizing interleukin-4 antibody therapy given only in the newborn period ameliorates inflammation and restores vascularity and β-cell function into adulthood, demonstrating a novel role for Th2 immune responses in the induction and progression of T2D. In the neonatal stage, inflammation and vascular changes are reversible and may define an important developmental window for therapeutic intervention to prevent adult-onset diabetes.

The 4-cysteine zinc-finger motif of the RNA polymerase regulator DksA serves as a thiol switch for sensing oxidative and nitrosative stress

We show that thiols in the 4-cysteine zinc-finger motif of DksA, an RNA polymerase accessory protein known to regulate the stringent response, sense oxidative and nitrosative stress. Hydrogen peroxide- or nitric oxide (NO)-mediated modifications of thiols in the DksA 4-cysteine zinc-finger motif release the metal cofactor and drive reversible changes in the α-helicity of the protein. Wild-type and relA spoT mutant Salmonella, but not isogenic dksA-deficient bacteria, experience the downregulation of r-protein and amino acid transport expression after NO treatment, suggesting that DksA can regulate gene expression in response to NO congeners independently of the ppGpp alarmone. Oxidative stress enhances the DksA-dependent repression of rpsM, while preventing the activation of livJ and hisG gene transcription that is supported by reduced, zinc-bound DksA. The inhibitory effects of oxidized DksA on transcription are reversible with dithiothreitol. Our investigations indicate that sensing of reactive species by DksA redox active thiols fine-tunes the expression of translational machinery and amino acid assimilation and biosynthesis in accord with the metabolic stress imposed by oxidative and nitrosative stress. Given the conservation of Cys(114) , and neighbouring hydrophobic and charged amino acids in DksA orthologues, phylogenetically diverse microorganisms may use the DksA thiol switch to regulate transcriptional responses to oxidative and nitrosative stress.

Protein microarray characterization of the S-nitrosoproteome

Nitric oxide (NO) mediates a substantial part of its physiologic functions via S-nitrosylation, however the cellular substrates for NO-mediated S-nitrosylation are largely unknown. Here we describe the S-nitrosoproteome using a high-density protein microarray chip containing 16,368 unique human proteins. We identified 834 potentially S-nitrosylated human proteins. Using a unique and highly specific labeling and affinity capture of S-nitrosylated proteins, 138 cysteine residues on 131 peptides in 95 proteins were determined, defining critical sites of NO's actions. Of these cysteine residues 113 are novel sites of S-nitrosylation. A consensus sequence motif from these 834 proteins for S-nitrosylation was identified, suggesting that the residues flanking the S-nitrosylated cysteine are likely to be the critical determinant of whether the cysteine is S-nitrosylated. We identify eight ubiquitin E3 ligases, RNF10, RNF11, RNF41, RNF141, RNF181, RNF208, WWP2, and UBE3A, whose activities are modulated by S-nitrosylation, providing a unique regulatory mechanism of the ubiquitin proteasome system. These results define a new and extensive set of proteins that are susceptible to NO regulation via S-nitrosylation. Similar approaches could be used to identify other post-translational modification proteomes.

Regulation of protein function and signaling by reversible cysteine S-nitrosylation

NO is a versatile free radical that mediates numerous biological functions within every major organ system. A molecular pathway by which NO accomplishes functional diversity is the selective modification of protein cysteine residues to form S-nitrosocysteine. This post-translational modification, S-nitrosylation, impacts protein function, stability, and location. Despite considerable advances with individual proteins, the in vivo biological chemistry, the structural elements that govern the selective S-nitrosylation of cysteine residues, and the potential overlap with other redox modifications are unknown. In this minireview, we explore the functional features of S-nitrosylation at the proteome level and the structural diversity of endogenously modified residues, and we discuss the potential overlap and complementation that may exist with other cysteine modifications.

Site specific identification of endogenous S-nitrosocysteine proteomes

Cysteine S-nitrosylation is a post-translational modification regulating protein function and nitric oxide signaling. Herein the selectivity, reproducibility, and sensitivity of a mass spectrometry-based proteomic method for the identification of endogenous S-nitrosylated proteins are outlined. The method enriches for either S-nitrosylated proteins or peptides through covalent binding of the cysteine sulfur with phenylmercury at pH=6.0. Phenylmercury reacts selectively and efficiently with S-nitrosocysteine since no reactivity can be documented for disulfides, sulfinic or sulfonic acids, S-glutathionylated, S-alkylated or S-sulfhydrylated cysteine residues. A specificity of 97±1% for the identification of S-nitrosocysteine peptides in mouse liver tissue is achieved by the inclusion of negative controls. The method enables the detection of 36 S-nitrosocysteine peptides starting with 5pmolS-nitrosocysteine/mg of total tissue protein. Both the percentage of protein molecules modified as well as the occupancy by S-nitrosylation can be determined. Overall, selective, sensitive and reproducible enrichment of S-nitrosylated proteins and peptides is achieved by the use of phenylmercury. The inclusion of appropriate negative controls secures the precise identification of endogenous S-nitrosylated sites and proteins in biological samples.

BIOLOGICAL SIGNIFICANCE

The current study describes a selective, sensitive and reproducible method for the acquisition of endogenously S-nitrosylated proteins and peptides. The acquisition of endogenous S-nitrosoproteomes provides robust data that is necessary for investigating the mechanism(s) of S-nitrosylation in vivo, the factors that govern its selectivity, the dependency of the modification on different isoforms of nitric oxide synthases (NOS), as well as the physiological functions of this protein modification. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Nitric oxide regulates mitochondrial fatty acid metabolism through reversible protein S-nitrosylation

Cysteine S-nitrosylation is a posttranslational modification by which nitric oxide regulates protein function and signaling. Studies of individual proteins have elucidated specific functional roles for S-nitrosylation, but knowledge of the extent of endogenous S-nitrosylation, the sites that are nitrosylated, and the regulatory consequences of S-nitrosylation remains limited. We used mass spectrometry-based methodologies to identify 1011 S-nitrosocysteine residues in 647 proteins in various mouse tissues. We uncovered selective S-nitrosylation of enzymes participating in glycolysis, gluconeogenesis, tricarboxylic acid cycle, and oxidative phosphorylation, indicating that this posttranslational modification may regulate metabolism and mitochondrial bioenergetics. S-nitrosylation of the liver enzyme VLCAD [very long chain acyl-coenzyme A (CoA) dehydrogenase] at Cys(238), which was absent in mice lacking endothelial nitric oxide synthase, improved its catalytic efficiency. These data implicate protein S-nitrosylation in the regulation of β-oxidation of fatty acids in mitochondria.

T2R38 taste receptor polymorphisms underlie susceptibility to upper respiratory infection

Innate and adaptive defense mechanisms protect the respiratory system from attack by microbes. Here, we present evidence that the bitter taste receptor T2R38 regulates the mucosal innate defense of the human upper airway. Utilizing immunofluorescent and live cell imaging techniques in polarized primary human sinonasal cells, we demonstrate that T2R38 is expressed in human upper respiratory epithelium and is activated in response to acyl-homoserine lactone quorum-sensing molecules secreted by Pseudomonas aeruginosa and other gram-negative bacteria. Receptor activation regulates calcium-dependent NO production, resulting in stimulation of mucociliary clearance and direct antibacterial effects. Moreover, common polymorphisms of the TAS2R38 gene were linked to significant differences in the ability of upper respiratory cells to clear and kill bacteria. Lastly, TAS2R38 genotype correlated with human sinonasal gram-negative bacterial infection. These data suggest that T2R38 is an upper airway sentinel in innate defense and that genetic variation contributes to individual differences in susceptibility to respiratory infection.

Mass spectrometry-based identification of S-nitrosocysteine in vivo using organic mercury assisted enrichment

Protein S-nitrosylation is considered as one of the molecular mechanisms by which nitric oxide regulates signaling events and protein function. The present review presents an updated method which allows for the site-specific detection of S-nitrosylated proteins in vivo. The method is based on enrichment of S-nitrosylated proteins or peptides using organomercury compounds followed by LC-MS/MS detection. Technical aspects for determining the reaction and binding efficiency of the mercury resin that assists enrichment of S-nitrosylated proteins are presented and discussed. In addition, emphasis is given to the specificity of the method by providing technical details for the generation of four chemically distinct negative controls. Finally it is provided an overview of the key steps for generation and evaluation of mass spectrometry derived data.

Immunoglobulins against tyrosine-nitrated epitopes in coronary artery disease

BACKGROUND

Several lines of evidence support a pathophysiological role of immunity in atherosclerosis. Tyrosine-nitrated proteins, a footprint of oxygen- and nitrogen-derived oxidants generated by cells of the immune system, are enriched in atheromatous lesions and in circulation of patients with coronary artery disease (CAD). However, the consequences of possible immune reactions triggered by the presence of nitrated proteins in subjects with clinically documented atherosclerosis have not been explored.

METHODS AND RESULTS

Specific immunoglobulins that recognize 3-nitrotyrosine epitopes were identified in human lesions, as well as in circulation of patients with CAD. The levels of circulating immunoglobulins against 3-nitrotyrosine epitopes were quantified in patients with CAD (n=374) and subjects without CAD (non-CAD controls, n=313). A 10-fold increase in the mean level of circulating immunoglobulins against protein-bound 3-nitrotyrosine was documented in patients with CAD (3.75±1.8 μg antibody Eq/mL plasma versus 0.36±0.8 μg antibody Eq/mL plasma), and was strongly associated with angiographic evidence of significant CAD.

CONCLUSIONS

The results of this cross-sectional study suggest that posttranslational modification of proteins via nitration within atherosclerotic plaque-laden arteries and in circulation serve as neo-epitopes for the elaboration of immunoglobulins, thereby providing an association between oxidant production and the activation of the immune system in CAD.

Lymphocyte development requires S-nitrosoglutathione reductase

NO is critical to immunity, but its role in the development of the immune system is unknown. In this study, we show that S-nitrosoglutathione reductase (GSNOR), a protein key to the control of protein S-nitrosylation, is important for the development of lymphocytes. Genetic deletion of GSNOR in mice results in significant decrease in both T and B lymphocytes in the periphery. In thymus, GSNOR deficiency causes excessive protein S-nitrosylation, increases apoptosis, and reduces the number of CD4 single-positive thymocytes. Lymphopenia and increase in S-nitrosylation and apoptosis in GSNOR-deficient mice are largely abolished by genetic deletion of inducible NO synthase. Furthermore, the protection of lymphocyte development by GSNOR is apparently intrinsic to hematopoietic cells. Thus, GSNOR, likely through regulation of S-nitrosylation and apoptosis, physiologically plays a protective role in the development of the immune system.

DNA protecting and genotoxic effects of olive oil related components in cells exposed to hydrogen peroxide

In search for compounds, able to protect nuclear DNA in cells exposed to oxidative stress, extracts from olive leaves, olive fruits, olive oil and olive mill waste water were tested by using the "single cell gel electrophoresis" methodology (comet assay). Jurkat cells in culture were exposed to continuously generated hydrogen peroxide (11.8+/-1.5 microM per min) by direct addition into the growth medium of the appropriate amount of the enzyme "glucose oxidase" in the presence or absence of the tested total extracts. The protective effects of the tested extracts or isolated compounds were evaluated from their ability to decrease hydrogen peroxide-induced formation of single strand breaks in the nuclear DNA, while the toxic effects were estimated from the increase of DNA damage when the extracts or isolated compounds were incubated directly with the cells. Significant protection was observed in extracts from olive oil and olive mill waste water. However, above a concentration of 100 microg/ml olive oil extracts exerted DNA damaging effects by themselves in the absence of any H2O2. Extracts from olive leaves and olive fruits although protective, were also able to induce DNA damage by themselves. Main compounds isolated from the above described total extracts, like oleuropein glucoside, tyrosol, hydroxytyrosol and caffeic acid, were tested in the same experimental system and found to exert cytotoxic (oleuropein glucoside), no effect (tyrosol) or protective effects (hydroxytyrosol and caffeic acid). In conclusion, cytoprotective as well as cytotoxic compounds with potential pharmaceutical properties were detected in extracts from olive oil related sources by using the comet assay methodology.

Protection by tropolones against H2O2-induced DNA damage and apoptosis in cultured Jurkat cells

Tropolones, the naturally occurring compounds responsible for the durability of heartwood of several cupressaceous trees, have been shown to possess both metal chelating and antioxidant properties. However, little is known about the ability of tropolone and its derivatives to protect cultured cells from oxidative stress-mediated damage. In this study, the effect of tropolones on hydrogen peroxide-induced DNA damage and apoptosis was investigated in cultured Jurkat cells. Tropolone, added to the cells 15 min before the addition of glucose oxidase, provided a dose dependent protection against hydrogen peroxide induced DNA damage. The IC50 value observed was about 15 microM for tropolone. Similar dose dependent protection was also observed with three other tropolone derivatives such as trimethylcolchicinic acid, purpurogallin and beta-thujaplicin (the IC50 values were 34, 70 and 74 microM, respectively), but not with colchicine and tetramethyl purpurogallin ester. Hydrogen peroxide-induced apoptosis was also inhibited by tropolone. However, in the absence of exogenous H2O2 but in the presence of non-toxic concentrations of exogenous iron (100 microM Fe3+), tropolone dramatically increased the formation of single strand breaks at molar ratios of tropolone to iron lower than 3 to 1, while, when the ratio increased over 3, no toxicity was observed. In conclusion, the results presented in this study indicate that the protection offered by tropolone against hydrogen peroxide-induced DNA damage and apoptosis was due to formation of a redox-inactive iron complex, while its enhancement of iron-mediated DNA damage at ratios of [tropolone]/[Fe3+] lower than 3, was due to formation of a lipophilic iron complex which facilitates iron transport through cell membrane in a redox-active form.

Does the calcein-AM method assay the total cellular 'labile iron pool' or only a fraction of it?

The calcein-AM (calcein-acetoxymethyl ester) method is a widely used technique that is supposed to assay the intracellular 'labile iron pool' (LIP). When cells in culture are exposed to this ester, it passes the plasma membrane and reacts with cytosolic unspecific esterases. One of the reaction products, calcein, is a fluorochrome and a hydrophilic alcohol to which membranes are non-permeable and which, consequently, is retained within the cytosol of cells. Calcein fluorescence is quenched following chelation of low-mass labile iron, and the degree of quenching gives an estimate of the amounts of chelatable iron. However, a requirement for the assay to be able to demonstrate cellular LIP in total is that such iron be localized in the cytosol and not in a membrane-limited compartment. For some time it has been known that a major part of cellular, redox-active, labile, low-mass iron is temporarily localized in the lysosomal compartment as a result of the autophagic degradation of ferruginous materials, such as mitochondrial complexes and ferritin. Even if some calcein-AM may escape cytosolic esterases and enter lysosomes to be cleaved by lysosomal acidic esterases, the resulting calcein does not significantly chelate iron at Collapse

Key Words

• autophagy
• calcein
• iron
• labile iron pool
• lysosomes
• oxidative stress
• calcein-am, calcein-acetoxymethyl ester
• dmem, dulbecco's modified eagle's medium
• lip, labile iron pool
• sih, salicylaldehyde isonicotinoyl hydrazone
• ssm, sulfide-silver method

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MESH Headings

• Cell Membrane Permeability
• Cytosol/enzymology
• Esterases/metabolism
• Esters/analysis
• Esters/metabolism
• Fluoresceins/analysis
• Fluoresceins/metabolism
• HeLa Cells
• Humans
• Hydrogen-Ion Concentration
• Iron/analysis
• Iron/metabolism
• Iron Chelating Agents/analysis
• Iron Chelating Agents/metabolism
• Kinetics
• Lysosomes/metabolism
• Oxidative Stress
• Phagocytosis
• Substrate Specificity

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Grants Collapse

Affiliation(s)

Margarita Tenopoulou *Laboratory of Biological Chemistry, University of Ioannina Medical School, 451 10 Ioannina, Greece
Tino Kurz †Department of Pharmacology, University of Linköping, SE-581 85 Linköping, Sweden
Paschalis-Thomas Doulias *Laboratory of Biological Chemistry, University of Ioannina Medical School, 451 10 Ioannina, Greece
Dimitrios Galaris *Laboratory of Biological Chemistry, University of Ioannina Medical School, 451 10 Ioannina, Greece
Ulf T. Brunk †Department of Pharmacology, University of Linköping, SE-581 85 Linköping, Sweden

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Involvement of heat shock protein-70 in the mechanism of hydrogen peroxide-induced DNA damage: the role of lysosomes and iron

Heat shock protein-70 (Hsp70) is the main heat-inducible member of the 70-kDa family of chaperones that assist cells in maintaining proteins functional under stressful conditions. In the present investigation, the role of Hsp70 in the molecular mechanism of hydrogen peroxide-induced DNA damage to HeLa cells in culture was examined. Stably transfected HeLa cell lines, overexpressing or lacking Hsp70, were created by utilizing constitutive expression of plasmids containing the functional hsp70 gene or hsp70-siRNA, respectively. Compared to control cells, the Hsp70-overexpressing ones were significantly resistant to hydrogen peroxide-induced DNA damage, while Hsp70-depleted cells showed an enhanced sensitivity. In addition, the "intracellular calcein-chelatable iron pool" was determined in the presence or absence of Hsp70 and found to be related to the sensitivity of nuclear DNA to H(2)O(2). It seems likely that the main action of Hsp70, at least in this system, is exerted at the lysosomal level, by protecting the membranes of these organelles against oxidative stress-induced destabilization. Apart from shedding additional light on the mechanistic details behind the action of Hsp70 during oxidative stress, our results indicate that modulation of cellular Hsp70 may represent a way to make cancer cells more sensitive to normal host defense mechanisms or chemotherapeutic drug treatment.

Role of compartmentalized redox-active iron in hydrogen peroxide-induced DNA damage and apoptosis

Jurkat cells in culture were exposed to oxidative stress in the form of continuously generated hydrogen peroxide, obtained by the addition of glucose oxidase to the medium. This treatment induced a rapid, dose-dependent increase in the ICIP (intracellular calcein-chelatable iron pool). Early destabilization of lysosomal membranes and subsequent nuclear DNA strand breaks were also observed, as evaluated by the Acridine Orange relocation test and the comet assay respectively. Somewhat later, these effects were followed by a lowered mitochondrial membrane potential, with release of cytochrome c and apoptosis-inducing factor. These events were all prevented if cells were pretreated with the potent iron chelator DFO (desferrioxamine) for a period of time (2-3 h) long enough to allow the drug to reach the lysosomal compartment following fluid-phase endocytosis. The hydrophilic calcein, a cleavage product of calcein acetoxymethyl ester following the action of cytosolic esterases, obviously does not penetrate intact lysosomal membranes, thus explaining why ICIP increased dramatically following lysosomal rupture. The rapid decrease in ICIP after addition of DFO to the medium suggests draining of cytosolic iron to the medium, rather than penetration of DFO through the plasma membrane. Most importantly, these observations directly connect oxidative stress and resultant DNA damage with lysosomal rupture and the release of redox-active iron into the cytosol and, apparently, the nucleus.

Endosomal and lysosomal effects of desferrioxamine: protection of HeLa cells from hydrogen peroxide-induced DNA damage and induction of cell-cycle arrest

The role of endosomal/lysosomal redox-active iron in H2O2-induced nuclear DNA damage as well as in cell proliferation was examined using the iron chelator desferrioxamine (DFO). Transient transfections of HeLa cells with vectors encoding dominant proteins involved in the regulation of various routes of endocytosis (dynamin and Rab5) were used to show that DFO (a potent and rather specific iron chelator) enters cells by fluid-phase endocytosis and exerts its effects by chelating redox-active iron present in the endosomal/lysosomal compartment. Endocytosed DFO effectively protected cells against H2O2-induced DNA damage, indicating the importance of endosomal/lysosomal redox-active iron in these processes. Moreover, exposure of cells to DFO in a range of concentrations (0.1 to 100 microM) inhibited cell proliferation in a fluid-phase endocytosis-dependent manner. Flow cytometric analysis of cells exposed to 100 microM DFO for 24 h showed that the cell cycle was transiently interrupted at the G2/M phase, while treatment for 48 h led to permanent cell arrest. Collectively, the above results clearly indicate that DFO has to be endocytosed by the fluid-phase pathway to protect cells against H2O2-induced DNA damage. Moreover, chelation of iron in the endosomal/lysosomal cell compartment leads to cell cycle interruption, indicating that all cellular labile iron is propagated through this compartment before its anabolic use is possible.

DNA damage and apoptosis in hydrogen peroxide-exposed Jurkat cells: bolus addition versus continuous generation of H(2)O(2)

Aspects of the molecular mechanism(s) of hydrogen peroxide-induced DNA damage and cell death were studied in the present investigation. Jurkat T-cells in culture were exposed either to low rates of continuously generated H(2)O(2) by the action of glucose oxidase or to a bolus addition of the same agent. In the first case, steady state conditions were prevailing, while in the latter, H(2)O(2) was removed by the cellular defense systems following first order kinetics. By using single-cell gel electrophoresis (also called comet assay), an initial increase in the formation of DNA single-strand breaks was observed in cells exposed to a bolus of 150 microM H(2)O(2). As the H(2)O(2) was exhausted, a gradual decrease in DNA damage was apparent, indicating the existence of an effective repair of single-strand breaks. Addition of 10 ng glucose oxidase in 100 microl growth medium (containing 1.5 x 10(5) cells) generated 2.0 +/- 0.2 microM H(2)O(2) per min. This treatment induced an increase in the level of single-strand breaks reaching the upper limit of detection by the methodology used and continued to be high for the following 6 h. However, when a variety of markers for apoptotic cell death (DNA cell content, DNA laddering, activation of caspases, PARP cleavage) were examined, only bolus additions of H(2)O(2) were able to induce apoptosis, while the continuous presence of this agent inhibited the execution of the apoptotic process no matter whether the inducer was H(2)O(2) itself or an anti-Fas antibody. These observations stress that, apart from the apparent genotoxic and proapoptotic effects of H(2)O(2), it can also exert antiapoptotic actions when present, even at low concentrations, during the execution of apoptosis.

Intracellular iron, but not copper, plays a critical role in hydrogen peroxide-induced DNA damage

The role of intracellular iron, copper, and calcium in hydrogen peroxide-induced DNA damage was investigated using cultured Jurkat cells. The cells were exposed to low rates of continuously generated hydrogen peroxide by the glucose/glucose oxidase system, and the formation of single strand breaks in cellular DNA was evaluated by the sensitive method, single cell gel electrophoresis or "comet" assay. Pre-incubation with the specific ferric ion chelator desferrioxamine (0.1-5.0 mM) inhibited DNA damage in a time- and dose-dependent manner. On the other hand, diethylenetriaminepentaacetic acid (DTPA), a membrane impermeable iron chelator, was ineffective. The lipophilic ferrous ion chelator 1,10-phenanthroline also protected against DNA damage, while its nonchelating isomer 1,7-phenanthroline provided no protection. None of the above iron chelators produced DNA damage by themselves. In contrast, the specific cuprous ion chelator neocuproine (2,9-dimethyl-1,10-phenanthroline), as well as other copper-chelating agents, did not protect against H(2)O(2)-induced cellular DNA damage. In fact, membrane permeable copper-chelating agents induced DNA damage in the absence of H(2)O(2). These results indicate that, under normal conditions, intracellular redox-active iron, but not copper, participates in H(2)O(2)-induced single strand break formation in cellular DNA. Since BAPTA/AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester), an intracellular Ca(2+)-chelator, also protected against H(2)O(2)-induced DNA damage, it is likely that intracellular Ca(2+) changes are involved in this process as well. The exact role of Ca(2+) and its relation to intracellular transition metal ions, in particular iron, needs to be further investigated.

SIN-1-induced DNA damage in isolated human peripheral blood lymphocytes as assessed by single cell gel electrophoresis (comet assay)

Human lymphocytes were exposed to increasing concentrations of SIN-1, which generates superoxide and nitric oxide, and the formation of single-strand breaks (SSB) in individual cells was determined by the single-cell gel electrophoresis assay (comet assay). A dose- and time-dependent increase in SSB formation was observed rapidly after the addition of SIN-1 (0.1-15 mM). Exposure of the cells to SIN-1 (5 mM) in the presence of excess of superoxide dismutase (0.375 mM) increased the formation of SSB significantly, whereas 1000 U/ml catalase significantly decreased the quantity of SSB. The simultaneous presence of both superoxide dismutase and catalase before the addition of SIN-1 brought the level of SSB to that of the untreated cells. Moreover, pretreatment of the cells with the intracellular Ca(2+)-chelator BAPTA/AM inhibited SIN-1-induced DNA damage, indicating the involvement of intracellular Ca(2+) changes in this process. On the other hand, pretreatment of the same cells with ascorbate or dehydroascorbate did not offer any significant protection in this system. The data suggest that H2O2-induced changes in Ca(2+) homeostasis are the predominant pathway for the induction of SSB in human lymphocytes exposed to oxidants.