Hydroethidine- and MitoSOX-derived red fluorescence is not a reliable indicator of intracellular superoxide formation: another inconvenient truth

Rapid and specific measurements of superoxide using fluorescence spectroscopy

Superoxide plays a key role in many pathological processes; however, detection of superoxide by one of the most common methods using dihydroethidium (DHE) may be unspecific because of overlapping fluorescence of the superoxide-specific product, 2-OH-ethidium (2OH-E), and the unspecific oxidation product, ethidium. Here, we show a new optimized fluorescence spectroscopy protocol that allows rapid and specific detection of superoxide in cell-free systems and intact cells using DHE. We defined new optimized fluorescent settings to measure the superoxide-specific product and minimize the interference of unspecific DHE oxidation products. Using this protocol, we studied real-time superoxide production by xanthine oxidase- and menadione-treated cultured cells. Specificity of the plate reader-based superoxide measurements was confirmed by the inhibition of fluorescence with superoxide dismutase and high-performance liquid chromatography (HPLC) analysis. We show that limitations of the HPLC-based analysis can be overcome by the optimized fluorescence spectroscopy.

Detection of reactive oxygen species derived from the family of NOX NADPH oxidases

NADPH oxidases (NOX) are superoxide anion radical (O(2)(-•))-generating enzymes. They form a family of seven members, each with a specific tissue distribution. They function as electron transport chains across membranes, using NADPH as electron donor to reduce molecular oxygen to O(2)(-•). NOX have multiple biological functions, ranging from host defense to inflammation and cellular signaling. Measuring NOX activity is crucial in understanding the roles of these enzymes in physiology and pathology. Many of the methods used to measure NOX activity are based on the detection of small molecules that react with NOX-generated O(2)(-•) or its direct dismutation product hydrogen peroxide (H(2)O(2)) to form fluorescent, luminescent, or colored products. Initial techniques were developed to measure the activity of the phagocyte isoform NOX2 during the oxidative burst of stimulated polymorphonuclear leukocytes, which generate large quantities of O(2)(-•). However, other members of the NOX family generate much less O(2)(-•) and hence H(2)O(2), and their activity is difficult to distinguish from other sources of these reactive species. In addition, O(2)(-•) and H(2)O(2) are reactive molecules and most probes are prone to artifacts and therefore should be used with appropriate controls and the data carefully interpreted. This review gives an overview of current methods used to measure NOX activity and NOX-derived O(2)(-•) and H(2)O(2) in cells, tissues, isolated systems, and living organisms, describing the advantages and caveats of many established methods with emphasis on more recent technologies and future perspectives.

Poly(ADP-ribosyl)ation is a survival mechanism in cigarette smoke-induced and hydrogen peroxide-mediated cell death

Cigarette smoking can contribute to the development of many human diseases such as cardiovascular disease, lung cancer, asthma, and chronic obstructive pulmonary disease. Thousands of compounds are present in cigarette smoke, including a large number of reactive oxygen species that can cause DNA damage, leading to the activation of poly(ADP-ribose) polymerase (PARP) enzymes. The PAR polymer is degraded by poly(ADP-ribose) glycohydrolase (PARG). Here we have investigated the effects of cigarette smoke extract (CSE) on A549 human lung epithelial cells. CSE induced DNA damage (comet assay), PAR accumulation (immunofluorescence and immunoblotting), impaired proliferation (clonogenic survival assay and electric cell-substrate impedance sensing measurement), and cell death (MTT reduction, propidium iodide uptake, lactate dehydrogenase release). CSE-induced cell death was also characterized by mitochondrial depolarization but massive translocation of apoptosis-inducing factor could not be observed. To investigate the role of PARylation in CSE-induced oxidative stress, PARP-1- and PARG-silenced A549 cells were used. Silencing of both PARP-1 and PARG sensitized cells to CSE-induced toxicity: PARP-1- and PARG-silenced cell lines exhibited reduced clonogenic survival, displayed a delayed repair of DNA breaks, and showed higher levels of cytotoxicity. CSE triggered the production of mitochondrial superoxide and hydrogen peroxide. Addition of superoxide dismutase increased, whereas catalase abolished, CSE-induced PAR formation. In summary, our data show that the superoxide-hydrogen peroxide-DNA breakage pathway activates the PAR cycle by PARP-1 and PARG, which serves as a survival mechanism in CSE-exposed cells. Our data also raise the possibility that the PARP-1/PARG status of smokers may be an important determinant of the efficiency of DNA repair in their lungs and of their susceptibility to CS-induced carcinogenesis.

Probing oxidative stress: Small molecule fluorescent sensors of metal ions, reactive oxygen species, and thiols

Oxidative stress is a common feature shared by many diseases, including neurodegenerative diseases. Factors that contribute to cellular oxidative stress include elevated levels of reactive oxygen species, diminished availability of detoxifying thiols, and the misregulation of metal ions (both redox-active iron and copper as well as non-redox active calcium and zinc). Deciphering how each of these components interacts to contribute to oxidative stress presents an interesting challenge. Fluorescent sensors can be powerful tools for detecting specific analytes within a complicated cellular environment. Reviewed here are several classes of small molecule fluorescent sensors designed to detect several molecular participants of oxidative stress. We focus our review on describing the design, function and application of probes to detect metal cations, reactive oxygen species, and intracellular thiol-containing compounds. In addition, we highlight the intricacies and complications that are often faced in sensor design and implementation.

Mitochondria-targeted antioxidants and metabolic modulators as pharmacological interventions to slow ageing

Populations in many nations today are rapidly ageing. This unprecedented demographic change represents one of the main challenges of our time. A defining property of the ageing process is a marked increase in the risk of mortality and morbidity with age. The incidence of cancer, cardiovascular and neurodegenerative diseases increases non-linearly, sometimes exponentially with age. One of the most important tasks in biogerontology is to develop interventions leading to an increase in healthy lifespan (health span), and a better understanding of basic mechanisms underlying the ageing process itself may lead to interventions able to delay or prevent many or even all age-dependent conditions. One of the putative basic mechanisms of ageing is age-dependent mitochondrial deterioration, closely associated with damage mediated by reactive oxygen species (ROS). Given the central role that mitochondria and mitochondrial dysfunction play not only in ageing but also in apoptosis, cancer, neurodegeneration and other age-related diseases there is great interest in approaches to protect mitochondria from ROS-mediated damage. In this review, we explore strategies of targeting mitochondria to reduce mitochondrial oxidative damage with the aim of preventing or delaying age-dependent decline in mitochondrial function and some of the resulting pathologies. We discuss mitochondria-targeted and -localized antioxidants (e.g.: MitoQ, SkQ, ergothioneine), mitochondrial metabolic modulators (e.g. dichloroacetic acid), and uncouplers (e.g.: uncoupling proteins, dinitrophenol) as well as some alternative future approaches for targeting compounds to the mitochondria, including advances from nanotechnology.

Analysis of kinetics of dihydroethidium fluorescence with superoxide using xanthine oxidase and hypoxanthine assay

Superoxide (O(2) (-)) is an important reactive oxygen species (ROS), and has an essential role in physiology and pathophysiology. An accurate detection of O(2) (-) is needed to better understand numerous vascular pathologies. In this study, we performed a mechanistic study by using the xanthine oxidase (XOD)/hypoxanthine (HX) assay for O(2) (-) generation and a O(2) (-) sensitive fluorescent dye dihydroethidium (DHE) for O(2) (-) measurement. To quantify O(2) (-) and DHE interactions, we measured fluorescence using a microplate reader. We conducted a detailed reaction kinetic analysis for DHE-O(2) (-) interaction to understand the effect of O(2) (-) self-dismutation and to quantify DHE-O(2) (-) reaction rate. Fluorescence of DHE and 2-hydroethidium (EOH), a product of DHE and O(2) (-) interaction, were dependent on reaction conditions. Kinetic analysis resulted in a reaction rate constant of 2.169 ± 0.059 × 10(3) M(-1) s(-1) for DHE-O(2) (-) reaction that is ~100× slower than the reported value of 2.6 ± 0.6 × 10(5) M(-1) s(-1). In addition, the O(2) (-) self-dismutation has significant effect on DHE-O(2) (-) interaction. A slower reaction rate of DHE with O(2) (-) is more reasonable for O(2) (-) measurements. In this manner, the DHE is not competing with superoxide dismutase and NO for O(2) (-). Results suggest that an accurate measurement of O(2) (-) production rate may be difficult due to competitive interference for many factors; however O(2) (-) concentration may be quantified.

Light-Activated Pharmaceuticals: Mechanisms and Detection

Photodynamic therapy relies on the interaction between light, oxygen and a photosensitizing agent. Its medical significance relates to the ability of certain agents, usually based on porphyrin or phthalocyanine structures, to localize somewhat selectively in neoplastic cells and their vasculature. Subsequent irradiation, preferably at a sufficiently high wavelength to have a significant pathway through tissues, results in a photophysical reaction whereby the excited state of the photosensitizing agent transfers energy to molecular oxygen and results in the formation of reactive oxygen species. Analogous reactive nitrogen species are also formed. These contain both nitrogen and oxygen atoms. The net result is both direct tumor cell death and a shutdown of the tumor vasculature. Other processes may also occur that promote the anti-tumor response but these are outside the scope of this review.

In vivo quantification reveals extensive natural variation in mitochondrial form and function in Caenorhabditis briggsae

We have analyzed natural variation in mitochondrial form and function among a set of Caenorhabditis briggsae isolates known to harbor mitochondrial DNA structural variation in the form of a heteroplasmic nad5 gene deletion (nad5Δ) that correlates negatively with organismal fitness. We performed in vivo quantification of 24 mitochondrial phenotypes including reactive oxygen species level, membrane potential, and aspects of organelle morphology, and observed significant among-isolate variation in 18 traits. Although several mitochondrial phenotypes were non-linearly associated with nad5Δ levels, most of the among-isolate phenotypic variation could be accounted for by phylogeographic clade membership. In particular, isolate-specific mitochondrial membrane potential was an excellent predictor of clade membership. We interpret this result in light of recent evidence for local adaptation to temperature in C. briggsae. Analysis of mitochondrial-nuclear hybrid strains provided support for both mtDNA and nuclear genetic variation as drivers of natural mitochondrial phenotype variation. This study demonstrates that multicellular eukaryotic species are capable of extensive natural variation in organellar phenotypes and highlights the potential of integrating evolutionary and cell biology perspectives.

Actions of "antioxidants" in the protection against atherosclerosis

This review addresses the role of oxidative processes in atherosclerosis and its resulting cardiovascular disease by focusing on the outcome of antioxidant interventions. Although there is unambiguous evidence for the presence of heightened oxidative stress and resulting damage in atherosclerosis, it remains to be established whether this represents a cause or a consequence of the disease. This critical question is complicated further by the increasing realization that oxidative processes, including those related to signaling, are part of normal cell function. Overall, the results from animal interventions suggest that antioxidants provide benefit neither generally nor consistently. Where benefit is observed, it appears to be achieved at least in part via modulation of biological processes such as increase in nitric oxide bioavailability and induction of protective enzymes such as heme oxygenase-1, rather than via inhibition of oxidative processes and lipid oxidation in the arterial wall. Exceptions to this may be situations of multiple/excessive stress, the relevance of which for humans is not clear. This interpretation is consistent with the overall disappointing outcome of antioxidant interventions in humans and can be rationalized by the spatial compartmentalization of cellular oxidative signaling and/or damage, complex roles of oxidant-producing enzymes, and the multifactorial nature of atherosclerosis.

Activation of peroxisome proliferator-activated receptor-β/-δ (PPARβ/δ) prevents endothelial dysfunction in type 1 diabetic rats

Endothelial dysfunction plays a key role in the pathogenesis of diabetic vascular disease. Herein, we have analyzed if the peroxisome proliferator-activated receptor-β/-δ (PPARβ/δ) agonist GW0742 exerts protective effects on endothelial function in type 1 diabetic rats. The rats were divided into 4 groups: control, control-treated (GW0742, 5 mg kg(-1)day(-1) for 5 weeks), diabetic (streptozotocin injection), and diabetic-treated. GW0742 administration in diabetic rats did not alter plasma glucose, systolic blood pressure, or heart rate, but reduced plasma triglyceride levels. The vasodilatation induced by acetylcholine was decreased in aortas from diabetic rats. GW0742 restored endothelial function, increasing eNOS phosphorylation. Superoxide production, NADPH oxidase activity, and mRNA expression of prepro endothelin-1, p22(phox), p47(phox), and NOX-1 were significantly higher in diabetic aortas, and GW0742 treatment prevented these changes. In addition, GW0742 prevented the endothelial dysfunction and the upregulation of prepro endothelin-1 and p47(phox) after the in vitro incubation of aortic rings with high glucose and these effects were prevented by the PPARβ/δ antagonist GSK0660. PPARβ/δ activation restores endothelial function in type 1 diabetic rats. This effect seems to be related to an increase in nitric oxide bioavailability as a result of reduced NADPH oxidase-driven superoxide production and downregulation of prepro endothelin-1.

Interleukin-6 counteracts therapy-induced cellular oxidative stress in multiple myeloma by up-regulating manganese superoxide dismutase

IL (interleukin)-6, an established growth factor for multiple myeloma cells, induces myeloma therapy resistance, but the resistance mechanisms remain unclear. The present study determines the role of IL-6 in re-establishing intracellular redox homoeostasis in the context of myeloma therapy. IL-6 treatment increased myeloma cell resistance to agents that induce oxidative stress, including IR (ionizing radiation) and Dex (dexamethasone). Relative to IR alone, myeloma cells treated with IL-6 plus IR demonstrated reduced annexin/propidium iodide staining, caspase 3 activation, PARP [poly(ADP-ribose) polymerase] cleavage and mitochondrial membrane depolarization with increased clonogenic survival. IL-6 combined with IR or Dex increased early intracellular pro-oxidant levels that were causally related to activation of NF-κB (nuclear factor κB) as determined by the ability of N-acetylcysteine to suppress both pro-oxidant levels and NF-κB activation. In myeloma cells, upon combination with hydrogen peroxide treatment, relative to TNF (tumour necrosis factor)-α, IL-6 induced an early perturbation in reduced glutathione level and increased NF-κB-dependent MnSOD (manganese superoxide dismutase) expression. Furthermore, knockdown of MnSOD suppressed the IL-6-induced myeloma cell resistance to radiation. MitoSOX Red staining showed that IL-6 treatment attenuated late mitochondrial oxidant production in irradiated myeloma cells. The present study provides evidence that increases in MnSOD expression mediate IL-6-induced resistance to Dex and radiation in myeloma cells. The results of the present study indicate that inhibition of antioxidant pathways could enhance myeloma cell responses to radiotherapy and/or chemotherapy.

Aldosterone, oxidative stress, and NF-κB activation in hypertension-related cardiovascular and renal diseases

The mineralocorticoid aldosterone regulates electrolyte and fluid balance and is involved in blood pressure homoeostasis. Classically, it binds to its intracellular mineralocorticoid receptor to induce expression of proteins influencing the reabsorption of sodium and water in the distal nephron. Aldosterone gained special attention when large clinical studies showed that blocking its receptor in patients with cardiovascular diseases reduced their mortality. These patients present increased plasma aldosterone levels. The exact mechanisms of the potential toxic effects of aldosterone leading to cardiovascular damage are not known yet. The observation of reduced nitric oxide bioavailability in hyperaldosteronism implied the generation of oxidative stress by aldosterone. Subsequent studies confirmed the increase of oxidative stress markers in patients with chronic heart failure and in animal models of hyperaldosteronism. The effects of reactive oxygen species have been related to the activation of transcription factors, such as NF-κB. This review summarizes the present-day knowledge of aldosterone-induced oxidative stress and NF-κB activation in humans and different experimental models.

Alterations in bioenergetic function induced by Parkinson's disease mimetic compounds: lack of correlation with superoxide generation

In vitro and in vivo models of Parkinson's disease (PD) suggest that increased oxidant production leads to mitochondrial dysfunction in dopaminergic neurons and subsequent cell death. However, it remains unclear if cell death in these models is caused by inhibition of mitochondrial function or oxidant production. The objective of this study was to determine the relationship between mitochondrial dysfunction and oxidant production in response to multiple PD neurotoxicant mimetics. MPP(+) caused a dose-dependent decrease in the basal oxygen consumption rate in dopaminergic N27 cells, indicating a loss of mitochondrial function. In parallel, we found that MPP(+) only modestly increased oxidation of hydroethidine as a diagnostic marker of superoxide production in these cells. Similar results were found using rotenone as a mitochondrial inhibitor, or 6-hydroxydopamine (6-OHDA) as a mechanistically distinct PD neurotoxicant, but not with exposure to paraquat. In addition, the extracellular acidification rate, used as a marker of glycolysis, was stimulated to compensate for oxygen consumption rate inhibition after exposure to MPP(+), rotenone, or 6-OHDA, but not paraquat. Together these data indicate that MPP(+), rotenone, and 6-OHDA dramatically shift bioenergetic function away from the mitochondria and towards glycolysis in N27 cells.

Peroxiredoxin 3 is a redox-dependent target of thiostrepton in malignant mesothelioma cells

Thiostrepton (TS) is a thiazole antibiotic that inhibits expression of FOXM1, an oncogenic transcription factor required for cell cycle progression and resistance to oncogene-induced oxidative stress. The mechanism of action of TS is unclear and strategies that enhance TS activity will improve its therapeutic potential. Analysis of human tumor specimens showed FOXM1 is broadly expressed in malignant mesothelioma (MM), an intractable tumor associated with asbestos exposure. The mechanism of action of TS was investigated in a cell culture model of human MM. As for other tumor cell types, TS inhibited expression of FOXM1 in MM cells in a dose-dependent manner. Suppression of FOXM1 expression and coincidental activation of ERK1/2 by TS were abrogated by pre-incubation of cells with the antioxidant N-acetyl-L-cysteine (NAC), indicating its mechanism of action in MM cells is redox-dependent. Examination of the mitochondrial thioredoxin reductase 2 (TR2)-thioredoxin 2 (TRX2)-peroxiredoxin 3 (PRX3) antioxidant network revealed that TS modifies the electrophoretic mobility of PRX3. Incubation of recombinant human PRX3 with TS in vitro also resulted in PRX3 with altered electrophoretic mobility. The cellular and recombinant species of modified PRX3 were resistant to dithiothreitol and SDS and suppressed by NAC, indicating that TS covalently adducts cysteine residues in PRX3. Reduction of endogenous mitochondrial TRX2 levels by the cationic triphenylmethane gentian violet (GV) promoted modification of PRX3 by TS and significantly enhanced its cytotoxic activity. Our results indicate TS covalently adducts PRX3, thereby disabling a major mitochondrial antioxidant network that counters chronic mitochondrial oxidative stress. Redox-active compounds like GV that modify the TR2/TRX2 network may significantly enhance the efficacy of TS, thereby providing a combinatorial approach for exploiting redox-dependent perturbations in mitochondrial function as a therapeutic approach in mesothelioma.

Croix CM, Watkins SC, Fleming AM, Champion HC, Mason RP, Freeman BA, Kelley EE. Obesity-induced tissue free radical generation: an in vivo immuno-spin trapping study

Assessment of tissue free radical production is routinely accomplished by measuring secondary by-products of redox reactions and/or diminution of key antioxidants such as reduced thiols. However, immuno-spin trapping, a newly developed immunohistochemical technique for detection of free radical formation, is garnering considerable interest as it allows for the visualization of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-adducted molecules. Yet, to date, immuno-spin trapping reports have utilized in vivo models in which successful detection of free radical adducts required exposure to lethal levels of oxidative stress not reflective of chronic inflammatory disease. To study the extents and anatomic locations of more clinically relevant levels of radical formation, we examined tissues from high-fat (HF) diet-fed mice, a model of low-grade chronic inflammation known to demonstrate enhanced rates of reactive species production. Mice subjected to 20 weeks of HF diet displayed increased free radical formation (anti-DMPO mean fluorescence staining) in skeletal muscle (0.863±0.06 units vs 0.512±0.07 units), kidney (0.076±0.0036 vs 0.043±0.0025), and liver (0.275±0.012 vs 0.135±0.014) compared to control mice fed normal laboratory chow (NC). Western blot analysis of tissue homogenates confirmed these results showing enhanced DMPO immunoreactivity in HF mice compared to NC samples. The obesity-related results were confirmed in a rat model of pulmonary hypertension and right heart failure in which intense immunodetectable radical formation was observed in the lung and right ventricle of monocrotaline-treated rats compared to saline-treated controls. Combined, these data affirm the utility of immuno-spin trapping as a tool for in vivo assessment of altered extents of macromolecule oxidation to radical intermediates under chronic inflammatory conditions.

Mitochondrial compartmentalization of redox processes

Knowledge of location and intracellular subcompartmentalization is essential for the understanding of redox processes, because oxidants, owing to their reactive nature, must be generated close to the molecules modified in both signaling and damaging processes. Here we discuss known redox characteristics of various mitochondrial microenvironments. Points covered are the locations of mitochondrial oxidant generation, characteristics of antioxidant systems in various mitochondrial compartments, and diffusion characteristics of oxidants in mitochondria. We also review techniques used to measure redox state in mitochondrial subcompartments, antioxidants targeted to mitochondrial subcompartments, and methodological concerns that must be addressed when using these tools.

Mitochondrial superoxide production contributes to vancomycin-induced renal tubular cell apoptosis

Vancomycin chloride (VCM), a glycopeptide antibiotic, is widely used for the therapy of infections caused by methicillin-resistant Staphylococcus aureus. However, nephrotoxicity is a major adverse effect in VCM therapy. In this study, we investigated the cellular mechanisms underlying VCM-induced renal tubular cell injury in cultured LLC-PK1 cells. VCM induced a concentration- and time-dependent cell injury in LLC-PK1 cells. VCM caused increases in the numbers of annexin V-positive/PI-negative cells and TUNEL-positive cells, indicating the involvement of apoptotic cell death in VCM-induced renal cell injury. The VCM-induced apoptosis was accompanied by the activation of caspase-9 and caspase-3/7 and reversed by inhibitors of these caspases. Moreover, VCM caused an increase in intracellular reactive oxygen species production and mitochondrial membrane depolarization, which were reversed by vitamin E. In addition, mitochondrial complex I activity was inhibited by VCM as well as by the complex I inhibitor rotenone, and rotenone mimicked the VCM-induced LLC-PK1 cell injury. These findings suggest that VCM causes apoptotic cell death in LLC-PK1 cells by enhancing mitochondrial superoxide production leading to mitochondrial membrane depolarization followed by the caspase activities. Moreover, mitochondrial complex I may play an important role in superoxide production and renal tubular cell apoptosis induced by VCM.

Succinobucol induces apoptosis in vascular smooth muscle cells

Probucol inhibits the proliferation of vascular smooth muscle cells in vitro and in vivo, and the drug reduces intimal hyperplasia and atherosclerosis in animals via induction of heme oxygenase-1 (HO-1). Because the succinyl ester of probucol, succinobucol, recently failed as an antiatherogenic drug in humans, we investigated its effects on smooth muscle cell proliferation. Succinobucol and probucol induced HO-1 and decreased cell proliferation in rat aortic smooth muscle cells. However, whereas inhibition of HO-1 reversed the antiproliferative effects of probucol, this was not observed with succinobucol. Instead, succinobucol but not probucol induced caspase activity and apoptosis, and it increased mitochondrial oxidation of hydroethidine to ethidium, suggestive of the participation of H(2)O(2) and cytochrome c. Also, succinobucol but not probucol converted cytochrome c into a peroxidase in the presence of H(2)O(2), and succinobucol-induced apoptosis was decreased in cells that lacked cytochrome c or a functional mitochondrial complex II. In addition, succinobucol increased apoptosis of vascular smooth muscle cells in vivo after balloon angioplasty-mediated vascular injury. Our results suggest that succinobucol induces apoptosis via a pathway involving mitochondrial complex II, H(2)O(2), and cytochrome c. These unexpected results are discussed in light of the failure of succinobucol as an antiatherogenic drug in humans.

Evaluation of diethyl-3-3'-(9,10-anthracenediyl)bis acrylate as a probe for singlet oxygen formation during photodynamic therapy

The cell-permeable anthracene analog diethyl-3-3'-(9,10-anthracenediyl)bis acrylate (DADB) was recently identified as a highly selective probe for singlet oxygen ((1)O(2)). Now, we show that DADB can be used to monitor (1)O(2) formation in cell culture during photodynamic therapy. An atypical property of DADB is that fluorescence emission is decreased upon oxidation. Using photosensitizers that target specific organelles, we determined that DADB could detect (1)O(2) whether formed in ER, mitochondria or lysosomes. DADB fluorescence was not, however, significantly altered when the photosensitizing agent was the palladium bacteriopheophorbide termed WST11, an agent reported to produce mainly oxygen radicals upon irradiation in an aqueous environment, whereas singlet oxygen was formed in organic solvents.

Metabolic state determines sensitivity to cellular stress in Huntington disease: normalization by activation of PPARγ

Impairments in mitochondria and transcription are important factors in the pathogenesis of Huntington disease (HD), a neurodegenerative disease caused by a polyglutamine expansion in the huntingtin protein. This study investigated the effect of different metabolic states and peroxisome proliferator-activated receptor γ (PPARγ) activation on sensitivity to cellular stressors such as H(2)O(2) or thapsigargin in HD. Striatal precursor cells expressing wild type (STHdh(Q7)) or mutant huntingtin (STHdh(Q111)) were prepared in different metabolic conditions (glucose vs. pyruvate). Due to the fact that STHdh(Q111) cells exhibit mitochondrial deficits, we expected that in the pyruvate condition, where ATP is generated primarily by the mitochondria, there would be greater differences in cell death between the two cell types compared to the glucose condition. Intriguingly, it was the glucose condition that gave rise to greater differences in cell death. In the glucose condition, thapsigargin treatment resulted in a more rapid loss of mitochondrial membrane potential (ΔΨm), a greater activation of caspases (3, 8, and 9), and a significant increase in superoxide/reactive oxygen species (ROS) in STHdh(Q111) compared to STHdh(Q7), while both cell types showed similar kinetics of ΔΨm-loss and similar levels of superoxide/ROS in the pyruvate condition. This suggests that bioenergetic deficiencies are not the primary contributor to the enhanced sensitivity of STHdh(Q111) cells to stressors compared to the STHdh(Q7) cells. PPARγ activation significantly attenuated thapsigargin-induced cell death, concomitant with an inhibition of caspase activation, a delay in ΔΨm loss, and a reduction of superoxide/ROS generation in STHdh(Q111) cells. Expression of mutant huntingtin in primary neurons induced superoxide/ROS, an effect that was significantly reduced by constitutively active PPARγ. These results provide significant insight into the bioenergetic disturbances in HD with PPARγ being a potential therapeutic target for HD.

The contribution of oxidative stress to drug-induced organ toxicity and its detection in vitro and in vivo

INTRODUCTION

Nowadays the 'redox hypothesis' is based on the fact that thiol/disulfide couples such as glutathione (GSH/GSSG), cysteine (Cys/CySS) and thioredoxin ((Trx-(SH)2/Trx-SS)) are functionally organized in redox circuits controlled by glutathione pools, thioredoxins and other control nodes, and they are not in equilibrium relative to each other. Although ROS can be important intermediates of cellular signaling pathways, disturbances in the normal cellular redox can result in widespread damage to several cell components. Moreover, oxidative stress has been linked to a variety of age-related diseases. In recent years, oxidative stress has also been identified to contribute to drug-induced liver, heart, renal and brain toxicity.

AREAS COVERED

This review provides an overview of current in vitro and in vivo methods that can be deployed throughout the drug discovery process. In addition, animal models and noninvasive biomarkers are described.

EXPERT OPINION

Reducing post-market drug withdrawals is essential for all pharmaceutical companies in a time of increased patient welfare and tight budgets. Predictive screens positioned early in the drug discovery process will help to reduce such liabilities. Although new and more efficient assays and models are being developed, the hunt for biomarkers and noninvasive techniques is still in progress.

Analysis of radicals and radical reaction products in cell signalling and biomolecular damage: the long hard road to gold-standard measures

The field of free radical biology and medicine continues to move at a tremendous pace, with a constant flow of ground-breaking discoveries. The following collection of papers in this issue of Biochemical Society Transactions highlights several key areas of topical interest, including the crucial role of validated measurements of radicals and reactive oxygen species in underpinning nearly all research in the field, the important advances being made as a result of the overlap of free radical research with the reinvigorated field of lipidomics (driven in part by innovations in MS-based analysis), the acceleration of new insights into the role of oxidative protein modifications (particularly to cysteine residues) in modulating cell signalling, and the effects of free radicals on the functions of mitochondria, extracellular matrix and the immune system. In the present article, we provide a brief overview of these research areas, but, throughout this discussion, it must be remembered that it is the availability of reliable analytical methodologies that will be a key factor in facilitating continuing developments in this exciting research area.

Measuring reactive oxygen and nitrogen species with fluorescent probes: challenges and limitations

The purpose of this position paper is to present a critical analysis of the challenges and limitations of the most widely used fluorescent probes for detecting and measuring reactive oxygen and nitrogen species. Where feasible, we have made recommendations for the use of alternate probes and appropriate analytical techniques that measure the specific products formed from the reactions between fluorescent probes and reactive oxygen and nitrogen species. We have proposed guidelines that will help present and future researchers with regard to the optimal use of selected fluorescent probes and interpretation of results.

Vision preservation during retinal inflammation by anthocyanin-rich bilberry extract: cellular and molecular mechanism

Anthocyanin-rich bilberry extract, a plant-derived antioxidant, has been utilized as a popular supplement for ocular health worldwide. However, it is unclear whether this extract has any biological effect on visual function, and the mechanism for such an effect is completely unknown. In this study, we generated a mouse model of endotoxin-induced uveitis (EIU) that shows retinal inflammation, as well as uveitis, by injecting lipopolysaccharide. We pretreated the mice with anthocyanin-rich bilberry extract and analyzed the effect on the retina. Anthocyanin-rich bilberry extract prevented the impairment of photoreceptor cell function, as measured by electroretinogram. At the cellular level, we found that the EIU-associated rhodopsin decreased and the shortening of outer segments in photoreceptor cells were suppressed in the bilberry-extract-treated animals. Moreover, the extract prevented both STAT3 activation, which induces inflammation-related rhodopsin decrease, and the increase in interleukin-6 expression, which activates STAT3. In addition to its anti-inflammatory effect, the anthocyanin-rich bilberry extract ameliorated the intracellular elevation of reactive oxygen species and activated NF-κB, a redox-sensitive transcription factor, in the inflamed retina. Our findings indicate that anthocyanin-rich bilberry extract has a protective effect on visual function during retinal inflammation.

Global profiling of reactive oxygen and nitrogen species in biological systems: high-throughput real-time analyses

Herein we describe a high-throughput fluorescence and HPLC-based methodology for global profiling of reactive oxygen and nitrogen species (ROS/RNS) in biological systems. The combined use of HPLC and fluorescence detection is key to successful implementation and validation of this methodology. Included here are methods to specifically detect and quantitate the products formed from interaction between the ROS/RNS species and the fluorogenic probes, as follows: superoxide using hydroethidine, peroxynitrite using boronate-based probes, nitric oxide-derived nitrosating species with 4,5-diaminofluorescein, and hydrogen peroxide and other oxidants using 10-acetyl-3,7-dihydroxyphenoxazine (Amplex® Red) with and without horseradish peroxidase, respectively. In this study, we demonstrate real-time monitoring of ROS/RNS in activated macrophages using high-throughput fluorescence and HPLC methods. This global profiling approach, simultaneous detection of multiple ROS/RNS products of fluorescent probes, developed in this study will be useful in unraveling the complex role of ROS/RNS in redox regulation, cell signaling, and cellular oxidative processes and in high-throughput screening of anti-inflammatory antioxidants.

Mitochondria-ros crosstalk in the control of cell death and aging

Reactive oxygen species (ROS) are highly reactive molecules, mainly generated inside mitochondria that can oxidize DNA, proteins, and lipids. At physiological levels, ROS function as “redox messengers” in intracellular signalling and regulation, whereas excess ROS induce cell death by promoting the intrinsic apoptotic pathway. Recent work has pointed to a further role of ROS in activation of autophagy and their importance in the regulation of aging. This review will focus on mitochondria as producers and targets of ROS and will summarize different proteins that modulate the redox state of the cell. Moreover, the involvement of ROS and mitochondria in different molecular pathways controlling lifespan will be reported, pointing out the role of ROS as a “balance of power,” directing the cell towards life or death.

Mitochondrial injury after mechanical stretch of cortical neurons in vitro: biomarkers of apoptosis and selective peroxidation of anionic phospholipids

Mechanical injury of neurites accompanied by rupture of mitochondrial membranes may lead to immediate nonspecific release and spreading of pro-apoptotic factors and activation of proteases, that is, execution of apoptotic program. In the current work, we studied the time course of the major biomarkers of apoptosis as they are induced by exposure of rat cortical neurons to mechanical stretch. By using transmission electron microscopy, we found that mitochondria in the neurites were damaged early (1 h) after mechanical stretch injury whereas somal mitochondria were significantly more resistant and demonstrated structural damage and degenerative mitochondrial changes at a later time point after stretch (12 h). We also report that the stretch injury caused immediate activation of reactive oxygen species production followed by selective oxidation of a mitochondria-specific phospholipid, cardiolipin, whose individual peroxidized molecular species have been identified and quantified by electrospray ionization mass spectrometry analysis. Most abundant neuronal phospholipids - phosphatidylcholine, phophatidylethanolamine - did not undergo oxidative modification. Simultaneously, a small-scale release of cytochrome c was observed. Notably, caspase activation and phosphatidylserine externalization - two irreversible apoptotic events designating a point of no return - are substantially delayed and do not occur until 6-12 h after the initial impact. The early onset of reactive oxygen species production and cytochrome c release may be relevant to direct stretch-induced damage to mitochondria. The delayed emergence of apoptotic neuronal death after the immediate mechanical damage to mitochondria suggests a possible window of opportunity for targeted therapies.

Measuring mitochondrial function in intact cardiac myocytes

Mitochondria are involved in cellular functions that go beyond the traditional role of these organelles as the power plants of the cell. Mitochondria have been implicated in several human diseases, including cardiac dysfunction, and play a role in the aging process. Many aspects of our knowledge of mitochondria stem from studies performed on the isolated organelle. Their relative inaccessibility imposes experimental difficulties to study mitochondria in their natural environment-the cytosol of intact cells-and has hampered a comprehensive understanding of the plethora of mitochondrial functions. Here we review currently available methods to study mitochondrial function in intact cardiomyocytes. These methods primarily use different flavors of fluorescent dyes and genetically encoded fluorescent proteins in conjunction with high-resolution imaging techniques. We review methods to study mitochondrial morphology, mitochondrial membrane potential, Ca(2+) and Na(+) signaling, mitochondrial pH regulation, redox state and ROS production, NO signaling, oxygen consumption, ATP generation and the activity of the mitochondrial permeability transition pore. Where appropriate we complement this review on intact myocytes with seminal studies that were performed on isolated mitochondria, permeabilized cells, and in whole hearts.

Oxidative chemistry of fluorescent dyes: implications in the detection of reactive oxygen and nitrogen species

HE (hydroethidine), a widely used fluorescent dye for detecting intracellular superoxide, undergoes specific oxidation and hydroxylation reactions. The reaction between HE and O2•− (superoxide radical) yields a diagnostic marker product, 2-hydroxyethidium. This is contrary to the popular notion that O2•− oxidizes HE to form ethidium. HE, however, undergoes a non-specific oxidation to form ethidium in the presence of other oxidants (hydroxyl radical, peroxynitrite and perferryl iron) and other dimeric products. The mitochondria-targeted HE analogue Mito-SOX® undergoes the same type of oxidative chemistry to form products similar to those formed from HE. On the basis of the oxidative chemical mechanism of HE and Mito-SOX®, we conclude that flurorescence microscopy or related techniques are not sufficient to measure the superoxide-specific hydroxylated products. HPLC methodologies are required to separate and identify these products. Peroxynitrite reacts rapidly and stoichiometrically with boronates to form specific products. Assays using fluorescent-based boronate probes will be more reliable for peroxynitrite determination than those using either dichlorodihydrofluorescein or dihydrorhodamine.

Cytometric assessment of mitochondria using fluorescent probes

Mitochondria are most important organelles in the survival of eukaryotic aerobic cells because they are the primary producers of ATP, regulators of ion homeostasis or redox state, and producers of free radicals. The key role of mitochondria in the generation of primordial ATP for the survival and proliferation of eukaryotic cells has been proven by extensive biochemical studies. In this context, it is crucial to understand the complexity of the mitochondrial compartment and its functionality and to develop experimental tools allowing the assessment of its nature and its function and metabolism. This review covers the role of the mitochondria in the cell, focusing on its structure, the mechanism of the mitochondrial respiratory chain, the maintenance of the transmembrane potential and the production of reactive oxygen species. The main probes used for mitochondrial compartment monitoring are described. In addition, various applications using mitochondrial-specific probes are detailed to illustrate the potential of flow and image cytometry in the study of the mitochondrial compartment. This review contains a panel of tools to explore mitochondria and to help researchers design experiments, determine the approach to be employed, and interpret their results.

Biophysical properties and functional consequences of reactive oxygen species (ROS)-induced ROS release in intact myocardium

Reactive oxygen species (ROS)-induced ROS release (RIRR) is a fundamental mechanism by which cardiac mitochondria respond to elevated ROS levels by stimulating endogenous ROS production in a regenerative, autocatalytic process that ultimately results in global oxidative stress (OS), cellular dysfunction and death. Despite elegant studies describing the phenomenon of RIRR under artificial conditions such as photo-induced oxidation of discrete regions within cardiomyocytes, the existence, biophysical properties and functional consequences of RIRR in intact myocardium remain unclear. Here, we used a semi-quantitative approach of optical superoxide (O(2)(-)) mapping using dihydroethidium (DHE) fluorescence to explore RIRR, its arrhythmic consequences and underlying mechanisms in intact myocardium. Initially, perfusion of rat hearts with 200 μM H(2)O(2) for 40 min (n = 4) elicited two distinct O(2)(-) peaks that were readily distinguished by their timing and amplitude. The first peak (P1), which was generated rapidly (within 5-8 min of H(2)O(2) perfusion) was associated with a relatively limited (10 ± 2%) rise in normalized O(2)(-) levels relative to baseline. In contrast, the second peak (P2) occurred 19-26 min following onset of H(2)O(2) perfusion and was associated with a significantly greater amplitude compared to P1. Spatio-temporal ROS mapping during P2 revealed active O(2)(-) propagation across the myocardium at a velocity of ~20 μm s(-1). Exposure of hearts (n = 18) to a short (10 min) episode of H(2)O(2) perfusion revealed consistent generation of P2 by high (≥200 μM, 8/8) but not lower (≤100 μM, 3/8) H(2)O(2) concentrations (P < 0.03). In these hearts, onset of P2 occurred following, not during, the 10 min OS protocol, consistent with RIRR. Importantly, P2 (+) hearts exhibited a markedly greater (by 3.8-fold, P < 0.001) arrhythmia score compared to P2 (-) hearts. To explore the mechanism underlying RIRR in intact myocardium, hearts were perfused with either cyclosporin A (CsA) or 4-chlorodiazepam (4-Cl-DZP) to inhibit the mitochondrial permeability transition pore (mPTP) or the inner membrane anion channel (IMAC), respectively. Surprisingly, perfusion with CsA failed to suppress (P = 0.75, n.s.) or even delay H(2)O(2)-induced P2 or the incidence of arrhythmias compared to untreated hearts. In sharp contrast, perfusion with 4-Cl-DZP markedly blunted O(2)(-) levels during P2, and suppressed the incidence of sustained ventricular tachycardia or ventricular fibrillation (VT/VF). Finally, perfusion of hearts with the synthetic superoxide dismutase/catalase mimetic EUK-134 completely abolished the H(2)O(2)-mediated RIRR response as well as the incidence of arrhythmias. These findings extend the concept of RIRR to the level of the intact heart, establish regenerative O(2)(-) production as the mediator of RIRR-related arrhythmias and reveal their strong dependence on IMAC and not the mPTP in this acute model of OS.

The circularly permuted yellow fluorescent protein cpYFP that has been used as a superoxide probe is highly responsive to pH but not superoxide in mitochondria: implications for the existence of superoxide ‘flashes’

The properties of a cpYFP [circularly permuted YFP (yellow fluorescent protein)] reported to act as a superoxide sensor have been re-examined in Arabidopsis mitochondria. We have found that the probe has high pH sensitivity and that dynamics in the cpYFP signal disappeared when the matrix pH was clamped by nigericin. In contrast, genetic and pharmacological manipulation of matrix superoxide had no detectable effect on the cpYFP signal. These findings question the existence of superoxide flashes in mitochondria.

Functional roles of superoxide and hydrogen peroxide generated by mitochondrial DNA mutation in regulating tumorigenicity of HepG2 cells

Mitochondria are a major source of reactive oxygen species (ROS). Recent studies have estimated that mitochondrial DNA mutations inducing the overproduction of ROS are associated with human cancer. However, a substantial challenge in elucidating their diverse roles in regulating tumorigenesis is the lack of methods for probing ROS in living systems with molecular specificity. In this study, we reported the application of two fluorescent probes, 2-chloro-1,3-dibenzothiazolinecyclohexene and naphthofluorescein disulfonate, which showed high selectivity for superoxide (O2(•-)) and hydrogen peroxide (H2O2). They were capable of detecting and visualizing O2(•-) and H2O2 overproduction caused by a mutation in the gene encoding nicotinamide adenine dinucleotide dehydrogenase subunit 6 (ND6) in HepG2 cells. The levels of O2(•-) and H2O2 in mitochondria isolated from HepG2 cells were found to be 0.63 ± 0.07 and 1.13 ± 0.05 μM, respectively. Using assays of tumorigenesis in mouse models, we found that treatment of the mice with different ROS scavengers suppressed tumour growth. These findings suggested that ROS generated by ND6 gene mutation do play an important role in regulating tumorigenesis and H2O2 may be a key modulator.

Extracellular ATP induces spikes in cytosolic free Ca(2+) but not in NADPH oxidase activity in neutrophils

In order to establish whether non-mitochondrial oxidase activity in human neutrophils is tightly related to cytosolic Ca(2+) concentration, we simultaneously measured Ca(2+) oscillations induced by ATP and oxidant production in single adherent neutrophils using confocal microscopy. ATP induced fast damped Ca(2+) spikes with a period of 15s and slower irregular spikes with a period greater than 50s. Spikes in Ca(2+) occurred in the absence of Ca(2+) influx, but the amplitude was damped by inhibition of Ca(2+) influx. Using the oxidation of hydroethidine as a cytosolic marker of oxidant production, we show that the generation of reactive oxygen species by neutrophils adherent to glass was accelerated by ATP. The step-up in NADPH oxidase activity followed the first elevation of cytosolic Ca(2+) but, despite subsequent spikes in Ca(2+) concentration, no oscillations in oxidase activity could be detected. ATP induced spikes in Ca(2+) in a very reproducible way and we propose that the Ca(2+) signal is an on-switch for oxidase activity, but the activity is apparently not directly correlated with spiking activity in cytosolic Ca(2+).

Plasmodium falciparum resistance to cytocidal versus cytostatic effects of chloroquine

With one exception (Gligorijevic et al., Mol Biochem Parasitol 2008;159:7-23.) all previous quantification of chloroquine (CQ) potency vs. P. falciparum has been by growth inhibition assays, meaning potency is defined as cytostatic potential and quantified by IC(50) values. In this study we investigate the cytocidal potency of CQ and other common quinoline antimalarial drugs (quantified as LD(50)). Similar to results from assays for cytostatic potency, we are able to readily distinguish drug resistant from drug sensitive P. falciparum parasites as well as different degrees of resistance. However, we find that fold-resistance to CQ and other quinoline drugs quantified via LD(50) ratios differs quite dramatically from fold resistance calculated via IC(50) ratios. Also, importantly, we find that verapamil chemoreversal of CQ resistance differs when quantified via cytocidal vs. cytostatic assays, as do patterns of "multidrug" resistance in well-known laboratory strains of P. falciparum. The results have important implications for development of new antimalarial drugs and for fully defining the genetic loci that confer clinically relevant antimalarial drug resistance phenomena.

Natural variation in life history and aging phenotypes is associated with mitochondrial DNA deletion frequency in Caenorhabditis briggsae

BACKGROUND

Mutations that impair mitochondrial functioning are associated with a variety of metabolic and age-related disorders. A barrier to rigorous tests of the role of mitochondrial dysfunction in aging processes has been the lack of model systems with relevant, naturally occurring mitochondrial genetic variation. Toward the goal of developing such a model system, we studied natural variation in life history, metabolic, and aging phenotypes as it relates to levels of a naturally-occurring heteroplasmic mitochondrial ND5 deletion recently discovered to segregate among wild populations of the soil nematode, Caenorhabditis briggsae. The normal product of ND5 is a central component of the mitochondrial electron transport chain and integral to cellular energy metabolism.

RESULTS

We quantified significant variation among C. briggsae isolates for all phenotypes measured, only some of which was statistically associated with isolate-specific ND5 deletion frequency. We found that fecundity-related traits and pharyngeal pumping rate were strongly inversely related to ND5 deletion level and that C. briggsae isolates with high ND5 deletion levels experienced a tradeoff between early fecundity and lifespan. Conversely, oxidative stress resistance was only weakly associated with ND5 deletion level while ATP content was unrelated to deletion level. Finally, mean levels of reactive oxygen species measured in vivo showed a significant non-linear relationship with ND5 deletion level, a pattern that may be driven by among-isolate variation in antioxidant or other compensatory mechanisms.

CONCLUSIONS

Our findings suggest that the ND5 deletion may adversely affect fitness and mitochondrial functioning while promoting aging in natural populations, and help to further establish this species as a useful model for explicit tests of hypotheses in aging biology and mitochondrial genetics.

Mechanisms of diesel-induced endothelial nitric oxide synthase dysfunction in coronary arterioles

BACKGROUND AND OBJECTIVE

Increased air pollutants correlate with increased incidence of cardiovascular disease potentially due to vascular dysfunction. We have reported that acute diesel engine exhaust (DE) exposure enhances vasoconstriction and diminishes acetylcholine (ACh)-induced dilation in coronary arteries in a nitric oxide synthase (NOS)-dependent manner. We hypothesize that acute DE inhalation leads to endothelial dysfunction by uncoupling NOS.

METHODS

Rats inhaled fresh DE (300 µg particulate matter/m3) or filtered air for 5 hr. After off-gassing, intraseptal coronary arteries were isolated and dilation to ACh recorded using videomicroscopy.

RESULTS

Arteries from DE-exposed animals dilated less to ACh than arteries from air-exposed animals. NOS inhibition did not affect ACh dilation in control arteries but increased dilation in the DE group, suggesting NOS does not normally contribute to ACh-induced dilation in coronary arteries but does contribute to endothelial dysfunction after DE inhalation. Cyclooxygenase (COX) inhibition did not affect ACh dilation in the DE group, but combined inhibition of NOS and COX diminished dilation in both groups and eliminated intergroup differences, suggesting that the two pathways interact. Superoxide scavenging increased ACh dilation in DE arteries, eliminating differences between groups. Tetrahydrobiopterin (BH4) supplementation with sepiapterin restored ACh-mediated dilation in the DE group in a NOS-dependent manner. Superoxide generation (dihydroethidium staining) was greater in DE arteries, and superoxide scavenging, BH4 supplementation, or NOS inhibition reduced the signal in DE but not air arteries.

CONCLUSION

Acute DE exposure appears to uncouple NOS, increasing reactive oxygen species generation and causing endothelial dysfunction, potentially because of depletion of BH4 limiting its bioavailability.

Chemical cytometry quantitates superoxide levels in the mitochondrial matrix of single myoblasts

Triphenylphosphonium hydroethidine (TPP-HE) is a membrane-permeable probe that reacts with superoxide and forms hydroxytriphenylphosphonium ethidium (OH-TPP-E(+)), a fluorescent product that has been previously used in qualitative measurements of superoxide production. In order to develop quantitative methods to measure superoxide, it is necessary to take into consideration the principles that drive TPP-HE accumulation into various subcellular compartments. In the mitochondria matrix, TPP-HE accumulation depends on the mitochondrial membrane potential, which varies from cell to cell. Here we address this issue by including rhodamine 123 (R123) as an internal mitochondrial membrane potential calibrant in chemical cytometry experiments. After loading with TPP-HE and R123, a single cell is lysed within a separation capillary and its contents are separated and detected by micellar electrokinetic capillary chromatography with laser-induced fluorescence detection (MEKC-LIF). Using theoretical arguments, we show that the ratio [OH-TPP-E(+)]/[R123] is adequate to obtain a relative quantitation of mitochondrial matrix superoxide levels for each analyzed cell. We applied this method to single skeletal muscle myoblasts and determined that the steady state superoxide levels in the mitochondrial matrix is approximately (0.29 +/- 0.10) x 10(-12) M. The development of this quantitative method is a critical step toward establishing the importance of reactive oxygen species in biological systems, including those relevant to aging and disease.

On the use of fluorescence probes for detecting reactive oxygen and nitrogen species associated with photodynamic therapy

Fluorescent probes are frequently employed for the detection of different reactive oxygen and nitrogen species formed during the irradiation of photosensitized cells and tissues. Investigators often interpret the results in terms of information provided with the different probes without examining specificity or determinants of fluorogenic reactions. We examine five fluorescent probes in a cell-free system: reduced 2',7'-dichlorofluorescein, dihydroethidine, dihydrorhodamine, 3'-(p aminophenyl) fluorescein (APF), and 4',5'-diaminofluorescein. Of these, only APF demonstrates a high degree of specificity for a single reactive species. There is a substantial influence of peroxidase activity on all fluorogenic interactions. The fluorescence of the photosensitizing agent also must be taken into account in evaluating results.

Asymmetric superoxide release inside and outside the mitochondria in skeletal muscle under conditions of aging and disuse

Superoxide released from mitochondria forms reactive oxygen species that can cause severe oxidative damage and have been associated with aging- and disuse-induced muscle dysfunction. Superoxide is released to both the exterior and the matrix of mitochondria, where oxidative damage is not necessarily the same. This complicates determining the role of mitochondrial superoxide in eliciting oxidative stress in skeletal muscle. A newly developed capillary electrophoretic method analyzes hydroxytriphenylphosphonium ethidium, a superoxide-specific product of triphenylphosphonium hydroethidine, released to outside the mitochondria (supernatant) and retained in the matrix (pellet). In this study, we investigated the mitochondrial superoxide production of soleus (type I) and semimembranosus (type II) muscles of Fischer 344 rats affected by aging (13 vs. 26 mo) and disuse (hindlimb unloading). In agreement with previous studies, overall superoxide production increased with aging and disuse. On the other hand, the new experimental method revealed that superoxide production outside the mitochondria of the soleus does not show a significant age-related increase. Another observation was that the superoxide production increase in the matrix occurs earlier (7 days of disuse) compared with the outside mitochondria (14 days of disuse) in both muscle types. These findings indicate that superoxide release is complex as it occurs asymmetrically at both sides of the mitochondrial inner membrane, and that such release has muscle type and temporal specificity. These findings are important to refine current concepts on oxidative stress associated with muscle aging and disuse.

Analysis of superoxide production in single skeletal muscle fibers

Due to their high energetic profile, skeletal muscle fibers are prone to damage by endogenous reactive oxygen species (ROS), thereby causing alterations in muscle function. Unfortunately, the complexity of skeletal muscle makes it difficult to measure and understand ROS production by fibers since other components (e.g., extracellular collagen and vascular vessels) may also generate ROS. Single cell imaging techniques are promising approaches to monitor ROS production in single muscle fibers, but usually the detection schemes for ROS are not specific. Single cell analysis by capillary electrophoresis (aka chemical cytometry) has the potential to separate and detect specific ROS reporters, but the approach is only suitable for small spherical cells that fit within the capillary lumen. Here, we report a novel method for the analysis of superoxide in single fibers maintained in culture for up to 48 h. Cultured muscle fibers in individual nanoliter-volume wells were treated with triphenylphosphonium hydroethidine (TPP-HE), which forms the superoxide specific reporter hydroxytriphenylphosphonium ethidium (OH-TPP-E(+)). After lysis of each fiber in their corresponding nanowell, the contents of each well were processed and analyzed by micellar electrokinetic capillary chromatography with laser-induced fluorescence detection (MEKC-LIF) making it possible to detect superoxide found in single fibers. Superoxide basal levels as well as changes due to fiber treatment with the scavenger, tiron, and the inducer, antimycin A, were easily monitored demonstrating the feasibility of the method. Future uses of the method include parallel single-fiber measurements aiming at comparing pharmacological treatments on the same set of fibers and investigating ROS production in response to muscle disease, disuse, exercise, and aging.

Heme synthesis increases artemisinin-induced radical formation and cytotoxicity that can be suppressed by superoxide scavengers

Artemisinin (ART) is a sesquiterpene lactone natural product that is widely used to treat multi-drug resistant strains of malaria. Artemisinin and its derivatives are also selectively cytotoxic to cancer cells, which can be modulated by altering heme synthesis. Cytotoxicity to cancer cells is thought to involve generation of oxidative stress, although conflicting data exist. We have analyzed reactive oxygen species (ROS) generation using the fluorescent probes 2',7'-dichlorodihydrofluorescein diacetate (DCF) and dihydroethidine (HET) upon exposure to dihydroartemisinin (DHA) in Molt-4 leukemia cells. HET fluorescence correlated with dose-dependent DHA-induced cytotoxicity, increased within 30 min of DHA exposure, and was significantly enhanced by increasing heme synthesis. Protein levels of copper, zinc-superoxide dismutase (CuZnSOD), manganese-superoxide dismutase (MnSOD), catalase, and glutathione peroxidases 1/2 were also found to increase with DHA exposure. 4-hydroxy-tempol (TEMPOL) and DF-Mn, MnSOD mimetics, could significantly inhibit ROS generation and reduce cell death. Production of superoxide appears to be a central mediator of cytotoxicity from DHA.