Oxidant-induced signaling: effects of peroxynitrite and singlet oxygen

Terpinen-4-ol from tea tree oil prevents Aspergillus flavus growth in postharvest wheat grain

Plant volatile organic compounds (PVOCs) have gained increasing attention for their role in preventing fungal spoilage and insect contamination in postharvest agro-products owing to their effectiveness and sustainability. In this study, the essential oil was extracted from fresh M. alternifolia (tea tree) leaves, and the fumigation vapor of tea tree oil (TTO) completely inhibited the growth of Aspergillus flavus on agar plates at a concentration of 1.714 μL/mL. Terpinen-4-ol was identified as the major component (40.76 %) of TTO volatiles analyzed using headspace gas chromatography-mass spectrometry. Terpinen-4-ol vapor completely inhibited the A. flavus growth on agar plates and 20 % moisture wheat grain at 0.556 and 1.579 μL/mL, respectively, indicating that terpinen-4-ol serves as the main antifungal constituent in TTO volatiles. The minimum inhibitory concentration of terpinen-4-ol in liquid-contact culture was 1.6 μL/mL. Terpinen-4-ol treatment caused depressed, wrinkled, and punctured mycelial morphology and destroyed the plasma membrane integrity of A. flavus. Metabolomics analysis identified significant alterations in 93 metabolites, with 79 upregulated and 14 downregulated in A. flavus mycelia exposed to 1.6 μL/mL terpinen-4-ol for 6 h, involved in multiple cellular processes including cell membrane permeability and integrity, the ABC transport system, pentose phosphate pathway, and the tricarboxylic acid cycle. Biochemical analysis and 2,7-dichlorofluorescein diacetate staining showed that terpinen-4-ol induced oxidative stress and mitochondrial dysfunction in A. flavus mycelia. This study provides new insights into the antifungal effects of the main TTO volatile compounds terpinen-4-ol on the growth of A. flavus.

Surviving the odds: From perception to survival of fungal phytopathogens under host-generated oxidative burst

Fungal phytopathogens pose a serious threat to global crop production. Only a handful of strategies are available to combat these fungal infections, and the increasing incidence of fungicide resistance is making the situation worse. Hence, the molecular understanding of plant-fungus interactions remains a primary focus of plant pathology. One of the hallmarks of host-pathogen interactions is the overproduction of reactive oxygen species (ROS) as a plant defense mechanism, collectively termed the oxidative burst. In general, high accumulation of ROS restricts the growth of pathogenic organisms by causing localized cell death around the site of infection. To survive the oxidative burst and achieve successful host colonization, fungal phytopathogens employ intricate mechanisms for ROS perception, ROS neutralization, and protection from ROS-mediated damage. Together, these countermeasures maintain the physiological redox homeostasis that is essential for cell viability. In addition to intracellular antioxidant systems, phytopathogenic fungi also deploy interesting effector-mediated mechanisms for extracellular ROS modulation. This aspect of plant-pathogen interactions is significantly under-studied and provides enormous scope for future research. These adaptive responses, broadly categorized into "escape" and "exploitation" mechanisms, are poorly understood. In this review, we discuss the oxidative stress response of filamentous fungi, their perception signaling, and recent insights that provide a comprehensive understanding of the distinct survival mechanisms of fungal pathogens in response to the host-generated oxidative burst.

Metabolome modulatory effects of Kigelia africana (Lam.) Benth. fruit extracts on oxidative stress, hyperlipidaemic biomarkers in STZ-induced diabetic rats and antidiabetic effects in 3T3 L1 adipocytes

OBJECTIVES

The management of diabetes is considered a global problem, and a cure is yet to be discovered. This study investigated the modulatory effect of Kigelia africana fruit on oxidative stress and hyperlipidaemic biomarkers in STZ-induced diabetic rats, profiled phytoconstituents using GC-TOF-MS and evaluated antidiabetic effects on 3T3 L1 adipocytes.

METHODS

Thirty male Wistar rats (120-150 g) were divided into six groups (n = 5). Diabetes was induced by a single intraperitoneal injection of STZ (60 mg/kg) and treated with 100, 200 and 400 of hexane fraction of KA for 28 days. Immunohistochemical evaluation was carried out using avidin-biotin immunoperoxidase (ABI) method. Catalase and SOD activities as well as the levels of total protein, albumin, bilirubin, triglyceride, cholesterol, and high-density lipoprotein were measured.

KEY FINDINGS

The expressions of oxidative stress and hyperlipidaemic biomarkers alongside fasting blood glucose concentrations were remarkedly decreased in KA-treated diabetic rats. Moreover, there was a significant increase in endocrine cell distribution, area covered with increase in β-cell mass, composition and morphology of KA-treated animals. Additionally, there was constant up-regulation in 3T3 L1 adipocytes due to the presence of phytoconstituents.

CONCLUSION

Kigelia africana fruit can act as a modulatory agent due to its ameliorative effects against oxidative stress.

Anti-inflammatory and cell proliferative effect of the 1270 nm laser irradiation on the BALB/c nude mouse model involves activation of the cell antioxidant system

Recently, many interdisciplinary community researchers have focused their efforts on study of the low-level light irradiation effects (photobiomodulation, PBM) as a promising therapeutic technology. Among the priorities, a search of new wavelength ranges of laser radiation to enhance the laser prospects in treatment of autoimmune and cancer diseases commonly accompanied by disorders in the antioxidant system of the body. The laser wavelengths within 1265-1270 nm corresponds to the maximum oxygen absorption band. Therefore, PBM effects on a model organism within this spectrum range are of particular interest for preclinical research. Here, we report comprehensive biomolecular studies of the changes in the BALB/c nude mice skin after an exposure to the continuous laser radiation at the 1270 nm wavelength and energy densities of 0.12 and 1.2 J/cm2. Such regime induces both local and systemic PBM effects, presumably due to the short-term increase in ROS levels, which in turn activate the cell antioxidative system.

Impact of ROS Generated by Chemical, Physical, and Plasma Techniques on Cancer Attenuation

For the last few decades, while significant improvements have been achieved in cancer therapy, this family of diseases is still considered one of the deadliest threats to human health. Thus, there is an urgent need to find novel strategies in order to tackle this vital medical issue. One of the most pivotal causes of cancer initiation is the presence of reactive oxygen species (ROS) inside the body. Interestingly, on the other hand, high doses of ROS possess the capability to damage malignant cells. Moreover, several important intracellular mechanisms occur during the production of ROS. For these reasons, inducing ROS inside the biological system by utilizing external physical or chemical methods is a promising approach to inhibit the growth of cancer cells. Beside conventional technologies, cold atmospheric plasmas are now receiving much attention as an emerging therapeutic tool for cancer treatment due to their unique biophysical behavior, including the ability to generate considerable amounts of ROS. This review summarizes the important mechanisms of ROS generated by chemical, physical, and plasma approaches. We also emphasize the biological effects and cancer inhibition capabilities of ROS.

Design, synthesis, and biological activities of 1-aryl-(3-(2-styryl)phenyl)prop-2-en-1-ones

A moderate elevation in reactive oxygen species (ROS) levels can generally be controlled in normal cells, but may lead to death of cancer cells as the ROS level in cancer cells is already elevated. Therefore, a ROS-generating compound can act as a selective chemotherapeutic agent for cancer cells that does not affect normal cells. In our previous study, a compound containing a Michael acceptor was selectively cytotoxic to cancer cells without affecting normal cells; therefore, we designed and synthesized 26 compounds containing a Michael acceptor. Their cytotoxicities against HCT116 human colon cancer cell lines were measured by using a clonogenic long-term survival assay. To derive the structural conditions required to obtain stronger cytotoxicity against cancer cells, the relationships between the half-maximal cell growth inhibitory concentration values of the synthesized compounds and their physicochemical properties were evaluated by Comparative Molecular Field Analysis and Comparative Molecular Similarity Indices Analysis. It was confirmed that the compound with the best half-maximal cell growth inhibitory concentration triggered apoptosis through ROS generation, which then led to stimulation of the caspase pathway.

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

The reactive oxygen species, generally labeled toxic due to high reactivity without target specificity, are gradually uncovered as signaling molecules involved in a myriad of biological processes. But one important feature of ROS roles in macromolecule movement has not caught attention until recent studies with technique advance and design elegance have shed lights on ROS signaling for intercellular and interorganelle communication. This review begins with the discussions of genetic and chemical studies on the regulation of symplastic dye movement through intercellular tunnels in plants (plasmodesmata), and focuses on the ROS regulatory mechanisms concerning macromolecule movement including small RNA-mediated gene silencing movement and protein shuttling between cells. Given the premise that intercellular tunnels (bridges) in mammalian cells are the key physical structures to sustain intercellular communication, movement of macromolecules and signals is efficiently facilitated by ROS-induced membrane protrusions formation, which is analogously applied to the interorganelle communication in plant cells. Although ROS regulatory differences between plant and mammalian cells exist, the basis for ROS-triggered conduit formation underlies a unifying conservative theme in multicellular organisms. These mechanisms may represent the evolutionary advances that have enabled multicellularity to gain the ability to generate and utilize ROS to govern material exchanges between individual cells in oxygenated environment.

Time-gated luminescence imaging of singlet oxygen photoinduced by fluoroquinolones and functionalized graphenes in Daphnia magna

Singlet oxygen (¹O₂) can be photogenerated by photoactive xenobiotics and is capable of causing adverse effects due to its electrophilicity and its high reactivity with biological molecules. Detection of the production and distribution of ¹O₂ in living organisms is therefore of great importance. In this study, a luminescent probe ATTA-Eu³⁺ combined with time-gated luminescence imaging was adopted to detect the distribution and temporal variation of ¹O₂ photoinduced by fluoroquinolone antibiotics and carboxylated/aminated graphenes in Daphnia magna. Results show that the xenobiotics generate ¹O₂ in living daphnids under simulated sunlight irradiation (SSR). The photogeneration of ¹O₂ by carboxylated/aminated graphenes was also confirmed by electron paramagnetic resonance spectroscopy. The strongest luminescence signals of ¹O₂ were observed in the hindgut of daphnids, and the signals in different areas of the daphnids (gut, thoracic legs and post-abdominal claw) displayed a similar trend of enhancement over irradiation time. Mean ¹O₂ concentrations at different regions of daphnids within one hour of SSR irradiation were estimated to be in the range of 0.5∼4.8μM. This study presented an efficient method for visualizing and quantifying the temporal and spatial distribution of ¹O₂ photogenerated by xenobiotics in living organisms, which can be employed for phototoxicity evaluation of xenobiotics.

Harnessing Evolutionary Toxins for Signaling: Reactive Oxygen Species, Nitric Oxide and Hydrogen Sulfide in Plant Cell Regulation

During the early periods of evolution, as well as in niche environments today, organisms have had to learn to tolerate the presence of many reactive compounds, such as reactive oxygen species, nitric oxide, and hydrogen sulfide. It is now known that such compounds are instrumental in the signaling processes in plant cells. There are enzymes which can make them, while downstream of their signaling pathways are coming to light. These include the production of cGMP, the activation of MAP kinases and transcription factors, and the modification of thiol groups on many proteins. However, organisms have also had to tolerate other reactive compounds such as ammonia, methane, and hydrogen gas, and these too are being found to have profound effects on signaling in cells. Before a holistic view of how such signaling works, the full effects and interactions of all such reactive compounds needs to be embraced. A full understanding will be beneficial to both agriculture and future therapeutic strategies.

Peroxynitrite: From interception to signaling

Peroxynitrite is a strong oxidant and nitrating species that mediates certain biological effects of superoxide and nitrogen monoxide. These biological effects include oxidative damage to proteins as well as the formation of 3-nitrotyrosyl moieties in proteins. As a consequence, such proteins may lose their activity, gain altered function, or become prone to proteolytic degradation - resulting in modulation of cellular protein turnover and in the modulation of signaling cascades. In analogy to hydrogen peroxide, peroxynitrite may be scavenged by selenoproteins like glutathione peroxidase-1 (GPx-1) or by selenocompounds with a GPx-like activity, such as ebselen; in further analogy to H2O2, peroxiredoxins have also been established as contributors to peroxynitrite reduction. This review covers three aspects of peroxynitrite biochemistry, (i) the interaction of selenocompounds/-proteins with peroxynitrite, (ii) peroxynitrite-induced modulation of cellular proteolysis, and (iii) peroxynitrite-induced modulation of cellular signaling.

Extracted Triterpenes from Antrodia cinnamomea Reduce the Inflammation to Promote the Wound Healing via the STZ Inducing Hyperglycemia-Diabetes Mice Model

This research evaluated the effects of triterpenes on the regulation of STZ-induced hyperglycaemic diabetes through an anti-inflammatory response. Diabetic mice were orally administered various concentrations of triterpenes on a daily basis. Weight gain, volume of drinking water, and liver and spleen weight were recorded and evaluated. These evaluations presented a positive regulation to the abnormal metabolism appearance compared to the diabetic mice. In the diabetic mice, the detection of adiponectin production or elevated levels of inflammatory factors such as CCL1 and TPO expression were found to reduce hyperglycaemia and thereby induce an inflammatory response. Moreover, to the best of our knowledge, hyperglycaemia impairs the tissue healing associated with an increased and prolonged inflammatory response. An investigation of the anti-inflammatory response in wound healing as affected by the triterpenes verified the promotion of wound recovery.

A singlet oxygen photosensitizer enables photoluminescent monitoring of singlet oxygen doses

Dichromophoric molecular dyad enables photosensitization and detection of sinlget oxygen.

Role of the extracytoplasmic function sigma factor CarQ in oxidative response of Bradyrhizobium japonicum

As a nitrogen-fixing bacterium, Bradyrhizobium japonicum can establish a symbiotic relationship with the soybean plant (Glycine max). To be a successful symbiont, B. japonicum must deal with plant defense responses, such as an oxidative burst. Our previous functional genomics study showed that carQ (bll1028) encoding extracytoplasmic function (ECF) sigma factor was highly expressed (107.8-fold induction) under oxidative stress. Little is known about the underlying mechanisms of how CarQ responds to oxidative stress. In this study, a carQ knock-out mutant was constructed using site-specific mutagenesis to identify the role of carQ in the oxidative response of B. japonicum. The carQ mutant showed a longer generation time than the wild type and exhibited significantly decreased survival at 10 mM H(2)O(2) for 10 min of exposure. Surprisingly, there was no significant difference in expression of oxidative stress-responsive genes such as katG and sod between the wild type and carQ mutant. The mutant also showed a significant increase in susceptibility to H(2)O(2) compared to the wild type in the zone inhibition assay. Nodulation phenotypes of the carQ mutant were distinguishable compared to those of the wild type, including lower numbers of nodules, decreased nodule dry weight, decreased plant dry weight, and a lower nitrogen fixation capability. Moreover, desiccation of mutant cells also resulted in significantly lower percent of survival in both early (after 4 h) and late (after 24 h) desiccation periods. Taken together, this information will provide an insight into the role of the ECF sigma factor in B. japonicum to deal with a plant-derived oxidative burst.

Insulin-like modulation of Akt/FoxO signaling by copper ions is independent of insulin receptor

Copper ions are known to induce insulin-like effects in various cell lines, stimulating the phosphoinositide 3'-kinase (PI3K)/Akt signaling cascade and leading to the phosphorylation of downstream targets, including FoxO transcription factors. The aim of this work was to study the role of insulin- and IGF1-receptors (IR and IGF1R) in insulin-like signaling induced by copper in HepG2 human hepatoma cells. Cells were exposed to Cu(II) at various concentrations for up to 60 min. While Akt and FoxO1a/FoxO3a were strongly phosphorylated in copper- and insulin-treated cells at all time points studied, only faint tyrosine phosphorylation of IR/IGF1R was detected in cells exposed to Cu(II) by either immunoprecipitation/immunoblot or by immunoblotting using phospho-specific antibodies, whereas insulin triggered strong phosphorylation at these sites. Pharmacological inhibition of IR/IGF1R modestly attenuated Cu-induced Akt and FoxO phosphorylation, whereas no attenuation of Cu-induced Akt activation was achieved by siRNA-mediated IR depletion. Cu(II)-induced FoxO1a nuclear exclusion was only slightly impaired by pharmacological inhibition of IR/IGF1R, whereas insulin-induced effects were blunted. In contrast, genistein, a broad-spectrum tyrosine kinase inhibitor, at concentrations not affecting IR/IGF1R, attenuated Cu(II)-induced Akt phosphorylation, pointing to the requirement of tyrosine kinases other than IR/IGF1R for Cu(II)-induced signaling.

Coupling of transcriptional response to oxidative stress and secondary metabolism regulation in filamentous fungi

To survive sudden and potentially lethal changes in their environment, filamentous fungi must sense and respond to a vast array of stresses, including oxidative stresses. The generation of reactive oxygen species, or ROS, is an inevitable aspect of existence under aerobic conditions. In addition, in the case of fungi with pathogenic lifestyles, ROS are produced by the infected hosts and serve as defense weapons via direct toxicity, as well as effectors in fungal cell death mechanisms. Filamentous fungi have thus developed complex and sophisticated responses to evade oxidative killing. Several steps are determinant in these responses, including the activation of transcriptional regulators involved in the control of the antioxidant machinery. Gathering and integrating the most recent advances in knowledge of oxidative stress responses in fungi are the main objectives of this review. Most of the knowledge coming from two models, the yeast Saccharomyces cerevisiae and fungi of the genus Aspergillus, is summarized. Nonetheless, recent information on various other fungi is delivered when available. Finally, special attention is given on the potential link between the functional interaction between oxidative stress and secondary metabolism that has been suggested in recent reports, including the production of mycotoxins.

Single cell responses to spatially controlled photosensitized production of extracellular singlet oxygen

The response of individual HeLa cells to extracellularly produced singlet oxygen was examined. The spatial domain of singlet oxygen production was controlled using the combination of a membrane-impermeable Pd porphyrin-dendrimer, which served as a photosensitizer, and a focused laser, which served to localize the sensitized production of singlet oxygen. Cells in close proximity to the domain of singlet oxygen production showed morphological changes commonly associated with necrotic cell death. The elapsed postirradiation "waiting period" before necrosis became apparent depended on: (1) the distance between the cell membrane and the domain irradiated, (2) the incident laser fluence and, as such, the initial concentration of singlet oxygen produced and (3) the lifetime of singlet oxygen. The data imply that singlet oxygen plays a key role in this process of light-induced cell death. The approach of using extracellularly generated singlet oxygen to induce cell death can provide a solution to a problem that often limits mechanistic studies of intracellularly photosensitized cell death: it can be difficult to quantify the effective light dose, and hence singlet oxygen concentration, when using an intracellular photosensitizer.

Reactive oxygen species in phytopathogenic fungi: signaling, development, and disease

Reactive oxygen species (ROS) play a major role in pathogen-plant interactions: recognition of a pathogen by the plant rapidly triggers the oxidative burst, which is necessary for further defense reactions. The specific role of ROS in pathogen defense is still unclear. Studies on the pathogen so far have focused on the importance of the oxidative stress response (OSR) systems to overcome the oxidative burst or of its avoidance by effectors. This review focuses on the role of ROS for fungal virulence and development. In the recent years, it has become obvious that (a) fungal OSR systems might not have the predicted crucial role in pathogenicity, (b) fungal pathogens, especially necrotrophs, can actively contribute to the ROS level in planta and even take advantage of the host's response, (c) fungi possess superoxide-generating NADPH oxidases similar to mammalian Nox complexes that are important for pathogenicity; however, recent data indicate that they are not directly involved in pathogen-host communication but in fungal differentiation processes that are necessary for virulence.

Secondary ketocarotenoid astaxanthin biosynthesis in algae: a multifunctional response to stress

Under stressful environments, many green algae such as Haematococcus pluvialis accumulate secondary ketocarotenoids such as canthaxanthin and astaxanthin. The carotenogenesis, responsible for natural phenomena such as red snows, generally accompanies larger metabolic changes as well as morphological modifications, i.e., the conversion of the green flagellated macrozoids into large red cysts. Astaxanthin accumulation constitutes a convenient way to store energy and carbon, which will be used for further synthesis under less stressful conditions. Besides this, the presence of high amount of astaxanthin enhances the cell resistance to oxidative stress generated by unfavorable environmental conditions including excess light, UV-B irradiation, and nutrition stress and, therefore, confers a higher survival capacity to the cells. This better resistance results from the quenching of oxygen atoms for the synthesis itself as well as from the antioxidant properties of the astaxanthin molecules. Therefore, astaxanthin synthesis corresponds to a multifunctional response to stress. In this contribution, the various biochemical, genetic, and molecular data related to the biosynthesis of ketocarotenoids by Haematococcus pluvialis and other taxa are reviewed and compared. A tentative regulatory model of the biochemical network driving astaxanthin production is proposed.

Loss of gap junctional intercellular communication in rat lung epithelial cells exposed to carbon or silica-based nanoparticles

The aim of this study was to investigate whether fine and ultrafine carbon black (fC and ufC), and fine and ultrafine silica (fS, ufS) particles affect gap junctional intercellular communication (GJIC) in rat lung epithelial cells. Exposure of cells to subcytotoxic doses of ufC, fS and ufS resulted in a 63%, 59% and 77% reduction of GJIC, respectively, as determined in a dye transfer assay. In contrast to ufC, fC did not significantly alter GJIC. Changes in subcellular localization of the major gap junction protein in RLE cells, connexin-43 (Cx43), and of β-catenin were observed in cells exposed to ufC, fS or ufS. The loss of GJIC was counteracted by N-acetyl cysteine and was largely prevented by specific inhibitors of epidermal growth factor receptor-dependent signaling, pointing to the crucial role of two known major mediators of nanoparticle action, namely reactive oxygen species and membrane-receptor signaling, in particle-induced modulation of GJIC.

Peroxynitrite stimulates pulmonary artery endothelial and smooth muscle cell proliferation: involvement of ERK and PKC

BACKGROUND

There is evidence that peroxynitrite is generated in pulmonary hypertension and we have therefore investigated whether peroxynitrite can cause proliferation of pulmonary artery cells.

METHODS

Bovine pulmonary artery endothelial (PAEC) and smooth muscle cells (PASMC) were exposed to peroxynitrite solution or to the peroxynitrite generating compound, 3-morpholinosydnonimine (SIN-1). Vascular cell proliferation was determined by cell count and (3)H-thymidine incorporation. Protein biochemistry was by western blot analysis.

RESULTS

Transient exposure to peroxynitrite stimulated the proliferation of PASMC (peroxynitrite 0.2 nM-2 μM) and PAEC (peroxynitrite 0.2 μM). Peroxynitrite 0.2 μM stimulated DNA synthesis in PASMC cell by 200 ± 22% and in PAEC by 137 ± 4%. DNA synthesis in PAEC and PASMC was also stimulated by the peroxynitrite generator SIN-1 2 μM. Cell proliferation was accompanied by activation of ERK, which peaked at 15 min and remained elevated for 12 h in PASMC. However peroxynitrite at the concentrations used in this study did not activate the stress pathways p38 mitogen activated protein kinase (MAPK) or Jun N-terminal kinase (JNK). Peroxynitrite-induced proliferation and ERK phosphorylation in PASMC were abolished by the peroxynitrite scavenger ebselen 5 μM. Peroxynitrite-induced proliferation and extracellular signal-regulated kinase (ERK) phosphorylation in PASMC was prevented by selective inhibitors of MAP kinase kinase (MEK) (U0126 5 μM, PD98059 50 μM), Raf-1 (Raf-1 kinase inhibitor 10 μM), Ras (FPT II and FPT III 10 μM) and protein kinase C (PKC) (GF109203X 10 μM). Inhibition of EGF or PDGF receptor signaling using AG-1296, AG-1478 or imatinib prevented peroxynitrite-induced cell proliferation and ERK phosphorylation in PASMC.

CONCLUSION

Peroxynitrite can stimulate proliferation of pulmonary artery cells, involving ERK, PKC and EGF or PDGF receptors.

Structure Effect on Antioxidant Activity of Catecholamines toward Singlet Oxygen and Other Reactive Oxygen Species in vitro

The reactivity of catecholamine neurotransmitters and the related metabolites were precisely investigated toward 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals and reactive oxygen species. Catecholamines reacted immediately with DPPH radicals, their reactivity being stronger than that of ascorbic acid as a reference. Superoxide scavenging activities of catecholamines determined by WST-1 and electron spin resonance (ESR) spin trapping methods were also high. Whereas tyrosine, the dopamine precursor showed no reactivity toward superoxide. The reactivity toward singlet oxygen was evaluated by observing specific photon emission from singlet oxygen. The results revealed that reactivity of catecholamines was markedly higher than that of sodium azide, and catechin as catechol reference. The reaction of catecholamines and singlet oxygen was further studied by ESR using 55-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping reagent and rose bengal as photosensitizer. DMPO-OH signal of epinephrine was significantly small compared to other catecholamines, catechin, and 4-methylcatechol as a reference compound and was as small as that of tyrosine. The signal formation was totally dependent on singlet oxygen, and the presence of catechol compounds. These results indicated that epinephrine is the most potent singlet oxygen quencher than other catecholamines, and the secondary amino group in its alkyl side chain could play a role in unique singlet oxygen quenching property of epinephrine.

Role of HuR and p38MAPK in ultraviolet B-induced post-transcriptional regulation of COX-2 expression in the human keratinocyte cell line HaCaT

COX-2 (cyclooxygenase-2) is a pivotal player in inflammatory processes, and ultraviolet radiation is a known stimulus for COX-2 expression in skin cells. Here, an induction of COX-2 expression in HaCaT human keratinocytes was observed only upon exposure of cells to UVB (280-320 nm) but not to UVA radiation (320-400 nm), as demonstrated by reverse transcription-PCR and Western blotting. Prostaglandin E(2) levels were elevated in cell culture supernatants of HaCaT cells exposed to UVB. COX-2 mRNA stability was dramatically increased by UVB irradiation. Both the stabilization of COX-2 mRNA and the enhancement of COX-2 steady-state mRNA and protein levels caused by UVB were prevented both by inhibition and small interfering RNA-induced depletion of p38(MAPK), a kinase strongly activated upon exposure to UVB, suggesting p38(MAPK)-dependent mRNA stabilization as a mechanism of UVB-induced COX-2 expression. A dramatic decrease in COX-2 expression induced by UVB was elicited by small interfering RNA-based depletion of a stress-responsive mRNA stabilizing protein regulated by p38(MAPK), i.e. HuR; UVB-induced elevation of COX-2 mRNA and protein levels coincided with an accumulation of HuR in the cytoplasm and was attenuated in cells depleted of HuR. Moreover, UVB-induced generation of prostaglandin E(2) by HaCaT cells was blunted by HuR depletion, suggesting that stress kinases (such as p38(MAPK)) as well as HuR are excellent targets for approaches aiming at interfering with induction of COX-2 expression by UVB.

The proteasome is an integral part of solar ultraviolet a radiation-induced gene expression

Solar ultraviolet (UV) A radiation is a well known trigger of signaling responses in human skin fibroblasts. One important consequence of this stress response is the increased expression of matrix metalloproteinase-1 (MMP-1), which causes extracellular protein degradation and thereby contributes to photoaging of human skin. In the present study we identify the proteasome as an integral part of the UVA-induced, intracellular signaling cascade in human dermal fibroblasts. UVA-induced singlet oxygen formation was accompanied by protein oxidation, the cross-linking of oxidized proteins, and an inhibition of the proteasomal system. This proteasomal inhibition subsequently led to an accumulation of c-Jun and phosphorylated c-Jun and activation of activator protein-1, i.e. transcription factors known to control MMP-1 expression. Increased transcription factor activation was also observed if the proteasome was inhibited by cross-linked proteins or lactacystin, indicating a general mechanism. Most importantly, inhibition of the proteasome was of functional relevance for UVA-induced MMP-1 expression, because overexpression of the proteasome or the protein repair enzyme methionine sulfoxide reductase prevented the UVA-induced induction of MMP-1. These studies show that an environmentally relevant stimulus can trigger a signaling pathway, which links intracellular and extracellular protein degradation. They also identify the proteasome as an integral part of the UVA stress response.

Singlet Oxygen-induced Attenuation of Growth Factor Signaling: Possible Role of Ceramides

Singlet oxygen, an electronically excited form of molecular oxygen, is a primary mediator of the activation of stress-activated protein kinases elicited by ultraviolet A (UVA; 320-400 nm). Here, the effects of singlet oxygen (1O2) on the extracellular signal-regulated kinase (ERK) 1/2 and Akt/protein kinase B pathways were analyzed in human dermal fibroblasts. While basal ERK 1/2 phosphorylation was lowered in cells exposed to either 1O2, UVA or photodynamic treatment, Akt was moderately activated by photochemically generated 1O2 in a phosphoinositide 3-kinase (PI3K)-dependent fashion, resulting in the phosphorylation of glycogen synthase kinase-3 (GSK3). The activation of ERK 1/2 and Akt as induced by stimulation with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) was inhibited by 1O2 generated intracellularly upon photoexcitation of rose Bengal (RB). Photodynamic therapy (PDT)-induced apoptosis is known to be associated with increased formation of ceramides. Likewise, both 1O2 and UVA induced ceramide generation in human skin fibroblasts. The attenuation of EGF- and PDGF-induced activation of ERK 1/2 and Akt by 1O2 was mimicked by stimulation of fibroblasts with the cell-permeable C2-ceramide. Interestingly, EGF-induced tyrosine phosphorylation of the EGF receptor was strongly attenuated by 1O2 but unimpaired by C2-ceramide, implying that, although ceramide formation may mediate the above attenuation of ERK and Akt phosphorylation induced by 1O2, mechanisms beyond ceramide formation exist that mediate impairment of growth factor signaling by singlet oxygen. In summary, these data point to a novel mechanism of 1O2 toxicity: the known 1O2-induced activation of proapoptotic kinases such as JNK and p38 is paralleled by the prevention of activation of growth factor receptor-dependent signaling and of anti-apoptotic kinases, thus shifting the balance towards apoptosis.

Zinc fingers as biologic redox switches?

Despite zinc ions being redox inert in biologic systems, zinc-finger structures act as redox-sensitive molecular switches controlling several crucial cellular processes. Oxidative or nitrosative stress, via modification of zinc finger cysteine thiols, leads to a release of Zn(2+) from these structures, causing not only a loss of zinc-finger function but also an increase of cytoplasmic or nuclear free Zn(2+) that may, in turn, stimulate and interfere with cellular signaling cascades. A signaling cascade stimulated by exposure of cells to zinc ions or to stressful stimuli that are reported to cause an intracellular release of zinc ions involves phosphoinositide 3'-kinases and the Ser/Thr protein kinase Akt, resulting in an inactivation of transcriptional regulators of the FoxO family. Possible modes of action of zinc ions to stimulate this signaling cascade and consequences of stimulation are discussed. Moreover, we present an overview on human diseases or disorders characterized by an intracellular Zn(2+) dyshomeostasis.

Consumption of oxygen by astaxanthin biosynthesis: a protective mechanism against oxidative stress in Haematococcus pluvialis (Chlorophyceae)

Haematococcus pluvialis, a unicellular green microalga, experiences photooxidative stress when exposed to excess photon flux density (PFD) relative to the capacity of photosynthesis, and particularly under other adverse environmental conditions (e.g., nutrient depletion, salinity, and excess heavy metals). Under stress, Haematococcus cells synthesize and accumulate large amounts of the secondary carotenoid astaxanthin stored in cytosolic lipid bodies. In this study, the transcriptional expression of five astaxanthin biosynthesis genes and two plastid terminal oxidase (PTOX) genes either in high PFD or in the presence of excessive sodium acetate and/or iron was determined by real-time reverse transcription PCR, and astaxanthin accumulation was measured by HPLC. Photosynthetic oxygen evolution, lipid peroxidation, and cell mortality were also investigated under these stress conditions. Our results indicate that the astaxanthin biosynthesis pathway may consume as much as 9.94% of the molecular oxygen evolved from photosynthesis under stress via at least two distinct routes: (1) extensive oxygen-dependent processes leading to astaxanthin formation, and (2) conversion of molecular oxygen into water using electrons derived from carotenogenic desaturation steps to PTOX via the photosynthetic plastoquinone (PQ) pool. Reduction of reactive oxygen species (ROS) production by reducing subcellular molecular oxygen substrates through the astaxanthin biosynthesis pathway may represent a novel protective mechanism to cope with oxidative stress. Reoxidation of the PQ pool by PTOX may further reduce photosynthetic electron transport chain-induced ROS formation.

Tocochromanols, plastoquinol, and other biological prenyllipids as singlet oxygen quenchers-determination of singlet oxygen quenching rate constants and oxidation products

Singlet oxygen quenching rate constants for tocopherol and tocotrienol homologues have been determined in organic solvents of different polarities, as well as for other biological prenyllipids such as plastoquinol, ubiquinol, and alpha-tocopherolquinol. The obtained results showed that the quenching activity of tocochromanols was mainly due to the chromanol ring of the molecule and the activity increased with the number of the methyl groups in the ring and solvent polarity. Among prenylquinols, alpha-tocopherolquinol was the most active scavenger of singlet oxygen followed by ubiquinol and plastoquinol. The oxidation products of tocopherols were identified as 8a-hydroperoxy-tocopherones which are converted to the corresponding tocopherolquinones under acidic conditions. The primary oxidation products of prenylquinols, containing unsaturated side chains, were the corresponding prenylquinones that were further oxidized to hydroxyl side-chain derivatives. In the case of plastochromanol, the gamma-tocotrienol homologue found in some seed oils, mainly the hydroxyl derivatives were formed, although 8a-hydroperoxy-gamma-tocopherones were also formed to a minor extent, both from plastochromanol and from its hydroxyl, side-chain derivatives. The obtained results were discussed in terms of the activity of different prenyllipids as singlet oxygen scavengers in vivo.

Enzymatic, but not non-enzymatic, 1O2-mediated peroxidation of polyunsaturated fatty acids forms part of the EXECUTER1-dependent stress response program in the flu mutant of Arabidopsis thaliana

The conditional flu mutant of Arabidopsis accumulates excess amounts of protochlorophyllide within plastid membranes in the dark and generates singlet oxygen upon light exposure. By varying the length of the dark period, the level of the photosensitizer protochlorophyllide may be modulated, and conditions have been established that either endorse the cytotoxicity of (1)O(2) or reveal its signaling role. Two criteria have been used to distinguish between these two modes of activity of (1)O(2): the impact of the EXECUTER1 mutation and the prevalence of either non-enzymatic or enzymatic lipid peroxidation. During illumination of etiolated flu seedlings, toxic effects of (1)O(2) prevail and non-enzymatic lipid peroxidation proceeds rapidly. In contrast, in light-grown flu plants that were subjected to an 8 h dark/light shift, lipid peroxidation occurred almost exclusively enzymatically. The resulting oxidation product, 13-hydroperoxy octadecatrienoic acid (13-HPOT), serves as a substrate for synthesis of 12-oxo phytodienoic acid (OPDA) and jasmonic acid (JA), both of which are known to control various metabolic and developmental processes in plants. Inactivation of the EXECUTER1 protein abrogates not only (1)O(2)-mediated cell death and growth inhibition of flu plants, but also enzymatic lipid peroxidation. However, inactivation of jasmonate biosynthesis in the aos/flu double mutant does not affect (1)O(2)-mediated growth inhibition and cell death. Hence, JA and OPDA do not act as second messengers during (1)O(2) signaling, but form an integral part of a stress-related signaling cascade activated by (1)O(2) that encompasses several signaling pathways known to be activated by abiotic and biotic stressors.

Glutathione production is regulated via distinct pathways in stressed and non-stressed cortical neurons

Peroxynitrite-mediated damage has been linked to numerous neurological and neurodegenerative diseases, including stroke, Alzheimer's and Parkinson's Diseases, amyotrophic lateral sclerosis and multiple sclerosis. Studies on the toxic effects of peroxynitrite in neurons have focused primarily on adverse effects resulting from the nitration of cellular proteins as the principal mode of toxicity while the consequences of the modulation of kinase pathways by peroxynitrite have received relatively less attention. Our results show that treatment of primary rat neurons with the peroxynitrite donor, SIN-1, leads to decreases in glutathione (GSH) levels and cell viability via a novel extracellular-signal-related kinase (ERK)/c-Myc phosphorylation pathway and a reduction in the nuclear expression of NF-E2-related factor-2 (Nrf2) that down-regulate the expression of glutamate cysteine ligase, the rate limiting enzyme for GSH synthesis. The flavonoid fisetin protects against the SIN-1-mediated alterations in ERK/c-Myc phosphorylation, nuclear Nrf2 levels, glutamate cysteine ligase levels, GSH concentration and cell viability. We also show that inhibition of mitogen-activated protein kinase kinase or Raf kinase can increase GSH levels in unstressed primary rat neurons through the same ERK/c-Myc phosphorylation pathway. Together, these results demonstrate that distinct signaling pathways modulate GSH metabolism in unstressed and stressed cortical neurons.

Cross-talk between singlet oxygen- and hydrogen peroxide-dependent signaling of stress responses in Arabidopsis thaliana

Upon a dark-to-light shift, the conditional fluorescent (flu) mutant of Arabidopsis releases singlet oxygen (1O2) within the plastid compartment. Distinct sets of nuclear genes are activated that are different from those induced by superoxide (O2*-)) and/or hydrogen peroxide (H2O2), suggesting that different types of reactive oxygen species activate distinct signaling pathways. It is not known whether the pathways operate separately or interact with each other. We have addressed this problem by modulating noninvasively the level of H2O2 in plastids by means of a transgenic line that overexpresses the thylakoid-bound ascorbate peroxidase (tAPX). The overexpression of the H2O2-specific scavenger reduced strongly the activation of nuclear genes in plants treated with the herbicide paraquat that in the light leads to the enhanced generation of O2*- and H2O2. In the flu mutant overexpressing tAPX, the intensity of 1O2-mediated cell death and growth inhibition was increased when compared with the flu parental line. Also, the expression of most of the nuclear genes that were rapidly activated after the release of 1O2 was significantly higher in flu plants overexpressing tAPX, whereas in wild-type plants, overexpression of tAPX did not lead to visible stress responses and had only a very minor impact on nuclear gene expression. The results suggest that H2O2 antagonizes the 1O2-mediated signaling of stress responses as seen in the flu mutant. This cross-talk between H2O2- and 1O2-dependent signaling pathways might contribute to the overall stability and robustness of wild-type plants exposed to adverse environmental stress conditions.

Singlet oxygen inactivates protein tyrosine phosphatase-1B by oxidation of the active site cysteine

Singlet oxygen ((1)O(2)), an electronically excited form of molecular oxygen, is a mediator of biological effects of ultraviolet A radiation, stimulating signaling cascades in human cells. We demonstrate here that (1)O(2) generated by photosensitization or by thermodecomposition of 3,3'-(1,4-naphthylidene)dipropionate-1,4-endoperoxide inactivates isolated protein tyrosine phosphatases (PTPases). PTPase activities of PTP1B or CD45 were abolished by low concentrations of (1)O(2), but were largely restored by post-treatment with dithiothreitol. Electrospray ionization mass spectrometry analysis of tryptic digests of PTP1B exposed to (1)O(2) revealed oxidation of active-site Cys215 as the only cysteine residue oxidized. In summary, (1)O(2) may activate signaling cascades by interfering with phosphotyrosine dephosphorylation.

Does the oxidation of methionine in thrombomodulin contribute to the hypercoaguable state of smokers and diabetics?

The leading cause of premature death in smokers is cardiovascular disease. Diabetics also suffer from increased cardiovascular disease. This results, in part, from the hypercoagulable state associated with these conditions. However, the molecular cause(s) of the elevated risk of cardiovascular disease and the prothrombotic state of smokers and diabetics remain unknown. It is well known that oxidative stress is increased in both conditions. In smokers, it is established that oxidation of methionine residues takes place in alpha(1)-antitrypsin in lungs and that this leads to emphysema. Thrombomodulin is a key regulator of blood clotting and is found on the endothelium. Oxidation of methionine 388 in thrombomodulin is known to slow the rate at which the thrombomodulin-thrombin complex activates protein C, a protein which, in turn, degrades the factors which activate thrombin and lead to clot formation. In analogy to the cause of emphysema, it is hypothesized that oxidation of this methionine is elevated in smokers relative to non-smokers and, perhaps, in conditions such as diabetes that impose oxidative stress on the body. Evidence for the hypothesis that such an oxidation and concomitant reduction in activated protein C levels would lead to elevated cardiovascular risk is presented.

An abortive apoptotic pathway induced by singlet oxygen is due to the suppression of caspase activation

Singlet oxygen causes the cytotoxic process of tumour cells in photodynamic therapy. The mechanism by which singlet oxygen damages cells is, however, not fully understood. To address this issue, we synthesized and used two types of endoperoxides, MNPE (1-methylnaphthalene-4-propionate endoperoxide) and NDPE (naphthalene-1,4-dipropionate endoperoxide), that generate defined amounts of singlet oxygen at 37 degrees C with similar half lives. MNPE, which is more hydrophobic than NDPE, induced the release of cytochrome c from mitochondria into the cytosol and exhibited cytotoxicity, but NDPE did not. RBL cells, a rat basophil leukaemia-derived line, that overexpress phospholipid hydroperoxide glutathione peroxidase in mitochondria were found to be highly resistant to the cytotoxic effect of MNPE. MNPE treatment induced much less DNA ladder formation and nuclear fragmentation in cells than etoposide treatment, even though these treatments induced a similar extent of cellular damage. Singlet oxygen inhibited caspase 9 and 3 activities directly and also suppressed the activation of the caspase cascade. Collectively, these data suggest that singlet oxygen triggers an apoptotic pathway by releasing cytochrome c from mitochondria via the peroxidation of mitochondrial components and results in cell death that is different from typical apoptosis, because of the abortive apoptotic pathway caused by impaired caspase activation.

Translocation of Ethanolamine Phosphoglyceride is Required for Initiation of Apoptotic Death in OLN-93 Oligodendroglial Cells

The possible interplay between extracellular signal-regulated protein kinase (ERK) activation and ethanolamine phosphoglycerides (PG) membrane bilayer translocation following oxidative stress (OS) (0.5 mM H2O2/0.05 mM Fe2+), was examined in oligodendroglia, OLN93, cells with altered plasma membrane PG composition. Cells supplemented with 50 microM docosahexaenoic acid (DHA, 22:6n3) to increase the number of potential double bond targets for OS in ethanolamine-PG (EPG) were compared to cells with diminished content of EPG, attained by the addition of 0.5 mM N,N-dimethylethanolamine (dEa). After 30 min OS, EPG translocation accompanied by sustained ERK activation and nuclear translocation culminating in apoptosis was found in DHA-supplemented cells in contrast to no EPG translocation, a brief ERK activation, but no nuclear translocation, and no cell death in DHA/dEa-supplemented cells. DHA/dEa-supplemented cells pretreated with the protein-tyrosine phosphatases inhibitor Na3VO4 followed by OS, although expressing a sustained ERK activation and nuclear translocation, failed to show apoptosis and lacked EPG translocation. In DHA-supplemented cells U0126, a MEK inhibitor, prevented ERK activation and EPG translocation and protected from cell death. These findings most likely indicate that ERK activation is an indispensable component for the signaling cascades leading to EPG translocation but only activation of the latter is leading to OS-induced apoptotic cell death.

Phosphoinositide 3-kinase signaling in the cellular response to oxidative stress

Oxidative stress is linked to the pathogenesis and pathobiochemistry of various diseases, including cancer, diabetes and cardiovascular disorders. The non-specific damaging effect of reactive oxygen species (ROS) generated during oxidative stress is involved in the development of diseases, as well as the activation of specific signaling cascades in cells exposed to the higher oxidant load. A cellular signaling cascade that is activated by several types of reactive oxygen species is the phosphoinositide 3'-kinase (PI 3-kinase)/protein kinase B (PKB) pathway, which regulates cellular survival and fuel metabolism, thus establishing a link between oxidative stress and signaling in neoplastic, metabolic or degenerative diseases. Several links of PI 3-kinase/PKB signaling to ROS are discussed in this review, with particular focus on the molecular mechanisms involved in the regulation of PI 3-kinase signaling by oxidative stress and important players such as (i) the glutathione and glutaredoxin system, (ii) the thioredoxin system and (iii) Ser/Thr- and Tyr phosphatases.

Hydrogen peroxide in the Burkitt's lymphoma cell line Raji provides protection against arsenic trioxide-induced apoptosis via the phosphoinositide-3 kinase signalling pathway

Many anticarcinogenic drugs kill tumour cells by inducing apoptosis. We examined the effects of hydrogen peroxide (H(2)O(2)) on arsenic trioxide (As(2)O(3))-induced cell killing. Low concentrations of H(2)O(2) (200 micromol/l) inhibited the ability of As(2)O(3) to induce apoptosis in the Burkitt's lymphoma cell line Raji. H(2)O(2) altered the form of cell death from apoptosis to pyknosis/necrosis and also lowered the degree of cell killing by As(2)O(3). H(2)O(2) was capable of preventing caspase-3 activation induced by As(2)O(3) in Raji cells. Incubation of cells with a phosphoinositide-3 kinase (PI-3K) inhibitor, wortmannin (100 nmol/l), blocked the effects of H(2)O(2) on As(2)O(3)-induced caspase-3 activation. In addition, the PI-3K inhibitor partially blocked the effects of H(2)O(2) on up-regulation of Bcl-2 and Bcl-X(L) protein expression, down-regulation of Bax protein expression, and phosphorylation of Bcl-2 and IkappaBalpha. This investigation demonstrated for the first time that low concentrations of H(2)O(2) provide protection against the in vivo of As(2)O(3)-induced apoptosis. PI-3K plays a crucial role in enhancing cell survival during H(2)O(2), inhibiting As(2)O(3)-induced apoptosis in the Burkitt's lymphoma cells. As(2)O(3)-induced cancer cell apoptosis may be enhanced by certain antioxidants in the treatment protocol.

Reactive oxygen species: metabolism, oxidative stress, and signal transduction

Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Whereas plants are surfeited with mechanisms to combat increased ROS levels during abiotic stress conditions, in other circumstances plants appear to purposefully generate ROS as signaling molecules to control various processes including pathogen defense, programmed cell death, and stomatal behavior. This review describes the mechanisms of ROS generation and removal in plants during development and under biotic and abiotic stress conditions. New insights into the complexity and roles that ROS play in plants have come from genetic analyses of ROS detoxifying and signaling mutants. Considering recent ROS-induced genome-wide expression analyses, the possible functions and mechanisms for ROS sensing and signaling in plants are compared with those in animals and yeast.

Rapid induction of distinct stress responses after the release of singlet oxygen in Arabidopsis

The conditional fluorescent (flu) mutant of Arabidopsis accumulates the photosensitizer protochlorophyllide in the dark. After a dark-to-light shift, the generation of singlet oxygen, a nonradical reactive oxygen species, starts within the first minute of illumination and was shown to be confined to plastids. Immediately after the shift, plants stopped growing and developed necrotic lesions. These early stress responses of the flu mutant do not seem to result merely from physicochemical damage. Peroxidation of chloroplast membrane lipids in these plants started rapidly and led to the transient and selective accumulation of a stereospecific and regiospecific isomer of hydroxyoctadecatrieonic acid, free (13S)-HOTE, that could be attributed almost exclusively to the enzymatic oxidation of linolenic acid. Within the first 15 min of reillumination, distinct sets of genes were activated that were different from those induced by superoxide/hydrogen peroxide. Collectively, these results demonstrate that singlet oxygen does not act primarily as a toxin but rather as a signal that activates several stress-response pathways. Its biological activity in Arabidopsis exhibits a high degree of specificity that seems to be derived from the chemical identity of this reactive oxygen species and/or the intracellular location at which it is generated.

3-Nitrotyrosine immunoreactivity in olfactory receptor neurons of patients with Alzheimer's disease: implications for impaired odor sensitivity

Olfactory sensory function is impaired in patients with the diagnosis of probable Alzheimer's disease (AD) compared to elderly controls, and the olfactory epithelium (OE) of AD patients exhibits several pathological changes characteristic of the AD brain. To confirm that the populations from whom our postmortem tissues are obtained exhibit similar decrements in sensory function, threshold testing was performed; probable AD patients had significantly higher olfactory thresholds than controls. To determine if oxidative stress contributes to decreased olfactory function in AD, we localized 3-nitrotyrosine (3-NT) immunoreactivity in OE obtained postmortem from patients with neuropathologically confirmed AD and age-matched controls with brains free of significant neurodegenerative pathology. In AD patients, immunoreactivity was localized in olfactory receptor neurons (ORNs), including dendritic knobs where ion channels that participate in sensory transduction are located, suggesting a direct mechanism for olfactory impairment. In controls, immunoreactivity occurred in blood vessel endothelium, suggesting age-related vascular dysfunction. Immunohistochemistry for CD68, a macrophage scavenger receptor, demonstrated activated macrophages, a source of free radicals contributing to 3-NT formation, in the OE of AD patients but not controls. These results demonstrate increased oxidative stress and modification of ORN proteins that may contribute directly to olfactory impairment in AD patients.

Role of copper, zinc, selenium and tellurium in the cellular defense against oxidative and nitrosative stress

The trace elements copper, zinc and selenium are linked together in cytosolic defense against reactive oxygen and nitrogen species. Copper, zinc-superoxide dismutase catalyzes the dismutation of superoxide to oxygen and hydrogen peroxide. The latter and other hydroperoxides are subsequently reduced by the selenoenzyme glutathione peroxidase (GPx). Cytosolic GPx can also act as a peroxynitrite reductase. The antioxidative functions of these trace elements are not confined to being constituents of enzymes: 1) copper and zinc ions may stimulate protective cellular stress-signaling pathways such as the antiapoptotic phosphoinositide-3-kinase/Akt cascade and may stabilize proteins, thereby rendering them less prone to oxidation; and 2) selenium does not only exist in the cell as selenocysteine (as in GPx) but also as selenomethionine, which is regularly present in low amounts in proteins in place of methionine. Selenomethionine catalyzes the reduction of peroxynitrite at the expense of glutathione. Also, low-molecular-weight organoselenium and organotellurium compounds of pharmacologic interest catalyze the reduction of hydroperoxides or peroxynitrite with various cellular reducing equivalents.

Singlet oxygen-induced signaling effects in mammalian cells

Singlet oxygen, an electronically excited form of molecular oxygen, may be generated photochemically or in dark reactions in vivo. Singlet oxygen is not only toxic to cells and impairs signaling events but is also capable of eliciting a cellular stress response. The signaling processes initiated in this response include the activation of mitogen-activated protein kinases. Two possible activation mechanisms of signaling pathways by singlet oxygen are the generation of positive regulators as well as the inactivation of negative regulators.