Oxidant stress distressed

Summary

The range of photon energies in solar radiation and the diverse cell and molecular targets in skin allow for participation of oxygen radicals and oxidative stress at several levels in the development of skin cancer: DNA damage and mutation, membrane damage, and intracellular signalling. The intense UVA component of sunlight (315–400 nm) is of particular interest because of deep penetration, generation of oxidative damage and having a mutational spectrum which overlaps that of the more carcinogenic UVB (280–315 nm). Many UV-induced mutagenic and signalling events are now understood at the molecular level, and significant protection from UV carcinogenesis has been obtained with antioxidants in experimental animals. There is little evidence to suggest, however, that similar results have been achieved in humans although the converse effect has been established, of elevated skin cancer risk following simultaneous exposure to sunlight and precursors of the pro-oxidant paraquat. The present difficulty in translating these findings to prevent human skin cancer may arise from deficiencies in the models used and incomplete information about the specific responses of the target cells relevant to solar UV.     

Good genes, oxidative stress and condition-dependent sexual signals

The immune and the detoxication systems of animals are characterized by allelic polymorphisms, which underlie individual differences in ability to combat assaults from pathogens and toxic compounds. Previous studies have shown that females may improve offspring survival by selecting mates on the basis of sexual ornaments and signals that honestly reveal health. In many cases the expression of these ornaments appears to be particularly sensitive to oxidative stress. Activated immune and detoxication systems often generate oxidative stress by an extensive production of reactive metabolites and free radicals. Given that tolerance or resistance to toxic compounds and pathogens can be inherited, female choice should promote the evolution of male ornaments that reliably reveal the status of the bearers' level of oxidative stress. Hence, oxidative stress may be one important agent linking the expression of sexual ornaments to genetic variation in fitness-related traits, thus promoting the evolution of female mate choice and male sexual ornamentation, a controversial issue in evolutionary biology ever since Darwin.     

Transducing oxidative stress to death signals in neurons

Accumulation of reactive oxygen species (ROS) has been associated with aging and neurodegenerative diseases. Nevertheless, how elevated ROS levels cause neurodegeneration is unclear. In this issue, Wakatsuki et al. (2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201506102) delineate how oxidative stress is transduced into death signals, leading to neuronal apoptosis and axonal degeneration.The human brain consumes ∼20% of the body’s energy in the resting state. Oxygen metabolism produces reactive oxygen species (ROS) as a byproduct. Under normal conditions, ROS regulate redox homeostasis and serve as important messengers in cell signaling. However, when environmental stressors exacerbate ROS generation or when detoxification mechanisms fail to remove excessive ROS, the imbalance results in abnormally high levels of ROS that become toxic to cells (referred to as oxidative stress). Neurons have high energy–demanding activities, which cause significant challenges for ROS detoxification, especially in highly specialized cellular compartments such as branchy dendrites and lengthy axons. In addition, because neurons are postmitotic cells and have limited capacity to regenerate in the adult central nervous system, they are especially prone to oxidative stress and its consequences (Mattson and Magnus, 2006).Oxidative stress has long been associated with human neurological disorders such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and Friedreich ataxia (Andersen, 2004; Calabrese et al., 2006). For example, mutations of known Parkinson’s disease–associated genes, including PAR7, PINK1, PARK2, SNCA (encoding α-synuclein), and LRRK2, can directly or indirectly impair mitochondrial functions and lead to increased ROS levels as well as vulnerability to oxidative stress (Dias et al., 2013). Similarly, mutations in the gene encoding superoxide dismutase 1 (SOD1), a key antioxidative enzyme in the cell, account for ∼20% of familial amyotrophic lateral sclerosis cases (Barber and Shaw, 2010). Oxidative stress can exert cytotoxic effects through the generation of peroxides and free radicals that damage DNA, proteins, and lipids. However, the pathogenic mechanisms and the exact signaling pathways by which oxidative stress causes neurodegeneration are elusive. In this issue, Wakatsuki et al. show that the E3 ubiquitin ligase zinc and ring finger 1 (ZNRF1) plays a critical role in mediating oxidative stress–induced neuronal cell death and axonal degeneration.In a previous study by the Araki group, Wakatsuki et al. (2011) demonstrated that ZNRF1 targets AKT for ubiquitin proteasome system (UPS)–dependent degradation in Wallerian degeneration, the progressive degeneration of the distal axonal segment that is separated from the neuronal cell body in nerve injury. Removal of AKT releases phosphorylation suppression on glycogen synthase kinase 3β (GSK3β), which subsequently phosphorylates and induces collapsin response mediator protein 2 (CRMP2) degradation. CRMP2 is required for microtubule stabilization. Thus, the ZNRF1–AKT–GSK3β–CRMP2 pathway mediates axon destruction in Wallerian degeneration (Wakatsuki et al., 2011). As ZNRF1 functions as a key mediator for axonal degeneration, it is reasonable to hypothesize that this pathway may also participate in another major form of neurodegeneration—neuronal cell death. With a particular interest in oxidative stress–induced neurodegeneration, in this study, the authors first used a mouse model of focal cerebral ischemia to demonstrate that CRMP2 phosphorylation is increased in ischemic neurons. They then applied 6-hydroxydopamine (6OHDA) and H₂O₂ (both are frequently used to induce cellular oxidative stress) in primary cultured cortical neurons. They found that oxidative stress triggers AKT ubiquitination and degradation, which is prevented by overexpressing a dominant-negative form of ZNRF1 or by RNAi knockdown of endogenous ZNRF1. These results indicated that the ZNRF1–AKT–GSK3β–CRMP2 pathway is indeed activated in neurons upon oxidative stress.How then is ZNRF1 activated by oxidative stress? An important clue came from the observation that ZNRF1 is highly phosphorylated in SH-SY5Y neuroblastoma cells when treated with 6OHDA (Wakatsuki et al., 2015). A web-based program predicted that ZNRF1 may be phosphorylated at the tyrosine residue Y103 by EGF receptor (EGFR) tyrosine kinase. Using a combination of primary neuronal culture, in vivo mouse models of cerebral ischemia and 6OHDA-induced brain lesions, and in vitro kinase assay, Wakatsuki et al. (2015) demonstrated that ZNRF1 is specifically phosphorylated at Y103 by EGFR in response to oxidative stress. This activation of ZNRF1 resulted in neuronal apoptosis, as evidenced by increases in caspase 3 cleavage, annexin V–positive staining, and lactate dehydrogenase release. Moreover, the authors found that application of antioxidants such as N-acetyl-l-cysteine and curcumin, down-regulation of EGFR activity by siRNA or via the EGFR inhibitor C56, and expression of the dominant-negative form ZNRF1 C184A or the phosphorylation-resistant mutant ZNRF1 Y103F all prevented 6OHDA-induced neuronal apoptosis. Furthermore, Wakatsuki et al. (2015) characterized the cellular signaling downstream of ZNRF1 and showed that EGFR-dependent ZNRF1 phosphorylation stimulated AKT ubiquitination and degradation. Expression of either a constitutively active form of AKT or a kinase-dead form of GSK3β potently suppressed 6OHDA-induced neuronal cell death (Wakatsuki et al., 2015). Together, these results demonstrate that EGFR-dependent phosphorylation of ZNRF1 at Y103 promotes degradation of AKT and resultant activation of GSK3β, which mediates oxidative stress–induced neuronal apoptosis (Fig. 1).Open in a separate windowFigure 1.The ZNRF1 signaling pathway mediates oxidative stress–induced neuronal apoptosis and axonal degeneration. Oxidative stress induced by treatment with 6OHDA or H₂O₂ or generated by NADPH oxidases in axons upon traumatic injury activates EGFR tyrosine kinase activity and leads to ZNRF1 phosphorylation at Y103. This stimulates the E3 ubiquitin ligase activity of ZNRF1, which ubiquitinates and targets AKT for degradation via the UPS. Degradation of AKT relieves the inhibitory phosphorylation of GSK3β, which then phosphorylates CRMP2 and subjects it to degradation. In axons, CRMP2 is required for microtubule stabilization, whose disassembly results in axonal degeneration. In soma, oxidative stress activates this same ZNRF1 signaling pathway, which causes cleavage and activation of caspase 3, leading to neuronal apoptosis.Does axonal degeneration also use oxidative stress–induced activation of ZNRF1 signaling? If so, where does oxidative stress come from in the first place? Another important finding of this study is that traumatic neural injury induces oxidative stress in axons by NADPH oxidases (Wakatsuki et al., 2015). Using an in vitro model of Wallerian degeneration by primary cultured dorsal root ganglion (DRG) neurons, Wakatsuki et al. (2015) showed that the levels of oxidative stress, the activity of the endogenous EGFR kinase, and the EGFR-dependent phosphorylation of ZNRF1 at Y103 in injured neurites are all robustly increased as early as 3 h after transection. In an attempt to identify the generators of oxidative stress in injured axons, Wakatsuki et al. (2015) examined the effects of inhibiting NADPH oxidase activity. They found that the NADPH oxidase inhibitors not only prevented injury-elicited elevation of oxidative stress, but also remarkably suppressed Wallerian degeneration. Knockdown of the NADPH oxidase catalytic subunits by RNAi in cultured DRG neurons confirmed these results and further pointed out that the NADPH oxidases NOX2, 3, 4, and DUOX2 may be particularly involved in this process (Wakatsuki et al., 2015). Consistent with their previous study (Wakatsuki et al., 2011) and similar to oxidative stress–induced neuronal apoptosis, interruption of the ZNRF1 signaling pathway at each step significantly delayed Wallerian degeneration of injured axons in cultured DRG neurons (Fig. 1). Finally, Wakatsuki et al. (2015) generated transgenic mice expressing the dominant-negative mutant ZNRF1 C184A and validated the hypothesis that blocking the ZNRF1 signaling cascade protects neurons from oxidative stress–induced cell death and axonal degeneration in vivo.Wakatsuki et al. (2015) used a combination of pharmacological, genetic, biochemical, immunohistological, and other approaches to unveil the involvement of the EGFR–ZNRF1–AKT–GSK3β–CRMP2 pathway in oxidative stress–induced neurodegeneration. This is a Herculean task, especially given that this is a multistep signaling cascade, and the authors made tremendous efforts to confirm their results in various experimental setups from both in vitro and in vivo models, which establish the oxidative stress–induced, EGFR-dependent activation of the ZNRF1 E3 ligase activity as a common signaling mechanism in both of the two major neurodegeneration forms—neuronal apoptosis and axonal degeneration (Fig. 1).The ubiquitin–proteasome-mediated degradation system plays an important role in regulating protein homeostasis and is involved in neurodegenerative diseases (McKinnon and Tabrizi, 2014; Zheng et al., 2014). In addition, emerging evidence has linked the UPS to Wallerian degeneration: inhibition of the UPS activity by both pharmacological and genetic methods remarkably suppressed axonal degeneration both in vitro and in vivo (Zhai et al., 2003). A recent study in Drosophila melanogaster revealed that highwire, an E3 ubiquitin ligase, promotes Wallerian degeneration by targeting the NAD⁺ biosynthetic enzyme nicotinamide mononucleotide adenyltransferase (Nmnat) for degradation (Xiong et al., 2012). And now, the study by Wakatsuki et al. (2015) exemplifies a mechanism by which E3 ligase ZNRF1 acts as a mediator to transduce oxidative stress to death signals in neurons. It should be noted, however, that the ZNRF1 signaling pathway is unlikely to be the only mechanism promoting neurodegeneration. For example, inhibition of ZNRF1 signaling offers axon protection for up to 48 h, whereas expression of the neuroprotective Wld^S protein (Lunn et al., 1989; Coleman and Freeman, 2010; Fang and Bonini, 2012) or down-regulation of the prodegenerative SARM1 gene (Osterloh et al., 2012; Gerdts et al., 2015; Yang et al., 2015) protects injured axons for up to 72 h (Wakatsuki et al., 2015). These results strongly suggest that there are other mechanisms transducing neural injury and oxidative stress to degenerative signals in neurons.The exciting findings by Wakatsuki et al. (2015) have raised a number of further questions. One immediate question is how EGFR senses oxidative stress. EGFR exists on the cell surface and dimerizes upon binding to its specific ligands that activate its intrinsic intracellular protein–tyrosine kinase activity (Herbst, 2004). In the context of cellular oxidative stress, how is EGFR activated? A second question is, in addition to the ZNRF1–AKT–GSK3β–CRMP2 pathway, is any other signaling pathway downstream of EGFR also activated? A recent study reported that the MAPK cascade is activated in the early response of axon injury (Yang et al., 2015). MAPK pathway activation is another well-known outcome of EGFR signaling (Nguyen et al., 2013). Of note, although Wakatsuki et al. (2011, 2015) argue that AKT phosphorylates and inhibits GSK3β, which subsequently stabilizes CRMP2 and microtubules to prevent axonal degeneration, Yang et al. (2015) claim that AKT promotes axonal survival by phosphorylation of MKK4 at serine 78, which suppresses MKK4-mediated activation of prodegenerative JNK signaling. Further study of EGFR signaling in oxidative stress and neural injury is needed to provide a better understanding of the regulatory mechanisms at different stages of the degenerative process. Third, although NADPH oxidases are involved in the elevation of oxidative stress in injured axons (Wakatsuki et al., 2015), it remains unclear whether this is because axon injury promotes the activity of NADPH oxidases or because the ROS detoxification system is impaired in injured axons and the NADPH oxidase activity is merely required to maintain a steady-state level of ROS. Fourth, what is the relationship between the ZNRF1 signaling pathway, Wld^S/Nmnat, and SARM1 in axonal degeneration? Does Wld^S/Nmnat or loss of function of SARM1 manifest axonal protection by blocking a step in the EGFR–ZNRF1–AKT–GSK3β–CRMP2 axis? Finally, because EGFR has been successfully targeted in the development of antitumor drugs, identification of specific inhibitors of the EGFR–ZNRF1 signaling cascade holds a high hope for the development of effective therapeutics treating neuronal and axonal degeneration in diseases and traumatic injury. And the ultimate question is, when?     

Estrogen, insulin, and dietary signals cooperatively regulate longevity signals to enhance resistance to oxidative stress in mice

To investigate the biological significance of a longevity mutation found in daf-2 of Caenorhabditis elegans, we generated a homologous murine model by replacing Pro-1195 of insulin receptors with Leu using a targeted knock-in strategy. Homozygous mice died in the neonatal stage from diabetic ketoacidosis, whereas heterozygous mice showed the suppressed kinase activity of the insulin receptor but grew normally without spontaneously developing diabetes during adulthood. We examined heterozygous insulin receptor mutant mice for longevity phenotypes. Under 80% oxygen, mutant female mice survived 33.3% longer than wild-type female mice, whereas mutant male mice survived 18.2% longer than wild-type male mice. These results suggested that mutant mice acquired more resistance to oxidative stress, but the benefit of the longevity mutation was more pronounced in females than males. Manganese superoxide dismutase activity in mutant mice was significantly upregulated, suggesting that the suppressed insulin signaling leads to an enhanced antioxidant defense. To analyze the molecular basis of the gender difference, we administered estrogen to mutant mice. It was found that the survival of mice under 80% oxygen was extended when they were administered estradiol. In contrast, mutant and wild-type female mice showed shortened survivals when their ovaries were removed. The influence of estrogen is remarkable in mutant mice compared with wild-type mice, suggesting that estrogen modulates insulin signaling in mutant mice. Furthermore, we showed additional extension of survival under oxidative conditions when their diet was restricted. Collectively, we show that three distinct signals; insulin, estrogen, and dietary signals work in independent and cooperative ways to enhance the resistance to oxidative stress in mice.     

Oxidant stress modulates murine allergic airway responses

The allergic inflammation occurring in asthma is believed to be accompanied by the production of free radicals. To investigate the role of free radicals and the cells affected we turned to a murine model of allergic inflammation produced by sensitization to ovalbumin with subsequent aerosol challenge. We examined oxidant stress by measuring and localizing the sensitive and specific marker of lipid peroxidation, the F2-isoprostanes. F2-isoprostanes in whole lung increased from 0.30 +/- 0.08 ng/lung at baseline to a peak of 0.061 +/- 0.09 ng/lung on the ninth day of daily aerosol allergen challenge. Increased immunoreactivity to 15-F2t-IsoP (8-iso-PGF2alpha) or to isoketal protein adducts was found in epithelial cells 24 h after the first aerosol challenge and at 5 days in macrophages. Collagen surrounding airways and blood vessels, and airway and vascular smooth muscle, also exhibited increased immunoreactivity after ovalbumin challenge. Dietary vitamin E restriction in conjunction with allergic inflammation led to increased whole lung F2-isoprostanes while supplemental vitamin E suppressed their formation. Similar changes in immunoreactivity to F2-isoprostanes were seen. Airway responsiveness to methacholine was also increased by vitamin E depletion and decreased slightly by supplementation with the antioxidant. Our findings indicate that allergic airway inflammation in mice is associated with an increase in oxidant stress, which is most striking in airway epithelial cells and macrophages. Oxidant stress plays a role in the production of airway responsiveness.     

Oxidant and antioxidant signalling in plants: a re-evaluation of the concept of oxidative stress in a physiological context

While the chemical nature of reactive oxygen species (ROS) dictates that they are potentially harmful to cells, recent genetic evidence suggests that in planta purely physicochemical damage may be much more limited than previously thought. The most potentially deleterious effect of ROS under most conditions is that at high concentrations they trigger genetically programmed cell suicide events. Moreover, because plants use ROS as second messengers in signal transduction cascades in processes as diverse as mitosis, tropisms and cell death, their accumulation is crucial to plant development as well as defence. Direct ROS signal transduction will ensue only if ROS escape destruction by antioxidants or are otherwise consumed in a ROS cascade. Thus, the major low molecular weight antioxidants determine the specificity of the signal. They are also themselves signal-transducing molecules that can either signal independently or further transmit ROS signals. The moment has come to re-evaluate the concept of oxidative stress. In contrast to this pejorative or negative term, implying a state to be avoided, we propose that the syndrome would be more usefully described as 'oxidative signalling', that is, an important and critical function associated with the mechanisms by which plant cells sense the environment and make appropriate adjustments to gene expression, metabolism and physiology.     

Rapid transmission of oxidative and nitrosative stress signals from roots to shoots in Arabidopsis.

Protein kinases play a central role in signal transduction pathways in eukaryotes. A highly conserved group of kinases, termed mitogen-activated-protein kinases (MAPKs) was shown to mediate many diverse stress responses. In plants, MAPKs were shown to function in resistance responses to many biotic and abiotic stresses. Here, we show that exposure of Arabidopsis roots to hydrogen peroxide or to nitric oxide resulted in rapid activation of protein kinases in the shoots that exhibited MAPK properties. The same pattern of kinases was induced by direct injection of these compounds into leaves, indicating accurate long-distance transmission of H2O2 and NO signals. These results are important for the understanding of redox signal transmission from the rhizosphere throughout the plant.     

Atherosclerosis and oxidative stress

This review focuses on the morphological features of atherosclerosis and the involvement of oxidative stress in the initiation and progression of this disease. There is now consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein in the vascular wall. Reactive oxygen species (ROS) are key mediators of signaling pathways that underlie vascular inflammation in atherogenesis, starting from the initiation of fatty streak development, through lesion progression, to ultimate plaque rupture. Plaque rupture and thrombosis result in the acute clinical complications of myocardial infarction and stroke. Many data support the notion that ROS released from nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, myeloperoxidase (MPO), xanthine oxidase (XO), lipoxygenase (LO), nitric oxide synthase (NOS) and enhanced ROS production from dysfunctional mitochondrial respiratory chain, indeed, have a causatory role in atherosclerosis and other vascular diseases. Moreover, oxidative modifications in the arterial wall can contribute to the arteriosclerosis when the balance between oxidants and antioxidants shifts in favour of the former. Therefore, it is important to consider sources of oxidants in the context of available antioxidants such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase and transferases thiol-disulfide oxidoreductases and peroxiredoxins. Here, we review also the mechanisms in which they are involved in order to accelerate the pace of the discovery and facilitate development of novel therapeutic approaches.     

Aging and oxidative stress

The scientific establishment has been discussing the relationship between aging and oxidative stress for quite some time now. While we are still far from a general agreement about this subject, there is an impressive amount of data collected that can be used to draw a compelling picture of the events that take place during the human aging process and their correlation with the oxidant status of the organism. In this review, we bring forth the results of some key studies that can help to elucidate the aging-oxidative stress puzzle, as well as to explain which are the fundamental events in this interplay and why their causal relationships remain so elusive. We also put forward here data on the systemic oxidative stress status of a group of 503 healthy human subjects. The data consist of the plasma levels of TBARS and of the nutritional antioxidants, alpha-tocopherol, beta-carotene and ascorbic acid, and of the activity of the antioxidant enzymes, Cu, Zn-superoxide dismutase, catalase and glutathione peroxidase, of red blood cells. The data indicate that a moderate situation of oxidative stress gradually develops during human aging.     

Hyperinsulinemia and oxidative stress

The aim of the study was to compare the effect of short-term hyperglycemia and short-term hyperinsulinemia on parameters of oxidative stress in Wistar rats. Twenty male rats (aged 3 months, average weight 325 g) were tested by hyperinsulinemic clamp (100 IU/l) at two different glycemia levels (6 and 12 mmol/l). Further 20 rats were used as a control group infused with normal saline (instead of insulin) and 30 % glucose simultaneously. Measured parameters of oxidative stress were malondialdehyd (MDA), reduced glutathion (GSH) and total antioxidant capacity (AOC). AOC remained unchanged during hyperglycemia and hyperinsulinemia. Malondialdehyde (as a marker of lipid peroxidation) decreased significantly (p<0.05) during the euglycemic hyperinsulinemic clamp, and increased significantly during isolated hyperglycemia without hyperinsulinemia. Reduced glutathion decreased significantly (p<0.05) during hyperglycemia without hyperinsulinemia. These results suggest that the short-term exogenous hyperinsulinemia reduced the production of reactive oxygen species (ROS) during hyperglycemia in an animal model compared with the control group.     

Microcirculation and oxidative stress

The microcirculation is a complex and integrated system, transporting oxygen and nutrients to the cells. The key component of this system is the endothelium, contributing to the local balance between pro and anti-inflammatory mediators, hemostatic balance, as well as vascular permeability and cell proliferation. A constant shear stress maintains vascular endothelium homeostasis while perturbed shear stress leads to changes in secretion of vasodilator and vasoconstrictor agents. Increased oxidative stress is a major pathogenetic mechanism of endothelial dysfunction by decreasing NO bioavailability, promoting inflammation and participating in activation of intracellular signals cascade, so influencing ion channels activation, signal transduction pathways, cytoskeleton remodelling, intercellular communication and ultimately gene expression. Targeting the microvascular inflammation and oxidative stress is a fascinating approach for novel therapies in order to decrease morbidity and mortality of chronic and acute diseases.     

Homocysteine and oxidative stress

Summary. Hyperhomocysteinemia is an independent risk factor for cardiovascular disease (ischemic disease, such as stroke and myocardial infarction, and arterial and venous thrombotic events) in the general population. We can assume that the association is causal, based on the example of homocystinuria, and on the evidence put forward by several basic science and epidemiological studies; however, the results of large intervention trials, which will grant further support to this hypothesis, are not yet available. In addition, the mechanisms underlying this relationship, and also explaining the several toxic effects of homocysteine, related or not to cardiovascular disease, are unclear. Oxidation is one of the most favored postulated mechanisms; others are nitrosylation, acylation, and hypomethylation. Regarding the relative importance of these mechanisms, each of these hold pros and cons, and these are weighed in order to propose a balance of evidence.     

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.     

Microcirculation and oxidative stress

The microcirculation is a complex and integrated system, transporting oxygen and nutrients to the cells. The key component of this system is the endothelium, contributing to the local balance between pro and anti-inflammatory mediators, hemostatic balance, as well as vascular permeability and cell proliferation. A constant shear stress maintains vascular endothelium homeostasis while perturbed shear stress leads to changes in secretion of vasodilator and vasoconstrictor agents. Increased oxidative stress is a major pathogenetic mechanism of endothelial dysfunction by decreasing NO bioavailability, promoting inflammation and participating in activation of intracellular signals cascade, so influencing ion channels activation, signal transduction pathways, cytoskeleton remodelling, intercellular communication and ultimately gene expression. Targeting the microvascular inflammation and oxidative stress is a fascinating approach for novel therapies in order to decrease morbidity and mortality of chronic and acute diseases.     

Oxidant stress but not thromboxane decreases with epoprostenol therapy

Epoprostenol has improved the outcome of patients with primary pulmonary hypertension (PPH); however, its mechanism of action remains poorly understood. Isoprostanes are easily measured markers of oxidant stress and can activate platelets leading to increased thromboxane A2 (TxA2) production. We hypothesized that oxidant stress is associated with increased TxA2 synthesis and that epoprostenol decreases oxidant stress and TxA2 production in patients with PPH. Morning urine samples were obtained from 19 patients with PPH. We measured urinary metabolites of the isoprostane, 8-iso-PGF2alpha (F2-IsoP-M), and of TxA2 (Tx-M) before and after treatment with epoprostenol in patients with PPH. Mean (+/-SE) levels of F2-IsoP-M were elevated at baseline in our patients, 863 +/- 97 pg/mg creatinine. During treatment with epoprostenol, values decreased to 636 +/- 77 pg/mg creatinine (P = 0.011), and there was a strong correlation between the change in F2-IsoP-M and follow-up pulmonary vascular resistance (R2 = 0.69, P < 0.001). Tx-M levels were markedly elevated at baseline and were unchanged with therapy. These results indicate that oxidant stress decreases with epoprostenol therapy and is associated with hemodynamic and clinical improvement. The failure of Tx-M to decrease with therapy suggests that epoprostenol does not exert a beneficial effect through inhibition of TxA2 production in patients with PPH.