Hepatitis C virus, oxidative stress and steatosis: current status and perspectives

Reactive oxygen and nitrogen species (ROS/RNS), whether produced endogenously as a consequence of normal cell functions or derived from external sources, pose a constant threat to cells living in an aerobic environment. When the production of ROS/RNS overrides the antioxidant capability of the target cells, oxidative damage may occur as a consequence of the interaction with DNA, protein, and lipids. Hepatitis C virus (HCV) is a major cause of viral hepatitis. Although the molecular mechanisms of HCV pathogenesis remain unclear, oxidative stress is emerging as a key step and a major initiator in the development and the progression of liver damage, and the evaluation of oxidative stress may be useful for a better understanding of the pathogenesis of hepatitis C. Liver steatosis is one of the most important histopathological features in patients with chronic hepatitis C. Both viral and host factors contribute to the development of steatosis, and putative defects caused by ROS/RNS may be involved through abnormalities in lipid metabolism. This review is aimed to offer an updated overview of the relationship between oxidative stress and HCV infection, focusing on the significance of ROS/RNS in the pathogenesis of liver disease. The potential role played by oxidative stress in the pathogenic mechanisms of HCV-related steatosis is also discussed.     

Influence of vitamin C and vitamin E on redox signaling: Implications for exercise adaptations

The exogenous antioxidants vitamin C (ascorbate) and vitamin E (α-tocopherol) often blunt favorable cell signaling responses to exercise, suggesting that redox signaling contributes to exercise adaptations. Current theories posit that this antioxidant paradigm interferes with redox signaling by attenuating exercise-induced reactive oxygen species (ROS) and reactive nitrogen species (RNS) generation. The well-documented in vitro antioxidant actions of ascorbate and α-tocopherol and characterization of the type and source of the ROS/RNS produced during exercise theoretically enable identification of redox-dependent mechanisms responsible for the blunting of favorable cell signaling responses to exercise. This review aimed to apply this reasoning to determine how the aforementioned antioxidants might attenuate exercise-induced ROS/RNS production. The principal outcomes of this analysis are (1) neither antioxidant is likely to attenuate nitric oxide signaling either directly (reaction with nitric oxide) or indirectly (reaction with derivatives, e.g., peroxynitrite); (2) neither antioxidant reacts appreciably with hydrogen peroxide, a key effector of redox signaling; (3) ascorbate but not α-tocopherol has the capacity to attenuate exercise-induced superoxide generation; and (4) alternate mechanisms, namely pro-oxidant side reactions and/or reduction of bioactive oxidized macromolecule adducts, are unlikely to interfere with exercise-induced redox signaling. Out of all the possibilities considered, ascorbate-mediated suppression of superoxide generation with attendant implications for hydrogen peroxide signaling is arguably the most cogent explanation for blunting of favorable cell signaling responses to exercise. However, this mechanism is dependent on ascorbate accumulating at sites rich in NADPH oxidases, principal contributors to contraction-mediated superoxide generation, and outcompeting nitric oxide and superoxide dismutase isoforms. The major conclusions of this review are: (1) direct evidence for interference of ascorbate and α-tocopherol with exercise-induced ROS/RNS production is lacking; (2) theoretical analysis reveals that both antioxidants are unlikely to have a major impact on exercise-induced redox signaling; and (3) it is worth considering alternate redox-independent mechanisms.     

Free radicals and antioxidants in normal physiological functions and human disease

Reactive oxygen species (ROS) and reactive nitrogen species (RNS, e.g. nitric oxide, NO(*)) are well recognised for playing a dual role as both deleterious and beneficial species. ROS and RNS are normally generated by tightly regulated enzymes, such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. Overproduction of ROS (arising either from mitochondrial electron-transport chain or excessive stimulation of NAD(P)H) results in oxidative stress, a deleterious process that can be an important mediator of damage to cell structures, including lipids and membranes, proteins, and DNA. In contrast, beneficial effects of ROS/RNS (e.g. superoxide radical and nitric oxide) occur at low/moderate concentrations and involve physiological roles in cellular responses to noxia, as for example in defence against infectious agents, in the function of a number of cellular signalling pathways, and the induction of a mitogenic response. Ironically, various ROS-mediated actions in fact protect cells against ROS-induced oxidative stress and re-establish or maintain "redox balance" termed also "redox homeostasis". The "two-faced" character of ROS is clearly substantiated. For example, a growing body of evidence shows that ROS within cells act as secondary messengers in intracellular signalling cascades which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. This review will describe the: (i) chemistry and biochemistry of ROS/RNS and sources of free radical generation; (ii) damage to DNA, to proteins, and to lipids by free radicals; (iii) role of antioxidants (e.g. glutathione) in the maintenance of cellular "redox homeostasis"; (iv) overview of ROS-induced signaling pathways; (v) role of ROS in redox regulation of normal physiological functions, as well as (vi) role of ROS in pathophysiological implications of altered redox regulation (human diseases and ageing). Attention is focussed on the ROS/RNS-linked pathogenesis of cancer, cardiovascular disease, atherosclerosis, hypertension, ischemia/reperfusion injury, diabetes mellitus, neurodegenerative diseases (Alzheimer's disease and Parkinson's disease), rheumatoid arthritis, and ageing. Topics of current debate are also reviewed such as the question whether excessive formation of free radicals is a primary cause or a downstream consequence of tissue injury.     

Diabetes,oxidative stress and therapeutic strategies

Background

Diabetes has emerged as a major threat to health worldwide.

Scope of Review

The exact mechanisms underlying the disease are unknown; however, there is growing evidence that excess generation of reactive oxygen species (ROS), largely due to hyperglycemia, causes oxidative stress in a variety of tissues. Oxidative stress results from either an increase in free radical production, or a decrease in endogenous antioxidant defenses, or both. ROS and reactive nitrogen species (RNS) are products of cellular metabolism and are well recognized for their dual role as both deleterious and beneficial species. In type 2 diabetic patients, oxidative stress is closely associated with chronic inflammation. Multiple signaling pathways contribute to the adverse effects of glucotoxicity on cellular functions. There are many endogenous factors (antioxidants, vitamins, antioxidant enzymes, metal ion chelators) that can serve as endogenous modulators of the production and action of ROS. Clinical trials that investigated the effect of antioxidant vitamins on the progression of diabetic complications gave negative or inconclusive results. This lack of efficacy might also result from the fact that they were administered at a time when irreversible alterations in the redox status are already under way. Another strategy to modulate oxidative stress is to exploit the pleiotropic properties of drugs directed primarily at other targets and thus acting as indirect antioxidants.

Major Conclusions

It appears important to develop new compounds that target key vascular ROS producing enzymes and mimic endogenous antioxidants.

General significance

This strategy might prove clinically relevant in preventing the development and/or retarding the progression of diabetes associated with vascular diseases.     

Oxidative mechanism of arsenic toxicity and carcinogenesis

Arsenic is a known toxin and carcinogen that is present in industrial settings and in the environment. The mechanisms of disease initiation and progression are not fully understood. In the last a few years, there has been increasing evidence of the correlation between the generation of reactive oxygen species (ROS), DNA damage, tumor promotion, and arsenic exposure. This article summarizes the current literature on the arsenic mediated generation of ROS and reactive nitrogen species (RNS) in various biological systems. This article also discusses the role of ROS and RNS in arsenic-induced DNA damage and activation of oxidative sensitive gene expression.     

Chemical and molecular mechanisms of antioxidants: experimental approaches and model systems

Free radicals derived from oxygen, nitrogen and sulphur molecules in the biological system are highly active to react with other molecules due to their unpaired electrons. These radicals are important part of groups of molecules called reactive oxygen/nitrogen species (ROS/RNS), which are produced during cellular metabolism and functional activities and have important roles in cell signalling, apoptosis, gene expression and ion transportation. However, excessive ROS attack bases in nucleic acids, amino acid side chains in proteins and double bonds in unsaturated fatty acids, and cause oxidative stress, which can damage DNA, RNA, proteins and lipids resulting in an increased risk for cardiovascular disease, cancer, autism and other diseases. Intracellular antioxidant enzymes and intake of dietary antioxidants may help to maintain an adequate antioxidant status in the body. In the past decades, new molecular techniques, cell cultures and animal models have been established to study the effects and mechanisms of antioxidants on ROS. The chemical and molecular approaches have been used to study the mechanism and kinetics of antioxidants and to identify new potent antioxidants. Antioxidants can decrease the oxidative damage directly via reacting with free radicals or indirectly by inhibiting the activity or expression of free radical generating enzymes or enhancing the activity or expression of intracellular antioxidant enzymes. The new chemical and cell-free biological system has been applied in dissecting the molecular action of antioxidants. This review focuses on the research approaches that have been used to study oxidative stress and antioxidants in lipid peroxidation, DNA damage, protein modification as well as enzyme activity, with emphasis on the chemical and cell-free biological system.     

Free radicals, chemokines, and cell injury in HIV-1 and SIV infections and alcoholic hepatitis

Oxidative stress has been observed in HIV-1 infection and alcoholic liver disease. The formation of reactive oxygen species (ROS) contributes to cell injury through apoptosis and/or necrosis and secretion of proinflammatory cytokines and chemokines. The major sources of ROS and chemokines are the Kupffer cells. During chronic ethanol consumption they are primed and activated for enhanced formation of pro-inflammatory factors, probably as a result of ethanol-induced translocation of gut-derived endotoxin into the circulation. Pro-inflammatory factors produced in the liver stimulate neutrophilic and/or lymphocytic infiltration to this organ. The presence of inflammatory cells in the liver may compromise the hepatocytes to injury by releasing cytotoxic factors, i.e., ROS, cytolytic proteases. Kupffer cells also interact with the glycoprotein 120 of SIV and HIV-1, which can induce oxidative stress and chemokine release. CD4+ lymphocytes that are infected with the virus generate intracellular ROS, which in turn leads to apoptosis and cell death. Downregulation of CD4+ cells contributes to immunodeficiency, while enhanced sequestration of inflammatory cells in the liver during chronic ethanol use with or without HIV-1/SIV may result in hepatocyte injury and exacerbation of alcoholic liver disease.     

Increased Reactive Oxygen Species Formation and Oxidative Stress in Rheumatoid Arthritis

BackgroundRheumatoid arthritis (RA) is an autoimmune inflammatory disorder. Highly reactive oxygen free radicals are believed to be involved in the pathogenesis of the disease. In this study, RA patients were sub-grouped depending upon the presence or absence of rheumatoid factor, disease activity score and disease duration. RA Patients (120) and healthy controls (53) were evaluated for the oxidant—antioxidant status by monitoring ROS production, biomarkers of lipid peroxidation, protein oxidation and DNA damage. The level of various enzymatic and non-enzymatic antioxidants was also monitored. Correlation analysis was also performed for analysing the association between ROS and various other parameters.MethodsIntracellular ROS formation, lipid peroxidation (MDA level), protein oxidation (carbonyl level and thiol level) and DNA damage were detected in the blood of RA patients. Antioxidant status was evaluated by FRAP assay, DPPH reduction assay and enzymatic (SOD, catalase, GST, GR) and non-enzymatic (vitamin C and GSH) antioxidants.ResultsRA patients showed a higher ROS production, increased lipid peroxidation, protein oxidation and DNA damage. A significant decline in the ferric reducing ability, DPPH radical quenching ability and the levels of antioxidants has also been observed. Significant correlation has been found between ROS and various other parameters studied.ConclusionRA patients showed a marked increase in ROS formation, lipid peroxidation, protein oxidation, DNA damage and decrease in the activity of antioxidant defence system leading to oxidative stress which may contribute to tissue damage and hence to the chronicity of the disease.     

Energy determinants GAPDH and NDPK act as genetic modifiers for hepatocyte inclusion formation

Genetic factors impact liver injury susceptibility and disease progression. Prominent histological features of some chronic human liver diseases are hepatocyte ballooning and Mallory-Denk bodies. In mice, these features are induced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) in a strain-dependent manner, with the C57BL and C3H strains showing high and low susceptibility, respectively. To identify modifiers of DDC-induced liver injury, we compared C57BL and C3H mice using proteomic, biochemical, and cell biological tools. DDC elevated reactive oxygen species (ROS) and oxidative stress enzymes preferentially in C57BL livers and isolated hepatocytes. C57BL livers and hepatocytes also manifested significant down-regulation, aggregation, and nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). GAPDH knockdown depleted bioenergetic and antioxidant enzymes and elevated hepatocyte ROS, whereas GAPDH overexpression decreased hepatocyte ROS. On the other hand, C3H livers had higher expression and activity of the energy-generating nucleoside-diphosphate kinase (NDPK), and knockdown of hepatocyte NDPK augmented DDC-induced ROS formation. Consistent with these findings, cirrhotic, but not normal, human livers contained GAPDH aggregates and NDPK complexes. We propose that GAPDH and NDPK are genetic modifiers of murine DDC-induced liver injury and potentially human liver disease.     

Down regulation of superoxide dismutases and glutathione peroxidase by reactive oxygen and nitrogen species

The levels of antioxidative enzymes are regulated by gene expressions as well as by post-translational modifications. Although their functions are to scavenge reactive oxygen (ROS) and nitrogen species (RNS), they may also be targets of various oxidants. When ROS and RNS modify the functions of antioxidative enzymes, especially glutathione peroxidase, they may induce apoptotic cell death in susceptible cells. It is conceivable, therefore, that at least a part of the apoptotic pathways mediated by ROS and RNS may be associated with modification of the redox regulation of cellular functions due to elevations of such substances. In this article we review recent findings about the effects of various oxidative conditions associated with alteration of these antioxidative enzymes and the concomitant cellular damage induced.     

Down regulation of superoxide dismutases and glutathione peroxidase by reactive oxygen and nitrogen species

The levels of antioxidative enzymes are regulated by gene expressions as well as by post-translational modifications. Although their functions are to scavenge reactive oxygen (ROS) and nitrogen species (RNS), they may also be targets of various oxidants. When ROS and RNS modify the functions of antioxidative enzymes, especially glutathione peroxidase, they may induce apoptotic cell death in susceptible cells. It is conceivable, therefore, that at least a part of the apoptotic pathways mediated by ROS and RNS may be associated with modification of the redox regulation of cellular functions due to elevations of such substances. In this article we review recent findings about the effects of various oxidative conditions associated with alteration of these antioxidative enzymes and the concomitant cellular damage induced.     

Mitochondrial superoxide mediates labile iron level: evidence from Mn-SOD-transgenic mice and heterozygous knockout mice and isolated rat liver mitochondria

Superoxide is the main reactive oxygen species (ROS) generated by aerobic cells primarily in mitochondria. It is also capable of producing other ROS and reactive nitrogen species (RNS). Moreover, superoxide has the potential to release iron from its protein complexes. Unbound or loosely bound cellular iron, known as labile iron, can catalyze the formation of the highly reactive hydroxyl radical. ROS/RNS can cause mitochondrial dysfunction and damage. Manganese superoxide dismutase (Mn-SOD) is the chief ROS-scavenging enzyme and thereby the primary antioxidant involved in protecting mitochondria from oxidative damage. To investigate whether mitochondrial superoxide mediates labile iron in vivo, the levels of labile iron were determined in the tissues of mice overexpressing Mn-SOD and heterozygous Mn-SOD-knockout mice. Furthermore, the effect of increased mitochondrial superoxide generation on labile iron levels was determined in isolated rat liver mitochondria exposed to various electron transport inhibitors. The results clearly showed that increased expression of Mn-SOD significantly lowered the levels of labile iron in heart, liver, kidney, and skeletal muscle, whereas decreased expression of Mn-SOD significantly increased the levels of labile iron in the same organs. In addition, the data showed that peroxidative damage to membrane lipids closely correlated with the levels of labile iron in various tissues and that altering the status of Mn-SOD did not alter the status of other antioxidant systems. Results also showed that increased ROS production in isolated liver mitochondria significantly increased the levels of mitochondrial labile iron. These findings constitute the first evidence suggesting that mitochondrial superoxide is capable of releasing iron from its protein complexes in vivo and that it could also release iron from protein complexes contained within the organelle.     

Metabolic syndrome and mitochondrial function: molecular replacement and antioxidant supplements to prevent membrane peroxidation and restore mitochondrial function

Metabolic syndrome consists of a cluster of metabolic conditions, such as hypertriglyceridemia, hyper-low-density lipoproteins, hypo-high-density lipoproteins, insulin resistance, abnormal glucose tolerance and hypertension, that-in combination with genetic susceptibility and abdominal obesity-are risk factors for type 2 diabetes, vascular inflammation, atherosclerosis, and renal, liver and heart disease. One of the defects in metabolic syndrome and its associated diseases is excess cellular oxidative stress (mediated by reactive oxygen and nitrogen species, ROS/RNS) and oxidative damage to mitochondrial components, resulting in reduced efficiency of the electron transport chain. Recent evidence indicates that reduced mitochondrial function caused by ROS/RNS membrane oxidation is related to fatigue, a common complaint of MS patients. Lipid replacement therapy (LRT) administered as a nutritional supplement with antioxidants can prevent excess oxidative membrane damage, restore mitochondrial and other cellular membrane functions and reduce fatigue. Recent clinical trials have shown the benefit of LRT plus antioxidants in restoring mitochondrial electron transport function and reducing moderate to severe chronic fatigue. Thus LRT plus antioxidant supplements should be considered for metabolic syndrome patients who suffer to various degrees from fatigue.     

The role of kupffer cell oxidant production in early ethanol-induced liver disease

Considerable evidence for a role of Kupffer cells in alcoholic liver disease has accumulated and they have recently been shown to be a predominant source of free radicals. Several approaches including pharmacological agents, knockout mice, and viral gene transfer have been used to fill critical gaps in understanding key mechanisms by which Kupffer cell activation, oxidant formation, and cytokine production lead to liver damage and subsequent pathogenesis. This review highlights new data in support of the hypothesis that Kupffer cells play a pivotal role in hepatotoxicity due to ethanol by producing oxidants via NADPH oxidase.     

Antioxidant responses to oxidant-mediated lung diseases

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated throughout the human body. Enzymatic and nonenzymatic antioxidants detoxify ROS and RNS and minimize damage to biomolecules. An imbalance between the production of ROS and RNS and antioxidant capacity leads to a state of "oxidative stress" that contributes to the pathogenesis of a number of human diseases by damaging lipids, protein, and DNA. In general, lung diseases are related to inflammatory processes that generate increased ROS and RNS. The susceptibility of the lung to oxidative injury depends largely on its ability to upregulate protective ROS and RNS scavenging systems. Unfortunately, the primary intracellular antioxidants are expressed at low levels in the human lung and are not acutely induced when exposed to oxidative stresses such as cigarette smoke and hyperoxia. However, the response of extracellular antioxidant enzymes, the critical primary defense against exogenous oxidative stress, increases rapidly and in proportion to oxidative stress. In this paper, we review how antioxidants in the lung respond to oxidative stress in several lung diseases and focus on the mechanisms that upregulate extracellular glutathione peroxidase.