Etsuo Niki

Do free radicals play causal role in atherosclerosis? Low density lipoprotein oxidation and vitamin E revisited

Lipid peroxidation induced by free radicals has been implicated in the pathogenesis of various diseases. Numerous in vitro and animal studies show that oxidative modification of low density lipoprotein (LDL) is an important initial event of atherosclerosis. Vitamin E and other antioxidants inhibit low density lipoprotein oxidation efficiently in vitro, however, human clinical trials with vitamin E have not yielded positive results. The mixed results for vitamin E effect may be ascribed primarily to the two factors. Firstly low density lipoprotein oxidation proceeds by multiple pathways mediated not only by free radicals but also by other non-radical oxidants and vitamin E is effective only against free radical mediated oxidation. Secondly, in contrast to animal experiments, vitamin E is given at the latter stage where oxidation is no more important. Free radicals must play causal role in pathogenesis of atherosclerosis and vitamin E should be effective if given at right time to right subjects.

Fatty liver induced by free radicals and lipid peroxidation

Abstract An excessive accumulation of fat in the liver leads to chronic liver injury such as non-alcoholic fatty liver disease (NAFLD), which is an important medical problem affecting many populations worldwide. Oxidative stress has been implicated in the pathogenesis of NAFLD, but the exact nature of active species and the underlying mechanisms have not been elucidated. It was previously found that the administration of free radical-generating azo compound to mice induced accumulation of fat droplet in the liver. The present study was performed aiming at elucidating the changes of lipid classes and fatty acid composition and also measuring the levels of lipid peroxidation products in the liver induced by azo compound administration to mouse. The effects of azo compound on the liver were compared with those induced by high fat diet, a well-established cause of NAFLD. Azo compounds given to mice either by intraperitoneal administration or by dissolving to drinking water induced triacylglycerol (TG) increase and concomitant phospholipid decrease in the liver, whose pattern was quite similar to that induced by high fat diet. Lipid peroxidation products such as hydroxyoctadecadienoic acid and hydroxyeicosatetraenoic acid were increased in the liver in association with the increase in TG. These results show that free radicals as well as high fat diet induce fatty liver by similar mechanisms, in which lipid peroxidation may be involved.

Plasma lipid oxidation induced by peroxynitrite, hypochlorite, lipoxygenase and peroxyl radicals and its inhibition by antioxidants as assessed by diphenyl-1-pyrenylphosphine.

Lipid oxidation has been implicated in the pathogenesis of many diseases. Lipids are oxidized in vivo by several different oxidants to give diverse products, in general lipid hydroperoxides as the major primary product. In the present study, the production of lipid hydroperoxides in the oxidation of mouse plasma induced by multiple oxidants was measured using diphenyl-1-pyrenylphosphine (DPPP) as a probe. DPPP itself is not fluorescent, but it reacts with lipid hydroperoxides stochiometrically to give highly fluorescent DPPP oxide and lipid hydroxides. The production of lipid hydroperoxides could be followed continuously in the oxidation of plasma induced by peroxynitrite, hypochlorite, 15-lipoxygenase, and peroxyl radicals with a microplate reader. A clear lag phase was observed in the plasma oxidation mediated by aqueous peroxyl radicals and peroxynitrite, but not in the oxidation induced by hypochlorite and lipoxygenase. The effects of several antioxidants against lipid oxidation induced by the above oxidants were assessed. The efficacy of antioxidants was dependent markedly on the type of oxidants. α-Tocopherol exerted potent antioxidant effects against peroxyl radical-mediated lipid peroxidation, but it did not inhibit lipid oxidation induced by peroxynitrite, hypochlorite, and 15-lipoxygenase efficiently, suggesting that multiple antioxidants with different selectivities are required for the inhibition of plasma lipid oxidation in vivo. This is a novel, simple and most high throughput method to follow plasma lipid oxidation induced by different oxidants and also to assess the antioxidant effects in biologically relevant settings.

Antioxidants: basic principles, emerging concepts, and problems.

The radical scavenging antioxidants play an essential role in the maintenance of health and prevention of diseases, and a thorough understanding of the action and capacity of antioxidants is critically important. Despite the assumption that antioxidants must exert beneficial effects against oxidative stress, many large-scale randomized controlled trials gave inconsistent and disappointing results on the prevention of chronic diseases. It is now generally accepted that there is no evidence to support the use of non-discriminative antioxidant supplements for prevention of diseases. On the other hand, recent data show that antioxidants may be effective in the prevention and/or treatment of diseases when the right antioxidant is given to the right subject at the right time for the right duration. Now it is accepted that reactive oxygen species (ROS) act as physiologically important signaling messengers as well as deleterious agents. The signaling ROS are produced in a subtly regulated manner, while many deleterious ROS are produced and react randomly. Free radical-mediated lipid peroxidation products which, in contrast to enzymatic oxidation products, are produced by non-specific mechanisms cause oxidative damage, but may also induce adaptive response to enhance the expression of antioxidant enzymes and compounds. This has raised a question if removal of too many ROS by supplementation of antioxidants may upset the cell signaling pathways and actually increase the risk of chronic diseases. However, it is unlikely that antioxidants impair physiologically essential signaling pathways.

Antioxidant action of fermented grain food supplement: Scavenging of peroxyl radicals and inhibition of plasma lipid oxidation induced by multiple oxidants

Unregulated oxidative modification of biological molecules induced by multiple oxidants in vivo has been implicated in the pathogenesis of various diseases. Accordingly, the role of antioxidants contained in foods in the maintenance of health and prevention of diseases has received much attention. The efficacy of antioxidants against oxidative stress depends on the nature of oxidants. In the present study, the antioxidant action of fermented grain food supplement, Antioxidant Biofactor (AOB), for scavenging peroxyl radical and inhibition of plasma lipid oxidation induced by multiple oxidants was measured. The antioxidant efficacy against lipid oxidation was assessed by the level of lipid hydroperoxides produced using diphenyl-1-pyrenylphosphine, which is not fluorescent per se but reacts with lipid hydroperoxides stoichiometrically to yield highly fluorescent diphenyl-1-pyrenylphosphine oxide. AOB acted as a potent peroxyl radical scavenger and suppressed lipid oxidation induced by peroxyl radical, peroxynitrite, hypochlorite, and singlet oxygen, but not by 15-lipoxygenase.

Inhibition of plasma lipid oxidation induced by peroxyl radicals, peroxynitrite, hypochlorite, 15-lipoxygenase, and singlet oxygen by clinical drugs

With increasing evidence showing the involvement of oxidative stress in the pathogenesis of various diseases, the effects of clinical drugs possessing antioxidant functions have received much attention. The unregulated oxidative modification of biological molecules leading to diseases is mediated by multiple oxidants including free radicals, peroxynitrite, hypochlorite, lipoxygenase, and singlet oxygen. The capacity of antioxidants to scavenge or quench oxidants depends on the nature of oxidants. In the present study, the antioxidant effects of several clinical drugs against plasma lipid oxidation induced by the aforementioned five kinds of oxidants were investigated from the production of lipid hydroperoxides, which have been implicated in the pathogenesis of various diseases. Troglitazone acted as a potent peroxyl radical scavenger, whereas probucol and edaravone showed only moderate reactivity and carvedilol, pentoxifylline, and ebselen did not act as radical scavenger. Probucol and edaravone suppressed plasma oxidation mediated by peroxynitrite and hypochlorite. Troglitazone and edaravone inhibited 15-lipoxygenase mediated plasma lipid oxidation, the IC50 being 20 and 34μM respectively. None of the drugs used in this study suppressed plasma lipid oxidation by singlet oxygen. This study shows that the antioxidant effects of drugs depend on the nature of oxidants and that antioxidants against multiple oxidants are required to cope with oxidative stress in vivo.

Unregulated Lipid Peroxidation in Neurological Dysfunction

Lipid peroxidation has been the subject of extensive studies focusing on its biochemical mechanisms, dynamics, product analysis, involvement in diseases, inhibition, and biological signaling. Lipid hydroperoxides are formed as major primary products, but they are substrates for various enzymes and they also undergo various secondary reactions. Recently, the biological roles of lipid peroxidation products have received much attention, not only for their pathological mechanisms, but also for their practical clinical applications as biomarkers. Hydroxyoctadecadienoic acid from linoleates, F 2 -isoprostanes from arachidonates, and neuroprostanes from docosahexanoates have been proposed as biomarkers for evaluating oxidative stress in vivo and in oxidative stress-related diseases. Although neurodegenerative disorders have various causes, there is much accumulated evidence to show that oxidative stress is involved in their pathologies. Moreover, the mechanism of reactive oxygen species production differs in each disease. Here, the mechanisms underlying Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, stroke, and Down syndrome are described.