Membrane cholesterol contents influence the protective effects of quercetin and rutin in erythrocytes damaged by oxidative stress

Flavonoids are potent scavengers of reactive oxygen species (ROS) that effectively prevent erythrocyte oxidation. Their antioxidant activities are governed by their structural characteristics and their ability to interact with and penetrate lipid bilayers. In order to gain a better understanding of the relationship between cholesterol contents and the antioxidant effectiveness of flavonoids against oxidative damage induced by ROS in cells, here we analyzed the integrity and structural stability of cholesterol-modified (enriched or depleted) and control erythrocytes exposed to tert-butyl hydroperoxide in the presence of quercetin or rutin. In control and cholesterol-enriched erythrocytes, quercetin provided greater protection against lipid peroxidation, ROS formation, and it preserved better cellular integrity than rutin. Both antioxidants suppressed the alterations in membrane fluidity and lipid losses with similar efficiency, reducing hemoglobin oxidation by 30% and GSH losses by 60% in the above-mentioned erythrocytes. Cholesterol depletion reduced the efficiency of the antioxidant power of both flavonoids against oxidative damage induced in the erythrocyte membrane, while a stronger degree of protection of GSH and hemoglobin contents was observed, mainly in the presence of rutin. These findings suggest a preferential incorporation of the antioxidants into the membranes from erythrocytes with normal and high cholesterol contents, whereas they would mainly be located in the cytoplasm of cholesterol-depleted erythrocytes.     

Interaction of ascorbate and alpha-tocopherol enhances antioxidant reserve of erythrocytes during anemia in visceral leishmaniasis

Visceral leishmaniasis (V.L.) is associated with enhanced lipid peroxidation along with impaired function of antioxidant defense system in erythrocytes. The effect of chronic treatment with ascorbate and alpha-tocopherol was studied on erythrocytes in hamsters infected with Leishmania donovani. Combination treatment with both antioxidants proved to be a potential suppressor of lipid hydroperoxide formation as well as hypotonic osmotic lysis during the leishmanial infection. Positive correlations between the depleted levels of erythrocyte ascorbate, GSH and alpha-tocopherol exhibit proportionate alterations in the nonenzymatic antioxidant levels at different stages of infection. Indirect measurement of transmembrane electron transfer as ferricyanide reduction suggests an active participation of endogenous contents of ascorbate and alpha-tocopherol in the protection against oxidative damage of membrane lipids. Cooperative behavior of both antioxidants in the ferricyanide reducing capacity was further evinced by resealing the ghosts in presence of exogenous ascorbate and alpha-tocopherol. Furthermore, intravesicular ascorbate serves in the defense of extravesicular ferricyanide induced oxidation of endogenous alpha-tocopherol. The results suggest an interacting role of ascorbate and alpha-tocopherol in maintaining the antioxidant reserve of erythrocytes during anemia in V.L.     

Protective effect of quercetin and rutin on photosensitized lysis of human erythrocytes in the presence of hematoporphyrin

Photosensitized hemolysis of human erythrocytes by hematoporphyrin was suppressed by flavonols such as quercetin and rutin at submillimolar concentrations. The suppression of photohemolysis was accompanied by inhibition of lipid peroxidation by the reagents. Quercetin and rutin were photooxidized in the presence of hematoporphyrin and the photooxidation was partially suppressed by 1 mM NaN3, a quencher of singlet molecular oxygen. Flavonols were also oxidized by radicals formed during degradation of lauroyl peroxide. These results indicate that flavonols can function as antioxidants in biological systems by terminating radical chain reactions and removing singlet molecular oxygen. A pharmacological function of flavonols, decrease of the increased permeability and fragility of capillary, was discussed in relation to their antioxidative functions.     

Phenolic compounds protect HepG2 cells from oxidative damage: relevance of glutathione levels

In the present work, the potential hepatoprotective effects of five phenolic compounds against oxidative damages induced by tert-butyl hydroperoxide (t-BHP) were evaluated in HepG2 cells in order to relate in vitro antioxidant activity with cytoprotective effects. t-BHP induced considerable cell damage in HepG2 cells as shown by significant LDH leakage, increased lipid peroxidation, DNA damage as well as decreased levels of reduced glutathione (GSH). All tested phenolic compounds significantly decreased cell death induced by t-BHP (when in co-incubation). If the effects of quercetin are given the reference value 1, the compounds rank in the following order according to inhibition of cell death: luteolin (4.0) > quercetin (1.0) > rosmarinic acid (0.34) > luteolin-7-glucoside (0.30) > caffeic acid (0.21). The results underscore the importance of the compound's lipophilicity in addition to its antioxidant potential for its biological activity. All tested phenolic compounds were found to significantly decrease lipid peroxidation and prevent GSH depletion induced by t-BHP, but only luteolin and quercetin significantly decreased DNA damage. Therefore, the lipophilicity of the natural antioxidants tested appeared to be of even greater importance for DNA protection than for cell survival. The protective potential against cell death was probably achieved mainly by preventing intracellular GSH depletion. The phenolic compounds studied here showed protective potential against oxidative damage induced in HepG2 cells. This could be beneficial against liver diseases where it is known that oxidative stress plays a crucial role.     

Quercetin metabolites inhibit copper ion-induced lipid peroxidation in rat plasma

The oxidative susceptibility of plasma obtained from rats after intragastric administration of quercetin was studied to know whether or not quercetin acts as an in vivo antioxidant after metabolic conversion. Quercetin was raised in the rat blood plasma essentially as glucuronide and/or sulfate conjugates. The plasma obtained from rats after quercetin administration was more resistant against copper sulfate-induced lipid peroxidation than the control plasma on the basis of the accumulation of cholesteryl ester hydroperoxides and the consumption of α-tocopherol. The results strongly suggest that some conjugated metabolites of quercetin act as effective antioxidants when plasma is subject to metal ion-induced lipid peroxidation.     

Antioxidant effect of red wine polyphenols on red blood cells

The protective effect of red wine polyphenols against hydrogen peroxide (H(2)O(2))-induced oxidation was investigated in normal human erythrocytes (RBCs). RBCs, preincubated with micromolar amounts of wine extract and challenged with H(2)O(2), were analyzed for reactive oxygen species (ROS), hemolysis, methemoglobin production, and lipid peroxidation. All these oxidative modifications were prevented by incubating the RBCs with oak barrel aged red wine extract (SD95) containing 3.5 mM gallic acid equivalent (GAE) of phenolic compounds. The protective effect was less apparent when RBCs were incubated with wines containing lower levels of polyphenols. Furthermore, resveratrol and quercetin, well known red wine antioxidants, showed lower antioxidant properties compared with SD95, indicating that interaction between constituents may bring about effects that are not necessarily properties of the singular components. Our findings demonstrate that the nonalcoholic components of red wine, mainly polyphenols, have potent antioxidant properties, supporting the hypothesis of a beneficial effect of red wine in oxidative stress in human system.     

Lipid peroxidation and antioxidants in hyperlipidemia and hypertension

Lipid peroxidation and lipid-derived oxidized products have been implicated in the pathogenesis of a variety of human diseases. To clarify the role of oxidative stress in essential hypertension and hypercholesterolemia the in vitro oxidative susceptibility of LDL, the antioxidant status and the lipid peroxide content of blood plasma were examined in hypercholesterolemic (HC), hypertensive (H), hypercholesterolemic/hypertensive (HH) and normolipidemic/normotensive subjects (N). Plasma ascorbate and lipid-soluble antioxidants were lower, while LDL oxidizability, CE-OOH and TL-OOH were higher in H, HC, and HH groups than in the N group. No difference was observed among groups for PL-OOH and isoprostanes. In summary, the results show that: 1) lipid- and water-soluble antioxidants are lower in hypercholesterolemic and hypertensive patients as compared to normal subjects, whereas the lipid peroxide content and the LDL susceptibility to oxidation were higher; 2) total cholesterol, LDL-cholesterol, apoB and CE-OOH were negatively correlated with the content of a-tocopherol; 3) there was a positive correlation between the content of lipid-soluble antioxidants and the resistance of LDL to oxidation; and 4) CE-OOH and TL-OOH were positively correlated with total cholesterol and LDL-cholesterol.     

Increase in vulnerability to oxidative damage in cholesterol-modified erythrocytes exposed to t-BuOOH

During the course of radical oxidation, cholesterol may exert seemingly contradictory effects. In order to gain a better understanding of the relationship between cholesterol levels and membrane susceptibility to oxidative damage induced by reactive oxygen species (ROS), here we analyze the integrity and structural stability of cholesterol-modified (enriched or depleted) and unmodified (control) erythrocytes exposed to tert-butyl hydroperoxide. The oxidant significantly increased ROS production, with almost complete oxidation of hemoglobin and a reduction in GSH content in the different erythrocyte groups at 2 mM concentration. These changes were accompanied by losses of cholesterol and total phospholipids, the main decreases being in phosphatidylethanolamine and phosphatidylcholine. The highest lipid loss was found in the cholesterol-depleted group. Fatty acid analyses revealed changes only in peroxidized cholesterol-modified erythrocytes, with decreases in linoleic and arachidonic acids. Fluorescence anisotropy studies showed an increase in the fluidity of the negatively charged surface of peroxidized control erythrocytes. Increased hemolysis and a positive correlation between cellular osmotic fragility and malondialdehyde contents were found in all peroxidized groups. These findings provide evidence that the modification of cholesterol levels in the erythrocyte membrane has provoking effects on peroxidation, with corresponding increases in oxidative damage in the treated cell, possibly as a consequence of lipid bilayer destabilization.     

Comparing beta-carotene,vitamin E and nitric oxide as membrane antioxidants

Singlet oxygen initiates lipid peroxidation via a nonfree radical mechanism by reacting directly with unsaturated lipids to form lipid hydroperoxides (LOOHs). These LOOHs can initiate free radical chain reactions leading to membrane leakage and cell death. Here we compare the ability and mechanism by which three small-molecule membrane antioxidants (beta-carotene, alpha-tocopherol and nitric oxide) inhibit lipid peroxidation in membranes. We demonstrate that beta-carotene provides protection against singlet oxygen-mediated lipid peroxidation, but does not slow free radical-mediated lipid peroxidation. Alpha-Tocopherol does not protect cells from singlet oxygen, but does inhibit free radical formation in cell membranes. Nitric oxide provides no direct protection against singlet oxygen exposure, but is an exceptional chain-breaking antioxidant as evident from its ability to blunt oxygen consumption during free radical-mediated lipid peroxidation. These three small-molecule antioxidants appear to have complementary mechanisms for the protection of cell membranes from detrimental oxidations.     

Effect of oral methionine on blood lipid peroxidation and antioxidants in alloxan-induced diabetic rats

Supplementation of thiol compounds has been suggested to protect against the toxic effects of reduced oxygen species by contributing to the thiol pool of the cell. The present study was designed to determine whether supplementation of methionine in the diet of diabetic animals protected against the oxidative stress in diabetic pathology. Oral methionine was administered at a dosage of 330 mg/100 g feed to diabetic rats. The effect was compared with the effect of insulin administration. Levels of lipid peroxides were measured in plasma, erythrocytes, and erythrocyte membrane. Anti-oxidants were measured in plasma. Diabetic condition was associated with increased lipid peroxidation and depletion in antioxidant levels. Although methionine did not affect the level of blood glucose and some of the antioxidants, it lowered the lipid peroxide content in blood. Erythrocyte lipid peroxidation activity was unaffected by methionine treatment. Administration of insulin lowered both plasma and erythrocyte lipid peroxide levels.     

Cholesterol-rich diets have different effects on lipid peroxidation, cholesterol oxides, and antioxidant enzymes in rats and rabbits

The objective of this study was to compare the effect of cholesterol feeding of rats and rabbits. The levels of lipid peroxidation products and oxysterols in the plasma of the two species plus the antioxidant enzyme activities in the liver and erythrocytes were measured to explain their different susceptibilities to atherosclerosis. Our study showed that rats are less susceptible than are rabbits to the atherogenic effect of a cholesterol-rich diet because of differences in lipid peroxidation products as well as antioxidant enzymes activities in their livers. In rabbits, cholesterol feeding produced severe hypercholesterolemia (43-fold increase) and increased plasma and liver lipid peroxidation. Total as well as the individual oxysterol contents of 7alpha-, 7beta-hydroxycholesterol, alpha-epoxy, beta-epoxycholesterol, cholestanetriol, 7-keto, and 27-hydroxycholesterol significantly increased in the plasma of hypercholesterolemic (HC) rabbits. Erythrocyte glutathione peroxidase (GSH-Px) activity significantly decreased whereas catalase activity significantly increased in HC rabbits. In rats cholesterol feeding increased the plasma cholesterol only twofold and had no effect on plasma or liver lipid peroxidation. Only 7alpha- and 7beta-hydroxycholesterol increased and no change was observed in any of the antioxidant enzymes activity in the erythrocytes. Although cholesterol feeding caused a 10-fold increase of liver cholesterol as ester in both rats and rabbits, the antioxidant enzyme GSH-Px and catalase activities in the liver significantly increased in rats but significantly decreased in rabbits. The increase of GSH-Px and catalase activities in the liver of cholesterol fed rats could have a protective role against oxidation, thus preventing the formation of lipid peroxidation and oxysterols.     

Ultraviolet-treated lipoproteins as a model system for the study of the biological effects of lipid peroxides on cultured cells. III. The protective effect of antioxidants (probucol, catechin, vitamin E) against the cytotoxicity of oxidized LDL occurs in two different ways

Comparison of the protective effect of three antioxidants (from three different chemical classes) against cell injury due to LDL oxidation, allowed us to clearly discriminate between two different lines of defence. The ultraviolet-induced lipid peroxidation of LDL was strongly inhibited by 10 mumol/l catechin and 25 mumol/l probucol, but only poorly by 100 mumol/l vitamin E. The ultraviolet-treated LDL protected by catechin or probucol (i.e. LDL irradiated by ultraviolet in the presence of effective concentrations of antioxidants inhibiting the lipid peroxidation) were much less 'cytotoxic' than unprotected ultraviolet-treated LDL. In contrast, LDL treated by ultraviolet in the presence of 100 mumol/l vitamin E were 'cytotoxic' similarly to unprotected LDL. The level of 'cytotoxicity' of LDL treated by ultraviolet in the presence of antioxidants (protected ultraviolet-treated LDL) was well correlated with their content in lipid peroxidation markers. Therefore these markers can be useful for predicting the 'cytotoxicity' of oxidized LDL and subsequently the protective effect of the tested antioxidants. The 'cytotoxicity' of unprotected ultraviolet-treated LDL (i.e. LDL irradiated by ultraviolet in the absence of exogenous antioxidant) can be effectively blocked by preincubation of the cells with antioxidants. Catechin (10 mumol/l) and vitamin E (100 mumol/l) are very effective cytoprotective agents, whereas probucol (up to 50 mumol/l) was completely ineffective under these experimental conditions. The cytoprotective effect of vitamin E was associated to a complete inhibition of the cellular TBARS formation induced by ultraviolet-treated LDL, whereas the cytoprotective effect of catechin was relatively independent on the TBARS inhibition. All these results showed that: (1) probucol (25 mumol/l) is very effective to prevent lipid peroxidation of LDL and their subsequent 'cytotoxicity', but it cannot protect cells against the 'cytotoxicity' of previously oxidized LDL; (2) vitamin E (100 mumol/l) prevents poorly the ultraviolet-induced lipid peroxidation of LDL, but is able to block simultaneously the cellular oxidative stress and the 'cytotoxicity' induced by previously oxidized LDL; and (3) catechin (10 mumol/l) exhibited two types of protective effects: it inhibits the lipid peroxidation of LDL (and their subsequent 'cytotoxicity') and very effectively protects the cells against 'toxicity' of previously oxidized LDL (with only little inhibition of the cellular oxidative stress).(ABSTRACT TRUNCATED AT 400 WORDS)     

Flavonoids can replace alpha-tocopherol as an antioxidant

Endogenous antioxidants such as the lipid-soluble vitamin E protect the cell membranes from oxidative damage. Glutathione seems to be able to regenerate alpha-tocopherol via a so-called free radical reductase. The transient protection by reduced glutathione (GSH) against lipid peroxidation in control liver microsomes is not observed in microsomes deficient in alpha-tocopherol. Introduction of antioxidant flavonoids, such as 7-monohydroxyethylrutoside, fisetin or naringenin, into the deficient microsomes restored the GSH-dependent protection, suggesting that flavonoids can take over the role of alpha-tocopherol as a chain-breaking antioxidant in liver microsomal membranes.     

Antioxidant effects of different extracts from Melissa officinalis, Matricaria recutita and Cymbopogon citratus

Considering the important role of oxidative stress in the pathogenesis of several neurological diseases, and the growing evidence of the presence of compounds with antioxidant properties in the plant extracts, the aim of the present study was to investigate the antioxidant capacity of three plants used in Brazil to treat neurological disorders: Melissa officinalis, Matricaria recutita and Cymbopogon citratus. The antioxidant effect of phenolic compounds commonly found in plant extracts, namely, quercetin, gallic acid, quercitrin and rutin was also examined for comparative purposes. Cerebral lipid peroxidation (assessed by TBARS) was induced by iron sulfate (10 μM), sodium nitroprusside (5 μM) or 3-nitropropionic acid (2 mM). Free radical scavenger properties and the chemical composition of plant extracts were assessed by 1′-1′ Diphenyl-2′ picrylhydrazyl (DPPH) method and by Thin Layer Chromatography (TLC), respectively. M. officinalis aqueous extract caused the highest decrease in TBARS production induced by all tested pro-oxidants. In the DPPH assay, M. officinalis presented also the best antioxidant effect, but, in this case, the antioxidant potencies were similar for the aqueous, methanolic and ethanolic extracts. Among the purified compounds, quercetin had the highest antioxidant activity followed by gallic acid, quercitrin and rutin. In this work, we have demonstrated that the plant extracts could protect against oxidative damage induced by various pro-oxidant agents that induce lipid peroxidation by different process. Thus, plant extracts could inhibit the generation of early chemical reactive species that subsequently initiate lipid peroxidation or, alternatively, they could block a common final pathway in the process of polyunsaturated fatty acids peroxidation. Our study indicates that M. officinalis could be considered an effective agent in the prevention of various neurological diseases associated with oxidative stress.     

Probucol prevents oxidative injury to endothelial cells

We evaluated the effect of the antioxidant, probucol, on the cytotoxic effects of oxidized low density lipoprotein (OX-LDL) or of cumene hydroperoxide (CumOOH) on cultured bovine endothelial cells (EC). The addition of CumOOH to EC caused the release of lactate dehydrogenase and the accumulation of thiobarbituric acid-reacting substances (TBARS), effects that were protected against by preincubation with either probucol or tocopherol. Similarly, preincubation of EC with those antioxidants protected against OX-LDL toxicity and the accumulation of TBARS. The content of probucol in EC measured by high performance liquid chromatography was directly correlated with the extent of protection against OX-LDL toxicity. We also found that treatment of EC with serum from patients receiving treatment with probucol resulted in the detection of probucol in the cells. We conclude that probucol is transported and incorporated into EC membranes to act as a radical-trapping antioxidant, protecting the EC against oxidative stress. Our results also indicate that lipid peroxidation in cellular membranes involves cell injury inflicted by OX-LDL.     

β-Amyloid Neurotoxicity In Vitro: Evidence of Oxidative Stress but Not Protection by Antioxidants

Abstract: Recent data from several groups suggest that the primary mechanism of β-amyloid neurotoxicity may be mediated by reactive oxygen species. To evaluate this hypothesis, we first compared the efficacy of antioxidant agents in preventing toxicity caused by oxidative insults (iron, hydrogen peroxide, and tert -butyl hydroperoxide) and β-amyloid peptides in cultured rat hippocampal neurons. Tested antioxidants (propyl gallate, Trolox, probucol, and promethazine) generally provided significant protection against oxidative insults but not β-amyloid peptides. Next, we examined whether β-amyloid causes oxidative stress, by comparing levels of lipid peroxidation after exposure to either iron or β-amyloid. In a cell-free system, iron but not β-amyloid generated lipid peroxidation. In culture, both insults caused rapid increases in lipid peroxidation, with iron inducing higher levels at later time points. Pretreatment with the antioxidant probucol significantly reduced lipid peroxidation caused by both insults but only attenuated iron toxicity, suggesting that lipid peroxidation does not contribute directly to cell death induced by β-amyloid. Finally, we observed that increasing basal levels of oxidative stress by pretreating cultures with subtoxic doses of iron significantly increased neuronal vulnerability to β-amyloid. The ability of β-amyloid to induce oxidative stress and the demonstration that oxidative stress potentiates β-amyloid toxicity support the clinical use of antioxidants for AD. However, these data do not support the theory that the primary mechanism of β-amyloid toxicity involves oxidative pathways, indicating a continued need to identify additional cellular responses to β-amyloid that underlie its neurodegenerative actions.     

Inhibition of the peroxidation of linoleic acid by the flavonoid quercetin within their complex with human serum albumin

This work provides a quantitative kinetic analysis of oxidative pathways involving linoleic acid and the common dietary antioxidant quercetin (flavonoid), both bound to human serum albumin (HSA). In particular, it is shown that quercetin, although embedded in drug site I, is oxidized as quickly as free quercetin under a flux of hydrophilic peroxyl radicals. This observation suggests that efficient charge relays are established between the periphery of HSA and bound quercetin. Moreover, the peroxidation of HSA-bound linoleic acid is shown to take place at some specific fatty acid binding sites once one to two critical HSA residues are themselves oxidized. Quercetin efficiently delays the onset of lipid peroxidation. The inhibition persists long after the total consumption of quercetin, in agreement with some quercetin oxidation products exerting a residual antioxidant activity. Consistently, HSA markedly increases the maximal concentration of a two-electron oxidation product of quercetin that is accumulated and then consumed in the course of the peroxidation. The additional observation of the faster consumption of the single Trp residue in the presence of quercetin suggests that HSA enhances the antioxidant activity of quercetin by regenerating some of its oxidation products retaining a H-donating activity.     

Effect of DL-alpha-lipoic acid on mitochondrial enzymes in aged rats.

Mitochondrial dysfunction appears to contribute to some of the loss of function accompanying ageing. Mitochondria from aged tissue use oxygen inefficiently impairing ATP synthesis and results in increased oxidant production. A high flux of oxidants not only damages mitochondria, but other important cell biomolecules as well. In the present investigation, the levels of lipid peroxidation, oxidized glutathione, non-enzymatic antioxidants and the activities of mitochondrial enzymes were measured in liver and kidney mitochondria of young and aged rats before and after lipoic acid supplementation. In both liver and kidney increase in the levels of mitochondrial lipid peroxidation and oxidized glutathione and decrease in the levels of antioxidants and the activities of mitochondrial enzymes were observed in aged rats. DL-alpha-lipoic acid supplemented aged rats showed a decrease in the levels of lipid peroxidation and oxidized glutathione and increase in the levels of reduced glutathione, vitamins C and E and the activities of mitochondrial enzymes like isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase, NADH-dehydrogenase and cytochrome-c-oxidase. Thus, lipoic acid reverses the age-associated decline in endogenous low molecular weight antioxidants and mitochondrial enzymes and, therefore, may lower the increased risk of oxidative damage that occurs during ageing. From our results it can be concluded that lipoic acid supplementation enhances the activities of mitochondrial enzymes and antioxidant status and thereby protects mitochondria from ageing.     

Role of calcium in AMF-mediated alleviation of the adverse impacts of cadmium stress in Bassia indica [Wight] A.J. Scott

The aim of this study was to evaluate cadmium stress induced changes in the growth, lipid peroxidation and antioxidant activity of Bassia indica associated with arbuscular mycorrhizal fungi (AMF) and their amelioration by calcium application. Cadmium stress can cause alterations in the physiological and biochemical processes in plants. A calcium application combined with an AMF treatment resulted in the reduction of lipid peroxidation and the production of hydrogen peroxide, thereby mediating the mitigation of cadmium induced oxidative stress. The activity of antioxidant enzymes increased with cadmium application, whereas AMF inoculation combined with a calcium application further enhanced their activity. An increase in the content of non-enzymatic antioxidants such as ascorbate, reduced glutathione (GSH), oxidized glutathione (GSSG) and S-nitrosoglutathione (GSNO) in AMF-inoculated and calcium-treated plants further suggests their role in strengthening the antioxidant defense system that results in maintained growth. The application of calcium combined with the AMF treatment caused a significant reduction in lipid peroxidation and in the production of hydrogen peroxide, thereby mediating the mitigation of the cadmium induced oxidative stress. Increased proline accumulation was clearly evident in stressed plants, and the calcium application as well as the AMF inoculation further induced proline synthesis, thereby providing efficient protection against cadmium stress by increasing the maintenance of the systemic resistance criteria.     

Protective effect of quercetin in primary neurons against Abeta(1-42): relevance to Alzheimer's disease

Quercetin, a flavonoid found in various foodstuffs, has antioxidant properties and increases glutathione (GSH) levels and antioxidant enzyme function. Considerable attention has been focused on increasing the intracellular GSH levels in many diseases, including Alzheimer's disease (AD). Amyloid beta-peptide [Abeta(1-42)], elevated in AD brain, is associated with oxidative stress and neurotoxicity. We aimed to investigate the protective effects of quercetin on Abeta(1-42)-induced oxidative cell toxicity in cultured neurons in the present study. Decreased cell survival in neuronal cultures treated with Abeta(1-42) correlated with increased free radical production measured by dichlorofluorescein fluorescence and an increase in protein oxidation (protein carbonyl, 3-nitrotyrosine) and lipid peroxidation (protein-bound 4-hydroxy-2-nonenal). Pretreatment of primary hippocampal cultures with quercetin significantly attenuated Abeta(1-42)-induced cytotoxicity, protein oxidation, lipid peroxidation and apoptosis. A dose-response study suggested that quercetin showed protective effects against Abeta(1-42) toxicity by modulating oxidative stress at lower doses, but higher doses were not only non-neuroprotective but also toxic. These findings provide motivation to test the hypothesis that quercetin may provide a promising approach for the treatment of AD and other oxidative-stress-related neurodegenerative diseases.