Macrophage-mediated oxidation of extracellular low density lipoprotein requires an initial binding of the lipoprotein to its receptor

Flavonoids: Antioxidant Powerhouses and Their Role in Nanomedicine

This study emphasizes the critical role of antioxidants in protecting human health by counteracting the detrimental effects of oxidative stress induced by free radicals. Antioxidants-found in various forms such as vitamins, minerals, and the phytochemicals abundant in fruits and vegetables-neutralize free radicals by stabilizing them through electron donation. Specifically, flavonoid compounds are highlighted as robust defenders, addressing oxidative stress and inflammation to avert chronic illnesses like cancer, cardiovascular diseases, and neurodegenerative diseases. This research explores the bioactive potential of flavonoids, shedding light on their role not only in safeguarding health, but also in managing conditions such as diabetes, cancer, cardiovascular diseases, and neurodegenerative diseases. This review highlights the novel integration of South African-origin flavonoids with nanotechnology, presenting a cutting-edge strategy to improve drug delivery and therapeutic outcomes. This interdisciplinary approach, blending traditional wisdom with contemporary techniques, propels the exploration of flavonoid-mediated nanoparticles toward groundbreaking pharmaceutical applications, promising revolutionary advancements in healthcare. This collaborative synergy between traditional knowledge and modern science not only contributes to human health, but also underscores a significant step toward sustainable and impactful biomedical innovations, aligning with principles of environmental conservation.

Plant Polyphenols and Their Potential Benefits on Cardiovascular Health: A Review

Fruits, vegetables, and other food items contain phytochemicals or secondary metabolites which may be considered non-essential nutrients but have medicinal importance. These dietary phytochemicals exhibit chemopreventive and therapeutic effects against numerous diseases. Polyphenols are secondary metabolites found in vegetables, fruits, and grains. These compounds exhibit several health benefits such as immune modulators, vasodilators, and antioxidants. This review focuses on recent studies on using dietary polyphenols to treat cardiovascular disorders, atherosclerosis, and vascular endothelium deficits. We focus on exploring the safety of highly effective polyphenols to ensure their maximum impact on cardiac abnormalities and discuss recent epidemiological evidence and intervention trials related to these properties. Kaempferol, quercetin, and resveratrol prevent oxidative stress by regulating proteins that induce oxidation in heart tissues. In addition, polyphenols modulate the tone of the endothelium of vessels by releasing nitric oxide (NO) and reducing low-density lipoprotein (LDL) oxidation to prevent atherosclerosis. In cardiomyocytes, polyphenols suppress the expression of inflammatory markers and inhibit the production of inflammation markers to exert an anti-inflammatory response. Consequently, heart diseases such as strokes, hypertension, heart failure, and ischemic heart disease could be prevented by dietary polyphenols.

Neuroprotective Potential of Flavonoids in Brain Disorders

Flavonoids are a large subgroup of polyphenols known to be sourced from over 6000 natural products, including fruits, vegetables, bark, and herbs. Due to their antioxidant properties, flavonoids have been implicated as a therapy source for many diseases and conditions, including inflammation, vasculitis, venous insufficiency, and hemorrhoids. Currently, some flavonoids are being researched for their antioxidant ability concerning neuroprotection. These flavonoids can penetrate the blood-brain barrier and, depending on the specific flavonoid, retain adequate bioavailability in certain brain regions. Further data suggest that flavonoids could have a strong anti-inflammatory effect in the brain, which not only could be a robust therapeutic source for known neuroinflammatory diseases such as Alzheimer's Disease or Parkinson's Disease but also could be a therapeutic source for ischemic or hemorrhagic conditions such as a stroke. While flavonoid toxicity exists, they are relatively safe and non-invasive drugs from natural origins. As such, exploring the known mechanisms and therapies may highlight and establish flavonoid therapy as a viable source of therapy for stroke patients. As stated, many flavonoids are already being isolated, purified, and implemented in both in vitro and in vivo experiments. As these flavonoids proceed to clinical trials, it will be important to understand how they function as a therapy, primarily as antioxidants, and by other secondary mechanisms. This review aims to elucidate those mechanisms and explore the neuroprotective role of flavonoids.

How Monocytes Contribute to Increased Risk of Atherosclerosis in Virologically-Suppressed HIV-Positive Individuals Receiving Combination Antiretroviral Therapy

Combination antiretroviral therapy (ART) is effective at suppressing HIV viremia to achieve persistently undetectable levels in peripheral blood in the majority of individuals with access and ability to maintain adherence to treatment. However, evidence suggests that ART is less effective at eliminating HIV-associated inflammation and innate immune activation. To the extent that residual inflammation and immune activation persist, virologically suppressed people living with HIV (PLWH) may have increased risk of inflammatory co-morbidities, and adjunctive therapies may need to be considered to reduce HIV-related inflammation and fully restore the health of virologically suppressed HIV+ individuals. Cardiovascular disease (CVD) is the single leading cause of death in the developed world and is becoming more important in PLWH with access to ART. Arterial disease due to atherosclerosis, leading to acute myocardial infarction (AMI) and stroke, is a major component of CVD. Atherosclerosis is an inflammatory disease, and epidemiological comparisons of atherosclerosis and AMI show a higher prevalence and suggest a greater risk in PLWH compared to the general population. The reasons for greater prevalence of CVD in PLWH can be broadly grouped into four categories: (a) the higher prevalence of traditional risk factors e.g., smoking and hypertension (b) dyslipidemia (also a traditional risk factor) caused by off-target effects of ART drugs (c) HIV-related inflammation and immune activation and (d) other undefined HIV-related factors. Management strategies aimed at reducing the impact of traditional risk factors in PLWH are similar to those for the general population and their effectiveness is currently being evaluated. Together with improvements in ART regimens and guidelines for treatment, and a greater awareness of its impact on CVD, the HIV-related risk of AMI and stroke is decreasing but remains elevated compared to the general community. Monocytes are key effector cells which initiate the formation of atherosclerotic plaques by migrating into the intima of coronary arteries and accumulating as foam cells full of lipid droplets. This review considers the specific role of monocytes as effector cells in atherosclerosis which progresses to AMI and stroke, and explores mechanisms by which HIV may promote an atherogenic phenotype and function independent of traditional risk factors. Altered monocyte function may represent a distinct HIV-related factor which increases risk of CVD in PLWH.

Beneficial effects of polyphenols on cardiovascular disease

In recent years, numerous studies have demonstrated the health benefits of polyphenols, and special attention has been paid to their beneficial effects against cardiovascular disease, the leading cause of death in the world today. Polyphenols present vasodilator effects and are able to improve lipid profiles and attenuate the oxidation of low density lipoproteins. In addition, they present clear anti-inflammatory effects and can modulate apoptotic processes in the vascular endothelium. It has been suggested that most of these effects are a consequence of the antioxidant properties of polyphenols, but this idea is not completely accepted, and many other mechanisms have been proposed recently to explain the health effects of these compounds. In fact, different signaling pathways have been linked to polyphenols. This review brings together some recent studies which establish the beneficial properties of polyphenols for cardiovascular disease and analyzes the mechanisms involved in these properties.

A new insight into resveratrol as an atheroprotective compound: inhibition of lipid peroxidation and enhancement of cholesterol efflux

Resveratrol, a polyphenolic constituent of red wine, is known for its anti-atherogenic properties and is thought to be beneficial in reducing the incidence of cardiovascular diseases (CVD). However, the mechanism of action by which it exerts its anti-atherogenic effect remains unclear. In this study, we investigated the relationship between the antioxidant effects of resveratrol and its ability to promote cholesterol efflux. We measured the formation of conjugated dienes and the rate of lipid peroxidation, and observed that resveratrol inhibited copper- and irradiation-induced LDL and HDL oxidation as observed by a reduction in oxidation rate and an increase in the lag phase (p<0.05). We used DPPH screening to measure free radical scavenging activity and observed that resveratrol (0-50microM) significantly reduced the content of free radicals (p<0.001). Respect to its effect on cholesterol homeostasis, resveratrol also enhanced apoA-1-mediated cholesterol efflux (r(2)=0.907, p<0.05, linear regression) by up-regulating ABCA-1 receptors, and reduced cholesterol influx or uptake in J774 macrophages (r(2)=0.89, p<0.05, linear regression). Incubation of macrophages (J774, THP-1 and MPM) with Fe/ascorbate ion, attenuated apoA-1 and HDL(3)-mediated cholesterol efflux whereas resveratrol (0-25microM) significantly redressed this attenuation in a dose-dependent manner (p<0.001). Resveratrol thus appears to be a natural antioxidant that enhances cholesterol efflux. These properties make it a potential natural antioxidant that could be used to prevent and treat CVD.

Antiatherogenicity of extra virgin olive oil and its enrichment with green tea polyphenols in the atherosclerotic apolipoprotein-E-deficient mice: enhanced macrophage cholesterol efflux

The antiatherogenic properties of extra virgin olive oil (EVOO) enriched with green tea polyphenols (GTPPs; hereafter called EVOO-GTPP), in comparison to EVOO, were studied in the atherosclerotic apolipoprotein-E-deficient (E0) mice. E0 mice (eight mice in each group) consumed EVOO or EVOO-GTPP (7 microl/mouse/day, for 2 months) by gavage feeding. The placebo group received only water. At the end of the study, blood samples, peritoneal macrophages and aortas were collected. Consumption of EVOO or EVOO-GTPP resulted in a minimal increase in serum total and high-density lipoprotein (HDL) cholesterol levels (by 12%) and in serum paraoxonase 1 activity (by 6% and 10%). EVOO-GTPP (but not EVOO) decreased the susceptibility of the mouse serum to AAPH-induced lipid peroxidation (by 18%), as compared to the placebo-treated mice. The major effect of both EVOO and EVOO-GTPP consumption was on HDL-mediated macrophage cholesterol efflux. Consumption of EVOO stimulated cholesterol efflux rate from mouse peritoneal macrophages (MPMs) by 42%, while EVOO-GTPP increased it by as much as 139%, as compared to MPMs from placebo-treated mice. Finally, the atherosclerotic lesion size of mice was significantly reduced by 11% or 20%, after consumption of EVOO or EVOO-GTPP, respectively. We thus conclude that EVOO possesses beneficial antiatherogenic effects, and its enrichment with GTPPs further improved these effects, leading to the attenuation of atherosclerosis development.

Antioxidative Effect of Conjugated Linolenic Acid in Diabetic and Non-Diabetic Blood: an in vitro Study

The present study examined the in vitro antioxidant activity of conjugated octadecatrienoic fatty acid (9cis, 11 trans, 13 trans-18:3), alpha-eleostearic acid present in karela seed oil (Momordica charantia) at about 55% level. The in vitro antioxidant properties of alpha-eleostearic acid are investigated on oxidative modification of human plasma, low-density lipoprotein (LDL) and erythrocyte membrane lipid. Blood samples are collected from diabetic and non-diabetic (normal) healthy individuals. alpha-eleostearic acid is added at 0.05% and 0.1% concentrations to plasma, LDL and erythrocyte membrane isolated from the respective blood samples and peroxidations are determined against control samples. A significant increase of respective peroxidation levels has been observed in diabetic control blood than the non-diabetic control blood. alpha-eleostearic acid has decreased lipid peroxidation level against control samples in a dose dependent manner. The present findings suggest that CLnA, 9cis, 11trans, 13trans-18:3 is a potentially effective antioxidant that can protect plasma, low density lipoprotein and erythrocyte membrane from oxidation which may be effective in reducing the risk of coronary heart disease in diabetes mellitus.

Contribution of xanthoma tissue-derived LDL density substances in the transformation of macrophages to foam cells

BACKGROUND

The source of accumulated lipids in the foam cells of xanthoma is primarily the lipoproteins existing in the lesional dermis.

OBJECTIVE

This study was designated to clarify the contribution of low density lipoprotein (LDL) density substances existing in xanthoma tissue to foam cell formation.

METHODS

An LDL density fraction was obtained from homogenized rabbit experimental xanthoma tissue. Biochemical and functional characteristics of xanthoma-extracted LDL density substance were examined. The in vivo foam cell-inducing ability of xanthoma-extracted LDL density substance was examined microscopically at the intradermal injection site.

RESULTS

Xanthoma-extracted LDL density substance showed more negatively charged mobility on agarose gel electrophoresis than plasma native LDL. A small amount of aggregated material remained at the origin on agarose gel electrophoresis. Xanthoma-extracted LDL density substance contained much higher level of lipid peroxides than native LDL. Mouse peritoneal macrophages internalized xanthoma-extracted LDL density substance extensively and transformed into foam cells by incubation with xanthoma-extracted LDL density substance. Intradermal injection of the xanthoma-extracted LDL density substance induced foam cell infiltration in the skin of a normolipemic rabbit.

CONCLUSION

LDL density substances prepared ex vivo from experimental xanthoma tissue contained lipid-protein complexes that have physiochemical properties of oxidized LDL. The lipid-protein complexes were incorporated into foam cells. The substances were considered to contribute to foam cell recruitment during the persistence of xanthoma lesions.

Dietary antioxidants and paraoxonases against LDL oxidation and atherosclerosis development

Oxidative modification of low-density lipoprotein (LDL) in the arterial wall plays a key role in the pathogenesis of atherosclerosis. Under oxidative stress LDL is exposed to oxidative modifications by arterial wall cells including macrophages. Oxidative stress also induces cellular-lipid peroxidation, resulting in the formation of 'oxidized macrophages', which demonstrate increased capacity to oxidize LDL and increased uptake of oxidized LDL. Macrophage-mediated oxidation of LDL depends on the balance between pro-oxidants and antioxidants in the lipoprotein and in the cells. LDL is protected from oxidation by antioxidants, as well as by a second line of defense--paraoxonase 1 (PON1), which is a high-density lipoprotein-associated esterase that can hydrolyze and reduce lipid peroxides in lipoproteins and in arterial cells. Cellular paraoxonases (PON2 and PON3) may also play an important protective role against oxidative stress at the cellular level. Many epidemiological studies have indicated a protective role for a diet rich in fruits and vegetables against the development and progression of cardiovascular disease. A large number of studies provide data suggesting that consumption of dietary antioxidants is associated with reduced risk for cardiovascular diseases. Basic research provides plausible mechanisms by which dietary antioxidants might reduce the development of atherosclerosis. These mechanisms include inhibition of LDL oxidation, inhibition of cellular lipid peroxidation and consequently attenuation of cell-mediated oxidation of LDL. An additional possible mechanism is preservation/increment of paraoxonases activity by dietary antioxidants. This review chapter presents recent data on the anti-atherosclerotic effects and mechanism of action of three major groups of dietary antioxidants-vitamin E, carotenoids and polyphenolic flavonoids.

Paraoxonases 1, 2, and 3, oxidative stress, and macrophage foam cell formation during atherosclerosis development

Paraoxonases PON1 and PON3, which are both associated in serum with HDL, protect the serum lipids from oxidation, probably as a result of their ability to hydrolyze specific oxidized lipids. The activity of HDL-associated PON1 seems to involve an activity (phospholipase A2-like activity, peroxidase-like activity, lactonase activity) which produces LPC. To study the possible role of PON1 in macrophage foam cell formation and atherogenesis we used macrophages from control mice, from PON1 knockout mice, and from PON1 transgenic mice. Furthermore, we analyzed PON1-treated macrophages and PON1-transfected cells to demonstrate the contribution of PON1 to the attenuation of macrophage cholesterol and oxidized lipid accumulation and foam cell formation. PON1 was shown to inhibit cholesterol influx [by reducing the formation of oxidized LDL (Ox-LDL), increasing the breakdown of specific oxidized lipids in Ox-LDL, and decreasing macrophage uptake of Ox-LDL]. PON1 also inhibits cholesterol biosynthesis and stimulates HDL-mediated cholesterol efflux from macrophages. PON2 and PON3 protect against oxidative stress, with PON2 acting mainly at the cellular level. Whereas serum PON1 and PON3 were inactivated under oxidative stress, macrophage PON2 expression and activity were increased under oxidative stress, probably as a compensatory mechanism against oxidative stress. Intervention to increase the paraoxonases (cellular and humoral) by dietary or pharmacological means can reduce macrophage foam cell formation and attenuate atherosclerosis development.

Demethylbellidifolin preserves endothelial function by reduction of the endogenous nitric oxide synthase inhibitor level

The present study examined the anti-oxidation and protective effects of demethylbellidifolin (DMB), a xanthone compound extracted from swertia davidi Franch, on endothelium. The relationship between the protective effects of DMB on endothelium and the level of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, was also determined in the present study. DMB significantly inhibited Cu(2+)-induced low-density lipoprotein (LDL) oxidation and scavenged DPPH radicals. DMB significantly attenuated the inhibition of endothelium-dependent vasodilator responses, induced by lysophosphatidycholine (LPC) in vitro and LDL in vivo, and increased release of lactate dehydrogenase induced by LDL in cultured endothelial cells. DMB significantly attenuated the increased concentration of malondialdehyde and ADMA, and the decreased level of nitric oxide induced by LDL in vivo and in cultured endothelial cells. DMB also significantly reduced the decreased activity of dimethylarginine dimethylaminohydrolase (DDAH) induced by LDL in cultured endothelial cells. In summary, the present results suggest that DMB protects endothelial damage induced by LPC in vitro and LDL in vivo or in endothelial cells, and the protective effect of DMB on the endothelium is related to reduction of ADMA concentration via an increase of DDAH activity by inhibition of lipid peroxidation.

Tissue Angiotensin-Converting-Enzyme (ACE) Deficiency Leads to a Reduction in Oxidative Stress and in Atherosclerosis

Background- Angiotensin II, produced by angiotensin-converting-enzyme (ACE), enhances oxidative stress and atherogenesis. In this study, we analyzed whether tissue ACE deficiency in ACE-knockout mice type-2 would affect their oxidative status. Moreover, by crossbreeding the ACE-knockout mice with atherosclerotic apolipoprotein E (apo E)-deficient (E0) mice, we questioned whether tissue ACE deficiency affects atherogenesis.

METHODS AND RESULTS

ACE-deficient mice type-2 (ACE+/-) exhibited reduced serum lipid peroxidation compared with ACE+/+ mice. Peritoneal macrophages from ACE+/- mice demonstrated lower oxidative status, as exhibited by decreases of 47%, 33% 56%, and 51%, in their lipid peroxides, superoxide release, dichlorofluorescein fluorescence, and LDL oxidation, respectively, compared with ACE+/+ mice. ACE+/- mice crossbred with E0 mice, resulting in atherosclerotic mice heterozygous for ACE (ACE+/-/E0 mice), exhibited reduced lipid peroxidation, increased paraoxonase activity, and lower macrophage LDL oxidation compared with E0 and ACE+/+/E0 mice. ACE+/-/E0 mice also exhibited reduced NADPH-induced aortic superoxide ion production by 52% and a reduction of 43% in their atherosclerotic lesion size compared with E0 mice. Finally, 2 animals genotyped as homozygous-knockout for both ACE and APOE genes (ACE-/-/E0), exhibited a striking reduction of 86% in their atherosclerotic lesion area compared with E0 mice.

CONCLUSIONS

Reduction of tissue ACE with the ACE-knockout mouse type-2 model inhibited oxidative stress and atherogenesis.

Effect of eplerenone, a selective aldosterone blocker, on blood pressure, serum and macrophage oxidative stress, and atherosclerosis in apolipoprotein E-deficient mice

Oxidative stress is involved in the pathogenesis of atherosclerosis, and angiotensin II (AT-II) induces oxidative stress and enhances atherogenesis. Aldosterone, which has an important role in the pathology of heart failure, has recently been implicated as a mediator of AT-II biologic activities. In this study, we analyzed whether administration of the selective aldosterone blocker eplerenone to atherosclerotic apolipoprotein E-deficient (E0) mice would affect their oxidative status and atherogenesis. Apolipoprotein E-deficient mice were administered chow containing eplerenone (200 mg/kg/day) for 3 months. Blood pressure, serum and macrophage oxidative status, and aortic atherosclerotic lesion area were evaluated in mice treated with eplerenone compared with untreated mice. Eplerenone administration significantly decreased systolic and diastolic blood pressure by 12% and 11%, respectively, compared with untreated mice. Serum susceptibility to lipid peroxidation decreased by as much as 26%, and serum paraoxonase activity increased by 28% in eplerenone-treated mice compared with untreated mice. Peritoneal macrophages from eplerenone-treated mice contained reduced levels of lipid peroxides, and their macrophage oxidation of low-density lipoprotein (LDL) and superoxide ion release were significantly reduced (by 17% and 43%, respectively), compared to untreated mice. Daily injections of AT-II (0.1 mL, 10(-)7M) during the final 3 weeks of the study in eplerenone-treated mice substantially attenuated the eplerenone-mediated reduction in macrophage superoxide release and LDL oxidation. Finally, the atherosclerotic lesion area in aortas of eplerenone-treated mice was significantly reduced (by 35%) versus untreated mice, and this effect was reversed by AT-II. Administration of the selective aldosterone blocker eplerenone significantly reduced oxidative stress and atherosclerosis progression in E0 mice. These data suggest that aldosterone could have a significant pro-oxidative role in the pathogenesis of atherosclerosis.

Renin-angiotensin system antagonism and lipid-lowering therapy in cardiovascular risk management

The renin-angiotensin system (RAS) and dyslipidaemia have been shown to be involved in the genesis and progression of atherosclerosis. Manipulation of the RAS has been effective in modifying human coronary artery disease progression. Similarly, the 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors or statins have been shown to reduce cholesterol and lower cardiovascular events in primary and secondary prevention trials in coronary artery disease. In addition to their primary mode of action, statins and blockers of the RAS possess common additional properties that include restoration of endothelial activity and inhibition of cellular proliferation. This article reviews the current data on the common properties of these classes of drugs in which the beneficial effects extend beyond their antihypertensive and lipid-lowering properties.

Wine flavonoids protect against LDL oxidation and atherosclerosis

We have previously shown that consumption of red wine, but not of white wine, by healthy volunteers, resulted in the enrichment of their plasma LDL with flavonoid antioxidants such as quercetin, the potent free radicals scavenger flavanol, which binds to the LDL via a glycosidic ether bond. This phenomenon was associated with a significant three-fold reduction in copper ion-induced LDL oxidation. The ineffectiveness of flavonoid-poor white wine could be overcome by grape's skin contact for 18 hours in the presence of alcohol, which extracts grape's skin flavonoids. Recently, we observed that the high antioxidant potency of Israeli red wine could be related to an increased content of flavonols, which are very potent antioxidants and their biosynthesis is stimulated by sunlight exposure. To find out the effect (and mechanisms) of red wine consumption on atherosclerosis, we used the apo E deficient (E(0)) mice. In these mice, red wine consumption for two months resulted in a 40% decrement in basal LDL oxidation, a similar decrement in LDL oxidizability and aggregation, a 35% reduction in lesion size, and a marked attenuation in the number and morphology of lesion's macrophage foam cells. Red wine consumption resulted in accumulation of flavonoids in the mouse macrophages and these cells oxidized LDL and took up LDL about 40% less than macrophages from placebo-treated mice. Finally, the activity of serum paraoxonase (which can hydrolyze specific lipid peroxides in oxidized LDL and in atherosclerotic lesions) was significantly increased following consumption of red wine by E(0) mice. Red wine consumption thus acts against the accumulation of oxidized LDL in lesions as a first line of defense (by a direct inhibition of LDL oxidation), and as a second line of defense (by paraoxonase elevation and removal of atherogenic lesion's and lipoprotein's oxidized lipids).

Flavonoids protect LDL from oxidation and attenuate atherosclerosis

Consumption of some plant-derived flavonoids results in their absorption and appearance in plasma and tissues. The inverse relationship between dietary flavonoids consumption and cardiovascular diseases may be associated with the ability of flavonoids to attenuate LDL oxidation, macrophage foam cell formation and atherosclerosis. The effect of flavonoids on arterial cell-mediated oxidation of LDL is determined by their accumulation in the lipoprotein and in arterial cells, such as macrophages. Flavonoids can reduce LDL lipid peroxidation by scavenging reactive oxygen/nitrogen species, chelation of transition metal ions and sparing of LDL-associated antioxidants. They can also reduce macrophage oxidative stress by inhibition of cellular oxygenases [such as nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase] or by activating cellular antioxidants (such as the glutathione system). Thus, plant flavonoids, as potent natural antioxidants that protect against lipid peroxidation in arterial cells and lipoproteins, significantly attenuate the development of atherosclerosis.

Macrophage-released proteoglycans enhance LDL aggregation: studies in aorta from apolipoprotein E-deficient mice

Aggregated low-density lipoprotein (LDL) was shown to be present in the atherosclerotic lesion, but the mechanism responsible for its formation in vivo is not known yet. To find out whether LDL aggregation occurs in the arterial wall during atherogenesis, LDLs were extracted from the aortas of apolipoprotein E-deficient (E(0)) mice during their aging (and the development of atherosclerosis), and were analyzed for their aggregation states, in comparison to LDLs isolated from aortas of control mice. LDL isolated from aortas of E(0) mice was already aggregated at 1 month of age and its aggregation state substantially increased with age, with 3-fold elevation at 6 months of age compared to younger, 1-month-old, mice. Only minimal aggregation could be detected in LDL derived from control mice. Electron microscopy examination revealed that LDL particles from aortas of the E(0) mice were heterogeneous in their size, ranging between 20 and 300 nm. The mouse aortic LDL contained proteoglycans (PGs) and their content increased with the age of the mice, with about 2-fold higher levels than those found in LDLs derived from aortas of control mice. Macrophage-released PGs were previously demonstrated to enhance LDL aggregation in vitro. However, their involvement in LDL aggregation in vivo has not been studied yet. Thus, we next studied the effect of arterial macrophage-released PGs on the susceptibility of plasma LDL to aggregation by Bacillus cereus sphingomyelinase (SMase). Foam cell macrophages were isolated from aortas of the atherosclerotic E(0) mice at 6 months of age and were found to be loaded with cholesterol and to contain oxidized lipids. To analyze the effect of macrophage-released PGs on LDL aggregation, PGs were prelabeled by cell incubation with [35S]sulfate, followed by incubation of macrophage-released PGs with E(0) mouse plasma LDL (200 microg protein/ml) for 1 h at 37 degrees C. [35S]Sulfated PGs were found to be LDL-associated and the susceptibility of PG-associated LDL to aggregation by SMase was increased by up to 45% in comparison to control LDL. Similar results demonstrating the involvement of PGs in LDL aggregation were obtained upon incubation of LDL with increasing concentrations of PGs that were isolated from the entire aorta of E(o) mice (rather than the isolated macrophages). The stimulatory effect of macrophage-released PGs on LDL aggregation was markedly reduced when the PGs were pretreated with the glycosaminoglycan-hydrolyzing enzymes, chondroitinase ABC or chondroitinase AC, and to a much lesser extent with heparinase. We thus conclude that macrophage-released chondroitin sulfate PG can contribute to the formation of atherogenic aggregated LDL in the arterial wall.

Oxidation of low-density lipoprotein in normotensive type 2 diabetic patients. Comparative effects of enalapril versus nifedipine: a randomized cross-over over study

The role of lipoprotein oxidation in promoting atherosclerosis is gaining recognition as its spectrum of effects is being unveiled. Accelerated atherosclerosis is a major cause of morbidity and mortality in diabetic patients. Treatment with ACE inhibitors reduces oxidation of low-density lipoprotein (LDL-ox) in hypertensive subjects, however, their effect on LDL-ox in diabetic patients is yet obscure. To evaluate the effect of the ACE inhibitor enalapril and the calcium channel blocker nifedipine on LDL oxidation in normotensive type 2 diabetic patients. A randomized single blinded cross-over study was conducted on 24 nonobese, metabolically stable, normotensive patients with type 2 diabetes who were randomly allocated to receive either enalapril, 10 mg/day, or nifedipine, 30 mg/day, for 4 weeks followed by a 2-week washout period. They were then crossed over to a 4-week course with the alternate drug. The oxidation of LDL was evaluated by three methods: dialdehyde analysis using the thiobarbituric acid reactive substances assay with and without the addition of CuSO(4) as well as determination of conjugated dienes in the LDL lipid extract. The propensity of the serum to oxidize LDL was reduced by enalapril by 17-28% depending on the laboratory method used (P=0.0001). Treatment with nifedipine resulted in a rise in LDL-ox of 7-11% as compared to baseline (P<0.05). The difference between the effects of enalapril and nifedipine was statistically significant with all three laboratory methods used (P=0.0001). Both drugs were equally effective in reducing systolic and diastolic blood pressure without affecting HbA(1c) levels and lipid profile. The albumin excretion rate was significantly reduced during treatment with enalapril returning to baseline levels during the washout period and the nifedipine treatment course. Our findings suggest that oxidation of LDL is attenuated by ACE inhibition and augmented by some calcium channel blockers. This observation may contribute insight into the underlying mechanism of the therapeutic effects of ACE inhibition in diabetic patients.

The rebound of lipoproteins after LDL-apheresis. Effects on chemical composition and LDL-oxidizability

The changes in low density lipoprotein (LDL) composition and oxidizability after LDL-apheresis (LA) using dextran sulfate cellulose columns were evaluated in 12 hypercholesterolemic men (mean+/-S.D. total cholesterol (TC) 9.7+/-1.8 mmol/l). After 10-20 months on biweekly LA combined with simvastatin 40 mg per day immediate pre-apheresis levels of TC, LDL-cholesterol, and apolipoprotein B were decreased to 5.3+/-1.3 mmol/l, 3.3+/-1.2 mmol/l, and 1.6+/-0.4 g/l, respectively, whereas apheresis induced mean acute reductions of 61, 78, and 76%, respectively. Measurements of copper-induced LDL-oxidizability in vitro showed an increased resistance against oxidation after LA until day 3 post-treatment: lag time (min) (day 0 (before LA) versus day 1 (post-LA)) 112+/-27 versus 130+/-26 (P=0.001), maximal rate of diene production (nmol/min per mg LDL) 11.1+/-2.7 versus 9.1+/-2.1 (P=0.001), and time to maximal diene production (min) 186+/-39 versus 209+/-35 (P=0. 001). Analysis of the chemical composition of LDL revealed a 25% (P<0.001) reduced content of cholesteryl esters and a decrease of the cholesterol to protein ratio of 1.20+/-0.25 to 0.70+/-0.22 (P<0. 001) through the 3rd day post-LA. Linoleic acid and arachidonic acid content of LDL decreased 11 and 18%, respectively, at the expense of palmitic acid. Vitamin E levels (mg/l) were significantly lowered due to reduction of the lipoprotein pool by apheresis; however, vitamin E content of LDL did not change in the days after apheresis when expressed per g protein or per micromol linoleic acid. The changes in fatty acid pattern were strongly associated with changes in LDL-oxidizability indices (P</=0.01). Thus, LA effectively decreased LDL pool size, inducing the presence of less buoyant lipoproteins, which were less susceptible to in vitro oxidation. This was not explained by changes in vitamin E levels, but by short-term changes in the fatty acids composition.

Essential involvement of 12-lipoxygenase in regiospecific andstereospecific oxidation of low density lipoprotein by macrophages

To establish a role of the 12-lipoxygenase on the generation of oxidized low density lipoprotein (LDL) in macrophages that leads to foam cell formation in atherosclerosis, we overexpressed 12-lipoxygenases in a macrophage-like cell line, J774A.1, that does not show intrinsic enzyme activity. When the 12-lipoxygenase-expressing cells were incubated with 400 microg.mL-1 LDL in Dulbecco's modified Eagle's medium at 37 degrees C for 12 h, LDL oxidation, as determined by thiobarbituric acid reactive substance, was markedly increased compared with the mock-transfected cells. Oxygenated products in the modified LDL were examined by HPLC before and after alkaline hydrolysis. Most of the oxygenated derivatives were of an esterified form, and the major product was identified as 13S-hydroxyoctadeca-9Z,11E-dienoic acid. These results clearly demonstrate that esterified fatty acids in LDL are oxygenated by the 12-lipoxygenases expressed in the J774A.1 cells. Furthermore, the oxidized LDL generated by intracellular 12-lipoxygenases was recognized by a scavenger receptor as assessed by macrophage degradation assay.

Oxidized low density lipoprotein: atherogenic and proinflammatory characteristics during macrophage foam cell formation. An inhibitory role for nutritional antioxidants and serum paraoxonase

Oxidative stress and inflammatory processes are of major importance in atherogenesis because they stimulate oxidized LDL (Ox-LDL)-induced macrophage cholesterol accumulation and foam cell formation, the hallmark of early atherosclerosis. Under oxidative stress, both blood monocytes and plasma lipoproteins invade the arterial wall, where they are exposed to atherogenic modifications. Oxidative stress stimulates endothelial secretion of monocyte chemoattractant protein 1 (MCP-1) and of macrophage colony stimulating factor (M-CSF), leading to monocyte adhesion and differentiation, respectively. LDL binds to extracellular matrix (ECM secreted by endothelial cells, smooth muscle cells and macrophages) proteoglycans, in a process that contributes to the enhanced susceptibility of the lipoprotein to oxidation by arterial wall macrophages. ECM-retained Ox-LDL is taken up by activated macrophages via their scavenger receptors. This leads to cellular cholesterol accumulation and enhanced atherogenesis. Protection of LDL against oxidation by antioxidants that can act directly on the LDL, or indirectly on the cellular oxidative machinery, or conversion of Ox-LDL to a non-atherogenic particle by HDL-associated paraoxonase (PON-1), can contribute to attenuation of atherosclerosis.

Macrophage enrichment with the isoflavan glabridin inhibits NADPH oxidase-induced cell-mediated oxidation of low density lipoprotein. A possible role for protein kinase C

Macrophage-mediated oxidation of low density lipoprotein (LDL) is considered to be of major importance in early atherogenesis; therefore, intervention means to inhibit this process are being extensively studied. In the present study, we questioned the ability of the isoflavan glabridin (from licorice) to accumulate in macrophages and to affect cell-mediated oxidation of LDL. We first performed in vitro studies, using mouse peritoneal macrophages (MPMs) and the J-774 A.1 macrophage-like cell line. Both cells accumulated up to 1.5 micrograms of glabridin/mg of cell protein after 2 h of incubation, and this process was time- and glabridin dose-dependent. In parallel, in glabridin-enriched cells, macrophage-mediated oxidation of LDL was inhibited by up to 80% in comparison with control cells. Glabridin inhibited superoxide release from MPMs in response to phorbol 12-myristate 13-acetate, or to LDL when added together with copper ions, by up to 60%. Translocation of P-47, a cytosolic component of NADPH oxidase to the plasma membrane was substantially inhibited. In glabridin-enriched macrophages, protein kinase C activity reduced by approximately 70%. All of the above effects of glabridin required the presence of the two hydroxyl groups on the flavonoid's B phenol ring. In order to assess the physiological significance of these results, we next performed in vivo studies, using the atherosclerotic apolipoprotein E-deficient (E0) mice. MPMs harvested from glabridin-treated E0 mice (20 micrograms/mouse/day for a period of 6 weeks) demonstrated reduced capability to oxidize LDL by 80% in comparison with placebo-treated mice. This latter phenomenon was associated with a reduction in the lesion oxysterols and a 50% reduction in the aortic lesion size. We thus conclude that glabridin accumulation in macrophages is associated with reduced cell-mediated oxidation of LDL and decreased activation of the NADPH oxidase system. These phenomena could be responsible for the attenuation of atherosclerosis in E0 mice, induced by glabridin.

Microsomal cytochromes P450 catalyze the oxidation of low density lipoprotein

Low density lipoprotein (LDL) oxidation is a major contributor to foam cell formation during early atherogenesis. Several oxygenases have been implicated in the process of LDL oxidation in the arterial wall, where the environment is relatively low in antioxidants, but the exact mechanism for LDL oxidation in vivo is not known. In the present study we sought to determine the ability of cytochrome P450 2E1 (P450 2E1) and other P450s, located in the liver and in other tissues, to oxidize LDL. Upon incubation of LDL (0.1 mg of protein/ml) with purified, reconstituted rabbit P450 2E1 in the presence of NADPH and the NADPH-cytochrome P450 reductase, time- and P450 2E1 concentration-dependent LDL oxidation was observed, as analyzed by determining the formation of peroxides, thiobarbituric acid reactive substances (TBARS), and conjugated dienes. Within 1 h of initiating the reaction, almost maximal oxidation was observed. NADPH, and active P450 2E1 enzyme were required for LDL oxidation to occur. The rate of P450 2E1-induced LDL oxidation was also dependent on the lipoprotein concentration. P450 2E1 could also oxidize pure phospholipids and cholesteryl ester, the major lipids in LDL. In the presence of catalase or superoxide dismutase (SOD), LDL oxidation was completely blocked, suggesting that hydrogen peroxide and superoxide are involved in P450 2E1-induced LDL oxidation. The ability of P450 2E1 to oxidize LDL was not unique to this enzyme, and could be observed with some other purified, cytochromes P450 in the reconstituted system such as rat P450 2B1 and human P450 3A4. Finally, microsomal membranes obtained from rats that were induced to express high levels of P450s 2B1, 2E1, and 1A1/2 were able to oxidize LDL, whereas little oxidation was seen with microsomes that were induced to express 3A2. We thus conclude that LDL can be oxidized by some cytochrome P450s and, as some of these enzymes are present in liver and in arterial wall, they may have a physio/pathological relevance to LDL oxidation and atherogenesis.

Lipid hydroperoxides inhibit nitric oxide production in RAW264.7 macrophages

The effects of oxidatively modified low density lipoprotein (oxLDL) on atherogenesis may be partly mediated by alterations in the production of nitric oxide (NO) by vascular cells. Lipid hydroperoxides (LOOH) and lysophosphatidylcholine (lysoPC) are the major primary products of LDL oxidation. The purpose of this study was to characterize the effects of oxLDL, LOOH and lysoPC on NO production and the expression of inducible nitric oxide synthase (iNOS) gene in lipopolysaccharide (LPS) stimulated macrophages. LDL was oxidized using an azo-initiator 2,2'-azobis (2-amidinopropane) HCl (ABAP) and octadecadienoic acid was oxidized by lipoxygenase to generate 13-hydroperoxyl octadecadienoic acid (13-HPODE). Our study showed that oxLDL markedly decreased the production of NO, the levels of iNOS protein and iNOS mRNA in LPS stimulated macrophages. The inhibition potential of oxLDL on NO production and iNOS gene expression depended on the levels of LOOH formed in oxLDL and was not due to oxLDL cytotoxicity. Furthermore, 13-HPODE markedly reduced NO production and iNOS protein levels, whereas lysoPC showed only slight reduction. The effects of 13-HPODE and lysoPC did not require an acetylated LDL carrier. Our results suggest that 13-HPODE is a much more potent inhibitor of NO production and iNOS gene expression than lysoPC in LPS stimulated RAW264.7 macrophages.

Macrophage released proteoglycans are involved in cell-mediated aggregation of LDL

Aggregated low density lipoprotein (LDL) is taken up by macrophages at enhanced rate, leading to macrophage cholesterol accumulation and foam cell formation. Since macrophages were shown to mediate self aggregation of modified forms of LDL, we sought to study the effect of macrophages on the susceptibility of native LDL to aggregation. Incubation of LDL (100 microg of protein/ml) with J-774A.1 macrophage-like cell line for 18 h at 37 degrees C, led to a 114 and 56% enhanced susceptibility of LDL to aggregation by vortexing and by Bacillus cereus SMase respectively. Macrophage conditioned media (MCMs) that were obtained from J-774A.1 cells also enhanced the susceptibility of LDL to aggregation by vortexing and SMase by 134 and 75% respectively, suggesting the involvement of macrophage secretory products in the enhanced aggregation of LDL. As proteoglycans were shown to be involved in lipoprotein aggregation, we analyzed the possible involvement of macrophage-released proteoglycans in LDL aggregation. Incubation of LDL (100 microg protein/ml) with 25 microg of proteoglycans that were isolated from MCM led to a dose-dependent enhanced susceptibility of LDL to aggregation by vortexing or by SMase by up to 62 and 77% respectively. The stimulatory effect of the MCMs on LDL aggregation was markedly reduced upon MCMs treatment with the glycosaminoglycan hydrolyzing enzyme chondroitinase ABC, chondroitinase AC, but not heparinase. On the contrary, incubation of LDL (100 microg of protein/ml) with increasing concentrations (up to 50 microg/ml) of chondroitin sulfate, or heparan sulfate enhanced the susceptibility of LDL to aggregation by up to 98 or by only 18% respectively, in comparison with non-treated LDL. Since macrophages under atherogenic conditions (cholesterol-loading, cellular lipid peroxidation and activation) demonstrate enhanced secretion of proteoglycans, we finally studied the effect of J-774A.1 macrophages on the susceptibility of native LDL to aggregation under the above atherogenic conditions. Incubation of LDL with cholesterol-loaded macrophages led to a 62% enhanced susceptibility of LDL to undergo aggregation by vortexing, in comparison with LDL that was incubated with non-loaded cells. Macrophage activation with phorbol myristate acetate (5 microM of PMA) also significantly increased cell-mediated aggregation of LDL by 50%, in comparison with non-activated cells. Lipid peroxidized macrophages obtained by cell treatment with either FeSO4 (50 microM), or angiotensin II (10(-7) M) enhanced the susceptibility of LDL to aggregation by 22 or by 39% respectively. These results suggest that under atherogenic conditions, macrophages release proteoglycans, and mainly chondroitin sulfate, which can contribute to cell-mediated formation of aggregated LDL, a potent inducer of macrophage foam cells which are the hallmark of early atherogenesis.

Macrophage foam cell formation during early atherogenesis is determined by the balance between pro-oxidants and anti-oxidants in arterial cells and blood lipoproteins

Atherosclerosis is a multifactorial disease, where more than one mechanism, along more than one step, contributes to macrophage cholesterol accumulation and foam cell formation, the hallmark of early atherogenesis. Arterial macrophages take up oxidized low-density lipoproteins (Ox-LDL), leading to cellular accumulation of cholesterol and oxysterols. Atherogenic modifications of LDL include, in addition to oxidation, retention and aggregation. Intervention to inhibit LDL oxidation can affect the above additional LDL modifications. Indeed, we have demonstrated in the atherosclerotic apolipoprotein E-deficient mice that consumption of vitamin E or of flavonoids from red wine or licorice decreased LDL oxidation, LDL retention, and LDL aggregation and attenuated macrophage foam cell formation and atherosclerosis. The balance between pro-oxidants and anti-oxidants in the LDL particle (such as cholesteryl ester vs. vitamin E), as well as in arterial wall macrophages (such as NADPH oxidase vs. glutathione), determines the extent of LDL oxidation. Antioxidants can protect LDL from oxidation not only by their binding to the lipoprotein, but also following their accumulation in cells of the arterial wall. Whereas antioxidants can prevent the formation of Ox-LDL, human serum paraoxonase (PON 1), an HDL-associated esterase that hydrolyzes organophosphates, can eliminate oxidized LDL (by hydrolysis of its lipid peroxides), which is formed when antioxidant protection is not sufficient. Ox-LDL, in turn, can inactivate paraoxonase activity. Thus, the combination of antioxidants together with active paraoxonase decreases the formation of Ox-LDL and preserves PON1's ability to hydrolyze this atherogenic lipoprotein and hence, to attenuate atherosclerosis.

Structural aspects of the inhibitory effect of glabridin on LDL oxidation

The inhibitory effects of glabridin, an isoflavan isolated from licorice (Glycyrrhiza glabra) root, and its derivatives on the oxidation of LDL induced by copper ions or mediated by macrophages were studied, in order to evaluate the contribution of the different parts of the isoflavan molecule to its antioxidant activity. The peak potential (E1/2) of the isoflavan derivatives, their radical scavenging capacity toward 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical and their ability to chelate heavy metals were also analyzed and compared to their inhibitory activity on LDL oxidation. In copper ion-induced LDL oxidation, glabridin (1), 4'-O-methylglabridin (2), hispaglabridin A (3), and hispaglabridin B (4), which have two hydroxyl groups at positions 2' and 4' or one hydroxyl at position 2' on ring B, successfully inhibited the formation of conjugated dienes, thiobarbituric acid reactive substances (TBARS) and lipid peroxides, and inhibited the electrophoretic mobility of LDL under oxidation. Compounds 1-3 exhibited similar activities, whereas compound 4 was less active. In macrophage-mediated LDL oxidation, the TBARS formation was also inhibited by these isoflavans (1-4) at a similar order of activity to that obtained in copper ion-induced LDL oxidation. On the other hand, 2'-O-methylglabridin (5), a synthesized compound, whose hydroxyl at 2'-position is protected and the hydroxyl at 4'-position is free, showed only minor inhibitory activity in both LDL oxidation systems. 2',4'-O-Dimethylglabridin (6), whose hydroxyls at 2'- and 4'-positions are both protected, was inactive. Resorcinol (7), which is identical to the phenolic B ring in glabridin, presented low activity in these oxidation systems. The isoflavene glabrene (8), which contains an additional double bond in the heterocyclic C ring, was the most active compound of the flavonoid derivatives tested in both oxidation systems. The peak potential of compounds 1-5 (300 microM), tested at pH 7.4, was similar (425-530 mV), and that for compound 6 and 8 was 1078 and 80 mV, respectively. Within 30 min of incubation, compounds 1, 2, 3, 4, 8 scavenged 31%, 16%, 74%, 51%, 86%, respectively, of DPPH radical, whereas compounds 5 and 6, which almost did not inhibit LDL oxidation, also failed to scavenge DPPH. None of the isoflavan derivatives nor the isoflavene compound were able to chelate iron, or copper ions. These results suggest that the antioxidant effect of glabridin on LDL oxidation appears to reside mainly in the 2' hydroxyl, and that the hydrophobic moiety of the isoflavan is essential to obtain this effect. It was also shown that the position of the hydroxyl group at B ring significantly affected the inhibitory efficiency of the isoflavan derivatives on LDL oxidation, but did not influence their ability to donate an electron to DPPH or their peak potential values.

Oxidized LDL damages endothelial cell monolayer and promotes thrombocyte adhesion

The influence of oxidized low density lipoprotein (LDL) on a human endothelial cell monolayer was examined. The resulting contraction of the oxidized LDL-damaged endothelial cells lets intercellular spaces become enlarged and therefore visible via light microscopy. Electron microscopy reveals that the structural damage facilitates thrombocyte adhesion and formation of microthrombi. Oxidized LDL appears to play a pivotal role in initiating and deteriorating thromboembolic complications.

Polyphenolic flavonoids inhibit macrophage-mediated oxidation of LDL and attenuate atherogenesis

Macrophage-mediated oxidation of LDL, a hallmark in early atherosclerosis, depends on the oxidative state of the LDL, and that of the macrophages. The LDL oxidative state is determined by the balance between the LDL polyunsaturated fatty acids and cholesterol, which are prone to oxidation, and the LDL associated antioxidants. Dietary consumption of nutrients rich in polyphenols, such as red wine or liquorice results in LDL enrichment with these polyphenolic flavonoids, and hence, subsequent LDL oxidation is reduced. In addition, enrichment of LDL with polyphenols results in a marked decrease in the susceptibility of the lipoprotein to aggregation (another lipoprotein atherogenic modification). The oxidative status of the macrophages depends on the balance between cellular oxygenases and antioxidants. Macrophage enrichment with polyphenolic flavonoids in vitro or in vivo also reduce macrophage oxidative state, and subsequently cell-mediated oxidation of LDL. The present review article summarizes our current data on these aspects of the antiatherogenic potential of polyphenolic flavonoids.

Human peritoneal monocytic cells: lipoprotein uptake and foam cell formation

Human peritoneal cells isolated from dialysis effluent have in vivo maturated human macrophages that could serve as a model for studying lipoprotein metabolism and foam cell formation. We previously characterized the low density lipoprotein (LDL) and acetylated LDL (acetyl-LDL) receptor activities of human total peritoneal cells. Now, we provide evidence that both LDL and acetyl-LDL stimulate acylCoA cholesterol:acyl transferase (ACAT) activity of peritoneal cells. Prolonged incubation of cells with LDL results in suppression of ACAT activity, while incubation with acetyl-LDL results in elevated and sustained enzyme activity. When human peritoneal cells were analyzed using flow cytometry, the cell population showed reactivity for CD2, CD4, CD8, CD20, CD14 and HLA-DR antigens. Purified human peritoneal mononuclear cells degraded LDL. Human peritoneal macrophages formed foam cells when exposed to LDL or acetyl-LDL in culture, and lipid deposition increased with incubation time. Macrophages incubated in the presence of butylated hydroxy toluene and LDL did not form foam cells.

Macrophage glutathione content and glutathione peroxidase activity are inversely related to cell-mediated oxidation of LDL: in vitro and in vivo studies

Macrophage-mediated oxidation of low-density lipoprotein (LDL) is thought to play a key role during early atherogenesis, and cellular oxygenases were shown to mediate this process. As macrophage antioxidants may also contribute to the extent of cell-mediated oxidation of LDL, we analyzed the role of cellular reduced glutathione (GSH) and glutathione peroxidase (GPx) in LDL oxidation. The present study examined the effect of the macrophage GSH-GPx status on the ability of the cells to oxidize LDL. Upon incubation of J-774 A.1 macrophages for 20 h at 37 degrees C with 50 microM of buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis, cellular GSH content and GPx activity were reduced by 89 and 50%, respectively, and this effect was associated with a twofold elevation in macrophage-mediated oxidation of LDL. The BSO-treated cells contained high levels of peroxides, and released 32% more superoxide anions than nontreated cells in response to their stimulation with LDL in the presence of copper ions. To increase macrophage GSH content and GPx activity we have used L-2-oxothiazolidine-4-carboxylic acid (OTC), which delivers cysteine residues to the cells for GSH synthesis, and also selenium, which activates GPx and increases cellular glutathione synthesis. GSH content and GPx activity in J-774 A.1 macrophages were increased by 80 and 50%, respectively, following cells incubation with 2 mM OTC for 20 h at 37 degrees C, and this was paralleled by a 47% inhibition in LDL oxidation by these cells. An inverse correlation was found between the extent of macrophage-mediated oxidation of LDL and cellular GSH content (r = .97), or GPx activity (r = .95). Upon incubation of J-774 A.1 macrophages with selenomethionine (10 ng/ml) for 1 week, cellular GSH content and GPx activity were increased by about twofold compared to control cells, and this effect was associated with a 30% reduction in cell-mediated oxidation of LDL. Dietary selenium supplementation (1 microg/d/mouse) to the atherosclerotic apolipoprotein E-deficient mice for a 6-month period, increased GSH content and GPx activity in the mice peritoneal macrophages by 36 and 30%, respectively, and this effect was associated with a 46% reduction in cell-mediated oxidation of LDL. Finally, the atherosclerotic lesion area in the aortas derived from these mice after selenium supplementation was found to be reduced by 30% compared to the lesion area found in nontreated mice. Our results demonstrate an inverse relationship between macrophage GSH content/GPx activity and cell-mediated oxidation of LDL. Intervention means to enhance the macrophage GSH-GPx status may thus contribute to attenuation of the atherosclerotic process.

Plasma LDL oxidation leads to its aggregation in the atherosclerotic apolipoprotein E-deficient mice

Two major modifications of low density lipoprotein (LDL) that can lead to macrophage cholesterol accumulation and foam cell formation include its oxidation and aggregation. To find out whether these modifications can already occur in vivo in plasma and whether they are related to each other, the oxidation and aggregation states of plasma LDL were analyzed in the apolipoprotein E-deficient (E degree) transgenic mice during their aging (and the development of atherosclerosis), in comparison to plasma LDL from control mice. Plasma LDL from the E degree mice was already minimally oxidized at 1 month of age in comparison to control mice LDL, and it further oxidized with age in the E degree mice but not in the control mice. At 6 months of age, the contents of the E degree mice LDL-associated cholesteryl ester hydroperoxides, thiobarbituric acid reactive substances, and conjugated dienes were higher by two, three, and twofold, respectively, in comparison to LDL from the young, 1-month-old E degree mice. We also investigated the LDL aggregation state in E degree mice. In the young E degree mice, LDL oxidation was shown in comparison to control mice, but in both groups of young mice their LDL was not aggregated. In the E degree mice, however, the LDL aggregation state substantially increased with age, by as much as 125% at 6 months of age compared to the 1-month-old mice, whereas no significant aggregation could be detected in plasma LDL from control mice at the same age. To question the possible effect of LDL oxidation on its subsequent aggregation, LDL oxidation was induced by either copper ions, or by the free radical generator 2,2-azobis-2-amidinopropane hydrochloride, or by hypochlorite. All these oxidative systems led to LDL oxidation (to different degrees) and resulted in a similar, substantial LDL aggregation. These oxidation systems also enhanced the susceptibility of LDL to aggregation (induced by vortexing) by 23%, 28%, or 40%, respectively. To further analyze the relationships between the lipoprotein oxidation and its aggregation, LDL (0.1 mg of protein/mL) was incubated with 5 mumol/L CuSO4 at 37 degrees C in the absence or presence of the antioxidant, vitamin E (25 mumol/L). In the absence of vitamin E, a time-dependent increment in LDL oxidation was noted, which reached a plateau after 2 hours of incubation. LDL aggregation, however, only started at this time point and reached a plateau after only 5 hours of incubation. In the presence of vitamin E, both LDL oxidation and its aggregation were reduced at all time points studied. We extended the vitamin E study to the in vivo situation, and the effect of vitamin E supplementation to the E degree mice (50 mg.kg-1.d-1 for a 3-month period) on their plasma LDL oxidation and aggregation states was studied. Vitamin E supplementation to these mice resulted in a 35% reduction in the LDL oxidation state and in parallel, the LDL aggregation state was also reduced by 23%. These reductions in LDL oxidation and aggregation states were accompanied by a 33% reduction in the aortic lesion area, in comparison to nontreated E degree mice. We conclude that in E degree mice, LDL oxidation, which already took place in the plasma, can lead to the lipoprotein aggregation. These modified forms of LDL were shown to be taken up by macrophages at an enhanced rate, leading to foam cell formation. Thus, the use of an appropriate antioxidant can inhibit the formation of both atherogenic forms of LDL.

Effect of bone marrow transplantation on lipoprotein metabolism and atherosclerosis in LDL receptor-knockout mice

The LDL receptor (LDLR) plays an important role in the removal of LDL and its precursors, the intermediate and very low density lipoproteins, from the blood circulation. The receptor is expressed on various cell types. In this study the relative importance of the LDLR on macrophages for lipoprotein metabolism and atherogenesis was assessed. For this purpose, irradiated LDLR-knockout (-/-) mice were transplanted with bone marrow of normal C57BL/6J mice. DNA analysis showed that the transplanted mice were chimeric. The transplantation resulted in a slight decrease of total serum cholesterol when compared with LDLR-/- mice that were transplanted with LDLR-/- bone marrow. This modest decrease, however, did not reach statistical significance at all time points examined. This decrease can be almost completely attributed to a decrease in LDL cholesterol. The specific lowering of LDL cholesterol could clearly be observed at 4 weeks after transplantation, but the decrease was less at 12 weeks after transplantation. Quantification of atherosclerotic lesions of mice fed a 1% cholesterol diet for 6 months revealed that there were no differences in mean lesion area between mice transplanted with wild-type bone marrow or LDLR-/- bone marrow. We anticipate that in LDLR-/- mice transplanted with wild-type bone marrow, the LDLR is downregulated by the relatively high concentrations of circulating cholesterol. In vitro incubations of peritoneal macrophages with 125I-LDL indicated that the LDLR of these cells could be downregulated by 25-hydroxycholesterol. Peritoneal macrophages isolated from LDLR-/- mice transplanted with wild-type bone marrow, in contrast to those transplanted with LDLR-/- bone marrow, were able to degrade 125I-LDL, indicating that the capacity to express functional LDLR was achieved. In conclusion, introduction of the LDLR into LDLR -/- mice via bone marrow transplantation resulted in only a relatively modest decrease of LDL cholesterol that became less pronounced at later time points, possibly due to downregulation of the LDLR. To utilize the LDLR in macrophages for effective cholesterol lowering, either the sterol-regulatory elements have to be "silenced" or a high-expression LDLR construct has to be introduced into macrophages, eg, via transplantation of in vitro transfected hematopoietic stem cells.

Direct copper reduction by macrophages. Its role in low density lipoprotein oxidation

Oxidation of low density lipoprotein (LDL) results in changes to the lipoprotein that are potentially atherogenic. Numerous studies have shown that macrophages cultured in vitro can promote LDL oxidation via a transition metal-dependent process, yet the exact mechanisms that are responsible for macrophage-mediated LDL oxidation are not understood. One contributing mechanism may be the ability of macrophages to reduce transition metals. Reduced metals (such as Fe(II) or Cu(I)) rapidly react with lipid hydroperoxides, leading to the formation of reactive lipid radicals and conversion of the reduced metal to its oxidized form. We demonstrate here the ability of macrophages to reduce extracellular iron and copper and identify a contributing mechanism. Evidence is provided that a proportion of cell-mediated metal reduction is due to direct trans-plasma membrane electron transport. Glucagon suppressed both macrophage-mediated metal reduction and LDL oxidation. Although metal reduction was augmented when cells were provided with a substrate for thiol production, thiol export was not a strict requirement for cell-mediated metal reduction. Similarly, while the metal-dependent acceleration of LDL oxidation by macrophages was augmented by thiol production, macrophages could still promote LDL oxidation when thiol export was minimized (by substrate limitation). This study identifies a novel mechanism that may contribute to macrophage-mediated LDL oxidation and may also reveal potential new strategies for the inhibition of this process.

Reduced susceptibility of low density lipoprotein (LDL) to lipid peroxidation after fluvastatin therapy is associated with the hypocholesterolemic effect of the drug and its binding to the LDL

Increased plasma cholesterol concentration in hypercholesterolemic patients is a major risk factor for atherosclerosis. The impaired removal of plasma low density lipoprotein (LDL) in these patients results in the presence of their LDL in the plasma for a long period of time and thus can contribute to its enhanced oxidative modification. In the present study we analyzed the effect of the hypocholesterolemic drug, fluvastatin, on plasma and LDL susceptibilities to oxidation during 24 weeks of therapy. Fluvastatin therapy (40 mg/day for 24 weeks) in 10 hypercholesterolemic patients resulted in 30%, 34% and 22% decrements in plasma levels of total cholesterol, LDL cholesterol and triglycerides, respectively. This effect has been achieved after only 4 weeks of therapy. We next studied the effect of fluvastatin therapy on LDL susceptibility to oxidation in vivo and in vitro. 2.2-Azobis, 2-amidinopropane hydrochloride (AAPH, 100 mM)-induced plasma lipid peroxidation was decreased by 70% and 77% after 12 weeks and 24 weeks of fluvastatin therapy respectively. The lag time required for the initiation of CuSO4 (10 microM)-induced LDL oxidation was prolonged by 1.2- and 2.5-fold, after 12 and 24 weeks of fluvastatin therapy respectively. We next analyzed the in vitro effect of fluvastatin on plasma and LDL susceptibilities to oxidation. Preincubation of plasma or LDLs that were obtained from normal subjects with 0.1 microgram/ml of fluvastatin, caused 20% or 57% reduction in AAPH-induced lipid peroxidation, respectively. Similarly, a 1.6- and 2.7-fold prolongation of the lag time required for CuSO4-induced LDL oxidation was found following LDL incubation with 0.1 and 1.0 microgram/ml of fluvastatin, respectively. To find out possible mechanisms that contribute to this inhibitory effect of fluvastatin on LDL oxidizability, we analyzed the antioxidative properties of fluvastatin. Fluvastatin did not scavenge free radicals and did not inhibit linoleic acid peroxidation. Fluvastatin also did not act as a chelator of copper ions. However, fluvastatin was shown to specifically bind mainly to the LDL surface phospholipids and this interaction altered the lipoprotein charge as evident from the 38% decrement in the electrophoretic mobility of fluvastatin-treated LDL, in comparison to nontreated LDL. The inhibitory effect of fluvastatin therapy on LDL oxidation probably involves both its stimulatory effect on LDL removal from the circulation, as well as a direct binding effect of the drug to the lipoprotein. We thus conclude that the antiatherogenic properties of fluvastatin may not be limited to its hypocholesterolemic effect, but could also be related to its ability to reduce LDL oxidizability.

Increased uptake of LDL by oxidized macrophages is the result of an initial enhanced LDL receptor activity and of a further progressive oxidation of LDL

Iron ions were recently shown to induce cellular lipid peroxidation in macrophages, and these oxidized cells can convert native low-density lipoprotein (LDL) to oxidized LDL (Ox-LDL). The present study demonstrates that deoxycholic acid (DCA) and angiotensin II (ANG-II) can also induce oxidative modification of macrophages via metal ions independent mechanisms. Furthermore, incubation of LDL (200 micrograms of protein/ml) for 24 h at 37 degrees C with DCA, ANG-II, as well as FeSO4-induced oxidized macrophages, resulted in oxidative modification of the lipoprotein as evidenced by increased TBARS formation in LDL (by 50, 105, and 258%, respectively), decreased TNBS reactivity (by 45, 56, and 42%, respectively), and increased cellular uptake (by 60, 166, and 230%, respectively). A positive correlation (n = .88) was found between the extent of the cellular lipid peroxidation and the increment in the cellular uptake of the LDL. The oxidative modification of LDL by oxidized macrophages was found to be a progressive process. Incubation of LDL with oxidized macrophages for increasing periods of time up to 24 h resulted in progressive increment in: (1) the electrophoretic mobility of the LDL; (2) the TBARS formation in LDL; (3) the cellular uptake of LDL by the oxidized macrophages via the Ox-LDL receptor. Upon fractionation on a heparin-sepharose column of LDL that was incubated for different periods of time with oxidized macrophages, a gradual increment in the unbound LDL fraction was obtained, up to 72% after 24 h of incubation. During the first hour of LDL incubation with the oxidized macrophages a twofold increase in the cellular uptake of LDL by these cells was detected, although no significant oxidation of the lipoprotein occurred during this short time period. This effect could be attributed to an increased number of LDL receptors on the cell surface of the oxidized macrophages. In conclusion, increased uptake of LDL by oxidized macrophages results from two routes: (1) enhanced uptake via the LDL receptor due to increased LDL receptor activity; (2) lipoprotein uptake via the Ox-LDL receptors due to cellular modification of LDL. Both of these processes lead to macrophage cholesterol accumulation and foam cell formation, and thus contribute to accelerated atherosclerosis under oxidative stress.

Activation of NADPH oxidase required for macrophage-mediated oxidation of low-density lipoprotein

Low-density lipoprotein (LDL) oxidation by arterial wall cells, a key event during early atherogenesis, was suggested to involve the activation of 15-lipoxygenase and/or nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. We sought to analyze the role of these oxygenases in macrophage-mediated oxidation of LDL under oxidative stress. Upon incubation of LDL with the J-774 A.1 macrophage-like cell line or with human monocyte-derived macrophages (HMDM) in the presence of 1 micromol/L CuSO4, the release of superoxide anions to the medium was demonstrated. Under these conditions, the cytosolic protein components of the NADPH oxidase complex, P-47 and P-67, translocated to the plasma membrane, indicating LDL-mediated activation of the NADPH oxidase complex. Under the above-mentioned experimental conditions, the macrophage 15-lipoxygenase was also activated, as determined by the release of 15-hydroxy-5,8,11,13-eicosatetraenoic acid (15-HETE) and 13-hydroxyoctadecadienoic acid (13-HODE) to the medium. Inhibition of the macrophage NADPH oxidase with apocynin or dismutation of superoxide anions, the product of NADPH oxidase activation, with superoxide dismutase (SOD) significantly inhibited macrophage-mediated oxidation of LDL (by 61% to 89%) under these conditions. Phorbol myristate acetate (PMA), which causes NADPH oxidase activation in J-774 A.1 macrophages, had no significant effect on 15-lipoxygenase activity, but still resulted in cell-mediated oxidation of LDL. Finally, HMDM from two patients with chronic granulomatous disease (CGD) that were shown to lack active NADPH oxidase, but to possess almost normal 15-lipoxygenase activity failed to oxidize LDL. We thus conclude that LDL-induced NADPH oxidase activation (under oxidative stress) is required for macrophage-mediated oxidation of LDL, whereas activation of 15-lipoxygenase may not be sufficient for LDL oxidation under these conditions.

Enhanced capacity of n-3 fatty acid-enriched macrophages to oxidize low density lipoprotein mechanisms and effects of antioxidant vitamins

We have investigated possible mechanisms by which n-3 fatty acid-enriched macrophages enhance the oxidation of low density lipoprotein (LDL), and the ability of antioxidant vitamins to prevent this. Macrophages were enriched with n-3 fatty acids (eicosapentaenoic acid, docosapentaenoic acid and docosahexaenoic acid) following incubation with fish oil. These macrophages produced large amount of TBARS in medium containing metals, and showed enhanced capacity to oxidize LDL (3-4 fold increase compared to control cells) and to accumulate the modified LDL. 5,8,11,14-eicosatetraynoic acid (ETYA, 15-lipoxygenase inhibitor) and superoxide dismutase (SOD) did not inhibit the enhanced capacity of n-3 fatty acid-enriched cells to oxidize LDL. However antioxidants, (vitamin E-enriched macrophages or vitamin C in the medium), inhibited this enhanced capacity. Medium conditioned by n-3 fatty acid-enriched cells had pro-oxidant effects on metal-initiated LDL oxidation. We conclude that n-3 fatty acid-enriched macrophages display increased oxidant capacity which is not inhibited by ETYA or SOD, and that antioxidant vitamins inhibit the enhanced capacity to oxidize LDL.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Angiotensin II-modified LDL is taken up by macrophages via the scavenger receptor, leading to cellular cholesterol accumulation

The incidence of myocardial infarction is significantly higher in hypertensive patients with increased plasma concentration of angiotensin (Ang) II. Ang II was shown to bind to LDL in vitro, and in the present study we showed its binding to LDL in vivo. Ang II (10(-7) mol/L) was incubated with LDL for 3 hours at 37 degrees C, followed by reseparation of the modified lipoprotein (Ang II-LDL) and its incubation with J-774 A.1 macrophages. Binding of Ang II to LDL significantly increased the lipoprotein protein degradation (by 25%) and its cell association (by 75%) compared with nontreated LDL. Unlike Ang II-LDL, both Ang I-LDL and Ang III-LDL were taken up by macrophages similar to native LDL. The lipid composition and size of Ang II-LDL were similar to those of native LDL, and it was not aggregated. Ang II-LDL was not oxidized, as the contents of malondialdehyde and peroxides were not different from those found in native LDL. On heparin-Sepharose column chromatography, Ang II-LDL was eluted in the void volume, like acetylated LDL (Ac-LDL) and unlike native LDL, which binds to heparin. The cellular degradation of Ang II-125I-labeled LDL by J-774 A.1 macrophages of Ang II-125I-labeled LDL by J-774 A.1 macrophages was studied in the presence of a 50-fold excess of nonlabeled native LDL, Ang II-LDL, Ac-LDL, or oxidized LDL (Ox-LDL). Whereas native LDL had no effect on the degradation of Ang II-125I-LDL by the macrophages, Ac-LDL, Ox-LDL, and Ang II-LDL reduced the cellular uptake of the lipoprotein by 77%, 82%, and 87%, respectively. Similarly, fucoidin but not free Ang II reduced macrophage degradation of the labeled Ang II-LDL. We conclude that Ang II can modify LDL to a form that is not oxidized or aggregated but is still taken up at an enhanced rate by macrophages via the scavenger receptor.