Oxidation of cholesterol moiety of low density lipoprotein in the presence of human endothelial cells or Cu+2 ions: identification of major products and their effects

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.

7-Ketocholesterol induces ROS-mediated mRNA expression of 12-lipoxygenase, cyclooxygenase-2 and pro-inflammatory cytokines in human mesangial cells: Potential role in diabetic nephropathy

7-Ketocholesterol (7-KCHO) is a highly proinflammatory oxysterol and plays an important role in the pathophysiology of diabetic nephropathy (DN). Lipoxygenases (LOXs) and cyclooxygenases (COXs) are also involved in the development of DN. The aim of this study was to clarify the effects of 7-KCHO on mRNA expression of LOXs and COXs as well as pro-inflammatory cytokines in human mesangial cells (HMC). We evaluated cell viability by WST-8 assay and measured mRNA expression by reverse transcription-polymerase chain reaction. Intracellular reactive oxygen species (ROS) production was evaluated by flow cytometry. Although 7-KCHO did not affect cell viability of HMC, 7-KCHO stimulated significant increases in mRNA expression of 12-LOX, COX-2 and pro-inflammatory cytokines. 7-KCHO also induced an increase in ROS production, while N-acetylcysteine partially suppressed the increase. The 12-LOX and COX-2 inhibitors also suppressed mRNA expression of cytokines. These findings may contribute to the elucidation of the molecular mechanism of the pathophysiology of DN.

oxLDL and eLDL Induced Membrane Microdomains in Human Macrophages

Background

Extravasation of macrophages and formation of lipid-laden foam cells are key events in the development and progression of atherosclerosis. The degradation of atherogenic lipoproteins subsequently leads to alterations in cellular lipid metabolism that influence inflammatory signaling. Especially sphingolipids and ceramides are known to be involved in these processes. We therefore analyzed monocyte derived macrophages during differentiation and after loading with enzymatically (eLDL) and oxidatively (oxLDL) modified low-density lipoproteins (LDL).

Methods

Primary human monocytes were isolated from healthy, normolipidemic blood donors using leukapheresis and counterflow elutriation. On the fourth day of MCSF-induced differentiation eLDL (40 μg/ml) or oxLDL (80 μg/ml) were added for 48h. Lipid species were analyzed by quantitative tandem mass spectrometry. Taqman qPCR was performed to investigate transcriptional changes in enzymes involved in sphingolipid metabolism. Furthermore, membrane lipids were studied using flow cytometry and confocal microscopy.

Results

MCSF dependent phagocytic differentiation of blood monocytes had only minor effects on the sphingolipid composition. Levels of total sphingomyelin and total ceramide remained unchanged, while lactosylceramides, cholesterylesters and free cholesterol decreased. At the species level most ceramide species showed a reduction upon phagocytic differentiation. Loading with eLDL preferentially increased cellular cholesterol while loading with oxLDL increased cellular ceramide content. Activation of the salvage pathway with a higher mRNA expression of acid and neutral sphingomyelinase, neutral sphingomyelinase activation associated factor and glucosylceramidase as well as increased surface expression of SMPD1 were identified as potentially underlying mechanisms. Moreover, flow-cytometric analysis revealed a higher cell-surface-expression of ceramide, lactosylceramide (CDw17), globotriaosylceramide (CD77), dodecasaccharide-ceramide (CD65s) and GM1 ganglioside upon oxLDL loading. ApoE in contrast to apoA-I preferentially bound to the ceramide enriched surfaces of oxLDL loaded cells. Confocal microscopy showed a co-localization of acid sphingomyelinase with ceramide rich membrane microdomains.

Conclusion

eLDL leads to the formation of lipid droplets and preferentially induces cholesterol/sphingomyelin rich membrane microdomains while oxLDL promotes the development of cholesterol/ceramide rich microdomains via activation of the salvage pathway.

Alzheimer disease beta-amyloid activity mimics cholesterol oxidase

The abnormal accumulation of amyloid beta-peptide (Abeta) in the form of senile (or amyloid) plaques is one of the main characteristics of Alzheimer disease (AD). Both cholesterol and Cu2+ have been implicated in AD pathogenesis and plaque formation. Abeta binds Cu2+ with very high affinity, forming a redox-active complex that catalyzes H2O2 production from O2 and cholesterol. Here we show that Abeta:Cu2+ complexes oxidize cholesterol selectively at the C-3 hydroxyl group, catalytically producing 4-cholesten-3-one and therefore mimicking the activity of cholesterol oxidase, which is implicated in cardiovascular disease. Abeta toxicity in neuronal cultures correlated with this activity, which was inhibited by Cu2+ chelators including clioquinol. Cell death induced by staurosporine or H2O2 did not elevate 4-cholesten-3-one levels. Brain tissue from AD subjects had 98% more 4-cholesten-3-one than tissue from age-matched control subjects. We observed a similar increase in the brains of Tg2576 transgenic mice compared with nontransgenic littermates; the increase was inhibited by in vivo treatment with clioquinol, which suggests that brain Abeta accumulation elevates 4-cholesten-3-one levels in AD. Cu2+-mediated oxidation of cholesterol may be a pathogenic mechanism common to atherosclerosis and AD.

Lipoprotein profile in limited systemic sclerosis

The objective of this study was to determine the lipoprotein profile of limited cutaneous systemic sclerosis (LcSSc). Fasting lipids were determined in 24 female patients and 24 healthy age-matched and sex-matched controls. Exclusion criteria were conditions that induce an altered lipid profile. Lipoprotein levels of risk were determined in accordance with the National Cholesterol Education Program (NCEP). Significantly lower levels of high-density lipoprotein (HDL) cholesterol (47.6+/-12.4 mg dL(-1) vs. 58.2+/-12.3 mg dL(-1); P=0.003) and total cholesterol (197.0+/-40.7 mg dL(-1) vs. 222.0+/-34.0 mg dL(-1); P=0.02) were observed in LcSSc patients than in controls. The presence of anti-centromere antibodies (ACA) was also associated with lower HDL levels (45.0+/-12.1 mg dL(-1)) compared to ACA-negative patients and controls (50.2+/-12.6 and 58.2+/-12.3 mg dL(-1), respectively, P=0.01). The only clinical variable associated with low HDL levels was pulmonary hypertension (PH) (33.6+/-2.3 mg dL(-1) vs. 49.6+/-11.9 mg dL(-1), P=0.01). No significant correlation was observed among HDL levels and ESR (r=-0.313; P=0.14), CRP (r=-0.296; P=0.16), or BMI (r=-0.263; P=0.21). Remarkably, a higher percentage of risk HDL levels was identified in LcSSc patients (41.6%) than in healthy controls (8.3%) (P=0.02). Our data suggest that LcSSc patients, particularly those who are ACA positive, have an adverse lipid profile characterized by low HDL levels, a known independent risk for CAD in women. The relevance of this finding for the development of atherosclerosis in this disease must be confirmed by epidemiological studies.

Omega-carboxyl variants of 7-ketocholesteryl esters are ligands for beta(2)-glycoprotein I and mediate antibody-dependent uptake of oxidized LDL by macrophages

beta(2)-Glycoprotein I (beta(2)-GPI) is a major antigen for anticardiolipin antibodies (aCL, Abs) present in patients with antiphospholipid syndrome. We recently reported that beta(2)-GPI specifically binds to oxidized LDL (oxLDL) and that the beta(2)-GPI's major ligand, oxLig-1 is 7-ketocholesteryl-9-carboxynonanoate (Kobayashi, K., E. Matsuura, Q. P. Liu, J. Furukawa, K. Kaihara, J. Inagaki, T. Atsumi, N. Sakairi, T. Yasuda, D. R. Voelker, and T. Koike. 2001. A specific ligand for beta(2)-glycoprotein I mediates autoantibody-dependent uptake of oxidized low density lipoprotein by macrophages. J. Lipid Res. 42: 697-709). In the present study, we demonstrate that omega-carboxylated 7-ketocholesteryl esters are critical for beta(2)-GPI binding. A positive ion mass spectrum of a novel ligand, designated oxLig-2, showed fragmented ions at m/z 383 and 441 in the presence of acetone, which share features of oxLig-1 and 7-ketocholesterol. In the negative ion mode, ions at m/z 627, 625, and 243 were observed. oxLig-2 was most likely 7-ketocholesteryl-12-carboxy (keto) dodecanoate. These ligands were recognized by beta(2)-GPI. Liposome binding to macrophages was significantly increased depending on the ligand's concentration, in the presence of beta(2)-GPI and an anti-beta(2)-GPI Ab. Synthesized variant, 7-ketocholesteryl-13-carboxytridecanoate (13-COOH-7KC), also showed a significant interaction with beta(2)-GPI and a similar binding profile with macrophages. Methylation of the carboxyl function diminished all of the specific ligand interactions with beta(2)-GPI. Thus, omega-carboxyl variants of 7-ketocholesteryl esters can mediate anti-beta(2)-GPI Ab-dependent uptake of oxLDL by macrophages, and autoimmune atherogenesis linked to beta(2)-GPI interaction with oxLDL.

Evidence of increased formation of products retaining strong antioxidant activity from estradiol-17beta oxidation in the presence of human plasma lipoproteins

Estradiol-17beta (E2) exhibits potent antioxidant effects that cause continuous suppression of metal-catalyzed oxidation of low-density-lipoprotein in vitro. We sought to learn whether unidentified oxidation products retaining strong antioxidant property may be generated from E2 incubated with lipoproteins and subjected to oxidation by reactive oxygen species generators. E2 oxidation was markedly stimulated in the presence of both LDL and high-density lipoprotein. We have isolated two novel products (less polar than E2), formed when E2 was oxidized with copper sulfate and hydrogen peroxide in the presence of lipoproteins). Both compounds had molecular weights of 306 on gas chromatography/mass spectrometry. They appear to be as strong as E2 in inhibiting LDL oxidation in vitro. Because of their increased hydrophobicity, they have the potential of being associated with LDL and offer promise as agents that can limit LDL oxidation, thereby contributing to cardioprotection.

Lymphatic transport of dietary cholesterol oxidation products, cholesterol and triacylglycerols in rats

Rats were fed on a diet containing 0.5% cholesterol oxidation products (oxysterols) or 0.5% cholesterol for 30 min, and their lymph was collected for 7 h. The amount of each of the individual oxysterols absorbed in the lymph depended on the ingested amounts, but the recovery was the highest for 5alpha,6alpha-epoxycholesterol (10.5%), this being followed by 7-ketocholesterol (5.8%), cholestanetriol (5.2%), 7beta-hydroxycholesterol (4.8%), 7alpha-hydroxycholesterol (3.4%), 5beta,6beta-epoxycholesterol (2.2%), and 25-hydroxycholesterol (1.8%). A diet enriched with oxysterol, but not cholesterol, resulted in increased transport of triacylglycerols in the lymph. These results suggest that the absorption rate of oxysterols depends on the type, and indicate that the effect of dietary oxysterols on the lymphatic transport of triacylglycerols differs from that of dietary cholesterol. It therefore remains to be determined which oxysterol was responsible for the triacyglycerol transport.

25-hydroxycholesterol induces lipopolysaccharide-tolerance and decreases a lipopolysaccharide-induced TNF-alpha secretion in macrophages

Several different oxysterols are formed when LDL is oxidized. The role of oxysterols in the inflammatory process in the atherosclerotic plaque is not totally elucidated. In this study we have investigated the effect of four different oxysterols on an LPS-induced TNF-alpha secretion in human macrophages. Cultured human macrophages were incubated with 7-keto-, 7beta-hydroxy-, 27-hydroxy- and 25-hydroxycholesterol for 24 h before exposure to endotoxin (LPS) for 3 h. All oxysterols, except 7-ketocholesterol, significantly decreased an LPS-induced TNF-alpha secretion. The most pronounced effect was obtained with 25-hydroxycholesterol, where the TNF-alpha secretion was reduced to 8%. This decreased effect was also found on the TNF-alpha mRNA level. The decreased LPS-induced TNF-alpha secretion coincided with an increased binding of the transcription factors Sp1 and Sp3 to the TNF-alpha promoter. In vitro studies of the TNF-alpha promoter suggests possible interactions between Sp1 and Sp3 and the NF-kappaB transcription factor complex that might affect the transcriptional initiation.

Selective distribution of oxysterols in atherosclerotic lesions and human plasma lipoproteins

The presence of oxidized sterols (oxysterols) in human serum and lesions has been linked to the initiation and progression of atherosclerosis. Data concerning the origin, identity and quantity of oxysterols in biological samples are controversial and inconsistent. This inconsistency may arise from different analytical methods or handling conditions used by different investigators. In the present study, oxysterol levels and distribution were analyzed by an optimized GC-MS method, in human atherosclerotic coronary and carotid lesions, in atherosclerotic apolipoprotein E deficient mice (E degrees mice) and in native and in vitro oxidized human low and high density lipoproteins. Oxysterol levels were analyzed with a limit of detection of 0.06 - 0.24 ng, with 25-hydroxycholesterol (25-OH) being the least sensitive. In human coronary and carotid lesions, obtained from endatherectomic samples, 27-hydroxycholesterol (27-OH) was the major oxysterol, with about 85% as sterols esterified to fatty acids. While total cholesterol and oxysterols levels were similar in both kinds of human lesions, oxysterol distribution was significantly different. In coronary lesions the mean levels of 27-OH and 7beta-hydroxycholesterol (7beta-OH) were 38% and 20% of total oxysterols, whereas in carotid lesions their mean levels were 66% and 5%, respectively. Unlike in human aortic lesions, 27-OH was entirely absent in E degrees mice, whereas the level of 7alpha-hydroxycholesterol (7alpha-OH) was 28% of the total oxysterols, vs. 5% in human coronary lesions. As 27-OH is an enzymatic product of cholesterol oxidation, this finding may indicate that such an enzymatic process does not take place in E degrees mice.

Analysis of 7-ketocholesterol in low density lipoprotein and fatty acid composition in erythrocyte membranes of patients on maintenance hemodialysis and healthy controls

We established a method to quantify 7-ketocholesterol (7-KC) in low density lipoprotein by using the heparin-citrate method and gas chromatography-mass spectrometry. We examined the concentration of 7-ketocholesterol in LDL using this method to assess the pathological conditions in uremic patients with hemodialysis and healthy controls. We also examined the fatty acid composition in erythrocyte membranes to estimate the modification of biological membranes. We showed that the concentrations of 7-KC/cholesterol in LDL were significantly increased in hemodialysis patients compared to healthy controls (3.68+/-0.45 vs. 2.41+/-0.19, P<0.05) and the ratio of polyunsaturated fatty acids to saturated fatty acids in erythrocyte membranes was significantly decreased in hemodialysis patients compared to healthy controls (0.499+/-0.014 vs. 0.655+/-0. 017, P<0.001). There was no significant difference in 7-KC concentration in LDL or fatty acid composition in erythrocyte membranes between pre- and post-intervention of hemodialysis. We concluded that hemodialysis patients are under oxidative stress, which modifies LDL and erythrocyte membranes and we speculated these modifications may participate in the process of atherosclerosis. We believe that the method to quantify 7-KC in this study is concise and reliable and may be used to investigate various diseases.

Plasma oxysterols and tocopherol in patients with diabetes mellitus and hyperlipidemia

The plasma levels of free oxysterols (7-ketocholesterol; 7alpha-hydroxy-, 7beta-hydroxy-, 25-hydroxy-, and 27-hydroxycholesterol; and 5alpha,6alpha-epoxycholestanol) in patients with diabetes mellitus and hypercholesterolemia were determined using gas chromatography-mass spectrometry with selective ion monitoring. We studied 39 patients with diabetes mellitus, 20 nondiabetic patients with hypercholesterolemia, and 37 normal controls. Plasma cholesterol levels in diabetic and hypercholesterolemic patients showed no statistical difference. Plasma 7-ketocholesterol was significantly higher in patients with diabetes (31.6+/-2.8 ng/mL) or hypercholesterolemia (52.3+/-5.9) than in the control group (22.4+/-1.2). The increased plasma cholesterol can be regarded as an oxidation substrate for the oxidant stress and the higher absolute levels of oxysterols in hypercholesterolemic plasma compared with the control plasma. This difference disappeared when 7-ketocholesterol was expressed in proportion to total cholesterol. The oxidizability of plasma cholesterol was evaluated by comparing the increased ratio of 7-ketocholesterol after CuSO4 oxidation to the ratio before. We demonstrated that the patients with diabetes showed increased oxidizability (77.5%) compared with the control (36.6%) or hyperlipemic group (45.3%), which is likely due to the lower amounts of alpha-tocopherol in the diabetics. Measurement of oxysterols may serve as a marker for in vivo oxidized lipoproteins in diabetes and hyperlipemia.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Oxysterols from oxidized LDL are cytotoxic but fail to induce hsp70 expression in endothelial cells

Oxidized low density lipoprotein (OxLDL) possesses several proatherogenic characteristics, among which a marked cytotoxicity. In vitro, cytotoxicity of OxLDL to endothelial cells is associated with an increase in the expression of the inducible form of heat shock protein 70 (hsp70), generally regarded as a cytoprotective protein. Oxidized derivatives of cholesterol which form upon LDL oxidation are cytotoxic. Moreover, most of the OxLDL cytotoxicity is due to its lipid moiety, in particular to oxysterols. In this report we demonstrate that although oxysterols identified in OxLDL are cytotoxic, they cannot trigger the increase in hsp70 expression observed with intact oxidized lipoproteins. We speculate therefore that oxysterols may represent the most toxic form of oxidized lipids in LDL because they cannot activate a rescue mechanism (i.e. the hsp response) and may contribute significantly to cell death within atherosclerotic plaques.

Major differences in oxysterol formation in human low density lipoproteins (LDLs) oxidized by *OH/O2*- free radicals or by copper

The aim of our study was to determine the oxysterol formation in low density lipoproteins (LDLs) oxidized by defined oxygen free radicals (*OH/O2*-). This was compared to the oxysterol produced upon the classical copper oxidation procedure. The results showed a markedly lower formation of oxysterols induced by *OH/O2*- free radicals than by copper and thus suggested a poor ability of these radicals to initiate cholesterol oxidation in LDLs. Moreover, the molecular species of cholesteryl ester hydroperoxides produced by LDL copper oxidation seemed more labile than those formed upon *OH/O2*(-)-induced oxidation, probably due to their degradation by reaction with copper ions.

Glycated hemoglobin and lipid peroxidation in erythrocytes of diabetic patients

In diabetes, glycation and subsequent browning (or glycoxidation) reactions are enhanced by elevated glucose concentrations. It is unclear whether the diabetic state per se also induces an increase in the generation of oxygen-derived free radicals (OFRs). However, there is some evidence that glycation itself may induce the formation of OFRs. OFRs cause oxidative damage to endogenous molecules, including cholesterol. 7-Oxocholesterol is known to be one of the major products of cholesterol oxidation. The level of cholesterol peroxidation products was assessed in erythrocyte membrane lipid by monitoring the peak height ratio of 7-oxocholesterol, one of the products of cholesterol peroxidation, to cholesterol with gas chromatography/mass spectrometry (GC/MS). The peak height ratio of 7-oxocholesterol to cholesterol was used as a biomarker of lipid peroxidation. The hemoglobin A1c (HbA1c) value, an index of glycemic stress, was measured by high-performance liquid chromatography. We examined the relationship between the levels of cholesterol peroxidation products and HbA1c in erythrocytes of diabetic and healthy subjects. There was a significantly increased ratio of 7-oxocholesterol to cholesterol in diabetic erythrocytes compared with control erythrocytes. The ratio of 7-oxocholesterol to cholesterol was significantly correlated with the level of HbA1c. This suggests that glycation of hemoglobin via chronic hyperglycemia is linked to cholesterol peroxidation in erythrocytes of both diabetic and healthy subjects.

Inhibition by cholesterol oxides of NO release from human vascular endothelial cells

Recent studies have demonstrated that, unlike cholesterol, cholesterol oxidized at position 7 can reduce the maximal endothelium-dependent relaxation of isolated rabbit aortas (Circulation. 1997;95:723-731). The aim of the current study was to determine whether cholesterol oxides reduce the release of nitric oxide (NO) from human umbilical vein endothelial cells (HUVECs). The amount of NO released by histamine-stimulated HUVECs was determined by differential pulse amperometry using a nickel porphyrin- and Nafion-coated carbon microfiber electrode. The effects of cholesterol (preserved from oxidation by butylated hydroxytoluene), 7-ketocholesterol, 7beta-hydroxycholesterol, 5alpha,6alpha-epoxycholesterol, 19-hydroxycholesterol (60 microg/mL), and alpha-lysophosphatidylcholine (10 microg/mL) were compared. Pretreatment of HUVECs with cholesterol, 5alpha,6alpha-epoxycholesterol, or 19-hydroxycholesterol did not alter histamine-activated NO production. In contrast, pretreatment with 7-ketocholesterol or 7beta-hydroxycholesterol significantly decreased NO release. The inhibitory effect of 7-ketocholesterol was time and dose dependent and was maintained in the presence of L-arginine. In the absence of serum, lysophosphatidylcholine also reduced NO production. In ionomycin-stimulated cells, pretreatment with 7-ketocholesterol did not inhibit NO release. These results demonstrate that cholesterol derivatives oxidized at the 7 position, the main products of low density lipoprotein oxidation, reduce histamine-activated NO release in HUVECs. Such an inhibitory effect of cholesterol oxides may account, at least in part, for the ability of oxidized low density lipoprotein to reduce the endothelium-dependent relaxation of arteries.

The antioxidative effects of the isoflavan glabridin on endogenous constituents of LDL during its oxidation

The effect of the consumption of glabridin, an isoflavan isolated from Glycyrrhiza glabra (licorice) root, on the susceptibility of low density lipoprotein (LDL) to oxidation was studied in atherosclerotic apolipoprotein E deficient (E[o] mice) and was compared with that of the known flavonoids, quercetin and catechin. Glabridin inhibitory activity on in vitro oxidation of human LDL was also investigated by determining the formation of lipid peroxides and oxysterols and the consumption of LDL-associated lipophilic antioxidants. Determination of the extent of LDL oxidation by measuring the formation of thiobabituric acid reactive substances (TBARS) after 2 h of LDL incubation with CuSO4 (10 microM) or 2,2'-azobis (2-amidino-propane) dihydrochloride (AAPH) (5 mM), revealed that glabridin or quercetin consumption resulted in a 53 and 54% reduction in copper ion induced oxidation, respectively, and a 95 and 83% reduction in AAPH induced LDL oxidation, respectively. No inhibition was obtained with consumption of catechin. About 80% of glabridin was found to bind to the LDL human particle. In the in vitro oxidation of LDL induced by AAPH (5 mM), glabridin inhibited the formation of TBARS, lipid peroxides and cholesteryl linoleate hydroperoxide (CLOOH) at all the concentrations tested (5-60 microM), while in oxidation induced by copper ions (10 microM), glabridin exhibited a pro-oxidant activity at concentrations lower than 20 microM, and a clear antioxidant activity at concentrations greater than 20 microM. Glabridin (30 microM) inhibited the formation of cholest-5-ene-3,7-diol (7-hydroxycholesterol), cholest-5-ene-3-ol-7-one (7-ketocholesterol) and cholestan-5,6-epoxy-3-ol (5,6-epoxycholesterol) after 6 h of AAPH induced LDL oxidation, by 55, 80 and 40%, respectively, and after 6 h of copper ion induced LDL oxidation, by 73, 94 and 52%, respectively. Glabridin also inhibited the consumption of beta-carotene and lycopene by 38 and 52%, respectively, after 0.5 h of LDL oxidation with AAPH, but failed to protect vitamin E. The in vivo and in vitro reduction of the susceptibility of LDL to oxidation obtained with glabridin, may be related to the absorption or binding of glabridin to the LDL particle and subsequent protection of LDL from oxidation by inhibiting the formation of lipid peroxides and oxysterols, and by protecting LDL associated carotenoids.

Differential effects of subcutaneous estrogen and progesterone on low-density lipoprotein size and susceptibility to oxidation in postmenopausal rhesus monkeys

OBJECTIVE

To study the differential effects of subcutaneous E2 alone or in combination with P on the susceptibility of low-density lipoprotein (LDL) cholesterol to oxidation in naturally postmenopausal diet-controlled rhesus monkeys.

DESIGN

Prospective, longitudinal controlled study.

SETTING

Oregon Health Sciences University, Portland, Oregon, and Oregon Regional Primate Research Center, Beaverton, Oregon.

PATIENT(S)

Five naturally postmenopausal rhesus monkeys.

INTERVENTION(S)

Estradiol was administered subcutaneously for the first 4 weeks, followed by E2 plus P for 4 weeks, followed by a third 4-week washout period.

MAIN OUTCOME MEASURE(S)

Changes in plasma lipoprotein levels and oxidation of LDL and serum concentrations of E2 and P.

RESULT(S)

Levels of LDL cholesterol fell after 4 weeks of treatment with E2, compared with baseline. The lag time to half maximal light absorbancy after 4 weeks of E2 treatment was significantly increased compared with baseline. The maximal absorbance values and the slope of the propagation phase after 4 weeks of treatment with E2 were decreased compared with baseline. After 4 weeks of combined E2 and P treatment, all values were comparable to baseline.

CONCLUSION(S)

These results suggest that subcutaneous E2 therapy appears to enhance LDL resistance to oxidation and that this effect is attenuated by the addition of the P.

Absorption of dietary cholesterol oxidation products and incorporation into rat lymph chylomicrons

Cholesterol oxidation products (oxysterols) induce macrophage lipid loading and accumulate in early arterial fatty streaks. The origin of lesion oxysterols has not been elucidated. The absorption of oxysterols from the diet and transport to the arterial wall by postprandial lipoprotein remnants may be a significant source. This study aimed to investigate the extent of oxysterol absorption and the effect on chylomicron composition. Cholesterol was heat-treated, causing 30% oxidation; the major oxidation products were 7 beta-hydroxycholesterol, 7-keto-cholesterol, 5 alpha,6 alpha-epoxycholesterol, and 5 beta,6 beta-epoxycholesterol. Conscious lymph-cannulated rats were given a bolus gastric infusion of 50 mg oxidized cholesterol or 50 mg purified cholesterol in a vehicle of triglyceride. In the rats given the oxidized cholesterol, 6% of the oxysterol load was absorbed and incorporated into lymph chylomicrons. Rats given pure cholesterol had no increase in oxysterols above baseline levels. The incorporation of oxysterols into lymph chylomicrons differed over time with 7 beta-hydroxycholesterol, having peak absorption at 3 h, followed by 7-ketocholesterol at 4 h and 5 alpha,6 alpha-epoxy-cholesterol at 5 h. The absorption of oxysterols in animals given the oxidized cholesterol gastric infusate was associated with lymph chylomicron compositional changes at 2-4 h. The oxidized cholesterol-treated group had a twofold increase in the cholesterol (890 +/- 84 micrograms vs. 440 +/- 83 microgram at 3 h) and triglyceride content (19.76 +/- 3.4 micrograms vs. 8.49 +/- 3.8 micrograms at 3 h). This led to a doubling of chylomicron size over this postprandial period, with particles having a mean diameter of 294 nm in the oxidized cholesterol-treated animals, compared to 179 nm in the purified cholesterol group. In conclusion, dietary oxysterols appear to influence postprandial lipoprotein particle size and composition. These changes may have effects on the clearance of chylomicrons from plasma, arterial delivery of oxysterols, and possible deposition in arterial lesions.

Both intracellular and extracellular vitamin C inhibit atherogenic modification of LDL by human vascular endothelial cells

Oxidative modification of LDL by vascular cells has been proposed as a mechanism by which LDL becomes atherogenic. Antioxidants that can prevent LDL oxidation may therefore act as antiatherogens. We used endothelial cells (ECs) from human aortas (HAECs), human saphenous veins (HSECs), and bovine aortas (BAECs) to investigate the role of intracellular and extracellular vitamin C (ascorbate) in EC-mediated LDL modification. Incubation of LDL (0.1 mg protein per milliliter) with confluent HAECs in Ham's F-10 medium led to time-dependent modification of the lipoprotein. In contrast, incubation of LDL with HAECs in medium 199, which does not contain redox-active transition metal ions, did not lead to LDL modification. Both HAEC-mediated and cell-free LDL modifications in Ham's F-10 medium were strongly inhibited in a time- and dose-dependent manner by physiological concentrations of ascorbate. Confluent HAECs cultured under conventional conditions contained very little intracellular ascorbate (< 0.5 nmol/mg protein) but could be loaded with up to 20 nmol ascorbate per milligram protein in a time- and concentration-dependent manner. Ascorbate-loaded HAECs exhibited a lower capacity to modify LDL than did non-ascorbate-loaded control cells. When LDL was incubated with HSECs instead of HAECs, similar time- and concentration-dependent inhibitory effects on LDL modification of intracellular and extracellular ascorbate were observed. In contrast to human ECs, BAECs did not take up vitamin C and therefore only coincubation but not preincubation with ascorbate inhibited BAEC-mediated LDL modification. Our data show that enrichment of human vascular ECs with vitamin C lowers their capacity to modify LDL. In addition, extracellular vitamin C strongly inhibits EC-mediated, metal ion-dependent atherogenic modification of LDL.

The effect of oxidizing cholesterol on gastrointestinal absorption, plasma clearance, tissue distribution, and processing by endothelial cells

Little is known about the absorption or metabolism of oxysterols. Toward better appreciating the metabolic consequences of oxidizing cholesterol, we compared labeled cholesterol with the labeled oxysterols 7 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, and 7-ketocholesterol prepared from [4-14C]cholesterol, [26,26,26,27,27,27-2H6]cholesterol, and [23,24,25,26,27-13C5] cholesterol. Gastrointestinal absorption of oxysterols in rats was 91.5 +/- 0.3% compared with 75 +/- 1.1% for cholesterol, determined by fecal collection (P < .001). When injected intravenously and followed by gas chromatography/mass spectrometry, 7 alpha-hydroxycholesterol was cleared at 23 times the rate of cholesterol. After 5 minutes, only 1.2 +/- 0.2% of 7 alpha-hydroxycholesterol remained in the plasma, whereas 28.0 +/- 1.7% of cholesterol and 40.0 +/- 2.5% of a triglyceride emulsion injected simultaneously were still present. [14C]7 alpha-Hydroxycholesterol injected intravenously was also rapidly cleared from plasma, was widely distributed in tissues and organs, and showed evidence of extensive metabolism at 5 minutes. The fractional rate of uptake of radiolabeled oxysterols by cultured endothelial cells was 15.7 times that of cholesterol (P < .001), and the fractional rate of efflux was 3.4 times that of cholesterol (P < .001). Oxysterols passed through endothelial cells grown on transwell membranes at a rate 4.3 times that of cholesterol (P < .001). Fractional oxysterol transport across the endothelial cell monolayer was increased 62 +/- 17% when HDL was added to the medium in the lower chamber (P = .003). Oxysterols were extensively metabolized to even more polar metabolites during endothelial cell transit. These properties of oxysterols potentially provide a mechanism for enhancing transport of cholesterol through tissues and preventing accumulation of cholesterol in those cells that can oxidize it.

Lipoprotein oxidation and progression of carotid atherosclerosis

BACKGROUND

Epidemiological studies and animal experiments have provided evidence supporting the role of lipid peroxidation in atherogenesis and cardiovascular diseases. Direct evidence linking lipid oxidation to atherosclerotic progression in humans, however, has been lacking. We investigated the association of lipid oxidation products with the progression of early carotid atherosclerosis in hypercholesterolemic men from eastern Finland.

METHODS AND RESULTS

Twenty subjects with a fast progression and 20 with no progression of carotid atherosclerosis in 3 years were selected from > 400 participants in the Kuopio Atherosclerosis Prevention Study. Progression of carotid atherosclerosis was assessed by high-resolution B-mode ultrasonography. Serum 7 beta-hydroxycholesterol, a major oxidation product of cholesterol in membranes and lipoproteins, and seven other cholesterol oxidation products were measured by isotope dilution-mass spectrometry, lipid hydroperoxides in LDL fluorometrically as thiobarbituric acid-reactive substances (TBARS) and oxidation susceptibility of LDL and VLDL kinetically. High concentrations of serum 7 beta-hydroxycholesterol (beta = 47, P = .0005), cigarette smoking (beta = .35, P = .0167), and LDL TBARS (beta = .23, P = .0862) and an increased oxidation susceptibility of VLDL + LDL (beta = .22 P = .1114) were the strongest predictors of a 3-year increase in carotid wall thickness of more than 30 variables tested in step-up least-squares regression models. A 10-variable model explained 60% of the atherosclerotic progression. In a multivariate logistic model, the risk of experiencing a fast progression increased by 80% (P = .013) per unit (microgram/L) of 7 beta-hydroxycholesterol.

CONCLUSIONS

The findings of this study provide further evidence to support an association between lipid oxidation and atherogenesis in humans.

Inhibitors of arterial relaxation among components of human oxidized low-density lipoproteins. Cholesterol derivatives oxidized in position 7 are potent inhibitors of endothelium-dependent relaxation

BACKGROUND

Oxidized low-density lipoproteins (LDLs) are known to impair arterial relaxation. The aim of the present study was to identify the components of oxidized LDL that may account for inhibition of endothelium-dependent relaxation.

METHODS AND RESULTS

LDLs from 12 healthy subjects were either maintained at 4 degrees C (native LDL) or incubated at 37 degrees C in the presence of copper sulfate (oxidized LDL). Unlike pretreatment with native LDL, pretreatment with oxidized LDL reduced significantly the acetylcholine-mediated relaxation of rabbit aortic segments compared with control segments incubated in Krebs' buffer (maximal relaxation [Emax], 72.0 +/- 6.7% versus 94.1 +/- 0.8%, respectively, P < .01; negative logarithm of the concentration required to produce a half-maximal relaxing effect [pD2], 6.6 +/- 0.1 versus 7.2 +/- 0.1, respectively, P < .001). The absolute difference between Emax values obtained with oxidized and native LDL (delta Emax) correlated significantly with the formation of 7 ketocholesterol, 7 alpha-hydroxycholesterol, and 7 beta-hydroxy-cholesterol. In contrast, delta Emax did not correlate with the amount of lipoperoxides or lysophosphatidylcholine formed, and the difference of pD2 values measured with oxidized and native LDL (delta pD2) did not correlate significantly with any of the oxidation-derived LDL compounds. When added individually, 7-ketocholesterol and 7 beta-hydroxycholesterol reduced Emax values but not pD2 values in a time- and concentration-dependent manner.

CONCLUSIONS

Cholesterol derivatives in oxidized LDL can reduce maximal arterial relaxation through a specific effect on vascular endothelial cells.

Characterization of cholesterol oxidation products formed by oxidative modification of low density lipoprotein

Oxidative modification of LDL is evidenced by alterations in both the protein and lipid components of the particle. Progressive oxidation of the apoprotein is associated with loss of specific amino acids and a gradual increase in electronegativity. Electronegative LDL has been isolated from human plasma (LDL-) by several groups using liquid chromatographic techniques and appears to be oxidized based on increased lipid peroxide levels and cholesterol oxidation products (ChOx). Formation of LDL- also takes place following Cu(2+)-induced oxidation. Cu(2+)-induced oxidation caused a small fraction of the normal unoxidized LDL (n-LDL) to convert to LDL-during the oxidative lag phase while minimal increases in conjugated dienes were apparent. After the lag phase, there was a further increase in LDL-, a rapid accumulation of conjugated dienes, and another more electronegative particle was formed (LDL2-). By the end of the lag phase, approximately 30% and 12% of the total LDL converted to LDL- and LDL2-, respectively. Nearly 40% of the total ChOx formed was present by the end of the lag period, accompanied by small increases in conjugated dienes. The major products accumulating during this time were 7-ketocholesterol, cholesterol-beta-epoxide and 7-alpha-hydroxycholesterol. Accumulation of predominated during the subsequent propagation phase. At the end of propagation phase there was a six fold increase in conjugated dienes and total ChOx increased eight-fold. It appears that a subpopulation of LDL rapidly converts to LDL-, representing a mildly oxidized but oxidant sensitive LDL population. Oxidation of cholesterol accompanies these early events in LDL oxidation with formation of specific ChOx.

A comparison of the kinetics of low-density lipoprotein oxidation initiated by copper or by azobis (2-amidinopropane)

This article describes the kinetics of low density lipoprotein (LDL) oxidation catalyzed by azobis (2-amidinopropane) dihydrochloride, ABAP, or by copper. The LDLs were isolated from nonhuman primates fed diets enriched in one of three types of fatty acids: saturated fatty acids, monounsaturated fatty acids, predominantly, oleic acid, or polyunsaturated fatty acids, predominantly linoleic acid. Oxidation was followed by monitoring the formation of conjugated diene hydroperoxides from polyunsaturated fatty acids (PUFA). For both copper and ABAP-initiated oxidation, the rate of LDL oxidation depended on the concentrations of initiator, PUFA, and LDL. Except for the dependence on PUFA concentration the rate of LDL oxidation was not directly influenced by the fatty acid composition of the LDL particle. The two initiators had very different dependence on initiator concentration. Because LDL particles are essentially small, lipid-rich droplets, the kinetic descriptions of LDL oxidation assumed: (1), that there was only one chain per particle, and (2) that the radical chain was terminated when a second radical either entered or was formed in the particle. When two LDL samples having very different lag times were mixed, the oxidation profile was bimodal. This finding demonstrated that the oxidation of native LDL particles was independent of the oxidation state of the other native LDL particles in solution, i.e., LDL particles do not rapidly exchange radicals, for example, hydroperoxyl radicals. Oxidation initiated by ABAP was proportional to [ABAP]0.5, suggesting that hydroperoxyl radical recombination between the lipid hydroperoxyl radical and the ABAP-hydroperoxyl radical was the chain-terminating step. The reciprocal of the rate of copper oxidation was linearly related to the reciprocal copper concentration, demonstrating that the binding of copper to LDL was necessary to initiate oxidation. This binding constant showed considerable variability among LDL samples. The kinetic descriptions of LDL oxidation reflect the differences in the mechanisms of initiation and termination.

The oxysterols cholest-5-ene-3 beta,4 alpha-diol, cholest-5-ene-3 beta,4 beta-diol and cholestane-3 beta,5 alpha,6 alpha-triol are formed during in vitro oxidation of low density lipoprotein, and are present in human atherosclerotic plaques

Isolated human low density lipoprotein (LDL) was oxidized with either cupric ions or soybean lipoxygenase and linoleic acid. Cholesterol oxidation products (oxysterols) were determined by isotope dilution gas chromatography-mass spectrometry. A new cholestane-3,5,6-triol isomer, cholestane-3 beta,5 alpha,6 alpha-triol, which has not previously been recognized as a cholesterol autoxidation product, was found at similar concentrations as the well-known cytotoxic cholestane-3 beta,5 alpha,6 beta-triol during both copper- and lipoxygenase-mediated LDL oxidation. Furthermore, two epimeric cholest-5-ene-3 beta,4-diols were identified in the oxidized LDL at similar concentrations. These two isomers were also identified in human atherosclerotic tissue in a ratio of 1:1 at a concentration more than 10-times higher than in non-atherosclerotic vessels. In vitro oxidation of LDL under an 18O2 atmosphere revealed that molecular oxygen was the only source of the oxygen functions at C-4 in the cholest-5-ene-3 beta,4-diols. Taken together, these findings suggest that the cholest-5-ene-3 beta,4-diols in atherosclerotic plaques are formed by autoxidation.

Oxidized low density lipoprotein inhibits lipopolysaccharide-induced binding of nuclear factor-kappaB to DNA and the subsequent expression of tumor necrosis factor-alpha and interleukin-1beta in macrophages

A large body of evidence suggests that oxidized LDL (oxLDL) has a role in atherogenesis. One effect is the impact on macrophage function. We have studied the effects of oxLDL and oxysterols on the binding of the transcription factors nuclear factor (NF)-kappaB and AP-1 to DNA. These transcription factors are involved in the regulation of several genes and expressed during activation of macrophages, for example by endotoxin (LPS). OxLDL did not induce binding of NF-kappaB. However, the LPS-induced response to NF-kappaB was substantially reduced after preincubation with oxLDL. Medium and highly oxidized LDL also decreased the constitutive DNA-binding of AP-1. Similar effects on AP-1-binding were seen with the oxysterols, 7beta-hydroxycholesterol, 24- hydroxy-, 25-hydroxy-, and 27-hydroxy-cholesterol. Our data therefore suggest an effect of oxLDL on the DNA-binding of AP-1, which might be mediated by the oxysterol content of oxLDL. A decreased LPS-induced TNF-alpha and IL-1beta mRNA and protein expression were found in macrophages incubated with oxLDL before LPS-exposure. These observations suggest that macrophages that internalize extensively oxidized LDL are suppressed in their response to inflammatory stimulation.

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.

Superior and distinct antioxidant effects of selected estrogen metabolites on lipid peroxidation

The effect of the estrogen metabolites, 4-hydroxyestrone and 17alpha-dihydroequilin (metabolites of estradiol-17beta and equilin, respectively), were examined for antioxidant effects on plasma and lipoprotein lipid peroxidation . Lipid peroxidation was evaluated by products of both fatty acid (thiobarbituric acid-reactive substances [TBARS]) and cholesterol (oxysterols) oxidation from lipoproteins or whole plasma. Although all estrogens significantly reduced lipid peroxidation, 4-hydroxyestrone was far more potent than either equilin or 17alpha-dihydroequilin in inhibiting TBARS formation in lipoproteins induced by Cu2+. Similar effects were also noted on TBARS formation in THP-l macrophages in culture. However, 17alpha-dihydroequilin (along with equilin) strongly inhibited oxysterol formation, whereas 4-hydroxyestrone was ineffective. These studies suggest that different estrogens might act preferentially on distinct lipid substrates in exhibiting antioxidant effects.

Identification of 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide in human oxidized LDL

Oxidized LDL exhibits cytotoxic activity towards endothelial cells, which is thought to be mediated by the products derived also from cholesterol oxidation, mainly involving the B-ring. By LC-PB-EI-MS and EI-MS/MS analysis, we have identified 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide among the cholesterol oxidation products in LDL incubated with micromolar concentrations of copper ions.

Active oxygen species and lysophosphatidylcholine are involved in oxidized low density lipoprotein activation of smooth muscle cell DNA synthesis

It has recently been shown that oxidative modification of LDL enhances the mitogenic effect of LDL on smooth muscle cell (SMC) DNA synthesis. However, because of its complex chemical structure, the mitogenic components have not been well characterized. Exposure of LDL to the oxidant Cu2+ is followed by a rapid accumulation of peroxides that peaks after 8 to 12 hours and a conversion of the phospholipid phosphatidylcholine into lysophosphatidylcholine that continues for up to 48 hours. Most of the mitogenic activity is formed during the first 4 hours of oxidation. Both superoxide dismutase and catalase effectively inhibit the mitogenic activity of oxidized LDL, suggesting involvement of reactive oxygen intermediates. In the presence of 1% serum, low concentrations of hydrogen peroxide activated SMC DNA synthesis in a dose-dependent manner, with a maximal effect at a concentration of 200 mumol/L, whereas higher concentrations were inhibitory. Lysophosphatidylcholine also enhanced SMC DNA synthesis, with a maximal stimulation at a concentration of 10 mumol/L. Oxysterols, which also accumulate in oxidized LDL, effectively inhibited DNA synthesis. These results demonstrate that oxidation of LDL is associated with formation of several substances affecting the growth of SMCs. Among these substances, low levels of reactive oxygen intermediates and lysophosphatidylcholine stimulate DNA synthesis, whereas at a higher concentration they, as well as oxysterols, are inhibitory.

Oxysterols present in atherosclerotic tissue decrease the expression of lipoprotein lipase messenger RNA in human monocyte-derived macrophages

The presence of oxysterols in macrophages isolated from atherosclerotic tissue and the effect of oxysterols on the regulation of lipoprotein lipase (LPL) mRNA were studied. Both rabbit and human macrophages, freshly isolated from atherosclerotic aorta, show about the same distribution of oxysterols, analyzed by isotope dilution mass spectrometry, except that all three preparations of human arterial-derived macrophages contained high levels of 27-hydroxycholesterol, which was not found in rabbit macrophages. To determine if oxysterols regulate LPL expression, human monocyte-derived macrophages were incubated with different oxysterols. Incubation with 7 beta-hydroxycholesterol and 25-hydroxycholesterol resulted in a 70-75% reduction of LPL mRNA, analyzed by quantitative RT-PCR. Cholesterol and other tested oxysterols showed no effect on macrophage LPL mRNA expression compared with control. LPL activity in the medium was also reduced after exposure of the macrophages to 7 beta-hydroxycholesterol and 25-hydroxycholesterol. In conclusion, we have demonstrated accumulation of oxysterols in macrophage-derived foam cells isolated from atherosclerotic aorta. There was suppression of LPL mRNA in human monocyte-derived macrophages after incubation with 7 beta-hydroxycholesterol and 25-hydroxycholesterol. It is tempting to suggest that an exposure to oxysterols may explain our earlier observation of a low level of LPL mRNA in arterial foam cells.

Time course of oxysterol formation during in vitro oxidation of low density lipoprotein

Cholesterol oxidation products (oxysterols) have been implicated in several aspects of atherogenesis; they affect key enzymes in cholesterol homeostasis, induce calcification in vascular cells and possess cytotoxic properties. Oxysterols are formed during oxidative modification of low density lipoprotein (LDL). Using a recently developed method based on isotope dilution-mass spectrometry, the kinetics of formation of oxysterols during oxidation of LDL by cupric ions or soybean lipoxygenase was studied. The same products, mainly 7- and 5-oxygenated cholesterol, were formed by the two oxidation methods. Virtually no side-chain oxidized oxysterols were formed. During the oxidations, preferentially esterified cholesterol was consumed and consumption of polyunsaturated fatty acids and formation of conjugated dienes preceded the appearance of oxysterols. Cholesterol 7-hydroperoxides potential cytotoxins, were present in LDL oxidized by copper or lipoxygenase.

Macrophage uptake of oxidized LDL inhibits lysosomal sphingomyelinase, thus causing the accumulation of unesterified cholesterol-sphingomyelin-rich particles in the lysosomes. A possible role for 7-Ketocholesterol

Macrophage uptake of oxidatively modified LDL (Ox-LDL), unlike the uptake of acetylated LDL (Ac-LDL), resulted in lysosomal accumulation of unesterified cholesterol (UC). As sphingomyelin (SM) binds UC with high affinity, we considered whether lysosomes also accumulate Ox-LDL-derived SM, and if such a phenomenon could be involved in the lysosomal trapping of Ox-LDL-derived UC. Incubation of J-774 A.1 macrophages with Ox-LDL increased the lysosomal accumulations of UC by 75% and SM by 63% compared with the effect of Ac-LDL. The addition of chlorpromazine, an inhibitor of lysosomal sphingomyelinase (SMase), to macrophages that were incubated with [3H]cholesteryl ester-labeled Ac-LDL also led to lysosomal accumulation of both SM and UC. 7-Ketocholesterol (7-KC), the major oxysterol in Ox-LDL, inhibited lysosomal SMase in a cell-free system. The addition of 7-KC to cells in the presence of [3H]choline- or [3H]cholesteryl ester-labeled Ac-LDL led to macrophage accumulation of SM or UC, respectively. Niemann-Pick type C disease (NP-C) is an inherited cholesterol-storage disease in which lysosomal SMase activity is attenuated after uptake of LDL. Incubation of monocyte-derived macrophages from two NP-C patients with Ac-LDL or Ox-LDL resulted in an accumulation of UC in the lysosomes, whereas normal monocyte-derived macrophages accumulate UC in their lysosomes after incubation with Ox-LDL but not Ac-LDL. These results suggest that inhibition of lysosomal SMase in NP-C cells or by 7-KC is required for lysosomal accumulation of UC. Analysis of the macrophage lysosomal extract (following cell incubation with Ox-LDL) by density-gradient ultracentrifugation and gel-filtration chromatography revealed the presence of a particle consisting of UC, SM, 7-KC, and apoB-100. We conclude that 7-KC in Ox-LDL can inhibit lysosomal SMase, thus leading to the accumulation of SM, which binds UC avidly and inhibits its further cellular processing out of the lysosome. As UC-SM particles of lysosomal origin exist in the atherosclerotic lesion, the formation of such particles may result from an impaired processing of Ox-LDL by arterial wall macrophages during early atherogenesis.

Increased susceptibility to oxidation of low-density lipoproteins isolated from patients with systemic sclerosis

OBJECTIVE

To examine the resistance to oxidation of low-density lipoproteins (LDL) from patients with systemic sclerosis (SSc) and primary Raynaud's phenomenon (RP) compared with healthy controls.

METHODS

Plasma LDL were isolated from patients with diffuse cutaneous and limited cutaneous SSc (dcSSc and lcSSc, respectively), patients with primary RP, and healthy control subjects. The lipoproteins were assessed for their resistance to oxidation in the presence of cupric ions, using spectrophotometric assays.

RESULTS

LDL from patients with dcSSc and lcSSc were more susceptible to oxidation than were those from healthy control subjects or patients with RP.

CONCLUSION

Our findings suggest that free radicals may play a role in the pathology of SSc.

EDTA differentially and incompletely inhibits components of prolonged cell-mediated oxidation of low-density lipoprotein

The extent to which cells can oxidize LDL may be underestimated because of the use of standard and arbitrary 24 hour in vitro incubations of cells with LDL. Such incubations have resulted in inconsistent results regarding the ability of cell-mediated LDL oxidation to generate relatively advanced oxidation products such as 7-ketocholesterol (7-KC). We studied prolonged oxidation of low density lipoprotein (LDL) by mouse peritoneal macrophages using HPLC measurement of cholesterol, cholesteryl esters and their oxidation products 7-KC and cholesteryl linoleate hydroperoxide (CL-OOH). Cell-mediated oxidation in Ham's F10 consistently followed the successive stages previously described during 24 hour-10 microM copper-mediated LDL oxidation, always generating 7-KC if allowed to proceed for sufficient time. The degree of inhibition of LDL oxidation achieved by metal chelators EDTA and DTPA at more advanced stages of cell-mediated LDL oxidation was not predictable from the published effects of such chelators upon early stages of metal-mediated and cell-mediated LDL oxidation. EDTA and DTPA only incompletely prevented the consumption of cholesteryl esters and the loss of performed CL-OOH when added after cell-mediated LDL oxidation was established, while effectively concurrently inhibiting the generation of 7-KC. These data indicate that progressive cell-mediated peroxidation of LDL cholesteryl esters and decomposition of CL-OOH may be less dependent upon a continuing supply of redox active metals than is the generation of 7-KC. In addition, they confirm the plausibility of prolonged cell-mediated oxidation of LDL as a source of oxysterols found in human atherosclerotic plaque, and imply that active redox cycling of metals is particularly important for their generation in vivo.

Cholest-3,5-dien-7-one formation in peroxidized human plasma as an indicator of lipoprotein cholesterol peroxidation potential

Lipoprotein peroxidation susceptibility is routinely evaluated using products of unsaturated fatty acids as markers (e.g., malonaldehyde). The significance and factors influencing peroxidation of cholesterol moiety of lipoproteins are relatively unknown due to lack of a reliable marker product which can be measured easily. Under the influence of Cu2+ ions, the major product of lipoprotein cholesterol peroxidation (isolated after saponification) was cholest-3-5-dien-7-one (CSD). Apart from gas-liquid chromatography, this compound lends itself for measurement by alternative methods. Due to lack of the 3 beta-hydroxyl group, CSD was separated from the rest of the oxysterols and cholesterol by passing through digitonin-coated silica-gel G and its concentration was determined by absorption at 283 nm. The recovery of CSD by this method exceeded by 87%. The formation of CSD was also sensitive to vitamin E and therefore could be used as an index of lipoprotein cholesterol susceptibility to peroxidation.

Antioxidant activity of thiocholesterol on copper-induced oxidation of low-density lipoprotein

The effect of thiocholesterol (SH-Chol) on the copper-induced in vitro oxidation of low-density lipoprotein (LDL; 1.019 < d < 1.063) was investigated. Among the antioxidants tested, including cysteine, glutathione, 2-mercaptoethanol, dithiothreitol, probucol, thiopalmitic acid, and SH-Chol, SH-Chol was the most effective antioxidant in copper-induced LDL oxidation. Also, SH-Chol completely inhibited the formation of oxysterols, i.e., 7-hydroxycholesterol and 7-ketocholesterol, in LDL particles and reduced 1,1-diphenyl-2-picrylhydrazyl used as stable free-radical model. Moreover, SH-Chol suppressed the degradation of endogenous alpha-tocopherol in LDL particles. These findings indicate that SH-Chol acts as antioxidant in the oxidative damage of LDL in vitro and as a free-radical scavenger in lipid peroxidation.

Apolipoprotein A-I-mediated efflux of sterols from oxidized LDL-loaded macrophages

Although oxidized low-density lipoprotein (Ox-LDL) can accumulate in macrophages in vitro, generating cholesterol-loaded cells, little attention has been paid to the capacity of such macrophages loaded with OxLDL to export cholesterol and oxidized sterol moieties. In vitro lipid-loaded cells were generated by incubating primary cultures of mouse peritoneal macrophages with acetylated LDL (AcLDL) or OxLDL for 24 hours. The cellular content of native cholesterol, individual cholesteryl esters, and 7-ketocholesterol was determined by high-performance liquid chromatography. These cells were then incubated with medium containing apolipoprotein (apo) A-I and albumin or albumin alone for up to 24 hours; cholesterol and oxidized sterol efflux were measured both in terms of intracellular depletion and extracellular accumulation. Macrophages loaded with AcLDL accumulated cholesterol and large quantities of cholesteryl esters, whereas OxLDL-loaded cells accumulated cholesterol, a number of oxidized compounds (predominantly 7-ketocholesterol), and a relatively small quantity of cholesteryl esters. AcLDL-derived cells released approximately 50% of their total cholesterol (unesterified and esterified) to apo A-I-containing medium over 24 hours in the form of unesterified cholesterol, whereas OxLDL-derived cells released approximately 30% of their total cholesterol and 7% of their total content of 7-ketocholesterol over the same period. There was minimal efflux of any sterol in the absence of apo A-I. The proportions of cholesterol and 7-ketocholesterol released by either AcLDL- or OxLDL-loaded cells were not reduced by inhibiting cellular acyl-CoA:cholesterol acyl transferase using Sandoz 58-035, despite substantial alterations in the proportions of both free cholesterol and (in OxLDL-loaded cells) free 7-ketocholesterol in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of oxidized lipids during modification of low-density lipoprotein by macrophages or copper

The oxidation of low-density lipoprotein (LDL) is implicated in atherosclerosis. Lipids and oxidized lipids were analysed by gas chromatography and gas chromatography-mass spectrometry in human LDL incubated with mouse peritoneal macrophages (MPM) or copper (II) sulphate in Ham's F-10 medium or medium alone (control). MPM-modification and copper-catalysed oxidation of LDL resulted in the formation of oxysterols, mainly cholest-5-en-3 beta,7 beta-diol (7 beta-OH-CHOL); 7%-19% of the initial cholesterol was converted to 7 beta-OH-CHOL in 24 h. 7 beta-OH-CHOL levels in control LDL were very low. The increase in 7 beta-OH-CHOL in MPM and copper-oxidized LDL was accompanied by decreases in linoleate and arachidonate and increases in the electrophoretic mobility and degradation of LDL protein by 'target' macrophages. The concerted occurrence of these processes and their similarity in both MPM-modification and copper-catalysed oxidation of LDL were suggested by the highly significant cross-correlations. The fall in polyunsaturated fatty acid (PUFA) was accompanied by a directly proportional increase in electrophoretic mobility of the LDL. Production of 7 beta-OH-CHOL and protein degradation by macrophages showed modest elevations during the initial steep fall in PUFA, and showed their greatest increases as the levels of PUFA slowly approached zero. The levels of 7 beta-OH-CHOL and the degradation of LDL by macrophages were directly proportional. The degradation of LDL by macrophages increased rapidly as the electrophoretic mobility of LDL was slowly approaching its maximum level.

7 beta-hydroperoxycholest-5-en-3 beta-ol, a component of human atherosclerotic lesions, is the primary cytotoxin of oxidized human low density lipoprotein

Modification of low density lipoprotein (LDL) by free radical oxidation renders this molecular complex cytotoxic. Oxidized lipoproteins exist in vivo in atherosclerotic lesions and in the plasma of diabetic animals, suggesting that lipoprotein-induced tissue damage may occur in certain diseases. We undertook purification and identification of the major cytotoxin in oxidized LDL. The lipid extract from oxidized LDL was subjected to multiple HPLC separations, and the fractions were assayed for cytotoxicity. Mass spectrometry and nuclear magnetic resonance identified the purified toxin as 7 beta-hydroperoxycholest-5-en-3 beta-ol (7 beta-OOH-Chol). This molecule accounted for approximately 90% of the cytotoxicity of the lipids of oxidized LDL. We also found 7 beta-OOH-Chol in human atherosclerotic lesions from endarterectomy specimens obtained immediately after excision. These results are consistent with the hypothesis that the oxidized LDL present in lesions has the capacity to induce cell and tissue injury, leading to progression of the disease and the generation of the necrotic core of the lesion.