Comparative atherogenic effects of cholesterol and cholesterol oxides

Linking phytosterols and oxyphytosterols from food to brain health: origins, effects, and underlying mechanisms

Phytosterols and their oxidation products, namely oxyphytosterols, are natural compounds present in plant foods. With increased intake of phytosterol-enriched functional food products, the exposure of both phytosterols and oxyphytosterols is rising. Over the past ten years, researches have been focused on their absorption and metabolism in human body, as well as their biological effects. More importantly, recent studies showed that phytosterols and oxyphytosterols can traverse the blood-brain barrier and accumulate in the brain. As brain health problems resulting from ageing being more serious, attenuating central nervous system (CNS) disorders with active compounds in food are becoming a hot topic. Phytosterols and oxyphytosterols have been shown to implicated in cognition altering and the pathologies of several CNS disorders, including Alzheimer's disease and multiple sclerosis. We will overview these findings with a focus on the contents of phytosterols and oxyphytosterols in food and their dietary intake, as well as their origins in the brain, and illustrate molecular pathways through which they affect brain health, in terms of inflammation, cholesterol homeostasis, oxidative stress, and mitochondria function. The existing scientific gaps of phytosterols and oxyphytosterols to brain health in knowledge are also discussed, highlighting research directions in the future.

Current knowledge on the mechanism of atherosclerosis and pro-atherosclerotic properties of oxysterols

Due to the fact that one of the main causes of worldwide deaths are directly related to atherosclerosis, scientists are constantly looking for atherosclerotic factors, in an attempt to reduce prevalence of this disease. The most important known pro-atherosclerotic factors include: elevated levels of LDL, low HDL levels, obesity and overweight, diabetes, family history of coronary heart disease and cigarette smoking. Since finding oxidized forms of cholesterol – oxysterols – in lesion in the arteries, it has also been presumed they possess pro-atherosclerotic properties. The formation of oxysterols in the atherosclerosis lesions, as a result of LDL oxidation due to the inflammatory response of cells to mechanical stress, is confirmed. However, it is still unknown, what exactly oxysterols cause in connection with atherosclerosis, after gaining entry to the human body e.g., with food containing high amounts of cholesterol, after being heated. The in vivo studies should provide data to finally prove or disprove the thesis regarding the pro-atherosclerotic prosperities of oxysterols, yet despite dozens of available in vivo research some studies confirm such properties, other disprove them. In this article we present the current knowledge about the mechanism of formation of atherosclerotic lesions and we summarize available data on in vivo studies, which investigated whether oxysterols have properties to cause the formation and accelerate the progress of the disease. Additionally we will try to discuss why such different results were obtained in all in vivo studies.

Effect of long-term ingestion of weakly oxidised flaxseed oil on biomarkers of oxidative stress in LDL-receptor knockout mice

The effect of oxidised fatty acids on atherosclerosis progression is controversial. Thus, our objective was to evaluate the effect of long-term consumption of weakly oxidised PUFA from flaxseed oil on oxidative stress biomarkers of LDL-receptor(-/-) mice. To test our hypothesis, mice were separated into three groups. The first group received a high-fat diet containing fresh flaxseed oil (CONT-), the second was fed the same diet prepared using heated flaxseed oil (OXID), and the third group received the same diet containing fresh flaxseed oil and had diabetes induced by streptozotocin (CONT+). Oxidative stress, aortic parameters and non-alcoholic fatty liver disease were assessed. After 3 months, plasma lipid profile, glucose levels, body weight, energy intake and dietary intake did not differ among groups. Likewise, oxidative stress, plasma malondialdehyde (MDA), hepatic MDA expressed as nmol/mg portion (ptn) and antioxidant enzymes did not differ among the groups. Hepatic linoleic acid, α-linolenic acid, arachidonic acid and EPA acid declined in the OXID and CONT+ groups. Aortic wall thickness, lumen and diameter increased only in the OXID group. OXID and CONT+ groups exhibited higher concentrations of MDA, expressed as μmol/mg ptn per %PUFA, when compared with the CONT- group. Our results suggest that ingestion of oxidised flaxseed oil increases hepatic MDA concentration and is potentially pro-atherogenic. In addition, the mean MDA value observed in all groups was similar to those reported in other studies that used xenobiotics as oxidative stress inducers. Thus, the diet applied in this study represents an interesting model for further research involving antioxidants.

Steroid plant hormones: effects outside plant kingdom

Brassinosteroids (BS) are the first group of steroid-hormonal compounds isolated from and acting in plants. Among numerous physiological effects of BS growth stimulation and adaptogenic activities are especially remarkable. In this review, we provide evidence that BS possess similar types of activity also beyond plant kingdom at concentrations comparable with those for plants. This finding allows looking at steroids from a new point of view: how common are the mechanisms of steroid bioregulation in different types of organisms from protozoa to higher animals.

Effect of dietary cholesterol and cholesterol oxides on blood cholesterol, lipids, and the development of atherosclerosis in rabbits

Two studies were conducted to determine the effects of dietary cholesterol (CHO) and cholesterol oxides (COPs) on the development of atherosclerosis and the changes in fatty acid and blood characteristics in rabbits. In the first study, forty male New Zealand white rabbits were divided into 5 groups and fed commercial rabbit chow with no added CHO or COPs, 1 g CHO, 0.9 g CHO + 0.1 g COPs, 0.8 g CHO + 0.2 g COPs, or 0.5 g CHO + 0.5 g COPs per kg diet. In the second study, 24 male New Zealand White rabbits were divided into 3 groups and fed a diet containing 2 g CHO, 1.6 g CHO + 0.4 g COPs, or 1.2 g CHO + 0.8 g COPs per kg diet. All diets induced atherosclerotic lesions in the rabbits' ascending thoracic aorta. The serum CHO and triglyceride levels (p < 0.05) increased significantly with the increased levels of CHO in the diets. Dietary CHO or COPs did not influence high-density lipoprotein CHO levels. The ratio of saturated fatty acid to unsaturated fatty acid increased as the level of dietary CHO and COPs increased.

Effects of dietary oxysterols on coronary arteries in hyperlipidaemic hamsters

The aim of this study was to evaluate the effect of dietary oxysterols on coronary atherosclerosis and vasospasm. Golden Syrian hamsters were fed three diets with different lipid contents for 3 months: (1) a normolipidaemic diet containing 25 g corn oil–fish oil (4:1, w/w)/kg (group Low L); (2) a hyperlipidaemic diet composed of the normolipidaemic diet supplemented with 150 g lard+30 g cholesterol/kg (group High L); (3) a third diet, similar to the hyperlipidaemic diet, in which 4 g cholesterol/kg was replaced by a mixture of oxysterols (group High L+OS). The oxysterol mixture contained (g/kg): 5,6α-epoxycholesterol 211, 5,6β-epoxycholesterol 179, 7α-hydroxycholesterol 67, 7β-hydroxycholesterol (7βOH) 185, 7-ketocholesterol (7 K) 235; and trace amounts of 7-hydroperoxycholesterols (approximately 30 g/kg). Atherosclerosis was evaluated by measuring myocardial Ca, oxysterols and acyl-CoA cholesterol acyl transferase (ACAT) activity; furthermore, coronary reactivity to sodium nitroprusside (5×10-6 m) was measured and the morphology of coronary arteries was visualized by transmission electron microscopy. Coronary spasm was determined by evaluating reactivity to serotonin (5×10-6 m). Feeding the high-lipid diet (group High L) increased the plasma level of 7βOH, 7 K and cholestanetriol. The presence of oxysterols in the diet (group High L+OS) further increased the concentrations of 7βOH and 7 K in the plasma. However, as evidenced by myocardial Ca, ACAT activity and coronary reactivity to sodium nitroprusside, severe atherosclerosis did not develop during the 3-month diet. 7 K was increased in myocardial lipids of groups High L and High L+OS. Electron microscopy did not show the development of atherosclerosis in group High L, whereas vascular wall thickening, endothelial damage and smooth muscle cell proliferation and migration occurred when oxysterols were present in the food. Serotonin (5×10-6 m) induced exacerbated coronary vasoconstriction in group High L that was completely reversed by dietary oxysterols. In conclusion, dietary oxysterols exhibit anti-spasmodic properties, but they cannot be used as agents against excess dietary lipid-induced coronary spasm because of their atherogenic properties.

Dietary cholesterol-oxidation products accumulate in serum and liver in apolipoprotein E-deficient mice, but do not accelerate atherosclerosis

There are conflicting reports regarding the effect of dietary cholesterol-oxidation products (oxysterols) on the development of atherosclerosis in experimental animals. To address this issue, apolipoprotein (Apo) E-deficient mice were fed a purified diet (AIN-93) or the same purified diet containing 0.2 g cholesterol or 0.2 g oxysterols/kg. The dietary oxysterols had no significant effect on the serum lipid levels. Although all of the diet-derived oxysterols (cholest-5-en-3beta,7alpha-diol, cholest-5-en-3beta,7beta-diol, cholestan-5alpha,6alpha-epoxy-3beta-ol, cholestan-5beta,6beta-epoxy-3beta-ol, cholestan-3beta, 5alpha, 6beta-triol, cholest-5-en-3beta-ol-7-one and cholest-5-en-3beta, 25-diol) accumulated in the serum and liver, only cholest-5-en-3beta-ol-7-one and cholestan-3beta, 5alpha, 6beta-triol accumulated significantly (P<0.05) in the aorta. The oxysterol diet did not result in elevation of the aortic cholesterol level or the lesion volume in the aortic valve. These present results indicate that exogenous oxysterols do not promote the development of atherosclerosis in ApoE-deficient mice.

Oxidized cholesterol in the diet accelerates the development of atherosclerosis in LDL receptor- and apolipoprotein E-deficient mice

The aim of the current study was to determine whether oxidized cholesterol in the diet accelerates atherosclerosis in low density lipoprotein receptor- (LDLR) and apolipoprotein E- (apo E) deficient mice. Mice were fed either a control diet or a diet containing oxidized cholesterol. For LDLR-deficient mice, the control diet consisted of regular mouse chow to which 1.0% cholesterol was added. The oxidized diet was identical to the control diet except that 5% of the added cholesterol was oxidized. In apo E-deficient mice, the control diet contained 0.15% cholesterol, whereas in the oxidized diet, 5% of the added cholesterol was oxidized. LDLR-deficient and apo E-deficient mice were fed the experimental diets for 7 and 4 months, respectively. In mice fed the oxidized-cholesterol diets, the levels of oxidized cholesterol in sera were increased. At the end of the experiment, aortas were removed and atherosclerosis was assessed. We found that in LDLR-deficient mice, feeding of an oxidized-cholesterol diet resulted in a 32% increase in fatty streak lesions (15.93+/-1.59% versus 21.00+/-1.38%, P<0.03). Similarly, in apo E-deficient mice, feeding of an oxidized-cholesterol diet increased fatty streak lesions by 38% (15.01+/-0.92% versus 20. 70+/-0.86%, P<0.001). The results of the current study thus demonstrate that oxidized cholesterol in the diet accelerates fatty streak lesion formation in both LDLR- and apo E-deficient mice.

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.

Cholesterol oxidation products induce vascular foam cell lesion formation in hypercholesterolemic New Zealand white rabbits

Circulating cholesterol oxidation products (ChOx) have long been implicated in the etiology of early atherosclerosis; however, direct in vivo evidence elucidating their role in atherogenesis is only recently becoming available. This study investigated ChOx effects on vascular lesion formation in New Zealand White rabbits under controlled hypercholesterolemic conditions. By closely monitoring plasma cholesterol levels and adjusting dietary cholesterol intake during a 78-day period, total plasma cholesterol exposures (cumulative plasma cholesterol levels over time) were controlled between 27 000 and 34 000 mg/dLxday (final plasma cholesterol concentration, 467+/-77 mg/mL), representing a threshold range for sudanophilic lesion formation in the aorta. Twenty injections of a ChOx mixture (70 mg per injection) were made bearing an oxysterol composition similar to that found in circulating oxidatively modified low density lipoprotein. At sacrifice, the ChOx-injected rabbits (n=5) had (1) significantly higher plasma ChOx levels, (2) significantly increased cholesterol content in the aortas, mainly as esterified cholesterol, and (3) significantly greater sudanophilic lesion size and frequency in the aortas compared with vehicle-injected control rabbits (n=5). The aortic cholesterol content and extent of sudanophilic lesion area were correlated significantly with total plasma ChOx exposure (P<0.003 and P<0.0001, respectively) but not with total cholesterol exposure. The results indicate that for moderate experimental hypercholesterolemia, a situation more relevant to physiological hypercholesterolemia in humans, circulating ChOx may play an important role in inducing formation of early atherosclerotic lesions. Because ChOx are often present in cholesterol-containing diets, foam cell lesion formation induced by ChOx rather than cholesterol cannot be overlooked.

Oxysterols and atherosclerosis

Oxysterols are present in human atherosclerotic plaque and are suggested to play an active role in plaque development. Moreover, the oxysterol:cholesterol ratio in plaque is much higher than in normal tissues or plasma. Oxysterols in plaque are derived both non-enzymically, either from the diet and/or from in vivo oxidation, or (e.g. 27-hydroxycholesterol) are formed enzymically during cholesterol catabolism. While undergoing many of the same reactions as cholesterol, such as being esterified by cells and in plasma, certain oxysterols in some animal and in vitro models exhibit far more potent effects than cholesterol per se. In vitro, oxysterols perturb several aspects of cellular cholesterol homeostasis (including cholesterol biosynthesis, esterification, and efflux), impair vascular reactivity and are cytotoxic and/or induce apoptosis. Injection of relatively large doses of oxysterols into animals causes acute angiotoxicity whereas oxysterol-feeding experiments have yielded contrary results as far as their atherogenicity is concerned. There is no direct evidence yet in humans that oxysterols contribute to atherogenesis. However, oxysterol levels are elevated in human low-density lipoprotein (LDL) subfractions that are considered potentially atherogenic and two recent studies have indicated that raised plasma levels of a specific oxysterol (7beta-hydroxycholesterol) may be associated with an increased risk of atherosclerosis. At the present time there are a number of significant and quite widespread problems with current literature which preclude more than a tentative suggestion that oxysterols have a causal role in atherogenesis. Further studies are necessary to definitively determine the role of oxysterols in atherosclerosis, and considering the wide-ranging tissue levels reported in the literature, special emphasis is needed on their accurate analysis, especially in view of the susceptibility of the parent cholesterol to artifactual oxidation.

Arterial injury by cholesterol oxidation products causes endothelial dysfunction and arterial wall cholesterol accumulation

Cholesterol oxidation products (ChOx) have been reported to cause acute vascular injury in vivo; however, the pharmacokinetics of ChOx after administration and the mechanisms by which they cause chronic vascular injury are not well understood. To further study the pharmacokinetics and atherogenic properties of ChOx, New Zealand White rabbits were injected intravenously (70 mg per injection, 20 injections per animal) with a ChOx mixture having a composition similar to that found in vivo during a 70-day period. Total ChOx concentrations in plasma peaked almost immediately after a single injection, declined rapidly, and returned to preinjection levels in 2 hours. After multiple injections, the ChOx concentrations rose gradually to levels 2- to 3-fold above baseline levels, increasing mostly in the cholesteryl ester fraction of LDL and VLDL. Rabbit serum and the isolated LDL/VLDL fraction containing elevated ChOx concentrations were cytotoxic to V79 fibroblasts and rabbit aortic endothelial cells. At the time of killing, cholesterol levels in the aortas from ChOx-injected rabbits were significantly elevated despite the fact that plasma cholesterol levels remained in the normal range. In addition, aortas from the ChOx-injected rabbits retained more 125I-labeled horseradish peroxidase, measured 20 minutes after intravenous injection. Transmural concentration profiles across the arterial wall also showed increased horseradish peroxidase accumulation in the inner half of the media from the thoracic aorta in ChOx-injected rabbits. In conclusion, ChOx injection resulted in accumulation of circulating ChOx and induced increased vascular permeability and accumulation of lipids and macromolecules. This study reveals that even under normocholesterolemic conditions, ChOx can cause endothelial dysfunction, increased macromolecular permeability, and increased cholesterol accumulation, parameters believed to be involved in the development of early atherosclerotic lesions.

Oxidized cholesterol in the diet accelerates the development of aortic atherosclerosis in cholesterol-fed rabbits

Oxidized lipoproteins may play a role in atherosclerosis. Recently, we have demonstrated that the levels of oxidized fatty acids in the circulation correlate directly with the quantity of oxidized fatty acids in the diet and that dietary oxidized fatty acids accelerate atherosclerosis in rabbits. The present study tests the hypothesis that oxidized cholesterol in the diet accelerates the development of atherosclerosis. Rabbits were fed a diet containing 0.33% nonoxidized cholesterol (control diet) or the same diet containing 0.33% cholesterol of which 5% was oxidized (oxidized diet). Serum cholesterol levels increased to a similar extent in both groups, with the majority of cholesterol in the beta-VLDL fraction. Moreover, in the serum beta-VLDL fraction and liver, there was a significant increase in the oxidized cholesterol levels. Most importantly, feeding a diet enriched in oxidized cholesterol resulted in a 100% increase in fatty streak lesions in the aorta. Western diets contain high concentrations of oxidized cholesterol products, and our results suggest that these foods may be a risk factor for atherosclerosis.

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.

Vitamin E inhibits low-density lipoprotein-induced adhesion of monocytes to human aortic endothelial cells in vitro

Monocyte adhesion to human aortic endothelial cells (ECs) is one of the early events in the development of atherogenesis. ECs were used to investigate the role of vitamin E in human monocyte adhesion to ECs in vitro. ECs incubated with 40 to 193 mg/dL of low-density lipoprotein cholesterol (LDL) for 22 hours exhibited increasing dose-dependent adherence for untreated, isolated human monocytes (P < .05). ECs exposed to the highest dose of LDL (193 mg/dL) but pretreated with 19 mumol/L alpha-tocopherol for 24 hours showed a trend to lower adherence for monocytes compared with non-treated ECs (4.4 +/- 1.2% versus 7.6 +/- 1.9%; P = .09). This effect of vitamin E became more significant (P < .05) when ECs were exposed to a lower level of LDL (40 mg/dL) or were pretreated with a higher level of alpha-tocopherol (42 mumol/L) and then exposed to 80 mg/dL LDL. Presupplementation of ECs with 15, 19, and 37 mumol/L alpha-tocopherol significantly (P < .05) reduced monocyte adhesion by 6 +/- 1%, 37 +/- 6%, and 69 +/- 17%, respectively. Levels of soluble intercellular adhesion molecule-1 (sICAM-1), one of the adhesion molecules for monocytes, increased after incubation of ECs with LDL 80 mg/dL (4.7 +/- 0.7 versus 6.4 +/- 1.2 ng/mL, respectively; P < .05). Treatment of ECs with alpha-tocopherol (42 mumol/L) significantly reduced induction of sICAM-1 by LDL to 2.2 +/- 2.3 ng/mL. After exposure to LDL, prostaglandin I2 production by ECs was diminished, whereas presupplementation of ECs with alpha-tocopherol partially reversed the LDL effect. Production of interleukin-1 beta was not detectable when ECs were treated with alpha-tocopherol, LDL, or alpha-tocopherol followed by LDL. Our findings indicate that vitamin E has an inhibitory effect on LDL-induced production of adhesion molecules and adhesion of monocytes to ECs via its antioxidant function and/or its direct regulatory effect on sICAM-1 expression.

Biological effects of oxysterols: current status

A review of relevant literature on biological activities of oxysterols (OS) and cholesterol is presented. The data clearly demonstrate manifold biological activities, often detrimental, for OS compared with little or no such activity of a deleterious nature for cholesterol itself. Cholesterol is perhaps the single most important compound in animal tissue and, as such, it is difficult to imagine it as a toxin or hazard. In contrast, OS exhibit cytotoxicity to a wide variety of cells leading to angiotoxic and atherogenic effects; alter vascular permeability to albumin; alter prostaglandin synthesis and stimulate platelet aggregation, an important process facilitating atherosclerosis and thrombosis; alter the functionality of low density lipoprotein (LDL) receptors, possibly stimulating hypercholesterolaemia; modify cholesteryl ester accumulation in various cells, inducing foam cell formation; and enrich the LDL particle in cholesteryl esters, possibly increasing its atherogenicity. Furthermore, OS are mutagenic and carcinogenic, although some have been studied as antitumour agents based on their cytotoxic properties. Moreover, numerous studies have implicated OS in membrane and enzyme alterations that are interrelated with many of the foregoing effects. The authors find that OS deserve much more attention than cholesterol itself in terms of research activity but that unfortunately the reverse is true with regard to funding.

Analysis of plasma cholesterol oxidation products using gas- and high-performance liquid chromatography/mass spectrometry

The application of gas chromatography and high-pressure liquid chromatography/mass spectrometry techniques for analysis of plasma cholesterol oxidation products is described. Cholesterol oxides that are widely identified in biological samples were subjected to gas (GC) and high-pressure liquid chromatographic (HPLC) separations, and their detection and characterization by mass spectrometry (MS) were compared. Analysis of cholesterol oxides from plasma samples revealed distinct advantages for each method according to the specific cholesterol oxide in question. Whereas HPLC/MS analysis of cholesterol oxides provided less resolution and lower sensitivity as compared to GC/MS, a distinct advantage was evident for direct measurements of cholesterol-7-hydroperoxides and 7-ketocholesterol. These two cholesterol oxides are particularly sensitive to storage in solvents, derivatization procedures, and analytical conditions used for GC analysis, which are minimized or avoided using the HPLC/MS conditions described. Analysis of human and rabbit plasma samples identified cholest-5-ene-3 beta, 7 beta-diol (7 beta-hydroxycholesterol); 5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol (cholesterol-5 alpha, 6 alpha-epoxide); 5 alpha-cholestane-3 beta, 5,6 beta-triol (cholestanetriol); 3 beta-hydroxycholest-5-ene-7-one (7-ketocholesterol); and 5,6 beta-epoxy-5 beta-cholestan-3 beta-ol (cholesterol-5 beta,6 beta-epoxide) as commonly occurring components (trivial names indicated in parentheses). The latter two compounds were dramatically increased in hypercholesterolemic samples and were found in approximately equal amounts in the free cholesterol and cholesteryl ester fractions. Although most of the plasma cholesterol oxides are found in the dietary cholesterol, others are not, particularly cholesterol-5 beta,6 beta-epoxide, suggesting that at least some of these compounds are formed by in vivo oxidation of cholesterol. Despite the readily measurable levels of the above cholesterol oxides, as well as other less prominent oxides, there was no evidence of cholesterol-7-hydroperoxides associated with plasma free cholesterol. Although several of the plasma cholesterol oxides may derive from cholesterol-7-hydroperoxides, it appears that the latter are either unstable and decompose in plasma, are metabolized to other cholesterol oxidation products, or break down during their isolation.

Biological activities of oxygenated sterols: physiological and pathological implications

Oxygenated derivatives of cholesterol (oxysterols) are widely distributed in nature, being found in the blood and tissues of animals and man as well as in foodstuff. They exhibit many biological activities which are of potential physiological, pathological or pharmacological importance. Many oxysterols have been found to be potent inhibitors of cholesterol biosynthesis and one or more oxysterols may play a role as the physiologic feedback regulator of cholesterol synthesis. Oxysterols also inhibit cell replication and have cytotoxic properties, effects which suggest that these sterols may participate in the regulation of cell proliferation and may be potentially useful as therapeutic agents for cancer. Furthermore, there is considerable evidence that oxysterols may be involved in the pathogenesis of atherosclerosis. Although the mechanism of action of oxysterols in all these instances is not well understood, the existence of cytosolic and microsomal proteins which bind oxysterols with high affinity and specificity suggests that this group of compounds may represent a family of intracellular regulatory molecules.

Angiotoxicity and atherogenicity of cholesterol oxides

Cholesterol in the diet can readily autoxidize and be absorbed and transported in plasma lipoproteins. Cholesterol oxides can also be endogenously produced in tissues via free-radical-induced reactions. Some cholesterol oxides, notably cholestane-3 beta, 5 alpha, 6 beta-triol and 25-hydroxycholesterol, have been shown to cause injury to vascular endothelial and smooth muscle cells, to alter LDL receptor function, to enhance cholesteryl ester accumulation, to inhibit prostacyclin production, and to induce experimental atherosclerosis alone or in combination with cholesterol. An epidemiological study examining relationships between atherosclerosis and plasma levels of cholesterol oxides as independent risk factors may provide additional insights regarding the roles of cholesterol oxides in atherogenesis.

The antioxidant butylated hydroxytoluene protects against atherosclerosis

Rabbits fed a 1% cholesterol diet with or without the antioxidant butylated hydroxytoluene (BHT) developed typical atherosclerotic lesions. The addition of BHT gave higher levels of total cholesterol (+40%), triglycerides (+250%), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) in plasma. Despite the lower plasma lipid levels, the degree of atherosclerosis of the aortic surface was considerably higher in rabbits fed cholesterol than in the group treated with cholesterol and BHT. The mean atherosclerotic involvement was 18.6 +/- 4.4% in the former group and 5.9 +/- 1.7% in the latter group (p = 0.02). In all animals, there was a high correlation between the area of the arterial lesion and cholesterol content (r = 0.96). Serum levels of cholesterol autooxidation products (7-ketocholesterol and cholesterol 5 alpha,6 alpha-epoxide) were lower in the group of rabbits treated with BHT (p less than 0.005). Serum levels of vitamin E were slightly higher in the BHT group. There was no significant difference in the clearance of beta-VLDL between the two treatment groups after using either beta-VLDL from cholesterol-fed animals or beta-VLDL from BHT-fed animals. The results are in accord with the contention that oxidative modification of lipoproteins is important for the development of atherosclerosis and that antioxidants may have a protective effect. At present, however, other explanations cannot be completely excluded, for example, effects of antioxidants on immunologic factors or monocyte adhesion.

Estimation of 26-hydroxycholesterol in serum by high-performance liquid chromatography and its measurement in patients with atherosclerosis

A method for analysing 26-hydroxycholesterol (26OHC) in serum and tissue samples using solid phase extraction and high-performance liquid chromatography is reported. This procedure was used to measure the levels of 26OHC in the sera of apparently healthy subjects and of 18 patients with angiographically proven atherosclerosis. Sixteen of the patients had levels within or below the range detected in the apparently healthy subjects (125-294 ng/ml), indicating that high 26OHC levels cannot be a major factor in the development of atherosclerosis. However, when the patients and the normal subjects were combined in a group, there was a significant positive correlation (r = 0.54, P less than 0.01) between serum cholesterol and serum 26OHC, and that correlation approached significance for each of the individual groups (P = 0.06 for each group). These results suggest that there is an association between cholesterol and 26OHC levels in human serum.

Antiatherogenic effect of olive and corn oils in cholesterol-fed rabbits with the same plasma cholesterol levels

Two groups of 18 rabbits were fed isocaloric, cholesterol-enriched diets for 8 weeks. The diet for one group was supplemented with 5% corn oil. The concentration of cholesterol in plasma was determined weekly and the amount of cholesterol in the diet was adjusted individually so that each rabbit had a mean plasma cholesterol concentration of about 45 mM during the experimental period. The aortic cholesterol concentrations were 122 +/- 29 and 193 +/- 38 (mean +/- SEM) mumol/g protein for the corn-oil group and the control group, respectively (p less than 0.05). In a similar experiment, each of 36 rabbits was given a mean plasma cholesterol level of about 20 mM over a period of 12 weeks. One-third of the rabbits received 10% to 15% corn oil, another third 10% to 15% olive oil, while the last third served as a control group. The aortic cholesterol concentrations were 98 +/- 25, 57 +/- 9, and 131 +/- 32 mumol/g protein, respectively. The value for the olive-oil group was significantly (p less than 0.01) lower than the value for the control group. The triglyceride concentrations and the distributions of cholesterol between HDL, LDL, and VLDL in plasma showed no significant differences between the plant-oil groups and their control groups. This suggests that plant oils have a direct effect on the aortic cholesterol metabolism.

Toxicity of enzymically-oxidized low-density lipoprotein

Intravenous injection of cholesterol oxidase into hyperlipidemic rabbits in which aortic atheromatous lesions have been induced by dietary means is lethal within hours, whereas injection of the same enzyme into normal rabbits has no visible adverse effect. The lethal effect of the enzyme is explicable by the finding that injection of cholesterol-oxidase treated low-density lipoprotein kills normal rabbits, in contrast to untreated low-density lipoprotein which does not. Enzymically oxidized low-density lipoprotein was also found to be cytotoxic for two human cell lines and for cultured bovine aortic endothelial cells. We suggest that in vivo enzymic conversion of low-density lipoprotein cholesterol to low-density lipoprotein cholestenone may possibly play a role in the initiation of atheromatous lesions in humans.

Cholesterol hydroperoxides inhibit calmodulin and suppress atherogenesis in rabbits

A mixture of cholesterol autoxidation products, prepared from an aged sample of cholesterol by recrystallization from methanol, inhibits calmodulin irreversibly in a Ca2+-dependent reaction. Inhibitory activity is lost after treatment with NaBH4, NaCNBH3, or NaI, from which we conclude that calmodulin inhibition is due to one or more cholesterol hydroperoxides. Partially purified cholesterol hydroperoxides, with or without cholesterol, were fed to young adult white rabbits. Cholesterol in the diet caused extensive atheroma formation in the aortas, but the addition of cholesterol hydroperoxides markedly reduced lesion formation. A cholesterol hydroperoxide preparation that was reduced by treatment with NaI was not effective in preventing atheroma formation. Cholesterol hydroperoxides did not lower cholesterol concentrations in blood plasma, liver, or heart.