Effects of dietary oxysterols on coronary arteries in hyperlipidaemic hamsters

The Cross-Talk Between the Peripheral and Brain Cholesterol Metabolisms

Cholesterol is an essential element for the development and normal function of the central nervous system. While peripheral cholesterol is influenced by liver metabolism and diet, brain cholesterol metabolism takes place in an isolated system due to the impermeability of the blood-brain barrier (BBB). However, cross-talk occurs between the brain and periphery, specifically through metabolites such as oxysterols that play key roles in regulating cholesterol balance. Several neurodegenerative conditions such as Alzheimer's disease or Parkinson's disease are considered to be affected by the loss of this balance. Also, the treatment of hypercholesterolemia needs to consider these discrete interferences between brain and peripheral cholesterol and the possible implications of each therapeutic approach. This is particularly important because of 27-hydroxycholesterol and 24-hydroxycholesterol, which can cross the BBB and are involved in cholesterol metabolism. This paper examines the metabolic pathways of cholesterol metabolism in the brain and periphery and focuses on the complex cross-talk between these metabolisms. Also, we emphasize the regulatory role of the BBB and the need for an integrated approach to cholesterol management.

Dietary cholesterol oxidation products: Perspectives linking food processing and storage with health implications

Dietary cholesterol oxidation products (COPs) are heterogeneous compounds formed during the processing and storage of cholesterol-rich foods, such as seafood, meat, eggs, and dairy products. With the increased intake of COPs-rich foods, the concern about health implications of dietary COPs is rising. Dietary COPs may exert deleterious effects on human health to induce several inflammatory diseases including atherosclerosis, neurodegenerative diseases, and inflammatory bowel diseases. Thus, knowledge regarding the effects of processing and storage conditions leading to formation of COPs is needed to reduce the levels of COPs in foods. Efficient methodologies to determine COPs in foods are also essential. More importantly, the biological roles of dietary COPs in human health and effects of phytochemicals on dietary COPs-induced diseases need to be established. This review summarizes the recent information on dietary COPs including their formation in foods during their processing and storage, analytical methods of determination of COPs, metabolic fate, implications for human health, and beneficial interventions by phytochemicals. The formation of COPs is largely dependent on the heating temperature, storage time, and food matrices. Alteration of food processing and storage conditions is one of the potent strategies to restrict hazardous dietary COPs from forming, including maintaining relatively low temperatures, shorter processing or storage time, and the appropriate addition of antioxidants. Once absorbed into the circulation, dietary COPs can contribute to the progression of several inflammatory diseases, where the absorbed dietary COPs may induce inflammation, apoptosis, and autophagy in cells in the target organs or tissues. Improved intake of phytochemicals may be an effective strategy to reduce the hazardous effects of dietary COPs.

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.

Current Knowledge about Oxysterols: A Review

For years food consumers have been warned that a cholesterol-rich diet may result in atherosclerosis. It is also well known that consumption of large amounts of phytosterols decreases concentration of low-density lipoproteins (LDLs) in blood (LDLs are regarded a key risk factor in development of cardiovascular diseases). However, no scientific evidence has unambiguously proved any direct connection between amount of consumed cholesterol and LDL level in blood. On the other hand, concentration of cholesterol oxidation products, oxysterols, seems to be indeed relevant; for example, they significantly impact appearance of atherosclerotic lesions (plaques). Phytosterols (like sitosterol or campasterol) decrease LDL level in blood, but on the other hand products of their oxidation are toxic. Therefore, it is worth to know influence of phytosterols on living organisms, processes which lead to their formation, and their levels in popular foodstuffs. This paper is an attempt to review literature data on the above aspects, as well as on impact on living organisms of oxidation products of popular sterols.

Oxysterols in biological systems: sources, metabolism and pathophysiological relevance

Oxysterols are the 27-carbon products of cholesterol oxidation by both enzymic and non-enzymic mechanisms. Their roles in cholesterol homeostasis, as well as in diseases in which oxidative damage and lipid peroxidation are implicated (e.g. atherosclerosis), have been investigated extensively. However, there are a number of important considerations regarding the physiological/pathophysiological functions and activities of the different oxysterols. First, in both normal and diseased tissues, the levels of oxysterols are very low when compared to the native sterol. Also, when assessing studies that have measured the levels of oxysterols in biological samples, there must be careful consideration as to the method of sample isolation, storage and sampling. This is because of the potential generation or loss of oxysterols during these procedures. Additionally, the relevance of in vitro studies which examine the effects of oxysterols upon cell function should be judged as to cellular oxysterol content (both in terms of the levels of oxysterol and the degree of esterification) resulting from the oxysterol treatment. We present evidence that the means by which oxysterol is delivered in vitro determines whether the oxysterol content reflects what has been found in vivo. Studies identifying the specific cellular targets of oxysterol indicate that several oxysterols may be regulators of cellular lipid metabolism via control of gene transcription.

Dietary oxysterols induce in vivo toxicity of coronary endothelial and smooth muscle cells

Dietary cholesterol oxidation products (COPs) were reported to exhibit in vitro toxicity toward vascular cells. The aim of this study was to determine whether dietary COPs induce in vivo toxicity toward coronary arteries and to evaluate their effect on the coronary reactivity. Golden Syrian hamsters were fed either a normolipidic diet or a hyperlipidic diet with or without a mixture of COPs (1.4 mg/kg/day). At the end of the feeding periods, cardiac mitochondria and cytosol were prepared to determine the subcellular distribution of cytochrome c. Oxidative phosphorylation was evaluated with glutamate, pyruvate or palmitoylcarnitine as a substrate. The main coronary artery was examined all along its length by transmission electron microscopy (TEM). Plasma sterol concentrations were determined. Furthermore, at the end of the 3-month feeding period, the hearts were perfused at constant pressure by the Langendorff method. The endothelium-dependent reactivity to acetylcholine was evaluated. The myocardial sterol concentration was also estimated. After a 15-day diet with dietary COPs, a release of cytochrome c into the cytosolic fraction of the whole heart occurred, which indicated apoptosis of one or several types of cardiac cells probably induced by excess circulating cholestanetriol. The morphological data obtained by TEM after three months of diet suggested that mainly vascular cells (endothelial and smooth muscle cells) were damaged by dietary COPs, whereas cardiomyocytes appeared healthy. Furthermore, the mitochondrial oxidation of palmitoylcarnitine was reduced and that of pyruvate was increased, suggesting some maintenance of energy metabolism. This strengthens the hypothesis of apoptosis. Several changes in coronary reactivity suggesting an increased NO production were observed. In conclusion, dietary COPs triggered in vivo apoptosis of coronary cells through the release of cytochrome c in the cytosol. This toxicity was counterbalanced by an increased endothelium-dependent dilation.

Purification and characterization of 7beta-hydroxysteroid dehydrogenase from rabbit liver microsomes

7beta-Hydroxysteroid dehydrogenase (7beta-HSD), a specific enzyme active in the metabolization of 7beta-hydroxycholesterol, was purified about 300-fold from male rabbit liver microsomes using ion exchange, hydroxylapatite, 2'5'ADP Sepharose 4B, and high-performance liquid chromatography on the basis of its catalytic activity. The specific activity of the purified enzyme was 276 nmol/min/mg protein. The molecular weight of the purified enzyme was 34,000. The preferred coenzyme was beta-NADP+. The optimum pH for oxidation was around 7.7 in potassium phosphate buffer, and 11.0 in glycine-NaOH buffer. The purified enzyme catalyzed the synthesis of not only 7beta-hydroxycholesterol but also corticosterone and hydrocortisone. Enzyme activities toward these three substrates accompanied all purification steps of 7beta-HSD. The amino acid sequence of the N-terminal of the purified enzyme showed that 7beta-HSD had sequence similarity to rabbit type I 11beta-hydroxysteroid dehydrogenase (11beta-HSD), indicating that 7beta-HSD may belong to the rabbit type I 11beta-HSD family and may play the same role in the metabolism of 11-hydroxysteroids and 7-hydroxysterols.

Effects of dietary 27-hydroxycholesterol on cholesterol metabolism and bile acid biosynthesis in the hamster

27-hydroxycholesterol (27OH-Chol) is an important endogenous oxysterol resulting from the action of sterol 27-hydroxylase (CYP27A1) on cholesterol in the liver and numerous extrahepatic tissues. It may act as a modulator of cholesterol and bile acid metabolism. The effects of 27OH-Chol on the main enzymes and receptors of cholesterol metabolism were investigated by feeding male hamsters a diet supplemented with 27OH-Chol (0.1% w/w) for 1 week. Intestinal scavenger class B, type I (SR-BI) protein level was decreased (–65%), but hepatic expression was increased (+34%). Liver 3β-hydroxy-3β-methyl glutaryl coenzyme A reductase (–58%), cholesterol 7α-hydroxylase (–54%), oxysterol 7α-hydroxylase (–44%), and sterol 12α-hydroxylase (–70%) activities were all decreased. Bile acid composition was changed (fourfold increase in the chenodeoxycholic/cholic acid ratio). This study demonstrates that dietary 27OH-Chol modulates major enzymes of cholesterol metabolism and alters the biliary bile acid profile, making it more hydrophobic, at least at this level of intake. Its effects on SR-BI protein levels are organ dependent. The properties of 27OH-Chol or its metabolites on cholesterol metabolism probably result from the activation of specific transcription factors. Key words: cholesterol 7α-hydroxylase (CYP7A1), sterol 12α-hydroxylase (CYP8B1), sterol 27-hydroxylase (CYP27A1), 3β-hydroxy-3β-methyl glutaryl coenzyme A reductase (HMGCoAR), scavenger receptor class B type I (SR-BI).

Impairment of the cytotoxic and oxidative activities of 7 beta-hydroxycholesterol and 7-ketocholesterol by esterification with oleate

Atherosclerosis involves inflammatory processes, as well as cytotoxic and oxidative reactions. In atherosclerotic plaques, these phenomena are revealed by the presence of dead cells, oxidized lipids, and oxidative DNA damage, but the molecules triggering these events are still unknown. As 7 beta-hydroxycholesterol and 7-ketocholesterol, which are present at elevated concentrations in atherosclerotic lesions, are strongly cytotoxic and pro-oxidative, their effects were determined on cell death, superoxide anion and nitric oxide production, lipid peroxidation, and oxidative DNA damage. 7-Ketocholesterol- and 7 beta-hydroxycholesterol-induced cell death leads to a loss of mitochondrial potential, to increased permeability to propidium iodide, and to morphological nuclear changes (swelling, fragmentation, and/or condensation of nuclei). These effects are preceded by the formation of cytoplasmic monodansylcadaverine-positive structures and are associated with a rapid enhancement of cells overproducing superoxide anions, a decrease in cells producing nitric oxide, lipid peroxidation (formation of malondialdehyde and 4-hydroxynonenal adducts, low ratio of [unsaturated fatty acids]/[saturated fatty acids]) as well as oxidative DNA damage (8-oxoguanine formation). Noteworthy, none of the cytotoxic features previously observed with 7 beta-hydroxycholesterol and 7-ketocholesterol were noted with cholesterol, 7 beta-hydroxycholesteryl-3-oleate and 7-ketocholesteryl-3-oleate, with the exception of a slight increase in superoxide anion production with 7 beta-hydroxycholesteryl-3-oleate. This finding supports the theory that 7 beta-hydroxycholesterol and 7-ketocholesterol could induce cytotoxic and oxidative processes observed in atherosclerotic lesions and that esterification of these compounds may contribute to reducing atherosclerosis progression.

Oxidized fats in foods

PURPOSE OF REVIEW

Lipid oxidation is the cause of important deteriorative changes in chemical, sensory and nutritional food properties. In particular, the question of whether oxidized fats in the diet may be detrimental to health is nowadays of the upmost concern, but finding an answer is not easy and requires careful consideration of different aspects of lipid oxidation.

RECENT FINDINGS

In this review, the most recent works on the formation, nature and evaluation of oxidized dietary lipids are addressed; important issues such as the difficulties encountered in estimating their intake and the relationships between oxidants and antioxidants in the diet are discussed, and the latest studies on health implications of oxidized lipids are summarized.

SUMMARY

The current literature reflects various important points. At present, there is no information on the intake of oxidized fats, which is essential to know if the amount of oxidized lipids in normal diets is sufficient to cause the physiological effects claimed. Recently, relevant advances in analytical methodologies for quantitation of specific oxidation compounds have been reported, although their application to improve the analytical definition of the oxidized substrate used in nutritional studies is still a goal to be reached. Alternatively, one of the most promising current tendencies in this field is the study of the molecular targets by which dietary oxidized lipids can influence health. Overall, more selected research based on coordinated multidisciplinary studies is needed to define the role of dietary oxidized fats in health.