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

Excipient-related impurities in liposome drug products

Liposomes are widely used in the pharmaceutical industry as drug delivery systems to increase the efficacy and reduce the off-target toxicity of active pharmaceutical ingredients (APIs). The liposomes are more complex drug delivery systems than the traditional dosage forms, and phospholipids and cholesterol are the major structural excipients. These two excipients undergo hydrolysis and/or oxidation during liposome preparation and storage, resulting in lipids hydrolyzed products (LHPs) and cholesterol oxidation products (COPs) in the final liposomal formulations. These excipient-related impurities at elevated concentrations may affect liposome stability and exert biological functions. This review focuses on LHPs and COPs, two major categories of excipient-related impurities in the liposomal formulations, and discusses factors affecting their formation, and analytical methods to determine these excipient-related impurities.

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.

Postprandial Dysmetabolism and Oxidative Stress in Type 2 Diabetes: Pathogenetic Mechanisms and Therapeutic Strategies

Postprandial dysmetabolism in type 2 diabetes (T2D) is known to impact the progression and evolution of this complex disease process. However, the underlying pathogenetic mechanisms still require full elucidation to provide guidance for disease prevention and treatment. This review focuses on the marked redox changes and inflammatory stimuli provoked by the spike in blood glucose and lipids in T2D individuals after meals. All the causes of exacerbated postprandial oxidative stress in T2D were analyzed, also considering the consequence of enhanced inflammation on vascular damage. Based on this in-depth analysis, current strategies of prevention and pharmacologic management of T2D were critically reexamined with particular emphasis on their potential redox-related rationale.

The intestinal bioavailability of vaccenic acid and activation of peroxisome proliferator-activated receptor-α and -γ in a rodent model of dyslipidemia and the metabolic syndrome

SCOPE

Evidence suggests a neutral to beneficial role of certain trans fatty acids (TFA) from natural ruminant sources. Trans11-18:1 (vaccenic acid, VA), the most predominant ruminant TFA and a precursor to conjugated linoleic acid, has been shown to improve atherogenic dyslipidemia and symptoms of hepatic steatosis in animal models. The objective of this study was to assess the intestinal bioavailability of various VA sources including synthetic free fatty acid (FFA) and natural ruminant triglyceride forms, as well as the mechanistic pathways that mediate VA's bioactivity.

METHODS AND RESULTS

VA acts as a partial agonist to both peroxisome proliferator-activated receptors (PPAR)-α and PPAR-γ in vitro, with similar affinity compared to commonly known PPAR agonists. It was further confirmed that VA at 30 and 100 μM concentrations suppressed cardiomyocyte hypertrophy vitro in a PPAR-α- and PPAR-γ-dependent manner. In vivo, feeding of VA (1%, w/w) resulted in increased mRNA and protein expression of PPAR-γ in the mucosa of JCR:LA-cp rats, a model of the metabolic syndrome (p < 0.01 and p < 0.05, respectively) compared to control. In addition, VA from a triglyceride source had greater intestinal bioavailability in vivo compared to VA provided in an FFA form (p < 0.01).

CONCLUSION

The activation of PPAR-α- and PPAR-γ-dependent pathways provides a mechanistic explanation of how VA improves blood lipids and related metabolic disorders during conditions of hyperlipidemia. This report also supports the consideration of differential reporting of industrially produced versus natural TFA on food nutrient labels.

Regulation of the expression of interleukin-8 induced by 25-hydroxycholesterol in retinal pigment epithelium cells

PURPOSE

This study aimed at elucidating the molecular mechanisms involved in the regulation of IL-8 production by several oxysterols in retinal pigment epithelium (RPE) cells.

METHODS

A human cell line from RPE (ARPE-19) was used to test the role of cholesterol and several oxysterols (25-OH, 7-KC and 7β-OH) in the expression and secretion of IL-8. Expression of IL-8 was assessed by real-time PCR, while IL-8 secretion was evaluated by ELISA. PI3K-, MEK1/2-, ERK1/2- and NF-κB-specific inhibitors were used to assess the specific role of the several players on the regulation of IL-8 production by oxysterols. A gene-reporter assay for AP-1 activity was also conducted to evaluate the putative role of this transcription factor on IL-8 expression induced by oxysterols.

RESULTS

Here, we demonstrate that 25-OH specifically increases transcription and secretion of the cytokine IL-8 in ARPE-19 cells. Indeed, treatment of ARPE-19 with 25-OH, but not with 7-KC, 7β-OH or cholesterol, induced the secretion of IL-8 from cells. 25-OH also induced the activation/phosphorylation of ERK1/2 through a mechanism dependent on MEK, ERK1/2 and PI3K kinase activity. Real-time PCR and ELISA experiments demonstrated that 25-OH increased transcription and secretion of IL-8 through a mechanism that is dependent on ERK1/2 and PI3K activity. Furthermore, 25-OH triggered the activation/phosphorylation of the AP-1 component c-Jun and, consistently, increased the transcriptional activity of AP-1. Additionally, we also found that 25-OH decreases the levels of IκB and increases the nuclear levels of NF-κB p65 subunit and that inhibition of NF-κB activity partially prevents the increased secretion of IL-8 induced by 25-OH.

CONCLUSIONS

The results presented in this study suggest a role for 25-OH in inducing IL-8 production through pathways that are likely to involve AP-1 and NF-κB in ARPE-19 cells. Our data may also provide new molecular targets for the treatment of AMD.

Rimonabant-mediated changes in intestinal lipid metabolism and improved renal vascular dysfunction in the JCR:LA-cp rat model of prediabetic metabolic syndrome

Rimonabant (SR141716) is a specific antagonist of the cannabinoid-1 receptor. Activation of the receptor initiates multiple effects on central nervous system function, metabolism, and body weight. The hypothesis that rimonabant has protective effects against vascular disease associated with the metabolic syndrome was tested using JCR:LA-cp rats. JCR:LA-cp rats are obese if they are cp/cp, insulin resistant, and exhibit associated micro- and macrovascular disease with end-stage myocardial and renal disease. Treatment of obese rats with rimonabant (10 mg.kg(-1).day(-1), 12-24 wk of age) caused transient reduction in food intake for 2 wk, without reduction in body weight. However, by 4 wk, there was a modest, sustained reduction in weight gain. Glycemic control improved marginally compared with controls, but at the expense of increased insulin concentration. In contrast, rimonabant normalized fasting plasma triglyceride and reduced plasma plasminogen activator inhibitor-1 and acute phase protein haptoglobin in cp/cp rats. Furthermore, these changes were accompanied by reduced postprandial intestinal lymphatic secretion of apolipoprotein B48, cholesterol, and haptoglobin. While macrovascular dysfunction and ischemic myocardial lesion frequency were unaffected by rimonabant treatment, both microalbuminuria and glomerular sclerosis were substantially reduced. In summary, rimonabant has a modest effect on body weight in freely eating obese rats and markedly reduces plasma triglyceride levels and microvascular disease, in part due to changes in intestinal metabolism, including lymphatic secretion of apolipoprotein B48 and haptoglobin. We conclude that rimonabant improves renal disease and intestinal lipid oversecretion associated with an animal model of the metabolic syndrome that appears to be independent of hyperinsulinemia or macrovascular dysfunction.

Trans-11 vaccenic acid reduces hepatic lipogenesis and chylomicron secretion in JCR:LA-cp rats

Trans-11 vaccenic acid (VA) is the predominant trans isomer in ruminant fat and a major precursor to the endogenous synthesis of cis9,trans11-conjugated linoleic acid in humans and animals. We have previously shown that 3-wk VA supplementation has a triglyceride (TG)-lowering effect in a rat model of dyslipidemia, obesity, and metabolic syndrome (JCR:LA-cp rats). The objective of this study was to assess the chronic effect (16 wk) of VA on lipid homeostasis in both the liver and intestine in obese JCR:LA-cp rats. Plasma TG (P < 0.001), total cholesterol (P < 0.001), LDL cholesterol (P < 0.01), and nonesterified fatty acid concentrations, as well as the serum haptoglobin concentration, were all lower in obese rats fed the VA diet compared with obese controls (P < 0.05). In addition, there was a decrease in the postprandial plasma apolipoprotein (apo)B48 area under the curve (P < 0.05) for VA-treated obese rats compared with obese controls. The hepatic TG concentration and the relative abundance of fatty acid synthase and acetyl-CoA carboxylase proteins were all lower (P < 0.05) in the VA-treated group compared with obese controls. Following acute gastrointestinal infusion of a VA-triolein emulsion in obese rats that had been fed the control diet for 3 wk, the TG concentration was reduced by 40% (P < 0.05) and the number of chylomicron (CM) particles (apoB48) in nascent mesenteric lymph was reduced by 30% (P < 0.01) relative to rats infused with a triolein emulsion alone. In conclusion, chronic VA supplementation significantly improved dyslipidemia in both the food-deprived and postprandial state in JCR:LA-cp rats. The appreciable hypolipidemic benefits of VA may be attributed to a reduction in both intestinal CM and hepatic de novo lipogenesis pathways.

Monitoring dynamic changes in lymph metabolome of fasting and fed rats by electrospray ionization-ion mobility mass spectrometry (ESI-IMMS)

Ambient pressure ion mobility time-of-flight mass spectrometry (IMMS) has recently emerged as a rapid and efficient analytical technique for applications to metabolomics. An important application of metabolomics is to monitor metabolome shifts caused by stress due to toxin exposure, nutritional changes, or disease. The research presented in this paper uses IMMS to monitor metabolic changes in rat lymph fluid caused by dietary stresses over time. Extracts of metabolites found in the lymph fluid collected from dietary stressed rats were subjected to analysis by electrospray (ESI) IMMS operated both in positive and negative ion detection mode. Metabolites detected were tentatively identified based on their mass to charge ratio (m/z). In one sample, 1180 reproducible tentative metabolite ions were detected in negative mode and 1900 reproducible tentative metabolite ions detected in positive mode. Only biologically reproducible ions, defined as metabolite ions that were measured in different rats under the same treatment, were analyzed to reduce the complexity of the data. A metabolite peak list including m/z, mobility, and intensity generated for each metabolome was used to perform principle component analysis (PCA). Dynamic changes in metabolomes were investigated using principle components PC1 and PC2 that described 62% of the variation of the system in positive mode and 81% of the variation of the system in negative mode. Analysis of variance (ANOVA) was performed for PC1 and PC2 and means were statistically evaluated. Profiles of intensities were compared for tentative metabolite ions detected at different times before and after the rats were fed to identify the metabolites that were changing the most. Mobility-mass correlation curves (MMCC) were investigated for the different classes of compounds.

Mechanisms involved in chylomicron remnant lipid uptake by macrophages

Although it is clear that chylomicron remnants are atherogenic, events leading to their internalization by macrophages are still debated. The lack of apoE (apolipoprotein E) in CRLPs (chylomicron remnant-like particles) reduces macrophage TAG (triacylglycerol) content by approx. 50%, suggesting that, as well as apoE-mediated endocytic uptake, apoE receptor-independent mechanisms are involved in the induction of foam cells by chylomicron remnants. Evaluation of the radioactivity associated with macrophages after incubation with CRLPs containing radiolabelled lipids suggests that the TAG and cholesterol carried by the particles have different kinetics of internalization. In addition, inhibition-based experiments indicate that cholesteryl ester-selective uptake and the extracellular lipoprotein lipase hydrolysis of TAG contribute to cholesterol and TAG accumulation respectively. Thus plasma TAG and cholesterol carried by remnant particles have to be considered two independent and non-interchangeable risk factors for athero-related diseases. In addition, the interaction between CRLPs and macrophages is modulated by dietary oxidized lipids and other lipophilic components. The presence of oxidized lipids, such as 7β-hydroxycholesterol and 7-oxocholesterol, the major cholesterol oxidation products found in atherosclerotic lesions, in CRLPs interferes with the mechanisms of their internalization, but does not cause quantitative changes of accumulated lipids, while the presence of the plant carotenoid, lycopene, or the antioxidant drug, probucol, enhances lipid accumulation in macrophages by increasing the rate of uptake of the particles and raising the intracellular synthesis of TAG. In conclusion, several mechanisms contribute to the macrophage uptake of postprandial lipoproteins, however, little is known of the balance and modulation between the different pathways.

Lipid metabolism and TNF-alpha secretion in response to dietary sterols in human monocyte derived macrophages

BACKGROUND

The postprandial phase is characterized by the circulation of atherogenic dietary-triacylglycerol rich lipoproteins. Little is known about the modulation of lipid and immune functions in macrophages by these particles or of the role of the oxysterols found in food such as 7beta-hydroxycholesterol and 7-ketocholesterol.

MATERIALS AND METHODS

Human macrophages were tested with different concentrations of chylomicron remnant-like particles (CRLP) with or without incorporated oxysterols to study their uptake by the cells, and their effects on cholesteryl ester and triacylglycerol synthesis and the secretion of inflammatory mediators, including tumour necrosis factor-alpha (TNF-alpha), interleukin 6 (IL-6) and interleukin 10 (IL-10).

RESULTS

Independently of the presence of oxysterols, CRLP caused cholesterol accumulation. However, the dose-dependent increase in [3H]cholesterol internalization by macrophages after incubation with [3H]cholesteryl ester-labelled CRLP was abolished by the presence of oxysterols in the particles. TNF-alpha secretion was decreased and that of IL-10 unaffected by CRLP independently of the presence of oxysterol. Exposure to CRLP containing 7beta-hydroxysterol, but not to CRLP or 7-ketosterol-containing CRLP, reduced IL-6 secretion with respect to cells not exposed to any particles. Because TNF-alpha levels have been related to scavenger receptor expression, we tested the uptake of modified LDL in macrophages exposed to human postprandial triacylglycerol-rich lipoproteins and found it to be markedly increased.

CONCLUSIONS

Cholesterol loading as a result of dietary lipids depresses the inflammatory response of macrophages and the presence of 7beta-hydroxysterol may exacerbate this effect. In addition, exposure to dietary lipids enhances scavenger receptor activity in macrophages. These results suggest that changes induced by dietary lipids in human macrophage function are related to an increased propensity of the cells to accumulate lipids during the postprandial phase.

Cholesterol-3-beta, 5-alpha, 6-beta-triol induced genotoxicity through reactive oxygen species formation

The mutagenicity of oxysterols, cholesterol-3beta,5alpha,6beta-triol (alpha-Triol), 7-keto-cholesterol (7-Keto) and cholesterol-5alpha,6alpha-epoxide (alpha-Epox) were examined by the Ames method and chromosome aberration test in this study. Only alpha-Triol concentration-dependently caused an increase of bacterial revertants in the absence of metabolic activating enzymes (S9), but not 7-keto and alpha-Epox. The mutagenic effect of alpha-Triol was reduced by the addition of S9. On the other hand, although alpha-Triol significantly induced chromosome aberration in CHO-K1 cells with and without S9. However, the addition of S9 reduced the degree of abnormal structure chromosome compared to without S9 mix. Catalase and superoxide dismutase (SOD) inhibited alpha-Triol induced increase of revertants in Salmonella typhimurium and chromosome aberration frequency in CHO cells, suggesting that reactive oxygen species (ROS) might be involved in the genotoxic effect of alpha-Triol. Treatment with alpha-Triol increased the ROS production in CHO cells, which could be attenuated by catalase and SOD. Results in this study suggested, for the first time that alpha-Triol, causes genotoxic effect in an ROS-dependent manner.

The fatty acid composition of chylomicron remnants influences their propensity to oxidate

BACKGROUND AND AIM

Although the replacement of saturated with unsaturated dietary fat has been advocated as a means of reducing the risk of cardiovascular disease, diets high in polyunsaturated fatty acids (PUFAs) may increase lipid peroxidation, thus contributing to the pathogenesis of atherosclerosis. As the susceptibility of individual fatty acids to oxidation directly depends on their degree of unsaturation, and the oxidative modification of lipoproteins may be an important determinant of atherogenesis, the aim of this study was to evaluate the susceptibility to auto-oxidation and copper-mediated oxidation of chylomicron remnants (CMRs) enriched in n-3 or n-6 PUFA.

METHODS AND RESULTS

The remnants were prepared in vitro from chylomicrons obtained from rats given an oral dose of fish or corn oil, using rat plasma containing lipoprotein lipase. Their propensity to oxidate and the extent of the oxidation were estimated by measuring the formation of conjugated dienes and the detrimental products of lipid peroxidation. The results showed that: 1) the corn oil CMRs contained a relatively high proportion of n-6 PUFA (mainly linoleic acid), whereas the fish oil CMRs contained more n-3 PUFA, mainly eicosapentanoic and docosahexaenoic acids; 2) n-3-rich CMRs have a significantly lower propensity to oxidate than n-6-rich CMRs despite their 50% lower alpha-tocopherol content and 40% higher unsaturation index.

CONCLUSION

Our data indicate that the precise allocation of n-3 PUFA within the lipid core of CMRs may play a pivotal role in lowering the susceptibility to oxidation of fish CMRs by overcoming the effects of unfavourable alpha-tocopherol concentration. Eating n-3 rather than n-6 PUFAs seems to make CMRs more resistant against free radical attack, which may contribute to attenuating their potential atherogenic properties.

Effects of dietary fat and oxidized cholesterol on gene expression in rat liver as assessed by cDNA expression array analysis

BACKGROUND

Specific oxysterols acting as ligands for nuclear transcription factors were shown to affect expression of genes involved in lipid metabolism. However, the various biological effects of oxysterols such as cytotoxicity, atherogenicity or mutagenicity suggest that other genes may be also affected by oxysterols than lipid metabolism.

AIM OF THE STUDY

The present study was conducted to investigate the effects of dietary oxidized cholesterol containing significant amounts of oxysterols and its interactions with different dietary fats on gene expression profiles as assessed by DNA array technology in rats.

METHODS

54 male Sprague-Dawley rats were assigned to six groups and were fed six semisynthetic diets, which varied in the type of dietary fat (coconut oil vs. salmon oil) and dietary cholesterol (none cholesterol vs. 5 g unoxidized cholesterol/kg vs. 5 g oxidized cholesterol/kg).

RESULTS

Changes in gene expression as observed in response to dietary oxidized cholesterol were strongly dependent on the type of fat. In the rats fed coconut oil, the expression of 7 genes (5 up- and 2 down-regulated) was altered by dietary oxidized cholesterol, while in the rats fed salmon oil, the expression of 50 genes (16 up- and 34 down-regulated) was altered. 29 genes (22 up- and 7 down-regulated) were identified as possible targets for an altered gene expression by dietary salmon oil as compared to dietary coconut oil.

CONCLUSION

The present study showed that dietary oxidized cholesterol transcriptionally affects many genes involved in xenobiotic metabolism and stress response--an effect that was amplified by the administration of fish oil as dietary fat.

Rapid hepatic metabolism of 7-ketocholesterol by 11beta-hydroxysteroid dehydrogenase type 1: species-specific differences between the rat, human, and hamster enzyme

The role of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) in the local activation of the glucocorticoid receptor by converting inactive 11-ketoglucocorticoids to active 11beta-hydroxyglucocorticoids is well established. Currently, 11beta-HSD1 is considered a promising target for treatment of obese and diabetic patients. Here, we demonstrate a role of 11beta-HSD1 in the metabolism of 7-ketocholesterol (7KC), the major dietary oxysterol. Comparison of recombinant 11beta-HSD1, transiently expressed in human embryonic kidney 293 cells, revealed the stereo-specific interconversion of 7KC and 7beta-hydroxycholesterol by rat and human 11beta-HSD1, whereas the hamster enzyme interconverted 7alpha-hydroxycholesterol, 7beta-hydroxycholesterol, and 7KC. In contrast to lysates, which efficiently catalyzed both oxidation and reduction, intact cells exclusively reduced 7KC. These findings were confirmed using rat and hamster liver homogenates, intact rat hepatocytes, and intact hamster liver tissue slices. Reduction of 7KC was abolished upon inhibition of 11beta-HSD1 by carbenoxolone (CBX) or 2'-hydroxyflavanone. In vivo, after gavage feeding rats, 7KC rapidly appeared in the liver and was converted to 7beta-hydroxycholesterol. CBX significantly decreased the ratio of 7beta-hydroxycholesterol to 7KC, supporting the evidence from cell culture experiments for 11beta-HSD1-dependent reduction of 7KC to 7beta-hydroxycholesterol. Upon inhibition of 11beta-HSD1 by CBX, 7KC tended to accumulate in the liver, and plasma 7KC concentration increased. Together, our results suggest that 11beta-HSD1 efficiently catalyzes the first step in the rapid hepatic metabolism of dietary 7KC, which may explain why dietary 7KC has little or no effect on the development of atherosclerosis.

Dietary oxidized cholesterol decreases expression of hepatic microsomal triglyceride transfer protein in rats

The aim of this study was to compare the effects of dietary oxidized cholesterol and pure cholesterol on plasma and very low density lipoprotein (VLDL) lipids and on some parameters of VLDL assembly and secretion in rats fed two different dietary fats. Four groups of male growing Sprague-Dawley rats were fed diets containing pure or oxidized cholesterol (5 g/kg diet) with either coconut oil or salmon oil as dietary fat (100 g/kg diet) for 35 days. Rats fed oxidized cholesterol supplemented diets had significantly lower concentrations of triglycerides and cholesterol in plasma and VLDL than rats fed pure cholesterol supplemented diets irrespective of the type of fat. In addition, rats fed oxidized cholesterol supplemented diets had significantly lower relative concentrations of microsomal triglyceride transfer protein messenger ribonucleic acid (mRNA) than rats fed pure cholesterol supplemented diets. In contrast, hepatic lipid concentrations and the relative concentration of apolipoprotein B mRNA were not influenced by the dietary factors investigated. Parameters of hepatic lipogenesis (relative mRNA concentration of sterol regulatory element binding protein-1c and activity of glucose-6-phosphat dehydrogenase) were significantly reduced by feeding fish oil compared to coconut oil, but were not affected by the type of cholesterol. In conclusion, the data of this study suggest, that dietary oxidized cholesterol affects VLDL assembly and/or secretion by reducing the synthesis of MTP but not by impairing hepatic lipogenesis or synthesis of apolipoprotein B.

Toxicity of cholesterol oxidation products to Caco-2 and HepG2 cells: modulatory effects of alpha- and gamma-tocopherol

Cholesterol can be oxidized to form a variety of cholesterol oxidation products also known as oxysterols. The aims of the present study were to compare the cytotoxic effects of four oxysterols, namely 25-hydroxycholesterol (25-OHC), 7beta-hydroxycholesterol (7beta-OHC), cholesterol-5beta,6beta-epoxide (beta-epox) and cholesterol-5alpha,6alpha-epoxide (alpha-epox), in two human cell culture models. Further, the ability of 10 and 100 micro m alpha- and gamma-tocopherol (alpha-TOC and gamma-TOC, respectively) to protect against oxysterol-induced cytotoxicity was also assessed. Human colonic adenocarcinoma Caco-2 and human hepatoma HepG2 cells were supplemented with increasing concentrations of 25-OHC, 7beta-OHC, beta-epox and alpha-epox (0-25 micro g ml(-1)) for 24, 48 or 96 h. Following 24-h and 48-h exposure, test media were replaced with normal growth media and the cells were maintained for 72 and 48 h, respectively. The 96-h exposure represented a constant challenge to the cells. Cytotoxicity was assessed using the neutral red uptake assay. The concentration of compound that inhibited cell viability by 50% (ic(50) value) was calculated. All four oxysterols investigated induced the greatest cytotoxic effects following 96 h of exposure. 25-Hydroxycholesterol exhibited the greatest cytotoxicity in both cell lines. Both beta-epox and alpha-epox were more toxic to HepG2 cells than to Caco-2 cells after the 48-h exposure. Pretreatment of cells with either alpha- or gamma-TOC did not protect against oxysterol-induced cytotoxicity. The caco-2 cells treated with the high concentration (100 micro m) of gamma-TOC were found to be more susceptible to oxysterol-induced toxicity under the conditions employed in this study.

Oxidized products of cholesterol: dietary and metabolic origin, and proatherosclerotic effects (review)

Cholesterol oxidation products, termed oxysterols, are increasingly considered of potential interest in the pathogenesis of atherosclerotic lesions. Of dietary or endogenous origin, oxysterols may occur in significant amounts in low density lipoprotein (LDL) particles, especially in hypercholesterolemic subjects. They likely contribute to the uptake of modified LDL by scavenger receptors and some of them finally accumulate in the subintimal space of major arteries; here cholesterol oxides may favor the perpetuation of a chronic inflammatory state, through their ability to trigger irreversible damage of vascular cells with consequent activation of phagocytes. Furthermore, practically all oxysterols of major pathophysiologic interest have been shown to markedly up-regulate expression and synthesis of adhesion molecules, inflammatory cytokines and chemokines. Cholesterol oxidation thus appears to be an important biochemical pathway through which it exerts toxic, inflammatory and finally atherogenic effects.

Effect of pure and oxidized cholesterol-rich diets on some biochemical parameters in rats

The present study was undertaken to evaluate the peroxidative damage and hypercholesterolemia induced in male Wistar albino rats by diets enriched either with 1% oxidized cholesterol (OC) (containing 49.8% of cholesterol oxidation products) or pure cholesterol (PC). The damage caused by the OC diet was revealed by a significant rise in red blood cell hemolysis, increased tissue lipid peroxidation and elevated aspartate amino transferase activity as compared with control and PC diets. Liver glutathione-S-transferase activity was decreased by both OC (P < 0.01) and PC (P < 0.05) diets, but glutathione was observed to be decreased only by the OC diet. Plasma triacylglycerol and cholesterol were increased significantly with both the OC and PC diets. Liver cholesterol and triacylglycerol were increased significantly with the OC diet only. These results indicate that the oxidative damage caused by the OC diet is much more pronounced than that caused by the PC diet.

Protection of superoxide-induced cholesterol oxidation by antioxidants in protic conditions

It was found in previous experiments that superoxide in the presence of added hydrogen peroxide in protic conditions produces oxysterols. The oxysterols formed under these conditions were 7beta-ketocholesterol, 7alpha-hydroxycholesterol and 7beta-hydroxycholesterol. In the present experiments, the inhibitory effects of three antioxidants, alpha-tocopherol (alpha-T), butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), on the oxidation of cholesterol in the presence of superoxide anion, water and hydrogen peroxide were investigated. It was found that BHA had the highest antioxidant activity on cholesterol oxidation, followed by alpha-T and BHT. The presence of antioxidants markedly retarded the formation of 7-ketocholesterol. The formation of 7beta-hydroxycholesterol or 7alpha-hydroxycholesterol was also reduced, but to a lesser degree.

Effect of dietary cholesterol oxidation products on the plasma clearance of chylomicrons in the rat

Oxidized cholesterols in the diet have been shown to exacerbate arterial cholesterol deposition and the development of atherosclerosis in animal models. Dietary oxidized cholesterols are absorbed through the intestine and incorporated into lymph chylomicrons. The aim of this study was to investigate the effect of oxidized cholesterols on the metabolism of nascent chylomicrons in vivo. It was shown that oxidized cholesterols markedly delay the clearance of chylomicrons from plasma compared to rats given TG alone. However, there was no difference in the clearance of chylomicrons containing oxidized cholesterols vs. purified cholesterol, although the presence of oxysterols did appear to exacerbate the removal of these particles from circulation. The impaired clearance of chylomicrons containing oxidized cholesterols was not due to impaired lipolysis and slower conversion to the remnant form. Moreover, the incorporation of oxidized cholesterols did not alter the hepatic or splenic uptake of chylomicrons compared to chylomicrons isolated from rats given purified cholesterol or TG alone. Collectively, the results of this study suggest that the exacerbated delay in clearance of chylomicron remnants enriched with oxysterols may be due to impaired uptake by tissues other than the liver and spleen. Apolipoprotein (apo) analysis showed that oxysterol incorporation reduced the apoE content and altered the apoC phenotype of chylomicrons, which may have an impact on the removal of chylomicron remnants from plasma. In conclusion, dietary oxysterols appear to have the potential to adversely affect chylomicron metabolism. Therefore, further investigations in humans are required to determine whether dietary oxidized cholesterols found in cholesterol-rich processed foods delay the clearance of postprandial remnants, which may contribute to and exacerbate the development of atherosclerosis.

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.

Oxidized fat in the diet, postprandial lipaemia and cardiovascular disease

Accumulating evidence suggests that oxidized fats and lipid oxidation products in the diet can contribute to the pathogenesis of atherosclerosis. The present review summarizes studies that show that oxidized fat and lipid oxidation products are present in human foods; that these compounds are absorbed by the intestine and appear in the blood circulation; and that these ingested substances can have deleterious cardiovascular effects in both humans and experimental animals. However, considerable additional research is required to establish the extent to which dietary fat oxidation poses a threat to human health and/or longevity.

alpha-Tocopherol, but not gamma-tocopherol inhibits 7 beta-hydroxycholesterol-induced apoptosis in human U937 cells

Oxysterols, particularly those oxidised at position 7, are toxic to cells in culture and have been shown to induce apoptosis in cell types such as vascular endothelial cells, smooth muscle cells and monocytes. The precise mechanism by which oxysterols induce apoptosis is unknown but may involve the generation of oxidative stress. In the present study we examined the ability of alpha-TOC, alpha-TOC acetate (alpha-TOCA) and gamma-TOC to protect against 7 beta-hydroxycholesterol (7 beta-OHC)-induced apoptosis of human monocytic U937 cells. 7 beta-OHC is one of the most commonly detected oxysterols in foods and its level in plasma has been positively associated with an increased risk of atherosclerosis. The present study demonstrates a significant decrease in cell membrane integrity and cellular glutathione levels when U937 cells were treated with 30 microM 7 beta-OHC. DNA fragmentation also occurred, as measured by agarose gel electrophoresis, and the number of apoptotic cells increased as assessed by nuclear morphology. Analysis by HPLC showed that there was a greater incorporation of gamma-TOC into U937 cells after a 48 h incubation, than either alpha-TOC or alpha-TOCA. However, despite the increased uptake of gamma-TOC, only alpha-TOC, and not gamma-TOC or alpha-TOCA was effective at inhibiting 7 beta-OHC-induced apoptosis in U937 cells.

Metabolism of an oxysterol, 7-ketocholesterol, by sterol 27-hydroxylase in HepG2 cells

7-Ketocholesterol (7K) is a quantitatively important oxysterol in both atherosclerotic lesions and macrophage foam cells. We reported recently that radiolabeled 7K delivered to rodents in a modified lipoprotein or chylomicron remnant-like emulsion, both cleared predominantly by the liver, was rapidly excreted into the intestine as water-soluble products, presumably bile acids. Herein, we aimed to elucidate the early or initial reactions in 7K metabolism. The hypothesis was tested that sterol 27-hydroxylase, a mitochondrial cytochrome P450 and the first enzyme of the acidic bile acid pathway, is responsible for the initial metabolism of 7K by HepG2 cells, a human hepatoblastoma cell-line. The 27-hydroxylated product of 7K (27OH-7K) was shown to be the initial, lipid-soluble product of 7K metabolism. It was produced in mitochondrial incubations and whole cells and was readily released into the media from cells. Intact cells generated metabolites of 7K that had undergone conversion from lipid-soluble precursors to water-soluble products rapidly and extensively. Their production was ablated with cyclosporin A, a sterol 27-hydroxylase inhibitor. Furthermore, we demonstrated the effectiveness of two novel selective inhibitors of this enzyme, GW273297X and GI268267X. These inhibitors also ablated the production of water-soluble products by cells; and the inhibitor of choice, GW273297X, decreased the production of 27OH-7K in mitochondrial preparations. This is the first study to demonstrate that sterol 27-hydroxylase plays an important role in the metabolism of oxysterols such as 7K in liver cells.

Red wine mitigates the postprandial increase of LDL susceptibility to oxidation

The aim of the present study was to verify the extent of oxidative stress induced by a meal at plasma and LDL level, and to investigate the capacity of red wine to counteract this action. In two different sessions, six healthy men ate the same test meal consisting of "Milanese" meat and fried potatoes. The meal was taken either with 400 ml red wine or with an isocaloric hydroalcoholic solution. Oxidative stress at plasma level was estimated through the measure of ascorbic acid, alpha-tocopherol, protein SH groups, uric acid, and antioxidant capacity, measured before and 1 and 3 h after the meal. The change in the resistance of LDL to oxidative modification was taken as an index of exposure to pro-oxidants. The susceptibility to Cu(II)-catalyzed oxidation of baseline and postprandial LDL was measured as conjugated dienes formation, tryptophan residues, and relative electrophoretic mobility. The experimental meal taken with wine provoked a significant increase in the total plasma antioxidant capacity and in the plasma concentration of alpha-tocopherol and SH groups. Postprandial LDL was more susceptible to metal-catalyzed oxidation than the homologous baseline LDL after the ethanol meal. On the contrary, postprandial LDL obtained after the wine meal was as resistant or more resistant to lipid peroxidation than fasting LDL.

Oxidation affects the regulation of hepatic lipid synthesis by chylomicron remnants

The effects of native and oxidized chylomicron remnants on lipid synthesis in normal and oxidatively stressed liver cells were investigated using MET murine hepatocytes (MMH cells), a nontransformed mouse hepatocyte cell line that maintains a highly differentiated hepatic phenotype in culture. Lipid synthesis was determined by measuring the incorporation of [(3)H]oleate into cholesteryl ester, triacylglycerol, and phospholipid by the cells. The formation of cholesteryl ester and phospholipid was decreased by chylomicron remnants in a dose-dependent manner, while triacylglycerol synthesis was increased. Exposure of MMH cells to mild oxidative stress by incubation with CuSO(4) (2.5 microM) for 24 h led to significantly increased incorporation of [(3)H]oleate into triacylglycerol and phospholipid, but not cholesteryl ester, in the absence of chylomicron remnants. In the presence of the lipoproteins, however, similar effects to those found in untreated cells were observed. Oxidatively modified chylomicron remnants prepared by incubation with CuSO(4) (10 microM, 18 h, 37 degrees C) did not influence cholesteryl ester or phospholipid synthesis in MMH cells, but had a similar effect to that found with native remnants on triacylglycerol synthesis. These findings show that hepatic lipid metabolism is altered by exposure to mild oxidative stress and by lipids from the diet delivered to the liver in chylomicron remnants, and these effects may play a role in the development of atherosclerosis.

7-Ketocholesterol delivered to mice in chylomicron remnant-like particles is rapidly metabolised, excreted and does not accumulate in aorta

Cholesterol oxidation products (oxysterols) have been implicated in atherogenesis due to their presence in atherosclerotic tissue and their potent effects in vitro. One of the major oxysterols currently of interest is 7-ketocholesterol (7K) and it has been suggested that the diet is an important source of this oxysterol. This investigation tested the hypothesis that 7K, delivered in a physiologically relevant vehicle, chylomicron remnant-like emulsion (CMR), would be metabolised and excreted by mice in a similar manner and to a similar extent as previously observed in rats when delivered in a chemically modified lipoprotein, acetylated low-density lipoprotein (acLDL). Indeed, the metabolism of 14C-7K delivered in CMR mirrored that of acLDL and was much more rapid than (3)H-cholesterol delivered simultaneously. The 7K-derived (14)C was cleared from the liver, appeared in the intestine and was excreted in the faeces. A substantial proportion of the 7K-derived (14)C in the intestine and faeces was aqueous-soluble, indicating metabolism to polar products, presumably bile acids. Moreover, while cholesterol-derived (3)H increased in the aorta, (14)C appeared transiently and there was no observable accumulation within 24 h. The data confirm our previous findings of rapid hepatic metabolism of 7K when delivered in acLDL and demonstrate that 7K delivered in a vehicle of dietary significance is similarly metabolised and excreted. Indeed, the data encourage further investigation into the contribution that dietary oxysterols may or may not make to atherogenesis.

Apolipoprotein A-I, cyclodextrins and liposomes as potential drugs for the reversal of atherosclerosis. A review

Several studies have revealed that high-density lipoprotein (HDL) is the most reliable predictor for susceptibility to cardiovascular disease. Since apolipoprotein A-I (apoA-I) is the major protein of HDL, it is worthwhile evaluating the potential of this protein to reduce the lipid burden of lesions observed in the clinic. Indeed, apoA-I is used extensively in cell culture to induce cholesterol efflux. However, while there is a large body of data emanating from in-vitro and cell-culture studies with apoA-I, little animal data and scant clinical trials examining the potential of this apolipoprotein to induce cholesterol (and other lipid) efflux exists. Importantly, the effects of oxysterols, such as 7-ketocholesterol (7KC), on cholesterol and other lipid efflux by apoA-I needs to be investigated in any attempt to utilise apoA-I as an agent to stimulate efflux of lipids. Lessons may be learnt from studies with other lipid acceptors such as cyclodextrins and phospholipid vesicles (PLVs, liposomes), by combination with other effluxing agents, by remodelling the protein structure of the apolipoprotein, or by altering the composition of the lipoprotein intended for administration in-vivo. Akin to any other drug, the usage of this apolipoprotein in a therapeutic context has to follow the traditional sequence of events, namely an evaluation of the biodistribution, safety and dose-response of the protein in animal trials in advance of clinical trials. Mass production of the apolipoprotein is now a simple process due to the advent of recombinant DNA technology. This review also considers the potential of cyclodextrins and PLVs for use in inducing reverse cholesterol transport in-vivo. Finally, the potential of cyclodextrins as delivery agents for nucleic acid-based constructs such as oligonucleotides and plasmids is discussed.

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.

Lymphatic absorption of phytosterol oxides in rats

Two of the main classes of oxyphytosterols (7-keto and epoxides) were synthesized from sitosterol and campesterol and given to mesenteric duct-cannulated adult male rats. Lymph was collected during 24 h and was analyzed for oxysterols. The results showed that the lymphatic recovery of the phytosterol oxides was low: 4.7% of the given dose for epoxy derivatives and 1.5% for 7-keto compounds. The campesterol oxides presented a better absorption than the sitosterol oxides. During the process of absorption, the epoxyphytostanols were also partly transformed in campestanetriol and stigmastanetriol.

7-Ketocholesterol

7-Ketocholesterol is a major oxidation product of cholesterol found in human atherosclerotic plaque and is more atherogenic than cholesterol in some animal studies. 7-Ketocholesterol can inhibit cholesterol 7 alpha-hydroxylase, the rate-limiting step in bile acid biosynthesis, as well as strongly inhibiting HMG-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. It has even been suggested that 7-ketocholesterol is formed enzymically as an endogenous regulator of cholesterol biosynthesis. However, when tested as a pharmacological cholesterol-lowering agent, inhibition of HMG-CoA reductase was rapidly overcome and the 7-ketocholesterol metabolised. In vitro, 7-ketocholesterol has wide-ranging and potent effects, most of which have the potential to contribute to atherosclerosis. For example, 7-ketocholesterol can be cytotoxic and can induce apoptosis in vascular cells. These effects, either individually or more likely, in combination, all implicate 7-ketocholesterol in the initiation and development of atherosclerosis, but further work is needed to establish whether or not its role is a direct causal one.

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.

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.