Toxicity of oxysterols to human monocyte-macrophages

Research Progress of Design Drugs and Composite Biomaterials in Bone Tissue Engineering

Bone, like most organs, has the ability to heal naturally and can be repaired slowly when it is slightly injured. However, in the case of bone defects caused by diseases or large shocks, surgical intervention and treatment of bone substitutes are needed, and drugs are actively matched to promote osteogenesis or prevent infection. Oral administration or injection for systemic therapy is a common way of administration in clinic, although it is not suitable for the long treatment cycle of bone tissue, and the drugs cannot exert the greatest effect or even produce toxic and side effects. In order to solve this problem, the structure or carrier simulating natural bone tissue is constructed to control the loading or release of the preparation with osteogenic potential, thus accelerating the repair of bone defect. Bioactive materials provide potential advantages for bone tissue regeneration, such as physical support, cell coverage and growth factors. In this review, we discuss the application of bone scaffolds with different structural characteristics made of polymers, ceramics and other composite materials in bone regeneration engineering and drug release, and look forward to its prospect.

Different effects of oxysterols on a model lipid raft - Langmuir monolayer study complemented with theoretical calculations

Three oxysterols (7β-hydroxycholesterol; 7β-OH, 7-ketocholesterol; 7-K and 25-hydroxycholesterol, 25-OH) differing in the site of oxidation (ring system versus chain) and kind of polar group (hydroxyl versus carbonyl) were studied in lipid raft environment using the Langmuir monolayer technique complemented with theoretical calculations. Experiments were performed for the unmodified raft system, composed of sphingomyelin (SM) and cholesterol (Chol), and in the next step the raft was modified by the incorporation of oxysterol in different proportions. In the examined three-component system (Chol:SM:oxysterol), apart from interactions between the lipid raft components, the affinity of Chol to its oxidized derivatives also plays an important role. 25-OH was found to enhance interactions between SM and Chol and thus stabilize the raft, contrary to 7β-OH and 7-K, which exterted the fluidizing effect as well as the destabilization of the raft. Different action of oxysterols on model raft was observed. 7β-OH and 7-K, which are highly potent inducers of cell dath caused raft destabilization, while 25-OH, which is the least toxic of the investigated oxysterols, was found to stabilize the raft.

Development of a Biomaterial Scaffold Integrated with Osteoinductive Oxysterol Liposomes to Enhance Hedgehog Signaling and Bone Repair

Bone repair requires the tightly regulated control of multiple intrinsic and extrinsic cell types and signaling pathways. One of the positive regulatory signaling pathways in membranous and endochondral bone healing is the Hedgehog (Hh) signaling family. Here, a novel therapeutic liposomal delivery vector was developed by self-assembly of an Hh-activating cholesterol analog with an emulsifier, along with the addition of Smoothened agonist (SAG) as a drug cargo, for the enhancement of Hh signaling in bone regeneration. The drug-loaded nanoparticulate agonists of Hh signaling were immobilized onto trabecular bone-mimetic apatite-coated 3D scaffolds using bioinspired polydopamine adhesives to ensure favorable microenvironments for cell growth and local therapeutic delivery. Results showed that SAG-loaded liposomes induced a significant and dose-dependent increase in Hh-mediated osteogenic differentiation, as evidenced by in vitro analysis of bone marrow stromal cells, and in vivo calvarial bone healing, as evidenced using all radiographic parameters and histomorphometric analyses. Moreover, favorable outcomes were achieved in comparison to standards of care, including collagen sponge-delivered rBMP2 or allograft bone. In summary, this study demonstrates using a nanoparticle packaged Hh small molecule as a widely applicable bone graft substitute for robust bone repair.

Liver x receptor alpha drives chemoresistance in response to side-chain hydroxycholesterols in triple negative breast cancer

Triple negative breast cancer (TNBC) is challenging to treat successfully because targeted therapies do not exist. Instead, systemic therapy is typically restricted to cytotoxic chemotherapy, which fails more often in patients with elevated circulating cholesterol. Liver x receptors are ligand-dependent transcription factors that are homeostatic regulators of cholesterol, and are linked to regulation of broad-affinity xenobiotic transporter activity in non-tumor tissues. We show that LXR ligands confer chemotherapy resistance in TNBC cell lines and xenografts, and that LXRalpha is necessary and sufficient to mediate this resistance. Furthermore, in TNBC patients who had cancer recurrences, LXRalpha and ligands were independent markers of poor prognosis and correlated with P-glycoprotein expression. However, in patients who survived their disease, LXRalpha signaling and P-glycoprotein were decoupled. These data reveal a novel chemotherapy resistance mechanism in this poor prognosis subtype of breast cancer. We conclude that systemic chemotherapy failure in some TNBC patients is caused by co-opting the LXRalpha:P-glycoprotein axis, a pathway highly targetable by therapies that are already used for prevention and treatment of other diseases.

Targeting the alternative bile acid synthetic pathway for metabolic diseases

The gut microbiota is profoundly involved in glucose and lipid metabolism, in part by regulating bile acid (BA) metabolism and affecting multiple BA-receptor signaling pathways. BAs are synthesized in the liver by multi-step reactions catalyzed via two distinct routes, the classical pathway (producing the 12α-hydroxylated primary BA, cholic acid), and the alternative pathway (producing the non-12α-hydroxylated primary BA, chenodeoxycholic acid). BA synthesis and excretion is a major pathway of cholesterol and lipid catabolism, and thus, is implicated in a variety of metabolic diseases including obesity, insulin resistance, and nonalcoholic fatty liver disease. Additionally, both oxysterols and BAs function as signaling molecules that activate multiple nuclear and membrane receptor-mediated signaling pathways in various tissues, regulating glucose, lipid homeostasis, inflammation, and energy expenditure. Modulating BA synthesis and composition to regulate BA signaling is an interesting and novel direction for developing therapies for metabolic disease. In this review, we summarize the most recent findings on the role of BA synthetic pathways, with a focus on the role of the alternative pathway, which has been under-investigated, in treating hyperglycemia and fatty liver disease. We also discuss future perspectives to develop promising pharmacological strategies targeting the alternative BA synthetic pathway for the treatment of metabolic diseases.

The acidic pathway of bile acid synthesis: Not just an alternative pathway☆

Over the last two decades, the prevalence of obesity, and metabolic syndromes (MS) such as non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM), have dramatically increased. Bile acids play a major role in the digestion, absorption of nutrients, and the body's redistribution of absorbed lipids as a function of their chemistry and signaling properties. As a result, a renewed interest has developed in the bile acid metabolic pathways with the challenge of gaining insight into novel treatment approaches for this rapidly growing healthcare problem. Of the two major pathways of bile acid synthesis in the liver, the foremost role of the acidic (alternative) pathway is to generate and control the levels of regulatory oxysterols that help control cellular cholesterol and lipid homeostasis. Cholesterol transport to mitochondrial sterol 27-hydroxylase (CYP27A1) by steroidogenic acute regulatory protein (StarD1), and the subsequent 7α-hydroxylation of oxysterols by oxysterol 7α-hydroxylase (CYP7B1) are the key regulatory steps of the pathway. Recent observations suggest CYP7B1 to be the ultimate controller of cellular oxysterol levels. This review discusses the acidic pathway and its contribution to lipid, cholesterol, carbohydrate, and energy homeostasis. Additionally, discussed is how the acidic pathway's dysregulation not only leads to a loss in its ability to control cellular cholesterol and lipid homeostasis, but leads to inflammatory conditions.

Flavonoids Ability to Disrupt Inflammation Mediated by Lipid and Cholesterol Oxidation

Flavonoids are plant secondary metabolites that act as protectants against harmful effects of UV-B radiation inasmuch as biotic stress, conferring at the same time pigmentation of fruits and leaves [67]. The term "flavonoid" refers to phenolics having a basic skeleton of diphenylpropane (C6-C3-C6), which consists of two aromatic rings linked through three carbons that usually form an oxygenated heterocycle [25, 52]. Flavonoids are broken down into several different sub-categories based on their chemical structure. The main subclasses commonly found in food items are: flavonols, flavones, flavanones, flavan-3-ols, proanthocyanidins, and anthocyanins [44, 67]. Figure 19.1 depicts the major classification of flavonoids according to their chemical structure. Their occurrence in food matrices has been extensively reviewed [39, 44], and has been subject of extensive research in the last decades. Table 19.1 contains a few examples of compounds from each of the subcategory, with the fruit (berry) in which they are commonly found. The monomeric unit of flavonoids can dimerize and polymerize to form other important high molecular weight molecules; this is the case of proanthocyanidins, that are polymers of flavan-3-ols or flavanols. Not only do these compounds act as plant protectants, but they can also be very beneficial to human health. Cohorts studies performed in the early '90 have shown that dietary consumption of flavonoids was inversely associated with morbidity and mortality from coronary heart disease [31, 32]. These findings have opened an intensive field of research on the effects of flavonoids and flavonoids-rich food extracts in cardiovascular diseases (CVD) progression, particularly in the modulating CVD-associated oxidative stress and inflammation. In this short review, we will summarize the current findings in flavonoids beneficial effects in preventing CVD through inhibition of initial stages of CVD progression. Given the magnitude of scientific literature in the field, we will focus on two strictly mechanistic aspects: inhibition of chemical-induced LDL oxidation, and the effect of flavonoids in the monocyte/macrophages activation pathways.

Synthesis of side-chain oxysterols and their enantiomers through cross-metathesis reactions of Δ22 steroids

A synthetic route that utilizes a cross-metathesis reaction with Δ²² steroids has been developed to prepare sterols with varying C-27 side-chains. Natural sterols containing hydroxyl groups at the 25 and (25R)-26 positions were prepared. Enantiomers of cholesterol and (3β,25R)-26-hydroxycholesterol (27-hydroxycholesterol) trideuterated at C-19 were prepared for future biological studies.

CO2 Plant Extracts Reduce Cholesterol Oxidation in Fish Patties during Cooking and Storage

Cholesterol oxidation products (COPs) in foods may pose risks for human health. Suitable antioxidants can reduce the formation of COPs in industrial products. Consumer awareness of food additives has brought a need for more natural alternatives. This is the first study on the effects of supercritical CO₂ extracts of rosemary, oregano, and an antimicrobial blend of seven herbs, tested at two levels (1 and 3 g/kg fish), against cholesterol oxidation in patties made of a widely consumed fish species, Atlantic salmon (Salmo salar), during baking and storage. Cholesterol oxidation was reduced by the extracts as indicated by lowered levels of 7α-hydroxycholesterol, 7β-hydroxycholesterol, and 7-ketocholesterol, which were quantified by GC-MS. The total amount of COPs was smaller in all of the cooked samples containing the plant extracts (<1 μg/g extracted fat) than in the cooked control (14 μg/g). Furthermore, the plant extracts exhibited protective effects also during cold storage for up to 14 days.

Protein Thiol Redox Signaling in Monocytes and Macrophages

SIGNIFICANCE

Monocyte and macrophage dysfunction plays a critical role in a wide range of inflammatory disease processes, including obesity, impaired wound healing diabetic complications, and atherosclerosis. Emerging evidence suggests that the earliest events in monocyte or macrophage dysregulation include elevated reactive oxygen species production, thiol modifications, and disruption of redox-sensitive signaling pathways. This review focuses on the current state of research in thiol redox signaling in monocytes and macrophages, including (i) the molecular mechanisms by which reversible protein-S-glutathionylation occurs, (ii) the identification of bona fide S-glutathionylated proteins that occur under physiological conditions, and (iii) how disruptions of thiol redox signaling affect monocyte and macrophage functions and contribute to atherosclerosis. Recent Advances: Recent advances in redox biochemistry and biology as well as redox proteomic techniques have led to the identification of many new thiol redox-regulated proteins and pathways. In addition, major advances have been made in expanding the list of S-glutathionylated proteins and assessing the role that protein-S-glutathionylation and S-glutathionylation-regulating enzymes play in monocyte and macrophage functions, including monocyte transmigration, macrophage polarization, foam cell formation, and macrophage cell death.

CRITICAL ISSUES

Protein-S-glutathionylation/deglutathionylation in monocytes and macrophages has emerged as a new and important signaling paradigm, which provides a molecular basis for the well-established relationship between metabolic disorders, oxidative stress, and cardiovascular diseases.

FUTURE DIRECTIONS

The identification of specific S-glutathionylated proteins as well as the mechanisms that control this post-translational protein modification in monocytes and macrophages will facilitate the development of new preventive and therapeutic strategies to combat atherosclerosis and other metabolic diseases. Antioxid. Redox Signal. 25, 816-835.

Antibodies Against Various Forms of Mildly Oxidized Low-Density Lipoprotein Are Not Associated With Carotid Intima-Media Thickness in Patients With Primary Hyperlipidemia

The carotid intima-media thickness (IMT) can reflect early atherosclerosis. Oxidative modification of low-density lipoprotein (LDL) leads to the formation of several immunogenic epitopes and different forms of antibodies against oxidized LDL (oxLDL). We investigated the possible relationship between autoantibody titers against various forms of mildly oxLDL and carotid IMT in patients (n=100) with primary hyperlipidemia. Three different types of mildly oxidized LDL—oxLDLL, oxLDLP, and oxLDLD—were prepared at the end of lag, propagation, and decomposition phases of oxidation, respectively. Similar types of oxLDL were also prepared from the same LDL preparations after inactivation of the LDL-associated platelet-activating factor acetylhydrolase (PAF-AH). These types were denoted as oxLDL(-)L, oxLDL(-)P, and oxLDL(-)D. OxLDL types are primarily enriched in lysophosphatidylcholine (lyso-PC) due to hydrolysis of oxidized phospholipids (oxPL) by PAF-AH. OxLDL(-) types are mainly enriched in intact oxPL due to the inactivation of the LDL-associated PAF-AH before oxidation. IgG autoantibodies against all types of oxLDL were determined and IMT was evaluated ultrasonographically. IMT values were significantly associated with age, systolic blood pressure and serum triglyceride levels, whereas no correlation was found between IMT values and antibody titers against all types of either oxLDL or oxLDL(-). We suggest that autoantibodies against various types of mildly oxidized LDL enriched either in lyso-PC or in oxPL are not associated with the extent of carotid atherosclerosis. This supports the concept that extensively oxidized LDL enriched in aldehydes rather than mildly oxidized LDL may play a prominent role in the early stage of atherosclerosis.

Structure activity relationship studies on cytotoxicity and the effects on steroid receptors of AB-functionalized cholestanes

Structure-activity relationship analysis and profiling of a library of AB-functionalized cholestane derivatives closely related to brassinosteroids (BRs) were performed to examine their antiproliferative activities and activities on steroid hormone receptors. Some of the compounds were found to have strong cytotoxic activity in several human normal and cancer cell lines. The presence of a 3-hydroxy or 3-oxo group and 2,3-vicinal diol or 3,4-vicinal diol moiety were found to be necessary for optimum biological activity, as well as a six-membered B ring. According to the profiling of all steroid receptors in both agonist and antagonist mode, the majority of the cholestanes were weakly active or inactive compared to the natural ligands. Estrogenic activity was detected for two compounds, two compounds possessed antagonistic properties on estrogen receptors and seven compounds showed agonistic activity. Two active cholestane derivatives were shown to strongly influence cell viability, proliferation, cell cycle distribution, apoptosis and molecular pathways responsible for these processes in hormone-sensitive/insensitive (MCF7/MDA-MB-468) breast cancer cell lines.

The Reactive Oxygen Species in Macrophage Polarization: Reflecting Its Dual Role in Progression and Treatment of Human Diseases

High heterogeneity of macrophage is associated with its functions in polarization to different functional phenotypes depending on environmental cues. Macrophages remain in balanced state in healthy subject and thus macrophage polarization may be crucial in determining the tissue fate. The two distinct populations, classically M1 and alternatively M2 activated, representing the opposing ends of the full activation spectrum, have been extensively studied for their associations with several disease progressions. Accumulating evidences have postulated that the redox signalling has implication in macrophage polarization and the key roles of M1 and M2 macrophages in tissue environment have provided the clue for the reasons of ROS abundance in certain phenotype. M1 macrophages majorly clearing the pathogens and ROS may be crucial for the regulation of M1 phenotype, whereas M2 macrophages resolve inflammation which favours oxidative metabolism. Therefore how ROS play its role in maintaining the homeostatic functions of macrophage and in particular macrophage polarization will be reviewed here. We also review the biology of macrophage polarization and the disturbance of M1/M2 balance in human diseases. The potential therapeutic opportunities targeting ROS will also be discussed, hoping to provide insights for development of target-specific delivery system or immunomodulatory antioxidant for the treatment of ROS-related diseases.

Impairment of Macrophage Cholesterol Efflux by Cholesterol Hydroperoxide Trafficking: Implications for Atherogenesis Under Oxidative Stress

OBJECTIVE

Oxidative stress associated with cardiovascular disease can produce various oxidized lipids, including cholesterol oxides, such as 7-hydroperoxide (7-OOH), 7-hydroxide (7-OH), and 7-ketone (7=O). Unlike 7=O and 7-OH, 7-OOH is redox active, giving rise to the others via potentially toxic-free radical reactions. We tested the novel hypothesis that under oxidative stress conditions, steroidogenic acute regulatory (StAR) family proteins not only deliver cholesterol to/into mitochondria of vascular macrophages, but also 7-OOH, which induces peroxidative damage that impairs early stage reverse cholesterol transport.

APPROACH AND RESULTS

Stimulation of human monocyte-derived THP-1 macrophages with dibutyryl-cAMP resulted in substantial upregulation of StarD1 and ATP-binding cassette (ABC) transporter, ABCA1. Small interfering RNA-induced StarD1 knockdown before stimulation had no effect on StarD4, but reduced ABCA1 upregulation, linking the latter to StarD1 functionality. Mitochondria in stimulated StarD1-knockdown cells internalized 7-OOH slower than nonstimulated controls and underwent less 7-OOH-induced lipid peroxidation and membrane depolarization, as probed with C11-BODIPY (4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-inda-cene-3-undecanoic acid) and JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolylcarbocyanine iodide), respectively. Major functional consequences of 7-OOH exposure were (1) loss of mitochondrial CYP27A1 activity, (2) reduced 27-hydroxycholesterol (27-OH) output, and (3) downregulation of cholesterol-exporting ABCA1 and ABCG1. Consistently, 7-OOH-challenged macrophages exported less cholesterol to apoA-I or high-density lipoprotein than did nonchallenged controls. StarD1-mediated 7-OOH transport was also found to be highly cytotoxic, whereas 7=O and 7-OH were minimally toxic.

CONCLUSIONS

This study describes a previously unrecognized mechanism by which macrophage cholesterol efflux can be incapacitated under oxidative stress-linked disorders, such as chronic obesity and hypertension. Our findings provide new insights into the role of macrophage redox damage/dysfunction in atherogenesis.

Oxidised plant sterols as well as oxycholesterol increase the proportion of severe atherosclerotic lesions in female LDL receptor+/ − mice

Oxysterols (oxidised cholesterol) may play a role in the pathogenesis of CVD. Similar to cholesterol, plant sterols are susceptible to oxidation. However, less is known about the potential atherogenicity of oxidised plant sterols (oxyphytosterols). In the present study, the atherogenicity of a mixture of oxyphytosterols was examined by feeding female LDL receptor-deficient (LDLR+/ −) mice for 35 weeks a control diet (atherogenic high-fat diet; n 9), an oxysterol diet (control diet+0·025 % (w/w) oxysterols; n 12) or an oxyphytosterol diet (control diet+0·025 % (w/w) oxyphytosterols; n 12). In the LDLR+/ − mice, serum levels of cholesterol, lipoprotein profiles, cholesterol exposure and inflammatory markers at the end of the experiment were comparable between the three diet groups. Nevertheless, the proportion of severe atherosclerotic lesions was significantly higher after oxysterol (41 %; P= 0·004) and oxyphytosterol (34 %; P= 0·011) diet consumption than after control diet consumption (26 %). Oxyphytosterol levels in the lesions were the highest in the oxyphytosterol group. Here, we show that not only dietary oxysterols but also dietary oxyphytosterols increase the proportion of severe atherosclerotic lesions. This suggests that plant sterols when oxidised may increase atherosclerotic lesion severity instead of lowering the size and severity of lesions when fed in their non-oxidised form. Therefore, this finding might give an indication as to where to find the answer in the current hot debate about the potential atherogenicity of plant sterols. However, to what extent these results can be extrapolated to the human situation warrants further investigation.

The association between biomarkers in the blood and carotid plaque composition-focusing on oxidized lipids, oxysterols and plaque status

Human atherosclerotic plaque is composed of a large mixture of elements, predominantly lipids and oxidized lipids, lipid-loaded macrophages and smooth muscle cells, forming foam cells. Plaque contents undergo dynamic changes during the plaque's progression, being in a constant interaction with the circulating blood. During the mutual interaction between blood and plaque and the specific biochemical processes occurring in both, specific molecules can be generated in the serum which might provide information on plaque status. This information, mostly on plaque vulnerability, is highly important for making appropriate treatment decisions before neurological symptoms appear. The present review summarizes plaque contents, mostly lipids, oxidized lipids, oxidized products of cholesterol (oxysterols), and covers the recent literature on their association with biomarkers in the blood and on the possibility of using them for providing information on plaque status.

Brain endogenous liver X receptor ligands selectively promote midbrain neurogenesis

Liver X receptors (Lxrα and Lxrβ) are ligand-dependent nuclear receptors critical for ventral midbrain neurogenesis in vivo. However, no endogenous midbrain Lxr ligand has so far been identified. Here we used LC/MS and functional assays to identify cholic acid as a new Lxr ligand. Moreover, 24(S),25-epoxycholesterol (24,25-EC) was found to be the most potent and abundant Lxr ligand in the developing mouse midbrain. Both Lxr ligands promoted neural development in an Lxr-dependent manner in zebrafish in vivo. Notably, each ligand selectively regulated the development of distinct midbrain neuronal populations. Whereas cholic acid increased survival and neurogenesis of Brn3a-positive red nucleus neurons, 24,25-EC promoted dopaminergic neurogenesis. These results identify an entirely new class of highly selective and cell type-specific regulators of neurogenesis and neuronal survival. Moreover, 24,25-EC promoted dopaminergic differentiation of embryonic stem cells, suggesting that Lxr ligands may thus contribute to the development of cell replacement and regenerative therapies for Parkinson's disease.

Reactive oxygen species and thiol redox signaling in the macrophage biology of atherosclerosis

SIGNIFICANCE

Despite the recent decline in the prevalence of cardiovascular diseases, atherosclerosis remains the leading cause of death in industrialized countries. Monocyte recruitment into the vessel wall is a rate-limiting step in atherogenesis. Death of macrophage-derived foam cells promotes lesion progression and the majority of acute complications of atherosclerotic disease (e.g., myocardial infarction) occur in lesions that are intensely infiltrated with monocyte-derived macrophages, underlining the critical roles monocytes and macrophages play in this complex chronic inflammatory disease.

RECENT ADVANCES

A rapidly growing body of literature supports a critical role for reactive oxygen species (ROS) in the regulation of monocyte and macrophage (dys)function associated with atherogenesis and macrophage death in atherosclerotic plaque.

CRITICAL ISSUES

In this review we highlight the important roles of NADHP oxidase 4 recently identified in monocytes and macrophages and the role of ROS and (thiol) redox signaling in different aspects of monocytes and macrophage biology associated with atherosclerosis.

FUTURE DIRECTIONS

Studies aimed at identifying the intracellular targets of ROS involved in redox signaling in macrophages and at elucidating the redox signaling mechanisms that control differentiation, activation, polarization, and death of monocytes and macrophages may ultimately lead to the development of novel preventive and therapeutic strategies for atherosclerosis.

Liver X receptor β and peroxisome proliferator-activated receptor δ regulate cholesterol transport in murine cholangiocytes

Nuclear receptors (NRs) play crucial roles in the regulation of hepatic cholesterol synthesis, metabolism, and conversion to bile acids, but their actions in cholangiocytes have not been examined. In this study, we investigated the roles of NRs in cholangiocyte physiology and cholesterol metabolism and flux. We examined the expression of NRs and other genes involved in cholesterol homeostasis in freshly isolated and cultured murine cholangiocytes and found that these cells express a specific subset of NRs, including liver X receptor (LXR) β and peroxisome proliferator-activated receptor (PPAR) δ. Activation of LXRβ and/or PPARδ in cholangiocytes induces ATP-binding cassette cholesterol transporter A1 (ABCA1) and increases cholesterol export at the basolateral compartment in polarized cultured cholangiocytes. In addition, PPARδ induces Niemann-Pick C1-like L1 (NPC1L1), which imports cholesterol into cholangiocytes and is expressed on the apical cholangiocyte membrane via specific interaction with a peroxisome proliferator-activated response element (PPRE) within the NPC1L1 promoter.

CONCLUSION

We propose that (1) LXRβ and PPARδ coordinate NPC1L1/ABCA1-dependent vectorial cholesterol flux from bile through cholangiocytes and (2) manipulation of these processes may influence bile composition with important applications in cholestatic liver disease and gallstone disease, two serious health concerns for humans.

Aging-related changes in blood-brain barrier integrity and the effect of dietary fat

BACKGROUND

Disturbances in blood-brain barrier (BBB) integrity contribute to the onset and progression of neurodegenerative diseases including Alzheimer's disease (AD) and vascular dementia (VaD). Aging is positively associated with AD and VaD risk, but this may reflect comorbidities or the effects of other chronic modulators of vascular function such as diet.

OBJECTIVE

To explore putative synergistic effects of aging with diet, in this study genetically unmanipulated mice were maintained on diets enriched in saturated fatty acids (SFA) or cholesterol and compared to mice provided with low-fat (LF) feed formula.

METHODS

The functional integrity of the BBB was assessed following 3, 6 and 12 months of dietary intervention commenced at 6 weeks of age, by determining the brain parenchymal extravasation of immunoglobulin G (IgG).

RESULTS

Mice maintained on the SFA- or cholesterol-enriched diet showed significant parenchymal IgG abundance following 3 months of feeding, concomitant with diminished expression of the tight junction protein occludin. LF control mice had essentially no evidence of BBB disturbances. Six months of SFA feeding exacerbated the difference in IgG abundance compared to the LF mice. At 12 months of feeding, the control LF mice also had significant parenchymal IgG that was comparable to mice fed the SFA- or cholesterol-enriched diet for 3 months. However, there may have been an adaptation to the fat-enriched diets because SFA and cholesterol did not exacerbate IgG parenchymal accumulation beyond 6 months of feeding.

CONCLUSION

Collectively, the study suggests that diets enriched in SFA or cholesterol accelerate the onset of BBB dysfunction that otherwise occurs with aging.

Free cholesterol-induced cytotoxicity a possible contributing factor to macrophage foam cell necrosis in advanced atherosclerotic lesions

A major characteristic of advanced atherosclerotic lesions is the necrotic, or lipid, core, which likely plays an important role in the clinical progression of these lesions. Recent data suggest that the necrotic core forms primarily as a consequence of macrophage foam cell necrosis. Lesional macrophages initially accumulate mostly cholesteryl esters, but macrophages in advanced lesions contain large amounts of unesterified, or free, cholesterol (FC). Although there are many theories as to why macrophage foam cells die in advanced lesions, the fact that a high FC:phospholipid (PL) ratio in cellular membranes can be toxic to cells suggests that FC-induced cytotoxicity may contribute to foam cell necrosis. The mechanism of FC cytotoxicity can be explained by disturbances in membrane protein function as a result of "stiffening" of the bilayer and by formation of intracellular FC crystals that can cause physical damage to cellular organelles. Macrophages appear to respond to FC loading by a fascinating adaptive response, namely the induction of PL biosynthesis, which initially keeps the cellular FC:PL ratio below toxic levels. Studies with cultured macrophages have demonstrated that a failure of this adaptive response leads to FC-induced foam cell cytotoxicity and necrosis, and thus a similar series of events in advanced atherosclerotic lesions could provide an explanation for the development of the necrotic core. (Trends Cardiovasc Med 1997;7: 256-263). © 1997, Elsevier Science Inc.

Heme and haemoglobin direct macrophage Mhem phenotype and counter foam cell formation in areas of intraplaque haemorrhage

PURPOSE OF REVIEW

Several studies have recently shown that haemoglobin drives a novel macrophage subset that is protected from foam cell formation.

RECENT FINDINGS

In a previously overlooked area, two centres have independently shown that heme and haemoglobin drive an atheroprotective macrophage subset. We compare and contrast the approaches and findings of the laboratories and discuss some of the underlying biology and implications, concentrating on the aspects of lipidological relevance.

SUMMARY

Treatments based on direct heme-mimetics or other agonists of this pathway have enormous potential for linked antioxidant protection via heme oxygenase 1 and reduced foam cell formation via liver X receptor, a potent combination for treating atherosclerosis.

Lipid rafts: a signalling platform linking lipoprotein metabolism to atherogenesis

Lipid rafts are microdomains of the plasma membrane which are enriched in cholesterol and sphingolipids. They serve as a platform for signal transduction, in particular during immune and inflammatory responses. As hypercholesterolemia and inflammation are two key elements of atherogenesis, it is conceivable that the cholesterol and cholesterol oxide content of lipid rafts might influence the inflammatory signalling pathways, thus modulating the development of atherosclerosis. In support of this emerging view, lipid rafts have been shown to be involved in several key steps of atherogenesis, such as the oxysterol-mediated apoptosis of vascular cells, the blunted ability of high density lipoproteins (HDL) to exert anti-inflammatory effects, and the exacerbated secretion of pro-inflammatory cytokines by immune cells. Additional studies are now required to address the relative contribution of lipid raft abnormalities to the pathophysiology of atherosclerosis and cardiovascular disease.

Genetic connections between neurological disorders and cholesterol metabolism

Cholesterol is an essential component of both the peripheral and central nervous systems of mammals. Over the last decade, evidence has accumulated that disturbances in cholesterol metabolism are associated with the development of various neurological conditions. In addition to genetically defined defects in cholesterol synthesis, which will be covered in another review in this Thematic Series, defects in cholesterol metabolism (cerebrotendinous xanthomatosis) and intracellular transport (Niemann Pick Syndrome) lead to neurological disease. A subform of hereditary spastic paresis (type SPG5) and Huntington's disease are neurological diseases with mutations in genes that are of importance for cholesterol metabolism. Neurodegeneration is generally associated with disturbances in cholesterol metabolism, and presence of the E4 isoform of the cholesterol transporter apolipoprotein E as well as hypercholesterolemia are important risk factors for development of Alzheimer's disease. In the present review, we discuss the links between genetic disturbances in cholesterol metabolism and the above neurological disorders.

Mitochondrial cholesterol transporter, StAR, inhibits human THP-1 monocyte-derived macrophage apoptosis

Steroidogenic acute regulatory protein (StAR) plays an important role in the maintenance of intracellular lipid homeostasis. Macrophages are the key cellular player in the pathophysiology of atherosclerosis. Imbalance of macrophage lipid homeostasis causes cellular apoptosis, which is the key process in the initiation of atherosclerosis. The present study has investigated the effects of StAR in the apoptotic process of human THP-1 derived macrophages induced by serum withdrawal or Ox-LDL. Overexpression of StAR significantly decreased the number of apoptotic macrophages by decreasing the expression of pro-apoptotic genes Caspase-3 and Bax mRNA and protein levels, as well as through increasing expression of anti-apoptotic gene Bcl-2 mRNA and protein levels in the absence and presence of Ox-LDL. The results indicate that StAR plays an important role in macrophage and foam cell apoptotic processing, which may provide a potential method for preventing atherosclerosis.

Dietary fats, cerebrovasculature integrity and Alzheimer's disease risk

An emerging body of evidence is consistent with the hypothesis that dietary fats influence Alzheimer's disease (AD) risk, but less clear is the mechanisms by which this occurs. Alzheimer's is an inflammatory disorder, many consider in response to fibrillar formation and extracellular deposition of amyloid-beta (Abeta). Alternatively, amyloidosis could notionally be a secondary phenomenon to inflammation, because some studies suggest that cerebrovascular disturbances precede amyloid plaque formation. Hence, dietary fats may influence AD risk by either modulating Abeta metabolism, or via Abeta independent pathways. This review explores these two possibilities taking into consideration; (i) the substantial affinity of Abeta for lipids and its ordinary metabolism as an apolipoprotein; (ii) evidence that Abeta has potent vasoactive properties and (iii) studies which show that dietary fats modulate Abeta biogenesis and secretion. We discuss accumulating evidence that dietary fats significantly influence cerebrovascular integrity and as a consequence altered Abeta kinetics across the blood-brain barrier (BBB). Specifically, chronic ingestion of saturated fats or cholesterol appears to results in BBB dysfunction and exaggerated delivery from blood-to-brain of peripheral Abeta associated with lipoproteins of intestinal and hepatic origin. Interestingly, the pattern of saturated fat/cholesterol induced cerebrovascular disturbances in otherwise normal wild-type animal strains is analogous to established models of AD genetically modified to overproduce Abeta, consistent with a causal association. Saturated fats and cholesterol may exacerbate Abeta induced cerebrovascular disturbances by enhancing exposure of vessels of circulating Abeta. However, presently there is no evidence to support this contention. Rather, SFA and cholesterol appear to more broadly compromise BBB integrity with the consequence of plasma protein leakage into brain, including lipoprotein associated Abeta. The latter findings are consistent with the concept that AD is a dietary-fat induced phenotype of vascular dementia, reflecting the extraordinary entrapment of peripherally derived lipoproteins endogenously enriched in Abeta. Rather than being the initiating trigger for inflammation in AD, accumulation of extracellular lipoprotein-Abeta may be a secondary amplifier of dietary induced inflammation, or possibly, simply be consequential. Clearly, delineating the mechanisms by which dietary fats increase AD risk may be informative in developing new strategies for prevention and treatment of AD.

Marked accumulation of 27-hydroxycholesterol in SPG5 patients with hereditary spastic paresis

Patients with a recessively inherited "pure" hereditary spastic paresis (SPG5) have mutations in the gene coding for the oxysterol 7 alpha hydroxylase (CYP7B1). One of the expected metabolic consequences of such mutations is accumulation of oxysterol substrates due to decreased enzyme activity. In accordance with this, we demonstrate here that four patients with the SPG5 disease have 6- to 9-fold increased plasma levels of 27-hydroxycholesterol. A much higher increase, 30- to 50-fold, was found in cerebrospinal fluid. The plasma levels of 25-hydroxycholesterol were increased about 100-fold. There were no measurable levels of this oxysterol in cerebrospinal fluid. The pattern of bile acids in serum was normal, suggesting a normal bile acid synthesis. The findings are discussed in relation to two transgenic mouse models with increased levels of 27-hydroxy cholesterol in the circulation but without neurological symptoms: the cyp27a1 transgenic mouse and the cyp7b1 knockout mouse. The absolute plasma levels of 27-hydroxycholesterol in the latter models are, however, only about 20% of those in the SPG5 patients. If the accumulation of 27-hydroxycholesterol is an important pathogenetic factor, a reduction of its levels may reduce or prevent the neurological symptoms. A possible strategy to achieve this is discussed.

Cytotoxic and apoptotic effects of single and mixed oxides of beta-sitosterol on HepG2-cells

While health implications caused by cholesterol oxidation products (COPs) seem to be generally accepted, research on phytosterol oxidation products (POPs) is still limited. Since POPs are commercially not available knowledge on their toxic activities is mainly derived from blends instead of pure compounds. Therefore the aim of the present study was to examine the cytotoxicity of three individual oxidation products of beta-sitosterol, 7-ketositosterol, 7beta-OH-sitosterol, 7alpha-OH-sitosterol, a mixture of 6beta-OH-3-keto-sitosterol/6alpha-OH-3-keto-sitosterol (ratio 4:3) and a mixture of polar oxides towards HepG2-cells. All tested compounds were found to reduce cell viability in a significant and concentration dependent way, particularly 7-keto- and 7alpha-OH-sitosterol showed to be highly active. Only for 7-ketositosterol an increase in early apoptotic cells was observed. Enhancement of O(2)(-) production was assessed for all oxides, whereas malondialdehyd (MDA) levels were increased by 7-keto- and 7alpha-OH-sitosterol only. However, cell death did not appear to be necessarily dependent on the generation of oxidative stress. Further no DNA strand breaks were observed with the COMET assay. By assessing the accumulation of single oxidation products in the cells a link between higher proportions of oxides inside the cells and their cytotoxic potential could be found.

Human carotid atherosclerotic plaque increases oxidative state of macrophages and low-density lipoproteins, whereas paraoxonase 1 (PON1) decreases such atherogenic effects

Human atherosclerotic plaque contains a variety of oxidized lipids, which can facilitate further oxidation. Paraoxonase 1 (PON1) is a high-density lipoprotein (HDL)-associated esterase (lipolactonase), exhibiting antiatherogenic properties. The aims of the present study were to examine the oxidizing potency of the human carotid plaque lipid extract (LE), and the antiatherogenic role of PON1 on LE oxidation competence. Human carotid plaques were extracted by organic solvent, and the extract was incubated with lipoprotein particles, with macrophages, or with probes sensitive to oxidative stress, with or without preincubation with PON1, followed by oxidative-stress assessment. Our findings imply that the LE oxidized LDL, macrophages, and exogenous probes and decreases HDL-mediated cholesterol efflux from macrophages, in a dose-dependent manner. Incubation of PON1 with LE significantly affects LE composition, reduces LE atherogenic properties, and decreases the extract's total peroxide concentration by 44%, macrophage oxidation by 25%, and probe oxidation by up to 52%. We conclude that these results expand our understanding of how the plaque itself accelerates atherogenesis and provides an important mechanism for attenuation of atherosclerosis development by the antioxidant action of PON1 on the atherosclerotic plaque.

Oxysterols: novel biologic roles for the 21st century

A major focus for the 21st century are the sterol intermediates in cholesterol synthesis and their metabolites. No longer considered inactive way stations in their transformation to cholesterol, both physiologic and pathophysiologic studies, though early in their development, indicate novel biologic roles for these sterols, and their oxysterol metabolites that bypass cholesterol, the expected end product. A major impetus for further inquiry is the recognition that in genetically determined errors in cholesterol synthesis such as Smith-Lemil-Opitz syndrome, the phenotypic effects on the developing fetus are not solely attributable to the lack of cholesterol but the accumulation of 7-dehydrocholesterol and its 27-hydroxy metabolite. This view is now supported by a new mouse model, the double knockout Insig1 & 2 (insulin-induced genes 1 & 2) in which lack of the protein product results in a greater production of lanosterol compared to cholesterol during fetal life with severe dysmorphic consequences. Further support can be derived from in vitro studies of the Sonic hedgehog signaling pathway, essential for normal morphogenesis in the central nervous system and perhaps other organs, which may require the local presence of oxysterols for full expression. Future studies that can delineate the specific role of a sterol intermediate or its metabolite require a paradigm shift away from the generic use of oxysterols as a class of compounds to a focus on specific sterols that can be expected in tissues and techniques for mimicking the local environment. Another class of oxysterols are those arising by photoxidation, now considered to be an expected event generated by the photons of visible blue light and therefore pari passu with normal vision. The sequence of events from peroxides of cholesterol to hydroxy and keto derivatives is the signature of singlet oxygen as opposed to free radicals and other mechanisms for generating reactive oxygen species. Perhaps surprisingly, the retina expresses CYP 27A1 and CYP 46A1, enzymes with broad substrate specificity for ring-modified sterols, implying that, in addition to a rich blood supply for disposing of potentially toxic oxysterols, they can be detoxified locally. Recognition that the retina has nuclear receptors similar to those found in other tissues raises the possibility that the sterols that are generated may function in their traditional role as ligands for modulating gene expression but other, nonligand, activities can be expected since other proteins such as the oxysterol-binding proteins exist and are considered to have biologic activities. To critically evaluate these potentially new biologic roles for oxysterols a need exists for the synthesis and utilization of the expected naturally occurring metabolites rather than available surrogates that may not be truly representative of their tissue effects and to utilize analytical techniques that can identify their existence at the expected concentrations in tissues.

Role of redox regulation and lipid rafts in macrophages during Ox-LDL-mediated foam cell formation

Hyperlipidemias and small dense LDLs in patients with high-triglyceride low-HDL syndromes lead to a prolonged half life of apoB-containing particles. This is associated with reactive oxygen species (ROS) activation and leads to formation of oxidized LDL (Ox-LDL). Generators of ROS in macrophages (MACs) include myeloperoxidase (MPO)-mediated respiratory burst and raft-associated NADPH-oxidase. The intracellular oxidant milieu is involved in cellular signaling pathways, like ion-transport systems, protein phosphorylation, and gene expression. Lipid oxidation through ROS can amplify foam cell formation through Ox-LDL uptake, leading to formation of ceramide (Cer)-rich lipid membrane microdomains, and is associated with expansion of the lysosomal compartment and an upregulation of ABCA1 and other genes of the AP3 secretory pathway. Ox-LDL may also affect cell-surface turnover of Cer-backbone sphingolipids and apoE-mediated uptake by LRP-family members. In contrast, HDL-mediated lipid efflux causes disruption of lipid membrane microdomains and prevents foam cell formation. Oxidation of HDL through MPO leads to a failure of lipid efflux and enhancement of MAC loading. Therefore, lipid rafts and oxidation processes are important in regulation of MAC foam cell formation and atherosclerosis, and the balance between oxidant and antioxidant intracellular systems is critically important for efficient MAC function.

Potential to inhibit growth of atherosclerotic plaque development through modulation of macrophage neopterin/7,8-dihydroneopterin synthesis

The rise in plasma neopterin observed with increasing severity of vascular disease is a strong indicator of the inflammatory nature of atherosclerosis. Plasma neopterin originates as the oxidation product of 7,8-dihydroneopterin secreted by gamma-interferon stimulated macrophages within atherosclerotic plaques. Neopterin is increasingly being used as a marker of inflammation during clinical management of patients with a range of disorders including atherosclerosis. Yet the role of 7,8-dihydroneopterin/neopterin synthesis during the inflammatory process and plaque formation remains poorly understood and controversial. This is partially due to the unresolved role oxidants play in atherosclerosis and the opposing roles of 7,8-dihydroneopterin/neopterin. Neopterin can act as pro-oxidant, enhancing oxidant damage and triggering apoptosis in a number of different cell types. Neopterin appears to have some cellular signalling properties as well as being able to chelate and enhance the reactivity of transition metal ions during Fenton reactions. In contrast, 7,8-dihydroneopterin is also a radical scavenger, reacting with and neutralizing a range of reactive oxygen species including hypochlorite, nitric oxide and peroxyl radicals, thus protecting lipoproteins and various cell types including macrophages. This has led to the suggestion that 7,8-dihydroneopterin is synthesized to protect macrophages from the oxidants released during inflammation. The oxidant/antioxidant activity observed in vitro appears to be determined both by the relative concentration of these compounds and the specific chemistry of the in vitro system under study. How these activities might influence or modulate the development of atherosclerotic plaque in vivo will be explored in this review.

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.

Cellular toxicity of oxycholesterols

Oxycholesterols (OS) are formed from cholesterol or its immediate precursors by enzymatic or free radical action in vivo, or they may be derived from food. OS exhibit a wide spectrum of biological activities. In OS cytotoxicity, several mechanisms seem to be involved: e.g. inhibition of HMG-CoA reductase activity, antiproliferative action, apoptosis induction, replacement of cholesterol by OS in membranes followed by changes in cellular membrane structure and functionality, and immune system functions alteration. Furthermore, OS may be mutagenic and carcinogenic and may serve as intracellular signaling or regulatory molecules. Here we review OS cellular activities with special attention to the cytotoxic action in vivo and in vitro using experimental models.

Multiplexed flow cytometric analyses of pro- and anti-inflammatory cytokines in the culture media of oxysterol-treated human monocytic cells and in the sera of atherosclerotic patients

BACKGROUND

Some oxysterols are identified in atheromatous plaques and in plasma of atherosclerotic patients. We asked whether they might modulate cytokine secretion on human monocytic cells. In healthy and atherosclerotic subjects, we also investigated the relationships between circulating levels of C-reactive protein (CRP), conventional markers of hyperlipidemia, some oxysterols (7beta-hydroxycholesterol, 7-ketocholesterol, and 25-hydroxycholesterol), and various cytokines.

METHODS

Different flow cytometric bead-based assays were used to quantify some cytokines (IL-1beta, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12, IL-13, IL-17, G-CSF, GM-CSF, IFN-gamma, MCP-1, MIP-1beta, or TNF-alpha) in the culture media of oxysterol-treated U937 and THP-1 cells, and in the sera of healthy and atherosclerotic subjects. CRP and markers of hyperlipidemia were determined with routine analytical methods. Oxysterols were quantified by gas chromatography/mass spectrometry. Flow cytometric and biochemical methods were used to measure IL-8 mRNA levels, intracellular IL-8 content, and protein phosphorylation in the mitogenic extracellular kinase/extracellular signal-regulated kinase1/2 (MEK/ERK1/2) signaling pathway.

RESULTS

All oxysterols investigated are potent in vitro inducers of MCP-1, MIP-1beta, TNF-alpha, and/or IL-8 secretion, the latter involving the MEK/ERK1/2 cell signaling pathway. In healthy and atherosclerotic subjects, no relationships were found between cytokines (IL-8, IL-1beta, IL-6, IL-10, TNF-alpha, IL-12, and MCP-1), CRP, conventional markers of hyperlipidemia, and oxysterols. However, in patients with arterial disorders of the lower limbs, small but statistically significant differences in the circulating levels of CRP, TNF-alpha, and IL-10 were observed comparatively to healthy subjects and according to the atherosclerotic stage considered.

CONCLUSIONS

Flow cytometric bead-based assays are well adapted to measure variations of cytokine secretion in the culture media of oxysterol-treated cells and in the sera of healthy and atherosclerotic subjects. They underline the in vitro proinflammatory properties of oxysterols and may permit to distinguish healthy and atherosclerotic subjects, as well as various atherosclerotic stages.

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.

Acyl-coenzyme A:cholesterol acyltransferase promotes oxidized LDL/oxysterol-induced apoptosis in macrophages

7-Ketocholesterol (7KC) is a cytotoxic component of oxidized low density lipoproteins (OxLDLs) and induces apoptosis in macrophages by a mechanism involving the activation of cytosolic phospholipase A2 (cPLA2). In the current study, we examined the role of ACAT in 7KC-induced and OxLDL-induced apoptosis in murine macrophages. An ACAT inhibitor, Sandoz 58-035, suppressed 7KC-induced apoptosis in P388D1 cells and both 7KC-induced and OxLDL-induced apoptosis in mouse peritoneal macrophages (MPMs). Furthermore, compared with wild-type MPMs, ACAT-1-deficient MPMs demonstrated significant resistance to both 7KC-induced and OxLDL-induced apoptosis. Macrophages treated with 7KC accumulated ACAT-derived [14C]cholesteryl and [3H]7-ketocholesteryl esters. Tandem LC-MS revealed that the 7KC esters contained primarily saturated and monounsaturated fatty acids. An inhibitor of cPLA2, arachidonyl trifluoromethyl ketone, prevented the accumulation of 7KC esters and inhibited 7KC-induced apoptosis in P388D1 cells. The decrease in 7KC ester accumulation produced by the inhibition of cPLA2 was reversed by supplementing with either oleic or arachidonic acid (AA); however, only AA supplementation restored the induction of apoptosis by 7KC. These results suggest that 7KC not only initiates the apoptosis pathway by activating cPLA2, as we have reported previously, but also participates in the downstream signaling pathway when esterified by ACAT to form 7KC-arachidonate.

The liver X receptor-{beta} is essential for maintaining cholesterol homeostasis in the testis

The liver X receptor (LXR)alpha and -beta has been found to play a central role in maintaining cellular cholesterol homeostasis. In this study we comprehensively investigated the effect of deleting LXRalpha and -beta on testicular morphology and function. In the absence of LXRbeta, excessive cholesterol accumulated in the Sertoli cells from 2.5 months, resulting in severe cellular disruption and dysregulation of spermatogenesis by 10 months of age. This correlated with gene expression analyses that clearly indicated that LXRbeta was the dominant transcript in the testis Although the LXRalpha(-/-) testis was normal, the LXRalpha(-/-)beta(-/-) testis presented with a more severe phenotype than the LXRbeta(-/-) mice, and males were infertile by 4 months of age, indicating LXRalpha may partially rescue the testicular phenotype. Although Leydig cells did not accumulate excessive cholesterol, declining serum and intratesticular androgen levels with age suggested that these cells were in fact less functional. Treatment of a Sertoli cell line with the LXR agonist T0901317 led to increased expression of known LXR target genes like ATP binding cassette-G1 and sterol regulatory binding protein-1c; similar results were observed in wild-type testis after in vivo administration, suggesting the LXR is functioning in the same way as in other tissues. Ordinarily increased levels of cholesterol activate intracellular sensors to decrease these levels; however, the increasing amount of cholesterol in the Sertoli cells indicates improper control of cholesterol metabolism when LXRbeta is absent. Although the precise molecular mechanism at this time remains unclear, our study highlights the crucial role for LXRbeta in retaining cholesterol homeostasis in Sertoli cells.

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.

Oxidized LDL triggers phosphatidylserine exposure in human monocyte cell lines by both caspase-dependent and -independent mechanisms

Monocytic cell lines have been extensively used to characterize and model various features of the atherogenic process. We found striking differences in the apoptotic pathways of U937 cells and THP-1 cells exposed to copper-oxidized LDL. While phosphatidylserine exposure occurred in both lines, caspase activation was only apparent in the THP-1 cells. OxLDL caused caspase activity to decrease below that seen in untreated U937 cells, and this corresponded with a loss in intracellular thiols. In conclusion, exposure of U937 cells to oxLDL did not trigger a conventional apoptosis response, but still resulted in phosphatidylserine externalization.

Synthesis of long-chain polyunsaturated fatty acids is inhibited in vivo in hypercholesterolemic rabbits and in vitro by oxysterols

Plasma total lipids, total cholesterol (cholesterol esters and free cholesterol) and oxysterol (mainly 7 beta-hydroxycholesterol (7 beta OH)) concentrations were significantly elevated in New Zealand rabbits fed a 2% cholesterol-containing diet with respect to controls fed the same diet without cholesterol. In addition, linoleic (18:2 n-6) and alpha-linolenic acid (18:3 n-3) plasma concentrations were significantly elevated in hypercholesterolemic rabbits, while concentrations of long-chain n-6 and n-3 derivatives were reduced. Studies in monocytic cell line THP-1 revealed that 7 beta OH markedly inhibited the conversion of 18:2 to 20:4 n-6 and of 18:3 to 22:6 n-3, indicating depression of the desaturation steps; in particular the inhibition was greater for the Delta 5 desaturation step. Furthermore, experiments of Real-Time PCR showed that 5-10 microM 7 beta OH decreased the Delta 5 gene expression. In conclusion, atherogenic oxysterols interfere with the production of long-chain polyunsaturated fatty acids from their precursors both in hypercholesterolemic rabbits and in cultured cells.

Iron, atherosclerosis, and neurodegeneration: a key role for cholesterol in promoting iron-dependent oxidative damage?

This article reviews the roles and interactions of iron, atherosclerosis, and neurodegeneration. It highlights the importance of cholesterol in promoting iron-dependent oxidative damage. An intriguing possibility is that hypercholesterolemia can increase brain iron load and both the aggregation of beta-amyloid and the ability of iron on plaques to catalyze oxidative damage. This could explain why hypercholesterolemia is a risk factor for Alzheimer's disease. Further work is necessary to study the mechanism of increased iron transport across the blood brain barrier in atherosclerosis.

Mechanisms of resistance to the cytotoxic effects of oxysterols in human leukemic cells

We have developed hematopoietic cells resistant to the cytotoxic effects of oxysterols. Oxysterol-resistant HL60 cells were generated by continuous exposure to three different oxysterols-25-hydroxycholesterol (25-OHC), 7-beta-hydroxycholesterol (7beta-OHC) and 7-keto-cholesterol (7kappa-C). We investigated the effects of 25-OHC, 7beta-OHC, 7kappa-C and the apoptotic agent staurosporine on these cells. The effect of the calcium channel blocker nifedipine on oxysterol cytotoxicity was also investigated. Differential display and real-time PCR were used to quantitate gene expression of oxysterol-sensitive and -resistant cells. Our results demonstrate that resistance to the cytotoxic effects of oxysterols is relatively specific to the type of oxysterol, and that the cytotoxicity of 25-OHC but not that of 7beta-OHC and 7kappa-C, appears to occur by a calcium dependent mechanism. Oxysterol-resistant cells demonstrated no significant difference in the expression of several genes previously implicated in oxysterol resistance, but expressed the bcl-2 gene at significantly lower levels than those observed in control cells. We identified three novel genes differentially expressed in resistant cells when compared to HL60 control cells. Taken together, the results of this study reveal potentially novel mechanisms of oxysterol cytotoxicity and resistance, and indicate that cytotoxicity of 25-OHC, 7beta-OHC and 7kappa-C occur by independent, yet overlapping mechanisms.

Comparison of the cytotoxic effects of beta-sitosterol oxides and a cholesterol oxide, 7beta-hydroxycholesterol, in cultured mammalian cells

Phytosterols are plant sterols found in foods such as oils, nuts and vegetables. Phytosterols, in the same way as cholesterol, contain a double bond and are susceptible to oxidation. The objective of the present study was to assess the potential toxic effects of beta-sitosterol oxides on U937 cells. The effects of increasing concentrations (0-120 microm) of beta-sitosterol oxides on cellular cytotoxicity, apoptosis, antioxidant status and genotoxicity was assessed over 12, 24 and 48 h exposure periods. Following 12 h, the viability of cells treated with 120 microm-beta-sitosterol oxides was reduced to 51.7 % relative to control. At 24 and 48 h, both 60 and 120 microm-beta-sitosterol oxides caused a significant decrease in cell viability. For comparison, a decrease in viability of cells treated with a cholesterol oxide, 7beta-hydroxycholesterol (7beta-OH, 30 microm), was evident at 24 h. An increase in apoptotic cells, assessed using Hoechst 33342, indicates that the mode of cell death in U937 cells following exposure to 7beta-OH (30 microm) and beta-sitosterol oxides (60 and 120 microm) was by apoptosis. The increase in apoptotic cells after 12 h following treatment with 120 microm-beta-sitosterol oxides was accompanied by a decrease in cellular glutathione. Similarly, 7beta-OH (30 microm) treatment resulted in decreased glutathione at 12 h. Catalase activity was not affected by any of the treatments. beta-Sitosterol oxides had no genotoxic effects on U937 and V79 cells as assessed by the comet and sister chromatid exchange assays respectively. In general, the results indicate that thermally oxidised derivatives of beta-sitosterol demonstrate similar biological effects as 7beta-OH in U937 cells, but at higher concentrations.

Mildly oxidised LDL induces more macrophage death than moderately oxidised LDL: roles of peroxidation, lipoprotein-associated phospholipase A2and PPARγ

Death of macrophages and smooth muscle cells (SMC) can lead to progression of atherosclerosis. Mildly oxidised low-density lipoprotein (mildly-oxLDL) induced more overall death and apoptosis than moderately oxidised LDL, in human monocyte-macrophages (HMM). Mildly-oxLDL also induced more overall death in human SMC than did moderately-oxLDL. Mildly-oxLDL contained more hydroperoxides, but less oxysterol, malondialdehyde and negative charge than moderately-oxLDL. Specific inhibition of lipoprotein-associated phospholipase A(2) (by SB222657) diminished death induction in HMM by both oxLDL types. Peroxisome proliferator-activated receptor gamma (PPARgamma) antagonist (GW9662) and agonist (ciglitazone) experiments suggested that non-hydrolysed, oxidised phospholipids in oxLDL activate PPARgamma as a cellular defence mechanism. These results may be relevant to LDL oxidation within atherosclerotic plaques and may suggest strategies for combating atherosclerosis progression.

Cellular apoptosis is associated with increased caveolin-1 expression in macrophages

Macrophage apoptosis is an important factor in determining the efficiency of the immune response, atherosclerotic lesion stability, and clearance of aged cells by phagocytosis. The involvement of caveolin-1 in the regulation of apoptosis has been previously suggested in fibroblasts and epithelial cells. Here we show that treatment of thioglycollate-elicited mouse peritoneal macrophages with various unrelated apoptotic agents, including simvastatin, camptothecin, or glucose deprivation, is associated with a specific and large increase in caveolin-1 expression. In contrast, caveolin-2 levels remain unaffected. Induction of apoptosis was measured by changes in cell morphology, annexin V-labeling, and DNA fragmentation. We demonstrate that caveolin-1 in macrophages is present in lipid rafts and colocalizes with phosphatidylserine (PS) at the cell surface of apoptotic macrophages. Our data suggest that caveolin-1 increase is an early event, closely accompanied by PS externalization and independent of caspase activation and nuclear DNA fragmentation. The increase in caveolin-1 levels does not require new protein synthesis, as cycloheximide does not prevent the apoptosis-mediated increase in caveolin-1 levels. We propose that increased levels of caveolin-1 characterize the apoptotic phenotype of macrophages. Caveolin-1 may be involved in the efficient externalization of PS at the surface of the apoptotic cells.

Oxysterol-induced toxicity in R28 and ARPE-19 cells

Studies have shown an intimate relationship between cholesterol and retinal diseases; we examined the effects of cholesterol oxides on cultured cells. Using the rat retinal precursor cell line R28 and the human RPE cell line ARPE-19, we investigated the potential cytotoxicity of cholesterol oxides. Cultured R28 and ARPE-19 cells were treated with either 25-hydroxycholesterol and 7-ketocholesterol (0-50 microg/ml). Cell viability was determined by the WST-1 colorimetric assay. Production of reactive oxygen intermediate (ROI) was assessed by a fluorescent probe-based assay (2',7'-dichlorodihydrofluorescein diacetate [H2DCFDA]). To detect the presence of apoptosis, DNA fragmentation gel analysis and Hoescht nuclear staining were performed. Both cholesterol oxides tested were toxic in a time- and dose-dependent fashion to the two cell lines used in this study. Treatment of R28 cells with either 25-hydroxycholesterol or 7-ketocholesterol at a concentration of 25 micro/ml resulted in greater than 50% loss of cell viability after 24 h. ARPE-19 cells were slightly less affected, with a loss of cell viability of approximately 20% and 40% after 24 h-exposure of 25-hydroxycholesterol and 7-ketocholesterol, respectively. DNA fragmentation and chromatin condensation demonstrated apoptotic events occurring in 7-ketocholesterol-treated cells. The fluorescent assay for ROI production showed that after an hour of exposure to 7-ketocholesterol, R28 cells responded with increased levels of ROIs, whereas no immediate production of ROIs were detected with treated ARPE-19 cells. These in vitro findings provide evidence that cholesterol oxides can directly damage cultured retinal and RPE cells. The oxysterol-induced oxidative stress in these cells may be a factor in the pathology of retinal degenerative diseases.

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.