The effect of high density lipoprotein phospholipid acyl chain composition on the efflux of cellular free cholesterol

Omega 3 fatty acids promote macrophage reverse cholesterol transport in hamster fed high fat diet

The aim of this study was to investigate macrophage reverse cholesterol transport (RCT) in hamster, a CETP-expressing species, fed omega 3 fatty acids (ω3PUFA) supplemented high fat diet (HFD). Three groups of hamsters (n = 6/group) were studied for 20 weeks: 1) control diet: Control, 2) HFD group: HF and 3) HFD group supplemented with ω3PUFA (EPA and DHA): HFω3. In vivo macrophage-to-feces RCT was assessed after an intraperitoneal injection of (3)H-cholesterol-labelled hamster primary macrophages. Compared to Control, HF presented significant (p<0.05) increase in body weight, plasma TG (p<0.01) and cholesterol (p<0.001) with an increase in VLDL TG and in VLDL and LDL cholesterol (p<0.001). Compared to HF, HFω3 presented significant decrease in body weight. HFω3 showed less plasma TG (p<0.001) and cholesterol (p<0.001) related to a decrease in VLDL TG and HDL cholesterol respectively and higher LCAT activity (p<0.05) compared to HF. HFω3 showed a higher fecal bile acid excretion (p<0.05) compared to Control and HF groups and higher fecal cholesterol excretion (p<0.05) compared to HF. This increase was related to higher gene expression of ABCG5, ABCA1 and SR-B1 in HFω3 compared to Control and HF groups (<0.05) and in ABCG1 and CYP7A1 compared to HF group (p<0.05). A higher plasma efflux capacity was also measured in HFω3 using (3)H- cholesterol labeled Fu5AH cells. In conclusion, EPA and DHA supplementation improved macrophage to feces reverse cholesterol transport in hamster fed HFD. This change was related to the higher cholesterol and fecal bile acids excretion and to the activation of major genes involved in RCT.

Unraveling the complexities of the HDL lipidome

Plasma high density lipoproteins (HDL) are small, dense, protein-rich particles compared with other lipoprotein classes; roughly half of total HDL mass is accounted for by lipid components. Phospholipids predominate in the HDL lipidome, accounting for 40-60% of total lipid, with lesser proportions of cholesteryl esters (30-40%), triglycerides (5-12%), and free cholesterol (5-10%). Lipidomic approaches have provided initial insights into the HDL lipidome with identification of over 200 individual molecular lipids species in normolipidemic HDL. Plasma HDL particles, however, reveal high levels of structural, compositional, and functional heterogeneity. Establishing direct relationships between HDL structure, composition, and atheroprotective functions bears the potential to identify clinically relevant HDL subpopulations. Furthermore, development of HDL-based therapies designed to target beneficial subspecies within the circulating HDL pool can be facilitated using this approach. HDL lipidomics can equally contribute to the identification of biomarkers of both normal and deficient HDL functionality, which may prove useful as biomarkers of cardiovascular risk. However, numerous technical issues remain to be addressed in order to make such developments possible. With all technical questions resolved, quantitative analysis of the molecular components of the HDL lipidome will contribute to expand our knowledge of cardiovascular and metabolic diseases.

Spacing of integrin ligands influences signal transduction in endothelial cells

The physical attributes of the extracellular matrix play a key role in endothelium function by modulating the morphology and phenotype of endothelial cells. Despite the recognized importance of matrix-cell interactions, it is currently not known how the arrangement of adhesive ligands affects the morphology, signal transduction processes, and migration of endothelial cells. We aimed to study how endothelial cells respond to the average spatial arrangement of integrin ligands. We designed functionalized silicon surfaces with average spacing ranging from nanometers to micrometers of the peptide arginine-glycine-aspartic acid (RGD). We found that endothelial cells adhered to and spread on surfaces independently of RGD-to-RGD spacing. In contrast, organization within focal adhesions (FAs) was extremely sensitive to ligand spacing, requiring a nanoscaled average RGD spacing of 44 nm to form lipid raft domains at FAs. The localized membrane organization strongly correlated with the signaling efficiencies of integrin activation and regulated vascular endothelial growth factor (VEGF)-induced signaling events. Importantly, this modulation in signal transduction directly affected the migratory ability of endothelial cells. We conclude that endothelial cells sense nanoscaled variations in the spacing of integrin ligands, which in turn influences signal transduction processes. Average RGD spacing similar to that found in fibronectin leads to lipid raft accumulation at FAs, enhances sensitivity to VEGF stimulation, and controls migration in endothelial cells.

Lipid profiling of lipoproteins by electrospray ionization tandem mass spectrometry

Lipoproteins are of fundamental importance for the lipid transport and cardiovascular disease. The function and metabolism of lipoproteins is intimately linked to the biophysical properties of their surface lipids. Although a number of disease associations were found for lipid species in plasma, only a few studies reported lipid profiles of lipoproteins. Here, we provide an overview of techniques for lipoprotein separation, methods for lipid species analysis based on electrospray ionization tandem mass spectrometry (ESI-MS/MS) as well as data from recent lipidomic studies on lipoprotein fractions. We also discuss the different analytical strategies and how lipid profiling can expand our understanding of the biology and structures of lipoproteins.

Synthetic dimyristoylphosphatidylcholine liposomes assimilating into high-density lipoprotein promote regression of atherosclerotic lesions in cholesterol-fed rabbits

We have reported recently that enrichment of high-density lipoprotein (HDL) with phosphatidylcholine (PC) liposomes is effective in solubilizing cholesterol from isolated human atherosclerotic plaques. In the present study, we investigated the in vivo effect of enrichment of HDL with PC on regression of diet-induced atherosclerosis in rabbits. As part of the study, a preliminary in vitro study on blood collected from the cholesterol-fed rabbits was performed to assess the capacity of the HDL density ( d > 1.063 g/mL) plasma fraction from cholesterol-fed rabbits to assimilate multilamellar liposomes of synthetic dimyristoylphosphatidylcholine (DMPC). This was compared with the capacities of egg- and soy-PC liposomes to be assimilated into the HDL density plasma fraction. The capacity of the HDL density fraction to absorb PC from DMPC liposomes (11.5 mg/mL) was more than 10 times greater than egg or soy liposomes. Therefore, DMPC liposomes were chosen to infuse into cholesterol-fed rabbits. Cholesterol-fed rabbits infused weekly with DMPC liposomes (300 mg/kg body weight) for five weeks had significantly decreased aortic cholesterol contents ( P < 0.05) compared with saline-infused cholesterol-fed controls. Atherosclerotic plaque volume, as measured by a type of new magnetic resonance imaging analysis, also decreased significantly ( P < 0.05) after DMPC treatment. The present findings suggest that the enrichment of HDL with PC via intravenous infusion of synthetic DMPC liposomes could be a potential therapeutic approach for atherosclerotic plaque regression.

Composition and lipid spatial distribution of HDL particles in subjects with low and high HDL-cholesterol

A low level of high density lipoprotein cholesterol (HDL-C) is a powerful risk factor for cardiovascular disease. However, despite the reported key role of apolipo-proteins, specifically, apoA-I, in HDL metabolism, lipid molecular composition of HDL particles in subjects with high and low HDL-C levels is currently unknown. Here lipidomics was used to study HDL derived from well-characterized high and low HDL-C subjects. Low HDL-C subjects had elevated triacylglycerols and diminished lysophosphatidylcholines and sphingomyelins. Using information about the lipid composition of HDL particles in these two groups, we reconstituted HDL particles in silico by performing large-scale molecular dynamics simulations. In addition to confirming the measured change in particle size, we found that the changes in lipid composition also induced specific spatial distributions of lipids within the HDL particles, including a higher amount of triacylglycerols at the surface of HDL particles in low HDL-C subjects. Our findings have important implications for understanding HDL metabolism and function. For the first time we demonstrate the power of combining molecular profiling of lipoproteins with dynamic modeling of lipoprotein structure.

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

HDL lipids and insulin resistance

There is renewed interest in high-density lipoproteins (HDLs) due to recent findings linking atherosclerosis to the formation of dysfunctional HDL. This article focuses on the universe of HDL lipids and their potential protective or proinflammatory roles in vascular disease and insulin resistance. HDL carries a wide array of lipids including sterols, triglycerides, fat-soluble vitamins, and a large number of phospholipids, including phosphatidylcholine, sphingomyelin, and ceramide with many biological functions. Ceramide has been implicated in the pathogenesis of insulin resistance and has many proinflammatory properties. In contrast, sphingosine-1-phosphate, which is transported mainly in HDL, has anti-inflammatory properties that may be atheroprotective and may account for some of the beneficial effects of HDL. However, the complexity of the HDL lipidome is only beginning to reveal itself. The emergence of new analytical technologies should rapidly increase our understanding of the function of HDL lipids and their role in disease states.

Effects of cholesterol on thermal stability of discoidal high density lipoproteins

Reverse cholesterol transport in plasma involves variations in HDL cholesterol concentration. To understand physicochemical and functional implications of such variations, we analyzed stability of reconstituted HDL containing human apolipoproteins (apoA-I, apoA-II, or apoC-I), phosphatidylcholines varying in chain length (12-18 carbons) and unsaturation (0 or 1), and 0-35 mol% cholesterol. Lipoprotein heat denaturation was monitored by circular dichroism for protein unfolding/dissociation and by light scattering for particle fusion. We found that cholesterol stabilizes relatively unstable complexes; for example, incorporation of 10-30 mol% cholesterol in apoC-I:dimyristoyl phosphatidylcholine complexes increased their kinetic stability by deltaDeltaG* congruent with 1 kcal/mol. In more stable complexes containing larger proteins and/or longer-chain lipids, incorporation of 10% cholesterol did not significantly alter the disk stability; however, 15% or more cholesterol destabilized the apoA-I-containing complexes and led to vesicle formation. Thus, cholesterol tends to stabilize less stable lipoproteins, apparently by enhancing favorable packing interactions, but in more stable lipoproteins, where such interactions are already highly optimized, the stabilizing effect of cholesterol decreases and, eventually, becomes destabilizing. These results help uncouple the functional roles of particle stability and chain fluidity and suggest that structural disorder in HDL surface, rather than chain fluidity, is an important physicochemical determinant of HDL function.

Effect of lipid-modifying therapies on the functional quality of high-density lipoproteins: implications for drug development

BACKGROUND

Increasing interest has focused on the development of therapeutic strategies to promote the biological activity of high-density lipoproteins (HDL) to achieve more effective prevention of cardiovascular disease. The highly publicized failure of raising HDL cholesterol with the cholesteryl ester transfer protein inhibitor, torcetrapib, has fueled immense discussion with regard to the potential impact of lipid modifying therapies on the functional quality of HDL particles.

OBJECTIVE/METHOD

To review the literature that has investigated the role of HDL functionality in protection against cardiovascular disease.

CONCLUSION

It remains to be unequivocally demonstrated that therapies that directly target HDL are cardioprotective in humans. Increasing attention on the functional quality of HDL will be essentinal for developing new biomarkers and medical therapies.

Low trans structured fat from flaxseed oil improves plasma and hepatic lipid metabolism in apo E(-/-) mice

The objective of this study was to explicate the effects of feeding low trans structured fat from flaxseed oil (LF) on plasma and hepatic lipid metabolism involved in apo E(-/-) mice. The animals were fed a commercial shortening (CS), commercial low trans fat (CL) and LF diet based on AIN-76 diet (10% fat) for 12 weeks. LF supplementation exerted a significant suppression in hepatic lipid accumulation with the concomitant decrease in liver weight. The LF significantly lowered plasma total cholesterol and free fatty acid whereas it significantly increased HDL-C concentration and the HDL-C/total-C ratio compared to the CS group. Reduction of hepatic lipid levels in the LF group was related with the suppression of hepatic enzyme activities for fatty acid and triglyceride synthesis, and cholesterol regulating enzyme activity compared to the CS and CL groups. Accordingly, low trans structured fat from flaxseed oil is highly effective for improving hyperlipidemia and hepatic lipid accumulation in apo E(-/-) mice.

Correlation of structural stability with functional remodeling of high-density lipoproteins: the importance of being disordered

High-density lipoproteins (HDLs) are protein-lipid assemblies that remove excess cell cholesterol and prevent atherosclerosis. HDLs are stabilized by kinetic barriers that decelerate protein dissociation and lipoprotein fusion. We propose that similar barriers modulate metabolic remodeling of plasma HDLs; hence, changes in particle composition that destabilize HDLs and accelerate their denaturation may accelerate their metabolic remodeling. To test this notion, we correlate existing reports on HDL-mediated cell cholesterol efflux and esterification, which are obligatory early steps in cholesterol removal, with our kinetic studies of HDL stability. The results support our hypothesis and show that factors accelerating cholesterol efflux and esterification in model discoidal lipoproteins (including reduced protein size, reduced fatty acyl chain length, and/or increased level of cis unsaturation) destabilize lipoproteins and accelerate their fusion and apolipoprotein dissociation. Oxidation studies of plasma spherical HDLs show a similar trend: mild oxidation by Cu(2+) or OCl(-) accelerates cell cholesterol efflux, protein dissociation, and HDL fusion, while extensive oxidation inhibits these reactions. Consequently, moderate destabilization may be beneficial for HDL functions by facilitating insertion of cholesterol and lipophilic enzymes, promoting dissociation of lipid-poor apolipoproteins, which are primary acceptors of cell cholesterol, and thereby accelerating HDL metabolism. Therefore, HDL stability must be delicately balanced to maintain the structural integrity of the lipoprotein assembly and ensure structural specificity necessary for interactions of HDL with its metabolic partners, while facilitating rapid HDL remodeling and turnover at key junctures of cholesterol transport. The inverse correlation between HDL stability and remodeling illustrates the functional importance of structural disorder in macromolecular assemblies stabilized by kinetic barriers.

Lipid profiling of FPLC-separated lipoprotein fractions by electrospray ionization tandem mass spectrometry

Glycerophospholipid and sphingolipid species and their bioactive metabolites are important regulators of lipoprotein and cell function. The aim of the study was to develop a method for lipid species profiling of separated lipoprotein classes. Human serum lipoproteins VLDL, LDL, and HDL of 21 healthy fasting blood donors were separated by fast performance liquid chromatography (FPLC) from 50 microl serum. Subsequently, phosphatidylcholine (PC), lysophosphatidylcholine, sphingomyelin (SM), ceramide (CER), phosphatidylethanolamine (PE), PE-based plasmalogen (PE-pl), cholesterol, and cholesteryl ester (CE) content of the separated lipoproteins was quantified by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Analysis of FPLC fractions with PAGE demonstrated that albumin partially coelutes with HDL fractions. However, analysis of an HDL deficient serum (Tangier disease) showed that only lysophosphatidylcholine, but none of the other lipids analyzed, exhibited a significant coelution with the albumin containing fractions. Approximately 60% of lipoprotein CER were found in LDL fractions and 60% of PC, PE, and plasmalogens in HDL fractions. VLDL, LDL, and HDL displayed characteristic lipid class and species pattern. The developed method provides a detailed lipid class and species composition of lipoprotein fractions and may serve as a valuable tool to identify alterations of lipoprotein lipid species profiles in disease with a reasonable experimental effort.

Effects of acyl chain length, unsaturation, and pH on thermal stability of model discoidal HDLs

HDLs prevent atherosclerosis by removing excess cell cholesterol. Lipid composition affects HDL functions in cholesterol removal, yet its effects on the disk stability remain unclear. We hypothesize that reduced length or increased cis-unsaturation of phosphatidylcholine acyl chains destabilize discoidal HDL and promote protein dissociation and lipoprotein fusion. To test this hypothesis, we determined thermal stability of binary complexes reconstituted from apoC-I and diacyl PCs containing 12-18 carbons with 0-2 cis-double bonds. Kinetic analysis using circular dichroism shows that, for fully saturated PCs, chain length increase by two carbons stabilizes lipoprotein by deltaDeltaG* (37 degrees C) congruent with 1.4 kcal/mol, suggesting that hydrophobic interactions dominate the disk stability; distinct effects of pH and salt indicate contribution of electrostatic interactions. Similarly, apoA-I-containing disks show increased stability with increasing chain length. Acyl chain unsaturation reduces disk stability. In summary, stability of discoidal HDL correlates directly with fatty acyl chain length and saturation: the longer and more fully saturated are the chains, the more extensive are the stabilizing lipid-protein and lipid-lipid interactions and the higher is the free energy barrier for protein dissociation and lipoprotein fusion. This sheds new light on the existing data of cholesterol efflux to discoidal HDL and suggests that moderate lipoprotein destabilization facilitates cholesterol insertion.

Antiatherogenic activity of extracts of Argania spinosa L. pericarp: beneficial effects on lipid peroxidation and cholesterol homeostasisThis article is one of a selection of papers published in this special issue (part 1 of 2) on the Safety and Efficacy of Natural Health Products

Prevention of lipoprotein oxidation by natural compounds may prevent atherosclerosis via reducing early atherogenesis. In this study, we investigated for the first time the beneficial properties of methanolic extract of argania pericarp (MEAP) towards atherogenesis by protecting human low-density lipoprotein (LDL) against oxidation while promoting high-density lipoprotein (HDL)-mediated cholesterol efflux. By measuring the formation of malondialdehyde (MDA) and conjugated diene as well as the lag phase and the progression rate of lipid peroxidation, the MEAP was found to possess an inhibitory effect. In addition, MEAP reduced the rate of disappearance of α-tocopherol as well as the apoB electrophoretic mobility in a dose-dependent manner. These effects are related to the free radical scavenging and copper-chelating effects of MEAP. In terms of cell viability, MEAP has shown a cytotoxic effect (0–40 μg/mL). Incubation of3H-cholesterol-loaded J774 macrophages with HDL in the presence of increasing concentrations of MEAP enhanced HDL-mediated cholesterol efflux independently of ABCA1 receptor pathways. Our findings suggest that argania seed pericarp provides a source of natural antioxidants that inhibit LDL oxidation and enhance cholesterol efflux and thus can prevent development of cardiovascular diseases.

New targets of high-density lipoprotein therapy

PURPOSE OF REVIEW

Increasing attention has focused on the development of therapeutic strategies to promote the biologic activity of HDL particles, which possess a number of functional properties that contribute to their role in cardioprotection. Currently available therapies raise levels of HDL-cholesterol by relatively modest amounts. This review describes experimental strategies that promote HDL activity.

RECENT FINDINGS

The functional quality of HDL may be more important than the absolute level of HDL-cholesterol found in the systemic circulation. This is supported by the observation that small rises in HDL-cholesterol with current therapies is associated with clinical benefit. This has major implications for the development of new therapies. A number of therapeutic strategies have been developed that promote reverse cholesterol transport, inhibit inflammatory events in the vessel wall, and modify remodeling of HDL particles within the systemic circulation.

SUMMARY

A number of emerging therapies appear to promote the biologic activity of HDL. These agents can be administered as acute infusions in the setting of acute ischemic syndromes or as oral therapy for chronic prevention of cardiovascular disease.

Reverse cholesterol transport is regulated by varying fatty acyl chain saturation and sphingomyelin content in reconstituted high-density lipoproteins

Because phospholipid composition of high-density lipoprotein (HDL) plays a vital role in its reverse cholesterol transport (RCT) function, we studied RCT in vitro (uptake and efflux) with reconstituted HDLs (rHDLs) containing phosphatidylcholine (PC) with fatty acids of increasing saturation levels (stearic, oleic, linoleic, linolenic) and without or with sphingomyelin (SM). Uptake significantly increased from basal value when the PC component included up to 50 mol % of oleic or linolenic acid, but did not change with linoleic acid. Increasing oleic and linoleic acids to 100 mol % significantly decreased uptake, but increasing linolenic acid to the same value did not affect it. Sphingomyelin in rHDL significantly decreased uptake, but only with PC-containing unsaturated fatty acids, and not with saturated fatty acid. Efflux was not affected in a dose-dependent manner when oleic or linoleic acid content was increased, but was significantly increased with levels of linolenic acid up to 25 mol % in PC, and was dramatically lowered with higher levels. Sphingomyelin in rHDL (PC/SM, 20:80, mol/mol) significantly increased efflux only with oleic or linoleic acid-containing rHDLs, compared with efflux without SM. In conclusion, enrichment of PC component up to 25 mol % as linolenic acid has a beneficial effect on RCT, whereas a higher percentage of it or other unsaturated fatty acids seems to be detrimental. In addition, high SM content decreases uptake with rHDL-containing unsaturated fatty acids, whereas it increases efflux for rHDL-containing oleic or linoleic acid. These results show for the first time the importance of SM in RCT in a well-defined in vitro system.

Localization of Fatty Acids with Selective Chain Length by Imaging Time-of-Flight Secondary Ion Mass Spectrometry

Localization of fatty acids in biological tissues was made by using TOF-SIMS (time-of-flight secondary ion mass spectrometry). Two cell-types with a specific fatty acid distribution are shown. In rat cerebellum, different distribution patterns of stearic acid (C18:0), palmitic acid (C16:0), and oleic acid (C18:1) were found. Stearic acid signals were observed accumulated in Purkinje cells with high intensities inside the cell, but not in the nucleus region. The signals colocalized with high intensity signals of the phosphocholine head group, indicating origin from phosphatidylcholine or sphingomyelin. In mouse intestine, high palmitic acid signals were found in the secretory crypt cells together with high levels of phosphorylinositol colocalized in the crypt region. Palmitic acid was also seen in the intestinal lumen that contains high amounts of mucine, which is known to be produced in the crypt cells. Linoleic acid signals (C18:2) were low in the crypt region and high in the villus region. Oleic acid signals were seen in the villi and stearic acid signals were ubiquitous with no specific localization in the intestine. We conclude that the results obtained by using imaging TOF-SIMS are consistent with known brain and intestine biochemistry and that the localization of fatty acids is specific in differentiated cells.

Marrubium vulgare extract inhibits human-LDL oxidation and enhances HDL-mediated cholesterol efflux in THP-1 macrophage

The objective of the present study was to elucidate the beneficial properties of aqueous extracts of Marrubium vulgare (AEM) towards cardiovascular disease by protecting human-LDL against lipid peroxidation and promoting HDL-mediated cholesterol efflux. Human-LDL were oxidised by incubation with CuSO(4) in the presence of increased concentrations of AEM (0-100 microg/ml). LDL lipid peroxidation was evaluated by conjugated diene formation, vitamin E disappearance as well as LDL-electrophoretic mobility. HDL-mediated cholesterol efflux assay was carried out in human THP-1 macrophages. Incubation of LDL with AEM significantly prolonged the lag phase (P=0.014), lowered the progression rate of lipid peroxidation (P=0.004), reduced the disappearance of vitamin E and the electrophoretic mobility in a dose-dependent manner. Also, incubation of HDL with AEM significantly increased HDL-mediated cholesterol efflux from THP-1 macrophages implicating an independent ATP binding cassette A1 (ABCA1) pathways. Our findings suggest that M. vulgare provides a source of natural antioxidants, which inhibit LDL oxidation and enhance reverse cholesterol transport and thus can prevent cardiovascular diseases development. These antioxidant properties increase the anti-atherogenic potential of HDL.

Detergent-mediated phospholipidation of plasma lipoproteins increases HDL cholesterophilicity and cholesterol efflux via SR-BI

Cellular cholesterol efflux is an early, obligatory step in reverse cholesterol transport, the putative antiatherogenic mechanism by which human plasma high-density lipoproteins (HDL) transport cholesterol from peripheral tissue to the liver for recycling or disposal. HDL-phospholipid content is the essential cholesterol-binding component of lipoproteins and therefore a major determinant of cholesterol efflux. Thus, increased phospholipidation of lipoproteins, particularly HDL, is one strategy for increasing cholesterol efflux. This study validates a simple, new detergent perturbation method for the phospholipidation of plasma lipoproteins; we have quantified the cholesterophilicity of human plasma lipoproteins and the effects of lipoprotein phospholipidation on cholesterophilicity and cellular cholesterol efflux mediated by the class B type I scavenger receptor (SR-BI). We determined that low-density lipoproteins (LDL) are more cholesterophilic than HDL and that LDL has a higher affinity for phospholipids than HDL whereas HDL has a higher phospholipid capacity than LDL. Phospholipidation of total human plasma lipoproteins enhances cholesterol efflux, an effect that occurs largely through the preferential phospholipidation of HDL. We conclude that increasing HDL phospholipid increases its cholesterophilicity, thereby making it a better acceptor of cellular cholesterol efflux. Phospholipidation of lipoproteins by detergent perturbation is a simple way to increase HDL cholesterophilicity and cholesterol efflux in a way that may be clinically useful.

Phenolic-extract from argan oil (Argania spinosa L.) inhibits human low-density lipoprotein (LDL) oxidation and enhances cholesterol efflux from human THP-1 macrophages

Argan oil is rich in unsaturated fatty acids, tocopherol and phenolic compounds. These protective molecules make further study of its cardiovascular diseases (CVDs) action interesting. Furthermore, no previous study has explored the antioxidant activity of argan oil in comparison with olive oil. The present study was conducted to evaluate the beneficial properties of Virgin argan oil phenolic extracts (VAO-PE) towards CVD by: (A) protecting human (low-density lipoprotein, LDL) against lipid peroxidation and (B) promoting high-density lipoprotein (HDL)-mediated cholesterol efflux. Human LDLs were oxidized by incubation with CuSO(4) in the presence of different concentrations of VAO-PE (0-320mug/ml). LDL lipid peroxidation was evaluated by conjugated diene and MDA formation as well as Vitamin E disappearance. Incubation of LDL with VAO-PE significantly prolonged the lag-phase and lowered the progression rate of lipid peroxidation (P<0.01) and reduced the disappearance of Vitamin E in a concentration-dependent manner. Incubation of HDL with VAO-PE significantly increased the fluidity of the HDL phospholipidic bilayer (P=0.0004) and HDL-mediated cholesterol efflux from THP-1 macrophages. These results suggest that Virgin argan oil provides a source of dietary phenolic antioxidants, which prevent cardiovascular diseases by inhibiting LDL-oxidation and enhancing reverse cholesterol transport. These properties increase the anti-atherogenic potential of HDL.

Impact of Short-Term Administration of High-Density Lipoproteins and Atorvastatin on Atherosclerosis in Rabbits

OBJECTIVE

This study investigates effects of short-term administration of high-density lipoproteins (HDL) and a statin on atherosclerosis in cholesterol-fed rabbits. Effects of HDL apolipoprotein and phospholipid composition have also been investigated.

METHODS AND RESULTS

Aortic atherosclerosis was established over 17 weeks in 46 rabbits by balloon denudation and cholesterol feeding. During the past 5 days of the cholesterol-feeding period, animals received: (1) no treatment; (2) oral atorvastatin 5 mg/kg on each of the 5 days; or (3) infusions of HDL (8 mg/kg apolipoprotein A-I) on days 1 and 3 of the treatment phase. After euthanization, lesion size and composition were assessed by histological and immunohistochemical analysis. HDL (but not atorvastatin) reduced lesion size by 36% (P<0.05). The ratio of smooth muscle cells to macrophages in the lesions increased 2.6-fold in animals infused with HDL (P<0.05) and 4-fold in those receiving atorvastatin (P<0.01). HDL and atorvastatin reduced matrix metalloproteinase (MMP)-9 expression by 42% (P<0.05) and 45% (P<0.03), respectively. HDL increased thrombomodulin expression 2-fold (P<0.03). The beneficial effects on lesion area and plaque cellular composition were influenced by HDL phospholipid and apolipoprotein composition.

CONCLUSIONS

Infusing small amounts of HDL rapidly reduces lesion size and is comparable to atorvastatin in promoting a stable plaque phenotype.

Cysteine mutants of human apolipoprotein A-I: a study of secondary structural and functional properties

Apolipoprotein A-I(Milano) (A-I(M)) (R173C), a natural mutant of human apolipoprotein A-I (apoA-I), and five other cysteine variants of apoA-I at residues 52 (S52C), 74 (N74C), 107 (K107C), 129 (G129C), and 195 (K195C) were generated. Cysteine residues were incorporated in each of the various helices at the same helical wheel position as for the substitution in A-I(M). The secondary structural properties of the monomeric mutants, their abilities to bind lipid and to promote cholesterol efflux from THP-1 macrophages, and the possibility of antiperoxidation were investigated. Results showed that the alpha helical contents of all of the cysteine mutants were similar to that of wild-type apoA-I (wtapoA-I). The cysteine variant of A-I(M) at residue 173 [A-I(M)(R173C)] exhibited weakened structural stability, whereas A-I(G129C) a more stable structure than wtapoA-I. A-I(G129C) and A-I(K195C) exhibited significantly impaired capabilities to bind lipid compared with wtapoA-I. A-I(K107C) possessed a higher capacity to promote cholesterol efflux from macrophages than wtapoA-I, and A-I(M)(R173C) and A-I(K195C) exhibited an impaired efflux capability. Neither A-I(M)(R173C) nor any other cysteine mutant could resist oxidation against lipoxygenase. In summary, in spite of the similar mutant position on the helix, these variants exhibited different structural features or biological activities, suggesting the potential influence of the local environment of mutations on the whole polypeptide chain.

Kinetic stabilization and fusion of apolipoprotein A-2:DMPC disks: comparison with apoA-1 and apoC-1

Denaturation studies of high-density lipoproteins (HDL) containing human apolipoprotein A-2 (apoA-2) and dimyristoyl phosphatidylcholine indicate kinetic stabilization. Circular dichroism (CD) and light-scattering melting curves show hysteresis and scan rate dependence, indicating thermodynamically irreversible transition with high activation energy E(a). CD and light-scattering data suggest that protein unfolding triggers HDL fusion. Electron microscopy, gel electrophoresis, and differential scanning calorimetry show that such fusion involves lipid vesicle formation and dissociation of monomolecular lipid-poor protein. Arrhenius analysis reveals two kinetic phases, a slower phase with E(a,slow) = 60 kcal/mol and a faster phase with E(a,fast) = 22 kcal/mol. Only the fast phase is observed upon repetitive heating, suggesting that lipid-poor protein and protein-containing vesicles have lower kinetic stability than the disks. Comparison of the unfolding rates and the melting data recorded by differential scanning calorimetry, CD, and light scattering indicates the rank order for the kinetic disk stability, apoA-1 > apoA-2 > apoC-1, that correlates with protein size rather than hydrophobicity. This contrasts with the tighter association of apoA-2 than apoA-1 with mature HDL, suggesting different molecular determinants for stabilization of model discoidal and plasma spherical HDL. Different effects of apoA-2 and apoA-1 on HDL fusion and stability may reflect different metabolic properties of apoA-2 and/or apoA-1-containing HDL.

The spatial organization of apolipoprotein A-I on the edge of discoidal high density lipoprotein particles: a mass specrometry study

The three-dimensional structure of human apoA-I on nascent, discoidal HDL particles has been debated extensively over the past 25 years. Recent evidence has demonstrated that the alpha-helical domains of apoA-I are arranged in a belt-like orientation with the long axis of the helices perpendicular to the phospholipid acyl chains on the disc edge. However, experimental information on the spatial relationships between apoA-I molecules on the disc is lacking. To address this issue, we have taken advantage of recent advances in mass spectrometry technology combined with cleavable cross-linking chemistry to derive a set of distance constraints suitable for testing apoA-I structural models. We generated highly homogeneous, reconstituted HDL particles containing two molecules of apoA-I. These were treated with a thiol-cleavable cross-linking agent, which covalently joined Lys residues in close proximity within or between molecules of apoA-I in the disc. The cross-linked discs were then exhaustively trypsinized to generate a discrete population of peptides. The resulting peptides were analyzed by liquid chromatography/mass spectrometry before and after cleavage of the cross-links, and resulting peaks were identified based on the theoretical tryptic cleavage of apoA-I. We identified at least 8 intramolecular and 7 intermolecular cross-links in the particle. The distance constraints are used to analyze three current models of apoA-I structure. The results strongly support the presence of the salt-bridge interactions that were predicted to occur in the "double belt" model of apoA-I, but a helical hairpin model containing the same salt-bridge docking interface is also consistent with the data.

Transfer of cholesterol and diacylglycerol from lipophorin to Bombyx mori ovarioles in vitro: role of the lipid transfer particle

The objective of this study was to characterize the transfer of diacylglycerol (DAG) and cholesterol from larval Bombyx mori lipophorin to ovarioles. Transfer studies were carried out by incubating pupal ovarioles (5-day) with [(3)H]-cholesterol and [(3)H]-DAG-labeled lipophorin under different conditions. Transfer of both cholesterol and DAG exhibited hyperbolic dependency on lipophorin concentration with apparent Km values of 0.83 +/- 0.17 mg/ml and 0.74 +/- 0.16 mg/ml, respectively. Pretreatment of ovarioles with anti-lipid transfer particle (LTP) IgG significantly inhibited transfer of labeled DAG to ovarioles (75%) and not cholesterol. Injection of B. mori pupae (day 4) with anti-LTP IgG significantly affected the weight (65%), number of eggs (49%), amount of lipid (74%), and protein (65%) of the adult ovaries. Matured eggs had a very faint yellow color and deformed shape compared to controls. The inhibitory effect demonstrates the active role LTP plays in growth of ovaries, development, and oogenesis. The effect on vitellogenin shortage on egg development and maturation was determined by implanting ovaries in male recipients that lack vitellogenin. An 80% decline in egg production was observed. However, the mature eggs were normal in shape, color, and lipid content. Thus, restricting lipid or protein delivery to developing ovaries would dramatically affect choriogenesis.

The role of apolipoprotein A-I helix 10 in apolipoprotein-mediated cholesterol efflux via the ATP-binding cassette transporter ABCA1

Recent studies of Tangier disease have shown that the ATP-binding cassette transporter A1 (ABCA1)/apolipoprotein A-I (apoA-I) interaction is critical for high density lipoprotein particle formation, apoA-I integrity, and proper reverse cholesterol transport. However, the specifics of this interaction are unknown. It has been suggested that amphipathic helices of apoA-I bind to a lipid domain created by the ABCA1 transporter. Alternatively, apoA-I may bind directly to ABCA1 itself. To better understand this interaction, we created several truncation mutants of apoA-I and then followed up with more specific point mutants and helix translocation mutants to identify and characterize the locations of apoA-I required for ABCA1-mediated cholesterol efflux. We found that deletion of residues 221-243 (helix 10) abolished ABCA1-mediated cholesterol efflux from cultured RAW mouse macrophages treated with 8-bromo-cAMP. Point mutations in helix 10 that affected the helical charge distribution reduced ABCA1-mediated cholesterol efflux versus the wild type. We noted a strong positive correlation between cholesterol efflux and the lipid binding characteristics of apoA-I when mutations were made in helix 10. However, there was no such correlation for helix translocations in other areas of the protein as long as helix 10 remained intact at the C terminus. From these observations, we propose an alternative model for apolipoprotein-mediated efflux.

Probing red cell membrane cholesterol movement with cyclodextrin

We probed the kinetics with which cholesterol moves across the human red cell bilayer and exits the membrane using methyl-beta-cyclodextrin as an acceptor. The fractional rate of cholesterol transfer (% s(-1)) was unprecedented, the half-time at 37 degrees C being ~1 s. The kinetics observed under typical conditions were independent of donor concentration and directly proportional to acceptor concentration. The rate of exit of membrane cholesterol fell hyperbolically to zero with increasing dilution. The energy of activation for cholesterol transfer was the same at high and low dilution; namely, 27-28 Kcal/mol. This behavior is not consistent with an exit pathway involving desorption followed by aqueous diffusion to acceptors nor with a simple one-step collision mechanism. Rather, it is that predicted for an activation-collision mechanism in which the reversible partial projection of cholesterol molecules out of the bilayer precedes their collisional capture by cyclodextrin. Because the entire membrane pool was transferred in a single first-order process under all conditions, we infer that the transbilayer diffusion (flip-flop) of cholesterol must have proceeded faster than its exit, i.e., with a half-time of <1 s at 37 degrees C.

Characterization of cholesterol transport from midgut to fat body in Manduca sexta larvae

Using in vitro methods, we investigated the transfer of cholesterol from larval Manduca sexta midgut to the hemolymph lipoprotein, lipophorin, and the transfer of cholesterol from lipophorin to larval fat body. In the midgut, transfer of free cholesterol shows saturation kinetics, but the apparent Km is higher than the measured Kd for the midgut lipophorin-receptor complex. In addition, the transfer is unaffected by suramin, which binds to the receptor and inhibits lipophorin binding, and by antibodies to the lipid transfer particle, which is required for export of diacylglycerol from the midgut to lipophorin. In the fat body, transfer of free cholesterol also shows saturation kinetics, and the apparent Km is higher than the measured Kd for the fat body lipophorin-receptor complex. Suramin and anti-lipid transfer particle antibodies exert only a small (20%) inhibitory effect. In both tissues it seems that the most likely mode of cholesterol transfer is via aqueous diffusion, which is also an important mechanism in vertebrate cells. Based on these results, we propose that cholesterol homeostasis in larval M. sexta is maintained by a mass action mechanism in which cholesterol is freely transferred between lipophorin and tissues depending on the needs of the tissues. This simple mechanism is ideally suited to insects, which can neither make cholesterol nor internalize lipophorin, the two mechanisms that vertebrate cells use to control their cholesterol content.

Trans fatty acids affect lipoprotein metabolism in rats

This study was designed to investigate the effects of oleic (CIS), palmitic (SAT) and trans fatty acids (TRANS) on cholesterol metabolism. Rats fed the TRANS diet had lower plasma total cholesterol (P < 0.005) and non-HDL-cholesterol (non HDL-C) concentrations (P < 0.005) compared with their CIS-fed counterparts. Plasma HDL-C was highest in rats fed the SAT diet (P = 0.01). An in vivo assay of reverse cholesterol transport (RCT) was performed whereby radiolabeled cholesterol was delivered to the liver as acetylated LDL and the reappearance of label into plasma and HDL was determined. Plasma radioactivity in TRANS-fed rats was lower than in their SAT-fed counterparts (P = 0.01), and consistent with the cholesterol distribution in plasma, the difference was due to lower [(3)H]-cholesterol in lower density lipoproteins. Despite diet-induced differences in the cholesterol and phospholipid concentrations and fatty acid composition of HDL, the amount of label in HDL did not differ among groups, suggesting that consumption of these diets resulted in HDL populations with similar capacity to participate in RCT. The present findings suggest that dietary trans fatty acids regulate the metabolism of apolipoprotein B-containing lipoproteins in rats and that the effect may be masked in species possessing high plasma cholesteryl ester transfer protein (CETP) activity. These results reinforce the important role of CETP activity in determining the distribution of plasma cholesterol in response to dietary trans fatty acids.

Cholesterol efflux from larval Manduca sexta fat body in vitro: high-density lipophorin as the acceptor

The objective of this study was to characterize the transfer of cholesterol from Manduca sexta larvae fat body to high-density lipophorin. [(3)H]-Cholesterol-labeled fat body was incubated with lipophorin under different conditions and cholesterol transfer was determined. Transfer rate exhibited a hyperbolic dependency on lipophorin concentration with an apparent K(m) of 3.6 mg/ml, which is consistent with either an aqueous diffusion mechanism of cholesterol transfer or a receptor-mediated process. Several results, including the high K(m), the high activation energy, and the lack of Ca(2+) dependence favor aqueous diffusion model. In addition, anti-lipid transfer particle antibodies had only a small inhibitory effect, suggesting it is not involved in cholesterol transfer. However, the transfer was inhibited in the presence of suramin, which would be consistent with a receptor-mediated process. The effects of suramin may be complex because it can change membrane properties when bound to the lipophorin receptor and affect the rate of cholesterol desorption. The preponderance of data suggests that the export of cholesterol from fat body to lipophorin follows a simple aqueous diffusion pathway. Although we cannot completely exclude some contribution from a receptor-mediated pathway, it seems that if such a pathway were present, it represents a minor route.

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.

Release of cellular cholesterol: molecular mechanism for cholesterol homeostasis in cells and in the body

Most mammalian somatic cells are unable to catabolize cholesterol and therefore need to export it in order to maintain sterol homeostasis. This mechanism may also function to reduce excessively accumulated cholesterol, which would thereby contribute to prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) has been believed to play a main role in this reaction based on epidemiological evidence and in vitro experimental data. At least two independent mechanisms are identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from cell surface. Cholesterol molecules desorbed from cells can be trapped by various extracellular acceptors including various lipoproteins and albumin, and extracellular cholesterol esterification mainly on HDL may provide a driving force for the net removal of cell cholesterol by maintaining a cholesterol gradient between lipoprotein surface and cell membrane. The other is apolipoprotein-mediated process to generate new HDL by removing cellular phospholipid and cholesterol. The reaction is initiated by the interaction of lipid-free or lipid-poor helical apolipoproteins with cellular surface resulting in assembly of HDL particles with cellular phospholipid and incorporation of cellular cholesterol into the HDL being formed. Thus, HDL has dual functions as an active cholesterol acceptor in the diffusion-mediated pathway and as an apolipoprotein carrier for the HDL assembly reaction. The impairment of the apolipoprotein-mediated reaction was found in Tangier disease and other familial HDL deficiencies to strongly suggest that this is a main mechanism to produce plasma HDL. The causative mutations for this defect was identified in ATP binding cassette transporter protein A1, as a significant step for further understanding of the reaction and cholesterol homeostasis.

High density lipoprotein phospholipid composition is a major determinant of the bi-directional flux and net movement of cellular free cholesterol mediated by scavenger receptor BI

The role of high density lipoprotein (HDL) phospholipid in scavenger receptor BI (SR-BI)-mediated free cholesterol flux was examined by manipulating HDL(3) phosphatidylcholine and sphingomyelin content. Both phosphatidylcholine and sphingomyelin enrichment of HDL enhanced the net efflux of cholesterol from SR-BI-expressing COS-7 cells but by two different mechanisms. Phosphatidylcholine enrichment of HDL increased efflux, whereas sphingomyelin enrichment decreased influx of HDL cholesterol. Although similar trends were observed in control (vector-transfected) COS-7 cells, SR-BI overexpression amplified the effects of phosphatidylcholine and sphingomyelin enrichment of HDL 25- and 2.8-fold, respectively. By using both phosphatidylcholine-enriched and phospholipase A(2)-treated HDL to obtain HDL with a graded phosphatidylcholine content, we showed that SR-BI-mediated cholesterol efflux was highly correlated (r(2) = 0.985) with HDL phosphatidylcholine content. The effects of varying HDL phospholipid composition on SR-BI-mediated free cholesterol flux were not correlated with changes in either the K(d) or B(max) values for high affinity binding to SR-BI. We conclude that SR-BI-mediated free cholesterol flux is highly sensitive to HDL phospholipid composition. Thus, factors that regulate cellular SR-BI expression and the local modification of HDL phospholipid composition will have a large impact on reverse cholesterol transport.

Decreased cellular cholesterol efflux is a common cause of familial hypoalphalipoproteinemia: role of the ABCA1 gene mutations

BACKGROUND

High density lipoproteins (HDL) are complex lipoprotein particles involved in reverse cholesterol (C) transport and are negatively associated with the risk for coronary artery disease (CAD). We have described a disorder of familial HDL deficiency (FHD) due to abnormal cellular cholesterol efflux. In the present study, we investigated cellular cholesterol efflux on skin fibroblast from 15 probands with moderate to severe hypoalphalipoproteinemia, including one subject with Tangier disease (TD). We performed family studies on eight of these probands (269 individuals) with familial hypoalphalipoproteinemia (defined as a HDL-C <5th%, and with no known cause of HDL deficiency). We have previously shown that four of our FHD patients and patients with TD have mutations at the ABC1 gene, demonstrating that FHD is a heterozygous form of TD.

METHODS

On each subject, we carried out detailed biochemical analysis and determined apoA-I-mediated cellular cholesterol efflux using 3H-cholesterol labeled skin fibroblasts from study subjects compared with controls. TD has also been associated with abnormal cellular cholesterol efflux. Cell fusion experiments with polyethylene glycol (PEG) were carried out with fibroblasts from a subject with TD and one with FHD in order to determine whether the Tangier cells can complement the FHD defect. In all subjects with a reduced cellular cholesterol efflux, exons of the ABCA1 gene were sequenced.

RESULTS

Familial forms of HDL deficiency, defined as HDL-C levels <5th percentile, are a heterogeneous group of lipoprotein disorders. A reduced cellular cholesterol efflux has been identified in eight subjects from seven kindred (7/14 or 50% of probands tested), being reduced by a mean 59% of controls (range 49-63%). In four of these subjects, a mutation at the ABCA1 gene locus was identified. In three other subjects an efflux defect was idenfified but no critical mutation at the ABCA1 gene locus has been identified. In the remaining subjects, (7/14), no efflux defect was identified. Complementation studies reveal that the FHD defect is not corrected by Tangier cells, confirming that FHD and TD represent a spectrum of the same genetic defect.

CONCLUSION

Familial hypoalphalipoproteinemia syndromes are phenotypically heterogeneous; one form is associated with abnormal cellular cholesterol efflux caused by heterozygous mutations at the ABCA1 gene, that defines familial HDL Deficiency while homozygous mutations or compound heterozygocity causes TD. Other forms are primary hypoalphalipoproteinemia of unknown cause, while the remaining cases are associated with hypertriglyceridemia with or without elevated apoB levels. We conclude that a cellular cholesterol defect is a relatively frequent cause of familial HDL deficiency and that a mutation at the ABCA1 gene can be identified in half of these patients.

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.

Effect of dietary omega-3 fatty acids and chronic ethanol consumption on reverse cholesterol transport in rats

We previously showed that chronic ethanol feeding leads to a decrease of apolipoprotein E (apoE) in high-density lipoprotein (HDL), whereas supplementing this diet with 2.8% of total dietary calories as omega3-fatty acids (omega3FAs) restores HDL-apoE to the control values. Since HDL containing apoE plays a major role in reverse cholesterol transport (RCT), we measured the effects chronic ethanol intake and omega3-FAs on RCT in the present study. Four groups of rats, control normal fat (CN), alcohol-normal fat (AN), control omega3FA fat (CF), and alcohol-omega3FA fat (AF), were fed their respective diets for 8 weeks, after which hepatocytes and HDLs from each group were evaluated for RCT capacity (cholesterol efflux from macrophages and uptake by liver cells). Compared with the control diet (CN), chronic ethanol (AN) feeding inhibited the cholesterol efflux capacity of HDL by 21% (P < .01), whereas omega3FA feeding (2.8% of total dietary calories) stimulated this capacity by 79% (P < .01) and 25% (P < .01) in CF and AF rats, respectively. With respect to cholesterol uptake by the liver, there were no significant 3-way or 4-way interactions between the 4 factors, HDL-alcohol, HDL-fish oil, hepatocyte-alcohol, and hepatocyte-fish oil. The main effects for HDL-alcohol, HDL-fish oil, and hepatocyte-alcohol were all highly significant (P = .0001, .0001, and .007, respectively). There was a significant HDL-alcohol and HDL-fish oil interaction (P = .0001). Hepatocyte-alcohol was not a factor in any 2-way interactions. Our study indicates no evidence of an interaction between the effects of omega3FAs and the effects of alcohol on hepatocytes in terms of RCT function. Thus, feeding as little as 2.8% of the total dietary calories as omega3FA not only restored the impaired RCT function of HDL caused by chronic ethanol intake, but also enhanced by severalfold the ability of HDL to promote RCT even in normal animals.

Cholesterol efflux regulatory protein, Tangier disease and familial high-density lipoprotein deficiency

Cellular cholesterol efflux, by which cholesterol is transported from peripheral cells to HDL acceptor molecules for transport to the liver, is the first step of reverse cholesterol transport. Two genetic disorders, Tangier disease and some cases of familial HDL deficiency, have defects of cellular cholesterol efflux. The recent discovery of mutations in the ABC1 gene, which encodes the cholesterol efflux regulatory protein, in both these disorders establishes cholesterol efflux regulatory protein as a rate-limiting factor in reverse cholesterol transport.

Bioanalysis of PUFA metabolism and lipid peroxidation in coronary atherosclerosis

Twenty eight men (age 34-77 years) who underwent an elective coronary angiography for coronary artery disease (CAD), were studied. They were divided into group A (luminal narrowing < 50%; n = 11) and group B (luminal narrowing > 50%; n = 17). Capillary gas chromatography was used for determination of fatty acids. Retinol and alpha-tocopherol were analyzed by reversed-phase high-performance liquid chromatography (HPLC), other parameters were determined spectrofluorometrically and spectrophotometrically. Severe coronary atherosclerosis in group B was associated with higher serum low density lipoprotein/high density lipoprotein (LDL/HDL) cholesterol ratio, triacylglycerols, and phospholipids (P < 0.05). Erythrocyte membrane fatty acids C14:0, C16:1 and C22:6n3 were significantly higher in group B (P < 0.05). We found significantly higher plasma polyunsaturated fatty acids (PUFA) C18:3n6 in group B, whereas plasma linoleic acid was not changed significantly. There was a significant increase of IDL-C18:0, LDL-C14:0 and HDL-C22:6n3 PUFA in group B. We conclude that disturbances in saturated fatty acids (SUFA) and PUFA metabolism are associated with coronary atherogenesis. Such abnormalities may include enhanced extrahepatic transport of C14:0 SUFA via LDL and its incorporation into cell membranes, and enhanced clearance of anti atherosclerotic C22:6n3 PUFA via serum HDL.

Structural organization of the N-terminal domain of apolipoprotein A-I: studies of tryptophan mutants

Site-directed mutagenesis and detailed fluorescence studies were used to study the structure and dynamics of recombinant human proapolipoprotein (proapo) A-I in the lipid free state and in reconstituted high-density lipoprotein (rHDL) particles. Five different mutants of proapoA-I, each containing a single tryptophan residue, were produced in bacteria corresponding to each of the naturally occurring Trp residues (position -3 in the pro-segment, 8, 50, 72, and 108) in the N-terminal half of the protein. Structural analyses indicated that the conservative Phe-Trp substitutions did not perturb the conformation of the mutants with respect to the wild-type protein. Steady-state fluorescence studies indicated that all of the Trp residues exist in nonpolar environments that are highly protected from solvent in both the lipid-free and lipid-bound forms. Time-resolved lifetime and anisotropy studies indicated that the shape of the monomeric form of proapoA-I is a prolate ellipsoid with an axial ratio of about 6:1. In addition, the region surrounding Trp 108 appears to be more mobile than the rest of the protein in the lipid-free state. However, in rHDL particles, no significant domain motion was detected for any of the Trp residues. The results presented in this work are consistent with a model for monomeric lipid-free proapoA-I in which the N-terminal half of the molecule is organized into a bundle of helices.