Human apolipoprotein A-I and A-I mimetic peptides: potential for atherosclerosis reversal

PURPOSE OF REVIEW: Recent publications related to the potential use of apolipoprotein (apo)A-I and apoA-I mimetic peptides in the treatment of atherosclerosis are reviewed. RECENT FINDINGS: A preliminary report indicating that infusion of apoA-IMilano into humans once weekly for 5 weeks caused a significant decrease in coronary artery atheroma volume has sparked great interest in the potential therapeutic use of apoA-I. Recent studies have revealed that HDL quality (e.g. HDL apolipoprotein and lipid content, including oxidized lipids, particle size and electrophoretic mobility, associated enzymatic activities, inflammatory/anti-inflammatory properties, and ability to promote cholesterol efflux) may be more important than HDL-cholesterol levels. Therefore, when developing new strategies to raise HDL-cholesterol concentrations by interfering with HDL metabolism, one must consider the quality of the resulting HDL. In animal models, raising HDL-cholesterol levels by administering oral phospholipids improved both the quantity and quality of HDL and was associated with lesion regression. An apoA-I mimetic peptide, namely 4F synthesized from D-amino acids (D-4F), administered orally to mice did not raise HDL-cholesterol concentrations but promoted the formation of pre-beta HDL containing increased paraoxonase activity, resulting in significant improvements in HDL's anti-inflammatory properties and ability to promote cholesterol efflux from macrophages in vitro. Oral D-4F also promoted reverse cholesterol efflux from macrophages in vivo. SUMMARY: The quality of HDL may be more important than HDL-cholesterol levels. ApoA-I and apoA-I mimetic peptides appear to have significant therapeutic potential in atherosclerosis.     

HDL metabolism in context: looking on the bright side

PURPOSE OF REVIEW: To review new data concerning HDL metabolism and cardiovascular disease, the concept of HDL 'functionality', and HDL kinetics in the metabolic syndrome. RECENT FINDINGS: HDL-apoA-I and apoA-II may be better predictors of cardiovascular disease than HDL-cholesterol. Cholesteryl ester transfer protein inhibition with torcetrapib does not benefit cardiovascular disease; whether this is related to 'congestion' of HDL transport or a specific off-target vasopressor effect remains unclear. Accelerated catabolism of HDL particles in metabolic syndrome could be due to increased hepatic secretion of apoB and apoC-III, hepatic steatosis, and low plasma adiponectin. The role of serum amyloid A and homocysteine is uncertain. In metabolic syndrome, therapies that could favourably alter HDL transport include weight loss, fish oils, higher dose statins, and fibrates; 'balancing feedback' may offset reduced catabolism of HDL, fenofibrate being the only agent hitherto shown to increase apoA-I production. SUMMARY: Elevating HDL-apoA-I and apoA-II may be a more important therapeutic objective than increased HDL-cholesterol. Recent studies underscore the potential value of studying HDL functionality, particularly in the metabolic syndrome. Reverse cholesterol transport can only be reliably probed at present by studying the kinetics of HDL particles or apolipoproteins; new methods are needed for investigating cellular and whole body cholesterol turnover. In metabolic syndrome, HDL-raising therapies have differential impact on HDL kinetics, the optimal endpoint being to increase transport and concentration with unchanged or accelerated catabolism.     

High-density lipoproteins as therapeutic targets

PURPOSE OF REVIEW: The concentration of cholesterol in HDL is an inverse predictor of future cardiovascular disease, with evidence mounting that therapies that increase HDL concentration are antiatherogenic. The best known antiatherogenic function of HDL particles relates to their ability to promote the efflux of cholesterol from cells. However, they also have antioxidant, antiinflammatory and antithrombotic properties. RECENT FINDINGS: The past year has seen the publication of several papers that highlight a potential major protective role of HDL in states of acute inflammation. Papers showing extremely promising results using novel inhibitors of cholesteryl ester transfer protein as HDL-raising agents have also appeared. Finally, the discovery that ATP-binding cassette transporter G1 (ABCG1) transports cell cholesterol to large HDL particles in the extracellular space has largely reconciled apparent inconsistencies between basic research indicating that small, pre-beta-migrating HDL particles are the antiatherogenic components of HDL and epidemiological research that implicates larger HDL particles as the protective fraction. SUMMARY: The finding that ABCG1 promotes the efflux of cholesterol from cells to large HDL particles also provides powerful circumstantial evidence that cholesteryl ester transfer protein inhibition (which increases HDL size) may enhance, rather than reduce, cholesterol efflux, and thus inhibit the development of atherosclerosis.     

High-density lipoproteins： an emerging target in the prevention of cardiovascular disease

High-density lipoproteins （HDLs） have been well established to protect against the development of atherosclerotic cardiovascular disease. It has become apparent that in addition to the promotion of reverse cholesterol transport, HDLs possess a number of additional functional properties that may contribute to their beneficial influence on the arterial wall. A number of exciting therapeutic strategies have been developed that target HDL and its ability to protect against the development of atherosclerotic plaque. This paper will review how the promotion of the functional properties of HDL inhibits the formation of atherosclerotic plaque and stabilises lesions in patients with established disease.     

Dual effect of hypochlorite in the modification of high density lipoproteins

HDL-cholesterol levels are inversely correlated to the risk of cardiovascular disease. In recent years the concept that not only the quantity, but also the quality of HDL is related to their atheroprotective function has gained momentum. In fact several studies have showed that HDL can shift their properties from anti-atherogenic to pro-atherogenic upon chemical or enzymatic "modification". However, not all kind of modifications affect the antiatherogenic properties of HDL. For example, tyrosylation of HDL improves its ability to remove cholesterol from cultured cells and inhibits mice atherosclerotic lesion formation; oxidation of HDL(3) with 15-lipoxygenase or with copper ions for short time induce the formation of pre-β-migrating particles that are highly effective as cholesterol acceptors from lipid laden cells. Myeloperoxidase modifies HDL and apoA-I and reduces their ability to promote ABCA1-mediated cholesterol efflux. In the present study we show that modification with low concentration HOCl (a myeloperoxidase product) induces the formation of pre-β-migrating particles, thus improving the function of HDL in the reverse cholesterol transport, without affecting the anti-inflammatory activity. At higher HOCl concentration, pre-β-migrating particles were not detectable and the anti-inflammatory properties of HDL were lost. These findings suggest that during early phases of inflammation, when a low HOCl concentration is generated, changes in HDL occur that increase their ability to remove cholesterol and sparing anti-inflammatory properties; later during acute inflammation, when higher HOCl concentration are present changes in HDL occur that severely decrease their ability to remove cholesterol from macrophages and to protect endothelial cells from pro-inflammatory stimuli.     

HDL revisited: new opportunities for managing dyslipoproteinaemia and cardiovascular disease

Low concentrations of high-density lipoprotein (HDL) cholesterol constitute a risk factor for coronary heart disease (CHD). There is increasing evidence that increasing HDL-cholesterol levels reduces cardiovascular risk. The phenotype of low HDL cholesterol with or without elevated triglycerides is common and it is characteristic of patients with central obesity, insulin resistance, hypertension and type 2 diabetes mellitus; conditions associated with increased cardiovascular risk and are part of the rubric of the metabolic syndrome. Epidemiological, experimental and clinical trial evidence suggests that there is a good rationale for raising HDL-cholesterol in these and other high-risk patients. The protective effect of HDL-cholesterol against atherosclerosis and cardiovascular disease is mediated by both enhanced reverse cholesterol transport (RCT) and by direct anti-atherosclerotic mechanisms. Recent studies have elucidated mechanisms whereby HDL acts to reduce cardiovascular risk, supporting the rationale for targeting of HDL with lipid-modifying therapy. Ongoing investigation of mechanisms by which HDL acts to reduce the risk of atherosclerosis will provide opportunities for the development of new therapeutic strategies to decrease the risk of atherosclerosis.     

Influence of the HDL receptor SR-BI on atherosclerosis

The scavenger receptor class B, type I (SR-BI) is an HDL receptor that mediates selective cholesterol uptake from HDL to cells. In rodents, SR-BI has a critical influence on plasma HDL-cholesterol concentration and structure, the delivery of cholesterol to steroidogenic tissues, female fertility, and biliary cholesterol concentration. SR-BI can also serve as a receptor for non-HDL lipoproteins and appears to play an important role in reverse cholesterol transport. Recent studies involving the manipulation of SR-BI expression in mice, either using adenovirus-mediated or transgenic hepatic overexpression or using homologous recombination for complete functional ablation, indicate that the expression of SR-BI protects against atherosclerosis. If SR-BI has a similar activity in humans, it may become an attractive target for therapeutic intervention.     

The paradox of dysfunctional high-density lipoprotein

PURPOSE OF REVIEW: This review addresses how, in atherosclerosis or systemic inflammation, HDL can lose its usual atheroprotective characteristics and even paradoxically assume proinflammatory properties. RECENT FINDINGS: Specific chemical and structural changes within HDL particles can impede reverse cholesterol transport, enhance oxidation of LDL, and increase vascular inflammation. HDL may be viewed as a shuttle that can be either anti-inflammatory or proinflammatory, depending on its cargo of proteins, enzymes, and lipids. Some therapeutic approaches that reduce coronary risk, such as statins and therapeutic lifestyle changes, can favorably moderate the characteristics of proinflammatory HDL. In addition, apolipoprotein A-I mimetic peptides and other compounds that target functional aspects of HDL may offer novel approaches to reduction in cardiovascular risk. SUMMARY: Current data suggest that under some conditions HDL can become dysfunctional and even proinflammatory, but this characterization can change with resolution of systemic inflammation or use of certain treatments.     

Effect of obesity on high-density lipoprotein metabolism

Reduced levels of high-density lipoproteins (HDL) in non-obese and obese states are associated with increased risk for the development of coronary artery disease. Therefore, it is imperative to determine the mechanisms responsible for reduced HDL in obese states and, conversely, to examine therapies aimed at increasing HDL levels in these individuals. This paper examines the multiple causes for reduced HDL in obese states and the effect of exercise and diet--two non-pharmacologic therapies--on HDL metabolism in humans. In general, the concentration of HDL-cholesterol is adversely altered in obesity, with HDL-cholesterol levels associated with both the degree and distribution of obesity. More specifically, intra-abdominal visceral fat deposition is an important negative correlate of HDL-cholesterol. The specific subfractions of HDL that are altered in obese states include the HDL2, apolipoprotein A-I, and pre-beta1 subfractions. Decreased HDL levels in obesity have been attributed to both an enhancement in the uptake of HDL2 by adipocytes and an increase in the catabolism of apolipoprotein A-I on HDL particles. In addition, there is a decrease in the conversion of the pre-beta1 subfraction, the initial acceptor of cholesterol from peripheral cells, to pre-beta2 particles. Conversely, as a means of reversing the decrease in HDL levels in obesity, sustained weight loss is an effective method. More specifically, weight loss achieved through exercise is more effective at raising HDL levels than dieting. Exercise mediates positive effects on HDL levels at least partly through changes in enzymes of HDL metabolism. Increased lipid transfer to HDL by lipoprotein lipase and reduced HDL clearance by hepatic triglyceride lipase as a result of endurance training are two important mechanisms for increases in HDL observed from exercise.     

Dysfunctional HDL: A novel important diagnostic and therapeutic target in cardiovascular disease?

High density lipoprotein (HDL) has many properties, which contribute to its atheroprotective role. However, some recent clinical trials have identified subjects with the progression of atherosclerosis despite normal levels of HDL cholesterol. This raises the question if all subfractions of HDL have the same properties. Moreover, recent investigations have shown that both acute and chronic inflammation may lead to structural and functional changes of HDL, which render the particles proinflammatory. Although therapeutic agents that increase HDL levels are now quite well established it is not clear whether they influence HDL quality. We review the current state of knowledge on the properties of HDL and factors/therapeutic agents which may restrain the transformation of normal HDL into dysfunctional HDL.     

Effects of the acute phase response on the concentration and density distribution of plasma lipids and apolipoproteins

Longitudinal studies were carried out in the rabbit model to determine alterations in the concentration and density distribution of plasma lipids and apolipoproteins during the acute phase response (APR) characterized by elevated levels of C-reactive protein (CRP) and serum amyloid A (SAA). Twelve hr after the intramuscular injection of croton oil, SAA was detectable in high density lipoprotein (HDL). At the height of the response (72 hr), HDL decreased while SAA became the major HDL apoprotein, up to 80% of the proteins in the higher density fractions. The SAA-enriched particles became denser (density of HDL3) but larger (size of HDL2), had slower electrophoretic mobility, and were depleted in apoA-I, cholesterol, triglyceride, and phospholipid. HDL-cholesterol decreased and was redistributed to other fractions while apoA-I disappeared from the circulation. During this time plasma triglycerides increased 6- to 10-fold while plasma cholesterol and phospholipids showed minimal changes. ApoB increased 5- to 6-fold while the apoB-containing particles shifted to higher density resulting in elevated IDL and then LDL during recovery. VLDL (d less than 1.006 g/ml) increased and acquired 30-40% of the plasma triglycerides, cholesterol, phospholipid, and apoB. SAA also increased in VLDL while apoE decreased.     

Structural and functional basis for increased HDL-cholesterol levels due to the naturally occurring V19L mutation in human apolipoprotein A-I

Several hereditary point mutations in human apolipoprotein A-I (apoA-I) have been associated with low HDL-cholesterol levels and/or increased coronary artery disease (CAD) risk. However, one apoA-I mutation, the V19L, recently identified in Icelanders, has been associated with increased HDL-cholesterol levels and decreased CAD risk. In an effort to gain mechanistic insight linking the presence of this mutation in apoA-I with the increase of HDL-cholesterol levels we evaluated the effect of V19L mutation on the conformational integrity and functional properties of apoA-I in lipid-free and lipidated form. ApoA-I[V19L] was found to be thermodynamically destabilized in lipid-free form and displays an increased capacity to associate with phospholipids compared to WT apoA-I. When associated to reconstituted HDL (rHDL), apoA-I[V19L] was more thermodynamically stabilized than WT apoA-I. ApoA-I[V19L] displayed normal capacity to promote ABCA1-mediated cholesterol efflux and to activate the enzyme LCAT, in lipid-free and rHDL-associated forms, respectively. Additionally, rHDL-associated apoA-I[V19L] showed normal capacity to promote ABCG1-mediated cholesterol efflux, but 45% increased capacity to promote SR-BI-mediated cholesterol efflux, while the SR-BI-mediated HDL-lipid uptake was normal. Overall, our findings show that the apoA-I V19L mutation does not affect the first steps of HDL biogenesis pathway. However, the increased capacity of apoA-I[V19L] to associate with phospholipids, in combination with the enhanced thermodynamic stability of lipoprotein-associated apoA-I[V19L] and increased capacity of apoA-I[V19L]-containing lipoprotein particles to accept additional cholesterol by SR-BI could account for the increased HDL-cholesterol levels observed in human carriers of the mutation.     

High-density lipoproteins (HDL): Novel function and therapeutic applications

The failure of high-density lipoprotein (HDL)-raising agents to reduce cardiovascular disease (CVD) together with recent findings of increased cardiovascular mortality in subjects with extremely high HDL-cholesterol levels provide new opportunities to revisit our view of HDL. The concept of HDL function developed to explain these contradictory findings has recently been expanded by a role played by HDL in the lipolysis of triglyceride-rich lipoproteins (TGRLs) by lipoprotein lipase. According to the reverse remnant-cholesterol transport (RRT) hypothesis, HDL critically contributes to TGRL lipolysis via acquirement of surface lipids, including free cholesterol, released from TGRL. Ensuing cholesterol transport to the liver with excretion into the bile may reduce cholesterol influx in the arterial wall by accelerating removal from circulation of atherogenic, cholesterol-rich TGRL remnants. Such novel function of HDL opens wide therapeutic applications to reduce CVD in statin-treated patients, which primarily involve activation of cholesterol flux upon lipolysis.     

Effect of moderate physical training on serum lipids and lipoproteins

Total cholesterol, triglycerides and HDL-, HDL2-, HDL3-cholesterol were evaluated in 81 subjects, 49 of them practised moderate physical activity, 32 were controls. Total cholesterol and triglycerides were evaluated enzymatically. The Abell method was applied to analyse HDL-cholesterol and the HDL3 subclass. A significant difference was observed between the plasmatic triglycerides of the active group and the control, indicating that even moderate physical activity can be associated with lower concentrations of triglycerides. The correlation coefficients calculated for the HDL-cholesterol and the HDL2-cholesterol subclass, respectively, with some anthropometric and physiological variables were, in most of the cases, not significant. The highest number of significant correlations was found in the HDL3-cholesterol subclass, but in that of the control group only.     

Effects of apoA-V on HDL and VLDL metabolism in APOC3 transgenic mice

Apolipoprotein A-V (apoA-V) and apoC-III are exchangeable constituents of VLDL and HDL. ApoA-V counteracts the effect of apoC-III on triglyceride (TG) metabolism with poorly defined mechanisms. To better understand the effects of apoA-V on TG and cholesterol metabolism, we delivered apoA-V cDNA into livers of hypertriglyceridemic APOC3 transgenic mice by adenovirus-mediated gene transfer. In response to hepatic apoA-V production, plasma TG levels were reduced significantly as a result of enhanced VLDL catabolism without alternations in VLDL production. This effect was associated with reduced apoC-III content in VLDL. Increased apoA-V production also resulted in decreased apoC-III and increased apoA-I content in HDL. Furthermore, apoA-V-enriched HDL was associated with enhanced LCAT activity and increased cholesterol efflux. This effect, along with apoE enrichment in HDL, contributed to HDL core expansion and alpha-HDL formation, accounting for significant increases in both the number and size of HDL particles. As a result, apoA-V-treated APOC3 transgenic mice exhibited decreased VLDL-cholesterol and increased HDL-cholesterol levels. ApoA-V-mediated reduction of apoC-III content in VLDL represents an important mechanism by which apoA-V acts to ameliorate hypertriglyceridemia in adult APOC3 transgenic mice. In addition, increased apoA-V levels accounted for cholesterol redistribution from VLDL to larger HDL particles. These data suggest that in addition to its TG-lowering effect, apoA-V plays a significant role in modulating HDL maturation and cholesterol metabolism.     

Genetics of HDL regulation in humans

PURPOSE OF REVIEW: To review gene regulation of HDL-cholesterol and discuss molecular abnormalities in HDL candidate genes that may lead to human pathologic states. RECENT FINDINGS: The inverse association between HDL-cholesterol and vascular disease, especially coronary heart disease, has long been recognized, but understanding gene regulation of HDL in humans gained considerable momentum following the identification of ABCA1 as playing a pivotal role in reverse cholesterol transport. Recent data suggest that potentially important targets for upregulating HDL in humans include upregulators of ABCA1 and APOA1 (e.g. peroxisome proliferator activated receptor and liver X receptor agonists) and downregulators of CETP (e.g. JTT-705). A host of other nuclear receptors under investigation in animal models may advance to human testing in the near future. SUMMARY: Disorders affecting HDL metabolism are complex because monogenic disorders causing low HDL do not necessarily correlate with premature vascular disease. To date, pathologic phenotypes have only been deduced among several HDL candidate genes. Understanding the genetic underpinnings associated with variant HDL and reverse cholesterol transport provides an exceptional opportunity to identify novel agents that may optimize this process and reduce vascular event rates beyond currently available LDL lowering therapies.     

Effects of high-density lipoprotein(2) on cholesterol transport and acyl-coenzyme A:cholesterol acyltransferase activity in P388D1 macrophages

High-density lipoproteins are the putative vehicles for cholesterol removal from monocyte-derived macrophages, which are an important cell type in all stages of atherosclerosis. The role of HDL(2), an HDL subclass that accounts for most variation in plasma HDL-cholesterol concentration, in cholesterol metabolism in monocyte-derived macrophages is not known. In this study, the dose-dependent effects of HDL(2) on cellular cholesterol mass, efflux, and esterification, and on cellular cholesteryl ester (CE) hydrolysis using the mouse macrophage P388D1 cell line was investigated. HDL(2) at low concentrations (40 microg protein/ml) decreased CE content without affecting cellular free cholesterol content (FC), CE hydrolysis, or cholesterol biosynthesis. In addition, HDL(2) at low concentrations reduced cellular acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity and increased FC efflux from macrophages. Thus, HDL(2) has two potential roles in reverse cholesterol transport. In one, HDL(2) is an acceptor of macrophage FC. In the other, more novel role, HDL(2) increases the availability of macrophage FC through the inhibition of ACAT. Elucidation of the mechanism by which HDL(2) inhibits ACAT could identify new therapeutic targets that enhance the transfer of cholesterol from macrophages to the liver.     

Emerging role of HDL in brain cholesterol metabolism and neurodegenerative disorders

High-density lipoproteins (HDL) play a key role in cholesterol homeostasis maintenance in the central nervous system (CNS), by carrying newly synthesized cholesterol from astrocytes to neurons, to support their lipid-related physiological functions. As occurs for plasma HDL, brain lipoproteins are assembled through the activity of membrane cholesterol transporters, undergo remodeling mediated by specific enzymes and transport proteins, and finally deliver cholesterol to neurons by a receptor-mediated internalization process. A growing number of evidences indicates a strong association between alterations of CNS cholesterol homeostasis and neurodegenerative disorders, in particular Alzheimer's disease (AD), and a possible role in this relationship may be played by defects in brain HDL metabolism. In the present review, we summarize and critically examine the current state of knowledge on major modifications of HDL and HDL-mediated brain cholesterol transport in AD, by taking into consideration the individual steps of this process. We also describe potential and encouraging HDL-based therapies that could represent new therapeutic strategies for AD treatment. Finally, we revise the main plasma and brain HDL modifications in other neurodegenerative disorders including Parkinson's disease (PD), Huntington's disease (HD), and frontotemporal dementia (FTD).     

Extracellular hydrophobic regions in scavenger receptor BI play a key role in mediating HDL-cholesterol transport

The binding of high density lipoprotein (HDL) to scavenger receptor BI (SR-BI) is responsible for whole-body cholesterol disposal via reverse cholesterol transport. The extracellular domain of SR-BI is required for HDL binding and selective uptake of HDL-cholesterol. We identified six highly hydrophobic regions in this domain that may be important for receptor activity and performed site-directed mutagenesis to investigate the importance of these regions in SR-BI-mediated cholesterol transport. Non-conservative mutation of the regions encompassing V67, L140/L142, V164 or V221 reduced hydrophobicity and impaired the ability of SR-BI to bind HDL, mediate selective uptake of HDL-cholesterol, promote cholesterol efflux, and enlarge the cholesterol oxidase-sensitive pool of membrane free cholesterol. In contrast, conservative mutations at V67, V164 or V221 did not affect the hydrophobicity or these cholesterol transport activities. We conclude that the hydrophobicity of N-terminal extracellular regions of SR-BI is critical for cholesterol transport, possibly by mediating receptor-ligand and/or receptor-membrane interactions.     

P2Y13 Receptor Regulates HDL Metabolism and Atherosclerosis In Vivo

High-density lipoprotein (HDL) is known to protect against atherosclerosis by promoting the reverse cholesterol transport. A new pathway for the regulation of HDL-cholesterol (HDL-c) removal involving F1-ATPase and P2Y13 receptor (P2Y13R) was described in vitro, and recently in mice. However, the physiological role of F1-ATPase/P2Y13R pathway in the modulation of vascular pathology i.e. in the development of atherosclerotic plaques is still unknown. We designed a specific novel agonist (CT1007900) of the P2Y13R that caused stimulation of bile acid secretion associated with an increased uptake of HDL-c in the liver after single dosing in mice. Repeated dose administration in mice, for 2 weeks, stimulated the apoA-I synthesis and formation of small HDL particles. Plasma samples from the agonist-treated mice had high efflux capacity for mobilization of cholesterol in vitro compared to placebo group. In apoE^−/− mice this agonist induced a decrease of atherosclerotic plaques in aortas and carotids. The specificity of P2Y13R pathway in those mice was assessed using adenovirus encoding P2Y13R-shRNA. These results demonstrate that P2Y13R plays a pivotal role in the HDL metabolism and could also be a useful therapeutic agent to decrease atherosclerosis. In this study, the up-regulation of HDL-c metabolism via activation of the P2Y13R using agonists could promote reverse cholesterol transport and promote inhibition of atherosclerosis progression in mice.