Formation of dysfunctional high-density lipoprotein by myeloperoxidase

Recent studies identify the presence of high-density lipoprotein (HDL) particles in patients with cardiovascular disease, which are "dysfunctional," lacking in typical atheroprotective properties, and promoting proinflammatory effects. The mechanisms for generating dysfunctional HDL have been unclear. New evidence points to a role for myeloperoxidase (MPO)-generated oxidants as participants in rendering HDL dysfunctional within human atherosclerotic plaque. Myeloperoxidase was recently shown to bind to HDL within human atherosclerotic lesions, and biophysical studies reveal MPO binding occurs via specific interactions with apolipoprotein (apo) A-I, the predominant protein of HDL. This likely facilitates the observed selective targeting of apoA-I for site-specific chlorination and nitration by MPO-generated reactive oxidants in vivo. One apparent consequence of MPO-catalyzed apoA-I oxidation includes the functional impairment of the ability of HDL to promote cellular cholesterol efflux via the adenosine triphosphate binding cassette-1 transport system. Myeloperoxidase-mediated loss of the atheroprotective functional properties of HDL may thus provide a novel mechanism linking inflammation and oxidative stress to the pathogenesis of atherosclerosis.     

HDL cholesterol efflux capacity and incident cardiovascular events

Background It is unclear whether high-density lipoprotein(HDL) cholesterol concentration plays a causal role in atherosclerosis. A more important factor may be HDL cholesterol efflux capacity, the ability of HDL to accept cholesterol from macrophages, which is a key step in reverse cholesterol transport. We investigated the epidemiology of cholesterol efflux capacity and its association with incident atherosclerotic cardiovascular disease outcomes in a large, multiethnic population cohort.Methods We measured HDL cholesterol level, HDL particle concentration, and cholesterol efflux capacity at baseline in 2924 adults free from cardiovascular disease who were participants in the Dallas Heart Study, a probabilitybased population sample. The primary end point was atherosclerotic cardiovascular disease, defined as a first nonfatal myocardial infarction, nonfatal stroke, or coronary revascularization or death from cardiovascular causes. The median follow-up period was 9.4 years.Results In contrast to HDL cholesterol level, which was associated with multiple traditional risk factors and metabolic variables, cholesterol efflux capacity had minimal association with these factors. Baseline HDL cholesterol level was not associated with cardiovascular events in an adjusted analysis(hazard ratio, 1.08;95% confidence interval [CI], 0.59 to 1.99). In a fully adjusted model that included traditional risk factors,HDL cholesterol level, and HDL particle concentration, there was a 67% reduction in cardiovascular risk in the highest quartile of cholesterol efflux capacity versus the lowest quartile(hazard ratio, 0.33; 95% CI, 0.19 to 0.55). Adding cholesterol efflux capacity to traditional risk factors was associated with improvement in discrimination and reclassification indexes.Conclusions Cholesterol efflux capacity, a new biomarker that characterizes a key step in reverse cholesterol transport,was inversely associated with the incidence of cardiovascular events in a population-based cohort.(From: N Engl J Med 2014; 371:2383-2393 December 18, 2014DOI: 10.1056 / NEJMoa1409065)     

Apolipoproteins AI and B as therapeutic targets

Currently the apolipoprotein B:AI ratio integrates information about the potential for cardiovascular disease (CVD) risk reduction better than any other lipid or lipoprotein index. Certainly it could, with benefit, replace serum cholesterol and HDL cholesterol in the estimation of CVD risk. Defining the therapeutic target of statin therapy in terms of serum apolipoprotein B (apo B) rather than LDL cholesterol could also help to optimize statin treatment. Deciding whether a therapeutic response is adequate also requires knowledge of whether there is persisting hypertriglyceridaemia, because this gives an indication of whether small dense LDL is likely to have been satisfactorily reduced. Raising low levels of HDL, probably best measured as apo AI, may also prove to be an important aim of treatment. This is, however, a more complex issue and also depends on the mechanism by which a particular therapy alters HDL levels and on whether the capacity of HDL to perform its anti-inflammatory and antioxidative functions is restored. A meta-analysis of randomized clinical trials of statins in which apo B and apo AI have been reported could provide valuable information.     

Herz und Diabetes

Antidiabetic therapies have to prove cardiovascular safety and efficacy in addition to blood glucose lowering potential. In recent studies metformin administration and dipeptidyl peptidase-4 (DPP-4) inhibition both reduced hypoglycemic and cardiovascular events in comparison to sulfonylurea. High-density lipoprotein (HDL) cholesterol has recently received much attention as a modulator of cardiovascular risk; however, drug-induced increase of HDL cholesterol was unable to reduce cardiovascular events in patients on optimal statin therapy. Therefore, HDL cholesterol levels remain an important risk predictor without being a therapeutic target. In the field of interventional cardiology the FREEDOM trial demonstrated bypass operations to be superior to drug-eluting stent implantation in patients with diabetes and advanced coronary heart disease.     

Cholesterol efflux pathways and other potential mechanisms involved in the athero-protective effect of high density lipoproteins

Plasma high density lipoprotein (HDL) levels bear a strong independent inverse relationship with atherosclerotic cardiovascular disease. Although HDL has anti-oxidant, anti-inflammatory, vasodilating and anti-thrombotic properties, the central anti-atherogenic activity of HDL is likely to be its ability to remove cholesterol and oxysterols from macrophage foam cells, smooth muscle cells and endothelial cells in the arterial wall. To some extent, the pleotropic athero-protective properties of HDL may be related to its ability to promote sterol and oxysterol efflux from arterial wall cells, as well as to detoxify oxidized phospholipids. In cholesterol-loaded macrophages, activation of liver X receptors (LXRs) leads to increased expression of adenosine triphosphate (ATP) binding cassetter transporter (ABCA1), ATP binding cassetter transporter gene (ABCG1) and apoE and promotes cholesterol efflux. ABCA1 stimulates cholesterol efflux to lipid-poor apolipoproteins, whilst ABCG1 promotes efflux of cholesterol and oxysterols to HDL. Despite some recent setbacks in the clinical arena, there is still intense interest in therapeutically targeting HDL and macrophage cholesterol efflux pathways, via treatments with niacin, cholesterol ester transfer protein inhibitors, LXR activators and infusions of apoA-1, phospholipids and peptides.     

New insights into the role of HDL as an anti-inflammatory agent in the prevention of cardiovascular disease

Several known functions of high-density lipoproteins (HDLs) may contribute to their ability to protect against atherosclerosis. The best known of these functions is the ability to promote cholesterol efflux from cells in a process that may minimize the accumulation of foam cells in the artery wall. However, HDLs have additional properties, including antioxidant, anti-inflammatory, and antithrombotic effects, that may also be anti-atherogenic. Recent in vivo studies in several animal models have demonstrated that HDLs can inhibit acute and chronic vascular inflammation. The fact that these effects can be achieved with very low doses of reconstituted discoidal HDL or even lipid-free apolipoprotein A-I suggests that they may reflect activity of a minor, highly active HDL subpopulation. These results have potentially important clinical implications in regard to managing the acute vascular inflammation states that accompany acute coronary syndrome and acute ischemic stroke.     

HDL therapy for the acute treatment of atherosclerosis

Although pharmacologic intervention to treat atherosclerosis originally focused on lowering LDL-cholesterol levels as a therapeutic target, a number of intervention trials have also highlighted the powerful effect of elevating HDL-cholesterol levels to reduce cardiovascular morbidity and mortality. Although the mechanism(s) by which HDL beneficially alters the atherosclerotic disease process is (are) still unknown, it is presumed that high levels of HDL facilitate the efflux of cholesterol from the arterial wall, thereby enhancing the transport of cholesterol and other lipids from arteries back to the liver for biliary excretion as fecal sterols and bile acids. It has therefore been hypothesized that through a rapid facilitation of HDL mediated cholesterol efflux from arteries by infusion of synthetic apolipoprotein A-I (apoA-I)/phospholipid (A-I/PL) complexes, HDL therapy could have an acute therapeutic application to treat cardiovascular disease at the site of action, namely the vulnerable, unstable atherosclerotic plaque. Single high dose infusions and repeated injections of lower doses of apoA-I variants or mimetics complexed to phospholipids have produced remarkable effects on the progression and regression of atherosclerosis in animal models. The positive results of these preclinical experiments have compelled researchers to perform exploratory studies in human subjects in which reconstituted HDL and synthetic A-I/PL complexes are infused through a peripheral vein. These clinical studies are testing the hypothesis and the potential use of synthetic HDL as a new treatment modality for acute coronary syndromes. Given that there is an unmet medical need for new and more effective therapies to elevate HDL-cholesterol levels and improve HDL function, a historical review, update and discussion of the preclinical and clinical studies which support the use of HDL therapy for reducing cardiovascular morbidity and mortality is warranted.     

Therapeutic Options to Further Lower C-Reactive Protein for Patients on Statin Treatment

Cardiovascular disease remains the leading cause of morbidity and mortality in developed nations. Inflammation plays an increasingly important role in the cardiovascular disease process. Recent statin trials have demonstrated a correlation between the inflammatory biomarker high-sensitivity C-reactive protein (hsCRP) and cardiovascular risk. This review describes the results of changes in hsCRP in the major statin outcome trials and the concept of a “dual target” for both low-density lipoprotein cholesterol and hsCRP. The effect on hsCRP of combination statin therapy with ezetimibe, fenofibric acid, niacin, and colesevelam is reviewed. Although some statin combination therapies have additional effects on CRP and other atherogenic lipids, evidence for their effect on cardiovascular risk beyond statin monotherapy is lacking. Ongoing placebo-controlled trials investigating statin combination therapy versus statin monotherapy may provide additional clues to the role of additional changes in lipids versus markers of inflammation on cardiovascular risk reduction. Until these trials are completed, current evidence suggests focusing on low-density lipoprotein cholesterol targets.     

Common genetic variation of the cholesteryl ester transfer protein gene strongly predicts future cardiovascular death in patients with coronary artery disease

OBJECTIVES: We sought to evaluate the association between cholesteryl ester transfer protein (CETP) genotypes and the risk of future cardiovascular mortality in patients with coronary artery disease (CAD). BACKGROUND: Polymorphisms of the CETP gene influence CETP activity and high-density lipoprotein (HDL) cholesterol concentration and might affect the long-term prognosis and response to statin therapy in patients with CAD. METHODS: We used serum samples and deoxyribonucleic acid collected at baseline from a prospective cohort of 1,211 patients with CAD prospectively followed up (median follow-up of 4.1 years), 82 of whom experienced a fatal cardiovascular event. The CETP/C-629A and I405V polymorphisms, CETP activity, and HDL cholesterol were determined. RESULTS: Patients carrying the -629A allele had significantly lower CETP activity and higher HDL cholesterol levels. There was a significant association between this polymorphism and the risk of future cardiovascular death. Mortality decreased from 10.8% in CC homozygotes to 4.6% in CA heterozygotes and 4.0% in AA homozygotes (p < 0.0001). This association was independent of potential confounders, particularly HDL cholesterol and CETP activity levels. The clinical benefit of statin therapy was restricted to CC homozygotes, in whom cardiovascular mortality was divided by half (p = 0.01 for treatment x genotype interaction). Similar trends were observed with the CETP/I405V polymorphism, but these effects seemed to be mainly the consequence of linkage disequilibrium with the CETP/C-629A polymorphism. CONCLUSIONS: In patients with CAD, the CETP/-629A allele had a strong protective effect on future mortality from cardiovascular causes, independent of its role on HDL cholesterol and CETP activity levels. Additionally, this common polymorphism appeared to predict which patients with CAD will experience a survival benefit from statin therapy.     

New developments in the treatment of low high-density lipoprotein cholesterol

Reduced levels of high-density lipoprotein (HDL) cholesterol represent an important risk factor for the development and progression of coronary artery disease. In recent years, clinical outcome studies have verified that statin therapy may reduce the risk of initial or recurrent cardiovascular events in subjects with elevated or “normal” cholesterol levels. Subgroup analysis has also revealed that patients with low HDL benefit from this therapy. Two recently presented outcome trials using fibrate therapy also demonstrated a potential role for these medications in subjects with low HDL. The use of various HDL raising agents, singly or in combination on arteriographic progression and their potential mechanisms of action are reviewed. The latter may be an important consideration in the treatment of high-risk patients with low HDL.     

Modification of HDL3 by mild oxidative stress increases ATP-binding cassette transporter 1-mediated cholesterol efflux

OBJECTIVE: Elevated levels of high-density lipoprotein (HDL) cholesterol are inversely related to the risk of cardiovascular disease. The anti-atherosclerotic function of HDL is mainly ascribed to its role in reverse cholesterol transport, and requires the integrity of HDL structure. Experimental evidence suggests that the ability of HDL to promote removal of excess cholesterol from peripheral cells is impaired upon oxidation. On the other hand, tyrosylation of HDL enhances its protective function, suggesting that not all forms of modified lipoprotein may be atherogenic. In the present study we investigated the effect of a mild oxidation of HDL(3) on its function as cholesterol acceptor. METHODS AND RESULTS: A mild oxidative stress (induced by 15 min exposure of HDL(3) to 1 microM Cu(++) or to 15-lipoxygenase) caused the formation of pre-beta-migrating particles. Compared to native lipoprotein, mildly modified HDL(3) induced a significant ATP-binding cassette transporter 1 (ABCA1)-mediated increase of cholesterol and phospholipids efflux from J774 macrophages. This effect was abolished by an inhibitor of ABCA1-mediated lipid efflux (glyburide) and was absent in Tangier fibroblasts. CONCLUSIONS: A mild oxidative modification of HDL(3) may improve its function as cholesterol acceptor, increasing ABCA1-mediated lipid efflux from macrophages, a process that may reduce foam cell formation.     

The role of high-density lipoprotein in inflammation

High-density lipoprotein (HDL) appears to have evolved as part of the innate immune system, which in part uses an enhanced oxidative state as a nonspecific means of protecting against many pathogens. In the absence of acute or chronic inflammation, HDL is anti-inflammatory in mice, rabbits, and humans. However, with the onset of a systemic inflammatory state such as what occurs in atherosclerosis, HDL becomes pro-inflammatory, enhancing the inflammatory response. The major apolipoprotein of HDL is apoA-I, which may be altered by oxidative processes in patients with atherosclerosis. As a result, HDL from such patients is less efficient in promoting cellular cholesterol efflux. The ability of HDL to inhibit the inflammatory properties of oxidized phospholipids and low-density lipoproteins is also significantly altered. In mice and monkeys, the administration of an apoA-I-mimetic peptide renders pro-inflammatory HDL anti-inflammatory, improves HDL-mediated cellular cholesterol efflux; in mice, it dramatically inhibits atherosclerosis. Understanding the role of HDL in inflammation may lead to new diagnostic and therapeutic approaches to atherosclerosis and other inflammatory conditions.     

Mechanisms of disease: proatherogenic HDL--an evolving field

It is well known that, in large populations, HDL-cholesterol levels are inversely related to the risk of atherosclerotic clinical events; however, in an individual, the predictive value of an HDL-cholesterol level is far from perfect. As a result, other HDL-associated factors have been investigated, including the quality and function of HDL in contradistinction to the level of HDL-cholesterol. Regarding their quality, HDL particles are highly heterogeneous and contain varying levels of antioxidants or pro-oxidants, which results in variation in HDL function. It has been postulated that HDL functions to promote reverse cholesterol transport. Recent studies support this role for HDL but also indicate that HDL is a modulator of systemic inflammation. In the absence of inflammation, HDL has a complement of antioxidant enzymes that work to maintain an anti-inflammatory state. In the presence of systemic inflammation, these antioxidant enzymes can be inactivated and HDL can accumulate oxidized lipids and proteins that make it proinflammatory. Under these conditions the main protein of HDL, apolipoprotein A-I, can be modified by reactive oxygen species. This modification impairs the ability of HDL to promote cholesterol efflux by the ATP-binding cassette transporter A-1 pathway. Animal studies and small-scale human studies suggest that measures of the quality and novel functions of HDL might provide an improved means of identifying subjects at increased risk for atherosclerotic events, compared with the current practice of only measuring HDL-cholesterol levels. The quality and function of HDL are also attractive targets for emerging therapies.     

ATP-binding cassette A1 protein and HDL homeostasis

For three decades, low-density lipoprotein (LDL) dominated research into cholesterol metabolism and atherosclerosis, whereas scant attention was paid to high-density lipoprotein (HDL), an equally important risk factor for cardiovascular disease. This low interest reflected the lack of knowledge about physiological HDL receptors. As a result, our understanding of HDL-cell interactions failed to develop alongside that of LDL, and mechanisms through which atheroprotective HDL promoted clearance of cholesterol from peripheral cells remained poorly-defined. Interest was kindled with the recognition that scavenger receptor class B, type I is the cell-surface protein in hepatocytes and steroidogenic tissues which selectively extracts cholesteryl esters from HDL. Greater impetus still was given by the discovery that mutations in the gene encoding the ATP-binding cassette transporter, class A1 (ABCA1) are the cause of Tangier disease, a rare recessive disorder with near-absent plasma HDL. The ABCA1 transmembrane protein is crucial for efficient efflux of cellular cholesterol and HDL maturation and has emerged as a promising therapeutic target for cardiovascular disease. The hope is that new drugs, regulating ABCA1 activity and HDL homeostasis, will accelerate cholesterol efflux from lipid-laden foam cells and thus promote regression of atherosclerotic lesions.     

Experimental Models for the Investigation of High-Density Lipoprotein–Mediated Cholesterol Efflux

Reduction of low-density lipoprotein cholesterol by statin therapy has only modestly decreased coronary heart disease (CHD)-associated mortality in developed countries, which has prompted the search for alternative therapeutic strategies for CHD. Epidemiologic and interventional studies have clearly established an inverse association between plasma levels of high-density lipoprotein (HDL) cholesterol and incidence of atherosclerosis. The atheroprotective benefits of HDL are not only dependent on HDL concentrations (quantity), but also on HDL function (quality). Therefore, several techniques have been recently developed to assess the different properties of HDL. Because reverse cholesterol transport (RCT) is considered a key player in the beneficial action of HDL, this review focuses on the different methods used to evaluate cholesterol efflux. Measuring the in vivo function of HDL could be of significant importance for both the clinical evaluation of an individual patient and to evaluate the effectiveness of different RCT-enhancing therapeutic approaches.     

High density lipoproteins and atherosclerosis: emerging aspects

High density lipoproteins (HDL) promote the efflux of excess cholesterol from peripheral tissues to the liver for excretion. This ability is responsible for the most relevant anti-atherogenic effect of HDL. The ability of HDL to promote cholesterol efflux results also in the modulation of a series of responses in the immune cells involved in atherosclerosis, including monocyte-macrophages, B and T lymphocytes. Furthermore, during inflammation, the composition of this class of lipoproteins varies to a large extent, thus promoting the formation of dysfunctional HDL. The aim of this review is to discuss the emerging role of HDL in modulating the activity of immune cells and immune-inflammatory mediators during atherogenesis.     

Effect of low-density lipoprotein apheresis on inflammatory and noninflammatory high-density lipoprotein cholesterol

Low-density lipoprotein (LDL) apheresis, a treatment for familial hypercholesterolemia, significantly decreases LDL cholesterol and inflammatory markers such as C-reactive protein, CD40 ligand, and tissue factor. LDL apheresis also decreases high-density lipoprotein (HDL) cholesterol, which might be considered therapeutically counterproductive because HDL is known to be anti-inflammatory. However, recent studies have shown that HDL also possesses proinflammatory properties, as seen in its ability to alter LDL-induced monocyte chemotactic activity. We examined the acute effects of LDL apheresis on inflammatory HDL activity in 13 patients with familial hypercholesterolemia and cardiovascular disease who had been receiving bi-weekly LDL apheresis treatments. Immediately before and immediately after treatment, each patient's plasma was collected for analysis of inflammatory HDL and full lipid profile. LDL apheresis reduced LDL by 52% (from 208 +/- 89 to 99 +/- 48 mg/dl, p <0.002), and HDL decreased by 16% (49 +/- 15 to 41 +/- 13 mg/dl, p <0.003). At the same time, inflammatory HDL activity (in migrated monocytes per high-power field) decreased from 22 +/- 4 to 14 +/- 2, a 37% acute reduction (p <0.003). Moreover, inflammatory HDL before HDL apheresis was highly correlated with its acute reduction (r(s) = 0.85, p <0.001). In conclusion, our findings indicate that, in addition to decreasing LDL, LDL apheresis also decreases inflammatory HDL. The clinical significance of reducing inflammatory HDL is currently unknown, and further research is needed to examine its potential benefit for cardiovascular disease.     

Emerging role of high density lipoproteins as a player in the immune system

High density lipoproteins (HDL) possess a number of physiological activities. The most studied and, perhaps, better understood is the ability of HDL to promote excess cholesterol efflux from peripheral tissues and transport to the liver for excretion, a mechanism believed to confer protection against atherosclerotic cardiovascular disease. The ability of HDL to modulate cholesterol bioavailability in the lipid rafts, membrane microdomains enriched in glycosphingolipids and cholesterol, is evolutionary conserved and affects the properties of cells involved in the innate and adaptive immune response, tuning inflammatory response and antigen presentation functions in macrophages as well as B and T cell activation. Also sphingosine-1 phosphate (S1P), a major active sphingolipid carried by HDL, is of relevance in the pathogenesis of several immuno-inflammatory disorders through the modulation of macrophage and lymphocyte functions. Furthermore, HDL influence the humoral innate immunity by modulating the activation of the complement system and the expression of pentraxin 3 (PTX3). Finally, in humans, HDL levels and functions are altered in several immune-mediated disorders, such as rheumatoid arthritis, systemic lupus eritematosus, Crohn's disease and multiple sclerosis as well as during inflammatory responses. Altogether these observations suggest that the effects of HDL in immunity could be related, to either the ability of HDL to modulate cholesterol content in immune cell lipid rafts and to their role as reservoir for several biologically active substances that may impact the immune system.     

ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins

The mechanisms responsible for the inverse relationship between plasma high-density lipoprotein (HDL) levels and atherosclerotic cardiovascular disease are poorly understood. The ATP-binding cassette transporter A1 (ABCA1) mediates efflux of cellular cholesterol to lipid-poor apolipoproteins but not to HDL particles that constitute the bulk of plasma HDL. We show that two ABC transporters of unknown function, ABCG1 and ABCG4, mediate isotopic and net mass efflux of cellular cholesterol to HDL. In transfected 293 cells, ABCG1 and ABCG4 stimulate cholesterol efflux to both smaller (HDL-3) and larger (HDL-2) subclasses but not to lipid-poor apoA-I. Treatment of macrophages with an liver X receptor activator results in up-regulation of ABCG1 and increases cholesterol efflux to HDL. RNA interference reduced the expression of ABCG1 in liver X receptor-activated macrophages and caused a parallel decrease in cholesterol efflux to HDL. These studies indicate that ABCG1 and ABCG4 promote cholesterol efflux from cells to HDL. ABCG1 is highly expressed in macrophages and probably mediates cholesterol efflux from macrophage foam cells to the major HDL fractions, providing a mechanism to explain the relationship between HDL levels and atherosclerosis risk.     

High-density lipoprotein function recent advances

Although high-density lipoproteins (HDL) possess many features that contribute to the association between elevated HDL cholesterol and protection from atherosclerosis, these lipoproteins may be modified in certain individuals and/or circumstances to become proinflammatory. The ability of HDL to inhibit or paradoxically to enhance vascular inflammation, lipid oxidation, plaque growth, and thrombosis reflects changes in specific enzyme and protein components. The anti-inflammatory and proinflammatory functional properties of HDL can now be assessed using cell-based and cell-free assays. Acute or chronic systemic inflammation and the metabolic syndrome appear to render HDL proinflammatory. In contrast, statins and experimental agents such as apolipoprotein A-1 mimetics render HDL more anti-inflammatory. Functional characterization of HDL is a promising method for enhanced assessment of cardiovascular risk and effectiveness of risk reduction.