Rationale and design of the dal-OUTCOMES trial: efficacy and safety of dalcetrapib in patients with recent acute coronary syndrome

High-Density Lipoprotein Subfractions Remodeling: A Critical Process for the Treatment of Atherosclerotic Cardiovascular Diseases

Numerous studies have shown that a low level of high-density lipoprotein cholesterol (HDL-C) is an independent biomarker of cardiovascular disease. High-density lipoprotein (HDL) is considered to be a protective factor for atherosclerosis (AS). Therefore, raising HDL-C has been widely recognized as a promising strategy to treat atherosclerotic cardiovascular diseases (ASCVD). However, several studies have found that increasing HDL-C levels does not necessarily reduce the risk of ASCVD. HDL particles are highly heterogeneous in structure, composition, and biological function. Moreover, HDL particles from atherosclerotic patients exhibit impaired anti-atherogenic functions and these dysfunctional HDL particles might even promote ASCVD. This makes it uncertain that HDL-raising therapy will prevent and treat ASCVD. It is necessary to comprehensively analyze the structure and function of HDL subfractions. We review current advances related to HDL subfractions remodeling and highlight how current lipid-modifying drugs such as niacin, statins, fibrates, and cholesteryl ester transfer protein inhibitors regulate cholesterol concentration of HDL and specific HDL subfractions.

The Impact of Aerobic Exercise on HDL Quantity and Quality: A Narrative Review

High-density lipoproteins comprise roughly 25-30% of the circulating proteins involved in the transport of lipids in circulation. These particles differ in size and lipid composition. Recent evidence suggests that the quality of HDL particles (which depends on shape, size and the composition of proteins and lipids determining HDL functionality) may be more important than their quantity. The functionality of HDL is mirrored by its cholesterol efflux activity, as well as its antioxidant (including the protection of LDL against oxidation), anti-inflammatory and antithrombotic properties. The results of many studies and meta-analyses imply the beneficial impact of aerobic exercise on HDL-C levels. Physical activity was found to be usually associated with an increase in HDL cholesterol and a decrease in LDL cholesterol and triglycerides. Exercise, apart from inducing quantitative alterations in serum lipids, exerts a beneficial impact on HDL particle maturation, composition and functionality. The Physical Activity Guidelines Advisory Committee Report underlined the importance of establishing a program recommending exercises that enable attainment of maximal advantage at the lowest level of risk. The aim of this manuscript is to review the impact of different types of aerobic exercise (various intensities and durations) on the level and quality of HDL.

The repeated setbacks of HIV vaccine development laid the groundwork for SARS-CoV-2 vaccines

The decades-long effort to produce a workable HIV vaccine has hardly been a waste of public and private resources. To the contrary, the scientific know-how acquired along the way has served as the critical foundation for the development of vaccines against the novel, pandemic SARS-CoV-2 virus. We retell the real-world story of HIV vaccine research – with all its false leads and missteps – in a way that sheds light on the current state of the art of antiviral vaccines. We find that HIV-related R&D had more than a general spillover effect. In fact, the repeated failures of phase 2 and 3 clinical trials of HIV vaccine candidates have served as a critical stimulus to the development of successful vaccine technologies today. We rebut the counterargument that HIV vaccine development has been no more than a blind alley, and that recently developed vaccines against COVID-19 are really descendants of successful vaccines against Ebola, MERS, and SARS. These successful vaccines likewise owe much to the vicissitudes of HIV vaccine development. We then discuss how the failures of HIV vaccine development have taught us how adapt SARS-CoV-2 vaccines to immune escape from emerging variants. Finally, we inquire whether recent advances in the development of vaccines against SARS-CoV-2 might in turn further the development of an HIV vaccine - what we describe as a reverse spillover effect.

Role of Adenylate Cyclase 9 in the Pharmacogenomic Response to Dalcetrapib: Clinical Paradigm and Molecular Mechanisms in Precision Cardiovascular Medicine

Following the neutral results of the dal-OUTCOMES trial, a genome-wide study identified the rs1967309 variant in the adenylate cyclase type 9 (ADCY9) gene on chromosome 16 as being associated with the risk of future cardiovascular events only in subjects taking dalcetrapib, a CETP (cholesterol ester transfer protein) modulator. Homozygotes for the minor A allele (AA) were protected from recurrent cardiovascular events when treated with dalcetrapib, while homozygotes for the major G allele (GG) had increased risk. Here, we present the current state of knowledge regarding the impact of rs1967309 in ADCY9 on clinical observations and biomarkers in dalcetrapib trials and the effects of mouse ADCY9 gene inactivation on cardiovascular physiology. Finally, we present our current model of the interaction between dalcetrapib and ADCY9 gene variants in the arterial wall macrophage, based on the intracellular role of CETP in the transfer of complex lipids from endoplasmic reticulum membranes to lipid droplets. Briefly, the concept is that dalcetrapib would inhibit CETP-mediated transfer of cholesteryl esters, resulting in a progressive inhibition of cholesteryl ester synthesis and free cholesterol accumulation in the endoplasmic reticulum. Reduced ADCY9 activity, by paradoxically leading to higher cyclic AMP levels and in turn increased cellular cholesterol efflux, could impart cardiovascular protection in rs1967309 AA patients. The ongoing dal-GenE trial recruited 6145 patients with the protective AA genotype and will provide a definitive answer to whether dalcetrapib will be protective in this population.

Endothelial Lipase Is Involved in Cold-Induced High-Density Lipoprotein Turnover and Reverse Cholesterol Transport in Mice

The physiologic activation of thermogenic brown and white adipose tissues (BAT/WAT) by cold exposure triggers heat production by adaptive thermogenesis, a process known to ameliorate hyperlipidemia and protect from atherosclerosis. Mechanistically, it has been shown that thermogenic activation increases lipoprotein lipase (LPL)-dependent hydrolysis of triglyceride-rich lipoproteins (TRL) and accelerates the generation of cholesterol-enriched remnants and high-density lipoprotein (HDL), which promotes cholesterol flux from the periphery to the liver. HDL is also subjected to hydrolysis by endothelial lipase (EL) (encoded by LIPG). Genome-wide association studies have identified various variants of EL that are associated with altered HDL cholesterol levels. However, a potential role of EL in BAT-mediated HDL metabolism has not been investigated so far. In the present study, we show that in mice, cold-stimulated activation of thermogenic adipocytes induced expression of Lipg in BAT and inguinal WAT but that loss of Lipg did not affect gene expression of thermogenic markers. Furthermore, in both wild type (WT) and Lipg-deficient mice, activation of thermogenesis resulted in a decline of HDL cholesterol levels. However, cold-induced remodeling of the HDL lipid composition was different between WT and Lipg-deficient mice. Notably, radioactive tracer studies with double-labeled HDL indicated that cold-induced hepatic HDL cholesterol clearance was lower in Lipg-deficient mice. Moreover, this reduced clearance was associated with impaired macrophage-to-feces cholesterol transport. Overall, these data indicate that EL is a determinant of HDL lipid composition, cholesterol flux, and HDL turnover in conditions of high thermogenic activity.

Dalcetrapib Reduces Risk of New-Onset Diabetes in Patients With Coronary Heart Disease

OBJECTIVE

Incident type 2 diabetes is common among patients with recent acute coronary syndrome and is associated with an adverse prognosis. Some data suggest that cholesteryl ester transfer protein (CETP) inhibitors reduce incident type 2 diabetes. We compared the effect of treatment with the CETP inhibitor dalcetrapib or placebo on incident diabetes in patients with recent acute coronary syndrome.

RESEARCH DESIGN AND METHODS

In the dal-OUTCOMES trial, 15,871 patients were randomly assigned to treatment with dalcetrapib 600 mg daily or placebo, beginning 4-12 weeks after an acute coronary syndrome. Absence of diabetes at baseline was based on medical history, no use of antihyperglycemic medication, and hemoglobin A_1c and serum glucose levels below diagnostic thresholds. Among these patients, incident diabetes after randomization was defined by any diabetes-related adverse event, new use of antihyperglycemic medication, hemoglobin A_1c ≥6.5%, or a combination of at least two measurements of serum glucose ≥7.0 mmol/L (fasting) or ≥11.1 mmol/L (random).

RESULTS

At baseline, 10,645 patients (67% of the trial cohort) did not have diabetes. During a median follow-up of 30 months, incident diabetes was identified in 403 of 5,326 patients (7.6%) assigned to dalcetrapib and in 516 of 5,319 (9.7%) assigned to placebo, corresponding to absolute risk reduction of 2.1%, hazard ratio of 0.77 (95% CI 0.68-0.88; P < 0.001), and a need to treat 40 patients for 3 years to prevent 1 incident case of diabetes. Considering only those with prediabetes at baseline, the number needed to treat for 3 years to prevent 1 incident case of diabetes was 25. Dalcetrapib also decreased the number of patients who progressed from normoglycemia to prediabetes and increased the number who regressed from diabetes to no diabetes.

CONCLUSIONS

In patients with a recent acute coronary syndrome, incident diabetes is common and is reduced substantially by treatment with dalcetrapib.

Association of high-density lipoprotein particle concentration with cardiovascular risk following acute coronary syndrome: A case-cohort analysis of the dal-Outcomes trial

BACKGROUND

High-density lipoprotein cholesterol (HDL-C) concentration is inversely related to risk of major adverse cardiovascular events (MACE) in epidemiologic studies but is a poorer predictor of MACE in patients with established coronary heart disease. HDL particle concentration (HDLP) has been proposed as a better predictor of risk. We investigated whether HDLP is associated with risk of MACE after acute coronary syndrome (ACS).

METHODS

The dal-Outcomes trial compared the CETP inhibitor dalcetrapib with placebo in patients with recent ACS. In a nested case-cohort analysis, total, large, medium, and small HDLPs were measured by nuclear magnetic resonance spectroscopy at baseline (4-12 weeks after ACS) in 476 cases with MACE and 902 controls. Hazard ratios (HRs; case-control) for 1-SD increment of HDLP or HDL-C at baseline were calculated with and without adjustment for demographic, clinical, laboratory, and treatment variables. Similarly, HRs for MACE were calculated for changes in HDLP or HDL-C from baseline to month 3 of assigned treatment.

RESULTS

Over median follow-up of 28 months, the risk of MACE was not associated with baseline HDLP (adjusted HR = 0.98, 95% CI = 0.84-1.15, P = .81), any HDLP subclass, or HDL-C. Dalcetrapib increased HDL-C and total, medium, and large HDLP and decreased small HDLP but had no effect on MACE compared with placebo. There were no association of risk of MACE with change in HDLP or HDL-C and no interaction with assigned study treatment.

CONCLUSIONS

Neither baseline HDLP nor the change in HDLP on treatment with dalcetrapib or placebo was associated with risk of MACE after ACS.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

Clinical Pharmacokinetics and Pharmacodynamics of Dalcetrapib

The cholesterol ester transfer protein (CETP) inhibitor dalcetrapib has been under evaluation for its potential to prevent cardiovascular (CV) events for almost two decades. The current clinical development program, representing new advances in precision medicine and focused on a genetically defined population with acute coronary syndrome (ACS), is supported by a large body of pharmacokinetic and pharmacodynamic data as well as substantial clinical experience in over 13,000 patients and volunteers. Dalcetrapib treatment of 600 mg/day produces significant inhibition of CETP activity, and has been utilized in phase II and III studies, including CV endpoint trials. Numerous studies have investigated the interactions between dalcetrapib and most drugs commonly prescribed to CV patients and have not demonstrated any clinically significant effects. Evaluations in patients with renal and hepatic impairment demonstrate a greater exposure to dalcetrapib than in the non-impaired population, but long-term clinical studies including patients with mild to moderate hepatic and renal dysfunction demonstrate no increase in adverse events. Safety pharmacology and toxicology studies as well as the clinical safety experience support the continuing development of dalcetrapib as an adjunct to ‘standard of care’ for the ACS population. This article provides a full review of the pharmacokinetics, as well as pharmacodynamics and pharmacology, of dalcetrapib in the context of a large clinical program.

Association of Lipoprotein(a) With Risk of Recurrent Ischemic Events Following Acute Coronary Syndrome: Analysis of the dal-Outcomes Randomized Clinical Trial

Importance

It is uncertain whether lipoprotein(a) [Lp(a)], which is associated with incident cardiovascular disease, is an independent risk factor for recurrent cardiovascular events after acute coronary syndrome (ACS).

Objective

To determine the association of Lp(a) concentration measured after ACS with the subsequent risk of ischemic cardiovascular events.

Design, Setting, and Participants

This nested case-cohort analysis was performed as an ad hoc analysis of the dal-Outcomes randomized clinical trial. This trial compared dalcetrapib, the cholesteryl ester transfer protein inhibitor, with placebo in patients with recent ACS and was performed between April 2008 and September 2012 at 935 sites in 27 countries. There were 969 case patients who experienced a primary cardiovascular outcome, and there were 3170 control patients who were event free at the time of a case event and had the same type of index ACS (unstable angina or myocardial infarction) as that of the respective case patients. Concentration of Lp(a) was measured by immunoturbidimetric assay. Data analysis for this present study was conducted from June 8, 2016, to April 21, 2017.

Interventions

Patients were randomly assigned to receive treatment with dalcetrapib, 600 mg daily, or matching placebo, beginning 4 to 12 weeks after ACS.

Main Outcomes and Measures

Death due to coronary heart disease, a major nonfatal coronary event (myocardial infarction, hospitalization for unstable angina, or resuscitated cardiac arrest), or fatal or nonfatal ischemic stroke.

Results

The mean (SD) age was 63 (10) years for the 969 case patients and 60 (9) years for the 3170 control patients, and both cohorts were composed of predominantly male (770 case patients [79%] and 2558 control patients [81%]; P = .40) and white patients (858 case patients [89%] and 2825 control patients [89%]; P = .62). At baseline, the median (interquartile range) Lp(a) level was 12.3 (4.7-50.9) mg/dL. There was broad application of evidence-based secondary prevention strategies after ACS, including use of statins in 4030 patients (97%). The cumulative distribution of baseline Lp(a) levels did not differ between cases and controls at P = .16. Case-cohort regression analysis showed no association of baseline Lp(a) level with risk of cardiovascular events. For a doubling of Lp(a) concentration, the hazard ratio (case to control) was 1.01 (95% CI, 0.96-1.06; P = .66) after adjustment for 16 baseline variables, including assigned study treatment.

Conclusions and Relevance

For patients with recent ACS who are treated with statins, Lp(a) concentration was not associated with adverse cardiovascular outcomes. These findings call into question whether treatment specifically targeted to reduce Lp(a) levels would thereby lower the risk for ischemic cardiovascular events after ACS.

Trial Registration

clinicaltrials.gov Identifier: NCT00658515.

Metformin ameliorates Ox-LDL-induced foam cell formation in raw264.7 cells by promoting ABCG-1 mediated cholesterol efflux

AIMS

The accumulation of lipids in macrophages contributes to the development of atherosclerosis. Cholesterol efflux of lipid-loaded macrophages mediated by ATP binding cassette (ABC) cholesterol transporters, on the other hand, has been shown to attenuate atherosclerosis progression in patients with unknown mechanism. We therefore sought to test the effect of metformin that reduced cardiovascular risk in diabetic patients independent of its hypoglycemia effect on cholesterol transport in murine raw264.7 macrophages.

MATERIALS AND METHODS

Mouse raw264.7 macrophages were loaded with Ox-LDL (50 μg/ml) for 24 h before incubated with metformin (15 μM) for 24 h. Foam cell formation was assessed by Oil red staining and BIODIPY fluorescent staining as well as cholesterol-ester quantification by commercial kit. Cholesterol uptake and expression of scavenger receptors were detected by flow-cytometry. Cholesterol efflux capacity was measured by fluorescent plate-reader and ABC transporters were detected by Western Blots. Cytokines were detected by ELISA in supernatants and normalized by cellular lysates.

KEY FINDINGS

Our results showed that metformin decreased oxidized low-density lipoprotein (Ox-LDL)-induced cholesterol accumulation and foam cell formation by increasing cholesterol efflux to HDL, which was associated with an upregulation of ABC transporter ABCG-1. Moreover, metformin increased Ox-LDL-impaired IL-10 secretion, an important anti-foam cell cytokine in atherosclerosis.

SIGNIFICANCE

Our data highlighted the therapeutic potential of targeting macrophage cholesterol efflux with new or existing drugs for the possible reduction of foam cell formation in the prevention and treatment of diabetes-accelerated atherosclerosis.

The role of monocytosis and neutrophilia in atherosclerosis

Monocytosis and neutrophilia are frequent events in atherosclerosis. These phenomena arise from the increased proliferation of hematopoietic stem and multipotential progenitor cells (HSPCs) and HSPC mobilization from the bone marrow to other immune organs and circulation. High cholesterol and inflammatory signals promote HSPC proliferation and preferential differentiation to the myeloid precursors (i.e., myelopoiesis) that than give rise to pro-inflammatory immune cells. These cells accumulate in the plaques thereby enhancing vascular inflammation and contributing to further lesion progression. Studies in animal models of atherosclerosis showed that manipulation with HSPC proliferation and differentiation through the activation of LXR-dependent mechanisms and restoration of cholesterol efflux may have a significant therapeutic potential.

Mendelian randomization analysis of cholesteryl ester transfer protein and subclinical atherosclerosis: A population-based study

BACKGROUND

Several trials to prevent cardiovascular disease by inhibiting cholesteryl ester transfer protein (CETP) have failed, except Randomized EValuation of the Effects of Anacetrapib through Lipid-modification. Thus far, it is unclear to what extent CETP is causally related to measures of atherosclerosis.

OBJECTIVE

The aim of the article was to study the causal relationship between genetically determined CETP concentration and carotid intima-media thickness (cIMT) in a population-based cohort study.

METHODS

In the Netherlands Epidemiology of Obesity study, participants were genotyped, and cIMT was measured by ultrasonography. We examined the relation between a weighted genetic risk score for CETP concentration, based on 3 single-nucleotide polymorphisms that have previously been shown to largely determine CETP concentration and cIMT using Mendelian randomization in the total population and in strata by sex, Framingham 10-year risk, (pre)diabetes, high-density lipoprotein cholesterol, triglycerides, and statin use.

RESULTS

We analyzed 5655 participants (56% women) with a mean age of 56 (range 44-66) years, body mass index of 26 (range 17-61) kg/m², and serum CETP of 2.47 (range 0.68-5.33) μg/mL. There was no evidence for a causal relation between genetically determined CETP and cIMT in the total population, but associations were differently directed in men (16 μm per μg/mL increase in genetically determined CETP; 95% confidence interval: -8, 39) and women (-8 μm; -25, 9). Genetically determined CETP appeared to be associated with cIMT in normoglycemic men (26 μm; -1, 52) and in (pre)diabetic women (48 μm; -2, 98).

CONCLUSION

In this population-based study, there was no causal relation between genetically determined CETP concentration and cIMT in the total population although we observed directionally differing effects in men and women. Stratified results suggested associations in individuals with different cardiometabolic risk factor profiles, which require replication.

Apolipoprotein A-I Modulates Atherosclerosis Through Lymphatic Vessel-Dependent Mechanisms in Mice

Background

Subcutaneously injected lipid‐free apoA‐I (apolipoprotein A‐I) reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without increasing high‐density lipoprotein–cholesterol concentrations. Lymphatic vessels are now recognized as prerequisite players in the modulation of cholesterol removal from the artery wall in experimental conditions of plaque regression, and particular attention has been brought to the role of the collecting lymphatic vessels in early atherosclerosis‐related lymphatic dysfunction. In the present study, we address whether and how preservation of collecting lymphatic function contributes to the protective effect of apoA‐I.

Methods and Results

Atherosclerotic Ldlr^−/− mice treated with low‐dose lipid‐free apoA‐I showed enhanced lymphatic transport and abrogated collecting lymphatic vessel permeability in atherosclerotic Ldlr^−/− mice when compared with albumin‐control mice. Treatment of human lymphatic endothelial cells with apoA‐I increased the adhesion of human platelets on lymphatic endothelial cells, in a bridge‐like manner, a mechanism that could strengthen endothelial cell–cell junctions and limit atherosclerosis‐associated collecting lymphatic vessel dysfunction. Experiments performed with blood platelets isolated from apoA‐I‐treated Ldlr^−/− mice revealed that apoA‐I decreased ex vivo platelet aggregation. This suggests that in vivo apoA‐I treatment limits platelet thrombotic potential in blood while maintaining the platelet activity needed to sustain adequate lymphatic function.

Conclusions

Altogether, we bring forward a new pleiotropic role for apoA‐I in lymphatic function and unveil new potential therapeutic targets for the prevention and treatment of atherosclerosis.

Serum inhibitory factor 1, high-density lipoprotein and cardiovascular diseases

PURPOSE OF REVIEW

The atheroprotective properties of HDL are supported by epidemiological and preclinical research. However, the results of interventional trials paradoxically indicate that drugs increasing HDL-cholesterol (HDL-C) do not reduce coronary artery disease (CAD) risk. Moreover, Mendelian randomization studies have shown no effect of HDL-C-modifying variants on CAD outcome. Thus, the protective effects of HDL particles are more governed by their functional status than their cholesterol content. In this context, any successful clinical exploitation of HDL will depend on the identification of HDL-related biomarkers, better than HDL-C level, for assessing cardiovascular risk and monitoring responses to treatment.

RECENT FINDINGS

Recent studies have enlightened the role of ecto-F1-ATPase as a cell surface receptor for apoA-I, the major apolipoprotein of HDL, involved in the important metabolic and vascular atheroprotective functions of HDL. In the light of these findings, the clinical relevance of ecto-F1-ATPase in humans has recently been supported by the identification of serum F1-ATPase inhibitor (IF1) as an independent determinant of HDL-C, CAD risk and cardiovascular mortality in CAD patients.

SUMMARY

Serum IF1 measurement might be used as a novel HDL-related biomarker to better stratify risk in high-risk populations or to determine pharmacotherapy.

Association of High-Density Lipoprotein Subclasses with Carotid Intima-Media Thickness: Shimane CoHRE Study

AIMS

Recent studies suggested that subclasses of high-density lipoprotein (HDL) may be a better biomarker to predict the risk of atherosclerotic disorders. We aimed to examine the association of HDL2- and HDL3-cholesterol (HDL2-C and HDL3-C) with carotid intima-media thickness (IMT) using a new method to quantify the HDL-C subclasses.

METHODS

Participants were 657 Japanese subjects (434 women) who received a health examination (mean age: 73 years). Serum samples were analyzed by the homogenous assay for HDL-C and HDL3-C. HDL2-C was calculated indirectly by subtracting HDL3-C from HDL-C. HDL3-C measured by this assay was well correlated with that measured by ultracentrifugation (r=0.898, p＜0.001). The maximum IMT (max-IMT) and plaque score (PS) were evaluated by ultrasonography following the standard protocol.

RESULTS

HDL3-C was associated with age both in men (r=－0.322, p＜0.0001) and women (r=－0.315, p＜0.0001). In a simple regression analysis, max-IMT showed an inverse association with HDL3-C, whereas no significant association was observed with HDL2-C. A multiple linear regression analysis indicated, however, that the association between HDL3-C and max-IMT was not significant in both aged and younger populations when age was included in the analysis. Further, not only HDL2-C but also HDL3-C was not a significant predictor of 'atherosclerotic arteries' defined as the max-IMT ≥1.5 mm. Similar results were observed in the analysis on PS.

CONCLUSIONS

Neither HDL3-C nor HDL2-C was significantly associated with carotid atherosclerosis in the Japanese population in this study.

Could high-density lipoprotein cholesterol predict increased cardiovascular risk?

PURPOSE OF REVIEW

Serum high-density lipoprotein (HDL) is considered to be protective against cardiovascular disease. However, there is emerging evidence that under certain conditions the HDL molecule can become dysfunctional and proinflammatory, paradoxically leading to increased risk of cardiovascular disease. This review will provide a brief outline of the potential mechanisms by which HDL can become atherogenic and summarize some of the clinical evidence on this topic.

RECENT FINDINGS

HDL metabolism, structure, and function in addition to its level can be profoundly altered under conditions of marked oxidative stress and chronic inflammation. These abnormalities, in turn, lead to impaired reverse cholesterol transport, increased systemic oxidative stress/inflammation, and endothelial dysfunction that subsequently may contribute to atherogenesis and progression of cardiovascular disease.

SUMMARY

Association of serum HDL cholesterol level with outcomes is not only dependent on its serum concentration but also on the qualities/properties of this lipoprotein at a given point in time. Hence, it is essential that future studies examining association of HDL with risk of cardiovascular disease take into account the complexities of HDL metabolism and function and address the impact of the HDL particle as a whole (quantity as well as various properties) on atherosclerosis and cardiovascular outcomes.

The role of HDL in plaque stabilization and regression: basic mechanisms and clinical implications

On the basis of studies that extend back to the early 1900s, regression and stabilization of atherosclerosis in humans has progressed from being a concept to one that is achievable. Successful attempts at regression generally applied robust measures to improve plasma lipoprotein profiles. Possible mechanisms responsible for lesion shrinkage include decreased retention of atherogenic apolipoprotein B within the arterial wall, efflux of cholesterol and other toxic lipids from plaques, emigration of lesional foam cells out of the arterial wall, and influx of healthy phagocytes that remove necrotic debris as well as other components of the plaque. Currently available clinical agents, however, still fail to stop most cardiovascular events. For years, HDL has been considered the 'good cholesterol.' Clinical intervention studies to causally link plasma HDL-C levels to decreased progression or to the regression of atherosclerotic plaques are relatively few because of the lack of therapeutic agents that can selectively and potently increase HDL-C. The negative results of studies that were carried out have led to uncertainty as to the role that HDL plays in atherosclerosis. It is becoming clearer, however, that HDL function rather than quantity is most crucial and, therefore, discovery of agents that enhance the quality of HDL should be the goal.

Infusional high-density lipoproteins therapies as a novel strategy for treating atherosclerosis

High-density lipoproteins (HDL) have received considerable interest as a target for the development of novel anti-atherosclerotic agents beyond conventional approaches to lipid lowering. While a number of approaches have focused on modifying remodeling and expression pathways implicated in the regulation of HDL levels, an additional approach involves simply infusions of delipidated HDL. Several groups have advanced HDL infusions to clinical development with intriguing signs suggesting potentially favorable impacts at the level of the artery wall. The findings of early studies of infusional HDL therapies will be reviewed.

Aldosterone Does Not Predict Cardiovascular Events Following Acute Coronary Syndrome in Patients Initially Without Heart Failure

Background

Aldosterone may have adverse effects in the myocardium and vasculature. Treatment with an aldosterone antagonist reduces cardiovascular risk in patients with acute myocardial infarction complicated by heart failure (HF) and left ventricular systolic dysfunction. However, most patients with acute coronary syndrome do not have advanced HF. Among such patients, it is unknown whether aldosterone predicts cardiovascular risk.

Methods and Results

To address this question, we examined data from the dal‐OUTCOMES trial that compared the cholesteryl ester transfer protein inhibitor dalcetrapib with placebo, beginning 4 to 12 weeks after an index acute coronary syndrome. Patients with New York Heart Association class II (with LVEF <40%), III, or IV HF were excluded. Aldosterone was measured at randomization in 4073 patients. The primary outcome was a composite of coronary heart disease death, nonfatal myocardial infarction, stroke, hospitalization for unstable angina, or resuscitated cardiac arrest. Hospitalization for HF was a secondary endpoint. Over a median follow‐up of 37 months, the primary outcome occurred in 366 patients (9.0%), and hospitalization for HF occurred in 72 patients (1.8%). There was no association between aldosterone and either the time to first occurrence of a primary outcome (hazard ratio for doubling of aldosterone 0.92, 95% confidence interval 0.78‐1.09, P=0.34) or hospitalization for HF (hazard ratio 1.38, 95% CI 0.96‐1.99, P=0.08) in Cox regression models adjusted for covariates.

Conclusions

In patients with recent acute coronary syndrome but without advanced HF, aldosterone does not predict major cardiovascular events.

Clinical Trial Registration

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00658515.

Atherosclerosis

In this chapter, we discuss the manner through which the immune system regulates the cardiovascular system in health and disease. We define the cardiovascular system and elements of atherosclerotic disease, the main focus in this chapter. Herein we elaborate on the disease process that can result in myocardial infarction (heart attack), ischaemic stroke and peripheral arterial disease. We have discussed broadly the homeostatic mechanisms in place that help autoregulate the cardiovascular system including the vital role of cholesterol and lipid clearance as well as the role lipid homeostasis plays in cardiovascular disease in the context of atherosclerosis. We then elaborate on the role played by the immune system in this setting, namely, major players from the innate and adaptive immune system, as well as discussing in greater detail specifically the role played by monocytes and macrophages.This chapter should represent an overview of the role played by the immune system in cardiovascular homeostasis; however further reading of the references cited can expand the reader's knowledge of the detail, and we point readers to many excellent reviews which summarise individual immune systems and their role in cardiovascular disease.

Effects of LDL Receptor Modulation on Lymphatic Function

Atherosclerosis is driven by the accumulation of immune cells and cholesterol in the arterial wall. Although recent studies have shown that lymphatic vessels play an important role in macrophage reverse cholesterol transport, the specific underlying mechanisms of this physiological feature remain unknown. In the current report, we sought to better characterize the lymphatic dysfunction that is associated with atherosclerosis by studying the physiological and temporal origins of this impairment. First, we assessed that athero-protected Pcsk9^−/− mice exhibited improved collecting lymphatic vessel function throughout age when compared to WT mice for up to six months, while displaying enhanced expression of LDLR on lymphatic endothelial cells. Lymphatic dysfunction was present before the atherosclerotic lesion formation in a mouse model that is predisposed to develop atherosclerosis (Ldlr^−/−; hApoB100^+/+). This dysfunction was presumably associated with a defect in the collecting lymphatic vessels in a non-specific cholesterol- but LDLR-dependent manner. Treatment with a selective VEGFR-3 agonist rescued this impairment observed early in the onset of this arterial disease. We suggest that LDLR modulation is associated with early atherosclerosis-related lymphatic dysfunction, and bring forth a pleiotropic role for PCSK9 in lymphatic function. Our study unveils new potential therapeutic targets for the prevention and treatment of atherosclerosis.

Antilipidemic Drug Therapy Today and in the Future

The armamentarium for the treatment of dyslipidemia today comprises six different modes of action with overall around 24 different drugs. The treatment of lipid disorders was revolutionized with the introduction of statins which have become the most important therapeutic option available today to reduce and prevent atherosclerosis and its detrimental consequences like cardiovascular diseases and stroke. With and optimized reduction of elevated LDL levels with statins, the risk for cardiovascular diseases (CVD) can be reduced by 30%, indicating a residual remaining risk of 70% for the development and progression of CVD notifying still a high medical need for more effective antilipidemic drugs. Consequently, the search for novel lipid-modifying drugs is still one of the most active areas in research and development in the pharmaceutical industry. Major focus lies on approaches to LDL-lowering drugs superior to statins with regard to efficacy, safety, and patient compliance and on approaches modifying plasma levels and functionality of HDL particles based on the clinically validated inverse relationship between high-plasma HDL levels and the risk for CVD. The available drugs today for the treatment of dyslipidemia are small organic molecules or nonabsorbable polymers for binding of bile acids to be applied orally. Besides small molecules for novel targets, biological drugs such as monoclonal antibodies, antisense or gene-silencing oligonucleotides, peptidomimetics, reconstituted synthetic HDL particles and therapeutic proteins are novel approaches in clinical development are which have to be applied by injection or infusion. The promising clinical results of several novel drug candidates, particularly for LDL cholesterol lowering with monoclonal antibodies raised against PCSK9, may indicate more than a decade after the statins, the entrance of new breakthrough therapies to treat lipid disorders.

Acute high-density lipoprotein therapies

PURPOSE OF REVIEW

Increasing interest has focused on the strategies that target the atheroprotective properties of HDL in order to reduce cardiovascular risk. The potential impact of strategies to acutely promote HDL functionality will be reviewed.

RECENT FINDINGS

Population and animal studies suggest that HDLs have a protective impact on atherosclerotic plaque. However, the failure of recent clinical trials of HDL cholesterol-raising agents has raised concerns that this may not be a viable strategy to reduce cardiovascular risk. Increasing attention has highlighted the importance of the functional quality, as opposed to quantity, of HDL with evidence of impaired HDL function in the setting of acute coronary syndromes (ACSs). The finding that infusing HDL in patients with recent acute ischemic events promotes the rapid regression of coronary atherosclerosis suggests a potentially useful strategy for ACS patients, although this remains to be fully established in large clinical outcome trials.

SUMMARY

Infusing HDL has favorable effects on coronary atherosclerosis in ACS patients, suggesting a potentially beneficial therapeutic strategy to acutely promote HDL functionality.

Cholesterol efflux capacity, macrophage reverse cholesterol transport and cardioprotective HDL

PURPOSE OF REVIEW

Randomized clinical trials provide strong evidence that pharmacological elevation of HDL-cholesterol (HDL-C) fails to reduce cardiovascular disease (CVD) risk in statin-treated humans. It is thus critical to identify new metrics that capture HDL's cardioprotective effects.

RECENT FINDINGS

We review recent evidence that HDL's cholesterol efflux capacity is a strong inverse predictor of incident and prevalent CVD in humans. In light of those findings, we assess the proposal that impaired macrophage cholesterol efflux to HDL contributes to disease risk. We also discuss recent studies implicating small HDL particles in cholesterol efflux from macrophages.

SUMMARY

These observations lay the foundation for a new approach to understanding mechanistically how HDL's functional properties help reduce CVD risk.

Baseline diabetes as a way to predict CV outcomes in a lipid-modifying trial: a meta-analysis of 330,376 patients from 47 landmark studies

BACKGROUND

Diabetes is a major cardiovascular risk factor. However, its influence on the rate of occurrence of cardiovascular (CV) events during a clinical trial that included a diabetes subgroup has not yet been quantified.

AIMS

To establish equations relating baseline diabetes prevalence and incident CV events, based on comparator arms data of major lipid-modifying trials.

METHODS

Meta-analysis of primary outcomes (PO) rates of key prospective trials, for which the baseline proportion of diabetics was reported, including studies having specifically reported CV outcomes within their diabetic subgroups.

RESULTS

47 studies, representing 330,376 patients (among whom 124,115 diabetics), were analyzed as regards the relationship between CV outcomes rates (including CHD) and the number of diabetics enrolled. Altogether, a total of 18,445 and 16,156 events occurred in the comparator and treatment arms, respectively. There were significant linear relationships between diabetes prevalence and both PO and CHD rates (%/year): y = 0.0299*x + 3.12 [PO] (p = 0.0128); and y = 0.0531*x + 1.54 [CHD] (p = 0.0094), baseline diabetes predicting PO rates between 3.12 %/year (no diabetic included) and 6.11 %/year (all patients diabetic); and CHD rates between 1.54 %/year (no diabetic) and 6.85 %/year (all patients diabetic). The slopes of the equations did not differ according to whether they were derived from primary or secondary prevention trials.

CONCLUSIONS

Absolute and relative CV risk associated with diabetes at inclusion can be readily predicted using linear equations relating diabetes prevalence to primary outcomes or CHD rates.

Salicylate improves macrophage cholesterol homeostasis via activation of Ampk

Atherosclerosis stems from imbalances in lipid metabolism and leads to maladaptive inflammatory responses. The AMP-activated protein kinase (Ampk) is a highly conserved serine/threonine kinase that regulates many aspects of lipid and energy metabolism, although its specific role in controlling macrophage cholesterol homeostasis remains unclear. We sought to address this question by testing the effects of direct Ampk activators in primary bone marrow-derived macrophages from Ampk β1-deficient (β1^−/−) mice. Macrophages from Ampk β1^−/− mice had enhanced lipogenic capacity and diminished cholesterol efflux, although cholesterol uptake was unaffected. Direct activation of Ampk β1 via salicylate (the unacetylated form of aspirin) or A-769662 (a small molecule activator), decreased the synthesis of FAs and sterols in WT but not Ampk β1^−/− macrophages. In lipid-laden macrophages, Ampk activation decreased cholesterol content (foam cell formation) and increased cholesterol efflux to HDL and apoA-I, effects that occurred in an Ampk β1-dependent manner. Increased cholesterol efflux was also associated with increased gene expression of the ATP binding cassette transporters, Abcg1 and Abca1. Moreover, in vivo reverse cholesterol transport was suppressed in mice that received Ampk β1^−/− macrophages compared with the WT control. Our data highlight the therapeutic potential of targeting macrophage Ampk with new or existing drugs for the possible reduction in foam cell formation during the early stages of atherosclerosis.

High-density lipoprotein: structural and functional changes under uremic conditions and the therapeutic consequences

High-density lipoprotein (HDL) has attracted interest as a therapeutic target in cardiovascular diseases in recent years. Although many functional mechanisms of the vascular protective effects of HDL have been identified, increasing the HDL plasma level has not been successful in all patient cohorts with increased cardiovascular risk. The composition of the HDL particle is very complex and includes diverse lipids and proteins that can be modified in disease conditions. In patients with chronic kidney disease (CKD), the accumulation of uremic toxins, high oxidative stress, and chronic micro-inflammatory conditions contribute to changes in the HDL composition and may also account for protein/lipid modifications. These conditions are associated with a decreased protective function of HDL. Therefore, the HDL quantity and the functional quality of the particle must be considered. This review summarizes the current knowledge of dyslipidemia in CKD patients, the effects of lipid-modulating therapy, and the structural modifications of HDL that are associated with dysfunction.

The clinical burden of type 2 diabetes in patients with acute coronary syndromes: prognosis and implications for short- and long-term management

Type 2 diabetes mellitus (T2DM) is associated with increased morbidity and mortality in patients with acute coronary syndromes (ACS). Cardiometabolic risk factors, including hyperglycaemia, insulin resistance, atherogenic dyslipidaemia, increased visceral fat and inflammation, are associated with increased risk in this population and represent potential targets for treatment. In this review, management strategies for patients with T2DM post-ACS, both in the acute-care setting and in the long-term, are discussed. Although the benefits of long-term, aggressive, multifactorial risk factor modification are well established, a significant burden of recurrent events remains and the search for novel strategies continues. Several studies are assessing the potential cardiovascular (CV) benefits and safety of various classes of newer agents. Of these, AleCardio (aleglitazar), Examination of Cardiovascular Outcomes With Alogliptin versus Standard of Care in Patients With Type 2 Diabetes Mellitus and Acute Coronary Syndrome (EXAMINE; alogliptin) and Evaluation of LIXisenatide in Acute Coronary Syndrome (ELIXA; lixisenatide) specifically address patients with type 2 diabetes post-ACS. The mechanisms of action of these new therapies and aims of the CV outcome studies are briefly reviewed. The prevalence of type 2 diabetes continues to increase worldwide highlighting the need for new strategies that address the complex underlying processes that drive atherosclerosis and CV events in this high-risk patient population.

CETP inhibitors downregulate hepatic LDL receptor and PCSK9 expression in vitro and in vivo through a SREBP2 dependent mechanism

BACKGROUND

CETP inhibitors block the transfer of cholesteryl ester from HDL-C to VLDL-C and LDL-C, thereby raising HDL-C and lowering LDL-C. In this study, we explored the effect of CETP inhibitors on hepatic LDL receptor (LDLR) and PCSK9 expression and further elucidated the underlying regulatory mechanism.

RESULTS

We first examined the effect of anacetrapib (ANA) and dalcetrapib (DAL) on LDLR and PCSK9 expression in hepatic cells in vitro. ANA exhibited a dose-dependent inhibition on both LDLR and PCSK9 expression in CETP-positive HepG2 cells and human primary hepatocytes as well as CETP-negative mouse primary hepatocytes (MPH). Moreover, the induction of LDLR protein expression by rosuvastatin in MPH was blunted by cotreatment with ANA. In both HepG2 and MPH ANA treatment reduced the amount of mature form of SREBP2 (SREBP2-M). In vivo, oral administration of ANA to dyslipidemic C57BL/6J mice at a daily dose of 50 mg/kg for 1 week elevated serum total cholesterol by approximately 24.5% (p < 0.05%) and VLDL-C by 70% (p < 0.05%) with concomitant reductions of serum PCSK9 and liver LDLR/SREBP2-M protein. Finally, we examined the in vitro effect of two other strong CETP inhibitors evacetrapib and torcetrapib on LDLR/PCSK9 expression and observed a similar inhibitory effect as ANA in a concentration range of 1-10 μM.

CONCLUSION

Our study revealed an unexpected off-target effect of CETP inhibitors that reduce the mature form of SREBP2, leading to attenuated transcription of hepatic LDLR and PCSK9. This negative regulation of SREBP pathway by ANA manifested in mice where CETP activity was absent and affected serum cholesterol metabolism.

Dusty punch cards and an eternal enigma: high-density lipoproteins and atherosclerosis

Epidemiological, clinical, and experimental evidence has accumulated during the last decades suggesting that high-density lipoproteins (HDLs) may protect from atherosclerosis and its clinical consequences. However, more than 55 years after the first description of the link between HDL and heart attacks, many facets of the biochemistry, function, and clinical significance of HDL remain enigmatic. This applies particularly to the completely unexpected results that became available from some recent clinical trials of nicotinic acid and of inhibitors of cholesteryl ester transfer protein (CETP). The concept that raising HDL cholesterol by pharmacological means would decrease the risk of vascular disease has therefore been challenged.

Decreased cholesterol efflux capacity in patients with low cholesteryl ester transfer protein plasma levels

BACKGROUND

Cholesteryl ester transfer protein (CETP) has been considered as a possible target for treatment of cardiovascular disease. However, first clinical studies employing CETP inhibitors have failed to demonstrate clinical benefit. Additionally, we have previously shown that low endogenous plasma levels of CETP are associated with increased mortality in coronary artery disease (CAD) patients. We hypothesized that low CETP plasma levels are associated with decreased high-density lipoprotein (HDL) function.

MATERIALS AND METHODS

Serum HDL efflux capacity was measured in 154 patients of the Ludwigshafen Risk and Cardiovascular Health (LURIC) study displaying extremely low (< 0·68 μg/mL, n = 77) or high (> 2·13 μg/mL, n = 77) CETP concentrations in their plasma, respectively. The LURIC study is a prospective observational study of patients referred to coronary angiography at baseline with a median follow-up time of 7·75 years. Primary and secondary endpoints were cardiovascular and all-cause mortality, respectively.

RESULTS

High CETP patients showed a significant increase in the capacity of their plasma to mediate cholesterol efflux from cholesterol laden macrophages when compared to the efflux capacity observed in low CETP patients (+ 5·4%, P = 0·015). As shown by multiregression analysis, the impact of CETP on cholesterol efflux capacity was independent from classical risk and lifestyle factors, as well as from lipid parameters including HDL cholesterol, LDL cholesterol and triglycerides.

CONCLUSIONS

Our findings indicate that low plasma concentrations of CETP might indeed lead to impaired HDL function within the reverse cholesterol transport pointing towards an atheroprotective role of CETP at least in patients with high risk of CAD.

The effect of cholesteryl ester transfer protein inhibition on lipids, lipoproteins, and markers of HDL function after an acute coronary syndrome: the dal-ACUTE randomized trial

AIMS

The effects of cholesteryl ester transfer protein (CETP) inhibition on lipids, inflammation, and markers of high-density lipoprotein (HDL) function, following an acute coronary syndrome (ACS), are unknown.

METHODS AND RESULTS

The dal-ACUTE study randomized 300 patients (1 : 1) to dalcetrapib 600 mg/day or placebo within 1 week of an ACS. The primary endpoint was per cent change in HDL-cholesterol (HDL-C) after 4 weeks. Secondary endpoints included apolipoprotein levels, markers of HDL function, and inflammation. Dalcetrapib treatment increased HDL-C and apolipoprotein A1 by 33.7 and 11.8%, respectively (both P < 0.001) and total cholesterol efflux by 9.5% (P = 0.003) after 4 weeks, principally via an increase in non-ATP-binding cassette transporter (ABC) A1-mediated efflux, without statistically significant changes in pre-β1-HDL levels. The increase in total efflux with dalcetrapib correlated most strongly with increases in apolipoprotein A1 and HDL-C (r = 0.46 and 0.43, respectively) rather than the increase in pre-β1-HDL (r = 0.32). Baseline and on-treatment ABCA1-mediated efflux correlated most strongly with pre-β1-HDL levels; in contrast, non-ABCA1-mediated efflux correlated better with apolipoprotein A1 and HDL-C levels.

CONCLUSIONS

High-density lipoprotein raised through CETP inhibition with dalcetrapib improves cholesterol efflux, principally via a non-ABCA1-mediated pathway. While HDL-C was increased by one-third, apolipoprotein A1 and total efflux were increased only by one-tenth, supporting the concept of dissociation between improvements in HDL function and HDL-C levels, which may be of relevance to ongoing trials and the development of therapeutic interventions targeting HDL.

[A history and review of cholesterol ester transfer protein inhibitors and their contribution to the understanding of the physiology and pathophysiology of high density lipoprotein]

There is irrefutable evidence that statins reduce the risk of cardiovascular events in a magnitude proportional to the intensity of the decrease in cholesterol transport by the low density lipoproteins. Despite this great advance there is still a residual risk of cardiovascular events. For this reason, an increase in the levels of high density lipoprotein is considered in order to boost the main action of this lipoprotein, which is reverse cholesterol transport. Distinct classes of evidence (epidemiological, genetic, and pathophysiological) show that the inhibition and/or modulation of cholesterol ester transfer protein increases plasma high density lipoprotein-cholesterol levels. The main reason for presenting this review is to look at the physiology of cholesterol ester transfer protein, its interrelationship with high density lipoproteins, and to give an update on the development of different cholesterol ester transfer protein inhibitor/modulator molecules.

Future of cholesteryl ester transfer protein (CETP) inhibitors: a pharmacological perspective

In almost 30 years since the introduction of HMG-CoA reductase inhibitors (statins), no other class of lipid modulators has entered the market. Elevation of high-density lipoprotein-cholesterol (HDL-C) via inhibiting cholesteryl ester transfer protein (CETP) is an attractive strategy for reducing the risk of cardiovascular events in high-risk patients. Transfer of triglyceride and cholesteryl ester (CE) between lipoproteins is mediated by CETP; thus inhibition of this pathway can increase the concentration of HDL-C. Torcetrapib was the first CETP inhibitor evaluated in phase III clinical trials. Because of off-target effects, torcetrapib raised blood pressure and increased the concentration of serum aldosterone, leading to higher cardiovascular events and mortality. Torcetrapib showed positive effects on cardiovascular risk especially in patients with a greater increase in HDL-C and apolipoprotein A-1 (apoA-1) levels. The phase III clinical trial of dalcetrapib, the second CETP inhibitor that has entered clinical development, was terminated because of ineffectiveness. Dalcetrapib is a CETP modulator that elevated HDL-C levels but did not reduce the concentration of low-density lipoprotein cholesterol (LDL-C). Both heterotypic and homotypic CE transfer between lipoproteins are mediated by some CETP inhibitors, including torcetrapib, anacetrapib, and evacetrapib, while dalcetrapib only affects the heterotypic CE transfer. Dalcetrapib has a chemical structure that is distinct from other CETP inhibitors, with a smaller molecular weight and a lack of trifluoride moieties. Moreover, dalcetrapib is a pro-drug that must be hydrolyzed to a pharmacologically active thiol form. Two other CETP inhibitors, anacetrapib and evacetrapib, are currently undergoing evaluation in phase III clinical trials. Both molecules have shown beneficial effects by increasing HDL-C and decreasing LDL-C concentration. The success of anacetrapib and evacetrapib remains to be confirmed upon the completion of phase III clinical trials in 2017 and 2015, respectively. Generally, the concentration of HDL-C has been considered a biomarker for the activity of CETP inhibitors. However, it is not clear whether a fundamental relationship exists between HDL-C levels and the risk of coronary artery diseases. The most crucial role for HDL is cholesterol efflux capacity in which HDL can reverse transport cholesterol from foam cells in atherosclerotic plaques. In view of the heterogeneity in HDL particle size, charge, and composition, the mere concentration of HDL-C may not be a good surrogate marker for HDL functionality. Recent clinical studies have reported that increased HDL functionality inversely correlates with the development of atherosclerotic plaque. Future development of CETP inhibitors may therefore benefit from the use of biomarkers of HDL functionality.

HDL and cholesterol: life after the divorce?

For decades, HDL and HDL-cholesterol (HDL-C) levels were viewed as synonymous, and modulation of HDL-C levels by drug therapy held great promise for the prevention and treatment of cardiovascular disease. Nevertheless, recent failures of drugs that raise HDL-C to reduce cardiovascular risk and the now greater understanding of the complexity of HDL composition and biology have prompted researchers in the field to redefine HDL. As such, the focus of HDL has now started to shift away from a cholesterol-centric view toward HDL particle number, subclasses, and other alternative metrics of HDL. Many of the recently discovered functions of HDL are, in fact, not strictly conferred by its ability to promote cholesterol flux but by the other molecules it transports, including a diverse set of proteins, small RNAs, hormones, carotenoids, vitamins, and bioactive lipids. Based on HDL's ability to interact with almost all cells and transport and deliver fat-soluble cargo, HDL has the remarkable capacity to affect a wide variety of endocrine-like systems. In this review, we characterize HDL's unique cargo and address the functional relevance and consequences of HDL transport and delivery of noncholesterol molecules to recipient cells and tissues.

Atherosclerosis and transit of HDL through the lymphatic vasculature

Key components of atherosclerotic plaque known to drive disease progression are macrophages and cholesterol. It has been widely understood, and bolstered by recent evidence, that the efflux of cholesterol from macrophage foam cells quells disease progression or even to promote regression. Following macrophage cholesterol efflux, cholesterol loaded onto HDL must be removed from the plaque environment. Here, we focus on recent evidence that the lymphatic vasculature is critical for the removal of cholesterol, likely as a component of HDL, from tissues including skin and the artery wall. We discuss the possibility that progression of atherosclerosis might in part be linked to sluggish removal of cholesterol from the plaque.

Function and distribution of apolipoprotein A1 in the artery wall are markedly distinct from those in plasma

BACKGROUND

Prior studies show that apolipoprotein A1 (apoA1) recovered from human atherosclerotic lesions is highly oxidized. Ex vivo oxidation of apoA1 or high-density lipoprotein (HDL) cross-links apoA1 and impairs lipid binding, cholesterol efflux, and lecithin-cholesterol acyltransferase activities of the lipoprotein. Remarkably, no studies to date directly quantify either the function or HDL particle distribution of apoA1 recovered from the human artery wall.

METHODS AND RESULTS

A monoclonal antibody (10G1.5) was developed that equally recognizes lipid-free and HDL-associated apoA1 in both native and oxidized forms. Examination of homogenates of atherosclerotic plaque-laden aorta showed >100-fold enrichment of apoA1 compared with normal aorta (P<0.001). Surprisingly, buoyant density fractionation revealed that only a minority (<3% of total) of apoA1 recovered from either lesions or normal aorta resides within an HDL-like particle (1.063≤d≤1.21). In contrast, the majority (>90%) of apoA1 within aortic tissue (normal and lesions) was recovered within the lipoprotein-depleted fraction (d>1.21). Moreover, both lesion and normal artery wall apoA1 are highly cross-linked (50% to 70% of total), and functional characterization of apoA1 quantitatively recovered from aorta with the use of monoclonal antibody 10G1.5 showed ≈80% lower cholesterol efflux activity and ≈90% lower lecithin-cholesterol acyltransferase activity relative to circulating apoA1.

CONCLUSIONS

The function and distribution of apoA1 in human aorta are quite distinct from those found in plasma. The lipoprotein is markedly enriched within atherosclerotic plaque, predominantly lipid-poor, not associated with HDL, extensively oxidatively cross-linked, and functionally impaired.

Multi-dimensional co-separation analysis reveals protein-protein interactions defining plasma lipoprotein subspecies

The distribution of circulating lipoprotein particles affects the risk for cardiovascular disease (CVD) in humans. Lipoproteins are historically defined by their density, with low-density lipoproteins positively and high-density lipoproteins (HDLs) negatively associated with CVD risk in large populations. However, these broad definitions tend to obscure the remarkable heterogeneity within each class. Evidence indicates that each class is composed of physically (size, density, charge) and compositionally (protein and lipid) distinct subclasses exhibiting unique functionalities and differing effects on disease. HDLs in particular contain upward of 85 proteins of widely varying function that are differentially distributed across a broad range of particle diameters. We hypothesized that the plasma lipoproteins, particularly HDL, represent a continuum of phospholipid platforms that facilitate specific protein-protein interactions. To test this idea, we separated normal human plasma using three techniques that exploit different lipoprotein physicochemical properties (gel filtration chromatography, ionic exchange chromatography, and preparative isoelectric focusing). We then tracked the co-separation of 76 lipid-associated proteins via mass spectrometry and applied a summed correlation analysis to identify protein pairs that may co-reside on individual lipoproteins. The analysis produced 2701 pairing scores, with the top hits representing previously known protein-protein interactions as well as numerous unknown pairings. A network analysis revealed clusters of proteins with related functions, particularly lipid transport and complement regulation. The specific co-separation of protein pairs or clusters suggests the existence of stable lipoprotein subspecies that may carry out distinct functions. Further characterization of the composition and function of these subspecies may point to better targeted therapeutics aimed at CVD or other diseases.