Proteomic diversity of high density lipoproteins: our emerging understanding of its importance in lipid transport and beyond

HDL particle number measured on the Vantera®, the first clinical NMR analyzer

OBJECTIVES

Nuclear magnetic resonance (NMR) spectroscopy has been successfully applied to the measurement of high-density lipoprotein (HDL) particles, providing particle concentrations for total HDL particle number (HDL-P), HDL subclasses (small, medium, large) and weighted, average HDL size for many years. Key clinical studies have demonstrated that NMR-measured HDL-P was more strongly associated with measures of coronary artery disease and a better predictor of incident cardiovascular disease (CVD) events than HDL-cholesterol (HDL-C). Recently, an NMR-based clinical analyzer, the Vantera(®), was developed to allow lipoprotein measurements to be performed in the routine, clinical laboratory setting. The aim of this study was to evaluate and report the performance characteristics for HDL-P quantified on the Vantera(®) Clinical Analyzer.

DESIGN AND METHODS

Assay performance was evaluated according to Clinical and Laboratory Standards Institute (CLSI) guidelines. In order to ensure that quantification of HDL-P on the Vantera(®) Clinical Analyzer was similar to the well-characterized HDL-P assay on the NMR profiler, a method comparison was performed.

RESULTS

The within-run and within-lab imprecision ranged from 2.0% to 3.9%. Linearity was established within the range of 10.0 to 65.0 μmol/L. The reference intervals were different between men (22.0 to 46.0 μmol/L) and women (26.7 to 52.9 μmol/L). HDL-P concentrations between two NMR platforms, Vantera(®) Clinical Analyzer and NMR Profiler, demonstrated excellent correlation (R(2) = 0.98).

CONCLUSIONS

The performance characteristics, as well as the primary tube sampling procedure for specimen analysis on the Vantera(®) Clinical Analyzer, suggest that the HDL-P assay is suitable for routine clinical applications.

Quantification of HDL proteins, cardiac events, and mortality in patients with type 2 diabetes on hemodialysis

BACKGROUND AND OBJECTIVES

Impairment of HDL function has been associated with cardiovascular events in patients with kidney failure. The protein composition of HDLs is altered in these patients, presumably compromising the cardioprotective effects of HDLs. This post hoc study assessed the relation of distinct HDL-bound proteins with cardiovascular outcomes in a dialysis population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The concentrations of HDL-associated serum amyloid A (SAA) and surfactant protein B (SP-B) were measured in 1152 patients with type 2 diabetes mellitus on hemodialysis participating in The German Diabetes Dialysis Study who were randomly assigned to double-blind treatment of 20 mg atorvastatin daily or matching placebo. The association of SAA(HDL) and SP-B(HDL) with cardiovascular outcomes was assessed in multivariate regression models adjusted for known clinical risk factors.

RESULTS

High concentrations of SAA(HDL) were significantly and positively associated with the risk of cardiac events (hazard ratio per 1 SD higher, 1.09; 95% confidence interval, 1.01 to 1.19). High concentrations of SP-B(HDL) were significantly associated with all-cause mortality (hazard ratio per 1 SD higher, 1.10; 95% confidence interval, 1.02 to 1.19). Adjustment for HDL cholesterol did not affect these associations.

CONCLUSIONS

In patients with diabetes on hemodialysis, SAA(HDL) and SP-B(HDL) were related to cardiac events and all-cause mortality, respectively, and they were independent of HDL cholesterol. These findings indicate that a remodeling of the HDL proteome was associated with a higher risk for cardiovascular events and mortality in patients with ESRD.

Intravenously injected human apolipoprotein A-I rapidly enters the central nervous system via the choroid plexus

Background

Brain lipoprotein metabolism is dependent on lipoprotein particles that resemble plasma high‐density lipoproteins but that contain apolipoprotein (apo) E rather than apoA‐I as their primary protein component. Astrocytes and microglia secrete apoE but not apoA‐I; however, apoA‐I is detectable in both cerebrospinal fluid and brain tissue lysates. The route by which plasma apoA‐I enters the central nervous system is unknown.

Methods and Results

Steady‐state levels of murine apoA‐I in cerebrospinal fluid and interstitial fluid are 0.664 and 0.120 μg/mL, respectively, whereas brain tissue apoA‐I is ≈10% to 15% of its levels in liver. Recombinant, fluorescently tagged human apoA‐I injected intravenously into mice localizes to the choroid plexus within 30 minutes and accumulates in a saturable, dose‐dependent manner in the brain. Recombinant, fluorescently tagged human apoA‐I accumulates in the brain for 2 hours, after which it is eliminated with a half‐life of 10.3 hours. In vitro, human apoA‐I is specifically bound, internalized, and transported across confluent monolayers of primary human choroid plexus epithelial cells and brain microvascular endothelial cells.

Conclusions

Following intravenous injection, recombinant human apoA‐I rapidly localizes predominantly to the choroid plexus. Because apoA‐I mRNA is undetectable in murine brain, our results suggest that plasma apoA‐I, which is secreted from the liver and intestine, gains access to the central nervous system primarily by crossing the blood–cerebrospinal fluid barrier via specific cellular mediated transport, although transport across the blood–brain barrier may also contribute to a lesser extent.

Quantification of HDL particle concentration by calibrated ion mobility analysis

BACKGROUND

It is critical to develop new metrics to determine whether HDL is cardioprotective in humans. One promising approach is HDL particle concentration (HDL-P), the size and concentration of HDL in plasma. However, the 2 methods currently used to determine HDL-P yield concentrations that differ >5-fold. We therefore developed and validated an improved approach to quantify HDL-P, termed calibrated ion mobility analysis (calibrated IMA).

METHODS

HDL was isolated from plasma by ultracentrifugation, introduced into the gas phase with electrospray ionization, separated by size, and quantified by particle counting. We used a calibration curve constructed with purified proteins to correct for the ionization efficiency of HDL particles.

RESULTS

The concentrations of gold nanoparticles and reconstituted HDLs measured by calibrated IMA were indistinguishable from concentrations determined by orthogonal methods. In plasma of control (n = 40) and cerebrovascular disease (n = 40) participants, 3 subspecies of HDL were reproducibility measured, with an estimated total HDL-P of 13.4 (2.4) μmol/L. HDL-C accounted for 48% of the variance in HDL-P. HDL-P was significantly lower in participants with cerebrovascular disease (P = 0.002), and this difference remained significant after adjustment for HDL cholesterol concentrations (P = 0.02).

CONCLUSIONS

Calibrated IMA accurately determined the concentration of gold nanoparticles and synthetic HDL, strongly suggesting that the method could accurately quantify HDL particle concentration. The estimated stoichiometry of apolipoprotein A-I determined by calibrated IMA was 3-4 per HDL particle, in agreement with current structural models. Furthermore, HDL-P was associated with cardiovascular disease status in a clinical population independently of HDL cholesterol.

Novel metabolic biomarkers of cardiovascular disease

Coronary heart disease (CHD) accounts for one in every six deaths in US individuals. Great advances have been made in identifying important risk factors for CHD, such as hypertension, diabetes mellitus, smoking and hypercholesterolaemia, which have led to major developments in therapy. In particular, statins represent one of the greatest successes in the prevention of CHD. While these standard risk factors are important, an obvious opportunity exists to take advantage of ongoing scientific research to better risk-stratify individuals and to identify new treatment targets. In this Review, we summarize ongoing scientific research in a number of metabolic molecules or features, including lipoproteins, homocysteine, calcium metabolism and glycaemic markers. We evaluate the current state of the research and the strength of evidence supporting each emerging biomarker. We also discuss whether the associations with CHD are strong and consistent enough to improve current risk stratification metrics, and whether these markers enhance our understanding of the underlying biology of CHD and thus point towards new treatment options.

Parallel reaction monitoring (PRM) and selected reaction monitoring (SRM) exhibit comparable linearity, dynamic range and precision for targeted quantitative HDL proteomics

High-density lipoprotein (HDL), a lipid nanoparticle containing many different low abundance proteins, is an attractive target for clinical proteomics because its compositional heterogeneity is linked to its cardioprotective effects. Selected reaction monitoring (SRM) is currently the method of choice for targeted quantification of proteins in such a complex biological matrix. However, model system studies suggest that parallel reaction monitoring (PRM) is more specific than SRM because many product ions can be used to confirm the identity of a peptide. We therefore compared PRM and SRM for their abilities to quantify proteins in HDL, using (15)N-labeled apolipoprotein A-I (HDL's most abundant protein) as the internal standard. PRM and SRM exhibited comparable linearity, dynamic range, precision, and repeatability for protein quantification of HDL. Moreover, the single internal standard protein performed as well as protein-specific peptide internal standards when quantifying 3 different proteins. Importantly, PRM and SRM yielded virtually identical quantitative results for 26 proteins in HDL isolated from 44 subjects. Because PRM requires less method development than SRM and is potentially more specific, our observations indicate that PRM in concert with a single isotope-labeled protein is a promising new strategy for quantifying HDL proteins in translational studies.

BIOLOGICAL SIGNIFICANCE

HDL, a complex matrix composed of lipids and proteins, is implicated in cardioprotection. Its cholesterol content correlates inversely with cardiovascular disease and it is the current metric to assess cardiovascular risk. However, the cholesterol content does not capture HDL's complexity and heterogeneity. Devising metrics that better capture HDL's cardioprotective effects, we developed an optimized method for quantification of HDL proteome, using PRM in concert with a single labeled protein as internal standard. The availability of a method that increases sample throughput without compromising the reproducibility, sensitivity, and accuracy could therefore point to better risk assessment for CVD or other diseases.

Understanding high-density lipoprotein function in disease: recent advances in proteomics unravel the complexity of its composition and biology

Although the epidemiology of high-density lipoprotein (HDL) cholesterol and cardiovascular risk has been consistent, pharmacologic interventions to increase HDL-cholesterol by delaying HDL catabolism did not translate into reduction in cardiovascular risk. HDL particles are small, protein-rich when compared to other plasma lipoprotein classes. Latest progresses in proteomics technology have dramatically increased our understanding of proteins carried by HDL. In addition to proteins with well-established functions in lipid transport, iron transport proteins, members of the complement pathway, and proteins involved in immune function and acute phase response were repeatedly identified on HDL particles. With the unraveling of the complexity of the HDL proteome, different laboratories have started to monitor its changes in various disease states. In addition, dynamic aspects of HDL subgroups are being discovered. These recent studies clearly illustrate the promise of HDL proteomics for deriving new biomarkers for disease diagnosis and to measure the effectiveness of current and future treatment regimens. This review summarizes recent advances in proteomics and lipidomics helping to understand HDL function in health and disease.

Antipsoriatic treatment extends beyond the skin: recovering of high-density lipoprotein function

Epidemiological and clinical studies have shown a consistent association of psoriasis with systemic metabolic disorders including an increased prevalence of diabetes, obesity and cardiovascular disease. Psoriasis is accompanied by systemic inflammation and low levels of high-density lipoprotein (HDL) cholesterol. Recent studies provided clear evidence that psoriasis affects HDL composition and function. HDL isolated from patients with psoriasis showed a significantly impaired capability to mobilize cholesterol from macrophages, a crucial step in reverse cholesterol transport and markedly lower paraoxonase activity, a protein that co-transports with HDL in serum with well-known anti-atherogenic properties. Of particular interest, successful antipsoriatic therapy significantly improved HDL composition and function independently of serum HDL cholesterol levels. These novel findings suggest that the conventional approaches of evaluating cardiovascular risk in psoriasis may be in need of refinement. As these data argue for a loss of beneficial activities of HDL in patients with psoriasis, altered HDL functionality should be considered when evaluating the lipid status of patients.

Restoration of renal function does not correct impairment of uremic HDL properties

Cardiovascular disease remains the leading cause of death in renal transplant recipients, but the underlying causative mechanisms for this important problem remain elusive. Recent work has indicated that qualitative alterations of HDL affect its functional and compositional properties in ESRD. Here, we systematically analyzed HDL from stable renal transplant recipients, according to graft function, and from patients with ESRD to determine whether structural and functional properties of HDL remain dysfunctional after renal transplantation. Cholesterol acceptor capacity and antioxidative activity, representing two key cardioprotective mechanisms of HDL, were profoundly suppressed in kidney transplant recipients independent of graft function and were comparable with levels in patients with ESRD. Using a mass spectroscopy approach, we identified specific remodeling of transplant HDL with highly enriched proteins, including α-1 microglobulin/bikunin precursor, pigment epithelium-derived factor, surfactant protein B, and serum amyloid A. In conclusion, this study demonstrates that HDL from kidney recipients is uniquely altered at the molecular and functional levels, indicating a direct pathologic role of HDL that could contribute to the substantial cardiovascular risk in the transplant population.

High-density lipoprotein subfractions: current views and clinical practice applications

High-density lipoprotein (HDL) is astonishingly complex, but the de facto standard for its measurement has been remarkably simple: total cholesterol content. It is time to prioritize higher-resolution HDL measurement techniques that capture better the biologically and clinically important characteristics of HDL. Scientific advances have ushered in a new era in which we view HDL in terms of its subfractions, particle structure, metabolism, and functional integration of its proteome and lipidome. HDL subfractions appear to be associated with function. In general, smaller, denser HDL3 is more tightly linked to favorable atheroprotective functions and clinical outcomes. Techniques to measure the cholesterol content or particle concentrations of HDL subfractions are available clinically. In the future, we anticipate subfractionating HDL based on its functional properties.

Obesity favors apolipoprotein E- and C-III-containing high density lipoprotein subfractions associated with risk of heart disease

Human HDLs have highly heterogeneous composition. Plasma concentrations of HDL with apoC-III and of apoE in HDL predict higher incidence of coronary heart disease (CHD). The concentrations of HDL-apoA-I containing apoE, apoC-III, or both and their distribution across HDL sizes are unknown. We studied 20 normal weight and 20 obese subjects matched by age, gender, and race. Plasma HDL was separated by sequential immunoaffinity chromatography (anti-apoA-I, anti-apoC-III, anti-apoE), followed by nondenaturing-gel electrophoresis. Mean HDL-cholesterol concentrations in normal weight and obese subjects were 65 and 50 mg/dl (P = 0.009), and total apoA-I concentrations were 119 and 118 mg/dl, respectively. HDL without apoE or apoC-III was the most prevalent HDL type representing 89% of apoA-I concentration in normal weight and 77% in obese (P = 0.01) individuals; HDL with apoE-only was 5% versus 8% (P = 0.1); HDL with apoC-III-only was 4% versus 10% (P = 0.009); and HDL with apoE and apoC-III was 1.5% versus 4.6% (P = 0.004). Concentrations of apoE and apoC-III in HDL were 1.5-2× higher in obese subjects (P ≤ 0.004). HDL with apoE or apoC-III occurred in all sizes among groups. Obese subjects had higher prevalence of HDL containing apoE or apoC-III, subfractions associated with CHD, whereas normal weight subjects had higher prevalence of HDL without apoE or apoC-III, subfractions with protective association against CHD.

HDL-mediated mechanisms of protection in cardiovascular disease

Low plasma levels of HDL-cholesterol (HDL-C) represent a strong and independent risk factor for cardiovascular disease. HDL particles display a wide spectrum of atheroprotective activities, which include effluxing cellular cholesterol, diminishing cellular death, decreasing vascular constriction, reducing inflammatory response, protecting from pathological oxidation, combating bacterial infection, lessening platelet activation, regulating gene expression by virtue of microRNAs, and improving glucose metabolism. It remains presently indeterminate as to whether some biological activities of HDL are more relevant for the protection of the endothelium from atherogenesis when compared with others. The multitude of such activities raises the question of a proper assay to assess HDL functionality ex vivo. Together with clear understanding of molecular mechanisms underlying atheroprotective properties of HDL, such assay will provide a basis to resolve the ultimate question of the HDL field to allow the development of efficient HDL-targeting therapies.

Atheroprotective role of high-density lipoprotein (HDL)-associated sphingosine-1-phosphate (S1P)

Numerous epidemiological studies documented an inverse relationship between plasma high-density lipoprotein (HDL) cholesterol levels and the extent of atherosclerotic disease. However, clinical interventions targeting HDL cholesterol failed to show clinical benefits with respect to cardiovascular risk reduction, suggesting that HDL components distinct from cholesterol may account for anti-atherogenic effects attributed to this lipoprotein. Sphingosine-1-phosphate (S1P)-a lysosphingolipid exerting its biological activity via binding to specific G protein-coupled receptors and regulating a wide array of biological responses in a variety of different organs and tissues including the cardiovascular system-has been identified as an integral constituent of HDL particles. In the present review, we discuss current evidence from epidemiological studies, experimental approaches in vitro, and animal models of atherosclerosis, suggesting that S1P contributes to atheroprotective effects exerted by HDL particles.

High-density lipoproteins and cerebrovascular integrity in Alzheimer's disease

Cerebrovascular dysfunction significantly contributes to the clinical presentation and pathoetiology of Alzheimer's disease (AD). Deposition and aggregation of β-amyloid (Aβ) within vascular smooth muscle cells leads to inflammation, oxidative stress, impaired vasorelaxation, and disruption of blood-brain barrier integrity. Midlife vascular risk factors, such as hypertension, cardiovascular disease, diabetes, and dyslipidemia, increase the relative risk for AD. These comorbidities are all characterized by low and/or dysfunctional high-density lipoproteins (HDL), which itself is a risk factor for AD. HDL performs a wide variety of critical functions in the periphery and CNS. In addition to lipid transport, HDL regulates vascular health via mediating vasorelaxation, inflammation, and oxidative stress and promotes endothelial cell survival and integrity. Here, we summarize clinical and preclinical data examining the involvement of HDL, originating from the circulation and from within the CNS, on AD and hypothesize potential synergistic actions between the two lipoprotein pools.

High-Density Lipoprotein

High-density lipoprotein (HDL) is a complex mixture of lipoproteins that is associated with many minor proteins and lipids that influence the function of HDL. Although HDL is a promising marker and potential therapeutic target based on its epidemiological data and the effects of healthy HDL in vitro in endothelial cells and macrophages, as well as based on infusion studies of reconstituted HDL in patients with hypercholesterolemia, it remains still uncertain whether or not HDL cholesterol–raising drugs will improve outcomes. Recent studies suggest that HDL becomes modified in patients with coronary artery disease or acute coronary syndrome because of oxidative processes that result in alterations in its proteome composition (proteome remodelling) leading to HDL dysfunction.

High density lipoproteins and ischemia reperfusion injury: the therapeutic potential of HDL to modulate cell survival pathways

The clinical importance of high density lipoproteins has grown in recent years with demonstrations of their impact on diverse pathological mechanisms implicated not only in vascular disease, but also in other physiological systems. This is related to the multiple functions associated with high-density lipoproteins (HDL), notably their ability to limit oxidant and inflammatory processes, which are common to different disease states. A second feature of particular clinical relevance is the possibility of synthesising a simplified form of HDL that exhibits some of the functions of the mature lipoprotein. The therapeutic potential of synthetic HDL is already under clinical scrutiny. To illustrate these points, the present chapter will discuss the role of HDL in limiting damage to the heart consequent to myocardial ischemia. It will review molecular survival pathways stimulated by HDL to combat oxidative stress and the potential of synthetic HDL to activate such pathways.

Biochemical and functional characterization of charge-defined subfractions of high-density lipoprotein from normal adults

High-density lipoprotein (HDL) is regarded as atheroprotective because it provides antioxidant and anti-inflammatory benefits and plays an important role in reverse cholesterol transport. In this paper, we outline a novel methodology for studying the heterogeneity of HDL. Using anion-exchange chromatography, we separated HDL from 6 healthy individuals into five subfractions (H1 through H5) with increasing charge and evaluated the composition and biologic activities of each subfraction. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis showed that apolipoprotein (apo) AI and apoAII were present in all 5 subfractions; apoCI was present only in H1, and apoCIII and apoE were most abundantly present in H4 and H5. HDL-associated antioxidant enzymes such as lecithin-cholesterol acyltransferase, lipoprotein-associated phospholipase A2, and paraoxonase 1 were most abundant in H4 and H5. Lipoprotein isoforms were analyzed in each subfraction by using matrix-assisted laser desorption-time-of-flight mass spectrometry. To quantify other proteins in the HDL subfractions, we used the isobaric tags for the relative and absolute quantitation approach followed by nanoflow liquid chromatography-tandem mass spectrometry analysis. Most antioxidant proteins detected were found in H4 and H5. The ability of each subfraction to induce cholesterol efflux from macrophages increased with increasing HDL electronegativity, with the exception of H5, which promoted the least efflux activity. In conclusion, anion-exchange chromatography is an attractive method for separating HDL into subfractions with distinct lipoprotein compositions and biologic activities. By comparing the properties of these subfractions, it may be possible to uncover HDL-specific proteins that play a role in disease.

Molecules that mimic apolipoprotein A-I: potential agents for treating atherosclerosis

Certain amphipathic α-helical peptides can functionally mimic many of the properties of full-length apolipoproteins, thereby offering an approach to modulate high-density lipoprotein (HDL) for combating atherosclerosis. In this Perspective, we summarize the key findings and advances over the past 25 years in the development of peptides that mimic apolipoproteins, especially apolipoprotein A-I (apoA-I). This assemblage of information provides a reasonably clear picture of the state of the art in the apolipoprotein mimetic field, an appreciation of the potential for such agents in pharmacotherapy, and a sense of the opportunities for optimizing the functional properties of HDL.

Functionally defective high-density lipoprotein and paraoxonase: a couple for endothelial dysfunction in atherosclerosis

The endothelium is the primary target for biochemical or mechanical injuries caused by the putative risk factors of atherosclerosis. Endothelial dysfunction represents the ultimate link between atherosclerotic risk factors that promote atherosclerosis. HDL-C is thought to exert at least some parts of its antiatherogenic facilities via stimulating endothelial NO production, nearby inhibiting oxidative stress and inflammation. HDL-C is capable of opposing LDL's inductive effects and avoiding the ox-LDL's inhibition of eNOS. Paraoxonase 1 (PON1) is an HDL-associated enzyme esterase which appears to contribute to the antioxidant and antiatherosclerotic capabilities of HDL-C. "Healthy HDL," namely the particle that contains the active Paraoxonase 1, has the power to suppress the formation of oxidized lipids. "Dysfunctional HDL," on the contrary, has reduced Paraoxonase 1 enzyme activity and not only fails in its mission but also potentially leads to greater formation of oxidized lipids/lipoproteins to cause endothelial dysfunction. The association of HDL-C PON1 and endothelial dysfunction depends largely on the molecules with exact damaging effect on NO synthase coupling. Loss of nitric oxide bioavailability has a pivotal role in endothelial dysfunction preceding the appearance of atherosclerosis. Analyses of HDL-C and Paraoxonase1 would be more important in the diagnosis and treatment of atherosclerosis in the very near future.

Serum IF1 concentration is independently associated to HDL levels and to coronary heart disease: the GENES study

HDL is strongly inversely related to cardiovascular risk. Hepatic HDL uptake is controlled by ecto-F1-ATPase activity, and potentially inhibited by mitochondrial inhibitor factor 1 (IF1). We recently found that IF1 is present in serum and correlates with HDL-cholesterol (HDL-C). Here, we have evaluated the relationship between circulating IF1 and plasma lipoproteins, and we determined whether IF1 concentration is associated with the risk of coronary heart disease (CHD). Serum IF1 was measured in 648 coronary patients ages 45-74 and in 669 matched male controls, in the context of a cross-sectional study on CHD. Cardiovascular risk factors were documented for each participant, including life-style habits and biological and clinical markers. In controls, multivariate analysis demonstrated that IF1 was independently positively associated with HDL-C and apoA-I (r = 0.27 and 0.28, respectively, P < 0.001) and negatively with triglycerides (r = -0.23, P < 0.001). Mean IF1 concentration was lower in CHD patients than in controls (0.43 mg/l and 0.53 mg/l, respectively, P < 0.001). In multivariate analyses, following adjustments on cardiovascular risk factors or markers, IF1 was negatively related to CHD (P < 0.001). This relationship was maintained after adjustment for HDL-C or apoA-I. This study identifies IF1 as a new determinant of HDL-C that is inversely associated with CHD.

Anti-psoriatic therapy recovers high-density lipoprotein composition and function

Psoriasis is a chronic inflammatory disorder associated with increased cardiovascular mortality. Psoriasis affects high-density lipoprotein (HDL) composition, generating dysfunctional HDL particles. However, data regarding the impact of anti-psoriatic therapy on HDL composition and function are not available. HDL was isolated from 15 psoriatic patients at baseline and after effective topical and/or systemic anti-psoriatic therapy and from 15 age- and sex-matched healthy controls. HDL from psoriatic patients showed a significantly impaired capability to mobilize cholesterol from macrophages (6.4 vs. 8.0% [(3)H]cholesterol efflux, P<0.001), low paraoxonase (217 vs. 350 μM(-1) minute(-1) mg(-1) protein, P=0.011) and increased Lp-PLA2 activities (19.9 vs. 12.1 nM(-1) minute(-1) mg(-1) protein, P=0.028). Of particular interest, the anti-psoriatic therapy significantly improved serum lecithin-cholesterol acyltransferase activity and decreased total serum lipolytic activity but did not affect serum levels of HDL-cholesterol. Most importantly, these changes were associated with a significantly improved HDL-cholesterol efflux capability. Our results provide evidence that effective anti-psoriatic therapy recovers HDL composition and function, independent of serum HDL-cholesterol levels, and support to the emerging concept that HDL function may be a better marker of cardiovascular risk than HDL-cholesterol levels.

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.

Volumetric determination of apolipoprotein stoichiometry of circulating HDL subspecies

Although HDL is inversely correlated with coronary heart disease, elevated HDL-cholesterol is not always protective. Additionally, HDL has biological functions that transcend any antiatherogenic role: shotgun proteomics show that HDL particles contain 84 proteins (latest count), many correlating with antioxidant and anti-inflammatory properties of HDL. ApoA-I has been suggested to serve as a platform for the assembly of these protein components on HDL with specific functions - the HDL proteome. However, the stoichiometry of apoA-I in HDL subspecies is poorly understood. Here we use a combination of immunoaffinity chromatography data and volumetric analysis to evaluate the size and stoichiometry of LpA-I and LpA-I,A-II particles. We conclude that there are three major LpA-I subspecies: two major particles, HDL[4] in the HDL3 size range (d = 85.0 ± 1.2 Å) and HDL[7] in the HDL2 size range (d = 108.5 ± 3.8 Å) with apoA-I stoichiometries of 3 and 4, respectively, and a small minor particle, HDL[1] (d = 73.8 ± 2.1Å) with an apoA-I stoichiometry of 2. Additionally, we conclude that the molar ratio of apolipoprotein to surface lipid is significantly higher in circulating HDL subspecies than in reconstituted spherical HDL particles, presumably reflecting a lack of phospholipid transfer protein in reconstitution protocols.

High density lipoproteins and endothelial functions: mechanistic insights and alterations in cardiovascular disease

Prospective population studies in the primary prevention setting have shown that reduced plasma levels of HDL cholesterol are associated with an increased risk of coronary disease and myocardial infarction. Experimental and translational studies have further revealed several potential anti-atherogenic effects of HDL, including protective effects on endothelial cell functions. HDL has been suggested to protect endothelial cell functions by prevention of oxidation of LDL and its adverse endothelial effects. Moreover, HDL from healthy subjects can directly stimulate endothelial cell production of nitric oxide and anti-inflammatory, anti-apoptotic, and anti-thrombotic effects as well as endothelial repair processes. However, several recent clinical trials using HDL cholesterol-raising agents, such as torcetrapib, dalcetrapib, and niacin, did not demonstrate a significant reduction of cardiovascular events in patients with coronary disease. Of note, growing evidence suggests that the vascular effects of HDL can be highly heterogeneous and vasoprotective properties of HDL are altered in patients with coronary disease. Characterization of underlying mechanisms and understanding of the clinical relevance of this "HDL dysfunction" is currently an active field of cardiovascular research. Notably, in some recent studies no clear association of higher HDL cholesterol levels with a reduced risk of cardiovascular events was observed in patients with already established coronary disease. A greater understanding of mechanisms of action of HDL and its altered vascular effects is therefore critical within the context of HDL-targeted therapies. In this review, we will address different effects of HDL on endothelial cell functions potentially relevant to atherosclerotic vascular disease and explore molecular mechanisms leading to "dysfunctional HDL".

Aging affects high-density lipoprotein composition and function

Most coronary deaths occur in patients older than 65years. Age associated alterations in the composition and function of high-density lipoproteins (HDL) may contribute to cardiovascular mortality. The effect of advanced age on the composition and function of HDL is not well understood. HDL was isolated from healthy young and elderly subjects. HDL composition, cellular cholesterol efflux/uptake, anti-oxidant properties and paraoxonase activity were assessed. We observed a 3-fold increase of the acute phase protein serum amyloid A, an increased content of complement C3 and proteins involved in endopeptidase/protease inhibition in HDL of elderly subjects, whereas levels of apolipoprotein E were significantly decreased. HDL from elderly subjects contained less cholesterol but increased sphingomyelin. Most importantly, HDL from elderly subjects showed defective antioxidant properties, lower paraoxonase 1 activity and was more rapidly taken up by macrophages, whereas cholesterol efflux capability was not altered. These findings suggest that aging alters HDL composition, resulting in functional impairment that may contribute to the onset/progression of cardiovascular disease.