Why targeting HDL should work as a therapeutic tool, but has not

Imbalance of APOB Lipoproteins and Large HDL in Type 1 Diabetes Drives Atherosclerosis

BACKGROUND

Individuals with type 1 diabetes (T1D) generally have normal or even higher HDL (high-density lipoprotein)-cholesterol levels than people without diabetes yet are at increased risk for atherosclerotic cardiovascular disease (CVD). Human HDL is a complex mixture of particles that can vary in cholesterol content by >2-fold. To investigate if specific HDL subspecies contribute to the increased atherosclerosis associated with T1D, we created mouse models of T1D that exhibit human-like HDL subspecies. We also measured HDL subspecies and their association with incident CVD in a cohort of people with T1D.

METHODS

We generated LDL receptor-deficient (Ldlr-/-) mouse models of T1D expressing human APOA1 (apolipoprotein A1). Ldlr-/-APOA1Tg mice exhibited the main human HDL subspecies. We also generated Ldlr-/-APOA1Tg T1D mice expressing CETP (cholesteryl ester transfer protein), which had lower concentrations of large HDL subspecies versus mice not expressing CETP. HDL particle concentrations and sizes and proteins involved in lipoprotein metabolism were measured by calibrated differential ion mobility analysis and targeted mass spectrometry in the mouse models of T1D and in a cohort of individuals with T1D. Endothelial transcytosis was analyzed by total internal reflection fluorescence microscopy.

RESULTS

Diabetic Ldlr-/-APOA1Tg mice were severely hyperglycemic and hyperlipidemic and had markedly elevated plasma APOB levels versus nondiabetic littermates but were protected from the proatherogenic effects of diabetes. Diabetic Ldlr-/-APOA1Tg mice expressing CETP lost the atheroprotective effect and had increased lesion necrotic core areas and APOB accumulation, despite having lower plasma APOB levels. The detrimental effects of low concentrations of larger HDL particles in diabetic mice expressing CETP were not explained by reduced cholesterol efflux. Instead, large HDL was more effective than small HDL in preventing endothelial transcytosis of LDL mediated by scavenger receptor class B type 1. Finally, in humans with T1D, increased concentrations of larger HDL particles relative to APOB100 negatively predicted incident CVD independently of HDL-cholesterol levels.

CONCLUSIONS

Our results suggest that the balance between APOB lipoproteins and the larger HDL subspecies contributes to atherosclerosis progression and incident CVD in the setting of T1D and that larger HDLs exert atheroprotective effects on endothelial cells rather than by promoting macrophage cholesterol efflux.

The association of appendicular lean mass and grip strength with low-density lipoprotein, very low-density lipoprotein, and high-density lipoprotein particle diameter: a Mendelian randomization study of the UK Biobank cohort

Aims

Reduced muscle mass and reduced strength are frequently associated with both alterations in blood lipids and poorer cardiometabolic outcomes in epidemiological studies; however, a causal association cannot be determined from such observations. Two-sample Mendelian randomization (MR) was applied to assess the association of genetically determined appendicular lean mass (ALM) and handgrip strength (HGS) with serum lipid particle diameter.

Methods and results

Mendelian randomization was implemented using summary-level data from the largest genome-wide association studies on ALM (n = 450 243), HGS (n = 223 315), and lipoprotein [low-density lipoprotein (LDL), very LDL (VLDL), and high-density lipoprotein (HDL)] particle diameters (n = 115 078). Inverse variance-weighted (IVW) method was used to calculate the causal estimates. Weighted median-based method, MR-Egger, and leave-one-out method were applied as sensitivity analysis. Greater ALM had a statistically significant positive effect on HDL particle diameter (MR-Egger: β = 0.055, SE = 0.031, P = 0.081; IVW: β = 0.068, SE = 0.014, P < 0.001) and a statistically significant negative effect on VLDL particle diameter (MR-Egger: β = -0.114, SE = 0.039, P = 0.003; IVW: β = -0.081, SE = 0.017, P < 0.001). Similarly, greater HGS had a statistically significant positive effect on HDL particle diameter (MR-Egger: β = 0.433, SE = 0.184, P = 0.019; IVW: β = 0.121, SE = 0.052, P = 0.021) and a statistically significant negative effect on VLDL particle diameter (MR-Egger: β = -0.416, SE = 0.163, P = 0.011; IVW: β = -0.122, SE = 0.046, P = 0.009). There was no statistically significant effect of either ALM or HGS on LDL particle diameter.

Conclusion

There were potentially causal associations between both increasing ALM and HGS and increasing HDL particle size and decreasing VLDL particle size. These causal associations may offer possibilities for interventions aimed at improving cardiovascular disease risk profile.

Associations of genetically determined lipid traits and lipid-modifying agents with the risk of diabetic retinopathy: A Mendelian randomization study

BACKGROUND AND AIMS

The evidence that dyslipidemia is associated with hyperglycemia calls for an investigation of whether dyslipidemia, as well as lipid-modifying agents, could affect the subsequent development of diabetic retinopathy (DR). Therefore, we aimed to address these unanswered questions by utilizing Mendelian randomization (MR) analysis.

METHODS

Genetic variants were selected from the UK Biobank as instruments to serve as proxies for lipid traits [high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglyceride (TG), apolipoprotein A-I (APOA-I) and apolipoprotein B (APOB)]. Univariable and multivariable MR analyses were performed to examine the associations of these lipid traits with DR and different levels of severity of DR. Based on the evidence for the effects of lipids on outcomes, we estimated the causal relevance of cholesteryl ester transfer protein (CETP) inhibitors in severe nonproliferative and proliferative DR using protein quantitative trait loci (pQTLs) and expression quantitative trait loci (eQTLs) as instruments.

RESULTS

Genetically determined HDL-C levels were inversely associated with the risk of severe nonproliferative DR (OR = 0.70, 95% CI = 0.52-0.94) and proliferative DR (OR = 0.90, 95% CI = 0.83-0.97) in the main analyses utilizing the inverse variance-weighted (IVW) MR method and a couple of sensitivity analyses. No association was noted between genetically proxied CETP inhibitors and DR.

CONCLUSIONS

This MR study suggests the casual protective roles of HDL-C in severe nonproliferative DR and proliferative DR, which calls for further studies to confirm these findings. Current lipid-modifying agents acting on HDL-C may not reduce the risk of DR and new treatments are required in the future.

A calorie-restricted diet with nuts favourably raises plasma high-density lipoprotein-cholesterol in overweight and obese patients with stable coronary heart disease: A randomised controlled trial

BACKGROUND

Weight loss and consumption of nuts may both improve plasma lipids and lipoproteins. However, their effects in patients with coronary artery disease (CAD) who are under treatment with statins remain unclear. This study was conducted to determine plasma lipid and lipoprotein responses to a low-calorie (25% of energy deficit) nuts containing diet (NELCD) compared with a nut-free energy-restricted diet (NFLCD) in stable CAD patients who were overweight or obese (n = 67).

METHODS

Plasma lipids and lipoproteins and ATP-binding cassette (ABC) A1 and ABCG1 mRNA expressions in peripheral blood mononuclear cells (PBMCs) isolated from patients were assessed at baseline and after 8 weeks.

RESULTS

Plasma high-density lipoprotein (HDL) cholesterol and apoA1 increased significantly in the NELCD group at the end of Week 8 compared with those in NFLCD group (in NELCD group, mean change and 95% confidence interval (CI) were 1.17 [-1.31, 3.66] mg/dL, P = 0.012, and 2.55 [-2.10, 7.20] mg/dL, P = 0.011, for HDL cholesterol and apoA1, respectively). In both groups, the concentrations of total cholesterol, low-density lipoprotein cholesterol and triglyceride reduced significantly at Week 8 compared with those in baseline with no significant difference between the two groups. ABCA1 and ABCG1 mRNA expressions non-significantly decreased in both groups.

CONCLUSION

This study indicates that inclusion of nuts as part of a calorie-restricted diet improves the concentrations of HDL cholesterol and apoA-1 in patients with stable, established CHD receiving statins.

APOL1 Genetic Variants Are Associated With Increased Risk of Coronary Atherosclerotic Plaque Rupture in the Black Population

[Figure: see text].

High-Density Lipoprotein Functionality as a New Pharmacological Target on Cardiovascular Disease: Unifying Mechanism That Explains High-Density Lipoprotein Protection Toward the Progression of Atherosclerosis

The formation of the atherosclerotic plaque that is characterized by the accumulation of abnormal amounts of cholesterol-loaded macrophages in the artery wall is mediated by both inflammatory events and alterations of lipid/lipoprotein metabolism. Reverse transport of cholesterol opposes the formation and development of atherosclerotic plaque by promoting high density lipoprotein (HDL)-mediated removal of cholesterol from peripheral macrophages and its delivery back to the liver for excretion into the bile. Although an inverse association between HDL plasma levels and the risk of cardiovascular disease (CVD) has been demonstrated over the years, several studies have recently shown that the antiatherogenic functions of HDL seem to be mediated by their functionality, not always associated with their plasma concentrations. Therefore, assessment of HDL function, evaluated as the capacity to promote cell cholesterol efflux, may offer a better prediction of CVD than HDL levels alone. In agreement with this idea, it has recently been shown that the assessment of serum cholesterol efflux capacity (CEC), as a metric of HDL functionality, may represent a predictor of atherosclerosis extent in humans. The purpose of this narrative review is to summarize the current evidence concerning the role of cholesterol efflux capacity that is important for evaluating CVD risk, focusing on pharmacological evidences and its relationship with inflammation. We conclude that HDL therapeutics are a promising area of investigation but strategies for identifying efficacy must move beyond the idea of simply raising static HDL-cholesterol levels and toward methods of measuring the dynamics of HDL particle remodeling and the generation of lipid-free apolipoprotein A-I (apoA-I). In this way, apoA-I, unlike mature HDL, can promote the greatest extent of cholesterol efflux relieving cellular cholesterol toxicity and the inflammation it causes.

Bariatric surgery as a model to explore the basis and consequences of the Reaven hypothesis: Small, dense low-density lipoprotein and interleukin-6

BACKGROUND

Reaven originally described the clustering of insulin resistance/hyperinsulinaemia, obesity (particularly visceral), altered cytokine levels, glucose intolerance, hypertriglyceridaemia and low high-density lipoprotein cholesterol. Subsequently, a potentially highly atherogenic small, dense low-density lipoprotein was also reported. We have studied the effect of bariatric surgery on this and other risk factors for atherosclerosis.

METHODS

Forty patients (20 with type 2 diabetes mellitus) undergoing bariatric surgery were studied before and 1 year after bariatric surgery.

RESULTS

Twelve months after bariatric surgery, median body mass index had decreased from 49.5 to 36.5 kg/m², fasting insulin from 21.3 to 7.8 mU/L and insulin resistance (homeostatic model assessment of insulin resistance) from 5.9 to 1.8 (all p < 0.001). Thirteen out of 20 patients had remission from type 2 diabetes mellitus. Highly sensitive C-reactive protein, interleukin-6, fasting triglycerides ( p < 0.001) and small, dense low-density lipoprotein ( p < 0.001) decreased, while high-density lipoprotein cholesterol increased ( p < 0.001) significantly, irrespective of having type 2 diabetes mellitus and/or being treated with statin therapy before surgery.

CONCLUSION

The association between marked weight loss and change in insulin resistance and hyperinsulinaemia with the change in small, dense low-density lipoprotein and interleukin-6 warrants further investigation. Bariatric surgery provides a model for investigating the mechanisms linking insulin resistance/hyperinsulinaemia to atherosclerosis.

Apolipoprotein AI prevents regulatory to follicular helper T cell switching during atherosclerosis

Regulatory T (Treg) cells contribute to the anti-inflammatory response during atherogenesis. Here we show that during atherogenesis Treg cells lose Foxp3 expression and their immunosuppressive function, leading to the conversion of a fraction of these cells into T follicular helper (Tfh) cells. We show that Tfh cells are pro-atherogenic and that their depletion reduces atherosclerosis. Mechanistically, the conversion of Treg cells to Tfh cells correlates with reduced expression of IL-2Rα and pSTAT5 levels and increased expression of IL-6Rα. In vitro, incubation of naive T cells with oxLDL prevents their differentiation into Treg cells. Furthermore, injection of lipid-free Apolipoprotein AI (ApoAI) into ApoE^−/− mice reduces intracellular cholesterol levels in Treg cells and prevents their conversion into Tfh cells. Together our results suggest that ApoAI, the main protein in high-density lipoprotein particles, modulates the cellular fate of Treg cells and thus influences the immune response during atherosclerosis.

Regulatory T (Treg) cells contribute to the anti-inflammatory response during atherogenesis. Here Gaddis et al. show that Apolipoprotein AI prevents the conversion of Treg cells into pro-atherogenic T follicular helper cells, and thus regulates the immune response during atherogenesis.

Apo A-I (Apolipoprotein A-I) Vascular Gene Therapy Provides Durable Protection Against Atherosclerosis in Hyperlipidemic Rabbits

OBJECTIVE

Gene therapy that expresses apo A-I (apolipoprotein A-I) from vascular wall cells has promise for preventing and reversing atherosclerosis. Previously, we reported that transduction of carotid artery endothelial cells with a helper-dependent adenoviral (HDAd) vector expressing apo A-I reduced early (4 weeks) fatty streak development in fat-fed rabbits. Here, we tested whether the same HDAd could provide long-term protection against development of more complex lesions.

APPROACH AND RESULTS

Fat-fed rabbits (n=25) underwent bilateral carotid artery gene transfer, with their left and right common carotids randomized to receive either a control vector (HDAdNull) or an apo A-I-expressing vector (HDAdApoAI). Twenty-four additional weeks of high-fat diet yielded complex intimal lesions containing lipid-rich macrophages as well as smooth muscle cells, often in a lesion cap. Twenty-four weeks after gene transfer, high levels of apo A-I mRNA (median ≥250-fold above background) were present in all HDAdApoAI-treated arteries. Compared with paired control HDAdNull-treated arteries in the same rabbit, HDAdApoAI-treated arteries had 30% less median intimal lesion volume (P=0.03), with concomitant reductions (23%-32%) in intimal lipid, macrophage, and smooth muscle cell content (P≤0.05 for all). HDAdApoAI-treated arteries also had decreased intimal inflammatory markers. VCAM-1 (vascular cell adhesion molecule-1)-stained area was reduced by 36% (P=0.03), with trends toward lower expression of ICAM-1 (intercellular adhesion molecule-1), MCP-1 (monocyte chemoattractant protein 1), and TNF-α (tumor necrosis factor-α; 13%-39% less; P=0.06-0.1).

CONCLUSIONS

In rabbits with severe hyperlipidemia, transduction of vascular endothelial cells with an apo A-I-expressing HDAd yields at least 24 weeks of local apo A-I expression that durably reduces atherosclerotic lesion growth and intimal inflammation.

Apolipoprotein C-III in the high-density lipoprotein proteome of cerebral lacunar infarction patients impairs its anti-inflammatory function

High-density lipoprotein (HDL) proteomic study has identified substantial changes associated with various disease states. In the current study, the HDL proteomes in patients with cerebral lacunar infarction (LACI) and control subjects were investigated. A total of 12 LACI patients without evident large vessel occlusions and 12 controls were enrolled in the study. The HDL fraction from each sample was isolated from the plasma by ultracentrifugation. The protemics of the HDL were investigated using nano liquid chromatography coupled to tandem mass spectrometry. There were 55 proteins identified as differentially expressed in the LACI and control groups. Among the 55 proteins, 33 were upregulated and 22 were downregulated in the patients with LACI. The identified proteins were associated with numerous molecular functions, including lipid and cholesterol transport, lipid metabolism, inflammatory response, the complement and coagulation pathway, metal ion metabolism, hemostasis and endopeptidase inhibitory activity. Serum amyloid A, apolipoprotein C (apoC-III) and apolipoprotein A-II (apoA-II) were selected to confirm the proteomics results via western blotting. HDL from the LACI patients exhibited an impaired ability to inhibit the binding of THP-1 cells to endothelial cells compared with the controls (P<0.01). ApoC-III-rich HDL also had a significantly reduced ability to inhibit the binding of THP-1 cells to endothelial cells (P<0.01). The expression of vascular cell adhesion molecule-1 protein by the endothelial cells exhibited a similar pattern of response to the different HDL samples. In conclusion, the present study demonstrates major modifications of the HDL proteome in patients with LACI. The ApoC-III enrichment of the HDL of patients with LACI may cause a reduction in the anti-inflammatory ability of HDL, which may contribute to the progression of the disease.

Automation of PRM-dependent D3-Leu tracer enrichment in HDL to study the metabolism of apoA-I, LCAT and other apolipoproteins

We developed an automated quantification workflow for PRM-enabled detection of D3-Leu labeled apoA-I in high-density lipoprotein (HDL) isolated from humans. Subjects received a bolus injection of D3-Leu and blood was drawn at eight time points over three days. HDL was isolated and separated into six size fractions for subsequent proteolysis and PRM analysis for the detection of D3-Leu signal from ∼0.03 to 0.6% enrichment. We implemented an intensity-based quantification approach that takes advantage of high-resolution/accurate mass PRM scans to identify the D3-Leu 2HM3 ion from non-specific peaks. Our workflow includes five modules for extracting the targeted PRM peak intensities (XPIs): Peak centroiding, noise removal, fragment ion matching using Δm/z windows, nine intensity quantification options, and validation and visualization outputs. We optimized the XPI workflow using in vitro synthesized and clinical samples of D0/D3-Leu labeled apoA-I. Three subjects' apoA-I enrichment curves in six HDL size fractions, and LCAT, apoA-II and apoE from two size fractions were generated within a few hours. Our PRM strategy and automated quantification workflow will expedite the turnaround of HDL apoA-I metabolism data in clinical studies that aim to understand and treat the mechanisms behind dyslipidemia.

Lipid-Free Apolipoprotein A-I Reduces Progression of Atherosclerosis by Mobilizing Microdomain Cholesterol and Attenuating the Number of CD131 Expressing Cells: Monitoring Cholesterol Homeostasis Using the Cellular Ester to Total Cholesterol Ratio

BACKGROUND

Atherosclerosis is a chronic inflammatory disorder whose development is inversely correlated with high-density lipoprotein concentration. Current therapies involve pharmaceuticals that significantly elevate plasma high-density lipoprotein cholesterol concentrations. Our studies were conducted to investigate the effects of low-dose lipid-free apolipoprotein A-I (apoA-I) on chronic inflammation. The aims of these studies were to determine how subcutaneously injected lipid-free apoA-I reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without sustained elevations in plasma high-density lipoprotein cholesterol concentrations.

METHODS AND RESULTS

Ldlr^-/- and Ldlr^-/- apoA-I^-/- mice were fed a Western diet for a total of 12 weeks. After 6 weeks, a subset of mice from each group received subcutaneous injections of 200 μg of lipid-free human apoA-I 3 times a week, while the other subset received 200 μg of albumin, as a control. Mice treated with lipid-free apoA-I showed a decrease in cholesterol deposition and immune cell retention in the aortic root compared with albumin-treated mice, regardless of genotype. This reduction in atherosclerosis appeared to be directly related to a decrease in the number of CD131 expressing cells and the esterified cholesterol to total cholesterol content in several immune cell compartments. In addition, apoA-I treatment altered microdomain cholesterol composition that shifted CD131, the common β subunit of the interleukin 3 receptor, from lipid raft to nonraft fractions of the plasma membrane.

CONCLUSIONS

ApoA-I treatment reduced lipid and immune cell accumulation within the aortic root by systemically reducing microdomain cholesterol content in immune cells. These data suggest that lipid-free apoA-I mediates beneficial effects through attenuation of immune cell lipid raft cholesterol content, which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation.

High-Density Lipoprotein Biogenesis: Defining the Domains Involved in Human Apolipoprotein A-I Lipidation

The first step in removing cholesterol from a cell is the ATP-binding cassette transporter 1 (ABCA1)-driven transfer of cholesterol to lipid-free or lipid-poor apolipoprotein A-I (apoA-I), which yields cholesterol-rich nascent high-density lipoprotein (nHDL) that then matures in plasma to spherical, cholesteryl ester-rich HDL. However, lipid-free apoA-I has a three-dimensional (3D) conformation that is significantly different from that of lipidated apoA-I on nHDL. By comparing the lipid-free apoA-I 3D conformation of apoA-I to that of 9-14 nm diameter nHDL, we formulated the hypothetical helical domain transitions that might drive particle formation. To test the hypothesis, ten apoA-I mutants were prepared that contained two strategically placed cysteines several of which could form intramolecular disulfide bonds and others that could not form these bonds. Mass spectrometry was used to identify amino acid sequence and intramolecular disulfide bond formation. Recombinant HDL (rHDL) formation was assessed with this group of apoA-I mutants. ABCA1-driven nHDL formation was measured in four mutants and wild-type apoA-I. The mutants contained cysteine substitutions in one of three regions: the N-terminus, amino acids 34 and 55 (E34C to S55C), central domain amino acids 104 and 162 (F104C to H162C), and the C-terminus, amino acids 200 and 233 (L200C to L233C). Mutants were studied in the locked form, with an intramolecular disulfide bond present, or unlocked form, with the cysteine thiol blocked by alkylation. Only small amounts of rHDL or nHDL were formed upon locking the central domain. We conclude that both the N- and C-terminal ends assist in the initial steps in lipid acquisition, but that opening of the central domain was essential for particle formation.

Microdomains, Inflammation, and Atherosclerosis

Elevated levels of cholesteryl ester (CE)-enriched apoB containing plasma lipoproteins lead to increased foam cell formation, the first step in the development of atherosclerosis. Unregulated uptake of low-density lipoprotein cholesterol by circulating monocytes and other peripheral blood cells takes place through scavenger receptors and over time causes disruption in cellular cholesterol homeostasis. As lipoproteins are taken up, their CE core is hydrolyzed by liposomal lipases to generate free cholesterol (FC). FC can be either re-esterified and stored as CE droplets or shuttled to the plasma membrane for ATP-binding cassette transporter A1-mediated efflux. Because cholesterol is an essential component of all cellular membranes, some FC may be incorporated into microdomains or lipid rafts. These platforms are essential for receptor signaling and transduction, requiring rapid assembly and disassembly. ATP-binding cassette transporter A1 plays a major role in regulating microdomain cholesterol and is most efficient when lipid-poor apolipoprotein AI (apoAI) packages raft cholesterol into soluble particles that are eventually catabolized by the liver. If FC is not effluxed from the cell, it becomes esterified, CE droplets accumulate and microdomain cholesterol content becomes poorly regulated. This dysregulation leads to prolonged activation of immune cell signaling pathways, resulting in receptor oversensitization. The availability of apoAI or other amphipathic α-helix-rich apoproteins relieves the burden of excess microdomain cholesterol in immune cells allowing a reduction in immune cell proliferation and infiltration, thereby stimulating regression of foam cells in the artery. Therefore, cellular balance between FC and CE is essential for proper immune cell function and prevents chronic immune cell overstimulation and proliferation.

Anthocyanin-rich black elderberry extract improves markers of HDL function and reduces aortic cholesterol in hyperlipidemic mice

Serum high-density lipoprotein-cholesterol (HDL-C) is a risk factor considered to be protective of atherosclerosis. However, atherosclerosis is an inflammatory disease and contributes to impairment in high-density lipoprotein (HDL) function, including reductions in HDL-C, HDL antioxidant and anti-inflammatory activities. Anthocyanins are polyphenols that have demonstrated antioxidant and anti-inflammatory properties. The objective of this study was to determine whether an anthocyanin-rich black elderberry extract (Sambucus nigra) (BEE) (13% anthocyanins) would protect against inflammation-related impairments in HDL function and atherosclerosis in apoE(-/-) mice, a mouse model of hyperlipidemia and HDL dysfunction. We fed an AIN-93M diet supplemented with 1.25% (w/w) BEE or control diet to 10 week old male apoE(-/-) mice for 6 weeks. The BEE fed to mice was rich in cyanidin 3-sambubioside (∼ 9.8% w/w) and cyanidin 3-glucoside (∼ 3.8% w/w). After 6 weeks, serum lipids did not differ significantly between groups, while aspartate transaminase (AST) and fasting glucose were reduced in BEE-fed mice. Hepatic and intestinal mRNA changes with BEE-feeding were consistent with an improvement in HDL function (Apoa1, Pon1, Saa1, Lcat, Clu) and a reduction in hepatic cholesterol levels (increased Ldlr and Hmgcr, reduced Cyp7a1). In BEE-fed mice, serum paraoxonase-1 (PON1) arylesterase activity was significantly higher. In addition, mice fed BEE had significantly lower serum chemokine (C-C motif) ligand 2 (CCL2) compared to control-fed mice. Notably, we observed significant reductions in total cholesterol content of the aorta of BEE-fed mice, indicating less atherosclerosis progression. This study suggests that black elderberry may have the potential to influence HDL dysfunction associated with chronic inflammation by impacting hepatic gene expression.

Dysfunctional High-Density Lipoprotein: An Innovative Target for Proteomics and Lipidomics

High-Density Lipoprotein-Cholesterol (HDL-C) is regarded as an important protective factor against cardiovascular disease, with abundant evidence of an inverse relationship between its serum levels and risk of cardiovascular disease, as well as various antiatherogenic, antioxidant, and anti-inflammatory properties. Nevertheless, observations of hereditary syndromes featuring scant HDL-C concentration in absence of premature atherosclerotic disease suggest HDL-C levels may not be the best predictor of cardiovascular disease. Indeed, the beneficial effects of HDL may not depend solely on their concentration, but also on their quality. Distinct subfractions of this lipoprotein appear to be constituted by specific protein-lipid conglomerates necessary for different physiologic and pathophysiologic functions. However, in a chronic inflammatory microenvironment, diverse components of the HDL proteome and lipid core suffer alterations, which propel a shift towards a dysfunctional state, where HDL-C becomes proatherogenic, prooxidant, and proinflammatory. This heterogeneity highlights the need for further specialized molecular studies in this aspect, in order to achieve a better understanding of this dysfunctional state; with an emphasis on the potential role for proteomics and lipidomics as valuable methods in the search of novel therapeutic approaches for cardiovascular disease.

What does procollagen C-endopeptidase enhancer protein 2 have to do with HDL-cholesteryl ester uptake? Or how I learned to stop worrying and love reverse cholesterol transport?

PURPOSE OF REVIEW

The purpose of this study is to provide an update on the role HDL apolipoprotein A-I plays in reducing the risk of cardiovascular disease (CVD) and how it relates to reverse cholesterol transport (RCT).

RECENT FINDINGS

Despite numerous studies showing that plasma HDL cholesterol concentrations are correlated with a reduced risk of CVD, pharmacologic elevation of HDL has not shown any beneficial effects to date. In contrast, studies correlating the measure of an individual's plasma cholesterol efflux capacity show greater promise as a tool for assessing CVD risk. Although ATP-binding cassette transporter 1-mediated lipidation of apoA-I is considered the principal source of plasma HDL, it represents only one side of the RCT pathway. Equally important is the second half of the RCT pathway in which the liver scavenger receptor class B1 selectively removes HDL cholesteryl esters for excretion. The combined action of the two enzyme systems is reflected in the overall steady-state concentration of plasma HDL cholesterol. For example, reduced ATP-binding cassette transporter 1-mediated production of nascent HDL lowers plasma HDL concentration, just as an increase in cholesteryl ester uptake by scavenger receptor class B1 reduces HDL levels. Thus, the complexity of intravascular HDL metabolism suggests that steady-state plasma HDL concentrations do not provide adequate information regarding an individual's HDL quality or function. Herein, we describe a new player, procollagen C-endopeptidase enhancer 2, which shows atheroprotective function and influences both sides of RCT by enhancing production and catabolism of HDL cholesteryl esters.

SUMMARY

The discovery of a new molecule, procollagen C-endopeptidase enhancer 2, implicated in the regulation of HDL cholesteryl ester concentrations suggests that the extracellular matrix and the proteins that regulate its function represent a new and as yet unexplored realm of HDL cholesterol metabolism.

Mass spectrometry meets the challenge of understanding the complexity of the lipoproteome: recent findings regarding proteins involved in dyslipidemia and cardiovascular disease

Despite the fact that link between dyslipidemia and atherosclerosis was made over 100 years ago, atherosclerosis remains a major cause of morbidity and mortality worldwide. Major efforts focus towards understanding lipid metabolism, particularly by studying its particle compartments in circulation: the lipoproteins. In recent years, mass spectrometry has played an integral role in the deep sequencing of the lipoproteome and in metabolism studies conducted in vivo. This review highlights the path of lipoprotein research towards state-of-the-art mass spectrometry with special emphasis on the method of selected reaction monitoring and its impact on apolipoprotein metabolism studies. Also presented is what is expected for the lipoprotein field with the recent advent of high resolution/accurate mass parallel reaction monitoring mass spectrometry. The benefits of high resolution/accurate mass measurements are demonstrated by example instrument workflows and by detailing a novel method to quantify very low levels of circulating proprotein convertase subtilisin-kexin type 9 in rabbit. It is anticipated that future clinical studies or clinical trials aimed to treat dyslipidemia by manipulating key regulatory proteins will benefit from the new and exciting opportunities afforded by the latest technologies in mass spectrometry.

Procollagen C-endopeptidase Enhancer Protein 2 (PCPE2) Reduces Atherosclerosis in Mice by Enhancing Scavenger Receptor Class B1 (SR-BI)-mediated High-density Lipoprotein (HDL)-Cholesteryl Ester Uptake

Studies in human populations have shown a significant correlation between procollagen C-endopeptidase enhancer protein 2 (PCPE2) single nucleotide polymorphisms and plasma HDL cholesterol concentrations. PCPE2, a 52-kDa glycoprotein located in the extracellular matrix, enhances the cleavage of C-terminal procollagen by bone morphogenetic protein 1 (BMP1). Our studies here focused on investigating the basis for the elevated concentration of enlarged plasma HDL in PCPE2-deficient mice to determine whether they protected against diet-induced atherosclerosis. PCPE2-deficient mice were crossed with LDL receptor-deficient mice to obtain LDLr(-/-), PCPE2(-/-) mice, which had elevated HDL levels compared with LDLr(-/-) mice with similar LDL concentrations. We found that LDLr(-/-), PCPE2(-/-) mice had significantly more neutral lipid and CD68+ infiltration in the aortic root than LDLr(-/-) mice. Surprisingly, in light of their elevated HDL levels, the extent of aortic lipid deposition in LDLr(-/-), PCPE2(-/-) mice was similar to that reported for LDLr(-/-), apoA-I(-/-) mice, which lack any apoA-I/HDL. Furthermore, LDLr(-/-), PCPE2(-/-) mice had reduced HDL apoA-I fractional clearance and macrophage to fecal reverse cholesterol transport rates compared with LDLr(-/-) mice, despite a 2-fold increase in liver SR-BI expression. PCPE2 was shown to enhance SR-BI function by increasing the rate of HDL-associated cholesteryl ester uptake, possibly by optimizing SR-BI localization and/or conformation. We conclude that PCPE2 is atheroprotective and an important component of the reverse cholesterol transport HDL system.

Improvement of hepatic bioavailability as a new step for the future of statin

Statins (HMG-CoA reductase inhibitors) are a group of highly efficient pharmacological agents used for reducing blood cholesterol level and prevention/treatment of cardiovascular disease. Adverse reactions during statin treatment affect quite significant numbers of patients (reportedly from 5% to 20%), with more side effects occurring at higher doses. Reduced statin dosing can be achieved by improved bioavailability of statins, which is fairly low due to poor aqueous solubility, low permeability and high molecular weight of some members of the statin family. Moreover, since hepatic cholesterologenesis is a main target of statin action and extrahepatic inhibition of HMG-CoA reductase has no effect on plasma lipids, hepatic bioavailability, in our opinion, becomes a new important modality of statins maximizing their potential effect on the plasma lipid profile and diminishing their extrahepatic toxicity. Therefore efficient delivery systems of statins into hepatocytes need to be developed and introduced. Uses of nano-emulsifying statin delivery systems which may include vectors of intrahepatic transport, in particular lycopene, are discussed. As a proof of concept, some preliminary results revealing the effect of a lycopene-containing nanoformulation of simvastatin (designated as Lyco-Simvastatin) on LDL in mildly hypercholesterolemic patients are shown.

Amyloid-Forming Properties of Human Apolipoproteins: Sequence Analyses and Structural Insights

Apolipoproteins are protein constituents of lipoproteins that transport cholesterol and fat in circulation and are central to cardiovascular health and disease. Soluble apolipoproteins can transiently dissociate from the lipoprotein surface in a labile free form that can misfold, potentially leading to amyloid disease. Misfolding of apoA-I, apoA-II, and serum amyloid A (SAA) causes systemic amyloidoses, apoE4 is a critical risk factor in Alzheimer's disease, and apolipoprotein misfolding is also implicated in cardiovascular disease. To explain why apolipoproteins are over-represented in amyloidoses, it was proposed that the amphipathic α-helices, which form the lipid surface-binding motif in this protein family, have high amyloid-forming propensity. Here, we use 12 sequence-based bioinformatics approaches to assess amyloid-forming potential of human apolipoproteins and to identify segments that are likely to initiate β-aggregation. Mapping such segments on the available atomic structures of apolipoproteins helps explain why some of them readily form amyloid while others do not. Our analysis shows that nearly all amyloidogenic segments: (i) are largely hydrophobic, (ii) are located in the lipid-binding amphipathic α-helices in the native structures of soluble apolipoproteins, (iii) are predicted in both native α-helices and β-sheets in the insoluble apoB, and (iv) are predicted to form parallel in-register β-sheet in amyloid. Most of these predictions have been verified experimentally for apoC-II, apoA-I, apoA-II and SAA. Surprisingly, the rank order of the amino acid sequence propensity to form amyloid (apoB>apoA-II>apoC-II≥apoA-I, apoC-III, SAA, apoC-I>apoA-IV, apoA-V, apoE) does not correlate with the proteins' involvement in amyloidosis. Rather, it correlates directly with the strength of the protein-lipid association, which increases with increasing protein hydrophobicity. Therefore, the lipid surface-binding function and the amyloid-forming propensity are both rooted in apolipoproteins' hydrophobicity, suggesting that functional constraints make it difficult to completely eliminate pathogenic apolipoprotein misfolding. We propose that apolipoproteins have evolved protective mechanisms against misfolding, such as the sequestration of the amyloidogenic segments via the native protein-lipid and protein-protein interactions involving amphipathic α-helices and, in case of apoB, β-sheets.

A metabolic view on menopause and ageing

The ageing of the global population calls for a better understanding of age-related metabolic consequences. Here we report the effects of age, sex and menopause on serum metabolites in 26,065 individuals of Northern European ancestry. Age-specific metabolic fingerprints differ significantly by gender and, in females, a substantial atherogenic shift overlapping the time of menopausal transition is observed. In meta-analysis of 10,083 women, menopause status associates with amino acids glutamine, tyrosine and isoleucine, along with serum cholesterol measures and atherogenic lipoproteins. Among 3,204 women aged 40-55 years, menopause status associates additionally with glycine and total, monounsaturated, and omega-7 and -9 fatty acids. Our findings suggest that, in addition to lipid alterations, menopause may contribute to future metabolic and cardiovascular risk via influencing amino-acid concentrations, adding to the growing evidence of the importance of amino acids in metabolic disease progression. These observations shed light on the metabolic consequences of ageing, gender and menopause at the population level.

Menopausal hormone treatment cardiovascular disease: another look at an unresolved conundrum

Cardiovascular disease (CVD) is the most common cause of death in women. Before the Women's Health Initiative (WHI) hormone trials, evidence favored the concept that menopausal hormone treatment (MHT) protects against CVD. WHI studies failed to demonstrate CVD benefit, with worse net outcomes for MHT versus placebo in the population studied. We review evidence regarding the relationship between MHT and CVD with consideration of mechanisms and risk factors for atherogenesis and cardiac events, results of observational case-control and cohort studies, and outcomes of randomized trials. Estrogen effects on CVD risk factors favor delay or amelioration of atherosclerotic plaque development but may increase risk of acute events when at-risk plaque is present. Long-term observational studies have shown ∼40% reductions in risk of myocardial infarction and all-cause mortality. Analyses of data from randomized control trials other than the WHI show a ∼30% cardioprotective effect in recently menopausal women. Review of the literature as well as WHI data suggests that younger and/or more recently menopausal women may have a better risk-benefit ratio than older or remotely menopausal women and that CVD protection may only occur after >5 years; WHI women averaged 63 years of age (12 years postmenopausal) and few were studied for >6 years. Thus, a beneficial effect of long-term MHT on CVD and mortality is still an open question and is likely to remain controversial for the foreseeable future.

Effects of native and myeloperoxidase-modified apolipoprotein a-I on reverse cholesterol transport and atherosclerosis in mice

OBJECTIVE

Preclinical and clinical studies have shown beneficial effects of infusions of apolipoprotein A-I (ApoA-I) on atherosclerosis. ApoA-I is also a target for myeloperoxidase-mediated oxidation, leading in vitro to a loss of its ability to promote ATP-binding cassette transporter A1-dependent macrophage cholesterol efflux. Therefore, we hypothesized that myeloperoxidase-mediated ApoA-I oxidation would impair its promotion of reverse cholesterol transport in vivo and the beneficial effects on atherosclerotic plaques.

APPROACH AND RESULTS

ApoA-I(-/-) or apolipoprotein E-deficient mice were subcutaneously injected with native human ApoA-I, oxidized human ApoA-I (myeloperoxidase/hydrogen peroxide/chloride treated), or carrier. Although early postinjection (8 hours) levels of total ApoA-I in plasma were similar for native versus oxidized human ApoA-I, native ApoA-I primarily resided within the high-density lipoprotein fraction, whereas the majority of oxidized human ApoA-I was highly cross-linked and not high-density lipoprotein particle associated, consistent with impaired ATP-binding cassette transporter A1 interaction. In ApoA-I(-/-) mice, ApoA-I oxidation significantly impaired reverse cholesterol transport in vivo. In advanced aortic root atherosclerotic plaques of apolipoprotein E-deficient mice, native ApoA-I injections led to significant decreases in lipid content, macrophage number, and an increase in collagen content; in contrast, oxidized human ApoA-I failed to mediate these changes. The decrease in plaque macrophages with native ApoA-I was accompanied by significant induction of their chemokine receptor CCR7. Furthermore, only native ApoA-I injections led to a significant reduction of inflammatory M1 and increase in anti-inflammatory M2 macrophage markers in the plaques.

CONCLUSIONS

Myeloperoxidase-mediated oxidation renders ApoA-I dysfunctional and unable to (1) promote reverse cholesterol transport, (2) mediate beneficial changes in the composition of atherosclerotic plaques, and (3) pacify the inflammatory status of plaque macrophages.