Sphingomyelinase Induces Aggregation and Fusion of Small Very Low–Density Lipoprotein and Intermediate-Density Lipoprotein Particles and Increases Their Retention to Human Arterial Proteoglycans

Computational analysis of the alpha-2 domain of apolipoprotein B - 100, a potential triggering factor in LDL aggregation

Atherosclerosis, the major underlying cause of cardiovascular disease, is believed to arise from the accumulation of low-density lipoprotein (LDL) in the arterial subendothelial space, ultimately leading to plaque formation. It is proposed that the accumulation of LDL is linked to its intrinsic aggregation propensity. Although the native LDL is not prone to aggregation, LDL(-), an electronegative LDL characterized in the plasma, has been shown to prime LDL aggregation in a domino-like behavior similar to amyloidogenic proteins. LDL(-) has also been observed to have a misfolded apolipoprotein B-100 (apo B-100), a huge protein consisting of 4563 amino acid residues. As misfolding of proteins is commonly associated with amyloid formation, apo B-100 is therefore being considered as the possible triggering factor in LDL aggregation. Previous computational studies have implicated the α2 domain to be the aggregation-prone region of apo B-100. In this study, the amyloidogenic properties of the α2 domain of apo B-100 were interrogated using both in silico and in vitro techniques. Since the crystal structure of the 570-amino acid α2 domain of apo B-100 is yet to be solved, we used several secondary structure prediction tools to model putative helical regions that make up the α2 domain. The stability of each of the 17 helices thus identified was further probed using molecular dynamics (MD), with the least stable of the helices considered as potentially amyloidogenic. In a 100 ns simulation window, helices k (YFEKLVGFIDDAVK), m (YHQFVDETNDKIREVTQRLNGEIQA), and p (QQELQRYLSLVGQVYS) were the least stable and appeared to transition to β-structures, the hallmark of amyloidogenesis. When the simulation was extended to longer times, only helices k and p formed stable β-sheets that persisted. Analysis of the data indicates that the final β-sheet conformation was stabilized by the π-π stacking interactions between the aromatic rings of Tyr-1 and Phe-8 for helix k and likely π-π stacking contacts between Arg-6 guanidino group and Tyr-15 ring for helix p. Based on the in silico work, we proceeded to synthesize and spectroscopically characterize helices k, m17-25 (QRLNGEIQA), and p. As expected, k and p formed detectable amyloids, with the latter appearing to be substantially more amyloidogenic based on kinetic aggregation assays. Amyloid fibrils formed by p were confirmed using circular dichroism spectroscopy and transmission electron microscopy. Data obtained could be exploited to further investigate the roles of peptides derived from the α2 domain helices of apo B-100 in triggering LDL aggregation. Based on preliminary data, one of the peptides designed based on this work reduced the aggregation of LDL.

Response to retention hypothesis as a source of targets for arterial wall-directed therapies to prevent atherosclerosis: A critical review

The subendothelial retention of circulating lipoproteins on extracellular matrix proteins and proteoglycans is one of the earliest events in the development of atherosclerosis. Multiple factors, including the size, type, composition, surrounding pH, and chemical modifications to lipoproteins, influence the electrostatic interactions between relevant moieties of the apolipoproteins on lipoproteins and the glycosaminoglycans of proteoglycans. The length and chemical composition of glycosaminoglycan chains attached to proteoglycan core proteins determine the extent of initial lipoprotein binding and retention in the artery wall. The phenomena of hyperelongation of glycosaminoglycan chains is associated with initial lipid retention and later atherosclerotic plaque formation. This review includes a summary of the current literature surrounding cellular mechanisms leading to GAG chain modification and lipid retention and discusses potential therapeutic strategies to target lipoprotein:proteoglycan interactions to prevent the development and progression of atherosclerosis.

Hegemony of inflammation in atherosclerosis and coronary artery disease

Inflammation drives coronary artery disease and atherosclerosis implications. Lipoprotein entry, retention, and oxidative modification cause endothelial damage, triggering innate and adaptive immune responses. Recruited immune cells orchestrate the early atherosclerotic lesions by releasing proinflammatory cytokines, expediting the foam cell formation, intraplaque haemorrhage, secretion of matrix-degrading enzymes, and lesion progression, eventually promoting coronary artery syndrome via various inflammatory cascades. In addition, soluble mediators disrupt the dynamic anti- and prothrombotic balance maintained by endothelial cells and pave the way for coronary artery disease such as angina pectoris. Recent studies have established a relationship between elevated levels of inflammatory markers, including C-reactive protein (CRP), interleukins (IL-6, IL-1β), and tumour necrosis factor-alpha (TNF-α) with the severity of CAD and the possibility of future cardiovascular events. High-sensitivity C-reactive protein (hs-CRP) is a marker for assessing systemic inflammation and predicting the risk of developing CAD based on its peak plasma levels. Hence, understanding cross-talk interactions of inflammation, atherogenesis, and CAD is highly warranted to recalculate the risk factors that activate and propagate arterial lesions and devise therapeutic strategies accordingly. Cholesterol-inflammation lowering agents (statins), monoclonal antibodies targeting IL-1 and IL-6 (canakinumab and tocilizumab), disease-modifying antirheumatic drugs (methotrexate), sodium-glucose transport protein-2 (SGLT2) inhibitors, colchicine and xanthene oxidase inhibitor (allopurinol) have shown promising results in reducing inflammation, regressing atherogenic plaque and modifying the course of CAD. Here, we review the complex interplay between inflammatory, endothelial, smooth muscle and foam cells. Moreover, the putative role of inflammation in atherosclerotic CAD, underlying mechanisms and potential therapeutic implications are also discussed herein.

Remnant cholesterol traits and risk of stroke: A multivariable Mendelian randomization study

Observational epidemiological studies have reported a relationship between remnant cholesterol and stroke. However, the results are inconclusive, and causality remains unclear due to confounding or reverse causality. Our objective in this study was to investigate the causal relevance of remnant cholesterol and the risk of stroke and its subtypes using the Mendelian randomization (MR) approach. Genome-wide association studies (GWASs) including 115,082 European individuals (UK Biobank) were used to identify instruments for remnant cholesterol, including intermediate-density lipoprotein (IDL) cholesterol and very-low-density lipoprotein (VLDL) cholesterol. Summary-level data for total stroke, intracerebral hemorrhage, subarachnoid hemorrhage, ischemic stroke (IS), and IS subtypes were obtained from GWAS meta-analyses conducted by the MEGASTROKE consortium. Univariable and multivariable MR analyses were performed. The GWAS identified multiple single-nucleotide polymorphisms after clumping for remnant cholesterol (n = 52), IDL cholesterol (n = 62), and VLDL cholesterol (n = 67). Assessed individually using MR, remnant cholesterol (weighted median: odds ratio [OR] 1.32 per 1-SD higher trait; 95% CI: 1.04-1.67; P = 0.024) had effect estimates consistent with a higher risk of LAS-IS, driven by IDL cholesterol (OR 1.32; 95% CI: 1.04-1.68; P = 0.022). In multivariable MR, IDL cholesterol (OR 1.46; 95% CI: 1.10-1.93; P = 0.009) retained a robust effect on LAS-IS after controlling for VLDL cholesterol and high-density lipoprotein cholesterol. The MR analysis did not indicate causal associations between remnant cholesterol and other stroke subtypes. This study suggests that remnant cholesterol is causally associated with the risk of LAS-IS driven by IDL cholesterol.

Modified Lipoproteins Induce Arterial Wall Inflammation During Atherogenesis

Circulating apolipoprotein B-containing lipoproteins, notably the low-density lipoproteins, enter the inner layer of the arterial wall, the intima, where a fraction of them is retained and modified by proteases, lipases, and oxidizing agents and enzymes. The modified lipoproteins and various modification products, such as fatty acids, ceramides, lysophospholipids, and oxidized lipids induce inflammatory reactions in the macrophages and the covering endothelial cells, initiating an increased leukocyte diapedesis. Lipolysis of the lipoproteins also induces the formation of cholesterol crystals with strong proinflammatory properties. Modified and aggregated lipoproteins, cholesterol crystals, and lipoproteins isolated from human atherosclerotic lesions, all can activate macrophages and thereby induce the secretion of proinflammatory cytokines, chemokines, and enzymes. The extent of lipoprotein retention, modification, and aggregation have been shown to depend largely on differences in the composition of the circulating lipoprotein particles. These properties can be modified by pharmacological means, and thereby provide opportunities for clinical interventions regarding the prevention and treatment of atherosclerotic vascular diseases.

Macrophages in Atherosclerosis, First or Second Row Players?

Macrophages represent a cell type that has been widely described in the context of atherosclerosis since the earliest studies in the 17th century. Their role has long been considered to be preponderant in the onset and aggravation of atherosclerosis, in particular by participating in the establishment of a chronic inflammatory state by the release of pro-inflammatory cytokines and by uncontrolled engorgement of lipids resulting in the formation of foam cells and later of the necrotic core. However, recent evidence from mouse models using an elegant technique of tracing vascular smooth muscle cells (VSMCs) during plaque development revealed that resident VSMCs display impressive plastic properties in response to an arterial injury, allowing them to switch into different cell types within the plaque, including mesenchymal-like cells, macrophage-like cells and osteochondrogenic-like cells. In this review, we oppose the arguments in favor or against the influence of macrophages versus VSMCs in all stages of atherosclerosis including pre-atherosclerosis, formation of lipid-rich foam cells, development of the necrotic core and the fibrous cap as well as calcification and rupture of the plaque. We also analyze the relevance of animal models for the investigation of the pathophysiological mechanisms of atherosclerosis in humans, and discuss potential therapeutic strategies targeting either VSMCs or macrophage to prevent the development of cardiovascular events. Overall, although major findings have been made from animal models, efforts are still needed to better understand and therefore prevent the development of atherosclerotic plaques in humans.

Neutral sphingomyelinase-2 and cardiometabolic diseases

Sphingolipids, in particular ceramides, play vital role in pathophysiological processes linked to metabolic syndrome, with implications in the development of insulin resistance, pancreatic ß-cell dysfunction, type 2 diabetes, atherosclerosis, inflammation, nonalcoholic steatohepatitis, and cancer. Ceramides are produced by the hydrolysis of sphingomyelin, catalyzed by different sphingomyelinases, including neutral sphingomyelinase 2 (nSMase2), whose dysregulation appears to underlie many of the inflammation-related pathologies. In this review, we discuss the current knowledge on the biochemistry of nSMase2 and ceramide production and its regulation by inflammatory cytokines, with particular reference to cardiometabolic diseases. nSMase2 contribution to pathogenic processes appears to involve cyclical feed-forward interaction with proinflammatory cytokines, such as TNF-α and IL-1ß, which activate nSMase2 and the production of ceramides, that in turn triggers the synthesis and release of inflammatory cytokines. We elaborate these pathogenic interactions at the molecular level and discuss the potential therapeutic benefits of inhibiting nSMase2 against inflammation-driven cardiometabolic diseases.

The role of triglycerides in the origin and progression of atherosclerosis

Hypertriglyceridaemia has been associated with cardiovascular disease risk in humans for several decades. However, only recently, data from basic research, as well as from genetic and observational studies, have suggested triglyceride-rich lipoproteins (TRLs) as causal factors for atherosclerotic cardiovascular disease. Novel findings highlighting the relevance of TRL-derived lipolytic products (remnant lipoprotein particles "RLPs"), rather than plasma triglycerides or TRL themselves, as the true mediators in atherosclerosis, have contributed to explain a causal relationship through a number of direct and indirect mechanisms. Thus, experimental studies in animal models and in vitro cell culture methods reveal that RLPs, having sizes below 70-80nm, enter the arterial wall and accumulate within the sub-endothelial space. They then become involved in the cholesterol deposition of cholesterol in the intima in addition to several pro-inflammatory and pro-apoptotic pathways. In this review, a summary is presented of current understanding of the pathophysiological mechanisms by which TRLs and their lipolytic derived RLP induce the formation and progression of atherosclerotic lesions, and actively contribute to cardiovascular disease.

NET-associated citrullinated histones promote LDL aggregation and foam cell formation in vitro

Neutrophils have been recently identified in the atherosclerotic lesion and they can release neutrophil extracellular trap (NET) under the pro-inflammatory conditions prevailing in the lesion. Citrullinated histones (Cit-histones) are the major type of citrullinated proteins associated with NET release. Since elevated levels of citrullinated proteins have been detected in inflammatory diseases including atherosclerosis, this study analysed the role played by NET and Cit-histones in different atherogenic events in vitro. First, neutrophil recruitment and NET release in the presence of low-density lipoprotein (LDL) and oxidised LDL (Ox-LDL) were analysed by Boyden's chamber method and microscopy respectively. Then, LDL oxidation and LDL aggregation in the presence of NET and Cit-histones were analysed spectroscopically. Foam cell formation in the presence of NET or Cit-histone was studied by both microscopic and spectroscopic methods. While neutrophil recruitment was facilitated by Ox-LDL and not by LDL, the extent of NET release was significantly increased in the presence of both LDL and Ox-LDL. In the presence of NET, LDL oxidation, aggregation and foam cell formation were found to be increased. Cit-histones were found to accelerate LDL aggregation and foam cell formation at higher citrulline levels. Altogether, the results suggest that both NET and NET-associated Cit-histone released at the lesion can play major roles as pro-atherogenic mediators. Inhibiting the action of NET or Cit-histone would, therefore, be beneficial in slowing down atherosclerotic progression.

Immunization with the Gly1127-Cys1140 amino acid sequence of the LRP1 receptor reduces atherosclerosis in rabbits. Molecular, immunohistochemical and nuclear imaging studies

Background: The LRP1 (CR9) domain and, in particular, the sequence Gly1127-Cys1140 (P3) plays a critical role in the binding and internalization of aggregated LDL (agLDL). We aimed to evaluate whether immunization with P3 reduces high-fat diet (HFD)-induced atherosclerosis. Methods: Female New Zealand White (NZW) rabbits were immunized with a primary injection and four reminder doses (R1-R4) of IrP (irrelevant peptide) or P3 conjugated to the carrier. IrP and P3-immunized rabbits were randomly divided into a normal diet group and a HFD-fed group. Anti-P3 antibody levels were determined by ELISA. Lipoprotein profile, circulating and tissue lipids, and vascular pro-inflammatory mediators were determined using standardized methods while atherosclerosis was determined by confocal microscopy studies and non-invasive imaging (PET/CT and Doppler ultrasonography). Studies treating human macrophages (hMΦ) and coronary vascular smooth muscle cells (hcVSMC) with rabbit serums were performed to ascertain the potential impact of anti-P3 Abs on the functionality of these crucial cells. Results: P3 immunization specifically induced the production of anti-P3 antibodies (Abs) and did not alter the lipoprotein profile. HFD strongly induced cholesteryl ester (CE) accumulation in the aorta of both the control and IrP groups, and their serum dose-dependently raised the intracellular CE of hMΦ and hcVSMC, promoting TNFR1 and phospho-NF-kB (p65) overexpression. These HFD pro-inflammatory effects were dramatically decreased in the aorta of P3-immunized rabbits and in hMΦ and hcVSMC exposed to the P3 rabbit serums. Microscopy studies revealed that P3 immunization reduced the percentage of lipids, macrophages, and SMCs in the arterial intima, as well as the atherosclerotic extent and lesion area in the aorta. PET/CT and Doppler ultrasonography studies showed that the average standardized uptake value (SUVmean) of the aorta and the arterial resistance index (ARI) of the carotids were more upregulated by HFD in the control and IrP groups than the P3 group. Conclusions: P3 immunization counteracts HFD-induced fatty streak formation in rabbits. The specific blockade of the LRP1 (CR9) domain with Anti-P3 Abs dramatically reduces HFD-induced intracellular CE loading and harmful coupling to pro-inflammatory signaling in the vasculature.

Low-density lipoprotein aggregation is inhibited by apolipoprotein J-derived mimetic peptide D-[113-122]apoJ

Mimetic peptides are promising therapeutic agents for atherosclerosis prevention. A 10-residue class G* peptide from apolipoprotein J (apoJ), namely, D-[113-122]apoJ, possesses anti-inflammatory and anti-atherogenic properties. This prompted us to determine its effect on the aggregation process of low-density lipoprotein (LDL) particles, an early event in the development of atherosclerosis. LDL particles with and without [113-122]apoJ peptide were incubated at 37 °C with sphingomyelinase (SMase) or were left to aggregate spontaneously at room temperature. The aggregation process was analyzed by size-exclusion chromatography (SEC), native gradient gel electrophoresis (GGE), absorbance at 405 nm, dynamic light scattering (DLS), and transmission electronic microscopy (TEM). In addition, circular dichroism was used to determine changes in the secondary structure of apoB, and SDS-PAGE was performed to assess apoB degradation. At an equimolar ratio of [113-122]apoJ peptide to apoB-100, [113-122]apoJ inhibited both SMase-induced or spontaneous LDL aggregation. All methods showed that [113-122]apoJ retarded the progression of SMase-induced LDL aggregation at long incubation times. No effect of [113-122]apoJ on apoB secondary structure was observed. Binding experiments showed that [113-122]apoJ presents low affinity for native LDL but binds readily to LDL during the first stages of aggregation. Laurdan fluorescence experiments showed that mild aggregation of LDL resulted in looser lipid packaging, which was partially prevented by D-[113-122]apoJ. These results demonstrate that [113-122]apoJ peptide prevents SMase-induced LDL aggregation at an equimolar ratio and opens the possibility for the use of this peptide as a therapeutic tool.

Molecular basis for the protective effects of low-density lipoprotein receptor-related protein 1 (LRP1)-derived peptides against LDL aggregation

Aggregated LDL is the first ligand reported to interact with the cluster II CR9 domain of low-density lipoprotein receptor-related protein 1 (LRP1). In particular, the C-terminal half of domain CR9, comprising the region Gly¹¹²⁷-Cys¹¹⁴⁰ exclusively recognizes aggregated LDL and it is crucial for aggregated LDL binding. Our aim was to study the effect of the sequence Gly¹¹²⁷-Cys¹¹⁴⁰ (named peptide LP3 and its retro-enantio version, named peptide DP3) on the structural characteristics of sphingomyelinase- (SMase) and phospholipase 2 (PLA₂)-modified LDL particles. Turbidimetry, gel filtration chromatography (GFC) and transmission electronic microscopy (TEM) analysis showed that LP3 and DP3 peptides strongly inhibited SMase- and PLA₂-induced LDL aggregation. Nondenaturing polyacrylamide gradient gel electrophoresis (GGE), agarose gel electrophoresis and high-performance thin-layer chromatography (HPTLC) indicated that LP3 and DP3 prevented SMase-induced alterations in LDL particle size, electric charge and phospholipid content, respectively, but not those induced by PLA₂. Western blot analysis showed that LP3 and DP3 counteracted changes in ApoB-100 conformation induced by the two enzymes. LDL proteomics (LDL trypsin digestion followed by mass spectroscopy) and computational modeling methods evidenced that peptides preserve ApoB-100 conformation due to their electrostatic interactions with a basic region of ApoB-100. These results demonstrate that LRP1-derived peptides are protective against LDL aggregation, even in conditions of extreme lipolysis, through their capacity to bind to ApoB-100 regions critical for ApoB-100 conformational preservation. These results suggests that these LRP1(CR9) derived peptides could be promising tools to prevent LDL aggregation induced by the main proteolytic enzymes acting in the arterial intima.

Pathways of smooth muscle foam cell formation in atherosclerosis

PURPOSE OF REVIEW

Smooth muscle cells (SMCs) are the major cell type in human atherosclerosis-prone arteries and take up excess lipids, thereby contributing to luminal occlusion. Here we provide a focused review on pathways by which smooth muscle cells (SMCs) can become foam cells in atherosclerosis.

RECENT FINDINGS

A synthesis of recent and older investigations provides key mechanistic insights into SMC foam cell formation. LDL and other apoB-containing lipoproteins are modified by a diverse array of oxidative, enzymatic, and nonenzymatic processes present in the arterial intima. These modifications of LDL all promote the aggregation of LDL (agLDL), a key finding from analysis of arterial lesion particles. Scavenger receptor and phagocytic capacity of SMCs can vary greatly, perhaps related to differences in SMC phenotype or in-vitro cell culture environments, and can be increased with exposure to cytokines, growth factors, and cholesterol. Macrophages promote the formation of SMC foam cells in direct or indirect co-culture models.

SUMMARY

SMCs contribute significantly to the foam cell population in atherosclerosis. Further investigation and identification of key mechanisms of SMC foam cell formation will help drive new therapeutics to reduce cardiovascular disease.

Contribution of acid sphingomyelinase to angiotensin II-induced vascular adventitial remodeling via membrane rafts/Nox2 signal pathway

AIMS

Vascular adventitial fibroblasts (AFs) in the vascular remodeling during atherosclerosis are increasing arousing attention. Acid sphingomyelinase (ASM) is a soluble glycoprotein which is involved in the development and progression of atherosclerosis. However, it remains unknown if ASM is expressed in vascular AFs and regulates vascular adventitial remodeling and underlying mechanisms.

MAIN METHODS AND KEY FINDINGS

ASM downregulation with gene silencing was used in the rat AFs treated with angiotensin (Ang) II, which is universally demonstrated to induce vascular adventitia remodeling. It was showed that ASM was indeed expressed in vascular AFs and ASM downregulation resulted in a significant decrease in the protein level of PCNA and collagen I and cell migration under Ang II stimulation. Such improvement of adventitial remodeling was not further augmented by Ang-(1-7), which is deemed as an endogenous Ang II blocker. We further found that ASM downregulation blocked the Nox2-dependent superoxide (O₂^-) generation, which regulated vascular remodeling in AFs under Ang II. ASM siRNA decreased the aggregation of membrane rafts (MRs) and the consequent recruiting of ceramide and Nox2 in MRs.

SIGNIFICANCE

In conclusion, these results suggested that ASM downregulation could improve vascular adventitial remodeling which was attributed to inhibiting MRs/Nox2 redox signaling pathway in AFs. Thus, these data supported the idea that ASM is a potential therapeutic target for diabetic vascular complication.

The effect of intakes of fish and Camelina sativa oil on atherogenic and anti-atherogenic functions of LDL and HDL particles: A randomized controlled trial

BACKGROUND AND AIMS

Omega-3 fatty acids are known to have several cardioprotective effects. Our aim was to investigate the effects of intakes of fish and Camelina sativa oil (CSO), rich in alpha-linolenic acid, on the atherogenic and anti-atherogenic functions of LDL and HDL particles.

METHODS

Altogether, 88 volunteers with impaired glucose metabolism were randomly assigned to CSO (10 g of alpha-linolenic acid/day), fatty fish (4 fish meals/week), lean fish (4 fish meals/week) or control group for 12 weeks. 79 subjects completed the study. The binding of lipoproteins to aortic proteoglycans, LDL aggregation and activation of endothelial cells by LDL and cholesterol efflux capacity of HDL were determined in vitro.

RESULTS

Intake of CSO decreased the binding of lipoproteins to aortic proteoglycans in a non-normalized model (p = 0.006). After normalizing with serum concentrations of non-HDL cholesterol, apolipoprotein B (apoB) or LDL cholesterol, which decreased in the CSO group, the change was no longer statistically significant. In the fish groups, there were no changes in the binding of lipoproteins to proteoglycans. Regarding other lipoprotein functions, there were no changes in any of the groups.

CONCLUSIONS

Intake of CSO decreases the binding of lipoproteins to aortic proteoglycans by decreasing serum LDL cholesterol concentration, which suggests that the level of apoB-containing lipoproteins in the circulation is the main driver of lipoprotein retention within the arterial wall. Intake of fish or CSO has no effects on other lipoprotein functions.

Metabolomic consequences of genetic inhibition of PCSK9 compared with statin treatment

Background

Both statins and PCSK9 inhibitors lower blood low-density lipoprotein cholesterol (LDL-C) levels to reduce risk of cardiovascular events. To assess potential differences between metabolic effects of these two lipid-lowering therapies, we performed detailed lipid and metabolite profiling of a large randomized statin trial and compared the results with the effects of genetic inhibition of PCSK9, acting as a naturally occurring trial.

Methods

228 circulating metabolic measures were quantified by nuclear magnetic resonance spectroscopy, including lipoprotein subclass concentrations and their lipid composition, fatty acids, and amino acids, for 5,359 individuals (2,659 on treatment) in the PROspective Study of Pravastatin in the Elderly at Risk (PROSPER) trial at 6-months post-randomization. The corresponding metabolic measures were analyzed in eight population cohorts (N=72,185) using PCSK9 rs11591147 as an unconfounded proxy to mimic the therapeutic effects of PCSK9 inhibitors.

Results

Scaled to an equivalent lowering of LDL-C, the effects of genetic inhibition of PCSK9 on 228 metabolic markers were generally consistent with those of statin therapy (R2=0.88). Alterations in lipoprotein lipid composition and fatty acid distribution were similar. However, discrepancies were observed for very-low-density lipoprotein (VLDL) lipid measures. For instance, genetic inhibition of PCSK9 had weaker effects on lowering of VLDL-cholesterol compared with statin therapy (54% vs. 77% reduction, relative to the lowering effect on LDL-C; P=2x10-7 for heterogeneity). Genetic inhibition of PCSK9 showed no significant effects on amino acids, ketones, or a marker of inflammation (GlycA) whereas statin treatment weakly lowered GlycA levels.

Conclusions

Genetic inhibition of PCSK9 had similar metabolic effects to statin therapy on detailed lipid and metabolite profiles. However, PCSK9 inhibitors are predicted to have weaker effects on VLDL lipids compared with statins for an equivalent lowering of LDL-C, which potentially translate into smaller reductions in cardiovascular disease risk.

Contribution of p62/SQSTM1 to PDGF-BB-induced myofibroblast-like phenotypic transition in vascular smooth muscle cells lacking Smpd1 gene

Accumulating evidence indicates a critical role of autophagy in regulating vascular smooth muscle cell (SMC) homeostasis in atherogenesis. However, little is known about the modulatory role of autophagy in PDGF-BB-induced SMC transition towards the synthetic phenotype and extracellular matrix remodeling. We recently demonstrated that acid sphingomyelinase (ASM, encoded by Smpd1 gene) controls autophagy maturation in coronary arterial SMCs. Here, we demonstrate that PDGF-BB stimulation causes a myofibroblast-like non-canonical synthetic phenotype transition in Smpd1^−/− SMCs. These non-canonical phenotypic changes induced by PDGF-BB in Smpd1^−/− SMCs were characterized by increased expression of fibroblast-specific protein (FSP-1), massive deposition of collagen type I, decreased cell size, elevated inflammatory status with enhanced cytokine release and adhesion molecule expression. Mechanistically, PDGF-BB induces prolonged Akt activation that causes decreased autophagosome biogenesis and thereby exaggerates p62/SQSTM1 accumulation in Smpd1^−/− SMCs. More importantly, Akt inhibition or p62/SQSTM1 gene silencing attenuates PDGF-BB-induced phenotypic changes in Smpd1^−/− SMCs. This first demonstration of a p62/SQSTM1-dependent myofibroblast-like phenotypic transition in Smpd1^−/− SMCs suggests that ASM-mediated autophagy pathway contributes to maintaining the arterial smooth muscle homeostasis in situation of vascular remodeling during atherosclerosis.

Extracellular Lipids Accumulate in Human Carotid Arteries as Distinct Three-Dimensional Structures and Have Proinflammatory Properties

Lipid accumulation is a key characteristic of advancing atherosclerotic lesions. Herein, we analyzed the ultrastructure of the accumulated lipids in endarterectomized human carotid atherosclerotic plaques using three-dimensional (3D) electron microscopy, a method never used in this context before. 3D electron microscopy revealed intracellular lipid droplets and extracellular lipoprotein particles. Most of the particles were aggregated, and some connected to needle-shaped or sheet-like cholesterol crystals. Proteomic analysis of isolated extracellular lipoprotein particles revealed that apolipoprotein B is their main protein component, indicating their origin from low-density lipoprotein, intermediate-density lipoprotein, very-low-density lipoprotein, lipoprotein (a), or chylomicron remnants. The particles also contained small exchangeable apolipoproteins, complement components, and immunoglobulins. Lipidomic analysis revealed differences between plasma lipoproteins and the particles, thereby indicating involvement of lipolytic enzymes in their generation. Incubation of human monocyte-derived macrophages with the isolated extracellular lipoprotein particles or with plasma lipoproteins that had been lipolytically modified in vitro induced intracellular lipid accumulation and triggered inflammasome activation in them. Taken together, extracellular lipids accumulate in human carotid plaques as distinct 3D structures that include aggregated and fused lipoprotein particles and cholesterol crystals. The particles originate from plasma lipoproteins, show signs of lipolytic modifications, and associate with cholesterol crystals. By inducing intracellular cholesterol accumulation (ie, foam cell formation) and inflammasome activation, the extracellular lipoprotein particles may actively enhance atherogenesis.

The Therapeutic Potential of Anti-Inflammatory Exerkines in the Treatment of Atherosclerosis

Although many cardiovascular (CVD) medications, such as antithrombotics, statins, and antihypertensives, have been identified to treat atherosclerosis, at most, many of these therapeutic agents only delay its progression. A growing body of evidence suggests physical exercise could be implemented as a non-pharmacologic treatment due to its pro-metabolic, multisystemic, and anti-inflammatory benefits. Specifically, it has been discovered that certain anti-inflammatory peptides, metabolites, and RNA species (collectively termed “exerkines”) are released in response to exercise that could facilitate these benefits and could serve as potential therapeutic targets for atherosclerosis. However, much of the relationship between exercise and these exerkines remains unanswered, and there are several challenges in the discovery and validation of these exerkines. This review primarily highlights major anti-inflammatory exerkines that could serve as potential therapeutic targets for atherosclerosis. To provide some context and comparison for the therapeutic potential of exerkines, the anti-inflammatory, multisystemic benefits of exercise, the basic mechanisms of atherosclerosis, and the limited efficacies of current anti-inflammatory therapeutics for atherosclerosis are briefly summarized. Finally, key challenges and future directions for exploiting these exerkines in the treatment of atherosclerosis are discussed.

Secretory sphingomyelinase in health and disease

Acid sphingomyelinase (ASM), a key enzyme in sphingolipid metabolism, hydrolyzes sphingomyelin to ceramide and phosphorylcholine. In mammals, the expression of a single gene, SMPD1, results in two forms of the enzyme that differ in several characteristics. Lysosomal ASM (L-ASM) is located within the lysosome, requires no additional Zn2+ ions for activation and is glycosylated mainly with high-mannose oligosaccharides. By contrast, the secretory ASM (S-ASM) is located extracellularly, requires Zn2+ ions for activation, has a complex glycosylation pattern and has a longer in vivo half-life. In this review, we summarize current knowledge regarding the physiology and pathophysiology of S-ASM, including its sources and distribution, molecular and cellular mechanisms of generation and regulation and relevant in vitro and in vivo studies. Polymorphisms or mutations of SMPD1 lead to decreased S-ASM activity, as detected in patients with Niemann-Pick disease B. Thus, lower serum/plasma activities of S-ASM are trait markers. No genetic causes of increased S-ASM activity have been identified. Instead, elevated activity is the result of enhanced release (e.g., induced by lipopolysaccharide and cytokine stimulation) or increased enzyme activation (e.g., induced by oxidative stress). Increased S-ASM activity in serum or plasma is a state marker of a wide range of diseases. In particular, high S-ASM activity occurs in inflammation of the endothelium and liver. Several studies have demonstrated a correlation between S-ASM activity and mortality induced by severe inflammatory diseases. Serial measurements of S-ASM reveal prolonged activation and, therefore, the measurement of this enzyme may also provide information on past inflammatory processes. Thus, S-ASM may be both a promising clinical chemistry marker and a therapeutic target.

The emerging role of acid sphingomyelinase in autophagy

Autophagy, the main intracellular process of cytoplasmic material degradation, is involved in cell survival and death. Autophagy is regulated at various levels and novel modulators of its function are being continuously identified. An intriguing recent observation is that among these modulators is the sphingolipid metabolising enzyme, Acid Sphingomyelinase (A-SMase), already known to play a fundamental role in apoptotic cell death participating in several pathophysiological conditions. In this review we analyse and discuss the relationship between autophagy and A-SMase describing how A-SMase may regulate it and defining, for the first time, the existence of an A-SMase-autophagy axis. The imbalance of this axis plays a role in cancer, nervous system, cardiovascular, and hepatic disorders.

Hyperlipoproteinemia type 3: the forgotten phenotype

Hyperlipoproteinemia type 3 (HLP3) is caused by impaired removal of triglyceride-rich lipoproteins (TGRL) leading to accumulation of TGRL remnants with abnormal composition. High levels of these remnants, called β-VLDL, promote lipid deposition in tuberous xanthomas, atherosclerosis, premature coronary artery disease, and early myocardial infarction. Recent genetic and molecular studies suggest more genes than previously appreciated may contribute to the expression of HLP3, both through impaired hepatic TGRL processing or removal and increased TGRL production. HLP3 is often highly amenable to appropriate treatment. Nevertheless, most HLP3 probably goes undiagnosed, in part because of lack of awareness of the relatively high prevalence (about 0.2% in women and 0.4-0.5% in men older than 20 years) and largely because of infrequent use of definitive diagnostic methods.

Apolipoprotein A-I mimetic peptide 4F blocks sphingomyelinase-induced LDL aggregation

Lipolytic modification of LDL particles by SMase generates LDL aggregates with a strong affinity for human arterial proteoglycans and may so enhance LDL retention in the arterial wall. Here, we evaluated the effects of apoA-I mimetic peptide 4F on structural and functional properties of the SMase-modified LDL particles. LDL particles with and without 4F were incubated with SMase, after which their aggregation, structure, and proteoglycan binding were analyzed. At a molar ratio of L-4F to apoB-100 of 2.5 to 20:1, 4F dose-dependently inhibited SMase-induced LDL aggregation. At a molar ratio of 20:1, SMase-induced aggregation was fully blocked. Binding of 4F to LDL particles inhibited SMase-induced hydrolysis of LDL by 10% and prevented SMase-induced LDL aggregation. In addition, the binding of the SMase-modified LDL particles to human aortic proteoglycans was dose-dependently inhibited by pretreating LDL with 4F. The 4F stabilized apoB-100 conformation and inhibited SMase-induced conformational changes of apoB-100. Molecular dynamic simulations showed that upon binding to protein-free LDL surface, 4F locally alters membrane order and fluidity and induces structural changes to the lipid layer. Collectively, 4F stabilizes LDL particles by preventing the SMase-induced conformational changes in apoB-100 and so blocks SMase-induced LDL aggregation and the resulting increase in LDL retention.

Bioactivity guided fractionation and hypolipidemic property of a novel HMG-CoA reductase inhibitor from Ficus virens Ait

BACKGROUND

The current perspective for the search of 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase inhibitor has been shifted towards a natural agent also having antioxidant property. Thus, this study was intended to isolate and identify the bioactive compounds from methanolic extract of Ficus virens bark (FVBM) and to evaluate their antioxidant, HMG-CoA reductase inhibitory and hypolipidemic activity.

METHODS

Bioactivity guided fractionation and isolation of bioactive compound from FVBM extract has been done to isolate and characterize the potent HMG-CoA reductase (HMGR) inhibitor with antioxidant activity by using repeated extensive column chromatography followed by spectroscopic methods, including Infrared (IR), 1H & 13C nuclear magnetic resonance (NMR) and Mass spectrum analysis. The in vitro HMGR inhibition and enzyme kinetic assay was determined using HMG-CoA as substrate. In addition, antioxidant activity of the new isolated compound, was measured using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay and FRAP value. In-silico molecular informatics of HMGR enzyme type inhibition and pharmacokinetics data of the new compound was further evaluated through molecular docking and ADME-T studies. Further, in-vivo hypolipidemic property of FVBM extract and newly isolated compound was also analyzed in triton-WR 1339 induced rats.

RESULTS

Thereby, we report the discovery of n-Octadecanyl-O-α-D-glucopyranosyl(6'→1″)-O-α-D-glucopyranoside (F18) as a novel HMG-CoA reductase inhibitor with strong antioxidant property. This inhibitor exhibited not only higher free radical scavenging activity but also marked HMG-CoA reductase inhibitory activity with an IC50 value of 84±2.8 ng/ml. This inhibitory activity concurred with kinetic study that showed inhibition constant (K i) of 84 ng/ml via an uncompetitive mode of inhibition. The inhibition was also corroborated by molecular docking analysis and in silico pharmacokinetics data. The in vivo study revealed that administration of FVBM extract (at higher dose, 100 mg/rat) and the inhibitor (1 mg/rat) to Triton WR-1339-induced hyperlipidemic rats significantly ameliorated the altered levels of plasma lipids and lipoproteins including hepatic HMG-CoA reductase activity; this effect was comparable to the effect of standard drug atorvastatin.

CONCLUSIONS

The in vitro, in silico and in vivo results clearly demonstrated the antioxidant potential and therapeutic efficacy of the inhibitor as an alternate drug against hyperlipidemia.

Glycosyltransferases, glycosylation and atherosclerosis

Cardiovascular diseases arising from atherosclerosis are currently the leading cause of mortality worldwide. Leukocyte recruitment is a key step for the successful initiation of atherosclerosis and occurs predominantly in the inflamed endothelium. Leukocyte recruitment is mediated by a group of adhesive molecules and chemokine receptors, which are often glycosylated protein. Recent studies demonstrated that post-translational glycosylation by glycosyltransferases is necessary for adhesive molecules and chemokine receptors activities. Several glycosyltransferases, such as α2,3-sialyltransferases IV, α1,3-fucosyltransferases IV and VII, core 2 β1,6-N-acetylglucosaminyltransferase-I, are considered to participate in the synthesis of glycosylation for adhesive molecules and chemokine receptors, and the initiation of atherosclerotic lesions. In this review, we will discuss new data concerning the roles of different glycosyltransferases in atherogenesis. The knowledge of glycosyltransferases in atherogenesis offers the opportunity to develop novel therapeutic strategies.

Sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is a novel nucleotide phosphodiesterase regulated by cholesterol in human macrophages

Cholesterol-loaded foam cell macrophages are prominent in atherosclerotic lesions and play complex roles in both inflammatory signaling and lipid metabolism, which are underpinned by large scale reprogramming of gene expression. We performed a microarray study of primary human macrophages that showed that transcription of the sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) gene is up-regulated after cholesterol loading. SMPDL3A protein expression in and secretion from primary macrophages are stimulated by cholesterol loading, liver X receptor ligands, and cyclic AMP, and N-glycosylated SMPDL3A protein is detectable in circulating blood. We demonstrate for the first time that SMPDL3A is a functional phosphodiesterase with an acidic pH optimum. We provide evidence that SMPDL3A is not an acid sphingomyelinase but unexpectedly is active against nucleotide diphosphate and triphosphate substrates at acidic and neutral pH. SMPDL3A is a major source of nucleotide phosphodiesterase activity secreted by liver X receptor-stimulated human macrophages. Extracellular nucleotides such as ATP may activate pro-inflammatory responses in immune cells. Increased expression and secretion of SMPDL3A by cholesterol-loaded macrophage foam cells in lesions may decrease local concentrations of pro-inflammatory nucleotides and potentially represent a novel anti-inflammatory axis linking lipid metabolism with purinergic signaling in atherosclerosis.

Control of autophagy maturation by acid sphingomyelinase in mouse coronary arterial smooth muscle cells: protective role in atherosclerosis

Recent studies have indicated a protective role of autophagy in regulating vascular smooth muscle cells homeostasis in atherogenesis, but the mechanisms controlling autophagy, particularly autophagy maturation, are poorly understood. Here, we investigated whether acid sphingomyelinase (ASM)-regulated lysosome function is involved in autophagy maturation in coronary arterial smooth muscle cells (CASMCs) in the pathogenesis of atherosclerosis. In coronary arterial wall of ASM-deficient (Smpd1⁻/⁻) mice on Western diet, there were high expression levels of both LC3B, a robust marker of autophagosomes (APs), and p62, a selective autophagy substrate, compared with those in wild-type (Smpd1⁺/⁺) mice. By Western blotting and flow cytometry, atherogenic stimulation of Smpd1⁺/⁺ CASMCs with 7-ketocholesterol was found to significantly enhance LC3B expression and increase the content of both APs and autophagolysosomes (APLs). In Smpd1⁻/⁻ CASMCs, such 7-ketocholesterol-induced increases in LC3B and p62 expression and APs were further augmented, but APLs formation was abolished. Analysis of fluorescence resonance energy transfer between fluorescence-labeled LC3B and Lamp1 (lysosome marker) showed that 7-ketocholesterol markedly induced fusion of APs with lysosomes in Smpd1⁺/⁺ CASMCs, which was abolished in Smpd1⁻/⁻ CASMCs. Moreover, 7-ketocholesterol-induced expression of cell dedifferentiation marker vimentin and proliferation was enhanced in Smpd1⁻/⁻ CASMCs compared with those in Smpd1⁺/⁺ CASMCs. Lastly, overexpression of ASM further increased APLs formation in Smpd1⁺/⁺ CASMCs and restored APLs formation in Smpd1⁻/⁻ CASMCs indicating that increased ASM expression is highly correlated with enhanced APLs formation. Taken together, our data suggest that the control of lysosome trafficking and fusion by ASM is essential to a normal autophagic flux in CASMCs, which implicates that the deficiency of ASM-mediated regulation of autophagy maturation may result in imbalance of arterial smooth muscle cell homeostasis and thus serve as an important atherogenic mechanism in coronary arteries.

KEY MESSAGES

Acid sphingomyelinase (ASM) controls autophagy maturation in smooth muscle cells. ASM maintains smooth muscle cell homeostasis and its contractile phenotype. ASM plays a protective role in smooth muscle dysfunction and atherosclerosis.

Automated measurement method for the determination of vitamin E in plasma lipoprotein classes

In a subendothelial space of atherosclerotic arteries, apolipoprotein B-containing lipoproteins are accumulated and oxidized, and the oxidized lipoproteins promote macrophage foam cell formation. Therefore, the analysis of vitamin E, a major antioxidant in lipoproteins, is important for understanding atherosclerotic pathogenesis. A new method for the automated measurement of vitamin-E (γ- and α-tocopherols) in plasma HDL, LDL, and VLDL was established by using anion-exchange-chromatography for separation of lipoproteins, reverse-phase-chromatography for separation of γ- and α-tocopherols in each of lipoproteins, and fluorescent detection. The within-day assay and between-day assay coefficients of variation for lipoprotein tocopherol levels were 4.73–12.84% and 7.00–14.73%, respectively. The γ- and α-tocopherol/cholesterol ratios of VLDL were higher in healthy plasma than in plasma of untreated patients with dyslipidemia, but the ratios of LDL and HDL were not different. This new estimated method can provide the reliable data of lipoprotein vitamin-E and would be useful for the clinical settings.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Modified lipoprotein-derived lipid particles accumulate in human stenotic aortic valves

In aortic stenosis plasma lipoprotein-derived lipids accumulate in aortic valves. Here, we first compared the lipid compositions of stenotic aortic valves and atherosclerotic plaque cores. Both pathological tissues were found to be enriched in cholesteryl linoleate, a marker of extracellularly accumulated lipoproteins. In addition, a large proportion of the phospholipids were found to contain arachidonic acid, the common precursor of a number of proinflammatory lipid mediators. Next, we isolated and characterized extracellular lipid particles from human stenotic and non-stenotic control valves, and compared them to plasma lipoproteins from the same subjects. The extracellular valvular lipid particles were isolated from 15 stenotic and 14 non-stenotic aortic valves. Significantly more apoB-100-containing lipid particles were found in the stenotic than in the non-stenotic valves. The majority of the lipid particles isolated from the non-stenotic valves had sizes (23±6.2 nm in diameter) similar to those of plasma low density lipoprotein (LDL) (22±1.5 nm), while the lipid particles from stenotic valves were not of uniform size, their sizes ranging from 18 to more than 500 nm. The lipid particles showed signs of oxidative modifications, and when compared to isolated plasma LDL particles, the lipid particles isolated from the stenotic valves had a higher sphingomyelin/phosphatidylcholine –ratio, and also higher contents of lysophosphatidylcholine and unesterified cholesterol. The findings of the present study reveal, for the first time, that in stenotic human aortic valves, infiltrated plasma lipoproteins have undergone oxidative and lipolytic modifications, and become fused and aggregated. The generated large lipid particles may contribute to the pathogenesis of human aortic stenosis.

Characterization of secretory sphingomyelinase activity, lipoprotein sphingolipid content and LDL aggregation in ldlr-/- mice fed on a high-fat diet

The propensity of LDLs (low-density lipoproteins) for aggregation and/or oxidation has been linked to their sphingolipid content, specifically the levels of SM (sphingomyelin) and ceramide. To investigate this association in vivo, ldlr (LDL receptor)-null mice (ldlr^−/−) were fed on a modified (atherogenic) diet containing saturated fats and cholesterol. The diet led to significantly elevated SM content in all serum lipoproteins. In contrast, ceramide increased only in the LDL particles. MS-based analyses of the lipid acyl chain composition revealed a marked elevation in C_16:0 fatty acid in SM and ceramide, consistent with the prevalence of palmitic acid in the modified diet. The diet also led to increased activity of the S-SMase [secretory SMase (sphingomyelinase)], a protein that is generated by ASMase (acid SMase) and acts on serum LDL. An increased macrophage secretion seemed to be responsible for the elevated S-SMase activity. ASMase-deficient mice (asm^−/−/ldlr^−/−) lacked S-SMase activity and were protected from diet-induced elevation in LDL ceramide. LDL from asm^−/−/ldlr^−/− mice fed on the modified diet were less aggregated and oxidized than LDL from asm^+/+/ldlr^−/− mice. When tested in vitro, the propensity for aggregation was dependent on the SM level: only LDL from animals on modified diet that have high SM content aggregated when treated with recombinant S-SMase. In conclusion, LDL-SM content and S-SMase activity are up-regulated in mice fed on an atherogenic diet. S-SMase mediates diet-induced changes in LDL ceramide content and aggregation. S-SMase effectiveness in inducing aggregation is dependent on diet-induced enrichment of LDL with SM, possibly through increased hepatic synthesis.

Conformational changes of apoB-100 in SMase-modified LDL mediate formation of large aggregates at acidic pH

During atherogenesis, the extracellular pH of atherosclerotic lesions decreases. Here, we examined the effect of low, but physiologically plausible pH on aggregation of modified LDL, one of the key processes in atherogenesis. LDL was treated with SMase, and aggregation of the SMase-treated LDL was followed at pH 5.5-7.5. The lower the pH, the more extensive was the aggregation of identically prelipolyzed LDL particles. At pH 5.5-6.0, the aggregates were much larger (size >1 µm) than those formed at neutral pH (100-200 nm). SMase treatment was found to lead to a dramatic decrease in α-helix and concomitant increase in β-sheet structures of apoB-100. Particle aggregation was caused by interactions between newly exposed segments of apoB-100. LDL-derived lipid microemulsions lacking apoB-100 failed to form large aggregates. SMase-induced LDL aggregation could be blocked by lowering the incubation temperature to 15°C, which also inhibited the changes in the conformation of apoB-100, by proteolytic degradation of apoB-100 after SMase-treatment, and by HDL particles. Taken together, sphingomyelin hydrolysis induces exposure of protease-sensitive sites of apoB-100, whose interactions govern subsequent particle aggregation. The supersized LDL aggregates may contribute to the retention of LDL lipids in acidic areas of atherosclerosis-susceptible sites in the arterial intima.

Size-selective uptake of colloidal low density lipoprotein aggregates by cultured white blood cells

This paper illustrates how principles of colloid science are useful in studying atherosclerosis. Accumulation of foam cells in the arterial intima is a key step in atherogenesis. The extent of foam cell formation is enhanced by low density lipoprotein (LDL) aggregates, and we have previously shown that the size of sphingomyelinase (Smase)-hydrolysis-induced aggregates depends directly on the concentration of ceramide generated in the LDL phospholipid monolayer, mediated by the hydrophobic effect. Here, we focus on the effect of LDL aggregate particle sizes on their subsequent uptake by macrophages. Our data show the first direct measurement of uptake as a function of aggregate size and the first direct comparison of uptake after Smase-catalyzed and vortex-mixing-mediated aggregation. Vortex-mixed aggregates with radii 20-77 nm showed maximal uptake approximately 118 microg sterol/mg protein at a 53 nm intermediate size, consistent with a mathematical model describing competition between aggregate surface area and volume. Smase-treated aggregates with radii 25-211 nm also showed maximal uptake at an intermediate size, approximately 58 microg sterol/mg protein for 132 nm particles, and fit a modified model that incorporated ceramide concentration expressed as aggregate size. This study shows that particle size is significant and composition may also be a factor in LDL uptake.

ApoCIII-enriched LDL in type 2 diabetes displays altered lipid composition, increased susceptibility for sphingomyelinase, and increased binding to biglycan

OBJECTIVE

Apolipoprotein CIII (apoCIII) is an independent risk factor for cardiovascular disease, but the molecular mechanisms involved are poorly understood. We investigated potential proatherogenic properties of apoCIII-containing LDL from hypertriglyceridemic patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

LDL was isolated from control subjects, subjects with type 2 diabetes, and apoB transgenic mice. LDL-biglycan binding was analyzed with a solid-phase assay using immunoplates coated with biglycan. Lipid composition was analyzed with mass spectrometry. Hydrolysis of LDL by sphingomyelinase was analyzed after labeling plasma LDL with [(3)H]sphingomyelin. ApoCIII isoforms were quantified after isoelectric focusing. Human aortic endothelial cells were incubated with desialylated apoCIII or with LDL enriched with specific apoCIII isoforms.

RESULTS

We showed that enriching LDL with apoCIII only induced a small increase in LDL-proteoglycan binding, and this effect was dependent on a functional site A in apoB100. Our findings indicated that intrinsic characteristics of the diabetic LDL other than apoCIII are responsible for further increased proteoglycan binding of diabetic LDL with high-endogenous apoCIII, and we showed alterations in the lipid composition of diabetic LDL with high apoCIII. We also demonstrated that high apoCIII increased susceptibility of LDL to hydrolysis and aggregation by sphingomyelinases. In addition, we demonstrated that sialylation of apoCIII increased with increasing apoCIII content and that sialylation of apoCIII was essential for its proinflammatory properties.

CONCLUSIONS

We have demonstrated a number of features of apoCIII-containing LDL from hypertriglyceridemic patients with type 2 diabetes that could explain the proatherogenic role of apoCIII.

Syndecan-4 mediates macrophage uptake of group V secretory phospholipase A2-modified LDL

We previously reported that LDL modified by group V secretory phospholipase A2 (GV-LDL) promotes macrophage foam cell formation through a mechanism independent of scavenger receptors SR-A and CD36, and dependent on cellular proteoglycans. This study investigates the role of syndecans, a family of cell surface proteoglycans known to mediate endocytosis through macropinocytosis, in macrophage uptake of GV-LDL. LY 294002, a phosphatidylinositol 3-kinase inhibitor, significantly reduced internalization of (125)I-labeled GV-LDL in J-774 macrophages, consistent with a macropinocytic uptake pathway. Using small, interfering RNA-directed gene silencing, we demonstrated a direct relationship between (125)I-labeled GV-LDL binding and the level of syndecan-3 and syndecan-4 expression in J-774 cells. However, (125)I-labeled GV-LDL uptake was significantly reduced only when syndecan-4 expression was suppressed. Peritoneal macrophages from syndecan-4-deficient mice exhibited markedly reduced uptake of fluorescently labeled GV-LDL compared with wild-type cells. Furthermore, cholesteryl ester accumulation induced by GV-LDL was dependent on syndecan-4 expression. Syndecan-4 expression and GV-LDL binding were significantly increased in J-774 cells treated with lipopolysaccharide, suggesting that GV-LDL uptake via this pathway may be enhanced during inflammation. Taken together, our data point to a novel role for syndecan-4 in mediating the uptake of GV-LDL, a process implicated in atherosclerotic lesion progression.

Deglycosylation of apo B-containing lipoproteins increase their ability to aggregate and to promote intracellular cholesterol accumulation in vitro

Sub-fractions of all apo B-100 containing lipoproteins (low density lipoproteins, very low density lipoproteins and intermediate density lipoproteins) with reduced contents of sialic acid were found in vivo in human blood. These lipoproteins were inclined to spontaneously form aggregates and were able to stimulate accumulation of cholesterol in cells cultured from human aortic intima. In vitro treatment of apo B-containing lipoproteins with 2,6- and 2,3-specific sialidases, alpha-mannosidase, endoglycosidases F1 or F2 or peptide-N-glycanase F also stimulated aggregation and increased the ability of these particles to potentiate cholesterol accumulation in cells of the intact human aortic intima. So, deglycosylation of various apo B-containing lipoproteins possibly occurs in the blood, decreases their resistance to aggregation and increases the ability of these particles to stimulate accumulation of cholesterol in human aortic intima cells, thereby increasing their atherogenic potential.

Vascular endothelium in atherosclerosis

Their strategic location between blood and tissue and their constitutive properties allow endothelial cells (EC) to monitor the transport of plasma molecules, by employing bidirectional receptor-mediated and receptor-independent transcytosis and endocytosis, and to regulate vascular tone, cellular cholesterol and lipid homeostasis. These cells are also involved in signal transduction, immunity, inflammation and haemostasis. Cardiovascular risk factors, such as hyperlipaemia/dyslipidaemia trigger the molecular machinery of EC to respond to insults by modulation of their constitutive functions followed by dysfunction and ultimately by injury and apoptosis. The gradual activation of EC consists initially in the modulation of two constitutive functions: (1) permeability, i.e. increased transcytosis of lipoproteins, and (2) biosynthetic activity, i.e. enhanced synthesis of the basement membrane and extracellular matrix. The increased transcytosis and the reduced efflux of beta-lipoproteins (betaLp) lead to their retention within the endothelial hyperplasic basal lamina as modified lipoproteins (MLp) and to their subsequent alteration (oxidation, glycation, enzymatic modifications). MLp generate chemoattractant and inflammatory molecules, triggering EC dysfunction (appearance of new adhesion molecules, secretion of chemokines, cytokines), characterised by monocyte recruitment, adhesion, diapedesis and residence within the subendothelium. In time, EC in the athero-prone areas alter their net negative surface charge, losing their non-thrombogenic ability, become loaded with lipid droplets and turn into foam cells. Prolonged and/or repeated exposure to cardiovascular risk factors can ultimately exhaust the protective effect of the endogenous anti-inflammatory system within EC. As a consequence, EC may progress to senescence, lose their integrity and detach into the circulation.

Increased lipolysis by secretory phospholipase A(2) group V of lipoproteins in diabetic dyslipidaemia

BACKGROUND

Lipolysis of lipoproteins by secretory phospholipase A(2) group V (sPLA(2)-V) promotes inflammation, lipoprotein aggregation and foam cell formation--all considered as atherogenic mechanisms.

OBJECTIVE

In this study, we compared the susceptibility to sPLA(2)-V lipolysis of VLDL and LDL from individuals with type 2 diabetes and the metabolic syndrome (T2D-MetS) and from healthy controls. Design. VLDL and LDL were isolated from 38 T2D-MetS subjects and 38 controls, treated pair-wise. Extent of sPLA(2)-V lipolysis was measured as release of nonesterified free fatty acids (NEFA). In a subset of the subjects, lipoprotein composition was determined as a relationship between lipid and apolipoprotein components.

RESULTS

Mean paired increase in sPLA(2)-V lipolysis after 1 h for T2D-MetS versus control was 2.0 micromol NEFA l(-1) for VLDL (P = 0.004) and 0.75 micromol NEFA l(-1) for LDL (P = 0.001). There were also substantial differences in lipoprotein composition between the groups. T2D-MetS VLDL had higher triglyceride and cholesterol contents than control VLDL. T2D-MetS LDL was smaller and contained more triglycerides and less cholesterol than control LDL. Both VLDL and LDL from T2D-MetS subjects also contained more apolipoprotein CIII per particle.

CONCLUSION

VLDL and LDL from T2D-MetS individuals were more susceptible to sPLA(2)-V lipolysis than those from control individuals. This may result in elevated levels of NEFA and lysophosphatidylcholine, both in circulation and in LDL, possibly contributing to the elevated inflammatory state and increased risk of cardiovascular diseases seen in these individuals.

Acid sphingomyelinase promotes lipoprotein retention within early atheromata and accelerates lesion progression

OBJECTIVE

The key initial step in atherogenesis is the subendothelial retention of apolipoprotein B-containing lipoproteins. Acid sphingomyelinase (acid SMase), an enzyme present extracellularly within the arterial wall, strongly enhances lipoprotein retention in model systems in vitro, and retained lipoproteins in human plaques are enriched in ceramide, a product of SMase. We now sought to test a direct causative role for acid SMase in lipoprotein retention and atherogenesis in vivo.

METHODS AND RESULTS

We studied atherogenesis and lipoprotein retention in Asm(-/-) versus Asm(+/+) mice on the Apoe(-/-) and Ldlr(-/-) backgrounds. Asm(-/-);Apoe(-/-) mice had a approximately 40% to 50% decrease in early foam cell aortic root lesion area compared with Asm(+/+);Apoe(-/-) mice (P<0.05) despite no difference in plasma cholesterol or lipoproteins. To assay lipoprotein retention in vivo, the two groups of mice were injected with fluorescently labeled Apoe(-/-) lipoproteins. Early foam cell lesions of Asm(-/-);Apoe(-/-) mice showed a striking 87% reduction in lipoprotein trapping (P<0.0001) compared with Asm(+/+);Apoe(-/-) lesions. Similar results were obtained with Ldlr(-/-) mice, including an 81% reduction in lipoprotein retention within Asm(-/-);Ldlr(-/-) lesions compared with Asm(+/+);Ldlr(-/-) lesions (P<0.0005).

CONCLUSIONS

These findings support a causal role for acid SMase in lipoprotein retention and lesion progression and provides further support for the response-to-retention model of atherogenesis.