Immunochemical analysis of the electronegative LDL subfraction shows that abnormal N-terminal apolipoprotein B conformation is involved in increased binding to proteoglycans

LDL binding to cell receptors and extracellular matrix is proatherogenic in obesity but improves after bariatric surgery

Obesity is a major global public health issue involving dyslipidemia, oxidative stress, inflammation, and increased risk of CVD. Weight loss reduces this risk, but the biochemical underpinnings are unclear. We explored how obesity and weight loss after bariatric surgery influence LDL interactions that trigger proatherogenic versus antiatherogenic processes. LDL was isolated from plasma of six patients with severe obesity before (basal) and 6-12 months after bariatric surgery (basal BMI = 42.7 kg/m2; 6-months and 12-months postoperative BMI = 34.1 and 30 kg/m2). Control LDL were from six healthy subjects (BMI = 22.6 kg/m2). LDL binding was quantified by ELISA; LDL size and charge were assessed by chromatography; LDL biochemical composition was determined. Compared to controls, basal LDL showed decreased nonatherogenic binding to LDL receptor, which improved postoperatively. Conversely, basal LDL showed increased binding to scavenger receptors LOX1 and CD36 and to glycosaminoglycans, fibronectin and collagen, which is proatherogenic. One year postoperatively, this binding decreased but remained elevated, consistent with elevated lipid peroxidation. Serum amyloid A and nonesterified fatty acids were elevated in basal and postoperative LDL, indicating obesity-associated inflammation. Aggregated and electronegative LDL remained elevated, suggesting proatherogenic processes. These results suggest that obesity-induced inflammation contributes to harmful LDL alterations that probably increase the risk of CVD. We conclude that in obesity, LDL interactions with cell receptors and extracellular matrix shift in a proatherogenic manner but are partially reversed upon postoperative weight loss. These results help explain why the risk of CVD increases in obesity but decreases upon weight loss.

Can Electronegative LDL Act as a Multienzymatic Complex?

Electronegative LDL (LDL(-)) is a minor form of LDL present in blood for which proportions are increased in pathologies with increased cardiovascular risk. In vitro studies have shown that LDL(-) presents pro-atherogenic properties, including a high susceptibility to aggregation, the ability to induce inflammation and apoptosis, and increased binding to arterial proteoglycans; however, it also shows some anti-atherogenic properties, which suggest a role in controlling the atherosclerotic process. One of the distinctive features of LDL(-) is that it has enzymatic activities with the ability to degrade different lipids. For example, LDL(-) transports platelet-activating factor acetylhydrolase (PAF-AH), which degrades oxidized phospholipids. In addition, two other enzymatic activities are exhibited by LDL(-). The first is type C phospholipase activity, which degrades both lysophosphatidylcholine (LysoPLC-like activity) and sphingomyelin (SMase-like activity). The second is ceramidase activity (CDase-like). Based on the complementarity of the products and substrates of these different activities, this review speculates on the possibility that LDL(-) may act as a sort of multienzymatic complex in which these enzymatic activities exert a concerted action. We hypothesize that LysoPLC/SMase and CDase activities could be generated by conformational changes in apoB-100 and that both activities occur in proximity to PAF-AH, making it feasible to discern a coordinated action among them.

Presence of Ceramidase Activity in Electronegative LDL

Electronegative low-density lipoprotein (LDL(-)) is a minor modified fraction of human plasma LDL with several atherogenic properties. Among them is increased bioactive lipid mediator content, such as lysophosphatidylcholine (LPC), non-esterified fatty acids (NEFA), ceramide (Cer), and sphingosine (Sph), which are related to the presence of some phospholipolytic activities, including platelet-activating factor acetylhydrolase (PAF-AH), phospholipase C (PLC), and sphingomyelinase (SMase), in LDL(-). However, these enzymes' activities do not explain the increased Sph content, which typically derives from Cer degradation. In the present study, we analyzed the putative presence of ceramidase (CDase) activity, which could explain the increased Sph content. Thin layer chromatography (TLC) and lipidomic analysis showed that Cer, Sph, and NEFA spontaneously increased in LDL(-) incubated alone at 37 °C, in contrast with native LDL(+). An inhibitor of neutral CDase prevented the formation of Sph and, in turn, increased Cer content in LDL(-). In addition, LDL(-) efficiently degraded fluorescently labeled Cer (NBD-Cer) to form Sph and NEFA. These observations defend the existence of the CDase-like activity's association with LDL(-). However, neither the proteomic analysis nor the Western blot detected the presence of an enzyme with known CDase activity. Further studies are thus warranted to define the origin of the CDase-like activity detected in LDL(-).

Autoimmune Rheumatic Diseases: An Update on the Role of Atherogenic Electronegative LDL and Potential Therapeutic Strategies

Atherosclerosis has been linked with an increased risk of atherosclerotic cardiovascular disease (ASCVD). Autoimmune rheumatic diseases (AIRDs) are associated with accelerated atherosclerosis and ASCVD. However, the mechanisms underlying the high ASCVD burden in patients with AIRDs cannot be explained only by conventional risk factors despite disease-specific factors and chronic inflammation. Nevertheless, the normal levels of plasma low-density lipoprotein (LDL) cholesterol observed in most patients with AIRDs do not exclude the possibility of increased LDL atherogenicity. By using anion-exchange chromatography, human LDL can be divided into five increasingly electronegative subfractions, L1 to L5, or into electropositive and electronegative counterparts, LDL (+) and LDL (-). Electronegative L5 and LDL (-) have similar chemical compositions and can induce adverse inflammatory reactions in vascular cells. Notably, the percentage of L5 or LDL (-) in total LDL is increased in normolipidemic patients with AIRDs. Electronegative L5 and LDL (-) are not recognized by the normal LDL receptor but instead signal through the lectin-like oxidized LDL receptor 1 (LOX-1) to activate inflammasomes involving interleukin 1β (IL-1β). Here, we describe the detailed mechanisms of AIRD-related ASCVD mediated by L5 or LDL (-) and discuss the potential targeting of LOX-1 or IL-1β signaling as new therapeutic modalities for these diseases.

Molecular and Cellular Mechanisms of Electronegative Lipoproteins in Cardiovascular Diseases

Dysregulation of glucose and lipid metabolism increases plasma levels of lipoproteins and triglycerides, resulting in vascular endothelial damage. Remarkably, the oxidation of lipid and lipoprotein particles generates electronegative lipoproteins that mediate cellular deterioration of atherosclerosis. In this review, we examined the core of atherosclerotic plaque, which is enriched by byproducts of lipid metabolism and lipoproteins, such as oxidized low-density lipoproteins (oxLDL) and electronegative subfraction of LDL (LDL(−)). We also summarized the chemical properties, receptors, and molecular mechanisms of LDL(−). In combination with other well-known markers of inflammation, namely metabolic diseases, we concluded that LDL(−) can be used as a novel prognostic tool for these lipid disorders. In addition, through understanding the underlying pathophysiological molecular routes for endothelial dysfunction and inflammation, we may reassess current therapeutics and might gain a new direction to treat atherosclerotic cardiovascular diseases, mainly targeting LDL(−) clearance.

The role of postprandial very-low-density lipoprotein in the development of atrial remodeling in metabolic syndrome

Background

Negatively charged very-low-density lipoprotein (VLDL-χ) in metabolic syndrome (MetS) patients exerts cytotoxic effects on endothelial cells and atrial myocytes. Atrial cardiomyopathy, manifested by atrial remodeling with a dilated diameter, contributes to atrial fibrillation pathogenesis and predicts atrial fibrillation development. The correlation of VLDL-χ with atrial remodeling is unknown. This study investigated the association between VLDL-χ and remodeling of left atrium.

Methods

Consecutively, 87 MetS and 80 non-MetS individuals between 23 and 74 years old (50.6% men) without overt cardiovascular diseases were included in the prospective cohort study. Blood samples were collected while fasting and postprandially (at 0.5, 1, 2, and 4 h after a unified meal). VLDL was isolated by ultracentrifugation; the percentile concentration of VLDL-χ (%) was determined by ultra-performance liquid chromatography. The correlations of left atrium diameter (LAD) with variables including VLDL-χ, LDL-C, HDL-C, triglycerides, glucose, and blood pressure, were analyzed by multiple linear regression models. A hierarchical linear model was conducted to test the independencies of each variable’s correlation with LAD.

Results

The mean LAD was 3.4 ± 0.5 cm in non-MetS subjects and 3.9 ± 0.5 cm in MetS patients (P < 0.01). None of the fasting lipid profiles were associated with LAD. VLDL-χ, BMI, waist circumference, hip circumference, and blood pressure were positively correlated with LAD (all P < 0.05) after adjustment for age and sex. Significant interactions between VLDL-χ and blood pressure, waist circumference, and hip circumference were observed. When adjusted for obesity- and blood pressure-related variables, 2-h postprandial VLDL-χ (mean 1.30 ± 0.61%) showed a positive correlation with LAD in MetS patients. Each 1% VLDL-χ increase was estimated to increase LAD by 0.23 cm.

Conclusions

Postprandial VLDL-χ is associated with atrial remodeling particularly in the MetS group. VLDL-χ is a novel biomarker and may be a therapeutic target for atrial cardiomyopathy in MetS patients.

Trial registration

ISRCTN 69295295. Retrospectively registered 9 June 2020.

Role of Low-Density Lipoprotein in Early Vascular Aging Associated With Systemic Lupus Erythematosus

OBJECTIVE

Patients with systemic lupus erythematosus (SLE) often have atherosclerotic complications at a young age but normal low-density lipoprotein (LDL) levels. This study was undertaken to investigate the role of LDL composition in promoting early vascular aging in SLE patients.

METHODS

Plasma LDL from 45 SLE patients (SLE-LDL) and from 37 normal healthy controls (N-LDL) was chromatographically divided into 5 subfractions (L1-L5), and the subfraction composition was analyzed. Correlations between subfraction levels and signs of early vascular aging were assessed. Mechanisms of lipid-mediated endothelial dysfunction were explored using in vitro assays and experiments in apoE^-/- mice.

RESULTS

The L5 percentage was increased 3.4 times in the plasma of SLE patients compared with normal controls. This increased percentage of SLE-L5 was positively correlated with the mean blood pressure (r = 0.27, P = 0.04), carotid intima-media thickness (IMT) (right carotid IMT, r = 0.4, P = 0.004; left carotid IMT, r = 0.36, P = 0.01), pulse wave velocity (r = 0.29, P = 0.04), and blood levels of CD16+ monocytes (r = 0.35, P = 0.004) and CX3CL1 cytokines (r = 0.43, P < 0.001) in SLE patients. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analysis revealed that plasma levels of lysophosphatidylcholine (LPC) and platelet-activating factor (PAF) were increased in SLE-LDL and in the SLE-L5 plasma subfraction. Injecting SLE-LDL, SLE-L5, or LPC into young, male apoE^-/- mice caused increases in plasma CX3CL1 levels, aortic fatty-streak areas, aortic vascular aging, and macrophage infiltration into the aortic wall, whereas injection of N-LDL or SLE-L1 had negligible effects (n = 3-8 mice per group). In vitro, SLE-L5 lipid extracts induced increases in CX3CR1 and CD16 expression in human monocytes; synthetic PAF and LPC had similar effects. Furthermore, lipid extracts of SLE-LDL and SLE-L5 induced the expression of CX3CL1 and enhanced monocyte-endothelial cell adhesion in assays with bovine aortic endothelial cells.

CONCLUSION

An increase in plasma L5 levels, not total LDL concentration, may promote early vascular aging in SLE patients, leading to premature atherosclerosis.

Biglycan and atherosclerosis: Lessons from high cardiovascular risk conditions

Atherosclerosis (ATH) is a chronic, dynamic, evolutive process involving morphological and structural subversion of artery walls, leading to the formation of atherosclerotic plaques. ATH generally initiates during the childhood, occurring as a result of a number of changes in the intima tunica and in the media of arteries. A key event occurring during the pathobiology of ATH is the accumulation of lipoproteins in the sub-intimal spaces mediated by extracellular matrix (ECM) molecules, especially by the chondroitin sulfate/dermatan sulfate (CS/DS) -containing proteoglycans (CS/DSPGs). Among them, the proteoglycan biglycan (BGN) is critically involved in the onset and progression of ATH and evidences show that BGN represents the missing link between the pro-atherogenic status induced by both traditional and non-traditional cardiovascular risk factors and the development and progression of vascular damage. In the light of these findings, the role of BGN in dyslipidemia, hypertension, cigarette smoking, diabetes, chronic kidney disease and inflammatory status is briefly analyzed and discussed in order to shed new light on the underlying mechanisms governing the association between BGN and ATH.

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.

Electronegative Low-Density Lipoprotein L5 Impairs Viability and NGF-Induced Neuronal Differentiation of PC12 Cells via LOX-1

There have been striking associations of cardiovascular diseases (e.g., atherosclerosis) and hypercholesterolemia with increased risk of neurodegeneration including Alzheimer's disease (AD). Low-density lipoprotein (LDL), a cardiovascular risk factor, plays a crucial role in AD pathogenesis; further, L5, a human plasma LDL fraction with high electronegativity, may be a factor contributing to AD-type dementia. Although L5 contributing to atherosclerosis progression has been studied, its role in inducing neurodegeneration remains unclear. Here, PC12 cell culture was used for treatments with human LDLs (L1, L5, or oxLDL), and subsequently cell viability and nerve growth factor (NGF)-induced neuronal differentiation were assessed. We identified L5 as a neurotoxic LDL, as demonstrated by decreased cell viability in a time- and concentration-dependent manner. Contrarily, L1 had no such effect. L5 caused cell damage by inducing ATM/H2AX-associated DNA breakage as well as by activating apoptosis via lectin-like oxidized LDL receptor-1 (LOX-1) signaling to p53 and ensuring cleavage of caspase-3. Additionally, sublethal L5 long-termly inhibited neurite outgrowth in NGF-treated PC12 cells, as evidenced by downregulation of early growth response factor-1 and neurofilament-M. This inhibitory effect was mediated via an interaction between L5 and LOX-1 to suppress NGF-induced activation of PI3k/Akt cascade, but not NGF receptor TrkA and downstream MAPK pathways. Together, our data suggest that L5 creates a neurotoxic stress via LOX-1 in PC12 cells, thereby leading to impairment of viability and NGF-induced differentiation. Atherogenic L5 likely contributes to neurodegenerative disorders.

Thermal stability of human plasma electronegative low-density lipoprotein: A paradoxical behavior of low-density lipoprotein aggregation

Low-density lipoprotein (LDL) aggregation is central in triggering atherogenesis. A minor fraction of electronegative plasma LDL, termed LDL(-), plays a special role in atherogenesis. To better understand this role, we analyzed the kinetics of aggregation, fusion and disintegration of human LDL and its fractions, LDL(+) and LDL(-). Thermal denaturation of LDL was monitored by spectroscopy and electron microscopy. Initially, LDL(-) aggregated and fused faster than LDL(+), but later the order reversed. Most LDL(+) disintegrated and precipitated upon prolonged heating. In contrast, LDL(-) partially retained lipoprotein morphology and formed soluble aggregates. Biochemical analysis of all fractions showed no significant degradation of major lipids, mild phospholipid oxidation, and an increase in non-esterified fatty acid (NEFA) upon thermal denaturation. The main baseline difference between LDL subfractions was higher content of NEFA in LDL(-). Since NEFA promote lipoprotein fusion, increased NEFA content can explain rapid initial aggregation and fusion of LDL(-) but not its resistance to extensive disintegration. Partial hydrolysis of apoB upon heating was similar in LDL subfractions, suggesting that minor proteins importantly modulate LDL disintegration. Unlike LDL(+), LDL(-) contains small amounts of apoA-I and apoJ. Addition of exogenous apoA-I to LDL(+) hampered lipoprotein aggregation, fusion and precipitation, while depletion of endogenous apoJ had an opposite effect. Therefore, the initial rapid aggregation of LDL(-) is apparently counterbalanced by the stabilizing effects of minor proteins such as apoA-I and apoJ. These results help identify key determinants for LDL aggregation, fusion and coalescence into lipid droplets in vivo.

LDL electronegativity index: a potential novel index for predicting cardiovascular disease

High cardiovascular risk conditions are frequently associated with altered plasma lipoprotein profile, such as elevated low-density lipoprotein (LDL) and LDL cholesterol and decreased high-density lipoprotein. There is, however, accumulating evidence that specific subclasses of LDL may play an important role in cardiovascular disease development, and their relative concentration can be regarded as a more relevant risk factor. LDL particles undergo multiple modifications in plasma that can lead to the increase of their negative charge. The resulting electronegative LDL [LDL(–)] subfraction has been demonstrated to be especially atherogenic, and became a subject of numerous recent studies. In this review, we discuss the physicochemical properties of LDL(–), methods of its detection, atherogenic activity, and relevance of the LDL electronegativity index as a potential independent predictor of cardiovascular risk.

Electronegative LDL: a circulating modified LDL with a role in inflammation

Electronegative low density lipoprotein (LDL(−)) is a minor modified fraction of LDL found in blood. It comprises a heterogeneous population of LDL particles modified by various mechanisms sharing as a common feature increased electronegativity. Modification by oxidation is one of these mechanisms. LDL(−) has inflammatory properties similar to those of oxidized LDL (oxLDL), such as inflammatory cytokine release in leukocytes and endothelial cells. However, in contrast with oxLDL, LDL(−) also has some anti-inflammatory effects on cultured cells. The inflammatory and anti-inflammatory properties ascribed to LDL(−) suggest that it could have a dual biological effect.

CD14 and TLR4 mediate cytokine release promoted by electronegative LDL in monocytes

AIMS

Electronegative LDL (LDL(-)), a minor modified LDL present in the circulation, induces cytokine release in monocytes. We aimed to determine the role of the receptor CD14 and toll-like receptors 2 and 4 (TLR2, TLR4) in the inflammatory action promoted by LDL(-) in human monocytes.

METHODS AND RESULTS

Monocytes were preincubated with antibodies to neutralize CD14, TLR2 and TLR4. The release of monocyte chemoattractant protein 1 (MCP1), and interleukin 6 and 10 (IL6 and IL10) promoted by LDL(-) was inhibited 70-80% by antiCD14 and antiTLR4, and 15-25% by antiTLR2. The involvement of CD14 and TLR4 was confirmed by gene silencing experiments. The human monocytic THP1 cell line overexpressing CD14 released more cytokines in response to LDL(-) than the same THP1 cell line without expressing CD14. VIPER, a specific inhibitor of the TLR4 signaling pathway, blocked 75-90% the cytokine release promoted by LDL(-). Cell binding experiments showed that monocytes preincubated with neutralizing antibodies presented lesser LDL(-) binding than non-preincubated monocytes The inhibitory capacity was antiCD14>antiTLR4>>antiTLR2. Cell-free experiments performed in CD14-coated microtiter wells confirmed that CD14 was involved in LDL(-) binding. When LDL(-) and lipopolysaccharide (LPS) were added simultaneously to monocytes, cytokine release was similar to that promoted by LDL(-) alone. Binding experiments showed that LDL(-) and LPS competed for binding to monocytes and to CD14 coated-wells.

CONCLUSIONS

CD14 and TLR4 mediate cytokine release induced by LDL(-) in human monocytes. The cross-competition between LPS and LDL(-) for the same receptors could be a counteracting action of LDL(-) in inflammatory situations.

The Induction of Cytokine Release in Monocytes by Electronegative Low-Density Lipoprotein (LDL) Is Related to Its Higher Ceramide Content than Native LDL

Electronegative low-density lipoprotein (LDL(−)) is a minor modified LDL subfraction that is present in blood. LDL(−) promotes inflammation and is associated with the development of atherosclerosis. We previously reported that the increase of cytokine release promoted by this lipoprotein subfraction in monocytes is counteracted by high-density lipoprotein (HDL). HDL also inhibits a phospholipase C-like activity (PLC-like) intrinsic to LDL(−). The aim of this work was to assess whether the inhibition of the PLC-like activity by HDL could decrease the content of ceramide (CER) and diacylglycerol (DAG) generated in LDL(−). This knowledge would allow us to establish a relationship between these compounds and the inflammatory activity of LDL(−). LDL(−) incubated at 37 °C for 20 h increased its PLC-like activity and, subsequently, the amount of CER and DAG. We found that incubating LDL(−) with HDL decreased both products in LDL(−). Native LDL was modified by lipolysis with PLC or by incubation with CER-enriched or DAG-enriched liposomes. The increase of CER in native LDL significantly increased cytokine release, whereas the enrichment in DAG did not show these inflammatory properties. These data point to CER, a resultant product of the PLC-like activity, as a major determinant of the inflammatory activity induced by LDL(−) in monocytes.

Electronegative low-density lipoprotein. A link between apolipoprotein B misfolding, lipoprotein aggregation and proteoglycan binding

PURPOSE OF REVIEW

Subendothelial retention of lipoproteins is considered the first step in the development of atherosclerosis, but the molecular mechanisms involved are poorly understood. Recent findings on the atherogenic properties of a minor electronegative fraction of LDL (LDL(-)) could contribute to a better understanding of this process.

RECENT FINDINGS

Circular dichroism, Trp-fluorescence and two-dimensional nuclear magnetic resonance have shown that apolipoprotein B (apoB) in LDL(-) has an abnormal, misfolded conformation. Immunochemical analysis revealed a different conformation, mainly in the N-terminal and C-terminal extremes. These alterations contribute to the high susceptibility to aggregation of LDL(-). Moreover, LDL(-) can seed the aggregation of native LDL, suggesting an amyloidogenic character that has been attributed to the amphipathic helix cluster in the α2-domain. A phospholipase C (PLC)-like activity associated to LDL(-) seems to play a major role in the LDL(-)-induced aggregation. The aggregation of LDL(-) increases its binding to proteoglycans because of the abnormal conformation of the N-terminal extreme of apoB.

SUMMARY

LDL(-) could play a relevant role in atherogenesis by acting as a priming factor that stimulates lipoprotein aggregation. This process, which appears to be mediated by a PLC-like activity intrinsic to LDL(-), increases the binding of LDL to proteoglycans and could promote subendothelial retention of these lipoproteins.

Kinetic analysis of thermal stability of human low density lipoproteins: a model for LDL fusion in atherogenesis

Fusion of modified LDL in the arterial wall promotes atherogenesis. Earlier we showed that thermal denaturation mimics LDL remodeling and fusion, and revealed kinetic origin of LDL stability. Here we report the first quantitative analysis of LDL thermal stability. Turbidity data show sigmoidal kinetics of LDL heat denaturation, which is unique among lipoproteins, suggesting that fusion is preceded by other structural changes. High activation energy of denaturation, E(a) = 100 ± 8 kcal/mol, indicates disruption of extensive packing interactions in LDL. Size-exclusion chromatography, nondenaturing gel electrophoresis, and negative-stain electron microscopy suggest that LDL dimerization is an early step in thermally induced fusion. Monoclonal antibody binding suggests possible involvement of apoB N-terminal domain in early stages of LDL fusion. LDL fusion accelerates at pH < 7, which may contribute to LDL retention in acidic atherosclerotic lesions. Fusion also accelerates upon increasing LDL concentration in near-physiologic range, which likely contributes to atherogenesis. Thermal stability of LDL decreases with increasing particle size, indicating that the pro-atherogenic properties of small dense LDL do not result from their enhanced fusion. Our work provides the first kinetic approach to measuring LDL stability and suggests that lipid-lowering therapies that reduce LDL concentration but increase the particle size may have opposite effects on LDL fusion.

Pluronic block copolymers inhibit low density lipoprotein self-association

Little is known about exogenous inhibitors of low-density lipoprotein (LDL) aggregation. The search for nontoxic and bioavailable inhibitors of LDL aggregation is of interest, especially considering that the suppression of the aggregation of LDL might represent a therapeutic approach. We hypothesized that amphiphilic copolymers of propylene oxide and ethylene oxide, the so-called Pluronic block copolymers, can be used to influence the aggregation of LDL. In this work we used Pluronic® P85, L61 and F68. A comparative study of the effects of Pluronic block copolymers with various hydrophilic-lipophilic properties on the aggregation process of LDL showed that Pluronic copolymers with strong hydrophobic properties (P85 and L61) at concentrations close to or greater than the respective critical concentration of micelle formation inhibited the aggregation process of LDL; however, the "hydrophilic" Pluronic F68 had no effect on the aggregation of LDL at any concentration. Thus, the study demonstrated for the first time that Pluronic® block copolymers inhibit LDL self-association. The possibility of modulating the aggregation of LDL by various Pluronic copolymers can be regarded as a prerequisite in the creation of new types of anti-atherosclerotic drugs.

Effect of improving glycemic control in patients with type 2 diabetes mellitus on low-density lipoprotein size, electronegative low-density lipoprotein and lipoprotein-associated phospholipase A2 distribution

The aim of this study was to determine the effect of intensified hypoglycemic therapy in patients with type 2 diabetes mellitus on the distribution of lipoprotein-associated phospholipase A2 (Lp-PLA2) activity between high-density lipoprotein and low-density lipoprotein (LDL) and its relation with the lipid profile and other qualitative properties of LDL. Forty-two patients with type 2 diabetes on the basis of poor glycemic control and normal or near normal LDL cholesterol were recruited. Lifestyle counseling and pharmacologic hypoglycemic therapy were intensified to improve glycemic control, but lipid-lowering therapy was unchanged. At 4 ± 2 months, glycosylated hemoglobin had decreased by a mean of 2.1%, but the only effect on the lipid profile were statistically significant decreases in nonesterified fatty acids and apolipoprotein B concentration. LDL size increased and the proportion of electronegative LDL decreased significantly. In parallel, total Lp-PLA2 activity decreased significantly, promoting a redistribution of Lp-PLA2 activity toward a higher proportion in high-density lipoprotein. Improvements in glycemic control led to more marked changes in Lp-PLA2 activity and distribution in patients with diabetes who had not received previous lipid-lowering therapy. In conclusion, optimizing glycemic control in patients with type 2 diabetes promotes atheroprotective changes, including larger LDL size, decreased electronegative LDL, and a higher proportion of Lp-PLA2 activity in high-density lipoprotein.

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.

Effects of electronegative VLDL on endothelium damage in metabolic syndrome

OBJECTIVE

Biochemical heterogeneity governs functional disparities among lipoproteins. We examined charge-defined VLDL subfractions in metabolic syndrome (MetS) to determine whether their increased electronegativity is associated with increased cytotoxicity and whether high concentrations of highly electronegative subfractions render VLDL harmful to the vascular endothelium.

RESEARCH DESIGN AND METHODS

Plasma VLDL of normal individuals (control subjects) (n = 13) and of those with MetS (n = 13) was resolved into subfractions with increasing negative charge (V1-V5) by anion-exchange chromatography. Human aortic endothelial cells were treated with V1-V5 or unfractionated VLDL.

RESULTS

Compared with the control subjects, individuals with MetS had a significantly higher percentage of V5 VLDL (V5/VLDL%) (34 ± 20 vs. 39 ± 11%, respectively; P < 0.05) and plasma V5 concentration ([V5]) (5.5 ± 4.4 vs. 15.2 ± 8.5 mg/dL, respectively; P < 0.001). Apolipoprotein (apo)B100 levels decreased and apoC levels increased from V1 to V5, indicating that V5 is apoC-rich VLDL. Regression analyses of all 26 individuals showed that [V5] was positively correlated with total cholesterol (P = 0.016), triglyceride (P < 0.000001), and V5/VLDL% (P = 0.002). Fasting plasma glucose, but not waist circumference, exhibited a positive trend (P = 0.058); plasma HDL cholesterol exhibited a weak inverse trend (P = 0.138). V5 (10 μg/mL) induced apoptosis in ~50% of endothelial cells in 24 h. V5 was the most rapidly (<15 min) internalized subfraction and induced the production of reactive oxygen species (ROS) in endothelial cells after 20 min. Unfractionated MetS VLDL, but not control VLDL, also induced ROS production and endothelial cell apoptosis.

CONCLUSIONS

In populations with increased risk of diabetes, the vascular endothelium is constantly exposed to VLDL that contains a high proportion of V5. The potential impact of V5-rich VLDL warrants further investigation.