Effects of phospholipase A(2) and its products on structural stability of human LDL: relevance to formation of LDL-derived lipid droplets

ApoB100 remodeling and stiffened cholesteryl ester core raise LDL aggregation in familial hypercholesterolemia patients

Patients with familial hypercholesterolemia (FH) exhibit a significant residual cardiovascular risk. A new cardiovascular risk factor is the susceptibility of individual LDL particles to aggregation. This study examined LDL aggregation and its relationship with LDL lipid composition and biophysical properties in patients with FH compared to controls. LDL aggregation was measured as the change in particle size, assessed by dynamic light scattering, after exposure to sphingomyelinase, which breaks down sphingomyelin in the LDL phospholipid layer. Dynamic light scattering and transmission electron microscopy showed that LDL in FH patients exhibited smaller size and greater susceptibility to aggregation. Biochemical analyses revealed a higher cholesteryl ester (CE)/ApoB100 ratio in LDL from FH patients. Differential scanning calorimetry showed that LDL from FH patients had higher transition temperatures, indicating a more ordered CE core. Fourier transform infrared spectroscopy revealed fewer flexible α-helices (1658 cm⁻1) and more stable α-helices (1651 cm⁻1) in ApoB100 of LDL from FH patients. These structural changes correlated with higher CE content and increased LDL aggregation. In conclusion, a more ordered CE core in smaller LDL particles, combined with a higher proportion of stable α-helices in ApoB100, promotes LDL aggregation in FH patients. These findings suggest ApoB100 conformational structure as a new potential therapeutic targets within LDL to reduce cardiovascular risk in FH patients.

Nonalcoholic Fatty Liver Disease Risk and Proprotein Convertase Subtilisin Kexin 9 in Familial Hypercholesterolemia Under Statin Treatment

BACKGROUND/OBJECTIVES

Fatty acids are involved in some hepatic disorders. The proprotein convertase subtilisin kexin 9 (PCSK9) inhibits the uptake of low-density lipoproteins (LDLs), which contain lipids, into the liver and may thus be associated with nonalcoholic fatty liver disease (NAFLD), a cardiovascular disorder (CVD) risk. Statins reduce blood LDL-cholesterol (LDL-C) levels and CVD risk and can attenuate the development of NAFLD while increasing blood PCSK9 levels.

METHODS

We investigated the correlation between PCSK9 and liver conditions in patients with familial hypercholesterolemia (FH), a CVD risk population with elevated blood LDL-C levels, under statin treatment. Blood tests for lipids, PCSK9, and liver function (aspartate aminotransferase [AST] and alanine aminotransferase [ALT]) were performed in patients with FH taking statins (n = 25, mean age = 57 years, 12% of males). The ALT:AST ratio was used as a marker of NAFLD risk.

RESULTS

The mean LDL-C level was 3.38 mmol/L, and the median PCSK9 level was 312 ng/mL. The median ALT:AST ratio was 0.88. A significant negative correlation was observed between the PCSK9 and ALT:AST ratio (β = -0.67, p < 0.05).

CONCLUSIONS

Their negative correlation might give a hypothetical insight into the effect of statin treatment on the development of NAFLD, in relation to PCSK9 behavior, in patients with FH.

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.

Aggregation Susceptibility of Low-Density Lipoproteins-A Novel Modifiable Biomarker of Cardiovascular Risk

Circulating low-density lipoprotein (LDL) particles enter the arterial intima where they bind to the extracellular matrix and become modified by lipases, proteases, and oxidizing enzymes and agents. The modified LDL particles aggregate and fuse into larger matrix-bound lipid droplets and, upon generation of unesterified cholesterol, cholesterol crystals are also formed. Uptake of the aggregated/fused particles and cholesterol crystals by macrophages and smooth muscle cells induces their inflammatory activation and conversion into foam cells. In this review, we summarize the causes and consequences of LDL aggregation and describe the development and applications of an assay capable of determining the susceptibility of isolated LDL particles to aggregate when exposed to human recombinant sphingomyelinase enzyme ex vivo. Significant person-to-person differences in the aggregation susceptibility of LDL particles were observed, and such individual differences largely depended on particle lipid composition. The presence of aggregation-prone LDL in the circulation predicted future cardiovascular events in patients with atherosclerotic cardiovascular disease. We also discuss means capable of reducing LDL particles' aggregation susceptibility that could potentially inhibit LDL aggregation in the arterial wall. Whether reductions in LDL aggregation susceptibility are associated with attenuated atherogenesis and a reduced risk of atherosclerotic cardiovascular diseases remains to be studied.

Why are cardiovascular diseases more common among patients with severe mental illness? The potential involvement of electronegative low-density lipoprotein (LDL) L5

Despite tremendous efforts of experimental and clinical studies and knowledge, the pathophysiology of severe mental illness (SMI), including bipolar disorder (BD), unipolar depression (mood disorders, MD), and schizophrenia (SCZ), remains poorly understood. Besides their chronic course and high prevalence in society, mental and somatic comorbidities are really serious problems; patients with these disorders have increased risk of cardiovascular (CV) diseases (CVD) including coronary artery diseases (CAD, i.e. myocardial infarction and angina), stroke, sudden cardiac death, hypertension, cardiomyopathy, arrhythmia, and thromboembolic disease. Although it is determined that triglycerides, cholesterol, glucose, and low-density lipoprotein (LDL) levels are increased in MD and SCZ, the underlying reason remains unknown. Considering this, we propose that electronegative LDL (L5) is probably the main crucial element to understanding CVD induced by SMI and to discovering novel remedial approaches for these diseases. When it is hypothesized that L5 is greatly presupposed in CV system abnormalities, it follows that the anti-L5 therapies and even antioxidant treatment options may open new therapeutic opportunities to prevent CVD diseases secondary to SMI. In this review article, we tried to bring a very original subject to the attention of readers who are interested in lipoprotein metabolism in terms of experimental, clinical, and cell culture studies that corroborate the involvement of L5 in physiopathology of CVD secondary to SMI and also the new therapeutic approaches for these disorders.

Binding to heparin triggers deleterious structural and biochemical changes in human low-density lipoprotein, which are amplified in hyperglycemia

Low-density lipoprotein (LDL) binding to arterial proteoglycans initiates LDL retention and modification in the arterial wall, triggering atherosclerosis. The details of this binding, its effectors, and its ramifications are incompletely understood. We combined heparin affinity chromatography with biochemical, spectroscopic and electron microscopic techniques to show that brief binding to heparin initiates irreversible pro-atherogenic remodeling of human LDL. This involved decreased structural stability of LDL and increased susceptibility to hydrolysis, oxidation and fusion. Furthermore, phospholipid hydrolysis, mild oxidation and/or glycation of LDL in vitro increase the proteolytic susceptibility of apoB and its heparin binding affinity, perhaps by unmasking additional heparin-binding sites. For LDL from hyperglycemic type-2 diabetic patients, heparin binding was particularly destabilizing and caused apoB fragmentation and LDL fusion. However, for similar patients whose glycemic control was restored upon therapy, LDL-heparin binding affinity was rectified and LDL structural stability was partially restored. These results complement previous studies of LDL binding to arterial proteoglycans and suggest that such interactions may produce a particularly pro-atherogenic subclass of electronegative LDL. In summary, binding to heparin alters apoB conformation, perhaps by partially peeling it off the lipid, and triggers pro-atherogenic LDL modifications including hydrolysis, oxidation, and destabilization. Furthermore, phospholipid lipolysis, mild oxidation and glycation of LDL in vitro strengthen its binding to heparin, which helps explain stronger binding observed in hyperglycemic LDL. Combined effects of hyperglycemia and heparin binding are especially deleterious but are largely rectified upon diabetes therapy. These findings help establish a mechanistic link between diabetes and atherosclerosis.

Electronegative LDL: An Active Player in Atherogenesis or a By- Product of Atherosclerosis?

Low-density lipoproteins (LDLs) are the major plasma carriers of cholesterol. However, LDL particles must undergo various molecular modifications to promote the development of atherosclerotic lesions. Modified LDL can be generated by different mechanisms, but as a common trait, show an increased electronegative charge of the LDL particle. A subfraction of LDL with increased electronegative charge (LDL(-)), which can be isolated from blood, exhibits several pro-atherogenic characteristics. LDL(-) is heterogeneous, due to its multiple origins but is strongly related to the development of atherosclerosis. Nevertheless, the implication of LDL(-) in a broad array of pathologic conditions is complex and in some cases anti-atherogenic LDL(-) properties have been reported. In fact, several molecular modifications generating LDL(-) have been widely studied, but it remains unknown as to whether these different mechanisms are specific or common to different pathological disorders. In this review, we attempt to address these issues examining the most recent findings on the biology of LDL(-) and discussing the relationship between this LDL subfraction and the development of different diseases with increased cardiovascular risk. Finally, the review highlights the importance of minor apolipoproteins associated with LDL(-) which would play a crucial role in the different properties displayed by these modified LDL particles.

Effects of triacylglycerol on the structural remodeling of human plasma very low- and low-density lipoproteins

Very low-density lipoprotein (VLDL) is the main plasma carrier of triacylglycerol that is elevated in pathological conditions such as diabetes, metabolic syndrome, obesity and dyslipidemia. How variations in triacylglycerol levels influence structural stability and remodeling of VLDL and its metabolic product, low-density lipoproteins (LDL), is unknown. We applied a biochemical and biophysical approach using lipoprotein remodeling by lipoprotein lipase and cholesterol ester transfer protein, along with thermal denaturation that mimics key aspects of lipoprotein remodeling in vivo. The results revealed that increasing the triacylglycerol content in VLDL promotes changes in the lipoprotein size and release of the exchangeable apolipoproteins. Similarly, increased triacylglycerol content in LDL promotes lipoprotein remodeling and fusion. These effects were observed in single-donor lipoproteins from healthy subjects enriched in exogenous triolein, in single-donor lipoproteins from healthy subjects with naturally occurring differences in endogenous triacylglycerol, and in LDL and VLDL from pooled plasma of diabetic and normolipidemic patients. Consequently, triacylglycerol-induced destabilization is a general property of plasma lipoproteins. This destabilization reflects a direct effect of triacylglycerol on lipoproteins. Moreover, we show that TG can act indirectly by increasing lipoprotein susceptibility to oxidation and lipolysis and thereby promoting the generation of free fatty acids that augment fusion. These in vitro findings are relevant to lipoprotein remodeling and fusion in vivo. In fact, fusion of LDL and VLDL enhances their retention in the arterial wall and, according to the response-to-retention hypothesis, triggers atherosclerosis. Therefore, enhanced fusion of triacylglycerol-rich lipoproteins suggests a new causative link between elevated plasma triacylglycerol and atherosclerosis.

[Atherogenic modification of low-density lipoproteins]

One of the first manifestations of atherosclerosis is accumulation of extra- and intracellular cholesterol esters in the arterial intima. Formation of foam cells is considered as a trigger in the pathogenesis of atherosclerosis. Low density lipoprotein (LDL) circulating in human blood is the source of lipids accumulated in the arterial walls. This review considered features and role in atherogenesis different modified forms of LDL: oxidized, small dense, electronegative and especially desialylated LDL. Desialylated LDL of human blood plasma is capable to induce lipid accumulation in cultured cells and it is atherogenic. LDL possesses numerous alterations of protein, carbohydrate and lipid moieties and therefore can be termed multiple-modified LDL. Multiple modification of LDL occurs in human blood plasma and represents a cascade of successive changes in the lipoprotein particle: desialylation, loss of lipids, reduction in the particle size, increase of surface electronegative charge, etc. In addition to intracellular lipid accumulation, stimulatory effects of naturally occurring multiple-modified LDL on other processes involved in the development of atherosclerotic lesions, namely cell proliferation and fibrosis, were shown.

Interactions between Phospholipids and Organic Phases: Insights into Lipoproteins and Nanoemulsions

The adsorption of phosphatidylcholines (PCs), dissolved in squalene or squalane as an organic phase, was studied at the interface with water. Using profile analysis tensiometry, the equilibrium adsorption isotherms, minimum molecular interfacial areas, and solubility limits were derived. For squalene, differences in PC solubility and interfacial adsorption were found, depending on PC saturation. Compared to saturated PCs, unsaturated PCs showed a 3-fold-lower interfacial density but up to a 28-fold-higher critical aggregation concentration (CAC). In addition, the solubility limit of unsaturated PC in squalene and in its saturated form squalane diverged by a factor of 739. These findings provided evidence for steric repulsion or π-π interactions of π bonds in both solvent and solute or both effects acting complementarily. In squalane, low solubilities but high interfacial densities were found for all investigated PCs. Changes in fatty acid chain lengths showed that the influence of the increases in entropy and enthalpy on solubility is much smaller than solvent/solute interactions. Oxidation products of squalene lowered the interfacial tension, but increasing concentrations of PC expelled them from the interface. The CAC of saturated PC was increased by oxidation products of squalene whereas that of unsaturated PCs was not. Our findings indicate that the oxidation of triglycerides in lipoprotein cores can lead to increased solubility of saturated phospholipids covering the lipoproteins, contributing to destabilization, coalescence, and terminally the formation of atherosclerotic plaques. The consideration of solvent/solute interactions in molecular modeling may contribute to the interfacial tension and the corresponding kinetic or thermodynamic stability of lipoproteins. Measured areas per molecule prove that PCs form monolayers of different interfacial densities at the squalene/water interface but multilayers at the squalane/water interface. These findings showed that combinations of solvent or solute saturation affect the outcome for nanoemulsions forming either expanded or condensed monolayers or multilayers.

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.

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

Low-density lipoproteins (LDLs, also known as 'bad cholesterol') are the major carriers of circulating cholesterol and the main causative risk factor of atherosclerosis. Plasma LDLs are 20- to 25-nm nanoparticles containing a core of cholesterol esters surrounded by a phospholipid monolayer and a single copy of apolipoprotein B (550 kDa). An early sign of atherosclerosis is the accumulation of LDL-derived lipid droplets in the arterial wall. According to the widely accepted 'response-to-retention hypothesis', LDL binding to the extracellular matrix proteoglycans in the arterial intima induces hydrolytic and oxidative modifications that promote LDL aggregation and fusion. This enhances LDL uptake by the arterial macrophages and triggers a cascade of pathogenic responses that culminate in the development of atherosclerotic lesions. Hence, LDL aggregation, fusion, and lipid droplet formation are important early steps in atherogenesis. In vitro, a variety of enzymatic and nonenzymatic modifications of LDL can induce these reactions and thereby provide useful models for their detailed analysis. Here, we summarize current knowledge of the in vivo and in vitro modifications of LDLs leading to their aggregation, fusion, and lipid droplet formation; outline the techniques used to study these reactions; and propose a molecular mechanism that underlies these pro-atherogenic processes. Such knowledge is essential in identifying endogenous and exogenous factors that can promote or prevent LDL aggregation and fusion in vivo and to help establish new potential therapeutic targets to decelerate or even block these pathogenic reactions.

Amyloidogenic mutations in human apolipoprotein A-I are not necessarily destabilizing - a common mechanism of apolipoprotein A-I misfolding in familial amyloidosis and atherosclerosis

High-density lipoproteins and their major protein, apolipoprotein A-I (apoA-I), remove excess cellular cholesterol and protect against atherosclerosis. However, in acquired amyloidosis, nonvariant full-length apoA-I deposits as fibrils in atherosclerotic plaques; in familial amyloidosis, N-terminal fragments of variant apoA-I deposit in vital organs, damaging them. Recently, we used the crystal structure of Δ(185-243)apoA-I to show that amyloidogenic mutations destabilize apoA-I and increase solvent exposure of the extended strand 44-55 that initiates β-aggregation. In the present study, we test this hypothesis by exploring naturally occurring human amyloidogenic mutations, W50R and G26R, within or close to this strand. The mutations caused small changes in the protein's α-helical content, stability, proteolytic pattern and protein-lipid interactions. These changes alone were unlikely to account for amyloidosis, suggesting the importance of other factors. Sequence analysis predicted several amyloid-prone segments that can initiate apoA-I misfolding. Aggregation studies using N-terminal fragments verified this prediction experimentally. Three predicted N-terminal amyloid-prone segments, mapped on the crystal structure, formed an α-helical cluster. Structural analysis indicates that amyloidogenic mutations or Met86 oxidation perturb native packing in this cluster. Taken together, the results suggest that structural perturbations in the amyloid-prone segments trigger α-helix to β-sheet conversion in the N-terminal ~ 75 residues forming the amyloid core. Polypeptide outside this core can be proteolysed to form 9-11 kDa N-terminal fragments found in familial amyloidosis. Our results imply that apoA-I misfolding in familial and acquired amyloidosis follows a similar mechanism that does not require significant structural destabilization or proteolysis. This novel mechanism suggests potential therapeutic interventions for apoA-I amyloidosis.

A comparison of five lipid extraction solvent systems for lipidomic studies of human LDL

Lipidome profile of fluids and tissues is a growing field as the role of lipids as signaling molecules is increasingly understood, relying on an effective and representative extraction of the lipids present. A number of solvent systems suitable for lipid extraction are commonly in use, though no comprehensive investigation of their effectiveness across multiple lipid classes has been carried out. To address this, human LDL from normolipidemic volunteers was used to evaluate five different solvent extraction protocols [Folch, Bligh and Dyer, acidified Bligh and Dyer, methanol (MeOH)-tert-butyl methyl ether (TBME), and hexane-isopropanol] and the extracted lipids were analyzed by LC-MS in a high-resolution instrument equipped with polarity switching. Overall, more than 350 different lipid species from 19 lipid subclasses were identified. Solvent composition had a small effect on the extraction of predominant lipid classes (triacylglycerides, cholesterol esters, and phosphatidylcholines). In contrast, extraction of less abundant lipids (phosphatidylinositols, lyso-lipids, ceramides, and cholesterol sulfates) was greatly influenced by the solvent system used. Overall, the Folch method was most effective for the extraction of a broad range of lipid classes in LDL, although the hexane-isopropanol method was best for apolar lipids and the MeOH-TBME method was suitable for lactosyl ceramides.

Low-density lipoprotein, its susceptibility to oxidation and the role of lipoprotein-associated phospholipase A2 and carboxyl ester lipase lipases in atherosclerotic plaque formation

An increased level of low-density lipoprotein (LDL) is a very well established risk factor of coronary artery disease (CAD). Unoxidized LDL is an inert transport vehicle of cholesterol and other lipids in the body and is thought to be atherogenic. Recently it has been appreciated that oxidized products of LDL are responsible for plaque formation properties previously attributed to the intact particle. The goal of this article is to review the recent understanding of the LDL oxidation pathway. The role of oxidized products and key enzymes (lipoprotein-associated phospholipase A2 and carboxyl ester lipase) are also extensively discussed in the context of clinical conditions.

Interfacial tension and surface pressure of high density lipoprotein, low density lipoprotein, and related lipid droplets

Lipid droplets play a central role in energy storage and metabolism on a cellular scale. Their core is comprised of hydrophobic lipids covered by a surface region consisting of amphiphilic lipids and proteins. For example, high and low density lipoproteins (HDL and LDL, respectively) are essentially lipid droplets surrounded by specific proteins, their main function being to transport cholesterol. Interfacial tension and surface pressure of these particles are of great interest because they are related to the shape and the stability of the droplets and to protein adsorption at the interface. Here we use coarse-grained molecular-dynamics simulations to consider a number of related issues by calculating the interfacial tension in protein-free lipid droplets, and in HDL and LDL particles mimicking physiological conditions. First, our results suggest that the curvature dependence of interfacial tension becomes significant for particles with a radius of ∼5 nm, when the area per molecule in the surface region is <1.4 nm(2). Further, interfacial tensions in the used HDL and LDL models are essentially unaffected by single apo-proteins at the surface. Finally, interfacial tensions of lipoproteins are higher than in thermodynamically stable droplets, suggesting that HDL and LDL are kinetically trapped into a metastable state.

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.

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.

Folded functional lipid-poor apolipoprotein A-I obtained by heating of high-density lipoproteins: relevance to high-density lipoprotein biogenesis

HDL (high-density lipoproteins) remove cell cholesterol and protect from atherosclerosis. The major HDL protein is apoA-I (apolipoprotein A-I). Most plasma apoA-I circulates in lipoproteins, yet ~5% forms monomeric lipid-poor/free species. This metabolically active species is a primary cholesterol acceptor and is central to HDL biogenesis. Structural properties of lipid-poor apoA-I are unclear due to difficulties in isolating this transient species. We used thermal denaturation of human HDL to produce lipid-poor apoA-I. Analysis of the isolated lipid-poor fraction showed a protein/lipid weight ratio of 3:1, with apoA-I, PC (phosphatidylcholine) and CE (cholesterol ester) at approximate molar ratios of 1:8:1. Compared with lipid-free apoA-I, lipid-poor apoA-I showed slightly altered secondary structure and aromatic packing, reduced thermodynamic stability, lower self-associating propensity, increased adsorption to phospholipid surface and comparable ability to remodel phospholipids and form reconstituted HDL. Lipid-poor apoA-I can be formed by heating of either plasma or reconstituted HDL. We propose the first structural model of lipid-poor apoA-I which corroborates its distinct biophysical properties and postulates the lipid-induced ordering of the labile C-terminal region. In summary, HDL heating produces folded functional monomolecular lipid-poor apoA-I that is distinct from lipid-free apoA-I. Increased adsorption to phospholipid surface and reduced C-terminal disorder may help direct lipid-poor apoA-I towards HDL biogenesis.

The crystal structure of the C-terminal truncated apolipoprotein A-I sheds new light on amyloid formation by the N-terminal fragment

Apolipoprotein A-I (apoA-I) is the main protein of plasma high-density lipoproteins (HDL, or good cholesterol) that remove excess cell cholesterol and protect against atherosclerosis. In hereditary amyloidosis, mutations in apoA-I promote its proteolysis and the deposition of the 9-11 kDa N-terminal fragments as fibrils in vital organs such as kidney, liver, and heart, causing organ damage. All known amyloidogenic mutations in human apoA-I are clustered in two residue segments, 26-107 and 154-178. The X-ray crystal structure of the C-terminal truncated human protein, Δ(185-243)apoA-I, determined to 2.2 Å resolution by Mei and Atkinson, provides the structural basis for understanding apoA-I destabilization in amyloidosis. The sites of amyloidogenic mutations correspond to key positions within the largely helical four-segment bundle comprised of residues 1-120 and 144-184. Mutations in these positions disrupt the bundle structure and destabilize lipid-free apoA-I, thereby promoting its proteolysis. Moreover, many mutations place a hydrophilic or Pro group in the middle of the hydrophobic lipid-binding face of the amphipathic α-helices, which will likely shift the population distribution from HDL-bound to lipid-poor/free apoA-I that is relatively unstable and labile to proteolysis. Notably, the crystal structure shows segment L44-S55 in an extended conformation consistent with the β-strand-like geometry. Exposure of this segment upon destabilization of the four-segment bundle probably initiates the α-helix to β-sheet conversion in amyloidosis. In summary, we propose that the amyloidogenic mutations promote apoA-I proteolysis by destabilizing the protein structure not only in the lipid-free but also in the HDL-bound form, with segment L44-S55 providing a likely template for the cross-β-sheet conformation.

Proteome of skeletal muscle lipid droplet reveals association with mitochondria and apolipoprotein a-I

The lipid droplet (LD) is a universal organelle governing the storage and turnover of neutral lipids. Mounting evidence indicates that elevated intramuscular triglyceride (IMTG) in skeletal muscle LDs is closely associated with insulin resistance and Type 2 Diabetes Mellitus (T2DM). Therefore, the identification of the skeletal muscle LD proteome will provide some clues to dissect the mechanism connecting IMTG with T2DM. In the present work, we identified 324 LD-associated proteins in mouse skeletal muscle LDs through mass spectrometry analysis. Besides lipid metabolism and membrane traffic proteins, a remarkable number of mitochondrial proteins were observed in the skeletal muscle LD proteome. Furthermore, imaging by fluorescence microscopy and transmission electronic microscopy (TEM) directly demonstrated that mitochondria closely adhere to LDs in vivo. Moreover, our results revealed for the first time that apolipoprotein A-I (apo A-I), the principal apolipoprotein of high density lipoprotein (HDL) particles, was also localized on skeletal muscle LDs. Further studies verified that apo A-I was expressed endogenously by skeletal muscle cells. In conclusion, we report the protein composition and characterization of skeletal muscle LDs and describe a novel LD-associated protein, apo A-I.