LDL phospholipid hydrolysis produces modified electronegative particles with an unfolded apoB-100 protein

Accurate and sensitive low-density lipoprotein (LDL) detection based on the proximity ligation assisted rolling circle amplification (RCA)

Metabolic-associated fatty liver disease (MAFLD) is one of the leading causes of mortality from chronic diseases worldwide, and it is strongly linked to dyslipidemia. Dyslipidemia typically presents as an elevated concentration of low density lipoprotein (LDL). Hence, accurate quantification of LDL particles is crucial for predicting the risks of cardiovascular illnesses. Nevertheless, conventional techniques can merely provide indirect measurements of LDL particle concentrations through the detection of cholesterol or proteins within LDL particles, and they often require significant effort and time. Therefore, an accurate and effective method for identifying intact LDL particles is highly desired. We have devised a method that allows for the measurement of LDL concentration without the need for isolation. This method relies on proximity ligation rolling circle amplification (RCA). This technique enables the direct and precise measurement of the concentration of "actual" LDL particles, rather than measuring the cholesterol content inside LDL. It has a detection limit of 7.3 μg dL-1, which also meets the requirements for analyzing lipoproteins in clinical samples. Hence, this platform exhibits immense potential in clinical applications and health management.

β-sheets in serum protein are independent risk factors for coronary lesions besides LDL-C in coronary heart disease patients

Background

Coronary heart disease (CHD) patients with standard low-density lipoprotein cholesterol (LDL-C) remain at risk of cardiovascular events, making it critical to explore new targets to reduce the residual risk. The relationship between β-sheet conformation and CHD is gaining attention. This study was designed to compare the coronary lesions in CHD patients with varying LDL-C and evaluate whether serum β-sheets are associated with coronary damage.

Methods

Two hundred and one patients diagnosed with stable CHD were recruited and divided into four groups according to LDL-C. Baseline information, coronary lesion-related indicators, and peripheral blood samples were collected. Serum β-sheet content was determined by thioflavin T fluorescence.

Results

The baseline information was comparable in CHD patients with different LDL-C. No difference was found in indicators relevant to coronary lesions among groups. The content of β-sheet was negatively correlated with LDL-C. Multiple linear regression revealed that serum β-sheet was positively correlated with coronary lesion when risk factors such as age, smoking, and LDL-C were controlled.

Conclusions

This is the first study that reports the serum β-sheet levels of CHD patients being gradually increased with decreasing LDL-C when coronary lesions were comparable. Serum β-sheet might exacerbate the coronary lesions in CHD patients independent of known risk factors such as LDL-C.

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.

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.

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.

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

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

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.

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.

Proteomic detection of nitroproteins as potential biomarkers for cardiovascular disease

Increased levels of reactive oxygen and nitrogen species are linked to many human diseases and can be formed as an indirect result of the disease process. The accumulation of specific nitroproteins which correlate with pathological processes suggests that nitration of protein tyrosine represents a dynamic and selective process, rather than a random event. Indeed, in numerous clinical disorders associated with an upregulation in oxidative stress, tyrosine nitration has been limited to certain cell types and to selective sites of injury. Additionally, proteomic studies show that only certain proteins are nitrated in selective tissue extracts. A growing list of nitrated proteins link the negative effects of protein nitration with their accumulation in a wide variety of diseases related to oxidation. Nitration of tyrosine has been demonstrated in diverse proteins such as cytochrome c, actin, histone, superoxide dismutase, α-synuclein, albumin, and angiotensin II. In vitro and in vivo aspects of redox-proteomics of specific nitroproteins that could be relevant to biomarker analysis and understanding of cardiovascular disease mechanism will be discussed within this review.

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

Electronegative LDL (LDL(-)) is a minor subfraction of modified LDL present in plasma. Among its atherogenic characteristics, low affinity to the LDL receptor and high binding to arterial proteoglycans (PGs) could be related to abnormalities in the conformation of its main protein, apolipoprotein B-100 (apoB-100). In the current study, we have performed an immunochemical analysis using monoclonal antibody (mAb) probes to analyze the conformation of apoB-100 in LDL(-). The study, performed with 28 anti-apoB-100 mAbs, showed that major differences of apoB-100 immunoreactivity between native LDL and LDL(-) concentrate in both terminal extremes. The mAbs Bsol 10, Bsol 14 (which recognize the amino-terminal region), Bsol 2, and Bsol 7 (carboxyl-terminal region) showed increased immunoreactivity in LDL(-), suggesting that both terminal extremes are more accessible in LDL(-) than in native LDL. The analysis of in vitro-modified LDLs, including LDL lipolyzed with sphingomyelinase (SMase-LDL) or phospholipase A(2) (PLA(2)-LDL) and oxidized LDL (oxLDL), suggested that increased amino-terminal immunoreactivity was related to altered conformation due to aggregation. This was confirmed when the aggregated subfractions of LDL(-) (agLDL(-)) and oxLDL (ag-oxLDL) were isolated and analyzed. Thus, Bsol 10 and Bsol 14 immunoreactivity was high in SMase-LDL, ag-oxLDL, and agLDL(-). The altered amino-terminal apoB-100 conformation was involved in the increased PG binding affinity of agLDL(-) because Bsol 10 and Bsol 14 blocked its high PG-binding. These observations suggest that an abnormal conformation of the amino-terminal region of apoB-100 is responsible for the increased PG binding affinity of agLDL(-).

Aggregated electronegative low density lipoprotein in human plasma shows a high tendency toward phospholipolysis and particle fusion

Aggregation and fusion of lipoproteins trigger subendothelial retention of cholesterol, promoting atherosclerosis. The tendency of a lipoprotein to form fused particles is considered to be related to its atherogenic potential. We aimed to isolate and characterize aggregated and nonaggregated subfractions of LDL from human plasma, paying special attention to particle fusion mechanisms. Aggregated LDL was almost exclusively found in electronegative LDL (LDL(-)), a minor modified LDL subfraction, but not in native LDL (LDL(+)). The main difference between aggregated (agLDL(-)) and nonaggregated LDL(-) (nagLDL(-)) was a 6-fold increased phospholipase C-like activity in agLDL(-). agLDL(-) promoted the aggregation of LDL(+) and nagLDL(-). Lipoprotein fusion induced by α-chymotrypsin proteolysis was monitored by NMR and visualized by transmission electron microscopy. Particle fusion kinetics was much faster in agLDL(-) than in nagLDL(-) or LDL(+). NMR and chromatographic analysis revealed a rapid and massive phospholipid degradation in agLDL(-) but not in nagLDL(-) or LDL(+). Choline-containing phospholipids were extensively degraded, and ceramide, diacylglycerol, monoacylglycerol, and phosphorylcholine were the main products generated, suggesting the involvement of phospholipase C-like activity. The properties of agLDL(-) suggest that this subfraction plays a major role in atherogenesis by triggering lipoprotein fusion and cholesterol accumulation in the arterial wall.

Electrochemical impedance spectroscopy to assess vascular oxidative stress

Vascular inflammatory responses are intimately linked with oxidative stress, favoring the development of pre-atherosclerotic lesions. We proposed that oxidized low density lipoprotein (oxLDL) and foam cell infiltrates in the subendothelial layer engendered distinct electrochemical properties that could be measured in terms of the electrochemical impedance spectroscopy (EIS). Concentric bipolar microelectrodes were applied to interrogate EIS of aortas isolated from fat-fed New Zealand White (NZW) rabbits and explants of human aortas. Frequency-dependent EIS measurements were assessed between 10 kHz and 100 kHz, and were significantly elevated in the pre-atherosclerotic lesions in which oxLDL and macrophage infiltrates were prevalent (At 100 kHz: aortic arch lesion=26.7±2.7 kΩ vs. control=15.8±2.4 kΩ; at 10 kHz: lesions=49.2±7.3 kΩ vs. control=27.6±2.7 kΩ, n=10, p<0.001). Similarly, EIS measurements were significantly elevated in the human descending aorta where pre-atherosclerotic lesions or fatty streaks were prominent. EIS measurements remained unchanged in spite of various depths of electrode submersion or orientation of the specimens. Hence, the concentric bipolar microelectrodes provided a reliable means to measure endoluminal electrochemical modifications in regions of pro-inflammatory with high spatial resolution and reproducibility albeit uneven lesion topography and non-uniform current distribution.

Phospholipolyzed LDL induces an inflammatory response in endothelial cells through endoplasmic reticulum stress signaling

Secreted phospholipases A2 (sPLA2s) are present in atherosclerotic plaques and are now considered novel attractive therapeutic targets and potential biomarkers as they contribute to the development of atherosclerosis through lipoprotein-dependent and independent mechanisms. We have previously shown that hGX-sPLA2-phospholipolyzed LDL (LDL-X) induces proinflammatory responses in human umbilical endothelial cells (HUVECs); here we explore the molecular mechanisms involved. Global transcriptional gene expression profiling of the response of endothelial cells exposed to either LDL or LDL-X revealed that LDL-X activates multiple distinct cellular pathways including the unfolded protein response (UPR). Mechanistic insight showed that LDL-X activates UPR through calcium depletion of intracellular stores, which in turn disturbs cytoskeleton organization. Treatment of HUVECs and aortic endothelial cells (HAECs) with LDL-X led to activation of all 3 proximal initiators of UPR: eIF-2alpha, IRE1alpha, and ATF6. In parallel, we observed a sustained phosphorylation of the p38 pathway resulting in the phosphorylation of AP-1 downstream targets. This was accompanied by significant production of the proinflammatory cytokines IL-6 and IL-8. Our study demonstrates that phospholipolyzed LDL uses a range of molecular pathways including UPR to initiate endothelial cell perturbation and thus provides an LDL oxidation-independent mechanism for the initiation of vascular inflammation in atherosclerosis.

Generation in human plasma of misfolded, aggregation-prone electronegative low density lipoprotein

Human plasma contains small amounts of a low density lipoprotein in which apoprotein is misfolded. Originally identified and isolated by means of anion-exchange chromatography, this component was subsequently described as electronegative low density lipoprotein (LDL)(-), with increased concentrations associated with elevated cardiovascular disease risk. It has been recognized recently as the trigger of LDL amyloidogenesis, which produces aggregates similar to subendothelial droplets observed in vivo in early atherogenesis. Although LDL(-) has been produced in vitro through various manipulations, the mechanisms involved in its generation in vivo remain obscure. By using a more physiological model, we demonstrate spontaneous, sustained and noticeable production of LDL(-) during incubation of unprocessed human plasma at 37 degrees C. In addition to a higher fraction of amyloidogenic LDL(-), LDL purified from incubated plasma contains an increased level of lysophospholipids and free fatty acids; analysis of LDL lipids packing shows their loosening. As a result, during plasma incubation, lipid destabilization and protein misfolding take place, and aggregation-prone particles are generated. All these phenomena can be prevented by inhibiting calcium-dependent secretory phospholipases A2. Our plasma incubation model, without removal of reaction products, effectively shows a lipid-protein interplay in LDL, where lipid destabilization after lipolysis threatens the apoprotein's structure, which misfolds and becomes aggregation-prone.

Role of protein tyrosine nitration in neurodegenerative diseases and atherosclerosis

Nitric oxide generates reactive nitrosative species, such as peroxynitrite (ONOO(-)) that may be involved in a number of diseases. ONOO(-) can mediate protein tyrosine nitration which causes structural changes of affected proteins and leads to their inactivation. Various proteomics and immunological methods including mass spectrometry combined with both liquid and 2-D PAGE, and immunodetection have been employed to identify and characterize nitrated proteins from pathological samples. This review presents the pahtobiological roles of the pathogenic posttranslational modification in neurodegenerative diseases and atherosclerosis.

Mediation of electronegative low-density lipoprotein signaling by LOX-1: a possible mechanism of endothelial apoptosis

The lectin-like oxidized LDL receptor LOX-1 mediates endothelial cell (EC) uptake of experimentally prepared copper-oxidized LDL (oxLDL). To confirm the atherogenic role of this receptor cloned against copper-oxLDL, we examined whether it mediates EC uptake of L5, an electronegative LDL abundant in dyslipidemic but not normolipidemic human plasma. Hypercholesterolemic (LDL-cholesterol, >160 mg/dL) human LDL was fractionated into L1-L5, increasingly electronegative, by ion-exchange chromatography. In cultured bovine aortic ECs (BAECs), L5 upregulated LOX-1 and induced apoptosis. Transfection of BAECs with LOX-1-specific small interfering RNAs (siLOX-1) minimized baseline LOX-1 production and restrained L5-induced LOX-1 upregulation. Internalization of labeled L1-L5 was monitored in BAECs and human umbilical venous ECs by fluorescence microscopy. LOX-1 knockdown with siLOX-1 impeded the endocytosis of L5 but not L1-L4. In contrast, blocking LDL receptor with RAP (LDL receptor-associated protein) stopped the internalization of L1-L4 but not L5. Although chemically different, L5 and oxLDL competed for EC entry through LOX-1. Via LOX-1, L5 signaling hampered Akt phosphorylation and suppressed EC expression of fibroblast growth factor-2 and Bcl-2. L5 also selectively inhibited Bcl-xL expression and endothelial nitric oxide synthase phosphorylation but increased synthesis of Bax, Bad, and tumor necrosis factor-alpha. Blocking Akt phosphorylation with wortmannin increased LOX-1 expression, suggesting a modulatory role of Akt in LOX-1 synthesis; L5 upregulated LOX-1 by dephosphorylating Akt. Because endothelial nitric oxide synthase and Bcl-2 activities are Akt-dependent, L5 impairs Akt-mediated growth and survival signals in vascular ECs by way of LOX-1. Thus, the L5/LOX-1 complex may play a critical role in atherogenesis and illuminate important targets for disease intervention.

LDL protein nitration: implication for LDL protein unfolding

Oxidatively- or enzymatically-modified low-density lipoprotein (LDL) is intimately involved in the initiation and progression of atherosclerosis. The in vivo modified LDL is electro-negative (LDL(-)) and consists of peroxidized lipid and unfolded apoB-100 protein. This study was aimed at establishing specific protein modifications and conformational changes in LDL(-) assessed by liquid chromatography/tandem mass spectrometry (LC/MS/MS) and circular dichroism analyses, respectively. The functional significance of these chemical modifications and structural changes were validated with binding and uptake experiments to- and by bovine aortic endothelial cells (BAEC). The plasma LDL(-) fraction showed increased nitrotyrosine and lipid peroxide content as well as a greater cysteine oxidation as compared with native- and total-LDL. LC/MS/MS analyses of LDL(-) revealed specific modifications in the apoB-100 moiety, largely involving nitration of tyrosines in the alpha-helical structures and beta(2) sheet as well as cysteine oxidation to cysteic acid in beta(1) sheet. Circular dichroism analyses showed that the alpha-helical content of LDL(-) was substantially lower ( approximately 25%) than that of native LDL ( approximately 90%); conversely, LDL(-) showed greater content of beta-sheet and random coil structure, in agreement with unfolding of the protein. These results were mimicked by treatment of LDL subfractions with peroxynitrite (ONOO(-)) or SIN-1: similar amino acid modifications as well as conformational changes (loss of alpha-helical structure and gain in beta-sheet structure) were observed. Both LDL(-) and ONOO(-)-treated LDL showed a statistically significant increase in binding and uptake to- and by BAEC compared to native LDL. We further found that most binding and uptake in control-LDL was through LDL-R with minimal oxLDL-R-dependent uptake. ONOO(-)-treated LDL was significantly bound and endocytosed by LOX-1, CD36, and SR-A with minimal contribution from LDL-R. It is suggested that lipid peroxidation and protein nitration may account for the mechanisms leading to apoB-100 protein unfolding and consequential increase in modified LDL binding and uptake to and by endothelial cells that is dependent on oxLDL scavenger receptors.

Novel phospholipolytic activities associated with electronegative low-density lipoprotein are involved in increased self-aggregation

Electronegative low-density lipoprotein (LDL(-)) is a minor LDL subfraction present in plasma with increased platelet-activating factor acetylhydrolase (PAF-AH) activity. This activity could be involved in the proinflammatory effects of LDL(-). Our aim was to study the presence of additional phospholipolytic activities in LDL(-). Total LDL was fractionated into electropositive (LDL(+)) and LDL(-) by anion-exchange chromatography, and phospholipolytic activities were measured by fluorometric methods. Phospholipolytic activity was absent in LDL(+) whereas LDL(-) presented activity against lysophosphatidylcholine (LPC, 82.4 +/- 34.9 milliunits/mg of apoB), sphingomyelin (SM, 53.3 +/- 22.5 milliunits/mg of apoB), and phosphatidylcholine (PC, 25.7 +/- 4.3 milliunits/mg of apoB). LDL(-), but not LDL(+), presented spontaneous self-aggregation at 37 degrees C in parallel to phospholipid degradation. This was observed in the absence of lipid peroxidation and suggests the involvement of phospholipolytic activity in self-aggregation of LDL(-). Phospholipolytic activity was not due to PAF-AH, apoE, or apoC-III and was not increased in LDL(+) modified by Cu (2+) oxidation, acetylation, or secretory phospholipase A 2 (PLA 2). However, LDL(-) efficiently degraded phospholipids of lipoproteins enriched in LPC, such as oxidized LDL or PLA 2-LDL, but not native or acetylated LDL. This finding supports that LPC is the best substrate for LDL(-)-associated phospholipolytic activity. These results reveal novel properties of LDL(-) that could play a significant role in its atherogenic properties.

Shear stress influences spatial variations in vascular Mn-SOD expression: implication for LDL nitration

Fluid shear stress modulates vascular production of endothelial superoxide anion (O2*-) and nitric oxide (*NO). Whether the characteristics of shear stress influence the spatial variations in mitochondrial manganese superoxide dismutase (Mn-SOD) expression in vasculatures is not well defined. We constructed a three-dimensional computational fluid dynamics model simulating spatial variations in shear stress at the arterial bifurcation. In parallel, explants of arterial bifurcations were sectioned from the human left main coronary bifurcation and right coronary arteries for immunohistolocalization of Mn-SOD expression. We demonstrated that Mn-SOD staining was prominent in the pulsatile shear stress (PSS)-exposed and atheroprotective regions, but it was nearly absent in the oscillatory shear stress (OSS)-exposed regions and lateral wall of arterial bifurcation. In cultured bovine aortic endothelial cells, PSS at mean shear stress (tau ave) of 23 dyn/cm2 upregulated Mn-SOD mRNA expression at a higher level than did OSS at tau ave = 0.02 dyn/cm2 +/- 3.0 dyn.cm(-2).s(-1) and at 1 Hz (PSS by 11.3 +/- 0.4-fold vs. OSS by 5.0 +/- 0.5-fold vs. static condition; P < 0.05, n = 4). By liquid chromatography and tandem mass spectrometry, it was found that PSS decreased the extent of low-density lipoprotein (LDL) nitration, whereas OSS increased nitration (P < 0.05, n = 4). In the presence of LDL, treatment with Mn-SOD small interfering RNA increased intracellular nitrotyrosine level (P < 0.5, n = 4), a fingerprint for nitrotyrosine formation. Our findings indicate that shear stress in the atheroprone versus atheroprotective regions regulates spatial variations in mitochondrial Mn-SOD expression with an implication for modulating LDL nitration.

Electronegative LDL impairs vascular endothelial cell integrity in diabetes by disrupting fibroblast growth factor 2 (FGF2) autoregulation

OBJECTIVE

L5, a circulating electronegative LDL identified in patients with hypercholesterolemia or type 2 diabetes, induces endothelial cell (EC) apoptosis by suppressing fibroblast growth factor (FGF)2 expression. FGF2 plays a pivotal role in endothelial regeneration and compensatory arteriogenesis. It is likely that vasculopathy and poor collateralization in diabetes is a result of FGF2 dysregulation.

RESEARCH DESIGN AND METHODS

To investigate this mechanism, we isolated L5 from type 2 diabetic patients. In cultured bovine aortic ECs (BAECs), L5 inhibited FGF2 transcription and induced apoptosis. Because FGF2 stimulates the phosphatidylinositol 3-kinase (PI3K)-Akt pathway, we examined whether FGF2 transcription is regulated by Akt through a feedback mechanism.

RESULTS

Diabetic L5 reduced FGF2 release to the medium but enhanced caspase-3 activity, with resultant apoptosis. Inhibition of PI3K with wortmannin or suppression of Akt activation with dominant-negative Akt inhibited FGF2 expression. Transfection of BAECs with FGF2 antisense cDNA depleted endogenous FGF2 protein. In these cells, not only was Akt phosphorylation inhibited, but FGF2 transcription was also critically impaired. In contrast, transfecting BAECs with FGF2 sense cDNA augmented Akt phosphorylation. Treatment with constitutively active Akt enhanced FGF2 expression. Augmentation of either FGF2 transcription or Akt phosphorylation rendered BAECs resistant to L5.

CONCLUSIONS

These findings suggest that FGF2 is the primary initiator of its own expression, which is autoregulated through a novel FGF2-PI3K-Akt loop. Thus, by disrupting FGF2 autoregulation in vascular ECs, L5 may impair reendothelialization and collateralization in diabetes.

Hemodynamics influences vascular peroxynitrite formation: Implication for low-density lipoprotein apo-B-100 nitration

Hemodynamics, specifically, fluid shear stress, modulates the focal nature of atherogenesis. Superoxide anion (O2(-.)) reacts with nitric oxide (.NO) at a rapid diffusion-limited rate to form peroxynitrite (O2(-.) + .NO-->ONOO(-)). Immunohistostaining of human coronary arterial bifurcations or curvatures, where OSS develops, revealed the presence of nitrotyrosine staining, a fingerprint of peroxynitrite; whereas in straight segments, where PSS occurs, nitrotyrosine was absent. We examined vascular nitrative stress in models of oscillatory (OSS) and pulsatile shear stress (PSS). Bovine aortic endothelial cells (BAEC) were exposed to fluid shear stress that simulates arterial blood flow: (1) PSS at a mean shear stress (tau(ave)) of 23 dyn cm(-2) and a temporal gradient (partial differential(tau)/partial differential(t)) at 71 dyn cm(-2) s(-1), and (2) OSS at tau(ave) = 0.02 dyn cm(- 2) and partial differential(tau)/partial differential(t) = +/- 3.0 dyn cm(-2) s(-1) at a frequency of 1 Hz. OSS significantly up-regulated one of the NADPH oxidase subunits (NOx4) expression accompanied with an increase in O2(-.) production. In contrast, PSS up-regulated eNOS expression accompanied with .NO production (total NO(2)(-) and NO(3)(-)). To demonstrate that O2(-.) and .NO are implicated in ONOO(-) formation, we added low-density lipoprotein cholesterol (LDL) to the medium in which BAEC were exposed to the above flow conditions. The medium was analyzed for LDL apo-B-100 nitrotyrosine by liquid chromatography electrospray ionization tandem mass spectrometry (LC/ESI/MS/MS). OSS induced higher levels of 3-nitrotyrosine, dityrosine, and o-hydroxyphenylalanine compared with PSS. In the presence of ONOO(-), specific apo-B-100 tyrosine residues underwent nitration in the alpha and beta helices: alpha-1 (Tyr(144)), alpha-2 (Tyr(2524)), beta-2 (Tyr(3295)), alpha-3 (Tyr(4116)), and beta-2 (Tyr(4211)). Hence, the characteristics of shear stress in the arterial bifurcations influenced the relative production of O2(-.) and .NO with an implication for ONOO(-) formation as evidenced by LDL protein nitration.

Untangling the role of amyloid in atherosclerosis

PURPOSE OF REVIEW

Amyloid deposits are a defining feature of several age-related and debilitating diseases. Their widespread presence in atherosclerotic plaques suggests a potential role in lesion development. This review discusses the proteins known to accumulate in atheroma and examines the evidence that amyloid-like structures activate macrophage signaling pathways linked to inflammation and prothrombotic potential.

RECENT FINDINGS

Numerous proteins that accumulate in atherosclerotic plaques form amyloid fibrils in vivo, including apolipoproteins, beta-amyloid, and alpha1-antitrypsin. In addition, oxidation or enzymatic modification of low-density lipoproteins induces a structural reorganization of the particle, including the acquisition of amyloid-like properties. Similarly, glycation of serum albumin, as observed in diabetes, is accompanied by the formation of aggregates with all the hallmarks of amyloid. Several receptors implicated in atherogenesis modulate the fate of amyloid fibrils by mediating their clearance (scavenger receptors A and B-I), activating inflammatory signaling cascades (receptor for advanced glycation endproducts), or both (CD36). Finally, recent studies indicate that amyloid deposition accelerates diet-induced atherosclerosis in mice.

SUMMARY

Given the substantial evidence that amyloid fibrils or preamyloidogenic species are cytotoxic, the aberrant deposition of amyloid in the intima may be pathologically important in vascular inflammation and the promotion of atherosclerosis.

17beta-Estradiol reverses shear-stress-mediated low density lipoprotein modifications

Within arterial bifurcations or branching points, oscillatory shear stress (OSS) induces oxidative stress mainly via the reduced nicotinamide adenine dinucleodtide phosphate (NADPH) oxidase system. It is unknown whether 17beta-estradiol (E(2)) can regulate OSS-mediated low-density lipoprotein (LDL) modifications. Bovine aortic endothelial cells were pretreated with E(2) at 5 nmol/L, followed by exposure to OSS (0 +/- 3.0 dynes/cm(2) s and 60 cycles/min) in a flow system. E(2) decreased OSS-mediated NADPH oxidase mRNA expression, and E(2)-mediated (.-)NO production was mitigated by the NO synthase inhibitor N(G)-nitro-l-argenine methyl ester. The rates of O(2)(-.) production in response to OSS increased steadily as determined by superoxide-dismutase-inhibited ferricytochrome c reduction; whereas, pretreatment with E(2) decreased OSS-mediated O(2)(-.) production (n = 4, p < 0.05). In the presence of native LDL (50 microg/mL), E(2) also significantly reversed OSS-mediated LDL oxidation as determined by high-performance liquid chromatography. In the presence of O(2)(-.) donor, xanthine oxidase (XO), E(2) further reversed XO-induced LDL lipid peroxidation (n = 3, p < 0.001). Mass spectra acquired in the m/z 400-1800 range, revealed XO-mediated LDL protein nitration involving tyrosine 2535 in the alpha-2 domains, whereas pretreatment with E(2) reversed nitration, as supported by the changes in nitrotyrosine intensities. Thus, E(2) plays an indirect antioxidative role. In addition to upregulation of endothelial (.-)NO synthase and downregulation of Nox4 expression, E(2) influences LDL modifications via lipid peroxidation and protein nitration.

Detection of electronegative low density lipoprotein (LDL-) in plasma and atherosclerotic lesions by monoclonal antibody-based immunoassays

OBJECTIVES

To produce a monoclonal antibody (MAb) against electronegative LDL (LDL-) for detecting this modified lipoprotein in blood plasma and tissues.

DESIGN AND METHODS

LDL- was isolated from human blood plasma and used as an antigen for immunization of Balb/c mice. Lymphocytes of immunized mice were fused with myeloma cells (SP2/0) to obtain the hybridomas. LDL- was detected in blood plasma and atherosclerotic lesions of humans and rabbits by MAb-based ELISA and immunohistochemistry, respectively.

RESULTS

LDL- concentrations were higher (P < 0.05) in the blood plasma of hypercholesterolemic subjects (HC, 248 +/- 77 mg/dL of total cholesterol) than in normolipidemic subjects (NL, 173 +/- 82 mg/dL of total cholesterol) and rabbits (HC, 250 +/- 15 mg/dL of cholesterol versus NL, 81 +/- 12 mg/dL of cholesterol). Moreover, LDL- was detected in the atherosclerotic lesions of humans and rabbits.

CONCLUSION

These MAb-based immunoassays are adequate to detect LDL- in biological samples and represent an important tool for investigating the role of LDL- in atherosclerosis.

Decrease in pH Strongly Enhances Binding of Native, Proteolyzed, Lipolyzed, and Oxidized Low Density Lipoprotein Particles to Human Aortic Proteoglycans

Binding of low density lipoprotein (LDL) to proteoglycans and modification of LDL are key processes in atherogenesis. Recently, it has been demonstrated that during atherogenesis the extracellular pH of atherosclerotic lesions decreases. We have examined the effect of the decreased pH on the binding of LDL to human aortic proteoglycans. The binding of native, oxidized, proteolyzed (alpha-chymotrypsin-treated), or lipolyzed (sphingomyelinase- or phospholipase A(2)-treated) LDL particles to proteoglycans were measured in microtiter well assays at pH 5.5-7.5. We found that the lower the pH, the higher the amount of binding of LDL to proteoglycans. At the lowest pH tested (pH 5.5), the amounts of proteoglycan-bound native, proteolyzed, sphingomyelinase-, and phospholipase A(2)-treated LDL were 20-, 23-, 30-, and 37-fold higher, respectively, than at pH 7.5. Interestingly, although oxidized LDL failed to bind to proteoglycans at neutral pH, there was significant binding at acidic pH. Binding of native and modified LDL to proteoglycans at pH 5.5 was blocked by 1 m NaCl, indicating that at neutral pH LDL binds to proteoglycans via ionic interactions. Inhibition of this binding by acetylation and cyclohexanedione treatment of LDL showed that the positively charged amino acids of apolipoprotein B-100, lysine, and arginine, respectively, mediated the ionic interaction. Taken together, our results suggest that in areas of atherosclerotic arterial intima where the extracellular pH decreases, retention of LDL by proteoglycans is enhanced, leading to extracellular accumulation of LDL and progression of the disease.

Oxidation of low-density lipoproteins induces amyloid-like structures that are recognized by macrophages

The macrophage scavenger receptor CD36 plays a key role in the initiation of atherosclerosis through its ability to bind to and internalize oxidized low-density lipoproteins (oxLDL). Prompted by recent findings that the CD36 receptor also recognizes amyloid fibrils formed by beta-amyloid and apolipoprotein C-II, we investigated whether the oxidation of low-density lipoproteins (LDL) generates characteristic amyloid-like structures and whether these structures serve as CD36 ligands. Our studies demonstrate that LDL oxidized by copper ions, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH), or ozone react with the diagnostic amyloid dyes thioflavin T and Congo Red and bind to serum amyloid P component (SAP), a universal constituent of physiological amyloid deposits. X-ray powder diffraction patterns for native LDL show a diffuse powder diffraction ring with maximum intensity corresponding to an atomic spacing of approximately 4.7 A, consistent with the spacing between beta-strands in a beta-sheet. Ozone treatment of LDL generates an additional diffuse powder diffraction ring with maximum intensity indicating a spacing of approximately 9.8 A. This distance is consistent with the presence of cross-beta-structure, a defining characteristic of amyloid. Evidence that these cross-beta-amyloid structures in oxLDL are recognized by macrophages is provided by the observation that SAP strongly inhibits the association and internalization of (125)I-labeled copper-oxidized LDL by peritoneal macrophages. The ability of SAP to bind to amyloid-like structures in oxLDL and prevent lipid uptake by macrophages highlights the potential importance of these structures and suggests an important preventative role for SAP in foam cell formation and early-stage atherosclerosis.