Pro-inflammatory action of LDL(−) on mononuclear cells is counteracted by increased IL10 production

Comparative Analysis of Atherogenic Lipoproteins L5 and Lp(a) in Atherosclerotic Cardiovascular Disease

PURPOSE OF REVIEW

Low-density lipoprotein (LDL) poses a risk for atherosclerotic cardiovascular disease (ASCVD). As LDL comprises various subtypes differing in charge, density, and size, understanding their specific impact on ASCVD is crucial. Two highly atherogenic LDL subtypes-electronegative LDL (L5) and Lp(a)-induce vascular cell apoptosis and atherosclerotic changes independent of plasma cholesterol levels, and their mechanisms warrant further investigation. Here, we have compared the roles of L5 and Lp(a) in the development of ASCVD.

RECENT FINDINGS

Lp(a) tends to accumulate in artery walls, promoting plaque formation and potentially triggering atherosclerosis progression through prothrombotic or antifibrinolytic effects. High Lp(a) levels correlate with calcific aortic stenosis and atherothrombosis risk. L5 can induce endothelial cell apoptosis and increase vascular permeability, inflammation, and atherogenesis, playing a key role in initiating atherosclerosis. Elevated L5 levels in certain high-risk populations may serve as a distinctive predictor of ASCVD. L5 and Lp(a) are both atherogenic lipoproteins contributing to ASCVD through distinct mechanisms. Lp(a) has garnered attention, but equal consideration should be given to L5.

Association between serum lipid and all-cause mortality in asthmatic populations: a cohort study

BACKGROUND

Presently, the majority of investigations primarily evaluate the association between lipid profiles and asthma. However, few investigations explore the connection between lipids and mortality related to the disease. This study aims to explore the association of serum lipids with all-cause mortality within asthmatic adults.

METHODS

The investigation included 3233 eligible patients with asthma from the NHANES (2011-2018). The potential associations were explored using three Cox proportional hazards models, restricted cubic splines (RCS), threshold effect models, and CoxBoost models. In addition, subgroup analyses were conducted to investigate these associations within distinct populations.

RESULTS

After controlling all covariables, the Cox proportional hazards model proved a 17% decrease in the probability of death for each increased unit of low-density lipoprotein-cholesterol (LDL-C) (mmol/L). Yet, there was no association seen between blood high-density lipoprotein cholesterol (HDL-C), total cholesterol, or triglyceride and all-cause mortality in asthmatics. The application of RCS and threshold effect models verified an inverse and linear association of LDL-C with all-cause mortality. According to the results from the CoxBoost model, LDL-C exhibited the most substantial impact on the follow-up status of asthmatics among the serum lipids.

CONCLUSION

Our investigation concluded that in American asthmatic populations, LDL-C levels were inversely and linearly correlated with mortality. However, no independent relationship was found between triglycerides, total cholesterol, or HDL-C and mortality.

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(-).

Electronegative LDL Induces M1 Polarization of Human Macrophages Through a LOX-1-Dependent Pathway

In response to environmental stimuli, monocytes undergo polarization into classically activated (M1) or alternatively activated (M2) states. M1 and M2 macrophages exert opposing pro- and anti-inflammatory properties, respectively. Electronegative low-density lipoprotein (LDL) (LDL(-)) is a naturally occurring mildly oxidized LDL found in the plasma of patients with hypercholesterolemia, diabetes, and acute myocardial infarction, and has been shown to involve in the pathogenesis of atherosclerosis. In this study, we examined the effects of LDL(-) on macrophage polarization and the involvement of lectin-like oxidized LDL receptor-1 (LOX-1) in this process. THP-1 macrophages were treated with native LDL (nLDL) or LDL(-), and then the expression of M1/M2-related surface markers and cytokines were evaluated. The results show that treatment with LDL(-) resulted in profound increase in proinflammatory cytokines, IL-1β, IL-6, and TNF-α, and M1-surface marker CD86; however, M2-related cytokines, IL-10 and TGF-β, and M2-surface marker CD206 were not changed by LDL(-). Untreated or nLDL-treated cells were used as control. LDL(-)-induced M1 polarization and secretion of proinflammatory cytokines were diminished in LOX-1 knockdown cells. Taken together, the results show that LDL(-) promotes differentiation of human monocytes to M1 macrophages through a LOX-1-dependent pathway, and explore the contribution of LDL(-) and LOX-1 to the development of chronic inflammation in atherosclerosis.

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.

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.

Electronegative LDL Promotes Inflammation and Triglyceride Accumulation in Macrophages

Electronegative low-density lipoprotein (LDL) (LDL(−)), a modified LDL that is present in blood and exerts atherogenic effects on endothelial cells and monocytes. This study aimed to determine the action of LDL(−) on monocytes differentiated into macrophages. LDL(−) and in vitro-modified LDLs (oxidized, aggregated, and acetylated) were added to macrophages derived from THP1 monocytes over-expressing CD14 (THP1-CD14). Then, cytokine release, cell differentiation, lipid accumulation, and gene expression were measured by ELISA, flow cytometry, thin-layer chromatography, and real-time PCR, respectively. LDL(−) induced more cytokine release in THP1-CD14 macrophages than other modified LDLs. LDL(−) also promoted morphological changes ascribed to differentiated macrophages. The addition of high-density lipoprotein (HDL) and anti-TLR4 counteracted these effects. LDL(−) was highly internalized by macrophages, and it was the major inductor of intracellular lipid accumulation in triglyceride-enriched lipid droplets. In contrast to inflammation, the addition of anti-TLR4 had no effect on lipid accumulation, thus suggesting an uptake pathway alternative to TLR4. In this regard, LDL(−) upregulated the expression of the scavenger receptors CD36 and LOX-1, as well as several genes involved in triglyceride (TG) accumulation. The importance and novelty of the current study is that LDL(−), a physiologically modified LDL, exerted atherogenic effects in macrophages by promoting differentiation, inflammation, and triglyceride-enriched lipid droplets formation in THP1-CD14 macrophages, probably through different receptors.

Electronegative LDL from Rabbits Fed with Atherogenic Diet Is Highly Proinflammatory

Electronegative low-density lipoprotein (LDL(-)) has been found in the plasma of familial hypercholesterolemia and acute myocardial infarction and has been implicated in atherosclerosis and cardiovascular disease. However, less is known about the involvement of LDL(-) in atherosclerosis-related inflammation. This study aims at investigating the inducibility of LDL(-) by atherogenic diet in rabbits and at exploring the proinflammatory potential of the diet-induced LDL(-) in macrophages. Rabbits were fed with an atherogenic diet; LDL was isolated from plasma by NaBr density gradient ultracentrifugation and was then resolved into nLDL and LDL(-) by anion-exchange chromatography. Isolated nLDL and LDL(-) were directly used or incubated with 10 μM CuSO₄ for 24 h to produce copper- (Cu-) ox-nLDL and Cu-ox-LDL(-). The effects of these LDLs on inflammation were evaluated in THP-1-derived macrophages. Macrophages were treated with nLDL, LDL(-), and extensively oxidized LDL (ox-LDL), then the levels of interleukin- (IL-) 1β, IL-6, and tumor necrosis factor- (TNF-) α in a culture medium were determined by ELISA, and the levels of total and phosphorylated IκB, p65, p38, JNK, and ERK in cell lysates were determined by Western blotting. The LDL(-) induced significantly higher levels of IL-1β, IL-6, and TNF-α in the medium. The levels of phosphorylated/total IκB, p65, p38, JNK, and ERK were also upregulated by LDL(-). In contrast, nLDL, Cu-ox-nLDL, and Cu-ox-LDL(-) exhibited much less effect. Knockdown of lectin-type oxidized LDL receptor- (LOX-) 1 resulted in significant reduction in LDL(-)-induced IL-1β, IL-6, and TNF-α. In addition, these LDL(-) effects were also markedly attenuated by inhibition of NF-κB and ERK1/2. The data suggested that LDL(-) induced inflammation through LOX-1-, NF-κB-, and ERK1/2-dependent pathways. Taken together, our results show that rabbits fed with atherogenic diet produce a highly proinflammatory LDL(-) that is more potent in inducing inflammation than nLDL and extensively oxidize LDL in macrophages. The results thus provide a novel link between diet-induced hypercholesterolemia and inflammation.

The Role of Distinctive Sphingolipids in the Inflammatory and Apoptotic Effects of Electronegative LDL on Monocytes

Electronegative low-density lipoprotein (LDL(-)) is a minor LDL subfraction that is present in blood with inflammatory and apoptotic effects. We aimed to evaluate the role of sphingolipids ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) in the LDL(-)-induced effect on monocytes. Total LDL was subfractioned into native LDL and LDL(-) by anion-exchange chromatography and their sphingolipid content evaluated by mass spectrometry. LDL subfractions were incubated with monocytes in the presence or absence of enzyme inhibitors: chlorpromazine (CPZ), d-erythro-2-(N-myristoyl amino)-1-phenyl-1-propanol (MAPP), and N,N-dimethylsphingosine (DMS), which inhibit Cer, Sph, and S1P generation, respectively. After incubation, we evaluated cytokine release by enzyme-linked immunosorbent assay (ELISA) and apoptosis by flow cytometry. LDL(-) had an increased content in Cer and Sph compared to LDL(+). LDL(-)-induced cytokine release from cultured monocytes was inhibited by CPZ and MAPP, whereas DMS had no effect. LDL(-) promoted monocyte apoptosis, which was inhibited by CPZ, but increased with the addition of DMS. LDL enriched with Sph increased cytokine release in monocytes, and when enriched with Cer, reproduced both the apoptotic and inflammatory effects of LDL(-). These observations indicate that Cer content contributes to the inflammatory and apoptotic effects of LDL(-) on monocytes, whereas Sph plays a more important role in LDL(-)-induced inflammation, and S1P counteracts apoptosis.

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.

Electronegative LDL induces MMP-9 and TIMP-1 release in monocytes through CD14 activation: Inhibitory effect of glycosaminoglycan sulodexide

OBJECTIVE

Electronegative LDL (LDL(-)) is involved in atherosclerosis through the activation of the TLR4/CD14 inflammatory pathway in monocytes. Matrix metalloproteinases (MMP) and their inhibitors (tissue inhibitors of metalloproteinase [TIMP]) are also crucially involved in atherosclerosis, but their modulation by LDL(-) has never been investigated. The aim of this study was to examine the ability of LDL(-) to release MMPs and TIMPs in human monocytes and to determine whether sulodexide (SDX), a glycosaminoglycan-based drug, was able to affect their secretion.

APPROACH AND RESULTS

Native LDL (LDL(+)) and LDL(-) separated by anion-exchange chromatography were added to THP1-CD14 monocytes in the presence or absence of SDX for 24 h. A panel of 9 MMPs and 4 TIMPs was analyzed in cell supernatants with multiplex immunoassays. The gelatinolytic activity of MMP-9 was assessed by gelatin zymography. LDL(-) stimulated the release of MMP-9 (13-fold) and TIMP-1 (4-fold) in THP1-CD14 monocytes, as well as the gelatinolytic activity of MMP-9. Co-incubation of monocytes with LDL(-) and SDX for 24 h significantly reduced both the release of MMP-9 and TIMP-1 and gelatinase activity. In THP1 cells not expressing CD14, no effect of LDL(-) on MMP-9 or TIMP-1 release was observed. The uptake of DiI-labeled LDL(-) was higher than that of DiI-LDL(+) in THP1-CD14 but not in THP1 cells. This increase was inhibited by SDX. Experiments in microtiter wells coated with SDX demonstrated a specific interaction of LDL(-) with SDX.

CONCLUSIONS

LDL(-) induced the release of MMP-9 and TIMP-1 in monocytes through CD14. SDX affects the ability of LDL(-) to promote TIMP-1 and MMP-9 release by its interaction with LDL(-).

Increased inflammatory effect of electronegative LDL and decreased protection by HDL in type 2 diabetic patients

BACKGROUND AND AIMS

Type 2 diabetic patients have an increased proportion of electronegative low-density lipoprotein (LDL(-)), an inflammatory LDL subfraction present in blood, and dysfunctional high-density lipoprotein (HDL). We aimed at examining the inflammatory effect of LDL(-) on monocytes and the counteracting effect of HDL in the context of type 2 diabetes.

METHODS

This was a cross-sectional study in which the population comprised 3 groups (n = 12 in each group): type 2 diabetic patients with good glycaemic control (GC-T2DM patients), type 2 diabetic patients with poor glycaemic control (PC-T2DM), and a control group. Total LDL, HDL, and monocytes were isolated from plasma of these subjects. LDL(-) was isolated from total LDL by anion-exchange chromatography. LDL(-) from the three groups of subjects was added to monocytes in the presence or absence of HDL, and cytokines released by monocytes were quantified by ELISA.

RESULTS

LDL(-) proportion and plasma inflammatory markers were increased in PC-T2DM patients. LDL(-) from PC-T2DM patients induced the highest IL1β, IL6, and IL10 release in monocytes compared to LDL(-) from GC-T2DM and healthy subjects, and presented the highest content of non-esterified fatty acids (NEFA). In turn, HDL from PC-T2DM patients showed the lowest ability to inhibit LDL(-)-induced cytokine release in parallel to an impaired ability to decrease NEFA content in LDL(-).

CONCLUSIONS

Our findings show an imbalance in the pro- and anti-inflammatory effects of lipoproteins from T2DM patients, particularly in PC-T2DM.

Serum Immune Mediators Independently Associate with Atherosclerosis in the Left (But Not Right) Carotid Territory of Older Individuals

BACKGROUND AND AIM

Disturbance in the carotid arteries strongly predicts cerebrovascular events and correlates with a systemic inflammatory milieu. We investigated the relationship of a profile of 10 circulating inflammatory mediators with measures of carotid intima-media thickness (cIMT) in elderly subjects, taking traditional risk factors into account.

METHODS

Clinical inspection for present and past chronic conditions and events, as well as biochemical and anthropometric measurements, was performed for patients in ambulatory setting. Scores of cIMT were obtained bilaterally in the distal common carotid artery wall. Serum concentrations of cytokines were assessed by bead-based, multiplexed flow cytometry immunoassays.

RESULTS

Correlation analysis between log-transformed cytokines levels implicated the mediators interleukin-1β (IL1β), IL6, IL8, IL10, and tumor necrosis factor-α (TNFα) (P ≤ .005) with scores of the left cIMT. Stepwise multivariate regression showed that TNFα, IL1β, and IL6 levels accounted for most of the variance in the cIMT scores. Comparison of cytokine levels across increasing tertiles of the left cIMT reproduced the positive association with TNFα and IL1β levels.

CONCLUSION

Five out of ten immune mediators independently correlated with cIMT of older subjects in a territory-sensitive manner. This possible contribution of immune mediators to an atherosclerotic process probably relates to the inflammaging process.

Inflammatory intracellular pathways activated by electronegative LDL in monocytes

AIMS

Electronegative LDL (LDL(-)) is a plasma LDL subfraction that induces cytokine release in monocytes through toll-like receptor 4 (TLR4) activation. However, the intracellular pathways induced by LDL(-) downstream TLR4 activation are unknown. We aimed to identify the pathways activated by LDL(-) leading to cytokine release in monocytes.

METHODS AND RESULTS

We determined LDL(-)-induced activation of several intracellular kinases in protein extracts from monocytes using a multikinase ELISA array. LDL(-) induced higher p38 mitogen-activated protein kinase (MAPK) phosphorylation than native LDL. This was corroborated by a specific cell-based assay and it was dependent on TLR4 and phosphoinositide 3-kinase (PI3k)/Akt pathway. P38 MAPK activation was involved in cytokine release promoted by LDL(-). A specific ELISA showed that LDL(-) activated cAMP response-element binding (CREB) in a p38 MAPK dependent manner. P38 MAPK was also involved in the nuclear factor kappa-B (NF-kB) and activating protein-1 (AP-1) activation by LDL(-). We found that NF-kB, AP-1 and CREB inhibitors decreased LDL(-)-induced cytokine release, mainly on MCP1, IL6 and IL10 release, respectively.

CONCLUSIONS

LDL(-) promotes p38 MAPK phosphorylation through TLR4 and PI3k/Akt pathways. Phosphorylation of p38 MAPK is involved in NF-kB, AP-1 and CREB activation, leading to LDL(-)-induced cytokine release in monocytes.

Electronegative LDL induces priming and inflammasome activation leading to IL-1β release in human monocytes and macrophages

BACKGROUND

Electronegative LDL (LDL(−)), a modified LDL fraction found in blood, induces the release of inflammatory mediators in endothelial cells and leukocytes. However, the inflammatory pathways activated by LDL(−) have not been fully defined. We aim to study whether LDL(−) induced release of the first-wave proinflammatory IL-1β in monocytes and monocyte-derived macrophages (MDM) and the mechanisms involved.

METHODS

LDL(−) was isolated from total LDL by anion exchange chromatography. Monocytes and MDM were isolated from healthy donors and stimulated with LDL(+) and LDL(−) (100 mg apoB/L).

RESULTS

In monocytes, LDL(−) promoted IL-1β release in a time-dependent manner, obtaining at 20 h-incubation the double of IL-1β release induced by LDL(−) than by native LDL. LDL(−)-induced IL-1β release involved activation of the CD14-TLR4 receptor complex. LDL(−) induced priming, the first step of IL-1β release, since it increased the transcription of pro-IL-1β (8-fold) and NLRP3 (3-fold) compared to native LDL. Several findings show that LDL(−) induced inflammasome activation, the second step necessary for IL-1β release. Preincubation of monocytes with K+ channel inhibitors decreased LDL(−)-induced IL-1β release. LDL(−) induced formation of the NLRP3-ASC complex. LDL(−) triggered 2-fold caspase-1 activation compared to native LDL and IL-1β release was strongly diminished in the presence of the caspase-1 inhibitor Z-YVAD. In MDM, LDL(−) promoted IL-1β release, which was also associated with caspase-1 activation.

CONCLUSIONS

LDL(−) promotes release of biologically active IL-1β in monocytes and MDM by induction of the two steps involved: priming and NLRP3 inflammasome activation.

SIGNIFICANCE

By IL-1β release, LDL(−) could regulate inflammation in atherosclerosis.

Low-density lipoprotein electronegativity is a novel cardiometabolic risk factor

BACKGROUND

Low-density lipoprotein (LDL) plays a central role in cardiovascular disease (CVD) development. In LDL chromatographically resolved according to charge, the most electronegative subfraction-L5-is the only subfraction that induces atherogenic responses in cultured vascular cells. Furthermore, increasing evidence has shown that plasma L5 levels are elevated in individuals with high cardiovascular risk. We hypothesized that LDL electronegativity is a novel index for predicting CVD.

METHODS

In 30 asymptomatic individuals with metabolic syndrome (MetS) and 27 healthy control subjects, we examined correlations between plasma L5 levels and the number of MetS criteria fulfilled, CVD risk factors, and CVD risk according to the Framingham risk score.

RESULTS

L5 levels were significantly higher in MetS subjects than in control subjects (21.9±18.7 mg/dL vs. 11.2±10.7 mg/dL, P:0.01). The Jonckheere trend test revealed that the percent L5 of total LDL (L5%) and L5 concentration increased with the number of MetS criteria (P<0.001). L5% correlated with classic CVD risk factors, including waist circumference, body mass index, waist-to-height ratio, smoking status, blood pressure, and levels of fasting plasma glucose, triglyceride, and high-density lipoprotein. Stepwise regression analysis revealed that fasting plasma glucose level and body mass index contributed to 28% of L5% variance. The L5 concentration was associated with CVD risk and contributed to 11% of 30-year general CVD risk variance when controlling the variance of waist circumference.

CONCLUSION

Our findings show that LDL electronegativity was associated with multiple CVD risk factors and CVD risk, suggesting that the LDL electronegativity index may have the potential to be a novel index for predicting CVD. Large-scale clinical trials are warranted to test the reliability of this hypothesis and the clinical importance of the LDL electronegativity index.

GFP-SCFV: expression and possible applications as a tool for experimental investigations of atherosclerosis

Experimental studies on atherosclerosis are crucial for investigating its pathophysiology, defining new therapeutic targets, and developing new drugs and diagnostic tools. Thus, many imaging markers have been developed and introduced in experimental studies. The main advantage of these new tools is that they allow the noninvasive diagnosis of atherosclerotic vascular disease. Here, we describe the cloning, expression, purification, and stabilization of a chimeric protein specifically designed to probe cells and tissues for the presence of LDL(-), a relevant marker of atherosclerosis. The DNA sequence that encodes the anti-LDL(-) scFv, previously obtained from a hybridoma secreting an anti-LDL(-) monoclonal antibody, was inserted into the bacterial vector pET-28a(+) in tandem with a DNA sequence encoding GFP. The recombinant protein was expressed in high yields in E. coli as inclusion bodies. The applicability of GFP-scFv was assessed by ELISA, which determined its affinity for LDL(-) and confocal microscopy, that showed macrophage uptake of the protein along with LDL(-). In conclusion, our data suggest that the anti-LDL(-) GFP-scFv chimeric protein could be useful in studies on atherogenesis as well as for developing diagnostic tools for atherosclerosis.

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.

Is zinc-α2-glycoprotein a cardiovascular protective factor for patients undergoing hemodialysis?

BACKGROUND

Zinc-α2-glycoprotein (ZAG) is a lipid mobilizing factor. Its anti-inflammatory action and expression pattern suggest that ZAG could act by protecting against the obesity-associated disorders. In hemodialysis (HD) patients, ZAG levels were described to be elevated but its effects on markers of inflammation and LDL oxidation are still unclear. We investigated the relationship between ZAG and markers of systemic inflammation and LDL atherogenic modification profile in HD patients.

METHODS

Forty-three patients regularly on HD were studied and compared to 20 healthy subjects. Plasma ZAG, adiponectin, electronegative LDL [LDL(-)], an atherosclerotic negatively charged LDL subfraction, and anti-LDL(-) autoantibodies levels were measured by ELISA. Markers of inflammation and atherogenic cell recruitment (TNF-α, interleukin-6, VCAM-1, ICAM-1, MCP-1 and PAI-1) were also determined.

RESULTS

Inflammatory markers and atherogenic cell recruitment were higher in HD patients when compared to healthy subjects. ZAG levels were also higher in HD patients (151.5 ± 50.1 mg/l vs 54.6 ± 23.0 mg/l; p<0.0001) and its levels were negatively correlated with TNF-α (r=-0.39; p=0.001) and VCAM-1 (r=-0.52; p<0.0001) and, positively correlated with anti-LDL(-) autoantibodies (r=0.38; p=0.016). On multivariate analyses, plasma ZAG levels were independently associated with VCAM-1 (p=0.01).

CONCLUSION

ZAG is inversely associated with markers of pro-atherogenic factors linked to systemic inflammation and oxidative stress. Thus, this adipokine may constitute a novel marker of a favorable metabolic profile regarding cardiovascular risk factors in HD population.

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(-).

Proteomic analysis of electronegative low-density lipoprotein

Low density lipoprotein is a heterogeneous group of lipoproteins that differs in lipid and protein composition. One copy of apolipoprotein (apo)B accounts for over 95% of the LDL protein, but the presence of minor proteins could disturb its biological behavior. Our aim was to study the content of minor proteins in LDL subfractions separated by anion exchange chromatography. Electropositive LDL [LDL(+)] is the native form, whereas electronegative LDL [LDL⁻] is a minor atherogenic fraction present in blood. LC-ESI MS/MS analysis of both LDL fractions identified up to 28 different proteins. Of these, 13 proteins, including apoB, were detected in all the analyzed samples. LDL⁻ showed a higher content of most minor proteins. Statistical analysis of proteomic data indicated that the content of apoE, apoA-I, apoC-III, apoA-II, apoD, apoF, and apoJ was higher in LDL⁻ than in LDL(+). Immunoturbidimetry, ELISA, or Western blot analysis confirmed these differences. ApoJ and apoF presented the highest difference between LDL(+) and LDL⁻ (>15-fold). In summary, the increased content of several apolipoproteins, and specifically of apoF and apoJ, could be related to the physicochemical characteristics of LDL⁻, such as apoB misfolding, aggregation, and abnormal lipid composition.

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.

HDL and electronegative LDL exchange anti- and pro-inflammatory properties

Electronegative LDL [LDL(-)] is a minor modified LDL subfraction present in blood with inflammatory effects. One of the antiatherogenic properties of HDL is the inhibition of the deleterious effects of in vitro modified LDL. However, the effect of HDL on the inflammatory activity of LDL(-) isolated from plasma is unknown. We aimed to assess the putative protective role of HDL against the cytokine released induced in monocytes by LDL(-). Our results showed that LDL(-) cytokine release was inhibited when LDL(-) was coincubated with HDL and human monocytes and also when LDL(-) was preincubated with HDL and reisolated prior to cell incubation. The addition of apoliprotein (apo)AI instead of HDL reproduced the protective behavior of HDL. HDL preincubated with LDL(-) promoted greater cytokine release than native HDL. Incubation of LDL(-) with HDL decreased the electronegative charge, phospholipase C-like activity, susceptibility to aggregation and nonesterified fatty acid (NEFA) content of LDL(-), whereas these properties increased in HDL. NEFA content in LDL appeared to be related to cytokine production because NEFA-enriched LDL induced cytokine release. HDL, at least in part through apoAI, inhibits phospholipase-C activity and cytokine release in monocytes, thereby counteracting the inflammatory effect of LDL(-). In turn, HDL acquires these properties and becomes inflammatory.

Electronegative LDL induction of apoptosis in macrophages: involvement of Nrf2

The aim of this study was to determine the apoptotic pathways and mechanisms involved in electronegative LDL [LDL(-)]-induced apoptosis in RAW 264.7 macrophages and the role of Nrf2 in this process. Incubation of RAW 264.7 macrophages with LDL(-) for 24 h resulted in dose-dependent cell death. Activated caspases were shown to be involved in the apoptosis induced by LDL(-); incubation with the broad caspase inhibitor z-VAD prevented apoptosis in LDL(-)-treated cells. CD95 (Fas), CD95 ligand (FasL), CD36 and the tumor necrosis factor (TNF) ligand Tnfsf10 were overexpressed in LDL(-)-treated cells. However, Bax, Bcl-2 and Mcl-1 protein levels remained unchanged after LDL(-) treatment. LDL(-) promoted hyperpolarization of the mitochondrial membrane, elevated reactive oxygen species (ROS) production and translocation of Nrf2 to the nucleus, a process absent in cells treated with native LDL. Elicited peritoneal macrophages from Nrf2-deficient mice exhibited an elevated apoptotic response after challenge with LDL(-), together with an increase in the production of ROS in the absence of alterations in CD36 expression. These results provide evidence that CD36 expression induced by LDL(-) is Nrf2-dependent. Also, it was demonstrated that Nrf2 acts as a compensatory mechanism of LDL(-)-induced apoptosis in macrophages.