Expression of human scavenger receptor class B type I in cultured human monocyte-derived macrophages and atherosclerotic lesions

MiR-125b downregulates macrophage scavenger receptor type B1 and reverse cholesterol transport

OBJECTIVE

To determine whether miR-125b regulates cholesterol efflux in vivo and in vitro through the regulation of scavenger receptor type B1 (SR-B1).

APPROACH AND RESULTS

We demonstrated that miR-125b is up-regulated in the human aortas of patients with CAD and is located in macrophages and vascular smooth muscle cells (VSMCs). We identified SCARB1 as a direct target of miR-125b by repressing the activity of the SCARB1 3'-untranslated region reporter construct. Moreover, the overexpression of miR-125b in both human and mouse macrophages as well as VSMCs was found to downregulated the expression of the SCARB1 and the SR-B1 protein levels, thereby impairing α-HDL-mediated macrophage cholesterol efflux in vitro. The in vivo reverse cholesterol transport (RCT) rate from non-cholesterol-loaded macrophages transfected with miR-125b to feces was also found to be decreased when compared with that of control mimic-transfected macrophages.

CONCLUSIONS

Together, these results provide evidence that miR-125b downregulates SCARB1 and SR-B1 in both human and mouse macrophages as well as VSMCs, thereby impairing macrophage cholesterol efflux in vitro and the whole macrophage-specific RCT pathway in vivo.

Oxidized LDLs as Signaling Molecules

Levels of oxidized low-density lipoproteins (oxLDLs) are usually low in vivo but can increase whenever the balance between formation and scavenging of free radicals is impaired. Under normal conditions, uptake and degradation represent the physiological cellular response to oxLDL exposure. The uptake of oxLDLs is mediated by cell surface scavenger receptors that may also act as signaling molecules. Under conditions of atherosclerosis, monocytes/macrophages and vascular smooth muscle cells highly exposed to oxLDLs tend to convert to foam cells due to the intracellular accumulation of lipids. Moreover, the atherogenic process is accelerated by the increased expression of the scavenger receptors CD36, SR-BI, LOX-1, and SRA in response to high levels of oxLDL and oxidized lipids. In some respects, the effects of oxLDLs, involving cell proliferation, inflammation, apoptosis, adhesion, migration, senescence, and gene expression, can be seen as an adaptive response to the rise of free radicals in the vascular system. Unlike highly reactive radicals, circulating oxLDLs may signal to cells at more distant sites and possibly trigger a systemic antioxidant defense, thus elevating the role of oxLDLs to that of signaling molecules with physiological relevance.

Human variant of scavenger receptor BI (R174C) exhibits impaired cholesterol transport functions

HDL and its primary receptor, scavenger receptor class B type I (SR-BI), work together to promote the clearance of excess plasma cholesterol, thereby protecting against atherosclerosis. Human variants of SR-BI have been identified in patients with high HDL-cholesterol levels, and at least one variant has been linked to cardiovascular disease. Therefore, while often regarded as beneficial, very high levels of HDL-cholesterol may result from impaired cholesterol clearance through SR-BI and contribute to cardiovascular risk. In this study, we characterized the function of a rare human variant of SR-BI, resulting in the substitution of arginine-174 with cysteine (R174C), which was previously identified in a heterozygous individual with high levels of HDL-cholesterol. We hypothesized that the R174C-SR-BI variant has impaired cholesterol transport functions, which were assessed in COS-7 cells after transient transfection with full-length WT or R174C-SR-BI. Although R174C-SR-BI was expressed at levels comparable to the WT receptor, HDL binding, cholesteryl hexadecyl ether uptake, free cholesterol efflux, and modulation of membrane cholesterol were disrupted in the presence of R174C-SR-BI. We further examined the role of salt bridges as a potential mechanism for R174C-SR-BI dysfunction. If translatable, this human variant could lead to increased plasma HDL-cholesterol levels, impaired cholesterol clearance, and increased cardiovascular disease risk.

High-Density Lipoprotein Therapy in Stroke: Evaluation of Endothelial SR-BI-Dependent Neuroprotective Effects

High-density lipoproteins (HDLs) display endothelial protective effects. We tested the role of SR-BI, an HDL receptor expressed by endothelial cells, in the neuroprotective effects of HDLs using an experimental model of acute ischemic stroke. After transient intraluminal middle cerebral artery occlusion (tMCAO), control and endothelial SR-BI deficient mice were intravenously injected by HDLs or saline. Infarct volume and blood-brain barrier (BBB) breakdown were assessed 24 h post tMCAO. The potential of HDLs and the role of SR-BI to maintain the BBB integrity was assessed by using a human cellular model of BBB (hCMEC/D3 cell line) subjected to oxygen-glucose deprivation (OGD). HDL therapy limited the infarct volume and the BBB leakage in control mice relative to saline injection. Interestingly, these neuroprotective effects were thwarted by the deletion of SR-BI in endothelial cells and preserved in mice deficient for SR-BI in myeloid cells. In vitro studies revealed that HDLs can preserve the integrity of the BBB in OGD conditions, and that this effect was reduced by the SR-BI inhibitor, BLT-1. The protection of BBB integrity plays a pivotal role in HDL therapy of acute ischemic stroke. Our results show that this effect is partially mediated by the HDL receptor, SR-BI expressed by endothelial cells.

Macrophage maturation from blood monocytes is altered in people with HIV, and is linked to serum lipid profiles and activation indices: A model for studying atherogenic mechanisms

People with HIV (PWH) are at increased risk for atherosclerotic cardiovascular disease (ASCVD). Proportions of vascular homing monocytes are enriched in PWH; however, little is known regarding monocyte-derived macrophages (MDMs) that may drive atherosclerosis in this population. We isolated PBMCs from people with and without HIV, and cultured these cells for 5 days in medium containing autologous serum to generate MDMs. Differential gene expression (DGE) analysis of MDMs from PWH identified broad alterations in innate immune signaling (IL-1β, TLR expression, PPAR βδ) and lipid processing (LXR/RXR, ACPP, SREBP1). Transcriptional changes aligned with the functional capabilities of these cells. Expression of activation markers and innate immune receptors (CD163, TLR4, and CD300e) was altered on MDMs from PWH, and these cells produced more TNFα, reactive oxygen species (ROS), and matrix metalloproteinases (MMPs) than did cells from people without HIV. MDMs from PWH also had greater lipid accumulation and uptake of oxidized LDL. PWH had increased serum levels of free fatty acids (FFAs) and ceramides, with enrichment of saturated FAs and a reduction in polyunsaturated FAs. Levels of lipid classes and species that are associated with CVD correlated with unique DGE signatures and altered metabolic pathway activation in MDMs from PWH. Here, we show that MDMs from PWH display a pro-atherogenic phenotype; they readily form foam cells, have altered transcriptional profiles, and produce mediators that likely contribute to accelerated ASCVD.

People with HIV (PWH) are at greater risk for developing cardiovascular disease (CVD) than the general public, but the mechanisms underlying this increased risk are poorly understood. Macrophages play key roles in the pathogenesis of atherosclerosis, and are potential targets for therapeutic intervention. Here, we investigate phenotypic and functional abnormalities in monocyte-derived macrophages (MDMs) isolated from PWH that may drive CVD risk in this population. MDMs were differentiated in the presence of autologous serum, enabling us to explore the contributions of serum components (lipids, inflammatory cytokines, microbial products) as drivers of altered MDM function. We link serum levels of inflammatory biomarkers and CVD-associated lipid species to MDM activation. Our study provides new insight into drivers of pro-atherogenic MDM phenotype in PWH, and identifies directions for future study and potential intervention strategies to mitigate CVD risk.

Lipid efflux mechanisms, relation to disease and potential therapeutic aspects

Lipids are hydrophobic and amphiphilic molecules involved in diverse functions such as membrane structure, energy metabolism, immunity, and signaling. However, altered intra-cellular lipid levels or composition can lead to metabolic and inflammatory dysfunction, as well as lipotoxicity. Thus, intra-cellular lipid homeostasis is tightly regulated by multiple mechanisms. Since most peripheral cells do not catabolize cholesterol, efflux (extra-cellular transport) of cholesterol is vital for lipid homeostasis. Defective efflux contributes to atherosclerotic plaque development, impaired β-cell insulin secretion, and neuropathology. Of these, defective lipid efflux in macrophages in the arterial walls leading to foam cell and atherosclerotic plaque formation has been the most well studied, likely because a leading global cause of death is cardiovascular disease. Circulating high density lipoprotein particles play critical roles as acceptors of effluxed cellular lipids, suggesting their importance in disease etiology. We review here mechanisms and pathways that modulate lipid efflux, the role of lipid efflux in disease etiology, and therapeutic options aimed at modulating this critical process.

Cholesterol transport system: An integrated cholesterol transport model involved in atherosclerosis

Atherosclerosis, the pathological basis of most cardiovascular disease (CVD), is closely associated with cholesterol accumulation in the arterial intima. Excessive cholesterol is removed by the reverse cholesterol transport (RCT) pathway, representing a major antiatherogenic mechanism. In addition to the RCT, other pathways are required for maintaining the whole-body cholesterol homeostasis. Thus, we propose a working model of integrated cholesterol transport, termed the cholesterol transport system (CTS), to describe body cholesterol metabolism. The novel model not only involves the classical view of RCT but also contains other steps, such as cholesterol absorption in the small intestine, low-density lipoprotein uptake by the liver, and transintestinal cholesterol excretion. Extensive studies have shown that dysfunctional CTS is one of the major causes for hypercholesterolemia and atherosclerosis. Currently, several drugs are available to improve the CTS efficiently. There are also several therapeutic approaches that have entered into clinical trials and shown considerable promise for decreasing the risk of CVD. In recent years, a variety of novel findings reveal the molecular mechanisms for the CTS and its role in the development of atherosclerosis, thereby providing novel insights into the understanding of whole-body cholesterol transport and metabolism. In this review, we summarize the latest advances in this area with an emphasis on the therapeutic potential of targeting the CTS in CVD patients.

A critical appraisal of the measurement of serum 'cholesterol efflux capacity' and its use as surrogate marker of risk of cardiovascular disease

The 'cholesterol efflux capacity (CEC)' assay is a simple in vitro measure of the capacities of individual sera to promote the first step of the reverse cholesterol transport pathway, the delivery of cellular cholesterol to plasma HDL. This review describes the cell biology of this model and critically assesses its application as a marker of cardiovascular risk. We describe the pathways for cell cholesterol export, current cell models used in the CEC assay with their limitations and consider the contribution that measurement of serum CEC provides to our understanding of HDL function in vivo.

Monocytes from HIV-infected individuals show impaired cholesterol efflux and increased foam cell formation after transendothelial migration

DESIGN

HIV-infected (HIV+) individuals have an increased risk of atherosclerosis and cardiovascular disease which is independent of antiretroviral therapy and traditional risk factors. Monocytes play a central role in the development of atherosclerosis, and HIV-related chronic inflammation and monocyte activation may contribute to increased atherosclerosis, but the mechanisms are unknown.

METHODS

Using an in-vitro model of atherosclerotic plaque formation, we measured the transendothelial migration of purified monocytes from age-matched HIV+ and uninfected donors and examined their differentiation into foam cells. Cholesterol efflux and the expression of cholesterol metabolism genes were also assessed.

RESULTS

Monocytes from HIV+ individuals showed increased foam cell formation compared with controls (18.9 vs. 0%, respectively, P = 0.004) and serum from virologically suppressed HIV+ individuals potentiated foam cell formation by monocytes from both uninfected and HIV+ donors. Plasma tumour necrosis factor (TNF) levels were increased in HIV+ vs. control donors (5.9 vs. 3.5 pg/ml, P = 0.02) and foam cell formation was inhibited by blocking antibodies to TNF receptors, suggesting a direct effect on monocyte differentiation to foam cells. Monocytes from virologically suppressed HIV+ donors showed impaired cholesterol efflux and decreased expression of key genes regulating cholesterol metabolism, including the cholesterol transporter ABCA1 (P = 0.02).

CONCLUSION

Monocytes from HIV+ individuals show impaired cholesterol efflux and are primed for foam cell formation following transendothelial migration. Factors present in HIV+ serum, including elevated TNF levels, further enhance foam cell formation. The proatherogenic phenotype of monocytes persists in virologically suppressed HIV+ individuals and may contribute mechanistically to increased atherosclerosis in this population.

Scavenger receptor class B, type I (Scarb1) deficiency promotes osteoblastogenesis but stunts terminal osteocyte differentiation

Scavenger receptor class B type I (SR-BI), the Scarb1 gene product, is a high-density lipoprotein (HDL) receptor which was shown to influence bone metabolism. Its absence in mice is associated with alterations of the glucocorticoid/adrenocorticotropic hormone axis, and translated in high bone mass and enhanced bone formation. Since the cellular alterations underlying the enhanced bone formation remain unknown, we investigated Scarb1-deficient marrow stromal cells (MSC) behavior in vitro. No difference in HDL3, cholesteryl ester (CE) or estradiol (E) association/binding was measured between Scarb1-null and wild-type (WT) cells. Scarb1 genic expression was down-regulated twofold following osteogenic treatment. Neither WT nor null cell proliferation was influenced by HDL3 exposure whereas this condition decreased genic expression of osteoblastic marker osterix (Sp7), and osteocyte markers sclerostin (Sost) and dentin matrix protein 1 (Dmp1) independently of genotype. Sost and Dmp1 basal expression in null cells was 40% and 50% that of WT cells; accordingly, osteocyte density was 20% lower in vertebrae from Scarb1-null mice. Genic expression of co-receptors for Wnt signaling, namely LDL-related protein (Lrp) 5 and Lrp8, was increased, respectively, by two- and threefold, and of transcription target-genes axis inhibition protein 2 (Axin2) and lymphoid enhancer-binding factor 1 (Lef1) over threefold. Gene expression of Wnt signaling agonist Wnt5a and of the antagonist dickkopfs-related protein 1 (Dkk1) were found to be increased 10- to 20-fold in null MSC. These data suggest alterations of Wnt pathways in Scarb1-deficient MSC potentially explaining their enhanced function, hence contributing to the high bone mass observed in these mice.

Selective transfection of microglia in the brain using an antibody-based non-viral vector

There are currently few approaches to transiently manipulate the expression of specific proteins in microglia of the brain. An antibody directed against an extracellular epitope of scavenger receptor class B, type I (SR-BI) was found to be selectively taken up by these cells in the brain. Other antibodies tested were not internalised by microglia. A vector was produced by linking the SR-BI antibody to polyethyleneimine and binding a DNA plasmid encoding green fluorescent protein. Infusions of this vector into the hippocampus produced a widespread transfection of cells, more than 80% of which were immunoreactive for microglial/macrophage markers. Transfection was not detected in cells expressing markers for astrocytes or neurons. Reporter gene expression was most prominent near the infusion site but was seen in tissue up to 4mm away. DNA bound to polyethyleneimine alone or to a vector containing a different antibody did not produce transfection in the brain. Single injections of the vector containing the SR-BI antibody into the brain also resulted in transfection of microglia, albeit with lower efficiency. Vector modifications to promote lysis of endosomes or entry of DNA into the nucleus did not increase efficiency. The findings clearly demonstrate the capacity of the SR-BI antibody to selectively target brain microglia. This approach offers considerable potential to deliver DNA and other molecules capable of modifying the function of these cells in vivo.

SR-BI in bone marrow derived cells protects mice from diet induced coronary artery atherosclerosis and myocardial infarction

SR-BI deficient mice that are also hypomorphic for apolipoprotein E expression develop diet induced occlusive coronary artery atherosclerosis, myocardial infarction and early death. To test the role of SR-BI in bone marrow derived cells, we used bone marrow transplantation to generate SR-BI-null; apoE-hypomorphic mice in which SR-BI expression was restored solely in bone marrow derived cells. SR-BI-null; apoE-hypomorphic mice were transplanted with SR-BI^+/+apoE-hypomorphic, or control, autologous SR-BI-null; apoE-hypomorphic bone marrow. Four weeks later, mice were fed a high-fat, high-cholesterol, cholate-containing diet to induce coronary artery atherosclerosis. Mice transplanted with autologous bone marrow developed extensive aortic atherosclerosis and severe occlusive coronary artery atherosclerosis after 4 weeks of feeding. This was accompanied by myocardial fibrosis and increased heart weights. In contrast, restoration of SR-BI expression in bone marrow derived-cells reduced diet induced aortic and coronary artery atherosclerosis, myocardial fibrosis and the increase in heart weights in SR-BI-null; apoE-hypomorphic mice. Restoration of SR-BI in bone marrow derived cells did not, however, affect steady state lipoprotein cholesterol levels, but did reduce plasma levels of IL-6. Monocytes from SR-BI-null mice exhibited a greater capacity to bind to VCAM-1 and ICAM-1 than those from SR-BI^+/+ mice. Furthermore, restoration of SR-BI expression in bone marrow derived cells attenuated monocyte recruitment into atherosclerotic plaques in mice fed high fat, high cholesterol cholate containing diet. These data demonstrate directly that SR-BI in bone marrow-derived cells protects against both aortic and CA atherosclerosis.

Hypolipidemic effect of SR‑BI gene delivery by combining cationic liposomal microbubbles and ultrasound in hypercholesterolemic rats

High-density lipoprotein (HDL) is a key mediator in reverse cholesterol transport and is involved in a mechanism known as 'selective lipid uptake', a process mediated by scavenger receptor B type I (SR‑BI), which is a HDL receptor. The aim of the present study was to investigate the therapeutic effect of the SR‑BI gene when delivered by combining cationic liposomal microbubbles (CLMs) and ultrasound (US) in hypercholesterolemic rats. Hypercholesterolemia was induced by administration of excessive doses of vitamin D3 and cholesterol in rats. The CLMs consisted of perfluoropropane gas encapsulated in a phospholipid shell using the sonication‑lyophilization method. The SR‑BI gene, mixed with the self‑made microbubbles, was transfected into hypercholesterolemic rat arteries using therapeutic US. SR‑BI protein expression was determined by western blot analysis 2 days post-transfection. Two weeks after transfection, total cholesterol (TC), triglyceride (TG), low-density lipoprotein (LDL) and HDL serum concentrations were measured. Transfection efficiency of the SR‑BI gene in the SR‑BI + US/CLM group increased 6‑7‑fold compared with the SR‑BI group. Two weeks after transfection, plasma lipid levels in treated hypercholesterolemic rats were observed to be significantly reduced compared with rats that did not receive treatment. However, no significant change was observed in the SR‑BI group compared with that in the SR‑BI + US/CLM group. Results of the present study indicate that the combination of US and CLMs loaded with the SR‑BI gene may exert a protective role in hypercholesterolemia.

The myeloperoxidase product hypochlorous acid generates irreversible high-density lipoprotein receptor inhibitors

OBJECTIVE

Elevated levels of advanced oxidation protein products have been described in several chronic inflammatory diseases, like chronic renal insufficiency, rheumatoid arthritis, and atherosclerosis. Recent findings revealed that advanced oxidation protein products are inhibitors of the major high-density lipoprotein receptor, scavenger receptor class B, type 1 (SR-BI). Here, we investigated which oxidation-induced structural alterations convert plasma albumin into a high-density lipoprotein-receptor inhibitor.

APPROACH AND RESULTS

Exposure of albumin to the physiological oxidant, hypochlorous acid, generated high-affinity SR-BI ligands. Protection of albumin-lysine residues before exposure to hypochlorous acid as well as regeneration of N-chloramines after oxidation of albumin completely prevented binding of oxidized albumin to SR-BI, indicating that modification of albumin-lysine residues is required to generate SR-BI ligands. Of particular interest, N-chloramines within oxidized albumin promoted irreversible binding to SR-BI, resulting in permanent receptor blockade. We observed that the SR-BI inhibitory activity of albumin isolated from chronic kidney disease patients correlated with the content of the myeloperoxidase-specific oxidation product 3-chlorotyrosine and was associated with alterations in the composition of high-density lipoprotein.

CONCLUSIONS

Given that several potential atheroprotective activities of high-density lipoprotein are mediated by SR-BI, the present results raise the possibility that oxidized plasma albumin, through permanent SR-BI blockade, contributes to the pathophysiology of cardiovascular disease.

Functions of scavenger receptor class B, type I in atherosclerosis

PURPOSE OF REVIEW

This review highlights the diverse roles of the high-affinity HDL receptor scavenger receptor class B, type I (SR-BI) in the modulation of global cholesterol homeostasis and vascular cell function, and the potential implications of these processes in atherosclerosis.

RECENT FINDINGS

SR-BI in the liver plays a critical role in reverse cholesterol transport and it dramatically impacts the characteristics of the HDL particle, and through reverse cholesterol transport it promotes an antiatherogenic lipid environment in the vascular wall. SR-BI in macrophages may influence their inflammatory phenotype. In endothelial cells, SR-BI mediates HDL-induced endothelial nitric oxide synthase activation and proliferation and migration, and in platelets SR-BI may be prothrombotic in the setting of dyslipidemia. Several polymorphisms of SR-BI have been reported in humans that influence receptor expression or function.

SUMMARY

In addition to regulating global lipid metabolism, SR-BI influences the functions of a variety of vascular cells relevant to atherosclerosis. Studies of SR-BI genetics in humans partially support the conclusions drawn from experimental models. However, because of the multiple functions of SR-BI, the diversity of cell types in which it is expressed, and the influence of the receptor on the characteristics of its own ligand, our understanding of the biology of the receptor is just emerging.

Identification of Candidate Genes Associated with Beef Marbling Using QTL and Pathway Analysis in Hanwoo (Korean Cattle)

Marbling from intramuscular fat is an important trait of meat quality and has an economic benefit for the beef industry. Quantitative trait loci (QTL) fine mapping was performed to identify the marbling trait in 266 Hanwoo steers using a 10K single nucleotide polymorphism panel with the combined linkage and linkage disequilibrium method. As a result, we found nine putative QTL regions for marbling: three on BTA6, two on BTA17, two on BTA22, and two on BTA29. We detected candidate genes for marbling within 1 cM of either side of the putative QTL regions. Additionally, to understand the functions of these candidate genes at the molecular level, we conducted a functional categorization using gene ontology and pathway analyses for those genes involved in lipid metabolism or fat deposition. In these putative QTL regions, we found 95 candidate genes for marbling. Using these candidate genes, we found five genes that had a direct interaction with the candidate genes. We also found SCARB1 as a putative candidate gene for marbling that involves fat deposition related to cholesterol transport.

Scavenger receptor class B type 1 gene polymorphisms and female fertility

PURPOSE OF REVIEW

Multiple studies have demonstrated a role for scavenger receptor class B type 1 (SR-B1) in female fertility. Recent studies have implicated specific SR-B1 gene polymorphisms in decreased progesterone production and suboptimal fertility outcomes.

RECENT FINDINGS

The lipoprotein receptor SR-B1 has been known to mediate selective uptake of lipids into steroidogenic tissues such as the ovaries. SR-B1 plays a major role in the ability of the corpus luteum to produce progesterone, which is known to play a key role in sustaining early pregnancy. Animal studies have demonstrated that deficiency in SR-B1 results in subfertility that can be restored with addition of SR-B1 function. Single-nucleotide polymorphisms in SCARB1, the gene encoding SR-B1, have been associated with human lipid levels. Women undergoing infertility treatment with low SR-B1 expression in granulosa cells were noted to have plasma estradiol levels half the normal levels and a significantly lower number of retrieved oocytes. In vitro, deficiency of SR-B1 is associated with lower progesterone secretion in human granulosa cells. Certain SR-B1 polymorphisms have been associated with lower follicular progesterone levels and a significantly lower clinical pregnancy rate.

SUMMARY

Deficiency of SR-B1, particularly due to single-nucleotide polymorphisms, could explain some features of female human infertility.

Estimated Creatinine Clearance, Homocysteine and High Sensitivity-C-Reactive Protein Levels Determination for Early Prediction of Nephropathy and Atherosclerosis Risk In Type 2 Diabetic Patients

Type 2 diabetes mellitus (T2D) patients are increased risk for cardiovascular disease (CVD) and chronic kidney disease (CKD). Many studies had demonstrated that CKD is significantly associated with CVD. We aim to indicate the using estimated creatinine clearance (eCrCl), homocysteine (tHcy), and high sensitivity-C-reactive protein (hs-CRP) levels, may have an impact on the interpretation risk for nephropathy and CVD. eCrCl was using the Cockroft-Gault formula, eCrCl levels were stratified according to the Kidney Disease Outcome Quality Initiative definition. We measured serum tHcy, hs-CRP, and the other biochemical variables in 54 T2D patients compared with 40 age matched healthy controls (NDM). T2D patients were significantly lower eCrCl than NDM (P < 0.05). T2D patients also showed significantly higher in tHcy, hs-CRP, and MDA levels than NDM subjects (P < 0.05). The eCrCl was significantly correlated with tHcy and hs-CRP levels in T2D patients (r = -0.504, P < 0.001; r = -0.282, P = 0.047). eCrCl had an impact on interpretation for CKD, especially in T2D patients. Decrease eCrCl concomitant with increased in tHcy, hs-CRP, and MDA levels may present a higher risk for future diabetic nephropathy and CVD.

Role of macrophage scavenger receptors in atherosclerosis

Accumulating evidence indicates that atherosclerosis is a chronic inflammatory disease. The key innate immune cells that are involved in the pathogenesis of atherosclerosis are circulating monocytes and plaque macrophages. Complex interplay between immune and metabolic processes results in pathological activity of these cells. The best understood pathological process mediated by macrophages is their inability to process modified lipoproteins properly resulting in the formation of foamy cells, which are a dangerous component of atherosclerotic plaques. Key molecules involved in the recognition and processing of modified lipoproteins are scavenger receptors (SR). This is a large family of surface expressed structurally heterogeneous receptors with a broad spectrum of endogenous and exogenous ligands. The common functional feature of SR is internalisation of extracellular components and targeting them for lysosomal degradation. However, these relatively simple functions can have complex consequences, since they are linked to diverse specific signalling pathways and to other membrane transport pathways. Moreover, scavenger receptors can co-operate with other types of receptors increasing the variability of the macrophage response to multiple extracellular ligands. At least some SRs respond to modified lipoproteins by amplification of inflammation and accumulation of macrophages in the plaque, while some SRs may support tolerogenic reactions. Outcome of different SR activities will be the decision of monocytes and macrophage to guard homeostatic balance, support atherosclerosis progression and plaque instability by inflammatory reactions, or support rapid fibrotic processes in the plaque that stabilise it. Despite the accumulating knowledge about the molecular mechanisms of scavenger receptor action, their role in the progression of atherosclerosis remains controversial. The activities of scavenger receptors that can contribute to each of these processes are a subject of current review.

Protein carbamylation renders high-density lipoprotein dysfunctional

Carbamylation of proteins through reactive cyanate has been demonstrated to predict an increased cardiovascular risk. Cyanate is formed in vivo by breakdown of urea and at sites of inflammation by the phagocyte protein myeloperoxidase. Because myeloperoxidase (MPO) associates with high-density lipoprotein (HDL) in human atherosclerotic intima, we examined in the present study whether cyanate specifically targets HDL. Mass spectrometry analysis revealed that protein carbamylation is a major posttranslational modification of HDL. The carbamyllysine content of lesion-derived HDL was more than 20-fold higher in comparison with 3-chlorotyrosine levels, a specific oxidation product of MPO. Notably, the carbamyllysine content of lesion-derived HDL was five- to eightfold higher when compared with lesion-derived low-density lipoprotein (LDL) or total lesion protein and increased with lesion severity. The carbamyllysine content of HDL, but not of LDL, correlated with levels of 3-chlorotyrosine, suggesting that MPO mediated carbamylation in the vessel wall. Remarkably, one carbamyllysine residue per HDL-associated apolipoprotein A-I was sufficient to induce cholesterol accumulation and lipid-droplet formation in macrophages through a pathway requiring the HDL-receptor scavenger receptor class B, type I. The present results raise the possibility that HDL carbamylation contributes to foam cell formation in atherosclerotic lesions.

Scavenger receptor SR-BI in macrophage lipid metabolism

OBJECTIVE

To investigate the mechanisms by which macrophage scavenger receptor BI (SR-BI) regulates macrophage cholesterol homeostasis and protects against atherosclerosis.

METHODS AND RESULTS

The expression and function of SR-BI was investigated in cultured mouse bone marrow-derived macrophages (BMM). SR-BI, the other scavenger receptors SRA and CD36 and the ATP-binding cassette transporters ABCA1 and ABCG1 were each distinctly regulated during BMM differentiation. SR-BI levels increased transiently to significant levels during culture. SR-BI expression in BMM was reversibly down-regulated by lipid loading with modified LDL; SR-BI was shown to be present both on the cell surface as well as intracellularly. BMM exhibited selective HDL CE uptake, however, this was not dependent on SR-BI or another potential candidate glycosylphosphatidylinositol anchored high density lipoprotein binding protein 1 (GPIHBP1). SR-BI played a significant role in facilitating bidirectional cholesterol flux in non lipid-loaded cells. SR-BI expression enhanced both cell cholesterol efflux and cholesterol influx from HDL, but did not lead to altered cellular cholesterol mass. SR-BI-dependent efflux occurred to larger HDL particles but not to smaller HDL(3). Following cholesterol loading, ABCA1 and ABCG1 were up-regulated and served as the major contributors to cholesterol efflux, while SR-BI expression was down-regulated.

CONCLUSION

Our results suggest that SR-BI plays a significant role in macrophage cholesterol flux that may partly account for its effects on atherogenesis.

Scavenger receptors: Implications in atherothrombotic disorders

Scavenger receptors are modified lipoprotein binding receptors, expressed on the surface of a variety of cells including endothelial, macrophages and platelets. The most extensively studied class B scavenger receptors comprise of CD36 and SR-BI and have been found to bind to native and modified LDL. Interaction of modified LDL to CD36 accelerates foam cell formation, the key step in atherosclerotic plaque deposition. Recently scavenger receptors have also been implicated in thrombosis. Platelet CD36 serves as a sensor of oxidative stress and modulator of platelet reactivity under hyperlipidemic conditions thus, inducing prothrombotic signals. In contrast, targeting platelet SR-BI corresponds to reduce platelet hyperreactivity in hyperlipidemia suggesting that targeting these receptors could be a promising strategy for the treatment of atherothrombotic disorders.

Carbamylated LDL

Nonenzymatic modification of protein by cyanate, that is, carbamylation, has received new attention due to its apparent relevance in atherosclerosis. For example, carbamylation of low-density lipoprotein (LDL) is an important mechanism that potentially impacts high-risk atherosclerotic individuals with increased urea (renal insufficiency) or thiocyanate (tobacco smoking). Carbamylated LDL (cLDL) is increased in patients with end-stage kidney disease, especially those with atherosclerosis. In addition, cLDL exhibits distinct cytotoxic effects when tested in vitro on endothelial cells, induces the expression of adhesion molecules, and aggravates the monocyte adhesion to endothelial cells. It also facilitates the proliferation of vascular smooth-muscle cell (VSMC). Studies of potential pharmacological interruption of these processes in vivo may lead to discoveries of novel therapies for atherosclerosis.

Relative roles of various efflux pathways in net cholesterol efflux from macrophage foam cells in atherosclerotic lesions

PURPOSE OF REVIEW

Cholesterol efflux mechanisms are essential for macrophage cholesterol homeostasis. HDL, an important cholesterol efflux acceptor, comprises a class of heterogeneous particles that induce cholesterol efflux via distinct pathways. This review focuses on the understanding of the different cholesterol efflux pathways and physiological acceptors involved, and their regulation in atherosclerotic lesions.

RECENT FINDINGS

The synergistic interactions of ATP-binding cassette transporters A1 and G1 as well as ATP-binding cassette transporter A1 and scavenger receptor class B type I are essential for cellular cholesterol efflux and the prevention of macrophage foam cell formation. However, the importance of aqueous diffusion should also not be underestimated. Significant progress has been made in understanding the mechanisms underlying ATP-binding cassette A1-mediated cholesterol efflux and regulation of its expression and trafficking. Conditions locally in the atherosclerotic lesion, for example, lipids, cytokines, oxidative stress, and hypoxia, as well as systemic factors, including inflammation and diabetes, critically influence the expression of cholesterol transporters on macrophage foam cells. Furthermore, HDL modification and remodeling in atherosclerosis, inflammation, and diabetes impairs its function as an acceptor for cellular cholesterol.

SUMMARY

Recent advances in the understanding of the regulation of cholesterol transporters and their acceptors in atherosclerotic lesions indicate that HDL-based therapies should aim to enhance the activity of cholesterol transporters and improve both the quantity and quality of HDL.

Distribution of smooth muscle cells and macrophages expressing scavenger receptor BI/II in atherosclerosis

AIM

Scavenger receptors type I and II (SRBI/II) have dual roles in both atherogenic and antiatherogenic functions through interactions with lipoproteins and their expression in macrophages; how-ever, the distribution and density of SRBI/II-positive macrophages and smooth muscle cells (SMCs) as well as their association with lipid metabolism-related proteins in atherosclerotic intima of the human aorta remain unclear.

METHODS

Autopsied aortic tissues were double-immunostained with SRBI/BII and smooth muscle actin or macrophage-specific antibodies. The density of SRBI/BII-positive SMCs and macrophages in intimal lesion was measured. They were also immunostained with antibodies against four apolipoproteins, four phospholipase A2s, and CETP.

RESULTS

SRBI/II was expressed in both macrophages and SMCs distributed in various intimal lesions. The density of SRBI/II-positive SMCs in intimal lesions significantly decreased with the advance of atherosclerosis, whereas the density of SRBI/II-positive macrophages significantly increased with atherosclerotic development. In addition, functional proteins, such as apolipoproteins, secretory phospholipase A2s, and CETP, were distributed in the intimal stroma around SRBI/II-positive cells in all lesion types.

CONCLUSION

The results indicated that SMCs are involved in lipid metabolism via SRBI/II expression mainly in the early stages of atherosclerosis evolution, and that SRBI/II-positive macrophages are mainly involved in advanced stages.

Pathways by which reconstituted high-density lipoprotein mobilizes free cholesterol from whole body and from macrophages

OBJECTIVE

Reconstituted high-density lipoprotein (rHDL) is of interest as a potential novel therapy for atherosclerosis because of its ability to promote free cholesterol (FC) mobilization after intravenous administration. We performed studies to identify the underlying molecular mechanisms by which rHDL promote FC mobilization from whole body in vivo and macrophages in vitro.

METHODS AND RESULTS

Wild-type (WT), SR-BI knockout (KO), ABCA1 KO, and ABCG1 KO mice received either rHDL or phosphate-buffered saline intravenously. Blood was drawn before and at several time points after injection for apolipoprotein A-I, phosphatidylcholine, and FC measurement. In WT mice, serum FC peaked at 20 minutes and rapidly returned toward baseline levels by 24 hours. Unexpectedly, ABCA1 KO and ABCG1 KO mice did not differ from WT mice regarding the kinetics of FC mobilization. In contrast, in SR-BI KO mice the increase in FC level at 20 minutes was only 10% of that in control mice (P<0.01). Bone marrow-derived macrophages from WT, SR-BI O, ABCA1 KO, and ABCG1 KO mice were incubated in vitro with rHDL and cholesterol efflux was determined. Efflux from SR-BI KO and ABCA1 KO macrophages was not different from WT macrophages. In contrast, efflux from ABCG1 KO macrophages was approximately 50% lower as compared with WT macrophages (P<0.001).

CONCLUSIONS

The bulk mobilization of FC observed in circulation after rHDL administration is primarily mediated by SR-BI. However, cholesterol mobilization from macrophages to rHDL is primarily mediated by ABCG1.

Scavenger receptor class B type I protein as an independent predictor of high-density lipoprotein cholesterol levels in subjects with hyperalphalipoproteinemia

CONTEXT

In mice, scavenger receptor class B, type I (SR-BI) receptor protein deficiency is associated with elevated high-density lipoprotein (HDL)-cholesterol (HDL-C) levels.

OBJECTIVE

Our objective was to determine the relationship between SR-BI protein and HDL-C levels in humans.

DESIGN

This was a prospective study of adults with hyperalphalipoproteinemia. Fasting blood was obtained for lipid and lipoprotein measurement, genomic DNA, and monocyte-derived macrophages. SR-BI protein levels were measured by Western blots, and SR-BI activity was measured by cholesteryl ester (CE) uptake of each donor's radiolabeled HDL with their monocyte-derived macrophages, or by degradation and specific cell association of dual-labeled HDL in vitro.

SETTING

The study was performed in a tertiary university teaching hospital.

RESULTS

The mean age was 57.2 +/- 10.9 yr (n = 65). SR-BI protein levels were inversely associated with HDL-C levels (P < 0.002), HDL particle size (P < 0.05), and positively associated with CE uptake (P < 0.004); there was no association with plasma apolipoprotein levels. SR-BI protein levels (P = 0.01) were independent predictors of HDL-C levels. Subjects who were carriers of the A allele for the rs4238001 (glycine to serine at position 2) polymorphism [single nucleotide polymorphism (SNP)] had lower SR-BI protein levels (P = 0.01), whereas carriers of the C allele for the rs2278986 SNP also had lower SR-BI protein levels (P = 0.02). Body mass index (P = 0.05), rs4238001 (P = 0.01), and rs2278986 (P = 0.01) SNPs were independent predictors of SR-BI protein levels. In vitro studies of murine macrophages stably expressing the glycine to serine at position 2 SNP showed less degradation (P < 0.0004) and specific cell association (P < 0.0004) of [(125)I, (3)H]-CE-labeled HDL.

CONCLUSIONS

SR-BI protein has an independent effect on HDL-C levels in women with hyperalphalipoproteinemia. Two SNPs were significantly associated with lower SR-BI protein levels.

Influence of aspirin on SR-BI expression in human carotid plaques

BACKGROUND

We recently showed that aspirin promotes scavenger receptor class-B type I (SR-BI) protein expression in vitro in primary human macrophages and in vivo in resident peritoneal macrophages of mice.

METHODS

We compared SR-BI and CD68 expression in carotid atherosclerotic specimens from endarterectomized patients with (n=38) or without (n=19) low-dose aspirin medication (100 mg/day) prior to endarterectomy.

RESULTS

We found no differences concerning expression of CD68, indicating that aspirin did not influence macrophage content within atherosclerotic plaques. However, aspirin increased the expression of SR-BI protein in the analyzed specimens. In human THP-1-derived macrophages, induction of SR-BI protein by aspirin was abrogated by concomitant pharmacological inhibition of nuclear factor-kappa B (NF-kappaB). In in vitro experiments employing cultured primary macrophages from NF-kappaB/p50 KO mice, aspirin was not able to influence SR-BI expression. Additionally, no considerable effects on SR-BI expression were observed in vivo in resident macrophages of NF-kappaB/p50 KO mice orally treated with low or high doses of aspirin, respectively.

CONCLUSIONS

We suggest that aspirin treatment might lead to enhanced expression of SR-BI in human plaque macrophages and that this effect is dependent on the presence of NF-kappaB.

Serum amyloid A: an acute-phase protein involved in tumour pathogenesis

The synthesis of acute-phase protein serum amyloid A (SAA) is largely regulated by inflammation- associated cytokines and a high concentration of circulating SAA may represent an ideal marker for acute and chronic inflammatory diseases. However, SAA is also synthesized in extrahepatic tissues, e.g. human carcinoma metastases and cancer cell lines. An increasing body of in vitro data supports the concept of involvement of SAA in carcinogenesis and neoplastic diseases. Accumulating evidence suggests that SAA might be included in a group of biomarkers to detect a pattern of physiological events that reflect the growth of malignancy and host response. This review is meant to provide a broad overview of the many ways that SAA could contribute to tumour development, and accelerate tumour progression and metastasis, and to gain a better understanding of this acute-phase reactant as a possible link between chronic inflammation and neoplasia.

Phosphatidylethanol mediates its effects on the vascular endothelial growth factor via HDL receptor in endothelial cells

BACKGROUND

Previous epidemiological studies have shown that light to moderate alcohol consumption has protective effects against coronary heart disease but the mechanisms of the beneficial effect of alcohol are not known. Ethanol may increase high density lipoprotein (HDL) cholesterol concentration, augment the reverse cholesterol transport, or regulate growth factors or adhesion molecules. To study whether qualitative changes in HDL phospholipids mediate part of the beneficial effects of alcohol on atherosclerosis by HDL receptor, we investigated whether phosphatidylethanol (PEth) in HDL particles affects the secretion of vascular endothelial growth factor (VEGF) by a human scavenger receptor CD36 and LIMPII analog-I (CLA-1)-mediated pathway.

METHODS

Human EA.hy 926 endothelial cells were incubated in the presence of native HDL or PEth-HDL. VEGF concentration and CLA-1 protein expression were measured. Human CLA-1 receptor-mediated mechanisms in endothelial cells were studied using CLA-1 blocking antibody and protein kinase inhibitors.

RESULTS

Phosphatidylethanol-containing HDL particles caused a 6-fold increase in the expression of CLA-1 in endothelial cells compared with the effect of native HDL. That emergent effect was mediated mainly through protein kinase C and p44/42 mitogen-activated protein kinase pathways. PEth increased the secretion of VEGF and that increase could be abolished by a CLA-1 blocking antibody.

CONCLUSIONS

High density lipoprotein particles containing PEth bind to CLA-1 receptor and thereby increase the secretion of VEGF from endothelial cells. Ethanol-induced protective effects against coronary heart disease may be explained, at least partly, by the effects of PEth-modified HDL particles on VEGF via CLA-1-mediated mechanisms in endothelial cells.

Human neutrophil peptides: a novel potential mediator of inflammatory cardiovascular diseases

The traditional view of atherosclerosis has recently been expanded from a predominantly lipid retentive disease to a coupling of inflammatory mechanisms and dyslipidemia. Studies have suggested a novel role for polymorphonuclear neutrophil (PMN)-dominant inflammation in the development of atherosclerosis. Human neutrophil peptides (HNPs), also known as alpha-defensins, are secreted and released from PMN granules upon activation and are conventionally involved in microbial killing. Current evidence suggests an important immunomodulative role for these peptides. HNP levels are markedly increased in inflammatory diseases including sepsis and acute coronary syndromes. They have been found within the intima of human atherosclerotic arteries, and their deposition in the skin correlates with the severity of coronary artery diseases. HNPs form complexes with LDL in solution and increase LDL binding to the endothelial surface. HNPs have also been shown to contribute to endothelial dysfunction, lipid metabolism disorder, and the inhibition of fibrinolysis. Given the emerging relationship between PMN-dominant inflammation and atherosclerosis, HNPs may serve as a link between them and as a biological marker and potential therapeutic target in cardiovascular diseases including coronary artery diseases and acute coronary syndromes.

Influence of apolipoprotein E genotype and dietary α-tocopherol on redox status and C-reactive protein levels in apolipoprotein E3 and E4 targeted replacement mice

The molecular basis of the positive association between apoE4 genotype and CVD remains unclear. There is directin vitroevidence indicating that apoE4 is a poorer antioxidant relative to the apoE3 isoform, with some indirectin vivoevidence also available. Therefore it was hypothesised that apoE4 carriers may benefit from α-tocopherol (α-Toc) supplementation. Targeted replacement mice expressing the human apoE3 and apoE4 were fed with a diet poor (0 mg/kg diet) or rich (200 mg/kg diet) in α-Toc for 12 weeks. Neither apoE genotype nor dietary α-Toc exerted any effects on the antioxidant defence system, including glutathione, catalase, superoxide dismutase, glutathione peroxidase and glutathione reductase activities. In addition, no differences were observed in mitogen-induced lymphocyte proliferation. α-Toc concentrations were modestly higher in plasma and lower in tissues of apoE4 compared with apoE3 mice, with the greatest differences evident in the lung, suggesting that an apoE4 genotype may reduce α-Toc delivery to tissues. A tendency towards increased plasma F2-isoprostanes in apoE4 mice was observed, while liver thiobarbituric acid-reactive substances did not differ between apoE3 and apoE4 mice. In addition, C-reactive protein (CRP) concentrations were reduced in apoE4 mice indicating that this positive effect on CRP may in part negate the increased CVD risk associated with an apoE4 genotype.

Hypochlorite-modified high-density lipoprotein acts as a sink for myeloperoxidase in vitro

AIMS

Myeloperoxidase (MPO), a cardiovascular risk factor in humans, is an in vivo catalyst for lipoprotein modification via intermediate formation of reactive chlorinating species. Among the different lipoprotein classes, anti-atherogenic high-density lipoprotein (HDL) represents a major target for modification by hypochlorous acid (HOCl), generated from H2O2 by MPO in the presence of physiological chloride concentrations. As MPO was identified as an HDL-associated protein that could facilitate selective oxidative modification of its physiological carrier, the aim of the present study was to investigate whether and to what extent modification of HDL by HOCl affects the binding affinity of MPO in vitro.

METHODS AND RESULTS

We show that binding affinity of 125I-labelled MPO to HDL markedly increases as a function of increasing extent of HOCl modification of HDL. In contrast to native HDL, HOCl-HDL potently inhibits MPO binding/uptake by endothelial cells and effectively attenuates metabolism of MPO by macrophages. Reduction of HDL-associated chloramines with methionine strongly impaired binding affinity of MPO towards HOCl-HDL. This indicates that N-chloramines generated by HOCl are regulators of the high-affinity interaction between HOCl-HDL and positively charged MPO. Most importantly, the presence of HOCl-HDL is almost without effect on the halogenating activity of MPO.

CONCLUSION

We propose that MPO-dependent modification of HDL and concomitant increase in the binding affinity for MPO could generate a vicious cycle of MPO transport to and MPO-dependent modification at sites of chronic inflammation.

Ox-LDL induces monocyte-to-macrophage differentiation in vivo: Possible role for the macrophage colony stimulating factor receptor (M-CSF-R)

Monocyte-to-macrophage differentiation and LDL oxidation play a pivotal role in early atherogenesis. We thus questioned possible mechanisms for oxidized LDL (Ox-LDL)-induced monocyte-to-macrophage differentiation in vivo. Mouse peritoneal mononuclear cells, that were isolated 1, 2, or 3 days after Ox-LDL intraperitoneal injection, gradually exhibited the characteristic macrophage morphology, along with the expression of the cell-surface antigen CD11b. Molecular mechanisms involved in Ox-LDL-induced differentiation were further investigated in vitro using the THP-1 monocytic cell line. THP-1 cells incubated with Ox-LDL in the presence of as low as 1 ng/ml of PMA differentiated into macrophages, as evidenced by morphologic, phenotypic, and functional properties. Stimulation of monocyte-to-macrophage differentiation was selective to Ox-LDL (and not native LDL), was dependent on the extent of LDL oxidation, and required Ox-LDL internalization by the cells. These effects of Ox-LDL could be attributed to its major oxysterols, 7-ketocholesterol and 7beta-hydroxycholesterol. Finally, the stimulation of monocyte differentiation to macrophages by Ox-LDL was shown to require the M-CSF-receptor, since blocking the binding to the receptor abolished Ox-LDL/7beta-hydroxycholesterol-induced differentiation. Furthermore, Ox-LDL/7beta-hydroxycholesterol elicited tyrosine phosphorylation and activation of the M-CSF-R. We thus conclude that Ox-LDL induces monocyte differentiation to macrophages in vivo and this phenomenon involves activation of the M-CSF-receptor.

Distinct mechanisms for OxLDL uptake and cellular trafficking by class B scavenger receptors CD36 and SR-BI

Modified forms of LDL, including oxidized low density lipoprotein (OxLDL), contribute to macrophage lipid accumulation in the vessel wall. Despite the pathophysiological importance of uptake pathways for OxLDL, the molecular details of OxLDL endocytosis by macrophages are not well understood. Studies in vitro demonstrate that the class B scavenger receptor CD36 mediates macrophage uptake and degradation of OxLDL. Although the closely related scavenger receptor class B type I (SR-BI) binds OxLDL with high affinity, evidence that SR-BI plays a role in OxLDL metabolism is lacking. In this study, we directly compared OxLDL uptake and degradation by CD36 and SR-BI. Our results indicate that although CD36 and SR-BI internalize OxLDL, SR-BI mediates significantly less OxLDL degradation. Endocytosis of OxLDL by both SR-BI and CD36 is independent of caveolae, microtubules, and actin cytoskeleton. However, OxLDL uptake by CD36, but not SR-BI, is dependent on dynamin. The analysis of chimeric SR-BI/CD36 receptors shows that the CD36 C-terminal cytoplasmic tail is necessary and sufficient for dynamin-dependent OxLDL internalization by class B scavenger receptors. These findings indicate that different mechanisms are involved in OxLDL uptake by SR-BI and CD36, which may segregate these two structurally homologous receptors at the cell surface, leading to differences in intracellular trafficking and degradation.

Cellular, molecular and clinical aspects of vitamin E on atherosclerosis prevention

Randomised clinical trials and epidemiologic studies addressing the preventive effects of vitamin E supplementation against cardiovascular disease reported both positive and negative effects, and recent meta-analyses of the clinical studies were rather disappointing. In contrast to that, many animal studies clearly show a preventive action of vitamin E in several experimental settings, which can be explained by the molecular and cellular effects of vitamin E observed in cell cultures. This review is focusing on the molecular effects of vitamin E on the cells playing a role during atherosclerosis, in particular on the endothelial cells, vascular smooth muscle cells, monocytes/macrophages, T cells, and mast cells. Vitamin E may act by normalizing aberrant signal transduction and gene expression in antioxidant and non-antioxidant manners; in particular, over-expression of scavenger receptors and consequent foam cell formation can be prevented by vitamin E. In addition to that, the cellular effects of alpha-tocopheryl phosphate and of EPC-K1, a composite molecule between alpha-tocopheryl phosphate and l-ascorbic acid, are summarized.

Soluble RAGE blocks scavenger receptor CD36-mediated uptake of hypochlorite-modified low-density lipoprotein

Engagement of the receptor for advanced glycation end products (RAGE) by its signal transduction ligands evokes inflammatory cell infiltration and activation of the vessel wall. However, soluble RAGE (sRAGE), the truncated form spanning the extracellular binding domain of RAGE, has potent anti-inflammatory properties by acting as a decoy for RAGE ligands. We now show that sRAGE binds with high affinity to atherogenic low-density lipoprotein (LDL) modified by hypochlorous acid (HOCl), the major oxidant generated by the myeloperoxidase-H2O2-chloride system of phagocytes activated during inflammation. We further demonstrate that sRAGE can be coprecipitated with HOCl-LDL from spiked serum. To determine the functional significance of sRAGE binding to HOCl-LDL, cell association studies with macrophages were performed. sRAGE effectively inhibited cellular uptake of HOCl-LDL and subsequent lipid accumulation. Using Chinese hamster ovary cells overexpressing class B scavenger receptor CD36 or SR-BI, two preferential scavenger receptors for HOCl-LDL, we demonstrate that sRAGE only interferes with CD36-mediated uptake of HOCl-LDL. The present findings indicate that sRAGE acts as a sink for HOCl-LDL, which is abundantly present in human atherosclerotic lesions. We propose that sRAGE represents a physiological antagonist that interferes with scavenger receptor-mediated cholesterol accumulation and foam cell formation of macrophages.

Adiponectin accelerates reverse cholesterol transport by increasing high density lipoprotein assembly in the liver

Plasma high density lipoprotein (HDL)-cholesterol levels are negatively correlated with the incidence of coronary artery disease. HDL plays an important role in protecting against atherosclerosis by removing cholesterol from atheroma and transporting it back to the liver. The ATP-binding cassette transporters (ABCA1 and ABCG1) and scavenger receptor BI (SR-BI) are thought to be one of the rate-limiting factors to generate HDL in the liver. Adiponectin (APN) secreted from adipocytes is also one of the important molecules to inhibit the development of atherosclerosis. Recently, it has been reported that plasma HDL-cholesterol levels are positively correlated with plasma APN concentrations in humans. Therefore, we investigated the association of APN with HDL assembly in the liver. Human hepatoma cell line, HepG2 cells, were incubated for 24h in the culture medium with the indicated concentrations of recombinant APN. APN enhanced the mRNA level of apolipoprotein A-I (apoA-I) in HepG2 cells and increased the secretion of apoA-I from the cells to the medium. Furthermore, APN increased both mRNA and protein levels of ABCA1, but not ABCG1 and SR-BI, in HepG2 cells. Taken together, the current study demonstrates that APN might protect against atherosclerosis by increasing HDL assembly through enhancing ABCA1 pathway and apoA-1 synthesis in the liver.

Glycoxidised LDL isolated from subjects with impaired glucose tolerance increases CD36 and peroxisome proliferator-activator receptor gamma gene expression in macrophages

AIMS/HYPOTHESIS

Glycoxidised LDL has been implicated in the pathogenesis of atherosclerosis, a major complication of diabetes. Since atherogenesis may occur at an early stage of diabetes, we investigated whether circulating LDL isolated from subjects with IGT (n = 20) showed an increased glycoxidation status and explored the proatherogenic effects of LDL samples on macrophages.

SUBJECTS AND METHODS

We investigated LDL modifications using GC-MS. Murine macrophages were incubated with LDL samples for 1 h, and then mRNA expression rates of the scavenger receptors CD36 and scavenger receptor class B type 1 (SCARB1, formerly known as SR-BI) and transcription factor peroxisome proliferator-activator receptor gamma (PPARgamma) were quantified by real-time RT-PCR.

RESULTS

The GC-MS experiments revealed that oxidative modifications of proline, arginine, lysine and tyrosine residues in apolipoprotein B100 were three- to fivefold higher in LDL samples from IGT subjects compared with those from NGT subjects (n = 20). Moreover, LDL glycoxidation estimated by both Nepsilon-(carboxymethyl)lysine (CML) and Nepsilon-(carboxyethyl)lysine (CEL) residues was increased more than ninefold in LDL from IGT subjects compared with samples from NGT subjects. Compared with NGT LDL, IGT LDL elicited a significantly higher CD36 (p < 0.05) and PPARG (p < 0.05) gene expression, whereas SCARB1 mRNA expression was not affected.

CONCLUSIONS/INTERPRETATION

These data suggest that IGT is associated with increased glycoxidation of circulating LDL, which might contribute to the conversion of macrophages into a proatherogenic phenotype.