Scavenging new insights into atherogenesis

Protective role of thymoquinone in hyperlipidemia-induced liver injury in LDL-R-/-mice

BACKGROUND

Hyperlipidemia, a heterogeneous group of disorders characterized by elevated plasma lipids in the blood, causes severe health problems, leading to fatty liver disease and nonalcoholic fatty liver disease. Thymoquinone, the major active chemical component of Nigella sativa, reportedly exerts a vast array of biological effects. Various studies have reported that Thymoquinone protects against liver injury.

AIMS

The aim of this study was to investigate the possible protective effects of Thymoquinone against liver injury in hyperlipidemia-induced LDL-R-/- mice.

METHODS

Eight-week-old male LDL-R-/- mice were randomly divided into three groups: a control group fed a normal diet and two groups fed a high-cholesterol diet or high-cholesterol diet mixed with Thymoquinone. All groups were fed different diets for 8 weeks. Blood samples were obtained from the inferior vena cava and collected in serum tubes. The samples were then stored at - 80 °C until used. Longitudinal sections of liver tissues were fixed in 10% formalin and then embedded in paraffin for histological evaluation. The remainder of the liver tissues were snap-frozen in liquid nitrogen for reverse transcription-polymerase chain reaction or western blotting.

RESULTS

Our results demonstrated that Thymoquinone administration significantly reduced liver histological alterations by hyperlipidemia. Thymoquinone mitigated hyperlipidemia-induced liver injury as indicated by the suppression of metabolic characteristics, liver biochemical parameters, pyroptosis indicators, a macrophage marker, and the phosphatidylinositide 3-kinase signaling pathway.

CONCLUSIONS

Thymoquinone is a potential therapeutic agent for hyperlipidemia-induced liver injury.

Plasma Membrane Localization of CD36 Requires Vimentin Phosphorylation; A Mechanism by Which Macrophage Vimentin Promotes Atherosclerosis

Vimentin is a type III intermediate filament protein expressed in cells of mesenchymal origin. Vimentin has been thought to function mainly as a structural protein and roles of vimentin in other cellular processes have not been extensively studied. Our current study aims to reveal functions of vimentin in macrophage foam cell formation, the critical stage of atherosclerosis. We demonstrated that vimentin null (Vim -/ - ) mouse peritoneal macrophages take up less oxidized LDL (oxLDL) than vimentin wild type (Vim +/+) macrophages. Despite less uptake of oxLDL in Vim -/ - macrophages, Vim +/+ and Vim -/ - macrophages did not show difference in expression of CD36 known to mediate oxLDL uptake. However, CD36 localized in plasma membrane was 50% less in Vim -/ - macrophages than in Vim +/+ macrophages. OxLDL/CD36 interaction induced protein kinase A (PKA)-mediated vimentin (Ser72) phosphorylation. Cd36 -/ - macrophages did not exhibit vimentin phosphorylation (Ser72) in response to oxLDL. Experiments using phospho-mimetic mutation of vimentin revealed that macrophages with aspartate-substituted vimentin (V72D) showed more oxLDL uptake and membrane CD36. LDL receptor null (Ldlr -/ - ) mice reconstituted with Vim -/ - bone marrow fed a western diet for 15 weeks showed 43% less atherosclerotic lesion formation than Ldlr -/ - mice with Vim +/+ bone marrow. In addition, Apoe -/ -Vim- / - (double null) mice fed a western diet for 15 weeks also showed 57% less atherosclerotic lesion formation than Apoe -/ - and Vim +/+mice. We concluded that oxLDL via CD36 induces PKA-mediated phosphorylation of vimentin (Ser72) and phosphorylated vimentin (Ser72) directs CD36 trafficking to plasma membrane in macrophages. This study reveals a function of vimentin in CD36 trafficking and macrophage foam cell formation and may guide to establish a new strategy for the treatment of atherosclerosis.

Deciphering Macrophage Phenotypes upon Lipid Uptake and Atherosclerosis

In the progression of atherosclerosis, macrophages are the key immune cells for foam cell formation. During hyperlipidemic condition, phagocytic cells such as monocytes and macrophages uptake oxidized low-density lipoproteins (oxLDLs) accumulated in subintimal space, and lipid droplets are accumulated in their cytosols. In this review, we discussed the characteristics and phenotypic changes of macrophages in atherosclerosis and the effect of cytosolic lipid accumulation on macrophage phenotype. Due to macrophage plasticity, the inflammatory phenotypes triggered by oxLDL can be re-programmed by cytosolic lipid accumulation, showing downregulation of NF-κB activation followed by activation of anti-inflammatory genes, leading to tissue repair and homeostasis. We also discuss about various in vivo and in vitro models for atherosclerosis research and next generation sequencing technologies for foam cell gene expression profiling. Analysis of the phenotypic changes of macrophages during the progression of atherosclerosis with adequate approach may lead to exact understandings of the cellular mechanisms and hint therapeutic targets for the treatment of atherosclerosis.

Lipid Accumulation and Chronic Kidney Disease

Obesity and hyperlipidemia are the most prevalent independent risk factors of chronic kidney disease (CKD), suggesting that lipid accumulation in the renal parenchyma is detrimental to renal function. Non-esterified fatty acids (also known as free fatty acids, FFA) are especially harmful to the kidneys. A concerted, increased FFA uptake due to high fat diets, overexpression of fatty acid uptake systems such as the CD36 scavenger receptor and the fatty acid transport proteins, and a reduced β-oxidation rate underlie the intracellular lipid accumulation in non-adipose tissues. FFAs in excess can damage podocytes, proximal tubular epithelial cells and the tubulointerstitial tissue through various mechanisms, in particular by boosting the production of reactive oxygen species (ROS) and lipid peroxidation, promoting mitochondrial damage and tissue inflammation, which result in glomerular and tubular lesions. Not all lipids are bad for the kidneys: polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) seem to help lag the progression of chronic kidney disease (CKD). Lifestyle interventions, especially dietary adjustments, and lipid-lowering drugs can contribute to improve the clinical outcome of patients with CKD.

Coenzyme Q10 protects against hyperlipidemia-induced cardiac damage in apolipoprotein E-deficient mice

BACKGROUND

Hyperlipidemia is a well-established risk factor for cardiac damage, which can lead to cardiovascular diseases. Many studies have shown that Coenzyme Q10(CoQ10) protects against cardiac damage in vivo. The aim of this study was to investigate the possible protective effects of CoQ10 against cardiac damage in apolipoprotein E-deficient (ApoE^-/-) mice.

METHODS

Eight-week-old male C57BL/6 and ApoE^-/- mice were randomly divided into four groups: C57BL/6 mice fed a normal diet (C57BL/6 group); C57BL/6 mice fed a normal diet + CoQ10 (C57BL/6 + CoQ10 group); ApoE^-/- mice fed a high-fat diet (ApoE^-/- HD group), and ApoE^-/- mice fed a high-fat diet + CoQ10 (ApoE^-/- HD + CoQ10 group). All groups were fed the different diets for 16 weeks. Blood samples were obtained from the inferior vena cava and collected in serum tubes. The samples were then stored at - 80 °C until used. Coronal sections of heart tissues were fixed in 10% formalin and then embedded in paraffin for histological evaluation. The remainder of the heart tissues was snap-frozen in liquid nitrogen for mRNA or immunohistochemical analysis.

RESULTS

The metabolic parameters such as total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-c), and triglycerides (TG) levels were lower in ApoE^-/-HD + CoQ10 mice than in ApoE^-/- HD mice. There were significant pathophysiological changes (H&E, PAS, Masson and CD68 staining) in ApoE^-/- mice in the HD group compared with those in the HD + CoQ10 group. CoQ10 reduced HD-induced cardiac tissue damage via autophagy (p62 and LC3), as evidenced by immunoblotting, immunohistochemistry, and RT-qPCR. CoQ10 also inhibited inflammation (IL-6 and TNF-α) gene expression in ApoE^-/- mice.

CONCLUSIONS

These results indicate that CoQ10 is a potential therapeutic target for cardiac damage caused by hyperlipidemia.

Thymoquinone reduces cardiac damage caused by hypercholesterolemia in apolipoprotein E-deficient mice

BACKGROUND

Hypercholesterolemia is a well-established risk factor for cardiac damage, which can lead to cardiovascular diseases. Many studies have shown that thymoquinone protected rats from doxorubicin-induced cardiotoxicity and cardiac damage. The aim of this study was to investigate the possible protective effects of thymoquinone against cardiac damage in apolipoprotein E knockout (ApoE-/-) mice.

METHODS

Eight-week-old male ApoE-/- mice were randomly divided into three groups: control group fed a normal diet (ND group), a high cholesterol diet (HD group) or HD mixed with thymoquinone (HD + TQ group). All groups were fed the different diets for 8 weeks. Blood samples were obtained from the inferior vena cava and collected in serum tubes. The samples were then stored at - 80 °C until used. Coronal sections of heart tissues were fixed in 10% formalin and then embedded in paraffin for histological evaluation. The remainder of the heart tissues was snap-frozen in liquid nitrogen for mRNA or immunohistochemical analysis.

RESULTS

The metabolic characteristics of total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-c), and high-sensitivity C-reactive protein (hs-CRP) were lower in ApoE-/-HD + TQ mice than in ApoE-/- HD mice. Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) gene and protein expression was lower in the heart tissue of ApoE-/-HD + TQ mice than in those of ApoE-/-HD mice. Furthermore, the levels of macrophages and pro-inflammatory cytokines were lower in the cardiac tissues of ApoE-/-HD + TQ mice than in those of ApoE-/-HD mice.

CONCLUSIONS

These results indicate that thymoquinone may provide a potential therapeutic target for cardiac damage caused by hypercholesterolemia.

Thymoquinone reduces kidney damage in apolipoprotein E-deficient mice fed a high-cholesterol diet

Hypercholesterolemia is a well-established risk factor for kidney injury that can lead to chronic kidney disease (CKD). Thymoquinone may be a potential therapeutic agent for kidney damage from hypercholesterolemia.

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

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

Porphyromonas gingivalis lipopolysaccharide increases lipid accumulation by affecting CD36 and ATP-binding cassette transporter A1 in macrophages

Porphyromonas gingivalis lipopolysaccharide (P. gingivalis LPS) promotes macrophage-derived foam cell formation, however, the mechanisms are not well established. In macrophages, lipid uptake is mediated by scavenger receptors including SR-A and CD36, while the cholesterol efflux is mediated by ATP-binding cassette transporter G1 (ABCG1), ABCA1 and SR-BI. We further investigated the mechanisms underlying the dysregulation by P. gingivalis LPS of these regulators resulting in the promotion of lipid accumulation in THP-1-derived macrophages. Our results showed that P. gingivalis LPS exacerbated lipid accumulation in oxidized low-density lipoprotein (oxLDL)-treated macrophages. However, cholesterol efflux was inhibited by P. gingivalis LPS in THP-1-derived macrophages. In oxLDL-untreated macrophages, P. gingivalis LPS treatment caused an increase in CD36 mRNA and protein levels, and a decrease in ABCA1 mRNA and protein levels, while having no effect on SR-A, SR-BI or ABCG1 expression. Upregulation of CD36 by P. gingivalis LPS resulted from activation of c-Jun/AP-1, and this was confirmed by the inhibition of increased CD36 expression after AP-1 inhibition using SP600125. However, the decreased protein stability of ABCA1 by P. gingivalis LPS was a result of increased calpain activity. Moreover, small hairpin RNA (shRNA) targeting heme oxygenase-1 (HO-1) augmented the P. gingivalis LPS-induced atherogenic effects on the expression of c-Jun/AP-1, CD36, ABCA1 and calpain activity. Accordingly, P. gingivalis LPS-regulated promotion of lipid accumulation in foam cells was also exacerbated by HO-1 shRNA. These results indicate that P. gingivalis LPS confers a exacerbation effect on the formation of foam cells by a novel HO-1-dependent mediation of cholesterol efflux and lipid accumulation in macrophages.

Highly upregulated expression of CD36 and MSR1 in circulating monocytes of patients with acute coronary syndromes

Acute Coronary Syndromes (ACS) are a group of disorders caused by the significant reduction of circulation in coronary arteries. The most common reason of the dysfunction is a blood clot formed in place of plaque rupture. The role of scavenger receptors in development and progression of atherosclerosis has been confirmed in many animal experiments, however the knowledge about contribution of the receptors in the development of ACS symptoms in humans still remains insufficient. The aim of this work was to define the expression of two scavenger receptors: CD36 and MSR1 in monocytes of patients with ACS after the onset of symptoms and after the 6 months of treatment. The analysis of CD36 and MSR1 expression was carried out with the use of real-time PCR and flow cytometry. Analyses of lipid and glucose concentration in blood and the level of inflammatory markers in plasma were performed additionally for all ACS patients. All data obtained during the research were analyzed using statistical tests, such as Mann Whitney test, Wilcoxon test, or correlation. In all patients with symptoms of ACS the amount of CD36 and MSR1 mRNA in circulating monocytes, as well as the density of both receptors on the cells surface was significantly higher. Re-analysis of subjects after 6 months of treatment, showed a significant decrease in the CD36 and MSR1 expression in all patients who received atorvastatin. The results of presented studies demonstrate that both investigated receptors are involved in the development and/or progression of ACS.

Insulin promotes macrophage foam cell formation: potential implications in diabetes-related atherosclerosis

The prevalence of atherosclerotic cardiovascular disease is higher in patients with type 2 diabetes, a disorder characterized by hyperinsulinemia and insulin resistance. The role of hyperinsulinemia as an independent participant in the atherogenic process has been controversial. In the current study, we tested the effect of insulin and the insulin sensitizer, adiponectin, on human macrophage foam cell formation. We found that both insulin and adiponectin increased the expression of the type 2 scavenger receptor CD36 by approximately twofold and decreased the expression of the ATP-binding cassette transporter ABCA1 by >80%. In both cases regulation was post-transcriptional. As a consequence of these changes, we found that oxidized LDL (oxLDL) uptake was increased by 80% and cholesterol efflux to apolipoprotein A1 (apoA1) was decreased by ∼25%. This led to two- to threefold more cholesterol accumulation over a 16-h period. As reported previously in studies of murine systems, scavenger receptor-A (SR-A) expression on human macrophages was downregulated by insulin and adiponectin. Insulin and adiponectin did not affect oxLDL-induced secretion of monocyte attractant protein-1 (MCP-1) and interleukin-6 (IL-6). These studies suggest that hyperinsulinemia could promote macrophage foam cell formation and thus may contribute to atherosclerosis in patients with type 2 diabetes.

Perilipin-1 in hemodialyzed patients: association with history of coronary heart disease and lipid profile

Perilipin-1 surrounds lipid droplets in both adipocytes and in atheroma plaque foam cells and controls access of lipases to the lipid core. In hemodialysis (HD) patients, dyslipidemia, malnutrition, inflammation and atherosclerosis are common. Thirty-six HD patients and 28 healthy volunteers were enrolled into the study. Ten HD patients suffered from coronary heart disease (CHD). Perilipin-1, triglycerides, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol (HDL-C), body mass index, albumin, geriatric nutritional risk index, normalized protein catabolic rate, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were measured. Perilipin-1 did not differ between HD patients and healthy volunteers. IL-6 and TNF-α were higher in HD patients. The evaluated nutritional markers and the markers of inflammation did not differ between HD patients with high perilipin-1 levels and HD patients with low perilipin-1 levels. Regarding the lipid profile, only HDL-C differed between HD patients with high perilipin-1 levels and HD patients with low perilipin-1 levels, and it was higher in the first subgroup. Perilipin-1 was significantly higher in HD patients without CHD. Perilipin-1 is detectable in the serum of HD patients and it is associated with increased HDL-C and decreased incidence of CHD.

In vivo examination of 111In-bis-5HT-DTPA to target myeloperoxidase in atherosclerotic ApoE knockout mice

PURPOSE

The aim of the study is to assess the feasibility of imaging specific activity of myeloperoxidase (MPO), a leukocyte-derived enzyme with important role in atherosclerosis, by SPECT/CT using a novel radiotracer, (111)In-bis-5-hydroxytryptamide-diethylenetriamine-pentaacetate ((111)In-bis-5HT-DTPA).

METHODS

Bis-5HT-DTPA was synthesized. Oligomerization of bis-5HT-DTPA in the presence of MPO/H(2)O(2) was studied and confirmed using MALDI-TOF. Apolipoprotein E knockout (ApoE KO) mice was used as an atherosclerosis-prone rodent model. Biodistribution assay and micro SPECT/CT imaging were carried out to prove the atherosclerosis targeting of (111)In-bis-5HT-DTPA in the ApoE KO mice.

RESULTS

MALDI-TOF spectrum showed that the 5HT base agent can self oligomerize after activating by MPO. From the biodistribution study, (111)In-bis-5HT-DTPA was quantified to be retained markedly higher while eliminated much slower in the aortas of the ApoE KO mice than that of the wild type (WT) mice within 1 h post-injection. The nuclear imaging showed significantly higher uptake in the aorta of the ApoE KO mice than that of the WT mice at least within 2 h post-injection.

CONCLUSION

This study described the pharmacokinetics and biodistribution of (111)In-bis-5HT-DTPA in ApoE KO mice and validated its utilization for early detection of atherosclerotic marker, MPO, in the aortic wall of atherosclerosis-prone rodent model.

Solid-phase immunoglobulins IgG and IgM activate macrophages with solid-phase IgM acting via a novel scavenger receptor a pathway

IgG may accelerate atherosclerosis via ligation of proinflammatory Fcγ receptors; however, IgM is unable to ligate FcγR and is often considered vasculoprotective. IgM aggravates ischemia-reperfusion injury, and solid-phase deposits of pure IgM, as seen with IgM-secreting neoplasms, are well known clinically to provoke vascular inflammation. We therefore examined the molecular mechanisms by which immunoglobulins can aggravate vascular inflammation, such as in atherosclerosis. We compared the ability of fluid- and solid-phase immunoglobulins to activate macrophages. Solid-phase immunoglobulins initiated prothrombotic and proinflammatory functions in human macrophages, including NF-κB p65 activation, H(2)O(2) secretion, macrophage-induced apoptosis, and tissue factor expression. Responses to solid-phase IgG (but not to IgM) were blocked by neutralizing antibodies to CD16 (FcγRIII), consistent with its known role. Macrophages from mice deficient in macrophage scavenger receptor A (SR-A; CD204) had absent IgM binding and no activation by solid-phase IgM. RNA interference-mediated knockdown of SR-A in human macrophages suppressed activation by solid-phase IgM. IgM binding to SR-A was demonstrated by both co-immunoprecipitation studies and the binding of fluorescently labeled IgM to SR-A-transfected cells. Immunoglobulins on solid-phase particles around macrophages were found in human plaques, increased in ruptured plaques compared with stable ones. These observations indicate that solid-phase IgM and IgG can activate macrophages and destabilize vulnerable plaques. Solid-phase IgM activates macrophages via a novel SR-A pathway.

Non-Gaussianity and dynamical trapping in locally activated random walks

We propose a minimal model of locally activated diffusion, in which the diffusion coefficient of a one-dimensional Brownian particle is modified in a prescribed way-either increased or decreased-upon each crossing of the origin. Such a local mobility decrease arises in the formation of atherosclerotic plaques due to diffusing macrophage cells accumulating lipid particles. We show that spatially localized mobility perturbations have remarkable consequences on diffusion at all scales, such as the emergence of a non-Gaussian multipeaked probability distribution and a dynamical transition to an absorbing static state. In the context of atherosclerosis, this dynamical transition can be viewed as a minimal mechanism that causes macrophages to aggregate in lipid-enriched regions and thereby to the formation of atherosclerotic plaques.

Current Concepts of the Role of Oxidized LDL Receptors in Atherosclerosis

Atherosclerosis is characterized by accumulation of lipids and inflammatory cells in the arterial wall. Oxidized low-density lipoprotein (ox-LDL) plays important role in the genesis and progression of atheromatous plaque. Various scavenger receptors have been recognized in the past two decades that mediate uptake of ox-LDL leading to formation of foam cells. Inhibition of scavenger receptor A and CD36 has been shown to affect progression of atherosclerosis by decreasing foam cell formation. Lectin-type oxidized LDL receptor 1 (LOX-1) participates at various steps involved in the pathogenesis of atherosclerosis, and in experimental studies its blockade has been shown to affect the progression of atherosclerosis at multiple levels. In this review, we summarize the role of ox-LDL and scavenger receptors in the formation of atheroma with emphasis on effects of LOX-1 blockade.

Increased atherosclerosis in mice deficient in perilipin1

Background

Perilipin1, a lipid droplet associated protein has an important role in the regulation of lipolysis and lipid storage in adipocytes. Perilipin1 is also expressed in foam cells of atheroma plaques and could therefore play a role in the accumulation of lipids in arterial wall and in the development of atherosclerosis. The aim of the study was to investigate this possible role of perilipin1 in atherogenesis.

Methods

Mice deficient in perilipin1 (Plin1-/-) were crossed with Ldlr-/- mice. Ldlr-/- and Plin1-/- Ldlr-/- mice received an atherogenic diet during 10 or 20 weeks. Blood pressure and plasma lipids concentrations were measured. Aortas were collected at the end of the atherogenic diet periods for quantification of atheroma lesions (en face method), histological and immunohistological studies

Results

Ldlr-/- and Plin1-/- Ldlr-/- mice had comparable blood pressure and plasma lipids levels. Plin1-/- Ldlr-/- mice had a lower body weight and decreased adiposity. The atherosclerotic lesion area in Plin1-/-Ldlr-/- mice was moderately increased after 10 weeks of atherogenic diet (ns) and significantly higher after 20 weeks (p < 0.01). Histology of atheroma plaques was comparable with no sign of increased inflammation in Plin1-/- Ldlr-/- mice.

Conclusion

Perilipin1 ablation in mice results in increased atherosclerosis independently of modifications of risk factors such as raised blood pressure or plasma lipids levels. These data strongly support an atheroprotective role for perilipin1.

Endothelin-1 exacerbates lipid accumulation by increasing the protein degradation of the ATP-binding cassette transporter G1 in macrophages

Endothelin-1 (ET-1), a potent proatherogenic vasoconstrictive peptide, is known to promote macrophage foam cell formation via mechanisms that are not fully understood. Excessive lipid accumulation in macrophages is a major hallmark during the early stages of atherosclerotic lesions. Cholesterol homeostasis is tightly regulated by scavenger receptors (SRs) and ATP-binding cassette (ABC) transporters during the transformation of macrophage foam cells. The aim of this study was to investigate the possible mechanisms by which ET-1 affects lipid accumulation in macrophages. Our results demonstrate that oxidized low-density lipoprotein (oxLDL) treatment increases lipid accumulation in rat bone marrow-derived macrophages. Combined treatment with ET-1 and oxLDL significantly exacerbated lipid accumulation in macrophages as compared to treatment with oxLDL alone. The results of Western blotting show that ET-1 markedly decreased the ABCG1 levels via ET type A and B receptors and activation of the phosphatidylinositol 3-kinase pathway; however, ET-1 had no effect on the protein expression of CD36, SR-BI, SR-A, or ABCA1. In addition, real-time PCR analysis showed that ET-1 treatment did not affect ABCG1 mRNA expression. We also found that ET-1 decreases ABCG1 possibly due to the enhancement of the proteosome/calpain pathway-dependent degradation of ABCG1. Moreover, ET-1 significantly reduced the efficiency of the cholesterol efflux in macrophages. Taken together, these findings suggest that ET-1 may impair cholesterol efflux and further exacerbate lipid accumulation during the transformation of macrophage foam cells.

Anti-atherogenic effect of berberine on LXRalpha-ABCA1-dependent cholesterol efflux in macrophages

Berberine, a botanical alkaloid purified from Cortidis rhizoma, has effects in cardiovascular diseases, yet the mechanism is not fully understood. Foam cells play a critical role in the progression of atherosclerosis. This study aimed to investigate the effect of berberine on the formation of foam cells by macrophages and the underlying mechanism. Treatment with berberine markedly suppressed oxidized low-density lipoprotein (oxLDL)-mediated lipid accumulation, which was due to an increase in cholesterol efflux. Berberine enhanced the mRNA and protein expression of ATP-binding membrane cassette transport protein A1 (ABCA1) but did not alter the protein level of ABCG1 or other scavenger receptors. Additionally, functional inhibition of ABCA1 with a pharmacological inhibitor or neutralizing antibody abrogated the effects of berberine on cholesterol efflux and lipid accumulation. Moreover, berberine induced the nuclear translocation and activation of liver X receptor alpha (LXRalpha) but not its protein expression. Knockdown of LXRalpha mRNA expression by small interfering RNA abolished the berberine-mediated protective effects on ABCA1 protein expression and oxLDL-induced lipid accumulation in macrophages. These data suggest that berberine abrogates the formation of foam cells by macrophages by enhancing LXRalpha-ABCA1-dependent cholesterol efflux.

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

Hydrolysis and oxidation of LDL stimulate LDL entrapment in the arterial wall and promote inflammation and atherosclerosis via various mechanisms including lipoprotein fusion and lipid droplet formation. To determine the effects of FFA on these transitions, we hydrolyzed LDL by phospholipase A(2) (PLA(2)), removed FFA by albumin, and analyzed structural stability of the modified lipoproteins. Earlier, we showed that heating induces LDL remodeling, rupture, and coalescence into lipid droplets resembling those found in atherosclerotic lesions. Here, we report how FFA affect these transitions. Circular dichroism showed that mild LDL lipolysis induces partial β-sheet unfolding in apolipoprotein B. Electron microscopy, turbidity, and differential scanning calorimetry showed that mild lipolysis promotes LDL coalescence into lipid droplets. FFA removal by albumin restores LDL stability but not the protein conformation. Consequently, FFA enhance LDL coalescence into lipid droplets. Similar effects of FFA were observed in minimally oxidized LDL, in LDL enriched with exogenous FFA, and in HDL and VLDL. Our results imply that FFA promote lipoprotein coalescence into lipid droplets and explain why LDL oxidation enhances such coalescence in vivo but hampers it in vitro. Such lipid droplet formation potentially contributes to the pro-atherogenic effects of FFA.

EGb761 ameliorates the formation of foam cells by regulating the expression of SR-A and ABCA1: role of haem oxygenase-1

AIMS

Accumulation of foam cells in the intima is a hallmark of early-stage atherosclerotic lesions. Ginkgo biloba extract (EGb761) has been reported to exert anti-oxidative and anti-inflammatory properties in atherosclerosis, yet the significance and the molecular mechanisms of action of EGb761 in the formation of macrophage foam cells are not fully understood.

METHODS AND RESULTS

Treatment with EGb761 resulted in a dose-dependent decrease in oxidized low-density lipoprotein (oxLDL)-mediated cholesterol accumulation in macrophages, a consequence that was due to a decrease in cholesterol uptake and an increase in cholesterol efflux. Additionally, EGb761 significantly down-regulated the mRNA and protein expression of class A scavenger receptor (SR-A) by decreasing expression of activator protein 1 (AP-1); however, EGb761 increased the protein stability of ATP-binding cassette transporter A1 (ABCA1) by reducing calpain activity without affecting ABCA1 mRNA expression. Small interfering RNA (siRNA) targeting haem oxygenase-1 (HO-1) abolished the EGb761-induced protective effects on the expression of AP-1, SR-A, ABCA1, and calpain activity. Accordingly, EGb761-mediated suppression of lipid accumulation in foam cells was also abrogated by HO-1 siRNA. Moreover, the lesion size of atherosclerosis was smaller in EGb761-treated, apolipoprotein E-deficient mice compared with the vehicle-treated mice, and the expression of HO-1, SR-A, and ABCA1 in aortas was modulated similar to that observed in macrophages.

CONCLUSION

These findings suggest that EGb761 confers a protection from the formation of foam cells by a novel HO-1-dependent regulation of cholesterol homeostasis in macrophages.

Niacin improves renal lipid metabolism and slows progression in chronic kidney disease

BACKGROUND

Mounting evidence points to lipid accumulation in the diseased kidney and its contribution to progression of nephropathy. We recently found heavy lipid accumulation and marked dysregulation of lipid metabolism in the remnant kidneys of rats with chronic renal failure (CRF). Present study sought to determine efficacy of niacin supplementation on renal tissue lipid metabolism in CRF.

METHODS

Kidney function, lipid content, and expression of molecules involved in cholesterol and fatty acid metabolism were determined in untreated CRF (5/6 nephrectomized), niacin-treated CRF (50 mg/kg/day in drinking water for 12 weeks) and control rats.

RESULTS

CRF resulted in hypertension, proteinuria, renal tissue lipid accumulation, up-regulation of scavenger receptor A1 (SR-A1), acyl-CoA cholesterol acyltransferase-1 (ACAT1), carbohydrate-responsive element binding protein (ChREBP), fatty acid synthase (FAS), acyl-CoA carboxylase (ACC), liver X receptor (LXR), ATP binding cassette (ABC) A-1, ABCG-1, and SR-B1 and down-regulation of sterol responsive element binding protein-1 (SREBP-1), SREBP-2, HMG-CoA reductase, PPAR-alpha, fatty acid binding protein (L-FABP), and CPT1A. Niacin therapy attenuated hypertension, proteinuria, and tubulo-interstitial injury, reduced renal tissue lipids, CD36, ChREBP, LXR, ABCA-1, ABCG-1, and SR-B1 abundance and raised PPAR-alpha and L-FABP.

CONCLUSIONS AND GENERAL SIGNIFICANCE

Niacin administration improves renal tissue lipid metabolism and renal function and structure in experimental CRF.

Interleukin-10 facilitates both cholesterol uptake and efflux in macrophages

Foam cell formation is a hallmark event during atherosclerosis. The current paradigm is that lipid uptake by scavenger receptor in macrophages initiates the chronic proinflammatory cascade and necrosis core formation that characterize atherosclerosis. We report here that a cytokine considered to be anti-atherogenic, interleukin-10 (IL10), promotes cholesterol uptake from modified lipoproteins in macrophages and its transformation into foam cells by increasing the expression of scavenger receptor CD36 and scavenger receptor A. Although uptake of modified lipoproteins is considered proatherogenic, we found that IL10 also increases cholesterol efflux from macrophages to protect against toxicity of free cholesterol accumulation in the cell. This process was PPARgamma-dependent and was mediated through up-regulation of ABCA1 (ATP-binding cassette transporter A1) protein expression. Importantly, expression of inflammatory molecules, such as tumor necrosis factor-alpha, intercellular adhesion molecule-1, and MMP9 as well as apoptosis were dramatically suppressed in lipid-laden foam cells treated with IL10. The notion that IL10 can mediate both the uptake of cholesterol from modified lipoproteins and the efflux of stored cholesterol suggests that the process of foam cell formation is not necessarily detrimental as long as mechanisms of cholesterol efflux and transfer to an exogenous acceptor are functioning robustly. Our results present a comprehensive antiatherogenic role of IL10 in macrophages, including enhanced disposal of harmful lipoproteins, inhibition of inflammatory molecules, and reduced apoptosis.

Renal mass reduction results in accumulation of lipids and dysregulation of lipid regulatory proteins in the remnant kidney

A significant reduction of renal mass results in proteinuria, glomerulosclerosis, and tubulointerstitial injury, culminating in end-stage chronic renal failure (CRF). The accumulation of lipids in the kidney can cause renal disease. Uptake of oxidized lipoproteins via scavenger receptors, reabsorption of filtered protein-bound lipids via the megalin-cubilin complex, and increased glucose load per nephron can promote lipid accumulation in glomerular, tubular, and interstitial cells in CRF. Cellular lipid homeostasis is regulated by lipid influx, synthesis, catabolism, and efflux. We examined lipid-regulatory factors in the remnant kidney of rats 11 wk after nephrectomy (CRF) or sham operation. CRF resulted in azotemia, proteinuria, lipid accumulation in the kidney, upregulation of megalin, cubilin, mediators of lipid influx (scavenger receptor class A and lectin-like oxidized receptor-1), lipid efflux (liver X receptor alpha/beta and ATP-binding cassette transporter), and fatty acid biosynthesis (carbohydrate-response element binding protein, fatty acid synthase, and acetyl-CoA carboxylase). However, factors involved in cholesterol biosynthesis (sterol regulatory element binding protein, 3-hydroxy-3-methylglutaryl coenzyme A reductase, SCAP, Insig-1, and Insig-2) and fatty acid oxidation (peroxisome proliferator-activated receptor, acyl-CoA oxidase, and liver-type fatty acid binding protein) were reduced in the remnant kidney. Thus CRF results in heavy lipid accumulation in the remnant kidney, which is mediated by upregulation of pathways involved in tubular reabsorption of filtered protein-bound lipids, influx of oxidized lipoproteins and synthesis of fatty acids, and downregulation of pathways involved in fatty acid catabolism.

Reverse cholesterol transport pathway in experimental chronic renal failure

BACKGROUND

Chronic renal failure (CRF) causes oxidative stress, inflammation, oxidation of lipoproteins, impaired maturation of HDL and accelerated atherosclerosis. Uptake of oxidized lipoproteins by macrophages via scavenger receptors (scavenger receptor class A type I--SR-AI, and lectin-like oxidized LDL receptor--LOX-1) leads to foam cell formation and atherosclerosis. HDL mitigates atherosclerosis by retrieving surplus cholesterol via ATP binding cassette transporter A1 (ABCA1) and ABCG1 transporters whose expression is regulated by liver X receptor (LXR). Free cholesterol reaching the surface of HDL is esterified by lecithin-cholesterol acyltransferase (LCAT) and sequestered in the core of HDL, thereby maximizing cholesterol uptake. In the liver, lipid-rich HDL unloads its lipid contents via reversible binding to SR-BI while lipid-poor HDL is degraded by the holo-receptor (ATP synthase beta-chain).

METHODS

Expression of the above molecules involved in reverse cholesterol/lipid transport was assessed in rats 8 weeks after 5/6 nephrectomy (CRF) or sham operation.

RESULTS

CRF caused heavy accumulation of neutral lipids, upregulation of SR-AI, LOX-1, LXRalpha/beta, ABCA1 and ABCG1 in the aorta, reduction in LCAT in the plasma and no significant change in either SR-BI or beta-chain ATP synthase in the liver.

CONCLUSIONS

Lipid accumulation despite upregulation of the efflux (LXR, ABCA1, ABCG1) system in the aorta in CRF is largely due to upregulation of influx (SR-AI and LOX-1) pathway and LCAT deficiency.

Resistin increases lipid accumulation by affecting class A scavenger receptor, CD36 and ATP-binding cassette transporter-A1 in macrophages

AIMS

Resistin promotes macrophage-foam cell formation, but the mechanisms are unclear. In macrophages, lipid uptake is regulated by scavenger receptors (SR-A and CD36), while the cholesterol efflux is regulated by SR-BI, ATP-binding cassette transporter-A1 (ABCA1) and ABCG1. We investigated the mechanisms underlying the dysregulation by resistin of these regulators leading to promotion of lipid accumulation in bone marrow-derived macrophages.

MAIN METHODS

Western blotting, real-time PCR and oil red O staining were performed.

KEY FINDINGS

Resistin exacerbated lipid accumulation in oxLDL-treated macrophages. Resistin treatment of oxLDL-untreated macrophages showed increased SR-A and CD36 mRNA and protein levels, and decreased ABCA1 protein level, while having no effect on SR-BI or ABCG1 expression. Up-regulation of SR-A and CD36 by resistin resulted from activation of AP-1 and PPARgamma, respectively, and this was confirmed by the lack of activation of either after AP-1 inhibition using curcumin or SP600125, or PPARgamma inhibition using GW9662, respectively. The down-regulation of ABCA1 by resistin was not accompanied by a reduced mRNA level or an activation of LXRalpha/RXR, but resulted from enhanced protein degradation as revealed by the abolition of the down-regulation after inhibition of the proteasome pathway using ALLN or MG-132. A combined inhibition by SP600125, GW9662 and ALLN prevented resistin-induced exacerbation of lipid accumulation in oxLDL-treated macrophages.

SIGNIFICANCE

Resistin promotes foam cell formation via dysregulation of SR-A, CD36 and ABCA1. SR-A and CD36 are transcriptionally up-regulated by resistin through AP-1 and PPARgamma, respectively, whereas ABCA1 is down-regulated by resistin through proteasome-mediated enhancement of protein degradation.

Acceleration of matrix metalloproteinase-1 production and activation of platelet-derived growth factor receptor β in human coronary smooth muscle cells by oxidized LDL and 4-hydroxynonenal

Increases in matrix metalloproteinases (MMPs) at atherosclerotic lesions are involved in the migration of smooth muscle cells (SMCs) into the intima and to the rupture of plaques, being implicated in the progression of atherosclerosis. The present study examined the mechanisms underlying the production of MMP-1, interstitial collagenase-1, induced by oxidized low-density lipoprotein (oxLDL) and 4-hydroxynonenal (4-HNE), factors proposed to play a pivotal role in atherogenesis, in human coronary SMCs. oxLDL promoted the production of MMP-1 with the preceding phosphorylation of extracellular signal-regulated kinase (ERK) 1/2. Immunoprecipitation of platelet-derived growth factor receptor beta (PDGFR-beta) revealed that oxLDL induced tyrosine phosphorylation of the receptor. Inhibition of the activation of PDGFR-beta and ERK1/2 resulted in a suppression of the production of MMP-1. Consistently, 4-HNE also elicited the production of MMP-1 with the preceding phosphorylation of PDGFR-beta and ERK1/2. The 4-HNE-induced production of MMP-1 was prevented when the activation of PDGFR-beta and ERK1/2 was inhibited. The present results suggest that the activation of PDGFR-beta and ERK1/2 is involved in the production of MMP-1 in oxLDL- and 4-HNE-stimulated human coronary SMCs.

Myocardial perfusion defects in patients with autoimmune diseases: a prospective study. Analysis of two diagnostic tests

A significant correlation between autoimmune diseases and premature or accelerated coronary atherosclerosis has been found. The objectives of the study were: (a) to evaluate myocardial perfusion defects in patients with autoimmune diseases by contrast echocardiography and nuclear imaging; and (b) to evaluate the prevalence of alterations in subclinical myocardial perfusion defects in autoimmune diseases. Myocardial perfusion in 37 patients was evaluated by contrast echocardiography at rest and with dobutamine and with nuclear imaging. The agreement between the two diagnostic tests at rest was 0.72 (P < 0.0001) and with dobutamine was 0.65 (P < 0.0001). The prevalence of abnormalities in myocardial perfusion in autoimmune diseases by contrast echocardiography and nuclear imaging was 27% and in patients with primary antiphospholipid syndrome was 30%. We concluded that there is a high level of agreement between contrast ecocardiography and nuclear imaging for assessment of myocardial perfusion defects in patients with autoimmune diseases, and their prevalence is similar to that reported in the literature.

Genes of cholesterol metabolism in human atheroma: overexpression of perilipin and genes promoting cholesterol storage and repression of ABCA1 expression

OBJECTIVE

Accumulation of cholesterol in foam cells of atheroma plaques depends on the balance between uptake and efflux of cholesterol. It may also depend on proteins surrounding lipid droplets, adipophilin, and perilipins. They favor triglyceride storage in adipocytes and could play a similar role for cholesterol in atheroma.

METHODS AND RESULTS

We measured in human atheroma and nearby macroscopically intact tissue (MIT) the expression of perilipin, adipophilin, and regulatory factors of cholesterol metabolism. We identified perilipin A in human arterial wall. Its expression was largely increased in atheroma compared with MIT, and perilipin was present in macrophages and vascular smooth muscle cells. Adipophilin, ACAT1, and CD36 were also overexpressed in atheroma. mRNA levels of low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and SREBP-2 were unchanged. With respect to efflux of cholesterol, the mRNA levels of NCEH and ABCA-1 were unchanged, whereas CLA-1 mRNA was slightly higher in atheroma. Importantly, immunoblotting of ABCA-1 showed a dramatic decrease of ABCA1 protein, the key molecule of cholesterol efflux, in atheroma compared with MIT.

CONCLUSIONS

We show the presence and induction of perilipin in atheroma. This overexpression and the coordinated modifications of expression of key regulatory factors for cholesterol metabolism could favor cholesterol accumulation.

The Mycobacterium tuberculosis cell wall component mycolic acid elicits pathogen-associated host innate immune responses

Recognition of conserved pathogen-associated molecular patterns constitutes a crucial step in the initiation of innate immune responses. We studied the contribution to the host-pathogen interaction of mycolic acid (MA), a major lipid component of the cell envelope of the macrophage intracellular pathogen Mycobacterium tuberculosis and other mycobacteria. MA administered to the peritoneal cavity or to the airways induced a unique macrophage morphotype, similar to the foamy macrophage derivatives observed in tuberculous granulomas and characterized by intracellular accumulation of neutral lipids and entry into mitosis. When assayed for production of inflammatory mediators, a conditioning rather than a direct activation of the MA-elicited foamy macrophages was observed. MA enabled production of IFN-gamma and myeloperoxidase, enhanced TNF-alpha production and suppressed IL-10 upon renewed exposure to innate triggers. Intratracheal instillation of MA mimicked additional features of the airway response to M. tuberculosis infection, namely a rapid but transient neutrophil influx and IL-6 production and a chronic IL-12 production. These MA-elicited cellular innate defenses and the accompanying formation of foamy macrophages identify for the first time the foamy macrophage morphotype as part of the host response to a pathogen-associated structure. Furthermore, these results characterize MA as a direct trigger of innate immunity, distinct from Toll-like receptor ligands.

Deletion of the alternatively spliced fibronectin EIIIA domain in mice reduces atherosclerosis

The alternatively spliced and highly conserved EIIIA domain of fibronectin (FN) is included in most FN of the extracellular matrix in embryos. In adults, both extracellular matrix and plasma FN essentially lack EIIIA. In diverse inflammatory situations however, EIIIA is specifically included by regulated RNA splicing. In atherosclerotic lesions, FN, including the EIIIA domain (EIIIA-FN), is abundant, whereas FN in the flanking vessel wall lacks EIIIA. Lesional EIIIA-FN is localized with endothelial cells and macrophage foam cells. To directly test the function of EIIIA-FN, we generated EIIIA-null (EIIIA–/–) mice that lack the EIIIA exon and crossed them with apolipoprotein E (ApoE)–null (ApoE–/–) mice that develop arterial wall lesions. Compared with ApoE–/– controls, EIIIA–/–ApoE–/– mice had significantly smaller lesions throughout the aortic tree. EIIIA-FN was increased in ApoE–/– plasma, and total plasma cholesterol was reduced in EIIIA–/–ApoE–/– mice, specifically in large lipoprotein particles, suggesting a functional role for plasma EIIIA-FN. To assess a role for macrophage EIIIA-FN in the vessel wall, we conducted in vitro foam cell assays. EIIIA–/–ApoE–/– macrophages accumulated significantly less intracellular lipid than control ApoE–/– cells. These results provide genetic evidence that suggests roles for EIIIA-FN in plasma lipoprotein metabolism and in foam cell formation.

Adipophilin Enhances Lipid Accumulation and Prevents Lipid Efflux From THP-1 Macrophages

Objective— Uptake of modified low-density lipoprotein (LDL) by macrophages through scavenger receptors results in lipid droplets accumulation and foam cell formation. Excess lipid deposition in macrophages has been reported to modulate expression of several genes including adipophilin. In this study, we investigated the function of adipophilin in lipid accumulation and cholesterol efflux in THP-1 macrophages.

Methods and Results— Adipophilin mRNA expression was 3.5-fold higher in human atherosclerotic plaques compared with healthy areas of the same arteries. Moreover, in the presence of acetylated LDL (AcLDL), triglycerides and cholesteryl esters were increased in macrophages overexpressing adipophilin by 40% and 67%, respectively, whereas their accumulation was reduced when endogenous cellular adipophilin was depleted using siRNA approach. In addition, neither overexpression nor downregulation of adipophilin altered expression of genes involved in lipid efflux. However, the affinity and the number of AcLDL receptors were not affected. After 24-hour incubation of lipid-loaded macrophages with apolipoprotein A-I, cholesterol efflux was reduced by 47% in adipophilin transfected cells versus control cells.

Conclusion— Our results showed that stimulation of adipophilin expression in macrophages by modified LDL promotes triglycerides and cholesterol storage and reduces cholesterol efflux. Therefore, adipophilin might contribute, in vivo, to lipid accumulation in the intima of the arterial wall.

Oxidized LDL activates phospholipase A2 to supply fatty acids required for cholesterol esterification

We examined the roles of phospholipase A2 (PLA2) in oxidized LDL (oxLDL)-induced cholesteryl ester formation in macrophages. In [3H]oleic acid-labeled RAW264.7 cells and mouse peritoneal macrophages, oxLDL induced [3H]cholesteryl oleate formation with an increase in free [3H]oleic acid and a decrease in [3H]phosphatidylcholine. The changes in these lipids were suppressed by methyl arachidonyl fluorophosphonate (MAFP), a cytosolic PLA2 (cPLA2) inhibitor. However, MAFP had no effect on the ACAT activity or the binding and/or uptake of oxLDL. Stimulation with oxLDL in the presence of [3H]cholesterol increased [3H]cholesteryl ester bearing fatty acyl chains derived from cellular and/or exogenous (oxLDL) lipids. The formation of cholesteryl ester under this condition was also inhibited by MAFP, and the inhibitory effect was reversed by adding oleic acid. While oxLDL did not affect the activity or amounts of cPLA2, preincubation with oxLDL enhanced the release of oleic acid and arachidonic acid induced by ionomycin in RAW264.7 cells. 13(S)-hydroxyoctadecadienoic acid, but not 7-ketocholesterol, also enhanced ionomycin-induced oleic acid release. These results suggest that oxLDL induces cPLA2 activation, which contributes, at least in part, to the supply of fatty acids required for the cholesteryl esterification, probably through the acceleration by oxidized lipids of the catalytic action of cPLA2 in macrophages.

Elimination of cholesterol ester from macrophage foam cells by adenovirus-mediated gene transfer of hormone-sensitive lipase

Cholesterol ester (CE)-laden foam cells are a hallmark of atherosclerosis. To determine whether stimulation of the hydrolysis of cytosolic CE can be used as a novel therapeutic modality of atherosclerosis, we overexpressed hormone-sensitive lipase (HSL) in THP-1 macrophage-like cells by adenovirus-mediated gene delivery, and we examined its effects on the cellular cholesterol trafficking. We show here that the overexpression of HSL robustly increased neutral CE hydrolase activity and completely eliminated CE in the cells that had been preloaded with CE by incubation with acetylated low density lipoprotein. In these cells, cholesterol efflux was stimulated in the absence or presence of high density lipoproteins, which might be at least partially explained by the increase in the expression of ABCA1. Importantly, these effects were achieved without the addition of acyl-CoA:cholesterol acyltransferase inhibitor, cAMP, or even high density lipoproteins. Furthermore, the uptake and degradation of acetylated low density lipoprotein was significantly reduced probably by decreased expression of scavenger receptor A and CD36. Notably, the cells with stimulated CE hydrolysis did not exhibit either buildup of free cholesterol or cytotoxicity. In conclusion, increased hydrolysis of CE by the overexpression of HSL leads to complete elimination of CE from THP-1 foam cells not only by increasing efflux but also by decreasing influx of cholesterol.

Stimulation by de novo-synthesized ceramide of phospholipase A(2)-dependent cholesterol esterification promoted by the uptake of oxidized low-density lipoprotein in macrophages

The involvement of cytosolic phospholipase A(2) (cPLA(2)) and ceramide in the accumulation of cholesteryl ester induced by the uptake of oxidized low-density lipoproteins (oxLDL) in macrophages was investigated. Uptake of oxLDL by [(3)H]oleic acid-labeled macrophages stimulated the formation of cholesteryl oleate, and this process was completely inhibited by a cPLA(2) inhibitor. Under the conditions, a time-dependent increase in ceramide was observed, while sphingomyelin levels were unaffected. The production of ceramide was completely inhibited by fumonisin B1, an inhibitor of the de novo synthesis of ceramide, and oxLDL-induced cholesteryl oleate formation was inhibited partially. Treatment of the cells with sphingomyelinase accelerated the formation of cholesteryl ester. Furthermore, sphingomyelinase or cell-permeable ceramide induced the release of oleic acid, and this was inhibited by a cPLA(2) inhibitor. These results suggest that activation of cPLA(2) is responsible for the formation of cholesteryl ester induced by the uptake of oxLDL in macrophages, and that de novo-synthesized ceramide is implicated, at least in part, in this process.

Novel 5' exon of scavenger receptor CD36 is expressed in cultured human vascular smooth muscle cells and atherosclerotic plaques

CD36, a member of the scavenger receptor family, is centrally involved in the uptake of oxidized low density lipoproteins (oxLDLs) from the bloodstream. During the atherosclerotic process, the lipid cargo of oxLDL accumulates in macrophages and smooth muscle cells (SMCs), inducing their pathological conversion to foam cells. Increased expression of CD36 occurs in human atherosclerotic lesions, and CD36 knockout mice show reduced uptake of modified LDLs and reduced atherosclerosis. Here, we describe a novel exon 1b and extended CD36 promoter in human SMCs. Exon 1b is specifically transcribed in activated aortic SMCs and mainly expressed in atherosclerotic plaques. Thus, switching to exon 1b transcription may be an important step for the activation of SMCs and their conversion to foam cells. Using an antisense oligonucleotide to exon 1b, we inhibit CD36 translation and highly reduce oxLDL uptake. The antisense to exon 1b does not affect CD36 in cell lines not expressing the new exon. The possibility of a novel antiatherosclerotic therapy and the use of exon 1b as a marker of atherosclerosis are discussed.

CD36 deficiency induced by antiretroviral therapy

BACKGROUND

The molecular basis of lipodystrophy, a syndrome associated with HIV antiretroviral (ARV) therapy, remains unknown.

OBJECTIVES

To examine whether ARV therapy might inhibit the expression of CD36, which is known to play an important role in fatty acid and glucose metabolism, and if this might contribute to the metabolic alterations associated with lipodystrophy.

DESIGN

The effects of ARV therapy on CD36 levels was examined in vivo in a prospective cohort of individuals treated with ARV therapy and in vitro in assays of human cell lines exposed to ARV drugs.

METHODS

Monocyte CD36 levels were assessed by flow cytometry at baseline and after 7 days of therapy in five healthy volunteers and 10 treatment-naive HIV-1-infected individuals. ARV therapy included protease inhibitors (ritonavir, nelfinavir or lopinavir/ritonavir). In addition, human cell lines (THP-1 and C32) were assessed for CD36 levels pre and post-ritonavir treatment.

RESULTS

Three of four healthy controls (one withdrew because of adverse effects) and 6 of 10 HIV-1-infected individuals had a 50 to > 90% decrease in monocyte CD36 levels after 7 days of therapy. One of ten HIV-infected subjects had a 30% decrease, and the remaining individuals had no change or an increase in CD36 levels. CD36 levels decreased significantly in human cell lines treated with ritonavir but not in those treated with zidovudine.

CONCLUSIONS

ARV therapy resulted in a marked decrease in CD36 in approximately 70% of our participants. Sustained ARV therapy-induced CD36 deficiency may contribute to insulin resistance and other metabolic complications of lipodystrophy.

The potential for novel anti-inflammatory therapies for coronary artery disease

Although drugs that lead to cholesterol and lipid lowering have proved to have significant effects in lowering cardiovascular morbidity and mortality, coronary artery disease remains a principal cause of death worldwide. There is a clear need to discover further therapeutic approaches to control this disease adequately. This review focuses on the mechanisms that have been implicated in the recruitment, activation and differentiation of inflammatory monocytes/macrophages in nascent vascular lesions into lipid-laden foam cells. These mechanisms might provide attractive targets for novel therapies for coronary artery disease.

Is there a genetic basis for resistance to atherosclerosis?

Atherosclerosis and its major clinical manifestation, coronary heart disease, is and will remain the main cause of mortality. Reviews on this subject dealt with factors that enhance development of atherosclerosis. This review deals with a new facet, that some individuals are less prone to develop atherosclerosis: (1) despite high cholesterol intake or (2) despite hypercholesterolemia with elevated low-density lipoprotein cholesterol (LDL-C) levels. The variability of response of plasma cholesterol to dietary intake was shown to be regulated by liver x receptor (LXR) that determines the rate of intestinal cholesterol absorption through the ATP-binding cassette (ABC) gene family. Other gene products, such as apolipoprotein-E (apo-E), scavenger receptor-B1 (SR-B1) and acyl coenzyme: cholesterol acyltransferase-2 (ACAT-2) affect cholesterol absorption also. The role of a genetic background for relative resistance to atherosclerosis is highlighted by subjects with familial hypercholesterolemia in whom high plasma cholesterol levels has not curtailed their expected life span. Studies in animals have shown that resistance to atherosclerosis in spite of hypercholesterolemia is affected by factors such as high-density lipoprotein (HDL) phospholipids that enhance reverse cholesterol transport, non-responsiveness to induction or lack of monocyte chemotactic protein-1 (MCP-1), C-C chemokine receptor 2 (CCR2), macrophage colony stimulating factor (MCSF), or vascular cell adhesion molecule-1 (VCAM-1). Since macrophages have been regarded as pro- or anti-atherogenic, evidence was collated that the high activity of scavenger receptors may contribute towards resistance to atherosclerosis if accompanied by adequate amounts of apo-E for cholesterol removal.

Class A scavenger receptors, macrophages, and atherosclerosis

The scope of this review is to discuss the new advances in our understanding of the role of scavenger receptor class A in the initiation and modulation of the atherosclerotic process. Through the approaches of gene manipulation in the mouse model, a substantial body of literature has accumulated that depicts scavenger receptor class A as a central player in atherogenesis. In studies of scavenger receptor class A overexpression in macrophages through bone marrow transplantation using transgenic donor material, recipient mice with hyperlipidemia caused either by apolipoprotein E or LDL receptor deficiency did not show convincing changes in the degree of atherosclerosis development compared with controls. Conversely, the deletion of the scavenger receptor class A gene in the mouse has shown, in a consistent and significant fashion, that this receptor serves a pro-atherogenic function under hyperlipidemic conditions, as both apolipoprotein E and LDL receptor-deficient mice had reduced atherosclerosis in the absence of scavenger receptor class A. In addition, we have recently shown that C57BL/6 mice are protected from diet-induced atherosclerosis when they lack scavenger receptor class A, and that the macrophage is the cell type responsible for the effect of scavenger receptor class A deficiency in reducing lesion formation in C57BL/6 and LDL receptor null mice. Together, these results demonstrate that macrophage scavenger receptor class A contributes significantly to atherosclerotic lesion formation, and suggest that the uptake of oxidized or modified lipoproteins by vessel wall macrophages is a central process in atherogenesis.

A cofactor of tRNA synthetase, p43, is secreted to up-regulate proinflammatory genes

An auxiliary factor of mammalian multi-aminoacyl-tRNA synthetases, p43, is thought to be a precursor of endothelial monocyte-activating polypeptide II (EMAP II) that triggers proinflammation in leukocytes and macrophages. In the present work, however, we have shown that p43 itself is specifically secreted from intact mammalian cells, while EMAP II is released only when the cells are disrupted. Secretion of p43 was also observed when its expression was increased. These results suggest that p43 itself should be a real cytokine secreted by an active mechanism. To determine the cytokine activity and active domain of p43, we investigated tumor necrosis factor (TNF) and interleukin-8 (IL-8) production from human monocytic THP-1 cells treated with various p43 deletion mutants. The full length of p43 showed higher cytokine activity than EMAP II, further supporting p43 as the active cytokine. p43 was also shown to activate MAPKs and NFkappaB, and to induce cytokines and chemokines such as TNF, IL-8, MCP-1, MIP-1alpha, MIP-1beta, MIP-2alpha, IL-1beta, and RANTES. Interestingly, the high level of p43 was observed in the foam cells of atherosclerotic lesions. Therefore, p43 could be a novel mediator of atherosclerosis development as well as other inflammation-related diseases.

Reduced atherosclerotic lesions in mice deficient for total or macrophage-specific expression of scavenger receptor-A

The absence of the scavenger receptor A (SR-A)-I/II has produced variable effects on atherosclerosis in different murine models. Therefore, we examined whether SR-AI/II deficiency affected atherogenesis in C57BL/6 mice, an inbred strain known to be susceptible to diet-induced atherosclerotic lesion formation, and whether the deletion of macrophage SR-AI/II expression would modulate lesion growth in C57BL/6 mice and LDL receptor (LDLR)(-/-) mice. SR-AI/II-deficient (SR-AI/II(-/-)) female and male mice on the C57BL/6 background were challenged with a butterfat diet for 30 weeks. No differences were detected in plasma lipids between SR-AI/II(-/-) and SR-AI/II(+/+) mice, whereas both female and male SR-AI/II(-/-) mice had a tremendous reduction (81% to 86%) in lesion area of the proximal aorta compared with SR-AI/II(+/+) mice. Next, to analyze the effect of macrophage-specific SR-AI/II deficiency in atherogenesis, female C57BL/6 mice were lethally irradiated, transplanted with SR-AI/II(-/-) or SR-AI/II(+/+) fetal liver cells, and challenged with the butterfat diet for 16 weeks. In a separate experiment, male LDLR(-/-) mice were reconstituted with SR-AI/II(-/-) or SR-AI/II(+/+) fetal liver cells and challenged with a Western diet for 10 weeks. No significant differences in plasma lipids and lipoprotein profiles were noted between the control and experimental groups in either experiment. SR-AI/II(-/-)-->C57BL/6 mice, however, had a 60% reduction in lesion area of the proximal aorta compared with SR-AI/II(+/+)-->C57BL/6 mice. A similar level of reduction (60%) in lesion area was noted in the proximal aorta and the entire aorta en face of SR-AI/II(-/-)-->LDLR(-/-) mice compared with SR-AI/II(+/+)-->LDLR(-/-) mice. These results demonstrate in vivo that SR-AI/II expression has no impact on plasma lipid levels and that macrophage SR-AI/II contributes significantly to atherosclerotic lesion formation.

The oxidative modification hypothesis of atherogenesis: an overview

The literature relating lipid and lipoprotein oxidation to atherosclerosis has expanded enormously in recent years. Papers on the "oxidative modification hypothesis" of atherogenesis have ranged from the most basic studies of the chemistry and enzymology of LDL oxidation, through studies of the biological effects of oxidized LDL on cultured cells, and on to in vivo studies of the effects of antioxidants on atherosclerosis in animals and humans. The data in support of this theory are mounting but many key questions remain unanswered. For example, while it is generally agreed that LDL undergoes oxidation and that oxidized LDL is present in arterial lesions, it is still not known how and where LDL gets oxidized in vivo nor which of its many biological effects demonstrable in vitro are relevant to atherogenesis in vivo. This brief review is not intended to be comprehensive but rather to offer a perspective and a context for this Forum. We discuss the strengths and weaknesses of each line of evidence, try to identify areas in which further research is needed, assess the relevance of the hypothesis to the human disease, and point to some of the potential targets for therapy.