Scavenger receptor regulation and atherosclerosis

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.

Vitamin E: Regulatory Role on Signal Transduction

Vitamin E modulates signal transduction pathways by several molecular mechanisms. As a hydrophobic molecule located mainly in membranes it contributes together with other lipids to the physical and structural characteristics such as membrane stability, curvature, fluidity, and the organization into microdomains (lipid rafts). By acting as the main lipid-soluble antioxidant, it protects other lipids such as mono- and poly-unsaturated fatty acids (MUFA and PUFA, respectively) against chemical reactions with reactive oxygen and nitrogen species (ROS and RNS, respectively) and prevents membrane destabilization and cellular dysfunction. In cells, vitamin E affects signaling in redox-dependent and redox-independent molecular mechanisms by influencing the activity of enzymes and receptors involved in modulating specific signal transduction and gene expression pathways. By protecting and preventing depletion of MUFA and PUFA it indirectly enables regulatory effects that are mediated by the numerous lipid mediators derived from these lipids. In recent years, some vitamin E metabolites have been observed to affect signal transduction and gene expression and their relevance for the regulatory function of vitamin E is beginning to be elucidated. In particular, the modulation of the CD36/FAT scavenger receptor/fatty acids transporter by vitamin E may influence many cellular signaling pathways relevant for lipid homeostasis, inflammation, survival/apoptosis, angiogenesis, tumorigenesis, neurodegeneration, and senescence. Thus, vitamin E has an important role in modulating signal transduction and gene expression pathways relevant for its uptake, distribution, metabolism, and molecular action that when impaired affect physiological and patho-physiological cellular functions relevant for the prevention of a number of diseases. © 2018 IUBMB Life, 71(4):456-478, 2019.

Association between plasma CD36 levels and incident risk of coronary heart disease among Danish men and women

BACKGROUND AND AIMS

CD36 is a cholesterol receptor involved in the uptake of oxidized low-density lipoprotein cholesterol and development of atherosclerotic plaques. Cross-sectional studies have shown correlations between plasma CD36 and atherosclerosis but no prospective study has examined the association yet. We prospectively examined the association between plasma CD36 levels and risk of incident coronary heart disease (CHD) in a Danish population.

METHODS

Plasma CD36 levels were measured in a case-cohort study nested within the Danish population-based cohort, the Diet, Cancer and Health Study. A total of 1963 incident CHD events occurred between baseline (1993-1997) and 2008, and a sub-cohort of 1759 participants were randomly selected as reference. Cox proportional hazard regression models were used to compute the hazard ratio (HR) and corresponding 95% confidence interval (CI).

RESULTS

After adjusting for CHD risk factors, including history of hypercholesterolemia and diabetes, elevated plasma CD36 levels were not associated with higher CHD risk in the total population, and the HR comparing the highest versus lowest tertile of CD36 levels was 1.02 (95% CI: 0.84-1.23). High CD36 levels were only found to be associated with risk of CHD in combination with prevalent diabetes (HR = 2.83, 95% CI: 1.08-7.45) vs. the joint reference group of lowest CD36 tertile and no diabetes.

CONCLUSIONS

Plasma CD36 levels were not predictive of CHD risk in the general population.

AMP-activated protein kinase attenuates oxLDL uptake in macrophages through PP2A/NF-κB/LOX-1 pathway

The differentiation of macrophages into lipid-laden foam cells is a hallmark in early-stage atherosclerosis. The developmental role of adenosine monophosphate-activated protein kinase (AMPK) in a transformation of foam cells, especially in macrophage cholesterol uptake that remains undetermined. Here we demonstrate that AMPK activation in response to IMM-H007 or AICAR resulted in a decrease in macrophage cholesterol uptake and thus inhibited foam cell formation in macrophages mediated by oxidized low-density lipoprotein (oxLDL). This functional change was caused by a downregulation of mRNA and protein expression of LOX-1 but not other scavenger receptors, including scavenger receptor-A (SR-A), CD36 and scavenger receptor-BI (SR-BI). The expression of LOX-1 was regulated by AMPK activation induced decreased phosphorylation of nuclear transcription factor NF-κB, since siRNA interference or dominant negative AMPK overexpression significantly promotes Ser536 dephosphorylation of NF-κB p65 and thus increases LOX-1 expression. Moreover, pharmacological AMPK activation was shown to promote protein phosphatase 2A (PP2A) activity and the specific PP2A inhibitor, okadaic acid, could prevent the effects of IMM-H007 or AICAR on NF-κB and LOX-1. In vivo, pharmacological AMPK activation reduced the lesion size of atherosclerosis and the expression of LOX-1 in aortas in apolipoprotein E-deficient mice. Our current findings suggest a novel mechanism of LOX-1 regulation by AMPK to attenuate macrophage oxLDL uptake and atherosclerosis.

Endothelial overexpression of LOX-1 increases plaque formation and promotes atherosclerosis in vivo

AIMS

Lectin-like oxLDL receptor-1 (LOX-1) mediates the uptake of oxidized low-density lipoprotein (oxLDL) in endothelial cells and macrophages. However, the different atherogenic potential of LOX-1-mediated endothelial and macrophage oxLDL uptake remains unclear. The present study was designed to investigate the in vivo role of endothelial LOX-1 in atherogenesis.

METHODS AND RESULTS

Endothelial-specific LOX-1 transgenic mice were generated using the Tie2 promoter (LOX-1TG). Oxidized low-density lipoprotein uptake was enhanced in cultured endothelial cells, but not in macrophages of LOX-1TG mice. Six-week-old male LOX-1TG and wild-type (WT) mice were fed a high-cholesterol diet (HCD) for 30 weeks. Increased reactive oxygen species production, impaired endothelial nitric oxide synthase activity and endothelial dysfunction were observed in LOX-1TG mice as compared with WT littermates. LOX-1 overexpression led to p38 phosphorylation, increased nuclear factor κB activity and subsequent up-regulation of vascular cell adhesion molecule-1, thereby favouring macrophage accumulation and aortic fatty streaks. Consistently, HCD-fed double-mutant LOX-1TG/ApoE(-/-) displayed oxidative stress and vascular inflammation with higher aortic plaques than ApoE(-/-) controls. Finally, bone marrow transplantation experiments showed that endothelial LOX-1 was sufficient for atherosclerosis development in vivo.

CONCLUSIONS

Endothelial-specific LOX-1 overexpression enhanced aortic oxLDL levels, thereby favouring endothelial dysfunction, vascular inflammation and plaque formation. Thus, LOX-1 may serve as a novel therapeutic target for atherosclerosis.

Human monocytes are severely impaired in base and DNA double-strand break repair that renders them vulnerable to oxidative stress

Monocytes are key players in the immune system. Crossing the blood barrier, they infiltrate tissues and differentiate into (i) macrophages that fight off pathogens and (ii) dendritic cells (DCs) that activate the immune response. A hallmark of monocyte/macrophage activation is the generation of reactive oxygen species (ROS) as a defense against invading microorganisms. How monocytes, macrophages, and DCs in particular respond to ROS is largely unknown. Here we studied the sensitivity of primary human monocytes isolated from peripheral blood and compared them with macrophages and DCs derived from them by cytokine maturation following DNA damage induced by ROS. We show that monocytes are hypersensitive to ROS, undergoing excessive apoptosis. These cells exhibited a high yield of ROS-induced DNA single- and double-strand breaks and activation of the ATR-Chk1-ATM-Chk2-p53 pathway that led to Fas and caspase-8, -3, and -7 activation, whereas macrophages and DCs derived from them were protected. Monocytes are also hypersensitive to ionizing radiation and oxidized low-density lipoprotein. The remarkable sensitivity of monocytes to oxidative stress is caused by a lack of expression of the DNA repair proteins XRCC1, ligase IIIα, poly(ADP-ribose) polymerase-1, and catalytic subunit of DNA-dependent protein kinase (DNA-PK(cs)), causing a severe DNA repair defect that impacts base excision repair and double-strand break repair by nonhomologous end-joining. During maturation of monocytes into macrophages and DCs triggered by the cytokines GM-CSF and IL-4, these proteins become up-regulated, making macrophages and DCs repair-competent and ROS-resistant. We propose that impaired DNA repair in monocytes plays a role in the regulation of the monocyte/macrophage/DC system following ROS exposure.

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.

Oxysterols in heart failure

Oxysterols are biologically active molecules that result from the oxidation of cholesterol. Several oxysterols are found in macrophages and macrophage-derived 'foam cells' in atherosclerotic tissue. Lipophilic oxysterols penetrate cell membranes and, therefore, their concentrations can reach harmful levels in endothelial and smooth muscle cells located in close proximity to the atherosclerotic plaques or inflammatory zones. New findings suggest that the effects of oxysterols on cardiomyocytes can lead to cell hypertrophy and death. This may make oxysterols one of the major factors precipitating morbidity in atherosclerosis-induced cardiac diseases and inflammation-induced heart complications. The pathological actions of oxysterols on muscle cells were shown to depend on dysfunctional Ca(2+) signaling; however, the mechanisms of the effects remain to be elucidated. Understanding the effects of oxysterols could lead to therapies that modulate malfunction of cardiomyocytes. This review discusses the experimental findings and the relevance of oxysterols to heart failure, and suggests strategies for important future investigations.

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.

The lectin-like oxidized low-density-lipoprotein receptor: a pro-inflammatory factor in vascular disease

Scavenger receptors are membrane glycoproteins that bind diverse ligands including lipid particles, phospholipids, apoptotic cells and pathogens. LOX-1 (lectin-like oxidized low-density lipoprotein receptor-1) is increasingly linked to atherosclerotic plaque formation. Transgenic mouse models for LOX-1 overexpression or gene knockout suggests that LOX-1 contributes to atherosclerotic plaque formation and progression. LOX-1 activation by oxidized LDL (low-density lipoprotein) binding stimulates intracellular signalling, gene expression and production of superoxide radicals. A key question is the role of leucocyte LOX-1 in pro-atherogenic lipid particle trafficking, accumulation and signalling leading to differentiation into foam cells, necrosis and plaque development. LOX-1 expression is elevated within vascular lesions and a serum soluble LOX-1 fragment appears diagnostic of patients with acute coronary syndromes. LOX-1 is increasingly viewed as a vascular disease biomarker and a potential therapeutic target in heart attack and stroke prevention.

Atherosclerosis and the Lectin-like OXidized low-density lipoprotein scavenger receptor

The Lectin-like OXidized low-density lipoprotein scavenger receptor (LOX-1) is implicated in vascular inflammation and atherosclerotic plaque initiation, progression, and destabilization. LOX-1 levels are elevated upon recognition of oxidized low-density lipoprotein, a key pro-atherogenic substance in the vasculature. Recent evidence indicates this gene product is a biomarker of inflammation and disease status. We review and assess the role of LOX-1 in atherosclerotic plaque formation, physiologic regulation, and as a biomarker and target in cardiovascular disease diagnosis and prevention.

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.

Antagonistic Effects of Oxidized Low Density Lipoprotein and α-Tocopherol on CD36 Scavenger Receptor Expression in Monocytes

Vitamin E deficiency increases expression of the CD36 scavenger receptor, suggesting specific molecular mechanisms and signaling pathways modulated by alpha-tocopherol. We show here that alpha-tocopherol down-regulated CD36 expression (mRNA and protein) in oxidized low density lipoprotein (oxLDL)-stimulated THP-1 monocytes, but not in unstimulated cells. Furthermore, alpha-tocopherol treatment of monocytes led to reduction of fluorescent oxLDL-3,3'-dioctadecyloxacarbocyanine perchlorate binding and uptake. Protein kinase C (PKC) appears not to be involved because neither activation of PKC by phorbol 12-myristate 13-acetate nor inhibition by PKC412 was affected by alpha-tocopherol. However, alpha-tocopherol could partially prevent CD36 induction after stimulation with a specific agonist of peroxisome proliferator-activated receptor-gamma (PPARgamma; troglitazone), indicating that this pathway is susceptible to alpha-tocopherol action. Phosphorylation of protein kinase B (PKB) at Ser473 was increased by oxLDL, and alpha-tocopherol could prevent this event. Expression of PKB stimulated the CD36 promoter as well as a PPARgamma element-driven reporter gene, whereas an inactive PKB mutant had no effect. Moreover, coexpression of PPARgamma and PKB led to additive induction of CD36 expression. Altogether, our results support the existence of PKB/PPARgamma signaling pathways that mediate CD36 expression in response to oxLDL. The activation of CD36 expression by PKB suggests that both lipid biosynthesis and fatty acid uptake are stimulated by PKB.

CD36 overexpression in ritonavir-treated THP-1 cells is reversed by alpha-tocopherol

Therapies with antiretroviral protease inhibitors (ARPI) are correlated with a higher risk for dyslipidemia, hypercholesterolemia, and atherosclerosis. The original aim of this study was to establish whether alpha-tocopherol can reduce CD36 scavenger receptor overexpression occurring after treatment of monocytes with the ARPI ritonavir. We show here that treatment of THP-1 monocytes with ritonavir increases total protein and surface expression of CD36; however, only weak changes are observed at the mRNA level, suggesting that CD36 overexpression occurs mainly at the posttranscriptional level. Concentrations of ritonavir that upregulate CD36 expression inhibit proteasome activity in THP-1 cells, indicating a possible regulatory role of the proteasome in CD36 overexpression. Similar to ritonavir, the proteasome inhibitor ALLN increases the CD36 surface expression on THP-1 cells. alpha-Tocopherol efficiently normalizes CD36 protein overexpression after ritonavir treatment and reduces oxLDL uptake. Furthermore, in THP-1 monocytes, alpha-tocopherol reverses the proteasome activity inhibited by ritonavir. This study indicates that an increased CD36 protein expression in THP-1 monocytes induced by ritonavir can be normalized by alpha-tocopherol. CD36 overexpression is caused by inhibition of proteasome activity by ritonavir, which is efficiently restored by alpha-tocopherol.

Beneficial effects of a novel IH636 grape seed proanthocyanidin extract and a niacin-bound chromium in a hamster atherosclerosis model

Atherosclerosis is a disease of the arteries in which fatty plaques develop on the inner arterial wall, which eventually obstructs blood flow. Identified risk factors for atherosclerosis include genetics, diet, lifestyle, smoking, circulating lipid and cholesterol levels, and molecular and circulating signals of chronic vascular inflammation. The link between flavonoids and atherosclerosis is based partly on the evidence that some flavonoids possess antioxidant properties and have been shown to be potent inhibitors of LDL oxidation in vitro. Hypercholesterolemia, a significant cardiovascular risk factor is prevalent in the American population. Grape seed proanthocyanidin extracts are known to exhibit a broad spectrum of chemopreventive and cardioprotective properties against oxidative stress. A recent study has shown that a combination of IH636 grape seed proanthocyanidin extract (GSPE) and a niacin-bound chromium (NBC) can decrease total cholesterol, LDL and oxidized LDL levels in hypercholesterolemic human subjects. In this study, we assessed the efficacy of GSPE supplementation in hamsters, singly and in combination with NBC, since these animals have a similar lipid profile to hypercholesterolemic humans when fed a hypercholesterolemic diet of 0.2% cholesterol and 10% coconut oil (HCD). After 10 weeks of feeding HCD, these animals developed foam cells, which is a biomarker of early stages of atherosclerosis. Atherosclerosis (% of aorta covered with foam cells) was reduced by approximately 50% and 63% following supplementation of these animals with 50 mg/kg and 100 mg/kg of GSPE, respectively, in conjunction with a HCD, while approximately 32% reduction was observed following supplementation of GSPE plus NBC. A range of 7-9 animals was used in each study group. GSPE alone and in combination with NBC exerted a pronounced effect on the cholesterol, and triglyceride levels, as well as oxidative lipid damage as demonstrated by the formation of thiobarbituric acid reactive substances (TBARS). This data demonstrates that GSPE and NBC may provide significant health benefits by dramatically ameliorating the incidence of atherosclerosis as demonstrated by reducing the formation of foam cells.

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.