The role of Siglec-1 and SR-BI interaction in the phagocytosis of oxidized low density lipoprotein by macrophages

Siglec-5 as a novel receptor mediates endothelial cells oxLDL transcytosis to promote atherosclerosis

BACKGROUND

Excessive subendothelial retention of oxidized low-density lipoprotein (oxLDL) and subsequent oxLDL engulfment by macrophages leads to the formation of foam cells and the development of atherosclerosis. Our previous study showed that the plasma level of sialic acid-binding immunoglobulin-like lectin 5 (Siglec-5) was a novel biomarker for the prognosis of atherosclerosis in diabetic patients. However, the role and underlying mechanisms of Siglec-5 in atherosclerosis have not been elucidated.

METHODS

The interaction between oxLDL and Siglec-5 was detected by fluorescence colocalization and coimmunoprecipitation. The effect of oxLDL on Siglec-5 expression was detected in endothelial cells and macrophages, and the effect of Siglec-5 on oxLDL transcytosis and uptake was investigated. Siglec-5 was overexpressed in mice using recombinant adeno-associated virus vector serotype 9 (rAAV9-Siglec-5) to evaluate the effect of Siglec-5 on oxLDL uptake and atherogenesis in vivo. In addition, the effects of Siglec-5 antibodies and soluble Siglec-5 proteins on oxLDL transcytosis and uptake and their role in atherogenesis were investigated in vivo and in vitro.

RESULTS

We found that oxLDL interacted with Siglec-5 and that oxLDL stimulated the expression of Siglec-5. Siglec-5 promotes the transcytosis and uptake of oxLDL, while both anti-Siglec-5 antibodies and soluble Siglec-5 protein attenuated oxLDL transcytosis and uptake. Interestingly, overexpression of Siglec-5 by recombinant adeno-associated viral vector serotype 9 (rAAV9-Siglec-5) promoted the retention of oxLDL in the aorta of C57BL/6 mice. Moreover, overexpression of Siglec-5 significantly accelerated the formation of atherosclerotic lesions in Apoe-/- mice. Moreover, both anti-Siglec-5 antibodies and soluble Siglec-5 protein significantly alleviated the retention of oxLDL in the aorta of rAAV9-Siglec-5-transfected C57BL/6 mice and the formation of atherosclerotic plaques in rAAV9-Siglec-5-transfected Apoe-/- mice.

CONCLUSION

Our results suggested that Siglec-5 was a novel receptor that mediated oxLDL transcytosis and promoted the formation of foam cells. Interventions that inhibit the interaction between oxLDL and Siglec-5, including anti-Siglec-5 antibody or soluble Siglec-5 protein treatment, may provide novel therapeutic strategies in treating atherosclerosis.

In vitro heterochronic parabiosis identifies pigment epithelium-derived factor as a systemic mediator of rejuvenation by young blood

Several decades of heterochronic parabiosis (HCPB) studies have demonstrated the restorative impact of young blood, and deleterious influence of aged blood, on physiological function and homeostasis across tissues, although few of the factors responsible for these observations have been identified. Here we develop an in vitro HCPB system to identify these circulating factors, using replicative lifespan (RLS) of primary human fibroblasts as an endpoint of cellular health. We find that RLS is inversely correlated with serum donor age and sensitive to the presence or absence of specific serum components. Through in vitro HCPB, we identify the secreted protein pigment epithelium-derived factor (PEDF) as a circulating factor that extends RLS of primary human fibroblasts and declines with age in mammals. Systemic administration of PEDF to aged mice reverses age-related functional decline and pathology across several tissues, improving cognitive function and reducing hepatic fibrosis and renal lipid accumulation. Together, our data supports PEDF as a systemic mediator of the effect of young blood on organismal health and homeostasis and establishes our in vitro HCPB system as a valuable screening platform for the identification of candidate circulating factors involved in aging and rejuvenation.

The Contribution of Serum Sialic Acid Binding Immunoglobulin-Like Lectin 1(sSIGLEC-1) as an IFN I Signature Biomarker in the Progression of Atherosclerosis in Egyptian Systemic Lupus Erythematosus (SLE) Patients

Clinical symptoms of systemic lupus erythematosus (SLE), such as atherosclerosis and related cardiovascular diseases, are caused by inflammatory cytokines and endothelial cell damage. The serum sialic acid binding immunoglobulin-like lectin 1 (sSIGLEC-1) is thought to be an alternative biomarker of IFN signature and may have a role in the pathogenesis of atherosclerosis. The aim of the study was to measure the levels of sSIGLEC-1 in the serum of SLE patients in comparison to a control group and examine the associations between sSIGLEC-1, SLEDAI, lipid profile, oxidized low density lipoprotein (oxLDL), and carotid intima media thickness (CIMT) to investigate whether sSIGLEC-1 participates in the development of atherosclerosis. sSIGLEC-1 levels were tested in 53 patients and 20 volunteers using ELISA kit. Duplex measurements were performed on all subjects to measure CIMT. SLE patients had significantly higher values of sSIGLEC-1 (P < 0.0001), total cholesterol (P = 0.029), triglycerides (P = 0.001), low density lipoprotein (P = 0.032), oxLDL (P = 0.001), right CIMT (P = 0.0099) and a significantly lower value of high-density lipoprotein (P = 0.04) when compared to controls. sSIGLEC-1 had significant positive correlations with right CIMT (r = 0.5, P < 0.0002) and oxLDL (r = 0.67, P < 0.0001) in all SLE patients. When compared to non-dyslipidemic patients, the dyslipidemic group exhibited significantly higher levels of all previous parameters except HDL and left CIMT. Circulating form of SIGLEC-1 accelerates atherosclerosis and provides a simple way to predict the occurrence of atherosclerosis in SLE patients.

RETRACTED: Deletion of the scavenger receptor Scarb1 in myeloid cells does not affect bone mass

The scavenger receptor class B member 1 (SR-B1 or Scarb1) is a glycosylated cell surface receptor for high density lipoproteins (HDL), oxidized low density lipoproteins (OxLDL), and phosphocholine-containing oxidized phospholipids (PC-OxPLs). Scarb1 is expressed in macrophages and has been shown to have both pro- and anti-atherogenic properties. It has been reported that global deletion of Scarb1 in mice leads to either high or low bone mass and that PC-OxPLs decrease osteoblastogenesis and increase osteoclastogenesis. PC-OxPLs decrease bone mass in 6-month-old mice and are critical pathogenetic factors in the bone loss caused by high fat diet or aging. We have investigated here whether Scarb1 expression in myeloid cells affects bone mass and whether PC-OxPLs exert their anti-osteogenic effects via activation of Scarb1 in macrophages. To this end, we generated mice with deletion of Scarb1 in LysM-Cre expressing cells and found that lack of Scarb1 did not affect bone mass in vivo. These results indicate that Scarb1 expression in cells of the myeloid/osteoclast lineage does not contribute to bone homeostasis. Based on this evidence, and earlier studies of ours showing that Scarb1 expression in osteoblasts does not affect bone mass, we conclude that Scarb1 is not an important mediator of the adverse effects on PC-OxPLs in osteoclasts or osteoblasts in 6-month-old mice.

The role of sialic acid-binding immunoglobulin-like-lectin-1 (siglec-1) in immunology and infectious disease

Siglec-1, also known as Sialoadhesin (Sn) and CD169 is highly conserved among vertebrates and with 17 immunoglobulin-like domains is Siglec-1 the largest member of the Siglec family. Expression of Siglec-1 is found primarily on dendritic cells (DCs), macrophages and interferon induced monocyte. The structure of Siglec-1 is unique among siglecs and its function as a receptor is also different compared to other receptors in this class as it contains the most extracellular domains out of all the siglecs. However, the ability of Siglec-1 to internalize antigens and to pass them on to lymphocytes by allowing dendritic cells and macrophages to act as antigen presenting cells, is the main reason that has granted Siglec-1's key role in multiple human disease states including atherosclerosis, coronary artery disease, autoimmune diseases, cell-cell signaling, immunology, and more importantly bacterial and viral infections. Enveloped viruses for example have been shown to manipulate Siglec-1 to increase their virulence by binding to sialic acids present on the virus glycoproteins allowing them to spread or evade immune response. Siglec-1 mediates dissemination of HIV-1 in activated tissues enhancing viral spread via infection of DC/T-cell synapses. Overall, the ability of Siglec-1 to bind a variety of target cells within the immune system such as erythrocytes, B-cells, CD8+ granulocytes and NK cells, highlights that Siglec-1 is a unique player in these essential processes.

Thirty-Five-Year History of Desialylated Lipoproteins Discovered by Vladimir Tertov

Atherosclerosis is one of the leading causes of death in developed and developing countries. The atherogenicity phenomenon cannot be separated from the role of modified low-density lipoproteins (LDL) in atherosclerosis development. Among the multiple modifications of LDL, desialylation deserves to be discussed separately, since its atherogenic effects and contribution to atherogenicity are often underestimated or, simply, forgotten. Vladimir Tertov is linked to the origin of the research related to desialylated lipoproteins, including the association of modified LDL with atherogenicity, autoimmune nature of atherosclerosis, and discovery of sialidase activity in blood plasma. The review will briefly discuss all the above-mentioned information, with a description of the current situation in the research.

Macrophage subsets in atherosclerosis as defined by single-cell technologies

Macrophages play a major role in the pathogenesis of atherosclerosis. Many studies have shone light on the different phenotypes and functions that macrophages can acquire upon exposure to local cues. The microenvironment of the atherosclerotic plaque contains a plethora of macrophage-controlling factors, such as cytokines, oxidised low-density lipoproteins and cell debris. Previous research has determined macrophage function within the plaque mainly by using immunohistochemistry and bulk analysis. The recent development and rapid progress of single-cell technologies, such as cytometry by time of flight and single-cell RNA sequencing, now enable comprehensive mapping of the wide range of cell types and their phenotypes present in atherosclerotic plaques. In this review we discuss recent advances applying these technologies in defining macrophage subsets residing in the atherosclerotic arterial wall of mice and men. Resulting from these studies, we describe three main macrophage subsets: resident-like, pro-inflammatory and anti-inflammatory foamy TREM2hi macrophages, which are found in both mouse and human atherosclerotic plaques. Furthermore, we discuss macrophage subset-specific markers and functions. More insights into the characteristics and phenotype of immune cells within the atherosclerotic plaque may guide future clinical approaches to treat disease. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

DBA/2J Haplotype on Distal Chromosome 2 Reduces Mertk Expression, Restricts Efferocytosis, and Increases Susceptibility to Atherosclerosis

OBJECTIVE

Arch atherosclerosis 4 (Aath4) is a quantitative trait locus for atherosclerotic plaque formation in the inner curve of the aortic arch previously identified in an F2 cross of Apoe^-/- mice on DBA/2J and 129S6 backgrounds. C-mer proto-oncogene tyrosine kinase (Mertk), coding for a ligand-activated transmembrane tyrosine kinase, is a candidate gene within the same chromosomal region. Our objective was to determine whether strain differences in Mertk influence plaque formation.

APPROACH AND RESULTS

To dissect the strain effects of Mertk on atherosclerosis, we first established a congenic mouse line (Aath4a^DBA/DBA ) in which a 5' region of Aath4 of DBA/2J, including Mertk, was backcrossed onto a 129S6-Apoe^-/- background. The resulting Aath4a^DBA/DBA male mice developed significantly larger plaques compared with control mice (Aath4a^129/129 ), proving that the DBA/2J allele of Aath4a is proatherogenic. Thioglycollate-elicited peritoneal macrophages from Aath4a^DBA/DBA mice express less than 50% of Mertk mRNA and cell-surface MERTK protein compared with those from the control mice. Moreover, both large and small peritoneal Aath4a^DBA/DBA macrophages showed reduced phagocytosis of apoptotic cells. When Mertk cDNAs from 129S6 and DBA/2J mice were overexpressed in HEK293T (human embryonic kidney 293T) cells, phagocytosis of apoptotic cells was equally enhanced in direct proportion to Mertk levels, indicating that phagocytosis is modulated by the amount of MERTK, but that it is not affected by MERTK amino acid differences between 129S6 and DBA/2J.

CONCLUSIONS

Reduced transcription of Mertk, rather than differences in MERTK protein structure, determines the reduced efficiency of apoptotic cell clearance in the Aath4a^DBA/DBA mice, which, in turn, contributes to their increased susceptibility to atherosclerosis.

Inhibition of siglec-1 by lentivirus mediated small interfering RNA attenuates atherogenesis in apoE-deficient mice

BACKGROUND

Siglec-1 is highly expressed on circulating monocytes and plaque macrophages in atherosclerotic patients, but the exact role of Siglec-1 in atherosclerosis has not been elucidated.

METHODS

Lentiviral vector containing small interfering RNA targeting Siglec-1 (Lv-shSiglec-1) or control vector (Lv-shNC) were injected intravenously into 6-week old Apoe^-/- mice. Then onset of atherosclerosis was observed.

RESULTS

Siglec-1 was highly expressed in aortic plaques and it can be down-regulated by Lv-shSiglec-1 injection. The plaque area and serum pro-inflammatory cytokine (IL-1β, IL-6, TNF-α and IL-17A) levels in Lv-shSiglec-1 mice were significantly lower than Lv-shNC mice, whereas IL-10 was higher. Moreover, plaque macrophages accumulation in aortic wall in Lv-shSiglec-1 mice was diminish, partly by decreased secretion of MCP-1/CXCL2 and CCR2/CXCR2 of aortas and monocytes, respectively. Furthermore, silencing of Siglec-1 can attenuate oxLDL uptake by peritoneal macrophages.

CONCLUSIONS

Inhibition of Siglec-1 can prevent atherosclerotic lesion formation by suppress monocytes-endothelial cells adhesion and macrophages accumulation.

NLRP3 inflammasome: a novel link between lipoproteins and atherosclerosis

INTRODUCTION

Pattern recognition receptor-mediated signaling pathways have recently been elucidated to bridge the innate immune system and atherosclerosis. NLRP3 is a member of the NLR family. Upon activation, it initiates IL-1β and IL-18 processing, a key step in the inflammatory process of atherosclerosis.

MATERIAL AND METHODS

We used three different types of lipoproteins, ox-LDL, ox-HDL, and HDL, in Thp-1 at the concentration of 50 mg/l, 100 mg/l, and 150 mg/l respectively. Using real-time polymerase chain reaction and western blot, ELISA detected the expression of NLRP3 and downstream cytokines. NLRP3 siRNA was constructed to down-regulate expression of the NLRP3 gene via the RNA interference technique. 150 mg/l of ox-LDL, ox-HDL and HDL was added to the Thp-1 cell line respectively. We observed the changes in the expression of caspase-1, IL-1β and IL-18 when the NLRP3 gene was down-regulated.

RESULTS

Ox-LDL and ox-HDL addition not only increases the expression of NLRP3, but also activates the NLRP3 downstream cytokines and caspase-1 and induces IL-1β and IL-18 secretion. Moreover, the effects of activation and induction are shown to have a dose-dependent manner. Expression of NLRP3 and its downstream inflammatory cytokines is reduced in the presence of HDL (p < 0.05). Furthermore, our data demonstrated that NLRP3 siRNA downregulates NLRP3 expression in mononuclear cells, thus leading to a dramatic reduction in the expression of caspase-1, IL-1β and IL-18 (p < 0.05).

CONCLUSIONS

The data suggest that activation of the NLRP3 inflammasome is a critical step in caspase-1 activation and IL-1β and IL-18 secretion. Interference with the NLRP3 inflammasome can significantly inhibit the generation of cytokines, thus impeding the pathogenesis of inflammation.

Bcl-2-associated transcription factor 1 interacts with fragile X-related protein 1

The absence of fragile X mental retardation protein (FMRP) causes fragile X syndrome (FXS), which is the leading cause of hereditary mental retardation. Fragile X-related protein 1 (FXR1P), which plays an important role in normal muscle development, is one of the two autosomal paralogs of FMRP. To understand the functions of FXR1P, we screened FXR1P-interacting proteins by using a yeast two-hybrid system. The fragile X-related gene 1 (FXR1) was fused to pGBKT7 and then used as the bait to screen the human fetal brain cDNA library. The screening results revealed 10 FXR1P-interacting proteins including Bcl-2-associated transcription factor 1 (BTF). The interaction between FXR1P and BTF was confirmed by using both β-galactosidase assay and growth test in selective media. Co-immunoprecipitation assay in mammalian cells was also carried out to confirm the FXR1P/BTF interaction. Moreover, we confirmed that BTF co-localized with FXR1P in the cytoplasm around the nucleus in rat vascular smooth muscle cells by using confocal fluorescence microscopy. These results provide clues to elucidate the relationship between FXR1P and FXS.