Lectin-like oxidized low-density lipoprotein receptor-1, a new promising target for the therapy of atherosclerosis?

Impaired vascular function in small resistance arteries of LOX-1 overexpressing mice on high-fat diet

AIMS

LOX-1 is a major vascular receptor for oxidized low-density lipoprotein (oxLDL). In this study, we analysed the impact of LOX-1 overexpression and high dietary fat intake on vascular function in small resistance arteries.

METHODS AND RESULTS

Relaxation of mesenteric arteries was measured using a wire myograph. Compared with the control group, mice overexpressing LOX-1 on a high-fat diet (FD) had preserved vascular smooth muscle relaxation, but impaired endothelium-dependent relaxation via NO. Vascular NO availability was decreased by exaggerated formation of reactive oxygen species and decreased endothelial NO synthase expression. Endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation via cytochrome P450 metabolites was increased in LOX-1 + FD animals, but did not completely compensate for the loss of NO. Currents of calcium-activated potassium channels with large conductance (BKCa channels) were measured by the voltage-clamp method. The BKCa current amplitudes were not altered in endothelial cells, but highly increased in vascular smooth muscle cells from resistance arteries of LOX-1-overexpressing mice on FD. BK(Ca) currents were activated by low-dose H2O2 and cytochrome P450 metabolites 11,12-EET and 14,15-EET as EDHF in control mice.

CONCLUSION

LOX-1 overexpression and FD caused functional changes in endothelial and vascular smooth muscle cells of small resistance arteries.

LOX-1-dependent transcriptional regulation in response to oxidized LDL treatment of human aortic endothelial cells

Oxidized low-density lipoprotein (OxLDL) has been implicated as a proatherogenic factor with a pathological role in the induction of endothelial dysfunction. Endothelial cells bind and uptake OxLDL primarily through the scavenger receptor lectin-like oxidized-low-density lipoprotein receptor-1 (LOX-1), which is believed to mediate critical effects of OxLDL in endothelial cells. To examine the biological events following LOX-1 activation by OxLDL, we used cDNA microarray analysis to globally analyze gene expression changes induced by OxLDL treatment of human aortic endothelial cell line (HAECT) cells overexpressing LOX-1. Consistent with reported functions of OxLDL, in control HAECT cells, OxLDL elicited gene changes in the oxidative stress pathway and other signaling pathways related to OxLDL. With OxLDL treatment, LOX-1-dependent gene expression changes associated with inflammation, cell adhesion, and signal transduction were observed. The transcripts of a number of cytokines and chemokines were induced, which included interleukin-8, CXCL2, CXCL3, and colony-stimulating factor-3. The secretion of these cytokines was confirmed by enzyme-linked immunosorbent assay analysis. In addition, our data revealed a novel link between LOX-1 and a number of genes, including Delta/notch-like epidermal growth factor repeat containing, stanniocalcin-1, cAMP response element modulator, and dual specificity phosphatase 1. Promoter analysis on the genes that changed as a result of LOX-1 activation by OxLDL allowed us to identify early growth response 1 and cAMP response element-binding protein as potential novel transcription factors that function downstream of LOX-1. Our study has enabled us to elucidate the gene expression changes following OxLDL activation of LOX-1 in endothelial cells and discover novel downstream targets for LOX-1.

CRP is a novel ligand for the oxidized LDL receptor LOX-1

C-reactive protein (CRP) is a risk factor for cardiovascular events and functions to amplify vascular inflammation through promoting endothelial dysfunction. Lectin-like oxidized low-density lipoprotein (oxLDL) receptor-1 (LOX-1) is the primary endothelial receptor for oxLDL, and both its expression and function are associated with vascular inflammation. As a scavenger receptor, LOX-1 is capable of binding to a variety of structurally unrelated ligands. Evidence is provided that demonstrates that CRP can act as a novel ligand for LOX-1. The direct interaction between these two proteins was demonstrated with purified protein in both ELISA and AlphaScreen assays. This interaction could be disrupted with known LOX-1 ligands, such as oxLDL and carrageenan. Moreover, the CRP interaction with cell surface-expressed LOX-1 was confirmed in cell-based immunofluorescent-binding studies. Mutagenesis studies demonstrated that the arginine residues forming the basic spine structure on the LOX-1 ligand-binding interface were dispensable for CRP binding, suggesting a novel ligand-binding mechanism for LOX-1, distinct from that used for oxLDL binding. The treatment of human endothelial cells with CRP led to the activation of proinflammatory genes including IL-8, ICAM-1, and VCAM-1. The inductions of these genes by CRP were LOX-1 dependent, as demonstrated by their attenuation in cells transfected with LOX-1 small-interfering RNA. Our study identifies and characterizes the direct interaction between LOX-1 and CRP and suggests that this interaction may mediate CRP-induced endothelial dysfunction.

Transmembrane signaling pathway mediates oxidized low-density lipoprotein-induced expression of plasminogen activator inhibitor-1 in vascular endothelial cells

Atherosclerotic cardiovascular disease is the number one cause of death for adults in Western society. Plasminogen activator inhibitor-1 (PAI-1), the major physiological inhibitor of plasminogen activators, has been implicated in both thrombogenesis and atherogenesis. Previous studies demonstrated that copper-oxidized low-density lipoprotein (C-oLDL) stimulated production of PAI-1 in vascular endothelial cells (EC). The present study examined the involvement of lectin-like oxidized LDL receptor-1 (LOX-1) and Ras/Raf-1/ERK1/2 pathway in the upregulation of PAI-1 in cultured EC induced by oxidized LDLs. The results demonstrated that C-oLDL or FeSO(4)-oxidized LDL (F-oLDL) increased the expression of PAI-1 or LOX-1 in human umbilical vein EC (HUVEC) or coronary artery EC (HCAEC). Treatment with C-oLDL significantly increased the levels of H-Ras mRNA, protein, and the translocation of H-Ras to membrane fraction in EC. LOX-1 blocking antibody, Ras farnesylation inhibitor (FTI-277), or small interference RNA against H-Ras significantly reduced C-oLDL or LDL-induced expression of H-Ras and PAI-1 in EC. Incubation with C-oLDL or F-oLDL increased the phosphorylation of Raf-1 and ERK1/2 in EC compared with LDL or vehicle. Treatment with Raf-1 inhibitor blocked Raf-1 phosphorylation and the elevation of PAI-1 mRNA level in EC induced by C-oLDL or LDL. Treatment with PD-98059, an ERK1/2 inhibitor, blocked C-oLDL or LDL-induced ERK1/2 phosphorylation or PAI-1 expression in EC. The results suggest that LOX-1, H-Ras, and Raf-1/ERK1/2 are implicated in PAI-1 expression induced by oxidized LDLs or LDL in cultured EC.

Vascular endothelium in atherosclerosis

Their strategic location between blood and tissue and their constitutive properties allow endothelial cells (EC) to monitor the transport of plasma molecules, by employing bidirectional receptor-mediated and receptor-independent transcytosis and endocytosis, and to regulate vascular tone, cellular cholesterol and lipid homeostasis. These cells are also involved in signal transduction, immunity, inflammation and haemostasis. Cardiovascular risk factors, such as hyperlipaemia/dyslipidaemia trigger the molecular machinery of EC to respond to insults by modulation of their constitutive functions followed by dysfunction and ultimately by injury and apoptosis. The gradual activation of EC consists initially in the modulation of two constitutive functions: (1) permeability, i.e. increased transcytosis of lipoproteins, and (2) biosynthetic activity, i.e. enhanced synthesis of the basement membrane and extracellular matrix. The increased transcytosis and the reduced efflux of beta-lipoproteins (betaLp) lead to their retention within the endothelial hyperplasic basal lamina as modified lipoproteins (MLp) and to their subsequent alteration (oxidation, glycation, enzymatic modifications). MLp generate chemoattractant and inflammatory molecules, triggering EC dysfunction (appearance of new adhesion molecules, secretion of chemokines, cytokines), characterised by monocyte recruitment, adhesion, diapedesis and residence within the subendothelium. In time, EC in the athero-prone areas alter their net negative surface charge, losing their non-thrombogenic ability, become loaded with lipid droplets and turn into foam cells. Prolonged and/or repeated exposure to cardiovascular risk factors can ultimately exhaust the protective effect of the endogenous anti-inflammatory system within EC. As a consequence, EC may progress to senescence, lose their integrity and detach into the circulation.

Combined effects of niacin and chromium treatment on vascular endothelial dysfunction in hyperlipidemic rats

Combined treatment with niacin and chromium has been found to have a protective effect against oxidative damage to different tissues of hyperlipidemic rats. But its effects on vascular endothelial dysfunction are less clear. This study was performed to investigate the effect of combined treatment with niacin and chromium on vascular endothelial dysfunction, with the aim of gaining insight to the mechanisms by detecting the expression levels of ox-LDL and LOX-1. Twenty-four male, 4-week-old Wistar rats were randomly divided into three groups: control group (CG; n = 8), high-fat group (HF; n = 8), and drug control group (DG; n = 8). In CG group, rats were fed with pellet chow. In HF group, rats were fed with high-fat diet for 12 weeks. In DG group, rats were fed with the same high-fat diet and treated with CrCl(3) x 6 H(2)O (250 microg/kg days) and niacin (100 mg/kg days) by gavage technique for 12 weeks. At the end of the 12th week, samples from aorta and blood were collected. In HF group, the serum levels of total cholesterol (TC), low-density lipoprotein (LDL), oxidized low-density lipoprotein (ox-LDL) and endothelin (ET) were higher, whereas the levels of high-density lipoprotein (HDL), serum NO were lower than those in CG group. The levels of serum TC, LDL, ox-LDL and ET decreased and HDL, NO levels increased in DG group when compared with HF group. The levels of LOX-1, ICAM-1 were also observed in abdominal artery. In HF group, the protein and mRNA expression of LOX-1, ICAM-1 were elevated comparing with CG group. In DG group, the protein and mRNA expression of LOX-1, ICAM-1 were decreased obviously, but still differed significantly from those in CG group. ox-LDL was related positively to TC, LDL, ET, ICAM-1 and LOX-1, but related negatively to NO and HDL. These findings indicated that combined treatment with niacin and chromium has potential therapeutic protection of endothelial function by down-regulating ox-LDL/LOX-1 signaling pathway.

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.

Homocysteine up-regulates vascular transmembrane chemokine CXCL16 and induces CXCR6+ lymphocyte recruitment in vitro and in vivo

Objective: Hyperhomocysteinemia induces endothelial dysfunction and promotes atherosclerotic vascular disease. Infiltrates of activated macrophages and lymphocytes are observed in human and experimental atherosclerotic lesions, their emigration being guided by endothelial-leukocyte adhesion molecules and chemoattractants. The CXC-chemokine CXCL16 functions as an adhesion molecule by interacting with its receptor (CXCR6) and also as a scavenger for oxidized low density lipoprotein (oxLDL). We investigated the modulation of CXCL16 on cultured endothelial cells (EC) and the recruitment of CXCR6⁺ lymphocytes in response to homocysteine (Hcy), in vitro and in vivo. Methods and Results: Hcy-stimulated EC show a significant increase in CXCL16 mRNA and protein expression. Incubation of EC with d,l-Hcy and l-Hcy significantly increased CXCR6⁺ lymphocyte adhesion to EC while l-Cysteine (l-Cys) had no effect. Furthermore, EC stimulation with Hcy increased uptake of DiI-oxLDL. An anti-CXCL16 monoclonal antibody, antioxidants (Tiron) and PPAR-γ agonists (Pioglitazone) considerably reduced CXCR6⁺ lymphocyte adhesion and uptake of DiI-oxLDL. Upon injection in the peritoneal cavities of mice, l-Hcy and not l-Cys, increased the number of CXCR6⁺ lymphocytes, which was reduced by coinjection with Pioglitazone or anti-human CXCL16 antibody. Conclusions: Hyperhomocysteinemia up-regulates CXCL16 leading to increased recruitment of CXCR6⁺ lymphocytes and scavenging of modified lipids via a potential involvement of a PPAR-γ-dependent mechanism. CXCL16 may therefore contribute to the formation and progression of atherosclerotic lesions under conditions of hyperhomocysteinemia.