Characterization of residues and sequences of the carbohydrate recognition domain required for cell surface localization and ligand binding of human lectin-like oxidized LDL receptor

LOX-1 variants modulate the severity of cardiovascular disease: state of the art and future directions

Atherosclerosis is one of the major causes of cerebral infarction and many other ischemic cardio-cerebrovascular diseases. Although large randomized clinical trials have highlighted the impressive benefits of lipid-lowering therapies, the 50-70% of patients who have achieved their lipid-lowering goal remain at high cardiovascular disease risk. For this reason, there is a need to investigate other markers of atherosclerosis progression. LOX-1 is a scavenger receptor that accepts oxidized low-density lipoproteins as major ligand and internalizes it by endocytosis favoring its retention in subendothelial layer and triggering a wide variety of proatherogenic events. However, other factors such as cytokines, shear stress, and advanced glycation end-products can upregulate LOX-1. LOX-1 is encoded by the OLR1 gene, located in the p12.3-p13 region of chromosome 12. OLR1 gene has different isoforms induced by splicing, or single-nucleotide polymorphisms (SNPs). According to some authors, the expression of these isoforms induces a different effect on atherosclerosis and cardiovascular disease. In particular, LOXIN, an isoform lacking part of the functional domain, exerts an important role in atherosclerosis protection. In other cases, studies on SNPs showed an association with more severe forms, like in the case of 3'UTR polymorphisms. The knowledge of these variants can give rise to the development of new preventive therapies and can lead to the identification of subjects at greater risk of cardiovascular event. In this review, we reported the state of the art regarding SNPs with known effects on OLR1 splicing and how LOX-1 variants modulate the severity of cardiovascular disease.

Bezafibrate mitigates oxidized-low density lipoprotein (ox-LDL)-induced the attachment of monocytes to endothelial cells: An implication in atherosclerosis

BACKGROUND

Oxidized forms of low-density lipoproteins (ox-LDL)-associated endothelial dysfunction and subsequent monocyte adhesion play an important role in the development of atherosclerosis (AS). Bezafibrate (BEZ) is a peroxisome proliferator-activated receptor (pan-PPAR) agonist licensed as a hypolipidemic drug. However, the effects of BEZ on endothelial dysfunction are less reported.

OBJECTIVES

In this study, we aim to investigate the protective effects of BEZ on ox-LDL-challenged vascular endothelial cells to evaluate its potential value in treating AS.

METHODS

Human aortic endothelial cells (HAECs) and THP-1 cells were used to establish an In Vitro AS model. Cell Counting Kit-8 (CCK-8) assay, Real-time PCR, Western blot analysis, and Enzyme-linked immunosorbent assay (ELISA) were used to test the data.

RESULTS

As expected, treatment with BEZ suppressed the expression of vascular endothelial growth factor A (VEGF-A), tissue factor (TF), Interleukin 12 (IL-12), tumor necrosis factor (TNF-α), and monocyte chemoattractant protein-1 (MCP-1). BEZ was also found to inhibit ox-LDL-induced expression of the endothelial adhesion molecules vascular cellular adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) in HAECs. Correspondingly, BEZ prevented attachment of THP-1 monocytes to ox-LDL-incubated HAECs. Mechanically, BEZ was found to prevent NF-κB activation by reducing the levels of nuclear NF-κB p65 and inhibiting luciferase activity of NF-κB.

CONCLUSION

Our study revealed the pharmacological function of BEZ in protecting endothelial dysfunction against ox-LDL, which may provide valuable insight for the clinical application of BEZ.

LOX-1 in Cardiovascular Disease: A Comprehensive Molecular and Clinical Review

LOX-1, ORL-1, or lectin-like oxidized low-density lipoprotein receptor 1 is a transmembrane glycoprotein that binds and internalizes ox-LDL in foam cells. LOX-1 is the main receptor for oxidized low-density lipoproteins (ox-LDL). The LDL comes from food intake and circulates through the bloodstream. LOX-1 belongs to scavenger receptors (SR), which are associated with various cardiovascular diseases. The most important and severe of these is the formation of atherosclerotic plaques in the intimal layer of the endothelium. These plaques can evolve into complicated thrombi with the participation of fibroblasts, activated platelets, apoptotic muscle cells, and macrophages transformed into foam cells. This process causes changes in vascular endothelial homeostasis, leading to partial or total obstruction in the lumen of blood vessels. This obstruction can result in oxygen deprivation to the heart. Recently, LOX-1 has been involved in other pathologies, such as obesity and diabetes mellitus. However, the development of atherosclerosis has been the most relevant due to its relationship with cerebrovascular accidents and heart attacks. In this review, we will summarize findings related to the physiologic and pathophysiological processes of LOX-1 to support the detection, diagnosis, and prevention of those diseases.

Radical Oxygen Species, Oxidized Low-Density Lipoproteins, and Lectin-like Oxidized Low-Density Lipoprotein Receptor 1: A Vicious Circle in Atherosclerotic Process

Atherosclerosis is a complex condition that involves the accumulation of lipids and subsequent plaque formation in the arterial intima. There are various stimuli, cellular receptors, and pathways involved in this process, but oxidative modifications of low-density lipoprotein (ox-LDL) are particularly important in the onset and progression of atherosclerosis. Ox-LDLs promote foam-cell formation, activate proinflammatory pathways, and induce smooth-muscle-cell migration, apoptosis, and cell death. One of the major receptors for ox-LDL is LOX-1, which is upregulated in several cardiovascular diseases, including atherosclerosis. LOX-1 activation in endothelial cells promotes endothelial dysfunction and induces pro-atherogenic signaling, leading to plaque formation. The binding of ox-LDLs to LOX-1 increases the generation of reactive oxygen species (ROS), which can induce LOX-1 expression and oxidize LDLs, contributing to ox-LDL generation and further upregulating LOX-1 expression. This creates a vicious circle that is amplified in pathological conditions characterized by high plasma levels of LDLs. Although LOX-1 has harmful effects, the clinical significance of inhibiting this protein remains unclear. Further studies both in vitro and in vivo are needed to determine whether LOX-1 inhibition could be a potential therapeutic target to counteract the atherosclerotic process.

"Affimer" synthetic protein scaffolds block oxidized LDL binding to the LOX-1 scavenger receptor and inhibit ERK1/2 activation

In multicellular organisms, a variety of lipid-protein particles control the systemic flow of triacylglycerides, cholesterol, and fatty acids between cells in different tissues. The chemical modification by oxidation of these particles can trigger pathological responses, mediated by a group of membrane proteins termed scavenger receptors. The lectin-like oxidized low-density lipoprotein (LOX-1) scavenger receptor binds to oxidized low-density lipoprotein (oxLDL) and mediates both signaling and trafficking outcomes. Here, we identified five synthetic proteins termed Affimers from a phage display library, each capable of binding recombinant LOX-1 extracellular (oxLDL-binding) domain with high specificity. These Affimers, based on a phytocystatin scaffold with loop regions of variable sequence, were able to bind to the plasma membrane of HEK293T cells exclusively when human LOX-1 was expressed. Binding and uptake of fluorescently labeled oxLDL by the LOX-1-expressing cell model was inhibited with subnanomolar potency by all 5 Affimers. ERK1/2 activation, stimulated by oxLDL binding to LOX-1, was also significantly inhibited (p < 0.01) by preincubation with LOX-1-specific Affimers, but these Affimers had no direct agonistic effect. Molecular modeling indicated that the LOX-1-specific Affimers bound predominantly via their variable loop regions to the surface of the LOX-1 lectin-like domain that contains a distinctive arrangement of arginine residues previously implicated in oxLDL binding, involving interactions with both subunits of the native, stable scavenger receptor homodimer. These data provide a new class of synthetic tools to probe and potentially modulate the oxLDL/LOX-1 interaction that plays an important role in vascular disease.

Autoimmune Rheumatic Diseases: An Update on the Role of Atherogenic Electronegative LDL and Potential Therapeutic Strategies

Atherosclerosis has been linked with an increased risk of atherosclerotic cardiovascular disease (ASCVD). Autoimmune rheumatic diseases (AIRDs) are associated with accelerated atherosclerosis and ASCVD. However, the mechanisms underlying the high ASCVD burden in patients with AIRDs cannot be explained only by conventional risk factors despite disease-specific factors and chronic inflammation. Nevertheless, the normal levels of plasma low-density lipoprotein (LDL) cholesterol observed in most patients with AIRDs do not exclude the possibility of increased LDL atherogenicity. By using anion-exchange chromatography, human LDL can be divided into five increasingly electronegative subfractions, L1 to L5, or into electropositive and electronegative counterparts, LDL (+) and LDL (-). Electronegative L5 and LDL (-) have similar chemical compositions and can induce adverse inflammatory reactions in vascular cells. Notably, the percentage of L5 or LDL (-) in total LDL is increased in normolipidemic patients with AIRDs. Electronegative L5 and LDL (-) are not recognized by the normal LDL receptor but instead signal through the lectin-like oxidized LDL receptor 1 (LOX-1) to activate inflammasomes involving interleukin 1β (IL-1β). Here, we describe the detailed mechanisms of AIRD-related ASCVD mediated by L5 or LDL (-) and discuss the potential targeting of LOX-1 or IL-1β signaling as new therapeutic modalities for these diseases.

LOX-1: Its cytotopographical variance and disease stress

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is a canonical receptor for oxidized LDL (oxLDL) among the known modified LDL particles. Topographical variance on LOX-1 expression in different cell types and its influence on the atherogenic potential of the particular cell type is the main focus of this review. Characteristic features of LOX-1 on the atherogenic potential of aortic endothelial cells, macrophages, platelets, and vascular smooth muscle cells have been discussed. Nonspecificity of ligands, besides oxLDL, is also the highlight of this review to show the chameleon characteristics in the functional activity of the receptor protein. Induction of LOX-1 has been reported in diseases like atherosclerosis, diabetes, and hypertension, as well as in the inflammatory response of immune reactions. The expression of LOX-1 is upregulated by the vicious cycle of stimulatory response from proatherogenic molecules.

LOX-1 and atherosclerotic-related diseases

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), a scavenger receptor of oxidized low-density lipoprotein (ox-LDL) found in various cells, plays a crucial role in the formation and progression of atherosclerotic plaques. Animal studies have suggested that LOX-1 mediates the balance between internalization and degeneration of endothelial cells, thereby contributing to various steps in the atherosclerotic process, from initiation to plaque rupture. Under pathological conditions, the extracellular domain of membrane bound LOX-1 can be largely proteolytically cleaved into a soluble form (sLOX-1), which is proportional and linked to the LOX-1 expression level. Circulating levels of sLOX-1 are regarded as a risk biomarker for plaque rupture and acute coronary syndrome (ACS). Recently, studies have shown that sLOX-1 is also elevated in patients with acute stroke and can be a predictive biomarker for acute stroke. With the discovery of the vital role of LOX-1 in atherosclerosis, there is growing focus on the influence of LOX-1 in atherosclerotic-related diseases, including coronary arterial disease(CAD), stroke, and other cardiovascular events. Genetic polymorphisms of LOX-1 have been investigated and have been found to modulate the risk of these diseases. Most polymorphisms have been found to be risk factors, except for the splicing isoform LOXIN. This review concludes with a discussion of the potential future applications of LOX-1 for atherosclerotic-related diseases.

The Ligands of C-Type Lectins

In this chapter, a comprehensive overview of the known ligands for the C-type lectins (CTLs) is provided. Emphasis has been placed on the chemical structure of the glycans that bind to the different CTLs and the amount of structural variation (or overlap) that each CTL can tolerate. In this way, both the synthetic carbohydrate chemist and the immunologist can more readily gain insight into the existing structure-activity space for the CTL ligands and, ideally, see areas of synergy that will help identify and refine the ligands for these receptors.

Inhibitory effects of Mycoepoxydiene on macrophage foam cell formation and atherosclerosis in ApoE-deficient mice

Background

Mycoepoxydiene (MED) is a polyketide that can be isolated from a marine fungus and is associated with various activities, including antitumor and anti-inflammatory functions. However, its effects on atherosclerosis remain unknown. Macrophage-derived foam cells play crucial roles in the initiation and progression of atherosclerotic plaques. In this study, we investigated the effects of MED on oxidized low-density lipoprotein (ox-LDL)-induced macrophage foam cell formation and activation, and on high fat diet (HFD)-induced atherosclerosis in ApoE-deficient (ApoE^−/−) mice.

Results

Our findings show that MED could significantly inhibit ox-LDL-induced macrophage foam cell formation and suppress the expression of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), which is a receptor for ox-LDL. Additionally, MED could significantly inhibit the secretion of proinflammatory cytokines, such as tumor necrosis factor (TNF-α), interleukin (IL)-6, and IL-1β. Mechanistically, MED inhibited NF-κB activation by blocking IκB-α degradation and reducing NF-κB DNA binding activity. Moreover, MED dramatically reduced the occurrence of HFD-induced atherosclerotic lesions in ApoE^−/− mice.

Conclusions

Our study shows that MED can inhibit macrophage foam cell formation and activation by inhibiting NF-κB activation, thereby protecting ApoE^−/− mice from HFD-induced atherosclerosis. Our findings suggest that MED might be a potential lead compound for the development of antiatherosclerotic therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s13578-015-0017-y) contains supplementary material, which is available to authorized users.

LOX-1, a new marker of risk and prognosis in coronary artery disease?

The development of atherosclerosis is caused by the accumulation of lipid, inflammatory cytokine production, and the large amount of inflammatory cells in the arterial wall. It is now established that the presence of oxidized low-density lipoproteins (ox-LDL) has an important role in the pathogenesis of the disease. There are many scavenger receptors for ox-LDL, among which LOX-1 seems to be important for the induction of endothelial dysfunction and the other subsequent events that lead to the formation of atheromatous plaque. Our findings indicate the presence of a regulatory role induced by the presence of ox-LDL on LOX-1 through the amplification of IL-6 synthesis. This mechanism contributes to the upregulation of the ORL-1 gene expression in presence of risk factors. Many authors have shown the possibility to use LOX-1 as a good marker for the diagnosis and prognosis of coronary artery disease because it is easy to measure and more sensitive than other markers commonly used in the routine of laboratory medicine.

The discovery of LOX-1, its ligands and clinical significance

LOX-1 is an endothelial receptor for oxidized low-density lipoprotein (oxLDL), a key molecule in the pathogenesis of atherosclerosis.The basal expression of LOX-1 is low but highly induced under the influence of proinflammatory and prooxidative stimuli in vascular endothelial cells, smooth muscle cells, macrophages, platelets and cardiomyocytes. Multiple lines of in vitro and in vivo studies have provided compelling evidence that LOX-1 promotes endothelial dysfunction and atherogenesis induced by oxLDL. The roles of LOX-1 in the development of atherosclerosis, however, are not simple as it had been considered. Evidence has been accumulating that LOX-1 recognizes not only oxLDL but other atherogenic lipoproteins, platelets, leukocytes and CRP. As results, LOX-1 not only mediates endothelial dysfunction but contributes to atherosclerotic plaque formation, thrombogenesis, leukocyte infiltration and myocardial infarction, which determine mortality and morbidity from atherosclerosis. Moreover, our recent epidemiological study has highlighted the involvement of LOX-1 in human cardiovascular diseases. Further understandings of LOX-1 and its ligands as well as its versatile functions will direct us to ways to find novel diagnostic and therapeutic approaches to cardiovascular disease.

LOX-1/LOXIN: the yin/yang of atheroscleorosis

Atherosclerosis is the first cause of death in industrialized countries. Together with traditional risk factors (male gender, hypercholesterolemia, hypertension, diabetes, smoking and age), non-traditional risk factors have also been described as predisposing to this disease. Among these, oxidized low density lipoproteins (OxLDL) have been described in correlation to many proatherogenic processes. Many of the effects of OxLDL are mediated by the lectin like oxidized low density lipoprotein receptor 1 (LOX-1), expressed on endothelial cells, macrophages, SMCs and platelets. LOX-1 is encoded by the lectin like oxidized low density lipoprotein receptor 1 (OLR1) gene, located in the p12.3-p13.2 region of human chromosome 12. Variations on this gene have been studied extensively both at the functional and epidemiological level. Despite the fact that functional roles for two variants have been demonstrated, the epidemiological studies have provided inconsistent and inconclusive results. Of particular interest, it has been demonstrated that a linkage disequilibirum block of SNPs located in the intronic sequence of the OLR1 gene modulates the alternative splicing of OLR1 mRNA, leading to different ratios of LOX-1 full receptor and LOXIN, an isoform lacking part of the functional domain. As demonstrated, LOXIN acts by blocking the negative effective of LOX-1 activation. Here we review the state of the art regarding LOX-1, LOXIN, and the functional effects that are associated with the interaction of these molecules.

Identification of 4-hydroxy-2-nonenal–histidine adducts that serve as ligands for human lectin-like oxidized LDL receptor-1

LOX-1 (lectin-like oxidized low-density lipoprotein receptor-1) is an endothelial scavenger receptor that is important for the uptake of OxLDL (oxidized low-density lipoprotein) and contributes to the pathogenesis of atherosclerosis. However, the precise structural motifs of OxLDL that are recognized by LOX-1 are unknown. In the present study, we have identified products of lipid peroxidation of OxLDL that serve as ligands for LOX-1. We used CHO (Chinese-hamster ovary) cells that stably express LOX-1 to evaluate the ability of BSA modified by lipid peroxidation to compete with AcLDL (acetylated low-density lipoprotein). We found that HNE (4-hydroxy-2-nonenal)-modified proteins most potently inhibited the uptake of AcLDL. On the basis of the findings that HNE-modified BSA and oxidation of LDL resulted in the formation of HNE–histidine Michael adducts, we examined whether the HNE–histidine adducts could serve as ligands for LOX-1. The authentic HNE–histidine adduct inhibited the uptake of AcLDL in a dose-dependent manner. Furthermore, we found the interaction of LOX-1 with the HNE–histidine adduct to have a dissociation constant of 1.22×10−8 M using a surface plasmon resonance assay. Finally, we showed that the HNE–histidine adduct stimulated the formation of reactive oxygen species and activated extracellular-signal-regulated kinase 1/2 and NF-κB (nuclear factor κB) in HAECs (human aortic endothelial cells); these signals initiate endothelial dysfunction and lead to atherosclerosis. The present study provides intriguing insights into the molecular details of LOX-1 recognition of OxLDL.

Lipid peroxidation modification of protein generates Nepsilon-(4-oxononanoyl)lysine as a pro-inflammatory ligand

4-Oxo-2(E)-nonenal (ONE), a peroxidation product of ω-6 polyunsaturated fatty acids, covalently reacts with lysine residues to generate a 4-ketoamide-type ONE-lysine adduct, N(ε)-(4-oxononanoyl)lysine (ONL). Using an ONL-coupled protein as the immunogen, we raised the monoclonal antibody (mAb) 9K3 directed to the ONL and conclusively demonstrated that the ONL was produced during the oxidative modification of a low density lipoprotein (LDL) in vitro. In addition, we observed that the ONL was present in atherosclerotic lesions, in which an intense immunoreactivity was mainly localized in the vascular endothelial cells and macrophage- and vascular smooth muscle cell-derived foam cells. Using liquid chromatography with on-line electrospray ionization tandem mass spectrometry, we also established a highly sensitive method for quantification of the ONL and confirmed that the ONL was indeed formed during the lipid peroxidation-mediated modification of protein in vitro and in vivo. To evaluate the biological implications for ONL formation, we examined the recognition of ONL by the scavenger receptor lectin-like oxidized LDL receptor-1 (LOX-1). Using CHO cells stably expressing LOX-1, we evaluated the ability of ONL to compete with the acetylated LDL and found that both the ONE-modified and ONL-coupled proteins inhibited the binding and uptake of the modified LDL. In addition, we demonstrated that the ONL-coupled protein was incorporated into differentiated THP-1 cells via LOX-1. Finally, we examined the effect of ONL on the expression of the inflammation-associated gene in THP-1 and observed that the ONL-coupled proteins significantly induced the expression of atherogenesis-related genes, such as the monocyte chemoattractant protein-1 and tumor necrosis factor-α, in a LOX-1-dependent manner. Thus, ONL was identified to be a potential endogenous ligand for LOX-1.

Surface plasmon resonance study on functional significance of clustered organization of lectin-like oxidized LDL receptor (LOX-1)

Lectin-like oxidized low-density lipoprotein (OxLDL) receptor 1 (LOX-1) is the major OxLDL receptor of vascular endothelial cells and is involved in an early step of atherogenesis. LOX-1 exists as a disulfide-linked homodimer on the cell surface, which contains a pair of the ligand-binding domains (CTLD; C-type lectin-like domain). Recent research using living cells has suggested that the clustered state of LOX-1 dimer on the cell is functionally required. These results questioned how LOX-1 exists on the cell to achieve OxLDL binding. In this study, we revealed the functional significance of the clustered organization of the ligand-binding domain of LOX-1 with surface plasmon resonance. Biotinylated CTLD was immobilized on a streptavidin sensor chip to make CTLD clusters on the surface. In this state, the CTLD had high affinity for OxLDL with a dissociation constant (K(D)) in the nanomolar range. This value is comparable to the K(D) measured for LOX-1 on the cell. In contrast, a single homodimeric LOX-1 extracellular domain had lower affinity for OxLDL in the supra-micromolar range of K(D). Monomeric CTLD showed marginal binding to OxLDL. In combination with the analyses on the loss-of-binding mutant W150A, we concluded that the clustered organization of the properly formed homodimeric CTLD is essential for the strong binding of LOX-1 to OxLDL.

Scavenger receptors and their potential as therapeutic targets in the treatment of cardiovascular disease

Scavenger receptors act as membrane-bound and soluble proteins that bind to macromolecular complexes and pathogens. This diverse supergroup of proteins mediates binding to modified lipoprotein particles which regulate the initiation and progression of atherosclerotic plaques. In vascular tissues, scavenger receptors are implicated in regulating intracellular signaling, lipid accumulation, foam cell development, and cellular apoptosis or necrosis linked to the pathophysiology of atherosclerosis. One approach is using gene therapy to modulate scavenger receptor function in atherosclerosis. Ectopic expression of membrane-bound scavenger receptors using viral vectors can modify lipid profiles and reduce the incidence of atherosclerosis. Alternatively, expression of soluble scavenger receptors can also block plaque initiation and progression. Inhibition of scavenger receptor expression using a combined gene therapy and RNA interference strategy also holds promise for long-term therapy. Here we review our current understanding of the gene delivery by viral vectors to cells and tissues in gene therapy strategies and its application to the modulation of scavenger receptor function in atherosclerosis.

Ellagic acid inhibits oxidized LDL-mediated LOX-1 expression, ROS generation, and inflammation in human endothelial cells

BACKGROUND

LOX-1, a lectin-like receptor on endothelial cells, facilitates the uptake of oxidized low-density lipoprotein (oxLDL). Expression of LOX-1 is involved in the pathobiological effects of oxLDL in endothelial cells, including reactive oxygen species (ROS) generation, suppression of endothelial nitric oxide synthase (eNOS) activity, and leukocytic adhesion. Moderate consumption of phenolic-enriched food may have a protective effect against the development of atherosclerosis via the antioxidant capacity of phenolic compounds at the endothelial level. In this study, we determined whether ellagic acid, a polyphenolic compound widely distributed in fruits and nuts, protects against oxLDL-induced endothelial dysfunction by modulating the LOX-1-mediated signaling pathway.

METHODS

Human umbilical vein endothelial cells (HUVECs) were pretreated with ellagic acid at doses of 5, 10, 15, and 20 μM for 2 hours and then incubated with oxLDL (150 μg/mL) for an additional 24 hours.

RESULTS

LOX-1 protein expression was markedly lower after exposure to oxLDL in HUVECs pretreated with ellagic acid or diphenyleneiodonium, a well-known inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, than in HUVECs exposed to oxLDL alone, suggesting that ellagic acid deactivates NADPH oxidase. We also found that oxLDL activated the membrane assembly of p47phox, Rac1, gp91 and p22phox, and the subsequent induction of ROS generation; however, ROS generation was markedly suppressed in cells pretreated with ellagic acid or anti-LOX-1 monoclonal antibody. In addition, oxLDL down-regulated eNOS and up-regulated inducible NO synthase (iNOS), thereby augmenting the formation of NO and protein nitrosylation. Furthermore, oxLDL induced the phosphorylation of p38 mitogen-activated protein kinase, activated the NF-κB-mediated inflammatory signaling molecules interleukin-(IL) 6 and IL-8 and the adhesion molecules intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin, and stimulated the adherence of THP-1 (a human acute monocytic leukemia cell line) to HUVECs. Pretreatment with ellagic acid, however, exerted significant cytoprotective effects in all events.

CONCLUSION

Findings from this study may provide insight into a possible molecular mechanism by which ellagic acid inhibits LOX-1-induced endothelial dysfunction. Our data indicate that ellagic acid exerts its protective effects by inhibiting NADPH oxidase-induced overproduction of superoxide, suppressing the release of NO by down-regulating iNOS, enhancing cellular antioxidant defenses, and attenuating oxLDL-induced LOX-1 up-regulation and eNOS down-regulation.

Lipid peroxidation generates body odor component trans-2-nonenal covalently bound to protein in vivo

trans-2-Nonenal is an unsaturated aldehyde with an unpleasant greasy and grassy odor endogenously generated during the peroxidation of polyunsaturated fatty acids. 2-Nonenal covalently modified human serum albumin through a reaction in which the aldehyde preferentially reacted with the lysine residues. Modified proteins were immunogenic, and a specific monoclonal antibody (mAb) 27Q4 that cross-reacted with the protein covalently modified with 2-nonenal was raised from mouse. To verify the presence of the protein-bound 2-nonenal in vivo, the mAb 27Q4 against the 2-nonenal-modified keyhole limpet hemocyanin was raised. It was found that a novel 2-nonenal-lysine adduct, cis- and trans-N(epsilon)-3-[(hept-1-enyl)-4-hexylpyridinium]lysine (HHP-lysine), constitutes an epitope of the antibody. The immunoreactive materials with mAb 27Q4 were detected in the kidney of rats exposed to ferric nitrilotriacetate, an iron chelate that induces free radical-mediated oxidative tissue damage. Using high performance liquid chromatography with on-line electrospray ionization tandem mass spectrometry, we also established a highly sensitive method for detection of the cis- and trans-HHP-lysine and confirmed that the 2-nonenal-lysine adducts were indeed formed during the lipid peroxidation-mediated modification of protein in vitro and in vivo. Furthermore, we examined the involvement of the scavenger receptor lectin-like oxidized low density lipoprotein receptor-1 in the recognition of 2-nonenal-modified proteins and established that the receptor recognized the HHP-lysine adducts as a ligand.

The lectin-like oxidized low-density lipoprotein receptor-1 and its soluble form: cardiovascular implications

The lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) is a multiligand receptor, whose repertoire of ligands includes oxidized low-density lipoprotein, advanced glycation endproducts, platelets, neutrophils, apoptotic/aged cells and bacteria. Sustained expression of LOX-1 by critical target cells, including endothelial cells, smooth muscle cells and macrophages in proximity to these ligands, sets the stage for chronic cellular activation and tissue damage suggesting the interaction of cellular LOX-1 with its ligands to contribute to the formation and development of atherosclerotic plaques. Studies with transgenic and knockout mouse models have elucidated in part the role of LOX-1 in the pathogenesis of atherosclerosis and cardiac remodeling. Recently, a circulating soluble form of LOX-1 (sLOX-1), corresponding solely to its extracellular domain, has been identified in human serum. Circulating levels of sLOX-1 are increased in inflammatory and atherosclerotic conditions and are associated with acute coronary syndrome, with the severity of coronary artery disease, and with serum biomarkers for oxidative stress and inflammation, suggesting that they could be a useful marker for vascular injury. However, many interesting questions have not yet been answered and in this review, we provide an updated overview of the literature on this receptor and on likely future directions.

Mediation of electronegative low-density lipoprotein signaling by LOX-1: a possible mechanism of endothelial apoptosis

The lectin-like oxidized LDL receptor LOX-1 mediates endothelial cell (EC) uptake of experimentally prepared copper-oxidized LDL (oxLDL). To confirm the atherogenic role of this receptor cloned against copper-oxLDL, we examined whether it mediates EC uptake of L5, an electronegative LDL abundant in dyslipidemic but not normolipidemic human plasma. Hypercholesterolemic (LDL-cholesterol, >160 mg/dL) human LDL was fractionated into L1-L5, increasingly electronegative, by ion-exchange chromatography. In cultured bovine aortic ECs (BAECs), L5 upregulated LOX-1 and induced apoptosis. Transfection of BAECs with LOX-1-specific small interfering RNAs (siLOX-1) minimized baseline LOX-1 production and restrained L5-induced LOX-1 upregulation. Internalization of labeled L1-L5 was monitored in BAECs and human umbilical venous ECs by fluorescence microscopy. LOX-1 knockdown with siLOX-1 impeded the endocytosis of L5 but not L1-L4. In contrast, blocking LDL receptor with RAP (LDL receptor-associated protein) stopped the internalization of L1-L4 but not L5. Although chemically different, L5 and oxLDL competed for EC entry through LOX-1. Via LOX-1, L5 signaling hampered Akt phosphorylation and suppressed EC expression of fibroblast growth factor-2 and Bcl-2. L5 also selectively inhibited Bcl-xL expression and endothelial nitric oxide synthase phosphorylation but increased synthesis of Bax, Bad, and tumor necrosis factor-alpha. Blocking Akt phosphorylation with wortmannin increased LOX-1 expression, suggesting a modulatory role of Akt in LOX-1 synthesis; L5 upregulated LOX-1 by dephosphorylating Akt. Because endothelial nitric oxide synthase and Bcl-2 activities are Akt-dependent, L5 impairs Akt-mediated growth and survival signals in vascular ECs by way of LOX-1. Thus, the L5/LOX-1 complex may play a critical role in atherogenesis and illuminate important targets for disease intervention.

Soluble lectin-like oxidized low density lipoprotein receptor-1 in type 2 diabetes mellitus

The lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) can be proteolytically cleaved and released as soluble forms (sLOX-1). We have determined serums LOX-1 in type 2 diabetes and evaluated the effect of glucose and advanced glycation end products (AGEs) on sLOX-1 in vitro and in vivo. Endothelial cells were incubated with glucose or AGEs, and sLOX-1 in cell medium was measured. Serum sLOX-1 was measured in 219 diabetic patients and 187 controls by ELISA. The effect of lowering glucose and AGEs on sLOX-1 was determined in 38 poorly controlled diabetic patients after improvement in glycemic control. Incubation of endothelial cells with AGE-BSA led to a dose-dependent increase in sLOX-1, whereas the effect of glucose on sLOX-1 was less marked. Serum sLOX-1 was 9% higher in diabetic patients compared with controls (P<0.01). In the poorly controlled patients, serum sLOX-1 decreased by 12.5% after improvement in glycemic control (P<0.05). The magnitude of reduction in sLOX-1 correlated with the improvement in hemoglobin A1c and AGEs but not with the reduction in oxidized LDL. sLOX-1 level is increased in type 2 diabetes. Both glucose and AGEs are important determinants of LOX-1 expression, and lowering glucose and AGEs leads to a reduction in sLOX-1.

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.

Macrophage scavenger receptors and host-derived ligands

The scavenger receptors are a large family of molecules that are structurally diverse and have been implicated in a range of functions. They are expressed by myeloid cells, selected endothelial cells and some epithelial cells and recognise many different ligands, including microbial pathogens as well as endogenous and modified host-derived molecules. This review will focus on the eight classes of scavenger receptors (class A-H) in terms of their structure, expression and recognition of host-derived ligands. Scavenger receptors have been implicated in a range of physiological and pathological processes, such as atherosclerosis and Alzheimer's disease, and function in adhesion and tissue maintenance. More recently, some of the scavenger receptors have been shown to mediate binding and endocytosis of chaperone proteins, such as the heat shock proteins, thereby playing an important role in antigen cross-presentation.

The splice variant LOXIN inhibits LOX-1 receptor function through hetero-oligomerization

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), encoded by the OLR1 gene, is a scavenger receptor that plays a central role in the pathogenesis of atherosclerosis. We have recently identified a truncated naturally occurring variant of the human receptor LOX-1, named LOXIN, which lacks part of the C-terminus lectin-like domain. In vivo and in vitro studies support that the new splicing isoform is protective against acute myocardial infarction. The mechanism by which LOXIN exerts its protective role is unknown. In this paper we report studies on the heterologous expression and functional characterization of LOXIN variant in mammalian fibroblasts and human endothelial cells. We found that LOXIN, when expressed in the absence of LOX-1, shows diminished plasma membrane localization and is deficient in ox-LDL ligand binding. When co-transfected with the full-length counterpart LOX-1, the two isoforms interact to form LOX-1 oligomers and their interaction leads to a decrease in the appearance of LOX-1 receptors in the plasma membrane and a marked impairment of ox-LDL binding and uptake. Co-immunoprecipitation studies confirmed the molecular LOX-1/LOXIN interaction and the formation of non-functional hetero-oligomers. Our studies suggest that hetero-oligomerization between naturally occurring isoforms of LOX-1 may represent a general paradigm for regulation of LOX-1 function by its variants.

Molecular dynamics simulation of human LOX-1 provides an explanation for the lack of OxLDL binding to the Trp150Ala mutant

Background

Dimeric lectin-like oxidized low-density lipoprotein receptor-1 LOX-1 is the target receptor for oxidized low density lipoprotein in endothelial cells. In vivo assays revealed that in LOX-1 the basic spine arginine residues are important for binding, which is lost upon mutation of Trp150 with alanine. Molecular dynamics simulations of the wild-type LOX-1 and of the Trp150Ala mutant C-type lectin-like domains, have been carried out to gain insight into the severe inactivating effect.

Results

The mutation does not alter the dimer stability, but a different dynamical behaviour differentiates the two proteins. As described by the residues fluctuation, the dynamic cross correlation map and the principal component analysis in the wild-type the two monomers display a symmetrical motion that is not observed in the mutant.

Conclusion

The symmetrical motion of monomers is completely damped by the structural rearrangement caused by the Trp150Ala mutation. An improper dynamical coupling of the monomers and different fluctuations of the basic spine residues are observed, with a consequent altered binding affinity.

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.

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

Endothelial activation and dysfunction induced by oxidized modified low-density lipoprotein (ox-LDL) is one of the key steps in the initiation of atherosclerosis. Recent studies have shown that a new lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) mediates the recognition and internalization of ox-LDL. LOX-1 is the main receptor for ox-LDL and may play an important role in the pathogenesis of hypertension, diabetes, and, especially, of atherosclerosis. The potential role of LOX-1 in the pathogenesis of atherosclerosis includes: endocytosis of ox-LDL, expression co-location with atherosclerosis enhanced by atherosclerosis-related risk factors, elevated LOX-1 protein in cardiovascular disease, effects related to atherosclerosis and eliminated by antiatherosclerotic drugs. Identification and regulation of LOX-1 and understanding its signal transduction pathways might improve our insight toward the pathogenesis of atherosclerosis and provide a selective treatment approach. LOX-1 might be a potential and promising target for the development of novel antiatherosclerotic drugs. However, due to limited knowledge about LOX-1, there are still many questions to be answered.

Lectin-like oxidized low-density lipoprotein receptor (LOX-1) functions as an oligomer and oligomerization is dependent on receptor density

Lectin-like oxidized low-density lipoprotein (LDL) receptor (LOX-1) exists as a homodimer formed by an intermolecular disulfide bond. Although the dimer is the minimum structural unit of LOX-1 on cell membranes, LOX-1 can form larger noncovalent oligomeric complexes. But, the functional unit of LOX-1 is not known. We quantitatively analyzed the correlation between cyan fluorescent protein-tagged LOX-1 expression and the fluorescence-labeled ligand (DiD-AcLDL) binding ability on each cell. The results clearly indicate that there is a threshold level of expression that enables LOX-1 to bind ligand. Above this threshold level, the ability of LOX-1 to bind ligand was proportional to its level of expression. Using the membrane impermeable crosslinker BS(3), we detected oligomers (primarily hexamers) only on the cell lines that stably expressed LOX-1 above the threshold level. In contrast, little oligomer or ligand binding was detected in cell lines expressing LOX-1 below the threshold level. Moreover, oligomerization was independent of ligand binding. These results indicate that the functional unit of LOX-1 is an oligomer and that oligomerization of LOX-1 is dependent on the receptor density on the plasma membrane.

Role of scavenger receptors in the binding and internalization of heat shock protein 70

Extracellular heat shock protein 70 (Hsp70) exerts profound effects both in mediating tumor rejection by Hsp70-based vaccines and in autoimmunity. Further progress in this area, however, awaits the identification of the cell surface receptors for extracellular Hsp70 that mediate its immune functions. We have examined a wide range of candidate Hsp70 receptors and find significant binding through two main families of cell surface proteins, including 1) the scavenger receptor (SR) family and 2) C-type lectins of the NK family. In addition, given that the anticancer effects of Hsp70 vaccines have been shown to involve uptake of Ags by APC exposed to Hsp70-tumor Ag complexes, we have examined the ability of the receptors identified here to internalize Hsp70-peptide complexes. Our findings indicate that three members of the SR family (lectin-like oxidized low density lipoprotein receptor 1; fasciclin, epidermal growth factor-like, laminin-type epidermal growth factor-like, and link domain-containing scavenger receptor-1; and SR expressed by endothelial cells-1) are able to bind Hsp70-peptide complexes and mediate its efficient internalization. Indeed, each of the SR was able to mediate efficient uptake of Hsp70 when transfected into Chinese hamster ovary cells previously null for uptake. Curiously, Hsp70 internalization occurs independently of the intracellular domains of the SR, and Hsp70 uptake could be detected when the entire intracellular domain of lectin-like oxidized low density lipoprotein receptor 1 or SR expressed by endothelial cells-1 was truncated. The existence of a wide repertoire of cell surface Hsp70-binding structures may permit intracellular responses to extracellular Hsp70 that are cell specific and discriminate between Hsp70 family members.

Dominant-negative lox-1 blocks homodimerization of wild-type lox-1-induced cell proliferation through extracellular signal regulated kinase 1/2 activation

C-type lectin-like oxidized low-density lipoprotein (Ox-LDL) receptor-1 (Lox-1) belongs to the same family as natural killer cell receptors Ly49A and CD94 and functionally undergoes dimerization. Although Lys262 and Lys263 in the C terminus of bovine (b)Lox-1 play an important role in the uptake of Ox-LDL, mutation of these residues has not been suggested to be a potential source of the dominant-negative property. We hypothesize that dominant-negative human (h)Lox-1 forms a heterodimer with Lox-1-wild-type (WT) and blocks Lox-1-WT-induced cell signaling. Based on the use of molecular imaging techniques with laser scanning confocal microscopy and immunoprecipitation in an hLox-1-expressing Chinese hamster ovary cell system, homodimerization of hLox-1-WT was localized in the cell membrane, and Ox-LDL activated extracellular signal regulated kinase (ERK)1/2 without the translocation of hLox-1-WT. Lys266 and Lys267 of hLox-1, corresponding with Lys262 and Lys263 of bLox-1, were mutated (hLox1-K266A/K267A), and the mutant receptor inhibited hLox-1-WT-induced thymidine incorporation and ERK1/2 activation. Although Ox-LDL binds to the dominant-negative mutant receptor and is taken up by cytoplasm, ERK1/2 activation was blocked by heterodimerization with the mutant receptor and hLox-1-WT in the cell membrane. In addition, in human coronary artery smooth muscle cells, which express hLox-1-WT, we confirmed that the activation of ERK1/2 and [3H]-thymidine incorporation was caused by the addition of Ox-LDL, and these actions were blocked by hLox1-K266A/K267A. In conclusion, the present findings constitute the first evidence that strategies aimed at blocking cell-proliferative pathways at the receptor level could be useful for impairing Lox-1-induced cell proliferation.

Biochemistry and cell biology of mammalian scavenger receptors

Scavenger receptors are integral membrane proteins that bind a wide variety of ligands including modified or oxidised low-density lipoproteins, apoptotic cells and pathogens. Modified low-density lipoprotein accumulation is thought to be an early event in vascular disease and thus scavenger receptor function is critical in this context. The scavenger receptor family has at least eight different subclasses (A-H) which bear little sequence homology to each other but recognize common ligands. Here we review our current understanding of the scavenger receptor subclasses with emphasis on their genetics, protein structure, biochemical properties, membrane trafficking, intracellular signalling and links to disease states. We also highlight emerging areas where scavenger receptors play roles in cell and animal physiology.

LOX-1 scavenger receptor mediates calcium-dependent recognition of phosphatidylserine and apoptotic cells

The LOX-1 (lectin-like oxidized low-density lipoprotein receptor-1) scavenger receptor regulates vascular responses to oxidized-low-density-lipoprotein particles implicated in atherosclerotic plaque formation. LOX-1 is closely related to C-type lectins, but the mechanism of ligand recognition is not known. Here we show that human LOX-1 recognizes a key cellular phospholipid, PS (phosphatidylserine), in a Ca2+-dependent manner, both in vitro and in cultured cells. A recombinant, folded and glycosylated LOX-1 molecule binds PS, but not other phospholipids. LOX-1 recognition of PS was maximal in the presence of millimolar Ca2+ levels. Mg2+ was unable to substitute for Ca2+ in LOX-1 binding to PS, indicating a Ca2+-specific requirement for bivalent cations. LOX-1-mediated recognition of PS-containing apoptotic bodies was dependent on Ca2+ and was decreased to background levels by bivalent-cation chelation, LOX-1-blocking antibodies or PS-containing liposomes. The LOX-1 membrane protein is thus a Ca2+-dependent phospholipid receptor, revealing novel recognition of phospholipids by mammalian lectins.

Essential role of cytoplasmic sequences for cell-surface sorting of the lectin-like oxidized LDL receptor-1 (LOX-1)

Lectin-like oxidized LDL receptor-1 (LOX-1) is an oxidized low-density lipoprotein (OxLDL) receptor found in endothelial cells and a member of the natural killer (NK) receptor gene complex. Here, we demonstrate that the ability of LOX-1 binding to OxLDL distinguishes it from other NK receptors. Domain swapping of the lectin-like domain between LOX-1 and the NK cell receptors CD94, NKG2D, and LY-49A demonstrated the crucial role of this domain for recognition of OxLDL by LOX-1, but not for the correct cell-surface sorting of LOX-1. Using LOX-1 GFP fusion constructs, we find that the combination of cytoplasmic and transmembrane domains of LOX-1 is sufficient to target the chimeric protein to the cell-surface. Using N-terminal deletions we determined that the correct cell-surface localization is dependent on a positively charged motif present in the cytosolic juxtamembrane region of LOX-1. Furthermore, the extracellular localization of the LOX-1 C-terminus is disrupted when we mutated the cytoplasmic basic amino acids, Lys-22, Lys-23 and Lys-25 to Glu. Collectively, these results indicate that the N-terminal cytoplasmic domain of LOX-1 determines the correct expression of the lectin domain on the cell-surface.

In Vivo and In Vitro Studies Support That a New Splicing Isoform of OLR1 Gene Is Protective Against Acute Myocardial Infarction

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), encoded by the OLR1 gene, is a scavenger receptor that plays a fundamental role in the pathogenesis of atherosclerosis. LOX-1 activation is associated with apoptosis of endothelial cells, smooth muscle cells (SMCs), and macrophages. This process is an important underlying mechanism that contributes to plaque instability and subsequent development of acute coronary syndromes. Independent association genetic studies have implicated OLR1 gene variants in myocardial infarction (MI) susceptibility. Because single nucleotide polymorphisms (SNPs) linked to MI are located in intronic sequences of the gene, it remains unclear as to how they determine their biological effects. Using quantitative real-time PCR and minigene approach, we show that intronic SNPs, linked to MI, regulate the expression of a new functional splicing isoform of the OLR1 gene, LOXIN, which lacks exon 5. Macrophages from subjects carrying the “non-risk” disease haplotype at OLR1 gene have an increased expression of LOXIN at mRNA and protein level, which results in a significant reduction of apoptosis in response to oxLDL. Expression of LOXIN in different cell types results in loss of surface staining, indicating that truncation of the C-terminal portion of the protein has a profound effect on its cellular trafficking. Furthermore, the proapoptotic effect of LOX-1 receptor in cell culture is specifically rescued by the coexpression of LOXIN in a dose-dependent manner. The demonstration that increasing levels of LOXIN protect cells from LOX-1 induced apoptosis sets a groundwork for developing therapeutic approaches for prevention of plaque instability.

Crystal Structure of Human Lectin-like, Oxidized Low-Density Lipoprotein Receptor 1 Ligand Binding Domain and Its Ligand Recognition Mode to OxLDL

Lectin-like, oxidized low-density lipoprotein (LDL) receptor 1, LOX-1, is the major receptor for oxidized LDL (OxLDL) in endothelial cells. We have determined the crystal structure of the ligand binding domain of LOX-1, with a short stalk region connecting the domain to the membrane-spanning region, as a homodimer linked by an interchain disulfide bond. In vivo assays with LOX-1 mutants revealed that the "basic spine," consisting of linearly aligned arginine residues spanning over the dimer surface, is responsible for ligand binding. Single amino acid substitution in the dimer interface caused a severe reduction in LOX-1 binding activity, suggesting that the correct dimer arrangement is crucial for binding to OxLDL. Based on the LDL model structure, possible binding modes of LOX-1 to OxLDL are proposed.

The lectin-like oxidized low-density lipoprotein receptor and its role in atherosclerosis

The lectin-like oxidized low-density lipoprotein receptor (LOX-1), a recently identified receptor that plays a role in the uptake of oxidized low-density lipoproteins into endothelial cells, has a pivotal role in the pathogenesis of atherosclerosis. The ways this receptor takes part in atherosclerosis is through uptake of oxidized low-density lipoproteins into endothelial cells, smooth muscle cells, and macrophages; decreasing nitric oxide production; increasing inflammatory cell recruitment; and increasing smooth muscle cell proliferation. LOX-1 is inducible and regulated by multiple factors known to underlie atherogenesis. Further understanding of this receptor may lead to potential molecular targets for prevention and treatment of atherosclerosis.

The 1.4 angstrom crystal structure of the human oxidized low density lipoprotein receptor lox-1

The lectin-like oxidized low density lipoprotein receptor-1 (Lox-1) mediates the recognition and internalization of oxidatively modified low density lipoprotein by vascular endothelial cells. This interaction results in a number of pro-atherogenic cellular responses that probably play a significant role in the pathology of atherosclerosis. The 1.4 angstrom crystal structure of the extracellular C-type lectin-like domain of human Lox-1 reveals a heart-shaped homodimer with a ridge of six basic amino acids extending diagonally across the apolar top of Lox-1, a central hydrophobic tunnel that extends through the entire molecule, and an electrostatically neutral patch of 12 charged residues that resides next to the tunnel at each opening. Based on the arrangement of critical binding residues on the Lox-1 structure, we propose a binding mode for the recognition of modified low density lipoprotein and other Lox-1 ligands.

Lipids from oxidized low-density lipoprotein modulate human trophoblast invasion: involvement of nuclear liver X receptors

Human embryonic implantation involves major invasion of the uterine wall and remodeling of the uterine arteries by extravillous cytotrophoblast cells (EVCT). Abnormalities in these early steps of placental development lead to poor placentation and fetal growth defects and are frequently associated with preeclampsia, a major complication of human pregnancy. We recently showed that oxidized low-density lipoproteins (oxLDLs) are present in situ in EVCT and inhibit cell invasion in a concentration-dependent manner. The aim of the present study was to better understand the mechanisms by which oxLDL modulate trophoblast invasion. We therefore investigated the presence of oxLDL receptors in our cell culture model of human invasive primary EVCT. We found using immunocytochemistry and immunoblotting that the lectin-like oxLDL receptor-1 was the scavenger receptor mainly expressed in EVCT and was probably involved in oxLDL uptake. We next examined the effect of low-density lipoprotein oxidative state on trophoblast invasion in vitro using EVCT cultured on Matrigel-coated Transwell. We demonstrated that only oxLDL containing a high proportion of oxysterols and phosphatidylcholine hydroperoxide derivatives that provide ligands for liver X receptor (LXR) and peroxisomal proliferator-activated receptor gamma (PPARgamma), respectively, reduced trophoblast invasion. We next investigated the presence and the role of these nuclear receptors and found that in addition to PPARgamma, human invasive trophoblasts express LXRbeta, and activation of these nuclear receptors by specific synthetic or natural ligands inhibited trophoblast invasion. Finally, using a PPARgamma antagonist, we suggest that LXRbeta, rather than PPARgamma, is involved in oxLDL-mediated inhibition of human trophoblast invasion in vitro.

Human lectin-like oxidized low-density lipoprotein receptor-1 functions as a dimer in living cells

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is a unique scavenger receptor that plays important roles in atherogenesis and has been thought to function as a monomer. Using coimmunoprecipitation studies, we demonstrate that human LOX-1 (hLOX-1) forms constitutive homo-interactions in vivo. Western blot analysis of cell lysates under nonreducing or reducing conditions revealed one clear immunoreactive species corresponding to the size of a putative receptor dimer or a monomer, respectively, consistent with the presence of disulfide-linked hLOX-1 complexes. Site-directed mutagenesis studies indicated that cysteine 140 has a key role in the formation of these disulfide-linked hLOX-1 dimers. Eliminating this intermolecular disulfide bond markedly impairs the recognition of Escherichia coli by hLOX-1. Furthermore, these dimers can act as a "structural unit" to form noncovalently associated oligomers, as demonstrated by a membrane-impermeant crosslinker, which resulted in immunoreactive species corresponding to the sizes of putative tetramers and hexamers. These results provide the first evidence for the existence of hLOX-1 dimers/oligomers.

Carbohydrate and Non-Carbohydrate Ligands for the C-Type Lectin-Like Receptors of Natural Killer Cells. A Review

The superfamily of C-type animal lectins is defined by a sequence motif of the carbohydrate- recognition domains (CRDs) and comprises seven groups of molecules. The soluble proteins are group I proteoglycans, group III collectins, and group VII containing the isolated CRDs. Type I membrane proteins include group IV selectins and group VI macrophage receptors and related molecules. Type II membrane proteins are group II hepatic lectins and group V natural killer cell receptors. The latter group has recently attracted considerable attention of the biomedical community. These receptors are arranged at the surface of lymphocytes as homo- or heterodimers composed of two polypeptides consisting of N-terminal peptide tails responsible for signaling, transmembrane domain, neck regions of varying length, and C-terminal lectin-like domains (CTLDs). Since this group is evolutionarily most distant from the rest of C-type animal lectins, the sequence of the C-terminal ligand-binding domain has diversified to accommodate other ligands than calcium or carbohydrates. These domains are referred to as natural killer domains (NKDs) forming a large percentage of CTLDs in vertebrates. Here are summarized the data indicating that calcium, carbohydrates, peptides, and large proteins such as major histocompatibility complex (MHC) class I can all be ligands for NKDs. The wide range of ligands that can be recognized by NKDs includes some new, unexpected compounds such as signal peptide-derived fragments, heat shock proteins, or oxidized lipids. The biological importance of this extended range of recognition abilities is also discussed. A review with 134 references.

Refolding and characterization of the functional ligand-binding domain of human lectin-like oxidized LDL receptor

Lectin-like oxidized low-density lipoprotein receptor (LOX-1), a type II membrane protein that can recognize a variety of structurally unrelated macromolecules, plays an important role in host defense and is implicated in atherogenesis. To understand the interaction between human LOX-1 and its ligands, in this study the functional C-type lectin-like domain (CTLD) of LOX-1 was reconstituted at high efficiency from inactive aggregates in Escherichia coli using a refolding technique based on an artificial chaperone. The CD spectra of the purified domain suggested that the domain has alpha-helical structure and the blue shift of Trp residues was observed on refolding of the domain. Like wild-type hLOX-1, the refolded CTLD domain was able to bind modified LDL. Thus, even though CTLD contains six Cys residues that form disulfide bonds, it recovered its specific binding ability on refolding. This suggests that the correct disulfide bonds in CTLD were formed by the artificial chaperone technique. Although the domain lacked N-glycosylation, it showed high affinity for its ligand in surface plasmon resonance experiments. Thus, unglycosylated CTLD is sufficient for binding modified LDL.

Genetic variation in lectin-like oxidized low-density lipoprotein receptor 1 (LOX1) gene and the risk of coronary artery disease

BACKGROUND

We examined the association of 3 polymorphisms in the lectin-like oxidized LDL receptor-1 (LOX1 or OLR1) gene with coronary artery disease in the Women's Ischemia Syndrome Evaluation (WISE) study population.

METHODS AND RESULTS

The WISE sample comprised 589 white and 122 black women who underwent angiography for suspected ischemia. The sample was divided into 3 groups: <20% stenosis (38.7%), 20% to 49% stenosis (24.9%), and >or=50% stenosis (35.3%). The three LOX1 polymorphisms (intron 4/G-->A, intron 5/T-->G, and 3' UTR/T-->C) were in linkage disequilibrium and thus behaved as a single polymorphism. The frequency of the 3'UTR/T allele was significantly higher in whites than blacks (49% versus 19%; P<0.0001). Among white women, the frequency of the 3'UTR/T allele carriers (TC+TT genotypes) increased gradually from 67.9% to 75.0% and 79.2% in the <20%, 20% to 49%, and >or=50% stenosis groups, respectively (chi2 trend=6.23; P=0.013). Logistic regression analyses indicated that APOE (odds ratio, 1.90; P=0.007) and LOX1 (odds ratio, 1.67; P=0.025) genotypes were independently associated with the risk of disease and that there was no interaction between the two genes. The 3'UTR/T allele carriers also had significantly higher IgG anti-oxLDL levels than individuals carrying the CC genotype (0.94+/-0.20 versus 0.86+/-0.16; P=0.032). Furthermore, our electrophoretic mobility shift assay data show that the 3'UTR polymorphic sequence affects the binding of a putative transcription factor in an allele-specific manner.

CONCLUSIONS

Our data suggest that common genetic variation in the LOX1 gene may be associated with the risk of coronary artery disease in white women.

Lectin-like Ox-LDL receptor is expressed in human INT-407 intestinal cells: involvement in the transcytosis of pancreatic bile salt-dependent lipase

We have recently shown that the pancreatic bile salt-dependent lipase (BSDL) can be taken up by intestinal cells and transported to the blood circulation. This mechanism likely involves (specific) receptor(s) able to bind BSDL and located at the apical intestinal cell membrane. In this study, using Int407 human intestinal cells cultured to form a tight epithelium, we attempted to characterize (the) BSDL receptor(s). We found that an apical 50-kDa protein was able to bind BSDL. Further, we have demonstrated that Int407 cells expressed the lectin-like oxidized-LDL receptor (LOX-1), the upregulation of which by oxidized-LDL potentiates the transcytosis of BSDL, whereas carrageenan and to a lesser extent polyinosinic acid and fucoidan decrease the enzyme transcytosis. The mAb JTX92, which blocks the LOX-1 receptor function, also impaired the BSDL transcytosis. To confirm these results, the cDNA encoding the human intestinal receptor LOX-1 has been cloned, inserted into vectors, and transfected into Int407 cells. Overexpression of LOX-1 by these cells leads to a substantial increase in the BSDL transcytosis. Globally, these data support the view that LOX-1 could be an intestinal receptor for BSDL, which is implicated in the transcytosis of this enzyme throughout Int407 cells.

LOX-1, the receptor for oxidized low-density lipoprotein identified from endothelial cells: implications in endothelial dysfunction and atherosclerosis

Lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (LOX-1) was initially identified as the major receptor for oxidized LDL (OxLDL) in endothelial cells. Its inducible expression in macrophages and smooth muscle cell was also observed. LOX-1 is a Type II membrane protein with a typical C-type lectin structure at the extracellular C-terminus. It can be cleaved by an unknown protease at the extracellular juxtamembrane region to release the soluble form of LOX-1. The extracellular domains of LOX-1 are post-translationally modified by N-linked glycosylation. Mutagenesis studies revealed that the lectin domain of LOX-1 is the functional domain that recognizes the LOX-1 ligand. The C-terminal end residues and several conserved positively charged residues spanning the lectin domain are essential for OxLDL binding. LOX-1 activation by OxLDL causes endothelial changes that are characterized by activation of nuclear factor-kappaB through an increased reactive oxygen species, subsequent induction of adhesion molecules, and endothelial apoptosis. In vitro, expression of LOX-1 is induced by many inflammatory cytokines, oxidative stress, hemodynamic stimuli, and OxLDL. In vivo, the expression is enhanced in pro-atherogenic settings including, hypertension, hyperlipidemia, and diabetes, and, indeed, is accumulated in the atherosclerotic and glomerulosclerotic lesions. LOX-1 binds multiple classes of ligands that are implicated in the pathogenesis of atherosclerosis. Besides OxLDL, LOX-1 can recognize apoptotic/aged cells, activated platelets, and bacteria, implying versatile physiological functions. Taken together, all these findings support the possible contribution of LOX-1 to the pathogenesis of vascular disorders, particularly atherosclerosis. Development of antagonists for LOX-1 might be a good therapeutic approach to vascular diseases.