Site-specific N-glycosylation identification of recombinant human lectin-like oxidized low density lipoprotein receptor-1 (LOX-1)

Human LOX-1/OLR 1 plays a key role in atherogenesis and endothelial dysfunction. The N-glycosylation of LOX-1 has been shown to affect its biological functions in vivo and modulate the pathogenesis of atherosclerosis. However, the N-glycosylation pattern of LOX-1 has not been described yet. The present study was aimed at elucidating the N-glycosylation of recombinant human LOX-1 with regard to N-glycan profile and N-glycosylation sites. Here, an approach using nonspecific protease (Pronase E) digestion followed by MALDI-QIT-TOF MS and multistage MS (MS(3)) analysis is explored to obtain site-specific N-glycosylation information of recombinant human LOX-1, in combination with glycan structure confirmation through characterizing released glycans using tandem MS. The results reveal that N-glycans structures as well as their corresponding attached site of LOX-1 can be identified simultaneously by direct MS analysis of glycopeptides from non-specific protease digestion. With this approach, one potential glycosylation site of recombinant human LOX-1 on Asn(139) is readily identified and found to carry heterogeneous complex type N-glycans. In addition, manual annotation of multistage MS data utilizing diagnostic ions, which were found to be particularly useful in defining the structure of glycopeptides and glycans was addressed for proper spectra interpretation. The findings described herein will shed new light on further research of the structure-function relationships of LOX-1?N-glycan.     

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.     

Lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) supports cell adhesion to fibronectin

A preliminary model has been calculated for the activating interaction of the interleukin 1 receptor (IL-1R) accessory protein IL-1RAcP with the ligand/receptor complex IL-1beta/IL-1R(I). First, IL-1RAcP was modeled on the crystal structure of IL-1R(I) bound to IL-1beta. Then, the IL-1RAcP model was docked using specific programs to the crystal structure of the IL-1beta/IL-1R(I) complex. Two types of models were predicted, with comparable probability. Experimental data obtained with the use of IL-1beta peptides and antibodies, and with mutated IL-1beta proteins, support the BACK model, in which IL-1RAcP establishes contacts with the back of IL-1R(I) wrapped around IL-1beta.     

Diabetes enhances lectin-like oxidized LDL receptor-1 (LOX-1) expression in the vascular endothelium: possible role of LOX-1 ligand and AGE

Diabetes mellitus accelerating atherosclerosis was associated with the enhanced glycoxidative modification of lipoproteins. LOX-1, the endothelial oxidized LDL receptor might be involved in the pathogenesis of diabetic atherosclerosis. In this study, we examined the vascular expression of LOX-1 in streptozotocin-induced diabetic rats. We found that LOX-1 was significantly increased in diabetic rat aorta compared with nondiabetic control. Immunohistochemistry revealed that the most distinctive staining of LOX-1 was in the endothelial cells, especially in the bifurcations of artery branches from aorta. In cultured aortic endothelial cells, diabetic rat serum and advanced glycation endproducts-BSA induced LOX-1 expression, while control rat serum along with high glucose did not. Applying a competitive inhibition assay, we found that LOX-1 ligand activity was accumulated in the diabetic rat serum, mainly in VLDL/LDL fractions. In addition, VLDL/LDL prominently increased LOX-1 among all the lipoprotein fractions of diabetic rat serum. In conclusion, diabetes markedly upregulated LOX-1 expression in the aortic endothelial cells. The enhanced glycoxidative modification of lipoproteins may contribute to the underlying mechanisms.     

Expression and localization of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) in murine and human placentas.

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is one of the scavenger receptors that recognizes oxidized low-density lipoprotein as a major ligand. The placenta is a major source of prooxidant during pregnancy, and the level of placental oxidative stress increases rapidly at the end of the first trimester and tapers off later in gestation. In our study, we evaluated placental expression of LOX-1 during different gestational stages in mice and humans. We used immunohistochemistry and ISH to identify LOX-1-expressing cells in murine and human placentas. In both species, higher expression of LOX-1 mRNA during early to midgestational stages compared with late gestation-corresponding to the increased oxidative stress in early pregnancy-was shown by real-time RT-PCR. In murine placenta, we showed that LOX-1-expressing cells were fibroblast-like stromal cells in metrial glands and decidua basalis and that they were glycogen trophoblast cells in the junctional and labyrinth zones. In the human, LOX-1 expression was detected in villous cytotrophoblasts in both first trimester and term placentas. These localization patterns of LOX-1 in murine and human placentas suggest the possible involvement of LOX-1 in high oxidative stress conditions of pregnancy.     

Advanced glycation end products serve as ligands for lectin-like oxidized low-density lipoprotein receptor-1(LOX-1): biochemical and binding characterizations assay

Advanced glycation end products (AGEs) are a class of complex heterogeneous compounds which accumulate with age and is known to be involved in the pathogenesis of several diseases from diabetes to atherosclerosis. AGEs serve as ligands for multiple receptors including scavenger receptor (SR-A), CD36, and SR-BIota. Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) plays an important role in both atherosclerosis and is found to be an endothelial cell receptor for AGEs. To explore the binding characterization of AGEs to LOX-1, AGEs were prepared by three different reducing sugars (d-glucose, d-fructose, and d-ribose) and the biochemical characterization including, free amino groups, free amine content, fructosamine residues, carbonyl content, fluorescence, and absorbance were determined. The binding activity was determined by FITC labeled AGEs using Chinese hamster ovary-K1 cells stably transfected with human LOX-1 gene. The obtained AGEs showed significant differences in the extent of side chain modifications, carbonyl content, fluorescence, and absorption models. All of the AGEs showed specific and saturable binding to hLOX-1-CHO-K1 cells. Furthermore, dose-dependent binding processes were observed. However, the maximal cellular binding of AGEs differs between the sugars (glucose > ribose > fructose). In addition, oxidized low-density lipoprotein (ox-LDL) could significantly inhibit the binding of AGEs to LOX-1 with different inhibitory efficiency. LOX-1 serves as receptor for AGEs which may give some insight into the role of LOX-1 in the pathogenesis of diabetes and related disorders.     

凝血素样氧化低密度脂蛋白受体结构与动脉粥样硬化

动脉粥样硬化(As)早期改变是内皮功能紊乱,新型受体-植物凝集素样氧化型低密度脂蛋白受体-1(LOX-1)在氧化低密度脂蛋白(Ox-LDL)诱导的内皮功能紊乱过程中起关键性作用.LOX-1主要在内皮细胞表达,其结构和功能与其他可吞噬ox-LDL的清道夫受体显著不同,但与自然杀伤细胞受体却高度一致.目前对LOX-1基因和蛋白结构以及功能尚不完全清楚.因此,进一步研究LOX-1的功能及其表达的调控机制,不仅有助于了解脂代谢和As发病机制,对心血管病防治也有十分重要的意义.本文对LOX-1其结构、功能及其调控因素等最新研究进行综述.     

Polycystic ovary syndrome and endothelial dysfunction: A potential role for soluble lectin-like oxidized low density lipoprotein receptor-1

The aims of this study were to investigate whether serum soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1), oxidized LDL (oxLDL), paraoxonase-1(PON-1) and hydroperoxide (LOOH) levels are altered in women with polycystic ovary syndrome (PCOS) and also to determine if hyperandrogenism, insulin resistance (IR) and Anti-Müllerian Hormone (AMH) are associated with endothelial dysfunction in PCOS. A total of 46 women with PCOS and 46 non-PCOS healthy controls were recruited. Women with PCOS had significantly higher sLOX-1, oxLDL and LOOH concentrations than non-PCOS women [6.16 (3.92−13.95) vs 1.37 (0.63−4.43) ng/mL, p < 0.001; 6.48 ± 1.03 vs 3.16 ± 1.02 μU/L, p < 0.001; 2.45 (1.45−3.45) vs 1.06 (0.64−1.56) μmol/L, p < 0.001]. The mean PON-1 level of PCOS group was lower than non-PCOS group (69.47 ± 10.75 vs 104.08 ± 21.43 U/mL, p < 0.001). There was no significant difference in terms of the sLOX-1, oxLDL, LOOH and PON-1 levels between normal weight and overweight PCOS women. On univariate logistic regression analysis, Ferriman-Gallwey scale (FGS), HOMA-IR and AMH were an independent predictors of high risk group of endothelial dysfunction markers (HR-EDm). Age and BMI were not associated with HR-EDm. When incorporated into the multivariate model, endotelial dysfunction markers independently correlated with clinical hyperandrogenism (FGS) but not with AMH. In conclusion, our results indicated that an increased concentration of sLOX-1 might be an early predictor of endothelial damage in patients with PCOS. Women with PCOS have elevated sLOX-1, oxLDL, LOOH and decreased PON-1 levels, independent of BMI. Endothelial dysfunction in women with PCOS is associated with hyperandrogenism. Further studies are required to confirm our findings.     

Cloning and expression of human lectin-like oxidized low density lipoprotein receptor-1 in<Emphasis Type="Italic"> Pichia pastoris</Emphasis>

Lectin-like oxidatively-modified LDL receptor-1 (LOX-1) is a major receptor for oxidized low-density lipoprotein (oxLDL) in aortic endothelial cells. Human LOX-1 (hLOX-1) gene (cDNA) was cloned from the monocytic leukemic cell line THP-1 and expressed in Pichia pastoris. The recombinant protein (rhLOX-1) was purified by his-tag affinity chromatography. Preliminary identification was performed by Western blot analysis and a ligand-receptor binding assay showed that the protein had specific oxLDL-binding activity.Revisions requested 21 September 2004; Revisions received 10 November 2004     

Structural implication for the impaired binding of W150A mutant LOX-1 to oxidized low density lipoprotein, OxLDL

Lectin-like oxidized lipoprotein (OxLDL) receptor 1, LOX-1, is the major OxLDL receptor expressed on vascular endothelial cells. We have previously reported the ligand-recognition mode of LOX-1 based on the crystal structure of the ligand binding domain (C-type lectin-like domain, CTLD) and surface plasmon resonance analysis, which suggested that the functional significance of the CTLD dimer (the 'canonical' dimer) is to harbor the characteristic "basic spine" on its surface. In this study, we have identified the key inter-domain interactions in retaining the canonical CTLD dimer by X-ray structural analysis of the inactive mutant W150A CTLD. The canonical CTLD dimer forms through tight hydrophobic interactions, in which W150 engages in a lock-and-key manner and represents the main interaction. The loss of the Trp ring by mutation to Ala prevents the formation of the canonical dimer, as elucidated from docking calculations using the crystal structure of W150A CTLD. The results emphasize that the canonically formed CTLD dimer is essential for LOX-1 to bind to OxLDL, which supports our proposed view that the basic spine surface present in the correctly formed dimer plays a primal role in OxLDL recognition. This concept provides insight into the pathogenic pattern recognized by LOX-1 as a member of the pattern recognition receptors.     

Requirements of basic amino acid residues within the lectin-like domain of LOX-1 for the binding of oxidized low-density lipoprotein.

Lectin-like OxLDL receptor-1 (LOX-1) was identified as the major receptor for oxidized low-density lipoprotein (OxLDL) in aortic endothelial cells. LOX-1 is a type II membrane protein that structurally belongs to the C-type lectin family. Here, we found that the lectin-like domain of LOX-1 is essential for ligand binding, but the neck domain is not. In particular, the large loop between the third and fourth cysteine of the lectin-like domain plays a critical role for OxLDL binding as well as C-terminal end residues. Alanine-directed mutagenesis of the basic amino acid residues around this region revealed that all of the basic residues are involved in OxLDL binding. Simultaneous mutations of these basic residues almost abolished the OxLDL-binding activity of LOX-1. Electrostatic interaction between basic residues in the lectin-like domain of LOX-1 and negatively charged OxLDL is critical for the binding activity of LOX-1.     

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.     

Effects of red wine polyphenolic compounds on paraoxonase-1 and lectin-like oxidized low-density lipoprotein receptor-1 in hyperhomocysteinemic mice

Hyperhomocysteinemia, or abnormally high plasma homocysteine (Hcy) concentration, has often been associated with vascular thrombosis and the development of premature atherosclerosis. Many studies have shown that moderate wine consumption has potential beneficial effects related to the prevention of atherosclerosis, in part attributed to the biological properties of polyphenolic components, mainly flavonoids. The aim of the present study is to determine the effects of a red wine polyphenolic extract (PE) administration on hyperhomocysteinemia due to cystathionine beta-synthase (CBS) deficiency and on the associated biochemical markers of hepatic and endothelial dysfunctions in mice. Red wine PE was added for 4 weeks to the drinking water of heterozygous CBS-deficient mice fed a high-methionine diet, a murine model of hyperhomocysteinemia. Red wine PE supplementation at low dose significantly reduced plasma Hcy levels and restored the hepatic and plasma-decreased paraoxonase-1 activity induced by chronic hyperhomocysteinemia. Moreover, aortic expression of proinflammatory cytokines and adhesion molecules and levels of soluble lectin-like oxidized low-density lipoprotein receptor-1 were reduced in hyperhomocysteinemic mice fed the red wine PE supplementation. These findings suggest that red wine PE administration in low quantities has beneficial effects on biochemical markers of endothelial dysfunction due to hyperhomocysteinemia.     

Surface-associated GroEL facilitates the adhesion of Escherichia coli to macrophages through lectin-like oxidized low-density lipoprotein receptor-1

The Escherichia coli homolog of GroEL, a 60 kDa heat shock protein (HSP), is a dominant protein produced not only in response to heat stress but also under in vitro growth condition. Beside its traditional cytoplasmic location, the surface exposures of GroEL have been observed in many pathogenic bacteria. To investigate the role of the surface-associated GroEL in the binding of E. coli to macrophages, we constructed a new strain of E. coli displaying GroEL on the outer membrane. We found that surface-associated GroEL increases the clearance ratio of E. coli by macrophages. It has been previously demonstrated that lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is the receptor for Hsp60 from different species. Our present results showed that GroEL on E. coli was recognized by LOX-1 on macrophages, leading to the phagocytosis of pathogen by macrophages. In addition, surface-associated GroEL made mice more susceptible to E. coli-induced peritonitis. These findings add to the research that clarifies the factors mediating bacterial adherence to host cells. Our results suggest that GroEL is a novel therapeutic target for modulating the immune response in infectious and inflammatory conditions.     

Chimeric structural stabilities in the coiled-coil structure of the NECK domain in human lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1)

LOX-1 (lectin-like oxidized low-density lipoprotein receptor 1) is the major oxidized LDL (OxLDL) receptor on endothelial cells. The extracellular part of LOX-1 comprises an 82-residue stalk region (NECK) and a C-type lectin-like ligand-binding domain (CTLD). The NECK displays sequence similarity to the coiled-coil region of myosin, having been suggested it adopts a rod-like structure. In this article, we report the structural analyses of human LOX-1 NECK using a variety of approaches including limited proteolysis, chemical cross-linking, circular dichroism (CD) and NMR. Our analysis reveals a unique structural feature of the LOX-1 NECK. Despite significant sequence similarity with the myosin coiled-coil, LOX-1 NECK does not form a uniform rod-like structure. Although not random, one-third of the N-terminal NECK is less structured than the remainder of the protein and is highly sensitive to cleavage by a variety of proteases. The coiled-coil structure is localized at the C-terminal part of the NECK, but is in dynamic equilibrium among multiple conformational states on a mus-ms time scale. This chimeric structural property of the NECK region may enable clustered LOX-1 on the cell surface to recognize OxLDL.     

Roles of lectin-like oxidized LDL receptor-1 and its soluble forms in atherogenesis

Lectin-like oxidized LDL receptor (LOX)-1 is a type II membrane protein that belongs to the C-type lectin family of molecules, which can act as a cell-surface endocytosis receptor for atherogenic oxidized LDL. LOX-1 can support binding, internalization and proteolytic degradation of oxidized LDL, but not of significant amounts of acetylated LDL, which is a well-known high-affinity ligand for class A scavenger receptors and scavenger receptor expressed by endothelial cells (SR-EC). LOX-1 is initially synthesized as a 40-kDa precursor protein with N-linked high mannose-type carbohydrate, which is further glycosylated and processed into a 50-kDa mature form. LOX-1 expression is not constitutive, but can be induced by proinflammatory stimuli, such as tumour necrosis factor-alpha, transforming growth factor-beta and bacterial endotoxin, as well as angiotensin II, oxidized LDL itself and fluid shear stress. In addition, LOX-1 expression is detectable in cultured macrophages and activated vascular smooth muscle cells. In vivo, endothelial cells that cover early atherosclerotic lesions, and intimal macrophages and smooth muscle cells in advanced atherosclerotic plaques can express LOX-1. Cell-surface LOX-1 can be cleaved through some protease activities that are associated with the plasma membrane, and released into the culture media. Purification of soluble LOX-1 and the N-terminal amino-acid sequencing identified the two cleavage sites (Arg86-Ser87 and Lys89-Ser90), both of which are located in the membrane proximal extracellular domain of LOX-1. Measurement of soluble LOX-1 in vivo may provide a novel diagnostic tool for the evaluation and prediction of atherosclerosis and vascular disease.