An alternative protein standard to measure activity of LOX-1 ligand containing apoB (LAB) - utilization of anti-LOX-1 single- chain antibody fused to apoB fragment

The Oxidized Lipoproteins In Vivo: Its Diversity and Behavior in the Human Circulation

A high concentration of low-density lipoproteins (LDLs) in circulation has been well-known as a major risk factor for cardiovascular diseases. The presence of oxidized LDLs (oxLDLs) in atherosclerotic lesions and circulation was demonstrated using anti-oxLDL monoclonal antibodies. The so-called “oxLDL hypothesis”, as a mechanism for atherosclerosis development, has been attracting attention for decades. However, the oxLDL has been considered a hypothetical particle since the oxLDL present in vivo has not been fully characterized. Several chemically modified LDLs have been proposed to mimic oxLDLs. Some of the subfractions of LDL, especially Lp(a) and electronegative LDL, have been characterized as oxLDL candidates as oxidized phospholipids that stimulate vascular cells. Oxidized high-density lipoprotein (oxHDL) and oxLDL were discovered immunologically in vivo. Recently, an oxLDL-oxHDL complex was found in human plasma, suggesting the involvement of HDLs in the oxidative modification of lipoproteins in vivo. In this review, we summarize our understanding of oxidized lipoproteins and propose a novel standpoint to understand the oxidized lipoproteins present in vivo.

Adiponectin forms a complex with atherogenic LDL and inhibits its downstream effects

Adiponectin, an adipocyte-derived protein, has antiatherogenic and antidiabetic effects, but how it confers the atherogenic effects is not well known. To study the antiatherogenic mechanisms of adiponectin, we examined whether it interacts with atherogenic low density lipoprotein (LDL) to attenuate LDL's atherogenicity. L5, the most electronegative subfraction of LDL, induces atherogenic responses similarly to copper-oxidized LDL (oxLDL). Unlike the native LDL endocytosed via the LDL receptor, L5 and oxLDL are internalized by cells via the lectin-like oxidized LDL receptor-1 (LOX-1). Using enzyme-linked immunosorbent assays (ELISAs), we showed that adiponectin preferentially bound oxLDL but not native LDL. In Chinese hamster ovary (CHO) cells transfected with the LOX-1 or LDL receptor, adiponectin selectively inhibited the uptake of oxLDL but not of native LDL, respectively. Furthermore, adiponectin suppressed the internalization of oxLDL in human coronary artery endothelial cells (HCAECs) and THP-1-derived macrophages. Western blot analysis of human plasma showed that adiponectin was abundant in L5 but not in L1, the least electronegative subfraction of LDL. Sandwich ELISAs with anti-adiponectin and anti-apolipoprotein B antibodies confirmed the binding of adiponectin to L5 and oxLDL. In LOX-1-expressing CHO cells, adiponectin inhibited cellular responses to oxLDL and L5, including nuclear factor-κB activation and extracellular signal-regulated kinas phosphorylation. In HCAECs, adiponectin inhibited oxLDL-induced endothelin-1 secretion and extracellular signal-regulated kinase phosphorylation. Conversely, oxLDL suppressed the adiponectin-induced activation of adenosine monophosphate-activated protein kinase in COS-7 cells expressing adiponectin receptor AdipoR1. Our findings suggest that adiponectin binds and inactivates atherogenic LDL, providing novel insight into the antiatherogenic mechanisms of adiponectin.

A Novel Cell-Free, Non-Fluorescent Method to Measure LOX-1-Binding Activity Corresponding to The Functional Activity of HDL

AIMS

A functional abnormality in high-density lipoprotein (HDL) particles rather than a quantitative abnormality in HDL cholesterol levels has been suggested to promote atherosclerosis. The modification of HDL may underlie functional changes to HDL such as gaining the ability to bind and activate the lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (LOX-1). We aimed to develop a novel method for measuring modified HDL on the basis of its binding to LOX-1.

METHODS

We designed a LOX-1 binding-based enzyme-linked immunosorbent assay (ELISA) with recombinant LOX-1 and anti-apoAI antibody. A lipid-free standard was devised by making a chimeric fusion protein containing anti-LOX-1 antibody and human apoAI fragment. We used this system to detect modified HDL, designated as LOX-1 ligand containing apoAI (LAA).

RESULTS

With our ELISA system, we detected HDL modified by copper oxidation, hypochlorous acid, 4-hydroxynonenal, and potassium cyanate, but not native HDL. Upon oxidation, HDL showed increased LOX-1 binding activity and decreased cholesterol efflux and paraoxonase-1 activities. In the ELISA, the chimeric fusion protein standard showed minimal variation in reference binding curves in contrast to copper-oxidized HDL preparations, suggesting better quality control of the chimeric fusion protein as the standard for measuring modified HDL activity. LAA was detectable in the plasma of healthy individuals and of mice fed a high-fat diet.

CONCLUSION

We have developed a novel ELISA by using recombinant LOX-1 and anti-apoAI antibody to measure the activity of modified HDL in plasma.

The Significance of Oxidized Low-Density Lipoprotein in Body Fluids as a Marker Related to Diseased Conditions

Oxidatively modified low-density lipoprotein (oxLDL) is known to be involved in various diseases, including cardiovascular diseases. The presence of oxLDL in the human circulatory system and in atherosclerotic lesions has been demonstrated using monoclonal antibodies. Studies have shown the significance of circulating oxLDL in various systemic diseases, including acute myocardial infarction and diabetic mellitus. Several different enzyme-linked immunosorbent assay (ELISA) procedures to measure oxLDL were utilized. Evidence has been accumulating that reveals changes in oxLDL levels under certain pathological conditions. Since oxLDL concentration tends to correlate with low-density lipoprotein (LDL)-cholesterol, the ratio of ox-LDL and LDL rather than oxLDL concentration alone has also been focused. In addition to circulating plasma, LDL and oxLDL are found in gingival crevicular fluid (GCF), where the ratio of oxLDL to LDL in GCF is much higher than in plasma. LDL and oxLDL levels in GCF show an increase in diabetic patients and periodontal patients, suggesting that GCF might be useful in examining systemic conditions. GCF oxLDL increased when the teeth were affected by periodontitis. It is likely that oxLDL levels in plasma and GCF could reflect oxidative stress and transfer efficacy in the circulatory system.

OxHDL controls LOX-1 expression and plasma membrane localization through a mechanism dependent on NOX/ROS/NF-κB pathway on endothelial cells

Systemic inflammatory diseases enhance circulating oxidative stress levels, which results in the oxidation of circulating high-density lipoprotein (oxHDL). Endothelial cell function can be negatively impacted by oxHDL, but the underlying mechanisms for this remain unclear. Some reports indicate that the lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is also a receptor for oxHDL. However, it is unknown if oxHDL induces increased LOX-1 expression at the plasma membrane, as an event that supports endothelial dysfunction. Therefore, the aims of this study were to determine if oxHDL induces plasma-membrane level changes in LOX-1 and, if so, to describe the underlying mechanisms in endothelial cells. Our results demonstrate that the incubation of arterial or vein endothelial cells with oxHDL (and not HDL) induces the increase of LOX-1 expression at the plasma membrane; effect prevented by LOX-1 inhibition. Importantly, same results were observed in endothelial cells from oxHDL-treated rats. Furthermore, the observed oxHDL-induced LOX-1 expression is abolished by the down-regulation of NOX-2 expression with siRNA (and no others NOX isoforms), by the pharmacological inhibition of NAD(P)H oxidase (with DPI or apocynin) or by the inhibition of NF-κB transcription factor. Coherently, LOX-1 expression is augmented by the incubation of endothelial cells with H₂O₂ or GSSG even in absence of oxHDL, indicating that the NOX-2/ROS/ NF-κB axis is involved. Interestingly, oxHDL incubation also increases TNF-α expression, cytokine that induces LOX-1 expression. Thus, our results suggest a positive feedback mechanism for LOX-1 receptor during inflammatory condition where an oxidative burst will generate oxHDL from native HDL, activating LOX-1 receptor which in turn will increase the expression of NOX-2, TNF-α and LOX-1 receptor at the plasma membrane. In conclusion, oxHDL-induced translocation of LOX-1 to the plasma membrane could constitute an induction mechanism of endothelial dysfunction in systemic inflammatory diseases.

Smoking cessation reduces the lectin-like low-density lipoprotein receptor index, an independent cardiovascular risk marker of vascular inflammation

Vessel wall inflammation promotes the destabilization of atherosclerotic plaques. The lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (LOX-1) expressed by vascular cells and monocytes. LOX index is calculated by multiplying LOX-1 ligand containing apolipoprotein B level with the soluble LOX-1. A high LOX index reflects an increased risk for stroke and myocardial infarction. However, the change in LOX index after smoking cessation and the relationship between smoking-related variables and LOX index are unknown. Relation of the clinical parameters to the LOX index was examined on 180 subjects (135 males and 45 females) at the first visit to our outpatient clinic for smoking cessation. The impact of smoking cessation on the LOX index was also determined in the 94 subjects (62 males and 32 females) who successfully stopped smoking. Sex-adjusted regression analysis and multivariate analysis identified three independent determinants of the LOX index, namely, low-density lipoprotein-cholesterol (LDL-C; β = 0.311, p < 0.001), high-sensitivity C-reactive protein (β = 0.358, p < 0.001), and expired carbon monoxide concentration reflecting smoking heaviness (β = 0.264, p = 0.003). Body mass index (BMI) significantly increased 3 months after the onset of smoking cessation (p < 0.001). However, the LOX index significantly decreased (p < 0.001), regardless of the rate of increase in BMI post-cessation. The LOX index is closely associated with smoking heaviness as well as dyslipidemia and an inflammation marker. Smoking cessation may induce a decrease in this cardiovascular risk marker, independently of weight gain.

Oxidized LDL (oxLDL) activates the angiotensin II type 1 receptor by binding to the lectin-like oxLDL receptor

The angiotensin II type 1 receptor (AT1) is a 7-transmembrane domain GPCR that when activated by its ligand angiotensin II, generates signaling events promoting vascular dysfunction and the development of cardiovascular disease. Here, we show that the single-transmembrane oxidized LDL (oxLDL) receptor (LOX-1) resides in proximity to AT1 on cell-surface membranes and that binding of oxLDL to LOX-1 can allosterically activate AT1-dependent signaling events. oxLDL-induced signaling events in human vascular endothelial cells were abolished by knockdown of AT1 and inhibited by AT1 blockade (ARB). oxLDL increased cytosolic G protein by 350% in Chinese hamster ovary (CHO) cells with genetically induced expression of AT1 and LOX-1, whereas little increase was observed in CHO cells expressing only LOX-1. Immunoprecipitation and in situ proximity ligation assay (PLA) assays in CHO cells revealed the presence of cell-surface complexes involving LOX-1 and AT1. Chimeric analysis showed that oxLDL-induced AT1 signaling events are mediated via interactions between the intracellular domain of LOX-1 and AT1 that activate AT1. oxLDL-induced impairment of endothelium-dependent vascular relaxation of vascular ring from mouse thoracic aorta was abolished by ARB or genetic deletion of AT1. These findings reveal a novel pathway for AT1 activation and suggest a new mechanism whereby oxLDL may be promoting risk for cardiovascular disease.

Inhibition of lectin-like oxidized low-density lipoprotein-1 receptor protects against plasma-mediated vascular dysfunction associated with pre-eclampsia

BACKGROUND

Pre-eclampsia (PE) is associated with vascular endothelial dysfunction and oxidative stress initiated by impaired trophoblast invasion. Oxidative stress modifies circulating low-density lipoprotein (LDL) to oxidized LDL (oxLDL). Lectin-like oxLDL receptor-1 (LOX-1) is a scavenger receptor for oxLDL. We hypothesized that plasma from patients with PE alters LOX-1 in normal human vessels during pregnancy, causing oxLDL-induced impairment of vascular function.

METHODS

Control-matched plasma was obtained from women with PE (n = 6). Oxidized LDL and soluble LOX-1 levels were determined by enzyme-linked immunoassay (ELISA). Remaining plasma was pooled and stored at -80ºC. Human omental arteries were incubated in 3% plasma from normal pregnant (NP) women or plasma from women with PE. Expression of LOX-1 in these vessels was determined by immunohistochemistry with antibodies against LOX-1. The omental vessels were exposed to oxLDL and the LOX-1 inhibitor TS20. Vascular function was assessed in response to the vasoconstrictor U46619 and the vasodilators bradykinin (BK) and sodium nitroprusside (SNP).

RESULTS

No significant differences in the concentrations of oxLDL or soluble LOX-1 (sLOX-1) were found in plasma from women with PE as compared with NP women. The expression of LOX-1 was not significantly different in either the NP or PE incubated omental vessels. Incubation of vessels from NP women in plasma from women with PE impaired their relaxation in response to BK as compared with that of NP vessels incubated in plasma from NP women. Exposure to oxLDL further impaired relaxation in NP vessels incubated with plasma from women with PE. Inhibition of LOX-1 protected against the impairment of vascular relaxation induced by plasma from women with PE.

CONCLUSION

Inhibition of LOX-1 prevents endothelial dysfunction in an in vitro model of PE and may prove useful as a therapeutic target in the treatment of PE.