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

Serum Soluble Lectin-like Oxidized Low-Density Lipoprotein Receptor-1 (sLOX-1) Is Associated with Atherosclerosis Severity in Coronary Artery Disease

Risk-factor-based scoring systems for atherosclerotic coronary artery disease (CAD) remain concerningly inaccurate at the level of the individual and would benefit from the addition of biomarkers that correlate with atherosclerosis burden directly. We hypothesized that serum soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1) would be independently associated with CAD and investigated this in the BioHEART study using 968 participants with CT coronary angiograms, which were scored for disease burden in the form of coronary artery calcium scores (CACS), Gensini scores, and a semi-quantitative soft-plaque score (SPS). Serum sLOX-1 was assessed by ELISA and was incorporated into regression models for disease severity and incidence. We demonstrate that sLOX-1 is associated with an improvement in the prediction of CAD severity when scored by Gensini or SPS, but not CACS. sLOX-1 also significantly improved the prediction of the incidence of obstructive CAD, defined as stenosis in any vessel >75%. The predictive value of sLOX-1 was significantly greater in the subgroup of patients who did not have any of the standard modifiable cardiovascular risk factors (SMuRFs). sLOX-1 is associated with CAD severity and is the first biomarker shown to have utility for risk prediction in the SMuRFless population.

Myeloperoxidase-Oxidized LDL Activates Human Aortic Endothelial Cells through the LOX-1 Scavenger Receptor

Cardiovascular disease as a result of atherosclerosis is a leading cause of death worldwide. Atherosclerosis is primarily caused by the dysfunction of vascular endothelial cells and the subendothelial accumulation of oxidized forms of low-density lipoprotein (LDL). Early observations have linked oxidized LDL effects in atherogenesis to the lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) scavenger receptor. It was shown that LOX-1 is upregulated by many inflammatory mediators and proatherogenic stimuli including cytokines, reactive oxygen species (ROS), hemodynamic blood flow, high blood sugar levels and, most importantly, modified forms of LDL. Oxidized LDL signaling pathways in atherosclerosis were first explored using LDL that is oxidized by copper (Cuox-LDL). In our study, we used a more physiologically relevant model of LDL oxidation and showed, for the first time, that myeloperoxidase oxidized LDL (Mox-LDL) may affect human aortic endothelial cell (HAEC) function through the LOX-1 scavenger receptor. We report that Mox-LDL increases the expression of its own LOX-1 receptor in HAECs, enhancing inflammation and simultaneously decreasing tubulogenesis in the cells. We hypothesize that Mox-LDL drives endothelial dysfunction (ED) through LOX-1 which provides an initial hint to the pathways that are initiated by Mox-LDL during ED and the progression of atherosclerosis.

Original contribution: sleeve gastrectomy reduces soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1) levels in patients with morbid obesity

BACKGROUND

Early diagnosis of subclinical cardiovascular disease (CVD) in patients with morbid obesity is important. We investigated the effects of sleeve gastrectomy (SG) on serum soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1), oxidized LDL (oxLDL), and other metabolic and inflammatory parameters associated with atherosclerosis in patients with morbid obesity.

METHODS

Body mass index (BMI) measurements and assays of metabolic and inflammatory markers were taken in patients in an SG surgery group and a healthy control group and compared at baseline and 12 months after SG. Correlations with changes in these parameters and variations in sLOX-1 were analyzed.

RESULTS

Metabolic and inflammatory marker values in the surgery (n = 20) and control (n = 20) groups were significantly different at baseline (p < 0.001). The majority of surgery group biomarker levels significantly decreased with mean BMI loss (- 11.8 ± 9.0, p < 0.001) at 12 months, trending toward control group values. Baseline albumin level as well as percentage reductions in oxLDL and the cholesterol retention fraction (CRF) were found to be significantly correlated with percentage reduction in sLOX-1 at 12 months following SG.

CONCLUSION

Metabolic and inflammatory biomarkers elevated at baseline significantly decreased after SG weight loss. Weight loss induced by SG may limit endothelial damage by reducing levels of oxLDL and LOX-1 as assessed by sLOX-1. These findings suggest that sLOX-1 may function as a marker of atherosclerotic disease states in patients with morbid obesity and that metabolic/bariatric surgery can play a meaningful role in CVD prevention.

The Mannose Receptor: From Endocytic Receptor and Biomarker to Regulator of (Meta)Inflammation

The mannose receptor is a member of the C-type lectin (CLEC) family, which can bind and internalize a variety of endogenous and pathogen-associated ligands. Because of these properties, its role in endocytosis as well as antigen processing and presentation has been studied intensively. Recently, it became clear that the mannose receptor can directly influence the activation of various immune cells. Cell-bound mannose receptor expressed by antigen-presenting cells was indeed shown to drive activated T cells towards a tolerogenic phenotype. On the other hand, serum concentrations of a soluble form of the mannose receptor have been reported to be increased in patients suffering from a variety of inflammatory diseases and to correlate with severity of disease. Interestingly, we recently demonstrated that the soluble mannose receptor directly promotes macrophage proinflammatory activation and trigger metaflammation. In this review, we highlight the role of the mannose receptor and other CLECs in regulating the activation of immune cells and in shaping inflammatory responses.

Dietary Apigenin Reduces Induction of LOX-1 and NLRP3 Expression, Leukocyte Adhesion, and Acetylated Low-Density Lipoprotein Uptake in Human Endothelial Cells Exposed to Trimethylamine-N-Oxide

By inducing vascular inflammation, trimethylamine-N-oxide (TMAO) is associated with endothelial dysfunction, atherosclerosis, and enhanced risk of cardiovascular diseases in humans. However, the underlying mechanisms are unknown. Expression of several genes related to arteriosclerosis, inflammasomes, and endothelial dysfunction was quantified by polymerase chain reaction after exposure to TMAO. LOX-1, ICAM-1, and NLRP3 were also quantified by Western blot, whereas leukocytic adhesion was examined using fluorescently labeled U937 cells. Scavenger receptors, adhesion molecules, and other genes associated with atherosclerosis were induced in endothelial cells exposed to TMAO. On the other hand, apigenin, a flavonoid that is abundant in parsley and celery, prevents initial arteriosclerosis events in endothelial cells. Apigenin reversed the effects of TMAO on mRNA expression of LOX-1, SREC, SR-PSOX, NLRP3, ASC, TXNIP, VCAM-1, ICAM-1, and MCP-1, as well as protein expression of LOX-1, the adhesion molecule ICAM-1, and the inflammasome protein NLRP3. Apigenin also suppressed leukocyte adhesion and uptake of acetylated low-density lipoprotein. The data indicate that expression of scavenger receptors and adhesion molecules in response to TMAO, along with formation of NLRP3 inflammasomes, may drive endothelial dysfunction through uptake of acetylated low-density lipoprotein and lymphocyte adhesion. Apigenin reverses these effects, implying that it may also prevent arteriosclerosis.

LOX-1 en las afecciones cardiovasculares, perspectivas terapéuticas futuras

El receptor de la lipoproteína de baja densidad oxidado tipo lectina 1 (LOX-1), también conocido como OLR-1, es un receptor scavenger (SR) clase E, que media la absorción del colesterol LDL en su forma oxidada, por las células vasculares. LOX-1 está involucrado en la disfunción endotelial, la adhesión de monocitos, la proliferación, migración y apoptosis de las células del músculo liso, la formación de células espumosas, la activación de plaquetas, así como la inestabilidad a nivel del endotelio vascular; todos eventos críticos en la patogénesis de la aterosclerosis. LOX-1 contribuyen a la inestabilidad de la placa ateroesclerótica y a las últimas secuelas clínicas de ruptura endotelial e isquemia tisular cardíaca potencialmente mortal. No existe en la actualidad ningún fármaco aprobado o en desarrollo clínico a partir de LOX-1, debido a sus complejos mecanismos biológicos no dilucidados completamente. Se han utilizado diversas terapias con el objetivo de inhibir la acción de LOX-1; medicamentos como: antioxidantes, estatinas, agentes antinflamatorios naturales, que actúen sobre su expresión, pero todos con eficacia moderada. También se ha evaluado la administración de anticuerpos anti-LOX-1 inhibe la aterosclerosis al disminuir eventos celulares. El diseño de fármacos enfocados en el conocimiento de las vías de señalización de LOX-1 y la aplicación de herramientas biotecnológicas permite el desarrollo de nuevas dianas terapéuticas basadas en la potencialidad que tienen los anticuerpos monoclonales. Con estos antecedentes el, receptor LOX-1, representa un objetivo terapéutico atractivo para el tratamiento de enfermedades ateroscleróticas humanas. La evidencia reciente indica que la acción sobre este SR es una posible estrategia para el tratamiento de la enfermedad vascular, explorando en esta revisión su papel y posibles futuras aplicaciones en el diagnóstico y la terapéutica.

Cross-Talk between Inflammatory Mediators and the Epithelial Mesenchymal Transition Process in the Development of Thyroid Carcinoma

There is strong association between inflammatory processes and their main metabolic mediators, such as leptin, adiponectin secretion, and low/high-density lipoproteins, with the cancer risk and aggressive behavior of solid tumors. In this scenario, cancer cells (CCs) and cancer stem cells (CSCs) have important roles. These cellular populations, which come from differentiated cells and progenitor stem cells, have increased metabolic requirements when it comes to maintaining or expanding the tumors, and they serve as links to some inflammatory mediators. Although the molecular mechanisms that are involved in these associations remain unclear, the two following cellular pathways have been suggested: 1) the mesenchymal-epithelial transition (MET) process, which permits the differentiation of adult stem cells throughout the acquisition of cell polarity and the adhesion to epithelia, as well to new cellular lineages (CSCs); and, 2) a reverse process, termed the epithelial-mesenchymal transition (EMT), where, in pathophysiological conditions (tissue injury, inflammatory process, and oxidative stress), the differentiated cells can acquire a multipotent stem cell-like phenotype. The molecular mechanisms that regulate both EMT and MET are complex and poorly understood. Especially, in the thyroid gland, little is known regarding MET/EMT and the role of CCs or CSCs, providing an exciting, new area of knowledge to be investigated. This article reviews the progress to date in research on the role of inflammatory mediators and metabolic reprogramming during the carcinogenesis process of the thyroid gland and the EMT pathways.

Emerging roles of calpain proteolytic systems in macrophage cholesterol handling

Calpains are Ca2+-dependent intracellular proteases that play central roles in the post-translational processing of functional proteins. In mammals, calpain proteolytic systems comprise the endogenous inhibitor calpastatin as well as 15 homologues of the catalytic subunits and two homologues of the regulatory subunits. Recent pharmacological and gene targeting studies in experimental animal models have revealed the contribution of conventional calpains, which consist of the calpain-1 and -2 isozymes, to atherosclerotic diseases. During atherogenesis, conventional calpains facilitate the CD36-dependent uptake of oxidized low-density lipoprotein (LDL), and block cholesterol efflux through ATP-binding cassette transporters in lesional macrophages, allowing the expansion of lipid-enriched atherosclerotic plaques. In addition, calpain-6, an unconventional non-proteolytic calpain, in macrophages reportedly potentiates pinocytotic uptake of native LDL, and attenuates the efferocytic clearance of apoptotic and necrotic cell corpses from the lesions. Herein, we discuss the recent progress that has been made in our understanding of how calpain contributes to atherosclerosis, in particular focusing on macrophage cholesterol handling.

Differential Effects of Low-Density Lipoprotein and Chylomicron Remnants on Lipid Accumulation in Human Macrophages

The effects of low-density lipoprotein (LDL) and chylomicron remnants on lipid accumulation in human monocyte–derived macrophages (HMDMs) and in macrophages derived from the human monocyte cell line THP-1 were compared. The HMDMs or THP-1 macrophages were incubated with LDL, oxidized LDL (oxLDL), chylomicron remnant–like particles (CMR-LPs), or oxidized CMR-LPs (oxCMR-LPs), and the amount and type of lipid accumulated were determined. As expected, the lipid content of both cell types was increased markedly by oxLDL but not LDL, and this was due to a rise in cholesterol, cholesteryl ester (CE), and triacylglycerol (TG) levels. In contrast, both CMR-LPs and oxCMR-LPs caused a considerable increase in cellular lipid in HMDMs and THP-1 macrophages, but in this case there was a greater rise in the TG than in the cholesterol or CE content. Lipid accumulation in response to oxLDL, CMR-LPs, and oxCMR-LPs was prevented by the ACAT inhibitor CI976 in HMDMs but not in THP-1 macrophages, where TG levels remained markedly elevated. The rate of incorporation of [3H]oleate into CE and TG in THP-1 macrophages was increased by oxLDL, CMR-LPs, and oxCMR-LPs, but incorporation into TG was increased to a greater extent with CMR-LPs and oxCMR-LPs compared with oxLDL. These results demonstrate that both CMR-LPs and oxCMR-LPs cause lipid accumulation in human macrophages comparable to that seen with oxLDL and that oxidation of the remnant particles does not enhance this effect. They also demonstrate that a greater proportion of the lipid accumulated in response to CMR-LPs compared with oxLDL is TG rather than cholesterol or CE and that this is associated with a higher rate of TG synthesis. This study, therefore, provides further evidence to suggest that chylomicron remnants have a role in foam cell formation that is distinct from that of oxLDL.

LOX-1 gene variants and maternal levels of plasma oxidized LDL and malondialdehyde in patients with gestational diabetes mellitus

PURPOSE

The aim of this study was to investigate the relationships between the maternal levels of oxidized LDL (ox-LDL), lipid peroxidation marker malondialdehyde (MDA) and LOX-1 3'UTR188C/T and K167N single nucleotide polymorphisms in pregnant Turkish women with gestational diabetes mellitus (GDM).

METHODS

116 pregnant women with GDM and 120 healthy pregnant women from the same geographic region were included in the study. Polymerase chain reaction-based restriction analysis was used to identify 3'UTR188C/T and K167N polymorphisms of the LOX-1 gene. Plasma ox-LDL and MDA levels were determined by enzyme-linked immunosorbent assay and spectrophotometric method in all study subjects, respectively.

RESULTS

Our results indicated that the distribution of the LOX-1 3'UTR188C/T and K167N genotypes and alleles did not differ significantly among subjects with or without GDM (p > 0.05). TT and NN genotype carriers are associated with some glucose metabolism parameters (p < 0.05). There were no significant differences among plasma ox-LDL and MDA levels with regard to LOX-1 3'UTR188C/T and K167N polymorphisms in GDM group and control subjects (p > 0.05). According to the combined genotype analysis of LOX-1 3'UTR 188 TT and K167N NN polymorphisms, plasma MDA and ox-LDL levels were significantly different between women with GDM and healthy subjects either with or without combined TT/NN genotype carriers (p < 0.001).

CONCLUSIONS

According to our results, ox-LDL and MDA levels were increased in GDM pregnant women and healthy pregnant women either with or without combined TT/NN genotype carriers, for our Turkish sample, these genotype carriers appear to be related with increased oxidative stress in patients with GDM.

Expression of transforming growth factor beta 1-related signaling proteins in irradiated vessels

AIM

Microvascular free tissue transfer is a standard method in head and neck reconstructive surgery. However, previous radiotherapy of the operative region is associated with an increased incidence in postoperative flap-related complications and complete flap loss. As transforming growth factor beta (TGF-β) 1 and galectin-3 are well known markers in the context of fibrosis and lectin-like oxidized low-density lipoprotein 1 (LOX-1) supports vascular atherosclerosis, the aim of this study was to evaluate the expression of TGF-β1 and related markers as well as LOX-1 in irradiated vessels.

MATERIALS AND METHODS

To evaluate the expression of galectin-3, Smad 2/3, TGF-β1, and LOX-1, 20 irradiated and 20 nonirradiated arterial vessels were used for immunohistochemical staining. We semiquantitatively assessed the ratio of stained cells/total number of cells (labeling index).

RESULTS

Expression of galectin-3, Smad 2/3, and TGF-β1 was significantly increased in previously irradiated vessels compared with nonirradiated controls. Furthermore, LOX-1 was expressed significantly higher in irradiated compared with nonirradiated vessels.

CONCLUSION

Fibrosis-related proteins like galectin-3, Smad 2/3, and TGF-β1 are upregulated after radiotherapy and support histopathological changes leading to vasculopathy of the irradiated vessels. Furthermore, postoperative complications in irradiated patients can be explained by increased endothelial dysfunction caused by LOX-1 in previously irradiated patients. Consequently, not only TGF-β1 but also galectin-3 inhibitors may decrease complications after microsurgical tissue transfer.

The association of lectin-like oxidized LDL receptor 1 (LOX-1) K167N and 3'UTR188CT polymorphisms with maternal plasma soluble LOX-1 levels and preeclampsia risk in Turkish population

PURPOSE

To investigate the main effect of polymorphisms in genes involved in endothelial pathophysiological mechanisms, LOX-1 K167N and 3'UTR188CT single nucleotide polymorphisms (SNPs) in relation to preeclampsia (PE) risk and possible interactions between the gene polymorphisms and plasma oxLDL and soluble LOX-1 (sLOX-1) levels on PE in Turkish population.

METHODS

LOX-1 K167N and 3'UTR188CT polymorphisms were studied in 113 pregnant women with preeclampsia and 96 healthy pregnant women by the PCR-RFLP techniques. sLOX-1 and oxLDL levels were determined by enzyme-linked immunosorbent assay (ELISA) in all study subjects.

RESULTS

Patients having LOX-1 3'UTR188CT (OR 3.55, 95% CI 1.89-6.67, P = 0.001) or 3'UTR188CC (OR 3.04, 95% CI 1.25-7.38, P = 0.012) genotype had a significantly higher risk of PE than those with 3'UTR188TT genotype. Also, patients having K167N KK (OR 2.73, 95% CI 1.33-5.61, P = 0.005) genotype had a significantly higher risk of PE than those with K167N NN genotype. LOX-1 3'UTR188TT and LOX-1 K167N NN genotype carriers were associated with significantly increased serum sLOX-1 level (P = 0.001). We further investigated the potential combined effect of these polymorphic variants on risk of PE development. According to the combined genotype analysis of LOX-1 3'UTR188TT and K167N NN polymorphisms, sLOX-1 and oxLDL levels also showed significant differences between PE patients and controls with or without combined TT/NN genotype carriers.

CONCLUSIONS

Our findings indicate that higher plasma sLOX-1 and oxLDL concentrations, and the LOX-1 3'UTR188C>T and LOX-1 K167N gene polymorphisms were significantly associated with risk of developing preeclampsia. Plasma sLOX-1 may be a potential therapeutic target in the treatment of preeclampsia.

Endothelial damage in white coat hypertension: role of lectin-like oxidized low-density lipoprotein-1

The aims of this study included an examination of soluble lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (sLOX-1) levels in hypertensive (HT) patients. Another aim examined sLOX-1 associations with oxidized LDL (oxLDL), nitric oxide synthase (eNOS) and nitric oxide (NOx). A final aim was to compare these parameters between HT patients, white-coat hypertensive (WCH) patients and healthy controls. The three groups, HT, WCH and controls, were comprised of 35 patients each. sLOX-1 and oxLDL levels were significantly increased in WCH and HT patients compared with controls. The eNOS activation was significantly lower in HT than in the control group. sLOX-1 and oxLDL levels were significantly negatively correlated with eNOS levels in the WCH and HT groups. Carotid intima-media thickness (CIMT) measurements were significantly higher in the WCH and HT groups compared with controls. There was a significant positive correlation between CIMT and sLOX-1 and oxLDL; however, there was a negative correlation with eNOS in WCH. Regression analysis revealed that sLOX-1 was the variable that had a significant effect on blood pressure (P<0.001, odds ratio (95% confidence interval=23.273 (5.843-92.688)). A possible endothelial impairment may act as a cardiovascular risk factor in WCH. Necessary measures should be considered in terms of atherosclerosis risk with HT, especially in early identification of endothelial damage by looking at sLOX-1 levels. We believe sLOX-1 levels are strong biomarkers for determining early endothelial damage in HT, and especially in WCH patients.

Soluble Lectin-Like Oxidized LDL Receptor 1 as a Possible Mediator of Endothelial Dysfunction in Patients With Metabolic Syndrome

BACKGROUND

Metabolic syndrome (MetS) defines a well-known cluster of metabolic disturbances associated with an increased risk of cardiovascular disease and diabetes. The aim of this study was to examine the distribution of soluble lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (sLOX-1) levels in patients with MetS, possible association of sLOX-1 with oxidized LDL (oxLDL), endothelial nitric oxide synthase (eNOS), nitric oxide (NOx), endothelin-1 (ET-1), paraoxonase 1 (PON1), and arylesterase (ARE) activities, and these parameters compared with healthy controls.

METHODS

A total of 55 patients (37 women, 18 men) with MetS and 29 healthy controls (19 women, 10 men) with a body mass index (BMI) less than 25 kg/m(2) were enrolled in the study.

RESULTS

sLOX-1, oxLDL, and ET-1 levels were significantly higher in patients with MetS than in control subjects (P = 0.023 P < 0.001, and P < 0.001, respectively). MetS patients have significantly lower eNOS and NOx levels, and PON1 and ARE activities than control subjects (P = 0.017, P < 0.004, P < 0.001, and P = 0.010, respectively). A positive correlation was observed between the sLOX-1 levels and the oxLDL, ET-1, BMI, glucose levels. ET-1 levels also exhibited significant negative correlation with ARE activity.

CONCLUSION

sLOX-1 levels are associated with cardiovascular risk factors, such as increased oxLDL, obesity, and diabetes, in patients with MetS. An increased concentration of sLOX-1 could be an early predictor of endothelial damage in MetS. In addition, it appears that oxLDL, ET-1, eNOS, NOx, PON1, and ARE activities may accurately reflect the levels of endothelial dysfunction in MetS patients.

Early Trypanosoma cruzi infection reprograms human epithelial cells

Trypanosoma cruzi, the causative agent of Chagas disease, has the peculiarity, when compared with other intracellular parasites, that it is able to invade almost any type of cell. This property makes Chagas a complex parasitic disease in terms of prophylaxis and therapeutics. The identification of key host cellular factors that play a role in the T. cruzi invasion is important for the understanding of disease pathogenesis. In Chagas disease, most of the focus is on the response of macrophages and cardiomyocytes, since they are responsible for host defenses and cardiac lesions, respectively. In the present work, we studied the early response to infection of T. cruzi in human epithelial cells, which constitute the first barrier for establishment of infection. These studies identified up to 1700 significantly altered genes regulated by the immediate infection. The global analysis indicates that cells are literally reprogrammed by T. cruzi, which affects cellular stress responses (neutrophil chemotaxis, DNA damage response), a great number of transcription factors (including the majority of NFκB family members), and host metabolism (cholesterol, fatty acids, and phospholipids). These results raise the possibility that early host cell reprogramming is exploited by the parasite to establish the initial infection and posterior systemic dissemination.

Simulative and experimental investigation on the cleavage site that generates the soluble human LOX-1

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is a scavenger receptor that mediates the recognition, the binding and internalization of ox-LDL. A truncated soluble form of LOX-1 (sLOX-1) has been identified that, at elevated levels, has been associated to acute coronary syndrome. Human sLOX-1 is the extracellular part of membrane LOX-1 which is cleaved in the NECK domain with a mechanism that has not yet been identified. Purification of human sLOX-1 has been carried out to experimentally identify the cleavage site region within the NECK domain. Molecular modelling and classical molecular dynamics simulation techniques have been used to characterize the structural and dynamical properties of the LOX-1 NECK domain in the presence and absence of the CTLD recognition region, taking into account the obtained proteolysis results. The simulative data indicate that the NECK domain is stabilized by the coiled-coil heptad repeat motif along the simulations, shows a definite flexibility pattern and is characterized by specific electrostatic potentials. The detection of a mobile inter-helix space suggests an explanation for the in vivo susceptibility of the NECK domain to the proteolytic cleavage, validating the assumption that the NECK domain sequence is composed of a coiled-coil motif destabilized in specific regions of functional significance.

The lectin-like oxidized low-density lipoprotein receptor-1 as therapeutic target for atherosclerosis, inflammatory conditions and longevity

INTRODUCTION

The lectin-like oxidized LDL receptor-1 (LOX-1) is a scavenger receptor and is regarded as a central element in the initiation of endothelial dysfunction and its further progression to atherosclerosis. Increasing numbers of studies suggest that therapeutic strategies to modulate LOX-1 will have a broad spectrum of applications ranging from cardiovascular diseases to longevity.

AREAS COVERED

The dual role of LOX-1 as a culprit molecule in the process of atherosclerosis and as a danger signal in various tissues is introduced. The structure of the receptor, its ligands and its modulation by known drugs, by natural products (e.g., statins, imipramine, salicylate-based drugs, procyanidins, curcumin) and by new strategies (antisenseRNA, miRNA, pyrrole-imidazol-polyamides, LOX-1 antibodies, lipid apheresis) are described.

EXPERT OPINION

Therapeutic approaches via transcript regulation, allowing a modulation of LOX-1, may be an easier and safer strategy than a blockade of the receptor. Considering the wide distribution of LOX-1 on different tissues, research on the mechanisms of LOX-1 modulation by drugs and natural products applying "omic"-technologies will not only allow a better understanding of the role of LOX-1 in the processes of atherosclerosis, inflammation and longevity but also support the development of specific LOX-1 modulators, avoiding the initiation of molecular mechanisms which lead to adverse events.

LOX-1 and obesity

Obesity is one of the most common lifestyle-related diseases. Being closely associated with insulin resistance, hypertension and dyslipidemia, it is also a component of metabolic syndrome and is involved in the development of atherosclerosis and cardiovascular and renal ailments. Obesity is also accompanied with a state of chronic inflammation. Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), a receptor for oxidized low density lipoprotein (ox-LDL), is expressed not only in endothelial cells, but also in macrophages, vascular smooth muscle cells, platelets and adipocytes. LOX-1 binds multiple ligands, has diverse physiological functions and plays a critical role in the signal transduction. It may well turn out to be a key, or very important, factor in the development of hypertension, diabetes mellitus and hyperlipidemia, the most important risk factors for atherosclerosis. In recent studies, LOX-1 upregulation appears to be a vital factor in obesity and its complications. This review summarizes current knowledge of obesity and the relationship between LOX-1 and obesity.

Lectin-like oxidized low-density lipoprotein receptor-1 modulates endothelial apoptosis in obstructive sleep apnea

BACKGROUND

Lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1) is the major receptor for oxidized low-density lipoprotein in endothelial cells, and its expression is enhanced in proatherogenic settings. The objective of this study was to investigate the association between LOX-1 in freshly harvested human venous endothelial cells and apoptotic circulating endothelial cells in patients with obstructive sleep apnea (OSA).

METHODS

We conducted a prospective, interventional study of 38 patients with newly diagnosed OSA free of disease and 12 healthy control subjects. Plasma LOX-1 (pLOX-1) levels were measured using a commercially available enzyme-linked immunosorbent assay. Protein expression of LOX-1 was quantified by immunofluorescence in freshly harvested venous endothelial cells before and after 8 weeks of continuous positive airway pressure (CPAP) therapy. Circulating apoptotic endothelial cells (CD146(+), CD45(-), and CD31(1)) were assessed concomitantly by flow cytometry.

RESULTS

pLOX-1 levels were higher in subjects with OSA than in control subjects (326.9 ± 267.1 pg/mL and 141.1 ± 138.6 g/mL, respectively; P = .004). Patients with OSA showed a threefold increase in baseline endothelial expression of LOX-1 relative to control subjects. CPAP therapy resulted in a significant decrease in endothelial LOX-1 expression only in CPAP-adherent patients. Circulating apoptotic endothelial cells correlated directly with baseline expression of LOX-1 (R(2) = 0.32, P = .01) after adjustment for age, BMI, and waist to hip ratio.

CONCLUSIONS

Increased expression of LOX-1 in vivo is associated with endothelial apoptosis. Adherence to CPAP therapy may reverse these derangements.

Pokkuri Death Syndrome; sudden cardiac death cases without coronary atherosclerosis in South Asian young males

Sudden death is one of the major concerns in forensic medicine. Especially when the deceased is a young subject without significant history, the case will be of major interest to the authorities. Sudden unexplained cardiac death has been known as "Pokkuri Death Syndrome" (PDS) in Japan, "Lai Tai" in Thailand, "Bangungut" in the Philippines, "Dream Disease" in Hawaii, and "Sudden Unexpected Nocturnal Death Syndrome" among South Asian immigrants in the USA. However, the clinical and pathological features of these sudden death cases, especially the characteristics of no coronary atherosclerosis, are surprisingly similar and mainly occur among Southeast Asian young males during sleep in the midnight. In this manuscript, we have reviewed the pathological characteristics and the possible mechanism of death in PDS cases, which were associated with significantly elevated remnant lipoproteins in plasma as revealed from our studies during the past 15 years in Japan. Although elevated plasma remnant lipoproteins have been known to be strongly atherogenic, coronary atherosclerosis was not observed in PDS cases. PDS cases were shown to be an interesting cardiovascular disease death discovered in forensic medicine research, which may suggest the difference between the occurrence of cardiovascular events and the severity of coronary atherosclerosis as separate factors. These observations in PDS cases suggest the possibility that the intervention could be more targeted to suppress the cardiovascular events rather than to slow down the progression of atherosclerosis, which is now most extensively targeted for the therapy of cardiovascular disease in Western countries.

Caloric restriction, aerobic exercise training and soluble lectin-like oxidized LDL receptor-1 levels in overweight and obese post-menopausal women

Background

Elevated circulating levels of soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1) have been observed in obese persons and are reduced by weight loss. However, it is not known if combining caloric restriction (CR) with exercise training is better in reducing sLOX-1 levels than CR alone.

Objective

We examined whether the addition of aerobic exercise to a weight loss intervention differentially affects sLOX-1 levels in 61 abdominally obese postmenopausal women randomly assigned to a CR only (n=22), CR + moderate-intensity exercise (n=22), or CR + vigorous-intensity exercise (n=17) intervention for 20 weeks. The caloric deficit was ~2,800 kcal/week for all groups.

Results

The intervention groups were similar at baseline with respect to body weight, body composition, lipids, and blood pressure. However, plasma sLOX-1 levels were higher in the CR only group (99.90 ± 8.23 pg/ml) compared to both the CR + moderate-intensity exercise (69.39 ± 8.23 pg/ml, p=0.01) and CR + vigorous-intensity exercise (72.83 ± 9.36 pg/ml, p=0.03) groups. All three interventions significantly reduced body weight (~14%), body fat, and waist and hip circumferences to a similar degree. These changes were accompanied by a 23% reduction in sLOX-1 levels overall (−19.00 ± 30.08 pg/ml, p<0.0001), which did not differ among intervention groups (p=0.13). Changes in body weight, body fat, and VO₂ max were not correlated with changes in sLOX-1 levels. In multiple regression analyses in all women combined, baseline sLOX-1 levels (β = − 0.70 ± 0.06, p<0.0001), age (β = 0.92 ± 0.43, p=0.03) and baseline BMI (β = 1.88 ± 0.66, p=0.006) were independent predictors of the change in sLOX-1 with weight loss.

Conclusions

Weight loss interventions of equal energy deficit have similar effects on sLOX-1 levels in overweight and obese postmenopausal women, with the addition of aerobic exercise having no added benefit when performed in conjunction with CR.

Impaired expression of genes regulating cholesterol efflux in human osteoarthritic chondrocytes

Altered lipid metabolism has been implicated as a critical player in osteoarthritis (OA). Our study aimed to investigate the expression of genes regulating cholesterol efflux in human chondrocytes and to study the effect of an LXR agonist on cholesterol efflux and lipid accumulation in osteoarthritic chondrocytes. ATP-binding-cassette transporter A1 (ABCA1), apolipoprotein A1 (ApoA1), and liver X receptors (LXRalpha and LXRbeta) mRNA expression levels were evaluated using real-time polymerase chain reaction (PCR) and ApoA1 protein levels by Western blot analysis in normal and osteoarthritic articular cartilage samples. Cholesterol efflux was evaluated in osteoarthritic chondrocytes radiolabeled with [1,2(n)-(3)H] cholesterol after LXR treatment, while intracellular lipid accumulation was studied after Oil-red-O staining. Cholesterol efflux gene expressions were significantly lower in osteoarthritic cartilage compared to normal. Treatment of osteoarthritic chondrocytes with the LXR agonist TO-901317 significantly increased ApoA1 and ABCA1 expression levels, as well as cholesterol efflux. Additionally, osteoarthritic chondrocytes presented intracellular lipids deposits, while no deposits were found after treatment with TO-901317. Our findings suggest that impaired expression of genes regulating cholesterol efflux may be a critical player in osteoarthritis, while the ability of the LXR agonist to facilitate cholesterol efflux suggests that it may be a target for therapeutic intervention in osteoarthritis.

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.

Host cholesterol and inflammation as common key regulators of toxoplasmosis and artherosclerosis development

Atherosclerosis and toxoplasmosis are two widely prevalent diseases worldwide. The relationship between these diseases is now being elucidated. Atherosclerosis is a disease with three main components: increased blood lipoprotein/cholesterol and their deposition in the arterial wall, an important Th1-mediated proinflammatory reaction and thrombogenic status. Toxoplasma gondii, in turn, is dependent on host cholesterol for optimal intracellular growth and replication. As a result, host cholesterol will be cleared from the blood, reducing plasma low-density lipoprotein, a crucial atherosclerosis risk factor. On the other hand, T. gondii infection elicits an important Th1 systemic inflammatory response in the host. Therefore, this additional proinflammatory stimulus may impose an enhanced pro-atherogenic environment in the host. As result, the association between these two diseases in one individual could change the course of atherosclerosis. In this review, we demonstrate that the host-parasite relationship is complex and that the outcome of each disease is dependent on the availability of intracellular cholesterol, as well as the intensity of the inflammatory reaction triggered by the parasite. We also discuss the possible clinical implications of these studies.

A case of myocardial infarction caused by coronary vasospasm: efficacy of soluble lectin-like oxidized LDL receptor-1 for distinguishing between vasospasm and plaque rupture

This case illustrates the benefits of using plasma levels of soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1) to distinguish between myocardial injuries caused by coronary vasospasm or coronary plaque rupture. A 62-year-old woman with a history of dyslipidemia was admitted due to chest pain of 1 hour's duration. Electrocardiography demonstrated mild ST segment elevation in leads V1-6, and blood chemistry studies on admission showed a slight increase in the serum level of troponin T; therefore, acute myocardial infarction was diagnosed. However, the plasma level of sLOX-1, a marker of plaque rupture, did not increase during hospitalization. Coronary angiography demonstrated normal coronary arteries, and intracoronary acetylcholine infusion provoked coronary artery vasospasm. We could therefore diagnose myocardial injury caused by coronary vasospasm. We propose that the cause of myocardial injury cannot be predicted with electrocardiography and measurement of troponin T but can be clarified by measuring plasma levels of sLOX-1 in the early stage of acute coronary syndrome.

Generation of monoclonal antibodies against a soluble form of lectin-like oxidized low-density lipoprotein receptor-1 and development of a sensitive chemiluminescent enzyme immunoassay

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), expressed prominently in atherosclerotic lesions, is cleaved and released as a soluble LOX-1 (sLOX-1), which is a specific biomarker to diagnose acute coronary syndrome (ACS) at an early stage. Although sLOX-1 levels in patient's blood were successfully measured with our previously established enzyme-linked immunosorbent assay (ELISA), the assay was not sensitive enough to detect normal serum levels of sLOX-1 in healthy human subjects. We therefore developed sensitive and specific monoclonal antibodies (mAbs) against sLOX-1 in order to establish a more sensitive immunoassay. Mice were immunized with recombinant human LOX-1 extracellular domain. mAbs were subsequently generated by standard myeloma cell fusion techniques with a novel screening method using time-resolved fluorescence immunoassay. Using two anti-human sLOX-1 mAbs and alkaline phosphatase as a label, a sandwich chemiluminescent enzyme immunoassay (CLEIA) was developed. In total, nine mAbs were obtained. The dissociation constant (K(d)) values of these mAbs for sLOX-1 were 0.12-1.32 nM. Characteristics of these mAbs were estimated and the best combination for CLEIA was selected. The newly established CLEIA could determine sLOX-1 levels as low as 8 pg/mL, and thus, was sensitive enough to measure serum sLOX-1 levels in normal human subjects and to evaluate subtle differences. Values for sLOX-1 measured by monoclonal CLEIA and polyclonal ELISA were highly correlated (r(2)=0.7594, p<0.0001). Area under the curve values of the receiver-operating characteristic curves in detecting ACS were 0.948 and 0.978 for monoclonal CLEIA and polyclonal ELISA, respectively. Thus, a more sensitive sLOX-1 CLEIA was established using newly developed mAbs against sLOX-1. In addition to its advantage in early diagnosis of ACS, this assay may also be useful in predicting cardiovascular disease risk in disease-free subjects.

Elevated soluble lectin-like oxidized LDL receptor-1 (sLOX-1) levels in obese postmenopausal women

We investigated the association between soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1) levels and obesity in older women. Fifty-one postmenopausal women (10 lean, 22 overweight, and 19 obese) were included in this small retrospective analysis. Plasma sLOX-1 levels were measured using a chemiluminescent enzyme-linked immunoassay. Plasma levels of sLOX-1 were significantly higher in obese women (55.33 +/- 4.49 pg/ml) compared to lean (30.91 +/- 6.19 pg/ml, P = 0.002) and overweight women (38.31 +/- 4.18 pg/ml, P = 0.017). Plasma sLOX-1 levels were positively associated with body weight, BMI, total body fat, and trunk fat. The relationship between sLOX-1 and BMI was attenuated after adjustment for age, hormone replacement therapy, and body fat. In conclusion, obese women have higher sLOX-1 levels, which may reflect increased LOX-1 expression in adipose tissue.

Variation in the human lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1) gene is associated with plasma soluble LOX-1 levels

The lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1) expressed on vascular cells plays a major role in atherogenesis by internalizing and degrading oxidized low-density lipoprotein. LOX-1 can be cleaved from the cell surface and released as soluble LOX-1 (sLOX-1), and elevated sLOX-1 levels may be indicative of atherosclerotic plaque instability. We examined associations between the LOX-1 gene 3'UTR-C/T and G501C polymorphisms and plasma sLOX-1 levels in 97 healthy older men and women. The frequencies for the 3'UTR-T and 501C alleles were 46 and 10%, respectively. Plasma sLOX-1 levels were significantly higher in the 3'UTR CC genotype group compared with both the CT (P=0.02) and TT genotype groups (P=0.002). Plasma sLOX-1 levels were also significantly higher in the 501GC genotype group compared with the GG genotype group (P=0.004). In univariate analyses, sLOX-1 levels were significantly associated with both the 3'UTR-C/T and G501C polymorphisms. These associations remained significant after adjusting for age, sex, race and body mass index. In conclusion, variation in the LOX-1 gene is associated with plasma sLOX-1 levels in older men and women.

Lectin-like oxidized LDL receptor-1 (LOX-1) expression is associated with atherosclerotic plaque instability—analysis in hypercholesterolemic rabbits

Lectin-like oxidized LDL receptor-1 (LOX-1), a cell-surface receptor for oxidized LDL (Ox-LDL), has been implicated in vascular cell dysfunction related to atherosclerotic plaque instability, according to cell culture experiments. In the present study, we investigated the relationship between LOX-1 expression and plaque instability in hypercholesterolemic rabbits by immunohistological analyses in vivo. We prepared thirty series of cross sections of the thoracic aorta from six myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits (12-24 months), in which seventy atherosclerotic plaques were observed. LOX-1, matrix metalloproteinase-9 (MMP-9), monocyte chemoattractant protein-1 (MCP-1) expression, apoptotic events, plaque instability index (an index of the morphological destabilization of atherosclerotic plaques) and fibromuscular cap thickness in each atherosclerotic plaque were determined by immunohistochemical staining, TUNEL staining and Azan-Mallory staining. LOX-1 expression was positively correlated with the plaque instability index and MMP-9 expression. LOX-1 expression was more prominent in atherosclerotic plaques with thinner fibromuscular cap (<100 microm). Furthermore, LOX-1 expression was shown in the macrophage-rich lipid core area where MCP-1 expression and apoptotic events were prominent. These results indicate that enhanced LOX-1 expression was associated with histologically unstable atherosclerotic plaques in hypercholesterolemic rabbits, suggesting the involvement of LOX-1 in the destabilization of atherosclerotic plaques in vivo.

Absence of peroxisome proliferator-activated receptor-alpha abolishes hypertension and attenuates atherosclerosis in the Tsukuba hypertensive mouse

Peroxisome proliferator-activated receptor-alpha is widely distributed in the vasculature where it is believed to exert pleiotropic antiatherogenic effects. Its role in the regulation of blood pressure is still unresolved; however, some evidence suggests that it may affect the renin-angiotensin system. We investigated its role in angiotensin II-induced hypertension in the Tsukuba hypertensive mouse (THM). This is a model of hypertension and atherosclerosis because of high angiotensin II and aldosterone levels as a result of the transgenic expression of the entire human renin-angiotensin system. Making the THM animals deficient in Peroxisome proliferator-activated receptor-alpha (THM/PPARKO) totally abolished hypertension and myocardial hypertrophy. This was accompanied by a reduction in plasma human active renin in THM/PPARKO mice compared with THM animals from 3525+/-128 mU/L to 1910+/-750 mU/L (P<0.05) and by a normalization of serum aldosterone (1.6+/-0.29 nmol/L versus 3.4+/-0.69 nmol/L; P=0.003). In the THM/PPARKO mice, the extent of atherosclerosis at the aortic sinus after a 12-week period on an atherogenic diet was decreased by >80%. In addition, the spontaneous formation of foam cells from peritoneal macrophages, a blood pressure-independent event, was reduced by 92% in the THM/PPARKO mice, suggesting protection from the usual oxidative stress in these animals, possibly because of lower prevailing angiotensin II levels. Finally, chronic fenofibrate treatment further elevated blood pressure in THM animals but not in THM/PPARKO animals. Taken together, these data indicate that peroxisome proliferator-activated receptor-alpha may regulate the renin-angiotensin system. They raise the possibility that its activation may aggravate hypertension and hasten atherosclerosis in the context of an activated renin-angiotensin system.

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.

Mulberry leaf aqueous fractions inhibit TNF-α-induced nuclear factor κB (NF-κB) activation and lectin-like oxidized LDL receptor-1 (LOX-1) expression in vascular endothelial cells

Mulberry (Morus Alba L., family Moraceae) leaf extracts have various biological effects including inhibition of oxidative modification of low-density lipoprotein (LDL), which is the major cause of atherosclerosis. Endothelial dysfunction elicited by oxidized LDL (Ox-LDL) has been implicated in atherogenesis. Lectin-like Ox-LDL receptor-1 (LOX-1), a cell-surface receptor for atherogenic Ox-LDL, appears to mediate Ox-LDL-induced inflammation, which may be crucial in atherogenesis. Previous studies revealed that expression of LOX-1 is highly inducible by proinflammatory stimuli, including tumor necrosis factor-alpha (TNF-alpha), lipopolysaccharide (LPS), and transforming growth factor-beta (TGF-beta). Therefore, we examined whether mulberry leaf aqueous fractions inhibit LOX-1 expression induced by proinflammatory stimuli. Pretreatment of cultured bovine aortic endothelial cells (BAECs) with mulberry leaf aqueous fractions inhibited TNF-alpha- and LPS-induced expression of LOX-1 at both protein and mRNA levels in a time- and concentration-dependent manner. In contrast, mulberry leaf aqueous fractions did not affect TGF-beta-induced LOX-1 expression. Furthermore, mulberry leaf aqueous fractions inhibited TNF-alpha-induced activation of nuclear factor-kappaB (NF-kappaB) and phosphorylation of inhibitory factor of NF-kappaB-alpha (IkappaB-alpha) in a time- and concentration-dependent fashion. Thus, mulberry leaf aqueous fractions suppress TNF-alpha- and LPS-induced LOX-1 gene expression, by inhibiting NF-kappaB activation.

The oxidative modification hypothesis of atherosclerosis: The comparison of atherogenic effects on oxidized LDL and remnant lipoproteins in plasma

A tremendous number of articles on oxidized LDL (Ox-LDL) and scavenger receptor in macrophage have been published since Steinberg proposed Ox-LDL hypothesis as the major cause of atherosclerosis. This hypothesis has provided strong support for the efficacy of LDL lowering drugs, indicating that lowering LDL means lowering Ox-LDL in vivo. This manuscript proposed a new oxidative modification hypothesis that remnant lipoproteins determined as remnant-like lipoprotein particles (RLP), not LDL are the major oxidized lipoproteins in plasma, resulting from the plasma concentration of these oxidized lipoproteins. Remnant lipoproteins may play a pivotal role for the initiation of atherosclerosis via lectin-like oxidized LDL receptor-1 (LOX-1) in endothelial cells. Isolated remnant lipoproteins were found to be oxidized or susceptible to be oxidized in plasma, not necessary to be further oxidized in vitro as Ox-LDL. High similarity of proatherogenic and proinflammatory properties of isolated Ox-LDL and remnant lipoporteins have been reported and predicted the presence of similar oxidized phospholipids in both lipoproteins as bioactive components. These results suggest the possibility that reducing plasma remnant lipoproteins rather than LDL should be the target for hyperlipidemic therapy especially in patients with metabolic syndrome for the prevention of endothelial dysfunction in the initiation of atherosclerosis.

Cytokines in Atherosclerosis: Pathogenic and Regulatory Pathways

Atherosclerosis is a chronic disease of the arterial wall where both innate and adaptive immunoinflammatory mechanisms are involved. Inflammation is central at all stages of atherosclerosis. It is implicated in the formation of early fatty streaks, when the endothelium is activated and expresses chemokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into the subendothelium. It also acts at the onset of adverse clinical vascular events, when activated cells within the plaque secrete matrix proteases that degrade extracellular matrix proteins and weaken the fibrous cap, leading to rupture and thrombus formation. Cells involved in the atherosclerotic process secrete and are activated by soluble factors, known as cytokines. Important recent advances in the comprehension of the mechanisms of atherosclerosis provided evidence that the immunoinflammatory response in atherosclerosis is modulated by regulatory pathways, in which the two anti-inflammatory cytokines interleukin-10 and transforming growth factor-β play a critical role. The purpose of this review is to bring together the current information concerning the role of cytokines in the development, progression, and complications of atherosclerosis. Specific emphasis is placed on the contribution of pro- and anti-inflammatory cytokines to pathogenic (innate and adaptive) and regulatory immunity in the context of atherosclerosis. Based on our current knowledge of the role of cytokines in atherosclerosis, we propose some novel therapeutic strategies to combat this disease. In addition, we discuss the potential of circulating cytokine levels as biomarkers of coronary artery disease.

Expression of lectin-like oxidized LDL receptor-1 in smooth muscle cells after vascular injury

Lectin-like oxidized LDL receptor-1 (LOX-1) is an oxidized LDL receptor, and its role in restenosis after angioplasty remains unknown. We used a balloon-injury model of rabbit aorta, and reverse transcription-polymerase chain reaction revealed that LOX-1 mRNA expression was modest in the non-injured aorta, reached a peak level 2 days after injury, and remained elevated until 24 weeks after injury. Immunohistochemistry and in situ hybridization showed that LOX-1 was not detected in the media of non-injured aorta but expressed in both medial and neointimal smooth muscle cells (SMC) at 2 and 24 weeks after injury. Low concentrations of ox-LDL (10 microg/mL) stimulated the cultured SMC proliferation, which was inhibited by antisense oligonucleotides of LOX-1 mRNA. Double immunofluorescence staining showed the colocalization of LOX-1 and proliferating cell nuclear antigen in human restenotic lesion. These results suggest that LOX-1 mediates ox-LDL-induced SMC proliferation and plays a role in neointimal formation after vascular injury.

Peroxisome proliferator-activated receptor alpha ligands activate transcription of lectin-like oxidized low density lipoprotein receptor-1 gene through GC box motif

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is a receptor for oxidized LDL. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors regulating transcription of various genes. We examined effects of PPAR ligands on LOX-1 expression and their transcriptional regulation in vascular endothelial cells. PPARalpha-specific ligands, such as fenofibrate and WY-14643, but not PPARgamma-specific ligands induced LOX-1 expression. Reduced expression of PPARalpha by antisense oligonucleotides directed to PPARalpha blocked fenofibrate-induced LOX-1 expression. Luciferase reporter gene assays with deletion and point mutations in the LOX-1 promoter revealed that transcriptional activity of LOX-1 gene by fenofibrate was localized in the -114/-106 GC box. Electrophoretic mobility shift assays with the radiolabeled GC box sequence showed inducible bands by PPARalpha ligands, which is competitively suppressed by unlabeled GC box motif and by an antibody to PPARalpha. In conclusion, PPARalpha appears to be one of the key regulators that induce LOX-1 expression, utilizing the GC box as a promoter.

Immunomodulation of atherosclerosis: implications for vaccine development

A number of studies have shown activation of the immune system throughout various stages of atherosclerosis. Recent observations have suggested that activation of immune responses may promote atherosclerosis on one hand by inducing and perpetuating arterial inflammation, whereas on the other hand, selective activation of certain immune functions may inhibit atherosclerosis and arterial inflammation. These observations suggest the possibility that selective suppression of proatherogenic immune responses or selective activation of antiatherogenic immune responses may provide new approaches for atherosclerosis prevention and treatment. Several antigens activating immune responses affecting development of atherosclerosis have been identified. These immune responses may be modulated by presenting the antigens together with different types of adjuvants as well as through the route of administration. In this review, we summarize recent experimental studies using immunomodulatory approaches for treatment of atherosclerosis.

Proinflammatory cytokines regulate LOX-1 expression in vascular smooth muscle cells

OBJECTIVE

Atherogenesis represents a type of chronic inflammation and involves elements of the immune response, eg, the expression of proinflammatory cytokines. In advanced atherosclerotic lesions, lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is expressed in endothelial cells, macrophages, and smooth muscle cells (SMCs). In vitro, the expression of LOX-1 is induced by inflammatory cytokines like TNF-alpha and transforming growth factor (TGF)-beta. Therefore, LOX-1 is thought to be upregulated locally in response to cytokines in vivo.

METHODS AND RESULTS

We determined by reverse-transcription polymerase chain reaction (PCR) and Western blot analysis whether the mediators of the acute phase response in inflammation, IL-1alpha, IL-1beta, and TNF-alpha, regulate LOX-1 expression in cultured SMC, and whether this regulation is influenced by peroxisome proliferator-activated receptor gamma (PPARgamma). We studied by immunohistochemistry whether these cytokines are spatially correlated with LOX-1 expression in advanced atherosclerotic lesions. We found upregulation of LOX-1 expression in SMC in a dose- and time-dependent manner after incubation with IL-1alpha, IL-1beta, and TNF-alpha. Simultaneous incubation with these cytokines at saturated concentrations had an additive effect on LOX-1 expression. The PPARgamma activator, 15d-PGJ(2), however, inhibited IL-1beta-induced upregulation of LOX-1. In the intima of atherosclerotic lesions regions of IL-1alpha, IL-1beta, and TNF-alpha expression corresponded to regions of LOX-1 expression.

CONCLUSIONS

We suppose that upregulated LOX-1 expression in SMC of advanced atherosclerotic lesions is a response to these proinflammatory cytokines. Moreover, the proinflammatory effects of these cytokines can be decreased by the antiinflammatory effect of PPARgamma.

Upregulation of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) in endothelial cells by nitric oxide deficiency

Endothelial cell dysfunction (ECD) is emerging as a common denominator for diverse cardiovascular abnormalities associated with inhibition of endothelial nitric oxide (NO) synthase (eNOS). Elevated levels of asymmetric dimethylarginine (ADMA), a potent eNOS inhibitor, are common in renal failure and may contribute to ECD. Through DNA microarray screening of genes modulated in human umbilical vein endothelial cells (HUVEC) by NG-nitro-l-arginine methyl ester (l-NAME), we found a 1.8-fold increase in low-density lipoprotein receptor-1 (LOX-1) expression. LOX-1 is a major endothelial receptor for oxidized low-density lipoproteins (OxLDL) and is assumed to play a role in the initiation and progression of atherosclerosis. Here, we confirmed the upregulation of LOX-1 mRNA and protein level by quantitative RT-PCR and Western blot analysis. Increased expression of LOX-1 was associated with the accumulation of DiI-labeled OxLDL (DiI-OxLDL) in ADMA- and l-NAME-pretreated HUVEC. To evaluate the contribution of LOX-1 in ADMA-induced accumulation of OxLDL by HUVEC, we used the competitive receptor inhibitor, soluble LOX-1. Treatment of HUVEC with soluble LOX-1 was associated with an approximately two- to threefold inhibition of DiI-OxLDL uptake in l-NAME- or ADMA-treated HUVEC. In conclusion, ADMA- or l-NAME-induced NO deficiency leads to the increased expression of LOX-1 mRNA and protein in HUVEC, which in turn results in the accumulation of OxLDL. Competition with LOX-1-soluble extracellular domain reduces OxLDL accumulation. In summary, elevated ADMA levels, i.e., in patients with renal failure, may be responsible for endothelial accumulation of OxLDL via upregulated LOX-1 receptor, thus contributing to endothelial lipidosis and dysfunction.

Lipoprotein receptors in the vascular wall

PURPOSE OF REVIEW

We will discuss the diverse roles of lipoprotein receptors that contribute to the maintenance and integrity of the vascular wall.

RECENT FINDINGS

Lipoprotein receptors function not only as transporters for cholesterol and other lipids. They also act as sensors and signal transducers through which the endothelium, macrophages and smooth muscle cells communicate with their environment.

SUMMARY

Traditionally, lipoprotein receptors were thought of merely as transporters of cholesterol and triglycerides to specific target cells, either for the purpose of delivery and redistribution of nutrients, or for the destruction or clearance of modified (oxidized) lipids by macrophages. Only recently have we begun to appreciate that the same receptors engage in a much more sophisticated and multi-faceted interaction with their environment. Inasmuch, they not only act as mere transporters, but as surprisingly versatile and adaptive signal transducers and modulators throughout the vessel wall. These recent findings now begin to reshape our thinking of how such structurally different and evolutionarily unrelated lipoprotein receptors orchestrate the response of the vessel wall to mechanical or metabolic damage.

Asymmetric dimethylarginine upregulates LOX-1 in activated macrophages: role in foam cell formation

Intimal infiltration by monocytes and accumulation of lipids represent a critical step in the formation of fatty streaks during atherogenesis. Because elevated plasma levels of asymmetric dimethylarginine (ADMA), a potent nitric oxide (NO) synthase (NOS) inhibitor, are prevalent in diverse cardiovascular diseases, the goal of this study was to examine the contribution of NO deficiency to macrophage lipid accumulation. Inhibition of NO synthesis in PMA-primed human monocytic leukemia HL-60 cells resulted in a twofold increase in expression of the receptor for oxidized LDL (OxLDL), termed the lectin-like OxLDL receptor (LOX-1). Blockade of inducible NOS in activated macrophages resulted in 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-OxLDL accumulation and imparted macrophages with a foamy appearance as detected with oil-red O lipid staining. ADMA (15 microM) or N(G)-nitro-l-arginine methyl ester (l-NAME, 300 microM), both of which suppress inducible NOS activity, increased oil-red staining 1.9- and 2.8-fold, respectively. Macrophages treated with ADMA or l-NAME showed a 2.4-fold increase in accumulation of DiI-OxLDL. To examine the role of LOX-1 in this process, we used small interfering RNA (siRNA) duplex-mediated LOX-1 gene silencing. LOX-1 expression was suppressed twofold by siRNA as shown by Western blot analysis. This suppression was associated with a two- to fourfold decrease in DiI-OxLDL uptake as identified by fluorescence microscopy and decreased oil-red O staining by activated macrophages. In conclusion, accumulation of ADMA (a competitive inhibitor of NOS) in patients with chronic renal failure may be responsible for upregulation of LOX-1 receptor and increased OxLDL uptake, thus contributing to lipidosis and foam cell formation. The data illustrate an additional nonendothelial mode of antiatherogenic action of NO: prevention of LOX-1 induction and lipid accumulation by macrophages.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

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.