Inverse relationship between circulating oxidized low density lipoprotein (oxLDL) and anti-oxLDL antibody levels in healthy subjects

Oxidized low density lipoprotein (oxLDL) has been implicated in the pathogenesis of atherosclerosis. Recent studies have shown that immunization of animals with oxLDL results in suppression of atherogenesis. Antibody against oxLDL (oxLDL Ab) is detectable in human sera, although its biological significance is not well established. We examined the relationship between oxLDL Ab titer and circulating oxLDL level in 130 healthy Japanese subjects. OxLDL was measured as apolipoprotein (apo) B-containing lipoproteins carrying oxidized phosphatidylcholines by a sensitive ELISA. IgG class oxLDL Ab titer was measured by ELISA. Plasma oxLDL concentration was very low and it corresponded on average to one to two out of 1000 apoB-containing lipoproteins in plasma. Plasma oxLDL correlated positively with LDL cholesterol and inversely with oxLDL Ab titer. These associations remained significant and independent in multiple regression analysis including age, gender, smoking, and high-density lipoprotein cholesterol. These data indicate that healthy subjects have a very low concentration of oxLDL in the circulation, and that oxLDL Ab titer is in an inverse relationship with plasma oxLDL concentration in this population. Although these results suggest that oxLDL Ab may play a role in maintaining the low level of plasma oxLDL, its role in atherogenesis awaits further studies.     

The Discovery of LOX-1, its Ligands and Clinical Significance

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

Reducing oxidized lipids to prevent cardiovascular disease

Despite significant success in reducing plasma cholesterol, especially low-density lipoprotein cholesterol, risks for cardiovascular disease (CVD) complications remain. Among these risks are circulating levels of oxidative modified lipoproteins, primarily oxidized low-density lipoproteins (oxLDL). The evidence supporting oxLDL as a potential target for therapeutic management to reduce metabolic complications and CVD events is reviewed in this report.     

Is plasma oxidized low-density lipoprotein, measured with the widely used antibody 4E6, an independent predictor of coronary heart disease among U.S. men and women?

OBJECTIVES: Our aim was to examine whether circulating oxidized low-density lipoprotein (oxLDL) is a predictor of coronary heart disease (CHD) independent of lipid markers and to compare oxLDL, apolipoprotein B100 (apoB), and total cholesterol (TC)/high-density lipoprotein-cholesterol (HDL-C) ratio as predictors of CHD. BACKGROUND: Measurement of circulating oxLDL with antibody 4E6, has been widely used in many studies; however, few large prospective studies have examined whether this marker is a predictor of CHD independent of lipids and compared oxLDL with other important lipid predictors. METHODS: After 6 years of follow-up among 18,140 men from the HPFS (Health Professionals Follow-up Study) and 8 years among 32,826 women from the Nurses' Health Study who provided blood samples at baseline, we identified incident nonfatal myocardial infarction or fatal CHD in 266 men and 235 women. Each case was matched with two control subjects by age, smoking, and time of blood draw. The oxLDL was measured via enzyme-linked immunosorbent assay with antibody 4E6 against oxidized apoB. RESULTS: Among both men and women, oxLDL was significantly related to risk of CHD in multivariate analysis before adjustment for any lipid markers. However, when oxLDL, LDL cholesterol, HDL-C, and triglycerides were mutually adjusted, oxLDL was no longer predictive. When oxLDL and apoB were mutually adjusted, only apoB was predictive of CHD. Similar results were found when oxLDL and TC/HDL-C ratio were mutually adjusted. CONCLUSIONS: Our results suggest that circulating oxLDL, measured with antibody 4E6, is not an independent overall predictor of CHD after adjustment of lipid markers and is less predictive in development of CHD than apoB and TC/HDL-C ratio.     

Microparticles and type 2 diabetes

Cell activation or apoptosis leads to plasma membrane blebbing and microparticles (MPs) release in the extracellular space. MPs are submicron membrane vesicles, which harbour a panel of oxidized phospholipids and proteins specific to the cells they derived from. MPs are found in the circulating blood of healthy volunteers. MPs levels are increased in many diseases, including cardiovascular diseases with high thrombotic risk. Exposure of negatively charged phospholipids and tissue factor confers a procoagulant potential to MPs. Elevation of plasma MPs levels, particularly those of endothelial origin, reflects cellular injury and appears now as a surrogate marker of vascular dysfunction. Recent studies demonstrate an elevation of circulating levels of MPs in diabetes. MPs could also be involved in the development of vascular complications in diabetes for they stimulate pro-inflammatory responses in target cells and promote thrombosis, endothelial dysfunction and angiogenesis. Thus, these studies provide new insight in the pathogenesis and treatment of vascular complications of diabetes.     

T lymphocytes from human atherosclerotic plaques recognize oxidized low density lipoprotein.

Atherosclerosis, an underlying cause of myocardial infarction, stroke, and other cardiovascular diseases, consists of focal plaques characterized by cholesterol deposition, fibrosis, and inflammation. The presence of activated T lymphocytes and macrophages and high expression of HLA class II molecules are indicative of a local immunologic activation in the atherosclerotic plaque, but the antigen(s) involved has not yet been identified. We established T-cell clones from human atherosclerotic plaques using polyclonal mitogens as stimuli and exposed the clones to potential antigens in the presence of autologous monocytes as antigen-presenting cells. Four of the 27 CD4+ clones responded to oxidized low density lipoprotein (oxLDL) by proliferation and cytokine secretion; this response was dependent on autologous antigen-presenting cells and restricted by HLA-DR. All clones that responded to oxLDL secreted interferon gamma upon activation, but only one produced interleukin 4, suggesting that the response to oxLDL results in immune activation and inflammation but may not be a strong stimulus to antibody production. No significant response to oxLDL could be detected in CD4+ T-cell clones derived from the peripheral blood of the same individuals. Together, the present data suggest that the inflammatory infiltrate in the atherosclerotic plaque is involved in a T-cell-dependent, autoimmune response to oxLDL.     

骨保护素与代谢综合征

骨保护素(OPG)是一种分泌型糖蛋白,属于肿瘤坏死因子受体超家族成员,不仅在骨代谢过程中起重要作用,而且是重要的血管调节因子,能够保护血管内皮细胞,抑制血管钙化和动脉粥样硬化.代谢综合征(MS)包括腹型肥胖、胰岛素抵抗、血脂异常、高血糖、高血压等.MS各组分均为心血管疾病的危险因素,2型糖尿病患者血OPG水平明显升高,但是OPG与MS其他各组分之间的关系日前还存在争议.     

Measurement of plasma oxidized low-density lipoprotein and its clinical implications

Oxidized low-density lipoprotein (OxLDL) has been shown to exist in human circulating plasma. Several groups including ours have developed methods for immunologically measuring OxLDL, which have been applied to several clinical, both cross-sectional and prospective, studies. These data clearly show that OxLDL levels correlate well with the severity of cardiovascular diseases. In particular, recent observations suggest that plasma OxLDL levels could be a useful marker for predicting future cardiovascular events; however, substantial differences exist among the different methods of OxLDL measurement. To evaluate the clinical data on circulating OxLDL, a proper understanding of the similarity, differences, and limitation of the methods is needed. This paper summarizes the characteristics of the methods used and recent clinical findings.     

Oxidized low-density lipoprotein biomarkers in atherosclerosis

The concept that the oxidation of lipoproteins is central in the pathogenesis of atherosclerosis was first reported over 25 years ago, initially by in vitro studies and subsequently through experimental models of atherosclerosis. The innate immune system plays a key role in atherogenesis as manifested by its atherosclerosis-modulating properties, the immunogenicity of oxidized low-density lipoprotein (LDL), and the presence of oxidized LDL autoantibodies in plasma and lesions of humans. In the past 10 years it has been possible to generate monoclonal antibodies to oxidized LDL to directly measure oxidized LDL in plasma. This has led to a rapidly accelerating pace of new reports on the relationship of circulating oxidized LDL and various cardiovascular pathologic processes, which are the focus of this review.     

Circulating oxidized low-density lipoprotein is associated with echolucent plaques in the femoral artery independently of hsCRP in 61-year-old men

OBJECTIVES: The aim of the study was to test the hypothesis that circulating markers of inflammation (high-sensitive C-reactive protein, hsCRP) and oxidative modification of lipids (oxidized low-density lipoprotein, oxLDL) were associated with the occurrence of echolucent rather than echogenic femoral artery plaques in a cross-sectional population based cohort of 513, 61-year-old men. BACKGROUND: The relationships between circulating oxLDL, hsCRP and the occurrence of echolucent plaques in the femoral artery have not previously been investigated. METHODS: The levels of circulating oxLDL and hsCRP were determined in plasma by ELISA. Plaque occurrence, size and echogenicity were measured by B-mode ultrasound in the right femoral artery. Assessment of plaque echogenicity was based on the classification (grades 1-4) proposed by Gray-Weale et al. RESULTS: A higher frequency of echolucent femoral plaques was observed in subjects with the metabolic syndrome and current smokers (p=0.01 and p<0.001, respectively) as well as with increasing levels of oxLDL and hsCRP (p=0.002 and p=0.005, respectively). In a multiple logistic regression analysis oxLDL and current smokers turned out to be independent associated with the presence of echolucent femoral artery plaques. CONCLUSIONS: The results of the present study support our hypothesis that circulating oxLDL is a marker of an unstable echolucent plaque phenotype in the femoral artery in man.     

Circulating miRNAs: Reflecting or Affecting Cardiovascular Disease?

MicroRNAs are a class of small, noncoding RNAs encoded by the metazoan genome that regulate protein expression. A collection of studies point to vital roles for microRNAs in the onset and development of cardiovascular diseases. So far, microRNAs have been considered as important intracellular mediators in maintaining proper cardiac function and hemostasis, and have been proposed as potential therapeutic targets in cardiovascular disease. The recent discovery that microRNAs circulate in a stable form in many body fluids, including blood, suggests that circulating microRNAs can serve as a new generation of biomarkers for cardiovascular diseases. In this review, we summarize the findings of studies focusing on circulating microRNAs present in human blood cells or plasma/serum, where they potentially could serve as diagnostic or prognostic markers for a variety of cardiovascular pathologies, including acute myocardial infarction, heart failure, coronary artery disease, stroke, diabetes and hypertension. The significance and limitations of microRNAs as the new biomarker generation for cardiovascular disease are also discussed.     

Dietary homocysteine promotes atherosclerosis in apoE-deficient mice by inducing scavenger receptors expression

Elevated plasma homocysteine (Hcy) levels have been recognized as an independent risk factor for atherosclerosis leading to cardiovascular diseases. However, the mechanisms contributing to atherosclerosis have not been delineated. Since, scavenger receptors mediated uptake of oxidized-LDL (oxLDL) by macrophages resulting in foam cell formation is an early event in atherosclerosis, we hypothesized that atherogenic effects of Hcy may be mediated via regulating expression of scavenger receptor(s). We have tested this hypothesis using apoE-/- female mice fed normal rodent chow (NC) diet or NC supplemented with Hcy in drinking water (9 g/L). Hcy-fed mice showed increased fatty streak lesions in aortic sinus/root compared to NC group without alterations in plasma lipid profiles. Similar findings were observed in the enface analysis of the descending aorta. To determine the molecular mechanisms underlying Hcy-mediated progression of fatty streak lesions, expression of scavenger receptors such as CD36 and lectin-like oxidized LDL binding protein-1 (LOX-1) in the aortic lesions were analyzed. Interestingly, Hcy-fed mice had increased immuno-positive staining for CD36 and LOX-1 in the atherosclerotic lesions compared to NC-fed mice. In vitro analyses showed neither Hcy nor HcyLDL directly affect the expression of CD36 and LOX-1 on mouse macrophages. However, Hcy supplementation in apoE-/- mice resulted in elevated oxLDL levels in plasma. Since oxLDL has been shown to upregulate the expression of CD36 and LOX-1, these findings suggest that Hcy may exert its atherogenic effect in part by elevating the levels of oxLDL. Interestingly, interaction of monocytes with Hcy-activated endothelial cells resulted in upregulation of CD36 expression on monocytes, suggesting a possible mechanism by which Hcy may upregulate CD36 expression at the lesion site. Further, these findings suggest a novel mechanism by which Hcy may promote atherogenesis.     

Oxidized LDL and anti-oxidized LDL antibodies in atherosclerosis – Novel insights and future directions in diagnosis and therapy,

We provide an up-to-date overview of current topics surrounding oxidized low-density lipoprotein (oxLDL) and its related antibodies in the quest to better identify the individuals at risk of cardiovascular disease and atherosclerotic plaques with unfavorable characteristics. We discuss the potential of oxLDL and anti-oxLDL antibodies as serum biomarkers of cardiovascular disease and emerging studies examining the targeting of arterial oxLDL for imaging and therapeutic delivery.     

CD36 plays an important role in the clearance of oxLDL and associated age-dependent sub-retinal deposits

Age-related macular degeneration (AMD) represents the major cause of vision loss in industrialized nations. Laminar deposits in Bruch's membrane (BM) are among the first prominent histopathologic features, along with drusen formation, and have been found to contain oxidized lipids. Increases in concentrations of oxidized LDL (oxLDL) in plasma are observed with age and high fat high (HFHC) cholesterol diet. CD36 is the principal receptor implicated in uptake of oxLDL, and is expressed in the retinal pigment epithelium (RPE). We determined if CD36 participates in oxLDL uptake in RPE and correspondingly in clearance of sub-retinal deposits. Uptake of oxLDL by RPEin vitro and in vivo was CD36-dependent. CD36 deficiency in mice resulted in age-associated accumulation of oxLDL and sub-retinal BM thickening, despite fed a regular diet. Conversely, treatment of HFHC-fed ApoE null mice with a CD36 agonist, EP80317 (300 μg/kg/day), markedly diminished thickening of BM, and partially preserved (in part) photoreceptor function. In conclusion, our data uncover a new role for CD36 in the clearance of oxidized lipids from BM and in the prevention of age-dependent sub-retinal laminar deposits.     

Plasma oxidized low-density lipoprotein as a prognostic predictor in patients with chronic congestive heart failure

OBJECTIVES: The aim of this study was to evaluate the relationship between plasma oxidized low-density lipoprotein (oxLDL), a marker of oxidative stress, and the prognosis of patients with chronic congestive heart failure (CHF). BACKGROUND: Oxidative stress appears to play a role in the pathophysiology of CHF. We have recently reported the usefulness of plasma oxLDL as a marker of oxidative stress in CHF patients with dilated cardiomyopathy. METHODS: We measured the plasma level of oxLDL by sandwich enzyme-linked immunosorbent assay using a specific monoclonal antibody against oxLDL in 18 age-matched normal subjects and in 84 patients with chronic CHF (New York Heart Association functional class II to IV) and monitored them prospectively for a mean follow-up period of 780 days. RESULTS: Plasma oxLDL level was significantly higher in severe CHF patients than in control subjects and mild CHF patients. A significant negative correlation existed between the plasma level of oxLDL and left ventricular ejection fraction (LVEF) and a significant positive correlation between the plasma level of oxLDL and plasma norepinephrine level. Twenty-six patients had cardiac events; 14 had cardiac death and 12 were hospitalized for heart failure or other cardiovascular events. Among 10 variables including LVEF and neurohumoral factors, only high plasma levels of brain natriuretic peptide and oxLDL were shown to be independent predictors of mortality. CONCLUSIONS: These results indicate that the plasma level of oxLDL is a useful predictor of mortality in patients with CHF, suggesting that oxidative stress plays an important role in the pathophysiology of CHF.     

Association between serum ferritin and circulating oxidized low-density lipoprotein levels in patients with type 2 diabetes

The oxidative modification of low-density lipoproteins (LDL) plays a central role in the initiation and acceleration of atherosclerosis. Iron plays a part in the formation of highly toxic free radicals such as hydroxide and superoxide anions, which can induce lipid peroxidation. We investigated whether serum iron status was associated with circulating oxidized LDL (oxLDL) levels in type 2 diabetic patients, in whom oxidative stress and susceptibility to lipid oxidation were supposedly increased. Serum ferritin levels were significantly correlated with plasma oxLDL concentrations in both male and female patients (p<0.02 and p<0.05, respectively). No correlation was detected between ferritin and LDL-cholesterol (LDL-C) concentrations despite the close correlation between LDL-C and oxLDL concentrations (p<0.0001). Stepwise regression analysis showed that ferritin concentration was an independent positive determinant of oxLDL level, in addition to triglyceride concentration, body mass index and sex. This is the first report to show that serum ferritin is associated with circulating oxLDL levels in patients with type 2 diabetes. Further work is required to establish a causative link between iron excess and the development of diabetic vascular complications.     

Is there more than C-reactive protein and fibrinogen? The prognostic value of soluble CD40 ligand, interleukin-6 and oxidized low-density lipoprotein with respect to coronary and cerebral vascular disease

Incidence of atherosclerosis and atherosclerosis-related complications will increase significantly in the coming decennia. Research identified many serum and plasma markers that are associated with cardiovascular disease. However, little is known about the prognostic value of these markers to identify patients at risk for future cardiovascular events. Therefore, we aimed to investigate the prognostic value of three of these markers (soluble CD40 ligand (sCD40L), interleukin-6 (IL-6) and oxidized low-density lipoprotein (oxLDL)) with respect to coronary vascular disease and stroke. For this reason the Medline database was searched for the period January 1999-January 2005. To be selected in our study, concentration of the marker had to be determined at baseline, follow-up period had to be longer than 3 months and an estimate of relative risk had to be available. Based on these criteria, 4 studies for sCD40L, 10 for IL-6 and 2 for oxLDL were selected. Relative risk estimates adjusted for potential confounders varied between 1.9 and 2.8 for sCD40L, between 1.1 and 3.1 for IL-6 and between 1.9 and 3.2 for oxLDL. In conclusion, this systematic review shows that sCD40L, IL-6 and oxLDL are associated with an increased relative risk of developing cardiovascular disease.     

Endogenous nitric oxide synthase inhibitors in the biology of disease: markers, mediators, and regulators?

The asymmetric methylarginines inhibit nitric oxide synthesis in vivo by competing with L-arginine at the active site of nitric oxide synthase. High circulating levels of asymmetric dimethylarginine predict adverse outcomes, specifically vascular events but there is now increasing experimental and epidemiological evidence that these molecules, and the enzymes that regulate this pathway, play a mechanistic role in cardiovascular diseases. Recent data have provided insight into the impact of altered levels of these amino acids in both humans and rodents, however these reports also suggest a simplistic approach based on measuring, and modulating circulating asymmetric dimethylarginine alone is inadequate. This review outlines the basic biochemistry and physiology of endogenous methylarginines, examines both the experimental and observational evidence for a role in disease pathogenesis, and examines the potential for therapeutic regulation of these molecules.     

C-reactive protein enhances LOX-1 expression in human aortic endothelial cells: relevance of LOX-1 to C-reactive protein-induced endothelial dysfunction

C-reactive protein (CRP), a characteristic inflammatory marker, is a powerful predictor of cardiovascular events. Recent data suggest that CRP may also promote atherogenesis through inducing endothelial dysfunction. Lectin-like oxidized low-density lipoprotein (oxLDL) receptor-1 (LOX-1) is a newly identified endothelial receptor for oxLDL that plays a pivotal role in oxLDL-induced endothelial dysfunction. Whether CRP may regulate endothelial LOX-1 and induce endothelial dysfunction through this receptor is unknown. In the present study, we studied the in vitro effect of CRP on LOX-1 expression in human aortic endothelial cells (HAECs) and the role of LOX-1 in CRP-induced human monocyte adhesion to endothelium and oxLDL uptake by endothelial cells. Incubation of HAECs with CRP enhanced, in a dose- and time-dependent manner, LOX-1 mRNA and protein levels. Induction of LOX-1 protein was already present at 5 microg/mL CRP and reached a maximum at 25 microg/mL. This effect was reduced by antibodies against CD32/CD64, endothelin-1 (ET-1) and interleukin-6 (IL-6). The extent of stimulation of LOX-1 achieved by CRP was comparable to that elicited by high glucose and IL-6 and remained unchanged in presence of these factors. Finally, CRP increased, through LOX-1, both human monocyte adhesion to endothelial cells and oxLDL uptake by these cells. We conclude that CRP enhances endothelial LOX-1 expression and propose a new mechanism by which CRP may promote endothelial dysfunction, that of inducing LOX-1.     

Estradiol,acting through estrogen receptor alpha,restores dimethylarginine dimethylaminohydrolase activity and nitric oxide production in oxLDL-treated human arterial endothelial cells

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase. ADMA accumulation, mainly due to a decreased dimethylarginine dimethylaminohydrolase (DDAH) activity, has been related to the development of cardiovascular diseases. We investigate whether estradiol prevents the changes induced by oxidized low density lipoprotein (oxLDL) on the DDAH/ADMA/NO pathway in human umbilical artery endothelial cells (HUAEC). HUAEC were exposed to estradiol, native LDL (nLDL), oxLDL and their combinations for 24 h. In some experiments, cells were also exposed to the unspecific estrogen receptor (ER) antagonist ICI 182780, the specific ERα antagonist MPP or specific agonists for ERα, ERβ and GPER. ADMA concentration was measured by HPLC and concentration of NO by amperometry. Protein expression and DDAH activity were measured by immunoblotting and an enzymatic method, respectively. oxLDL, but not nLDL, increased ADMA concentration with a concomitant decrease on DDAH activity. oxLDL reduced eNOS protein and NO production. Estradiol alone had no effects on DDAH/ADMA/NO pathway, but increased the attenuated endothelial NO production induced by oxLDL by reduction in ADMA and preventing loss of eNOS protein levels. ICI 182780 and MPP completely abolished these effects of estradiol on oxLDL-exposed cells. ERα agonist, but not ERβ and GPER agonists, mirrored estradiol effects on NO production. In conclusion, estradiol restores (1) DDAH activity, and therefore ADMA levels, and (2) NO production impaired by oxLDL in HUAEC acting through ERα.