Characterization of a specific monoclonal antibody 9F5-3a and the development of assay system for oxidized HDL

Oxidized high-density lipoprotein promotes CD36 palmitoylation and increases lipid uptake in macrophages

Oxidized high-density lipoprotein (oxHDL) reduces the ability of cells to mediate reverse cholesterol transport and also shows atherogenic properties. Palmitoylation of cluster of differentiation 36 (CD36), an important receptor mediating lipoprotein uptake, is required for fatty acid endocytosis. However, the relationship between oxHDL and CD36 has not been described in mechanistic detail. Here, we demonstrate using acyl-biotin exchange analysis that oxHDL activates CD36 by increasing CD36 palmitoylation, which promotes efficient uptake in macrophages. This modification increased CD36 incorporation into plasma lipid rafts and activated downstream signaling mediators, such as Lyn, Fyn, and c-Jun N-terminal kinase, which elicited enhanced oxHDL uptake and foam cell formation. Furthermore, blocking CD36 palmitoylation with the pharmacological inhibitor 2-bromopalmitate decreased cell surface translocation and lowered oxHDL uptake in oxHDL-treated macrophages. We verified these results by transfecting oxHDL-induced macrophages with vectors expressing wildtype or mutant CD36 (mCD36) in which the cytoplasmic palmitoylated cysteine residues were replaced. We show that cells containing mCD36 exhibited less palmitoylated CD36, disrupted plasma membrane trafficking, and reduced protein stability. Moreover, in ApoE^−/−CD36^−/− mice, lipid accumulation at the aortic root in mice receiving the mCD36 vector was decreased, suggesting that CD36 palmitoylation is responsible for lipid uptake in vivo. Finally, our data indicated that palmitoylation of CD36 was dependent on DHHC6 (Asp-His-His-Cys) acyltransferase and its cofactor selenoprotein K, which increased the CD36/caveolin-1 interaction and membrane targeting in cells exposed to oxHDL. Altogether, our study uncovers a causal link between oxHDL and CD36 palmitoylation and provides insight into foam cell formation and atherogenesis.

Structure and Dynamics of Oxidized Lipoproteins In Vivo: Roles of High-Density Lipoprotein

Oxidative modification of lipoproteins is implicated in the occurrence and development of atherosclerotic lesions. Earlier studies have elucidated on the mechanisms of foam cell formation and lipid accumulation in these lesions, which is mediated by scavenger receptor-mediated endocytosis of oxidized low-density lipoprotein (oxLDL). Mounting clinical evidence has supported the involvement of oxLDL in cardiovascular diseases. High-density lipoprotein (HDL) is known as anti-atherogenic; however, recent studies have shown circulating oxidized HDL (oxHDL) is related to cardiovascular diseases. A modified structure of oxLDL, which was increased in the plasma of patients with acute myocardial infarction, was characterized. It had two unique features: (1) a fraction of oxLDL accompanied oxHDL, and (2) apoA1 was heavily modified, while modification of apoB, and the accumulation of oxidized phosphatidylcholine (oxPC) and lysophosphatidylcholine (lysoPC) was less pronounced. When LDL and HDL were present at the same time, oxidized lipoproteins actively interacted with each other, and oxPC and lysoPC were transferred to another lipoprotein particle and enzymatically metabolized rapidly. This brief review provides a novel view on the dynamics of oxLDL and oxHDL in circulation.

Protective effect of rat aldo-keto reductase (AKR1C15) on endothelial cell damage elicited by 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (HNE), a major reactive product of lipid peroxidation, is believed to play a central role in atherogenic actions triggered by oxidized lipoproteins. An aldo-keto reductase (AKR) 1C15 efficiently reduces HNE and is distributed in many rat tissues including endothelial cells. In this study, we investigated whether AKR1C15 acts as a protective factor against endothelial damage elicited by HNE and oxidized lipoproteins. Treatment of rat endothelial cells with HNE provoked apoptosis through reactive oxygen species (ROS) formation, mitochondrial dysfunction and caspase activation in the cells. AKR1C15 converted HNE into less toxic 1,4-dihydroxy-2-nonene, and its overexpression markedly decreased the susceptibility of the cells to HNE. The forced expression of AKR1C15 also significantly suppressed the loss of cell viability caused by oxidized low-density lipoprotein and its lipidic fraction. Furthermore, the treatment of the cells with sublethal concentrations of HNE resulted in up-regulation of AKR1C15, which was partially abrogated by the ROS inhibitors. Collectively, these data indicate an anti-atherogenic function of AKR1C15 through the protection of endothelial cells from damage elicited by toxic lipids such as HNE.

Oxidized low-density lipoprotein

Oxidized low-density lipoprotein (Ox-LDL) has been studied for over 25 years. Numerous pro- and anti-atherogenic properties have been attributed to Ox-LDL. Yet, Ox-LDL has neither been defined nor characterized, as its components and composition change depending on its source, method of preparation, storage, and use. It contains unoxidized and oxidized fatty acid derivatives both in the ester and free forms, their decomposition products, cholesterol and its oxidized products, proteins with oxidized amino acids and cross-links, and polypeptides with varying extents of covalent modification with lipid oxidation products, and many others. It seems to exist in vivo in some form not yet fully characterized. Until its pathophysiological significance, and how it is generated in vivo are determined, the nature of its true identity will be only of classical interest. In this review, its components, their biological actions and methods of preparation will be discussed.

A sensitive and specific ELISA detects methionine sulfoxide-containing apolipoprotein A-I in HDL

Oxidized HDL has been proposed to play a key role in atherogenesis. A wide range of reactive intermediates oxidizes methionine residues to methionine sulfoxide (MetO) in apolipoprotein A-I (apoA-I), the major HDL protein. These reactive species include those produced by myeloperoxidase, an enzyme implicated in atherogenesis. The aim of the present study was to develop a sensitive and specific ELISA for detecting MetO residues in HDL. We therefore immunized mice with HPLC-purified human apoA-I containing MetO(86) and MetO(112) (termed apoA-I(+32)) to generate a monoclonal antibody termed MOA-I. An ELISA using MOA-I detected lipid-free apoA-I(+32), apoA-I modified by 2e-oxidants (hydrogen peroxide, hypochlorous acid, peroxynitrite), and HDL oxidized by 1e- or 2e-oxidants and present in buffer or human plasma. Detection was concentration dependent, reproducible, and exhibited a linear response over a physiologically plausible range of concentrations of oxidized HDL. In contrast, MOA-I failed to recognize native apoA-I, native apoA-II, apoA-I modified by hydroxyl radical or metal ions, or LDL and methionine-containing proteins other than apoA-I modified by 2e-oxidants. Because the ELISA we have developed specifically detects apoA-I containing MetO in HDL and plasma, it should provide a useful tool for investigating the relationship between oxidized HDL and coronary artery disease.

Establishment and evaluation of 2 monoclonal antibodies against oxidized apolipoprotein A-I (apoA-I) and its application to determine blood oxidized apoA-I levels

BACKGROUND

Apolipoprotein A-I (apoA-I) is the major lipoprotein component of high-density lipoprotein(HDL), and plays an important role in reverse cholesterol transport. Its function is known to be influenced by oxidation.

METHODS

Using H2O2-or chloramine T-oxidized apoA-I as antigen, we prepared 2 kinds of monoclonal antibodies, and established an ELISA system for the measurement of oxidized apoA-I.

RESULTS

The 2 monoclonal antibodies obtained, 7D3 and 98A7, exhibited different reactivity characteristics. The serum level of oxidized apoA-I was higher in patients with either inflammatory disease or diabetes than in healthy individuals, and suggested a diversity of oxidized apoA-I.

CONCLUSION

The 2 monoclonal antibodies are useful for the determination of oxidized apoA-I and study of diverse oxidized HDLs.

NF-kappa B activation in endothelial cells treated with oxidized high-density lipoprotein

We first determined whether oxidized high-density lipoprotein (ox-HDL) activates transcription factor nuclear factor-kappa B (NF-kappa B) in cultured human umbilical vein endothelial cells (HUVECs). Treatment for 7h with 100 microg/ml ox-HDL elicited a marked downregulation of I kappa B alpha and upregulation of the phosphorylated form of I kappa B alpha in HUVECs in a manner dependent on the dose of ox-HDL. Electrophoretic mobility shift assay in nuclear fraction from HUVECs showed translocation of NF-kappa B to the nucleus and binding of NF-kappa B to NF-kappa B consensus oligonucleotides during ox-HDL exposure for 7h, suggesting that ox-HDL brings about NF-kappa B activation in endothelial cells. To clarify the mechanism of NF-kappa B activation in HUVECs treated with ox-HDL, we investigated the effect of ox-HDL treatment on intracellular production of reactive oxygen species (ROS) in HUVECs. Ox-HDL induced a significant dose-dependent increase in ROS production during 4h incubation and this enhanced production of ROS was inhibited in the presence of probucol or diphenylene iodonium (DPI), an inhibitor of NADPH oxidase. In addition, pretreatment with probucol or DPI suppressed the phosphorylation and degradation of I kappa B alpha protein induced by ox-HDL, demonstrating that increased generation of ROS by ox-HDL may be associated with NF-kappa B activation. Pretreatment with antibody against oxidized low-density lipoprotein receptor-1 (LOX-1) significantly suppressed the ox-HDL-induced downregulation of I kappa B alpha, suggesting that LOX-1 mediates NF-kappa B activation in endothelial cells stimulated with ox-HDL. Taking all of the above findings together, ox-HDL activates NF-kappa B via binding to LOX-1 on the cell surface, followed by enhancement of intracellular ROS production in endothelial cells.

Glycated high-density lipoprotein regulates reactive oxygen species and reactive nitrogen species in endothelial cells

Nonenzymatic glycosylation of plasma proteins may contribute to the excess risk of developing atherosclerosis in patients with diabetes mellitus. Although it is believed that high-density lipoprotein (HDL) is glycosylated at an increased level in diabetic individuals, little is known about a possible linkage between glycated HDL and endothelial dysfunction in diabetes. To clarify whether glucose-modified HDL affects the function of endothelial cells, we first examined herein the level of H(2)O(2) generation from cultured human aortic endothelial cells (HAECs) exposed to a glycated oxidized HDL (gly-ox-HDL) prepared in vitro. Incubation for 48 hours with 100 microg/mL of gly-ox-HDL induced significant release of H(2)O(2) from cells and gly-ox-HDL-induced H(2)O(2) formation was inhibited in the presence of diphenyleneiodonium, an inhibitor of NADPH oxidase. In addition, stimulation of HAECs with gly-ox-HDL for 48 hours elicited a marked downregulation of catalase and Cu(2+), Zn(2+)-superoxide dismutase (CuZn-SOD), suggesting H(2)O(2) formation by gly-ox-HDL to be due to a disturbance involving oxidant and antioxidant enzymes in the cells. Treatment of HAECs with gly-ox-HDL attenuated the expression of endothelial nitric oxide synthase (eNOS), but not inducible nitric oxide synthase (iNOS), and this was followed by decreased production of nitric oxide (NO) by the cells. Furthermore, in vitro experiments with glycated HDL (gly-HDL) in the presence of 2 mmol/L EDTA and Cu(2+)-oxidized HDL suggested the effect of gly-HDL on endothelial function to be possibly potentiated by additional oxidative modification. Taking all of the above findings together, gly-ox-HDL may lead to the deterioration of vascular function through altered production of reactive oxygen species and reactive nitrogen species in endothelial cells.

Detection of oxidized high-density lipoprotein

This paper reviews working procedures for the separation and detection of oxidized high-density lipoproteins (ox-HDL) and their constituents. It begins with an introductory overview of structural alterations of the HDL particle and its constituents generated during oxidation. The main body of the review delineates various procedures for the isolation and detection of ox-HDL as well as the purification and separation of phosphatidylcholine metabolites and denatured apolipoproteins in the particle. The useful methods published more recently are picked up and the utility of the separation techniques is described. The last section covers a clinical evaluation of changes in these factors in ox-HDL as well as future directions of ox-HDL research.

Serum lipid metabolism abnormalities and change in lipoprotein contents in patients with advanced-stage renal disease

BACKGROUND

Arteriosclerosis is the major cause of death in patients with chronic renal failure. There is much interest in the lipid metabolism of patients treated with hemodialysis.

METHODS

We analyzed low-density lipoproteins (LDL) and high-density lipoproteins (HDL) in chronic renal failure (CRF) patients according to patients on hemodialysis (HD), patients with diabetic nephropathy before initiation of dialysis (DN), and patients with chronic glomerulonephritis in the conservative stage (CGN); and compared the lipid metabolic abnormalities in patients on hemodialysis and those not yet on hemodialysis. We also analyzed the qualitative abnormalities of LDL and HDL and their relationship with the pathological stages.

RESULTS

Electrophoretic patterns identified small LDL particles and small HDL particles in the three groups, and the degree of denaturation was more enhanced in CRF patients in the conservative stage than in HD patients. For LDL susceptibility to oxidation LDL (oxLDL) by addition of Cu(2+), the lag time was approximately 57 min in healthy controls and CGN patients, but was prolonged to approximately 75 min in HD and DN patients. For HDL susceptibility to oxidation HDL (oxHDL), HD, DN and CGN patients showed lag times shorter than those found in healthy control subjects. These results showed that LDL and HDL in the serum of CRF patients were in a state of enhanced susceptibility to oxidative modification. In Western blot analysis using anti-human-denatured LDL and anti-human-oxidized HDL monoclonal antibodies, bands of low molecular oxLDL at 150-197 kDa were detected in all CRF patients, with marked tailing in CGN patients. Similarly, bands of small oxHDL particles at 110 and 120 kDa were found in HD, DN and CGN patients.

CONCLUSIONS

Oxidative modification of both LDL and HDL occurs in patients with advanced CRF resulting in small lipoproteins. Increased production of oxLDL and oxHDL is the main cause of lipid metabolic abnormality in CRF patients.

Glycated high-density lipoprotein induces apoptosis of endothelial cells via a mitochondrial dysfunction

Glycation of plasma proteins may contribute to an excess risk of developing atherosclerosis in patients with diabetes mellitus. Although it is believed that high-density lipoprotein (HDL) is nonenzymatically glycosylated at an increased level in diabetic individuals, little is known about a possible linkage between glycated HDL and endothelium dysfunction in diabetes. This study set out to clarify whether glucose-modified HDL affects the function of endothelial cells by examining the apoptosis of cultured human aortic endothelial cells (HAECs) exposed to a glycated-oxidized HDL (gly-ox-HDL) prepared in vitro. Incubation of HAECs with 100 microg/ml of gly-ox-HDL for 48 h showed apoptotic features, such as cell shrinkage, membrane blebbing, and concentration and fragmentation of the nucleus, and the degree of apoptosis was dose-dependent on the glucose used in the preparation of gly-ox-HDL. Stimulation of HAECs with gly-ox-HDL elicited a marked increase in caspase 3 activity and the expressions of active caspase 3 and caspase 9, whereas concomitant treatment with a caspase 3 inhibitor significantly blocked gly-ox-HDL-induced apoptosis of HAECs. The release of cytochrome c into cytosols markedly increased in HAECs during the treatment with gly-ox-HDL. The increased expressions of Bax and Bad were detected in HAECs incubated for 24 h with gly-ox-HDL, but gly-ox-HDL failed to interfere with the expression of Bcl-2 and Bcl-x. Moreover, in vitro experiments with HDL (gly-HDL) glycated in the presence of 2 mM EDTA and Cu(2+)-oxidized HDL suggested that the apoptotic effect of gly-ox-HDL on endothelial cells might be due to an additional oxidative modification of gly-HDL. Taken altogether, additional oxidation of HDL under hyperglycemic conditions may induce endothelial apoptosis through a mitochondrial dysfunction, following the deterioration of vascular function.

The peroxisome proliferator-activated receptor alpha (PPAR alpha) regulates the plasma thiobarbituric acid-reactive substance (TBARS) level

We investigated whether liver expression of the peroxisome proliferator-activated receptor alpha (PPAR alpha) gene is related to the plasma thiobarbituric acid-reactive substance (TBARS) level, as well as to plasma cholesterol (TC) level and plasma triglyceride (TG) level in rats fed a high fat chow containing a variety of fatty acids. Only the plasma TBARS level showed a significant negative correlation with the liver PPAR alpha mRNA level (TC, R = 0.001, p = 0.9967; TG, R = 0.248, p = 0.1276; TBARS, R = 0.439, p = 0.0046). Although further studies are needed to clarify whether the increase of the liver PPAR alpha mRNA level confers a reduction in plasma TBARS levels, it is likely that PPAR alpha activity plays a regulatory role in the pathogenesis of hyperlipidemia and atherosclerosis.