Oxidation of low-density lipoprotein by human monocyte-macrophages results in toxicity to the oxidising culture

Modified low density lipoprotein and lipoprotein-containing circulating immune complexes as diagnostic and prognostic biomarkers of atherosclerosis and type 1 diabetes macrovascular disease

In atherosclerosis; blood low-density lipoproteins (LDL) are subjected to multiple enzymatic and non-enzymatic modifications that increase their atherogenicity and induce immunogenicity. Modified LDL are capable of inducing vascular inflammation through activation of innate immunity; thus, contributing to the progression of atherogenesis. The immunogenicity of modified LDL results in induction of self-antibodies specific to a certain type of modified LDL. The antibodies react with modified LDL forming circulating immune complexes. Circulating immune complexes exhibit prominent immunomodulatory properties that influence atherosclerotic inflammation. Compared to freely circulating modified LDL; modified LDL associated with the immune complexes have a more robust atherogenic and proinflammatory potential. Various lipid components of the immune complexes may serve not only as diagnostic but also as essential predictive markers of cardiovascular events in atherosclerosis. Accumulating evidence indicates that LDL-containing immune complexes can also serve as biomarker for macrovascular disease in type 1 diabetes.

Redistribution of metal ions to control low density lipoprotein oxidation in Ham's F10 medium

The study of cell-mediated low density lipoprotein (LDL) oxidation has traditionally been undertaken using Ham's F10 media due to its high metal content and low levels of antioxidants. Although there has been no acknowledged change to this media in recent years by the suppliers, Ham's F10 medium has been found to be extremely inconsistent in its promotion of LDL oxidation in the absence of cells. This variability contrasts with the relatively consistent rates of THP-1 cell-mediated LDL oxidation. This study has now shown that the variability in cell-free LDL oxidation is medium-dependent and not an artefact of experimental protocol. It presents evidence that suggests the variable rates of cell-free LDL oxidation are caused by iron auto-oxidation during storage of the Ham's F10 medium. The medium can be standardized by removal of all transition metals, by treatment with Chelex, before the addition of known amounts of iron or copper. This treatment generates a cell culture medium that only allows very slow LDL oxidation in the absence of cells.

Oxysterols and oxysterol binding proteins: role in lipid metabolism and atherosclerosis

Oxidized derivatives of cholesterol have been investigated actively for decades in the context of the oxidative hypothesis of atherosclerosis. Oxysterols arise in our tissues as a result of enzymatic or non-enzymatic oxidation reactions and are also obtained from dietary sources. Even though these compounds are found enriched in the atherosclerotic lesions in arterial walls, the plasma concentrations of oxysterols cannot, in the light of current knowledge, be regarded as a risk factor for atherosclerotic disease. However, oxysterols may still have important local effects in the arterial wall as factors that regulate the cellular lipid homeostasis and possibly the maturation of the lesions. Work during the past few years has revealed that oxysterols have a potential as signaling molecules that may play important roles in lipid metabolism, especially the reverse cholesterol transport process. This finding has recently moved oxysterols and the protein mediators of their biological effects, liver X receptors and cytosolic oxysterol binding proteins, into the center stage of atherosclerosis research.

A novel method of following oxidation of low-density lipoprotein using a sensitive fluorescent probe, diphenyl-1-pyrenylphosphine

Diphenyl-1-pyrenylphosphine (DPPP), which reacts with lipid hydroperoxide stoichiometrically to yield a fluorescent product DPPP oxide (DPPP=O) and the corresponding hydroxide, was used as a fluorescent probe for lipid peroxidation in low-density lipoprotein (LDL). DPPP was successfully incorporated into LDL using the dispersion reagent Pluronic F-127. Incorporation of DPPP into LDL was confirmed by gel filtration chromatography. Reaction of DPPP with hydroperoxide within an LDL particle was examined by monitoring the increase in fluorescence intensity of the LDL. It was found that lipid-soluble hydroperoxides such as methyl linoleate hydroperoxide preferably reacted with DPPP, whereas hydrogen peroxide did not. Fluorescence was increased at the early stages in the oxidation of DPPP-labeled LDL by an azo radical initiator or human neutrophils. LDL, which was labeled with DPPP or DPPP=O, was taken up by cells such as THP-1-derived macrophages and human umbilical vein endothelial cells. The fluorescence of DPPP=O could be observed in cells using fluorescence microscopy equipped with a cooled charge coupled device camera in a nondestructive manner. The present study shows that DPPP is a sensitive, selective, and quantitative probe for monitoring LDL oxidation and visualizing intracellular oxidation.

Inhibition of lipoprotein-associated phospholipase A2 diminishes the death-inducing effects of oxidised LDL on human monocyte-macrophages

The death of macrophages contributes to atheroma formation. Oxidation renders low-density lipoprotein (LDL) cytotoxic to human monocyte-macrophages. Lipoprotein-associated phospholipase A2 (Lp-PLA2), also termed platelet-activating factor acetylhydrolase, hydrolyses oxidised phospholipids. Inhibition of Lp-PLA2 by diisopropyl fluorophosphate or Pefabloc (broad-spectrum serine esterase/protease inhibitors), or SB222657 (a specific inhibitor of Lp-PLA2) did not prevent LDL oxidation, but diminished the ensuing toxicity and apoptosis induction when the LDL was oxidised, and inhibited the rise in lysophosphatidylcholine levels that occurred in the inhibitors' absence. Hydrolysis products of oxidised phospholipids thus account for over a third of the cytotoxic and apoptosis-inducing effects of oxidised LDL on macrophages.

Antioxidant BO-653 and human macrophage-mediated LDL oxidation

Oxidation of LDL is now widely accepted to be involved in atherogenesis. The aim of this study was to examine the effect of BO-653, a strong radical scavenger and antioxidant, on oxidation of LDL by human macrophages in vitro. Fifty microg/ml LDL protein was incubated with macrophages in Ham's F10 medium, supplemented with additional Fe2+, for up to 48 h. Then the medium was analysed by LDL agarose gel electrophoresis, the thiobarbituric acid assay and gas chromatography. In the absence of added exogenous antioxidants, after 24h LDL oxidation produced 30.48 nmoles MDA equivalents/mg LDL protein and a relative electrophoretic mobility of 4.74. Linoleic acid (18:2), arachidonic acid (20:4) and cholesterol were depleted and 7beta-hydroxycholesterol was generated. BO-653 completely inhibited this cell-mediated oxidation of LDL in concentrations as low as 5 microM, being more effective than either alpha-tocopherol or probucol, which completely inhibited oxidation at 200 and 80 microM and only partially at 80 and 8 microM, respectively. This inhibition of cell-mediated LDL oxidation was not due to toxicity, as alpha-tocopherol, probucol and BO-653 were not toxic for the macrophages at the concentrations tested. Eighty microM alpha-tocopherol, 8 microM probucol and 5 microM BO-653 significantly reduced the toxicity to the oxidising culture caused by LDL oxidation. The results show that in this system BO-653 is a more effective antioxidant than alpha-tocopherol or probucol.

Cell-mediated oxidation of LDL: comparison of different cell types of the atherosclerotic lesion

The three major cell types of the human atherosclerotic lesion--macrophages (Mø), smooth muscle cells (SMC) and endothelial cells (EC)--were compared for their ability to oxidise low density lipoprotein (LDL) in vitro under identical conditions. Near-confluent cultures were incubated for up to 48 h with 50 microg protein/ml LDL in Ham's F10 medium supplemented with 7 microM Fe2+. All three cell types oxidised LDL readily using our culture conditions. After 24 and 48 h, the degree of LDL oxidation was in the order: Mø > SMC > EC when based on cell growth area and EC > SMC > Mo when based on cellular DNA content. However, LDL oxidation in vitro progressed more slowly between 24 and 48 h, probably due to increasing toxicity to the cells and/or depletion of polyunsaturated fatty acids. We therefore compared the time of onset of LDL oxidation. The earliest increase in LDL oxidation was always apparent with SMC. Gas chromatography revealed that LDL oxidation by all three cell types followed a similar pattern. The polyunsaturated fatty acids linoleic acid (18:2) and arachidonic acid (20:4) were depleted (to 10.3-18.1% and 4.5-24.7% respectively, compared to native LDL), whereas the content of stearic acid (18:0) and oleic acid (18:1) remained unchanged. Cholesterol was depleted (to 54.1-75.6% of native LDL) with a concomitant rise in 7 -hydroxycholesterol (to 60.6-128.1 microg/mg LDL). This corresponds to a conversion of 4.9, 9.5 and 10.4% of LDL cholesterol in EC-, SMC- and Mo-modified LDL respectively. All three cell types showed significant toxicity in the oxidising culture after 24h. The possible relevance to LDL oxidation in atherosclerosis is discussed.

Toxicity of polyunsaturated fatty acid esters for human monocyte-macrophages: the anomalous behaviour of cholesteryl linolenate

We have investigated the toxicity to human monocytemacrophages, and susceptibility to oxidation, of different individual dietary fatty acids in cholesterol esters and triglycerides, added to the cell cultures as coacervates with bovine serum albumin. Toxicity was assessed using release of radioactivity from cells preloaded with tritiated adenine. Lipid oxidation was measured by gas chromatography (GC). The triglycerides showed a direct relationship between toxicity and increasing unsaturation, which in turn correlated with increasing susceptibility to oxidation. Triolein (18:1; omega-9) and trilinolein (18:2; omega-6) were non-toxic. Trilinolenin (18:3; omega-3) was toxic only after prolonged incubation. Triarachidonin (20:4; omega-6), trieicosapentaenoin (20:5; omega-3) and tridocosahexaenoin (22:6; omega-3) were profoundly and rapidly toxic. There was a similar relationship between toxicity and increasing unsaturation for most of the cholesterol esters, but cholesteryl linolenate was apparently anomalous, being non-toxic in spite of possessing three double bonds and being extensively oxidised. Probucol and DL-alpha-tocopherol conferred protection against the toxicity of cholesteryl arachidonate and triarachidonin. The oxidation in these experiments was largely independent of the presence of cells. GC indicated that formation of 7-oxysterols might contribute to the toxicity of cholesteryl linoleate. The toxicity of triglycerides suggests that polyunsaturated fatty acid peroxidation products are also toxic. Possible mechanisms of cytotoxicity and relevance to atherosclerosis are discussed.

Effect of glucose-mediated LDL oxidation on the P388D1 macrophage-like cell line

Oxidised human low density lipoprotein (LDL) is thought to play a role in the development of atherosclerosis. Recent reports suggest that glucose-derived oxidants are capable of oxidising LDL. In this report, the effect of glucose-mediated oxidation of LDL upon the macrophage like cell line, P388D(1), was examined. Glucose-mediated oxidation of LDL was assessed by changes in the electrophoretic mobility of LDL and by analysis of lipid content using gas chromatography. The presence of Cu(II) (0.5 microM) was essential for the oxidation of LDL. The oxidation was potentiated by glucose in a dose- and time-dependent manner. At the concentration of LDL used (1 mg/ml), high concentrations of glucose (up to 500 mM) were required to oxidise LDL. The electrophoretic mobility of LDL correlated with the degree of lipid oxidation; both correlated with an inhibitory effect of oxidised LDL upon P388D(1) DNA synthesis. Diethylenetriaminepentaacetic acid (DETAPAC), a transition metal chelator, and aminoguanidine (AMG), an anti-glycation agent, inhibited the oxidation of LDL and attenuated the effects on DNA synthesis. Thus, glucose can mediate transition metal-dependent oxidation of LDL to a level that can affect P388D(1) cells, a mechanism which might have relevance to accelerated atherosclerosis in diabetic patients.

Foam cell apoptosis and the development of the lipid core of human atherosclerosis

A characteristic feature of the advanced atherosclerotic lesion is the acellular lipid core, which appears to result at least partly from the death of macrophage foam cells. This study shows that foam cell death at the edge of the lipid core includes both necrosis and apoptosis and that remnants of apoptotic nuclei are present within the lipid core. Apoptotic cells were identified by transmission electron microscopy and by nick end-labelling using terminal deoxynucleotidyl transferase (TUNEL). Some TUNEL-positive cells also expressed proliferating cell nuclear antigen (PCNA). The cause of foam cell death in atherogenesis is unknown, but oxidized low-density lipoprotein (LDL) can cause macrophage apoptosis in vitro and might therefore play a role in the formation and enlargement of the lipid core.