Toxicity of enzymically-oxidized low-density lipoprotein

Introducing inducible fluorescent split cholesterol oxidase to mammalian cells

Cholesterol oxidase (COase) is a bacterial enzyme catalyzing the first step in the biodegradation of cholesterol. COase is an important biotechnological tool for clinical diagnostics and production of steroid drugs and insecticides. It is also used for tracking intracellular cholesterol; however, its utility is limited by the lack of an efficient temporal control of its activity. To overcome this we have developed a regulatable fragment complementation system for COase cloned from Chromobacterium sp. The enzyme was split into two moieties that were fused to FKBP (FK506-binding protein) and FRB (rapamycin-binding domain) pair and split GFP fragments. The addition of rapamycin reconstituted a fluorescent enzyme, termed split GFP-COase, the fluorescence level of which correlated with its oxidation activity. A rapid decrease of cellular cholesterol induced by intracellular expression of the split GFP-COase promoted the dissociation of a cholesterol biosensor D4H from the plasma membrane. The process was reversible as upon rapamycin removal, the split GFP-COase fluorescence was lost, and cellular cholesterol levels returned to normal. These data demonstrate that the split GFP-COase provides a novel tool to manipulate cholesterol in mammalian cells.

Molecular mechanisms for Alzheimer's disease: implications for neuroimaging and therapeutics

Alzheimer's disease is a progressive neurodegenerative disorder characterised by the gradual onset of dementia. The pathological hallmarks of the disease are beta-amyloid (Abeta) plaques, neurofibrillary tangles, synaptic loss and reactive gliosis. The current therapeutic effort is directed towards developing drugs that reduce Abeta burden or toxicity by inhibiting secretase cleavage, Abeta aggregation, Abeta toxicity, Abeta metal interactions or by promoting Abeta clearance. A number of clinical trials are currently in progress based on these different therapeutic strategies and they should indicate which, if any, of these approaches will be efficacious. Current diagnosis of Alzheimer's disease is made by clinical, neuropsychologic and neuroimaging assessments. Routine structural neuroimaging evaluation with computed tomography and magnetic resonance imaging is based on non-specific features such as atrophy, a late feature in the progression of the disease, hence the crucial importance of developing new approaches for early and specific recognition at the prodromal stages of Alzheimer's disease. Functional neuroimaging techniques such as functional magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography and single photon emission computed tomography, possibly in conjunction with other related Abeta biomarkers in plasma and CSF, could prove to be valuable in the differential diagnosis of Alzheimer's disease, as well as in assessing prognosis. With the advent of new therapeutic strategies there is increasing interest in the development of magnetic resonance imaging contrast agents and positron emission tomography and single photon emission computed tomography radioligands that will permit the assessment of Abeta burden in vivo.

Differential toxicities of air (mO-LDL) or copper-oxidized LDLs (Cu-LDL) toward endothelial cells

In vivo low density protein (LDL) oxidation is a progressive phenomenon leading to the presence of minimally and highly oxidized LDLs in the subendothelial arterial space. Oxidized LDLs have been reported to be cytotoxic against endothelial cells. The goal of this study was to determine which of the minimally and highly oxidized LDLs were the most cytotoxic against bovine aortic endothelial cells (BAEC). Both the morphological aspect of the cells themselves, and LDH or MTT tests revealed that mO- or Cu-LDLs had similar cytotoxicity with up to 8 hours of oxidation, showing no relation with the level of LDL oxidation; for longer oxidation times, Cu-LDL cytotoxicity decreased. This phenomenon is linked to their different oxidation kinetics. Moreover, in the initial hours following BAEC incubation with mO- or Cu-LDLs, total cell glutathione dropped, whereas after 16 hours of incubation, highly oxidized Cu-LDL increased the glutathione level in the cell. The biphasic evolution of glutathione concentration corresponds to an autoprotective mechanism of cells against oxidized LDL cytotoxicity. This study suggests that the specific chemical characteristics of the different types of oxidized LDLs should always be precisely described in future assays devoted to studying the biological effects of what are known under the generic term as "oxidized LDLs". This precaution should prevent any confusion in interpreting different studies.

An excess concentration of oxysterols in the plasma is cytotoxic to cultured endothelial cells

To test if there is an excess concentration of oxysterols in the plasma of the patients with cardiovascular disease, we analyzed the oxysterol content in the plasma from 105 cardiac catheterized patients with angina and 80+/-8% stenosis in their coronary arteries. The result showed that the plasma contained a significantly higher concentration of oxysterols than did plasma from 105 age- and sex-matched, non-catheterized and angina-free controls (P<0.05). We used endothelial cells (ECs) cultured in medium containing either [3H]thymidine, [3H]mevalonolactone or 45Ca(2+) to determine how the plasma from the patients influences cell growth and function. We found that less [3H]thymidine (P<0.05), less [3H]mevalonolactone (P<0.05) and more 45Ca(2+) (P<0.001) was incorporated into ECs cultured in the plasma from 36 patients with 83+/-4% stenosis than from the 36 controls. When synthetic 7beta-hydroxycholesterol, cholesterol 5beta,6beta-epoxide, cholesterol 5alpha,6alpha-epoxide and 7-ketocholesterol were added to the plasma from the controls, the influx of 45Ca(2+) into ECs then equaled that in the plasma of patients. The enhanced incorporation of 45Ca(2+) into the ECs cultured in the plasma both from the patients and from controls with added synthetic oxysterols substantiates in vitro the hypothesis that oxysterols increase the influx of calcium into cells. These data indicated that an excess of oxysterols in the plasma of the patients was cytotoxic to the cultured cells.

Oxysterols and atherosclerosis

Oxysterols are present in human atherosclerotic plaque and are suggested to play an active role in plaque development. Moreover, the oxysterol:cholesterol ratio in plaque is much higher than in normal tissues or plasma. Oxysterols in plaque are derived both non-enzymically, either from the diet and/or from in vivo oxidation, or (e.g. 27-hydroxycholesterol) are formed enzymically during cholesterol catabolism. While undergoing many of the same reactions as cholesterol, such as being esterified by cells and in plasma, certain oxysterols in some animal and in vitro models exhibit far more potent effects than cholesterol per se. In vitro, oxysterols perturb several aspects of cellular cholesterol homeostasis (including cholesterol biosynthesis, esterification, and efflux), impair vascular reactivity and are cytotoxic and/or induce apoptosis. Injection of relatively large doses of oxysterols into animals causes acute angiotoxicity whereas oxysterol-feeding experiments have yielded contrary results as far as their atherogenicity is concerned. There is no direct evidence yet in humans that oxysterols contribute to atherogenesis. However, oxysterol levels are elevated in human low-density lipoprotein (LDL) subfractions that are considered potentially atherogenic and two recent studies have indicated that raised plasma levels of a specific oxysterol (7beta-hydroxycholesterol) may be associated with an increased risk of atherosclerosis. At the present time there are a number of significant and quite widespread problems with current literature which preclude more than a tentative suggestion that oxysterols have a causal role in atherogenesis. Further studies are necessary to definitively determine the role of oxysterols in atherosclerosis, and considering the wide-ranging tissue levels reported in the literature, special emphasis is needed on their accurate analysis, especially in view of the susceptibility of the parent cholesterol to artifactual oxidation.

Oxidation of macrophage membrane cholesterol by intracellular Rhodococcus equi

Phagocytic uptake by cultured mouse macrophages (PD388D1) of a virulent strain (ATCC 33701) of Rhodococcus equi producing substantial cholesterol oxidase was accompanied by intracellular survival of the bacteria, and enzymatic oxidation of macrophage membrane cholesterol. A non-virulent strain (4219) lacking cholesterol oxidase was largely eliminated from the macrophages and did not bring about oxidation of membrane cholesterol. When R. equi 33701 was co-phagocytosed with Corynebacterium pseudotuberculosis there was a significant enhancement (10-fold) in the amount of oxidation product (4-cholesten-3-one) generated. R. equi and C. pseudotuberculosis are cooperative partners in the hemolysis of sheep erythrocytes, traceable to the cholesterol oxidase of the former, and phospholipase D of the latter. Results are discussed relative to the role of cooperative cytotoxins in damage to host tissue by bacterial pathogens.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Biological effects of oxysterols: current status

A review of relevant literature on biological activities of oxysterols (OS) and cholesterol is presented. The data clearly demonstrate manifold biological activities, often detrimental, for OS compared with little or no such activity of a deleterious nature for cholesterol itself. Cholesterol is perhaps the single most important compound in animal tissue and, as such, it is difficult to imagine it as a toxin or hazard. In contrast, OS exhibit cytotoxicity to a wide variety of cells leading to angiotoxic and atherogenic effects; alter vascular permeability to albumin; alter prostaglandin synthesis and stimulate platelet aggregation, an important process facilitating atherosclerosis and thrombosis; alter the functionality of low density lipoprotein (LDL) receptors, possibly stimulating hypercholesterolaemia; modify cholesteryl ester accumulation in various cells, inducing foam cell formation; and enrich the LDL particle in cholesteryl esters, possibly increasing its atherogenicity. Furthermore, OS are mutagenic and carcinogenic, although some have been studied as antitumour agents based on their cytotoxic properties. Moreover, numerous studies have implicated OS in membrane and enzyme alterations that are interrelated with many of the foregoing effects. The authors find that OS deserve much more attention than cholesterol itself in terms of research activity but that unfortunately the reverse is true with regard to funding.

Oxidizability and subsequent cytotoxicity of chylomicrons to monocytic U937 and endothelial cells are dependent on dietary fatty acid composition

Oxidized chylomicrons may be a metabolic factor involved in the injury of the arterial wall and may constitute a potential link between postprandial lipemia and atherogenesis. It was of interest to study the influence of dietary fatty acid composition on the oxidizability and subsequent cytotoxicity of chylomicrons on cultured cells. Human chylomicrons were obtained from healthy volunteers 3 h after ingestion of a triglyceride-rich meal containing mainly either polyunsaturated fatty acids (soya oil) or monounsaturated fatty acids (olive oil) or saturated fatty acids (partly hydrogenated palm oil). Polyunsaturated fatty acid (PUFA)-rich chylomicrons exhibited a high oxidizability, whereas chylomicrons enriched with monounsaturated or saturated fatty acids were relatively resistant to oxidation. The cytotoxicity of various types of chylomicrons submitted to oxidation has been tested comparatively on cultured human monocytic U937 cells and endothelial cells. Chylomicrons enriched with saturated and monounsaturated fatty acids were not or only slightly cytotoxic to cultured cells, whereas PUFA-rich chylomicrons (highly susceptible to oxidation) were highly cytotoxic. The influence of cholesterol on the oxidizability and subsequent cytotoxicity of PUFA-rich chylomicrons has been investigated by using comparatively a soya diet supplemented or not with cholesterol. PUFA-rich cholesterol-rich chylomicrons were slightly more oxidizable and more cytotoxic than PUFA-rich (cholesterol-poor) chylomicrons, thus suggesting that the cytotoxicity of PUFA-rich chylomicrons may be due to oxidation derivatives of PUFA (for the major part) and to oxysterols (for a minor part). Furthermore, the cytotoxic effects of oxidized PUFA-rich chylomicrons and of mildly oxidized LDL were in similar range (even higher for PUFA-rich chylomicrons when expressed per lipoprotein particle), thus suggesting that oxidized PUFA-rich chylomicrons may play a nonnegligible role in cytotoxic events occurring during atherogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation of F2-isoprostanes during oxidation of human low-density lipoprotein and plasma by peroxynitrite

F2-Isoprostanes are novel bioactive prostaglandin F2-like compounds produced by nonenzymatic free radical-catalyzed peroxidation of arachidonic acid. F2-Isoprostanes are initially formed in situ on phospholipids and subsequently released. Quantification of the F2-isoprostanes has been found to represent a valuable and reliable marker of lipid peroxidation. Oxidative modification of low-density lipoprotein (LDL) is a key process for the recognition of LDL by the scavenger receptors on macrophages. The oxidative mechanism responsible for the modification of LDL in vivo remains unclear, but an attractive candidate is the powerful oxidant peroxynitrite, which can be formed by reaction of nitric oxide and superoxide in the vessel wall. To further explore the potential role of peroxynitrite in the oxidative modification of plasma lipids, we investigated whether incubation of LDL and plasma with peroxynitrite or SIN-1, which decomposes to form nitric oxide and superoxide, catalyzes the formation of F2-isoprostanes. Incubation of LDL with peroxynitrite (0.125 to 1 mmol/L) or SIN-1 (0.5 and 1 mmol/L) induced a concentration-dependent increase in the formation of F2-isoprostanes, reaching a maximum of 5.5 +/- 2.05-fold (SEM) and 18.2 +/- 4.0-fold above control values, respectively. The increase of F2-isoprostanes induced by SIN-1 was essentially completely inhibited by superoxide dismutase. Incubation of plasma with peroxynitrite or SIN-1 yielded similar results. These results indicate that peroxynitrite can induce the formation of F2-isoprostanes in lipoproteins. Since F2-isoprostanes can exert potent biological activity such as vasoconstriction, they may contribute to the vascular pathobiology associated with atherosclerosis.

alpha-Tocopherol and trolox block the early intracellular events (TBARS and calcium rises) elicited by oxidized low density lipoproteins in cultured endothelial cells

Low-density lipoproteins (LDLs), treated by UV-C radiations under conditions permitting mildly oxidized LDL (6 +/- 2 nmol TBARS/mg apoB, without major structural or functional alteration of apoB), have been used for studying their cytotoxicity to cultured bovine aortic endothelial cells and the cytoprotective effect of various analogs of alpha-tocopherol. Toxic doses of oxidized LDL evoked intracellular events, such as cellular thiobarbituric acid reactive substances (TBARS) and a sustained peak of [Ca2+]i (cytosolic calcium). The sustained [Ca2+]i peak seems to be directly involved in the genesis of cell injury leading to cell death in contrast to cellular TBARS, which seems to be either an earlier step of signal transduction or a side effect, as shown by inhibiting the [Ca2+]i rise by ethylene glycol-O,O'-bis(amino ethyl)-N1N1N'1N'-tetraacetic acid (EGTA) added just before the time of the [Ca2+]i peak. When alpha-tocopherol or trolox (a short-chain, water-soluble analog of alpha-tocopherol) were added to the culture medium simultaneously with oxidized LDL, they were able to increase the resistance of endothelial cells against the cytotoxic effect of oxidized LDL, whereas alpha-tocopheryl acetate and alpha-tocopheryl succinate were almost completely ineffective because of the liberation of only very low levels of alpha-tocopherol. Trolox exhibited a more potent cytoprotective effect than alpha-tocopherol (IC50: 1 +/- 0.2 and 8 +/- 2 mumol/l for trolox and alpha-tocopherol, respectively). As shown by preincubating cells with effective concentrations of alpha-tocopherol or trolox, the cytoprotective effect was completely independent of any inhibition of LDL oxidation and was remanent for 2 d with alpha-tocopherol or for 3-4 d with trolox. Cytoprotective concentrations of trolox and alpha-tocopherol did not inhibit LDL uptake but acted at the cellular level by blocking the formation of cellular TBARS and the sustained [Ca2+]i peak as well. The potential relevance of these data in relation to the prevention of atherosclerosis is discussed.

The effect of peroxides on the vascular endothelium of isolated pig aorta in vitro

The effect of peroxide on endothelial cells (perfused pig aorta) was examined using an in vitro perfusion model. Hydrogen peroxide was added to the perfusion medium (pig serum together with a buffer solution) which was expected to lead to an increased oxidation of lipids and lipoproteins. Oxidation processes of this type play a decisive role in the pathogenesis and progression of arteriosclerosis. The aim of the present investigation was to demonstrate by introducing hydrogen peroxide (H2O2) in varying concentrations (0.5-1.5%), the destructive impact of peroxides on the endothelium, while these cells are believed to play a key role in the pathogenesis of arteriosclerosis. The extent of endothelial cell impairment was assessed by means of silver staining visualisation of endothelial cell borders as well as light- and scanning-electronmicroscopic investigation. It was discovered that the endothelial cells show increasing impairment after 10 h of perfusion due to the effect of peroxide (hydrogen peroxide).

Cytotoxicity of cholesterol oxidase to cells of hypercholesterolemic guinea pigs

1. A high cholesterol diet caused guinea pig erythrocytes to become sensitive to lysis by cholesterol oxidase (CO), a protein not hemolytic to normal cells. 2. Lysis was associated with conversion of membrane cholesterol to its oxidation product (delta-4-cholesten-3-one). 3. Intravenous injection of CO to hypercholesterolemic guinea pigs produced a reduction in serum cholesterol, but was not lethal as it was in rabbits. 4. Homogenized spleen, liver and kidney from the hyperlipidemic animals were sensitive to in vitro cholesterol oxidation while tissues from non-lipemic animals were resistant to modification.