Tissue reaction to atheroma lipids

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.

Oxysterol-induced endothelial cell dysfunction in culture

Cholesterol oxidation products (oxysterols), such as cholestan-3 beta,5 alpha,6 beta-triol (Triol), may be atherogenic by altering the barrier function of the vascular endothelium. We have shown that incubation of endothelial cell monolayers with Triol increased transendothelial albumin transfer (i.e., decreased barrier function) in a concentration- and time-dependent manner. Such dysfunction of endothelium could result from alterations in membrane characteristics, including changes in membrane-associated enzyme activities. To test this hypothesis, endothelial monolayers were treated with 20 microM Triol and the activities of selected membrane enzymes were measured at 0, 2, 4, 6, 12 and 24 hours. Calcium-adenosine triphosphatase (Ca(++)-ATPase) and sodium, potassium, magnesium-adenosine triphosphatase (Na+, K+, Mg(++)-ATPase) activities were significantly increased after 4 or 2 hours incubation with 20 microM Triol, respectively. 5'-nucleotidase activity was significantly elevated only after a 24-hour exposure to Triol, whereas there was no change in angiotensin-converting enzyme (ACE) activity in response to 20 microM Triol treatment at any time studied. Compared with all concentrations tested 40 microM Triol increased Ca(++)-ATPase activity most markedly, with a significant increase already after a 2-hour exposure. No major morphological changes were noted until 12 hours of exposure to 20 microM Triol; obvious cellular damage was observed by 24 hours. Cultures treated with Triol for 24 hours showed significant signs of toxicity, measured by an elevated [3H]adenine release, compared with control cultures. These data demonstrate that Triol alters the activity of certain membrane-bound enzymes, particularly Na+, K+, Mg(++)-ATPase and Ca(++)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular damage in mouse peritoneal macrophages exposed to cholesteryl linoleate

Mouse peritoneal macrophages readily oxidize cholesteryl linoleate/bovine serum albumin emulsions to produce soluble lipid oxidation products, some of the latter being thought to cause cell damage. Mouse peritoneal macrophages were therefore incubated in the presence of cholesteryl linoleate/bovine serum albumin emulsion with and without the addition of dl-alpha tocopherol. The macrophages were observed morphologically and cell damage was estimated by three methods: trypan blue exclusion, lactate dehydrogenase release and tritiated adenine release. All the methods showed significant cell damage which was reduced in the presence of physiological levels of dl-alpha tocopherol. Cholesteryl oleate/bovine serum albumin, which is taken up by macrophages but is not oxidized, was not toxic. dl-Alpha tocopherol was itself toxic in higher concentrations. This self-inflicted macrophage damage might explain the onset of necrosis in atherosclerotic plaques.

Mutagenic characterization of cholesterol epoxides in Chinese hamster V79 cells

The uptake, metabolism and alkylating properties of the diastereomeric cholesterol epoxides were studied using Chinese hamster lung fibroblasts (V79 cells). Specific emphasis is given to the comparative cyto- and geno-toxic effects of cholesterol 5 beta,6 beta-epoxide (beta CE) and cholesterol 5 alpha,6 alpha-epoxide (alpha CE) and data are provided for the first time indicating that beta CE can induce more 6-thioguanine-resistant cells than alpha CE. Cholesterol 5 beta,6 beta-epoxide induced colonies of cells resistant to 6-thioguanine at 2-3-fold the frequencies observed with the alpha-isomer, but neither compound produced ouabain-resistant colonies. The cytotoxicity (LD50) of alpha CE was estimated to be 45-50 microM whereas beta CE displayed an LD50 of 25-29 microM. Inhibition of DNA synthesis (IC50) was observed over the same dose ranges as the LD50 for each epoxide isomer. The epoxides were assimilated by cells to an equal extent, however, beta CE was metabolized to cholestane 3 beta,5 alpha-6 beta-triol twice as rapidly as the alpha-isomer. Both epoxides reacted with 4-(4'-nitrobenzyl)-pyridine to a similar extent, and with identical nucleophilic selectivity at pH 7.4, but their alkylating activity was estimated on this basis to be two orders of magnitude less than methyl methanesulfonate. Binding experiments with the DNA or cultured V79 cells or with calf-thymus DNA indicated that interactions were noncovalent and DNA binding did not correlate with the potency of the epoxides to induce the 6-thioguanine-resistant phenotype. Our results could be interpreted as indicating that both cholesterol epoxide isomers are weak mutagens or that they might induce some epigenetic event repressing the hypoxanthine guanine-phosphoribosyltransferase gene. The similarity of the epoxides' alkylating activity and their DNA-binding properties are inconsistent with their different potencies in inducing the 6-thioguanine-resistant phenotype, suggesting that the mechanism leading to this phenotype is not necessarily the result of DNA alkylation.

Effects of cholesterol autoxidation derivatives on hexose transport in cultured aortic smooth muscle cells

Several cholesterol autoxidation derivatives known to be cytotoxic to arterial smooth muscle cells both in vivo and in vitro were shown to inhibit hexose transport in these cells in culture. Cholestane-3 beta, 5 alpha, 6 beta-triol was the most potent inhibitory sterol. The rapid onset of inhibition (15 min) and the reversibility of the effect upon removal of the sterol from the tissue culture medium suggests that the effect may be due to incorporation of the sterol into the plasma membrane. 25-Hydroxycholesterol, a potent inhibitor of sterol biosynthesis in cultured arterial smooth muscle cells, did not affect hexose transport up to 8 hr of incubation. The cytotoxic effect of 25-hydroxycholesterol, therefore, may be a consequence of the reduced sterol biosynthesis caused by this sterol.

Macrophages, oxidised lipids and atherosclerosis

The suggestion has frequently been made that lipid-laden blood monocytes might contribute to atherosclerosis by emigrating into the arterial intima. In spite of much evidence that this occurs, the mechanism has never attracted widespread support as being of major importance, mainly because of the apparently small numbers of monocytes involved in this traffic, compared to the larger numbers of smooth muscle cells in the lesion. Recent observations suggest that some at least of the macrophages within the early lesion may be oxidising their lipid contents. Because some oxidised lipids are known to be toxic to cells, it is proposed that the production of oxidised lipids by macrophages may cause the death of these and other cells in the intima; and that-cell death begins the vicious circle of injury and further lipid accumulation which characterise the enlarging plaque.

Ultrastructure of experimental coronary artery atherosclerosis in cynomolgus macaques. A comparison with the lesions of other primates

We studied the ultrastructure of the left anterior descending coronary artery in 6 female cynomolgus macaques fed atherogenic food containing 0.5% cholesterol for 6 months, and in 2 female cynomolgus macaques that had received food low in cholesterol. Animals given the atherogenic food had coronary artery lesions of a characteristic two-level architecture consisting of a lipid-poor upper cap, and of a lipid-rich lower core. The cap consisted of layers of smooth muscle cells that were rich in rough endoplasmic reticulum and contained few myofilaments and relatively few lipid droplet inclusions. The core consisted mainly of macrophages overloaded with droplet inclusions (macrophage foam cells), droplet-laden smooth muscle cells, and extracellular lipid and cell debris. Some macrophage foam cells were in mitosis. The largest lipid cores contained multinucleate giant cells with large intracellular crystal clefts. Dead macrophage foam cells were the source of the extracellular lipid and debris particles. Intimal cores often extended into the adjacent media. The adventitia was involved in the intima-media lesions in 3 of the animals. Compared with the coronary artery lesions of rhesus macaques and patas monkeys given similar atherogenic diets, cynomolgus monkey lesions were morphologically closer to human atherosclerotic plaques with respect to their stratification into a cap and a core.

Cerebral atheroembolism. An animal model

Human atheromatous material was injected into the cerebral vasculature of anaesthetized rabbits via the left common carotid artery. The lethality of varying dosages was determined and the distribution and general character of occlusive vascular lesions which developed were analyzed by light and transmission electron microscopy. It was found that a dose exceeding 55 mg of the atheromatous material (125 mg/ml saline) was lethal in New Zealand white male rabbits weighing between 3 and 5 kg. In nonsurviving animals, parts of the Circle of Willis and usually one or more of its major tributaries were occluded. Some surviving animals exhibited signs of neurologic deficit evidenced by motor dysfunction. Occlusive vascular lesions found in surviving animals were predominantly localized in ipsilateral cortical and subcortical vessels within the distribution territory of the middle cerebral artery. The character of occlusive lesions showed strong evidence of thrombosis. These results demonstrated that this experimental system may be useful as a model for the study of blood-atheroembolic vascular reactions, cerebral infarction development and the testing of agents potentially prophylactic against the development or stabilization of occlusive lesions.

Connective tissue responses to oxysterols

We studied the effect of sterols and oxidized sterols on promoting tissue inflammation and necrosis, and on causing cell death in tissue culture. The cells studied were mouse fibroblasts and macrophages, and pig vascular smooth muscle. The lipid preparations were dibromide-purified cholesterol, air-oxidized cholesterol, 25-hydroxycholesterol, cholesterol-5alpha, 6 beta-epoxide and cholestane 3 beta, 5 alpha, 6 beta-triol. Of these, the triol was most active in its cytopathic effect on cells in culture, in size of granuloma formation and in producing necrosis. The epoxide, 25-hydroxycholesterol and air-oxidized cholesterol produced rather less effect on tissues and cultures than the triol, but more than purified cholesterol and the control. The results are considered in relation to the presence of oxysterols in stored foodstuffs and their possible presence over the surface of cholesterol crystals in the tissues.

The effects and morphology of atheroembolism in limb arteries: an experimental study

Eroded atheromatous plaques which have released atheromatous material into the circulation are commonly found at autopsy in the abdominal aorta and common iliac arteries in man. In an attempt to study this phenomenon quantitatively, human atheromatous material in suspension was injected intothe femoral and common iliac arteries in rabbits. The weight of atheromatous material injected was varied. There was a critical load for the common iliac artery beyond which there was progressive ascending thrombosis and death of the animal. In rabbits weighing 3 to 5 kg, up to 0.2 g of atheromatous material could be injected intra-arterially with survival of the animal and without producing observable effects. As the amount was increased there was a narrow zone in which survival of the animal with a limp occurred. Beyond 0.2 g, death resulted from ascending thrombosis of the peripheral limb arteries. This eventually reached the bifurcation of the oarta and obstructed the opposite common iliac artery. A thrombus rapidly formed around the atheromatous material. Examination of the atherothrombus by light, transmission electron and scanning electron microscopy showed that it contained cholesterol crystals, lipid material, many platelets, leucocytes and red cells bound together within a fibrin net. The atheromatous material was for the most part in the core of the thrombus.