Mosaicism in female hybrid hares heterozygous for glucose-6-phosphate dehydrogenase (G-6-PD). III. Changes in the ratios of G-6-PD types in skin fibroblast cultures exposed to 25-hydroxycholesterol

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Angiotoxicity and atherogenicity of cholesterol oxides

Cholesterol in the diet can readily autoxidize and be absorbed and transported in plasma lipoproteins. Cholesterol oxides can also be endogenously produced in tissues via free-radical-induced reactions. Some cholesterol oxides, notably cholestane-3 beta, 5 alpha, 6 beta-triol and 25-hydroxycholesterol, have been shown to cause injury to vascular endothelial and smooth muscle cells, to alter LDL receptor function, to enhance cholesteryl ester accumulation, to inhibit prostacyclin production, and to induce experimental atherosclerosis alone or in combination with cholesterol. An epidemiological study examining relationships between atherosclerosis and plasma levels of cholesterol oxides as independent risk factors may provide additional insights regarding the roles of cholesterol oxides in atherogenesis.

Mosaicism in female hybrid hares heterozygous for glucose-6-phosphate dehydrogenase. VII. Evidence for selective advantage of one phenotype over the other in ditypic samples from aortas of hares fed cholesterol oxidation products

In a previous study the authors demonstrated monotypic foci, all of timidus type, in the atherosclerotic lesions and normal aortic tissue of glucose-6-phosphate dehydrogenase (G-6-PD) mosaic hares fed 25-hydroxycholesterol or triol in addition to a high-cholesterol diet. Other hares fed the high-cholesterol diet without the cholesterol oxidation product additives did not develop monotypic foci. In the current study the question asked was whether ditypic samples from lesions or aortic mediae of the same hares fed the additives would show a higher percentage of timidus type G-6-PD than the hares fed the cholesterol alone. The results showed a significantly higher percentage of timidus type in both lesions and mediae in the groups fed either 25-hydroxycholesterol or triol than in the group fed cholesterol alone. The observations suggest that the change from ditypism to monotypism reported earlier with these cholesterol oxidation products was the result of progressive elimination of the europaeus type to leave only the timidus G-6-PD. This is consistent with the "selective advantage of one phenotype over the other" hypothesis for explaining the observed monotypism as opposed to the "genetic transformation" hypothesis.

Mosaicism in female hybrid hares heterozygous for glucose-6-phosphate dehydrogenase. V. The recovery of DNA synthesis of hare fibroblasts after ultraviolet irradiation

The effect of uv irradiation on the recovery of DNA synthesis is examined in a population of hare fibroblasts exhibiting heterozygosity with reference to the X-linked enzyme, glucose-6-phosphate-dehydrogenase (G-6-PD). These cells have been grown from skin explants of a hybrid female cross between Lepus timidus (female) and L. europaeus (male), the former carrying the G-6-PD gene for the slow-moving "T" variant and the latter with the fast-moving "E" variant gene. The hybrid, therefore, exhibits genetic mosaicism due to random inactivation in each cell, of one of the two X chromosomes in the embryonic stage. Exponentially growing cells from 13 fibroblast strains, comprising a wide range of E to T ratios, were exposed to moderately low dose of uv irradiation (6 J/m2). The recovery in DNA synthesis during the 2- to 8-h postirradiation period was calculated as the mean percentage rates of [3H]thymidine incorporated during the time as compared to the unirradiated zero-time controls. The results show a statistically significant positive correlation as determined by linear regression analysis between the levels of E and the rate of recovery in DNA synthesis. This is valid also at the higher dose of uv (21 J/m2). These results strengthen our earlier observations with 25-hydroxycholesterol that in the in vitro system the cell expressing the E variant is perhaps more resistant to cytotoxic agents. This also indicates that various factors contribute to the development of monotypism which include cell growth, cell death, mutation, and selection, to name a few.

Mosaicism in female hybrid hares heterozygous for glucose-6-phosphate dehydrogenase. VI. Production of monotypism in the aortas of 4 of 10 mosaic hares fed cholesterol oxidation products

The normal aortic tissue of black women heterozygous for glucose-6-phosphate dehydrogenase (G-6-PD) usually consists of two cell phenotypes (mosaicism). By electrophoresis two G-6-PD types are demonstrated (ditypism). Advanced atherosclerotic lesions from such women not infrequently yield samples displaying only one G-6-PD type (monotypism). Possible causes for the monotypism include (1) monoclonal origin of the lesion and (2) selective growth and/or survival advantage of one phenotype over the other. We have been attempting to produce monotypism in the aortas of a hybrid hare model that displays G-6-PD mosaicism in the normal state. In the current study 14 G-6-PD mosaic hares were fed a high-cholesterol diet for 6 to 17 months. All developed extensive moderately severe (as compared to humans) atherosclerotic lesions. No monotypic samples were found. Ten hares were fed (in addition to the standard high-cholesterol diet) small doses of one of two cholesterol oxidation products (25-hydroxycholesterol or triol) for 6 to 21 months. Four of the ten developed monotypic foci in either atherosclerotic lesions or normal aortic tissue or both. The reason for giving the oxidation products was because they are cytotoxic for cells in vitro and one (25-hydroxycholesterol) had been shown to be more toxic for one phenotype than the other with cultured fibroblasts from the mosaic hares (not infrequently resulting in the culture becoming monotypic). We suggest that the monotypism observed in vivo was probably produced by a mechanism similar to that operating in vitro.

Cholesterol-induced atherosclerosis. Clonal characteristics of arterial lesions in the hybrid hare

Utilizing the observation that a majority of human atherosclerotic fibrous plaques show monoclonal characteristics, we carried out this study to determine the clonal characteristics of cholesterol-induced atherosclerosis in the hybrid hare. If this is a valid model for human atherosclerosis, the lesions produced in the aorta should be monoclonal. Glucose-6-phosphate dehydrogenase (G-6-PD) was used as an X-linked cellular marker in the female hybrid hare (Lepus timidus X Lepus europaeus), which is heterozygous for electrophoretically separable isoenzymes of G-6-PD. Hares were fed cholesterol over either a 6-month or a 16-month period, and the easily dissectable lesions in the aorta and common iliac arteries were assayed for isoenzyme activity at these times. Of the 93 lesions assayed, all had polyclonal characteristics except a single monoclonal lesion found in an animal fed cholesterol over a 16-month period. Hares fed over the 16-month period showed lesions with isoenzyme patterns having a significantly higher contribution of L. timidus isoenzyme than those found in underlying media. This suggested that a selection of cells with the L. timidus X-chromosome had taken place, but the degree of this selection was not great enough to allow any of the lesions to be defined as monoclonal.

Mosaicism in female hybrid hares heterozygous for glucose-6-phosphate dehydrogenase. IV. Aortic atherosclerosis in hybrid hares fed alternating cholesterol-supplemented and nonsupplemented diets

Hybrid hares provide a model for induction of aortic atherosclerosis. The basic dietary regimen consisted of cholesterol-supplemented and nonsupplemented commercial pellets alternating at 1-week intervals for up to 18 months. Controls were fed nonsupplemented pellets. Control aorta features included branch cushions, intimal thickenings unrelated to branches, and several longitudinal layers of smooth muscle in the outer media. Branch cushions extended longitudinally in the main vessel, and had multidirectional, streamlike bundles of smooth muscle in the intima and media. Intimal thickenings unrelated to branches occurred as longitudinal patches, most frequently in the ventral quadrant. The adjacent media had a regular array of cells. Raised lesions concentrated at branch orifices and in the ventral quadrant. Lesions involved up to 80% of the thoracic and 40% of the abdominal aorta by 5 months. Predominant lesions consisted of superficial fibromuscular tissue and deeper foam cells and stainable lipids. Some of these lesions were atheromata with fibrous cap. Fibromuscular plaques, 120 to 150 micron thick, had a few superficial foam cells and hydropic degeneration. Foam-cell lesions were also observed. Data obtained suggest the importance of branch cushions and intimal thickenings unrelated to branches, and of the periods on nonsupplemented pellets.