| For medical practitioners and the general public - Cholesterol Journal Article Catalog. |
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| Oxidatively modified HDLs are potent inhibitors of cholesterol biosynthesis in human skin fibroblasts Ghiselli, G., L. Giorgini, et al. (1992), Arterioscler Thromb 12(8): 929-35. Abstract: Several biological properties of lipoproteins are modified by oxidative reactions. Modified lipoproteins are rapidly degraded by macrophages, and this is likely to be a major pathway for the formation of foam cells in the early phases of atherosclerosis. The effect of modification on other aspects of cholesterol homeostasis has, however, received lesser attention. In this study, the influence of copper ion- as well as rat aortic smooth muscle cell-oxidation-modified high density lipoprotein (HDL) on cholesterol biosynthesis in human skin fibroblasts has been investigated. Modified lipoproteins eluted at higher ionic strength than did control HDL on a Mono-Q 5/5 anion-exchange column. However, only copper ion-modified HDLs displayed greater electrophoretic mobility than did control lipoproteins on agarose gel electrophoresis. Both control and modified HDLs decreased cholesterol esterification in fibroblasts. On the other hand, whereas control HDLs were virtually ineffective in modulating cholesterol biosynthesis, modified HDLs had a significant suppressing effect. This was observed in normal as well as low density lipoprotein (LDL) receptor-defective fibroblasts, which are unresponsive to the LDL-mediated downregulation of cholesterol synthesis. These results are consistent with the concept that oxidative modification of HDLs drastically alters their effect on cholesterol homeostasis in fibroblasts. The data furthermore suggest the existence of a lipoprotein pathway for cholesterol biosynthesis regulation that is independent of the LDL receptor-mediated pathway. Downregulation of cholesterol biosynthesis would be a new function for oxidatively modified lipoproteins. |
| Oxidative-modified and acetylated low-density lipoproteins differ in their effects on cholesterol synthesis and stimulate synthesis of apolipoprotein E in rat peritoneal macrophages by different mechanisms Zhang, M. Y. and R. C. Lin (1994), Metabolism 43(12): 1523-30. Abstract: Apolipoprotein (apo) E plays an important role in the recognition of lipoproteins by cellular lipoprotein receptors. Unlike other apolipoproteins, apo E is expressed by many extrahepatic tissues including macrophages (M phi). Resident M phi express low levels of apo E. However, their synthesis of apo E is substantially increased after M phi have been incubated with acetylated low-density lipoprotein (LDL). But acetylation of LDL is not known to occur in vivo. On the other hand, modification of LDL by oxidation and by enzymatic action is believed to happen physiologically. In this report, we compared the effects of various modified LDLs on the synthesis of apo E by M phi. Freshly isolated human LDL was modified by (1) repeated addition of acetic anhydride (Ac-LDL); (2) incubation with 20 mumol/L CuSO4 at 37 degrees C for 24 hours (Ox-LDL); and (3) incubation with phospholipase C at 37 degrees C for 1 hour (PI-LDL). Resident peritoneal M phi were collected by lavage from rats and allowed to attach to plastic culture dishes. Although native LDL had no effect, treatment with Ac-, Ox-, and PI-LDL (50 micrograms/mL each) was found to increase medium apo E by (-fold) 4.19 +/- 0.26, 4.20 +/- 0.34, and 2.02 +/- 0.20 (mean +/- SEM, n = 5), respectively, as compared with untreated cells. Northern blot analysis revealed that cellular apo E mRNA was increased in parallel to apo E protein by Ac-LDL and PI-LDL. However, increases of apo E protein and mRNA by Ox-LDL were not equal.(ABSTRACT TRUNCATED AT 250 WORDS) |
| Oxidized cholesterol in oxidized low density lipoprotein may be responsible for the inhibition of LPS-induced nitric oxide production in macrophages Liu, S. X., Y. Chen, et al. (1998), Atherosclerosis 136(1): 43-9. Abstract: Previous work has shown that oxidized low density lipoprotein (Ox-LDL) inhibited lipopolysacchride (LPS)-induced nitric oxide (NO) production in macrophages. In this paper, the role of different components of Ox-LDL in the inhibitory effect was studied by measuring nitrite in media. Ox-LDL inhibited LPS-induced NO production in macrophage cell line J774.A1. When compared with Ox-LDL, native and acetylated LDL had a lesser effect on NO production. Pre-clearance of lipid hydroperoxides (LOOH) in Ox-LDL and removal of soluble thiobarbituric acid reactive substances (TBARS) in Ox-LDL solution by dialysis had no influence on the inhibitory effect of Ox-LDL. The protein moiety of Ox-LDL had no effect on NO production, but the lipid moiety inhibited NO production to about the same extent as intact Ox-LDL. Linoleic acid and phosphatidylcholine, the main components of LDL lipid, whether oxidized separately or together, had no effect on NO production. However, if linoleic acid and cholesterol oxidized together, there was very strong inhibition of NO production. Cholesterol oxidized alone also had some inhibitory effect. These results suggest that oxidized cholesterol in Ox-LDL might be responsible for the inhibition of NO production in macrophages. |
| Oxidized cholesterol in the diet accelerates the development of aortic atherosclerosis in cholesterol-fed rabbits Staprans, I., X. M. Pan, et al. (1998), Arterioscler Thromb Vasc Biol 18(6): 977-83. Abstract: Oxidized lipoproteins may play a role in atherosclerosis. Recently, we have demonstrated that the levels of oxidized fatty acids in the circulation correlate directly with the quantity of oxidized fatty acids in the diet and that dietary oxidized fatty acids accelerate atherosclerosis in rabbits. The present study tests the hypothesis that oxidized cholesterol in the diet accelerates the development of atherosclerosis. Rabbits were fed a diet containing 0.33% nonoxidized cholesterol (control diet) or the same diet containing 0.33% cholesterol of which 5% was oxidized (oxidized diet). Serum cholesterol levels increased to a similar extent in both groups, with the majority of cholesterol in the beta-VLDL fraction. Moreover, in the serum beta-VLDL fraction and liver, there was a significant increase in the oxidized cholesterol levels. Most importantly, feeding a diet enriched in oxidized cholesterol resulted in a 100% increase in fatty streak lesions in the aorta. Western diets contain high concentrations of oxidized cholesterol products, and our results suggest that these foods may be a risk factor for atherosclerosis. |
| Oxidized cholesterol in the diet accelerates the development of atherosclerosis in LDL receptor- and apolipoprotein E-deficient mice Staprans, I., X. M. Pan, et al. (2000), Arterioscler Thromb Vasc Biol 20(3): 708-14. Abstract: The aim of the current study was to determine whether oxidized cholesterol in the diet accelerates atherosclerosis in low density lipoprotein receptor- (LDLR) and apolipoprotein E- (apo E) deficient mice. Mice were fed either a control diet or a diet containing oxidized cholesterol. For LDLR-deficient mice, the control diet consisted of regular mouse chow to which 1.0% cholesterol was added. The oxidized diet was identical to the control diet except that 5% of the added cholesterol was oxidized. In apo E-deficient mice, the control diet contained 0.15% cholesterol, whereas in the oxidized diet, 5% of the added cholesterol was oxidized. LDLR-deficient and apo E-deficient mice were fed the experimental diets for 7 and 4 months, respectively. In mice fed the oxidized-cholesterol diets, the levels of oxidized cholesterol in sera were increased. At the end of the experiment, aortas were removed and atherosclerosis was assessed. We found that in LDLR-deficient mice, feeding of an oxidized-cholesterol diet resulted in a 32% increase in fatty streak lesions (15.93+/-1.59% versus 21.00+/-1.38%, P<0.03). Similarly, in apo E-deficient mice, feeding of an oxidized-cholesterol diet increased fatty streak lesions by 38% (15.01+/-0.92% versus 20. 70+/-0.86%, P<0.001). The results of the current study thus demonstrate that oxidized cholesterol in the diet accelerates fatty streak lesion formation in both LDLR- and apo E-deficient mice. |
| Oxidized cholesterol in the diet is a source of oxidized lipoproteins in human serum Staprans, I., X. M. Pan, et al. (2003), J Lipid Res 44(4): 705-15. Abstract: The aim of this study was to determine in humans whether oxidized cholesterol in the diet is absorbed and contributes to the pool of oxidized lipids in circulating lipoproteins. When a meal containing 400 mg cholestan-5alpha,6alpha-epoxy-3beta-ol (alpha-epoxy cholesterol) was fed to six controls and three subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol in serum was found in chylomicron/chylomicron remnants (CM/RM) and endogenous (VLDL, LDL, and HDL) lipoproteins. In controls, alpha-epoxy cholesterol in CM/RM was decreased by 10 h, whereas in endogenous lipoproteins it remained in the circulation for 72 h. In subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol was mainly in CM/RM. In vitro incubation of the CM/RM fraction containing alpha-epoxy cholesterol with human LDL and HDL that did not contain alpha-epoxy cholesterol resulted in a rapid transfer of oxidized cholesterol from CM/RM to both LDL and HDL. In contrast, no transfer was observed when human serum was substituted with rat serum, suggesting that cholesteryl ester transfer protein is mediating the transfer. Thus, alpha-epoxy cholesterol in the diet is incorporated into the CM/RM fraction and then transferred to LDL and HDL, contributing to lipoprotein oxidation. Moreover, LDL containing alpha-epoxy cholesterol displayed increased susceptibility to further copper oxidation in vitro. It is possible that oxidized cholesterol in the diet accelerates atherosclerosis by increasing oxidized cholesterol levels in circulating LDL and chylomicron remnants. |
| Oxidized cholesterol modulates age-related change in lipid metabolism in rats Osada, K., T. Kodama, et al. (1995), Lipids 30(5): 405-13. Abstract: For three weeks, male Sprague-Dawley rats at either four weeks (young) or eight months (adult) of age were pair-fed one of the purified diets free of or containing either 0.2% of oxidized cholesterol mixture (cholesterol oxidation products) or 0.2% of cholesterol. Although the food intake was similar, dietary oxidized cholesterol lowered body weight gain in young rats, but did not increase relative liver weight, in contrast to the enlargement seen with dietary cholesterol. Oxidized cholesterol, compared to cholesterol, tended to reduce the activity of hepatic 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol 7 alpha-hydroxylase, particularly the latter in aged rats, and prevented the rise in the concentration of liver cholesterol at both ages. It also tended to increase the activity of hepatic delta 6 desaturase, particularly in young rats. Moreover, oxidized cholesterol in relation to cholesterol influenced liver and serum lipid concentrations in different ways, and increased lipid peroxidation at both ages. The ratio of splenic CD4+/CD8+ T-lymphocytes increased with age, but the influence of cholesterol and oxidized cholesterol was comparable. Thus, oxidized cholesterol may specifically disturb growth and age-related changes in the lipid metabolism in rats. |
| Oxidized fatty acids promote atherosclerosis only in the presence of dietary cholesterol in low-density lipoprotein receptor knockout mice Khan-Merchant, N., M. Penumetcha, et al. (2002), J Nutr 132(11): 3256-62. Abstract: Studies suggest that heated oils contribute to the presence of oxidized components in the circulating lipoproteins and to the development of atherosclerosis in animals. We evaluated the effects of 11-13 wk of consumption of a well defined dietary oxidized fatty acid, 13-hydroxylinoleic acid (13-HODE) (8 mg), on atherosclerotic lesion development and plasma cholesterol concentrations in mice fed diets varying in fat and cholesterol contents. LDL receptor knockout mice were used in two feeding studies. In study 1, oxidized fatty acid consumption in association with a high fat diet increased aortic lesion areas by >100% (P < 0.05). Surprisingly, oxidized fatty acid intake also tended to increase plasma total cholesterol (P = 0.12) and LDL cholesterol (P < 0.05) as well as oxidative stress as measured by higher levels of autoantibodies to oxidatively modified proteins (P = 0.008). However, in mice fed a nonpurified diet, oxidized fatty acids were not atherogenic and may even have been beneficial, as indicated by a lower plasma triglyceride (TG) concentration (P < 0.05). In study 2, mice were fed either a high fat, medium fat or low fat diet to evaluate whether the increase in aortic lesions due to oxidized fatty acid consumption in study 1 was a result of the associated higher plasma total and LDL cholesterol concentrations. In study 2, 13-HODE-treated mice in the medium and low fat diet groups but not those fed the high fat diet had larger atherosclerotic lesions (P < 0.05). Additionally, plasma total and LDL cholesterol as well as TG were not affected by HODE treatment. However, the total cholesterol:HDL cholesterol ratio was higher in treated mice (P < 0.05) and HDL cholesterol was lower in HODE-treated mice that were fed the low fat diet (P < 0.05). Our results suggest that, in mice fed cholesterol, oxidized fatty acids may be atherogenic, both in terms of increased oxidative stress (as seen in study 1) and by increasing the atherogenicity of the plasma cholesterol profile. |
| Oxidized LDL activates phospholipase A2 to supply fatty acids required for cholesterol esterification Akiba, S., Y. Yoneda, et al. (2003), J Lipid Res 44(9): 1676-85. Abstract: We examined the roles of phospholipase A2 (PLA2) in oxidized LDL (oxLDL)-induced cholesteryl ester formation in macrophages. In 3Holeic acid-labeled RAW264.7 cells and mouse peritoneal macrophages, oxLDL induced 3Hcholesteryl oleate formation with an increase in free 3Holeic acid and a decrease in 3Hphosphatidylcholine. The changes in these lipids were suppressed by methyl arachidonyl fluorophosphonate (MAFP), a cytosolic PLA2 (cPLA2) inhibitor. However, MAFP had no effect on the ACAT activity or the binding and/or uptake of oxLDL. Stimulation with oxLDL in the presence of 3Hcholesterol increased 3Hcholesteryl ester bearing fatty acyl chains derived from cellular and/or exogenous (oxLDL) lipids. The formation of cholesteryl ester under this condition was also inhibited by MAFP, and the inhibitory effect was reversed by adding oleic acid. While oxLDL did not affect the activity or amounts of cPLA2, preincubation with oxLDL enhanced the release of oleic acid and arachidonic acid induced by ionomycin in RAW264.7 cells. 13(S)-hydroxyoctadecadienoic acid, but not 7-ketocholesterol, also enhanced ionomycin-induced oleic acid release. These results suggest that oxLDL induces cPLA2 activation, which contributes, at least in part, to the supply of fatty acids required for the cholesteryl esterification, probably through the acceleration by oxidized lipids of the catalytic action of cPLA2 in macrophages. |
| Oxidized LDL increase free cholesterol and fail to stimulate cholesterol esterification in murine macrophages Roma, P., A. L. Catapano, et al. (1990), Biochem Biophys Res Commun 171(1): 123-31. Abstract: Oxidatively modified low density lipoproteins (Ox-LDL) may be involved in determining the formation of foam cells by inducing cellular cholesteryl ester accumulation. We studied the effect of copper oxidized LDL (Ox-LDL) on cholesterol accumulation and esterification in murine macrophages. Ox-LDL (44 micrograms/ml of lipoprotein cholesterol) increased the total cholesterol content of the cells from 29 to 69 micrograms/mg cell protein. Free cholesterol accounted for 85% of this increase. Acetyl LDL (Ac-LDL) (38 micrograms/ml of lipoprotein cholesterol), raised total cellular cholesterol content to a similar extent (76 micrograms/mg cell protein), however only 25% of the accumulated cholesterol was unesterified. When ACAT activity was determined after incubation of J774 cell with Ox- or Ac-LDL, Ox-LDL were 12 times less effective than Ac-LDL in stimulating cholesteryl ester formation. This was not due to an inhibition of ACAT by Ox-LDL since these lipoproteins failed to inhibit pre activated enzyme in cholesteryl ester-loaded macrophages. The uptake of 125I-Ox-LDL: was 175% that of 125I-Ac-LDL, while degradation was only 20%. All together these data suggest an altered intracellular processing of Ox-LDL, which may be responsible for free cholesterol accumulation. |
| Oxidized lipids in the diet accelerate the development of fatty streaks in cholesterol-fed rabbits Staprans, I., J. H. Rapp, et al. (1996), Arterioscler Thromb Vasc Biol 16(4): 533-8. Abstract: Studies have indicated that oxidized lipoproteins may play a role in atherosclerosis. We have recently demonstrated that the levels of oxidized lipoproteins in the circulation can be directly correlated to the quantity of oxidized lipids in the diet. The present study tested the hypothesis that dietary oxidized lipids accelerate the development of atherosclerosis. For 12 to 14 weeks, 36 male New Zealand White rabbits were fed a low-cholesterol (0.25%) diet containing either 5% unoxidized corn oil (control diet) or 5% oxidized corn oil (oxidized-lipid diet). Serum cholesterol levels increased to a similar extent in both groups, with the majority of the cholesterol in the beta-migrating very low density lipoprotein (beta-VLDL) fraction. Beta-VLDL from control animals contained 3.86+/- 0.57 versus 9.07 +/- 2.14 nmol conjugated dienes per micromol cholesterol (P<.05) in rabbits fed the oxidized-lipid diet. No difference in oxidized lipid levels was detected in LDL. Most important, feeding a diet rich in oxidized-lipid resulted in a 100% increase in fatty streak lesions in the aorta. Additionally, rabbits that were fed the oxidized-lipid++ diet had a >100% increase in total cholesterol in the pulmonary artery that was primarily due to an increase in cholesteryl ester. Oxidized lipids are frequently present in the typical US diet, and our results suggest that consumption of these foods may be an important risk factor for atherosclerosis. |
| Oxidized low density lipoprotein leads to macrophage accumulation of unesterified cholesterol as a result of lysosomal trapping of the lipoprotein hydrolyzed cholesteryl ester Maor, I. and M. Aviram (1994), J Lipid Res 35(5): 803-19. Abstract: The early atherosclerotic lesion is comprised of foam cell macrophages filled with cholesteryl ester (CE), unesterified cholesterol (UC), and cholesterol oxides. Upon incubation of macrophages with oxidized low density lipoprotein (Ox-LDL), they accumulate UC rather than CE, which was shown to accumulate after incubation of cells with acetylated LDL (Ac-LDL). Using lipoproteins that were doubly labeled in their CE as well as in their protein moieties, we have demonstrated that lysosomal hydrolysis of the Ox-LDL CE was similar to the hydrolysis of the CE in Ac-LDL or native LDL whereas, as shown previously, a markedly impaired degradation of the protein moiety of Ox-LDL was observed. Cell fractionation revealed that the UC was derived from the hydrolyzed CE in Ox-LDL and was trapped in the macrophage lysosomal fraction, whereas in cells incubated with Ac-LDL, the lipoprotein UC was rapidly transported to the microsomal and cytosolic compartments. Lysosomal accumulation of Ox-LDL-derived UC could be related to the effect of the oxysterols in Ox-LDL, as oxidation of Ac-LDL or incubation of macrophages with Ac-LDL in the presence of oxysterols, in comparison to cell incubation with Ac-LDL, resulted in lysosomal accumulation of unesterified cholesterol. As a consequence of lysosomal trapping of Ox-LDL-derived UC, its availability to esterification was markedly impaired (by 6-fold), in comparison to the cholesterol esterification rate of Ac-LDL-derived UC. However, when the cholesterol esterification was expressed per lysosomal released UC, cellular cholesterol esterification rate of Ox-LDL-derived UC was found to be similar to that of Ac-LDL-derived UC. High density lipoprotein (HDL)-mediated efflux of the Ox-LDL-derived cholesterol from macrophages was similar to that found for Ac-LDL-derived cholesterol after 24 h of cell incubation with HDL3. Major defects in the cellular metabolism of Ox-LDL-derived 7-ketocholesterol were also found and could be related to its lysosomal trapping (together with the UC), its limited capacity to be esterified, and a 40% reduction in its HDL-mediated efflux from macrophages, in comparison to the efflux of the Ox-LDL-derived UC. We conclude that upon incubation of macrophages with Ox-LDL, lysosomal hydrolysis of the lipoprotein CE is not impaired but cellular accumulation of the Ox-LDL-derived UC occurs as a result of trapping of the hydrolyzed CE in the macrophage lysosomal compartment which may be related to the effect of oxysterols in Ox-LDL. |
| Oxidized low-density lipoprotein and high-density lipoprotein cholesterol modulate coronary arterial remodeling: an intravascular ultrasound study Yoneyama, S., K. Arakawa, et al. (2003), Clin Cardiol 26(1): 31-5. Abstract: BACKGROUND: Oxidized low-density lipoprotein (oxLDL) not only plays an important role in plaque formation, but also impairs the endothelium-dependent relaxation. Constrictive remodeling rather than intimal hyperplasia mainly contributes to restenosis after balloon angioplasty. Probucol (powerful antioxidant) reduced restenosis rate by improving constrictive remodeling. Thus, oxLDL may modulate coronary arterial remodeling. HYPOTHESIS: The study was designed for using intravascular ultrasound to test the hypothesis that arterial constrictive remodeling (CR) was associated with oxLDL in patients with coronary artery disease. METHODS: Intravascular ultrasound was performed in 36 patients with de novo atherosclerotic coronary. Remodeling was defined and evaluated as follows: remodeling index (RI) = lesion vessel area (VA)/(proximal reference VA + distal reference VA)/2. Constrictive remodeling (CR) was defined as remodeling index (RI) < 0.9. Neutral and expansive remodeling (NER) was defined as RI > or = 0.9. The level of plasma ox-LDL was measured by sandwich ELISA using the monoclonal antibody (DLH3)-recognized oxidatively modified lipoproteins and the antihuman apoprotein B monoclonal antibody. RESULTS: Neutral and expansive remodeling was found in 24 lesions, and CR in 12 lesions. Remodeling index was significantly lower in the CR group than in the NER group (0.8 +/- 0.1 vs. 1.0 +/- 0.1, p < 0.001). The level of oxLDL in the CR group was significantly higher than that in the NER group (24.0 +/- 12.1 vs. 16.4 +/- 6.2 U/ml, p < 0.05). The level of high-density lipoprotein-cholesterol (HDL-C) in the CR group was significantly lower than that in the NER group (40.5 +/- 4.8 vs. 46.2 +/- 10.6 mg/ml, p < 0.05). There was a statistically significant correlation between the value of HDL-C/ ox-LDL and the RI (r = -0.48, p < 0.005). CONCLUSIONS: Oxidized LDL and HDL-C were associated with arterial remodeling in de novo atherosclerotic lesions. |
| Oxolanosterol oximes: dual-action inhibitors of cholesterol biosynthesis Frye, L. L., K. P. Cusack, et al. (1994), J Lipid Res 35(8): 1333-44. Abstract: A series of oxolanosterol oximes and oxime ethers have been prepared as potential dual-action inhibitors of cholesterol biosynthesis. The synthesis of these oximes along with the evaluation of their ability to inhibit lanosterol 14 alpha-methyl demethylase (P450DM) and to suppress 3-hydroxy-3-methylglutaryl coenzyme. A reductase (HMGR) activity is presented. 3 beta-Hydroxylanost-7-en-15-one 15-oxime XIX was found to be an effective inhibitor of P450DM in rat liver microsomal preparations. In 14Cacetate incorporation studies using Chinese hamster ovary (CHO) cells, compound XIX was found to cause a dramatic reduction in the incorporation of acetate into C27 sterols with a concomitant increase in radiolabeled C30 sterols which is consistent with the inhibition of P450DM. In addition, 15-oxime XIX was shown to suppress HMGR activity in both wild-type CHO and P450DM-deficient (AR45) cells, indicating that suppression of HMGR is independent of any effects of this oxime on P450DM. In both cell lines, parallel declines in HMGR activity and HMGR protein levels were observed suggesting that compound XIX suppresses HMGR activity by regulation of gene expression. These results demonstrate that, as predicted, 15-oxime XIX is indeed a dual-action inhibitor of cholesterol biosynthesis which causes both the inhibition of P450DM and a reduction in HMGR activity. |
| Oxygen access to the active site of cholesterol oxidase through a narrow channel is gated by an Arg-Glu pair Coulombe, R., K. Q. Yue, et al. (2001), J Biol Chem 276(32): 30435-41. Abstract: Cholesterol oxidase is a monomeric flavoenzyme that catalyzes the oxidation and isomerization of cholesterol to cholest-4-en-3-one. Two forms of the enzyme are known, one containing the cofactor non-covalently bound to the protein and one in which the cofactor is covalently linked to a histidine residue. The x-ray structure of the enzyme from Brevibacterium sterolicum containing covalently bound FAD has been determined and refined to 1.7-A resolution. The active site consists of a cavity sealed off from the exterior of the protein. A model for the steroid substrate, cholesterol, can be positioned in the pocket revealing the structural factors that result in different substrate binding affinities between the two known forms of the enzyme. The structure suggests that Glu(475), located at the active site cavity, may act as the base for both the oxidation and the isomerization steps of the catalytic reaction. A water-filled channel extending toward the flavin moiety, inside the substrate-binding cavity, may act as the entry point for molecular oxygen for the oxidative half-reaction. An arginine and a glutamate residue at the active site, found in two conformations are proposed to control oxygen access to the cavity from the channel. These concerted side chain movements provide an explanation for the biphasic mode of reaction with dioxygen and the ping-pong kinetic mechanism exhibited by the enzyme. |
| Oxygenation of desmosterol and cholesterol in cell cultures Saucier, S. E., A. A. Kandutsch, et al. (1990), J Lipid Res 31(12): 2179-85. Abstract: In order to determine whether hydration of the delta 24 bond of desmosterol contributes to the formation of the regulatory oxysterol, 25-hydroxycholesterol, 3Hdesmosterol was incubated with two cultured cell lines and the labeled products were analyzed. Small amounts of 25-hydroxycholesterol were formed with Chinese hamster lung (Dede) cell cultures, but not with mouse fibroblast (L) cell cultures. Apparently, desmosterol was converted into cholesterol, a process that does not occur in L cells, before 25-hydroxycholesterol takes place. No reliable evidence could be obtained for hydration of the delta 24 bond or for the reverse reaction upon incubation of 3H25-hydroxycholesterol. Oxygenation of desmosterol occurred in both Dede and L cell cultures to give a mixture of 24(R)- and 24(S)-25-epoxy-cholesterol. This reaction, along with the production of 7-oxygenated sterols, may account for low levels of HMG-CoA reductase repressor activity previously found to be associated with delta 24 sterols. |
| Oxylanosterols as modifiers of cholesterol biosynthesis Trzaskos, J. M. (1995), Prog Lipid Res 34(2): 99-116. Abstract: In this review, the thinking and strategy that lead to the design of mechanism-based inhibitors of cholesterol biosynthesis have been recounted. This work began with a purely biochemical perspective on the mechanism of lanosterol demethylation. The final efforts focused on pharmacology and drug design thus bringing the basic science effort to a practical application. Most recently, a series of 15-oxalanosterols, which act as pure suppressors of HMG-CoA reductase lacking lanosterol demethylase inhibition properties has been identified. These molecules also lower serum cholesterol and show promise as potential agents for clinical evaluation. The utility of these compounds and validation of our hypothesis will have to await further testing. |
| Oxysterols and apoptosis: evidence for gene regulation outside the cholesterol pathway Thompson, E. B. and S. Ayala-Torres (1999), Crit Rev Biochem Mol Biol 34(1): 25-32. |
| Oxysterols, but not cholesterol, inhibit human immunodeficiency virus replication in vitro Moog, C., A. M. Aubertin, et al. (1998), Antivir Chem Chemother 9(6): 491-6. Abstract: Oxysterols, oxygenated derivatives of cholesterol selected for their cytostatic activity and their inhibitory effect on cholesterol synthesis, have been investigated for their anti-human immunodeficiency virus (HIV) activity in vitro. The three oxysterols tested, 7 beta-hydroxycholesterol (7 beta-OHC), 25-hydroxycholesterol (25-OHC) and 7 beta, 25-dihydroxycholesterol (7,25-OHC), inhibit viral replication at micromolar concentrations. The selectivity indexes for 7 beta-OHC and 25-OHC are quite modest (2 to 8) but reproducible; the dihydroxycholesterol 7,25-OHC exhibited antiviral properties at concentrations 13- to 25-fold lower than the highest concentration tested at which no toxicity was measurable. Oxysterols are naturally occurring compounds, and we speculate on their physiological relevance in HIV-infected individuals. |
| Oxysterols, cholesterol biosynthesis, and vascular endothelial cell monolayer barrier function Boissonneault, G. A., B. Hennig, et al. (1991), Proc Soc Exp Biol Med 196(3): 338-43. Abstract: A spectrum of cholesterol oxidation derivatives (oxysterols) is generated in food products exposed to heat or radiation in the presence of oxygen. One of these derivatives (cholestan-3 beta,5 alpha,6 beta-triol) was shown to compromise the selective barrier function of cultured vascular endothelial cell monolayers, an action that may initiate atherosclerotic lesion formation. This study sought to investigate the relationship of cholesterol synthesis inhibition by several naturally occurring oxysterols to depression of vascular endothelial cell monolayer barrier function, determined as an increase in albumin transfer across cultured endothelial monolayers. All oxysterols tested caused a variable time- and dose-dependent elevation in trans-endothelial albumin transfer, and they were also able to inhibit cholesterol biosynthesis to varying degrees. Pure cholesterol was without effect on both counts. The correlation between the increase in albumin transfer related to oxysterol exposure and the ability of oxysterols to suppress cholesterol biosynthesis was, however, poor. Moreover, mevinolin, a water-soluble competitive inhibitor of cholesterol synthesis, reduced the rate of cholesterol synthesis to 0.9% of control but did not significantly increase albumin transfer. Cholestan-3 beta,5 alpha,6 beta-triol caused a 660% elevation in albumin transfer while cholesterol synthesis remained at 11% of control. We conclude that changes in endothelial barrier function caused by exposure to the oxysterols examined, but not pure cholesterol, are probably related to factors other than the well-known action of cholesterol biosynthesis inhibition. These findings may have implications in the development of atherosclerosis. |