Failure of vitamin E to protect cultured human arterial smooth muscle cells against oxysterol-induced cytotoxicity

Chemical Compositional Changes in Over-Oxidized Fish Oils

A recent study has reported that the administration during gestation of a highly rancid hoki liver oil, obtained by oxidation through sustained exposure to oxygen gas and incident light for 30 days, causes newborn mortality in rats. This effect was attributed to lipid hydroperoxides formed in the omega-3 long-chain polyunsaturated fatty acid-rich oil, while other chemical changes in the damaged oil were overlooked. In the present study, the oxidation condition employed to damage the hoki liver oil was replicated, and the extreme rancidity was confirmed. A detailed analysis of temporal chemical changes resulting from the sustained oxidative challenge involved measures of eicosapentaenoic acid/docosahexaenoic acid (EPA/DHA) omega-3 oil oxidative quality (peroxide value, para-anisidine value, total oxidation number, acid value, oligomers, antioxidant content, and induction time) as well as changes in fatty acid content, volatiles, isoprostanoids, and oxysterols. The chemical description was extended to refined anchovy oil, which is a more representative ingredient oil used in omega-3 finished products. The present study also analyzed the effects of a different oxidation method involving thermal exposure in the dark in contact with air, which is an oxidation condition that is more relevant to retail products. The two oils had different susceptibility to the oxidation conditions, resulting in distinct chemical oxidation signatures that were determined primarily by antioxidant protection as well as specific methodological aspects of the applied oxidative conditions. Unique isoprostanoids and oxysterols were formed in the over-oxidized fish oils, which are discussed in light of their potential biological activities.

The oxysterol 27-hydroxycholesterol increases β-amyloid and oxidative stress in retinal pigment epithelial cells

Background

Alzheimer's disease (AD) and age-related macular degeneration (AMD) share several pathological features including β-amyloid (Aβ) peptide accumulation, oxidative damage, and cell death. The causes of AD and AMD are not known but several studies suggest disturbances in cholesterol metabolism as a culprit of these diseases. We have recently shown that the cholesterol oxidation metabolite 27-hydroxycholesterol (27-OHC) causes AD-like pathology in human neuroblastoma SH-SY5Y cells and in organotypic hippocampal slices. However, the extent to which and the mechanisms by which 27-OHC may also cause pathological hallmarks related to AMD are ill-defined. In this study, the effects of 27-OHC on AMD-related pathology were determined in ARPE-19 cells. These cells have structural and functional properties relevant to retinal pigmented epithelial cells, a target in the course of AMD.

Methods

ARPE-19 cells were treated with 0, 10 or 25 μM 27-OHC for 24 hours. Levels of Aβ peptide, mitochondrial and endoplasmic reticulum (ER) stress markers, Ca²⁺ homeostasis, glutathione depletion, reactive oxygen species (ROS) generation, inflammation and cell death were assessed using ELISA, Western blot, immunocytochemistry, and specific assays.

Results

27-OHC dose-dependently increased Aβ peptide production, increased levels of ER stress specific markers caspase 12 and gadd153 (also called CHOP), reduced mitochondrial membrane potential, triggered Ca²⁺ dyshomeostasis, increased levels of the nuclear factor κB (NFκB) and heme-oxygenase 1 (HO-1), two proteins activated by oxidative stress. Additionally, 27-OHC caused glutathione depletion, ROS generation, inflammation and apoptotic-mediated cell death.

Conclusions

The cholesterol metabolite 27-OHC is toxic to RPE cells. The deleterious effects of this oxysterol ranged from Aβ accumulation to oxidative cell damage. Our results suggest that high levels of 27-OHC may represent a common pathogenic factor for both AMD and AD.

Oxysterols in heart failure

Oxysterols are biologically active molecules that result from the oxidation of cholesterol. Several oxysterols are found in macrophages and macrophage-derived 'foam cells' in atherosclerotic tissue. Lipophilic oxysterols penetrate cell membranes and, therefore, their concentrations can reach harmful levels in endothelial and smooth muscle cells located in close proximity to the atherosclerotic plaques or inflammatory zones. New findings suggest that the effects of oxysterols on cardiomyocytes can lead to cell hypertrophy and death. This may make oxysterols one of the major factors precipitating morbidity in atherosclerosis-induced cardiac diseases and inflammation-induced heart complications. The pathological actions of oxysterols on muscle cells were shown to depend on dysfunctional Ca(2+) signaling; however, the mechanisms of the effects remain to be elucidated. Understanding the effects of oxysterols could lead to therapies that modulate malfunction of cardiomyocytes. This review discusses the experimental findings and the relevance of oxysterols to heart failure, and suggests strategies for important future investigations.

Effects of apigenin, lycopene and astaxanthin on 7β-hydroxycholesterol-induced apoptosis and Akt phosphorylation in U937 cells

Oxysterols arise from the enzymic or non-enzymic oxidation of cholesterol and have been shown to be cytotoxic to certain cell lines. In particular, apoptosis induced by the oxysterol 7β-hydroxycholesterol (7β-OH) has been associated with the generation of oxidative stress, cytochrome c release and caspase activation. Due to the fundamental importance of apoptosis in pathological processes, the identification of substances capable of modulating this form of cell death is now actively researched. The objective of the present study was to investigate if apigenin, lycopene and astaxanthin could inhibit 7β-OH-induced apoptosis in U937 cells. Pretreatment with 0·1 μm-astaxanthin protected against apoptosis, while lycopene did not oppose the adverse effects of 7β-OH. At low concentrations, apigenin did not protect against oxysterol-induced apoptosis; however, at higher concentrations it intensified cell death. Additionally, we investigated the effect of 7β-OH, apigenin and astaxanthin on the activation of the serine threonine kinase Akt (phosphorylated Akt:Akt ratio) to determine whether the effect on cell viability and growth was linked to the Akt signalling pathway. Akt activation was decreased in the oxysterol-treated cells compared with control cells; however, this did not attain significance. Interestingly, activation of Akt was significantly reduced compared with control cells following incubation with apigenin and astaxanthin both in the absence and in the presence of 7β-OH. Our data suggest that apigenin, lycopene and astaxanthin failed to protect against 7β-OH-induced apoptosis, and the decrease in cell viability and the increase in apoptotic nuclei induced by the antioxidants appear to be associated with down regulation of Akt activity.

27-Hydroxycholesterol causes remodeling in endothelial cell membrane lipid composition comparable to remodeling in the failed vein grafts of CABG patients

Our objective is to determine if vascular remodeling in CABG patients is related to oxysterols, therefore, we compared failed vein grafts from 18 patients, available after a second coronary artery bypass grafting (CABG), with human endothelial cells (ECs). The ECs were cultured in minimum essential medium (MEM) with or without 27-hydroxycholesterol (27OHC), one of the oxysterol products of oxidatively modified low density lipoproteins (ox-LDL), as an agent to alter molecular mechanisms in vascular cells. Significant changes in phospholipid composition, in fatty acid profile and in calcium concentration were found in the failed vein compared to the native saphenous vein from the same (CABG) patient. The failed vein contained significantly less phosphatidylethanolamine, more sphingomyelin, less arachidonic acid, more linoleic acid and more calcium than the native saphenous vein. Comparable changes in phospholipid composition, in fatty acid profile and increased calcium influx were reproduced in ECs cultured in medium containing 27OHC indicating that an oxysterol is an agent that can alter the lipid composition of vascular cell membranes. Our study indicates that a lipid agent, as well as protein agents that have previously been linked to the process of vascular remodeling, may be fundamental to many vascular diseases.

Cholestane-3β, 5α, 6β-triol promotes vascular smooth muscle cells calcification

Oxysterols found in atherosclerotic plaque may be associated with vascular calcification. We investigated the effect of oxysterol cholestane-3beta, 5alpha, 6beta-triol (Triol) on in vitro calcification of rat vascular smooth muscle cells (VSMCs). In vitro calcification was induced by incubation of VSMCs with beta-glycerophosphate. Calcifying nodule formation, calcium deposition in extracellular matrix, and alkaline phosphatase (ALP) activity were measured as indices of calcification. Because apoptotic bodies can serve as nucleation sites for calcification, apoptosis of calcifying VSMCs was determined by Hoechst 33258 staining, TUNEL, and FITC-labeled annexin V/PI double staining. The calcium deposition and ALP activity in calcifying VSMCs were much higher than those in non-calcifying VSMCs. Triol increased calcifying nodule formation, calcium deposition, ALP activity, and apoptosis of nodular cells in calcifying VSMCs. As determined by 2,7-dichlorofluorescein fluorescence, Triol induced the generation of reactive oxygen species (ROS) in calcifying VSMCs dose- and time-dependently. Triol-induced increases in calcium deposition, ALP activity, apoptosis, and ROS generation were all attenuated by antioxidant vitamin C plus vitamin E (VC + VE). The results demonstrated that Triol promoted VSMCs calcification through direct increase of ALP activity and apoptosis, probably by ROS-related mechanism.

27-Hydroxycholesterol inhibits neutral sphingomyelinase in cultured human endothelial cells

To study the effect of 27-hydroxycholesterol (27OHC) on the catabolism of sphingomyelin, we cultured endothelial cells (ECs) from human umbilical veins with 27OHC, then measured activities of acid sphingomyelinase (ASMase) and neutral sphingomyelinase (NSMase) and sphingomyelin consumption by using [14C]sphingomyelin, and determined NSMase mRNA expressions by RT-PCR method. The results indicated that [14C]sphingomyelin accumulated in cells treated with 27OHC, and that the activities of both NSMase and ASMase were inhibited in ECs cultured with 27OHC. To further study the effect of 27OHC on NSMase, we used desipramine, an inhibitor of ASMase, to exclude the possible interference of ASMase's residual activity at neutral condition. Also, we observed the significant inhibition of NSMase activity by using glutathione, an inhibitor of NSMase, but found no further impact when 27OHC was added later. To determine whether the inhibition of NSMase activity was directly due to the effect of 27OHC, we exposed cell homogenate to 27OHC, and found no inhibitive effect of 27OHC on the activity of NSMase. All of our data confirmed that 27OHC had only an indirect inhibitive effect on NSMase. Our finding that no change of the NSMase mRNA expression by 27OHC indicated that the inhibitive effect of 27OHC on NSMase activity occurred at a post-transcriptional level. We suggest that an altered membrane fluidity caused by 27OHC could be involved in the inhibited activity of NSMase.

Toxicity of cholesterol oxidation products to Caco-2 and HepG2 cells: modulatory effects of alpha- and gamma-tocopherol

Cholesterol can be oxidized to form a variety of cholesterol oxidation products also known as oxysterols. The aims of the present study were to compare the cytotoxic effects of four oxysterols, namely 25-hydroxycholesterol (25-OHC), 7beta-hydroxycholesterol (7beta-OHC), cholesterol-5beta,6beta-epoxide (beta-epox) and cholesterol-5alpha,6alpha-epoxide (alpha-epox), in two human cell culture models. Further, the ability of 10 and 100 micro m alpha- and gamma-tocopherol (alpha-TOC and gamma-TOC, respectively) to protect against oxysterol-induced cytotoxicity was also assessed. Human colonic adenocarcinoma Caco-2 and human hepatoma HepG2 cells were supplemented with increasing concentrations of 25-OHC, 7beta-OHC, beta-epox and alpha-epox (0-25 micro g ml(-1)) for 24, 48 or 96 h. Following 24-h and 48-h exposure, test media were replaced with normal growth media and the cells were maintained for 72 and 48 h, respectively. The 96-h exposure represented a constant challenge to the cells. Cytotoxicity was assessed using the neutral red uptake assay. The concentration of compound that inhibited cell viability by 50% (ic(50) value) was calculated. All four oxysterols investigated induced the greatest cytotoxic effects following 96 h of exposure. 25-Hydroxycholesterol exhibited the greatest cytotoxicity in both cell lines. Both beta-epox and alpha-epox were more toxic to HepG2 cells than to Caco-2 cells after the 48-h exposure. Pretreatment of cells with either alpha- or gamma-TOC did not protect against oxysterol-induced cytotoxicity. The caco-2 cells treated with the high concentration (100 micro m) of gamma-TOC were found to be more susceptible to oxysterol-induced toxicity under the conditions employed in this study.

Effects of bile salts on cholestan-3beta,5alpha,6beta-triol-induced apoptosis in dog gallbladder epithelial cells

Oxysterols are cytotoxic agents. The gallbladder epithelium is exposed to high concentrations of oxysterols, and so elucidating the mechanisms of cytotoxicity in this organ may enhance our understanding of the pathogenesis of biliary tract disorders. We investigated the cytotoxic effects of the oxysterol cholestan-3beta,5alpha,6beta-triol (TriolC) on dog gallbladder epithelial cells. Apoptosis was the major form of cytotoxicity, as determined by analysis of nuclear morphologic changes and by multiparameter flow cytometry. Hydrophobic bile salts are known to have cytotoxic effects, whereas hydrophilic bile salts have cytoprotective effects. We therefore examined whether the hydrophobic bile acid taurodeoxycholic acid (TDC) and the hydrophilic bile acid tauroursodeoxycholic acid (TUDC) had modifying effects on oxysterol-induced cytotoxicity. TriolC caused an increase in the number of apoptotic cells from 14+/-11% (control) to 48+/-12% of total cells (P<0.01). After combining TriolC with TDC, cell apoptosis increased to 63+/-16% (P<0.05), whereas after addition of TUDC, the number of apoptotic cells decreased to 31+/-12% (P<0.05) of total cells. In summary, oxysterols such as TriolC induce apoptosis. Hydrophobic bile salts enhance TriolC-induced apoptosis, whereas hydrophilic bile salts diminish TriolC-induced apoptosis. These results suggest that interactions between oxysterols and bile salts play a role in the pathophysiology of biliary tract disorders.

Cholestan-3beta,5alpha,6beta-triol, but not 7-ketocholesterol, suppresses taurocholate-induced mucin secretion by cultured dog gallbladder epithelial cells

In order to investigate oxysterol-mediated effects on the biliary system, we studied the effects of cholestan-3beta,5alpha,6beta-triol (TriolC) and 7-ketocholesterol (7KC) on gallbladder epithelial cells. We compared their cell proliferation effects in cultured dog gallbladder epithelial cells (DGBE) to their effects in cultured human pulmonary artery endothelial cells (HPAE). Oxysterols inhibited cell proliferation in a dose-dependent fashion. Oxysterols inhibited cell growth to 50% of control at a higher dose for DGBE cells than for HPAE cells. TriolC was more cytotoxic than 7KC. We also investigated the effect of oxysterols on bile salt-induced mucin secretion by DGBE cells. TriolC suppressed mucin secretion by DGBE cells, whereas 7KC did not. These findings support the hypothesis that biliary oxysterols affect gallbladder mucosal function.

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.

7-Ketocholesterol

7-Ketocholesterol is a major oxidation product of cholesterol found in human atherosclerotic plaque and is more atherogenic than cholesterol in some animal studies. 7-Ketocholesterol can inhibit cholesterol 7 alpha-hydroxylase, the rate-limiting step in bile acid biosynthesis, as well as strongly inhibiting HMG-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. It has even been suggested that 7-ketocholesterol is formed enzymically as an endogenous regulator of cholesterol biosynthesis. However, when tested as a pharmacological cholesterol-lowering agent, inhibition of HMG-CoA reductase was rapidly overcome and the 7-ketocholesterol metabolised. In vitro, 7-ketocholesterol has wide-ranging and potent effects, most of which have the potential to contribute to atherosclerosis. For example, 7-ketocholesterol can be cytotoxic and can induce apoptosis in vascular cells. These effects, either individually or more likely, in combination, all implicate 7-ketocholesterol in the initiation and development of atherosclerosis, but further work is needed to establish whether or not its role is a direct causal one.

Arterial injury by cholesterol oxidation products causes endothelial dysfunction and arterial wall cholesterol accumulation

Cholesterol oxidation products (ChOx) have been reported to cause acute vascular injury in vivo; however, the pharmacokinetics of ChOx after administration and the mechanisms by which they cause chronic vascular injury are not well understood. To further study the pharmacokinetics and atherogenic properties of ChOx, New Zealand White rabbits were injected intravenously (70 mg per injection, 20 injections per animal) with a ChOx mixture having a composition similar to that found in vivo during a 70-day period. Total ChOx concentrations in plasma peaked almost immediately after a single injection, declined rapidly, and returned to preinjection levels in 2 hours. After multiple injections, the ChOx concentrations rose gradually to levels 2- to 3-fold above baseline levels, increasing mostly in the cholesteryl ester fraction of LDL and VLDL. Rabbit serum and the isolated LDL/VLDL fraction containing elevated ChOx concentrations were cytotoxic to V79 fibroblasts and rabbit aortic endothelial cells. At the time of killing, cholesterol levels in the aortas from ChOx-injected rabbits were significantly elevated despite the fact that plasma cholesterol levels remained in the normal range. In addition, aortas from the ChOx-injected rabbits retained more 125I-labeled horseradish peroxidase, measured 20 minutes after intravenous injection. Transmural concentration profiles across the arterial wall also showed increased horseradish peroxidase accumulation in the inner half of the media from the thoracic aorta in ChOx-injected rabbits. In conclusion, ChOx injection resulted in accumulation of circulating ChOx and induced increased vascular permeability and accumulation of lipids and macromolecules. This study reveals that even under normocholesterolemic conditions, ChOx can cause endothelial dysfunction, increased macromolecular permeability, and increased cholesterol accumulation, parameters believed to be involved in the development of early atherosclerotic lesions.

Modulation of cholestane-3 beta,5 alpha,6 beta-triol toxicity by butylated hydroxytoluene, alpha-tocopherol and beta-carotene in newborn rat kidney cells in vitro

Cholesterol oxidation products (COP) have been reported to influence vital cellular processes such as cell growth, cell proliferation, membrane function and de novo sterol biosynthesis. The objectives of the present study were: (1) to develop an in vitro model using newborn rat kidney (NRK) cells to investigate the actions of COP; (2) to investigate the effect of COP on cell viability, endogenous antioxidant enzymes activities, i.e. superoxide dismutase (EC 1.15.1.1; SOD) and catalase (EC 1.11.1.6; CAT), and the extent of lipid peroxidation in this model; (3) to determine whether the addition of 100-1000 nM-alpha-tocopherol, beta-carotene or butylated hydroxytoluene (BHT) could protect against COP-induced cytotoxicity. NRK cells were cultured in the presence of various concentrations (5-50 microM) of cholesterol or cholestane-3 beta,5 alpha,6 beta-triol (cholestantriol) for a period of 24 h. Cholesterol over the range 5-50 microM did not induce cytotoxicity as indicated by the neutral-red-uptake assay or the lactate dehydrogenase (EC 1.1.1.27)-release assay. However, cell viability was compromised by the addition of > 10 microM-cholestantriol (P < 0.05). The addition of beta-carotene (100-1000 nM) did not increase cell viability significantly in cholestantriol-supplemented cells. However, the addition of alpha-tocopherol (1000 nM) and BHT (1000 nM) significantly increased percentage cell viability above that of the cholestantriol-supplemented cells but not back to control levels. SOD and CAT activities in NRK cells significantly decreased (P < 0.05) following incubation with cholestantriol. The addition of > 750 nM-alpha-tocopherol, beta-carotene or BHT returned SOD and CAT activities to that of the control. Lipid peroxidation was significantly induced (P < 0.05) in the presence of cholestantriol. Supplementation of the cells with alpha-tocopherol (250, 500 or 1000 nM) or BHT (750 or 1000 nM) resulted in a reduction in the extent of lipid peroxidation (P < 0.05). The addition of beta-carotene over the concentration range of 250-1000 nM did not reduce lipid peroxidation significantly compared with cells exposed to cholestantriol alone. These findings suggest that addition of exogenous antioxidants may be beneficial in the prevention of COP-induced toxicity in vitro.