Oxysterols in biological systems: sources, metabolism and pathophysiological relevance

Redox control of endoplasmic reticulum function

The lumen of the endoplasmic reticulum constitutes a separate intracellular compartment with a special proteome and metabolome. The redox conditions of the organelle are also characteristically different from those of the other subcellular compartments. The luminal environment has been considered more oxidizing than the cytosol due to the presence of oxidative protein folding. However, recent observations suggest that redox systems in reduced and oxidized states are present simultaneously. The concerted action of membrane transporters and oxidoreductase enzymes maintains the oxidized state of the thiol-disulfide and the reduced state of the pyridine nucleotide redox systems, which are prerequisites for the normal redox reactions localized in the organelle. The powerful thiol-oxidizing machinery of oxidative protein folding continuously challenges the local antioxidant defense. Alterations of the luminal redox conditions, either in oxidizing or reducing direction, affect protein processing, are sensed by the accumulation of misfolded/unfolded proteins, and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce programmed cell death if these attempts fail. Recent findings strongly support the involvement of redox-based endoplasmic reticulum stress in a plethora of human diseases, either as causative agents or as complications.

Cholesterol oxidation in the retina: implications of 7KCh formation in chronic inflammation and age-related macular degeneration

This review will discuss the formation and potential implications of 7-ketocholesterol (7KCh) in the retina. 7KCh is a proinflammatory oxysterol known to be present in high amounts in oxidized LDL deposits associated with atheromatous plaques. 7KCh is generated in situ in these lipoprotein deposits where it can accumulate and reach very high concentrations. In normal primate retina, 7KCh has been found associated with lipoprotein deposits in the choriocapillaris, Bruch's membrane, and the retinal pigment epithelium (RPE). In photodamaged rats, 7KCh has been found in the neural retina in areas of high mitochondrial content, ganglion cells, photoreceptor inner segments and synapses, and the RPE. Intermediates found by LCMS indicate 7KCh is formed via a free radical-mediated mechanism catalyzed by iron. 7KCh seems to activate several kinase signaling pathways that work via nuclear factor κB and cause the induction of vascular endothelial growth factor, interleukin (IL)-6, and IL-8. There seems to be little evidence of 7KCh metabolism in the retina, although some form of efflux mechanism may be active. The chronic mode of formation and the potent inflammatory properties of 7KCh indicate it may be an "age-related" risk factor in aging diseases such as atherosclerosis, Alzheimer's, and age-related macular degeneration.

7-ketocholesterol-induced inflammation: involvement of multiple kinase signaling pathways via NFκB but independently of reactive oxygen species formation

PURPOSE

7-Ketocholesterol (7KCh) accumulates in oxidized lipoprotein deposits and is known to be involved in macrophage foam cell formation and atherosclerosis. 7-KCh is present in the primate retina and is associated with oxidized lipoprotein deposits located in the choriocapillaris, Bruch's membrane, and retinal pigment epithelium (RPE). 7-KCh can also be formed in the retina as a consequence of light-induced iron release. The purpose of this study was to examine the signaling pathways involved in the 7KCh-mediated inflammatory response focusing on three cytokines, VEGF, IL-6, and IL-8.

METHODS

ARPE-19 cells were treated with 7KCh solubilized in hydroxypropyl-β-cyclodextrin. Cytokines were quantified by qRT-PCR (mRNA) and ELISA (protein) using commercially available products. NFκB activation was determined by IκBα mRNA induction.

RESULTS

Treatment of ARPE-19 cells with 15 μM 7KCh markedly induced the expression of VEGF, IL-6, and IL-8. No increase in NOX-4 expression or ROS formation was detected. 7KCh induced the phosphorylation of ERK1/2 and p38MAPK, and inhibitors to these kinases markedly reduced the cytokine expression but did not affect the IκBα mRNA expression. By contrast, inhibition of PI3K and PKCζ significantly decreased the cytokine and IκBα mRNA expression. Inhibition of the IκB kinase complex essentially ablated all cytokine induction.

CONCLUSIONS

7KCh induces cytokines via three kinase signaling pathways, AKT-PKCζ-NFκB, p38 MAPK, and ERK. The MAPK/ERK pathways seem to preferentially enhance cytokine induction downstream from NFκB activation. The results of this study suggest that 7KCh activates these pathways through interactions in the plasma membrane, but the mechanism(s) remains unknown.

Iron nanoparticles increase 7-ketocholesterol-induced cell death, inflammation, and oxidation on murine cardiac HL1-NB cells

OBJECTIVE

To evaluate the cytotoxicity of iron nanoparticles on cardiac cells and to determine whether they can modulate the biological activity of 7-ketocholesterol (7KC) involved in the development of cardiovascular diseases. Nanoparticles of iron labeled with Texas Red are introduced in cultures of nonbeating mouse cardiac cells (HL1-NB) with or without 7-ketocholesterol 7KC, and their ability to induce cell death, pro-inflammatory and oxidative effects are analyzed simultaneously.

STUDY DESIGN

Flow cytometry (FCM), confocal laser scanning microscopy (CLSM), and subsequent factor analysis image processing (FAMIS) are used to characterize the action of iron nanoparticles and to define their cytotoxicity which is evaluated by enhanced permeability to SYTOX Green, and release of lactate deshydrogenase (LDH). Pro-inflammatory effects are estimated by ELISA in order to quantify IL-8 and MCP-1 secretions. Pro-oxidative effects are measured with hydroethydine (HE).

RESULTS

Iron Texas Red nanoparticles accumulate at the cytoplasmic membrane level. They induce a slight LDH release, and have no inflammatory or oxidative effects. However, they enhance the cytotoxic, pro-inflammatory and oxidative effects of 7KC. The accumulation dynamics of SYTOX Green in cells is measured by CLSM to characterize the toxicity of nanoparticles. The emission spectra of SYTOX Green and nanoparticles are differentiated, and corresponding factor images specify the possible capture and cellular localization of nanoparticles in cells.

CONCLUSION

The designed protocol makes it possible to show how Iron Texas Red nanoparticles are captured by cardiomyocytes. Interestingly, whereas these fluorescent iron nanoparticles have no cytotoxic, pro-inflammatory or oxidative activities, they enhance the side effects of 7KC.

Influence of storage conditions on cholesterol oxidation in dried egg pasta

The oxidative characteristics of three different egg coproducts, namely, pasteurized eggs obtained from hens bred with organic methods (POE), pasteurized eggs from conventional breeding (PCE) and pasteurized spray-dried eggs (SPCE) obtained from conventional breeding, were analyzed. SPCE samples showed the highest content of peroxide (PV) and cholesterol oxides (COPs). In contrast, pasteurized eggs from organic breeding had the lowest index of oxidation. The three egg coproducts were used to produce dried egg pasta (spaghetti). The spaghetti was stored for 12 months at room temperature using typical pasta packaging (polypropylene foil) both under light, typical of retail conditions, and in the dark. Peroxide values and cholesterol oxidation were monitored at time 0 and then quarterly for 12 months. The oxidative parameters were significantly different in various egg coproducts, but the peroxide values of pasta were in the range of 3.0-3.5 mequiv of O(2)/kg of fat, with no differences in the types of pasta prepared with the various egg coproducts. Samples stored in the dark did not show any significant variations in peroxide values. However, PCE, SPCE and POE spaghetti stored in typical packaging increased the PV by 742.8, 846.7 and 625.7%, respectively. The pasta at time 0 showed COP values of about 50 microg of COPs/g of fat. During storage, COP values increased significantly. PCE, SPCE and POE spaghetti stored in the dark showed a content of total cholesterol oxides that was 2.0, 2.0, and 1.5 times lower than that of samples stored with typical pasta packaging.

Cholesterol oxides inhibit cholesterol esterification by lecithin: cholesterol acyl transferase

Cholesterol oxides are atherogenic and can affect the activity of diverse important enzymes for the lipidic metabolism. The effect of 7β-hydroxycholesterol, 7-ketocholesterol, 25-hydroxycholesterol, cholestan-3β,5α,6β-triol,5,6β-epoxycholesterol, 5,6α-epoxycholesterol and 7α-hydroxycholesterol on esterification of cholesterol by lecithin:cholesterol acyl transferase (LCAT, EC 2.3.1.43) and the transfer of esters of cholesterol oxides from high density lipoprotein (HDL) to low density lipoproteins (LDL) and very low density lipoproteins (VLDL) by cholesteryl ester transfer protein (CETP) was investigated. HDL enriched with increasing concentrations of cholesterol oxides was incubated with fresh plasma as source of LCAT. Cholesterol and cholesterol oxides esterification was followed by measuring the consumption of respective free sterol and oxysterols. Measurements of cholesterol and cholesterol oxides were done by gas-chromatography. 14C-cholesterol oxides were incorporated into HDL2 and HDL3 subfractions and then incubated with fresh plasma containing LCAT and CETP. The transfer of cholesterol oxide esters was followed by measuring the 14C-cholesterol oxide-derived esters transferred to LDL and VLDL. All the cholesterol oxides studied were esterified by LCAT after incorporation into HDL particles, competing with cholesterol by LCAT. Cholesterol esterification by LCAT was inversely related to the cholesterol oxide concentration. The esterification of 14C-cholesterol oxides was higher in HDL3 and the transfer of the derived esters was greater from HDL2 to LDL and VLDL. The results suggest that cholesterol esterification by LCAT is inhibited in cholesterol oxide-enriched HDL particles. Moreover, the cholesterol oxides-derived esters are efficiently transferred to LDL and VLDL. Therefore, we suggest that cholesterol oxides may exert part of their atherogenic effect by inhibiting cholesterol esterification on the HDL surface and thereby disturbing reverse cholesterol transport.

Biomarcadores de peroxidação lipídica na aterosclerose

A aterosclerose é caracterizada por uma resposta inflamatória crônica da parede arterial, iniciada por uma lesão do endotélio, cuja etiologia está relacionada à modificação oxidativa da lipoproteína de baixa densidade. O objetivo deste trabalho é apresentar os principais metabólitos envolvidos nos processos bioquímicos de peroxidação lipídica, discutindo as vantagens e desvantagens dos métodos empregados para a mensuração dos biomarcadores de peroxidação lipídica relacionados com a aterosclerose. A avaliação da oxidação das lipoproteínas pode ser realizada pela determinação dos produtos gerados durante a peroxidação lipídica, como os isoprostanos, hidroperóxidos lipídicos, aldeídos, fosfolípides oxidados e os produtos da oxidação do colesterol. A suscetibilidade das partículas de lipoproteína de baixa densidade à oxidação pode ser avaliada in vitro, após a indução da peroxidação lipídica por azoiniciadores radicalares lipossolúveis, hidrossolúveis, ou mais comumente, pelos íons cobre. Por outro lado, as modificações da lipoproteína de baixa densidade, pela ação das lipoxigenases e peroxidases, ou oxidação não-enzimática, resultam no aumento da carga negativa destas partículas e podem contribuir para a geração in vivo de uma subfração de lipoproteína de baixa densidade minimamente oxidada, denominada lipoproteína de baixa densidade eletronegativa (lipoproteína de baixa densidade). A determinação das concentrações desta partícula pode ser realizada em plasma por cromatografia líquida ou por imunoensaios..Diversos métodos podem ser utilizados para a avaliação dos biomarcadores de peroxidação lipídica in vivo e in vitro, porém, a definição do marcador mais adequado, depende de uma avaliação criteriosa das vantagens, desvantagens e particularidades de cada análise, levando-se em consideração os objetivos do estudo que será conduzido.

The antioxidant properties of serum albumin

Free radicals are a normal component of cellular oxygen metabolism in mammals. However, free radical-associated damage is an important factor in many pathological processes. Glycation and oxidative damage cause protein modifications, frequently observed in numerous diseases. Albumin represents a very abundant and important circulating antioxidant. This review brings together recent insights on albumin antioxidant properties. First, it focuses on the different activities of albumin concerning protein antioxidation. In particular, we describe the role of albumin in ligand binding and free radical-trapping activities. In addition, physiological and pathological situations that modify the antioxidant properties of albumin are reported.

Oxidised cholesterol is more hypercholesterolaemic and atherogenic than non-oxidised cholesterol in hamsters

The present study was to test the relative hypercholesterolaemic and atherogenic potency of oxidised cholesterol (OxC) and non-oxidised cholesterol in hamsters. An OxC mixture, prepared by heating pure cholesterol (100 g) at 160 degrees C in air for 72 h, contained 78 % cholesterol and 22 % OxC. Fifty Golden Syrian hamsters were randomly divided into five groups of ten animals and fed the control diet, a 0.05 % cholesterol diet (C-0.05), a 0.10 % cholesterol diet (C-0.1), a 0.05 % OxC mixture diet (OxC-0.05) or a 0.10 % OxC mixture diet (OxC-0.1), respectively. The OxC-0.05 and OxC-0.1 groups were more hypercholesterolaemic and had serum total cholesterol 22 and 12 % higher than the corresponding C-0.05 and C-0.1 hamsters (P < 0.05). The OxC-0.1 group demonstrated greater deposition of cholesterol and had a larger area of atherosclerotic plaque in the aorta than the corresponding C-0.1 hamsters (P < 0.05). Similarly, the aorta in the OxC-0.1 group showed greater inhibition on acetylcholine-induced relaxation compared with that in the C-0.1 hamsters. It was concluded that OxC was much more hypercholesterolaemic and atherogenic than cholesterol.

Antioxidant effects of 2,3-dihydro-5-hydroxy-4,6-di-tert-butyl-2,2-dipentylbenzofuran and α-tocopherol in hyperlipidemic mice as evaluated by hydroxyoctadecadienoic acid and 7-hydroxycholesterol

It has been hypothesized that oxidative modification of low density lipoprotein plays a key role in the pathogenesis of atherosclerosis. In order to elucidate the role of lipid oxidation and its inhibition in vivo, apolipoprotein E and alpha-tocopherol (alphaT) transfer protein double knockout (ApoE(-/-)alpha-TTP(-/-)) and apolipoprotein E knockout (ApoE(-/-)) mice fed with a vitamin E-depleted diet and a diet containing 0.002 wt.% alphaT, respectively, were used with or without the treatment of a synthetic antioxidant 2,3-dihydro-5-hydroxy-4,6-di-tert-butyl-2,2-dipentylbenzofuran (BO-653, 0.2 wt.%). The lipid oxidation markers of total hydroxylinoleic acid (tHODE), 8-iso-prostaglandin F(2alpha), and 7-hydroxycholesterol (t7-OHCh) in the blood, liver, and brain were inclusively measured with or without an excessive cholesterol-feeding (Ch-diet). The tHODE levels were elevated by Ch-diet in the plasma and brain of ApoE(-/-)alpha-TTP(-/-) mice and in the liver of ApoE(-/-) mice without BO-653. The levels of t7-OHCh in the liver were also increased due to the Ch-diet, and the ratio of t7-OHCh to the parent compound cholesterol was reduced to the control levels by BO-653. In summary, it was demonstrated by biomarkers, tHODE and t7-OHCh, that the added BO-653 in their diets exerted antioxidative effects in vivo under the condition of reduced vitamin E.

Cholesterol is more susceptible to oxidation than linoleates in cultured cells under oxidative stress induced by selenium deficiency and free radicals

Polyunsaturated fatty acids and their esters are known to be susceptible to free-radical mediated oxidation, while cholesterol is more resistant to oxidation. The present study focused on the relative susceptibilities of linoleates and cholesterol in Jurkat cells under oxidative stress induced by selenium deficiency and free radical insult, as assessed by total hydroxyoctadecadienoic acids (tHODE) and total 7-hydroxycholesterol (t7-OHCh) measured after reduction and saponification. It was observed that the levels of tHODE and t7-OHCh significantly increased by both oxidative insults. The increased amounts of t7-OHCh were higher than those of tHODE in both selenium-deficient and free radical-treated cells. These results suggest that, in contrast to plasma oxidation where cholesterol is much more resistant to oxidation than linoleates, cellular cholesterol is more susceptible to oxidation than cellular linoleates.

Synthesis of the oxysterol, 24(S), 25-epoxycholesterol, parallels cholesterol production and may protect against cellular accumulation of newly-synthesized cholesterol

AIM

The effects of 24(S),25-epoxycholesterol (24,25EC) on aspects of cholesterol homeostasis is well-documented. When added to cells, 24,25EC decreases cholesterol synthesis and up-regulates cholesterol efflux genes, including ABCA1. Synthesis of 24,25EC occurs in a shunt of the mevalonate pathway which also produces cholesterol. Therefore, 24,25EC synthesis should be subject to the same negative feedback regulation as cholesterol synthesis. To date, no role has been ascribed to 24,25EC in light of the fact that increased accumulation of cholesterol should decrease formation of this oxysterol through feedback inhibition. This leads to the intriguing paradox: why inhibit production of an apparently important regulator of cholesterol homeostasis when it is needed most?

METHODS

We used a combination of pharmacological and genetic approaches in Chinese Hamster Ovary cell-lines to investigate this paradox. Endogenous synthesis of 24,25EC was manipulated using partial inhibition of the enzyme, Oxidosqualene Cyclase. Changes in cholesterol and 24,25EC synthesis were determined using metabolic labelling with [1-14C]-acetate, thin-layer chromatography and phosphorimaging. Transcriptional effects mediated via SREBP and LXR were analysed by luciferase reporter assays.

RESULTS

We showed that cholesterol addition to cells lead to a rapid and preferential inhibition of 24,25EC synthesis. Addition of 24,25EC resulted in parallel inhibition of 24,25EC and cholesterol synthesis. Furthermore, we used a variety of approaches to examine the relationship between cholesterol and 24,25EC synthesis, including cell-lines with different rates of cholesterol synthesis, varying cholesterol synthetic rates by pre-treatment with a statin, or lipoprotein cholesterol loading of macrophages. In all cases, we showed that 24,25EC synthesis faithfully tracked cholesterol synthesis. Moreover, changes in 24,25EC synthesis exerted downstream effects, reducing SREBP transcriptional activity whilst increasing ABCA1 and LXR transcriptional activity.

CONCLUSION

Our results show that 24,25EC synthesis parallels cholesterol synthesis, consistent with this oxysterol functioning as a safety valve to protect against the accumulation of newly-synthesised cholesterol (as opposed to exogenously-derived cholesterol). Considering that 24,25EC is capable of being produced in all cholesterogenic cells, we propose that production of 24,25EC may represent a ubiquitous defence mechanism.