25-Hydroxycholesterol activates a cytochrome c release-mediated caspase cascade

Etiology of lipid-laden macrophages in the lung

Uniquely positioned as sentinel cells constantly exposed to the environment, pulmonary macrophages are vital for the maintenance of the lung lining. These cells are responsible for the clearance of xenobiotics, pathogen detection and clearance, and homeostatic functions such as surfactant recycling. Among the spectrum of phenotypes that may be expressed by macrophages in the lung, the pulmonary lipid-laden phenotype is less commonly studied in comparison to its circulatory counterpart, the atherosclerotic lesion-associated foam cell, or the acutely activated inflammatory macrophage. Herein, we propose that lipid-laden macrophage formation in the lung is governed by lipid acquisition, storage, metabolism, and export processes. The cellular balance of these four processes is critical to the maintenance of homeostasis and the prevention of aberrant signaling that may contribute to lung pathologies. This review aims to examine mechanisms and signaling pathways that are involved in lipid-laden macrophage formation and the potential consequences of this phenotype in the lung.

Aortic Valve Stenosis and Mitochondrial Dysfunctions: Clinical and Molecular Perspectives

Calcific aortic stenosis is a disorder that impacts the physiology of heart valves. Fibrocalcific events progress in conjunction with thickening of the valve leaflets. Over the years, these events promote stenosis and obstruction of blood flow. Known and common risk factors are congenital defects, aging and metabolic syndromes linked to high plasma levels of lipoproteins. Inflammation and oxidative stress are the main molecular mediators of the evolution of aortic stenosis in patients and these mediators regulate both the degradation and remodeling processes. Mitochondrial dysfunction and dysregulation of autophagy also contribute to the disease. A better understanding of these cellular impairments might help to develop new ways to treat patients since, at the moment, there is no effective medical treatment to diminish neither the advancement of valve stenosis nor the left ventricular function impairments, and the current approaches are surgical treatment or transcatheter aortic valve replacement with prosthesis.

25-hydroxycholesterol down-regulates oxysterol binding protein like 2 (OSBPL2) via the p53/SREBF2/NFYA signaling pathway

Oxysterol Binding Protein Like 2 (OSBPL2) is a lipid-binding protein implicated in various cellular processes. Previous studies have shown that depression of OSBPL2 significantly increases the level of cellular 25-hydroxycholesterol (25-OHC) which regulates the expression of lipid-metabolism-related genes. However, whether 25-OHC can regulate the expression of OSBPL2 remains unanswered. This study aimed to explore the molecular mechanism of 25-OHC regulating the expression of OSBPL2. Using dual-luciferase reporter assay, we found a decrease of nuclear transcription factor Y subunit alpha (NFYA) bound with OSBPL2 promoter when HeLa cells were treated with 25-OHC. Furthermore, transcriptome sequencing and RNA interference results revealed that the p53/sterol regulatory element binding transcription factor 2 (SREBF2) signaling pathway was involved in the NFYA-dependent transcription of OSBPL2 induced by 25-OHC. Based on these results, we concluded that pleomorphic adenoma gene 1 (PLAG1) and NFYA participated in the basal transcription of OSBPL2 and that 25-OHC decreased the transcription of OSBPL2 via the p53/SREBF2/NFYA signaling pathway. 25-OHC will accumulate over time in OSBPL2 knockdown cells. These results may provide a new insight into the deafness caused by OSBPL2 mutation.

25-HC decreases the sensitivity of human gastric cancer cells to 5-fluorouracil and promotes cells invasion via the TLR2/NF-κB signaling pathway

Hyperlipidemia is associated with metastasis in patients with gastric cancer (GC). 25-Hydroxycholesterol (25-HC) is a type of oxysterol which is synthesized from cholesterol and is involved in a number of processes, including inflammation, immune responses and cancer development. However, the role of 25-HC in gastric cancer remains unknown. In the present study, we demonstrated that 25-HC had no effects on GC cell proliferation and apoptosis, whereas it decreased the sensitivity of GC cells to 5-fluorouracil (5-FU), as demonstrated by the increased cell proliferation and the decreased cell apoptosis. On the other hand, exposure to 2.5-10 µM of 25-HC significantly promoted GC invasion, both in vitro (using AGS and MGC-803 GC cell lines) and in vivo (in an animal model), accompanied by the upregulation of the expression levels of matrix metalloproteinases (MMPs). Further investigations revealed that the promotion of GC invasion was, at least in part due to the activation of Toll-like receptor 2 (TLR2)/nuclear factor (NF)-κB signaling. Our results demonstrated that 25-HC promoted GC cells invasion by upregulating TLR2/NF-κB-mediated MMP expression. Thus, on the whole, the findings of this study suggest a novel mechanism of hyperlipidemia-induced GC progression.

Current knowledge on the mechanism of atherosclerosis and pro-atherosclerotic properties of oxysterols

Due to the fact that one of the main causes of worldwide deaths are directly related to atherosclerosis, scientists are constantly looking for atherosclerotic factors, in an attempt to reduce prevalence of this disease. The most important known pro-atherosclerotic factors include: elevated levels of LDL, low HDL levels, obesity and overweight, diabetes, family history of coronary heart disease and cigarette smoking. Since finding oxidized forms of cholesterol – oxysterols – in lesion in the arteries, it has also been presumed they possess pro-atherosclerotic properties. The formation of oxysterols in the atherosclerosis lesions, as a result of LDL oxidation due to the inflammatory response of cells to mechanical stress, is confirmed. However, it is still unknown, what exactly oxysterols cause in connection with atherosclerosis, after gaining entry to the human body e.g., with food containing high amounts of cholesterol, after being heated. The in vivo studies should provide data to finally prove or disprove the thesis regarding the pro-atherosclerotic prosperities of oxysterols, yet despite dozens of available in vivo research some studies confirm such properties, other disprove them. In this article we present the current knowledge about the mechanism of formation of atherosclerotic lesions and we summarize available data on in vivo studies, which investigated whether oxysterols have properties to cause the formation and accelerate the progress of the disease. Additionally we will try to discuss why such different results were obtained in all in vivo studies.

Oxysterols and 4-hydroxy-2-nonenal contribute to atherosclerotic plaque destabilization

A growing bulk of evidence suggests that cholesterol oxidation products, known as oxysterols, and 4-hydroxy-2-nonenal (HNE), the major proatherogenic components of oxidized low density lipoproteins (oxLDLs), significantly contribute to atherosclerotic plaque progression and destabilization, with eventual plaque rupture. These oxidized lipids are involved in various key steps of this complex process, mainly thanks to their ability to induce inflammation, oxidative stress, and apoptosis. This review summarizes the current knowledge of the effects induced by these compounds on vascular cells, after their accumulation in the arterial wall and in the atherosclerotic plaque.

The role of oxysterols in vascular ageing

The ageing endothelium progressively loses its remarkable and crucial ability to maintain homeostasis of the vasculature, as it acquires a proinflammatory phenotype. Cellular and structural changes gradually accumulate in the blood vessels, and markedly in artery walls. Most changes in aged arteries are comparable to those occurring during the atherogenic process, the latter being more marked: pro-oxidant and proinflammatory molecules, mainly deriving from or triggered by oxidized low density lipoproteins (oxLDLs), are undoubtedly a major driving force of this process. Oxysterols, quantitatively relevant components of oxLDLs, are likely candidate molecules in the pathogenesis of vascular ageing, because of their marked pro-oxidant, proinflammatory and proapoptotic properties. An increasing bulk of experimental data point to the contribution of a variety of oxysterols of pathophysiological interest, also in the age-related genesis of endothelium dysfunction, intimal thickening due to lipid accumulation, and smooth muscle cell migration and arterial stiffness due to increasing collagen deposition and calcification. This review provides an updated analysis of the molecular mechanisms whereby oxysterols accumulating in the wall of ageing blood vessels may 'activate' endothelial and monocytic cells, through expression of an inflammatory phenotype, and 'convince' smooth muscle cells to proliferate, migrate and, above all, to act as fibroblast-like cells.

Oxysterol-induced apoptosis of smooth muscle cells is under the control of a soluble adenylyl cyclase

AIMS

Apoptosis of vascular smooth muscle cells (VSMC) in advanced atherosclerotic plaques is an important cause of plaque instability. Oxysterols have been suggested as important inducers of apoptosis in VSMC, but the precise mechanism is still poorly understood. Here we aimed to analyse the role of the soluble adenylyl cyclase (sAC).

METHODS AND RESULTS

VSMC derived from rat aorta were treated with either 25-hydroxycholesterol or 7-ketocholesterol for 24 h. Apoptosis was detected by TUNEL staining and caspases cleavage. Oxysterols treatment led to the activation of the mitochondrial pathway of apoptosis (cytochrome c release and caspase-9 cleavage) and mitochondrial ROS formation, which were suppressed by the pharmacological inhibition or knockdown of sAC. Scavenging ROS with N-acetyl-l-cysteine prevented oxysterol-induced apoptosis. Analyses of the downstream pathway suggest that protein kinase A (PKA)-dependent phosphorylation and the mitochondrial translocation of the pro-apoptotic protein Bax is a key link between sAC and oxysterol-induced ROS formation and apoptosis. To distinguish between intra-mitochondrial and extra-mitochondrial/cytosolic sAC pools, sAC was overexpressed in mitochondria or in the cytosol. sAC expression in the cytosol, but not in mitochondria, significantly promoted apoptosis and ROS formation during oxysterol treatment.

CONCLUSION

These results suggest that the sAC/PKA axis plays a key role in the oxysterol-induced apoptosis of VSMC by controlling mitochondrial Bax translocation and ROS formation and that cytosolic sAC, rather than the mitochondrial pool, is involved in the apoptotic mechanism.

p38MAPK and Rho-dependent kinase are involved in anoikis induced by anicequol or 25-hydroxycholesterol in DLD-1 colon cancer cells

Anchorage-independent growth is evidence of the malignant transformation of cells. We previously reported the characterization of anicequol, a novel inhibitor of the anchorage-independent growth of tumor cells, and here we show that the effects of 25-hydroxycholesterol (25-HC) on colon cancer cells were very similar to those of anicequol. By analyzing the effects of inhibitors and performing RNA interference experiments, we found that p38 mitogen-activated protein kinase (p38MAPK) was involved in anicequol- and 25-HC-induced anoikis in DLD-1 cells. In addition, Rho-associated, coiled-coil containing protein kinase (ROCK) was also associated with anoikis induced by anicequol or 25-HC. Taken together, our findings suggest that activation of the p38MAPK and ROCK pathways might provide a new therapeutic strategy against cancer, and raise the possibility that tumor metastasis is influenced by 25-HC under physiological conditions.

25-Hydroxycholesterol exerts both a cox-2-dependent transient proliferative effect and cox-2-independent cytotoxic effect on bovine endothelial cells in a time- and cell-type-dependent manner

Background

25-hydroxycholesterol (25-OHC) is a product of oxidation of dietary cholesterol present in human plasma. 25-OHC and other oxidized forms of cholesterol are implicated in modulating inflammatory responses involved in development of atherosclerosis and colon carcinogenesis.

Methods

Primary lymphatic, venous and arterial endothelial cells isolated from bovine mesentery (bmLEC, bmVEC, bmAEC) were treated with 25-OHC and tested for several different cellular parameters.

Results

We found 25-OHC to be a potent inducer of cyclooxygenase-2 (Cox-2, prostaglandin G-H synthase-2) expression in bovine mesenteric lymphatic, venous, and arterial endothelial cells. The induction of Cox-2 expression in endothelial cells by 25-OHC led to an initial increase in cellular proliferation that was inhibited by the Cox-2 selective inhibitor celecoxib (Celebrex). Prolonged exposure to 25-OHC was cytotoxic. Furthermore, endothelial cells induced to express Cox-2 by 25-OHC were more sensitive to the effects of the Cox-2 selective inhibitor celecoxib (Celebrex). These results suggest that some effects of 25-OHC on cells may be dependent on Cox-2 enzymatic activity.

Conclusions

Cox-2 dependent elevating effects of 25-OHC on endothelial cell proliferation was transient. Prolonged exposure to 25-OHC caused cell death and enhanced celecoxib-induced cell death in a cell-type dependent manner. The lack of uniform response by the three endothelial cell types examined suggests that our model system of primary cultures of bmLECs, bmVECs, and bmAECs may aid the evaluation of celecoxib in inhibiting proliferation of different types of tumour-associated endothelial cells.

Liver X Receptor activation downregulates AKT survival signaling in lipid rafts and induces apoptosis of prostate cancer cells

Cholesterol is a structural component of lipid rafts within the plasma membrane. These domains, used as platforms for various signaling molecules, regulate cellular processes including cell survival. Cholesterol contents are tightly correlated with the structure and function of lipid rafts. Liver X receptors (LXRs) have a central role in the regulation of cholesterol homeostasis within the cell. Therefore, we investigated whether these nuclear receptors could modulate lipid raft signaling and consequently alter prostate cancer (PCa) cell survival. Treatment with the synthetic LXR agonist T0901317 downregulated the AKT survival pathway and thus induced apoptosis of LNCaP PCa cells in both xenografted nude mice and cell culture. The decrease in tumor cholesterol content resulted from the upregulation of ABCG1 and the subsequent increase in reverse cholesterol transport. RNA interference experiments showed that these effects were mediated by LXRs. Atomic force microscopy scanning of the inner plasma membrane sheet showed smaller and thinner lipid rafts after LXR stimulation, associated with the downregulation of AKT phosphorylation in these lipid rafts. Replenishment of cell membranes with exogenous cholesterol antagonized these effects, showing that cholesterol is a key modulator in this process. Altogether, pharmacological modulation of LXR activity could thus reduce prostate tumor growth by enhancing apoptosis in a lipid raft-dependent manner.

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.

Oncogenic Ras, but not (V600E)B-RAF, protects from cholesterol depletion-induced apoptosis through the PI3K/AKT pathway in colorectal cancer cells

Cholesterol is necessary for proliferation and survival of transformed cells. Here we analyse the effect of cholesterol depletion on apoptosis and the mechanisms underlying this effect in colorectal cancer cells carrying oncogenic Ras or (V600E)B-RAF mutations. We show that chronic cholesterol depletion achieved with lipoprotein-deficient serum (LPDS) and 25-hydroxycholesterol (25-HC) treatment results in a significant increase in apoptosis in HT-29 and Colo-205 cells containing the (V600E)B-RAF mutation, but not in HCT-116 and LoVo cells harbouring the (G13D)Ras mutation, or BE cells, which possess two mutations, (G13D)Ras and (G463V)B-RAF. We also demonstrate that oncogenic Ras protects from apoptosis induced by cholesterol depletion through constitutive activation of the phosphatidylinositol-3 kinase (PI3K)/AKT pathway. The specific activation of the PI3K/AKT pathway by overexpression of the (V12)RasC40 mutant or a constitutively active AKT decreases the LPDS plus 25-HC-induced apoptosis in HT-29 cells, whereas PI3K inhibition or abrogation of AKT expression renders HCT-116 sensitive to cholesterol depletion-induced apoptosis. Moreover, our data show that LPDS plus 25-HC increases the activity of c-Jun N-terminal kinase proteins only in HT-29 cells and that the inhibition of this kinase blocks the apoptosis induced by LPDS plus 25-HC. Finally, we demonstrate that AKT hyperactivation by oncogenic Ras protects from apoptosis, preventing the activation of c-Jun N-terminal kinase by cholesterol depletion. Thus, our data demonstrate that low levels of cholesterol induce apoptosis in colorectal cancer cells without oncogenic Ras mutations. These results reveal a novel molecular characteristic of colon tumours containing Ras or B-RAF mutations and should help in defining new targets for cancer therapy.

Oxysterol activation of phosphatidylcholine synthesis involves CTP:phosphocholine cytidylyltransferase α translocation to the nuclear envelope

In addition to suppressing cholesterol synthesis and uptake, oxysterols also activate glycerophospholipid and SM (sphingomyelin) synthesis, possibly to buffer cells from excess sterol accumulation. In the present study, we investigated the effects of oxysterols on the CDP-choline pathway for PtdCho (phosphatidylcholine) synthesis using wild-type and sterol-resistant CHO (Chinese-hamster ovary) cells expressing a mutant of SCAP [SREBP (sterol-regulatory-element-binding protein) cleavage-activating protein] (CHO-SCAP D443N). [3H]Choline-labelling experiments showed that 25OH (25-hydroxycholesterol), 22OH (22-hydroxycholesterol) and 27OH (27-hydroxycholesterol) increased PtdCho synthesis in CHO cells as a result of CCTα (CTP:phosphocholine cytidylyltransferase α) translocation and activation at the NE (nuclear envelope). These oxysterols also activate PtdCho synthesis in J774 macrophages. in vitro, CCTα activity was stimulated 2- to 2.5-fold by liposomes containing 5 mol% 25OH, 22OH or 27OH. Inclusion of up to 5 mol% cholesterol did not further activate CCTα. 25OH activated CCTα in CHO-SCAP D443N cells leading to a transient increase in PtdCho synthesis and accumulation of CDP-choline. CCTα translocation to the NE and intranuclear tubules in CHO-SCAP D443N cells was complete after 1 h exposure to 25OH compared with only partial translocation by 4–6 h in CHO-Mock cells. These enhanced responses in CHO-D443N cells were sterol-dependent since depletion with cyclodextrin or lovastatin resulted in reduced sensitivity to 25OH. However, the lack of effect of cholesterol on in vitro CCT activity indicates an indirect relationship or involvement of other sterols or oxysterol. We conclude that translocation and activation of CCTα at nuclear membranes by side-chain hydroxylated sterols are regulated by the cholesterol status of the cell.

25-hydroxycholesterol induces mitochondria-dependent apoptosis via activation of glycogen synthase kinase-3beta in PC12 cells

25-hydroxycholesterol (25-OH-chol) induces apoptosis in many cell types. The present study investigated the possible involvement of mitochondria-dependent apoptotic signalling molecules in the death of PC12 cells treated with 25-OH-chol. 25-OH-chol increased the production of reactive oxygen species and opened mitochondrial permeability transition pore, resulting in release of cytochrome c and subsequent activation of caspase-9 and -3. 25-OH-chol induced the activation of c-Jun N-terminal kinase (JNK) and glycogen synthase kinase-3beta (GSK-3beta). The JNK inhibitor SP600125 attenuated the activation of caspase-9 and -3 and reduced 25-OH-chol-induced cell death. GSK inhibitors SB415286 and SB216763 significantly down-regulated JNK activity and attenuated the cytotoxicity of 25-hydroxycholesterol. However, SP600125 did not alter the activity of GSK-3beta. The results indicate that 25-OH-chol induces cell death via activation of GSK-3beta and subsequent up-regulation of JNK. Pharmacological intervention of GSK-3beta-JNK-caspase signalling pathway may be useful for the reduction of cytotoxicity of oxysterols.

Cannabinoid (CB2) receptor deficiency reduces the susceptibility of macrophages to oxidized LDL/oxysterol-induced apoptosis

Macrophage apoptosis is an important process in the pathophysiology of atherosclerosis. Oxidized low-density lipoproteins (OxLDL) are a major component of lesions and potently induce macrophage apoptosis. Cannabinoid receptor 2 (CB2), the predominant macrophage cannabinoid receptor, modulates several macrophage processes associated with ongoing atherosclerosis; however, the role of CB2 in macrophage apoptosis is unknown. To determine if CB2 influences a macrophage apoptotic pathway relevant to atherosclerosis, we examined the effect of CB2 deficiency on OxLDL-induced macrophage apoptosis. In situ terminal transferase-mediated dUTP nick end labeling (TUNEL) analysis of resident peritoneal macrophages detected significantly fewer apoptotic CB2(-/-) macrophages than CB2(+/+) macrophages after incubation with OxLDL (27.9 +/- 4.7% vs. 61.9 +/- 8.5%, P < 0.001) or 7-ketocholesterol (7KC) (18.9 +/- 10.5% vs. 54.1 +/- 6.9%, P < 0.001), an oxysterol component of OxLDL. Caspase-3 activity; proteolytic conversion of procaspase-3; and cleavage of a caspase-3 substrate, PARP, were also diminished in 7KC-treated CB2(-/-) macrophages. Furthermore, the deactivation of the prosurvival kinase, Akt, in response to 7KC was impaired in CB2(-/-) macrophages. These results suggest that CB2 expression increases the susceptibility of macrophages to OxLDL-induced apoptosis, in part, by modulating the effect of oxysterols on the Akt survival pathway and that CB2 may influence atherosclerosis by modulating lesional macrophage apoptosis.

Prevention of 7-ketocholesterol-induced mitochondrial damage and cell death by calmodulin inhibition

Oxysterols such as 7-ketocholesterol and 25-hydroxycholesterol formed under enhanced oxidative stress in the brain are suggested to induce neuronal cell death. The present study investigated the effect of calmodulin antagonists (trifluoperazine, W-7 and calmidazolium) against the cytotoxicity of 7-ketocholesterol in relation to the mitochondria-mediated cell death process and oxidative stress. PC12 cells exposed to 7-ketocholesterol revealed nuclear damage, decrease in the mitochondrial transmembrane potential, cytosolic accumulation of cytochrome c, activation of caspase-3, increase in the formation of reactive oxygen species and depletion of GSH. N-Acetylcysteine, trolox, carboxy-PTIO and Mn-TBAP reduced the cytotoxic effect of 7-ketocholesterol. Calmodulin antagonists attenuated the 7-ketocholesterol-induced nuclear damage, formation of the mitochondrial permeability transition and cell viability loss in PC12 cells. The results suggest that calmodulin antagonists may prevent the 7-ketocholesterol-induced viability loss in PC12 cells by suppressing formation of the mitochondrial permeability transition, leading to the release of cytochrome c and subsequent activation of caspase-3. The effects seem to be ascribed to their depressant action on the formation of reactive oxygen species and depletion of GSH. The findings suggest that calmodulin inhibition may exhibit a protective effect against the neurotoxicity of 7-ketocholesterol.

Differential Modulation of 7-Ketocholesterol Toxicity Against PC12 Cells by Calmodulin Antagonists and Ca2+ Channel Blockers

The present study assessed the influence of intracellular Ca2+ and calmodulin against the neurotoxicity of oxysterol 7-ketocholesterol in relation to the mitochondria-mediated cell death process and oxidative stress in PC12 cells. Calmodulin antagonists calmidazolium and W-7 prevented the 7-ketocholesterol-induced mitochondrial damage, leading to caspase-3 activation and cell death, whereas Ca2+ channel blocker nicardipine, mitochondrial Ca2+ uptake inhibitor ruthenium red, and cell permeable Ca2+ chelator BAPTA-AM did not reduce it. Exposure of PC12 cells to 7-ketocholesterol caused elevation of intracellular Ca2+ levels. Unlike cell injury, calmodulin antagonists, nicardipine, and BAPTA-AM prevented the 7-ketocholesterol-induced elevations of intracellular Ca2+ levels. The results show that the cytotoxicity of 7-ketocholesterol seems to be modulated by calmodulin rather than changes in intracellular Ca2+ levels. Calmodulin antagonists may prevent the cytotoxicity of 7-ketocholesterol by suppressing the mitochondrial permeability transition formation, which is associated with the increased formation of reactive oxygen species and the depletion of GSH.

Induction of apoptosis by 25-hydroxycholesterol in adult rat Leydig cells: Protective effect of 17β-estradiol

Testicular macrophages can convert cholesterol into 25-hydroxycholesterol which strongly stimulates Leydig cell testosterone production. We demonstrated that 25-hydroxycholesterol reduced cholesterol biosynthesis in adult rat Leydig cells. This oxysterol can also be cytotoxic. As hydroxylated cholesterol can induce apoptosis in various cells, we investigated cell death produced by 25-hydroxycholesterol. Apoptosis was characterized by TUNEL assay and by DAPI test. Addition of 25-hydroxycholesterol, during 24h, induced a dose dependent increase of apoptosis. This effect was reduced by a treatment with a caspase-3 inhibitor (Ac-DEVD-CHO). 25-Hydroxycholesterol is known to stimulate testosterone production, but an increase of intracellular or culture medium testosterone level does not modify significantly the percentage of apoptotic cells. In contrast, addition of 17beta-estradiol (2 nM) induced a decrease of apoptotic cells. These data suggested that this oxysterol can be used by rat Leydig cells in culture for sterol metabolism, but also induces apoptosis which could be inhibited by 17beta-estradiol.

7-Ketocholesterol enhances 1-methyl-4-phenylpyridinium-induced mitochondrial dysfunction and cell death in PC12 cells

The present study investigated the promoting effect of oxysterol 7-ketocholesterol against the cytotoxicity of 1-methyl-4-phenylpyridinium (MPP(+)) in differentiated PC12 cells. 7-Ketocholesterol significantly enhanced the MPP(+)-induced nuclear damage, decrease in the mitochondrial transmembrane potential, cytosolic accumulation of cytochrome c, activation of caspase-3, increase in the formation of reactive oxygen species and depletion of GSH. N-Acetylcysteine, ascorbate, trolox, carboxy-PTIO and Mn-TBAP reduced the cytotoxic effect of MPP(+) in the presence of 7-ketocholesterol. The results indicate that 7-ketocholesterol shows a synergistic effect against the cytotoxic effect of MPP(+). 7-Ketocholesterol may enhance the MPP(+)-induced viability loss in PC12 cells by promoting the mitochondrial membrane permeability change, release of cytochrome c and subsequent activation of caspase-3, which is associated with the increased formation of reactive oxygen species and depletion of GSH. The findings suggest that 7-ketocholesterol as a promoting agent for the formation of mitochondrial permeability transition may enhance the toxic neuronal cell injury.

RhoA and p38 MAPK mediate apoptosis induced by cellular cholesterol depletion

Cholesterol is essential for cell viability, and homeostasis of cellular cholesterol is crucial to various cell functions. Here we examined the effect of cholesterol depletion on apoptosis and the mechanisms underlying this effect in NIH3T3 cells. We show that chronic cholesterol depletion achieved with lipoprotein-deficient serum (LPDS) and 25-hydroxycholesterol (25-HC) treatment resulted in a significant increase in cellular apoptosis and caspase-3 activation. This effect is not due to a deficiency of nonsterol isoprenoids, intermediate metabolites of the cholesterol biosynthetic pathway, but rather to low cholesterol levels, since addition of cholesterol together with LPDS and 25-HC nearly abolished apoptosis, whereas addition of farnesyl pyrophosphate or geranylgeranyl-pyrophosphate did not reverse the cell viability loss induced by LPDS plus 25-HC treatment. These effects were accompanied by an increase in ERK, JNK and p38 MAPK activity. However, only the inhibition of p38 MAPK with the specific inhibitor SB203580 or the overexpression of a kinase defective MKK6 resulted in a significant decrease in apoptosis and caspase-3 cleavage induced by cholesterol depletion. Furthermore, LPDS plus 25-HC increased RhoA activity, and this effect was reversed by addition of exogenous cholesterol. Finally, overexpression of the dominant negative N19RhoA inhibited p38 MAPK phosphorylation and apoptosis induced by low cholesterol levels. Together, our results demonstrate that cholesterol depletion induces apoptosis through a RhoA- and p38 MAPK-dependent mechanism.

Apoptotic effects of 25-hydroxycholesterol in immature rat Sertoli cells: Prevention by 17β-estradiol

The aim of the present study was to determine whether or not apoptosis occurs in Sertoli cells in presence of 25-hydroxycholesterol, an oxysterol derived from cholesterol-containing foods or endogenous oxidation. Here, we provide evidence that 25-hydroxycholesterol can induce cultured Sertoli cells of immature rat to undergo apoptosis. The cell death was identified by analysis of fragmented DNA detected using enzyme-immunoassay. After 48 h of treatment with 50 microM of 25-hydroxycholesterol, apoptosis increased by 70% in Sertoli cells. Moreover, 50 microM of 25-hydroxycholesterol inhibited the incorporation of [14C] acetate into cholesterol by 70%. Addition of mevanolate to prevent isoprenoid deficiency do not inhibit the apoptosis generated by 25-hydroxycholesterol. In contrast, this increase of DNA fragmentation was reversed by addition of caspase-3 inhibitors as Ac-DEVD-CHO or Ac-ESMD-CHO. Bcl-2 mRNA level in the Sertoli cells decreased by 60% after 24 h exposure to 25-hydroxycholesterol. In parallel, Bax mRNA level increased by 40% in the Sertoli cells incubated in presence of 50 microM of 25-hydroxycholesterol. Physiological concentrations of 17beta-estradiol (10 or 100 nM) elicited a significant protection on apoptosis generated by 25-hydroxycholesterol in Sertoli cells. Our results show that the 25-hydroxycholesterol would control the cholesterol synthesis without toxic effect in immature rat Sertoli cells, these cells being able to protect themselves by estradiol production.

Differential inhibition of oxidized LDL-induced apoptosis in human endothelial cells treated with different flavonoids

High plasma level of cholesterol is a well-known risk factor for atherosclerotic diseases. Oxidized LDL induces cellular and nuclear damage that leads to apoptotic cell death. We tested the hypothesis that flavonoids may function as antioxidants with regard to LDL incubated with 5 microm-Cu(2+) alone or in combination with human umbilical vein endothelial cells (HUVEC). Cytotoxicity and formation of thiobarbituric acid-reactive substances induced by Cu(2+)-oxidized LDL were examined in the presence of various subtypes of flavonoid. Flavanols, flavonols and flavanones at a non-toxic dose of 50 microm markedly inhibited LDL oxidation by inhibiting the formation of peroxidative products. In contrast, the flavones luteolin and apigenin had no such effect, with >30 % of cells killed after exposure to 0.1 mg LDL/ml. Protective flavonoids, especially (-)-epigallocatechin gallate, quercetin, rutin and hesperetin, inhibited HUVEC nuclear condensation and fragmentation induced by Cu(2+)-oxidized LDL. In addition, immunochemical staining and Western blot analysis revealed that anti-apoptotic Bcl-2 expression was enhanced following treatment with these protective flavonoids. However, Bax expression and caspase-3 cleavage stimulated by 18 h incubation with oxidized LDL were reduced following treatment with these protective flavonoids. The down-regulation of Bcl-2 and up-regulation of caspase-3 activation were reversed by the cytoprotective flavonoids, (-)-epigallocatechin gallate, quercetin and hesperetin, at >/=10 microm. These results suggest that flavonoids may differentially prevent Cu(2+)-oxidized LDL-induced apoptosis and promote cell survival as potent antioxidants. Survival potentials of certain flavonoids against cytotoxic oxidized LDL appeared to stem from their disparate chemical structure. Furthermore, dietary flavonoids may have therapeutic potential for protecting the endothelium from oxidative stress and oxidized LDL-triggered atherogenesis.

Acyl-coenzyme A:cholesterol acyltransferase promotes oxidized LDL/oxysterol-induced apoptosis in macrophages

7-Ketocholesterol (7KC) is a cytotoxic component of oxidized low density lipoproteins (OxLDLs) and induces apoptosis in macrophages by a mechanism involving the activation of cytosolic phospholipase A2 (cPLA2). In the current study, we examined the role of ACAT in 7KC-induced and OxLDL-induced apoptosis in murine macrophages. An ACAT inhibitor, Sandoz 58-035, suppressed 7KC-induced apoptosis in P388D1 cells and both 7KC-induced and OxLDL-induced apoptosis in mouse peritoneal macrophages (MPMs). Furthermore, compared with wild-type MPMs, ACAT-1-deficient MPMs demonstrated significant resistance to both 7KC-induced and OxLDL-induced apoptosis. Macrophages treated with 7KC accumulated ACAT-derived [14C]cholesteryl and [3H]7-ketocholesteryl esters. Tandem LC-MS revealed that the 7KC esters contained primarily saturated and monounsaturated fatty acids. An inhibitor of cPLA2, arachidonyl trifluoromethyl ketone, prevented the accumulation of 7KC esters and inhibited 7KC-induced apoptosis in P388D1 cells. The decrease in 7KC ester accumulation produced by the inhibition of cPLA2 was reversed by supplementing with either oleic or arachidonic acid (AA); however, only AA supplementation restored the induction of apoptosis by 7KC. These results suggest that 7KC not only initiates the apoptosis pathway by activating cPLA2, as we have reported previously, but also participates in the downstream signaling pathway when esterified by ACAT to form 7KC-arachidonate.

Chlamydia pneumoniae augments the oxidized low-density lipoprotein-induced death of mouse macrophages by a caspase-independent pathway

Chlamydia pneumoniae is a common respiratory pathogen that is associated with an increased risk of cardiovascular disease. However, the mechanisms by which C. pneumoniae contributes to cardiovascular disease have not been determined yet. C. pneumoniae infection may accelerate the death of cells within atherosclerotic lesions and contribute to the formation of unstable lesions. To test this hypothesis, the impact of C. pneumoniae infection on the death of lipid-loaded mouse macrophages was investigated. It was observed that RAW 264.7 cells are highly susceptible to the toxic effects of oxidized low-density lipoprotein (LDL) and exhibit markers of cell death within 24 h of treatment with as little as 5 microg/ml oxidized LDL. Subsequent infection with either live C. pneumoniae or heat-killed or UV-inactivated C. pneumoniae at a low multiplicity of infection for 24 to 72 h stimulated both additional binding of annexin V and the uptake of propidium iodide. Thus, C. pneumoniae augments the effects of oxidized LDL on cell death independent of a sustained infection. However, unlike oxidized LDL, C. pneumoniae infection does not activate caspase 3 or induce formation of the mitochondrial transition pore or the fragmentation of DNA, all of which are classical markers of apoptosis. Furthermore, primary bone marrow macrophages isolated from mice deficient in Toll-like receptor 2 (TLR-2) but not TLR-4 are resistant to C. pneumoniae-induced death. These data suggest that C. pneumoniae kills cells by a caspase-independent pathway and that the process is potentially mediated by activation of TLR-2.

Tocotrienols reduce 25-hydroxycholesterol-induced monocyte-endothelial cell interaction by inhibiting the surface expression of adhesion molecules

The migration of circulating monocytes into the subendothelial space occurs through the expressing of some adhesion molecules on endothelial cells. In the present study, using human aortic endothelial cells (HAECs), we investigated whether a model compound for oxysterols, 25-hydroxycholesterol, can enhance the monocyte adherence to HAECs exposed to 25-hydroxycholesterol via increasing expression of vascular cell adhesion molecule-1 (VCAM-1). We also aimed to determine the in vitro effects of tocotrienols on the enhanced interaction between monocytes and endothelial cells. We found that 25-hydroxycholesterol enhances surface expression determined by ELISA, induces VCAM-1 mRNA expression by real time-PCR, and stimulates adhesiveness of HAECs to U937 monocytic cells in a dose-dependent fashion. The combination treatment with anti-VCAM-1 and anti-CD11b monoclonal antibodies significantly reduced the monocyte adherence to 25-hydroxycholesterol-stimulated HAECs. Compared to alpha-tocopherol, tocotrienols displayed a more profound inhibitory effect on adhesion molecule expression and monocytic cell adherence. We observed that delta-tocotrienol exerted a most profound inhibitory action on monocytic cell adherence when compared to alpha-tocopherol and alpha-, beta-, and gamma-tocotrienols. Tocotrienols accumulated in HAECs to levels approximately 25-95-fold greater than that of alpha-tocopherol. In conclusion, these results indicate that a model compound 25-hydroxycholesterol can enhance the interaction between monocytes and HAECs, and that tocotrienols had a profound inhibitory effect on monocytic cell adherence to HAECs relative to alpha-tocopherol via inhibiting the VCAM-1 expression. These superior inhibitory effects of tocotrienols may be dependent on their intracellular accumulation.

Oxysterols from human bile induce apoptosis of canine gallbladder epithelial cells in monolayer culture

Oxysterols have been detected in various mammalian organs and blood. Biliary epithelium is exposed to high concentrations of cholesterol, and we have identified three keto-oxysterols (cholest-4-en-3-one, cholesta-4,6-dien-3-one, cholesta-3,5-dien-7-one) in human bile and gallstones. Because the effects of oxysterols on biliary physiology are not well defined, we investigated their biological effects on dog gallbladder epithelial cells. Enriched medium (culture medium containing taurocholate and lecithin and cholesterol +/- various oxysterols) was applied to confluent monolayers of dog gallbladder epithelial cells in culture. Cytotoxicity and apoptosis were studied by morphological analysis and flow cytometry. Oxysterols in the mitochondrial fraction were identified by gas chromatography/mass spectrometry, whereas release of cytochrome c from mitochondria was assayed by spectrophotometry and Western blot analysis. Compared with cells treated with culture medium or with enriched medium containing cholesterol, oxysterol-treated cells showed significantly increased apoptosis (P < 0.05). Exogenously applied oxysterols were recovered from the mitochondrial fraction. Cytochrome c release from mitochondria was increased significantly by cholest-4-en-3-one, cholesta-4,6-dien-3-one, and 5beta-cholestan-3-one (all P < 0.05). Thus oxysterols recovered from human bile and gallstones induce apoptosis of biliary epithelium via a mitochondrial-dependent pathway and may play a role in the pathogenesis of chronic inflammation and carcinogenesis in the gallbladder.

AKT/protein kinase B regulation of BCL family members during oxysterol-induced apoptosis

Cells of the vasculature, including macrophages, smooth muscle cells, and endothelial cells, exhibit apoptosis in culture upon treatment with oxidized low density lipoprotein, as do vascular cells of atherosclerotic plaque. Several lines of evidence support the hypothesis that the apoptotic component of oxidized low density lipoprotein is one or more oxysterols, which have been shown to induce apoptosis through the mitochondrial pathway. Activation of the mitochondrial pathway of apoptosis is regulated by members of the BCL family of proteins. In this study, we demonstrate that, in the murine macrophage-like cell line P388D1, oxysterols (25-hydroxycholesterol and 7-ketocholesterol) induced the degradation of the prosurvival protein kinase AKT (protein kinase B). This led, in turn, to the activation of the BCL-2 homology-3 domain-only proteins BIM and BAD and down-regulation of the anti-apoptotic multi-BCL homology domain protein BCL-xL. These responses would be expected to activate the pro-apoptotic multi-BCL homology domain proteins BAX and BAK, leading to the previously reported release of cytochrome c observed during oxysterol-induced apoptosis. Somewhat surprisingly, small interfering RNA knockdown of BAX resulted in a complete block of the induction of apoptosis by 25-hydroxycholesterol.

Oxidized low-density lipoprotein-induced apoptosis

Cultured cells are able to oxidize low-density lipoproteins (LDL) and oxidized LDL (oxLDL), which are present in atherosclerosis areas, exhibit a variety of biological properties potentially involved in atherogenesis. This review is focused on the toxicity of oxLDL, more precisely on the toxic compounds generated during LDL oxidation, the features and the mechanisms of cell death (apoptosis or necrosis) induced by oxLDL. After internalization, toxic oxidized lipids, namely lipid peroxides, oxysterols and aldehydes, induce modifications of cell proteins, elicit oxidative stress, lipid peroxidation and alter various signaling pathways and gene expression. These events may participate in the toxic effect, and converge to trigger an intense, delayed and sustained calcium peak which elicits either apoptosis or necrosis processes. OxLDL-induced apoptosis involves both mitochondrial and death-receptor (Fas/FasL) apoptotic pathways, thereby activating the classical caspase cascade and subsequent biochemical and morphological apoptotic features. When apoptosis is blocked by overexpression of Bcl-2, oxLDL trigger necrosis through a calcium-dependent pathway. Apoptosis occurring in atherosclerotic areas is potentially involved in endothelial cell lining defects, necrotic core formation and plaque rupture or erosion which may trigger atherothrombotic events. However, the precise role of oxLDL in apoptosis/necrosis occurring in vivo in atherosclerotic plaques remains to be clarified.

Enrichment of LDL with EPA and DHA decreased oxidized LDL-induced apoptosis in U937 cells

Oxidized LDL (oxLDL) may contribute to the accumulation of apoptotic cells in atherosclerotic plaques. Although it is well established in monophasic chemical systems that the highly unsaturated EPA and DHA will oxidize more readily than FA that contain fewer double bonds, our previous studies showed that enrichment of LDL, which has discrete polar and nonpolar phases, with these FA did not increase oxidation. The objective of this study was to compare the extent of apoptosis induced by EPA/DHA-rich oxLDL to that induced by EPA/DHA-non-rich oxLDL in U937 cells. LDL was obtained from one healthy subject three times before and after supplementation for 5 wk with 15 g/d of fish oil (FO), an amount easily obtainable from a diet that contains fatty fish. After supplementation, an EPA/DHA-rich LDL was obtained. Oxidative susceptibility of LDL, as determined by measuring the formation of conjugated dienes and the accumulation of cholesteryl ester hydroperoxides, was not higher in EPA/DHA-rich LDL. The oxLDL-induced cell apoptosis was detected by the activation of caspase-3, the translocation of PS to the outer surface of the plasma membrane using the Annexin V-fluorescein isothiocyanate binding assay, and the presence of chromatin condensation and nuclear fragmentation using the 4,6-diamidino-2-phenylindole staining assay. All three measures showed that after FO supplementation, EPA/DHA-rich oxLDL-induced cell apoptosis decreased. The decrease was not related to the concentration of lipid hydroperoxides. This study suggests that a possible protective effect of EPA/DHA-rich diets on atherosclerosis may be through lessening cell apoptosis in the arterial wall.

Mechanisms of oxysterol-induced apoptosis

The rationale for the present review is that oxysterols found in oxidized LDL (oxLDL) play a role in atherogenesis. This perspective is based on studies that show that induction of apoptosis in vascular cells is an important process in atherogenesis, that apoptosis can be induced by oxLDL, and that the oxysterol component of oxLDL is responsible for its proapoptotic activity. The evidence for these concepts is reviewed, as are studies on the mechanisms by which oxysterols can induce apoptosis. An elevation in intracellular calcium appears to be an early signal transduction event that leads to apoptosis through both the extrinsic and intrinsic apoptotic pathways.