Oxysterol-induced apoptosis in human monocytic cell lines

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.

Characterization of lipoprotein supplement and influence of its oxidized lipid content on cell culture performance and monoclonal antibody production by a SP2/0 hybridoma cell line

A challenging aspect with the use of the Sp2/0 hybridoma cell line in commercial manufacturing processes of recombinant therapeutic proteins is their exogenous lipids requirement for cell proliferation and optimal protein secretion. Lipids are commonly provided to the culture using serum or serum-derivatives, such as lipoprotein supplement. The batch-to-batch variability of these non-chemically defined raw-materials is known to impact cell culture process performance. Lipoprotein supplement variability and its impact on fed-batch production of a recombinant monoclonal antibody (mAb) expressed in Sp2/0 cells were studied using 36 batches from the same vendor. Several batches were associated with early viability drops leading to low process performance during fed-batch production. Increased caspase-3 activity (an indicator of apoptosis) was correlated to viability drops when low-performing batches were used. Addition of an antioxidant to the culture limited the increase in caspase-3 activity. Physicochemical characterization of batches confirmed that lipoproteins are mainly composed of lipids and proteins; no clear correlation between low-performing batches and lipoprotein supplement composition was observed. Controlled lipoprotein oxidation leads to lipoprotein solution browning, increasing absorbance at 276 nm and results in poor process performance. Because low-performing batches absorb more at 276 nm than other batches, oxidized lipids were suspected to be the root cause of low-performing batches. This study increased the understanding of lipoprotein supplement composition, its sensitivity to oxidation and its impact on process performance.

Can oxysterols work in anti-glioblastoma therapy? Model studies complemented with biological experiments

Despite the progress made in recent years in the field of oncology, the results of glioblastoma treatment remain unsatisfactory. In this paper, cholesterol derivatives - oxysterols - have been investigated in the context of their anti-cancer activity. First, the influence of three oxysterols (7-K, 7β-OH and 25-OH), differing in their chemical structure, on the properties of a model membrane imitating glioblastoma multiforme (GBM) cells was investigated. For this purpose, the Langmuir monolayer technique was applied. The obtained results clearly show that oxysterols modify the structure of the membrane by its stiffening, with the 7-K effect being the most pronounced. Next, the influence of 7-K on the nanomechanical properties of glioblastoma cells (U-251 line) was verified with AFM. It has been shown that 7-K has a dose-dependent cytotoxic effect on glioblastoma cells leading to the induction of apoptosis as confirmed by viability tests. Interestingly, significant changes in membrane structure, characteristic for phospholipidosis, has also been observed. Based on our results we believe that oxysterol-induced apoptosis and phospholipidosis are related and may share common signaling pathways. Dysregulation of lipids in phospholipidosis inhibit cell proliferation and may play key roles in the induction of apoptosis by oxysterols. Moreover, anticancer activity of these compounds may be related to the immobilization of cancer cells as a result of stiffening effect caused by oxysterols. Therefore, we believe that oxysterols are good candidates as new therapeutic molecules as an alternative to the aggressive treatment of GBM currently in use.

Oxiapoptophagy: A type of cell death induced by some oxysterols

Oxysterols are oxidized forms of cholesterol generated from cholesterol by auto-oxidation, enzymatic processes, or both. Some of them (7-ketocholesterol, 7β-hydroxycholesterol and 24(S)-hydroxycholesterol), when used at cytotoxic concentrations on different cell types from different species (mesenchymal bone marrow cells, monocytic cells and nerve cells), induce a type of cell death associated with OXIdative stress and several characteristics of APOPTOsis and autoPHAGY, defined as oxiapoptophagy. Oxidative stress is associated with overproduction of ROS, increased antioxidant enzyme activities, lipid peroxidation and protein carbonylation. Apoptosis is associated with activation of the mitochondrial pathway, opening of the mitochondrial permeability pore, loss of mitochondrial membrane potential, caspase-3 activation, PARP degradation, nuclear condensation and/or fragmentation. Autophagy is characterized by autophagic vacuoles revealed by monodansylcadaverine staining and transmission electron microscopy, plus increased ratio of LC-3II/LC-3I. In addition, morphological, topographical and functional changes of the peroxisome are observed. LINKED ARTICLES: This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Role of oxysterol-binding protein-related proteins in malignant human tumours

The oxysterol-binding protein-related protein (ORP) family is a group of proteins that mediate oxysterol metabolism and bioactivity in cells. ORPs constitute a large family of lipid transfer proteins. Much of the current evidence indicates that certain members of the family of oxysterol-binding proteins (OSBPs) can lead to cancer. Many studies have revealed the putative roles of OSBPs in various cancer types. However, the exact effects and mechanisms of action of members of the OSBP/ORP family in cancer initiation and progression are currently unclear. This review focuses on ORP family members that can accelerate human tumour cell proliferation, migration, and invasion. The mechanisms and functions of various ORPs are introduced in detail. We also attempt to identify the roles of these proteins in malignant tumours with the ultimate aim of determining the exact role of the OSBP/ORP family in human tumour cells.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

The Roles of Cholesterol and Its Metabolites in Normal and Malignant Hematopoiesis

Hematopoiesis is sustained throughout life by hematopoietic stem cells (HSCs) that are capable of self-renewal and differentiation into hematopoietic progenitor cells (HPCs). There is accumulating evidence that cholesterol homeostasis is an important factor in the regulation of hematopoiesis. Increased cholesterol levels are known to promote proliferation and mobilization of HSCs, while hypercholesterolemia is associated with expansion of myeloid cells in the peripheral blood and links hematopoiesis with cardiovascular disease. Cholesterol is a precursor to steroid hormones, oxysterols, and bile acids. Among steroid hormones, 17β-estradiol (E2) induces HSC division and E2-estrogen receptor α (ERα) signaling causes sexual dimorphism of HSC division rate. Oxysterols are oxygenated derivatives of cholesterol and key substrates for bile acid synthesis and are considered to be bioactive lipids, and recent studies have begun to reveal their important roles in the hematopoietic and immune systems. 27-Hydroxycholesterol (27HC) acts as an endogenous selective estrogen receptor modulator and induces ERα-dependent HSC mobilization and extramedullary hematopoiesis. 7α,25-dihydroxycholesterol (7α,25HC) acts as a ligand for Epstein-Barr virus-induced gene 2 (EBI2) and directs migration of B cells in the spleen during the adaptive immune response. Bile acids serve as chemical chaperones and alleviate endoplasmic reticulum stress in HSCs. Cholesterol metabolism is dysregulated in hematologic malignancies, and statins, which inhibit de novo cholesterol synthesis, have cytotoxic effects in malignant hematopoietic cells. In this review, recent advances in our understanding of the roles of cholesterol and its metabolites as signaling molecules in the regulation of hematopoiesis and hematologic malignancies are summarized.

Rhotekin 2 silencing inhibits proliferation and induces apoptosis in human osteosarcoma cells

Human osteosarcoma is the most frequent primary malignant of bone, and often occurs in adolescents. However, molecular mechanism of this disease remains unclear. In the present study, we found that the level of Rhotekin 2 (RTKN2) was up-regulated in osteosarcoma tissues and cell lines. In addition, silencing of RTKN2 of human osteosarcoma cell lines U2OS, inhibited proliferation, and induced G₁ phase cell cycle arrest via reducing the level of the cyclin-dependent kinase 2 (CDK2). Furthermore, RTKN2 knockdown in the U2OS cells induced apoptosis by increasing the level of Bax and decreasing the level of Bcl2. These results suggested that RTKN2 is involved in the progression of human osteosarcoma, and may be a potential therapeutic target.

27-hydroxycholesterol decreases cell proliferation in colon cancer cell lines

Colorectal cancer (CRC) is the third most diagnosed cancer in the western world, affecting 1 out of approximately 22 people in their lifetime. Several epidemiological studies suggest a positive association between high plasma cholesterol levels and colorectal cancer. However, the molecular mechanisms by which cholesterol may alter the risk of colorectal cancer (CRC) are ill-defined as the cholesterol lowering drugs statins do not appear to decrease a patient's risk of developing colorectal cancer. Cholesterol is metabolized to active derivatives including cholesterol oxidization products (COP), known as oxysterols, which have been shown to alter cellular proliferation. These metabolites and not cholesterol per se, may therefore affect the risk of developing colorectal cancer. The cholesterol metabolite or the oxysterol 27-hydroxycholesterol (27-OHC) is the most abundant oxysterol in the plasma and has been shown to be involved in the pathogenesis of several cancers including breast and prostate cancer. However, the role of 27-OHC in colorectal cancer has not been investigated. We treated Caco2 and SW620, two well characterized colon cancer cells with low, physiological and high concentrations of 27-OHC, and found that 27-OHC reduces cellular proliferation in these cells. We also found that the effects of 27-OHC on cell proliferation are not due to cellular cytotoxicity or apoptotic cellular death. Additionally, 27-OHC-induced reduction in cell proliferation is independent of actions on its target nuclear receptors, liver-X-receptors (LXR) and estrogen receptors (ER) activation. Instead, our study demonstrates that 27-OHC significantly decreases AKT activation, a major protein kinase involved in the pathogenesis of cancer as it regulates cell cycle progression, protein synthesis, and cellular survival. Our data shows that treatment with 27-OHC substantially decreases the activation of AKT by reducing levels of its active form, p-AKT, in Caco2 cells but not SW620 cells. All-together, our results show for the first time that the cholesterol metabolite 27-OHC reduces cell proliferation in colorectal cancer cells.

Knockdown of Rhotekin 2 expression suppresses proliferation and induces apoptosis in colon cancer cells

Colon cancer is one of the most common malignant tumors in the human body, ranking second as a gastrointestinal tumor. It has a high incidence in Europe, America and China and more than 1 million new cases of colon cancer are reported worldwide each year. The incidence of colon cancer in China has increased from 12/0.1 million in the early 1970s to 56/0.1 million at present with an annual growth rate of 4.2%, which far exceeds the international level (2%). Rhotekin (RTKN) 2, a Rho-guanosine triphosphatase (GTPase) effector, has been reported to be anti-apoptotic. However, the molecular mechanism underlying the biological function of RTKN2 in colon cancer remains unknown. The present study investigated whether the mRNA expression level of RTKN2 was markedly higher in 30 human colon cancer specimens compared with adjacent non-cancerous tissues. The results showed that the protein expression level of RTKN2 was significantly higher in SW480 and HCT116 cells, compared with HIEC cells. Knockdown of RTKN2 in the SW480 and HCT116 colon cancer cells, by lentivirus-mediated RNA interference led to the notable inhibition of cell proliferation and cell cycle progression, by reducing the expression levels of the PCDA, Cyclin D1 and c-myc cell cycle-associated proteins. The inhibitory effect of RTKN2 silencing on the proliferation of colon cancer cells may be partially realized by inhibiting the Wnt/β-catenin signaling pathway. Furthermore, the silencing of RTKN2 in the cells induced apoptosis by reducing the expression level of Bax and increasing the expression level of Bcl2. These results show that RTKN2 is involved in the carcinogenesis and progression of human colon cancer, indicating that RTKN2 may be a molecular target in colon cancer therapy.

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.

The Role of Oxysterols in Human Cancer

Oxysterols are oxygenated derivatives of cholesterol formed in the human body or ingested in the diet. By modulating the activity of many proteins [e.g., liver X receptors (LXRs), oxysterol-binding proteins (OSBPs), some ATP-binding cassette (ABC) transporters], oxysterols can affect many cellular functions and influence various physiological processes (e.g., cholesterol metabolism, membrane fluidity regulation, intracellular signaling pathways). Therefore, the role of oxysterols is also important in pathological conditions (e.g., atherosclerosis, diabetes mellitus type 2, neurodegenerative disorders). Finally, current evidence suggests that oxysterols play a role in malignancies such as breast, prostate, colon, and bile duct cancer. This review summarizes the physiological importance of oxysterols in the human body with a special emphasis on their roles in various tumors.

Differential Effects of Mixtures of Cholesterol Oxidation Products on Bovine Aortic Endothelial Cells and Human Monocytic U937 Cells

Cholesterol oxidation products or oxysterols are of interest due to their hypothesized role in the development of atherosclerosis. The objective of the present study was to assess the cytotoxic effects of mixtures of oxysterols: 25-hydroxycholesterol (25-OHC), 7 β-hydroxycholesterol (7 β-OHC), and cholesterol-5 β,6 β-epoxide ( β-epox) on two cell types associated with the atherosclerotic process, bovine aortic endothelial (BAE) cells and human monocytic U937 cells. Cells were exposed to 25-OHC, 7 β-OHC, or β-epox, or equimolar mixtures (30 μM) of 25-OHC and 7 β-OHC, 25-OHC and β-epox, or 7 β-OHC and β-epox for 48 h. Cell viability was assessed using the fluorescein diacetate/ethidium bromide (FDA/ EtBr) assay and nuclear morphology following staining with Hoechst 33342. 25-OHC was the least toxic of the oxysterols and did not induce apoptosis in either cell line. Both 7 β-OHC and β-epox treatments were cytotoxic and induced apoptosis in the cells. Cotreatment with 25-OHC did not alter the toxicity of 7 β-OHC and β-epox in U937 cells but did decrease the percentage apoptotic cell death. In contrast, in the BAE cells cotreatment with 25-OHC had a slight protective effect on 7 β-OHC and β-epox–induced toxicities and a marked decrease in apoptotic cell death. The 7 β-OHC and β-epox mixture induced a significant increase in apoptotic cell death in U937 cells but decreased this mode of cell death in the BAE cells. The effects of oxysterols on glutathione levels also differed between the cells with changes noted in U937 and not in BAE cells. Results demonstrate interactive effects when oxysterols are studied as mixtures rather than single compounds in vitro.

Silencing of RTKN2 by siRNA suppresses proliferation, and induces G1 arrest and apoptosis in human bladder cancer cells

Human bladder cancer is the most common urological malignancy in China. One of the causes of carcinogenesis in the cancer may be gene mutation. Therefore, the present study investigated the expression levels of Rhotekin 2 (RTKN2), a Rho effector protein, in human bladder cancer tissues and cell lines, and examined the effect of RTKN2 on the proliferation, cell cycle, apoptosis and invasion of human bladder cancer cell lines. The mRNA expression levels of RTKN2 in 30 human bladder cancer tissue samples were significantly higher, compared with those in 30 normal human bladder tissue samples. The protein expression levels of RTKN2 was markedly higher in T24 and 5637 cells, compared with those in four other human bladder cancer cell lines. The silencing of RTKN2 by small interfering (si)RNA inhibited cell proliferation and arrested cell cycle at the G1 phase, via reducing the expression levels of the MCM10, CDK2, CDC24A and CDC6 cell cycle‑associated proteins in the T24 and 5637 cells. Furthermore, RTKN2 knockdown in the cells led to cell apoptosis and the suppression of invasion. These results suggested that RTKN2 is involved in the carcinogenesis and progression of human bladder cancer, indicating that RTKN2 may be a molecular target in cancer therapy.

MicroRNA: a connecting road between apoptosis and cholesterol metabolism

Resistance to apoptosis leads to tumorigenesis and failure of anti-cancer therapy. Recent studies also highlight abrogated lipid/cholesterol metabolism as one of the root causes of cancer that can lead to metastatic transformations. Cancer cells are dependent on tremendous supply of cellular cholesterol for the formation of new membranes and continuation of cell signaling. Cholesterol homeostasis network tightly regulates this metabolic need of cancer cells on cholesterol and other lipids. Genetic landscape is also shared between apoptosis and cholesterol metabolism. MicroRNAs (miRNAs) are the new fine tuners of signaling pathways and cellular processes and are known for their ability to post-transcriptionally repress gene expression in a targeted manner. This review summarizes the current knowledge about the cross talk between apoptosis and cholesterol metabolism via miRNAs. In addition, we also emphasize herein recent therapeutic modulations of specific miRNAs and their promising potential for the treatment of deadly diseases including cancer and cholesterol related pathologies. Understanding of the impact of miRNA-based regulation of apoptosis and metabolic processes is still at its dawn and needs further research for the development of future miRNA-based therapies. As both these physiological processes affect cellular homeostasis, we believe that this comprehensive summary of miRNAs modulating both apoptosis and cholesterol metabolism will open uncharted territory for scientific exploration and will provide the foundation for discovering novel drug targets for cancer and metabolic diseases.

Oxysterols and mechanisms of survival signaling

Oxysterols, a family of oxidation products of cholesterol, are increasingly drawing attention of scientists to their multifaceted biochemical properties, several of them of clear relevance to human pathophysiology. Taken up by cells through both vesicular and non-vesicular ways or often generated intracellularly, oxysterols contribute to modulate not only the inflammatory and immunological response but also cell viability, metabolism and function by modulating several signaling pathways. Moreover, they have been recognized as elective ligands for the most important nuclear receptors. The outcome of such a complex network of intracellular reactions promoted by these cholesterol oxidation products appears to be largely dependent not only on the type of cells, the dynamic conditions of the cellular and tissue environment but also on the concentration of the oxysterols. Here focus has been given to the cascade of molecular events exerted by relatively low concentrations of certain oxysterols that elicit survival and functional signals in the cells, with the aim to contribute to further expand the knowledge about the biological and physiological potential of the biochemical reactions triggered and modulated by oxysterols.

Steroidal constituents from the edible sea urchin Diadema savignyi Michelin induce apoptosis in human cancer cells

Bioassay-directed fractionation and purification were used to isolate 12 steroids (1-12) from a CH(2)Cl(2) extract of the edible Vietnamese sea urchin Diadema savignyi Michelin. The cytotoxic activity of the CH(2)Cl(2) extract and 12 steroids was evaluated in three human cancer cell lines (HL-60, PC-3, and SNU-C5). Relative to the effects of the positive control, mitoxantrone, the CH(2)Cl(2) extract (with an inhibitory concentration of 50% [IC(50)] values ranging from 1.37±0.15 to 3.11±0.15 μg/mL) and compounds 2 (with IC(50) values ranging from 5.29±0.11 to 6.80±0.67 μM) and 11 (with IC(50) values ranging from 4.95±0.07 to 6.99±0.28 μM) exhibited potent cytotoxic effects against all three tested human cancer cell lines. In addition, the CH(2)Cl(2) extract and compounds 2 and 11 were found to induce apoptosis. The induction of apoptosis was accompanied by alterations of the apoptosis-related protein expression, inactivation of ERK1/2 mitogen-activated protein kinase signaling, and decreased c-Myc expression. These data suggest that compounds 2 and 11 from the edible sea urchin D. savignyi may have potential for the treatment of colon cancer, leukemia, and prostate cancer as complementary cancer remedies.

Anti-inflammatory components of the Vietnamese starfish Protoreaster nodosus

BACKGROUND

In the present study, we examined the inhibitory effects of a methanolic extract, dichloromethane fraction, water layer, and polyhydroxylated sterols (1-4) isolated from the Vietnamese starfish Protoreaster nodosus on pro-inflammatory cytokine (IL-12 p40, IL-6, and TNF-α) production in LPS-stimulated bone marrow-derived dendritic cells (BMDCs) using enzyme-linked immunosorbent assays (ELISA).

RESULTS

The methanolic extract and dichloromethane fraction exerted potent inhibitory effects on the production of all three pro-inflammatory cytokines, with IC50 values ranging from 0.60 ± 0.01 to 26.19 ± 0.64 μg/mL. Four highly pure steroid derivatives (1-4) were isolated from the dichloromethane fraction and water layer of P. nodosus. Potent inhibitory activities were also observed for (25S) 5α-cholestane-3β,4β,6α,7α,8β,15α,16β,26-octol (3) on the production of IL-12 p40 and IL-6 (IC50s = 3.11 ± 0.08 and 1.35 ± 0.03 μM), and for (25S) 5α-cholestane-3β,6α,8β,15α,16β,26-hexol (1) and (25S) 5α-cholestane-3β,6α,7α,8β,15α,16β,26-heptol (2) on the production of IL-12 p40 (IC50s = 0.01 ± 0.00 and 1.02 ± 0.01 μM). Moreover, nodososide (4) exhibited moderate inhibitory effects on IL-12 p40 and IL-6 production.

CONCLUSION

This is the first report of the anti-inflammatory activity from the starfish P. nodosus. The main finding of this study is the identification oxygenated steroid derivatives from P. nodosus with potent anti-inflammatory activities that may be developed as therapeutic agents for inflammatory diseases.

Evaluation of cytotoxic and apoptotic effects of individual and mixed 7-ketophytosterol oxides on human intestinal carcinoma cells

Phytosterol oxidation products (POPs) are constituents of the human diet. Definitive information on the toxic or biological effects of POPs is limited and in some cases contradictory. This study evaluates the cytotoxicity of four individual 7-ketophytosterol oxides, including 7-ketositosterol (7K-SI), 7-ketocampesterol (7K-CA), 7-ketobrassicasterol (7K-BR), 7-ketostigmasterol (7K-ST), and a mixture of 7-ketophytosterols (7K-MIX) toward a human intestinal carcinoma (HIC) cell line. Results showed that all tested compounds reduced cell proliferation in a dose-dependent manner; especially 7K-SI and 7K-CA exhibited higher activities. Both compounds increased early apoptotic cells and caused cell cycle arrest in the G1 phase with cell accumulation in the S phase. No evidence of cell death was observed induced by 7K-ST and 7K-MIX. Furthermore, 7K-SI, 7K-CA, and 7K-BR induced apoptosis by enhancing caspase-3 activity and the modulatory effects of Bcl-2, while 7K-ST and 7K-MIX did not involve caspase-3 activation and Bcl-2 down-regulation.

Oxidative stability of omega-3 polyunsaturated fatty acids enriched eggs

Omega-3 polyunsaturated fatty acids (n-3 PUFA) enriched eggs have a growing market share in the egg industry. This study examined the stability of n-3 PUFA enriched eggs fortified with antioxidants (vitamin E or organic Selenium [Sel-Plex] or both) following cooking and storage. The total fat content was not affected by cooking or simulated retail storage conditions, whereas, n-3 fatty acids were reduced. The content of n-3 fatty acids in boiled eggs was higher than in fried eggs. Lipid oxidation was significantly affected by the different cooking methods. Fried eggs contained higher levels of malondialdehyde (MDA, 2.02 μg/kg) and cholesterol oxidation products (COPs, 13.58 μg/g) compared to boiled (1.44 and 10.15 μg/kg) and raw eggs (0.95 and 9.03 μg/kg, respectively, for MDA and COPs). Supplementation of antioxidants reduced the formation of MDA by 40% and COPs by 12% in fried eggs. Although the content of MDA was significantly increased after 28 days of storage, COPs were not affected by storage. Our study indicated that the n-3 PUFA in enriched eggs was relatively stable during storage and home cooking in the presence of antioxidants.

The association between biomarkers in the blood and carotid plaque composition-focusing on oxidized lipids, oxysterols and plaque status

Human atherosclerotic plaque is composed of a large mixture of elements, predominantly lipids and oxidized lipids, lipid-loaded macrophages and smooth muscle cells, forming foam cells. Plaque contents undergo dynamic changes during the plaque's progression, being in a constant interaction with the circulating blood. During the mutual interaction between blood and plaque and the specific biochemical processes occurring in both, specific molecules can be generated in the serum which might provide information on plaque status. This information, mostly on plaque vulnerability, is highly important for making appropriate treatment decisions before neurological symptoms appear. The present review summarizes plaque contents, mostly lipids, oxidized lipids, oxidized products of cholesterol (oxysterols), and covers the recent literature on their association with biomarkers in the blood and on the possibility of using them for providing information on plaque status.

Evidence of cell damage induced by major components of a diet-compatible mixture of oxysterols in human colon cancer CaCo-2 cell line

Cholesterol oxidation products, termed oxysterols, have been shown to be more reactive than unoxidized cholesterol, possessing marked pro-inflammatory and cytotoxic effects in a number of cells and tissues. Oxysterols, absorbed with the diet as products of cholesterol auto-oxidation, have recently been suggested to potentially interfere with homeostasis of the mucosal intestinal epithelium, by promoting and sustaining irreversible damage. However, the treatment of colon cancer cells with a diet-compatible mixture of oxysterols does not elicit the same responses than individual components added to the cells at the same concentrations at which they are present in the mixture. Sixty μM oxysterol mixture showed a slight pro-apoptotic effect on human colon cancer CaCo-2 cell line, evaluated in terms of caspase-3 and caspase-7 activation; conversely, 7α-hydroxycholesterol, 7β-hydroxycholesterol and 5α,6α-epoxycholesterol were identified to be able to induce a significant pro-apoptotic effect if added to cell culture singly; 7β-hydroxycholesterol had stronger action than other compounds. The enhanced production of reactive oxygen species through up-regulation of the colonic NADPH-oxidase isoform NOX1 appeared to be the key event in oxysterol-induced apoptosis in these colon cancer cells. As regards pro-inflammatory effects of oxysterols, IL-8 and MCP-1 were evaluated for their chemotactic activity. Only MCP-1 production was significantly induced by 7β-hydroxycholesterol, as well as by cholesterol and oxysterol mixture. However, oxysterol-induced inflammation appeared to be NOX1-independent, suggesting a secondary role of this enzyme in inducing inflammation in colon cancer cells. A selective cell death induced by specific oxysterols against colon cancer cells, mainly exploiting their ability to activate NOX1 in generating oxidative reactions, might represent a promising field of investigation in colorectal cancer, and might bring new insights on strategies in anticancer therapy.

Mass spectrometric detection of cholesterol oxidation in bovine sperm

We report on the presence and formation of cholesterol oxidation products (oxysterols) in bovine sperm. Although cholesterol is the most abundant molecule in the membrane of mammalian cells and is easily oxidized, this is the first report on cholesterol oxidation in sperm membranes as investigated by state-of-the-art liquid chromatographic and mass spectrometric methods. First, oxysterols are already present in fresh semen samples, showing that lipid peroxidation is part of normal sperm physiology. After chromatographic separation (by high-performance liquid chromatography), the detected oxysterol species were identified with atmospheric pressure chemical ionization mass spectrometry in multiple-reaction-monitoring mode that enabled detection in a broad and linear concentration range (0.05-100 pmol for each oxysterol species detected). Second, exposure of living sperm cells to oxidative stress does not result in the same level and composition of oxysterol species compared with oxidative stress imposed on reconstituted vesicles from protein-free sperm lipid extracts. This suggests that living sperm cells protect themselves against elevated oxysterol formation. Third, sperm capacitation induces the formation of oxysterols, and these formed oxysterols are almost completely depleted from the sperm surface by albumin. Fourth, and most importantly, capacitation after freezing/thawing of sperm fails to induce both the formation of oxysterols and the subsequent albumin-dependent depletion of oxysterols from the sperm surface. The possible physiological relevance of capacitation-dependent oxysterol formation and depletion at the sperm surface as well as the omission of this after freezing/thawing semen is discussed.

Mechanisms of oxysterol-induced carcinogenesis

Oxysterols are oxidation products of cholesterol that are generated by enzymatic reactions mediated by cytochrome P450 family enzymes or by non-enzymatic reactions involving reactive oxygen and nitrogen species. Oxysterols play various regulatory roles in normal cellular processes such as cholesterol homeostasis by acting as intermediates in cholesterol catabolism. Pathological effects of oxysterols have also been described, and various reports have implicated oxysterols in several disease states, including atherosclerosis, neurological disease, and cancer. Numerous studies show that oxysterols are associated with various types of cancer, including cancers of the colon, lung, skin, breast and bile ducts. The molecular mechanisms whereby oxysterols contribute to the initiation and progression of cancer are an area of active investigation. This review focuses on the current state of knowledge regarding the role of oxysterols in carcinogenesis. Mutagenicity of oxysterols has been described in both nuclear and mitochondrial DNA. Certain oxysterols such as cholesterol-epoxide and cholestanetriol have been shown to be mutagenic and genotoxic. Oxysterols possess pro-oxidative and pro-inflammatory properties that can contribute to carcinogenesis. Oxysterols can induce the production of inflammatory cytokines such as interleukin-8 and interleukin-1β. Certain oxysterols are also involved in the induction of cyclo-oxygenase-2 expression. Inflammatory effects can also be mediated through the activation of liver-X-receptor, a nuclear receptor for oxysterols. Thus, several distinct molecular mechanisms have been described showing that oxysterols contribute to the initiation and progression of cancers arising in various organ systems.

Red wine prevents the postprandial increase in plasma cholesterol oxidation products: a pilot study

Moderate wine consumption has been shown to lower cardiovascular risk. One of the mechanisms could involve the control of postprandial hyperlipaemia, a well-defined risk factor for atherosclerosis, reasonably by reducing the absorption of lipid oxidised species from the meal. The objective of the present study was to investigate whether wine consumption with the meal is able to reduce the postprandial increase in plasma lipid hydroperoxides and cholesterol oxidation products, in human subjects. In two different study sessions, twelve healthy volunteers consumed the same test meal rich in oxidised and oxidisable lipids (a double cheeseburger), with 300 ml of water (control) or with 300 ml of red wine (wine). The postprandial plasma concentration of cholesterol oxidation products was measured by GC–MS. The control meal induced a significant increase in the plasma concentration of lipid hydroperoxides and of two cholesterol oxidation products, 7-β-hydroxycholesterol and 7-ketocholesterol. The postprandial increase in lipid hydroperoxides and cholesterol oxidation products was fully prevented by wine when consumed with the meal. In conclusion, the present study provides evidence that consumption of wine with the meal could prevent the postprandial increase in plasma cholesterol oxidation products.

Pro-oxidant and proapoptotic effects of cholesterol oxidation products on human colonic epithelial cells: a potential mechanism of inflammatory bowel disease progression

With the aim of investigating whether cholesterol oxidation products could contribute to the pathogenesis of the intestinal epithelial barrier dysfunction that occurs in human inflammatory bowel disease (IBD), differentiated versus undifferentiated CaCo-2 cells, an accepted model for human intestinal epithelial cells, were challenged with a dietary-representative mixture of oxysterols. Only differentiated colonic cells were susceptible to the proapoptotic action of the oxysterol mixture, checked both by enzymatic and by morphological methods, mainly because of a very low AKT phosphorylation pathway compared to the undifferentiated counterparts. Enhanced production of reactive oxygen species by a colonic NADPH oxidase hyperactivation seemed to represent the key event in oxysterol-induced up-regulation of the mitochondrial pathway of programmed death of differentiated CaCo-2 cells. These in vitro findings point to the pro-oxidant and cytotoxic potential of cholesterol oxidation products, of both dietary and endogenous origin, as an important mechanism of induction and/or worsening of the functional impairment of enteric mucosa that characterizes IBD.

RTKN2 Induces NF-KappaB Dependent Resistance to Intrinsic Apoptosis in HEK Cells and Regulates BCL-2 Genes in Human CD4(+) Lymphocytes

The gene for Rhotekin 2 (RTKN2) was originally identified in a promyelocytic cell line resistant to oxysterol-induced apoptosis. It is differentially expressed in freshly isolated CD4⁺ T-cells compared with other hematopoietic cells and is down-regulated following activation of the T-cell receptor. However, very little is known about the function of RTKN2 other than its homology to Rho-GTPase effector, rhotekin, and the possibility that they may have similar roles. Here we show that stable expression of RTKN2 in HEK cells enhanced survival in response to intrinsic apoptotic agents; 25-hydroxy cholesterol and camptothecin, but not the extrinsic agent, TNFα. Inhibitors of NF-KappaB, but not MAPK, reversed the resistance and mitochondrial pro-apoptotic genes, Bax and Bim, were down regulated. In these cells, there was no evidence of RTKN2 binding to the GTPases, RhoA or Rac2. Consistent with the role of RTKN2 in HEK over-expressing cells, suppression of RTKN2 in primary human CD4⁺ T-cells reduced viability and increased sensitivity to 25-OHC. The expression of the pro-apoptotic genes, Bax and Bim were increased while BCL-2 was decreased. In both cell models RTKN2 played a role in the process of intrinsic apoptosis and this was dependent on either NF-KappaB signaling or expression of downstream BCL-2 genes. As RTKN2 is a highly expressed in CD4⁺ T-cells it may play a role as a key signaling switch for regulation of genes involved in T-cell survival.

Cytotoxic and apoptotic effects of single and mixed oxides of beta-sitosterol on HepG2-cells

While health implications caused by cholesterol oxidation products (COPs) seem to be generally accepted, research on phytosterol oxidation products (POPs) is still limited. Since POPs are commercially not available knowledge on their toxic activities is mainly derived from blends instead of pure compounds. Therefore the aim of the present study was to examine the cytotoxicity of three individual oxidation products of beta-sitosterol, 7-ketositosterol, 7beta-OH-sitosterol, 7alpha-OH-sitosterol, a mixture of 6beta-OH-3-keto-sitosterol/6alpha-OH-3-keto-sitosterol (ratio 4:3) and a mixture of polar oxides towards HepG2-cells. All tested compounds were found to reduce cell viability in a significant and concentration dependent way, particularly 7-keto- and 7alpha-OH-sitosterol showed to be highly active. Only for 7-ketositosterol an increase in early apoptotic cells was observed. Enhancement of O(2)(-) production was assessed for all oxides, whereas malondialdehyd (MDA) levels were increased by 7-keto- and 7alpha-OH-sitosterol only. However, cell death did not appear to be necessarily dependent on the generation of oxidative stress. Further no DNA strand breaks were observed with the COMET assay. By assessing the accumulation of single oxidation products in the cells a link between higher proportions of oxides inside the cells and their cytotoxic potential could be found.

Human carotid atherosclerotic plaque increases oxidative state of macrophages and low-density lipoproteins, whereas paraoxonase 1 (PON1) decreases such atherogenic effects

Human atherosclerotic plaque contains a variety of oxidized lipids, which can facilitate further oxidation. Paraoxonase 1 (PON1) is a high-density lipoprotein (HDL)-associated esterase (lipolactonase), exhibiting antiatherogenic properties. The aims of the present study were to examine the oxidizing potency of the human carotid plaque lipid extract (LE), and the antiatherogenic role of PON1 on LE oxidation competence. Human carotid plaques were extracted by organic solvent, and the extract was incubated with lipoprotein particles, with macrophages, or with probes sensitive to oxidative stress, with or without preincubation with PON1, followed by oxidative-stress assessment. Our findings imply that the LE oxidized LDL, macrophages, and exogenous probes and decreases HDL-mediated cholesterol efflux from macrophages, in a dose-dependent manner. Incubation of PON1 with LE significantly affects LE composition, reduces LE atherogenic properties, and decreases the extract's total peroxide concentration by 44%, macrophage oxidation by 25%, and probe oxidation by up to 52%. We conclude that these results expand our understanding of how the plaque itself accelerates atherogenesis and provides an important mechanism for attenuation of atherosclerosis development by the antioxidant action of PON1 on the atherosclerotic plaque.

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.

In vitrofolate supplementation alleviates oxidative stress, mitochondria-associated death signalling and apoptosis induced by 7-ketocholesterol

Folate has recently been proposed as a new antioxidant. Folate supplementation may have a protective effect in counteracting oxidant-induced apoptotic damage. The present studies were undertaken to examine whether there is a direct link between folate levels, antioxidant capability and reduced apoptotic damage. Using anin vitrocellular model of 7-ketocholesterol (KC)-induced apoptosis, U937 cells were pre-cultured with a folate-deficient medium supplemented with various levels of folate (2–1500μmol/l) before treatment with 7-KC. Apoptotic markers, mitochondria-associated death signals and levels of reactive oxygen species were assayed. After treatment with 7-KC for 30h, low and high levels of folate supplementation significantly (P<0.05) reduced nuclear DNA loss. Only high levels of folate supplementation (>1000μmol/l) were effective in counteracting 7-KC-promoted apoptotic membrane phosphatidylserine exposure and DNA laddering. The attenuation of 7-KC-induced apoptotic damage by high-dose folate supplementation coincided with a partial normalization of mitochondria membrane potential dissipation, a suppression of cytochromecrelease and an inhibition of procaspase 3 activation. The prevention of mitochondrial dysfunctions and apoptotic processes was associated with antioxidant actions of high-dose folate by a marked scavenging of intracellular superoxide. Collectively, our present results demonstrate thatin vitrofolate supplementation exerts differentially protective effects against 7-KC-induced damage. High-dose supplementation alleviates oxidative stress, mitochondria-associated death signalling and apoptosis induced by 7-KC. However, thein vivorelevance is not clear and requires further study.

Cytotoxicity of myeloperoxidase/nitrite-oxidized low-density lipoprotein toward endothelial cells is due to a high 7beta-hydroxycholesterol to 7-ketocholesterol ratio

Oxygenated cholesterols (oxysterols) formed during oxidation of low-density lipoprotein (LDL) are associated with endothelial dysfunction and atherogenesis. We compared the profile of oxysterols in modified human LDL obtained on reaction with myeloperoxidase/H2O2 plus nitrite (MPO/H2O2/nitrite-oxLDL) with that on Cu2+ -catalyzed oxidation. The 7beta-hydroxycholesterol/7-ketocholesterol ratio was markedly higher in MPO/H2O2/nitrite-oxLDL than in Cu2+ -oxidized LDL (7.9 +/- 3.0 versus 0.94 +/- 0.10). Like MPO/H2O2/nitrite-oxLDL, 7beta-hydroxycholesterol was cytotoxic toward endothelial cells through eliciting oxidative stress. Cytotoxicity was accompanied by DNA fragmentation and was prevented by the NADPH oxidase inhibitor apocynin, suggesting stimulation of NADPH oxidase-mediated O2-* formation. 7-Ketocholesterol was only cytotoxic when added alone, whereas a 1:1-mixture with 7beta-hydroxycholesterol surprisingly was noncytotoxic. We conclude from our data that (i) 7beta-hydroxycholesterol is a pivotal cytotoxic component of oxidized LDL, (ii) 7-ketocholesterol protects against 7beta-hydroxycholesterol in oxysterol mixtures or oxLDL, (iii) the 7beta-hydroxycholesterol/7-ketocholesterol ratio is a crucial determinant for cytotoxicity of oxidized LDL species and oxysterol mixtures, and (iv) the low share of 7-ketocholesterol explains the higher cytotoxicity of MPO/H2O2/nitrite-oxLDL than other forms of oxidized LDL. The dietary polyphenol (-)-epicatechin inhibited not only formation but also cytotoxic actions of both oxLDL and oxysterols.

Oxysterol mixtures, in atheroma-relevant proportions, display synergistic and proapoptotic effects

Apoptotic cells in atheroma lesions may contribute to plaque development and instability. Oxysterols constitute the major toxic component in oxLDL and are present in mixed forms in human atheroma lesions. However, the cellular effects of oxysterols have been mostly studied individually. In the present study, we investigated the cytotoxic effects of 7beta-hydroxycholesterol (7betaOH), 7-ketocholesterol (7keto), 25-hydroxycholesterol (25OH), and 27-hydroxycholesterol (27OH) on U937 monocytic cells, both individually and in atheroma-relevant mixtures mimicking the oxysterol composition reported in human atheroma lesions. Apoptosis and necrosis were studied by examining cell morphology, phosphatidylserine exposure, caspase activation, and the terminal dUTP nick end-labeling technique. Cellular reactive oxygen species and total amount of reduced thiols were measured by using fluorescence probes and 5,5'-dithiobis-(2-nitrobenzoic acid), respectively. We found that 7betaOH and 7keto induced caspase activation, ROS production, cellular thiol depletion, permeabilization of lysosomal and mitochondrial membranes, and cell death. 25OH and 27OH did not cause any of the above alterations, whereas 7betaOH and 7keto exerted synergistic toxic effects. Although single 25OH or 27OH exhibited quenching effects on both 7betaOH- and 7keto-induced cell death, the combination of all four oxysterols in atheroma-relevant proportions was proapoptotic. Our findings indicate that the major oxysterols accumulated in human atheroma are proapoptotic and may contribute to atherosclerotic lesion development.

Comparison of the cytotoxic, pro-oxidant and pro-inflammatory characteristics of different oxysterols

Oxidized low-density lipoproteins play important roles in the development of atherosclerosis and contain several lipid-derived, bioactive molecules which are believed to contribute to atherogenesis. Of these, some cholesterol oxidation products, referred to as oxysterols, are suspected to favor the formation of atherosclerotic plaques involving cytotoxic, pro-oxidant and pro-inflammatory processes. Ten commonly occurring oxysterols (7alpha-, 7beta-hydroxycholesterol, 7-ketocholesterol, 19-hydroxycholesterol, cholesterol-5alpha,6alpha-epoxide, cholesterol-5beta,6beta-epoxide, 22R-, 22S-, 25-, and 27-hydroxycholesterol) were studied for both their cytotoxicity and their ability to induce superoxide anion production (O2*-) and IL-8 secretion in U937 human promonocytic leukemia cells. Cytotoxic effects (phosphatidylserine externalization, loss of mitochondrial potential, increased permeability to propidium iodide, and occurrence of cells with swollen, fragmented and/or condensed nuclei) were only identified with 7beta-hydroxycholesterol, 7-ketocholesterol and cholesterol-5beta,6beta-epoxide, which also induce lysosomal destabilization associated or not associated with the formation of monodansylcadaverine-positive cytoplasmic structures. No relationship between oxysterol-induced cytotoxicity and HMG-CoA reductase activity was found. In addition, the highest O2*- overproduction quantified with hydroethidine was identified with 7beta-hydroxycholesterol, 7-ketocholesterol and cholesterol-5beta,6beta-epoxide, with cholesterol-5alpha, 6alpha-epoxide and 25-hydroxycholesterol. The highest capacity to simultaneously stimulate IL-8 secretion (quantified by ELISA and by using a multiplexed, particle-based flow cytometric assay) and enhance IL-8 mRNA levels (determined by RT-PCR) was observed with 7beta-hydroxycholesterol and 25-hydroxycholesterol. None of the effects observed for the oxysterols were detected for cholesterol. Therefore, oxysterols may have cytotoxic, oxidative, and/or inflammatory effects, or none whatsoever.

Cellular toxicity of oxycholesterols

Oxycholesterols (OS) are formed from cholesterol or its immediate precursors by enzymatic or free radical action in vivo, or they may be derived from food. OS exhibit a wide spectrum of biological activities. In OS cytotoxicity, several mechanisms seem to be involved: e.g. inhibition of HMG-CoA reductase activity, antiproliferative action, apoptosis induction, replacement of cholesterol by OS in membranes followed by changes in cellular membrane structure and functionality, and immune system functions alteration. Furthermore, OS may be mutagenic and carcinogenic and may serve as intracellular signaling or regulatory molecules. Here we review OS cellular activities with special attention to the cytotoxic action in vivo and in vitro using experimental models.

Cytotoxic effect of 7beta-hydroxycholesterol on human NCI-H460 lung cancer cells

The cytotoxic activity of 7beta-hydroxycholesterol (7beta-OHC) was evaluated on human NCI-H460 lung cancer cells. 7beta-OHC decreased clonogenic survival of NCI-H460 in a dose dependent pattern. 7beta-OHC induced apoptosis in NCI-H460, with the characteristic features like increase in sub-G(1) hypodiploid (apoptotic) cells, and apoptotic body formation, as evidenced by flow cytometry and fluorescence microscope, respectively. Apoptosis was also associated with loss of mitochondrial transmembrane potential, and the activation of caspases 9 and 3. 7beta-OHC resulted in generation of reactive oxygen species (ROS) during apoptosis. On the whole, the results indicated that 7beta-OHC inhibited the proliferation of NCI-H460 cells through apoptosis via caspase activation.

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.

Comparison of the apoptotic processes induced by the oxysterols 7β-hydroxycholesterol and cholesterol-5β,6β-epoxide

Oxysterols have been shown to induce apoptosis in a variety of cell lines. The mechanism of oxysterol-induced apoptosis is mainly known at the post-mitochondrial level. The aim of the present study was to compare the pathway of apoptosis induced by the oxysterols 7beta-hydroxycholesterol (7beta-OH) and cholesterol-5beta,6beta-epoxide (beta-epoxide) in U937 cells. To this end, we employed a range of inhibitors of apoptosis; a broad-spectrum caspase inhibitor, a specific caspase-3 inhibitor and an inhibitor of cytochrome c release and the antioxidants; trolox, ebselen and resveratrol. The three inhibitors of apoptosis prevented cell death induced by 7beta-OH; however, in beta-epoxide-treated cells, the inhibitor of cytochrome c release did not protect against apoptosis. The cellular antioxidant glutathione was depleted in 7beta-OH-treated cells but not in cells incubated with beta-epoxide. Trolox, a water-soluble synthetic analogue of alpha-tocopherol, prevented 7beta-OH-induced apoptosis but did not protect against cell death induced by beta-epoxide. Ebselen and resveratrol did not protect U937 cells against apoptosis induced by either 7beta-OH or beta-epoxide. Our results suggest that differences occur in the pathways of apoptosis induced by 7beta-OH and beta-epoxide in U937 cells.

Trojan horse-like behavior of a biologically representative mixture of oxysterols

Oxysterols, 27-carbon atoms cholesterol oxidation products, are consistently detectable in minimally oxidized low density lipoproteins (oxLDLs) and accumulate in the core of fibrotic plaques. Several oxysterols of pathophysiological interest have been shown to possess many and diverse biochemical activities. In particular, 7-ketocholesterol (7K), a major cholesterol oxide both in oxLDLs and in atherosclerotic lesions, is able to lead vascular cells to apoptosis. Indeed, when 7K is added to cells of the macrophage lineage, in a concentration range actually detectable in hypercholesterolemic patients, a marked apoptotic effect was observed. However, when identical concentrations of 7K are given to the same cells in a mixture with other oxysterols, also detectable in human low density lipoprotein (LDL), cell apoptosis was dramatically reduced. Of note, identical amounts of unoxidized cholesterol did not show any significant pro-apoptotic effect. With the aim to investigate the mechanisms underlying the quenching of 7K-dependent apoptosis by the oxysterol mixture, we found that the combined oxysterol mixture counteracted the ability of 7K given alone to strongly increase the steady-state level of reactive oxygen species (ROS) in macrophages as well as the up-regulation of the pro-apoptotic factor p21 and the triggering of the mitochondria-dependent pathway of apoptosis. Competition among oxysterols, apparently at NADPH oxidase level, diminishes the macrophage ROS production and direct toxicity that is evoked by defined oxysterols, as for instance, 7-ketocholesterol.

Mechanisms of resistance to the cytotoxic effects of oxysterols in human leukemic cells

We have developed hematopoietic cells resistant to the cytotoxic effects of oxysterols. Oxysterol-resistant HL60 cells were generated by continuous exposure to three different oxysterols-25-hydroxycholesterol (25-OHC), 7-beta-hydroxycholesterol (7beta-OHC) and 7-keto-cholesterol (7kappa-C). We investigated the effects of 25-OHC, 7beta-OHC, 7kappa-C and the apoptotic agent staurosporine on these cells. The effect of the calcium channel blocker nifedipine on oxysterol cytotoxicity was also investigated. Differential display and real-time PCR were used to quantitate gene expression of oxysterol-sensitive and -resistant cells. Our results demonstrate that resistance to the cytotoxic effects of oxysterols is relatively specific to the type of oxysterol, and that the cytotoxicity of 25-OHC but not that of 7beta-OHC and 7kappa-C, appears to occur by a calcium dependent mechanism. Oxysterol-resistant cells demonstrated no significant difference in the expression of several genes previously implicated in oxysterol resistance, but expressed the bcl-2 gene at significantly lower levels than those observed in control cells. We identified three novel genes differentially expressed in resistant cells when compared to HL60 control cells. Taken together, the results of this study reveal potentially novel mechanisms of oxysterol cytotoxicity and resistance, and indicate that cytotoxicity of 25-OHC, 7beta-OHC and 7kappa-C occur by independent, yet overlapping mechanisms.

Oxysterol mixtures prevent proapoptotic effects of 7-ketocholesterol in macrophages: implications for proatherogenic gene modulation

Oxysterols are common components of oxidized low-density lipoprotein and accumulate in the core of fibrotic plaques as a mixture of cholesterol and cholesteryl ester oxidation products. The proapoptotic effects of a biologically representative mixture of oxysterols was compared with equimolar amounts of 7-ketocholesterol and unoxidized cholesterol. The oxysterol mixture in a concentration range actually detectable in hypercholesterolemic patients did not stimulate programmed cell death in cultivated murine macrophages. Unoxidized cholesterol also produced no effect. By contrast, when given alone, 7-ketocholesterol strongly stimulated the mitochondrial pathway of apoptosis with cytochrome c release, caspase-9 activation, and eventually caspase-3 activation. Subsequent experiments showed that when 7-ketocholesterol was administered to cells together with another oxysterol, namely 7betaOH-cholesterol, the strong proapoptotic effect of 7-ketocholesterol was markedly attenuated. As regards the mechanism underlying this quenching, we found that the combined oxysterol treatment counteracted the ability of 7-ketocholesterol, when administered alone, to strongly up-regulate the steady-state levels of reactive oxygen species (ROS) without interfering with sterol uptake. Furthermore, this increase in intracellular ROS appeared to be responsible for the up-regulation of proapoptotic factor, p21, after treatment with 7-ketocholesterol but not in cells challenged with the oxysterol mixture. Competition among oxysterols, apparently at the level of NADPH oxidase, diminishes the ROS induction and direct toxicity that is evoked by specific oxysterols. As a consequence, a more subtle gene modulation by oxysterols becomes facilitated in vascular cells.

Generation of an oxidative stress precedes caspase activation during 7?-hydroxycholesterol-induced apoptosis in U937 cells

The oxysterol 7beta-hydroxycholesterol (7beta-OH) has been shown to induce apoptosis in a number of cell lines. Though not fully elucidated, the mechanism through which this oxysterol induces cell death is thought to involve the generation of an oxidative stress leading to perturbation of the mitochondrion and release of cytochrome c into the cytosol. Cytochrome c together with Apaf-1 causes activation of the initiator caspase, caspase-9, which in turn activates caspase-3 ultimately leading to the degradation of poly(ADP-ribose) polymerase (PARP). The objective of the present study was to investigate the signalling pathway in 7beta-OH-induced apoptosis in U937 cells, a human monocytic blood cell line known to undergo apoptosis upon treatment with 7beta-OH, over a time course of 48 h. Apoptosis was evident after 24 h incubation. Glutathione levels were decreased after 6 h and this was coupled with an increase in SOD activity. Through western blot analysis we examined expression of caspase-3, -8, and -9 and cleavage of the caspase-3 substrate PARP. The sequence proceeded with activation of caspase-9 after 9 h, caspase-3 at the 12 h timepoint, and cleavage of PARP after 24 h treatment with 7beta-OH. Caspase-8 did not appear to play a major role in this particular apoptotic pathway.

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.