Oyxsterols induce membrane procoagulant activity in monocytic THP-1 cells

Therapeutic Role of Sirtuins Targeting Unfolded Protein Response, Coagulation, and Inflammation in Hypoxia-Induced Thrombosis

Thrombosis remains one of the leading causes of morbidity and mortality across the world. Many pathological milieus in the body resulting from multiple risk factors escort thrombosis. Hypoxic condition is one such risk factor that disturbs the integrity of endothelial cells to cause an imbalance between anticoagulant and procoagulant proteins. Hypoxia generates reactive oxygen species (ROS) and triggers inflammatory pathways to augment the coagulation cascade. Hypoxia in cells also activates unfolded protein response (UPR) signaling pathways in the endoplasmic reticulum (ER), which tries to restore ER homeostasis and function. But the sustained UPR linked with inflammation, generation of ROS and apoptosis stimulates the severity of thrombosis in the body. Sirtuins, a group of seven proteins, play a vast role in bringing down inflammation, oxidative and ER stress and apoptosis. As a result, sirtuins might provide a therapeutic approach towards the treatment or prevention of hypoxia-induced thrombosis. Sirtuins modulate hypoxia-inducible factors (HIFs) and counteract ER stress-induced apoptosis by attenuating protein kinase RNA-like endoplasmic reticulum kinase (PERK)/Eukaryotic translation initiation factor 2α (eIF2α) pathway activation. It prevents ER-stress mediated inflammation by targeting X-Box Binding Protein 1 (XBP1) and inhibiting nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κβ) signaling through deacetylation. Sirtuins also obstruct nucleotide-binding domain, leucine-rich-containing family, pyrin domain containing 3 (NLRP3) inflammasome activation to reduce the expression of several pro-inflammatory molecules. It protects cells against oxidative stress by targeting nuclear factor erythroid 2-related factor 2 (Nrf2), glutathione (GSH), forkhead box O3 (FOXO3), superoxide dismutase (SOD), catalase (CAT), peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), glucose-6-phosphate dehydrogenase (G6PD), phosphoglucomutase-2 (PGAM2), and NF-κB, to name few. This review, thus, discusses the potential role of sirtuins as a new treatment for hypoxia-induced thrombosis that involves an intersection of UPR and inflammatory pathways in its pathological manifestation.

Coalescence of phospholipid membranes as a possible origin of anticoagulant effect of serum proteins

Interactions between phospholipid membranes (made of palmitoyloleoylphosphatidylcholine, cardiolipin and cholesterol) after addition of beta2 glycoprotein I (beta2GPI) or anti-beta2GPI antibodies or a mixture of both were studied by observing giant phospholipid vesicles under the phase contrast microscope. Both, negatively charged and neutral vesicles coalesced into complexes and adhered to the bottom of the observation chamber in the presence of beta2GPI in solution. Anti-beta2GPIs alone or previously mixed with beta2GPI caused coalescence of charged but not neutral vesicles, i.e. for neutral membranes the effect of beta2GPI was abolished by the presence of anti-beta2GPIs. Since the presence of the above adhesion mediators can prevent fragmentation of the membrane we propose a (new) possible anticoagulant mechanism for some serum proteins by preventing the release of prothrombogenic microexovesicles into circulation.

Murine macrophage P2X7 receptors support rapid prothrombotic responses

Non-apoptotic externalization of phosphatidylserine (PS) can act as a reactive surface for the efficient assembly of the prothrombinase complex leading to thrombin generation and coagulation. Here we show that extracellular ATP, acting at the macrophage P2X(7) receptor, drives the rapid Ca(2+)-dependent formation and release of PS-rich microvesicles that enhance the assembly of the prothrombinase complex and subsequent formation of thrombin. Incubation with P2X(7) receptor antagonists (KN-62 and Brilliant Blue G) attenuates ATP induced prothrombotic responses. Consistent with the hypothesis that exposed PS enhances prothrombinase activity; pre-incubation with annexin V blocks the increase in thrombin formation. The rapid translocation of PS and formation of pro-thrombotic microvesicles occurs in the absence of cell lysis. These data demonstrate that the pro-inflammatory P2X(7) receptor can also support and propagate rapid increases in thrombin formation.

The effect of oxysterols, individually and as a representative mixture from food, on in vitro cultured bovine ovarian granulosa cells

The cytotoxicity of five oxysterols identified in cooked fish, 7-ketocholesterol, 7beta-hydroxycholesterol, cholesterol 5alpha,6alpha-epoxide, cholestanetriol and 4-cholesten-3-one, was investigated in primary cultures of bovine ovarian granulosa cells. Cells were exposed to the oxysterols individually and to a mixture of the same oxysterols for 24 h. Cell viability as determined by trypan blue exclusion and mitochondrial integrity (3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide (MTT) reduction) was reduced by 0.5 and 2.5 microM 7beta-hydroxycholesterol, cholesterol 5alpha,6alpha-epoxide, cholestanetriol and 4-cholesten-3-one, but not by 0.5 or 2.5 microM 7-ketocholesterol under the same culture conditions. A mixture of 7-ketocholesterol, 7beta-hydroxycholesterol, cholesterol 5alpha,6alpha-epoxide, cholestanetriol and 4-cholesten-3-one 0.5 microM each; 2.5 microM oxysterol in total) did not change cell viability relative to controls. Lipid peroxidation, as determined by thiobarbituric acid reactive substances assay, was unaffected by a 24-hour exposure of granulosa cells to individual oxysterols but was increased slightly by the oxysterol mixture. The specific activities of antioxidant enzymes superoxide dismutase and catalase were increased to different extents (1.17- to 6.43-fold), relative to controls, by the administration of individual oxysterols and the oxysterol mixture. These results indicate that while some individual oxysterols can induce cytotoxic effects and defensive responses in bovine ovarian granulosa cells, administration of the same oxysterols as a mixture does not elicit the same responses. In addition, the oxysterols tested exerted a pro-apoptotic effect on granulosa cells when administered individually at concentrations of 0.5 and 2.5 microM, but not when administered as a 2.5 microM oxysterol mixture. The results suggest that major oxysterols are not universally cytotoxic, they may complete with other oxysterols for receptor sites, and that the simultaneous presence of several different oxysterol species may reduce the adverse effects of individual oxysterols.

Vessel wall apoptosis and atherosclerotic plaque instability

Atherosclerotic cardiovascular disease remains the leading cause of death in the industrialized world. Most cardiovascular deaths result from acute coronary syndromes, including unstable angina pectoris and acute myocardial infarction. Coronary syndromes often arise from acute coronary thrombosis, itself commonly a result of disruption or rupture of the fibrous cap of a lipid-laden atherosclerotic plaque. Despite this huge clinical burden of atherosclerotic plaque instability, our understanding of the molecular mechanisms mediating atherosclerotic plaque disruption and rupture, at a cellular level, remains limited. Placed in a clinical context, this review discusses our current understanding of the molecular basis for atherosclerotic plaque instability, with particular emphasis on the process of apoptosis-or programmed cell death-seen increasingly as playing a key role in a number of cell types within the vessel wall.

Annexin A5 down-regulates surface expression of tissue factor: a novel mechanism of regulating the membrane receptor repertoir

Phosphatidylserine (PtdSer) is exposed on the external leaflet of the plasma membrane during apoptosis. The protein annexin A5 (anxA5) shows high affinity for PtdSer. When anxA5 binds to the PtdSer-expressing membranes during apoptosis, it crystallizes as an extended two-dimensional network and activates thereby a novel portal of cell entry that results in the internalization of the PtdSer-expressing membrane patches. This novel pathway of cell entry is potentially involved in the regulation of the surface expression of membrane receptors. In this study we report the regulation of surface expression of the initiator of blood coagulation tissue factor (TF) by this novel pathway of cell entry. AnxA5 induces the internalization of tissue factor expressed on the surface of apoptotic THP-1 macrophages. This down-regulation depends on the abilities of anxA5 to bind to PtdSer and to form a two-dimensional crystal at the membrane. We furthermore show that THP-1 cells produce and externalize anxA5 that cause the internalization of TF in an autocrine type of mechanism. We extended our in vitro work to the in vivo situation and show in a mouse model that anxA5 causes the down-regulation of TF expression by smooth muscle cells of the media of the carotid artery that was mechanically injured. In conclusion, anxA5 down-regulates surface-expressed TF by activating the novel portal of cell entry. This mechanism may be part of a more general autocrine function of anxA5 to regulate the plasma membrane receptor repertoir under stress conditions associated with the surface expression of PtdSer.

Different quantitative apoptotic traits in coronary atherosclerotic plaques from patients with stable angina pectoris and acute coronary syndromes

BACKGROUND

Apoptosis in human atherosclerotic coronary plaques possibly causes plaque destabilization by contributing to the weakening and breaking down of the fibrous cap. We tested the hypothesis that apoptosis is quantitatively increased in unstable atherosclerotic plaques.

METHODS AND RESULTS

We analyzed the expression of apoptotic genes such as BAX, CASP1, FAS, FAS L, FOS, MDM2, NFkB2, P53, PCNA, TERT, and XRCC1 in coronary plaques collected with directional coronary atherectomy from 15 patients with stable angina and 15 with acute coronary syndromes without ST elevation (ACS). Total RNA was extracted and cDNA was amplified with a specific set of primers and TaqMan probes. Apoptosis was also revealed by DNA laddering. To clarify the source of mRNAs, we performed in situ reverse transcriptase-polymerase chain reaction coupled with immunocytochemistry and found a substantial overlap between the mRNAs of the above genes and vascular smooth muscle cells. Gene expression analysis showed that the proapoptotic genes (ie, BAX, CASP1, FAS, FAS L, FOS, NFkB2, P53, PCNA) were significantly more expressed (P<0.001) in ACS plaques, whereas the antiapoptotic genes (ie, MDM2, TERT, XRCC1) were more transcribed (P<0.001) in stable angina plaques. Total gDNA gel electrophoresis identified a laddering pattern in the ACS plaques as evidence of end-point apoptosis. Western blotting substantially confirmed the above data.

CONCLUSIONS

Our findings support the idea that ACS plaques are committed to apoptosis through an established meshwork of gene activation and inactivation, whereas stable angina plaques retain active cell homeostasis and repair mechanisms.

Synthesis of long-chain polyunsaturated fatty acids is inhibited in vivo in hypercholesterolemic rabbits and in vitro by oxysterols

Plasma total lipids, total cholesterol (cholesterol esters and free cholesterol) and oxysterol (mainly 7 beta-hydroxycholesterol (7 beta OH)) concentrations were significantly elevated in New Zealand rabbits fed a 2% cholesterol-containing diet with respect to controls fed the same diet without cholesterol. In addition, linoleic (18:2 n-6) and alpha-linolenic acid (18:3 n-3) plasma concentrations were significantly elevated in hypercholesterolemic rabbits, while concentrations of long-chain n-6 and n-3 derivatives were reduced. Studies in monocytic cell line THP-1 revealed that 7 beta OH markedly inhibited the conversion of 18:2 to 20:4 n-6 and of 18:3 to 22:6 n-3, indicating depression of the desaturation steps; in particular the inhibition was greater for the Delta 5 desaturation step. Furthermore, experiments of Real-Time PCR showed that 5-10 microM 7 beta OH decreased the Delta 5 gene expression. In conclusion, atherogenic oxysterols interfere with the production of long-chain polyunsaturated fatty acids from their precursors both in hypercholesterolemic rabbits and in cultured cells.

Delphinidin, an active compound of red wine, inhibits endothelial cell apoptosis via nitric oxide pathway and regulation of calcium homeostasis

1. Epidemiological studies have suggested that moderate consumption of natural dietary polyphenolic compounds might reduce the risk of cardiovascular disease and also protect against cancer. The present study investigates the effects of delphinidin, an anthocyanin present in red wine, on bovine aortic endothelial cells apoptosis. 2. Based on flow cytometry, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling analysis and detection of mitochondrial cytochrome c release, we show that delphinidin (10(-2) g l(-1)) alone had no effect either on necrosis or on apoptosis, but it significantly reduced apoptosis elicited by actinomycin D (1 micro g ml(-1), 24 h) and 7beta-hydroxycholesterol (10 micro g ml(-1), 18 h). 3. The protective effect of delphinidin was abolished by inhibitors of nitric oxide-synthase (NOS) (L-NA, 100 micro M and SMT, 100 micro M), guanylyl cyclase (ODQ, 100 micro M) and MAP kinase (PD98059, 30 micro M). 4. Western blot analysis and protein detection by confocal microscopy demonstrate that the antiapoptotic effect of delphinidin was associated with an increased endothelial NOS expression mediated by a MAP kinase pathway. 5. Finally, delphinidin alone had no effect on cytosolic-free calcium ([Ca(2+)](i)), but normalized the changes in [Ca(2+)](i) produced by actinomycin D towards the control values, suggesting that the antiapoptotic effect of delphinidin is associated with the maintenance of [Ca(2+)](i) in the physiological range. 6. All of the observed effects of delphinidin may preserve endothelium integrity, the alteration of which lead to pathologies including cardiovascular diseases, such as atherosclerosis, and is often associated with cancers. In conclusion, the protective effect of delphinidin against endothelial cell apoptosis contributes to understand the potential benefits of a consumption rich in polyphenols.

Impairment of the cytotoxic and oxidative activities of 7 beta-hydroxycholesterol and 7-ketocholesterol by esterification with oleate

Atherosclerosis involves inflammatory processes, as well as cytotoxic and oxidative reactions. In atherosclerotic plaques, these phenomena are revealed by the presence of dead cells, oxidized lipids, and oxidative DNA damage, but the molecules triggering these events are still unknown. As 7 beta-hydroxycholesterol and 7-ketocholesterol, which are present at elevated concentrations in atherosclerotic lesions, are strongly cytotoxic and pro-oxidative, their effects were determined on cell death, superoxide anion and nitric oxide production, lipid peroxidation, and oxidative DNA damage. 7-Ketocholesterol- and 7 beta-hydroxycholesterol-induced cell death leads to a loss of mitochondrial potential, to increased permeability to propidium iodide, and to morphological nuclear changes (swelling, fragmentation, and/or condensation of nuclei). These effects are preceded by the formation of cytoplasmic monodansylcadaverine-positive structures and are associated with a rapid enhancement of cells overproducing superoxide anions, a decrease in cells producing nitric oxide, lipid peroxidation (formation of malondialdehyde and 4-hydroxynonenal adducts, low ratio of [unsaturated fatty acids]/[saturated fatty acids]) as well as oxidative DNA damage (8-oxoguanine formation). Noteworthy, none of the cytotoxic features previously observed with 7 beta-hydroxycholesterol and 7-ketocholesterol were noted with cholesterol, 7 beta-hydroxycholesteryl-3-oleate and 7-ketocholesteryl-3-oleate, with the exception of a slight increase in superoxide anion production with 7 beta-hydroxycholesteryl-3-oleate. This finding supports the theory that 7 beta-hydroxycholesterol and 7-ketocholesterol could induce cytotoxic and oxidative processes observed in atherosclerotic lesions and that esterification of these compounds may contribute to reducing atherosclerosis progression.

Intrinsic pathway of blood coagulation contributes to thrombogenicity of atherosclerotic plaque

Thrombosis is the major mechanism underlying acute complications of atherosclerosis. Although thrombogenicity of atherosclerotic plaques has been ascribed to activation of the extrinsic pathway of blood coagulation, in the present study we investigated contribution of the intrinsic factor VIII (fVIII)-dependent pathway. We found that in vitro exposure of human macrophages and smooth muscle cells (SMCs) to atherogenic oxidized low-density lipoprotein (oxLDL) enhances their ability to support activity of 2 major complexes of the intrinsic pathway, Xase and prothrombinase, leading to a 20- and 10-fold increase in thrombin formation, respectively. In contrast, human aortic endothelial cells were less responsive to oxLDL. The increase in the intrinsic procoagulant activity was related to formation of additional fVIII binding sites due to enhanced translocation of phosphatidylserine to the outer surface of oxLDL-treated cells and a 5-fold higher affinity of interaction between components of the Xase complex, activated factors VIII and IX. Processes occurring at early apoptotic stages, including changes in the cell membrane induced by free radicals, may be related to activation of the intrinsic pathway as suggested by effects of inhibitors of early apoptosis on thrombin formation. Immunohistochemical studies on human atherectomy specimens revealed the presence of fVIII in the vicinity of macrophages and SMCs in atheromatous regions with massive deposits of oxLDL, supporting the possible involvement of the intrinsic pathway in thrombus formation in vivo. Our data predict that the intrinsic pathway significantly enhances thrombogenicity of atherosclerotic lesions after removal of the endothelial layer and exposure of SMCs and macrophages to blood flow.

Possible pathogenic role of muscle cell dysfunction in motor neuron death in spinal muscular atrophy

We have previously shown that myofibers formed by fusion of muscle satellite cells from spinal muscular atrophy (SMA) I or II undergo degeneration 1 to 3 weeks after innervation by rat embryonic spinal cord explants, whereas normal myofibers survive for several months. In the "muscle component" of the coculture, the only cells responsible for the degeneration are the SMA muscle satellite cells. Moreover, SMA muscle satellite cells do not fuse as rapidly as do normal muscle satellite cells. To determine whether death of muscle cells precedes that of motor neurons, we studied the origin and kinetics of release of apoptotic microparticles. In SMA cocultures, motor neuron apoptosis occurred before myofiber degeneration becomes visible, indicating that SMA myofibers were unable to sustain survival of motor neurons. In normal cocultures, motor neuron apoptosis occurred 4 days after innervation. However, it did not continue beyond 2 days. These results strengthen the hypothesis that SMA is due to a defect in neurotrophic muscle cell function.

Abnormalities in hemostasis in acute promyelocytic leukemia

Our understanding of the hemostatic abnormalities in acute promyelocytic leukemia (APL) has undergone remarkable changes over the past three decades. Hemorrhagic complications lead to significant morbidity and mortality in patients with APL. Up to 30% of early deaths are due to hemorrhagic complications. Our concept of the coagulation changes in APL have evolved from a simple belief that the bleeding was due to disseminated intravascular coagulation (DIC) from underlying infection, to a knowledge that the APL cells themselves are responsible for the bleeding. We have observed that apoptotic APL cells have increased generation of thrombin and therefore contribute to the hypercoagulability and DIC. In addition, excessive fibrinolysis has been recognized to play an important role. The advent of a new therapeutic approach with all-trans-retinoic acid has improved overall survival and has greatly shortened the hemorrhagic phase of the disease, but only to produce a new complication, the retinoic acid syndrome. Its pathogenesis and management still remain to be the challenge for the present.

Thrombogenic role of cells undergoing apoptosis

Apoptosis is involved in many biological processes, especially during chemotherapy in cancer patients. Chemotherapy is also associated with an increased risk of thrombosis. The relationship between thrombogenicity and apoptosis was studied in various human tumour cell lines and non-tumour cell lines. Apoptosis was induced by the chemotherapeutic agent camptothecin and by Fas ligand, then quantified by staining with fluorescein isothiocyanate-conjugated annexin V and propidium iodide. A significant correlation between thrombin generation and degree of apoptosis was observed (P < 0.0005). Addition of anti-tissue factor antibody in excess or of tissue factor pathway inhibitor partially inhibited thrombin generation, suggesting that tissue factor activation was responsible for this process. A statistical correlation between tissue factor activity and degree of apoptosis was also found (P < 0.005). Both thrombin generation and tissue factor activity were blocked by the addition of annexin V, which binds and inhibits phosphatidylserine. This indicates that the exteriorization and exposure of phosphatidylserine on the cell surface membrane during apoptosis were essential for both thrombin generation and tissue factor activation.

Current perspective on the role of apoptosis in atherothrombotic disease

Thrombus formation on a disrupted atherosclerotic plaque is a threatening event that leads to vessel occlusion and acute ischemia. In this current perspective, we present evidence for apoptosis as a major determinant of the thrombogenicity of the plaque lipid core and a potential contributor to plaque erosion and associated thrombosis. Moreover, apoptosis may directly affect blood thrombogenicity through the release of apoptotic cells and microparticles into the bloodstream.

Epicatechin and its in vivo metabolite, 3'-O-methyl epicatechin, protect human fibroblasts from oxidative-stress-induced cell death involving caspase-3 activation

There is considerable current interest in the cytoprotective effects of natural antioxidants against oxidative stress. In particular, epicatechin, a major member of the flavanol family of polyphenols with powerful antioxidant properties in vitro, has been investigated to determine its ability to attenuate oxidative-stress-induced cell damage and to understand the mechanism of its protective action. We have induced oxidative stress in cultured human fibroblasts using hydrogen peroxide and examined the cellular responses in the form of mitochondrial function, cell-membrane damage, annexin-V binding and caspase-3 activation. Since one of the major metabolites of epicatechin in vivo is 3'-O-methyl epicatechin, we have compared its protective effects with that of epicatechin. The results provide the first evidence that 3'-O-methyl epicatechin inhibits cell death induced by hydrogen peroxide and that the mechanism involves suppression of caspase-3 activity as a marker for apoptosis. Furthermore, the protection elicited by 3'-O-methyl epicatechin is not significantly different from that of epicatechin, suggesting that hydrogen-donating antioxidant activity is not the primary mechanism of protection.

Rapid, transient phosphatidylserine externalization induced in host cells by infection with Chlamydia spp

Chlamydia organisms are obligate intracellular bacterial pathogens responsible for a range of human diseases. Persistent infection or reinfection with Chlamydia trachomatis leads to scarring of ocular or genital tissues, and Chlamydia pneumoniae infection is associated with the development of atherosclerosis. We demonstrate that C. trachomatis and C. pneumoniae infection in vitro elicits the externalization of the lipid phosphatidylserine on the surface of human epithelial, endothelial, granulocytic, and monocytic cells. Phosphatidylserine externalization is associated with cellular development, differentiation, and death. Infection-induced phosphatidylserine externalization was immediate, transient, calcium dependent, and infectious dose dependent and was unaffected by a broad-spectrum caspase inhibitor. Chlamydia-infected cells accelerated plasma clotting and increased the macrophage phagocytosis of infected cells that was phosphatidylserine dependent. The rapid externalization of phosphatidylserine by infected cells may be an important factor in the pathogenesis of chlamydial infections.

The Pathogenetic Role of Apoptosis in Hypercoagulable States

Hypercoagulable states are common disorders with high risk of thrombosis associated with cardiovascular and malignant diseases. The pathogenesis of hypercoagulability is multifactorial. The basic physiological mechanism is the imbalance between anticoagulant activities and procoagulant activities in hemostatic system. In this review, we discuss the correlation between apoptosis and thrombogenesis in hypercoagulable states. Some cell-associated cofactors in coagulation system, including phosphatidylserine, tissue factor, thrombomodulin and cancer procoagulant, are regulated during apoptosis of various cell types. Vascular endothelial cells may act as one of the most important aspects affecting the balance of anticoagulant and procoagulant activities. When endothelial cells are activated or induced to undergo apoptosis by a number of physiological factors, such as inflammatory cytokines and bacterial lipopolysaccharide, the procoagulant activities of endothelial cells are enhanced. Other cell types such as apoptotic vascular smooth muscle cells, monocytes and macrophages may also contribute to the pathogenesis in atherosclerosis. Apoptotic tumor cells, which express high level of procoagulant activities, may act as a direct trigger for coagulation activation.

Isolation of a somatic cell mutant resistant to the induction of apoptosis by oxidized low density lipoprotein

Oxidized low density lipoprotein (oxLDL) induces apoptosis in macrophages, smooth muscle cells, and endothelial cells. To elucidate the molecular mechanism of oxLDL-induced cytotoxicity and determine its tissue specificity, we have used Chinese hamster ovary (CHO)-K1 cells expressing human CD36 (CHO/CD36). Expression of CD36 rendered these cells susceptible to killing by oxLDL. This cytotoxicity was due to the induction of apoptosis. Therefore, CD36 expression is the only requirement for oxLDL-induced apoptosis. Oxysterols apparently mediate the cytotoxicity of oxLDL in macrophage foam cells and endothelial cells. 25-Hydroxycholesterol, at concentrations higher than 1 microg/ml, killed CHO-K1 cells, by apoptosis, in medium supplemented with serum as a source of cholesterol. These effects were not seen in a 25-hydroxycholesterol-resistant CHO/CD36 mutant (OX(R)), which was otherwise capable of undergoing apoptosis in response to staurosporine. This mutant was also resistant to killing by oxLDL, suggesting that oxysterols are at least partially responsible for the toxic effects of oxLDL. Oxysterol-induced apoptosis did not involve regulation of sterol regulatory element-binding protein proteolysis or the cholesterol biosynthetic pathway. 25-Hydroxycholesterol stimulated calcium uptake by CHO-K1 cells within 2 min after addition. Treatment of CHO or THP-1 (macrophage) cells with the calcium channel blocker nifedipine prevented 25-hydroxycholesterol induction of apoptosis. OX(R) showed no enhanced calcium uptake in response to 25-hydroxycholesterol.

Human monoclonal anti-phospholipid antibodies selectively bind to membrane phospholipid and beta2-glycoprotein I (beta2-GPI) on apoptotic cells

The ability of an anti-phospholipid (LJ1) and an anti-beta2-GPI (RSP-57) human MoAb to bind to apoptotic but not viable cells was demonstrated in this study. Both MoAbs were derived from patients with systemic lupus erythematosus and anti-phospholipid antibody syndrome. The parallel analysis of the specificity and affinity of four anti-phospholipid human MoAbs suggests that the binding of LJ1 MoAb to apoptotic cells is a specific property of this MoAb. RSP-57 MoAb recognizes apoptotic cells through beta2-GPI which becomes available for binding after the interaction with negatively charged phospholipids. This observation provides evidence that the binding of human anti-phospholipid antibodies to apoptotic cells occurs in both a beta2-GPI-dependent and independent way and involves a restricted group of epitopes. The finding that LJ1 and RSP-57 MoAbs bind apoptotic cells underlines the property of these MoAbs to act as cell membrane markers of apoptosis. Major pathological implications derive from the observation that LJ1 and RSP-57 MoAbs recognize epitopes expressed on 'early' apoptotic cells. The interference with the in vivo clearance and processing of apoptotic cells is a potential pathogenic mechanism of these antibodies.

LDL increases inactive tissue factor on vascular smooth muscle cell surfaces: hydrogen peroxide activates latent cell surface tissue factor

BACKGROUND

Tissue factor, which is required for the initiation of the extrinsic coagulation cascade, is known to be upregulated in cells within atherosclerotic lesions, including smooth muscle cells. Tissue factor expression on the smooth muscle cell surface could be of pathological significance as a contributor to plaque growth, thrombus formation, and the acute coronary syndrome after plaque rupture.

METHODS AND RESULTS

In this study, we show that LDL increased tissue factor mRNA and cell surface protein in smooth muscle cells without a marked increase in surface tissue factor activity. Hydrogen peroxide activated tissue factor on the cell surface but did not increase tissue factor mRNA or cell surface protein. Sequentially added LDL and hydrogen peroxide increased mRNA, cell surface protein, and activity; surface activity was greater than that observed with hydrogen peroxide alone. The action of hydrogen peroxide did not involve a regulatory mechanism associated with the cytoplasmic tail of tissue factor because a truncated tissue factor lacking the cytoplasmic tail was activated by hydrogen peroxide.

CONCLUSIONS

These results suggest a novel 2-step pathway for increased tissue factor activity on smooth muscle cell surfaces in which lipoproteins regulate synthesis of a latent tissue factor and oxidants activate the protein complex.

Alpha-tocopherol inhibits oxidative stress induced by cholestanetriol and 25-hydroxycholesterol in porcine ovarian granulosa cells

The cytotoxicity of oxysterols including 7-ketocholesterol, alpha-epoxide, cholestanetriol and 25-hydroxycholesterol and the possible protecting effect of alpha-tocopherol on cholestanetriol and 25-hydroxycholesterol-induced cytotoxicity were investigated in primary cultures of porcine ovarian granulosa cells. Cell viability as determined by % trypan blue staining and mitochondrial function as determined using 3-[4,5-dimethylthiazol-2-yl]-2,5- diphenyltetrazolium bromide (MTT) reduction were decreased significantly after 24 h exposure to 2.5 microM alpha-epoxide, cholestanetriol and 25-hydroxycholesterol. 7-Ketocholesterol (2.5 microM) did not affect cell viability or mitochondrial function under the same culture conditions. The specific activities of catalase and superoxide dismutase, two antioxidant defense enzymes were increased significantly (p < 0.01) following 24 h exposure to 2.5 microM concentrations of cholestanetriol while only superoxide dismutase was increased in 25-hydroxycholesterol-treated cells (p < 0.001). Specific activity of glutathione peroxidase was unchanged relative to control cells. Levels of thiobarbituric acid reactive substances remained unchanged after exposure to 7-ketocholesterol, alpha-epoxide, cholestanetriol, 25-hydroxycholesterol and cholesterol. Administration of 1 microM alpha-tocopherol to the culture medium significantly improved cell viability and restored both superoxide dismutase and catalase activities to control levels in cholestanetriol -treated cells and only superoxide dismutase in 25-hydroxycholesterol-treated cells. These studies suggest that the cytotoxic nature of physiologically relevant concentrations of cholestanetriol and 25-hydroxycholesterol in granulosa cells is in part due to oxidative stress, but it may be reduced in the presence of alpha-tocopherol.

Distribution of exogenous 25-hydroxycholesterol in Hep G2 cells between two different pools

Binding of [26,27-(3)H]25-hydroxycholesterol (25HC) to human hepatoma Hep G2 cells was saturated within 120 min. Two intracellular pools of 25HC were identified in a pulse-chase experiment: (i) an exchangeable pool which was in dynamic equilibrium with 25HC in the medium (t(1/2) of reversible exchange 15 min) and (ii) an unexchangeable pool which remained in cells during incubation in medium containing LPDS. 25HC from the exchangeable pool inhibits cholesterol biosynthesis, decreases the HMG CoA reductase mRNA level and stimulates cholesterol acylation. 25HC from the unexchangeable pool was partially bound to cytosolic proteins and apparently utilized for metabolic transformation. Incubation of Hep G2 cells with [26,27-(3)H]25HC in the presence of a 30-fold molar excess of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one was found to cause (i) 2-fold decrease in the binding of [26,27-(3)H]25HC to cytosolic proteins (sedimentation constant of radioactive complex was 4-5 S) and (ii) the 35% inhibition of 25HC transformation to polar metabolites.

Shed membrane microparticles with procoagulant potential in human atherosclerotic plaques: a role for apoptosis in plaque thrombogenicity

BACKGROUND

The specific role of apoptosis in human atherosclerosis remains unknown. During apoptotic cell death, phosphatidylserine exposure on the cell surface confers a high tissue-factor (TF)-dependent procoagulant activity.

METHODS AND RESULTS

In this study, we examined the role of apoptotic cell death in the promotion of plaque thrombogenicity. TF expression and its relation to apoptosis was analyzed in 16 human atherosclerotic plaques by the use of immunohistochemical techniques. The presence of shed membrane apoptotic microparticles was analyzed in extracts from 6 human atherosclerotic plaques and 3 underlying arterial walls. The microparticles were captured by annexin V and their amounts estimated with respect to their phospholipid content by use of a prothrombinase assay. The prothrombogenic potential of the microparticles was further assessed by the measurement of total and microparticle-dependent TF activity in the extracts. The cell origin of the microparticles was determined after capture by specific antibodies. We were able to detect marked TF expression in the plaques in close proximity to apoptotic cells and debris, suggesting a potential interaction between TF and the apoptotic cell surfaces. High levels of shed membrane apoptotic microparticles were detected in extracts from atherosclerotic plaques but not in the underlying arterial walls (29.5+/-3.7 nmol/L phosphatidylserine equivalent versus 1.3+/-0.4 nmol/L, respectively, P<0.02). The microparticles were mainly of monocytic and lymphocytic origin and retained 97+/-2% of total TF activity, indicating a direct causal relationship between shed membrane microparticles and procoagulant activity of plaque extracts.

CONCLUSIONS

These results indicate that shed membrane microparticles with procoagulant potential are produced in human atherosclerotic plaques. Apoptosis could be a critical determinant of plaque thrombogenicity after plaque rupture.

Apoptosis and antiphospholipid antibodies

OBJECTIVE

To analyze the potential links between antiphospholipid antibodies (aPL) and apoptosis in the pathogenesis of the antiphospholipid antibody syndrome (APS).

METHODS

A review was undertaken of the most relevant scientific literature on apoptosis and autoimmune phenomena. Experimental and human pathology were reviewed to substantiate the hypothesis that apoptosis is involved in the generation of aPL.

RESULTS

Several considerations suggest that exposure of phospholipids (PL) during apoptosis may be a driving antigenic stimulus to the production of aPL. Furthermore, the molecular PL-protein complexes formed during apoptosis are targeted by "pathogenic" aPL. The binding and the clearance of apoptotic cells by these autoantibodies likely further enhances the aPL immune response. Experimental models and human pathology suggest that a restricted genetic background is key to the development of this immune response.

CONCLUSIONS

Abnormalities of apoptosis observed in the course of autoimmune conditions likely provide an antigenic stimulus to the production of aPL.

Effects of oxysterols upon macrophage and lymphocyte functions in vitro

Oxygenated derivatives of cholesterol (oxysterols), found in high concentrations in atherosclerotic lesions, are potent immunosuppressive agents inhibiting T-cell responses to different stimuli. The action of oxysterols on macrophage functions and macrophage-lymphocyte interaction has been poorly investigated. In this work, the effects of 25-hydroxycholesterol (25-OHCh) and 7-ketocholesterol (7-KCh) upon some functions of murine peritoneal macrophage (PM), such as generation of reactive oxygen intermediates (ROI), secretion of neopterin and interleukin-1 (IL-1)-like activity, Fc-receptor (FcR) activity, and murine and human lymphocyte functions, participating in lymphocyte-macrophage interactions, such as macrophage-activating factor (MAF) and Ia-inducing factor (IaIF) secretion, were studied in vitro. 7-KCh in concentration of 5 micrograms/mL culture medium only, but not 25-OHCh, significantly inhibited ROI generation by zymosan-stimulated PM. Pretreatment of PM for 22 h with 25-OHCh and 7-KCh led to the decrease of IL-1-like activity secretion. 25-OHCh and 7-KCh inhibited both FcR-dependent binding and phagocytosis of sheep red blood cells (SRBC). Oxysterols did not change both spontaneous and lipopolysaccharide-stimulated secretion of neopterin by PM. 25-OHCh dose-dependently and more efficiently than 7-KCh inhibited murine splenocyte secretion of MAF, which activity was determined by the ability of splenocyte-conditioned medium to stimulate ROI generation in PM. Both 25-OHCh and 7-KCh inhibited significantly proliferative activity of human mixed lymphocyte culture (MLC), as well as lymphocyte secretion of IaIF, which stimulates the expression of HLA antigens in cultured human monocytes. Purified Ch did not alter these parameters. These data showed, that some inflammatory functions of macrophages and lymphocytes may be modified by such environmental conditions as the presence of oxysterols.

Biochemistry and pathology of radical-mediated protein oxidation

Radical-mediated damage to proteins may be initiated by electron leakage, metal-ion-dependent reactions and autoxidation of lipids and sugars. The consequent protein oxidation is O2-dependent, and involves several propagating radicals, notably alkoxyl radicals. Its products include several categories of reactive species, and a range of stable products whose chemistry is currently being elucidated. Among the reactive products, protein hydroperoxides can generate further radical fluxes on reaction with transition-metal ions; protein-bound reductants (notably dopa) can reduce transition-metal ions and thereby facilitate their reaction with hydroperoxides; and aldehydes may participate in Schiff-base formation and other reactions. Cells can detoxify some of the reactive species, e.g. by reducing protein hydroperoxides to unreactive hydroxides. Oxidized proteins are often functionally inactive and their unfolding is associated with enhanced susceptibility to proteinases. Thus cells can generally remove oxidized proteins by proteolysis. However, certain oxidized proteins are poorly handled by cells, and together with possible alterations in the rate of production of oxidized proteins, this may contribute to the observed accumulation and damaging actions of oxidized proteins during aging and in pathologies such as diabetes, atherosclerosis and neurodegenerative diseases. Protein oxidation may also sometimes play controlling roles in cellular remodelling and cell growth. Proteins are also key targets in defensive cytolysis and in inflammatory self-damage. The possibility of selective protection against protein oxidation (antioxidation) is raised.

The significance of shed membrane particles during programmed cell death in vitro, and in vivo, in HIV-1 infection

The plasma membrane remodeling, including the early transverse redistribution of phosphatidylserine, is a general feature occurring in cells in which a death program has been induced. In most cases, studies of this kind have focused mainly on cells. In this study, we report a clear correlation between the degree of apoptosis induced by a variety of agents in several types of cultured cells and the amount of shed membrane microparticles captured in the corresponding supernatants by insolubilized annexin V, a protein showing a strong affinity for phosphatidylserine. Such particles carry membrane antigens specific of the cells they stem from, and through which capture is also feasible. Homologous circulating microparticles were captured in peripheral blood from individuals with HIV-1 infection. A substantial proportion bore CD4 antigen. In some cases, CD4+ particles could be detected even in the absence of circulating CD4+ T cells, testifying to the presence of such resident cells in lymphoid tissues. These results suggest that shed membrane particles are one of the hallmarks of programmed cell death, of particular interest when the corresponding cells are hardly accessible.

Apoptotic Vascular Endothelial Cells Become Procoagulant

Whereas unperturbed endothelial cells provide potent anticoagulant properties, exposure to inflammatory and atherogenic stimuli can rapidly lead to a procoagulant behavior. Because recent studies provide evidence that apoptosis of vascular cells may occur under conditions such as atherosclerosis and inflammation, we investigated whether apoptotic endothelial cells may contribute to the development of a prothrombotic state. In this report, it is shown that both adherent and detached apoptotic human umbilical vein endothelial cells (HUVECs) become procoagulant. Apoptosis was induced by staurosporine, a nonspecific protein kinase inhibitor, or by culture in suspension with serum deprivation. Both methods resulted in similar findings. As assessed by flow cytometric determination of annexin V binding, HUVECs undergoing cell death exhibited typically a more rapid exposure of membrane phosphatidylserine (PS) than DNA fragmentation. Depending on the stage of apoptosis, this redistribution of phospholipids was found to induce an increase of the activity of the intrinsic tenase complex by 25% to 60%. Although apoptotic cells did not show antigenic or functional tissue factor (TF ) activity, when preactivated with lipopolysaccharide, TF procoagulant activity increased by 50% to 70%. At 8 hours after apoptosis induction, antigenic thrombomodulin, heparan sulfates, and TF pathway inhibitor decreased by about 83%, 80%, and 59%, respectively. The functional activity of these components was reduced by about 36%, 52%, and 39%, respectively. Moreover, the presence of apoptotic HUVECs led to a significant increase of thrombin formation in recalcified citrated plasma. In conclusion, apoptotic HUVECs, either adherent or in suspension, become procoagulant by increased expression of PS and the loss of anticoagulant membrane components.