Phagocytosis of apoptotic cells by macrophages is impaired in atherosclerosis

Pallbearer and friends: lending a hand in apoptotic cell clearance

Engulfment and prompt removal of apoptotic cells occurs from embryogenesis throughout the lifespan of multicellular organisms. A new player, Pallbearer, has recently been identified in Drosophila as being important for efficient engulfment by macrophages. Pallbearer is a component of the SCF E3 ubiquitin ligase complex involved in the ubiquitylation of proteins targeted for proteasomal degradation. This work provides the first link between the cellular processes of ubiquitylation/proteasomal degradation and the ability to clear apoptotic cells efficiently.

Selective depletion of macrophages in atherosclerotic plaques via macrophage-specific initiation of cell death

Macrophages play a central role in atherosclerotic plaque destabilization, leading to acute coronary syndromes and sudden death. Removal of macrophages from plaques via pharmacological therapy may therefore represent a promising approach to stabilize vulnerable, rupture-prone lesions. In this review, we summarize the current therapeutic means to induce macrophage cell death in atherosclerotic plaques without affecting smooth muscle cell viability, and their potential pitfalls.

Journey to the grave: signaling events regulating removal of apoptotic cells

Programmed cell death is critical both for organ formation during development and during adult life, when billions of cells must be removed every day. The culmination of the apoptotic process is the specific recognition and engulfment of the apoptotic cell by a phagocyte. A number of recent studies have revealed a series of evolutionarily conserved proteins that link corpse recognition to membrane movement, facilitating the internalization of the target and its subsequent degradation. Two potential signaling modules have been identified: one involving the CED-12/ELMO and CED-5/Dock180 proteins, which function as a bipartite guanine nucleotide exchange factor (GEF) for Rac1, and a second involving CED-1/LRP1 (a potential engulfment receptor) and the adaptor protein CED-6/GULP. Recognition of the apoptotic cell modulates cytokine secretion by the phagocyte, resulting in an anti-inflammatory state distinct from that induced by necrotic cells. The recent molecular delineation of the phagocytic process and the identification of novel signaling proteins involved in engulfment have provided an exciting new platform for future studies into this biologically important process.

The phosphatidylserine receptor mediates phagocytosis by vascular smooth muscle cells

Apoptosis participates in every step of atherogenesis, but the process of clearance of apoptotic cells by phagocytosis has been underestimated. Rapid removal of apoptotic cells is critical for tissue homeostasis, in order to avoid accumulation of necrotic material and subsequent inflammation in the pathological vascular wall. We have demonstrated by RT-PCR, western blot and immunocytofluorescence that vascular smooth muscle cells (VSMCs) express the phosphatidylserine receptor (PSR). We then tested the involvement of PSR in the ability of VSMCs to bind and engulf apoptotic cells. We used a model of senescent erythrocytes, which expose PS after 4 days of culture (85% of cells relative to 8% in freshly isolated erythrocytes). The pseudo-peroxidase activity of haemoglobin contained within erythrocytes allowed us to quantify per se both binding and phagocytosis by VSMCs. We have also shown by light and confocal microscopy that VSMCs were able to ingest aged erythrocytes. Addition of a blocking antibody or transfection of VSMCs by a siRNA directed against PSR reduced the binding and engulfment of aged erythrocytes by more than 90%. These results suggest that PSR is involved in phagocytosis of PS-presenting cells. Incubation of aged erythrocytes with VSMCs also significantly increased the expression of PSR, suggesting that the tethering/ingestion of apoptotic cells triggers this process. Immunostaining for PSR in complicated atherosclerotic plaques shows positivity in the media and macrophage-rich areas. The mechanisms underlying phagocytosis and involving PSR in vivo, within the pathological arterial wall, deserve further investigation.

The macrophage at the crossroads of insulin resistance and atherosclerosis

The macrophage has emerged as an important player in the pathogenesis of both atherosclerosis and insulin resistance. Cross-talk between inflammatory macrophages and adipocytes may be involved in insulin resistance in peripheral tissues. Defective insulin signaling in cells of the arterial wall including macrophages may promote the development of atherosclerosis. Insulin resistant macrophages are more susceptible to endoplasmic reticulum stress and apoptosis in response to various stimuli such as nutrient deprivation, free cholesterol loading, and oxidized LDL. Increased apoptosis of insulin resistant macrophages and impaired phagocytic clearance of apoptotic cells by insulin resistant macrophages in atherosclerotic lesions may lead to enhanced postapoptotic necrosis, larger lipid-rich cores, increased inflammation, and more complex vulnerable plaques.

Proteomics analysis of human coronary atherosclerotic plaque: a feasibility study of direct tissue proteomics by liquid chromatography and tandem mass spectrometry

Cardiovascular disease presents significant variations in human populations with respect to the atherosclerotic plaque progression, inflammation, thrombosis, and rupture. To gain a more comprehensive picture of the pathogenic mechanism of atherosclerosis and the variations seen in patients, efficient methods to identify proteins from the normal and diseased arteries need to be developed. To accomplish this goal, we tested the feasibility and efficiency of protein identification by a recently developed method, termed direct tissue proteomics (DTP). We analyzed frozen and paraformaldehyde-fixed archival coronary arteries with the DTP method. We also validated the distinct expression of four proteins by immunohistochemistry. In addition, we demonstrated the compatibility of the DTP method with laser capture microdissection and the possibility of monitoring specific cytokines and growth factors by the absolute quantification of abundance method. Major findings from this feasibility study are that 1) DTP can be used to efficiently identify proteins from paraformaldehyde-fixed, paraffin-embedded, and frozen coronary arteries; 2) approximately twice the number of proteins were identified from the frozen sections when compared with the paraformaldehyde-fixed sections; 3) laser capture microdissection is compatible with DTP; and 4) detection of low abundance cytokines and growth factors in the coronary arteries required selective reaction monitoring experiments coupled to absolute quantification of abundance. The analysis of 35 human coronary atherosclerotic samples allowed identification of a total of 806 proteins. The present study provides the first large scale proteomics map of human coronary atherosclerotic plaques.

Selective clearance of macrophages in atherosclerotic plaques by the protein synthesis inhibitor cycloheximide

Macrophages are an essential component of unstable atherosclerotic plaques and play a pivotal role in the destabilization process. We have demonstrated previously that local delivery of the mammalian target of rapamycin (mTOR) inhibitor everolimus selectively clears macrophages in rabbit plaques. Because mTOR controls mRNA translation, inhibition of protein synthesis might induce selective macrophage cell death. We therefore investigated in the present study the effect of the protein synthesis inhibitor cycloheximide on macrophage and smooth muscle cell (SMC) viability. In vitro studies with cultured macrophages and SMCs showed that cycloheximide induced selective apoptosis of macrophages in a concentration- and time-dependent manner. Moreover, macrophages could be selectively depleted in rabbit carotid artery rings with collar-induced atherosclerotic plaques after in vitro treatment with cycloheximide. Local in vivo administration of cycloheximide via osmotic minipumps to rabbit carotid arteries with collar-induced atherosclerotic plaques significantly reduced the macrophage but not the SMC content. Cycloheximide-treated plaques showed signs of apoptosis (increased terminal deoxynucleotidyl transferase end labeling and fluorescein isothiocyanate-Val-Ala-dl-Asp(O-methyl)-fluoromethylketone labeling) that did not colocalize with SMCs. Organ chamber studies demonstrated that the functionality of SMCs and the endothelium were not influenced by cycloheximide treatment. All together, these findings demonstrate that cycloheximide decreases the macrophage load in atherosclerotic plaques by induction of apoptosis without changing SMC content or contractility.

Cellular origins and thrombogenic activity of microparticles isolated from human atherosclerotic plaques

OBJECTIVES

In this study, we evaluated the cellular origins and thrombogenic potential of microparticles.

BACKGROUND

Human atherosclerotic plaques contain submicron vesicles (microparticles) released during cell activation or apoptosis.

METHODS

Microparticles were purified from plaques and platelet-free plasma from 26 patients undergoing carotid endarterectomy. Flow cytometry analysis revealed the presence of large amounts of microparticles in plaques but not in healthy vessels.

RESULTS

Most plaque microparticles originated from leukocytes, of which 29 +/- 5% were macrophages, 15 +/- 3% lymphocytes, and 8 +/- 1% granulocytes. Plaques microparticles also derived from erythrocytes (27 +/- 4%), smooth muscle (13 +/- 4%) and endothelial cells (8 +/- 2%), but not from platelets. Plaques from asymptomatic and symptomatic patients showed no differences in microparticle origins. Microparticles were at least 200-fold more concentrated in plaque than in plasma. Plasma microparticles were primarily platelet-derived in contrast with those of plaque and showed no smooth muscle cell origin. Both plaque and plasma microparticles exposed tissue factor and generated thrombin, but this activity was twice as high in microparticles isolated from plaques, reflecting the thrombogenic contribution of the individual classes of microparticles.

CONCLUSIONS

These results demonstrate that microparticles are more abundant and more thrombogenic in human atherosclerotic plaques than in plasma. The different cellular origins of plaque and plasma microparticles might explain the increased thrombogenic activity of plaque microparticles.

Imaging apoptosis for detecting plaque instability: rendering death a brighter facade

The relatively poor correlation between the risk of atherosclerotic plaque rupture and the degree of luminal obstruction before this event implies a strong imperative for in vivo detection of the processes underlying progressive plaque destabilization. In addition to the morphologic characteristics, apoptosis and inflammation comprise two important indicators of plaque instability. Apoptotic macrophage death results in enlargement of the plaque necrotic core and positive vascular remodelling, whereas apoptosis of the smooth muscle cells leads to attenuation of the fibrous cap. Imaging of apoptotic cells with annexin A5 provides an opportunity for the non-invasive assessment of cell death, and hence plaque vulnerability. The clinical detection of apoptosis could therefore promote the development of novel intervention strategies.

Enhanced VDUP-1 gene expression by PPARγ agonist induces apoptosis in human macrophage

The fate and phenotype of lesion macrophages is regulated by cellular oxidative stress. Thioredoxin-1 (Trx-1) plays a major role in the regulation of cellular redox balance, with resultant effects on gene expression and cellular responses including cell growth and death. Trx-1 activity is inhibited by interaction with vitamin D-upregulated protein-1 (VDUP-1). Peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed by human monocyte-derived macrophages (HMDM) and PPARgamma agonism has been reported to decrease expression of inflammatory genes and to promote apoptosis of these cells. To determine whether VDUP-1 may be involved in regulating the effects of PPARgamma agonists in macrophages, we investigated the effect of a synthetic PPARgamma agonist (GW929) on the expression of VDUP-1 in HMDM. GW929 concentration-dependently increased HMDM expression of VDUP-1 (mRNA and protein). Transfection of different fragments of the VDUP-1 promoter as well as gel shift analysis revealed the presence of functional PPARgamma response elements (PPRE) in the promoter. Under conditions in which PPAR agonism altered levels of VDUP-1, caspase-3 activity, and macrophage apoptosis were also elevated. The results suggest that PPARgamma activation stimulates apoptosis in human macrophages by altering the cellular redox balance via regulation of VDUP-1.

Oxidized low-density lipoprotein induces apoptotic insults to mouse cerebral endothelial cells via a Bax-mitochondria-caspase protease pathway

Cerebral endothelial cells (CECs) are crucial components of the blood-brain barrier. Oxidized low-density lipoprotein (oxLDL) can induce cell injuries. In this study, we attempted to evaluate the effects of oxLDL on mouse CECs and its possible mechanisms. Mouse CECs were isolated from brain tissues and identified by immunocytochemical staining of vimentin and Factor VIII. oxLDL was prepared from LDL oxidation by copper sulfate. Exposure of mouse CECs to oxLDL decreased cell viability in concentration- and time-dependent manners. oxLDL time-dependently caused shrinkage of cell morphologies. Administration of oxLDL to CECs induced DNA fragmentation in concentration- and time-dependent manners. Analysis of the cell cycle revealed that oxLDL concentration- and time-dependently increased the proportion of CECs which underwent apoptosis. Analysis of confocal microscopy and immunoblot revealed that oxLDL significantly increased cellular and mitochondrial Bax levels as well as the translocation of this proapoptotic protein from the cytoplasm to mitochondria. In parallel with the increase in the levels and translocation of Bax, oxLDL time-dependently decreased the mitochondrial membrane potential. Exposure of mouse CECs to oxLDL decreased the amounts of mitochondrial cytochrome c, but enhanced cytosolic cytochrome c levels. The amounts of intracellular reactive oxygen species were significantly augmented after oxLDL administration. Sequentially, oxLDL increased activities of caspase-9, -3, and -6 in time-dependent manners. Pretreatment with Z-VEID-FMK, an inhibitor of caspase-6, significantly decreased caspase-6 activity and the oxLDL-induced DNA fragmentation and cell apoptosis. This study showed that oxLDL induces apoptotic insults to CECs via signal-transducing events, including enhancing Bax translocation, mitochondrial dysfunction, cytochrome c release, increases in intracellular reactive oxygen species, and cascade activation of caspase-9, -3, and -6. Therefore, oxLDL can damage the blood-brain barrier through induction of CEC apoptosis via a Bax-mitochondria-caspase protease pathway.

The emerging role of vascular smooth muscle cell apoptosis in atherosclerosis and plaque stability

Accelerated atherosclerosis is both a cause and a consequence of chronic renal failure. Vascular smooth muscle cells (VSMCs) are an important component of atherosclerotic plaques, responsible for promoting plaque stability in advanced lesions. In contrast, VSMC apoptosis has been implicated in a number of deleterious consequences of atherosclerosis, including plaque rupture, vessel remodelling, co- agulation, inflammation and calcification. Although VSMC apoptosis occurs in association with these processes, its precise contribution to these diseases is unknown, given that apoptosis frequently accompanies vessel injury or alterations to flow. Using transgenic mice with selective induction of VSMC apoptosis, a recent study has precisely determined the direct consequences of VSMC apoptosis in both normal vessels and atherosclerotic plaques. Surprisingly, normal arteries can withstand huge cell losses with little change in active or passive properties. Normal vessels demonstrate highly efficient clearance of apoptotic bodies, even in the absence of professional phagocytes. In contrast, VSMC apoptosis alone is sufficient to induce multiple features of vulnerability to rupture in plaques. This study identifies VSMC apoptosis as a critical process determining plaque stability and thus the most important consequence of atherosclerosis, plaque rupture.

The inflammatory cytokine response of cholesterol-enriched macrophages is dampened by stimulated pinocytosis

Two features of advanced atherosclerotic lesions are large numbers of macrophages and a heightened state of inflammation. Some of the macrophages appear to be enriched with free cholesterol (FCMphis), and we have shown that this process induces the synthesis and secretion of inflammatory cytokines, including TNF-alpha and IL-6. However, lesions contain many other macrophages that are not FC-enriched (non-FCMphis). Therefore, we sought to understand how the interaction of these two populations of macrophages would influence the inflammatory response. We show here that non-FCMphis possess a robust ability to deplete TNF-alpha and IL-6 secreted by FCMphis. The mechanism involves enhanced pinocytic uptake and lysosomal degradation of the FCMphi-secreted cytokines by the non-FCMphis. The FCMphis contribute directly to this process by secreting pinocytosis-stimulatory factors that act on non-FCMphis but not on the FCMphis themselves. One of these pinocytosis-stimulatory factors is M-CSF, which is induced by a process involving cholesterol trafficking to the endoplasmic reticulum and signaling through PI-3K and ERK MAPK pathways. However, one or more other FCMphi-secreted factors are also required for stimulating pinocytosis in non-FCMphis. Thus, FCMphis secrete inflammatory cytokines as well as factors that promote the eventual pinocytosis and degradation of these cytokines by neighboring macrophages. This process may normally serve to prevent prolonged or disseminated effects of inflammatory cytokines during inflammation. Moreover, possible perturbation of stimulated pinocytosis during the progression of advanced atherosclerosis may contribute to the heightened inflammatory state of these lesions.

Matrix metalloproteinases and atherogenesis in dependence of age

BACKGROUND

Changes in the proteoglycan metabolism of the intima of arteries belong to the initial lesions of atherosclerosis (AS). The accumulation of proteoglycans, alterations of pericellular glycoproteins and modulations of collagen turnover also play a fundamental role in the progression of AS. They influence lipid retention, cell behavior and calcinosis. The decisive role played by the matrix metalloproteinases (MMPs) and their inhibiting factors (tissue inhibitors of metalloproteinases [TIMPs]) in these processes is not yet fully understood and therefore the subject of this overview. The causes of the abrupt change of a long-term existing stabile AS to a vulnerable plaque as well as the participation of age-related vascular wall remodeling in the progression of AS also remain open questions.

DISCUSSION

Apart from the well-known risk factors for AS, less well-known influences like the disturbances of gene expression in vascular smooth muscle cells affect an MMP/TIMP imbalance. The various consequences of this imbalance range from intima cell proliferation as an early change in AS as well as accelerated progression to the destabilization of fibrous plaques by increased collagenolysis as well as the formation of aneurysms. Infectious or toxic influences may trigger these mechanisms; an involvement of age-related vessel wall changes should also be considered. The prognostic significance of circulating MMP concentrations for the existence of instabile plaques are of great interest, as is the plaque stabilizing effect of statins by suppression of MMPs.

CONCLUSIONS

MMPs navigate the behavior of vascular wall cells in different AS stages, in adaptive remodeling, in normal aging and in non-atherosclerotic vessel disease. The clinical relevance of a disturbance in the MMP/TIMP balance is demonstrated firstly by the initiation of AS due to migration and proliferation of intima cells and secondly in the collagenolysis, necrotic transformation and apoptosis of existing fibrous lesions resulting in instabile rupture proned plaques. Investigations into the genetic typing of MMPs and the results of experimental gene deficiency models have significantly contributed to the clarification of these facts.

Burying the dead: the impact of failed apoptotic cell removal (efferocytosis) on chronic inflammatory lung disease

Apoptosis and the removal of apoptotic cells (termed efferocytosis) are tightly coupled with the regulation of normal lung structure, both in the developing and adult organism. Processes that disrupt or uncouple this balance have the potential to alter normal cell turnover, ultimately resulting in the induction of lung pathology and disease. Apoptotic cells are increased in several chronic inflammatory lung diseases, including cystic fibrosis (CF), non-CF bronchiectasis, COPD, and asthma. While this may well be due to the enhanced induction of apoptosis, increasing data suggest that the clearance of dying cells is also impaired. Because efferocytosis appears to be a key regulatory checkpoint for the innate immune system, the adaptive immune system, and cell proliferation, the failure of this highly conserved process may contribute to disease pathogenesis by impeding both the resolution of inflammation and the maintenance of alveolar integrity. The recognition of impaired efferocytosis as a contributor to chronic inflammation may ultimately direct us toward the identification of new disease biomarkers, as well as novel therapeutic approaches.

Activation of the canonical Wnt/beta-catenin pathway enhances monocyte adhesion to endothelial cells

Monocyte adhesion to vascular endothelium has been reported to be one of the early processes in the development of atherosclerosis. In an attempt to develop strategies to prevent or delay atherosclerosis progression, we analyzed effects of the Wnt/beta-catenin signaling pathway on monocyte adhesion to various human endothelial cells. Adhesion of fluorescein-labeled monocytes to various human endothelial cells was analyzed under a fluorescent microscope. Unlike sodium chloride, lithium chloride enhanced monocyte adhesion to endothelial cells in a dose-dependent manner. We further demonstrated that inhibitors for glycogen synthase kinase (GSK)-3beta or proteosome enhanced monocyte-endothelial cell adhesion. Results of semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) indicated that activation of Wnt/beta-catenin pathway did not change expression levels of mRNA for adhesion molecules. In conclusion, the canonical Wnt/beta-catenin pathway enhanced monocyte-endothelial cell adhesion without changing expression levels of adhesion molecules.

Suppression of atherosclerotic plaque progression and instability by tissue inhibitor of metalloproteinase-2: involvement of macrophage migration and apoptosis

BACKGROUND

Matrix metalloproteinase (MMP)-associated extracellular matrix degradation is thought to contribute to the progression and rupture of atherosclerotic plaques. However, direct evidence of this concept remains elusive. We hypothesized that overexpression of tissue inhibitor of metalloproteinase (TIMP)-1 or TIMP-2 would attenuate atherosclerotic plaque development and instability in high fat-fed apolipoprotein E-knockout (apoE(-/-)) mice.

METHODS AND RESULTS

Seventy male apoE(-/-) mice (n=10/group) fed a high-fat diet for 7 weeks were injected intravenously with first-generation adenoviruses expressing the gene for human TIMP-1 (RAdTIMP-1) or TIMP-2 (RAdTIMP-2) or a control adenovirus (RAd66) and were fed a high-fat diet for a further 4 weeks. Analysis of brachiocephalic artery plaques revealed that RAdTIMP-2 but not RAdTIMP-1 infection resulted in a marked reduction (48+/-13%, P<0.05) in lesion area compared with that in control animals. Markers associated with plaque instability, assessed by smooth muscle cell and macrophage content and the presence of buried fibrous caps, were significantly reduced by RAdTIMP-2. Effects on lesion size were not sustained with first-generation adenoviruses, but murine TIMP-2 overexpression mediated by helper-dependent adenoviral vectors exerted significant effects on plaques assessed 11 weeks after infection. In an attempt to determine the mechanism of action, we treated macrophages and macrophage-derived foam cells with exogenous TIMP-2 in vitro. TIMP-2 significantly inhibited migration and apoptosis of macrophages and foam cells, whereas TIMP-1 failed to exert similar effects.

CONCLUSIONS

Overexpression of TIMP-2 but not TIMP-1 inhibits atherosclerotic plaque development and destabilisation, possibly through modulation of macrophage and foam cell behavior. Helper-dependent adenovirus technology is required for these effects to be maintained long term.

Minimally Oxidized LDL Offsets the Apoptotic Effects of Extensively Oxidized LDL and Free Cholesterol in Macrophages

OBJECTIVE

Lipid-loaded macrophage-derived foam cells populate atherosclerotic lesions and produce many pro-inflammatory and plaque-destabilizing factors. An excessive accumulation of extensively oxidized low-density lipoprotein (OxLDL) or free cholesterol (FC), both of which are believed to be major lipid components of macrophages in advanced lesions, rapidly induces apoptosis in macrophages. Indeed, there is evidence of macrophage death in lesions, but how the surviving macrophages avoid death induced by OxLDL, FC, and other factors is not known.

METHODS AND RESULTS

Minimally oxidized LDL (mmLDL), which is an early product of progressive LDL oxidation in atherosclerotic lesions, countered OxLDL-induced or FC-induced apoptosis and stimulated macrophage survival both in cell culture and in vivo. DNA fragmentation and caspase-3 activity in OxLDL-treated peritoneal macrophages were significantly reduced by coincubation with mmLDL. In a separate set of experiments, mmLDL significantly reduced annexin V binding to macrophages in which apoptosis was induced by FC loading. In both cellular models, mmLDL activated a pro-survival PI3K/Akt signaling pathway, and PI3K inhibitors, wortmannin and LY294002, eliminated the pro-survival effect of mmLDL. Immunohistochemical examination demonstrated phospho-Akt in murine atherosclerotic lesions.

CONCLUSIONS

Minimally oxidized LDL, an early form of oxidized LDL in atherosclerotic lesions, may contribute to prolonged survival of macrophage foam cells in lesions via a PI3K/Akt-dependent mechanism.

Cytokines in Atherosclerosis: Pathogenic and Regulatory Pathways

Atherosclerosis is a chronic disease of the arterial wall where both innate and adaptive immunoinflammatory mechanisms are involved. Inflammation is central at all stages of atherosclerosis. It is implicated in the formation of early fatty streaks, when the endothelium is activated and expresses chemokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into the subendothelium. It also acts at the onset of adverse clinical vascular events, when activated cells within the plaque secrete matrix proteases that degrade extracellular matrix proteins and weaken the fibrous cap, leading to rupture and thrombus formation. Cells involved in the atherosclerotic process secrete and are activated by soluble factors, known as cytokines. Important recent advances in the comprehension of the mechanisms of atherosclerosis provided evidence that the immunoinflammatory response in atherosclerosis is modulated by regulatory pathways, in which the two anti-inflammatory cytokines interleukin-10 and transforming growth factor-β play a critical role. The purpose of this review is to bring together the current information concerning the role of cytokines in the development, progression, and complications of atherosclerosis. Specific emphasis is placed on the contribution of pro- and anti-inflammatory cytokines to pathogenic (innate and adaptive) and regulatory immunity in the context of atherosclerosis. Based on our current knowledge of the role of cytokines in atherosclerosis, we propose some novel therapeutic strategies to combat this disease. In addition, we discuss the potential of circulating cytokine levels as biomarkers of coronary artery disease.

Cell death in the cardiovascular system

Cell death is important for both development and tissue homeostasis in the adult. As such, it is tightly controlled and deregulation is associated with diverse pathologies; for example, regulated cell death is involved in vessel remodelling during development or following injury, but deregulated death is implicated in pathologies such as atherosclerosis, aneurysm formation, ischaemic and dilated cardiomyopathies and infarction. We describe the mechanisms of cell death and its role in the normal physiology and various pathologies of the cardiovascular system.

Cholesterol-induced Apoptotic Macrophages Elicit an Inflammatory Response in Phagocytes, Which Is Partially Attenuated by the Mer Receptor

Macrophage apoptosis and the ability of phagocytes to clear these apoptotic cells are important processes in advanced atherosclerosis. Phagocytic clearance not only disposes of dead cells but usually elicits an anti-inflammatory response. To study this process in a model of advanced lesional macrophage death, macrophages rendered apoptotic by free cholesterol loading (FC-AMs) were incubated briefly with fresh macrophages ("phagocytes"). FC-AMs were promptly ingested by the phagocytes, which was dependent upon actin polymerization and the phagocyte Mer receptor. Surprisingly, this brief exposure to FC-AMs triggered a modest proinflammatory response in the phagocytes: tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-1beta were induced, whereas the levels of transforming growth factor-beta and IL-10 were not increased. This response required cell contact between the FC-AMs and phagocytes but not FC-AM ingestion. TNF-alpha and IL-1beta induction required one or more proteins on the FC-AM surface and was dependent on signaling through extracellular signal-regulated kinase-1/2 mitogen-activated protein kinase and nuclear factor-kappaB in the phagocytes. TNF-alpha production was markedly greater when Mer-defective phagocytes were used, indicating that Mer attenuated the inflammatory response. Interestingly, a more typical anti-inflammatory response was elicited when phagocytes were exposed to macrophages rendered apoptotic by oxidized low density lipoprotein or UV radiation. Thus, the proinflammatory milieu of advanced atherosclerotic lesions may be promoted, or at least not dampened, by contact between FC-induced apoptotic macrophages and neighboring phagocytes prior to apoptotic cell ingestion.

Comparison of Apoptosis Detection Markers Combined with Macrophage Immunostaining to Study Phagocytosis of Apoptotic Cells in Situ

Efficient phagocytosis of cells undergoing apoptosis by macrophages is important to prevent immunological responses and development of chronic inflammatory disorders such as systemic lupus erythematosus, cystic fibrosis and atherosclerosis. To study phagocytosis of apoptotic cells (AC) by macrophages in tissue, we validated different apoptosis markers (DNA fragmentation, caspase-3 activation and cleavage of its substrate poly(ADP-ribose)polymerase-1) in combination with macrophage immunostaining. Human tonsils were used as a model because they show a high apoptosis frequency under physiological conditions as well as efficient phagocytosis of AC by macrophages. On the other hand, advanced human atherosclerotic plaques were examined since plaques show severely impaired phagocytosis of AC. Our results demonstrate that the presence of non-phagocytized terminal deoxynucleotidyl transferase end labelling (TUNEL)-positive AC represents a suitable marker of poor phagocytosis by macrophages in situ. Other markers for apoptosis, such as cleavage of caspase-3 or PARP-1, should not be used to assess phagocytosis efficiency, because activation of the caspase cascade and cleavage of their substrates can occur in AC when they have not yet been phagocytized by macrophages.

Role of cholesterol and lipid organization in disease

Membrane lipids are essential for biological functions ranging from membrane trafficking to signal transduction. The composition of lipid membranes influences their organization and properties, so it is not surprising that disorders in lipid metabolism and transport have a role in human disease. Significant recent progress has enhanced our understanding of the molecular and cellular basis of lipid-associated disorders such as Tangier disease, Niemann-Pick disease type C and atherosclerosis. These insights have also led to improved understanding of normal physiology.

Consequences and Therapeutic Implications of Macrophage Apoptosis in Atherosclerosis

Macrophage apoptosis occurs throughout all stages of atherosclerosis, yet new findings in vivo suggest that the consequences of this event may be very different in early versus late atherosclerotic lesions. In early lesions, where phagocytic clearance of apoptotic cells appears to be efficient, macrophage apoptosis is associated with diminished lesion cellularity and decreased lesion progression. In late lesions, however, a number of factors may contribute to defective phagocytic clearance of apoptotic macrophages, leading to secondary necrosis of these cells and a proinflammatory response. The cumulative effect of these late lesional events is generation of the necrotic core, which, in concert with proatherogenic effects of residual surviving macrophages, promotes further inflammation, plaque instability, and thrombosis. Thus, the ability or lack thereof of lesional phagocytes to safely clear apoptotic macrophages may be an important determinant of acute atherothrombotic clinical events. Further understanding of the mechanisms involved in macrophage apoptosis and phagocytic clearance might lead to novel therapeutic strategies directed against the progression of advanced plaques.

Lipoprotein Retention—and Clues for Atheroma Regression

Subendothelial retention of apoB-lipoproteins is the key initiating event in atherosclerosis, provoking a cascade of pathogenic responses. Dissection of the molecular participants provides fresh insight into how this major killer might be reversed. Efflux of harmful lipids derived from retained lipoproteins may be crucial in promoting beneficial remodeling of lesions.

Lipoprotein-associated phospholipase A2 as a target of therapy

PURPOSE OF REVIEW

Considerable discussion continues regarding the precise role that secreted lipoprotein-associated phospholipase A2 (Lp-PLA2), also called platelet-activating factor acetylhydrolase, plays in atherosclerosis. Since interest in this enzyme as a putative drug target has been based primarily upon its association with low-density lipoprotein (LDL) in human plasma, this review will focus on Lp-PLA2 and human coronary heart disease.

RECENT FINDINGS

Recent reports have linked Lp-PLA2 enrichment not only to the most atherogenic of LDL particles but also to the most advanced, rupture-prone, plaques. Electronegative LDL has been shown to be highly enriched in Lp-PLA2; and in advanced atheroma, Lp-PLA2 levels are highly upregulated, colocalizing with macrophages in both the necrotic core and fibrous cap. Lp-PLA2 is well placed, whether on an oxidation susceptible LDL particle or in the highly oxidative environment of an advanced rupture-prone plaque, to hydrolyse oxidized phospholipid and generate significant quantities of the two pro-inflammatory mediators, lysophosphatidylcholine and oxidized nonesterified fatty acid. Several studies have confirmed that Lp-PLA2 is an independent risk factor for cardiovascular events (i.e. myocardial infarction and stroke). Although epidemiology studies consistently support a relationship between plasma Lp-PLA2 levels and susceptibility to coronary heart disease this is not the case for Lp-PLA2 polymorphisms. Two clinical studies have linked the Ala-379-->Val polymorphism with a reduced risk of myocardial infarction, but functional differences between the AA and VV polymorphs have yet to be demonstrated.

SUMMARY

Lp-PLA2 is intimately associated with several aspects of human atherogenesis. Although various lipid-lowering therapies, such as statins, have been shown to reduce plasma levels of Lp-PLA2, none has been studied in terms of its ability to lower the large macrophage-mediated upregulation of Lp-PLA2 within advanced plaques.