Consequences and Therapeutic Implications of Macrophage Apoptosis in Atherosclerosis

Contrasting Inflammation Resolution during Atherosclerosis and Post Myocardial Infarction at the Level of Monocyte/Macrophage Phagocytic Clearance

In cardiovascular disorders including advanced atherosclerosis and myocardial infarction (MI), increased cell death and tissue destabilization is associated with recruitment of inflammatory monocyte subsets that give rise to differentiated macrophages. These phagocytic cells clear necrotic and apoptotic bodies and promote inflammation resolution and tissue remodeling. The capacity of macrophages for phagocytosis of apoptotic cells (efferocytosis), clearance of necrotic cell debris, and repair of damaged tissue are challenged and modulated by local cell stressors that include increased protease activity, oxidative stress, and hypoxia. The effectiveness, or lack thereof, of phagocyte-mediated clearance, in turn is linked to active inflammation resolution signaling pathways, susceptibility to atherothrombosis and potentially, adverse post MI cardiac remodeling leading to heart failure. Previous reports indicate that in advanced atherosclerosis, defective efferocytosis is associated with atherosclerotic plaque destabilization. Post MI, the role of phagocytes and clearance in the heart is less appreciated. Herein we contrast the roles of efferocytosis in atherosclerosis and post MI and focus on how targeted modulation of clearance and accompanying resolution and reparative signaling may be a strategy to prevent heart failure post MI.

Sida rhomboidea.Roxb aqueous extract down-regulates in vivo expression of vascular cell adhesion molecules in atherogenic rats and inhibits in vitro macrophage differentiation and foam cell formation

The present study evaluates efficacy of Sida rhomboidea.Roxb (SR) leaves extract in ameliorating experimental atherosclerosis using in vitro and in vivo experimental models. Atherogenic (ATH) diet fed rats recorded significant increment in the serum total cholesterol (TC), triglycerides (TG), low-density lipoprotein (LDL), very LDL (VLDL), autoantibody against oxidized LDL (Ox-LDL), markers of LDL oxidation and decrement in high-density lipoprotein (HDL) along with increment in aortic TC and TG. The ex vivo LDL oxidation assay revealed an increased susceptibility of LDL isolated from ATH rats to undergo copper mediated oxidation. These set of changes were minimized by simultaneous co-supplementation of SR extract to ATH diet fed rats. Histopathology of aorta and immunolocalization studies recorded pronounced atheromatous plaque formation, vascular calcification, significant elastin derangements and higher expression of macrophage surface marker (F4/80), vascular cell adhesion molecule-1 (VCAM-1) and p-selectin in ATH rats. Whereas, ATH+SR rats depicted minimal evidence of atheromatous plaque formation, calcium deposition, distortion/defragmentation of elastin and accumulation of macrophages along with lowered expression of VCAM-1 and P-selectin compared to ATH rats. Further, monocyte to macrophage differentiation and in vitro foam cell formation were significantly attenuated in presence of SR extract. In conclusion, SR extract has the potency of controlling experimental atherosclerosis and can be used as promising herbal supplement in combating atherosclerosis.

Role and function of macrophages in the metabolic syndrome

Macrophages are key innate immune effector cells best known for their role as professional phagocytes, which also include neutrophils and dendritic cells. Recent evidence indicates that macrophages are also key players in metabolic homoeostasis. Macrophages can be found in many tissues, where they respond to metabolic cues and produce pro- and/or anti-inflammatory mediators to modulate metabolite programmes. Certain metabolites, such as fatty acids, ceramides and cholesterol crystals, elicit inflammatory responses through pathogen-sensing signalling pathways, implicating a maladaptation of macrophages and the innate immune system to elevated metabolic stress associated with overnutrition in modern societies. The outcome of this maladaptation is a feedforward inflammatory response leading to a state of unresolved inflammation and a collection of metabolic pathologies, including insulin resistance, fatty liver, atherosclerosis and dyslipidaemia. The present review summarizes what is known about the contributions of macrophages to metabolic diseases and the signalling pathways that are involved in metabolic stress-induced macrophage activation. Understanding the role of macrophages in these processes will help us to develop therapies against detrimental effects of the metabolic syndrome.

Bcl-x inactivation in macrophages accelerates progression of advanced atherosclerotic lesions in Apoe(-/-) mice

OBJECTIVE

Bcl-x is the most abundantly expressed member of the Bcl-2 gene family in macrophages, but its role in macrophage apoptosis during atherogenesis is unknown.

METHODS AND RESULTS

We previously reported dual pro- and antiatherogenic effects of macrophage survival in early versus advanced atherosclerotic lesions, respectively, potentially reflecting growing impairment of efferocytosis during plaque progression. Here, we specifically inactivated Bcl-x in macrophages and evaluated its impact on atherosclerotic lesion formation in Apoe(-/-) mice at various stages of the disease. Bcl-x deficiency in macrophages increased their susceptibility to apoptosis, resulting in the depletion of tissue macrophages in vivo, including its major pool, Küppfer cells in the liver. We also observed increased cholesterol levels that were, however, not associated with any acceleration of early atherosclerotic plaque progression. This observation suggests that the atheroprotective effect of macrophage apoptosis at that stage of disease was counterbalanced by enhanced cholesterol levels. Bcl-x KO(mac)/Apoe(-/-) mice exhibited significantly larger advanced lesions than control mice. These lesions showed vulnerable traits. Such enhanced lesion size may occur as a result not only of apoptotic cell accumulation but also of elevated cholesterol levels.

CONCLUSIONS

Modulation of macrophage resistance to apoptosis through targeted deletion of Bcl-x has a major impact on the entire macrophage cell population in the body, including Küpffer cells. Macrophage survival may, therefore, not only influence atherosclerotic plaque development and vulnerability but also cholesterol metabolism.

Pharmacological modulation of cell death in atherosclerosis: a promising approach towards plaque stabilization?

Despite tremendous advances over the last 15 years in identifying vulnerable atherosclerotic plaques, the incidence of death and disability caused by such lesions still remains the number one health threat in developed countries. Therefore, new systemic or focal therapies aimed at decreasing the overall burden of disease, and a change to a more benign phenotype, are needed. Because cell death is a prominent feature of advanced atherosclerotic plaques with a major impact on plaque destabilization, an increasing number of compounds targeting the apoptotic or autophagic machinery in atherosclerosis are being explored, predominantly at the preclinical level. This review will provide an overview of these compounds, with a focus on both inhibition and stimulation of cell death, to prevent acute coronary syndromes and sudden cardiac death.

Emerging role of mast cells and macrophages in cardiovascular and metabolic diseases

Mast cells are essential in allergic immune responses. Recent discoveries have revealed their direct participation in cardiovascular diseases and metabolic disorders. Although more sophisticated mechanisms are still unknown, data from animal studies suggest that mast cells act similarly to macrophages and other inflammatory cells and contribute to human diseases through cell-cell interactions and the release of proinflammatory cytokines, chemokines, and proteases to induce inflammatory cell recruitment, cell apoptosis, angiogenesis, and matrix protein remodeling. Reduced cardiovascular complications and improved metabolic symptoms in animals receiving over-the-counter antiallergy medications that stabilize mast cells open another era of mast cell biology and bring new hope to human patients suffering from these conditions.

Atherosclerosis, caveolae and caveolin-1

Atherosclerosis is a disease of the blood vessel characterized by the development of an arterial occlusion containing lipid and cellular deposits. Caveolae are 50-100 nm cell surface plasma membrane invaginations that are believed to play an important role in the regulation of cellular signaling and transport of molecules among others. These organelles are enriched in sphingolipids and cholesterol and are characterized by the presence of the protein caveolin-1. Caveolin-1 and caveolae are present in most of the cells involved in the development of atherosclerosis. The current literature suggests a rather complex role for caveolin-1 in this disease, with evidence of either pro- or anti-atherogenic functions depending on the cell type examined. In the present chapter, the various roles of caveolae and caveolin-1 in the development of atherosclerosis are examined.

Carotid plaque vulnerability: a positive feedback between hemodynamic and biochemical mechanisms

BACKGROUND AND PURPOSE

Rupture of atherosclerotic plaques is one of the main causes of ischemic strokes. The aim of this study was to investigate carotid plaque vulnerability markers in relation to blood flow direction and the mechanisms leading to plaque rupture at the upstream side of carotid stenoses.

METHODS

Frequency and location of rupture, endothelial erosion, neovascularization, and hemorrhage were determined in longitudinal sections of 80 human carotid specimens. Plaques were immunohistochemically analyzed for markers of vulnerability. Plaque geometry was measured to reconstruct shape profiles of ruptured versus stable plaques and to perform computational fluid dynamics analyses.

RESULTS

In 86% of ruptured plaques, rupture was observed upstream. In this region, neovascularization and hemorrhage were increased, along with increased immunoreactivity of vascular endothelial and connective tissue growth factor, whereas endothelial erosion was more frequent downstream. Proteolytic enzymes, mast cell chymase and cathepsin L, and the proapoptotic protein Bax showed significantly higher expression upstream as compared with the downstream shoulder of atherosclerotic lesions. Comparison of geometric profiles for ruptured and stable plaques showed increased longitudinal asymmetry of fibrous cap and lipid core thickness in ruptured plaques. The specific geometry of plaques ruptured upstream induced increased levels of shear stress and increased pressure drop between the upstream and the downstream plaque shoulders.

CONCLUSIONS

Vulnerability of the upstream plaque region is associated with enhanced neovascularization, hemorrhage, and cap thinning induced by proteolytic and proapoptotic mechanisms. These processes are reflected in structural plaque characteristics, analyses of which could improve the efficacy of vascular diagnostics and prevention.

Plaque Rupture and Thrombosis Are Reduced by Lowering Cholesterol Levels and Crystallization With Ezetimibe and Are Correlated With Fluorodeoxyglucose Positron Emission Tomography

Objective—

This study evaluated effects of lipid lowering with ezetimibe on plaque burden and associated cholesterol crystallization and inflammation in a rabbit model of plaque disruption and thrombosis.

Methods and Results—

Atherosclerotic rabbits (Group I, n=10 without; Group II, n=12 with ezetimibe, 1 mg/kg per day) were pharmacologically triggered for plaque disruption. Fluorodeoxyglucose positron emission tomography, RAM 11 macrophage staining, and serum inflammatory markers detected arterial inflammation. Serum and aortic wall cholesterol levels were measured, and thrombus area was planimetered. Cholesterol crystal density on aortic surface was scored (0 to +3) by scanning electron microscopy. Serum and aortic wall cholesterol, plaque area, and thrombosis area were significantly lower in Group II versus Group I (83.4±106.4 versus 608±386 mg/dL, P =0.002; 3.12±1.40 versus 9.39±5.60 mg/g, P =0.003; 10.84±1.6 versus 17.48±1.8 mm 2 , P <0.001; and 0.05±0.15 versus 0.72±0.58 mm 2 , P =0.01, respectively). There were significant correlations between crystal density and plaque area ( r =0.75, P <0.003) and between crystal density and RAM 11 ( r =0.82, P <0.001). Scanning electron microscopy demonstrated that there were fewer crystals in Group II versus Group I (+1.2±0.61 versus +2.4±0.63, P <0.001) and less inflammation detected by fluorodeoxyglucose positron emission tomography and RAM 11 ( P <0.004 and P <0.04, respectively).

Conclusion—

Lowering cholesterol levels with ezetimibe reduced plaque burden, crystallization, and inflammation, preventing plaque disruption and thrombosis.

The role of macrophages and dendritic cells in the clearance of apoptotic cells in advanced atherosclerosis

Accumulating evidence supports the notion that defective phagocytic clearance of dying cells, or defective "efferocytosis," is causally linked to the progression of advanced atherosclerosis. In advanced atherosclerotic lesions, defective efferocytosis leads to post-apoptotic necrosis, expansion of plaque necrotic cores, and susceptibility to atherothrombosis. Both macrophages and DC-like efferocytes are juxtaposed near expanding necrotic cores, where they engage apoptotic cells. In this Viewpoint, we discuss how reduced efferocytosis by macrophages and CD11c(HI) DC-like cells may combine to reduce overall plaque stability and therefore promote susceptibility to acute atherothrombosis.

Microparticles, Vascular Function, and Atherothrombosis

Membrane-shed submicron microparticles (MPs) are released after cell activation or apoptosis. High levels of MPs circulate in the blood of patients with atherothrombotic diseases, where they could serve as a useful biomarker of vascular injury and a potential predictor of cardiovascular mortality and major adverse cardiovascular events. Atherosclerotic lesions also accumulate large numbers of MPs of leukocyte, smooth muscle cell, endothelial, and erythrocyte origin. A large body of evidence supports the role of MPs at different steps of atherosclerosis development, progression, and complications. Circulating MPs impair the atheroprotective function of the vascular endothelium, at least partly, by decreased nitric oxide synthesis. Plaque MPs favor local inflammation by augmenting the expression of adhesion molecule, such as intercellular adhesion molecule -1 at the surface of endothelial cell, and monocyte recruitment within the lesion. In addition, plaque MPs stimulate angiogenesis, a key event in the transition from stable to unstable lesions. MPs also may promote local cell apoptosis, leading to the release and accumulation of new MPs, and thus creating a vicious circle. Furthermore, highly thrombogenic plaque MPs could increase thrombus formation at the time of rupture, together with circulating MPs released in this context by activated platelets and leukocytes. Finally, MPs also could participate in repairing the consequences of arterial occlusion and tissue ischemia by promoting postischemic neovascularization.

Emerging applications of nanotechnology for the diagnosis and management of vulnerable atherosclerotic plaques

An estimated 16 million people in the United States have coronary artery disease (CAD), and approximately 325,000 people die annually from cardiac arrest. About two-thirds of unexpected cardiac deaths occur without prior recognition of cardiac disease. A vast majority of these deaths are attributable to the rupture of 'vulnerable atherosclerotic plaques'. Clinically, plaque vulnerability is typically assessed through imaging techniques, and ruptured plaques leading to acute myocardial infarction are treated through angioplasty or stenting. Despite significant advances, it is clear that current imaging methods are insufficiently capable for elucidating plaque composition--which is a key determinant of vulnerability. Further, the exciting improvement in the treatment of CAD afforded by stenting procedures has been buffered by significant undesirable host-implant effects, including restenosis and late thrombosis. Nanotechnology has led to some potential solutions to these problems by yielding constructs that interface with plaque cellular components at an unprecedented size scale. By leveraging the innate ability of macrophages to phagocytose nanoparticles, contrast agents can now be targeted to plaque inflammatory activity. Improvements in nano-patterning procedures have now led to increased ability to regenerate tissue isotropy directly on stents, enabling gradual regeneration of normal, physiologic vascular structures. Advancements in immunoassay technologies promise lower costs for biomarker measurements, and in the near future, may enable the addition of routine blood testing to the clinician's toolbox--decreasing the costs of atherosclerosis-related medical care. These are merely three examples among many stories of how nanotechnology continues to promise advances in the diagnosis and treatment of vulnerable atherosclerotic plaques.

IgE stimulates human and mouse arterial cell apoptosis and cytokine expression and promotes atherogenesis in Apoe-/- mice

IgE has a key role in the pathogenesis of allergic responses through its ability to activate mast cells via the receptor FcεR1. In addition to mast cells, many cell types implicated in atherogenesis express FcεR1, but whether IgE has a role in this disease has not been determined. Here, we demonstrate that serum IgE levels are elevated in patients with myocardial infarction or unstable angina pectoris. We found that IgE and the FcεR1 subunit FcεR1α were present in human atherosclerotic lesions and that they localized particularly to macrophage-rich areas. In mice, absence of FcεR1α reduced inflammation and apoptosis in atherosclerotic plaques and reduced the burden of disease. In cultured macrophages, the presence of TLR4 was required for FcεR1 activity. IgE stimulated the interaction between FcεR1 and TLR4, thereby inducing macrophage signal transduction, inflammatory molecule expression, and apoptosis. These IgE activities were reduced in the absence of FcεR1 or TLR4. Furthermore, IgE activated macrophages by enhancing Na+/H+ exchanger 1 (NHE1) activity. Inactivation of NHE1 blocked IgE-induced macrophage production of inflammatory molecules and apoptosis. Cultured human aortic SMCs (HuSMCs) and ECs also exhibited IgE-induced signal transduction, cytokine expression, and apoptosis. In human atherosclerotic lesions, SMCs and ECs colocalized with IgE and TUNEL staining. This study reveals what we believe to be several previously unrecognized IgE activities that affect arterial cell biology and likely other IgE-associated pathologies in human diseases.

OxLDL-mediated survival of macrophages does not require LDL internalization or signalling by major pattern recognition receptors

Macrophages play a key role in the pathogenesis of atherosclerosis, in part by destabilizing plaques. We and others have shown that low concentrations of oxidized LDL (oxLDL) inhibit macrophage apoptosis. As oxLDL is present in lesions, this may be a mechanism by which macrophage populations in the intima are expanded. We have previously shown that oxLDL activates prosurvival signalling pathways such as the phosphoinositide 3-kinase (PI3K) pathway in bone marrow derived macrophages (BMDMs). However, little is known about more upstream signalling events especially at the receptor level. The endocytic pattern recognition receptors (PRRs), scavenger receptor A (SR-A) and CD36, are the main receptors on macrophages for uptake of oxLDL and are therefore important in foam cell formation. The signalling PRRs such as toll-like receptor (TLR) 2 and 4 also bind some types of oxLDL. This study was done to determine if any of the known PRRs are required for the anti-apoptotic effects of oxLDL in BMDMs. To do this, we tested the effect of oxLDL on viability of BMDMs lacking both SR-A and CD36 or lacking TLR2, TLR4, CD14, FcγRIIb, or RAGE. Our results indicate that none of these receptors are essential for activating the oxLDL prosurvival pathway. Furthermore, we show that the anti-apoptotic effect is not dependent on the uptake of oxLDL.

Monocyte and macrophage dynamics during atherogenesis

Vascular inflammation is associated with and in large part driven by changes in the leukocyte compartment of the vessel wall. Here, we focus on monocyte influx during atherosclerosis, the most common form of vascular inflammation. Although the arterial wall contains a large number of resident macrophages and some resident dendritic cells, atherosclerosis drives a rapid influx of inflammatory monocytes (Ly-6C(+) in mice) and other monocytes (Ly-6C(-) in mice, also known as patrolling monocytes). Once in the vessel wall, Ly-6C(+) monocytes differentiate to a phenotype consistent with inflammatory macrophages and inflammatory dendritic cells. The phenotype of these cells is modulated by lipid uptake, Toll-like receptor ligands, hematopoietic growth factors, cytokines, and chemokines. In addition to newly recruited macrophages, it is likely that resident macrophages also change their phenotype. Monocyte-derived inflammatory macrophages have a short half-life. After undergoing apoptosis, they may be taken up by surrounding macrophages or, if the phagocytic capacity is overwhelmed, can undergo secondary necrosis, a key event in forming the necrotic core of atherosclerotic lesions. In this review, we discuss these and other processes associated with monocytic cell dynamics in the vascular wall and their role in the initiation and progression of atherosclerosis.

Recent advances on the role of cytokines in atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial wall driven by innate and adaptive immune responses. Inflammation controls the development and the destabilization of arterial plaque. 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 have provided evidence for a dual role of cytokines: proinflammatory and T helper-1-related cytokines promote the development and progression of the disease, whereas antiinflammatory and regulatory T cell-related cytokines exert clear antiatherogenic activities. This review focuses on recent advances regarding the role of cytokines, with the exception of chemokines, in the development, progression, and complications of atherosclerosis.

Peroxisome proliferator-activated receptor gamma induces apoptosis and inhibits autophagy of human monocyte-derived macrophages via induction of cathepsin L: potential role in atherosclerosis

Macrophages play a pivotal role in the pathophysiology of atherosclerosis. These cells express cathepsin L (CatL), a cysteine protease that has been implicated in atherogenesis and the associated arterial remodeling. In addition, macrophages highly express peroxisome proliferator-activated receptor (PPAR) γ, a transcription factor that regulates numerous genes important for lipid and lipoprotein metabolism, for glucose homeostasis, and inflammation. Hence, PPARγ might affect macrophage function in the context of chronic inflammation such as atherogenesis. In the present study, we examined the effect of PPARγ activation on the expression of CatL in human monocyte-derived macrophages (HMDM). Activation of PPARγ by the specific agonist GW929 concentration-dependently increased the levels of CatL mRNA and protein in HMDM. By promoter analysis, we identified a functional PPAR response element-like sequence that positively regulates CatL expression. In addition, we found that PPARγ-induced CatL promotes the degradation of Bcl2 without affecting Bax protein levels. Consistently, degradation of Bcl2 could be prevented by a specific CatL inhibitor, confirming the causative role of CatL. PPARγ-induced CatL was found to decrease autophagy through reduction of beclin 1 and LC3 protein levels. The reduction of these proteins involved in autophagic cell death was antagonized either by the CatL inhibitor or by CatL knockdown. In conclusion, our data show that PPARγ can specifically induce CatL, a proatherogenic protease, in HMDM. In turn, CatL inhibits autophagy and induces apoptosis. Thus, the proatherogenic effect of CatL could be neutralized by apoptosis, a beneficial phenomenon, at least in the early stages of atherosclerosis.

Cysteine Protease Cathepsins in Atherosclerosis and Abdominal Aortic Aneurysm

Extracellular matrix remodeling is an important mechanism in the initiation and progression of cardiovascular diseases. Cysteine protease cathepsins are among the important proteases that affect major events in the pathogenesis of atherosclerosis and abdominal aortic aneurysm, including smooth muscle cell transmigration through elastic lamina, macrophage foam cell formation, vascular cell and macrophage apoptosis, and plaque rupture. These events have been studied in cathepsin deficiencies and cathepsin inhibitor deficiencies in mice and have provided invaluable insights regarding the roles of cathepsins in cardiovascular diseases. Pharmacological inhibitions for cathepsins are under evaluation for other human diseases and may be used as clinical treatments for cardiovascular diseases in the near future. This article reviews different mechanisms for cathepsins in atherosclerosis and abdominal aortic aneurysm that could be targeted by selective cathepsin inhibitors.

A requirement for the p85 PI3K adapter protein BCAP in the protection of macrophages from apoptosis induced by endoplasmic reticulum stress

Macrophages are innate immune cells that play key roles in regulation of the immune response and in tissue injury and repair. In response to specific innate immune stimuli, macrophages may exhibit signs of endoplasmic reticulum (ER) stress and progress to apoptosis. Factors that regulate macrophage survival under these conditions are poorly understood. In this study, we identified B cell adapter protein (BCAP), a p85 PI3K-binding adapter protein, in promoting survival in response to the combined challenge of LPS and ER stress. BCAP was unique among nine PI3K adapter proteins in being induced >10-fold in response to LPS. LPS-stimulated macrophages incubated with thapsigargin, a sarcoplasmic/endoplasmic reticulum calcium ATPase inhibitor that induces ER stress, underwent caspase-3 activation and apoptosis. Macrophages from BCAP(-/-) mice exhibited increased apoptosis in response to these stimuli. BCAP-deficient macrophages demonstrated decreased activation of Akt, but not ERK, and, unlike BCAP-deficient B cells, expressed normal amounts of the NF-κB subunits, c-Rel and RelA. Retroviral transduction of BCAP-deficient macrophages with wild-type BCAP, but not a Y4F BCAP mutant defective in binding the SH2 domain of p85 PI3K, reversed the proapoptotic phenotype observed in BCAP-deficient macrophages. We conclude that BCAP is a nonredundant PI3K adapter protein in macrophages that is required for maximal cell survival in response to ER stress. We suggest that as macrophages engage their pathogenic targets, innate immune receptors trigger increased expression of BCAP, which endows them with the capacity to withstand further challenges from ongoing cellular insults, such as ER stress.

Monocyte trafficking in acute and chronic inflammation

Environmental signals at the site of inflammation mediate rapid monocyte mobilization and dictate differentiation programs whereby these cells give rise to macrophages or dendritic cells. Monocytes participate in tissue healing, clearance of pathogens and dead cells, and initiation of adaptive immunity. However, recruited monocytes can also contribute to the pathogenesis of infection and chronic inflammatory disease, such as atherosclerosis. Here, we explore monocyte trafficking in the context of acute inflammation, relying predominantly on data from microbial infection models. These mechanisms will be compared to monocyte trafficking during chronic inflammation in experimental models of atherosclerosis. Recent developments suggest that monocyte trafficking shares common themes in diverse inflammatory diseases; however, important differences exist between monocyte migratory pathways in acute and chronic inflammation.

Apolipoprotein L6, induced in atherosclerotic lesions, promotes apoptosis and blocks Beclin 1-dependent autophagy in atherosclerotic cells

Inflammatory cytokine-regulated apoptosis and autophagy play pivotal roles in plaque rupture and thrombosis of atherosclerotic lesions. However, the molecular interplay between apoptosis and autophagy in vascular cells has not been investigated. Our prior study showed that human apolipoprotein L6 (ApoL6), a pro-apoptotic BH3-only member of the Bcl-2 family, was one of the downstream targets of interferon-γ (INFγ), which sensitizes atherosclerotic lesion-derived cells (LDCs) to Fas-induced apoptosis. To investigate whether ApoL6 plays a causal role in atherosclerotic apoptosis and autophagy, in this study, we demonstrate that IFNγ treatment itself strongly induces ApoL6, and ApoL6 is highly expressed and partially co-localized with activated caspase 3 in activated smooth muscle cells in atherosclerotic lesions. In addition, overexpression of ApoL6 promotes reactive oxygen species (ROS) generation, caspase activation, and subsequent apoptosis, which can be blocked by pan caspase inhibitor and ROS scavenger. Knockdown of ApoL6 expression by siApoL6 suppresses INFγ- and Fas-mediated apoptosis. Further, ApoL6 binds Bcl-X(L), one of the most abundant anti-death proteins in LDCs. Interestingly, forced ApoL6 expression in LDCs induces degradation of Beclin 1, accumulation of p62, and subsequent attenuation of LC3-II formation and translocation and thus autophagy, whereas siApoL6 treatment reverts the phenotype. Taken together, our results suggest that ApoL6 regulates both apoptosis and autophagy in SMCs. IFNγ-initiated, ApoL6-induced apoptosis in vascular cells may be an important factor causing plaque instability and a potential therapeutic target for treating atherosclerosis and cardiovascular disease.

High phosphate diet reduces atherosclerosis formation in apolipoprotein E-deficient mice

Although higher serum phosphate level is a risk factor for cardiovascular diseases in general population as well as chronic kidney disease patients, it has not been clarified whether higher phosphate can affect atherosclerotic plaque formation. In this study, we investigated the effect of prolonged-intake of different concentrations of phosphate on atherosclerosis formation using apolipoprotein E-deficient mice. Apolipoprotein E-deficient mice were fed with high fat diet including 0.6%, 1.2% or 1.8% phosphate. After 20-week treatment, atherosclerotic plaque formation in aorta in 1.8% phosphate diet group was unexpectedly less than that in the other groups. To elucidate mechanisms of suppression of plaque formation by high phosphate diet, we hypothesized that high phosphate diet may modify a profile of monocytes/macrophages suppressing plaque formation. We confirmed that elevated peripheral monocytes (CD11b+, F4/80+ cell numbers) in apolipoprotein E-deficient mice were decreased by feeding with 1.8% P diet. In addition, ex vivo study indicated that high dose of phosphate induced macrophage apoptosis. These observations suggest that excess phosphate intake decreased atherosclerosis formation, at least in part, by changing the profile of peripheral monocytes or inducing apoptosis of macrophages in apolipoprotein E-deficient mice.

Lipoprotein associated phospholipase A(2): role in atherosclerosis and utility as a biomarker for cardiovascular risk

Atherosclerosis and its clinical manifestations are widely prevalent throughout the world. Atherogenesis is highly complex and modulated by numerous genetic and environmental risk factors. A large body of basic scientific and clinical research supports the conclusion that inflammation plays a significant role in atherogenesis along the entire continuum of its progression. Inflammation adversely impacts intravascular lipid handling and metabolism, resulting in the development of macrophage foam cell, fatty streak, and atheromatous plaque formation. Given the enormous human and economic cost of myocardial infarction, ischemic stroke, peripheral arterial disease and amputation, and premature death and disability, considerable effort is being committed to refining our ability to correctly identify patients at heightened risk for atherosclerotic vascular disease and acute cardiovascular events so that they can be treated earlier and more aggressively. Serum markers of inflammation have emerged as an important component of risk factor burden. Lipoprotein-associated phospholipase A2 (Lp-PLA(2)) potentiates intravascular inflammation and atherosclerosis. A variety of epidemiologic studies support the utility of Lp-PLA(2) measurements for estimating and further refining cardiovascular disease risk. Drug therapies to inhibit Lp-PLA(2) are in development and show considerable promise, including darapladib, a specific molecular inhibitor of the enzyme. In addition to substantially inhibiting Lp-PLA(2) activity, darapladib reduces progression of the necrotic core volume of human coronary artery atheromatous plaque. The growing body of evidence points to an important role and utility for Lp-PLA(2) testing in preventive and personalized clinical medicine.

A reporter for tracking the UPR in vivo reveals patterns of temporal and cellular stress during atherosclerotic progression

Progression of human arteriosclerosis is associated with and promoted by induction of the endoplasmic reticulum (ER) stress pathway known as the unfolded protein response (UPR). Most studies that assess UPR markers in atherosclerosis rely on methodologies that suffer from low signal sensitivity, nonspecific immunohistochemistry, or inability to resolve differences between cellular subsets. To accurately monitor the UPR independently of artifacts generated postmortem, we describe here the first in vivo reporter for ER stress during atherosclerosis. Mice transgenic for the fluorescent XBP-1 ER stress indicator Erai were bred onto the Ldlr(-/-) background and fed an atherogenic diet. Subsequently, ERAI fluorescence at aortic roots was quantified and colocalized with lesional cell type. We found that the ERAI fluorescent signal increased as a function of time on the atherogenic diet and, in advanced lesions, was found close to necrotic cores. The majority of ERAI fluorescence localized to macrophages, and to a lesser extent, to intimal smooth muscle cells and patches of endothelial cells. These mice provide a valuable tool to monitor activation of the UPR in atherosclerosis and will be useful for future studies investigating relationships between pharmacologic and genetic modulators of UPR and atherosclerosis.

Sphingosine-1-Phosphate-Specific G Protein-Coupled Receptors as Novel Therapeutic Targets for Atherosclerosis

Atherosclerosis is a chronic inflammatory process involving complex interactions of modified lipoproteins, monocyte-derived macrophages or foam cells, lymphocytes, endothelial cells (ECs), and vascular smooth muscle cells. Sphingosine-1-phosphate (S1P), a biologically active blood-borne lipid mediator, exerts pleiotropic effects such as cell proliferation, migration and cell-cell adhesion in a variety of cell types via five members of S1P-specific high-affinity G protein-coupled receptors (S1P₁-S1P₅). Among them, S1P₁, S1P₂ and S1P₃ are major receptor subtypes which are widely expressed in various tissues. Available evidence suggest that S1P and HDL-bound S1P exert atheroprotective effects including inhibition of leukocyte adhesion and stimulation of endothelial nitric oxide synthase (eNOS) in endothelial cells (ECs) through the activation of G_i signaling pathway via S1P₃ and probably S1P₁, although there is still controversy. FTY720, the phosphorylation product of which is a high-affinity agonist for all S1P receptors except S1P₂ and act as an immunosuppressant by downregulating S1P₁ on lymphocytes, inhibits atherosclerosis in LDL receptor-null mice and apoE-null mice through the inhibition of lymphocyte and macrophage functions and probably stimulation of EC functions, without influencing plasma lipid concentrations. In contrast to S1P₁ and S1P₃, S1P₂ facilitates atherosclerosis by activating G_12/13-Rho-Rho kinase (ROCK) in apoE-null mice. S1P₂ mediates transmigration of monocytes into the arterial intima, oxidized LDL accumulation and cytokine secretion in monocyte-derived macrophages, and eNOS inhibition and cytokine secretion in ECs through Rac inhibition, NF-κB activation and 3′-specific phosphoinositide phosphatase (PTEN) stimulation downstream of G_12/13-Rho-ROCK. Systemic long-term administration of a selective S1P₂-blocker remarkably inhibits atherosclerosis without overt toxicity. Thus, multiple S1P receptors positively and negatively regulate atherosclerosis through multitudes of mechanisms. Considering the essential and multi-faceted role of S1P₂ in atherogenesis and the impact of S1P₂ inactivation on atherosclerosis, S1P₂ is a particularly promising therapeutic target for atherosclerosis.

Caveolae-dependent endocytosis is required for class A macrophage scavenger receptor-mediated apoptosis in macrophages

SR-A (class A macrophage scavenger receptor) is a transmembrane receptor that can bind many different ligands, including modified lipoproteins that are relevant to the development of vascular diseases. However, the precise endocytic pathways of SR-A/mediated ligands internalization are not fully characterized. In this study, we show that the SR-A/ligand complex can be endocytosed by both clathrin- and caveolae-dependent pathways. Internalizations of SR-A-lipoprotein (such as acLDL) complexes primarily go through clathrin-dependent endocytosis. In contrast, macrophage apoptosis triggered by SR-A-fucoidan internalization requires caveolae-dependent endocytosis. The caveolae-dependent process activates p38 kinase and JNK signaling, whereas the clathrin-mediated endocytosis elicits ERK signaling. Our results suggest that different SR-A endocytic pathways have distinct functional consequences due to the activation of different signaling cascades in macrophages.

The emerging role of autophagy in the pathophysiology of diabetes mellitus

An emerging body of evidence supports a role for autophagy in the pathophysiology of type 1 and type 2 diabetes mellitus. Persistent high concentrations of glucose lead to imbalances in the antioxidant capacity within the cell resulting in oxidative stress-mediated injury in both disorders. An anticipated consequence of impaired autophagy is the accumulation of dysfunctional organelles such as mitochondria within the cell. Mitochondria are the primary site of the production of reactive oxygen species (ROS), and an imbalance in ROS production relative to the cytoprotective action of autophagy may lead to the accumulation of ROS. Impaired mitochondrial function associated with increased ROS levels have been proposed as mechanisms contributing to insulin resistance. In this article we review and interpret the literature that implicates a role for autophagy in the pathophysiology of type 1 and type 2 diabetes mellitus as it applies to β-cell dysfunction, and more broadly to organ systems involved in complications of diabetes including the cardiovascular, renal and nervous systems.

Methods and models for monitoring UPR-associated macrophage death during advanced atherosclerosis

During advanced atherosclerosis, chronic activation of the endoplasmic reticulum (ER) stress pathway, otherwise known as the unfolded protein response (UPR), is strongly associated with atherosclerotic plaque destabilization, the precursor to acute myocardial infarction and sudden death. Destabilized or vulnerable plaques are characterized by features that include thinning of a protective collagenous cap at the interface between the plaque and the vascular lumen and expansion of the necrotic core, a lipid-rich graveyard of dead macrophages. The cell biology of advanced plaque progression is complex and includes multiple cellular stressors that combine to promote chronic inflammation and progressive plaque deterioration. Several of these stressors converge at the ER, leading to activation of the UPR in multiple cell types. In macrophages, prolonged UPR activation triggers apoptosis, which when coupled to defective phagocytic clearance of these dying cells, leads to secondary necrosis and expansion of the plaque necrotic core. Though much insight has been gained recently on the role of the UPR in atherosclerosis, future studies are warranted to determine the cell-type specific contributions of ER stress to athero-progression and the therapeutic potential of UPR modulation. For such objectives to be met, reliable and standardized methodology must be utilized and developed. This chapter summarizes our current understanding of ER stress-induced macrophage apoptosis in atheromata and outlines both in vitro and in vivo methodologies to quantify the UPR in the context of experimental murine-advanced atherosclerosis.

Atherosclerosis in chronic kidney disease: the role of macrophages

Patients with chronic kidney disease (CKD) are at increased risk of atherosclerotic cardiovascular disease and loss of renal parenchyma accelerates atherosclerosis in animal models. Macrophages are central to atherogenesis because they regulate cholesterol traffic and inflammation in the arterial wall. CKD influences macrophage behavior at multiple levels, rendering them proatherogenic. Even at normal creatinine levels, macrophages from uninephrectomized Apoe(-/-) mice are enriched in cholesterol owing to downregulation of cholesterol transporter ATP-binding cassette subfamily A member 1 levels and activation of nuclear factor κB, which leads to impaired cholesterol efflux. Interestingly, treatment with an angiotensin-II-receptor blocker (ARB) improves these effects. Moreover, atherosclerotic aortas from Apoe(-/-) mice transplanted into renal-ablated normocholesterolemic recipients show plaque progression and increased macrophage content instead of the substantial regression seen in recipient mice with intact kidneys. ARBs reduce atherosclerosis development in mice with partial renal ablation. These results, combined with the clinical benefits of angiotensin-converting-enzyme (ACE) inhibitors and ARBs in patients with CKD, suggest an important role for the angiotensin system in the enhanced susceptibility to atherosclerosis seen across the spectrum of CKD. The role of macrophages could explain why these therapies may be effective in end-stage renal disease, one of the few conditions in which statins show no clinical benefit.

Selective macrophage ascorbate deficiency suppresses early atherosclerosis

To test whether severe ascorbic acid deficiency in macrophages affects progression of early atherosclerosis, we used fetal liver cell transplantation to generate atherosclerosis-prone apolipoprotein E-deficient (apoE(-/-)) mice that selectively lacked the ascorbate transporter (SVCT2) in hematopoietic cells, including macrophages. After 13 weeks of chow diet, apoE(-/-) mice lacking the SVCT2 in macrophages had surprisingly less aortic atherosclerosis, decreased lesion macrophage numbers, and increased macrophage apoptosis compared to control-transplanted mice. Serum lipid levels were similar in both groups. Peritoneal macrophages lacking the SVCT2 had undetectable ascorbate; increased susceptibility to H(2)O(2)-induced mitochondrial dysfunction and apoptosis; decreased expression of genes for COX-2, IL1β, and IL6; and decreased lipopolysaccharide-stimulated NF-κB and antiapoptotic gene expression. These changes were associated with decreased expression of both the receptor for advanced glycation end products and HIF-1α, either or both of which could have been the proximal cause of decreased macrophage activation and apoptosis in ascorbate-deficient macrophages.

Recombinant TFPI-2 enhances macrophage apoptosis through upregulation of Fas/FasL

Tissue factor pathway inhibitor-2 (TFPI-2) is a Kunitz-type serine proteinase inhibitor with inhibitory activity toward activated factor XI, plasma kallikrein, plasmin, certain matrix metalloproteinases, and the tissue factor-activated factor VII complex. In addition, TFPI-2 has other functions such as promoting cell migration and inducing apoptosis. In the present study, we investigated if TFPI-2 induced apoptosis in cultured U937-derived macrophages and the possible signal pathways that involved in the apoptotic process. Apoptotic DNA fragment detection and caspase-3,9 activity measurements indicated that rTFPI-2 promoted U937-derived macrophage apoptosis. Hoechst 33342 assay and flow cytometry further showed that rTFPI-2 induced apoptosis in cultured macrophages in a dose-dependent manner. Because death receptors of the TNF family such as Fas are the best-understood death pathways that recruit Fas-associated death domain (FADD) and procaspase-8 to the receptor in macrophages, we investigated the expression of Fas and its ligand (FasL) and downstream signal caspase-8 by Western blot analysis. The results indicated that the process of apoptosis triggered by rTFPI-2 was, at least in part, actively conducted by U937-derived macrophages possibly through Fas/FasL signal pathway. In brief, rTFPI-2 may have the potential usefulness in inducing macrophages apoptosis, which suggest TFPI-2 might have antiatherogenic effects.

Mechanisms of failed apoptotic cell clearance by phagocyte subsets in cardiovascular disease

Recent evidence in humans indicate that defective phagocytic clearance of dying cells is linked to progression of advanced atherosclerotic lesions, the precursor to atherothrombosis, ischemic heart disease, and leading cause of death in the industrialized world. During atherogenesis, apoptotic cell turnover in the vascular wall is counterbalanced by neighboring phagocytes with high clearance efficiency, thereby limiting cellularity and maintaining lesion integrity. However, as lesions mature, phagocytic removal of apoptotic cells (efferocytosis) becomes defective, leading to secondary necrosis, expansion of plaque necrotic cores, and susceptibility to rupture. Recent genetic causation studies in experimental rodents have implicated key molecular regulators of efferocytosis in atherosclerotic progression. These include MER tyrosine kinase (MERTK), milk fat globule-EGF factor 8 (MFGE8), and complement C1q. At the cellular level, atheromata are infiltrated by a heterogenous population of professional phagocytes, comprised of monocytes, differentiated macrophages, and CD11c(+) dendritic-like cells. Each cell type is characterized by disparate clearance efficiencies and varying activities of key phagocytic signaling molecules. It is in this context that we outline a working model whereby plaque necrosis and destabilization is jointly promoted by (1) direct inhibition of core phagocytic signaling pathways and (2) expansion of phagocyte subsets with poor clearance capacity. Towards identifying targets for promoting efficient apoptotic cell clearance and resolving inflammation in atherosclerosis and during ischemic heart disease and post myocardial infarction, this review will discuss potential in vivo suppressors of efferocytosis at each stage of clearance and how these putative interventional targets may differentially affect uptake at the level of vascular phagocyte subsets.

Serum opacity factor enhances HDL-mediated cholesterol efflux, esterification and anti inflammatory effects

Serum opacity factor (SOF) is a streptococcal protein that disrupts the structure of human high density lipoproteins (HDL) releasing lipid-free apo A-I while forming a large cholesteryl ester-rich particle and a small neo HDL. Given its low cholesterol and high phospholipid contents, we tested the hypotheses that neo HDL is a better substrate for cholesterol esterification via lecithin:cholesterol acyltransferase (LCAT), better than HDL as an acceptor of THP-1 macrophage cholesterol efflux, and improves reduction of oxidized LDL-induced production of inflammatory markers. We observed that both cholesterol efflux and esterification were improved by recombinant (r)SOF treatment of whole plasma and that the underlying cause of the improved cholesterol esterification in plasma and macrophage cholesterol efflux to rSOF-treated plasma was due to the rSOF-mediated conversion of HDL to neo HDL. Moreover, the reduction of secretion of TNF-α and IL-6 by THP-1 cells by neo HDL was twice that of HDL. Studies in BHK cells overexpressing cholesterol transporters showed that efflux to neo HDL occurred primarily via ABCA1 not ABCG1. Thus, rSOF improves two steps in reverse cholesterol transport with a concomitant reduction in the release of macrophage markers of inflammation. We conclude that rSOF catalyzes a novel reaction that might be developed as a new therapy that prevents or reverses atherosclerosis via improved reverse cholesterol transport.

Engineered annexin A5 variants have impaired cell entry for molecular imaging of apoptosis using pretargeting strategies

Phosphatidylserine (PS) on apoptotic cells is a target for diagnosis and therapy using annexin A5 (anxA5). Pretargeting is a strategy developed to improve signal to background ratio for molecular imaging and to minimize undesired side effects of pharmacological and radiotherapy. Pretargeting relies on accessibility of the target finder on the surface of the target cell. anxA5 binds PS and crystallizes in a two-dimensional network covering the PS-expressing cell surface. Two-dimensional crystallization is the driving force for anxA5 internalization by PS-expressing cells. Here, we report structure/function analysis of anxA5 internalization. Guided by structural bioinformatics including protein-protein docking, we revealed that the amino acids Arg(63), Lys(70), Lys(101), Glu(138), Asp(139), and Asn(160) engage in intermolecular salt bridges within the anxA5 trimer, which is the basic building block of the two-dimensional network. Disruption of the salt bridges by site-directed mutagenesis does not affect PS binding but inhibits trimer formation and cell entry of surface-bound anxA5. The anxA5 variants with impaired internalization are superior molecular imaging agents in pretargeting strategies as compared with wild-type anxA5.

Macrophage pro-inflammatory cytokine secretion is enhanced following interaction with autologous platelets

BACKGROUND

Macrophages are the dominant phagocyte at sites of wound healing and inflammation, and the cellular and acellular debris encountered by macrophages can have profound effects on their inflammatory profile. Following interaction with apoptotic cells, macrophages are known to switch to an anti-inflammatory phenotype. Activated platelets, however, are also a major component of inflammatory lesions and have been proposed to be pro-inflammatory mediators. In the present study, we tested the hypothesis that macrophage interaction with activated platelets results in an inflammatory response that differs from the response following phagocytosis of apoptotic cells.

METHODS

Human monocyte-derived macrophages (hMDMs) were co-incubated with autologous activated platelets (AAPs) and the platelet-macrophage interaction was examined by electron microscopy and flow cytometry. The cytokines TNF-α, IL-6, and IL-23 were also measured during LPS-activated hMDM co-incubation with AAPs, which was compared to co-incubation with apoptotic lymphocytes. Cytokine secretion was also compared to platelets pre-treated with the gluococorticoid dexamethasone.

RESULTS

Macrophages trapped and phagocytized AAPs utilizing a mechanism that was significantly inhibited by the scavenger receptor ligand fucoidan. LPS-induced macrophage secretion of TNF-α, IL-6, and IL-23 was inhibited by co-incubation with apoptotic cells, but enhanced by co-incubation with AAPs. The platelet-dependent enhancement of LPS-induced cytokines could be reversed by pre-loading the platelets with the glucocorticoid dexamethasone.

CONCLUSIONS

The interaction of human macrophages with autologous platelets results in scavenger-receptor-mediated platelet uptake and enhancement of LPS-induced cytokines. Therefore, the presence of activated platelets at sites of inflammation may exacerbate pro-inflammatory macrophage activation. The possibility of reversing macrophage activation with dexamethasone-loaded platelets is a promising therapeutic approach to treating unresolved inflammation.

Atherogenic lipids and lipoproteins trigger CD36-TLR2-dependent apoptosis in macrophages undergoing endoplasmic reticulum stress

Macrophage apoptosis in advanced atheromata, a key process in plaque necrosis, involves the combination of ER stress with other proapoptotic stimuli. We show here that oxidized phospholipids, oxidized LDL, saturated fatty acids (SFAs), and lipoprotein(a) trigger apoptosis in ER-stressed macrophages through a mechanism requiring both CD36 and Toll-like receptor 2 (TLR2). In vivo, macrophage apoptosis was induced in SFA-fed, ER-stressed wild-type but not Cd36⁻(/)⁻ or Tlr2⁻(/)⁻ mice. For atherosclerosis, we combined TLR2 deficiency with that of TLR4, which can also promote apoptosis in ER-stressed macrophages. Advanced lesions of fat-fed Ldlr⁻(/)⁻ mice transplanted with Tlr4⁻(/)⁻Tlr2⁻(/)⁻ bone marrow were markedly protected from macrophage apoptosis and plaque necrosis compared with WT →Ldlr⁻(/)⁻ lesions. These findings provide insight into how atherogenic lipoproteins trigger macrophage apoptosis in the setting of ER stress and how TLR activation might promote macrophage apoptosis and plaque necrosis in advanced atherosclerosis.

The role of endoplasmic reticulum stress in the progression of atherosclerosis

Prolonged activation of the endoplasmic reticulum (ER) stress pathway known as the unfolded protein response (UPR) can lead to cell pathology and subsequent tissue dysfunction. There is now ample evidence that the UPR is chronically activated in atherosclerotic lesional cells, particularly advanced lesional macrophages and endothelial cells. The stressors in advanced lesions that can lead to prolonged activation of the UPR include oxidative stress, oxysterols, and high levels of intracellular cholesterol and saturated fatty acids. Importantly, these arterial wall stressors may be especially prominent in the settings of obesity, insulin resistance, and diabetes, all of which promote the clinical progression of atherosclerosis. In the case of macrophages, prolonged ER stress triggers apoptosis, which in turn leads to plaque necrosis if the apoptotic cells are not rapidly cleared. ER stress-induced endothelial cell apoptosis may also contribute to plaque progression. Another potentially important proatherogenic effect of prolonged ER stress is activation of inflammatory pathways in macrophages and, perhaps in response to atheroprone shear stress, endothelial cells. Although exciting work over the last decade has begun to shed light on the mechanisms and in vivo relevance of ER stress-driven atherosclerosis, much more work is needed to fully understand this area and to enable an informed approach to therapeutic translation.

Induction of the unfolded protein response after monocyte to macrophage differentiation augments cell survival in early atherosclerotic lesions

Endoplasmic reticulum (ER) stress causes macrophage cell death within advanced atherosclerotic lesions, thereby contributing to necrotic core formation and increasing the risk of atherothrombotic disease. However, unlike in advanced lesions, the appearance of dead/apoptotic macrophages in early lesions is less prominent. Given that activation of the unfolded protein response (UPR) is detected in early lesion-resident macrophages and can enhance cell survival against ER stress, we investigated whether UPR activation occurs after monocyte to macrophage differentiation and confers a cytoprotective advantage to the macrophage. Human peripheral blood monocytes were treated with monocyte colony-stimulating factor to induce macrophage differentiation, as assessed by changes in ultrastructure and scavenger receptor expression. UPR markers, including GRP78, GRP94, and spliced XBP-1, were induced after macrophage differentiation and occurred after a significant increase in de novo protein synthesis. UPR activation after differentiation reduced macrophage cell death by ER stress-inducing agents. Further, GRP78 overexpression in macrophages was sufficient to reduce ER stress-induced cell death. Consistent with these in vitro findings, UPR activation was observed in viable lesion-resident macrophages from human carotid arteries and from the aortas of apoE(-/-) mice. However, no evidence of apoptosis was observed in early lesion-resident macrophages from the aortas of apoE(-/-) mice. Thus, our findings that UPR activation occurs during macrophage differentiation and is cytoprotective against ER stress-inducing agents suggest an important cellular mechanism for macrophage survival within early atherosclerotic lesions.

Sphingosine-1-phosphate receptor-2 deficiency leads to inhibition of macrophage proinflammatory activities and atherosclerosis in apoE-deficient mice

Sphingosine-1-phosphate (S1P) is a biologically active sphingolipid that has pleiotropic effects in a variety of cell types including ECs, SMCs, and macrophages, all of which are central to the development of atherosclerosis. It may therefore exert stimulatory and inhibitory effects on atherosclerosis. Here, we investigated the role of the S1P receptor S1PR2 in atherosclerosis by analyzing S1pr2-/- mice with an Apoe-/- background. S1PR2 was expressed in macrophages, ECs, and SMCs in atherosclerotic aortas. In S1pr2-/-Apoe-/- mice fed a high-cholesterol diet for 4 months, the area of the atherosclerotic plaque was markedly decreased, with reduced macrophage density, increased SMC density, increased eNOS phosphorylation, and downregulation of proinflammatory cytokines compared with S1pr2+/+Apoe-/- mice. Bone marrow chimera experiments indicated a major role for macrophage S1PR2 in atherogenesis. S1pr2-/-Apoe-/- macrophages showed diminished Rho/Rho kinase/NF-κB (ROCK/NF-κB) activity. Consequently, they also displayed reduced cytokine expression, reduced oxidized LDL uptake, and stimulated cholesterol efflux associated with decreased scavenger receptor expression and increased cholesterol efflux transporter expression. S1pr2-/-Apoe-/- ECs also showed reduced ROCK and NF-κB activities, with decreased MCP-1 expression and elevated eNOS phosphorylation. Pharmacologic S1PR2 blockade in S1pr2+/+Apoe-/- mice diminished the atherosclerotic plaque area in aortas and modified LDL accumulation in macrophages. We conclude therefore that S1PR2 plays a critical role in atherogenesis and may serve as a novel therapeutic target for atherosclerosis.

Sphingosine-1-phosphate receptor-2 function in myeloid cells regulates vascular inflammation and atherosclerosis

OBJECTIVE

Sphingomyelin deposition and metabolism occurs in the atherosclerotic plaque, leading to the formation of sphingosine-1-phosphate (S1P), which activates G protein-coupled receptors to regulate vascular and immune cells. The role of S1P receptors in atherosclerosis has not been examined.

METHODS AND RESULTS

We tested the hypothesis that S1P receptor-2 (S1PR2) regulates atherosclerosis. Apoe(-/-) S1pr2(-/-) mice showed greatly attenuated atherosclerosis compared with the Apoe(-/-) mice. Bone marrow transplant experiments indicate that S1PR2 function in the hematopoietic compartment is critical. S1PR2 is expressed in bone marrow-derived macrophages and in macrophage-like foam cells in atherosclerotic plaques. Reduced macrophage-like foam cells were found in the atherosclerotic plaques of Apoe(-/-)S1pr2(-/-) mice, suggesting that S1PR2 retains macrophages in atherosclerotic plaques. Lipoprotein profiles, plasma lipids, and oxidized low-density lipoprotein uptake by bone marrow-derived macrophages were not altered by the S1pr2 genotype. In contrast, endotoxin-induced inflammatory cytokine (interleukin [IL]-1β, IL-18) levels in the serum of S1PR2 knockout mice were significantly reduced. Furthermore, treatment of wild-type mice with S1PR2 antagonist JTE-013 suppressed IL-1β and IL-18 levels in plasma.

CONCLUSIONS

These data suggest that S1PR2 signaling in the plaque macrophage regulates macrophage retention and inflammatory cytokine secretion, thereby promoting atherosclerosis.

Drug treatment of hyperlipidaemia: a guide to the rational use of lipid-lowering drugs

Mammalian sterol and lipid metabolism depends on a large number of highly evolved biochemical and histological processes responsible for the absorption, distribution and steady-state anabolic/catabolic handling of these substances. Lipoproteins are complex polymolecular assemblies comprising phospholipids, cholesterol and cholesterol esters, triglycerides and a variety of apolipoproteins. The primary function of lipoproteins is to facilitate the systemic distribution of sterols and lipids. Abnormalities in lipoprotein metabolism are quite common and are attributable to a large number of genetic mutations, metabolic derangements such as insulin resistance or thyroid dysfunction, and excess availability of cholesterol and fat from dietary sources. Dyslipidaemic states facilitate endothelial dysfunction and atherogenesis. Dyslipidaemia is recognized as a risk factor for cardiovascular disease in both men and women, and people of all racial and ethnic groups throughout the world. Dyslipidaemia is modifiable with dietary change and the use of medications that impact on lipid metabolism through a variety of mechanisms. Reducing atherogenic lipoprotein burden in serum is associated with significant and meaningful reductions in risk for a variety of cardiovascular endpoints, including myocardial infarction, ischaemic stroke, development of peripheral arterial disease and mortality. This review provides an overview on how to best position lipid-lowering drugs when attempting to normalize serum lipid profiles and reduce risk for cardiovascular disease. HMG-CoA reductase inhibitors (statins) are widely accepted to be the agents of choice for reducing serum levels of low-density lipoprotein cholesterol (LDL-C) in both the primary and secondary prevention settings. Ezetimibe and bile acid sequestrants are both effective agents for reducing LDL-C, either used alone or in combination with statins. The statins, fibric acid derivatives (fibrates) and niacin raise high-density lipoprotein cholesterol to different extents depending upon genetic and metabolic background. Fibrates, niacin and omega-3 fish oils are efficacious therapies for reducing serum triglycerides. Combinations of these drugs are frequently required for normalizing mixed forms of dyslipidaemia.