Pivotal advance: macrophages become resistant to cholesterol-induced death after phagocytosis of apoptotic cells

Mechanism of efferocytosis in atherosclerosis

Atherosclerosis (AS) is a chronic inflammatory vascular disease that occurs in the intima of large and medium-sized arteries with the immune system's involvement. It is a common pathological basis for high morbidity and mortality of cardiovascular diseases. Abnormal proliferation of apoptotic cells and necrotic cells leads to AS plaque expansion, necrotic core formation, and rupture. In the early stage of AS, macrophages exert an efferocytosis effect to engulf and degrade apoptotic, dead, damaged, or senescent cells by efferocytosis, thus enabling the regulation of the organism. In the early stage of AS, macrophages rely on this effect to slow down the process of AS. However, in the advanced stage of AS, the efferocytosis of macrophages within the plaque is impaired, which leads to the inability of macrophages to promptly remove the apoptotic cells (ACs) from the organism promptly, causing exacerbation of AS. Moreover, upregulation of CD47 expression in AS plaques also protects ACs from phagocytosis by macrophages, resulting in a large amount of residual ACs in the plaque, further expanding the necrotic core. In this review, we discussed the molecular mechanisms involved in the process of efferocytosis and how efferocytosis is impaired and regulated during AS, hoping to provide new insights for treating AS.

Dysregulated cellular metabolism in atherosclerosis: mediators and therapeutic opportunities

Accumulating evidence over the past decades has revealed an intricate relationship between dysregulation of cellular metabolism and the progression of atherosclerotic cardiovascular disease. However, an integrated understanding of dysregulated cellular metabolism in atherosclerotic cardiovascular disease and its potential value as a therapeutic target is missing. In this Review, we (1) summarize recent advances concerning the role of metabolic dysregulation during atherosclerosis progression in lesional cells, including endothelial cells, vascular smooth muscle cells, macrophages and T cells; (2) explore the complexity of metabolic cross-talk between these lesional cells; (3) highlight emerging technologies that promise to illuminate unknown aspects of metabolism in atherosclerosis; and (4) suggest strategies for targeting these underexplored metabolic alterations to mitigate atherosclerosis progression and stabilize rupture-prone atheromas with a potential new generation of cardiovascular therapeutics.

Mechanism of Efferocytosis in Determining Ischaemic Stroke Resolution-Diving into Microglia/Macrophage Functions and Therapeutic Modality

After ischaemic cerebral vascular injury, efferocytosis-a process known as the efficient clearance of apoptotic cells (ACs) by various phagocytes in both physiological and pathological states-is crucial for maintaining central nervous system (CNS) homeostasis and regaining prognosis. The mechanisms of efferocytosis in ischaemic stroke and its influence on preventing inflammation progression from secondary injury were still not fully understood, despite the fact that the fundamental process of efferocytosis has been described in a series of phases, including AC recognition, phagocyte engulfment, and subsequent degradation. The genetic reprogramming of macrophages and brain-resident microglia after an ischaemic stroke has been equated by some researchers to that of the peripheral blood and brain. Based on previous studies, some molecules, such as signal transducer and activator of transcription 6 (STAT6), peroxisome proliferator-activated receptor γ (PPARG), CD300A, and sigma non-opioid intracellular receptor 1 (SIGMAR1), were discovered to be largely associated with aspects of apoptotic cell elimination and accompanying neuroinflammation, such as inflammatory cytokine release, phenotype transformation, and suppressing of antigen presentation. Exacerbated stroke outcomes are brought on by defective efferocytosis and improper modulation of pertinent signalling pathways in blood-borne macrophages and brain microglia, which also results in subsequent tissue inflammatory damage. This review focuses on recent researches which contain a number of recently discovered mechanisms, such as studies on the relationship between benign efferocytosis and the regulation of inflammation in ischaemic stroke, the roles of some risk factors in disease progression, and current immune approaches that aim to promote efferocytosis to treat some autoimmune diseases. Understanding these pathways provides insight into novel pathophysiological processes and fresh characteristics, which can be used to build cerebral ischaemia targeting techniques.

The role of efferocytosis-fueled macrophage metabolism in the resolution of inflammation

The phagocytosis of dying cells by macrophages, termed efferocytosis, is a tightly regulated process that involves the sensing, binding, engulfment, and digestion of apoptotic cells. Efferocytosis not only prevents tissue necrosis and inflammation caused by secondary necrosis of dying cells, but it also promotes pro-resolving signaling in macrophages, which is essential for tissue resolution and repair following injury or inflammation. An important factor that contributes to this pro-resolving reprogramming is the cargo that is released from apoptotic cells after their engulfment and phagolysosomal digestion by macrophages. The apoptotic cell cargo contains amino acids, nucleotides, fatty acids, and cholesterol that function as metabolites and signaling molecules to bring about this re-programming. Here, we review efferocytosis-induced changes in macrophage metabolism that mediate the pro-resolving functions of macrophages. We also discuss various strategies, challenges, and future perspectives related to drugging efferocytosis-fueled macrophage metabolism as strategy to dampen inflammation and promote resolution in chronic inflammatory diseases.

Cardiac macrophages and emerging roles for their metabolism after myocardial infarction

Interest in cardioimmunology has reached new heights as the experimental cardiology field works to tap the unrealized potential of immunotherapy for clinical care. Within this space is the cardiac macrophage, a key modulator of cardiac function in health and disease. After a myocardial infarction, myeloid macrophages both protect and harm the heart. To varying degrees, such outcomes are a function of myeloid ontogeny and heterogeneity, as well as functional cellular plasticity. Diversity is further shaped by the extracellular milieu, which fluctuates considerably after coronary occlusion. Ischemic limitation of nutrients constrains the metabolic potential of immune cells, and accumulating evidence supports a paradigm whereby macrophage metabolism is coupled to divergent inflammatory consequences, although experimental evidence for this in the heart is just emerging. Herein we examine the heterogeneous cardiac macrophage response following ischemic injury, with a focus on integrating putative contributions of immunometabolism and implications for therapeutically relevant cardiac injury versus cardiac repair.

The regulation of efferocytosis signaling pathways and adipose tissue homeostasis in physiological conditions and obesity: Current understanding and treatment options

Obesity is associated with changes in the resolution of acute inflammation that contribute to the clinical complications. The exact mechanisms underlying unresolved inflammation in obesity are not fully understood. Adipocyte death leads to pro-inflammatory adipose tissue macrophages, stimulating additional adipocyte apoptosis. Thus, a complex and tightly regulated process to inhibit inflammation and maintain homeostasis after adipocyte apoptosis is needed to maintain health. In normal condition, a specialized phagocytic process (efferocytosis) performs this function, clearing necrotic and apoptotic cells (ACs) and controlling inflammation. For efficient and continued efferocytosis, phagocytes must internalize multiple ACs in physiological conditions and handle the excess metabolic burden in adipose tissue. In obesity, this control is lost and can be an important hallmark of the disease. In this regard, the deficiency of efferocytosis leads to delayed resolution of acute inflammation and can result in ongoing inflammation, immune system dysfunction, and insulin resistance in obesity. Hence, efficient clearance of ACs by M2 macrophages could limit long-term inflammation and ensue clinical complications, such as cardiovascular disease and diabetes. This review elaborates upon the molecular mechanisms to identify efferocytosis regulators in obesity, and the mechanisms that can improve efferocytosis and reduce obesity-related complications, such as the use of pharmacological agents and regular exercise.

Defective efferocytosis of vascular cells in heart disease

The efficient phagocytic clearance of dying cells and apoptotic cells is one of the processes that is essential for the maintenance of physiologic tissue function and homeostasis, which is termed "efferocytosis." Under normal conditions, "find me" and "eat me" signals are released by apoptotic cells to stimulate the engulfment and efferocytosis of apoptotic cells. In contrast, abnormal efferocytosis is related to chronic and non-resolving inflammatory diseases such as atherosclerosis. In the initial steps of atherosclerotic lesion development, monocyte-derived macrophages display efficient efferocytosis that restricts plaque progression; however, this capacity is reduced in more advanced lesions. Macrophage reprogramming as a result of the accumulation of apoptotic cells and augmented inflammation accounts for this diminishment of efferocytosis. Furthermore, defective efferocytosis plays an important role in necrotic core formation, which triggers plaque rupture and acute thrombotic cardiovascular events. Recent publications have focused on the essential role of macrophage efferocytosis in cardiac pathophysiology and have pointed toward new therapeutic strategies to modulate macrophage efferocytosis for cardiac tissue repair. In this review, we discuss the molecular and cellular mechanisms that regulate efferocytosis in vascular cells, including macrophages and other phagocytic cells and detail how efferocytosis-related molecules contribute to the maintenance of vascular hemostasis and how defective efferocytosis leads to the formation and progression of atherosclerotic plaques.

Drugging the efferocytosis process: concepts and opportunities

The daily removal of billions of apoptotic cells in the human body via the process of efferocytosis is essential for homeostasis. To allow for this continuous efferocytosis, rapid phenotypic changes occur in the phagocytes enabling them to engulf and digest the apoptotic cargo. In addition, efferocytosis is actively anti-inflammatory and promotes resolution. Owing to its ubiquitous nature and the sheer volume of cell turnover, efferocytosis is a point of vulnerability. Aberrations in efferocytosis are associated with numerous inflammatory pathologies, including atherosclerosis, cancer and infections. The recent exciting discoveries defining the molecular machinery involved in efferocytosis have opened many avenues for therapeutic intervention, with several agents now in clinical trials.

Crosstalk Between Macrophages and Vascular Smooth Muscle Cells in Atherosclerotic Plaque Stability

Most acute cardiovascular events are due to plaque rupture, with atheromas containing large necrotic cores and thin fibrous caps being more susceptible to rupture and lesions with small necrotic cores and thick fibrous caps being more protected from rupture. Atherosclerotic plaques are comprised various extracellular matrix proteins, modified lipoprotein particles, and cells of different origins, that is, vascular cells and leukocytes. Although much has been revealed about the mechanisms that lead to plaque instability, several key areas remain incompletely understood. This In-Focus Review highlights processes related to cellular crosstalk and the role of the tissue microenvironment in determining cell function and plaque stability. Recent advances highlight critical underpinnings of atherosclerotic plaque vulnerability, particularly impairments in the ability of macrophages to clear dead cells and phenotypic switching of vascular smooth muscle cells. However, these processes do not occur in isolation, as crosstalk between macrophages and vascular smooth muscle cells and interactions with their surrounding microenvironment play a significant role in determining plaque stability. Understanding these aspects of cellular crosstalk within an atherosclerotic plaque may shed light on how to modify cell behavior and identify novel approaches to transform rupture-prone atheromas into stable lesions.

Macrophages, Metabolism and Heterophagy in the Heart

The heart is a never-stopping engine that relies on a formidable pool of mitochondria to generate energy and propel pumping. Because dying cardiomyocytes cannot be replaced, this high metabolic rate creates the challenge of preserving organelle fitness and cell function for life. Here, we provide an immunologist's perspective on how the heart solves this challenge, which is in part by incorporating macrophages as an integral component of the myocardium. Cardiac macrophages surround cardiomyocytes and capture dysfunctional mitochondria that these cells eject to the milieu, effectively establishing a client cell-support cell interaction. We refer to this heterologous partnership as heterophagy. Notably, this process shares analogies with other biological systems, is essential for proteostasis and metabolic fitness of cardiomyocytes, and unveils a remarkable degree of dependence of the healthy heart on immune cells for everyday function.

Good Cholesterol Gone Bad? HDL and COVID-19

The transmissible respiratory disease COVID-19, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has affected millions of people worldwide since its first reported outbreak in December of 2019 in Wuhan, China. Since then, multiple studies have shown an inverse correlation between the levels of high-density lipoprotein (HDL) particles and the severity of COVID-19, with low HDL levels being associated with an increased risk of severe outcomes. Some studies revealed that HDL binds to SARS-CoV-2 particles via the virus's spike protein and, under certain conditions, such as low HDL particle concentrations, it facilitates SARS-CoV-2 binding to angiotensin-converting enzyme 2 (ACE2) and infection of host cells. Other studies, however, reported that HDL suppressed SARS-CoV-2 infection. In both cases, the ability of HDL to enhance or suppress virus infection appears to be dependent on the expression of the HDL receptor, namely, the Scavenger Receptor Class B type 1 (SR-B1), in the target cells. SR-B1 and HDL represent crucial mediators of cholesterol metabolism. Herein, we review the complex role of HDL and SR-B1 in SARS-CoV-2-induced disease. We also review recent advances in our understanding of HDL structure, properties, and function during SARS-CoV-2 infection and the resulting COVID-19 disease.

Phagocytic function of tumor-associated macrophages as a key determinant of tumor progression control: a review

Tumor-associated macrophage (TAM) phagocytic activity is emerging as a new mechanism to harness for cancer treatment. Currently, many approaches are investigated at the preclinical level and some modalities have now reached clinical trials, including the targeting of the phagocytosis inhibitor CD47. The rationale for increasing TAM phagocytic activity is to improve innate anticancer immunity, and to promote T-cell mediated adaptive immune responses. In this context, a clear understanding of the impact of TAM phagocytosis on both innate and adaptive immunity is critical. Indeed, uncertainties persist regarding the capacity of TAM to present tumor antigens to CD8 T cells by cross-presentation. This process is critical for an optimal cytotoxic T-cell immune response and can be mediated by dendritic cells but also potentially by macrophages. In addition, the engulfment of cancer cells affects TAM functionality, as apoptotic cell uptake (a process termed efferocytosis) promotes macrophage anti-inflammatory functions. Because of the abundance of TAM in most solid tumors and the common use of apoptosis inducers such as radiotherapy to treat patients with cancer, efferocytosis potentially affects the overall immune balance within the tumor microenvironment (TME). In this review, we will discuss how cancer cell phagocytosis by TAM impacts antitumor immunity. First, we will focus on the potential of the phagocytic activity of TAM per se to control tumor progression. Second, we will examine the potential of TAM to act as antigen presenting cells for tumor specific CD8 T cells, considering the different characteristics of this process in the tumor tissue and at the molecular level. Finally, we will see how phagocytosis and efferocytosis affect TAM functionality and how these mechanisms impact on antitumor immunity. A better understanding of these aspects will enable us to better predict and interpret the consequences of cancer therapies on the immune status of the TME. Future cancer treatment regimens can thereby be designed to not only impact directly on cancer cells, but also to favorably modulate TAM phagocytic activity to benefit from the potential of this central immune player to achieve more potent therapeutic efficacy.

Unconjugated p-cresol activates macrophage macropinocytosis leading to increased LDL uptake

Patients with chronic kidney disease (CKD) and end-stage renal disease suffer from increased cardiovascular events and cardiac mortality. Prior studies have demonstrated that a portion of this enhanced risk can be attributed to the accumulation of microbiota-derived toxic metabolites, with most studies focusing on the sulfonated form of p-cresol (PCS). However, unconjugated p-cresol (uPC) itself was never assessed due to rapid and extensive first-pass metabolism that results in negligible serum concentrations of uPC. These reports thus failed to consider the host exposure to uPC prior to hepatic metabolism. In the current study, not only did we measure the effect of altering the intestinal microbiota on lipid accumulation in coronary arteries, but we also examined macrophage lipid uptake and handling pathways in response to uPC. We found that atherosclerosis-prone mice fed a high-fat diet exhibited significantly higher coronary artery lipid deposits upon receiving fecal material from CKD mice. Furthermore, treatment with uPC increased total cholesterol, triglycerides, and hepatic and aortic fatty deposits in non-CKD mice. Studies employing an in vitro macrophage model demonstrated that uPC exposure increased apoptosis whereas PCS did not. Additionally, uPC exhibited higher potency than PCS to stimulate LDL uptake and only uPC induced endocytosis- and pinocytosis-related genes. Pharmacological inhibition of varying cholesterol influx and efflux systems indicated that uPC increased macrophage LDL uptake by activating macropinocytosis. Overall, these findings indicate that uPC itself had a distinct effect on macrophage biology that might have contributed to increased cardiovascular risk in patients with CKD.

Metabolic Consequences of Efferocytosis and its Impact on Atherosclerosis

Billions of cells undergo apoptosis daily and are swiftly removed by macrophages through an evolutionarily conserved program termed "efferocytosis". Consequently, macromolecules within an apoptotic cell significantly burden a phagocyte with nutrients, such as lipids, oligonucleotides, and amino acids. In response to this nutrient overload, metabolic reprogramming must occur for the process of efferocytosis to remain non-phlogistic and to execute successive rounds of efferocytosis. The inability to undergo metabolic reprogramming after efferocytosis drives inflammation and impairs its resolution, often promoting many chronic inflammatory diseases. This is particularly evident for atherosclerosis, as metabolic reprogramming alters macrophage function in every stage of atherosclerosis, from the early formation of benign lesions to the progression of clinically relevant atheromas and during atherosclerosis regression upon aggressive lipid-lowering. This Review focuses on the metabolic pathways utilized upon apoptotic cell ingestion, the consequences of these metabolic pathways in macrophage function thereafter, and the role of metabolic reprogramming during atherosclerosis. Due to the growing interest in this new field, I introduce a new term, "efferotabolism", as a means to define the process by which macrophages break down, metabolize, and respond to AC-derived macromolecules. Understanding these aspects of efferotabolism will shed light on novel strategies to combat atherosclerosis and compromised inflammation resolution.

Intracellular and Intercellular Aspects of Macrophage Immunometabolism in Atherosclerosis

Macrophage immunometabolism, the changes in intracellular metabolic pathways that alter the function of these highly plastic cells, has been the subject of intense interest in the past few years, in part because macrophage immunometabolism plays important roles in atherosclerosis and other inflammatory diseases. In this review article, part of the Compendium on Atherosclerosis, we introduce the concepts of (1) intracellular immunometabolism-the canonical pathways of intrinsic cell activation leading to changes in intracellular metabolism, which in turn alter cellular function; and (2) intercellular immunometabolism-conditions in which intermediates of cellular metabolism are transferred from one cell to another, thereby altering the function of the recipient cell. The recent discovery that the metabolite cargo of dead and dying cells ingested through efferocytosis by macrophages can alter metabolic pathways and downstream function of the efferocyte is markedly changing the way we think about macrophage immunometabolism. Metabolic transitions of macrophages contribute to their functions in all stages of atherosclerosis, from lesion initiation to formation of advanced lesions characterized by necrotic cores, to lesion regression following aggressive lipid lowering. This review article discusses recent advances in our understanding of these different aspects of macrophage immunometabolism in atherosclerosis. With the increasing understanding of the roles of macrophage immunometabolism in atherosclerosis, new exciting concepts and potential targets for intervention are emerging.

Efferocytosis in health and disease

The clearance of apoptotic cells by professional and non-professional phagocytes - a process termed 'efferocytosis' - is essential for the maintenance of tissue homeostasis. Accordingly, defective efferocytosis underlies a growing list of chronic inflammatory diseases. Although much has been learnt about the mechanisms of apoptotic cell recognition and uptake, several key areas remain incompletely understood. This Review focuses on new discoveries related to how phagocytes process the metabolic cargo they receive during apoptotic cell uptake; the links between efferocytosis and the resolution of inflammation in health and disease; and the roles of efferocytosis in host defence. Understanding these aspects of efferocytosis sheds light on key physiological and pathophysiological processes and suggests novel therapeutic strategies for diseases driven by defective efferocytosis and impaired inflammation resolution.

Effect of soluble cleavage products of important receptors/ligands on efferocytosis: Their role in inflammatory, autoimmune and cardiovascular disease

Efferocytosis, the clearance of apoptotic cells (ACs), is a physiologic, multifaceted and dynamic process and a fundamental mechanism for the preservation of tissue homeostasis by avoiding unwanted inflammation and autoimmune responses through special phagocytic receptors. Defective efferocytosis is associated with several disease states, including cardiovascular disease and impaired immune surveillance, as occurs in cancer and autoimmune disease. A major cause of defective efferocytosis is non-functionality of surface receptors on either the phagocytic cells or the ACs, such as TAM family tyrosine kinase, which turns to a soluble form by cleavage/shedding or alternative splicing. Recently, soluble forms have featured prominently as potential biomarkers, indicative of prognosis and enabling targeted therapy using several commonly employed drugs and inhibitors, such as bleomycin, dexamethasone, statins and some matrix metalloproteinase inhibitors such as TAPI-1 and BB3103. Importantly, to design drug carriers with enhanced circulatory durability, the adaptation of soluble forms of physiological receptors/ligands has been purported. Research has shown that soluble forms are more effective than antibody forms in enabling targeted treatment of certain conditions, such as autoimmune diseases. In this review, we sought to summarize the current knowledge of these soluble products, how they are generated, their interactions, roles, and their potential use as biomarkers in prognosis and treatment related to inflammatory, cardiovascular, and autoimmune diseases.

Sonodynamic therapy-induced foam cells apoptosis activates the phagocytic PPARγ-LXRα-ABCA1/ABCG1 pathway and promotes cholesterol efflux in advanced plaque

In advanced atherosclerotic plaques, defective efferocytosis of apoptotic foam cells and decreased cholesterol efflux contribute to lesion progression. In our previous study, we demonstrated that 5-aminolevulinic acid (ALA)-mediated sonodynamic therapy (SDT) could induce foam cells apoptosis via the mitochondrial-caspase pathway. In the current research, we sought to explore ALA-SDT-induced apoptosis of phagocytes and the effects of cholesterol efflux and efferocytosis in advanced apoE^-/- mice plaque.

Methods: apoE^-/- mice fed western diet were treated with ALA-SDT and sacrificed at day 1, day 3, day 7 and day 28 post treatment. THP-1 macrophage-derived foam cells were treated with ALA-SDT. 5 hours later, the supernatant was collected and added to fresh foam cells (phagocytes). Then, the lipid area, efferocytosis, cholesterol efflux, anti-inflammatory reactions and PPARγ-LXRα-ABCA1/ABCG1 pathway were detected in plaque in vivo and in phagocytes in vitro.

Results: We found that ALA-SDT induced foam cells apoptosis coupled with efferocytosis and upregulation of Mer tyrosine kinase (MerTK) both in vivo and in vitro. The lipid content in plaque decreased as early as 1 day after ALA-SDT and this tendency persisted until 28 days. The enhancement of phagocytes cholesterol efflux was accompanied by an approximately 40% decrease in free cholesterol and a 24% decrease in total cholesterol in vitro. More importantly, anti-inflammatory factors such as TGFβ and IL-10 were upregulated by ALA-SDT treatment. Finally, we found that PPARγ-LXRα-ABCA1/ABCG1 pathway was activated both in vivo and in vitro by ALA-SDT, which could be blocked by PPARγ siRNA.

Conclusions: Activation of PPARγ-LXRα-ABCA1/ABCG1 pathway induced by ALA-SDT treatment engages a virtuous cycle that enhances efferocytosis, cholesterol efflux and anti-inflammatory reactions in advanced plaque in vivo and in phagocytes in vitro.

Effects of garlic on brachial endothelial function and capacity of plasma to mediate cholesterol efflux in patients with coronary artery disease

Objective:

This study investigated the effects of garlic on brachial endothelial function and THP-1 macrophage cholesterol efflux (CE) and examined whether garlic modulates ATP-binding cassette (ABC) A1 and ABCG1 mRNA expressions in peripheral blood mononuclear cells (PBMCs) isolated from patients with coronary artery disease (CAD).

Methods:

In this randomized, placebo-controlled trial, patients with CAD were randomly divided into two groups: those receiving garlic powder or placebo tablets twice daily for 3 months. Brachial flow-mediated dilation (FMD) was assessed using ultrasound. Fasting blood samples were collected before and after period and PBMC and plasma were isolated. Human THP-1 monocytes were differentiated into macrophages, labeled with 3H-cholesterol, and incubated with plasma samples, and CE was assessed. ABCA1 and ABCG1 mRNA expressions were determined in PBMCs.

Results:

After 3 months, brachial FMD values significantly improved (50.7%) in the garlic group compared with those in the placebo group (p=0.016). High-sensitive C-reactive protein (hs-CRP) levels significantly decreased in the garlic group, but the difference between the two groups was not statistically significant. No significant difference was observed with regard to CE and ABCA1 and ABCG1 mRNA expressions in PBMCs. CE was negatively correlated with hs-CRP levels.

Conclusion:

Short-term treatment with garlic may improve the endothelial function and may affect hs-CRP levels; however, it could neither significantly improve THP-1 macrophage CE nor affect ABCA1 or ABCG1 expressions in PBMCs.

The Role of Efferocytosis in Atherosclerosis

The necrotic core has long been a hallmark of the vulnerable atherosclerotic plaque. Although apoptotic cells are cleared quickly in almost all other tissue beds, their removal appears to be significantly impaired in the diseased blood vessel. Emerging evidence indicates that this phenomenon is caused by a defect in efferocytosis, the process by which apoptotic tissue is recognized for engulfment by phagocytic cells such as macrophages. Genetic and experimental data suggest that efferocytosis is impaired during atherogenesis caused by dysregulation of so-called eat me ligands, which govern the edibility of cells undergoing programmed cell death. The following is a summary of recent data indicating that efferocytosis is a major unappreciated driver of lesion expansion but also a reversible defect that can potentially be targeted as a means to prevent plaque progression.

The many ways tissue phagocytes respond to dying cells

Apoptosis is an important component of normal tissue physiology, and the prompt removal of apoptotic cells is equally essential to avoid the undesirable consequences of their accumulation and disintegration. Professional phagocytes are highly specialized for engulfing apoptotic cells. The recent ability to track cells that have undergone apoptosis in situ has revealed a division of labor among the tissue resident phagocytes that sample them. Macrophages are uniquely programmed to process internalized apoptotic cell-derived fatty acids, cholesterol and nucleotides, as a reflection of their dominant role in clearing the bulk of apoptotic cells. Dendritic cells carry apoptotic cells to lymph nodes where they signal the emergence and expansion of highly suppressive regulatory CD4 T cells. A broad suppression of inflammation is executed through distinct phagocyte-specific mechanisms. A clever induction of negative regulatory nodes is notable in dendritic cells serving to simultaneously shut down multiple pathways of inflammation. Several of the genes and pathways modulated in phagocytes in response to apoptotic cells have been linked to chronic inflammatory and autoimmune diseases such as atherosclerosis, inflammatory bowel disease and systemic lupus erythematosus. Our collective understanding of old and new phagocyte functions after apoptotic cell phagocytosis demonstrates the enormity of ways to mediate immune suppression and enforce tissue homeostasis.

Eating the Dead to Keep Atherosclerosis at Bay

Atherosclerosis is the primary cause of coronary heart disease (CHD), ischemic stroke, and peripheral arterial disease. Despite effective lipid-lowering therapies and prevention programs, atherosclerosis is still the leading cause of mortality in the United States. Moreover, the prevalence of CHD in developing countries worldwide is rapidly increasing at a rate expected to overtake those of cancer and diabetes. Prominent risk factors include the hardening of arteries and high levels of cholesterol, which lead to the initiation and progression of atherosclerosis. However, cell death and efferocytosis are critical components of both atherosclerotic plaque progression and regression, yet, few currently available therapies focus on these processes. Thus, understanding the causes of cell death within the atherosclerotic plaque, the consequences of cell death, and the mechanisms of apoptotic cell clearance may enable the development of new therapies to treat cardiovascular disease. Here, we review how endoplasmic reticulum stress and cholesterol metabolism lead to cell death and inflammation, how dying cells affect plaque progression, and how autophagy and the clearance of dead cells ameliorates the inflammatory environment of the plaque. In addition, we review current research aimed at alleviating these processes and specifically targeting therapeutics to the site of the plaque.

2016 Russell Ross Memorial Lecture in Vascular Biology: Molecular-Cellular Mechanisms in the Progression of Atherosclerosis

Atherosclerosis is initiated by the subendothelial accumulation of apoB-lipoproteins, which initiates a sterile inflammatory response dominated by monocyte-macrophages but including all classes of innate and adaptive immune cells. These inflammatory cells, together with proliferating smooth muscle cells and extracellular matrix, promote the formation of subendothelial lesions or plaques. In the vast majority of cases, these lesions do not cause serious clinical symptoms, which is due in part to a resolution-repair response that limits tissue damage. However, a deadly minority of lesions progress to the point where they can trigger acute lumenal thrombosis, which may then cause unstable angina, myocardial infarction, sudden cardiac death, or stroke. Many of these clinically dangerous lesions have hallmarks of defective inflammation resolution, including defective clearance of dead cells (efferocytosis), necrosis, a defective scar response, and decreased levels of lipid mediators of the resolution response. Efferocytosis is both an effector arm of the resolution response and an inducer of resolution mediators, and thus its defect in advanced atherosclerosis amplifies plaque progression. Preclinical causation/treatment studies have demonstrated that replacement therapy with exogenously administered resolving mediators can improve lesional efferocytosis and prevent plaque progression. Work in this area has the potential to potentiate the cardiovascular benefits of apoB-lipoprotein-lowering therapy.

Active membrane cholesterol as a physiological effector

Sterols associate preferentially with plasma membrane sphingolipids and saturated phospholipids to form stoichiometric complexes. Cholesterol in molar excess of the capacity of these polar bilayer lipids has a high accessibility and fugacity; we call this fraction active cholesterol. This review first considers how active cholesterol serves as an upstream regulator of cellular sterol homeostasis. The mechanism appears to utilize the redistribution of active cholesterol down its diffusional gradient to the endoplasmic reticulum and mitochondria, where it binds multiple effectors and directs their feedback activity. We have also reviewed a broad literature in search of a role for active cholesterol (as opposed to bulk cholesterol or lipid domains such as rafts) in the activity of diverse membrane proteins. Several systems provide such evidence, implicating, in particular, caveolin-1, various kinds of ABC-type cholesterol transporters, solute transporters, receptors and ion channels. We suggest that this larger role for active cholesterol warrants close attention and can be tested easily.

Extensive macrophage accumulation in young and old Niemann-Pick C1 model mice involves the alternative, M2, activation pathway and inhibition of macrophage apoptosis

We have studied the pathophysiology of lung disease which occurs in two mouse models of Niemann-Pick C1 disease. We utilized Npc1(-/-) mice transgenic for normal gene expression in glia or neurons and glia at ages several fold the usual and a mouse model of the juvenile form of NPC1, a point mutation, at one age to confirm some findings. Lung weights, as per cent of body weight, increase much more than liver and spleen weights. Although pulmonary function parameters only vary for hysteresis between young and older Npc1(-/-) mice, they are markedly different than those found in normal control mice. Cholesterol accumulation continued in the older mice but sphingosine-1-phosphate was not increased. Bronchoalveolar lavage (BAL) showed a massive increase (26×) in the number of macrophages. Histologic examination from the older, transgenic Npc1(-/-) mice showed small foci of alveolar proteinosis and evidence of hemorrhage, as well as dense macrophage accumulation. A large subset of macrophages was immunopositive for Fizz1 or arginase-1, markers of the alternative activation pathway, while no Fizz1 or arginase-1 positive macrophages were found in wild-type mice. The percentage of marker positive macrophages was relatively stable at 5-10% at various ages and within the 2 transgenic models. Phosphohistone H3 and Ki67 showed low levels of proliferation of these macrophages. Apoptosis was prominent within lung capillary endothelial cells, but limited within macrophages. Thus, activation of the alternative pathway is involved in Niemann-Pick C1 associated pulmonary macrophage accumulation, with low proliferation of these cells balanced by low levels of apoptosis.

High-density lipoprotein cholesterol: origins and the path ahead

PURPOSE OF REVIEW

We provide a historical perspective of how high-density lipoprotein (HDL) cholesterol became a clinical standard, the evidence in favor of HDL function as a more appropriate indication of HDL's antiatherogenic nature, and the options ahead.

RECENT FINDINGS

Recent studies have demonstrated a strong relationship between the cholesterol efflux capacity of plasma and prevalent cardiovascular disease (CVD) and CVD event risk, indicating the utility of HDL function as a diagnostic/prognostic of CVD.

SUMMARY

We will present how HDL cholesterol came to be the standard proxy of HDL function, the key observations that drew its clinical relevance into question, and the pros and cons of commercially available approaches to measuring HDL function.

Macrophage defense mechanisms against intracellular bacteria

Macrophages and neutrophils play a decisive role in host responses to intracellular bacteria including the agent of tuberculosis (TB), Mycobacterium tuberculosis as they represent the forefront of innate immune defense against bacterial invaders. At the same time, these phagocytes are also primary targets of intracellular bacteria to be abused as host cells. Their efficacy to contain and eliminate intracellular M. tuberculosis decides whether a patient initially becomes infected or not. However, when the infection becomes chronic or even latent (as in the case of TB) despite development of specific immune activation, phagocytes have also important effector functions. Macrophages have evolved a myriad of defense strategies to combat infection with intracellular bacteria such as M. tuberculosis. These include induction of toxic anti-microbial effectors such as nitric oxide and reactive oxygen intermediates, the stimulation of microbe intoxication mechanisms via acidification or metal accumulation in the phagolysosome, the restriction of the microbe's access to essential nutrients such as iron, fatty acids, or amino acids, the production of anti-microbial peptides and cytokines, along with induction of autophagy and efferocytosis to eliminate the pathogen. On the other hand, M. tuberculosis, as a prime example of a well-adapted facultative intracellular bacterium, has learned during evolution to counter-balance the host's immune defense strategies to secure survival or multiplication within this otherwise hostile environment. This review provides an overview of innate immune defense of macrophages directed against intracellular bacteria with a focus on M. tuberculosis. Gaining more insights and knowledge into this complex network of host-pathogen interaction will identify novel target sites of intervention to successfully clear infection at a time of rapidly emerging multi-resistance of M. tuberculosis against conventional antibiotics.

Oxysterols as non-genomic regulators of cholesterol homeostasis

Tight regulation of cellular and plasma cholesterol is crucial to proper cellular functioning because excess free cholesterol is toxic to cells and is associated with atherosclerosis and heart disease. Cellular cholesterol homeostasis is regulated by enzymatically formed oxygenated cholesterol derivatives termed oxysterols. Although the effects of oxysterols on transcriptional pathways are well described, the non-transcriptional mechanisms through which oxysterols acutely modulate cellular cholesterol levels are less well understood. We present emerging evidence suggesting that the membrane biophysical properties of oxysterols underlie their acute cholesterol-regulatory functions and discuss the relevance of these acute effects to cholesterol overload in physiological and pathophysiological states.

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.

Homozygosity for an allele encoding deacetylated FoxO1 protects macrophages from cholesterol-induced inflammation without increasing apoptosis

OBJECTIVE

Insulin resistance renders macrophages more prone to cholesterol-induced apoptosis by promoting nuclear localization of transcription factor forkhead box transcription factor (Fox) O1. However, FoxO1 also decreases macrophage inflammation, raising the question of how the balance between proapoptotic and antiinflammatory effects is determined. We sought to identify the mechanism whereby FoxO1 dampens inflammation without promoting apoptosis. We hypothesized that nutrient-dependent FoxO1 acetylation plays a role in this process.

METHODS AND RESULTS

We generated knock-in mice bearing alleles that encode constitutively deacetylated FoxO1 and studied the ex vivo response of primary peritoneal macrophages. We show that macrophages derived from mice homozygous for constitutively deacetylated FoxO1 alleles retain antiinflammatory properties in response to free cholesterol loading, without increasing apoptosis. Deacetylated FoxO1 inhibits free cholesterol-induced Akt phosphorylation and increases levels of the nuclear factor-κB precursor p105, decreasing nuclear translocation of nuclear factor-κB p65 subunit and dampening mitogen-activated protein/extracellular signal-regulated kinase activation to prevent inflammation.

CONCLUSIONS

Deacetylated FoxO1 regulates p105 to prevent macrophage inflammation without causing apoptosis, suggesting a potential novel therapeutic approach to atherosclerosis through FoxO1 deacetylation.

Plasma CD93 concentration is a potential novel biomarker for coronary artery disease

OBJECTIVES

A common nonsynonymous single nucleotide polymorphism (SNP) in the CD93 gene (rs3746731, Pro541Ser) has been associated with risk of coronary artery disease (CAD). CD93 is a transmembrane glycoprotein, which is detectable in soluble form in human plasma. We investigated whether the concentration of soluble CD93 in plasma is related to risk of myocardial infarction (MI) and CAD, using a case-control study of premature MI (n = 764) and a nested case-control analysis of a longitudinal cohort study of 60-year-old subjects (analysis comprising 844 of 4232 subjects enrolled at baseline). In addition, SNPs in the CD93 gene were studied in relation to plasma CD93 concentration and CD93 mRNA expression.

METHODS AND RESULTS

A sensitive and specific enzyme-linked immunosorbent assay was established for determination of the plasma CD93 concentration. Subjects were divided into three groups according to tertiles of the distribution of CD93 concentration. Lower odds ratios for risk of MI and incidence of CAD were observed in the middle CD93 tertile (142-173 μg L(-1) ): odds ratio (95% confidence interval), 0.69 (0.49-0.97) and 0.61 (0.40-0.94), respectively. These associations were independent of traditional CAD risk factors. The minor allele of a SNP in the 3' untranslated region of CD93 (rs2749812) was associated with increased plasma CD93 concentrations (P = 0.03) and increased CD93 mRNA expression levels (P = 0.02).

CONCLUSION

The results of the present study suggest that the concentration of soluble CD93 in plasma is a potential novel biomarker for CAD, including MI.

Interleukin-6 protects human macrophages from cellular cholesterol accumulation and attenuates the proinflammatory response

Cholesterol-laden monocyte-derived macrophages are phagocytic cells characteristic of early and advanced atherosclerotic lesions. Interleukin-6 (IL-6) is a macrophage secretory product that is abundantly expressed in atherosclerotic plaques but whose precise role in atherogenesis is unclear. The capacity of macrophages to clear apoptotic cells, through the efferocytosis mechanism, as well as to reduce cellular cholesterol accumulation contributes to prevent plaque progression and instability. By virtue of its capacity to promote cellular cholesterol efflux from phagocyte-macrophages, ABCA1 was reported to reduce atherosclerosis. We demonstrated that lipid loading in human macrophages was accompanied by a strong increase of IL-6 secretion. Interestingly, IL-6 markedly induced ABCA1 expression and enhanced ABCA1-mediated cholesterol efflux from human macrophages to apoAI. Stimulation of ABCA1-mediated cholesterol efflux by IL-6 was, however, abolished by selective inhibition of the Jak-2/Stat3 signaling pathway. In addition, we observed that the expression of molecules described to promote efferocytosis, i.e. c-mer proto-oncogene-tyrosine kinase, thrombospondin-1, and transglutaminase 2, was significantly induced in human macrophages upon treatment with IL-6. Consistent with these findings, IL-6 enhanced the capacity of human macrophages to phagocytose apoptotic cells; moreover, we observed that IL-6 stimulates the ABCA1-mediated efflux of cholesterol derived from the ingestion of free cholesterol-loaded apoptotic macrophages. Finally, the treatment of human macrophages with IL-6 led to the establishment of an anti-inflammatory cytokine profile, characterized by an increased secretion of IL-4 and IL-10 together with a decrease of that of IL-1β. Taken together, our results indicate that IL-6 favors the elimination of excess cholesterol in human macrophages and phagocytes by stimulation of ABCA1-mediated cellular free cholesterol efflux and attenuates the macrophage proinflammatory phenotype. Thus, high amounts of IL-6 secreted by lipid laden human macrophages may constitute a protective response from macrophages to prevent accumulation of cytotoxic-free cholesterol. Such a cellular recycling of free cholesterol may contribute to reduce both foam cell formation and the accumulation of apoptotic bodies as well as intraplaque inflammation in atherosclerotic lesions.

Metabolic and functional relevance of HDL subspecies

PURPOSE OF REVIEW

Our purpose is to review recent findings highlighting the metabolic and functional diversity of HDL subspecies.

RECENT FINDINGS

HDL heterogeneity - both structural and functional - is the main focus of this review. Recent work indicates that the metabolism and functionality of HDL particles differ greatly among HDL subspecies. With the introduction of new and improved methodology (e.g., proteomics), new aspects of the structural complexity and functionality of HDL have been revealed. It has been confirmed that HDL functions - including, but not limited to decreasing inflammation, apoptosis, macrophage adhesion to the endothelium and insulin resistance - are due to HDL's ability to remove cholesterol from cells (reverse cholesterol transport). A new level of HDL complexity has recently been revealed by investigating the lipid composition of HDL with gas chromatography, gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry. There are about 100 different HDL-associated proteins; however, there are many more lipid species potentially associated with HDL particles.

SUMMARY

The most important recent findings disclose that HDL is more complex than previously thought. HDL subclasses differ in physical-chemical properties, protein and lipid composition, metabolism, physiological functions and pathophysiological significance. The staggering complexity of HDL demands significantly more investigation before we can truly begin to understand HDL metabolism and function in humans.

Cholesterol efflux capacity, high-density lipoprotein function, and atherosclerosis

BACKGROUND

High-density lipoprotein (HDL) may provide cardiovascular protection by promoting reverse cholesterol transport from macrophages. We hypothesized that the capacity of HDL to accept cholesterol from macrophages would serve as a predictor of atherosclerotic burden.

METHODS

We measured cholesterol efflux capacity in 203 healthy volunteers who underwent assessment of carotid artery intima-media thickness, 442 patients with angiographically confirmed coronary artery disease, and 351 patients without such angiographically confirmed disease. We quantified efflux capacity by using a validated ex vivo system that involved incubation of macrophages with apolipoprotein B-depleted serum from the study participants.

RESULTS

The levels of HDL cholesterol and apolipoprotein A-I were significant determinants of cholesterol efflux capacity but accounted for less than 40% of the observed variation. An inverse relationship was noted between efflux capacity and carotid intima-media thickness both before and after adjustment for the HDL cholesterol level. Furthermore, efflux capacity was a strong inverse predictor of coronary disease status (adjusted odds ratio for coronary disease per 1-SD increase in efflux capacity, 0.70; 95% confidence interval [CI], 0.59 to 0.83; P<0.001). This relationship was attenuated, but remained significant, after additional adjustment for the HDL cholesterol level (odds ratio per 1-SD increase, 0.75; 95% CI, 0.63 to 0.90; P=0.002) or apolipoprotein A-I level (odds ratio per 1-SD increase, 0.74; 95% CI, 0.61 to 0.89; P=0.002). Additional studies showed enhanced efflux capacity in patients with the metabolic syndrome and low HDL cholesterol levels who were treated with pioglitazone, but not in patients with hypercholesterolemia who were treated with statins.

CONCLUSIONS

Cholesterol efflux capacity from macrophages, a metric of HDL function, has a strong inverse association with both carotid intima-media thickness and the likelihood of angiographic coronary artery disease, independently of the HDL cholesterol level. (Funded by the National Heart, Lung, and Blood Institute and others.).

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.

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

Lipid signaling in the atherogenesis context

Normally macrophages localized in the arterial vessel wall perform the "reverse transfer" of cholesterol, which includes endocytosis of low density lipoproteins (LDL), cholesterol transfer to newly formed high density lipoprotein particles, and their following elimination by the liver. The homeostatic function of macrophages for cholesterol involves a system of lipid sensors. Oxysterol sensors LXRs, oxysterol and cholesterol sensors INSIG and SCAP acting through controlled transcription factors SREBP, as well as sensors for oxidized fatty acids and their derivatives, PPAR, are the best studied. Activation of LXR and PPAR is also accompanied by inhibition of macrophage functions related to inflammation. Accumulation of oxidized and otherwise modified LDL in the subendothelial space induced by endothelium injury, infection, or other pathogenic factors instead of stimulation of the homeostatic functions of macrophages leads to their weakening with a concurrent increase in the inflammatory potential of these cells. These shifts seem to drive the transformation of macrophages into foam cells, which form the core of sclerotic plaques. The intervention of another lipid sensor, TLR4, can trigger such a radical change in the functional activity of macrophages. The interaction of modified LDL with this signaling receptor results in inhibition of the homeostatic oxysterol signaling, induction of additional LDL transporters, and activation of the phagocytic function of macrophages. The re-establishment of cholesterol homeostasis under these circumstances can be achieved by administration of LXR and PPARgamma agonists. Therefore, it is urgent to design ligands with reduced side effects.

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.

ABCA1 and ABCG1 protect against oxidative stress-induced macrophage apoptosis during efferocytosis

RATIONALE

Antiatherogenic effects of plasma high-density lipoprotein (HDL) include the ability to inhibit apoptosis of macrophage foam cells. The ATP-binding cassette transporters ABCA1 and ABCG1 have a major role in promoting cholesterol efflux from macrophages to apolipoprotein A-1 and HDL and are upregulated during the phagocytosis of apoptotic cells (efferocytosis).

OBJECTIVE

The goal of this study was to determine the roles of ABCA1 and ABCG1 in preserving the viability of macrophages during efferocytosis.

METHODS AND RESULTS

We show that despite similar clearance of apoptotic cells, peritoneal macrophages from Abca1(-/-)Abcg1(-/-), Abcg1(-/-), and, to a lesser extent, Abca1(-/-) mice are much more prone to apoptosis during efferocytosis compared to wild-type cells. Similar findings were observed following incubations with oxidized phospholipids, and the ability of HDL to protect against oxidized phospholipid-induced apoptosis was markedly reduced in Abca1(-/-)Abcg1(-/-) and Abcg1(-/-) cells. These effects were independent of any role of ABCA1 and ABCG1 in mediating oxidized phospholipid efflux but were reversed by cyclodextrin-mediated cholesterol efflux. The apoptotic response observed in Abca1(-/-)Abcg1(-/-) macrophages after oxidized phospholipid exposure or engulfment of apoptotic cells was dependent on an excessive oxidative burst secondary to enhanced assembly of NADPH oxidase (NOX)2 complexes, leading to sustained Jnk activation which turned on the apoptotic cell death program. Increased NOX2 assembly required Toll-like receptors 2/4 and MyD88 signaling, which are known to be enhanced in transporter deficient cells in a lipid raft-dependent fashion.

CONCLUSIONS

We identified a new beneficial role of ABCA1, ABCG1 and HDL in dampening the oxidative burst and preserving viability of macrophages following exposure to oxidized phospholipids and/or apoptotic cells.

Macrophage death and defective inflammation resolution in atherosclerosis

A key event in atherosclerosis is a maladaptive inflammatory response to subendothelial lipoproteins. A crucial aspect of this response is a failure to resolve inflammation, which normally involves the suppression of inflammatory cell influx, effective clearance of apoptotic cells and promotion of inflammatory cell egress. Defects in these processes promote the progression of atherosclerotic lesions into dangerous plaques, which can trigger atherothrombotic vascular disease, the leading cause of death in industrialized societies. In this Review I provide an overview of these concepts, with a focus on macrophage death and defective apoptotic cell clearance, and discuss new therapeutic strategies designed to boost inflammation resolution in atherosclerosis.

Macrophage apoptosis in atherosclerosis: consequences on plaque progression and the role of endoplasmic reticulum stress

Atherothrombotic vascular diseases, such as myocardial infarction and stroke, are the leading causes of death in the industrialized world. The immediate cause of these diseases is acute occlusive thrombosis in medium-sized arteries feeding critical organs. Thrombosis is triggered by the rupture or erosion of a minority of atherosclerotic plaques that have advanced to a particular stage of "vulnerability." Vulnerable plaques are characterized by certain key features, such as inflammation, thinning of a protective collagenous cap, and a lipid-rich necrotic core consisting of macrophage debris. A number of cellular events contribute to vulnerable plaque formation, including secretion of pro-inflammatory, procoagulant, and proteolytic molecules by macrophages as well as the death of macrophages, intimal smooth muscles cells, and possibly endothelial cells. The necrotic core in particular is a key factor in plaque vulnerability, because macrophage debris promotes inflammation, plaque instability, and thrombosis. Plaque necrosis arises from a combination of lesional macrophage apoptosis and defective clearance of these dead cells, a process called efferocytosis. This review focuses on how macrophage apoptosis, in the setting of defective efferocytosis, contributes to necrotic core formation and how a process known to be prominent in advanced lesions--activation of ER stress signal-transduction pathways--contributes to macrophage apoptosis in these plaques.

Apoptotic body engulfment by hepatic stellate cells promotes their survival by the JAK/STAT and Akt/NF-kappaB-dependent pathways

BACKGROUND/AIMS

We have previously shown that phagocytosis of apoptotic bodies (AB) by hepatic stellate cells (HSC) is profibrogenic. As HSC survival is central to the progression of liver fibrosis, our goal was to investigate if phagocytosis induces HSC survival.

METHODS

Apoptosis of phagocytosing HSC was studied in the presence of known apoptotic agents. The JAK/STAT- and PI3K/Akt-dependent pathways, NF-kappaB activation and expression of the anti-apoptotic proteins Mcl-1 and A1 were evaluated. Apoptosis was assessed after blocking A1 by an siRNA approach.

RESULTS

Phagocytosing HSC were resistant to FasL/cycloheximide or TRAIL-induced apoptosis. Inhibition of the JAK/STAT or PI3K-mediated pathways induced apoptosis of HSC. Phagocytosis induced JAK1/STAT3 phosphorylation, and this was prevented by inhibiting JAK. Translocation of STAT3 to the nucleus was also blocked by JAK inhibition. Mcl-1 expression was upregulated in a JAK-dependent manner. PI3K-dependent phosphorylation of Akt depended on NADPH oxidase activity and superoxide production. NF-kappaB activation and subsequent upregulation of A1 was observed, and A1 inhibition induced apoptosis of HSC.

CONCLUSION

Phagocytosis of AB promotes HSC survival by two pathways, of which the A1 dependent is more significant. This represents a new mechanism by which engulfment of AB contributes to the propagation of liver fibrosis.

Mechanisms and consequences of efferocytosis in advanced atherosclerosis

Throughout atherosclerotic lesion development, intimal macrophages undergo apoptosis, a form of death that usually prevents cellular necrosis. In advanced atherosclerotic lesions, however, these apoptotic macrophages become secondarily necrotic and coalesce over time into a key feature of vulnerable plaques, the necrotic core. This event is critically important, as necrotic core formation in these advanced atheromata is thought to promote plaque disruption and ultimately, acute atherothrombotic vascular disease. Increasing evidence suggests that the mechanism behind postapoptotic macrophage necrosis in advanced atherosclerosis is defective phagocytic clearance or "efferocytosis" of the apoptotic cells. Thus, understanding the cellular and molecular mechanisms of efferocytosis in atherosclerosis and why efferocytosis becomes defective in advanced lesions is an important goal. Molecular-genetic causation studies in mouse models of advanced atherosclerosis have provided evidence that several molecules known to be involved in efferocytosis, including TG2, MFG-E8, complement C1q, Mertk, lysoPC, and Fas, play important roles in the clearance of apoptotic cells in advanced plaques. These and future insights into the molecular mechanisms of defective efferocytosis in advanced atheromata may open the way for novel therapeutic strategies for atherothrombotic vascular disease, the leading cause of death in the industrialized world.

Diabetes mellitus and apoptosis: inflammatory cells

Since the early observation that similarities between thyroiditis and insulitis existed, the important role played by inflammation in the development of diabetes has been appreciated. More recently, experiments have shown that inflammation also plays a prominent role in the development of target organ damage arising as complications, with both elements of the innate and the adaptive immune system being involved, and that cytokines contributing to local tissue damage may arise from both infiltrating and resident cells. This review will discuss the experimental evidence that shows that inflammatory cell-mediated apoptosis contributes to target organ damage, from beta cell destruction to both micro- and macro-vascular disease complications, and also how alterations in leukocyte turnover affects immune function.

Forkhead transcription factors (FoxOs) promote apoptosis of insulin-resistant macrophages during cholesterol-induced endoplasmic reticulum stress

OBJECTIVE

Endoplasmic reticulum stress increases macrophage apoptosis, contributing to the complications of atherosclerosis. Insulin-resistant macrophages are more susceptible to endoplasmic reticulum stress-associated apoptosis probably contributing to macrophage death and necrotic core formation in atherosclerotic plaques in type 2 diabetes. However, the molecular mechanisms of increased apoptosis in insulin-resistant macrophages remain unclear.

RESEARCH DESIGN AND METHODS

The studies were performed in insulin-resistant macrophages isolated from insulin receptor knockout or ob/ob mice. Gain- or loss-of-function approaches were used to evaluate the roles of forkhead transcription factors (FoxOs) in endoplasmic reticulum stress-associated macrophage apoptosis.

RESULTS

Insulin-resistant macrophages showed attenuated Akt activation and increased nuclear localization of FoxO1 during endoplasmic reticulum stress induced by free cholesterol loading. Overexpression of active FoxO1 or FoxO3 failed to induce apoptosis in unchallenged macrophages but exacerbated apoptosis in macrophages with an active endoplasmic reticulum stress response. Conversely, macrophages with genetic knockouts of FoxO1, -3, and -4 were resistant to apoptosis in response to endoplasmic reticulum stress. FoxO1 was shown by chromatin immunoprecipitation and promoter expression analysis to induce inhibitor of kappaBepsilon gene expression and thereby to attenuate the increase of nuclear p65 and nuclear factor-kappaB activity during endoplasmic reticulum stress, with proapoptotic and anti-inflammatory consequences.

CONCLUSIONS

Decreased Akt and increased FoxO transcription factor activity during the endoplasmic reticulum stress response leads to increased apoptosis of insulin-resistant macrophages. FoxOs may have a dual cellular function, resulting in either proapoptotic or anti-inflammatory effects in an endoplasmic reticulum stress-modulated manner. In the complex plaque milieu, the ultimate effect is likely to be an increase in macrophage apoptosis, plaque inflammation, and destabilization.