Effects of High Fat Feeding and Diabetes on Regression of Atherosclerosis Induced by Low-Density Lipoprotein Receptor Gene Therapy in LDL Receptor-Deficient Mice

Trained immunity in monocyte/macrophage: Novel mechanism of phytochemicals in the treatment of atherosclerotic cardiovascular disease

Atherosclerosis (AS) is the pathology of atherosclerotic cardiovascular diseases (ASCVD), characterized by persistent chronic inflammation in the vessel wall, in which monocytes/macrophages play a key role. It has been reported that innate immune system cells can assume a persistent proinflammatory state after short stimulation with endogenous atherogenic stimuli. The pathogenesis of AS can be influenced by this persistent hyperactivation of the innate immune system, which is termed trained immunity. Trained immunity has also been implicated as a key pathological mechanism, leading to persistent chronic inflammation in AS. Trained immunity is mediated via epigenetic and metabolic reprogramming and occurs in mature innate immune cells and their bone marrow progenitors. Natural products are promising candidates for novel pharmacological agents that can be used to prevent or treat cardiovascular diseases (CVD). A variety of natural products and agents exhibiting antiatherosclerotic abilities have been reported to potentially interfere with the pharmacological targets of trained immunity. This review describes in as much detail as possible the mechanisms involved in trained immunity and how phytochemicals of this process inhibit AS by affecting trained monocytes/macrophages.

Loss of PRMT2 in myeloid cells in normoglycemic mice phenocopies impaired regression of atherosclerosis in diabetic mice

The regression, or resolution, of inflammation in atherosclerotic plaques is impaired in diabetes. However, the factors mediating this effect remain incomplete. We identified protein arginine methyltransferase 2 (PRMT2) as a protein whose expression in macrophages is reduced in hyperglycemia and diabetes. PRMT2 catalyzes arginine methylation to target proteins to modulate gene expression. Because PRMT2 expression is reduced in cells in hyperglycemia, we wanted to determine whether PRMT2 plays a causal role in the impairment of atherosclerosis regression in diabetes. We, therefore, examined the consequence of deleting PRMT2 in myeloid cells during the regression of atherosclerosis in normal and diabetic mice. Remarkably, we found significant impairment of atherosclerosis regression under normoglycemic conditions in mice lacking PRMT2 (Prmt2^-/-) in myeloid cells that mimic the decrease in regression of atherosclerosis in WT mice under diabetic conditions. This was associated with increased plaque macrophage retention, as well as increased apoptosis and necrosis. PRMT2-deficient plaque CD68+ cells under normoglycemic conditions showed increased expression of genes involved in cytokine signaling and inflammation compared to WT cells. Consistently, Prmt2^-/- bone marrow-derived macrophages (BMDMs) showed an increased response of proinflammatory genes to LPS and a decreased response of inflammation resolving genes to IL-4. This increased response to LPS in Prmt2^-/- BMDMs occurs via enhanced NF-kappa B activity. Thus, the loss of PRMT2 is causally linked to impaired atherosclerosis regression via a heightened inflammatory response in macrophages. That PRMT2 expression was lower in myeloid cells in plaques from human subjects with diabetes supports the relevance of our findings to human atherosclerosis.

Efferocytosis induces macrophage proliferation to help resolve tissue injury

Apoptotic cell clearance by macrophages (efferocytosis) promotes resolution signaling pathways, which can be triggered by molecules derived from the phagolysosomal degradation of apoptotic cells. We show here that nucleotides derived from the hydrolysis of apoptotic cell DNA by phagolysosomal DNase2a activate a DNA-PKcs-mTORC2/Rictor pathway that increases Myc to promote non-inflammatory macrophage proliferation. Efferocytosis-induced proliferation expands the pool of resolving macrophages in vitro and in mice, including zymosan-induced peritonitis, dexamethasone-induced thymocyte apoptosis, and atherosclerosis regression. In the dexamethasone-thymus model, hematopoietic Rictor deletion blocked efferocytosing macrophage proliferation, apoptotic cell clearance, and tissue resolution. In atherosclerosis regression, silencing macrophage Rictor or DNase2a blocked efferocyte proliferation, apoptotic cell clearance, and plaque stabilization. In view of previous work showing that other types of apoptotic cell cargo can promote resolution in individual efferocytosing macrophages, the findings here suggest that signaling-triggered apoptotic cell-derived nucleotides can amplify this benefit by increasing the number of these macrophages.

Cardiovascular disease in diabetes, beyond glucose

Despite the decades-old knowledge that diabetes mellitus is a major risk factor for cardiovascular disease, the reasons for this association are only partially understood. While this association is true for both type 1 and type 2 diabetes, different pathophysiological processes may be responsible. Lipids and other risk factors are indeed important, whereas the role of glucose is less clear. This lack of clarity stems from clinical trials that do not unambiguously show that intensive glycemic control reduces cardiovascular events. Animal models have provided mechanisms that link diabetes to increased atherosclerosis, and evidence consistent with the importance of factors beyond hyperglycemia has emerged. We review clinical, pathological, and animal studies exploring the pathogenesis of atherosclerosis in humans living with diabetes and in mouse models of diabetes. An increased effort to identify risk factors beyond glucose is now needed to prevent the increased cardiovascular disease risk associated with diabetes.

Cardiovascular benefits of Momordica charantia in cholesterol-fed Wistar rats

Background

Momordica. charantia is popularly used as a medicinal herb in ethnomedicine for the management of cardiovascular diseases. In this study, we evaluated the beneficial effects of the ethanolic extract of M. charantia (Linn.) in experimentally induced cardiovascular disorders using cholesterol-fed Wistar rat.

Methods

Seventy-two experimental rats were randomly assigned into nine 9 groups of 8 rats each and treated as follows: Rats in group A (control) were given distilled water only; Rats in group B were given 30 mg/kg of cholesterol dissolved in coco-nut oil (cholesterol solution); Rats in group C were given cholesterol solution and 100 mg/kg Atorvastatin; Rats in group D were given 250 mg/kg of M. charantia and cholesterol solution; Rats in group E were given 500 mg/kg of M. charantia and cholesterol solution; Rats in group F were given 250 mg/kg. M. charantia; Rats in group G were given 500 mg/kg M. charantia; Rats in group H were given 1 ml of coconut oil; Rats in group I were given 100 mg/kg of Atorvastatin.

Results

Mean LDL-cholesterol was significantly (P < 0.05) lower in groups F, E and H as compared with the control groups. Histological analysis of the heart and aortic branch of the experimental rats show that cholesterol administration induced myocardial degeneration, vascular ulceration and stenosis in the aorta and heavy perivascular infiltrates of inflammatory cells. However, these deleterious effects were ameliorated upon treatment with Momordica charantia and Atorvastatin as compared with the control groups.

Conclusion

Our findings indicate the possible cardiovascular benefits of M. charantia.

ODC (Ornithine Decarboxylase)-Dependent Putrescine Synthesis Maintains MerTK (MER Tyrosine-Protein Kinase) Expression to Drive Resolution

OBJECTIVE

ODC (ornithine decarboxylase)-dependent putrescine synthesis promotes the successive clearance of apoptotic cells (ACs) by macrophages, contributing to inflammation resolution. However, it remains unknown whether ODC is required for other arms of the resolution program. Approach and Results: RNA sequencing of ODC-deficient macrophages exposed to ACs showed increases in mRNAs associated with heightened inflammation and decreases in mRNAs related to resolution and repair compared with WT (wild type) macrophages. In zymosan peritonitis, myeloid ODC deletion led to delayed clearance of neutrophils and a decrease in the proresolving cytokine, IL (interleukin)-10. Nanoparticle-mediated silencing of macrophage ODC in a model of atherosclerosis regression lowered IL-10 expression, decreased efferocytosis, enhanced necrotic core area, and reduced fibrous cap thickness. Mechanistically, ODC deletion lowered basal expression of MerTK (MER tyrosine-protein kinase)-an AC receptor-via a histone methylation-dependent transcriptional mechanism. Owing to lower basal MerTK, subsequent exposure to ACs resulted in lower MerTK-Erk (extracellular signal-regulated kinase) 1/2-dependent IL-10 production. Putrescine treatment of ODC-deficient macrophages restored the expression of both MerTK and AC-induced IL-10.

CONCLUSIONS

These findings demonstrate that ODC-dependent putrescine synthesis in macrophages maintains a basal level of MerTK expression needed to optimally resolve inflammation upon subsequent AC exposure. Graphic Abstract: A graphic abstract is available for this article.

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.

Macrophage Metabolism of Apoptotic Cell-Derived Arginine Promotes Continual Efferocytosis and Resolution of Injury

Continual efferocytic clearance of apoptotic cells (ACs) by macrophages prevents necrosis and promotes injury resolution. How continual efferocytosis is promoted is not clear. Here, we show that the process is optimized by linking the metabolism of engulfed cargo from initial efferocytic events to subsequent rounds. We found that continual efferocytosis is enhanced by the metabolism of AC-derived arginine and ornithine to putrescine by macrophage arginase 1 (Arg1) and ornithine decarboxylase (ODC). Putrescine augments HuR-mediated stabilization of the mRNA encoding the GTP-exchange factor Dbl, which activates actin-regulating Rac1 to facilitate subsequent rounds of AC internalization. Inhibition of any step along this pathway after first-AC uptake suppresses second-AC internalization, whereas putrescine addition rescues this defect. Mice lacking myeloid Arg1 or ODC have defects in efferocytosis in vivo and in atherosclerosis regression, while treatment with putrescine promotes atherosclerosis resolution. Thus, macrophage metabolism of AC-derived metabolites allows for optimal continual efferocytosis and resolution of injury.

HDL and Reverse Cholesterol Transport

Cardiovascular disease, with atherosclerosis as the major underlying factor, remains the leading cause of death worldwide. It is well established that cholesterol ester-enriched foam cells are the hallmark of atherosclerotic plaques. Multiple lines of evidence support that enhancing foam cell cholesterol efflux by HDL (high-density lipoprotein) particles, the first step of reverse cholesterol transport (RCT), is a promising antiatherogenic strategy. Yet, excitement towards the therapeutic potential of manipulating RCT for the treatment of cardiovascular disease has faded because of the lack of the association between cardiovascular disease risk and what was typically measured in intervention trials, namely HDL cholesterol, which has an inconsistent relationship to HDL function and RCT. In this review, we will summarize some of the potential reasons for this inconsistency, update the mechanisms of RCT, and highlight conditions in which impaired HDL function or RCT contributes to vascular disease. On balance, the evidence still argues for further research to better understand how HDL functionality contributes to RCT to develop prevention and treatment strategies to reduce the risk of cardiovascular disease.

Monocytes and Macrophages as Protagonists in Vascular Complications of Diabetes

With the increasing prevalence of diabetes worldwide, vascular complications of diabetes are also on the rise. Diabetes results in an increased risk of macrovascular complications, with atherosclerotic cardiovascular disease (CVD) being the leading cause of death in adults with diabetes. The exact mechanisms for how diabetes promotes CVD risk are still unclear, although it is evident that monocytes and macrophages are key players in all stages of atherosclerosis both in the absence and presence of diabetes, and that phenotypes of these cells are altered by the diabetic environment. Evidence suggests that at least five pro-atherogenic mechanisms involving monocytes and macrophages contribute to the accelerated atherosclerotic lesion progression and hampered lesion regression associated with diabetes. These changes include (1) increased monocyte recruitment to lesions; (2) increased inflammatory activation; (3) altered macrophage lipid accumulation and metabolism; (4) increased macrophage cell death; and (5) reduced efferocytosis. Monocyte and macrophage phenotypes and mechanisms have been revealed mostly by different animal models of diabetes. The roles of specific changes in monocytes and macrophages in humans with diabetes remain largely unknown. There is an ongoing debate on whether the changes in monocytes and macrophages are caused by altered glucose levels, insulin deficiency or insulin resistance, lipid abnormalities, or combinations of these factors. Current research in humans and mouse models suggests that reduced clearance of triglyceride-rich lipoproteins and their remnants is one important mechanism whereby diabetes adversely affects macrophages and promotes atherosclerosis and CVD risk. Although monocytes and macrophages readily respond to the diabetic environment and can be seen as protagonists in diabetes-accelerated atherosclerosis, they are likely not instigators of the increased CVD risk.

Glucose lowering by SGLT2-inhibitor empagliflozin accelerates atherosclerosis regression in hyperglycemic STZ-diabetic mice

Diabetes worsens atherosclerosis progression and leads to a defect in repair of arteries after cholesterol reduction, a process termed regression. Empagliflozin reduces blood glucose levels via inhibition of the sodium glucose cotransporter 2 (SGLT-2) in the kidney and has been shown to lead to a marked reduction in cardiovascular events in humans. To determine whether glucose lowering by empagliflozin accelerates atherosclerosis regression in a mouse model, male C57BL/6J mice were treated intraperitoneally with LDLR- and SRB1- antisense oligonucleotides and fed a high cholesterol diet for 16 weeks to induce severe hypercholesterolemia and atherosclerosis progression. At week 14 all mice were rendered diabetic by streptozotocin (STZ) injections. At week 16 a baseline group was sacrificed and displayed substantial atherosclerosis of the aortic root. In the remaining mice, plasma cholesterol was lowered by switching to chow diet and treatment with LDLR sense oligonucleotides to induce atherosclerosis regression. These mice then received either empagliflozin or vehicle for three weeks. Atherosclerotic plaques in the empagliflozin treated mice were significantly smaller, showed decreased lipid and CD68⁺ macrophage content, as well as greater collagen content. Proliferation of plaque resident macrophages and leukocyte adhesion to the vascular wall were significantly decreased in empagliflozin-treated mice. In summary, plasma glucose lowering by empagliflozin improves plaque regression in diabetic mice.

Circular RNA profile in coronary artery disease

Circular RNAs (circRNAs) are potential biomarkers and therapeutic targets of coronary artery disease due to their high stability, covalently closed structure. And implied roles in gene regulation. The aim of this study was to identify and characterize circRNAs from human coronary arteries. Epicardial coronary arteries were removed during the autopsy of an 81-year-old man who died from heart attack. The natural history and histological classification of atherosclerotic lesions in coronary artery segments were analyzed by hematoxylin and eosin staining, and their circRNA expression profiles were characterized by RNA sequencing. RNA sequencing identified 1259 annotated and 381 novel circRNAs. Combined with the results of histologic examination, intersection analysis identified 54 upregulated and 12 downregulated circRNAs, representing 4.0% of the total number. Coronary artery segments with or without severe atherosclerosis showed distinctly different circRNA profiles on the basis of hierarchical clustering. Our results suggest that these 66 circRNAs contribute to the pathology underlying coronary artery atherosclerosis and may serve as diagnostic or therapeutic targets in coronary artery disease.

Macrophages in Atherosclerosis Regression

Macrophages play a central role in the development of atherosclerotic cardiovascular disease (ASCVD), which encompasses coronary artery disease, peripheral artery disease, cerebrovascular disease, and aortic atherosclerosis. In each vascular bed, macrophages contribute to the maintenance of the local inflammatory response, propagate plaque development, and promote thrombosis. These central roles, coupled with their plasticity, makes macrophages attractive therapeutic targets in stemming the development of and stabilizing existing atherosclerosis. In the context of ASCVD, classically activated M1 macrophages initiate and sustain inflammation, and alternatively activated M2 macrophages resolve inflammation. However, this classification is now considered an oversimplification, and a greater understanding of plaque macrophage physiology in ASCVD is required to aid in the development of therapeutics to promote ASCVD regression. Reviewed herein are the macrophage phenotypes and molecular regulators characteristic of ASCVD regression, and the current murine models of ASCVD regression.

Naringenin enhances the regression of atherosclerosis induced by a chow diet in Ldlr-/- mice

BACKGROUND AND AIMS

Naringenin is a citrus-derived flavonoid with lipid-lowering and insulin-sensitizing effects leading to athero-protection in Ldlr^-/- mice fed a high-fat diet. However, the ability of naringenin to promote atherosclerosis regression is unknown. In the present study, we assessed the capacity of naringenin to enhance regression in Ldlr^-/- mice with diet-induced intermediate atherosclerosis intervened with a chow diet.

METHODS

Male Ldlr^-/- mice were fed a high-fat, cholesterol-containing (HFHC) diet for 12 weeks to induce intermediate atherosclerosis and metabolic dysfunction. Subsequently, a group of these mice were sacrificed for baseline analyses and the remainder either 1) continued on the HFHC diet, 2) switched to a chow diet or 3) switched to chow diet supplemented with naringenin.

RESULTS

After 12 weeks induction, intermediate lesions developed in the aortic sinus. Intervention with chow alone slowed lesion growth, while intervention with naringenin-supplemented chow completely halted lesion growth. Lesions were characterized by features of improved morphology. Compared to chow alone, naringenin reduced plaque macrophages and modestly increased smooth muscle cells. Investigating processes that contributed to improved plaque morphology, we showed naringenin further reduced plasma triglycerides and cholesterol compared to chow alone. Furthermore, elevated monocytosis and myelopoiesis were further corrected by intervention with naringenin compared to chow alone. Metabolically, naringenin enhanced the correction of insulin resistance, hepatic steatosis and obesity compared to chow alone, potentially contributing to enhanced regression.

CONCLUSIONS

Naringenin supplementation to chow enhances atherosclerosis regression in male Ldlr^-/- mice. These studies further underscore the potential therapeutic utility of naringenin.

Effects of Pharmacological Thermogenic Adipocyte Activation on Metabolism and Atherosclerotic Plaque Regression

Thermogenic adipocytes burn nutrients in order to produce heat. Upon activation, brown adipose tissue (BAT) clears vast amounts of lipids and glucose from the circulation and thus substantially lowers plasma lipid levels. As a consequence, BAT activation protects from the development of atherosclerosis. However, it is unclear if pharmacologic activation of BAT can be exploited therapeutically to reduce plaque burden in established atherosclerotic disease. Here we study the impact of thermogenic adipose tissues on plaque regression in a mouse model of atherosclerosis. Thermogenic adipocytes in atherosclerotic low-density lipoprotein (LDL) receptor (LDLR)-deficient mice were pharmacologically activated by dietary CL316,243 (CL) treatment for 4 weeks and the outcomes on metabolically active tissues, plasma lipids and atherosclerosis were analyzed. While the chronic activation of thermogenic adipocytes reduced adiposity, increased browning of white adipose tissue (WAT), altered liver gene expression, and reduced plasma triglyceride levels, atherosclerotic plaque burden remained unchanged. Our findings suggest that despite improving adiposity and plasma triglycerides, pharmacologic activation of thermogenic adipocytes is not able to reverse atherosclerosis in LDLR-deficient mice.

10,12 Conjugated Linoleic Acid-Driven Weight Loss Is Protective against Atherosclerosis in Mice and Is Associated with Alternative Macrophage Enrichment in Perivascular Adipose Tissue

The dietary fatty acid 10,12 conjugated linoleic acid (10,12 CLA) promotes weight loss by increasing fat oxidation, but its effects on atherosclerosis are less clear. We recently showed that weight loss induced by 10,12 CLA in an atherosclerosis-susceptible mouse model with characteristics similar to human metabolic syndrome is accompanied by accumulation of alternatively activated macrophages within subcutaneous adipose tissue. The objective of this study was to evaluate whether 10,12 CLA-mediated weight loss was associated with an atheroprotective phenotype. Male low-density lipoprotein receptor deficient (Ldlr^−/−) mice were made obese with 12 weeks of a high-fat, high-sucrose diet feeding (HFHS: 36% fat, 36% sucrose, 0.15% added cholesterol), then either continued on the HFHS diet with or without caloric restriction (CR), or switched to a diet with 1% of the lard replaced by either 9,11 CLA or 10,12 CLA for 8 weeks. Atherosclerosis and lipid levels were quantified at sacrifice. Weight loss in mice following 10,12 CLA supplementation or CR as a weight-matched control group had improved cholesterol and triglyceride levels, yet only the 10,12 CLA-treated mice had improved en face and aortic sinus atherosclerosis. 10,12 CLA-supplemented mice had increased lesion macrophage content, with enrichment of surrounding perivascular adipose tissue (PVAT) alternative macrophages, which may contribute to the anti-atherosclerotic effect of 10,12 CLA.

Endoplasmic Reticulum Stress in Metabolic Disorders

Metabolic disorders have become among the most serious threats to human health, leading to severe chronic diseases such as obesity, type 2 diabetes, and non-alcoholic fatty liver disease, as well as cardiovascular diseases. Interestingly, despite the fact that each of these diseases has different physiological and clinical symptoms, they appear to share certain pathological traits such as intracellular stress and inflammation induced by metabolic disturbance stemmed from over nutrition frequently aggravated by a modern, sedentary life style. These modern ways of living inundate cells and organs with saturating levels of sugar and fat, leading to glycotoxicity and lipotoxicity that induce intracellular stress signaling ranging from oxidative to ER stress response to cope with the metabolic insults (Mukherjee, et al., 2015). In this review, we discuss the roles played by cellular stress and its responses in shaping metabolic disorders. We have summarized here current mechanistic insights explaining the pathogenesis of these disorders. These are followed by a discussion of the latest therapies targeting the stress response pathways.

Insights From Pre-Clinical and Clinical Studies on the Role of Innate Inflammation in Atherosclerosis Regression

Atherosclerosis, the underlying cause of coronary artery (CAD) and other cardiovascular diseases, is initiated by macrophage-mediated immune responses to lipoprotein and cholesterol accumulation in artery walls, which result in the formation of plaques. Unlike at other sites of inflammation, the immune response becomes maladaptive and inflammation fails to resolve. The most common treatment for reducing the risk from atherosclerosis is low density lipoprotein cholesterol (LDL-C) lowering. Studies have shown, however, that while significant lowering of LDL-C reduces the risk of heart attacks to some degree, there is still residual risk for the majority of the population. We and others have observed “residual inflammatory risk” of atherosclerosis after plasma cholesterol lowering in pre-clinical studies, and that this phenomenon is clinically relevant has been dramatically reinforced by the recent Canakinumab Anti-inflammatory Thrombosis Outcomes Study (CANTOS) trial. This review will summarize the role of the innate immune system, specifically macrophages, in atherosclerosis progression and regression, as well as the pre-clinical and clinical models that have provided significant insights into molecular pathways involved in the resolution of plaque inflammation and plaque regression. Partnered with clinical studies that can be envisioned in the post-CANTOS period, including progress in developing targeted plaque therapies, we expect that pre-clinical studies advancing on the path summarized in this review, already revealing key mechanisms, will continue to be essential contributors to achieve the goals of dampening plaque inflammation and inducing its resolution in order to maximize the therapeutic benefits of conventional risk factor modifications, such as LDL-C lowering.

Regression of atherosclerosis: lessons learned from genetically modified mouse models

PURPOSE OF REVIEW

Regression, or reversal, of atherosclerosis has become an important clinical objective. The development of consistent models of murine atherosclerosis regression has accelerated this field of research. The purpose of this review is to highlight recent mouse studies that reveal molecular mechanisms as well as therapeutics targeted for regression.

RECENT FINDINGS

Atherosclerosis regression does not involve the same mechanisms as progression in reverse order. Distinct molecular processes within the plaque characterize regression. These processes remained elusive until the advent of murine regression models including aortic transplant, the Reversa mouse, gene complementation and dietary intervention. Studies revealed that depletion of plaque macrophages is a quintessential characteristic of regression, driven by reduced monocyte recruitment into plaques, increased egress of macrophages from plaques and reduced macrophage proliferation. In addition, regression results in polarization of remaining plaque macrophages towards an anti-inflammatory phenotype, smaller necrotic cores and promotion of an organized fibrous cap. Furthermore, type 1 diabetes hinders plaque regression, and several therapeutic interventions show promise in slowing plaque progression or inducing regression.

SUMMARY

Mouse models of atherosclerosis regression have accelerated our understanding of the molecular mechanisms governing lesion resolution. These insights will be valuable in identifying therapeutic targets aimed at atherosclerosis regression.

Novel Reversible Model of Atherosclerosis and Regression Using Oligonucleotide Regulation of the LDL Receptor

RATIONALE

Animal models have been used to explore factors that regulate atherosclerosis. More recently, they have been used to study the factors that promote loss of macrophages and reduction in lesion size after lowering of plasma cholesterol levels. However, current animal models of atherosclerosis regression require challenging surgeries, time-consuming breeding strategies, and methods that block liver lipoprotein secretion.

OBJECTIVE

We sought to develop a more direct or time-effective method to create and then reverse hypercholesterolemia and atherosclerosis via transient knockdown of the hepatic LDLR (low-density lipoprotein receptor) followed by its rapid restoration.

METHODS AND RESULTS

We used antisense oligonucleotides directed to LDLR mRNA to create hypercholesterolemia in wild-type C57BL/6 mice fed an atherogenic diet. This led to the development of lesions in the aortic root, aortic arch, and brachiocephalic artery. Use of a sense oligonucleotide replicating the targeted sequence region of the LDLR mRNA rapidly reduced circulating cholesterol levels because of recovery of hepatic LDLR expression. This led to a decrease in macrophages within the aortic root plaques and brachiocephalic artery, that is, regression of inflammatory cell content, after a period of 2 to 3 weeks.

CONCLUSIONS

We have developed an inducible and reversible hepatic LDLR knockdown mouse model of atherosclerosis regression. Although cholesterol reduction decreased early en face lesions in the aortic arches, macrophage area was reduced in both early and late lesions within the aortic sinus after reversal of hypercholesterolemia. Our model circumvents many of the challenges associated with current mouse models of regression. The use of this technology will potentially expedite studies of atherosclerosis and regression without use of mice with genetic defects in lipid metabolism.

Monocyte-Macrophages and T Cells in Atherosclerosis

Atherosclerosis is an arterial disease process characterized by the focal subendothelial accumulation of apolipoprotein-B-containing lipoproteins, immune and vascular wall cells, and extracellular matrix. The lipoproteins acquire features of damage-associated molecular patterns and trigger first an innate immune response, dominated by monocyte-macrophages, and then an adaptive immune response. These inflammatory responses often become chronic and non-resolving and can lead to arterial damage and thrombosis-induced organ infarction. The innate immune response is regulated at various stages, from hematopoiesis to monocyte changes and macrophage activation. The adaptive immune response is regulated primarily by mechanisms that affect the balance between regulatory and effector T cells. Mechanisms related to cellular cholesterol, phenotypic plasticity, metabolism, and aging play key roles in affecting these responses. Herein, we review select topics that shed light on these processes and suggest new treatment strategies.

Apolipoprotein A-I Modulates Atherosclerosis Through Lymphatic Vessel-Dependent Mechanisms in Mice

Background

Subcutaneously injected lipid‐free apoA‐I (apolipoprotein A‐I) reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without increasing high‐density lipoprotein–cholesterol concentrations. Lymphatic vessels are now recognized as prerequisite players in the modulation of cholesterol removal from the artery wall in experimental conditions of plaque regression, and particular attention has been brought to the role of the collecting lymphatic vessels in early atherosclerosis‐related lymphatic dysfunction. In the present study, we address whether and how preservation of collecting lymphatic function contributes to the protective effect of apoA‐I.

Methods and Results

Atherosclerotic Ldlr^−/− mice treated with low‐dose lipid‐free apoA‐I showed enhanced lymphatic transport and abrogated collecting lymphatic vessel permeability in atherosclerotic Ldlr^−/− mice when compared with albumin‐control mice. Treatment of human lymphatic endothelial cells with apoA‐I increased the adhesion of human platelets on lymphatic endothelial cells, in a bridge‐like manner, a mechanism that could strengthen endothelial cell–cell junctions and limit atherosclerosis‐associated collecting lymphatic vessel dysfunction. Experiments performed with blood platelets isolated from apoA‐I‐treated Ldlr^−/− mice revealed that apoA‐I decreased ex vivo platelet aggregation. This suggests that in vivo apoA‐I treatment limits platelet thrombotic potential in blood while maintaining the platelet activity needed to sustain adequate lymphatic function.

Conclusions

Altogether, we bring forward a new pleiotropic role for apoA‐I in lymphatic function and unveil new potential therapeutic targets for the prevention and treatment of atherosclerosis.

Arterial Calcification in Diabetes Mellitus: Preclinical Models and Translational Implications

Diabetes mellitus increasingly afflicts our aging and dysmetabolic population. Type 2 diabetes mellitus and the antecedent metabolic syndrome represent the vast majority of the disease burden-increasingly prevalent in children and older adults. However, type 1 diabetes mellitus is also advancing in preadolescent children. As such, a crushing wave of cardiometabolic disease burden now faces our society. Arteriosclerotic calcification is increased in metabolic syndrome, type 2 diabetes mellitus, and type 1 diabetes mellitus-impairing conduit vessel compliance and function, thereby increasing the risk for dementia, stroke, heart attack, limb ischemia, renal insufficiency, and lower extremity amputation. Preclinical models of these dysmetabolic settings have provided insights into the pathobiology of arterial calcification. Osteochondrogenic morphogens in the BMP-Wnt signaling relay and transcriptional regulatory programs driven by Msx and Runx gene families are entrained to innate immune responses-responses activated by the dysmetabolic state-to direct arterial matrix deposition and mineralization. Recent studies implicate the endothelial-mesenchymal transition in contributing to the phenotypic drift of mineralizing vascular progenitors. In this brief overview, we discuss preclinical disease models that provide mechanistic insights-and point to challenges and opportunities to translate these insights into new therapeutic strategies for our patients afflicted with diabetes mellitus and its arteriosclerotic complications.

Macrophage Phenotype and Function in Different Stages of Atherosclerosis

The remarkable plasticity and plethora of biological functions performed by macrophages have enticed scientists to study these cells in relation to atherosclerosis for >50 years, and major discoveries continue to be made today. It is now understood that macrophages play important roles in all stages of atherosclerosis, from initiation of lesions and lesion expansion, to necrosis leading to rupture and the clinical manifestations of atherosclerosis, to resolution and regression of atherosclerotic lesions. Lesional macrophages are derived primarily from blood monocytes, although recent research has shown that lesional macrophage-like cells can also be derived from smooth muscle cells. Lesional macrophages take on different phenotypes depending on their environment and which intracellular signaling pathways are activated. Rather than a few distinct populations of macrophages, the phenotype of the lesional macrophage is more complex and likely changes during the different phases of atherosclerosis and with the extent of lipid and cholesterol loading, activation by a plethora of receptors, and metabolic state of the cells. These different phenotypes allow the macrophage to engulf lipids, dead cells, and other substances perceived as danger signals; efflux cholesterol to high-density lipoprotein; proliferate and migrate; undergo apoptosis and death; and secrete a large number of inflammatory and proresolving molecules. This review article, part of the Compendium on Atherosclerosis, discusses recent advances in our understanding of lesional macrophage phenotype and function in different stages of atherosclerosis. With the increasing understanding of the roles of lesional macrophages, new research areas and treatment strategies are beginning to emerge.

A histopathological analysis of the epidemiology of coronary atherosclerosis: an autopsy study

BACKGROUND

Atherosclerosis accounts for a large proportion of cardiovascular system associated morbidity and mortality. We studied the possible association between the histopathological changes of the coronary atherosclerotic lesions and the risk of sudden cardiac death (SCD) using autopsy cases.

METHODS

We performed an autopsy analysis (n = 13, 4 women, 9 men mean age 67.5 years; age range 56-93 years) of SCD which occurred in patients aged over 50 years during March 2010 to December 2013. The following variables were considered: sex, age, medical history, autopsy findings to macroscopic and histological evaluation of the heart. The autopsies were performed according to standard techniques. In all subjects, the heart was dissected following standard autopsy protocol and a 5 cm section of the right coronary artery (RCA) in the atrio-ventricular groove from its origin, a 5 cm segment of the left anterior descending artery (LADA) distal to the origin of the circumflex artery, but including the region of origin of the circumflex branch and left coronary artery (LCA) from its origin till the circumflex branch were excised, dissected out, fixed in 10% formalin, marked for identification and sent for histopathological analysis.

RESULTS

Atherosclerotic plaques were identified in 6.5% of specimens, 69.34% of males and 30.66% of female. Such plaques were typically concentric and more represented with necrosis, calcification, cholesterol crystals, and giant cells, as well as had a higher inflammatory cell count. Furthermore, intima and media thickness of coronary arteries were significantly higher in studied specimens with visualize the connective tissue layers of the adventitia and the fatty acid containing adipose cells in the periadventitial tissue. Furthermore, the degree of microscopic lesion of atherosclerosis increased proportionally with the increase in the intensity of lipid deposition and with the percentage of collagen in the atherosclerotic plaques.

CONCLUSION

In this study, age estimate to be a risk factor for coronary atherosclerosis in individuals more than 50 years old and may be used to predict SCD. Altogether, an enhanced understanding of the pathobiologic processes responsible for atherosclerotic changes might allow for early identification of a high-risk coronary plaque and thereby provide a rationale for innovative diagnostic and/or therapeutic strategies for the management of coronary patients and prevention of acute coronary syndromes.