Current Drug Targets - Volume 9, Issue 3, 2008

In this comprehensive collection of reviews of various aspects of atherosclerosis and atherosclerosis biology in mouse models, the first issue dealt with risk factors such as diet, genetics, gender, obesity, diabetes, hypertension, metabolic syndrome and acute inflammation. The second issue was devoted to many putative mechanisms contributing to atherosclerosis. Among the topics reviewed were: the interaction of modified lipoproteins with Toll-like receptors influencing atherosclerosis; the role of oxidative stress in the stimulating atherogenesis; the family of lipases (hepatic, lipoprotein, endothelial) produced by a variety of atherosclerosis relevant cells and capable of influencing lipoprotein homeostasis; the recruitment of blood cells to the vessel wall during atherogenesis; the central role of monocyte derived foam cells in atherosclerosis; the cytokines produced by the variety of cell types among innate and adaptive immune systems that influence atherosclerosis; the role of nuclear receptors in regulating lipoprotein metabolism and macrophage biology; the important effects of apoptosis of cells of the atherosclerotic lesions particularly in influencing the fate of these lesions; and the role of the adaptive immune system in regulating lipoprotein homeostasis and atherogenesis. In this the third and final issue of the journal devoted to murine atherosclerosis, a variety of additional parameters and potential complications of the atherosclerotic lesion is examined in depth. First an extensive review by Veniant, Beigneux, Bensadoun, Fong and Young is devoted to the influence of lipoprotein size on atherosclerosis susceptibility as determined from genetics studies. Modified lipoproteins that are thought to play an important role in initiating the process of atherosclerosis are also capable of serving as neoantigens, some of which are also seen on the surface of apoptotic cells. The natural antibodies to these neo-antigens are in evidence in atherosclerosis models and may be useful markers of the oxidative aspects of the life history of the atherosclerotic lesion. These antibodies are discussed in detail in the review by Binder and Witztum. The atherosclerotic lesion, despite its complexity and chronicity, is nevertheless a dynamic lesion capable of being reversed or modified particularly by the removal of its lipid content. At least in the mouse, lesion reversibility is surprisingly dynamic. HDL is an agent that affords a major atheroprotective capacity via processes that extend beyond reverse cholesterol transport. These processes are reviewed by Feig, Shamir and Fisher. Based upon the notable atheroprotective and anti-inflammatory properties of HDL, investigators have developed apolipoprotein A-I small peptide mimetics that can be surprisingly effective in reversing atherogenesis and can have other actions on vascular pathobiology in mouse models. This story is reviewed by Navab, Anantharamaiah, Reddy, Van Lenten and Fogelman. In human cardiovascular biology, much attention is paid to the unstable plaque which upon rupture provides a nidus for full thrombus formation and obstruction of vascular channels. One of the limitations of the murine models of atherosclerosis is the difficulty of studying the unstable plaque upon which a thrombus supervenes. However, Rosenfeld, Averill, Bennett and Schwartz have carefully studied the advanced atherosclerotic lesion of apoE deficient mice fed chow. They demonstrate complex lesions with intra-plaque hemorrhage which apparently occurs over ruptured lateral fatty streaks but does not result in occlusive thrombi. They warn against regarding this as a model of the human ruptured plaque which generally involves the fibrous cap and may be accompanied by occlusive thrombi. Despite these observations, a fascinating model of coronary thrombosis and myocardial infarction is seen in the double knockout mouse involving apoE and SRB1 deficiency. Work with this model is carefully reviewed by Braun, Rigotti and Trigatti. Other potential complications of atherosclerosis may involve cartilaginous metaplasia and calcification. These complications have been explored by Hsu, Tintut, and Demer. Several investigators have studied the role of proteins involved in hemostasis on atherogenesis. In the light of the prominence of fibrin deposits and fibrin degradation products in plaques, this topic has been reviewed by Iwaki, Ploplis and Castellino. Surgical intervention with organ transplantation and stenting of atherosclerotic lesions, which involves injury to the plaque surface, may result in blood vessel pathology, either arteriosclerosis or atherosclerosis. This graft atherosclerosis is reviewed by Hu and Xu. Intimal hyperplasia may be a feature of atherosclerosis, but injury induced intimal hyperplasia is probably a separate entity being dominated by proliferating smooth muscle cells. This topic is expertly explored in the review by Hui. This collection involving 28 separate reviews has extensively explored currently available information on murine atherosclerosis and related pathobiology. Represented in this collection are the many faces of approaches to atherosclerosis from the initial lesion to advanced and complicated plaques, from lipoprotein homeostasis to lipid storage in lesions, from macrophages, T cells and smooth muscle cells to advanced lesions containing extracellular lipid and matrix proteins, from simple lipid lesions to complex and varied lesion phenotypes. All of these changes are influenced by the risk factors, diet, gender, blood pressure and genetics. What emerges from this extensive set of reviews is that we still have a great deal to learn if we are to be able to therapeutically modify the fate of lesions. This looms prominently on the horizon and further studies of these models will undoubtedly reveal many surprises and many opportunities. However one needs to hardly remind oneself of the fact that as useful as murine models may be, they do not accurately mimic what we see in pathologically significant human atherosclerotic plaques. In the initial overview introducing this collection of reviews I pointed out some of the most obvious differences in atherosclerosis relevant factors and phenotypes between mice and humans. The study of human atherosclerosis remains the ultimate arbiter for understanding the application of what we have learned from the mouse models to the therapy of human vascular disease. I am grateful to Dr Catherine A Reardon for her help with this editorial.

High plasma levels of the apo-B-containing lipoproteins are casually implicated in the pathogenesis of atherosclerosis. This finding, backed by decades of animal and human studies, has sparked interest in defining which classes of apo-B-containing lipoprotein particles are most atherogenic. Although small LDL particles and larger remnant lipoproteins both appear to be atherogenic, it has been difficult to discern which particles are the most atherogenic. Here, we summarize several mouse models that have provided insights into this issue. The influence of lipoprotein size on susceptibility to atherosclerosis was examined by studying the phenotypes of two strains of mice with virtually identical levels of plasma cholesterol—Ldlr-/-Apob100/100 and Apoe-/-Apob100/100 mice. The Ldlr-/-Apob100/100 mice, where the cholesterol is in small LDL particles, had far more atherosclerosis than Apoe-/-Apob100/100 mice, where virtually all of the cholesterol was in larger, VLDL-sized particles. Another intriguing animal model is the Gpihbp1-deficient mouse. GPIHBP1 is an endothelial cell platform for lipolysis, and mice lacking this protein have an accumulation of large, triglyceride-rich lipoproteins. Defining the extent of atherosclerosis in these mice should provide new insights into the atherogenicity of large, triglyceride-rich lipoproteins.

Natural antibodies are preformed antibodies that are present even in naive germ-free mice in the absence of any exogenous antigenic exposure. Consistent with their specificities for microbial antigens, natural antibodies play an important non-redundant role in the first line defense against bacterial and viral infections. On the other hand natural antibodies have also been shown to have specificities for self antigens, and therefore have been proposed to provide important homeostatic “house-keeping” functions. Many of the recognized self-antigens may in fact be stress-induced self-antigens, such as oxidation-specific epitopes that accumulate during atherogenesis as well as in many other inflammatory settings, and natural antibodies could protect from the impact of the pathological accumulation of these self-antigens. In this review we will discuss the specific example of the prototypic natural antibody T15/EO6, which is increased in atherosclerotic mice and mediates atheroprotection, and discuss the potential role of natural antibodies in atherogenesis in general.

The risk of atherosclerosis is inversely related to circulating levels of high density lipoprotein (HDL) cholesterol. Notably, in large-scale epidemiologic studies, this association is independent of plasma levels of low density lipoprotein cholesterol levels. Pharmacologic agents, such as fibrates and niacin that increase HDL cholesterol levels have been associated with decreased cardiovascular events and beneficial effects on the coronary and carotid arteries. Furthermore, there is evidence that the risk of restenosis following vascular interventions is inversely related to HDL levels. This review considers the available data from mainly murine models on potential mechanisms by which HDL may exert these anti-atherogenic effects, namely through its role in reverse cholesterol transport, its effects on endothelial cells, and its anti-inflammatory/anti-oxidant activities. In addition to discussing a role for HDL in retarding atherosclerosis progression, we will also review how HDL may play a role in promoting regression of atherosclerotic lesions.

The mouse has proven to be an excellent model for testing apolipoprotein mimetic peptides as agents to treat a variety of vascular inflammatory conditions including atherosclerosis, cognitive dysfunction associated with arteriole inflammation, chronic rejection of transplanted hearts, and scleroderma. The mechanism of action appears to relate to the ability of these peptides to preferentially bind pro-inflammatory oxidized lipids and is independent of the chirality of the peptides since peptides synthesized from either D- or L-amino acids appear to be equally effective.

The innominate artery is a predilection site for atherosclerotic lesion formation in hyperlipidemic mice. The lesions at this site in chow-fed apo E-/- mice progress from fatty streaks through stages that include atheroma with large necrotic areas, fibro-fatty nodules containing chondrocyte-like cells and highly calcified, acellular plaques. The advanced lesions in the innominate arteries of the apo E-/- mice exhibit a reproducible frequency of intra-plaque hemorrhage that occurs primarily as a result of fissures through lateral fatty streaks that form adjacent to or on top of the established plaques. However, this plaque disruption is not equivalent to plaque rupture in human lesions where there is rupture of well formed fibrous caps. The plaque disruption in the lesions of the chow-fed apo E-/- mice also do not lead to formation of occlusive thrombi, the predominant marker of plaque rupture in humans. Thus, although the lesions in the innominate arteries of hyperlipidemic mice progress to very advanced stages of the disease, they are not, in our opinion a model in which to study the mechanisms of plaque rupture in humans. The advanced lesions in the innominate arteries of the apo E-/- mice may however be adequate models for studying vascular fibrosis and calcification.

The most widely used mouse models for atherosclerosis are LDL receptor knockout (KO) mice and apolipoprotein E (apoE) KO mice fed standard chow diets or lipid-supplemented diets. Unfortunately, these do not usually exhibit myocardial infarction or other features of human cardiovascular disease such as occlusive coronary artery disease, cardiac dysfunction and/or reduced lifespan. Surgical models of myocardial infarction are successfully used for drug testing analyses during acute ischemia, but do not allow investigation of underlying mechanisms related to atherosclerotic coronary artery disease. Recently, experts in the pharmaceutical industry as well as some at the US Food and Drug Administration have identified inadequate animal models as being one of the major hurdles in drug discovery and development. There is an important need for additional well-characterized, genetically manipulable, small animal models that mimic many features of human coronary heart disease (CHD), which would provide investigators in academia and in the pharmaceutical industry with a better system to unravel the pathophysiology of atherosclerotic CHD and to evaluate preclinical drug candidates. Here we will review recently developed mouse models of occlusive CHD, focusing on mice lacking expression of the HDL receptor, SR-BI in the context of reduced expression of apoE.

Vascular calcification is associated with increased cardiovascular morbidity and mortality and has long been associated with advanced atherosclerotic lesions. While vascular calcification is considered a surrogate marker for atherosclerosis, the mechanisms that link the two are poorly understood. The consensus of recent research is that active regulatory processes govern vascular calcification, and much focus has been placed on elucidating the phenomenon of atherosclerotic calcification. Building upon extensive in vitro work and the previous development of atherosclerotic murine models, several groups have developed murine models of atherosclerotic calcification. From imposing chronic renal failure to developing double-knockout mice, this recent work has provided insight into the pathophysiology of mineralized matrix formation in atherosclerotic lesions, as well as development of potential therapies to prevent or inhibit progression of calcified plaque. The aim is to briefly review current understanding of the molecular basis for atherosclerotic calcification and to discuss some murine models that may be useful in advancing knowledge of its mechanisms.

Atherosclerosis is a self-sustaining inflammatory fibroproliferative disease that progresses in discrete stages and involves a number of cell types and effector molecules. The potential importance of the coagulation, anticoagulation, and fibrinolytic systems in atherosclerosis is based on the observation that fibrin deposits and fibrin degradation products are resident in atherosclerotic plaques. A number of investigations have been conducted to probe the relationships between components of the hemostasis system and atherosclerosis; and these types of studies proliferated after the availability of mice genetically manipulated to emphasize the impact of genes of interest. In order to summarize recent progress in this area, this review is focused on mice lacking individual hemostasis genes and their contributions to steps of the atherosclerotic process.

The use of animal models in the study of arteriosclerosis is essential for better understanding of the pathogenesis, improvement in diagnosis, prevention and therapy of the diseases in humans. Recently numerous investigators started to use mouse models to study the pathogenesis of cardiovascular diseases. This species is particularly valuable which is believed to have some advantages over other strains, because of the availability of well-defined genetic systems of transgenic and knockout mice. Concomitantly, we have established the mouse models for vein grafts and transplant arteriosclerosis. By using these models, we have learned much knowledge concerning the pathogenesis of the disease and possible therapeutic intervention has been gained. One of most important findings is that proteins or molecules influencing apoptosis, inflammation or proliferation of vascular smooth muscle cells or endothelial cells, have been found to enhance or inhibit neointimal lesion formation in knockout or mutant mice in these models. Furthermore, the findings on the origins of endothelial and smooth muscle cells in lesions of vein graft and transplant atherosclerosis provided basic information that have challenged the traditional hypothesies. Using these models, it has also been demonstrated that stem cells identified in blood and bone marrow as well as the vasculature contribute to the atherosclerotic lesion formation, supporting the importance of murine models of vessel grafts in understanding the mechanisms of the vascular diseases. The present review updates the progress of the research in this field, by summarizing data of using mouse models of vessel grafts, and provides a perspective analysis on the future directions.

The most commonly used procedures to induce arterial injury in mice are carotid artery ligation with cessation of blood flow and mechanically-induced denudation of endothelium in the carotid or the femoral arteries. Both procedures result in neointimal hyperplasia after two to three weeks. A survey of various inbred strains of mice shows that strainspecific differences in susceptibility to injury-induced neointimal hyperplasia are different than those for susceptibility to diet-induced atherosclerosis, with strains identified as susceptible to both neointimal hyperplasia and atherosclerosis, resistant to both, susceptible to atherosclerosis but resistant to neointimal hyperplasia, or resistant to atherosclerosis but susceptible to neointimal hyperplasia. Inflammatory cells such as T and B lymphocytes, which are contributory to atherosclerosis, are protective against injury-induced neointimal hyperplasia. In contrast, the infiltration of monocytes into the site of injury and their differentiation to macrophages favor neointimal hyperplasia similar to their pathogenic role in atherosclerosis. The regulatory role of lymphocytes and macrophages in neointimal hyperplasia is related to the production of cytokines such as interferon-γ and tumor necrosis factor-α, respectively. Interestingly, inducible nitric oxide synthase (iNOS) activity appears to inhibit neointimal hyperplasia in the endothelial denudation model but contributes to neointimal hyperplasia when arterial injury is induced by periadventitial cuff placement. The difference appears to be due to the time required for endothelial recovery and the participation of inflammatory cells. Thus, although arterial injury-induced neointimal hyperplasia results in similar vascular occlusion as progressive atherosclerosis, the pathology and mechanism of the two disease processes are quite different.