Role of leukocyte-specific LDL receptors on plasma lipoprotein cholesterol and atherosclerosis in mice

Research methods for animal models of atherosclerosis (Review)

Atherosclerosis is a chronic inflammatory disease that threatens human health and lives by causing vascular stenosis and plaque rupture. Various animal models have been employed for elucidating the pathogenesis, drug development and treatment validation studies for atherosclerosis. To the best of our knowledge, the species used for atherosclerosis research include mice, rats, hamsters, rabbits, pigs, dogs, non-human primates and birds, among which the most commonly used ones are mice and rabbits. Notably, apolipoprotein E knockout (KO) or low-density lipoprotein receptor KO mice have been the most widely used animal models for atherosclerosis research since the late 20th century. Although the aforementioned animal models can form atherosclerotic lesions, they cannot completely simulate those in humans with respect to lesion location, lesion composition, lipoprotein composition and physiological structure. Hence, an appropriate animal model needs to be selected according to the research purpose. Additionally, it is necessary for atherosclerosis research to include quantitative analysis results of atherosclerotic lesion size and plaque composition. Laboratory animals can provide not only experimental tissues for in vivo studies but also cells needed for in vitro experiments. The present review first summarizes the common animal models and their practical applications, followed by focus on mouse and rabbit models and elucidating the methods to quantify atherosclerotic lesions. Finally, the methods of culturing endothelial cells, macrophages and smooth muscle cells were elucidated in detail and the experiments involved in atherosclerosis research were discussed.

Bone Marrow Transplantation in Mice to Study the Role of Hematopoietic Cells in Atherosclerosis

Hematopoietic stem cell transplantation or bone marrow transplantation is a common approach to reconstitute the immune system of mice that have been subjected to marrow-ablative doses of radiation. This method can be used in the field of atherosclerosis to assess the contribution of hematopoietic cells of a desired genotype to disease pathogenesis. The engraftment of atherosclerosis-prone mice with donor cells that contain genetic alterations in cells of the innate or adaptive immune system has been invaluable to define the role of multiple gene products in atherosclerosis. Here, we describe the different steps involved in the bone marrow transplantation protocol along with specific guidelines regarding the theoretical and technical details of the procedure.

Use of Mouse Models in Atherosclerosis Research

Cardiovascular disease is the major cause of death in most developed nations and the social and economic burden of this disease is quite high. Atherosclerosis is a major underlying basis for most cardiovascular diseases including myocardial infarction and stroke. Genetically modified mouse models, particularly mice deficient in apoprotein E or the LDL receptor, have been widely used in preclinical atherosclerosis studies to gain insight into the mechanisms underlying this pathology. This chapter reviews several mouse models of atherosclerosis progression and regression as well as the role of immune cells in disease progression and the genetics of murine atherogenesis.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Disruption of tissue-specific fucosyltransferase VII, an enzyme necessary for selectin ligand synthesis, suppresses atherosclerosis in mice

A hallmark feature of atherosclerosis is that circulating mononuclear cells adhere to the endothelium and migrate into the subendothelial space. This adhesion is mediated by molecules such as selectins that are expressed on the surfaces of both leukocytes and endothelial cells. In this study, we have determined the role of tissue-specific fucosyltransferase VII (FucT-VII), an enzyme necessary for selectin ligand synthesis, in the development of atherosclerosis. We adopted a scheme of transplanting either FucT-VII(-/-)GFP(+) bone marrow into lethally irradiated low-density lipoprotein receptor low density lipoprotein receptor mice or FucT-VII(+/+) GFP(+) bone marrow into FucT-VII(-/-), low density lipoprotein receptor double-mutant mice to evaluate the roles of E- and P-selectin ligands versus L-selectin ligands, respectively, in diet-induced atherosclerosis. GFP was used to track the transplanted cells. Our results indicate that, compared with controls, selective disruption of E- and P-selectin ligand synthesis resulted in a significant reduction in atherosclerosis. Selective disruption of L-selectin ligand production did not reduce atherosclerosis as robustly as disruption of E- and P-selectin ligands. In both groups, however, there was a significant reduction in the accumulation of macrophages in the lesion. These studies indicate that selectin ligands, particularly those for E- and P-selectins, play an important role in the pathogenesis of atherosclerosis by regulating macrophage accumulation in atherosclerotic lesions.

Toll-like receptors in atherosclerosis

At one time, atherosclerosis was thought to be a simple lipid storage disease. However, it is now recognized as a chronic and progressive inflammation of the arterial wall. Gene deletion experiments in murine models of atherosclerosis that reduce the inflammatory process also reduce disease severity. Identifying the initiators and mediators of that inflammation can provide promising avenues for prevention or therapy. Two prominent risk factors, hyperlipidaemia and infectious disease, point to innate immune mechanisms as potential contributors to proatherogenic inflammation. The TLRs (Toll-like receptors), pro-inflammatory sensors of pathogens, are potential links between inflammation, infectious disease and atherosclerosis. A mechanism for hyperlipidaemic initiation of sterile inflammation can be postulated because oxidized lipoproteins or their component oxidized lipids have been identified as TLR ligands. Moreover, infectious agents are correlated with atherosclerosis risk. We have identified a role for TLR2 in atherosclerosis in mice deficient in low-density lipoprotein receptor. We observed that proatherogenic TLR2 responses to unknown endogenous or unknown endemic exogenous agonists are mediated by non-BMDC (bone-marrow-derived cells), which can include endothelial cells. In contrast, the proatherogenic TLR2 responses to the defined synthetic exogenous agonist Pam3 CSK4 are mediated at least in part by BMDC, which can include lymphocytes, monocytes/macrophages and dendritic cells. TLR2-mediated cell activation in response to endogenous and exogenous agents is proatherogenic in hyperlipidaemic mice.

Role of apolipoprotein E genotype in coronary artery disease

Recent gene-targeting technology has provided good animal models that provide insight into the pathology of complex diseases such as atherosclerosis. The apolipoprotein E gene polymorphism is one of the most extensively studied in cardiovascular medicine. The scope of the present review is to briefly outline the biochemical characteristics and the genetic variation of apolipoprotein E. Apolipoprotein E is best known for its role in modulating lipoprotein metabolism as a ligand for cellular receptors. Other functions unrelated to lipid transport are becoming known, including reverse cholesterol transport, immunoregulation and modulation of cell growth. This review will examine recent work that addresses how apolipoprotein E participates in atherosclerosis. Genotypic variation of apolipoprotein E has been associated with certain phenotypes regarding vascular disease, such as the presence of atherosclerosis and coronary heart disease outcomes. This article will also review evidence regarding the association between apolipoprotein E gene polymorphisms and coronary artery disease based upon experimental and clinical studies.

Role of apolipoproteins in gammadelta and NKT cell-mediated innate immunity

Recent findings reveal unanticipated connections between the fields of lipid metabolism and immunology. They concern gammadelta and NKT cells, nonconventional T cell populations that do not recognize protein antigens and are involved in immunity against cancer, defense against infections, or in regulation of classical immune responses. In this review, we summarize data linking perturbations of apolipoprotein levels and nonconventional T cells with inflammatory processes such as autoimmune diseases or atherosclerosis. We integrate and discuss recent findings on the implication of apolipoproteins in antigen recognition by gammadelta and NKT cells, with emphasis on apolipoproteins A-I and E. These findings also provide indications that apolipoproteins influence antitumor immunosurveillance.

Modulation of atherosclerosis in mice by Toll-like receptor 2

Epidemiologic evidence has established a relationship between microbial infection and atherosclerosis. Mammalian TLRs provide clues on the mechanism of this inflammatory cascade. TLR2 has a large ligand repertoire that includes bacterial-derived exogenous and possibly host-derived endogenous ligands. In atherosclerosis-susceptible low-density lipoprotein receptor-deficient (Ldlr-/-) mice, complete deficiency of TLR2 led to a reduction in atherosclerosis. However, with BM transplantation, loss of TLR2 expression from BM-derived cells had no effect on disease progression. This suggested that an unknown endogenous TLR2 agonist influenced lesion progression by activating TLR2 in cells that were not of BM cell origin. Moreover, with intraperitoneal administration of a synthetic TLR2/TLR1 agonist, Pam3CSK4, disease burden was dramatically increased in Ldlr-/- mice. A complete deficiency of TLR2 in Ldlr-/- mice, as well as a deficiency of TLR2 only in BM-derived cells in Ldlr-/- mice, led to striking protection against Pam3CSK4-mediated atherosclerosis, suggesting a role for BM-derived cell expression of TLR2 in transducing the effects of an exogenous TLR2 agonist. These studies support the concept that chronic or recurrent microbial infections may contribute to atherosclerotic disease. Additionally, these data suggest the presence of host-derived endogenous TLR2 agonists.

Thematic review series: The Immune System and Atherogenesis. Cytokine regulation of macrophage functions in atherogenesis

This review will focus on the role of cytokines in the behavior of macrophages, a prominent cell type of atherosclerotic lesions. Once these macrophages have immigrated into the vessel wall, they propagate the development of atherosclerosis by modifying lipoproteins, accumulating intracellular lipids, remodeling the extracellular environment, and promoting local coagulation. The numerous cytokines that have been detected in atherosclerosis, combined with the expression of large numbers of cytokine receptors on macrophages, are consistent with this axis being an important contributor to lesion development. Given the vast literature on cytokine-macrophage interactions, this review will be selective, with an emphasis on the major cytokines that have been detected in atherosclerotic lesions and their effects on properties that are relevant to lesion formation and maturation. There will be an emphasis on the role of cytokines in regulating lipid metabolism by macrophages. We will provide an overview of the major findings in cell culture and then put these in the context of in vivo studies.

Reduced macrophage apoptosis is associated with accelerated atherosclerosis in low-density lipoprotein receptor-null mice

OBJECTIVE

The majority of apoptotic cells in atherosclerotic lesions are macrophages. However, the pathogenic role of macrophage apoptosis in the development of atherosclerosis remains unclear. Elevated expression of Bax, one of the pivotal proapoptotic proteins of the Bcl-2 family, has been found in human atherosclerotic plaques. Activation of Bax also occurs in free cholesterol-loaded and oxysterol-treated mouse macrophages. In this study, we examined the effect of Bax deficiency in bone marrow-derived leukocytes on the development of atherosclerosis in low-density lipoprotein receptor-null (LDLR-/-) mice.

METHODS AND RESULTS

Fourteen 8-week-old male LDLR-/- mice were lethally irradiated and reconstituted with either wild-type (WT) C57BL6 or Bax-null (Bax-/-) bone marrow. Three weeks later, the mice were challenged with a Western diet for 10 weeks. No differences were found in the plasma cholesterol level between the WT and Bax-/- group. However, quantitation of cross sections from proximal aorta revealed a 49.2% increase (P=0.0259) in the mean lesion area of the Bax-/- group compared with the WT group. A 53% decrease in apoptotic macrophages in the Bax-/- group was found by TUNEL staining (P<0.05).

CONCLUSIONS

The reduction of apoptotic activity in macrophages stimulates atherosclerosis in LDLR-/- mice, which is consistent with the hypothesis that macrophage apoptosis suppresses the development of atherosclerosis.

Participation of innate and acquired immunity in atherosclerosis

Coronary artery disease, the major manifestation of atherosclerosis, is the leading cause of death in the Western world. However, the pathogenesis of atherosclerosis is still poorly understood. Controversy exists regarding the participation of innate immunity involving macrophages and natural killer (NK) cells vs antigen-specific acquired immunity involving lymphocytes. Macrophages predominate in atherosclerotic lesions. NK cells, although smaller in number, are present as well. Furthermore, T lymphocytes that participate in acquired immunity are frequently observed in lesions and can modulate lesion progression. By using mouse models of hyperlipidemia, our laboratory is addressing in vivo the participation of both innate inflammatory responses and acquired immune responses in atherosclerosis.

Apolipoprotein E and atherosclerosis

Apolipoprotein E plays a key protective role in atherosclerosis. Its capacity to safeguard against this disease can be attributed to at least three distinct functions. First, plasma apolipoprotein E maintains overall plasma cholesterol homeostasis by facilitating efficient hepatic uptake of lipoprotein remnants. Second, lesion apolipoprotein E in concert with apolipoprotein A-I facilitates cellular cholesterol efflux from macrophage foam cells within the intima of the lesion. Third, lesion apolipoprotein E directly modifies both macrophage- and T lymphocyte-mediated immune responses that contribute to this chronic inflammatory disease.

Lipoprotein receptors in acute myelogenous leukemia: failure to detect increased low-density lipoprotein (LDL) receptor numbers in cell membranes despite increased cellular LDL degradation

The high-affinity degradation of low-density lipoprotein (LDL) is enhanced 3- to 100-fold in leukemic blood cells from patients with acute myelogenous leukemia (AML), suggesting an increased cellular LDL receptor expression. There are, however, inconsistencies regarding the published properties of LDL receptor regulation in AML cells, and previous data on this are indirect. In the present study the aim was to determine whether the LDL receptor number is increased in AML cells. The LDL receptor number was assayed by ligand blot with rabbit 125I-labeled beta-very-low-density lipoprotein (beta-VLDL) of transferred, SDS-polyacrylamide-gel-electrophoresis-separated AML cell membranes. Samples from 10 patients, six with AML, one with chronic myelogenous leukemia in blast crisis, and three with acute lymphoblastic leukemia, were investigated. The LDL receptor expression was strongly suppressed in all samples to levels lower than that of normal mononuclear cells. This was despite the fact that cells from one patient with AML of M4 subtype had a 50- to 100-fold higher 125I labeled LDL degradation compared with normal cells. Immunoblots with antibodies against gp330/megalin and the LDL-receptor-related protein (LRP) and ligand blot using 125I-labeled 39-kd receptor-associated protein (RAP) could not detect gp330/megalin or VLDL receptors. The LRP was abundant in AML samples of M4 and M5b subtype, as determined from both RAP ligand blot and immunoblot using an LRP-specific antibody. It is concluded that LDL receptors are suppressed in AML cells. It is possible that the high degradation of 125I-labeled LDL present in type M4 and M5 AML cells may involve another lipoprotein receptor.

A leukocyte homologue of the IL-8 receptor CXCR-2 mediates the accumulation of macrophages in atherosclerotic lesions of LDL receptor-deficient mice

Chronic macrophage-mediated inflammation is central to atherosclerosis. A role of the monocyte chemotactic and activating C-C chemokine JE/monocyte chemotactic protein-1 has been proposed. However, the human C-X-C chemokines growth-regulated oncogene (GROalpha) and IL-8, and their shared receptor, CXCR-2, also can be expressed at sites of chronic inflammation. Because we detected CXCR-2 in the intima of human atherosclerotic lesions, we examined the role of leukocyte CXCR-2 expression in affecting lesion cellularity. Atherosclerosis-susceptible LDL receptor-deficient mice were irradiated, successfully repopulated with bone marrow cells that either lacked or expressed mIL-8RH (the homologue of CXCR-2), and fed an atherogenic diet for 16 wk. In recipients of mIL-8RH+/+ marrow, mIL-8RH colocalized with densely accumulated intimal MOMA-2 positive macrophages. In contrast, lesions in recipients of mIL-8RH-/- marrow lacked mIL-8RH, had little intimal MOMA-2 staining, and were less extensive. The mIL-8RH ligand KC/GROalpha was detected in the intima of all aortic atherosclerotic lesions. Thus, the capacity of leukocytes to express mIL-8RH, and associated intralesional expression of its ligands such as KC/GROalpha, mediated the intimal accumulation of macrophages in atherosclerotic lesions of LDL receptor-deficient mice.