Atherosclerotic Lesions In apoE-deficient Mice Research Articles

Serum Amyloid A Binding to CLA-1 (CD36 and LIMPII Analogous-1) Mediates Serum Amyloid A Protein-induced Activation of ERK1/2 and p38 Mitogen-activated Protein Kinases

Serum amyloid A protein (SAA) is an acute-phase reactant, known to mediate pro-inflammatory cellular responses. This study reports that CLA-1 (CD36 and LIMPII Analogous-1; human orthologue of the Scavenger Receptor Class B Type I (SR-BI)) mediates SAA uptake and downstream SAA signaling. Flow cytometry experiments revealed more than a 5-fold increase of Alexa-488 SAA uptake in HeLa cells stably transfected with CLA-1. Alexa 488-HDL uptake directly correlated with SAA uptake when determined in several CLA-1 stably transfected HeLa cell clones expressing various levels of CLA-1. SAA directly binds to CLA-1 as determined by cross-linking and colocalization of anti-CLA-1 antibody with SAA. SAA was co-internalized with transferrin to the endocytic recycling compartment pointing to a potential site of SAA metabolism. Alexa-488 SAA uptake in the CLA-1-overexpressing HeLa cells, as well as in THP-1 monocyte cell line, can be efficiently blocked by unlabeled SAA, high density lipoprotein, and other CLA-1 ligands. At the same time, markedly enhanced levels of phosphorylation of the mitogen-activated protein kinases (MAPKs), ERK1/2, and p38, were observed in cells stably transfected with CLA-1 cells following SAA stimulation when compared with mock transfected cells. The levels of the SAA-induced interleukin-8 (IL-8) secretion by CLA-1-overexpressing cells also significantly exceeded (5- to 10-fold) those detected for control cells. Synthetic amphipathic peptides possessing a structural alpha-helical motif inhibited SAA-induced activation of both MAPKs and IL-8 secretion in THP-1 cells. The results of this study demonstrate for the first time that CLA-1 functions as an endocytic SAA receptor and is involved in SAA-mediated cell signaling events associated with the immune-related and inflammatory effects of SAA.

Imaging of Aortic Atherosclerotic Lesions by 125I-LDL, 125I-Oxidized-LDL, 125I-HDL and 125I-BSA

Objective: The aim of the present study was to compare the accumulation of ¹²⁵I-labeled low-density lipoproteins (LDL), oxidized LDL (oxLDL), HDL and BSA in advanced atherosclerotic lesions of apoE-deficient mice. Methods:¹²⁵I-lipoproteins or ¹²⁵I-BSA were injected into the tail vein of apoE-deficient mice. Blood clearance of ¹²⁵I-lipoproteins and ¹²⁵I-BSA and their accumulation in atherosclerotic lesions were assayed. Results: Blood clearance of ¹²⁵I-LDL and ¹²⁵I-HDL was moderate, and approximately 30% of the injected lipoproteins were present in plasma 24 h following injection. oxLDL was removed much faster from plasma, and less than 10% of ¹²⁵I-oxLDL was present in the circulation 30 min after ¹²⁵I-oxLDL injection. The clearance of ¹²⁵I-BSA from the circulation was slower than the lipoprotein clearance. The highest accumulation of LDL, oxLDL, HDL and BSA was detected in atherosclerotic lesions in the aortic arch and abdominal aorta, while lower accumulation was detected in the less atherosclerotic descending thoracic aorta. Conclusion: Our findings demonstrate that both ¹²⁵I-HDL and ¹²⁵I-BSA as well as ¹²⁵I-LDL are accumulated in atherosclerotic plaques and that they can be used for the detection of atherosclerotic lesions.

The adhesion receptor CD44 promotes atherosclerosis by mediating inflammatory cell recruitment and vascular cell activation.

Atherosclerosis causes most acute coronary syndromes and strokes. The pathogenesis of atherosclerosis includes recruitment of inflammatory cells to the vessel wall and activation of vascular cells. CD44 is an adhesion protein expressed on inflammatory and vascular cells. CD44 supports the adhesion of activated lymphocytes to endothelium and smooth muscle cells. Furthermore, ligation of CD44 induces activation of both inflammatory and vascular cells. To assess the potential contribution of CD44 to atherosclerosis, we bred CD44-null mice to atherosclerosis-prone apoE-deficient mice. We found a 50-70% reduction in aortic lesions in CD44-null mice compared with CD44 heterozygote and wild-type littermates. We demonstrate that CD44 promotes the recruitment of macrophages to atherosclerotic lesions. Furthermore, we show that CD44 is required for phenotypic dedifferentiation of medial smooth muscle cells to the "synthetic" state as measured by expression of VCAM-1. Finally, we demonstrate that hyaluronan, the principal ligand for CD44, is upregulated in atherosclerotic lesions of apoE-deficient mice and that the low-molecular-weight proinflammatory forms of hyaluronan stimulate VCAM-1 expression and proliferation of cultured primary aortic smooth muscle cells, whereas high-molecular-weight forms of hyaluronan inhibit smooth muscle cell proliferation. We conclude that CD44 plays a critical role in the progression of atherosclerosis through multiple mechanisms.

Inhibition of transforming growth factor-beta signaling accelerates atherosclerosis and induces an unstable plaque phenotype in mice.

Atherosclerosis is a disease of the arterial wall that seems to be tightly modulated by the local inflammatory balance. Whereas a large body of evidence supports a role for proinflammatory mediators in disease progression, the understanding of the role of the antiinflammatory component in the modulation of plaque progression is only at its beginning. TGF-beta1, -beta2, and -beta3 are cytokines/growth factors with broad activities on cells and tissues in the cardiovascular system and have been proposed to play a role in the pathogenesis of atherosclerosis. However, no study has examined the direct role of TGF-beta in the development and composition of advanced atherosclerotic lesions. In the present study, we show that inhibition of TGF-beta signaling using a neutralizing anti-TGF-beta1, -beta2, and -beta3 antibody accelerates the development of atherosclerotic lesions in apoE-deficient mice. Moreover, inhibition of TGF-beta signaling favors the development of lesions with increased inflammatory component and decreased collagen content. These results identify a major protective role for TGF-beta in atherosclerosis.

Does Homocysteine Promote Atherosclerosis?

The hypothesis that homocysteine is atherogenic was proposed more than 30 years ago by Kilmer McCully,1 who observed vascular lesions in children with inherited disorders of methionine metabolism. Since McCully’s pioneering observations in 1969, a large number of epidemiological studies have confirmed that an elevation of total plasma homocysteine (tHcy) is prevalent in patients with stroke, myocardial infarction, peripheral vascular disease, and venous thrombosis.2 A significant association between hyperhomocysteinemia and clinical cardiovascular events has been observed in several large prospective studies, although a few prospective studies have failed to demonstrate this association.2 Nevertheless, hyperhomocysteinemia is now considered by many an independent risk factor for atherosclerotic vascular disease.3 See p 1470 Homocysteine is a thiol amino acid, but only a small fraction ( 15 μmol/L and may be caused by genetic defects, renal insufficiency, certain drugs, or nutritional deficiencies of folate, vitamin B6, or vitamin B12.5 Even a mild elevation of plasma tHcy to levels within the high-to-normal range (10 to 15 μmol/L) may increase cardiovascular risk.2 Because plasma tHcy can often be lowered by oral administration of folic acid or combinations of B vitamins, there is growing enthusiasm for treatment of hyperhomocysteinemia as a strategy for prevention of cardiovascular disease and …

Freunds adjuvant alone is antiatherogenic in apoE-deficient mice and specific immunization against TNFα confers no additional benefit

TNFα participates in the pathogenesis of atherosclerosis. The effect of immunization against TNFα on development of advanced vascular lesions in atherosclerosis-susceptible apoE-deficient mice was investigated. At 5–7 weeks of age, animals received immunization with either Freunds adjuvant and a recombinant antigenic TNFα molecule (TNF106), Freunds adjuvant alone, or no immunization. All mice received a Western-type high-fat diet for 12 weeks. Aortic sinus lesion area was determined by microscopic morphometry, the total aortic arch cholesterol content was determined by gas chromatography, and antibodies against TNFα, malondialdehyde-modified low density lipoprotein, or heat shock protein 60, were assessed by ELISAs. Immunization with TNF106 induced high-titered circulating antibodies against TNFα ( n=23), and these antibodies were not detected in mice immunized with Freunds adjuvant alone ( n=22), or in non-immunized control animals ( n=25). After 12 weeks, the atherosclerotic lesion size was significantly reduced in immunized animals, whether they had been immunized with TNF106 or Freunds adjuvant alone, and the total lesional cholesterol content was decreased in mice immunized with TNF106. There were no correlations between circulating antibody titers and plaque size, total aortic arch cholesterol content, or plasma lipid levels, respectively. Administration of Freunds adjuvant alone can thus reduce formation of mature atherosclerotic lesions in apoE-deficient mice and this response is not modified by specific immunization against TNFα.

Expression of Interleukin-15 in Mouse and Human Atherosclerotic Lesions

Atherosclerotic lesions are characterized by prominent macrophage and T-cell infiltration and atherosclerosis is widely recognized as an inflammatory disease. The cytokine interleukin-15 (IL-15) has T-cell chemotactic and pro-inflammatory properties and promotes the recruitment of T cells to sites of inflammation. We have therefore examined IL-15 expression in the atherosclerotic ApoE-deficient mouse model as well as in human atherosclerotic lesions. In gene expression arrays, a transcript corresponding to IL-15 mRNA was elevated in atherosclerotic aortas of ApoE-deficient mice fed a Western diet for 10 and 20 weeks, corresponding to lesions of the fatty streak and fibrofatty plaque stage, respectively. Immunostaining for IL-15 localized to aortic smooth muscle cells in nonatherosclerotic C57BL/6 mice, whereas both macrophages and smooth muscle cells stained positive for IL-15 in atherosclerotic lesions of ApoE-deficient mice. Finally, advanced atherosclerotic lesions of human carotid arteries were immunostained to determine whether IL-15 is involved in human disease. IL-15 protein was present also in the human lesions with a distribution primarily overlapping that of macrophages. In conclusion, IL-15 is up-regulated in both human and animal atherosclerotic lesions and may contribute to the recruitment of T cells and their activation during atherogenesis.

Evidence that dendritic cells infiltrate atherosclerotic lesions in apolipoprotein E-deficient mice.

Earlier we reported that atherosclerotic lesions of apoE-deficient mice contained cells which stained positively with anti-S-100 antibody and that cells exhibiting the ultrastructural features of dendritic cells were present in the aortic lesions. These observations suggested that dendritic cells might be involved in mouse atherosclerosis. By employing DEC-205 and MIDC-8 antibodies specific for dendritic cells, the present study has established that dendritic cells indeed accumulate in atherosclerotic lesions of apoE-deficient mice. Finding dendritic cells infiltrating atherosclerotic lesions in apoE-deficient mice offers the possibility of investigating the migratory routes of dendritic cells and their involvement in T-cell activation.

Angiotensin II-induced hypertension accelerates the development of atherosclerosis in apoE-deficient mice.

Angiotensin II may contribute to the development and progression of atherosclerotic lesions because of its growth and proinflammatory effects. We sought to determine whether angiotensin II-induced hypertension would augment and accelerate the development of atherosclerotic lesions in apoE-deficient mice. Angiotensin II (0.7 mg x kg(-1) x d(-1) SC) was administered to apoE-deficient mice via osmotic minipumps. The animals were placed on either standard chow or an atherogenic diet. After 8 weeks, the mean atherosclerotic lesion area in the descending thoracic and abdominal aortas of animals on a standard chow diet was 0.4+/-0.1% compared with 5.2+/-1.2% in those animals maintained on an atherogenic diet (P:<0.0001). In angiotensin II-treated animals on standard chow, the mean lesion area was increased to 11.0+/-2.3%, which was further increased to 69.9+/-9.4% (P:<0.0001) in angiotensin II-treated animals on an atherogenic diet. Similar findings were obtained when tissues from the ascending aorta were analyzed. At 8 weeks in mice receiving a standard chow diet, angiotensin II dramatically increased the atherosclerotic lesion area by 840+/-83 microm(2) (P:<0.0001). Animals on a high-fat diet had a similar marked increase in lesion area in response to angiotensin II (217+/-19 microm(2), P:<0.0001). In contrast, when hypertension was induced with norepinephrine, only a modest effect on the atherosclerotic lesion area was observed. Angiotensin II-induced hypertension specifically increased the development of atherosclerosis in apoE knockout mice. This response was seen in animals receiving either standard chow or an atherogenic diet. These studies demonstrate the profound effect of angiotensin II on the development of atherosclerosis.

Corresponding distributions of increased endothelin-B receptor expression and increased endothelin-1 expression in the aorta of apolipoprotein E-deficient mice with advanced atherosclerosis.

Endothelin (ET)-1 causes proliferation of vascular smooth muscle cells (VSMC). Although it has been reported that stimulation of ET(B) receptors as well as ET(A) receptors promote proliferation of VSMC, the precise distribution of each receptor subtype in atherosclerotic vessels is unknown. Previous studies demonstrated that apolipoprotein E (apoE)-deficient mice have hypercholesterolaemia and develop severe atherosclerosis. To investigate the pathophysiological roles of vascular ET system in atherosclerosis, we examined preproET-1 messenger ribonucleic acid expression in the aorta of apoE-deficient mice, and performed immunohistochemical staining for ET-1 and each ET receptor subtype (ET(A) and ET(B) receptors) in the atherosclerotic lesions of these mice. The level of preproET-1 mRNA in the aorta was significantly higher in the apoE-deficient mice than in the control mice. Strong ET-1 staining was observed in the macrophage-foam cells, intimal and medial VSMC in the atherosclerotic lesions of the apoE-deficient mice. In addition, in the atherosclerotic lesions, strong ET(B) receptor staining was observed in the macrophage-foam cells, intimal and medial VSMC, which distribution corresponded closely to that of ET-1. ET(A) receptor staining was observed in the medial VSMC of both groups, but not in the macrophage-foam cells of the apoE-deficient mice. ET(A) receptor staining in the medial VSMC was stronger in the apoE-deficient mice than in the control mice. These results suggest that the vascular ET system, including ET-1 and ET receptors, is activated in the atherosclerotic lesions of apoE-deficient mice. Since the distribution of strong ET(B) receptor staining corresponded closely to that of ET-1, it is suggested that the ET system, mediated by ET(B) receptors, has an important role in the pathophysiology of the atherosclerotic lesions of apoE-deficient mice.

Iron and the Genetics of Cardiovascular Disease

The hypothesis that iron depletion protects against ischemic heart disease was proposed in 1981 as an explanation for the sex difference in heart disease rates.1 2 3 4 5 The “iron hypothesis” has generated significant debate, especially in the 1990s after publication of supportive findings from Finland.6 Confusion has been introduced into the debate by inappropriate study designs in tests of the hypothesis. A recurring weakness is that stored iron is measured by inappropriate methods. In addition, evidence against a particular mechanism may be erroneously taken as evidence against the core hypothesis. For example, in a particular study design, a finding of no association of stored iron with coronary atherosclerosis would not invalidate an association of stored iron with cardiovascular mortality. Recent studies relevant to the iron hypothesis have been reviewed.5 7 8 9 10 11 12 In 1994, Ascherio and Willett13 recognized that the iron hypothesis cannot be rejected on the basis of available data, stating that “[s]tronger evidence is needed before the hypothesis is rejected that greater iron stores increase the incidence of coronary heart disease or death from myocardial infarction.” Indeed, nothing in data made available since 1994 excludes the possibility that iron depletion has a large protective effect, large enough to explain the low incidence of myocardial infarction in menstruating women. A 1997 editorial14 on the iron hypothesis suggests that this once-controversial idea has become more acceptable to many scientists. Gillum14 noted that “this important hypothesis and related research fronts have led to many valuable studies like that of Kiechl et al [see Reference 99 ], which will greatly enhance our understanding of atherosclerotic vascular disease and may lead to innovations useful in clinical medicine and public health, perhaps in heretofore unanticipated ways.” Mechanisms of protection have not been …

Rapid regression of atherosclerosis induced by liver-directed gene transfer of ApoE in ApoE-deficient mice.

Apolipoprotein E (apoE) is a multifunctional protein synthesized by the liver and tissue macrophages. ApoE-deficient mice have severe hyperlipidemia and develop accelerated atherosclerosis on a chow diet. Both liver-derived and macrophage-derived apoEs have been shown to reduce plasma lipoprotein levels and slow the progression of atherosclerosis in apoE-deficient mice, but regression of atherosclerosis has not been demonstrated in this model. We utilized second-generation adenoviruses to achieve hepatic expression of human apoE in chow-fed, apoE-deficient mice with established atherosclerotic lesions of different stages. As expected, hepatic expression of human apoE3 significantly reduced plasma cholesterol levels. Liver-derived apoE also accumulated substantially within preexisting atherosclerotic lesions, indicating that plasma apoE gained access to the arterial intima. Hepatic expression of human apoE3 for 6 weeks resulted in significant quantitative regression of both early fatty streak lesions as well as advanced, complex lesions in both the aortic root and the aortic arch. In addition, hepatic expression of apoE induced substantial morphological changes in lesions, including decreased foam cells and increased smooth muscle cells and extracellular matrix content. In parallel, human apoE4 and apoE2 were also expressed in the liver by using recombinant adenoviruses. ApoE4 reduced cholesterol levels to the same extent as did apoE3 and also prevented progression but did not induce significant regression of preexisting lesions. ApoE2 reduced cholesterol levels to a lesser degree than did apoE3 and apoE4 and lesion progression was reduced, but regression was not induced. In summary, (1) regression of preexisting atherosclerotic lesions in apoE-deficient mice can be rapidly induced by hepatic expression of apoE, despite the absence of macrophage-derived apoE; (2) the morphological changes seen in this model of regression resemble those in other animal models, induced over longer periods of time; (3) liver-derived apoE gained access to and was retained by intimal atherosclerotic lesions; and (4) apoE4 was less effective in inducing regression, despite its effects on plasma lipoproteins that were similar to those of apoE3. The rapid regression of preexisiting atherosclerotic lesions induced by apoE gene transfer in apoE-deficient mice could provide a convenient murine model for investigation of the molecular events associated with atherosclerosis regression.

Adenoviral-mediated overexpression of tissue inhibitor of metalloproteinase-1 induces reduction in atherosclerotic lesions in apoe-deficient mice

Adenoviral-mediated overexpression of tissue inhibitor of metalloproteinase-1 induces reduction in atherosclerotic lesions in apoe-deficient mice

The emergence of mouse models of atherosclerosis and their relevance to clinical research.

Due to the ability to introduce or mutate genes, the mouse has become the most common experimental animal model for atherosclerosis research. Wildtype mice on a chow diet do not get atherosclerosis. Three ways to induce atherosclerosis in mice are discussed: diet-induced, apoE deficiency-induced, and LDL receptor-deficiency induced. The atherosclerotic lesions in apoE-deficient mice have been well characterized, and they resemble human lesions in their sites of predilection and progression to the fibroproliferative stage. These mouse models of atherosclerosis are being used to identify genes which modify atherosclerosis susceptibility and in the development of antiatherogenic therapies.

"Tall oil"-derived phytosterols reduce atherosclerosis in ApoE-deficient mice.

We investigated the effects of a "tall oil"-derived phytosterol mixture (TODPM) on the formation of atherosclerotic lesions in apoE-deficient mice. TODPM was added at 2% (wt/wt) to the chow of nine mice; the control group had six animals. The diet of all animals contained 9% (wt/wt) fat and 0.15% (wt/wt) cholesterol. After 4 weeks, plasma total cholesterol levels were significantly reduced in the TODPM-treated mice (26.6 versus 42.0 mmol/L, P < .0001). The mean body weight of the TODPM-supplemented group was significantly higher at week 5 and throughout the study (29.4 versus 27.7 g, P < .05). The experiment was terminated at 18 weeks. Histological examination showed mature atherosclerotic lesions composed of foam cells underlying the endothelium, a mosaic of extracellular glycosaminoglycans, numerous apparently proliferative smooth muscle cells, and foci of cholesterol clefts in the control animals. By contrast, the TODPM-treated mice showed only early lesions containing mainly superficial foam cells. As assessed by morphometry, the lesion area in the aortic sinuses of TODPM-treated animals was less than half that of control animals (P < .0001). This reduced lesion area was accompanied by a substantial reduction in all lesional components, reflecting a delay in progression of atheromatous changes. A strong positive correlation (r = .69, P < .01) was found between plasma total cholesterol levels and lesion area in the aortic sinuses. TODPM also prevented the occurrence of xanthomatosis. We conclude that supplementation of a cholesterol-enriched diet with TODPM significantly lowers plasma cholesterol and retards development of atherosclerosis in apoE-deficient mice, suggesting a therapeutic potential for the mixture of phytosterols studied.