Atherosclerosis in the apolipoprotein-E-deficient mouse: a decade of progress

Conjugated linoleic acid and atherosclerosis: studies in animal models

Conjugated linoleic acids (CLA) are isomeric forms of linoleic acid (LA) containing two conjugated sites of unsaturation. The most abundant dietary form of CLA is the cis-9,trans-11 (c-9,t-11) isomer that is found in the fatty tissues and milk of ruminant animals. CLA can also be acquired by ingestion of supplements, which are usually equimolar mixtures of the c-9,t-11 and t-10,c-12 CLA. For more than a decade, the potential for CLA to modify atherosclerosis in animal models has been examined. However, to date, the studies have failed to reach consensus on whether CLA can be effective in reducing the incidence or severity of atherosclerotic lesions, or whether or not plasma lipid and lipoprotein levels can be improved with CLA supplementation. This review will examine the evidence for and against a role for CLA in atherosclerosis, with a focus on the rabbit, the hamster, and the apoE-deficient mouse.

Role of hydrogen sulfide in the development of atherosclerotic lesions in apolipoprotein E knockout mice

OBJECTIVE

We explored the effect of hydrogen sulfide (H(2)S) on atherosclerotic progression, particularly on intracellular adhesion molecule-1 (ICAM-1) in apolipoprotein-E knockout (apoE(-/-)) mice and human umbilical vein endothelial cells (HUVECs).

METHODS AND RESULTS

ApoE(-/-) mice were treated with sodium hydrosulfide (NaHS) or DL-propargylglycine (PPG); HUVECs were pretreated with NaHS. Compared with control mice, apoE(-/-) mice showed decreased plasma H(2)S level and aortic H(2)S production but increased plasma ICAM-1 and aortic ICAM-1 protein and mRNA. Compared with apoE(-/-) mice, apoE(-/-)+NaHS mice showed increased plasma H(2)S level, but decreased size of atherosclerotic plaque and plasma and aortic ICAM-1 levels, whereas apoE(-/-)+PPG mice showed decreased plasma H(2)S level but enlarged plaque size and increased plasma and aortic ICAM-1 levels. NaHS suppressed ICAM-1 expression in tumor necrosis factor (TNF)-alpha-treated HUVECs. NaHS inhibited IkappaB degradation and NF-kappaB nuclear translocation in HUVECs treated with TNF-alpha.

CONCLUSIONS

The vascular CSE/H(2)S pathway was disturbed in apoE(-/-) mice. H(2)S exerted an antiatherogenic effect and inhibited ICAM-1 expression in apoE(-/-) mice. H(2)S inhibited ICAM-1 expression in TNF-alpha-induced HUVECs via the NF-kappaB pathway.

Overactive endocannabinoid signaling impairs apolipoprotein E-mediated clearance of triglyceride-rich lipoproteins

The endocannabinoid (EC) system regulates food intake and energy metabolism. Cannabinoid receptor type 1 (CB1) antagonists show promise in the treatment of obesity and its metabolic consequences. Although the reduction in adiposity resulting from therapy with CB1 antagonists may not account fully for the concomitant improvements in dyslipidemia, direct effects of overactive EC signaling on plasma lipoprotein metabolism have not been documented. The present study used a chemical approach to evaluate the direct effects of increased EC signaling in mice by inducing acute elevations of endogenously produced cannabinoids through pharmacological inhibition of their enzymatic hydrolysis by isopropyl dodecylfluorophosphonate (IDFP). Acute IDFP treatment increased plasma levels of triglyceride (TG) (2.0- to 3.1-fold) and cholesterol (1.3- to 1.4-fold) in conjunction with an accumulation in plasma of apolipoprotein (apo)E-depleted TG-rich lipoproteins. These changes did not occur in either CB1-null or apoE-null mice, were prevented by pretreatment with CB1 antagonists, and were not associated with reduced hepatic apoE gene expression. Although IDFP treatment increased hepatic mRNA levels of lipogenic genes (Srebp1 and Fas), there was no effect on TG secretion into plasma. Instead, IDFP treatment impaired clearance of an intravenously administered TG emulsion, despite increased postheparin lipoprotein lipase activity. Therefore, overactive EC signaling elicits an increase in plasma triglyceride levels associated with reduced plasma TG clearance and an accumulation in plasma of apoE-depleted TG-rich lipoproteins. These findings suggest a role of CB1 activation in the pathogenesis of obesity-related hypertriglyceridemia and underscore the potential efficacy of CB1 antagonists in treating metabolic disease.

Effects of social isolation and environmental enrichment on atherosclerosis in ApoE-/- mice

Social support and a stimulating environment have been suggested to reduce stress reactions and cardiovascular risk. The aim of this study was to assess the role of environmental enrichment and social interaction for development of atherosclerosis in atherosclerosis prone mice. Male ApoE-/- mice were divided into four groups and followed during 20 weeks: (i) enriched environment (E, n=12), (ii) deprived environment (ED, n=12), (iii) enriched environment with exercise (E-Ex, n=12) and (iv) socially deprived by individual housing (SD, n=10). Plasma lipid and cytokine concentrations were measured. Atherosclerosis was quantified in cross-sections of innominate artery and en face in thoracic aorta. Plaque area was significantly increased in SD mice in the innominate artery (P<0.05 vs. all other groups), but not in the thoracic aorta. Plasma lipids were increased in SD mice (P<0.001 vs. all for total cholesterol, P<0.05 vs. E and P<0.01 vs. ED for triglycerides). Plasma concentration of granulocyte-colony stimulating factor (G-CSF) was decreased in SD mice compared to E mice (P<0.05). Thus, social isolation increased atherosclerosis and plasma lipids in ApoE-/- mice. Reduction in plasma G-CSF levels may hamper endothelial regeneration in the atherosclerotic process. While environmental enrichment did not affect atherosclerosis, social isolation accelerated atherosclerosis.

Atherosclerosis development in apolipoprotein E-null mice deficient for CD69

AIMS

Atherosclerosis is a chronic inflammatory disease regulated by immune mechanisms. CD69 is a cell surface receptor rapidly induced after leukocyte activation at sites of chronic inflammation. Genetic disruption of CD69 in the mouse aggravates collagen-induced arthritis (CIA), and partial depletion of CD69-expressing cells with anti-CD69 monoclonal antibody (mAb) prevents CIA development in wild-type mice, suggesting that this receptor negatively modulates immune and inflammatory responses. It has been recently reported that CD69 is upregulated in a large subset of T cells in atherosclerosis-prone apolipoprotein E-null mice (apoE(-/-)). In this study, we investigated whether altering CD69 function affects atherosclerosis development.

METHODS AND RESULTS

We studied native and diet-induced atherosclerosis in apoE(-/-) and doubly deficient apoE(-/-)CD69(-/-) mice and performed expression studies in tissues and primary cells derived from these animals. Plasma cholesterol level was unaffected by CD69 genetic inactivation. Although this genetic manipulation led to an elevated production of interferon gamma and interleukin 10 by activated T cells, apoE(-/-) and apoE(-/-)CD69(-/-) mice fed control and high-fat diet exhibited atheromas of similar size and composition when analysed at different stages of the disease. Likewise, anti-CD69 mAb treatment had no effect on plasma cholesterol and atherosclerosis burden in fat-fed apoE(-/-) mice.

CONCLUSION

In contrast to previous studies highlighting the protective function of CD69 against CIA, an autoimmune inflammatory disease, our results rule out a significant role for CD69 against atherosclerosis in apoE(-/-) mice, an experimental disease model featuring a local inflammatory response triggered and sustained by alterations in lipid homeostasis.

Apple polyphenols and fibers attenuate atherosclerosis in apolipoprotein E-deficient mice

Atherosclerosis, which is closely linked to nutritional habits, is a major cause of mortality in Western countries. Most of the previous investigations carried out on health effects of apples have been focused on their capacity to lower lipid concentration as well as on their antioxidant effects. The aim of the present study was to investigate the antiatherosclerotic effects of apple polyphenols and fibers. A crude apple polyphenol extract and low-viscosity apple fibers isolated from cider apples were administered separately or in association with the diet of apo E-deficient mice. After 4 months of supplementation, lipemia and oxidative stress biomarkers were measured and atheroslerotic lesions assessed by histomorphometry. Total plasmatic cholesterol and triacylgycerol levels were not affected by supplementation, and hepatic cholesterol level was lower in the group supplemented with both fibers and polyphenols. Uric acid concentrations and antioxidant capacity (FRAP) in plasma were reduced in all groups supplemented with polyphenols or fibers. The mean lesion area was reduced by 17, 38, and 38%, respectively, for the polyphenol, fiber, and polyphenol + fiber groups. Apple constituents supplied at nutritional doses therefore limit the development of atherosclerotic lesions in the aorta of apo E-deficient mice. On the basis of the results, we hypothesize that apple fibers and polyphenols may play a role in preventing atherosclerosis disease by decreasing uric acid plasma level.

Human apolipoprotein E expression from mouse skeletal muscle by electrotransfer of nonviral DNA (plasmid) and pseudotyped recombinant adeno-associated virus (AAV2/7)

Plasma apolipoprotein E (apoE) has multiple atheroprotective actions. However, although liver-directed adenoviral gene transfer of apoE reverses hypercholesterolemia and inhibits atherogenesis in apoE-deficient (apoE(-/-)) mice, safety considerations have revived interest in nonviral DNA (plasmid) and nonpathogenic adeno-associated viral (AAV) vectors. Here, we assess the effectiveness of these two delivery vehicles by minimally invasive intramuscular injection. First, we constructed AAV2-based expression plasmids harboring human apoE3 cDNA, driven by two muscle-specific promoters (CK6 and C5-12) and one ubiquitous promoter (CAG); each efficiently expressed apoE3 in transfected cultured C2C12 mouse myoblasts, although muscle-specific promoters were active only in differentiated multinucleate myotubes. Second, a pilot study verified that electrotransfer of the CAG-driven plasmid (p.CAG.apoE3) into tibialis anterior muscles, pretreated with hyaluronidase, of apoE(-/-) mice significantly enhanced (p < 0.001) local intramuscular expression of apoE3. However, in a 7-day experiment, the CK6- and C5-12-driven plasmids produced less apoE3 in muscle than did p.CAG.apoE3 (0.61 +/- 0.38 and 0.45 +/- 0.38 vs. 13.38 +/- 7.46 microg of apoE3 per muscle, respectively), but plasma apoE3 levels were below our detection limit (<15 ng/ml) in all mice and did not reverse the hyperlipidemia. Finally, we showed that intramuscular injection of a cross-packaged AAV serotype 7 viral vector, expressing human apoE3 from the CAG promoter, resulted in increasing levels of apoE3 in plasma over 4 weeks, although the concentration reached (1.40 +/- 0.35 microg/ml) was just below the threshold level needed to reduce the hypercholesterolemia. We conclude that skeletal muscle can serve as an effective secretory platform to express the apoE3 transgene, but that improved gene transfer vectors are needed to achieve full therapeutic levels of plasma apoE3 protein.

Apolipoprotein E-/- mice have delayed skeletal muscle healing after hind limb ischemia-reperfusion

INTRODUCTION

Classic studies of limb ischemia-reperfusion injury have been performed using young healthy mice. However, patients with peripheral vascular disease are older and often exhibit metabolic derangements that may delay healing after revascularization. Mice with genetic deletion of apolipoprotein E (ApoE(-/-)) have been used as a model in various experimental scenarios of hypercholesterolemia. These experiments evaluated the inflammatory response and changes in skeletal muscle morphology during the acute and chronic phases of limb ischemia-reperfusion injury in aged ApoE(-/-) mice.

METHODS

Age-matched ApoE(-/-) and wild-type (Wt) mice underwent 1.5 hours of unilateral hind limb ischemia, followed by 1, 7, or 14 days of reperfusion (DR). Histologic analysis of skeletal muscle fiber injury was assessed at 1DR. Morphologic evidence of muscular fiber maturation was assessed at 14DR. Levels of MyoD and myogenin, markers of skeletal muscle differentiation, were assessed at 7 and 14DR using Western blots. Markers of inflammation, including myeloperoxidase, macrophage inflammatory protein-2 (MIP-2), monocyte chemotactic protein-1 (MCP-1), and osteopontin, were assayed using enzyme-linked immunosorbent assay and chemokine (C-C motif) receptor 2 (CCR2) using Western blots at 1, 7, and 14DR. After 1DR, tissue adenosine 5'-triphosphate (ATP) levels were measured to assess metabolic activity. Unpaired t test and Mann-Whitney test were used for comparisons.

RESULTS

Histologic evaluation of skeletal muscle after 1DR showed no difference in the degree of injury between Wt and ApoE(-/-) mice. However, at 14DR, ApoE(-/-) mice had higher percentage of immature muscle fibers than Wt mice. Myogenin level was lower in the ApoE(-/-) mice at 7DR. Injured skeletal muscle of ApoE(-/-) mice had lower levels of myeloperoxidase than Wt mice at 7 DR and higher levels of MCP-1 at 14DR. There was no difference in the levels of tissue ATP, MIP-2, osteopontin, or CCR2 at all experimental intervals.

CONCLUSION

Although there was no difference between the injured muscle of Wt and ApoE(-/-) mice during the acute phase of reperfusion, ApoE(-/-) mice showed delay in skeletal muscle healing during the chronic phase of reperfusion. This lag in muscle regeneration was associated with lower levels of myogenin at 7DR and an increased level of MCP-1 at 14DR in the ApoE(-/-) mice. The delay in skeletal muscle healing in the ApoE(-/-) mice may have broader implications for poor tissue healing and functional recovery in elderly patients who have vascular risk factors such as hypercholesterolemia.

Proteomic and Metabolomic Analysis of Smooth Muscle Cells Derived From the Arterial Media and Adventitial Progenitors of Apolipoprotein E–Deficient Mice

We have recently demonstrated that stem cell antigen 1–positive (Sca-1 + ) progenitors exist in the vascular adventitia of apolipoprotein E–deficient (apoE −/− ) mice and contribute to smooth muscle cell (SMC) accumulation in vein graft atherosclerosis. Using a combined proteomic and metabolomic approach, we now characterize these local progenitors, which participate in the formation of native atherosclerotic lesions in chow-fed apoE −/− mice. Unlike Sca-1 + progenitors from embryonic stem cells, the resident Sca-1 + stem cell population from the vasculature acquired a mature aortic SMC phenotype after platelet-derived growth factor-BB stimulation. It shared proteomic and metabolomic characteristics of apoE −/− SMCs, which were clearly distinct from wild-type SMCs under normoxic and hypoxic conditions. Among the differentially expressed proteins were key enzymes in glucose metabolism, resulting in faster glucose consumption and a compensatory reduction in baseline interleukin-6 secretion. The latter was associated with a marked upregulation of insulin-like growth factor binding proteins (IGFBPs) 3 and 6. Notably, reconstitution of interleukin-6 to levels measured in the conditioned medium of wild-type SMCs attenuated the elevated IGFBP expression in apoE −/− SMCs and their vascular progenitors. This coregulation of apoE, interleukin-6, and IGFBPs was replicated in wild-type SMCs from hypercholesterolemic mice and confirmed by silencing apoE expression in SMCs from normocholesterolemic mice. In summary, we provide evidence that Sca-1 + progenitors contribute to native atherosclerosis in apoE −/− mice, that apoE deficiency and hypercholesterolemia alter progenitor cell behavior, and that inflammatory cytokines such as interleukin-6 act as metabolic regulators in SMCs of hyperlipidemic mice.

Antiatherosclerotic effects of small-molecular-weight compounds enhancing endothelial nitric-oxide synthase (eNOS) expression and preventing eNOS uncoupling

Many cardiovascular diseases are associated with reduced levels of bioactive nitric oxide (NO) and an uncoupling of oxygen reduction from NO synthesis in endothelial NO synthase (eNOS uncoupling). In human endothelial EA.hy 926 cells, two small-molecular-weight compounds with related structures, 4-fluoro-N-indan-2-yl-benzamide (CAS no. 291756-32-6; empirical formula C16H14FNO; AVE9488) and 2,2-difluoro-benzo[1,3]dioxole-5-carboxylic acid indan-2-ylamide (CAS no. 450348-85-3; empirical formula C17H13F2NO3; AVE3085), enhanced eNOS promoter activity in a concentration-dependent manner; with the responsible cis-element localized within the proximal 263 base pairs of the promoter region. RNA interference-mediated knockdown of the transcription factor Sp1 significantly reduced the basal activity of eNOS promoter, but it did not prevent the transcription activation by the compounds. Enhanced transcription of eNOS by AVE9488 in primary human umbilical vein endothelial cells was associated with increased levels of eNOS mRNA and protein expression, as well as increased bradykinin-stimulated NO production. In both wild-type C57BL/6J mice and apolipoprotein E-knockout (apoE-KO) mice, treatment with AVE9488 resulted in enhanced vascular eNOS expression. In apoE-KO mice, but not in eNOS-knockout mice, treatment with AVE9488 reduced cuff-induced neointima formation. A 12-week treatment with AVE9488 or AVE3085 reduced atherosclerotic plaque formation in apoE-KO mice, but not in apoE/eNOS-double knockout mice. Aortas from apoE-KO mice showed a significant generation of reactive oxygen species. This was partly prevented by nitric-oxide inhibitor N(omega)-nitro-l-arginine methyl ester, indicating eNOS uncoupling. Treatment of mice with AVE9488 enhanced vascular content of the essential eNOS cofactor (6R)-5,6,7,8-tetrahydro-l-biopterin and reversed eNOS uncoupling. The combination of an up-regulated eNOS expression and a reversal of eNOS uncoupling is probably responsible for the observed vasoprotective properties of this new type of compounds.

Role of cathepsin K in structural changes in brachiocephalic artery during progression of atherosclerosis in apoE-deficient mice

Cathepsin K, a potent extracellular matrix degrading cysteine protease, has been linked to the pathogenesis of osteoporosis, arthritis, cardiovascular and respiratory diseases. Here, we report the effects of cathepsin K deficiency (ctsK-/-) on atherosclerotic plaque formation in brachiocephalic arteries in an aggressive atherosclerosis model using apoE-deficient mice on cholate-containing high fat diet (HFD). On this diet, apoE-/- mice displayed severe lesions with buried fibrous caps after 8 weeks, whereas the apoE-/-ctsK-/- mice revealed a significantly decreased number of buried fibrous caps accompanied by increased collagen content in plaque areas and fibrous cap thickness. After 16 weeks of HFD, ctsK-/- mice had smaller plaque areas and maintained the structure of the tunica media in terms of their smooth muscle cell content and elastic lamina integrity. Overall macrophage content in the tunica media was lower in ctsK-/- mice but higher in the plaque area after 8 weeks of HFD. Decreased apoptosis rates in atherosclerotic plaques in brachiocephalic arteries of cathepsin K-deficient indicated a lower level of inflammation. In conclusion, cathepsin K deficiency appears to increase lesion stability in brachiocephalic arteries by maintaining the integrity of the tunica media and by decreasing plaque vulnerability to rupture.

Microarray gene profiling of laser-captured cells: a new tool to study atherosclerosis in mice

Genetically modified mice susceptible to atherosclerosis are widely used in atherosclerosis research. Although the atherosclerotic lesions in these animals show similarities to those in humans, comprehensive expression profile analysis of these lesions is limited by their very small size. In this communication, we have developed an approach to analyze global gene expression in mouse lesions by a combination of (a) laser capture microdissection (LCM) to isolate RNA from specific lesions, and (b) an efficient RNA amplification method that reliably yields sufficient quantities of high quality cRNA for quantitative real-time PCR (qPCR), as well as for microarray analysis. The RNA passed multiple quality controls and the expression profile observed in lesional cells compared with the whole artery encompasses genes that are characteristic of a macrophage/foam cell population. We believe that this method represents a useful new tool for the unbiased analysis of global gene expression of specific sub-regions in atherosclerotic lesions in different rodent models.

Squalene in a sex-dependent manner modulates atherosclerotic lesion which correlates with hepatic fat content in apoE-knockout male mice

BACKGROUND

Squalene is an intermediate of cholesterol biosynthesis which can be obtained from the diet where it is abundant, for example, in olive oil. The effect of this isoprenoid on the development of atherosclerosis was investigated on apoE-knockout mice.

METHODS AND RESULTS

Two groups of animals, separated according to sex, were fed on standard chow diet: the control group receiving only vehicle and the second group an aqueous solution of squalene to provide a dose of 1g/kg/day in male and female mice. This treatment was maintained for 10 weeks. At the end of this period, plasma lipid parameters, oxidative stress markers and hepatic fat were measured as well as cross-sectional lesion area of aortic root in both groups. Data showed that in males squalene feeding reduced atherosclerotic lesion area independently of plasma lipids and activation of circulating monocytes. In contrast, squalene intake did not decrease lesion area in females, despite reducing plasma cholesterol and triglycerides, isoprostane and percentage of Mac-1 expressing white cells. In males, atherosclerotic lesion area was positively and significantly associated with hepatic fat content and the plasma triglycerides were also strongly associated with liver weight.

CONCLUSIONS

These results indicate that administration of squalene modulates lesion development in a gender specific manner, and that accumulation of hepatic fat by liver is highly correlated with lesion progression in males. Hence, squalene administration could be used as a safe alternative to correct hepatic steatosis and atherosclerosis particularly in males.

Ambient air particulate matter exposure and tissue factor expression in atherosclerosis

Recent studies have suggested a link between inhaled particulate matter (PM) exposure and atherogenesis. We investigated tissue factor (TF) expression with ambient fine particulate matter (diameter < 2.5 microm, PM(2.5)) exposure and in response to in vitro exposure to fine and ultrafine PM in cultured human bronchial epithelial cells, vascular smooth muscle cells (hSMCs), and monocytes. ApoE-/- mice, fed with normal chow (NC) or high-fat chow (HFC), were exposed to concentrated PM(2.5) or filtered air (FA) for 6 mo (6 h/day, 5 day/wk, n = 28). Following in vivo ultrasound bio-microscopy (UBM) assessment of plaque area, macrophage infiltration (CD68) and TF expression in the aorta were quantified. Cultured cells were incubated with size-fractionated PM from cascade impactors, or with standard reference PM material (SRM, number 1649a) and assayed for TF protein, mRNA, and activity. UBM-derived plaque areas were 7 +/- 1% larger in the PM(2.5)-HFC than the FA-HFC group (p = .04), but not significantly different between the PM(2.5)-NC and FA-NC groups (p = .07). Immunohistochemistry revealed increased TF (15 +/- 3% vs. 8 +/- 2%, p < .01) and macrophage infiltration (19 +/- 2% vs. 14 +/- 3%, p < .01) in the plaques of PM(2.5)-HFC compared with FA-HFC groups. Impactor-collected PM(2.5) and ultrafine particles consistently increased TF protein in bronchial epithelial cells, monocytes, and hSMCs. TF mRNA expression increased rapidly (within 1 h) in response to SRM PM. We conclude that in vivo and in vitro exposure to ambient air PM(2.5) induces TF expression.

Nanoparticle PET-CT imaging of macrophages in inflammatory atherosclerosis

BACKGROUND

Macrophages participate centrally in atherosclerosis, and macrophage markers (eg, CD68, MAC-3) correlate well with lesion severity and therapeutic modulation. On the basis of the avidity of lesional macrophages for polysaccharide-containing supramolecular structures such as nanoparticles, we have developed a new positron emission tomography (PET) agent with optimized pharmacokinetics to allow in vivo imaging at tracer concentrations.

METHODS AND RESULTS

A dextranated and DTPA-modified magnetofluorescent 20-nm nanoparticle was labeled with the PET tracer 64Cu (1 mCi/0.1 mg nanoparticles) to yield a PET, magnetic resonance, and optically detectable imaging agent. Peak PET activity 24 hours after intravenous injection into mice deficient in apolipoprotein E with experimental atherosclerosis mapped to areas of high plaque load identified by computed tomography such as the aortic root and arch and correlated with magnetic resonance and optical imaging. Accumulated dose in apolipoprotein E-deficient aortas determined by gamma counting was 260% and in carotids 392% of respective wild-type organs (P<0.05 both). Autoradiography of aortas demonstrated uptake of the agent into macrophage-rich atheromata identified by Oil Red O staining of lipid deposits. The novel nanoagent accumulated predominantly in macrophages as determined by fluorescence microscopy and flow cytometry of cells dissociated from aortas.

CONCLUSIONS

This report establishes the capability of a novel trimodality nanoparticle to directly detect macrophages in atherosclerotic plaques. Advantages include improved sensitivity; direct correlation of PET signal with an established biomarker (CD68); ability to readily quantify the PET signal, perform whole-body vascular surveys, and spatially localize and follow the trireporter by microscopy; and clinical translatability of the agent given similarities to magnetic resonance imaging probes in clinical trials.

Coronary artery disease progression is associated with increased resistance of hearts and myocytes to cardiac insults

OBJECTIVE

To investigate whether coronary artery disease alters vulnerability of hearts and myocytes to cardiac insults. To address this issue, we developed an experimental model of coronary artery disease.

DESIGN

Prospective, experimental study.

SETTING

University experimental research laboratories.

SUBJECTS

Apolipoprotein E knockout mice.

INTERVENTIONS

Male apolipoprotein E knockout mice, aged 8 wks, were fed either a normal or high-fat diet.

MEASUREMENTS AND MAIN RESULTS

High-fat feeding for 24 wks induced atherosclerosis in the coronary arteries, was associated with myocardial infarction, and produced evidence of myocardial metabolic anaerobic stress when compared with apolipoprotein E knockout mice fed normal diet. Myocytes and hearts from both groups had similar morphometric and hemodynamic characteristics. During global ischemia, hearts with coronary disease had shorter time to enter into rigor and developed greater ischemic contracture. They were markedly more resistant to reperfusion injury than nondiseased hearts, as shown by cardiac function, release of cardiac enzymes, and metabolic preservation. An increase in prosurvival signaling was detected in diseased hearts, as shown by a higher ratio of phospho-Akt/total Akt than in nondiseased hearts. Myocytes from diseased heart exposed to metabolic inhibition and reperfusion had fewer arrhythmias than myocytes from nondiseased heart. These differences are not due to high-fat feeding, as hearts of wild-type mice fed this diet were more, not less, vulnerable to cardiac insults.

CONCLUSION

This work suggests that chronic partial ischemia associated with progression of coronary artery disease preconditions myocytes and hearts against subsequent acute cardiac insults.

Tumor necrosis factor alpha and adalimumab differentially regulate CD36 expression in human monocytes

In chronic inflammatory diseases, such as rheumatoid arthritis, inflammation acts as an independent cardiovascular risk factor and the use of anti-inflammatory drugs, such as anti-tumor necrosis factor alpha (anti-TNFα), may decrease this risk. The phagocytosis of oxidized low density lipoproteins (LDLs) accumulated in the subendothelium by mononuclear cells influences atherosclerosis and depends on CD36 expression. We investigated the role of TNFα and adalimumab, a human anti-TNFα monoclonal antibody widely used in human pathology, in CD36 expression in human monocytes. Human monocytes were prepared by adherence from whole-blood buffy-coat fractions from healthy donors. CD36 expression was assessed by RT-PCR and flow cytometry, with various TNFα or adalimumab concentrations. Implication of peroxisome proliferator-activated receptor (PPAR)γ in the regulation of CD36 expression was assessed using specific inhibitor or gel shift assays. The impact of redox signaling was investigated using quantification of reactive oxygen species, antioxidant and a NADPH oxidase inhibitor. The F(ab')2 fragment of adalimumab was isolated and its effect was analyzed. TNFα inhibits both CD36 membrane expression and mRNA expression. This inhibition involves a reduction in PPARγ activation. In contrast, adalimumab increases both CD36 membrane expression and mRNA expression. This induction is independent of the Fc portion of adalimumab and involves redox signaling via NADPH oxidase activation. CD36 expression on human monocytes is inhibited by TNFα and independently increased by adalimumab. These data highlight that pro-inflammatory cytokines and their specific neutralization influence the expression of cellular receptors implicated in atherosclerosis. Further studies are needed to investigate the clinical implications of these results in accelerated atherosclerosis observed in rheumatoid arthritis.

Blocking thrombospondin-1/CD47 signaling alleviates deleterious effects of aging on tissue responses to ischemia

OBJECTIVE

Decreased blood flow secondary to peripheral vascular disease underlies a significant number of chronic diseases that account for the majority of morbidity and mortality among the elderly. Blood vessel diameter and blood flow are limited by the matricellular protein thrombospondin-1 (TSP1) through its ability to block responses to the endogenous vasodilator nitric oxide (NO). In this study we investigate the role TSP1 plays in regulating blood flow in the presence of advanced age and atherosclerotic vascular disease.

METHODS AND RESULTS

Mice lacking TSP1 or CD47 show minimal loss of their resistance to ischemic injury with age and increased preservation of tissue perfusion immediately after injury. Treatment of WT and apolipoprotein E-null mice using therapeutic agents that decrease CD47 or enhance NO levels reverses the deleterious effects of age- and diet-induced vasculopathy and results in significantly increased tissue survival in models of ischemia.

CONCLUSIONS

With increasing age and diet-induced atherosclerotic vascular disease, TSP1 and its receptor CD47 become more limiting for blood flow and tissue survival after ischemic injury. Drugs that limit TSP1/CD47 regulation of blood flow could improve outcomes from surgical interventions in the elderly and ameliorate vascular complications attendant to aging.

Important differences between human and mouse APOE gene promoters: limitation of mouse APOE model in studying Alzheimer's disease

Apolipoprotein E (ApoE), encoded by the apolipoprotein E gene (APOE), plays an important role in the pathogenesis of Alzheimer's disease (AD). The APOE epsilon4 variant is strongly associated with AD. APOE promoter polymorphisms have also been reported to associate with higher AD risk. Mouse models of APOE expression have long been used to study the pathogenesis of AD. Elucidating the role of the APOE gene in AD requires understanding of how its regulation differs between mouse and human APOE genes, and how the differences influence AD risk. We compared the structure and function of both the human APOE gene promoter (hAPOEP) and mouse APOE gene promoter (mAPOEP) regions. Homology is less than 40% at 180 bp or more upstream of the two species' transcription start site (TSS, +1). Functional analysis revealed both similarities and important differences between the two sequences, significantly affected by human versus rodent cell line origin. We likewise probed nuclear extracts from several cell lines of different origins (astrocytic, glial, and neuronal) and mouse brain with specific hAPOEP and mAPOEP fragments. Each fragment shared DNA-protein interactions with the other but, notably, also bound distinct factors, demonstrated by gel shift and southwestern analyses. We determined possible identities for these distinct factors. These results suggest that regulation of mouse and human APOE genes may be sufficiently unique to justify the use of both the human APOE promoter sequence in transgenic rodent models and non-rodent AD models for studying factors involved in AD pathogenesis.

Systems biology of macrophages

Cells and tissues function in context. Under a given growth or survival medium they perform tasks, replicate and die. Given a stimulus they respond by invoking myriad biomolecular networks that result in a specified cellular outcome. At any given instant it can be argued that the cell is in a "state" defined by its components--their concentrations and locations, the interactions between components--that are modulated in space and time, and the complex circuitry--that involves a large number of interacting networks and a snapshot of the dynamical processes--such as gene expression, cell cycle, transport of components, etc. At present, we can measure, using high and low throughput methods, several cellular components in a context-dependent manner and obtain a partial picture of cellular networks and dynamical processes. Are these measurements sufficient to answer important biological questions and help reconstruct a systems-level understanding of a mammalian cell? This chapter will address systems biology strategies developed to address this question and demonstrate the power of integration of diverse cellular data for answering interesting biological questions in macrophages. We will use this systems biology approach to address the following questions: (1) How good are macrophage cell lines in addressing phenotypic biology of primary macrophages? (2) How do signals associated with inflammatory molecules regulate gene transcription in macrophages? (3) How can we combine proteomic and other cellular measurements to characterize the repertoire of upstream signaling networks invoked by macrophages? (4) How do designed knockdowns of proteins influence cellular phenotypes?

Atherosclerosis and vascular aging as modifiers of tumor progression, angiogenesis, and responsiveness to therapy

It is rarely considered that age-related common vascular co-morbidities may affect therapeutic outcomes of antiangiogenic therapy in cancer. Indeed, the accepted model of human disease consists of 4- to 8-week-old (young) tumor-bearing, but otherwise healthy, experimental mice, yet human cancers are diagnosed and treated in later decades of life when atherosclerosis and vascular diseases are highly prevalent. Here we present evidence that tumor growth and angiogenesis are profoundly altered in mice affected by natural aging and with genetically induced atherosclerosis (in ApoE(-/-) mice). Thus, transplantable tumors (Lewis lung carcinoma and B16F1) grew at higher rates in young (4 to 8 weeks old) ApoE(+/+) and ApoE(-/-) nonatherosclerotic syngeneic recipients than in their old (12 to 18 months old) or atherosclerotic (old/ApoE(-/-)) counterparts. These age-related changes were paralleled by reduced tumor vascularity, lower expression of tumor endothelial marker 1, increased acute tumor hypoxia, depletion of circulating CD45(-)/VEGFR(+) cells, and impaired endothelial sprouting ex vivo. Exposure of tumor-bearing mice to metronomic therapy with cyclophosphamide exerted antimitotic effects on tumors in young hosts, but this effect was reduced in atherosclerotic mice. Collectively, our results suggest that vascular aging and disease may affect tumor progression, angiogenesis, and responses to therapy.

C-terminal apolipoprotein E-derived peptide, Ep1.B, displays anti-atherogenic activity

OBJECTIVE

Apolipoprotein E (ApoE) is a lipid transport protein with expanded functions in cellular responses to tissue injury, immune regulation and cell growth. ApoE directs vascular changes that contribute to arterial protection as evidenced by the fact that isoforms of ApoE and ApoE deficiency correlate closely with accelerated plaque growth. The N-terminus of the ApoE protein has well-characterized functions, displaying lipid-binding and anti-atherogenic activity, whereas the function of the C-terminus is only partially defined. We have assessed the effects of a 14 amino acid C-terminal ApoE peptide, termed Ep1.B (239-252), on intimal neoplasia in animal models. This peptide is a fragment of a naturally processed peptide (236-252) of murine ApoE.

METHODS AND RESULTS

Ep1.B injection reduced neointimal hyperplasia after vascular surgery in rats and mice. When given during early plaque progression in ApoE-deficient mice, Ep1.B injections also prevented plaque growth. Treatment with Ep1.B did not, however, reduce established plaque growth in older mice. Peptides with alanine substitution of amino acid 249, Ep1.N, and with complete sequence reversal, Ep1.R, did not consistently inhibit plaque growth.

CONCLUSION

A naturally processed C-terminal ApoE peptide, Ep1.B, has anti-atherogenic activity indicating a role for this naturally metabolized peptide in vascular wound healing and lipid homeostasis.

Apolipoprotein E-deficient lipoproteins induce foam cell formation by downregulation of lysosomal hydrolases in macrophages

Apolipoprotein E (apoE) deficiency has been suggested to induce foam cell formation. Using lipoproteins obtained from wild-type mice and apoE-deficient mice expressing apoB-48 but not apoB-100, we studied apoE-deficient lipoprotein-induced changes in lipoprotein catabolism and protein expression in mouse peritoneal macrophages (MPMs). Our data demonstrate that incubation of MPMs with apoE-deficient lipoproteins induced intracellular lipoprotein, cholesteryl ester, and triglyceride accumulation, which was associated with a time-related decline in apoE-deficient lipoprotein degradation in MPMs. Confocal microscopy analysis indicated that the accumulated lipids were localized in lysosomes. ApoE-deficient lipoproteins reduced the protein levels of lysosomal acid lipase, cathepsin B, and cation-dependent mannose 6 phosphate receptor (MPR46). Exogenous apoE reduced apoE-deficient lipoprotein-induced lipid accumulation and attenuated the suppressive effect of apoE-deficient lipoproteins on lysosomal hydrolase and MPR46 expression. Although oxidized lipoproteins also increased lipid contents in MPMs, exogenous apoE could not attenuate oxidized lipoprotein-induced lipid accumulation. Our in vivo studies also showed that feeding apoE-deficient mice a high-fat diet resulted in cholesteryl ester and triglyceride accumulation and reduced lysosomal hydrolase expression in MPMs. These data suggest that apoE-deficient lipoproteins increase cellular lipid contents through pathways different from those activated by oxidized lipoproteins and that reducing lysosomal hydrolases in macrophages might be a mechanism by which apoE-deficient lipoproteins result in intralysosomal lipoprotein accumulation, thereby inducing foam cell formation.

Biomarkers of atherosclerosis and the potential of MRI for the diagnosis of vulnerable plaque

Atherosclerosis is a chronic inflammatory vascular disease. As it is an inflammation process, many cellular and molecular events are involved at each step of the progression of atherosclerosis from an early fatty streak lesion to a highly dangerous rupture-prone plaque. Magnetic resonance imaging (MRI) is a well-established diagnostic tool for many kinds of chronic inflammation in various systems and organs, and recent improvements in spatial resolution and contrast strategies make it a promising technique for the characterization of inflammatory vessel walls. The first part of this review will briefly introduce the main cellular and molecular processes involved in atherosclerotic lesions; the second part will focus on the use of high-resolution MRI and present-generation contrast agents for plaque characterization; and the third part will present some recent and ongoing cellular and molecular MRI studies of atherosclerosis.

Mouse models for atherosclerosis and pharmaceutical modifiers

Atherosclerosis is a multifactorial highly-complex disease with numerous etiologies that work synergistically to promote lesion development. The ability to develop preventive and ameliorative treatments will depend on animal models that mimic the human subject metabolically and pathophysiologically and will develop lesions comparable to those in humans. The mouse is the most useful, economic, and valid model for studying atherosclerosis and exploring effective therapeutic approaches. Among the most widely used mouse models for atherosclerosis are apolipoprotein E-deficient (ApoE-/-) and LDL receptor-deficient (LDLr-/-) mice. An up-and-coming model is the ApoE*3Leiden (E3L) transgenic mouse. Here, we review studies that have explored how and to what extent these mice respond to compounds directed at treatment of the risk factors hypercholesterolemia, hypertriglyceridemia, hypertension, and inflammation. An important outcome of this survey is that the different models used may differ markedly from one another in their response to a specific experimental manipulation. The choice of a model is therefore of critical importance and should take into account the risk factor to be studied and the working spectrum of the compounds tested.

Oxidatively damaged DNA and inflammation in the liver of dyslipidemic ApoE-/- mice exposed to diesel exhaust particles

Epidemiological studies have shown that exposure to air pollution particles is associated with cardiovascular diseases, whereas the role in the initiation of atherosclerosis is unresolved. Atherosclerosis is considered to be an inflammatory disease that also involves oxidative stress. Here we investigated effects of oxidative stress elicited by diesel exhaust particles (DEP) in the aorta, liver, and lung of dyslipidemic ApoE(-/-) mice at the age when visual plaques appear in the aorta (11-13 weeks). DEP was administrated by intraperitoneal injection (0, 50, 500 and 5,000 microg DEP/kg bodyweight) in order to omit vascular effects secondary to pulmonary inflammation. The mice were killed either 6 or 24h after the administration. Inflammation was measured as the expression of inducible nitric oxide synthase (iNOS) and serum nitric oxide and DNA damage was measured by the comet assay. The expression of iNOS mRNA was increased in the liver 6h after the administration. The level of oxidized purine bases, determined as formamidopyrimidine DNA glycosylase sites was increased by 67% (95% CI: 11-124%) in the liver after 24h in the mice administrated with only 50 microg/kg bodyweight. However, there was no indication of systemic inflammation determined as the serum concentration of nitric oxide and iNOS expression, and DNA damage was not increased in the aorta. These observations indicate that intraperitoneal DEP injection does not induce inflammation or oxidatively damaged DNA in the lung and aorta, whereas a direct effect in terms of inflammation and oxidized DNA was observed in the liver of dyslipidemic ApoE(-/-) mice.

Swings in blood glucose levels accelerate atherogenesis in apolipoprotein E-deficient mice

The aim of this study was to investigate the effect of fluctuations in blood glucose levels on atherogenesis. Apolipoprotein (apo) E-deficient mice fed maltose twice daily were used as a model of repetitive postprandial glucose spikes. We investigated the number of macrophages adherent to the endothelium and the area of fibrotic arteriosclerotic lesions, with and without administration of miglitol, an alpha-glucosidase inhibitor. Macrophage adhesion to endothelial cells in thoracic aorta was quantitated by the en face method for optimal observation of endothelial surface after immunohistochemical staining for Mac-2. The area of arteriosclerotic lesions was measured in elastica van Giesson-stained proximal aorta. The number of adherent macrophages increased at 1 week after commencement of maltose feeding and the size of arteriosclerotic lesion increased at 5 weeks after such feeding. These increases were prevented by simultaneous use of miglitol. Our data demonstrated that glucose fluctuations accelerate atherogenesis. This was independent of changes in serum cholesterol level in vivo. Reduction of glucose fluctuation by alpha-glucosidase inhibitor efficiently controlled the progression of atherosclerosis.

Deficiency of Niemann-Pick C1 Like 1 Prevents Atherosclerosis in ApoE −/− Mice

Objective— The objective of this study was to determine whether the deficiency of Niemann-Pick C1 Like 1 (Npc1l1) prevents atherosclerosis in apoE null mice.

Methods and Results— Npc1l1 −/− /apoE null −/− mice were generated and found to have a significant reduction in cholesterol absorption (−77%) compared with wild-type or apoE −/− mice. Npc1l1/apoE −/− mice were fed a chow or Western diet for 24 weeks, then lipoprotein, hepatic, and biliary cholesterol, and atherosclerosis development was compared with apoE −/− , Npc1l1 −/− , wild-type, and ezetimibe-treated apoE −/− mice. Chylomicron remnant/ VLDL cholesterol levels were reduced 80% to 90% in both chow and Western diet-fed Npc1l1/apoE −/− mice relative to apoE −/− mice. Male Npc1l1 −/− and Npc1l1/apoE −/− mice were completely resistant to diet induced hypercholesterolemia, and both male and female mice were completely resistant to increases in hepatic and biliary cholesterol levels. Atherosclerosis was reduced 99% in aortic lesion surface area, 94% to 97% in innominate artery intimal lesion area, and >90% in aortic root lesion area in both male and female Npc1l1/apoE −/− mice relative to apoE −/− mice.

Conclusions— Lack of Npc1l1, the molecular target of the cholesterol absorption inhibitor ezetimibe, in apoE −/− mice results in a significant reduction in cholesterol absorption and plasma cholesterol levels, and causes a nearly complete protection from the development of atherosclerosis, under both cholesterol-fed and non-cholesterol-fed conditions.

Atherosclerosis and the protective role played by different proteins in apolipoprotein E-deficient mice

The aim of the present study was to analyze the early events in atherogenesis and the role of pro- or anti-atherosclerotic proteins in the development of atherosclerotic lesions. We used apolipoprotein E-deficient (E(0)) mice that spontaneously develop hypercholesterolemia and atherosclerotic lesions in the aorta in a time-dependent manner. Aortas of mice aged 6, 8, 10 and 12 weeks were examined to determine histopathological changes. In mice aged 8-12 weeks, developing atherosclerotic lesions were present in different regions of the aortas. These lesions protruded into the lumen of the vessel and showed lipid deposits, lipid-filled macrophages and extensive accumulation of collagen and elastic fibers throughout the entire arterial wall. A parallel immunohistochemical study included analysis of three proteins known to be involved in atherosclerosis, i.e. inducible nitric oxide synthase (iNOS, NOS2), vascular endothelial growth factor (VEGF) and matrix metalloproteinase-2 (MMP2). Increased immunolabelling of iNOS and VEGF accompanied atherosclerosis development in E(0) mice aged 8, 10 and 12 weeks. On the contrary, immunolabelling for MMP2 was negative in E(0) mice aged 10 and 12 weeks. Our results indicate morphological alterations in the Tunica intima and Tunica media of atherosclerotic aortas and possible protective roles for iNOS and VEGF proteins against atherosclerosis development. These data may be relevant for developing therapeutic strategies for atherosclerosis development.

Experimental models of thrombosis and atherosclerosis

Atherothrombosis is a complex disease which includes two different pathologies: atherosclerosis, the process of plaque formation in the arterial wall and thrombosis, the formation of a blood clot mostly at the site of a ruptured atherosclerotic lesion. Animal models for both pathologies have been useful to understand their aetiology and their evolution and they were used to evaluate the efficacy of new treatments. Numerous models to study venous and arterial thrombosis have been described. Thus in the rat, venous thrombosis induced by lesion/stasis, e.g. in the vena cava, and arterial thrombosis by lesioning of the vessel wall are frequently used. The resulting blood clot formation is measured either directly (weight of the thrombus) or indirectly (reduction in blood flow). More complex models have been developed in large animals such as dogs and pigs in order to examine coronary thrombosis; the principle always being the arterial lesion that causes the thrombus formation. The effect of the TP-receptor antagonist terutroban (S 18886) on different thrombosis models has been evaluated and this has allowed to conclude on the powerful anti-thrombotic effects of this agent and has contributed to its progression into clinical development. In the past the most frequently used model of atherosclerosis was the hypercholesterolemic rabbit; both plaque formation and its consequences on vascular, endothelial, function have been largely studied in this model. More recently genetically engineered mouse models of atherosclerosis have been introduced and they are now largely studied to characterize the disease and to evaluate new drugs. The two models mostly used are the ApoE(-/-) and the LDL receptor(-/-) mice. Studies with terutroban have illustrated that this TP-receptor antagonist prevents lesion formation in mouse and rabbit models illustrating its interesting anti-atherosclerotic properties and demonstrating the role played by endothelial TP-receptors in atherogenesis. In conclusion, experimental models to study atherosclerosis and thrombosis have been developed and used to study the etiology and the evolution of atherothrombotic disease. They have also been of great value to predict anti-thrombotic and/or anti-atherosclerotic properties of new substances such as terutroban, that may become novel treatments for this complex cardiovascular disease.

Conditional animal models for the study of lipid metabolism and lipid disorders

The advent of technologies that allow conditional mutagenesis has revolutionized our ability to explore gene functions and to establish animal models of human diseases. Both aspects have proven to be of particular importance in the study of lipid-related disorders. Classical approaches to gene inactivation by conventional gene targeting strategies have been successfully applied to generate animal models like the LDL receptor- and the apolipoprotein E-knockout mice, which are still widely used to study diverse aspects of atherosclerosis, lipid transport, and neurodegenerative disease. In many cases, however, simply inactivating the gene of interest has resulted in early lethal or complex phenotypes which are difficult to interpret. In recent years, additional tools have therefore been developed that allow the spatiotemporally controlled manipulation of the genome, as described in detail in Part I of this volume. Our aim is to provide an exemplary survey of the application of different conditional mutagenesis techniques in lipid research in order to illustrate their potential to unravel physiological functions of a broad range of genes involved in lipid homeostasis.

Hemodynamics in the mouse aortic arch as assessed by MRI, ultrasound, and numerical modeling

Mice are widely used to study arterial disease in humans, and the pathogenesis of arterial diseases is known to be strongly influenced by hemodynamic factors. It is, therefore, of interest to characterize the hemodynamic environment in the mouse arterial tree. Previous measurements have suggested that many relevant hemodynamic variables are similar between the mouse and the human. Here we use a combination of Doppler ultrasound and MRI measurements, coupled with numerical modeling techniques, to characterize the hemodynamic environment in the mouse aortic arch at high spatial resolution. We find that the hemodynamically induced stresses on arterial endothelial cells are much larger in magnitude and more spatially uniform in the mouse than in the human, an effect that can be explained by fluid mechanical scaling principles. This surprising finding seems to be at variance with currently accepted models of the role of hemodynamics in atherogenesis and the known distribution of atheromatous lesions in mice.

Detecting and assessing macrophages in vivo to evaluate atherosclerosis noninvasively using molecular MRI

We investigated the ability of targeted immunomicelles to detect and assess macrophages in atherosclerotic plaque using MRI in vivo. There is a large clinical need for a noninvasive tool to assess atherosclerosis from a molecular and cellular standpoint. Macrophages play a central role in atherosclerosis and are associated with plaques vulnerable to rupture. Therefore, macrophage scavenger receptor (MSR) was chosen as a target for molecular MRI. MSR-targeted immunomicelles, micelles, and gadolinium-diethyltriaminepentaacetic acid (DTPA) were tested in ApoE-/- and WT mice by using in vivo MRI. Confocal laser-scanning microscopy colocalization, macrophage immunostaining and MRI correlation, competitive inhibition, and various other analyses were performed. In vivo MRI revealed that at 24 h postinjection, immunomicelles provided a 79% increase in signal intensity of atherosclerotic aortas in ApoE-/- mice compared with only 34% using untargeted micelles and no enhancement using gadolinium-DTPA. Confocal laser-scanning microscopy revealed colocalization between fluorescent immunomicelles and macrophages in plaques. There was a strong correlation between macrophage content in atherosclerotic plaques and the matched in vivo MRI results as measured by the percent normalized enhancement ratio. Monoclonal antibodies to MSR were able to significantly hinder immunomicelles from providing contrast enhancement of atherosclerotic vessels in vivo. Immunomicelles provided excellent validated in vivo enhancement of atherosclerotic plaques. The enhancement seen is related to the macrophage content of the atherosclerotic vessel areas imaged. Immunomicelles may aid in the detection of high macrophage content associated with plaques vulnerable to rupture.

Inulin attenuates atherosclerosis in apolipoprotein E-deficient mice

Effects of different inulin-type fructan fractions were studied on atherosclerotic plaque formation in male apo E-deficient mice. Thirty-two mice were randomly divided into four groups and received either a semi-purified sucrose-based diet (control group), or diets in which sucrose was replaced in part by various inulin-type fructans (10 g/100 g): long-chain inulin, oligofructose, or an oligofructose-enriched inulin for 16 weeks. The presence of atherosclerotic plaques was assessed by histomorphometry in the aortic sinus. The apo E-deficient mice fed long-chain inulin or an oligofructose-enriched inulin had about 35 % and 25 % less atherosclerotic lesion area compared with the control group, respectively. Feeding long-chain inulin significantly reduced plasma cholesterol concentrations (P<0.001), and the three inulin-type fructans reduced triacylglycerol (TAG) concentrations compared with the control group (P<0.001). Both the long-chain inulin and an oligofructose-enriched inulin significantly lowered hepatic cholesterol concentrations compared with the control diet (P<0.05). Hepatic TAG concentrations were significantly lower in all three groups fed the fructan-supplemented diets v. the control group (P<0.0001). The results of the present study suggest that inhibition of atherosclerotic plaque formation is more potent in the presence of long-chain inulin, either alone or in combination with oligofructose (an oligofructose-enriched inulin), and that this probably is related to changes in lipid metabolism.

Prothrombotic Effects of Hyperhomocysteinemia and Hypercholesterolemia in ApoE-Deficient Mice

Objective— We tested the hypothesis that hyperhomocysteinemia and hypercholesterolemia promote arterial thrombosis in mice.

Methods and Results— Male apolipoprotein E ( Apoe )-deficient mice were fed one of four diets: control, hyperhomocysteinemic (HH), high fat (HF), or high fat/hyperhomocysteinemic (HF/HH). Total cholesterol was elevated 2-fold with the HF or HF/HH diets compared with the control or HH diets ( P <0.001). Plasma total homocysteine (tHcy) was elevated (12 to 15 μmol/L) with the HH or HF/HH diets compared with the control or HF diets (4 to 6 μmol/L; P <0.001). Aortic sinus lesion area correlated strongly with total cholesterol ( P <0.001) but was independent of tHcy. At 12 weeks of age, the time to thrombotic occlusion of the carotid artery after photochemical injury was >50% shorter in mice fed the HF diets, with or without hyperhomocysteinemia, compared with the control diet ( P <0.05). At 24 weeks of age, carotid artery thrombosis was also accelerated in mice fed the HH diet ( P <0.05). Endothelium-dependent nitric oxide–mediated relaxation of carotid artery rings was impaired in mice fed the HF, HH, or HF/HH diets compared with the control diet ( P <0.05).

Conclusions— Hyperhomocysteinemia and hypercholesterolemia, alone or in combination, produce endothelial dysfunction and increased susceptibility to thrombosis in Apoe-deficient mice.

A mouse model for human atherosclerosis: long-term histopathological study of lesion development in the aortic arch of apolipoprotein E-deficient (E0) mice

Apolipoprotein E-deficient mice, since their introduction in the early 1990s, have proved to be a very popular model for studying spontaneous hypercholesterolemia and the subsequent development of atherosclerotic lesions. The pathogenesis of atherosclerotic lesions in these mice mimics that found in humans on a very short time-scale. Atherosclerotic lesion development is especially prominent in the aortic arch. We have followed the progressive histopathological development of atherosclerotic lesions in the aortic arch of apolipoprotein E-deficient mice aged from 6 weeks to 18 months in 1 microm epoxy-resin sections stained with alkaline toluidine blue, which gives greatly improved resolution over wax sections. During the early stages of lesion formation, lipid-filled macrophages appear in the subendothelium, and accumulate leading to "fatty streaks". Macrophage degeneration and the formation of lipid pools are accompanied by accumulation of cholesterol deposits. Disruptions of elastic laminae of the Tunica media are accompanied by structural changes in the myocytes. More advanced lesions involve fibrous cap development, calcification of the vessel wall and progressive occlusion of the lumen. Unstable plaque may also be found. Various approaches for quantitative determination of lesion size are considered. The study provides a histopathological baseline for spontaneous atherosclerosis associated with hypercholesterolemia, which can be used in connection with experimental interventional studies on the efficacy of drugs or foodstuffs in retardation of atherosclerosis.

Selective effect of conjugated linoleic acid isomers on atherosclerotic lesion development in apolipoprotein E knockout mice

Research suggests that conjugated linoleic acid (CLA) may inhibit atherosclerosis, but there are contradictory results in different animal models fed heterogeneous mixtures of CLA isomers. This study addressed the hypothesis that the individual CLA isomers may exert different atherogenic properties. ApoE(-/-) mice were fed isocaloric, isonitrogenous westernized diets containing 0.15% cholesterol and enriched with 1% (w/w) cis-9,trans-11-CLA (c9,t11-CLA), trans-10,cis-12-CLA (t10,c12-CLA) or linoleic acid (control diet) for 12 weeks. At the end of the dietary intervention, the effects of CLA isomers on the development of atherosclerotic vascular lesions, lipid metabolism, inflammation and oxidative stress were assessed. The t10,c12-CLA diet had a profound pro-atherogenic effect, whereas c9,t11-CLA impeded the development of atherosclerosis. En face aortic lesion assessment showed more dorsal and lumbar extensions presenting atherosclerotic foci after the t10,c12-CLA diet. Furthermore, animals fed t10,c12-CLA had pronounced hyperlipidemia, higher 8-iso-prostaglandin F(2alpha) levels, higher vulnerable atherosclerotic plaque with a lower smooth muscle and fibre contents and higher macrophage content and activation, assayed as plasma chitotriosidase compared to the control or c9,t11-CLA dietary groups. Plasma chitotriosidase activity was more closely associated with the extent of the plaque than with MOMA staining or than monocyte chemoattractant protein-1 levels. Our results demonstrate that CLA isomers differentially modulate the development of atherosclerosis, c9,t11-CLA impedes, whereas t10,c12-CLA promotes atherosclerosis. These opposing effects may be ascribed to divergent effects on lipid, oxidative, inflammatory and fibro muscular components of this pathology. Plasma chitotriosidase is a better indicator of dietary fat interventions that alter plaque monocyte activity in this murine model.

The cis-9,trans-11 isomer of conjugated linoleic acid (CLA) lowers plasma triglyceride and raises HDL cholesterol concentrations but does not suppress aortic atherosclerosis in diabetic apoE-deficient mice

OBJECTIVE

Reduction in atherosclerosis has been reported in experimental animals fed mixtures of conjugated linoleic acid (CLA). In this study, the major naturally occurring CLA isomer (cis-9,trans-11) was tested in an atherosclerosis-prone mouse model.

METHODS

In a model of insulin deficient apoE deficient mice, 16 animals were fed for 20 weeks with supplemental CLA (09.%, w/w) and compared with a similar number of mice of this phenotype. A control comparison was made of metabolic changes in non-diabetic apoE deficient mice that develop little atherosclerosis over 20 weeks. At 20 weeks, plasma lipids were measured and aortic atherosclerosis quantified by Sudan staining in the arch, thoracic and abdominal segments.

RESULTS

The diabetic apoE deficient mice developed marked dyslipidemia, primarily as cholesterol-enriched chylomicron and VLDL-sized lipoproteins and atherosclerosis in the aortic arch. However, there were no significant differences between CLA fed and non-CLA fed mice in either phenotype in plasma cholesterol concentration (in diabetic: 29.4+/-7.7 and 29.5+/-5.9 mmol/L, respectively) or in the area of aortic arch atherosclerosis (in diabetic: 24.8+/-10.3 and 27.6+/-7.7%, respectively). However, among diabetic mice the triglyceride concentration in triglyceride-rich lipoproteins was significantly lower in those fed CLA (for plasma 2.2+/-0.8 to 1.1+/-0.3 mmol/L; P<0.001), a significant difference that was seen also in the non-diabetic mice in which HDL cholesterol increased significantly with CLA (0.35+/-0.12-0.56+/-0.15 mmol/L).

CONCLUSION

In this atherosclerosis-prone model, the diabetic apoE deficient mouse, supplemental 0.9% CLA (cis-9,trans-11) failed to reduce the severity of aortic atherosclerosis, although plasma triglyceride concentration was substantially lowered and HDL cholesterol raised.

Mechanisms for cellular cholesterol transport: defects and human disease

This review summarizes the mechanisms of cellular cholesterol transport and monogenic human diseases caused by defects in intracellular cholesterol processing. In addition, selected mouse models of disturbed cholesterol trafficking are discussed. Current pharmacological strategies to prevent atherosclerosis are largely based on altering cellular cholesterol balance and are introduced in this context. Finally, because of the organizing potential of cholesterol in membranes, disturbances in cellular cholesterol transport have implications for a wide variety of human diseases, of which selected examples are given.

Homozygous disruption of Pctp modulates atherosclerosis in apolipoprotein E-deficient mice

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic phospholipid binding protein and a member of the steroidogenic acute regulatory-related transfer domain superfamily. Its tissue distribution includes liver and macrophages. PC-TP regulates hepatic lipid metabolism, and its absence in cholesterol-loaded macrophages is associated with reduced ATP binding cassette transporter A1-mediated lipid efflux and increased susceptibility to apoptosis induced by unesterified cholesterol. To explore a role for PC-TP in atherosclerosis, we prepared PC-TP-deficient/apolipoprotein E-deficient (Pctp(-/-)/Apoe(-/-)) mice and littermate Apoe(-/-) controls. At 16 weeks, atherosclerosis was increased in chow-fed male, but not female, Pctp(-/-)/Apoe(-/-) mice. This effect was associated with increases in plasma lipid concentrations. By contrast, no differences in atherosclerosis were observed between male or female Pctp(-/-)/Apoe(-/-) mice and Apoe(-/-) controls fed a Western-type diet for 16 weeks. At 24 weeks, atherosclerosis in chow-fed male Pctp(-/-)/Apoe(-/-) mice tended to be reduced in proportion to plasma cholesterol. The attenuation of atherosclerosis in female Pctp(-/-)/Apoe(-/-) mice fed chow or the Western-type diet for 24 weeks was not attributable to changes in plasma cholesterol or triglyceride concentrations. These findings suggest that PC-TP modulates the development of atherosclerosis, in part by regulating plasma lipid concentrations.

Xanthine oxidase inhibitor tungsten prevents the development of atherosclerosis in ApoE knockout mice fed a Western-type diet

Hyperlipidemia enhances xanthine oxidase (XO) activity. XO is an important source of reactive oxygen species (ROS). Since ROS are thought to promote atherosclerosis, we hypothesized that XO is involved in the development of atherosclerosis. ApoE(-/-) mice were fed a Western-type (WD) or control diet. In subgroups, tungsten (700 mg/L) was administered to inhibit XO. XO is a secreted enzyme which is formed in the liver as xanthine dehydrogenase (XDH) and binds to the vascular endothelium. High expression of XDH was found in the liver and WD increased liver XDH mRNA and XDH protein expression. WD induced the conversion of XDH to the radical-forming XO. Moreover, WD increased the hepatic expression of CD40, demonstrating activation of hepatic cells. Aortic tissue of ApoE(-/-) mice fed a WD for 6 months exhibited marked atherosclerosis, attenuated endothelium-dependent relaxation to acetylcholine, increased vascular oxidative stress, and mRNA expression of the chemokine KC. Tungsten treatment had no effect on plasma lipids but lowered the plasma XO activity. In animals fed a control diet, tungsten had no effect on radical formation, endothelial function, or atherosclerosis development. In mice fed a WD, however tungsten attenuated the vascular superoxide anion formation, prevented endothelial dysfunction, and attenuated KC mRNA expression. Most importantly, tungsten treatment largely prevented the development of atherosclerosis in the aorta of ApoE(-/-) mice on WD. Therefore, tungsten, potentially via the inhibition of XO, prevents the development of endothelial dysfunction and atherosclerosis in ApoE(-/-) mice on WD.

Noninvasive vascular cell adhesion molecule-1 imaging identifies inflammatory activation of cells in atherosclerosis

BACKGROUND

Noninvasive imaging of adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1) may identify early stages of inflammation in atherosclerosis. We hypothesized that a novel, second-generation VCAM-1-targeted agent with enhanced affinity had sufficient sensitivity to enable real-time detection of VCAM-1 expression in experimental atherosclerosis in vivo, to quantify pharmacotherapy-induced reductions in VCAM-1 expression, and to identify activated cells in human plaques.

METHODS AND RESULTS

In vivo phage display in apolipoprotein E-deficient mice identified a linear peptide affinity ligand, VHPKQHR, homologous to very late antigen-4, a known ligand for VCAM-1. This peptide was developed into a multivalent agent detectable by MRI and optical imaging (denoted VINP-28 for VCAM-1 internalizing nanoparticle 28, with 20 times higher affinity than previously reported for VNP). In vitro, VINP-28 targeted all cell types expressing VCAM-1. In vivo, MRI and optical imaging in apolipoprotein E-deficient mice (n=28) after injection with VINP-28 or saline revealed signal enhancement in the aortic root of mice receiving VINP-28 (P<0.05). VINP-28 colocalized with endothelial cells and other VCAM-1-expressing cells, eg, macrophages, and was spatially distinct compared with untargeted control nanoparticles. Atheromata of atorvastatin-treated mice showed reduced VINP-28 deposition and VCAM-1 expression. VINP-28 enhanced early lesions in juvenile mice and resected human carotid artery plaques.

CONCLUSIONS

VINP-28 allows noninvasive imaging of VCAM-1-expressing endothelial cells and macrophages in atherosclerosis and spatial monitoring of anti-VCAM-1 pharmacotherapy in vivo and identifies inflammatory cells in human atheromata. This clinically translatable agent could noninvasively detect inflammation in early, subclinical atherosclerosis.

A 1,3-diacylglycerol-rich oil induces less atherosclerosis and lowers plasma cholesterol in diabetic apoE-deficient mice

OBJECTIVE

Recent studies have demonstrated that 1,3-diacylglycerol (1,3-DAG) has several metabolic advantages over triacylglycerol (TAG) in humans and in animal models despite both oils having a similar fatty acid composition. In our current study, we have examined the effects of long-term feeding of a 1,3-DAG-rich oil on the dyslipidemia and atherosclerosis in the experimental model of the diabetic apolipoprotein E (apoE)-deficient mouse that develops accelerated atherosclerosis.

METHODS AND RESULTS

Diets containing 1,3-DAG-rich oil or TAG oil were administered to control non-diabetic apoE-dificient and diabetic apoE-deficient mice for 20 weeks. In diabetic apoE-deficient mice, 1,3-DAG reduced the extent of atherosclerotic lesions in the aortic arch and thoracic aorta by 37 and 44%, respectively, compared to TAG. Further, in diabetic apoE-deficient mice, plasma total cholesterol and triglyceride levels were significantly lower in the 1,3-DAG-fed group than in the TAG-fed group. This occurred partially through an apparent reduction in the size of triglyceride-rich lipoproteins but not apparently by reducing the number of lipoprotein particles. By contrast the control non-diabetic apoE-deficient mice showed no differential responses to the type of oil at least over 20 weeks.

CONCLUSIONS

We have demonstrated that dietary 1,3-DAG-rich oil reduced atherosclerosis in diabetic apoE-deficient mice, and was associated with reduction in plasma cholesterol especially within larger triglyceride-rich lipoproteins.

Histopathological Examination and Analysis of Mortality in DBA/2 Mouse Vasculitis Induced with CAWS, a Water-soluble Extracellular Polysaccharide Fraction Obtained from Candida albicans

CAWS, a water-soluble extracellular polysaccharide fraction obtained from the culture supernatant of Candida albicans, is one of the fungal pathogen-associated molecular patterns (PAMPs). It has been reported to show potent activity inducing arteritis and coronaritis in mice. Especially, CAWS-induced arteritis has a 100% incidence and severe mortality in the DBA/2 mouse strain. This artificial vasculitis was reported to provide a good murine model of Kawasaki disease and other inflammatory vascular disease. However, severe mortality was observed only in DBA/2 mice, which is a CAWS-sensitive strain. In this study, to clarify the mechanisms of CAWS-induced arteritis and mortality, we investigated microscopic histopathological changes in cardiovascular tissues in DBA/2 mice. Severe inflammatory infiltration was observed from the external elastic lamina in the aorta and proximal coronary arteries within 1 week after CAWS administration. Severe stenosis of the aorta and coronary arteries was observed more than 3 weeks after CAWS administration. Fibrinoid necrosis was observed in these vessel walls. All CAWS-treated mice died between the fifth and twelfth week after administration. Severe inflammatory change with aortic valve transformation suggested that CAWS-treated mice died of valvular endocarditis or cardiac dysfunction. Based on the simple induction method and complete incidence, these data suggest that CAWS-induced arteritis is a good model of not only Kawasaki disease but also other cardiovascular diseases such as valvular endocarditis.

Connexin37 protects against atherosclerosis by regulating monocyte adhesion

A genetic polymorphism in the human gene encoding connexin37 (CX37, encoded by GJA4, also known as CX37) has been reported as a potential prognostic marker for atherosclerosis. The expression of this gap-junction protein is altered in mouse and human atherosclerotic lesions: it disappears from the endothelium of advanced plaques but is detected in macrophages recruited to the lesions. The role of CX37 in atherogenesis, however, remains unknown. Here we have investigated the effect of deleting the mouse connexin37 (Cx37) gene (Gja4, also known as Cx37) on atherosclerosis in apolipoprotein E-deficient (Apoe(-/-)) mice, an animal model of this disease. We find that Gja4(-/-)Apoe(-/-) mice develop more aortic lesions than Gja4(+/+)Apoe(-/-) mice that express Cx37. Using in vivo adoptive transfer, we show that monocyte and macrophage recruitment is enhanced by eliminating expression of Cx37 in these leukocytes but not by eliminating its expression in the endothelium. We further show that Cx37 hemichannel activity in primary monocytes, macrophages and a macrophage cell line (H36.12j) inhibits leukocyte adhesion. This antiadhesive effect is mediated by release of ATP into the extracellular space. Thus, Cx37 hemichannels may control initiation of the development of atherosclerotic plaques by regulating monocyte adhesion. H36.12j macrophages expressing either of the two CX37 proteins encoded by a polymorphism in the human GJA4 gene show differential ATP-dependent adhesion. These results provide a potential mechanism by which a polymorphism in CX37 protects against atherosclerosis.