Influence of hypercholesterolemia and adventitial inflammation on the development of aortic aneurysm in rabbits

Aortic aneurysms: current pathogenesis and therapeutic targets

Aortic aneurysm is a chronic disease characterized by localized expansion of the aorta, including the ascending aorta, arch, descending aorta, and abdominal aorta. Although aortic aneurysms are generally asymptomatic, they can threaten human health by sudden death due to aortic rupture. Aortic aneurysms are estimated to lead to 150,000 ~ 200,000 deaths per year worldwide. Currently, there are no effective drugs to prevent the growth or rupture of aortic aneurysms; surgical repair or endovascular repair is the only option for treating this condition. The pathogenic mechanisms and therapeutic targets for aortic aneurysms have been examined over the past decade; however, there are unknown pathogenic mechanisms involved in cellular heterogeneity and plasticity, the complexity of the transforming growth factor-β signaling pathway, inflammation, cell death, intramural neovascularization, and intercellular communication. This review summarizes the latest research findings and current pathogenic mechanisms of aortic aneurysms, which may enhance our understanding of aortic aneurysms.

The mechanism and therapy of aortic aneurysms

Aortic aneurysm is a chronic aortic disease affected by many factors. Although it is generally asymptomatic, it poses a significant threat to human life due to a high risk of rupture. Because of its strong concealment, it is difficult to diagnose the disease in the early stage. At present, there are no effective drugs for the treatment of aneurysms. Surgical intervention and endovascular treatment are the only therapies. Although current studies have discovered that inflammatory responses as well as the production and activation of various proteases promote aortic aneurysm, the specific mechanisms remain unclear. Researchers are further exploring the pathogenesis of aneurysms to find new targets for diagnosis and treatment. To better understand aortic aneurysm, this review elaborates on the discovery history of aortic aneurysm, main classification and clinical manifestations, related molecular mechanisms, clinical cohort studies and animal models, with the ultimate goal of providing insights into the treatment of this devastating disease. The underlying problem with aneurysm disease is weakening of the aortic wall, leading to progressive dilation. If not treated in time, the aortic aneurysm eventually ruptures. An aortic aneurysm is a local enlargement of an artery caused by a weakening of the aortic wall. The disease is usually asymptomatic but leads to high mortality due to the risk of artery rupture.

Macrophage elastase derived from adventitial macrophages modulates aortic remodeling

Abdominal aortic aneurysm (AAA) is pathologically characterized by intimal atherosclerosis, disruption and attenuation of the elastic media, and adventitial inflammatory infiltrates. Although all these pathological events are possibly involved in the pathogenesis of AAA, the functional roles contributed by adventitial inflammatory macrophages have not been fully documented. Recent studies have revealed that increased expression of matrix metalloproteinase-12 (MMP-12) derived from macrophages may be particularly important in the pathogenesis of both atherosclerosis and AAA. In the current study, we developed a carrageenan-induced abdominal aortic adventitial inflammatory model in hypercholesterolemic rabbits and evaluated the effect of adventitial macrophage accumulation on the aortic remodeling with special reference to the influence of increased expression of MMP-12. To accomplish this, we compared the carrageenan-induced aortic lesions of transgenic (Tg) rabbits that expressed high levels of MMP-12 in the macrophage lineage to those of non-Tg rabbits. We found that the aortic medial and adventitial lesions of Tg rabbits were greater in degree than those of non-Tg rabbits, with the increased infiltration of macrophages and prominent destruction of elastic lamellae accompanied by the frequent appearance of dilated lesions, while the intimal lesions were slightly increased. Enhanced aortic lesions in Tg rabbits were focally associated with increased dilation of the aortic lumens. RT-PCR and Western blotting revealed high levels of MMP-12 in the lesions of Tg rabbits that were accompanied by elevated levels of MMP-2 and -3, which was caused by increased number of macrophages. Our results suggest that adventitial inflammation constitutes a major stimulus to aortic remodeling and increased expression of MMP-12 secreted from adventitial macrophages plays an important role in the pathogenesis of vascular diseases such as AAA.

Mechanisms of abdominal aortic aneurysm progression: A review

Abdominal aortic aneurysm (AAA) is a common disease associated with significant cardiovascular morbidity and mortality. Up to now, there is still controversy on the choice of treatment method of AAA. Even so, the mechanisms of AAA progression are poorly defined, making targeting new therapies problematic. Current evidence favors an interaction of the hemodynamic microenvironment with local and systemic immune responses. In this review, we aim to provide an update of mechanisms in AAA progression, involving hemodynamics, perivascular adipose tissue, adventitial fibroblasts, vasa vasorum remodeling, intraluminal thrombus, and distribution of macrophage subtypes.

Meibum lipid composition in type 2 diabetics with dry eye

PURPOSE

The purpose of this investigation was to analyze and compare the composition of meibum between type 2 diabetics with dry eye disease (DED) and control subjects to better reveal the pathologic mechanisms of the meibomian gland degeneration (MGD) and DED in type 2 diabetes mellitus (T2DM).

METHODS

90 subjects were divided into the following 4 groups: DM-DED group: T2DM patients with DED (n = 30); DM control group: DM patients without DED (n = 18); DED group: DED patients without DM (n = 26); naive control group: normal subjects (n = 16). The lipid composition of meibum samples collected from these subjects was analyzed by high-pressure liquid chromatography-mass spectrometry (HPLC-MS) system. The content of lipid features from 12 major lipid classes was compared among the 4 groups.

RESULTS

A significantly lower level of triacylglycerols (TG) and wax esters (WE) was found between DM-DED patients and normal controls (P < 0.01), whereas the level of Cholesteryl Ester (CE) in DM-DED patients increased compared with DED patients (P < 0.05). The level of (O-acyl)-omega-hydroxy fatty acids (OAHFA) in DM-DED patients was significantly lower than that in normal controls (P < 0.01). An opposite higher level of phospholipids (PLs) was observed in DM-DED patients than that in normal controls (P < 0.01).

CONCLUSIONS

T2DM could influence the expression of meibum lipids to further aggravate DED and MGD. Lower expression of TG,WE and OAHFA, higher expression of CE and PLs were discovered in meibum lipids of T2DM-DED.

Pathophysiology of Aortic Aneurysms: Insights from Animal Studies

Aortic aneurysms are defined as dilations of the aorta greater than 50 percent. Currently, the only effective treatment for aortic aneurysms is surgical repair, which is recommended only to those that meet criteria. There is no available pharmaceutical therapy to slow aneurysm growth and thus prevent lethal rupture. The development of a number of murine models has allowed in depth studies of various cellular and extracellular components of aneurysm pathophysiology. The identification of key therapeutic targets has resulted in several clinical trials evaluating pharmaceutical candidates to treat aneurysm progression. In this review, we focus on providing recent updates on developments in murine models of aortic aneurysm. In addition, we discuss recent studies of the various cellular and extracellular components of the aorta along with the abutting aortic structures that contribute to aneurysm development and progression.

SPECT/CT imaging of apoptosis in aortic aneurysm with radiolabeled duramycin

The objective of this research was to estimate whether a ^[99mTc]duramycin probe can be used for apoptosis imaging in patients with aortic aneurysm (AA). Vascular smooth muscle cell (SMC) apoptosis has an important influence on AA development. Thus, non-invasive imaging of SMC apoptosis may be able to evaluate AA progress and risk stratification. SMCs were treated with hydrogen peroxide (H₂O₂; 200 μΜ) or culture medium as a control. Apoptosis was measured using flow cytometry and ^[99mTc]duramycin to detect the binding efficiency to apoptotic SMCs. C57/BL6 mice were administered angiotensin-II and beta-aminopropionitrile (BAPN) subcutaneously to establish an AA model, or saline for controls. Aortic specimens underwent pathological evaluation and their aortic diameters were measured after 6 weeks. Micro-SPECT/CT scanning of ^[99mTc]duramycin and ¹⁸F-FDG PET detection were performed. SMCs treated with H₂O₂ showed more apoptosis compared with the control group (67.2 ± 3.8% vs. 16.1 ± 0.6%, P < 0.01). The experimental group showed a high rate of AA formation (70%) compared with no AA formation in the control group. The average aorta diameter was higher and ^[99mTc]duramycin uptake at the AA site was higher in the experimental group compared with the control group. Compared with the normal aorta in the control group, AA in experiment group had more severe medial degeneration, elastic fiber reduction and fracture, and collagen degeneration. TUNEL staining verified the higher apoptosis rate at the AA site in experiment group compared with the control group (63.9 ± 3.7% in ascending AA, 66.4 ± 4.0% in thoracic AA, vs. 3.5 ± 0.3% in normal aorta, P < 0.01). ^[99mTc]Duramycin may be an effective probe to evaluate apoptosis in AA.

Liposomal Nanotherapy for Treatment of Atherosclerosis

Atherosclerosis is a chronic disease that can lead to life-threatening events such as myocardial infarction and stroke, is characterized by the build-up of lipids and immune cells within the arterial wall. It is understood that inflammation is a hallmark of atherosclerosis and can be a target for therapy. In support of this concept, an injectable nanoliposomal formulation encapsulating fluocinolone acetonide (FA), a corticosteroid, is developed that allows for drug delivery to atherosclerotic plaques while reducing the systemic exposure to off-target tissues. In this study, FA is successfully incorporated into liposomal nanocarriers of around 100 nm in size with loading efficiency of 90% and the formulation exhibits sustained release up to 25 d. The anti-inflammatory effect and cholesterol efflux capability of FA-liposomes are demonstrated in vitro. In vivo studies carried out with an apolipoprotein E-knockout (Apoe^-/- ) mouse model of atherosclerosis show accumulation of liposomes in atherosclerotic plaques, colocalization with plaque macrophages and anti-atherogenic effect over 3 weeks of treatment. This FA-liposomal-based nanocarrier represents a novel potent nanotherapeutic option for atherosclerosis.

Hyperlipidemia induces meibomian gland dysfunction

PURPOSE

To investigate the pathological changes of the meibomian gland (MG) and ocular surface in Apolipoprotein E knockout (ApoE^-/-) mice and to investigate the association of meibomian gland dysfunction (MGD) with hyperlipidemia.

METHODS

Total plasma cholesterol was measured in different ages of ApoE^-/- and wild type (WT) mice, whilst the ocular surfaces were observed by slit-lamp biomicroscopy. MG sections were subjected to H&E staining, Oil Red O staining, TUNEL assay and immunostaining. Quantitate RT-PCR and Western blot analyses were performed to detect the relative gene expression in MGs. The 5-month-old ApoE^-/- mice were administered with rosiglitazone or GW9662 + rosiglitazone via oral gavage for 2 months to determine their effect on MG pathological change.

RESULTS

We found eyelid abnormality, MG dropout, abnormal MG acinar morphology, dilated MG duct and plugging of the MG orifice in ApoE^-/- mice. MG acini in ApoE^-/- mice showed exaggerated lipid accumulation. Abnormal keratinization increased in MG duct, accompanied with decreased proliferation and increased apoptosis in ApoE^-/- mice. Inflammatory cells infiltrated into the surrounding microenvironment of MG acini, and the NF-κB signaling pathway was activated in MG acinar cells. Oxidative stress was evident in MG acinar cells of ApoE^-/- mice. Further investigation showed downregulation of PPAR-γ in MG acinar cells of ApoE^-/- mice. PPAR-γ agonist rosiglitazone treatment reduced the morbidity of eyelid, as well as corneal pathological changes and MG inflammation in ApoE^-/- mice.

CONCLUSION

MGD and hyperlipidemia are closely associated in ApoE^-/- mice, which represent a new model to study the pathophysiology of MGD related to dyslipidemia.

Comprehensive intravascular imaging of atherosclerotic plaque in vivo using optical coherence tomography and fluorescence lifetime imaging

Comprehensive imaging of both the structural and biochemical characteristics of atherosclerotic plaque is essential for the diagnosis and study of coronary artery disease because both a plaque's morphology and its biochemical composition affect the level of risk it poses. Optical coherence tomography (OCT) and fluorescence lifetime imaging (FLIm) are promising optical imaging methods for characterizing coronary artery plaques morphologically and biochemically, respectively. In this study, we present a hybrid intravascular imaging device, including a custom-built OCT/FLIm system, a hybrid optical rotary joint, and an imaging catheter, to visualize the structure and biochemical composition of the plaque in an atherosclerotic rabbit artery in vivo. Especially, the autofluorescence lifetime of the endogenous tissue molecules can be used to characterize the biochemical composition; thus no exogenous contrast agent is required. Also, the physical properties of the imaging catheter and the imaging procedures are similar to those already used clinically, facilitating rapid translation into clinical use. This new intravascular imaging catheter can open up new opportunities for clinicians and researchers to investigate and diagnose coronary artery disease by simultaneously providing tissue microstructure and biochemical composition data in vivo without the use of exogenous contrast agent.

Rabbit aortic aneurysm model with enlarging diameter capable of better mimicking human aortic aneurysm disease

The self-healing phenomenon can be found in the elastase-induced abdominal aortic aneurysm (AAA) model, and an enlarging AAA model was successfully induced by coarctation. Unfortunately, aortic coarctation in these enlarging models is generally not found in human AAA disease. This study aimed to create an experiment model of enlarging AAA in rabbits to better mimic human aortic aneurysm disease. Eighty-four male New Zealand white rabbits were randomly divided into three equal groups: two aneurysm groups (A and B) and a SHAM group. Aneurysm group rabbits underwent extrinsic aortic stenosis below the right renal artery and received a 10-minute incubation of 60 μl elastase (1 unit/μl). Absorbable suture was used in Group A and nonabsorbable cotton thread was used in Group B. A sham operation was performed in the SHAM group. Aortic diameter was measured after 1, 3, 7, and 15 weeks; thereafter animals were sacrificed for histopathological, immunohistochemical and quantitative studies. Two rabbits died at 29 and 48 days, respectively, after operation in Group B. All aneurysms formed and enlarged progressively by 3 weeks in the Aneurysm groups. However, diameter enlargement in Group A was significantly lower than that in Group B at 7 weeks. Aneurysm groups developed intimal hyperplasia; intima-media thickness (IMT) increased significantly by week 7, and aortic media thickness and intima-media ratio (IMR) increased significantly by week 15. Marked destruction of elastin fibers and smooth muscle cells (SMCs) occurred 1 week later and increased progressively thereafter. Intimal hyperplasia and SMCs content in Group A increased significantly by week 15 compared with Group B. Aneurysm groups exhibited strong expression of matrix metalloproteinases 2 and 9 and RAM11 by week 1, and decreased progressively thereafter. In conclusion, this novel rabbit AAA model enlarges progressively without coarctation and is capable of better mimicking human aortic aneurysm disease.

Animal Models in the Research of Abdominal Aortic Aneurysms Development

Abdominal aortic aneurysm (AAA) is a prevalent and potentially life threatening disease. Many animal models have been developed to simulate the natural history of the disease or test preclinical endovascular devices and surgical procedures. The aim of this review is to describe different methods of AAA induction in animal models and report on the effectiveness of the methods described in inducing an analogue of a human AAA. The PubMed database was searched for publications with titles containing the following terms “animal” or ‘‘animal model(s)’’ and keywords “research”, “aneurysm(s)’’, “aorta”, “pancreatic elastase’’, “Angiotensin”, “AngII” “calcium chloride” or “CaCl2”. Starting date for this search was set to 2004, since previously bibliography was already covered by the review of Daugherty and Cassis (2004). We focused on animal studies that reported a model of aneurysm development and progression. A number of different approaches of AAA induction in animal models has been developed, used and combined since the first report in the 1960’s. Although specific methods are successful in AAA induction in animal models, it is necessary that these methods and their respective results are in line with the pathophysiology and the mechanisms involved in human AAA development. A researcher should know the advantages/disadvantages of each animal model and choose the appropriate model.

Carbonic anhydrase IX deposits are associated with increased ascending aortic dilatation

OBJECTIVES

Carbonic anhydrase IX (CA IX) expression is induced by local hypoxia. We studied whether CA IX deposits associate with ascending aortic dilatation.

DESIGN

Aortic wall histology, CA IX expression, presence of leukocytes, plasma cells, macrophages, endothelial cells, smooth muscle cells, cell proliferation, elastin and collagen were studied in histological specimens collected from 30 patients who underwent surgery for ascending aorta. The samples were grouped according to presence of CA IX deposits.

RESULTS

Twenty out of 30 patients had CA IX-positive deposits within the adventitia, whereas 10 specimens remained negative. Adventitial inflammation was increased in CA IX-positive samples as compared with CA IX-negative ones (p < 0.01). The mean diameter of the ascending aorta at the sinotubular junction increased significantly in patients with CA IX-positive staining as compared with CA IX-negative cases (63 ± 3 vs 53 ± 2 mm, p < 0.02). Receiver operating characteristic curve analysis confirmed the association of CA IX positivity with increased ascending aortic dilatation (AUC 0.766; S.E. 0.090; p = 0.020; 95% C.I. 0.590-0.941).

CONCLUSIONS

Positive CA IX staining in certain aortic specimens suggests that increased CA activity may contribute to ascending aortic dilatation.

An update on the etiology of abdominal aortic aneurysms: implications for future diagnostic testing

Abdominal aortic aneurysm (AAA) disease is multifactorial with both environmental and genetic risk factors. The current research in AAA revolves around genetic profiles and expression studies in both human and animal models. Variants in genes involved in extracellular matrix degradation, inflammation, the renin-angiotensin system, cell growth and proliferation and lipid metabolism have been associated with AAA using a variety of study designs. However, the results have been inconsistent and without a standard animal model for validation. Thus, despite the growing body of knowledge, the specific variants responsible for AAA development, progression and rupture have yet to be determined. This review explores some of the more significant genetic studies to provide an overview of past studies that have influenced the current understanding of AAA etiology. Expanding our understanding of disease pathogenesis will inform research into novel diagnostics and therapeutics and ultimately to improve outcomes for patients with AAA.

Recent Advances in the Development of Experimental Animal Models Mimicking Human Aortic Aneurysms

Aortic aneurysm is a common and life-threatening disease that can cause death from rupture. Current therapeutic options are limited to surgical or endovascular procedures because no pharmacological approaches have been proven to decrease the chance of expansion or rupture. The best approach to the management of aortic aneurysm would be the understanding and prevention of the processes involved in disease occurrence, progression, and rupture. There is a need for animal models that can reproduce the pathophysiological features of human aortic aneurysm, and several such models have been studied. This review will emphasize recent advances in animal models used in the determination of mechanisms and treatments of aortic aneurysms.

Different long-term outcomes of abdominal aortic aneurysm and intracranial aneurysm models: hemodynamic change may also play an essential role in the initiation and progression of abdominal aortic aneurysm in rabbits

Self-healing phenomenon was found in the periarterial elastase-induced abdominal aortic aneurysm (AAA) in rabbit. This kind of aneurysm model does not progress and heals spontaneously in the long term, which is quite different from the performance of AAA disease in human. In order to better mimic human AAA and overcome this shortcoming of traditional AAA model in rabbit, we studied the pathogenesis of cerebral aneurysm (CA) model in small animal, which shows an excellent long-term patency and progressive enlargement. We found that hemodynamic conditions, such as turbulence flow, high blood flow, and shear stress, play an important role in the formation and progression of CA. So, we hypothesize that hemodynamic change may also play an essential role in the initiation and progression of rabbit AAA, and self-healing will be overcome if hemodynamic condition changes by coarctation of infra-renal aorta after elastase incubation.

Reproducible porcine model of thoracic aortic aneurysm

BACKGROUND

Thoracic aortic aneurysms (TAAs) develop secondary to abnormal aortic extracellular matrix remodeling, resulting in a weakened and dilated aortic wall that progressed to rupture if left unattended. Currently, no diagnostic/prognostic tests are available for the detection of TAA disease. This is largely driven by the lack of a large animal model, which would permit longitudinal/mechanistic studies. Accordingly, the objective of the present study was to establish a reproducible porcine model of aortic dilatation, which recapitulates the structural and biochemical changes observed during human TAA development.

METHODS AND RESULTS

Descending TAAs were induced in Yorkshire pigs (20-25 kg; n=7) through intra-adventitial injections of collagenase (5 mL, 0.35 mg/mL) and periadventitial application of crystalline CaCl2 (0.5 g). Three weeks after TAA induction, aortas were harvested and tissue was collected for biochemical and histological measurements. A subset of animals underwent MRI preoperatively and at terminal surgery. Results were compared with sham-operated controls (n=6). Three weeks after TAA induction, aortic luminal area increased by 38 ± 13% (P=0.018 versus control). Aortic structural changes included elastic lamellar degradation and decreased collagen content. The protein abundance of matrix metalloproteinases 3, 8, 9, and 12 increased in TAA tissue homogenates, whereas tissue inhibitors of metalloproteinases 1 and 4 decreased.

CONCLUSIONS

These data demonstrate aortic dilatation, aortic medial degeneration, and alterations in matrix metalloproteinase/tissue inhibitors of metalloproteinase abundance, consistent with TAA formation. This study establishes for the first time a large animal model of TAA that recapitulates the hallmarks of human disease and provides a reproducible test bed for examining diagnostic, prognostic, and therapeutic strategies.

Micromanaging abdominal aortic aneurysms

The contribution of abdominal aortic aneurysm (AAA) disease to human morbidity and mortality has increased in the aging, industrialized world. In response, extraordinary efforts have been launched to determine the molecular and pathophysiological characteristics of the diseased aorta. This work aims to develop novel diagnostic and therapeutic strategies to limit AAA expansion and, ultimately, rupture. Contributions from multiple research groups have uncovered a complex transcriptional and post-transcriptional regulatory milieu, which is believed to be essential for maintaining aortic vascular homeostasis. Recently, novel small noncoding RNAs, called microRNAs, have been identified as important transcriptional and post-transcriptional inhibitors of gene expression. MicroRNAs are thought to "fine tune" the translational output of their target messenger RNAs (mRNAs) by promoting mRNA degradation or inhibiting translation. With the discovery that microRNAs act as powerful regulators in the context of a wide variety of diseases, it is only logical that microRNAs be thoroughly explored as potential therapeutic entities. This current review summarizes interesting findings regarding the intriguing roles and benefits of microRNA expression modulation during AAA initiation and propagation. These studies utilize disease-relevant murine models, as well as human tissue from patients undergoing surgical aortic aneurysm repair. Furthermore, we critically examine future therapeutic strategies with regard to their clinical and translational feasibility.

Development of a novel rabbit model of abdominal aortic aneurysm via a combination of periaortic calcium chloride and elastase incubation

Background

The purpose of this study was to introduce a novel, simple and effective technique for creating a reliable rabbit model of abdominal aortic aneurysm (AAA) via a combination of periaortic calcium chloride (CaCl₂) and elastase incubation.

Methods

Forty-eight New Zealand white rabbits were divided into four groups. The AAA model was developed via a 20-minute periaortic incubation of CaCl₂ (0.5 mol/L) and elastase (1 Unit/µL) in a 1.5-cm aortic segment (Group CE). A single incubation of CaCl₂ (Group C) or elastase (Group E) and a sham operation group (Sham Group) were used for the controls. Diameter was measured by serial digital subtraction angiography imaging on days 5, 15 and 30. Animals were sacrificed on day 5 and day 30 for histopathological and immunohistochemical studies.

Results

All animals in Group CE developed aneurysm, with an average dilation ratio of 65.3%±8.9% on day 5, 86.5%±28.7% on day 15 and 203.6%±39.1% on day 30. No aneurysm was found in Group C, and only one aneurysm was seen on day 5 in Group E. Group CE exhibited less intima-media thickness, endothelial recovery, elastin and smooth muscle cell (SMC) content, but stronger expression of matrix metalloproteinase-2, matrix metalloproteinase-9 and RAM11 compared to the controls.

Conclusions

The novel rabbit model of AAA created by using a combination of periaortic CaCl₂ and elastase incubation is simple and effective to perform and is valuable for elucidating AAA mechanisms and therapeutic interventions in experimental studies.

Novel mechanisms of abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are a common but asymptomatic disease that has high susceptibility to rupture. Current therapeutic options are limited to surgical procedures because no pharmacological approaches have been proven to decrease either expansion or rupture of human AAAs. The current dearth of effective medical treatment is attributed to insufficient understanding of the mechanisms underlying the initiation, propagation and rupture of AAAs. This review will emphasize recent advances in mechanistic studies that may provide insights into potential pharmacological treatments for this disease. While we primarily focus on recent salient findings, we also discuss mechanisms that continue to be controversial depending on models under study. Despite the progress on exploring mechanisms of experimental AAAs, ultimate validation of mechanisms will require completion of prospective double-blinded clinical trials. In addition, we advocate increased emphasis of collaborative studies using animal models and human tissues for determination of mechanisms that explore expansion and rupture of existing AAAs.

Balanced mineralization in the arterial system: possible role of osteoclastogenesis/osteoblastogenesis in abdominal aortic aneurysm and stenotic disease

Arterial calcification is the result of the same highly organized processes as seen in bone, which rely on a delicate balance between osteoblasts and osteoclasts. Although previously understood as passive precipitation, evidence has accumulated to suggest that arterial calcification is the result of organized, regulated processes bearing many similarities to osteogenesis in bone, including the presence of subpopulations of arterial wall cells that retain osteoblastic lineage potential. These cells have the potential to form mineralized nodules and express osteoblast markers, including bone morphogenetic protein-2, osteocalcin, osteopontin, and alkaline phosphatase. By contrast, osteoclast-like cells mediate the catabolic process of mineral resorption. Recent data shows that cells positive for tartrate-resistant acid phosphatase, a major marker for osteoclasts, have been histologically identified in atherosclerotic lesions and are referred to as osteoclast-like cells. Evidence has accumulated to suggest that initial arterial calcification through passive precipitation of calcium phosphate initiates balanced mineralization regulated by osteoclast-like and osteoblast-like cells. Subsequently, various pathogenic conditions may trigger an imbalance between osteoblastogenesis and osteoclastogenesis, leading to either calcification in stenotic/occlusive disease or destruction of the extracellular matrix in aneurysmal disease. Further elucidation of these newly emerging concepts could lead to a novel therapeutic approach to arterial stenotic/occlusive disease and/or abdominal aortic aneurysm.

The calcium chloride-induced rodent model of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) affects ∼5% men aged over 65 years and is an important cause of death in this population. Research into AAA pathogenesis has been fuelled by the need to identify new diagnostic biomarkers and therapeutic targets for this disease. One animal model of AAA involves peri-vascular application of calcium chloride (CaCl(2)) onto the infra-renal aorta of mice and rats to induce extracellular matrix remodelling. Twenty-three studies assessing CaCl(2)-induced AAA and six studies assessing AAA induced by a modified CaCl(2) method were identified. In the current report the preparation and pathological features of this AAA model are discussed. We also compared this animal model to human AAA. CaCl(2)-induced AAA shows the following pathological characteristics typically found in human AAA: calcification, inflammatory cell infiltration, oxidative stress, neovascularisation, elastin degradation and vascular smooth muscle cell apoptosis. A number of mechanisms involved in CaCl(2)-induced AAA have been identified which may be relevant to the pathogenesis of human AAA. Key molecules include c-Jun N-terminal kinase, peroxisome proliferator-activated receptor-γ, chemokine (C-C motif) receptor 2, group x secretory phospholipase A2 and plasminogen. CaCl(2)-induced AAA does not display aortic thrombus, atherosclerosis and rupture which are classical features of human AAA. Advantages of the CaCl(2)-induced AAA technique include (1) it can be applied to wild type mice making assessment of transgenic rodent models more straight forward and rapid; and (2) CaCl(2)-induced AAAs are usually developed in the infra-renal abdominal aorta, which is the most common location of human AAA. Currently findings obtained from the CaCl(2)-induced AAA model or other animal models of AAA have not been translated into the human situation. It is hoped that this deficiency will be corrected over the next decade with a number of clinical trials currently examining novel treatment options for AAA patients.

The role of lysyl oxidase family members in the stabilization of abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are a major cause of morbidity and mortality in the United States today. We employed a model for AAA development using apolipoprotein E knock out mice fed a high-fat diet and treated with ANG II and β-aminopropionitrile (β-APN) for 4 wk. ANG II induces hypertension and atherosclerotic disease, whereas β-APN inhibits the activity of the lysyl oxidase/ lysyl oxidase-like protein (LOX/LOXL) family members. LOX/LOXL family members crosslink collagen and elastin in the extracellular matrix and therefore contribute to the integrity and stabilization of a healthy vessel wall. In this model, cotreatment with ANG II and β-APN caused a 90% AAA incidence and increased atherosclerotic lesion formation from less than 5% to greater than 25% after 4 wk. In more atheroprotected mouse strains (C57BL/6 and BalbC), cotreatment with ANG II and β-APN caused 50% and 40% AAA incidence, respectively. These data demonstrate the importance of LOX/LOXL to the stability of the vessel wall. Therapeutic strategies to overexpress LOX/LOXL enzymes or to support the crosslinking of soluble matrix proteins in a polymeric scaffold are a promising opportunity to achieve stabilization of AAAs.

Accelerated aneurysmal dilation associated with apoptosis and inflammation in a newly developed calcium phosphate rodent abdominal aortic aneurysm model

OBJECTIVE

The calcium chloride (CaCl(2)) model is a widely accepted rodent model for abdominal aortic aneurysms (AAAs). Calcium deposition, mainly consisting of calcium phosphate (CaPO(4)) crystals, has been reported to exist in human and experimental aneurysms. CaPO(4) crystals have been used for in vitro DNA transfection by mixing CaCl(2) and phosphate-buffered saline (PBS). Here, we describe accelerated aneurysm formation resulting from a modification of the CaCl(2) model.

METHODS

A modified CaCl(2) model, the CaPO(4) model, was created by applying PBS onto the mouse infrarenal aorta after CaCl(2) treatment. Morphologic, histologic, and immunohistochemical analyses were performed on arteries treated with the CaPO(4) model and the conventional CaCl(2) model as the control. In vitro methods were performed using a mixture of CaCl(2) and PBS to create CaPO(4) crystals. CaPO(4)- induced apoptosis of primary cultured mouse vascular smooth muscle cells (VSMCs) was measured by DNA fragmentation enzyme-linked immunosorbent assay.

RESULTS

The CaPO(4) model produces AAA, defined as an increase of ≥50% in the diameter of the aorta, faster than in the CaCl(2) model. The CaPO(4) model showed significantly larger aneurysmal dilation at 7, 28, and 42 days, as reflected by a maximum diameter (measured in mm) fold-change of 1.69 ± 0.07, 1.99 ± 0.14, and 2.13 ± 0.09 vs 1.22 ± 0.04, 1.48 ± 0.07, and 1.68 ± 0.06 in a CaCl(2) model, respectively (n = 6; P < .05). A semiquantitative grading analysis of elastin fiber integrity at 7 days revealed a significant increase in elastin degradation in the CaPO(4) model compared with the CaCl(2) model (2.7 ± 0.2 vs 1.5 ± 0.2; n = 6; P < .05). A significantly higher level of apoptosis occurred in the CaPO(4) model (apoptosis index at 1, 2, and 3 days postsurgery: 0.26 ± 0.14, 0.37 ± 0.14, and 0.33 ± 0.08 vs 0.012 ± 0.10, 0.15 ± 0.02, and 0.12 ± 0.05 in the conventional CaCl(2) model; n = 3; P < .05). An enhancement of macrophage infiltration and calcification was also observed at 3 and 7 days in the CaPO(4) model. CaPO(4) induced approximately 3.7 times more apoptosis in VSMCs than a mixture of CaCl(2) (n = 4; P < .0001) in vitro.

CONCLUSIONS

The CaPO(4) model accelerates aneurysm formation with the enhancement of apoptosis, macrophage infiltration, and calcium deposition. This modified model, with its rapid and robust dilation, can be used as a new model for AAAs.

Animal models of cardiovascular diseases

Cardiovascular diseases are the first leading cause of death and morbidity in developed countries. The use of animal models have contributed to increase our knowledge, providing new approaches focused to improve the diagnostic and the treatment of these pathologies. Several models have been developed to address cardiovascular complications, including atherothrombotic and cardiac diseases, and the same pathology have been successfully recreated in different species, including small and big animal models of disease. However, genetic and environmental factors play a significant role in cardiovascular pathophysiology, making difficult to match a particular disease, with a single experimental model. Therefore, no exclusive method perfectly recreates the human complication, and depending on the model, additional considerations of cost, infrastructure, and the requirement for specialized personnel, should also have in mind. Considering all these facts, and depending on the budgets available, models should be selected that best reproduce the disease being investigated. Here we will describe models of atherothrombotic diseases, including expanding and occlusive animal models, as well as models of heart failure. Given the wide range of models available, today it is possible to devise the best strategy, which may help us to find more efficient and reliable solutions against human cardiovascular diseases.

Experimental models of abdominal aortic aneurysms

Despite being a leading cause of death in the West, the pathophysiology of abdominal aortic aneurysms (AAA) is still incompletely understood. Pharmacotherapy to reduce the growth of small AAAs is limited and techniques for repairing aneurysms continue to evolve. Experimental models play a key role in AAA research, as they allow a detailed evaluation of the pathogenesis of disease progression. This review focuses on in vivo experimental models, which have improved our understanding of the potential mechanisms of AAA development and contributed to the advancement of new treatments.

The Influence of Diabetes on Degree of Abdominal Aortic Aneurysm Tissue Inflammation

Abdominal aortic aneurysm (AAA) progression and disease resistance are related to transmural degenerative processes and an inflammatory infiltration (INF). Diabetes is associated with low prevalence and growth rate of AAA. We sought to characterize INF in established AAA (INFAAA), in diabetic patients. From 89 male patients aged 52 to 83 years, aneurysm specimens obtained at open asymptomatic nonruptured AAA repair were graded for INF and immunostained using antibodies against T-lymphocytes (CD3) and macrophages (CD68). Diabetic patients had an odds ratio (OR) 3.8, 95% confidence interval ([CI] 1.14-12.96), P = .03, of experiencing above-median INFAAA. These associations were affected by serum glucose (SG) levels (OR 3.6, 95% CI [0.72-18.77]; P = .1). Macrophage subpopulations higher in diabetic patients (1.44 ± 0.78 versus 0.98 ± 0.76; P = .02) were correlated with SG (r = .21, P = .044). Abdominal aortic aneurysms in diabetic patients are associated with higher INF. Macrophage densities are correlated with SG.

Vascular smooth muscle cell peroxisome proliferator-activated receptor-γ deletion promotes abdominal aortic aneurysms

OBJECTIVE

Peroxisome proliferator-activated receptor-γ (PPARγ) plays an important role in the vasculature; however, the role of PPARγ in abdominal aortic aneurysms (AAA) is not well understood. We hypothesized that PPARγ in smooth muscle cells (SMCs) attenuates the development of AAA. We also investigated PPARγ-mediated signaling pathways that may prevent the development of AAA.

METHODS

We determined whether periaortic application of CaCl(2) renders vascular SMC-selective PPARγ knockout (SMPG KO) mice more susceptible to destruction of normal aortic wall architecture.

RESULTS

There is evidence of increased vessel dilatation in the abdominal aorta 6 weeks after 0.25M periaortic CaCl(2) application in SMPG KO mice compared with littermate controls (1.4 ± 0.3 mm [n = 8] vs 1.1 ± 0.2 mm [n = 7]; P = .000119). Results from SMPG KO mice indicate medial layer elastin degradation was greater 6 weeks after abluminal application of CaCl(2) to the abdominal aorta (P < .01). Activated cathepsin S, a potent elastin-degrading enzyme, was increased in SMPG KO mice vs wild-type controls. To further identify a role of PPARγ signaling in reducing the development of AAA, we demonstrated that adenoviral-mediated PPARγ overexpression in cultured rat aortic SMCs decreases (P = .022) the messenger RNA levels of cathepsin S. In addition, a chromatin immunoprecipitation assay detected PPARγ bound to a peroxisome proliferator-activated receptor response element (PPRE) -141 to -159 bp upstream of the cathepsin S gene sequence in mouse aortic SMCs. Also, adenoviral-mediated PPARγ overexpression and knockdown in cultured rat aortic SMCs decreases (P = .013) and increases (P = .018) expression of activated cathepsin S. Finally, immunohistochemistry demonstrated a greater inflammatory infiltrate in SMPG KO mouse aortas, as evidenced by elevations in F4/80 and tumor necrosis factor-α expression.

CONCLUSION

In this study, we identify PPARγ as an important contributor in attenuating the development of aortic aneurysms by demonstrating that loss of PPARγ in vascular SMCs promotes aortic dilatation and elastin degradation. Thus, PPARγ activation may be potentially promising medical therapy in reducing the risk of AAA progression and rupture.

Elevation of ADAM10, ADAM17, MMP-2 and MMP-9 expression with media degeneration features CaCl2-induced thoracic aortic aneurysm in a rat model

PURPOSE

This study was designed to establish a rat model of thoracic aortic aneurysm (TAA) by calcium chloride (CaCl(2))-induced arterial injury and to explore the potential role of a disintegrin and metalloproteinase (ADAM), matrix metalloproteinases (MMPs) and their endogenous inhibitors (TIMPs) in TAA formation.

METHODS

Thoracic aorta of male Sprague-Dawley rats was exposed to 0.5M CaCl(2) or normal saline (NaCl). After 12weeks, animals were euthanized, and CaCl(2)-treated, CaCl(2)-untreated (n=12) and NaCl-treated aortic segments (n=12) were collected for histological and molecular assessments. MMP-TIMP and ADAM mRNAs were semi-quantitatively analyzed and protein expressions were determined by immunohistochemistry.

RESULTS

Despite similar external diameters among CaCl(2)-treated, non-CaCl(2)-treated and NaCl-treated segments, aneurymal alteration (n=6, 50%), media degeneration with regional disruption, fragmentation of elastic fiber, and increased collagen deposition (n=12, 100%) were demonstrated in CaCl(2)-treated segments. MMP-2, MMP-9, ADAM-10 and ADAM-17 mRNA levels were increased in CaCl(2)-treated segments (all p<0.01), with trends of elevation in CaCl(2)-untreated segments, as compared with NaCl-treated segments. Immunohistochemistry displayed significantly increased expressions of MMP-2, MMP-9, ADAM-10 and ADAM-17 (all p<0.01) in intima and media for CaCl(2)-treated segments. TIMP mRNA and tissue levels did not differ obviously among the three aortic segments.

CONCLUSION

This study establishes a TAA model by periarterial CaCl(2) exposure in rats, and demonstrates a significant elevation of expression of MMP-2, MMP-9, ADAM10 and ADAM17 in the pathogenesis of vascular remodeling.

Aldosterone receptor inhibition alters the viscoelastic biomechanical behavior of the aortic wall

Dynamic mechanical disturbances in the aortic wall may lead to progressive aortic dilation and possibly aneurysmal formation. Here, we investigated the previously unexplored effects of aldosterone inhibition on aortic wall viscoelastic properties in hyperlipidemic rabbits. Thirty-six New Zealand male rabbits, fed a standard diet for four weeks, were separated into three groups: control (C; n = 10), standard diet; eplerenone (A; n = 12), hyperlipidemic diet plus 100 mg/kg/d eplerenone (last 4 weeks); and vehicle (V; n = 14), hyperlipidemic diet (no eplerenone). After eight weeks, animals were sacrificed and rectangular strips from the aortic wall, cut in radial and axial orientations, were prepared. Fresh, saline-wetted strips at 37 degrees C were subjected to cyclic sinusoidal elongation from zero to 20% of the resting length at a frequency of 1 Hz. The dynamic biomechanical viscoelastic characteristics, 'elastin phase' low modulus (E(L)), 'collagen phase' high modulus (E(H)) and dissipated-energy index, were determined. Aortic tissue preparations were also examined histologically. Eplerenone increased aldosterone concentrations but did not affect blood pressure, cholesterol or potassium concentrations. There was a significant reduction of E(H) (from 3.40 to 1.80 MPa; P < 0.01) and E(L) (from 0.46 to 0.27 MPa; P < 0.05) in group A in the radial direction compared with group C. In the axial direction E(L) significantly increased in group A compared with group V (from 0.42 to 1.11 MPa; P < 0.01). Energy dissipation was not significantly different among groups, although there was a trend toward higher values in group A for both directions. Histological assessments revealed no significant differences in collagen or elastic fibers among groups. In conclusion, aldosterone receptor inhibition altered the viscoelastic properties of the aortic wall in hyperlipidemic rabbits without detectable microscopic changes in elastic or collagen fibers, an effect that progressively might predispose to dilation and/or aneurysmal formation.

Alterations in membrane type-1 matrix metalloproteinase abundance after the induction of thoracic aortic aneurysm in a murine model

Thoracic aortic aneurysms (TAAs) develop as a result of dysregulated extracellular matrix remodeling mediated by several matrix metalloproteinases (MMPs). Membrane type-1 MMP (MT1-MMP) is the prototypical member of a unique family of membrane-bound MMPs, possessing multiple substrates and functions. The present study tested the hypothesis that MT1-MMP expression, abundance, and activity would be elevated during TAA development and that this protease is produced primarily by mesenchymal cells within the thoracic aorta. Descending thoracic aortas were harvested from C57BL/6J mice at multiple time points (2, 4, 8, and 16 wk, n = 15 each) post-TAA induction (0.5M CaCl(2), 15 min) and compared with reference controls (n = 15). The expression and abundance of MT1-MMP, MMP-2, and tissue inhibitor of metalloproteinase (TIMP)-2 were assessed by quantitative PCR and immunoblot analysis. MT1-MMP activity was determined by fluorescent peptide assay. MT1-MMP was localized within the aortic wall by immunohistochemistry. MT1-MMP abundance and localization in live animals (8 wk post-TAA induction vs. control) was determined by micro-ultrasound imaging with an MT1-MMP-targeted microbubble contrast agent. Aortic diameter was increased 172 +/- 7% at 16 wk post-TAA induction (P < 0.05). MT1-MMP and MMP-2 mRNA levels were elevated at 2 wk post-TAA induction (P < 0.05). MT1-MMP protein abundance increased progressively to a maximum of 178 +/- 26% at 16 wk post-TAA induction, whereas MMP-2 and TIMP-2 peaked at 2 wk post-TAA induction (526 +/- 93% and 376 +/- 48%, respectively, P < 0.05). MT1-MMP colocalized with fibroblasts, and MT1-MMP-targeted contrast binding was elevated in 8-wk TAA-induced mice versus control mice (217 +/- 53% vs. 81 +/- 8%, P < 0.05). In conclusion, these novel results suggest that MT1-MMP plays a dynamic multifunctional role in TAA development and, therefore, may provide a significant target for therapeutic strategies.

Animal models of abdominal aortic aneurysm and their role in furthering management of human disease

Abdominal aortic aneurysm is a common degenerative disorder associated with sudden death due to aortic rupture. Current therapy is limited to open surgical repair of the aorta or endovascular placement of covered stents to exclude the abdominal aortic aneurysm from the circulation. A number of different animal models have been developed in order to study abdominal aortic aneurysm in an effort to advance current management deficiencies. Large animal models have been mostly used to assist in developing novel methods to surgically treat abdominal aortic aneurysms. Small animal models, particularly those developed in rodents, have been employed to further the understanding of the mechanisms involved in abdominal aortic aneurysm in order to identify potential new medical treatments. It is expected that findings from these animal models will contribute importantly to new treatments for human abdominal aortic aneurysm. This review explores the animal models which are used in abdominal aortic aneurysm research and highlights their advantages and disadvantages.

The Influence of Total Plasma Homocysteine and Traditional Atherosclerotic Risk Factors on Degree of Abdominal Aortic Aneurysm Tissue Inflammation

Objective: Modulating effects of genetic and environmental risk factors on severity of human abdominal aortic aneurysm (AAA) tissue inflammation remain unclear. We investigated the influence of total plasma homocysteine (tHcy) and traditional atherosclerotic risk factors (ARF) on degree of AAA tissue inflammation. Methods: Aneurysm specimens were obtained from 89 male patients aged 52 to 83 years, underwent asymptomatic not ruptured AAA (mean diameter 5.5 cm) open repair and graded for degree of histologic inflammation. Multivariate analysis was used to determine the association of tHcy and ARF, with degree of inflammation. Results: Current cigarette smoking, odds ratio (OR) 4.4, 95% confidence interval 1.3 to 15.2, P = .01 and no other ARF, neither tHcy levels OR 0.9 (0.9-1.02), P = .2 were associated with high-grade tissue inflammation. Conclusion: These results provide evidence against a major effect of tHcy levels on AAA tissue inflammation, while current cigarette smoking is a significant modulating factor.

Recent advances in molecular mechanisms of abdominal aortic aneurysm formation

Abdominal aortic aneurysm (AAA) is an increasingly common clinical condition with fatal implications. It is associated with advanced age, male gender, cigarette smoking, atherosclerosis, hypertension, and genetic predisposition. Although significant evidence has emerged in the last decade, the molecular mechanisms of AAA formation remain poorly understood. Currently, the treatment for AAA remains primarily surgical with the lone innovation of endovascular therapy. With advances in the human genome, understanding precisely which molecules and genes mediate AAA development and blocking their activity at the molecular level could lead to important new discoveries and therapies. This review summarizes recent updates in molecular mechanisms of AAA formation, including animal models, autoimmune components, infection, key molecules and cytokines, mechanical forces, genetics, and pharmacotherapy. This review will be helpful to those who want to recognize the newest endeavors within the field and identify possible lines of investigation in AAA.

Transforming growth factor-beta signaling in thoracic aortic aneurysm development: a paradox in pathogenesis

Thoracic aortic aneurysms (TAAs) are potentially devastating, and due to their asymptomatic behavior, pose a serious health risk characterized by the lack of medical treatment options and high rates of surgical morbidity and mortality. Independent of the inciting stimuli (biochemical/mechanical), TAA development proceeds by a multifactorial process influenced by both cellular and extracellular mechanisms, resulting in alterations of the structure and composition of the vascular extracellular matrix (ECM). While the role of enhanced ECM proteolysis in TAA formation remains undisputed, little attention has been focused on the upstream signaling events that drive the remodeling process. Recent evidence highlighting the dysregulation of transforming growth factor-beta (TGF-beta) signaling in ascending TAAs from Marfan syndrome patients has stimulated an interest in this intracellular signaling pathway. However, paradoxical discoveries have implicated both enhanced TGF-beta signaling and loss of function TGF-beta receptor mutations, in aneurysm formation; obfuscating a clear functional role for TGF-beta in aneurysm development. In an effort to elucidate this subject, TGF-beta signaling and its role in vascular remodeling and pathology will be reviewed, with the aim of identifying potential mechanisms of how TGF-beta signaling may contribute to the formation and progression of TAA.

Toward cell therapy for vascular calcification: osteoclast-mediated demineralization of calcified elastin

BACKGROUND

Elastin-oriented vascular calcification is a clinically significant feature, which involves formation of ectopic bone-like structures. Taking advantage of the similarities between arterial calcification and bone regulation, our hypothesis was that therapeutic approaches for limitation of vascular calcification could be developed using site-specific delivery of autologous osteoclasts. In the present paper, we tested the hypothesis that bone-marrow-derived osteoclasts have the ability to demineralize calcified elastin, without significant alterations in elastin integrity.

METHODS

Active, multinucleated osteoclasts were obtained by in vitro maturation of rat bone-marrow-derived progenitor cells in the presence of vitamin D(3) and retinoic acid. Cell phenotype was validated by staining for tartrate-resistant acid phosphatase, formation of resorption pits on hydroxyapatite-coated disks, and RT-PCR for identification of cathepsin K gene expression. Calcified aortic elastin was seeded with osteoclasts and calcium, and phosphorous levels were monitored in gels and culture media to detect demineralization of elastin. Soluble elastin peptides were also monitored in culture media for elastin degradation. For in vivo experiments, pure aortic elastin was coimplanted with allogenic osteoclasts subdermally into rats, and the degree of elastin calcification and degradation was evaluated using mineral analysis and desmosine quantitation.

RESULTS

Bone-marrow-derived osteoclasts reduced mineral content of calcified elastin in vitro by 80%. Moreover, in vivo implantation of allogenic osteoclasts in the vicinity of calcifying elastin limited elastin mineralization by almost 50%, in the absence of detectable elastin degradation.

CONCLUSIONS

Osteoclasts have the ability to demineralize calcified elastin, without significant alterations in elastin integrity.

Elastin degradation and calcification in an abdominal aorta injury model: role of matrix metalloproteinases

BACKGROUND

Elastin calcification is a widespread feature of vascular pathology, and circumstantial evidence exists for a correlation between elastin degradation and calcification. We hypothesized that matrix metalloproteinase (MMP)-mediated vascular remodeling plays a significant role in elastin calcification.

METHODS AND RESULTS

In the present studies, we determined that short-term periadventitial treatment of the rat abdominal aorta with low concentrations of calcium chloride (CaCl2) induced chronic degeneration and calcification of vascular elastic fibers in the absence of aneurysm formation and inflammatory reactions. Furthermore, the rate of progression of calcification depended on the application method and concentration of CaCl2 applied periarterially. Initial calcium deposits, associated mainly with elastic fibers, were persistently accompanied by elastin degradation, disorganization of aortic extracellular matrix, and moderate levels of vascular cell apoptosis. Application of aluminum ions (known inhibitors of elastin degradation) before the CaCl2-mediated injury significantly reduced elastin calcification and abolished both extracellular matrix degradation and apoptosis. We also found that MMP-knockout mice were resistant to CaCl2-mediated aortic injury and did not develop elastin degeneration and calcification.

CONCLUSIONS

Collectively, these data strongly indicate a correlation between MMP-mediated elastin degradation and vascular calcification.

Matrix metalloproteinases and atherosclerosis

Atherosclerosis is a major cause of coronary heart disease, and matrix metalloproteinases (MMPs) play an important role in atherosclerosis by degrading the extracellular matrix, which results in cardiovascular remodeling. Recent studies have identified enhanced expression of MMPs in the atherosclerotic lesion and their contribution to weakening of the vascular wall by degrading the extracellular matrix. The transcription, enzyme processing, and specific inhibition of MMPs by tissue inhibitors of matrix metalloproteinase (TIMPs) regulate these effects. These processes are also modified by inflammatory cytokines and cell-cell contact signaling. Both animal experiments and clinical sample analysis have shown that balance in expression and activation of MMPs and inhibition by TIMPs is critical for the development of stenotic and aneurysmal change. Polymorphism in the MMP gene promoter contributes to inter-individual differences in susceptibility to coronary heart disease. The development of therapeutic drugs specifically targeting MMPs may thus be useful for the prevention of atherosclerotic lesion progression, plaque rupture, and restenosis.

Effects of atherosclerotic plaque on the enlargement of an experimental model of abdominal aortic aneurysm in rabbits

PURPOSE

Abdominal aortic aneurysm (AAA) usually is associated with atherosclerosis. We attempted to create an abdominal aortic aneurysm with atherosclerotic plaque in rabbits to investigate the relationship between atherosclerosis and enlargement of AAA.

METHODS

An isolated segment of rabbit abdominal aorta was perfused with pancreatic elastase. Animals were fed a cholesterol-enriched diet to induce atherosclerotic plaque formation. Eight animals received perfusion with elastase and were fed a cholesterol-enriched diet (group EC), eight animals received perfusion with elastase and were fed a normal diet (group EN), eight animals received perfusion with saline and were fed a cholesterol-enriched diet (group SC), and eight animals received perfusion with saline and were fed a normal diet (group SN). Four animals served as a sham group. Each animal was killed after aortography at 4 or 12 weeks. The perfused segment was excised and examined histologically.

RESULTS

No animal treated with saline and fed normal diet (groups SN) developed either an aneurysm or atherosclerosis. Atherosclerotic plaque was observed in animals fed the cholesterol-enriched diet (groups SC and EC) at 4 weeks, and the plaque had thickened further by 12 weeks. All animals treated with elastase (groups EN and EC) developed an AAA. Maximum internal diameter in groups EN and EC (4.1 mm) was equal at 4 weeks, but at 12 weeks, the diameter was less for group EC than for group EN (4.0 mm versus 4.8 mm, P<0.05).

CONCLUSION

Cholesterol-enriched diet following the intra-luminal perfusion of an isolated aortic segment with elastase produced an AAA with atherosclerotic plaque in rabbits. It is likely that the thickened intima with atheroma suppressed continued enlargement of the aneurysm in this model.

A murine model of thoracic aortic aneurysms

BACKGROUND

The mechanisms of thoracic aortic aneurysm (TAA) formation are poorly understood, mainly due to the lack of a useful and reproducible model. Accordingly, the goal of this study was to test the hypothesis that abluminal calcium chloride (CaCl(2)) application could create TAAs in the mouse.

MATERIALS AND METHODS

Adult 129/SvE mice (n = 8) were anesthetized and their thoracic aortas exposed via left thoracotomy. CaCl(2) (0.5M) was applied to the distal descending thoracic aorta for 15 min followed by chest closure. At 4 weeks, the perfusion-fixed aorta was harvested from the root to the renal arteries. Diameter measurements were made using confocal microscopy, and wall thickness was measured from hematoxylin and eosin-stained sections.

RESULTS

The control (n = 15) distal descending thoracic aortic diameter was 0.60 +/- 0.04 mm and increased by 25% (0.76 +/- 0.06 mm, P < 0.05) following CaCl(2) treatment. Control aortic wall thickness was 48 +/- 9 mum and decreased by 47% in corresponding CaCl(2)-exposed segments (25 +/- 8 mum, P < 0.05). The diameter and wall thickness of the ascending aorta (used as an internal control) were not significantly different between groups. Picrosirius red staining of the TAA showed adventitial collagen breakdown and disruption of lamellar organization.

CONCLUSIONS

We conclude that abluminal application of CaCl(2) to the thoracic aorta reliably produces dilation, wall-thinning, and disruption of mural architecture, the hallmark signs of aneurysm formation. To our knowledge, these findings describe for the first time the generation of a reproducible model of isolated TAA formation in a murine system.

Provocation of experimental aortic inflammation and dilatation by inflammatory mediators and Chlamydia pneumoniae

BACKGROUND

The macrophage appears to have a key role in the inflammation and proteolysis associated with the growth and development of abdominal aortic aneurysms. The role of inflammatory mediators and Chlamydia pneumoniae in stimulating the influx of macrophages and dilatation of the abdominal aorta was investigated in an experimental model.

METHODS

Periaortic application of calcium chloride solution (and monocyte chemoattractant protein (MCP) 1, a cocktail of cytokines or C. pneumoniae) to the abdominal aorta of New Zealand White rabbits was performed at laparotomy. Some animals were fed a cholesterol-rich diet. The diameter of the aorta was measured by ultrasonography and after perfusion fixation, 3 weeks after laparotomy. Aortic sections were stained with RAM-11 to identify macrophages for counting. The presence of C. pneumoniae DNA was confirmed using the polymerase chain reaction.

RESULTS

Aortic macrophage influx in response to MCP-1, thioglycollate or C. pneumoniae was more than doubled in the cholesterol-fed animals. In response to human recombinant MCP-1 (1 microg) the mean(s.d.) macrophage count increased from 79(19) to 340(215) per unit area (P < 0.02). Even in cholesterol-fed animals, application of MCP-1 (recombinant human or rabbit form) was not associated with aortic dilatation. Application of thioglycollate 0.1 mol/l, or live or formalin-inactivated C. pneumoniae (0.5 x 108 organisms), was associated with a similar increase in macrophages to that caused by MCP-1 and a significant (approximately twofold) increase in aortic diameter after 3 weeks.

CONCLUSION

Macrophage influx into rabbit abdominal aorta, without macrophage activation, is insufficient to cause experimental aortic dilatation. C. pneumoniae antigens appeared to stimulate aortic dilatation, probably by specific activation of macrophages.

Development of a symmetric canine abdominal aortic aneurysm model with clinical relevance for endovascular graft studies

This study was designed to develop a large-animal model to study and validate transluminally placed endovascular grafts and related techniques for treating abdominal aortic aneurysms with minimal trauma. In four dogs, a segment of infrarenal abdominal aorta was dissected and an endarterectomy was performed through an aortotomy to remove the entire intima of this controlled aortic segment. The opening was patched with chemically processed bovine pericardium, and an identical patch was applied after aortotomy of the opposite side of the aorta. Aortic diameter and flow were measured before and after the procedure. Angiograms were obtained just before retrieval. All dogs recovered quickly with no complications. Aortic diameter increased approximately 2.7-fold. Aneurysm size and shape remained stable until the end of the experiments at 4 or 8 weeks. Gross and histologic studies for 4- and 8-week specimens were similar. Factor VIII/von Willebrand factor staining showed complete reendothelialization of the endarterectomized wall, with a moderate degree of intimal hyperplasia. The patch material retained its acellular nature and its surface was covered with thrombus or fibrinous material mixed with blood cells and inflammatory cells. Thus, this model is feasible and suitable for endovascular graft healing studies.

A quantitative study of the aorta of the New Zealand rabbit (Oryctolagus cuniculus L.)

Ten segments of the aorta of the rabbit were studied quantitatively. The thoracic and abdominal aorta of the rabbit were measured from proximal to distal and parallel to the reduced diameter, demonstrating a decreasing thickness of the tunica intima and tunica media that becomes physiologically apparent during adulthood in these commonly used laboratory animals. The ratio of intimal to medial thickness, multiplied by 100, was calculated and found to be between 2.56 and 3.74 for the thoracic aorta and 4.03 and 5.62 for the abdominal aorta of the rabbit. These findings are important for the better understanding of the development of atherosclerosis.

Atherosclerotic arterial remodeling and the localization of macrophages and matrix metalloproteases 1, 2 and 9 in the human coronary artery

Atherosclerotic luminal narrowing is determined by plaque mass and the mode of geometrical remodeling. Recently, we reported that the type of atherosclerotic remodeling is associated with the presence of histological markers for plaque vulnerability. Inflammation and matrix degrading proteases (MMPs) may play a role in both plaque vulnerability and in expansive arterial remodeling. The aim of the present study was to investigate the association between the remodeling mode and the localization of macrophages and MMPs in coronary atherosclerotic segments. From 36 atherosclerotic coronary arteries, 45 and 51 segments were selected with a vessel area that was >10% smaller and larger compared with the adjacent segments, respectively. No significant difference in staining for macrophages was observed between segments with expansive and constrictive remodeling. More MMP-2 and MMP-9 staining was observed in plaques of expansively remodeled segments compared with constrictively remodeled segments. In general, MMP-staining was less evident in the adventitial layer compared with the plaque. Zymography revealed more active MMP-2 in expansively remodeled segments compared with constrictively remodeled segments (340+/-319 vs. 199+/-181 (adjusted counts/mm(2)), respectively, P=0.019). Zymography did not show differences in inactive MMP-2 or MMP-9 among groups. It might be postulated that MMPs within the plaque play a causal role not only in plaque vulnerability but also in de novo atherosclerotic remodeling.