Elsevier

Cardiovascular Pathology

Volume 18, Issue 2, March–April 2009, Pages 67-76
Cardiovascular Pathology

Original Article
Identification of tissue factor in experimental aortic valve sclerosis

https://doi.org/10.1016/j.carpath.2007.12.014Get rights and content

Abstract

Background

Aortic valve sclerosis (AVS) shares epidemiological and histological similarities with atherosclerosis. Tissue factor (TF), the main initiator of blood coagulation, is present in atherosclerotic plaques and contributes to their thrombogenicity. We aimed to analyze valvular TF expression in addition to other components of atherosclerosis in two models of AVS.

Methods

Forty-five rabbits were randomly assigned to receive either normal chow (Ctrl, n=15), or 1% cholesterol-enriched chow alone (Hyperchol, n=15) or associated with vitamin D2 (VitD, n=15), for 12 weeks. Aortic valve (AV) performance, leaflet structure, cellular and lipid infiltration, and TF expression were assessed using Doppler, histology, and immunohistochemistry, respectively, and TF activity was evaluated in AV leaflets.

Results

Hyperchol and VitD animals developed abnormal leaflet thickening, with a significant alteration of AV performance in VitD animals. Leaflet thickening was related to the development of fatty plaque neolesions on the aortic side of the leaflets, displaying extracellular matrix disorganization, lipid and cellular infiltration, and calcification in VitD animals. TF was found on the leaflet aortic side in Ctrl animals and was identified in AVS lesions in both Hyperchol and VitD animals. TF immunostaining area and valvular activity increased significantly across the three groups.

Conclusions

Experimental AVS lesions that are present on the aortic side of leaflets display numerous characteristics of vascular atherosclerosis, including TF expression. Identification of TF associated with other components of the atherosclerotic process in AVS lesions strengthens the link between atherosclerosis and AVS.

Introduction

Aortic valve sclerosis (AVS), the most frequent valve disease in developed countries, is present in 25% of people over 65 years of age and leads to aortic stenosis in 2% of these individuals [1]. The mechanisms leading to the development of aortic valve (AV) lesions were traditionally believed to be degenerative, induced by time-dependent wear and tear of the leaflets with passive accumulation of calcium in the setting of sclerosis [2]. However, atherosclerotic risk factors have been associated with aortic sclerosis in epidemiological studies [1], [3], suggesting that valve sclerosis might be an atherosclerotic-like process. Moreover, histological analysis has revealed atherosclerotic-like lesions in aortic leaflets, characterized by accumulation of macrophages, T lymphocytes, oxidized low-density lipoproteins (LDL), and extracellular lipids at the aortic side of the leaflets [4], [5], [6].

Tissue factor (TF), the receptor for activated factor VII, is the main initiator of blood coagulation. In physiological conditions, TF is absent from vascular intima and media cells but is present in adventitial fibroblasts [7]. In contrast, TF is widely expressed in atherosclerotic plaques, especially in macrophages, foam cells, and the extracellular matrix and is considered to be the main contributor to the thrombogenicity of atherosclerotic plaques [8], [9]. After fibrous cap disruption, TF triggers thrombus formation leading to arterial lumen occlusion and/or downstream embolization. Hence, TF plays a major role in cell migration [10] and the development of atherosclerotic lesions [11].

Recently, two models of AVS in New Zealand White (Cegav, France) rabbits have been described, the first induced by a high-cholesterol diet [12] and the second by a high-cholesterol diet associated with vitamin D2 (VitD), which was responsible for more severe lesions with an hemodynamic stenotic effect [13]. The development of aortic sclerosis in these animals allows the investigation of the early stages of the disease.

Identification of TF in AVS lesions in these models might strengthen the proposed link between AVS and atherosclerosis. Accordingly, this study aimed to evaluate valvular TF expression and localization in these 2 experimental models of AVS, in addition to other components of atherosclerosis.

Section snippets

Methods

Forty-five male New Zealand White rabbits with an initial body weight of 3.0–3.5 kg were used in this study. All experiments were conducted in compliance with the position of the American Physiological Society on research animal use.

Biochemical and physiological parameters

As shown in Table 1, blood total cholesterol levels were significantly higher in the Hyperchol and VitD groups, when compared to the Ctrl group (both P<.0001). Calcium levels were significantly higher in the VitD group than in the Ctrl and Hyperchol groups (P=.001 and P=.017, respectively), reflecting VitD treatment efficiency.

Physiological measurements and aortic valve hemodynamic assessment

As shown in Table 1, no significant differences were observed with respect to animal weight, blood pressure, and heart rate between the three groups at the time of

Discussion

This study clearly indicates that rabbits given a high cholesterol diet with and without VitD develop AVS lesions localized on the aortic side of the leaflets, with the most severe lesions observed in animals receiving VitD. These lesions resemble those observed in atherosclerotic plaques and include extracellular matrix remodeling, lipid deposition, and macrophage infiltration. Furthermore, while low levels of TF were detected with myofibroblasts within the fibrosa and aortic subendothelium of

Acknowledgments

The authors acknowledge the assistance of Dr. Erwan Floch in manuscript editing, Cécile Lecointe in animal experiments, and the histology laboratory of Lille II University and Sigma Aldrich Corporation for technical assistance with histological staining.

This work was realized at the Institut Federatif de Recherche 114, and supported by grants from University of Lille 2 (EA2693), the Conseil Régional Nord pas de Calais, and Inserm ESPRI ERI-9 and FEDER R04026EE.

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