Vascular
Differential expression of integrin α5β1 in human abdominal aortic aneurysm and healthy aortic tissues and its significance in pathogenesis

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Abstract

Background

Abdominal aortic aneurysm (AAA) is a common aged disease of human aorta with increasing incidence. It is characterized by dramatic vascular remodeling via proteolysis and degradation of matrix proteins. Integrins are important cellular receptors for matrix proteins, which may have an association with pathological remodeling. The present study was undertaken to analyze the expression of integrin subunits in human aneurysmal aortas and with healthy aortic tissues as controls.

Materials and methods

The expression of integrin genes in AAA specimens and healthy human aortic tissues was detected by RT-PCR technique. The correlation of variation and distribution of smooth muscle cells (SMCs) and integrin protein expression in the corresponding tissues were studied immunohistochemically.

Results

The gene transcripts coding for integrin α4, α5, αV, β1, β3, β5, and β6 subunits were constitutively expressed in the normal aortas. Only gene expressions of integrin α5 and β1 were significantly decreased by 81% and 85%, respectively, in AAA specimens (P < 0.005) when compared with healthy aortic specimens. No age dependence of the expression of integrin α5β1 genes was found. Significant reduction of medial SMC density was confirmed in corresponding AAA compared with control aortas. Immunoreactivity of integrin α5β1 receptor was found to be exclusively localized within the medial layer of the parallel normal aortic sections, whereas this protein was absent in the destructive media of aneurysmal aortic sections.

Conclusions

The marked decrease in integrin α5β1 expressions was unique to aneurysmal aortic tissues and correlated to a decrease in density of SMCs, which are the major cells in maintaining the structure stability of normal aortas. As integrin α5β1 specifically binds fibronectin and collagen, those results may suggest that the absence of integrin α5β1 activity impair matrix protein attachment and alter the architecture in aortic media thereby lead to the deformity of aorta and aneurysm formation.

Introduction

The biochemical and pathological changes associated with abdominal aortic aneurysms (AAAs) have been extensively studied. Dramatic remodeling of the extracellular matrix 1, 2 is thought to be the principal pathological process. These changes are accompanied by an inflammatory infiltrate [3] and an increased production of matrix metalloproteinases (MMPs) 4, 5. However, no specific causative gene has been identified.

Aortic dilation and weakening are caused by the mechanical failure of extracellular matrix proteins, which are responsible for the structural integrity of the arterial wall [2]. Researchers have sought enzymes capable of causing such structural changes. The primary finding is an increase in the levels of matrix metalloproteinases (MMPs). However, a recent report showed that the overall balance of both MMPs and their inhibitors (TIMPs) in aneurysmal tissues may be preserved [6]. Although proteolytic degradation of extracellular matrix may contribute to the structural changes found in the aneurysmal arteries, other pathogenetic mechanisms may pertain.

Cell adhesion to the extracellular matrix (ECM) is specifically mediated by integrins, which belong to a family of heterodimeris transmembrane proteins comprising of at least 16 α and 8 β subunits in mammals [7]. Different combinations of α and β subunits determine the specificity for extracellular matrix proteins binding as well as intracellular signaling events 7, 8. Ruoslahti and Reed [9] showed that integrins are important receptors for the extracellular matrix in the reorganization of the cytoskeleton, lymphocyte recirculation, thrombosis, tumor formation, and metastasis. Integrin signaling has also been shown to influence tumor invasiveness by regulating matrix metalloproteinase expression [10]. However, the involvement of integrin in the pathogenesis of AAAs is still unknown. The present study was undertaken to explore the specific expression of integrins within healthy and aneurysmal aortic walls. This study may provide evidence for the involvement of specific integrin receptor expressions in the weakening of the aortic walls that contribute to the pathogenesis of aneurysms.

Section snippets

Human aortic tissues collection

The average diameter of the AAA was 6.5 cm (range, 5–7 cm). The mean age was 72 years (range, 66–83 years) for eight patients with AAAs and 60.8 years (range, 41–71 years) for seven aged organ donors.

All studies were performed with the approval of our local ethics committee. Eight aneurysmal aortic specimens without adherent thrombus were obtained in the operating room from patients undergoing elective aortic aneurysm surgery. Fifteen full-thickness walls of the infrarenal aorta were obtained

Integrins mRNAs in normal and aneurysmal aortas

α4, αV, β3, β5, and β6 Integrins subunits genes were found to be expressed constitutively in all control specimens and AAA specimens. Fig. 1 shows the summary of the relative expression of tested integrin subunits in all healthy aortic tissues and AAA specimens after normalization with the β-actin expression of the same RNA template. In AAA specimens, the expression levels of α5 and β1 genes were decreased 81% and 85%, respectively, and statistically significant (P < 0.005) compared with seven

Discussion

Integrity of aortic tissue requires coordinated synthesis of structural proteins [13]. Previous studies have demonstrated that the degradation of extracellular matrix proteins by matrix metalloproteinases (MMPs) in the aortic wall is critical for the development of aneurysms in human aortas 14, 15, 16, 17. However, the molecular events involved in the pathogenesis of AAA remain obscure.

Recently, experimental results from Hyne’s laboratory have established that integrins, which belong to a group

Acknowledgements

The authors are grateful to Lisa Wong and Silvana Lau for their invaluable assistance in the collection of human aortic tissues in the present study.

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