Rupture Angiogenesis
Objective
Considerable research has been applied to the aetiology of abdominal aortic aneurysm (AAA) expansion; however the mechanism of rupture remains poorly defined. Proteolytic activities of matrix metalloproteinases (MMPs) have been implicated in aneurysm wall weakening and rupture. MMPs play key pro-angiogenic roles during the process of neovascularisation such as the proteolytic degradation of extracellular matrix to facilitate endothelial cell migration. Although medial neovascularisation (MNV) is a known histopathological characteristic of AAAs, the role of angiogenesis in aneurysm rupture has never been investigated. This study assessed the hypothesis that increased MNV is associated with aneurysm rupture.
Methods
Paired biopsies were obtained from the rupture site and anterior sac (control) in 12 ruptured AAAs. Further controls were obtained from anterior sac of 10 non-ruptured AAAs. Using CD31 immunostaining, MNV was quantified as microvessel density in the medial layer. Microvessel maturity index (percentage of microvessels coated with smooth muscle cells) was assessed using double immunostaining to CD31/a-smooth muscle actin. Macrophages were identified by CD68 immunostaining and degree of inflammatory infiltrate was quantified using an image analysis software. The expression of candidate pro-angiogenic mediators (relative to 18srRNA) was quantified using quantitative real time-PCR.
Results
Compared to anterior sac, MNV was increased (p<0.01) in rupture site biopsies and consisted of smaller diameter (p<0.01) and more immature microvessels (p<0.01). Rupture site biopsies had increased mRNA expression (p<0.05) of vascular endothelial growth factor (VEGF), hypoxia-inducible factor (HIF)-1a, av-integrin, transforming growth factor-b1 (TGFb1) and basic fibroblast factor (bFGF). VEGF mRNA expression correlated with microvessel density (Pearson correlation coefficient r2=0.361, p=0.005). There were no significant differences in expression of VEGF receptor-2, VE-Cadherin, monocyte chemoattractant protein-1, vimentin, hepatocyte growth factor or CD68; and the extent of inflammatory infiltrate in areas of medial neovascularisation was not significantly different between rupture site and anterior sac.
Conclusions
This study demonstrated increased medial neovascularisation at the aneurysm rupture site in association with overexpression of the pro-angiogenic cytokines VEGF, HIF-1a, av-integrin, TGF-b1 and bFGF. HIF-1a overexpression suggests that the increased angiogenesis was a homeostatic response to hypoxia. Further investigations into whether this angiogenic response is a causative factor of aneurysm rupture are needed.
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