Transition from atherosclerosis to aortic aneurysm in humans coincides with an increased expression of RAS components
Introduction
Abdominal aortic aneurysm (AAA) is a complex vascular disorder which is most frequently associated with atherosclerosis [1]. Atherosclerotic lesions often, but not necessarily, proceed to aneurysmal disease [1]. AAAs are normally symptomless and only diagnosed by chance or in the event of rupture. Since overall mortality of AAA rupture still accounts to 80–90%, it would be desirable to diagnose patients early enough to forward them to elective AAA repair, and to better define patients at risk with respect to the mechanisms underlying the progression from atherosclerosis to AAA. Moreover, there is still need for more specific and non-invasive ways of treatment and/or prevention.
Angiotensin II (Ang II), the main effector peptide of the renin-angiotensin system (RAS), is known to play a critical role in vascular remodelling. It is produced systemically by the “circulating” RAS, and locally by “tissue” RAS [2] with the vessel wall being one of the first tissues for which a local RAS was demonstrated [2], [3]. Systemically or locally produced Ang II can bind to AT1- or AT2 receptors with AT1 receptors mainly localized within the adventitia and vascular smooth muscle cells and the AT2 receptor preferably localized on endothelial cells [4]. The “classical” enzymatic cascade leading to Ang II generation from the precursor angiotensinogen comprises cleavage of angiotensinogen by renin resulting in Ang I and subsequent cleavage of Ang I by ACE generating Ang II (supplementary online material: Fig. 1). However, the formation of Ang II by peptidases/proteinases different from ACE and renin has been recognized as an alternative Ang II-generating pathway. For example, while cathepsin D converts angiotensinogen to Ang I, cathepsin G, tissue plasminogen activator and tonin convert angiotensinogen directly to Ang II, and chymase and cathepsin A convert Ang I to Ang II [5]. These alternate Ang II-formation pathways appear to be important for the production of Ang II on a tissue level and, therefore, in the development of vascular disease. Some of these proteases may be brought to the diseased site within a tissue by infiltrating cells such as leukocytes (e.g. cathepsin D and cathepsin G is secreted by monocytes and neutrophils), T-cells (also secreting cathepsin D and cathepsin G) or mast cells (an important source of chymase, cathepsin D and cathepsin G and even renin) [6], [7], [8], [9].
There is evolving evidence that Ang II plays a key role in the initiation and propagation of atherosclerosis and AAA [1], [10]. This is supported by studies in animals and humans, which have consistently demonstrated that application of Ang II promotes the formation of both, atherosclerosis and AAA, and that ACE inhibitors [11], [12], [13] and AT1-receptor blockers [14], [15] can decelerate the progression of atherosclerosis and AAA formation independently of blood pressure reduction. Atherosclerosis and AAA are both characterized by a chronic inflammatory reaction, matrix degradation, and vascular tissue remodelling. The inflammatory reaction is initiated and maintained by activated macrophages, and activation of macrophages may be a central mechanism of action by which Ang II promotes both diseases. However, little is known about differences in RAS activation and expression between atherosclerosis and AAA, particularly in humans. Moreover, studies investigating the role of Ang II production in human aneurysm are limited and partly contradictory. While some authors have shown that human aneurysmal tissue possesses an increased ability to generate Ang II via ACE and chymase compared to normal tissue [3], [16], the comparison of the vascular aneurysmal wall from patients with ruptured or unruptured cerebral aneurysms suggested that a decreased expression of local RAS components plays a role in the pathogenesis of the disease [17].
The aim of the present study was to investigate how local RAS components are regulated in the aneurysmatic aorta compared to atherosclerotic and healthy tissues, and – based on differential expression patterns – to identify those molecules of the RAS cascade, which may contribute to the process of aneurysm formation from atherosclerotic lesions.
Section snippets
Tissue samples
Human AAA segments were obtained from patients undergoing elective repair (n = 10). The average age was 70.5 years (range, 53–81 years). The average diameter of the aneurysmal lesions estimated by CT scan and/or angiography was 5.8 cm (range, 4.5–8.0 cm). Eighty percent of patients were men and 20% women. Nonaneurysmal aortic specimens were obtained at autopsy from nine subjects with atherosclerosis of the aorta (advanced lesions, type IV, average age, 61.6 years; range, 46–84 years; 80% men, 20%
Angiotensinogen
Western blot analysis of tissue samples from healthy, atherosclerotic and aneurysmal tissue using an antibody directed against angiotensinogen revealed a double positive band at ∼60 kDa which corresponds to the molecular weight of angiotensinogen. The expression of angiotensinogen was significantly increased in the atherosclerotic tissues (1.7-fold) and even stronger in the AAA (5.4-fold) as compared to normal aorta, respectively (Fig. 2 of supplementary online material). Since the antibody we
Discussion
Abdominal aortic aneurysm often originates from atherosclerotic lesions [1], [10]. In order to early recognize and treat patients at risk to develop AAA and in order to find more specific and non-invasive ways of treatment and/or prevention, it would be helpful to learn more about the mechanisms underlying transition from atherosclerosis to AAA.
Angiotensin II, the main effector hormone of the RAS, is involved in the pathogenesis of both, atherosclerosis and AAA [1], [10]. This study compared
Conclusions
Summarizing our results, most components of the RAS are up-regulated in atherosclerosis and even more so in AAA. This suggests that a further activation of the RAS, which even exceeds the activation already present in atherosclerosis, may contribute to the development of AAA from atherosclerotic lesions.
Some components which take part in Ang II generation, namely chymase, cathepsin G and cathepsin D, additionally possess features such as chemotaxis or proteolysis which may accessorily
Acknowledgment
This study was supported by a grant of the German Ministry of Education and Research (BMBF) as part of the Competence Network of National Genomic Research (NGFN-2) for cardiovascular diseases.
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