Review of Current Theories for Abdominal Aortic Aneurysm Pathogenesis

Vulnerable areas of atherosclerotic plaques often contain lipid-laden macrophages and display matrix metalloproteinase activity. We hypothesized that reactive oxygen species released by macrophage-derived foam cells could trigger activation of latent proforms of metalloproteinases in the vascular interstitium. We showed that in vivo generated macrophage foam cells produce superoxide, nitric oxide, and hydrogen peroxide after isolation from hypercholesterolemic rabbits. Effects of these reactive oxygens and that of peroxynitrite, likely to result from simultaneous production of nitric oxide and superoxide, were tested in vitro using metalloproteinases secreted by cultured human vascular smooth muscle cells. Enzymes in culture media or affinity-purified (pro-MMP-2 and MMP-9) were examined by SDS-PAGE zymography, Western blotting, and enzymatic assays. Under the conditions used, incubation with xanthine/xanthine oxidase increased the amount of active gelatinases, while nitric oxide donors had no noticeable effect. Incubation with peroxynitrite resulted in nitration of MMP-2 and endowed it with collagenolytic activity. Hydrogen peroxide treatment showed a catalase-reversible biphasic effect (gelatinase activation at concentrations of 4 microM, inhibition at > or = 10-50 microM). Thus, reactive oxygen species can modulate matrix degradation in areas of high oxidant stress and could therefore contribute to instability of atherosclerotic plaques.

In a population-based study of 6,386 men and women aged 25--84 years in Tromsø, Norway, in 1994--1995, the authors assessed the age- and sex-specific distribution of the abdominal aortic diameter and the prevalence of and risk factors for abdominal aortic aneurysm. Renal and infrarenal aortic diameters were measured with ultrasound. The mean infrarenal aortic diameter increased with age. The increase was more pronounced in men than in women. The age-related increase in the median diameter was less than that in the mean diameter. An aneurysm was present in 263 (8.9%) men and 74 (2.2%) women (p < 0.001). The prevalence of abdominal aortic aneurysm increased with age. No person aged less than 48 years was found with an abdominal aortic aneurysm. Persons who had smoked for more than 40 years had an odds ratio of 8.0 for abdominal aortic aneurysm (95% confidence interval: 5.0, 12.6) compared with never smokers. Low serum high density lipoprotein cholesterol was associated with an increased risk for abdominal aortic aneurysm. Other factors associated with abdominal aortic aneurysm were a high level of plasma fibrinogen and a low blood platelet count. Antihypertensive medication (ever use) was significantly associated with abdominal aortic aneurysm, but high systolic blood pressure was a risk factor in women only. This study indicates that risk factors for atherosclerosis are also associated with increased risk for abdominal aortic aneurysm.

The role of inflammation in the pathogenesis of abdominal aortic aneurysms (AAA) is well established. The inflammatory process leads to protease-mediated degradation of the extracellular matrix and apoptosis of smooth muscle cells (SMC), which are the predominant matrix synthesizing cells of the vascular wall. These processes act in concert to progressively weaken the aortic wall, resulting in dilatation and aneurysm formation. Oxidative stress is invariably increased in, and contributes importantly to, the pathophysiology of inflammation. Moreover, reactive oxygen species (ROS) play a key role in regulation of matrix metalloproteinases and induction of SMC apoptosis. ROS may also contribute to the pathogenesis of hypertension, a risk factor for AAA. Emerging evidence suggests that ROS and reactive nitrogen species (RNS) are associated with AAA formation in animal models and in humans. Although experimental data are limited, several studies suggest that modulation of ROS production or activity may suppress AAA formation and improve experimental outcome in rodent models. Although a number of enzymes can produce injurious ROS in the vasculature, increasing evidence points toward a role for NADPH oxidase as a source of oxidative stress in the pathogenesis of AAA.