No abstract

Abdominal aortic aneurysm (AAA) is one of a number of diseases associated with a prominent inflammatory cell infiltrate and local destruction of structural matrix macromolecules. This chronic infiltrate is predominately composed of macrophages and T lymphocytes. Activated macrophages produce a variety of cytokines, including TNF-α. Elevated levels of TNF-α were observed in patients with AAA, suggesting that TNF-α may play a role in the pathogenic mechanisms of AAA. In the present study, we investigated the role of TNF-α in AAA formation. By studying a murine aneurysm model, we found that both mRNA and protein levels of TNF-α were increased in aneurysm tissue compared with normal aortic tissues. Therefore, we tested the response of mice lacking expression of TNF-α. These mice were resistant to aneurysm formation. Our results show that TNF-α deficiency attenuates matrix metalloproteinase (MMP) 2 and MMP-9 expression and macrophage infiltration into the aortic tissue. These data suggest that TNF-α plays a central role in regulating matrix remodeling and inflammation in the aortic wall leading to AAA. In addition, we investigated the pharmacological inhibition of AAA. A Food and Drug Administration-approved TNF-α antagonist, infliximab, inhibited aneurysm growth. Our data also show that infliximab treatment attenuated elastic fiber disruption, macrophage infiltration, and MMP-2 and MMP-9 expression in aortic tissue. This study confirms that a strategy of TNF-α antagonism may be an important therapeutic strategy for treating AAA.

This study demonstrates that doxycycline significantly delays aneurysm rupture in MFS-like mice by inhibiting expression of tissue MMP-2 and MMP-9 and thus, degradation of the elastic matrix. The results suggest that MMPs contribute to the progression of thoracic aneurysm in MFS and that doxycycline has the potential to significantly alter the course of the disease.

Rationale Aneurysm and dissection of the ascending thoracic aorta are the main cardiovascular complication of Marfan Syndrome (MFS) resulting in premature death. Studies using mouse models of MFS have shown that activation of transforming growth factor (TGF)-β and the concomitant up-regulation of matrix metalloproteinases (MMPs) contribute to aneurysm development. Our previous study showed that doxycycline delayed aneurysm rupture in a mouse model of MFS, Fbn1mgR/mgR. Losartan has been shown to prevent aneurysms in another mouse model of MFS, Fbn1 C1039G/+, through inhibition of the Erk1/2 pathway. However, the role of MMP-2 in MFS and effect of losartan on the lifespan of MFS mice remain unknown. Objective We investigated the role of MMP-2 in MFS and compared the effects of losartan and doxycycline on aortic dilatation and survival in Fbn1mgR/mgR mice. Methods and Results By life table analysis, we found that losartan and doxycycline improved the survival of Fbn1mgR/mgR mice. Gelatin zymography and Western blot data showed that only doxycycline inhibited MMP-2 expression while both drugs decreased Erk1/2 phosphorylation. When combined, only one of 9 mice died within the 30 week study; aortic histology and diameter were normalized and the effects on Smad2 phosphorylation was additive. To further explore the role of MMP-2 in MFS, we created MMP-2-deficient Fbn1mgR/mgR mice. MMP-2 deletion inhibited activation of TGF-β and phosphorylation of Erk1/2 and Smad2 and prolonged the lifespan of the mice. Conclusions These studies demonstrated that inhibition of MMP-2 by doxycycline delayed the manifestations of MFS, in part, through its ability to decrease active TGF-β and the noncanonical signaling cascade downstream of TGF-β. This study further suggested that targeting TGF-β signaling at different points might be a more effective strategy for inhibiting disease progression.

Reactive oxygen species (ROS) are increased in human abdominal aortic aneurysms (AAA). NADPH oxidases are the predominant source of superoxide anion (O 2 − ) in the vasculature. Inducible nitric oxide synthase (iNOS) produces significant amount of nitric oxide (NO) during inflammatory processes. We hypothesized that ROS produced by NADPH oxidases and iNOS play an important role in aneurysm formation. We examined this hypothesis using selective blockade of NADPH oxidases and iNOS in a murine model of AAA. Mice, including C57BL/6, iNOS knockout (iNOS −/− ) mice, and its background matched control (C57BL/6), underwent AAA induction by periaortic application of CaCl 2 . Aortic diameter was measured at aneurysm induction and harvest. Beginning 1 week prior to aneurysm induction and continuing to aortic harvest 6 weeks later, one group of the C57BL/6 mice were treated with orally administered apocynin (NADPH oxidase inhibitor). Control mice were given water. The mean diameter and change in diameter of each group were compared with concurrent controls. Aortic levels of the NO metabolite, NO x N(NO 2 and NO 3 ), are significantly increased in CaCl 2 -treated wild type mice. INOS −/− mice are resistant to aneurysm induction. This is associated with reduced expression of matrix metalloproteinase (MMP)-2 and MMP-9 and decreased production of NO x in the aortic tissues. Inhibition of NADPH oxidase by apocynin also blocked aneurysm formation. In conclusion, both iNOS deficiency and NADPH oxidase inhibition suppress aneurysm formation in association with decreased NO x levels. These studies suggest that both the NADPH oxidase and iNOS pathways contribute to ROS production and AAA development.