Catalase overexpression in aortic smooth muscle prevents pathological mechanical changes underlying abdominal aortic aneurysm formation

Abstract

The causality of the associations between cellular and mechanical mechanisms of abdominal aortic aneurysm (AAA) formation has not been completely defined. Because reactive oxygen species are established mediators of AAA growth and remodeling, our objective was to investigate oxidative stress-induced alterations in aortic biomechanics and microstructure during subclinical AAA development. We investigated the mechanisms of AAA in an angiotensin II (ANG II) infusion model of AAA in apolipoprotein E-deficient (apoE(-/-)) mice that overexpress catalase in vascular smooth muscle cells (apoE(-/-)xTg(SMC-Cat)). At baseline, aortas from apoE(-/-)xTg(SMC-Cat) exhibited increased stiffness and the microstructure was characterized by 50% more collagen content and less elastin fragmentation. ANG II treatment for 7 days in apoE(-/-) mice altered the transmural distribution of suprarenal aortic circumferential strain (quantified by opening angle, which increased from 130 ± 1° at baseline to 198 ± 8° after 7 days of ANG II treatment) without obvious changes in the aortic microstructure. No differences in aortic mechanical behavior or suprarenal opening angle were observed in apoE(-/-)xTg(SMC-Cat) after 7 days of ANG II treatment. These data suggest that at the earliest stages of AAA development H(2)O(2) is functionally important and is involved in the control of local variations in remodeling across the vessel wall. They further suggest that reduced elastin integrity at baseline may predispose the abdominal aorta to aneurysmal mechanical remodeling.

Fig. 1.

Mechanical behavior of aortas of apolipoprotein E-deficient (apoE^−/−) mice vs. apoE^−/− mice that overexpress catalase in vascular smooth muscle cells (apoE^−/−xTg^SMC-Cat) at baseline as acquired by fixed length aortic inflation and opening angle measurement. A and B: pressure vs. diameter and pressure-dependent compliance vs. pressure behavior showed that aortas from apoE^−/− mice are more compliant than aortas from apoE^−/−xTg^SMC-Cat mice. C: mean circumferential stress (σ_θ) vs. mid-wall circumferential Green strain (E̅_θ) behavior of apoE^−/−xTg^SMC-Cat aortas shifted to the left of apoE^−/− aortas, indicating that the aortic material properties of the groups differed at baseline. Mean circumferential stress and strain were calculated using ex vivo and histological mean morphological measurements provided in Table 1. D: mean circumferential Green strain vs. normalized radius [E_θ(r)] showed that the distribution of strain across the aortic wall of both groups was constant and that the strain magnitude in the apoE^−/− aortas was greater than in apoE^−/−xTg^SMC-Cat. Data were analyzed by ANOVA using a Bonferroni's post hoc analysis. *P < 0.05.

Fig. 2.

Morphology of apoE^−/− and apoE^−/−xTg^SMC-Cat aortas at baseline. A: total collagen content per aortic dry weight decreased by 41% in apoE^−/− compared with apoE^−/−xTg^SMC-Cat. B: apoE^−/− adventitia thickness was 32% less than apoE^−/−xTg^SMC-Cat. C and D: representative images of the aortic wall stained with picrosirius red and imaged under polarized light. ApoE^−/− image shows red and green illumination indicating presence of type I and type III collagen, respectively. ApoE^−/−xTg^SMC-Cat aortic cross section showed intense red illumination indicating a greater abundance of type I collagen. E: elastin fragmentation in the abdominal aortic wall was greater in apoE^−/− compared with apoE^−/−xTg^SMC-Cat aortas. Elastin fragmentation was expressed as the number of breaks in elastic laminae per medial area. Medial area was defined as the area enclosed by the inner and outer elastic laminae measured from aortic sections. *P < 0.05; n = 5 for apoE^−/−; n = 3 for apoE^−/−xTg^SMC-Cat.

Fig. 3.

Mechanical response of apoE^−/− aortas after ANG II treatment for 7 days. A and B: ANG II treatment had no effect on the pressure vs. diameter and compliance vs. pressure behavior of aortas from apoE^−/− mice (n = 5–6). C: mean circumferential stress (σ_θ) vs. mean mid-wall circumferential Green strain (E̅_θ) changed only at high E̅_θ values, where aortas from treated apoE^−/− mice showed reduced circumferential stress. D: mean circumferential Green strain [E_θ(r)] vs. normalized radius showed the distribution of strain across the aortic wall in apoE^−/− aortas changed after ANG II treatment to reach higher strain at the outer wall radius. ANG II treatment did not affect the strain distribution of aortas from apoE^−/−xTg^SMC-Cat mice; n = 3; for apoE^−/−; n = 5–8 for apoE^−/− + ANG II; n = 4 for apoE^−/−xTg^SMC-Cat; n = 5 for apoE^−/−xTg^SMC-Cat + ANG II.

Fig. 4.

Opening angle of apoE^−/− and apoE^−/− + ANG II aortas and morphology of apoE^−/− and apoE^−/−xTg^SMC-Cat aortas after ANG II treatment for 7 days. A: apoE^−/− abdominal aortic opening angle (α) increased after ANG II treatment. B: collagen content decreased in the apoE^−/−xTg^SMC-Cat group and was unchanged in the apoE^−/− after ANG II treatment. C: adventitial thickness in apoE^−/−xTg^SMC-Cat aortas decreased to the same level as apoE^−/−. D: elastin fragmentation in the abdominal aortic wall of apoE^−/− mice remained higher than in aortas from apoE^−/−xTg^SMC-Cat mice, with no change from baseline in either group. *P < 0.05; n = 3 for apoE^−/−; n = 5–8 for apoE^−/− + ANG II; n = 4 for apoE^−/−xTg^SMC-Cat; n = 5 for apoE^−/−xTg^SMC-Cat + ANG II.