Angiotensin II promotes atherosclerotic lesions and aneurysms in apolipoprotein E-deficient mice

Abstract

Increased plasma concentrations of angiotension II (Ang II) have been implicated in atherogenesis. To examine this relationship directly, we infused Ang II or vehicle for 1 month via osmotic minipumps into mature apoE(-/-) mice. These doses of Ang II did not alter arterial blood pressure, body weight, serum cholesterol concentrations, or distribution of lipoprotein cholesterol. However, Ang II infusions promoted an increased severity of aortic atherosclerotic lesions. These Ang II-induced lesions were predominantly lipid-laden macrophages and lymphocytes; moreover, Ang II promoted a marked increase in the number of macrophages present in the adventitial tissue underlying lesions. Unexpectedly, pronounced abdominal aortic aneurysms were present in apoE(-/-) mice infused with Ang II. Sequential sectioning of aneurysmal abdominal aorta revealed two major characteristics: an intact artery that is surrounded by a large remodeled adventitia, and a medial break with pronounced dilation and more modestly remodeled adventitial tissue. Although no atherosclerotic lesions were visible at the medial break point, the presence of hyperlipidemia was required because infusions of Ang II into apoE(+/+) mice failed to generate aneurysms. These results demonstrate that increased plasma concentrations of Ang II have profound and rapid effects on vascular pathology when combined with hyperlipidemia, in the absence of hemodynamic influences.

Figure 1

Arterial blood pressure determined using a catheter in the femoral artery of anesthetized apoE^–/– mice. Mice were anesthetized after 28 days of infusion of vehicle or the stated dose of Ang II. Points represent the mean of at least seven observations, and bars represent SEM.

Figure 2

Distribution of lipoprotein cholesterol in serum during infusion of vehicle (a), Ang II at 500 ng/min/kg (b), or Ang II at 1,000 ng/min/kg (c). Serum (50 μL) was resolved with a Sepharose 6B column, and cholesterol content of fractions was analyzed with commercially available assay kits. Points represent the mean of five chromatographic separations that were performed on serum from individual mice, and bars represent SEM.

Figure 3

Percent of intimal area covered by grossly discernible atherosclerotic lesions in the thoracic region. Circles represent the values for individual animals, and bars are the means for the groups. Infusion of Ang II significantly increased the percent of lesion area in the thoracic aorta. ^AStatistical difference of P > 0.05 from the control group using a Wilcoxon’s rank sum test.

Figure 4

Characteristics of atherosclerotic lesions from vehicle-infused apoE^–/– mice (a) and Ang II–infused mice (b–d). Tissues were immunostained for the presence of macrophages (a–c) using an antiserum purchased from Accurate Chemical and Scientific Corp. (1:10,000 dilution). Lymphocytes were immunostained (d) using a CD90 antibody from BioSource International (1:30 dilution). Tissue sections were counterstained with hematoxylin. a–c, ×200; d ×400. An arrow indicates the medial-intimal boundary.

Figure 5

An example of the aneurysms formed in the abdominal aorta of apoE^–/– mice infused with Ang II. The aortic segments shown are from approximately the last intercostal branch to the ileal bifurcation. The aorta on the right is an example from an apoE^–/– mouse infused with AngII (1,000 ng/min/kg) for 28 days. The aorta on the left is an example from an age- and gender-matched apoE^–/– mouse infused with vehicle for the same interval.

Figure 6

Characteristics of aneurysmal tissue in the abdominal aorta of apoE^–/– mice infused with Ang II. Sections are stained or immuno-stained with the following methods: (a) Verhoeff ; (b) Gimori; (c) Oil Red O; (d) presence of macrophages using antisera from Accurate Chemical and Scientific Corp. (dilution 1:10,000) and counterstained with hematoxylin; (e) Oil Red O; and (f) presence of macrophages as in d. ×200.