No abstract

Background-Hypercholesterolemia-induced atherosclerosis is attenuated by either pharmacological antagonism of AT 1 receptors or AT 1A receptor deficiency. However, the mechanism underlying the pronounced responses to angiotensin II (Ang II) antagonism has not been determined. We hypothesized that hypercholesterolemia stimulates the production of angiotensin peptides to provide a rationale for the profound effect of AT 1A receptor deficiency on atherogenesis. Methods and Results-Atherosclerotic lesions were analyzed in LDL receptor-deficient mice. Immunocytochemical analysis demonstrated that atherosclerotic lesions contained all the components of the conventional pathway for Ang II synthesis. AT 1A receptor deficiency caused a marked decrease in atherosclerotic lesion size in both the aortic root and arch of male and female mice, without a discernible effect on composition. AT 1A receptor deficiency-induced reductions in atherosclerosis were independent of systolic blood pressure and measurements of oxidation and chemoattractants. Aortic AT 2 receptor mRNA expression was not altered in AT 1A receptor-deficient mice, and AT 2 receptor deficiency had no effect on lesion area or cellular composition. Hypercholesterolemia greatly augmented the systemic renin-angiotensin system, as demonstrated by large increases in plasma concentrations of angiotensinogen and angiotensin peptides (Ang II, III, IV, and 4 -8). These increases were ablated in hypercholesterolemic AT 1A receptor-deficient mice. Conclusions-AT 1A receptor deficiency had a striking effect in reducing hypercholesterolemia-induced atherosclerosis in LDL receptor-negative mice. Hypercholesterolemia was associated with increased systemic angiotensinogen and angiotensin peptides, which were reduced in AT 1A receptor-deficient mice. These results demonstrate that hypercholesterolemia-induced stimulation of angiotensin peptide production provides a basis for the marked effect of AT 1A receptor deficiency in reducing atherosclerosis.

AngII (angiotensin II) induces atherosclerosis and AAAs (abdominal aortic aneurysms) through multiple proposed mechanisms, including chemotaxis. Therefore, we determined the effects of whole-body deficiency of the chemokine receptor CCR2 (CC chemokine receptor 2) on these diseases. To meet this objective, apoE (apolipoprotein E)−/− mice that were either CCR2+/+ or CCR2−/−, were infused with either saline or AngII (1000 ng·kg−1 of body weight·min−1) for 28 days via mini-osmotic pumps. Deficiency of CCR2 markedly attenuated both atherosclerosis and AAAs, unrelated to systolic blood pressure or plasma cholesterol concentrations. During the course of the present study, we also observed that AngII infusion led to large dilatations that were restricted to the ascending aortic region of apoE−/− mice. The aortic media in most of the dilated area was thickened. In regions of medial thickening, distinct elastin layers were discernable. There was an expansion of the distance between elastin layers in a gradient from the intimal to the adventitial aspect of the media. This pathology differed in a circumscribed area of the anterior region of ascending aortas in which elastin breaks were focal and almost transmural. All regions of the ascending aorta of AngII-infused mice had diffuse medial macrophage accumulation. Deficiency of CCR2 greatly attenuated the AngII-induced lumen dilatation in the ascending aorta. This new model of ascending aortic aneurysms has pathology that differs markedly from AngII-induced atherosclerosis or AAAs, but all vascular pathologies were attenuated by CCR2 deficiency.

The role of the renin angiotensin system (RAS) in atherosclerosis is complex because of the involvement of multiple peptides and receptors. Renin is the rate-limiting enzyme in the production of all angiotensin peptides. To determine the effects of renin inhibition on atherosclerosis, we administered the novel renin inhibitor aliskiren over a broad dose range to fat-fed LDL receptor-deficient (Ldlr -/-) mice. Renin inhibition resulted in striking reductions of atherosclerotic lesion size in both the aortic arch and the root. Subsequent studies demonstrated that cultured macrophages expressed all components of the RAS. To determine the role of macrophage-derived angiotensin in the development of atherosclerosis, we transplanted renin-deficient bone marrow to irradiated Ldlr -/-mice and observed a profound decrease in the size of atherosclerotic lesions. In similar experiments, transplantation of bone marrow deficient for angiotensin II type 1a receptors failed to influence lesion development. We conclude that renin-dependent angiotensin production in macrophages does not act in an autocrine/paracrine manner. Furthermore, in vitro studies demonstrated that coculture with renin-expressing macrophages augmented monocyte adhesion to endothelial cells. Therefore, although previous work suggests that angiotensin peptides have conflicting effects on atherogenesis, we found that renin inhibition profoundly decreased lesion development in mice.

Objective-Angiotensin II (AngII) infusion into hypercholesterolemic mice accelerates atherosclerosis and promotes formation of abdominal aortic aneurysms (AAAs). The purpose of this study was to define whether AngII interacts with receptors on infiltrating versus resident cells in promoting vascular pathologies. Methods and Results-Male LDL receptorϪ/Ϫ mice, that were either AT1a receptor ϩ/ϩ or Ϫ/Ϫ, were fed a fat enriched diet and infused with either saline or AngII. AngII-induced augmentation of atherosclerosis and formation of AAAs was ablated in AT1a receptorϪ/Ϫ mice. Bone marrow transplantation studies were performed to determine the role of AT1a receptors expressed on infiltrating cells. AT1a receptor ϩ/ϩ and Ϫ/Ϫ mice were irradiated and repopulated with bone marrow-derived stem cells of either genotype. These 4 groups of chimeric mice were infused with either saline or AngII. Repopulation of irradiated AT1a receptor ϩ/ϩ mice with Ϫ/Ϫ bone marrow-derived cells resulted in modest reductions in AngII-induced atherosclerosis. Unexpectedly, AT1a receptor-deficient recipient mice were dramatically protected from AngII-induced vascular pathologies, irrespective of donor genotype. Conclusion-

Rationale: Human studies and mouse models have provided evidence for angiotensin II (Ang II)-based mechanisms as an underlying cause of aneurysms localized to the ascending aorta. In agreement with this associative evidence, we have published recently that Ang II infusion induces aneurysmal pathology in the ascending aorta.Objective: The aim of this study was to define the role of angiotensin II type 1a (AT 1a ) receptors and their cellular location in Ang II-induced ascending aortic aneurysms (AAs). Methods and Results: Male LDL receptor؊/؊ mice were fed a saturated fat-enriched diet for 1 week before osmotic mini-pump implantation and infused with either saline or Ang II (1000 ng/kg per minute) for 28 days. Intimal surface areas of ascending aortas were measured to quantify ascending AAs. Whole body AT 1a receptor deficiency ablated Ang II-induced ascending AAs (P<0.001). To determine the role of AT 1a receptors on leukocytes, LDL receptor ؊/؊ ؋AT 1a receptor ؉/؉ or AT 1a receptor ؊/؊ mice were irradiated and repopulated with bone marrow-derived cells isolated from either AT 1a receptor ؉/؉ or AT 1a receptor ؊/؊ mice. Deficiency of AT 1a receptors in bone marrow-derived cells had no effect on Ang II-induced ascending AAs. To determine the role of AT 1a receptors on vascular wall cells, we developed AT 1a receptor floxed mice with depletion on either smooth muscle or endothelial cells using Cre driven by either SM22 or Tek, respectively. AT 1a receptor deletion in smooth muscle cells had no effect on ascending AAs. In contrast, endothelial-specific depletion attenuated this pathology. Key Words: ascending aneurysm Ⅲ angiotensin II Ⅲ AT 1a receptor Ⅲ Tek-cre A scending aortic aneurysm (AA) is an asymptomatic expansion of this restricted region in which rupture has catastrophic consequences. 1 It has now become apparent that ascending AAs have a significantly higher incidence than originally thought. Furthermore, studies and communities in which autopsies are routinely performed have demonstrated that the incidence of aortic aneurysms is increasing. 2 Recent human and experimental studies have inferred a role for angiotensin II (Ang II) in the development of ascending AAs. [3][4][5][6] Ascending AAs can be generated in transgenic mice that express a common mutation of fibrillin-1 present in patients with Marfan syndrome. There are also several other genes that have been associated with AAs. 4 The aortic dilation that is localized to the ascending region is prevented by the administration of the AT 1 receptor antagonist, losartan. 5 Retrospective analysis of pharmacological treatments given to individuals with Marfan syndrome demonstrated that administration of losartan attenuated dilation of the ascending aorta. 3 Prospective evaluations of angiotensin receptor antagonists are currently being performed in populations afflicted with Marfan syndrome. 7 Ang II has diverse effects that could be implicated in aneurysm formation in the ascending aorta. Ang II exerts its bioactive effects mainly via stimulation o...

Infusion of ANG II in hyperlipidemic mice augments atherosclerosis and causes formation of abdominal aortic aneurysms (AAAs). The purpose of this study was to define the contribution of ANG II-induced hypertension to these vascular pathologies. Male apolipoprotein E (apoE)- and LDL receptor (LDLr)-deficient mice were infused with ANG II (1,000 ng.kg(-1).min(-1)) or norepinephrine (NE; 5.6 mg.kg(-1).day(-1)) for 28 days. Infusion of ANG II or NE increased mean arterial pressure (MAP; ANG II, 133 +/- 2.8; NE, 129 +/- 13 mmHg) to a similar extent compared with baseline blood pressures (MAP, 107 +/- 2 mmHg). Abdominal aortic width increased in both apoE-deficient (apoE(-/-)) or LDLr-deficient (LDLr(-/-)) mice infused with ANG II (apoE(-/-): 1.4 +/- 0.1; LDLr(-/-): 1.6 +/- 0.2 mm). In contrast, NE did not change diameters of abdominal aortas (apoE(-/-): 0.91 +/- 0.03; LDLr(-/-): 0.87 +/- 0.02 mm). Similarly, atherosclerotic lesions in aortic arches were much greater in mice infused with ANG II compared with NE. At a subpressor infusion rate of ANG II (500 ng.kg(-1).min(-1)), AAAs developed in 50% of apoE(-/-) mice. Alternatively, administration of hydralazine (250 mg/l) to ANG II-infused apoE(-/-) mice (1,000 ng.kg(-1).min(-1)) lowered systolic blood pressure (day 28: ANG II, 157 +/- 6; ANG II/hydralazine, 135 +/- 6 mmHg) but did not prevent AAA formation or atherosclerosis. These results demonstrate that infusion of ANG II to hyperlipidemic mice induces AAAs and augments atherosclerosis independent of increased blood pressure.

Angiotensin II (AngII) infusion initiates abdominal aortic aneurysm (AAA) development due to medial disruption and results in luminal dilation and thrombus formation. The objective of this study was to determine whether AAA progressed during protracted AngII infusion. Male apoE(-/-) mice were infused with AngII using miniosmotic pumps. On day 27, suprarenal aortic luminal diameters were ultrasonically measured to identify mice exhibiting AAAs. Mice were designated to three groups with similar mean luminal dilation. Group 1 mice were sacrificed on day 28. Group 2 and 3 mice were subsequently infused with saline or AngII, respectively, for an additional 56 days. In Group 2, saline infusion-after the initial 28 days of AngII infusion-led to an immediate decrease in systolic blood pressure. Over the subsequent 56 days of saline infusion, there were no aneurysm-related deaths or significant changes in luminal diameter. In contrast, continuous AngII infusion in Group 3 maintained persistently increased systolic blood pressure, with aneurysmal rupture-associated deaths, increased luminal diameters, and tissue remodeling. Aortic aneurysmal segments that expanded during continuous AngII infusion exhibited macrophage accumulation in regions of medial disruption, predominantly on the adventitial aspect. Macrophages immunostained for CD206 more than for iNOS, consistent with an M2 phenotype. In conclusion, prolonged AngII infusion promotes AAA expansion, and is associated with enhanced rupture rates and increased macrophage infiltration.