Impaired Vascular Contractility and Aortic Wall Degeneration in Fibulin-4 Deficient Mice: Effect of Angiotensin II Type 1 (AT1) Receptor Blockade

Abstract

Medial degeneration is a key feature of aneurysm disease and aortic dissection. In a murine aneurysm model we investigated the structural and functional characteristics of aortic wall degeneration in adult fibulin-4 deficient mice and the potential therapeutic role of the angiotensin (Ang) II type 1 (AT1) receptor antagonist losartan in preventing aortic media degeneration. Adult mice with 2-fold (heterozygous Fibulin-4+/R) and 4-fold (homozygous Fibulin-4R/R) reduced expression of fibulin-4 displayed the histological features of cystic media degeneration as found in patients with aneurysm or dissection, including elastin fiber fragmentation, loss of smooth muscle cells, and deposition of ground substance in the extracellular matrix of the aortic media. The aortic contractile capacity, determined by isometric force measurements, was diminished, and was associated with dysregulation of contractile genes as shown by aortic transcriptome analysis. These structural and functional alterations were accompanied by upregulation of TGF-β signaling in aortas from fibulin-4 deficient mice, as identified by genome-scaled network analysis as well as by immunohistochemical staining for phosphorylated Smad2, an intracellular mediator of TGF-β. Tissue levels of Ang II, a regulator of TGF-β signaling, were increased. Prenatal treatment with the AT1 receptor antagonist losartan, which blunts TGF-β signaling, prevented elastic fiber fragmentation in the aortic media of newborn Fibulin-4R/R mice. Postnatal losartan treatment reduced haemodynamic stress and improved lifespan of homozygous knockdown fibulin-4 animals, but did not affect aortic vessel wall structure. In conclusion, the AT1 receptor blocker losartan can prevent aortic media degeneration in a non-Marfan syndrome aneurysm mouse model. In established aortic aneurysms, losartan does not affect aortic architecture, but does improve survival. These findings may extend the potential therapeutic application of inhibitors of the renin-angiotensin system to the preventive treatment of aneurysm disease.

Differences in genetic signaling, and not mechanical properties of the wall, are linked to ascending aortic aneurysms in fibulin-4 knockout mice

“…We chose several genes from the microarray results for verification of expression by qPCR. Based on previous research involving fibulin-4 mutations, we focused on genes involved in SMC migration and proliferation, ERK1/2 activation (34), inflammation (57,65), and ECM remodeling (31) ( Table 2). Genes were chosen that were up-or downregulated in Fbln4 Ϫ/Ϫ compared with Fbln4 ϩ/ϩ at both AA and DA locations.…”

“…TGF-␤ phosphorylation of ShcA also activates ERK1/2 signaling (47). TGF-␤ activity has been linked to fibulin-4 mutations (31,57,65), although the mechanism of interaction between TGF-␤ and fibulin-4 is unclear. Our study suggests that wall thickening and aneurysm development in Fbln4 Ϫ/Ϫ AA may be due to proliferation and migration of SMCs through an activated ERK1/2 pathway that depends on cyclic stretch and upstream signaling molecules such as Hbegf, Ereg, IGF-1, and TGF-␤.…”

“…Mutations in SMC contractile proteins cause TAAs (26,61,89) and altered SMC phenotype and/or contractility is observed in TAAs caused by other genetic defects (12,34,38,57). SMCs, ECM, and signaling molecules in the arterial wall are a tightly integrated system, and the emerging genetic evidence suggests that disruptions at many points of the system (i.e., SMC contraction, specific ECM components, and TGF-␤ signaling) can lead to TAAs.…”

Hypertension

“…31 Both the transforming growth factor-β-induced canonical (pSmad2/3) and noncanonical (mitogen-activated protein kinases) signaling pathways are upregulated in thoracic aortic aneurysms mouse models, and their suppression may underlie the effectiveness of AT 1 receptor blockade in these models. 31,32 As a consequence of these exciting new findings, multiple trials now investigate the effectiveness of RAAS blockers in Marfan's syndrome. 29 An important issue will be to what degree ACE inhibition (which does not result in AT 2 receptor stimulation) differs from AT 1 receptor antagonism.…”

Involvement of the renin–angiotensin system in abdominal and thoracic aortic aneurysms

“…Furthermore, fibulin-4 deficiency in mice leads to TAAs that are associated with the suppression of SMC-specific contractile proteins and degenerative changes that impair the integrity of the aortic structure [91]. A recent study has also provided evidence that AT 1 receptor blockade with losartan attenuates aortic wall abnormality in fibulin-4-deficient mice [92]. This finding indicates that the AngII-AT 1 receptor activation contributes to the structural destruction of the aortic wall induced by depletion of fibulin-4.…”