Alterations in aortic cellular constituents during thoracic aortic aneurysm development: myofibroblast-mediated vascular remodeling

Inhibition of transforming growth factor β worsens elastin degradation in a murine model of Kawasaki disease

Kawasaki disease (KD) is an acute inflammatory illness marked by coronary arteritis. However, the factors increasing susceptibility to coronary artery lesions are unknown. Because transforming growth factor (TGF) β increases elastin synthesis and suppresses proteolysis, we hypothesized that, in contrast to the benefit observed in aneurysms forming in those with Marfan syndrome, inhibition of TGF-β would worsen inflammatory-induced coronary artery lesions. By using a murine model of KD in which injection of Lactobacillus casei wall extract (LCWE) induces coronary arteritis, we show that LCWE increased TGF-β signaling in the coronary smooth muscle cells beginning at 2 days and continuing through 14 days, the point of peak coronary inflammation. By 42 days, LCWE caused fragmentation of the internal and external elastic lamina. Blocking TGF-β by administration of a neutralizing antibody accentuated the LCWE-mediated fragmentation of elastin and induced an overall loss of medial elastin without increasing the inflammatory response. We attributed these increased pathological characteristics to a reduction in the proteolytic inhibitor, plasminogen activator inhibitor-1, and an associated threefold increase in matrix metalloproteinase 9 activity compared with LCWE alone. Therefore, our data demonstrate that in the coronary arteritis associated with KD, TGF-β suppresses elastin degradation by inhibiting plasmin-mediated matrix metalloproteinase 9 activation. Thus, strategies to block TGF-β, used in those with Marfan syndrome, are unlikely to be beneficial and could be detrimental.

Endothelial cell-specific deficiency of Ang II type 1a receptors attenuates Ang II-induced ascending aortic aneurysms in LDL receptor-/- mice

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.

CONCLUSIONS

Ang II infusion promotes aneurysms in the ascending aorta via stimulation of AT(1a) receptors that are expressed on endothelial cells.

Cellular phenotype transformation occurs during thoracic aortic aneurysm development

OBJECTIVE

Thoracic aortic aneurysms result from dysregulated remodeling of the vascular extracellular matrix, which may occur as a result of altered resident cellular function. The present study tested the hypothesis that aortic fibroblasts undergo a stable change in cellular phenotype during thoracic aortic aneurysm formation.

METHODS

Primary murine aortic fibroblasts were isolated from normal and thoracic aortic aneurysm-induced aortas (4 weeks post induction with 0.5 mol/L CaCl(2) 15 minutes) by the outgrowth method. Normal and thoracic aortic aneurysm cultures were examined using a focused polymerase chain reaction array to determine fibroblast-specific changes in gene expression in the absence and presence of biological stimulation (endothelin-1, phorbol-12-myristate-13-acetate, angiotensin-II). The relative expression of 38 genes, normalized to 4 housekeeping genes, was determined, and genes displaying a minimum 2-fold increase/decrease or genes with significantly different normalized cycle threshold values were considered to have altered expression.

RESULTS

At steady state, thoracic aortic aneurysm fibroblasts revealed elevated expression of several matrix metalloproteinases (Mmp2, Mmp11, Mmp14), collagen genes/elastin (Col1a1, Col1a2, Col3a1, Eln), and other matrix proteins, as well as decreased expression of Mmp3, Timp3, and Ltbp1. Moreover, gene expression profiles in thoracic aortic aneurysm fibroblasts were different than normal fibroblasts after equivalent biological stimuli.

CONCLUSIONS

This study demonstrated for the first time that isolated primary aortic fibroblasts from thoracic aortic aneurysm-induced mice possess a unique and stable gene expression profile, and when challenged with biological stimuli, induce a transcriptional response that is different from normal aortic fibroblasts. Together, these data suggest that aortic fibroblasts undergo a stable phenotypic change during thoracic aortic aneurysm development, which may drive the enhancement of extracellular matrix proteolysis in thoracic aortic aneurysm progression.

Role of myofibroblasts in vascular remodelling: focus on restenosis and aneurysm

Myofibroblasts (MFs) are contractile cells deriving from a multiplicity of resident cells and/or circulating progenitors that are known to play a key role in wound healing. They were first discovered and analysed in the early 1970s in granulation tissue. Since their first identification, the role of MF and their mechanisms of differentiation have been highlighted in a number of diseases, including organ fibrosis and tumours, with particular attention devoted to the liver, kidney, and pulmonary fibrosis. The aim of this review is to summarize the current evidence for the role played by MFs in two frequent vascular diseases related to the remodelling of the vascular wall: the different forms of arterial restenosis and the most common forms of thoracic aortic aneurysm. The in-depth knowledge of the molecular pathways involved in MF differentiation, contraction, and survival/apoptosis could contribute to the identification of novel therapeutic strategies for anti-fibrotic and anti-remodelling therapy of vascular diseases in which these cells are involved.

Alterations in membrane type-1 matrix metalloproteinase abundance after the induction of thoracic aortic aneurysm in a murine model

Thoracic aortic aneurysms (TAAs) develop as a result of dysregulated extracellular matrix remodeling mediated by several matrix metalloproteinases (MMPs). Membrane type-1 MMP (MT1-MMP) is the prototypical member of a unique family of membrane-bound MMPs, possessing multiple substrates and functions. The present study tested the hypothesis that MT1-MMP expression, abundance, and activity would be elevated during TAA development and that this protease is produced primarily by mesenchymal cells within the thoracic aorta. Descending thoracic aortas were harvested from C57BL/6J mice at multiple time points (2, 4, 8, and 16 wk, n = 15 each) post-TAA induction (0.5M CaCl(2), 15 min) and compared with reference controls (n = 15). The expression and abundance of MT1-MMP, MMP-2, and tissue inhibitor of metalloproteinase (TIMP)-2 were assessed by quantitative PCR and immunoblot analysis. MT1-MMP activity was determined by fluorescent peptide assay. MT1-MMP was localized within the aortic wall by immunohistochemistry. MT1-MMP abundance and localization in live animals (8 wk post-TAA induction vs. control) was determined by micro-ultrasound imaging with an MT1-MMP-targeted microbubble contrast agent. Aortic diameter was increased 172 +/- 7% at 16 wk post-TAA induction (P < 0.05). MT1-MMP and MMP-2 mRNA levels were elevated at 2 wk post-TAA induction (P < 0.05). MT1-MMP protein abundance increased progressively to a maximum of 178 +/- 26% at 16 wk post-TAA induction, whereas MMP-2 and TIMP-2 peaked at 2 wk post-TAA induction (526 +/- 93% and 376 +/- 48%, respectively, P < 0.05). MT1-MMP colocalized with fibroblasts, and MT1-MMP-targeted contrast binding was elevated in 8-wk TAA-induced mice versus control mice (217 +/- 53% vs. 81 +/- 8%, P < 0.05). In conclusion, these novel results suggest that MT1-MMP plays a dynamic multifunctional role in TAA development and, therefore, may provide a significant target for therapeutic strategies.