Loss of Smooth Muscle α-Actin Leads to NF-κB-Dependent Increased Sensitivity to Angiotensin II in Smooth Muscle Cells and Aortic Enlargement

RATIONALE

Mutations in ACTA2, encoding the smooth muscle isoform of α-actin, cause thoracic aortic aneurysms, acute aortic dissections, and occlusive vascular diseases.

OBJECTIVE

We sought to identify the mechanism by which loss of smooth muscle α-actin causes aortic disease.

METHODS AND RESULTS

Acta2^-/- mice have an increased number of elastic lamellae in the ascending aorta and progressive aortic root dilation as assessed by echocardiography that can be attenuated by treatment with losartan, an angiotensin II (AngII) type 1 receptor blocker. AngII levels are not increased in Acta2^-/- aortas or kidneys. Aortic tissue and explanted smooth muscle cells from Acta2^-/- aortas show increased production of reactive oxygen species and increased basal nuclear factor κB signaling, leading to an increase in the expression of the AngII receptor type I a and activation of signaling at 100-fold lower levels of AngII in the mutant compared with wild-type cells. Furthermore, disruption of smooth muscle α-actin filaments in wild-type smooth muscle cells by various mechanisms activates nuclear factor κB signaling and increases expression of AngII receptor type I a.

CONCLUSIONS

These findings reveal that disruption of smooth muscle α-actin filaments in smooth muscle cells increases reactive oxygen species levels, activates nuclear factor κB signaling, and increases AngII receptor type I a expression, thus potentiating AngII signaling in vascular smooth muscle cells without an increase in the exogenous levels of AngII.

Abstract 458: Loss of Smooth Muscle α-actin in Mice Results in Thoracic Aortic Aneurysms via Increased Reactive Oxygen Species, Increased Nox4,and Increased Angiotensin II type 1 Receptor-Mediated Signaling

Objective: ACTA2 mutations cause 10-14% of familial thoracic aortic aneurysms and dissections. Mice deficient in smooth muscle α-actin (Acta2-/-) develop root and ascending thoracic aortic enlargement associated with thickening of the aortic media and fragmentation and disarray of elastic fibers. We hypothesized that blocking AT1 activation would block the aortic pathology and prevent aortic enlargement in Acta2-/- mice.

Methods and Results: Beginning at 4 weeks of age, Acta2-/- mice were treated with losartan or placebo (n≥10) for 6 months and echocardiograms were performed at baseline and every other month. The aortic root in Acta2-/- mice was found to undergo progressive dilatation. After 6 months of treatment, there was no difference in the diameter of the aortic root between wild-type (WT) mice and the losartan treated mice (p=0.44). Histologic analysis of Acta2-/- aortas demonstrated medial thickening and fragmentation of elastic fibers which was normalized by treatment with losartan. Gene expression of matrix metalloproteinase-2 and -9 (Mmp2, Mmp9), along with lumican and decorin, interleukin-6 (Il6) and phosphorylation of RelA (a subunit of nuclear factor κB, NF-κB) was increased in Acta2-/- aortas, and was corrected by treatment with losartan. NADPH oxidase 4 and AT1a mRNA was increased in Acta2-/- aortic smooth muscle cells (SMCs) and aortas. Increase of AT1a was blocked by lowering reactive oxygen species (ROS) with N-actetyl cysteine (NAC). Angiotensin II (AngII) dose response studies suggested Acta2-/- aortic SMCs had increased sensitivity to Ang II. Additionally, Acta2-/- SMCs had increased ROS compared to WT by flow cytometry (P<0.05). Increased ROS was also observed in aortic samples derived from Acta2-/- mice. Blocking ROS using NAC attenuated aneurysm formation by reducing the expression of Mmps and IL6 and reducing signaling through AT1 in Acta2-/- mice. Genetic deletion of AT1a attenuated ROS and expression of Mmps and IL6 levels in the Acta2-/-aortas but not to WT levels.

Conclusions: Our results demonstrate that complete loss of α-SMA leads to aortic dilation and pathologic changes by increasing SMC ROS levels, thus increasing sensitivity to AngII, which results in NF-κB activation and increased expression of Mmps and IL6.

Aortic remodeling after transverse aortic constriction in mice is attenuated with AT1 receptor blockade

OBJECTIVE

Although hypertension is the most common risk factor for thoracic aortic diseases, it is not understood how increased pressures on the ascending aorta lead to aortic aneurysms. We investigated the role of angiotensin II type 1 receptor activation in ascending aortic remodeling in response to increased biomechanical forces using a transverse aortic constriction (TAC) mouse model.

APPROACH AND RESULTS

Two weeks after TAC, the increased biomechanical pressures led to ascending aortic dilatation and thickening of the medial and adventitial layers of the aorta. There was significant adventitial hyperplasia and inflammatory responses in TAC ascending aortas were accompanied by increased adventitial collagen, elevated inflammatory and proliferative markers, and increased cell density attributable to accumulation of myofibroblasts and macrophages. Treatment with losartan significantly blocked TAC-induced vascular inflammation and macrophage accumulation. However, losartan only partially prevented TAC-induced adventitial hyperplasia, collagen accumulation, and ascending aortic dilatation. Increased Tgfb2 expression and phosphorylated-Smad2 staining in the medial layer of TAC ascending aortas were effectively blocked with losartan. In contrast, the increased Tgfb1 expression and adventitial phospho-Smad2 staining were only partially attenuated by losartan. In addition, losartan significantly blocked extracellular signal-regulated kinase activation and reactive oxygen species production in the TAC ascending aorta.

CONCLUSIONS

Inhibition of the angiotensin II type 1 receptor using losartan significantly attenuated the vascular remodeling associated with TAC but did not completely block the increased transforming growth factor-β1 expression, adventitial Smad2 signaling, and collagen accumulation. These results help to delineate the aortic transforming growth factor-β signaling that is dependent and independent of the angiotensin II type 1 receptor after TAC.

Smooth muscle hyperplasia due to loss of smooth muscle α-actin is driven by activation of focal adhesion kinase, altered p53 localization and increased levels of platelet-derived growth factor receptor-β

Mutations in ACTA2, encoding the smooth muscle cell (SMC)-specific isoform of α-actin (α-SMA), cause thoracic aortic aneurysms and dissections and occlusive vascular diseases, including early onset coronary artery disease and stroke. We have shown that occlusive arterial lesions in patients with heterozygous ACTA2 missense mutations show increased numbers of medial or neointimal SMCs. The contribution of SMC hyperplasia to these vascular diseases and the pathways responsible for linking disruption of α-SMA filaments to hyperplasia are unknown. Here, we show that the loss of Acta2 in mice recapitulates the SMC hyperplasia observed in ACTA2 mutant SMCs and determine the cellular pathways responsible for SMC hyperplasia. Acta2(-/-) mice showed increased neointimal formation following vascular injury in vivo, and SMCs explanted from these mice demonstrated increased proliferation and migration. Loss of α-SMA induced hyperplasia through focal adhesion (FA) rearrangement, FA kinase activation, re-localization of p53 from the nucleus to the cytoplasm and increased expression and ligand-independent activation of platelet-derived growth factor receptor beta (Pdgfr-β). Disruption of α-SMA in wild-type SMCs also induced similar cellular changes. Imatinib mesylate inhibited Pdgfr-β activation and Acta2(-/-) SMC proliferation in vitro and neointimal formation with vascular injury in vivo. Loss of α-SMA leads to SMC hyperplasia in vivo and in vitro through a mechanism involving FAK, p53 and Pdgfr-β, supporting the hypothesis that SMC hyperplasia contributes to occlusive lesions in patients with ACTA2 missense mutations.

Exome sequencing identifies SMAD3 mutations as a cause of familial thoracic aortic aneurysm and dissection with intracranial and other arterial aneurysms

RATIONALE

Thoracic aortic aneurysms leading to acute aortic dissections (TAAD) can be inherited in families in an autosomal dominant manner. As part of the spectrum of clinical heterogeneity of familial TAAD, we recently described families with multiple members that had TAAD and intracranial aneurysms or TAAD and intracranial and abdominal aortic aneurysms inherited in an autosomal dominant manner.

OBJECTIVE

To identify the causative mutation in a large family with autosomal dominant inheritance of TAAD with intracranial and abdominal aortic aneurysms by performing exome sequencing of 2 distantly related individuals with TAAD and identifying shared rare variants.

METHODS AND RESULTS

A novel frame shift mutation, p. N218fs (c.652delA), was identified in the SMAD3 gene and segregated with the vascular diseases in this family with a logarithm of odds score of 2.52. Sequencing of 181 probands with familial TAAD identified 3 additional SMAD3 mutations in 4 families, p.R279K (c.836G>A), p.E239K (c.715G>A), and p.A112V (c.235C>T), resulting in a combined logarithm of odds score of 5.21. These 4 mutations were notably absent in 2300 control exomes. SMAD3 mutations were recently described in patients with aneurysms osteoarthritis syndrome and some of the features of this syndrome were identified in individuals in our cohort, but these features were notably absent in many SMAD3 mutation carriers.

CONCLUSIONS

SMAD3 mutations are responsible for 2% of familial TAAD. Mutations are found in families with TAAD alone, along with families with TAAD, intracranial aneurysms, abdominal aortic and bilateral iliac aneurysms segregating in an autosomal dominant manner.

Paucity of skeletal manifestations in Hispanic families with FBN1 mutations

Marfan syndrome (MFS) is an autosomal dominant condition with pleiotropic manifestations involving the skeletal, ocular, and cardiovascular systems. The diagnosis is based primarily on clinical involvement of these and other systems, referred to as the Ghent criteria. We have identified three Hispanic families from Mexico with cardiovascular and ocular manifestations due to novel FBN1 mutations but with paucity of skeletal features. The largest family, hMFS001, had a frameshift mutation in exon 24 (3075delC) identified as the cause of aortic disease in the family. Assessment of eight affected adults revealed no major skeletal manifestation of MFS. Family hMFS002 had a missense mutation (R1530C) in exon 37. Four members fulfilled the criteria for ocular and cardiovascular phenotype but lacked skeletal manifestations. Family hMFS003 had two consecutive missense FBN1 mutations (C515W and R516G) in exon 12. Eight members fulfilled the ocular criteria for MFS and two members had major cardiovascular manifestations, however none of them met criteria for skeletal system. These data suggest that individuals of Hispanic descent with FBN1 mutations may not manifest skeletal features of the MFS to the same extent as Caucasians. We recommend that echocardiogram, ocular examination and FBN1 molecular testing be considered for any patients with possible MFS even in the absence of skeletal features, including Hispanic patients.

Colombian study to assess the use of noninvasive determination of the endothelium-mediated vasodilation (CANDEV) II: does location of the occlusion device affects the accuracy of the diagnosis?

Endothelial dysfunction (ED), which is often evaluated by flow-mediated vasodilation (FMV) in the brachial artery, has been postulated as a predictor of cardiac events. Although the upper and forearm location of the occlusion device have been used for FMV evaluation, currently there is no consensus whether they provide the same information. The main goal of this study was to evaluate if both techniques have the same accuracy to differentiate subjects with and without cardiovascular risk factors (CRFs). A cross-sectional study in 124 subjects was performed. The volunteers were divided in two groups: 62 subjects (20 women and 42 men) with at least one CRF and 62 (20 women and 42 men) healthy subjects without CRFs. FMV measurements using the cuff in the upper arm and forearm with intervals of 30 min were taken. In all subjects, %FMV with the cuff located in the upper arm was 10.13 +/- 4.5 and 9.8 +/- 4.1 with the cuff located below the elbow. In healthy subjects without CRFs the %FMV in the upper arm was 12.19 +/- 4.0 versus 12.31 +/- 3.4 in the upper forearm, in CRF subjects it was 8.08 +/- 4.0 vs 7.29 +/- 3.2., respectively. FMV was not affected by the location of the cuff in maintaining the test ability and accuracy to differentiate subjects with and without CRFs.