Ying and Yang of Stat3 in pathogenesis of aortic dissection.

Aortic dissection (AD) is a medical emergency, in which acute destruction of aortic wall occurs with unknown etiology. Recent studies have uncovered the critical role of inteleukin-6 (IL-6) and inflammatory cells including macrophages in the disease mechanism of AD. IL-6 activates janus kinase and signal transducer and activator of transcription 3 (STAT3) to alter the gene expression program in many cell types, thus regulating various aspects of inflammatory response. We found that in human AD tissue, STAT3 was activated in infiltrating macrophages and in medial smooth muscle cells (SMCs), suggesting that STAT3 may regulate the response of these cell types. However, it is unknown how Stat3 regulates the cell type-specific response in pathogenesis of AD. The role of STAT3 was examined in genetically modified mice in which STAT3 sensitivity was enhanced specifically in macrophages or in SMCs by tissue-specific deletion of suppressor of cytokine signaling 3 (Socs3), a negative regulator of STAT3. Macrophage-specific deletion of Socs3 caused acute enhancement of STAT3 activation, M1-dominant differentiation of macrophages, suppression of tissue repair response of SMCs, and exaggerated AD. In contrast, SMC-specific deletion of Socs3 caused chronic STAT3 activation and low-grade inflammatory response in aortic walls, activation of fibroblasts, M2-dominant differentiation of macrophages, increase in adventitial collagen deposition, resulting in the protection of aorta from AD by reinforcing the tensile strength of the aortic walls. Therefore, STAT3 regulates the balance between the destruction and the reinforcement of the aortic tissue, depending on the cell types and the time course of STAT3 activation, which ultimately regulates the development of AD. Elucidating such a dynamic mechanism to regulate the aortic tissue integrity would be essential to decipher the molecular pathogenesis of AD.

Therapeutic Effect of Rapamycin on Aortic Dissection in Mice.

Aortic dissection (AD) is a serious clinical condition that is unpredictable and frequently results in fatal outcome. Although rapamycin, an inhibitor of mechanistic target of rapamycin (mTOR), has been reported to be effective in preventing aortopathies in mouse models, its mode of action has yet to be clarified. A mouse AD model that was created by the simultaneous administration of ß-aminopropionitrile (BAPN) and angiotensin II (AngII) for 14 days. Rapamycin treatment was started either at day 1 or at day 7 of BAPN+AngII challenge, and continued throughout the observational period. Rapamycin was effective both in preventing AD development and in suppressing AD progression. On the other hand, gefitinib, an inhibitor of growth factor signaling, did not show such a beneficial effect, even though both rapamycin and gefitinib suppressed cell cycle activation in AD. Rapamycin suppressed cell cycle-related genes and induced muscle development-related genes in an AD-related gene expression network without a major impact on inflammation-related genes. Rapamycin augmented the activation of Akt1, Akt2, and Stat3, and maintained the contractile phenotype of aortic smooth muscle cells. These findings indicate that rapamycin was effective both in preventing the development and in suppressing the progression of AD, indicating the importance of the mTOR pathway in AD pathogenesis.

Genetic Deletion of Socs3 in Smooth Muscle Cells Ameliorates Aortic Dissection in Mice.

Aortic dissection (AD) is the acute destruction of aortic wall and is reportedly induced by inflammatory response. Here we investigated the role of smooth muscle Socs3 (a negative regulator of Janus kinases/signal transducer and activator of transcription signaling) in AD pathogenesis using a mouse model generated via ß-aminopropionitrile and angiotensin II infusion. Socs3 deletion specifically in smooth muscle cells yielded a chronic inflammatory response of the aortic wall, which was associated with increased fibroblasts, reinforced aortic tensile strength, and less-severe tissue destruction. Although an acute inflammatory response is detrimental in AD, smooth muscle-regulated inflammatory response seemed protective against AD.

MRTF-A promotes angiotensin II-induced inflammatory response and aortic dissection in mice.

Aortic dissection (AD) is a major cause of acute aortic syndrome with high mortality due to the destruction of aortic walls. Although recent studies indicate the critical role of inflammation in the disease mechanism of AD, it is unclear how inflammatory response is initiated. Here, we demonstrate that myocardin-related transcription factor A (MRTF-A), a signal transducer of humoral and mechanical stress, plays an important role in pathogenesis of AD in a mouse model. A mouse model of AD was created by continuous infusion of angiotensin II (AngII) that induced MRTF-A expression and caused AD in 4 days. Systemic deletion of Mrtfa gene resulted in a marked suppression of AD development. Transcriptome and gene annotation enrichment analyses revealed that AngII infusion for 1 day caused pro-inflammatory and pro-apoptotic responses before AD development, which were suppressed by Mrtfa deletion. AngII infusion for 1 day induced pro-inflammatory response, as demonstrated by expressions of Il6, Tnf, and Ccl2, and apoptosis of aortic wall cells, as detected by TUNEL staining, in an MRTF-A-dependent manner. Pharmacological inhibition of MRTF-A by CCG-203971 during AngII infusion partially suppressed AD phenotype, indicating that acute suppression of MRTF-A is effective in preventing the aortic wall destruction. These results indicate that MRTF-A transduces the stress of AngII challenge to the pro-inflammatory and pro-apoptotic responses, ultimately leading to AD development. Intervening this pathway may represent a potential therapeutic strategy.

High Salt Intake Worsens Aortic Dissection in Mice: Involvement of IL (Interleukin)-17A-Dependent ECM (Extracellular Matrix) Metabolism.

OBJECTIVE: Aortic dissection (AD) is a fatal disease that occurs suddenly without preceding clinical signs or symptoms. Although high salt intake is a proposed risk factor for cardiovascular diseases, the relationship between AD and high salt intake has not been clarified. We examined the effect of high-salt challenge on a mouse AD model. Approach and Results: AD was induced in male mice by continuous infusion of ß-aminopropionitrile and Ang II (angiotensin II). High-salt challenge exacerbated aortic wall destruction in AD. Deletion of Il17a (IL-17KO [IL (interleukin)-17A knockout]) did not affect the AD phenotype at baseline, but it abolished the high salt-induced worsening of the aortic destruction. Unexpectedly, aortas of IL-17KO mice exhibited global changes in ECM (extracellular matrix)-related genes without alteration of proinflammatory genes, altered architecture of collagen fibers, and reduced stiffness before AD induction. The aortas of IL-17KO mice were less sensitive to AD-inducing stimuli, as shown by the induction of phenotypic modulation markers SMemb and vimentin, suggesting a reduced stress response. The aortas of IL-17KO mice had a higher population of smooth muscle cells with nuclear-localized phosphorylated Smad2, indicative of TGFß (transforming growth factor-beta) signal activation. Consistently, pretreatment of smooth muscle cells in culture with IL-17A blunted the activation of Smad2 by TGFß1. CONCLUSIONS: These findings indicate that high salt intake has a worsening effect on AD in the context of high aortic wall stiffness, which is under the control of IL-17A through ECM metabolism. Therefore, salt restriction may represent a low-cost and practical way to reduce AD risk.

Involvement of B Cells, Immunoglobulins, and Syk in the Pathogenesis of Abdominal Aortic Aneurysm.

BACKGROUND: Abdominal aortic aneurysm (AAA) is a potentially life-threatening disease that is common in older individuals. Currently, therapeutic options are limited to surgical interventions. Although it has long been known that AAA tissue is enriched in B cells and immunoglobulins, their involvement in AAA pathogenesis remains controversial. METHODS AND RESULTS: We investigated the role of B cells and immunoglobulins in a murine model of AAA, induced with a periaortic application of CaCl2, and in human AAA. Both human and mouse AAA tissue showed B-cell infiltration. Mouse AAA tissue showed deposition of IgG and activation of Syk, a key molecule in B-cell activation and immunoglobulin function, which were localized to infiltrating cells including B cells and macrophages. B-cell-deficient muMT mice showed suppression of AAA development that was associated with reduced activation of Syk and less expression of matrix metalloproteinase-9. Administration of exogenous immunoglobulins restored the blunted Syk activation and AAA development in muMT mice. Additionally, exogenous immunoglobulins induced interleukin-6 and metalloproteinase-9 secretions in human AAA tissue cultures. Furthermore, administration of R788, a specific Syk inhibitor, suppressed AAA expansion, reduced inflammatory response, and reduced immunoglobulin deposition in AAA tissue. CONCLUSIONS: From these results, we concluded that B cells and immunoglobulins participated in AAA pathogenesis by promoting inflammatory and tissue-destructive activities. Finally, we identified Syk as a potential therapeutic target.

Role of Macrophage Socs3 in the Pathogenesis of Aortic Dissection.

BACKGROUND: Aortic dissection (AD) is a life-threatening medical emergency caused by the abrupt destruction of the intimomedial layer of the aortic walls. Given that previous studies have reported the involvement of proinflammatory cytokine interleukin-6 in AD pathogenesis, we investigated the role of signal transduction and activator of transcription 3 signaling, a downstream pathway of interleukin-6 in macrophages in pathogenesis of AD. METHODS AND RESULTS: We characterized the pathological and molecular events triggered by aortic stress, which can lead to AD. Aortic stress on the suprarenal aorta because of infrarenal aorta stiffening and angiotensin II infusion for 1 week caused focal medial rupture at the branching point of the celiac trunk and superior mesenteric artery. This focal medial rupture healed in 6 weeks in wild-type (WT) mice, but progressed to AD in mice with macrophage-specific deletion of Socs3 gene (mSocs3-KO). mSocs3-KO mice showed premature activation of cell proliferation, an inflammatory response, and skewed differentiation of macrophages toward the tissue-destructive phenotype. Concomitantly, they showed aberrant phenotypic modulation of smooth muscle cells and transforming growth factor beta signaling, which are likely to participate in tissue repair. Human AD samples revealed signal transduction and activator of transcription 3 activation in adventitial macrophages adjacent to the site of tissue destruction. CONCLUSIONS: These findings suggest that AD development is preceded by focal medial rupture, in which macrophage Socs3 maintains proper inflammatory response and differentiation of SMCs, thus promoting fibrotic healing to prevent tissue destruction and AD development. Understanding the sequence of the pathological and molecular events preceding AD development will help predict and prevent AD development and progression.

The role of IL-6 in pathogenesis of abdominal aortic aneurysm in mice.

Although the pathogenesis of abdominal aortic aneurysm (AAA) remains unclear, evidence is accumulating to support a central role for inflammation. Inflammatory responses are coordinated by various soluble cytokines of which IL-6 is one of the major proinflammatory cytokines. In this study we examined the role of IL-6 in the pathogenesis of experimental AAA induced by a periaortic exposure to CaCl2 in mice. We now report that the administration of MR16-1, a neutralizing monoclonal antibody specific for the mouse IL-6 receptor, mildly suppressed the development of AAA. The inhibition of IL-6 signaling provoked by MR16-1 also resulted in a suppression of Stat3 activity. Conversely, no significant changes in either NFκB activity, Jnk activity or the expression of matrix metalloproteinases (Mmp) -2 and -9 were identified. Transcriptome analyses revealed that MR16-1-sensitive genes encode chemokines and their receptors, as well as factors that regulate vascular permeability and cell migration. Imaging cytometric analyses then consistently demonstrated reduced cellular infiltration for MR16-1-treated AAA. These results suggest that IL-6 plays an important but limited role in AAA pathogenesis, and primarily regulates cell migration and infiltration. These data would also suggest that IL-6 activity may play an important role in scenarios of continuous cellular infiltration, possibly including human AAA.

Tenascin C protects aorta from acute dissection in mice.

Acute aortic dissection (AAD) is caused by the disruption of intimomedial layer of the aortic walls, which is immediately life-threatening. Although recent studies indicate the importance of proinflammatory response in pathogenesis of AAD, the mechanism to keep the destructive inflammatory response in check is unknown. Here, we report that induction of tenascin-C (TNC) is a stress-evoked protective mechanism against the acute hemodynamic and humoral stress in aorta. Periaortic application of CaCl2 caused stiffening of abdominal aorta, which augmented the hemodynamic stress and TNC induction in suprarenal aorta by angiotensin II infusion. Deletion of Tnc gene rendered mice susceptible to AAD development upon the aortic stress, which was accompanied by impaired TGFß signaling, insufficient induction of extracellular matrix proteins and exaggerated proinflammatory response. Thus, TNC works as a stress-evoked molecular damper to maintain the aortic integrity under the acute stress.