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

Promising Novel Therapies in the Treatment of Aortic and Visceral Aneurysms

Aortic and visceral aneurysms affect large arterial vessels, including the thoracic and abdominal aorta, as well as visceral arterial branches, such as the splenic, hepatic, and mesenteric arteries, respectively. Although these clinical entities have not been equally researched, it seems that they might share certain common pathophysiological changes and molecular mechanisms. The yet limited published data, with regard to newly designed, novel therapies, could serve as a nidus for the evaluation and potential implementation of such treatments in large artery aneurysms. In both animal models and clinical trials, various novel treatments have been employed in an attempt to not only reduce the complications of the already implemented modalities, through manufacturing of more durable materials, but also to regenerate or replace affected tissues themselves. Cellular populations like stem and differentiated vascular cell types, large diameter tissue-engineered vascular grafts (TEVGs), and various molecules and biological factors that might target aspects of the pathophysiological process, including cell-adhesion stabilizers, metalloproteinase inhibitors, and miRNAs, could potentially contribute significantly to the treatment of these types of aneurysms. In this narrative review, we sought to collect and present relevant evidence in the literature, in an effort to unveil promising biological therapies, possibly applicable to the treatment of aortic aneurysms, both thoracic and abdominal, as well as visceral aneurysms.

Peptide Amphiphile Supramolecular Nanofibers Designed to Target Abdominal Aortic Aneurysms

An abdominal aortic aneurysm (AAA) is a localized dilation of the aorta located in the abdomen that poses a severe risk of death when ruptured. The cause of AAA is not fully understood, but degradation of medial elastin due to elastolytic matrix metalloproteinases is a key step leading to aortic dilation. Current therapeutic interventions are limited to surgical repair to prevent catastrophic rupture. Here, we report the development of injectable supramolecular nanofibers using peptide amphiphile molecules designed to localize to AAA by targeting fragmented elastin, matrix metalloproteinase 2 (MMP-2), and membrane type 1 matrix metalloproteinase. We designed four targeting peptide sequences from X-ray crystallographic data and incorporated them into PA molecules via solid phase peptide synthesis. After coassembling targeted and diluent PAs at different molar ratios, we assessed their ability to form nanofibers using transmission electron microscopy and to localize to AAA in male and female Sprague-Dawley rats using light sheet fluorescence microscopy. We found that three formulations of the PA nanofibers were able to localize to AAA tissue, but the MMP-2 targeting PA substantially outperformed the other nanofibers. Additionally, we demonstrated that the MMP-2 targeting PA nanofibers had an optimal dose of 5 mg (∼12 mg/kg). Our results show that there was not a significant difference in targeting between male and female Sprague-Dawley rats. Given the ability of the MMP-2 targeting PA nanofiber to localize to AAA tissue, future studies will investigate potential diagnostic and targeted drug delivery applications for AAA.

Molecular Imaging of Aortic Aneurysm and Its Translational Power for Clinical Risk Assessment

Aortic aneurysms (AAs) are dilations of the aorta, that are often fatal upon rupture. Diagnostic radiological techniques such as ultrasound (US), magnetic resonance imaging (MRI), and computed tomography (CT) are currently used in clinical practice for early diagnosis as well as clinical follow-up for preemptive surgery of AA and prevention of rupture. However, the contemporary imaging-based risk prediction of aneurysm enlargement or life-threatening aneurysm-rupture remains limited as these are restricted to visual parameters which fail to provide a personalized risk assessment. Therefore, new insights into early diagnostic approaches to detect AA and therefore to prevent aneurysm-rupture are crucial. Multiple new techniques are developed to obtain a more accurate understanding of the biological processes and pathological alterations at a (micro)structural and molecular level of aortic degeneration. Advanced anatomical imaging combined with molecular imaging, such as molecular MRI, or positron emission tomography (PET)/CT provides novel diagnostic approaches for in vivo visualization of targeted biomarkers. This will aid in the understanding of aortic aneurysm disease pathogenesis and insight into the pathways involved, and will thus facilitate early diagnostic analysis of aneurysmal disease. In this study, we reviewed these molecular imaging modalities and their association with aneurysm growth and/or rupture risk and their limitations. Furthermore, we outline recent pre-clinical and clinical developments in molecular imaging of AA and provide future perspectives based on the advancements made within the field. Within the vastness of pre-clinical markers that have been studied in mice, molecular imaging targets such as elastin/collagen, albumin, matrix metalloproteinases and immune cells demonstrate promising results regarding rupture risk assessment within the pre-clinical setting. Subsequently, these markers hold potential as a future diagnosticum of clinical AA assessment. However currently, clinical translation of molecular imaging is still at the onset. Future human trials are required to assess the effectivity of potentially viable molecular markers with various imaging modalities for clinical rupture risk assessment.

miR-133a Replacement Attenuates Thoracic Aortic Aneurysm in Mice

Background Thoracic aortic aneurysms (TAAs) occur because of abnormal remodeling of aortic extracellular matrix and are accompanied by the emergence of proteolytically active myofibroblasts. The microRNA miR-133a regulates cellular phenotypes and is reduced in clinical TAA specimens. This study tested the hypothesis that miR-133a modulates aortic fibroblast phenotype, and overexpression by lentivirus attenuates the development of TAA in a murine model. Methods and Results TAA was induced in mice. Copy number of miR-133a was reduced in TAA tissue and linear regression analysis confirmed an inverse correlation between aortic diameter and miR-133a. Analyses of phenotypic markers revealed an mRNA expression profile consistent with myofibroblasts in TAA tissue. Fibroblasts were isolated from the thoracic aortae of mice with/without TAA. When compared with controls, miR-133a was reduced, migration was increased, adhesion was reduced, and the ability to contract a collagen disk was increased. Overexpression/knockdown of miR-133a controlled these phenotypes. After TAA induction in mice, a single tail-vein injection of either miR-133a overexpression or scrambled sequence (control) lentivirus was performed. Overexpression of miR-133a attenuated TAA development. The pro-protein convertase furin was confirmed to be a target of miR-133a by luciferase reporter assay. Furin was elevated in this murine model of TAA and repressed by miR-133a replacement in vivo resulting in reduced proteolytic activation. Conclusions miR-133a regulates aortic fibroblast phenotype and over-expression prevented the development of TAA in a murine model. These findings suggest that stable alterations in aortic fibroblasts are associated with development of TAA and regulation by miR-133a may lead to a novel therapeutic strategy.

Numerical knockouts-In silico assessment of factors predisposing to thoracic aortic aneurysms

Myriad risk factors–including uncontrolled hypertension, aging, and diverse genetic mutations–contribute to the development and enlargement of thoracic aortic aneurysms. Detailed analyses of clinical data and longitudinal studies of murine models continue to provide insight into the natural history of these potentially lethal conditions. Yet, because of the co-existence of multiple risk factors in most cases, it has been difficult to isolate individual effects of the many different factors or to understand how they act in combination. In this paper, we use a data-informed computational model of the initiation and progression of thoracic aortic aneurysms to contrast key predisposing risk factors both in isolation and in combination; these factors include localized losses of elastic fiber integrity, aberrant collagen remodeling, reduced smooth muscle contractility, and dysfunctional mechanosensing or mechanoregulation of extracellular matrix along with superimposed hypertension and aortic aging. In most cases, mild-to-severe localized losses in cellular function or matrix integrity give rise to varying degrees of local dilatations of the thoracic aorta, with enlargement typically exacerbated in cases wherein predisposing risk factors co-exist. The simulations suggest, for the first time, that effects of compromised smooth muscle contractility are more important in terms of dysfunctional mechanosensing and mechanoregulation of matrix than in vessel-level control of diameter and, furthermore, that dysfunctional mechanobiological control can yield lesions comparable to those in cases of compromised elastic fiber integrity. Particularly concerning, therefore, is that loss of constituents such as fibrillin-1, as in Marfan syndrome, can compromise both elastic fiber integrity and mechanosensing.

Aneurysms are local dilatations of the arterial wall that are responsible for significant disability and death. Detailed analyses of clinical data continue to provide insight into the natural history of these potentially lethal conditions, with myriad risk factors–including uncontrolled hypertension, aging, and diverse genetic mutations–contributing to their development and enlargement. Yet, because of the co-existence of these risk factors in most cases, it has been difficult to isolate individual effects or to understand how they act in combination. In this paper, we use a computational model of the initiation and progression of thoracic aortic aneurysms to contrast key predisposing factors both in isolation and in combination as well as with superimposed hypertension and aging. The present study recovers many findings from mouse models but with new and important observations that promise to guide in vivo and ex vivo studies as we seek to understand and eventually better treat these complex, multi-factorial lesions, with data-informed patient-specific computations eventually the way forward.

High-Affinity Binding of LDL Receptor-Related Protein 1 to Matrix Metalloprotease 1 Requires Protease:Inhibitor Complex Formation

Matrix metalloprotease (MMP) activation contributes to the degradation of the extracellular matrix (ECM), resulting in a multitude of pathologies. Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifaceted endocytic and signaling receptor that is responsible for internalization and lysosomal degradation of diverse proteases, protease inhibitors, and lipoproteins along with numerous other proteins. In this study, we identified MMP-1 as a novel LRP1 ligand. Binding studies employing surface plasmon resonance revealed that both proMMP-1 and active MMP-1 bind to purified LRP1 with equilibrium dissociation constants (K_D) of 19 and 25 nM, respectively. We observed that human aortic smooth muscle cells readily internalize and degrade ¹²⁵I-labeled proMMP-1 in an LRP1-mediated process. Our binding data also revealed that all tissue inhibitors of metalloproteases (TIMPs) bind to LRP1 with K_D values ranging from 23 to 33 nM. Interestingly, the MMP-1/TIMP-1 complex bound to LRP1 with an affinity (K_D = 0.6 nM) that was 30-fold higher than that of either component alone, revealing that LRP1 prefers the protease:inhibitor complex as a ligand. Of note, modification of lysine residues on either proMMP-1 or TIMP-1 ablated the ability of the MMP-1/TIMP-1 complex to bind to LRP1. LRP1's preferential binding to enzyme:inhibitor complexes was further supported by the higher binding affinity for proMMP-9/TIMP-1 complexes than for either of these two components alone. LRP1 has four clusters of ligand-binding repeats, and MMP-1, TIMP-1, and MMP-1/TIMP-1 complexes bound to cluster III most avidly. Our results reveal an important role for LRP1 in controlling ECM homeostasis by regulating MMP-1 and MMP-9 levels.

Induction of thoracic aortic dissection: a mini-review of β-aminopropionitrile-related mouse models

Thoracic aortic dissection (TAD) is one of the most lethal aortic diseases due to its acute onset, rapid progress, and high rate of aortic rupture. The pathogenesis of TAD is not completely understood. In this mini-review, we introduce three emerging experimental mouse TAD models using β-aminopropionitrile (BAPN) alone, BAPN for a prolonged duration (four weeks) and then with added infusion of angiotensin II (AngII), or co-administration of BAPN and AngII chronically. We aim to provide insights into appropriate application of these three mouse models, thereby enhancing the understanding of the molecular mechanisms of TAD.

Mapping Semaphorins and Netrins in the Pathogenesis of Human Thoracic Aortic Aneurysms

Thoracic aortic aneurysm (TAA) is a complex life-threatening disease characterized by extensive extracellular matrix (ECM) fragmentation and persistent inflammation, culminating in a weakened aorta. Although evidence suggests defective canonical signaling pathways in TAA, the full spectrum of mechanisms contributing to TAA is poorly understood, therefore limiting the scope of drug-based treatment. Here, we used a sensitive RNA sequencing approach to profile the transcriptomic atlas of human TAA. Pathway analysis revealed upregulation of key matrix-degrading enzymes and inflammation coincident with the axonal guidance pathway. We uncovered their novel association with TAA and focused on the expression of Semaphorins and Netrins. Comprehensive analysis of this pathway showed that several members were differentially expressed in TAA compared to controls. Immunohistochemistry revealed that Semaphorin4D and its receptor PlexinB1, similar to Netrin-1 proteins were highly expressed in damaged areas of TAA tissues but faintly detected in the vessel wall of non-diseased sections. It should be considered that the current study is limited by its sample size and the use of internal thoracic artery as control for TAA for the sequencing dataset. Our data determines important neuronal regulators of vascular inflammatory events and suggest Netrins and Semaphorins as potential key contributors of ECM degradation in TAA.

Matrix Metalloproteinases, Vascular Remodeling, and Vascular Disease

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that degrade various proteins in the extracellular matrix (ECM). Typically, MMPs have a propeptide sequence, a catalytic metalloproteinase domain with catalytic zinc, a hinge region or linker peptide, and a hemopexin domain. MMPs are commonly classified on the basis of their substrates and the organization of their structural domains into collagenases, gelatinases, stromelysins, matrilysins, membrane-type (MT)-MMPs, and other MMPs. MMPs are secreted by many cells including fibroblasts, vascular smooth muscle (VSM), and leukocytes. MMPs are regulated at the level of mRNA expression and by activation through removal of the propeptide domain from their latent zymogen form. MMPs are often secreted in an inactive proMMP form, which is cleaved to the active form by various proteinases including other MMPs. MMPs degrade various protein substrates in ECM including collagen and elastin. MMPs could also influence endothelial cell function as well as VSM cell migration, proliferation, Ca²⁺ signaling, and contraction. MMPs play a role in vascular tissue remodeling during various biological processes such as angiogenesis, embryogenesis, morphogenesis, and wound repair. Alterations in specific MMPs could influence arterial remodeling and lead to various pathological disorders such as hypertension, preeclampsia, atherosclerosis, aneurysm formation, as well as excessive venous dilation and lower extremity venous disease. MMPs are often regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs), and the MMP/TIMP ratio often determines the extent of ECM protein degradation and tissue remodeling. MMPs may serve as biomarkers and potential therapeutic targets for certain vascular disorders.

Matrix Metalloproteinase Inhibitors as Investigational and Therapeutic Tools in Unrestrained Tissue Remodeling and Pathological Disorders

Matrix metalloproteinases (MMPs) are zinc-dependent proteolytic enzymes that degrade various proteins in the extracellular matrix (ECM). MMPs may also regulate the activity of membrane receptors and postreceptor signaling mechanisms and thereby affect cell function. The MMP family includes collagenases, gelatinases, stromelysins, matrilysins, membrane-type MMPs, and other MMPs. Inactive proMMPs are cleaved by other MMPs or proteases into active MMPs, which interact with various protein substrates in ECM and cell surface. MMPs regulate important biological processes such as vascular remodeling and angiogenesis and may be involved in the pathogenesis of cardiovascular disorders such as hypertension, atherosclerosis, and aneurysm. The role of MMPs is often assessed by measuring their mRNA expression, protein levels, and proteolytic activity using gel zymography. MMP inhibitors are also used to assess the role of MMPs in different biological processes and pathological conditions. MMP activity is regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs), and the MMP/TIMP balance could determine the net MMP activity, ECM turnover, and tissue remodeling. Also, several synthetic MMP inhibitors have been developed. Synthetic MMP inhibitors include a large number of zinc-binding globulins (ZBGs), in addition to non-ZBGs and mechanism-based inhibitors. MMP inhibitors have been proposed as potential tools in the management of osteoarthritis, cancer, and cardiovascular disorders. However, most MMP inhibitors have broad-spectrum actions on multiple MMPs and could cause undesirable musculoskeletal side effects. Currently, doxycycline is the only MMP inhibitor approved by the Food and Drug Administration. New generation biological and synthetic MMP inhibitors may show greater MMP specificity and fewer side effects and could be useful in targeting specific MMPs, reducing unrestrained tissue remodeling, and the management of MMP-related pathological disorders.

Regulation of membrane type-1 matrix metalloproteinase activity and intracellular localization in clinical thoracic aortic aneurysms

OBJECTIVE

Membrane type-1 matrix metalloproteinase (MT1-MMP) is elevated during thoracic aortic aneurysm (TAA) development in mouse models, and plays an important role in the activation of matrix metalloproteinase (MMP)-2 and the release of matrix- bound transforming growth factor-β. In this study, we tested the hypothesis that MT1-MMP is subject to protein kinase C (PKC)-mediated regulation, which alters intracellular trafficking and activity with TAAs.

METHODS

Levels of MMP-2, native and phosphorylated MT1-MMP, and PKC-δ were measured in aortic tissue from patients with small TAAs (<5 cm; n = 8) and large TAAs (>6.5 cm; n = 8), and compared with values measured in normal controls (n = 8). Cellular localization of green fluorescent protein (GFP)-tagged MT1-MMP was assessed in aortic fibroblasts isolated from control and 4-week TAA mice. The effects of PKC-mediated phosphorylation on MT1-MMP cellular localization and function (active MMP-2 vs phospo-Smad2 abundance) were assessed after treatment with a PKC activator (phorbol-12-myristate-13-acetate [PMA], 100 nM) with and without a PKC-δ-specific inhibitor (röttlerin, 3 μM).

RESULTS

Compared with controls, MT1-MMP abundance was increased in aortas from both TAA groups. Active MMP-2 was increased only in the large TAA group. The abundances of phosphorylated MT1-MMP and activated PKC-δ were enhanced in the small TAA group compared with the large TAA group. MT1-MMP was localized on the plasma membrane in aortic fibroblasts from control mice and in endosomes from TAA mice. Treatment with PMA induced MT1-MMP-GFP internalization, enhanced phospho-Smad2, and reduced MMP-2 activation, whereas röttlerin pretreatment inhibited these effects.

CONCLUSIONS

Phosphorylation of MT1-MMP mediates its activity through directing cellular localization, shifting its role from MMP-2 activation to intracellular signaling. Thus, targeted inhibition of MT1-MMP may have therapeutic relevance as an approach to attenuating TAA development.

Ascending Aortic Proaneurysmal Genetic Mutations with Antiatherogenic Effects

Thoracic aortic aneurysms are common and are associated with a high morbidity and mortality. Despite this lethal diagnosis, there is an increasing body of evidence to suggest that the diagnosis of an aneurysm, specifically in the ascending thoracic aorta, may significantly reduce the risk of developing systemic atherosclerosis. Clinical observations in the operating room have shown pristine blood vessels in patients undergoing surgery for thoracic aortic aneurysms. There is now evidence that both the carotid intima-media thickness and arterial calcification, which are early and late signs of atherosclerosis respectively, are decreased in those with thoracic aortic aneurysms. These clinical studies are supported by molecular, genetic, and pharmacological evidence. Two principle mechanisms have been identified to explain the relationship of a proaneurysmal state conferring protection from atherosclerosis. These include an excess proteolytic balance of matrix metalloproteinase activity, leading to fragmentation of elastic lamellae and disordered collagen deposition. In addition, transforming growth factor β modulates vascular smooth muscle cells, extracellular matrix, and leukocytes. This confers protection from the initial plaque formation and, later provides stability to the plaque possibly through alteration of the types I and II transforming growth factor β receptor ratio. Furthermore, studies are now beginning to establish an important role for statins and estradiol in modulating these complex pathways. In the future, as our understanding of these complex mechanisms underlying aneurysmal protection against atherosclerosis increases, corresponding therapies may be developed to offer protection from atherosclerosis.

Divergent Roles of Matrix Metalloproteinase 2 in Pathogenesis of Thoracic Aortic Aneurysm

Objective—

Aortic aneurysm, focal dilation of the aorta, results from impaired integrity of aortic extracellular matrix (ECM). Matrix metalloproteinases (MMPs) are traditionally known as ECM-degrading enzymes. MMP2 has been associated with aneurysm in patients and in animal models. We investigated the role of MMP2 in thoracic aortic aneurysm using 2 models of aortic remodeling and aneurysm.

Approach and Results—

Male 10-week-old MMP2-deficient (MMP2 −/− ) and wild-type mice received angiotensin II (Ang II, 1.5 mg/kg/day) or saline (Alzet pump) for 4 weeks. Although both genotypes exhibited dilation of the ascending aorta after Ang II infusion, MMP2 −/− mice showed more severe dilation of the thoracic aorta and thoracic aortic aneurysm. The Ang II–induced increase in elastin and collagen (mRNA and protein) was markedly suppressed in MMP2 −/− thoracic aorta and smooth muscle cells, whereas only mRNA levels were reduced in MMP2 −/− -Ang II abdominal aorta. Consistent with the absence of MMP2, proteolytic activities were lower in MMP2 −/− -Ang II compared with wild-type-Ang II thoracic and abdominal aorta. MMP2-deficiency suppressed the activation of latent transforming growth factor-β and the Smad2/3 pathway in vivo and in vitro. Intriguingly, MMP2 −/− mice were protected against CaCl 2 -induced thoracic aortic aneurysm, which triggered ECM degradation but not synthesis.

Conclusions—

This study reveals the dual role of MMP2 in ECM degradation, as well as ECM synthesis. Moreover, the greater susceptibility of the thoracic aorta to impaired ECM synthesis, compared with vulnerability of the abdominal aorta to aberrant ECM degradation, provides an insight into the regional susceptibility of the aorta to aneurysm development.

Reproducible porcine model of thoracic aortic aneurysm

BACKGROUND

Thoracic aortic aneurysms (TAAs) develop secondary to abnormal aortic extracellular matrix remodeling, resulting in a weakened and dilated aortic wall that progressed to rupture if left unattended. Currently, no diagnostic/prognostic tests are available for the detection of TAA disease. This is largely driven by the lack of a large animal model, which would permit longitudinal/mechanistic studies. Accordingly, the objective of the present study was to establish a reproducible porcine model of aortic dilatation, which recapitulates the structural and biochemical changes observed during human TAA development.

METHODS AND RESULTS

Descending TAAs were induced in Yorkshire pigs (20-25 kg; n=7) through intra-adventitial injections of collagenase (5 mL, 0.35 mg/mL) and periadventitial application of crystalline CaCl2 (0.5 g). Three weeks after TAA induction, aortas were harvested and tissue was collected for biochemical and histological measurements. A subset of animals underwent MRI preoperatively and at terminal surgery. Results were compared with sham-operated controls (n=6). Three weeks after TAA induction, aortic luminal area increased by 38 ± 13% (P=0.018 versus control). Aortic structural changes included elastic lamellar degradation and decreased collagen content. The protein abundance of matrix metalloproteinases 3, 8, 9, and 12 increased in TAA tissue homogenates, whereas tissue inhibitors of metalloproteinases 1 and 4 decreased.

CONCLUSIONS

These data demonstrate aortic dilatation, aortic medial degeneration, and alterations in matrix metalloproteinase/tissue inhibitors of metalloproteinase abundance, consistent with TAA formation. This study establishes for the first time a large animal model of TAA that recapitulates the hallmarks of human disease and provides a reproducible test bed for examining diagnostic, prognostic, and therapeutic strategies.

Early cell changes and TGFβ pathway alterations in the aortopathy associated with bicuspid aortic valve stenosis

Previous studies on BAV (bicuspid aortic valve)-related aortopathy, whose aetiology is still debated, have focused mainly on severe dilatations. In the present study, we aimed to detect earlier signs of aortopathy. Specimens were collected from the 'concavity' (lesser curvature) and the 'convexity' (greater curvature) of mildly dilated AAs (ascending aortas; diameter ≤4 cm) with stenotic TAV (tricuspid aortic valve) or BAV and from donor normal aortas. Specimens were submitted to morphometry, immunohistochemistry and differential gene-expression analysis, focusing on SMC (smooth muscle cell) phenotype, remodelling, MF (myofibroblast) differentiation and TGFβ (transforming growth factor β) pathway. Smoothelin and myocardin mRNAs decreased in all the samples from patients, with the exception of those from BAV convexity, where a change in orientation of smoothelin-positive SMCs and an increase of α-SMA (α-smooth muscle actin) mRNA occurred. Dilated aortas from BAV and TAV patients showed both shared and distinct alterations concerning the TGFβ pathway, including an increased TGFβ and TGFβR2 (TGFβ receptor 2) expression in both groups and a decreased TGFβR1 expression in BAV samples only. Despite a decrease of the mRNA coding for the ED-A (extra domain-A) isoform of FN (fibronectin) in the BAV convexity, the onset of the expression of the corresponding protein in the media was observed in dilated aortas, whereas the normal media from donors was negative for this isoform. This discrepancy could be related to modifications in the intima, normally expressing ED-A FN and showing an altered structure in mild aortic dilatations in comparison with donor aorta. Our results suggest that changes in SMC phenotype and, likely, MF differentiation, occur early in the aortopathy associated with valve stenosis. The defective expression of TGFβR1 in BAV might be a constitutive feature, while other changes we reported could be influenced by haemodynamics.

Pressure overload-dependent membrane type 1-matrix metalloproteinase induction: relationship to LV remodeling and fibrosis

Increased myocardial extracellular matrix collagen represents an important structural milestone during the development of left ventricular (LV) pressure overload (PO); however, the proteolytic pathways that contribute to this process are not fully understood. This study tested the hypothesis that membrane type 1-matrix metalloproteinase (MT1-MMP) is directly induced at the transcriptional level in vivo during PO and is related to changes in LV collagen content. PO was induced in vivo by transverse aortic constriction in transgenic mice containing the full length human MT1-MMP promoter region ligated to luciferase (MT1-MMP Prom mice). MT1-MMP promoter activation (luciferase expression), expression, and activity; collagen volume fraction (CVF); and left atrial dimension were measured at 1 (n = 8), 2 (n = 12), and 4 (n = 17) wk following PO. Non-PO mice (n = 10) served as controls. Luciferase expression increased by fivefold at 1 wk, fell at 2 wk, and increased again by ninefold at 4 wk of PO (P < 0.05). MT1-MMP expression and activity increased at 1 wk, fell at 2 wk, and increased again at 4 wk after PO. CVF increased at 1 wk, remained unchanged at 2 wk, and increased by threefold at 4 wk of PO (P < 0.05). There was a strong positive correlation between CVF and MT1-MMP activity (r = 0.80, P < 0.05). Left atrial dimension remained unchanged at 1 and 2 wk but increased by 25% at 4 wk of PO. When a mechanical load was applied in vitro to LV papillary muscles isolated from MT1-MMP Prom mice, increased load caused MT1-MMP promoter activation to increase by twofold and MT1-MMP expression to increase by fivefold (P < 0.05). These findings challenge the canonical belief that PO suppresses overall matrix proteolytic activity, but rather supports the concept that certain proteases, such as MT1-MMP, play a pivotal role in PO-induced matrix remodeling and fibrosis.

Matrix metalloproteinase inhibitors as investigative tools in the pathogenesis and management of vascular disease

Matrix metalloproteinases (MMPs) are proteolytic enzymes that degrade various components of the extracellular matrix (ECM). MMPs could also regulate the activity of several non-ECM bioactive substrates and consequently affect different cellular functions. Members of the MMPs family include collagenases, gelatinases, stromelysins, matrilysins, membrane-type MMPs, and others. Pro-MMPs are cleaved into active MMPs, which in turn act on various substrates in the ECM and on the cell surface. MMPs play an important role in the regulation of numerous physiological processes including vascular remodeling and angiogenesis. MMPs may also be involved in vascular diseases such as hypertension, atherosclerosis, aortic aneurysm, and varicose veins. MMPs also play a role in the hemodynamic and vascular changes associated with pregnancy and preeclampsia. The role of MMPs is commonly assessed by measuring their gene expression, protein amount, and proteolytic activity using gel zymography. Because there are no specific activators of MMPs, MMP inhibitors are often used to investigate the role of MMPs in different physiologic processes and in the pathogenesis of specific diseases. MMP inhibitors include endogenous tissue inhibitors (TIMPs) and pharmacological inhibitors such as zinc chelators, doxycycline, and marimastat. MMP inhibitors have been evaluated as diagnostic and therapeutic tools in cancer, autoimmune disease, and cardiovascular disease. Although several MMP inhibitors have been synthesized and tested both experimentally and clinically, only one MMP inhibitor, i.e., doxycycline, is currently approved by the Food and Drug Administration. This is mainly due to the undesirable side effects of MMP inhibitors especially on the musculoskeletal system. While most experimental and clinical trials of MMP inhibitors have not demonstrated significant benefits, some trials still showed promising results. With the advent of new genetic and pharmacological tools, disease-specific MMP inhibitors with fewer undesirable effects are being developed and could be useful in the management of vascular disease.

Selective microRNA suppression in human thoracic aneurysms: relationship of miR-29a to aortic size and proteolytic induction

BACKGROUND

Increasing evidence points to a direct role for altered microRNA (miRNA or miR) expression levels in cardiovascular remodeling and disease progression. Although alterations in miR expression levels have been directly linked to cardiac hypertrophy, fibrosis, and remodeling, their role in regulating gene expression during thoracic aortic aneurysm (TAA) development has yet to be explored.

METHODS AND RESULTS

The present study examined miR expression levels in aortic tissue specimens collected from patients with ascending TAAs by quantitative real-time PCR, and observed decreased miR expression (miRs -1, -21, -29a, -133a, and -486) as compared with normal aortic specimens. A significant relationship between miR expression levels (miRs -1, -21, -29a, and -133a) and aortic diameter was identified; as aortic diameter increased, miR expression decreased. Through the use of a bioinformatics approach, members of the matrix metalloproteinase (MMP) family, proteins involved in TAA development, were examined for putative miR binding sites. MMP-2 and MMP-9 were identified as potential targets for miR-29a and miR-133a, respectively, and MMP-2 was subsequently verified as a miR-29a target in vitro. A significant inverse relationship between miR-29a and total MMP-2 was then identified in the clinical TAA specimens.

CONCLUSIONS

These findings demonstrate altered miR expression patterns in clinical TAA specimens, suggesting that the loss of specific miR expression may allow for the elaboration of specific MMPs capable of driving aortic remodeling during TAA development. Importantly, these data suggest that these miRs have biological and clinical relevance to the behavior of TAAs and may provide significant targets for therapeutic and diagnostic applications.

Matrix metalloproteinases and descending aortic aneurysms: parity, disparity, and switch

Central to the pathologic changes in developing aortic aneurysms are alterations in the abundance and activity of proteases, of which the most important for aneurysm production comprise the matrix metalloproteinase (MMP) family. In this review, literature demonstrating the role of MMPs in the development of aortic aneurysms is presented, with emphasis on the parity and disparity between the thoracic and abdominal aorta. Furthermore, the role of embryologic cellular origins and evidence of phenotypic switch will be addressed in terms of how this process alters MMP production during aneurysm development.