Small mechanical strains selectively suppress matrix metalloproteinase-1 expression by human vascular smooth muscle cells

Identification of Estrogen Signaling in a Prioritization Study of Intraocular Pressure-Associated Genes

Elevated intraocular pressure (IOP) is the only modifiable risk factor for primary open-angle glaucoma (POAG). Herein we sought to prioritize a set of previously identified IOP-associated genes using novel and previously published datasets. We identified several genes for future study, including several involved in cytoskeletal/extracellular matrix reorganization, cell adhesion, angiogenesis, and TGF-β signaling. Our differential correlation analysis of IOP-associated genes identified 295 pairs of 201 genes with differential correlation. Pathway analysis identified β-estradiol as the top upstream regulator of these genes with ESR1 mediating 25 interactions. Several genes (i.e., EFEMP1, FOXC1, and SPTBN1) regulated by β-estradiol/ESR1 were highly expressed in non-glaucomatous human trabecular meshwork (TM) or Schlemm’s canal (SC) cells and specifically expressed in TM/SC cell clusters defined by single-cell RNA-sequencing. We confirmed ESR1 gene and protein expression in human TM cells and TM/SC tissue with quantitative real-time PCR and immunofluorescence, respectively. 17β-estradiol was identified in bovine, porcine, and human aqueous humor (AH) using ELISA. In conclusion, we have identified estrogen receptor signaling as a key modulator of several IOP-associated genes. The expression of ESR1 and these IOP-associated genes in TM/SC tissue and the presence of 17β-estradiol in AH supports a role for estrogen signaling in IOP regulation.

Mechanisms of the Osteogenic Switch of Smooth Muscle Cells in Vascular Calcification: WNT Signaling, BMPs, Mechanotransduction, and EndMT

Characterized by the hardening of arteries, vascular calcification is the deposition of hydroxyapatite crystals in the arterial tissue. Calcification is now understood to be a cell-regulated process involving the phenotypic transition of vascular smooth muscle cells into osteoblast-like cells. There are various pathways of initiation and mechanisms behind vascular calcification, but this literature review highlights the wingless-related integration site (WNT) pathway, along with bone morphogenic proteins (BMPs) and mechanical strain. The process mirrors that of bone formation and remodeling, as an increase in mechanical stress causes osteogenesis. Observing the similarities between the two may aid in the development of a deeper understanding of calcification. Both are thought to be regulated by the WNT signaling cascade and bone morphogenetic protein signaling and can also be activated in response to stress. In a pro-calcific environment, integrins and cadherins of vascular smooth muscle cells respond to a mechanical stimulus, activating cellular signaling pathways, ultimately resulting in gene regulation that promotes calcification of the vascular extracellular matrix (ECM). The endothelium is also thought to contribute to vascular calcification via endothelial to mesenchymal transition, creating greater cell plasticity. Each of these factors contributes to calcification, leading to increased cardiovascular mortality in patients, especially those suffering from other conditions, such as diabetes and kidney failure. Developing a better understanding of the mechanisms behind calcification may lead to the development of a potential treatment in the future.

Mechanosensing and Mechanoregulation of Endothelial Cell Functions

Vascular endothelial cells (ECs) form a semiselective barrier for macromolecules and cell elements regulated by dynamic interactions between cytoskeletal elements and cell adhesion complexes. ECs also participate in many other vital processes including innate immune reactions, vascular repair, secretion, and metabolism of bioactive molecules. Moreover, vascular ECs represent a unique cell type exposed to continuous, time-dependent mechanical forces: different patterns of shear stress imposed by blood flow in macrovasculature and by rolling blood cells in the microvasculature; circumferential cyclic stretch experienced by the arterial vascular bed caused by heart propulsions; mechanical stretch of lung microvascular endothelium at different magnitudes due to spontaneous respiration or mechanical ventilation in critically ill patients. Accumulating evidence suggests that vascular ECs contain mechanosensory complexes, which rapidly react to changes in mechanical loading, process the signal, and develop context-specific adaptive responses to rebalance the cell homeostatic state. The significance of the interactions between specific mechanical forces in the EC microenvironment together with circulating bioactive molecules in the progression and resolution of vascular pathologies including vascular injury, atherosclerosis, pulmonary edema, and acute respiratory distress syndrome has been only recently recognized. This review will summarize the current understanding of EC mechanosensory mechanisms, modulation of EC responses to humoral factors by surrounding mechanical forces (particularly the cyclic stretch), and discuss recent findings of magnitude-specific regulation of EC functions by transcriptional, posttranscriptional and epigenetic mechanisms using -omics approaches. We also discuss ongoing challenges and future opportunities in developing new therapies targeting dysregulated mechanosensing mechanisms to treat vascular diseases. © 2019 American Physiological Society. Compr Physiol 9:873-904, 2019.

Estrogen receptor beta and truncated variants enhance the expression of transfected MMP-1 promoter constructs in response to specific mechanical loading

Background

Joint diseases such as osteoarthritis (OA) predominantly afflict post-menopausal women, suggesting a pertinent role for female hormones. Estrogen receptor beta (ER-β) has been detected in connective tissues of the knee joint suggesting that these tissues are responsive to the hormone estrogen. Matrix metalloproteinase-1 (MMP-1) activity contributes to cartilage degradation, a key factor leading to OA development in synovial joints. Two polymorphic forms of MMP-1 exist due to a deletion/insertion of the guanine residue in the promoter, and the 2G allelic variant of MMP-1 exhibits more activity than the 1G allele. Previous studies have demonstrated that the polymorphic forms of the human MMP-1 are influenced by the modulating effects of estrogen receptor isoforms. In addition to hormonal influences, physiological factors such as altered mechanical loading are also contributory features of OA. In the present study, the combined influence of biomechanical and hormonal variables on the activity of MMP-1 isoforms was evaluated. We hypothesized that the combined effects of ER-β and sheer stress will differentially activate the two allelic forms of MMP-1 in a hormone-independent manner.

Methods

HIG-82 synoviocytes were transiently transfected with 1G or 2G alleles (±) ER-β and subjected to either shear or equibiaxial stress. Next, 1G/2G promoter activity was measured to determine the combined influence of physiological stimuli. Truncated ER-β constructs were used to determine the importance of different domains of ER-β on 1G/2G activation.

Results

The 2G allele exhibited a constitutively higher activity than the 1G allele, which was further increased when the transfected cells were subject to shear stress, but not equibiaxial stress. Moreover, the combination of ER-β and shear stress further increased the activity levels of the 1G/2G allelic variants. Additionally, select AF-2 truncated ER-β variants led to increased activity levels for the 2G allele, indicating the AF-1 domain was likely involved in the response to mechanical stimulation.

Conclusions

These results suggest that the 1G/2G alleles of MMP-1 are influenced by specific mechanical stimuli like shear stress, as well as the ER-β receptor. These findings contribute to the potential allelic involvement in connective tissue diseases such as OA in females compared to males.

Role of NF-kappaB in flow-induced vascular remodeling

Vascular remodeling associated with increased blood flow involves reactive oxygen species (ROS)-dependent activation of matrix metalloproteinases (MMPs). To investigate the potential role of NF-kappaB in this process, human umbilical vein endothelial cells were subjected to different flow conditions during a 24-h period. Normal (15 dynes/cm(2)) and high (30 dynes/cm(2)) shear stress induced IkappaBalpha degradation and NF-kappaB p65 phosphorylation, and activated MMP-2 and MMP-9. These effects were blunted in cells incubated with the NF-kappaB inhibitor pyrrolidine dithio-carbamate (PDTC). In mice, creation of a carotid artery-jugular vein arteriovenous fistula (AVF) increased carotid blood flow sixfold, triggering the increase in carotid diameter from 459 +/- 8 microm (before AVF) to 531 +/- 13 and 669 +/- 21 microm (7 and 21 days after AVF). ROS production and NF-kappaB activity were enhanced in fistulated carotids, but only the latter was blocked by PDTC, although PDTC blocked ROS production in vitro. In PDTC-treated mice, changes in carotid caliber and shear stress matched controls at 7 days, but carotids enlarged only marginally thereafter, reaching only 578 +/- 8 microm at 21 days (p < 0.01 vs. untreated). Similarly, both MMP-9 expression and activity were abrogated by PDTC at 3 weeks. Hence, induction of NF-kappaB by shear stress contributes to MMP induction and allows long-term flow-induced vascular enlargement.

Small GTPases in mechanosensitive regulation of endothelial barrier

Alterations in vascular permeability are defining feature of diverse processes including atherosclerosis, inflammation, ischemia/reperfusion injury, and ventilator-induced lung injury. Clinical observations and experimental studies support an essential role of mechanical forces in pathophysiologic regulation of lung barrier. Accumulating data demonstrate that decreased levels of blood flow and increased cyclic stretch of lung tissues associated with lung mechanical ventilation at high tidal volumes increase vascular permeability, activate inflammatory cytokine production, alveolar flooding, leukocyte infiltration, and hypoxemia, and increase morbidity and mortality. Potential synergism between pathologic mechanical stimulation and inflammatory molecules resulting in vascular leak and lung injury becomes increasingly recognized. This review will discuss a role of Rho family of small GTPases in the mechanochemical regulation of pulmonary endothelial permeability associated with ventilator induced lung injury.

Myocardial Matrix Remodeling and the Matrix Metalloproteinases: Influence on Cardiac Form and Function

It is now becoming apparent that dynamic changes occur within the interstitium that directly contribute to adverse myocardial remodeling following myocardial infarction (MI), with hypertensive heart disease and with intrinsic myocardial disease such as cardiomyopathy. Furthermore, a family of matrix proteases, the matrix metalloproteinases (MMPs) and the tissue inhibitors of MMPs (TIMPs), has been recognized to play an important role in matrix remodeling in these cardiac disease states. The purpose of this review is fivefold: 1) to examine and redefine the myocardial matrix as a critical and dynamic entity with respect to the remodeling process encountered with MI, hypertension, or cardiomyopathic disease; 2) present the remarkable progress that has been made with respect to MMP/TIMP biology and how it relates to myocardial matrix remodeling; 3) to evaluate critical translational/clinical studies that have provided a cause-effect relationship between alterations in MMP/TIMP regulation and myocardial matrix remodeling; 4) to provide a critical review and analysis of current diagnostic, prognostic, and pharmacological approaches that utilized our basic understanding of MMP/TIMPs in the context of cardiac disease; and 5) most importantly, to dispel the historical belief that the myocardial matrix is a passive structure and supplant this belief that the regulation of matrix protease pathways such as the MMPs and TIMPs will likely yield a new avenue of diagnostic and therapeutic strategies for myocardial remodeling and the progression to heart failure.

Role of Matrix Metalloproteinases in Early Hypertensive Vascular Remodeling

Hypertension is associated with vascular remodeling characterized by rearrangement of extracellular matrix proteins. To evaluate how matrix metalloproteinase (MMP)-9 contributes to the progression of hypertensive vascular disease in vivo, wild-type (wt) or MMP-9(-/-) mice were treated with angiotensin II (Ang II; 1 microg/kg per minute, by minipump) plus a 5% NaCl diet during 10 days. Baseline blood pressure was equivalent in wt and knockout mice, but Ang II treatment increased systolic blood pressure to a greater extent (P<0.05) in MMP-9(-/-) mice (94+/-6 to 134+/-6 mm Hg; P<0.001) than in wt animals (93+/-4 to 114+/-6 mm Hg; P<0.01). In wt mice, Ang II treatment increased the carotid artery pressure-diameter relationship significantly, and maximal diameter reached 981+/-19 microm (P<0.01 versus sham; 891+/-10 microm). In contrast, in MMP-9(-/-) mice, carotid artery compliance was actually reduced after Ang II (P<0.05), and maximal diameter only reached 878+/-13 microm. Ang II treatment induced MMP-2 and increased carotid media thickness equally in both phenotypes. However, MMP-9 induction and in situ gelatinase activity were only enhanced in Ang II-treated wt mice, and vessels from these mice also produced more collagen I breakdown products than their MMP-9(-/-) counterparts (P<0.05). Inversely, staining for collagen IV was particularly enhanced in vessels from MMP-9(-/-) mice treated with Ang II. These results demonstrate the following: (1) the onset of Ang II-induced hypertension is accompanied by increased MMP-9 activity in conductance vessels; (2) absence of MMP-9 activity results in vessel stiffness and increased pulse pressure; and (3) MMP-9 activation is associated with a beneficial role early on in hypertension by preserving vessel compliance and alleviating blood pressure increase.

Molecular basis of the effects of mechanical stretch on vascular smooth muscle cells

The pulsatile nature of blood pressure and flow creates hemodynamic stimuli in the forms of cyclic stretch and shear stress, which exert continuous influences on the constituents of the blood vessel wall. Vascular smooth muscle cells (VSMCs) use multiple sensing mechanisms to detect the mechanical stimulus resulting from pulsatile stretch and transduce it into intracellular signals that lead to modulations of gene expression and cellular functions, e.g., proliferation, apoptosis, migration, and remodeling. The cytoskeleton provides a structural framework for the VSMC to transmit mechanical forces between its luminal, abluminal, and junctional surfaces, as well as its interior, including the focal adhesion sites, the cytoplasm, and the nucleus. VSMCs also respond differently to the surrounding structural environment, e.g., two-dimensional versus three-dimensional matrix. In vitro studies have been conducted on cultured VSMCs on deformable substrates to elucidate the molecular mechanisms by which the cells convert mechanical inputs into biochemical events, eventually leading to functional responses. The knowledge gained from research on mechanotransduction in vitro, in conjunction with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes involved in vascular remodeling and adaptation in health and disease.

Mechanical Loading Modulates the Differentiation State of Vascular Smooth Muscle Cells

The cause underlying the onset of stenosis after vascular reconstruction is not well understood. In the present study, we evaluated the effect of mechanical unloading on the differentiation state of human vascular smooth muscle cells (hVSMCs) using a tissue-engineered vascular media (TEVM). hVSMCs cultured in a mechanically loaded three-dimensional environment, known as a living tissue sheet, had a higher differentiated state than cells grown on plastic. When the living tissue sheet was detached from its support, the release of the residual stress resulted in a mechanical unloading and cells within the extracellular matrix (ECM) dedifferentiated as shown by downregulation of differentiation markers. The relaxed living tissue sheet can be rolled onto a tubular mandrel to form a TEVM. The rolling procedure resulted in the reintroduction of a mechanical load leading to a cohesive compacted tissue. During this period, cells gradually redifferentiated and aligned circumferentially to the tubular support. Our results suggest that differentiation of hVSMCs can be driven by mechanical loading and may occur simultaneously in the absence of other cell types. The extrapolation of our results to the clinical context suggests the hypothesis that hVSMCs may adopt a proliferative phenotype resulting from the mechanical unloading of explanted blood vessels during vascular reconstruction. Therefore, we propose that this mechanical unloading may play an important role in the onset of vascular graft stenosis.

Matrix metalloproteinases, the pros and cons, in liver fibrosis

Residing in the space of Disse within loose extracellular matrix (ECM) resembling that in basement membranes, the hepatic stellate cells (HSC) remain in quiescence as vitamin A storage cells. In response to liver injury HSC undergo morphologic and functional trans-differentiation, converting from vitamin A-storing, star-like cells into contractile myofibroblastic cells, a process called activation. Accompanying cellular activation, the ECM components in the space of Disse switch from matrices rich in type-IV collagen and laminin, into condensed interstitial ECM, indicating that proteolytic degradation may occur to change the microenvironment in sinusoids as well as the fate of HSC. Indeed, matrix metalloproteinases (MMP), a family of ECM degradative enzymes, are promptly expressed by HSC in response to diverse hepatic toxins. In vitro experiments also demonstrated the role of MMP in activation of HSC cultured in 3-D ECM. Conversely, MMP may also contribute to regression of liver fibrosis through cleavage of the fibrillar ECM and promotion of apoptosis among the activated HSC. Thus, MMP play dual roles both bad and good in liver fibrosis, depending on the timing.

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.

Tissue inhibitors of metalloproteinases-1 (TIMP-1) and -2(TIMP-2) are major serum factors that stimulate the TIMP-1 gene in human gingival fibroblasts

We demonstrate in this study that both TIMP-1 and TIMP-2 are major serum factors that stimulate the induction of TIMP-1 mRNA in quiescent human gingival fibroblasts (Gin-1 cells) at mid-G1 (6-9 h after serum stimulation) of the cell cycle, but not that of TIMP-2. When we chased the secretion of both TIMP proteins into culture medium containing 10% FCS freed of both TIMPs, TIMP-2 secretion rose to the level in 10% FCS after 24 h, but TIMP-1 secretion remained at a fairly low level even after 3 days, thus reflecting a contrastive difference in the induction of both TIMP mRNAs. The stimulating activity of TIMP-1 on the expression of the TIMP-1 gene switched over to inhibitory activity, when the TIMP-1 concentration in the culture medium exceeded about 30 ng/ml. The depletion of TIMP-1 and TIMP-2 from FCS affected remarkably the induction of c-jun and c-fos mRNAs, but not that of c-ets-1 mRNA. TIMP-1 and TIMP-2-dependent expression of AP-1 protein was further demonstrated by using nuclear extracts of Gin-1 cells in an electrophoretic mobility shift assay.

An introductory review of cell mechanobiology

Mechanical loads induce changes in the structure, composition, and function of living tissues. Cells in tissues are responsible for these changes, which cause physiological or pathological alterations in the extracellular matrix (ECM). This article provides an introductory review of the mechanobiology of load-sensitive cells in vivo, which include fibroblasts, chondrocytes, osteoblasts, endothelial cells, and smooth muscle cells. Many studies have shown that mechanical loads affect diverse cellular functions, such as cell proliferation, ECM gene and protein expression, and the production of soluble factors. Major cellular components involved in the mechanotransduction mechanisms include the cytoskeleton, integrins, G proteins, receptor tyrosine kinases, mitogen-activated protein kinases, and stretch-activated ion channels. Future research in the area of cell mechanobiology will require novel experimental and theoretical methodologies to determine the type and magnitude of the forces experienced at the cellular and sub-cellular levels and to identify the force sensors/receptors that initiate the cascade of cellular and molecular events.

Abnormal cardiac wall motion and early matrix metalloproteinase activity

Activation of matrix metalloproteinases (MMPs) in the heart is known to facilitate cardiac remodeling and progression to failure. We hypothesized that regional dyskinetic wall motion of the left ventricle would stimulate activation of MMPs. Abnormal wall motion at a target site on the anterior lateral wall of the left ventricle was induced by pacing atrial and ventricular sites of five open-chest anesthetized dogs. Changes in shortening at the left ventricular (LV) pacing site and at a remote site at the anterior base of the left ventricle were monitored with piezoelectric crystals. Simultaneous atrial and ventricular pacing resulted in abnormal motion at the LV pacing site, yielding early shortening and late systolic lengthening, whereas the shortening pattern at the remote site remained unaffected. Assessment of global myocardial MMP activity showed a sevenfold increase in substrate cleavage ( P < 0.02) at the LV pacing site relative to the remote site. Gelatin zymography revealed increases in 92-kDa MMP-9 activity and 86-kDa MMP-9 activity at the LV pacing site relative to the remote site, whereas MMP-2 activity was unaffected. Abnormal wall motion was associated with increases in collagen degradation (∼2-fold; P < 0.03), plasmin activity (∼1.5-fold; P < 0.05), nitrotyrosine levels (∼20-fold; P = 0.05), and inflammatory infiltrate (∼2-fold; P < 0.02) relative to the remote site. Results indicate that regional dyskinesis induced by epicardial activation is sufficient to stimulate significant MMP activity in the heart, suggesting that abnormal wall motion is a stimulus for MMP activation.

Estimation of the axial wall strains induced by an arterial stenosis at peak flow

In the last 30 years, thousands of basic or clinical studies have been devoted to atherosclerosis or to the problem of restenosis after angioplasty. In these studies, axial stresses in the vessel wall have received practically no attention, contrary to circumferential stress and purely biological aspects. Based on a recent article describing how arterial stenoses can induce a considerable increase in axial wall stress during flow systole in the region immediately proximal to the stenosis entrance, we have used a simple (theoretical) spring model and data available in the literature on the mechanical properties of arteries to investigate the relative wall elongations (axial strains) resulting from the systolic increases in axial stress generated by the stenosis. The model shows that high axial wall strains are tightly limited to the stenosis entrance if the axial wall forces generating the supplementary stress are strongly absorbed by the tissues surrounding the vessel. Inversely, if this absorption is weak, the zone of high strains extends over a longer vessel segment upstream of the stenosis entrance. The maximum strain value, which is always situated at the stenosis entrance, appears to be relatively independent of the presence or absence of surrounding tissues. The simulation also shows that in a 3 mm coronary artery presenting a 75% diameter stenosis, the axial strain at the stenosis entrance can exceed 10% at peak flow, depending on the respective axial elasticities of vessel wall and surrounding tissues. In a more severe stenosis, or in case of a pathologically high systolic pressure, the maximum strain value might even exceed 20%. Since abnormal axial strains have been shown to induce abnormal biological processes in smooth muscle cells cultures, it is quite conceivable that such axial strains are deleterious, at least in arterial segments whose length normally does not vary.

Influence of external uniaxial cyclic strain on oriented fibroblast-seeded collagen gels

This study investigates the influence of cyclic tensile strain, applied to fully contracted fibroblast-seeded collagen constructs. The constructs were preloaded to either 2 or 10 mN. The preloaded constructs were subsequently subjected to a further 10% cyclic strain (0-10%) at 1 Hz, using a triangular waveform, or were cultured in the preloaded state. In all cases cellular viability was maintained during the conditioning period. Cell proliferation was enhanced by the application of cyclic strain within constructs preloaded to both 2 and 10 mN. Collagen synthesis was enhanced by cyclic strain within constructs preloaded at 2 mN only. The profile of matrix metalloproteinase (MMP) expression, determined by zymography, was broadly similar in constructs preloaded at 2 mN with or without the application of cyclic strain. By contrast, constructs preloaded at 10 mN and subjected to cyclic strain expressed enhanced levels of staining for latent MMP-1, latent MMP-9, and both latent and active MMP-2, when compared with the other conditioning regimens. The structural stiffness of constructs preloaded at 2 mN and subjected to cyclic strain was enhanced compared with control specimens, reflecting the increase in collagen synthesis. By contrast, the initial failure loads for cyclically strained constructs preloaded at 10 mN were reduced, potentially because of enhanced catabolic activity.

Pressure-induced matrix metalloproteinase-9 contributes to early hypertensive remodeling

BACKGROUND

High blood pressure causes a change in vascular wall structure involving altered extracellular matrix composition, but how this process occurs is not fully understood.

METHODS AND RESULTS

Using mouse carotid arteries maintained in organ culture for 3 days, we detected increased gelatin zymographic activity of matrix metalloproteinase (MMP)-2 (168+/-13%, P<0.05) in vessels kept at low intraluminal pressure (10 mm Hg) compared with vessels at 80 mm Hg (100%), whereas in vessels maintained at high pressure (150 mm Hg), both MMP-2 and MMP-9 activity was induced (182+/-32%, P<0.05, and 194+/-21%, P<0.01, respectively). MMPs were detected in endothelial and smooth muscle cells by immunohistochemistry and in situ gelatin zymography. In vessels at 150 mm Hg, MMP activation was associated with a shift in the pressure-diameter curve toward greater distensibility (P<0.01) compared with vessels at 80 mm Hg. However, distensibility was not altered in vessels at 10 mm Hg, in which only activated MMP-2 was detected. The role of MMPs in high pressure-induced vessel distensibility was confirmed by use of the MMP inhibitor FN-439, which prevented the shift in the pressure-diameter relationship. Furthermore, in carotid arteries from MMP-9-deficient mice, the pressure-dependent increase in MMP-2 and in situ gelatinolytic activity were maintained, but the upward shift in the pressure-diameter curve was abolished.

CONCLUSIONS

MMP-9 seems to play a key role in the early stages of hypertensive vascular remodeling.

Platelet-derived growth factor-BB and Ets-1 transcription factor negatively regulate transcription of multiple smooth muscle cell differentiation marker genes

Platelet-derived growth factor (PDGF)-BB, a potent mitogen for mesenchymal cells, also downregulates expression of multiple smooth muscle (SM) cell (SMC)-specific markers. However, there is conflicting evidence whether PDGF-BB represses SMC marker expression at a transcriptional or posttranscriptional level, and little is known regarding the mechanisms responsible for these effects. Results of the present studies provide clear evidence that PDGF-BB treatment strongly repressed SM alpha-actin, SM myosin heavy chain (MHC), and SM22alpha promoters in SMCs. Of major significance for resolving previous controversies in the field, we found PDGF-BB-induced repression of SMC marker gene promoters in subconfluent, but not postconfluent, cultures. Treatment of postconfluent SMCs with a tyrosine phosphatase inhibitor restored PDGF-BB-induced repression, whereas treatment of subconfluent SMCs with a tyrosine kinase blocker abolished PDGF-BB-induced repression, suggesting that a tyrosine phosphorylation event mediates cell density-dependent effects. On the basis of previous observations that Ets-1 transcription factor is upregulated within phenotypically modulated neointimal SMCs, we tested whether Ets-1 would repress SMC marker expression. Consistent with this hypothesis, results of cotransfection experiments indicated that Ets-1 overexpression reduced transcriptional activity of SMC marker promoter constructs in SMCs, whereas it increased activity of SM alpha-actin promoter in endothelial cells. PDGF-BB treatment increased expression of Ets-1 in cultured SMCs, and SM alpha-actin mRNA expression was reduced in multiple independent clones of SMCs stably transfected with an Ets-1-overexpressing construct. Taken together, results of these experiments provide novel insights regarding possible mechanisms whereby PDGF-BB and Ets-1 may contribute to SMC phenotypic switching associated with vascular injury.

CITED2-mediated regulation of MMP-1 and MMP-13 in human chondrocytes under flow shear

CITED2 (CBP/p300-interacting transactivator with ED-rich tail 2) is a member of the Cited family of nuclear regulators, previously known as mrg1 (melanocyte-specific gene-related gene 1). CITED2 is inducible by varying stimuli including lipopolysaccharide, hypoxia, and cytokines such as interleukin 9 and interferon gamma. Using the immortalized human chondrocyte cell line, C-28/I2, we investigated whether CITED2 could be responsive to mechanical stimuli, and if so, whether CITED2 could mediate shear-driven regulation of matrix metalloproteinase (MMP) genes. The C-28/I2 cells were cultured under flow shear at 1-20 dyn/cm2, and the role of CIT-ED2 in regulation of MMPs was examined using the plasmids encoding sense and antisense CITED2 DNA sequences. The results showed that flow shear at 5 dyn/cm2 increased CITED2 mRNA and protein levels and down-regulated MMP-1 and MMP-13 mRNA and protein levels as well as enzyme activities. Consistent with the coordinated expression patterns of CITED2 and MMPs, overexpression of CITED2 repressed MMP-1 and MMP-13 mRNA levels and activities, whereas antisense CITED2 plasmids prevented the shear-induced down-regulation of MMP expression. Interleukin-1beta induced the formation of p300-Ets-1 complexes without affecting expression of CITED2. Transforming growth factor-beta as well as flow shear at 5 dyn/cm2 stimulated not only the expression of CITED2 but also the association of CIT-ED2 with p300 by dissociating Ets-1 from p300. These results indicate that CITED2 plays a major role in shear-induced down-regulation of MMP-1 and MMP-13 via a transforming growth factor-beta-dependent pathway.

Magnitude-dependent regulation of pulmonary endothelial cell barrier function by cyclic stretch

Ventilator-induced lung injury syndromes are characterized by profound increases in vascular leakiness and activation of inflammatory processes. To explore whether excessive cyclic stretch (CS) directly causes vascular barrier disruption or enhances endothelial cell sensitivity to edemagenic agents, human pulmonary artery endothelial cells (HPAEC) were exposed to physiologically (5% elongation) or pathologically (18% elongation) relevant levels of strain. CS produced rapid (10 min) increases in myosin light chain (MLC) phosphorylation, activation of p38 and extracellular signal-related kinase 1/2 MAP kinases, and actomyosin remodeling. Acute (15 min) and chronic (48 h) CS markedly enhanced thrombin-induced MLC phosphorylation (2.1-fold and 3.2-fold for 15-min CS at 5 and 18% elongation and 2.1-fold and 3.1-fold for 48-h CS at 5 and 18% elongation, respectively). HPAEC preconditioned at 18% CS, but not at 5% CS, exhibited significantly enhanced thrombin-induced reduction in transendothelial electrical resistance but did not affect barrier protective effect of sphingosine-1-phosphate (0.5 microM). Finally, expression profiling analysis revealed a number of genes, including small GTPase rho, apoptosis mediator ZIP kinase, and proteinase activated receptor-2, to be regulated by CS in an amplitude-dependent manner. Thus our study demonstrates a critical role for the magnitude of CS in regulation of agonist-mediated pulmonary endothelial cell permeability and strongly suggests phenotypic regulation of HPAEC barrier properties by CS.

Region- and type-specific induction of matrix metalloproteinases in post-myocardial infarction remodeling

BACKGROUND

Induction of matrix metalloproteinases (MMPs) contributes to adverse remodeling after myocardial infarction (MI). Whether a region- and type-specific distribution of MMPs occurs within the post-MI myocardium remained unknown.

METHODS AND RESULTS

Ten sheep were instrumented with a sonomicrometry array to measure dimensions in 7 distinct regions corresponding to the remote, transition, and MI regions. Eight sheep served as reference controls. The relative abundance of representative MMP types and the tissue inhibitors of the MMPs (TIMPs) was quantified by immunoblotting. Segment length increased from baseline in the remote (24.9+/-5.4%), transition (18.0+/-2.9%), and MI (53.8+/-11.0%) regions at 8 weeks after MI (P<0.05) and was greatest in the MI region (P<0.05). Region- and type-specific changes in MMPs occurred after MI. For example, MMP-1 and MMP-9 abundance was unchanged in the remote, fell to 3+/-2% in the transition, and was undetectable in the MI region (P<0.05). MMP-13, MMP-8, and MT1-MMP increased by >300% in the transition and MI regions (P<0.05). TIMP abundance decreased significantly in the transition region after MI and fell to undetectable levels within the MI region.

CONCLUSIONS

The unique findings of this study were 2-fold. First, changes in regional geometry after MI were associated with changes in MMP levels. Second, a region-specific portfolio of MMPs was induced after MI and was accompanied by a decline in TIMP levels, indicative of a loss of MMP inhibitory control. Targeting the regional imbalance between specific MMPs and TIMPs within the post-MI myocardium holds therapeutic potential.

A central role for the nuclear factor-kappaB pathway in anti-inflammatory and proinflammatory actions of mechanical strain

Mechanical signals play an integral role in bone homeostasis. These signals are observed at the interface of bone and teeth, where osteoblast-like periodontal ligament (PDL) cells constantly take part in bone formation and resorption in response to applied mechanical forces. Earlier, we reported that signals generated by tensile strain of low magnitude (TENS-L) are antiinflammatory, whereas tensile strain of high magnitude (TENS-H) is proinflammatory and catabolic. In this study, we examined the mechanisms of intracellular actions of the antiinflammatory and proinflammatory signals generated by TENS of various magnitudes. We show that both low and high magnitudes of mechanical strain exploit nuclear factor (NF)-kappaB as a common pathway for transcriptional inhibition/activation of proinflammatory genes and catabolic processes. TENS-L is a potent inhibitor of interleukin (IL)-1 beta-induced I-kappaBbeta degradation and prevents dissociation of NF-kB from cytoplasmic complexes and thus its nuclear translocation. This leads to sustained suppression of IL-1beta-induced NF-kappaB transcriptional regulation of proinflammatory genes. In contrast, TENS-H is a proinflammatory signal that induces I-kappaBbeta degradation, nuclear translocation of NF-kappaB, and transcriptional activation of proinflammatory genes. These findings are the first to describe the largely unknown intracellular mechanism of action of applied tensile forces in osteoblast-like cells and have critical implications in bone remodeling.

Uniaxial strain upregulates matrix-degrading enzymes produced by human vascular smooth muscle cells

Arteries remodel in response to environmental changes. We investigated whether mechanical strain modulates production of matrix metalloproteinase (MMP)-2 and -9 by cultured vascular smooth muscle cells (SMC). MMP-2 and MMP-9 expression were tested using human saphenous vein SMC cultured on silicone membranes at rest or subjected to physiological levels (5%) of stationary or cyclical (1 Hz) uniaxial strain. Compared with control, stationary strain significantly increased MMP-2 mRNA levels at all time points, whereas cyclic strain decreased it after 48 h. Both secreted and cell-associated pro-MMP-2 levels were increased by stationary strain at all times (P < 0.01), whereas cyclic strain decreased secreted levels after 48 h (P < 0.02). MMP-9 mRNA levels and pro-MMP-9 protein were increased after 48 h of stationary stretch (P < 0.01) compared with both no strain and cyclic strain. Our study indicates that vascular SMC show a selective response to different types of strain. We suggest that local increases in stationary mechanical strain resulting from stenting, hypertension, or atherosclerosis may lead to enhanced matrix degradation by SMC.

Coronary artery stretch versus deep injury in the development of in-stent neointima

OBJECTIVE

To investigate the relative importance of stent induced arterial stretch and deep injury to the development of in-stent neointima.

SETTING

Normal porcine coronary arteries

METHODS

30 BiodivYsio stents (Biocompatibles) were deployed at a stent to artery ratio of 1.25:1 (a moderate injury) and harvested at 28 days. Multiple serial cross sections were analysed morphometrically and the neointimal areas were correlated with the type and degree of injury.

RESULTS

Arterial stretch occurred in 78% of struts (77% of sections) and produced moderate neointimal growth (neointimal area 1.93 (0.13) mm2). Deep injury (rupture of the internal elastic lamina) occurred in 20% of struts (23% of sections) and produced a 1.7-fold increase in neointimal area (3.33 (0.41) mm2) compared with stretch only (p = 0.0002). With even deeper injury (rupture of the external elastic lamina), there was a 2.6-fold increase in neointimal area (5.01 (0.48) mm2) compared with stretch only (p = 0.02). A new injury score, incorporating both stretch and deep injury, correlated with neointimal area (r = 0.60, p < 0.001).

CONCLUSIONS

Stretch of the coronary artery in a stent is common, and a major contributor to neointima formation, even in the absence of deep injury. Deep injury is, however, a more potent stimulus to neointima formation than stretch. Greater degrees of stretch are associated with thicker neointima. Where neither deep injury nor stretch are seen, the stent has no effect upon the development of neointima.

Mechanical regulation of IGF-I and IGF-binding protein gene transcription in bladder smooth muscle cells

Mechanical forces are well known to modulate smooth muscle cell growth and synthetic phenotype. The signals controlling this process are complex and potentially involve changes in the expression of peptide growth factor genes such as those of the insulin-like growth factor (IGF) system. This study was designed to investigate the mechanical regulation of IGF-I and the binding proteins for IGF (IGFBPs) in smooth muscle cells cultured on a deformable surface and subjected to cyclic stretch. Using the RNase protection assay, we found that the application of a cyclic biaxial strain to cells induced a 2.5- to 4-fold increase in IGF-I mRNA levels after 8 h and an even greater increase after 16-24 h of stretch. This change was not affected by variations in the magnitude of the applied strain but was attenuated ( approximately 40%) when cells were treated with antagonists for angiotensin II receptors. Furthermore, the transcript levels of the three major IGF binding proteins produced in smooth muscle cells, e.g., IGFBP-2, IGFBP-4, and IGFBP-5, varied between stretched and control cells. Both IGFBP-2 and IGFBP-4 mRNA levels were consistently reduced in stretched cells but remained comparable to those of the control cells when the angiotensin II transducing pathway was blocked by inhibitors prior to the application of mechanical strain. Conversely, the gene expression of IGFBP-5 was upregulated in stretched cells, and neutralizing antibodies to IGF-I blocked this activation. Similarly, pharmacologic inhibition of the phosphatidylinositol 3-kinase, an important component of the IGF receptor transduction pathway, inhibited IGFBP-5 gene expression in stretched cells. These results suggest that the downstream effects of mechanical strain on IGF-I and IGFBP transcript levels are mediated, to greater or lesser extent, either through an angiotensin II tranducing pathway or via a feedback loop involving the autocrine secretion of IGF-I itself.

Cartilage tissue remodeling in response to mechanical forces

Recent studies suggest that there are multiple regulatory pathways by which chondrocytes in articular cartilage sense and respond to mechanical stimuli, including upstream signaling pathways and mechanisms that may lead to direct changes at the level of transcription, translation, post-translational modifications, and cell-mediated extracellular assembly and degradation of the tissue matrix. This review focuses on the effects of mechanical loading on cartilage and the resulting chondrocyte-mediated biosynthesis, remodeling, degradation, and repair of this tissue. The effects of compression and tissue shear deformation are compared, and approaches to the study of mechanical regulation of gene expression are described. Of particular interest regarding dense connective tissues, recent experiments have shown that mechanotransduction is critically important in vivo in the cell-mediated feedback between physical stimuli, the molecular structure of newly synthesized matrix molecules, and the resulting macroscopic biomechanical properties of the tissue.

Vascular smooth muscle cells and pericytes express MMP-1, MMP-9, TIMP-1 and type I procollagen in inflammatory bowel disease

AIMS

Matrix metalloproteinases (MMPs) are involved in tissue remodelling, which is one of the important aspects of inflammatory disease. To assess the balance between the matrix degradation and production, we analysed the in situ expression of MMP-1, -3, -8 and -9, tissue inhibitor of metalloproteinases (TIMP)-1 and -2, and type I procollagen (PC-I) in inflammatory bowel disease.

METHODS AND RESULTS

Immunohistochemistry using frozen sections was performed in 17 patients with ulcerative colitis (UC) and 16 with Crohn's disease (CD). In both UC and CD, MMPs and TIMPs were expressed by inflammatory cells as well as by fibroblastic cells most prominently in actively inflamed areas in ulcer bases, but sparsely in intact inflamed mucosa in both UC and CD. In UC, inflamed mucosa with erosions expressed these substances focally. Fibroblasts also expressed PC-I. We identified that vascular smooth muscle cells of venules in ulcer bases expressed MMP-1 and -9, TIMP-1 and PC-I. These venules also expressed E-selectin, a cell adhesion molecule to facilitate the leucocyte extravasation, and vascular endothelial growth factor (VEGF) receptor 2, consistent with their property of newly formed vessels.

CONCLUSIONS

Our results suggest that MMPs are involved in the tissue remodelling, angiogenesis and promotion of leucocyte extravasation in the actively inflamed area in the ulcer base in both UC and CD. MMP-1 expression in the mucosa may be related to the initial step of ulceration in UC. Therapeutic manipulation of extracellular matrix turnover would be an effective therapy to alleviate active inflammation and accelerate ulcer healing.

Mechanical strain induces specific changes in the synthesis and organization of proteoglycans by vascular smooth muscle cells

In the mechanically active environment of the artery, cells sense mechanical stimuli and regulate extracellular matrix structure. In this study, we explored the changes in synthesis of proteoglycans by vascular smooth muscle cells in response to precisely controlled mechanical strains. Strain increased mRNA for versican (3.2-fold), biglycan (2.0-fold), and perlecan (2.0-fold), whereas decorin mRNA levels decreased to a third of control levels. Strain also increased versican, biglycan, and perlecan core proteins, with a concomitant decrease in decorin core protein. Deformation did not alter the hydrodynamic size of proteoglycans as evidenced by molecular sieve chromatography but increased sulfate incorporation in both chondroitin/dermatan sulfate proteoglycans and heparan sulfate proteoglycans (p < 0.05 for both). Using DNA microarrays, we also identified the gene for the hyaluronan-linking protein TSG6 as mechanically induced in smooth muscle cells. Northern analysis confirmed a 4.0-fold increase in steady state mRNA for TSG6 following deformation. Size exclusion chromatography under associative conditions showed that versican-hyaluronan aggregation was enhanced following deformation. These data demonstrate that mechanical deformation increases specific vascular smooth muscle cell proteoglycan synthesis and aggregation, indicating a highly coordinated extracellular matrix response to biomechanical stimulation.

p38 mitogen-activated kinase is a bidirectional regulator of human fibroblast collagenase-1 induction by three-dimensional collagen lattices

When fibroblasts are cultured in contracting collagen matrices, matrix metalloproteinase-1 (MMP-1, collagenase-1) is induced. In the present study we demonstrate that p38alpha mitogen-activated protein kinase (p38alpha MAPK) plays a bi-directional role in the MMP-1 response to contracting floating collagen lattices (fl-coll). fl-coll, but not attached collagen lattices (att-coll), co-ordinately increased expression of MMP-1 and activities of p38alpha and MKK3/6 (MAPK kinase 3/6). However, treatment of primary fibroblasts cultured in fl-coll with increasing doses of SB203580, an inhibitor of p38alpha and p38beta, caused a bipolar pattern of MMP-1 expression. Partial inhibition of p38 MAPK activity resulted in the lowest level of MMP-1 expression, whereas total inhibition of p38 activity led to MMP-1 levels as high as in the absence of inhibitor. The activation/inhibition of p38alpha was apparently responsible for the observed phenomena, as supported by three lines of evidence. (1) p38alpha was the predominant isoform sensitive to SB203580 in primary fibroblasts. (2) Fibroblasts transfected with increasing dose of a dominant negative p38alpha (p38DN) similarly demonstrated the bipolar pattern of MMP-1 expression induced by fl-coll. (3) The bipolar MMP-1 expression occurred during the gradual, linear inhibition of p38alpha kinase activity by both inhibitors, SB203580 and p38DN. Nuclear factor-kappaB (NF-kappaB), a previously identified positive regulator of MMP-1 expression induced by fl-coll [Xu, Zutter, Santoro and Clark (1998) J. Cell Biol. 140, 709-719] was mediated by fl-coll-activated p38alpha. However, the fl-coll-induced expression of MMP-1 facilitated by p38alpha suppression was maintained independent of NF-kappaB activity, suggesting the existence of a p38alpha-dependent antagonistic pathway. We conclude that fl-coll-induced MMP-1 expression is the net outcome of opposing effects mediated by p38alpha. Therefore, the level of p38alpha kinase activity may provide a fine-tuned control of MMP-1 gene expression in response to biomechanical signals.

Messenger-RNA expression of matrix metalloproteinases, tissue inhibitors of metalloproteinases, and transcription factors in rheumatic synovial cells under mechanical stimuli

In an effort to elucidate the role of mechanical stimuli in rheumatoid arthritis, we determined mRNA levels of matrix metalloproteinase (MMP)-1, MMP-3, MMP-13, tissue inhibitor of metalloproteinase (TIMP)-1 and TIMP-2, and three transcription factors (c-fos, ets-1, and ets-2) under two mechanical shearing conditions as well as simulated unloading. Human synovial cell cultures (MH7A and RA99-01), derived from rheumatoid arthritis patients, were grown for 1 h under mechanical stimuli and the transcript level was assayed by the reverse transcription-polymerase-chain reaction procedure. First, gentle shearing, estimated at approximately 1 dyn/cm(2), induced a consistent decrease in mRNA level of MMP-1, MMP-3, MMP-13, and ets-1 and an increase in the transcript level of TIMP-1, TIMP-2, c-fos, and ets-2. Second, intermediate shearing, estimated at approximately 6 dyn/cm(2), elevated the mRNA level of all MMPs, TIMPs, and the three transcription factors. Third, minimum mRNA level of c-fos, ets-1, and ets-2 was achieved under control conditions at rest, gentle shearing, and simulated unloading, respectively. These in vitro results support a stimulus-dependent transcriptional regulation of MMPs, TIMPs, and transcription factors in cell cultures, suggesting a potential role of shear stress in tissue degradation and prevention in rheumatic joints.

Physiological cyclic stretch directs L-arginine transport and metabolism to collagen synthesis in vascular smooth muscle

Application of cyclic stretch (10% at 1 hertz) to vascular smooth muscle cells (SMC) increased L-arginine uptake and this was associated with a specific increase in cationic amino acid transporter-2 (CAT-2) mRNA. In addition, cyclic stretch stimulated L-arginine metabolism by inducing arginase I mRNA and arginase activity. In contrast, cyclic stretch inhibited the catabolism of L-arginine to nitric oxide (NO) by blocking inducible NO synthase expression. Exposure of SMC to cyclic stretch markedly increased the capacity of SMC to generate L-proline from L-arginine while inhibiting the formation of polyamines. The stretch-mediated increase in L-proline production was reversed by methyl-L-arginine, a competitive inhibitor of L-arginine transport, by hydroxy-L-arginine, an arginase inhibitor, or by the ornithine aminotransferase inhibitor L-canaline. Finally, cyclic stretch stimulated collagen synthesis and the accumulation of type I collagen, which was inhibited by L-canaline. These results demonstrate that cyclic stretch coordinately stimulates L-proline synthesis by regulating the genes that modulate the transport and metabolism of L-arginine. In addition, they show that stretch-stimulated collagen production is dependent on L-proline formation. The ability of hemodynamic forces to up-regulate L-arginine transport and direct its metabolism to L-proline may play an important role in stabilizing vascular lesions by promoting SMC collagen synthesis.

Molecular responses of rat tracheal epithelial cells to transmembrane pressure

Smooth muscle constriction in asthma causes the airway to buckle into a rosette pattern, folding the epithelium into deep crevasses. The epithelial cells in these folds are pushed up against each other and thereby experience compressive stresses. To study the epithelial cell response to compressive stress, we subjected primary cultures of rat tracheal epithelial cells to constant elevated pressures on their apical surface (i.e., a transmembrane pressure) and examined changes in the expression of genes that are important for extracellular matrix production and maintenance of smooth muscle activation. Northern blot analysis of RNA extracted from cells subjected to transmembrane pressure showed induction of early growth response-1 (Egr-1), endothelin-1, and transforming growth factor-beta1 in a pressure-dependent and time-dependent manner. Increases in Egr-1 protein were detected by immunohistochemistry. Our results demonstrate that airway epithelial cells respond rapidly to compressive stresses. Potential transduction mechanisms of transmembrane pressure were also investigated.

Collagen degradation and platelet-derived growth factor stimulate the migration of vascular smooth muscle cells

Cell migration is a key event in many biological processes and depends on signals from both extracellular matrix and soluble motogenic factors. During atherosclerotic plaque development, vascular smooth muscle cells migrate from the tunica media to the intima through a basement membrane and interstitial collagenous matrix and proliferate to form a neointima. Matrix metalloproteinases have previously been implicated in neointimal formation and in this study smooth muscle cell adhesion and migration on degraded collagen have been evaluated. Vascular smooth muscle cells adhered to native intact collagen type I and to its first degradation by-product, 3/4 fragment (generated by collagenase-3 cleavage), unwound at 35 degrees C to mimic physiological conditions. PDGF-BB pre-treatment induced a fourfold stimulation of smooth muscle cell motility on the collagen 3/4 fragment whereas no increase in smooth muscle cell motility on collagen type I was observed. Cell migration on collagen type I was mediated by alpha2 integrin, whereas PDGF-BB-stimulated migration on the 3/4 collagen fragment was dependent on alphavbeta3 integrin. alphavbeta3 integrin was organised in clusters concentrated at the leading and trailing edges of the cells and was only expressed when cells were exposed to the 3/4 collagen fragment. Tyrphostin A9, an inhibitor of PDGF receptor-beta tyrosine kinase activity, resulted in complete abolition of migration of PDGF-BB treated cells on collagen type I and 3/4 fragment. These results strongly support the hypothesis that the cellular migratory response to soluble motogens can be regulated by proteolytic modification of the extracellular matrix.

Mechanotransduction and arterial smooth muscle cells: new insight into hypertension and atherosclerosis

Vascular cells depend on multiple stimuli to maintain a biomechanically and biologically stable environment. Mechanical stresses contribute significantly to multiple cellular processes that regulate vascular structure and function. For example, fluid shear stresses control endothelial cell molecular responses. Less attention has focused on responses of the smooth muscle cell, the 'other' major vascular cell, to mechanical stimuli, in part because of the experimental difficulties in applying precisely controlled deformation. With the advent of new bioengineered devices, combined with modern technologies for studying molecular expression, we are beginning to understand how the smooth muscle cell responds to and controls the biomechanical environment. These studies will help us to understand vascular diseases where vascular mechanics plays a prominent role, such as hypertension, aneurysm formation and atherosclerotic plaque rupture.

Biomechanical regulation of human monocyte/macrophage molecular function

When the monocyte infiltrates a tissue, adhesion to the extracellular matrix provides structural anchors, and the cell may be deformed through these attachments. To test the hypothesis that human monocytes/macrophages are mechanically responsive, we studied the effects of small cyclic mechanical deformations on cultured human monocytes/macrophages. When monocytes/macrophages were subjected to 4% strain at 1 Hz for 24 hours, neither matrix metalloproteinase (MMP)-1 nor MMP-3 was induced; however, in the presence of phorbol myristate acetate, strain augmented MMP-1 expression by 5.1 +/- 0.7-fold (P < 0.05) and MMP-3 expression by 1. 6 +/- 0.1-fold (P < 0.05). In contrast, MMP-9 expression was not changed by mechanical strain in the presence or absence of phorbol myristate acetate. Deformation rapidly induced the immediate early response genes c-fos and c-jun. In addition, mechanical deformation induced the transcription factor PU.1, an ets family member that is essential in monocyte differentiation, as well as mRNA for the M-CSF receptor. These studies demonstrate that human monocytes/macrophages respond to mechanical deformation with selective augmentation of MMPs, induction of immediate early genes, and induction of the M-CSF receptor. In addition to enhancing the proteolytic activity of macrophages within repairing tissues, cellular deformation within tissues may play a role in monocyte differentiation.

Coronary stent symmetry and vascular injury determine experimental restenosis

OBJECTIVE

To assess the impact of stent symmetry on restenosis using the coronary overstretch sheep model.

METHODS

Neointimal thickness, injury index, and percentage diameter and area stenosis were calculated by digital morphometry. The standard deviation of the angular burden was used to assess stent symmetry for each section.

MATERIALS

15 healthy Merino sheep (63-75 kg) underwent implantation of 30 slotted tube stents (7 mm). Restenosis was induced by calculated overstretch of the coronary artery. Twenty eight days after implantation, stents were excised and underwent histological examination using quantitative digital morphometry.

RESULTS

The severity of vessel injury was positively correlated with neointimal thickness and with percentage diameter and area stenosis (p < 0.001). Mean neointimal thickness and mean vascular injury per cross section were strongly related to the standard deviation of angular burden, with correlation coefficients of 0.6 and 0.8, respectively (p < 0.001).

CONCLUSIONS

The well known relation between vascular injury and restenosis was confirmed, and a new relation was discovered between stent asymmetry and restenosis. If these results apply to human coronary arteries, restenosis may also be dependent on the degree of asymmetric stent expansion. These results should influence the development of new stent designs to reduce asymmetric stent expansion, leading to a more homogeneous strain distribution in stented coronary segments.

Transcriptional profile of mechanically induced genes in human vascular smooth muscle cells

Vascular smooth muscle cells must monitor and respond to their mechanical environment; however, the molecular response of these cells to mechanical stimuli remains incompletely defined. By applying a highly uniform biaxial cyclic strain to cultured cells, we used DNA microarray technology to describe the transcriptional profile of mechanically induced genes in human aortic smooth muscle cells. We first identified vascular endothelial growth factor (VEGF) as a mechanically induced gene in these cells; VEGF served as a positive control for these experiments. We then used a DNA microarray with 5000 genes with putative functions to identify additional mechanically induced genes. Surprisingly, relatively few genes are mechanically induced in human aortic smooth muscle cells. Only 3 transcripts of 5000 were induced >2.5-fold: cyclooxygenase-1, tenascin-C, and plasminogen activator inhibitor-1. Downregulated transcripts included matrix metalloproteinase-1 and thrombomodulin. The transcriptional profile of mechanically induced genes in human aortic smooth muscle cells suggests a response of defense against excessive deformation. These data also demonstrate that in addition to identifying large clusters of genes that respond to a given stimulus, DNA microarray technology may be used to identify a small subset of genes that comprise a highly specific molecular response.

Diminished matrix metalloproteinase 2 (MMP-2) in ectomesenchyme-derived tissues of the Patch mutant mouse: regulation of MMP-2 by PDGF and effects on mesenchymal cell migration

Platelet-derived growth factors (PDGF) regulate cell proliferation, survival, morphology, and migration, as well as deposition and turnover of the extracellular matrix. Important roles for the A form of PDGF (PDGF-A) during connective tissue morphogenesis have been highlighted by the murine Patch mutation, which includes a deletion of the alpha subunit of the PDGF receptor. Homozygous (Ph/Ph) embryos exhibit multiple connective tissue defects including cleft face (involving the first branchial arch and frontonasal processes), incomplete heart septation, and heart valve abnormalities before they die in utero. Analyses of the cell biology underlying the defects in Ph/Ph embryos have revealed a deficit in a matrix metalloproteinase (MMP-2) and one of its activators (MT-MMP) that are likely to be involved in cell migration and tissue remodeling, two processes necessary for normal cardiac and craniofacial development. Morphogenesis of these structures requires infiltration of ectomesenchymal precursors and their subsequent deposition and remodeling of extracellular matrix components. First branchial arch and heart tissue from E10.5 embryos were examined by gelatin zymography and RT-PCR in order to characterize the expression of MMPs in these tissues. Of the MMPs examined, only MMP-2 and one of its activators, MT-MMP, were expressed in the first arch and heart at this stage of development. Tissues from Ph/Ph embryos exhibited a significant decrease in both MMP-2 and MT-MMP compared to tissues from normal embryos of the same developmental stage. In order to assess whether this decrease affects the motile activity of mesenchymal cells, cell migration from Ph/Ph branchial arch explants was compared to migration from normal arch tissue and found to be significantly less. In addition, the migratory ability of branchial arch cells from normal explants could be reduced in a similar manner using a specific MMP inhibitor. Although it is still unclear whether the MMP-2 reduction is a direct result of the absence of response of Ph/Ph cells to PDGF-A treatment of normal branchial arch cells in vitro with recombinant PDGF-AA significantly upregulated MMP-2 protein. Together, these results suggest that PDGF-A regulates MMP-2 expression and activation during normal development and that faulty proteinase expression may be at least partially responsible for the developmental defects exhibited by Ph/Ph embryos.

Mechanical stretching of human saphenous vein grafts induces expression and activation of matrix-degrading enzymes associated with vascular tissue injury and repair

After coronary artery bypass surgery, saphenous vein graft occlusion occurs through tissue remodeling. Although a likely trigger, the role of preparative mechanical injury incurred by the graft is not yet understood. We studied the early effects of simple mechanical injury on human saphenous vein grafts by exposing them to longitudinal stretch, a deformation which potentially occurs during surgery. We then maintained ex vivo for up to 7 days matched pairs of experimentally stretched and nonstretched (control) vein segments and examined the expression and activation of matrix metalloproteinases (MMPs) and integrin alphav, molecules implicated in vascular remodeling. At peak expression on day 3, stretched vein secreted 177 +/- 16% active MMP-2 (P < 0.01), 161 +/- 36% (P < 0.05) pro-MMP-9, and contained 206 +/- 18% (P < 0.01) alphav, a receptor for active MMP-2, compared to control. In situ gelatinase activity was present in the intima and adventitia of stretched veins, but not of control, and correlated spatially with expression of alphav. Stretch also increased severalfold cell proliferation (1.27 +/- 0.4 vs. 0.23 +/- 0.05% in control, P < 0.05), as assessed by bromodeoxyuridine incorporation. Furthermore, we found that cell proliferation colocalized with gelatinase activity and alphav in the adventitia. Our results show that a single longitudinal stretch of vein grafts produces significant changes in the expression and activation of key molecules in vascular remodeling. We also found support for the notion that the adventitial layer contributes to vein graft remodeling.

AP-1 mediates stretch-induced expression of HB-EGF in bladder smooth muscle cells

Mechanical induction of growth factor synthesis may mediate adaptive responses of smooth muscle cells (SMC) to increases in physical load. We previously demonstrated that cyclic mechanical stretch induces expression of the SMC, fibroblast, and epithelial cell mitogen heparin-binding epidermal growth factor-like growth factor (HB-EGF) in bladder SMC, an observation that suggests that this growth factor may be involved in compensatory bladder hypertrophy. In the present study we provide evidence that the activator protein-1 (AP-1) transcription factor plays a critical role in this mechanoinduction process. Rat bladder SMC were transiently transfected with a series of 5' deletion mutants of a promoter-reporter construct containing 1. 7 kb of the mouse HB-EGF promoter that was previously shown to be stretch responsive. The stretch-mediated increase in promoter activity was completely ablated with deletion of nucleotide positions -1301 to -881. Binding of AP-1, as evaluated by electrophoretic mobility shift assay, to a synthetic oligonucleotide containing an AP-1 binding site increased in response to stretch, and binding was inhibited by excess unlabeled DNA corresponding to nucleotides -993 to -973 from the HB-EGF promoter, a region that contains a previously recognized composite AP-1/Ets site. Stretch-induced promoter activity was significantly inhibited by site-directed mutagenesis of the AP-1 or Ets components of this site. Consistent with the promoter and gel-shift studies, curcumin, an inhibitor of AP-1 activation, suppressed the HB-EGF mRNA induction after stretch. Stretch also specifically increased mRNA levels for matrix metalloproteinase (MMP)-1, the promoter of which contains a functional AP-1 element, but not for MMP-2, the promoter of which does not contain an AP-1 element. The stretch response of the MMP-1 gene was also completely inhibited by curcumin. Collectively, these findings indicate that AP-1-mediated transcription plays an important role in the regulation of gene expression in bladder muscle in response to mechanical forces.