Tyrosine phosphorylation of pp125FAK and paxillin in aortic endothelial cells induced by mechanical strain

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Feeling Things Out: Bidirectional Signaling of the Cell-ECM Interface, Implications in the Mechanobiology of Cell Spreading, Migration, Proliferation, and Differentiation

Biophysical cues stemming from the extracellular environment are rapidly transduced into discernible chemical messages (mechanotransduction) that direct cellular activities-placing the extracellular matrix (ECM) as a potent regulator of cell behavior. Dynamic reciprocity between the cell and its associated matrix is essential to the maintenance of tissue homeostasis and dysregulation of both ECM mechanical signaling, via pathological ECM turnover, and internal mechanotransduction pathways contribute to disease progression. This review covers the current understandings of the key modes of signaling used by both the cell and ECM to coregulate one another. By taking an outside-in approach, the inherent complexities and regulatory processes at each level of signaling (ECM, plasma membrane, focal adhesion, and cytoplasm) are captured to give a comprehensive picture of the internal and external mechanoregulatory environment. Specific emphasis is placed on the focal adhesion complex which acts as a central hub of mechanical signaling, regulating cell spreading, migration, proliferation, and differentiation. In addition, a wealth of available knowledge on mechanotransduction is curated to generate an integrated signaling network encompassing the central components of the focal adhesion, cytoplasm and nucleus that act in concert to promote durotaxis, proliferation, and differentiation in a stiffness-dependent manner.

Extracellular Matrix in Regulation of Contractile System in Cardiomyocytes

The contractile apparatus of cardiomyocytes is considered to be a stable system. However, it undergoes strong rearrangements during heart development as cells progress from their non-muscle precursors. Long-term culturing of mature cardiomyocytes is also accompanied by the reorganization of their contractile apparatus with the conversion of typical myofibrils into structures of non-muscle type. Processes of heart development as well as cell adaptation to culture conditions in cardiomyocytes both involve extracellular matrix changes, which appear to be crucial for the maturation of contractile apparatus. The aim of this review is to analyze the role of extracellular matrix in the regulation of contractile system dynamics in cardiomyocytes. Here, the remodeling of actin contractile structures and the expression of actin isoforms in cardiomyocytes during differentiation and adaptation to the culture system are described along with the extracellular matrix alterations. The data supporting the regulation of actin dynamics by extracellular matrix are highlighted and the possible mechanisms of such regulation are discussed.

Pulmonary Endothelial Mechanical Sensing and Signaling, a Story of Focal Adhesions and Integrins in Ventilator Induced Lung Injury

Patients with critical illness such as acute lung injury often undergo mechanical ventilation in the intensive care unit. Though lifesaving in many instances, mechanical ventilation often results in ventilator induced lung injury (VILI), characterized by overdistension of lung tissue leading to release of edemagenic agents, which further damage the lung and contribute to the mortality and progression of pulmonary inflammation. The endothelium is particularly sensitive, as VILI associated mechanical stress results in endothelial cytoskeletal rearrangement, stress fiber formation, and integrity loss. At the heart of these changes are integrin tethered focal adhesions (FAs) which participate in mechanosensing, structure, and signaling. Here, we present the known roles of FA proteins including c-Src, talin, FAK, paxillin, vinculin, and integrins in the sensing and response to cyclic stretch and VILI associated stress. Attention is given to how stretch is propagated from the extracellular matrix through integrins to talin and other FA proteins, as well as signaling cascades that include FA proteins, leading to stress fiber formation and other cellular responses. This unifying picture of FAs aids our understanding in an effort to prevent and treat VILI.

Endothelial Cell Mechanotransduction in the Dynamic Vascular Environment

The vascular endothelial cells (ECs) that line the inner layer of blood vessels are responsible for maintaining vascular homeostasis under physiological conditions. In the presence of disease or injury, ECs can become dysfunctional and contribute to a progressive decline in vascular health. ECs are constantly exposed to a variety of dynamic mechanical stimuli, including hemodynamic shear stress, pulsatile stretch, and passive signaling cues derived from the extracellular matrix. This review describes the molecular mechanisms by which ECs perceive and interpret these mechanical signals. The translational applications of mechanosensing are then discussed in the context of endothelial-to-mesenchymal transition and engineering of vascular grafts.

Emerging Role of Plasma Membranes in Vascular Endothelial Mechanosensing

Vascular endothelial cells (ECs) maintain circulatory system homeostasis by changing their functions in response to changes in hemodynamic forces, including shear stress and stretching. However, it is unclear how ECs sense changes in shear stress and stretching and transduce these changes into intracellular biochemical signals. The plasma membranes of ECs have recently been shown to respond to shear stress and stretching differently by rapidly changing their lipid order, fluidity, and cholesterol content. Such changes in the membranes' physical properties trigger the activation of membrane receptors and cell responses specific to each type of force. Artificial lipid-bilayer membranes show similar changes in lipid order in response to shear stress and stretching, indicating that they are physical phenomena rather than biological reactions. These findings suggest that the plasma membranes of ECs act as mechanosensors; in response to mechanical forces, they first alter their physical properties, modifying the conformation and function of membrane proteins, which then activates downstream signaling pathways. This new appreciation of plasma membranes as mechanosensors could help to explain the distinctive features of mechanotransduction in ECs involving shear stress and stretching, which activate a variety of membrane proteins and multiple signal transduction pathways almost simultaneously.

Fak-Mapk, Hippo and Wnt signalling pathway expression and regulation in distraction osteogenesis

OBJECTIVES

To investigate the mechanism of mechanical stimulation in bone formation and regeneration during distraction osteogenesis.

MATERIALS AND METHODS

In this study, microarray technology was used to investigate the time course of bone-related molecular changes in distraction osteogenesis in rats. Real-time PCR and Western-blot analyses were used to confirm the expression of genes identified in microarrays. Meanwhile, we used a lentivirus vector to inhibit Fak expression, in order to identify the osteogenic effect of Fak and Fak-Mapk pathway during distraction osteogenesis.

RESULTS

Several components of the Wnt and Hippo pathways were found to be up- or down-regulated during distraction osteogenesis by microarray. Meanwhile, it was found that Fak, Src, Raf-1, Erk1, Jnk and p38-Mapk were up-regulated during gradual distraction, compared with consolidation. To further determine whether Fak-Mapk pathway played an important role in distraction osteogenesis, Fak was disrupted with a lentivirus vector. The expressions levels of p-Fak, p-Erk1/2, p-JNK and p-p38Mapk were decreased. Meanwhile, a poor early and late osteogenesis effect was found in the shRNA-Fak group.

CONCLUSION

It was inferred that the mechanical stimulus induces increased expression of Fak and activates Fak-Mapk pathway, by activation of Erk, Jnk and p38-Mapk pathway, and that Fak at least, in part, plays an important role in maintaining osteogenic effect by activating Fak-Mapk pathway during distraction osteogenesis.

Ultraviolet Irradiation-Induced Volume Alteration of Corneal Epithelial Cells

Purpose

The purpose of the study is to understand how extracellular stresses, such as ultraviolet (UV) irradiation, affect corneal epithelial cells. Cell volume changes, damage to corneal epithelial integrity, and cellular responses were assessed after exposure to UVC stresses.

Methods

Primary human and rabbit corneal epithelial cells were exposed to UVC light in culture conditions. Ultraviolet C irradiation–induced changes in cell size and volume were measured by real-time microscopy and self-quenching of the fluorescent dye calcein, respectively. The effects of UVC irradiation on Src and focal adhesion kinase (FAK) phosphorylation and FAK-dependent integrin signaling were detected by ELISA, immunoblotting, and immunostaining.

Results

Ultraviolet C irradiation induced both size and volume shifts in human and rabbit corneal epithelial cells. Ultraviolet C irradiation-induced decrease of cell volume elicited activation of Src and FAK, characterized by increased phosphorylations of Src^Y416, FAK^Y397, and FAK^Y925. In addition, immunostaining studies showed UVC irradiation–induced increases in phosphorylation of FAK and formation of integrin β5 clustering. Application of Kv channel blockers, including 4-aminopyridine (4-AP), α-DTX, and depressing substance-1 (BDS-1), effectively suppressed UVC irradiation–induced cell volume changes, and subsequently inhibited UVC irradiation–induced phosphorylation of Src/FAK, and formation of integrin β5 clustering, suggesting UVC irradiation–induced volume changes and Src/FAK activation. Hyperosmotic pressure–induced volume decreases were measured in comparison with effects of UVC irradiation on volume and Src/FAK activation. However, Kv channel blocker, 4-AP, had no effect on hyperosmotic pressure–induced responses.

Conclusions

The present study demonstrates that UVC irradiation–induced decreases in cell volume lead to Src/FAK activation due to a rapid loss of K ions through membrane Kv channels.

Allosteric regulation of focal adhesion kinase by PIP₂ and ATP

Focal adhesion kinase (FAK) is a nonreceptor tyrosine kinase that regulates cell signaling, proliferation, migration, and development. A major mechanism of regulation of FAK activity is an intramolecular autoinhibitory interaction between two of its domains--the catalytic and FERM domains. Upon cell adhesion to the extracellular matrix, FAK is being translocated toward focal adhesion sites and activated. Interactions of FAK with phosphoinositide phosphatidylinsositol-4,5-bis-phosphate (PIP₂) are required to activate FAK. However, the molecular mechanism of the activation remains poorly understood. Recent fluorescence resonance energy transfer experiments revealed a closure of the FERM-kinase interface upon ATP binding, which is reversed upon additional binding of PIP₂. Here, we addressed the allosteric regulation of FAK by performing all-atom molecular-dynamics simulations of a FAK fragment containing the catalytic and FERM domains, and comparing the dynamics in the absence or presence of ATP and PIP₂. As a major conformational change, we observe a closing and opening motion upon ATP and additional PIP₂ binding, respectively, in good agreement with the fluorescence resonance energy transfer experiments. To reveal how the binding of the regulatory PIP₂ to the FERM F2 lobe is transduced to the very distant F1/N-lobe interface, we employed force distribution analysis. We identified a network of mainly charged residue-residue interactions spanning from the PIP₂ binding site to the distant interface between the kinase and FERM domains, comprising candidate residues for mutagenesis to validate the predicted mechanism of FAK activation.

Polarized actin structural dynamics in response to cyclic uniaxial stretch

Endothelial cell (EC) alignment to directional flow or stretch supports anti-inflammatory functions, but mechanisms controlling polarized structural adaptation in response to physical cues remain unclear. This study aimed to determine whether factors associated with early actin edge ruffling implicated in cell polarization are prerequisite for stress fiber (SF) reorientation in response to cyclic uniaxial stretch. Time-lapse analysis of EGFP-actin in confluent ECs showed that onset of either cyclic uniaxial or equibiaxial stretch caused a non-directional increase in edge ruffling. Edge activity was concentrated in a direction perpendicular to the stretch axis after 60 min, consistent with the direction of SF alignment. Rho-kinase inhibition caused reorientation of both stretch-induced edge ruffling and SF alignment parallel to the stretch axis. Arp2/3 inhibition attenuated stretch-induced cell elongation and disrupted polarized edge dynamics and microtubule organizing center reorientation, but it had no effect on the extent of SF reorientation. Disrupting localization of p21-activated kinase (PAK) did not prevent stretch-induced SF reorientation, suggesting that this Rac effector is not critical in regulating stretch-induced cytoskeletal remodeling. Overall, these results suggest that directional edge ruffling is not a primary mechanism that guides SF reorientation in response to stretch; the two events are coincident but not causal.

Paxillin and focal adhesion kinase colocalise in human skeletal muscle and its associated microvasculature

Focal adhesion kinase (FAK) and paxillin are functionally linked hormonal- and mechano-sensitive proteins. We aimed to describe paxillin's subcellular distribution using widefield and confocal immunofluorescence microscopy and test the hypothesis that FAK and paxillin colocalise in human skeletal muscle and its associated microvasculature. Percutaneous muscle biopsies were collected from the m. vastus lateralis of seven healthy males, and 5-μm cryosections were stained with anti-paxillin co-incubated with anti-dystrophin to identify the sarcolemma, anti-myosin heavy chain type I for fibre-type differentiation, anti-dihydropyridine receptor to identify T-tubules, lectin UEA-I to identify the endothelium of microvessels and anti-α-smooth muscle actin to identify vascular smooth muscle cells (VSMC). Colocalisation of anti-paxillin with anti-dystrophin or anti-FAK was quantified using Pearson's correlation coefficient on confocal microscopy images. Paxillin was primarily present in (sub)sarcolemmal regions of skeletal muscle fibres where it colocalised with dystrophin (r = 0.414 ± 0.026). The (sub)sarcolemmal paxillin immunofluorescence intensity was ~2.4-fold higher than in sarcoplasmic regions (P < 0.001) with sarcoplasmic paxillin immunofluorescence intensity ~10 % higher in type I than in type II fibres (P < 0.01). In some longitudinally orientated fibres, paxillin formed striations that corresponded to the I-band region. Paxillin immunostaining was highest in endothelial and VSMC and distributed heterogeneously in both cell types. FAK and paxillin colocalised at (sub)sarcolemmal regions and within the microvasculature (r = 0.367 ± 0.036). The first images of paxillin in human skeletal muscle suggest paxillin is present in (sub)sarcolemmal and I-band regions of muscle fibres and within the microvascular endothelium and VSMC. Colocalisation of FAK and paxillin supports their suggested role in hormonal and mechano-sensitive signalling.

Mechanosensitive properties in the endothelium and their roles in the regulation of endothelial function

: Vascular endothelial cells (ECs) line the luminal surface of blood vessels, which are exposed constantly to mechanical stimuli, such as fluid shear stress, cyclic strain, and blood pressure. In recent years, more and more evidence indicates that ECs sense these mechanical stimuli and subsequently convert mechanical stimuli into intracellular signals. The properties of ECs that sense the mechanical stimuli are defined as mechanosensors. There are a variety of mechanosensors that have been identified in ECs. These mechanosensors play an important role in regulating the function of the endothelium and vascular function, including blood pressure. This review focuses on the mechanosensors that have been identified in ECs and on the roles that mechanosensors play in the regulation of endothelium function, and in the regulation of vascular function.

Integrin - dependent mechanotransduction in mechanically stimulated human annulus fibrosus cells: evidence for an alternative mechanotransduction pathway operating with degeneration

Intervertebral disc (IVD) cells derived from degenerate tissue respond aberrantly to mechanical stimuli, potentially due to altered mechanotransduction pathways. Elucidation of the altered, or alternative, mechanotransduction pathways operating with degeneration could yield novel targets for the treatment of IVD disease. Our aim here was to investigate the involvement of RGD-recognising integrins and associated signalling molecules in the response to cyclic tensile strain (CTS) of human annulus fibrosus (AF) cells derived from non-degenerate and degenerate IVDs. AF cells from non-degenerate and degenerate human IVDs were cyclically strained with and without function blocking RGD – peptides with 10% strain, 1.0 Hz for 20 minutes using a Flexercell® strain device. QRT-PCR and Western blotting were performed to analyse gene expression of type I collagen and ADAMTS -4, and phosphorylation of focal adhesion kinase (FAK), respectively. The response to 1.0 Hz CTS differed between the two groups of AF cells, with decreased ADAMTS -4 gene expression and decreased type I collagen gene expression post load in AF cells derived from non-degenerate and degenerate IVDs, respectively. Pre-treatment of non-degenerate AF cells with RGD peptides prevented the CTS-induced decrease in ADAMTS -4 gene expression, but caused an increase in expression at 24 hours, a response not observed in degenerate AF cells where RGD pre-treatment failed to inhibit the mechano-response. In addition, FAK phosphorylation increased in CTS stimulated AF cells derived from non-degenerate, but not degenerate IVDs, with RGD pre-treatment inhibiting the CTS – dependent increase in phosphorylated FAK. Our findings suggest that RGD -integrins are involved in the 1.0 Hz CTS – induced mechano-response observed in AF cells derived from non-degenerate, but not degenerate IVDs. This data supports our previous work, suggesting an alternative mechanotransduction pathway may be operating in degenerate AF cells.

Netrin-1 attracts axons through FAK-dependent mechanotransduction

The mechanism by which extracellular cues influence intracellular biochemical cascades that guide axons is important, yet poorly understood. Because of the mechanical nature of axon extension, we explored whether the physical interactions of growth cones with their guidance cues might be involved. In the context of mouse spinal commissural neuron axon attraction to netrin-1, we found that mechanical attachment of netrin-1 to the substrate was required for axon outgrowth, growth cone expansion, axon attraction and phosphorylation of focal adhesion kinase (FAK) and Crk-associated substrate (CAS). Myosin II activity was necessary for traction forces >30 pN on netrin-1. Interestingly, while these myosin II-dependent forces on netrin-1 substrates or beads were needed to increase the kinase activity and phosphorylation of FAK, they were not necessary for netrin-1 to increase CAS phosphorylation. When FAK kinase activity was inhibited, the growth cone's ability to recruit additional adhesions and to generate forces >60 pN on netrin-1 was disrupted. Together, these findings demonstrate an important role for mechanotransduction during chemoattraction to netrin-1 and that mechanical activation of FAK reinforces interactions with netrin-1 allowing greater forces to be exerted.

Microtubule depolymerization induces traction force increase through two distinct pathways

Traction forces increase after microtubule depolymerization; however, the signaling mechanisms underlying this, in particular the dependence upon myosin II, remain unclear. We investigated the mechanism of traction force increase after nocodazole-induced microtubule depolymerization by applying traction force microscopy to cells cultured on micropatterned polyacrylamide hydrogels to obtain samples of homogeneous shape and size. Control cells and cells treated with a focal adhesion kinase (FAK) inhibitor showed similar increases in traction forces, indicating that the response is independent of FAK. Surprisingly, pharmacological inhibition of myosin II did not prevent the increase of residual traction forces upon nocodazole treatment. This increase was abolished upon pharmacological inhibition of FAK. These results suggest two distinct pathways for the regulation of traction forces. First, microtubule depolymerization activates a myosin-II-dependent mechanism through a FAK-independent pathway. Second, microtubule depolymerization also enhances traction forces through a myosin-II-independent, FAK-regulated pathway. Traction forces are therefore regulated by a complex network of complementary signals and force-generating mechanisms.

Integrated morphodynamic signalling of the mammary gland

The mammary gland undergoes a spectacular series of changes as it develops, and maintains a remarkable capacity to remodel and regenerate for several decades. Mammary morphogenesis has been investigated for over 100 years, motivated by the dairy industry and cancer biologists. Over the past decade, the gland has emerged as a major model system in its own right for understanding the cell biology of tissue morphogenesis. Multiple signalling pathways from several cell types are orchestrated together with mechanical cues and cell rearrangements to establish the pattern of the mammary gland. The integrated mechanical and molecular pathways that control mammary morphogenesis have implications for the developmental regulation of other epithelial organs.

Upregulation of paxillin and focal adhesion signaling follows Dystroglycan Complex deletions and promotes a hypertensive state of differentiation

Anchorage to matrix is mediated for many cells not only by integrin-based focal adhesions but also by a parallel assembly of integral and peripheral membrane proteins known as the Dystroglycan Complex. Deficiencies in either dystrophin (mdx mice) or γ-sarcoglycan (γSG(-/-) mice) components of the Dystroglycan Complex lead to upregulation of numerous focal adhesion proteins, and the phosphoprotein paxillin proves to be among the most prominent. In mdx muscle, paxillin-Y31 and Y118 are both hyper-phosphorylated as are key sites in focal adhesion kinase (FAK) and the stretch-stimulatable pro-survival MAPK pathway, whereas γSG(-/-) muscle exhibits more erratic hyper-phosphorylation. In cultured myotubes, cell tension generated by myosin-II appears required for localization of paxillin to adhesions while vinculin appears more stably integrated. Overexpression of wild-type (WT) paxillin has no obvious effect on focal adhesion density or the physical strength of adhesion, but WT and a Y118F mutant promote contractile sarcomere formation whereas a Y31F mutant shows no effect, implicating Y31 in striation. Self-peeling of cells as well as Atomic Force Microscopy (AFM) probing of cells with or without myosin-II inhibition indicate an increase in cell tension within paxillin-overexpressing cells. However, prednisolone, a first-line glucocorticoid for muscular dystrophies, decreases cell tension without affecting paxillin at adhesions, suggesting a non-linear relationship between paxillin and cell tension. Hypertension that results from upregulation of integrin adhesions is thus a natural and treatable outcome of Dystroglycan Complex down-regulation.

Regulation of Epithelial-Mesenchymal Transition by Transmission of Mechanical Stress through Epithelial Tissues

Epithelial-mesenchymal transition (EMT) is a phenotypic shift wherein epithelial cells lose or loosen attachments to their neighbors and assume a mesenchymal-like morphology. EMT drives a variety of developmental processes, but may also be adopted by tumor cells during neoplastic progression. EMT is regulated by both biochemical and physical signals from the microenvironment, including mechanical stress, which is increasingly recognized to play a major role in development and disease progression. Biological systems generate, transmit and concentrate mechanical stress into spatial patterns; these gradients in mechanical stress may serve to spatially pattern developmental and pathologic EMTs. Here we review how epithelial tissues generate and respond to mechanical stress gradients, and highlight the mechanisms by which mechanical stress regulates and patterns EMT.

Focal adhesion kinase regulation of mechanotransduction and its impact on endothelial cell functions

Vascular endothelial cells lining the blood vessels form the interface between the bloodstream and the vessel wall and as such they are continuously subjected to shear and cyclic stress from the flowing blood in the lumen. Additional mechanical stimuli are also imposed on these cells in the form of substrate stiffness transmitted from the extracellular matrix components in the basement membrane, and additional mechanical loads imposed on the lung endothelium as the result of respiration or mechanical ventilation in clinical settings. Focal adhesions (FAs) are complex structures assembled at the abluminal endothelial plasma membrane which connect the extracellular filamentous meshwork to the intracellular cytoskeleton and hence constitute the ideal checkpoint capable of controlling or mediating transduction of bidirectional mechanical signals. In this review we focus on focal adhesion kinase (FAK), a component of FAs, which has been studied for a number of years with regards to its involvement in mechanotransduction. We analyzed the recent advances in the understanding of the role of FAK in the signaling cascade(s) initiated by various mechanical stimuli with particular emphasis on potential implications on endothelial cell functions.

The involvement of interleukin-1 and interleukin-4 in the response of human annulus fibrosus cells to cyclic tensile strain: an altered mechanotransduction pathway with degeneration

Introduction

Recent evidence suggests that intervertebral disc (IVD) cells derived from degenerative tissue are unable to respond to physiologically relevant mechanical stimuli in the 'normal' anabolic manner, but instead respond by increasing matrix catabolism. Understanding the nature of the biological processes which allow disc cells to sense and respond to mechanical stimuli (a process termed 'mechanotransduction') is important to ascertain whether these signalling pathways differ with disease. The aim here was to investigate the involvement of interleukin (IL)-1 and IL-4 in the response of annulus fibrosus (AF) cells derived from nondegenerative and degenerative tissue to cyclic tensile strain to determine whether cytokine involvement differed with IVD degeneration.

Methods

AF cells were isolated from nondegenerative and degenerative human IVDs, expanded in monolayers and cyclically strained in the presence or absence of the cytokine inhibitors IL-1 receptor antagonist (IL-1Ra) or IL-4 receptor antibody (IL-4RAb) with 10% strain at 1.0 Hz for 20 minutes using a Flexcell strain device. Total RNA was extracted from the cells at time points of baseline control and 1 or 24 hours poststimulation. Quantitative real-time polymerase chain reaction was used to analyse the gene expression of matrix proteins (aggrecan and type I collagen) and enzymes (matrix metalloproteinase 3 (MMP3) and a disintegrin and metalloproteinase with a thrombospondin type 1 motif 4 (ADAMTS4)).

Results

Expression of catabolic genes (MMP3 and ADAMTS4) decreased in AF cells derived from nondegenerative tissue in response to 1.0-Hz stimulation, and this decrease in gene expression was inhibited or increased following pretreatment of cells with IL-1Ra or IL-4RAb respectively. Treatment of AF cells derived from degenerative tissue with an identical stimulus (1.0-Hz) resulted in reduced anabolic gene expression (aggrecan and type I collagen), with IL-1Ra or IL-4RAb pretreatment having no effect.

Conclusions

Both IL-1 and IL-4 are involved in the response of AF cells derived from nondegenerative tissue to 1.0-Hz cyclic tensile strain. Interestingly, the altered response observed at 1.0-Hz in AF cells from degenerative tissue appears to be independent of either cytokine, suggesting an alternative mechanotransduction pathway in operation.

Mammary gland ECM remodeling, stiffness, and mechanosignaling in normal development and tumor progression

Cells of the mammary gland are in intimate contact with other cells and with the extracellular matrix (ECM), both of which provide not only a biochemical context, but a mechanical context as well. Cell-mediated contraction allows cells to sense the stiffness of their microenvironment, and respond with appropriate mechanosignaling events that regulate gene expression and differentiation. ECM composition and organization are tightly regulated throughout development of the mammary gland, resulting in corresponding regulation of the mechanical environment and proper tissue architecture. Mechanical regulation is also at play during breast carcinoma progression, as changes in ECM deposition, composition, and organization accompany breast carcinoma. These changes result in stiffer matrices that activate mechanosignaling pathways and thereby induce cell proliferation, facilitate local tumor cell invasion, and promote progression. Thus, understanding the role of forces in the mammary gland is crucial to understanding both normal developmental and pathological processes.

Endogenous patterns of mechanical stress are required for branching morphogenesis

Spatial patterning of cell behaviors establishes the regional differences within tissues that collectively develop branched organs into their characteristic treelike shapes. Here we show that the pattern of branching morphogenesis of three-dimensional (3D) engineered epithelial tissues is controlled in part by gradients of endogenous mechanical stress. We used microfabrication to build model mammary epithelial tissues of defined geometry that branched in a stereotyped pattern when induced with growth factors. Branches initiated from sites of high mechanical stress within the tissues, as predicted numerically and measured directly using 3D traction force microscopy. Branch sites were defined by activation of focal adhesion kinase (FAK), inhibition of which disrupted morphogenesis. Stress, FAK activation, and branching were all altered by manipulating cellular contractility, matrix stiffness, intercellular cohesion and tissue geometry. These data suggest that the pattern and magnitude of mechanical stress across epithelial tissues cooperate with biochemical signals to specify branching pattern.

Mechanical stretch decreases FAK phosphorylation and reduces cell migration through loss of JIP3-induced JNK phosphorylation in airway epithelial cells

JNK is a nonreceptor kinase involved in the early events that signal cell migration after injury. However, the linkage to early signals required to initiate the migration response to JNK has not been defined in airway epithelial cells, which exist in an environment subjected to cyclic mechanical strain (MS). The present studies demonstrate that the JNK/stress-activated protein kinase-associated protein 1 (JSAP1; also termed JNK-interacting protein 3, JIP3), a scaffold factor for MAPK cascades that links JNK activation to focal adhesion kinase (FAK), are both associated and activated following mechanical injury in 16HBE14o- human airway epithelial cells and that both FAK and JIP3 phosphorylation seen after injury are decreased in cells subjected to cyclic MS. Overexpression of either wild-type (WT)-FAK or WT-JIP3 enhanced phosphorylation and kinase activation of JNK and reduced the inhibitory effect of cyclic MS. These results suggest that cyclic MS impairs signaling of cell migration after injury via a pathway that involves FAK-JIP3-JNK.

Endothelial cell-laminin interaction: modulation of LDH expression involves alpha6beta4 integrin-FAK-p38MAPK pathway

One of the possible mechanisms of the angiogenic effect of laminin (Ln) involves modulation of the biological activity of VEGF by regulating poly ADP ribosylation (PAR). PAR modification of VEGF was found to be related with the changes in NAD(+) associated with a shift in LDH isoenzymes. Further investigations on LDH gene expression in HUVECs suggested that the effect of Ln was mediated through alpha(6)beta(4) integrin-FAK-src-p38 MAPK pathway. This was evidenced by (a) co-immunoprecipitation of beta(4) integrin with alpha(6) subunit, (b) activation by tyrosine phosphorylation of beta(4) integrin and FAK, (c) co-immunoprecipitation of FAK with beta(4) and with adapter protein, src, (d) increased phosphorylation of p38 MAPK in cells maintained on Ln and (e) blocking of effect of Ln on LDH-B gene expression by inhibition of p38 MAPK. Increase in serine phosphorylation of c-fos and c-jun and higher levels of heterodimers of AP-1 in the nucleus in cells maintained on Ln suggested activation of AP-1 transcription factor. These results provide evidence for modulation of endothelial cell function relevant to angiogenesis by Ln through alpha(6)beta(4) integrin.

The effect of different frequencies of stretch on human dermal keratinocyte proliferation and survival

The effects of cyclic stretch on mitogen-activated protein kinase (MAPK) and apoptosis in keratinocytes are not well understood. The aim of this study is to compare the effect of high frequency repetitive (HF) stretch to intermittent (I) stretch on human dermal keratinocytes proliferation and survival. Cultured human dermal keratinocytes were exposed to either repetitive HF or I stretch. Cell number was measured by coulter counter, DNA synthesis was assessed by BrdU staining, and apoptosis was assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining. The activation of p38 MAPK, ERK 1/2, and AKT was assessed by immunoblotting. p38 MAPK, ERK 1/2, and AKT exhibited no change after HF stretch, while AKT and Homo sapiens BCL-2-antagonist of cell death (BAD) were significantly activated after I stretch. After experiencing I stretch for 2 d, keratinocyte proliferation rates were significantly decreased. This decrease was most likely not due to apoptosis as TUNEL-positive cells only increased for cells treated with an AKT inhibitor.

Extracellular matrix rigidity promotes invadopodia activity

Invadopodia are actin-rich subcellular protrusions with associated proteases used by cancer cells to degrade extracellular matrix (ECM) [1]. Molecular components of invadopodia include branched actin-assembly proteins, membrane trafficking proteins, signaling proteins, and transmembrane proteinases [1]. Similar structures exist in nontransformed cells, such as osteoclasts and dendritic cells, but are generally called podosomes and are thought to be more involved in cell-matrix adhesion than invadopodia [2-4]. Despite intimate contact with their ECM substrates, it is unknown whether physical or chemical ECM signals regulate invadopodia function. Here, we report that ECM rigidity directly increases both the number and activity of invadopodia. Transduction of ECM-rigidity signals depends on the cellular contractile apparatus [5-7], given that inhibition of nonmuscle myosin II, myosin light chain kinase, and Rho kinase all abrogate invadopodia-associated ECM degradation. Whereas myosin IIA, IIB, and phosphorylated myosin light chain do not localize to invadopodia puncta, active phosphorylated forms of the mechanosensing proteins p130Cas (Cas) and focal adhesion kinase (FAK) are present in actively degrading invadopodia, and the levels of phospho-Cas and phospho-FAK in invadopodia are sensitive to myosin inhibitors. Overexpression of Cas or FAK further enhances invadopodia activity in cells plated on rigid polyacrylamide substrates. Thus, in invasive cells, ECM-rigidity signals lead to increased matrix-degrading activity at invadopodia, via a myosin II-FAK/Cas pathway. These data suggest a potential mechanism, via invadopodia, for the reported correlation of tissue density with cancer aggressiveness.

Thrombospondin-1-induced migration is functionally dependent upon focal adhesion kinase

Vascular smooth muscle cell migration is important in vascular disease. Previously, we showed thrombospondin-1 activates focal adhesion kinase in these cells. We hypothesized that focal adhesion kinase is important for thrombspondin-1-induced vascular smooth muscle cell migration. Bovine aortic smooth muscle cells were transfected with FAK397, FAK-wild type, pcDNA, or beta-Gal plasmids. Migration was assessed with thrombospondin-1 or serum-free medium in quiescent transfected cells or quiescent cells pretreated with the focal adhesion kinase inhibitor, geldanamycin. Number of cells migrated per 5 fields (x400) were recorded. Antihemagglutinin immunoprecipitation and Western blot were used to examine thrombospondin-1-induced focal adhesion kinase phosphorylation in transfected cells. FAK397 transfection inhibited thrombospondin-1-induced focal adhesion kinase phosphorylation and migration (P < .05). Geldanamycin inhibited thrombospondin-1-induced smooth muscle cell migration (P < .05). In conclusion, vascular smooth muscle cells transfected with FAK397 inhibited thrombosponin-1-induced migration and tyrosine phosphorylation. Further, geldanamycin also inhibited migration. These results suggest focal adhesion kinase is involved in thrombospondin-1-induced vascular smooth muscle cell migration.

Effect of different frequencies of tensile strain on human dermal fibroblast proliferation and survival

The aim of this study is to compare the effect of a high-frequency repetitive (HF) stretch or an intermittent (I) stretch on the cell proliferation and survival of human dermal fibroblasts and to determine the activation of any relevant signal pathways. Cultured human dermal fibroblasts were exposed to either HF or I stretch. Cell number was measured by counting, while DNA synthesis was assessed by 5-bromo-2'-deoxyuridine (BrdU) staining and apoptosis by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining. To investigate the potential mechanisms of repetitive strain on the proliferation and survival of fibroblasts, the activation of relevant transduction pathways, such as p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK)1/2, AKT, and BAD, was assessed by Western blot. In addition, the effect of inhibition of these pathways on the fibroblast response was also studied. After either HF or I stretch for 7 days, fibroblast number was significantly decreased and there were less BrdU-positive cells. The numbers of apoptotic and/or necrotic fibroblasts were not affected. p38 MAPK and ERK1/2 were significantly activated after HF stretch, but AKT and BAD were significantly activated after I stretch. The inhibitors of p38 MAPK and MAPK/ERK kinase as well as dominant-negative AKT reduced cell number after both HF and I stretch but these pathways were not critical for the stretch-induced decrease in cell number.

Integrins regulate mouse embryonic stem cell self-renewal

Extracellular matrix (ECM) components regulate stem-cell behavior, although the exact effects elicited in embryonic stem (ES) cells are poorly understood. We previously developed a simple, defined, serum-free culture medium that contains leukemia inhibitory factor (LIF) for propagating pluripotent mouse embryonic stem (mES) cells in the absence of feeder cells. In this study, we determined the effects of ECM components as culture substrata on mES cell self-renewal in this culture medium, comparing conventional culture conditions that contain serum and LIF with gelatin as a culture substratum. mES cells remained undifferentiated when cultured on type I and type IV collagen or poly-D-lysine. However, they differentiated when cultured on laminin or fibronectin as indicated by altered morphologies, the activity of alkaline phosphatase decreased, Fgf5 expression increased, and Nanog and stage-specific embryonic antigen 1 expression decreased. Under these conditions, the activity of signal transducer and activator of transcription (STAT)3 and Akt/protein kinase B (PKB), which maintain cell self-renewal, decreased. In contrast, the extracellular signal-regulated kinase (ERK)1/2 activity, which negatively controls cell self-renewal, increased. In the defined conditions, mES cells did not express collagen-binding integrin subunits, but they expressed laminin- and fibronectin-binding integrin subunits. The expression of some collagen-binding integrin subunits was downregulated in an LIF concentration-dependent manner. Blocking the interactions between ECM and integrins inhibited this differentiation. Conversely, the stimulation of ECM-integrin interactions by overexpressing collagen-binding integrin subunits induced differentiation of mES cells cultured on type I collagen. The results of the study indicated that inactivation of the integrin signaling is crucial in promoting mouse embryonic stem cell self-renewal. Disclosure of potential conflicts of interest is found at the end of this article.

Mechanical strain regulates endothelial cell patterning in vitro

Blood vessels of the vertebrate circulatory system typically exhibit tissue-specific patterning. However, the cues that guide the development of these patterns remain unclear. We investigated the effect of cyclic uniaxial strain on vascular endothelial cell dynamics and sprout formation in vitro in two-dimensional (2D) and three-dimensional (3D) culture systems under the influence of growth factors. Cells preferentially aligned and moved in the direction perpendicular to the major strain axis in monolayer culture, and mechanical strain also regulated the spatial location of cell proliferation in 2D cell culture. Cells in 3D cell culture could be induced to form sprouts by exposure to appropriate growth factor combinations (vascular endothelial growth factor and hepatocyte growth factor), and the strain direction regulated the directionality of this process. Moreover, cyclic uniaxial strain inhibited branching of the structures formed by endothelial cells and increased their thickness. Taken together, these data support the importance of external mechanical stimulation in the regulation of endothelial cell migration, proliferation, and differentiation into primitive vessels.

Phosphoinositide 3-kinase, Src, and Akt modulate acute ventilation-induced vascular permeability increases in mouse lungs

To determine the role of phosphoinositide 3-OH kinase (PI3K) pathways in the acute vascular permeability increase associated with ventilator-induced lung injury, we ventilated isolated perfused lungs and intact C57BL/6 mice with low and high peak inflation pressures (PIP). In isolated lungs, filtration coefficients (K(f)) increased significantly after ventilation at 30 cmH(2)O (high PIP) for successive periods of 15, 30 (4.1-fold), and 50 (5.4-fold) min. Pretreatment with 50 microM of the PI3K inhibitor, LY-294002, or 20 microM PP2, a Src kinase inhibitor, significantly attenuated the increase in K(f), whereas 10 microM Akt inhibitor IV significantly augmented the increased K(f). There were no significant differences in K(f) or lung wet-to-dry weight (W/D) ratios between groups ventilated with 9 cmH(2)O PIP (low PIP), with or without inhibitor treatment. Total lung beta-catenin was unchanged in any low PIP isolated lung group, but Akt inhibition during high PIP ventilation significantly decreased total beta-catenin by 86%. Ventilation of intact mice with 55 cmH(2)O PIP for up to 60 min also increased lung vascular permeability, indicated by increases in lung lavage albumin concentration and lung W/D ratios. In these lungs, tyrosine phosphorylation of beta-catenin and serine/threonine phosphorylation of Akt, glycogen synthase kinase 3beta (GSK3beta), and ERK1/2 increased significantly with peak effects at 60 min. Thus mechanical stress activation of PI3K and Src may increase lung vascular permeability through tyrosine phosphorylation, but simultaneous activation of the PI3K-Akt-GSK3beta pathway tends to limit this permeability response, possibly by preserving cellular beta-catenin.

Src and focal adhesion kinase mediate mechanical strain-induced proliferation and ERK1/2 phosphorylation in human H441 pulmonary epithelial cells

Pulmonary epithelial cells are exposed to repetitive deformation during physiological breathing and mechanical ventilation. Such deformation may influence pulmonary growth, development, and barotrauma. Although deformation stimulates proliferation and activates extracellular signal-regulated kinases (ERK1/2) in human pulmonary epithelial H441 cells, the upstream mechanosensors that induce ERK activation are poorly understood. We investigated whether c-Src or focal adhesion kinase (FAK) mediates cyclic mechanical strain-induced ERK1/2 activation and proliferation in human pulmonary epithelial (NCI-H441) cells. The H441 and A549 cells were grown on collagen I-precoated membranes and were subjected to an average 10% cyclic mechanical strain at 20 cycles/min. Cyclic strain activated Src within 2 min by increasing phosphorylation at Tyr(418), followed by rapid phosphorylation of FAK at Tyr(397) and Tyr(576) and ERK1/2 at Thr(202)/Tyr(204) (n = 5, P < 0.05). Twenty-four (A549 cells) and 24-72 h (H441 cells) of cyclic mechanical strain increased cell numbers compared with static culture. Twenty-four hours of cyclic strain also increased H441 FAK, Src, and ERK phosphorylation without affecting total FAK, Src, or ERK protein. The mitogenic effect was blocked by Src (10 micromol/l PP2 or short interfering RNA targeted to Src) or MEK (50 micromol/l PD-98059) inhibition. PP2 also blocked strain-induced phosphorylation of FAK-Tyr(576) and ERK-Thr(202)/Tyr(204) but not FAK-Tyr(397). Reducing FAK by FAK-targeted short interfering RNA blocked mechanical strain-induced mitogenicity and significantly attenuated strain-induced ERK activation but not strain-induced Src phosphorylation. Together, these results suggest that repetitive mechanical deformation induced by ventilation supports pulmonary epithelial proliferation by a pathway involving Src, FAK, and then ERK signaling.

Capillary pressure-induced lung injury: fact or fiction?

Lung capillary pressure in healthy humans at rest ranges between 6 and 10 mmHg. At maximal effort or in pathophysiological conditions such as left sided heart disease or massive pulmonary vasoconstriction, for example in high-altitude pulmonary disease, capillary pressure may be markedly elevated. Increased capillary pressure directly affects transendothelial fluid dynamics and thus results in the formation of hydrostatic lung edema. Excessive pressure increases may cause capillary stress failure. Recent studies, however, suggest that the microvascular response to lung capillary hypertension is more complex. Pressure, strain and shear stress cause dysfunction of the capillary endothelium characterized by an imbalanced release of vasoactive mediators. Endothelial dysfunction evokes a multicellular response with features of vasoconstriction, inflammation, and vascular leakage, thrombosis, and remodeling. These active cellular reactions contribute to the pathophysiological process and may be specifically targeted by new therapeutic strategies.

Stress-induced corneal epithelial apoptosis mediated by K+ channel activation

One of the functional roles of the corneal epithelial layer is to protect the cornea, lens and other underlying ocular structures from damages caused by environmental insults. It is important for corneal epithelial cells to maintain this function by undergoing continuous renewal through a dynamic process of wound healing. Previous studies in corneal epithelial cells have provided substantial evidence showing that environmental insults, such as ultraviolet (UV) irradiation and other biohazards, can induce stress-related cellular responses resulting in apoptosis and thus interrupt the dynamic process of wound healing. We found that UV irradiation-induced apoptotic effects in corneal epithelial cells are started by the hyperactivation of K+ channels in the cell membrane resulting in a fast loss of intracellular K+ ions. Recent studies provide further evidence indicating that these complex responses in corneal epithelial cells are resulted from the activation of stress-related signaling pathways mediated by K+ channel activity. The effect of UV irradiation on corneal epithelial cell fate shares common signaling mechanisms involving the activation of intracellular responses that are often activated by the stimulation of various cytokines. One piece of evidence for making this distinction is that at early times UV irradiation activates a Kv3.4 channel in corneal epithelial cells to elicit activation of c-Jun N-terminal kinase cascades and p53 activation leading to cell cycle arrest and apoptosis. The hypothetic model is that UV-induced potassium channel hyperactivity as an early event initiates fast cell shrinkages due to the loss of intracellular potassium, resulting in the activation of scaffolding protein kinases and cytoskeleton reorganizations. This review article presents important control mechanisms that determine Kv channel activity-mediated cellular responses in corneal epithelial cells, involving activation of stress-induced signaling pathways, arrests of cell cycle progression and/or induction of apoptosis.

Effects of 17beta-estradiol on growth and apoptosis in human vascular endothelial cells: influence of mechanical strain and tumor necrosis factor-alpha

Vascular endothelial cell (EC) integrity is key to arterial health; endothelial dysfunction is linked to atherogenesis. Atherosclerosis shows a male preponderance, possibly related to the protective effect of estrogens in women. This study examined the effect of estrogens on growth, apoptosis and adhesion molecule expression in cultured human EC. The effects of 17beta-estradiol (E2) were studied in human umbilical vein endothelial cells (HUVEC) under normal culture conditions, and following exposure to cyclic mechanical strain or tumor necrosis factor alpha (TNFalpha). E2 enhanced HUVEC growth in serum-enriched media, in a concentration-dependent manner. This up-regulation of EC growth by E2 was associated with an increase in telomerase activity, assessed by PCR-based TRAP analysis. Cyclic strain enhanced [(3)H]-thymidine incorporation into DNA, and increased activation of mitogen-activated protein (MAP) kinase ERK1/2 and expression of early growth genes (Egr-1 and Sp-1); E2 attenuated the strain-induced ERK1/2 activation but not the early growth gene expression or DNA synthesis. TNFalpha (20 ng/mL) induced apoptosis in HUVEC, causing a decrease in DNA synthesis, increase in floating and Annexin-V-stained cell numbers, and morphological changes. TNFalpha also upregulated ERK1/2 activity and expression of adhesion molecules (ICAM-1, VCAM-1 and E-selectin). E2 significantly attenuated the effects of TNFalpha on ERK1/2 activity, apoptosis, and E-selectin expression in the cells. Thus, estradiol enhances growth and reduces TNFalpha-induced apoptosis in EC; enhanced EC growth may be mediated via upregulation of telomerase activity. These effects are possible cellular mechanisms underlying female gender-associated cardiovascular protection.

Podocytes are sensitive to fluid shear stress in vitro

Podocytes are exposed to mechanical forces arising from glomerular capillary pressure and filtration. It has been shown that stretch affects podocyte biology in vitro and plays a significant role in the development of glomerulosclerosis in vivo. However, whether podocytes are sensitive to fluid shear stress is completely unknown. In the present study, we therefore exposed cells of a recently generated conditionally immortalized mouse podocyte cell line to defined fluid shear stress in a flow chamber, mimicking flow of the glomerular ultrafiltrate over the surface of podocytes in Bowman's space. Shear stress above 0.25 dyne/cm(2) resulted in dramatic loss of podocytes but not of proximal tubular epithelial cells (LLC-PK(1) cells) after 20 h. At 0.015-0.25 dyne/cm(2), lamellipodia formation in podocytes was enhanced and the actin nucleation protein cortactin was redistributed to the cell margins. Shear stress further diminished stress fibers and the presence of vinculin in focal adhesions. Linear zonula occludens-1 distribution at cell-cell contacts remained unaffected at low shear stress. At 0.25 dyne/cm(2), the monolayer was broken up and remaining cell-cell contacts were reinforced by F-actin and alpha-actinin. Because the cytoskeletal changes induced by shear stress suggested the involvement of tyrosine kinases (TKs), we tested several TK inhibitors that were all without effect on podocyte number under static conditions. At 0.25 dyne/cm(2), however, the TK inhibitors genistein and AG 82 were associated with marked podocyte loss. Our data demonstrate that podocytes are highly sensitive to fluid shear stress. Shear stress induces a reorganization of the actin cytoskeleton and activates specific tyrosine kinases that are required to withstand fluid shear stress.

Force meets chemistry: Analysis of mechanochemical conversion in focal adhesions using fluorescence recovery after photobleaching

Mechanotransduction--the process by which mechanical forces are converted into changes of intracellular biochemistry--is critical for normal cell and tissue function. Integrins facilitate mechanochemical conversion by transferring physical forces from the extracellular matrix, across the cell surface, and to cytoskeletal-associated proteins within focal adhesions. It is likely that force alters biochemistry at these sites by altering molecular binding affinities of a subset of focal adhesion proteins, but this has been difficult to quantify within living cells. Here, we describe how the fluorescence recovery after photobleaching (FRAP) technique can be adapted and used in conjunction with mathematical models to directly measure force-dependent alterations in molecular binding and unbinding rate constants of individual focal adhesion proteins in situ. We review these recent findings, and discuss the strengths and limitations of this approach for analysis of mechanochemical signaling in focal adhesions and other cellular structures. The ability to quantify molecular binding rate constants in the physical context of the living cytoplasm should provide new insight into the molecular basis of cellular mechanotransduction. It also may facilitate future efforts to bridge biological experimentation and mathematical modeling in our quest for a systems biology level description of cell regulation.

Stromagenesis: the changing face of fibroblastic microenvironments during tumor progression

During tumorigenesis, reciprocal changes in stromal fibroblasts and tumor cells induce changes to the neoplastic microenvironmental landscape. In stromagenesis, both the complex network of bi-directional stromal fibroblastic signaling pathways and the stromal extracellular matrix are modified. The presence of a 'primed' stroma during the early, reversible stage of tumorigenesis is optimal for stromal-directed therapeutic intervention. Three-dimensional (3D) cell culture systems have been developed that mimic the in vivo microenvironment. These systems provide unique experimental tools to identify early alterations in stromagenesis that are supportive of tumor progression with the ultimate goal of blocking neoplastic permissiveness and restoring normal phenotypes.

Cyclic strain-mediated regulation of vascular endothelial cell migration and tube formation

Hemodynamic forces exerted by blood flow (cyclic strain, shear stress) affect the initiation and progression of angiogenesis; however, the precise signaling mechanism(s) involved are unknown. In this study, we examine the role of cyclic strain in regulating bovine aortic endothelial cell (BAEC) migration and tube formation, indices of angiogenesis. Considering their well-documented mechanosensitivity, functional inter-dependence, and involvement in angiogenesis, we hypothesized roles for matrix metalloproteinases (MMP-2/9), RGD-dependent integrins, and urokinase plasminogen activator (uPA) in this process. BAECs were exposed to equibiaxial cyclic strain (5% strain, 1Hz for 24h) before their migration and tube formation was assessed by transwell migration and collagen gel tube formation assays, respectively. In response to strain, both migration and tube formation were increased by 1.83+/-0.1- and 1.84+/-0.1-fold, respectively. Pertussis toxin, a Gi-protein inhibitor, decreased strain-induced migration by 45.7+/-32% and tube formation by 69.8+/-13%, whilst protein tyrosine kinase (PTK) inhibition with genistein had no effect. siRNA-directed attenuation of endothelial MMP-9 (but not MMP-2) expression/activity decreased strain-induced migration and tube formation by 98.6+/-41% and 40.7+/-31%, respectively. Finally, integrin blockade with cRGD peptide and siRNA-directed attenuation of uPA expression reduced strain-induced tube formation by 85.7+/-15% and 84.7+/-31%, respectively, whilst having no effect on migration.

CONCLUSIONS

Cyclic strain promotes BAEC migration and tube formation in a Gi-protein-dependent PTK-independent manner. Moreover, we demonstrate for the first time a putative role for MMP-9 in both strain-induced events, whilst RGD-dependent integrins and uPA appear only to be involved in strain-induced tube formation.

Strain-induced vascular endothelial cell proliferation requires PI3K-dependent mTOR-4E-BP1 signal pathway

The aim of this study was to determine whether the phosphatidylinositol 3-kinase (PI3K)-dependent mammalian target of rapamycin (mTOR)-eukaryotic initiation factor 4E binding protein 1 (4E-BP1) signal pathway and S6 kinase (S6K), the major element of the mTOR pathway, play a role in the enhanced vascular endothelial cell (EC) proliferation induced by cyclic strain. Bovine aortic ECs were subjected to an average of 10% strain at a rate of 60 cycles/min for ≤24 h. Cyclic strain-induced EC proliferation was reduced by pretreatment with rapamycin but not the MEK1 inhibitor PD-98059. The PI3K inhibitors wortmannin and LY-294002 also attenuated strain-induced EC proliferation and strain-induced activation of S6K. Rapamycin but not PD-98059 prevented strain-induced S6K activation, and PD-98059 but not rapamycin prevented strain-induced activation of extracellular signal-regulated kinases 1 and 2. Cyclic strain also activated 4E-BP1, which could be inhibited by PI3K inhibitors. These data suggest that the PI3K-dependent S6K-mTOR-4E-BP1 signal pathway may be critically involved in strain-induced bovine aortic EC proliferation.