Selective adapter recruitment and differential signaling networks by VEGF vs. shear stress

Androgen receptor activation inhibits endothelial cell migration in vitro and angiogenesis in vivo

Our previous research revealed that androgen receptor (AR) activation reduces endothelial cell proliferation via non-genomic pathways. We hypothesized that AR activation might also affect endothelial cell migration, a critical step in angiogenesis. Our data demonstrates that treatment of human umbilical vein endothelial cells (HUVECs) with AR agonists, metribolone (R1881) or dihydrotestosterone (DHT), results in a dose-dependent reduction in migration, which can be reversed by AR antagonists or AR knockdown. Mechanistically, R1881 inhibits HUVEC migration by suppressing RhoA activity through the cSrc/FAK/paxillin pathway and promoting RhoA degradation via RhoA-p27 complex formation, ultimately resulting in RhoA ubiquitination. Transfection with constitutively active RhoA-V14 rescues the inhibitory effect of R1881 on HUVEC migration. Furthermore, R1881 elevates intracellular vascular endothelial growth factor (VEGF) and connective tissue growth factor (CTGF) levels but reduces VEGF secretion from HUVECs. This reduction is attributed to the formation of VEGF-CTGF complexes in the cytosol induced by R1881. Transfection with RhoA-V14 reduces CTGF levels and VEGF-CTGF complex formation, leading to enhanced VEGF secretion. Pre-treatment with WP631, a CTGF inhibitor, mitigates the R1881-induced reduction in VEGF secretion and HUVECs migration. In vivo assessments using zebrafish angiogenesis and mouse matrigel plug assays validate the anti-angiogenic effects of R1881. These findings provide insight into the molecular mechanisms through which AR activation modulates endothelial cell migration and angiogenesis.

Biomechanical forces and force-triggered drug delivery in tumor neovascularization

Tumor angiogenesis is one of the typical hallmarks of tumor occurrence and development, and tumor neovascularization also exhibits distinct characteristics from normal blood vessels. As the number of cells and matrix inside the tumor increases, the biomechanical force is enhanced, specifically manifested as solid stress, fluid stress, stiffness, and topology. This mechanical microenvironment also provides shelter for tumors and intensifies angiogenesis, providing oxygen and nutritional support for tumor progression. During tumor development, the biomechanical microenvironment also emerges, which in turn feeds back to regulate the tumor progression, including tumor angiogenesis, and biochemical and biomechanical signals can regulate tumor angiogenesis. Blood vessels possess inherent sensitivity to mechanical stimuli, but compared to the extensive research on biochemical signal regulation, the study of the regulation of tumor neovascularization by biomechanical signals remains relatively scarce. Biomechanical forces can affect the phenotypic characteristics and mechanical signaling pathways of tumor blood vessels, directly regulating angiogenesis. Meanwhile, they can indirectly regulate tumor angiogenesis by causing an imbalance in angiogenesis signals and affecting stromal cell function. Understanding the regulatory mechanism of biomechanical forces in tumor angiogenesis is beneficial for better identifying and even taming the mechanical forces involved in angiogenesis, providing new therapeutic targets for tumor vascular normalization. Therefore, we summarized the composition of biomechanical forces and their direct or indirect regulation of tumor neovascularization. In addition, this review discussed the use of biomechanical forces in combination with anti-angiogenic therapies for the treatment of tumors, and biomechanical forces triggered delivery systems.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Molecular Tension Probes to Quantify Cell-Generated Mechanical Forces

Living cells generate, sense, and respond to mechanical forces through their interaction with neighboring cells or extracellular matrix, thereby regulating diverse cellular processes such as growth, motility, differentiation, and immune responses. Dysregulation of mechanosensitive signaling pathways is found associated with the development and progression of various diseases such as cancer. Yet, little is known about the mechanisms behind mechano-regulation, largely due to the limited availability of tools to study it at the molecular level. The recent development of molecular tension probes allows measurement of cellular forces exerted by single ligandreceptor interaction, which has helped in revealing the hitherto unknown mechanistic details of various mechanosensitive processes in living cells. Here, we provide an introductory overview of two methods based on molecular tension probes, tension gauge tether (TGT), and molecular tension fluorescence microscopy (MTFM). TGT utilizes the irreversible rupture of double-stranded DNA tether upon application of force in the piconewton (pN) range, whereas MTFM utilizes the reversible extension of molecular springs such as polymer or single-stranded DNA hairpin under applied pN forces. Specifically, the underlying principle of how molecular tension probes measure cell-generated mechanical forces and their applications to mechanosensitive biological processes are described.

Editorial: Understanding molecular interactions that underpin vascular mechanobiology

Cells are exposed to a variety of mechanical forces in their daily lives, especially endothelial cells that are stretched from vessel distention and are exposed to hemodynamic shear stress from a blood flow. Exposure to excessive forces can induce a disease, but the molecular details on how these cells perceive forces, transduce them into biochemical signals and genetic events, i.e., mechanotransduction, and integrate them into physiological or pathological changes remain unclear. However, seminal studies in endothelial cells over the past several decades have begun to elucidate some of these signals. These studies have been highlighted in APL Bioengineering and elsewhere, describing a complex temporal pattern where forces are sensed immediately by ion channels and force-dependent conformational changes in surface proteins, followed by biochemical cascades, cytoskeletal contraction, and nuclear remodeling that can affect long-term changes in endothelial morphology and fate. Key examples from the endothelial literature that have established these pathways include showing that integrins and Flk-1 or VE-cadherin act as shear stress transducers, activating downstream proteins such as Cbl and Nckβ or Src, respectively. In this Editorial, we summarize a recent literature highlighting these accomplishments, noting the engineering tools and analysis methods used in these discoveries while also highlighting unanswered questions.

Sinner or Saint?: Nck Adaptor Proteins in Vascular Biology

The Nck family of modular adaptor proteins, including Nck1 and Nck2, link phosphotyrosine signaling to changes in cytoskeletal dynamics and gene expression that critically modulate cellular phenotype. The Nck SH2 domain interacts with phosphotyrosine at dynamic signaling hubs, such as activated growth factor receptors and sites of cell adhesion. The Nck SH3 domains interact with signaling effectors containing proline-rich regions that mediate their activation by upstream kinases. In vascular biology, Nck1 and Nck2 play redundant roles in vascular development and postnatal angiogenesis. However, recent studies suggest that Nck1 and Nck2 differentially regulate cell phenotype in the adult vasculature. Domain-specific interactions likely mediate these isoform-selective effects, and these isolated domains may serve as therapeutic targets to limit specific protein-protein interactions. In this review, we highlight the function of the Nck adaptor proteins, the known differences in domain-selective interactions, and discuss the role of individual Nck isoforms in vascular remodeling and function.

Histopathologic findings of vascular damage after mechanical thrombectomy using stent retriever in canine models

Although mechanical thrombectomy is a powerful predictor of stroke outcome, it induces vessel wall injury in the acute phase. This study aimed to analyze the degree and the condition of recovery of wall injury after the acute phase via angiography and histopathological analysis of autopsied canine models. Digital subtraction angiography (DSA) and embolization with autologous thrombus were performed in six canines. The model of arterial occlusion was effective in all target vessels. Mechanical thrombectomy was performed in completely occluded vessels using stent retriever. Follow-up angiographic and histopathologic evaluations were performed 1 month later. Complete recanalization using stent retriever was achieved in four cases. Slight residual vessel narrowing after recanalization and moderate narrowing was observed in one case each. Histopathological analysis showed that inflammation, hemorrhage, and device-induced medial injury were not observed in any of the cases. Severe intimal proliferation (grade 4), marked diffuse thrombosis (grade 4), and weak vascular endothelial cell loss (grade 1) were observed in one case and weak endovascular proliferation was observed in one case. Although successful complete recanalization was achieved with a single mechanical thrombectomy attempt and no change was observed in the follow-up DSA, special attention should be paid to postoperative follow-up, as device-induced intimal proliferation, diffuse thrombosis, and endothelial cell loss may remain after 1 month.

Remodeling of the Microvasculature: May the Blood Flow Be With You

The vasculature ensures optimal delivery of nutrients and oxygen throughout the body, and to achieve this function it must continually adapt to varying tissue demands. Newly formed vascular plexuses during development are immature and require dynamic remodeling to generate well-patterned functional networks. This is achieved by remodeling of the capillaries preserving those which are functional and eliminating other ones. A balanced and dynamically regulated capillary remodeling will therefore ensure optimal distribution of blood and nutrients to the tissues. This is particularly important in pathological contexts in which deficient or excessive vascular remodeling may worsen tissue perfusion and hamper tissue repair. Blood flow is a major determinant of microvascular reshaping since capillaries are pruned when relatively less perfused and they split when exposed to high flow in order to shape the microvascular network for optimal tissue perfusion and oxygenation. The molecular machinery underlying blood flow sensing by endothelial cells is being deciphered, but much less is known about how this translates into endothelial cell responses as alignment, polarization and directed migration to drive capillary remodeling, particularly in vivo. Part of this knowledge is theoretical from computational models since blood flow hemodynamics are not easily recapitulated by in vitro or ex vivo approaches. Moreover, these events are difficult to visualize in vivo due to their infrequency and briefness. Studies had been limited to postnatal mouse retina and vascular beds in zebrafish but new tools as advanced microscopy and image analysis are strengthening our understanding of capillary remodeling. In this review we introduce the concept of remodeling of the microvasculature and its relevance in physiology and pathology. We summarize the current knowledge on the mechanisms contributing to capillary regression and to capillary splitting highlighting the key role of blood flow to orchestrate these processes. Finally, we comment the potential and possibilities that microfluidics offers to this field. Since capillary remodeling mechanisms are often reactivated in prevalent pathologies as cancer and cardiovascular disease, all this knowledge could be eventually used to improve the functionality of capillary networks in diseased tissues and promote their repair.

Nck1, But Not Nck2, Mediates Disturbed Flow-Induced p21-Activated Kinase Activation and Endothelial Permeability

Background

Alteration in hemodynamic shear stress at atheroprone sites promotes endothelial paracellular pore formation and permeability. The molecular mechanism remains unknown.

Methods and Results

We show that Nck (noncatalytic region of tyrosine kinase) deletion significantly ameliorates disturbed flow‐induced permeability, and selective isoform depletion suggests distinct signaling mechanisms. Only Nck1 deletion significantly reduces disturbed flow‐induced paracellular pore formation and permeability, whereas Nck2 depletion has no significant effects. Additionally, Nck1 re‐expression, but not Nck2, restores disturbed flow‐induced permeability in Nck1/2 knockout cells, confirming the noncompensating roles. In vivo, using the partial carotid ligation model of disturbed flow, Nck1 knockout prevented the increase in vascular permeability, as assessed by Evans blue and fluorescein isothiocyanate dextran extravasations and leakage of plasma fibrinogen into the vessel wall. Domain swap experiments mixing SH2 (phosphotyrosine binding) and SH3 (proline‐rich binding) domains between Nck1 and Nck2 showed a dispensable role for SH2 domains but a critical role for the Nck1 SH3 domains in rescuing disturbed flow‐induced endothelial permeability. Consistent with this, both Nck1 and Nck2 bind to platelet endothelial adhesion molecule‐1 (SH2 dependent) in response to shear stress, but only Nck1 ablation interferes with shear stress–induced PAK2 (p21‐activated kinase) membrane translocation and activation. A single point mutation into individual Nck1 SH3 domains suggests a role for the first domain of Nck1 in PAK recruitment to platelet endothelial cell adhesion molecule‐1 and activation in response to shear stress.

Conclusions

This work provides the first evidence that Nck1 but not the highly similar Nck2 plays a distinct role in disturbed flow‐induced vascular permeability by selective p21‐activated kinase activation.

Endothelial mechanobiology

Lining the luminal surface of the vasculature, endothelial cells (ECs) are in direct contact with and differentially respond to hemodynamic forces depending on their anatomic location. Pulsatile shear stress (PS) is defined by laminar flow and is predominantly located in straight vascular regions, while disturbed or oscillatory shear stress (OS) is localized to branch points and bifurcations. Such flow patterns have become a central focus of vascular diseases, such as atherosclerosis, because the focal distribution of endothelial dysfunction corresponds to regions exposed to OS, whereas endothelial homeostasis is maintained in regions defined by PS. Deciphering the mechanotransduction events that occur in ECs in response to differential flow patterns has required the innovation of multidisciplinary approaches in both in vitro and in vivo systems. The results from these studies have identified a multitude of shear stress-regulated molecular networks in the endothelium that are implicated in health and disease. This review outlines the significance of scientific findings generated in collaboration with Dr. Shu Chien.

Mechanical Forces in Tumor Angiogenesis

A defining hallmark of cancer and cancer development is upregulated angiogenesis. The vasculature formed in tumors is structurally abnormal, not organized in the conventional hierarchical arrangement, and more permeable than normal vasculature. These features contribute to leaky, tortuous, and dilated blood vessels, which act to create heterogeneous blood flow, compression of vessels, and elevated interstitial fluid pressure. As such, abnormalities in the tumor vasculature not only affect the delivery of nutrients and oxygen to the tumor, but also contribute to creating an abnormal tumor microenvironment that further promotes tumorigenesis. The role of chemical signaling events in mediating tumor angiogenesis has been well researched; however, the relative contribution of physical cues and mechanical regulation of tumor angiogenesis is less understood. Growing research indicates that the physical microenvironment plays a significant role in tumor progression and promoting abnormal tumor vasculature. Here, we review how mechanical cues found in the tumor microenvironment promote aberrant tumor angiogenesis. Specifically, we discuss the influence of matrix stiffness and mechanical stresses in tumor tissue on tumor vasculature, as well as the mechanosensory pathways utilized by endothelial cells to respond to the physical cues found in the tumor microenvironment. We also discuss the impact of the resulting aberrant tumor vasculature on tumor progression and therapeutic treatment.

Shear stress-induced angiogenesis in mouse muscle is independent of the vasodilator mechanism and quickly reversible

AIM

Is modulation of skeletal muscle capillary supply by altering blood flow due to a presumptive shear stress response per se, or dependent on the vasodilator mechanism?

METHODS

The response to four different vasodilators, and cotreatment with blockers of NO and prostaglandin synthesis, was compared. Femoral artery blood flow was correlated with capillary-to-fibre ratio (C:F) and protein levels of putative angiogenic compounds.

RESULTS

All vasodilators induced a similar increase in blood flow after 14 days, with a similar effect on C:F (1.62 ± 0.05, 1.60 ± 0.01, 1.57 ± 0.06, 1.57 ± 0.07, respectively, all P < 0.05 vs. control 1.20 ± 0.01). Concomitant inhibitors revealed differential effects on blood flow and angiogenesis, demonstrating that a similar response may have different signalling origins. The time course of this response with the most commonly used vasodilator, prazosin, showed that blood flow increased from 0.40 mL min^-1 to 0.61 mL min^-1 by 28 days (P < 0.05), dropped within 1 week after the cessation of treatment (0.54 mL min^-1 ; P < 0.05) and returned to control levels by 6 weeks. In parallel with FBF, capillary rarefaction began within 1 week (P < 0.05), giving C:F values similar to control by 2 weeks. Of the dominant signalling pathways, prazosin decreased muscle VEGF, but increased its cognate receptor Flk-1 (both P < 0.01); levels of eNOS varied with blood flow (P < 0.05), and Ang-1 initially increased, while its receptor Tie-2 was unchanged, with only modest changes in the antiangiogenic factor TSP-1.

CONCLUSION

Hyperaemia-induced angiogenesis, likely in response to elevated shear stress, is independent of the vasodilator involved, with a rapid induction and quick regression following the stimulus withdrawal.

ERK activation in endothelial cells is a novel marker during neovasculogenesis

Vasculogenesis is essential during early development to construct networks transporting oxygen, blood and nutrients. Tip and stalk cells are specialized endothelial cells involved in novel vessel formation because of their behavior such as sprouting as a leading cell and following tip cell. However, the spatiotemporal details determining the emergence of these cells are unknown. Here, we first show that the ERK activity in endothelial cells represents the precursor of tip and stalk cells for vasculogenesis in zebrafish. We identified that tip and stalk cells for intersegmental vessel (ISV) formation were already specialized in the dorsal aorta (DA) before sprouting. Furthermore, similar specialization was observed in tip cells during parachordal vessel (PAV) formation in lymphangiogenesis. We also identified that the ERK activity was required for specialized cells to emerge from existing blood vessels. Our data show that the ERK activity is a novel marker for determining the emergence of cells in both angiogenesis and lymphangiogenesis.

Diffusion phenomena of cells and biomolecules in microfluidic devices

Biomicrofluidics is an emerging field at the cross roads of microfluidics and life sciences which requires intensive research efforts in terms of introducing appropriate designs, production techniques, and analysis. The ultimate goal is to deliver innovative and cost-effective microfluidic devices to biotech, biomedical, and pharmaceutical industries. Therefore, creating an in-depth understanding of the transport phenomena of cells and biomolecules becomes vital and concurrently poses significant challenges. The present article outlines the recent advancements in diffusion phenomena of cells and biomolecules by highlighting transport principles from an engineering perspective, cell responses in microfluidic devices with emphases on diffusion- and flow-based microfluidic gradient platforms, macroscopic and microscopic approaches for investigating the diffusion phenomena of biomolecules, microfluidic platforms for the delivery of these molecules, as well as the state of the art in biological applications of mammalian cell responses and diffusion of biomolecules.

Analysing calcium dependent and independent regulation of eNOS in endothelium triggered by extracellular signalling events

The vascular endothelium, the intima of blood vessels, coordinately interacts with several biochemical factors expressing endothelial nitric oxide synthase (eNOS) to produce nitric oxide (NO), a potent endogenous vasodilator. The present study investigated the regulation of eNOS by multiple molecular signal transduction pathways, namely vascular endothelial growth factor (VEGF-A) and shear stress which are implicated in the process of angiogenesis and vascular remodelling respectively. In response to signal transduction upstream by VEGF-A and shear stress, different signalling pathways mediated by kinases and intracellular calcium potentiates eNOS activation leading to nitric oxide release. Our study revealed a distinct pattern of eNOS activation driven by VEGF-A and shear stress, maintaining the signalling specificity of the respective pathways. A transient response to eNOS activation was observed under VEGF-A and shear stress stimulus when mediated by calcium dependent cascades, whereas a sustained response was produced by calcium independent vascular signalling kinases. Furthermore, we found that the basal arterial shear stress enhanced eNOS activity when stimulated synergistically even at low VEGF-A levels which might be utilized to facilitate specific endothelial cell functions. Moreover, our study revealed that the presence of PI3K imparted transient behaviour to PLCγ1 supporting the hypothesis that regression and formation of tube structures are mediated by PLCγ1 and PI3K respectively in endothelial cells. This fact is corroborated by the absence of transient behaviour when PI3K is inhibited. We therefore obtained subtle insights into the control mechanism governing the role of specific signalling proteins which are obligate for the regulation of endothelial cell function and the consequent modulation of the nitric oxide release pattern.

Mechanotransduction at focal adhesions: from physiology to cancer development

Living cells are continuously exposed to mechanical cues, and can translate these signals into biochemical information (e.g. mechanotransduction). This process is crucial in many normal cellular functions, e.g. cell adhesion, migration, proliferation, and survival, as well as the progression of diseases such as cancer. Focal adhesions are the major sites of interactions between extracellular mechanical environments and intracellular biochemical signalling molecules/cytoskeleton, and hence focal adhesion proteins have been suggested to play important roles in mechanotransduction. Here, we overview the current molecular understanding in mechanotransduction occurring at focal adhesions. We also introduce recent studies on how extracellular matrix and mechanical microenvironments contribute to the development of cancer.

Shear stress during early embryonic stem cell differentiation promotes hematopoietic and endothelial phenotypes

Pluripotent embryonic stem cells (ESCs) are a potential source for cell-based tissue engineering and regenerative medicine applications, but their translation into clinical use will require efficient and robust methods for promoting differentiation. Fluid shear stress, which can be readily incorporated into scalable bioreactors, may be one solution for promoting endothelial and hematopoietic phenotypes from ESCs. Here we applied laminar shear stress to differentiating ESCs using a 2D adherent parallel plate configuration to systematically investigate the effects of several mechanical parameters. Treatment similarly promoted endothelial and hematopoietic differentiation for shear stress magnitudes ranging from 1.5 to 15 dyne/cm(2) and for cells seeded on collagen-, fibronectin- or laminin-coated surfaces. Extension of the treatment duration consistently induced an endothelial response, but application at later stages of differentiation was less effective at promoting hematopoietic phenotypes. Furthermore, inhibition of the FLK1 protein (a VEGF receptor) neutralized the effects of shear stress, implicating the membrane protein as a critical mediator of both endothelial and hematopoietic differentiation by applied shear. Using a systematic approach, studies such as these help elucidate the mechanisms involved in force-mediated stem cell differentiation and inform scalable bioprocesses for cellular therapies.

Fibroblast growth factor 2 induces the precocious development of endothelial cell networks in bovine luteinising follicular cells

The transition from follicle to corpus luteum represents a period of intense angiogenesis; however, the exact roles of angiogenic factors during this time remain to be elucidated. Thus, the roles of vascular endothelial growth factor (VEGF) A, fibroblast growth factor (FGF) 2 and LH in controlling angiogenesis were examined in the present study. A novel serum-free luteinising follicular angiogenesis culture system was developed in which progesterone production increased during the first 5 days and was increased by LH (P < 0.01). Blockade of signalling from FGF receptors (SU5402; P < 0.001) and, to a lesser extent, VEGF receptors (SU1498; P < 0.001) decreased the development of endothelial cell (EC) networks. Conversely, FGF2 dose-dependently (P < 0.001) induced the precocious transition of undeveloped EC islands into branched networks associated with a twofold increase in the number of branch points (P < 0.001). In contrast, VEGFA had no effect on the area of EC networks or the number of branch points. LH had no effect on the area of EC networks, but it marginally increased the number of branch points (P < 0.05) and FGF2 production (P < 0.001). Surprisingly, progesterone production was decreased by FGF2 (P < 0.01) but only on Day 5 of culture. Progesterone production was increased by SU5402 (P < 0.001) and decreased by SU1498 (P < 0.001). These results demonstrate that FGF and VEGF receptors play a fundamental role in the formation of luteal EC networks in vitro, which includes a novel role for FGF2 in induction of EC sprouting.

Vascular potency of Sus scrofa bone marrow-derived mesenchymal stem cells: a progenitor source of medial but not endothelial cells

Short-term thrombotic occlusion and compliance mismatch hamper clinical use of synthetic small-diameter tissue engineered vascular grafts. It is felt that preconditioning of the graft with intimal (endothelial) and medial (vascular smooth muscle) cells contributes to patency of the graft. Autologous, non-vessel-derived cells are preferred because of systemic vascular pathology and immunologic concerns. We tested in a porcine model whether cultured bone marrow-derived mononuclear cells, also referred to as mesenchymal stem cells (MSC), are a potential source of intimal or medial cells in vascular tissue engineering. We show that MSC cultured in endothelial medium do not gain an endothelial phenotype or functional characteristics, even after enrichment for CD31, culturing under flow, treatment with additional growth factors (vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF)-2), or co-culture with microvascular endothelial cells (EC). On the other hand, we show that MSC cultured in MSC medium, but not in smooth muscle cell medium, show phenotypical and functional characteristics of vascular smooth muscle cells. We conclude that bone marrow-derived MSCs can be used as a bona fide source of medial, but not EC in small-diameter vascular tissue engineering.

VEGFR2 signalling contributes to increased endothelial susceptibility to TNF-α under chronic non-uniform shear stress

OBJECTIVES

Vascular endothelial growth factor receptor 2 (VEGFR2), a tyrosine kinase receptor activated by VEGF and shear stress, is critically involved in endothelial mechanotransduction. We investigated the role of VEGFR2 in non-uniform shear stress-induced endothelial susceptibility to inflammatory stimuli.

METHODS

Endothelial cells (ECs) were exposed to non-uniform shear stress, followed by stimulation with TNF-α. ECs were transfected with siRNAs against VEGFR2. Alternatively, ECs were treated with blocking antibody against VEGFR2, or with inhibitors of VEGFR2 (ZM 323881), PI3K (LY 294002), or Src-kinase (PP2). THP-1 monocytes were used for dynamic adhesion assays. Endothelial protein expression was determined by immunofluorescence.

RESULTS

siRNA against VEGFR2 decreased VEGFR2 protein expression by 40% as determined by Western blotting. In endothelial cells exposed to non-uniform shear stress, VEGFR2 knockdown inhibited TNF-α-induced NF-κB translocation to the nucleus, and the upregulation of VCAM-1 and E-selectin. Consequently, monocytic cell recruitment to endothelium under non-uniform shear stress conditions was reduced. Similar effects were observed by blocking VEGFR2 activity using a specific inhibitor ZM 323881, or an antibody against VEGFR2 before TNF-α stimulation. Inhibition of PI3K with LY 294002 significantly reduced non-uniform shear stress-induced endothelial susceptibility to TNF-α, whereas blocking Src-kinase with PP2 was ineffective.

CONCLUSION

VEGFR2 is critically involved in adhesion molecule induction and monocytic cell recruitment to endothelium in response to non-uniform shear stress and TNF-α. Targeting the mechanosensory cascade can prevent endothelial activation in atherosclerosis-prone regions.

Fluid forces control endothelial sprouting

During angiogenesis, endothelial cells (ECs) from intact blood vessels quickly infiltrate avascular regions via vascular sprouting. This process is fundamental to many normal and pathological processes such as wound healing and tumor growth, but its initiation and control are poorly understood. Vascular endothelial cell growth factor (VEGF) can promote vessel dilation and angiogenic sprouting, but given the complex nature of vascular morphogenesis, additional signals are likely necessary to determine, for example, which vessel segments sprout, which dilate, and which remain quiescent. Fluid forces exerted by blood and plasma are prime candidates that might codirect these processes, but it is not known whether VEGF cooperates with mechanical fluid forces to mediate angiogenesis. Using a microfluidic tissue analog of angiogenic sprouting, we found that fluid shear stress, such as exerted by flowing blood, attenuates EC sprouting in a nitric oxide-dependent manner and that interstitial flow, such as produced by extravasating plasma, directs endothelial morphogenesis and sprout formation. Furthermore, positive VEGF gradients initiated sprouting but negative gradients inhibited sprouting, promoting instead sheet-like migration analogous to vessel dilation. These results suggest that ECs integrate signals from fluid forces and local VEGF gradients to achieve such varied goals as vessel dilation and sprouting.

X-linked inhibitor of apoptosis protein controls alpha5-integrin-mediated cell adhesion and migration

The association of integrins with caveolin-1 regulates cell adhesion. However, the vascular ramifications of this association remain to be clearly determined. We recently reported that the X chromosome-linked inhibitor of apoptosis protein (XIAP)-caveolin-1 interaction is critical to endothelial cell survival. Thus, we hypothesized that XIAP performs a crucial function in integrin/caveolin-1-mediated endothelial cell survival. In this study, we demonstrated that XIAP is recruited into the alpha(5)-integrin complex via caveolin-1 binding and mediates cell adhesion. We also determined that XIAP is critical to shear stress-stimulated ERK activation in an alpha(5)-integrin-dependent manner but is not important to VEGF-induced ERK activation. This differential activation of ERK is partly attributable to unique localizations of the receptors. Furthermore, we confirmed that XIAP is an essential molecule in the efficient recruitment of focal adhesion kinase (FAK) into the alpha(5)-integrin-associated complex. This alpha(5)-integrin-caveolin-1-XIAP-FAK multicomplex regulates endothelial cell migration via a mechanism that involves shear-dependent ERK activation. Together, our results indicate that XIAP stabilizes the alpha(5)-integrin-associated focal adhesion complex, thereby further regulating endothelial cell adhesion and migration. The findings of this study provide us with greater insight into the molecular mechanisms underlying the control of vascular function by integrins.

Effects of tensile strain and fluid flow on osteoarthritic human chondrocyte metabolism in vitro

This study examined the hypothesis that tensile strain and fluid flow differentially influence osteoarthritic human chondrocyte metabolism. Primary high-density monolayer chondrocytes cultures were exposed to varying magnitudes of tensile strain and fluid-flow using a four-point bending system. Metabolic changes were quantified by real-time PCR measurement of aggrecan, IL-6, SOX-9, and type II collagen gene expression, and by determination of nitric oxide levels in the culture medium. A linear regression model was used to investigate the roles of strain, fluid flow, and their interaction on metabolic activity. Aggrecan, type II collagen, and SOX9 mRNA expression were negatively correlated to increases in applied strain and fluid flow. An effect of the strain on the induction of nitric oxide release and IL-6 gene expression varied by level of fluid flow (and visa versa). This interaction between strain and fluid flow was negative for nitric oxide and positive for IL-6. These results confirm that articular chondrocyte metabolism is responsive to tensile strain and fluid flow under in vitro loading conditions. Although the articular chondrocytes reacted to the mechanically applied stress, it was notable that there was a differential effect of tensile strain and fluid flow on anabolic and catabolic markers.

Mechanobiology and the microcirculation: cellular, nuclear and fluid mechanics

Endothelial cells are stimulated by shear stress throughout the vasculature and respond with changes in gene expression and by morphological reorganization. Mechanical sensors of the cell are varied and include cell surface sensors that activate intracellular chemical signaling pathways. Here, possible mechanical sensors of the cell including reorganization of the cytoskeleton and the nucleus are discussed in relation to shear flow. A mutation in the nuclear structural protein lamin A, related to Hutchinson-Gilford progeria syndrome, is reviewed specifically as the mutation results in altered nuclear structure and stiffer nuclei; animal models also suggest significantly altered vascular structure. Nuclear and cellular deformation of endothelial cells in response to shear stress provides partial understanding of possible mechanical regulation in the microcirculation. Increasing sophistication of fluid flow simulations inside the vessel is also an emerging area relevant to the microcirculation as visualization in situ is difficult. This integrated approach to study--including medicine, molecular and cell biology, biophysics and engineering--provides a unique understanding of multi-scale interactions in the microcirculation.

Live cell imaging of mechanotransduction

Mechanical forces play important roles in the regulation of cellular functions, including polarization, migration and stem cell differentiation. Tremendous advancement in our understanding of mechanotransduction has been achieved with the recent development of imaging technologies and molecular biosensors. In particular, genetically encoded biosensors based on fluorescence resonance energy transfer (FRET) technology have been widely developed and applied in the field of mechanobiology. In this article, we will provide an overview of the recent progress of FRET application in mechanobiology, specifically mechanotransduction. We first introduce fluorescent proteins and FRET technology. We then discuss the mechanotransduction processes in different cells including stem cells, with a special emphasis on the important signalling molecules involved in mechanotransduction. Finally, we discuss methods that can allow the integration of simultaneous FRET imaging and mechanical stimulation to trigger signalling transduction. In summary, FRET technology has provided a powerful tool for the study of mechanotransduction to advance our systematic understanding of the molecular mechanisms by which cells respond to mechanical stimulation.

VEGFR-2 silencing by small interference RNA (siRNA) suppresses LPA-induced epithelial ovarian cancer (EOC) invasion

BACKGROUND

The VEGF-VEGF receptor (VEGFR) signaling axis has emerged as a promising target for cancer therapy, attributing to its vital role in tumor angiogenesis and growth. We have previously reported the regulation of epithelial ovarian cancer (EOC) invasion and migration by VEGF and the implication of VEGF-VEGFR-2 axis in lysophosphatidic acid (LPA)-induced EOC invasion. However, the expression profile of VEGF and VEGFRs in EOC, their association with tumor aggressiveness, and their regulation by LPA remain unclear.

OBJECTIVES AND METHODS

In this study, we examined the expression of VEGFR-1, VEGFR-2, neuropilin-1 (NRP-1), NRP-2, VEGF(121), and VEGF(165) in established EOC cell lines and assessed their correlation with cell invasiveness. Moreover, using an ovarian cancer tissue qPCR array, we analyzed VEGFR-2 expression across a panel of 48 tissues with different disease stages and histological grades. We also tested the effect of LPA on VEGF and VEGFR-2 expression and examined whether blocking VEGFR-2 by RNA interference (RNAi) affects LPA-induced EOC invasion.

RESULTS

We show that VEGF and VEGFR-2 expression correlates with cell invasiveness and VEGFR-2 expression in ovarian cancer tissues correlate with tumor grade. In addition, LPA, at 20 muM, significantly induced the expression of VEGF(121), VEGF(165), and VEGFR-2 in SKOV3 and DOV13 cells (P<0.05). VEGFR-2 small interference RNA (siRNA) transfection remarkably decreased LPA's invasion-promoting effect (P<0.001) in SKOV3 cells without significantly decreasing SKOV3 cells' basal invasiveness. In DOV13 cells, VEGFR-2 silencing significantly decreases both the basal level cell invasion and LPA's invasion promoting effect (P<0.001).

CONCLUSION

These results suggest that decreasing VEGFR-2 expression by RNAi may prove to be an effective method to reduce the metastatic potential of EOC cells exposed to elevated levels of LPA.

FGF2 is crucial for the development of bovine luteal endothelial networks in vitro

The development of the corpus luteum requires angiogenesis, and involves the complex interplay between factors such as vascular endothelial growth factor A (VEGFA), fibroblast growth factor 2 (FGF2) and platelet-derived growth factor (PDGF). However, the relative role of these factors remains to be elucidated. This study used a new physiologically relevant mixed luteal cell culture system to test the hypotheses that: a) FGF2 and VEGFA are critical for bovine luteal angiogenesis; and b) local luteal PDGF signalling stimulates the formation of endothelial networks. Cells were treated with receptor tyrosine kinase inhibitors against VEGFA (SU1498), FGF2 (SU5402) or PDGF (AG1295) activity. After 9 days in culture, endothelial cells were immunostained for von Willebrand factor (VWF) and quantified by image analysis. Highly organised intricate endothelial networks were formed in the presence of exogenous VEGFA and FGF2. The inhibition of FGF2 activity reduced the total area of VWF staining versus controls (>95%; P<0.001). Inhibition of VEGF and PDGF activity reduced the endothelial network formation by more than 60 and 75% respectively (P<0.05). Progesterone production increased in all treatments from day 1 to 7 (P<0.001), and was unaffected by FGF2 or PDGF receptor kinase inhibition (P>0.05), but was reduced by the VEGF receptor inhibitor on days 5 and 7 (P<0.001). In conclusion, this study confirmed that VEGF signalling regulates both bovine luteal angiogenesis and progesterone production. However, FGF2 was crucial for luteal endothelial network formation. Also, for the first time, this study showed that local luteal PDGF activity regulates bovine luteal endothelial network formation in vitro.

Fluorescence proteins, live-cell imaging, and mechanobiology: seeing is believing

Fluorescence proteins (FPs) have been widely used for live-cell imaging in the past decade. This review summarizes the recent advances in FP development and imaging technologies using FPs to monitor molecular localization and activities and gene expressions in live cells. We also discuss the utilization of FPs to develop molecular biosensors and the principles and application of advanced technologies such as fluorescence resonance energy transfer (FRET), fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging microscopy (FLIM), and chromophore-assisted light inactivation (CALI). We present examples of the application of FPs and biosensors to visualize mechanotransduction events with high spatiotemporal resolutions in live cells. These live-cell imaging technologies, which represent a frontier area in biomedical engineering, can shed new light on the mechanisms regulating mechanobiology at cellular and molecular levels in normal and pathophysiological conditions.

Rapid signal transduction in living cells is a unique feature of mechanotransduction

It is widely postulated that mechanotransduction is initiated at the local force-membrane interface by inducing local conformational changes of proteins, similar to soluble ligand-induced signal transduction. However, all published reports are limited in time scale to address this fundamental issue. Using a FRET-based cytosolic Src reporter in a living cell, we quantified changes of Src activities as a local stress via activated integrins was applied. The stress induced rapid (<0.3 s) activation of Src at remote cytoplasmic sites, which depends on the cytoskeletal prestress. In contrast, there was no Src activation within 12 s of soluble epidermal growth factor (EGF) stimulation. A 1.8-Pa stress over a focal adhesion activated Src to the same extent as 0.4 ng/ml EGF at long times (minutes), and the energy levels for mechanical stimulation and chemical stimulation were comparable. The effect of both stress and EGF was less than additive. Nanometer-scale cytoskeletal deformation analyses revealed that the strong activation sites of Src by stress colocalized with large deformation sites of microtubules, suggesting that microtubules are essential structures for transmitting stresses to activate cytoplasmic proteins. These results demonstrate that rapid signal transduction via the prestressed cytoskeleton is a unique feature of mechanotransduction.

Synergism of biochemical and mechanical stimuli in the differentiation of human placenta-derived multipotent cells into endothelial cells

There have been intensive studies on the differentiation of endothelial progenitor cells (EPCs) into endothelial cells. We investigated the endothelial differentiation of placenta-derived multipotent cells (PDMCs), a population of CD34(-)/CD133(-)/Flk-1(-) cells. PDMCs were cultured in basal media or media containing endothelial growth factors (EGM), including vascular endothelial growth factor (VEGF), for 3 days and then subjected to shear stress of 6 or 12dyn/cm(2) for 24h. Culture of PDMCs in EGM under static conditions resulted in significant increases in VEGF receptor-1 (Flt-1) and receptor-2 (Flk-1) expression. Application of shear stress at 12dyn/cm(2) to these cells led to significant increases in their expression of von Willebrand Factor and platelet-endothelial cell adhesion molecule-1 at both the gene and protein levels. Shear stress at 6dyn/cm(2) had lesser effects. Uptakes of acetylated low-density lipoproteins as well as formation of tube-like structures on Matrigel were significantly increased after subjecting to shear stress of 12dyn/cm(2) for 24h. Our findings suggest that the combined use of endothelial growth factors and high shear stress is synergistic for the endothelial differentiation of PDMCs.

Stimulus interval, rate and direction differentially regulate phosphorylation for mechanotransduction in neonatal cardiac myocytes

The effect of interval, direction and rate of strain on mechanotransduction in neonatal rat cardiomyocytes is determined for focal adhesion kinase (Y397pFAK), extracellular signal-regulated kinase ERK1/2 (Thr(183)/Tyr(185)) and paxillin (pY31) and phosphorylation time courses to 10% strain assessed. Cells are non-responsive at 5 min but recover at 15 min (P<0.03) with FAK nuclear translocation by 30 min. Cyclic biaxial strain increased phosphorylation from slower to faster rates (P<0.05). Uniaxial strain to groove-aligned myocytes increased FAK and ERK1/2 phosphorylation transversely more than longitudinally (P<0.05). Mechanotransduction may have a refractory period of 5 min and differentiate directions and rates of strain.