Activation of NF-kappaB nuclear transcription factor by flow in human endothelial cells

NIK Is a Mediator of Inflammation and Intimal Hyperplasia in Endothelial Denudation-Induced Vascular Injury

Neointimal hyperplasia is the main cause of vascular graft failure in the medium term. NFκB is a key mediator of inflammation that is activated during neointimal hyperplasia following endothelial injury. However, the molecular mechanisms involved in NFκB activation are poorly understood. NFκB may be activated through canonical (transient) and non-canonical (persistent) pathways. NFκB-inducing kinase (NIK, MAP3K14) is the upstream kinase of the non-canonical pathway. We have now explored the impact of NIK deficiency on neointimal hyperplasia following guidewire-induced endothelial cell injury and on local inflammation by comparing NIK activity-deficient alymphoplasia mice (NIKaly/aly) with control wild-type (NIK+/+) mice. Guidewire-induced endothelial cell injury caused neointimal hyperplasia and luminal stenosis and upregulated the local expression of NIK and the NFκB target chemokines monocyte chemoattractant protein-1 (MCP-1/CCL2) and chemokine ligand 5 (RANTES/CCL5). Immunohistochemistry disclosed the infiltration of the media and intima by F4/80 positive macrophages. The intima/media ratio and percentage of stenosis were milder in the NIKaly/aly than in the NIK+/+ mice. Additionally, the gene expression for MCP-1 and RANTES was lower and F4/80+ cell infiltration was milder in the NIKaly/aly than in the NIK+/+ mice. Finally, circulating MCP-1 levels were lower in the NIKaly/aly than in the NIK+/+ mice, reflecting milder systemic inflammation. In conclusion, NIK is a driver of vascular wall inflammation and stenosis following guidewire-induced endothelial cell injury. NIK targeting may be a novel therapeutic approach to limit arterial stenosis following endothelial cell injury.

Transcriptional Profiling of Human Endothelial Cells Unveils PIEZO1 and Mechanosensitive Gene Regulation by Prooxidant and Inflammatory Inputs

PIEZO1 is a mechanosensitive cation channel implicated in shear stress-mediated endothelial-dependent vasorelaxation. Since altered shear stress patterns induce a pro-inflammatory endothelial environment, we analyzed transcriptional profiles of human endothelial cells to determine the effect of altered shear stress patterns and subsequent prooxidant and inflammatory conditions on PIEZO1 and mechanosensitive-related genes (MRG). In silico analyses were validated in vitro by assessing PIEZO1 transcript levels in both the umbilical artery (HUAEC) and vein (HUVEC) endothelium. Transcriptional profiling showed that PIEZO1 and some MRG associated with the inflammatory response were upregulated in response to high (15 dyn/cm2) and extremely high shear stress (30 dyn/cm2) in HUVEC. Changes in PIEZO1 and inflammatory MRG were paralleled by p65 but not KLF or YAP1 transcription factors. Similarly, PIEZO1 transcript levels were upregulated by TNF-alpha (TNF-α) in diverse endothelial cell types, and pre-treatment with agents that prevent p65 translocation to the nucleus abolished PIEZO1 induction. ChIP-seq analysis revealed that p65 bonded to the PIEZO1 promoter region, an effect increased by the stimulation with TNF-α. Altogether this data showed that NF-kappa B activation via p65 signaling regulates PIEZO1 expression, providing a new molecular link for prooxidant and inflammatory responses and mechanosensitive pathways in the endothelium.

The Inflammatory Profile of CTEPH-Derived Endothelial Cells Is a Possible Driver of Disease Progression

Chronic thromboembolic pulmonary hypertension (CTEPH) is a form of pulmonary hypertension characterized by the presence of fibrotic intraluminal thrombi and causing obliteration of the pulmonary arteries. Although both endothelial cell (EC) dysfunction and inflammation are linked to CTEPH pathogenesis, regulation of the basal inflammatory response of ECs in CTEPH is not fully understood. Therefore, in the present study, we investigated the role of the nuclear factor (NF)-κB pro-inflammatory signaling pathway in ECs in CTEPH under basal conditions. Basal mRNA levels of interleukin (IL)-8, IL-1β, monocyte chemoattractant protein-1 (MCP-1), C-C motif chemokine ligand 5 (CCL5), and vascular cell adhesion molecule-1 (VCAM-1) were upregulated in CTEPH-ECs compared to the control cells. To assess the involvement of NF-κB signaling in basal inflammatory activation, CTEPH-ECs were incubated with the NF-κB inhibitor Bay 11-7085. The increase in pro-inflammatory cytokines was abolished when cells were incubated with the NF-κB inhibitor. To determine if NF-κB was indeed activated, we stained pulmonary endarterectomy (PEA) specimens from CTEPH patients and ECs isolated from PEA specimens for phospho-NF-κB-P65 and found that especially the vessels within the thrombus and CTEPH-ECs are positive for phospho-NF-κB-P65. In summary, we show that CTEPH-ECs have a pro-inflammatory status under basal conditions, and blocking NF-κB signaling reduces the production of inflammatory factors in CTEPH-ECs. Therefore, our results show that the increased basal pro-inflammatory status of CTEPH-ECs is, at least partially, regulated through activation of NF-κB signaling and potentially contributes to the pathophysiology and progression of CTEPH.

Transcriptome comparisons of in vitro intestinal epithelia grown under static and microfluidic gut-on-chip conditions with in vivo human epithelia

Gut-on-chip devices enable exposure of cells to a continuous flow of culture medium, inducing shear stresses and could thus better recapitulate the in vivo human intestinal environment in an in vitro epithelial model compared to static culture methods. We aimed to study if dynamic culture conditions affect the gene expression of Caco-2 cells cultured statically or dynamically in a gut-on-chip device and how these gene expression patterns compared to that of intestinal segments in vivo. For this we applied whole genome transcriptomics. Dynamic culture conditions led to a total of 5927 differentially expressed genes (3280 upregulated and 2647 downregulated genes) compared to static culture conditions. Gene set enrichment analysis revealed upregulated pathways associated with the immune system, signal transduction and cell growth and death, and downregulated pathways associated with drug metabolism, compound digestion and absorption under dynamic culture conditions. Comparison of the in vitro gene expression data with transcriptome profiles of human in vivo duodenum, jejunum, ileum and colon tissue samples showed similarities in gene expression profiles with intestinal segments. It is concluded that both the static and the dynamic gut-on-chip model are suitable to study human intestinal epithelial responses as an alternative for animal models.

NF-κB inhibition prevents acute shear stress-induced inflammation in the saphenous vein graft endothelium

The long saphenous vein (LSV) is commonly used as a conduit in coronary artery bypass grafting. However, long term patency remains limited by the development of vascular inflammation, intimal hyperplasia and accelerated atherosclerosis. The impact of acute exposure of venous endothelial cells (ECs) to acute arterial wall shear stress (WSS) in the arterial circulation, and the subsequent activation of inflammatory pathways, remain poorly defined. Here, we tested the hypothesis that acute exposure of venous ECs to high shear stress is associated with inflammatory responses that are regulated by NF-κB both in-vitro and ex-vivo. Analysis of the LSV endothelium revealed that activation of NF-κB occurred within 30 min after exposure to arterial rates of shear stress. Activation of NF-κB was associated with increased levels of CCL2 production and enhanced binding of monocytes in LSVECs exposed to 6 h acute arterial WSS. Consistent with this, ex vivo exposure of LSVs to acute arterial WSS promoted monocyte interactions with the vessel lumen. Inhibition of the NF-κB pathway prevented acute arterial WSS-induced CCL2 production and reduced monocyte adhesion, both in vitro and in human LSV ex vivo, demonstrating that this pathway is necessary for the induction of the acute arterial WSS-induced pro-inflammatory response. We have identified NF-κB as a critical regulator of acute endothelial inflammation in saphenous vein in response to acute arterial WSS. Localised endothelial-specific inhibition of the NF-κB pathway may be beneficial to prevent vein graft inflammation and consequent failure.

Endothelial Cell Biomechanical Responses are Dependent on Both Fluid Shear Stress and Tensile Strain

Introduction

The goal of this study was to investigate how concurrent shear stress and tensile strain affect endothelial cell biomechanical responses.

Methods

Human coronary artery endothelial cells were exposed to concurrent pulsatile shear stress and cyclic tensile strain in a programmable shearing and stretching device. Three shear stress-tensile strain conditions were used: (1) pulsatile shear stress at 1 Pa and cyclic tensile strain at 7%, simulating normal stress/strain conditions in a healthy coronary artery; (2) shear stress at 3.7 Pa and tensile strain at 3%, simulating pathological stress/strain conditions near a stenosis; (3) shear stress at 0.7 Pa and tensile strain at 5%, simulating pathological stress/strain conditions in a recirculation zone. Cell morphology was quantified using immunofluorescence microscopy. Cell surface PECAM-1 phosphorylation, ICAM-1 expression, ERK1/2 and NF-κB activation were measured using ELISA or Western blot.

Results

Simultaneous stimulation from pulsatile shear stress and cyclic tensile strain induced a significant increase in cell area, compared to that induced by shear stress or tensile strain alone. The combined stimulation caused significant increases in PECAM-1 phosphorylation. The combined stimulation also significantly enhanced EC surface ICAM-1 expression (compared to that under shear stress alone) and transcriptional factor NF-κB activation (compared to that under control conditions).

Conclusion

Pulsatile shear stress and cyclic tensile strain could induce increased but not synergistic effect on endothelial cell morphology or activation. The combined mechanical stimulation can be relayed from cell membrane to nucleus. Therefore, to better understand how mechanical conditions affect endothelial cell mechanotransduction and cardiovascular disease development, both shear stress and tensile strain need to be considered.

Disturbed flow induces a sustained, stochastic NF-κB activation which may support intracranial aneurysm growth in vivo

Intracranial aneurysms are associated with disturbed velocity patterns, and chronic inflammation, but the relevance for these findings are currently unknown. Here, we show that (disturbed) shear stress induced by vortices is a sufficient condition to activate the endothelial NF-kB pathway, possibly through a mechanism of mechanosensor de-activation. We provide evidence for this statement through in-vitro live cell imaging of NF-kB in HUVECs exposed to different flow conditions, stochastic modelling of flow induced NF-kB activation and induction of disturbed flow in mouse carotid arteries. Finally, CFD and immunofluorescence on human intracranial aneurysms showed a correlation similar to the mouse vessels, suggesting that disturbed shear stress may lead to sustained NF-kB activation thereby offering an explanation for the close association between disturbed flow and intracranial aneurysms.

Deciphering Nuclear Mechanobiology in Laminopathy

Extracellular mechanical stimuli are translated into biochemical signals inside the cell via mechanotransduction. The nucleus plays a critical role in mechanoregulation, which encompasses mechanosensing and mechanotransduction. The nuclear lamina underlying the inner nuclear membrane not only maintains the structural integrity, but also connects the cytoskeleton to the nuclear envelope. Lamin mutations, therefore, dysregulate the nuclear response, resulting in abnormal mechanoregulations, and ultimately, disease progression. Impaired mechanoregulations even induce malfunction in nuclear positioning, cell migration, mechanosensation, as well as differentiation. To know how to overcome laminopathies, we need to understand the mechanisms of laminopathies in a mechanobiological way. Recently, emerging studies have demonstrated the varying defects from lamin mutation in cellular homeostasis within mechanical surroundings. Therefore, this review summarizes recent findings highlighting the role of lamins, the architecture of nuclear lamina, and their disease relevance in the context of nuclear mechanobiology. We will also provide an overview of the differentiation of cellular mechanics in laminopathy.

Endothelial Activation: The Ang/Tie Axis in Sepsis

Sepsis, a dysregulated host response to infection that causes life-threatening organ dysfunction, is a highly heterogeneous syndrome with no specific treatment. Although sepsis can be caused by a wide variety of pathogenic organisms, endothelial dysfunction leading to vascular leak is a common mechanism of injury that contributes to the morbidity and mortality associated with the syndrome. Perturbations to the angiopoietin (Ang)/Tie2 axis cause endothelial cell activation and contribute to the pathogenesis of sepsis. In this review, we summarize how the Ang/Tie2 pathway is implicated in sepsis and describe its prognostic as well as therapeutic utility in life-threatening infections.

Mechanotransduction in tumor progression: The dark side of the force

Broders-Bondon et al. review the pathological mechanical properties of tumor tissues and how abnormal mechanical signals result in oncogenic biochemical signals during tumor progression.

Cancer has been characterized as a genetic disease, associated with mutations that cause pathological alterations of the cell cycle, adhesion, or invasive motility. Recently, the importance of the anomalous mechanical properties of tumor tissues, which activate tumorigenic biochemical pathways, has become apparent. This mechanical induction in tumors appears to consist of the destabilization of adult tissue homeostasis as a result of the reactivation of embryonic developmental mechanosensitive pathways in response to pathological mechanical strains. These strains occur in many forms, for example, hypervascularization in late tumors leads to high static hydrodynamic pressure that can promote malignant progression through hypoxia or anomalous interstitial liquid and blood flow. The high stiffness of tumors directly induces the mechanical activation of biochemical pathways enhancing the cell cycle, epithelial–mesenchymal transition, and cell motility. Furthermore, increases in solid-stress pressure associated with cell hyperproliferation activate tumorigenic pathways in the healthy epithelial cells compressed by the neighboring tumor. The underlying molecular mechanisms of the translation of a mechanical signal into a tumor inducing biochemical signal are based on mechanically induced protein conformational changes that activate classical tumorigenic signaling pathways. Understanding these mechanisms will be important for the development of innovative treatments to target such mechanical anomalies in cancer.

Mechanotransduction in Blood and Lymphatic Vascular Development and Disease

The blood and lymphatic vasculatures are hierarchical networks of vessels, which constantly transport fluids and, therefore, are exposed to a variety of mechanical forces. Considering the role of mechanotransduction is key for fully understanding how these vascular systems develop, function, and how vascular pathologies evolve. During embryonic development, for example, initiation of blood flow is essential for early vascular remodeling, and increased interstitial fluid pressure as well as initiation of lymph flow is needed for proper development and maturation of the lymphatic vasculature. In this review, we introduce specific mechanical forces that affect both the blood and lymphatic vasculatures, including longitudinal and circumferential stretch, as well as shear stress. In addition, we provide an overview of the role of mechanotransduction during atherosclerosis and secondary lymphedema, which both trigger tissue fibrosis.

Platelets modulate endothelial cell response to dynamic shear stress through PECAM-1

INTRODUCTION

Both vascular endothelial cells and platelets are sensitive to blood flow induced shear stress. We have recently reported that platelet-endothelial cell interaction could greatly affect platelet activation under flow. In the present study, we aimed to investigate how platelet-endothelial cell interaction affected endothelial cell inflammatory responses under flow.

MATERIALS AND METHODS

Human coronary artery endothelial cells were exposed to normal or low pulsatile shear stress with or without the presence of platelets. Following shear exposure, endothelial cell ICAM-1 expression was measured using ELISA, Western blot and PCR; cell surface PECAM-1 expression/phosphorylation was measured using ELISA. Platelet adhesion to endothelial cells was quantified using immunofluorescence microscopy. To determine the role of PECAM-1 in platelet-endothelial cell interaction, endothelial cell PECAM-1 expression was suppressed using siRNA.

RESULTS

Pathological low shear stress induced a significant increase in endothelial cell ICAM-1 expression, at both protein and mRNA levels. Platelet adhesion to endothelial cells increased significantly under low shear stress, co-localizing with PECAM-1 at endothelial cell junctions. The presence of platelets inhibited low shear stress-induced ICAM-1 upregulation. When endothelial cell PECAM-1 expression was suppressed, platelet adhesion to endothelial cells under low shear stress decreased significantly; endothelial cell ICAM-1 expression was not affected by shear stress, with or without platelets.

CONCLUSIONS

These results suggested that PECAM-1 could mediate platelet adhesion to endothelial cells under shear stress. Platelets binding to endothelial cells interfered with endothelial cell mechanotransduction through PECAM-1, affecting endothelial cell inflammatory responses towards pathological shear flow.

Glycocalyx Degradation Induces a Proinflammatory Phenotype and Increased Leukocyte Adhesion in Cultured Endothelial Cells under Flow

Leukocyte adhesion to the endothelium is an early step in the pathogenesis of atherosclerosis. Effective adhesion requires the binding of leukocytes to their cognate receptors on the surface of endothelial cells. The glycocalyx covers the surface of endothelial cells and is important in the mechanotransduction of shear stress. This study aimed to identify the molecular mechanisms underlying the role of the glycocalyx in leukocyte adhesion under flow. We performed experiments using 3-D cell culture models, exposing human abdominal aortic endothelial cells to steady laminar shear stress (10 dynes/cm2 for 24 hours). We found that with the enzymatic degradation of the glycocalyx, endothelial cells developed a proinflammatory phenotype when exposed to uniform steady shear stress leading to an increase in leukocyte adhesion. Our results show an up-regulation of ICAM-1 with degradation compared to non-degraded controls (3-fold increase, p<0.05) and we attribute this effect to a de-regulation in NF-κB activity in response to flow. These results suggest that the glycocalyx is not solely a physical barrier to adhesion but rather plays an important role in governing the phenotype of endothelial cells, a key determinant in leukocyte adhesion. We provide evidence for how the destabilization of this structure may be an early and defining feature in the initiation of atherosclerosis.

Molecular controls of arterial morphogenesis

Formation of arterial vasculature, here termed arteriogenesis, is a central process in embryonic vascular development as well as in adult tissues. Although the process of capillary formation, angiogenesis, is relatively well understood, much remains to be learned about arteriogenesis. Recent discoveries point to the key role played by vascular endothelial growth factor receptor 2 in control of this process and to newly identified control circuits that dramatically influence its activity. The latter can present particularly attractive targets for a new class of therapeutic agents capable of activation of this signaling cascade in a ligand-independent manner, thereby promoting arteriogenesis in diseased tissues.

Nuclear envelope proteins Nesprin2 and LaminA regulate proliferation and apoptosis of vascular endothelial cells in response to shear stress

The dysfunction of vascular endothelial cells (ECs) influenced by flow shear stress is crucial for vascular remodeling. However, the roles of nuclear envelope (NE) proteins in shear stress-induced EC dysfunction are still unknown. Our results indicated that, compared with normal shear stress (NSS), low shear stress (LowSS) suppressed the expression of two types of NE proteins, Nesprin2 and LaminA, and increased the proliferation and apoptosis of ECs. Targeted small interfering RNA (siRNA) and gene overexpression plasmid transfection revealed that Nesprin2 and LaminA participate in the regulation of EC proliferation and apoptosis. A protein/DNA array was further used to detect the activation of transcription factors in ECs following transfection with target siRNAs and overexpression plasmids. The regulation of AP-2 and TFIID mediated by Nesprin2 and the activation of Stat-1, Stat-3, Stat-5 and Stat-6 by LaminA were verified under shear stress. Furthermore, using Ingenuity Pathway Analysis software and real-time RT-PCR, the effects of Nesprin2 or LaminA on the downstream target genes of AP-2, TFIID, and Stat-1, Stat-3, Stat-5 and Stat-6, respectively, were investigated under LowSS. Our study has revealed that NE proteins are novel mechano-sensitive molecules in ECs. LowSS suppresses the expression of Nesprin2 and LaminA, which may subsequently modulate the activation of important transcription factors and eventually lead to EC dysfunction.

Hemodynamic regulation of reactive oxygen species: implications for vascular diseases

SIGNIFICANCE

Arterial blood vessels functionally and structurally adapt to altering hemodynamic forces in order to accommodate changing needs and to provide stress homeostasis. This ability is achieved at the cellular level by converting mechanical stimulation into biochemical signals (i.e., mechanotransduction). Physiological mechanical stress helps maintain vascular structure and function, whereas pathologic or aberrant stress may impair cellular mechano-signaling, and initiate or augment cellular processes that drive disease.

RECENT ADVANCES

Reactive oxygen species (ROS) may represent an intriguing class of mechanically regulated second messengers. Chronically enhanced ROS generation may be induced by adverse mechanical stresses, and is associated with a multitude of vascular diseases. Although a causal relationship has clearly been demonstrated in large numbers of animal studies, an effective ROS-modulating therapy still remains to be established by clinical studies.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This review article focuses on the role of various mechanical forces (in the form of laminar shear stress, oscillatory shear stress, or cyclic stretch) as modulators of ROS-driven signaling, and their subsequent effects on vascular biology and homeostasis, as well as on specific diseases such as arteriosclerosis, hypertension, and abdominal aortic aneurysms. Specifically, it highlights the significance of the various NADPH oxidase (NOX) isoforms as critical ROS generators in the vasculature. Directed targeting of defined components in the complex network of ROS (mechano-)signaling may represent a key for successful translation of experimental findings into clinical practice.

Circulating Bmp10 acts through endothelial Alk1 to mediate flow-dependent arterial quiescence

Blood flow plays crucial roles in vascular development, remodeling and homeostasis, but the molecular pathways required for transducing flow signals are not well understood. In zebrafish embryos, arterial expression of activin receptor-like kinase 1 (alk1), which encodes a TGFβ family type I receptor, is dependent on blood flow, and loss of alk1 mimics lack of blood flow in terms of dysregulation of a subset of flow-responsive arterial genes and increased arterial endothelial cell number. These data suggest that blood flow activates Alk1 signaling to promote a flow-responsive gene expression program that limits nascent arterial caliber. Here, we demonstrate that restoration of endothelial alk1 expression to flow-deprived arteries fails to rescue Alk1 activity or normalize arterial endothelial cell gene expression or number, implying that blood flow may play an additional role in Alk1 signaling independent of alk1 induction. To this end, we define cardiac-derived Bmp10 as the crucial ligand for endothelial Alk1 in embryonic vascular development, and provide evidence that circulating Bmp10 acts through endothelial Alk1 to limit endothelial cell number in and thereby stabilize the caliber of nascent arteries. Thus, blood flow promotes Alk1 activity by concomitantly inducing alk1 expression and distributing Bmp10, thereby reinforcing this signaling pathway, which functions to limit arterial caliber at the onset of flow. Because mutations in ALK1 cause arteriovenous malformations (AVMs), our findings suggest that an impaired flow response initiates AVM development.

The flow dependency of Tie2 expression in endotoxemia

RATIONALE

Tie2 is predominantly expressed by endothelial cells and is involved in vascular integrity control during sepsis. Changes in Tie2 expression during sepsis development may contribute to microvascular dysfunction. Understanding the kinetics and molecular basis of these changes may assist in the development of therapeutic intervention to counteract microvascular dysfunction.

OBJECTIVE

To investigate the molecular mechanisms underlying the changes in Tie2 expression upon lipopolysaccharide (LPS) challenge.

METHODS AND RESULTS

Studies were performed in LPS and pro-inflammatory cytokine challenged mice as well as in mice subjected to hemorrhagic shock, primary endothelial cells were used for in vitro experiments in static and flow conditions. Eight hours after LPS challenge, Tie2 mRNA loss was observed in all major organs, while loss of Tie2 protein was predominantly observed in lungs and kidneys, in the capillaries. A similar loss could be induced by secondary cytokines TNF-α and IL-1β. Ang2 protein administration did not affect Tie2 protein expression nor was Tie2 protein rescued in LPS-challenged Ang2-deficient mice, excluding a major role for Ang2 in Tie2 down regulation. In vitro, endothelial loss of Tie2 was observed upon lowering of shear stress, not upon LPS and TNF-α stimulation, suggesting that inflammation related haemodynamic changes play a major role in loss of Tie2 in vivo, as also hemorrhagic shock induced Tie2 mRNA loss. In vitro, this loss was partially counteracted by pre-incubation with a pharmacologically NF-кB inhibitor (BAY11-7082), an effect further substantiated in vivo by pre-treatment of mice with the NF-кB inhibitor prior to the inflammatory challenge.

CONCLUSIONS

Microvascular bed specific loss of Tie2 mRNA and protein in vivo upon LPS, TNFα, IL-1β challenge, as well as in response to hemorrhagic shock, is likely an indirect effect caused by a change in endothelial shear stress. This loss of Tie2 mRNA, but not Tie2 protein, induced by TNFα exposure was shown to be controlled by NF-кB signaling. Drugs aiming at restoring vascular integrity in sepsis could focus on preventing the Tie2 loss.

Mechanotransduction in embryonic vascular development

A plethora of biochemical signals provides spatial and temporal cues that carefully orchestrate the complex process of vertebrate embryonic development. The embryonic vasculature develops not only in the context of these biochemical cues, but also in the context of the biomechanical forces imparted by blood flow. In the mature vasculature, different blood flow regimes induce distinct genetic programs, and significant progress has been made toward understanding how these forces are perceived by endothelial cells and transduced into biochemical signals. However, it cannot be assumed that paradigms that govern the mature vasculature are pertinent to the developing embryonic vasculature. The embryonic vasculature can respond to the mechanical forces of blood flow, and these responses are critical in vascular remodeling, certain aspects of sprouting angiogenesis, and maintenance of arterial-venous identity. Here, we review data regarding mechanistic aspects of endothelial cell mechanotransduction, with a focus on the response to shear stress, and elaborate upon the multifarious effects of shear stress on the embryonic vasculature. In addition, we discuss emerging predictive vascular growth models and highlight the prospect of combining signaling pathway information with computational modeling. We assert that correlation of precise measurements of hemodynamic parameters with effects on endothelial cell gene expression and cell behavior is required for fully understanding how blood flow-induced loading governs normal vascular development and shapes congenital cardiovascular abnormalities.

Activation of NF-kappaB by fluid shear stress, but not TNF-alpha, requires focal adhesion kinase in osteoblasts

When bone is mechanically loaded fluid shear stress (FSS) is generated as a result of the movement of interstitial fluid across the membranes of osteoblasts and osteocytes. This external mechanical loading stimulates changes in the activity of cytoplasmic signaling molecules and alters gene expression in bone cells. This process, referred to as mechanotransduction, is vital for maintaining bone health in vivo by regulating the balance between bone formation and bone resorption. This current study focuses on the role of focal adhesions, sites of integrin-mediated cellular attachment to the extracellular matrix, and their proposed function as mechanosensors in bone cells. We examined the role of a key component of focal adhesions and of mechanotransduction, focal adhesion kinase (FAK) in regulation of FSS- and tumor necrosis factor-alpha (TNF-alpha)-induced activation of nuclear factor-kappa B (NF-kappaB) signaling in osteoblasts. Immortalized FAK(+/+) and FAK(-)(/)(-) osteoblasts were exposed to periods of oscillatory fluid shear stress (OFF) and NF-kappaB activation was analyzed. We determined that FAK is required for OFF-induced nuclear translocation and activation of NF-kappaB in osteoblasts. In addition we found that OFF-induced phosphorylation of the IkappaB kinases (IKKalpha/beta) in both FAK(+/+) and FAK(-/-) osteoblasts, but only FAK(+/+) osteoblasts demonstrated the resulting degradation of NF-kappaB inhibitors IkappaBalpha and IkappaBbeta. OFF did not induce the degradation of IkappaBepsilon or the processing of p105 in either FAK(+/+) and FAK(-/-) osteoblasts. To compare the role of FAK in mediating OFF-induced mechanotransduction to the well characterized activation of NF-kappaB by inflammatory cytokines, we exposed FAK(+/+) and FAK(-/-) osteoblasts to TNF-alpha. Interestingly, FAK was not required for TNF-alpha induced NF-kappaB activation in osteoblasts. In addition we determined that TNF-alpha treatment did not induce the degradation of IkappaBbeta as did OFF. These data indicate a novel relationship between FAK and NF-kappaB activation in osteoblast mechanotransduction and demonstrates that the mechanism of FSS-induced NF-kappaB activation in osteoblasts differs from the well characterized TNF-alpha-induced activation.

Low intensity shear stress increases endothelial ELR+ CXC chemokine production via a focal adhesion kinase-p38{beta} MAPK-NF-{kappa}B pathway

CXC chemokines with a glutamate-leucine-arginine (ELR) tripeptide motif (ELR(+) CXC chemokines) play an important role in leukocyte trafficking into the tissues. For reasons that are not well elucidated, circulating leukocytes are recruited into the tissues mainly in small vessels such as capillaries and venules. Because ELR(+) CXC chemokines are important mediators of endothelial-leukocyte interaction, we compared chemokine expression by microvascular and aortic endothelium to investigate whether differences in chemokine expression by various endothelial types could, at least partially, explain the microvascular localization of endothelial-leukocyte interaction. Both in vitro and in vivo models indicate that ELR(+) CXC chemokine expression is higher in microvascular endothelium than in aortic endothelial cells. These differences can be explained on the basis of the preferential activation of endothelial chemokine production by low intensity shear stress. Low shear activated endothelial ELR(+) CXC chemokine production via cell surface heparan sulfates, beta(3)-integrins, focal adhesion kinase, the mitogen-activated protein kinase p38beta, mitogen- and stress-associated protein kinase-1, and the transcription factor.

p21-activated kinase signaling regulates oxidant-dependent NF-kappa B activation by flow

Disturbed blood flow induces inflammatory gene expression in endothelial cells, which promotes atherosclerosis. Flow stimulates the proinflammatory transcription factor nuclear factor (NF)-kappaB through integrin- and Rac-dependent production of reactive oxygen species (ROS). Previous work demonstrated that NF-kappaB activation by flow is matrix-specific, occurring in cells on fibronectin but not collagen. Activation of p21-activated kinase (PAK) followed the same matrix-dependent pattern. We now show that inhibiting PAK in cells on fibronectin blocked NF-kappaB activation by both laminar and oscillatory flow in vitro and at sites of disturbed flow in vivo. Constitutively active PAK rescued flow-induced NF-kappaB activation in cells on collagen. Surprisingly, PAK was not required for flow-induced ROS production. Instead, PAK modulated the ability of ROS to activate the NF-kappaB pathway. These data demonstrate that PAK controls NF-kappaB activation by modulating the sensitivity of cells to ROS.

Unusual transduction response of progenitor-derived and mature endothelial cells exposed to laminar pulsatile shear stress

Progenitor-derived endothelial cells (PDECs) isolated from human umbilical cord blood generate a great hope in the fields of vascular tissue engineering. Endothelial cells subjected to shear stress convert mechanical stimuli into intracellular signals that affect cellular functions. It is essential to ensure that PDECs are able to sense shear stress as mature endothelial cells from human saphenous veins (HSVECs) do with mitogen-activated protein (MAP) kinase and nuclear factor (NF)-kappaB signal transduction pathways. HSVECs and PDECs were seeded on glass slides coated with gelatin and exposed to 12 dyn/cm2 in a parallel-plate flow chamber. In both cell types, shear stress activated extracellular signal-related kinase (ERK)1/2 with a rapid time course (maximum 5 min) followed by a reduced phosphorylation, and p38 pathway. c-Jun N-terminal protein kinase (JNK) phosphorylation is observed only in PDECs. With respect to NF-kappaB translocation to the nucleus, the NF-kappaB pathway is not activated by flow in HSVECs and PDECs although interleukin-1alpha (IL-1alpha) activates this pathway in both cell types. In our experimental conditions, shear stress does not modify the nuclear translocation of NF-kappaB in HSVECs after IL-1alpha stimulation. It can be stated that PDECs are shear stress sensitive and capable of signal transduction as mature HSVECs are, despite the unusual transduction response of both cell types.

Reversal of LPS induced endothelial cell TNF synthesis and increased permeability with microencapsulated antisense oligomers to NF-kappaB

Endothelial cells form the barrier between the circulation and interstitial space. Changes in permeability of endothelial cells allow penetration of inflammatory cells such as polymorphonuclear cells and macrophages to respond to infections and other inflammatory stimuli. Endothelial cells have also been shown to be phagocytic and produce pro-inflammatory cytokines such as TNF. It is the purpose of this study to evaluate endothelial cell phagocytosis of albumin microspheres containing antisense oligonucluetide to NF-kappaB (MASO), the effect of MASO on TNF synthesis after LPS stimulation and the effect of TNF inhibition on the permeability of endothelial cells in vitro. Results were (1) endothelial cells avidly phagocytozed albumin miocrospheres 1.0 and 1.7 microm in size, (2) phagocytosis of microspheres was potentiated by LPS, (3) TNF is synthesized by endothelial cells in cell culture with the peak concentrations occurring 4 h after stimulation with LPS, (4) MASO results in high intracellular concentration of oligomer, (5) MASO inhibits TNF synthesis to a greater extent than equivalent amounts of NF-kappaB antisense in solution and (6) the inhibition of TNF by MASO significantly decreases the permeability of albumin through endothelial cells in vitro.

NFKB1 promoter variation implicates shear-induced NOS3 gene expression and endothelial function in prehypertensives and stage I hypertensives

In endothelial cells, NF-kappaB is an important intracellular signaling molecule by which changes in wall shear stress are transduced into the nucleus to initiate downstream endothelial nitric oxide synthase (NOS3) gene expression. We investigated whether NF-kappa light-chain gene enhancer in B cells 1 (NFKB1) promoter polymorphism ((-94)NFKB1 I/D, where I is the insertion allele and D is the deletion allele) was associated with 1) NOS3 gene expression in endothelial cells under physiological levels of unidirectional laminar shear stress (LSS) and 2) endothelial function in prehypertensive and stage I hypertensive individuals before and after a 6-mo supervised endurance exercise intervention. Competitive EMSAs revealed that proteins present in the nuclei of endothelial cells preferentially bound to the I allele NFKB1 promoter compared with the D allele. Reporter gene assays showed that the I allele promoter had significantly higher activity than the D allele. In agreement with these observations, homozygous II genotype cells had higher p50 expression levels than homozygous DD genotype cells. Cells with the homozygous II genotype showed a greater increase in NOS3 protein expression than did homozygous DD genotype cells under LSS. Functional experiments on volunteers confirmed higher baseline reactive hyperemic forearm blood flow, and, furthermore, the subgroup analysis revealed that DD homozygotes were significantly less prevalent in the exercise responder group compared with II and ID genotypes. We conclude that the (-94)NFKB1 I/D promoter variation contributes to the modulation of vascular function and adaptability to exercise-induced flow shear stress, most likely due to differences in NFKB1 gene transactivity.

Nitric oxide and mitochondrial signaling: from physiology to pathophysiology

Nitric oxide (NO) has been known for many years to bind to cytochrome C oxidase, the terminal acceptor in the mitochondrial electron transport chain, in competition with oxygen. This interaction may be significant in vivo and explain some of the biological actions of NO. In this article we review the evidence showing that binding of NO to cytochrome C oxidase elicits intracellular signaling events, including the diversion of oxygen to nonrespiratory substrates and the generation of reactive oxygen species. We discuss findings indicating that these NO-elicited events act as triggers by which mitochondria modulate signal transduction cascades involved in the induction of cellular defense mechanisms and adaptive responses. We also discuss instances in which the effects of NO on the electron transport chain might lead to mitochondrial dysfunction and pathology.

Low shear stress preferentially enhances IKK activity through selective sources of ROS for persistent activation of NF-kappaB in endothelial cells

NF-kappaB signaling pathway has been known to play a major role in the pathological process of atherogenesis. Unlike high shear stress, in which the NF-kappaB activity is transient, our earlier studies have demonstrated a persistent activation of NF-kappaB in response to low shear stress in human aortic endothelial cells. These findings partially explained why low shear regions that exist at bifurcations of arteries are prone to atherosclerosis, unlike the relatively atheroprotective high shear regions. In the present study, we further investigated 1) the role of NF-kappaB signaling kinases (IKKalpha and beta) that may be responsible for the sustained activation of NF-kappaB in low shear stress and 2) the regulation of these kinases by reactive oxygen species (ROS). Our results demonstrate that not only is a significant proportion of low shear-induced-kinase activity is contributed by IKKbeta, but it is also persistently induced for a prolonged time frame. The IKK activity (both alpha and beta) is blocked by apocynin (400 microM), a specific NADPH oxidase inhibitor, and diphenyleneiodonium chloride (DPI; 10 microM), an inhibitor of flavin-containing oxidases like NADPH oxidases. Determination of ROS also demonstrated an increased generation in low shear stress that could be blocked by DPI. These results suggest that the source of ROS generation in endothelial cells in response to low shear stress is NADPH oxidase. The DPI-inhibitable component of ROS is the primary regulator of specific upstream kinases that determine the persistent NF-kappaB activation selectively in low shear-induced endothelial cells.

Endothelin-1 and angiotensin II contribute to BNP but not c-fos gene expression response to elevated load in isolated mice hearts

The early events in the cardiac hypertrophic process induced by hemodynamic load include activation of B-type natriuretic peptide (BNP) and c-fos gene expression. However, it is unknown whether stretch acts directly or through local paracrine factors to trigger changes in cardiac gene expression. Herein we studied the involvement of endothelin-1 (ET-1) and angiotensin II (Ang II) in load-induced activation of left ventricular BNP and c-fos gene expression using an in vitro stretch model in isolated perfused adult mice hearts. Two-hour stretch induced by increasing coronary flow rate from 2 to 5 ml/min increased the expression of BNP and c-fos genes by 1.9- and 1.5-fold, respectively (P<0.001 and P<0.05). A mixed ET(A/B) receptor antagonist bosentan attenuated the BNP gene expression response to load by 58% (P<0.005). A similar 53% inhibition was observed with the selective ET(A) receptor blocker BQ-123 (P<0.05). Type 1 Ang II receptor antagonist CV-11974 decreased the activation of BNP gene expression by 50% (P<0.05). In contrast, the activation of c-fos gene expression was not inhibited by antagonists of ET(A/B) and AT(1) receptors. Our results show that ET-1 and Ang II play a key role in the induction of BNP, but not c-fos gene expression in response to load in intact adult murine hearts.

Endothelial mechanotransduction, nitric oxide and vascular inflammation

Numerous aspects of vascular homeostasis are modulated by nitric oxide and reactive oxygen species (ROS). The production of these is dramatically influenced by mechanical forces imposed on the endothelium and vascular smooth muscle. In this review, we will discuss the effects of mechanical forces on the expression of the endothelial cell nitric oxide synthase, production of ROS and modulation of endothelial cell glutathione. We will also review data that exercise training in vivo has a similar effect as laminar shear on endothelial function and discuss the clinical relevance of these basic findings.

Molecular basis of the effects of shear stress on vascular endothelial cells

Blood vessels are constantly exposed to hemodynamic forces in the form of cyclic stretch and shear stress due to the pulsatile nature of blood pressure and flow. Endothelial cells (ECs) are subjected to the shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular functions, e.g., proliferation, apoptosis, migration, permeability, and remodeling, as well as gene expression. The ECs use multiple sensing mechanisms to detect changes in mechanical forces, leading to the activation of signaling networks. The cytoskeleton provides a structural framework for the EC to transmit mechanical forces between its luminal, abluminal and junctional surfaces and its interior, including the cytoplasm, the nucleus, and focal adhesion sites. Endothelial cells also respond differently to different modes of shear forces, e.g., laminar, disturbed, or oscillatory flows. In vitro studies on cultured ECs in flow channels have been conducted to investigate the molecular mechanisms by which cells convert the mechanical input into biochemical events, which eventually lead to functional responses. The knowledge gained on mechano-transduction, with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes in vascular remodeling and adaptation in health and disease.

The subendothelial extracellular matrix modulates NF-kappaB activation by flow: a potential role in atherosclerosis

Atherosclerotic plaque forms in regions of the vasculature exposed to disturbed flow. NF-κB activation by fluid flow, leading to expression of target genes such as E-selectin, ICAM-1, and VCAM-1, may regulate early monocyte recruitment and fatty streak formation. Flow-induced NF-κB activation is downstream of conformational activation of integrins, resulting in new integrin binding to the subendothelial extracellular matrix and signaling. Therefore, we examined the involvement of the extracellular matrix in this process. Whereas endothelial cells plated on fibronectin or fibrinogen activate NF-κB in response to flow, cells on collagen or laminin do not. In vivo, fibronectin and fibrinogen are deposited at atherosclerosis-prone sites before other signs of atherosclerosis. Ligation of integrin α2β1 on collagen prevents flow-induced NF-κB activation through a p38-dependent pathway that is activated locally at adhesion sites. Furthermore, altering the extracellular matrix to promote p38 activation in cells on fibronectin suppresses NF-κB activation, suggesting a novel therapeutic strategy for treating atherosclerosis.