Is PECAM-1 a mechanoresponsive molecule?

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.

PIEZO1 and PECAM1 interact at cell-cell junctions and partner in endothelial force sensing

Two prominent concepts for the sensing of shear stress by endothelium are the PIEZO1 channel as a mediator of mechanically activated calcium ion entry and the PECAM1 cell adhesion molecule as the apex of a triad with CDH5 and VGFR2. Here, we investigated if there is a relationship. By inserting a non-disruptive tag in native PIEZO1 of mice, we reveal in situ overlap of PIEZO1 with PECAM1. Through reconstitution and high resolution microscopy studies we show that PECAM1 interacts with PIEZO1 and directs it to cell-cell junctions. PECAM1 extracellular N-terminus is critical in this, but a C-terminal intracellular domain linked to shear stress also contributes. CDH5 similarly drives PIEZO1 to junctions but unlike PECAM1 its interaction with PIEZO1 is dynamic, increasing with shear stress. PIEZO1 does not interact with VGFR2. PIEZO1 is required in Ca2+-dependent formation of adherens junctions and associated cytoskeleton, consistent with it conferring force-dependent Ca2+ entry for junctional remodelling. The data suggest a pool of PIEZO1 at cell junctions, the coming together of PIEZO1 and PECAM1 mechanisms and intimate cooperation of PIEZO1 and adhesion molecules in tailoring junctional structure to mechanical requirement.

The impact of fluid status and decremental PEEP strategy on cardiac function and lung and kidney damage in mild-moderate experimental acute respiratory distress syndrome

BACKGROUND

We evaluated the effects of abrupt versus gradual PEEP decrease, combined with standard versus high-volume fluid administration, on cardiac function, as well as lung and kidney damage in an established model of mild-moderate acute respiratory distress syndrome (ARDS).

METHODS

Wistar rats received endotoxin intratracheally. After 24 h, they were treated with Ringer's lactate at standard (10 mL/kg/h) or high (30 mL/kg/h) dose. For 30 min, all animals were mechanically ventilated with tidal volume = 6 mL/kg and PEEP = 9 cmH2O (to keep alveoli open), then randomized to undergo abrupt or gradual (0.2 cmH2O/min for 30 min) PEEP decrease from 9 to 3 cmH2O. Animals were then further ventilated for 10 min at PEEP = 3 cmH2O, euthanized, and their lungs and kidneys removed for molecular biology analysis.

RESULTS

At the end of the experiment, left and right ventricular end-diastolic areas were greater in animals treated with high compared to standard fluid administration, regardless of PEEP decrease rate. However, pulmonary arterial pressure, indicated by the pulmonary acceleration time (PAT)/pulmonary ejection time (PET) ratio, was higher in abrupt compared to gradual PEEP decrease, independent of fluid status. Animals treated with high fluids and abrupt PEEP decrease exhibited greater diffuse alveolar damage and higher expression of interleukin-6 (a pro-inflammatory marker) and vascular endothelial growth factor (a marker of endothelial cell damage) compared to the other groups. The combination of standard fluid administration and gradual PEEP decrease increased zonula occludens-1 expression, suggesting epithelial cell preservation. Expression of club cell-16 protein, an alveolar epithelial cell damage marker, was higher in abrupt compared to gradual PEEP decrease groups, regardless of fluid status. Acute kidney injury score and gene expression of kidney injury molecule-1 were higher in the high versus standard fluid administration groups, regardless of PEEP decrease rate.

CONCLUSION

In the ARDS model used herein, decreasing PEEP abruptly increased pulmonary arterial hypertension, independent of fluid status. The combination of abrupt PEEP decrease and high fluid administration led to greater lung and kidney damage. This information adds to the growing body of evidence that supports gradual transitioning of ventilatory patterns and warrants directing additional investigative effort into vascular and deflation issues that impact lung protection.

Recruitment and maturation of the coronary collateral circulation: Current understanding and perspectives in arteriogenesis

The coronary collateral circulation is a rich anastomotic network of primitive vessels which have the ability to augment in size and function through the process of arteriogenesis. In this review, we evaluate the current understandings of the molecular and cellular mechanisms by which this process occurs, specifically focussing on elevated fluid shear stress (FSS), inflammation, the redox state and gene expression along with the integrative, parallel and simultaneous process by which this occurs. The initiating step of arteriogenesis occurs following occlusion of an epicardial coronary artery, with an increase in FSS detected by mechanoreceptors within the endothelium. This must occur within a 'redox window' where an equilibrium of oxidative and reductive factors are present. These factors initially result in an inflammatory milieu, mediated by neutrophils as well as lymphocytes, with resultant activation of a number of downstream molecular pathways resulting in increased expression of proteins involved in monocyte attraction and adherence; namely vascular cell adhesion molecule 1 (VCAM-1), monocyte chemoattractant protein 1 (MCP-1) and transforming growth factor beta (TGF-β). Once monocytes and other inflammatory cells adhere to the endothelium they enter the extracellular matrix and differentiate into macrophages in an effort to create a favourable environment for vessel growth and development. Activated macrophages secrete inflammatory cytokines such as tumour necrosis factor-α (TNF-α), growth factors such as fibroblast growth factor-2 (FGF-2) and matrix metalloproteinases. Finally, vascular smooth muscle cells proliferate and switch to a contractile phenotype, resulting in an increased diameter and functionality of the collateral vessel, thereby allowing improved perfusion of the distal myocardium subtended by the occluded vessel. This simultaneously reduces FSS within the collateral vessel, inhibiting further vessel growth.

Mechanotransduction and Uterine Blood Flow in Preeclampsia: The Role of Mechanosensing Piezo 1 Ion Channels

There is a large increase in uterine arterial blood flow during normal pregnancy. Structural and cellular adjustments occur in the uterine vasculature during pregnancy to accommodate this increased blood flow through a complex adaptive process that is dependent on multiple coordinated and interactive influences and this process is known as "vascular remodeling." The etiology of preeclampsia involves aberrant placentation and vascular remodeling leading to reduced uteroplacental perfusion. The placental ischemia leads to development of hypertension and proteinuria in the mother, intrauterine growth restriction, and perinatal death in the fetus. However, the underlying source of the deficient vascular remodeling and the subsequent development of preeclampsia remain to be fully understood. Mechanoreceptors in the vascular system convert mechanical force (shear stress) to biochemical signals and feedback mechanisms. This review focuses on the Piezo 1 channel, a mechanosensitive channel that is sensitive to shear stress in the endothelium; it induces Ca2+ entry which is linked to endothelial nitric oxide synthase (eNOS) activation as the mechanoreceptor responsible for uterine vascular dilatation during pregnancy. Here we describe the downstream signaling pathways involved in this process and the possibility of a deficiency in expression of Piezo 1 in preeclampsia leading to the abnormal vascular dysfunction responsible for the pathophysiology of the disease. The Piezo 1 ion channel is expressed in the endothelium and vascular smooth muscle cells (VSMCs) of small-diameter arteries. It plays a role in the structural remodeling of arteries and is involved in mechanotransduction of hemodynamic shear stress by endothelial cells (ECs).

Darwinian evolution and cardiovascular remodeling

Mechanotransduction, MT, is an ancient evolutionary legacy existing in every living species and involving complex rearrangements of multiple proteins in response to a mechanical stress. MT includes three different interrelated processes: mechanosensation, mechanotransmission, and mechanoresponse. Each process is specifically adapted to a given tissue and stress. Both cardiac and arterial remodeling involve MT. Physiological or pathological cardiac remodeling, CR, is firstly a beneficial mechanoresponse, MR, which allows the heart to recover to a normal economy, better adapted to the new working conditions. Nevertheless, exercise-induced cardiac remodeling is more a coming-back to normal conditions than a superimposed event. On the longer term, the MR creates fibrosis which accounts, in part, for the reduced cardiac output in the CR. In the hypertension-induced arterial remodeling, arterial MR allows the vessels to maintain a normal circumferential constraint before an augmented arterial pressure. In atherogenesis: (i) The presence of atheroma in several animal species and atherosclerosis in ancient civilizations suggests more basic predispositions. (ii) The atherosclerotic plaques preferably develop at predictable arterial sites of disturbed blood flow showing that MT is involved in the initial steps of atherogenesis.

PECAM1 regulates flow-mediated Gab1 tyrosine phosphorylation and signaling

Endothelial dysfunction, characterized by impaired activation of endothelial nitric oxide (NO) synthase (eNOS) and ensued decrease of NO production, is a common mechanism of various cardiovascular pathologies, including hypertension and atherosclerosis. Laminar blood flow-mediated specific signaling cascades modulate vascular endothelial cells (ECs) structure and functions. We have previously shown that flow-stimulated Gab1 (Grb2-associated binder-1) tyrosine phosphorylation mediates eNOS activation in ECs, which in part confers laminar flow atheroprotective action. However, the molecular mechanisms whereby flow regulates Gab1 tyrosine phosphorylation and its downstream signaling events remain unclear. Here we show that platelet endothelial cell adhesion molecule-1 (PECAM1), a key molecule in an endothelial mechanosensing complex, specifically mediates Gab1 tyrosine phosphorylation and its downstream Akt and eNOS activation in ECs upon flow rather than hepatocyte growth factor (HGF) stimulation. Small interfering RNA (siRNA) targeting PECAM1 abolished flow- but not HGF-induced Gab1 tyrosine phosphorylation and Akt, eNOS activation as well as Gab1 membrane translocation. Protein-tyrosine phosphatase SHP2, which has been shown to interact with Gab1, was involved in flow signaling and HGF signaling, as SHP2 siRNA diminished the flow- and HGF-induced Gab1 tyrosine phosphorylation, membrane localization and downstream signaling. Pharmacological inhibition of PI3K decreased flow-, but not HGF-mediated Gab1 phosphorylation and membrane localization as well as eNOS activation. Finally, we observed that flow-mediated Gab1 and eNOS phosphorylation in vivo induced by voluntary wheel running was reduced in PECAM1 knockout mice. These results demonstrate a specific role of PECAM1 in flow-mediated Gab1 tyrosine phosphorylation and eNOS signaling in ECs.

Inhibition of lung tumor growth in nude mice by siRNACD31 targeting PECAM-1

Small interfering RNA (siRNA) provides a promising therapeutic approach in the silencing of disease-causing genes. In the present study, the use of 2′-O-methyl-modified siRNA-cluster of differentiation 31 (siRNA^CD31), with cationic liposome RNA interference (RNAi)-mate as a carrier, effectively silenced the platelet endothelial cell molecule 1 (PECAM-1) gene of murine hemangioendothelioma cells in vitro. In vivo, 2′-O-methyl-modified siRNA^CD31 carried by RNAi-mate was successfully delivered, targeting the PECAM-1 gene in the vasculature of nude mouse lung carcinoma xenografts. The growth of the lung carcinoma xenografts was inhibited by the 2′-O-methyl-modified siRNA^CD31 and RNAi-mate complexes, and the expression of the PECAM-1 protein was downregulated, with a simultaneous decrease in vascular endothelial growth factor (VEGF) protein in the lung carcinoma xenografts. 2′-O-methyl-modified siRNA^CD31-RNAi-mate complexes may provide a potential therapeutic strategy in lung carcinoma treatment. The effect of PECAM-1 on VEGF expression may possibly be attributed to the function of PECAM-1 signal transduction.

PECAM-1 phosphorylation and tissue factor expression in HUVECs exposed to uniform and disturbed pulsatile flow and chemical stimuli

INTRODUCTION

We analyzed the relationship between platelet endothelial cell adhesion molecule-1 (PECAM-1) activation and tissue factor (TF) RNA expression in human umbilical vein endothelial cells (HUVECs) exposed to mechanical and chemical stimulation.

METHODS

Fifty percent confluent and 100% confluent HUVEC cultures were exposed to pulsatile forward flow, as a model for uniform flow, or pulsatile to-and-fro flow, as a model for disturbed flow, using a parallel-plate flow chamber system for up to 4 hours in the presence or absence of 4 U/mL thrombin. Protein lysates were immunoprecipitated for PECAM-1 and then immunoblotted with anti-phospho-tyrosine antibody. TF RNA expression was determined using quantitative reverse transcription polymerase chain reaction.

RESULTS

HUVECs exposed to disturbed flow induced higher TF expression at 4 hours than HUVECs exposed to uniform flow in sparse cultures (16.8 ± 5.8 vs 5.1 ± 1.2; P < .05). HUVECs exposed to disturbed flow and thrombin induced higher TF RNA expression at 4 hours than cultures exposed to uniform flow and thrombin in both confluent (47.0 ± 6.0 vs 30.2 ± 4.9; P < .05) and sparse (72.3 ± 10.7 vs 49.8 ± 4.7; P < .05) cultures. In confluent HUVEC cultures, PECAM-1 is minimally phosphorylated by disturbed and uniform flow, while in sparse HUVEC cultures, PECAM-1 phosphorylation at 15 minutes is greater in both disturbed and uniform flow (2.0 ± 0.2 and 2.1 ± 0.4 respectively; P < .05). Thrombin treatment of static HUVECs exhibited greater PECAM-1 phosphorylation at 15 minutes in confluent compared with sparse cultures (3.0 ± 0.5 vs 2.3 ± 0.1; P < .05). PECAM-1 phosphorylation of HUVECs exposed to both flow and thrombin is significantly higher in sparse cultures compared with either flow or thrombin stimulation alone but was suppressed in confluent cultures.

CONCLUSIONS

The significantly higher TF RNA expression induced by disturbed flow and cell confluence indicates that suppression of PECAM-1 phosphorylation may be an important contributory mechanical signal pathway that promotes TF expression when HUVECs are exposed to disturbed flow.

Vascular endothelial growth factor (VEGF) and platelet (PF-4) factor 4 inputs modulate human microvascular endothelial signaling in a three-dimensional matrix migration context

The process of angiogenesis is under complex regulation in adult organisms, particularly as it often occurs in an inflammatory post-wound environment. As such, there are many impacting factors that will regulate the generation of new blood vessels which include not only pro-angiogenic growth factors such as vascular endothelial growth factor, but also angiostatic factors. During initial postwound hemostasis, a large initial bolus of platelet factor 4 is released into localized areas of damage before progression of wound healing toward tissue homeostasis. Because of its early presence and high concentration, the angiostatic chemokine platelet factor 4, which can induce endothelial anoikis, can strongly affect angiogenesis. In our work, we explored signaling crosstalk interactions between vascular endothelial growth factor and platelet factor 4 using phosphotyrosine-enriched mass spectrometry methods on human dermal microvascular endothelial cells cultured under conditions facilitating migratory sprouting into collagen gel matrices. We developed new methods to enable mass spectrometry-based phosphorylation analysis of primary cells cultured on collagen gels, and quantified signaling pathways over the first 48 h of treatment with vascular endothelial growth factor in the presence or absence of platelet factor 4. By observing early and late signaling dynamics in tandem with correlation network modeling, we found that platelet factor 4 has significant crosstalk with vascular endothelial growth factor by modulating cell migration and polarization pathways, centered around P38α MAPK, Src family kinases Fyn and Lyn, along with FAK. Interestingly, we found EphA2 correlational topology to strongly involve key migration-related signaling nodes after introduction of platelet factor 4, indicating an influence of the angiostatic factor on this ambiguous but generally angiogenic signal in this complex environment.

Biomechanical force in blood development: extrinsic physical cues drive pro-hematopoietic signaling

The hematopoietic system is dynamic during development and in adulthood, undergoing countless spatial and temporal transitions during the course of one's life. Microenvironmental cues in the many unique hematopoietic niches differ, characterized by distinct soluble molecules, membrane-bound factors, and biophysical features that meet the changing needs of the blood system. Research from the last decade has revealed the importance of substrate elasticity and biomechanical force in determination of stem cell fate. Our understanding of the role of these factors in hematopoiesis is still relatively poor; however, the developmental origin of blood cells from the endothelium provides a model for comparison. Many endothelial mechanical sensors and second messenger systems may also determine hematopoietic stem cell fate, self renewal, and homing behaviors. Further, the intimate contact of hematopoietic cells with mechanosensitive cell types, including osteoblasts, endothelial cells, mesenchymal stem cells, and pericytes, places them in close proximity to paracrine signaling downstream of mechanical signals. The objective of this review is to present an overview of the sensors and intracellular signaling pathways activated by mechanical cues and highlight the role of mechanotransductive pathways in hematopoiesis.

Nanotopography-guided tissue engineering and regenerative medicine

Human tissues are intricate ensembles of multiple cell types embedded in complex and well-defined structures of the extracellular matrix (ECM). The organization of ECM is frequently hierarchical from nano to macro, with many proteins forming large scale structures with feature sizes up to several hundred microns. Inspired from these natural designs of ECM, nanotopography-guided approaches have been increasingly investigated for the last several decades. Results demonstrate that the nanotopography itself can activate tissue-specific function in vitro as well as promote tissue regeneration in vivo upon transplantation. In this review, we provide an extensive analysis of recent efforts to mimic functional nanostructures in vitro for improved tissue engineering and regeneration of injured and damaged tissues. We first characterize the role of various nanostructures in human tissues with respect to each tissue-specific function. Then, we describe various fabrication methods in terms of patterning principles and material characteristics. Finally, we summarize the applications of nanotopography to various tissues, which are classified into four types depending on their functions: protective, mechano-sensitive, electro-active, and shear stress-sensitive tissues. Some limitations and future challenges are briefly discussed at the end.

Acute and chronic shear stress differently regulate endothelial internalization of nanocarriers targeted to platelet-endothelial cell adhesion molecule-1

Intracellular delivery of nanocarriers (NC) is controlled by their design and target cell phenotype, microenvironment, and functional status. Endothelial cells (EC) lining the vascular lumen represent an important target for drug delivery. Endothelium in vivo is constantly or intermittently (as, for example, during ischemia-reperfusion) exposed to blood flow, which influences NC-EC interactions by changing NC transport properties, and by direct mechanical effects upon EC mechanisms involved in NC binding and uptake. EC do not internalize antibodies to marker glycoprotein PECAM(CD31), yet internalize multivalent NC coated with PECAM antibodies (anti-PECAM/NC) via a noncanonical endocytic pathway distantly related to macropinocytosis. Here we studied the effects of flow on EC uptake of anti-PECAM/NC spheres (~180 nm diameter). EC adaptation to chronic flow, manifested by cellular alignment with flow direction and formation of actin stress fibers, inhibited anti-PECAM/NC endocytosis consistent with lower rates of anti-PECAM/NC endocytosis in vivo in arterial compared to capillary vessels. Acute induction of actin stress fibers by thrombin also inhibited anti-PECAM/NC endocytosis, demonstrating that formation of actin stress fibers impedes EC endocytic machinery. In contrast, acute flow without stress fiber formation, stimulated anti-PECAM/NC endocytosis. Anti-PECAM/NC endocytosis did not correlate with the number of cell-bound particles under flow or static conditions. PECAM cytosolic tail deletion and disruption of cholesterol-rich plasmalemma domains abrogated anti-PECAM/NC endocytosis stimulation by acute flow, suggesting complex regulation of a flow-sensitive endocytic pathway in EC. The studies demonstrate the importance of the local flow microenvironment for NC uptake by the endothelium and suggest that cell culture models of nanoparticle uptake should reflect the microenvironment and phenotype of the target cells.

Role of PECAM-1 in arteriogenesis and specification of preexisting collaterals

RATIONALE

Hemodynamic forces caused by the altered blood flow in response to an occlusion lead to the induction of collateral remodeling and arteriogenesis. Previous work showed that platelet endothelial cell adhesion molecule (PECAM)-1 is a component of a mechanosensory complex that mediates endothelial cell responses to shear stress.

OBJECTIVE

We hypothesized that PECAM-1 plays an important role in arteriogenesis and collateral remodeling.

METHODS AND RESULTS

PECAM-1 knockout (KO) and wild-type littermates underwent femoral artery ligation. Surprisingly, tissue perfusion and collateral-dependent blood flow were significantly increased in the KO mice immediately after surgery. Histology confirmed larger caliber of preexisting collaterals in the KO mice. Additionally, KO mice showed blunted recovery of perfusion from hindlimb ischemia and reduced collateral remodeling, because of deficits in shear stress-induced signaling, including activation of the nuclear factor κB pathway and inflammatory cell accumulation. Partial recovery was associated with normal responses to circumferential wall tension in the absence of PECAM-1, as evidenced by the upregulation of ephrin B2 and monocyte chemoattractant protein-1, which are 2 stretch-induced regulators of arteriogenesis, both in vitro and in vivo.

CONCLUSIONS

Our findings suggest a novel role for PECAM-1 in arteriogenesis and collateral remodeling. Furthermore, we identify PECAM-1 as the first molecule that determines preexisting collateral diameter.

Endothelial Cell Membrane Sensitivity to Shear Stress is Lipid Domain Dependent

Blood flow-associated shear stress causes physiological and pathophysiological biochemical processes in endothelial cells that may be initiated by alterations in plasma membrane lipid domains characterized as liquid-ordered (l(o)), such as rafts or caveolae, or liquid-disordered (l(d)). To test for domain-dependent shear sensitivity, we used time-correlated single photon counting instrumentation to assess the photophysics and dynamics of the domain-selective lipid analogues DiI-C(12) and DiI-C(18) in endothelial cells subjected to physiological fluid shear stress. Under static conditions, DiI-C(12) fluorescence lifetime was less than DiI-C(18) lifetime and the diffusion coefficient of DiI-C(12) was greater than the DiI-C(18) diffusion coefficient, confirming that DiI-C(12) probes l(d), a more fluid membrane environment, and DiI-C(18) probes the l(o) phase. Domains probed by DiI-C(12) exhibited an early (10 s) and transient decrease of fluorescence lifetime after the onset of shear while domains probed by DiI-C(18) exhibited a delayed decrease of fluorescence lifetime that was sustained for the 2 min the cells were subjected to flow. The diffusion coefficient of DiI-C(18) increased after shear imposition, while that of DiI-C(12) remained constant. Determination of the number of molecules (N) in the control volume suggested that DiI-C(12)-labeled domains increased in N immediately after step-shear, while N for DiI-C(18)-stained membrane transiently decreased. These results demonstrate that membrane microdomains are differentially sensitive to fluid shear stress.

Proteomic Analysis of Vascular Endothelial Cells-Effects of Laminar Shear Stress and High Glucose

This study directly measured the relative protein levels in bovine aortic endothelial cells (BAEC) that were cultured for two weeks in normal (5 mM, NG) or high (22 mM, HG) glucose and then were subjected to laminar shear stress at 0 or 15 dynes/cm(2). Membrane preparations were labeled with one of the four isobaric tagging reagents (iTRAQ), followed by LC-MS/MS analysis. The results showed that HG and/or shear stress induced alterations in various membrane associated proteins involving many signaling pathways. While shear stress induced an increase in heat shock proteins and protein ubiquitination, which remained enhanced in HG, the effects of shear stress on the mechanosensing and protein phosphorylation pathways were altered by HG. These results were validated by Western blot analysis, suggesting that HG importantly modulates shear stress-mediated endothelial function.

Molecular mechanisms responsible for the atheroprotective effects of laminar shear stress

The endothelium lining the inner surface of blood vessels of the cardiovascular system is constantly exposed to hemodynamic shear stress. The interaction between endothelial cells and hemodynamic shear stress has critical implications for atherosclerosis. Regions of arterial narrowing, curvatures, and bifurcations are especially susceptible to atherosclerotic lesion formation. In such areas, endothelial cells experience low, or oscillatory, shear stress. Corresponding changes in endothelial cell structure and function make them susceptible to the initiation and development of atherosclerosis. In contrast, blood flow with high laminar shear stress activates signal transductions as well as gene and protein expressions that play important roles in vascular homeostasis. In response to laminar shear stress, the release of vasoactive substances such as nitric oxide and prostacyclin decreases permeability to plasma lipoproteins as well as the adhesion of leukocytes, and inhibits smooth muscle cell proliferation and migration. In summary, different flow patterns directly determine endothelial cell morphology, metabolism, and inflammatory phenotype through signal transduction and gene and protein expression. Thus, high laminar shear stress plays a key role in the prevention of atherosclerosis through its regulation of vascular tone and long-term maintenance of the integrity and function of endothelial cells.

PECAM-1 is necessary for flow-induced vascular remodeling

OBJECTIVE

Vascular remodeling is a physiological process that occurs in response to long-term changes in hemodynamic conditions, but may also contribute to the pathophysiology of intima-media thickening (IMT) and vascular disease. Shear stress detection by the endothelium is thought to be an important determinant of vascular remodeling. Previous work showed that platelet endothelial cell adhesion molecule-1 (PECAM-1) is a component of a mechanosensory complex that mediates endothelial cell (EC) responses to shear stress.

METHODS AND RESULTS

We tested the hypothesis that PECAM-1 contributes to vascular remodeling by analyzing the response to partial carotid artery ligation in PECAM-1 knockout mice and wild-type littermates. PECAM-1 deficiency resulted in impaired vascular remodeling and significantly reduced IMT in areas of low flow. Inward remodeling was associated with PECAM-1-dependent NFkappaB activation, surface adhesion molecule expression, and leukocyte infiltration as well as Akt activation and vascular cell proliferation.

CONCLUSIONS

PECAM-1 plays a crucial role in the activation of the NFkappaB and Akt pathways and inflammatory cell accumulation during vascular remodeling and IMT. Elucidation of some of the signals that drive vascular remodeling represent pharmacologically tractable targets for the treatment of restenosis after balloon angioplasty or stent placement.

Stress fracture healing: fatigue loading of the rat ulna induces upregulation in expression of osteogenic and angiogenic genes that mimic the intramembranous portion of fracture repair

Woven bone is formed in response to fatigue-induced stress fractures and is associated with increased local angiogenesis. The molecular mechanisms that regulate this woven bone formation are unknown. Our objective was to measure the temporal and spatial expression of osteo- and angiogenic genes in woven bone formation in response to increasing levels of fatigue-induced damage. We used the rat forelimb compression model to produce four discrete levels of fatigue damage in the right ulna of 115 male Fischer rats. Rats were killed at 0 (1 h), 1, 3 and 7 days after loading. Using qRT-PCR, we quantified gene expression associated with osteogenesis (BMP2, Msx2, Runx2, Osx, BSP, Osc), cell proliferation (Hist4), and angiogenesis (VEGF, PECAM-1) from the central half of the ulna. The spatial distribution of BMP2, BSP and PCNA was assessed by immunohistochemistry or in situ hybridization in transverse histological sections 1, 4, and 7 mm distal to the ulnar mid-diaphysis. One hour after loading, BMP2 was significantly upregulated in neurovascular structures in the medial ulnar periosteum. Expression of angiogenic markers (VEGF, PECAM-1) increased significantly between Day 0 and 1 and, as with BMP2 expression, remained upregulated through Day 7. While Osx and BSP were upregulated on Day 1, the other osteogenic genes (Msx2, Runx2, Osx, BSP and Osc) were induced on Day 3 in association with the initiation of periosteal woven bone formation and continued through Day 7. The magnitude of osteogenic gene expression, particularly matrix genes (BSP, Osc) was significantly proportional the level of fatigue damage. The woven bone response to fatigue injury is remarkably similar to the "intramembranous" portion of fracture repair - rapid formation of periosteal woven bone characterized by early BMP2 expression, cell proliferation, and upregulation of osteogenic genes. We speculate that woven bone repair of fatigue damage may be an abbreviated fracture response without the requirement for endochondral repair. We conclude that bone fatigue repair is a process similar to intramembranous fracture repair characterized by increases in the expression of genes associated with angiogenesis, cell proliferation and osteoblastogenesis, and that the response from the local vasculature precedes the osteogenic response to fatigue loading.

Hemodynamic shear stress and the endothelium in cardiovascular pathophysiology

Endothelium lining the cardiovascular system is highly sensitive to hemodynamic shear stresses that act at the vessel luminal surface in the direction of blood flow. Physiological variations of shear stress regulate acute changes in vascular diameter and when sustained induce slow, adaptive, structural-wall remodeling. Both processes are endothelium-dependent and are systemically and regionally compromised by hyperlipidemia, hypertension, diabetes and inflammatory disorders. Shear stress spans a range of spatiotemporal scales and contributes to regional and focal heterogeneity of endothelial gene expression, which is important in vascular pathology. Regions of flow disturbances near arterial branches, bifurcations and curvatures result in complex spatiotemporal shear stresses and their characteristics can predict atherosclerosis susceptibility. Changes in local artery geometry during atherogenesis further modify shear stress characteristics at the endothelium. Intravascular devices can also influence flow-mediated endothelial responses. Endothelial flow-induced responses include a cell-signaling repertoire, collectively known as mechanotransduction, that ranges from instantaneous ion fluxes and biochemical pathways to gene and protein expression. A spatially decentralized mechanism of endothelial mechanotransduction is dominant, in which deformation at the cell surface induced by shear stress is transmitted as cytoskeletal tension changes to sites that are mechanically coupled to the cytoskeleton. A single shear stress mechanotransducer is unlikely to exist; rather, mechanotransduction occurs at multiple subcellular locations.

Regulation of G protein-coupled receptor activities by the platelet-endothelial cell adhesion molecule, PECAM-1

It is becoming increasingly evident that the cell-cell junction is a major signaling center. Here we show that the Galphaq/11 subunit of heterotrimeric G proteins forms a complex with platelet-endothelial cell adhesion molecule 1 (PECAM-1), a junctional protein that has been shown to be involved in mechanosignaling in endothelial cells. To understand the role of PECAM-1 in this complex, we determined the critical regions of PECAM-1 involved in this interaction. By expressing truncated forms of PECAM-1 in human embryonic kidney (HEK293) cells, we found that the cytoplasmic domain of PECAM-1 is not required for its association with Galphaq/11. Domain swapping of PECAM-1 with intracellular cell adhesion molecule 1 (ICAM-1), a protein that does not form a complex with Galphaq/11, provides evidence that the extracellular domain of PECAM-1 is critical for this interaction. This result also suggests that PECAM-1 does not directly interact with Galphaq/11. Coexpression of bradykinin receptor B2 (BKRB2), a Galphaq/11-coupled receptor, with PECAM-1 enhances formation of the PECAM-1-Galphaq/11 complex, suggesting an interaction between PECAM-1 and BKRB2. Co-immunoprecipitation experiments indicate that these two molecules indeed form a complex when expressed in HEK293 cells. Activation of ERK1/2 by bradykinin in HUVEC is enhanced when PECAM-1 expression is inhibited by transfection of small interference RNA against PECAM-1. Taken together, our results provide evidence of interaction of PECAM-1 with BKRB2 and of its possible role in regulating G protein-coupled receptor (GPCR) and G protein functions.

Finite-element stress analysis of a multicomponent model of sheared and focally-adhered endothelial cells

Hemodynamic forces applied at the apical surface of vascular endothelial cells may be redistributed to and amplified at remote intracellular organelles and protein complexes where they are transduced to biochemical signals. In this study we sought to quantify the effects of cellular material inhomogeneities and discrete attachment points on intracellular stresses resulting from physiological fluid flow. Steady-state shear- and magnetic bead-induced stress, strain, and displacement distributions were determined from finite-element stress analysis of a cell-specific, multicomponent elastic continuum model developed from multimodal fluorescence images of confluent endothelial cell (EC) monolayers and their nuclei. Focal adhesion locations and areas were determined from quantitative total internal reflection fluorescence microscopy and verified using green fluorescence protein-focal adhesion kinase (GFP-FAK). The model predicts that shear stress induces small heterogeneous deformations of the endothelial cell cytoplasm on the order of <100 nm. However, strain and stress were amplified 10-100-fold over apical values in and around the high-modulus nucleus and near focal adhesions (FAs) and stress distributions depended on flow direction. The presence of a 0.4 microm glycocalyx was predicted to increase intracellular stresses by approximately 2-fold. The model of magnetic bead twisting rheometry also predicted heterogeneous stress, strain, and displacement fields resulting from material heterogeneities and FAs. Thus, large differences in moduli between the nucleus and cytoplasm and the juxtaposition of constrained regions (e.g. FAs) and unattached regions provide two mechanisms of stress amplification in sheared endothelial cells. Such phenomena may play a role in subcellular localization of early mechanotransduction events.

Vascular endothelium and blood flow

Major advances have been made over the last decade towards the elucidation of the molecular mechanisms involved in the endothelium-dependent regulation of vascular tone and blood flow. While the primary endothelium-derived vasodilator autacoid is nitric oxide, it is clear that epoxyeicosatrienoic acids and other endothelium-derived hyperpolarising factors, as well as endothelin-1 and reactive oxygen species, play a significant role in the regulation of vascular tone and gene expression. This review is intended as an overview of the signalling mechanisms that link haemodynamic stimuli (such as shear stress and cyclic stretch) and endothelial cell perturbation to the activation of enzymes generating vasoactive autacoids.

Peroxynitrite reduces the endothelium-derived hyperpolarizing factor component of coronary flow-mediated dilation in PECAM-1-knockout mice

Platelet endothelial cell adhesion molecule 1 (PECAM-1) is capable of transducing signals in endothelial cells exposed to shear; however, the biological consequences of this signal transduction are unknown. Because shear stress elicits flow-mediated dilation (FMD), we examined whether steady-state FMD in mouse coronary arteries (MCAs) is affected in the PECAM-1 knockout (KO) mouse. MCAs were isolated from wild-type (WT) or KO mice and prepared for videomicroscopy, histofluorescence, Western blotting, and immunohistochemistry. FMD was examined in the absence and presence of N(omega)-nitro-l-arginine methyl ester (l-NAME) and l-NAME+indomethacin (INDO). FMD was reduced in KO relative to WT MCAs, but the l-NAME-inhibitable portion of FMD was similar between the two. The INDO-sensitive component of FMD was diminished in KO MCAs. In contrast, the residual component of dilation, presumably because of endothelium-derived hyperpolarizing factor (EDHF), was abolished in KO MCAs. Histofluorescence showed relatively more superoxide (O2-.; oxy-ethidium fluorescence) and peroxide production (dihydrochlorofluorescene fluoresecence) in KO MCAs at rest. Flow augmented O2-. and peroxide production in WT MCAs but had little effect on KO MCAs. Enhanced nitric oxide generation was observed in arteries from KO mice, accompanied with increased eNOS S1177 phosphorylation. In vessels from KO mice, treatment with ebselen decreased peroxynitrite (ONOO-) formation and improved the reduced FMD, largely due to restoration of the presumed EDHF component. These results suggest that PECAM-1 is necessary for normal FMD in the mouse coronary circulation. In the absence of this adhesion and signaling molecule, ONOO- production is increased concomitant with a reduction in both the EDHF and INDO-sensitive components of FMD.

PECAM-1 Mediates NO-Dependent Dilation of Arterioles to High Temporal Gradients of Shear Stress

OBJECTIVE

In response to changes in wall shear stress (WSS) the vascular endothelium releases several factors, among others nitric oxide. On the basis of studies of endothelial cells in culture, suggesting that platelet endothelial cell adhesion molecule-1 (PECAM-1) is specifically involved in sensing and coupling high temporal gradients of fluid shear stress with activation of eNOS, we hypothesized that dilations of isolated skeletal muscle arterioles from PECAM-1 knockout mice (PECAM-KO) will be reduced to rapid increases in WSS elicited by increases in perfusate flow.

METHODS AND RESULTS

Small and large step increases in flow resulted in substantial dilations in arterioles of WT mice (45+/-4%), but they were markedly reduced in arterioles of PECAM-KO mice (22+/-5%). The initial slope of dilations, when WSS increased rapidly, was greater in vessels of WT than those of PECAM-KO mice (slopes: 0.378 and 0.094, respectively), whereas the second phase of dilations, when flow/shear stress was steady, was similar in the 2 groups (slopes: 0.085 and 0.094, respectively). Inhibition of eNOS significantly reduced the initial phase of dilations in arterioles from WT, but not from those of PECAM-KO mice. The calcium ionophore A23187 elicited similar NO-mediated dilation in both WT and PECAM-KO mice.

CONCLUSIONS

In isolated arterioles of PECAM-KO mice activation of eNOS and consequent dilation by agonists is maintained, but the dilation to high temporal gradients of wall shear stress elicited by increases in perfusate flow is reduced. Thus, we propose that PECAM-1 plays an important role in the ability of the endothelium to sense and couple high temporal gradients of wall shear stress to NO-mediated arteriolar dilation during sudden changes in blood flow in vivo.

Reduced expression of platelet endothelial cell adhesion molecule-1 in bone marrow cells in mice after skeletal unloading

One week of tail suspension significantly decreased the expression of PECAM-1 in mouse tibial bone marrow cells but not those of a number of other vascular factors. Anti-PECAM-1 antibody suppressed both ALP+ CFU-f formation and ALP production under co-culture of the osteoblastic cell line and the PECAM-1+ endothelial cell line. This study suggests that the reduced ALP activity after skeletal unloading is related to downregulation of PECAM-1 expression in bone marrow cells in mice.

INTRODUCTION

Vascular factors play a role in bone development and regeneration. We tested the hypothesis that skeletal unloading reduces osteogenic potential by inhibiting the molecules related to angiogenesis and/or vasculogenesis in bone marrow cells.

MATERIALS AND METHODS

Eight-week-old male mice were assigned to three groups after acclimatization for 1 week: ground control (GC), tail suspension (TS), and reloading after 7-day TS (RL). Bilateral tibial and humeral samples were used for analyses. MC3T3-E1, a mouse osteoblastic cell line, and EOMA and ISOS-1, mouse endothelial cell lines, were also used.

RESULTS

Flow cytometric analysis revealed that 7-day TS significantly decreased the expression of platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) in tibial bone marrow cells, but not those of angiopoietin-1, angiopoietin-2, Flk-1 (vascular endothelial growth factor receptor-2), and vascular endothelial cadherin. The expression of PECAM-1 in tibial marrow cells was reduced at day 3 of TS to 80% and still showed significantly low levels at day 7 of TS to 72% of that at the respective days of GC. This decreased expression of PECAM-1 after 7-day TS showed the GC level at 5-day reloading after 7-day TS. However, the expression of PECAM-1 in humeral marrow cells (internal bone marrow control) after TS and RL remained unchanged and equivalent to that of GC. The expression level of PECAM-1 mRNA was significantly lower at day 7 of TS to 62% of that in GC. Double labeling analyses revealed that PECAM-1+ cells mostly consisted of endothelial cells and partially of granulocytes. In bone marrow cell cultures, the formation of alkaline phosphatase (ALP)+ colony forming units-fibroblastic was significantly reduced in the presence of anti-PECAM-1 antibody in the medium compared with the presence of immunoglobulin G (0.025 times as much as ALP production with immunoglobulin G). ALP production by cultured MC3T3-E1 was enhanced in combination with PECAM-1+ EOMA (1.8 times as much as ALP production by MC3T3-E1 alone), but not in combination with PECAM-1- ISOS-1. Anti-PECAM-1 antibody inhibited the increase in ALP production under co-culture with EOMA.

CONCLUSIONS

Our data show that the reduced ALP activity after skeletal unloading is closely correlated with reduced expression of PECAM-1 in bone marrow cells. We speculate that the loss of osteogenic potential after skeletal unloading is caused by the suppression of PECAM-1 signaling on endothelial cellular surface.

MAPKs (ERK1/2, p38) and AKT can be phosphorylated by shear stress independently of platelet endothelial cell adhesion molecule-1 (CD31) in vascular endothelial cells

PECAM-1 (CD31) is a member of the Ig superfamily of cell adhesion molecules and is expressed on endothelial cells (EC) as several circulating blood elements including platelets, polymorphonuclear leukocytes, monocytes, and lymphocytes. PECAM-1 tyrosine phosphorylation has been observed following mechanical stimulation of EC but its role in mechanosensing is still incompletely understood. The aim of this study was to investigate the involvement of PECAM-1 in signaling cascades in response to fluid shear stress (SS) in vascular ECs. PECAM-1-deficient (KO) and PECAM-reconstituted murine microvascular ECs, 50 and 100% confluent bovine aortic EC (BAEC), and human umbilical vein EC (HUVEC) transfected with antisense PECAM-1 oligonucleotides were exposed to oscillatory SS (14 dynes/cm2) for 0, 5, 10, 30 or 60 min. The tyrosine phosphorylation level of PECAM-1 immunoprecipitated from SS-stimulated PECAM-reconstituted, but not PECAM-1-KO, murine ECs increased. Although PECAM-1 was phosphorylated in 100% confluent BAEC and HUVEC, its phosphorylation level in 50% confluent BAECs or HUVEC was not detected by SS. Likewise PECAM-1 phosphorylation was robust in the wild type and scrambled-transfected HUVEC but not in the PECAM-1 antisense-HUVEC. ERK(1/2), p38 MAPK, and AKT were activated by SS in all cell types tested, including the PECAM-1-KO murine ECs, 50% confluent BAECs, and HUVEC transfected with antisense PECAM-1. This suggests that PECAM-1 may not function as a major mechanoreceptor for activation of MAPK and AKT in ECs and that there are likely to be other mechanoreceptors in ECs functioning to detect shear stress and trigger intercellular signals.

Mechanics and function in heart morphogenesis

For years, biomechanical engineers have studied the physical forces involved in morphogenesis of the heart. In a parallel stream of research, molecular and developmental biologists have sought to identify the molecular pathways responsible for embryonic heart development. Recently, several studies have shown that these two avenues of research should be integrated to explain how genes expressed in the heart regulate early heart function and affect physical morphogenetic steps, as well as to conversely show how early heart function affects the expression of genes required for morphogenesis. This review combines the perspectives of biomechanical engineering and developmental biology to lay out an integrated view of the role of mechanical forces in heart development.

Platelet endothelial cell adhesion molecule in cell signaling and thrombosis

Platelet endothelial cell adhesion molecule (PECAM-1) is a member of the superfamily of immunoglobulins. This cell adhesion molecule has been implicated to mediate the adhesion and trans-endothelial migration of T lymphocytes/monocytes into the vascular wall, a critical step in the initiation of atherogenesis. Current thinking, however, posits that PECAM-1 by virtue of being a scaffolding molecule may well play a role in several signal transduction reactions. As a consequence, this cell adhesion molecule may be responsible for several biological and pathophysiological functions such as thrombosis, and inflammation. Evidence has also been put forward for a potential role of PECAM-1 in apoptosis and atherosclerosis. This article focuses on the structure of PECAM-1 and its role in intracellular signaling and implications in health and disease.

Not just angiogenesis--wider roles for the angiopoietins

Since the discovery of the angiopoietins, much interest has been focused on their biological actions and their potential use as therapeutic targets. It is generally accepted that the angiopoietins play an important role in angiogenesis and hence are described as angiogenic factors. However, it is becoming increasingly clear that this is not their only role and it is likely that the angiopoietins have important roles in a wider range of biological and pathological functions.

Insulin-like growth factor 1-induced interleukin-1 receptor II overrides the activity of interleukin-1 and controls the homeostasis of the extracellular matrix of cartilage

OBJECTIVE

We examined the effect of the insulin-like growth factor 1 (IGF-1)/IGF receptor I (IGFRI) autocrine/paracrine anabolic pathway on the extracellular matrix (ECM) of human chondrocytes and the mechanism by which IGF-1 reverses the catabolic effects of interleukin-1 (IL-1).

METHODS

Phenotypically stable human articular cartilage cells were obtained from normal cartilage and maintained in culture in alginate beads for 1 week to reach equilibrium of accumulated cell-associated matrix (CAM) compounds. Levels of CAM components aggrecan and type II collagen (CII) and levels of intracellular IGF-1, IL-1alpha, and IL-1beta and their respective plasma membrane-bound receptors IGFRI, IL-1 receptor I (IL-1RI), and the decoy receptor IL-1RII were assayed using flow cytometry to investigate the relationship between the autocrine/paracrine pathways and the homeostasis of ECM molecules in the CAM. The effects of IGF-1 on the expression of IGF-1, IL-1alpha, and IL-1beta and their respective receptor systems, the aggrecan core protein, and CII were determined by flow cytometry.

RESULTS

Cause-effect relationship experiments showed that IGF-1 up-regulates the levels of IGF-1, IGFRI, aggrecan, and CII in the CAM. No effects on the expression of IL-1alpha and IL-1beta and their signaling receptor IL-1RI were observed. However, IGF-1 was able to reverse IL-1beta-mediated degradation of aggrecan and the repression of the aggrecan synthesis rate. Interestingly, levels of aggrecan and CII in the CAM strongly correlated not only with IGF-1, but also with IL-1RII, which acts as a decoy receptor for IL-1alpha and IL-1beta. This suggests that IGF-1 and IL-1RII may cooperate in regulating ECM homeostasis. Additional experiments demonstrated that IGF-1 up-regulated IL-1RII, thereby overriding the catabolic effects of IL-1.

CONCLUSION

These findings reveal a new paradigm by which IGF-1 influences chondrocyte metabolism, by reversing the IL-1-mediated catabolic pathway through up-regulation of its decoy receptor.

Fluid shear stress and the vascular endothelium: for better and for worse

As blood flows, the vascular wall is constantly subjected to physical forces, which regulate important physiological blood vessel responses, as well as being implicated in the development of arterial wall pathologies. Changes in blood flow, thus generating altered hemodynamic forces are responsible for acute vessel tone regulation, the development of blood vessel structure during embryogenesis and early growth, as well as chronic remodeling and generation of adult blood vessels. The complex interaction of biomechanical forces, and more specifically shear stress, derived by the flow of blood and the vascular endothelium raise many yet to be answered questions:How are mechanical forces transduced by endothelial cells into a biological response, and is there a "shear stress receptor"?Are "mechanical receptors" and the final signaling pathways they evoke similar to other stimulus-response transduction systems?How do vascular endothelial cells differ in their response to physiological or pathological shear stresses?Can shear stress receptors or shear stress responsive genes serve as novel targets for the design of diagnostic and therapeutic modalities for cardiovascular pathologies?The current review attempts to bring together recent findings on the in vivo and in vitro responses of the vascular endothelium to shear stress and to address some of the questions raised above.

Relationship between lymph node sinuses with blood and lymphatic metastasis of gastric cancer

AIM: To elucidate the relationship between lymph node sinuses with blood and lymphatic metastasis of gastric cancer.

METHODS: Routine autopsy was carried out in the randomly selected 102 patients (among them 100 patients died of various diseases, and 2 patients died of non-diseased reasons), their superficial lymph nodes locating in bilateral necks (include supraclavicle), axilla, inguina, thorax, and abdomen were sampled. Haematoxylin-Eosin staining was performed on 10% formalin-fixed and paraffin-embedded lymph node tissue sections (5 μm). The histological patterns of the lymph sinuses containing blood were observed under light microscope. The expression of CD31, a marker for endothelial cell, was detected both in blood and non-blood containing lymph node sinuses with the method of immunohistochemistry.

RESULTS: Among the 1322 lymph nodes sampled from the autopsies of 100 diseased cases, lymph node sinuses containing blood were found in 809 lymph nodes sampled from 91 cases, but couldn’t be seen in the lymph nodes sampled from the non-diseased cases. According to histology, we divided the blood containing lymph node sinuses into five categories: vascular-opening sinus, blood-deficient sinus, erythrophago-sinus, blood-abundant sinus, vascular-formative sinus. Immunohistochemical findings showed that the expression of CD31 was strongly positive in vascular-formative sinuses and some vascular-opening sinuses while it was faint in blood-deficient sinuses, erythrophago-sinuses and some vascular-opening sinuses. It was almost negative in blood-abundant sinus and non-blood containing sinus.

CONCLUSION: In the state of disease, the phenomenon of blood present in the lymph sinus is not uncommon. Blood could possibly enter into the lymph sinuses through the lymphaticovenous communications between the veins and the sinuses in the node. Lymph circulation and the blood circulation could communicate with each other in the lymph node sinuses. The skipping and distal lymphatic metastasis of gastric cancer may have some connection with the blood containing lymph node sinuses.

Expression of inducible nitric oxide synthase in human umbilical vein endothelial cells during primary culture

The adaptive response of endothelial cells to stress may lead to the upregulation of nitric oxide (NO) production. Herein, we report inducible nitric oxide synthase (iNOS) induction in primary cultures of human umbilical vein endothelial cells (HUVEC). The enzyme expression was earlier observed in 12-h cultures, reaching maximal levels after 3 days and decreasing when cells become confluent. The time course of NO production by HUVEC paralleled iNOS expression during the whole culture period, indicating that enzyme was functionally active. Conversely, iNOS induction could not be further detected in HUVEC subcultures passed once from cells presenting maximal levels of iNOS expression in the primary culture. Induction of iNOS in HUVEC was not related to lipopolysaccharide contamination, since the enzyme expression was not affected in the presence of polymyxin B added to primary cultures. Further analysis showed that aminoguanidine, a specific iNOS inhibitor, did not affect cell proliferation, suggesting that the NO produced by HUVEC may not be directly related to cell growth. Platelet endothelial cell adhesion molecule-1 expression was upregulated during cell confluence, in contrast to the decrease of iNOS expression and activity. The data suggest that iNOS expression may be a molecular mechanism mediating the adaptive response of endothelial cells to culture environment.

Evidence for a role of platelet endothelial cell adhesion molecule-1 in endothelial cell mechanosignal transduction: is it a mechanoresponsive molecule?

Fluid shear stress (FSS) induces many forms of responses, including phosphorylation of extracellular signal-regulated kinase (ERK) in endothelial cells (ECs). We have earlier reported rapid tyrosine phosphorylation of platelet endothelial cell adhesion molecule-1 (PECAM-1) in ECs exposed to FSS. Osmotic changes also induced similar PECAM-1 and ERK phosphorylation with nearly identical kinetics. Because both FSS and osmotic changes should mechanically perturb the cell membrane, they might activate the same mechanosignaling cascade. When PECAM-1 is tyrosine phosphorylated by FSS or osmotic changes, SHP-2 binds to it. Here we show that ERK phosphorylation by FSS or osmotic changes depends on PECAM-1 tyrosine phosphorylation, SHP-2 binding to phospho-PECAM-1, and SHP-2 phosphatase activity. In ECs under flow, detectable amounts of SHP-2 and Gab1 translocated from the cytoplasm to the EC junction. When magnetic beads coated with antibodies against the extracellular domain of PECAM-1 were attached to ECs and tugged by magnetic force for 10 min, PECAM-1 associated with the beads was tyrosine phosphorylated. ERK was also phosphorylated in these cells. Binding of the beads by itself or pulling on the cell surface using poly-l-coated beads did not induce phosphorylation of PECAM-1 and ERK. These results suggest that PECAM-1 is a mechanotransduction molecule.

VEGF receptor 2 and the adherens junction as a mechanical transducer in vascular endothelial cells

Blood-flow interactions with the vascular endothelium represents a specialized example of mechanical regulation of cell function that has important physiological and pathophysiological cardiovascular consequences. Yet, the mechanisms of mechanostransduction are not understood fully. This study shows that shear stress induces a rapid induction as well as nuclear translocation of the vascular endothelial growth factor (VEGF) receptor 2 and promotes the binding of the VEGF receptor 2 and the adherens junction molecules, VE-cadherin and beta-catenin, to the endothelial cytoskeleton. These changes are accompanied by the formation of a complex containing the VEGF receptor 2-VE-cadherin-beta-catenin. In endothelial cells lacking VE-cadherin, shear stress did not augment nuclear translocation of the VEGF receptor 2 and phosphorylation of Akt1 and P38 as well as transcriptional induction of a reporter gene regulated by a shear stress-responsive promoter. These results suggest that VEGF receptor 2 and the adherens junction act as shear-stress cotransducers, mediating the transduction of shear-stress signals into vascular endothelial cells.