αvβ3 Integrins Mediate Flow-Induced NF-κB Activation, Proinflammatory Gene Expression, and Early Atherogenic Inflammation

Integrin activation by the lipid molecule 25-hydroxycholesterol induces a proinflammatory response

Integrins are components of cell-matrix adhesions, and function as scaffolds for various signal transduction pathways. So far no lipid ligand for integrin has been reported. Here we show that a lipid, oxysterol 25-hydroxycholesterol (25HC), directly binds to α5β1 and αvβ3 integrins to activate integrin-focal adhesion kinase (FAK) signaling. Treatment of macrophages and epithelial cells with 25HC results in an increase in activated αvβ3 integrin in podosome and focal adhesion matrix adhesion sites. Moreover, activation of pattern recognition receptor on macrophages induces secretion of 25HC, triggering integrin signaling and the production of proinflammatory cytokines such as TNF and IL-6. Thus, the lipid molecule 25HC is a physiologically relevant activator of integrins and is involved in positively regulating proinflammatory responses. Our data suggest that extracellular 25HC links innate immune inflammatory response with integrin signaling.

Shear-Induced CCN1 Promotes Atheroprone Endothelial Phenotypes and Atherosclerosis

BACKGROUND

Atherosclerosis occurs preferentially at the blood vessels encountering blood flow turbulence. The matricellular protein CCN1 is induced in endothelial cells by disturbed flow, and is expressed in advanced atherosclerotic lesions in patients and in the Apoe^-/- mouse model. The role of CCN1 in atherosclerosis remains undefined.

METHODS

To assess the function of CCN1 in vivo, knock-in mice carrying the integrin α6β1-binding-defective mutant allele Ccn1-dm on the Apoe^-/- background were tested in an atherosclerosis model generated by carotid artery ligation. Additionally, CCN1-regulated functional phenotypes of human umbilical vein endothelial cells, or primary mouse aortic endothelial cells isolated from wild-type and Ccn1 ^dm/dm mice, were investigated in the in vitro shear stress experiments under unidirectional laminar shear stress (12 dyn/cm²) versus oscillatory shear stress (±5 dyn/cm²) conditions.

RESULTS

We found that Ccn1 expression was upregulated in the arterial endothelium 3 days after ligation before any detectable structural changes, and intensified with the progression of atherosclerotic lesions. Compared with Apoe^-/- controls, Ccn1 ^dm/dm/ Apoe^-/- mice were remarkably resistant to ligation-induced plaque formation (n=6). These mice exhibited lower oxidative stress, expression of endothelin-1 and monocyte chemoattractant protein-1, and monocyte homing. CCN1/α6β1 critically mediated flow-induced activation of the pleiotropic transcription factor nuclear factor-κB and therefore the induction of atheroprone gene expression in the mouse arterial endothelium after ligation (n=6), or in cultured human umbilical vein endothelial cells or primary mouse aortic endothelial cells exposed to oscillatory shear stress (n=3 in triplicate). Interestingly, the activation of nuclear factor-κB by CCN1/α6β1 signaling prompted more production of CCN1 and α6β1. Blocking CCN1-α6β1 binding by the Ccn1-dm mutation or by T1 peptide (derived from an α6β1-binding sequence of CCN1) disrupted the positive-feedback regulation between CCN1/α6β1 and nuclear factor-κB, and prevented flow-induced atheroprone phenotypic alterations in endothelial cells or atherosclerosis in mice.

CONCLUSIONS

These data demonstrate a causative role of CCN1 in atherosclerosis via modulating endothelial phenotypes. CCN1 binds to its receptor integrin α6β1 to activate nuclear factor-κB, thereby instigating a vicious circle to persistently promote atherogenesis. T1, a peptide antagonist selectively targeting CCN1-α6β1, can be further optimized for developing T1-mimetics to treat atherosclerosis.

c-Abl regulates YAPY357 phosphorylation to activate endothelial atherogenic responses to disturbed flow

Local flow patterns determine the uneven distribution of atherosclerotic lesions. This research aims to elucidate the mechanism of regulation of nuclear translocation of Yes-associated protein (YAP) under oscillatory shear stress (OSS) in the atheroprone phenotype of endothelial cells (ECs). We report here that OSS led to tyrosine phosphorylation and strong, continuous nuclear translocation of YAP in ECs that is dependent on integrin α5β1 activation. YAP overexpression in ECs blunted the anti-atheroprone effect of an integrin α5β1-blocking peptide (ATN161) in Apoe-/- mice. Activation of integrin α5β1 induced tyrosine, but not serine, phosphorylation of YAP in ECs. Blockage of integrin α5β1 with ATN161 abolished the phosphorylation of YAP at Y357 induced by OSS. Mechanistic studies showed that c-Abl inhibitor attenuated the integrin α5β1-induced YAP tyrosine phosphorylation. Furthermore, the phosphorylation of c-Abl and YAPY357 was significantly increased in ECs in atherosclerotic vessels of mice and in human plaques versus normal vessels. Finally, bosutinib, a tyrosine kinase inhibitor, markedly reduced the level of YAPY357 and the development of atherosclerosis in Apoe-/- mice. The c-Abl/YAPY357 pathway serves as a mechanism for the activation of integrin α5β1 and the atherogenic phenotype of ECs in response to OSS, and provides a potential therapeutic strategy for atherogenesis.

3D micro-environment regulates NF-κβ dependent adhesion to induce monocyte differentiation

Differentiation of monocytes entails their relocation from blood to the tissue, hence accompanied by an altered physicochemical micro-environment. While the mechanism by which the biochemical make-up of the micro-environment induces differentiation is known, the fluid-like to gel-like transition in the physical micro-environment is not well understood. Monocytes maintain non-adherent state to prevent differentiation. We establish that irrespective of the chemical makeup, a 3D gel-like micro-environment induces a positive-feedback loop of adhesion-MAPK-NF-κβ activation to facilitate differentiation. In 2D fluid-like micro-environment, adhesion alone is capable of inducing differentiation via the same positive-feedback signaling. Chemical inducer treatment in fluid-like micro-environment, increases the propensity of monocyte adhesion via a brief pulse of p-MAPK. The adhesion subsequently elicit differentiation, establishing that adhesion is both necessary and sufficient to induce differentiation in 2D/3D micro-environment. MAPK, and NF-κβ being key molecules of multiple signaling pathways, we hypothesize that biochemically inert 3D gel-like micro-environment would also influence other cellular functions.

Piezo1 and Gq/G11 promote endothelial inflammation depending on flow pattern and integrin activation

Atherosclerosis preferentially develops in areas of disturbed flow. Albarrán-Juárez et al. provide evidence that this depends on at least two different endothelial mechanosignaling pathways, a flow direction-independent pathway involving Piezo1 and G_q/G₁₁, as well as integrin signaling, which is only initiated in response to disturbed flow.

The vascular endothelium is constantly exposed to mechanical forces, including fluid shear stress exerted by the flowing blood. Endothelial cells can sense different flow patterns and convert the mechanical signal of laminar flow into atheroprotective signals, including eNOS activation, whereas disturbed flow in atheroprone areas induces inflammatory signaling, including NF-κB activation. How endothelial cells distinguish different flow patterns is poorly understood. Here we show that both laminar and disturbed flow activate the same initial pathway involving the mechanosensitive cation channel Piezo1, the purinergic P2Y₂ receptor, and G_q/G₁₁-mediated signaling. However, only disturbed flow leads to Piezo1- and G_q/G₁₁-mediated integrin activation resulting in focal adhesion kinase-dependent NF-κB activation. Mice with induced endothelium-specific deficiency of Piezo1 or Gα_q/Gα₁₁ show reduced integrin activation, inflammatory signaling, and progression of atherosclerosis in atheroprone areas. Our data identify critical steps in endothelial mechanotransduction, which distinguish flow pattern-dependent activation of atheroprotective and atherogenic endothelial signaling and suggest novel therapeutic strategies to treat inflammatory vascular disorders such as atherosclerosis.

p21-activated kinase 4 as a switch between caspase-8 apoptosis and NF-κB survival signals in response to TNF-α in hepatocarcinoma cells

PAK4 is overexpressed in a variety of human cancers and considered a promising candidate for therapeutic target. However, its functions remain poorly understood, especially in liver carcinogenesis which could be triggered by inflammation. In the present study, endogenous PAK4 was knockdown using siRNA in HepG2 and SK-Hep1 cells. The two cell lines performed reduced cell viability, altered cell cycle composed of decreased S and arrest in G2, and apoptosis. Meanwhile, expression of NF-κB p65 in the nuclei and caspase-8 activity did not show significant differences from control. However, after treating cells with TNF-α, an inflammatory cytokine, we investigated repressed nuclear expression and localization of NF-κB p65, and induced apoptosis with increased caspase-8 activity in PAK4-knockdown cells. The findings revealed that ablation of PAK4 inhibited cell viability via blocking cell cycle and progressing apoptosis. The apoptosis was partially dependent upon caspase-8 concomitant with attenuated NF-κB survival signal due to stimulus of TNF-α. It suggests that PAK4 as target is a switch between caspase-8 apoptosis and NF-κB survival signals induced by TNF-α in hepatocarcinoma cells.

De novo designed transmembrane peptides activating the α5β1 integrin

Computationally designed transmembrane α-helical peptides (CHAMP) have been used to compete for helix-helix interactions within the membrane, enabling the ability to probe the activation of the integrins αIIbβ3 and αvβ3. Here, this method is extended towards the design of CHAMP peptides that inhibit the association of the α5β1 transmembrane (TM) domains, targeting the Ala-X3-Gly motif within α5. Our previous design algorithm was performed alongside a new workflow implemented within the widely used Rosetta molecular modeling suite. Peptides from each computational approach activated integrin α5β1 but not αVβ3 in human endothelial cells. Two CHAMP peptides were shown to directly associate with an α5 TM domain peptide in detergent micelles to a similar degree as a β1 TM peptide does. By solution-state nuclear magnetic resonance, one of these CHAMP peptides was shown to bind primarily the integrin β1 TM domain, which itself has a Gly-X3-Gly motif. The second peptide associated modestly with both α5 and β1 constructs, with slight preference for α5. Although the design goal was not fully realized, this work characterizes novel CHAMP peptides activating α5β1 that can serve as useful reagents for probing integrin biology.

Low levels of physiological interstitial flow eliminate morphogen gradients and guide angiogenesis

Convective transport can significantly distort spatial concentration gradients. Interstitial flow is ubiquitous throughout living tissue, but our understanding of how interstitial flow affects concentration gradients in biological processes is limited. Interstitial flow is of particular interest for angiogenesis because pathological and physiological angiogenesis is associated with altered interstitial flow, and both interstitial flow and morphogen gradients (e.g., vascular endothelial growth factor, VEGF) can potentially stimulate and guide new blood vessel growth. We designed an in vitro microfluidic platform to simulate 3D angiogenesis in a tissue microenvironment that precisely controls interstitial flow and spatial morphogen gradients. The microvascular tissue was developed from endothelial colony forming cell-derived endothelial cells extracted from cord blood and stromal fibroblasts in a fibrin extracellular matrix. Pressure in the microfluidic lines was manipulated to control the interstitial flow. A mathematical model of mass and momentum transport, and experimental studies with fluorescently labeled dextran were performed to validate the platform. Our data demonstrate that at physiological interstitial flow (0.1-10 μm/s), morphogen gradients were eliminated within hours, and angiogenesis demonstrated a striking bias in the opposite direction of interstitial flow. The interstitial flow-directed angiogenesis was dependent on the presence of VEGF, and the effect was mediated by αvβ3 integrin. We conclude that under physiological conditions, growth factors such as VEGF and fluid forces work together to initiate and spatially guide angiogenesis.

Vascular Smooth Muscle Cells and Arterial Stiffening: Relevance in Development, Aging, and Disease

The cushioning function of large arteries encompasses distension during systole and recoil during diastole which transforms pulsatile flow into a steady flow in the microcirculation. Arterial stiffness, the inverse of distensibility, has been implicated in various etiologies of chronic common and monogenic cardiovascular diseases and is a major cause of morbidity and mortality globally. The first components that contribute to arterial stiffening are extracellular matrix (ECM) proteins that support the mechanical load, while the second important components are vascular smooth muscle cells (VSMCs), which not only regulate actomyosin interactions for contraction but mediate also mechanotransduction in cell-ECM homeostasis. Eventually, VSMC plasticity and signaling in both conductance and resistance arteries are highly relevant to the physiology of normal and early vascular aging. This review summarizes current concepts of central pressure and tensile pulsatile circumferential stress as key mechanical determinants of arterial wall remodeling, cell-ECM interactions depending mainly on the architecture of cytoskeletal proteins and focal adhesion, the large/small arteries cross-talk that gives rise to target organ damage, and inflammatory pathways leading to calcification or atherosclerosis. We further speculate on the contribution of cellular stiffness along the arterial tree to vascular wall stiffness. In addition, this review provides the latest advances in the identification of gene variants affecting arterial stiffening. Now that important hemodynamic and molecular mechanisms of arterial stiffness have been elucidated, and the complex interplay between ECM, cells, and sensors identified, further research should study their potential to halt or to reverse the development of arterial stiffness.

Flow pattern-dependent endothelial cell responses through transcriptional regulation

Blood flow provides endothelial cells (ECs) lining the inside of blood vessels with mechanical stimuli as well as humoral stimuli. Fluid shear stress, the frictional force between flowing blood and ECs, is recognized as an essential mechanical cue for vascular growth, remodeling, and homeostasis. ECs differentially respond to distinct flow patterns. High laminar shear flow leads to inhibition of cell cycle progression and stabilizes vessels, whereas low shear flow or disturbed flow leads to increased turnover of ECs and inflammatory responses of ECs prone to atherogenic. These differences of EC responses dependent on flow pattern are mainly ascribed to distinct patterns of gene expression. In this review, we highlight flow pattern-dependent transcriptional regulation in ECs by focusing on KLF2 and NFκB, major transcription factors responding to laminar flow and disturbed flow, respectively. Moreover, we introduce roles of a new flow-responsive transcriptional co-regulator, YAP, in blood vessel maintenance and discuss how these transcriptional regulators are spatiotemporally regulated by flow and then regulate EC functions in normal and pathological conditions.

Molecular Sensors of Blood Flow in Endothelial Cells

Mechanical stress from blood flow has a significant effect on endothelial physiology, with a key role in initiating vasoregulatory signals. Disturbances in blood flow, such as in regions of disease-associated stenosis, arterial branch points, and sharp turns, can induce proatherogenic phenotypes in endothelial cells. The disruption of vascular homeostasis as a result of endothelial dysfunction may contribute to early and late stages of atherosclerosis, the underlying cause of coronary artery disease. In-depth knowledge of the mechanobiology of endothelial cells is essential to identifying mechanosensory complexes involved in the pathogenesis of atherosclerosis. In this review, we describe different blood flow patterns and summarize current knowledge on mechanosensory molecules regulating endothelial vasoregulatory functions, with clinical implications. Such information may help in the search for novel therapeutic approaches.

The arterial microenvironment: the where and why of atherosclerosis

The formation of atherosclerotic plaques in the large and medium sized arteries is classically driven by systemic factors, such as elevated cholesterol and blood pressure. However, work over the past several decades has established that atherosclerotic plaque development involves a complex coordination of both systemic and local cues that ultimately determine where plaques form and how plaques progress. Although current therapeutics for atherosclerotic cardiovascular disease primarily target the systemic risk factors, a large array of studies suggest that the local microenvironment, including arterial mechanics, matrix remodelling and lipid deposition, plays a vital role in regulating the local susceptibility to plaque development through the regulation of vascular cell function. Additionally, these microenvironmental stimuli are capable of tuning other aspects of the microenvironment through collective adaptation. In this review, we will discuss the components of the arterial microenvironment, how these components cross-talk to shape the local microenvironment, and the effect of microenvironmental stimuli on vascular cell function during atherosclerotic plaque formation.

Integrin signaling in atherosclerosis

Atherosclerosis, a chronic lipid-driven inflammatory disease affecting large arteries, represents the primary cause of cardiovascular disease in the world. The local remodeling of the vessel intima during atherosclerosis involves the modulation of vascular cell phenotype, alteration of cell migration and proliferation, and propagation of local extracellular matrix remodeling. All of these responses represent targets of the integrin family of cell adhesion receptors. As such, alterations in integrin signaling affect multiple aspects of atherosclerosis, from the earliest induction of inflammation to the development of advanced fibrotic plaques. Integrin signaling has been shown to regulate endothelial phenotype, facilitate leukocyte homing, affect leukocyte function, and drive smooth muscle fibroproliferative remodeling. In addition, integrin signaling in platelets contributes to the thrombotic complications that typically drive the clinical manifestation of cardiovascular disease. In this review, we examine the current literature on integrin regulation of atherosclerotic plaque development and the suitability of integrins as potential therapeutic targets to limit cardiovascular disease and its complications.

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.

Blood Brothers: Hemodynamics and Cell-Matrix Interactions in Endothelial Function

SIGNIFICANCE

Alterations in endothelial function contribute to a variety of vascular diseases. In pathological conditions, the endothelium shows a reduced ability to regulate vasodilation (endothelial dysfunction) and a conversion toward a proinflammatory and leaky phenotype (endothelial activation). At the interface between the vessel wall and blood, the endothelium exists in a complex microenvironment and must translate changes in these environmental signals to alterations in vessel function. Mechanical stimulation and endothelial cell interactions with the vascular matrix, as well as a host of soluble factors, coordinately contribute to this dynamic regulation.

RECENT ADVANCES

Blood hemodynamics play an established role in the regulation of endothelial function. However, a growing body of work suggests that subendothelial matrix composition similarly and coordinately regulates endothelial cell phenotype such that blood flow affects matrix remodeling, which affects the endothelial response to flow.

CRITICAL ISSUES

Hemodynamics and soluble factors likely affect endothelial matrix remodeling through multiple mechanisms, including transforming growth factor β signaling and alterations in cell-matrix receptors, such as the integrins. Likewise, differential integrin signaling following matrix remodeling appears to regulate several key flow-induced responses, including nitric oxide production, regulation of oxidant stress, and activation of proinflammatory signaling and gene expression. Microvascular remodeling responses, such as angiogenesis and arteriogenesis, may also show coordinated regulation by flow and matrix.

FUTURE DIRECTIONS

Identifying the mechanisms regulating the dynamic interplay between hemodynamics and matrix remodeling and their contribution to the pathogenesis of cardiovascular disease remains an important research area with therapeutic implications across a variety of conditions. Antioxid. Redox Signal. 25, 415-434.

Endothelial erosion of plaques as a substrate for coronary thrombosis

Myocardial infarction is a prevalent, life-threatening consequence of athero-thrombosis. Post-mortem histology and intravascular imaging in live patients have shown that approximately one third of myocardial infarctions are caused by a thrombus overlying an intact, non-ruptured atherosclerotic plaque. Histology identifies erosion of luminal endothelial cells from smooth muscle and proteoglycan-rich, thick fibrous cap atheromas as the underlying pathology. Unlike plaque ruptures, endothelial erosions tend to occur on thick-capped atherosclerotic plaques and may or may not be associated with inflammation. Smoking and female gender are strong risk factors for erosion. Multiple mechanisms may contribute to endothelial erosion, including endothelial dysfunction, TLR signalling, leukocyte activation and modification of sub-endothelial matrix by endothelial or smooth muscle cells, which may trigger loss of adhesion to the extracellular matrix or endothelial apoptosis. Diagnosis of endothelial erosion by intravascular imaging, especially high resolution optical coherence tomography, may influence treatment strategies, offering prognostic value and utility as an endpoint in trials of agents designed to preserve an intact coronary endothelium.