Sequential activation of RhoA and FAK/paxillin leads to ATP release and actin reorganization in human endothelium

An optogenetic model reveals cell shape regulation through FAK and fascin

Cell shape regulation is important, but the mechanisms that govern shape are not fully understood, in part due to limited experimental models in which cell shape changes and underlying molecular processes can be rapidly and non-invasively monitored in real time. Here, we used an optogenetic tool to activate RhoA in the middle of mononucleated macrophages to induce contraction, resulting in a side with the nucleus that retained its shape and a non-nucleated side that was unable to maintain its shape and collapsed. In cells overexpressing focal adhesion kinase (FAK; also known as PTK2), the non-nucleated side exhibited a wide flat morphology and was similar in adhesion area to the nucleated side. In cells overexpressing fascin, an actin-bundling protein, the non-nucleated side assumed a spherical shape and was similar in height to the nucleated side. This effect of fascin was also observed in fibroblasts even without inducing furrow formation. Based on these results, we conclude that FAK and fascin work together to maintain cell shape by regulating adhesion area and height, respectively, in different cell types. This article has an associated First Person interview with the first author of the paper.

Network Pharmacology Reveals the Mechanism of Activity of Tongqiao Huoxue Decoction Extract Against Middle Cerebral Artery Occlusion-Induced Cerebral Ischemia-Reperfusion Injury

Several clinical therapies such as tissue repair by replacing injured tissues with functional ones have been reported; however, there is great potential for exploring traditional herbal-induced regeneration with good safety. Tongqiao Huoxue Decoction (TQHXD), a well-known classical traditional Chinese medicinal formula, has been widely used for clinical treatment of stroke. However, biological activity and mechanisms of action of its constituents toward conferring protection against cerebral ischemia-reperfusion (I/R) injury remain unclear. In this present study, we evaluated TQHXD quality using HPLC; then, it was screened for its potential active ingredients using a series of indices, such as their drug-likeness and oral bioavailability. Subsequently, we analyzed the potential mechanisms of TQHXD anti-I/R using gene ontology functional enrichment analyses. The network pharmacological approach enabled us to screen 265 common targets associated with I/R, indicating that TQHXD had remarkable protective effects on infarction volume, neurological function scores, and blood-brain barrier (BBB) injury. In addition, TQHXD significantly promoted the recovery of regional cerebral blood flow (rCBF) 7 days after reperfusion compared to rats in the vehicle group. Immunofluorescence results revealed a significantly higher CD34 expression in TQHXD-treated rats 7 days after reperfusion. TQHXD is not merely effective but eventually develops a secretory profile composed of VEGF and cerebral blood flow, a typical signature termed the angiogenesis-associated phenotype. Mechanistically, our data revealed that TQHXD (6 g/kg) treatment resulted in a marked increase in expression of p-focal adhesion kinase (FAK) and p-Paxillin proteins. However, Ki8751-mediated inhibition of VEGFR2 activity repealed its angiogenesis and protective effects and decreased both p-FAK and p-Paxillin protein levels. Taken together, these findings affirmed the potential of TQHXD as a drug for the management of stroke, which might be exerted by increasing the angiogenesis via the VEGF pathway. Therefore, these results provide proof-of-concept evidence that angiogenesis is a major contributor to TQHXD-treated I/R and that TQHXD is a promising traditional ethnic medicine for the management of this condition.

P2Y Purinergic Receptors, Endothelial Dysfunction, and Cardiovascular Diseases

Purinergic G-protein-coupled receptors are ancient and the most abundant group of G-protein-coupled receptors (GPCRs). The wide distribution of purinergic receptors in the cardiovascular system, together with the expression of multiple receptor subtypes in endothelial cells (ECs) and other vascular cells demonstrates the physiological importance of the purinergic signaling system in the regulation of the cardiovascular system. This review discusses the contribution of purinergic P2Y receptors to endothelial dysfunction (ED) in numerous cardiovascular diseases (CVDs). Endothelial dysfunction can be defined as a shift from a “calm” or non-activated state, characterized by low permeability, anti-thrombotic, and anti-inflammatory properties, to a “activated” state, characterized by vasoconstriction and increased permeability, pro-thrombotic, and pro-inflammatory properties. This state of ED is observed in many diseases, including atherosclerosis, diabetes, hypertension, metabolic syndrome, sepsis, and pulmonary hypertension. Herein, we review the recent advances in P2Y receptor physiology and emphasize some of their unique signaling features in pulmonary endothelial cells.

Dynamic actin remodeling in response to lysophosphatidic acid

Lysophosphatidic acid (LPA) is a multifunctional regulator of actin cytoskeleton that exerts a dramatic impact on the actin cytoskeleton to build a platform for diverse cellular processes including growth cone guidance, neurite retraction and cell motility. It has been implicated in the formation and dissociation of complexes between actin and actin binding proteins, supporting its role in actin remodeling. Several studies point towards its ability to facilitate formation of special cellular structures including focal adhesions and actin stress fibres by phosphoregulation of several actin associated proteins and their multiple regulatory kinases and phosphatases. In addition, multiple levels of crosstalk among the signaling cascades activated by LPA, affect actin cytoskeleton-mediated cell migration and chemotaxis which in turn play a crucial role in cancer metastasis. In the current review, we have attempted to highlight the role of LPA as an actin modulator which functions by controlling activities of specific cellular proteins that underlie mechanisms employed in cytoskeletal and pathophysiological events within the cell. Further studies on the actin affecting/remodeling activity of LPA in different cell types will no doubt throw up many surprises essential to gain a full understanding of its contribution in physiological processes as well as in diseases.Communicated by Ramaswamy H. Sarma.

Paracrine signalling of AGR2 stimulates RhoA function in fibroblasts and modulates cell elongation and migration

The most prominent cancer-associated fibroblasts (CAFs) in tumor stroma is known to form a protective structure to support tumor growth. Anterior gradient-2 (AGR2), a tumor secretory protein is believed to play a pivotal role during tumor microenvironment (TME) development. Here, we report that extracellular AGR2 enhances fibroblasts elongation and migration significantly. The early stimulation of RhoA showed the association of AGR2 by upregulation of G1-S phase-regulatory protein cyclin D1 and FAK phosphorylation through fibroblasts growth factor receptor (FGFR) and vascular endothelial growth factor receptor (VEGFR). Our finding indicates that secretory AGR2 alters fibroblasts elongation, migration, and organization suggesting the secretory AGR2 as a potential molecular target that might be responsible to alter fibroblasts infiltration to support tumor growth.

Altering integrin engagement regulates membrane localization of Kir2.1 channels

How ion channels localize and distribute on the cell membrane remains incompletely understood. We show that interventions that vary cell adhesion proteins and cell size also affect the membrane current density of inward-rectifier K+ channels (Kir2.1; encoded by KCNJ2) and profoundly alter the action potential shape of excitable cells. By using micropatterning to manipulate the localization and size of focal adhesions (FAs) in single HEK293 cells engineered to stably express Kir2.1 channels or in neonatal rat cardiomyocytes, we establish a robust linear correlation between FA coverage and the amplitude of Kir2.1 current at both the local and whole-cell levels. Confocal microscopy showed that Kir2.1 channels accumulate in membrane proximal to FAs. Selective pharmacological inhibition of key mediators of protein trafficking and the spatially dependent alterations in the dynamics of Kir2.1 fluorescent recovery after photobleaching revealed that the Kir2.1 channels are transported to the cell membrane uniformly, but are preferentially internalized by endocytosis at sites that are distal from FAs. Based on these results, we propose adhesion-regulated membrane localization of ion channels as a fundamental mechanism of controlling cellular electrophysiology via mechanochemical signals, independent of the direct ion channel mechanogating.

ROS-dependent activation of RhoA/Rho-kinase in pulmonary artery: Role of Src-family kinases and ARHGEF1

The role of reactive oxygen species (ROS) in smooth muscle contraction is poorly understood. We hypothesised that G-protein coupled receptor (GPCR) activation and hypoxia induce Rho-kinase activity and contraction in rat intra-pulmonary artery (IPA) via stimulation of ROS production and subsequent Src-family kinase (SrcFK) activation. The T-type prostanoid receptor agonist U46619 induced ROS production in pulmonary artery smooth muscle cells (PASMC). U46619 also induced c-Src cysteine oxidation, SrcFK auto-phosphorylation, MYPT-1 and MLC₂₀ phosphorylation and contraction in IPA, and all these responses were inhibited by antioxidants (ebselen, Tempol). Contraction and SrcFK/MYPT-1/MLC₂₀ phosphorylations were also inhibited by combined superoxide dismutase and catalase, or by the SrcFK antagonist PP2, while contraction and MYPT-1/MLC₂₀ phosphorylations were inhibited by the Rho guanine nucleotide exchange factor (RhoGEF) inhibitor Y16. H₂O₂ and the superoxide-generating quinoledione LY83583 both induced c-Src oxidation, SrcFK auto-phosphorylation and contraction in IPA. LY83583 and H₂O₂-induced contractions were inhibited by PP2, while LY83583-induced contraction was also inhibited by antioxidants and Y16. SrcFK auto-phosphorylation and MYPT-1/MLC₂₀ phosphorylation was also induced by hypoxia in IPA and this was blocked by mitochondrial inhibitors rotenone and myxothiazol. In live PASMC, sub-cellular translocation of RhoA and the RhoGEF ARHGEF1 was triggered by both U46619 and LY83583 and this translocation was blocked by antioxidants and PP2. RhoA translocation was also inhibited by an ARHGEF1 siRNA. U46619 enhanced ROS-dependent co-immunoprecipitation of ARHGEF1 with c-Src. Our results demonstrate a link between GPCR-induced cytosolic ROS or hypoxia-induced mitochondrial ROS and SrcFK activity, Rho-kinase activity and contraction. ROS and SrcFK activate RhoA via ARHGEF1.

Paxillin: a crossroad in pathological cell migration

Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.

Mechanosensitive Molecular Networks Involved in Transducing Resistance Exercise-Signals into Muscle Protein Accretion

Loss of skeletal muscle myofibrillar protein with disease and/or inactivity can severely deteriorate muscle strength and function. Strategies to counteract wasting of muscle myofibrillar protein are therefore desirable and invite for considerations on the potential superiority of specific modes of resistance exercise and/or the adequacy of low load resistance exercise regimens as well as underlying mechanisms. In this regard, delineation of the potentially mechanosensitive molecular mechanisms underlying muscle protein synthesis (MPS), may contribute to an understanding on how differentiated resistance exercise can transduce a mechanical signal into stimulation of muscle accretion. Recent findings suggest specific upstream exercise-induced mechano-sensitive myocellular signaling pathways to converge on mammalian target of rapamycin complex 1 (mTORC1), to influence MPS. This may e.g. implicate mechanical activation of signaling through a diacylglycerol kinase (DGKζ)-phosphatidic acid (PA) axis or implicate integrin deformation to signal through a Focal adhesion kinase (FAK)-Tuberous Sclerosis Complex 2 (TSC2)-Ras homolog enriched in brain (Rheb) axis. Moreover, since initiation of translation is reliant on mRNA, it is also relevant to consider potentially mechanosensitive signaling pathways involved in muscle myofibrillar gene transcription and whether some of these pathways converge with those affecting mTORC1 activation for MPS. In this regard, recent findings suggest how mechanical stress may implicate integrin deformation and/or actin dynamics to signal through a Ras homolog gene family member A protein (RhoA)-striated muscle activator of Rho signaling (STARS) axis or implicate deformation of Notch to affect Bone Morphogenetic Protein (BMP) signaling through a small mother of decapentaplegic (Smad) axis.

Shear-Sensitive Genes in Aortic Valve Endothelium

SIGNIFICANCE

Currently, calcific aortic valve disease (CAVD) is only treatable through surgical intervention because the specific mechanisms leading to the disease remain unclear. In this review, we explore the forces and structure of the valve, as well as the mechanosensors and downstream signaling in the valve endothelium known to contribute to inflammation and valve dysfunction.

RECENT ADVANCES

While the valvular structure enables adaptation to dynamic hemodynamic forces, these are impaired during CAVD, resulting in pathological systemic changes. Mechanosensing mechanisms-proteins, sugars, and membrane structures-at the surface of the valve endothelial cell relay mechanical signals to the nucleus. As a result, a large number of mechanosensitive genes are transcribed to alter cellular phenotype and, ultimately, induce inflammation and CAVD. Transforming growth factor-β signaling and Wnt/β-catenin have been widely studied in this context. Importantly, NADPH oxidase and reactive oxygen species/reactive nitrogen species signaling has increasingly been recognized to play a key role in the cellular response to mechanical stimuli. In addition, a number of valvular microRNAs are mechanosensitive and may regulate the progression of CAVD.

CRITICAL ISSUES

While numerous pathways have been described in the pathology of CAVD, no treatment options are available to avoid surgery for advanced stenosis and calcification of the aortic valve. More work must be focused on this issue to lead to successful therapies for the disease.

FUTURE DIRECTIONS

Ultimately, a more complete understanding of the mechanisms within the aortic valve endothelium will lead us to future therapies important for treatment of CAVD without the risks involved with valve replacement or repair. Antioxid. Redox Signal. 25, 401-414.

Matrix stiffness exerts biphasic control over monocyte-endothelial adhesion via Rho-mediated ICAM-1 clustering

Leukocyte-endothelial adhesion is a critical early step in chronic vascular inflammation associated with diabetes, emphysema, and aging. Importantly, these conditions are also marked by abnormal subendothelial matrix crosslinking (stiffness). Yet, whether and how abnormal matrix stiffness contributes to leukocyte-endothelial adhesion remains poorly understood. Using a co-culture of human monocytic cells and human microvascular endothelial cells (ECs) grown on matrices of tunable stiffness, we demonstrate that matrix stiffness exerts biphasic control over monocyte-EC adhesion, with both matrix softening and stiffening eliciting a two-fold increase in this adhesive interaction. This preferential endothelial adhesivity on softer and stiffer matrices was consistent with a significant increase in α-actinin-4-associated endothelial ICAM-1 clustering, a key determinant of monocyte-EC adhesion. Further, the enhanced ICAM-1 clustering on soft and stiff matrices correlated strongly with an increase in Rho activity and ROCK2 expression. Importantly, inhibition of Rho/ROCK activity blocked the effects of abnormal matrix stiffness on ICAM-1 clustering and monocyte-EC adhesion. Thus, these findings implicate matrix stiffness-dependent ICAM-1 clustering as an important regulator of vascular inflammation and provide the rationale for closely examining mechanotransduction pathways as new molecular targets for anti-inflammatory therapy.

Fyn Mediates High Glucose-Induced Actin Cytoskeleton Reorganization of Podocytes via Promoting ROCK Activation In Vitro

Fyn, a member of the Src family of tyrosine kinases, is a key regulator in cytoskeletal remodeling in a variety of cell types. Recent studies have demonstrated that Fyn is responsible for nephrin tyrosine phosphorylation, which will result in polymerization of actin filaments and podocyte damage. Thus detailed involvement of Fyn in podocytes is to be elucidated. In this study, we investigated the potential role of Fyn/ROCK signaling and its interactions with paxillin. Our results presented that high glucose led to filamentous actin (F-actin) rearrangement in podocytes, accompanied by paxillin phosphorylation and increased cell motility, during which Fyn and ROCK were markedly activated. Gene knockdown of Fyn by siRNA showed a reversal effect on high glucose-induced podocyte damage and ROCK activation; however, inhibition of ROCK had no significant effects on Fyn phosphorylation. These observations demonstrate that in vitro Fyn mediates high glucose-induced actin cytoskeleton remodeling of podocytes via promoting ROCK activation and paxillin phosphorylation.

A bibenzyl from Dendrobium ellipsophyllum inhibits migration in lung cancer cells

Metastatic cancer cells have been shown to have aggressive behaviors accounting for the high incidence of chemotherapeutic failure and mortality. Because migration and invasion are crucial behaviors for cancer cell dissemination, promising compounds exhibiting potential antimigration effects are of interest for metastasis-based therapeutic approaches. This study aimed to evaluate the activity of a bibenzyl, 4,5,4'-trihydroxy-3,3'-dimethoxybibenzyl (TDB), isolated from Dendrobium ellipsophyllum Tang and Wang, in the suppression of migration in human lung cancer cells. TDB at nontoxic concentrations (1 and 5 µM) significantly inhibited the motility of lung cancer cells in scratch-wound assay. Chemotaxis-induced migration and invasion assays also revealed that the cell motility dramatically diminished in the cells treated with 1-5 µM TDB. Western blot analysis provided the underlying molecular mechanism, showing that TDB reduced such cell migration and invasion by decreasing migration-regulating proteins, including integrins αv, α4, β1, β3 and β5, as well as downstream signaling proteins, such as activated focal adhesion kinase (pFAK), activated Ras-related C3 botulinum toxin substrate 1 (Rac1-GTP) and cell division control protein 42 (Cdc42). As the presence of cellular protrusion, called filopodia, has been indicated as a hallmark of migrating cells, we showed that the reduction of the mentioned proteins correlated well with the disappearance of filopodia. In summary, this study demonstrates the promising activity of TDB and its mechanism in the inhibition of lung cancer cell migration, which might be useful for encouraging the development of this compound for antimetastatic approaches.

Hypoxia is an effective stimulus for vesicular release of ATP from human umbilical vein endothelial cells

Introduction

Hypoxia induces dilatation of the umbilical vein by releasing autocoids from endothelium; prostaglandins (PGs), adenosine and nitric oxide (NO) have been implicated. ATP is vasoactive, thus we tested whether hypoxia releases ATP from primary Human Umbilical Vein Endothelial Cells (HUVEC).

Methods

HUVEC were grown on inserts under no-flow conditions. ATP was assayed by luciferin–luciferase and visualised by quinacrine labeling. Intracellular Ca²⁺ ([Ca²⁺]_i) was imaged with Fura-2.

Results

ATP release occurred constitutively and was increased by hypoxia (PO₂: 150–8 mmHg), ∼10-fold more from apical, than basolateral surface. Constitutive ATP release was decreased, while hypoxia-induced release was abolished by brefeldin or monensin A, inhibitors of vesicular transport, and LY294002 or Y27632, inhibitors of phosphoinositide 3-kinases (PI₃K) and Rho-associated protein kinase (ROCK). ATP release was unaffected by NO donor, but increased by calcium ionophore, by >60-fold from apical, but <25% from basolateral surface. Hypoxia induced a small increase in [Ca²⁺]_i compared with ATP (10 μM); hypoxia inhibited the ATP response. Quinacrine-ATP fluorescent loci in the perinuclear space, were diminished by hypoxia and monensin, whereas brefeldin A increased fluorescence intensity, consistent with inhibition of anterograde transport.

Discussion.

Hypoxia within the physiological range releases ATP from HUVEC, particularly from apical/adluminal surfaces by exocytosis, via an increase in [Ca²⁺]_i, PI₃K and ROCK, independently of NO. We propose that hypoxia releases ATP at concentrations sufficient to induce umbilical vein dilation via PGs and NO and improve fetal blood flow, but curbs amplification of ATP release by autocrine actions of ATP, so limiting its pro-inflammatory effects.

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Hypoxia releases ATP from Human umbilical vein endothelial cells (HUVEC).

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This ATP release is preferentially from apical surfaces.

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Polarised ATP release is also triggered by Ca²⁺ ionophore.

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Hypoxia-induced ATP release occurs from vesicles, as visualised by quinacrine.

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It is attenuated by inhibitors of vesicular trafficking, PI₃K and ROCK.

Lysophosphatidic acid (LPA) signalling in cell migration and cancer invasion: a focussed review and analysis of LPA receptor gene expression on the basis of more than 1700 cancer microarrays

Lysophosphatidic acid (LPA) is a ubiquitously present signalling molecule involved in diverse cellular processes such as cell migration, proliferation and differentiation. LPA acts as an autocrine and/or paracrine signalling molecule via different G-protein-coupled LPA receptors (LPARs) that trigger a broad range of intracellular signalling cascades, especially the RHOA pathway. Mounting evidence suggests a crucial role of the LPA/LPAR-axis in cancer cell metastasis and promising studies are underway to investigate the therapeutic potential of LPAR-antagonists. This review summarises current knowledge on how LPA promotes cytoskeletal remodelling to enhance the migratory and invasive properties of cells, which may ultimately contribute to cancer metastasis. Furthermore, we provide comprehensive transcriptome analyses of published microarrays of more than 350 normal tissues and more than 1700 malignant tissues to define the expression signatures of LPARs and the LPA-generating enzymes autotaxin (ATX) and lipase member 1 (LIPI). These analyses demonstrate that ATX is highly expressed in a variety of carcinomas and sarcomas, whereas LIPI is almost exclusively overexpressed in highly aggressive Ewing's sarcomas, which underscores the potential contribution of LPA in metastatic disease. In addition, these analyses show that different cancer entities display distinct expression signatures of LPARs that distinguish them from one another. Finally, we discuss current approaches to specifically target the LPA/LPAR circuits in experimental cancer therapy.

Lysophosphatidic acid in atherosclerotic diseases

Lysophosphatidic acid (LPA) is a potent bioactive phospholipid. As many other biological active lipids, LPA is an autacoid: it is formed locally on demand, and it acts locally near its site of synthesis. LPA has a plethora of biological activities on blood cells (platelets, monocytes) and cells of the vessel wall (endothelial cells, smooth muscle cells, macrophages) that are all key players in atherosclerotic and atherothrombotic processes. The specific cellular actions of LPA are determined by its multifaceted molecular structures, the expression of multiple G-protein coupled LPA receptors at the cell surface and their diverse coupling to intracellular signalling pathways. Numerous studies have now shown that LPA has thrombogenic and atherogenic actions. Here, we aim to provide a comprehensive, yet concise, thoughtful and critical review of this exciting research area and to pinpoint potential pharmacological targets for inhibiting thrombogenic and atherogenic activities of LPA. We hope that the review will serve to accelerate knowledge of basic and clinical science, and to foster drug development in the field of LPA and atherosclerotic/atherothrombotic diseases.

Comparing the differential effects of LPA on the barrier function of human pulmonary endothelial cells

Lysophosphatidic acid (LPA) is a class of bioactive lyso-phospholipid that mediates most of its biological effects through a family of G protein-coupled receptors of which six have been identified. The role of the LPA pathway in driving chronic lung diseases such as idiopathic pulmonary fibrosis (IPF) has gained considerable academic and industry attention. Modulation of the pulmonary artery endothelial barrier function by the LPA1 receptor has been shown to drive pulmonary fibrosis in murine models of disease. The purpose of this study was (i) to assess the effect of LPA on the barrier function of human pulmonary arterial (HPAEC) and microvascular (HMVEC) endothelial cells and (ii) to identify the LPA receptor subtype(s) responsible for changes in human pulmonary endothelial cell permeability using LPA receptor antagonists and siRNA technology. Analysis of the LPA receptor subtype expression demonstrated predominant expression of LPA2 and LPA6 receptor subtypes in both HPAECs and HMVECs. HPAECs also exhibit low expression of LPA1, LPA3, and LPA4 receptor subtypes. Treatment of cells with increasing concentrations of LPA caused loss of barrier function in HPAECs but not HMVECs, despite both cell types exhibiting very similar LPA receptor expression profiles. The LPA-mediated loss of barrier function in HPAECs appears to be independent of the LPA1 receptor and likely to be mediated via the LPA6 receptor although we cannot exclude an additional role for the LPA2 and LPA4 receptors in mediating these effects. These results suggest cell-specific mechanisms exist in human pulmonary endothelial cells to permit regulation of barrier function downstream of LPA receptors. More importantly, our data indicate that selective LPA1 receptor antagonism may be insufficient for therapeutic use in pulmonary diseases where impaired endothelial barrier function is related to disease initiation and progression.

New insights regarding the regulation of chemotaxis by nucleotides, adenosine, and their receptors

The directional movement of cells can be regulated by ATP, certain other nucleotides (e.g., ADP, UTP), and adenosine. Such regulation occurs for cells that are "professional phagocytes" (e.g., neutrophils, macrophages, certain lymphocytes, and microglia) and that undergo directional migration and subsequent phagocytosis. Numerous other cell types (e.g., fibroblasts, endothelial cells, neurons, and keratinocytes) also change motility and migration in response to ATP, other nucleotides, and adenosine. In this article, we review how nucleotides and adenosine modulate chemotaxis and motility and highlight the importance of nucleotide- and adenosine-regulated cell migration in several cell types: neutrophils, microglia, endothelial cells, and cancer cells. We also discuss difficulties in conducting experiments and drawing conclusions regarding the ability of nucleotides and adenosine to modulate the migration of professional and non-professional phagocytes.

Osmosensory mechanisms in cellular and systemic volume regulation

Perturbations of cellular and systemic osmolarity severely challenge the function of all organisms and are consequently regulated very tightly. Here we outline current evidence on how cells sense volume perturbations, with particular focus on mechanisms relevant to the kidneys and to extracellular osmolarity and whole body volume homeostasis. There are a variety of molecular signals that respond to perturbations in cell volume and osmosensors or volume sensors responding to these signals. The early signals of volume perturbation include integrins, the cytoskeleton, receptor tyrosine kinases, and transient receptor potential channels. We also present current evidence on the localization and function of central and peripheral systemic osmosensors and conclude with a brief look at the still limited evidence on pathophysiological conditions associated with deranged sensing of cell volume.

Lysophosphatidic acid receptor 1 modulates lipopolysaccharide-induced inflammation in alveolar epithelial cells and murine lungs

Lysophosphatidic acid (LPA), a bioactive phospholipid, plays an important role in lung inflammation by inducing the release of chemokines and lipid mediators. Our previous studies have shown that LPA induces the secretion of interleukin-8 and prostaglandin E(2) in lung epithelial cells. Here, we demonstrate that LPA receptors contribute to lipopolysaccharide (LPS)-induced inflammation. Pretreatment with LPA receptor antagonist Ki16425 or downregulation of LPA receptor 1 (LPA(1)) by small-interfering RNA (siRNA) attenuated LPS-induced phosphorylation of p38 MAPK, I-κB kinase, and I-κB in MLE12 epithelial cells. In addition, the blocking of LPA(1) also suppressed LPS-induced IL-6 production. Furthermore, LPS treatment promoted interaction between LPA(1) and CD14, a LPS coreceptor, in a time- and dose-dependent manner. Disruption of lipid rafts attenuated the interaction between LPA(1) and CD14. Mice challenged with LPS increased plasma LPA levels and enhanced expression of LPA receptors in lung tissues. To further investigate the role of LPA receptors in LPS-induced inflammation, wild-type, or LPA(1)-deficient mice, or wild-type mice pretreated with Ki16425 were intratracheally challenged with LPS for 24 h. Knock down or inhibition of LPA(1) decreased LPS-induced IL-6 release in bronchoalveolar lavage (BAL) fluids and infiltration of cells into alveolar space compared with wild-type mice. However, no significant differences in total protein concentration in BAL fluids were observed. These results showed that knock down or inhibition of LPA(1) offered significant protection against LPS-induced lung inflammation but not against pulmonary leak as observed in the murine model for lung injury.

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

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

Non-Edg family LPA receptors: the cutting edge of LPA research

Lysophosphatidic acid (LPA) is a bioactive lipid mediator with diverse physiological and pathological actions on many types of cells. Originally, LPA was thought to elicit its biological functions through three subtypes of endothelial differentiation gene (Edg) family G protein-coupled receptors (LPA1, LPA2 and LPA3) until our group identified a fourth subtype, LPA4. The discovery of this receptor, which is structurally distinct from the Edg family LPA receptors, led to the identification of two additional LPA receptors, LPA5 and LPA6, homologous to LPA4. These 'non-Edg family' LPA receptors now provide a new framework for understanding the diverse functions of LPA, including vascular development, platelet activation and hair growth. In this review, we summarize the identification, intracellular signalling and biological functions of this novel cluster of LPA receptors.

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

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

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

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

Nucleotide receptors as targets in the pharmacological enhancement of dermal wound healing

With a growing interest of the involvement of extracellular nucleotides in both normal physiology and pathology, it has become evident that P2 receptor agonists and antagonists may have therapeutic potential. The P2Y2 receptor agonists (diquafosol tetrasodium and denufosol tetrasodium) are in the phase 3 of clinical trials for dry eye and cystic fibrosis, respectively. The thienopyridine derivatives clopidogrel and ticlopidine (antagonists of the platelet P2Y12 receptor) have been used in cardiovascular medicine for nearly a decade. Purines and pyrimidines may be of therapeutic potential also in wound healing since ATP and UTP have been shown to have many hallmarks of wound healing factors. Recent studies have demonstrated that extracellular nucleotides take part in all phases of wound repair: hemostasis, inflammation, tissue formation, and tissue remodeling. This review is focused on the potent purines and pyrimidines which regulate many physiological processes important for wound healing.

Nucleotide release by airway epithelia

The purinergic events regulating the airways' innate defenses are initiated by the release of purines from the epithelium, which occurs constitutively and is enhanced by chemical or mechanical stimulation. While the external triggers have been reviewed exhaustively, this chapter focuses on current knowledge of the receptors and signaling cascades mediating nucleotide release. The list of secreted purines now includes ATP, ADP, AMP and nucleotide sugars, and involves at least three distinct mechanisms reflecting the complexity of airway epithelia. First, the constitutive mechanism involves ATP translocation to the ER/Golgi complex as energy source for protein folding, and fusion of Golgi-derived vesicles with the plasma membrane. Second, goblet cells package ATP with mucins into granules, which are discharged in response to P2Y(2)R activation and Ca(2+)-dependent signaling pathways. Finally, non-mucous cells support a regulated mechanism of ATP release involving protease activated receptor (PAR)-elicited G(12/13) activation, leading to the RhoGEF-mediated exchange of GDP for GTP on RhoA, and cytoskeleton rearrangement. Together, these pathways provide fine tuning of epithelial responses regulated by purinergic signaling events.

P2Y1 and P2Y13 purinergic receptors mediate Ca2+ signaling and proliferative responses in pulmonary artery vasa vasorum endothelial cells

Extracellular ATP and ADP have been shown to exhibit potent angiogenic effects on pulmonary artery adventitial vasa vasorum endothelial cells (VVEC). However, the molecular signaling mechanisms of extracellular nucleotide-mediated angiogenesis remain not fully elucidated. Since elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) is required for cell proliferation and occurs in response to extracellular nucleotides, this study was undertaken to delineate the purinergic receptor subtypes involved in Ca(2+) signaling and extracellular nucleotide-mediated mitogenic responses in VVEC. Our data indicate that stimulation of VVEC with extracellular ATP resulted in the elevation of [Ca(2+)](i) via Ca(2+) influx through plasma membrane channels as well as Ca(2+) mobilization from intracellular stores. Moreover, extracellular ATP induced simultaneous Ca(2+) responses in both cytosolic and nuclear compartments. An increase in [Ca(2+)](i) was observed in response to a wide range of purinergic receptor agonists, including ATP, ADP, ATPγS, ADPβS, UTP, UDP, 2-methylthio-ATP (MeSATP), 2-methylthio-ADP (MeSADP), and BzATP, but not adenosine, AMP, diadenosine tetraphosphate, αβMeATP, and βγMeATP. Using RT-PCR, we identified mRNA for the P2Y1, P2Y2, P2Y4, P2Y13, P2Y14, P2X2, P2X5, P2X7, A1, A2b, and A3 purinergic receptors in VVEC. Preincubation of VVEC with the P2Y1 selective antagonist MRS2179 and the P2Y13 selective antagonist MRS2211, as well as with pertussis toxin, attenuated at varying degrees agonist-induced intracellular Ca(2+) responses and activation of ERK1/2, Akt, and S6 ribosomal protein, indicating that P2Y1 and P2Y13 receptors play a major role in VVEC growth responses. Considering the broad physiological implications of purinergic signaling in the regulation of angiogenesis and vascular homeostasis, our findings suggest that P2Y1 and P2Y13 receptors may represent novel and specific targets for treatment of pathological vascular remodeling involving vasa vasorum expansion.

Deleted in liver cancer 2 suppresses cell growth via the regulation of the Raf-1-ERK1/2-p70S6K signalling pathway

BACKGROUND

Deleted in liver cancer 2 (DLC2) gene, a putative tumour suppressor gene, encodes a Rho GTPase-activating protein (RhoGAP) with GAP activity specific for RhoA. It exhibits tumour suppressor functions and inhibits tumour cell proliferation, migration as well as transformation.

AIMS

In this study, we aimed to investigate the underlying mechanisms of the DLC2 gene in suppressing cell migration and cell growth. HepG2 hepatoma cells were stably transfected with the DLC2γ isoform, which contains the RhoGAP domain.

METHODS AND RESULTS

On performing immunofluorescence staining and Western blot analysis, the expression of the focal adhesion protein paxillin was found to be much reduced in DLC2γ-stable clones. Upon flow cytometric analysis of the cell cycle profiles, the DLC2γ-stable clones were shown to have a higher population of cells arrested at the G1 phase than the EGFP vector-stable clone, suggesting that downregulation of RhoA activity in DLC2γ-stable clones inhibited cell cycle progression. In the DLC2γ-stable clone, the levels of Raf-1 and extracellular signal-regulated kinase (ERK) 1/2 were decreased as compared with those of the parental HepG2, EGFP vector and DLC2γ-GAP defective mutant-stable clones. Furthermore, the ribosomal kinase p70S6K, a downstream target of ERK1/2, was suppressed in the DLC2-stable clones. On the contrary, when DLC2 was knocked down by siRNA in HepG2 cells, the expression levels of phospho-p70S6K and phospho-ERK1/2 were upregulated.

CONCLUSION

Our data show that DLC2 inhibits the activity of Raf-1-ERK1/2-p70S6K via its RhoGAP function, resulting in the suppression of cell growth. Further studies on the molecular signalling between DLC2 and p70S6K may provide an insight into its growth suppressor function.

Activation of the TASK-2 channel after cell swelling is dependent on tyrosine phosphorylation

The swelling-activated K(+) currents (I(K,vol)) in Ehrlich ascites tumor cells (EATC) has been reported to be through the two-pore domain (K(2p)), TWIK-related acid-sensitive K(+) channel 2 (TASK-2). The regulatory volume decrease (RVD), following hypotonic exposure in EATC, is rate limited by I(K,vol) indicating that inhibition of RVD reflects inhibition of TASK-2. We find that in EATC the tyrosine kinase inhibitor genistein inhibits RVD by 90%, and that the tyrosine phosphatase inhibitor monoperoxo(picolinato)-oxo-vanadate(V) [mpV(pic)] shifted the volume set point for inactivation of the channel to a lower cell volume. Swelling-activated K(+) efflux was impaired by genistein and the Src kinase family inhibitor 4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) and enhanced by the tyrosine phosphatase inhibitor mpV(pic). With the use of the TASK-2 inhibitor clofilium, it is demonstrated that mpV(pic) increased the volume-sensitive part of the K(+) efflux 1.3 times. To exclude K(+) efflux via a KCl cotransporter, cellular Cl(-) was substituted with NO(3)(-). Also under these conditions K(+) efflux was completely blocked by genistein. Thus tyrosine kinases seem to be involved in the activation of the volume-sensitive K(+) channel, whereas tyrosine phosphatases appears to be involved in inactivation of the channel. Overexpressing TASK-2 in human embryonic kidney (HEK)-293 cells increased the RVD rate and reduced the volume set point. TASK-2 has tyrosine sites, and precipitation of TASK-2 together with Western blotting and antibodies against phosphotyrosines revealed a cell swelling-induced, time-dependent tyrosine phosphorylation of the channel. Even though we found an inhibiting effect of PP2 on RVD, neither Src nor the focal adhesion kinase (FAK) seem to be involved. Inhibitors of the epidermal growth factor receptor tyrosine kinases had no effect on RVD, whereas the Janus kinase (JAK) inhibitor cucurbitacin inhibited the RVD by 40%. It is suggested that the cytokine receptor-coupled JAK/STAT pathway is upstream of the swelling-induced phosphorylation and activation of TASK-2 in EATC.

The effects of the small GTPase RhoA on the muscarinic contraction of airway smooth muscle result from its role in regulating actin polymerization

The small GTPase RhoA increases the Ca(2+) sensitivity of smooth muscle contraction and myosin light chain (MLC) phosphorylation by inhibiting the activity of MLC phosphatase. RhoA is also a known regulator of cytoskeletal dynamics and actin polymerization in many cell types. In airway smooth muscle (ASM), contractile stimulation induces MLC phosphorylation and actin polymerization, which are both required for active tension generation. The objective of this study was to evaluate the primary mechanism by which RhoA regulates active tension generation in intact ASM during stimulation with acetylcholine (ACh). RhoA activity was inhibited in canine tracheal smooth muscle tissues by expressing the inactive RhoA mutant, RhoA T19N, in the intact tissues or by treating them with the cell-permeant RhoA inhibitor, exoenzyme C3 transferase. RhoA inactivation reduced ACh-induced contractile force by approximately 60% and completely inhibited ACh-induced actin polymerization but inhibited ACh-induced MLC phosphorylation by only approximately 20%. Inactivation of MLC phosphatase with calyculin A reversed the reduction in MLC phosphorylation caused by RhoA inactivation, but calyculin A did not reverse the depression of active tension and actin polymerization caused by RhoA inactivation. The MLC kinase inhibitor, ML-7, inhibited ACh-induced MLC phosphorylation by approximately 80% and depressed active force by approximately 70% but did not affect ACh-induced actin polymerization, demonstrating that ACh-stimulated actin polymerization occurs independently of MLC phosphorylation. We conclude that the RhoA-mediated regulation of ACh-induced contractile tension in ASM results from its role in mediating actin polymerization rather than from effects on MLC phosphatase or MLC phosphorylation.

PI3K, Rho, and ROCK play a key role in hypoxia-induced ATP release and ATP-stimulated angiogenic responses in pulmonary artery vasa vasorum endothelial cells

We recently reported that vasa vasorum expansion occurs in the pulmonary artery (PA) adventitia of chronically hypoxic animals and that extracellular ATP is a pro-angiogenic factor for isolated vasa vasorum endothelial cells (VVEC). However, the sources of extracellular ATP in the PA vascular wall, as well as the molecular mechanisms underlying its release, remain elusive. Studies were undertaken to explore whether VVEC release ATP in response to hypoxia and to determine signaling pathways involved in this process. We found that hypoxia (1-3% O2) resulted in time- and O2-dependent ATP release from VVEC. Preincubation with the inhibitors of vesicular transport (monensin, brefeldin A, and N-ethylmaleimide) significantly decreased ATP accumulation in the VVEC conditioned media, suggesting that hypoxia-induced ATP release occurs through vesicular exocytosis. Additionally, both hypoxia and exogenously added ATP resulted in the activation of PI3K and accumulation of GTP-bound RhoA in a time-dependent manner. Pharmacological inhibition of PI3K and ROCK or knockout of RhoA by small interfering RNA significantly abolished hypoxia-induced ATP release from VVEC. Moreover, RhoA and ROCK play a critical role in ATP-induced increases in VVEC DNA synthesis, migration, and tube formation, indicating a functional contribution of PI3K, Rho, and ROCK to both the autocrine mechanism of ATP release and ATP-mediated angiogenic activation of VVEC. Taken together, our findings provide novel evidence for the signaling mechanisms that link hypoxia-induced increases in extracellular ATP and vasa vasorum expansion.

Thrombin promotes release of ATP from lung epithelial cells through coordinated activation of rho- and Ca2+-dependent signaling pathways

Extracellular ATP controls key aspects of lung function via activation of epithelial cell purinergic receptors, but how ATP is released from cells remains poorly understood. To identify mechanistic components upstream of ATP release, we examined the effect of selected G protein coupled-receptor activation on ATP release from lung epithelial cells. The protease-activated receptor (PAR) agonist thrombin elicited a rapid Ca(2+)-dependent release of ATP from A549 cells. In contrast, the P2Y(2) receptor agonist UTP caused negligible ATP release, despite promoting a robust Ca(2+) response. Agonist-elicited ATP release was associated with Rho activation and was reduced in cells transfected with dominant negative mutants of p115-Rho GEF or RhoA, and by inhibitors of Rho kinase (ROCK). However, RhoA activation alone did not promote ATP release if temporally separated from Ca(2+) mobilization. PAR3 was the only PAR subtype detected in A549 cells by reverse transcription-PCR. Transfection of cells with human PAR3 cDNA increased thrombin-promoted ATP release, inositol phosphate formation, and RhoA activation. Conversely, small interference RNA against PAR3 diminished thrombin-evoked responses. Thrombin-elicited ATP release was accompanied by an enhanced cellular uptake of propidium iodide in a Ca(2+)- and ROCK-dependent manner and was inhibited by connexin/pannexin hemichannel blockers. Our data suggest that thrombin promotes ATP release from A549 cells via Rho- and Ca(2+)-dependent activation of connexin/pannexin hemichannels. The relevance of these findings is highlighted by the observation that exposure of primary cultures of well differentiated human bronchial epithelial cells to thrombin resulted in robust ATP release, which was inhibited by ROCK inhibitors and by connexin/pannexin hemichannel blockers.

Identification and characterization of a novel lysophosphatidic acid receptor, p2y5/LPA6

p2y5 is an orphan G protein-coupled receptor that is closely related to the fourth lysophosphatidic acid (LPA) receptor, LPA4. Here we report that p2y5 is a novel LPA receptor coupling to the G13-Rho signaling pathway. "LPA receptor-null" RH7777 and B103 cells exogenously expressing p2y5 showed [3H]LPA binding, LPA-induced [35S]guanosine 5'-3-O-(thio)triphosphate binding, Rho-dependent alternation of cellular morphology, and Gs/13 chimeric protein-mediated cAMP accumulation. LPA-induced contraction of human umbilical vein endothelial cells was suppressed by small interfering RNA knockdown of endogenously expressed p2y5. We also found that 2-acyl-LPA had higher activity to p2y5 than 1-acyl-LPA. A recent study has suggested that p2y5 is an LPA receptor essential for human hair growth. We confirmed that p2y5 is a functional LPA receptor and propose to designate this receptor LPA6.

ATP release from non-excitable cells

All cells release nucleotides and are in one way or another involved in local autocrine and paracrine regulation of organ function via stimulation of purinergic receptors. Significant technical advances have been made in recent years to quantify more precisely resting and stimulated adenosine triphosphate (ATP) concentrations in close proximity to the plasma membrane. These technical advances are reviewed here. However, the mechanisms by which cells release ATP continue to be enigmatic. The current state of knowledge on different suggested mechanisms is also reviewed. Current evidence suggests that two separate regulated modes of ATP release co-exist in non-excitable cells: (1) a conductive pore which in several systems has been found to be the channel pannexin 1 and (2) vesicular release. Modes of stimulation of ATP release are reviewed and indicate that both subtle mechanical stimulation and agonist-triggered release play pivotal roles. The mechano-sensor for ATP release is not yet defined.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Involvement of heparan sulfate proteoglycan in sensing hypotonic stress in bovine aortic endothelial cells

Hypotonic stress (HTS) induces various responses in vascular endothelium, but the molecules involved in sensing HTS are not known. To investigate a possible role of heparan sulfate proteoglycan (HSPG) in sensing HTS, we compared the responses of control bovine aortic endothelial cells (BAECs) with those of cells treated with heparinase III, which exclusively degrades HSPG. Tyrosine phosphorylation of 125 kDa FAK induced by HTS (-30%) in control cells was abolished in heparinase III-treated BAECs. The amplitude of the volume-regulated anion channel (VRAC) current, whose activation is regulated by tyrosine kinase, was significantly reduced by the treatment with heparinase III. Also, HTS-induced ATP release through the VRAC pore and the concomitant Ca(2+) transients were significantly reduced in the heparinase III-treated BAECs. In contrast, exogenously applied ATP evoked similar Ca(2+) transients in both control and heparinase III-treated BAECs. The transient formation of actin stress fibers induced by HTS in control cells was absent in heparinase III-treated BAECs. Lysophosphatidic acid (LPA) also induced FAK phosphorylation, actin reorganization and ATP release in control BAECs, but heparinase III did not affect these LPA-induced responses. We conclude from these observations that HSPG is one of the sensory molecules of hypotonic cell swelling in BAECs.

Hypergravity induces ATP release and actin reorganization via tyrosine phosphorylation and RhoA activation in bovine endothelial cells

Mechanical stresses regulate physiological and pathological functions of vascular endothelial cells. We examined, in this study, the effects of hypergravity on endothelial functions. Hypergravity (3 G) applied by low speed centrifuge immediately induced a membrane translocation of small G-protein RhoA and tyrosine phosphorylation of 125 kDa FAK in bovine aortic endothelial cells (BAECs). Hypergravity also induced a transient reorganization of actin fibers in 3 min, which was inhibited by Rho-kinase inhibitor (Y27632) and tyrosine kinase inhibitors (herbimycin A and tyrphostin 46). Furthermore, the extracellular ATP concentration ([ATP]o) was increased by 2 G and 3 G hypergravity in 5 min, and the inhibitors of Rho-kinase, tyrosine kinase, and volume-regulated anion channels (VRAC; verapamil, tamoxifen and fluoxetine) significantly suppressed [ATP]o elevation. Application of 3 G hypergravity for 1 h increased the nuclear uptake of BrdU, which was inhibited by Rho-kinase inhibitor and VARC inhibitors. Furthermore, intermittent application of 3 G hypergravity for 1 or 2 h/day stimulated endothelial migration in 5 days, and this was inhibited by suramin, a P2 antagonist. Collectively, these results indicate that hypergravity induces ATP release and actin reorganization via RhoA activation and FAK phosphorylation, thereby activating cell proliferation and migration in BAECs. These also suggest that gravity can be regarded as an extracorporeal signal that could significantly affect endothelial functions.

Rho-family GTPases modulate Ca(2+) -dependent ATP release from astrocytes

Previously, we reported that activation of G protein-coupled receptors (GPCR) in 1321N1 human astrocytoma cells elicits a rapid release of ATP that is partially dependent on a G(q)/phophospholipase C (PLC)/Ca(2+) mobilization signaling cascade. In this study we assessed the role of Rho-family GTPase signaling as an additional pathway for the regulation of ATP release in response to activation of protease-activated receptor-1 (PAR1), lysophosphatidic acid receptor (LPAR), and M3-muscarinic (M3R) GPCRs. Thrombin (or other PAR1 peptide agonists), LPA, and carbachol triggered quantitatively similar Ca(2+) mobilization responses, but only thrombin and LPA caused rapid accumulation of active GTP-bound Rho. The ability to elicit Rho activation correlated with the markedly higher efficacy of thrombin and LPA, relative to carbachol, as ATP secretagogues. Clostridium difficile toxin B and Clostridium botulinum C3 exoenzyme, which inhibit Rho-GTPases, attenuated the thrombin- and LPA-stimulated ATP release but did not decrease carbachol-stimulated release. Thus the ability of certain G(q)-coupled receptors to additionally stimulate Rho-GTPases acts to strongly potentiate a Ca(2+)-activated ATP release pathway. However, pharmacological inhibition of Rho kinase I/II or myosin light chain kinase did not attenuate ATP release. PAR1-induced ATP release was also reduced twofold by brefeldin treatment suggesting the possible mobilization of Golgi-derived, ATP-containing secretory vesicles. ATP release was also markedly repressed by the gap junction channel inhibitor carbenoxolone in the absence of any obvious thrombin-induced change in membrane permeability indicative of hemichannel gating.

High tidal volume mechanical ventilation with hyperoxia alters alveolar type II cell adhesion

Patients with acute respiratory distress syndrome undergoing mechanical ventilation may be exposed to both high levels of stretch and high levels of oxygen. We hypothesized that the combination of high stretch and hyperoxia promotes loss of epithelial adhesion and impairs epithelial repair mechanisms necessary for restoration of barrier function. We utilized a model of high tidal volume mechanical ventilation (25 ml/kg) with hyperoxia (50% O(2)) in rats to investigate alveolar type II (AT2) cell adhesion and focal adhesion signaling. AT2 cells isolated from rats exposed to hyperoxia and high tidal volume mechanical ventilation (MVHO) exhibited significantly decreased cell adhesion and reduction in phosphotyrosyl levels of focal adhesion kinase (FAK) and paxillin compared with control rats, rats exposed to hyperoxia without ventilation (HO), or rats ventilated with normoxia (MV). MV alone increased phosphorylation of p130(Cas). RhoA activation was increased by MV, HO, and the combination of MV and HO. Treatment of MVHO cells with keratinocyte growth factor (KGF) for 1 h upon isolation reduced RhoA activity and restored attachment to control levels. Attachment and migration of control AT2 cells was significantly decreased by constitutively active RhoA or a kinase inactive form of FAK (FRNK), whereas expression of dominant negative RhoA in cells from MVHO-treated rats restored cell adhesion. Mechanical ventilation with hyperoxia promotes changes in focal adhesion proteins and RhoA in AT2 cells that may be deleterious for cell adhesion and migration.

Long-term treatment with TGFbeta1 impairs mechanotransduction in bovine aortic endothelial cells

BACKGROUND AND PURPOSE

Vascular endothelial cells play a role in the physiological response to mechanical stress. Transforming growth factor beta1 (TGFbeta1) induces morphological changes in endothelial cells, and this may alter their mechanosensitive responses. The aim of this study was to examine the effects of TGFbeta1 on hypotonic stress (HTS)-induced responses in bovine aortic endothelial cells (BAECs).

EXPERIMENTAL APPROACH

Cultured BAECs were treated with 3 ng ml(-1) TGFbeta1 for 24 h (24h-TGFbeta1) or 7 days (7d-TGFbeta1). Cytosolic actin fibres were stained with rhodamine-phalloidin. Intracellular Ca2+ concentration was measured using fura2. Tyrosine phosphorylation and RhoA expression were assessed by Western blotting. Expression of RhoA mRNA was assessed by real-time PCR.

KEY RESULTS

BAECs developed pseudopod-like processes within 24 h and showed a fibroblast-like appearance after 7 days. HTS induced Ca2+ transients via endogenous ATP release in both control and 24h-TGFbeta1 BAECs but not in 7d-TGFbeta1 BAECs. We have previously shown that HTS-induced ATP release is mediated by sequential activation of RhoA and tyrosine kinases. The basal amount of membrane-bound RhoA was significantly lower in 7d-TGFbeta1 than in 24h-TGFbeta1 or control BAECs. HTS increased the membrane-bound RhoA to the same fractional level in 24h-TGFbeta1 and control BAECs, but its net maximal amount was significantly lower in 7d-TGFbeta1. HTS-induced downstream signals of RhoA activation, i.e. the tyrosine phosphorylation of FAK and paxillin, were markedly suppressed in 7d-TGFbeta1 BAECs.

CONCLUSIONS AND IMPLICATIONS

These results indicate that long-term treatment with TGFbeta1 does not impair mechanoreception in BAECs but impairs mechanotransduction by affecting RhoA membrane translocation.

Hypertonicity triggers RhoA-dependent assembly of myosin-containing striated polygonal actin networks in endothelial cells

Endothelial cells respond to mechanical stresses of the circulation with cytoskeletal rearrangements such as F-actin stress fiber alignment along the axis of fluid flow. Endothelial cells are exposed to hypertonic stress in the renal medulla or during mannitol treatment of cerebral edema. We report here that arterial endothelial cells exposed to hypertonic stress rearranged F-actin into novel actin-myosin II fibers with regular 0.5-microm striations, in which alpha-actinin colocalizes with actin. These striated fibers assembled over hours into three-dimensional, irregular, polygonal actin networks most prominent at the cell base, and occasionally surrounding the nucleus in a geodesic-like structure. Hypertonicity-induced assembly of striated polygonal actin networks was inhibited by cytochalasin D, blebbistatin, cell ATP depletion, and intracellular Ca(2+) chelation but did not require intact microtubules, regulatory volume increase, or de novo RNA or protein synthesis. Striated polygonal actin network assembly was insensitive to inhibitors of MAP kinases, tyrosine kinases, or phosphatidylinositol 3-kinase, but was prevented by C3 exotoxin, by the RhoA kinase inhibitor Y-27632, and by overexpressed dominant-negative RhoA. In contrast, overexpression of dominant-negative Rac or of dominant-negative cdc42 cDNAs did not prevent striated polygonal actin network assembly. The actin networks described here are novel in structure, as striated actin-myosin structures in nonmuscle cells, as a cellular response to hypertonicity, and as a cytoskeletal regulatory function of RhoA. Endothelial cells may use RhoA-dependent striated polygonal actin networks, possibly in concert with cytoskeletal load-bearing elements, as a contractile, tension-generating component of their defense against isotropic compressive forces.

Pivotal role of integrin alpha5beta1 in hypotonic stress-induced responses of human endothelium

We have previously reported that both hypotonic stress (HTS) and lysophosphatidic acid (LPA) induce ATP release and a transient reorganization of actin through sequential activation of RhoA/Rho-kinase and focal adhesion kinase F-actin (FAK)/paxillin in human umbilical cord vein endothelial cells (HUVECs). LPA is known to induce the activation of RhoA via its specific receptors, but the mechanisms by which HTS initiates these intracellular signals are not known. The present study aimed to identify the molecule(s) that are unique to the sensing and/or transducing the mechanical stress. Reverse transcriptase-polymerase chain reaction revealed the expression of several integrin subunits in HUVECs. Anti-integrin alpha5beta1 antibody (Ab), but not anti-integrin alpha2, alpha6, alpha v, or beta4 antibodies, inhibited HTS-induced RhoA translocation, tyrosine phosphorylation of FAK and paxillin, ATP release, and actin reorganization. However, the LPA-induced ATP release and actin reorganization were not inhibited by any of these anti-integrin antibodies, indicating that integrin alpha5beta1 plays a pivotal role in the HTS-induced but not in the LPA-induced responses. It is therefore reasonable to assume that this particular subtype of integrin is involved in the initiation of the responses induced by mechanical stimuli in HUVECs.

ATP participates in the regulation of microvessel permeability

We demonstrated previously that stimulation of the P2Y receptor enhanced the macromolecular permeability of cultured endothelial cell monolayers via the paracellular pathway. To determine whether the P2Y receptor participates in the regulation of permeability in intact microvessels, we have examined the effects of exogenous and endogenous ATP on the permeation of the surface tissue of perfused rat tail caudal artery using a fluorescein isothiocyanate-dextran (FD-4; MW 4400; 1.0 mg mL(-1)). The permeation of FD-4 was assessed by a confocal fluorescence imaging system. We found that 2-methylthioadenosine 5'-triphosphate, a P2Y receptor agonist, enhanced the fluorescence intensity of FD-4 in the surface of the rat caudal artery tissue and that it was inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, a P2 receptor antagonist. Also, noradrenaline, a sympathetic neurotransmitter, and bradykinin, an inflammatory autacoid, enhanced the fluorescence intensity of FD-4 in the surface tissue of the rat caudal artery. The enhancement by noradrenaline was significantly inhibited by the P2 receptor antagonist. In addition, noradrenaline and bradykinin caused the release of ATP, ADP, AMP and adenosine from the endothelium of the rat caudal artery. These results indicated that the exogenous and endogenous ATP increased the macromolecular permeability of blood capillaries via the P2Y receptor. Such purinergic regulation of endothelial permeability may function in physiological and pathophysiological conditions.

STAT3-independent inhibition of lysophosphatidic acid-mediated upregulation of connective tissue growth factor (CTGF) by cucurbitacin I

Cucurbitacins are recognised as anti-tumour agents because of their interference with STAT3 signalling, but may also affect the integrity of the actin cytoskeleton. In the present study the effect of cucurbitacin I was investigated in fibroblasts. In these cells, cucurbitacin I interfered with lysophosphatidic acid (LPA) signalling. It inhibited tyrosine phosphorylation of focal adhesion proteins and induction of connective tissue growth factor (CTGF), a potent profibrotic protein. Inhibition of Src family kinases with PP2, but not the inactive analogue PP3, also interfered with LPA-mediated tyrosine phosphorylation and induction of CTGF. Jak2-STAT3 signalling seemed to be the connecting link, because CTGF induction was sensitive to AG490, an inhibitor of Jak2, and cucurbitacin I, an inhibitor of Jak2 and STAT3. However, LPA did not activate tyrosine phosphorylation of STAT3. Furthermore, cucurbitacin I was as effective in STAT3 knock out cells as in control cells. Therefore, the inhibitory effect of cucurbitacin I was not related to inhibition of STAT3. Immunocytochemical analysis of cucurbitacin I-treated cells revealed disassembly of F-actin fibres, reorganisation into F-actin patches and resolution of focal adhesions. The phenotypic changes resembled changes observed after treatment of the cells with cytochalasin D, which has been shown to interfere with CTGF induction. Concentrations of cucurbitacin I, which have been shown to target Jak2-STAT3 signalling, thus, profoundly affect the actin cytoskeleton and may therefore modulate cell morphology, migration, adherence and gene expression also in non-tumour cells.