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

RGD Peptidomimetic MMAE-Conjugate Addressing Integrin αVβ3-Expressing Cells with High Targeting Index

Small molecule-drug conjugates (SMDCs) mimicking the RGD sequence (-Arg-Gly-Asp-) with a non-peptide moiety require a pharmacophore-independent attachment site. A library of 36 sulfonamide-modified RGD mimetics with nM to pM affinity for integrin α_V β₃ was synthesized and analysed via DAD mapping. The best structure of the conjugable RGD mimetic was used and a linker was attached to an aromatic ring by Negishi cross-coupling. The product retained high affinity and selectivity for integrin α_V β₃ . The conjugable RGD mimetic was then attached to an enzymatically cleavable GKGEVA linker equipped with a self-immolative PABC and the antimitotic drug monomethyl auristatin E (MMAE). The resulting SMDC preferred binding to integrin α_V β₃ over α₅ β₁ in a ratio of 1 : 4519 (ELISA) and showed selectivity for α_V β₃ -positive WM115 cells over α_V β₃ -negative M21-L cells in the in vitro cell adhesion assay as well as in cell viability assays with a targeting index of 134 (M21-L/WM115).

Substrate Stiffness-Driven Membrane Tension Modulates Vesicular Trafficking via Caveolin-1

Liver fibrosis, a condition characterized by extensive deposition and cross-linking of extracellular matrix (ECM) proteins, is idiosyncratic in cases of chronic liver injury. The dysregulation of ECM remodeling by hepatic stellate cells (HSCs), the main mediators of fibrosis, results in an elevated ECM stiffness that drives the development of chronic liver disease such as cirrhosis and hepatocellular carcinoma. Tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) is a key element in the regulation of ECM remodeling, which modulates the degradation and turnover of ECM components. We have previously reported that a rigid, fibrotic-like substrate can impact TIMP-1 expression at the protein level in HSCs without altering its mRNA expression. While HSCs are known to be highly susceptible to mechanical stimuli, the mechanisms through which mechanical cues regulate TIMP-1 at the post-translational level remain unclear. Here, we show a mechanism of regulation of plasma membrane tension by matrix stiffness. We found that this effect is orchestrated by the β1 integrin/RhoA axis and results in elevated exocytosis and secretion of TIMP-1 in a caveolin-1- and dynamin-2-dependent manner. We then show that TIMP-1 and caveolin-1 expression increases in cirrhosis and hepatocellular carcinoma. These conditions are associated with fibrosis, and this effect can be recapitulated in 3D fibrosis models consisting of hepatic stellate cells encapsulated in a self-assembling polypeptide hydrogel. This work positions stiffness-dependent membrane tension as a key regulator of enzyme secretion and function and a potential target for therapeutic strategies that aim at modulating ECM remodeling in chronic liver disease.

Distinct CD40L receptors mediate inflammasome activation and secretion of IL-1β and MCP-1 in cultured human retinal pigment epithelial cells

CD40L signaling occurs in several diseases with inflammatory components, including ocular and retinal diseases. However, it has never been evaluated as a pathogenic mechanism in age-related macular degeneration (AMD) or as an inducer of inflammasome formation in any cell type. mRNA and protein levels of CD40, IL-1β, NALP1, NALP3, caspase-1, and caspase-5 were determined by RT-PCR, qPCR, and Western blot. CD40L receptor (CD40, α5β1, and CD11b) expression was determined by Western and immunofluorescent staining. IL-1β, IL-18, and MCP-1 secretions were determined by ELISA. NALP1 and NALP3 inflammasome formation were determined by Co-IP. Experiments were conducted on primary human retinal pigment epithelial (hRPE) cells from four different donors. Human umbilical vein endothelial (HUVEC) and monocytic leukemia (THP-1) cells demonstrated the general applicability of our findings. In hRPE cells, CD40L-induced NALP1 and NALP3 inflammasome activation, cleavage of caspase-1 and caspase-5, and IL-1β and IL-18 secretion. Interestingly, neutralizing CD11b and α5β1 antibodies, but not CD40, reduced CD40L-induced IL-1β secretion in hRPE cells. Similarly, CD40L treatment also induced HUVEC and THP-1 cells to secret IL-1β through CD11b and α5β1. Additionally, the CD40L-induced IL-1β secretion acted in an autocrine/paracrine manner to feed back and induce hRPE cells to secrete MCP-1. This study is the first to show that CD40L induces inflammasome activation in any cell type, including hRPE cells, and that this induction is through CD11b and α5β1 cell-surface receptors. These mechanisms likely play an important role in many retinal and non-retinal diseases and provide compelling drug targets that may help reduce pro-inflammatory processes.

Rho Kinase Inhibitor Fasudil Suppresses the Vasculogenic Mimicry of B16 Mouse Melanoma Cells Both In Vitro and In Vivo

The aim of this study was to investigate the biologic role of the Rho kinase inhibitor fasudil in the vasculogenic mimicry (VM) of B16 mouse melanoma cells. It was previously reported that RhoA plays a critical role in angiogenesis by coordinating endothelial cell cytoskeleton remodeling and promoting endothelial cell motility. Although RhoA has been implicated in the regulation of angiogenesis, little has been described regarding its control of these tumor cell-lined channels. In this study, we established an in vitro model of VM using 3-dimensional cell culturing of mouse B16 melanoma cells and studied VM in vivo by transplanting B16 cells into C57/BL mice. Next, we explored the effect of RhoA and Rho-associated, coiled-coil containing protein kinase (ROCK) on VM formation using the Rho kinase inhibitor fasudil. We provide direct evidence that fasudil leads to reduced vascular-like channels in Matrigel. Additional experiments suggested that fasudil prevents both initial cellular architecture changes and cell migration in vitro. Finally, we provide in-depth evidence for the underlying mechanisms of fasudil-induced VM destruction using the Rho-GTPase agonist lysophosphatidic acid. In vivo studies revealed that fasudil reduced B16 melanoma cell xenograft tumor growth without causing significant toxicity in mice. Fasudil-treated tumors also displayed fewer VM channels. These results suggest that fasudil may be an emerging therapeutic option for targeting cancer VM.

Real-time imaging of ATP release induced by mechanical stretch in human airway smooth muscle cells

Airway smooth muscle (ASM) cells within the airway walls are continually exposed to mechanical stimuli, and exhibit various functions in response to these mechanical stresses. ATP acts as an extracellular mediator in the airway. Moreover, extracellular ATP is considered to play an important role in the pathophysiology of asthma and chronic obstructive pulmonary disease. However, it is not known whether ASM cells are cellular sources of ATP secretion in the airway. We therefore investigated whether mechanical stretch induces ATP release from ASM cells. Mechanical stretch was applied to primary human ASM cells cultured on a silicone chamber coated with type I collagen using a stretching apparatus. Concentrations of ATP in cell culture supernatants measured by luciferin-luciferase bioluminescence were significantly elevated by cyclic stretch (12 and 20% strain). We further visualized the stretch-induced ATP release from the cells in real time using a luminescence imaging system, while acquiring differential interference contrast cell images with infrared optics. Immediately after a single uniaxial stretch for 1 second, strong ATP signals were produced by a certain population of cells and spread to surrounding spaces. The cyclic stretch-induced ATP release was significantly reduced by inhibitors of Ca(2+)-dependent vesicular exocytosis, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester, monensin, N-ethylmaleimide, and bafilomycin. In contrast, the stretch-induced ATP release was not inhibited by a hemichannel blocker, carbenoxolone, or blockade of transient receptor potential vanilloid 4 by short interfering RNA transfection or ruthenium red. These findings reveal a novel property of ASM cells: mechanically induced ATP release may be a cellular source of ATP in the airway.

Development of vascular smooth muscle contractility by endothelium-derived transforming growth factor β proteins

It is well established that the release of vasodilators and vasoconstrictors from vascular endothelium regulates vascular smooth muscle contraction. In this report, we investigate the role of the endothelium in the development and maintenance of constitutive vascular contractility. For that purpose, contractile activity of cultured bovine aortic smooth muscle cells (BASMCs) embedded in collagen gels was monitored by changes in gel diameter. After culturing for 5 days, ATP- and high KCl solution-induced contractions were significantly enhanced in the gels that were overlaid with bovine aortic endothelial cells (BAECs) or were cultured with conditioned medium of cultured BAECs. ATP-induced Ca(2+) transients, recorded in BASMCs cultured with conditioned medium of BAECs, were markedly augmented, but high KCl-induced Ca(2+) transients were not affected. BASMCs in control gels were spindle shaped, and those in endothelium-treated gels were more elongated and interconnected. The endothelial conditioned medium also strongly affected the intracellular distribution of actin fibers. Conditioned medium of BAECs contained TGFβ1 and TGFβ2. The TGFβ receptor antagonist SB431542 as well as simultaneous treatment with TGFβ1 and TGFβ2 neutralizing antibodies completely reversed the above effects of endothelial conditioned medium on BASMCs. BAECs medium induced phosphorylation of Smad2 and increased ATP-induced phosphorylation of myosin light chain in BASMCs. The present results indicate that the release of TGFβ1 and TGFβ2 from vascular endothelium affects the contractility of vascular smooth muscle cells by altering their morphology and agonist-induced Ca(2+) mobilization.

Endothelium-dependent epithelial-mesenchymal transition of tumor cells: exclusive roles of transforming growth factor β1 and β2

BACKGROUND

Induction of epithelial-mesenchymal transition (EMT) is essential for the metastasis of tumor cells and maintaining their stemness. This study aimed to examine whether endothelial cells, which are most closely located to tumor cells in vivo, play a role in inducing EMT in tumor cells or not.

METHODS

Concentrated culture medium of bovine aortic endothelial cells (BAECs) was applied to tumor cell lines (A549 and PANC-1) and epithelial cell line (NMuMg). Cadherin conversion, expressions of α-smooth muscle actin and ZO-1, actin fiber formation and cell migration were examined as hallmarks of the induction of EMT in these cell lines. Transforming growth factor β (TGFβ) antibodies were used to neutralize TGFβ1, TGFβ2 and TGFβ3. Expression and release of TGFβ proteins in BAECs as well as in porcine and human endothelial cells were assessed by Western blotting and ELISA, respectively.

RESULTS

Conditioned medium of BAEC induced EMT in the examined cell lines. All endothelial cells from various species and locations expressed TGFβ1 and TGFβ2 proteins and much lower level of TGFβ3 protein. Conditioned medium from these endothelial cells contained TGFβ1 and TGFβ2, but TGFβ3 could not be detected. Neutralizing antibody against each of TGFβ1 or TGFβ2 did not reverse endothelium-dependent EMT, but simultaneous neutralization of both TGFβ1 and TGFβ2 completely abolished it.

CONCLUSIONS

Endothelial cells may play a role in the induction and maintenance of EMT in tumor cells by constitutively releasing TGFβ1 and TGFβ2.

GENERAL SIGNIFICANCE

The present results provide a novel strategy of the inhibition of tumor metastasis by targeting vascular endothelium.

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.

Hypotonic swelling-induced activation of PKN1 mediates cell survival in cardiac myocytes

Hypotonic cell swelling in the myocardium is induced by pathological conditions, including ischemia-reperfusion, and affects the activities of ion transporters/channels and gene expression. However, the signaling mechanism activated by hypotonic stress (HS) is not fully understood in cardiac myocytes. A specialized protein kinase cascade, consisting of Pkc1 and MAPKs, is activated by HS in yeast. Here, we demonstrate that protein kinase N1 (PKN1), a serine/threonine protein kinase and a homolog of Pkc1, is activated by HS (67% osmolarity) within 5 min and reaches peak activity at 60 min in cardiac myocytes. Activation of PKN1 by HS was accompanied by Thr(774) phosphorylation and concomitant activation of PDK1, a potential upstream regulator of PKN1. HS also activated RhoA, thereby increasing interactions between PKN1 and RhoA. PP1 (10(-5) M), a selective Src family tyrosine kinase inhibitor, significantly suppressed HS-induced activation of RhoA and PKN1. Constitutively active PKN1 significantly increased the transcriptional activity of Elk1-GAL4, an effect that was inhibited by dominant negative MEK. Overexpression of PKN1 significantly increased ERK phosphorylation, whereas downregulation of PKN1 inhibited HS-induced ERK phosphorylation. Downregulation of PKN1 and inhibition of ERK by U-0126 both significantly inhibited the survival of cardiac myocytes in the presence of HS. These results suggest that a signaling cascade, consisting of Src, RhoA, PKN1, and ERK, is activated by HS, thereby promoting cardiac myocyte survival.

Transforming growth factor beta1 alters calcium mobilizing properties and endogenous ATP release in A549 cells: possible implications for cell migration

We examined the effects of transforming growth factor beta(1) (TGFbeta(1)) on cellular functions in human lung cancer cell line A549. Treatment of A549 cells with 1 ng/ml TGFbeta(1) for more than 3 days altered their morphology from an epithelial cobblestone-like appearance to a fibroblast-like one, reduced the expression of E-cadherin mRNA and protein, and induced the formation of F-actin fibers. These hallmarks indicate that TGFbeta(1) induced the epithelial-mesenchymal transition in A549 cells. Migration of TGFbeta(1)-treated A549 cells, which was quantified by the wound-healing assay, was markedly accelerated by 3 microM ATPgammaS, a non-hydrolyzable ATP analogue. ATPgammaS-induced migration of TGFbeta(1)-treated A549 cells was reversed by the P2 antagonist suramin. In contrast, migration of control A549 cells was not altered by ATPgammaS. TGFbeta(1)-treated A549 cells showed an augmentation of ATP-induced Ca(2+) transients, thapsigargin-induced Ca(2+) transients, and store-operated Ca(2+) entry compared with those in control cells. Basal level of the extracellular ATP concentration was significantly lower in TGFbeta(1)-treated A549 cells than in control cells. We conclude from these results that TGFbeta(1) augments ATP-induced Ca(2+) mobilization, which leads to the acceleration of migration, in A549 cells but, it markedly reduces endogenous ATP release. This implies that the actions of ATP would become a novel therapeutic target for inhibiting cancer cell migration.

CFTR Cl- channel functional regulation by phosphorylation of focal adhesion kinase at tyrosine 407 in osmosensitive ion transporting mitochondria rich cells of euryhaline killifish

Cystic fibrosis transmembrane conductance regulator (CFTR) anion channels are the regulated exit pathway in Cl(-) secretion by teleost mitochondria rich salt secreting (MR) cells of the gill and opercular epithelia of euryhaline teleosts. By confocal light immunocytochemistry, immunogold transmission electron microscopy (TEM), and co-immunoprecipitation, using regular and phospho-antibodies directed against conserved sites, we found that killifish CFTR (kfCFTR) and the tyrosine kinase focal adhesion kinase (FAK) phosphorylated at Y407 (FAK pY407) are colocalized in the apical membrane and in subjacent membrane vesicles of MR cells. We showed previously that basolateral FAK pY407, unlike other FAK phosphorylation sites, is osmosensitive and dephosphorylates during hypotonic shock of epithelial cells (Marshall et al., 2008). In the present study, we found that hypotonic shock and the alpha(2)-adrenergic agonist clonidine (neither of which affects cAMP levels) rapidly and reversibly inhibit Cl(-) secretion by isolated opercular membranes, simultaneous with dephosphorylation of FAK pY407, located in the apical membrane. FAK pY407 is rephosphorylated and Cl(-) secretion rapidly restored by hypertonic shock as well as by forskolin and isoproterenol, which operate via cAMP and protein kinase A. We conclude that hormone mediated, cAMP dependent and osmotically mediated, cAMP independent pathways converge on a mechanism to activate CFTR and Cl(-) secretion, possibly through tyrosine phosphorylation of CFTR by FAK.

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.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.