Phosphotyrosine-containing proteins are concentrated in focal adhesions and intercellular junctions in normal cells

RBP Image Database: A resource for the systematic characterization of the subcellular distribution properties of human RNA binding proteins

RNA binding proteins (RBPs) are central regulators of gene expression implicated in all facets of RNA metabolism. As such, they play key roles in cellular physiology and disease etiology. Since different steps of post-transcriptional gene expression tend to occur in specific regions of the cell, including nuclear or cytoplasmic locations, defining the subcellular distribution properties of RBPs is an important step in assessing their potential functions. Here, we present the RBP Image Database, a resource that details the subcellular localization features of 301 RBPs in the human HepG2 and HeLa cell lines, based on the results of systematic immuno-fluorescence studies conducted using a highly validated collection of RBP antibodies and a panel of 12 markers for specific organelles and subcellular structures. The unique features of the RBP Image Database include: (i) hosting of comprehensive representative images for each RBP-marker pair, with ∼250,000 microscopy images; (ii) a manually curated controlled vocabulary of annotation terms detailing the localization features of each factor; and (iii) a user-friendly interface allowing the rapid querying of the data by target or annotation. The RBP Image Database is freely available at https://rnabiology.ircm.qc.ca/RBPImage/.

CK2β Is a Gatekeeper of Focal Adhesions Regulating Cell Spreading

CK2 is a hetero-tetrameric serine/threonine protein kinase made up of two CK2α/αʹ catalytic subunits and two CK2β regulatory subunits. The free CK2α subunit and the tetrameric holoenzyme have distinct substrate specificity profiles, suggesting that the spatiotemporal organization of the individual CK2 subunits observed in living cells is crucial in the control of the many cellular processes that are governed by this pleiotropic kinase. Indeed, previous studies reported that the unbalanced expression of CK2 subunits is sufficient to drive epithelial to mesenchymal transition (EMT), a process involved in cancer invasion and metastasis. Moreover, sub-stoichiometric expression of CK2β compared to CK2α in a subset of breast cancer tumors was correlated with the induction of EMT markers and increased epithelial cell plasticity in breast carcinoma progression. Phenotypic changes of epithelial cells are often associated with the activation of phosphotyrosine signaling. Herein, using phosphotyrosine enrichment coupled with affinity capture and proteomic analysis, we show that decreased expression of CK2β in MCF10A mammary epithelial cells triggers the phosphorylation of a number of proteins on tyrosine residues and promotes the striking activation of the FAK1-Src-PAX1 signaling pathway. Moreover, morphometric analyses also reveal that CK2β loss increases the number and the spatial distribution of focal adhesion signaling complexes that coordinate the adhesive and migratory processes. Together, our findings allow positioning CK2β as a gatekeeper for cell spreading by restraining focal adhesion formation and invasion of mammary epithelial cells.

Ephrin receptor A2, the epithelial receptor for Epstein-Barr virus entry, is not available for efficient infection in human gastric organoids

Epstein-Barr virus (EBV) is best known for infection of B cells, in which it usually establishes an asymptomatic lifelong infection, but is also associated with the development of multiple B cell lymphomas. EBV also infects epithelial cells and is associated with all cases of undifferentiated nasopharyngeal carcinoma (NPC). EBV is etiologically linked with at least 8% of gastric cancer (EBVaGC) that comprises a genetically and epigenetically distinct subset of GC. Although we have a very good understanding of B cell entry and lymphomagenesis, the sequence of events leading to EBVaGC remains poorly understood. Recently, ephrin receptor A2 (EPHA2) was proposed as the epithelial cell receptor on human cancer cell lines. Although we confirm some of these results, we demonstrate that EBV does not infect healthy adult stem cell-derived gastric organoids. In matched pairs of normal and cancer-derived organoids from the same patient, EBV only reproducibly infected the cancer organoids. While there was no clear pattern of differential expression between normal and cancer organoids for EPHA2 at the RNA and protein level, the subcellular location of the protein differed markedly. Confocal microscopy showed EPHA2 localization at the cell-cell junctions in primary cells, but not in cancer cell lines. Furthermore, histologic analysis of patient tissue revealed the absence of EBV in healthy epithelium and presence of EBV in epithelial cells from inflamed tissue. These data suggest that the EPHA2 receptor is not accessible to EBV on healthy gastric epithelial cells with intact cell-cell contacts, but either this or another, yet to be identified receptor may become accessible following cellular changes induced by inflammation or transformation, rendering changes in the cellular architecture an essential prerequisite to EBV infection.

Phosphorylation of RIAM Activates Its Adaptor Function in Mediating Integrin Signaling

Integrins are cellular receptors that regulate cell adhesion and many other cellular functions. Integrins can be activated via an "inside-out pathway" that is promoted by RAP1 GTPase. RAP1-GTP-Interacting Adaptor Molecular (RIAM) mediates integrin activation by linking RAP1 GTPase to talin, an integrin activator. RIAM's function in integrin signaling is tightly regulated. In this commentary, we review recent studies of the molecular mechanisms underlying RIAM autoinhibition via both intramolecular interaction and oligomer assembly, and the phosphorylation-dependent activation of RIAM.

Adherens Junctions on the Move-Membrane Trafficking of E-Cadherin

Cadherin-based adherens junctions are conserved structures that mediate epithelial cell-cell adhesion in invertebrates and vertebrates. Despite their pivotal function in epithelial integrity, adherens junctions show a remarkable plasticity that is a prerequisite for tissue architecture and morphogenesis. Epithelial cadherin (E-cadherin) is continuously turned over and undergoes cycles of endocytosis, sorting and recycling back to the plasma membrane. Mammalian cell culture and genetically tractable model systems such as Drosophila have revealed conserved, but also distinct, mechanisms in the regulation of E-cadherin membrane trafficking. Here, we discuss our current knowledge about molecules and mechanisms controlling endocytosis, sorting and recycling of E-cadherin during junctional remodeling.

Regulation of Endothelial Adherens Junctions by Tyrosine Phosphorylation

Endothelial cells form a semipermeable, regulated barrier that limits the passage of fluid, small molecules, and leukocytes between the bloodstream and the surrounding tissues. The adherens junction, a major mechanism of intercellular adhesion, is comprised of transmembrane cadherins forming homotypic interactions between adjacent cells and associated cytoplasmic catenins linking the cadherins to the cytoskeleton. Inflammatory conditions promote the disassembly of the adherens junction and a loss of intercellular adhesion, creating openings or gaps in the endothelium through which small molecules diffuse and leukocytes transmigrate. Tyrosine kinase signaling has emerged as a central regulator of the inflammatory response, partly through direct phosphorylation and dephosphorylation of the adherens junction components. This review discusses the findings that support and those that argue against a direct effect of cadherin and catenin phosphorylation in the disassembly of the adherens junction. Recent findings indicate a complex interaction between kinases, phosphatases, and the adherens junction components that allow a fine regulation of the endothelial permeability to small molecules, leukocyte migration, and barrier resealing.

The genesis of tyrosine phosphorylation

Tyrosine phosphorylation of proteins was discovered in 1979, but this posttranslational modification had been "invented" by evolution more than a billion years ago in single-celled eukaryotic organisms that were the antecedents of the first multicellular animals. Because sophisticated cell-cell communication is a sine qua non for the existence of multicellular organisms, the development of cell-surface receptor systems that use tyrosine phosphorylation for transmembrane signal transduction and intracellular signaling seems likely to have been a crucial event in the evolution of metazoans. Like all types of protein phosphorylation, tyrosine phosphorylation serves to regulate proteins in multiple ways, including causing electrostatic repulsion and inducing allosteric transitions, but the most important function of phosphotyrosine (P.Tyr) is to serve as a docking site that promotes a specific interaction between a tyrosine phosphorylated protein and another protein that contains a P.Tyr-binding domain, such as an SH2 or PTB domain. Such docking interactions are essential for signal transduction downstream from receptor tyrosine kinases (RTKs) on the cell surface, which are activated on binding a cognate extracellular ligand, and, as a consequence, elicit specific cellular outcomes.

RPTPα controls epithelial adherens junctions, linking E-cadherin engagement to c-Src-mediated phosphorylation of cortactin

Epithelial junctions are fundamental determinants of tissue organization, subject to regulation by tyrosine phosphorylation. Homophilic binding of E-cadherin activates tyrosine kinases, such as Src, that control junctional integrity. Protein tyrosine phosphatases (PTPs) also contribute to cadherin-based adhesion and signaling, but little is known about their specific identity or functions at epithelial junctions. Here, we report that the receptor PTP RPTPα (human gene name PTPRA) is recruited to epithelial adherens junctions at the time of cell-cell contact, where it is in molecular proximity to E-cadherin. RPTPα is required for appropriate cadherin-dependent adhesion and for cyst architecture in three-dimensional culture. Loss of RPTPα impairs adherens junction integrity, as manifested by defective E-cadherin accumulation and peri-junctional F-actin density. These effects correlate with a role for RPTPα in cellular (c)-Src activation at sites of E-cadherin engagement. Mechanistically, RPTPα is required for appropriate tyrosine phosphorylation of cortactin, a major Src substrate and a cytoskeletal actin organizer. Expression of a phosphomimetic cortactin mutant in RPTPα-depleted cells partially rescues F-actin and E-cadherin accumulation at intercellular contacts. These findings indicate that RPTPα controls cadherin-mediated signaling by linking homophilic E-cadherin engagement to cortactin tyrosine phosphorylation through c-Src.

Abl regulates planar polarized junctional dynamics through β-catenin tyrosine phosphorylation

Interactions between epithelial cells are mediated by adherens junctions that are dynamically regulated during development. Here we show that the turnover of β-catenin is increased at cell interfaces that are targeted for disassembly during Drosophila axis elongation. The Abl tyrosine kinase is concentrated at specific planar junctions and is necessary for polarized β-catenin localization and dynamics. abl mutant embryos have decreased β-catenin turnover at shrinking edges, and these defects are accompanied by a reduction in multicellular rosette formation and axis elongation. Abl promotes β-catenin phosphorylation on the conserved tyrosine 667 and expression of an unphosphorylatable β-catenin mutant recapitulates the defects of abl mutants. Notably, a phosphomimetic β-catenin(Y667E) mutation is sufficient to increase β-catenin turnover and rescue axis elongation in abl deficient embryos. These results demonstrate that the asymmetrically localized Abl tyrosine kinase directs planar polarized junctional remodeling during Drosophila axis elongation through the tyrosine phosphorylation of β-catenin.

Large-scale phosphotyrosine proteomic profiling of rat renal collecting duct epithelium reveals predominance of proteins involved in cell polarity determination

Although extensive phosphoproteomic information is available for renal epithelial cells, previous emphasis has been on phosphorylation of serines and threonines with little focus on tyrosine phosphorylation. Here we have carried out large-scale identification of phosphotyrosine sites in pervanadate-treated native inner medullary collecting ducts of rat, with a view towards identification of physiological processes in epithelial cells that are potentially regulated by tyrosine phosphorylation. The method combined antibody-based affinity purification of tyrosine phosphorylated peptides coupled with immobilized metal ion chromatography to enrich tyrosine phosphopeptides, which were identified by LC-MS/MS. A total of 418 unique tyrosine phosphorylation sites in 273 proteins were identified. A large fraction of these sites have not been previously reported on standard phosphoproteomic databases. All results are accessible via an online database: http://helixweb.nih.gov/ESBL/Database/iPY/. Analysis of surrounding sequences revealed four overrepresented motifs: [D/E]xxY*, Y*xxP, DY*, and Y*E, where the asterisk symbol indicates the site of phosphorylation. These motifs plus contextual information, integrated using the NetworKIN tool, suggest that the protein tyrosine kinases involved include members of the insulin- and ephrin-receptor kinase families. Analysis of the gene ontology (GO) terms and KEGG pathways whose protein elements are overrepresented in our data set point to structures involved in epithelial cell-cell and cell-matrix interactions ("adherens junction," "tight junction," and "focal adhesion") and to components of the actin cytoskeleton as major sites of tyrosine phosphorylation in these cells. In general, these findings mesh well with evidence that tyrosine phosphorylation plays a key role in epithelial polarity determination.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Trask phosphorylation defines the reverse mode of a phosphotyrosine signaling switch that underlies cell anchorage state

Phosphotyrosine signaling in anchored epithelial cells constitutes a spacially ordained signaling program that largely functions to promote integrin-linked focal adhesion complexes, serving to secure cell anchorage to matrix and as a bidirectional signaling hub that coordinates the physical state of the cell and its environment with cellular functions including proliferation and survival. Cells release their adhesions during processes such as mitosis, migration, or tumorigenesis, but the fate of signaling through tyrosine phosphorylation in unanchored cells remains poorly understood. In an examination of epithelial cells in the unanchored state, we find abundant phosphotyrosine signaling, largely recommitted to an anti-adhesive function mediated through the Src family phosphorylation of their transmembrane substrate Trask/CDCP1/gp140. Src-Trask phosphorylation inhibits integrin clustering and focal adhesion assembly and signaling, defining an active phosphotyrosine signaling program underlying the unanchored state. Src-Trask signaling and Src-focal adhesion signaling inactivate each other, constituting two opposing modes of phosphotyrosine signaling that define a switch underline cell anchorage state. Src kinases are prominent drivers of both signaling modes, identifying their position at the helm of adhesion signaling capable of specifying anchorage state through substrate selection. These experimental studies along with concurring phylogenetic evidence suggest that phosphorylation on tyrosine is a signaling function fundamentally linked with the regulation of integrins.

Differences in the mechanism of collagen lattice contraction by myofibroblasts and smooth muscle cells

Both rat derived vascular smooth muscle cells (SMC) and human myofibroblasts contain α smooth muscle actin (SMA), but they utilize different mechanisms to contract populated collagen lattices (PCLs). The difference is in how the cells generate the force that contracts the lattices. Human dermal fibroblasts transform into myofibroblasts, expressing α-SMA within stress fibers, when cultured in lattices that remain attached to the surface of a tissue culture dish. When attached lattices are populated with rat derived vascular SMC, the cells retain their vascular SMC phenotype. Comparing the contraction of attached PCLs when they are released from the culture dish on day 4 shows that lattices populated with rat vascular SMC contract less than those populated with human myofibroblast. PCL contraction was evaluated in the presence of vanadate and genistein, which modify protein tyrosine phosphorylation, and ML-7 and Y-27632, which modify myosin ATPase activity. Genistein and ML-7 had no affect upon either myofibroblast or vascular SMC-PCL contraction, demonstrating that neither protein tyrosine kinase nor myosin light chain kinase was involved. Vanadate inhibited myofibroblast-PCL contraction, consistent with a role for protein tyrosine phosphatase activity with myofibroblast-generated forces. Y-27632 inhibited both SMC and myofibroblast PCL contraction, consistent with a central role of myosin light chain phosphatase.

Locomotion guidance by extracellular matrix is adaptive and can be restored by a transient change in Ca2+ level

Navigation of cell locomotion by gradients of soluble factors can be desensitized if the concentration of the chemo-attractant stays unchanged. It remains obscure if the guidance by immobilized extracellular matrix (ECM) as the substrate is also adaptive and if so, how can the desensitized ECM guidance be resensitized. When first interacting with a substrate containing micron-scale fibronectin (FBN) trails, highly motile fish keratocytes selectively adhere and migrate along the FBN paths. However, such guided motion become adaptive after about 10 min and the cells start to migrate out of the ECM trails. We found that a burst increase of intracellular calcium created by an uncaging technique immediately halts the undirected migration by disrupting the ECM-cytoskeleton coupling, as evidenced by the appearance of retrograde F-actin flow. When the motility later resumes, the activated integrin receptors render the cell selectively binding to the FBN path and reinitiates signaling events, including tyrosine phosphorylation of paxillin, that couple retrograde F-actin flow to the substrate. Thus, the calcium-resensitized cell can undergo a period of ECM-navigated movement, which later becomes desensitized. Our results also suggest that endogenous calcium transients as occur during spontaneous calcium oscillations may exert a cycling resensitization-desensitization control over cell's sensing of substrate guiding cues.

Imaging morphogenesis, in Xenopus with Quantum Dot nanocrystals

Mesoderm migration is a well studied morphogenetic movement that takes place during Xenopus gastrulation. The study of mesoderm migration and other morphogenetic movements has been primarily based on in vitro assays due to the inability to image deep tissue movements in the opaque embryo. We are the first to report the use of Near Infra Red Quantum Dots (NIR QD's) to image mesoderm migration in vivo with single cell resolution and provide quantitative in vivo data regarding migration rates. In addition we use QD's to address the function of the focal adhesion kinase (FAK) in this movement. Inhibition of FAK blocks mesoderm spreading and migration both in vitro and in vivo without affecting convergent extension highlighting the molecular differences between the two movements. These results provide new insights about the role of FAK and of focal adhesions during gastrulation and provide a new tool for the study of morphogenesis in vivo.

FAK mediates the inhibition of glioma cell migration by truncated 24 kDa FGF-2

A truncated form of 24kDa FGF-2 consisting of 86 NH(2)-terminal amino acids (ATE+31) inhibits cell migration in vitro and tumor development and angiogenesis in vivo. Focal adhesion kinase (FAK) is phosphorylated on tyrosine and serine sites after cell stimulation by growth factors. This study examined the effect of ATE+31 on FAK phosphorylation in human glioma cells. FAK and Pyk phosphorylation were evaluated at serines known to be involved with cell migration. We demonstrated that ATE+31 at 3 x 10(-11)M decreases phosphorylation levels of Tyr(407)-FAK and Ser(732)-FAK in the presence of platelet-derived growth factor (PDGF), that ATE+31 in the presence of PDGF alters the distribution of FAK and other phosphotyrosine proteins in the adhesion contacts, and that ATE+31 in the presence of PDGF has no effect on the activation of Pyk2. These data suggest that the inhibition of cell migration by ATE+31 occurs via Tyr(407)-FAK and Ser(732)-FAK.

Genetic and cell biological analysis of integrin outside-in signaling

Integrins are cell surface transmembrane receptors that recognize and bind to extracellular matrix proteins and counter receptors. Binding of activated integrins to their ligands induces a vast number of structural and signaling changes within the cell. Large, multimolecular complexes assemble onto the cytoplasmic tails of activated integrins to engage and organize the cytoskeleton, and activate signaling pathways that ultimately lead to changes in gene expression. Additionally, integrin-mediated signaling intersects with growth factor-mediated signaling through various levels of cross-talk. This review discusses recent work that has tremendously broadened our understanding of the complexity of integrin-mediated signaling.

Action of the colony-stimulating factor, CSF-1

Colony-stimulating factor 1 (CSF-1) is a glycoprotein growth factor that specifically regulates the survival, proliferation and differentiation of mononuclear phagocytes and their precursors via a cell surface receptor selectively expressed on these cell types. The purified receptor is a single glycosylated polypeptide, Mr 165 000, which exhibits CSF-1-dependent autophosphorylation in tyrosine. CSF-1 alone regulates cells of the mononuclear phagocytic series (CSF-1-dependent colony-forming unit [CFU-C]----monoblast----promonocyte----monocyte----macrophage). However, the presence of a multipotent haemopoietic cell growth factor, haemopoietin-1, permits CSF-1 to stimulate precursors of CFU-C to proliferate and differentiate to macrophages. Precursors of CFU-C possess low levels of the CSF-1 receptor but there is an increase in receptor levels on CFU-C just before their differentiation to adherent, proliferating mononuclear phagocytes. As the timing of this developmentally associated increase in receptor expression coincides with the acquisition of responsiveness to CSF-1 alone, it is an early indicator of determination to the mononuclear phagocytic lineage.

Calreticulin affects fibronectin-based cell-substratum adhesion via the regulation of c-Src activity

Calreticulin is an endoplasmic reticulum Ca2+-storage protein, which influences gene expression and cell adhesion. In this study, we show that calreticulin induces fibronectin gene expression and matrix deposition, leading to differences in cell spreading and focal adhesion formation in cells differentially expressing calreticulin. We further show that these effects of calreticulin occur via a c-Src-regulated pathway and that c-Src activity is inversely related to calreticulin abundance. Since c-Src is an important regulator of focal contact turnover, we investigated the effect of c-Src inhibition on cells differentially expressing calreticulin. Inhibition of c-Src rescued the poorly adhesive phenotype of the calreticulin-underexpressing cells in that they became well spread, commenced formation of numerous focal contacts, and deposited a rich fibronectin matrix. Importantly, we show that c-Src activity is dependent on releasable Ca2+ from the endoplasmic reticulum, thus implicating Ca2+-sensitive pathways that are affected by calreticulin in cell-substratum adhesion. We propose that calreticulin affects fibronectin synthesis and matrix assembly via the regulation of fibronectin gene expression. In parallel, calcium-dependent effects of calreticulin on c-Src activity influence the formation and/or stability of focal contacts, which are instrumental in matrix assembly and remodeling.

Vanadate ingestion enhances the organization and collagen fibril diameters of rat healing medical collateral ligaments

Although an injured medial collateral ligament (MCL) will naturally heal, the quality of healing tissue is inferior to the uninjured MCL tissue. Previous studies have shown promising results of sodium orthovanadate (vanadate) in enhancing the quality of rat skin wounds. This study therefore investigated whether vanadate enhances the quality of the rat healing MCL in terms of the collagen fibril organization and diameter. Six mature male Sprague-Dawley rats, with weight ranges of 475-505 g and ages of 25 weeks, were used in this study. Three rats in the experimental group received vanadate (0.2 mg/ml) in their saline drinking water (150 mM NaCl), whereas three rats in the control group were only given saline water. Three weeks after transection, the rat MCLs were harvested for hematoxylin and eosin (H&E) staining and transmission electron microscopy. It was found that vanadate promoted organization of collagen fibrils and significantly increased the diameters of collagen fibrils by 14% in healing MCL (P<0.001). These results indicate that application of vanadate may be a promising tissue engineering approach to enhance the quality of healing tissues such as injured MCLs.

Quantitative kinetic analysis of gene expression during human osteoblastic adhesion on orthopaedic materials

Little information was found in the literature about the expression on hydroxyapatite (HA) materials of genes specific of cellular adhesion molecules although more were found on titanium-based substrates. Hence, the goal of this work was to study by a kinetic approach from 30 min to 4 days the adhesion of Saos-2 cells on microporous (mHA) and non-microporous hydroxyapatite (pHA) in comparison to polished titanium. Our strategy associated the visualization of adhesion proteins inside the cells by immunohistochemistry and the quantitative expression of genes at mRNA level by real-time PCR. The cell morphology was assessed using scanning electron microscopy and the number of cells thanks to biochemical techniques. The cellular attachment was the highest on mHA from 30 min to 24 h although the cell growth on mHA was the lowest after 4 days. Generally, the Saos-2 osteoblastic cells morphology on mHA was radically different than on other surfaces with the particularity of the cytoplasmic edge, which appeared un-distinguishable from the surface. The revelation by specific antibodies of proteins of the cytoskeleton (actin) and the focal adhesions (FAK, phosphotyrosine) confirmed that adhesion and spreading were different on the 3 materials. The actin stress fibres were less numerous and shorter on mHA ceramics. Cells had more focal contacts after 4 h on mHA compared to other substrates but less after 24 h. The highest values of total proteins were extracted from mHA at 0.5 and 24 h and from pHA at 1, 4, and 96 h. The alphav and beta1 integrin, actin, FAK, and ERK gene expression were found to be different with adhesion time and with materials. C-jun expression was comparable on mHA, titanium and plastic but was largely higher than on pHA at 0.5 and 1 h. On the contrary, c-fos expression was the highest on pHA after 0.5 h and the lowest after 1h. This difference between c-fos and c-jun expression on pHA after 0.5 h could be related to the fact that these two genes may differ in their signalling pathways. The expression of the alkaline phosphatase gene after 4 days was lower on mHA compared to other materials demonstrating that the microstructure of the mHA ceramic was not favourable to Saos-2 cells differentiation. Finally, it was demonstrated in this study that HA and titanium surfaces influence as well gene expression at early times of adhesion as the synthesis of adhesion proteins but also proliferation and differentiation phases. Indeed, the signal transduction pathways involved in adhesion of Saos-2 cells on HA and titanium were confirmed by the sequential expression of alphav and beta1 integrins, FAK, and ERK genes followed by the expression of c-jun and c-fos genes for proliferation and alkaline phosphatase gene for differentiation.

Analysis of Chlamydia caviae entry sites and involvement of Cdc42 and Rac activity

In epithelial cells, endocytic activity is mostly dedicated to nutrient and macromolecule uptake. To invade these cells, Chlamydiaceae, like other pathogens, have evolved strategies that utilise the existing endocytic machineries and signalling pathways, but little is known about the host cell molecules involved. In this report, we show that within five minutes of infection of HeLa cells by Chlamydia caviae GPIC strain several events take place in the immediate vicinity of invasive bacteria: GM1-containing microdomains cluster, tyrosine-phosphorylated proteins accumulate, and intense actin polymerization occurs. We show that actin polymerization is controlled by the small GTPases Cdc42 and Rac, which become activated upon infection. Expression of dominant negative forms of these GTPases inhibits C. caviae entry and leads to abnormal actin polymerization. In contrast, the small GTPase Rho does not seem essential for bacterial entry. Finally, phosphatidylinositol 3-kinase activity is also required for internalization of C. caviae, probably downstream of the other molecular events reported here. We present the first scheme of the events occurring at the sites of invasion of epithelial cells by a member of the Chlamydiaceae family.

Nerve growth factor (NGF) induces a rapid and sustained downregulation of the focal adhesion kinase (FAK)

1. Exposure of PC12 cells to nerve growth factor (NGF) induces an early tyrosine phosphorylation of many proteins, a number of which is still unidentified. Although NGF is known to bind to and activate the receptor tyrosine kinase TrkA, many downstream targets of NGF signaling may be possibly phosphorylated by nonreceptor tyrosine kinases such as c-Src and focal adhesion kinase (FAK). 2. In the present study, exposure of TrkA-overexpressing PC12 cells to NGF is found to cause a rapid and sustained loss in the recovery of a subpopulation of nominally active FAK (i.e., being autophosphorylated on the positive site of regulation). 3. Consistent with the possibility that NGF induces the proteolysis of FAK via recruitment of Src family kinases, the use of various phosphorylation site-specific anti-FAK antibodies revealed an NGF-inducible and PP1-sensitive accumulation of a putative fragment (i.e., p62) of FAK. Significantly, the mitogenic epidermal growth factor (EGF) failed to induce the downregulation of FAK and the accumulation of tyrosine phosphorylated p62. Such differential response of FAK to NGF and EGF may shape the specificity by which these growth factors control the status of cell-matrix adhesion and the adhesion-driven signaling.

Cytoskeletal reorganization dependence of signaling by the gonadotropin-releasing hormone receptor

Activation of classical G protein-coupled receptors (GPCRs) like the mammalian gonadotropin-releasing hormone receptor (GnRHR) typically stimulates heterotrimeric G protein molecules that subsequently activate downstream effectors. Receptor activation of heterotrimeric G protein pathways primarily controls intermediary cell metabolism by elevation or diminution of soluble cytoplasmic second messenger molecules. We have demonstrated here that stimulation of the GnRHR also results in a dramatic change in both cell adhesion and superstructural morphology. Gonadotropin-releasing hormone (GnRH) receptor activation rapidly increases the capacity of HEK293 cells expressing the GnRHR to remain matrix-adherent in the face of fluid insults. Coinciding with this profound elevation in matrix adherence, we demonstrated a GnRH-induced alteration in both cell morphology and the de novo generation of polymerized actin structures. GnRH induction of cytoskeletal remodeling was correlated with significant increases in the tyrosine phosphorylation status of a series of cytoskeletal associated proteins, e.g. focal adhesion kinase (FAK), c-Src, and microtubule-associated protein kinase (MAPK or ERK1/2). The activation of the distal downstream effector ERK1/2 was demonstrated to be sensitive to the disrupters of cytoskeletal rearrangement, cytochalasin D and latrunculin B. In addition to the sensitivity of ERKs to cytoskeletal integrity, GnRH-induced FAK and c-Src kinase activation were sensitive to these agents and the fibronectin-integrin antagonistic RGDS peptide. Activation of ERK was dependent on its protein-protein assembly with FAK and c-Src at focal adhesion complexes. Induction of the cell remodeling event leading to this signaling complex assembly occurred primarily via GnRHR activation of the monomeric G protein Rac but not RhoA. These findings demonstrated a clear divergence of GnRHR signaling via the Rac monomeric G protein focal adhesion signaling complex assembly and cytoskeletal remodeling independent of the classical heterotrimeric G protein-controlled phospholipase C-beta pathway.

Systemic vanadate ingestion modulates rat tendon repair

The chronic ingestion of vanadate prevents the appearance of myofibroblasts within granulation tissue of full excision wounds in rats, yet these wounds close at an optimal rate. Myofibroblasts are reported in the repair of transected tendons. Here we investigate tendon repair in the absence of myofibroblasts. Vanadate in saline drinking water was given to rats in the experimental group, while rats in the control group received saline alone. The Achilles tendon of the left leg of each rat was transected and suture repaired. On day 10, both repaired tendons and uninjured tendons from the right leg were harvested and processed for histology. By immunohistology the repaired tendons of control rats had myofibroblasts (fibroblasts with alpha smooth muscle actin positive stress fibers), while myofibroblasts were absent in healing tendons from vanadate-treated rats. By transmission electron microscopy and polarized light optics, repaired tendons of control rats demonstrated thin, loosely packed, immature collagen fiber bundles. Collagen fiber bundles from healing tendons of the vanadate-treated group were thicker, uniformly packed, and more mature. The chronic ingestion of vanadate promotes the more rapid organization of collagen fiber bundles of healing transected tendons in the absence of myofibroblasts.

Topical vanadate optimizes collagen organization within granulation tissue

Systemic ingestion of vanadate, a nonspecific inhibitor of tyrosine phosphatases, doubles wound breaking strength, enhances the packing of collagen fibers, and prevents the appearance of myofibroblasts in granulation tissue. Will the local application of vanadate mimic the systemic effects? Pairs of polyvinyl alcohol sponges, each with a central reservoir and attached injection port, were subcutaneously implanted in rats. Daily, one implant received 0.2 ml of saline and the other received 0.2 ml of 0.03 mM vanadate in saline. On day 7, harvested sponges had equivalent wet weights. The vanadate-treated sponges had fibroblasts separated by connective tissue, with a more intense birefringence of the collagen fibers. Transmission electron microscopy showed collagen more uniformly packed in the vanadate treated sponges where collagen fibers were equally spaced and had equal diameters. By immunohistology, myofibroblasts, defined by the expression of alpha-smooth muscle actin within stress fibers, were absent in vanadate-treated granulation tissue. The expression of alpha-smooth muscle actin was restricted to smooth muscle cells of blood vessels. Controls had densely packed alpha-smooth muscle actin staining myofibroblasts, weak birefringence, and randomly spaced collagen fibers with irregular diameters. We conclude that the local application of vanadate prevents the appearance of myofibroblasts and optimizes the organization of collagen fibers in developing granulation tissue.

Nephrocystin-conserved domains involved in targeting to epithelial cell-cell junctions, interaction with filamins, and establishing cell polarity

Nephrocystin is the protein product of the gene mutated in juvenile nephronophthisis, an autosomal recessive cystic kidney disease afflicting children and young adults. Because the normal cellular function of nephrocystin is largely unknown, the molecular defects underlying disease pathogenesis remain obscure. Analysis of nephrocystin amino acid sequences from human and other species revealed three distinct conserved domains including Src homology 3 and coil-coil domains in the N-terminal region, as well as a large highly conserved C-terminal region bearing no obvious homology to other proteins and hence referred to as the "nephrocystin homology domain" (NHD). The objective of this study was to gain insight into nephrocystin function by defining functional properties of the conserved domains. We analyzed a series of nephrocystin deletion mutants expressed in Madin-Darby canine kidney and COS-7 cells. This analysis revealed previously unrecognized functional attributes of the NHD, including abilities to promote both self-association and epithelial cell-cell junctional targeting. We further observed that Madin-Darby canine kidney cell lines stably expressing a nephrocystin mutant with a deletion of the Src homology 3 domain have reduced ability to establish tight junctions as measured by transepithelial electrical resistance. Finally, from a two-hybrid screen and coimmunoprecipitation studies we identified members of the filamin family of actin-binding proteins as having the capacity to interact with the NHD. These findings support a functional role for nephrocystin as a docking protein involved in organizing a protein complex to regulate the actin cytoskeleton at sites of epithelial cell-cell adhesion and further suggest that these properties are important for establishing epithelial cell polarity.

Sensing the environment: a historical perspective on integrin signal transduction

Cell adhesion mediated by integrin receptors has a critical function in organizing cells in tissues and in guiding haematopoietic cells to their sites of action. However, integrin adhesion receptors have broader functions in regulating cell behaviour through their ability to transduce bi-directional signals into and out of the cell and to engage in reciprocal interactions with other cellular receptors. This historical perspective traces the key findings that have led to our current understanding of these important functions of integrins.

Nitric oxide increases albumin permeability of isolated rat glomeruli via a phosphorylation-dependent mechanism

Nitric oxide (NO) has been implicated in the induction of proteinuria in acute inflammatory glomerulonephritis and in the increased vascular permeability seen in various other disease conditions. The complicated interactions of NO with other factors in vivo hinder analysis of the mechanisms involved. By use of a recently introduced method for measuring albumin permeability (P(a)) in isolated glomeruli, the question of whether NO has a direct effect on the permeability barrier of glomerular tufts was examined and the potential mechanisms were explored. Exposure of isolated glomeruli to three NO donors, s-nitroso-N-acetyl-penicillamine (SNAP), (Z)-1-[-2-(aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NONOate), and sodium nitroprusside, all increased the P(a). This action of NO was time- and concentration-dependent and could be mimicked by 8-bromoguanosine 3', 5'-cyclic monophosphate. Western blot analysis of the proteins from NO donor-treated glomeruli revealed an increase of phosphotyrosine levels of proteins of molecular mass about 120 and 70 kD. The demonstration that pretreatment of glomeruli with the tyrosine kinase inhibitor, genistein, could largely prevent the effect of SNAP and DETA-NONOate confirmed the crucial role of tyrosine phosphorylation in the NO-induced increase of P(a). Furthermore, the tyrosine phosphatase inhibitor, phenylarsine oxide (PAO), could mimic the action of NO on P(a). NO-enhanced tyrosine phosphorylation was further confirmed by immunofluorescence staining, where positive cells in SNAP- and PAO-treated glomeruli were much more frequent than that in controls. By use of dual-label staining in combination with podocyte specific marker, nephrin, it was observed that most of the phosphorylated positive cells corresponded to podocytes. These results suggest that NO impairs the glomerular permeability barrier through a tyrosine phosphorylation-dependent mechanism.

pp60c-src and related tyrosine kinases: a role in the assembly and reorganization of matrix adhesions

Activation of tyrosine kinases during integrin-mediated cell-matrix adhesion is involved both in the regulation of focal contact assembly and in the initiation of signaling processes at the cell-matrix adhesive interface. In order to determine the role of pp60c-src and related kinases in these processes, we have compared the dynamic reorganization of phosphotyrosine, vinculin, focal adhesion kinase and tensin in cells with altered expression of Src-family kinases. Both null cells for pp60c-src and triple knockout cells for pp60c-src, pp59fyn, and pp62c-yes exhibited decreased phosphotyrosine levels in focal contacts when compared with wild-type cells. pp60c-src-null cells also exhibited faster assembly of cell-matrix adhesions and a more exuberant recruitment of FAK to these sites. Tensin, which normally segregates into fibrillar adhesions was localized in large focal contacts in the two mutant cell lines, suggesting involvement of pp60c-src in the segregation of focal contacts and fibrillar adhesions. Moreover, treatment of wild-type cells with tyrphostin AG1007, which inhibits both pp60c-src and FAK activity, induced accumulation of tensin in peripheral focal adhesions. These findings demonstrate that Src family kinases, and pp60c-src in particular, have a central role in regulating protein dynamics at cell-matrix interfaces, both during early stages of interaction and in mature focal contacts.

Vanadate ingestion increases the gain in wound breaking strength and leads to better organized collagen fibers in rats during healing

Repair of incision wounds closed by suturing is evaluated by the progressive gain in wound breaking strength. Previously the closure of open wounds in rats ingesting vanadate, an inhibitor of tyrosine phosphate phosphatases, was shown to occur with deposition of more uniformly organized collagen fiber bundles. The hypothesis of this study was that deposition of more uniformly organized collagen fibers would enhance the gain in wound breaking strength of incisional wounds. Six adult rats received vanadate-supplemented saline drinking water for 1 week before placement of two 6-cm, parallel, suture-closed wounds on their backs. Six control rats received identical wounds and were given saline drinking water. The drinking water regimen was continued for 1 week after wounding, and then wound strength was tested with a tensiometer and tissue samples were obtained for histologic evaluation. Wound breaking strength doubled in vanadate-treated rats compared with controls. Bright-field and polarized light microscopy showed that the connective tissue matrix of granulation tissue from control rats was oriented perpendicular to the surface of the skin. In contrast, the connective tissue matrix of granulation tissue from vanadate-treated rats was oriented parallel to the skin surface. The gap in granulation tissue between the edges of the wounds in the vanadate-treated rats was greater than that in controls. Electron microscopy showed that wounds in the vanadate-treated contained uniform collagen fibers that were 20 percent greater in diameter and more evenly spaced than they were in controls. It is proposed that these changes in the organization of collagen fibers within incisional wounds were responsible for the increased wound breaking strength observed in rats ingesting vanadate.

Interleukin-1beta gene transcription in U937 cells is modulated by type I collagen and cytoskeletal integrity via distinct signaling pathways

Type I collagen (Col), an extracellular matrix molecule highly expressed in injured tissues, stimulates interleukin-1beta (IL-1beta) expression in monocytic cells. Using U937 cells transfected with the human IL-1beta gene promoter connected to a reporter gene, we examined how the organizational state of the cytoskeleton modulates the expression of IL-1beta after Col stimulation. We found the same degree of stimulation of IL-1beta gene transcription in cells exposed to Col presented in different fashions (i.e., soluble Col, Col-coated plate, three-dimensional Col lattice), suggesting that stimulation of IL-1beta is independent of the mode of presentation of Col. The Col-stimulated response was associated with induction of the transcription factor activator protein-1 (AP-1) and was abolished by a protein kinase C (PKC) inhibitor, by a mitogen-activated protein kinase (MAPK) inhibitor, and by cotransfection of cells with a competing AP-1 oligo. Disruption of cytoskeletal organization with colchicine or cytochalasin B stimulated IL-1beta gene transcription and enhanced the cells' response to Col. The effects of cytochalasin and colchicine were inhibited by the PKC inhibitor but were not affected by the MAPK inhibitor or the AP-1 oligo. These findings suggest that the cytoskeletal integrity modulates the constitutive and Col-stimulated transcription of the IL-1beta gene via distinct signaling mechanisms.

Disruption of cell-substrate adhesion activates the protein tyrosine kinase pp60(c-src)

Treatment of confluent chicken embryo fibroblasts (CEFs) with trypsin results in a dose- and time-dependent increase in c-Src protein tyrosine kinase (PTK) activity. A similar, but less marked, increase in c-Src PTK activity occurs upon incubation of CEFs in calcium-free phosphate-buffered saline, which also causes a decrease in cell-substrate adhesion. The increase in c-Src PTK activity following disruption of cell-substrate adhesion correlates with a decrease in the phosphorylation of c-Src at the regulatory site, Tyr527. The phosphotyrosine phosphatase inhibitor phenylarsine oxide blocks the increase in c-Src PTK activity seen following treatment with trypsin and the morphological changes associated with the disruption of cell-substrate adhesion. In contrast, disruption of cell-substrate adhesion causes a decrease in FAK PTK activity that rapidly returns to control levels when the cells are plated on fibronection-coated dishes. Treatment of cells with cytochalasin D, which disrupts actin filaments but not cell-substrate adhesion, causes only a slight increase in c-Src PTK activity. Thus, these studies demonstrate a ligand-independent mechanism for the activation of c-Src that is consistent with its role in both cell adhesion and cell motility. Furthermore, these data suggest that similar to adhesion, loss of adhesion is not a passive process but can activate specific signaling pathways that may have significant effects on cellular function.

Crk-associated substrate p130(Cas) interacts with nephrocystin and both proteins localize to cell-cell contacts of polarized epithelial cells

Crk-associated substrate (p130(Cas), Cas) is a docking protein first recognized as having elevated phosphotyrosine content in mammalian cells transformed by v-Src and v-Crk oncoproteins. Subsequent studies have implicated Cas in the control of normal cell behavior through its roles in integrin-mediated signal transduction and organization of the actin cytoskeleton at sites of cell adhesion. In this study, we sought to gain new insight into normal Cas function by identifying previously unrecognized interacting proteins. A yeast two-hybrid screen using the C-terminal region of Cas as a bait identified the Src homology 3 (SH3) domain of the mouse "nephrocystin" protein-orthologous to a human protein whose loss of function leads to the cystic kidney disease familial juvenile nephronophthisis. The putative full-length mouse and partial canine nephrocystin sequences were deduced from cDNA clones. Additional studies using epitope-tagged mouse nephrocystin indicated that nephrocystin and Cas can interact in mammalian cells and revealed that both proteins prominently localize at or near sites of cell-cell contact in polarized Madin-Darby canine kidney epithelial cells. Our findings provide novel insight into the normal cellular activities regulated by both Cas and nephrocystin, and raise the possibility that these proteins have a related function in polarized epithelial cells.

c-Src association with and phosphorylation of p58gag, a membrane- and microfilament-associated retroviral Gag-like protein in a xenotransplantable rat mammary tumor

The retroviral Gag-like protein p58gag expressed in a highly metastatic ascites rat mammary adenocarcinoma has been implicated in cell surface changes contributing to xenotransplantability. p58gag is present in the cells in a plasma membrane- and microfilament-associated signal transduction particle containing Src and is phosphorylated on tyrosine. Overlay analyses and affinity chromatography with glutathione S-transferase (GST) fusion proteins of Src homology-3 (SH3) domains showed direct binding of the Src but not the Crk SH3 domain to p58gag. This association was confirmed by co-immunoprecipitation of partially purified p58gag from ascites cell lysates with platelet Src. Further, a GST-p58gag fusion protein bound full length c-Src from either platelets or c-Src-expressing insect cells. The GST-p58gag fusion protein, but not GST, was phosphorylated by platelet or insect cell-expressed c-Src, but not by a kinase negative c-Src variant. The binding of GST-p58gag to c-Src was almost completely abolished by a 50-fold excess of the GST-SH3 domain of Src, and a parallel decrease in tyrosine phosphorylation of p58gag was observed. These results demonstrate that p58gag is tyrosine-phosphorylated as a consequence of its specific association with c-Src via its SH3 domain. These observations suggest a mechanism by which Gag proteins may contribute to retroviral maturation or pathogenesis through binding and relocalization of SH3 domain-containing proteins such as Src-like tyrosine kinases to sites of association of microfilaments with the plasma membrane.

Del1 induces integrin signaling and angiogenesis by ligation of alphaVbeta3

Del1 is a novel extracellular matrix protein encoding three Notch-like epidermal growth factor repeats, an RGD motif, and two discoidin domains. Del1 is expressed in an endothelial cell-restricted pattern during early development. In studies reported here, recombinant baculovirus Del1 protein was shown to promote alphavbeta3-dependent endothelial cell attachment and migration. Attachment of endothelial cells to Del1 was associated with clustering of alphavbeta3, the formation of focal complexes, and recruitment of talin and vinculin into these complexes. These events were shown to be associated with phosphorylation of proteins in the focal complexes, including the time-dependent phosphorylation of p125(FAK), MAPK, and Shc. When recombinant Del1 was evaluated in an in ovo chick chorioallantoic membrane assay, it was found to have potent angiogenic activity. This angiogenic activity was inhibited by a monoclonal antibody directed against alphavbeta3, and an RAD mutant Del1 protein was inactive. Thus Del1 provides a unique autocrine angiogenic pathway for the embryonic endothelium, and this function is mediated in part by productive ligation of integrin alphavbeta3.

Regulation of fibroblast motility by the protein tyrosine phosphatase PTP-PEST

The protein tyrosine phosphatase PTP-PEST is a cytosolic enzyme that displays a remarkable degree of selectivity for tyrosine-phosphorylated p130(Cas) as a substrate, both in vitro and in intact cells. We have investigated the physiological role of PTP-PEST using Rat1 fibroblast-derived stable cell lines that we have engineered to overexpress PTP-PEST. These cell lines exhibit normal levels of tyrosine phosphorylation of the majority of proteins but have significantly lower levels of tyrosine phosphorylation of p130(Cas) than control cells. Initial cellular events occurring following integrin-mediated attachment to fibronectin (cell attachment and spreading) are essentially unchanged in cells overexpressing PTP-PEST; similarly, the extent and time course of mitogen-activated protein kinase activation in response to integrin engagement is unchanged. In contrast, the reduced phosphorylation state of p130(Cas) is associated with a considerably reduced rate of cell migration and a failure of cells overexpressing PTP-PEST to accomplish the normally observed redistribution of p130(Cas) to the leading edge of migrating cells. Furthermore, cells overexpressing PTP-PEST demonstrate significantly reduced levels of association of p130(Cas) with the Crk adaptor protein. Our results suggest that one physiological role of PTP-PEST is to dephosphorylate p130(Cas), thereby controlling tyrosine phosphorylation-dependent signaling events downstream of p130(Cas) and regulating cell migration.

Bacterial lipopolysaccharide disrupts endothelial monolayer integrity and survival signaling events through caspase cleavage of adherens junction proteins

Bacterial lipopolysaccharide or endotoxin induces actin reorganization, increased paracellular permeability, and endothelial cell detachment from the underlying extracellular matrix in vitro. We studied the effect of endotoxin on transendothelial albumin flux and detachment of endothelial cells cultured on gelatin-impregnated filters. The endotoxin-induced changes in endothelial barrier function and detachment occurred at doses and times that were compatible with endotoxin-induced apoptosis. Since the actin cytoskeleton and cell-cell and cell-matrix adhesion all participate in the regulation of the paracellular pathway and cell-matrix interactions, we studied whether protein components of the actin-linked adherens junctions were modified in response to endotoxin. Components of cell-cell (beta- and gamma-catenin) and cell-matrix (focal adhesion kinase and p130(Cas)) adherens junctions were cleaved by caspases activated during apoptosis with dose and time requirements that paralleled those seen for barrier dysfunction and detachment. Cleavage of focal adhesion kinase led to its dissociation from the focal adhesion-associated signaling protein, paxillin, resulting in reduced paxillin tyrosine phosphorylation. Inhibition of caspase-mediated cleavage of these proteins protected against detachment but not opening of the paracellular pathway. Therefore, endotoxin-induced disruption of endothelial monolayer integrity and survival signaling events is mediated, in part, through caspase cleavage of adherens junction proteins.

Specific intercellular binding of the beta-amyloid precursor protein to the presenilins induces intercellular signaling: its significance for Alzheimer's disease

Genetic evidence has implicated three proteins, the beta-amyloid precursor protein (beta-APP) and the two homologous presenilins (PS-1 and PS-2), in the etiology of Alzheimer's disease (AD). How these three proteins jointly contribute to AD, however, is not clear. Nor is any of their normal physiological functions known. Herein, we demonstrate, confirming a prediction made earlier, that beta-APP and either PS-1 or PS-2 act as a specific membrane-bound ligand binding intercellularly with either of its two membrane receptors. This results in a cell-cell adhesion, after which rapid transient increases in protein tyrosine kinase activity and protein tyrosine phosphorylation occur coordinately inside one or both of the two adherent cells. The spectrum of proteins modified by tyrosine phosphorylation differs depending on whether PS-1 or PS-2 is involved in the specific intercellular binding to beta-APP, which implies that PS-1 and PS-2 have distinct, rather than redundant, functions in normal physiology. The relevance of this intercellular interaction and signaling process to AD is discussed.

Endothelial cell spreading on type IV collagen and spreading-induced FAK phosphorylation is regulated by Ca2+ influx

The interaction of endothelial cells with their basement membrane and local stroma is highly regulated. The observation that CAI, an inhibitor of Ca++ influx, inhibited human umbilical vein endothelial cell (HUVEC) adhesion suggested that Ca++ influx was a regulator of HUVEC-matrix interaction. Exposure of HUVEC cells to CAI or SK&F 96365, another Ca++ influx inhibitor, selectively blocked spreading but not attachment on type IV collagen but not type I collagen. Ca++ influx blockade also prevented spreading-induced FAK phosphorylation and kinase activity and secondary paxillin phosphorylation. No inhibitory effect was observed when the cells spread on type I collagen. The inhibitory effect of CAI on spreading and spreading-associated FAK phosphorylation and kinase activity was reversible. These data indicate that HUVEC cells have a selective requirement for Ca++ influx for spreading and downstream signaling on basement membrane type IV collagen.

Hormonal regulation of focal adhesions in bovine adrenocortical cells: induction of paxillin dephosphorylation by adrenocorticotropic hormone

A study of bovine adrenocortical cell shape on adrenocorticotropic hormone (ACTH) challenge showed that the cells round up and develop arborized processes. This effect was found to be (1) specific for ACTH because angiotensin II and basic fibroblast growth factor have no effect; (2) mediated by a cAMP-dependent pathway because forskolin reproduces the effect of the hormone; (3) inhibited by sodium orthovanadate, a phosphotyrosine phosphatase inhibitor, but unchanged by okadaic acid, a serine/threonine phosphatase inhibitor; and (4) correlated with a complete loss of focal adhesions. Biochemical studies of the focal-adhesion-associated proteins showed that pp125fak, vinculin (110 kDa) and paxillin (70 kDa) were detected in the Triton X-100-insoluble fraction from adrenocortical cells. During cell adhesion on fibronectin as substratum, two major phosphotyrosine-containing proteins of molecular masses 125 and 68 kDa were immunodetected in the same fraction. A dramatic decrease in the extent of tyrosine phosphorylation of these proteins was observed within 60 min after treatment with ACTH. No change in pp125fak tyrosine phosphorylation nor in Src activity was detected. In contrast, paxillin was found to be tyrosine-dephosphorylated in a time-dependent manner in ACTH-treated cells. Sodium orthovanadate completely prevented the effect of ACTH. These observations suggest a possible role for phosphotyrosine phosphatases in hormone-dependent cellular regulatory processes.

Identification of tyrosine phosphorylated adhesion proteins in human cancer cells

Tyrosine phosphorylation is a form of signal transduction that regulates cell growth, differentiation, migration, and survival. This knowledge has promoted much interest in the role of tyrosine kinases and phosphatases in regulating cell behavior during development and tumorigenesis. However, it is generally less well appreciated that tyrosine phosphorylated proteins are enriched within sites of cell adhesion, particularly in transformed cells. To identify these, we developed a panel of monoclonal antibodies specific for tyrosine phosphorylated proteins in breast cancer cells, using extensive modifications of existing technologies for immunization, somatic fusion, and antibody screening. Mice were immunized with a complex mixture of phosphotyrosine containing proteins using the newly developed RIMMS method. By increasing the sensitivity of antigen recognition, we isolated reagents specific for a wide diversity of tyrosine phosphorylated adhesion proteins in breast cancer cells.

Increased tyrosine phosphorylation causes redistribution of adherens junction and tight junction proteins and perturbs paracellular barrier function in MDCK epithelia

Polarized monolayers of strain II Madin-Darby canine kidney cells (MDCK II) were treated with vanadate/H2O2, known inhibitors of protein tyrosine phosphatase activity. Vanadate/H2O2 treatment resulted in a rapid increase in paracellular permeability as revealed by decreased transepithelial resistance and increased permeability to inulin. These alterations in epithelial barrier function coincided with increased phosphotyrosine immunofluorescence in the vicinity of intercellular junctions and with redistribution of F-actin, the adherens junction protein E-cadherin and the tight junction protein ZO-1. The effects of vanadate/H2O2 on intercellular junction permeability and protein distribution were completely blocked by the specific protein tyrosine kinase (PTK) inhibitor tyrphostin 25 and partially inhibited by the alternative PTK inhibitor genistein. The relative potency of these two inhibitors in blocking the effects of vanadate/H2O2 on intercellular junctions correlated with their abilities to inhibit tyrosine phosphorylation. The potent ser/thr protein kinase inhibitor staurosporine had only a small influence on the vanadate/H2O2-induced increase in paracellular permeability and did not affect the observed redistribution of intercellular junction proteins or phosphotyrosine immunofluorescence. The relative potencies of these distinct protein kinase inhibitors in reversing the effects of vanadate/H2O2 indicate that these effects are directly related to tyrosine phosphorylation. In conclusion, our data provide evidence that enhanced tyrosine phosphorylation of intercellular junction proteins in MDCK epithelia increases paracellular permeability and can also induce prominent reorganization of the junctional complex.