Requirement for diacylglycerol and protein kinase C in HeLa cell-substratum adhesion and their feedback amplification of arachidonic acid production for optimum cell spreading

Zebrafish Dkk3a protein regulates the activity of myf5 promoter through interaction with membrane receptor integrin α6b

Myogenic regulatory factor Myf5 plays important roles in muscle development. In zebrafish myf5, a microRNA (miR), termed miR-3906 or miR-In300, was reported to silence dickkopf-3-related gene (dkk3r or dkk3a), resulting in repression of myf5 promoter activity. However, the membrane receptor that interacts with ligand Dkk3a to control myf5 expression through signal transduction remains unknown. To address this question, we applied immunoprecipitation and LC-MS/MS to screen putative membrane receptors of Dkk3a, and Integrin α6b (Itgα6b) was finally identified. To further confirm this, we used cell surface binding assays, which showed that Dkk3a and Itgα6b were co-expressed at the cell membrane of HEK-293T cells. Cross-linking immunoprecipitation data also showed high affinity of Itgα6b for Dkk3a. We further proved that the β-propeller repeat domains of Itgα6b are key segments bound by Dkk3a. Moreover, when dkk3a and itgα6b mRNAs were co-injected into embryos, luciferase activity was up-regulated 4-fold greater than that of control embryos. In contrast, the luciferase activities of dkk3a knockdown embryos co-injected with itgα6b mRNA and itgα6b knockdown embryos co-injected with dkk3a mRNA were decreased in a manner similar to that in control embryos, respectively. Knockdown of itgα6b resulted in abnormal somite shape, fewer somitic cells, weaker or absent myf5 expression, and reduced the protein level of phosphorylated p38a in somites. These defective phenotypes of trunk muscular development were similar to those of dkk3a knockdown embryos. We demonstrated that the secreted ligand Dkk3a binds to the membrane receptor Itgα6b, which increases the protein level of phosphorylated p38a and activates myf5 promoter activity of zebrafish embryos during myogenesis.

Non-specific effects of leukotriene synthesis inhibitors on HeLa cell physiology

We examined the effects of various leukotriene synthesis inhibitors on calcium signalling in HeLa cells, before and after transfection with BLT(1). All of the inhibitors studied were found to reduce increases in intracellular calcium concentration induced by BLT(1), but also by an ionophore or activation of various G-protein coupled receptors, regardless of BLT(1) expression. In order to explore the mechanism of these apparently general effects we examined HeLa cell expression of leukotriene receptors and biosynthetic enzymes and found that the genes for key leukotriene synthesis enzymes and all of the leukotriene receptors were not expressed. Leukotrienes are involved in the pathology of a variety of cancers, and for HeLa cells leukotrienes have been reported to be important for aspects of the carcinogenic phenotype. We find that leukotriene synthesis inhibitors have non-specific effects, so careful controls are necessary to avoid interpreting non-specific effects as evidence for leukotriene involvement.

Multinucleated giant cell formation exhibits features of phagocytosis with participation of the endoplasmic reticulum

Macrophage fusion leading to formation of multinucleated giant cells during chronic inflammation is poorly understood in mechanism and physiological significance. To address this, we developed a system of human macrophage fusion that utilizes IL-4, IL-13, or alpha-tocopherol to generate large foreign body-type giant cells (FBGC). Extending our previously demonstrated requirements for F-actin and mannose receptor (MR) activity, we find that macrophage fusion exhibits further features of a phagocytic process. Pharmacological inhibition of IL-4-induced FBGC formation indicates critical roles for vacuolar-type ATPase, microtubules, the endoplasmic reticulum (ER), and calcium-independent phospholipase A(2) (iPLA(2)), but not calcium-dependent PLA(2) (cPLA(2)), secretory PLA(2) (sPLA(2)), cyclooxygenase, or lipoxygenase. Immunocytochemistry confirms iPLA(2) expression and absence of cPLA(2) or sPLA(2) expression in macrophages/FBGC. As markers of ER-mediated phagocytosis, calnexin and calregulin are detectable on non-permeabilized fusing macrophages and also concentrated at fusion interfaces where they co-localize with actin in permeabilized macrophages/FBGC. Furthermore, ER markers co-localize with concanavalin A reactivity on non-permeabilized fusing macrophages, suggesting that the ER may present MR ligand during fusion events. These data demonstrate for the first time that the mechanism of macrophage fusion leading to formation of multinucleated giant cells exhibits multiple features of phagocytosis with potential participation of the ER.

The PINCH-ILK-parvin complexes: assembly, functions and regulation

Cell-extracellular matrix (ECM) adhesion is mediated by transmembrane cell adhesion receptors (e.g., integrins) and receptor proximal cytoplasmic proteins. Over the past several years, studies using biochemical, structural, cell biological and genetic approaches have provided important evidence suggesting crucial roles of integrin-linked kinase (ILK), PINCH and CH-ILKBP/actopaxin/affixin/parvin (abbreviated as parvin herein) in ECM control of cell behavior. One general theme emerging from these studies is that the formation of ternary protein complexes consisting of ILK, PINCH and parvin is pivotal to the functions of PINCH, ILK and parvin proteins. In addition, recent studies have begun to uncover the molecular mechanisms underlying the assembly, functions and regulation of the PINCH-ILK-parvin (PIP) complexes. The PIP complexes provide crucial physical linkages between integrins and the actin cytoskeleton and transduce diverse signals from ECM to intracellular effectors. Among the challenges of future studies are to define the functions of different PIP complexes in various cellular processes, identify additional partners of the PIP complexes that regulate and/or mediate the functions of the PIP complexes, and determine the roles of the PIP complexes in the pathogenesis of human diseases involving abnormal cell-ECM adhesion and signaling.

Differential regulation of alpha6beta4 integrin by PKC isoforms in murine skin keratinocytes

In mammalian epidermis, alpha6beta4 integrin is expressed exclusively on the basal layer localized to the hemidesmosomes, where it interacts extracellularly with the laminin-5 ligand. During differentiation, loss of alpha6beta4 is associated with keratinocyte detachment from the basement membrane and upward migration. The protein kinase C (PKC) family of isoforms participates in regulation of integrin function and is linked to skin differentiation. Exposure of primary murine keratinocytes to PKC activators specifically downregulates alpha6beta4 expression. Utilizing recombinant adenoviruses, we selectively overexpressed skin PKC isoforms in primary keratinocytes. PKCdelta and PKCzeta induced downregulation of alpha6beta4 protein expression, leading to reduced keratinocyte attachment to laminin-5 and enhanced gradual detachment from the underlying matrix. In contrast, PKCalpha upregulated alpha6beta4 protein expression, leading to increased keratinocyte attachment to laminin-5 and to the underlying matrix. Altogether, these results suggest distinct roles for specific PKC isoforms in alpha6beta4 functional regulation during the early stages of skin differentiation.

5-lipoxygenase and cyclooxygenase regulate wound closure in NIH/3T3 fibroblast monolayers

Wound healing involves multiple cell signaling pathways, including those regulating cell-extracellular matrix adhesion. Previous work demonstrated that arachidonate oxidation to leukotriene B(4) (LTB(4)) by 5-lipoxygenase (5-LOX) signals fibroblast spreading on fibronectin, whereas cyclooxygenase-2 (COX-2)-catalyzed prostaglandin E(2) (PGE(2)) formation facilitates subsequent cell migration. We investigated arachidonate metabolite signaling in wound closure of perturbed NIH/3T3 fibroblast monolayers. We found that during initial stages of wound closure (0-120 min), all wound margin cells spread into the wound gap perpendicularly to the wound long axis. At regular intervals, between 120 and 300 min, some cells elongated to project across the wound and meet cells from the opposite margin, forming distinct cell bridges spanning the wound that act as foci for later wound-directed cell migration and resulting closure. 5-LOX inhibition by AA861 demonstrated a required LTB(4) signal for initial marginal cell spreading and bridge formation, both of which must precede wound-directed cell migration. 5-LOX inhibition effects were reversible by exogenous LTB(4). Conversely, COX inhibition by indomethacin reduced directed migration into the wound but enhanced early cell spreading and bridge formation. Exogenous PGE(2) reversed this effect and increased cell migration into the wound. The differential effects of arachidonic acid metabolites produced by LOX and COX were further confirmed with NIH/3T3 fibroblast cell lines constitutively over- and underexpressing the 5-LOX and COX-2 enzymes. These data suggest that two competing oxidative enzymes in arachidonate metabolism, LOX and COX, differentially regulate sequential aspects of fibroblast wound closure in vitro.

Cdc42 and RhoA are differentially regulated during arachidonate-mediated HeLa cell adhesion

Cell adhesion to extracellular matrix requires stimulation of an eicosanoid signaling pathway through the metabolism of arachidonate by 5-lipoxygenase to leukotrienes and cyclooxygenase-1/2 to prostaglandins, as well as activation of the small GTPase signaling pathway involving Cdc42 and Rho. These signaling pathways direct remodeling of the actin cytoskeleton during the adhesion process, specifically the polymerization of actin during cell spreading and the bundling of actin filaments when cells migrate. However, few studies linking these signaling pathways have been described in the literature. We have previously shown that HeLa cell adhesion to collagen requires oxidation of arachidonic acid (AA) by lipoxygenase for actin polymerization and cell spreading, and cyclooxygenase for bundling actin filaments during cell migration. We demonstrate that small GTPase activity is required for HeLa cell spreading upon gelatin, and that Cdc42 is activated while Rho is downregulated during the spreading process. Using constitutively active and dominant negative expression studies, we show that Cdc42 is required for HeLa cell spreading and migration, while activated RhoA is antagonistic towards spreading. Constitutively active RhoA promotes cell migration and increases the degree of actin bundling in HeLa cells. Further, we demonstrate that activation of either the AA oxidation pathway or the small GTPase pathway cannot rescue inhibition of spreading when the alternate pathway is blocked. Our results suggest (1) both the eicosanoid signaling pathway and small GTPase activation are required during HeLa cell adhesion, and (2) these signaling pathways converge to properly direct remodeling of the actin cytoskeleton during HeLa cell spreading and migration upon collagen.

Skeletal muscle LIM protein 1 regulates integrin-mediated myoblast adhesion, spreading, and migration

The skeletal muscle LIM protein 1 (SLIM1) is highly expressed in skeletal and cardiac muscle, and its expression is downregulated significantly in dilated human cardiomyopathy. However, the function of SLIM1 is unknown. In this study, we investigated the intracellular localization of SLIM1. Endogenous and recombinant SLIM1 localized to the nucleus, stress fibers, and focal adhesions in skeletal myoblasts plated on fibronectin, collagen, or laminin. However, after inhibition of integrin signaling either by plating on poly-l-lysine or by soluble RGD peptide, SLIM1 localized diffusely in the cytosol, with decreased nuclear expression. Disruption of the actin cytoskeleton by cytochalasin D did not inhibit nuclear localization of SLIM1 in integrin-activated cells. Green fluorescent protein-tagged SLIM1 shuttled in the nucleus of untransfected NIH 3T3 cells, in a heterokaryon fusion assay. Overexpression of SLIM1 in Sol8 myoblasts inhibited cell adhesion and promoted cell spreading and migration. These studies show SLIM1 localizes in an integrin-dependent manner to the nucleus and focal adhesions where it functions downstream of integrin activation to promote cell spreading and migration.

Assembly of the PINCH-ILK-CH-ILKBP complex precedes and is essential for localization of each component to cell-matrix adhesion sites

PINCH, integrin-linked kinase (ILK) and calponin homology-containing ILK-binding protein (CH-ILKBP) form a ternary complex that plays crucial roles at cell-extracellular matrix adhesion sites. To understand the mechanism underlying the complex formation and recruitment to cell-adhesion sites we have undertaken a combined structural, mutational and cell biological analysis. Three-dimensional structure-based point mutations identified specific PINCH and ILK sites that mediate the complex formation. Analyses of the binding defective point mutants revealed that the assembly of the PINCH-ILK-CH-ILKBP complex is essential for their localization to cell-extracellular matrix adhesion sites. The formation of the PINCH-ILK-CH-ILKBP complex precedes integrin-mediated cell adhesion and spreading. Furthermore, inhibition of protein kinase C, but not that of actin polymerization, inhibited the PINCH-ILK-CH-ILKBP complex formation, suggesting that the PINCH-ILK-CH-ILKBP complex likely serves as a downstream effector of protein kinase C in the cellular control of focal adhesion assembly. Finally, we provide evidence that the formation of the PINCH-ILK-CH-ILKBP complex, while necessary, is not sufficient for ILK localization to cell-extracellular matrix adhesion sites. These results provide new insights into the molecular mechanism underlying the assembly and regulation of cell-matrix adhesion structures.

Alpha4 integrins and the immune response

The alpha4 integrins (alpha4beta1 and alpha4beta7) play multiple roles in the immune system. Alpha4 integrins impact hematopoiesis, leukocyte trafficking in immune surveillance and inflammation, and leukocyte activation and survival. To perform these functions, alpha4 integrins act as both adhesive and signaling receptors. Paxillin, a signaling adapter molecule, binds directly to the alpha4 subunit cytoplasmic domain, and its binding is regulated by serine phosphorylation of the alpha4 subunit. This regulated interaction of paxillin with the alpha4 subunit is likely to regulate the diverse functions of alpha4 integrins in the immune system. Furthermore, this protein-protein interaction may provide novel targets for the modulation of the immune response.

A lipid-regulated docking site on vinculin for protein kinase C

During cell spreading, binding of actin-organizing proteins to acidic phospholipids and phosphorylation are important for localization and activity of these proteins at nascent cell-matrix adhesion sites. Here, we report on a transient interaction between the lipid-dependent protein kinase Calpha and vinculin, an early component of these sites, during spreading of HeLa cells on collagen. In vitro binding of protein kinase Calpha to vinculin tail was found dependent on free calcium and acidic phospholipids but independent of a functional kinase domain. The interaction was enhanced by conditions that favor the oligomerization of vinculin. Phosphorylation by protein kinase Calpha reached 1.5 mol of phosphate/mol of vinculin tail and required the C-terminal hydrophobic hairpin, a putative phosphatidylinositol 4,5-bisphosphate-binding site. Mass spectroscopy of peptides derived from in vitro phosphorylated vinculin tail identified phosphorylation of serines 1033 and 1045. Inhibition of C-terminal phospholipid binding at the vinculin tail by mutagenesis or deletion reduced the rate of phosphorylation to < or =50%. We suggest a possible mechanism whereby phospholipid-regulated conformational changes in vinculin may lead to exposure of a docking site for protein kinase Calpha and subsequent phosphorylation of vinculin and/or vinculin interaction partners, thereby affecting the formation of cell adhesion complexes.

Modulation of cell-substrate adhesion by arachidonic acid: lipoxygenase regulates cell spreading and ERK1/2-inducible cyclooxygenase regulates cell migration in NIH-3T3 fibroblasts

Adhesion of cells to an extracellular matrix is characterized by several discrete morphological and functional stages beginning with cell-substrate attachment, followed by cell spreading, migration, and immobilization. We find that although arachidonic acid release is rate-limiting in the overall process of adhesion, its oxidation by lipoxygenase and cyclooxygenases regulates, respectively, the cell spreading and cell migration stages. During the adhesion of NIH-3T3 cells to fibronectin, two functionally and kinetically distinct phases of arachidonic acid release take place. An initial transient arachidonate release occurs during cell attachment to fibronectin, and is sufficient to signal the cell spreading stage after its oxidation by 5-lipoxygenase to leukotrienes. A later sustained arachidonate release occurs during and after spreading, and signals the subsequent migration stage through its oxidation to prostaglandins by newly synthesized cyclooxygenase-2. In signaling migration, constitutively expressed cyclooxygenase-1 appears to contribute approximately 25% of prostaglandins synthesized compared with the inducible cyclooxygenase-2. Both the second sustained arachidonate release, and cyclooxygenase-2 protein induction and synthesis, appear to be regulated by the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)1/2. The initial cell attachment-induced transient arachidonic acid release that signals spreading through lipoxygenase oxidation is not sensitive to ERK1/2 inhibition by PD98059, whereas PD98059 produces both a reduction in the larger second arachidonate release and a blockade of induced cyclooxygenase-2 protein expression with concomitant reduction of prostaglandin synthesis. The second arachidonate release, and cyclooxygenase-2 expression and activity, both appear to be required for cell migration but not for the preceding stages of attachment and spreading. These data suggest a bifurcation in the arachidonic acid adhesion-signaling pathway, wherein lipoxygenase oxidation generates leukotriene metabolites regulating the spreading stage of cell adhesion, whereas ERK 1/2-induced cyclooxygenase synthesis results in oxidation of a later release, generating prostaglandin metabolites regulating the later migration stage.

Identification of a novel interaction between integrin beta1 and 14-3-3beta

Integrins are cell surface receptors for extracellular matrix, which play important roles in a variety of biological processes. 14-3-3 proteins are a highly conserved family of cytoplasmic proteins that associate with several intracellular signaling molecules in regulation of various cellular functions. Here, we report identification of an interaction between the integrin beta1 cytoplasmic domain and 14-3-3beta by using the yeast two-hybrid screen. Like several other proteins, the integrin beta1 cytoplasmic domain associated with 14-3-3beta by a non-phosphoserine mechanism. The 14-3-3beta/integrin beta1 interaction was confirmed by in vitro binding assays as well as co-precipitation in vivo. Furthermore, we found that 14-3-3beta co-localized with integrin beta1 during the early stage of cell spreading on fibronectin, suggesting a potential role of the 14-3-3beta/integrin beta1 interaction in the regulation of cell adhesion. Using tetracycline-regulated expression system, we showed that overexpression of 14-3-3beta stimulated cell spreading and migration on fibronectin but not on poly-L-lysine. However, the induced expression of 14-3-3beta did not affect tyrosine phosphorylation of FAK or its substrates, p130(cas) and paxillin, suggesting that 14-3-3beta regulated integrin-mediated cell spreading and migration by FAK-independent mechanisms. Taken together, these results identify an interaction between integrin and 14-3-3 proteins and suggest a potentially novel cellular function for 14-3-3 proteins in the regulation of integrin-mediated cell adhesion and signaling events.

Role of endoproteolytic processing in the adhesive and signaling functions of alphavbeta5 integrin

Some integrin alpha subunits undergo a post-translational cleavage in their extracellular domain. However, the role of this cleavage in integrin function is unclear. Enzymes involved in this maturation belong to the subtilisin-like endoprotease family (convertases). To understand the role of the alpha subunit cleavage in integrin function, we have designed stable transfectants (PDX39P cells) expressing alpha(1)-PDX, a convertase inhibitor. Immunoprecipitation of cell surface proteins from PDX39P showed that alpha(3), alpha(6) and alpha(v) integrins lack endoproteolytic cleavage. We have compared adhesion between PDX39P cells and mock-transfected cells on different extracellular matrix proteins. No difference in adhesion could be observed on laminin-1 and type I collagen, while attachment of PDX39P cells to vitronectin (ligand of the alpha(v)beta(5) integrin) was dramatically reduced. The reduced adhesion of PDX39P cells was not due to changes in integrin affinity as determined by solid-phase receptor assay in a cell-free environment. Intracellular signaling pathways activated by alpha(v) integrin ligation were also affected in PDX39P cells. It thus seems that the absence of endoproteolytic cleavage of alpha(v) integrins has important consequences on signal transduction pathways leading to alterations in integrin function such as cell adhesion.

Alpha4 integrins in cardiovascular development and diseases

Alpha4 integrins (alpha4beta1 and alpha4beta7) have a restricted distribution pattern and are critical for the development and diseases of the cardiovascular system. alpha4 integrins support unique biological properties such as promoting cell migration and inhibiting cell spreading and focal adhesion formation. We have found that the alpha4 integrin subunit directly and tightly binds to a signaling adapter molecule, paxillin, and disruption of the alpha4-paxillin interaction interferes with many of alpha4-dependent biological functions. Consequently, the interaction of alpha4 integrins with paxillin may play an important role in regulating alpha4-mediated functions. This review focuses on what we have known about the alpha4-paxillin interaction and discusses the possible mechanism of regulation for this interaction.

Structural and molecular characterization of dynein in a gall-midge insect having motile sperm with only the outer arm

The dipteran Monarthropalpus flavus possesses a peculiar sperm axoneme, characterized by multiple rows of microtubular doublets linked by the outer dynein arms only, lacking any equivalent of the central pair/radial spoke complex. The structure of these dynein molecules was studied by electron microscopy (EM). Using the quick-freeze, deep-etch method of EM, they were found to be similar to outer dynein arms described previously. Two globular "heads," each subdivided by a cleft, are clearly discernible. "Stalks" extend from proximal head to contact the B-tubule of the adjacent doublet. Unlike the situation in vertebrate sperm, the stalks sometimes branch into two thinner strands that contact the B-tubule at different sites. Treatment of demembranated sperm cells with ATP and vanadate induces conformational changes in the dynein outer arms. These are interpreted as the result of rotation of the dynein head with respect to what is observed in axonemes in rigor condition (after ATP depletion). SDS-PAGE indicates that the high-molecular-weight complement of this molecule comprises a single heavy chain. Specific dynein heavy chain-related DNA sequences corresponding to the catalytic-phosphate binding region were amplified by RT-PCR. Only one axonemal dynein sequence was identified among all amplified fragments. Southern blot analysis performed on genomic DNA using this sequence as a probe identified two hybridizing genes, only one of which is able to encode a functional product. Thus, genetic analysis indicates that this axonemal outer arm dynein is a homodymer of a single heavy chain subunit. In vivo, spermatozoa of this species are stored in a rolled configuration in female spermatheca, where they move rapidly with a wave-like motion. This movement could not be reproduced in vitro, except when spermatozoa were constrained in a bent configuration by some mechanical impediment. We propose that, in the absence of both the central pair/radial spoke complex and the inner arms, a curvature-dependent activation acts to trigger motility in these spermatozoa.

Activation of protein kinase C is required for the stable attachment of adherent platelets to collagen but is not needed for the initial rapid adhesion under flow conditions

We have investigated the role of protein kinase C (PKC) in the initial events of alpha(2)beta(1)-integrin-mediated platelet adhesion to collagen under flow conditions. Although adhesion caused activation of PKC, as evidenced by pleckstrin phosphorylation, the PKC inhibitors GF 109203X and Gö 6976 had no effect on adhesion, even though they prevented pleckstrin phosphorylation. The initial kinetics and extent of platelet adhesion to collagen (<5 seconds) and tyrosine phosphorylation of p125(FAK) and p72(syk) were not influenced by the PKC inhibitors, whereas adhesion to polylysine was prevented. These results indicate that adhesion to collagen and polylysine involve different mechanisms and requirements for PKC activation. Pretreatment with GF 109203X destabilized collagen-adherent platelets, accelerating their detachment, which was associated with tyrosine dephosphorylation of p125(FAK). Thus, although PKC activation was not required for rapid platelet adhesion to collagen, it appears to play an important role in stabilizing the attachment of adherent platelets to collagen. We also examined the effect of PKC activation by the phorbol ester phorbol 12-myristate 13-acetate (PMA) on platelet adhesion to collagen. PMA at 100 nmol/L strongly potentiated adhesion and tyrosine phosphorylation of p125(FAK) and p72(syk) and activated beta(1)-integrins, as determined by increased exposure of the 15/7 epitope. The PMA-stimulated adhesion was partially blocked by an anti-alpha(2)beta(1) antibody, was completely inhibited by GF 109203X, and was not correlated with the extent of pleckstrin phosphorylation. Therefore, strong PKC activation may lead to inside-out signaling, enhancing the role of beta(1)-integrins in adhesion. Pleckstrin phosphorylation does not appear to be involved in the initial phase of basic or PMA-stimulated adhesion but may help stabilize the adherent platelets.

Cell-matrix adhesions differentially regulate fascin phosphorylation

Cell adhesion to individual macromolecules of the extracellular matrix has dramatic effects on the subcellular localization of the actin-bundling protein fascin and on the ability of cells to form stable fascin microspikes. The actin-binding activity of fascin is down-regulated by phosphorylation, and we used two differentiated cell types, C2C12 skeletal myoblasts and LLC-PK1 kidney epithelial cells, to examine the hypothesis that cell adhesion to the matrix components fibronectin, laminin-1, and thrombospondin-1 differentially regulates fascin phosphorylation. In both cell types, treatment with the PKC activator 12-tetradecanoyl phorbol 13-acetate (TPA) or adhesion to fibronectin led to a diffuse distribution of fascin after 1 h. C2C12 cells contain the PKC family members alpha, gamma, and lambda, and PKCalpha localization was altered upon cell adhesion to fibronectin. Two-dimensional isoelectric focusing/SDS-polyacrylamide gels were used to determine that fascin became phosphorylated in cells adherent to fibronectin and was inhibited by the PKC inhibitors calphostin C and chelerythrine chloride. Phosphorylation of fascin was not detected in cells adherent to thrombospondin-1 or to laminin-1. LLC-PK1 cells expressing green fluorescent protein (GFP)-fascin also displayed similar regulation of fascin phosphorylation. LLC-PK1 cells expressing GFP-fascin S39A, a nonphosphorylatable mutant, did not undergo spreading and focal contact organization on fibronectin, whereas cells expressing a GFP-fascin S39D mutant with constitutive negative charge spread more extensively than wild-type cells. In contrast, C2C12 cells coexpressing S39A fascin with endogenous fascin remained competent to form microspikes on thrombospondin-1, and cells that expressed fascin S39D attached to thrombospondin-1 but did not form microspikes. Blockade of PKCalpha activity by TPA-induced down-regulation led to actin association of wild-type fascin in fibronectin-adherent C2C12 and LLC-PK1 cells but did not alter the distribution of S39A or S39D fascins. The association of fascin with actin in fibronectin-adherent cells was also evident in the presence of an inhibitory antibody to integrin alpha5 subunit. These novel results establish matrix-initiated PKC-dependent regulation of fascin phosphorylation at serine 39 as a mechanism whereby matrix adhesion is coupled to the organization of cytoskeletal structure.

Tumor invasion: role of growth factor-induced cell motility

Cancer progression to the invasive and metastatic stage represents the most formidable barrier to successful treatment. To develop rational therapies, we must determine the molecular bases of these transitions. Cell motility is one of the defining characteristics of invasive tumors, enabling tumors to migrate into adjacent tissues or transmigrate limiting basement membranes and extracellular matrices. Invasive tumor cells have been demonstrated to present dysregulated cell motility in response to extracellular signals from growth factors and cytokines. Recent findings suggest that this growth factor receptor-mediated motility is one of the most common aberrations in tumor cells leading to invasiveness and represents a cellular behavior distinct from-adhesion-related haptokinetic and haptotactic migration. This review focuses on the emerging understanding of the biochemical and biophysical foundations of growth factor-induced cell motility and tumor cell invasiveness, and the implications for development of targeted agents, with particular emphasis on signaling from the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptors, as these have most often been associated with tumor invasion. The nascent models highlight the roles of various intracellular signaling pathways including phospholipase C-gamma (PLC gamma), phosphatidylinositol (PI)3'-kinase, mitogen-activated protein (MAP) kinase, and actin cytoskeleton-related events. Development of novel agents against tumor invasion will require not only a detailed appreciation of the biochemical regulatory elements of motility but also a paradigm shift in our approach to and assessment of cancer therapy.

Beta1-integrins mediate Ca2+-signalling and T cell spreading via divergent pathways

Interaction of Jurkat T-lymphocytes with two extracellular matrix (ECM) proteins of the basement membrane, laminin or collagen type IV, combined with poly-L-lysine resulted in a strong adhesion, a highly increased intracellular Ca2+-concentration ([Ca2]i), as compared to cells on laminin or collagen type IV alone and in spreading of the cells. The strong adhesion was independent of an increase in [Ca2+]i, was not mediated by a beta1-integrin, and was due to charge interaction between the positively charged polyaminoacid and the negatively charged cell surface. The latter was confirmed by substitution of poly-L-lysine by other positively charged polyaminoacids. In contrast, Ca+-signalling and spreading of the cells adhering to laminin or collagen type IV combined with poly-L-lysine was completely blocked by anti-beta1 mAb. However, spreading of the cells was independent of an increase in [Ca2+]i suggesting divergent signal transduction pathways leading to Ca2+-signalling and spreading of the cells. We elucidated these signal transduction pathways by inhibition of key enzymes involved. The tyrosine kinase inhibitor genistein blocked Ca2+-signalling as well as spreading, whereas inhibitors of PKC (calphostin C, GF109203x), PLCgamma (U73122) and PLA2 (bromophenacyl-bromide (BPB), 3-[4-octadecyl)benzoyl]acrylic acid (OBAA)) selectively blocked spreading of the cells.

The beta1-integrin cytosolic domain optimizes phospholipase A2-mediated arachidonic acid release required for NIH-3T3 cell spreading

Inhibition of PLA2 activity and rescue by addition of exogenous AA was used to demonstrate that AA production is essential for integrin-mediated NIH-3T3 murine cell spreading. Both AA release and cell spreading after attachment to a FN substrate were inhibited by the PLA2 inhibitor mepacrine. AA release was essential for signaling spreading since the inhibition of spreading induced by mepacrine was overcome by exogenous AA. Cells ectopically expressing full-length chicken beta1-integrins both released AA and spread fully on a substrate of anti-chicken beta1-integrin monoclonal antibody, and inhibition of PLA2 by mepacrine suppressed both spreading and AA release. Exogenous AA also reversed this mepacrine-induced inhibition of spreading. The role of the beta1-integrin cytosolic domain in AA release was examined by comparing responses of cells expressing full-length chicken beta1-integrins versus cells expressing a deletion mutant chicken beta1-integrin with a truncated cytosolic domain. Cells expressing a truncated chicken beta1-integrin released significantly less AA and failed to spread on the anti-chicken beta1-integrin antibody substrate. Furthermore, clustering full-length receptors with soluble antibody stimulated greater AA release than clustering of receptors having truncated cytosolic domains. These data suggest the beta1-integrin cytosolic domain is required for optimal PLA2 activation to produce AA necessary for cell spreading.

Protein kinase C regulates integrin-induced activation of the extracellular regulated kinase pathway upstream of Shc

Adhesion of fibroblasts to extracellular matrices via integrin receptors is accompanied by extensive cytoskeletal rearrangements and intracellular signaling events. The protein kinase C (PKC) family of serine/threonine kinases has been implicated in several integrin-mediated events including focal adhesion formation, cell spreading, cell migration, and cytoskeletal rearrangements. However, the mechanism by which PKC regulates integrin function is not known. To characterize the role of PKC family kinases in mediating integrin-induced signaling, we monitored the effects of PKC inhibition on fibronectin-induced signaling events in Cos7 cells using pharmacological and genetic approaches. We found that inhibition of classical and novel isoforms of PKC by down-regulation with 12-0-tetradeconoyl-phorbol-13-acetate or overexpression of dominant-negative mutants of PKC significantly reduced extracellular regulated kinase 2 (Erk2) activation by fibronectin receptors in Cos7 cells. Furthermore, overexpression of constitutively active PKCalpha, PKCdelta, or PKCepsilon was sufficient to rescue 12-0-tetradeconoyl-phorbol-13-acetate-mediated down-regulation of Erk2 activation, and all three of these PKC isoforms were activated following adhesion. PKC was required for maximal activation of mitogen-activated kinase kinase 1, Raf-1, and Ras, tyrosine phosphorylation of Shc, and Shc association with Grb2. PKC inhibition does not appear to have a generalized effect on integrin signaling, because it does not block integrin-induced focal adhesion kinase or paxillin tyrosine phosphorylation. These results indicate that PKC activity enhances Erk2 activation in response to fibronectin by stimulating the Erk/mitogen-activated protein kinase pathway at an early step upstream of Shc.

Evidence that atypical protein kinase C-lambda and atypical protein kinase C-zeta participate in Ras-mediated reorganization of the F-actin cytoskeleton

Expression of transforming Ha-Ras L61 in NIH3T3 cells causes profound morphological alterations which include a disassembly of actin stress fibers. The Ras-induced dissolution of actin stress fibers is blocked by the specific PKC inhibitor GF109203X at concentrations which inhibit the activity of the atypical aPKC isotypes lambda and zeta, whereas lower concentrations of the inhibitor which block conventional and novel PKC isotypes are ineffective. Coexpression of transforming Ha-Ras L61 with kinase-defective, dominant-negative (DN) mutants of aPKC-lambda and aPKC-zeta, as well as antisense constructs encoding RNA-directed against isotype-specific 5' sequences of the corresponding mRNA, abrogates the Ha-Ras-induced reorganization of the actin cytoskeleton. Expression of a kinase-defective, DN mutant of cPKC-alpha was unable to counteract Ras with regard to the dissolution of actin stress fibers. Transfection of cells with constructs encoding constitutively active (CA) mutants of atypical aPKC-lambda and aPKC-zeta lead to a disassembly of stress fibers independent of oncogenic Ha-Ras. Coexpression of (DN) Rac-1 N17 and addition of the phosphatidylinositol 3'-kinase (PI3K) inhibitors wortmannin and LY294002 are in agreement with a tentative model suggesting that, in the signaling pathway from Ha-Ras to the cytoskeleton aPKC-lambda acts upstream of PI3K and Rac-1, whereas aPKC-zeta functions downstream of PI3K and Rac-1. This model is supported by studies demonstrating that cotransfection with plasmids encoding L61Ras and either aPKC-lambda or aPKC-zeta results in a stimulation of the kinase activity of both enzymes. Furthermore, the Ras-mediated activation of PKC-zeta was abrogated by coexpression of DN Rac-1 N17.

Extracellular calcium regulates HeLa cell morphology during adhesion to gelatin: role of translocation and phosphorylation of cytosolic phospholipase A2

Attachment of HeLa cells to gelatin induces the release of arachidonic acid (AA), which is essential for cell spreading. HeLa cells spreading in the presence of extracellular Ca2+ released more AA and formed more distinctive lamellipodia and filopodia than cells spreading in the absence of Ca2+. Addition of exogenous AA to cells spreading in the absence of extracellular Ca2+ restored the formation of lamellipodia and filopodia. To investigate the role of cytosolic phospholipase A2 (cPLA2) in regulating the differential release of AA and subsequent formation of lamellipodia and filopodia during HeLa cell adhesion, cPLA2 phosphorylation and translocation from the cytosol to the membrane were evaluated. During HeLa cell attachment and spreading in the presence of Ca2+, all cPLA2 became phosphorylated within 2 min, which is the earliest time cell attachment could be measured. In the absence of extracellular Ca2+, the time for complete cPLA2 phosphorylation was lengthened to <4 min. Maximal translocation of cPLA2 from cytosol to membrane during adhesion of cells to gelatin was similar in the presence or absence of extracellular Ca2+ and remained membrane associated throughout the duration of cell spreading. The amount of total cellular cPLA2 translocated to the membrane in the presence of extracellular Ca2+ went from <20% for unspread cells to >95% for spread cells. In the absence of Ca2+ only 55-65% of the total cPLA2 was translocated to the membrane during cell spreading. The decrease in the amount translocated could account for the comparable decrease in the amount of AA released by cells during spreading without extracellular Ca2+. Although translocation of cPLA2 from cytosol to membrane was Ca2+ dependent, phosphorylation of cPLA2 was attachment dependent and could occur both on the membrane and in the cytosol. To elucidate potential activators of cPLA2, the extracellular signal-related protein kinase 2 (ERK2) and protein kinase C (PKC) were investigated. ERK2 underwent a rapid phosphorylation upon early attachment followed by a dephosphorylation. Both rates were enhanced during cell spreading in the presence of extracellular Ca2+. Treatment of cells with the ERK kinase inhibitor PD98059 completely inhibited the attachment-dependent ERK2 phosphorylation but did not inhibit cell spreading, cPLA2 phosphorylation, translocation, or AA release. Activation of PKC by phorbol ester (12-O-tetradecanoylphorbol-13-acetate) induced and attachment-dependent phosphorylation of both cPLA2 and ERK2 in suspension cells. However, in cells treated with the PKC inhibitor Calphostin C before attachment, ERK2 phosphorylation was inhibited, whereas cPLA2 translocation and phosphorylation remained unaffected. In conclusion, although cPLA2-mediated release of AA during HeLa cell attachment to a gelatin substrate was essential for cell spreading, neither ERK2 nor PKC appeared to be responsible for the attachment-induced cPLA2 phosphorylation and the release of AA.

Epidermal growth factor receptor-mediated motility in fibroblasts

Cell motility is induced by many growth factors acting through cognate receptors with intrinsic tyrosine kinase activity (RPTK). However, most of the links between receptor activation and the biophysical processes of cell motility remain undeciphered. We have focused on the mechanisms by which the EGF receptor (EGFR) actuates fibroblast cell motility in an attempt to define this integrated process in one system. Our working model is that divergent, but interconnected pathways lead to the biophysical processes necessary for cell motility: cytoskeleton reorganization, membrane extension, formation of new adhesions to substratum, cell contraction, and release of adhesions at the rear. We postulate that for any given growth factor some of the pathways/processes will be actively signaled and rate-limiting, while others will be permissive due to background low-level activation. Certain couplings have been defined, such as PLCgamma and actin modifying proteins being involved in cytoskeletal reorganization and lamellipod extension and MEK being implicated in detachment from substratum. Others are suggested by complementary investigations in integrin-mediated motility, including rac in membrane protrusion, rho in new adhesions, myosin II motors in contraction, and calpain in detachment, but have yet to be placed in growth factor-induced motility. Our model postulates that many biochemical pathways will be shared between chemokinetic and haptokinetic motility but that select pathways will be activated only during RPTK-enhanced motility.

Extracellular matrix survival signals transduced by focal adhesion kinase suppress p53-mediated apoptosis

In many malignant cells, both the anchorage requirement for survival and the function of the p53 tumor suppressor gene are subverted. These effects are consistent with the hypothesis that survival signals from extracellular matrix (ECM) suppress a p53-regulated cell death pathway. We report that survival signals from fibronectin are transduced by the focal adhesion kinase (FAK). If FAK or the correct ECM is absent, cells enter apoptosis through a p53-dependent pathway activated by protein kinase C lambda/iota and cytosolic phospholipase A2. This pathway is suppressible by dominant-negative p53 and Bcl2 but not CrmA. Upon inactivation of p53, cells survive even if they lack matrix signals or FAK. This is the first report that p53 monitors survival signals from ECM/FAK in anchorage- dependent cells.

Activation of Rac and Cdc42 by integrins mediates cell spreading

Adhesion to ECM is required for many cell functions including cytoskeletal organization, migration, and proliferation. We observed that when cells first adhere to extracellular matrix, they spread rapidly by extending filopodia-like projections and lamellipodia. These structures are similar to the Rac- and Cdc42-dependent structures observed in growth factor-stimulated cells. We therefore investigated the involvement of Rac and Cdc42 in adhesion and spreading on the ECM protein fibronectin. We found that integrin-dependent adhesion led to the rapid activation of p21-activated kinase, a downstream effector of Cdc42 and Rac, suggesting that integrins activate at least one of these GTPases. Dominant negative mutants of Rac and Cdc42 inhibit cell spreading in such a way as to suggest that integrins activate Cdc42, which leads to the subsequent activation of Rac; both GTPases then contribute to cell spreading. These results demonstrate that initial integrin-dependent activation of Rac and Cdc42 mediates cell spreading.

Effect of microtubule disruption on cell adhesion and spreading

Microtubules have been involved in a variety of cellular processes. In this study, we examined the role of the microtubular system in the adhesion and spreading of the adenocarcinoma cell line HT29-D4. Disruption of microtubules by nocodazole or navelbine resulted in an increase in cell adhesion to purified ECM proteins. This enhanced cell adhesion is mediated by integrins, but is not attributable to quantitative changes in the number of integrin receptors at the cell surface, as determined by flow cytometric analysis. In contrast to attachment, spreading of HT29-D4 cells was reduced by nocodazole treatment in a dose-dependent manner. Thus, microtubule depolymerization appears to increase initial attachment of cells to extracellular matrix, while impeding subsequent cell spreading.

Syndecan-4 proteoglycan cytoplasmic domain and phosphatidylinositol 4,5-bisphosphate coordinately regulate protein kinase C activity

Phosphatidylinositol 4,5-bisphosphate (PIP2) is involved in the organization of the actin cytoskeleton by regulating actin-associated proteins. The transmembrane heparan sulfate proteoglycan syndecan-4 also plays a critical role in protein kinase C (PKC) signaling in the formation of focal adhesions and actin stress fibers. The cytoplasmic domain of syndecan-4 core protein directly interacts with and potentiates PKCalpha activity, and it can directly interact with the phos- phoinositide PIP2. We, therefore, investigated whether the interaction of inositol phosphates and inositol phospholipids with syndecan-4 could regulate PKC activity. Data from in vitro kinase assays using purified PKCalpha beta gamma show that in the absence of phosphatidylserine and diolein, PIP2 increased the extent of autophosphorylation of PKCalpha beta gamma and partially activated it to phosphorylate both histone III-S and an epidermal growth factor receptor peptide. This activity was dose-dependent, and its calcium dependence varied with PKC isotype/source. Addition of the cytoplasmic syndecan-4 peptide, but not equivalent syndecan-1 or syndecan-2 peptides, potentiated the partial activation of PKCalpha beta gamma by PIP2, resulting in activity greater than that observed with phosphatidylserine, diolein, and calcium. This study indicates that syndecan-4 cytoplasmic domain may bind both PIP2 and PKCalpha, localize them to forming focal adhesions, and potentiate PKCalpha activity there.

Direct association of protein-tyrosine phosphatase PTP-PEST with paxillin

Tyrosine phosphorylation of focal adhesion-associated proteins may be involved in the regulation of the cytoskeleton and in the control of signals for growth and survival. The focal adhesion kinase (FAK) functions in regulating tyrosine phosphorylation of several of these proteins, including paxillin, tensin, and p130(cas). Protein- tyrosine phosphatases, the counterparts of protein-tyrosine kinases, also presumably regulate phosphorylation of these proteins. We have tested the hypothesis that FAK intimately associates with a protein-tyrosine phosphatase. Protein-tyrosine phosphatase activity associated with the recombinant C-terminal domain of FAK in vitro and could be coimmunoprecipitated with both FAK and paxillin from lysates of chicken embryo cells. However, the interaction with FAK appeared to be indirect and mediated via paxillin. The protein-tyrosine phosphatase was subsequently identified as protein-tyrosine phosphatase-PEST, a nonreceptor protein-tyrosine phosphatase. The C-terminal noncatalytic domain of protein-tyrosine phosphatase-PEST directly bound to paxillin in vitro. The association of both a protein-tyrosine kinase and a protein-tyrosine phosphatase with paxillin suggests that paxillin may play a critical role in the regulation of the phosphotyrosine content of proteins in focal adhesions.

Arachidonate initiated protein kinase C activation regulates HeLa cell spreading on a gelatin substrate by inducing F-actin formation and exocytotic upregulation of beta 1 integrin

HeLa cell spreading on a gelatin substrate requires the activation of protein kinase C (PKC), which occurs as a result of cell-attachment-induced activation of phospholipase A2 (PLA2) to produce arachidonic acid (AA) and metabolism of AA by lipoxyginase (LOX). The present study examines how PKC activation affects the actin- and microtubule-based cytoskeletal machinery to facilitate HeLa cell spreading on gelatin. Cell spreading on gelatin is contingent on PKC induction of both actin polymerization and microtubule-facilitated exocytosis, which is based on the following observations. There is an increase in the relative content of filamentous (F)-actin during HeLa cell spreading, and treating HeLa cells with PKC-activating phorbol esters such as 12-O-tetradecanoyl phorbol 13-acetate (TPA) further increases the relative content of F-actin and the rate and extent to which the cells spread. Conversely, inhibition of PKC by calphostin C blocked both cell spreading and the increase of F-actin content. The increased F-actin content induced by PKC activators also was observed in suspension cells treated with TPA, and the kinetics of F-actin were similar to that for PKC activation. In addition, PKC epsilon, which is the PKC isoform most involved in regulating HeLa cell spreading in response to AA production, is more rapidly translocated to the membrane in response to TPA treatment than is the increase in F-actin. Blocking the activities of either PLA2 or LOX inhibited F-actin formation and cell spreading, both of which were reversed by TPA treatment. This result is consistent with AA and a LOX metabolite of AA as being upstream second messengers of activation of PKC and its regulation of F-actin formation and cell spreading. PKC appears to activate actin polymerization in the entire body of the cell and not just in the region of cell-substrate adhesion because activated PKC was associated not only with the basolateral plasma membrane domain contacting the culture dish but also with the apical plasma membrane domain exposed to the culture medium and with an intracellular membrane fraction. In addition to the facilitation of F-actin formation, activation of PKC induces the exocytotic upregulation of beta 1 integrins from an intracellular domain to the cell surface, possibly in a microtubule-dependent manner because the upregulation is inhibited by Nocodazole. The results support the concept that cell-attachment-induced AA production and its metabolism by LOX results in the activation of PKC, which has a dual role in regulating the cytoskeletal machinery during HeLa cell spreading. One is through the formation of F-actin that induces the structural reorganization of the cells from round to spread, and the other is the exocytotic upregulation of collagen receptors to the cell surface to enhance cell spreading.

Nuclear-cytoplasmic shuttling of the focal contact protein, zyxin: a potential mechanism for communication between sites of cell adhesion and the nucleus

Integrin-dependent cell adhesion to specific extracellular matrix molecules has been demonstrated to trigger dramatic changes in gene expression that can affect cell fate. However, little is understood about the molecular mechanism by which events at sites of cell- substratum adhesion are communicated to the cell interior to regulate the transcriptional apparatus. By analogy to classical mechanisms of cell surface receptor function, it seems likely that some components of the integrin-activated signal transduction machinery will be colocalized with cell adhesion molecules. Zyxin is a low abundance phosphoprotein that accumulates with integrins at sites of cell-substratum attachment. Here we show that zyxin exhibits a functional nuclear export signal that is required to keep zyxin concentrated in the cytoplasm and is sufficient to direct nuclear proteins to the cytosol. Furthermore, we demonstrate that native zyxin shuttles between the nucleus and sites of cell adhesion in fibroblasts and is thus an excellent candidate for relaying information between these two compartments.

Phosphatidylinositol 3-kinase is required for integrin-stimulated AKT and Raf-1/mitogen-activated protein kinase pathway activation

Cell attachment to fibronectin stimulates the integrin-dependent interaction of p85-associated phosphatidylinositol (PI) 3-kinase with integrin-dependent focal adhesion kinase (FAK) as well as activation of the Ras/mitogen-activated protein (MAP) kinase pathway. However, it is not known if this PI 3-kinase-FAK interaction increases the synthesis of the 3-phosphorylated phosphoinositides (3-PPIs) or what role, if any, is played by activated PI 3-kinase in integrin signaling. We demonstrate here the integrin-dependent accumulation of the PI 3-kinase products, PI 3,4-bisphosphate [PI(3,4)P2] and PI(3,4,5)P3, as well as activation of AKT kinase, a serine/threonine kinase that can be stimulated by binding of PI(3,4)P2. The PI 3-kinase inhibitors wortmannin and LY294002 significantly decreased the integrin-induced accumulation of the 3-PPIs and activation of AKT kinase, without having significant effects on the levels of PI(4,5)P2 or tyrosine phosphorylation of paxillin. These inhibitors also reduced cell adhesion/spreading onto fibronectin but had no effect on attachment to polylysine. Interestingly, integrin-mediated Erk-2, Mek-1, and Raf-1 activation, but not Ras-GTP loading, was inhibited at least 80% by wortmannin and LY294002. In support of the pharmacologic results, fibronectin activation of Erk-2 and AKT kinases was completely inhibited by overexpression of a dominant interfering p85 subunit of PI 3-kinase. We conclude that integrin-mediated adhesion to fibronectin results in the accumulation of the PI 3-kinase products PI(3,4)P2 and PI(3,4,5)P3 as well as the PI 3-kinase-dependent activation of the kinases Raf-1, Mek-1, Erk-2, and AKT and that PI 3-kinase may function upstream of Raf-1 but downstream of Ras in integrin activation of Erk-2 MAP and AKT kinases.

Multimerization of the cytoplasmic domain of syndecan-4 is required for its ability to activate protein kinase C

The transmembrane proteoglycan syndecan-4, which is a coreceptor with integrins in cytoskeleton-matrix interactions, appears to be multimerized in vivo. Both purified and recombinant core proteins form sodium dodecyl sulfate-resistant oligomers, and we now report that a synthetic peptide corresponding to the central region of syndecan-4 cytoplasmic domain (4V) also oligomerizes. The degree of oligomerization correlates with the previously reported ability to bind protein kinase C (PKC) and regulate its activity. Only multimeric recombinant syndecan-4 core protein, but not the monomeric protein, potentiated the activity of PKCalpha, and only oligomeric syndecan-4 cytoplasmic peptides were active. Changes in peptide sequence caused parallel loss of stable oligomeric status and ability to regulate a mixture of PKCalphabetagamma activity. A synthetic peptide encompassing the whole cytoplasmic domain of syndecan-4 (4L) containing a membrane-proximal basic sequence did not form higher order oligomers and could not regulate the activity of PKCalphabetagamma unless induced to aggregate by phosphatidylinositol 4,5-bisphosphate. Oligomerization and PKC regulatory activity of the 4V peptide were both increased by addition of N-terminal cysteine and reduced by phosphorylation of the cysteine thiol group. Concentration of syndecan-4 at sites of focal adhesion formation may enhance multimerization and both localize PKC and potentiate its activity to induce stable complex formation.

The membrane-spanning proteoglycan NG2 binds to collagens V and VI through the central nonglobular domain of its core protein

NG2 is a membrane-spanning proteoglycan with a primary structure unique among cell surface or extracellular matrix proteins. To characterize the interaction between NG2 and extracellular matrix proteins, we have used a eukaryotic expression system to produce and purify several recombinant fragments covering not only the entire ectodomain of NG2 but also distinct subdomains of the molecule. Using a solid phase binding assay with various extracellular matrix proteins, we have identified two main ligands for NG2, namely, collagens V and VI. Consistent with previous models of glycosaminoglycan attachment, roughly 50% of the recombinant NG2 fragments containing the central domain have chondroitin sulfate chains attached to the protein core. These glycosaminoglycan chains are not directly involved in collagen binding, since chondroitinase-treated fragments exhibit an unimpaired ability to bind to both collagens. Using more restricted recombinant fragments of NG2, we mapped the binding site for both collagens to the central domain of NG2. Electron microscopy after rotary shadowing of native NG2 molecules indicates that this extended nonglobular domain provides a flexible connection joining the two N- and C-terminal globular regions of NG2. Rotary shadowing of mixtures of NG2 and collagen V or VI confirms a direct interaction between the molecules and indicates that the collagens align with the central region of NG2, giving the appearance of a rod between the N- and C-terminal globules.

A three-dimensional collagen lattice induces protein kinase C-zeta activity: role in alpha2 integrin and collagenase mRNA expression

A three-dimensional collagen lattice can provide skin fibroblasts with a cell culture environment that simulates normal dermis. Such a collagen matrix environment regulates interstitial collagenase (type I metalloproteinase [MMP-1], collagenase-1) and collagen receptor alpha2 subunit mRNA expression in both unstimulated or platelet-derived growth factor-stimulated dermal fibroblasts (Xu, J., and R.A.F. Clark. 1996. J. Cell Biol. 132:239-249). Here we report that the collagen gel can signal protein kinase C (PKC)-zeta activation in human dermal fibroblasts. An in vitro kinase assay demonstrated that autophosphorylation of PKC-zeta immunoprecipitates was markedly increased by a collagen matrix. In contrast, no alteration in PKC-zeta protein levels or intracellular location was observed. DNA binding activity of nuclear factor kappaB (NF-kappaB), a downstream regulatory target of PKC-zeta, was also increased by fibroblasts grown in collagen gel. The composition of the NF-kappaB/Rel complexes that contained p50, was not changed. The potential role of PKC-zeta in collagen gel-induced mRNA expression of collagen receptor alpha2 subunit and human fibroblast MMP-1 was assessed by the following evidence. Increased levels of alpha2 and MMP-1 mRNA in collagen gel-stimulated fibroblasts were abrogated by bisindolylmaleimide GF 109203X and calphostin C, chemical inhibitors for PKC, but retained when cells were depleted of 12-myristate 13-acetate (PMA)-inducible PKC isoforms by 24 h of pretreatment with phorbol PMA. Antisense oligonucleotides complementary to the 5' end of PKC-zeta mRNA sequences significantly reduced the collagen lattice-stimulated alpha2 and MMP-1 mRNA levels. Taken together, these data indicate that PKC-zeta, a PKC isoform not inducible by PMA or diacylglycerol, is a component of collagen matrix stimulatory pathway for alpha2 and MMP-1 mRNA expression. Thus, a three-dimensional collagen lattice maintains the dermal fibroblast phenotype, in part, through the activation of PKC-zeta.

Macrophage arachidonate release via both the cytosolic Ca(2+)-dependent and -independent phospholipases is necessary for cell spreading

We have observed that phospholipase A2 (PLA2) activation and arachidonate (AA) release are essential for monocyte/macrophage adherence and spreading. In this study, we addressed the relationship between AA release and cell adherence/spreading in murine resident peritoneal macrophages, and the roles of specific PLA2S in these processes. The PLA2-specific inhibitors, (E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (BEL, specific for the Ca(2+)-independent PLA2 (iPLA2)) and methyl arachidonoyl fluorophosphonate (MAFP, specific for the Ca(2+)-dependent phospholipase (cPLA2)) inhibited AA release and cell spreading in a correlated fashion but only modestly decreased cell adherence. Cell spreading was normalized by the addition of AA to PLA2-inhibited cells. AA release during spreading was also inhibited by Ca2+ depletion or protein kinase C (PKC) inhibition, and was accompanied by increased (but transient) phosphorylation of cPLA2-Inhibition of macrophage spreading, however, only partially inhibited AA release. Moreover, constitutive AA release was seen in fully spread macrophages which was inhibited by BEL, but not MAFP or Ca2+ depletion. BEL also reversed the phenotype of fully spread cells. These data suggest that macrophage spreading requires the release of AA by the iPLA2 (which appears to be constitutively active) and cPLA2 (which appears to be stimulated by adherence/spreading). Maintenance of macrophage spreading, in contrast, appears to be principally dependent on the iPLA2.

Integrin regulation of c-Abl tyrosine kinase activity and cytoplasmic-nuclear transport

The product of the c-abl protooncogene is a nonreceptor tyrosine kinase found in both the cytoplasm and the nucleus. We report herein that cell adhesion regulates the kinase activity and subcellular localization of c-Abl. When fibroblastic cells are detached from the extracellular matrix, kinase activity of both cytoplasmic and nuclear c-Abl decreases, but there is no detectable alteration in the subcellular distribution. Upon adhesion to the extracellular matrix protein fibronectin, a transient recruitment of a subset of c-Abl to early focal contacts is observed coincident with the export of c-Abl from the nucleus to the cytoplasm. The cytoplasmic pool of c-Abl is reactivated within 5 min of adhesion, but the nuclear c-Abl is reactivated after 30 min, correlating closely with its return to the nucleus and suggesting that the active nuclear c-Abl originates in the cytoplasm. In quiescent cells where nuclear c-Abl activity is low, the cytoplasmic c-Abl is similarly regulated by adhesion but the nuclear c-Abl is not activated upon cell attachment. These results show that c-Abl activation requires cell adhesion and that this tyrosine kinase can transmit integrin signals to the nucleus where it may function to integrate adhesion and cell cycle signals.

Focal adhesions, contractility, and signaling

Focal adhesions are sites of tight adhesion to the underlying extracellular matrix developed by cells in culture. They provided a structural link between the actin cytoskeleton and the extracellular matrix and are regions of signal transduction that relate to growth control. The assembly of focal adhesions is regulated by the GTP-binding protein Rho. Rho stimulates contractility which, in cells that are tightly adherent to the substrate, generates isometric tension. In turn, this leads to the bundling of actin filaments and the aggregation of integrins (extracellular matrix receptors) in the plane of the membrane. The aggregation of integrins activates the focal adhesion kinase and leads to the assembly of a multicomponent signaling complex.

Regulation of intracellular pH by phospholipase A2 and protein kinase C upon neutrophil adhesion to solid substrata

Adhesion to solid substrata has been shown to increase intracellular pH (pH(i)) of fibroblasts and of other cells (FEBS Lett. (1988) 234, 449-450; Proc. Natl. Acad. Sci. USA (1989) 86, 4525-4529; J. Biol. Chem. (1990) 265, 1327-1332; Exp. Cell Res. (1992) 200, 211-214; FEBS Lett. (1995) 374, 17-20). We have found that the inhibitors of PLA2, 4-bromophenacyl bromide and manoalide, completely blocked the increase of pH(i) and spreading of neutrophils upon adhesion to solid substrata. Inhibition of phospholipase C with neomycin or removal of extracellular Ca2+ affects neither neutrophil spreading nor their pH(i). Inhibition of PKC with H-7 or staurosporin increased pH(i). PMA, an activator of PKC, dramatically decreased pH(i) but did not impair the spreading of neutrophils. The effect of arachidonic acid, a product of PLA2 activity, on neutrophil pH(i) and spreading was similar to that of PMA. H-7, an inhibitor of PKC, partially blocked the effect of arachidonic acid (AA) on pH(i). BW755C, an inhibitor of AA metabolism by cyclooxygenase or lipoxygenase, affected neither the pH(i) nor cell spreading. We propose that the increase of pH(i) upon neutrophil adhesion is mediated by PLA2 activity, while PKC decreased pH(i). AA produced by PLA2 activates PKC, thus forming a feedback regulation of pH(i).

Integrin-dependent signal transduction

Integrins are receptor molecules for extracellular matrix molecules (e.g., the beta(1) family), serum components (alpha(v) family) and immunoglobulin family adhesion molecules (beta(2) family). Integrin-dependent adhesion has also been shown to have metabolic consequences. Adhesion to a variety of extracellular matrix proteins, such as fibronectin, collagen, and laminin, is a potent regulator of cell growth, differentiation, and gene expression. Ligand binding or aggregation of integrin receptors initiates a number of metabolic changes including activation of serine/threonine and tyrosine kinases, increased Ca2+ influx, increased cytoplasmic alkalinization, and altered inositol lipid metabolism. In some instances activation of transcription factors and induction of gene expression have also been demonstrated. Components of key signaling pathways involving integrins are beginning to be identified. Some studies have shown that integrins form multi-component complexes with signal transduction molecules. Elucidating the interactions of the signal transduction molecules with each other and with the integrin cytoplasmic domains will be key to understanding the initial events of signal transduction through the integrins.

Differential translocation of protein kinase C epsilon during HeLa cell adhesion to a gelatin substratum

The spreading of HeLa cells, following attachment to a collagen or gelatin substratum, requires the activation of protein kinase C (PKC). Membrane-bound PKC was previously shown to be activated during cell attachment and in response to the activation of a series of lipid second messengers turned on by the ligation of beta1-integrin collagen receptors. HeLa cells express the alpha, gamma, epsilon, zeta, lambda, and iota isozymes of PKC as determined by Western blotting with specific antibodies. Only PKCepsilon redistributed from the cytosol to the membrane during cell adhesion. Most of the PKCepsilon in cells that were in suspension was in the cytosolic fraction. During cell attachment to a gelatin matrix, all of the PKCepsilon moved out of the cytosol, with most going to the membrane fraction. After the cells became fully spread, PKCepsilon began to reappear in the cytosol. Translocation of PKCepsilon was not observed during the adhesion of cells to culture dishes where cells nonspecifically attach but do not spread. The conventional PKCalpha and -gamma isozymes were translocated from the cytosol to the membrane only when phorbol ester was present at a concentration that increases the rate and extent of cell spreading. Under normal conditions, i.e. in the absence of phorbol ester, PKCepsilon appears to be the PKC isozyme responsible for the regulation of HeLa cell adhesion to the extracellular matrix.

ADP-ribosylation of the G protein Rho inhibits integrin regulation of tumor cell growth

Using a gastric derived tumor line, we investigated the involvement of beta 1 integrin and Rho in cell growth regulation in response to collagen. The addition of C3 exoenzyme from clostridium botulinum to specifically ribosylate and inhibit the function of the rho gene products inhibited cellular proliferation in a dose-dependent fashion. C3 exoenzyme exhibited broad cytostatic activity toward a number of tumor lines and induced G0/G1 accumulation, cyclin A inhibition, and pronounced alterations in cell morphology. Integrin-mediated adhesion to collagen led to the expression of the cyclin A gene whose expression could be blocked using anti-beta 1 integrin monoclonal antibodies. Phospholipid levels were induced upon beta 1 integrin-mediated adhesion to collagen, and the phospholipid induction was inhibited by either antibodies to beta 1 integrin or pretreatment of cells with C3 exoenzyme. Significant reduction in phospholipid levels correlated with proliferation for a panel of tumor lines deprived of adhesion to substrate. These results implicate a novel role for integrins and Rho in the regulation of tumour growth in response to matrix.

Rapid regulation of neurite outgrowth and retraction by phospholipase A2-derived arachidonic acid and its metabolites

Arachidonic acid and lipoxygenase metabolites have been proposed to act as retrograde synaptic messengers and as early mediators of neuronal injury, but few studies have analyzed their roles in controlling neurite behavior within a time window of minutes to hours. Phospholipase A2 inhibitors (BPB, ONO-RS-082, quinacrine and AACOCF3) and the lipoxygenase inhibitor AA861 delayed the initial outgrowth of NG108-15 cell neurites on laminin. Inhibitors of diacylglycerol lipase (RHC 80267), cyclooxygenase (indomethacin) and free radicals (N-acetyl cysteine and vitamin E) did not produce similar effects. Phospholipase A2 and lipoxygenase inhibitors also prevented acute neurite retraction in response to lysophosphatidic acid and eight other agents tested, and decreased F-actin staining at cell margins. Conversely, exogenous arachidonic acid (1 microM) enhanced the responses of neurites in outgrowth and retraction assays. Phospholipase A2 and lipoxygenase pathways appear to have a general role in maintaining the ability of neurites to respond rapidly to external stimuli, possibly via regulating the ability of the cytoskeleton to remodel.

Protein kinase C regulates the recruitment of syndecan-4 into focal contacts

Cell surface heparan sulfate proteoglycans have been implicated as co-receptors facilitating cell adhesion and growth factor binding. Recent studies on the role of a family of transmembrane heparan sulfate proteoglycans, syndecans, in cell adhesion has identified one member, syndecan-4, to be present within focal contacts. The current study investigates the mechanisms regulating the association of syndecan-4 with focal contacts based upon its immunolocalization with vinculin in quiescent, serum-stimulated, and 12-0-tetradecanoylphorbol 13-acetate (TPA)-induced cultures. In quiescent cells, syndecan-4 did not localize to focal contacts. However, activation of protein kinase C by TPA or serum induces the active recruitment of syndecan-4 into focal contacts. This induction preferentially localizes syndecan-4 to focal contacts behind the leading lamella, the subnuclear region, and along the trailing edge of migratory cells. Focal contacts in either freshly adhered cells or in the leading lamellae of migrating cells did not stain for syndecan-4. In addition to the observed subcellular distribution and recruitment, syndecan-4 was observed to co-localize with endogenously synthesized fibronectin fibrils within focal contacts as well as with fibrils present in the matrix. These findings suggest that protein kinase C activation results in syndecan-4 recruitment to focal contacts and its association with sites of matrix deposition.

Convergence of integrin and growth factor receptor signaling pathways within the focal adhesion complex

Extracellular matrix controls capillary endothelial cell sensitivity to soluble mitogens by binding integrin receptors and thereby activating a chemical signaling response that rapidly integrates with growth factor-induced signaling mechanisms. Here we report that in addition to integrins, growth factor receptors and multiple molecules that transduce signals conveyed by both types of receptors are immobilized on the cytoskeleton (CSK) and spatially integrated within the focal adhesion complex (FAC) at the site of integrin binding. FACs were rapidly induced in round cells and physically isolated from the remainder of the CSK after detergent-extraction using magnetic microbeads coated with fibronectin or a synthetic RGD-containing peptide. Immunofluorescence microscopy revealed that multiple signaling molecules (e.g., pp60c-src, pp125FAK, phosphatidylinositol-3-kinase, phospholipase C-gamma, and Na+/H+ antiporter) involved in both integrin and growth factor receptor signaling pathways became associated with the CSK framework of the FAC within 15 min after binding to beads coated with integrin ligands. Recruitment of tyrosine kinases to the FAC was also accompanied by a local increase in tyrosine phosphorylation, as indicated by enhanced phosphotyrosine staining at the site of integrin binding. In contrast, neither recruitment of signaling molecules nor increased phosphotyrosine staining was observed when cells bound to beads coated with a control ligand (acetylated low density lipoprotein) that ligates transmembrane scavenger receptors, but does not induce FAC formation. Western blot analysis confirmed that FACs isolated using RGD-beads were enriched for pp60c-src, pp125FAK, phospholipase C-gamma, and the Na+/H+ antiporter when compared with intact CSK or basal cell surface preparations that retained lipid bilayer. Isolated FACs were also greatly enriched for the high affinity fibroblast growth factor receptor flg. Most importantly, isolated FACs continued to exhibit multiple chemical signaling activities in vitro, including protein tyrosine kinase activities (pp60c-src and pp125FAK) as well as the ability to undergo multiple sequential steps in the inositol lipid synthesis cascade. These data suggest that many of the chemical signaling events that are induced by integrins and growth factor receptors in capillary cells may effectively function in a "solid-state" on insoluble CSK scaffolds within the FAC and that the FAC may represent a major site for signal integration between these two regulatory pathways. Future investigations into the biochemical and biophysical basis of signal transduction may be facilitated by this method, which results in isolation of FACs that retain the CSK framework as well as multiple associated chemical signaling activities.

Alpha v beta 5 integrin receptor-mediated endocytosis of vitronectin is protein kinase C-dependent

Previous studies have demonstrated that the alpha v beta 5 integrin receptor functions in the endocytosis and degradation of matrix-bound vitronectin by human skin fibroblasts (Panetti, T. S., and McKeown-Longo, P. J. (1993) J. Biol. Chem. 268, 11988-11993; Panetti, T. S., and McKeown-Longo, P. J. (1993) J. Biol. Chem. 268, 11492-11495). These earlier studies demonstrated that vitronectin degradation was inhibited by either antibodies to the beta 5 integrin or exogenous heparin, suggesting that both integrin receptors and cell surface heparan sulfate proteoglycans are involved in the endocytosis and degradation of vitronectin. The present study was done to define intracellular signaling pathways involved in endocytosis of vitronectin and to evaluate the relative contribution of cell surface heparan sulfate proteoglycans and the alpha v beta 5 integrin in the activation of these signaling pathways. The addition of the phorbol ester phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, to monolayers of human skin fibroblasts, increased vitronectin degradation. Staurosporine and calphostin C, inhibitors of protein kinase C, blocked internalization and subsequent degradation of vitronectin, while KT5720, an inhibitor of protein kinase A, had no effect on the degradation of vitronectin. PMA was also able to reverse the inhibition of vitronectin degradation seen when cells were pretreated with heparinase or incubated with exogenous heparin. In contrast, the inhibitory effect of either RGD peptides or anti-alpha v beta 5 antibodies on vitronectin degradation were not overcome by the addition of PMA. These data suggest that the internalization of vitronectin from the matrix is mediated by the alpha v beta 5 integrin following activation of protein kinase C.