Paxillin and focal adhesion signalling

Expression of the LIM proteins paxillin and Hic-5 in human tissues

The LIM domain is a protein-protein interaction motif critically involved in a variety of fundamental biological processes, including cytoskeletal organization, cell lineage specification, and organ development. In this study we examined the expression of the LIM proteins paxillin and Hic-5 in adult human tissues by immunohistochemistry and immunoblotting. Paxillin expression was widespread and observed both in non-muscle and muscle tissues. Of the latter, paxillin was mainly expressed in multinuclear striated muscle. In contrast, Hic-5 showed restricted expression and was expressed in muscle tissues, mainly in mononuclear smooth muscle. Taken together with previous findings, it appears likely that the counterbalance between paxillin and Hic-5 may be deeply involved in muscle differentiation.

Biological aspects of signal transduction by cell adhesion receptors

Cell adhesion receptors such as integrins, cadherins, selectins, and immunoglobulin family receptors profoundly modulate many signal transduction cascades. In this review we examine aspects of adhesion receptor signaling and how this impinges on key biological processes. We have chosen to focus on cell migration and on programmed cell death. We examine many of the cytoplasmic signaling molecules that interface with adhesion receptors, including focal adhesion kinase (FAK), phosphatidylinositol-3-kinase (PI3K), and elements of the Erk/MAP kinase pathway. In many cases these molecules impinge on both the regulation of cell movement and on control of apoptosis.

Live-cell monitoring of tyrosine phosphorylation in focal adhesions following microtubule disruption

Tyrosine phosphorylation of focal adhesion components is involved in the regulation of focal adhesion formation and turnover, yet the underlying molecular mechanisms are still poorly defined. In the present study, we have used quantitative fluorescence microscopy to investigate the dynamic relationships between the incorporation of new components into growing focal adhesions and tyrosine phosphorylation of these sites. For this purpose, a new approach for monitoring phosphotyrosine levels in live cells was developed, based on a 'phosphotyrosine reporter' consisting of yellow fluorescent protein fused to two consecutive phosphotyrosine-binding Src-homology 2 (SH2)-domains derived from pp60(c-Src). This YFP-dSH2 localized to cell-matrix adhesions and its intensity was linearly correlated with that of an anti-phosphotyrosine antibody labeling. The differential increase in vinculin and phosphotyrosine levels was examined in live cells by two-color time-lapse movies of CFP-vinculin and YFP-dSH2. In this study, focal adhesion growth was triggered by microtubule disruption, which was previously shown to stimulate focal adhesion development by inducing cellular contraction. We show here that, 2 minutes after addition of the microtubule-disrupting drug nocodazole, the local densities of the focal adhesion-associated proteins vinculin, paxillin and focal adhesion kinase (FAK) are significantly elevated and the focal adhesion area is increased, whereas elevation in tyrosine phosphorylation inside the growing adhesions occurs only a few minutes later. Phosphotyrosine and FAK density reach their maximum levels after 10 minutes of treatment, whereas vinculin and paxillin levels as well as focal adhesion size continue to grow, reaching a plateau at about 30 minutes. Our findings suggest that protein recruitment and growth of focal adhesions are an immediate and direct result of increased contractility induced by microtubule disruption, whereas tyrosine phosphorylation is activated later.

Epidermal growth factor-induced epithelio-mesenchymal transition in human breast carcinoma cells

PMC42-LA cells display an epithelial phenotype: the cells congregate into pavement epithelial sheets in which E-cadherin and beta-catenin are localized at cell-cell borders. They abundantly express cytokeratins, although 5% to 10% of the cells also express the mesenchymal marker vimentin. Stimulation of PMC42-LA cells with epidermal growth factor (EGF) leads to epithelio-mesenchymal transition-like changes including up-regulation of vimentin and down-regulation of E-cadherin. Vimentin expression is seen in virtually all cells, and this increase is abrogated by treatment of cells with an EGF receptor antagonist. The expression of the mesenchyme-associated extracellular matrix molecules fibronectin and chondroitin sulfate proteoglycan also increase in the presence of EGF. PMC42-LA cells adhere rapidly to collagen I, collagen IV, and laminin-1 substrates and markedly more slowly to fibronectin and vitronectin. EGF increases the speed of cell adhesion to most of these extracellular matrix molecules without altering the order of adhesive preference. EGF also caused a time-dependent increase in the motility of PMC42-LA cells, commensurate with the degree of vimentin staining. The increase in motility was at least partly chemokinetic, because it was evident both with and without chemoattractive stimuli. Although E-cadherin staining at cell-cell junctions disappeared in response to EGF, beta-catenin persisted at the cell periphery. Further analysis revealed that N-cadherin was present at the cell-cell junctions of untreated cells and that expression was increased after EGF treatment. N- and E-cadherin are not usually coexpressed in human carcinoma cell lines but can be coexpressed in embryonic tissues, and this may signify an epithelial cell population prone to epithelio-mesenchymal-like responses.

Hic-5 communicates between focal adhesions and the nucleus through oxidant-sensitive nuclear export signal

hic-5 was originally isolated as an H(2)O(2)-inducible cDNA clone whose product was normally found at focal adhesions. In this study, we found that Hic-5 accumulated in the nucleus in response to oxidants such as H(2)O(2). Other focal adhesion proteins including paxillin, the most homologous to Hic-5, remained in the cytoplasm. Mutation analyses revealed that the C- and N-terminal halves of Hic-5 contributed to its nuclear localization in a positive and negative manner, respectively. After the finding that leptomycin B (LMB), an inhibitor of nuclear export signal (NES), caused Hic-5 to be retained in the nucleus, Hic-5 was demonstrated to harbor NES in the N-terminal, which was sensitive to oxidants, thereby regulating the nuclear accumulation of Hic-5. NES consisted of a leucine-rich stretch and two cysteines with a limited similarity to Yap/Pap-type NES. In the nucleus, Hic-5 was suggested to participate in the gene expression of c-fos. Using dominant negative mutants, we found that Hic-5 was actually involved in endogenous c-fos gene expression upon H(2)O(2) treatment. Hic-5 was thus proposed as a focal adhesion protein with the novel aspect of shuttling between focal adhesions and the nucleus through an oxidant-sensitive NES, mediating the redox signaling directly to the nucleus.

Regulation of the formation of osteoclastic actin rings by proline-rich tyrosine kinase 2 interacting with gelsolin

Osteoclast activation is important for bone remodeling and is altered in multiple bone disorders. This process requires cell adhesion and extensive actin cytoskeletal reorganization. Proline-rich tyrosine kinase 2 (PYK2), a major cell adhesion-activated tyrosine kinase in osteoclasts, plays an important role in regulating this event. The mechanisms by which PYK2 regulates actin cytoskeletal organization and osteoclastic activation remain largely unknown. In this paper, we provide evidence that PYK2 directly interacts with gelsolin, an actin binding, severing, and capping protein essential for osteoclastic actin cytoskeletal organization. The interaction is mediated via the focal adhesion-targeting domain of PYK2 and an LD motif in gelsolin's COOH terminus. PYK2 phosphorylates gelsolin at tyrosine residues and regulates gelsolin bioactivity, including decreasing gelsolin binding to actin monomer and increasing gelsolin binding to phosphatidylinositol lipids. In addition, PYK2 increases actin polymerization at the fibroblastic cell periphery. Finally, PYK2 interacts with gelsolin in osteoclasts, where PYK2 activation is required for the formation of actin rings. Together, our results suggest that PYK2 is a regulator of gelsolin, revealing a novel PYK2-gelsolin pathway in regulating actin cytoskeletal organization in multiple cells, including osteoclasts.

High tidal volume ventilation induces proinflammatory signaling in rat lung endothelium

Alveolar overdistension during mechanical ventilation causes leukocyte sequestration, leading to lung injury. However, underlying endothelial cell (EC) mechanisms are undefined. In a new approach, we exposed isolated blood-perfused rat lungs to high tidal volume ventilation (HV) for 2 h, then obtained fresh lung endothelial cells (FLEC) by immunosorting at 4 degrees C. Immunoblotting experiments indicated that as compared with FLEC derived from lungs ventilated at low volume (LV), HV markedly enhanced tyrosine phosphorylation (TyrP). The tyrosine kinase blocker, genistein, inhibited this response. HV also induced focal adhesion (FA) formation in FLEC, as detected by immunofluorescent aggregates of the alpha(v)beta(3) integrin that co-localized with aggregations of focal adhesion kinase (FAK). Immunoprecipitation and blotting experiments revealed that HV increased TyrP of the FA protein, paxillin. In addition, HV induced a paxillin-associated P-selectin expression on FLEC that was also inhibited by genistein. However, HV did not increase lung water. These results indicate that in HV, EC signaling in situ causes FA formation and induces TyrP-dependent P-selectin expression. These signaling mechanisms may promote leukocyte-mediated responses in HV.

Protein phosphatase-2A restricts migration of Lewis lung carcinoma cells by modulating the phosphorylation of focal adhesion proteins

Compared to metastatic Lewis lung carcinoma (LLC) cells, nonmetastatic LLC cells have increased levels of activity of the protein phosphatase PP-2A, which functions to limit their migration through transwell chambers. Inhibition of PP-2A in nonmetastatic LLC stimulates their transmigration to levels similar to those of metastatic LLC cells. Studies to define the signaling pathways intermediate between diminished PP-2A activity and stimulated migration showed that inhibiting PP-2A activity resulted in paxillin serine hyperphosphorylation and tyrosine dephosphorylation. Paxillin was important for the stimulated migration because the increased transmigration in response to PP-2A inhibition was dampened by expression of mutant paxillin at the LIM3 S457 and S481 residues. Inhibition of PP-2A also led to the dissolution of FAK/Src/paxillin focal adhesion complexes, which was also dependent on paxillin S457 and S481 residues. In addition, inhibition of PP-2A resulted in dephosphorylation of Src inhibitory Y527 residue, suggesting increased Src activity. The stimulated transmigration of cells with diminished PP-2A was in part dependent on this Src activity. These studies show the importance of PP-2A in limiting tumor cell migration through its modulation of proteins of the focal adhesions.

A study of compatibility between cells and biopolymeric surfaces through quantitative measurements of adhesive forces

The mechanism of cell adhesion to biomaterials or components of the extracellular matrix is an important topic in the field of tissue engineering and related biotechnological processes. Many factors affect cell adhesion, and many biochemical and biological studies have attempted to identify their roles in the adhesion mechanism. Systematic studies of this nature require quantification of the adhesive force of a cell to identify the effect of a specific factor. However, most studies of cell adhesive force have used qualitative approaches. We propose a new technique for quantifying the force by which cells adhere to various biomaterial surfaces, which utilizes the relationship between the deflection of a cantilever beam and the required force. A micropipette was used as the cantilever beam. This technique was used to measure the attachment forces of chondrocytes seeded on three different biodegradable polymers commonly used in tissue engineering and medicine: poly epsilon-carprolactone (PCL), poly(L-lactide) (PLLA) and poly(lactic-co-glycolic acid) (PGLA, L/G = 75:25). The bond between the cells and the three polymers was evaluated using the quantified adhesive forces. The adhesive forces were also measured 8, 12, 24 h and 5 days after seeding the chondrocytes on the polymer surfaces. Results of statistical analysis showed that the cells attached to the PLLA had the strongest average attachment force for up to 24 h after seeding (P < 0.05).

The guanine nucleotide exchange factor C3G is necessary for the formation of focal adhesions and vascular maturation

The Ras signalling pathway has major roles in normal cell function and oncogenesis. C3G is a guanine nucleotide exchange factor for members of the Ras family of GTPases. We generated a mouse strain with a hypomorphic C3G allele. C3G(gt/gt) mutant embryos died of vascular defects around E11.5 due to haemorrhage and vascular integrity defects. Vascular supporting cells did not develop appropriately. C3G-deficient fibroblasts responded to PDGF-BB abnormally, exhibited cell adhesion defects and lacked paxillin and integrin-beta1-positive cell adhesions. In contrast, integrin-beta3-positive cell adhesions formed normally. These results show that C3G is required for (1) vascular myogenesis, (2) the formation of paxillin- and integrin beta1-positive, but not integrin beta3-positive, cell adhesions and (3) normal response to PDGF, necessary for vascular myogenesis.

Calpain mediates progressive plasma membrane permeability and proteolysis of cytoskeleton-associated paxillin, talin, and vinculin during renal cell death

The goal of the present study was to determine the role of calpain in changes in plasma membrane permeability and cytoskeleton-associated paxillin, vinculin, talin, and alpha-actinin levels during acute renal cell death. The mitochondrial inhibitor antimycin A or hypoxia produced graded plasma membrane permeability in renal proximal tubules (RPTs), first allowing propidium iodide (PI, molecular mass 668 Da) influx and then lactate dehydrogenase (LDH, molecular mass 130 kDa) release. Cytoskeleton-associated paxillin levels decreased concomitantly with PI influx and before LDH release, whereas cytoskeleton-associated talin and vinculin levels decreased concomitantly with LDH release. Cytoskeleton-associated alpha-actinin levels did not change during antimycin A exposure or hypoxia. Purified micro-calpain cleaved paxillin, talin, vinculin, but not alpha-actinin. The dissimilar calpain inhibitors 3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid (PD150606) or chloroacetic acid N'-[6,7-dichloro-4-phenyl)-3-oxo-3,4-dihydroquinoxalin-2-yl] hydrazide (SJA7029) preserved cytoskeleton-associated paxillin, talin, and vinculin levels and prevented PI influx and LDH release in antimycin A-exposed or hypoxic RPTs. These results suggest that calpain mediates increased plasma membrane permeability and hydrolysis of cytoskeleton-associated paxillin, vinculin, and talin during renal cell death.

Transcriptional activation of the c-fos gene by a LIM protein, Hic-5

Hic-5 is a member of LIM family proteins with a striking similarity to paxillin and localizes primarily in the focal adhesion. We recently reported that Hic-5 translocated to the nucleus under oxidative stress and was involved in transcriptional regulation. In the present study, we extended these findings to show that transcription of c-fos gene was up-regulated by overexpression of Hic-5. In clonal stable transformants established from human immortalized fibroblasts by transfection of an expression vector of Hic-5, the constitutive level of c-fos mRNA was well correlated with that of Hic-5. In reporter assays using the luciferase gene under control of the human c-fos 5(')-upstream region from -2.2kb to +1, expression of Hic-5, that was engineered to accumulate in the nucleus, stimulated the transcriptional activity of the c-fos enhancer. From experiments using various deletions and point mutations, it was revealed that multiple sequences including GC/Sp1, Ets, and ERE/AP-1 elements found around the -1.3kb region were responsible for the activation by Hic-5. Hic-5 itself did not bind to these elements in a sequence specific manner, but p300 appeared to be involved in the induction of c-fos. These results suggest that Hic-5 participates in the transcriptional regulation of c-fos as a scaffold in transcriptional complexes.

Cellular signaling during the macrophage invasion by Trypanosoma cruzi

We have reported that protein tyrosine kinases play an important role in the invasion of Trypanosoma cruzi into primary resident macrophages. In the present study we carry out immunofluorescence assays, using monoclonal anti-phosphotyrosine antibodies, to reveal an accumulation of tyrosine-phosphorylated residues at the site of parasite association with the macrophage surface, colocalizing with host cell F-actin-rich domains. SDS-PAGE analysis of macrophage cell line IC-21 tyrosine phosphoproteins, labeled with [(35)S] L-methionine, revealed several peptides with increased levels of phosphorylation upon interaction with the parasite. Among them, were detected bands of 140, 120, 112, 94, 73, 67, and 56 kDa that match the molecular weights of proteins described as being tyrosine phosphorylated during events that lead to actin assembly in mononuclear phagocytes. The pretreatment of IC-21 macrophages with the tyrosine kinase inhibitor tyrphostin 23 inhibited trypomastigote uptake showing that tyrosine phosphorylation is important for the parasite penetration in this particular cell line. Immunofluorescence microscopy, using antibodies against p85, the regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase), placed this enzyme also in the same sites, in accordance to what is reported for phagocytosis. We suggest that once the components of T. cruzi trypomastigotes surface are recognized by macrophage receptors, they trigger the activation of a tyrosine phosphorylation cascade, PI 3-kinase recruitment, and assembly of actin filaments at the site of initial cell-to-cell contact, resembling the events described during phagocytosis. These achievements support the model for a phagocytic-like actin-dependent invasion mechanism for T. cruzi trypomastigotes into macrophages.

Localized suppression of RhoA activity by Tyr31/118-phosphorylated paxillin in cell adhesion and migration

RhoA activity is transiently inhibited at the initial phase of integrin engagement, when Cdc42- and/or Rac1-mediated membrane spreading and ruffling predominantly occur. Paxillin, an integrin-assembly protein, has four major tyrosine phosphorylation sites, and the phosphorylation of Tyr31 and Tyr118 correlates with cell adhesion and migration. We found that mutation of Tyr31/118 caused enhanced activation of RhoA and premature formation of stress fibers with substantial loss of efficient membrane spreading and ruffling in adhesion and migration of NMuMG cells. These phenotypes were similar to those induced by RhoA(G14V) in parental cells, and could be abolished by expression of RhoA(T19N), Rac1(G12V), or p190RhoGAP in the mutant-expressing cells. Phosphorylated Tyr31/118 was found to bind to two src homology (SH)2 domains of p120RasGAP, with coprecipitation of endogenous paxillin with p120RasGAP. p190RhoGAP is known to be a major intracellular binding partner for the p120RasGAP SH2 domains. We found that Tyr31/118-phosphorylated paxillin competes with p190RhoGAP for binding to p120RasGAP, and provides evidence that p190RhoGAP freed from p120RasGAP efficiently suppresses RhoA activity during cell adhesion. We conclude that Tyr31/118-phosphorylated paxillin serves as a template for the localized suppression of RhoA activity and is necessary for efficient membrane spreading and ruffling in adhesion and migration of NMuMG cells.

Insulin stimulates spreading of skeletal muscle cells involving the activation of focal adhesion kinase, phosphatidylinositol 3-kinase and extracellular signal regulated kinases

Insulin plays an important role in muscle cell survival and proliferation. However, there is no report showing the role of insulin in spreading of muscle cells. In the present report, we showed that insulin enhances muscle cell spreading concomitant with enhanced tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin. Moreover, insulin can stimulate the cell spreading even in presence of integrin alpha5 blockers although to a lesser extent as compared to control. Cell adhesion was not dependent on insulin and serum, and decreased in presence of integrin blockers. We found direct association of FAK with affinity purified insulin receptors using in vitro kinase assay. The increase in FAK tyrosine phosphorylation was associated with increase in its kinase activity and further supported by increased phosphotyrosine accumulation on focal adhesions and increased membrane localization of FAK after stimulation by insulin. Moreover, insulin-mediated muscle cell spreading was dependent upon phosphatidylinositol 3-kinase (PI 3-kinase) activity. PI 3-kinase activity was found to be associated with FAK and the FAK associated PI 3-kinase activity enhanced when cells were plated in presence of insulin. We also observed activation of MAP kinases, i.e., ERK-1/-2 during insulin mediated muscle cell spreading. In conclusion, FAK, PI 3-kinase, and MAP kinase are important components of pathway(s) that regulate insulin stimulated muscle cell spreading.

Regulation of microtubule-organizing center orientation and actomyosin cytoskeleton rearrangement during immune interactions

The reorganization of membrane, cytoskeletal and signaling molecules during immune interactions is critical for the generation of immune response. At the initiation of the T cell-antigen presenting cell (APC) interaction, antigen-independent weak adhesion forces allow the scanning of the APC surface by the T cell receptor for specific antigens. The stabilization of T cell-APC conjugates involves the segregation of membrane and intracellular signaling proteins, driven by reorganization of membrane microdomains and cytoskeletal changes. In early T cell-APC cognate interactions, the microtubular cytoskeleton undergoes drastic changes that lead to microtubule-organizing center (MTOC) reorientation to the vicinity of the cell-cell contact area. Recent data on the dynamics of MTOC redistribution and its influence in T cell-APC conjugate stabilization, together with the description of an increasing number of signaling molecules associated to this complex, underscore the key role of MTOC translocation in the T cell response. We focus on the mechanisms that control the early MTOC reorientation during T cell-APC interaction and the relevance of this process to T cell activation.

Interaction of POB1, a downstream molecule of small G protein Ral, with PAG2, a paxillin-binding protein, is involved in cell migration

POB1 was previously identified as a RalBP1-binding protein. POB1 and RalBP1 function downstream of small G protein Ral and regulate receptor-mediated endocytosis. To look for additional functions of POB1, we screened for POB1-binding proteins using a yeast two-hybrid method and found that POB1 interacts with mouse ASAP1, which is a human PAG2 homolog. PAG2 is a paxillin-associated protein with ADP-ribosylation factor GTPase-activating protein activity. POB1 formed a complex with PAG2 in intact cells. The carboxyl-terminal region containing the proline-rich motifs of POB1 directly bound to the carboxyl-terminal region including the SH3 domain of PAG2. Substitutions of Pro(423) and Pro(426) with Ala (POB1(PA)) impaired the binding of POB1 to PAG2. Expression of PAG2 inhibited fibronectin-dependent migration and paxillin recruitment to focal contacts of CHO-IR cells. Co-expression with POB1 but not with POB1(PA) suppressed the inhibitory action of PAG2 on cell migration and paxillin localization. These results suggest that POB1 interacts with PAG2 through its proline-rich motif, thereby regulating cell migration.

Secretin regulates paracellular permeability in canine gastric monolayers by a Src kinase-dependent pathway

Previous studies found that epidermal growth factor (EGF) decreased paracellular permeability in gastric mucosa, but the other physiological regulators and the molecular mechanisms mediating these responses remain undefined. We investigated the role of secretin and Src in regulating paracellular permeability because secretin regulates gastric chief cell function and Src mediates events involving the cytoskeletal-membrane interface, respectively. Confluent monolayers were formed from canine gastric epithelial cells in short-term culture on Transwell filter inserts. Resistance was monitored in the presence of secretin with or without specific kinase inhibitors. Tyrosine phosphorylation of Src at Tyr(416) was measured with a site-specific phosphotyrosine antibody. Basolateral, but not apical, secretin at concentrations from 1 to 100 nM dose dependently increased resistance; this response was rapid and sustained over hours. PP2 (10 microM), a selective Src tyrosine kinase inhibitor, but not the inactive isomer PP3, abolished the increase in resistance by secretin but only modestly attenuated apical EGF effects. AG-1478 (100 nM), a specific EGF receptor tyrosine kinase inhibitor, attenuated the resistance increase to EGF but not secretin. Secretin, but not EGF, induced tyrosine phosphorylation of Src at Tyr(416) in a dose-dependent fashion, with the maximal response observed at 1 min. PP2, but not PP3, dramatically inhibited this tyrosine phosphorylation. Secretin increases paracellular resistance in gastric mucosa through a Src-mediated pathway, while the effect of EGF is Src independent. Src appears to mediate the physiological effects of this G(s)-coupled receptor in primary epithelial cells.

Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions

Remodeling of extracellular matrices occurs during development, wound healing, and in a variety of pathological processes including atherosclerosis, ischemic injury, and angiogenesis. Thus, identifying factors that control the balance between matrix deposition and degradation during tissue remodeling is essential for understanding mechanisms that regulate a variety of normal and pathological processes. Using fibronectin-null cells, we found that fibronectin polymerization into the extracellular matrix is required for the deposition of collagen-I and thrombospondin-1 and that the maintenance of extracellular matrix fibronectin fibrils requires the continual polymerization of a fibronectin matrix. Further, integrin ligation alone is not sufficient to maintain extracellular matrix fibronectin in the absence of fibronectin deposition. Our data also demonstrate that the retention of thrombospondin-1 and collagen I into fibrillar structures within the extracellular matrix depends on an intact fibronectin matrix. An intact fibronectin matrix is also critical for maintaining the composition of cell-matrix adhesion sites; in the absence of fibronectin and fibronectin polymerization, neither alpha5beta1 integrin nor tensin localize to fibrillar cell-matrix adhesion sites. These data indicate that fibronectin polymerization is a critical regulator of extracellular matrix organization and stability. The ability of fibronectin polymerization to act as a switch that controls the organization and composition of the extracellular matrix and cell-matrix adhesion sites provides cells with a means of precisely controlling cell-extracellular matrix signaling events that regulate many aspects of cell behavior including cell proliferation, migration, and differentiation.

[Regulation of gene expression by active oxygen species]

Reactive oxygen species (ROS) are generated from cells stimulated by various cytokines, hormones, and stresses, and regulate cellular functions such as gene expression and cell growth. They affect activities of many types of molecular targets, including signaling molecules and transcription factors. Early-response genes (c-fos, egr-I and JE) that encode transcription factors are induced by ROS, and activities of their products are modulated by ROS through redox-based mechanisms. We isolated a novel gene, hic-5, that was induced by hydrogen peroxide and encodes a focal adhesion protein. hic-5 was found to translocate to the nucleus in cells treated with ROS and regulates several cellular genes. We propose that hic-5 is a key element in the transduction of signals from the cell surface to the nucleus under oxidative stress.

Cell interactions with three-dimensional matrices

Signaling and other cellular functions differ in three-dimensional compared with two-dimensional systems. Cell adhesion structures can evolve in vitro towards in-vivo-like adhesions with distinct biological activities. In this review, we examine recent advances in studies of interactions of fibroblasts with collagen gels and fibronectin-containing matrices that mimic in vivo three-dimensional microenvironments. These three-dimensional systems are illuminating mechanisms of cell-matrix interactions in living organisms.

Protein phosphatase-2A modulates the serine and tyrosine phosphorylation of paxillin in Lewis lung carcinoma tumor variants

Cellular adhesion and motility, processes regulated by focal adhesion assembly and disassembly, can influence a tumor cell's ability to metastasize. Focal adhesion dynamics are, in turn, influenced by the serine and tyrosine phosphorylation state of paxillin. Using Lewis lung carcinoma (LLC) tumor variants, this study shows the importance of the serine/threonine protein phosphatase-2A (PP-2A) in maintaining adherence and restricting tumor cell motility, and modulating the serine and tyrosine phosphorylation of paxillin. Treating non-metastatic LLC-C8 tumor variants with okadaic acid to inhibit PP-2A activity resulted in cell rounding and increased motility. These effects on motility and adherence were accompanied by increased serine and decreased tyrosine phosphorylation of paxillin. These results suggest PP-2A regulation of paxillin phosphorylation may have a role in controlling tumor cell adherence and motility.

Calcium mobilization evoked by hepatocellular swelling is linked to activation of phospholipase Cgamma

Recovery from swelling of hepatocytes and selected other epithelia is triggered by intracellular Ca(2+) release from the endoplasmic reticulum, which leads to fluid and electrolyte efflux through volume-sensitive K(+) and Cl(-) channels. The aim of this study was to determine the mechanisms responsible for swelling-mediated hepatocellular Ca(2+) mobilization. Swelling of HTC rat hepatoma cells, evoked by exposure to hypotonic medium, elicited transient increases in intracellular levels of inositol 1,4,5-trisphosphate (IP(3)) and cytosolic [Ca(2+)]. The latter was attenuated by inhibition of phospholipase C (PLC) with and by IP(3) receptor blockade with 2-aminoethoxydiphenyl borate, but it was unaffected by ryanodine, an inhibitor of intracellular Ca(2+)-induced Ca(2+) release channels. Hypotonic swelling was associated with a transient increase in tyrosine phosphorylation of PLCgamma, with kinetics that paralleled the increases in intracellular IP(3) levels and cytosolic [Ca(2+)]. Confocal imaging of HTC cells exposed to hypotonic medium revealed a swelling-induced association of tyrosine-phosphorylated PLCgamma with the plasma membrane. These findings suggest that activation of PLCgamma by hepatocellular swelling leads to the generation of IP(3) and stimulates discharge of Ca(2+) from the endoplasmic reticulum via activation of IP(3) receptors. By extension, these data support the concept that tyrosine phosphorylation of PLCgamma represents a critical step in adaptive responses to hepatocellular swelling.

alpha4beta1 integrin regulates lamellipodia protrusion via a focal complex/focal adhesion-independent mechanism

alpha4beta1 integrin plays an important role in cell migration. We show that when ectopically expressed in Chinese hamster ovary cells, alpha4beta1 is sufficient and required for promoting protrusion of broad lamellipodia in response to scratch-wounding, whereas alpha5beta1 does not have this effect. By time-lapse microscopy of cells expressing an alpha4/green fluorescent protein fusion protein, we show that alpha4beta1 forms transient puncta at the leading edge of cells that begin to protrude lamellipodia in response to scratch-wounding. The cells expressing a mutant alpha4/green fluorescent protein that binds paxillin at a reduced level had a faster response to scratch-wounding, forming alpha4-positive puncta and protruding lamellipodia much earlier. While enhancing lamellipodia protrusion, this mutation reduces random motility of the cells in Transwell assays, indicating that lamellipodia protrusion and random motility are distinct types of motile activities that are differentially regulated by interactions between alpha4beta1 and paxillin. Finally, we show that, at the leading edge, alpha4-positive puncta and paxillin-positive focal complexes/adhesions do not colocalize, but alpha4beta1 and paxillin colocalize partially in ruffles. These findings provide evidence for a specific role of alpha4beta1 in lamellipodia protrusion that is distinct from the motility-promoting functions of alpha5beta1 and other integrins that mediate cell adhesion and signaling events through focal complexes and focal adhesions.

Protein phosphatase-2A regulates endothelial cell motility and both the phosphorylation and the stability of focal adhesion complexes

Solid cancers must stimulate expansion of the vascular network for continued growth. The process of angiogenesis involves endothelial cell migration so as to reorganize into vessel structures. The extent of cellular motility is regulated in part by the balance between serine/threonine kinases and protein phosphatases. In the present study, we show a decline in the activity of the serine/threonine phosphatase PP-2A in endothelial cells whose motility is stimulated by exposure to medium conditioned by either murine LLC cells or human HNSCC cells. Inhibition of endothelial cell PP-2A pharmacologically by treatment with okadaic acid also stimulated endothelial cell motility. Identification of mechanisms by which PP-2A inhibition might stimulate endothelial cell motility focused on proteins of the focal adhesions. Inhibition of PP-2A caused hyperphosphorylation of the paxillin serine residues and dephosphorylation of its tyrosine residues, dissolution of FAK/Src/paxillin complexes and decreased phosphorylation of the inhibitory Y529 residue of Src, suggesting increased Src activity. Inhibition of Src activity prevented the stimulation of PP-2A-inhibited cell motility. Our results suggest an interrelationship between tumor inhibition of PP-2A, dissolution of focal adhesion complexes and stimulated motility of endothelial cells.

Mitogen-induced, FAK-dependent tyrosine phosphorylation of the SSeCKS scaffolding protein

The ability of mitogens to rapidly induce tyrosine phosphorylation of cellular proteins has been taken as evidence of participation in subsequent signaling pathways. SSeCKS, a major protein kinase C (PKC) substrate with protein scaffolding and tumor suppressive properties, becomes tyrosine phosphorylated in NIH3T3 and rodent embryo fibroblasts after short-term treatment with epidermal growth factor (EGF), platelet-derived growth factor (PDGF), or fetal calf serum in the presence of pervanadate, but not by treatment with insulin or insulin-like growth factor-1. The relative phosphotyrosine level on SSeCKS was higher in actively dividing cells than in confluent cultures. Tyrosine phosphorylation of SSeCKS was apparent in cells deficient in Src, Fyn, Yes, or Abl tyrosine kinases or in NIH3T3 cells expressing a temperature-sensitive v-Src allele, but not in FAK-deficient embryo fibroblasts. Purified FAK or Src enzyme failed to directly phosphorylate SSeCKS in vitro. EGF failed to induce SSeCKS tyrosine phosphorylation in FAK-/- fibroblasts, indicating that the EGF receptor is probably not the direct kinase of SSeCKS. Phosphorylation under these conditions was rescued by the transient reexpression of wt-FAK but not FAK mutated at Y397, a major autophosphorylation and SH2-based docking site. Adhesion of FAK+/+ cells to fibronectin failed to significantly induce SSeCKS tyrosine phosphorylation although FAK was activated, suggesting that SSeCKS phosphorylation is mediated through a growth factor receptor-FAK rather than an integrin-FAK pathway. Moreover, PDGF could induce SSeCKS tyrosine phosphorylation in the absence of FAK activation, suggesting a role for FAK SH2-based docking rather than kinase activity. Immunofluorescence analysis showed that in FAK-/- cells, SSeCKS costains along F-actin stress fibers, in contrast to FAK+/+ cells, where most SSeCKS stains at the cell edge and along a cortical cytoskeletal matrix. This correlated with increased coprecipitation of SSeCKS with biotin-phalloidin-bound F-actin from FAK-/- compared to FAK+/+ cell lysates. Similarly, bacterially expressed, unphosphorylated SSeCKS cosedimented with F-actin in ultracentrifugation assays. These data suggest that mitogen-induced, FAK-dependent tyrosine phosphorylation of SSeCKS modulates its binding to the actin-based cytoskeleton, suggesting a role for SSeCKS in mitogen-induced cytoskeletal reorganization.

The focal adhesion protein paxillin regulates contraction in canine tracheal smooth muscle

The adapter protein paxillin localizes to the focal adhesions of adherent cells and has been implicated in the regulation of cytoskeletal organization and cell motility. Paxillin undergoes tyrosine phosphorylation in response to the contractile stimulation of tracheal smooth muscle. We therefore hypothesized that paxillin may be involved in regulating smooth muscle contraction. Tracheal smooth muscle strips were treated with paxillin antisense oligonucleotides to inhibit the expression of paxillin protein selectively. Paxillin antisense or sense was introduced into muscle strips by reversible permeabilization and strips were incubated with antisense or sense for 3 days. Paxillin antisense selectively depressed paxillin expression, but it did not affect the expression of vinculin, focal adhesion kinase, myosin light chain kinase, myosin heavy chain or myosin light chain. Tension development in response to stimulation with ACh or KCl was markedly depressed in paxillin-depleted muscle strips. Active force and paxillin protein expression were restored by incubation of antisense-treated strips in the absence of oligonucleotides. The depletion of paxillin did not inhibit the increase in intracellular free Ca2+, myosin light chain phosphorylation or myosin ATPase activity in response to contractile stimulation. The concentration of G-actin was significantly lower in unstimulated paxillin-depleted smooth muscle tissues than in normal tissues. While stimulation with acetylcholine caused a decrease in G-actin in normal muscle strips, it caused little change in the G-actin concentration in paxillin-depleted muscle strips, suggesting that paxillin is necessary for normal actin dynamics in smooth muscle. We conclude that paxillin is required for active tension development in smooth muscle, but that it does not regulate increases in intracellular Ca2+, myosin light chain phosphorylation or myosin ATPase activity during contractile stimulation. Paxillin may be important in regulating actin filament dynamics and organization during smooth muscle contraction.

Roles for the tubulin- and PTP-PEST-binding paxillin LIM domains in cell adhesion and motility

Cell dynamics mediated through cell-extracellular matrix contacts, such as adhesion and motility involve the precise regulation of large complexes of structural and signaling molecules called focal adhesions (FAs). Paxillin is a multi-domain FA adaptor protein containing five amino-terminal paxillin leucine-aspartate repeat (LD) motifs and four carboxyl-terminal Lin-11 Isl-1 and Mec-3 (LIM) domains. The LD motifs support paxillin binding to actopaxin, integrin linked kinase (ILK), FA kinase (FAK), paxillin kinase linker (PKL) and vinculin. Of the LIM domains, LIM2 and 3 comprise the paxillin FA-targeting motif, with phosphorylation of these domains modulating paxillin targeting and cell adhesion to fibronectin (Fn). The identity of the paxillin FA targeting partner remains to be determined; however, the LIM domains mediate interactions with tubulin and the protein-tyrosine phosphatase (PTP)-PEST. PTP-PEST binding requires both LIM3 and 4, whereas, the precise LIM target of tubulin binding is not known. In this report, we demonstrate that the individual paxillin LIM2 and 3 domains support specific binding to tubulin and suggest a potential role for this interaction in the regulation of paxillin sub-cellular compartmentalization. In addition, expression of paxillin molecules with mutations in the tubulin- and PTP-PEST-binding LIM domains differentially impaired Chinese hamster ovary K1 (CHO.K1) cell adhesion and migration to Fn. Perturbation of LIM3 or 4 inhibited adhesion while mutation of LIM2 or 4 decreased cell motility. Interestingly, expression of tandem LIM2-3 inhibited cell adhesion and spreading while LIM3-4 stimulated a well-spread polarized phenotype. These data offer further support for a critical role for paxillin in cell adhesion and motility.

A fragment of paxillin binds the alpha 4 integrin cytoplasmic domain (tail) and selectively inhibits alpha 4-mediated cell migration

The alpha(4) integrins play important roles in embryogenesis, hematopoiesis, cardiac development, and the immune responses. The alpha(4) integrin subunit is indispensable for these biological processes, possibly because the alpha(4) subunit regulates cellular functions differently from other integrin alpha subunits. We have previously reported that the alpha(4) cytoplasmic domain directly and tightly binds paxillin, an intracellular signaling adaptor molecule, and this interaction accounts for some of the unusual functional responses to alpha(4) integrin-mediated cell adhesion. We also have identified a conserved 9-amino acid region (Glu(983)-Tyr(991)) in the alpha(4) cytoplasmic domain that is sufficient for paxillin binding, and an alanine substitution at either Glu(983) or Tyr(991) within this region disrupted the alpha(4)-paxillin interaction and reversed the effects of the alpha(4) cytoplasmic domain on cell spreading and migration. In the current study, we have mapped the alpha(4)-binding site within paxillin using mutational analysis, and examined its effects on the alpha(4) tail-mediated functional responses. Here we report that sequences between residues Ala(176) and Asp(275) of paxillin are sufficient for binding to the alpha(4) tail. We found that the alpha(4) tail, paxillin, and FAT, the focal adhesion targeting domain of pp125(FAK), could form a ternary complex and that the alpha(4)-binding paxillin fragment, P(Ala(176)-Asp(275)), specifically blocked paxillin binding to the alpha(4) tail more efficiently than it blocked binding to FAT. Furthermore, when expressed in cells, this alpha(4)-binding paxillin fragment specifically inhibited the alpha(4) tail-stimulated cell migration. Thus, paxillin binding to the alpha(4) tail leads to enhanced cell migration and inhibition of the alpha(4)-paxillin interaction selectively blocks the alpha4-dependent cellular responses.

Molecular and immunohistochemical analysis of signaling adaptor protein Crk in human cancers

Crk is a signaling adaptor protein which is mostly composed of SH2 and SH3 domains, and has been shown to play a pivotal role in cell proliferation, differentiation, and migration. Because Crk was originally isolated as an avian sarcoma virus CT10 encoding oncoprotein v-Crk, we examined a potential role for c-Crk in the carcinogenesis of human cancers. First, to analyze gene mutations of c-Crk, we isolated a human bacterial artificial chromosome clone containing Crk genome and exon/intron structures. However, polymerase chain reaction-single strand conformation polymorphism methods failed to show any genomic mutations in the Crk exon which could be related to carcinogenesis. Second, immunohistochemical analysis of c-Crk-II demonstrated that the levels of c-Crk-II were significantly elevated in most of the tumors, particularly in the colon and lung cancers. Furthermore, immunoblot analysis using human lung cancer cell lines revealed that the expression levels of c-Crk-II were correlated to growth rates of cells. The elevated expression levels of c-Crk-II might be related to the development of human cancers.

Ligation of HLA class I molecules on endothelial cells induces phosphorylation of Src, paxillin, and focal adhesion kinase in an actin-dependent manner

The development of chronic rejection is the major limitation to long-term allograft survival. HLA class I Ags have been implicated to play a role in this process because ligation of class I molecules by anti-HLA Abs stimulates smooth muscle cell and endothelial cell proliferation. In this study, we show that ligation of HLA class I molecules on the surface of human aortic endothelial cells stimulates phosphorylation of Src, focal adhesion kinase, and paxillin. Signaling through class I stimulated Src phosphorylation and mediated fibroblast growth factor receptor (FGFR) translocation to the nucleus. In contrast, Src kinase activity was not involved in class I-mediated transfer of FGFR from cytoplasmic stores to the cell surface. Inhibition of Src protein kinase activity blocked HLA class I-stimulated tyrosine phosphorylation of paxillin and focal adhesion kinase. Furthermore, HLA class I-mediated phosphorylation of the focal adhesion proteins and FGFR expression was inhibited by cytochalasin D and latrunculin A, suggesting a role for the actin cytoskeleton in the signaling process. These findings indicate that anti-HLA Abs have the capacity to transduce activation signals in endothelial cells that may promote the development of chronic rejection.

Lymphocyte binding to MAdCAM-1 via alpha4beta7 integrin activates a signal transduction pathway involving tyrosine phosphorylation of paxillin and p105(Cas-L)

Alpha4beta7 integrin mediates lymphocyte trafficking to mucosal lymphoid organs by interacting with the mucosal vascular addressin MAdCAM-1. While the structural basis for the alpha4beta7 integrin-MAdCAM-1 interaction has been well characterized, less is known about the signal transduction pathways that regulate the alpha4beta7 integrin-mediated lymphocyte interaction with MAdCAM-1-expressing endothelial cells. Here we demonstrate that ligation of alpha4beta7 integrin with MAdCAM-1 induces a prominent tyrosine phosphorylation of paxillin and a 105-kDa protein (p105) that is reactive with an anti-p130(Cas) antibody, in the mouse T-cell line TK-1. Cloning and expression of a full-length cDNA encoding the mouse p105(Cas-L) revealed that the p105 molecule is a mouse ortholog of p105(Cas-L). We also demonstrated that crosslinking of alpha4beta7 integrin with MAdCAM-1 induces the rapid tyrosine phosphorylation of paxillin and p105(Cas-L) in normal lymphocytes and that PMA stimulation enhances the tyrosine phosphorylation of p105(Cas-L) but not of paxillin. These results suggest that intracellular signals initiated by alpha4beta7 integrin involve the tyrosine phosphorylation of paxillin and p105(Cas-L), which are differentially regulated, at least in part, by mechanisms that are PMA-sensitive or -insensitive.

Paxillin-dependent paxillin kinase linker and p21-activated kinase localization to focal adhesions involves a multistep activation pathway

The precise temporal-spatial regulation of the p21-activated serine-threonine kinase PAK at the plasma membrane is required for proper cytoskeletal reorganization and cell motility. However, the mechanism by which PAK localizes to focal adhesions has not yet been elucidated. Indirect binding of PAK to the focal adhesion protein paxillin via the Arf-GAP protein paxillin kinase linker (PKL) and PIX/Cool suggested a mechanism. In this report, we demonstrate an essential role for a paxillin-PKL interaction in the recruitment of activated PAK to focal adhesions. Similar to PAK, expression of activated Cdc42 and Rac1, but not RhoA, stimulated the translocation of PKL from a generally diffuse localization to focal adhesions. Expression of the PAK regulatory domain (PAK1-329) or the autoinhibitory domain (AID 83-149) induced PKL, PIX, and PAK localization to focal adhesions, indicating a role for PAK scaffold activation. We show PIX, but not NCK, binding to PAK is necessary for efficient focal adhesion localization of PAK and PKL, consistent with a PAK-PIX-PKL linkage. Although PAK activation is required, it is not sufficient for localization. The PKL amino terminus, containing the PIX-binding site, but lacking paxillin-binding subdomain 2 (PBS2), was unable to localize to focal adhesions and also abrogated PAK localization. An identical result was obtained after PKLDeltaPBS2 expression. Finally, neither PAK nor PKL was capable of localizing to focal adhesions in cells overexpressing paxillinDeltaLD4, confirming a requirement for this motif in recruitment of the PAK-PIX-PKL complex to focal adhesions. These results suggest a GTP-Cdc42/GTP-Rac triggered multistep activation cascade leading to the stimulation of the adaptor function of PAK, which through interaction with PIX provokes a functional PKL PBS2-paxillin LD4 association and consequent recruitment to focal adhesions. This mechanism is probably critical for the correct subcellular positioning of PAK, thereby influencing the ability of PAK to coordinate cytoskeletal reorganization associated with changes in cell shape and motility.

Different phenotypes in human prostate cancer: alpha6 or alpha3 integrin in cell-extracellular adhesion sites

The distribution of alpha6/alpha3 integrin in adhesion complexes at the basal membrane in human normal and cancer prostate glands was analyzed in 135 biopsies from 61 patients. The levels of the polarized alpha6/alpha3 integrin expression at the basal membrane of prostate tumor glands were determined by quantitative immunohistochemistry. The alpha6/alpha3 integrin expression was compared with Gleason sum score, pathological stage, and preoperative serum prostate-specific antigen (PSA). The associations were assessed by statistical methods. Eighty percent of the tumors expressed the alpha6 or alpha3 integrin and 20% was integrin-negative. Gleason sum score, but not serum PSA, was associated with the integrin expression. Low Gleason sum score correlated with increased integrin expression, high Gleason sum score with low and negative integrin expression. Three prostate tumor phenotypes were distinguished based on differential integrin expression. Type I coexpressed both alpha6 and alpha3 subunits, type II exclusively expressed alpha6 integrin, and type III expressed alpha3 integrin only. Fifteen cases were further examined for the codistribution of vinculin, paxillin, and CD 151 on frozen serial sections using confocal laser scanning microscopy. The alpha6/alpha3 integrins, CD151, paxillin, and vinculin were present within normal glands. In prostate carcinoma, alpha6 integrin was colocalized with CD 151, but not with vinculin or paxillin. In tumor phenotype I, the alpha6 subunit did not colocalize with the alpha subunit indicating the existence of two different adhesion complexes. Human prostate tumors display on their cell surface the alpha6beta1 and/or alpha3beta1 integrins. Three tumor phenotypes associated with two different adhesion complexes were identified, suggesting a reorganization of cell adhesion structures in prostate cancer.

Involvement of integrins in fimbriae-mediated binding and invasion by Porphyromonas gingivalis

Interaction between the major fimbriae of Porphyromonas gingivalis and gingival epithelial cells is important for bacterial adhesion and invasion. In this study, we identified integrins as an epithelial cell cognate receptor for P. gingivalis fimbriae. Immunoprecipitation and direct binding assays revealed a physical association between recombinant fimbrillin and beta1 integrins. In vitro adhesion and invasion assays demonstrated inhibition of binding and invasion of P. gingivalis by beta1 integrin antibodies. In contrast, invasion of a fimbriae-deficient mutant of P. gingivalis was not affected by integrin antibodies. Infection of gingival epithelial cells with wild-type P. gingivalis induced tyrosine phosphorylation of the 68 kDa focal adhesion protein paxillin, whereas the fimbriae-deficient mutant failed to evoke similar changes. Interestingly, activation of paxillin was not accompanied by an increase in the phosphorylation of focal adhesion kinase (FAK). These results provide evidence that P. gingivalis fimbriae promote adhesion to gingival epithelial cells through interaction with beta1 integrins, and this association represents a key step in the induction of the invasive process and subsequent cell responses to P. gingivalis infection.

Linkage of caspase-mediated degradation of paxillin to apoptosis in Ba/F3 murine pro-B lymphocytes

We have cloned the complete cDNA from mouse paxillin, a 68-kDa adapter protein found in focal adhesions. We found that paxillin was degraded by caspases in Ba/F3 cell apoptosis induced by withdrawal of interleukin-3 (IL-3), a survival factor for this cell, and by ionizing radiation. Also, paxillin was degraded in vitro by incubation with recombinant caspase-3. Western blot analyses of degradation products of overexpressed green fluorescence protein-tagged paxillin and site-specific mutants demonstrated that Asp-102 and Asp-301 were early caspase cleavage sites, and Asp-5, Asp-146, Asp-165, and Asp-222 were late cleavage sites. Overexpression of paxillin delayed apoptosis of Ba/F3 after IL-3 withdrawal. Furthermore, this anti-apoptotic effect of paxillin was augmented by a triple mutation in aspartic acids at caspase cleavage sites. These results suggest that paxillin plays a critical role in cell survival signaling and that the cleavage of paxillin by caspases might be an important event for focal adhesion disassembly during cell apoptosis, contributing to detachment, rounding, and death.

Approaches to studying cellular signaling: a primer for morphologists

Many research projects will lead to understanding tissue and/or cell responses to extracellular influences either from soluble factors or the surrounding extracellular matrix. These types of investigations will require the understanding of signal transduction. This particular cell biological field has literally exploded with information and new technical approaches in the past 10 years. This article is directed toward investigators interested in using these new approaches to study their systems. An overview of the general principles of signal transduction events including the types of receptors and intracellular signaling events is followed by an introduction to methods for visualizing signal transduction. This is followed by an introduction to biochemical analysis and an example of combining several approaches to understanding a tissue response to extracellular matrix stimulus.

STAT3 deficiency in keratinocytes leads to compromised cell migration through hyperphosphorylation of p130(cas)

We previously reported that STAT3 plays a crucial role in transducing a signal for migration of keratinocytes (Sano, S., Itami, S., Takeda, K., Tarutani, M., Yamaguchi, Y., Miura, H., Yoshikawa, K., Akira, S., and Takeda, J. (1999) EMBO J. 18, 4657-4668). To clarify the role of STAT3 in signaling the migration, we studied the intracellular signaling pathway through an integrin receptor in STAT3-deficient keratinocytes. STAT3-deficient keratinocytes demonstrated increased adhesiveness and fast spreading on a collagen matrix. Staining with anti-phosphotyrosine antibody revealed that STAT3-deficient keratinocytes had an increased number of tyrosyl-hyperphosphorylated focal adhesions. Analyses with immunoprecipitation revealed that p130(cas) was constitutively hyperphosphorylated on tyrosine residues, while other focal adhesion molecules such as focal adhesion kinase and paxillin were not. Transfection of STAT3-deficient keratinocytes with an adenoviral vector encoding the wild-type Stat3 gene reversed not only impaired migration but also the increased tyrosine phosphorylation of p130(cas). These results strongly suggest that STAT3 in keratinocytes plays a critical role in turnover of tyrosine phosphorylation of p130(cas), modulating cell adhesiveness to the substratum leading to growth factor-dependent cell migration.

Syndesmos, a syndecan-4 cytoplasmic domain interactor, binds to the focal adhesion adaptor proteins paxillin and Hic-5

Syndecan-4 and integrins are the primary transmembrane receptors of focal adhesions in cells adherent to extracellular matrix molecules. Syndesmos is a cytoplasmic protein that interacts specifically with the cytoplasmic domain of syndecan-4, and it co-localizes with syndecan-4 in focal contacts. In the present study we sought possible interactors with syndesmos. We find that syndesmos interacts with the focal adhesion adaptor protein paxillin. The binding of syndesmos to paxillin is direct, and these interactions are triggered by the activation of protein kinase C. Syndesmos also binds the paxillin homolog, Hic-5. The connection of syndecan-4 with paxillin through syndesmos parallels the connection between paxillin and integrins and may thus reflect the cooperative signaling of these two receptors in the assembly of focal adhesions and actin stress fibers.

Inhibition of anchorage-independent growth of transformed NIH3T3 cells by epithelial protein lost in neoplasm (EPLIN) requires localization of EPLIN to actin cytoskeleton

Epithelial protein lost in neoplasm (EPLIN) is a cytoskeleton-associated protein characterized by the presence of a single centrally located lin-11, isl-1, and mec-3 (LIM) domain. We have reported previously that EPLIN is down-regulated in transformed cells. In this study, we have investigated whether ectopic expression of EPLIN affects transformation. In untransformed NIH3T3 cells, retroviral-mediated transduction of EPLIN did not alter the cell morphology or growth. NIH3T3 cells expressing EPLIN, however, failed to form colonies when transformed by the activated Cdc42 or the chimeric nuclear oncogene EWS/Fli-1. This suppression of anchorage-independent growth was not universal because EPLIN failed to inhibit the colony formation of Ras-transformed cells. Interestingly, the localization of EPLIN to the actin cytoskeleton was maintained in the EWS/Fli-1- or Cdc42-transformed cells, but not in Ras-transformed cells where it was distributed heterogeneously in the cytoplasm. Using truncated EPLIN constructs, we demonstrated that the NH(2)-terminal region of EPLIN is necessary for both the localization of EPLIN to the actin cytoskeleton and suppression of anchorage-independent growth of EWS/Fli-1-transformed cells. The LIM domain or the COOH-terminal region of EPLIN could be deleted without affecting its cytoskeletal localization or ability to suppress anchorage-dependent growth. Our study indicates EPLIN may function in growth control by associating with and regulating the actin cytoskeleton.

SRC catalytic but not scaffolding function is needed for integrin-regulated tyrosine phosphorylation, cell migration, and cell spreading

Src family kinases (SFKs) are crucial for signaling through a variety of cell surface receptors, including integrins. There is evidence that integrin activation induces focal adhesion kinase (FAK) autophosphorylation at Y397 and that Src binds to and is activated by FAK to carry out subsequent phosphorylation events. However, it has also been suggested that Src functions as a scaffolding molecule through its SH2 and SH3 domains and that its kinase activity is not necessary. To examine the role of SFKs in integrin signaling, we have expressed various Src molecules in fibroblasts lacking other SFKs. In cells plated on fibronectin, FAK could indeed autophosphorylate at Y397 independently of Src but with lower efficiency than when Src was present. This step was promoted by kinase-inactive Src, but Src kinase activity was required for full rescue. Src kinase activity was also required for phosphorylation of additional sites on FAK and for other integrin-directed functions, including cell migration and spreading on fibronectin. In contrast, Src mutations in the SH2 or SH3 domain greatly reduced binding to FAK, Cas, and paxillin but had little effect on tyrosine phosphorylation or biological assays. Furthermore, our indirect evidence indicates that Src kinase activity does not need to be regulated to promote cell migration and FAK phosphorylation. Although Src clearly plays important roles in integrin signaling, it was not concentrated in focal adhesions. These results indicate that the primary role of Src in integrin signaling is as a kinase. Indirect models for Src function are proposed.

Epidermal growth factor stimulates serine/threonine phosphorylation of the focal adhesion protein paxillin in a MEK-dependent manner in normal rat kidney cells

Epidermal growth factor (EGF)-stimulated proliferation of renal epithelial cells plays an important role in the recovery of kidney tubule epithelia following exposure to insult. Numerous studies have demonstrated that tyrosine phosphorylation of the focal adhesion protein paxillin mediates in part the effects of growth factors on cell growth, migration, and organization of the actin-based cytoskeleton. The experiments in this report were designed to determine the effect of EGF on paxillin phosphorylation in normal rat kidney (NRK) epithelial cells. Interestingly, treatment of NRK cells with EGF stimulated paxillin serine/threonine phosphorylation, which caused a reduction in the mobility of paxillin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The EGF-stimulated mobility shift of paxillin was independent of an intact cytoskeleton, phosphatidylinositol 3-kinase (PI 3-kinase) activation, protein kinase C (PKC) activation, and cellular adhesion. However, inhibitors of the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase abrogated the EGF-stimulated change in paxillin mobility. In addition, the EGF-stimulated change in paxillin serine/threonine phosphorylation was not accompanied by a profound reorganization of the actin cytoskeleton. These results identify paxillin as a component EGF signaling in renal epithelial cells and implicate members of the MAP kinase pathway as critical regulators of paxillin serine/threonine phosphorylation.

Further characterization of the interaction between the cytoskeletal proteins talin and vinculin

The cytoskeletal protein talin, which is thought to couple integrins to F-actin, contains three binding sites (VBS1-VBS3) for vinculin, a protein implicated in the negative regulation of cell motility and whose activity is modulated by an intramolecular interaction between the vinculin head (Vh) and vinculin tail (Vt) domains. In the present study we show that recombinant talin polypeptides containing the three VBSs (VBS1, residues 498-636; VBS2, residues 727-965; and VBS3, residues 1943-2157) each bind tightly to the same or overlapping sites within vinculin(1-258). A short synthetic talin VBS3 peptide (residues 1944-1969) was sufficient to inhibit binding of a (125)I-labelled talin VBS3 polypeptide to vinculin(1-258), and NMR spectroscopy confirmed that this peptide forms a 1:1 complex in slow exchange with vinculin(1-258). Binding of the (125)I-labelled VBS3 polypeptide was markedly temperature dependent, but was not inhibited by 1 M salt or 10% (v/v) 2-methyl-2-propanol. Attempts to further define the talin-binding site within vinculin(1-258) using a gel-blot assay were unsuccessful, but near maximal talin-binding activity was retained by a construct spanning vinculin residues 1-131 in a yeast two-hybrid assay. Interestingly, the talin VBS3 polypeptide was a potent inhibitor of the Vh-Vt interaction, and the VBS3 synthetic peptide was able to expose the actin-binding site in intact vinculin, which is otherwise masked by the Vh-Vt interaction. The results suggest that under certain conditions, talin may be an effective activator of vinculin.

The paxillin LD motifs

Adapter/scaffold proteins, through their multidomain structure, perform a fundamental role in facilitating signal transduction within cells. Paxillin is a focal adhesion adapter protein implicated in growth factor- as well as integrin-mediated signaling pathways. The amino-terminus of paxillin contains five leucine-rich sequences termed LD motifs. These paxillin LD motifs are highly conserved between species as well as within the paxillin superfamily. They mediate interactions with several structural and regulatory proteins important for coordinating changes in the actin cytoskeleton associated with cell motility and cell adhesion as well as in the regulation of gene expression.

Possible involvement of hic-5, a focal adhesion protein, in the differentiation of C2C12 myoblasts

Hic-5, a focal adhesion protein, has been implicated in cellular senescence and differentiation. In this study, we examined its involvement in myogenic differentiation. The hic-5 expression level in growing C2C12 myoblasts increased slightly on the first day and then gradually decreased until no hic-5 was detectable after 7 days of differentiation. In vivo, its expression level declined in the thigh and the calf skeletal muscle of mouse embryos after birth. The introduction of an antisense expression vector of hic-5 into C2C12 cells decreased the number of clones expressing the myosin heavy chain (MHC) upon exposure to the differentiation medium. In the cloned cells with low levels of hic-5, the efficiency of myotube formation was significantly reduced. The expression levels of MyoD, myogenin, MHC and p21 were also reduced in these clones. The results suggested that hic-5 plays a role in the initial stage of myogenic differentiation.

Molecular dissection of actopaxin-integrin-linked kinase-Paxillin interactions and their role in subcellular localization

Paxillin is a focal adhesion adapter protein involved in integrin signaling. We have recently reported that the paxillin LD1 motif acts as a binding interface for both the actin-binding protein actopaxin and the serine/threonine integrin-linked kinase (ILK). In this report we demonstrate the direct association between actopaxin and ILK and dissect the role of the respective interactions in their subcellular localization. Co-immunoprecipitation experiments were employed to map the binding sites on ILK and actopaxin. ILK binds to the CH2 domain of actopaxin. However, an actopaxin CH2 domain mutant defective for paxillin binding (paxillin binding subdomain mutant) retains the capacity to bind ILK, indicating that paxillin and ILK binding sites on actopaxin are distinct. Actopaxin binds to the C terminus of ILK. Despite the direct binding between actopaxin and ILK, mutation analysis confirmed a primary role for paxillin in their localization to focal adhesions. Interestingly, an ILK mutant (E359K) that was previously reported to act as dominant negative for ILK function was unable to bind actopaxin or paxillin and failed to localize to focal adhesions. This mutant also exhibited in vitro kinase activity comparable with wild-type ILK. Taken together, these data suggest that normal ILK signaling is dependent on efficient localization involving multiple protein interactions.

Paxillin null embryonic stem cells are impaired in cell spreading and tyrosine phosphorylation of focal adhesion kinase

Paxillin is a focal-adhesion associated protein implicated in the regulation of integrin signaling and organization of the actin cytoskeleton. Paxillin associates with numerous signaling molecules including adaptor molecules (p130Cas, CRK), kinases (FAK, Pyk2, PAK and SRC), tyrosine phosphatases (PTP-PEST), ARF-GAP proteins (p95pkl, PAG3) and papillomavirus E6 oncoproteins. Although paxillin is tyrosine phosphorylated in cellular processes such as cell attachment and spreading, little direct evidence is available about paxillin's role in these events. Targeted gene disruption was used to generate paxillin null mouse embryonic stem (ES) cells and paxillin null differentiated cells. Paxillin null ES cells exhibit delayed spreading on integrin binding substrates fibronectin and laminin, and there is reduced tyrosine phosphorylation of Focal Adhesion Kinase (FAK). Both of these phenotypes are recovered in paxillin knockout cells upon exogenous re-expression of paxillin. The individual LD motifs of paxillin that are binding sites for FAK, vinculin and ARF-GAP proteins, as well as tyrosine residues that when phosphorylated create binding sites for CRK family members, are dispensable for FAK phosphorylation and early cell spreading. These results demonstrate that paxillin contributes to attachment-dependent tyrosine phosphorylation of FAK and early cell spreading in ES cells.

The Promise of Integrins as Effective Targets for Anticancer Agents

This review will briefly describe integrin function, address why integrins are attractive targets for chemotherapeutic drug design, and discuss some ongoing studies aimed at inhibiting integrin activity. Integrins are cell surface heterodimeric receptors. They modulate many cellular processes including: growth, death (apoptosis), adhesion, migration, and invasion by activating several signaling pathways. Many potential chemotherapeutic agents target integrins directly (eg, polypeptides, monoclonal antibodies, adenovirus vectors). These agents may be clinically useful in controlling the metastatic spread of cancer.

EDG-1 links the PDGF receptor to Src and focal adhesion kinase activation leading to lamellipodia formation and cell migration

Sphingosine-1-phosphate (SPP), formed by sphingosine kinase, is the ligand for EDG-1, a GPCR important for cell migration and vascular maturation. Here we show that cytoskeletal rearrangements, lamellipodia extensions, and cell motility induced by platelet-derived growth factor (PDGF) are abrogated in EDG-1 null fibroblasts. However, EDG-1 appears to be dispensable for mitogenicity and survival effects, even those induced by its ligand SPP and by PDGF. Furthermore, PDGF induced focal adhesion formation and activation of FAK, Src, and stress-activated protein kinase 2, p38, were dysregulated in the absence of EDG-1. In contrast, tyrosine phosphorylation of the PDGFR and activation of extracellular signal regulated kinase (ERK1/2), important for growth and survival, were unaltered. Our results suggest that EDG-1 functions as an integrator linking the PDGFR to lamellipodia extension and cell migration. PDGF, which stimulates sphingosine kinase, leading to increased SPP levels in many cell types, also induces translocation of sphingosine kinase to membrane ruffles. Hence, recruitment of sphingosine kinase to the cell's leading edge and localized formation of SPP may spatially and temporally stimulate EDG-1, resulting in activation and integration of downstream signals important for directional movement toward chemoattractants, such as PDGF. These results may also shed light on the vital role of EDG-1 in vascular maturation.