Roles played by a subset of integrin signaling molecules in cadherin-based cell-cell adhesion

Integrins can intercommunicate with cadherins. Here, we examined their possible relationship by use of small interfering RNA–mediated protein knockdown in HeLa cells. We found that a subset of integrin signaling molecules, namely Fak and paxillin, but not p130 Crk-associated substrate or proline-rich tyrosine kinase 2, participate in processes regulating N-cadherin–based cell–cell adhesion. Paxillin was found to be required primarily for the recruitment of Fak to robust focal adhesions. Our results suggest that at least some signals involving Fak are linked to a mechanism down-regulating Rac1 activity at the cell periphery, which appears to be important for the formation of N-cadherin–based adhesions in motile cells. Our analyses simultaneously exemplified the essential role of Fak in the maintenance of cell–cell adhesions in collective cell migration, a type of migration occurring in embryonic development and carcinoma invasion.

Phosphorylation of paxillin by p38MAPK is involved in the neurite extension of PC-12 cells

Cell adhesions play an important role in neurite extension. Paxillin, a focal adhesion adaptor protein involved in focal adhesion dynamics, has been demonstrated to be required for neurite outgrowth. However, the molecular mechanism by which paxillin regulates neurite outgrowth is unknown. Here, we show that paxillin is phosphorylated by p38MAPK in vitro and in nerve growth factor (NGF)–induced PC-12 cells. Ser 85 (Ser 83 for endogenous paxillin) is identified as one of major phosphorylation sites by phosphopeptide mapping and mass spectrometry. Moreover, expression of the Ser 85 → Ala mutant of paxillin (pax_S85A) significantly inhibits NGF-induced neurite extension of PC-12 cells, whereas expression of wild-type (wt) paxillin does not influence neurite outgrowth. Further experiments indicate that cells expressing pax_S85A exhibit small, clustered focal adhesions which are not normally seen in cells expressing wt paxillin. Although wt paxillin and pax_S85A have the same ability to bind vinculin and focal adhesion kinase, wt paxillin more efficiently associates with Pyk2 than pax_S85A. Thus, phosphorylation of paxillin is involved in NGF-induced neurite extension of PC-12 cells, probably through regulating focal adhesion organization.

FAK and paxillin: regulators of N-cadherin adhesion and inhibitors of cell migration?

FAK and paxillin are important components in integrin-regulated signaling. New evidence suggests that these two proteins function in crosstalk between cell–matrix and cell–cell adhesions. Further, new insight suggests that under some conditions these proteins inhibit cell migration, in contrast to their established roles in several cell systems as positive regulators of cell adhesion and migration.

Calpain-mediated proteolysis of talin regulates adhesion dynamics

Dynamic regulation of adhesion complexes is required for cell migration and has therefore emerged as a key issue in the study of cell motility. Recent progress has been made in defining some of the molecular mechanisms by which adhesion disassembly is regulated, including the contributions of adhesion adaptor proteins and tyrosine kinases. However, little is known about the potential contribution of proteolytic mechanisms to the regulation of adhesion complex dynamics. Here, we show that proteolysis of talin by the intracellular calcium-dependent protease calpain is critical for focal adhesion disassembly. We have generated a single point mutation in talin that renders it resistant to proteolysis by calpain. Quantification of adhesion assembly and disassembly rates demonstrates that calpain-mediated talin proteolysis is a rate-limiting step during adhesion turnover. Furthermore, we demonstrate that disassembly of other adhesion components, including paxillin, vinculin and zyxin, is also dependent on the ability of calpain to cleave talin, suggesting a general role for talin proteolysis in regulating adhesion turnover. Together, these findings identify calpain-mediated proteolysis of talin as a mechanism by which adhesion dynamics are regulated.

Vinculin modulation of paxillin-FAK interactions regulates ERK to control survival and motility

Cells lacking vinculin are highly metastatic and motile. The reasons for this finding have remained unclear. Both enhanced survival and motility are critical to metastasis. Here, we show that vinculin null (vin-/-) cells and cells expressing a vinculin Y822F mutant have increased survival due to up-regulated activity of extracellular signal-regulated kinase (ERK). This increase is shown to result from vinculin's modulation of paxillin-FAK interactions. A vinculin fragment (amino acids 811-1066) containing the paxillin binding site restored apoptosis and suppressed ERK activity in vin-/- cells. Both vinY822F and vin-/- cells exhibit increased interaction between paxillin and focal adhesion kinase (FAK) and increased paxillin and FAK phosphorylation. Transfection with paxillin Y31FY118F dominant-negative mutant in these cells inhibits ERK activation and restores apoptosis. The enhanced motility of vin-/- and vinY822F cells is also shown to be due to a similar mechanism. Thus, vinculin regulates survival and motility via ERK by controlling the accessibility of paxillin for FAK interaction.

Pxl1p, a paxillin-like protein in Saccharomyces cerevisiae, may coordinate Cdc42p and Rho1p functions during polarized growth

Rho-family GTPases Cdc42p and Rho1p play critical roles in the budding process of the yeast Saccharomyces cerevisiae. However, it is not clear how the functions of these GTPases are coordinated temporally and spatially during this process. Based on its ability to suppress cdc42-Ts mutants when overexpressed, a novel gene PXL1 was identified. Pxl1p resembles mammalian paxillin, which is involved in integrating various signaling events at focal adhesion. Both proteins share amino acid sequence homology and structural organization. When expressed in yeast, chicken paxillin localizes to the sites of polarized growth as Pxl1p does. In addition, the LIM domains in both proteins are the primary determinant for targeting the proteins to the cortical sites in their native cells. These data strongly suggest that Pxl1p is the "ancient paxillin" in yeast. Deletion of PXL1 does not produce any obvious phenotype. However, Pxl1p directly binds to Rho1p-GDP in vitro, and inhibits the growth of rho1-2 and rho1-3 mutants in a dosage-dependent manner. The opposite effects of overexpressed Pxl1p on cdc42 and rho1 mutants suggest that the functions of Cdc42p and Rho1p may be coordinately regulated during budding and that Pxl1p may be involved in this coordination.

Regulation of Vascular Endothelial Growth Factor Receptor 2-mediated Phosphorylation of Focal Adhesion Kinase by Heat Shock Protein 90 and Src Kinase Activities

Exposure of endothelial cells to vascular endothelial growth factor (VEGF) induced tyrosine phosphorylation of focal adhesion kinase (FAK) on site Tyr(407), an effect that required the association of VEGF receptor 2 (VEGFR2) with HSP90. The association of VEGFR2 with HSP90 involved the last 130 amino acids of VEGFR2 and was blocked by geldanamycin, a specific inhibitor of HSP90. Moreover, geldanamycin inhibited the VEGF-induced activation of the small GTPase RhoA, which resulted in an inhibition of phosphorylation of FAK on site Tyr(407). In this context, the inhibition of RhoA kinase (ROCK) with Y27632 or by expression of dominant negative forms of RhoA or ROCK impaired the VEGF-induced phosphorylation of Tyr(407) within FAK. In contrast to phosphorylation of Tyr(861), the phosphorylation of site Tyr(407) was insensitive to Src kinase inhibition by 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP2). We also found that the recruitment of paxillin to FAK was inhibited by geldanamycin but not by PP2, whereas both geldanamycin and PP2 inhibited the recruitment of vinculin to FAK. In accordance, the recruitment of paxillin and vinculin to FAK was inhibited in cells that express the mutant FAK-Y407F, whereas the expression of the mutant Y861F inhibited the recruitment of paxillin but not of vinculin. Importantly, cell migration was abolished in cells in which the signal from the VEGFR2-HSP90 pathway was blocked by the expression of Delta130VEGFR2, a deletant of VEGFR2 that does not associate with HSP90. Our findings underscore for the first time the key role played by the VEGFR2-HSP90-RhoA-ROCK-FAK/Tyr(407) pathway in transducing the VEGF signal that leads to the assembly of focal adhesions and endothelial cell migration.

ILK is required for the assembly of matrix-forming adhesions and capillary morphogenesis in endothelial cells

Integrins play a key role in regulating endothelial cell survival, migration and differentiated function during angiogenic blood-vessel remodeling. Integrin-linked kinase (ILK) is a multidomain protein that interacts with the cytoplasmic tail of integrin β subunits and is thought to participate in integrin-mediated signal transduction. We report here that attenuation of ILK expression in cultured bovine aortic endothelial cells by RNA interference had marked effects on surface distribution of α5β1 integrin and the organization of cell-matrix adhesions characterized by the disappearance of fibrillar (3D-like) adhesions that are rich in α5β1 and paxillin, and associated fibrillar fibronectin matrix. This defect was not caused by a decrease in fibronectin mRNA levels or by intracellular retention of the protein. Adhesion to surface-adsorbed matrix proteins based on β1 and β3 integrin was enhanced following ILK depletion, whereas cell spreading, migration and multilayer alignment into capillary-like structures on Matrigel were impaired. We conclude that ILK is an important regulator of the endothelial phenotype and vascular network formation by directing the assembly and/or maturation of α5β1-competent matrix-forming adhesions.

Extracellular matrix molecules regulate endothelial cell migration stimulated by lysophosphatidic acid

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are lipids that bind G-protein coupled receptors and differentially promote transmigration of endothelial cells.

OBJECTIVE

To determine if endothelial cell transmigration stimulated by LPA, not S1P, is dependent on the extracellular matrix.

METHODS

Bovine pulmonary artery (BPAE) endothelial cell transmigration and locomotion were measured using a modified-Boyden chamber and video microscopy, respectively. Results were related to strength of adhesion and characteristics of cell adhesive contacts.

RESULTS AND CONCLUSIONS

BPAEs responded to LPA by transmigration through gelatin- or collagen-coated filters, but not through fibronectin-, vitronectin-, or fibrinogen-coated filters. Fewer cells adhered to collagen or gelatin than to fibronectin in a static cell adhesion assay or after application of a g-force to detach cells. Video microscopy revealed that S1P stimulates large lamellipodia on two-dimensional fibronectin substrate. LPA stimulated lamellipodia on fibronectin, but the trailing edge remained attached, resulting in sting ray-shaped cells in video microscopy. LPA-treated cells on gelatin released the trailing edge. To understand how the extracellular matrix may regulate endothelial cell shape during movement, we surveyed changes in focal adhesion proteins. More Hic-5, a paxillin homolog, was detected in the detergent insoluble fraction of BPAEs attached to gelatin than fibronectin. No such difference was found in paxillin. In BPAEs, Hic-5 was localized to smaller punctate structures on fibronectin and longer, thinner focal adhesions on gelatin. These results indicated that localization of Hic-5 and strength of adhesion correlate with endothelial cell transmigration stimulated by LPA, but not with transmigration stimulated by S1P.

Effect of enamel matrix derivative on periodontal ligament cells in vitro is diminished by Porphyromonas gingivalis

BACKGROUND

Enamel matrix derivative (EMD) has been shown to possess a mitogenic effect to induce effective periodontal regeneration, however, it is unclear whether periodontal pathogens can modulate the effect of EMD. The present study examined the influence of Porphyromonas gingivalis on EMD-stimulated periodontal ligament (PDL) cells.

METHODS

P. gingivalis ATCC33277 and its mutants deficient in fimbriae (delta fimA) or gingipains (delta rgpA delta rgpB, delta kgp, and delta rgpA delta rgpB delta kgp) were employed. PDL cells were grown on EMD-coated dishes and infected with P. gingivalis wild strain or a mutant. Cell migration and proliferation were then evaluated with an in vitro wound healing assay. The expression of transforming growth factor-beta1 (TGF-beta1) and insulin-like growth factor I (IGF-I) mRNA by PDL cells was examined. Further, the degradation and phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) as well as paxillin in infected PDL cells were estimated using Western blot analysis.

RESULTS

P. gingivalis ATCC33277 inhibited the migration and proliferation of PDL cells on EMD-coated dishes, and the mutants delta fimA, delta rgpA delta rgpB, and delta kgp showed the same effects. Further, each of these organisms diminished the expression of TGF-beta1 and IGF-I mRNA, as well as the phosphorylation of ERK1/2 from EMD-stimulated PDL cells. In addition, total paxillin protein was markedly degraded by both the wild-type strain and each of the mutants except for delta rgpA delta rgpB delta kgp, which showed a negligible effect in all of the assays with EMD-stimulated PDL cells.

CONCLUSION

These results suggest that P. gingivalis diminishes the effect of EMD on PDL cells in vitro through a cooperative action of gingipains.

Cardiomyocyte apoptosis triggered by RAFTK/pyk2 via Src kinase is antagonized by paxillin

Altered cellular adhesion and apoptotic signaling in cardiac remodeling requires coordinated regulation of multiple constituent proteins that comprise cytoskeletal focal adhesions. One such protein activated by cardiac remodeling is related adhesion focal tyrosine kinase (RAFTK, also known as pyk2). Adenoviral-mediated expression of RAFTK in neonatal rat cardiomyocytes involves concurrent increases in phosphorylation of Src, c-Jun N-terminal kinase, and p38 leading to characteristic apoptotic changes including cleavage of poly(ADP-ribose) polymerase, caspase-3 activation, and increased DNA laddering. DNA laddering was decreased by mutation of the Tyr(402) Src-binding site in RAFTK, suggesting a central role for Src activity in apoptotic cell death that was confirmed by adenoviral-mediated Src expression. Multiple apoptotic signaling cascades are recruited by RAFTK as demonstrated by prevention of apoptosis using caspase-3 inhibitor IV (caspase-3 specific inhibitor), PP2 (Src-specific kinase inhibitor), or Csk (cellular negative regulator for Src), as well as dominant negative constructs for p38beta or MKP-1. These RAFTK-mediated phenotypic characteristics are prevented by concurrent expression of wild-type or a phosphorylation-deficient paxillin mutated at Tyr(31) and Tyr(118). Wild-type or mutant paxillin protein accumulation in the cytoplasm has no overt effect upon cell structure, but paxillin accumulation prevents losses of myofibril organization as well as focal adhesion kinase, vinculin, and paxillin protein levels mediated by RAFTK. Apoptotic signaling cascade inhibition by paxillin indicates interruption of signaling proximal to but downstream of RAFTK activity. Chronic RAFTK activation in cardiac remodeling may represent a maladaptive reactive response that can be modulated by paxillin, opening up novel possibilities for inhibition of cardiomyocyte apoptosis and structural degeneration in heart failure.

Connective tissue growth factor [CTGF]/CCN2 stimulates mesangial cell migration through integrated dissolution of focal adhesion complexes and activation of cell polarization

Connective tissue growth factor [CTGF]/CCN2 is a prototypic member of the CCN family of regulatory proteins. CTGF expression is up-regulated in a number of fibrotic diseases, including diabetic nephropathy, where it is believed to act as a downstream mediator of TGF-beta function; however, the exact mechanisms whereby CTGF mediates its effects remain unclear. Here, we describe the role of CTGF in cell migration and actin disassembly in human mesangial cells, a primary target in the development of renal glomerulosclerosis. The addition of CTGF to primary mesangial cells induced cell migration and cytoskeletal rearrangement but had no effect on cell proliferation. Cytoskeletal rearrangement was associated with a loss of focal adhesions, involving tyrosine dephosphorylation of focal adhesion kinase and paxillin, increased activity of the protein tyrosine phosphatase SHP-2, with a concomitant decrease in RhoA and Rac1 activity. Conversely, Cdc42 activity was increased by CTGF. These functional responses were associated with the phosphorylation and translocation of protein kinase C-zeta to the leading edge of migrating cells. Inhibition of CTGF-induced protein kinase C-zeta activity with a myristolated PKC-zeta inhibitor prevented cell migration. Moreover, transient transfection of human mesangial cells with a PKC-zeta kinase inactive mutant (dominant negative) expression vector also led to a decrease in CTGF-induced migration compared with wild-type. Furthermore, CTGF stimulated phosphorylation and activation of GSK-3beta. These data highlight for the first time an integrated mechanism whereby CTGF regulates cell migration through facilitative actin cytoskeleton disassembly, which is mediated by dephosphorylation of focal adhesion kinase and paxillin, loss of RhoA activity, activation of Cdc42, and phosphorylation of PKC-zeta and GSK-3beta. These changes indicate that the initial stages of CTGF mediated mesangial cell migration are similar to those involved in the process of cell polarization. These findings begin to shed mechanistic light on the renal diabetic milieu, where increased CTGF expression in the glomerulus contributes to cellular dysfunction.

Interaction between Dab1 and CrkII is promoted by Reelin signaling

Reelin-induced Dab1 tyrosine phosphorylation has been implicated in the regulation of neuronal positioning during brain development. The downstream consequences of Dab1 tyrosine phosphorylation are not fully understood, however. Here we identify CrkII, CrkL and Dock1 in complexes bound to tyrosine-phosphorylated Dab1, through mass spectrometry. The CrkII-Dab1 interaction requires tyrosine phosphorylation of Dab1 at residues 220 or 232 and is promoted by Reelin treatment of embryonic forebrain neurons. Unlike other CrkII binding proteins, such as paxillin and p130Cas, expression of Dab1 interfered with CrkII-dependent cell migration of Nara Bladder Tumor II (NBT-II) cells, in a tyrosine phosphorylation-site dependent manner. Overexpression of CrkIIGFP rescued the migration of these cells, suggesting that Dab1 makes Crk a limiting factor for migration. The Dock1-Dab1 association is indirect and requires CrkII. In organisms such as Drosophila melanogaster and Caenorhabditis elegans, signaling complexes, which contain Crk and Dock1 family members are conserved and act through Rac. We show that a rough-eye phenotype in Drosophila caused by exogenous expression of tyrosine-phosphorylated mouse Dab1RFP is partially rescued by a loss-of-function mutation in myoblast city, a Dock1-like gene in Drosophila. We propose a model that tyrosine-phosphorylated Dab1 engages the conserved Crk-Dock1-Rac signaling cassette, but when bound to Dab1 this signaling complex does not support migration.

Overexpression of Hyperactive Integrin-Linked Kinase Leads to Increased Cellular Radiosensitivity

Integrin-linked kinase (ILK), bound to the cytoplasmic tails of integrin beta1, beta2, and beta3, is thought to signal through AKT and glycogen synthase kinase-3beta (GSK-3beta) for survival and proliferation regulation. To determine the role of ILK in the cellular radiation response, stably transfected A549 lung cancer cells overexpressing either wild-type (ILK-wk) or hyperactive ILK (ILK-hk) were studied for survival, signaling, proliferation, and examined in immunofluorescence and adhesion assays. Strong radiosensitization was observed in ILK-hk in contrast to ILK-wk mutants and empty vector controls. ILK small interfering RNA transfections showed radioresistance similar to irradiation on fibronectin. AKT, GSK-3beta-cyclin D1, mitogen-activated protein kinase kinase 1/2-mitogen-activated protein kinase, and c-Jun NH2-terminal kinase signaling was dysregulated in irradiated ILK-hk mutants. Immunofluorescence stainings of ILK-hk cells indicated disturbed ILK and paxillin membrane localization with concomitant decrease in focal adhesions. Profound ILK-hk-dependent changes in morphology were characterized by spindle-like cell shape, cell size reduction, increased cell protrusions, strong formation of membranous f-actin rings, and significantly reduced adhesion to matrix proteins. Additionally, ILK-wk and ILK-hk overexpression impaired beta1-integrin clustering and protein Tyr-phosphorylation. Taken together, the data provide evidence that ILK signaling modulates the cellular radiation response involving diverse signaling pathways and through changes in f-actin-based processes such as focal adhesion formation, cell adhesion, and spreading. Identification of ILK and its signaling partners as potential targets for tumor radiosensitization might promote innovative anticancer strategies by providing insight into the mechanism of cell adhesion-mediated radioresistance, oncogenic transformation, and tumor growth and spread.

Activation of Rac1 by paxillin-Crk-DOCK180 signaling complex is antagonized by Rap1 in migrating NBT-II cells

Induction of epithelial cell motility is a fundamental morphogenetic event that is recapitulated during carcinoma metastasis. Random motility of NBT-II carcinoma cells on collagen critically depends on paxillin phosphorylation at Tyr-31 and Tyr-118, the binding sites for the adapter protein CrkII. Two constitutive partners of CrkII are the exchange factors DOCK180 and C3G. CrkII bound to DOCK180 formed a signaling complex with phosphorylated paxillin that was necessary for cell migration as inferred from the inhibition caused by a DOCK180-interfering mutant. DOCK180, which acts predominantly on the Rho family GTPase Rac1, restored cell locomotion in cells expressing Phe-31/118 paxillin mutants deficient in Rac1 GTP-loading, suggesting that formation of paxillin-Crk-DOCK180 signaling complex controls collagen-dependent migration mainly through Rac1 activation. In migrating cells, CrkII constitutive association with C3G was not sufficient to stimulate its GDP/GTP exchange activity toward the Ras family GTPase Rap1. However, when constitutively active RapV12 was overexpressed, it negatively regulated cell motility. Activation of the C3G/Rap1 signaling pathway resulted in down-regulation of the paxillin-Crk-DOCK180 complex and reduction of Rac1-GTP, suggesting that Rap1 activation could suppress the Rac1 signaling pathway in epithelial cells.

Hyperosmotic stress induces rapid focal adhesion kinase phosphorylation at tyrosines 397 and 577. Role of Src family kinases and Rho family GTPases

Hyperosmotic stress induced by treatment of Swiss 3T3 cells with the non-permeant solutes sucrose or sorbitol, rapidly and robustly stimulated endogenous focal adhesion kinase (FAK) phosphorylation at Tyr-397, the major autophosphorylation site, and at Tyr-577, within the kinase activation loop. Hyperosmotic stress-stimulated FAK phosphorylation at Tyr-397 occurred via a Src-independent pathway, whereas Tyr-577 phosphorylation was completely blocked by exposure to the Src family kinase inhibitor PP-2. Inhibition of p38 MAP kinase or phosphatidylinositol 3-kinases did not prevent FAK phosphorylation stimulated by hyperosmotic stress. Overexpression of N17 RhoA did not reduce hyperosmotic stress-mediated localization of phosphorylated FAK to focal contacts and treatment with the Rho-associated kinase inhibitor Y-27632 did not prevent FAK translocation and tyrosine phosphorylation in response to hyperosmotic stress. Overexpression of N17 Rac only slightly altered the hyperosmotic stress-mediated localization of phosphorylated FAK to focal contacts. In contrast, overexpression of the N17 mutant of Cdc42 disrupted hyperosmotic stress-stimulated FAK Tyr-397 localization to focal contacts. Additionally, treatment of cells with Clostridium difficile toxin B potently inhibited hyperosmotic stress-induced FAK tyrosine phosphorylation. Furthermore, FAK null fibroblasts compared with their FAK containing controls show markedly increased sensitivity, manifest by subsequent apoptosis, to sustained hyperosmotic stress. Our results indicate that FAK plays a fundamental role in protecting cells from hyperosmotic stress, and that the pathway(s) that mediates FAK autophosphorylation at Tyr-397 in response to osmotic stress can be distinguished from the pathways utilized by many other stimuli, including neuropeptides and bioactive lipids (Rho- and Rho-associated kinase-dependent), tyrosine kinase receptor agonists (phosphatidylinositol 3-kinase-dependent), and integrins (Src-dependent).

Ligand stimulation of CD155alpha inhibits cell adhesion and enhances cell migration in fibroblasts

CD155 (poliovirus receptor) localizes in cell-matrix adhesions and cell-cell junctions, but its role in the regulation of cell adhesion and cell motility has not been investigated. We identified a conserved immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic domain of human CD155alpha. The ITIM was tyrosine-phosphorylated upon binding of anti-CD155 monoclonal antibody D171, poliovirus, and DNAM-1 (CD226) to human CD155alpha, and recruited SH2-domain-containing tyrosine phosphatase-2 (SHP-2). After CD155alpha stimulation with its ligands, cell adhesion was inhibited and cell motility was enhanced, effects that were associated with the phosphorylation of ITIM by Src kinases and accompanied by dephosphorylation of focal adhesion kinase and paxillin. These effects were abolished by introducing a point-mutation in Y398F into the ITIM of CD155alpha and by coexpression of a dominant negative SHP-2 mutant with CD155alpha. These results suggest that CD155alpha plays a role in the regulation of cell adhesion and cell motility.

Activated signal transducer and activator of transcription (STAT) 3: localization in focal adhesions and function in ovarian cancer cell motility

Constitutive activation of the Janus-activated kinase/signal transducer and activator of transcription (STAT) pathway promotes the proliferation and survival of cancer cells in culture and is associated with various cancers, including those of the ovary. We found that constitutively activated STAT3 levels correlated with aggressive clinical behavior of ovarian carcinoma specimens. Furthermore, inhibition of STAT3 reduced the motility of ovarian cancer cells in vitro. Surprisingly, we found that activated STAT3 localized not only to nuclei but also to focal adhesions in these cells. Activated STAT3 coimmunoprecipitated with phosphorylated paxillin and focal adhesion kinase and required paxillin and Src for its localization to focal adhesions. These results suggest that Janus-activated kinase/STAT signaling may contribute to ovarian cancer cell invasiveness.

Shiga toxin binding to globotriaosyl ceramide induces intracellular signals that mediate cytoskeleton remodeling in human renal carcinoma-derived cells

Shiga toxin is a bacterial toxin consisting of A and B subunits. Generally, the essential cytotoxicity of the toxin is thought to be mediated by the A subunit, which possesses RNA cleavage activity and thus induces protein synthesis inhibition. We previously reported, however, that the binding of the Shiga toxin 1-B subunit to globotriaosyl ceramide, a functional receptor for Shiga toxin, induces intracellular signals in a manner that is dependent on glycolipid-enriched membrane domains, or lipid rafts. Although the precise role of this signaling mechanism is not known, here we report that Shiga-toxin-mediated intracellular signals induce cytoskeleton remodeling in ACHN cells derived from renal tubular epithelial carcinoma. Using confocal laser scanning microscopy, we observed that Shiga toxin 1-B treatment induces morphological changes in ACHN cells in a time-dependent manner. In addition, the morphological changes were accompanied by the redistribution of a number of proteins, including actin, ezrin, CD44, vimentin, cytokeratin, paxillin, FAK, and α- and γ-tubulins, all of which are involved in cytoskeletal organization. The transient phosphorylation of ezrin and paxillin was also observed during the course of protein redistribution. Experiments using inhibitors for a variety of kinases suggested the involvement of lipid rafts, Src family protein kinase, PI 3-kinase, and RHO-associated kinase in Shiga toxin 1-B-induced ezrin phosphorylation. Shiga toxin 1-B-induced cytoskeletal remodeling should provide an in vitro model that can be used to increase our understanding of the pathogenesis of Shiga-toxin-mediated cell injury and the role of lipid-raft-mediated cell signaling in cytoskeletal remodeling.

RACK1 inhibits the serum- and anchorage-independent growth of v-Src transformed cells

Cancer cells are capable of serum- and anchorage-independent growth, and focus formation on monolayers of normal cells. Previously, we showed that RACK1 inhibits c-Src kinase activity and NIH3T3 cell growth. Here, we show that RACK1 partially inhibits v-Src kinase activity, and the serum- and anchorage-independent growth of v-Src transformed cells, but has no effect on focus formation. RACK1-overexpressing v-Src cells show disassembly of podosomes, which are actin-rich structures that are distinctive to fully transformed cells. Together, our results demonstrate that RACK1 overexpression in v-Src cells partially reverses the transformed phenotype of the cells. Our results identify an endogenous inhibitor of the oncogenic Src tyrosine kinase and of cell transformation.

Phosphorylation of paxillin tyrosines 31 and 118 controls polarization and motility of lymphoid cells and is PMA-sensitive

Tyrosine phosphorylation of paxillin regulates actin cytoskeleton-dependent changes in cell morphology and motility in adherent cells. In this report we investigated the involvement of paxillin tyrosine phosphorylation in the regulation of actin cytoskeleton-dependent polarization and motility of a non-adherent IL-3-dependent murine pre-B lymphocytic cell line Baf3. We also assessed the effect of phorbol myristate acetate (PMA), a phorbol ester analogous to those currently in clinical trials for the treatment of leukemia, on paxillin phosphorylation. Using tyrosine-to-phenylalanine phosphorylation mutants of paxillin and phosphospecific antibody we demonstrated that IL-3 stimulated phosphorylation of paxillin tyrosine residues 31 and 118, whereas the tyrosines 40 and 181 were constitutively phosphorylated. Phosphorylation of paxillin residues 31 and 118 was required for cell polarization and motility. In the presence of IL-3, PMA dramatically reduced the phosphorylation of residues 31 and 118, which was accompanied by inhibition of cell polarization and motility. This PMA effect was partially recapitulated by expression of exogenous tyrosine 31 and 118 mutants of paxillin. We also demonstrated that PMA inhibited the IL-3-induced and activation-dependent tyrosine phosphorylation of focal adhesion kinase. Thus, our results indicate that phosphorylation of paxillin tyrosine residues 31 and 118 regulates actin-dependent polarization and motility of pre-B Baf3 cells, both of which could be inhibited by PMA. They also suggest that inhibition of upstream signaling by PMA contributes to the decrease of paxillin phosphorylation and subsequent changes in cell morphology.

Differential regulation of focal adhesion kinase and paxillin phosphorylation by the small GTP-binding protein Rho in human corneal epithelial cells

PURPOSE

Lysophosphatidic acid (LPA), which activates the small guanosine triphosphate (GTP)-binding protein Rho, was previously shown to promote migration of the rabbit corneal epithelium in culture. The signaling pathway responsible for this effect of LPA was examined in this study with a human corneal epithelial (HCE) cell line.

METHODS

The activation of Rho was detected with a pull-down assay. Tyrosine phosphorylation of paxillin and of focal adhesion kinase (FAK) were examined both by immunofluorescence staining and by immunoprecipitation and immunoblot analyses. Expression of integrins alpha5 and beta1 was evaluated by immunoblot analysis.

RESULTS

Incubation of cells with LPA (10 micro M) for 2 min resulted in marked activation of Rho, and this effect was blocked by pretreatment with the Rho inhibitor exoenzyme C3 (2 micro g/ml) for 24 h. Tyrosine phosphorylation of both paxillin and FAK was detected in HCE cells under basal conditions by immunofluorescence staining, immunoprecipitation, and immunoblot analyses. LPA induced a concentration- and time-dependent increase in the tyrosine phosphorylation of paxillin, which was maximal at a concentration of 10 micro M and a time of 2 min. Exoenzyme C3 inhibited LPA-induced paxillin phosphorylation. Neither LPA nor exoenzyme C3 affected tyrosine phosphorylation of FAK or expression of integrins alpha5 and beta1.

CONCLUSIONS

LPA induces Rho activation and the consequent tyrosine phosphorylation of paxillin in HCE cells, and these effects likely contribute to the promotion of corneal epithelial migration by this agent.

Early molecular events in the assembly of the focal adhesion-stress fiber complex during fibroblast spreading

Cell adhesion to the extracellular matrix triggers the formation of integrin-mediated contact and reorganization of the actin cytoskeleton. Examination of nascent adhesions, formed during early stages of fibroblast spreading, reveals a variety of forms of actin-associated matrix adhesions. These include: (1). small ( approximately 1 microm), dot-like, integrin-, vinculin-, paxillin-, and phosphotyrosine-rich structures, with an F-actin core, broadly distributed over the ventral surfaces of the cells; (2). integrin-, vinculin-, and paxillin-containing "doublets" interconnected by short actin bundles; (3). arrays of actin-vinculin complexes. Such structures were formed by freshly plated cells, as well as by cells recovering from latrunculin treatment. Time-lapse video microscopy of such cells, expressing GFP-actin, indicated that long actin cables are formed by an end-to-end lining-up and apparent fusion of short actin bundles. All these structures were prominent during cell spreading, and persisted for up to 30-60 min after plating. Upon longer incubation, they were gradually replaced by stress fibers, associated with focal adhesions at the cell periphery. Direct examination of paxillin and actin reorganization in live cells revealed alignment of paxillin doublets, forming long and highly dynamic actin bundles, undergoing translocation, shortening, splitting, and convergence. The mechanisms underlying the assembly and reorganization of actin-associated focal adhesions and the involvement of mechanical forces in regulating their dynamic properties are discussed.

EphB1-mediated Cell Migration Requires the Phosphorylation of Paxillin at Tyr-31/Tyr-118

Interactions between Eph receptors and their membrane-bound ligands (ephrins) are of critical importance for key developmental processes such as boundary formation or vascular development. Their downstream signaling pathways are intricate and heterogeneous at several levels, the combined effect being a highly complex and flexible system. Here we demonstrate that activated EphB1 induces tyrosine phosphorylation of the focal adhesion protein paxillin at Tyr-31 and Tyr-118 and is recruited to paxillin-focal adhesion kinase (FAK) complexes. Pretreatment with the specific Src inhibitor PP2, or expression of dominant-negative, kinase-dead c-Src abrogates EphB1-induced tyrosine phosphorylation of paxillin. Cells transfected with the paxillin mutant Y31F/Y118F displayed a reduced migration in response to ephrin B2 stimulation. Furthermore, expression of an LD4 deletion mutant (paxillin DeltaLD4) significantly reduces EphB1-paxillin association, paxillin tyrosine phosphorylation, as well as EphB1-dependent cell migration. Finally, mutation of the Nck-binding site of EphB1 (Y594F) interrupts the interaction between Nck, paxillin, and EphB1. These data suggest a model in which ligand-activated EphB1 forms a signaling complex with Nck, paxillin, and focal adhesion kinase and induces tyrosine phosphorylation of paxillin in a c-Src-dependent manner to promote cell migration.

Ectodomain Shedding of SHPS-1 and Its Role in Regulation of Cell Migration

SHPS-1 is a transmembrane protein whose cytoplasmic region undergoes tyrosine phosphorylation and then binds the protein-tyrosine phosphatase SHP-2. Formation of the SHPS-1-SHP-2 complex is implicated in regulation of cell migration. In addition, SHPS-1 and its ligand CD47 constitute an intercellular recognition system that contributes to inhibition of cell migration by cell-cell contact. The ectodomain of SHPS-1 has now been shown to be shed from cells in a reaction likely mediated by a metalloproteinase. This process was promoted by activation of protein kinase C or of Ras, and the released ectodomain exhibited minimal CD47-binding activity. Metalloproteinases catalyzed the cleavage of a recombinant SHPS-1-Fc fusion protein in vitro, and the primary cleavage site was localized to the juxtamembrane region of SHPS-1. Forced expression of an SHPS-1 mutant resistant to ectodomain shedding impaired cell migration, cell spreading, and reorganization of the actin cytoskeleton. It also increased the tyrosine phosphorylation of paxillin and FAK triggered by cell adhesion. These results suggest that shedding of the ectodomain of SHPS-1 plays an important role in regulation of cell migration and spreading by this protein.