Integrin and FAK-mediated MAPK activation is required for cyclic strain mitogenic effects in Caco-2 cells

Rhythmic strain stimulates Caco-2 proliferation. We asked whether mitogen-activated protein kinase (MAPK) activation mediates strain mitogenicity and characterized upstream signals regulating MAPK. Caco-2 cells were subjected to strain on collagen I-precoated membranes or antibodies to integrin subunits. Twenty-four hours of cyclic strain increased cell numbers compared with static conditions. MAPK-extracellular signal-regulated kinase (ERK) kinase inhibition (20 microM PD-98059) blocked strain mitogenicity. p38 Inhibition (10 microM SB-202190) did not. Strain rapidly and time-dependently activated focal adhesion kinase (FAK), paxillin, ERK1 and 2, and p38 on collagen. c-Jun NH(2)-terminal kinase (JNK)1 and 2 exhibited delayed activation. Similar activation occurred when Caco-2 cells were subjected to strain on a substrate of functional antibody to the alpha2-, alpha3-, alpha6-, or beta1-integrin subunits but not on a substrate of functional antibody to the alpha5-subunit. FAK inhibition by FAK397 transfection blocked ERK2 and JNK1 activation by in vitro kinase assays, but pharmacological protein kinase C inhibition did not block ERK1 or 2 activation by strain. Strain-induced ERK signals mediate strain's mitogenic effects and may require integrins and FAK activation.

Abnormal localisation and hyperclustering of (alpha)(V)(beta)(3) integrins and associated proteins in Src-deficient or tyrphostin A9-treated osteoclasts

The non-receptor tyrosine kinase Src was shown to be essential for osteoclast function in vivo. We have previously reported that engagement of (alpha)(v)(beta)(3) integrin in osteoclasts induces tyrosine phosphorylation and activation of the adhesion kinase PYK2 and the adaptor protein p130(Cas) in a Src-dependent manner. The objective of this study was to analyse the role of c-Src in the (alpha)(v)(beta)(3) integrin-dependent recruitment of signalling and cytoskeletal molecules in osteoclasts during bone resorption. Using prefusion osteoclasts (pOCs) obtained from cocultures of osteoblasts and spleen cells isolated from Src(-/-) mice or their normal littermates, we found: (1) similar expression levels and ligand binding affinities of (alpha)(v)(beta)(3) integrins in Src(-/-) and Src(+/?) pOCs, (2) reduced adhesion and spreading of Src(-/-) pOCs, (3) defective organisation of the microfilament proteins, F-actin, vinculin and paxillin, and of PYK2 and p130(Cas) in the sealing zone of Src(-/-)OCLs, and (4) hyperclustering of (alpha)(v)(beta)(3) integrins together with microfilament and signalling proteins in the basal membrane of Src-deficient OCLs. In normal OCLs, the tyrosine kinase inhibitor tyrphostin A9 inhibits actin ring formation, bone resorption and tyrosine phosphorylation of several proteins, including c-Src. Furthermore, tyrphostin A9 induced similar hyperclustering of (alpha)(v)(beta)(3) integrins in osteoclasts as observed in Src(-/-) OCLs. Taken together, these findings suggest that normal localisation of (alpha)(v)(beta)(3) and recruitment of its downstream effectors to the appropriate compartments of the osteoclast during resorption depend on Src kinase activity.

Separate but linked functions of conventional myosins modulate adhesion and neurite outgrowth

The potential functional diversity of closely related myosin isoforms found in eukaryotic cells is not yet understood in detail. We have previously provided evidence from functional knockouts of Neuro-2A neuroblastoma cells that myosin IIB is essential for neurite outgrowth. Here we investigate the role of non-muscle myosin IIA in the same cell line. We show that suppression of myosin IIA transcript and protein expression, brought about through exposure to isoform-specific antisense oligonucleotides, caused a rearrangement of the actin cytoskeleton and loss of cell adhesion. This also led to disruption of focal contacts, as evidenced by coincident reduction in paxillin and vinculin immunofluorescence, but did not diminish transcript expression. All effects were fully reversible. Before myosin IIA antisense-induced detachment, neurite outgrowth remained unaffected. By contrast, antisense oligonucleotides directed against myosin IIB transcripts had no effect on adhesion but severely attenuated neurite outgrowth. We infer that the two main isoforms of neuronal conventional myosin, myosins IIA and IIB, have separate but linked functions during neuronal adhesion and neurite outgrowth.

Rat seminiferous epithelium contains a unique junction (Ectoplasmic specialization) with signaling properties both of cell/cell and cell/matrix junctions

The seminiferous epithelium contains unique actin related cell-cell junctions, termed ectoplasmic specializations (ESs). Turnover of these junctions is fundamental to sperm release and to movement of spermatocytes from basal to adluminal compartments of the epithelium during spermatogenesis. In this study we report several novel observations related to the spatial and temporal distribution of integrin-related signaling molecules at ESs. We confirm the presence of beta(1)-integrin at these sites and further demonstrate co-localization of integrin linked kinase (ILK). beta(1)-Integrin and ILK were shown by immunoprecipitation to associate in whole cell lysates of seminiferous epithelium. This observation provides the first evidence for a direct beta(1)-integrin/ILK interaction in noncultured epithelium. Pan-cadherin and beta-catenin antibodies did not react at ESs. Rather, antibodies reacted with desmosome-like junctions that are present both at basal junctional complexes between Sertoli cells and at sites of attachment to spermatogenic cells. Focal adhesion kinase (FAK), a known integrin-associated molecule, did not codistribute with beta(1)-integrins and did not associate with these adhesion molecules in immunoprecipitation studies. Although FAK was expressed in the epithelium, it appeared to be limited to the cytoplasm of early spermatogenic cells. Significantly, polyclonal antibodies against phosphotyrosine-containing residues reacted strongly at ESs, with highest levels detected during sperm release and turnover of basal junction complexes. Our observations indicate that ESs share cell signaling features both of cell-cell junctions and of cell-extracellular matrix junctions.

Actopaxin, a new focal adhesion protein that binds paxillin LD motifs and actin and regulates cell adhesion

Paxillin is a focal adhesion adapter protein involved in the integration of growth factor- and adhesion-mediated signal transduction pathways. Paxillin LD motifs have been demonstrated to bind to several proteins associated with remodeling of the actin cytoskeleton including the focal adhesion kinase, vinculin, and a complex of proteins comprising p95PKL, PIX, and PAK (Turner, C.E., M. C. Brown, J.A. Perrotta, M.C. Riedy, S.N. Nikolopoulos, A.R. McDonald, S. Bagrodia, S. Thomas, and P.S. Leventhal. 1999. J. Cell Biol. 145:851-863). In this study, we report the cloning and initial characterization of a new paxillin LD motif-binding protein, actopaxin. Analysis of the deduced amino acid sequence of actopaxin reveals a 42-kD protein with two calponin homology domains and a paxillin-binding subdomain (PBS). Western blotting identifies actopaxin as a widely expressed protein. Actopaxin binds directly to both F-actin and paxillin LD1 and LD4 motifs. It exhibits robust focal adhesion localization in several cultured cell types but is not found along the length of the associated actin-rich stress fibers. Similar to paxillin, it is absent from actin-rich cell-cell adherens junctions. Also, actopaxin colocalizes with paxillin to rudimentary focal complexes at the leading edge of migrating cells. An actopaxin PBS mutant incapable of binding paxillin in vitro cannot target to focal adhesions when expressed in fibroblasts. In addition, ectopic expression of the PBS mutant and/or the COOH terminus of actopaxin in HeLa cells resulted in substantial reduction in adhesion to collagen. Together, these results suggest an important role for actopaxin in integrin-dependent remodeling of the actin cytoskeleton during cell motility and cell adhesion.

A switch from p130Cas/Crk to Gab1/Crk signaling correlates with anchorage independent growth and JNK activation in cells transformed by the Met receptor oncoprotein

Cell transformation is associated with anchorage independent growth and morphological changes characterized by reduced adhesion and spreading. The molecular signals that control these events are poorly understood. The Met receptor tyrosine kinase is deregulated in human tumors and an oncogenic derivative of this receptor transforms cells. In this paper we demonstrate that fibroblasts transformed by the Met oncoprotein display decreased cell spreading consistent with the loss of actin stress fibers and vinculin staining focal adhesions. In contrast to control cells, focal adhesion kinase, p130Cas and paxillin are weakly or not detectably tyrosine phosphorylated in Met transformed cells. Moreover, although paxillin and p130Cas associate with the Crk adapter protein in control cells, they fail to associate with Crk in Met transformed cells, yet these cells are motile and capable of wound closure to the same extent as control cells. In Met transformed cells, Crk predominantly associates with the Cbl and Gab1docking proteins in a tyrosine phosphorylation dependent manner. The coupling of Gab1, but not Cbl, with Crk is retained in cells grown in suspension and enhances JNK activation. We propose that the loss of adhesion dependent signals required for cell cycle progression is compensated through Met induced Gab1/Crk signals.

Paxillin and focal adhesion signalling

To facilitate a rapid response to environmental change, cells use scaffolding - or adaptor - proteins to recruit key components of their signal-transduction machinery to specific subcellular locations. Paxillin is a multi-domain adaptor found at the interface between the plasma membrane and the actin cytoskeleton. Here it provides a platform for the integration and processing of adhesion- and growth factor-related signals.

Changes in cytoskeletal organization in polyoma middle T antigen-transformed fibroblasts: involvement of protein phosphatase 2A and src tyrosine kinases

The major transforming activity of polyomavirus, middle T antigen, targets several cellular regulatory effectors including protein phosphatase 2A and src tyrosine kinases. Although transformed cells exhibit profound morphological changes, little is known about how middle T antigen-induced changes in the cellular regulatory environment specifically affect the cytoskeleton. We have investigated these changes in 10T(1/2) mouse fibroblasts transformed with polyoma middle T antigen. Immunofluorescence microscopy revealed that expression of middle T antigen (Pym T cells) depleted the stable (acetylated) microtubule array and increased the sensitivity of dynamic (tyrosinated) microtubules to nocodazole-induced disassembly. These effects were associated with a modest but statistically significant (P</=0.05) increase in recovery of protein phosphatase 2A activity with microtubules. Middle T antigen expression also depleted the normal cellular complement of actin stress fibers and focal adhesions, in parallel with changes in the distribution of src tyrosine kinases. Herbimycin A promoted recovery of paxillin and phosphotyrosine into nascent focal adhesion sites, in addition to restoring normal src tyrosine kinase distribution. However, herbimycin A did not restore actin stress fibers or parental-type microtubules to Pym T cells. We suggest that regulation of the microtubule array by middle T antigen may occur through direct effects including redistribution of protein phosphatase 2A as well as indirect effects such as altered interactions with actin-based stress fibers.

Association of focal adhesion kinase with fibronectin and paxillin is required for precartilage condensation of chick mesenchymal cells

We show that tyrosine phosphorylation of FAK was increased as precartilage condensation occurred, followed by a subsequent decrease in proliferation of in vitro micromass culture of wing bud mesenchymal cells. FAK was associated with fibronectin and paxillin, which were maximal at day 3 of culture. FAK was also associated with signaling molecules such as PLC-gamma and PI3-kinase through c-Src. The beta1 integrin antibody and several inhibitors of signaling molecules such as herbimycin A, U73122, LY294002, as well as cytochalasin D, an actin depolymerizing agent, remarkably decreased tyrosine phosphorylation of FAK and its association with fibronectin and paxillin during condensation. resulting in a marked inhibition of condensation and chondrogenesis. Taken together, our findings suggest that beta1 integrin-mediated interaction of mesenchymal cells and fibronectin signals to accelerate the precartilage condensation through tyrosine phosphorylation of FAK and its association with paxillin. This signaling pathway is required for precartilage condensation and subsequent cartilage nodule formation in chondrogenesis.

Integrin engagement, the actin cytoskeleton, and c-Src are required for the calcitonin-induced tyrosine phosphorylation of paxillin and HEF1, but not for calcitonin-induced Erk1/2 phosphorylation

We have previously shown that in a HEK-293 cell line that overexpresses the C1a isoform of the calcitonin receptor (C1a-HEK), calcitonin induces the tyrosine phosphorylation of the focal adhesion-associated proteins HEF1 (a p130(Cas)-like docking protein), paxillin, and focal adhesion kinase and that it also stimulates the phosphorylation and activation of Erk1 and Erk2. We report here that cell attachment to the extracellular matrix, an intact actin cytoskeleton, and c-Src are absolutely required for the calcitonin-induced phosphorylation of focal adhesion-associated proteins. In contrast to the phosphorylation of paxillin and HEF1 in cells attached to fibronectin-coated dishes, calcitonin failed to stimulate the phosphorylation of paxillin and HEF1 in suspended cells, in cells attached to poly-d-lysine-coated dishes, and in attached cells pretreated with the RGD-containing peptide GRGDS. Overexpression of wild-type c-Src increased calcitonin-induced paxillin and HEF1 phosphorylation, whereas overexpression of kinase-dead Src or Src lacking a functional SH2 domain inhibited the calcitonin-stimulated tyrosine phosphorylation of these proteins. Overexpression of Src lacking the SH3 domain did not affect the calcitonin-induced phosphorylation of paxillin and HEF1. In contrast to the regulation of paxillin and HEF1 phosphorylation, the calcitonin-induced phosphorylation of Erk1 and Erk2 did not appear to involve c-Src and was only partially dependent on cell adhesion to the extracellular matrix and an intact actin cytoskeleton. Furthermore, inhibition of Erk1 and Erk2 phosphorylation had no effect on the calcitonin-induced phosphorylation of paxillin and HEF1. Thus, in C1a-HEK cells, the calcitonin receptor is coupled to the tyrosine phosphorylation of focal adhesion-associated proteins and to Erk1/2 phosphorylation by mechanisms that are in large part independent.

Involvement of Rb family proteins, focal adhesion proteins and protein synthesis in senescent morphogenesis induced by hydrogen peroxide

Early passage human diploid fibroblasts develop senescent morphology prematurely within a week after a 2-hour pulse treatment with low or mild dose H(2)O(2). We test here the role of cell cycle checkpoints, cytoskeletal proteins and de novo protein synthesis in senescent morphogenesis following H(2)O(2) treatment. H(2)O(2) treatment causes transient elevation of p53 protein and prolonged inhibition of Rb hyperphosphorylation. Expression of human papillomaviral E6 gene prevented elevation of p53 but did not affect senescent morphogenesis. Expression of human papillomaviral E7 gene reduced the level of Rb protein and prevented induction of senescent morphology by H(2)O(2). The mutants of the E7 gene, in which the Rb family protein binding site was destroyed, could not reduce Rb protein or prevent H(2)O(2) from inducing senescent morphology. Senescent-like cells showed enhanced actin stress fibers. In untreated cells, vinculin and paxillin preferentially distributed along the edge of the cells. In contrast, vinculin and paxillin distributed randomly and sporadically throughout senescent-like cells. E7 expression prevented enhancement of actin filament formation and redistribution of vinculin or paxillin. Neither wild-type nor E7 cells showed changes in the protein level of actin, vinculin or paxillin measured by western blot after H(2)O(2) treatment. Finally, depletion of methionine in the culture medium after H(2)O(2) treatment prevented senescent morphogenesis without affecting dephosphorylation of Rb protein. Our results suggest that senescent morphology likely develops by a program involving activated Rb family proteins, enhancement of actin stress fibers, redistribution of focal adhesion proteins and de novo protein synthesis.

Src homology 2 domain substitution modulates the kinase and transforming activities of the Fes protein-tyrosine kinase

The c-fes proto-oncogene encodes a Mr 93,000 protein-tyrosine kinase (Fes) that is strongly expressed in myeloid cells and has been implicated in myelomonocytic differentiation. Fes autophosphorylation and transforming activity are highly restrained after ectopic expression in fibroblasts, indicating tight negative regulation of Fes kinase activity in vivo. Here we investigated the regulatory role of the Fes Src homology 2 (SH2) domain by producing a series of chimeric constructs in which the Fes SH2 domain was replaced with those of the transforming oncogenes v-Fps and v-Src or by the NH2-terminal SH2 domain of the Ras GTPase-activating protein. Wild-type and chimeric Fes proteins readily underwent tyrosine autophosphorylation in vitro and produced identical cyanogen bromide phosphopeptide cleavage patterns, indicating that the SH2 substitutions did not influence overall kinase activity or autophosphorylation site selection. However, metabolic labeling of Rat-2 fibroblasts expressing each construct showed that only the Fes/Src SH2 chimera was active in vivo. Consistent with this result, the Fes/Src SH2 domain chimera exhibited potent transforming activity in fibroblasts and enhanced differentiation-inducing activity in K-562 myeloid leukemia cells. In addition, the Fes/Src SH2 chimera exhibited constitutive localization to focal adhesions in Rat-2 fibroblasts and induced the attachment and spreading of TF-1 myeloid cells. These data demonstrate a central role for the SH2 domain in the regulation of Fes kinase activity and biological function in vivo.

Cell adhesion molecule CEACAM1 associates with paxillin in granulocytes and epithelial and endothelial cells

CEACAM1 functions as an epithelial tumor suppressor and as an angiogenic growth factor. In the present study, utilizing differentially (serine/threonine or tyrosine) phosphorylated cytoplasmic domains of CEACAM1 and CEACAM3 as bait to isolate associated proteins from granulocyte extracts, we have identified human paxillin as a binding partner of the tyrosine-phosphorylated cytoplasmic CEACAM1 domain. CEACAM1-paxillin complexes were coimmunoprecipitated from extracts of granulocytes, the colonic cell line HT29, and HUVECs. We identified phosphorylated Tyr-488-a residue in the cytoplasmic CEACAM1 domain known to be essential for the tumor suppressive effect-to be necessary for this association. The CEACAM1-paxillin interaction was confirmed using laser scanning confocal microscopy analyses in granulocytes and HT29 cells, where CEACAM1 colocalizes with paxillin at the plasma membrane. In HUVECs a highly polarized expression pattern and colocalization of paxillin and CEACAM1 was observed. These findings support the findings that CEACAM1 is linked to the actin-based cytoskeleton.

Matrix-specific FAK and MAPK reorganization during Caco-2 cell motility

We have previously reported that Caco-2 cell motility redistributes FAK, paxillin, and activates p38. However, the subcellular organization of these intracellular signals during cell migration is unclear. We, therefore, investigated the organization of actin, FAK, paxillin, and activated ERK and activated p38 during Caco-2 motility across collagen I, fibronectin, laminin, and tissue culture treated glass. Differential density seeding generated homogeneous static and migrating populations. Expression of actin, FAK, paxillin, phospho-ERK, and phospho-p38 were examined by immunofluorescent staining in static and motile cells. Actin was concentrated toward the peri-nuclear central area of cells migrating on matrix proteins studied. Actin immunoreactivity was decreased in the leading edge of lamellipodia. FAK immunoreactivity was weaker in migrating cells than in static cells on the same matrix. FAK was expressed along cell-cell contacts of both cell populations, but absent in migrating lamellipodia of matrix-cultured cells. Paxillin staining was diffuse in static cells but organized toward migrating lamellipodia in a radial manner. Like FAK, phosphorylated ERK was expressed in the central region of migrating cells but was dramatically decreased at areas of cell-cell contact and free lamellipodia. Fibronectin exerted the greatest effect on ERK activation in all matrix proteins studied. In contrast, phosphorylated p38 staining was stronger in migrating cells on matrix than in static cells on the same matrix. Phosphorylated p38 was expressed in the nuclear of migrating cells and disappeared in the cell-cell contact side and free lamellipodia. Interestingly, the reorganization of these proteins was distinctly different on tissue culture treated glass without a physiologic matrix substrate. For instance, FAK staining increased rather than decreased in motile cells on plastic, and lamellipodial FAK staining could be discerned. Matrix may influence Caco-2 biology during migration not only by triggering intracellular phosphorylation events but also by reorganizing the cytoskeleton and the subcellular localization of these intracellular signals.

Increased migration in late G(1) phase in cultured smooth muscle cells

Migration and proliferation of smooth muscle cells (SMC) contribute to neointimal formation after arterial injury. However, the relation between migration and proliferation in these cells is obscure. To discriminate between migration and proliferation, we employed a migration assay of SMC at different phases of the cell cycle. Serum-deprived SMC were synchronized in different phases of the cell cycle by addition of serum for various periods of time. Migration induced by platelet-derived growth factor B-chain homodimer was maximal in SMC that were predominantly in the late G(1) (G(1b)) phase. In addition, in nonsynchronized SMC, 65-75% of SMC that had migrated were in the G(1b) phase. Phosphorylated myosin light chain was enriched around the cell periphery in SMC in the G(1b) phase compared with SMC in the other cell cycle phases. Interestingly, the Triton X-100-insoluble fraction of myosin was remarkably decreased in G(1b)-enriched SMC. These findings suggest that migratory activity of SMC may be coupled with the G(1b) phase. The phosphorylation and retention of myosin might explain some of the properties responsible for increased migration.

[D-Arg(1),D-Trp(5,7,9),Leu(11)]Substance P inhibits bombesin-induced mitogenic signal transduction mediated by both G(q) and G(12) in Swiss 3T3cells

Substance P (SP) analogues including [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP are broad spectrum neuropeptide antagonists and potential anticancer agents, but their mechanism of action is not fully understood. Here, we examined the mechanism of action of [d-Arg(1), d-Trp(5,7,9),Leu(11)]SP as an inhibitor of G protein-coupled receptor (GPCR)-mediated signal transduction and cellular DNA synthesis in Swiss 3T3 cells. Addition of [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP, at 10 micrometer, caused a striking rightward shift in the dose-response curves of DNA synthesis induced by bombesin, bradykinin, or vasopressin and markedly inhibited the activation of p42(mapk) (ERK-2) and p44(mapk) (ERK-1) induced by these GPCR agonists. In addition, this SP analogue also prevented the protein kinase C-dependent activation of protein kinase D induced by these agonists. [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP, at a concentration (10 micrometer) that inhibited these G(q)-mediated events, also prevented GPCR agonist-induced responses mediated through the G proteins of the G(12) subfamily. These include bombesin-induced assembly of focal adhesions, formation of parallel arrays of actin stress fibers, increase in the tyrosine phosphorylation of focal adhesion kinase (FAK), p130(Cas), and paxillin, and formation of a complex between FAK and Src. We conclude that [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP acts as a mitogenic antagonist of neuropeptide GPCRs blocking signal transduction via both G(q) and G(12).

Overexpression of PAX6(5a) in lens fiber cells results in cataract and upregulation of (alpha)5(beta)1 integrin expression

The PAX6 gene, a key regulator of eye development, produces two major proteins that differ in paired domain structure: PAX6 and PAX6(5a). It is known that an increase in the PAX6(5a) to PAX6 ratio leads to multiple ocular defects in humans. Here, transgenic mice were created that overexpress human PAX6(5a) in the lens. These mice develop cataracts with abnormalities in fiber cell shape as well as fiber cell/lens capsule and fiber cell/fiber cell interactions. While the structure of the actin cytoskeleton appeared relatively normal, the cataractous lens expresses increased amounts of paxillin and p120(ctn) as well as large aggregates of (alpha)5(beta)1 integrin in the dysgenic fiber cells. The elevated amounts of these proteins in the transgenic lens correlated well with elevated levels of their respective mRNAs. To investigate the role of Pax-6(5a) in the upregulation of these genes, a series of gel shift experiments using truncated proteins and consensus oligonucleotides demonstrated the complexity of Pax-6 and Pax-6(5a) binding to DNA, aiding our identification of potential binding sites in the human (α)5- and (beta)1-integrin promoters. Consequent gel shift analysis demonstrated that these putative regulatory sequences bind Pax-6 and/or Pax-6(5a) in lens nuclear extracts, suggesting that the human (alpha)5 and (beta)1 integrin promoters contain PAX6/PAX6(5a) binding sites and maybe directly regulated by this transcription factor in the transgenic lens. We conclude that these transgenic mice are good models to study a type of human cataract and for identifying batteries of genes that are directly or indirectly regulated by both forms of Pax-6.

Differences in hyaluronic acid-mediated functions and signaling in arterial, microvessel, and vein-derived human endothelial cells

Hyaluronic acid (HA), a nonsulfated glycosaminoglycan, regulates cell adhesion and migration. Small HA fragments (3-25 disaccharide units) induce neovascularization. We investigated the effect of HA and a HA fragment (10-15 disaccharide units, F1) on primary human endothelial cells (ECs). Human pulmonary ECs (HPAEC) and lung microvessel ECs (HMVEC-L) bound HA (K(d) approximately 1 and 2.3 nm, respectively) and expressed 17,780 and 16,690 HA binding sites, respectively. Both ECs showed HA-mediated cell adhesion; however, HMVEC-L was 1.5-fold better. Human umbilical vein ECs neither bound HA nor showed HA-mediated adhesion. All three ECs expressed CD44 ( approximately 110 kDa). The expression of receptor for HA-mediated motility (RHAMM) (approximately 80 kDa) was the highest in HMVEC-L, followed by HPAEC and human umbilical vein ECs. RHAMM, not CD44, bound HA in all three ECs. F1 was better than HA and stimulated a 2. 5- and 1.8-fold mitogenic response in HMVEC-L and HPAEC, respectively. Both HA and F1 induced tyrosine phosphorylation of p125(FAK), paxillin, and p42/44 ERK in HMVEC-L and HPAEC, which was blocked by an anti-RHAMM antibody. These results demonstrate that RHAMM is the functional HA receptor in primary human ECs. Heterogeneity exists among primary human ECs of different vascular origins, with respect to functional HA receptor expression and function.

Regulation of anchorage-dependent signal transduction by protein kinase A and p21-activated kinase

Activation of the canonical mitogen-activated protein kinase (MAPK) cascade by soluble mitogens is blocked in non-adherent cells. It is also blocked in cells in which the cAMP-dependent protein kinase (PKA) is activated. Here we show that inhibition of PKA allows anchorage-independent stimulation of the MAPK cascade by growth factors. This effect is transient, and its duration correlates with sustained tyrosine phosphorylation of paxillin and focal-adhesion kinase (FAK) in non-adherent cells. The effect is sensitive to cytochalasin D, implicating the actin cytoskeleton as an important factor in mediating this anchorage-independent signalling. Interestingly, constitutively active p21-activated kinase (PAK) also allows anchorage-independent MAPK signalling. Furthermore, PKA negatively regulates PAK in vivo, and whereas the induction of anchorage-independent signaling resulting from PKA suppression is blocked by dominant negative PAK, it is markedly prolonged by constitutively active PAK. These observations indicate that PKA and PAK are important regulators of anchorage-dependent signal transduction.

Coupling of PAK-interacting exchange factor PIX to GIT1 promotes focal complex disassembly

The p21-activated kinase PAK is targeted to focal complexes (FCs) through interactions with the SH3 domains of the PAK-interacting exchange factor PIX and Nck. PIX is a Rac GTP exchange factor that also binds the G-protein-coupled receptor kinase-interacting protein known as GIT1. Overexpression of GIT1 in fibroblasts or epithelial cells causes a loss of paxillin from FCs and stimulates cell motility. This is due to the direct interaction of a C-terminal 125-residue domain of GIT1 with paxillin, under the regulation of PIX. In its activated state, GIT1 can promote FC disassembly independent of actin-myosin contractile events. Additionally, GIT directly couples to a key component of FCs, focal adhesion kinase (FAK), via a conserved Spa2 homology domain. We propose that GIT1 and FAK cooperate to promote motility both by directly regulating focal complex dynamics and by the activation of Rac.

Tyrosine phosphorylation of paxillin alpha is involved in temporospatial regulation of paxillin-containing focal adhesion formation and F-actin organization in motile cells

Temporal and spatial regulation of actin-based cytoskeletal organization and focal adhesion formation play an essential role in cell migration. Here, we show that tyrosine phosphorylation of a focal adhesion protein, paxillin, crucially participates in these regulations. We found that tyrosine phosphorylation of paxillin was a prominent event upon integrin activation during epithelial-mesenchymal trans-differentiation and cell migration. Four major tyrosine phosphorylation sites were identified, and two of them were highly inducible upon integrin activation. Paxillin exhibits three distinct subcellular localizations as follows: localization along the cell periphery colocalized with circumferential actin meshworks, macroaggregation at focal adhesions connected to actin stress fibers, and diffuse cytoplasmic distribution. Tyrosine phosphorylation of paxillin localized at the cell periphery and focal adhesions was shown using phosphorylation site-specific antibodies. Mutations in the phosphorylation sites affected the peripheral localization of paxillin and paxillin-containing focal adhesion formation during cell migration and cell-cell collision, accompanied by altered actin organizations. Our analysis indicates that phosphorylation of multiple tyrosines in paxillin alpha is necessary for the proper function of paxillin and is involved in the temporospatial regulation of focal adhesion formation and actin cytoskeletal organization in motile cells.

Pyk2 and FAK regulate neurite outgrowth induced by growth factors and integrins

Integration of signalling pathways initiated by receptor tyrosine kinases and integrins is essential for growth-factor-mediated biological responses. Here we show that co-stimulation of growth-factor receptors and integrins activates the focal-adhesion kinase (FAK) family to promote outgrowth of neurites in PC12 and SH-SY5Y cells. Pyk2 and FAK associate with adhesion-based complexes that contain epidermal growth factor (EGF) receptors, through their carboxy- and amino-terminal domains. Expression of the C-terminal domain of Pyk2 or of FAK is sufficient to block neurite outgrowth, but not activation of extracellular-signal-regulated kinase (ERK). Moreover, activation and autophosphorylation of Pyk2/FAK, as well as of effectors of their adhesion-targeting domains, such as paxillin, are important for propagation of signals that control neurite formation. Thus, Pyk2/FAK have important functions in signal integration proximal to integrin/growth-factor receptor complexes in neurons.

Paxillin binding is not the sole determinant of focal adhesion localization or dominant-negative activity of focal adhesion kinase/focal adhesion kinase-related nonkinase

The carboxy-terminal 150 residues of the focal adhesion kinase (FAK) comprise the focal adhesion-targeting sequence, which is responsible for its subcellular localization. The mechanism of focal adhesion targeting has not been fully elucidated. We describe a mutational analysis of the focal adhesion-targeting sequence of FAK to further examine the mechanism of focal adhesion targeting and explore additional functions encoded by the carboxy-terminus of FAK. The results demonstrate that paxillin binding is dispensable for focal adhesion targeting of FAK. Cell adhesion-dependent tyrosine phosphorylation strictly correlated with the ability of mutants to target to focal adhesions. Focal adhesion targeting was also a requirement for maximal FAK-dependent tyrosine phosphorylation of paxillin and FAK-related nonkinase (FRNK)-dependent inhibition of endogenous FAK function. However, there were additional requirements for these latter functions because we identified mutants that target to focal adhesions, yet are defective for the induction of paxillin phosphorylation or the dominant-negative function of FRNK. Furthermore, the paxillin-binding activity of FRNK mutants did not correlate with their ability to inhibit FAK, suggesting that FRNK has other targets in addition to paxillin.

Calcium signaling induced by adhesion mediates protein tyrosine phosphorylation and is independent of pHi

Our goal was to evaluate early signaling events that occur as epithelial cells make initial contact with a substrate and to correlate them with phosphorylation. The corneal epithelium was chosen to study signaling events that occur with adhesion because it represents a simple system in which the tissue adheres to a basal lamina, is avascular, and is bathed by a tear film in which changes in the local environment are hypothesized to alter signaling. To perform these experiments we developed a novel adhesion assay to capture the changes in intracellular Ca(2+) and pH that occur as a cell makes its initial contact with a substrate. The first transient cytosolic Ca(2+) peak was detected only as the cell made contact with the substrate and was demonstrated using fluorimetric assays combined with live cell imaging. We demonstrated that this transient Ca(2+) peak always preceded a cytoplasmic alkalization. When the intracellular environment was modified, the initial response was altered. Pretreatment with 1,2-bis(o-aminophenoxy)ethane-N,N, N'N'-tetraacetic acid (BAPTA), an intracellular chelator, inhibited Ca(2+) mobilization, whereas benzamil altered the duration of the oscillations. Thapsigargin caused an initial Ca(2+) release followed by a long attenuated response. An inositol triphosphate analog induced a large initial response, whereas heparin inhibited Ca(2+) oscillations. Inhibitors of tyrosine phosphorylation did not alter the initial mobilization of cytosolic Ca(2) but clearance of cytosolic Ca(2+) was inhibited. Exposing corneal epithelial cells to BAPTA, benzamil, or thapsigargin also attenuated the phosphorylation of the focal adhesion protein paxillin. However, although heparin inhibited Ca(2+) oscillations, it did not alter phosphorylation of paxillin. These studies demonstrate that the initial contact that a cell makes with a substrate modulates the intracellular environment, and that changes in Ca(2+) mobilization can alter later signaling events such as the phosphorylation of specific adhesion proteins. These findings may have implications for wound repair and development.

Paxillin localizes to the lymphocyte microtubule organizing center and associates with the microtubule cytoskeleton

Paxillin is a focal adhesion-associated protein that functions as a multi-domain adapter protein, binding several structural and signaling molecules. alpha-Tubulin was identified as an interacting protein in a two-hybrid screen using the paxillin C-terminal LIM domain as a bait. In vitro binding assays with glutathione S-transferase-paxillin demonstrated an interaction of alpha-tubulin with the C terminus of paxillin. Another member of the tubulin family, gamma-tubulin, bound to both the N and the C terminus of paxillin. The interaction between paxillin and both alpha- and gamma-tubulin in vivo was confirmed by co-immunoprecipitation from human T lymphoblasts. Immunofluorescence studies revealed that, in adherent T cells, paxillin localized to sites of cell-matrix interaction as well as to a large perinuclear region. Confocal microscopy revealed that this region corresponds to the lymphocyte microtubule organizing center, where paxillin colocalizes with alpha- and gamma-tubulin. The localization of paxillin to this area was observed in cells in suspension as well as during adhesion to integrin ligands. These data constitute the first characterization of the interaction of paxillin with the microtubule cytoskeleton, and suggest that paxillin, in addition to its well established role at focal adhesions, could also be associated with the lymphocyte microtubule network.