The Rho guanine nucleotide exchange factor ARHGEF5 promotes tumor malignancy via epithelial-mesenchymal transition.

Epithelial tumor cells often acquire malignant properties, such as invasion/metastasis and uncontrolled cell growth, by undergoing epithelial-mesenchymal transition (EMT). However, the mechanisms by which EMT contributes to malignant progression remain elusive. Here we show that the Rho guanine nucleotide exchange factor (GEF) ARHGEF5 promotes tumor malignancy in a manner dependent on EMT status. We previously identified ARHGEF5, a member of the Dbl family of GEFs, as a multifunctional mediator of Src-induced cell invasion and tumor growth. In the present study, ARHGEF5 was upregulated during tumor growth factor-ß-induced EMT in human epithelial MCF10A cells, and promoted cell migration by activating the Rho-ROCK pathway. ARHGEF5 was necessary for the invasive and in vivo metastatic activity of human colorectal cancer HCT116 cells. These findings underscore the crucial role of ARHGEF5 in cell migration and invasion/metastasis. An in vivo tumorigenesis assay revealed that ARHGEF5 had the potential to promote tumor growth via the phosphatidylinositol 3-kinase (PI3K) pathway. However, ARHGEF5 was not required for tumor growth in epithelial-like human colorectal cancer HCT116 and HT29 cells, whereas the growth of mesenchymal-like SW480 and SW620 cells depended on ARHGEF5. Induction of EMT by tumor necrosis factor-α or Slug in HCT116 cells resulted in the dependence of tumor growth on ARHGEF5. In these mesenchymal-like cells, Akt was activated via ARHGEF5 and its activity was required for tumor growth. Analysis of a transcriptome data set revealed that the combination of ARHGEF5 upregulation and E-cadherin downregulation or Snail upregulation was significantly correlated with poor prognosis in patients with colorectal cancers. Taken together, our findings suggest that EMT-induced ARHGEF5 activation contributes to the progression of tumor malignancy. ARHGEF5 may serve as a potential therapeutic target in a subset of malignant tumors that have undergone EMT.

ZEB1 induces EPB41L5 in the cancer mesenchymal program that drives ARF6-based invasion, metastasis and drug resistance.

Onset of the cancer mesenchymal program is closely associated with cancer malignancy and drug resistance. Among the different epithelial-mesenchymal transition (EMT)-associated transcriptional factors, ZEB1 has a key role in inducing the mesenchymal phenotypes and stem cell-like properties of different breast cancer cells. ARF6 and its effector AMAP1 are frequently overexpressed in breast cancer cells, and promote invasion, metastasis and drug resistance. EPB41L5 is induced during EMT, and mediates the disruption of E-cadherin-based cell-cell adhesion and the promotion of focal adhesion dynamics. Here we show that EPB41L5 is an integral component of the ARF6-based pathway, which is induced by ZEB1. We found that EPB41L5 is expressed at high levels in malignant breast cancer cells and binds to AMAP1. ZEB1 induced EPB41L5 both in cancer cells and normal cells. This relationship was recaptured with The Cancer Genome Atlas RNASeq data set, and correlated with the poor outcome of the patients. In contrast, diversified events, such as tumor growth factor ß1 stimulation, expression of SNAI1 and TP53 mutation, can each cause the induction of ZEB1 and EPB41L5, depending on the cellular context. Our results demonstrated that the ZEB1-EPB41L5 axis is at the core of the cancer mesenchymal program that drives ARF6-based invasion, metastasis and drug resistance of significant populations of primary breast cancers, and is tightly correlated with the poor outcomes of patients.

Involvement of EphA2-mediated tyrosine phosphorylation of Shp2 in Shp2-regulated activation of extracellular signal-regulated kinase.

Shp2 is a positive regulator for Erk activation downstream of receptor tyrosine kinases for growth factors. It has been controversial how Shp2 induces Erk activation. We here demonstrate that EphA2 is responsible for Shp2-mediated Erk activation by phosphorylating Tyr542 and Tyr580 of Shp2 in the cells stimulated with growth factors. In NMuMG mammary epithelial cells stimulated with hepatocyte growth factor (HGF), HGF-dependent Erk phosphorylation was prolonged only in the presence of EphA2. This Erk activation paralleled the phosphorylation of Tyr542/580 of Shp2 and the association of Grb2 with Shp2, suggesting the positive signal involving Grb2 signal to activate Ras-Erk pathway. Immunohistochemical studies of mammary cancer specimens revealed that the cancer progression was associated with both Tyr580 phosphorylation of Shp2 and increased expression of EphA2, which were also correlated with increased Erk phosphorylation. Overexpression of either Shp2Thr468Met (a phosphatase-defective mutant found in Lentigines, Electrocardiographic abnormalities, Ocular hypertelorism, Pulmonary stenosis, Abnormal genitalia, Retardation of growth and sensorineural Deafness (LEOPARD) syndrome) or Shp2Asn308Asp (a phosphatase-active mutant found in Noonan syndrome) with EphA2 exhibited comparable activation of Erk and stronger activation than wild-type Shp2, suggesting the phosphatase-independent Erk activation. Expression of Shp2Thr468Met with Tyr542/580Phe mutations resulted in the suppression of Erk activation. Phosphatase-active and -inactive, and wild-type Shp2s bound equally to Grb2, suggesting that phosphorylation of Tyr542/580 of Shp2 was essential but not sufficient for Shp2-mediated Erk activation. We found that Gab1 (Grb2-associated binder 1) was involved in the mutant Shp2-mediated Erk activation. Zebrafish injected with Shp2Thr468Met mRNA showed cardiac edema, whereas those depleted of EphA2b showed less phenotype, suggesting that EphA2 might partly account for the phenotype of LEOPARD syndrome. Collectively, tyrosine phosphorylation of Shp2 by EphA2 contributes to the phosphatase-independent Shp2-mediated activation of Erk and might be involved in Shp2-associated diseases.

A novel muscle LIM-only protein is generated from the paxillin gene locus in Drosophila.

Paxillin is a protein containing four LIM domains, and functions in integrin signaling. We report here that two transcripts are generated from the paxillin gene locus in Drosophila; one encodes a protein homolog of the vertebrate Paxillin (DPxn37), and the other a protein with only three LIM domains, partly encoded by its own specific exon (PDLP). At the myotendinous junctions of Drosophila embryos where integrins play important roles, both DPxn37 and PDLP are highly expressed with different patterns; DPxn37 is predominantly concentrated at the center of the junctions, whereas PDLP is highly enriched at neighboring sides of the junction centers, primarily expressed in the mesodermal myotubes. Northern blot analysis revealed that DPxn37 is ubiquitously expressed throughout the life cycle, whereas PDLP expression exhibits a biphasic pattern during development, largely concomitant with muscle generation and remodeling. Our results collectively reveal that a unique system exists in Drosophila for the generation of a novel type of LIM-only protein, highly expressed in the embryonic musculature, largely utilizing the Paxillin LIM domains.

Different modes and qualities of tyrosine phosphorylation of Fak and Pyk2 during epithelial-mesenchymal transdifferentiation and cell migration: analysis of specific phosphorylation events using site-directed antibodies.

Integrin signaling is activated during epithelial-mesenchymal transdifferentiation (EMT) and cell migration, processes serving as models for carcinogenesis. We have shown that paxillin and p130Cas become highly tyrosine phosphorylated during these processes in NMuMG cells. Here, we examined the regulation of Fak and Pyk2, kinases implicated in this phosphorylation. Pyk2 became phosphorylated at the major autophosphorylation site (Tyr-402) and the potential Grb2-binding site (Tyr-881) during EMT. In contrast, phosphorylation of Fak at the corresponding autophosphorylation site (Tyr-397) occurred even in sedentary epithelial cells, whereas phosphorylation at Tyr-407 and Tyr-861 was induced during EMT. During cell migration, these phosphorylation events, except Fak Tyr-397, were augmented further, and phosphorylation of Fak Tyr-577 and the corresponding Pyk2 Tyr-580, both within the kinase activation loops, was also induced. In all cases, phosphorylation of the putative Grb2-binding site in Fak (Tyr-925) was almost undetectable. Although Fak and Pyk2 have several phosphorylation sites in common, Tyr-407 and Tyr-861 are unique to Fak. Our results revealed that Fak and Pyk2 are non-equivalent in the tyrosine phosphorylation events and thereby likely to evoke different downstream signaling cascades during EMT and cell migration of NMuMG cells. We also show that Fak Tyr-397 phosphorylation occurs exclusively at the cytoplasm, but not at focal contacts, in the sedentary epithelial cells. In contrast, all other tyrosine phosphorylated forms of Fak and Pyk2 are predominantly localized to focal adhesions and the cell periphery in motile cells, all colocalized with paxillin and p130Cas.

PAG3/Papalpha/KIAA0400, a GTPase-activating protein for ADP-ribosylation factor (ARF), regulates ARF6 in Fcgamma receptor-mediated phagocytosis of macrophages.

The Fcgamma receptor (FcgammaR)-mediated phagocytosis of macrophages is a complex process where remodeling of both the actin-based cytoskeleton and plasma membrane occur coordinately. Several different families of small GTPases are involved. We have isolated a GTPase-activating protein (GAP) for ADP-ribosylation factor (ARF), paxillin-associated protein with ARFGAP activity (PAG)3/Papalpha/KIAA0400, from mature monocytes and macrophage-like cells. Mammalian ARFs fall into three classes, and the class III isoform (ARF6) has been shown to be involved in FcgammaR-mediated phagocytosis. Here we report that PAG3 is enriched together with ARF6 and F-actin at phagocytic cups formed beneath immunoglobulin G-opsonized beads in P388D1 macrophages, in which overexpression of ARF6, but not ARF1 (class I) or ARF5 (class II), inhibits the phagocytosis. Overexpression of PAG3, but not its GAP-inactive mutant, attenuated the focal accumulation of F-actin and blocked phagocytosis, although surface levels of the FcgammaRs were not affected. Other ubiquitously expressed ARFGAPs, G protein-coupled receptor kinase interactors GIT2 and GIT2-short/KIAA0148, which we have shown to exhibit GAP activity for ARF1 in COS-7 cells, did not accumulate at the phagocytic cups or inhibit phagocytosis. Moreover, cooverexpression of ARF6, but not ARF1 or ARF5, restored the phagocytic activity of PAG3-overexpressing cells. We propose that PAG3 acts as a GAP for ARF6 and is hence involved in FcgammaR-mediated phagocytosis in mouse macrophages.

An ADP-ribosylation factor GTPase-activating protein Git2-short/KIAA0148 is involved in subcellular localization of paxillin and actin cytoskeletal organization.

Paxillin acts as an adaptor protein in integrin signaling. We have shown that paxillin exists in a relatively large cytoplasmic pool, including perinuclear areas, in addition to focal complexes formed at the cell periphery and focal adhesions formed underneath the cell. Several ADP-ribosylation factor (ARF) GTPase-activating proteins (GAPs; ARFGAPs) have been shown to associate with paxillin. We report here that Git2-short/KIAA0148 exhibits properties of a paxillin-associated ARFGAP and appears to be colocalized with paxillin, primarily at perinuclear areas. A fraction of Git2-short was also localized to actin-rich structures at the cell periphery. Unlike paxillin, however, Git2-short did not accumulate at focal adhesions underneath the cell. Git2-short is a short isoform of Git2, which is highly homologous to p95PKL, another paxillin-binding protein, and showed a weaker binding affinity toward paxillin than that of Git2. The ARFGAP activities of Git2 and Git2-short have been previously demonstrated in vitro, and we provided evidence that at least one ARF isoform, ARF1, is an intracellular substrate for the GAP activity of Git2-short. We also showed that Git2-short could antagonize several known ARF1-mediated phenotypes: overexpression of Git2-short, but not its GAP-inactive mutant, caused the redistribution of Golgi protein beta-COP and reduced the amounts of paxillin-containing focal adhesions and actin stress fibers. Perinuclear localization of paxillin, which was sensitive to ARF inactivation, was also affected by Git2-short overexpression. On the other hand, paxillin localization to focal complexes at the cell periphery was unaffected or even augmented by Git2-short overexpression. Therefore, an ARFGAP protein weakly interacting with paxillin, Git2-short, exhibits pleiotropic functions involving the regulation of Golgi organization, actin cytoskeletal organization, and subcellular localization of paxillin, all of which need to be coordinately regulated during integrin-mediated cell adhesion and intracellular signaling.

Interaction of paxillin with p21-activated Kinase (PAK). Association of paxillin alpha with the kinase-inactive and the Cdc42-activated forms of PAK3.

p21-activated kinases (PAKs) are implicated in integrin signalings, and have been proposed to associate with paxillin indirectly. We show here that paxillin can bind directly to PAK3. We examined several representative focal adhesion proteins, and found that paxillin is the sole protein that associates with PAK3. PAK3 associated with the alpha and beta isoforms of paxillin, but not with gamma. We also show that paxillin alpha associated with both the kinase-inactive and the Cdc42-activated forms of PAK3 in vivo, without affecting the activation states of the kinase. A number of different functions have been ascribed to PAKs; and PAKs can bind directly to growth factor signaling-adaptor molecule, Nck, and a guanine nucleotide exchanger, betaPIX. Our results revealed that paxillin alpha can compete with Nck and betaPIX in the binding of PAK3. Moreover, paxillin alpha can be phosphorylated by PAK3 at serine. Therefore, paxillin alpha, but not gamma, appears to be capable of linking both the kinase-inactive and activated forms of PAK3 to integrins independent of Nck and betaPIX, as Nck links PAK1 to growth factor receptors. Our results also revealed that paxillin is involved in highly complexed protein-protein interactions in integrin signaling.

Meltrin alpha cytoplasmic domain interacts with SH3 domains of Src and Grb2 and is phosphorylated by v-Src.

Meltrin alpha/ADAM12 is a member of the ADAM/MDC family proteins characterized by the presence of metalloprotease and disintegrin domains. This protein also contains a single transmembrane domain and a relatively long cytoplasmic domain containing several proline-rich sequences. These sequences are compatible with the consensus sequences for binding the Src homology 3 (SH3) domains. To determine whether the proline-rich sequences interact with SH3 domains in several proteins, binding of recombinant SH3 domains to the meltrin alpha cytoplasmic domain was analysed by pull-down assays. The SH3 domains of Src and Yes bound strongly, but that of Abl or phosphatidylinositol 3-kinase p85 subunit did not. Full-length Grb2/Ash bound strongly, whereas its N-terminal SH3 domain alone did less strongly. Src and Grb2 in bovine brain extracts also bound to meltrin alpha cytoplasmic domain on affinity resin. Furthermore, immunoprecipitation with a monoclonal antibody to meltrin alpha resulted in coprecipitation of Src and Grb2 with meltrin alpha in cell extracts, suggesting that Src and Grb2 are associated in vivo with meltrin alpha cytoplasmic domain. This notion was also supported by the findings that exogenously expressed meltrin cytoplasmic domain coexisted with Src and Grb2 on the membrane ruffles. The C-terminal Tyr901 of meltrin alpha was phosphorylated both in vitro and in cultured cells by v-Src. These results may imply that meltrin alpha cytoplasmic domain is involved in a signal transduction for some biological function through the interaction with SH3-containing proteins.

Paxillin alpha and Crk-associated substrate exert opposing effects on cell migration and contact inhibition of growth through tyrosine phosphorylation.

Protein tyrosine phosphorylation accompanies and is essential for integrin signaling. We have shown that tyrosine phosphorylation of paxillin alpha and Crk-associated substrate (p130(Cas)) is a prominent event on integrin activation in normal murine mammary gland epithelial cells. Tyrosine phosphorylation of p130(Cas) has been demonstrated to facilitate cell migration. We show here that tyrosine phosphorylation of paxillin alpha acts to reduce haptotactic cell migrations as well as transcellular invasive activities in several different experimental cell systems, whereas tyrosine phosphorylation of p130(Cas) exerts opposing effects to those of paxillin alpha. Each of the phosphorylation-null mutants acts as a dominant negative for each phenotype. Moreover, we found that overexpression of paxillin alpha reduced the cell saturation density of normal murine mammary gland cells, whereas overexpression of p130(Cas) increased it. These effects also seemed to depend on tyrosine phosphorylation events. Cell growth rates and morphologies at growing phases were not significantly altered, nor were cells transformed. Addition of epidermal growth factor increased saturation density of the paxillin alpha-overexpressing cells, whereas no further increment was observed in p130(Cas)-overexpressing cells. We propose that tyrosine phosphorylation of paxillin alpha and p130(Cas) exerts opposing effects on several integrin-mediated cellular events, possibly through different signaling pathways.

Inhibition of cytokinesis by a lipid metabolite, psychosine.

Although a number of cellular components of cytokinesis have been identified, little is known about the detailed mechanisms underlying this process. Here, we report that the lipid metabolite psychosine (galactosylsphingosine), derived from galactosylceramide, induced formation of multinuclear cells from a variety of nonadherent and adherent cells due to inhibition of cytokinesis. When psychosine was added to the human myelomonocyte cell line U937, which was the most sensitive among the cell lines tested, cleavage furrow formed either incompletely or almost completely. However, abnormal contractile movement was detected in which the cellular contents of one of the hemispheres of the contracting cell were transferred into its counterpart. Finally, the cleavage furrow disappeared and cytokinesis was reversed. Psychosine treatment also induced giant clots of actin filaments in the cells that probably consisted of small vacuoles with filamentous structures, suggesting that psychosine affected actin reorganization. These observations could account for the formation of multinuclear globoid cells in the brains of patients with globoid cell leukodystrophy, a neurological disorder characterized by the accumulation of psychosine due to galactosylceramidase deficiency.

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.

A new paxillin-binding protein, PAG3/Papalpha/KIAA0400, bearing an ADP-ribosylation factor GTPase-activating protein activity, is involved in paxillin recruitment to focal adhesions and cell migration.

Paxillin acts as an adaptor molecule in integrin signaling. Paxillin is localized to focal contacts but seems to also exist in a relatively large cytoplasmic pool. Here, we report the identification of a new paxillin-binding protein, PAG3 (paxillin-associated protein with ADP-ribosylation factor [ARF] GTPase-activating protein [GAP] activity, number 3), which is involved in regulation of the subcellular localization of paxillin. PAG3 bound to all paxillin isoforms and was induced during monocyte maturation, at which time paxillin expression is also increased and integrins are activated. PAG3 was diffusely distributed in the cytoplasm in premature monocytes but became localized at cell periphery in mature monocytes, a fraction of which then colocalized with paxillin. PAG3, on the other hand, did not accumulate at focal adhesion plaques, suggesting that PAG3 is not an integrin assembly protein. PAG3 was identical to KIAA0400/Papalpha, which was previously identified as a Pyk2-binding protein bearing a GAP activity toward several ARFs in vitro. Mammalian ARFs fall into three classes, and we showed that all classes could affect subcellular localization of paxillin. We also examined possible interaction of PAG3 with ARFs and showed evidence that at least one of them, ARF6, seems to be an intracellular substrate for GAP activity of PAG3. Moreover, overexpression of PAG3, but not its GAP-inactive mutant, inhibited paxillin recruitment to focal contacts and hampered cell migratory activities, whereas cell adhesion activities were almost unaffected. Therefore, our results demonstrate that paxillin recruitment to focal adhesions is not mediated by simple cytoplasmic diffusion; rather, PAG3 appears to be involved in this process, possibly through its GAP activity toward ARF proteins. Our result thus delineates a new aspect of regulation of cell migratory activities.

A truncated isoform of the PP2A B56 subunit promotes cell motility through paxillin phosphorylation.

Both F10 and BL6 sublines of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells are metastatic after subcutaneous injection. Retrotransposon insertion was found to produce an N-terminally truncated form (Deltagamma1) of the B56gamma1 regulatory subunit isoform of protein phosphatase (PP) 2A in BL6 cells, but not in F10 cells. We found an interaction of paxillin with PP2A C and B56gamma subunits by co-immunoprecipitation. B56gamma1 co-localized with paxillin at focal adhesions, suggesting a role for this isoform in targeting PP2A to paxillin. In this regard, Deltagamma1 behaved similarly to B56gamma1. However, the Deltagamma1-containing PP2A heterotrimer was insufficient for the dephosphorylation of paxillin. Transfection with Deltagamma1 enhanced paxillin phosphorylation on serine residues and recruitment into focal adhesions, and cell spreading with an actin network. In addition, Deltagamma1 rendered F10 cells as highly metastatic as BL6 cells. These results suggest that mutations in PP2A regulatory subunits may cause malignant progression.

Tyrosine phosphorylation and subcellular localization of focal adhesion proteins during in vitro decidualization of human endometrial stromal cells.

Human endometrial stromal cells undergo in vitro decidualization when treated with progesterone and estrogen. Using this model, we previously reported specific changes in the c-Src kinase activity and tyrosine phosphorylation of several proteins during in vitro decidualization. Focal adhesion kinase (FAK) and paxillin are known to form a complex with c-Src at the focal contacts and to participate in the integrin-mediated signal transduction as c-Src substrates. We here examined the tyrosine phosphorylation and subcellular localization of the focal adhesion proteins in stromal cells isolated from human endometrium. We found, however, that the total levels of FAK and paxillin tyrosine phosphorylation were not markedly changed during decidualization or after steroid withdrawal. In our culture system numerous multicellular nodules were developed in cultures of decidualized stromal cells, within whose nodules the focal contacts were found to disappear. Moreover, disruption of the focal contacts was accompanied by disorganization of the actin-based cytoskeleton. These findings suggest that tyrosine phosphorylation of the endometrial paxillin and FAK is not tightly regulated by the kinase activity of c-Src during in vitro decidualization. The escape from regulation by c-Src may be in part due to the dissociation of the focal adhesion proteins/c-Src complex caused by the breakdown of the focal adhesion plaques as well as the loss of the actin-based cytoskeletal architecture.

Transglutaminase type 1 and its cross-linking activity are concentrated at adherens junctions in simple epithelial cells.

Transglutaminase type 1 was identified as a tyrosine-phosphorylated protein from the isolated junctional fraction of the mouse liver. This enzyme was reported to be involved in the covalent cross-linking of proteins in keratinocytes, but its expression and activity in other cell types have not been examined. Northern blotting revealed that transglutaminase type 1 was expressed in large amounts in epithelial tissues (lung, liver, and kidney), which was also confirmed by immunoblotting with antibodies raised against mouse recombinant protein. Immunoblotting of the isolated junctional fraction revealed that transglutaminase type 1 was concentrated in the fraction not only as a 97-kDa form but also as forms of various molecular masses cross-linked to other proteins. In agreement with this finding, endogenous transglutaminase type 1 was immunofluorescently colocalized with E-cadherin in cultured simple epithelial cells. In the liver and kidney, immunoelectron microscopy revealed that transglutaminase type 1 was concentrated, albeit not exclusively, at cadherin-based adherens junctions. Furthermore, by in vitro and in vivo labeling, transglutaminase cross-linking activity was also shown to be concentrated at intercellular junctions of simple epithelial cells. These findings suggested that the formation of covalently cross-linked multimolecular complexes by transglutaminase type 1 is an important mechanism for maintenance of the structural integrity of simple epithelial cells, especially at cadherin-based adherens junctions.

Hypoxia induces activation and subcellular translocation of focal adhesion kinase (p125(FAK)) in cultured rat cardiac myocytes.

We previously reported that hypoxia caused rapid activation of RAS/mitogen-activated protein kinase (MAPK) pathway, two other stress-activated MAPK family members, stress-activated protein kinase (SAPK) and p38MAPK, and Src family tyrosine kinases, p60(c-src) and p59(c-fyn) in cultured rat cardiac myocytes. In this study, to elucidate how hypoxia affects adhesive interaction between cardiac myocytes and extracellular matrix (ECM), we investigated the molecular mechanism of the activation of focal adhesion-associated tyrosine kinases p125(FAK) and paxillin. Here, we show that hypoxia induced tyrosine phosphorylation of p125(FAK) and paxillin and that hypoxia-induced activation of p125(FAK) was accompanied by its increased association with adapter proteins Shc and GRB2, and non-receptor type tyrosine kinase p60(c-src). Furthermore, hypoxia caused subcellular translocation of p125(FAK) from perinuclear sites to the focal adhesions. These results strongly suggest that p125(FAK) is one of the most important components in hypoxia-induced intracellular signaling in cardiac myocytes and may play a pivotal role in adhesive interaction between cardiac myocytes and ECM.

Activation of c-Src kinase is associated with in vitro decidualization of human endometrial stromal cells.

Tyrosine phosphorylation of cellular proteins, controlled coordinately by tyrosine kinases and phosphatases, is a critical element in signal transduction pathways involved in the regulation of biological responses including cell growth and differentiation. Decidualization is a dramatic progesterone-induced differentiation of the estrogen-primed endometrium, which is crucial for embryo implantation and maintenance of pregnancy. Here we have shown that the kinase activity of c-Src was increased, accompanied by altered tyrosine phosphorylation of several cellular proteins, during in vitro decidualization of human endometrial stromal cells. Withdrawal of both estrogen and progesterone from the cultures of decidualized stromal cells reduced c-Src kinase activity to the basal level and also changed the pattern of tyrosine phosphorylation of the several cellular proteins to the unstimulated state. The kinase activity of endometrial c-Src appeared to inversely correlate with the level of its tyrosine phosphorylation. Moreover, although the endometrial stromal cells expressed another src-family kinase, Fyn, the activity of the Fyn kinase was almost undetectable during decidualization and thereafter upon steroid withdrawal. Our findings suggest that the activation of c-Src kinase may be a normal physiological event associated with decidualization, being specifically involved in the signaling cascades mediated by ovarian hormone stimulation.

Vascular endothelial growth factor induces activation and subcellular translocation of focal adhesion kinase (p125FAK) in cultured rat cardiac myocytes.

Vascular endothelial growth factor (VEGF) has been proposed to be among the candidate factors with the most potential to play a role in ischemia-induced collateral vessel formation. Recently, we found that VEGF activated the mitogen-activated protein kinase cascade in cultured rat cardiac myocytes. To elucidate how VEGF affects adhesive interaction of cardiac myocytes with the extracellular matrix (ECM), one of the important cell functions, we investigated the molecular mechanism of activation of focal adhesion-related proteins, especially focal adhesion kinase (p125(FAK)), in cultured rat cardiac myocytes. We found that the 2 VEGF receptors, KDR/Flk-1 and Flt-1, were expressed in cardiac myocytes and that KDR/Flk-1 was significantly tyrosine phosphorylated on VEGF stimulation. VEGF induced tyrosine phosphorylation and activation of p125(FAK) as well as tyrosine phosphorylation of paxillin; this was accompanied by subcellular translocation of p125(FAK) from perinuclear sites to the focal adhesions. This VEGF-induced activation of p125(FAK) was inhibited partially by the tyrosine kinase inhibitors genistein and tyrphostin. Activation of p125(FAK) was accompanied by its increased association with adapter proteins GRB2, Shc, and nonreceptor type tyrosine kinase p60(c-src). Furthermore, we confirmed that VEGF induced a significant increase in adhesive interaction between cardiac myocytes and ECM using an electric cell-substrate impedance sensor. These results strongly suggest that p125(FAK) is one of the most important components in VEGF-induced signaling in cardiac myocytes, playing a critical role in adhesive interaction between cardiac myocytes and ECM.

The LIM domains of hic-5 protein recognize specific DNA fragments in a zinc-dependent manner in vitro.

hic-5 protein is a member of the LIM protein family, containing four LIM domains in its C-terminal region. It is mainly localized in focal adhesions and shows striking similarity to paxillin in its LIM domains, although the function of these LIM domains has remained elusive. In the present study, we found that full-length and the C-terminal half of hic-5 protein, including four LIM domains, bound to DNA in a zinc-dependent manner in vitro . Mouse genomic fragments that specifically bound to the hic-5 protein were isolated by successive rounds of hic-5 protein-DNA complex immunoprecipitation and PCR amplification. Seven independent clones were isolated, which contained high amounts of G+A and/or a long A/T tract. A DNA binding protein blot assay revealed the specificity of the interaction between hic-5 protein and the DNA fragment. Using a series of truncated forms of the hic-5 LIM domains, each of the four LIM domains was found to contribute to DNA binding in a distinctive manner.