Focal adhesions require catalytic activity of Src family kinases to mediate integrin-matrix adhesion

RACK1-mediated integration of adhesion and insulin-like growth factor I (IGF-I) signaling and cell migration are defective in cells expressing an IGF-I receptor mutated at tyrosines 1250 and 1251

The scaffolding protein receptor for activated C kinase (RACK1) has been proposed to mediate the integration of insulin-like growth factor I receptor (IGF-IR) and adhesion signaling. Here we investigated the mechanism of this integration of signaling, by using an IGF-IR mutant (Y1250F/Y1251F) that is deficient in anti-apoptotic and transforming function. RACK1 was found to associate with the IGF-IR only in adherent cells and did not associate with the IGF-IR in nonadherent cells, lymphocytic cells, or cells expressing the Y1250F/Y1251F mutant. In R- cells transiently expressing the Y1250F/Y1251F mutant RACK1 became constitutively associated with beta1 integrin and did not associate with Shc, Src, or Shp2. This was accompanied by the loss of formation of a complex containing the IGF-IR, RACK1, and beta1 integrin; loss of migratory capacity; enhanced Src and FAK activity; enhanced Akt phosphorylation; and decreased p38 mitogen-activated protein kinase activity. Shc was not phosphorylated in response to IGF-I in cells expressing the Y1250F/Y1251F mutant and remained associated with protein phosphatase 2A. Similar alterations in signaling were observed in cells that were stimulated with IGF-I in nonadherent cultures. Our data suggest that disruption of RACK1 scaffolding function in cells expressing the Y1250F/Y1251F mutant results in the loss of adhesion signals that are necessary to regulate Akt activity and to promote turnover of focal adhesions and cell migration.

Activation of a signaling cascade by cytoskeleton stretch

Cells sense and respond to mechanical force. However, the mechanisms of transduction of extracellular matrix (ECM) forces to biochemical signals are not known. After removing the cell membrane and soluble proteins by Triton X-100 extraction, we found that the remaining complex (Triton cytoskeletons) activated Rap1 upon stretch. Rap1 guanine nucleotide exchange factor, C3G, was required for this activation; C3G as well as the adaptor protein, CrkII, in cell extract bound to Triton cytoskeletons in a stretch-dependent manner. CrkII binding, which was Cas dependent, correlated with stretch-dependent tyrosine phosphorylation of proteins in Triton cytoskeletons including Cas at the contacts with ECM. These in vitro findings were compatible with in vivo observations of stretch-enhanced phosphotyrosine signals, accumulation of CrkII at cell-ECM contacts, and CrkII-Cas colocalization. We suggest that mechanical force on Triton cytoskeletons activates local tyrosine phosphorylation, which provides docking sites for cytosolic proteins, and initiates signaling to activate Rap1.

Focal adhesion regulation of cell behavior

Focal adhesions lie at the convergence of integrin adhesion, signaling and the actin cytoskeleton. Cells modify focal adhesions in response to changes in the molecular composition, two-dimensional (2D) vs. three-dimensional (3D) structure, and physical forces present in their extracellular matrix environment. We consider here how cells use focal adhesions to regulate signaling complexes and integrin function. Furthermore, we examine how this regulation controls complex cellular behaviors in response to matrices of diverse physical and biochemical properties. One event regulated by the physical structure of the ECM is phosphorylation of focal adhesion kinase (FAK) at Y397, which couples FAK to several signaling pathways that regulate cell proliferation, survival, migration, and invasion.

kette and blown fuse interact genetically during the second fusion step of myogenesis in Drosophila

Drosophila myoblast fusion proceeds in two steps. The first one gives rise to small syncytia, the muscle precursor cells, which then recruit further fusion competent myoblasts to reach the final muscle size. We have identified Kette as an essential component for myoblast fusion. In kette mutants, founder cells and fusion-competent myoblasts are determined correctly and overcome the very first fusion. But then, at the precursor cell stage, fusion is interrupted. At the ultrastructural level, fusion is characterised by cell-cell recognition, alignment, formation of prefusion complexes, electron dense plaques and membrane breakdown. In kette mutants, electron dense plaques of aberrant length accumulate and fusion is interrupted owing to a complete failure of membrane breakdown. Furthermore, we show that kette interacts genetically with blown fuse (blow) which is known to be required to proceed from prefusion complexes to the formation of the electron dense plaques. Interestingly, a surplus of Kette can replace Blow function during myogenesis. We propose a model in which Dumbfounded/Sticks and stones-dependent cell adhesion is mediated over Rolling Pebbles, Myoblast city, Crk, Blown fuse and Kette, and thus induces membrane fusion.

SHEP1 Function in Cell Migration Is Impaired by a Single Amino Acid Mutation That Disrupts Association with the Scaffolding Protein Cas but Not with Ras GTPases

SHEP1 is a signaling protein that contains a guanine nucleotide exchange factor-like domain, which binds Ras family GTPases and also forms a stable complex with the scaffolding protein Crk-associated substrate (Cas). SHEP1 and Cas have several common functions, such as increasing c-Jun N-terminal kinase activity, promoting T cell activation, and regulating the actin cytoskeleton. However, it is unclear whether a physical association between SHEP1 and Cas is required for these activities. We reported previously that SHEP1 is tyrosine-phosphorylated downstream of the EphB2 receptor; in this study, we further demonstrate that activated EphB2 inhibits SHEP1 association with Cas. To investigate whether phosphorylation negatively regulates the SHEP1-Cas complex, we have identified by mass spectrometry several SHEP1 tyrosine phosphorylation sites downstream of EphB2; of particular interest among them is tyrosine 635 in the Cas association/exchange factor domain. Mutation of this tyrosine to glutamic acid, but not to phenylalanine, disrupts Cas binding to SHEP1 without inhibiting Ras GTPase binding. The glutamic acid mutation also makes SHEP1 unable to promote Cas-Crk association, membrane ruffling, and cell migration toward epidermal growth factor (EGF), implying that these activities of SHEP1 depend upon a physical interaction with Cas. Association with Cas also seems to be necessary for EGF-induced SHEP1 tyrosine phosphorylation, which is mediated by a Src family kinase. It is noteworthy that EGF stimulation does not cause dissociation of SHEP1 from Cas. These data show that SHEP1 regulates membrane ruffling and cell migration and that binding to Cas is probably critical for these functions. Furthermore, the SHEP1-Cas complex may have different roles downstream of EphB2 and the EGF receptor.

Phosphorylated guanine nucleotide exchange factor C3G, induced by pervanadate and Src family kinases localizes to the Golgi and subcortical actin cytoskeleton

BACKGROUND

The guanine nucleotide exchange factor C3G (RapGEF1) along with its effector proteins participates in signaling pathways that regulate eukaryotic cell proliferation, adhesion, apoptosis and embryonic development. It activates Rap1, Rap2 and R-Ras members of the Ras family of GTPases. C3G is activated upon phosphorylation at tyrosine 504 and therefore, determining the localization of phosphorylated C3G would provide an insight into its site of action in the cellular context.

RESULTS

C3G is phosphorylated in vivo on Y504 upon coexpression with Src or Hck, two members of the Src family tyrosine kinases. Here we have determined the subcellular localization of this protein using antibodies specific to C3G and Tyr 504 phosphorylated C3G (pY504 C3G). While exogenously expressed C3G was present mostly in the cytosol, pY504 C3G formed upon Hck or Src coexpression localized predominantly at the cell membrane and the Golgi complex. Tyrosine 504-phosphorylated C3G showed colocalization with Hck and Src. Treatment of Hck and C3G transfected cells with pervanadate showed an increase in the cytosolic staining of pY504 C3G suggesting that tyrosine phosphatases may be involved in dephosphorylating cytosolic phospho-C3G. Expression of Src family kinases or treatment of cells with pervanadate resulted in an increase in endogenous pY504 C3G, which was localized predominantly at the Golgi and the cell periphery. Endogenous pY504 C3G at the cell periphery colocalized with F-actin suggesting its presence at the subcortical actin cytoskeleton. Disruption of actin cytoskeleton by cytochalasin D abolished phospho-C3G staining at the periphery of the cell without affecting its Golgi localization.

CONCLUSIONS

These findings show that tyrosine kinases involved in phosphorylation of C3G are responsible for regulation of its localization in a cellular context. We have demonstrated the localization of endogenous C3G modified by tyrosine phosphorylation to defined subcellular domains where it may be responsible for restricted activation of signaling pathways.

C3G-mediated suppression of oncogene-induced focus formation in fibroblasts involves inhibition of ERK activation, cyclin A expression and alterations of anchorage-independent growth

We showed previously that exogenous overexpression of C3G, a guanine nucleotide releasing factor (GEF) for Rap1 and R-Ras proteins, blocks the focus-forming activity of cotransfected, activated, sis, ras and v-raf oncogenes in NIH 3T3 cells. In this report, we show that C3G also interferes with dbl and R-Ras focus-forming activity and demonstrate that the transformation suppressor ability of C3G maps to its Crk-binding region (SH3-b domain). Using full-length C3G and C3GDeltaCat mutant, lacking catalytic domain, we showed here that overexpression of cotransfected C3G or C3GDeltaCat inhibited oncogenic Hraslys12-mediated phosphorylation of ERK, without altering Ras and Raf-1 kinase activation. We also showed that, overexpressed C3G and C3GdeltaCat inhibited the viability of oncogenic Ras-induced colonies in soft agar, indicating that C3G interferes with the anchorage-independent growth of Ras-transformed cells in a Rap1-independent manner. Consistent with both observations, overexpression of exogenous C3G and C3GDeltaCat also caused downregulation of Ras-induced cyclin A expression. Altogether, our results indicate that C3G interferes with at least two separate aspects of oncogenic transformation - cell cycle progression and loss of contact inhibition - and that these inhibitory effects probably account for its transformation suppressor activity.

Activation of a Dab1/CrkL/C3G/Rap1 pathway in Reelin-stimulated neurons

During brain development, many neurons migrate long distances before settling and differentiating. These migrations are coordinated to ensure normal development. The secreted protein Reelin controls the locations of many types of neurons, and its absence causes the classic "Reeler" phenotype. Reelin action requires tyrosine phosphorylation of the intracellular protein Dab1 by Src-family kinases. However, little is known about signaling pathways downstream of Dab1. Here, we identify several proteins in embryonic brain extract that bind to tyrosine-phosphorylated, but not non-phosphorylated, Dab1. Of these, the Crk-family proteins (CrkL, CrkI, and CrkII ), bind significant quantities of Dab1 when embryonic cortical neurons are exposed to Reelin. CrkL binding to Dab1 involves two tyrosine phosphorylation sites, Y220 and 232, that are critical for proper positioning of migrating cortical plate neurons. CrkL also binds C3G, an exchange factor (GEF) for the small GTPase Rap1 that is activated in other systems by tyrosine phosphorylation. We report that Reelin stimulates tyrosine phosphorylation of C3G and activates Rap1. C3G and Rap1 regulate adhesion of fibroblasts and other cell types. Regulation of Crk/CrkL, C3G, and Rap1 by Reelin may be involved in coordinating neuron migrations during brain development.

TGF-beta1 (transforming growth factor-beta1)-mediated adhesion of gastric carcinoma cells involves a decrease in Ras/ERKs (extracellular-signal-regulated kinases) cascade activity dependent on c-Src activity

Signalling by integrin-mediated cell anchorage to extracellular matrix proteins is co-operative with other receptor-mediated signalling pathways to regulate cell adhesion, spreading, proliferation, survival, migration, differentiation and gene expression. It was observed that an anchorage-independent gastric carcinoma cell line (SNU16) became adherent on TGF-beta1 (transforming growth factor beta1) treatment. To understand how a signal cross-talk between integrin and TGF-beta1 pathways forms the basis for TGF-beta1 effects, cell adhesion and signalling activities were studied using an adherent subline (SNU16Ad, an adherent variant cell line derived from SNU16) derived from the SNU16 cells. SNU16 and SNU16Ad cells, but not integrin alpha5-expressing SNU16 cells, showed an increase in adhesion on extracellular matrix proteins after TGF-beta1 treatment. This increase was shown to be mediated by an integrin alpha3 subunit, which was up-regulated in adherent SNU16Ad cells and in TGF-beta1-treated SNU16 cells, compared with the parental SNU16 cells. After TGF-beta1 treatment of SNU16Ad cells on fibronectin, Tyr-416 phosphorylation of c-Src was increased, but Ras-GTP loading and ERK1/ERK2 (extracellular-signal-regulated kinases 1 and 2) activity were decreased, which showed a dependence on c-Src family kinase activity. Studies on adhesion and signalling activities using pharmacological inhibitors or by transient-transfection approaches showed that inhibition of ERK1/ERK2 activity increased TGF-beta1-mediated cell adhesion slightly, but not the basal cell adhesion significantly, and that c-Src family kinase activity and decrease in Ras/ERKs cascade activity were required for the TGF-beta1 effects. Altogether, the present study indicates that TGF-beta1 treatment causes anchorage-independent gastric carcinoma cells to adhere by an increase in integrin alpha3 level and a c-Src family kinase activity-dependent decrease in Ras/ERKs cascade activity.

Urokinase-type plasminogen activator receptor is involved in mediating the apoptotic effect of cleaved high molecular weight kininogen in human endothelial cells

Cleaved high molecular weight kininogen (HKa) has been shown to inhibit in vivo neovascularization and induce apoptosis of endothelial cells. We have shown that HKa-induced apoptosis correlated with its antiadhesive effect and was regulated by extracellular matrix (ECM) proteins. In this study, we identified the urokinase-type plasminogen activator receptor (uPAR) as a target of HKa activity at the endothelial cell surface. Anti-uPAR antibodies blocked the apoptotic effect of HKa. Further studies revealed that uPAR formed a signaling complex containing integrin alpha(v)beta3 or alpha5beta1, caveolin, and Src kinase Yes in endothelial cells. HKa physically disrupted the formation of this complex in a manner that paralleled its apoptotic effect. For the first time, our results provide a mechanistic explanation for the previous observation that HKa selectively induces apoptosis of endothelial cells grown on vitronectin, but not cells grown on fibronectin. These data also resolve the controversial role of uPAR in mediating the apoptotic and antiadhesive activities of HKa.

Temporal dissection of beta1-integrin signaling indicates a role for p130Cas-Crk in filopodia formation

Invasin-promoted spreading of beta1-integrin-deficient cells, transfected with the beta1A- or beta1B-integrin splice variants, were used to dissect early beta1-integrin signaling events. The beta1B isoform, which has a different membrane-distal part of the cytoplasmic tail from beta1A, is defective in signaling and function. When plated on surfaces coated with the high affinity ligand invasin, beta1B-integrin-expressing cells spread by forming filopodia with distinct adhesive phosphotyrosine complexes at the tips, without signs of lamellipodia. This suggested that the beta1B-integrin mediated a partial signaling sufficient for formation of filopodia but insufficient for lamellipodia formation. When screening for proteins present in the distal filopodial phosphotyrosine complexes of beta1B cells, p130Cas and the filopodia proteins vasodilator-stimulated phosphoprotein and talin were found, whereas the typical focal complex proteins focal adhesion kinase, paxillin, and vinculin were not. Invasin-promoted adhesion induced complex formation of p130Cas and the adapter Crk. Moreover, Crk together with Dock180 were present at the filopodial tips of beta1B-integrin-expressing cells, and there was a prominent Rac1 activation. Expression of dominant negative variants of p130Cas or CrkII blocked beta1B-integrin-mediated filopodia formation, indicating that this signaling scaffold is central in this process.

Enhanced adhesion of monocytes via reverse signaling triggered by decoy receptor 3

Decoy receptor 3 (DcR3), a newly identified soluble protein belonging to the tumor necrosis factor receptor (TNFR) superfamily, is a receptor for Fas ligand (FasL), LIGHT and TL1A. It has been demonstrated that DcR3 is frequently overexpressed by malignant tumors arising from lung, gastrointestinal tract, neuronal glia and virus-associated leukemia. Recently, we demonstrated that DcR3 is able to modulate the differentiation and activation of dendritic cells (DCs), and that DcR3-treated DCs skew naive T cell differentiation towards a Th2 phenotype. In this study, we further demonstrate that DcR3 is able to induce actin reorganization and enhance the adhesion of monocytes and THP-1 cells by activating multiple signaling molecules, such as protein kinase C (PKC), phosphatidylinositol 3-kinase (PI3K), focal adhesion kinase (FAK) and Src kinases. This provides the first evidence that the soluble DcR3, like other immobilized members of TNFR superfamily, is able to trigger 'reverse signaling' to modulate cell function.

Oxidative stress activates both Src-kinases and their negative regulator Csk and induces phosphorylation of two targeting proteins for Csk: caveolin-1 and paxillin

Csk negatively regulates Src family kinases (SFKs). In lymphocytes, Csk is constitutively active, and is transiently inactivated in response to extracellular stimuli, allowing activation of SFKs. In contrast, both SFKs and Csk were inactive in unstimulated mouse embryonic fibroblasts, and both were activated in response to oxidative stress. Csk modulated the oxidative stress-induced, but not the basal SFK activity in these cells. These data indicate that Csk may be more important for the return of Src-kinases to the basal state than for the maintenance of basal activity in some cell types. Csk must be targeted to its SFK substrates through an SH2-domain-mediated interaction with a phosphoprotein. Our data indicate that caveolin-1 is one of these targeting proteins. SFKs bind to caveolin-1 and phosphorylate it in response to oxidative stress and insulin. Csk binds specifically to the phosphorylated caveolin-1 and attenuates its stress-induced phosphorylation. Importantly, phosphocaveolin was one of two major phosphoproteins associated with Csk after incubation with peroxide or insulin. Paxillin was the other. Activation/rapid attenuation of SFKs by Csk is required for actin remodeling. Caveolin-1 is phosphorylated at the ends of actin fibers at points of contact between the actin cytoskeleton and the plasma membrane, where it could in part mediate this attenuation.

Apoptotic cues from the extracellular matrix: regulators of angiogenesis

A variety of factors cooperate to regulate neovessel formation and persistence. Proangiogenic growth factors have remained an area of intense interest due to their capacity to promote endothelial cell (EC) proliferation and to initiate the angiogenic program. These growth factors are associated with increased cell survival, yet paradoxically, angiogenic ECs are more susceptible to apoptosis than quiescent ECs. Survival is regulated by cooperation between growth factor receptors and integrins, which are in turn governed by the composition of the local extracellular matrix (ECM). Integrin-mediated signaling is altered or disrupted by the presence of soluble, rather than matrix-bound ligands, thus providing a means by which ECM remodeling can influence both integrin- and growth factor-mediated events. Ultimately, the collaboration of these factors determines whether ECs survive or die, thereby regulating neovascularization.

Synergistic promotion of c-Src activation and cell migration by Cas and AND-34/BCAR3

The adapter molecule p130Cas (Cas) plays a role in cellular processes such as proliferation, survival, cell adhesion, and migration. The ability of Cas to promote migration has been shown to be dependent upon its carboxyl terminus, which contains a bipartite binding site for the protein tyrosine kinase c-Src (Src). The association between Src and Cas enhances Src kinase activity, and like Cas, Src plays an important role in cell proliferation and migration. In this study, we show that Src and Cas function cooperatively to promote cell migration in a manner that depends upon kinase-active Src. Another carboxyl-terminal binding partner of Cas, AND-34/BCAR3 (AND-34), functions synergistically with Cas to enhance Src activation and cell migration. The carboxyl-terminal guanine nucleotide exchange factor domain of AND-34, as well as the activity of its putative target Rap1, contribute to these events. A mechanism through which AND-34 may regulate Cas-dependent cell migration is suggested by the finding that Cas becomes redistributed from focal adhesions to lamellipodia located at the leading edge of AND-34 overexpressing cells. These data thus provide insight into how Cas and AND-34 may function together to stimulate Src signaling pathways and promote cell migration.

Src-CrkII-C3G-dependent activation of Rap1 switches growth hormone-stimulated p44/42 MAP kinase and JNK/SAPK activities

We demonstrate here that growth hormone (GH) stimulates the activation of Rap1 and Rap2 in NIH-3T3 cells. Full activation of Rap1 and Rap2 by GH necessitated the combined activity of both JAK2 and c-Src kinases, although c-Src was predominantly required. GH-stimulated Rap1 and Rap2 activity was also demonstrated to be CrkII-C3G-dependent. GH stimulated the tyrosine phosphorylation of C3G, which again required the combined activity of JAK2 and c-Src. C3G tyrosine residue 504 was required for GH-stimulated Rap activation. Activated Rap1 inhibited GH-stimulated activation of RalA and subsequent GH-stimulated p44/42 MAP kinase activity and Elk-1-mediated transcription. In addition, we demonstrated that C3G-Rap1 mediated CrkII enhancement of GH-stimulated JNK/SAPK activity. We have therefore identified a linear JAK2-independent pathway switching GH-stimulated p44/42 MAP kinase and JNK/SAPK activities.

Does Rap1 deserve a bad Rap?

The Ras superfamily of small G proteins is remarkable for both its diversity and physiological functions. One member, Rap1, has been implicated in a particularly wide range of biological processes, from cell proliferation and differentiation to cell adhesion. But the diversity of Rap1 has lead to contradictory reports of its effects. Originally identified as an antagonist of Ras-induced transformation, Rap1 can oppose other actions of Ras including regulation of cell growth and differentiation, integrin-dependent responses and synaptic plasticity. Furthermore, recent evidence confirms that Rap1, like Ras, can activate the MAP kinase cascade (ERK) in several cell types. These diverse functions of Rap1 underscore that the activation and action of Rap1 are regulated by complex factors that are cell-type specific.

RPTP-alpha acts as a transducer of mechanical force on alphav/beta3-integrin-cytoskeleton linkages

Cell motility on ECM critically depends on the cellular response to force from the matrix. We find that force-dependent reinforcement of alphav/beta3-integrin-mediated cell-matrix connections requires the receptor-like tyrosine phosphatase alpha (RPTPalpha). RPTPalpha colocalizes with alphav-integrins at the leading edge during early spreading, and coimmunoprecipitates with alphav-integrins during spreading on fibronectin and vitronectin. RPTPalpha-dependent activation of Src family kinases, in particular activation of Fyn, is required for the force-dependent formation of focal complexes and strengthening of alphav/beta3-integrin-cytoskeleton connections during the initial phase of ECM contact. These observations indicate that Src family kinases have distinct functions during adhesion site assembly, and that RPTPalpha is an early component in force-dependent signal transduction pathways leading to the assembly of focal complexes on both fibronectin and vitronectin.

Csk regulates integrin-mediated signals: involvement of differential activation of ERK and Akt

Csk phosphorylates Src family tyrosine kinases and down-regulates their activities in vitro and in vivo. To gain insight into the integrin-mediated cellular functions of this negative regulator of the Src family, we examined integrin-mediated signals in Csk-deficient fibroblasts (Csk(-) cells) and their stable transfectants expressing re-introduced Csk (Csk(-)/Csk cells). Integrin-mediated activation of extracellular signal-regulated kinase/mitogen-activated protein (ERK/MAP) kinase in Csk(-)/Csk cells upon adhesion to fibronectin or laminin-10/11 was down-regulated, whereas Akt activation increased. Interestingly, the suppression of ERK-MAP kinase activation in Csk(-)/Csk cells was restored by overexpression of a dominant-negative Akt. In agreement with these results, Csk(-)/Csk cells were more resistant to apoptosis induced by serum depletion, but were less proliferative, compared with Csk(-) cells. These results, taken together, demonstrate that Csk is an important regulator of integrin-mediated signaling and cellular behavior.

Translocation of CrkL to focal adhesions mediates integrin-induced migration downstream of Src family kinases

The adapter protein Crk-Like (CrkL) can associate with the Src substrate p130(Cas) (Cas). The biological role of CrkL downstream of Cas, however, has been largely obscure. Consistent with the ability of CrkL to biochemically associate with Cas, we found that Src triggers translocation of CrkL to focal adhesions (FAs) in a manner dependent on Cas. Forced localization of CRKL to FAs (FA-CRKL) by itself was sufficient to induce activation of Rac1 and Cdc42 and rescued haptotaxis defects of mouse embryonic fibroblasts (MEFs) lacking Src, Yes, and Fyn, three broadly expressed Src family members required for integrin-induced migration. Consistent with Rac1 activation, FA-CRKL induced cotranslocation of a Rac1 activator, Dock1, to focal adhesions. These results therefore indicate a role for CrkL in mediating Src signaling by activating small G proteins at focal adhesions. Furthermore, MEFs lacking CrkL show impaired integrin-induced migration despite expression of a closely related protein, Crk-II, in these cells. These results therefore provide formal evidence that CrkL plays a specific role in integrin-induced migration as a downstream mediator of Src.

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

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

Adaptor protein Crk is required for ephrin-B1-induced membrane ruffling and focal complex assembly of human aortic endothelial cells

Endothelial cell migration is an essential step in vasculogenesis and angiogenesis, in which receptor tyrosine kinases play a pivotal role. We investigated the mechanism by which ephrin-B1 promotes membrane ruffling in human aortic endothelial cells, because membrane ruffling heralds cell body migration. We especially focused on the role of Crk adaptor protein in EphB-mediated signaling. Using DsRed-tagged Crk and a fluorescent time-lapse microscope, we showed that Crk was recruited to the nascent focal complex after ephrin-B1 stimulation. Furthermore, we found that p130(Cas), but not paxillin, recruited Crk to the nascent focal complex. The necessity of Crk in ephrin-B1-induced membrane ruffling was shown both by the overexpression of dominant negative Crk mutants and by the depletion of Crk by using RNA interference. Then, we examined the role of two major downstream molecules of Crk, Rac1 and Rap1. The dominant negative mutant of Rac1 completely inhibited ephrin-B1-induced membrane ruffling and focal complex assembly. In contrast, rap1GAPII, a negative regulator of Rap1, did not inhibit ephrin-B1-induced membrane ruffling. However, in rap1GAPII-expressing cells, ephrin-B1 did not induce membrane spreading, probably due to instability of the focal complex. These results indicated that Crk plays a critical role in Rac1-induced membrane ruffling and Rap1-mediated nascent focal complex stabilization contributing to ephrin-B1-induced human aortic endothelial cells migration.

Galpha and Gbeta gamma require distinct Src-dependent pathways to activate Rap1 and Ras

The Src tyrosine kinase is necessary for activation of extracellular signal-regulated kinases (ERKs) by the beta-adrenergic receptor agonist, isoproterenol. In this study, we examined the role of Src in the stimulation of two small G proteins, Ras and Rap1, that have been implicated in isoproterenol's signaling to ERKs. We demonstrate that the activation of isoproterenol of both Rap1 and Ras requires Src. In HEK293 cells, isoproterenol activates Rap1, stimulates Rap1 association with B-Raf, and activates ERKs, all via PKA. In contrast, the activation by isoproterenol of Ras requires Gbetagamma subunits, is independent of PKA, and results in the phosphoinositol 3-kinase-dependent activation of AKT. Interestingly, beta-adrenergic stimulation of both Rap1 and ERKs, but not Ras and AKT, can be blocked by a Src mutant (SrcS17A) that is incapable of being phosphorylated and activated by PKA. Furthermore, a Src mutant (SrcS17D), which mimics PKA phosphorylation at serine 17, stimulates Rap1 activation, Rap1/B-Raf association, and ERK activation but does not stimulate Ras or AKT. These data suggest that Rap1 activation, but not that of Ras, is mediated through the direct phosphorylation of Src by PKA. We propose that the beta(2)-adrenergic receptor activates Src via two independent mechanisms to mediate distinct signaling pathways, one through Galpha(s) to Rap1 and ERKs and the other through Gbetagamma to Ras and AKT.

Bombesin-dependent pro-MMP-9 activation in prostatic cancer cells requires beta1 integrin engagement

Bombesin-like peptides, including the mammalian homologue gastrin-releasing peptide, are highly expressed and secreted by neuroendocrine cells in prostate carcinoma tissues and are likely to be related to the progression of this neoplastic disease. Previously, we demonstrated that bombesin increased migration and protease expression in androgen-independent cells. In this work we show that bombesin is able to activate pro-MMP-9 through a mechanism involving the beta1 integrin subunit. In fact, MMP-9 processing was evident only when beta1 integrin was engaged with specific adhesive substrates, such as type I collagen, or when cells were seeded on dishes coated with antibodies against beta1 integrin, resulting in activation of the surface ligand. When exogenous pro-MMP-9 was added to PC3 cells, MMP-9 active forms were produced within 30 min by bombesin-treated cultures while control cultures expressed activated forms only after a longer time and at lower levels. MMP-9 activation required cytoskeleton integrity since this effect was abolished by cytochalasin D. Engagement of beta1 integrin caused an increased membrane-linked uPA activity which was required for MMP-9 activation. The cross talk between bombesin- and beta1-integrin-engaged signals seems to be crucial for the modulation of both membrane-linked uPA activity and MMP-9 activation and triggers complex intracellular signaling pathways requiring activation of tyrosine kinase activity, including that of src and PI3K. The beta1 integrin may be considered an important mechanism by which bombesin induces MMP-9 activation. This finding supports the idea that cellular responses to growth factors may be driven by cell-matrix interactions and stresses the role of neuroendocrine factors in prostate carcinoma progression.

Get a ligand, get a life: integrins, signaling and cell survival

Programmed cell death is crucial for the development and maintenance of multicellular organisms. The decision to live, or to die, depends, at the cellular level, upon the cell's interaction with extracellular cues that trigger cell signaling pathways promoting survival or death. The extracellular matrix (ECM) influences the execution of the apoptotic program through the actions of adhesion receptors. Among these, integrins initiate a variety of downstream signaling events in response to ECM ligation. Integrins directly activate survival pathways via the PI 3-kinase and MAPK pathways and act as essential cofactors for their stimulation by growth factors. Conversely, elevated integrin expression in the absence of appropriate ligands, or in the presence of natural or synthetic antagonists, can promote apoptosis under otherwise permissive growth conditions. Integrins thus act in a crucial biosensory role, coordinating survival or death responses as a function of ECM composition. This dual function provides an elegant mechanism through which tissue-remodeling events may regulate cell death or survival in a temporal, ECM-governed manner.

Src-/- fibroblasts are defective in their ability to disassemble focal adhesions in response to phorbol ester/hyaluronan treatment

Exogenous hyaluronan promotes a rapid recruitment of Src to lamellae of mutant active H-ras transformed fibroblasts and an Src- and RHAMM (CD168)-dependent increase in random motility. These responses are accompanied by a loss of vinculin-positive lamellae focal adhesions. Nontransformed immortalized wild-type fibroblasts (WT) do not increase random motility in response to hyaluronan alone, but do increase motility in response to a combination of PMA treatment followed by hyaluronan. PMA treatment alone increases the number of lamellae/cell, percentage of cells with lamellae and number of focal adhesions/lamellae. Subsequent addition of hyaluronan does not affect the number of lamellae/cell but reduces both the number of focal adhesion/lamellae and the percentage of cells forming focal adhesion-positive lamellae. These effects are prevented by blocking RHAMM antibodies and mimicked by agonist RHAMM antibodies. Src-/- fibroblasts exhibit a limited response to PMA but do not increase motility or disassemble focal adhesions in response to a subsequent addition of HA. Rescue of Src-/- fibroblasts with either SrcA or c-Src restores response to close to WT levels. These results suggest that Src activity is uniquely required for both PMA and PMA-induced hyaluronan regulation of random motility and focal adhesion turnover.