Regulation of integrin-mediated p130(Cas) tyrosine phosphorylation in human B cells. A role for p59(Fyn) and SHP2

FAK-Dependent Cell Motility and Cell Elongation

Fibroblastic cells show specific substrate selectivity for typical cell–substrate adhesion. However, focal adhesion kinase (FAK) contributes to controlling the regulation of orientation and polarity. When fibroblasts attach to micropatterns, tyrosine-phosphorylated proteins and FAK are both detected along the inner border between the adhesive micropatterns and the nonadhesive glass surface. FAK likely plays important roles in regulation of cell adhesion to the substrate, as FAK is a tyrosine-phosphorylated protein that acts as a signal transduction molecule at sites of cell–substrate attachment, called focal adhesions. FAK has been suggested to play a role in the attachment of cells at adhesive micropatterns by affecting cell polarity. Therefore, the localization of FAK might play a key role in recognition of the border of the cell with the adhesive micropattern, thus regulating cell polarity and the cell axis. This review discusses the regulation and molecular mechanism of cell proliferation and cell elongation by FAK and its associated signal transduction proteins.

Carboxy-terminal domain of Cas differentially modulates c-Jun expression, DNA synthesis, and membrane ruffling induced by insulin, EGF, and IGF-1

p130 Crk-associated substrate (Cas) is an adaptor protein associating with many other signaling proteins and regulates a various biological processes including cell adhesion, migration, and growth factor stimulation. However, the exact functional role of Cas in growth factor signaling pathway was poorly understood. Here we investigated the role of Cas and its domains in the effects of insulin, EGF, and IGF-1 on c-Jun gene expression, DNA synthesis, cytoskeletal reorganization. We found that microinjection of anti-Cas antibody and C-terminal domain of Cas (Cas-CT) specifically inhibited EGF-induced, but not insulin- or IGF-1-induced, c-Jun expression. Cell cycle progression and cytoskeleton reorganization induced by insulin and EGF, but not by IGF-1, were inhibited by microinjected anti-Cas and Cas-CT. In contrast, microinjection of the substate domain (Cas-SD) of Cas did not have any inhibitory effects. These results revealed that the Cas-CT is differentially implicated in insulin and EGF-mediated, but not IGF-1-mediated, c-Jun expression, DNA synthesis and membrane ruffling.

Probing the interaction between U24 and the SH3 domain of Fyn tyrosine kinase

The putative membrane protein U24 from HHV-6A shares a seven-residue sequence identity (which includes a PxxP motif) with myelin basic protein (MBP), a protein responsible for the compaction of the myelin sheath in the central nervous system. U24 from HHV-6A also shares a PPxY motif with U24 from the related virus HHV-7, allowing them both to block early endosomal recycling. Recently, MBP has been shown to have protein-protein interactions with a range of proteins, including proteins containing SH3 domains. Given that this interaction is mediated by the proline-rich segment in MBP, and that similar proline-rich segments are found in U24, we investigate here whether U24 also interacts with SH3 domain-containing proteins and what the nature of that interaction might be. The implications of a U24-Fyn tyrosine kinase SH3 domain interaction are discussed in terms of the hypothesis that U24 may function like MBP through molecular mimicry, potentially contributing to the disease state of multiple sclerosis or other demyelinating disorders.

Focal adhesion protein expression in human diffuse large B-cell lymphoma

AIMS

Focal adhesions have been associated with poor prognosis in multiple cancer types, but their prognostic value in diffuse large B-cell lymphoma (DLBCL) has not been evaluated. The aim of this study was to investigate the expression patterns and the prognostic value of the focal adhesion proteins FAK, Pyk2, p130Cas and HEF1 in DLBCL.

METHODS AND RESULTS

Focal adhesion protein expression was examined using immunohistochemistry in normal lymphoid tissues and in 60 DLBCL patient samples. Kaplan-Meier survival and Cox regression analysis were performed to evaluate the correlation of focal adhesion protein expression with patient prognosis. FAK, Pyk2, p130Cas and HEF1 expression was mostly found in the germinal centres of normal human lymphoid tissues. When assessed in DLBCL samples, FAK, Pyk2, p130Cas and HEF1 were highly expressed in 45%, 34%, 42% and 45% of the samples, respectively. Multivariate Cox analysis revealed that decreased FAK expression was a significant independent predictor of poorer disease outcome.

CONCLUSIONS

FAK expression is an independent prognostic factor in DLBCL. Our results suggest that the addition of FAK immunostaining to the current immunohistochemical algorithms may facilitate risk stratification of DLBCL patients.

Oligodendroglial p130Cas is a target of Fyn kinase involved in process formation, cell migration and survival

Oligodendrocytes are the myelinating glial cells of the central nervous system. In the course of brain development, oligodendrocyte precursor cells migrate, scan the environment and differentiate into mature oligodendrocytes with multiple cellular processes which recognize and ensheath neuronal axons. During differentiation, oligodendrocytes undergo dramatic morphological changes requiring cytoskeletal rearrangements which need to be tightly regulated. The non-receptor tyrosine kinase Fyn plays a central role in oligodendrocyte differentiation and myelination. In order to improve our understanding of the role of oligodendroglial Fyn kinase, we have identified Fyn targets in these cells. Purification and mass-spectrometric analysis of tyrosine-phosphorylated proteins in response to overexpressed active Fyn in the oligodendrocyte precursor cell line Oli-neu, yielded the adaptor molecule p130Cas. We analyzed the function of this Fyn target in oligodendroglial cells and observed that reduction of p130Cas levels by siRNA affects process outgrowth, the thickness of cellular processes and migration behavior of Oli-neu cells. Furthermore, long term p130Cas reduction results in decreased cell numbers as a result of increased apoptosis in cultured primary oligodendrocytes. Our data contribute to understanding the molecular events taking place during oligodendrocyte migration and morphological differentiation and have implications for myelin formation.

A system for reconstructing B cell antigen receptor signaling in the mouse myeloma J558L cell line

B cell antigen receptor (BCR) signaling is positively and negatively regulated by various cell surface receptors such as CD19 and CD45. Functional analysis of these receptors has been performed using gene targeting technology, which is a valid approach to elucidate their functions. However, this type of analysis is restricted when multiple molecules are evaluated simultaneously. From a different perspective, synthetic biology provides a high degree of freedom for analyzing various molecules. Here we developed a system to reconstruct BCR signaling using the J558L myeloma cell line in combination with the protein-based Ca(2+) indicator YC3.60. BCR-reconstituted J558L cells harboring YC3.60 (J558Lμv11 cells) permitted monitoring of Ca(2+) mobilization. Reconstituting CD19 in J558Lμv11 cells resulted in detectable BCR-induced Ca(2+) mobilization but with kinetics different from that of CD45-expressing cells. Furthermore, we evaluated the validity of the J558L system by proteomic analysis of tyrosine-phosphorylated proteins after antigen stimulation. Identification of more than 100 BCR-induced tyrosine-phosphorylated proteins in J558Lμv11 cells revealed a similarity to that observed in B cells, and a novel member, non-receptor protein tyrosine kinase Fer, was found. Thus, this reconstruction system using J558L cells appeared to be valid for comprehensively investigating BCR signaling.

Proline substitutions and threonine pseudophosphorylation of the SH3 ligand of 18.5-kDa myelin basic protein decrease its affinity for the Fyn-SH3 domain and alter process development and protein localization in oligodendrocytes

The developmentally regulated myelin basic proteins (MBPs), which arise from the golli (gene of oligodendrocyte lineage) complex, are highly positively charged, intrinsically disordered, multifunctional proteins having several alternatively spliced isoforms and posttranslational modifications, and they play key roles in myelin compaction. The classic 18.5-kDa MBP isoform has a proline-rich region comprising amino acids 92-99 (murine sequence -T(92)PRTPPPS(99)-) that contains a minimal SH3 ligand domain. We have previously shown that 18.5-kDa MBP binds to several SH3 domains, including that of Fyn, a member of the Src family of tyrosine kinases involved in a number of signaling pathways during CNS development. To determine the physiological role of this binding as well as the role of phosphorylation of Thr92 and Thr95, in the current study we have produced several MBP variants specifically targeting phosphorylation sites and key structural regions of MBP's SH3 ligand domain. Using isothermal titration calorimetry, we have demonstrated that, compared with the wild-type protein, these variants have lower affinity for the SH3 domain of Fyn. Moreover, overexpression of N-terminal-tagged GFP versions in immortalized oligodendroglial N19 and N20.1 cell cultures results in aberrant elongation of membrane processes and increased branching complexity and inhibits the ability of MBP to decrease Ca(2+) influx. Phosphorylation of Thr92 can also cause MBP to traffic to the nucleus, where it may participate in additional protein-protein interactions. Coexpression of MBP with a constitutively active form of Fyn kinase resulted in membrane process elaboration, a phenomenon that was abolished by point amino acid substitutions in MBP's SH3 ligand domain. These results suggest that MBP's SH3 ligand domain plays a key role in intracellular protein interactions in vivo and may be required for proper membrane elaboration of developing oligodendrocytes and, further, that phosphorylation of Thr92 and Thr95 can regulate this function.

Fyn promotes proliferation, differentiation, survival and function of osteoclast lineage cells

c-Src and Lyn are the only Src family kinases (SFKs) with established activity in osteoclasts (OCs). c-Src promotes function via cytoskeletal organization of the mature resorptive cell while Lyn is a negative regulator of osteoclastogenesis. We establish that Fyn, another SFK, also impacts the OC, but in a manner distinctly different than c-Src and Lyn. Fyn deficiency principally alters cells throughout the osteoclastogenic process, resulting in diminished numbers of resorptive polykaryons. Arrested OC formation in the face of insufficient Fyn reflects reduced proliferation of precursors, in response to M-CSF and retarded RANK ligand (RANKL)-induced differentiation, attended by suppressed activation of the osteoclastogenic signaling molecules, c-Jun, and NF-κB. The anti-apoptotic properties of RANKL are also compromised in cells deleted of Fyn, an event mediated by increased Bim expression and failed activation of Akt. The defective osteoclastogenesis of Fyn-/- OCs dampens bone resorption, in vitro. Finally, while Fyn deficiency does not regulate basal osteoclastogenesis, in vivo, it reduces that stimulated by RANKL by ~2/3. Thus, Fyn is a pro-resorptive SFK, which exerts its effects by prompting proliferation and differentiation while attenuating apoptosis of OC lineage cells.

p130CAS is required for netrin signaling and commissural axon guidance

Netrins are an important family of axon guidance cues. Here, we report that netrin-1 induces tyrosine phosphorylation of p130(CAS) (Crk-associated substrate). Our biochemical studies indicate that p130(CAS) is downstream of the Src family kinases and upstream of the small GTPase Rac1 and Cdc42. Inhibition of p130(CAS) signaling blocks both the neurite outgrowth-promoting activity and the axon attraction activity of netrin-1. p130(CAS) RNA interference inhibits the attraction of commissural axons in the spinal cord by netrin-1 and causes defects in commissural axon projection in the embryo. These results demonstrate that p130(CAS) is a key component in the netrin signal transduction pathway and plays an important role in guiding commissural axons in vivo.

Src-family tyrosine kinase fyn phosphorylates phosphatidylinositol 3-kinase enhancer-activating Akt, preventing its apoptotic cleavage and promoting cell survival

Phosphatidylinositol 3-kinase enhancer-activating Akt (PIKE-A) binds Akt and upregulates its kinase activity, preventing apoptosis. PIKE-A can be potently phosphorylated on tyrosine residues 682 and 774, leading to its resistance to caspase cleavage. However, the upstream tyrosine kinases responsible for PIKE-A phosphorylation and subsequent physiological significance remain unknown. Here, we show that PIKE-A can be cleaved by the active apoptosome at both D474 and D592 residues. Employing fyn-deficient mouse embryonic fibroblast cells and tissues, we demonstrate that fyn is essential for phosphorylating PIKE-A and protects it from apoptotic cleavage. Active but not kinase-dead fyn interacts with PIKE-A and phosphorylates it on both Y682 and Y774 residues. Tyrosine phosphorylation in PIKE-A is required for its association with active fyn but not for Akt. Mutation of D into A in PIKE-A protects it from caspase cleavage and promotes cell survival. Thus, this finding provides a molecular mechanism accounting for the antiapoptotic action of src-family tyrosine kinase.

HEF1-aurora A interactions: points of dialog between the cell cycle and cell attachment signaling networks

Regulated timing of cell division cycles, and geometrical precision in the planar orientation of cell division, are critical during organismal development and remain important for the maintenance of polarized structures in adults. Mounting evidence suggests that these processes are coordinated at the centrosome through the action of proteins that mediate both cell cycle and cell attachment. Our recent work identifying HEF1 as an activator of the Aurora A kinase suggests a novel hub for such integrated signaling. We suggest that defects in components of the machinery specifying the temporal and spatial integration of cell division may induce cancer and other diseases through pleiotropic effects on cell migration, proliferation, apoptosis, and genomic stability.

Anchorage-dependent cell growth: tyrosine kinases and phosphatases meet redox regulation

Recent data have provided new insight concerning the regulation of nontransformed cell proliferation in response to both soluble growth factors and adhesive cues. Nontransformed cells are anchorage-dependent for the execution of the complete mitotic program and cannot avoid the concomitant signals starting from mitogenic molecules, as growth factors, and adhesive agents belonging to the extracellular matrix. Protein tyrosine kinases (PTKs) and phosphotyrosine phosphatases (PTPs) together with soluble small molecules have been included among intracellular signal transducers of growth factor and extracellular matrix receptors. Reactive oxygen species retain a key role during both growth factor and integrin receptor signaling, and these second messengers are recognized to be a synergistic point of confluence for anchorage-dependent growth signaling. Redox-regulated proteins include PTPs and PTKs, although with opposite regulation of enzymatic activity. Transient oxidation of PTPs leads to their inactivation, through the formation of an intramolecular S-S bridge. Conversely, oxidation of PTKs leads to their activation, either by direct SH modification or, indirectly, by concomitant inhibition of PTPs that leads to sustained activation of PTKs. This review will focus on the redox regulation of PTPs and PTKs during anchorage-dependent cell growth and its implications for tumor biology.

Src phosphorylates Cas on tyrosine 253 to promote migration of transformed cells

Cas is a member of the focal adhesion complex. Phosphorylation of Cas by Src is an important event leading to cell transformation. Using mass spectrometry, we have mapped 11 sites in Cas that are phosphorylated by Src. These sites are all located between residues 132 and 414 of Cas, in a region that is required for binding to a number of other proteins including Crk. We tested synthetic peptides modeled on Cas phosphorylation sites, and found that the sequence containing tyrosine 253 was phosphorylated by Src most efficiently. Using cells derived from Cas-deficient mice, we confirmed that Cas greatly enhanced the ability of Src to transform cells. Phosphorylation of Cas on tyrosine 253 was not required for Src to increase growth rate, suppress contact inhibition, or suppress anchorage dependence. Yet, in contrast to these growth characteristics, phosphorylation of Cas on tyrosine 253 was required for Src to promote cell migration. Thus, a single phosphorylation site on this focal adhesion adaptor protein can effectively separate cell migration from other transformed growth characteristics.

Anchorage-independent phosphorylation of p130(Cas) protects lung adenocarcinoma cells from anoikis

The regulation and function of the signaling adaptor protein p130(Cas) in tumor cell anchorage-independent survival, or anoikis resistance, were investigated in human lung adenocarcinoma cells. The tyrosine phosphorylation and function of p130(Cas) during cell detachment were analyzed in tumor cells and compared with that of normal epithelial cells. Cell detachment trigged rapid dephosphorylation of p130(Cas) in the nontumorigenic and anoikis-sensitive normal epithelial cells, but had no effect on the tyrosine phosphorylation of p130(Cas) in the anoikis-resistant lung adenocarcinoma cells. Further analysis revealed that the total tyrosine kinase activities associated with p130(Cas) in the lung tumor cells are anchorage-independent and are significantly higher than that in the normal cells, in which the p130(Cas)-associated tyrosine kinase activities are anchorage-dependent. Analysis of two known p130(Cas)-associated tyrosine kinases FAK and Src indicated that the regulation of tyrosine phosphorylation of FAK and Src are altered in the tumor cells. Inhibition of Src specifically abolished phosphorylation of p130(Cas) and induced anoikis. Furthermore, overexpression of dominant-negative forms of p130(Cas) also induced apoptosis. Taken together, these data suggest that p130(Cas) mediates a cell survival signal from cell-matrix interaction. Alterations in tumor cells that lead to constitutive phosphorylation of p130(Cas) can prevent cells from anoikis, hence contribute to tumor cell anchorage independence and metastasis.

The role of Aktand RAFTK in beta1 integrin mediated survival of precursor B-acute lymphoblastic leukemia cells

In this study, we report that the related adhesion focal tyrosine kinase RAFTK, is an upstream kinase in beta1 integrin mediated activation of Akt. Stimulation through beta1 integrins by fibronectin reversed apoptosis induced by adriamycin. Inhibitors of phosphatidylinositol 3-kinase (PI3 kinase)/Akt (LY 294002), tyrosine kinases (Herbimycin-A) and the cytotoxic agent adriamycin induced apoptosis of REH cells. beta1 integrin ligation induced activation of Akt, and tyrosine phosphorylation of RAFTK and FAK, but not SYK in REH cells. This suggested that RAFTK and FAK activation might be linked to Akt activation. Evidence that RAFTK is a modulator of Akt came from phorbol myristic acetate (PMA) stimulation. RAFTK and Akt were activated but FAK was not. Using fibroblasts from FAK -/- mice, which express high levels of RAFTK, fibronectin plating enhanced Akt activation. Pretreatment of REH cells with a P13 kinase/Akt inhibitor LY 294002 did not inhibit RAFTK tyrosine phosphorylation showing that RAFTK is upstream of P13k/Akt. Further evidence for a link between RAFTK tyrosine phosphorylation and Akt activation was the observation that the p85 subunit of P13 kinase associated with RAFTK following integrin ligation in REH cells. These results suggest that RAFTK plays an anti-apoptotic role through the activation of Akt.

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

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

Mechanisms of CAS substrate domain tyrosine phosphorylation by FAK and Src

Tyrosine phosphorylation of CAS (Crk-associated substrate, p130(Cas)) has been implicated as a key signaling step in integrin control of normal cellular behaviors, including motility, proliferation, and survival. Aberrant CAS tyrosine phosphorylation may contribute to cell transformation by certain oncoproteins, including v-Crk and v-Src, and to tumor growth and metastasis. The CAS substrate domain (SD) contains 15 Tyr-X-X-Pro motifs, which are thought to represent the major tyrosine phosphorylation sites and to function by recruiting downstream signaling effectors, including c-Crk and Nck. CAS makes multiple interactions, direct and indirect, with the tyrosine kinases Src and focal adhesion kinase (FAK), and as a result of this complexity, several plausible models have been proposed for the mechanism of CAS-SD phosphorylation. The objective of this study was to provide experimental tests of these models in order to determine the most likely mechanism(s) of CAS-SD tyrosine phosphorylation by FAK and Src. In vitro kinase assays indicated that FAK has a very poor capacity to phosphorylate CAS-SD, relative to Src. However, FAK expression along with Src was found to be important for achieving high levels of CAS tyrosine phosphorylation in COS-7 cells, as well as recovery of CAS-associated Src activity toward the SD. Structure-functional studies for both FAK and CAS further indicated that FAK plays a major role in regulating CAS-SD phosphorylation by acting as a docking or scaffolding protein to recruit Src to phosphorylate CAS, while a secondary FAK-independent mechanism involves Src directly bound to the CAS Src-binding domain (SBD). Our results do not support models in which FAK either phosphorylates CAS-SD directly or phosphorylates CAS-SBD to promote Src binding to this site.

Proteolysis of the docking protein HEF1 and implications for focal adhesion dynamics

The dynamic regulation of focal adhesions is implicated in cellular processes of proliferation, differentiation, migration, and apoptosis. The focal adhesion-associated docking protein HEF1 is cleaved by caspases during both mitosis and apoptosis. Common to both of these cellular processes is the loss of focal adhesions, transiently during mitosis and permanently during apoptosis. The proteolytic processing of HEF1 during both mitosis and apoptosis therefore posits a general role for HEF1 as a sensor of altered adhesion states. In this study, we find that HEF1 undergoes proteolytic processing specifically in response to cellular detachment, while HEF1 proteolysis is prevented by specific integrin receptor ligation and focal adhesion formation. We show that overexpression of a C-terminal caspase-derived 28-kDa HEF1 peptide causes cellular rounding that is demonstrably separable from apoptosis. Mutation of the divergent helix-loop-helix motif found in 28-kDa HEF1 significantly reduces the induction of apoptosis by this peptide, while deletion of the amino-terminal 28 amino acids of 28-kDa HEF1 completely abrogates the induction of apoptosis. Conversely, these mutations have no effect on the rounding induced by 28-kDa HEF1. Finally, we detect a novel focal adhesion targeting domain located in the C terminus of HEF1 and show that this activity is necessary for HEF1-induced cell spreading. Together, these data suggest that proteolytic and other posttranslational modifications of HEF1 in response to loss of adhesion serve to modulate the disassembly of focal adhesions.

Inhibition of osteoclast function by adenovirus expressing antisense protein-tyrosine kinase 2

Osteoclast activation is initiated by adhesion to bone, cytoskeletal rearrangement, formation of the sealing zone, and formation of the polarized ruffled membrane. Previous findings suggest that protein-tyrosine kinase 2 (PYK2), a cytoplasmic kinase related to focal adhesion kinase, participates in these events. This study examines the role of PYK2 in adhesion-mediated signaling and osteoclast function, using PYK2 antisense. We produced a recombinant adenovirus containing a 300-base pair reversed 5'-coding region of PYK2 and used full-length PYK2 as a control. Murine osteoclast-like cells or their mononuclear precursors were generated in a co-culture of bone marrow and osteoblasts. Infection with antisense adenovirus significantly reduced the expression of endogenous PYK2 protein relative to uninfected cells or to cells infected with sense PYK2 and caused: 1) a reduction in osteoclast formation in vitro; 2) inhibition of cell spreading and of actin ring formation in osteoclasts plated on glass or bone and of attachment and spreading of osteoclast precursors plated on vitronectin; 3) inhibition of bone resorption in vitro; 4) marked reduction in p130(Cas) tyrosine phosphorylation; and 5) no change in alpha(v)beta(3) integrin expression or c-Src tyrosine phosphorylation. Taken together, these findings support the hypothesis that PYK2 plays a central role in the adhesion-dependent cytoskeletal organization and sealing zone formation required for osteoclastic bone resorption.

Rhodocytin induces platelet aggregation by interacting with glycoprotein Ia/IIa (GPIa/IIa, Integrin alpha 2beta 1). Involvement of GPIa/IIa-associated src and protein tyrosine phosphorylation

Although glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) has established its role as a collagen receptor, it remains unclear whether GPIa/IIa mediates activation signals. In this study, we show that rhodocytin, purified from the Calloselasma rhodostoma venom, induces platelet aggregation, which can be blocked by anti-GPIa monoclonal antibodies. Studies with rhodocytin-coupled beads and liposomes loaded with recombinant GPIa/IIa demonstrated that rhodocytin directly binds to GPIa/IIa independently of divalent cations. In vitro kinase assays and Western blotting of GPIa immunoprecipitates revealed that Src and Lyn constitutively associate with GPIa/IIa and that Src activity increases transiently after rhodocytin stimulation. Src specifically associates with p130 Crk-associated substrate (Cas) in a manner dependent upon Cas phosphorylation, suggesting that Src is responsible for Cas tyrosine phosphorylation. While all these phenomena occur early after rhodocytin stimulation in a cAMP-resistant manner, tyrosine phosphorylation of Syk and phospholipase Cgamma2, intracellular Ca(2+) mobilization, and platelet aggregation occur later in a cAMP-sensitive manner. Cytochalasin D, which interferes with actin polymerization and blocks receptor clustering, inhibits all the rhodocytin-mediated signals we examined in this study. We suggest that rhodocytin, by clustering GPIa/IIa, activates GPIa/IIa-associated Src, which then mediates downstream activation signals.

Integrin signalling: a new Cas(t) of characters enters the stage

Cellular morphology is determined by the organization of the intracellular actin cytoskeleton, which is influenced by external and internal cues. Focal adhesions are sites at which the actin cytoskeleton is linked to the extracellular matrix by integrin receptor complexes. In addition to providing structural tethering points for cells, integrin receptor complexes transduce signals that influence a broad range of cellular processes, including migration, proliferation, transformation and apoptosis. The Cas proteins (p130Cas, HEF1/Cas-L and Efs/Sin), a family of docking proteins containing multiple interaction domains, are important components of integrin receptor signalling and have been implicated in all of these processes.

Cas mediates transcriptional activation of the serum response element by Src

The Src substrate p130(Cas) is a docking protein containing an SH3 domain, a substrate domain that contains multiple consensus SH2 binding sites, and a Src binding region. We have examined the possibility that Cas plays a role in the transcriptional activation of immediate early genes (IEGs) by v-Src. Transcriptional activation of IEGs by v-Src occurs through distinct transcriptional control elements such as the serum response element (SRE). An SRE transcriptional reporter was used to study the ability of Cas to mediate Src-induced SRE activation. Coexpression of v-Src and Cas led to a threefold increase in SRE-dependent transcription over the level induced by v-Src alone. Cas-dependent activation of the SRE was dependent on the kinase activity of v-Src and the Src binding region of Cas. Signaling to the SRE is promoted by a serine-rich region within Cas and inhibited by the Cas SH3 domain. Cas-dependent SRE activation was accompanied by an increase in the level of active Ras and in the activity of the mitogen-activated protein kinase (MAPK) Erk2; these changes were blocked by coexpression of dominant-negative mutants of the adapter protein Grb2. SRE activation was abrogated by coexpression of dominant-negative mutants of Ras, MAPK kinase (Mek1), and Grb2. Coexpression of Cas with v-Src enhanced the association of Grb2 with the adapter protein Shc and the protein tyrosine phosphatase Shp-2; coexpression of Shc or Shp-2 mutants significantly reduced SRE activation by Cas and v-Src. Cas-induced Grb2 association with Shp-2 and Shc may account for the Cas-dependent activation of the Ras/Mek/Erk pathway and SRE-dependent transcription. 14-3-3 proteins may also play a role in Cas-mediated signaling to the SRE. Overexpression of Cas was found to modestly enhance epidermal growth factor (EGF)-induced activation of the SRE. A Cas mutant lacking the Src binding region did not potentiate the EGF response, suggesting that Cas enhances EGF signaling by binding to endogenous cellular Src or another Src family member. These observations implicate Cas as a mediator of Src-induced transcriptional activation.

The next wave: protein tyrosine phosphatases enter T cell antigen receptor signalling

Recent years have seen an exponentially increasing interest in the molecular mechanisms of signal transduction. Much of the focus has been on protein tyrosine kinase-mediated signalling, while the study of protein tyrosine phosphatases has lagged behind. We predict that the phosphatases will become a "hot topic" in the field within the next few years. This review summarizes the current state-of-the-art in our understanding of the structure, regulation and role of protein tyrosine phosphatases in T lymphocyte activation.

Protein tyrosine phosphatases as negative regulators of mitogenic signaling

The regulation of tyrosine phosphorylation represents a key mechanism governing cell proliferation. In fibroblasts, inputs from both growth factor and extracellular matrix receptors are required for cell division. Triggering such receptors induces a wave of tyrosine phosphorylation on key signaling molecules, culminating in the activation of cyclin-dependent kinases and cell cycle progression. In general, protein tyrosine kinases stimulate, while protein tyrosine phosphatases inhibit, such cell proliferation pathways. The role of protein tyrosine kinases in mitogenesis has been extensively studied, but the identity and targets of the protein tyrosine phosphatases that regulate cell growth are not well described. In this review, I will survey recent advances in the identification and regulation of protein tyrosine phosphatases that downregulate cell proliferation.

Membrane-targeting of signalling molecules by SH2/SH3 domain-containing adaptor proteins

SH2/SH3 domain-containing adaptor proteins play a critical role in regulating tyrosine kinase signalling pathways. The major function of these adaptors, such as Grb2, Nck, and Crk, is to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. In recent years dozens of novel proteins have emerged that are capable of associating with the SH2 and the SH3 domains of adaptors. In this review, the author attempts to summarise these novel binding partners of Grb2, Nck, and Crk, and to discuss current controversies regarding function and regulation of protein multicomplexes held together by SH2/SH3 adaptor molecules at the plasma membrane.

Signaling through focal adhesion kinase

Integrin receptor binding to extracellular matrix proteins generates intracellular signals via enhanced tyrosine phosphorylation events that are important for cell growth, survival, and migration. This review will focus on the functions of the focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) and its role in linking integrin receptors to intracellular signaling pathways. FAK associates with several different signaling proteins such as Src-family PTKs, p130Cas, Shc, Grb2, PI 3-kinase, and paxillin. This enables FAK to function within a network of integrin-stimulated signaling pathways leading to the activation of targets such as the ERK and JNK/mitogen-activated protein kinase pathways. Focus will be placed on the structural domains and sites of FAK tyrosine phosphorylation important for FAK-mediated signaling events and how these sites are conserved in the FAK-related PTK, Pyk2. We will review what is known about FAK activation by integrin receptor-mediated events and also non-integrin stimuli. In addition, we discuss the emergence of a consensus FAK substrate phosphorylation sequence. Emphasis will also be placed on the role of FAK in generating cell survival signals and the cleavage of FAK during caspase-mediated apoptosis. An in-depth discussion will be presented of integrin-stimulated signaling events occurring in the FAK knockout fibroblasts (FAK-) and how these cells exhibit deficits in cell migration. FAK re-expression in the FAK- cells confirms the role of this PTK in the regulation of cell morphology and in promoting cell migration events. In addition, these results reinforce the potential role for FAK in promoting an invasive phenotype in human tumors.

NSP1 defines a novel family of adaptor proteins linking integrin and tyrosine kinase receptors to the c-Jun N-terminal kinase/stress-activated protein kinase signaling pathway

As part of a program to further understand the mechanism by which extracellular signals are coordinated and cell-specific outcomes are generated, we have cloned a novel class of related adaptor molecules (NSP1, NSP2, and NSP3) and have characterized in more detail one of the members, NSP1. NSP1 has an Shc-related SH2 domain and a putative proline/serine-rich SH3 interaction domain. Treatment of cells with epidermal growth factor or insulin leads to NSP1 phosphorylation and increased association with a hypophosphorylated adaptor protein, p130(Cas). In contrast, cell contact with fibronectin results in Cas phosphorylation and a transient dissociation of NSP1 from p130(Cas). Increased expression of NSP1 in 293 cells induces activation of JNK1, but not of ERK2. Consistent with this observation, NSP1 increases the activity of an AP-1-containing promoter. Thus, we have described a novel family of adaptor proteins, one of which may be involved in the process by which receptor tyrosine kinase and integrin receptors control the c-Jun N-terminal kinase/stress-activated protein kinase pathway.

Transmembrane tyrosine phosphatase LAR induces apoptosis by dephosphorylating and destabilizing p130Cas

BACKGROUND

LAR is a transmembrane receptor-like protein tyrosine phosphatase (PTP). Genetic studies of Drosophila LAR suggest that LAR may function to regulate cell adhesions or adhesion-mediated signal transduction. The over-expression of LAR in mammalian tissue culture cells does not affect cell adhesion but induces caspase-dependent apoptosis. This study investigates molecular mechanisms of LAR-induced apoptosis by searching for in vivo substrates of LAR which are responsible for LAR-induced apoptosis.

RESULTS

The over-expression of LAR in tissue culture cells specifically decreased the steady state protein level of p130Cas, a multifunctional signal assembly protein in signal transduction, by reducing the tyrosine phosphorylation and protein stability of p130Cas. The reduction of p130Cas protein level could be inhibited by tyrosine phosphatase inhibitors. Phosphatase domain-deleted mutant LARs had no effect on p130Cas. LAR also preferentially dephosphorylated p130Cas in vitro. Subcellularly, LAR and p130Cas were co-localized along stress fibres and at focal adhesions. LAR over-expression eliminated p130Cas from focal adhesions without affecting focal adhesion assembly. Restoring the level of p130Cas alleviated LAR-induced apoptosis.

CONCLUSIONS

p130Cas is an in vivo substrate of LAR. LAR specifically dephosphorylates and destabilizes p130Cas and may play a role in regulating cell adhesion-mediated cell survival. The function of p130Cas in focal adhesions may not be to regulate focal adhesion assembly and cell adhesion but rather to transduce the cell adhesion-generated signals which are essential for cell survival.

Fyn, a Src family tyrosine kinase

Fyn is a 59 kDa member of the Src family of tyrosine kinases. The protein is synthesized and N-myristoylated on cytosolic polysomes and then rapidly targeted to the plasma membrane, where it is palmitoylated. Dually acylated Fyn clusters in caveolae-like membrane microdomains, and can interact with a variety of other signaling molecules. Fyn's biological functions are diverse, and include signaling via the T cell receptor, regulation of brain function, as well as adhesion mediated signaling. Alteration of the levels of Fyn in appropriate target tissues may lead to better treatments for alcoholism and autoimmune disease.

Human immunodeficiency virus tat modulates the Flk-1/KDR receptor, mitogen-activated protein kinases, and components of focal adhesion in Kaposi's sarcoma cells

Kaposi's sarcoma (KS) spindle cell growth and spread have been reported to be modulated by various cytokines as well as the human immunodeficiency virus (HIV) gene product Tat. Recently, HIV-1 Tat has been shown to act like a cytokine and bind to the Flk-1/KDR receptor for the vascular endothelial growth factor A (VEGF-A), which is expressed by KS cells. We have characterized signal transduction pathways stimulated by HIV-1 Tat upon its binding to surface receptors on KS cells. We observed that stimulation in KS 38 spindle cells resulted in tyrosine phosphorylation and activation of the Flk-1/KDR receptor. We also report that HIV-1 Tat treatment enhanced the phosphorylation and association of proteins found in focal adhesions, such as the related adhesion focal tyrosine kinase RAFTK, paxillin, and p130(cas). Further characterization revealed the activation of mitogen-activated protein kinase, c-Jun amino-terminal kinase (JNK), and Src kinase. HIV-1 Tat contains a basic domain which can interact with growth factor tyrosine kinase receptors and a classical RGD sequence which may bind to and activate the surface integrin receptors for fibronectin and vitronectin. We observed that stimulation of KS cells with basic as well as RGD sequence-containing Tat peptides resulted in enhanced phosphorylation of RAFTK and activation of MAP kinase. These studies reveal that Tat stimulation activates a number of signal transduction pathways that are associated with cell growth and migration.

Phosphorylation of p130Cas by angiotensin II is dependent on c-Src, intracellular Ca2+, and protein kinase C

p130Cas is a signaling molecule that was initially found to be tyrosine-phosphorylated in v-Crk and v-Src transformed cells. We characterized the regulation of p130Cas tyrosine phosphorylation in vascular smooth muscle cells by angiotensin II (Ang II). This ligand induced a transient increase in p130Cas tyrosine phosphorylation, which was sensitive to the actin polymerization inhibitor cytochalasin D and to the intracellular Ca2+ chelator BAPTA-AM but not the Ca2+ channel blocker verapamil. The Ang II-induced tyrosine phosphorylation of p130Cas was also dependent on an active Src family tyrosine kinase, since it could be blocked by the Src kinase inhibitors geldanamycin and PP1. Ang II treatment resulted in the ability of p130Cas to bind at least 11 different phosphate-containing proteins. Analysis of these proteins revealed that protein kinase Calpha and the cell adhesion signaling molecule pp120 formed temporal associations with p130Cas in response to Ang II. c-Src was found to associate with p130Cas in a manner that was independent of Ang II treatment. Inhibition of protein kinase C by either calphostin C or phorbol 12-myristate 13-acetate downregulation inhibited the Ang II-induced tyrosine phosphorylation of p130Cas. These results are the first to demonstrate that the tyrosine phosphorylation of p130Cas by Ang II is transduced by the Src, intracellular Ca2+, protein kinase C signaling pathway.

SHP-1 associates with both platelet-derived growth factor receptor and the p85 subunit of phosphatidylinositol 3-kinase

The Src homology 2 (SH2)-containing protein tyrosine phosphatase 1, SHP-1, is highly expressed in all hematopoietic cells as well as in many non-hematopoietic cells, particularly in some malignant epithelial cell lines. In hematopoietic cells, SHP-1 negatively regulates multiple cytokine receptor pathways. The precise function and the targets of SHP-1 in non-hematopoietic cells, however, are largely unknown. Here we demonstrate that SHP-1 associates with both the tyrosine-phosphorylated platelet-derived growth factor (PDGF) receptor and the p85 subunit of phosphatidylinositol 3-kinase in MCF-7 and TRMP cells. Through the use of mutant PDGF receptors and performing peptide competition for immunoprecipitation, it was determined that SHP-1 independently associates with the PDGF receptor and p85 and that its N-terminal SH2 domain is directly responsible for the interactions. Overexpression of SHP-1 in TRMP cells transfected with the PDGF receptor markedly inhibited PDGF-induced c-fos promoter activation, whereas the expression of three catalytically inactive SHP-1 mutants increased the c-fos promoter activation in response to PDGF stimulation. These results indicate that SHP-1 might negatively regulate PDGF receptor-mediated signaling in these cells. Identification of the association of SHP-1 with the PDGF receptor and p85 in MCF-7 and TRMP cells furthers our understanding of the function of SHP-1 in non-hematopoietic cells.

The related adhesion focal tyrosine kinase differentially phosphorylates p130Cas and the Cas-like protein, p105HEF1

The related adhesion focal tyrosine kinase (RAFTK) is tyrosine-phosphorylated following beta1 integrin or B cell antigen receptor stimulation in human B cells. Two substrates that are tyrosine-phosphorylated following integrin ligation in B cells are p130(Cas) and the Cas family member human enhancer of filamentation 1 (HEF1), both of which can associate with RAFTK. In this report we observed that RAFTK was involved in the phosphorylation of these two proteins. While a catalytically active RAFTK was required for both p130(Cas) and HEF1, phosphorylation of p130(Cas), but not of HEF1, was dependent on an intact autophosphorylation site (Tyr402) on RAFTK. To determine if RAFTK phosphorylated p130(Cas) and HEF1 directly or through an intermediate, we assayed the ability of RAFTK and of a Tyr402 mutant to phosphorylate purified HEF1 and p130(Cas) domains. RAFTK was able to phosphorylate the substrate domains of both p130(Cas) and HEF1, but only the C-terminal domain of p130(Cas). Furthermore, Tyr402, which mediates the binding of RAFTK to c-Src kinase, was required for the phosphorylation of the C-terminal domain of p130(Cas). These data suggest that RAFTK itself is sufficient for HEF1 phosphorylation, whereas a cooperation between RAFTK and Src kinases is required for the complete phosphorylation of p130(Cas).