From the Second Department of Internal Medicine, Kobe
University School of Medicine, 7-5-1 Kusunoki-cho,
Chuo-ku, Kobe 650, Japan, the ¶ Ben May Institute for Cancer
Research and Center for Molecular Oncology, University of Chicago,
Chicago, Illinois 60637, and the
Department of Oncology,
Institute of Medical Science, Tokyo University,
4-6-1 Shirokanedai, Minato-ku, Tokyo 108, Japan
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ABSTRACT |
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SHPS-1 is a receptor-like glycoprotein that undergoes tyrosine phosphorylation and binds SHP-2, an Src homology 2 domain containing protein tyrosine phosphatase, in response to various mitogens. Cell adhesion to extracellular matrix proteins such as fibronectin and laminin also induced the tyrosine phosphorylation of SHPS-1 and its association with SHP-2. These responses were markedly reduced in cells overexpressing the Csk kinase or in cells that lack focal adhesion kinase or the Src family kinases Src or Fyn. However, unlike Src, focal adhesion kinase did not catalyze phosphorylation of the cytoplasmic domain of SHPS-1 in vitro. Overexpression of a catalytically inactive SHP-2 markedly inhibited activation of mitogen-activated protein (MAP) kinase in response to fibronectin stimulation without affecting the extent of tyrosine phosphorylation of focal adhesion kinase or its interaction with the docking protein Grb2. Overexpression of wild-type SHPS-1 did not enhance fibronectin-induced activation of MAP kinase. These results indicate that the binding of integrins to the extracellular matrix induces tyrosine phosphorylation of SHPS-1 and its association with SHP-2, and that such phosphorylation of SHPS-1 requires both focal adhesion kinase and an Src family kinase. In addition to its role in receptor tyrosine kinase-mediated MAP kinase activation, SHP-2 may play an important role, partly through its interaction with SHPS-1, in the activation of MAP kinase in response to the engagement of integrins by the extracellular matrix.
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INTRODUCTION |
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The binding of integrins to extracellular matrix (ECM)1 proteins such as fibronectin and laminin contributes to cell adhesion to the ECM and evokes a variety of cellular responses (1-3). Activation of integrins induces cell cycle progression (4), enhances growth factor-mediated cell proliferation and DNA synthesis (5), and promotes cell migration (6). These responses are thought to be mediated by integrin-induced tyrosine phosphorylation of various proteins (1-3) that may couple the integrin signal to activation of Ras and MAP kinase. Fak has been suggested to mediate integrin-induced activation of the Ras-MAP kinase signaling cascade (7, 8). Fak is a non-receptor-type protein tyrosine kinase that colocalizes with integrins at focal contacts and is autophosphorylated in response to cell adhesion to the ECM (9, 10). The integrin-induced autophosphorylation of Fak results in its interaction with the SH2 domain of Src kinase (7, 11), after which Src may catalyze the tyrosine phosphorylation of both the kinase domain and the COOH-terminal Tyr925 residue of Fak (7, 8). Phosphorylation of Tyr925 of Fak creates a binding site for the SH2 domain of Grb2, which constitutively binds to the guanine nucleotide exchange protein SOS (12) and may link integrin engagement to the activation of the Ras-MAP kinase cascade (7, 8). However, the existence of an additional pathway for integrin-induced activation of MAP kinase that is independent of Fak or Grb2 binding to Fak has recently been suggested (13, 14). In contrast to the apparent participation of protein tyrosine kinases in integrin activation of Ras and MAP kinase, it remains unknown whether a PTPase also is important in this process.
SHP-2 is a non-transmembrane PTPase that contains two SH2 domains (15-17). A dominant negative mutant of SHP-2 inhibited insulin- or EGF-stimulated activation of Ras and MAP kinase, DNA synthesis, or cell proliferation (18-23). Furthermore, microinjection of RNA encoding a dominant negative SHP-2 into Xenopus oocytes resulted in severe posterior truncation and inhibited FGF- and activin-mediated mesoderm induction (24). Corkscrew, a putative Drosophila homolog of SHP-2, is thought to be required for Ras1 activation or to function in conjunction with Ras1 during signaling by the Sevenless receptor tyrosine kinase (25, 26). Most recently, disruption of the mouse Shp-2 gene was shown to result in failure of normal gastrulation and subsequent death in the early stages of embryogenesis, as well as in a marked impairment in FGF-induced MAP kinase activation (27). Thus, SHP-2 may play a crucial role in intracellular signaling elicited by various growth factors and hormones, probably by contributing to activation of the Ras-MAP kinase pathway.
SHP-2 binds directly to growth factor receptors, including the platelet-derived growth factor, EGF receptor, c-KIT, and the erythropoietin receptor, in response to receptor stimulation with the corresponding ligand and undergoes tyrosine phosphorylation (28-33). Furthermore, although SHP-2 does not interact directly with the insulin receptor, it binds through its SH2 domains to tyrosine-phosphorylated docking proteins such as IRS1, IRS2, and GAB1 in response to insulin (34-36). Corkscrew also binds to another docking protein Drosophila DOS (37). These docking proteins possess multiple tyrosine phosphorylation sites that serve as binding sites for the SH2 domains of various signaling molecules, but they lack any catalytic activity. Thus, in response to ligand stimulation of various receptors, SHP-2 appears to be recruited either to the receptors themselves or to receptor docking proteins.
We have recently identified a membrane glycoprotein, SHPS-1 (38, 39) (also known as SIRP (40) and BIT (41)), that possesses three immunoglobulin (Ig)-like domains in its putative extracellular region and four YXX(L/V/I) motifs, which are potential tyrosine phosphorylation and SH2 domain binding sites, in its cytoplasmic region. Various mitogens, including serum, insulin, lysophosphatidic acid, and EGF, induce tyrosine phosphorylation of SHPS-1 and its subsequent association with SHP-2 in cultured cells (38, 40, 42). Thus, in addition to IRS and GAB1 proteins, SHPS-1 also may recruit SHP-2 from the cytosol to a site near the plasma membrane in response to growth factor stimulation. We have also detected both tyrosine phosphorylation of SHPS-1 and its association with SHP-2 in unstimulated, serum-deprived cells, possibly reflecting the effects of cell adhesion (38). However, the precise mechanism by which cell adhesion induces the tyrosine phosphorylation of SHPS-1 and the physiological roles of the interaction of SHPS-1 with SHP-2 in cell adhesion-triggered signal transduction remain unclear.
We have now shown that the binding of integrins to ECM components such as fibronectin and laminin induces tyrosine phosphorylation of SHPS-1 and its subsequent association with SHP-2. This cell adhesion-induced phosphorylation of SHPS-1 appears to require both Fak and an Src family kinase. Furthermore, the formation of the SHPS-1·SHP-2 complex may contribute to MAP kinase activation in response to fibronectin stimulation.
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EXPERIMENTAL PROCEDURES |
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Cells and Antibodies-- Rat-1 fibroblasts were cultured in DMEM supplemented with 10% FBS. CHO cells that overexpress human insulin receptors (CHO-IR cells) were maintained in Ham's F-12 medium supplemented with 10% FBS. Before transfection experiments, COS-7 cells were also cultured in DMEM supplemented with 10% FBS.
Rat-1 cells that overexpress a catalytically inactive SHP-2 mutant were generated as described (21). Briefly, a point mutation that changed Cys459 to Ser was introduced into human SHP-2 cDNA by site-directed mutagenesis, and the mutant cDNA was cloned into the EcoRI site of the mammalian expression vector pSRCell Adhesion, Immunoprecipitation, and Immunoblot Analysis-- Before all experiments, cells were incubated for 12 h with DMEM in the absence of serum. Cells were detached by incubation for 15 min at 37 °C in phosphate-buffered saline without Ca2+ and Mg2+, collected by centrifugation, and washed twice with serum-free DMEM. They were then suspended in DMEM, replated on polystyrene dishes (~2.5 × 107 per 10-cm dish) coated with fibronectin or other ECM proteins (Falcon), and incubated at 37 °C for various times in serum-free DMEM. In some experiments, cytochalasin D (Sigma) was added 10 min before replating of the cells.
The attached cells were then lysed on ice in 0.5 to 1 ml of ice-cold lysis buffer (20 mM Tris-HCl (pH 7.6), 140 mM NaCl, 2.6 mM CaCl2, 1 mM MgCl2, 1% Nonidet P-40, 10% (v/v) glycerol) containing 1 mM phenylmethylsulfonyl fluoride and 1 mM sodium vanadate. The lysates were centrifuged at 10,000 × g for 15 min at 4 °C, and the resulting supernatants were subjected to immunoprecipitation and immunoblot analysis. Total cell protein in lysates was standardized with the use of a protein assay kit (Bio-Rad) before immunoprecipitation. Immunoprecipitation was performed by incubation of cell lysates for 4 h at 4 °C with various antibodies bound to protein G-Sepharose beads (2 µg of antibody per 20 µl of beads) (Amersham Pharmacia Biotech). The beads were then washed twice with 1 ml of WG buffer (50 mM Hepes-NaOH (pH 7.6), 150 mM NaCl, 0.1% Triton X-100) and resuspended in SDS sample buffer. Gel electrophoresis and immunoblot analysis, with PY20 or other antibodies and an ECL detection kit (Amersham Pharmacia Biotech) were performed as described previously (21, 45).Immune Complex Kinase Assays--
A GST fusion protein
(GST-SHPS-1-cyto) containing the cytoplasmic portion of SHPS-1 (amino
acids 373-481) was generated and purified as described previously
(38). Src kinase was immunoprecipitated from Rat-1 cell lysates with a
mAb to v-Src as described above. The immune complex was washed twice
with kinase assay buffer (50 mM Hepes-NaOH (pH 7.6), 3 mM MnCl2, 10 mM MgCl2,
1 mM dithiothreitol, 10 µM ATP) and incubated
with GST-SHPS-1-cyto (~1 µg) for 30 min at 24 °C in kinase assay
buffer (50 µl) in the absence or presence of
[-32P]ATP (0.1 mCi/ml, 6000 Ci/mmol) (ICN). For
examination of the phosphorylation of SHPS-1 by Fak, COS-7 cells (in
10-cm dishes) were transiently transfected with 10 µg of the pRC CMV
expression vector containing cDNA that encoded HA epitope-tagged
mouse Fak (kindly provided by S. K. Hanks) (46) with the use of 18 µl of LipofectAMINE. After 48 h, the transfected cells were
harvested, lysed, and subjected to immunoprecipitation with mAb 12CA5.
The precipitates were then subjected to the phosphorylation reaction with GST-SHPS-1-cyto or poly(Glu, Tyr) (1 mg/ml) (Sigma) as described above. After each phosphorylation reaction, the samples were
centrifuged at 10,000 × g for 5 min at 4 °C, and
the resulting supernatants were mixed with SDS sample buffer, boiled,
and subjected to gel electrophoresis and autoradiography. The
immunoprecipitated Src kinase and HA-tagged Fak were also subjected to
immunoblot analysis with antibodies to the corresponding protein
tyrosine kinase to determine the amount of each protein in the immune
complex, or to autoradiography for determination of
autophosphorylation.
Determination of MAP Kinase Activation-- MAP kinase activation was monitored by immunoblot analysis of cell lysates with antibodies to p44 and p42 MAP kinase (New England BioLabs) that recognize the enzymes only when they are activated by phosphorylation of Tyr204.
MAP kinase activation was also monitored by using the direct in vitro kinase assay as described previously (21). Briefly, after attached or suspension, cells were lysed in 500 µl of lysis buffer (25 mM Tris-HCl (pH 7.4), 25 mM NaCl, 1 mM sodium orthovanadate, 10 mM NaF, 10 mM sodium pyrophosphate, 0.5 mM EGTA, 10 nM okadaic acid (Wako Chemicals), and 1 mM phenylmethylsulfonyl fluoride). The lysates were incubated for 2 h at 4 °C with ![]() |
RESULTS |
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Effects of Interaction of Integrins with ECM Proteins on the Tyrosine Phosphorylation of SHPS-1 and Its Association with SHP-2-- The effects of cell adhesion on tyrosine phosphorylation of SHPS-1 and its interaction with SHP-2 were investigated by detaching serum-deprived Rat-1 cells and then replating them on fibronectin-coated dishes. After incubation for 0-120 min, cells were lysed and subjected to immunoprecipitation with mAb 2F34 to rat SHPS-1 followed by immunoblot analysis with either mAb PY20 to phosphotyrosine or antibodies to SHP-2. Cell detachment resulted in the loss of tyrosine phosphorylation of SHPS-1 and its association with SHP-2 normally apparent in adherent cells (Fig. 1). After replating of the cells onto fibronectin-coated dishes, both the tyrosine phosphorylation of SHPS-1 and its association with SHP-2 were evident within 15 min, as the cells started to spread, and were maximal at 60-120 min. Similar time courses of tyrosine phosphorylation of SHPS-1 and its association with SHP-2 were also observed when CHO-IR cells were plated on fibronectin-coated dishes (data not shown).
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Roles of an Src Family Kinase and Fak in Tyrosine Phosphorylation of SHPS-1 in Response to Fibronectin Stimulation-- The binding of integrins to ECM proteins such as fibronectin induces autophosphorylation of Fak, its association with Src kinase, and the subsequent tyrosine phosphorylation of various proteins localized at focal contacts (9-11). Given that SHPS-1 may serve as a substrate for v-Src kinase (19, 38), Src or another Src family kinase such as Fyn might be responsible for the tyrosine phosphorylation of SHPS-1 in response to fibronectin stimulation. To investigate this hypothesis, we examined the effects of overexpression of wild-type or kinase-inactive mutant Csk proteins on fibronectin-induced tyrosine phosphorylation of SHPS-1. Csk is an Src-like tyrosine kinase that inhibits the activity of Src family kinases by phosphorylating a COOH-terminal tyrosine residue (48, 49). The wild-type Csk cDNA and the mutant Csk cDNA, which contained a point mutation that changed Lys222 to Arg, were transfected separately into CHO-IR cells, and cell lines that overexpress the wild-type (Csk-WT cells) or catalytically inactive (Csk-K/R cells) Csk were established. The Lys222 to Arg mutation completely abolished the kinase activity of recombinant Csk (data not shown). The amount of total Csk protein in Csk-WT and Csk-K/R cells was ~15 and 18 times that in the parental CHO-IR cells, respectively (Fig. 3A). Replating of parental CHO-IR cells on fibronectin-coated dishes induced the tyrosine phosphorylation of SHPS-1 (Fig. 3B) and its association with SHP-2 (data not shown). Overexpression of Csk-WT markedly reduced the extent of tyrosine phosphorylation of SHPS-1 induced by fibronectin stimulation, whereas overexpression of Csk-K/R enhanced the effect of fibronectin on the tyrosine phosphorylation of SHPS-1.
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Effects of Fibronectin Stimulation on MAP Kinase Activation in Rat-1 Cells That Overexpress a Catalytically Inactive SHP-2 or Wild-type SHPS-1-- To investigate the physiological consequences of formation of the SHPS-1·SHP-2 complex in response to fibronectin stimulation, we examined the effects of overexpression of a catalytically inactive SHP-2 or wild-type SHPS-1 on fibronectin-induced activation of MAP kinase. Because we have found that Rat-1 cells are more suitable than CHO cells for studying fibronectin activation of MAP kinase, we generated Rat-1 cell lines that overexpress a catalytically inactive SHP-2 (Rat-1-SHP-2-C/S cells) or wild-type SHPS-1 (Rat-1-SHPS-1-WT cells). The levels of expression of SHP-2 and SHPS-1 in parental cells, Rat-1-SHP-2-C/S cells, and Rat-1-SHPS-1-WT cells are shown in Fig. 6A. Immunoblot analysis with mAb PY20 of cell lysates (Fig. 6A) or of immunoprecipitates prepared with mAb 2F34 (data not shown) revealed that the extent of tyrosine phosphorylation of the ~120-kDa SHPS-1 was markedly increased in Rat-1-SHP-2-C/S cells compared with that in parental cells, consistent with previous observations with CHO cells (38, 42). Replating of parental Rat-1 cells on fibronectin-coated dishes resulted in activation of MAP kinase within 15 min, with the maximal response apparent at 30 min (Fig. 6, B and C). In contrast, the extent of fibronectin-induced MAP kinase activation was markedly reduced in Rat-1-SHP-2-C/S cells. Furthermore, the overexpression of wild-type SHPS-1 did not markedly enhance MAP kinase activation in response to fibronectin.
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DISCUSSION |
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In the present study, we have characterized the cell adhesion-induced tyrosine phosphorylation of SHPS-1 and its subsequent association with SHP-2. Cell adhesion to ECM components such as fibronectin and laminin, but not to poly-L-lysine, a nonspecific substrate, markedly increased the extent of tyrosine phosphorylation of SHPS-1. Because fibronectin and laminin are specific ligands for integrins (47), our results indicate that the engagement of integrins by ECM proteins may induce this response. The binding of integrins to the ECM has previously been shown to increase the extent of tyrosine phosphorylation of various proteins, including Fak, paxillin, tensin, and p130cas (9, 51-53), that are localized at focal adhesion contacts. Thus, SHPS-1 might be a member of the group of proteins that are localized at focal adhesion contacts and undergo tyrosine phosphorylation in response to the interaction of integrins with the ECM. The fibronectin-stimulated tyrosine phosphorylation of SHPS-1 was inhibited by cytochalasin D. Integrin aggregation has been previously shown to induce localized membrane accumulation of SHP-2, and this effect is inhibited by cytochalasin D or tyrosine kinase inhibitors such as herbimycin A or genistein (54). Thus, our data suggest that tyrosine phosphorylation of SHPS-1 contributes, at least in part, to integrin-induced membrane accumulation of SHP-2, as postulated by Miyamoto et al. (54).
We have also explored the mechanism by which cell adhesion to fibronectin stimulates the tyrosine phosphorylation of SHPS-1. Overexpression of Csk markedly inhibited this effect of fibronectin, whereas the expression of a kinase-inactive Csk enhanced this response. Csk inhibits the activity of Src family kinases by phosphorylating a COOH-terminal tyrosine residue in these enzymes (48, 49). Thus, the overexpression of Csk may reduce the extent of fibronectin-induced tyrosine phosphorylation of SHPS-1 by inhibiting the activity of an Src family kinase. The overexpression of the kinase-negative Csk may enhance this effect of fibronectin by competitively inhibiting the action of endogenous Csk and thereby increasing the activity of an Src family kinase. This notion is also supported by our observation that the extent of fibronectin-stimulated tyrosine phosphorylation of SHPS-1 was reduced in Src- or Fyn-deficient cells, suggesting that Src and Fyn may contribute to the tyrosine phosphorylation of SHPS-1 in response to cell adhesion.
Fibronectin stimulation also failed to induce tyrosine phosphorylation of SHPS-1 in Fak-deficient cells. Engagement of integrins by the ECM induces autophosphorylation of Fak and the consequent interaction of Fak with Src kinase, mediated by binding of the SH2 domain of Src to phosphorylated Tyr397 of Fak (11, 55). This interaction of Fak with Src is thought to recruit Src to focal adhesion contacts and to enhance its kinase activity (7, 8). Src may then promote the tyrosine phosphorylation of the kinase domain of Fak and thereby increase Fak protein tyrosine kinase activity (46). It is therefore possible that Fak activated as a result of fibronectin stimulation could directly catalyze the tyrosine phosphorylation of SHPS-1. However, Fak, unlike Src, did not phosphorylate the cytoplasmic portion of SHPS-1 in vitro. Together, these observations suggest that Fak is essential for tyrosine phosphorylation of SHPS-1 in response to fibronectin but that Fak does not directly catalyze this reaction. The importance of Fak may lie in either its enhancement of the catalytic activity of Src or its recruitment of Src to focal contacts. Because SHPS-1 appears to be an effective substrate for Src kinase in vitro, activated Src family kinases may catalyze the tyrosine phosphorylation of SHPS-1 in vivo. Regardless, both Src family kinases and Fak appear to be required for fibronectin-stimulated tyrosine phosphorylation of SHPS-1 and its association with SHP-2. In contrast to the phosphorylation of SHPS-1, loss of Fak did not inhibit, but rather enhanced, fibronectin-stimulated tyrosine phosphorylation of paxillin, as described previously (50). This observation suggests that fibronectin-induced tyrosine phosphorylation of SHPS-1 and that of paxillin are mediated by different mechanisms, even though tyrosine phosphorylation of paxillin is induced by integrin stimulation and catalyzed by a transforming variant of Src kinase (56).
SHP-2 has been suggested to mediate activation of the Ras-MAP kinase cascade in response to various growth factors, including insulin, EGF, platelet-derived growth factor, and FGF (16, 17). Thus, our observation that cell adhesion to fibronectin induces tyrosine phosphorylation of SHPS-1 and its association with SHP-2 prompted us to examine the role of the SHPS-1·SHP-2 complex in fibronectin-induced activation of MAP kinase. We have now shown that expression of a catalytically inactive SHP-2 markedly inhibited MAP kinase activation in response to fibronectin stimulation, suggesting that SHP-2 may mediate the integrin-induced activation of MAP kinase as well as that induced by receptor protein tyrosine kinases. Fibronectin activation of MAP kinase is thought to be triggered by autophosphorylation of Fak- and Src-mediated phosphorylation of Tyr925 of Fak, which creates a binding site for the SH2 domain of Grb2 (7, 8). However, the expression of catalytically inactive SHP-2 did not substantially inhibit either the tyrosine phosphorylation of Fak or its association with Grb2 in response to fibronectin stimulation. Thus, SHP-2 may mediate fibronectin activation of MAP kinase through an as yet unidentified mechanism that is independent of the interaction of Fak with Grb2.
The overexpression of wild-type SHPS-1 did not markedly enhance MAP kinase activation in response to fibronectin stimulation, although it did increase the fibronectin-induced association of SHPS-1 with SHP-2 compared with that apparent in the parental cells (data not shown). However, it remains possible that endogenous SHPS-1 present in Rat-1 cells is sufficient for maximal MAP kinase activation in response to fibronectin, so that overexpression of exogenous SHPS-1 cannot further enhance this effect. The expression of a membrane-targeted form of Corkscrew, a Drosophila homolog of SHP-2, can bypass Sevenless tyrosine kinase function to mediate development of the R7 photoreceptor (25), suggesting that recruitment of Corkscrew or SHP-2 to the plasma membrane is crucial for its function. Therefore, SHPS-1 likely plays an important role in recruitment of SHP-2 from the cytosol to a site near the plasma membrane in response to fibronectin stimulation. A phosphotyrosyl peptide containing the sequence surrounding the binding sites for the SH2 domains of SHP-2 in SHPS-1 stimulates the PTPase activity of recombinant SHP-2 in vitro (57), suggesting that binding of SHP-2 to tyrosine-phosphorylated SHPS-1 in response to fibronectin stimulation may increase the catalytic activity of SHP-2 in vivo. SHP-2 may subsequently dephosphorylate SHPS-1 and activate an unknown factor, which also associates with SHPS-1 or is located near the plasma membrane, by catalyzing its tyrosine dephosphorylation, resulting in activation of the Ras-MAP kinase cascade. Identification of this putative factor should facilitate our understanding of the mechanism of SHP-2-mediated activation of the Ras-MAP kinase cascade.
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ACKNOWLEDGEMENTS |
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We thank M. Okada for kindly providing the rat Csk cDNA; P. Soriano for permission to use the knock-out mice cells; and S. K. Hanks for the expression vector encoding HA-tagged Fak.
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FOOTNOTES |
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* This work was supported by a grant-in-aid for cancer research and a grant-in-aid for scientific research from the Ministry of Education, Science, Sports, and Culture of Japan; Grant 96-22809 from the Princess Takamatsu Cancer Research Fund; a grant from the Yamanouchi Foundation for Research on Metabolic Disorders; a grant from the Ciba-Geigy Foundation (Japan) for the Promotion of Science; National Institutes of Health Grants HD 24875 and HD 25326; and a grant from Kirin Brewery Co.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ To whom correspondence should be addressed: Second Dept. of Internal Medicine, Kobe University School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650, Japan. Tel.: 81-78-341-7451 (ext. 5522); Fax: 81-78-382-2080; E-mail: matozaki{at}med.kobe-u.ac.jp.
1 The abbreviations used are: ECM, extracellular matrix; MAP, mitogen-activated protein; Fak, focal adhesion kinase; SH2, Src homology 2; PTPase, protein tyrosine phosphatase; EGF, epidermal growth factor; FGF, fibroblast growth factor; DMEM, Dulbecco's modified Eagle's medium supplemented; FBS, fetal bovine serum; CHO, Chinese hamster ovary; IR, insulin receptor; mAb, monoclonal antibody; GST, glutathione S-transferase; HRP, horseradish peroxidase; HA, hemagglutinin; Csk, COOH-terminal Src kinase.
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REFERENCES |
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