Epidermal growth factor-induced association of the SHPTP2 protein tyrosine phosphatase with a 115-kDa phosphotyrosine protein

Downregulation of galectin-3 by EGF mediates the apoptosis of HepG2 cells

Epidermal growth factor (EGF) in high concentrations induces apoptosis of the tumor cells which express high levels of epidermal growth factor receptor. However, the precise mechanism for this induction is not clear. Galectin-3 is the most probable candidate for mediating this effect, as it is known to induce anti-apoptotic activity in a variety of tumor cells exposed to diverse apoptotic stimuli. In this study, we determined whether galectin-3 plays a role in high concentrations of EGF-induced apoptosis of HepG2 cells. We found that EGF in high concentrations led to the growth inhibition of HepG2 cells, which were associated with promotion of cell death. High concentrations of EGF suppressed cytoplasmic expression of galectin-3. Moreover, we demonstrated overexpression of galectin-3 could reduce EGF-induced apoptosis in HepG2 cells. Our study demonstrated for the first time that downregulation of cytoplasmic galectin-3 was essential for high concentrations of EGF-induced apoptosis in HepG2 cells.

Transcriptional Regulation of Signal Regulatory Protein α1 Inhibitory Receptors by Epidermal Growth Factor Receptor Signaling

Signal regulatory protein (SIRP) alpha1 is a membrane glycoprotein and a member of the SIRP receptor family. These transmembrane receptors have been shown to exert negative effects on signal transduction by receptor tyrosine kinases via immunoreceptor tyrosine-based inhibitory motifs in the carboxyl domain. Previous work has demonstrated that SIRPs negatively regulate many signaling pathways leading to reduction in tumor migration, survival, and cell transformation. Thus, modulation of SIRP expression levels or activity could be of great significance in the field of cancer therapy. The aim of the present study was to determine the factors that regulate levels of SIRPalpha1 in human glioblastoma cells that frequently overexpress the epidermal growth factor receptor (EGFR) because SIRPs have been shown to negatively regulate EGFR signaling. Northern blot analysis and immunoprecipitation assays showed variable expression levels of endogenous SIRPalpha transcripts in nine well-characterized glioblastoma cell lines. We examined SIRPalpha1 regulation in U87MG and U373MG cells in comparison with clonal derivatives that express a truncated form of erbB2, which negatively regulates EGFR signaling by inducing the formation of nonfunctional heterodimeric complexes. Mutant erbB2-expressing cells contained more SIRPalpha1 mRNA when compared with the parental cells in presence or absence of serum. Similarly, immunoprecipitation assays showed increased SIRPalpha1 protein levels in erbB-inhibited cells when compared with parental cells. Messenger RNA stability assays revealed that the increased mRNA levels in EGFR-inhibited cells were due to an induction of transcription. Consistent with this finding, expression of the erbB2 mutant receptor up-regulated SIRPalpha1 promoter activity in all cell lines tested. Interestingly, pharmacological inhibition of the kinase activities of EGFR, erbB2, and src and activation of mitogen-activated protein kinase, but not phosphatidylinositol 3'-kinase, significantly up-regulated SIRPalpha1 promoter activity. Based on these observations, we hypothesize that down-modulation of EGFR signaling leads to transcriptional up-regulation of the inhibitory SIRPalpha1 gene. These data may be important in the application of erbB-inhibitory strategies and for design of therapies for the treatment of glial tumors and other epithelial malignancies.

Increased susceptibility to ischemia-induced brain damage in transgenic mice overexpressing a dominant negative form of SHP2

Cell culture studies have established SH2 domain-containing protein tyrosine phosphatase-2 (SHP2) as an important factor in growth factor and cytokine-activated signaling pathways. However, the significance of SHP2 in the mammalian central nervous system (CNS) is not known since early embryonic lethality occurs in shp2 null mice. To bypass this embryonic lethality, transgenic animals containing a catalytically inactive mutant of SHP2 (SHP2-CS) under the control of a nestin intron II/thymidine kinase minimal promoter were generated. In the developing CNS of these animals, although high-level transgene expression was detected in the neuroepithelium, there was no obvious abnormality in progenitor cell proliferation or migration. In the adult brain, high-level transgene expression was detected in the subventricular zone, rostral migratory stream, dentate gyrus of hippocampus, and cerebellum. Because SHP2 function is likely important in cell survival pathways, we used a focal cerebral ischemia model to examined whether SHP2 is important during CNS injury. Ischemia-induced damage and neuronal death was found to be significantly greater in nestin-SHP2-CS mice than in wild-type littermates. These findings indicate that SHP2 is a required factor in signaling pathway(s) important for neuronal survival.

Inhibition of EGFR-mediated phosphoinositide-3-OH kinase (PI3-K) signaling and glioblastoma phenotype by signal-regulatory proteins (SIRPs)

Several growth factors and cytokines, including EGF, are known to induce tyrosine phosphorylation of Signal Regulatory Proteins (SIRPs). Consistent with the idea that increased phosphorylation activates SIRP function, we overexpressed human SIRPalpha1 in U87MG glioblastoma cells in order to examine how SIRPalpha1 modulates EGFR signaling pathways. Endogenous EGFR proteins are overexpressed in U87MG cells and these cells exhibit survival and motility phenotypes that are influenced by EGFR kinase activity. Overexpression of the SIRPalpha1 cDNA diminished EGF-induced phosphoinositide-3-OH kinase (PI3-K) activation in U87MG cells. Reduced EGF-stimulated activation of PI3-K was mediated by interactions between carboxyl terminus of SIRPalpha1 and the Src homology-2 (SH2)-containing phosphotyrosine phosphatase, SHP2. SIRPalpha1 overexpression also reduced the EGF-induced association between SHP2 and the p85 regulatory subunit of PI3-K. Inhibition of transformation and enhanced apoptosis following gamma-irradiation were observed in SIRPalpha1-overexpressing U87MG cells, and enhanced apoptosis was associated with reduced levels of bcl-xL protein. Furthermore, SIRPalpha1-overexpressing U87MG cells displayed reduced cell migration and cell spreading that was mediated by association between SIRPalpha1 and SHP2. However, SIRPalpha1-overexpressing U87MG clonal derivatives exhibited no differences in cell growth or levels of mitogen-activated protein kinase (MAPK) activation. These data reveal a pathway that negatively regulates EGFR-induced PI3-K activation in glioblastoma cells and involves interactions between SHP2 and tyrosine phosphorylated SIRPalpha1. These results also suggest that negative regulation of PI3-K pathway activation by the SIRP family of transmembrane receptors may diminish EGFR-mediated motility and survival phenotypes that contribute to transformation of glioblastoma cells. Oncogene (2000) 19, 3999 - 4010.

Mechanism of extracellular signal-regulated kinase (ERK)-1 and ERK-2 activation by vanadium pentoxide in rat pulmonary myofibroblasts

Vanadium pentoxide (V(2)O(5)) is a cause of occupational asthma and chronic bronchitis, yet the molecular mechanisms through which V(2)O(5) exerts its effects on cell function are unclear. In this study we investigated the potential of V(2)O(5) to activate the extracellular signal-regulated kinases 1 and 2 (ERK-1/2) in rat pulmonary myofibroblasts. Treatment of myofibroblasts with V(2)O(5) resulted in the activation of ERK-1/2, yet the inert metal titanium dioxide had no effect on ERK-1/2 activation. V(2)O(5)-induced ERK-1/2 activation was abolished by pretreatment with forskolin or PD98059, indicating a dependence on Raf and mitogen-activated protein (MAP) kinase kinase, respectively. Depletion of conventional protein kinase C activity with phorbol 12-myristate 13-acetate did not inhibit V(2)O(5)-induced ERK-1/2 activation. ERK-1/2 activation by V(2)O(5) was inhibited > 70% with the epidermal growth factor receptor (EGF-R) tyrosine kinase inhibitor AG1478. Phosphorylation of the 170-kD EGF-R by V(2)O(5) was detected after immunoprecipitation with an anti-EGF-R antibody followed by phosphotyrosine Western blotting. V(2)O(5) strongly tyrosine-phosphorylated a 115-kD protein (p115) and activation of p115 was inhibited 60 to 70% by AG1478, indicating that this protein was an EGF-R substrate. Phosphorylation of p115 was also observed in EGF-stimulated cells. Immunoprecipitation of V(2)O(5)- or EGF-treated cell lysates with an antibody against Src homology 2 protein tyrosine phosphatase (SH-PTP2) identified p115 as a SH-PTP2-binding protein. Pretreatment of cells with the antioxidant N-acetyl-L-cysteine blocked V(2)O(5)-induced MAP kinase activation and p115 phosphorylation > 90%. These data suggest that V(2)O(5) activation of ERK-1/2 is oxidant-dependent and mediated through tyrosine phosphorylation of EGF-R and an EGF-R substrate which we identified as a 115-kD SH-PTP2-binding protein.

Activated mutants of SHP-2 preferentially induce elongation of Xenopus animal caps

In Xenopus ectodermal explants (animal caps), fibroblast growth factor (FGF) evokes two major events: induction of ventrolateral mesodermal tissues and elongation. The Xenopus FGF receptor (XFGFR) and certain downstream components of the XFGFR signal transduction pathway (e.g., members of the Ras/Raf/MEK/mitogen-activated protein kinase [MAPK] cascade) are required for both of these processes. Likewise, activated versions of these signaling components induce mesoderm and promote animal cap elongation. Previously, using a dominant negative mutant approach, we showed that the protein-tyrosine phosphatase SHP-2 is necessary for FGF-induced MAPK activation, mesoderm induction, and elongation of animal caps. Taking advantage of recent structural information, we now have generated novel, activated mutants of SHP-2. Here, we show that expression of these mutants induces animal cap elongation to an extent comparable to that evoked by FGF. Surprisingly, however, activated mutant-induced elongation can occur without mesodermal cytodifferentiation and is accompanied by minimal activation of the MAPK pathway and mesodermal marker expression. Our results implicate SHP-2 in a pathway(s) directing cell movements in vivo and identify potential downstream components of this pathway. Our activated mutants also may be useful for determining the specific functions of SHP-2 in other signaling systems.

The linoleic acid metabolite, 13-HpODE augments the phosphorylation of EGF receptor and SHP-2 leading to their increased association

In previous studies with Syrian hamster embryo fibroblasts, we found that a specific lipoxygenase metabolite of linoleic acid, 13(S)-hydroperoxyoctadecadienoic acid (HpODE), enhanced epidermal growth factor (EGF) signal transduction in a tumor suppressor gene plus phenotype (supB+); with a diminished response to 13(S)-HpODE in a tumor suppressor gene minus phenotype (supB-). This differential response was attributed to differences in the rate of EGF receptor (EGFR) dephosphorylation. To further define the molecular basis for these observations, in this report we examine the interaction of phosphorylated EGFR with the SH2 domain-containing protein tyrosine phosphatase, SHP-2, a positive modulator of EGF dependent cell growth. SHP-2 associated with phosphorylated EGFR to a greater extent in supB+ cells when compared to supB-. This differential association could not be accounted for by differences between suppressor gene phenotypes in SHP-2 protein level or mutations in the molecular sequence. The addition of 13(S)-HpODE stimulated a concentration-dependent increase in EGF-dependent phosphorylation of SHP-2 and its association with EGFR. A more dramatic response was observed in the supB+ cells. Differences in SHP-2 interaction with EGFR may account, in part, for phenotypic differences in the growth rates and responsiveness to EGF between the supB+ and supB- cells. EGFR-SHP-2 association appears to play an important role in the regulation of EGFR signal transduction.

Gab-Family Adapter Proteins Act Downstream of Cytokine and Growth Factor Receptors and T- and B-Cell Antigen Receptors

We previously found that the adapter protein Gab1 (110 kD) is tyrosine-phosphorylated and forms a complex with SHP-2 and PI-3 kinase upon stimulation through either the interleukin-3 receptor (IL-3R) or gp130, the common receptor subunit of IL-6–family cytokines. In this report, we identified another adapter molecule (100 kD) interacting with SHP-2 and PI-3 kinase in response to various stimuli. The molecule displays striking homology to Gab1 at the amino acid level; thus, we named it Gab2. It contains a PH domain, proline-rich sequences, and tyrosine residues that bind to SH2 domains when they are phosphorylated. Gab1 is phosphorylated on tyrosine upon stimulation through the thrombopoietin receptor (TPOR), stem cell factor receptor (SCFR), and T-cell and B-cell antigen receptors (TCR and BCR, respectively), in addition to IL-3R and gp130. Tyrosine phosphorylation of Gab2 was induced by stimulation through gp130, IL-2R, IL-3R, TPOR, SCFR, and TCR. Gab1 and Gab2 were shown to be substrates for SHP-2 in vitro. Overexpression of Gab2 enhanced the gp130 or Src-related kinases–mediated ERK2 activation as that of Gab1 did. These data indicate that Gab-family molecules act as adapters for transmitting various signals.

Formation of distinct signalling complexes involving phosphatidylinositol 3-kinase activity with stimulation of epidermal growth factor or insulin-like growth factor-I in human skin fibroblasts

We recently described a better correlation of DNA synthesis with phosphatidylinositol (PI) 3-kinase than with mitogen-activated protein (MAP) kinase stimulated by insulin-like growth factor (IGF)-1 or epidermal growth factor (EGF) in human skin fibroblasts (Takahashi et al., 1997, Endocrinology 138:741-750). IGF-I-induced PI 3-kinase activation is generally mediated via insulin receptor substrate (IRS)-1, but EGF-induced PI 3-kinase activation is mediated by various signalling molecules such as ErbB3 and c-Cbl in different cells. We therefore investigated the mechanism regulating PI 3-kinase in human skin fibroblasts by comparing complexes involving PI 3-kinase when stimulated by IGF-I or EGF and found that p115 and p105, which were tyrosine-phosphorylated by EGF stimulation and associated with SHP-2, were also associated with the p85 subunit of PI 3-kinase by EGF. Anti-SHP-2 and anti-p85 subunits of PI 3-kinase antibodies did not coprecipitate tyrosine-phosphorylated EGF receptor or ErbB3; in addition, p115 and p105 appeared to be distinct from tyrosine-phosphorylated c-Cbl. Thus, tyrosine-phosphorylated p115 and p105 may provide a novel platform recruiting p85, which may simultaneously bind to SHP-2. In contrast, tyrosine phosphorylation of p115 or p 105 was undetectable by immunoblot with IGF-I stimulation, and PI 3-kinase activity was mediated via IRS-1 phosphorylated with IGF-I stimulation, little of which was associated with SHP-2. Thus, EGF and IGF-I cause formation of a distinct signalling complex which associates with p85 subunit of PI 3-kinase.

B Cell Antigen Receptor (BCR)-Mediated Formation of a SHP-2-pp120 Complex and Its Inhibition by FeγRIIB1-BCR Coligation

Accumulating evidence indicates that the Src homology 2-containing tyrosine phosphatase 2 (SHP-2) plays an important role in signal transduction through receptor tyrosine kinase and cytokine receptors. In most models, SHP-2 appears to be a positive mediator of signaling. However, coligation of FcγRIIB1 with B cell Ag receptors (BCR) inhibits BCR-mediated signaling by a mechanism that may involve recruitment of phosphatases SHP-1, SHP-2, and the SH2 containing inositol 5′phosphatase (SHIP) to the phosphorylated FcγRIIB1 immunoreceptor tyrosine-based inhibitory motif. The role of SHP-2 in BCR-mediated cell activation and in FcγRIIB1-mediated inhibitory signaling is unclear. In this study we assessed the association of SHP-2 with phosphotyrosine-containing cellular protein(s) before and after stimulation through these receptors. BCR stimulation induced the association of SHP-2 with a single major tyrosyl-phosphorylated molecule (pp120) that had an apparent molecular mass of 120 kDa. Coligation of FcγRIIB1 with BCR led to a rapid decrease in SHP-2 association with pp120. Analysis of the subcellular localization of pp120 showed that the complex of SHP-2 and tyrosyl-phosphorylated p120 occurs predominantly in the cytosol. Furthermore, the binding of the two molecules was mediated by the interaction of tyrosyl-phosphorylated p120 with the SHP-2 N-terminal SH2 domain. These findings indicate that SHP-2 and pp120 function in BCR signaling, and this function may be inhibited by FcγRIIB1 signaling.

A common requirement for the catalytic activity and both SH2 domains of SHP-2 in mitogen-activated protein (MAP) kinase activation by the ErbB family of receptors. A specific role for SHP-2 in map, but not c-Jun amino-terminal kinase activation

The ErbB family of receptors, which include the epidermal growth factor receptor (EGFR), ErbB2, ErbB3, and ErbB4 mediate the actions of a family of bioactive polypeptides. EGF signals through EGFR, whereas heregulin (HRG) signaling is initiated through binding to either ErbB3 or ErbB4. In this report we studied the role of protein-tyrosine phosphatase SHP-2 in ErbB-mediated activation of mitogen-activated protein kinase (MAPK) by overexpressing SHP-2 mutants in COS-7 cells. We demonstrate that enzymatic activity and both NH2- and COOH-terminal SH2 domains of SHP-2 are required for EGF-induced MAPK activation, but not for c-Jun amino-terminal kinase stimulation or MAPK activation which occurred in response to myristoylated son of sevenless, activated Ras, or phorbol ester. Dominant-negative forms of SHP-2 had no effect on EGF-stimulated interaction of GRB2 with EGFR or SHC, nor did they influence phosphorylation of SHC and SHC/EGFR association. The same mutant SHP-2 structures that inhibited EGF-mediated stimulation of MAPK also blocked HRG alpha/beta-induced MAPK activation. EGF or HRG beta caused SHP-2 SH2 domains to engage multiple phosphotyrosine proteins, and mutation of either domain disrupted these associations. These results demonstrate that SHP-2 performs a common and essential function(s) in ligand-stimulated MAPK activation by the ErbB family of receptors.

Genetic analysis of protein tyrosine phosphatases

Genetic analysis has enhanced our understanding of the biological roles of many protein tyrosine kinases (PTKs). More recently, studies utilizing both spontaneous mutants and mutants induced by homologous recombination techniques have begun to yield key insights into the role of specific protein tyrosine phosphatases (PTPs) and to suggest how PTKs and PTPs interact. Specific PTPs in Saccharomyces cerevesiae and Schizomyces pombe regulate MAP kinase pathways. Several Drosophila receptor PTPs control axonal targeting pathways, whereas the non-receptor PTP Corkscrew (Csw), plays an essential positive signaling role in multiple developmental pathways directed by receptor PTKs. The vertebrate homolog of Csw, SHP-2, also is required for growth factor signaling and normal development. Finally, very recent studies of other mammalian PTPs suggest that they have critical roles in processes as diverse as hematopoiesis and liver and pituitary development.

Involvement of the Src homology 2-containing tyrosine phosphatase SHP-2 in growth hormone signaling

Growth hormone (GH) signaling requires activation of the GH receptor (GHR)-associated tyrosine kinase, JAK2. JAK2 activation by GH is believed to facilitate initiation of various pathways including the Ras, mitogen-activated protein kinase, STAT, insulin receptor substrate (IRS), and phosphatidylinositol 3-kinase systems. In the present study, we explore the biochemical and functional involvement of the Src homology 2 (SH2)-containing protein-tyrosine phosphatase, SHP-2, in GH signaling. GH stimulation of murine NIH 3T3-F442A fibroblasts, cells that homologously express GHRs, resulted in tyrosine phosphorylation of SHP-2. As assessed specifically by anti-SHP-2 coimmunoprecipitation and by affinity precipitation with a glutathione S-transferase fusion protein incorporating the SH2 domains of SHP-2, GH induced formation of a complex of tyrosine phosphoproteins including SHP-2, GHR, JAK2, and a glycoprotein with properties consistent with being a SIRP-alpha-like molecule. A reciprocal binding assay using IM-9 cells as a source of SHP-1 and SHP-2 revealed specific association of SHP-2 (but not SHP-1) with a glutathione S-transferase fusion incorporating GHR cytoplasmic domain residues 485-620, but only if the fusion was first rendered tyrosine-phosphorylated. GH-dependent tyrosine phosphorylation of SHP-2 was also observed in murine 32D cells (which lack IRS-1 and -2) stably transfected with the GHR. Further, GH-dependent anti-SHP-2 coimmunoprecipitation of the Grb2 adapter protein was detected in both 3T3-F442A and 32D-rGHR cells, indicating that biochemical involvement of SHP-2 in GH signaling may not require IRS-1 or -2. Finally, GH-induced transactivation of a c-Fos enhancer-driven luciferase reporter in GHR- and JAK2-transfected COS-7 cells was significantly reduced when a catalytically inactive SHP-2 mutant (but not wild-type SHP-2) was coexpressed; in contrast, expression of a catalytically inactive SHP-1 mutant allowed modestly enhanced GH-induced transactivation of the reporter in comparison with that found with expression of wild-type SHP-1. Collectively, these biochemical and functional data imply a positive role for SHP-2 in GH signaling.

Structural determinants of SHP-2 function and specificity in Xenopus mesoderm induction

SHP-2 is a positive component of many receptor tyrosine kinase signaling pathways. The related protein-tyrosine phosphatase (PTP) SHP-1 usually acts as a negative regulator. The precise domains utilized by SHP-2 to transmit positive signals in vivo and the basis for specificity between SHP-1 and SHP-2 are not clear. In Xenopus, SHP-2 is required for mesoderm induction and completion of gastrulation. We investigated the effects of SHP-2 mutants and SHP-2/SHP-1 chimeras on basic fibroblast growth factor-induced mesoderm induction. Both SH2 domains and the PTP domain are required for normal SHP-2 function in this pathway. The N-terminal SH2 domain is absolutely required, whereas the C-terminal SH2 contributes to wild-type function. The C-terminal tyrosyl phosphorylation sites and proline-rich region are dispensable, arguing against adapter models of SHP-2 function. Although the SH2 domains contribute to SHP-2 specificity, studies of SHP chimeras reveal that substantial specificity resides in the PTP domain. Thus, PTP domains exhibit biologically relevant specificity in vivo, and noncatalytic and catalytic domains of PTPs contribute to specificity in a combinatorial fashion.

Interleukin-3 induces association of the protein-tyrosine phosphatase SHP2 and phosphatidylinositol 3-kinase with a 100-kDa tyrosine-phosphorylated protein in hemopoietic cells

We have observed previously the co-immunoprecipitation of the p85 subunit of phosphatidylinositol-3 kinase (PI3K) and SHP2 in murine lymphohemopoietic cells after stimulation with interleukin-3. We have investigated this interaction in more detail and now report the identification of a potentially novel 100-kDa protein (termed p100), which is inducibly phosphorylated on tyrosine after interleukin-3 treatment and which co-immunoprecipitates with both p85 PI3K and SHP2. The Src homology region 2 domains of both p85 and SHP2 appear to mediate their interactions with p100. Sequential precipitation analyses suggest that these interactions are direct and do not involve Grb2, and that the same p100 protein, or a portion of it, interacts with both p85 and SHP2, implying that p100 may serve to link these two proteins. Far Western blotting with both full-length p85 and isolated p85 Src homology region 2 domains supports this view. Interestingly, p100 also appears to be a substrate for the SHP2 phosphatase activity. In addition, p100 is precipitated by Grb2-glutathione S-transferase fusion proteins, an interaction largely mediated by the Grb2 SH3 domains. p100 appears to be distinct from JAK2, Vav, STAT5, and c-Cbl. Although largely cytosolic, p100 can be detected associated with SHP2 and PI3K in crude membrane fractions after interleukin-3 stimulation. We propose that p100 plays a role as an adaptor molecule, linking PI3K and SHP2 in IL-3 signaling.

Epidermal growth factor stimulates the tyrosine phosphorylation of SHPS-1 and association of SHPS-1 with SHP-2, a SH2 domain-containing protein tyrosine phosphatase

SHPS-1 is a 120 kDa glycosylated receptor-like protein that contains immunoglobulin-like domains in its extracellular region and four potential tyrosine phosphorylation for SH2 domain binding sites in its cytoplasmic region. Epidermal growth factor (EGF) stimulated the rapid tyrosine phosphorylation of SHPS-1 and subsequent association of SHPS-1 with SHP-2, a protein tyrosine phosphatase containing SH2 domains, in Chinese hamster ovary cells overexpressing human EGF receptors. In the cells overexpressing SHPS-1, the tyrosine phosphorylation of SHPS-1 was more evident than that observed in parent cells. However, overexpression of SHPS-1 alone did not affect the activation of MAP kinase in response to EGF. These results suggest that SHPS-1 may be involved in the recruitment of SHP-2 from the cytosol to the plasma membrane in response to EGF.

Kinase-deficient forms of Jak1 and Tyk2 inhibit interferon alpha signaling in a dominant manner

Signaling by interferon alpha (IFN alpha), an extracellular factor that mediates a number of anti-viral and growth-suppressive effects, requires two members of the Janus family of tyrosine kinases (JAK family): Jak1 and Tyk2. IFN alpha treatment of cells induces the rapid tyrosine phosphorylation of these two kinases, two subunits of the IFN alpha receptor, and two members of the signal transducer and activator of transcription (STAT) family of latent transcription factors. These proteins are believed to be direct substrates of one or both JAKs. Though the requirement for both Jak1 and Tyk2 in the IFN alpha-signaling cascade is well established, the order of activation and the relative contribution of the two kinases has not been elucidated completely. To address these questions, we have employed kinase-deficient mutants of both enzymes. Both mutant kinases suppress transcriptional activation as measured by an IFN alpha-dependent reporter-gene assay. Furthermore, in transient-transfection assays, the kinase-deficient versions of Tyk2 and Jak1 can act independently to inhibit STAT phosphorylation. Thus, kinase-deficient versions of JAK can act in a dominant-negative fashion to suppress IFN alpha signaling. The effects of the overexpressed mutant kinases on the phosphorylation of the kinases themselves, however, are unequal, suggesting that Jak1 functions upstream of Tyk2.

Characterization of two SHP-2-associated binding proteins and potential substrates in hematopoietic cells

Multiple studies have demonstrated an important role for the Src homology 2-containing tyrosine phosphatase 2 (SHP-2) in receptor tyrosine kinase-regulated cell proliferation and differentiation. Recent studies have identified potential SHP-2 substrates which mediate these effects. SHP-2 also is implicated in several cytokine receptor signaling pathways and in Bcr-Abl transformation. However, its precise role and targets in normal and abnormal hematopoietic cells remain to be determined. We identified two novel tyrosyl-phosphorylated proteins associated with SHP-2 in hematopoietic cells. The first, a 97-kDa cytosolic protein (p97), associates inducibly with SHP-2 upon cytokine stimulation and constitutively in Bcr-Abl-transformed cells. In contrast, p135, a 135-kDa transmembrane glycoprotein, forms a distinct complex with SHP-2, independent of cytokine stimulation or Bcr-Abl transformation. Far Western analysis reveals that SHP-2, via its Src homology 2 domains, can interact directly with either protein. In vitro dephosphorylation experiments, as well as transient transfection studies using wild type and mutant SHP-2 constructs, suggest that p97 and p135 also are SHP-2 substrates. Our results indicate that SHP-2 forms at least two separate complexes in hematopoietic cells and point to new potential SHP-2 targets.

Characterization of a novel tyrosine phosphorylated 100-kDa protein that binds to SHP-2 and phosphatidylinositol 3'-kinase in myeloid cells

Fms is a tyrosine kinase-containing receptor for macrophage colony-stimulating factor (M-CSF) that regulates survival, growth, and differentiation of cells along the monocyte/macrophage lineage. M-CSF stimulation of murine myeloid FDC-P1 cells expressing Fms resulted in the tyrosine phosphorylation of a number of signal transduction proteins, including an unidentified 100-kDa protein. This 100-kDa protein associated with the tyrosine phosphatase SHP-2 but not with the related phosphatase SHP-1. The kinetics of tyrosine phosphorylation of p100 and SHP-2 suggest that p100 may be a direct substrate of SHP-2. p100 bound directly to the SH2 domains of both SHP-2 and the p85 subunit of phosphatidylinositol 3'-kinase. The 100-kDa protein did not appear to bind directly to Fms, Ship, Cbl, Shc, or Grb2, although all of these proteins were coimmunoprecipitated with p85 after M-CSF stimulation. Association of p100 with SHP-2 and p85 did not require the major autophosphorylation sites on Fms nor binding of p85 to Fms. A tyrosine phosphorylated protein of 100 kDa also coprecipitated with SHP-2 from several other myeloid cell lines after M-CSF stimulation but was not seen in immunoprecipitates from Rat2 fibroblasts expressing Fms. Stimulation of FDC-P1 cells with additional cytokines also resulted in coprecipitation of a 100-kDa protein with SHP-2. p100 may therefore be a common component of the signaling pathways of cytokine receptors in myeloid cells.

Role of phosphatases in lymphocyte activation

Many lymphocyte signaling pathways are regulated by protein tyrosyl phosphorylation, which is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Substantial progress has been made in defining the functions of lymphocyte PTPs. Individual PTPs can enhance or diminish cell signaling levels. The transmembrane PTP CD45 is a key positive element in multiple lymphocyte signaling pathways in vivo. New insights into the function of individual CD45 isoforms have emerged. Anti-CD45 antibodies with potent immunosuppressant activity have been identified, suggesting that CD45 may be a propitious target for drug design. Progress has also been made in elucidating the function and targets of specific nontransmembrane PTPs, particularly those with Src homology 2 domains.

Protein tyrosine phosphatases in signal transduction

Protein-tyrosyl phosphorylation, regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs), is a key cellular control mechanism. Until recently, little was known about PTPs. However, the past two years have witnessed an explosion of information about PTP structure, regulation and function. Crystal structures of several PTPs have provided insights into enzymatic mechanisms and regulation and suggested the design of 'substrate-trapping' mutants. Candidate homophilic and heterophilic ligands for transmembrane PTPs have been identified, and roles for transmembrane PTPs in regulating cell-cell interactions have been suggested. Finally, progress has been made in understanding signaling by Src homology 2 domain containing PTPs and PTPs controlling yeast osmoregulatory pathways.

A family of proteins that inhibit signalling through tyrosine kinase receptors

Phosphotyrosine phosphatases are critical negative or positive regulators in the intracellular signalling pathways that result in growth-factor-specific cell responses such as mitosis, differentiation, migration, survival, transformation or death. The SH2-domain-containing phosphotyrosine phosphatase SHP-2 is a positive signal transducer for several receptor tyrosine kinases (RTKs) and cytokine receptors. To investigate its mechanism of action we purified a tyrosine-phosphorylated glycoprotein which in different cell types associates tightly with SHP-2 and appears to serve as its substrate. Peptide sequencing in conjunction with complementary DNA cloning revealed a new gene family of at least fifteen members designated signal-regulatory proteins (SIRPs). They consist of two subtypes distinguished by the presence or absence of a cytoplasmic SHP-2-binding domain. The transmembrane polypeptide SIRP alpha1 is a substrate of activated RTKs and in its tyrosine-phosphorylated form binds SHP-2 through SH2 interactions and acts as its substrate. It also binds SHP-1 and Grb2 in vitro and has negative regulatory effects on cellular responses induced by growth factors, oncogenes or insulin. Our findings indicate that proteins belonging to the SIRP family generally regulate signals defining different physiological and pathological processes.

Mechanism of inhibition of protein-tyrosine phosphatases by vanadate and pervanadate

Vanadate and pervanadate (the complexes of vanadate with hydrogen peroxide) are two commonly used general protein-tyrosine phosphatase (PTP) inhibitors. These compounds also have insulin-mimetic properties, an observation that has generated a great deal of interest and study. Since a careful kinetic study of the two inhibitors has been lacking, we sought to analyze their mechanisms of inhibition. Our results show that vanadate is a competitive inhibitor for the protein-tyrosine phosphatase PTP1B, with a Ki of 0.38+/-0.02 microM. EDTA, which is known to chelate vanadate, causes an immediate and complete reversal of the inhibition due to vanadate when added to an enzyme assay. Pervanadate, by contrast, inhibits by irreversibly oxidizing the catalytic cysteine of PTP1B, as determined by mass spectrometry. Reducing agents such as dithiothreitol that are used in PTP assays to keep the catalytic cysteine reduced and active were found to convert pervanadate rapidly to vanadate. Under certain conditions, slow time-dependent inactivation by vanadate was observed; since catalase blocked this inactivation, it was ascribed to in situ generation of hydrogen peroxide and subsequent formation of pervanadate. Implications for the use of these compounds as inhibitors and rationalization for some of their in vivo effects are considered.

A novel membrane glycoprotein, SHPS-1, that binds the SH2-domain-containing protein tyrosine phosphatase SHP-2 in response to mitogens and cell adhesion

Protein tyrosine phosphatases (PTPases), such as SHP-1 and SHP-2, that contain Src homology 2 (SH2) domains play important roles in growth factor and cytokine signal transduction pathways. A protein of approximately 115 to 120 kDa that interacts with SHP-1 and SHP-2 was purified from v-src-transformed rat fibroblasts (SR-3Y1 cells), and the corresponding cDNA was cloned. The predicted amino acid sequence of the encoded protein, termed SHPS-1 (SHP substrate 1), suggests that it is a glycosylated receptor-like protein with three immunoglobulin-like domains in its extracellular region and four YXX(L/V/I) motifs, potential tyrosine phosphorylation and SH2-domain binding sites, in its cytoplasmic region. Various mitogens, including serum, insulin, and lysophosphatidic acid, or cell adhesion induced tyrosine phosphorylation of SHPS-1 and its subsequent association with SHP-2 in cultured cells. Thus, SHPS-1 may be a direct substrate for both tyrosine kinases, such as the insulin receptor kinase or Src, and a specific docking protein for SH2-domain-containing PTPases. In addition, we suggest that SHPS-1 may be a potential substrate for SHP-2 and may function in both growth factor- and cell adhesion-induced cell signaling.

Characterization of a 115-kDa protein that binds to SH-PTP2, a protein-tyrosine phosphatase with Src homology 2 domains, in Chinese hamster ovary cells

SH-PTP2, a non-transmembrane-type protein-tyrosine phosphatase with two Src homology 2 domains, was previously shown to play a positive signaling role in the insulin-induced activation of Ras and mitogen-activated protein kinase. SH-PTP2 was shown to associate with a 115-kDa tyrosine-phosphorylated protein (pp115), as well as with insulin receptor substrate 1, in insulin-stimulated Chinese hamster ovary cells that overexpress human insulin receptors (CHO-IR cells). In vivo and in vitro binding experiments revealed that SH-PTP2 bound to pp115 through one or both of its SH2 domains. The pp115 protein was partially purified from insulin-stimulated CHO-IR cells that overexpress a catalytically inactive SH-PTP2 by a combination of immunoaffinity and lectin-affinity chromatography. A monoclonal antibody to pp115 was then generated by injecting the partially purified protein into mice. Experiments with this monoclonal antibody revealed that pp115 is a transmembrane protein with a domain exposed on the cell surface and that it binds to SH-PTP2 in response to insulin. The insulin receptor kinase appeared to phosphorylate pp115 on tyrosine residues both in vivo and in vitro. The extent of tyrosine phosphorylation of pp115 associated with SH-PTP2 was greatly increased in CHO-IR cells that overexpress catalytically inactive SH-PTP2 compared with that observed in CHO-IR cells overexpressing wild-type SH-PTP2. Furthermore, recombinant SH-PTP2 preferentially dephosphorylated pp115 in vitro, indicating that SH-PTP2 may catalyze the dephosphorylation of phosphotyrosine residues in pp115 after it binds to this protein. These results suggest that pp115 may act as a transmembrane anchor to which SH-PTP2 binds in response to insulin. Furthermore, pp115 may be a physiological substrate for both the insulin receptor kinase and SH-PTP2.

The SH2 domain-containing tyrosine phosphatase PTP1D is required for interferon alpha/beta-induced gene expression

Interferons (IFNs) induce early response genes by stimulating Janus family (Jak) tyrosine kinases, leading to tyrosine phosphorylation of Stat (signal transducer and activator of transcription) proteins. Previous studies demonstrated that a protein-tyrosine phosphatase (PTP) is required for activation of the ISGF3 transcription complex by IFNalpha/beta, but the specific PTP responsible remained unidentified. We now show that the SH2 domain containing tyrosine phosphatase PTP1D (also designated as SHPTP2, SHPTP3, PTP2C, or Syp) is constitutively associated with the IFNalpha/beta receptor and becomes tyrosine-phosphorylated in response to ligand. Furthermore, transient expression of a phosphatase-inactive mutant or the COOH-terminal SH2 domain of PTP1D causes a dominant negative effect on IFNalpha/beta-induced early response gene expression. These results provide strong evidence that PTP1D functions as a positive regulator of the IFNalpha/beta-induced Jak/Stat signal transduction pathway.

A tyrosine-phosphorylated 110-120-kDa protein associates with the C-terminal SH2 domain of phosphotyrosine phosphatase-1D in T cell receptor-stimulated T cells

The role of cytosolic phosphotyrosine phosphatases (PTPase) in T cell receptor (TCR)-mediated signaling was investigated. PTPase activity was detected in a purified immunocomplex comprising aggregated TCR from the cell surface of Jurkat T cells. Since TCR aggregation results in phosphorylation of critical immunoreceptor tyrosine-based activation motifs (ITAM) in the TCR zeta chain, a doubly tyrosine-phosphorylated synthetic peptide containing the membrane-proximal zeta chain ITAM (zeta p ITAM) was used to characterize TCR zeta-associated PTPases. PTPase activity was detected in stable association with zeta p ITAM and the SH2 domain-containing PTPase PTP-1D (Syp, SH-PTP2) was identified in this complex. TCR stimulation resulted in increased total PTPase activity and PTP-1D protein in zeta p ITAM precipitates. TCR stimulation did not result in the tyrosine phosphorylation of PTP-1D but caused the rapid and transient tyrosine phosphorylation of a 110-120-kDa protein which associated selectively with the C-terminal SH2 domain of PTP-1D. This currently unidentified phosphotyrosine protein may be involved in localizing PTP-1D to the TCR following receptor stimulation.