Protein-tyrosine phosphatase 1D modulates its own state of tyrosine phosphorylation

CLEC12B is a melanocytic gene regulating the color of the skin

Pigmentation of the human skin is a complex process regulated by many genes. However, only a few have a profound impact on melanogenesis. Transcriptome analysis of pigmented skin compared to vitiligo skin devoid of melanocytes, allowed us to unravel CLEC12B as a melanocytic gene. We demonstrated that CLEC12B, a C-type lectin receptor, is highly expressed in melanocytes, and that its expression is decreased in dark skin compared to white skin. CLEC12B directly recruits and activates SHP-1 and SHP-2 through its ITIM domain and promotes CREB degradation, leading to downregulation of the downstream MITF pathway. CLEC12B ultimately controls melanin production and pigmentation in vitro and in a model of reconstructed human epidermis. The identification of CLEC12B in melanocyte demonstrates that C-type lectin receptors exert function beyond immunity and inflammation. It also provides insights in the understanding of the melanocyte biology and regulation of melanogenesis.

CLEC12B Decreases Melanoma Proliferation by Repressing Signal Transducer and Activator of Transcription 3

The potential role of CLEC12B, a gene predominantly expressed by skin melanocytes discovered through transcriptomic analysis, in melanoma is unknown. In this study, we show that CLEC12B expression is lower in melanoma and melanoma metastases than in melanocytes and benign melanocytic lesions and that its decrease correlates with poor prognosis. We further show that CLEC12B recruits SHP2 phosphatase through its immunoreceptor tyrosine-based inhibition motif domain, inactivates signal transducer and activator of transcription 1/3/5, increases p53/p21/p27 expression/activity, and modulates melanoma cell proliferation. The growth of human melanoma cells overexpressing CLEC12B in nude mice after subcutaneous injection is significantly decreased compared with that in the vehicle control group and is associated with decreased signal transducer and activator of transcription 3 phosphorylation and increased p53 levels in the tumors. Reducing the level of CLEC12B had the opposite effect. We show that CLEC12B represses the activation of the signal transducer and activator of transcription pathway and negatively regulates the cell cycle, providing a proliferative asset to melanoma cells.

Systems biology of IL-6, IL-12 family cytokines

Interleukin-6-type cytokines play important roles in the communication between cells of multicellular organisms. They are involved in the regulation of complex cellular processes such as proliferation and differentiation and act as key player during inflammation and immune response. A major challenge is to understand how these complex non-linear processes are connected and regulated. Systems biology approaches are used to tackle this challenge in an iterative process of quantitative experimental and mathematical analyses. Here we review quantitative experimental studies and systems biology approaches dealing with the function of Interleukin-6-type cytokines in physiological and pathophysiological conditions. These approaches cover the analyses of signal transduction on a cellular level up to pharmacokinetic and pharmacodynamic studies on a whole organism level.

Role of SHP2 phosphatase in KIT-induced transformation: identification of SHP2 as a druggable target in diseases involving oncogenic KIT

Intracellular mechanism(s) that contribute to promiscuous signaling via oncogenic KIT in systemic mastocytosis and acute myelogenous leukemia are poorly understood. We show that SHP2 phosphatase is essential for oncogenic KIT-induced growth and survival in vitro and myeloproliferative disease (MPD) in vivo. Genetic disruption of SHP2 or treatment of oncogene-bearing cells with a novel SHP2 inhibitor alone or in combination with the PI3K inhibitor corrects MPD by disrupting a protein complex involving p85α, SHP2, and Gab2. Importantly, a single tyrosine at position 719 in oncogenic KIT is sufficient to develop MPD by recruiting p85α, SHP2, and Gab2 complex to oncogenic KIT. Our results demonstrate that SHP2 phosphatase is a druggable target that cooperates with lipid kinases in inducing MPD.

Protein tyrosine phosphatase SHP2 mediates chronic insulin-induced endothelial inflammation

OBJECTIVE

Insulin promotes adhesion of leukocytes to the endothelium through increased expression of surface adhesion molecules. We determined whether src-homology domain-2-containing protein tyrosine phosphatase 2 (SHP2), a downstream effecter of insulin signaling, is involved in insulin-induced endothelial inflammation.

METHODS AND RESULTS

In human umbilical vein-derived endothelial cells, treatment with insulin (100 nmol/L) increased Tyr(542) phosphorylation, activity, and subsequently expression of SHP2. Increase in SHP2 accompanied a parallel decrease in the availability of the anti-inflammatory molecule, NO. This consequently enhanced the expression of cell adhesion molecules. Decrease in NO index was caused by endothelial NO synthase (eNOS) uncoupling and increased arginase activity. Among the 2 isoforms, insulin treatment induced the expression of arginase II. Inactivation of endogenous SHP2 via NSC87877 [8-hydroxy-7-(6-sulfonapthalen-2-yl)-diazenyl-quinoline-5-sulfonic acid] and its knockdown by small interfering RNA decreased arginase activity by blocking arginase II expression; however, it failed to restore eNOS coupling. Inactivation of SHP2 also abrogated insulin-mediated leukocyte adhesion by blocking the expression of adhesion molecules. Finally, downregulation of endogenous arginase II blocked insulin-mediated endothelial inflammation.

CONCLUSIONS

SHP2 mediates chronic insulin-induced endothelial inflammation by limiting the production of NO in an eNOS-independent and arginase-II-dependent manner.

Protein tyrosine phosphatases expression during development of mouse superior colliculus

Protein tyrosine phosphatases (PTPs) are key regulators of different processes during development of the central nervous system. However, expression patterns and potential roles of PTPs in the developing superior colliculus remain poorly investigated. In this study, a degenerate primer-based reverse transcription-polymerase chain reaction (RT-PCR) approach was used to isolate seven different intracellular PTPs and nine different receptor-type PTPs (RPTPs) from embryonic E15 mouse superior colliculus. Subsequently, the expression patterns of 11 PTPs (TC-PTP, PTP1C, PTP1D, PTP-MEG2, PTP-PEST, RPTPJ, RPTPε, RPTPRR, RPTPσ, RPTPκ and RPTPγ) were further analyzed in detail in superior colliculus from embryonic E13 to postnatal P20 stages by quantitative real-time RT-PCR, Western blotting and immunohistochemistry. Each of the 11 PTPs exhibits distinct spatiotemporal regulation of mRNAs and proteins in the developing superior colliculus suggesting their versatile roles in genesis of neuronal and glial cells and retinocollicular topographic mapping. At E13, additional double-immunohistochemical analysis revealed the expression of PTPs in collicular nestin-positive neural progenitor cells and RC-2-immunoreactive radial glia cells, indicating the potential functional importance of PTPs in neurogenesis and gliogenesis.

Molecular mechanism for SHP2 in promoting HER2-induced signaling and transformation

The Src homology phosphotyrosyl phosphatase 2 (SHP2) plays a positive role in HER2-induced signaling and transformation, but its mechanism of action is poorly understood. Given the significance of HER2 in breast cancer, defining a mechanism for SHP2 in the HER2 signaling pathway is of paramount importance. In the current report we show that SHP2 positively modulates the Ras-extracellular signal-regulated kinase 1 and 2 and the phospoinositide-3-kinase-Akt pathways downstream of HER2 by increasing the half-life the activated form of Ras. This is accomplished by dephosphorylating an autophosphorylation site on HER2 that serves as a docking platform for the SH2 domains of the Ras GTPase-activating protein (RasGAP). The net effect is an increase in the intensity and duration of GTP-Ras levels with the overall impact of enhanced HER2 signaling and cell transformation. In conformity to these findings, the HER2 mutant that lacks the SHP2 target site exhibits an enhanced signaling and cell transformation potential. Therefore, SHP2 promotes HER2-induced signaling and transformation at least in part by dephosphorylating a negative regulatory autophosphorylation site. These results suggest that SHP2 might serve as a therapeutic target against breast cancer and other cancers characterized by HER2 overexpression.

Temporal dynamics of tyrosine phosphorylation in insulin signaling

The insulin-signaling network regulates blood glucose levels, controls metabolism, and when dysregulated, may lead to the development of type 2 diabetes. Although the role of tyrosine phosphorylation in this network is clear, only a limited number of insulin-induced tyrosine phosphorylation sites have been identified. To address this issue and establish temporal response, we have, for the first time, carried out an extensive, quantitative, mass spectrometry-based analysis of tyrosine phosphorylation in response to insulin. The study was performed with 3T3-L1 adipocytes stimulated with insulin for 0, 5, 15, and 45 min. It has resulted in the identification and relative temporal quantification of 122 tyrosine phosphorylation sites on 89 proteins. Insulin treatment caused a change of at least 1.3-fold in tyrosine phosphorylation on 89 of these sites. Among the responsive sites, 20 were previously known to be tyrosine phosphorylated with insulin treatment, including sites on the insulin receptor and insulin receptor substrate-1. The remaining 69 responsive sites have not previously been shown to be altered by insulin treatment. They were on proteins with a wide variety of functions, including components of the trafficking machinery for the insulin-responsive glucose transporter GLUT4. These results show that insulin-elicited tyrosine phosphorylation is extensive and implicate a number of hitherto unrecognized proteins in insulin action.

Mutation of Thr466 in SHP2 abolishes its phosphatase activity, but provides a new substrate-trapping mutant

Most classical phosphotyrosyl phosphatases (PTPs), including the Src homology phosphotyrosyl phosphatase 2 (SHP2) possess a Thr or a Ser residue immediately C-terminal to the invariant Arg in the active site consensus motif (H/V-C-X5-R-S/T), also known as the "signature motif". SHP2 has a Thr (Thr466) at this position, but its importance in catalysis has not been investigated. By employing site-directed mutagenesis, phosphatase assays and substrate-trapping studies, we demonstrate that Thr466 is critical for the catalytic activity of SHP2. Its mutation to Ala abolishes phosphatase activity, but provides a new substrate-trapping mutant. We further show that the nucleophilic Cys459 is not involved in substrate trapping by Thr466Ala-SHP2 (T/A-SHP2). Mutation of Thr466 does not cause significant structural changes in the active site as revealed by the trapping of the epidermal growth factor receptor (EGFR), the physiological substrate of SHP2, and by orthovanadate competition experiments. Based on these results and previous other works, we propose that the role of Thr466 in the catalytic process of SHP2 could be stabilizing the sulfhydryl group of Cys459 in its reduced state, a state that enables nucleophilic attack on the phosphate moiety of the substrate. The T/A-SHP2 harbors a single mutation and specifically interacts with the EGFR. Since the nucleophilic Cys459 and the proton donor Asp425 are intact in the T/A-SAHP2, it offers an excellent starting material for solving the structure of SHP2 in complex with its physiological substrate.

Pinpointing phosphotyrosine-dependent interactions downstream of the collagen receptor DDR1

Activation of the receptor tyrosine kinase DDR1 by collagen results in robust and sustained phosphorylation, however little is known about its downstream mediators. Using phosphopeptide mapping and site-directed mutagenesis, we here identified multiple tyrosine phosphorylation sites within DDR1. We found that Nck2 and Shp-2, two SH2 domain-containing proteins, bind to DDR1 in a collagen-dependent manner. The binding site of Shp-2 was mapped to tyrosine-740 of DDR1 within an ITIM-consensus sequence. Lastly, ablation of DDR1 in the mouse mammary gland resulted in delocalized expression of Nck2, suggesting that defects observed during alveologenesis are caused by the lack of the DDR1-Nck2 interaction.

Mutual regulation of protein-tyrosine phosphatase 20 and protein-tyrosine kinase Tec activities by tyrosine phosphorylation and dephosphorylation

PTP20, also known as HSCF/protein-tyrosine phosphatase K1/fetal liver phosphatase 1/brain-derived phosphatase 1, is a cytosolic protein-tyrosine phosphatase with currently unknown biological relevance. We have identified that the nonreceptor protein-tyrosine kinase Tec-phosphorylated PTP20 on tyrosines and co-immunoprecipitated with the phosphatase in a phosphotyrosine-dependent manner. The interaction between the two proteins involved the Tec SH2 domain and the C-terminal tyrosine residues Tyr-281, Tyr-303, Tyr-354, and Tyr-381 of PTP20, which were also necessary for tyrosine phosphorylation/dephosphorylation. Association between endogenous PTP20 and Tec was also tyrosine phosphorylation-dependent in the immature B cell line Ramos. Finally, the Tyr-281 residue of PTP20 was shown to be critical for deactivating Tec in Ramos cells upon B cell receptor ligation as well as dephosphorylation and deactivation of Tec and PTP20 itself in transfected COS7 cells. Taken together, PTP20 appears to play a negative role in Tec-mediated signaling, and Tec-PTP20 interaction might represent a negative feedback mechanism.

Molecular mechanism for a role of SHP2 in epidermal growth factor receptor signaling

The Src homology 2-containing phosphotyrosine phosphatase (SHP2) is primarily a positive effector of receptor tyrosine kinase signaling. However, the molecular mechanism by which SHP2 effects its biological function is unknown. In this report, we provide evidence that defines the molecular mechanism and site of action of SHP2 in the epidermal growth factor-induced mitogenic pathway. We demonstrate that SHP2 acts upstream of Ras and functions by increasing the half-life of activated Ras (GTP-Ras) in the cell by interfering with the process of Ras inactivation catalyzed by Ras GTPase-activating protein (RasGAP). It does so by inhibition of tyrosine phosphorylation-dependent translocation of RasGAP to the plasma membrane, to its substrate (GTP-Ras) microdomain. Inhibition is achieved through the dephosphorylation of RasGAP binding sites at the level of the plasma membrane. We have identified Tyr992 of the epidermal growth factor receptor (EGFR) to be one such site, since its mutation to Phe renders the EGFR refractory to the effect of dominant-negative SHP2. To our knowledge, this is the first report to outline the site and molecular mechanism of action of SHP2 in EGFR signaling, which may also serve as a model to describe its role in other receptor tyrosine kinase signaling pathways.

Tyrosyl phosphorylation of Shp2 is required for normal ERK activation in response to some, but not all, growth factors

The protein-tyrosine phosphatase Shp2 is required for normal activation of the ERK mitogen-activated protein kinase in multiple receptor tyrosine kinase signaling pathways. In fibroblasts, Shp2 undergoes phosphorylation at two C-terminal tyrosyl residues in response to some (fibroblast growth factor and platelet-derived growth factor (PDGF)) but not all (epidermal growth factor and insulin-like growth factor) growth factors. Whereas the catalytic activity of Shp2 is required for all Shp2 actions, the effect of tyrosyl phosphorylation on Shp2 function has been controversial. To clarify the role of Shp2 tyrosyl phosphorylation, we infected Shp2-mutant fibroblasts with retroviruses expressing wild type Shp2 or mutants of either (Y542F or Y580F) or both (Y542F,Y580F) C-terminal tyrosines. Compared with wild type cells, ERK activation was decreased in Y542F- or Y580F-infected cells in response to fibroblast growth factor and PDGF but not the epidermal growth factor. Mutation of both phosphorylation sites resulted in a further decrease in growth factor-evoked ERK activation, although not to the level of the vector control. Immunoblot analyses confirm that Tyr-542 and Tyr-580 are the major sites of Shp2 tyrosyl phosphorylation and that Tyr-542 is the major Grb2 binding site. However, studies with antibodies specific for individual Shp2 phosphorylation sites reveal unexpected complexity in the mechanism of Shp2 tyrosyl phosphorylation by different receptor tyrosine kinases. Moreover, because Y580F mutants retain nearly wild type Grb2-binding ability, yet exhibit defective PDGF-evoked ERK activation, our results show that the association of Grb2 with Shp2 is not sufficient for promoting full ERK activation in response to these growth factors, thereby arguing strongly against the "Grb2-adapter" model of Shp2 action.

EGFR and FGFR signaling through FRS2 is subject to negative feedback control by ERK1/2

Fibroblast growth factor (FGF) receptor substrate 2 (FRS2) is a membrane-anchored docking protein that has been shown to play an important role in linking FGF, nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF) receptors with the Ras/mitogen-activated protein (MAP) kinase signaling cascade. Here we provide evidence that FRS2 can also play a role in epidermal growth factor (EGF) signaling. Upon EGF stimulation, FRS2 mediates enhanced MAPK activity and undergoes phosphorylation on tyrosine as well as serine/threonine residues. This involves the direct interaction of the FRS2 PTB domain with the EGFR and results in a significantly altered mobility of FRS2 in SDS-PAGE which is also observed in FGF stimulated cells. This migration shift of FRS2 is completely abrogated by U0126, a specific MAPK kinase 1 (MEK1) inhibitor, suggesting that ERK1/2 acts as serine/threonine kinase upstream of FRS2. Indeed, we show that the central portion of FRS2 constitutively associates with ERK1/2, whereas the FRS2 carboxy-terminal region serves as substrate for ERK2 phosphorylation in response to EGF and FGF stimulation. Notably, tyrosine phosphorylation of FRS2 is enhanced when ERK1/2 activation is inhibited after both EGF and FGF stimulation. These results indicate a ligand-stimulated negative regulatory feedback loop in which activated ERK1/2 phosphorylates FRS2 on serine/threonine residues thereby down-regulating its tyrosine phosphorylation. Our findings support a broader role of FRS2 in EGFR-controlled signaling pathways in A-431 cells and provide insight into a molecular mechanism for ligand-stimulated feedback regulation with FRS2 as a central regulatory switch point.

The adaptor function of SHP-2 downstream of the prolactin receptor is required for the recruitment of p29, a substrate of SHP-2

SHP-2, a cytosolic protein tyrosine phosphatase with two SH2 domains and multiple tyrosine phosphorylation sites, contributes to signal transduction as an enzyme and/or adaptor molecule. Here we demonstrate that prolactin (PRL) stimulation of the PRL-responsive Nb2 cells, a rat lymphoma cell line, and T47D cells, a human breast cancer cell line, lead to the complex formation of SHP-2 and growth factor receptor-bound protein-2 (grb2). Using transient co-overexpression studies of the prolactin receptor (PRLR) and several tyrosine to phenylalanine mutants of SHP-2, we show that grb2 associates with SHP-2 through the C-terminal tyrosine residues of SHP-2, Y(546) and Y(584). Furthermore, in this study, we found a highly phosphorylated, 29-kDa protein (p29), a substrate of SHP-2. The recruitment of p29 to SHP-2 requires the carboxy-terminal tyrosine residues of SHP-2 (Y(546) and Y(584)). Together, our results indicate that SHP-2 may function as an adaptor molecule downstream of the PRLR and highlight a new recruitment mechanism of SHP-2 substrates.

Nitric oxide-induced motility in aortic smooth muscle cells: role of protein tyrosine phosphatase SHP-2 and GTP-binding protein Rho

We have previously reported that SHP-2 upregulation is necessary for NO-stimulated motility in differentiated rat aortic smooth muscle cells. We now test the hypothesis that upregulation of SHP-2 is necessary and sufficient to stimulate cell motility. Overexpression of SHP-2 via recombinant adenoviral vector stimulated motility to the same extent as NO, whereas the expression of C463S-SHP-2, the dominant-negative SHP-2 allele, blocked the motogenic effect of NO. On the basis of previous studies, we next tested the hypothesis that NO decreases RhoA activity and that this event is necessary and sufficient to explain NO-induced motogenesis. We found that NO decreased RhoA activity in a concentration-dependent manner. Moreover, a dominant-negative SHP-2 allele, DSH2, blocked the NO-induced inhibition of RhoA activity, indicating that upregulation of SHP-2 is necessary for this event. Expression of G14V-RhoA, the constitutively active RhoA allele, decreased cell motility and blocked the motogenic effect of NO, whereas the expression of T19N-RhoA, the dominant-negative RhoA allele, increased cell motility to an extent similar to that induced by NO. Dominant-negative RhoA reversed the effect of dominant-negative SHP-2, indicating that RhoA functions downstream from SHP-2. To investigate events downstream from RhoA, we treated cells with fasudil, a selective Rho kinase inhibitor, and found that it increased cell motility. These results indicate that upregulation of SHP-2, leading to downregulation of RhoA, which is followed by decreased Rho kinase activity, is a sequence of events necessary and sufficient to explain NO-induced cell motility in differentiated aortic smooth muscle cells. The results may be of relevance to in vivo events such as neointimal formation, angiogenesis, and vasculogenesis.

Phosphorylation and activation of protein tyrosine phosphatase (PTP) 1B by insulin receptor

We have previously reported a direct in vivo interaction between the activated insulin receptor and protein-tyrosine phosphatase-1B (PTP1B), which leads to an increase in PTP1B tyrosine phosphorylation. In order to determine if PTP1B is a substrate for the insulin receptor tyrosine kinase, the phosphorylation of the Cys 215 Ser, catalytically inactive mutant PTP1B (CS-PTP1B) was measured in the presence of partially purified and activated insulin receptor. In vitro, the insulin receptor tyrosine kinase catalyzed the tyrosine phosphorylation of PTP1B. 53% of the total cellular PTP1B became tyrosine phosphorylated in response to insulin in vivo. Tyrosine phosphorylation of PTP1B by the insulin receptor was absolutely dependent upon insulin-stimulated receptor autophosphorylation and required an intact kinase domain, containing insulin receptor tyrosines 1146, 1150 and 1151. Tyrosine phosphorylation of wild type PTP1B by the insulin receptor kinase increased phosphatase activity of the protein. Intermolecular transdephosphorylation was demonstrated both in vitro and in vivo, by dephosphorylation of phosphorylated CS-PTP1B by the active wild type enzyme either in a cell-free system or via expression of the wild type PTP1B into Hirc-M cell line, which constitutively overexpress the human insulin receptor and CS-PTP1B. These results suggest that PTP1B is a target protein for the insulin receptor tyrosine kinase and PTP1B can regulate its own phosphatase activity by maintaining the balance between its phosphorylated (the active form) and dephosphorylated (the inactive form) state.

Modulation of insulin-stimulated glycogen synthesis by Src Homology Phosphatase 2

We have examined the requirement of the protein tyrosine phosphatase Src Homology Phosphatase 2 (SHP2) for insulin-stimulated glycogen synthesis. To this end, 3T3L1 fibroblasts were stably transfected with either wild type or a catalytically inactive C463A-mutant of SHP2, and analysed for insulin-induced glycogen synthesis, tyrosine phosphorylation of the insulin receptor and IRS-1, and activation of phosphatidylinositol 3'-kinase (PI 3'-kinase). Glycogen synthesis was stimulated 9.1+/-0.9-fold by insulin in untransfected cells. In cells expressing the dominant-negative C463A-SHP2 mutant, the stimulation of glycogen synthesis by insulin was strongly enhanced (18.7+/-2.7-fold stimulation), while this response was impaired in cells overexpressing wild-type SHP2 (6.6+/-1.1-fold stimulation). When exploring the early post-receptor signalling pathways that contribute to glycogen synthesis, we found that insulin stimulated the tyrosine phosphorylation of IRS-1, and the activation of IRS-1-associated PI 3'-kinase more strongly in C463A-SHP2 expressing 3T3L1-cells (18.1+/-4.7-fold) than in parental 3T3L1 cells (6.8+/-0.5-fold). In 3T3L1 cells overexpressing wild-type SHP2, the insulin stimulation of IRS-1 tyrosine phosphorylation and the activation of PI 3'-kinase (4.5+/-1.0-fold) were impaired. An enhanced activity of SHP2 leads to negative modulation of insulin signalling by reducing the tyrosine phosphorylation of IRS-1 and the concomitant activation of PI 3'-kinase. This results in an impaired ability of insulin to stimulate glycogen synthesis.

The protein-tyrosine phosphatase SHP-2 is required during angiotensin II-mediated activation of cyclin D1 promoter in CHO-AT1A cells

Angiotensin II (Ang II) binds to specific G protein-coupled receptors and is mitogenic in Chinese hamster ovary (CHO) cells stably expressing a rat vascular angiotensin II type 1A receptor (CHO-AT(1A)). Cyclin D1 protein expression is regulated by mitogens, and its assembly with the cyclin-dependent kinases induces phosphorylation of the retinoblastoma protein pRb, a critical step in G(1) to S phase cell cycle progression contributing to the proliferative responses. In the present study, we found that in CHO-AT(1A) cells, Ang II induced a rapid and reversible tyrosine phosphorylation of various intracellular proteins including the protein-tyrosine phosphatase SHP-2. Ang II also induced cyclin D1 protein expression in a phosphatidylinositol 3-kinase and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK)-dependent manner. Using a pharmacological and a co-transfection approach, we found that p21(ras), Raf-1, phosphatidylinositol 3-kinase and also the catalytic activity of SHP-2 and its Src homology 2 domains are required for cyclin D1 promoter/reporter gene activation by Ang II through the regulation of MAPK/ERK activity. Our findings suggest for the first time that SHP-2 could play an important role in the regulation of a gene involved in the control of cell cycle progression resulting from stimulation of a G protein-coupled receptor independently of epidermal growth factor receptor transactivation.

SHP2 association with VE-cadherin complexes in human endothelial cells is regulated by thrombin

Thrombin-mediated changes in endothelial cell adherens junctions modulate vascular permeability. We demonstrate that the nonreceptor protein-tyrosine phosphatase SHP2 co-precipitates with VE-cadherin complexes in confluent, quiescent human umbilical vein endothelial cells. Ligand-binding blots using a SHP2-glutathione S-transferase fusion peptide established that SHP2 associates selectively with beta-catenin in VE-cadherin complexes. Thrombin treatment of human umbilical vein endothelial cells promotes SHP2 tyrosine phosphorylation and dissociation from VE-cadherin complexes. The loss of SHP2 from the cadherin complexes correlates with a dramatic increase in the tyrosine phosphorylation of beta-catenin, gamma-catenin, and p120-catenin complexed with VE-cadherin. We propose that thrombin regulates the tyrosine phosphorylation of VE-cadherin-associated beta-catenin, gamma-catenin, and p120-catenin by modulating the quantity of SHP2 associated with VE-cadherin complexes. Such changes in adherens junction complex composition likely underlie thrombin-elicited alterations in endothelial monolayer permeability.

Shp-2 specifically regulates several tyrosine-phosphorylated proteins in brain-derived neurotrophic factor signaling in cultured cerebral cortical neurons

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophins, promotes differentiation and survival and regulates plasticity of various types of neurons. BDNF binds to TrkB, a receptor tyrosine kinase, which results in the activation of a variety of signaling molecules to exert the various functions of BDNF. Shp-2, a Src homology 2 domain-containing cytoplasmic tyrosine phosphatase, is involved in neurotrophin signaling in PC12 cells and cultured cerebral cortical neurons. To examine the roles of Shp-2 in BDNF signaling in cultured rat cerebral cortical neurons, the wild-type and phosphatase-inactive mutant (C/S mutant) forms of Shp-2 were ectopically expressed in cultured neurons using recombinant adenovirus vectors. We found that several proteins tyrosine-phosphorylated in response to BDNF showed enhanced levels of tyrosine phosphorylation in cultured neurons infected with C/S mutant adenovirus in comparison with those infected with the wild-type Shp-2 adenovirus. In addition, in immunoprecipitates with anti-Shp-2 antibody, we also observed at least four proteins that displayed enhanced phosphorylation in response to BDNF in cultured neurons infected with the C/S mutant adenovirus. We found that the Shp-2-binding protein, brain immunoglobulin-like molecule with tyrosine-based activation motifs (BIT), was strongly tyrosine-phosphorylated in response to BDNF in cultured neurons expressing the C/S mutant of Shp-2. In contrast, the level of BDNF-induced phosphorylation of mitogen-activated protein kinase and coprecipitated proteins with anti-Trk and Grb2 antibodies did not show any difference between neurons infected with these two types of Shp-2. Furthermore, the survival effect of BDNF was enhanced by the wild type of Shp-2, although it was not influenced by the C/S mutant of Shp-2. These results indicated that in cultured cerebral cortical neurons Shp-2 is specifically involved in the regulation of several tyrosine-phosphorylated proteins, including BIT, in the BDNF signaling pathway. In addition, the phosphatase Shp-2 may not influence the level of BDNF-induced activation of mitogen-activated protein kinase in cultured cortical neurons. Further, Shp-2 may have potential to positively regulate BDNF-promoting neuronal survival.

SHP-1 regulation of p62(DOK) tyrosine phosphorylation in macrophages

SHP-1 plays key roles in the modulation of hematopoietic cell signaling. To ascertain the impact of SHP-1 on colony-stimulating factor-1 (CSF-1)-mediated survival and proliferative signaling, we compared the CSF-1 responses of primary bone marrow macrophages (BMM) from wild-type and SHP-1-deficient motheaten (me/me) mice. CSF-1-induced protein tyrosine phosphorylation levels were similar in wild-type and me/me BMM, except for the constitutive hyperphosphorylation of a 62-kDa phosphoprotein (pp62) in me/me macrophages. pp62 was identified as the RASGAP-associated p62(DOK) and was shown to be the major CSF-1R-associated tyrosine-phosphorylated protein in CSF-1-treated BMM. p62(DOK) was found to be constitutively associated with SHP-1 in BMM and in 293T cells, co-expressing p62(dok) and either wild-type or catalytically inert SHP-1 (SHP-1 C453S). In both cell types, the interaction of SHP-1 with p62(DOK) occurred independently of p62(DOK) tyrosine phosphorylation, but only the tyrosine-phosphorylated p62(DOK) was bound by SHP-1 C453S in a far Western analysis. These findings suggest a constitutive association of SHP-1 and p62(DOK) that is either conformation-dependent or indirect as well as a direct, inducible association of the SHP-1 catalytic domain with tyrosine-phosphorylated p62(DOK). p62(DOK) hyperphosphorylation is not associated with altered CSF-1-induced RAS signaling or proliferation. However, whereas wild-type macrophages undergo cell death following CSF-1 removal, me/me macrophages exhibit prolonged survival in the absence of growth factor. Thus, p62(DOK) is a major SHP-1 substrate whose tyrosine phosphorylation correlates with growth factor-independent survival in macrophages.

Leptin receptor activation of SH2 domain containing protein tyrosine phosphatase 2 modulates Ob receptor signal transduction

Leptin exerts its weight-reducing effects by binding to its receptor and activating signal transduction in hypothalamic neurons and other cell types. To identify the components of the leptin signal transduction pathway, an approach was developed in which bacterially expressed phosphorylated fragments of Ob receptor b (Ob-Rb) were used as affinity agents. Leptin binding to the Ob-Rb form of the leptin receptor leads to tyrosyl phosphorylation of the cytoplasmic domain of its receptor. Two of the three cytoplasmic tyrosines of Ob-Rb, at positions 985 and 1138, are phosphorylated after leptin treatment. Affinity chromatography using a tyrosine-phosphorylated fragment spanning Tyr 985 of Ob-Rb was used to identify proteins that bind to this site. The SH2 domain containing protein tyrosine phosphatase 2 (SHP-2) was isolated from bovine and mouse hypothalamus by using this method. After cotransfection of Ob-Rb, Janus kinase 2 (JAK2), and SHP-2 into 293T cells, leptin results in direct binding of SHP-2 to the phosphorylated Tyr 985. The bound SHP-2 is itself tyrosine phosphorylated after leptin treatment. SHP-2 is not phosphorylated after leptin treatment when a Y-->F 985 receptor mutant is cotransfected. In the absence of SHP-2 phosphorylation, the level of JAK2 phosphorylation was increased. Tyrosyl phosphorylation of the leptin receptor and signal transducer and activater of transcription 3 (STAT3) are not affected by phosphorylation of SHP-2. These data suggest that activation of SHP-2 by the leptin receptor results in a decreased phosphorylation of JAK2 and may act to attenuate leptin signal transduction. The method used in this report can in principle be used to isolate additional components of the leptin, or other, signal transduction pathway.

Receptor protein tyrosine phosphatase alpha participates in the m1 muscarinic acetylcholine receptor-dependent regulation of Kv1.2 channel activity

The phosphorylation state of a given tyrosine residue is determined by both protein tyrosine kinase (PTK) and protein tyrosine phosphatase (PTP) activities. However, little is known about the functional interaction of these opposing activities at the level of an identified effector molecule. G protein-coupled receptors (GPCRs), including the m1 muscarinic acetylcholine receptor (mAChR), regulate a tyrosine kinase activity that phosphorylates and suppresses current generated by the Kv1.2 potassium channel. We examined the possibility that PTPs also participate in this signaling pathway since the tyrosine phosphatase inhibitor vanadate increases the extent of both Kv1.2 phosphorylation and suppression. We show that an endogenous transmembrane tyrosine phosphatase, receptor tyrosine phosphatase alpha (RPTPalpha), becomes tyrosine phosphorylated and co-immunoprecipitates with Kv1.2 in a manner dependent on m1 receptor activation. The N- and C-termini of Kv1.2 are shown to bind RPTPalpha in vitro. Overexpression of RPTPalpha in Xenopus oocytes increases resting Kv1.2 current. Biochemical and electrophysiological analysis reveals that recruiting RPTPalpha to Kv1.2 functionally reverses the tyrosine kinase-induced phosphorylation and suppression of Kv1.2 current in mammalian cells. Taken together, these results identify RPTPalpha as a new target of m1 mAChR signaling and reveal a novel regulatory mechanism whereby GPCR-mediated suppression of a potassium channel depends on the coordinate and parallel regulation of PTK and PTP activities.

Activation of the protein tyrosine phosphatase SHP2 via the interleukin-6 signal transducing receptor protein gp130 requires tyrosine kinase Jak1 and limits acute-phase protein expression

Stimulation of the interleukin-6 (IL-6) signalling pathway occurs via the IL-6 receptor-glycoprotein 130 (IL-6R-gp130) receptor complex and results in the regulation of acute-phase protein genes in liver cells. Ligand binding to the receptor complex leads to tyrosine phosphorylation and activation of Janus kinases (Jak), phosphorylation of the signal transducing subunit gp130, followed by recruitment and phosphorylation of the signal transducer and activator of transcription factors STAT3 and STAT1 and the src homology domain (SH2)-containing protein tyrosine phosphatase (SHP2). The tyrosine phosphorylated STAT factors dissociate from the receptor, dimerize and translocate to the nucleus where they bind to enhancer sequences of IL-6 target genes. Phosphorylated SHP2 is able to bind growth factor receptor bound protein (grb2) and thus might link the Jak/STAT pathway to the ras/raf/mitogen-activated protein kinase pathway. Here we present data on the dose-dependence, kinetics and kinase requirements for SHP2 phosphorylation after the activation of the signal transducer, gp130, of the IL-6-type family receptor complex. When human fibrosarcoma cell lines deficient in Jak1, Jak2 or tyrosine kinase 2 (Tyk2) were stimulated with IL-6-soluble IL-6R complexes it was found that only in Jak1-, but not in Jak 2- or Tyk2-deficient cells, SHP2 activation was greatly impaired. It is concluded that Jak1 is required for the tyrosine phosphorylation of SHP2. This phosphorylation depends on Tyr-759 in the cytoplasmatic domain of gp130, since a Tyr-759-->Phe exchange abrogates SHP2 activation and in turn leads to elevated and prolonged STAT3 and STAT1 activation as well as enhanced acute-phase protein gene induction. Therefore, SHP2 plays an important role in acute-phase gene regulation.

Interleukin-6-type cytokine signalling through the gp130/Jak/STAT pathway

The family of cytokines signalling through the common receptor subunit gp130 comprises interleukin (IL)-6, IL-11, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and cardiotrophin-1. These so-called IL-6-type cytokines play an important role in the regulation of complex cellular processes such as gene activation, proliferation and differentiation. The current knowledge on the signal-transduction mechanisms of these cytokines from the plasma membrane to the nucleus is reviewed. In particular, we focus on the assembly of receptor complexes after ligand binding, the activation of receptor-associated kinases of the Janus family, and the recruitment and phosphorylation of transcription factors of the STAT family, which dimerize, translocate to the nucleus, and bind to enhancer elements of respective target genes leading to transcriptional activation. The important players in the signalling pathway, namely the cytokines and the receptor components, the Janus kinases Jak1, Jak2 and Tyk2, the signal transducers and activators of transcription STAT1 and STAT3 and the tyrosine phosphatase SHP2 [SH2 (Src homology 2) domain-containing tyrosine phosphatase] are introduced and their structural/functional properties are discussed. Furthermore, we review various mechanisms involved in the termination of the IL-6-type cytokine signalling, namely the action of tyrosine phosphatases, proteasome, Jak kinase inhibitors SOCS (suppressor of cytokine signalling), protein inhibitors of activated STATs (PIAS), and internalization of the cytokine receptors via gp130. Although all IL-6-type cytokines signal through the gp130/Jak/STAT pathway, the comparison of their physiological properties shows that they elicit not only similar, but also distinct, biological responses. This is reflected in the different phenotypes of IL-6-type-cytokine knock-out animals.

SHP-2, SH2-containing protein tyrosine phosphatase-2

SHP-2 is an ubiquitously expressed cytosolic protein tyrosine phosphatase composed of two amino-terminal SH2 domains, a central phosphatase domain and a carboxy-terminal tail. Upon activation of cells with different stimuli, SHP-2 is recruited to the plasma membrane where it can associate with a number of tyrosine phosphorylated molecules, including receptor tyrosine kinases (e.g. growth factor receptors), multisite adapter proteins and cell adhesion molecules. SHP-2 is thought to function as a positive mediator of signals generated by activated membrane receptor complexes although the number and diversity of binding partners and substrates identified thus far suggests that it may have other functions. It is likely that several negative regulatory influences exist but that these are obscured by its positive function making the investigation of the inhibitory effects of this phosphatase difficult. The positive regulatory role of SHP-2 in signal cascades leading to cell growth suggests involvement in tumorigenesis, raising the possibility that SHP-2 may be a target in the treatment of some forms of cancer.

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.

PSTPIP: a tyrosine phosphorylated cleavage furrow-associated protein that is a substrate for a PEST tyrosine phosphatase

We have investigated proteins which interact with the PEST-type protein tyrosine phosphatase, PTP hematopoietic stem cell fraction (HSCF), using the yeast two-hybrid system. This resulted in the identification of proline, serine, threonine phosphatase interacting protein (PSTPIP), a novel member of the actin- associated protein family that is homologous to Schizosaccharomyces pombe CDC15p, a phosphorylated protein involved with the assembly of the actin ring in the cytokinetic cleavage furrow. The binding of PTP HSCF to PSTPIP was induced by a novel interaction between the putative coiled-coil region of PSTPIP and the COOH-terminal, proline-rich region of the phosphatase. PSTPIP is tyrosine phosphorylated both endogenously and in v-Src transfected COS cells, and cotransfection of dominant-negative PTP HSCF results in hyperphosphorylation of PSTPIP. This dominant-negative effect is dependent upon the inclusion of the COOH-terminal, proline-rich PSTPIP-binding region of the phosphatase. Confocal microscopy analysis of endogenous PSTPIP revealed colocalization with the cortical actin cytoskeleton, lamellipodia, and actin-rich cytokinetic cleavage furrow. Overexpression of PSTPIP in 3T3 cells resulted in the formation of extended filopodia, consistent with a role for this protein in actin reorganization. Finally, overexpression of mammalian PSTPIP in exponentially growing S. pombe results in a dominant-negative inhibition of cytokinesis. PSTPIP is therefore a novel actin-associated protein, potentially involved with cytokinesis, whose tyrosine phosphorylation is regulated by PTP HSCF.

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.

Both SH2 domains are involved in interaction of SHP-1 with the epidermal growth factor receptor but cannot confer receptor-directed activity to SHP-1/SHP-2 chimera

The previously demonstrated functional and physical interaction of the SH2 domain protein-tyrosine phosphatase SHP-1 with the epidermal growth factor (EGF) receptor (Tomic, S., Greiser, U., Lammers, R., Kharitonenkov, A., Imyanitov, E., Ullrich, A., and Böhmer, F. D. (1995) J. Biol. Chem. 270, 21277-21284) was investigated with respect to the involved structural elements of SHP-1. Various mutants of SHP-1 were transiently expressed in 293 or COS-7 cells and analyzed for their capacity to associate with immobilized autophosphorylated EGF receptor in vitro and to dephosphorylate coexpressed EGF receptor in intact cells. Inactivating point mutation of the C-terminal SH2 domain reduced the association weakly, point mutation of the N-terminal SH2 domain reduced association strongly and the respective double mutation abolished association totally. The capacity of SHP-1 to dephosphorylate coexpressed EGF receptor was impaired by all point mutations. Truncation of the N-terminal or of both SH2 domains strongly reduced or abolished association, respectively, but the truncated SHP-1 derivatives still dephosphorylated coexpressed EGF receptor effectively. Various chimeric protein-tyrosine phosphatases constructed from SHP-1 and the closely homologous SHP-2 dephosphorylated the EGF receptor when they contained the catalytic domain of SHP-1. As native SHP-2, the chimera lacked activity toward the receptor when they contained the catalytic domain of SHP-2, despite their capacity to associate with the receptor and to dephosphorylate an artificial phosphopeptide. We conclude that the differential interaction of SHP-1 and SHP-2 with the EGF receptor is due to the specificity of the respective catalytic domains rather than to the specificity of the SH2 domains. Functional interaction of native SHP-1 with the EGF receptor requires association mediated by both SH2 domains.

Insulin-induced tyrosine dephosphorylation of paxillin and focal adhesion kinase requires active phosphotyrosine phosphatase 1D

Insulin stimulation of fibroblasts rapidly induces the tyrosine dephosphorylation of proteins of 68 kDa and 125 kDa, in addition to the tyrosine phosphorylation of the insulin receptor beta-chain, insulin receptor substrates 1 and 2, and Shc. Using specific antibodies, the 68 kDa and 125 kDa proteins were identified as paxillin and focal adhesion kinase (pp125FAK) respectively. We have examined whether dephosphorylation of paxillin and pp125FAK requires interaction of the cells with the extracellular matrix. For this, cells were grown on poly(L-lysine) plates, and the tyrosine phosphorylation of pp125FAK and paxillin was increased by addition of lysophosphatidic acid. Under these conditions, insulin still induced the complete dephosphorylation of pp125FAK and paxillin, indicating that this process can occur independently of the interaction of integrins with extracellular matrix proteins. We also studied whether dephosphorylation of pp125FAK and paxillin results from the action of a phosphotyrosine phosphatase. It was found that phenylarsine oxide, a phosphotyrosine phosphatase inhibitor, prevented the insulin-induced dephosphorylation of pp125FAK and paxillin. Furthermore, this insulin-induced dephosphorylation was also impaired in cells expressing a dominant-negative mutant of phosphotyrosine phosphatase 1D (PTP 1D). Thus we have identified paxillin as a target for dephosphorylation by insulin. In addition, we have obtained evidence that the insulin-mediated dephosphorylation of paxillin and pp125FAK requires active PTP 1D.

Protein tyrosine phosphatases in T-cell development, apoptosis and signalling

The study of phosphatases was viewed as a rather esoteric subject for immunologists until eight years ago, when the discovery that CD45 is a protein tyrosine phosphatase (PTPase) began to make the topic respectable. Now, as reviewed by Julie Frearson and Denis Alexander, PTPases are increasingly being shown to play key roles in the molecular physiology of haematopoietic cells and some have been shown to regulate critical events in T-cell development and signalling.