Regulation of ACh receptor clustering by the tyrosine phosphatase Shp2

At the vertebrate neuromuscular junction (NMJ), postsynaptic aggregation of muscle acetylcholine receptors (AChRs) depends on the activation of MuSK, a muscle-specific tyrosine kinase that is stimulated by neural agrin and regulated by muscle-intrinsic tyrosine kinases and phosphatases. We recently reported that Shp2, a tyrosine phosphatase containing src homology two domains, suppressed MuSK-dependent AChR clustering in cultured myotubes, but how this effect of Shp2 is controlled has remained unclear. In this study, biochemical assays showed that agrin-treatment of C2 mouse myotubes enhanced the tyrosine phosphorylation of signal regulatory protein alpha1 (SIRPalpha1), a known activator of Shp2, and promoted SIRPalpha1's interaction with Shp2. Moreover, in situ experiments revealed that treatment of myotubes with the Shp2-selective inhibitor NSC-87877 increased spontaneous and agrin-induced AChR clustering, and that AChR clustering was also enhanced in myotubes ectopically expressing inactive (dominant-negative) Shp2; in contrast, AChR clustering was reduced in myotubes expressing constitutively active Shp2. Significantly, expression of truncated (nonShp2-binding) and full-length (Shp2-binding) forms of SIRPalpha1 in myotubes also increased and decreased AChR clustering, respectively, and coexpression of truncated SIRPalpha1 with active Shp2 and full-length SIRPalpha1 with inactive Shp2 reversed the actions of the exogenous Shp2 proteins on AChR clustering. These results suggest that SIRPalpha1 is a novel downstream target of MuSK that activates Shp2, which, in turn, suppresses AChR clustering. We propose that an inhibitory loop involving both tyrosine kinases and phosphatases sets the level of agrin/MuSK signaling and constrains it spatially to help generate high-density AChR clusters selectively at NMJs.     

A novel function for the protein tyrosine phosphatase Shp2 during lung branching morphogenesis

Branching morphogenesis of many organs, including the embryonic lung, is a dynamic process in which growth factor mediated tyrosine kinase receptor activation is required, but must be tightly regulated to direct ramifications of the terminal branches. However, the specific regulators that modulate growth factor signaling downstream of the tyrosine kinase receptor remain to be determined. Herein, we demonstrate for the first time an important function for the intracellular protein tyrosine phosphatase Shp2 in directing embryonic lung epithelial morphogenesis. We show that Shp2 is specifically expressed in embryonic lung epithelial buds, and that loss of function by the suppression of Shp2 mRNA expression results in a 53% reduction in branching morphogenesis. Furthermore, by intra-tracheal microinjection of a catalytically inactive adenoviral Shp2 construct, we provide direct evidence that the catalytic activity of Shp2 is required for proper embryonic lung branch formation. We demonstrate that Shp2 activity is required for FGF10 induced endodermal budding. Furthermore, a loss of Shp2 catalytic activity in the embryonic lung was associated with a reduction in ERK phosphorylation and epithelial cell proliferation. However, epithelial cell differentiation was not affected. Our results show that the protein tyrosine phosphatase Shp2 plays an essential role in modulating growth factor mediated tyrosine kinase receptor activation in early embryonic lung branching morphogenesis.     

Shp2 protein tyrosine phosphatase inhibitor activity of estramustine phosphate and its triterpenoid analogs

Shp2 protein tyrosine phosphate (PTP) is a novel target for anticancer drug discovery. We identified estramustine phosphate as a Shp2 PTP inhibitor from the National Cancer Institute Approved Oncology Drug set. A focused structure-activity relationship study indicated that the 17-phosphate group is required for the Shp2 PTP inhibitor activity of estramustine phosphate. A search for estramustine phosphate analogs led to identification of two triterpenoids, enoxolone, and celastrol, having Shp2 PTP inhibitor activity. With the previously reported PTP1B inhibitor trodusquemine, our study reveals steroids and triterpenoids with negatively charged phosphate, carboxylate, or sulfonate groups as novel pharmacophores of selective PTP inhibitors.     

Prolongation of insulin-induced activation of mitogen-activated protein kinases ERK 1/2 and phosphatidylinositol 3-kinase by vanadyl sulfate, a protein tyrosine phosphatase inhibitor

Vanadium salts such as vanadyl sulfate (VS), potent inhibitors of protein tyrosine phosphatases, have been shown to mimic, augment, and prolong insulin's action. However, the molecular mechanism of responses to these salts is not clear. In the present studies, we examined if VS-induced effects on insulin action are associated with enhancement or augmentation in the activation state of key components of the insulin signaling pathway. Treatment of insulin receptor-overexpressing cells with insulin or VS resulted in a time-dependent transient increase in phosphorylation and activation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) that peaked at about 5 min, then declined rapidly to about baseline within 30 min. However, when the cells were treated with VS before stimulation with insulin, sustained ERK 1/2 phosphorylation and activation were observed well beyond 60 min. VS treatment also prolonged the insulin-stimulated activation of phosphatidylinositol 3-kinase (PI3-K), which was associated with sustained interaction between insulin receptor substrate-1 (IRS-1) and the p(85 alpha) subunit of phosphatidylinositol 3-kinase (PI3-K) in response to insulin. These data indicate that prolongation of insulin-stimulated ERK 1/2 and PI3-K activation by VS is due to a more stable complex formation of IRS-1 with the p(85 alpha) subunit which may, in turn, be responsible for its ability to enhance and extend the biological effects of insulin.     

The protein tyrosine phosphatase, Shp2, is required for the complete activation of the RAS/MAPK pathway by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) and other neurotrophins induce a unique prolonged activation of mitogen-activated protein kinase (MAPK) compared with growth factors. Characterization and kinetic and spatial modeling of the signaling pathways underlying this prolonged MAPK activation by BDNF will be important in understanding the physiological role of BDNF in many complex systems in the nervous system. In addition to Shc, fibroblast growth factor receptor substrate 2 (FRS2) is required for the BDNF-induced activation of MAPK. BDNF induces phosphorylation of FRS2. However, BDNF does not induce phosphorylation of FRS2 in cells expressing a deletion mutant of TrkB (TrkBDeltaPTB) missing the juxtamembrane NPXY motif. This motif is the binding site for SHC. NPXY is the consensus sequence for phosphotyrosine binding (PTB) domains, and notably, FRS2 and SHC contain PTB domains. This NPXY motif, which contains tyrosine 484 of TrkB, is therefore the binding site for both FRS2 and SHC. Moreover, the proline containing region (VIENP) of the NPXY motif is also required for FRS2 and SHC phosphorylation, which indicates this region is an important component of FRS2 and SHC recognition by TrkB. Previously, we had found that the phosphorylation of FRS2 induces association of FRS2 and growth factor receptor binding protein 2 (Grb2). Now, we have intriguing data that indicates BDNF induces association of the SH2 domain containing protein tyrosine phosphatase, Shp2, with FRS2. Moreover, the PTB association motif of TrkB containing tyrosine 484 is required for the BDNF-induced association of Shp2 with FRS2 and the phosphorylation of Shp2. These results imply that FRS2 and Shp2 are in a BDNF signaling pathway. Shp2 is required for complete MAPK activation by BDNF, as expression of a dominant negative Shp2 in cells attenuates BDNF-induced activation of MAPK. Moreover, expression of a dominant negative Shp2 attenuates Ras activation showing that the protein tyrosine phosphatase is required for complete activation of MAPKs by BDNF. In conclusion, Shp2 regulates BDNF signaling through the MAPK pathway by regulating either Ras directly or alternatively, by signaling components upstream of Ras. Characterization of MAPK signaling controlled by BDNF is likely to be required to understand the complex physiological role of BDNF in neuronal systems ranging from the regulation of neuronal growth and survival to the regulation of synapses.     

Differential regulation of endoplasmic reticulum stress by protein tyrosine phosphatase 1B and T cell protein tyrosine phosphatase

Protein-tyrosine phosphatase 1B (PTP1B) and T cell protein-tyrosine phosphatase (TCPTP) are closely related intracellular phosphatases implicated in the control of glucose homeostasis. PTP1B and TCPTP can function coordinately to regulate protein tyrosine kinase signaling, and PTP1B has been implicated previously in the regulation of endoplasmic reticulum (ER) stress. In this study, we assessed the roles of PTP1B and TCPTP in regulating ER stress in the endocrine pancreas. PTP1B and TCPTP expression was determined in pancreases from chow and high fat fed mice and the impact of PTP1B and TCPTP over- or underexpression on palmitate- or tunicamycin-induced ER stress signaling assessed in MIN6 insulinoma β cells. PTP1B expression was increased, and TCPTP expression decreased in pancreases of mice fed a high fat diet, as well as in MIN6 cells treated with palmitate. PTP1B overexpression or TCPTP knockdown in MIN6 cells mitigated palmitate- or tunicamycin-induced PERK/eIF2α ER stress signaling, whereas PTP1B deficiency enhanced ER stress. Moreover, PTP1B deficiency increased ER stress-induced cell death, whereas TCPTP deficiency protected MIN6 cells from ER stress-induced death. ER stress coincided with the inhibition of Src family kinases (SFKs), which was exacerbated by PTP1B overexpression and largely prevented by TCPTP knockdown. Pharmacological inhibition of SFKs ameliorated the protective effect of TCPTP deficiency on ER stress-induced cell death. These results demonstrate that PTP1B and TCPTP play nonredundant roles in modulating ER stress in pancreatic β cells and suggest that changes in PTP1B and TCPTP expression may serve as an adaptive response for the mitigation of chronic ER stress.     

Redox regulation of protein tyrosine phosphatase activity by hydroxyl radical

Substantial evidence suggests that transient production of reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) is an important signaling event triggered by the activation of various cell surface receptors. Major targets of H₂O₂ include protein tyrosine phosphatases (PTPs). Oxidation of the active site Cys by H₂O₂ abrogates PTP catalytic activity, thereby potentially furnishing a mechanism to ensure optimal tyrosine phosphorylation in response to a variety of physiological stimuli. Unfortunately, H₂O₂ is poorly reactive in chemical terms and the second order rate constants for the H₂O₂-mediated PTP inactivation are ~ 10 M^− 1 s^− 1, which is too slow to be compatible with the transient signaling events occurring at the physiological concentrations of H₂O₂. We find that hydroxyl radical is produced from H₂O₂ solutions in the absence of metal chelating agent by the Fenton reaction. We show that the hydroxyl radical is capable of inactivating the PTPs and the inactivation is active site directed, through oxidation of the catalytic Cys to sulfenic acid, which can be reduced by low molecular weight thiols. We also show that hydroxyl radical is a kinetically more efficient oxidant than H₂O₂ for inactivating the PTPs. The second-order rate constants for the hydroxyl radical-mediated PTP inactivation are at least 2–3 orders of magnitude higher than those mediated by H₂O₂ under the same conditions. Thus, hydroxyl radical generated in vivo may serve as a more physiologically relevant oxidizing agent for PTP inactivation. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases.     

Regulated coupling of the Neu receptor to phosphatidylinositol 3'-kinase and its release by oncogenic activation.

The neu protooncogene encodes a tyrosine kinase receptor that is involved in the regulation of normal growth and malignant transformation. To circumvent the use of the incompletely characterized ligand of Neu, we constructed a chimeric protein composed of the ligand-binding domain of the epidermal growth factor receptor and the transmembrane and cytoplasmic portions of Neu. By expressing this Neu-epidermal growth factor receptor chimera (termed NEC), we found that following stimulation by the heterologous ligand, the tyrosine kinase of Neu became associated with a phosphatidylinositol (PI) kinase activity. The association was dependent on the concentration of the ligand and was almost maximal within 30 s after ligand binding. The lipid kinase was identified as a type I PI 3'-kinase on the basis of its inhibition by Nonidet P-40 and high pressure liquid chromatography of the phosphorylated product. To confirm the identification of PI 3'-kinase as an effector of Neu, we raised antibodies to the alpha-isoform of the regulatory subunit of PI 3'-kinase (p85). Using these antibodies, it was possible to directly demonstrate ligand-dependent formation of a tyrosine-phosphorylated complex of NEC and PI 3'-kinase. Apparently, both PI 3'-kinase and phospholipase C gamma, another substrate of the Neu kinase, simultaneously associated with the same activated NEC molecule. Nevertheless, immunofluorescence localization of PI 3'-kinase revealed no significant cellular redistribution of the enzyme after activation of the Neu kinase. Interestingly, PI 3'-kinase was localized primarily to the cell nucleus and to confined regions of the plasma membrane. Analysis of mutants of the Neu protein indicated that the oncogenic point-mutated Neu (Glu664) was permanently coupled to PI 3'-kinase; but two nontransforming versions of the oncoprotein, a kinase-defective protein and a carboxyl-terminally deleted Neu, were devoid of the constitutive association with PI 3'-kinase. Hence, we concluded that phosphatidylinositol 3'-kinase is a physiological substrate of the Neu receptor, but the regulation of this coupling is released upon oncogenic activation.     

Negative regulation of T cell activation by placental protein 14 is mediated by the tyrosine phosphatase receptor CD45

CD45 is the major protein tyrosine phosphatase receptor on T cell surfaces that functions as both a positive and a negative regulator of T cell receptor (TCR) signaling. Although CD45 is required for the activation of TCR-associated Src family kinases, it also dephosphorylates phosphoproteins involved in the TCR-signaling cascade. This study links CD45 to the inhibitory activity of placental protein 14 (PP14), a major soluble protein of pregnancy that is now known to be a direct modulator of T cells and to function by desensitizing TCR signaling. PP14 and CD45 co-capped with each other, pointing to a physical linkage between the two. Interestingly, however, the binding of PP14 to T cell surfaces was not restricted to CD45 alone, with evidence showing that PP14 binds to other surface molecules in a carbohydrate-dependent fashion. Notwithstanding the broader molecular binding potential of PP14, its interaction with CD45 appeared to have special functional significance. Using transfected derivatives of the HPB.ALL mutant T cell line that differ in CD45 expression, we established that the inhibitory effects of PP14 are dependent upon the expression of intact CD45 on T cell surfaces. Based upon these findings, we propose a new immunoregulatory model for PP14, wherein one of its surface molecular targets, CD45, mediates its T cell inhibitory activity, accounting for the intriguing capacity of PP14 to elevate TCR activation thresholds and thereby down-regulate T cell activation.     

A model of activation of the protein tyrosine phosphatase SHP-2 by the human leptin receptor

Signalling through the leptin receptor has been shown to activate the SH2 domain-containing tyrosine phosphatase SHP-2 through tyrosine phosphorylation. The human leptin receptor contains five tyrosine residues in the cytoplasmic domain that may become phosphorylated. We show here using BIAcore studies, wherein binding of peptides to SHP-2 was detected, that peptides corresponding to sequences containing phosphotyrosines 974 and 986 (LR974P and LR986P, respectively) from the leptin receptor cytoplasmic domain were the only two peptides that bound to the enzyme. Binding of LR974P to SHP-2 was inhibited in a dose-dependent fashion by orthovanadate, whereas binding of LY986P was not, indicating that the enzyme binds to these peptides through different sites. Only the leptin receptor-derived peptide corresponding to tyrosine 974 was dephosphorylated by recombinant purified SHP-2. Time courses of the reaction were complex, and fitted a two exponent rate equation. Preincubation of SHP-2 with LR986P markedly activated the enzyme at early time points and time courses of the activated enzyme fitted a single exponential first order rate equation. We propose that LR974P binds to the active site of SHP-2, whereas LR986P may bind to the N- and C-terminal SH2 domains of SHP-2, thus activating the phosphatase activity. These data support a model in which SHP-2 binds to phosphotyrosine 986 in the activated leptin receptor and is activated to dephosphorylate phosphotyrosine 974, downregulating signalling events emanating from SH2 domain-containing proteins that bind here.     

Site-selective regulation of platelet-derived growth factor beta receptor tyrosine phosphorylation by T-cell protein tyrosine phosphatase

The platelet-derived growth factor (PDGF) beta receptor mediates mitogenic and chemotactic signals. Like other tyrosine kinase receptors, the PDGF beta receptor is negatively regulated by protein tyrosine phosphatases (PTPs). To explore whether T-cell PTP (TC-PTP) negatively regulates the PDGF beta receptor, we compared PDGF beta receptor tyrosine phosphorylation in wild-type and TC-PTP knockout (ko) mouse embryos. PDGF beta receptors were hyperphosphorylated in TC-PTP ko embryos. Fivefold-higher ligand-induced receptor phosphorylation was observed in TC-PTP ko mouse embryo fibroblasts (MEFs) as well. Reexpression of TC-PTP partly abolished this difference. As determined with site-specific phosphotyrosine antibodies, the extent of hyperphosphorylation varied among different autophosphorylation sites. The phospholipase Cgamma1 binding site Y1021, previously implicated in chemotaxis, displayed the largest increase in phosphorylation. The increase in Y1021 phosphorylation was accompanied by increased phospholipase Cgamma1 activity and migratory hyperresponsiveness to PDGF. PDGF beta receptor tyrosine phosphorylation in PTP-1B ko MEFs but not in PTPepsilon ko MEFs was also higher than that in control cells. This increase occurred with a site distribution different from that seen after TC-PTP depletion. PDGF-induced migration was not increased in PTP-1B ko cells. In summary, our findings identify TC-PTP as a previously unrecognized negative regulator of PDGF beta receptor signaling and support the general notion that PTPs display site selectivity in their action on tyrosine kinase receptors.     

Regulation of RhoA-dependent ROCKII activation by Shp2

Contractile forces mediated by RhoA and Rho kinase (ROCK) are required for a variety of cellular processes, including cell adhesion. In this study, we show that RhoA-dependent ROCKII activation is negatively regulated by phosphorylation at a conserved tyrosine residue (Y722) in the coiled-coil domain of ROCKII. Tyrosine phosphorylation of ROCKII is increased with cell adhesion, and loss of Y722 phosphorylation delays adhesion and spreading on fibronectin, suggesting that this modification is critical for restricting ROCKII-mediated contractility during these processes. Further, we provide evidence that Shp2 mediates dephosphorylation of ROCKII and, therefore, regulates RhoA-induced cell rounding, indicating that Shp2 couples with RhoA signaling to control ROCKII activation during deadhesion. Thus, reversible tyrosine phosphorylation confers an additional layer of control to fine-tune RhoA-dependent activation of ROCKII.     

Regulation of EphA2 receptor endocytosis by SHIP2 lipid phosphatase via phosphatidylinositol 3-Kinase-dependent Rac1 activation

Endocytosis of Eph receptors is critical for a number of biological processes, including modulating axon growth cone collapse response and regulating cell surface levels of receptor in epithelial cells. In particular, ephrin-A ligand stimulation of tumor cells induces EphA2 receptor internalization and degradation, a process that has been explored as a means to reduce tumor malignancy. However, the mechanism and regulation of ligand-induced Eph receptor internalization are not well understood. Here we show that SHIP2 (Src homology 2 domain-containing phosphoinositide 5-phosphatase 2) is recruited to activated EphA2 via a heterotypic sterile alpha motif (SAM)-SAM domain interaction, leading to regulation of EphA2 internalization. Overexpression of SHIP2 inhibits EphA2 receptor endocytosis, whereas suppression of SHIP2 expression by small interfering RNA-mediated gene silencing promotes ligand-induced EphA2 internalization and degradation. SHIP2 regulates EphA2 endocytosis via phosphatidylinositol 3-kinase-dependent Rac1 activation. Phosphatidylinositol 3,4,5-trisphosphate levels are significantly elevated in SHIP2 knockdown cells, phosphatidylinositol 3-kinase inhibitor decreases phosphatidylinositol 3,4,5-trisphosphate levels and suppresses increased EphA2 endocytosis. Ephrin-A1 stimulation activates Rac1 GTPase, and the Rac1-GTP levels are further increased in SHIP2 knockdown cells. A dominant negative Rac1 GTPase effectively inhibited ephrin-A1-induced EphA2 endocytosis. Together, our findings provide evidence that recruitment of SHIP2 to EphA2 attenuates a positive signal to receptor endocytosis mediated by phosphatidylinositol 3-kinase and Rac1 GTPase.     

Allosteric activation of PTEN phosphatase by phosphatidylinositol 4,5-bisphosphate

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is a tumor suppressor that is lost in many human tumors and encodes a phosphatidylinositol phosphate phosphatase specific for the 3-position of the inositol ring. Here we report a novel mechanism of PTEN regulation. Binding of di-C8-phosphatidylinositol 4,5-P2 (PI(4,5)P2) to PTEN enhances phosphatase activity for monodispersed substrates, PI(3,4,5)P3 and PI(3,4)P2. PI(5)P also is an activator, but PI(4)P, PI(3,4)P2, and PI(3,5)P2 do not activate PTEN. Activation by exogenous PI(4,5)P2 is more apparent with PI(3,4)P2 as a substrate than with PI(3,4,5)P3, probably because hydrolysis of PI(3,4)P2 yields PI(4)P, which is not an activator. In contrast, hydrolysis of PI(3,4,5)P3 yields a potent activator, PI(4,5)P2, creating a positive feedback loop. In addition, neither di-C4-PI(4,5)P2 nor inositol trisphosphate-activated PTEN. Hence, the interaction between PI(4,5)P2 and PTEN requires specific, ionic interactions with the phosphate groups on the inositol ring as well as hydrophobic interactions with the fatty acid chains, likely mimicking the physiological interactions that PTEN has with the polar surface head groups and the hydrophobic core of phospholipid membranes. Mutations of the apparent PI(4,5)P2-binding motif in the PTEN N terminus severely reduced PTEN activity. In contrast, mutation of the C2 phospholipid-binding domain had little effect on PTEN activation. These results suggest a model in which a PI(4,5)P2 monomer binds to PTEN, initiates an allosteric conformational change and, thereby, activates PTEN independent of membrane binding.     

Hematopoietic protein tyrosine phosphatase suppresses extracellular stimulus-regulated kinase activation

The mitogen-activated protein kinases (MAPKs) are signaling molecules that become enzymatically activated through phosphorylation by diverse stimuli. Hematopoietic cytokines, growth factors, and stimulated lymphocyte antigen receptors may activate specific MAPKs by altering the balance of MAPK-activating protein kinases and the protein phosphatases that target their activation sites. Hematopoietic protein tyrosine phosphatase (HePTP) is a hematopoiesis-specific cytoplasmic protein tyrosine phosphatase whose expression is induced by mitogenic stimuli. To investigate the role of HePTP in hematopoietic development, we constructed mice deficient in this phosphatase using the technique of homologous recombination. Primary lymphocytes from HePTP(-/-) mice show enhanced activation of extracellular stimulus-regulated kinase (ERK) after both phorbol myristate acetate (PMA) and anti-CD3-mediated T-cell receptor (TCR) stimulation, suggesting a true physiological relationship between these two molecules. Activation of MEK, the physiological activator of ERK, by anti-CD3 or PMA is not affected by HePTP deletion. The distribution of hematopoietic lineages in bone marrow and peripheral blood samples and the in vitro proliferative capacity of bone marrow progenitors from HePTP deletion mice do not deviate from those of matched littermate controls. Similarly, lymphocyte activation and development are indistinguishable in HePTP(-/-) mice and controls. We conclude that HePTP is a physiological regulator of ERK on the basis of these studies and hypothesize that its deletion is well compensated for in the developing mouse through reduction of ERK targets or enhancement of physiologically opposed signaling pathways.     

Dendritic cell-specific ablation of the protein tyrosine phosphatase Shp1 promotes Th1 cell differentiation and induces autoimmunity

Dendritic cells (DCs) promote immune responses to foreign Ags and immune tolerance to self-Ags. Deregulation of DCs is implicated in autoimmunity, but the molecules that regulate DCs to protect against autoimmunity have remained unknown. In this study, we show that mice lacking the protein tyrosine phosphatase Shp1 specifically in DCs develop splenomegaly associated with more CD11c(+) DCs. Splenic DCs from the mutant mice showed upregulation of CD86 and CCR7 expression and of LPS-induced production of proinflammatory cytokines. The mice manifested more splenic Th1 cells, consistent with the increased ability of their DCs to induce production of IFN-γ by Ag-specific T cells in vitro. The number of splenic CD5(+)CD19(+) B-1a cells and the serum concentrations of Igs M and G2a were also increased in the mutant mice. Moreover, aged mutant mice developed glomerulonephritis and interstitial pneumonitis together with increased serum concentrations of autoantibodies. Shp1 is thus a key regulator of DC functions that protects against autoimmunity.     

Conditional deletion of Shp2 tyrosine phosphatase in thymocytes suppresses both pre-TCR and TCR signals

It is well known that T cell differentiation and maturation in the thymus is tightly controlled at multiple checkpoints. However, the molecular mechanism for the control of this developmental program is not fully understood. A number of protein tyrosine kinases, such as Zap-70, Lck, and Fyn, have been shown to promote signals required for thymocyte development, whereas a tyrosine phosphatase Src homology domain-containing tyrosine phosphatase (Shp)1 has a negative effect in pre-TCR and TCR signaling. We show in this study that Shp2, a close relative of Shp1, plays a positive role in T cell development and functions. Lck-Cre-mediated deletion of Shp2 in the thymus resulted in a significant block in thymocyte differentiation/proliferation instructed by the pre-TCR at the beta selection step, and reduced expansion of CD4(+) T cells. Furthermore, mature Shp2(-/-) T cells showed decreased TCR signaling in vitro. Mechanistically, Shp2 acts to promote TCR signaling through the ERK pathway, with impaired activation of ERK kinase observed in Shp2(-/-) T cells. Thus, our results provide physiological evidence that Shp2 is a common signal transducer for pre-TCR and TCR in promoting T cell maturation and proliferation.     

Shp2, a novel oncogenic tyrosine phosphatase and potential therapeutic target for human leukemia

Shp2, encoded by the PTPNll gene in human, is a ubiquitously expressed protein tyrosine phosphatase that contains two N-terminal Src homology 2 （SH2） domains （N-SH2, C-SH2, respectively）, a catalytic protein-tyrosine phosphatase （PTP） domain, and a C-terminal tail with tyrosyl phosphorylation sites and a prolyl-rich motif [1]. The progress of our understanding of biological functions of Shp2 has clearly shown that Shp2 plays an important role not only in biology of normal hematopoietic cells and other mammalian cells, but also in the development of leukemia and other tumors. Most recently, PTPNll gene has been firmly established as the first proto-oncogene that encodes a protein tyrosine phosphatase [1-3]. In the hematopoietic system, most if not all function of Shp2 is to act as a positive component that is essential for proliferation and/or survival of hematopoietic cells through regulation of signaling pathways involving Erk, Akt and STATS [ 1-4]. Over the past few years, a number of disease-associated Shp2 mutants have been identified in human leukemia and other malignancies [1, 3, 4]. Recently, studies from our laboratories and others strongly suggest that dysregulation of wild-type Shp2 enzyme may be involved in the pathogenesis of adult leukemia [4-7]. These findings not only provide new insights into the role of Shp2 in leukemogenesis and other tumors, but also suggest new therapeutic targets for anti-leukemia drugs.