SHP1 and SHP2 protein-tyrosine phosphatases associate with betac after interleukin-3-induced receptor tyrosine phosphorylation. Identification of potential binding sites and substrates

Suppression of JAK-STAT signaling by Epstein-Barr virus tegument protein BGLF2 through recruitment of SHP1 phosphatase and promotion of STAT2 degradation

Some lytic proteins encoded by Epstein-Barr virus (EBV) suppress host interferon (IFN) signaling to facilitate viral replication. In this study we sought to identify and characterize EBV proteins antagonizing IFN signaling. The induction of IFN-stimulated genes (ISGs) by IFN-β was effectively suppressed by EBV. A functional screen was therefore performed to identify IFN-antagonizing proteins encoded by EBV. EBV tegument protein BGLF2 was identified as a potent suppressor of JAK-STAT signaling. This activity was found to be independent of its stimulatory effect on p38 and JNK pathways. Association of BGLF2 with STAT2 resulted in more pronounced K48-linked polyubiquitination and proteasomal degradation of the latter. Mechanistically, BGLF2 promoted the recruitment of SHP1 phosphatase to STAT1 to inhibit its tyrosine phosphorylation. In addition, BGLF2 associated with cullin 1 E3 ubiquitin ligase to facilitate its recruitment to STAT2. Consequently, BGLF2 suppressed ISG induction by IFN-β. Furthermore, BGLF2 also suppressed type II and type III IFN signaling, although the suppressive effect on type II IFN response was milder. When pre-treated with IFN-β, host cells became less susceptible to primary infection of EBV. This phenotype was reversed when expression of BGLF2 was enforced. Finally, genetic disruption of BGLF2 in EBV led to more pronounced induction of ISGs. Taken together, our study unveils the roles of BGLF2 not only in the subversion of innate IFN response but also in lytic infection and reactivation of EBV. Importance Epstein-Barr virus (EBV) is an oncogenic virus associated with the development of lymphoid and epithelial malignancies. EBV has to subvert interferon-mediated host antiviral response to replicate and cause diseases. It is therefore of great interest to identify and characterize interferon-antagonizing proteins produced by EBV. In this study we perform a screen to search for EBV proteins that suppress the action of interferons. We further show that BGLF2 protein of EBV is particularly strong in this suppression. This is achieved by inhibiting two key proteins STAT1 and STAT2 that mediate the antiviral activity of interferons. BGLF2 recruits a host enzyme to remove the phosphate group from STAT1 thereby inactivating its activity. BGLF2 also redirects STAT2 for degradation. A recombinant virus in which BGLF2 gene has been disrupted can activate host interferon response more robustly. Our findings reveal a novel mechanism by which EBV BGLF2 protein suppresses interferon signaling.

A novel partially open state of SHP2 points to a "multiple gear" regulation mechanism

The protein tyrosine phosphatase SHP2 mediates multiple signal transductions in various cellular pathways, controlled by a variety of upstream inputs. SHP2 dysregulation is causative of different types of cancers and developmental disorders, making it a promising drug target. However, how SHP2 is modulated by its different regulators remains largely unknown. Here, we use single-molecule fluorescence resonance energy transfer and molecular dynamics simulations to investigate this question. We identify a partially open, semiactive conformation of SHP2 that is intermediate between the known open and closed states. We further demonstrate a “multiple gear” regulatory mechanism, in which different activators (e.g., insulin receptor substrate-1 and CagA), oncogenic mutations (e.g., E76A), and allosteric inhibitors (e.g., SHP099) can shift the equilibrium of the three conformational states and regulate SHP2 activity to different levels. Our work reveals the essential role of the intermediate state in fine-tuning the activity of SHP2, which may provide new opportunities for drug development for relevant cancers.

Role of the JAK-STAT Pathway in Bovine Mastitis and Milk Production

Simple Summary

The cytokine-activated Janus kinase (JAK)—signal transducer and activator of transcription (STAT) pathway has an important role in the regulation of immunity and inflammation. In addition, the signaling of this pathway has been reported to be associated with mammary gland development and milk production. Because of such important functions, the JAK-STAT pathway has been widely targeted in both human and animal diseases as a therapeutic agent. Recently, the JAK2, STATs, and inhibitors of the JAK-STAT pathway, especially cytokine signaling suppressors (SOCSs), have been reported to be associated with milk production and mastitis-resistance phenotypic traits in dairy cattle. Thus, in the current review, we attempt to overview the development of the JAK-STAT pathway role in bovine mastitis and milk production.

Abstract

The cytokine-activated Janus kinase (JAK)—signal transducer and activator of transcription (STAT) pathway is a sequence of communications between proteins in a cell, and it is associated with various processes such as cell division, apoptosis, mammary gland development, lactation, anti-inflammation, and immunity. The pathway is involved in transferring information from receptors on the cell surface to the cell nucleus, resulting in the regulation of genes through transcription. The Janus kinase 2 (JAK2), signal transducer and activator of transcription A and B (STAT5 A & B), STAT1, and cytokine signaling suppressor 3 (SOCS3) are the key members of the JAK-STAT pathway. Interestingly, prolactin (Prl) also uses the JAK-STAT pathway to regulate milk production traits in dairy cattle. The activation of JAK2 and STATs genes has a critical role in milk production and mastitis resistance. The upregulation of SOCS3 in bovine mammary epithelial cells inhibits the activation of JAK2 and STATs genes, which promotes mastitis development and reduces the lactational performance of dairy cattle. In the current review, we highlight the recent development in the knowledge of JAK-STAT, which will enhance our ability to devise therapeutic strategies for bovine mastitis control. Furthermore, the review also explores the role of the JAK-STAT pathway in the regulation of milk production in dairy cattle.

SHP-2 in Lymphocytes' Cytokine and Inhibitory Receptor Signaling

Somewhat counterintuitively, the tyrosine phosphatase SHP-2 (SH2 domain-containing protein tyrosine phosphatase-2) is crucial for the activation of extracellular signal-regulated kinase (ERK) downstream of various growth factor receptors, thereby exerting essential developmental functions. This phosphatase also deploys proto-oncogenic functions and specific inhibitors have recently been developed. With respect to the immune system, the role of SHP-2 in the signaling of cytokines relevant for myelopoiesis and myeloid malignancies has been intensively studied. The function of this phosphatase downstream of cytokines important for lymphocytes is less understood, though multiple lines of evidence suggest its importance. In addition, SHP-2 has been proposed to mediate the suppressive effects of inhibitory receptors (IRs) that sustain a dysfunctional state in anticancer T cells. Molecules involved in IR signaling are of potential pharmaceutical interest as blockade of these inhibitory circuits leads to remarkable clinical benefit. Here, we discuss the dichotomy in the functions ascribed to SHP-2 downstream of cytokine receptors and IRs, with a focus on T and NK lymphocytes. Further, we highlight the importance of broadening our understanding of SHP-2′s relevance in lymphocytes, an essential step to inform on side effects and unanticipated benefits of its therapeutic blockade.

The molecular details of cytokine signaling via the JAK/STAT pathway

More than 50 cytokines signal via the JAK/STAT pathway to orchestrate hematopoiesis, induce inflammation and control the immune response. Cytokines are secreted glycoproteins that act as intercellular messengers, inducing proliferation, differentiation, growth, or apoptosis of their target cells. They act by binding to specific receptors on the surface of target cells and switching on a phosphotyrosine-based intracellular signaling cascade initiated by kinases then propagated and effected by SH2 domain-containing transcription factors. As cytokine signaling is proliferative and often inflammatory, it is tightly regulated in terms of both amplitude and duration. Here we review molecular details of the cytokine-induced signaling cascade and describe the architectures of the proteins involved, including the receptors, kinases, and transcription factors that initiate and propagate signaling and the regulatory proteins that control it.

Liraglutide Attenuates Nonalcoholic Fatty Liver Disease through Adjusting Lipid Metabolism via SHP1/AMPK Signaling Pathway

A glucagon-like peptide-1 (GLP-1) receptor agonist liraglutide (LR) had been experimentally and clinically shown to ameliorate nonalcoholic fatty liver disease (NAFLD). This study aimed to investigate the beneficial effect of LR on NAFLD in vivo and in vitro and its underlying molecular mechanism. The effects of LR were examined on the high-fat diet-induced in vivo model in mice and in vitro model of NAFLD in human HepG2 cells. Liver tissues and HepG2 cells were procured for measuring lipid metabolism, histological examination, and western blot analysis. LR administration significantly lowered the serum lipid profile and lipid disposition in vitro and in vivo because of the altered expression of enzymes on hepatic gluconeogenesis and lipid metabolism. Moreover, LR significantly decreased Src homology region 2 domain-containing phosphatase-1 (SHP1) and then increased the expression of phosphorylated-AMP-activated protein kinase (p-AMPK). However, the overexpression of SHP1 mediated by lentivirus vector reversed LR-induced improvement in lipid deposition. Moreover, SHP1 silencing could further increase the expression of p-AMPK to ameliorate lipid metabolism and relative lipogenic gene induced by LR. In addition, abrogation of AMPK by Compound C eliminated the protective effects of LR on lipid metabolism without changing the expression of SHP1. LR markedly prevented NAFLD through adjusting lipid metabolism via SHP1/AMPK signaling pathway.

CD22: A Regulator of Innate and Adaptive B Cell Responses and Autoimmunity

CD22 (Siglec 2) is a receptor predominantly restricted to B cells. It was initially characterized over 30 years ago and named “CD22” in 1984 at the 2nd International workshop in Boston (1). Several excellent reviews have detailed CD22 functions, CD22-regulated signaling pathways and B cell subsets regulated by CD22 or Siglec G (2–4). This review is an attempt to highlight recent and possibly forgotten findings. We also describe the role of CD22 in autoimmunity and the great potential for CD22-based immunotherapeutics for the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE).

Shp1 function in myeloid cells

The motheaten mouse was first described in 1975 as a model of systemic inflammation and autoimmunity, as a result of immune system dysregulation. The phenotype was later ascribed to mutations in the cytoplasmic tyrosine phosphatase Shp1. This phosphatase is expressed widely throughout the hematopoietic system and has been shown to impact a multitude of cell signaling pathways. The determination of which cell types contribute to the different aspects of the phenotype caused by global Shp1 loss or mutation and which pathways within these cell types are regulated by Shp1 is important to further our understanding of immune system regulation. In this review, we focus on the role of Shp1 in myeloid cells and how its dysregulation affects immune function, which can impact human disease.

Gamma tocotrienol targets tyrosine phosphatase SHP2 in mammospheres resulting in cell death through RAS/ERK pathway

Background

There is increasing evidence supporting the concept of cancer stem cells (CSCs), which are responsible for the initiation, growth and metastasis of tumors. CSCs are thus considered the target for future cancer therapies. To achieve this goal, identifying potential therapeutic targets for CSCs is essential.

Methods

We used a natural product of vitamin E, gamma tocotrienol (gamma-T3), to treat mammospheres and spheres from colon and cervical cancers. Western blotting and real-time RT-PCR were employed to identify the gene and protein targets of gamma-T3 in mammospheres.

Results

We found that mammosphere growth was inhibited in a dose dependent manner, with total inhibition at high doses. Gamma-T3 also inhibited sphere growth in two other human epithelial cancers, colon and cervix. Our results suggested that both Src homology 2 domain-containing phosphatase 1 (SHP1) and 2 (SHP2) were affected by gamma-T3 which was accompanied by a decrease in K- and H-Ras gene expression and phosphorylated ERK protein levels in a dose dependent way. In contrast, expression of self-renewal genes TGF-beta and LIF, as well as ESR signal pathways were not affected by the treatment. These results suggest that gamma-T3 specifically targets SHP2 and the RAS/ERK signaling pathway.

Conclusions

SHP1 and SHP2 are potential therapeutic targets for breast CSCs and gamma-T3 is a promising natural drug for future breast cancer therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1614-1) contains supplementary material, which is available to authorized users.

The molecular regulation of Janus kinase (JAK) activation

The JAK (Janus kinase) family members serve essential roles as the intracellular signalling effectors of cytokine receptors. This family, comprising JAK1, JAK2, JAK3 and TYK2 (tyrosine kinase 2), was first described more than 20 years ago, but the complexities underlying their activation, regulation and pleiotropic signalling functions are still being explored. Here, we review the current knowledge of their physiological functions and the causative role of activating and inactivating JAK mutations in human diseases, including haemopoietic malignancies, immunodeficiency and inflammatory diseases. At the molecular level, recent studies have greatly advanced our knowledge of the structures and organization of the component FERM (4.1/ezrin/radixin/moesin)-SH2 (Src homology 2), pseudokinase and kinase domains within the JAKs, the mechanism of JAK activation and, in particular, the role of the pseudokinase domain as a suppressor of the adjacent tyrosine kinase domain's catalytic activity. We also review recent advances in our understanding of the mechanisms of negative regulation exerted by the SH2 domain-containing proteins, SOCS (suppressors of cytokine signalling) proteins and LNK. These recent studies highlight the diversity of regulatory mechanisms utilized by the JAK family to maintain signalling fidelity, and suggest alternative therapeutic strategies to complement existing ATP-competitive kinase inhibitors.

Th2 cytokine antagonists: potential treatments for severe asthma

INTRODUCTION

Asthma is a major disease burden worldwide. Treatment with steroids and long acting β-agonists effectively manage symptoms in many patients but do not treat the underlying cause of disease and have serious side effects when used long term and in children. Therapies targeting the underlying causes of asthma are urgently needed. T helper type 2 (Th2) cells and the cytokines they release are clinically linked to the presentation of all forms of asthma. They are the primary drivers of mild to moderate and allergic asthma. They also play a pathogenetic role in exacerbations and more severe asthma though other factors are also involved. Much effort using animal models and human studies has been dedicated to the identification of the pathogenetic roles of these cells and cytokines and whether inhibition of their activity has therapeutic benefit in asthma.

AREAS COVERED

We discuss the current status of Th2 cytokine antagonists for the treatment of asthma. We also discuss the potential for targeting Th2-inducing cytokines, Th2 cell receptors and signaling as well as the use of Th2 cell antagonists, small interfering oligonucleotides, microRNAs, and combination therapies.

EXPERT OPINION

Th2 antagonists may be most effective in particular asthma subtypes/endotypes where specific cytokines are known to be active through the analysis of biomarkers. Targeting common receptors and pathways used by these cytokines may have additional benefit. Animal models have been valuable in identifying therapeutic targets in asthma, however the results from such studies need to be carefully interpreted and applied to appropriately stratified patient cohorts in well-designed clinical studies and trials.

Deficiency of SHP1 leads to sustained and increased ERK activation in mast cells, thereby inhibiting IL-3-dependent proliferation and cell death

SHP-1 plays an important role for the regulation of signaling from various hematopoietic cell receptors. In this study, we examined IL-3-induced cell proliferation and IL-3 depletion-induced apoptosis in bone marrow-derived mast cells (BMMC) established from motheaten (me) that lack SHP-1 expression, viable motheaten (me(v)) expressing phosphatase-deficient SHP-1, and wild-type (WT) mice. When BMMC were stimulated with IL-3, increased ERK activation was evident in resting state and sustained in me-BMMC relative to WT-BMMC. ERK is known to be involved in the regulation of cell proliferation and apoptosis in some cells. In accordance with sustained ERK activation, apoptosis was decreased in me- and me(v)-BMMC compared with WT-BMMC. In contrast to the predicted role of ERK as a pro-survival molecule, IL-3-induced cell proliferation was much lower in me- and me(v)-BMMC than WT-BMMC. Stimulation with lower concentration of IL-3 or addition of PD98059, a MEK inhibitor, to the culture resulted in the suppression of decreased apoptosis and cell proliferation in me- and me(v)-BMMC. Collectively, these results suggest that SHP-1 positively regulates IL-3-dependent mast cell proliferation and apoptosis by inhibiting ERK activity through its phosphatase activity. Furthermore, our results indicate that ERK would act as a negative regulator for cell proliferation and induce apoptosis when its activity is highly increased.

CoA Synthase is phosphorylated on tyrosines in mammalian cells, interacts with and is dephosphorylated by Shp2PTP

CoA Synthase (CoASy, 4'-phosphopantetheine adenylyltransferase/dephospho-CoA kinase) mediates two final stages of de novo coenzyme A (CoA) biosynthesis in higher eukaryotes. Unfortunately very little is known about regulation of this important metabolic pathway. In this study, we demonstrate that CoASy interacts in vitro with Src homology-2 (SH2) domains of a number of signaling proteins, including Src homology-2 domains containing protein tyrosine phosphatase (Shp2PTP). Complexes between CoASy and Shp2PTP exist in vivo in mammalian cells and this interaction is regulated in a growth-factor-dependent manner. We have also demonstrated that endogenous CoASy is phosphorylated on tyrosine residues in vivo, and that cytoplasmic protein tyrosine kinases can mediate this phosphorylation in vitro and in vivo. Importantly, Shp2PTP-mediated CoASy in vitro dephosphorylation leads to an increase in CoASy enzymatic phosphopantetheine adenylyltransferase (PPAT) activity. We therefore argue that CoASy is a novel potential substrate of Shp2PTP and phosphorylation of CoASy at tyrosine residue(s) could represent unrecognized before mechanism of modulation intracellular CoA level in response to hormonal and (or) other extracellular stimuli.

The Shc-binding site of the betac subunit of the GM-CSF/IL-3/IL-5 receptors is a negative regulator of hematopoiesis

Tyrosine and serine phosphorylation of the common beta chain (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is widely viewed as a general mechanism that provides positive inputs by coupling the receptor to signaling pathways that stimulate several cellular functions. We show here that despite the known action of Tyr577 in beta(c) to recruit Shc-PI-3 kinase (PI3K) pathway members, Tyr577 plays, surprisingly, a negative regulatory role in cell function, and that this is mediated, at least in part, through the uncoupling of SH2-containing inositol 5'-phosphatase (SHIP) from beta(c). Fetal liver cells from beta(c)/beta(IL-3)(-/-) mice expressing human GM-CSF receptor alpha chain and beta(c) Tyr577Phe mutant showed enhanced colony formation and expansion of progenitor cells in response to GM-CSF. Dissection of these activities revealed that basal survival was increased, as well as cytokine-stimulated proliferation. As expected, the recruitment and activation of Shc was abolished, but interestingly, Gab-2 and Akt phosphorylation increased. Significantly, the activation of PI3K was enhanced and prolonged, accompanied by loss of SHIP activity. These results reveal a previously unrecognized negative signaling role for Tyr577 in beta(c) and demonstrate that uncoupling Shc from cytokine receptors enhances PI3K signaling as well as survival and proliferation.

Sequence specificity of SHP-1 and SHP-2 Src homology 2 domains. Critical roles of residues beyond the pY+3 position

A combinatorial phosphotyrosyl (pY) peptide library was screened to determine the amino acid preferences at the pY+4 to pY+6 positions for the four SH2 domains of protein-tyrosine phosphatases SHP-1 and SHP-2. Individual binding sequences selected from the library were resynthesized and their binding affinities and specificities to various SH2 domains were further evaluated by SPR studies, stimulation of SHP-1 and SHP-2 phosphatase activity, and in vitro pulldown assays. These studies reveal that binding of a pY peptide to the N-SH2 domain of SHP-2 is greatly enhanced by a large hydrophobic residue (Trp, Tyr, Met, or Phe) at the pY+4 and/or pY+5 positions, whereas binding to SHP-1 N-SH2 domain is enhanced by either hydrophobic or positively charged residues (Arg, Lys, or His) at these positions. Similar residues at the pY+4 to pY+6 positions are also preferred by SHP-1 and SHP-2 C-SH2 domains, although their influence on the overall binding affinities is much smaller compared with the N-SH2 domains. A structural model was generated to qualitatively interpret the contribution of the pY+4 and pY+5 residues to the overall binding affinity. Examination of pY motifs from known SHP-1 and SHP-2-binding proteins shows that many of the pY motifs contain a hydrophobic or positively charged residue(s) at the pY+4 and pY+5 positions.

A Novel Recognition System for MHC Class I Molecules Constituted by PIR

The paired immunoglobulin (Ig)-like receptors (PIRs) represent a typical receptor pair of the Ig-like receptor family in which various combinations of ligand-receptor interaction provide a positive and negative regulation of immune cells, thus enabling those cells to respond properly to extrinsic stimuli. Activating PIR-A and inhibitory PIR-B are expressed in a wide range of cells in the murine immune system, such as B cells, mast cells, macrophages, and dendritic cells, mostly in a pair-wise fashion. PIRs bind to MHC class I molecules expressed ubiquitously on hematopoietic as well as nonhematopoietic cells. The unbalanced binding of PIR-A and PIR-B to MHC class I molecules may lead to the perturbation of cell development, regulation, and function as observed in PIR-B-deficient mice. Thus, PIR-A and PIR-B are indispensable for the regulation of cellular signaling and important for homeostasis of the immune system.

A negative role of SHP-2 tyrosine phosphatase in growth factor-dependent hematopoietic cell survival

SHP-2 tyrosine phosphatase is highly expressed in hematopoietic cells; however, the function of SHP-2 in hematopoietic cell processes is not fully understood. Recent identification of SHP-2 mutations in childhood leukemia further emphasizes the importance of SHP-2 regulation in hematopoietic cells. We previously reported that SHP-2 played a positive role in IL-3-induced activation of Jak2 kinase in a catalytic-dependent manner. Interestingly, enforced expression of wild-type (WT) SHP-2 in Ba/F3 cells enhanced growth factor deprivation-induced apoptosis. Biochemical analyses revealed that although IL-3 activation of Jak2 kinase was increased, tyrosyl phosphorylation of its downstream substrate STAT5 was disproportionately decreased by the overexpression of SHP-2. Following IL-3 deprivation, the tyrosyl phosphorylation of STAT5 that is required for its antiapoptotic activity was rapidly diminished in SHP-2 overexpressing cells. As a result, reduction of the putative downstream targets of STAT5-Bcl-X(L) and pim-1 was accelerated by overexpression of SHP-2. Further investigation showed that SHP-2 associated with STAT5, and that it was indeed able to dephosphorylate STAT5. Finally, overexpression of SHP-2 in primary bone marrow hematopoietic progenitor cells compromised their differentiative and proliferative potential, and enhanced growth factor withdrawal-induced cell death. And, the effect of SHP-2 overexpression on growth factor-dependent survival was diminished in STAT5-deficient hematopoietic cells. Taken together, these results suggest that SHP-2 tyrosine phosphatase negatively regulates hematopoietic cell survival by dephosphorylation of STAT5.

Characterization of phosphotyrosine binding motifs in the cytoplasmic domain of B and T lymphocyte attenuator required for association with protein tyrosine phosphatases SHP-1 and SHP-2

B and T lymphocytes express receptors providing positive and negative co-stimulatory signals. We recently identified a novel co-stimulatory molecule, B and T lymphocyte attenuator (BTLA), which exerts inhibitory effects on B and T lymphocytes. The cytoplasmic domain of murine and human BTLA share three conserved tyrosine-based signaling motifs, a Grb-2 recognition consensus, and two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Phosphorylation of the cytoplasmic domain of BTLA induced the association with the protein tyrosine phosphatases SHP-1 and SHP-2. Association of SHP-1 and SHP-2 to other receptors can involve recruitment to either a single receptor ITIM or to two receptor ITIMs. Here, we analyzed the requirements of BTLA interaction with SHP-1 and SHP-2 in a series of murine and human BTLA mutants. For human BTLA, mutations of either Y257 or Y282, but not Y226, abrogated association with both SHP-1 and SHP-2. For murine BTLA, mutation of either Y274 or Y299, but not Y245, also abrogated association with both SHP-1 and SHP-2. These results indicate that for both murine and human BTLA, association with SHP-1 or SHP-2 requires both of conserved ITIM motifs and does not involve the conserved Grb-2 consensus. Thus, similar to the bisphosphoryl tyrosine-based activation motif (BTAM) by which the Grb-2 associated binder (Gab1), PDGF receptor, and PECAM-1 recruit SHP-2, BTLA also relies on dual ITIMs for its association with the phosphatases SHP-1 and SHP-2.

Regulation of interleukin-3-induced substrate phosphorylation and cell survival by SHP-2 (Src-homology protein tyrosine phosphatase 2)

The cytosolic SHP-2 (Src homology protein tyrosine phosphatase 2) has previously been implicated in IL-3 (interleukin-3) signalling [Bone, Dechert, Jirik, Schrader and Welham (1997) J. Biol. Chem. 272, 14470 -14476; Craddock and Welham (1997) J. Biol. Chem. 272, 29281-29289; Welham, Dechert, Leslie, Jirik and Schrader (1994) J. Biol. Chem. 269, 23764-23768; Qu, Nguyen, Chen and Feng (2001) Blood 97, 911-914]. To investigate the role of SHP-2 in IL-3 signalling in greater detail, we have inducibly expressed WT (wild-type) or two potentially substrate-trapping mutant forms of SHP-2, generated by mutation of Asp-425 to Ala (D425A) or Cyst-459 to Ser (C459S), in IL-3-dependent BaF/3 cells. Effects on IL-3-induced tyrosine phosphorylation, signal transduction and functional responses were examined. Expression of C459S SHP-2 protected the beta-chain of the murine IL-3R (IL-3 receptor), the adaptor protein Gab2 (Grb2-associated binder 2), and a cytosolic protein of 48 kDa from tyrosine dephosphorylation, consistent with them being bona fide substrates of SHP-2 in IL-3 signalling. The tyrosine phosphorylation of a 135 kDa transmembrane protein was also protected upon expression of C459S SHP-2. We have identified the inhibitory immunoreceptor PECAM-1 (platelet endothelial cell adhesion molecule-1)/CD31 (cluster determinant 31) as a component of this 135 kDa substrate and also show that IL-3 can induce tyrosine phosphorylation of PECAM-1. Expression of WT, C459S and D425A forms of SHP-2 had little effect on IL-3-driven proliferation or STAT5 (signal transduction and activators of transcription) phosphorylation or activation of protein kinase B. However, expression of WT SHP-2 increased ERK (extracellular-signal-regulated kinase) activation. Interestingly, expression of C459S SHP-2 decreased ERK activation at later times after IL-3 stimulation, but potentiated IL-3-induced activation of Jun N-terminal kinases. In addition, expression of C459S SHP-2 decreased cell survival in suboptimal IL-3 and upon IL-3 withdrawal. These findings indicate that SHP-2 plays an important role in mediating the anti-apoptotic effect of IL-3 and raises the possibility that PECAM-1 participates in the modulation of cytokine-induced signals.

Catalytic-dependent and -independent roles of SHP-2 tyrosine phosphatase in interleukin-3 signaling

SHP-2 tyrosine phosphatase is highly expressed in hematopoietic cells, however, the function of SHP-2 in hematopoietic cell signaling is not well understood. Here we focus on the role of SHP-2 phosphatase in the signal transduction of interleukin (IL)-3, a cytokine involved in hematopoietic cell survival, proliferation, and differentiation. We established immortalized SHP-2(-/-) hematopoietic cell pools and showed that IL-3-induced proliferative response was diminished in SHP-2(-/-) cells. Moreover, inhibition of the catalytic activity of SHP-2 in wild-type (WT) bone marrow hematopoietic progenitor cells and Ba/F3 cells by overexpression of catalytically inactive SHP-2 mutant suppressed their differentiative and proliferative responses to IL-3, demonstrating an important positive role for SHP-2 in IL-3 signal transduction. Further biochemical analyses revealed that IL-3-induced Jak/Stat, Erk, and PI3 kinase pathways in SHP-2(-/-) cells were impaired and reintroduction of WT SHP-2 into mutant cells partially restored IL-3 signaling. Interestingly, in catalytically inactive SHP-2-overexpressing Ba/F3 cells, although IL-3-induced activation of Jak2 and Erk kinases was reduced and shortened, PI3 kinase activation remained unaltered. Taken together, these results suggest that SHP-2 tyrosine phosphatase plays multiple roles in IL-3 signal transduction, functioning in both catalytic-dependent and -independent manners in the Jak/Stat, Erk, and PI3 kinase pathways.

Role of the protein tyrosine phosphatase SHP-1 (Src homology phosphatase-1) in the regulation of interleukin-3-induced survival, proliferation and signalling

The tyrosine phosphatase SHP-1 (Src homology phosphatase-1) has been widely implicated as a negative regulator of signalling in immune cells. We have investigated in detail the role of SHP-1 in interleukin-3 (IL-3) signal transduction by inducibly expressing wild-type (WT), C453S (substrate-trapping) and R459M (catalytically inactive) forms of SHP-1 in the IL-3-dependent cell line BaF/3. Expression of WT SHP-1 had little impact on IL-3-induced proliferation, but enhanced apoptosis following IL-3 withdrawal. Expression of R459M SHP-1 increased the proliferative response of BaF/3 cells to IL-3 and increased cell survival at low doses of IL-3 and following IL-3 withdrawal. Investigation into the biochemical consequences resulting from expression of these SHP-1 variants demonstrated that the beta chain of the IL-3 receptor (Aic2A) was hypo-phosphorylated in cells expressing WT SHP-1 and hyper-phosphorylated in those expressing R459M SHP-1. Further, ectopic expression of the trapping mutant, C453S SHP-1, protected Aic2A from dephosphorylation, suggesting that Aic2A is a SHP-1 substrate in BaF/3 cells. Examination of overall levels of tyrosine phosphorylation demonstrated that they were not perturbed in these transfectants. Activation-specific phosphorylation of STAT (signal transducer and activator of transcription) 5a/b, protein kinase B and ERK (extracellular-signal-regulated kinase)-1 and -2 was also unaffected by expression of WT or R459M SHP-1. However, overall levels of IL-3-induced tyrosine phosphorylation of STAT5 were reduced upon expression of WT SHP-1 and increased when R459M SHP-1 was expressed, consistent with STAT5 being a potential SHP-1 substrate. These results demonstrate that SHP-1 acts to negatively regulate IL-3-driven survival and proliferation, potentially via regulation of tyrosine phosphorylation of Aic2A and STAT5.

SHP-2 Is a Dual-specificity Phosphatase Involved in Stat1 Dephosphorylation at Both Tyrosine and Serine Residues in Nuclei

Signal transducer and activator of transcription (STAT) proteins are both tyrosine- and serine-phosphorylated, mediating signal transduction and gene regulation. Following gene regulation, STAT activity in the nucleus is then terminated by a nuclear protein phosphatase(s), which remains unidentified. Using novel antibody arrays to screen the Stat1-specific protein phosphatase(s), we identified a SHP-2-Stat1 interaction in the A431 cell nucleus. SHP-2 and Stat1 nuclear localization and their association in response to either epidermal growth factor or interferon-gamma (IFNgamma) were confirmed by immunofluorescent staining and affinity precipitation assays. The SHP-2 C-terminal region containing protein-tyrosine phosphatase activity interacted with the C-terminal SH2 transcriptional activation domain of Stat1. In SHP-2-/- mouse fibroblast cells, Stat1 phosphorylation at both the tyrosine residue Tyr(701) and the serine residue Ser(727) by IFNgamma was enhanced and prolonged. Consistently, purified GST-SHP-2 dephosphorylated Stat1 at both tyrosine and serine residues when immunoprecipitated phospho-Stat1 or a peptide corresponding to the sequence surrounding Tyr(P)(701) or Ser(P)(727) of Stat1 was used as the substrate. Overexpression of SHP-2 in 293T cells inhibited IFNgamma-dependent Stat1 phosphorylation and suppressed Stat1-dependent induction of luciferase activity. Our findings demonstrate that SHP-2 is a dual-specificity protein phosphatase involved in Stat1 dephosphorylation at both tyrosine and serine residues and plays an important role in modulating STAT function in gene regulation.

A new high affinity binding site for suppressor of cytokine signaling-3 on the erythropoietin receptor

Erythropoietin (Epo) is a hematopoietic cytokine that is crucial for the differentiation and proliferation of erythroid progenitor cells. Epo acts on its target cells by inducing homodimerization of the erythropoietin receptor (EpoR), thereby triggering intracellular signaling cascades. The EpoR encompasses eight tyrosine motifs on its cytoplasmic tail that have been shown to recruit a number of regulatory proteins. Recently, the feedback inhibitor suppressor of cytokine signaling-3 (SOCS-3), also referred to as cytokine-inducible SH2-containing protein 3 (CIS-3), has been shown to act on Epo signaling by both binding to the EpoR and the EpoR-associated Janus kinase 2 (Jak2) [Sasaki, A., Yasukawa, H., Shouda, T., Kitamura, T., Dikic, I. & Yoshimura, A. (2000) J. Biol. Chem 275, 29338-29347]. In this study tyrosine 401 was identified as a binding site for SOCS-3 on the EpoR. Here we show that human SOCS-3 binds to pY401 with a Kd of 9.5 microm while another EpoR tyrosine motif, pY429pY431, can also interact with SOCS-3 but with a ninefold higher affinity than we found for the previously reported motif pY401. In addition, SOCS-3 binds the double phosphorylated motif pY429pY431 more potently than the respective singly phosphorylated tyrosines indicating a synergistic effect of these two tyrosine residues with respect to SOCS-3 binding. Surface plasmon resonance analysis, together with peptide precipitation assays and model structures of the SH2 domain of SOCS-3 complexed with EpoR peptides, provide evidence for pY429pY431 being a new high affinity binding site for SOCS-3 on the EpoR.

Molecular interactions of SHP1 and SHP2 in IL-3-signalling

SHP1 and SHP2 tyrosine phosphatases have both been implicated in signalling pathways downstream of the interleukin-3 (IL-3) receptor. We have investigated the co-association of SHP1 and SHP2 with tyrosine-phosphorylated proteins in IL-3-dependent BaF/3 cells. We demonstrate that both SHP1 and SHP2 associate with Aic2A (beta chain of the IL-3 receptor), Gab2 and the paired inhibitory receptor B (PIR-B). The individual SH2 domains of SHP2 can independently bind Gab2, potentially important for the adapter function of SHP2. Association of both phosphatases with Aic2A and Gab2 increases upon IL-3 treatment. Recruitment of SHP1 to PIR-B also increases in response to IL-3, suggesting a functional link between inhibitory and cytokine receptor signalling. Aic2A is a rapid target for dephosphorylation following IL-3 stimulation and substrate-trapping versions of both phosphatases identify Aic2A and Gab2 as substrates for SHP1 and SHP2. These studies suggest that SH2-domain interactions are important for targetting these phosphatases to their substrates.

Proteasomal regulation of betac signaling reveals a novel mechanism for cytokine receptor heterotypic desensitization

IL-5, IL-3, and GM-CSF are hematopoietic cytokines that are key mediators of the allergic inflammatory response. The receptors for these three cytokines consist of a cytokine-specific alpha (Ralpha) chain and a shared common beta (betac) chain. Herein, we demonstrate that agonistic ligation of these receptor subunits rapidly induces proteasomal degradation of the betac, but not the Ralpha, cytoplasmic domain, resulting in termination of signal transduction and yielding a truncated betac isoform ligated to the Ralpha subunit. Proteasomal degradation of the betac cytoplasmic domain was also a prerequisite for endocytosis and lysosomal degradation of the ligated receptor subunits. Moreover, proteasome-dependent termination of signaling induced by one betac-engaging cytokine resulted in cellular desensitization to signal transduction by subsequent stimulation with another betac-engaging cytokine. These data provide the first evidence for ligand-dependent proteasomal degradation of the betac cytoplasmic domain, and they establish a novel mechanism for heterotypic desensitization of shared cytokine receptor signaling.

Identification and characterization of Magmas, a novel mitochondria-associated protein involved in granulocyte-macrophage colony-stimulating factor signal transduction

OBJECTIVE

The aim of this study was to identify granulocyte-macrophage colony-stimulating factor (GM-CSF) responsive genes.

MATERIALS AND METHODS

Potential GM-CSF responsive genes were identified by comparing the mRNA expression pattern of the murine myeloid cell line PGMD1 grown in either interleukin-3 (IL-3) or GM-CSF by differential display. Human and murine cDNA clones of one of the bands having increased expression in GM-CSF were isolated. mRNA expression of the gene was examined by Northern blot. Immunohistochemistry and studies with a green fluorescent fusion protein were used to determine its intracellular location. Growth factor-stimulated proliferation of PGMD1 cells transfected with constitutively expressed sense and anti-sense cDNA constructs of the gene was measured by 3H-thymidine incorporation.

RESULTS

A gene, named Magmas (mitochondria-associated granulocyte macrophage CSF signaling molecule), was shown to be rapidly induced when cells were switched from IL-3 to GM-CSF. Analysis of the amino acid sequence of Magmas showed it contained a mitochondrial signal peptide, but not any other known functional domains. The human and murine clones encode nearly identical 13-kDa proteins that localized to the mitochondria. Magmas mRNA expression was observed in all tissues examined. PGMD1 cells that overexpressed Magmas proliferated similarly to untransfected cells when cultured in IL-3 or GM-CSF. In contrast, cells with reduced protein levels grew normally in IL-3, but had impaired proliferation in GM-CSF.

CONCLUSION

Magmas is a mitochondrial protein involved in GM-CSF signal transduction.

Reduction of hematopoietic cell-specific tyrosine phosphatase SHP-1 gene expression in natural killer cell lymphoma and various types of lymphomas/leukemias : combination analysis with cDNA expression array and tissue microarray

To investigate the lymphomagenesis of NK/T lymphoma, we comprehensively and systematically analyzed the expression pattern of the human NK/T cell line (NK-YS) genome by cDNA expression array and tissue microarray. We detected significant changes in the gene expression of NK-YS cell line: an increase in 18 and a decrease in 20 genes compared to normal NK cells or peripheral blood mononuclear cells. Among these genes, we found a strong decrease in hematopoietic cell specific protein-tyrosine-phosphatase SH-PTP1 (SHP1) mRNA by cDNA expression array and reverse transcriptase-polymerase chain reaction. Further analysis with standard immunohistochemistry and tissue microarray, which used 207 paraffin-embedded specimens of various kinds of malignant lymphomas, showed that 100% of NK/T lymphoma specimens and more than 95% of various types of malignant lymphoma were negative for SHP1 protein expression. On the other hand, SHP1 protein was strongly expressed in the mantle zone and interfollicular zone lymphocytes in reactive lymphoid hyperplasia specimens. In addition, various kinds of hematopoietic cell lines, particularly the highly aggressive lymphoma/leukemia lines, lacked SHP1 expression in vitro, suggesting that loss of SHP1 expression may be related to not only malignant transformation, but also tumor cell aggressiveness. SHP1 expression could not be induced in either of two NK/T cell lines by phorbol ester, suggesting that genetic impairment or modification with methylation of SHP1 DNA could be one of the critical events in the pathogenesis of NK/T lymphoma. This evidence strongly suggests that loss of SHP1 gene expression plays an important role in multistep tumorigenesis, possibly as an anti-oncogene in the wide range of lymphomas/leukemias as well as NK/T lymphomas.

Phosphoinositide 3-kinase-dependent regulation of interleukin-3-induced proliferation: involvement of mitogen-activated protein kinases, SHP2 and Gab2

We have demonstrated previously that class I(A) phosphoinositide 3-kinases play a major role in regulation of interleukin-3 (IL)-3-dependent proliferation. Investigations into the downstream targets involved have identified the MAPK cascade as a target. Expression of Deltap85 and incubation with LY294002 both inhibited IL-3-induced activation of Mek, Erk1, and Erk2. This was most pronounced during the initial phase of Erk activation. The Mek inhibitor, PD98059, blocked IL-3-driven proliferation, an effect enhanced by Deltap85 expression, suggesting that inhibition of Mek and Erks by Deltap85 contributes to the decrease in IL-3-induced proliferation in these cells but that additional pathways may also be involved. To investigate the mechanism leading to decreased activation of Erks, we investigated effects on SHP2 and Gab2, both implicated in IL-3 regulation of Erk activation. Expression of Deltap85 led to a reduction in SHP2 tyrosine phosphorylation and its ability to interact with Grb2 and Gab2 but increased overall tyrosine phosphorylation of Gab2. LY294002 did not perturb SHP2 interactions, potentially related to differences in the effects of these inhibitors on levels of phosphoinositides. These results imply that the regulation of Erks by class I(A) phosphoinositide 3-kinase may contribute to IL-3-driven proliferation and that both SHP2 and Gab2 are possibly involved in this regulation.

SHP2 and cbl participate in alpha-chemokine receptor CXCR4-mediated signaling pathways

Stromal cell-derived factor (SDF)-1alpha and its receptor, CXCR4, play an important role in cell migration, embryonic development, and human immunodeficiency virus infection. However, the cellular signaling pathways that mediate these processes are not fully elucidated. We and others have shown that the binding of SDF-1alpha to CXCR4 activates phosphatidylinositol-3 kinase (PI-3 kinase), p44/42 mitogen-associated protein kinase, and the transcription factor nuclear factor-kappaB, and it also enhances the tyrosine phosphorylation and association of proteins involved in the formation of focal adhesions. In this study, we examined the role of phosphatases in CXCR4-mediated signaling pathways. We observed significant inhibition of SDF-1alpha-induced migration by phosphatase inhibitors in CXCR4-transfected pre-B lymphoma L1.2 cells, Jurkat T cells, and peripheral blood lymphocytes. Further studies revealed that SDF-1alpha stimulation induced robust tyrosine phosphorylation in the SH2-containing phosphatase SHP2. SHP2 associated with the CXCR4 receptor and the signaling molecules SHIP, cbl, and fyn. Overexpression of wild-type SHP2 increased SDF-1alpha-induced chemotaxis. Enhanced activation of fyn and lyn kinases and the tyrosine phosphorylation of cbl were also observed. In addition, SDF-1alpha stimulation enhanced the association of cbl with PI-3 kinase, Crk-L, and 14-3-3beta proteins. Our results suggest that CXCR4-mediated signaling is regulated by SHP2 and cbl, which collectively participate in the formation of a multimeric signaling complex.

Negative regulation of Ros receptor tyrosine kinase signaling. An epithelial function of the SH2 domain protein tyrosine phosphatase SHP-1

Male "viable motheaten" (me(v)) mice, with a naturally occurring mutation in the gene of the SH2 domain protein tyrosine phosphatase SHP-1, are sterile. Known defects in sperm maturation in these mice correlate with an impaired differentiation of the epididymis, which has similarities to the phenotype of mice with a targeted inactivation of the Ros receptor tyrosine kinase. Ros and SHP-1 are coexpressed in epididymal epithelium, and elevated phosphorylation of Ros in the epididymis of me(v) mice suggests that Ros signaling is under control of SHP-1 in vivo. Phosphorylated Ros strongly and directly associates with SHP-1 in yeast two-hybrid, glutathione S-transferase pull-down, and coimmunoprecipitation experiments. Strong binding of SHP-1 to Ros is selective compared to six other receptor tyrosine kinases. The interaction is mediated by the SHP-1 NH(2)-terminal SH2 domain and Ros phosphotyrosine 2267. Overexpression of SHP-1 results in Ros dephosphorylation and effectively downregulates Ros-dependent proliferation and transformation. We propose that SHP-1 is an important downstream regulator of Ros signaling.

Determination of the binding specificity of the SH2 domains of protein tyrosine phosphatase SHP-1 through the screening of a combinatorial phosphotyrosyl peptide library

A method for the rapid identification of high-affinity ligands to Src homology-2 (SH2) domains is reported. A phosphotyrosyl (pY) peptide library containing completely randomized residues at positions -2 to +3 relative to the pY was synthesized on TentaGel resin, with a unique peptide sequence on each resin bead (total 2.5 x 10(6) different sequences). The library was screened against the biotinylated N- and C-terminal SH2 domains of protein tyrosine phosphatase SHP-1, and the beads that carry high-affinity ligands of the SH2 domains were identified using an enzyme-linked assay involving a streptavidin-alkaline phosphatase conjugate. Peptide ladder sequencing of the selected beads using matrix-assisted laser desorption ionization mass spectrometry revealed consensus sequences for both SH2 domains. The N-terminal SH2 domain strongly selects for peptides with a leucine at the -2 position; at the C-terminal side of the pY residue, it can recognize two distinct classes of peptides with consensus sequences of LXpY(M/F)X(F/M) and LXpYAXL (X = any amino acid), respectively. The C-terminal SH2 domain exhibits almost exclusive selectivity for peptides of the consensus sequence, (V/I/L)XpYAX(L/V). Several representative sequences selected from the library were individually synthesized and tested for binding to the SH2 domains by surface plasmon resonance and for their ability to stimulate the catalytic activity of SHP-1. Both experiments have demonstrated that the selected peptides are capable of binding to the SH2 domains with dissociation constants (K(D)) in the low micromolar range.

New role for Shc in activation of the phosphatidylinositol 3-kinase/Akt pathway

Most, if not all, cytokines activate phosphatidylinositol 3-kinase (PI-3K). Although many cytokine receptors have direct binding sites for the p85 subunit of PI-3K, others, such as the interleukin-3 (IL-3) receptor beta common chain (betac) and the IL-2 receptor beta chain (IL-2Rbeta), lack such sites, leaving the mechanism by which they activate PI-3K unclear. Here, we show that the protooncoprotein Shc, which promotes Ras activation by recruiting the Grb2-Sos complex in response to stimulation of cytokine stimulation, also signals to the PI-3K/Akt pathway. Analysis of Y-->F and "add-back" mutants of betac shows that Y577, the Shc binding site, is the major site required for Gab2 phosphorylation in response to cytokine stimulation. When fused directly to a mutant form of IL-2Rbeta that lacks other cytoplasmic tyrosines, Shc can promote Gab2 tyrosyl phosphorylation. Mutation of the three tyrosyl phosphorylation sites of Shc, which bind Grb2, blocks the ability of the Shc chimera to evoke Gab2 tyrosyl phosphorylation. Overexpression of mutants of Grb2 with inactive SH2 or SH3 domains also blocks cytokine-stimulated Gab2 phosphorylation. The majority of cytokine-stimulated PI-3K activity associates with Gab2, and inducible expression of a Gab2 mutant unable to bind PI-3K markedly impairs IL-3-induced Akt activation and cell growth. Experiments with the chimeric receptors indicate that Shc also signals to the PI-3K/Akt pathway in response to IL-2. Our results suggest that cytokine receptors lacking direct PI-3K binding sites activate Akt via a Shc/Grb2/Gab2/PI-3K pathway, thereby regulating cell survival and/or proliferation.

FDF03, a novel inhibitory receptor of the immunoglobulin superfamily, is expressed by human dendritic and myeloid cells

In this study, we describe human FDF03, a novel member of the Ig superfamily expressed as a monomeric 44-kDa transmembrane glycoprotein and containing a single extracellular V-set Ig-like domain. Two potential secreted isoforms were also identified. The gene encoding FDF03 mapped to chromosome 7q22. FDF03 was mostly detected in hemopoietic tissues and was expressed by monocytes, macrophages, and granulocytes, but not by lymphocytes (B, T, and NK cells), indicating an expression restricted to cells of the myelomonocytic lineage. FDF03 was also strongly expressed by monocyte-derived dendritic cells (DC) and preferentially by CD14+/CD1a- DC derived from CD34+ progenitors. Moreover, flow cytometric analysis showed FDF03 expression by CD11c+ blood and tonsil DC, but not by CD11c- DC precursors. The FDF03 cytoplasmic tail contained two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like sequences. When overexpressed in pervanadate-treated U937 cells, FDF03 was tyrosine-phosphorylated and recruited Src homology-2 (SH2) domain-containing protein tyrosine phosphatase (SHP)-2 and to a lesser extent SHP-1. Like engagement of the ITIM-bearing receptor LAIR-1/p40, cross-linking of FDF03 inhibited calcium mobilization in response to CD32/FcgammaRII aggregation in transfected U937 cells, thus demonstrating that FDF03 can function as an inhibitory receptor. However, in contrast to LAIR-1/p40, cross-linking of FDF03 did not inhibit GM-CSF-induced monocyte differentiation into DC. Thus, FDF03 is a novel ITIM-bearing receptor selectively expressed by cells of myeloid origin, including DC, that may regulate functions other than that of the broadly distributed LAIR-1/p40 molecule.

Beta-chemokine receptor CCR5 signals through SHP1, SHP2, and Syk

The beta-chemokine receptor CCR5 has been shown to modulate cell migration, proliferation, and immune functions and to serve as a co-receptor for the human immunodeficiency virus. We and others have shown that CCR5 activates related adhesion focal tyrosine kinase (RAFTK)/Pyk2/CAK-beta. In this study, we further characterize the signaling molecules activated by CCR5 upon binding to its cognate ligand, macrophage inflammatory protein-1beta (MIP1beta). We observed enhanced tyrosine phosphorylation of the phosphatases SHP1 and SHP2 upon MIP1beta stimulation of CCR5 L1.2 transfectants and T-cells derived from peripheral blood mononuclear cells. Furthermore, we observed that SHP1 associated with RAFTK. However, using a dominant-negative phosphatase-binding mutant of RAFTK (RAFTK(m906)), we found that RAFTK does not mediate SHP1 or SHP2 phosphorylation. SHP1 and SHP2 also associated with the adaptor protein Grb2 and the Src-related kinase Syk. Pretreatment of CCR5 L1.2 transfectants or T-cells with the phosphatase inhibitor orthovanadate markedly abolished MIP1beta-induced chemotaxis. Syk was also activated upon MIP1beta stimulation of CCR5 L1.2 transfectants or T-cells and associated with RAFTK. Overexpression of a dominant-negative Src-binding mutant of RAFTK (RAFTK(m402)) significantly attenuated Syk activation, whereas overexpression of wild-type RAFTK enhanced Syk activity, indicating that RAFTK acts upstream of CCR5-mediated Syk activation. Taken together, these results suggest that MIP1beta stimulation mediated by CCR5 induces the formation of a signaling complex consisting of RAFTK, Syk, SHP1, and Grb2.

Shc associates with the IL-3 receptor beta subunit, SHIP and Gab2 following IL-3 stimulation. Contribution of Shc PTB and SH2 domains

p46(Shc) and p52(Shc) become heavily tyrosine phosphorylated in response to interleukin 3 (IL-3) treatment. We have investigated the potential of Shc to integrate IL-3 signalling pathways and demonstrate that Shc associates with the beta subunits of the human (betac) and murine (Aic2A) IL-3 receptors, SHIP and Gab2 following IL-3 stimulation. The interaction between Shc and the IL-3 receptor beta chains was direct, mediated by both the SH2 and PTB domains. Interaction with SHIP was via the Shc PTB domain and the Shc SH2 domain mediated the interaction with Gab2. Phosphopeptide competition studies suggest that the SH2 domain interacts primarily with tyrosine 612 of betac (610 of Aic2A), and the PTB domain with tyrosine 577 of betac (575 of Aic2A). PTB binding to IL-3R beta chains was of highest affinity, and appeared to play the primary role in binding. These findings suggest that Shc may play an important role in coordinately integrating IL-3 signalling pathways.

Association of Lyn tyrosine kinase to the GM-CSF and IL-3 receptor common betac subunit and role of Src tyrosine kinases in DNA synthesis and anti-apoptosis

BACKGROUND

After GM-CSF or IL-3 stimulation, the activation of JAK2 tyrosine kinase and members of the Src family of tyrosine kinases takes place, followed by phosphorylation of betac tyrosine residues and the recruitment of SH2 containing molecules to the receptor complex. The exact role of Src kinases such as Lyn in this and other downstream signal transduction events remains unclear.

RESULTS

We investigated the association of Lyn kinase with betac using synthetic peptides derived from the eight betac tyrosine residues and the Box 1 motif. We found that Lyn kinase GST fusion proteins bind to peptides corresponding to the membrane proximal region of betac and to peptides containing specific betac derived phosphorylated tyrosine residues. We also determined that betac tyrosine residues Y1,2 as well as Y7 and Y8 can act as substrates of Lyn. We further analysed the role of the Src kinases in DNA synthesis and anti-apoptosis downstream of GM-CSF by using the Src kinase inhibitor PP1 in murine BA/F3 cells stably expressing a series of mutant betac receptors.

CONCLUSIONS

Lyn binds to betac derived peptides through multiple interactions, and may play an important role in betac phosphorylation. Src family kinases also play an essential role in GM-CSF mediated DNA synthesis, as well as an important role in anti-apoptosis in response to GM-CSF.

Thrombin-induced association of SHP-2 with multiple tyrosine-phosphorylated proteins in human platelets

SH2 domain containing phosphatase-2 (SHP-2) has an important regulatory role in a variety of cell types. However, little is known concerning its function in platelets. We show here that, in thrombin-stimulated human platelets, SHP-2 undergoes a time-dependent association with platelet endothelial cell adhesion molecule-1 (PECAM-1) and four low molecular weight phosphoproteins which are attenuated by the Src kinase inhibitor PP1. The low molecular weight proteins, which may be transmembrane proteins, are shown to bind exclusively to the N-terminal SH2 domain of SHP-2 and are therefore possible activators of the phosphatase. In addition, SHP-2 phosphatase activity is shown to be increased following thrombin stimulation or cross-linking of PECAM.

Identification of a 14-3-3 Binding Sequence in the Common β Chain of the Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), Interleukin-3 (IL-3), and IL-5 Receptors That Is Serine-Phosphorylated by GM-CSF

The common β chain (βc) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is the major signaling subunit of these receptors coupling ligand binding to multiple biological activities. It is thought that these multiple functions arise as a consequence of the recruitment of specific signaling molecules to tyrosine-phosphorylated residues in the cytoplasmic domain of βc. However, the contribution of serine phosphorylation in βc to the recruitment of signaling molecules is not known. We show here the identification of a phosphoserine motif in the cytoplasmic domain of βc that interacts with the adaptor protein 14-3-3ζ. Coimmunoprecipitation and pull-down experiments with a glutathione S-transferase (GST):14-3-3ζ fusion protein showed that 14-3-3 directly associates with βc but not the GM-CSF receptor  chain. C-terminal truncation mutants of βcfurther showed that a region between amino acids 544 and 626 in βc was required for its association with 14-3-3ζ. This region contains the sequence 582HSRSLP587, which closely resembles the RSXSXP (where S is phosphorylated) consensus 14-3-3 binding site identified in a number of signaling molecules, including Raf-1. Significantly, substitution of582HSRSLP587 for EFAAAA completely abolished interaction of βc with GST–14-3-3ζ. Furthermore, the interaction of βc with GST–14-3-3 was greatly reduced in the presence of a peptide containing the 14-3-3 binding site, but only when 585Ser was phosphorylated. Direct binding experiments showed that the peptide containing phosphorylated 585Ser bound 14-3-3ζ with an affinity of 150 nmol/L. To study the regulation of 585S phosphorylation in vivo, we raised antibodies that specifically recognized 585Ser-phosphorylated βc. Using these antibodies, we showed that GM-CSF stimulation strongly upregulated 585Ser phosphorylation in M1 myeloid leukemic cells. The proximity of the SHC-binding site (577Tyr) to the 14-3-3–binding site (582HSRSLP587) and their conservation between mouse, rat, and human βc but not in other cytokine receptors suggest that they form a distinct motif that may subserve specialized functions associated with the GM-CSF, IL-3, and IL-5 receptors.

Identification of a 14-3-3 Binding Sequence in the Common β Chain of the Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), Interleukin-3 (IL-3), and IL-5 Receptors That Is Serine-Phosphorylated by GM-CSF

The common β chain (βc) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is the major signaling subunit of these receptors coupling ligand binding to multiple biological activities. It is thought that these multiple functions arise as a consequence of the recruitment of specific signaling molecules to tyrosine-phosphorylated residues in the cytoplasmic domain of βc. However, the contribution of serine phosphorylation in βc to the recruitment of signaling molecules is not known. We show here the identification of a phosphoserine motif in the cytoplasmic domain of βc that interacts with the adaptor protein 14-3-3ζ. Coimmunoprecipitation and pull-down experiments with a glutathione S-transferase (GST):14-3-3ζ fusion protein showed that 14-3-3 directly associates with βc but not the GM-CSF receptor  chain. C-terminal truncation mutants of βcfurther showed that a region between amino acids 544 and 626 in βc was required for its association with 14-3-3ζ. This region contains the sequence 582HSRSLP587, which closely resembles the RSXSXP (where S is phosphorylated) consensus 14-3-3 binding site identified in a number of signaling molecules, including Raf-1. Significantly, substitution of582HSRSLP587 for EFAAAA completely abolished interaction of βc with GST–14-3-3ζ. Furthermore, the interaction of βc with GST–14-3-3 was greatly reduced in the presence of a peptide containing the 14-3-3 binding site, but only when 585Ser was phosphorylated. Direct binding experiments showed that the peptide containing phosphorylated 585Ser bound 14-3-3ζ with an affinity of 150 nmol/L. To study the regulation of 585S phosphorylation in vivo, we raised antibodies that specifically recognized 585Ser-phosphorylated βc. Using these antibodies, we showed that GM-CSF stimulation strongly upregulated 585Ser phosphorylation in M1 myeloid leukemic cells. The proximity of the SHC-binding site (577Tyr) to the 14-3-3–binding site (582HSRSLP587) and their conservation between mouse, rat, and human βc but not in other cytokine receptors suggest that they form a distinct motif that may subserve specialized functions associated with the GM-CSF, IL-3, and IL-5 receptors.

Regulation and function of protein kinase B and MAP kinase activation by the IL-5/GM-CSF/IL-3 receptor

Interleukin (IL)-3, IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) regulate proliferation, differentiation and apoptosis of target cells. Receptors for these cytokines consist of a cytokine-specific alpha subunit and a common shared beta c subunit. Tyrosine phosphorylation of the beta c is thought to play a critical role in mediating signal transduction events. We have examined the effect of mutation of beta c tyrosines on the activation of multiple signal transduction pathways. Activation of protein kinase B (PKB) required JAK2 and was inhibited by dominant-negative phosphatidylinositol 3-kinase (P13K). Overexpression of JAK2 was sufficient to activate both protein kinase B (PKB) and extracellular regulated kinase-1 (ERK1). Tyrosine 577 and 612 were found to be critical for the activation of PKB and ERK1, but not activation of STAT transcription factors. Activation of both PKB and ERK have been implicated in the regulation of proliferation and apoptosis. We generated GM-CSFR stable cell lines expressing receptor mutants to evaluate their effect on these processes. Activation of both PKB and ERK was perturbed, while STAT activation remained unaffected. Tyrosines 577 and 612 were necessary for optimal proliferation, however, mutation of these tyrosine residues did not affect GM-CSF mediated rescue from apoptosis. These data demonstrate that while phosphorylation of beta c tyrosine residues 577 and 612 are important for optimal cell proliferation, rescue from apoptosis can be mediated by alternative signalling routes apparently independent of PKB or ERK activation.

Bovine leukemia virus transmembrane protein gp30 physically associates with the down-regulatory phosphatase SHP-1

In B lymphocytes, the down-regulatory phosphatase SHP-1 associates with CD22 and CD32b (also known as FcgammaRIIB) and acts as a critical negative regulator of B-cell receptor signaling. Bovine leukemia virus, a retrovirus of the HTLV/BLV group, causes persistently increased numbers of peripheral blood B lymphocytes, known as persistent lymphocytosis (PL) and, in some animals, progression to B-cell leukemia and/or lymphoma. Here, we show that SHP-1 associates with the bovine leukemia virus transmembrane protein, gp30. This interaction is either direct or indirect. The interaction is dependent on tyrosine phosphorylation, and the interaction increases after cell stimulation with sodium pervanadate. The gp30-SHP-1 interaction is seen in all of the BLV-infected, PL animals tested, but is not seen in uninfected animals or in most BLV-infected, non-PL animals, which do not express significant quantities of gp30. However, one BLV-infected, non-PL animal expressed large quantities of gp30, yet no gp30-SHP-1 interaction was detected, suggesting that there may be other factors in cells from the PL animals that facilitate the gp30-SHP-1 interaction. The association of gp30 and SHP-1 suggests the hypothesis that gp30 may act as a decoy to sequester SHP-1, resulting in up-regulation of B-cell receptor signaling. The implication of this could be a novel mechanism of viral activation of lymphocytes by removal of a down-regulatory phosphatase.

IL-3 and IL-4 Activate Cyclic Nucleotide Phosphodiesterases 3 (PDE3) and 4 (PDE4) by Different Mechanisms in FDCP2 Myeloid Cells

In FDCP2 myeloid cells, IL-4 activated cyclic nucleotide phosphodiesterases PDE3 and PDE4, whereas IL-3, granulocyte-macrophage CSF (GM-CSF), and phorbol ester (PMA) selectively activated PDE4. IL-4 (not IL-3 or GM-CSF) induced tyrosine phosphorylation of insulin-receptor substrate-2 (IRS-2) and its association with phosphatidylinositol 3-kinase (PI3-K). TNF-α, AG-490 (Janus kinase inhibitor), and wortmannin (PI3-K inhibitor) inhibited activation of PDE3 and PDE4 by IL-4. TNF-α also blocked IL-4-induced tyrosine phosphorylation of IRS-2, but not of STAT6. AG-490 and wortmannin, not TNF-α, inhibited activation of PDE4 by IL-3. These results suggested that IL-4-induced activation of PDE3 and PDE4 was downstream of IRS-2/PI3-K, not STAT6, and that inhibition of tyrosine phosphorylation of IRS molecules might be one mechnism whereby TNF-α could selectively regulate activities of cytokines that utilized IRS proteins as signal transducers. RO31-7549 (protein kinase C (PKC) inhibitor) inhibited activation of PDE4 by PMA. IL-4, IL-3, and GM-CSF activated mitogen-activated protein (MAP) kinase and protein kinase B via PI3-K signals; PMA activated only MAP kinase via PKC signals. The MAP kinase kinase (MEK-1) inhibitor PD98059 inhibited IL-4-, IL-3-, and PMA-induced activation of MAP kinase and PDE4, but not IL-4-induced activation of PDE3. In FDCP2 cells transfected with constitutively activated MEK, MAP kinase and PDE4, not PDE3, were activated. Thus, in FDCP2 cells, PDE4 can be activated by overlapping MAP kinase-dependent pathways involving PI3-K (IL-4, IL-3, GM-CSF) or PKC (PMA), but selective activation of PDE3 by IL-4 is MAP kinase independent (but perhaps IRS-2/PI3-K dependent).

Dissociation of apoptosis from proliferation, protein kinase B activation, and BAD phosphorylation in interleukin-3-mediated phosphoinositide 3-kinase signaling

Interleukin-3 (IL-3) acts as both a growth and survival factor for many hemopoietic cells. IL-3 treatment of responsive cells leads to the rapid and transient activation of Class IA phosphoinositide-3-kinases (PI3Ks) and the serine/threonine kinase Akt/protein kinase B (PKB) and phosphorylation of BAD. Each of these molecules has been implicated in anti-apoptotic signaling in a wide range of cells. Using regulated expression of dominant-negative p85 (Deltap85) in stably transfected IL-3-dependent BaF/3 cells, we have specifically investigated the role of class IA PI3K in IL-3 signaling. The major functional consequence of Deltap85 expression in these cells is a highly reproducible, dramatic reduction in IL-3-induced proliferation. Expression of Deltap85 reduces IL-3-induced PKB phosphorylation and activation and phosphorylation of BAD dramatically, to levels seen in unstimulated cells. Despite these reductions, the levels of apoptosis observed in the same cells are very low and do not account for the reduction in IL-3-dependent proliferation we observe. These results show that Deltap85 inhibits both PKB activity and BAD phosphorylation without significantly affecting levels of apoptosis, suggesting that there are targets other than PKB and BAD that can transmit survival signals in these cells. Our data indicate that the prime target for PI3K action in IL-3 signaling is at the level of regulation of proliferation.

Negative Regulation of Myeloid Cell Proliferation and Function by the SH2 Domain-Containing Tyrosine Phosphatase-1

The SH2 domain containing tyrosine phosphatase SHP-1 has been implicated in the regulation of a multiplicity of signaling pathways involved in hemopoietic cell growth, differentiation, and activation. A pivotal contribution of SHP-1 in the modulation of myeloid cell signaling cascades has been revealed by the demonstration that SHP-1 gene mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten mice. To investigate the role of SHP-1 in regulation of myeloid leukocytes, an HA epitope-tagged dominant negative (interfering) SHP-1 (SHP-1C453S) was expressed in the myelo-monocytic cell line U937 using the pcDNA3 vector. Overexpression of this protein in SHP-1C453S transfectants was demonstrated by Western blot analysis and by detection of decreased specific activity. Growth, proliferation, and IL-3-induced proliferative responses were substantially increased in the SHP-1C453S-overexpressing cells relative to those in control cells. The results of cell cycle analysis also revealed that the proportion of cells overexpressing SHP-1C453S in S phase was greater than that of control cells. The SHP-1C453S-expressing cells also displayed diminished rates of apoptosis as detected by flow cytometric analysis of propidium iodide-stained cells and terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end-labeling assay. While motility and phagocytosis were not affected by SHP-1C453S overexpression, adhesion and the oxidative burst in response to PMA were enhanced in the SHP-1C453S compared with those in the vector alone transfectants. Taken together, these results suggest that SHP-1 exerts an important negative regulatory influence on cell proliferation and activation while promoting spontaneous cell death in myeloid cells.

Cloning of p97/Gab2, the major SHP2-binding protein in hematopoietic cells, reveals a novel pathway for cytokine-induced gene activation

Several components in cytokine signaling remain unidentified. We report the cloning and initial characterization of one such component, p97, a widely expressed scaffolding protein distantly related to Drosophila DOS and mammalian Gab1. Upon cytokine, growth factor, or antigen receptor stimulation, p97 becomes tyrosyl phosphorylated and associates with several SH2 domain-containing proteins, including SHP2. Expression of p97 mutants unable to bind SHP2 blocks cytokine-induced c-fos promoter activation, inhibiting Elk1-mediated and STAT5-mediated transactivation. Surprisingly, such mutants do not inhibit MAPK activation. Our results identify p97 as an important regulator of receptor signaling that controls a novel pathway to immediate-early gene activation and suggest multiple functions for SHP2 in cytokine receptor signaling.

Regulation of proliferation, differentiation and survival by the IL-3/IL-5/GM-CSF receptor family

The receptors for the I1-3/IL-5/GM-CSF cytokine family are composed of a heterodimeric complex of a cytokine-specific alpha chain and a common beta chain (betac). Binding of IL-3/IL-5/GM-CSF to their respective receptors rapidly induces activation of multiple intracellular signalling pathways, including the Ras-Raf-ERK, the JAK/STAT, the phosphatidylinositol 3-kinase PKB, and the JNK/SAPK and p38 signalling pathways. This review focuses on recent advancements in understanding how these different signalling pathways are activated by IL-3/IL-5/GM-CSF receptors, and how the individual pathways contribute to the pleiotropic effects of IL-3/IL-5/GM-CSF on their target cells, including proliferation, differentiation, survival, and effector functions.

Cytoplasmic domains of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor beta chain (hbetac) responsible for human GM-CSF-induced myeloid cell differentiation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates differentiation, survival, and proliferation of myeloid progenitor cells. The biologic actions of GM-CSF are mediated by its binding to the alpha and beta subunits of the GM-CSF receptor (GM-CSFRalpha and betac, respectively). To determine whether identical regions of the betac protein mediate both cell growth and differentiation, we expressed cDNA constructs encoding the human wild-type (897 amino acids) and truncated betac (hbetac) subunits along with the wild-type human GM-CSFRalpha subunit in the murine WT19 cell line, an FDC-P1-derived cell line that differentiates toward the monocytic lineage in response to murine GM-CSF. Whereas the WT19 cell line carrying the C-terminal deleted hbetac subunit of 627 amino acids was still able to grow in human GM-CSF (hGM-CSF), 681 amino acids of the hbetac were necessary for cell differentiation. The addition of hGM-CSF to WT19 cell lines containing the hbetac627 subunit stimulated the phosphorylation of ERK (extracellular signal-regulated kinase) and induced the tyrosine-phosphorylation of SHP-2 and STAT5, suggesting that the activation of these molecules is insufficient to mediate the induction of differentiation. A point mutation of tyrosine 628 to phenylalanine (Y628F) within hbetac681 abolished the ability of hGM-CSF to induce differentiation. Our results indicate that the signals required for hGM-CSF-induced differentiation and cell growth are mediated by different regions of the hbetac subunit.