Protein tyrosine phosphatase containing SH2 domains: characterization, preferential expression in hematopoietic cells, and localization to human chromosome 12p12-p13. Mol Cell Biol. 1992;12:836-846. [PMID: 1732748 DOI: 10.1128/mcb.12.2.836]

Pathogenesis of acute myeloid leukaemia and inv(16)(p13;q22): a paradigm for understanding leukaemogenesis?

Acute myeloid leukaemia (AML) has been proposed to arise from the collaboration between two classes of mutation, a class I, or proliferative, mutation and a class II, or blocking, mutation. A limitation of this so-called 'two-hit' hypothesis has been the lack of identifiable proliferative and blocking mutations in most AML cases. However, it is now known that the CBFbeta-MYH11 fusion gene in AML and inv(16), by disrupting the normal transcription factor activity of core binding factor (CBF), functions as a class II mutation. In addition, nearly 70% of patients with AML and inv(16) are known to possess mutually exclusive mutations of the receptor tyrosine kinases (RTKs), c-KIT and FLT3, as well as RAS genes, that provide a class I, or proliferative, signal. AML and inv(16), therefore, is one of the best understood of the acute leukaemias at the genetic level and so provides a paradigm for the 'two-hit' hypothesis of leukaemogenesis. This paper reviews the recent advances in the molecular pathology of AML and inv(16) and discusses possible therapeutic implications of the current pathogenetic model.

EGF-stimulation activates the nuclear localization signal of SHP-1

Protein tyrosine phosphatase SHP-1 plays a critical role in the regulation of a variety of intracellular signaling pathways. SHP-1 is predominantly expressed in the cells of hematopoietic origin, and is recognized as a negative regulator of lymphocyte development and activation. SHP-1 consists of two Src homology 2 (SH2) domains and one protein tyrosine phosphatase (PTP) domain followed by a highly basic C-terminal tail containing tyrosyl phosphorylation sites. It is unclear how the C-terminal tail regulates SHP-1 function. We report the examination of the subcellular localization of a variety of truncated or mutated SHP-1 proteins fused with enhanced green fluorescent protein (EGFP) protein at either the N-terminal or the C-terminal end in different cell lines. Our data demonstrate that a nuclear localization signal (NLS) is located in the C-terminal tail of SHP-1 and the signal is primarily defined by three amino-acid residues (KRK) at the C-terminus. This signal is generally blocked in the native protein and can be exposed by fusing EGFP at the appropriate position or by domain truncation. We have also revealed that this NLS of SHP-1 is triggered by epidermal growth factor (EGF) stimulation and mediates translocation of SHP-1 from the cytosol to the nucleus in COS7 cell lines. These results not only demonstrate the importance of the C-terminal tail of SHP-1 in the regulation of nuclear localization, but also provide insights into its role in SHP-1-involved signal transduction pathways.

Receptor tyrosine kinases in normal and malignant haematopoiesis

Haematopoiesis is controlled by a number of growth factors and cytokines, a number of which act through binding to high-affinity receptor tyrosine kinases (RTKs). Approximately 20 different RTK classes have been identified, all of which share a similar structure that includes a ligand binding extracellular domain, a single transmembrane domain and an intracellular tyrosine kinase domain. Recent studies have linked an increasing number of mutations in the RTKs to the pathogenesis of both acute and chronic leukaemia. For example, the FLT3 receptor, a RTK class III, is the most commonly mutated gene in acute myeloid leukaemia, while c-kit mutations are strongly linked to the development of mast cell malignancy. This review summarizes the RTK classes that are known to be expressed on normal haematopoietic tissue and highlights the many 'gain-of-function' mutations involved in leukaemogenesis. It is to be hoped that this knowledge will provide important new insights for targeted therapy in leukaemia.

The ability of protein tyrosine phosphatase SHP-1 to suppress NFkappaB can be inhibited by dominant negative mutant of SIRPalpha

In contrast with hematopoietic cells and fibroblasts, which express mainly one form of protein tyrosine phosphatase (PTP) SHP-1 or SHP-2, epithelial cells like A431, HeLa, and 293 express both forms of PTP. These two PTP regulate NFkappaB activity differently; SHP-1 inhibits and SHP-2 stimulates NFkappaB activation. In epithelial cells the process of NFkappaB activation depends on the combination of two PTP activities. The activity of PTP SHP-1 dominates in this tandem according to our data. The signal regulatory protein (SIRPalpha) is the adapter and the substrate of PTP SHP-1 and SHP-2. We investigated the role of SIRPalpha and its dominant negative mutant in PTP activities in 293 cells. The overexpression of wild-type SIRPalpha suppresses the activities of both PTP, but has a stronger effect on PTP SHP-2, especially when this protein is overexpressed in 293 cells. In contrast with wild-type SIRPalpha, its dominant negative mutant acts predominantly against PTP SHP-1, and can be detected in the complex with PTP SHP-1. The expression of dominant negative mutant of SIRPalpha has an effect similar to the expression of dominant negative PTP SHP-1 in the process of NFkappaB activation.

Granulocyte colony-stimulating factor and its receptor in normal hematopoietic cell development and myeloid disease

Hematopoiesis, the process of blood cell formation, is orchestrated by cytokines and growth factors that stimulate the expansion of different progenitor cell subsets and regulate their survival and differentiation into mature blood cells. Granulocyte colony-stimulating factor (G-CSF) is the major hematopoietic growth factor involved in the control of neutrophil development. G-CSF is now applied on a routine basis in the clinic for treatment of congenital and acquired neutropenias. G-CSF activates a receptor of the hematopoietin receptor superfamily, the G-CSF receptor (G-CSF-R), which subsequently triggers multiple signaling mechanisms. Here we review how these mechanisms contribute to the specific responses of hematopoietic cells to G-CSF and how perturbations in the function of the G-CSF-R are implicated in various types of myeloid disease.

A novel SHP-1/Grb2-dependent mechanism of negative regulation of cytokine-receptor signaling: contribution of SHP-1 C-terminal tyrosines in cytokine signaling

SHP-1, an src homology 2 (SH2) domain containing protein tyrosine phosphatase, functions as a negative regulator of signaling downstream of cytokine receptors, receptor tyrosine kinases and receptor complexes of the immune system. Dephosphorylation of receptors and/or receptor-associated kinases has been described as the mechanism for the function of SHP-1. Here we demonstrate a novel mechanism by which SHP-1 down-regulates the Janus kinase-2 (Jak2)/signal transducer and activator of transcription-5 (Stat5) pathway downstream of the prolactin receptor (PRLR) and the erythropoietin receptor (EPOR) in a catalytic activity-independent manner. Structural/functional analysis of SHP-1 defined the C-terminal tyrosine residues (Y278, Y303, Y538, Y566) within growth factor receptor-bound protein 2 (Grb-2) binding motif to be responsible for delivering the inhibitory effects. Our results further indicate that these tyrosine residues, via recruitment of the adaptor protein Grb-2, are required for targeting the inhibitory protein suppressor of cytokine signaling-1 (SOCS-1) to Jak2 kinase. Finally, loss of SOCS-1 expression in SOCS-1(-/-) mouse embryonic fibroblast (MEF) cells led to attenuation in SHP-1 function to down-regulate PRL-induced Stat5 activation. All together, our results indicate that SHP-1 inhibits PRLR and EPOR signaling by recruitment and targeting of SOCS-1 to Jak2, highlighting a new mechanism of SHP-1 regulation of cytokine-receptor signaling.

Focal cerebral ischemia upregulates SHP-1 in reactive astrocytes in juvenile mice

The role of the tyrosine phosphatase SHP-1 in the hematopoietic system has been well studied; however, its role in the central nervous system (CNS) response to injury is not well understood. Previous studies in our laboratory have demonstrated increased immunoreactivity for SHP-1 in a subset of reactive astrocytes that do not appear to enter the cell cycle following deafferentation of the chicken auditory brainstem. In order to determine whether mammalian astrocytes also upregulate SHP-1 immunoreactivity following CNS injury, a mouse model of focal cerebral ischemia was utilized to study SHP-1 expression. The brains of 3-week-old mice were analyzed at four time points following permanent middle cerebral artery occlusion (MCAO): 1, 3, 7, and 14 days. Our results demonstrate consistent infarct volumes within surgical groups, and infarct volumes decrease as a function of time from 1 day (maximum infarct volume) to 14 days (minimum infarct volume) post-MCAO. In addition, SHP-1 protein levels are upregulated following cerebral ischemia and this increase peaks at 7 days post-MCAO. Analysis of confocal images further reveals that immunoreactivity for SHP-1 occurs predominantly in GFAP+ reactive astrocytes, although a small percentage of F4-80+ microglia are also double labeled for SHP-1 at early times post-MCAO. These SHP-1+ reactive astrocytes do not appear to enter the cell cycle (as defined by PCNA immunoreactivity), confirming our previous studies in the avian auditory brainstem. These results suggest that SHP-1 plays an important role in the regulation of glial activation and proliferation in the ischemic CNS.

TCR ligand discrimination is enforced by competing ERK positive and SHP-1 negative feedback pathways

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

Abnormal splicing of SHP-1 protein tyrosine phosphatase in human T cells. Implications for lymphomagenesis

OBJECTIVE

SHP-1 protein tyrosine phosphatase has been implicated in suppressing B-lymphocyte and myeloid cell malignancies; however, there are little data on this role of SHP-1 in T-lymphocyte malignancies. We examined malignant human T cells to identify any abnormalities of SHP-1 that would support a role for this molecule in suppressing T lymphomagenesis.

MATERIALS AND METHODS

Human T-lymphocyte cell lines and primary blood cells were used to examine the expression of SHP-1 mRNA and protein. Reverse transcriptase polymerase chain reaction was used to amplify particular portions of the SHP-1 mRNA for cloning and sequencing. Gene transfer was used to examine the effects of SHP-1 on cell growth and morphology. Glutathione S-transferase (GST) fusion proteins were generated and used to determine SHP-1-associated proteins.

RESULTS

Leukemia- and lymphoma-derived cell lines were identified that did not express SHP-1 protein. Examination of the mRNA from these and other T-cell lines, and from normal peripheral blood mononuclear cells (PBMCs), revealed three distinct transcripts by restriction enzymes, reverse transcriptase polymerase chain reaction, and Southern blot analysis. In addition to the expected wild-type transcript, two novel transcripts were identified. One was a deletion transcript found only in Jurkat leukemia-derived cells, predicted to encode for a 7-kDa protein containing most of the amino-terminal SH2 domain. The second contained an 88-nucleotide insert that is the unspliced second intron resulting in a frame shift and the formation of a noncoding transcript. This mRNA was found in all cells examined but was the only transcript detected in the cell lines lacking SHP-1 protein. Expressing wild-type SHP-1 in these cell lines resulted in a change in the morphology of the cells with a concomitant decrease in their growth. GST fusion constructs showed the 7-kDa variant able to associate with an identical array of proteins as wild-type SHP-1, suggesting that it could compete with the wild-type SHP-1 for substrates. This protein was detectable in the cell line expressing its corresponding mRNA and was able to induce significant changes in cell morphology when transfected into a cell line expressing wild-type SHP-1; however, it did not induce any changes in cell growth.

CONCLUSIONS

These data are the first to show the existence of multiple transcripts of SHP-1 in human transformed T lymphocytes and normal PBMCs and supports previous work showing that alternate forms of SHP-1 mRNA are a common finding in other cells. We also show the lack of splicing out of an intron as a novel mechanism of regulation of SHP-1 protein expression in both normal and transformed T cells. Moreover, we provide the first evidence showing a protein product detectable in cells that is translated from an alternatively spliced form of SHP-1 mRNA, a variant truncated SHP-1 protein having potential biologic relevance. This report provides evidence supporting the concept that SHP-1 can negatively regulate growth of malignant human T cells and that lack of SHP-1 protein or function may be associated with lymphomagenesis.

Dynamic Changes in the Expression of Protein Tyrosine Phosphatases During Preimplantation Mouse Development: Semi-Quantification by Real-Time PCR

Protein tyrosine phosphatase (PTP) expression was examined in preimplantation mouse embryos. We previously reported that SHP-2, LAR, PTPT9, SHP-1, and mRPTPB were expressed in preimplantation mouse embryos. Here, we examined changes in the expression levels of these PTPs during preimplantation development. cDNA was obtained by reverse transcription of embryo mRNA, amplified with 10 PCR cycles, and then subjected to real-time fluorescence-monitored PCR. Experiments with an mRNA dilution series revealed that the data obtained matched the quantities of mRNA used. The measurements obtained with real-time fluorescence-monitored PCR showed that the expression of each PTP mRNA changed dynamically, and that each had a different expression pattern. This suggests that PTPs are involved in the regulation of growth and differentiation during preimplantation development.

Anticancer activity of sodium stibogluconate in synergy with IFNs

Cancer cell resistance limits the efficacy of IFNs. In this study, we show that sodium stibogluconate (SSG) and IFN-alpha synergized to overcome IFN-alpha resistance in various human cancer cell lines in culture and eradicated IFN-alpha-refractory WM9 human melanoma tumors in nude mice with no obvious toxicity. SSG enhanced IFN-alpha-induced Stat1 tyrosine phosphorylation, inactivated intracellular SHP-1 and SHP-2 that negatively regulate IFN signaling, and induced cellular protein tyrosine phosphorylation in cancer cell lines. These effects are consistent with inactivation of phosphatases as the basis of SSG anticancer activity. Characterization of SSG by chromatography revealed that only selective compounds in SSG were effective protein tyrosine phosphatase inhibitors. These observations suggest the potential of SSG as a clinically usable protein tyrosine phosphatase inhibitor in cancer treatment and provide insights for developing phosphatase-targeted therapeutics.

Shp-2 positively regulates brain-derived neurotrophic factor-promoted survival of cultured ventral mesencephalic dopaminergic neurons through a brain immunoglobulin-like molecule with tyrosine-based activation motifs/Shp substrate-1

To examine the roles of Shp-2, a cytoplasmic tyrosine phosphatase, in neuronal survival, we generated and used recombinant adenoviruses expressing wild type and phosphatase-inactive (C/S), phosphatase domain-deficient (delta P) and constitutively active (D61A and E76A) mutants of Shp-2. We found that wild-type Shp-2 enhanced brain-derived neurotrophic factor (BDNF)-promoted survival of cultured ventral mesencephalic dopaminergic neurons. In contrast, the C/S and delta P mutants of Shp-2 did not affect survival. In addition, the constitutively active D61A and E76A mutants mimicked BDNF and promoted survival. Furthermore, to examine the effects of BIT/SHPS-1, a substrate of Shp-2, on the BDNF-promoted survival, we generated adenovirus vectors expressing wild-type BIT/SHPS-1 and its 4F mutant in which all tyrosine residues in the cytoplasmic domain of BIT/SHPS-1 were replaced with phenylalanine. We found that BDNF-promoted survival of cultured mesencephalic dopaminergic neurons was enhanced by expression of the 4F mutant but not of wild-type BIT/SHPS-1. In addition, we found that co-expression of wild-type BIT/SHPS-1 with Shp-2 significantly enhanced the survival-promoting effect of BDNF on cultured mesencephalic dopaminergic neurons. These results indicated that Shp-2 positively regulates the survival-promoting effect of BDNF on cultured ventral mesencephalic dopaminergic neurons. Dephosphorylation of BIT/SHPS-1 by Shp-2 may participate in BDNF-stimulated survival signaling.

Absence of CD5 dramatically reduces progression of pulmonary inflammatory lesions in SHP-1 protein-tyrosine phosphatase-deficient 'viable motheaten' mice

Mice homozygous for the viable motheaten (Hcph(me-v)) mutation are deficient in SHP-1 protein-tyrosine phosphatase, resulting in severe systemic autoimmunity and immune dysfunction. A high percentage of B-cells in viable motheaten mice express the cell surface glycoprotein CD5, in contrast to wild type mice that express CD5 on only a small percentage of B-cells. CD5(+) B-cells have been associated with autoantibody production. To determine the role of CD5 in the development of the inflammatory disease in me(v)/ me(v) mice, we created a stock of CD5(null)me(v)/ me(v) mice. The longevity of CD5(null)me(v)/ me(v) mice was increased 69% in comparison to me(v)/ me(v) mice on a similar (B6;129) background. The increased lifespan was associated with a marked reduction in pulmonary inflammation. Flow cytometry analysis of spleen cells from CD5(null)me(v)/ me(v) mice at 9-12 weeks of age revealed significant decreases in percentages of IgM/B220 double positive B-cells, Mac-1/Gr-1 double positive cells and CD4(+) T-cells compared with me(v)/ me(v) mice. CD5(null)me(v)/ me(v) mice also had significantly lower serum IgM levels in comparison to me(v)/ me(v) mice. Study of CD5(null)me(v)/ me(v) mice may provide further insight into the role of CD5 in cell signaling and may help explain the observed association of CD5(+) B-cells with autoimmune disease.

A bipartite NLS at the SHP-1 C-terminus mediates cytokine-induced SHP-1 nuclear localization in cell growth control

SHP-1 protein tyrosine phosphatase is a critical regulator of signaling in hematopoietic cells as illustrated by the lethal hematopoietic disorders in SHP-1-deficient mice. We and others have shown in previous studies that SHP-1 regulates membrane receptor signaling: it binds via its N-terminal region SH2 domains to tyrosine phosphorylated membrane receptors to dephosphorylate key substrates in the receptor complexes. Here we demonstrate that the SHP-1 C-terminal region contains a bipartite NLS that mediates SHP-1 nuclear localization in response to cytokines. This NLS was located within amino acids 576-595 of the PTPase and, when fused by itself to EGFP, targeted the fluorescent protein into the nuclei of transiently transfected NIH3T3 fibroblasts and Bac1.2f5 macrophage cells. When positioned within SHP-1, the activity of the NSL was under tight regulation as indicated by the predominant cytoplasmic distribution of the EGFP/SHP-1 fusion protein in NIH3T3 transfectants and the exclusive cytoplasmic localization of the endogenous SHP-1 in hematopoietic cell line PBLC-1. Activation of the NLS in SHP-1 by IL-4 was demonstrated by increased nuclear localization of the EGFP/SHP-1 fusion protein in NIH3T3 transfectants and of the endogenous SHP-1 protein in PBCL-1 cells at 4, 6 and 8 h post-IL-4 stimulation. SHP-1 nuclear localization in PBCL-1 cells was also induced by IL-7 in a similar manner, suggesting it as a common event in cytokine signaling. In comparison to that of the wild-type phosphatase, an SHP-1 mutant lacking the NLS showed only approximately half of the activity in inhibiting proliferation of NIH3T3 transfectants. These results provide evidence of cytokine-regulated SHP-1 nuclear localization mediated by a bipartite NLS and suggest that SHP-1 regulates nuclear signaling in cell growth control.

Negative regulation of the SHPTP1 protein tyrosine phosphatase by protein kinase C delta in response to DNA damage

The SHPTP1 protein tyrosine phosphatase is activated by the c-Abl and Lyn tyrosine kinases in the cellular response to genotoxic stress. However, signaling mechanisms involved in the negative regulation of SHPTP1 are unknown. This study demonstrates that protein kinase C delta (PKCdelta) associates with SHPTP1. The PKCdelta catalytic domain binds directly to SHPTP1. The results also demonstrate that PKCdelta is required, at least in part, for phosphorylation and inactivation of SHPTP1. The phosphatase activity of SHPTP1 was attenuated by coincubation with PKCdelta in vitro. In addition, treatment of U-937 human myeloid leukemia cells with 1-beta-D-arabinofuranosylcytosine (ara-C) was associated with induction of the PKCdelta kinase function and inhibition of SHPTP1 activity. Down-regulation of SHPTP1 by ara-C was blocked by the PKCdelta inhibitor rottlerin but not by the PKCalpha and -beta inhibitor Gö6976. Moreover, transient coexpression studies with a dominant-negative mutant of PKCdelta demonstrate that the kinase activity of PKCdelta is required for the down-regulation of SHPTP1. These findings support the functional interaction between PKCdelta and SHPTP1 in the cellular response to DNA damage.

Sodium stibogluconate is a potent inhibitor of protein tyrosine phosphatases and augments cytokine responses in hemopoietic cell lines

Using in vitro protein tyrosine phosphatase (PTPase) assays, we found that sodium stibogluconate, a drug used in treatment of leishmaniasis, is a potent inhibitor of PTPases Src homology PTPase1 (SHP-1), SHP-2, and PTP1B but not the dual-specificity phosphatase mitogen-activated protein kinase phosphatase 1. Sodium stibogluconate inhibited 99% of SHP-1 activity at 10 micrograms/ml, a therapeutic concentration of the drug for leishmaniasis. Similar degrees of inhibition of SHP-2 and PTP1B required 100 micrograms/ml sodium stibogluconate, demonstrating differential sensitivities of PTPases to the inhibitor. The drug appeared to target the SHP-1 domain because it showed similar in vitro inhibition of SHP-1 and a mutant protein containing the SHP-1 PTPase domain alone. Moreover, it forms a stable complex with the PTPase: in vitro inhibition of SHP-1 by the drug was not removed by a washing process effective in relieving the inhibition of SHP-1 by the reversible inhibitor suramin. The inhibition of cellular PTPases by the drug was suggested by its rapid induction of tyrosine phosphorylation of cellular proteins in Baf3 cells and its augmentation of IL-3-induced Janus family kinase 2/Stat5 tyrosine phosphorylation and proliferation of Baf3 cells. The augmentation of the opposite effects of GM-CSF and IFN-alpha on TF-1 cell growth by the drug indicated its broad activities in the signaling of various cytokines. These data represent the first evidence that sodium stibogluconate inhibits PTPases and augments cytokine responses. Our results provide novel insights into the pharmacological effects of the drug and suggest potential new therapeutic applications.

Role of the tyrosine phosphatase SHP-1 in K562 cell differentiation

The erythro-megakaryoblastic leukemia cell line K562 undergoes erythroid or myeloid differentiation in response to treatment with various inducing agents. We observed that expression of the SH2-containing protein tyrosine phosphatase SHP-1 was induced upon exposure of K562 cells to differentiating agents. Under the same conditions, expression of SHP-2, a close relative of SHP-1, and the more distantly related PTP-1 B remained unchanged. Induction of SHP-1 expression correlates with dephosphorylation of a specific and limited set of tyrosyl phosphoproteins, suggesting that dephosphorylation of these proteins may be important for the differentiation process. Importantly, expression of exogenous SHP-1 inhibits K562 proliferation and alters the adhesion properties of these cells, indicating a more differentiated phenotype. Moreover, SHP-1 is found in a complex with both p210 Bcr-Abl and p190 Bcr-Abl, suggesting that it may regulate Bcr-Abl or Bcr-Abl-associated phosphotyrosyl proteins. Our results indicate that induction of SHP-1 expression is important for K562 differentiation in response to various inducers and raise the possibility that functional inactivation of SHP-1 may play a role in progression to blast crisis in chronic myelogenous leukemia.

A functional nuclear localization sequence in the C-terminal domain of SHP-1

The Src homology 2 domain-containing protein tyrosine phosphatases SHP-1 and SHP-2 play an important role in many intracellular signaling pathways. Both SHP-1 and SHP-2 have been shown to interact with a diverse range of cytosolic and membrane-bound signaling proteins. Generally, SHP-1 and SHP-2 perform opposing roles in signaling processes; SHP-1 acts as a negative regulator of transduction in hemopoietic cells, whereas SHP-2 acts as a positive regulator. Intriguingly, SHP-1 has been proposed to play a positive regulating role in nonhemopoietic cells, although the mechanisms for this are not understood. Here we show that green fluorescent protein-tagged SHP-1 is unexpectedly localized within the nucleus of transfected HEK293 cells. In contrast, the highly related SHP-2 protein is more abundant within the cytoplasm of transfected cells. In accordance with this, endogenous SHP-1 is localized within the nucleus of several other nonhemopoietic cell types, whereas SHP-2 is distributed throughout the cytoplasm. In contrast, SHP-1 is confined to the cytoplasm of hemopoietic cells, with very little nuclear SHP-1 evident. Using chimeric SHP proteins and mutagenesis studies, the nuclear localization signal of SHP-1 was identified within the C-terminal domain of SHP-1 and found to consist of a short cluster of basic amino acids (KRK). Although the KRK motif resembles half of a bipartite nuclear localization signal, it appears to function independently and is absolutely required for nuclear import. Our findings show that SHP-1 and SHP-2 are distinctly localized within nonhemopoietic cells, with the localization of SHP-1 differing dramatically between nonhemopoietic and hemopoietic cell lineages. This implies that SHP-1 nuclear import is a tightly regulated process and indicates that SHP-1 may possess novel nuclear targets.

Involvement of SHP-1 tyrosine phosphatase in TCR-mediated signaling pathways in lipid rafts

To elucidate the process of TCR-mediated signaling pathways in lipid rafts, we constructed a chimeric molecule that localizes activated SHP-1 to rafts. Raft targeting of activated SHP-1 in Jurkat-derived transfectants completely inhibited the expression of CD69 and transcriptional factors after TCR cross-linking. Whereas the inducible tyrosine phosphorylation of TCR zeta and ZAP-70 and the kinase activity of Lck were intact, phosphorylated LAT was rapidly dephosphorylated by raft targeting of activated SHP-1, leading to defects in LAT activation and subsequent downstream signaling events. Intriguingly, recruitment of endogenous SHP-1 to rafts and its association with LAT were dramatically increased after TCR engagement, suggesting that SHP-1 is involved in raft-mediated T cell activation.

Adrenocorticotrophic hormone stimulates phosphotyrosine phosphatase SHP2 in bovine adrenocortical cells: phosphorylation and activation by cAMP-dependent protein kinase

During activation of adrenocortical cells by adrenocorticotrophic hormone (ACTH), tyrosine dephosphorylation of paxillin is stimulated and this correlates with protrusion of filopodial structures and a decreased number of focal adhesions. These effects are inhibited by Na(3)VO(4), a phosphotyrosine phosphatase inhibitor [Vilgrain, Chinn, Gaillard, Chambaz and Feige (1998) Biochem. J. 332, 533-540]. However, the tyrosine phosphatases involved in these processes remain to be identified. In this study, we provide evidence that the Src homology domain (SH)2-containing phosphotyrosine phosphatase (SHP)2, but not SHP1, is expressed in adrenocortical cells and is phosphorylated upon ACTH challenge. ACTH (10(-8) M) treatment of (32)P-labelled adrenocortical cells resulted in an increase in phosphorylated SHP2. By probing SHP2-containing immunoprecipitates with an antibody to phosphoserine we found that SHP2 was phosphorylated on serine in ACTH-treated cells in a dose- and time-dependent manner. Furthermore, using an in vitro kinase assay, we showed that SHP2 was a target for cAMP-dependent protein kinase (PKA). Serine was identified as the only target amino acid phosphorylated in SHP2. Phosphorylation of SHP2 by PKA resulted in a dramatic stimulation of phosphatase activity measured either with insulin receptor substrate-1 or with the synthetic peptide [(32)P]poly(Glu/Tyr) as substrate. In an in-gel assay of SHP2-containing immunoprecipitates, phosphorylated in vitro by PKA or isolated from adrenocortical cells treated with 10 nM ACTH, a pronounced activation of SHP2 activity was shown. These observations clearly support the idea that a PKA-mediated signal transduction pathway contributes to SHP2 regulation in adrenocortical cells and point to SHP2 as a possible mediator of the effects of ACTH.

Lack of phosphotyrosine phosphatase SHP-1 expression in malignant T-cell lymphoma cells results from methylation of the SHP-1 promoter

SHP-1 is an important negative regulator of signaling by several receptors including receptors for interleukin-2 (IL-2R) and other cytokines. SHP-1 acts by dephosphorylating the receptors and receptor-associated kinases such as IL-2R-associated Jak3 kinase. We found that SHP-1 protein was not detectable or greatly diminished in most (six of seven) T cell lines derived from various types of T cell lymphomas and all (eight of eight) cutaneous T-cell lymphoma tissues with a transformed, large-cell morphology. All T-cell lymphoma lines tested (eight of eight) expressed diminished amounts or no detectable SHP-1 mRNA. These T cell lines did not, however, carry any mutations in the SHP-1 gene-coding, splice-junction, and promoter regions. Importantly, SHP-1 DNA promoter region in the T cell lines was resistant to digestion with three different methylation-sensitive restriction enzymes. This resistance was reversed by treatment of the cells with a demethylating agent, 5-deoxyazacytidine. The treatment resulted also in the expression of SHP-1 mRNA and, less frequently, SHP-1 protein. The expression of SHP-1 protein was associated with dephosphorylation of the Jak3 kinase. These results show that lack of SHP-1 expression is frequent in malignant T cells and results from methylation of the SHP-1 gene promoter. Furthermore, they indicate that SHP-1 loss may play a role in the pathogenesis of T cell lymphomas by permitting persistence of signals generated by IL-2R and, possibly, other receptor complexes.

RNA hyperediting and alternative splicing of hematopoietic cell phosphatase (PTPN6) gene in acute myeloid leukemia

The SH2 domain-containing tyrosine phosphatase PTPN6 (SHP-1, PTP1C, HCP) is a 68 kDa cytoplasmic protein primarily expressed in hematopoietic cell development, proliferation and receptor-mediated mitogenic signaling pathways. By means of direct dephosphorylation, it down-regulates a broad spectrum of growth-promoting receptors, including the Kit tyrosine kinase, activated to elicit a prominent cascade of intracellular events by stem cell factor binding. The pivotal contribution of PTPN6 in modulating myeloid cell signaling has been revealed by the finding that shp-1 mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten (me/me) and motheaten viable (me(v)/me(v)) mice. Association of PTPN6 with c-Kit and negative modulation of the myeloid leukocyte signal transduction pathways prompted us to examine the expression of the protein tyrosine phosphatase PTPN6 gene in CD34(+)/CD117(+) blasts from acute myeloid leukemia patients. We identified and cloned cDNAs representing novel PTPN6 mRNA species, derived from aberrant splicing within the N-SH2 domain leading to retention of intron 3. Sequence analysis of cDNA clones revealed multiple A-->G editing conversions. The editing of PTPN6 mRNA mainly occurred as an A-->G conversion of A(7866), which represents the putative branch site in IVS3 of PTPN6 mRNA. Evidence that editing of A(7866) abrogates splicing has been obtained in vitro by using an edited clone and its backward clone generated by site-directed mutagenesis. The level of the aberrant intron-retaining splice variant, evaluated by semi-quantitative RT-PCR, was lower in CD117(+)-AML bone marrow mononuclear cells at remission than at diagnosis, suggesting the involvement of post-transcriptional PTPN6 processing in leukemogenesis.

Src homology region 2 (SH2) domain-containing phosphatase-1 dephosphorylates B cell linker protein/SH2 domain leukocyte protein of 65 kDa and selectively regulates c-Jun NH2-terminal kinase activation in B cells

Src homology region 2 (SH2) domain-containing phosphatase-1 (SHP-1) is a cytosolic protein tyrosine phosphatase containing two SH2 domains in its NH2 terminus. That immunological abnormalities of the motheaten and viable motheaten mice are caused by mutations in the gene encoding SHP-1 indicates that SHP-1 plays important roles in lymphocyte differentiation, proliferation, and activation. To elucidate molecular mechanisms by which SHP-1 regulates BCR-mediated signal transduction, we determined SHP-1 substrates in B cells using the substrate-trapping approach. When the phosphatase activity-deficient form of SHP-1, in which the catalytic center cysteine (C453) was replaced with serine (SHP-1-C/S), was introduced in WEHI-231 cells, tyrosine phosphorylation of a protein of about 70 kDa was strongly enhanced. Immunoprecipitation and Western blot analyses revealed that this protein is the B cell linker protein (BLNK), also named SH2 domain leukocyte protein of 65 kDa, and that upon tyrosine phosphorylation BLNK binds to SHP-1-C/S in vitro. In vitro kinase assays demonstrated that hyperphosphorylation of BLNK in SHP-1-C/S-expressing cells was not due to enhanced activity of Lyn or Syk. Furthermore, BCR-induced activation of c-Jun NH2-terminal kinase was shown to be significantly enhanced in SHP-1-C/S transfectants. Taken collectively, our results suggest that BLNK is a physiological substrate of SHP-1 in B cells and that SHP-1 selectively regulates c-Jun NH2-terminal kinase activation.

Myeloid specific human CD33 is an inhibitory receptor with differential ITIM function in recruiting the phosphatases SHP-1 and SHP-2

CD33 is a myeloid specific member of the sialic acid-binding receptor family and is expressed highly on myeloid progenitor cells but at much lower levels in differentiated cells. Human CD33 has two tyrosine residues in its cytoplasmic domain (Y340 and Y358). When phosphorylated, these tyrosines could function as docking sites for the phosphatases, SHP-1 and/or SHP-2, enabling CD33 to function as an inhibitory receptor. Here we demonstrate that CD33 is tyrosine phosphorylated in the presence of the phosphatase inhibitor, pervanadate, and recruits SHP-1 and SHP-2. Co-expression studies suggest that the Src-family kinase Lck is effective at phosphorylating Y340, but not Y358, suggesting that these residues may function in the selective recruitment of adapter molecules and have distinct functions. Further support for overlapping, but nonredundant, roles for Y340 and Y358 comes from peptide-binding studies that revealed the recruitment of both SHP-1 and SHP-2 to Y340 but only SHP-2 to Y358. Analysis using mutants of SHP-1 demonstrated that binding Y340 of CD33 was primarily to the amino Src homology-2 domain of SHP-1. The potential of CD33 to function as an inhibitory receptor was demonstrated by its ability to down-regulate CD64-induced calcium mobilization in U937. The dependence of this inhibition on SHP-1 was demonstrated by blocking CD33-mediated effects with dominant negative SHP-1. This result implies that CD33 is an inhibitory receptor and also that SHP-1 phosphatase has a significant role in mediating CD33 function. Further studies are essential to identify the receptor(s) that CD33 inhibits in vivo and its function in myeloid lineage development.

Myeloid specific human CD33 is an inhibitory receptor with differential ITIM function in recruiting the phosphatases SHP-1 and SHP-2

CD33 is a myeloid specific member of the sialic acid-binding receptor family and is expressed highly on myeloid progenitor cells but at much lower levels in differentiated cells. Human CD33 has two tyrosine residues in its cytoplasmic domain (Y340 and Y358). When phosphorylated, these tyrosines could function as docking sites for the phosphatases, SHP-1 and/or SHP-2, enabling CD33 to function as an inhibitory receptor. Here we demonstrate that CD33 is tyrosine phosphorylated in the presence of the phosphatase inhibitor, pervanadate, and recruits SHP-1 and SHP-2. Co-expression studies suggest that the Src-family kinase Lck is effective at phosphorylating Y340, but not Y358, suggesting that these residues may function in the selective recruitment of adapter molecules and have distinct functions. Further support for overlapping, but nonredundant, roles for Y340 and Y358 comes from peptide-binding studies that revealed the recruitment of both SHP-1 and SHP-2 to Y340 but only SHP-2 to Y358. Analysis using mutants of SHP-1 demonstrated that binding Y340 of CD33 was primarily to the amino Src homology-2 domain of SHP-1. The potential of CD33 to function as an inhibitory receptor was demonstrated by its ability to down-regulate CD64-induced calcium mobilization in U937. The dependence of this inhibition on SHP-1 was demonstrated by blocking CD33-mediated effects with dominant negative SHP-1. This result implies that CD33 is an inhibitory receptor and also that SHP-1 phosphatase has a significant role in mediating CD33 function. Further studies are essential to identify the receptor(s) that CD33 inhibits in vivo and its function in myeloid lineage development.

The SH2 domain-containing protein tyrosine phosphatase SHP-1 is induced by granulocyte colony-stimulating factor (G-CSF) and modulates signaling from the G-CSF receptor

The SH2 domain-containing protein tyrosine phosphatase SHP-1 is expressed widely in the hematopoietic system. SHP-1 has been shown to negatively control signal transduction from many cytokine receptors by direct docking to either the receptor itself, or to members of the Jak family of tyrosine kinases which are themselves part of the receptor complex. Motheaten and viable motheaten mice, which are deficient in SHP-1, have increased myelopoiesis and show an accumulation of morphologically and phenotypically immature granulocytes, suggesting a role for SHP-1 in granulocytic differentiation. Here, we report that SHP-1 protein levels are up-regulated during the granulocyte colony-stimulating factor (G-CSF)-mediated granulocytic differentiation of myeloid 32D cells. Enforced expression of SHP-1 in these cells leads to decreased proliferation and enhanced differentiation, while introduction of a catalytically inactive mutant produces increased proliferation and results in a delay of differentiation. In vitro binding revealed that the SH2 domains of SHP-1 are unable to associate directly with tyrosine-phosphorylated G-CSF receptor (G-CSF-R). Furthermore, over-expression of SHP-1 in Ba/F3 cells expressing a G-CSF-R mutant lacking all cytoplasmic tyrosines also inhibited proliferation. Together, these data suggest that SHP-1 directly modulates G-CSF-mediated responses in hematopoietic cells via a mechanism that does not require docking to the activated G-CSF-R.

Protein-tyrosine phosphatases in the vessel wall: differential expression after acute arterial injury

Many protein-tyrosine phosphatases (PTPases) have now been identified, but little is known about PTPase expression and regulation in vascular tissue and in vascular disease. Polymerase chain reaction (PCR) amplification and cDNA fingerprinting of PTPase catalytic domains, combined with random sequencing of PCR product libraries, identified 18 (8 receptor-like and 10 cytosolic) PTPases in the rat carotid artery and revealed differential expression of 5 of these PTPases during neointima formation after balloon catheter injury. In situ hybridization was used to localize mRNA expression in vessel cross sections for the 5 differentially expressed PTPases. This revealed that for 3 PTPases (SHP1, CD45, and PTPbeta), differential transcript abundance was due to appearance/loss of the cell types by which they were expressed (leukocytes for SHP1 and CD45, endothelial cells for PTPbeta). However, mRNA expression of 2 PTPases (PTPL1 and PTP1B) was specifically upregulated by proliferating and migrating smooth muscle cells (SMCs) in characteristic temporal and regional patterns in response to vessel damage. Quantitative PCR analysis showed that PTP1B and PTPL1 were induced approximately 30-fold and approximately 60-fold, respectively, by 2 weeks after injury in the damaged vessels compared with the uninjured vessels. PTP1B was rapidly upregulated in the media after vessel injury and remained highly expressed in the developing neointima. By contrast, PTPL1 expression did not increase dramatically until the SMCs had migrated into the intima. The differential expression of PTP1B and PTPL1 by SMCs after injury suggests roles for these PTPases in the regulation of vessel wall remodeling.

SH2-Containing protein tyrosine phosphatase-1 (SHP-1) association with Jak2 in UT-7/Epo cells

We have investigated the interaction of the SH2-containing protein tyrosine phosphatase-1 (SHP-1) and Jak2 in an erythropoietin (Epo)-dependent human leukemia cell line, UT-7/Epo, using reciprocal immunoprecipitation and immunoblotting. The Epo-induced kinetics and dose response on phosphorylated Jak2 in anti-SHP-1 precipitates of UT-7/Epo cell lysates were similar to those in direct anti-Jak2 precipitates, suggesting that Jak2 coprecipitated with SHP-1. Furthermore, immunoblotting with anti-Jak2 and anti-SHP-1 antibodies indicated that SHP-1 appeared to be constitutively associated with non-tyrosine-phosphorylated Jak2 in UT-7/Epo cells in the absence of Epo and without phosphorylation of the Epo receptor (EpoR). Competition studies with C-terminal SHP-1 and Jak2 peptides decreased the amounts of SHP-1 and Jak2 detected in immunoprecipitates supporting the specific coprecipitation of SHP-1 and Jak2. In the presence of a recombinant GST-fusion protein containing both the N-terminal and C-terminal SH2 domains of SHP-1, anti-GST precipitated the fusion protein but not cellular Jak2. These studies suggest that SHP-1 and Jak2 are constitutively associated in UT-7/EPO cells. The association is not dependent upon Epo and is not mediated via SHP-1 SH2 binding. Sequential double immunoprecipitation demonstrated that only a small portion of intracellular Jak2 and SHP-1 molecules are constitutively associated. This partial association pattern may allow a more flexible and diverse regulation of Jak2 and SHP-1 activities. Whether Jak2 and SHP-1 are directly associated with each other or are part of a larger complex needs further investigation.

Human 70-kDa SHP-1L differs from 68-kDa SHP-1 in its C-terminal structure and catalytic activity

The tyrosine phosphatase SHP-1 functions as a negative regulator in hematopoietic cell development, proliferation, and receptor-mediated cellular activation. In Jurkat T cells, a major 68-kDa band and a minor 70-kDa band were immunoprecipitated by a monoclonal antibody against the SHP-1 protein-tyrosine phosphatase domain, while an antibody against the SHP-1 C-terminal 19 amino acids recognized only the 68-kDa SHP-1. The SDS-gel-purified 70-kDa protein was subjected to tryptic mapping and microsequencing, which was followed by molecular cloning. It revealed that the 70-kDa protein, termed SHP-1L, is a C-terminal alternatively spliced form of SHP-1. SHP-1L is 29 amino acids longer than SHP-1, and its 66 C-terminal amino acids are different from SHP-1. The C terminus of SHP-1L contains a proline-rich motif PVPGPPVLSP, a potential Src homology 3 domain-binding site. In contrast to SHP-1, tyrosine phosphorylation of SHP-1L is not detected upon stimulation in Jurkat T cells. This is apparently due to the lack of a single in vivo tyrosine phosphorylation site, which only exists in the C terminus of SHP-1 (Y564). COS cell-expressed glutathione S-transferase-SHP-1L can dephosphorylate tyrosine-phosphorylated ZAP70. At pH 7.4, SHP-1L was shown to be more active than SHP-1 in the dephosphorylation of ZAP70. At pH 5.4, SHP-1L and SHP-1 exhibited similar catalytic activity. It is likely that these two isoforms play different roles in the regulation of hematopoietic cell signal transduction.

Molecular and functional characterization of IRp60, a member of the immunoglobulin superfamily that functions as an inhibitory receptor in human NK cells

In this study we describe the functional and molecular characterization of IRp60 (inhibitory receptor protein 60), an inhibitory receptor expressed on all human NK cells. The IRp60 molecule has been identified by the generation of three novel monoclonal antibodies (mAb). Cross-linking of IRp60 by specific mAb strongly inhibits the spontaneous cytotoxicity of NK cells as well as the NK-mediated cytolytic activity induced via different non-HLA-specific or HLA-specific activating receptors. IRp60 is a 60-kDa glycoprotein that, upon sodium pervanadate treatment, becomes tyrosine phosphorylated and associates with the SH2-containing phosphatases SHP-1 and SHP-2. The IRp60 gene is located on human chromosome 17 and encodes a molecule belonging to the immunoglobulin (Ig) superfamily characterized by a single V-type Ig-like domain in the extracellular portion. The cytoplasmic tail contains three classical immunoreceptor tyrosine-based inhibitory motifs. Southern blot analysis revealed cross-hybridization with monkey and mouse genomic DNA, thus suggesting that IRp60 may be conserved among different species. Moreover, based on the use of different anti-IRp60 mAb, we could identify two IRp60 allelic variants. Since IRp60 is also expressed by other cell types, including T cell subsets, monocytes and granulocytes, it may play a more general role in the negative regulation of different leukocyte populations.

The tyrosine phosphatase SHP-1 is a negative regulator of osteoclastogenesis and osteoclast resorbing activity: increased resorption and osteopenia in me(v)/me(v) mutant mice

Naturally occuring inactivating mutations of the Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 (SHP-1) in mice give rise to the motheaten (me) phenotype. me/me mice have multiple hematopoietic abnormalities, suggesting that this phosphatase plays an important role in hematopoiesis. SHP-1 binds to and is activated by several hematopoietic surface receptors, including the colony-stimulating factor type 1 receptor. We have examined the role of SHP-1 in osteoclastogenesis and osteoclast function using mice with the viable motheaten (me(v)/me(v)) mutation, which has markedly decreased SHP-1 activity. Histomorphometric analysis of 6-week-old me(v)/me(v) mice and control littermates showed a marked osteopenia with an increase in bone resorption indices. The number of formed osteoclast-like cells (OCLs) in cocultures of me(v)/me(v) hematopoietic cells with normal osteoblasts was significantly increased. In contrast, the number of OCLs formed in the coculture of normal bone marrow cells with the me(v)/me(v) osteoblasts was not significantly different from controls. The bone-resorbing activity of me(v)me(v) OCLs and authentic osteoclasts was also found to be increased. Finally, Western blotting of proteins from me(v)/me(v) and control OCLs revealed an overall increase in tyrosine phosphorylation in the me(v)/me(v) lysates. These in vivo and in vitro results suggest that SHP-1 is a negative regulator of bone resorption, affecting both the formation and the function of osteoclasts.

Dephosphorylation of ZAP-70 and inhibition of T cell activation by activated SHP1

Studies with motheaten mice, which lack the SHP1 protein tyrosine phosphatase, indicate that this enzyme plays an important negative role in T cell antigen receptor (TCR) signaling. The physiological substrates for SHP1 in T lymphocytes, however, have remained unclear or controversial. To define these targets for SHP1 we have compared the effects of constitutively active and inactive mutants of SHP1 on TCR signaling. Expression of wild-type SHP1 had a very small effect on the TCR-induced tyrosine phosphorylation of ZAP-70 and Syk, even when SHP1 was overexpressed 20 - 100-fold over endogenous SHP1. Inactive SHP1-D421A and wild-type SHP2 were without effects. Constitutively active SHP1-DeltaSH2 had a more pronounced effect on ZAP-70 and Syk, even when expressed at near physiological levels. SHP1-DeltaSH2 also inhibited events downstream of ZAP-70 and Syk, such as activation of the mitogen-activated protein kinase Erk2 and the transcriptional activation of the interleukin-2 gene. In contrast, a constitutively active SHP2-DeltaSH2 had no statistically significant effect (although it caused a slight augmentation in some individual experiments). None of the constructs influenced the anti-CD3-induced tyrosine phosphorylation of the TCR zeta-chain or phospholipase Cgamma1, indicating that Src family kinase function was intact. Taken together, our findings support the notion that ZAP-70 and Syk can be direct substrates for SHP1 in intact cells. However, the two SH2 domains of SHP1 did not facilitate its recognition of ZAP-70 and Syk as substrates in intact cells. Therefore, we suggest that SHP1 is not actively recruited to inhibit TCR signaling induced by ligation of this receptor alone. Instead, we propose that ligation of a distinct inhibitory receptor leads to the recruitment of SHP1 via its SH2 domains, activation of SHP1 and subsequently inhibition of TCR signals if the inhibitory receptor is juxtaposed to the TCR.

Development of lupus-like autoimmune diseases by disruption of the PD-1 gene encoding an ITIM motif-carrying immunoreceptor

PD-1, a 55 kDa transmembrane protein containing an immunoreceptor tyrosine-based inhibitory motif, is induced in lymphocytes and monocytic cells following activation. Aged C57BL/6(B6)-PD-1(-/-) congenic mice spontaneously developed characteristic lupus-like proliferative arthritis and glomerulonephritis with predominant IgG3 deposition, which were markedly accelerated by introduction of a Fas mutation (lpr). Introduction of a PD-1 null mutation into the 2C-TCR (anti-H-2Ld) transgenic mice of the H-2(b/d) background resulted in the chronic and systemic graft-versus-host-like disease. Furthermore, CD8+ 2C-TCR+ PD-1(-/-) T cells exhibited markedly augmented proliferation in vitro in response to H-2d allogenic cells. Collectively, it is suggested that PD-1 is involved in the maintenance of peripheral self-tolerance by serving as a negative regulator of immune responses.

p27KIP1 deletions in childhood acute lymphoblastic leukemia

The p27KIP1 gene, which encodes a cyclin-dependent kinase (CDK) inhibitor, has been assigned to chromosome band 12p12, a region often affected by cytogenetically apparent deletions or translocations in childhood acute lymphoblastic leukemia (ALL). As described here, fluorescence in situ hybridization (FISH) analysis of 35 primary ALL samples with cytogenetic evidence of 12p abnormalities revealed hemizygous deletions of p27KIP1 in 29 cases. Further analysis of 19 of these cases with two additional gene-specific probes from the 12p region (hematopoietic cell phosphatase, HCP and cyclin D2, CCND2) showed that p27KIP1 is located more proximally on the short arm of chromosome 12 and is deleted more frequently than either HCP or CCND2. Of 16 of these cases with hemizygous deletion of p27KIP1, only eight showed loss of HCP or CCND2, whereas loss of either of the latter two loci was uniformly associated with loss of p27KIP1. Missense mutations or mutations leading to premature termination codons were not detected in the coding sequences of the retained p27KIP1 alleles in any of the 16 ALL cases examined, indicating a lack of homozygous inactivation. By Southern blot analysis, one case of primary T-cell ALL had hemizygous loss of a single p27KIP1 allele and a 34.5-kb deletion, including the second coding exon of the other allele. Despite homozygous inactivation of p27KP1 in this case, our data suggest that haploinsufficiency for p27KIP1 is the primary consequence of 12p chromosomal deletions in childhood ALL. The oncogenic role of reduced, but not absent, levels of p27KIP1 is supported by recent studies in murine models and evidence that this protein not only inhibits the activity of complexes containing CDK2 and cyclin E, but also promotes the assembly and catalytic activity of CDK4 or CDK6 in complexes with cyclin D.

Murine SHP-1 splice variants with altered Src homology 2 (SH2) domains. Implications for the SH2-mediated intramolecular regulation of SHP-1

SHP-1 is a protein-tyrosine phosphatase with two Src homology 2 (SH2) domains. These SH2 domains determine which proteins SHP-1 associates with, but they also autoregulate the activity of the catalytic domain. In this report, we find that the murine SHP-1 transcript is processed to yield a series of alternatively spliced in-frame transcripts, the majority of which exclude exons encoding one or the other SH2 domain. We have examined the corresponding protein isoforms in several ways. First, our measurements of V(max) and K(m) under different conditions indicate that the SH2 variants have elevated activity because of lessened autoregulation. Second, to ascertain whether regulation by the SH2 domains reflects intra- or intermolecular effects, we analyzed the state of SHP-1 by high performance liquid chromatography and sucrose density gradient centrifugation. Our results showed that SHP-1 is a monomer and, thus, is regulated in an intramolecular manner. Third, our analyses detected shape differences between SHP-1 and the active splice variant protein deleted of the amino-terminal SH2 domain; i.e. SHP-1 was globular and resistant to proteolytic digestion, while the splice variant protein was "rod-shaped" and more susceptible to proteolytic digestion.

Deficiency of SHP-1 protein-tyrosine phosphatase activity results in heightened osteoclast function and decreased bone density

Mice homozygous for the motheaten (Hcphme) or viable motheaten (Hcphme-v) mutations are deficient in functional SHP-1 protein-tyrosine phosphatase and show severe defects in hematopoiesis. Comparison of femurs from mev/mev mice revealed significant decreases in bone mineral density (0.33 +/- 0.03 mg/mm3 for mev/mevversus 0.41 +/- 0.01 mg/mm3 for controls) and mineral content (1.97 +/- 0.36 mg for mev/mevversus 10.64 +/- 0.67 for controls) compared with littermate controls. Viable motheaten mice also showed reduced amounts of trabecular bone and decreased cortical thickness. These bone abnormalities were associated with a 14% increase in numbers of multinucleated osteoclasts and an increase in osteoclast resorption activity. In co-cultures of normal osteoblasts with mutant or control bone marrow cells, numbers of osteoclasts developing from mutant mice were increased compared with littermate control mice. Although mev/mev osteoclasts develop in the absence of colony-stimulating factor (CSF)-1, nevertheless cultured osteoclasts show increased size in the presence of CSF-1. CSF-1-deficient osteopetrosis (op/op) mutant mice develop severe osteosclerosis. However, doubly homozygous mev/mevop/op mice show an expansion of bone marrow cavities and reduced trabecular bone mass compared with op/op mice. Western blot analysis showed that several proteins that were markedly hyperphosphorylated on tyrosine residues were detected in the motheaten osteoclasts, including a novel 126-kd phosphotyrosine protein. The marked hyperphosphorylation of a 126-kd protein in motheaten osteoclasts suggests that this protein depends on SHP-1 for dephosphorylation. These findings demonstrate that the decreased SHP-1 catalytic activity in me/me and mev/mev mice results in an increased population of activated osteoclasts and consequent reduction in bone density.

Protein-tyrosine phosphatases in development

One of the most important mechanisms of eukaryotic signalling is protein phosphorylation on tyrosine residues, which plays a pivotal role in development by regulating cell proliferation, differentiation and migration. Cellular phosphotyrosine (P.Tyr) levels are regulated by the antagonistic activities of the protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). We have good insight into the function of PTKs at the molecular level and into the role of PTK-mediated signalling in development. Intuitively, PTPs and PTKs are equally important in development. Over the past decade, much emphasis has been placed on elucidation of the function of PTPs, which has led to good insights into the mechanism of PTP-mediated dephosphorylation. Although still relatively little is known about the role of PTPs in cell signalling and development, evidence is now emerging that several PTPs are crucial for proper development. Here I will introduce PTP-mediated signalling and discuss recent findings regarding the function of PTPs in development.

Defective expression of the SHP-1 phosphatase in polycythemia vera

The SHP-1 phosphatase associates with the receptors for erythropoietin, stem cell factor, and interleukin-3, and negatively regulates the mitogenic signals generated during engagement by their respective ligands. The erythroid progenitors of patients with polycythemia vera are hypersensitive to the mitogenic effects of these growth factors despite the fact that the numbers and binding affinities for their receptors are not increased. To determine whether post-receptor signaling defects may account for growth factor-hypersensitivity in polycythemia vera, we determined the expression of SHP-1 in highly purified erythroid progenitors from polycythemia vera patients. Our data demonstrate that in approximately 60% of the patients, expression of SHP-1 in the colony forming unit-erythroid population is diminished. The decreased expression of the protein may result from a transcriptional defect, as suggested by the diminished SHP-1 mRNA expression in the erythroid progenitors of these patients. Studies to determine the level of maturation of polycythemia vera and normal cells indicated that there was no difference between the two at early colony forming unit-erythroid stage of differentiation although polycythemia vera cells showed retarded differentiation kinetics at late colony forming unit-erythroid stage of differentiation. Furthermore, SHP-1 expression in normal colony forming unit-erythroid demonstrated downregulation of mRNA and protein levels during terminal differentiation, suggesting that its function is required for growth control during the early stages of erythropoiesis. These results indicate an important role for SHP-1 in the regulation of normal human erythroid progenitors and suggest that defective expression of the protein may contribute to the pathogenesis of polycythemia vera.

Specificity of the SH2 Domains of SHP-1 in the Interaction with the Immunoreceptor Tyrosine-Based Inhibitory Motif-Bearing Receptor gp49B

Inhibitory receptors on hemopoietic cells critically regulate cellular function. Despite their expression on a variety of cell types, these inhibitory receptors signal through a common mechanism involving tyrosine phosphorylation of the immunoreceptor tyrosine-based inhibitory motif (ITIM), which engages Src homology 2 (SH2) domain-containing cytoplasmic tyrosine or inositol phosphatases. In this study, we have investigated the proximal signal-transduction pathway of an ITIM-bearing receptor, gp49B, a member of a newly described family of murine NK and mast cell receptors. We demonstrate that the tyrosine residues within the ITIMs are phosphorylated and serve for the association and activation of the cytoplasmic tyrosine phosphatase SHP-1. Furthermore, we demonstrate a physiologic association between gp49B and SHP-1 by coimmunoprecipitation studies from NK cells. To address the mechanism of binding between gp49B and SHP-1, binding studies involving glutathione S-transferase SHP-1 mutants were performed. Utilizing the tandem SH2 domains of SHP-1, we show that either SH2 domain can interact with phosphorylated gp49B. Full-length SHP-1, with an inactivated amino SH2 domain, also retained gp49B binding. However, binding to gp49B was disrupted by inactivation of the carboxyl SH2 domain of full-length SHP-1, suggesting that in the presence of the phosphatase domain, the carboxyl SH2 domain is required for the recruitment of phosphorylated gp49B. Thus, gp49B signaling involves SHP-1, and this association is dependent on tyrosine phosphorylation of the gp49B ITIMs, and an intact SHP-1 carboxyl SH2 domain.

The low-molecular-weight phosphotyrosine protein phosphatase, when overexpressed, reduces the mitogenic response to macrophage colony-stimulating factor and tyrosine phosphorylation of its receptor

The interference of low-molecular-weight phosphotyrosine protein phosphatase with the macrophage response to macrophage colony-stimulating factor was investigated. This paper shows that this phosphatase, already known to be involved in platelet-derived growth factor receptor signaling, is physiologically expressed in murine macrophages and dephosphorylates in vitro macrophage colony-stimulating factor receptor molecules immunoprecipitated from macrophage colony-stimulating factor-stimulated macrophages. We obtained the first demonstration that a phosphotyrosine-specific protein phosphatase dephosphorylates the macrophage colony-stimulating factor receptor in vivo and reduces the mitogenic response to macrophage colony-stimulating factor. The data indicate that low-molecular-weight phosphotyrosine protein phosphatase is a negative regulator of macrophage colony-stimulating factor receptor signaling.

Tyrosine unphosphorylated platelet SHP-1 is a substrate for calpain

The platelet phosphotyrosine phosphatase (PTP) SHP-1 is tyrosine phosphorylated during thrombin-induced activation. Stimulation of platelets by the ionophore A23187 in the presence of CaCl2 induced a calpain dependent cleavage of SHP-1. SHP-1 proteolysis was undetectable during thrombin-induced stimulation. When SHP-1 was tyrosine phosphorylated by thrombin, further addition of A23187 failed to induce its cleavage. In the presence of tyrphostin to inhibit thrombin-induced SHP-1 tyrosine phosphorylation, SHP-1 was cleaved. Thus, only the tyrosine unphosphorylated form of SHP-1 was a substrate for calpain. A23187 induced the disappearance of all platelet phosphotyrosine proteins and a two-fold increase in PTP activity, both inhibited by pervanadate, a PTP inhibitor, but unaffected by calpeptin, a calpain inhibitor. The data show that SHP-1 is either tyrosine phosphorylated or cleaved by calpain, and suggest that SHP-1 cleavage does not contribute to A23187-induced PTP activity.

Establishment and characterization of pro-B cell lines from motheaten mutant mouse defective in SHP-1 protein tyrosine phosphatase

Mice homozygous for the motheaten (Hcph(me)) mutation lack a functional SHP-1 protein tyrosine phosphatase, show severe immunologic dysregulation and die at an early age. Severe pneumonitis in me/me mice is associated with abnormal proliferation of macrophages and granulocytes. Overgrowth of macrophages in long term cultures of me/me bone marrow has prevented analyses of lymphopoiesis in vitro. To establish hematopoietic cell lines from me/me mice, we cultured me/me bone marrow with the PA6 stromal cell line in the presence of antagonistic antibody against the receptor (c-Fms) for macrophage colony stimulating factor (M-CSF). In these cultures, overgrowth of M-CSF-dependent macrophages was suppressed by the antagonistic antibody and other hemopoietic cell lineages were generated efficiently from me/me bone marrow. By using this culture system, we established me/me pro-B cell clones (MEBs) with rearranged DH-JH but not VH-DJH. The growth of MEB clones required IL-7 and c-Kit ligand, corresponding to normal pro-B cells which express SHP-1. MEB cells were sensitive to starvation by either IL-7 or c-Kit ligand, resulting in apoptotic death. The present culture system, which supports hematopoiesis of me/me bone marrow, provides useful tools for the determination of the role of SHP-1 in signal transduction of B lymphopoiesis.

Signal-Regulatory Protein Is Selectively Expressed by Myeloid and Neuronal Cells

Signal-regulatory proteins (SIRP) are transmembrane glycoproteins with three extracellular Ig-like domains, closely related to Ag receptors Ig, TCR, and MHC, and a cytoplasmic domain with two immunoreceptor with tyrosine-based inhibition motifs that can interact with src homology 2 domain-containing phosphatases. SIRP have previously been shown to inhibit signaling through receptor tyrosine kinases, but their physiologic function is unknown. Here we demonstrate by expression cloning that the mAbs ED9, ED17, and MRC-OX41 recognize rat SIRP. In addition, we show for the first time that rat SIRP is selectively expressed by myeloid cells (macrophages, monocytes, granulocytes, dendritic cells) and neurons. Moreover, SIRP ligation induces nitric oxide production by macrophages. This implicates SIRP as a putative recognition/signaling receptor in both immune and nervous systems.

Epitope mapping of SHP-1 monoclonal antibodies using peptide phage display

We have characterized the binding epitopes of four monoclonal antibodies for SHP-1, an SH2 domain containing protein tyrosine phosphatase, using two phage displayed random peptide libraries. Three of the antibodies are directed against the phosphatase domain of the molecule and the fourth is toward the NH2-terminal part of the second SH2 domain. The first two antibodies recognize the sequence NANY, amino acid 305 to amino acid 308, numbered in the non haematopoietic form of human SHP-1 sequence. The third antibody binds the sequence P Y W P (amino acids 365 to 368) located toward the middle of the phosphatase domain of the enzyme. The fourth antibody is directed against the first two amino acids, W Y (amino acids 112 and 113), of the second SH2 domain. The specificities of these antibodies are demonstrated by ELISA and western blot using different protein constructs expressed in bacteria. All the antibodies can detect wild type SHP-1, expressed in 293 cells, by western blot analysis, both under denaturing conditions as well as following renaturation. The data presented here show that the antibodies characterized in this study are raised against linear epitopes and suggest that these epitopes are accessible from the outside in the native SHP-1 molecule.