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

SHP2 as a primordial epigenetic enzyme expunges histone H3 pTyr-54 to amend androgen receptor homeostasis

Mutations that decrease or increase the activity of the tyrosine phosphatase, SHP2 (encoded by PTPN11), promotes developmental disorders and several malignancies by varying phosphatase activity. We uncovered that SHP2 is a distinct class of an epigenetic enzyme; upon phosphorylation by the kinase ACK1/TNK2, pSHP2 was escorted by androgen receptor (AR) to chromatin, erasing hitherto unidentified pY54-H3 (phosphorylation of histones H3 at Tyr54) epigenetic marks to trigger a transcriptional program of AR. Noonan Syndrome with Multiple Lentigines (NSML) patients, SHP2 knock-in mice, and ACK1 knockout mice presented dramatic increase in pY54-H3, leading to loss of AR transcriptome. In contrast, prostate tumors with high pSHP2 and pACK1 activity exhibited progressive downregulation of pY54-H3 levels and higher AR expression that correlated with disease severity. Overall, pSHP2/pY54-H3 signaling acts as a sentinel of AR homeostasis, explaining not only growth retardation, genital abnormalities and infertility among NSML patients, but also significant AR upregulation in prostate cancer patients.

Antagonizing roles of SHP1 in the pathogenesis of Helicobacter pylori infection

BACKGROUND

SHP1 has been documented as a tumor suppressor and it was thought to play an antagonistic role in the pathogenesis of Helicobacter pylori infection. In this study, the exact mechanism of this antagonistic action was studied.

MATERIALS AND METHODS

AGS, MGC803, and GES-1 cells were infected with H. pylori, intracellular distribution changes of SHP1 were first detected by immunofluorescence. SHP1 overexpression and knockdown were then constructed in these cells to investigate its antagonistic roles in H. pylori infection. Migration and invasion of infected cells were detected by transwell assay, secretion of IL-8 was examined via ELISA, the cells with hummingbird-like alteration were determined by microexamination, and activation of JAK2/STAT3, PI3K/Akt, and ERK pathways were detected by immunoblotting. Mice infection model was established and gastric pathological changes were evaluated. Finally, the SHP1 activator sorafenib was used to analyze the attenuating effect of SHP1 activation on H. pylori pathogenesis in vitro and in vivo.

RESULTS

The sub-localization of SHP1 changed after H. pylori infection, specifically that the majority of the cytoplasmic SHP1 was transferred to the cell membrane. SHP1 inhibited H. pylori-induced activation of JAK2/STAT3 pathway, PI3K/Akt pathway, nuclear translocation of NF-κB, and then reduced EMT, migration, invasion, and IL-8 secretion. In addition, SHP1 inhibited the formation of CagA-SHP2 complex by dephosphorylating phosphorylated CagA, reduced ERK phosphorylation and the formation of CagA-dependent hummingbird-like cells. In the mice infection model, gastric pathological changes were observed and increased IL-8 secretion, indicators of cell proliferation and EMT progression were also detected. By activating SHP1 with sorafenib, a significant curative effect against H. pylori infection was obtained in vitro and in vivo.

CONCLUSIONS

SHP1 plays an antagonistic role in H. pylori pathogenesis by inhibiting JAK2/STAT3 and PI3K/Akt pathways, NF-κB nuclear translocation, and CagA phosphorylation, thereby reducing cell EMT, migration, invasion, IL-8 secretion, and hummingbird-like changes.

SHP-1 phosphatase acts as a coactivator of PCK1 transcription to control gluconeogenesis

We previously reported that the protein-tyrosine phosphatase SHP-1 (PTPN6) negatively regulates insulin signaling, but its impact on hepatic glucose metabolism and systemic glucose control remains poorly understood. Here, we use co-immunoprecipitation assays, chromatin immunoprecipitation sequencing, in silico methods, and gluconeogenesis assay, and found a new mechanism whereby SHP-1 acts as a coactivator for transcription of the phosphoenolpyruvate carboxykinase 1 (PCK1) gene to increase liver gluconeogenesis. SHP-1 is recruited to the regulatory regions of the PCK1 gene and interacts with RNA polymerase II. The recruitment of SHP-1 to chromatin is dependent on its association with the transcription factor signal transducer and activator of transcription 5 (STAT5). Loss of SHP-1 as well as STAT5 decrease RNA polymerase II recruitment to the PCK1 promoter and consequently PCK1 mRNA levels leading to blunted gluconeogenesis. This work highlights a novel nuclear role of SHP-1 as a key transcriptional regulator of hepatic gluconeogenesis adding a new mechanism to the repertoire of SHP-1 functions in metabolic control.

SHP-1 dephosphorylates histone H2B to facilitate its ubiquitination during transcription

Dynamic regulation of phosphorylation and dephosphorylation of histones is essential for eukaryotic transcription, but the enzymes engaged in histone dephosphorylation are not fully explored. Here, we show that the tyrosine phosphatase SHP-1 dephosphorylates histone H2B and plays a critical role during transition from the initiation to the elongation stage of transcription. Nuclear-localized SHP-1 is associated with the Paf1 complex at chromatin and dephosphorylates H2B at tyrosine 121. Moreover, knockout of SHP-1, or expression of a mutant mimicking constitutive phosphorylation of H2B Y121, leads to a reduction in genome-wide H2B ubiquitination, which subsequently causes defects in RNA polymerase II-dependent transcription. Mechanistically, we demonstrate that Y121 phosphorylation precludes H2B's interaction with the E2 enzyme, indicating that SHP-1-mediated dephosphorylation of this residue may be a prerequisite for efficient H2B ubiquitination. Functionally, we find that SHP-1-mediated H2B dephosphorylation contributes to maintaining basal autophagic flux in cells through the efficient transcription of autophagy and lysosomal genes. Collectively, our study reveals an important modification of histone H2B regulated by SHP-1 that has a role during eukaryotic transcription.

Melittin Tryptophan Substitution with a Fluorescent Amino Acid Reveals the Structural Basis of Selective Antitumor Effect and Subcellular Localization in Tumor Cells

Melittin is a membrane-active peptide with strong anticancer activity against various cancers. Despite decades of research, the role of the singular Trp in the anticancer activity and selectivity of melittin remains poorly understood. Here, we propose a theranostic solution based on the substitution of Trp19 with a noncanonical fluorescent amino acid (DapAMCA). The introduction of DapAMCA residue in melittin stabilized the helical structure of the peptide, as evaluated by circular dichroism spectra and molecular dynamics simulations. In vitro hemolytic and anticancer activity assays revealed that introducing DapAMCA residue in melittin changed its mode of action with the cell membrane, resulting in reduced hemolytic toxicity and an improved the selectivity index (SI), with up to a five-fold increase compared to melittin. In vitro fluorescence imaging of DapAMCA-labeled melittin (MELFL) in cancer cells demonstrated high membrane-penetrating activity, with strong nuclear and nucleolar localization ability. These findings provide implications for novel anticancer therapies based on Trp-substituted designs and nuclear/nucleolar targeted therapy.

The Discovery of Potent SHP2 Inhibitors with Anti-Proliferative Activity in Breast Cancer Cell Lines

Despite available treatments, breast cancer is the leading cause of cancer-related death. Knowing that the tyrosine phosphatase SHP2 is a regulator in tumorigenesis, developing inhibitors of SHP2 in breast cells is crucial. Our study investigated the effects of new compounds, purchased from NSC, on the phosphatase activity of SHP2 and the modulation of breast cancer cell lines’ proliferation and viability. A combined ligand-based and structure-based virtual screening protocol was validated, then performed, against SHP2 active site. Top ranked compounds were tested via SHP2 enzymatic assay, followed by measuring IC50 values. Subsequently, hits were tested for their anti-breast cancer viability and proliferative activity. Our experiments identified three compounds 13030, 24198, and 57774 as SHP2 inhibitors, with IC50 values in micromolar levels and considerable selectivity over the analogous enzyme SHP1. Long MD simulations of 500 ns showed a very promising binding mode in the SHP2 catalytic pocket. Furthermore, these compounds significantly reduced MCF-7 breast cancer cells’ proliferation and viability. Interestingly, two of our hits can have acridine or phenoxazine cyclic system known to intercalate in ds DNA. Therefore, our novel approach led to the discovery of SHP2 inhibitors, which could act as a starting point in the future for clinically useful anticancer agents.

Inhibitory affinity modulation of FcγRIIA ligand binding by glycosphingolipids by inside-out signaling

The interaction of the human FcγRIIA with immune complexes (ICs) promotes neutrophil activation and thus must be tightly controlled to avoid damage to healthy tissue. Here, we demonstrate that a fungal-derived soluble β-1,3/1,6-glucan binds to the glycosphingolipid long-chain lactosylceramide (LacCer) to reduce FcγRIIA-mediated recruitment to immobilized ICs under flow, a process requiring high-affinity FcγRIIA-immunoglobulin G (IgG) interactions. The inhibition requires Lyn phosphorylation of SHP-1 phosphatase and the FcγRIIA immunotyrosine-activating motif. β-glucan reduces the effective 2D affinity of FcγRIIA for IgG via Lyn and SHP-1 and, in vivo, inhibits FcγRIIA-mediated neutrophil recruitment to intravascular IgG deposited in the kidney glomeruli in a glycosphingolipid- and Lyn-dependent manner. In contrast, β-glucan did not affect FcγR functions that bypass FcγR affinity for IgG. In summary, we have identified a pathway for modulating the 2D affinity of FcγRIIA for ligand that relies on LacCer-Lyn-SHP-1-mediated inhibitory signaling triggered by β-glucan, a previously described activator of innate immunity.

Okubo et al. demonstrate that β-glucan binding to the glycosphingolipid lactosylceramide engages a Lyn kinase to SHP-1 phosphatase pathway that reduces FcγRIIA binding propensity for IgG, which suggests FcγRIIA affinity regulation by “inside-out” signaling. The β-glucan-lactosylceramide-Lyn axis prevents FcγRIIA-dependent neutrophil recruitment in vitro and to intravascular IgG deposits following glomerulonephritis.

Cytocompatibility of stabilized black phosphorus nanosheets tailored by directly conjugated polymeric micelles for human breast cancer therapy

The novel procedure of few-layer black phosphorus (FLBP) stabilization and functionalisation was here proposed. The cationic polymer PLL and non-ionic PEG have been involved into encapsulation of FLBP to allow sufficient time for further nanofabrication process and overcome environmental degradation. Two different spacer chemistry was designed to bind polymers to tumor-homing peptides. The efficiency of functionalisation was examined by RP-HPLC, microscopic (TEM and SEM) and spectroscopic (FT-IR and Raman) techniques as well supported by ab-initio modelling. The cell and dose dependent cytotoxicity of FLBP and its bioconjugates was evaluated against HB2, MCF-7 and MDA-MB-231 cell lines. Functionalisation allowed not only for improvement of environmental stability, but also enhances therapeutic effect by abolished the cytotoxicity of FLBP against HB2 cell line. Moreover, modification of FLBP with PLL caused increase of selectivity against highly aggressive breast cancer cell lines. Results indicate the future prospect application of black phosphorus nanosheets as nanocarrier, considering its unique features synergistically with conjugated polymeric micelles.

HoxA10 Facilitates SHP-1-Catalyzed Dephosphorylation of p38 MAPK/STAT3 To Repress Hepatitis B Virus Replication by a Feedback Regulatory Mechanism

Hepatitis B virus (HBV) infection is the leading cause of chronic hepatitis B (CHB), liver cirrhosis (LC), and hepatocellular carcinoma (HCC). This study reveals a distinct mechanism underlying the regulation of HBV replication. HBV activates homeobox A10 (HoxA10) in human hepatocytes, leukocytes, peripheral blood mononuclear cells (PBMCs), HepG2-NTCP cells, leukocytes isolated from CHB patients, and HBV-associated HCC tissues. HoxA10 in turn represses HBV replication in human hepatocytes, HepG2-NTCP cells, and BALB/c mice. Interestingly, we show that during early HBV infection, p38 mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 3 (STAT3) were activated to facilitate HBV replication; however, during late HBV infection, HoxA10 was induced to attenuate HBV replication. Detailed studies reveal that HoxA10 binds to p38 MAPK, recruits SH2-containing protein tyrosine phosphatase 1 (SHP-1) to facilitate SHP-1 in catalyzing dephosphorylation of p38 MAPK/STAT3, and thereby attenuates p38 MAPK/STAT3 activation and HBV replication. Furthermore, HoxA10 binds to the HBV enhancer element I (EnhI)/X promoter, competes with STAT3 for binding of the promoter, and thereby represses HBV transcription. Taken together, these results show that HoxA10 attenuates HBV replication through repressing the p38 MAPK/STAT3 pathway by two approaches: HoxA10 interacts with p38 MAPK and recruits SHP-1 to repress HBV replication, and HoxA10 binds to the EnhI/X promoter and competes with STAT3 to attenuate HBV transcription. Thus, the function of HoxA10 is similar to the action of interferon (IFN) in terms of inhibition of HBV infection; however, the mechanism of HoxA10-mediated repression of HBV replication is different from the mechanism underlying IFN-induced inhibition of HBV infection.IMPORTANCE Two billion people have been infected with HBV worldwide; about 240 million infected patients developed chronic hepatitis B (CHB), and 650,000 die each year from liver cirrhosis (LC) or hepatocellular carcinoma (HCC). This work elucidates a mechanism underlying the control of HBV replication. HBV infection activates HoxA10, a regulator of cell differentiation and cancer progression, in human cells and patients with CHB and HCC. HoxA10 subsequently inhibits HBV replication in human tissue culture cells and mice. Additionally, HoxA10 interacts with p38 MAPK to repress the activation of p38 MAPK and STAT3 and recruits and facilitates SHP-1 to catalyze the dephosphorylation of p38 MAPK and STAT3. Moreover, HoxA10 competes with STAT3 for binding of the HBV X promoter to repress HBV transcription. Thus, this work reveals a negative regulatory mechanism underlying the control of HBV replication and provides new insights into the development of potential agents to control HBV infection.

Beneficial and paradoxical roles of selenium at nutritional levels of intake in healthspan and longevity

Accumulation of genome and macromolecule damage is a hallmark of aging, age-associated degeneration, and genome instability syndromes. Although processes of aging are irreversible, they can be modulated by genome maintenance pathways and environmental factors such as diet. Selenium (Se) confers its physiological functions mainly through selenoproteins, but Se compounds and other proteins that incorporate Se nonspecifically also impact optimal health. Bruce Ames proposed that the aging process could be mitigated by a subset of low-hierarchy selenoproteins whose levels are preferentially reduced in response to Se deficiency. Consistent with this notion, results from two selenotranscriptomic studies collectively implicate three low-hierarchy selenoproteins in age or senescence. Experimental evidence generally supports beneficial roles of selenoproteins in the protection against damage accumulation and redox imbalance, but some selenoproteins have also been reported to unexpectedly display harmful functions under sporadic conditions. While longevity and healthspan are usually thought to be projected in parallel, emerging evidence suggests a trade-off between longevity promotion and healthspan deterioration with damage accumulation. We propose that longevity promotion under conditions of Se deficiency may be attributed to 1) stress-response hormesis, an advantageous event of resistance to toxic chemicals at low doses; 2) reduced expression of selenoproteins with paradoxical functions to a lesser extent. In particular, selenoprotein H is an evolutionally conserved nuclear selenoprotein postulated to confer Se functions in redox regulation, genome maintenance, and senescence. This review highlights the need to pinpoint roles of specific selenoproteins and Se compounds in healthspan and lifespan for a better understanding of Se contribution at nutritional levels of intake to healthy aging.

Actin retrograde flow controls natural killer cell response by regulating the conformation state of SHP-1

Natural killer (NK) cells are a powerful weapon against viral infections and tumor growth. Although the actin-myosin (actomyosin) cytoskeleton is crucial for a variety of cellular processes, the role of mechanotransduction, the conversion of actomyosin mechanical forces into signaling cascades, was never explored in NK cells. Here, we demonstrate that actomyosin retrograde flow (ARF) controls the immune response of primary human NK cells through a novel interaction between β-actin and the SH2-domain-containing protein tyrosine phosphatase-1 (SHP-1), converting its conformation state, and thereby regulating NK cell cytotoxicity. Our results identify ARF as a master regulator of the NK cell immune response. Since actin dynamics occur in multiple cellular processes, this mechanism might also regulate the activity of SHP-1 in additional cellular systems.

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.

The tyrosine phosphatase SHP-1 regulates hypoxia inducible factor-1α (HIF-1α) protein levels in endothelial cells under hypoxia

Introduction

The tyrosine phosphatase SHP-1 negatively influences endothelial function, such as VEGF signaling and reactive oxygen species (ROS) formation, and has been shown to influence angiogenesis during tissue ischemia. In ischemic tissues, hypoxia induced angiogenesis is crucial for restoring oxygen supply. However, the exact mechanism how SHP-1 affects endothelial function during ischemia or hypoxia remains unclear. We performed in vitro endothelial cell culture experiments to characterize the role of SHP-1 during hypoxia.

Results

SHP-1 knock-down by specific antisense oligodesoxynucleotides (AS-Odn) increased cell growth as well as VEGF synthesis and secretion during 24 hours of hypoxia compared to control AS-Odn. This was prevented by HIF-1α inhibition (echinomycin and apigenin). SHP-1 knock-down as well as overexpression of a catalytically inactive SHP-1 (SHP-1 CS) further enhanced HIF-1α protein levels, whereas overexpression of a constitutively active SHP-1 (SHP-1 E74A) resulted in decreased HIF-1α levels during hypoxia, compared to wildtype SHP-1. Proteasome inhibition (MG132) returned HIF-1α levels to control or wildtype levels respectively in these cells. SHP-1 silencing did not alter HIF-1α mRNA levels. Finally, under hypoxic conditions SHP-1 knock-down enhanced intracellular endothelial reactive oxygen species (ROS) formation, as measured by oxidation of H₂-DCF and DHE fluorescence.

Conclusions

SHP-1 decreases half-life of HIF-1α under hypoxic conditions resulting in decreased cell growth due to diminished VEGF synthesis and secretion. The regulatory effect of SHP-1 on HIF-1α stability may be mediated by inhibition of endothelial ROS formation stabilizing HIF-1α protein. These findings highlight the importance of SHP-1 in hypoxic signaling and its potential as therapeutic target in ischemic diseases.

Tumor antigen ROR1 targeted drug delivery mediated selective leukemic but not normal B-cell cytotoxicity in chronic lymphocytic leukemia

Selective cytotoxicity to cancer cells without compromising their normal counterparts pose a huge challenge for traditional drug design. Here we developed a tumor antigen-targeted delivery of immunonanoparticle carrying a novel non-immunosuppressive FTY720 derivative OSU-2S with potent cytotoxicity against leukemic B cells. OSU-2S induces activation of protein phosphatase 2A (PP2A), phosphorylation and nuclear translocation of SHP1(S591) and deregulation of multiple cellular processes in chronic lymphocytic leukemia (CLL) resulting in potent cytotoxicity. To preclude OSU-2S-mediated effects on these ubiquitous phosphatases in unintended cells and avoid potential adverse effects, we developed an OSU-2S-targeted delivery of immunonanoparticles (2A2-OSU-2S-ILP), that mediated selective cytotoxicity of CLL but not normal B cells through targeting receptor tyrosine kinase ROR1 expressed in leukemic but not normal B cells. Developing a novel spontaneous CLL mouse model expressing human ROR1 (hROR1) in all leukemic B cells, we demonstrate the therapeutic benefit of enhanced survival with 2A2-OSU-2S-ILP in vivo. The newly developed non-immunosuppressive OSU-2S, its delivery using human CLL directed immunonanoparticles and the novel transgenic (Tg) mouse model of CLL that expresses hROR1 exclusively in leukemic B cell surface are highly innovative and can be applied to CLL and other ROR1+ malignancies including mantle cell lymphoma and acute lymphoblastic leukemia.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Protein tyrosine phosphatases as novel targets in breast cancer therapy

Breast cancer is linked to hyperactivation of protein tyrosine kinases (PTKs), and recent studies have unveiled that selective tyrosine dephosphorylation by protein tyrosine phosphatases (PTPs) of specific substrates, including PTKs, may activate or inactivate oncogenic pathways in human breast cancer cell growth-related processes. Here, we review the current knowledge on the involvement of PTPs in breast cancer, as major regulators of breast cancer therapy-targeted PTKs, such as HER1/EGFR, HER2/Neu, and Src. The functional interplay between PTKs and PTK-activating or -inactivating PTPs, and its implications in novel breast cancer therapies based on targeting of specific PTPs, are discussed.

SHP-1 as a critical regulator of Mycoplasma pneumoniae-induced inflammation in human asthmatic airway epithelial cells

Asthma is a chronic inflammatory disease in which airway epithelial cells are the first line of defense against exposure of the airway to infectious agents. Src homology protein (SHP)-1, a protein tyrosine phosphatase, is a negative regulator of signaling pathways that are critical to the development of asthma and host defense. We hypothesize that SHP-1 function is defective in asthma, contributing to the increased inflammatory response induced by Mycoplasma pneumoniae, a pathogen known to exacerbate asthma. M. pneumoniae significantly activated SHP-1 in airway epithelial cells collected from nonasthmatic subjects by bronchoscopy with airway brushing but not in cells from asthmatic subjects. In asthmatic airway epithelial cells, M. pneumoniae induced significant PI3K/Akt phosphorylation, NF-κB activation, and IL-8 production compared with nonasthmatic cells, which were reversed by SHP-1 overexpression. Conversely, SHP-1 knockdown significantly increased IL-8 production and PI3K/Akt and NF-κB activation in the setting of M. pneumoniae infection in nonasthmatic cells, but it did not exacerbate these three parameters already activated in asthmatic cells. Thus, SHP-1 plays a critical role in abrogating M. pneumoniae-induced IL-8 production in nonasthmatic airway epithelial cells through inhibition of PI3K/Akt and NF-κB activity, but it is defective in asthma, resulting in an enhanced inflammatory response to infection.

Contribution of SHP-1 protein tyrosine phosphatase to osmotic regulation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor TonEBP/OREBP, resulting in increased expression of osmoprotective genes, including those responsible for accumulation of organic osmolytes and heat-shock proteins. Phosphorylation of TonEBP/OREBP contributes to its activation. Several of the kinases that are involved were previously identified, but the phosphatases were not. In the present studies we screened a genomewide human phosphatase siRNA library in human embryonic kidney (HEK)293 cells for effects on TonEBP/OREBP transcriptional activity. We found that siRNAs against 57 phosphatases significantly alter TonEBP/OREBP transcriptional activity during normotonicity (290 mosmol/kg) or hypertonicity (500 mosmol/kg, NaCl added) or both. Most siRNAs increase TonEBP/OREBP activity, implying that the targeted phosphatases normally reduce that activity. We further studied in detail SHP-1, whose knockdown by its specific siRNA increases TonEBP/OREBP transcriptional activity at 500 mosmol/kg. We confirmed that SHP-1 is inhibitory by overexpressing it, which reduces TonEBP/OREBP transcriptional activity at 500 mosmol/kg. SHP-1 dephosphorylates TonEBP/OREBP at a known regulatory site, Y143, both in vivo and in vitro. It inhibits TonEBP/OREBP by both reducing TonEBP/OREBP nuclear localization, which is Y143 dependent, and by lowering high NaCl-induced TonEBP/OREBP transactivating activity. SHP-1 coimmunoprecipitates with TonEBP/OREBP and vice versa, suggesting that they are physically associated in the cell. High NaCl inhibits the effect of SHP-1 on TonEBP/OREBP by increasing phosphorylation of SHP-1 on Ser591, which reduces its phosphatase activity and localization to the nucleus. Thus, TonEBP/OREBP is extensively regulated by phosphatases, including SHP-1, whose inhibition by high NaCl increases phosphorylation of TonEBP/OREBP at Y143, contributing to the nuclear localization and activation of TonEBP/OREBP.

Homophilic adhesion and CEACAM1-S regulate dimerization of CEACAM1-L and recruitment of SHP-2 and c-Src

Carcinoembryonic antigen (CEA)-related cell adhesion molecule 1 (CAM1 [CEACAM1]) mediates homophilic cell adhesion and regulates signaling. Although there is evidence that CEACAM1 binds and activates SHP-1, SHP-2, and c-Src, knowledge about the mechanism of transmembrane signaling is lacking. To analyze the regulation of SHP-1/SHP-2/c-Src binding, we expressed various CFP/YFP-tagged CEACAM1 isoforms in epithelial cells. The supramolecular organization of CEACAM1 was examined by cross-linking, coclustering, coimmunoprecipitation, and fluorescence resonance energy transfer. SHP-1/SHP-2/c-Src binding was monitored by coimmunoprecipitation and phosphotyrosine-induced recruitment to CEACAM1-L in cellular monolayers. We find that trans-homophilic CEACAM1 binding induces cis-dimerization by an allosteric mechanism transmitted by the N-terminal immunoglobulin-like domain. The balance of SHP-2 and c-Src binding is dependent on the monomer/dimer equilibrium of CEACAM1-L and is regulated by trans-binding, whereas SHP-1 does not bind under physiological conditions. CEACAM1-L homodimer formation is reduced by coexpression of CEACAM1-S and modulated by antibody ligation. These data suggest that transmembrane signaling by CEACAM1 operates by alteration of the monomer/dimer equilibrium, which leads to changes in the SHP-2/c-Src-binding ratio.

Knockdown of protein tyrosine phosphatase SHP-1 inhibits G1/S progression in prostate cancer cells through the regulation of components of the cell-cycle machinery

SHP-1, a haematopoietic cell-specific tyrosine phosphatase, is also expressed in human prostate. In this study, we report that SHP-1 depletion in PC-3 cells induced by small interfering RNAs causes G1 phase cell-cycle arrest accompanied by changes in some components of the cell-cycle machinery. SHP-1 knockdown increases p27(Kip1) (p27) protein stability, its nuclear localization and p27 gene transcription. These effects could be mediated by PI3K-AKT pathway as SHP-1 interacts with PI3K regulating its activity and p110 catalytic subunit phosphorylation. The increase in p27 protein stability could also because of reduced cyclin-dependent kinase (CDK2) activity. SHP-1 knockdown decreases the CDK6 levels, inducing retinoblastoma protein hypophosphorylation, downregulation of cyclin E and thereby a decrease in the CDK2 activity. However, the codepletion of SHP-1 and p27 does not produce re-entry into the cycle, implying that p27 is not required to maintain cell-cycle arrest induced by SHP-1 depletion. The maintenance of the PC-3 cell anti-proliferative response after p27 loss could be because of mislocalization of CDK2 induced by SHP-1 knockdown. This study shows that SHP-1 depletion promotes cell-cycle arrest by modulating the activity of cell-cycle regulators and suggests that SHP-1 may be required for the proper functioning of events governing cell-cycle progression.

The nuclear localization of 3'-phosphoinositide-dependent kinase-1 is dependent on its association with the protein tyrosine phosphatase SHP-1

3'-Phosphoinositide-dependent protein kinase-1 (PDK1), the direct upstream kinase of Akt, can localize to the nucleus during specific signalling events. The mechanism used for its import into the nucleus, however, remains unresolved as it lacks a canonical nuclear localization signal (NLS). Expression of activated Src kinase in C6 glioblastoma cells promotes the association of tyrosylphosphorylated PDK1 with the NLS-containing tyrosine phosphatase SHP-1 as well as the nuclear localization of both proteins. A constitutive nucleo-cytoplasmic SHP-1:PDK1 shuttling complex is supported by several lines of evidence including (i) the distribution of both proteins to similar subcellular compartments following manipulation of the nuclear pore complex, (ii) the nuclear retention of SHP-1 upon overexpression of a PDK1 protein bearing a disrupted nuclear export signal (NES), and (iii) the exclusion of PDK1 from the nucleus upon overexpression of SHP-1 lacking the NLS or following siRNA-mediated knock-down of SHP-1. The latter case results in a perinuclear distribution of PDK1 that corresponds with the distribution of PIP3 (phosphatidylinositol 3,4,5-triphosphate), while a PDK1 protein bearing a mutated PH domain that abrogates PIP3-binding is excluded from the nucleus. Our data suggest that the SHP-1:PDK1 complex is recruited to the nuclear membrane by binding to perinuclear PIP3, whereupon SHP-1 (and its NLS) facilitates active import. Export from the nucleus relies on PDK1 (and its NES). The intact complex contributes to Src kinase-induced, Akt-sensitive podial formation in C6 cells.

Activation of Cdk2 stimulates proteasome-dependent truncation of tyrosine phosphatase SHP-1 in human proliferating intestinal epithelial cells

SHP-1 is expressed in the nuclei of intestinal epithelial cells (IECs). Increased SHP-1 expression and phosphatase activity coincide with cell cycle arrest and differentiation in these cells. Suspecting the tumor-suppressive properties of SHP-1, a yeast two-hybrid screen of an IEC cDNA library was conducted using the full-length SHP-1 as bait. Characterization of many positive clones revealed sequences identical to a segment of the Cdk2 cDNA sequence. Interaction between SHP-1 and Cdk2 was confirmed by co-immunoprecipitations whereby co-precipitated Cdk2 phosphorylated SHP-1 protein. Inhibition of Cdk2 (roscovitine) or proteasome (MG132) was associated with an enhanced nuclear punctuate distribution of SHP-1. Double labeling localization studies with signature proteins of subnuclear domains revealed a co-localization between the splicing factor SC35 and SHP-1 in bright nucleoplasmic foci. Using Western blot analyses with the anti-SHP-1 antibody recognizing the C terminus, a lower molecular mass species of 45 kDa was observed in addition to the full-length 64-65-kDa SHP-1 protein. Treatment with MG132 led to an increase in expression of the full-length SHP-1 protein while concomitantly leading to a decrease in the levels of the lower mass 45-kDa molecular species. Further Western blots revealed that the 45-kDa protein corresponds to the C-terminal portion of SHP-1 generated from proteasome activity. Mutational analysis of Tyr(208) and Ser(591) (a Cdk2 phosphorylation site) residues on SHP-1 abolished the expression of the amino-truncated 45-kDa SHP-1 protein. In conclusion, our results indicate that Cdk2-associated complexes, by targeting SHP-1 for proteolysis, counteract the ability of SHP-1 to block cell cycle progression of IECs.

TIMP1 induces CD44 expression and the activation and nuclear translocation of SHP1 during the late centrocyte/post-germinal center B cell differentiation

Tissue inhibitor of metalloproteinase-1 (TIMP1) is a survival factor of germinal center (GC) B cells, and its over-expression is correlated with aggressive B cell lymphomas and classical Hodgkin lymphomas. We previously demonstrated that TIMP1 down-regulates B-cell receptor and BCL6, and activates interleukins-6,-10 (ILs)/signal transducer and activator of transcription-3 (STAT3) signaling in GC B cells. The activation of ILs/STAT3 signaling can amplify CD44 function, and vice versa, and induce protein-tyrosine phosphatase SHP1 activity by a negative feedback mechanism. Here, we show that TIMP1 up-regulates cell surface CD44 (standard and variants 3 and 7-10) and induces the activity and nuclear localization of SHP1 in an Epstein Barr virus (EBV)-negative Burkitt lymphoma cell line, the neoplastic counterpart of GC centroblasts. These results suggest that TIMP1 functions as a differentiating and survival factor of GC B cells by modulating CD44 and SHP1 in the late centrocyte/post-GC stage, regardless of EBV infection.

Identification of a novel lipid raft-targeting motif in Src homology 2-containing phosphatase 1

The tyrosine phosphatase Src homology 2-containing phosphatase 1 (SHP-1) is a key negative regulator of TCR-mediated signaling. Previous studies have shown that in T cells a fraction of SHP-1 constitutively localizes to membrane microdomains, commonly referred to as lipid rafts. Although this localization of SHP-1 is required for its functional regulation of T cell activation events, how SHP-1 is targeted to the lipid rafts was unclear. In this study, we identify a novel, six-amino acid, lipid raft-targeting motif within the C terminus of SHP-1 based on several biochemical and functional observations. First, mutations of this motif in the context of full-length SHP-1 result in the loss of lipid raft localization of SHP-1. Second, this motif alone restores raft localization when fused to a mutant of SHP-1 (SHP-1 DeltaC) that fails to localize to rafts. Third, a peptide encompassing the 6-mer motif directly binds to phospholipids whereas a mutation of this motif abolishes lipid binding. Fourth, whereas full-length SHP-1 potently inhibits TCR-induced tyrosine phosphorylation of specific proteins, expression of a SHP-1-carrying mutation within the 6-mer motif does not. Additionally, although SHP-1 DeltaC was functionally inactive, the addition of the 6-mer motif restored its functionality in inhibiting TCR-induced tyrosine phosphorylation. Finally, this 6-mer mediated targeting of SHP-1 lipid rafts was essential for the function of this phosphatase in regulating IL-2 production downstream of TCR. Taken together, these data define a novel 6-mer motif within SHP-1 that is necessary and sufficient for lipid raft localization and for the function of SHP-1 as a negative regulator of TCR signaling.

Rapid T cell receptor-mediated SHP-1 S591 phosphorylation regulates SHP-1 cellular localization and phosphatase activity

Since the tyrosine phosphatase SHP-1 plays a major role in regulating T cell signaling, we investigated regulation thereof by Ser/Thr phosphorylation. We found that T cell receptor (TCR) stimulation induced fast (<or=1 min) and transient phosphorylation of SHP-1 S591 in both Jurkat and human peripheral blood T-cells (PBT). Phosphorylation of S591 in T-cells could be mediated artificially by a constitutive active PKC-theta construct, but the dose dependence of inhibition by PKC inhibitors indicated that PKCs were not the relevant basophilic kinase in the physiological response. S591 phosphorylation inhibited phosphatase function since a S591D mutant had lower activity than the S591A mutant. Additional evidence that S591 phosphorylation alters SHP-1 function was provided by studies of Jurkat cells stably expressing SHP-1 wild type or mutants. In those cells, S591D mutation reduced the capacity of transfected SHP-1 to inhibit TCR-induced phosphorylation of PLC-gamma1. Interestingly, SHP-1 Y536 phosphorylation (previously shown to augment phosphatase activity) was also induced in PBT by TCR signal but at a much later time compared with S591 ( approximately 30 min). S591 phosphorylation also altered cellular distribution of SHP-1 because: 1) SHP-1 in lipid rafts and a sheared membrane fraction was hypophosphorylated; 2) In stably transfected Jurkat cell lines, S591D mutant protein had reduced presence in both lipid raft and the sheared membrane fraction; 3) S591 phosphorylation prevented nuclear localization of a C-terminal GFP tagged SHP-1 construct. Our studies also shed light on an additional mechanism regulating SHP-1 nuclear localization, namely conformational autoinhibition. These findings highlight elegant regulation of SHP-1 by sequential phosphorylation of serine then tyrosine.

Nonreceptor Protein-Tyrosine Phosphatases in Immune Cell Signaling

Tyrosyl phosphorylation plays a critical role in multiple signaling pathways regulating innate and acquired immunity. Although tyrosyl phosphorylation is a reversible process, we know much more about the functions of protein-tyrosine kinases (PTKs) than about protein-tyrosine phosphatases (PTPs). Genome sequencing efforts have revealed a large and diverse superfamily of PTPs, which can be subdivided into receptor-like (RPTPs) and nonreceptor (NRPTPs). The role of the RPTP CD45 in immune cell signaling is well known, but those of most other PTPs remain poorly understood. Here, we review the mechanism of action, regulation, and physiological functions of NRPTPs in immune cell signaling. Such an analysis indicates that PTPs are as important as PTKs in regulating the immune system.

Investigations into the regulation and function of the SH2 domain-containing protein-tyrosine phosphatase, SHP-1

Our laboratory is interested in identifying genes relevant to diseases. Our approach is to use spontaneous mouse mutants with immunological defects and decipher the molecular basis of the phenotypes. In the early 1990s, our attention was focused on the motheaten and viable motheaten mouse mutants. We used these mutant mice as a model system for elucidating the genetic and cellular events contributing to expression of normal hematopoietic and immune function. Our initial goal was to identify the gene responsible for the motheaten and viable motheaten phenotype. In 1993, we and others reported that both motheaten and viable motheaten mice have mutations in the SHP-1 gene. Currently, there are more than 600 publications involving SHP-1. In this review, rather than summarizing all these studies, we highlight work involving SHP-1 that were/are carried out in our and our collaborators' laboratories.

Subcellular localization and differentiation-induced redistribution of the protein tyrosine phosphatase PTP-BL in Neuroblastoma cells

1. In cells of epithelial origin the protein tyrosine phosphatase PTP-BL is predominantly localized at the apical membrane of polarized cells. This large submembranous multidomain PTP is also expressed in cells of neuronal origin. We studied the localization of PTP-BL in mouse neuroblastoma cells utilizing EGFP-tagged versions of the protein. 2. In proliferating Neuro-2a cells, immunofluorescence and immuno-electron microscopy revealed a submembranous FERM domain-dependent localization at cell-cell boundaries for EGFP-PTP-BL. Additionally, significant amounts of EGFP-PTP-BL are located in the cytoplasm as well as in nuclei. Upon serum depletion-induced differentiation of Neuro-2a cells, a partial shift of EGFP-PTP-BL from a cortical localization to cytoskeleton-like F-actin-positive structures is observed. Parallel biochemical studies corroborate this finding and reveal a serum depletion-induced shift of EFGP-PTP-BL from a membrane(-associated) fraction to an NP40-soluble cytoskeletal fraction. 3. Different pools of PTP-BL-containing protein complexes can be discerned in neuronal cells, reflecting distinct molecular microenvironments in which PTP-BL may exert its function.

A SHPing tale: perspectives on the regulation of SHP-1 and SHP-2 tyrosine phosphatases by the C-terminal tail

Protein tyrosine phosphorylation is a ubiquitous signalling mechanism and is regulated by a balance between the action of kinases and phosphatases. The SH2 domain-containing phosphatases SHP-1 and SHP-2 are the best studied of the classical non-receptor tyrosine phosphatases, but it is intriguing that despite their close sequence and structural homology these two phosphatases play quite different cellular roles. In particular, whereas SHP-1 plays a largely negative signalling role suppressing cellular activation, SHP-2 plays a largely positive signalling role. Major sequence differences between the two molecules are apparent in the approximately 100 amino acid residues at the extreme C-terminus of the proteins, beyond the phosphatase catalytic domain. Here we review how the differences in the tails of these proteins may regulate their activities and explain some of their functional differences.

Tyrosine phosphatase SHP-1 acts at different stages of development to regulate hematopoiesis

Mice lacking SHP-1 exhibit a plethora of perturbations in their hematopoietic and immune systems. To reveal the primary effects resulting from SHP-1 deficiency, we used embryonic stem (ES) cells to study the role of SHP-1 in developmental hematopoiesis. We expressed wild-type (WT) and dominant-negative (R459M) forms of SHP-1 in ES cells and used ES/OP-9 coculture and embryoid body development followed by hematopoietic colony assays to demonstrate that SHP-1 acts at multiple stages of hematopoietic differentiation to alter lineage balance. Expression of WT SHP-1 reduced myeloid colony numbers while increasing the numbers of secondary embryoid bodies and mixed hematopoietic colonies obtained. Conversely, expression of R459M SHP-1 resulted in a significant increase in the numbers and sizes of myeloid colonies observed while reducing the numbers of colonies derived from undifferentiated cells or hematopoietic precursor cells. Confining the expression of WT or R459M SHP-1 to the early phases of differentiation decreased and increased progenitor cell numbers, respectively, and influenced colony formation. Overall, our results are consistent with SHP-1 acting during multiple stages of hematopoietic development, and they suggest that the increases in granulocytes and macrophages observed in motheaten mice arise as the result of a cell autonomous effect early during development.

Phosphatidylinositol phosphate kinase type Igamma directly associates with and regulates Shp-1 tyrosine phosphatase

Tyrosine phosphorylation plays a critical role in many regulatory aspects of cellular signaling, and dephosphorylation of phosphotyrosine residues is crucial for termination of signals initiated by tyrosine kinases. Previous work has shown that the tyrosine kinase Src phosphorylates Tyr644 on phosphatidylinositol phosphate kinase type I (PIPKI) gamma661 in a focal adhesion kinase-dependent manner. Phosphorylation of this residue is essential for high affinity binding of PIPKI gamma661 to the focal adhesion protein talin and for targeting of PIPKI gamma661 to focal adhesions. A yeast two-hybrid screen performed with the C-terminal 178-amino acid tail of PIPKI gamma661 identified an interaction with the phosphatase domain of the tyrosine phosphatase Shp-1. The interaction between PIPKI gamma661 and Shp-1 was confirmed via co-immunoprecipitation from HEK293 cell lysates. In addition, Src-phosphorylated PIPKI gamma661 is a substrate for Shp-1, and Shp-1 modulates both the association between PIPKI gamma661 and talin and the targeting of PIPKI gamma661 to focal adhesions in mammalian cells. Finally, we showed that Shp-1 phosphatase activity is inhibited by the product of PIPKI gamma661, phosphatidylinositol 4,5-bisphosphate, in vitro. These combined results suggest a model in which the reciprocal actions of Src tyrosine kinase and Shp-1 tyrosine phosphatase dynamically regulate the association between PIPKI gamma661 and talin.

Abnormal Th1 cell differentiation and IFN-gamma production in T lymphocytes from motheaten viable mice mutant for Src homology 2 domain-containing protein tyrosine phosphatase-1

Src homology 2 domain-containing protein tyrosine phosphatase-1 (SHP-1) plays an important role in T and B lymphocyte signaling; however, the function of SHP-1 in Th cell differentiation, in particular, the Th1 response, has not been defined. In this study, we provide evidence that SHP-1 phosphatase negatively regulates Th1 cell development and IFN-gamma production. Compared with the wild-type control, anti-CD3-activated mouse T lymphocytes carrying the motheaten viable mutation in the SHP-1 gene produced a significantly increased amount of IFN-gamma in the presence of IL-12. This increase was also seen at the basal level without IL-12 addition. Similarly, Th1 cell differentiation and proliferation of anti-CD3-activated SHP-1 mutant lymph node cells in the presence or absence of IL-12 were markedly enhanced, indicating a negative role for SHP-1 phosphatase in such lymphocyte activities. Interestingly, IL-12-induced activation of Jak2 and STAT4, critical components for IL-12-mediated cellular responses, was shortened or attenuated in mutant T cells. Together these results suggest that SHP-1 negatively regulates Th1 cell development and functions through a mechanism that is not directly related to IL-12 signaling.

Inhibition of protein-tyrosine phosphatases by mild oxidative stresses is dependent on S-nitrosylation

Previous studies have shown that a Ca(2+)-dependent nitric-oxide synthase (NOS) is activated as part of a cellular response to low doses of ionizing radiation. Genetic and pharmacological inhibitor studies linked this NO signaling to the radiation-induced activation of ERK1/2. Herein, a mechanism for the radiation-induced activation of Tyr phosphorylation-dependent pathways (e.g. ERK1/2) involving the inhibition of protein-Tyr phosphatases (PTPs) by S-nitrosylation is tested. The basis for this mechanism resides in the redox-sensitive active site Cys in PTPs. These studies also examined oxidative stress induced by low concentrations of H(2)O(2). S-Nitrosylation of total cellular PTP and immunopurified SHP-1 and SHP-2 was detected as protection of PTP enzymatic activity from alkylation by N-ethylmaleimide and reversal by ascorbate. Both radiation and H(2)O(2) protected PTP activity from alkylation by a mechanism reversible by ascorbate and inhibited by NOS inhibitors or expression of a dominant negative mutant of NOS-1. Radiation and H(2)O(2) stimulated a transient increase in cytoplasmic free [Ca(2+)]. Radiation, H(2)O(2), and the Ca(2+) ionophore, ionomycin, also stimulated NOS activity, and this was associated with an enhanced S-nitrosylation of the active site Cys(453) determined by isolation of S-nitrosylated wild type but not active site Cys(453) --> Ser SHP-1 mutant by the "biotin-switch" method. Thus, one consequence of oxidative stimulation of NO generation is S-nitrosylation and inhibition of PTPs critical in cellular signal transduction pathways. These results support the conclusion that a mild oxidative signal is converted to a nitrosative one due to the better redox signaling properties of NO.

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.

Regulation of SHP-1 Tyrosine Phosphatase in Human Platelets by Serine Phosphorylation at Its C Terminus

SHP-1 is a Src homology 2 (SH2) domain-containing tyrosine phosphatase that plays an essential role in negative regulation of immune cell activity. We describe here a new model for regulation of SHP-1 involving phosphorylation of its C-terminal Ser591 by associated protein kinase Calpha. In human platelets, SHP-1 was found to constitutively associate with its substrate Vav1 and, through its SH2 domains, with protein kinase Calpha. Upon activation of either PAR1 or PAR4 thrombin receptors, the association between the three proteins was retained, and Vav1 became phosphorylated on tyrosine and SHP-1 became phosphorylated on Ser591. Phosphorylation of SHP-1 was mediated by protein kinase C and negatively regulated the activity of SHP-1 as demonstrated by a decrease in the in vitro ability of SHP-1 to dephosphorylate Vav1 on tyrosine. Protein kinase Calpha therefore critically and negatively regulates SHP-1 function, forming part of a mechanism to retain SHP-1 in a basal active state through interaction with its SH2 domains, and phosphorylating its C-terminal Ser591 upon cellular activation leading to inhibition of SHP-1 activity and an increase in the tyrosine phosphorylation status of its substrates.

Dysmyelination and reduced myelin basic protein gene expression by oligodendrocytes of SHP-1-deficient mice

We have shown previously that myelin-forming oligodendrocytes express the protein tyrosine phosphatase SHP-1 and that myelin formation was decreased in SHP-1-deficient motheaten mice compared to that in normal littermates. These studies suggested a potential importance for SHP-1 in oligodendrocyte and myelin development. To address further this possibility, we analyzed myelin formation by microscopy and myelin basic protein (MBP) gene expression in motheaten mice at ages when myelination occurs in the developing central nervous system (CNS). Furthermore, we correlate these findings with MBP gene expression in oligodendrocytes grown in vitro. We have found that CNS myelination was significantly reduced in SHP-1-deficient mice relative to their normal littermates at multiple times during the active period of myelination. Under electron microscopy, greater numbers of axons in spinal cords of motheaten mice were either unmyelinated or had thinner myelin sheathes compared to those in matched areas of normal littermates. Accordingly, MBP protein and mRNA levels were reduced in SHP-1-deficient mice compared to that in the CNS of normal littermates. In vitro, O1(+) oligodendrocytes from motheaten mice expressed much less MBP than O1(+) oligodendrocytes of normal littermates indicating an alteration in oligodendrocyte differentiation. The latter correlated with reduced MBP mRNA relative to cerebroside galactosyl transferase (CGT) gene mRNA in SHP-1-deficient oligodendrocytes in purified cultures. We propose that SHP-1 is a critical regulator of developmental signals leading to terminal differentiation and myelin sheath formation by oligodendrocytes.

Effective dephosphorylation of Src substrates by SHP-1

The protein-tyrosine phosphatase SHP-1 is a negative regulator of multiple signal transduction pathways. We observed that SHP-1 effectively antagonized Src-dependent phosphorylations in HEK293 cells. This occurred by dephosphorylation of Src substrates, because Src activity was unaffected in the presence of SHP-1. One reason for efficient dephosphorylation was activation of SHP-1 by Src. Recombinant SHP-1 had elevated activity subsequent to phosphorylation by Src in vitro, and SHP-1 variants with mutated phosphorylation sites in the C terminus, SHP-1 Y538F, and SHP-1 Y538F,Y566F were less active toward Src-generated phosphoproteins in intact cells. A second reason for efficient dephosphorylation is the substrate selectivity of SHP-1. Pull-down experiments with different GST-SHP-1 fusion proteins revealed efficient interaction of Src-generated phosphoproteins with the SHP-1 catalytic domain rather than with the SH2 domains. Phosphopeptides that correspond to good Src substrates were efficiently dephosphorylated by SHP-1 in vitro. Phosphorylated "optimal Src substrate" AEEEIpYGEFEA (where pY is phosphotyrosine) and a phosphopeptide corresponding to a recently identified Src phosphorylation site in p120 catenin, DDLDpY(296)GMMSD, were excellent SHP-1 substrates. Docking of these phosphopeptides into the catalytic domain of SHP-1 by molecular modeling was consistent with the biochemical data and explains the efficient interaction. Acidic residues N-terminal of the phosphotyrosine seem to be of major importance for efficient substrate interaction. Residues C-terminal of the phosphotyrosine probably contribute to the substrate selectivity of SHP-1. We propose that activation of SHP-1 by Src and complementary substrate specificities of SHP-1 and Src may lead to very transient Src signals in the presence of SHP-1.

Progressive vascular changes in a transgenic mouse model of squamous cell carcinoma

Phage display was used to identify homing peptides for blood vessels in a mouse model of HPV16-induced epidermal carcinogenesis. One peptide, CSRPRRSEC, recognized the neovasculature in dysplastic skin but not in carcinomas. Two other peptides, with the sequences CGKRK and CDTRL, preferentially homed to neovasculature in tumors and, to a lesser extent, premalignant dysplasias. The peptides did not home to vessels in normal skin, other normal organs, or the stages in pancreatic islet carcinogenesis in another mouse model. The CGKRK peptide may recognize heparan sulfates in tumor vessels. The dysplasia-homing peptide is identical to a loop in kallikrein-9 and may bind a kallikrein inhibitor or substrate. Thus, characteristics of the angiogenic vasculature distinguish premalignant and malignant stages of skin tumorigenesis.

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.

ERBB receptor tyrosine kinases and cellular radiation responses

Ionizing radiation induces in autocrine growth-regulated carcinoma and malignant glioma cells powerful cytoprotective responses that confer relative resistance to consecutive radiation exposures. Understanding the mechanisms of these responses should provide new molecular targets for tumor radiosensitization. ERBB and other receptor Tyr kinases have been identified as immediate early response gene products that are activated by radiation within minutes, as by their physiological growth factor ligands, and induce secondary stimulation of cytoplasmic protein kinase cascades. The simultaneous activation of all receptor Tyr kinases and nonreceptor Tyr kinases leads to complex cytoprotective responses including increased cell proliferation, reduced apoptosis and enhanced DNA repair. Since these responses contribute to cellular radioresistance, ERBB1, the most extensively studied ERBB receptor, is examined as a target for tumor cell radiosensitization. The three methods of ERBB1 inhibition include blockade of growth factor binding by monoclonal antibody against the ligand-binding domain, inhibition of the receptor Tyr kinase-mediating receptor activation, and overexpression of a dominant-negative epidermal growth factor receptor-CD533 that lacks the COOH-terminal 533 amino acids and forms nonfunctional heterodimeric complexes with wild-type receptors. All the three approaches enhance radiation toxicity in vitro and in vivo. The different mechanisms of inhibition have contributed to the understanding of cellular responses to radiation, vary in relative effectiveness and pose different challenges for translation.

The 'Shp'ing news: SH2 domain-containing tyrosine phosphatases in cell signaling

Src homology-2 (SH2) domain-containing phosphatases (Shps) are a small, highly conserved subfamily of protein-tyrosine phosphatases, members of which are present in both vertebrates and invertebrates. The mechanism of regulation of Shps by ligand binding is now well understood. Much is also known about the normal signaling pathways regulated by each Shp and the consequences of Shp deficiency. Recent studies have identified mutations in human Shp2 as the cause of the inherited disorder Noonan syndrome. Shp2 mutations might also contribute to the pathogenesis of some leukemias. In addition, Shp2 might be a key virulence determinant for the important human pathogen Helicobacter pylori. Despite these efforts, however, the key targets of each Shp have remained elusive. Identifying these substrates remains a major challenge for future research.

Identification of Shp-2 as a Stat5A phosphatase

Stat5A, a member of the signal transducers and activators of transcription (Stat) family, is activated upon a single tyrosine phosphorylation. Although much is known about the activation process, the mechanism by which the tyrosine-phosphorylated Stat5A proteins are inactivated is largely unknown. In this report, we demonstrate that down-regulation of the tyrosine-phosphorylated Stat5A was via dephosphorylation. Using tyrosine-phosphorylated peptides derived from Stat5A, we were able to purify protein-tyrosine phosphatase Shp-2 from cell lysates. Shp-2, but not Shp-1, specifically interacted with Stat5A in vivo, and the interaction was tyrosine phosphorylation-dependent. Moreover, Shp-2 was able to accelerate Stat5A dephosphorylation, and dephosphorylation of Stat5A was dramatically delayed in Shp-2-deficient cells. Therefore, we conclude that Shp-2 is a Stat5A phosphatase, which down-regulates the active Stat5A in vivo.

Negative regulation of beta-catenin signaling by tyrosine phosphatase SHP-1 in intestinal epithelial cells

Protein-tyrosine phosphatase SHP-1 is expressed at high levels in hematopoietic cells and at moderate levels in many other cell types including epithelial cells. Although SHP-1 has been shown to be a negative regulator of multiple signaling pathways in hematopoietic cells, very little is known about the biological role of SHP-1 in epithelial cells. In order to elucidate the mechanism(s) responsible for the loss of proliferative potential once committed intestinal epithelial cells begin to differentiate, the role and regulation of SHP-1 were analyzed in both intact epithelium as well as in well established intestinal cell models recapitulating the crypt-villus axis in vitro. Results show that SHP-1 was expressed in the nuclei of all intestinal epithelial cell models as well as in epithelial cells of intact human fetal jejunum and colon. Expression and phosphatase activity levels of SHP-1 were much more elevated in confluent growth-arrested intestinal epithelial cells and in differentiated enterocytes as well. Overexpression of SHP-1 in intestinal epithelial crypt cells significantly inhibited dhfr, c-myc, and cyclin D1 gene expression but did not interfere with c-fos gene expression. In contrast, a mutated inactive form of SHP-1 had no effect on these genes. SHP-1 expression significantly decreased beta-catenin/TCF-dependent transcription in intestinal epithelial crypt cells. Immunoprecipitation experiments revealed that beta-catenin is one of the main binding partners and a substrate for SHP-1. Taken together, our results indicate that SHP-1 may be involved in the regulation of beta-catenin transcriptional function and in the negative control of intestinal epithelial cell proliferation.

Crystal structure of human protein-tyrosine phosphatase SHP-1

SHP-1 is a cytosolic protein-tyrosine phosphatase that behaves as a negative regulator in eukaryotic cellular signaling pathways. To understand its regulatory mechanism, we have determined the crystal structure of the C-terminal truncated human SHP-1 in the inactive conformation at 2.8-A resolution and refined the structure to a crystallographic R-factor of 24.0%. The three-dimensional structure shows that the ligand-free SHP-1 has an auto-inhibited conformation. Its N-SH2 domain blocks the catalytic domain and keeps the enzyme in the inactive conformation, which supports that the phosphatase activity of SHP-1 is primarily regulated by the N-SH2 domain. In addition, the C-SH2 domain of SHP-1 has a different orientation from and is more flexible than that of SHP-2, which enables us to propose an enzymatic activation mechanism in which the C-SH2 domains of SHPs could be involved in searching for phosphotyrosine activators.

Regulation of the dephosphorylation of Stat6. Participation of Tyr-713 in the interleukin-4 receptor alpha, the tyrosine phosphatase SHP-1, and the proteasome

Signal transducer and activator of transcription 6 (Stat6) plays an important role in interleukin (IL)-4-induced responses. To analyze the regulation of Stat6 phosphorylation, cells were cultured in the continuous presence of IL-4 or after a pulse and washout. In the continual presence of IL-4, Stat6 remained phosphorylated for an extended period. After IL-4 removal and inhibition of the Janus family kinase, tyrosine-phosphorylated Stat6 decayed at a rate dependent upon the length of IL-4 stimulation. The decay of tyrosine-phosphorylated Stat6 was similar in the presence or absence of either cycloheximide or actinomycin D. In the absence of functional Src homology-containing phosphatase-1 (SHP-1), the early loss of tyrosine-phosphorylated Stat6 was substantially reduced. Furthermore, the rate of loss of tyrosine-phosphorylated Stat6 in cells expressing a mutation of the human IL-4 receptor alpha in the immunoreceptor tyrosine-based inhibitory motif sequence (Y5F) was dramatically decreased compared with wild-type cells. The early rate of decay was similar in the presence or absence of MG132, an inhibitor of the proteasome, but the later rate of decay was decreased 5-fold. These results suggest that the loss of tyrosine phosphorylation of Stat6 is regulated by the action of SHP-1 and the proteasome but is not dependent on new protein synthesis.

Prolactin induces SHP-2 association with Stat5, nuclear translocation, and binding to the beta-casein gene promoter in mammary cells

The Src homology 2 (SH2) domain containing protein-tyrosine phosphatase SHP-2 contributes to prolactin receptor (PRLR) signal transduction to beta-casein gene promoter activation. We report for the first time that SHP-2 physically associates with the signal transducer and activator of transcription-5a (Stat5a), an important mediator of PRLR signaling to milk protein gene activation, in the mouse mammary HC11 and the human breast cancer T47D cells when stimulated with prolactin (PRL) and human growth hormone, respectively. In addition, overexpression studies indicate that the carboxyl-terminal SH2 domain of SHP-2 is required to maintain tyrosine phosphorylation of Stat5 and its interaction with SHP-2. Furthermore, we demonstrate by nuclear co-immunoprecipitation and indirect immunofluorescence studies that PRL stimulation of mammary cells leads to the nuclear translocation of SHP-2 as a complex with Stat5a. This process was found to involve the catalytic activity of the phosphatase. Finally, using the Stat5 GAS (gamma-activated sequence) element of the beta-casein gene promoter in electrophoretic mobility shift assays, we demonstrate that PRL induces the SHP-2-Stat5a complex to bind to DNA. The presence of the phosphatase in the protein-bound DNA complex was verified by using polyclonal antisera to SHP-2. Our studies indicate a tight physical and functional interaction between SHP2 and Stat5 required for regulation and perpetuation of PRL-mediated signaling in mammary cells and suggest a potential role for SHP-2 in the nucleus.

A nick-sensing DNA 3'-repair enzyme from Arabidopsis

DNA single-strand breaks, a major cause of genome instability, often produce unconventional end groups that must be processed to restore terminal moieties suitable for reparative DNA gap filling or ligation. Here, we describe a bifunctional repair enzyme from Arabidopsis (named AtZDP) that recognizes DNA strand breaks and catalyzes the removal of 3'-end-blocking lesions. The isolated C-terminal domain of AtZDP is by itself competent for 3'-end processing, but not for strand break recognition. The N-terminal domain instead contains three Cys(3)-His zinc fingers and recognizes various kinds of damaged double-stranded DNA. Gapped DNA molecules are preferential targets of AtZDP, which bends them by approximately 73 degrees upon binding, as measured by atomic force microscopy. Potential partners of AtZDP were identified in the Arabidopsis genome using the human single-strand break repairosome as a reference. These data identify a novel pathway for single-strand break repair in which a DNA-binding 3'-phosphoesterase acts as a "nick sensor" for damage recognition, as the catalyst of one repair step, and possibly as a nucleation center for the assembly of a fully competent repair complex.

The C-terminal C1 cassette of the N -methyl-D-aspartate receptor 1 subunit contains a bi-partite nuclear localization sequence

The N -methyl-D-aspartate receptor (NMDAR) is a multimeric transmembrane protein composed of at least two subunits. One subunit, NR1, is derived from a single gene and can be subdivided into three regions: the N-terminal extracellular domain, the transmembrane regions, and the C-terminal intracellular domain. The N-terminal domain is responsible for Mg2+ metal ion binding and channel activity, while the transmembrane domains are important for ion channel formation. The intracellular C-terminal domain is involved in regulating receptor activity and subcellular localization. Our recent experiments indicated that the intracellular C-terminal domain, when expressed independently, localizes almost exclusively in the nucleus. An examination of the amino acid sequence reveals the presence of a putative nuclear localization sequence (NLS) in the C1 cassette of the NR1 intracellular C-terminus. Using an expression vector designed to test whether a putative NLS sequence is a valid, functional NLS, we have demonstrated that a bi-partite NLS does in fact exist within the NR1-1 C-terminus. Computer algorithms identified a putative helix-loop-helix motif that spanned the C0C1 cassettes of the C-terminus. These data suggest that the NR1 subunit may represent another member of a family of transmembrane proteins that undergo intramembrane proteolysis, releasing a cytosolic peptide that is actively translocated to the nucleus leading to alterations in gene regulation.