Expression of dominant negative mutant SHPTP2 attenuates phosphatidylinositol 3'-kinase activity via modulation of phosphorylation of insulin receptor substrate-1

Protein and fat intake interacts with the haplotype of PTPN11_rs11066325, RPH3A_rs886477, and OAS3_rs2072134 to modulate serum HDL concentrations in middle-aged people

BACKGROUND & AIMS

Low serum HDL cholesterol (HDL-C) concentration is a risk factor for cardiovascular diseases and it is influenced by genetic and environmental factors. We hypothesized that genetic variants that decrease serum HDL-C concentrations may interact with nutrient intakes in ways that increase or decrease the risk of cardiovascular disease.

METHODS

Candidate genetic variants that can lower serum HDL-C concentrations were explored by genome-wide association studies (GWAS), after adjusting for covariates, in the Ansan/Ansung cohort (n = 8842) from KoGES. The best genetic variants were selected and used to form a haplotype. According to the haplotype frequencies of SNPs, they were divided into major allele, heterozygote allele, and minor allele. The association of haplotype with serum HDL-C levels was determined using logistic regression after adjusting for confounding factors. Interaction of the haplotype with nutrient intake was also determined.

RESULTS

PTPN11_rs11066325, RPH3A_rs886477 and OAS3_rs2072134 were selected to modulate serum HDL-C levels from GWAS(P = 1.09E-09, 7.04E-10, and 1.27E-09, respectively). The adjusted odds ratios (ORs) for a decrease in serum HDL-C concentration in the minor-allele group of the haplotype were elevated by 1.534 fold, compared to the major-allele group of the haplotype. Furthermore, the adjusted ORs for serum LDL cholesterol and levels increased by 1.645 in the minor-alleles compared to the major-alleles of the haplotype without a significant change of serum cholesterol levels. Interestingly, the adjusted ORs for serum triglyceride were lower in the minor-alleles than in the major-alleles. The haplotype had a significant interaction with the intake of protein, fat, saturated fatty acids (SAF) and polyunsaturated fatty acids (PUFA; P < 0.05). In particular, the minor alleles of the haplotype decreased serum HDL-C levels compared to the major-alleles in the high intake of protein, fat, SFA, and PUFA, not in the low intake.

CONCLUSIONS

People carrying the minor-allele of haplotypes should avoid diets that are high in protein and fat, especially rich in SFA and PUFA.

SHP-2 in Lymphocytes' Cytokine and Inhibitory Receptor Signaling

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

The effect of hematocrit and hemoglobin on the risk of ischemic heart disease: A Mendelian randomization study

Hematocrit and hemoglobin affect viscosity, and have been considered as risk factors for ischemic heart disease (IHD), although observations are inconsistent; randomized controlled trials targeting hematocrit or hemoglobin have not been definitive. To clarify their role, the risk of IHD was assessed according to genetically determined hematocrit and hemoglobin. We applied single nucleotide polymorphisms (SNPs) strongly determining hematocrit and hemoglobin, from a genome wide association study, to a large case (64,746) control (130,681) study of coronary artery disease, CARDIoGRAMplusC4D, to obtain unconfounded estimates using instrumental variable analysis by combining the Wald estimators for each SNP taking into account any correlation between SNPs using weighted generalized linear regression. Hematocrit was positively associated with IHD, odds ratio (OR) 1.07 per %, 95% confidence interval (CI) 1.03 to 1.11, before and after excluding SNPs from gene regions directly functionally relevant to IHD. However, hematocrit was not associated with IHD (OR 0.99, 0.94 to 1.04) after also excluding SNPs associated with lipids at genome wide significance. Hemoglobin was not associated with IHD (OR 1.06 per g/dL, 0.97 to 1.15) which was similar (OR 1.02, 0.94 to 1.11) after excluding SNPs from gene regions directly functionally relevant to IHD. Hemoglobin was negatively associated with IHD after also excluding SNPs associated with lipids at genome wide significance (OR 0.86, 0.78 to 0.94). In conclusion, hematocrit shares genetic determinants with IHD, but whether the genes contribute to IHD via hematocrit or other mechanisms is not entirely clear. Higher Hemoglobin is unlikely to cause IHD.

SHP2 sails from physiology to pathology

Over the two past decades, mutations of the PTPN11 gene, encoding the ubiquitous protein tyrosine phosphatase SHP2 (SH2 domain-containing tyrosine phosphatase 2), have been identified as the causal factor of several developmental diseases (Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NS-ML), and metachondromatosis), and malignancies (juvenile myelomonocytic leukemia). SHP2 plays essential physiological functions in organism development and homeostasis maintenance by regulating fundamental intracellular signaling pathways in response to a wide range of growth factors and hormones, notably the pleiotropic Ras/Mitogen-Activated Protein Kinase (MAPK) and the Phosphoinositide-3 Kinase (PI3K)/AKT cascades. Analysis of the biochemical impacts of PTPN11 mutations first identified both loss-of-function and gain-of-function mutations, as well as more subtle defects, highlighting the major pathophysiological consequences of SHP2 dysregulation. Then, functional genetic studies provided insights into the molecular dysregulations that link SHP2 mutants to the development of specific traits of the diseases, paving the way for the design of specific therapies for affected patients. In this review, we first provide an overview of SHP2's structure and regulation, then describe its molecular roles, notably its functions in modulating the Ras/MAPK and PI3K/AKT signaling pathways, and its physiological roles in organism development and homeostasis. In the second part, we describe the different PTPN11 mutation-associated pathologies and their clinical manifestations, with particular focus on the biochemical and signaling outcomes of NS and NS-ML-associated mutations, and on the recent advances regarding the pathophysiology of these diseases.

Polymorphisms of PTPN11 gene could influence serum lipid levels in a sex-specific pattern

Background

Previous studies have reported that different genotypes of PTPN11 gene (protein tyrosine phosphatase, non-receptor 11) were associated with different levels of serum lipids. The aim of this study was to explore the relationship between single nucleotide polymorphisms (SNPs) of PTPN11 and serum lipids in Northeast Chinese.

Methods

A total of 1003 subjects, 584 males and 419 females, were included in the study and their serum lipids were determined. Five htSNPs (rs2301756, rs12423190, rs12229892, rs7958372 and rs4767860) of PTPN11 gene were genotyped using TaqMan assay method.

Results

All of the five SNPs were in Hardy-Weinberg equilibrium. The male subjects had higher triglyceride (TG), higher low-density lipoprotein cholesterol (LDL-C) and lower high-density lipoprotein cholesterol (HDL-C) level than females. In males, rs4767860 was found to be associated with serum TG and total cholesterol (TC) levels and rs12229892 was associated with TC level. However, these significant associations could not be observed in females. In females, rs2301756 was found to be associated with TG and rs7958372 was associated with LDL-C level. Haplotype analysis showed that the GCGTG haplotype was associated with slightly higher TG level and ATGCG with higher TC level.

Conclusions

SNPs of PTPN11 may play a role in serum lipids in a sex-specific pattern. However, more studies are needed to confirm the conclusion and explore the underlying mechanism.

Hepatocyte-specific Ptpn6 deletion protects from obesity-linked hepatic insulin resistance

The protein-tyrosine phosphatase Shp1 negatively regulates insulin action on glucose homeostasis in liver and muscle, but its potential role in obesity-linked insulin resistance has not been examined. To investigate the role of Shp1 in hepatic insulin resistance, we generated hepatocyte-specific Shp1 knockout mice (Ptpn6(H-KO)), which were subjected to extensive metabolic monitoring throughout an 8-week standard chow diet (SD) or high-fat diet (HFD) feeding. We report for the first time that Shp1 expression is upregulated in metabolic tissues of HFD-fed obese mice. When compared with their Shp1-expressing Ptpn6(f/f) littermates, Ptpn6(H-KO) mice exhibited significantly lowered fasting glycemia and heightened hepatic insulin sensitivity. After HFD feeding, Ptpn6(H-KO) mice developed comparable levels of obesity as Ptpn6(f/f) mice, but they were remarkably protected from liver insulin resistance, as revealed by euglycemic clamps and hepatic insulin signaling determinations. Although Ptpn6(H-KO) mice still acquired diet-induced peripheral insulin resistance, they were less hyperinsulinemic during a glucose tolerance test because of reduced insulin secretion. Ptpn6(H-KO) mice also exhibited increased insulin clearance in line with enhanced CC1 tyrosine phosphorylation in liver. These results show that hepatocyte Shp1 plays a critical role in the development of hepatic insulin resistance and represents a novel therapeutic target for obesity-linked diabetes.

Regulation of Src homology 2-containing protein tyrosine phosphatase by advanced glycation end products: the role on atherosclerosis in diabetes

Advanced glycation end products (AGEs), among the most important causes of atherosclerosis in diabetes mellitus, stimulate the proliferation of smooth muscle cells (SMCs). Smooth muscle cells are central in the formation of atherosclerotic lesions, where they show both increased migration and accelerated proliferation. In investigating how AGEs stimulate SMC proliferation, we focused on protein tyrosine phosphatase, especially Src homology 2-containing protein tyrosine phosphatase (SHP2), which is considered important in regulating cell proliferation. Advanced glycation end products increased activity of SHP2 in the membrane fraction of rat aortic SMCs compared with control bovine serum albumin (P < .05). Upon characterizing the genomic and promoter structure of SHP2, we detected nuclear factor-kappaB (NF-kappaB) binding sites in the promoter area. Advanced glycation end product stimulation increased luciferase activity in cells transfected with SHP2 promoter region including NF-kappaB binding sites (P < .05) and increased SHP2 expression (P < .05). These data indicate that AGE stimulation appears to activate NF-kappaB. Activated NF-kappaB binds to sites on the SHP2 promoter, resulting in increased SHP2 expression, SHP2 activity, and, ultimately, SMC proliferation. It suggests that AGE stimulation induces SMC proliferation via SHP2, underscoring the importance of control of AGE for suppressing macroangiopathy in diabetes mellitus.

SHP-2 and PI3-kinase genes PTPN11 and PIK3R1 may influence serum apoB and LDL cholesterol levels in normal women

Insulin regulates apoB metabolism via activation of PI3K or regulation of MTP via MAPK/ERK signalling. SHP-2 enhances both pathways through increased IRS-1 phosphorylation. We hypothesized that variants in the SHP-2 gene PTPN11 and PI3K p85alpha subunit gene PIK3R1 may influence fasting levels of plasma apoB and/or LDL cholesterol. We tested association of tagging SNPs (tSNPs) in each gene with serum lipids in a large sample of unselected population-based Caucasian female twins (n=2771, mean age 47.4+/-12.5 years) and then tested interaction between tSNPs in determining apoB and LDL levels. PTPN11 tSNP rs11066322 was associated with apoB (P=0.007) and rs11066320 was associated with LDL cholesterol (P=0.016). PIK3R1 tSNP rs251406 was associated with apoB (P=0.0003) and rs706713 was associated with LDL cholesterol (P=0.009). PTPN11 tSNP rs11066322 interacted with PIK3R1 tSNP rs251406 in determining serum apoB levels (P=0.012) and with PIK3R1 tSNP rs40318 in determining LDL cholesterol levels (P=0.009). Association of single tSNPs with both apoB and LDL cholesterol as well as interactions between the two genes suggest that variants influencing SHP-2 activity may modulate the acute pathway by which insulin regulates these lipids.

Increased insulin-stimulated Akt pSer473 and cytosolic SHP2 protein abundance in human skeletal muscle following acute exercise and short-term training

The purpose of the present study was to determine in human skeletal muscle whether a single exercise bout and 7 days of consecutive endurance (cycling) training 1) increased insulin-stimulated Akt pSer(473) and 2) altered the abundance of the protein tyrosine phosphatases (PTPases), PTP1B and SHP2. In healthy, untrained men (n = 8; 24 +/- 1 yr), glucose infusion rate during a hyperinsulinemic euglycemic clamp, when compared with untrained values, was not improved 24 h following a single 60-min bout of endurance cycling but was significantly increased ( approximately 30%; P < 0.05) 24 h following completion of 7 days of exercise training. Insulin-stimulated Akt pSer(473) was approximately 50% higher (P < 0.05) 24 h following the acute bout of exercise, with this effect remaining after 7 days of training (P < 0.05). Insulin-stimulated insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation were not altered 24 h after acute exercise and short-term training. Insulin did not acutely regulate the localization of the PTPases, PTP1B or SHP2, although cytosolic protein abundance of SHP2 was increased (P < 0.05; main effect) 24 h following acute exercise and short-term training. In conclusion, insulin-sensitive Akt pSer(473) and cytosolic SHP2 protein abundance are higher after acute exercise and short-term training, and this effect appears largely due to the residual effects of the last bout of prior exercise. The significance of exercise-induced alterations in cytosolic SHP2 and insulin-stimulated Akt pSer(473) on the improvement in insulin sensitivity requires further elucidation.

Recruitment of the tyrosine phosphatase Src homology 2 domain tyrosine phosphatase-2 to the p85 subunit of phosphatidylinositol-3 (PI-3) kinase is required for insulin-like growth factor-I-dependent PI-3 kinase activation in smooth muscle cells

IGF-I stimulates smooth muscle cell (SMC) migration and the phosphatidylinositol-3 (PI-3) kinase pathway plays an important role in mediating the IGF-I-induced migratory response. Prior studies have shown that the tyrosine phosphatase Src homology 2 domain tyrosine phosphatase (SHP)-2 is necessary to activate PI-3 kinase in response to growth factors and expression of a phosphatase inactive form of SHP-2 (SHP-2/C459S) impairs IGF-I-stimulated cell migration. However, the mechanism by which SHP-2 phosphatase activity or the recruitment of SHP-2 to other signaling molecules contributes to IGF-I stimulated PI-3 kinase activation has not been determined. SMCs that had stable expression of SHP-2/C459S had reduced cell migration and Akt activation in response to IGF-I, compared with SMC-expressing native SHP-2. Similarly in cells expressing native SHP-2, IGF-I induced SHP-2 binding to p85, whereas in cells expressing SHP-2/C459S, there was no increase. Because the C459S substitution results in loss of the ability of SHP-2 to disassociate from its substrates, making it inaccessible not only to p85 but also the other proteins, a p85 mutant in which tyrosines 528 and 556 were changed to phenylalanines was prepared to determine whether this would disrupt the p85/SHP-2 interaction and whether the loss of this specific interaction would alter IGF-I stimulated the cell migration. Substitution for these tyrosines in p85 resulted in loss of SHP-2 recruitment and was associated with a reduction in association of the p85/p110 complex with insulin receptor substrate-1. Cells stably expressing this p85 mutant also showed a decrease in IGF-I-stimulated PI-3 kinase activity and cell migration. Preincubation of cells with a cell-permeable peptide that contains the tyrosine556 motif of p85 also disrupted SHP-2 binding to p85 and inhibited the IGF-I-induced increase in cell migration. The findings indicate that tyrosines 528 and 556 in p85 are required for SHP-2 association. SHP-2 recruitment to p85 is required for IGF-I-stimulated association of the p85/p110 complex with insulin receptor substrate-1 and for the subsequent activation of the PI-3 kinase pathway leading to increased cell migration.

The src homology 2 domain-containing tyrosine phosphatase 2 regulates primary T-dependent immune responses and Th cell differentiation

The src homology 2 domain-containing tyrosine phosphatase 2 (SHP2) plays an important role in development and in growth factor receptor signaling pathways, yet little is known of its role in the immune system. We generated mice expressing a dominant-negative version of the protein, SHP2(CS), specifically in T cells. In SHP2(CS) mice, T cell development appears normal with regard to both negative and positive selection. However, SHP2(CS) T cells express higher levels of activation markers, and aged mice have elevated serum Abs. This is associated with a marked increase in IL-4, IL-5, and IL-10 secretion by SHP2(CS) T cells in vitro. In addition, primary thymus-dependent B cell responses are deficient in SHP2(CS) mice. We show that whereas TCR-induced linker for activation of T cells phosphorylation is defective, CTLA-4 and programmed death-1 signaling are not affected by SHP2(CS) expression. Our results suggest that a key action of wild-type SHP2 is to suppress differentiation of T cells to the Th2 phenotype.

Supplement of tetrahydrobiopterin by a gene transfer of GTP cyclohydrolase I cDNA improves vascular dysfunction in insulin-resistant rats

Deficiency of tetrahydrobiopterin (BH4) in the vascular tissue contributes to endothelial dysfunction in the insulin-resistant state. We intended to develop a new gene transfer method by overexpression of its biosynthetic enzyme, GTP cyclohydrolase I (GTP-CH1). The GTP-CH1 cDNA was inserted into a pCAGGS vector, and then plasmid DNA was mixed with atelocollagen, and the aliquot was injected into thigh muscles of insulin-resistant Zucker fatty rats. After 4 weeks, pteridine derivative levels, superoxide anion (O2-), activity of endothelial nitric oxide synthase (eNOS), and endothelium-dependent relaxation were evaluated in the aortas obtained from Zucker lean or fatty rats. The BH4 contents and GTP-CH1 activity in Zucker fatty rats were 50%-55% less than those of Zucker lean rats. However, those impairments were significantly improved by a plasmid DNA injection, and aortic BH4 content reached more than 80% of the level of Zucker lean rats. Increased A23187-stimulated O2- production as well as decreased eNOS activity and endothelial function in insulin-resistant Zucker fatty rats were improved by a plasmid DNA injection to a level similar to that in Zucker lean rats. These findings suggest that intramuscular GTP-CH1 gene transfer using atelocollagen serves as a useful method of long-term systemic delivery of BH4 and the treatment of endothelial dysfunction.

Shp2 is required for protein kinase C-dependent phosphorylation of serine 307 in insulin receptor substrate-1

The function of insulin receptor substrate-1 (IRS-1), a key molecule of insulin signaling, is modulated by phosphorylation at multiple serine/threonine residues. Phorbol ester stimulation of cells induces phosphorylation of two inhibitory serine residues in IRS-1, i.e. Ser-307 and Ser-318, suggesting that both sites may be targets of protein kinase C (PKC) isoforms. However, in an in vitro system using a broad spectrum of PKC isoforms (alpha, beta1, beta2, delta, epsilon, eta, mu), we detected only Ser-318, but not Ser-307 phosphorylation, suggesting that phorbol ester-induced phosphorylation of this site in intact cells requires additional signaling elements and serine kinases that link PKC activation to Ser-307 phosphorylation. As we have observed recently that the tyrosine phosphatase Shp2, a negative regulator of insulin signaling, is a substrate of PKC, we studied the role of Shp2 in this context. We found that phorbol ester-induced Ser-307 phosphorylation is reduced markedly in Shp2-deficient mouse embryonic fibroblasts (Shp2-/-) whereas Ser-318 phosphorylation is unaltered. The Ser-307 phosphorylation was rescued by transfection of mouse embryonic fibroblasts with wild-type Shp2 or with a phosphatase-inactive Shp2 mutant, respectively. In this cell model, tumor necrosis factor-alpha-induced Ser-307 phosphorylation as well depended on the presence of Shp2. Furthermore, Shp2-dependent phorbol ester effects on Ser-307 were blocked by wortmannin, rapamycin, and the c-Jun NH2-terminal kinase (JNK) inhibitor SP600125. This suggests an involvement of the phosphatidylinositol 3-kinase/mammalian target of rapamycin cascade and of JNK in this signaling pathway resulting in IRS-1 Ser-307 phosphorylation. Because the activation of these kinases does not depend on Shp2, it is concluded that the function of Shp2 is to direct these activated kinases to IRS-1.

Protein tyrosine phosphatases: their role in insulin action and potential as drug targets

Protein tyrosine phosphatases (PTPases) are the enzymes responsible for the selective dephosphorylation of tyrosine residues. PTPases function to regulate a wide array of biological responses mediated by growth factors and other stimuli by balancing the cellular level of phosphotyrosine in concert with their counterparts, protein tyrosine kinases. The important roles which PTPases play in regulating intracellular signalling and, ultimately, biological function along with the recent availability of information regarding their structural features has highlighted them as potential targets for pharmacological modulation. This is demonstrated by the increased level of activity directed towards the identification of novel small-molecule PTPase inhibitors. The rationale and potential utility of this drug discovery approach is discussed here, with particular emphasis on its application for the treatment of insulin resistance and Type 2 diabetes.

Evidence for a Causal Role of the Renin-Angiotensin System in Vascular Dysfunction Associated With Insulin Resistance

Excess production of superoxide anion in response to angiotensin II plays a central role in the transduction of signal molecules and the regulation of vascular tone. We examined the ability of insulin resistance to stimulate superoxide anion production and investigated the identity of the oxidases responsible for its production. Rats were fed diets containing 60% fructose (fructose-fed rats) or 60% starch (control rats) for 8 weeks. In aortic homogenates from fructose-fed rats, the superoxide anion generated in response to NAD(P)H was more than 2-fold higher than that of control rats. Pretreatment of the aorta from fructose-fed rats with inhibitors of NADPH oxidase significantly reduced superoxide anion production. In the isolated aorta, contraction induced by angiotensin II was more potent in fructose-fed rats compared with control rats. Losartan normalized blood pressure, NAD(P)H oxidase activity, endothelial function, and angiotensin II-induced vasoconstriction in fructose-fed rats. To elucidate the molecular mechanisms of the enhanced constrictor response to angiotensin II, expressions of angiotensin II receptor and subunits of NADPH oxidase were examined with the use of angiotensin II type 1a receptor knockout (AT1a KO) mice. Expression of AT1a receptor mRNA was enhanced in fructose-fed mice, whereas expression of either AT1b or AT2 was unaltered. In addition, protein expression of each subunit of NADPH oxidase was increased in fructose-fed mice, whereas the expression was significantly decreased in fructose-fed AT1a KO mice. The novel observation of insulin resistance-induced upregulation of AT1 receptor expression could explain the association of insulin resistance with endothelial dysfunction and hypertension.

KIR2DL4 is an IL-2-regulated NK cell receptor that exhibits limited expression in humans but triggers strong IFN-gamma production

Killer cell Ig-like receptor (KIR)2DL4 (2DL4, CD158d) was previously described as the only KIR expressed by every human NK cell. It is also structurally atypical among KIRs because it possesses a basic transmembrane residue, which is characteristic of many activating receptors, but also contains a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). We expressed epitope-tagged 2DL4 in an NK-like cell line to study receptor function. Three distinct 2DL4 cDNA clones were analyzed: one encoding the "conventional" 2DL4 with the cytoplasmic ITIM (2DL4.1) and two encoding different cytoplasmic truncated forms lacking the ITIM (2DL4.2 and 2DL4(*)). Surprisingly, one truncated receptor (2DL4.2), which is the product of a prevalent human 2DL4 allele, was not expressed on the cell surface, indicating that some individuals may lack functional 2DL4 protein expression. Conversely, both 2DL4.1 and 2DL4(*) were expressed on the cell surface and up-regulated by IL-2. Analysis of primary NK cells with anti-2DL4 mAb confirmed the lack of surface expression in a donor with the 2DL4.2 genotype. Donors with the 2DL4.1 genotype occasionally expressed receptor only on CD56(high) NK cells, although their expression was up-regulated by IL-2. Interestingly, Ab engagement of epitope-tagged 2DL4 triggered rapid and robust IFN-gamma production, but weak redirected cytotoxicity in an NK-like cell line, which was the opposite pattern to that observed upon engagement of another NK cell activating receptor, NKp44. Importantly, both 2DL4.1 and 2DL4(*) exhibited similar activation potential, indicating that the ITIM does not influence 2DL4.1 activating function. The unique activation properties of 2DL4 suggest linkage to a distinct signaling pathway.

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

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

The association between integrin-associated protein and SHPS-1 regulates insulin-like growth factor-I receptor signaling in vascular smooth muscle cells

Growth factor signaling is usually analyzed in isolation without considering the effect of ligand occupancy of transmembrane proteins other than the growth factor receptors themselves. In smooth muscle cells, the transmembrane protein Src homology 2 domain containing protein tyrosine phosphatase substrate-1 (SHPS-1) has been shown to be an important regulator of insulin-like growth factor-I (IGF-I) signaling. SHPS-1 is phosphorylated in response to IGF-I, leading to recruitment of Src homology 2 domain tyrosine phosphatase (SHP-2). Subsequently, SHP-2 is transferred to IGF-I receptor and regulates the duration of IGF-I receptor phosphorylation. Whether ligand occupancy of SHPS-1 influences SHPS-1 phosphorylation or SHP-2 recruitment, thereby altering growth factor signaling, is unknown. Previous studies have shown that integrin associated protein (IAP) associates with SHPS-1. We undertook these studies to determine whether this interaction controlled SHPS-1 phosphorylation and/or SHP-2 recruitment and thereby regulated IGF-I signaling. Disruption of IAP-SHPS-1 binding, by using an IAP monoclonal antibody or cells expressing mutant forms of IAP that did not bind to SHPS-1, inhibited IGF-I-stimulated SHPS-1 phosphorylation and SHP-2 recruitment. This was associated with a lack of SHP-2 transfer to IGF-I receptor and sustained receptor phosphorylation. This resulted in an inability of IGF-I to stimulate sustained mitogen-activated protein kinase activation, cell proliferation, and cell migration. The effect was specific for IGF-I because disruption of the IAP-SHPS-1 interaction had no effect on platelet-derived growth factor-stimulated SHPS-1 phosphorylation or cell migration. In summary, our results show that 1) ligand occupancy of SHPS-1 is a key determinant of its ability to be phosphorylated after IGF-I stimulation, and 2) the interaction between IAP and SHPS-1 is an important regulator of IGF-I signaling because disruption of the results in impaired SHP-2 recruitment and subsequent inhibition of IGF-I-stimulated cell proliferation and migration.

Src homology region 2-containing protein tyrosine phosphatase-2 (SHP-2) can play a direct role in the inhibitory function of killer cell Ig-like receptors in human NK cells

The inhibitory forms of killer cell Ig-like receptors (KIR) are MHC class I-binding receptors that are expressed by human NK cells and prevent their attack of normal cells. Substantial evidence indicates that the mechanism of KIR-mediated inhibition involves recruitment of the protein tyrosine phosphatase, Src homology region 2-containing protein tyrosine phosphatase (SHP)-1, to phosphorylated immunoreceptor tyrosine-based inhibitory motifs (ITIMs). However, the functional significance of parallel recruitment of a SHP-1-related phosphatase, SHP-2, to KIR ITIMs has not been addressed. In the present study, our results with mutant forms of a classical KIR, KIR3DL1, show a direct correlation between SHP-2 recruitment and functional inhibition of target cell conjugation and cytotoxicity. In addition, KIR3DL1 inhibition of target cell cytotoxicity is blocked by overexpression of a dominant-negative form of SHP-2. Finally, KIR3DL1 fused directly with the catalytic domain of SHP-2 inhibits both target cell conjugation and cytotoxicity responses. These results strongly indicate that SHP-2 catalytic activity plays a direct role in inhibitory KIR functions, and SHP-2 inhibits NK cell activation in concert with SHP-1.

Role of SRC homology 2-containing tyrosine phosphatase 2 on proliferation of rat smooth muscle cells

OBJECTIVE

Src homology 2-containing phosphotyrosine phosphatase 2 (SHP2) is ubiquitously expressed and believed to function as part of a positive signaling pathway mediating growth factor-induced protein tyrosine phosphorylation. Proliferation of aortic vascular smooth muscle cells (SMCs) is an important contributor to atherosclerosis. We examined the effect of SHP2 expression on SMC proliferative activity.

METHODS AND RESULTS

SHP2 was abundant in cultured aortic SMCs, and SHP2 staining was markedly increased in the thickened aortic intima in rats with balloon-induced injury. We obtained several SMC clones by using geneticin screening. Endogenous SHP2 expression varied among individual clones. Significant positive relationships were observed between SHP2 expression and bromodeoxyuridine uptake in SMCs stimulated by FBS, platelet-derived growth factor, or insulin-like growth factor-1. In SMCs transiently transfected with SHP2, FBS stimulation significantly increased bromodeoxyuridine uptake beyond the uptake by control SMCs.

CONCLUSIONS

Increased SHP2 expression in SMCs may accelerate aortic atherosclerosis by increasing cell growth.

Receptor-specific regulation of phosphatidylinositol 3'-kinase activation by the protein tyrosine phosphatase Shp2

Receptor tyrosine kinases (RTKs) play distinct roles in multiple biological systems. Many RTKs transmit similar signals, raising questions about how specificity is achieved. One potential mechanism for RTK specificity is control of the magnitude and kinetics of activation of downstream pathways. We have found that the protein tyrosine phosphatase Shp2 regulates the strength and duration of phosphatidylinositol 3'-kinase (PI3K) activation in the epidermal growth factor (EGF) receptor signaling pathway. Shp2 mutant fibroblasts exhibit increased association of the p85 subunit of PI3K with the scaffolding adapter Gab1 compared to that for wild-type (WT) fibroblasts or Shp2 mutant cells reconstituted with WT Shp2. Far-Western analysis suggests increased phosphorylation of p85 binding sites on Gab1. Gab1-associated PI3K activity is increased and PI3K-dependent downstream signals are enhanced in Shp2 mutant cells following EGF stimulation. Analogous results are obtained in fibroblasts inducibly expressing dominant-negative Shp2. Our results suggest that, in addition to its role as a positive component of the Ras-Erk pathway, Shp2 negatively regulates EGF-dependent PI3K activation by dephosphorylating Gab1 p85 binding sites, thereby terminating a previously proposed Gab1-PI3K positive feedback loop. Activation of PI3K-dependent pathways following stimulation by other growth factors is unaffected or decreased in Shp2 mutant cells. Thus, Shp2 regulates the kinetics and magnitude of RTK signaling in a receptor-specific manner.

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

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

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

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

Thrombopoietin induces phosphoinositol 3-kinase activation through SHP2, Gab, and insulin receptor substrate proteins in BAF3 cells and primary murine megakaryocytes

Thrombopoietin (TPO) is a recently characterized member of the hematopoietic growth factor family that serves as the primary regulator of megakaryocyte (MK) and platelet production. The hormone acts by binding to the Mpl receptor, the product of the cellular proto-oncogene c-mpl. Although many downstream signaling targets of TPO have been identified in cell lines, primary MKs, and platelets, the molecular mechanism(s) by which many of these molecules are activated remains uncertain. In this report we demonstrate that the TPO-induced activation of phosphoinositol 3-kinase (PI3K), a signaling intermediate vital for cellular survival and proliferation, occurs through its association with inducible signaling complexes in both BaF3 cells engineered to express Mpl (BaF3/Mpl) and in primary murine MKs. Although a direct association between PI3K and Mpl could not be demonstrated, we found that several proteins, including SHP2, Gab2, and IRS2, undergo phosphorylation and association in BaF3/Mpl cells in response to TPO stimulation, complexes that recruit and enhance the enzymatic activity of PI3K. To verify the physiological relevance of the complex, SHP2-Gab2 association was disrupted by overexpressing a dominant negative SHP2 construct. TPO-induced Akt phosphorylation was significantly decreased in transfected cells suggesting an important role of SHP2 in the complex to enhance PI3K activity. In primary murine MKs, TPO also induced phosphorylation of SHP2, its association with p85 and enhanced PI3K activity, but in contrast to the results in cell lines, neither Gab2 nor IRS2 are phosphorylated in MKs. Instead, a 100-kDa tyrosine-phosphorylated protein (pp100) co-immunoprecipitated with the regulatory subunit of PI3K. These findings support a model where PI3K activity is dependent on its recruitment into TPO-induced multiphosphoprotein complexes, implicate the existence of a scaffolding protein in primary MKs distinct from the known Gab and IRS proteins, and suggest that, in contrast to erythroid progenitor cells that employ Gab1 in PI3K signaling complexes, utilization of an alternate member of the Gab/IRS family could be responsible for specificity in TPO signaling.

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

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

Increased energy expenditure, decreased adiposity, and tissue-specific insulin sensitivity in protein-tyrosine phosphatase 1B-deficient mice

Protein-tyrosine phosphatase 1B (PTP-1B) is a major protein-tyrosine phosphatase that has been implicated in the regulation of insulin action, as well as in other signal transduction pathways. To investigate the role of PTP-1B in vivo, we generated homozygotic PTP-1B-null mice by targeted gene disruption. PTP-1B-deficient mice have remarkably low adiposity and are protected from diet-induced obesity. Decreased adiposity is due to a marked reduction in fat cell mass without a decrease in adipocyte number. Leanness in PTP-1B-deficient mice is accompanied by increased basal metabolic rate and total energy expenditure, without marked alteration of uncoupling protein mRNA expression. In addition, insulin-stimulated whole-body glucose disposal is enhanced significantly in PTP-1B-deficient animals, as shown by hyperinsulinemic-euglycemic clamp studies. Remarkably, increased insulin sensitivity in PTP-1B-deficient mice is tissue specific, as insulin-stimulated glucose uptake is elevated in skeletal muscle, whereas adipose tissue is unaffected. Our results identify PTP-1B as a major regulator of energy balance, insulin sensitivity, and body fat stores in vivo.

A high fructose diet affects the early steps of insulin action in muscle and liver of rats

A high fructose diet induces insulin resistance in rats, although the exact molecular mechanism involved is unknown. In this study, we used immunoprecipitation and immunoblotting to examine the levels and phosphorylation status of the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1), as well as the association of the IRS-1 with phosphatidylinositol 3-kinase (PI 3-kinase), and phosphotyrosine phosphatase (SHP2) in the liver and muscle of rats fed a control or high fructose diet for 28 d. There were no differences in IR and the IRS-1 protein levels in the liver and muscle of rats fed the control and high fructose diets. However, tyrosine-phosphorylation of the insulin receptor after insulin stimulation was reduced to 71 +/- 2% (P < 0.05) of control in the liver of the fructose-fed rats. In samples previously immunoprecipitated with anti-IRS-1 antibody and blotted with antiphosphotyrosine antibody, the insulin-stimulated IRS-1 phosphorylation levels in the liver and muscle of the fructose-fed group were only 70 +/- 6% (P < 0.05) and 76 +/- 5% (P < 0.05) of those of control rats, respectively. The insulin-stimulated IRS-1 association with PI 3-kinase was reduced to 84 +/- 3% (P < 0.05) in the liver and to 84 +/- 4% (P < 0.05) in the muscle of the fructose-fed group compared with control rats. Insulin-stimulated IRS-1 association with SHP2 was reduced to 79 +/- 5% (P < 0.05) in liver of the fructose-fed rats. These data suggest that changes in the early steps of insulin signal transduction may have an important role in the insulin resistance observed in these rats.

Expression of a dominant negative SHP-2 in transgenic mice induces insulin resistance

To elucidate the roles of SHP-2, we generated transgenic (Tg) mice expressing a dominant negative mutant lacking protein tyrosine phosphatase domain (DeltaPTP). On examining two lines of Tg mice identified by Southern blot, the transgene product was expressed in skeletal muscle, liver, and adipose tissues, and insulin-induced association of insulin receptor substrate 1 with endogenous SHP-2 was inhibited, confirming that DeltaPTP has a dominant negative property. The intraperitoneal glucose loading test demonstrated an increase in blood glucose levels in Tg mice. Plasma insulin levels in Tg mice after 4 h fasting were 3 times greater with comparable blood glucose levels. To estimate insulin sensitivity by a constant glucose, insulin, and somatostatin infusion, steady state blood glucose levels were higher, suggesting the presence of insulin resistance. Furthermore, we observed the impairment of insulin-stimulated glucose uptake in muscle and adipocytes in the presence of physiological concentrations of insulin. Moreover, tyrosine phosphorylation of insulin receptor substrate-1 and stimulation of phosphatidylinositol 3-kinase and Akt kinase activities by insulin were attenuated in muscle and liver. These results indicate that the inhibition of endogenous SHP-2 function by the overexpression of a dominant negative mutant may lead to impaired insulin sensitivity of glucose metabolism, and thus SHP-2 may function to modulate insulin signaling in target tissues.

Dissociation of PTPase levels from their modulation of insulin receptor signal transduction

Protein tyrosine phosphatases have been implicated in the regulation of receptor tyrosine kinase signalling pathways, including that of the insulin receptor. Here, cell density-dependent changes in PTPase expression have been exploited to investigate the relationship between cellular PTPase levels and the insulin receptor signal transduction pathway. Increasing cell density (20%, 50%, and >90%) in the rat McA-RH7777 hepatoma cell line resulted in increased protein expression of the receptor-like PTPase LAR (14-fold), and the nonreceptor PTPases PTP1B (11-fold) and SHP2 (10-fold). Each of these PTPases has previously been implicated in regulating insulin receptor signal transduction. Despite these marked increases, maximum insulin receptor autophosphorylation as well as receptor expression actually increased 2-fold. MAP kinase also increased approximately 2-fold as a function of cell density and paralleled increases in expression levels. Neither sensitivity nor maximum responsiveness to insulin were decreased at increasing cell densities as assessed by activation of PI 3-kinase. Duration of response was also unimpaired. These results suggest that expression levels of relevant PTPases are not the primary determinant in their modulation of insulin receptor kinase activity. Restricted accessibility at the molecular level or involvement of accessory proteins may be more critical parameters.

Discrete expression of insulin receptor substrate-4 mRNA in adult rat brain

The insulin receptor substrate (IRS) proteins are a family of important regulatory adapters that mediate the coupling between receptor-associated tyrosine kinases and downstream effectors including phosphatidylinositol-3'-kinases (PI-3-Ks). In the present study, the distribution of IRS-4 mRNA was determined in rat brain by in situ hybridization. IRS-4 mRNA was widely expressed throughout the hypothalamus, with the most dense labeling observed in the medial preoptic nucleus, ventromedial hypothalamus, and arcuate nucleus. In contrast, and unlike IRS-1 or IRS-2, expression of IRS-4 mRNA in other forebrain and midbrain regions was much more restricted. The expression of IRS-4 mRNA in the hypothalamus suggests a specific role for this factor in the signaling of one or more receptors involved hypothalamic functions including feeding, lactation, sexual and parental behaviors.

Dual signaling role of the protein tyrosine phosphatase SHP-2 in regulating expression of acute-phase plasma proteins by interleukin-6 cytokine receptors in hepatic cells

One of the major actions of interleukin-6 (IL-6) is the transcriptional activation of acute-phase plasma proteins (APP) genes in liver cells. Signaling by the IL-6 receptor is mediated through the signal transducing subunit gp130 and involves the activation of Janus-associated kinases (JAKs), signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein (MAP) kinase. Functional analysis of gp130 in rat hepatoma cells by using transduced chimeric G-CSFR-gp130 receptor constructs demonstrates that SHP-2, the Src homology 2 (SH2) domain-containing protein tyrosine phosphatase, acts as a negative regulator of the JAK/STAT signaling in part by downregulating JAK activity, thereby indirectly moderating the induction of STAT3-dependent APP genes. This study shows that in hepatoma cells, the recruitment and tyrosine phosphorylation of SHP-2, but not SHC, is the primary signaling event associated with the activation of MAP kinases (ERK1/2) by gp130. Overexpression of truncated SHP-2 that lacks Grb2-interacting sites, but not the full-length catalytically inactive SHP-2, reduces ERK activation by IL-6, confirming the signal-mediating role of SHP-2. Activation of ERK1/2 is correlated with induction of the immediate-early response genes. Stimulation of the c-fos, c-jun, and egr-1 genes is essentially absent in cells expressing gp130 with a Y759F mutation, which is unable to recruit SHP-2. Interestingly, both JAK/STAT and SHP-2 pathways regulate the induction of the junB gene. Moreover, disengagement of SHP-2 from gp130 signaling not only enhances APP gene induction but also further reduces cell proliferation, in part correlated with the attenuated expression of immediate-early response genes. These results suggest that IL-6 regulation of APP genes is affected by SHP-2 in two ways: SHP-2 acts as a phosphatase on the JAK/STAT pathway and serves as linker to the MAP kinase pathway, which in turn moderates APP production.

The phosphotyrosine phosphatase SHP-2 participates in a multimeric signaling complex and regulates T cell receptor (TCR) coupling to the Ras/mitogen-activated protein kinase (MAPK) pathway in Jurkat T cells

Src homology 2 (SH2) domain-containing phosphotyrosine phosphatases (SHPs) are increasingly being shown to play critical roles in protein tyrosine kinase-mediated signaling pathways. The role of SHP-1 as a negative regulator of T cell receptor (TCR) signaling has been established. To further explore the function of the other member of this family, SHP-2, in TCR-mediated events, a catalytically inactive mutant SHP-2 was expressed under an inducible promoter in Jurkat T cells. Expression of the mutant phosphatase significantly inhibited TCR-induced activation of the extracellular-regulated kinase (ERK)-2 member of the mitogen-activated protein kinase (MAPK) family, but had no effect on TCR-zeta chain tyrosine phosphorylation or TCR-elicited Ca2+ transients. Inactive SHP-2 was targeted to membranes resulting in the selective increase in tyrosine phosphorylation of three membrane-associated candidate SHP-2 substrates of 110 kD, 55-60 kD, and 36 kD, respectively. Analysis of immunoprecipitates containing inactive SHP-2 also indicated that the 110-kD and 36-kD Grb-2-associated proteins were putative substrates for SHP-2. TCR-stimulation of Jurkat T cells expressing wild-type SHP-2 resulted in the formation of a multimeric cytosolic complex composed of SHP-2, Grb-2, phosphatidylinositol (PI) 3'-kinase, and p110. A significant proportion of this complex was shown to be membrane associated, presumably as a result of translocation from the cytosol. Catalytically inactive SHP-2, rather than the wild-type PTPase, was preferentially localized in complex with Grb-2 and the p85 subunit of PI 3'-kinase, suggesting that the dephosphorylating actions of SHP-2 may regulate the association of these signaling molecules to the p110 complex. Our results show that SHP-2 plays a critical role in linking the TCR to the Ras/MAPK pathway in Jurkat T cells, and also provide some insight into the molecular interactions of SHP-2 that form the basis of this signal transduction process.

Interleukin-3 induces association of the protein-tyrosine phosphatase SHP2 and phosphatidylinositol 3-kinase with a 100-kDa tyrosine-phosphorylated protein in hemopoietic cells

We have observed previously the co-immunoprecipitation of the p85 subunit of phosphatidylinositol-3 kinase (PI3K) and SHP2 in murine lymphohemopoietic cells after stimulation with interleukin-3. We have investigated this interaction in more detail and now report the identification of a potentially novel 100-kDa protein (termed p100), which is inducibly phosphorylated on tyrosine after interleukin-3 treatment and which co-immunoprecipitates with both p85 PI3K and SHP2. The Src homology region 2 domains of both p85 and SHP2 appear to mediate their interactions with p100. Sequential precipitation analyses suggest that these interactions are direct and do not involve Grb2, and that the same p100 protein, or a portion of it, interacts with both p85 and SHP2, implying that p100 may serve to link these two proteins. Far Western blotting with both full-length p85 and isolated p85 Src homology region 2 domains supports this view. Interestingly, p100 also appears to be a substrate for the SHP2 phosphatase activity. In addition, p100 is precipitated by Grb2-glutathione S-transferase fusion proteins, an interaction largely mediated by the Grb2 SH3 domains. p100 appears to be distinct from JAK2, Vav, STAT5, and c-Cbl. Although largely cytosolic, p100 can be detected associated with SHP2 and PI3K in crude membrane fractions after interleukin-3 stimulation. We propose that p100 plays a role as an adaptor molecule, linking PI3K and SHP2 in IL-3 signaling.

The IRS-signalling system during insulin and cytokine action

The discovery of the first intracellular substrate for insulin, IRS-1, redirected the field of diabetes research and has led to many important advances in our understanding of insulin action. Detailed analysis of IRS-1 demonstrates structure/function relationships for this modular docking molecule, including mechanisms of substrate recognition and signal propagation. Recent work has also identified other structurally similar molecules, including IRS-2, the Drosophila protein, DOS, and the Grb2-binding protein, Gab1, suggesting that this intracellular signalling strategy is conserved evolutionarily and is utilized by an expanding number of receptor systems. In fact, IRS-1 itself has been shown to be important in other growth factor and cytokine signalling systems, including growth hormone and several interleukins. Analysis of mice lacking IRS-1 confirms an important physiological role for this protein in glucose metabolism and general cell growth in the intact animal. Disregulation of the signalling pathways integrated by the IRS proteins may contribute to the pathophysiology of non-insulin-dependent diabetes mellitus or other diseases.

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

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

Reduced cell attachment and phosphorylation of focal adhesion kinase associated with expression of a mutant insulin receptor

Insulin signaling results in rapid changes to the cell cytoskeleton, and it has recently been shown that insulin stimulates the dephosphorylation of the cytoskeletal-associated tyrosine kinase, focal adhesion kinase (pp125(FAK)). We report here that mutation of two tryptic cleavage sites (Lys164 and Lys582 --> Asn; 2N) in the insulin receptor alpha-subunit results in a cell-line (CHO.2N-10) with altered morphology associated with an increase in cell size, a decrease in cell adhesiveness, and a decrease in pp125(FAK) tyrosine phosphorylation in the absence of insulin (45.2 +/- 9.7% compared to nontransfected Chinese hamster ovary (CHO) cells). In contrast to pp125(FAK), paxillin phosphorylation was similar in all cell lines despite lower levels (61.0 +/- 10.4% compared to CHO cells) of paxillin protein in CHO.2N-10 cells. We observed comparable protein levels of pp125(FAK) and the structural focal adhesion protein, vinculin, in all cell lines. Despite underphosphorylation of pp125(FAK) in the basal state, insulin stimulation of CHO.2N-10 cells still resulted in dephosphorylation of pp125(FAK). CHO.2N-10 and CHO.T (overexpress wild-type insulin receptor) cells have similar insulin binding characteristics, insulin-stimulated autokinase and peptide phosphorylation, and insulin-stimulated pp185/IRS-1 phosphorylation. Our results suggest that the insulin receptor may play an important role in cell-matrix interactions, such as modulating cell adhesion and inducing cell architecture changes.