Requirement of Shp-2 tyrosine phosphatase in lymphoid and hematopoietic cell development

Structural and functional effects of disease-causing amino acid substitutions affecting residues Ala72 and Glu76 of the protein tyrosine phosphatase SHP-2

Mutations of the protein tyrosine phosphatase SHP-2 are implicated in human diseases, causing Noonan syndrome (NS) and related developmental disorders or contributing to leukemogenesis depending on the specific amino acid substitution involved. SHP-2 is composed by a catalytic (PTP) and two regulatory (N-SH2 and C-SH2) domains that bind to signaling partners and control the enzymatic activity by limiting the accessibility of the catalytic site. Wild type SHP-2 and four disease-associated mutants recurring in hematologic malignancies (Glu76Lys and Ala72Val) or causing NS (Glu76Asp and Ala72Ser), with affected residues located in the PTP-interacting region of the N-SH2 domain, were analyzed by molecular dynamics simulations and in vitro biochemical assays. Simulations demonstrate that mutations do not affect significantly the conformation of the N-SH2 domain. Rather they destabilize the interaction of this domain with the catalytic site, with more evident effects in the two leukemia associated mutants. Consistent with this structural evidence, mutants exhibit an increased level of basal phosphatase activity in the order Glu76Lys > Ala72Val > Glu76Asp > Ala72Ser > WT. The experimental data also show that the mutants with higher basal activity are more responsive to an activating phosphopeptide. A thermodynamic analysis demonstrates that an increase in the overall phosphopeptide affinity of mutants can be explained by a shift in the equilibrium between the inactive and active SHP-2 structure. These data support the view that an increase in the affinity of SHP-2 for its binding partners, caused by destabilization of the closed, inactive conformation, rather than protein basal activation per se, would represent the molecular mechanism, leading to pathogenesis in these mutants.

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

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

Licensing of natural killer cells by self-major histocompatibility complex class I

Natural killer (NK) cells have potent capacities to immediately kill cellular targets and produce cytokines that may potentially damage normal self-tissues unless they are kept in check. Such tolerance mechanisms are incompletely understood. Here we discuss recent studies suggesting that NK cells undergo a host major histocompatibility complex (MHC) class I-dependent functional maturation process, termed 'licensing'. Ironically, licensing directly involves inhibitory receptors that recognize target cell MHC class I molecules and block activation of NK cells in effector responses. This process results in two types of tolerant NK cells: functionally competent (licensed) NK cells, whose effector responses are inhibited by self-MHC class I molecules through the same receptors that conferred licensing, and functionally incompetent (unlicensed) NK cells.

PTPN11 is the first identified proto-oncogene that encodes a tyrosine phosphatase

Elucidation of the molecular mechanisms underlying carcinogenesis has benefited tremendously from the identification and characterization of oncogenes and tumor suppressor genes. One new advance in this field is the identification of PTPN11 as the first proto-oncogene that encodes a cytoplasmic tyrosine phosphatase with 2 Src-homology 2 (SH2) domains (Shp2). This tyrosine phosphatase was previously shown to play an essential role in normal hematopoiesis. More recently, somatic missense PTPN11 gain-of-function mutations have been detected in leukemias and rarely in solid tumors, and have been found to induce aberrant hyperactivation of the Ras-Erk pathway. This progress represents another milestone in the leukemia/cancer research field and provides a fresh view on the molecular mechanisms underlying cell transformation.

SHP-2 phosphatase is required for hematopoietic cell transformation by Bcr-Abl

SHP-2 phosphatase forms a stable protein complex with and is heavily tyrosine-phosphorylated by the oncogenic tyrosine kinase Bcr-Abl. However, the role of SHP-2 in Bcr-Abl-mediated leukemogenesis is unclear. In the present report, we provide evidence that SHP-2 is required for hematopoietic cell transformation by Bcr-Abl. In vitro biological effects of Bcr-Abl transduction were diminished in SHP-2Delta/Delta hematopoietic cells, and the leukemic potential of Bcr-Abl-transduced SHP-2Delta/Delta cells in recipient animals was compromised. Further analyses showed that Bcr-Abl protein (p210) was degraded, and its oncogenic signaling was greatly decreased in SHP-2Delta/Delta cells. Treatment with proteasome inhibitors or reintroduction of SHP-2 restored p210 level in Bcr-Abl-transduced SHP-2Delta/Delta cells. Subsequent investigation revealed that SHP-2 interacted with heat shock protein 90, an important chaperone protein protecting p210 from proteasome-mediated degradation. The role of SHP-2 in the stability of p210 is independent of its catalytic activity. Blockade of SHP-2 expression in p210-expressing cells by antisense or small-interfering RNA approaches decreased p210 level, causing cell death. Inhibition of SHP-2 enzymatic activity by overexpression of catalytically inactive SHP-2 mutant did not destabilize p210 but enhanced serum starvation-induced apoptosis, suggesting that SHP-2 also plays an important role in downstream signaling of p210 kinase. These studies identified a novel function of SHP-2 and suggest that SHP-2 might be a useful target for controlling Bcr-Abl-positive leukemias.

Mutations of the PTPN11 and RAS genes in rhabdomyosarcoma and pediatric hematological malignancies

PTPN11 has been identified as a causative gene in Noonan syndrome (NS), responsible for about 50% of cases of NS. Given the association between NS and an increased risk of some malignancies, notably leukemia and probably some solid tumors including neuroblastoma (NB) and rhabdomyosarcoma (RMS), recent studies have reported that gain-of-function somatic mutations in PTPN11 occur in some hematological malignancies, especially de novo juvenile myelomonocytic leukemia (JMML) and in some solid tumors such as NB, although at a low frequency. In a screen for mutations of PTPN11 in 7 cell lines and 30 fresh tumors of RMS and in 25 cell lines and 40 fresh tumors of NB, we identified a missense mutation (A72T) in an embryonal RMS patient. In the RMS samples, we also detected mutations of NRAS in 1 cell line and 1 patient; both mutations were in embryonal RMSs and had no PTPN11 mutations. No mutations of PTPN11 were detected in NB. In 95 leukemia cell lines and 261 fresh leukemia samples including 22 JMMLs, 9 kinds of missense mutations were detected in 17 leukemia samples, which included 11 (50.0%) mutations in JMML samples and lower frequencies in other hematological malignancies. Furthermore, we identified 4 (18.2%) NRAS mutations and 1 (4.5%) KRAS mutation in 5 JMML samples, 1 of which had a concomitant PTPN11 mutation. Our data suggest that mutations of PTPN11 as well as RAS play a role in the pathogenesis of not only myeloid hematological malignancies but also a subset of RMS malignancies.

Inherited predispositions and hyperactive Ras in myeloid leukemogenesis

Identifying the molecular basis for inherited cancer predispositions reveals genes that when mutated, play a critical role in the earliest stages of tumorigenesis. Although rare, inherited predispositions to myeloid leukemias have led to a greater understanding of pathways important for myeloid proliferation and maturation. In particular, elucidating why children with neurofibromatosis type 1 (NF1) and Noonan syndrome (NS) are predisposed to juvenile myelomonocytic leukemia (JMML) has uncovered a critical role of hyperactive Ras signaling in normal myeloid growth and leukemogenesis. Here, we review studies of human samples and experiments performed in genetically engineered strains of mice investigating the molecular and biochemical basis of aberrant growth in JMML. These strains model human disease features and provide an opportunity to investigate novel therapeutic strategies that may ultimately cure JMML and other myeloid malignancies characterized by hyperactive Ras.

Dendritic Cell-Based Tumor Vaccines and Antigen Presentation Attenuators

Dendritic cell (DC)-based tumor vaccines are being extensively tested to treat cancer patients. Although the results of most DC-based clinical trials have been disappointing, recent advances in the basic molecular understanding of positive and negative regulation of antigen presentation and immune responses can form a basis to enhance the efficacy of DC-based vaccines. Here we describe the new understanding of the importance of Toll-like receptor, tumor necrosis factor receptor, and cytokine receptor signaling in activation of innate and adaptive immunity. In particular, we describe the emerging importance of hardwired negative regulators, such as cytokine signaling regulators, as antigen presentation attenuators (APAs), providing a new strategy to break self-tolerance and enhance the potency of tumor vaccines by inhibiting APAs.

How do natural killer cells find self to achieve tolerance?

Natural killer (NK) cells provide innate defense against tumors and infections by virtue of potent capacities to immediately kill cellular targets and produce cytokines. These effector functions may potentially damage normal self-tissues unless they are kept in check by tolerance mechanisms that need clarification. Here, we discuss recent studies indicating that the NK cells acquire functional competence directly through engagement of their MHC-specific receptors by self-MHC. Ironically, these receptors were first identified in terms of recognizing target cell MHC class I molecules and inhibiting NK cells in effector responses. Other studies of NK cell tolerance are also discussed. Although these studies begin to clarify the means by which NK cell tolerance is achieved, much more investigation is needed because NK cell tolerance is relevant to clinical observations in patients with infections and cancer.

Reduction of Shp-2 Expression by Small Interfering RNA Reduces Murine Embryonic Stem Cell-Derived In Vitro Hematopoietic Differentiation

Shp-2 is a member of a small family of cytoplasmic Src homology 2 (SH2) domain-containing protein tyrosine phosphatases. Although Shp-2 has been shown to be necessary for hematopoiesis using a mouse model expressing a mutant residual protein (Shp-2(delta/delta)), we used small interfering RNA (siRNA) to reduce Shp-2 expression and examined the consequences on embryonic stem cell (ESC)-derived hemangioblast, primitive, and definitive hematopoietic development. We found that at a concentration of 50 nM, Shp-2 siRNA effectively diminished Shp-2 expression in differentiating embryoid bodies. Hemangioblast, primitive, and definitive hematopoietic progenitor formation was decreased significantly after transfection with Shp-2 siRNA but not with scrambled siRNA. Because Shp-2 is involved in signals emanating from the basic fibroblast growth factor (bFGF) receptor, we asked whether Shp-2 functions in bFGF-mediated hemangioblast development. Reduction of Shp-2 expression using siRNA, but not scrambled siRNA, blocked the bFGF-induced increase in hemangioblast development. Using siRNA as an independent method of reducing Shp-2 function, in contrast to the mutant mouse model (Shp-2(delta/delta)) previously used, we demonstrate that Shp-2 is required in hemangioblast, primitive, and definitive progenitor hematopoietic development and that Shp-2 is integrally necessary for bFGF-mediated hemangioblast production.

SHP2 forecast for the immune system: fog gradually clearing

The src homology 2 (SH2) domain containing tyrosine phosphatase SHP2 (also referred to as SHP-2) is ubiquitously expressed in mammalian tissues and has been shown to be essential for embryonic development, haematopoiesis and signalling downstream of a variety of growth factors. Dysregulation of SHP2 function or expression has recently been implicated in the pathogenesis of human diseases involving haematopoietic cell lineages. New findings also demonstrate the involvement of SHP2 in the regulation of immune responses through its effects on cytokine and inhibitory receptor signalling pathways, and novel transgenic models are providing valuable insights into the role of SHP2 in T cells.

Diversity and functional consequences of germline and somatic PTPN11 mutations in human disease

Germline mutations in PTPN11, the gene encoding the protein tyrosine phosphatase SHP-2, cause Noonan syndrome (NS) and the clinically related LEOPARD syndrome (LS), whereas somatic mutations in the same gene contribute to leukemogenesis. On the basis of our previously gathered genetic and biochemical data, we proposed a model that splits NS- and leukemia-associated PTPN11 mutations into two major classes of activating lesions with differential perturbing effects on development and hematopoiesis. To test this model, we investigated further the diversity of germline and somatic PTPN11 mutations, delineated the association of those mutations with disease, characterized biochemically a panel of mutant SHP-2 proteins recurring in NS, LS, and leukemia, and performed molecular dynamics simulations to determine the structural effects of selected mutations. Our results document a strict correlation between the identity of the lesion and disease and demonstrate that NS-causative mutations have less potency for promoting SHP-2 gain of function than do leukemia-associated ones. Furthermore, we show that the recurrent LS-causing Y279C and T468M amino acid substitutions engender loss of SHP-2 catalytic activity, identifying a previously unrecognized behavior for this class of missense PTPN11 mutations.

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.

Effects of a leukemia-associated gain-of-function mutation of SHP-2 phosphatase on interleukin-3 signaling

Mutations in SHP-2 phosphatase that cause hyperactivation of its catalytic activity have been identified in human leukemias, particularly juvenile myelomonocytic leukemia, which is characterized by hypersensitivity of myeloid progenitor cells to granulocyte macrophage colony-stimulating factor and interleukin (IL)-3. However, the molecular mechanisms by which gain-of-function (GOF) mutations of SHP-2 induce hematopoietic malignancies are not fully understood. Our previous studies have shown that SHP-2 plays an essential role in IL-3 signal transduction in both catalytic-dependent and -independent manners and that overexpression (5-6-fold) of wild type (WT) SHP-2 attenuates IL-3-mediated hematopoietic cell function through accelerated dephosphorylation of STAT5. These results raised the possibility that SHP-2-associated leukemias are not solely attributed to the increased catalytic activity of GOF mutant SHP-2. GOF mutant SHP-2 must have gained additional capacities. To test this possibility, we investigated effects of a GOF mutation of SHP-2 (SHP-2 E76K) on hematopoietic cell function and IL-3 signal transduction by comparing with those of overexpressed WT SHP-2. Our results showed that SHP-2 E76K mutation caused myeloproliferative disease in mice, while overexpression of WT SHP-2 decreased hematopoietic potential of the transduced cells in recipient animals. The E76K mutation in the N-terminal Src homology 2 domain increased interactions of mutant SHP-2 with Grb2, Gab2, and p85, leading to hyperactivation of IL-3-induced Erk and phosphatidylinositol 3-kinase (PI3K) pathways. In addition, despite the substantial increase in the catalytic activity, dephosphorylation of STAT5 by SHP-2 E76K was dampened. Furthermore, catalytically inactive SHP-2 E76K with an additional C459S mutation retained the capability to increase the interaction with Gab2 and to enhance the activation of the PI3K pathway. Taken together, these studies suggest that in addition to the elevated catalytic activity, fundamental changes in physical and functional interactions between GOF mutant SHP-2 and signaling partners also play an important role in SHP-2-related leukemigenesis.

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.

Noonan syndrome and related disorders: genetics and pathogenesis

Noonan syndrome is a pleiomorphic autosomal dominant disorder with short stature, facial dysmorphia, webbed neck, and heart defects. In the past decade, progress has been made in elucidating the pathogenesis of this disorder using a positional cloning approach. Noonan syndrome is now known to be a genetically heterogeneous disorder with nearly one half of cases caused by gain-of-function mutations in PTPN11, the gene encoding the protein tyrosine phosphatase SHP-2. Similar germ line mutations cause two related genetic disorders, Noonan-like disorder with multiple giant cell lesion syndrome and LEOPARD syndrome, and somatic PTPN11 mutations can underlie certain pediatric hematopoietic malignancies, including juvenile myelomonocytic, acute lymphoblastic, and acute myelogenous leukemias. A mouse model of PTPN11-related Noonan syndrome was recently generated, providing a reagent for studying disease pathogenesis in greater depth as well as experimenting with novel therapeutic strategies.

SHP-2 Phosphatase Regulates DNA Damage-induced Apoptosis and G2/M Arrest in Catalytically Dependent and Independent Manners, Respectively

SHP-2, a tyrosine phosphatase implicated in diverse signaling pathways induced by growth factors and cytokines, is also involved in DNA damage-triggered signaling and cellular responses. We previously demonstrated that SHP-2 played an important role in DNA damage-induced apoptosis and G2/M cell cycle checkpoint. In the present studies, we have provided evidence that SHP-2 functions in DNA damage apoptosis and G2/M arrest in catalytically dependent and independent manners, respectively. Mutant embryonic fibroblasts with the Exon 3 deletion mutation in SHP-2 showed decreased apoptosis and diminished G2/M arrest in response to cisplatin treatment. Wild type (WT), but not catalytically inactive mutant SHP-2 (SHP-2 C459S), rescued the apoptotic response of the mutant cells. Interestingly, both WT and SHP-2 C459S efficiently restored the G2/M arrest response. Furthermore, inhibition of the catalytic activity of endogenous SHP-2 in WT cells by overexpression of SHP-2 C459S greatly decreased cell death but not G2/M arrest induced by cisplatin. Biochemical analyses revealed that activation of c-Abl kinase was decreased in SHP-2 C459S-overexpressing cells. However, DNA damage-induced translocation of Cdc25C from the nucleus to the cytoplasm was fully restored in both WT and SHP-2 C459S "rescued" cells. Additionally, we demonstrated that the role of SHP-2 in DNA damage-induced cellular responses was independent of the tumor suppressor p53. Embryonic stem cells with the SHP-2 deletion mutation showed markedly decreased sensitivity to cisplatin-induced apoptosis, attributed to impaired induction of p73 but not p53. In agreement with these results, DNA damage-induced apoptosis and G2/M arrest were also decreased in SHP-2/p53 double mutant embryonic fibroblasts. Collectively, these studies have further defined the mechanisms by which SHP-2 phosphatase regulates DNA damage responses.

Overexpression of Shp2 tyrosine phosphatase is implicated in leukemogenesis in adult human leukemia

Shp2 tyrosine phosphatase plays a critical role in hematopoiesis, and dominant active mutations have been detected in the human gene PTPN11, encoding Shp2, in child leukemia patients. We report here that although no such mutations were detected in 44 adult leukemia patients screened, Shp2 expression levels were significantly elevated in primary leukemia cells and leukemia cell lines, as compared with normal hematopoietic progenitor cells. The Shp2 protein amounts correlated well with the hyperproliferative capacity but were inversely associated with the differentiation degree of leukemia cells. Suppression of Shp2 expression induced apoptosis and inhibition of leukemic cell clonogenic growth. Notably, the majority of Shp2 was preferentially localized to the plasma membrane and was constitutively phosphorylated on tyrosine in leukemia cells, and also in normal hematopoietic cells following mitogenic stimulation. Based on these results, we propose that aberrantly increased expression of Shp2 may contribute, collaboratively with other factors, to leukemogenesis.

Somatic PTPN11 mutations in childhood acute myeloid leukaemia

Somatic mutations in PTPN11, the gene encoding the transducer SHP-2, have emerged as a novel class of lesions that upregulate RAS signalling and contribute to leukaemogenesis. In a recent study of 69 children and adolescents with de novo acute myeloid leukaemia (AML), we documented a non-random distribution of PTPN11 mutations among French-American-British (FAB) subtypes. Lesions were restricted to FAB-M5 cases, where they were relatively common (four of 12 cases). Here, we report on the results of a molecular screening performed on 181 additional unselected patients, enrolled in participating institutions of the Associazione Italiana Ematologia Oncologia Pediatrica-AML Study Group, to provide a more accurate picture of the prevalence, spectrum and distribution of PTPN11 mutations in childhood AML and to investigate their clinical relevance. We concluded that PTPN11 defects do not represent a frequent event in this heterogeneous group of malignancies (4.4%), although they recur in a considerable percentage of patients with FAB-M5 (18%). PTPN11 lesions rarely occur in other subtypes. Within the FAB-M5 group no clear association of PTPN11 mutations with any clinical variable was evident. Nearly two third of the patients with this subtype were found to harbour an activating mutation in PTPN11, NRAS, KRAS2 or FLT3.

Receptor-stimulated oxidation of SHP-2 promotes T-cell adhesion through SLP-76-ADAP

Receptor-stimulated generation of intracellular reactive oxygen species (ROS) modulates signal transduction, although the mechanism(s) is unclear. One potential basis is the reversible oxidation of the active site cysteine of protein tyrosine phosphatases (PTPs). Here, we show that activation of the antigen receptor of T cells (TCR), which induces production of ROS, induces transient inactivation of the SH2 domain-containing PTP, SHP-2, but not the homologous SHP-1. SHP-2 is recruited to the LAT-Gads-SLP-76 complex and directly regulates the phosphorylation of key signaling proteins Vav1 and ADAP. Furthermore, the association of ADAP with the adapter SLP-76 is regulated by SHP-2 in a redox-dependent manner. The data indicate that TCR-mediated ROS generation leads to SHP-2 oxidation, which promotes T-cell adhesion through effects on an SLP-76-dependent signaling pathway to integrin activation.

Germ-line and somatic PTPN11 mutations in human disease

Reversible protein tyrosyl phosphorylation of cell surface receptors and downstream intracellular transducers is a major regulatory mechanism used to modulate cellular responses to extracellular stimuli, and its deregulation frequently drives aberrant cell proliferation, survival and/or differentiation. SHP-2 is a cytoplasmic Src-homology 2 domain-containing protein tyrosine phosphatase that plays an important role in intracellular signaling and is required during development and hematopoiesis. Germ-line missense mutations in PTPN11, the gene coding SHP-2, have been discovered as a major molecular event underlying Noonan syndrome, an autosomal dominant trait characterized by short stature, dysmorphic facies, and congenital heart defects, as well as in other closely related developmental disorders. More recently, a distinct class of missense mutations in the same gene has been identified to occur as a somatic event contributing to myeloid and lymphoid malignancies. This review focuses on the role of SHP-2 in signal transduction, development and hematopoiesis, as well as on the consequences of SHP-2 gain-of-function.

Protein tyrosine phosphatases and the immune response

Reversible tyrosine phosphorylation of proteins is a key regulatory mechanism for numerous important aspects of eukaryotic physiology and is catalysed by kinases and phosphatases. Together, cells of the immune system express at least half of the 107 protein tyrosine phosphatase (PTP) genes in the human genome, most of which encode multidomain proteins that contain protein- and phospholipid-interaction domains. Here, we discuss the diverse but specific, and important, roles that PTPs have in immune cells, focusing mainly on T and B cells, and we highlight recent evidence that even subtle alterations in PTPs can cause immune dysfunction and human disease.

Cytokine functions in the formative stages of a lymphocyte's life

Five core cytokines that control lymphocyte differentiation and maintenance have been identified and studied in depth. IL-7 sits at the apex of this cytokine hierarchy in terms of functional significance during lymphocyte development. The IL-7-dominant phase of lymphopoiesis is preceded by the actions of c-Kit ligand (also called stem cell factor; SCF) and fetal liver kinase 2 ligand (Flk-2L); the function of both of these cytokines is essential for the maintenance and development of the progenitor compartment of multiple lineages. IL-7 activity is complemented by two cytokines whose receptors share components of the IL-7 receptor: thymic stromal lymphopoietin (TSLP) and IL-15. The influences of these core cytokines on precursor lymphocyte subsets overlap during development and are often synergistic. Recent studies are beginning to uncover the molecular mechanisms of these interrelated core cytokine functions.

SHP-2 and myeloid malignancies

PURPOSE OF REVIEW

This review focuses on the non-receptor Src-homology 2 domain-containing protein tyrosine phosphatase SHP-2 and its role in signal transduction, hematopoiesis, and leukemogenesis. Specifically, we discuss the role of inherited and somatic mutations that result in SHP-2 gain-of-function in human disease, including myeloid malignancies.

RECENT FINDINGS

Up-regulation of RAS signaling is a major perturbation that drives the aberrant growth of malignant myeloid cells. Leukemia-associated SHP-2 mutations define a novel type of molecular events resulting in hyperactive RAS function.

SUMMARY

SHP-2 plays an important role in intracellular signaling elicited by growth factors, hormones, and cytokines, and it is required during development and hematopoiesis. Gain of function mutations in PTPN11, the gene encoding SHP-2, is observed in Noonan syndrome and related development disorders, as well as in myeloid malignancies. Fully characterizing the incidence and spectrum of PTPN11 mutations in hematologic malignancies, and in other forms of cancer, is an area of active investigation.

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

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

Noonan syndrome-associated SHP2/PTPN11 mutants cause EGF-dependent prolonged GAB1 binding and sustained ERK2/MAPK1 activation

Noonan syndrome is a developmental disorder with dysmorphic facies, short stature, cardiac defects, and skeletal anomalies, which can be caused by missense PTPN11 mutations. PTPN11 encodes Src homology 2 domain-containing tyrosine phosphatase 2 (SHP2 or SHP-2), a protein tyrosine phosphatase that acts in signal transduction downstream to growth factor, hormone, and cytokine receptors. We compared the functional effects of three Noonan syndrome-causative PTPN11 mutations on SHP2's phosphatase activity, interaction with a binding partner, and signal transduction. All SHP2 mutants had significantly increased basal phosphatase activity compared to wild type, but that activity varied significantly between mutants and was further increased after epidermal growth factor stimulation. Cells expressing SHP2 mutants had prolonged extracellular signal-regulated kinase 2 activation, which was ligand-dependent. Binding of SHP2 mutants to Grb2-associated binder-1 was increased and sustained, and tyrosine phosphorylation of both proteins was prolonged. Coexpression of Grb2-associated binder-1-FF, which lacks SHP2 binding motifs, blocked the epidermal growth factor-mediated increase in SHP2's phosphatase activity and resulted in a dramatic reduction of extracellular signal-regulated kinase 2 activation. Taken together, these results document that Noonan syndrome-associated PTPN11 mutations increase SHP2's basal phosphatase activity, with greater activation when residues directly involved in binding at the interface between the N-terminal Src homology 2 and protein tyrosine phosphatase domains are altered. The SHP2 mutants prolonged signal flux through the RAS/mitogen-activated protein kinase (ERK2/MAPK1) pathway in a ligand-dependent manner that required docking through Grb2-associated binder-1 (GAB1), leading to increased cell proliferation.

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.

Physical proximity and functional interplay of PECAM-1 with the Fc receptor Fc gamma RIIa on the platelet plasma membrane

We and others have recently defined that Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1/CD31) functions as a negative regulator of platelet-collagen interactions involving the glycoprotein VI/Fc receptor gamma chain (GPVI/FcR-gamma chain) signaling pathway.1,2 In this study, we hypothesized that PECAM-1 may be physically and functionally associated with Fc gamma RIIa on the platelet membrane. The functional relationship between PECAM-1 and Fc gamma RIIa was assessed by determining the effect of anti-PECAM-1 monoclonal antibody Fab fragments on Fc gamma RIIa-mediated platelet aggregation and heparin-induced thrombocytopenia (HITS)-mediated platelet aggregation. Preincubation of washed platelets with monoclonal antibody fragments of 2BD4 directed against PECAM-1 and IV.3 directed against Fc gamma RIIa completely blocked Fc gamma RIIa-mediated platelet aggregation and HITS-mediated platelet aggregation, whereas anti-CD151 antibody had no blocking effect. Coengagement of Fc gamma RIIa and PECAM-1 resulted in negative regulation of Fc gamma RIIa-mediated phospholipase C gamma 2 activation, calcium mobilization, and phosphoinositide 3-kinase-dependent signaling pathways. In addition, the physical proximity of Fc gamma RIIa and PECAM-1 was confirmed by using fluorescence resonance energy transfer and coimmunoprecipitation studies. These results indicate that PECAM-1 and Fc gamma RIIa are colocalized on the platelet membrane and PECAM-1 down-regulates Fc gamma RIIa-mediated platelet responses.

Blocking the function of tyrosine phosphatase SHP-2 by targeting its Src homology 2 domains

SHP-2 is an Src homology 2 (SH2) domain-containing tyrosine phosphatase with crucial functions in cell signaling and major pathological implications. It stays inactive in the cytosol and is activated by binding through its SH2 domains to tyrosine-phosphorylated receptors on the cell surface. One such cell surface protein is PZR, which contains two tyrosine-based inhibition motifs responsible for binding of SHP-2. We have generated a glutathione S-transferase fusion protein carrying the tandem tyrosine-based inhibition motifs of PZR, and the protein was tyrosine-phosphorylated by co-expressing c-Src in Escherichia coli cells. The purified phosphoprotein displays a strong binding to SHP-2 and causes its activation in vitro. However, when introduced into NIH 3T3 cells by using a protein delivery reagent, it effectively inhibited the activation of ERK1/2 induced by growth factors and serum but not by phorbol ester, in reminiscence of the effects caused by expression of dominant negative SHP-2 mutants and deletion of functional SHP-2. The data suggest that the exogenously introduced PZR protein specifically binds SHP-2, blocks its translocation, and renders it functionally incompetent. This is further supported by the fact that the phosphorylated PZR protein had no inhibitory effects on fibroblasts derived from mice expressing only a mutant SHP-2 protein lacking most of the N-terminal SH2 domain. This study thus provides a novel and highly specific method to interrupt the function of SHP-2 in cells.

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

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

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.

Protein tyrosine phosphatases: the quest for negative regulators of insulin action

Type 2 diabetes is increasing at an alarming rate worldwide, and there has been a considerable effort in several laboratories to identify suitable targets for the design of drugs against the disease. To this end, the protein tyrosine phosphatases that attenuate insulin signaling by dephosphorylating the insulin receptor (IR) have been actively pursued. This is because inhibiting the phosphatases would be expected to prolong insulin signaling and thereby facilitate glucose uptake and, presumably, result in a lowering of blood glucose. Targeting the IR protein tyrosine phosphatase, therefore, has the potential to be a significant disease-modifying strategy. Several protein tyrosine phosphatases (PTPs) have been implicated in the dephosphorylation of the IR. These phosphatases include PTPalpha, LAR, CD45, PTPepsilon, SHP2, and PTP1B. In most cases, there is evidence for and against the involvement of the phosphatases in insulin signaling. The most convincing data, however, support a critical role for PTP1B in insulin action. PTP1B knockout mice are not only insulin sensitive but also maintain euglycemia (in the fed state), with one-half the level of insulin observed in wild-type littermates. Interestingly, these mice are also resistant to diet-induced obesity when fed a high-fat diet. The insulin-sensitive phenotype of the PTP1B knockout mouse is reproduced when the phosphatase is also knocked down with an antisense oligonucleotide in obese mice. Thus PTP1B appears to be a very attractive candidate for the design of drugs for type 2 diabetes and obesity.

Positive and negative regulation of T-cell activation through kinases and phosphatases

The sequence of events in T-cell antigen receptor (TCR) signalling leading to T-cell activation involves regulation of a number of protein tyrosine kinases (PTKs) and the phosphorylation status of many of their substrates. Proximal signalling pathways involve PTKs of the Src, Syk, Csk and Tec families, adapter proteins and effector enzymes in a highly organized tyrosine-phosphorylation cascade. In intact cells, tyrosine phosphorylation is rapidly reversible and generally of a very low stoichiometry even under induced conditions due to the fact that the enzymes removing phosphate from tyrosine-phosphorylated substrates, the protein tyrosine phosphatases (PTPases), have a capacity that is several orders of magnitude higher than that of the PTKs. It follows that a relatively minor change in the PTK/PTPase balance can have a major impact on net tyrosine phosphorylation and thereby on activation and proliferation of T-cells. This review focuses on the involvement of PTKs and PTPases in positive and negative regulation of T-cell activation, the emerging theme of reciprocal regulation of each type of enzyme by the other, as well as regulation of phosphotyrosine turnover by Ser/Thr phosphorylation and regulation of localization of signal components.

Role of protein tyrosine phosphatases in T cell activation

The last decade has seen an exponentially increasing interest in the molecular mechanisms of signal transduction. In T cells, much of the focus has been on protein tyrosine kinase (PTK)-mediated signaling from the T cell receptor (TCR) and cytokine receptors, while the study of protein tyrosine phosphatases (PTPases) has lagged behind. However, recent discoveries have revealed that several PTPases play important roles in many different aspects of T cell physiology. We predict that the phosphatases will become a 'hot topic' in the field within the next few years. This review summarizes the current understanding of the regulation and biology of PTPases in T lymphocyte activation.

src homology 2 domain-containing tyrosine phosphatase SHP-1 controls the development of allergic airway inflammation

Th2 cells are generated from naive CD4 T cells upon T cell receptor (TCR) recognition of antigen and IL-4 stimulation and play crucial roles in humoral immunity against infectious microorganisms and the pathogenesis of allergic and autoimmune diseases. A tyrosine phosphatase, SHP-1, that contains src homology 2 (SH2) domains is recognized as a negative regulator for various intracellular signaling molecules, including those downstream of the TCR and the IL-4 receptor. Here we assessed the role of SHP-1 in Th1/Th2 cell differentiation and in the development of Th2-dependent allergic airway inflammation by using a natural SHP-1 mutant, the motheaten mouse. CD4 T cells appear to develop normally in the heterozygous motheaten (me/+) thymus even though they express decreased amounts of SHP-1 (about one-third the level of wild-type thymus). The me/+ naive splenic CD4 T cells showed enhanced activation by IL-4 receptor-mediated signaling but only marginal enhancement of TCR-mediated signaling. Interestingly, the generation of Th2 cells was increased and specific cytokine production of mast cells was enhanced in me/+ mice. In an OVA-induced allergic airway inflammation model, eosinophilic inflammation, mucus hyperproduction, and airway hyperresponsiveness were enhanced in me/+ mice. Thus, SHP-1 may have a role as a negative regulator in the development of allergic responses, such as allergic asthma.

Role of the SHP-2 tyrosine phosphatase in cytokine-induced signaling and cellular response

Cytokines and growth factors are important extracellular regulatory proteins. They exert their biological functions through binding to their cognate receptors on the cell surface and triggering intracellular signaling cascades. However, the intracellular signaling mechanisms of cytokines and growth factors are not well understood. Accumulating evidence has shown that protein phosphorylation and dephosphorylation carried out by protein kinases and protein phosphatases are fundamental biochemical events in intracellular signal transduction. SHP-2, a Src homology (SH) 2 domain-containing protein tyrosine phosphatase (PTP), is widely involved in a variety of signaling pathways triggered by cytokines and growth factors, including the MAP kinase, Jak-Stat, and PI3 kinase pathways. Recent studies have clearly demonstrated that this phosphatase plays an important role in transducing signals relayed from the cell surface to the nucleus, and is a critical intracellular regulator in cytokine and growth factor-induced cell survival, proliferation, and differentiation.

Role of the docking protein Gab2 in beta(1)-integrin signaling pathway-mediated hematopoietic cell adhesion and migration

Gab2, a newly identified pleckstrin homology domain-containing docking protein, is a major binding protein of SHP-2 tyrosine phosphatase in interleukin (IL)-3-stimulated hematopoietic cells. Its signaling mechanism remains largely unknown. We report here an important regulatory role for Gab2 in beta(1) integrin signaling pathway that mediates hematopoietic cell adhesion and migration. Cross-linking of the beta(1) integrin on Ba/F3 cells induced rapid tyrosine phosphorylation of Gab2 and its association with Syk kinase, SHP-2 phosphatase, and the p85 subunit of phosphatidylinositol (PI)-3 kinase. In addition, Gab2 was also constitutively associated with SHP-1 phosphatase via its C-terminal Src homology 2 domain. Overexpression of the pleckstrin homology domain or a mutant Gab2 molecule lacking SHP-2 binding sites resulted in significant reductions in Ba/F3 cell adhesion and migration. Biochemical analyses revealed that enforced expression of Gab2 mutant molecules dramatically reduced beta(1)-integrin ligation-triggered PI3 kinase activation, whereas Erk kinase activation remained unaltered. Furthermore, transduction of primary hematopoietic progenitor cells from viable motheaten mice with these mutant Gab2 molecules also significantly ameliorated their enhanced migration capacity associated with the SHP1 gene mutation. Taken together, these results suggest an important signaling role for Gab2 in regulating hematopoietic cell adhesion and migration.

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.

Signaling pathways recruited by the cardiotrophin-like cytokine/cytokine-like factor-1 composite cytokine: specific requirement of the membrane-bound form of ciliary neurotrophic factor receptor alpha component

Ciliary neurotrophic factor (CNTF) is a cytokine supporting the differentiation and survival of a number of neural cell types. Its receptor complex consists of a ligand-binding component, CNTF receptor (CNTFR), associated with two signaling receptor components, gp130 and leukemia inhibitory factor receptor (LIFR). Striking phenotypic differences between CNTF- and CNTFR-deficient mice suggest that CNTFR serves as a receptor for a second developmentally important ligand. We recently demonstrated that cardiotrophin-like cytokine (CLC) associates with the soluble orphan receptor cytokine-like factor-1 (CLF) to form a heterodimeric cytokine that displayed activities only on cells expressing the tripartite CNTF receptor on their surface. In this present study we examined the membrane binding of the CLC/CLF composite cytokine and observed a preferential interaction of the cytokine with the CNTFR subunit. Signaling pathways recruited by the CLC/CLF complex in human neuroblastoma cell lines were also analyzed in detail. The results obtained showed an activation of Janus kinases (JAK1, JAK2, and TYK2) leading to a tyrosine phosphorylation of the gp130 and LIFR. The phosphorylated signaling receptors served in turn as docking proteins for signal transducing molecules such as STAT3 and SHP-2. In vitro analysis revealed that the gp130-LIFR pathway could also stimulate the phosphatidylinositol 3-kinase and the mitogen-activated protein kinase pathways. In contrast to that reported before for CNTF, soluble CNTFR failed to promote the action CLC/CLF, and an absolute requirement of the membrane form of CNTFR was required to generate a functional response to the composite cytokine. This study reinforces the functional similarity between CNTF and the CLC/CLF composite cytokine defining the second ligand for CNTFR.