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Abstract

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References

SHP-2 is a cytoplasmic protein tyrosine phosphatase (PTP) that contains two Src homology 2 (SH2) domains. Although PTPs are generally considered to be negative regulators on the basis of their ability to oppose the effects of protein tyrosine kinases, SHP-2 is unusual in that it promotes the activation of the Ras-MAPK signaling pathway by receptors for various growth factors and cytokines. The molecular basis for the activation of SHP-2 is also unique: In the basal state, the NH2-terminal SH2 domain of SHP-2 interacts with the PTP domain, resulting in autoinhibition of PTP activity; the binding of SHP-2 via its SH2 domains to tyrosine-phosphorylated growth factor receptors or docking proteins, however, results in disruption of this intramolecular interaction, leading to exposure of the PTP domain and catalytic activation. Indeed, SHP-2 proteins with artificial mutations in the NH2-terminal SH2 domain have been shown to act as dominant active mutants in vitro. Such activating mutations of PTPN11 (human SHP-2 gene) were subsequently identified in individuals with Noonan syndrome, a human developmental disorder that is sometimes associated with juvenile myelomonocytic leukemia. Furthermore, somatic mutations of PTPN11 were found to be associated with pediatric leukemia. SHP-2 is also thought to participate in the development of other malignant disorders, but in a manner independent of such activating mutations. Biochemical and functional studies of SHP-2 and genetic analysis of PTPN11 in human disorders have thus converged to provide new insight into the pathogenesis of cancer as well as potential new targets for cancer treatment. (Cancer Sci 2009; 100: 1786–1793)

Mutations of Ras genes are highly prevalent (30%) in human malignancies, occurring at particularly high frequencies in colon and pancreatic cancers.(1) Ras was first implicated in oncogenesis by pioneering studies in the 1980s by Weinberg and colleagues, who showed that transfection of NIH 3T3 mouse fibroblasts with DNA derived from human bladder carcinoma cell lines (T24 and EJ) resulted in their transformation.(2) Furthermore, they as well as other groups found that the DNA indeed contained the H-Ras gene with a point mutation at codon 12. Mutations of Ras genes were subsequently identified in a variety of sporadic human cancers.(1) In parallel with these genetic studies of human cancer, it was shown that Ras genes encode small GTP-binding proteins and that the mutations found in human malignancies result in constitutive activation of these proteins.(1,2) Such gain-of-function mutations of Ras were thus concluded to be important for cell transformation (Fig. 1). In the early 1990s, it became evident that Ras is an essential component of the signaling pathway by which growth factors stimulate cell proliferation. The binding of growth factors to their receptor tyrosine kinases (RTKs) triggers receptor autophosphorylation and the consequent recruitment of an adaptor protein, designated growth factor receptor-bound protein 2 (Grb2), which forms a constitutive complex with Son of Sevenless (Sos), a guanine nucleotide exchange factor that catalyzes conversion of the inactive, GDP-bound form of Ras to the active, GTP-bound form.(3,4) The GTP-bound form of Ras in turn activates the Raf-MEK-MAPK cascade, which promotes cell proliferation, differentiation, or survival (Fig. 1). The combination of genetic analysis of Ras genes in cancer and biochemical characterization of Ras proteins provided important insight both into a signaling pathway that mediates the stimulatory effect of growth factors on cell proliferation as well as into the mechanism of cell transformation by specific Ras gene mutations.

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Figure 1. The function of Ras and its deregulation. Ras is an essential component of the signaling pathway that underlies growth factor-induced cell proliferation, differentiation, or survival. Growth factors stimulate the tyrosine kinase (TK) activity and consequent autophosphorylation of their receptors, resulting in recruitment of the adaptor protein Grb2, which forms a constitutive complex with Sos, a guanine nucleotide exchange factor that catalyzes conversion of the GDP-bound (inactive) form of Ras to the GTP-bound (active) form. Activated Ras in turn induces activation of the Raf-MEK-MAPK cascade, which leads to changes in gene expression that are required for the induction of cell proliferation, differentiation, or survival. Neurofibromin (encoded by the tumor suppressor gene NF1) shows sequence similarity to the catalytic domain of GTPase-activating proteins (GAPs) that negatively regulate Ras by increasing its intrinsic GTPase activity. SHP-2, a cytoplasmic protein tyrosine phosphatase (PTP), promotes the activation of Ras by regulating signaling upstream of Ras. Mutations of Ras genes that result in constitutive activation of the encoded protein (gain-of-function mutation) induce activation of the Raf-MEK-MAPK cascade in the absence of growth factor stimulation, resulting in cell transformation and development of cancer. Loss of NF1 (loss-of-function mutation) also induces constitutive activation of Ras and gives rise to neurofibromatosis type 1 (NF1) as well as to cancer. Mutations of PTPN11 (human SHP-2 gene) that result in constitutive activation of the encoded phosphatase (gain-of-function mutation) promote Ras activation and cause Noonan syndrome (NS) as well as leukemia.

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The role of deregulated activation of Ras in cancer development was also revealed by genetic analysis of neurofibromatosis type 1 (NF1), a genetic disorder characterized by an increased susceptibility to malignancies including neurofibrosarcoma, pheochromocytoma, and juvenile leukemia.(5) The gene responsible for this condition, NF1, is a tumor suppressor gene that encodes the protein neurofibromin, the central domain of which shows extensive sequence similarity to the catalytic domain of GTPase-activating proteins (GAPs) for Ras, such as p120 RasGAP, which negatively regulate Ras by increasing the rate of its intrinsic GTPase activity.(4) Loss of neurofibromin would thus be expected to mimic the effect of an activating mutation of Ras in malignant transformation of cells (Fig. 1). However, other observations suggest that the tumor suppressor function of neurofibromin is more complex.(5)

Recent studies have revealed a third mechanism for deregulation of Ras in the development of malignancies. Germline mutations of the gene PTPN11, which encodes SHP-2, a cytoplasmic protein tyrosine phosphatase (PTP), have been identified in individuals with Noonan syndrome (NS), a human developmental disorder that is sometimes associated with juvenile myelomonocytic leukemia (JMML) (Fig. 1). Furthermore, somatic mutations of PTPN11 were also identified in individuals with pediatric leukemia. In contrast to Ras and neurofibromin, the biochemical analysis of SHP-2, which was found to promote Ras activation, took place before the genetic analysis of the corresponding gene and its role in human disease. Indeed, mutations of SHP-2 that were shown to result in constitutive activation of this PTP by biochemical studies were subsequently found to coincide with those identified in individuals with NS or leukemia.

Physiological functions of SHP-2

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References

SHP-2 was identified in the early 1990s by several groups on the basis of its sequence similarity to the catalytic domain of known PTPs and with the use of PCR amplification.(6–10) SHP-2 contains two tandem Src homology 2 (SH2) domains, a single PTP domain, and a COOH-terminal hydrophobic tail with two tyrosine phosphorylation sites (Fig. 2a). Exposure of cells to a variety of extracellular stimuli triggers the binding of SHP-2 via its SH2 domains both to tyrosine-phosphorylated receptors for growth factors such as platelet-derived growth factor (PDGF) as well as to tyrosine-phosphorylated docking proteins including insulin receptor substrates (IRSs), signal regulatory protein α (SIRPα; also known as SHP substrate-1 ([SHPS-1]), Grb2-associated binder proteins (Gabs), and fibroblast growth factor receptor substrate (FRS) (Fig. 2b).(11–17) Such interactions are important both for activation of the PTP activity of SHP-2 (see below) and for its recruitment to sites near the plasma membrane where potential substrate proteins may be located.(18)

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Figure 2. Structure and function of SHP-2. (a) Domain organization of human SHP-2. SHP-2 contains two tandem SH2 domains (N-SH2 and C-SH2), a single protein tyrosine phosphatase (PTP) domain, and a COOH-terminal hydrophobic tail that includes tyrosine phosphorylation sites. The residue numbers of amino acids that delineate the various domains(51) or correspond to the phosphorylation sites are indicated. (b) In response to extracellular stimuli, SHP-2 binds via its SH2 domains either to autophosphorylated growth factor receptors (such as that for platelet-derived growth factor [PDGF]) or to docking proteins (such as insulin receptor substrates [IRSs], Grb2-associated binder proteins [Gabs], fibroblast growth factor receptor substrate [FRS], and signal regulatory protein α[SIRPα; also known as SHP substrate-1, SHPS-1]) that are tyrosine-phosphorylated by activated receptor tyrosine kinases (RTKs) or by Src family kinases (SFKs). Such interactions result in the activation of SHP-2 and its consequent promotion of Ras activation, leading to cell growth or differentiation. SHP-2 also participates in the regulation of cell adhesion and migration by controlling the activity of Rho. (c) Models proposed for the activation of Ras by SHP-2. In model a, SHP-2 promotes Ras activation by dephosphorylating tyrosine-phosphorylated sites of growth factor receptors that bind p120 RasGAP (GAP). Dephosphorylation of these sites prevents inhibition of Ras activation by p120 RasGAP. According to model b, SHP-2 promotes the activation of SFKs by dephosphorylating Cbp/PAG. Dephosphorylation of Cbp prevents the access of Csk (a negative regulator of SFKs) to SFKs, which may promote Ras activation by an as yet unclear mechanism (dotted arrow). In model c, SHP-2 promotes Ras-MAPK activation by dephosphorylating Sprouty, a negative regulator of Ras. Tyrosine-phosphorylated Sprouty binds the Grb2–Sos complex and thereby prevents its interaction with Ras. SHP-2 dephosphorylates Sprouty in response to growth factor stimulation, thereby preventing its interaction with Grb2.

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Although PTPs are conventionally thought to be negative regulators on the basis of the fact that they reverse the effects of protein tyrosine kinases, biochemical and genetic analyses indicate that SHP-2 promotes Ras activation by growth factors and cytokines. The first indication of such a role for SHP-2 in vertebrates came from studies showing that forced expression of a catalytically inactive mutant of SHP-2 prevented activation of Ras(19) and MAPK in cultured mammalian cells as well as in Xenopus.(19–22) In addition, Drosophila (Csw) and Caenorhabditis elegans (PTP-2) orthologs of SHP-2 were implicated as mediators of Ras activation downstream of RTKs.(23,24) Homozygous SHP-2 mutant mice, in which amino acid residues 46 to 110, including most of the NH2-terminal SH2 domain of the protein, were deleted, were found to die as embryos as a result of a defect in gastrulation and abnormal mesoderm patterning.(25,26) Fibroblast growth factor- or epidermal growth factor-induced activation of MAPK was also found to be attenuated in fibroblasts from these mice.(25,27) In addition, this same deletion mutation of the NH2-terminal SH2 domain of SHP-2 suppressed the development of embryonic stem cell-derived hematopoietic cells.(28) These observations thus supported the notion that SHP-2 positively regulates cell growth and differentiation by promoting activation of the Ras–MAPK pathway (Fig. 2b).

The PTP activity of SHP-2 is indeed now thought to be required for full activation of Ras, with SHP-2 being thought to regulate an upstream element necessary for Ras activation.(19,29) However, the precise mechanism by which SHP-2 promotes Ras activation remains unclear. Three principal models have been proposed to date (Fig. 2c). The first model (model a, Fig. 2c), which is based on observations with cultured mammalian cells, proposes that SHP-2 promotes Ras activation through dephosphorylation of tyrosine-phosphorylated sites required for the binding of p120 RasGAP either to RTKs for growth factors such as PDGF and epidermal growth factor or to the docking protein Gab-1, thereby preventing inhibition by p120 RasGAP of Ras activation.(30–32) Similarly, Csw (Drosophila SHP-2), a Drosophila ortholog of SHP-2, has been proposed to promote Ras activation through dephosphorylation of the binding site for RasGAP on the RTK Torso.(33) The second model (model b, Fig. 2c) for Ras activation by SHP-2 proposes that SHP-2 promotes the activation of Src family kinases (SFKs) through dephosphorylation either of Csk binding protein (Cbp; also known as phosphoprotein associated with glycosphingolipid-enriched membrane microdomains [PAG]), a membrane-anchored phosphoprotein that binds COOH-terminal Src kinase (Csk),(34,35) or of paxillin, thereby preventing the access of Csk, a negative regulator of SFKs, to these enzymes.(36,37) The third model (model c, Fig. 2c) proposes that SHP-2 promotes Ras-MAPK activation through dephosphorylation of Sprouty, a negative regulator of the Ras-MAPK pathway whose tyrosine phosphorylation is indispensable for its inhibitory effect.(38) Tyrosine-phosphorylated Sprouty binds Grb2, preventing recruitment of the Grb2-Sos complex to FRS.(38,39) SHP-2 dephosphorylates Sprouty in response to growth factor stimulation, thereby preventing its inhibitory effect on Ras-MAPK activation. Such regulation of Sprouty by Csw (Drosophila SHP-2) has also been demonstrated.(40)

In addition to the regulation of mitogenic signaling, SHP-2 is implicated in the regulation of cell adhesion and migration, in part through control of the activity of the small GTP-binding protein Rho (Fig. 2b).(41–46) Studies with dominant negative, dominant active, or loss-of-function mutants of SHP-2 have thus shown that the regulation by SHP-2 of RTK- or integrin-dependent cell adhesion and migration is mediated, at least in part, by Rho, and that SHP-2 negatively regulates Rho activity in these processes.(44,45,47) SHP-2 was also found to function in a spatially and temporally specific manner as a positive or negative regulator of Rho activity in integrin-mediated cell adhesion and migration.(46) Although the mechanism by which SHP-2 regulates Rho activity is not completely understood, it is thought to involve regulation by SHP-2 of Vav2, a guanine nucleotide exchange factor for Rho family proteins.(44)

Intramolecular regulation of the PTP activity of SHP-2

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References

Biochemical and enzymatic studies have revealed that SHP-2 possesses only low PTP activity in its basal state. In contrast, synthetic phosphopeptides corresponding to the binding sites for the SH2 domains of SHP-2 on the PDGF receptor or IRS-1 were shown to markedly increase the PTP activity of SHP-2 in vitro,(48–50) suggesting that the activity of SHP-2 is regulated by an autoinhibitory mechanism mediated by its SH2 domains (Fig. 3a). Consistent with this notion, the crystal structure of SHP-2 indicated that the NH2-terminal SH2 domain indeed interacts with the PTP domain in the basal state, likely resulting in autoinhibition of PTP activity (Fig. 3b).(51) This conformation is maintained by hydrogen bonding between D61 and C459 residues through one water molecule as well as involving N58, G60, A72, G503, and Q506.(51) Furthermore, occupation of the NH2-terminal SH2 domain of SHP-2 by phosphotyrosine-containing ligands (such as tyrosine-phosphorylated RTKs or docking proteins) was proposed to disrupt the intramolecular interaction between the SH2 domain and the PTP domain, resulting in the latter domain becoming available for interaction with substrate (Fig. 3a). The residues of the NH2-terminal SH2 domain that interact with the PTP domain were found not to overlap with those essential for phosphotyrosine binding.(51,52) Indeed, two independent forms of SHP-2 (D61A and E76A) with mutations of residues in the NH2-terminal SH2 domain that interact with the catalytic domain manifested an increased PTP activity in an in vitro assay as well as functioned as dominant active mutants in elongation of animal caps of Xenopus embryos.(52)

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Figure 3. Intramolecular regulation of the protein tyrosine phosphatase (PTP) activity of SHP-2 and the distribution of SHP-2 mutations associated with Noonan syndrome (NS) or juvenile myelomonocytic leukemia (JMML). (a) Mechanism for regulation of the PTP activity of SHP-2. In the basal state, the NH2-terminal SH2 domain of SHP-2 interacts with the PTP domain (closed form), resulting in autoinhibition of PTP activity. In response to extracellular stimuli, SHP-2 binds via its SH2 domains to tyrosine-phosphorylated growth factor receptors or docking proteins such as insulin receptor substrate (IRS), resulting in its adoption of an open conformation (open form) that is catalytically active. PH, pleckstrin homology domain. (b) A ribbon diagram of the crystal structure of human SHP-2 is shown in the left panel. The NH2- and COOH-terminal SH2 domains are shown in brown and green, respectively. The PTP domain is shown in blue. The circled region in the left panel is depicted in the right panel, which shows amino acids that participate in the formation of hydrogen bonds (dotted lines) that mediate the interaction of the NH2-terminal SH2 domain with the PTP domain. Red sphere, oxygen; white sphere, carbon; blue sphere, nitrogen; yellow sphere, sulfur. (c) Distribution of residues of SHP-2 that are frequently mutated in NS or JMML.

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Association of PTPN11 mutations with NS and pediatric leukemia

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References

In 2001, PTPN11 (human SHP-2 gene) was identified as the susceptibility gene for NS.(53) NS is an autosomal dominant disorder with an estimated prevalence of 1 in 1000 to 2500 live births.(54) The main clinical features of NS are short stature, facial dysmorphia, and congenital heart defects. NS is also associated with two childhood leukemias, JMML and acute lymphoblastic leukemia, although these leukemias affect only a small percentage of NS patients. Missense mutations of PTPN11 have been found to be present in ~50% of individuals with a clinical diagnosis of NS.(53,54) Furthermore, the residues of SHP-2 commonly mutated in NS (including G60, D61, Y62, Y63, T73, Q79, N308, and G503) (Fig. 3c) either participate directly in the interaction between the NH2-terminal SH2 domain and the PTP domain or are located in close proximity to these interacting residues, suggesting that the pathogenesis of NS is related to a loss of autoinhibition of PTP activity resulting from disruption of the intramolecular interaction between the SH2 and PTP domains (Fig. 4a). Furthermore, in addition to the germline mutations of PTPN11 associated with NS, somatic mutations of PTPN11 have been identified in a substantial proportion (34%) of JMML patients without NS.(55) Somatic mutations of PTPN11 are also found in a small percentage of children with myelodysplastic syndrome (~10%), acute myeloid leukemia (AML) (~5%), or B-precursor acute lymphoblastic leukemia (~7%).(55–58) The residues of SHP-2 commonly mutated in JMML (D61, E69, A72, and E76 in the NH2-terminal SH2 domain) partly overlap with but are not identical to those associated with NS (Fig. 3c).(54) In contrast to the childhood condition, PTPN11 mutations appear to be rare in adult AML.(54,59)

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Figure 4. Roles of SHP-2 in human cancer. (a) Mutations of SHP-2 associated with Noonan syndrome (NS) or juvenile myelomonocytic leukemia (JMML) disrupt the intramolecular interaction between the NH2-terminal SH2 domain and the protein tyrosine phosphatase (PTP) domain and thereby result in a loss of autoinhibition of PTP activity. The constitutive activation of SHP-2 in the absence of growth factor stimulation results in aberrant activation of the Ras-MAPK pathway, which in turn leads to the development of NS or leukemia. (b) Gab-2, a pleckstrin homology (PH) domain-containing docking protein, binds and activates SHP-2 in response to a variety of cytokines and growth factors. Overexpression of Gab-2 in human breast cancer may result in hyperactivation of SHP-2 and consequent aberrant activation of the Ras-MAPK pathway. (c) CagA in Helicobacter pylori (H. pylori)-infected gastric epithelial cells undergoes tyrosine phosphorylation by Src family kinases (SFKs). Tyrosine-phosphorylated CagA serves as a docking protein for SHP-2 and thereby triggers aberrant activation of SHP-2 and the Ras-MAPK pathway and subsequent development of gastric cancer.

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Enzymatic analysis has revealed that the PTP activity of either NS- or leukemia-associated SHP-2 mutants is indeed greater than that of wild-type SHP-2.(55,60–63) Moreover, SHP-2 mutants associated with sporadic JMML as well as those related to NS-associated JMML manifest higher PTP activity than do those related to NS alone.(55,61–63) Forced expression of these various SHP-2 mutants in cultured mammalian cells together with growth factor stimulation resulted in prolonged MAPK activation in some studies(55,60) but not in others.(61,64,65) Mutations of PTPN11 that result in constitutive activation of the PTP activity of SHP-2 thus appear to induce aberrant activation of Ras and development of NS (Fig. 4a). The higher level of PTP activity conferred by certain mutations of PTPN11 may result in a higher level of Ras activation, leading to the development of JMML or other types of pediatric leukemia. Such a notion is also supported by evidence that gain-of-function mutations of K-Ras or N-Ras, or a homozygous loss of NF1, are also associated with NS or sporadic JMML.(66–69)

Further insight into the molecular mechanism by which mutation of PTPN11 causes NS or pediatric leukemia has been provided by the generation of ‘knock-in’ mice expressing the NS-associated mutation D61G.(65) Homozygous D61G mice were found to die in utero, whereas heterozygous (D61G/+) mice exhibited features characteristic of NS, including short stature, craniofacial abnormalities, and multiple cardiac defects.(65) The D61G/+mice also developed mild myeloproliferative disease with peripheral blood leukocytosis as well as mild myeloid hyperplasia in the spleen and bone marrow (BM). In addition, BM from D61G/+mice yielded growth factor-independent myeloid colonies, and BM progenitor cells showed increased sensitivity to interleukin (IL)-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) in a colony formation assay.(65) Given that the D61G mutation is found not only in NS but also (albeit infrequently) in sporadic JMML,(54,70) these observations provided the first evidence that mutations of PTPN11 indeed cause NS as well as myeloproliferative disease in vivo. Moreover, forced expression of leukemia-associated mutants of SHP-2 (E76K or D61Y), but not that of wild-type SHP-2, was found to promote transformation of BM cells or fetal liver cells.(71,72) The transformation potency of SHP-2 mutants associated with leukemia (either NS-related or sporadic) was shown to be much higher than that of NS-specific mutants.(71) Forced expression of leukemia-associated SHP-2 mutants also conferred the property of cytokine-independent formation of myeloid colonies as well as hypersensitivity to GM-CSF or IL-3 stimulation in hematopoietic cells.(71–73) Furthermore, forced expression of SHP-2 mutants (D61V, D61Y, or E76K) in macrophage progenitors enhanced the GM-CSF-induced activation of MAPK,(73) and mast cells derived from BM expressing mutant SHP-2 (D61Y or E76K) showed increased basal and IL-3-induced activity of MAPK and Akt or hyperphosphorylation of signal transducer and activator of transcription 5 (Stat5).(71) The transformation activity of the leukemia-associated SHP-2 mutant E76K was shown to require PTP activity as well as the ability of the SH2 domains to bind a tyrosine-phosphorylated target such as Gab-2.(71) Activating mutations of Ras or NF1 deficiency also cause JMML-like myeloproliferative disease as well as hypersensitivity of hematopoietic cells to GM-CSF and IL-3.(74,75) Together, these various observations indicate that hyperactivation by aberrantly activated Ras of GM-CSF- or IL-3-induced signaling pathways is responsible, at least in part, for the development of NS-associated as well as sporadic JMML.

Recent evidence suggests that the effect of leukemia-associated SHP-2 mutants on myeloid cell transformation involves inactivation of interferon consensus sequence-binding protein (ICSBP), also known as interferon regulatory factor (IRF)-8, a transcription factor that positively regulates NF1 transcription.(76) The tyrosine phosphorylation of ICSBP is required for its effect on NF1 transcription, and ICSBP was shown to be a substrate for a constitutively active SHP-2 mutant.(77) Dephosphorylation of ICSBP by constitutively active mutants of SHP-2 in myeloid progenitors may thus inhibit the transactivation activity of ICSBP and thereby down-regulate the abundance of NF1, resulting in hyperactivation of Ras signaling and excessive cell proliferation.(77) Although ICSBP deficiency itself induces myeloproliferative disease that progresses to AML over time in mice, expression of an active mutant (E76K) of SHP-2 cooperates with ICSBP deficiency to accelerate progression of AML.(76)

Cancer development associated with up-regulation of SHP-2 docking proteins

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References

SHP-2 is also implicated in cancer development by a mechanism different from that for leukemia caused by PTPN11 mutation (Fig. 4b). Gab-2, a pleckstrin homology domain-containing docking protein in hematopoietic cells, binds and activates SHP-2 in response to a variety of cytokines and is important for recruitment of SHP-2 to sites near the plasma membrane.(16) Forced expression of Gab-2 promotes proliferation of MCF10A human mammary cells, and expression of Gab-2 with an activated form of human EGFR-related 2 (HER2), a receptor-type protein tyrosine kinase, confers an invasive phenotype on these cells.(78) Such effects of Gab-2 require its binding site for SHP-2 and activation of MAPK. Furthermore, the expression of Gab-2 is increased in human breast cancer,(78,79) whereas mutation of PTPN11 is infrequent in this and other types of solid tumor.(59,80) Hyperactivation of SHP-2 due to an increased abundance of Gab-2 might thus give rise to breast cancer as a result of aberrant activation of the Ras-MAPK pathway.

Another example of cancer development attributable to increased expression of a SHP-2 docking protein might be that of gastric cancer associated with CagA-positive Helicobacter pylori (H. pylori) (Fig. 4c).(81) Infection with CagA-positive H. pylori is a risk factor for the development of gastric cancer, and the CagA protein, which is directly injected by the bacterium into gastric epithelial cells and undergoes tyrosine phosphorylation at its EPIYA motifs by SFKs, behaves as a docking protein, like Gab or IRS, for SHP-2.(82) Indeed, forced expression of CagA in gastric epithelial cells promotes sustained activation of MAPK.(83) Transgenic mice expressing CagA manifest gastric hyperplasia, with some of these animals also developing polyps or adenocarcinomas in the stomach.(84) These transgenic mice also manifest leukocytosis as well as IL-3 and GM-CSF hypersensitivity, and some of the animals develop myeloid leukemia(84) phenotypes similar to those of mice transplanted with BM cells expressing leukemia-associated mutants of SHP-2.(71)

Concluding remarks

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References

The convergence of biochemical analysis of SHP-2 with genetic analysis of PTPN11 in NS and leukemia has shown that SHP-2 is an important regulator of Ras, acting downstream of growth factor or cytokine receptors, and that gain-of-function mutations of SHP-2 give rise to human cancer. Following the discovery of mutations of PTPN11 in NS, Aoki's group has found germline mutations of H-Ras in Costello syndrome and those of K-Ras or BRAF in cardio-facio-cutaneous (CFC) syndrome, respectively.(85,86) Both syndromes are autosomal dominant disorders characterized by symptoms, such as facial dysmorphia, congenital heart defects, and mental retardation, that overlap with those of NS.(1,87) Moreover, patients with Costello syndrome have an increased risk for malignancy development.(87) Germline loss-of-function mutations of SPRED1 are also associated with an autosomal dominant disorder in humans that resembles NF1.(88) Spred1 is a member of the Sprouty-Spred family of proteins that act as negative regulators of the Ras–MAPK pathway. On the basis of these various observations, it has been proposed that disorders attributable to germline mutations of molecules that participate in the Ras-MAPK pathway should be termed ‘Ras-MAPK syndromes’.(87) The discovery of germline mutations of PTPN11 associated with NS has been key to the development of this concept.

It is now clear that the activity of Ras is aberrantly up-regulated as a consequence of various gene alterations both in sporadic cancers (such as pancreatic, lung, and colon cancers) and in Ras-MAPK syndromes (such as NS, NF1, and Costello syndrome). However, the reason why the different gene mutations that underlie the aberrant activation of Ras give rise to such different conditions remains unclear. Given that Ras activates multiple downstream signaling pathways in addition to the Raf-MEK-MAPK cascade, including those mediated by phosphoinositide 3-kinase or Rho family proteins,(89) the pathways activated by Ras in response to different gene mutations might be distinct and thereby give rise to the development of different conditions. Identification of aberrantly activated signaling molecules downstream of Ras in the various types of sporadic cancer or Ras-MAPK syndromes might provide new therapeutic targets for cancer treatment.

References

  1. Top of page
  2. Abstract
  3. Physiological functions of SHP-2
  4. Intramolecular regulation of the PTP activity of SHP-2
  5. Association of PTPN11 mutations with NS and pediatric leukemia
  6. Cancer development associated with up-regulation of SHP-2 docking proteins
  7. Concluding remarks
  8. Acknowledgment
  9. References
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