Functions of Shp2 in cancer

1Src homology region 2 (SH2)-containing protein tyrosine phosphatase 2 (Shp2), encoded by the ptpn11gene, is a non-receptor phosphotyrosine phosphatase. Shp2 is ubiquitously expressed in various vertebrate cells. Shp2 contains one protein tyrosine phosphatase (PTP) catalytic domain and two SH2 domains. Two tandem-arranged SH2 domains are found in the N-terminal region of Shp2 and a phosphatase domain is located in the C-terminal domain of Shp2 –. The N-SH2 domain is a conformational switch that binds and inhibits phosphatase or binds phosphoproteins and activates enzymes; whereas the C-SH2 domain contributes binding energy and specificity; however, the C-SH2 domain does not play a direct role in activation. Furthermore, Shp2 contains two tyrosine residues (Y542 and Y580), which can be phosphorylated in the presence of extracellular stimulation. Bennett et al. first identified Shp2 as a major phosphorylation site in response to platelet-derived growth factor (PDGF). Since then, many stimuli, including some cytokines and growth factors, have been found to activate Shp2. Ptpn11 is also the first identified proto-oncogene that encodes a tyrosine phosphatase and it has been extensively investigated in the field of cancer. Ptpn11-related phosphatase activity is implicated in the regulation of intracellular signalling activity –. Experimental and clinical data have also indicated that Shp2 promotes tumour progression in many types of cancer.

2Shp2 is closely related to cancer; for this reason, researchers have focused on the role of Shp2 in various types of cancer, and results have shown that Shp2 may be a new target of anti-neoplastic drugs. Since Zhou and Agazie first proposed that Shp2 is upregulated in breast cancer cells, various regulatory mechanisms of Shp2 in breast cancer have been found. For instance, Aceto et al. screened Shp2 signature genes, which are simultaneously activated in a large subset of human primary breast tumours associated with invasive behaviour and poor prognosis; this result provided new insights into the signalling cascades influencing tumour-initiating cells and a rationale to target Shp2 in breast cancer. Furthermore, Shp2 interacts with growth factor receptor (GFR)-bound protein 2/Grb2 associated binding protein 1 (Grb2/Gab1), participates in signal transducer and activator of transcription 1 (Stat1) regulation and promotes signal transduction of breast cancer markers, such as human epidermal growth factor (EGF) receptor 2 (Her2) to control tumour development. In this manner, the important position of Shp2 in breast cancer is established. Therefore, the function of this protein in gastric and lung cancers has been investigated,. Shp2 may also play an important role in the progression of oral squamous cell carcinoma (OSCC). Moreover, Shp2 expression is negatively correlated with patient prognosis; Shp2 further promotes tumourigenesis in laryngeal cancer; and the mitogen-activated protein kinase (MAPK) pathway is involved in Shp2-induced growth of laryngeal cancer cells. Another study found that Shp2 expression is also correlated with human papillomavirus infected cervical cancer, and it also promotes cell proliferation by inhibiting interferon (IFN)-β production.

3Ptpn11 is a crucial oncogene that has been extensively investigated. Nevertheless, ptpn11/Shp2 exhibits a tumour-suppressing function in liver cancer; Yang et al. has suggested that ptpn11 suppresses tumourigenesis in cartilage, and this finding indicated that the function of ptpn11 is tissue specific. Shp2 deficiency is oncogenic in cartilage cell population characterized by cathepsin K expression. In these cells, extracellular regulated protein kinases (Erk) normally repress the expression of the growth stimulator Indian hedgehog and the production of parathyroid hormone-related protein. Monitoring of the tumour microenvironment and other compensatory pathways should be strengthened to avoid the abuse of pathway inhibitors.

4Based on the structure of Shp2, phosphorylation sites and phosphatase activity have been commonly investigated. As a phosphatase, its activity is also implicated in diverse cancers via Shp2 mutants. In this article, this implication is introduced. Several studies have focused on the extent of Shp2 phosphorylation activation, not its phosphatase activity. A previous report has suggested that phosphorylated Tyr 542 and Tyr 580 can interact intramolecularly with N-SH2 and C-SH2 domains, respectively; thus, this interaction, prevents basal inhibition of phosphatase activity. This type of relationship has established the association between phosphatase activity and Shp2 phosphorylation.

5Receptor tyrosine kinase (RTK) activates a series of signal transduction pathways and affects tumour progression. Shp2 is a phosphatase; hence, RTK signalling related to this phosphatase has been commonly explored. Furthermore, previous studies demonstrated that Shp2 is a critical mediator involved in the activation of the small G protein Ras-Erk (Ras-Erk) signalling pathway,.

6The expression of Shp2 catalytically inactive mutant C459S inhibits Erk activation in response to insulin but not in response to 12-O-tetradecanoyl phorbol-13-acetate. Likewise, Shp2 can be phosphorylated by stimulation with growth factors or cytokines, such as EGF, and hepatocyte growth factor, to activate the Ras-Erk signalling pathway. Glycoprotein gp130 (CD130) and GFR interact with Shp2 to mediated signalling; as such, the mechanism by which Shp2 is phosphorylated by various exogenous stimuli has been widely studied. Furthermore, one (or both) of these tyrosines should be phosphorylated to activate Shp2. Correspondingly, Miura suggested that tyrosine phosphorylation of Shp2, not phosphatase activity of Shp2 is implicated in Erk activation. The molecular mechanism of Shp2 phosphorylation activity has been investigated, particularly kinase, which stimulates Shp2 phosphorylation. EphA2 is identified as a tyrosine kinase that phosphorylates Tyr542 and Tyr580 of Shp2 to enhance and prolong Erk activation in cells stimulated by growth factors. Thus, whether Shp2 phosphatase activity or phosphorylation level is necessary to activate the Erk pathway remains controversial. Moreover, whether these factors cause different phenomena under various stimuli remains unknown.

7Although experimental data have shown that Shp2 plays a key role in the activation of the Erk signalling, Tseng et al. demonstrated that Shp2 influences IFN-γ resistance but does not affect hyperproliferation or Erk activation in gastric cancer by participating in PI3K-Akt signalling.

8Grb2 controls fibroblast GFR 2 (FGFR2) phosphorylation by inhibiting receptor kinase and Shp2 phosphatase activity. Thus, Shp2 is a critical mediator of the activation of the PI3K-Akt signalling pathway. Epidermal growth factor induces rapid and transient interaction of Shp2 with Gab1; in turn, this interaction mediates association with EGFR and activation of PI3K.

9Grb2 should be considered in studies in involving Shp2, because the interaction between Grb2 and Shp2 is essential to activate the downstream pathway of Erk,,. Activated Shp2 recruits Grb2; phosphorylated Tyr580 of Shp2 functions as the main binding site of Grb2, thereby activating Ras in response to growth factor. In addition, Grb2 controls FGFR2 phosphorylation by inhibiting receptor kinase and Shp2 phosphatase activity.

10Activated Shp2 can downregulate tyrosine phosphorylation of Stat3, which promotes the Noonan syndrome (NS) and juvenile myelomonocytic leukaemia (JMML). Although Stat3 activation is essential for cancer progression, especially breast cancer,, however, Shp2 is a proto-oncogene promotes breast cancer; these conflicting results indicated that an unknown mechanism should be further investigated. Shou et al. proposed that the negative regulation of Shp2-Stat3 and positive promotion of the Shp2-Ras pathways are synergistic, but the mechanism is still unclear. We deduce that Shp2 is an adaptor protein that recruits the Grb2/Sos component and activates MAPK, and the scaffolding role is dependent on tyrosine phosphorylation. Its phosphorylation enhances the phosphatase activity of Shp2, so p-Shp2 can bind to some substrates to function.

11Shp2 mutation has been detected in several diseases, such as NS, childhood leukaemia, and human malignancies,. Ptpn11 gene mutations are common in patients with NS and LEOPARD syndrome (LS), two developmental disorders with pleiomorphic phenotypes. These conditions are mainly caused by mutations of the ptpn11 gene that catalytically inactivates tyrosine phosphatase Shp2 in LS but activates this phosphatase in NS. Two recurrent mutations, namely, Tyr279Cys and Thr468Met, have also been identified in patients with LS; Thr468Met mutation is used to construct animal models of LS. The mutation Gln506Pro is in the PTP domain of Shp2. This region is a common site of mutation, in which Shp2 is activated to a great extent when residues directly involved in binding at the interface between the N-terminal Src homology 2 and PTP domains are altered. Such mutants prolong signal flux via theErk2/MAPK1 pathway; this mechanism requires docking via Grb2-associated binder-1 (Gab1), thereby promoting cell proliferation.

12Shp2 mutants are related to cancer. Mutations in ptpn11 occur at low frequencies in several human cancers, particularly neuroblastoma and acute myelogenous leukaemia (AML). Leukaemia-associated mutant Shp2-E76K is one of the most common and active ptpn11 mutation found in leukaemia and solid tumours. Shp2-E76K is associated with Gab1 in the lungs of transgenic mice. When activated, Shp2 mutants promote lung tumourigenesis; thus, Shp2 mutants are essential for tumour maintenance in the mouse model of non-small cell lung cancer (NSCLC). PHPS1 (specific inhibitor of Shp2) efficiently inhibits Erk1/2 activation by Shp2-E76K, a leukaemia-associated Shp2 mutant, and blocks the anchorage-independent growth of various human tumour cell lines. Shp2-Q51E, a dominant-negative loss of function mutation, increases cell migration and causes hypertrophic cardiomyopathy by dysregulating mTOR signalling.

13Although several mutants, such as D61G, Y279C, N308D, T468M and E76K have been studied and compared with Shp2-N308D, Shp2-E76K possesses higher phosphatase activity. Moreover, the frequency of mutation in tumours is not very high. Nevertheless, this result provided the basis for studying Shp2 activity. Furthermore, gene mutation is an effective mechanism to understand gene functions. In cancer, the phosphatase activity of Shp2 is a result of mutations and may serve as a switch in different signal stimulations to reveal different pathways.

14Shp2 mediates epithelial mesenchymal transition (EMT) and is upregulated in breast cancer cells. Various regulatory mechanisms of Shp2 in breast cancer have also been found. Shp2 depletion prevents invasion in vivo, and Shp2 knockdown in established breast tumours inhibits growth and impedes metastasis. In triple-negative breast cancer (TNBC) cells, Shp2 affects motility in vivo, as well as cell migration and invasion in vitro, by activating several SRC-family kinases and downstream targets. In CXC chemokine ligand-12 -induced chemotaxis and chemoinvasion in breast cancer cells, Shp2 functions as a kind of component of the multimeric complex that mediates these processes. Shp2 also modulates the activity of focal adhesion kinase (Fak) by dephosphorylating pTyr397 to mediate lamellipodia persistence and cell polarity; in turn, cell migration in MDA-MB231 and MDA-MB468 basal-like and TNBC cell line is promoted. Shp2 overexpression is positively related to Her2 overexpression, high tumour grade and lymph node metastasis.

15In other cancers, such as OSCC, Shp2 overexpression is associated with advanced tumour clinical stages and lymph node metastasis ex vivo. The knockdown of Shp2 expression in vitro inhibits OSCC cell viability and invasion. Furthermore, Shp2 promotes invasion and metastasis of oral cancer cells; this result indicated that the Shp2-ERK1/2-Snail/Twist1 pathway is possibly implicated in oral cancer invasion and metastasis. Clinical data have also suggested that Shp2 expression in NSCLC exhibits high specificity and sensitivity, and this expression is closely related to lymph node metastasis. Shp2 expression may promote invasion and metastasis of NSCLC via angiogenesis and via the lymphatic system,. Transforming growth factor-β1-induced EMT in lung epithelial A549 cells is partially blocked when Shp2 is decreased by transfected siRNA; tyrosine phosphatase Shp2 is involved in EMT, and Hook1 represses EMT by regulating Shp2 activation. The Shp2-Hook1 (hook microtubule-tethering protein 1) complex may also play important roles in tumour metastases by regulating EMT. The mRNA levels of Shp2 are significantly higher in gastric cancer tissues than those in normal gastric mucosa. In addition, Shp2 expression is significantly correlated with tumour differentiation, clinical classification and lymph node metastasis. The migration of anaplastic large cell lymphoma cells is reduced by Shp2 shRNA. These findings showed that Shp2 is directly involved in nucleophosmin/anaplastic lymphoma kinase lymphomagenesis, highlighting its critical role in lymphoma cell proliferation and migration. Moreover, the interaction with Y580-Shp2 localizes Fyn to receptor sites required for α6β4-dependent carcinoma invasion. The knockdown of Shp2 significantly increases podosome rosette formation in Src-transformed fibroblasts by selectively suppressing the tyrosine phosphorylation of Src substrate Tks5, a scaffolding protein necessary to form podosome; this finding may elucidate the mechanism by which tumour metastasis is promoted.

16In cancer research, the apoptotic role of Shp2 was first discovered in multiple myeloma cells. Chauhan et al., demonstrated that activated Shp2 inhibits the activation of a related adhesion focal tyrosine kinase, also known as Pyk2, and Shp2 is involved in the process by which interleukin-6 (IL-6) blocks apoptosis induced by dexamethasone. Leukaemia, a Shp2-associated disease, has been intensively investigated. The suppression of Shp2 expression induces apoptosis and inhibits leukaemic cell clonogenic growth. In addition, the knockdown of Shp2 expression in vitro induces OSCC cell apoptosis by regulating the expression of apoptosis-related proteins. The activation of Shp2 PTP is synergized with IFN consensus sequence binding protein haploinsufficiency to facilitate cytokine-induced myeloproliferation, apoptosis resistance and rapid progression to AML in a murine bone marrow transplantation model.

17Yang et al., showed that tyrosine phosphatase Shp2 prevents apoptosis in tumour stem cells by activating Erk. The expression of gain of function (GOF) mutation Shp2-E76K, the most common and active ptpn11mutation found in leukaemia and solid tumours, suppresses the apoptosis pathway. Shp2-D61Y or Shp2-E76K-expressing hematopoietic cells also reduce apoptosis, as indicated by Annexin-V staining results, and they produces increased progenitor colonies after 48 hrs in minimal media compared with cells transduced with an empty vector or wildtype of Shp2. These results proved that the phosphatase activity of Shp2 plays a key role in controlling apoptosis.

18Shp2 is associated with apoptosis suppression. However, liver cancer cells showed anoikis when treated with arecoline; furthermore, IL-6 expression and Stat3 phosphorylation provide protection against anoikis; caspase-3 activity is increased and Shp2 is inhibited by arecoline. In another study, the knockdown of Shp2 inhibits sorafenib-induced Tyr(705) p-Stat3 dephosphorylation and increases tumour cell apoptosis in cholangiocarcinoma cells. These results were consistent with that of Feng et al., who demonstrated that ptpn11/Shp2acts as a tumour suppressor in hepatocellular carcinogenesis. These conflicting results are observed in a U2OS osteosarcoma cell line, in which the knockdown of Shp2 expression with small interfering RNA in apoptotic cells increases cell viability and rescues cells from retinoblastoma/transcription factor E2F-associated apoptotic response to inhibition of cleavage of both caspase-8 and caspase-3. Tumour necrosis factor-induced EC apoptosis is possibly reduced significantly in Shp2-knockout EC by regulating apoptosis signal-regulating kinase 1 phosphorylation and stability in response to cytokines.

19In addition, Shp2 plays an essential role in controlling the survival and maintenance of hematopoietic stem cells by decreasing apoptosis in vivo,. Shp2-knockout mice exhibit a remarkable reduction in surfactant proteins with increased alveolar epithelial apoptosis. Furthermore, Cat.G induces Shp2 activation that leads to Fak tyrosine dephosphorylation and promotes cardiomyocyte anoikis. The inactivation of Shp2 sensitizes cells to epigallocatechin gallate (EGCG)-mediated death, and mouse embryonic fibroblasts without functional Shp2 undergo massive apoptosis after these factors are treated with EGCG; thus, Shp2 serves as a negative regulator of EGCG-induced apoptosis. Shp2 may contribute to Erk5 activation by participating in Src kinase activation and by docking to PDGF receptor beta, such that PDGF-BB fails to suppress caspase-3 activation and inhibit apoptotic nuclear morphological changes. Shp2 is needed to prevent pulmonary arterial hypertension-pulmonary artery smooth muscle cells apoptosis. These results may help explain Shp2-regulated apoptosis.

20Shp2 is demonstrated as a tumour-promoting gene by regulating invasion and apoptosis; Shp2 is also a factor that promotes cell proliferation –. In cancer, Shp2 regulates multivariate signalling regulation to control proliferation in glioma cells. Tyrosine phosphatase Shp2 also promotes the proliferation of breast carcinoma. These phenomena provide the basis for investigating the role of Shp2 in the cell cycle.

21Yang et al. suggested that GOF Shp2 mutants promote hematopoietic progenitor cell cycle progression and survival. In another report, agents targeting cell cycle or promoting apoptosis were found to have therapeutic potential in JMML. Microinjection of antibodies can block the interaction of the SH2 domains of the PI3K p85α subunit with tyrosine phosphorylated intracellular targets that inhibit DNA synthesis; thus, when antibodies to Shp2 are injected during the first 15 min. of the G1 phase, DNA synthesis is inhibited. Shp2 promotes the growth of glioblastoma cells by suppressing cellular senescence. Shp2 and Stat5 function as proximal effectors of the Kit oncogene, and cell survival is driven by the Shp2/Erk pathway; conversely, G1/S transition during the cell cycle is accelerated by the Kit/Stat5 and Kit/PI3K/Akt pathways. Shp2 is also involved in radioresistance by controlling cell cycle distribution in nasopharyngeal carcinoma cell lines. In HeLa cell line, Shp2 depletion arrests checkpoint-mediated cell; these results indicated the importance of Shp2 in checkpoint control and revealed a novel link between Shp2 and cell cycle checkpoints. These findings may also explain the function of Shp2 in cell proliferation and provide a new direction for novel drug research; however, additional evidence should be obtained to support these data.

22The DNA replication function of Shp2 is still a new research direction. In a previous study, ptpn11 (Shp2) is involved in the p53 pathway, including anti-apoptotic pathways, structural loss, and DNA replication. Further detailed studies have revealed that Shp2 is necessary to maintain checkpoints after DNA damage is induced by cisplatin or ionizing radiation in HeLa cells; Shp2 is also activated after cells are exposed to replicative stress and DNA damage, and Shp2 depletion impairs checkpoint kinase 1 activation and checkpoint-mediated DNA repair. In another study, Shp2 is related to DNA damage 45G (GADD45G), a stress sensor with multiple implications in various biological processes; this stress sensor is downregulated in a broad spectrum of cancers. The ectopic expression of GADD45G induces senescence in hepatocellular carcinoma (HCC) cells and suppresses tumour growth in vivo. The knockdown of Shp2 efficiently counteracts GADD45G-induced senescence. In clinical HCC specimens, GADD45G expression is inversely correlated with phosphorylated Stat3 expression in tumour cells and disease progression; this result was consistent with the relationship of Shp2 with Stat3,.

23Shp2 is involved in the treatment of the EGFR signalling pathway of H1975 cells, in which II-B08, a specific Shp2 inhibitor, is used to reduce Erk1/2 activation; therefore targeting Shp2 may represent an effective strategy for the treatment of EGFR inhibitor resistant NSCLCs. Gastric cancer cell line AGS does not respond to IFN-γ, and PI3K/AKT mediates IFN-γ resistance. Tseng et al. reported that IFN-γ resistance is regulated by Pten/Akt/GSK-3β/Shp2 signalling in hyperproliferating gastric cancer cells.

24Traditional radiotherapy and chemotherapy are modestly effective cancer treatments; nevertheless, recent advances in targeted therapies have provided a noticeable benefit to patients. However, patients eventually develop resistance to drugs. Combination therapy directed to a complementing target may significantly improve treatment results. Although the role of Shp2 in drug resistance has been partially revealed, Shp2 is involved in key signalling pathways. Therefore, the inhibitor of Shp2 may be used in combination therapy drugs.

25Cancer poses health risks to humans. In cancer, Shp2 plays different roles in various tumours and different microenvironments. Although great progress has been observed in studies focusing on Shp2-related mechanisms, specific processes involved in such mechanisms should be further investigated. Taken together, Figure1 is the summary of signalling pathways and functions of Shp2.

26Based on the role of Shp2 in tumours, various Shp2 inhibitors have been discovered to target Shp2 for cancer treatments. Inhibitors of tyrosine phosphatase Shp2 have been widely studied because of its broad role in cancer. For instance, cryptotanshinone can be potentially used directly or developed to treat ptpn11-associated malignancies, mouse myeloid progenitors and leukaemic cells caused by E76K mutation are sensitive to this inhibitor. II-B08 can inhibit Shp2 and strongly bind to the receptor. Shp2 inhibitor II-B08 enhances the effects of dasatinib on human and mouse mastocytoma cells. Furthermore, computer-aided drug designs are used to discover Shp2 inhibitors. Shp2 inhibitors play only a partial role, but these inhibitors have shown promising results for developing drugs to treat cancers.

27Although the detailed mechanism of Shp2 in cancer progression needs further investigation and the activity of Shp2 in tumours has been analysed to provide a theoretical basis for cancer treatment, better research ideas and more definitive results may help develop successful therapeutic strategies for this deadly disease. Targeted studies have revealed that a combination of inhibitors may be required to effectively block a given function in cancer research. Studies that broaden our understanding of the functions of Shp2 could lead to a re-evaluation of the role in determining clinical outcome. However, future studies of the clinical importance should be carefully designed to explain conflicting viewpoints. Drugs should be used with caution as a result of the different functions of Shp2 in various signalling pathways and cancer types. Ultimately, future studies should focus on confirming the effects of Shp2 on tumours in different tumour micro-environments, as well as the signalling pathway, including the substrate of Shp2 phosphatase activity. The overall effects of microenvironments should be studied by combining several factors.