Sprouty and cancer: The first terms report

The FRS2 family of docking/scaffolding adaptor proteins as therapeutic targets of cancer treatment

BACKGROUND

There are two members--FRS2alpha and FRS2beta--in the fibroblast growth factor receptor substrate 2 (FRS2) family of docking/scaffolding adaptor proteins. These proteins function downstream of certain kinds of receptor tyrosine kinases (RTKs) that are important for tumorigenesis. FRS2alpha acts as a control centre for fibroblast growth factor receptor signalling and encourages tumorigenesis, while FRS2beta regulates EGFR signalling negatively, and might have a tumour suppressive role. Therefore, both proteins could be good therapeutic targets for the treatment of cancer.

OBJECTIVE

To examine the physiological and pathological roles of FRS2, especially in cancer, and describe their potential value as therapeutic targets.

METHODS

A review of relevant literature.

RESULTS/CONCLUSIONS

Although it is still difficult to develop small compounds to modify functions of FRS2 adaptor proteins, such compounds may be useful as the next generation of molecular targeting drugs. Combination therapy with RTK-targeting drugs and FRS2-targeting drugs may be useful for cancer treatment in the near future.

Intermolecular interactions of Sprouty proteins and their implications in development and disease

Receptor tyrosine kinase (RTK) signaling is spatially and temporally regulated by a number of positive and negative regulatory mechanisms. These regulatory mechanisms control the amplitude and duration of the signals initiated at the cell surface to have a normal or aberrant biological outcome in development and disease, respectively. In the past decade, the Sprouty (Spry) family of proteins has been identified as modulators of RTK signaling in normal development and disease. This review summarizes recent advances concerning the biological activities modulated by Spry family proteins, their interactions with signaling proteins, and their involvement in cardiovascular diseases and cancer. The diversity of mechanisms in the regulation of Spry expression and activity in cell systems emphasizes the crucial role of Spry proteins in development and growth across the animal kingdom.

Kit K641E oncogene up-regulates Sprouty homolog 4 and trophoblast glycoprotein in interstitial cells of Cajal in a murine model of gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GIST) are thought to derive from the interstitial cells of Cajal (ICC) or an ICC precursor. Oncogenic mutations of the receptor tyrosine kinase KIT are present in most GIST. KIT K642E was originally identified in sporadic GIST and later found in the germ line of a familial GIST cohort. A mouse model harbouring a germline Kit K641E mutant was created to model familial GIST. The expression profile was investigated in the gastric antrum of the Kit(K641E) murine GIST model by microarray, quantitative PCR and immunofluorescence. Gja1/Cx43, Gpc6, Gpr133, Pacrg, Pde3a, Prkar2b, Prkcq/Pkce, Rasd2, Spry4 and Tpbg/5T4 were found to be up-regulated. The proteins encoded by Gja1/Cx43, Pde3a, Prkcq/Pkce were localized in Kit-ir ICC in wild-type and Kit(K641E) animals while Spry4 and Tpbg/5T4 were detected in Kit-ir cells only in Kit(K641E), but not in Kit(WT/WT) animals. Most up-regulated genes in this mouse model belong to the gene expression profile of human GIST but also to the profile of normal Kit(+) ICC in the mouse small intestine. Spry4 and Tpbg/5T4 may represent candidates for targeted therapeutic approaches in GIST with oncogenic KIT mutations.

Expression of sprouty2 inhibits B-cell proliferation and is epigenetically silenced in mouse and human B-cell lymphomas

B-cell lymphoma is the most common immune system malignancy. TCL1 transgenic mice (TCL1-tg), in which TCL1 is ectopically expressed in mature lymphocytes, develop multiple B- and T-cell leukemia and lymphoma subtypes, supporting an oncogenic role for TCL1 that probably involves AKT and MAPK-ERK signaling pathway augmentation. Additional, largely unknown genetic and epigenetic alterations cooperate with TCL1 during lymphoma progression. We examined DNA methylation patterns in TCL1-tg B-cell tumors to discover tumor-associated epigenetic changes, and identified hypermethylation of sprouty2 (Spry2). Sprouty proteins are context-dependent negative or positive regulators of MAPK-ERK pathway signaling, but their role(s) in B-cell physiology or pathology are unknown. Here we show that repression of Spry2 expression in TCL1-tg mouse and human B-cell lymphomas and cell lines is associated with dense DNA hypermethylation and was reversed by inhibition of DNA methylation. Spry2 expression was induced in normal splenic B cells by CD40/B-cell receptor costimulation and regulated a negative feedback loop that repressed MAPK-ERK signaling and decreased B-cell viability. Conversely, loss of Spry2 function hyperactivated MAPK-ERK signaling and caused increased B-cell proliferation. Combined, these results implicate epigenetic silencing of Spry2 expression in B lymphoma progression and suggest it as a companion lesion to ectopic TCL1 expression in enhancing MAPK-ERK pathway signaling.

Sprouty4 negatively regulates protein kinase C activation by inhibiting phosphatidylinositol 4,5-biphosphate hydrolysis

Sproutys have been shown to negatively regulate growth factor-induced extracellular signal-regulated kinase (ERK) activation, and suggested to be an anti-oncogene. However, molecular mechanism of the suppression has not yet been clarified completely. Sprouty4 inhibits vascular endothelial growth factor (VEGF)-A-induced ERK activation, but not VEGF-C-induced ERK activation. It has been shown that VEGF-A-mediated ERK activation is strongly dependent on protein kinase C (PKC), whereas that by VEGF-C is dependent on Ras. This suggests that Sprouty4 inhibits the PKC pathway more specifically than the Ras pathway. In this study, we confirmed that Sprouty4 suppressed various signals downstream of PKC, such as phosphorylation of MARCKS and protein kinase D (PKD), as well as PKC-dependent nuclear factor (NF)-kappaB activation. Furthermore, Sprouty4 suppressed upstream signals of PKC, such as Ca(2+) mobilization, phosphatidylinositol 4,5-biphosphate (PIP(2)) breakdown and inositol 1,4,5-triphosphate (IP(3)) production in response to VEGF-A. Those effects were dependent on the C-terminal cysteine-rich region, but not on the N-terminal region of Sprouty4, which is critical for the suppression of fibroblast growth factor (FGF)-mediated ERK activation. Sprouty4 overexpression or deletion of the Sprouty4 gene did not affect phospholipase C (PLC) gamma-1 activation, which is an enzyme that catalyzes PIP(2) hydrolysis. Moreover, Sprouty4 inhibited not only VEGF-A-mediated PIP(2) hydrolysis but also inhibited the lysophosphatidic acid (LPA)-induced PIP(2) breakdown that is catalyzed by PLC beta/epsilon activated by G-protein coupled receptor (GPCR). Taken together, Sprouty4 has broader suppression activity for various stimuli than previously thought; it may function as an inhibitor for various types of PLC-dependent signaling as well as for ERK activation.

Derailed endocytosis: an emerging feature of cancer

Once engaged by soluble or matrix-anchored ligands, cell surface proteins are commonly sorted to lysosomal degradation through several endocytic pathways. Defective vesicular trafficking of growth factor receptors, as well as unbalanced recycling of integrin- and cadherin-based adhesion complexes, has emerged in the past 5 years as a multifaceted hallmark of malignant cells. In line with the cooperative nature of endocytic machineries, multiple oncogenic alterations underlie defective endocytosis, such as altered ubiquitylation (Cbl and Nedd4 ubiquitin ligases, for example), altered cytoskeletal interactions and alterations to Rab family members. Pharmaceutical interception of the propensity of tumour cells to derail their signalling and their adhesion receptors may constitute a novel target for cancer therapy.

Down-regulation of Sprouty2 via p38 MAPK plays a key role in the induction of cellular apoptosis by tumor necrosis factor-alpha

Mammalian Sprouty2 (Spry2) is a key regulator of the receptor tyrosine kinase/ERK signaling pathway and involved in many biological processes, including cell growth, migration, and tumor suppression. Here, we demonstrated that the intracellular protein level of Spry2 was significantly down-regulated by tumor necrosis factor-alpha (TNF-alpha) in both murine Swiss 3T3 fibroblasts and MLE15 lung epithelial cells. Although TNF-alpha activates multiple signaling cascades, only the inhibitor of p38 MAPK pathway blocked TNF-alpha-induced Spry2 down-regulation. Moreover, since both the mRNA level and protein half-life of Spry2 were unaltered by TNF-alpha treatment, this indicated the possible involvement of a translational mechanism in mediating the inhibitory effect of TNF-alpha. Importantly, rescue of the TNF-alpha-induced down-regulation of Spry2 by gene overexpression led to reverse of the apoptotic effect of TNF-alpha in Swiss 3T3 cells. To our knowledge, this study is the first that reported the association of Spry2 with TNF-alpha signaling pathway.

Regulation of growth factor signaling by FRS2 family docking/scaffold adaptor proteins

The FRS2 family of adaptor/scaffold proteins has two members, FRS2alpha and FRS2beta. Both proteins contain N-terminal myristylation sites for localization on the plasma membrane and a PTB domain for binding to limited species of receptor tyrosine kinases (RTKs), including the FGF receptor, the neurotophin receptor, RET, and ALK. Activation of these RTKs allows FRS2 proteins to become phosphorylated of tyrosine residues and then bind to Grb2 and Shp2, a SH2 domain-containing adaptor and a tyrosine phosphatase, respectively. Subsequently, Shp2 activates a Ras/ERK pathway and Grb2 activates a Ras/ERK, phosphatidyl inositol (PI)-3 kinase and ubiquitination/degradation pathways by binding to SOS, Gab1, and Cbl via the SH3 domains of Grb2. FRS2alpha acts as 'a conning center' in FGF signaling mainly because it induces sustained levels of activation of ERK via Shp2-binding sites and Grb2-binding sites, though the contribution of the former is greater. Indeed, FRS2alpha knockout mice and mice with mutated Shp2-binding sites exhibit a variety of phenotypes due to defects in FGF signaling in vivo. Although FRS2beta binds to the EGF receptor, it does not induce tyrosine phosphorylation on the receptor. Instead, it inhibits EGF signaling, resulting in inhibition of EGF-induced cell proliferation and cell transformation. Based on these findings, the involvement of FRS2 proteins in tumorigenesis should be studied extensively to be validated as candidate biomarkers for the effectiveness of treatments targeting RTKs such as the FGF receptor and EGF receptor.

Modulation of endocrine pancreas development but not beta-cell carcinogenesis by Sprouty4

Sprouty (Spry) proteins modulate signal transduction pathways elicited by receptor tyrosine kinases (RTK). Depending on cell type and the particular RTK, Spry proteins exert dual functions: They can either repress RTK-mediated signaling pathways, mainly by interfering with the Ras/Raf/mitogen-activated protein kinase pathway or sustaining RTK signal transduction, for example by sequestering the E3 ubiquitin-ligase c-Cbl and thus preventing ubiquitylation, internalization, and degradation of RTKs. Here, by the inducible expression of murine Spry4 in pancreatic beta cells, we have assessed the functional role of Spry proteins in the development of pancreatic islets of Langerhans in normal mice and in the Rip1Tag2 transgenic mouse model of beta-cell carcinogenesis. beta cell-specific expression of mSpry4 provokes a significant reduction in islet size, an increased number of alpha cells per islet area, and impaired islet cell type segregation. Functional analysis of islet cell differentiation in cultured PANC-1 cells shows that mSpry4 represses adhesion and migration of differentiating pancreatic endocrine cells, most likely by affecting the subcellular localization of the protein tyrosine phosphatase PTP1B. In contrast, transgenic expression of mSpry4 during beta-cell carcinogenesis does not significantly affect tumor outgrowth and progression to tumor malignancy. Rather, tumor cells seem to escape mSpry4 transgene expression.

Epigenetic inactivation of the ERK inhibitor Spry2 in B-cell diffuse lymphomas

Spry2 has been characterized as a negative regulator of the extracellular-regulated kinase (ERK) pathway. In this study we analysed whether epigenetic alterations of hSpry2 promoter occur in human lymphoid/hematopoietic malignancies. Our results revealed that hSpry2 promoter was hypermethylated in the HT cell line derived from a B-cell diffuse lymphoma, which correlated with decreased hSpry2 expression. We detected deregulation of the ERK pathway in these cells, but not in other blood cell lines expressing hSpry2. In addition, the ectopic overexpression of hSpry2 in HT cells drastically reduced the activation of ERK upon phorbol 12-myristate-13-acetate stimulation. Nude mice inoculated with HT mock cells developed tumors seven times larger than those from HT-hSpry2-transfected cells. We found hypermethylation of hSpry2 promoter in 37% (26 cases out of 71) of primary tumors from patients with B-cell diffuse lymphoma but none in normal B lymphocytes from 37 healthy individuals. Finally, we detected that hSpry2 promoter hypermethylation was associated with a significant decrease in the 5-year survival rate. These data suggest that hSpry2 could be important in lymphoid malignancies.

Integration of genomic analysis and in vivo transfection to identify sprouty 2 as a candidate tumor suppressor in liver cancer

Hepatocellular carcinoma (HCC) is 1 of the leading causes of cancer-related deaths worldwide, yet the molecular genetics underlying this malignancy are still poorly understood. In our study, we applied statistical methods to correlate human HCC gene expression data obtained from complementary DNA (cDNA) microarrays and corresponding DNA copy number variation data obtained from array-based comparative genomic hybridization. We have thus identified 76 genes that are up-regulated and show frequent DNA copy number gain, and 37 genes that are down-regulated and show frequent DNA copy loss in human HCC samples. Among these down-regulated genes is Sprouty2 (Spry2), a known inhibitor of receptor tyrosine kinases. We investigated the potential role of Spry2 in HCC by expressing dominant negative Spry2 (Spry2Y55F) and activated beta-catenin (DeltaN90-beta-catenin) in the mouse liver through hydrodynamic injection and sleeping beauty-mediated somatic integration. When stably expressed in mouse hepatocytes, Spry2Y55F cooperates with DeltaN90-beta-catenin to confer a neoplastic phenotype in mice. Tumor cells show high levels of expression of phospho-extracellular signal-regulated kinase (ERK), as well as deregulation of genes involved in cell proliferation, apoptosis, and angiogenesis.

CONCLUSION

We identified a set of candidate oncogenes and tumor suppressor genes for human HCC. Our study provides evidence that inhibition of Spry activity cooperates with other oncogenes to promote liver cancer in mouse models, and Spry2 may function as a candidate tumor suppressor for HCC development in vivo. In addition, we demonstrate that the integration of genomic analysis and in vivo transfection is a powerful tool to identify genes that are important during hepatic carcinogenesis.

Many roads lead to oncogene-induced senescence

Oncogene-induced senescence is a mechanism of tumor suppression that restricts the progression of benign tumors. Important advances have been made toward elucidating the mechanisms that regulate this response; however, there is presently no unified model that integrates all current findings. DNA damage, replicative stress, reactive oxygen species, heterochromatin formation and negative feedback signaling networks have all been proposed to play an integral role in promoting senescence in response to various oncogenic insults. In all cases, these signals have been shown to function through Rb and p53, but utilize different intermediaries. Thus, it appears that senescence is not triggered by a single, linear series of events, but instead is regulated by a complex signaling network. Accordingly, multiple proteins may cooperate to establish a senescence response, but the limiting signal(s) may be dictated by the initiating genetic alteration and/or tissue type. This review will focus on integrating current models and will highlight data that provide new insight into the signals that function to suppress human tumor development.

Tesk1 interacts with Spry2 to abrogate its inhibition of ERK phosphorylation downstream of receptor tyrosine kinase signaling

The Sprouty (Spry) proteins function as inhibitors of the Ras-ERK pathway downstream of various receptor tyrosine kinases. In this study, we have identified Tesk1 (testicular protein kinase 1) as a novel regulator of Spry2 function. Endogenous Tesk1 and Spry2 exist in a complex in cell lines and mouse tissues. Tesk1 coexpression relocalizes Spry2 to vesicles including endosomes, inhibiting its translocation to membrane ruffles upon growth factor stimulation. Independent of its kinase activity, Tesk1 binding leads to a loss of Spry2 function as an inhibitor of ERK phosphorylation and reverses inhibition of basic fibroblast growth factor (bFGF)- and nerve growth factor-induced neurite outgrowth in PC12 cells by Spry2. Furthermore, depletion of endogenous Tesk1 in PC12 cells leads to a reduction in neurite outgrowth induced by bFGF. Tesk1 nullifies the inhibitory effect of Spry2 by abrogating its interaction with the adaptor protein Grb2 and interfering with its serine dephosphorylation upon bFGF and FGF receptor 1 stimulation by impeding its binding to the catalytic subunit of protein phosphatase 2A. A construct of Tesk1 that binds to Spry2 but does not localize to the vesicles does not interfere with its function, highlighting the importance of subcellular localization of Tesk1 in this context. Conversely, Tesk1 does not affect interaction of Spry2 with the E3 ubiquitin ligase, c-Cbl, and consequently, does not affect its inhibition of Cbl-mediated ubiquitination of the epidermal growth factor receptor. By selectively modulating the downstream effects of Spry2, Tesk1 may thus serve as a molecular determinant of the signaling outcome.

Sprouty2 regulates growth and differentiation of human neuroblastoma cells through RET tyrosine kinase

The Sprouty (SPRY) family of proteins includes important regulators of downstream signaling initiated by receptor tyrosine kinases. In the present study, we investigated the role of SPRY proteins in intracellular signaling via the RET receptor tyrosine kinase activated by glial cell line-derived neurotrophic factor (GDNF). Expression of SPRY1, SPRY2, SPRY3 and SPRY4 in HEK293T cells transfected with RET and GDNF receptor family alpha1 (GFRalpha1) genes significantly reduced sustained ERK activation as well as ELK-1 activation. Because expression of SPRY2 was efficiently induced by GDNF in TGW human neuroblastoma cells expressing RET and GFRalpha1, we further investigated the role of SPRY2 in the growth and differentiation of TGW cells. Expression of wild-type SPRY2 (WT-SPRY2) decreased the growth of TGW cells. In contrast, expression of a dominant negative form of SPRY2 (MT-SPRY2, with a mutated tyrosine residue) enhanced cell proliferation. In addition, expression of WT-SPRY2 reduced GDNF-dependent neurite outgrowth of TGW cells, whereas expression of MT-SPRY2 enhanced it. Taken together, our results suggest that SPRY2 regulates GDNF-dependent proliferation and differentiation of TGW neuroblastoma cells mediated by RET tyrosine kinase.

Sprouty-2 regulates oncogenic K-ras in lung development and tumorigenesis

Somatic activation of Ras occurs frequently in human cancers, including one-third of lung cancers. Activating Ras mutations also occur in the germline, leading to complex developmental syndromes. The precise mechanism by which Ras activation results in human disease is uncertain. Here we describe the phenotype of a mouse engineered to harbor a germline oncogenic K-rasG12D mutation. This mouse exhibits early embryonic lethality due to a placental trophoblast defect. Reconstitution with a wild-type placenta rescues the early lethality, but mutant embryos still succumb to cardiovascular and hematopoietic defects. In addition, mutant embryos demonstrate a profound defect in lung branching morphogenesis associated with striking up-regulation of the Ras/mitogen-activated protein kinase (MAPK) antagonist Sprouty-2 and abnormal localization of MAPK activity within the lung epithelium. This defect can be significantly suppressed by lentiviral short hairpin RNA (shRNA)-mediated knockdown of Sprouty-2 in vivo. Furthermore, in the context of K-rasG12D-mediated lung tumorigenesis, Sprouty-2 is also up-regulated and functions as a tumor suppressor to limit tumor number and overall tumor burden. These findings indicate that in the lung, Sprouty-2 plays a critical role in the regulation of oncogenic K-ras, and implicate counter-regulatory mechanisms in the pathogenesis of Ras-based disease.

Getting a first clue about SPRED functions

Spreds form a new protein family with an N-terminal Enabled/VASP homology 1 domain (EVH1), a central c-Kit binding domain (KBD) and a C-terminal Sprouty-related domain (SPR). They are able to inhibit the Ras-ERK signalling pathway after various mitogenic stimulations. In mice, Spred proteins are identified as regulators of bone morphogenesis, hematopoietic processes, allergen-induced airway eosinophilia and hyperresponsiveness. They inhibit cell motility and metastasis and have a high potential as tumor markers and suppressors of carcinogenesis. Moreover, in vertebrates, XtSpreds help together with XtSprouty proteins to coordinate gastrulation and mesoderm specification. Here, we give an overview of this new field and summarize the domain functions, binding partners, expression patterns and the cellular localizations, regulations and functions of Spred proteins and try to give perspectives for future scientific directions.

The power and the promise of oncogene-induced senescence markers

Recently, it has been shown that oncogene-induced senescence (OIS) occurs during the early stages of tumorigenesis. Senescent tumour cells are abundant within premalignant neoplastic lesions, whereas they are scarce in malignant tumours. This association of senescence with the premalignant stages of tumour progression opens the possibility of using senescence markers as diagnostic and prognostic tools. Moreover, some chemotherapeutic protocols induce senescence in tumour cells and, consequently, senescence markers could help to monitor treatment response.