Tyrosine phosphorylation of Sprouty proteins regulates their ability to inhibit growth factor signaling: a dual feedback loop

Bimodal expression of Sprouty2 during the cell cycle is mediated by phase-specific Ras/MAPK and c-Cbl activities

Sprouty2 is an important inhibitor of cell proliferation and signal transduction. In this study, we found a bimodal expression of Sprouty2 protein during cell cycle progression after exit from quiescence, whereas elevated Sprouty4 expression in the G1 phase stayed high throughout the rest of the cell cycle. Induction of the mitogen-activated protein kinase via activated Ras was crucial for increased Sprouty2 expression at the G0/G1 transition. Following the first peak, accelerated proteasomal protein degradation caused a transient attenuation of Sprouty2 abundance during late G1. Since the decline in its expression was abolished by dominant negative c-Cbl and the timely restricted interaction between Sprouty2 and c-Cbl disappeared at the second peak of Sprouty2 expression, we conclude that the second phase in the cell cycle-specific expression profile of Sprouty2 is solely dependent on ubiquitination by c-Cbl. Our results suggest that Sprouty2 abundance is the result of strictly coordinated activities of Ras and c-Cbl.

Sprouty proteins inhibit receptor-mediated activation of phosphatidylinositol-specific phospholipase C

PLCγ03B3 binds Spry1 and Spry2. Overexpression of Spry decreased PLCγ03B3 activity and IP₃ and DAG production, whereas Spry-deficient cells yielded more IP₃. Spry overexpression inhibited T-cell receptor signaling and Spry1 null T-cells hyperproliferated with TCR ligation. Through action of PLCγ03B3, Spry may influence signaling through multiple receptors.

Sprouty (Spry) proteins are negative regulators of receptor tyrosine kinase signaling; however, their exact mechanism of action remains incompletely understood. We identified phosphatidylinositol-specific phospholipase C (PLC)-γ as a partner of the Spry1 and Spry2 proteins. Spry–PLCγ interaction was dependent on the Src homology 2 domain of PLCγ and a conserved N-terminal tyrosine residue in Spry1 and Spry2. Overexpression of Spry1 and Spry2 was associated with decreased PLCγ phosphorylation and decreased PLCγ activity as measured by production of inositol (1,4,5)-triphosphate (IP₃) and diacylglycerol, whereas cells deficient for Spry1 or Spry1, -2, and -4 showed increased production of IP₃ at baseline and further increased in response to growth factor signals. Overexpression of Spry 1 or Spry2 or small-interfering RNA-mediated knockdown of PLCγ1 or PLCγ2 abrogated the activity of a calcium-dependent reporter gene, suggesting that Spry inhibited calcium-mediated signaling downstream of PLCγ. Furthermore, Spry overexpression in T-cells, which are highly dependent on PLCγ activity and calcium signaling, blocked T-cell receptor-mediated calcium release. Accordingly, cultured T-cells from Spry1 gene knockout mice showed increased proliferation in response to T-cell receptor stimulation. These data highlight an important action of Spry, which may allow these proteins to influence signaling through multiple receptors.

Functional interaction between Env oncogene from Jaagsiekte sheep retrovirus and tumor suppressor Sprouty2

Background

Jaagsiekte sheep retrovirus (JSRV) is a type D retrovirus capable of transforming target cells in vitro and in vivo. The Envelope (Env) gene from JSRV and from related retroviruses can induce oncogenic transformation, although the detailed mechanism is yet to be clearly understood. Host cell factors are envisaged to play a critical determining role in the regulation of Env-mediated cell transformation.

Results

JSRV Env-mediated transformation of a lung adenocarcinoma cell line induced rapid proliferation, anchorage-independent growth and tumor formation, but completely abrogated the migration ability. An analysis of the signaling scenario in the transformed cells suggested the involvement of the ERK pathway regulated by Sprouty2 in cell migration, and the PI3K-Akt and STAT3 pathways in proliferation and anchorage-independence. On the other hand, in a normal lung epithelial cell line, Env-mediated transformation only decreased the migration potential while the other functions remained unaltered. We observed that Env induced the expression of a tumor suppressor, Sprouty2, suggesting a correlation between Env-effect and Sprouty2 expression. Overexpression of Sprouty2 per se not only decreased the migratory potential and tumor formation potential of the target cells but also made them resistant to subsequent Env-mediated transformation. On the other hand, over expression of the functional mutants of Sprouty2 had no inhibitory effect, confirming the role of Sprouty2 as a tumor suppressor.

Conclusions

Our studies demonstrate that Env and Sprouty2 have a functional relationship, probably through shared signaling network. Sprouty2 functions as a tumor suppressor regulating oncogenic transformation of cells, and it therefore has the potential to be exploited as a therapeutic anti-cancer agent.

Sprouty-4 inhibits transformed cell growth, migration and invasion, and epithelial-mesenchymal transition, and is regulated by Wnt7A through PPARgamma in non-small cell lung cancer

Sprouty proteins are potent receptor tyrosine kinase inhibitors that antagonize growth factor signaling and are involved in lung development. However, little is known about the regulation or targets of Sprouty-4 (Spry4) in lung cancer. Our study aimed to determine the role of Spry4 in non-small cell lung cancer (NSCLC). We found that Spry4 mRNA expression was decreased in NSCLC cell lines and in dysplastic lung cell lines compared with a nontransformed cell line, suggesting that Spry4 has tumor-suppressing activity. When Spry4 was stably transfected into H157 and H2122 NSCLC cell lines, decreased migration and invasion were observed. Matrix metalloproteinase-9 activity was decreased, and the expression of matrix metalloproteinase inhibitors TIMP1 and CD82 were increased. Stable expression of Spry4 led to reduced cell growth and reduced anchorage-independent growth in NSCLC cell lines, along with upregulation of tumor suppressors p53 and p21. Changes in epithelial and mesenchymal markers indicated that Spry4 expression induces a reversal of the epithelial to mesenchymal transition characteristic of tumor cells. Treatment of a nontransformed lung epithelial cell line with short hairpin RNA to Spry4 led to the decreased expression of epithelial markers and increased cell growth, supporting the concept of Spry4 acting as a tumor suppressor. We showed that the activity of the Spry4 promoter is increased by Wnt7A/Fzd9 signaling through peroxisome proliferator-activated receptor gamma. These data present previously undescribed targets of Spry4 and suggest that Spry4 is a downstream target of Wnt7A/Fzd 9 signaling. Spry4 may have efficacy in the treatment of NSCLC.

CrkL is a novel target of Sprouty2 in fibroblast growth factor signaling

Sprouty, an inhibitor of receptor tyrosine kinase signaling, plays an important role in the regulation of a wide variety of biological processes. Although it is established that the Sprouty inhibitory activity is induced by tyrosine phosphorylation in response to stimuli, its action mechanisms have not been fully elucidated. Here, we report identification of a novel target of Sprouty. We find that Sprouty1 and Sprouty2 bind to the adaptor protein CrkL in a stimulus-dependent manner. Biochemical analyses show that the binding requires tyrosine phosphorylation of Sprouty and that both the SH2 domain and the N-terminal SH3 domain of CrkL are necessary for the binding. In fibroblast growth factor-stimulated NIH3T3 cells, CrkL binding to Sprouty2 occurs concomitantly with tyrosine phosphorylation of Sprouty2, which occurs slowly but is sustained. Importantly, our results show that tyrosine-phosphorylated Sprouty2 suppresses Rap1 activation. These results taken together indicate that Sprouty2 acts as an inhibitor of CrkL-Rap1 signaling.

Cbl controls EGFR fate by regulating early endosome fusion

Amino acid residues 1 to 434 of the E3 ubiquitin ligase Cbl control signaling of the epidermal growth factor receptor (EGFR) by enhancing its ubiquitination, down-regulation, and lysosomal degradation. This region of Cbl comprises a tyrosine kinase-binding domain, a linker region, a really interesting new gene finger (RF), and a subset of the residues of the RF tail. In experiments with full-length alanine substitution mutants, we demonstrated that the RF tail of Cbl regulated biochemically distinct checkpoints in the endocytosis of EGFR. The Cbl- and ubiquitin-dependent degradation of the regulator of internalization hSprouty2 was compromised by the Val(431)--> Ala mutation, whereas the Cbl- and EGFR-dependent dephosphorylation or degradation of the endosomal trafficking regulator Hrs was compromised by the Phe(434)--> Ala mutation. Deregulated phosphorylation of Hrs correlated with inhibition of the fusion of early endosomes and of the degradation of EGFR. This study provides the first evidence that Cbl regulates receptor fate by controlling the fusion of sorting endosomes. We postulate that it does so by modulating the abundance of tyrosine-phosphorylated Hrs.

HECT domain-containing E3 ubiquitin ligase Nedd4 interacts with and ubiquitinates Sprouty2

Sprouty (Spry) proteins are important regulators of receptor tyrosine kinase signaling in development and disease. Alterations in cellular Spry content have been associated with certain forms of cancers and also in cardiovascular diseases. Thus, understanding the mechanisms that regulate cellular Spry levels are important. Herein, we demonstrate that Spry1 and Spry2, but not Spry3 or Spry4, associate with the HECT domain family E3 ubiquitin ligase, Nedd4. The Spry2/Nedd4 association involves the WW domains of Nedd4 and requires phosphorylation of the Mnk2 kinase sites, Ser(112) and Ser(121), on Spry2. The phospho-Ser(112/121) region on Spry2 that binds WW domains of Nedd4 is a novel non-canonical WW domain binding region that does not contain Pro residues after phospho-Ser. Endogenous and overexpressed Nedd4 polyubiquitinate Spry2 via Lys(48) on ubiquitin and decrease its stability. Silencing of endogenous Nedd4 increased the cellular Spry2 content and attenuated fibroblast growth factor-elicited ERK1/2 activation that was reversed when elevations in Spry2 levels were prevented by Spry2-specific small interfering RNA. Mnk2 silencing decreased Spry2-Nedd4 interactions and also augmented the ability of Spry2 to inhibit fibroblast growth factor signaling. This is the first report demonstrating the regulation of cellular Spry content and its ability to modulate receptor tyrosine kinase signaling by a HECT domain-containing E3 ubiquitin ligase.

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.

Cell density-dependent inhibition of epidermal growth factor receptor signaling by p38alpha mitogen-activated protein kinase via Sprouty2 downregulation

Contact inhibition is a fundamental process in multicellular organisms aimed at inhibiting proliferation at high cellular densities through poorly characterized intracellular signals, despite availability of growth factors. We have previously identified the protein kinase p38alpha as a novel regulator of contact inhibition, as p38alpha is activated upon cell-cell contacts and p38alpha-deficient cells are impaired in both confluence-induced proliferation arrest and p27(Kip1) accumulation. Here, we establish that p27(Kip1) plays a key role downstream of p38alpha to arrest proliferation at high cellular densities. Surprisingly, p38alpha does not directly regulate p27(Kip1) expression levels but leads indirectly to confluent upregulation of p27(Kip1) and cell cycle arrest via the inhibition of mitogenic signals originating from the epidermal growth factor receptor (EGFR). Hence, confluent activation of p38alpha uncouples cell proliferation from mitogenic stimulation by inducing EGFR degradation through downregulation of the EGFR-stabilizing protein Sprouty2 (Spry2). Accordingly, confluent p38alpha-deficient cells fail to downregulate Spry2, providing them in turn with sustained EGFR signaling that facilitates cell overgrowth and oncogenic transformation. Our results provide novel mechanistic insight into the role of p38alpha as a sensor of cell density, which induces confluent cell cycle arrest via the Spry2-EGFR-p27(Kip1) network.

Roles of cell adhesion molecules nectin and nectin-like molecule-5 in the regulation of cell movement and proliferation

In response to chemoattractants, migrating cells form protrusions, such as lamellipodia and filopodia, and structures, such as ruffles over lamellipodia, focal complexes and focal adhesions at leading edges. The formation of these leading edge structures is essential for directional cell movement. Nectin-like molecule-5 (Necl-5) interacts in cis with PDGF receptor and integrin alpha(v)beta(3), and enhances the activation of signalling molecules associated with these transmembrane proteins, which results in the formation of leading edge structures and enhancement of directional cell movement. When migrating cells come into contact with each other, cell-cell adhesion is initiated, resulting in reduced cell velocity. Necl-5 first interacts in trans with nectin-3. This interaction is transient and induces down-regulation of Necl-5 expression at the cell surface, resulting in reduced cell movement. Cell proliferation is also suppressed by the down-regulation of Necl-5, because the inhibitory effect of Necl-5 on Sprouty2, a negative regulator of the Ras signalling, is diminished. PDGF receptor and integrin alpha(v)beta(3), which have interacted with Necl-5, then form a complex with nectin, which initiates cell-cell adhesion and recruits cadherin to the nectin-based cell-cell adhesion sites to form stable adherens junctions. The formation of adherens junctions stops cell movement, in part through inactivation of integrin alpha(v)beta(3) caused by the trans-interaction of nectin. Thus, nectin and Necl-5 play key roles in the regulation of cell movement and proliferation.

Sprouty2-mediated inhibition of fibroblast growth factor signaling is modulated by the protein kinase DYRK1A

Raf-MEK-extracellular signal-regulated kinase (Erk) signaling initiated by growth factor-engaged receptor tyrosine kinases (RTKs) is modulated by an intricate network of positive and negative feedback loops which determine the specificity and spatiotemporal characteristics of the intracellular signal. Well-known antagonists of RTK signaling are the Sprouty proteins. The activity of Sprouty proteins is modulated by phosphorylation. However, little is known about the kinases responsible for these posttranslational modifications. We identify DYRK1A as one of the protein kinases of Sprouty2. We show that DYRK1A interacts with and regulates the phosphorylation status of Sprouty2. Moreover, we identify Thr75 on Sprouty2 as a DYRK1A phosphorylation site in vitro and in vivo. This site is functional, since its mutation enhanced the repressive function of Sprouty2 on fibroblast growth factor (FGF)-induced Erk signaling. Further supporting the idea of a functional interaction, DYRK1A and Sprouty2 are present in protein complexes in mouse brain, where their expression overlaps in several structures. Moreover, both proteins copurify with the synaptic plasma membrane fraction of a crude synaptosomal preparation and colocalize in growth cones, pointing to a role in nerve terminals. Our results suggest, therefore, that DYRK1A positively regulates FGF-mitogen-activated protein kinase signaling by phosphorylation-dependent impairment of the inhibitory activity of Sprouty2.

Sprouty 2 disturbs FGFR3 degradation in thanatophoric dysplasia type II: a severe form of human achondroplasia

Thanatophoric dysplasia is a member of the achondroplasia family of human skeletal dysplasias, which result from FGFR3 mutations that exaggerate this receptor's inhibitory influence on chondrocyte proliferation and differentiation in the skeletal growth plate. We have previously reported that defective lysosomal degradation of activated receptor contributes to the gain-of-function of the mutant FGFR3. We now provide evidence that this disturbance is mediated by the receptor's kinase activity and involves constitutive induction and activation of Spry2. Our findings suggest that activated Spry2 may interfere with c-Cbl-mediated ubiquitination of FGFR3 by sequestering c-Cbl. They provide novel insight into the pathogenesis of this group of human skeletal dysplasias and identify a mechanism that potentially could be targeted therapeutically.

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.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

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.

Sprouty proteins, masterminds of receptor tyrosine kinase signaling

Angiogenesis relies on endothelial cells properly processing signals from growth factors provided in both an autocrine and a paracrine manner. These mitogens bind to their cognate receptor tyrosine kinases (RTKs) on the cell surface, thereby activating a myriad of complex intracellular signaling pathways whose outputs include cell growth, migration, and morphogenesis. Understanding how these cascades are precisely controlled will provide insight into physiological and pathological angiogenesis. The Sprouty (Spry) family of proteins is a highly conserved group of negative feedback loop modulators of growth factor-mediated mitogen-activated protein kinase (MAPK) activation originally described in Drosophila. There are four mammalian orthologs (Spry1-4) whose modulation of RTK-induced signaling pathways is growth factor- and cell context-dependent. Endothelial cells are a group of highly differentiated cell types necessary for defining the mammalian vasculature. These cells respond to a plethora of growth factors and express all four Spry isoforms, thus highlighting the complexity that is required to form and maintain vessels in mammals. This review describes Spry functions in the context of endothelial biology and angiogenesis, and provides an update on Spry-interacting proteins and Spry mechanisms of action.

A novel role of Sprouty 2 in regulating cellular apoptosis

Sprouty (SPRY) proteins modulate receptor-tyrosine kinase signaling and, thereby, regulate cell migration and proliferation. Here, we have examined the role of endogenous human SPRY2 (hSPRY2) in the regulation of cellular apoptosis. Small inhibitory RNA-mediated silencing of hSPRY2 abolished the anti-apoptotic action of serum in adrenal cortex adenocarcinoma (SW13) cells. Silencing of hSPRY2 decreased serum- or epidermal growth factor (EGF)-elicited activation of AKT and ERK1/2 and reduced the levels of EGF receptor. Silencing of hSPRY2 also inhibited serum-induced activation of p90RSK and decreased phosphorylation of pro-apoptotic protein BAD (BCL2-antagonist of cell death) by p90RSK. Inhibiting both the ERK1/2 and AKT pathways abolished the ability of serum to protect against apoptosis, mimicking the effects of silencing hSPRY2. Serum transactivated the EGF receptor (EGFR), and inhibition of the EGFR by a neutralizing antibody attenuated the anti-apoptotic actions of serum. Consistent with the role of EGFR and perhaps other growth factor receptors in the anti-apoptotic actions of serum, the tyrosine kinase binding domain of c-Cbl (Cbl-TKB) protected against down-regulation of the growth factor receptors such as EGFR and preserved the anti-apoptotic actions of serum when hSpry2 was silenced. Additionally, silencing of Spry2 in c-Cbl null cells did not alter the ability of serum to promote cell survival. Moreover, reintroduction of wild type hSPRY2, but not its mutants that do not bind c-Cbl or CIN85 into SW13 cells after endogenous hSPRY2 had been silenced, restored the anti-apoptotic actions of serum. Overall, we conclude that endogenous hSPRY2-mediated regulation of apoptosis requires c-Cbl and is manifested by the ability of hSPRY2 to sequester c-Cbl and thereby augment signaling via growth factor receptors.

Evidence that sprouty 2 is necessary for sarcoma formation by H-Ras oncogene-transformed human fibroblasts

Sprouty 2 (Spry2) acts as an inhibitor of receptor tyrosine kinase signaling in various cellular contexts. Interestingly, Spry2 also prevents the c-Cbl-induced degradation of epidermal growth factor receptor (EGFR). We compared human fibroblasts malignantly transformed by overexpression of H-Ras(V12) oncogene to their nontransformed parental cells and found that the malignant cells express a high level of Spry2. These cells also exhibited an increase in the level of EGFR compared with their precursor cells. We found that intact EGFR was required if H-Ras-transformed cells were to grow in the absence of exogenous growth factors or form large colonies in agarose. When we decreased expression of Spry2, using a Spry2-specific shRNA, the H-Ras(V12)-transformed fibroblasts could no longer form large colonies in agarose, grow in reduced levels of serum, or form tumors in athymic mice. The level of active H-Ras in these cells remained unaltered. A similar, but less pronounced, effect in tumor formation was observed when Spry2 was down-regulated in human patient-derived fibrosarcoma cell lines. In H-Ras-transformed cells Spry2 sustained the level and the downstream signaling activity of EGFR. In the parental, non-H-Ras-transformed fibroblasts, expression of Spry2 resulted in the inhibition of H-Ras and ERK activation, suggesting that the positive effect of Spry2 in tumor formation is specific to H-Ras transformation. Co-immunoprecipitation studies with H-Ras-transformed cells revealed that Spry2 and H-Ras interact and that H-Ras interacts with Spry2-binding partners, c-Cbl and CIN85, in a Spry2-dependent manner. These data show that Spry2 plays a critical role in the ability of H-Ras-transformed cells to form tumors in athymic mice.

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.

Down-regulation of Sprouty2 in non-small cell lung cancer contributes to tumor malignancy via extracellular signal-regulated kinase pathway-dependent and -independent mechanisms

Sprouty (Spry) proteins function as inhibitors of receptor tyrosine kinase signaling mainly by interfering with the Ras/Raf/mitogen-activated protein kinase cascade, a pathway known to be frequently deregulated in human non-small cell lung cancer (NSCLC). In this study, we show a consistently lowered Spry2 expression in NSCLC when compared with the corresponding normal lung epithelium. Based on these findings, we investigated the influence of Spry2 expression on the malignant phenotype of NSCLC cells. Ectopic expression of Spry2 antagonized mitogen-activated protein kinase activity and inhibited cell migration in cell lines homozygous for K-Ras wild type, whereas in NSCLC cells expressing mutated K-Ras, Spry2 failed to diminish extracellular signal-regulated kinase (ERK) phosphorylation. Nonetheless, Spry2 significantly reduced cell proliferation in all investigated cell lines and blocked tumor formation in mice. Accordingly, a Spry2 mutant unable to inhibit ERK phosphorylation reduced cell proliferation significantly but less pronounced compared with the wild-type protein. Therefore, we conclude that Spry2 interferes with ERK phosphorylation and another yet unidentified pathway. Our results suggest that Spry2 plays a role as tumor suppressor in NSCLC by antagonizing receptor tyrosine kinase-induced signaling at different levels, indicating feasibility for the usage of Spry in targeted gene therapy of NSCLC.

Sprouty2 inhibits BDNF-induced signaling and modulates neuronal differentiation and survival

Sprouty (Spry) proteins are ligand-inducible inhibitors of receptor tyrosine kinases-dependent signaling pathways, which control various biological processes, including proliferation, differentiation and survival. Here, we investigated the regulation and the role of Spry2 in cells of the central nervous system (CNS). In primary cultures of immature neurons, the neurotrophic factor BDNF (brain-derived neurotrophic factor) regulates spry2 expression. We identified the transcription factors CREB and SP1 as important regulators of the BDNF activation of the spry2 promoter. In immature neurons, we show that overexpression of wild-type Spry2 blocks neurite formation and neurofilament light chain expression, whereas inhibition of Spry2 by a dominant-negative mutant or small interfering RNA favors sprouting of multiple neurites. In mature neurons that exhibit an extensive neurite network, spry2 expression is sustained by BDNF and is downregulated during neuronal apoptosis. Interestingly, in these differentiated neurons, overexpression of Spry2 induces neuronal cell death, whereas its inhibition favors neuronal survival. Together, our results imply that Spry2 is involved in the development of the CNS by inhibiting both neuronal differentiation and survival through a negative-feedback loop that downregulates neurotrophic factors-driven signaling pathways.

Regulation of platelet-derived growth factor-induced Ras signaling by poliovirus receptor Necl-5 and negative growth regulator Sprouty2

Necl-5, known as a poliovirus receptor and up-regulated in many cancer cells, enhances platelet-derived growth factor (PDGF)-induced activation of Ras-Raf-MEK-ERK signaling, but not PDGF-induced tyrosine phosphorylation of PDGF receptor, resulting in facilitation of cell proliferation. Here, we showed that Necl-5 interacted with Sprouty2, known to be a negative regulator of growth factor-induced signaling, and reduced the inhibitory effect of Sprouty2 on PDGF-induced Ras signaling. Necl-5 was reported to be down-regulated by its trans-interaction with nectin-3 upon cell-cell contact, initiating cooperative cell-cell adhesion with cadherin. This down-regulation of Necl-5 caused tyrosine phosphorylation of Sprouty2 by c-Src, which was activated by PDGF receptor in response to PDGF, and inhibited PDGF-induced Ras signaling. Thus, Necl-5 and Sprouty2 cooperatively regulate PDGF-induced Ras signaling. The roles of Necl-5 and Sprouty2 in contact inhibition for cell proliferation are also discussed.

Negative Regulation of Receptor Tyrosine Kinase (RTK) Signaling: A Developing Field

Trophic factors control cellular physiology by activating specific receptor tyrosine kinases (RTKs). While the over activation of RTK signaling pathways is associated with cell growth and cancer, recent findings support the concept that impaired down-regulation or deactivation of RTKs may also be a mechanism involved in tumor formation. Under this perspective, the molecular determinants of RTK signaling inhibition may act as tumor-suppressor genes and have a potential role as tumor markers to monitor and predict disease progression. Here, we review the current understanding of the physiological mechanisms that attenuate RTK signaling and discuss evidence that implicates deregulation of these events in cancer.

Direct binding of PP2A to Sprouty2 and phosphorylation changes are a prerequisite for ERK inhibition downstream of fibroblast growth factor receptor stimulation

In the context of fibroblast growth factor (FGF) signaling, Sprouty2 (Spry2) is the most profound inhibitor of the Ras/ERK pathway as compared with other Spry isoforms. An exclusive, necessary, but cryptic PXXPXR motif in the C terminus of Spry2 is revealed upon stimulation. The activation of Spry2 appears to be linked to sequences in the N-terminal half of the protein and correlated with a bandshifting seen on SDS-PAGE. The band-shifting is likely caused by changes in the phosphorylation status of key Ser and Thr residues following receptor stimulation. Dephosphorylation of at least two conserved Ser residues (Ser-112 and Ser-115) within a conserved Ser/Thr sequence is accomplished upon stimulation by a phosphatase that binds to Spry2 around residues 50-60. We show that human Spry2 co-immunoprecipitates with both the catalytic and the regulatory subunits of protein phosphatase 2A (PP2A-C and PP2A-A, respectively) in cells upon FGF receptor (FGFR) activation. PP2A-A binds directly to Spry2, but not to Spry2Delta50-60 (Delta50-60), and the activity of PP2A increases with both FGF treatment and FGFR1 overexpression. c-Cbl and PP2A-A compete for binding centered around Tyr-55 on Spry2. We show that there are at least two distinct pools of Spry2, one that binds PP2A and another that binds c-Cbl. c-Cbl binding likely targets Spry2 for ubiquitin-linked destruction, whereas the phosphatase binding and activity are necessary to dephosphorylate specific Ser/Thr residues. The resulting change in tertiary structure enables the Pro-rich motif to be revealed with subsequent binding of Grb2, a necessary step for Spry2 to act as a Ras/ERK pathway inhibitor in FGF signaling.

Spred-2 steady-state levels are regulated by phosphorylation and Cbl-mediated ubiquitination

Spred proteins modulate growth factor receptor signaling by inhibiting the Ras-MAPK cascade. Here, we show that Spred-1, Spred-2, and Spred-3 are ubiquitinated in HEK293T cells stimulated with epidermal growth factor (EGF) or pervanadate. Spred-2 tyrosines Y228 and/or Y231 in the Kit binding domain were identified as putative phosphorylation site(s) critical for Spred-2 ubiquitination. Depletion of Cbl and Cbl-b E3 ubiquitin ligases by RNA interference, or overexpression of a Cbl dominant inhibitory mutant (Cbl-N), inhibited Spred-2 ubiquitination, while conversely, wild type Cbl enhanced Spred-2 ubiquitination. Interaction of Spred-2 with Cbl-N was detectable by co-immunoprecipitation and required the Cbl SH2 domain and Spred-2 Y228 and Y231 residues. Studies on endogenous Spred-2 in ME4405 melanoma cells showed that pervanadate induced Spred-2 ubiquitination and a marked reduction in Spred-2 steady-state levels that was partially blocked by the proteasomal inhibitor, MG-132. These results suggest a role for Spred-2 tyrosine phosphorylation and ubiquitination in controlling Spred-2 expression levels.

Raf-1 kinase activation is uncoupled from downstream MEK/ERK pathway in cells treated with Src tyrosine kinase inhibitor PP2

Recently we demonstrated that PP2 (4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), a potent and selective inhibitor of the Src-family tyrosine kinase, markedly enhanced Ras-independent activation of Raf-1 by the combination of phorbol myristate acetate (PMA) and hydrogen peroxide (H(2)O(2)). We report here that Raf-1 knockdown cells were significantly more sensitive to treatment of PP2 than control cells. This PP2-induced growth inhibition was found to be linked to decreased ERK and p38 activity. Interestingly, the growth of Sprouty knockdown cells appeared to be inhibited at earlier time points of PP2 treatment when compared with control cells. Unexpectedly, siRNA-mediated knockdown of Spry2, which is known to modulate the Ras/Raf/MAPK signal through feedback regulation, resulted in decreased Raf-1 kinase activity. PP2 had limited effect on the ability of PMA/H(2)O(2) to induce significant phosphorylation of MEK/ERK proteins in both Spry2 knockdown and control cells, indicating that PP2-mediated activation of Raf-1 did not potentiate signaling through the downstream MEK/ERK pathway. Taken together our results suggest that Raf-1 signaling may be bypassed in PP2-treated cells by uncoupling from downstream MEK/ERK pathway.

An essential role for FGF receptor signaling in lens development

Since the days of Hans Spemann, the ocular lens has served as one of the most important developmental systems for elucidating fundamental processes of induction and differentiation. More recently, studies in the lens have contributed significantly to our understanding of cell cycle regulation and apoptosis. Over 20 years of accumulated evidence using several different vertebrate species has suggested that fibroblast growth factors (FGFs) and/or fibroblast growth factor receptors (FGFRs) play a key role in lens development. FGFR signaling has been implicated in lens induction, lens cell proliferation and survival, lens fiber differentiation and lens regeneration. Here we will review and discuss historical and recent evidence suggesting that (FGFR) signaling plays a vital and universal role in multiple aspects of lens development.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

The VASP-Spred-Sprouty domain puzzle

Sprouty-related proteins with an EVH1 domain (Spreds) belong to a new protein family harboring a conserved N-terminal EVH1 domain, which is related to the VASP (vasodilator-stimulated phosphoprotein) EVH1 domain (Enabled/VASP homology 1 domain) and a C-terminal Sprouty-related domain, typical for Sprouty proteins. Spreds were, like Sproutys, initially discovered as inhibitors of the Ras/MAPK pathway, and the SPR (Sprouty-related) domains of both protein families seem to be very important for many protein interactions and cellular processes. VASP was initially characterized as a proline-rich substrate of protein kinases A and G in human platelets and later shown to be a scaffold protein, regulating both signal transduction pathways and the actin filament system. The VASP-EVH1 domain is known to bind specifically to a FP(4) binding motif, which is, for example, present in the focal adhesion proteins vinculin and zyxin. In this review we give a structural and functional overview on these three protein families and ask whether nature plays a modular protein domain puzzle with stable exchangeable elements or if these closely related domains have various functions when pasted in a different protein context.

A Src homology 3-binding sequence on the C terminus of Sprouty2 is necessary for inhibition of the Ras/ERK pathway downstream of fibroblast growth factor receptor stimulation

Because the Sprouty (Spry) proteins were shown to be inhibitors of the mainstream Ras/ERK pathway, there has been considerable interest in ascertaining their mechanism of action especially since a possible role as tumor suppressors for these inhibitory proteins has been suggested. We compared the ability of the mammalian Spry isoforms to inhibit the Ras/ERK pathway in the context of fibroblast growth factor receptor (FGFR) signaling. Spry2 is considerably more inhibitory than Spry1 or Spry4, and this correlates with the binding to Grb2 via a C-terminal proline-rich sequence that is found exclusively on Spry2. This PXXPXR motif binds directly to the N-terminal Src homology domain 3 of Grb2, and when added onto the C terminus of Spry4 the resultant chimera inhibits the Ras/ERK pathway. The ability to inhibit neurite outgrowth in PC-12 cells correlates with the propensity of Spry isoforms and engineered constructs to inhibit the phosphorylation of ERK1/2. The PXXPXR motif is cryptic in unstimulated cells, and it is postulated that Spry2 undergoes a conformational change following FGFR stimulation, enabling the subsequent interaction with Grb2. We present evidence that Spry2 can compete with the RasGEF (guanine nucleotide exchange factor) SOS1 for binding to Grb2, resulting in the inhibition of phosphorylation of ERK1/2.

ERK MAP kinase in G cell cycle progression and cancer

The extracellular-signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase signaling pathway plays an important role in various cellular responses, including cell proliferation, cell differentiation and cell survival. Recent studies have identified a number of Ras/ERK signaling-related proteins, such as scaffold proteins and inhibitors. These proteins modulate ERK signaling and thereby could give variations in ERK signaling outputs that regulate cell fate decisions. Here we focus on the role of ERK signaling in cell cycle progression from G0/G1 to S phase and cancer.

A functional interaction between sprouty proteins and caveolin-1

Growth factor-mediated signal transduction cascades can be regulated spatio-temporally by signaling modulators, such as Sprouty proteins. The four mammalian Sprouty family members are palmitoylated phosphoproteins that can attenuate or potentiate numerous growth factor-induced signaling pathways. Previously, we have shown that Sprouty-1 and Sprouty-2 associate with Caveolin-1, the major structural protein of caveolae. Like Sprouty, Caveolin-1 inhibits growth factor-induced mitogen-activated protein kinase activation. Here, we demonstrate that all four mammalian Sprouty family members physically interact with Caveolin-1. The C terminus of Caveolin-1 is the major Sprouty-binding site, whereas Sprouty binds Caveolin-1 via its conserved C-terminal domain. A single point mutation in this domain results in loss of Caveolin-1 interaction. Moreover, we demonstrate that the various Sprouty isoforms differ dramatically in their cooperation with Caveolin-1-mediated inhibition of mitogen-activated protein kinase activation and that such cooperation is also highly dependent on the type of growth factor investigated and on cell density. Together, the data suggest that the Sprouty/Caveolin-1 interaction modulates signaling in a growth factor- and Sprouty isoform-specific manner.

Dual effects of Sprouty1 on TCR signaling depending on the differentiation state of the T cell

Sprouty (Spry) is known to be a negative feedback inhibitor of growth factor receptor signaling through inhibition of the Ras/MAPK pathway. Several groups, however, have reported a positive role for Spry involving sequestration of the inhibitory protein c-Cbl. Thus, Spry may have various functions in the regulation of receptor-mediated signaling depending on the context. In the immune system, the function of Spry is unknown. In this study, we investigated the role of Spry1 in T cell activation. Spry1, among the four mammalian homologs, was specifically induced by TCR signaling of CD4(+) murine T cells. In fully differentiated Th1 clones, overexpressed Spry1 inhibited TCR signaling and decreased IL-2 production while reducing expression with specific siRNA transfection had the opposite effect, increasing IL-2 production. In contrast, in naive T cells, Spry1 overexpression enhanced TCR signaling, and increased proliferation and IL-2 production, while siRNA transfection again had the opposite effect, reducing IL-2 production following activation. The enhancing effect in naive cells was abrogated by preactivation of the T cells with Ag and APC, indicating that the history of exposure to Ag is correlated with a hierarchy of T cell responsiveness to Spry1. Furthermore, both the NF-AT and MAPK pathways were influenced by Spry1, implying a different molecular mechanism from that for growth factor receptor signaling. Thus, Spry1 uses a novel mechanism to bring about differential effects on TCR signaling through the same receptor, depending on the differentiation state of the T cell.

FGF regulated gene-expression and neuronal differentiation in the developing midbrain-hindbrain region

The neuroectodermal tissue close to the midbrain-hindbrain boundary (MHB) is an important secondary organizer in the developing neural tube. This so-called isthmic organizer (IsO) secretes signaling molecules, such as fibroblast growth factors (FGFs), which regulate cellular survival, patterning and proliferation in the midbrain and rhombomere 1 (R1) of the hindbrain. We have previously shown that FGF-receptor 1 (FGFR1) is required for the normal development of this brain region in the mouse embryo. Here, we have compared the gene expression profiles of midbrain-R1 tissues from wild-type embryos and conditional Fgfr1 mutants, in which FGFR1 is inactivated in the midbrain and R1. Loss of Fgfr1 results in the downregulation of several genes expressed close to the midbrain-hindbrain boundary and in the disappearance of gene expression gradients in the midbrain and anterior hindbrain. Our screen identified several previously uncharacterized genes which may participate in the development of midbrain-R1 region. Our results also show altered neurogenesis in the midbrain and R1 of the Fgfr1 mutants. Interestingly, the neuronal progenitors in midbrain and R1 show different responses to the loss of signaling through FGFR1.

Sprouty4, a suppressor of tumor cell motility, is down regulated by DNA methylation in human prostate cancer

PURPOSE

Alterations of fibroblast growth factors (FGFs) and their receptors contribute to prostate cancer progression by enhancing cellular proliferation, survival, and motility. The Sprouty gene family negatively regulates FGF signaling and may limit the ability of FGFs to enhance tumor progression. Sprouty1 is down regulated in human prostate cancers and Sprouty1 expression can markedly inhibit prostate cancer proliferation in vitro. Sprouty4 has been shown to negatively regulate both proliferation and cell migration in other systems. We therefore examined whether Sprouty4 expression was altered in prostate cancer.

EXPERIMENTAL DESIGN

Expression of Sprouty4 was examined by in situ hybridization and quantitative RT-PCR. Methylation of the Sprouty4 gene promoter was assessed using bisulfite modification and sequencing. The effect of Sprouty4 expression on cell migration was determined using an in vitro wounding assay.

RESULTS

By in situ hybridization Sprouty4 is expressed in normal prostatic epithelial cells and is decreased in a subset of prostate cancers. Quantitative RT-PCR confirms that Sprouty4 expression is decreased in approximately one half of prostate cancers. Analysis of the 5'-regulatory region revealed a CpG island approximately 1 kb upstream of the transcription initiation site, the proximal portion of which was preferentially methylated in prostate cancer tissues. More than one half of all prostate cancer DNAs were methylated in this region and methylation was significantly correlated with decreased Sprouty4 expression as determined by quantitative RT-PCR. When overexpressed in prostate cancer cell lines, Sprouty4 did not inhibit cell proliferation but did inhibit cell migration.

CONCLUSIONS

Sprouty4 expression is down regulated in human prostate cancer by DNA methylation and this decreased expression may contribute to increased cell migration.

Sprouty genes are expressed in osteoblasts and inhibit fibroblast growth factor-mediated osteoblast responses

Fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) are major regulators of skeletal growth and development. Signal transduction via FGFRs is complex and mediates proliferation, differentiation, or migration depending upon the cellular context. Members of the Spry gene family antagonize the FGFR signal transduction pathway and inhibit lung morphogenesis, angiogenesis, and chondrogenesis. We examined the expression of Spry2 in the osteoblastic MC3T3-E1 cell line. MC3T3-E1 cells express Spry2 in response to FGF1 stimulation. Treatment of MC3T3-E1 cells with FGF1 results in the expression of Spry2 in a manner consistent with an early response gene. Pharmacological inhibitors of mitogen-activated protein kinase activation inhibit FGF1-induced expression of Spry2 mRNA. Transient overexpression of Spry2 in MC3T3-E1 resulted in decreased FGF1-mediated extracellular signal-regulated kinase phosphorylation and FGF1-stimulated osteopontin promoter activity. Furthermore, we show that Spry2 interacts with Raf-1 in a glutathione-S-transferase pulldown assay and that this interaction may involve multiple sites. Finally, Spry2 expression precedes the onset of the expression of osteoblast differentiation markers in an in vitro assay of primary osteoblast differentiation. Taken together, these results indicate that Spry2 expression is an early response to stimulation by FGF1 in MC3T3-E1 cells and acts as a feedback inhibitor of FGF1-induced osteoblast responses, possibly through interaction with Raf1.

Sprouty proteins are in vivo targets of Corkscrew/SHP-2 tyrosine phosphatases

Drosophila Corkscrew protein and its vertebrate ortholog SHP-2(now known as Ptpn11) positively modulate receptor tyrosine kinase (RTK)signaling during development, but how these tyrosine phosphatases promote tyrosine kinase signaling is not well understood. Sprouty proteins are tyrosine-phosphorylated RTK feedback inhibitors, but their regulation and mechanism of action are also poorly understood. Here, we show that Corkscrew/SHP-2 proteins control Sprouty phosphorylation and function. Genetic experiments demonstrate that Corkscrew/SHP-2 and Sprouty proteins have opposite effects on RTK-mediated developmental events in Drosophilaand an RTK signaling process in cultured mammalian cells, and the genes display dose-sensitive genetic interactions. In cultured cells, inactivation of SHP-2 increases phosphorylation on the critical tyrosine of Sprouty 1. SHP-2 associates in a complex with Sprouty 1 in cultured cells and in vitro,and a purified SHP-2 protein dephosphorylates the critical tyrosine of Sprouty 1. Substrate-trapping forms of Corkscrew bind Sprouty in cultured Drosophila cells and the developing eye. These results identify Sprouty proteins as in vivo targets of Corkscrew/SHP-2 tyrosine phosphatases and show how Corkscrew/SHP-2 proteins can promote RTK signaling by inactivating a feedback inhibitor. We propose that this double-negative feedback circuit shapes the output profile of RTK signaling events.

Regulation of sprouty stability by Mnk1-dependent phosphorylation

Sprouty (Spry) proteins are negative feedback modulators of receptor tyrosine kinase pathways in Drosophila melanogaster and mammals. Mammalian Spry proteins have been shown to undergo tyrosine and serine phosphorylation in response to growth factor stimulation. While several studies have addressed the function of tyrosine phosphorylation of Spry, little is known about the significance of Spry serine phosphorylation. Here we identify mitogen-activated protein kinase-interacting kinase 1 (Mnk1) as the kinase that phosphorylates human Spry2 (hSpry2) on serines 112 and 121. Mutation of these serine residues to alanine or inhibition of Mnk1 activity increases the rate of ligand-induced degradation of hSpry2. Conversely, enhancement of serine phosphorylation achieved through either the inhibition of cellular phosphatases or the expression of active Mnk1 results in the stabilization of hSpry2. Previous studies have shown that growth factor stimulation induces the proteolytic degradation of hSpry2 by stimulating tyrosine phosphorylation on hSpry2, which in turn promotes c-Cbl binding and polyubiquitination. A mutant of hSpry2 that is deficient in serine phosphorylation displays enhanced tyrosine phosphorylation and c-Cbl binding, indicating that serine phosphorylation stabilizes hSpry2 by exerting an antagonistic effect on tyrosine phosphorylation. Moreover, loss of serine phosphorylation and the resulting enhanced degradation of hSpry2 impair its capacity to antagonize fibroblast growth factor-induced extracellular signal-regulated kinase activation. Our results imply that Mnk1-mediated serine phosphorylation of hSpry2 constitutes a regulatory mechanism to extend the temporal range of Spry activity.

K644E/M FGFR3 mutants activate Erk1/2 from the endoplasmic reticulum through FRS2 alpha and PLC gamma-independent pathways

Fibroblast growth factor receptors 3 (FGFR3) with K644M/E substitutions are associated to the severe skeletal dysplasias: severe achondroplasia with developmental delay and achanthosis nigricans(SADDAN) and thanatophoric dysplasia(TDII). The high levels of kinase activity of the FGFR3-mutants cause uncompleted biosynthesis that results in the accumulation of the immature/mannose-rich, phosphorylated receptors in the endoplasmic reticulum (ER) and STATs activation. Here we report that FGFR3 mutants activate Erk1/2 from the ER through an FRS2-independent pathway: instead, a multimeric complex by directly recruiting PLCgamma, Pyk2 and JAK1 is formed. The Erk1/2 activation from the ER however, is PLCgamma-independent, since preventing the PLCgamma/FGFR3 interaction by the Y754F substitution does not inhibit Erks. Furthermore, Erk1/2 activation is abrogated upon treatment with the Src inhibitor PP2, suggesting a role played by a Src family member in the pathway from the ER. Finally we show that the intrinsic kinase activity by mutant receptors is required to allow signaling from the ER. Overall these results highlight how activated FGFR3 exhibits signaling activity in the early phase of its biosynthesis and how segregation in a sub-cellular compartment can affect the FGFR3 multi-faceted capacity to recruit specific substrates.

Sprouty proteins: multifaceted negative-feedback regulators of receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTKs) control a wide variety of processes in multicellular organisms, including proliferation, differentiation, migration and survival. Their activity is tightly controlled through the coordinated action of both positive and negative regulators that function at multiple levels of the signal transduction cascade, and at different time points within the growth-factor-induced response. When this process goes awry, the outcome can be developmental defects and malignancy. Sprouty (Spry) proteins represent a major class of ligand-inducible inhibitors of RTK-dependent signaling pathways. New biochemical and genetic evidence indicates specific roles of the Spry genes in development and multiple modes of action of the Spry proteins in regulation of the RTK-induced response.

Current challenges in quantitative modeling of epidermal growth factor signaling

Over the last decade, epidermal growth factor (EGF) signaling has been used repeatedly as a test-bed for pioneering computational systems biology. Recent breakthroughs in our molecular understanding of EGF signaling pose new challenges for mathematical modeling strategies. Three key areas emerge as particularly relevant: the pervasive importance of compartmentalization and endosomal trafficking; the complexity of signalosome complexes; and the regulatory influence of diffusion and spatiality. Each one of them demands a drastic change in current computational approaches. We discuss recent developments in the field that address these emerging aspects in a new generation of more realistic - and potential more useful - models of EGF signaling.

The Cbl RING finger C-terminal flank controls epidermal growth factor receptor fate downstream of receptor ubiquitination

Evolutionarily conserved sequences of the E3/protein-ubiquitin ligase Cbl regulate epidermal growth factor receptor (EGF-R) signaling and degradation. These sequences encompass Cbl's tyrosine kinase-binding domain, linker region, RING finger (RF), and an uncharacterized flank C-terminal to the RF (residues 420-436). The latter domain, designated the RF tail, extends beyond Cbl's ubiquitin-conjugating enzyme (Ubc)-binding domain and has no known function. We report structure-function studies evaluating the impact of Cbl RF tail truncations on EGF-R fate in HEK 293 cells. All of the truncation mutants exhibit greatly reduced binding to activated EGF-R and lack proline-rich sequences that mediate direct Cbl association with SH3 proteins such as Grb2, yet a subset of mutants collectively enhances EGF-R ubiquitination, downregulation, and degradation. Significantly, EGF-R degradation correlates better with RF tail-dependent degradation of the Cbl substrate Sprouty2 than with EGF-R ubiquitination: expression of the RF tail truncation mutant Cbl 1-433 enhanced EGF-R ubiquitination while impeding Sprouty2 degradation, and Cbl 1-433 failed to enhance EGF-R downregulation or degradation. Our results suggest that EGF-R fate is controlled by a checkpoint downstream of receptor ubiquitination whose regulation by the Cbl RF tail may require Sprouty2 degradation.

Modulation of Fgf8 activity during vertebrate brain development

In recent years much emphasis has been placed on investigation of the precise control of FGF signaling during brain development. Such control is achieved in part by regulatory elements that determine the domains and levels of expression of genes coding for the diverse FGF ligands via specific molecular signaling pathways. There is new knowledge on the operation of such mechanisms in regions of the neural tube involved in the correct patterning of adjacent territories (known as secondary organizers of neural tube pattern). In the present minireview we intend to summarize recent evidence and emerging conclusions on potent modulators that govern the activity of Fgf8 signals in the developing vertebrate brain, focusing our attention on the best known secondary organizer, the isthmic organizer.

FRS2-dependent SRC activation is required for fibroblast growth factor receptor-induced phosphorylation of Sprouty and suppression of ERK activity

Activation of signalling by fibroblast growth factor receptor leads to phosphorylation of the signalling attenuator human Sprouty 2 (hSpry2) on residue Y55. This event requires the presence of the signalling adaptor fibroblast growth factor receptor substrate 2 (FRS2). The phosphorylation of hSpry2 is therefore mediated by an intermediate kinase. Using a SRC family kinase-specific inhibitor and mutant cells, we show that hSpry2 is a direct substrate for SRC family kinases, including SRC itself. Activation of SRC via fibroblast growth factor signalling is dependent upon FRS2 and fibroblast growth factor receptor kinase activity. SRC forms a complex with hSpry2 and this interaction is enhanced by hSpry2 phosphorylation. Phosphorylation of hSpry2 is required for hSpry2 to inhibit activation of the extracellular signal-regulated kinase pathway. These results show that recruitment of SRC to FRS2 leads to activation of signal attenuation pathways.