Protein-tyrosine phosphatase-1B (PTP1B) mediates the anti-migratory actions of Sprouty

Targeting Mitochondrial ATP-Synthase: Evolving Role of Chromium as a Regulator of Carbohydrate and Fat Metabolism

The micronutrient trivalent chromium, 3 + (Cr(III)), is postulated to play a role in carbohydrate, lipid, and protein metabolism. Although the mechanisms by which chromium mediates its actions are largely unknown, previous studies have suggested that pharmacological doses of chromium improve cardiometabolic symptoms by augmenting carbohydrate and lipid metabolism. Activation of AMP-activated protein kinase (AMPK) was among the many mechanisms proposed to explain the salutary actions of chromium on carbohydrate metabolism. However, the molecular pathways leading to the activation of AMPK by chromium remained elusive. In an elegant series of studies, Sun and coworkers recently demonstrated that chromium augments AMPK activation by binding to the beta-subunit of ATP synthase and inhibiting its enzymatic activity. This mini-review attempts to trace the evolving understanding of the molecular mechanisms of chromium leading to the hitherto novel pathway unraveled by Sun and coworkers and its potential implication to our understanding of the biological actions of chromium.

Spatial and temporal alterations in protein structure by EGF regulate cryptic cysteine oxidation

Stimulation of plasma membrane receptor tyrosine kinases (RTKs), such as the epidermal growth factor receptor (EGFR), locally increases the abundance of reactive oxygen species (ROS). These ROS then oxidize cysteine residues in proteins to potentiate downstream signaling. Spatial confinement of ROS is an important regulatory mechanism of redox signaling that enables the stimulation of different RTKs to oxidize distinct sets of downstream proteins. To uncover additional mechanisms that specify cysteines that are redox regulated by EGF stimulation, we performed time-resolved quantification of the EGF-dependent oxidation of 4200 cysteine sites in A431 cells. Fifty-one percent of cysteines were statistically significantly oxidized by EGF stimulation. Furthermore, EGF induced three distinct spatiotemporal patterns of cysteine oxidation in functionally organized protein networks, consistent with the spatial confinement model. Unexpectedly, protein crystal structure analysis and molecular dynamics simulations indicated widespread redox regulation of cryptic cysteine residues that are solvent exposed only upon changes in protein conformation. Phosphorylation and increased flux of nucleotide substrates served as two distinct modes by which EGF specified the cryptic cysteine residues that became solvent exposed and redox regulated. Because proteins that are structurally regulated by different RTKs or cellular perturbations are largely unique, these findings suggest that solvent exposure and redox regulation of cryptic cysteine residues contextually delineate redox signaling networks.

Hypoxia inducible factors regulate the transcription of the sprouty2 gene and expression of the sprouty2 protein

Receptor Tyrosine Kinase (RTK) signaling plays a major role in tumorigenesis and normal development. Sprouty2 (Spry2) attenuates RTK signaling and inhibits processes such as angiogenesis, cell proliferation, migration and survival, which are all upregulated in tumors. Indeed in cancers of the liver, lung, prostate and breast, Spry2 protein levels are markedly decreased correlating with poor patient prognosis and shorter survival. Thus, it is important to understand how expression of Spry2 is regulated. While prior studies have focused on the post-translation regulation of Spry2, very few studies have focused on the transcriptional regulation of SPRY2 gene. Here, we demonstrate that in the human hepatoma cell line, Hep3B, the transcription of SPRY2 is inhibited by the transcription regulating hypoxia inducible factors (HIFs). HIFs are composed of an oxygen regulated alpha subunit (HIF1α or HIF2α) and a beta subunit (HIF1β). Intriguingly, silencing of HIF1α and HIF2α elevates SPRY2 mRNA and protein levels suggesting HIFs reduce the transcription of the SPRY2 promoter. In silico analysis identified ten hypoxia response elements (HREs) in the proximal promoter and first intron of SPRY2. Using chromatin immunoprecipitation (ChIP), we show that HIF1α/2α bind near the putative HREs in the proximal promoter and intron of SPRY2. Our studies demonstrated that not only is the SPRY2 promoter methylated, but silencing HIF1α/2α reduced the methylation. ChIP assays also showed DNA methyltransferase1 (DNMT1) binding to the proximal promoter and first intron of SPRY2 and silencing HIF1α/2α decreased this association. Additionally, silencing of DNMT1 mimicked the HIF1α/2α silencing-mediated increase in SPRY2 mRNA and protein. While simultaneous silencing of HIF1α/2α and DNMT1 increased SPRY2 mRNA a little more, the increase was not additive suggesting a common mechanism by which DNMT1 and HIF1α/2α regulate SPRY2 transcription. Together these data suggest that the transcription of SPRY2 is inhibited by HIFs, in part, via DNMT1- mediated methylation.

PTP1B inhibitor promotes endothelial cell motility by activating the DOCK180/Rac1 pathway

Promoting endothelial cell (EC) migration is important not only for therapeutic angiogenesis, but also for accelerating re-endothelialization after vessel injury. Several recent studies have shown that inhibition of protein tyrosine phosphatase 1B (PTP1B) may promote EC migration and angiogenesis by enhancing the vascular endothelial growth factor receptor-2 (VEGFR2) signalling. In the present study, we demonstrated that PTP1B inhibitor could promote EC adhesion, spreading and migration, which were abolished by the inhibitor of Rac1 but not RhoA GTPase. PTP1B inhibitor significantly increased phosphorylation of p130Cas, and the interactions among p130Cas, Crk and DOCK180; whereas the phosphorylation levels of focal adhesion kinase, Src, paxillin, or Vav2 were unchanged. Gene silencing of DOCK180, but not Vav2, abrogated the effects of PTP1B inhibitor on EC motility. The effects of PTP1B inhibitor on EC motility and p130Cas/DOCK180 activation persisted in the presence of the VEGFR2 antagonist. In conclusion, we suggest that stimulation of the DOCK180 pathway represents an alternative mechanism of PTP1B inhibitor-stimulated EC motility, which does not require concomitant VEGFR2 activation as a prerequisite. Therefore, PTP1B inhibitor may be a useful therapeutic strategy for promoting EC migration in cardiovascular patients in which the VEGF/VEGFR functions are compromised.

Sprouty2 Drives Drug Resistance and Proliferation in Glioblastoma

Glioblastoma multiforme (GBM) is notoriously resistant to therapy, and the development of a durable cure will require the identification of broadly relevant regulators of GBM cell tumorigenicity and survival. Here, we identify Sprouty2 (SPRY2), a known regulator of receptor tyrosine kinases (RTK), as one such regulator. SPRY2 knockdown reduced proliferation and anchorage-independent growth in GBM cells and slowed xenograft tumor growth in mice. SPRY2 knockdown also promoted cell death in response to coinhibition of the epidermal growth factor receptor (EGFR) and the c-MET receptor in GBM cells, an effect that involved regulation of the ability of the p38 mitogen-activated protein kinase (MAPK) to drive cell death in response to inhibitors. Analysis of data from clinical tumor specimens further demonstrated that SPRY2 protein is definitively expressed in GBM tissue, that SPRY2 expression is elevated in GBM tumors expressing EGFR variant III (EGFRvIII), and that elevated SPRY2 mRNA expression portends reduced GBM patient survival. Overall, these results identify SPRY2 and the pathways it regulates as novel candidate biomarkers and therapeutic targets in GBM.

IMPLICATIONS

SPRY2, counter to its roles in other cancer settings, promotes glioma cell and tumor growth and cellular resistance to targeted inhibitors of oncogenic RTKs, thus making SPRY2 and the cell signaling processes it regulates potential novel therapeutic targets in glioma.

The developing story of Sprouty and cancer

Sprouty proteins are evolutionarily conserved modulators of MAPK/ERK pathway. Through interacting with an increasing number of effectors, mediators, and regulators with ultimate influence on multiple targets within or beyond ERK, Sprouty orchestrates a complex, multilayered regulatory system and mediates a crosstalk among different signaling pathways for a coordinated cellular response. As such, Sprouty has been implicated in various developmental and physiological processes. Evidence shows that ERK is aberrantly activated in malignant conditions. Accordingly, Sprouty deregulation has been reported in different cancer types and shown to impact cancer development, progression, and metastasis. In this article, we have tried to provide an overview of the current knowledge about the Sprouty physiology and its regulatory functions in health, as well as an updated review of the Sprouty status in cancer. Putative implications of Sprouty in cancer biology, their clinical relevance, and their proposed applications are also revisited. As a developing story, however, role of Sprouty in cancer remains to be further elucidated.

Sprouty 2 protein, but not Sprouty 4, is an independent prognostic biomarker for human epithelial ovarian cancer

Sprouty proteins are evolutionary-conserved modulators of receptor tyrosine kinase signaling, deregulation of which has been implicated in the pathophysiology of cancer. In the present study, the expression status of Spry2 and Spry4 proteins and its clinical relevance in human epithelial ovarian cancer (EOC) were investigated retrospectively. We examined the immunohistochemical expression of Spry2 and Spry4 in matched tumor and normal tissue samples from 99 patients. The expression of ERK, p-ERK, Ki67, fibroblast growth factor-2, vascular endothelial growth factor and interleukin-6 and their correlation with Sprouty homologs were also evaluated. Moreover, the correlation between Spry2 and Spry4 and the clinicopathological characteristics were analyzed along with their predictive value for overall survival (OS) and disease-free survival (DFS). Our data indicated significant downregulation of Spry2 and Spry4 in tumor tissues (p < 0.0001). A significant inverse correlation was evident between Spry2 and p-ERK/ERK (p = 0.048), Ki67 (p = 0.011), disease stage (p = 0.013), tumor grade (p = 0.003), recurrence (p < 0.001) and post-treatment ascites (p = 0.001), individually. It was found that Spry2 low-expressing patients had significantly poorer OS (p = 0.002) and DFS (p = 0.004) than those with high expression of Spry2. Multivariate analysis showed that high Spry2 (p = 0.018), low stage (p = 0.049) and no residual tumor (p =0.006) were independent prognostic factors for a better OS. With regard to DFS, high Spry2 (p = 0.044) and low stage (p = 0.046) remained as independent predictors. In conclusion, we report for the first time significant downregulation of Spry2 and Spry4 proteins in human EOC. Spry2 expression was revealed to significantly impact tumor behavior with predictive value as an independent prognostic factor for survival and recurrence.

Protein tyrosine phosphatase PTP1B is involved in hippocampal synapse formation and learning

ER-bound PTP1B is expressed in hippocampal neurons, and accumulates among neurite contacts. PTP1B dephosphorylates ß-catenin in N-cadherin complexes ensuring cell-cell adhesion. Here we show that endogenous PTP1B, as well as expressed GFP-PTP1B, are present in dendritic spines of hippocampal neurons in culture. GFP-PTP1B overexpression does not affect filopodial density or length. In contrast, impairment of PTP1B function or genetic PTP1B-deficiency leads to increased filopodia-like dendritic spines and a reduction in mushroom-like spines, while spine density is unaffected. These morphological alterations are accompanied by a disorganization of pre- and post-synapses, as judged by decreased clustering of synapsin-1 and PSD-95, and suggest a dynamic synaptic phenotype. Notably, levels of ß-catenin-Tyr-654 phosphorylation increased ∼5-fold in the hippocampus of adult PTP1B^−/− (KO) mice compared to wild type (WT) mice and this was accompanied by a reduction in the amount of ß-catenin associated with N-cadherin. To determine whether PTP1B-deficiency alters learning and memory, we generated mice lacking PTP1B in the hippocampus and cortex (PTP1B^fl/fl–Emx1-Cre). PTP1B^fl/fl–Emx1-Cre mice displayed improved performance in the Barnes maze (decreased time to find and enter target hole), utilized a more efficient strategy (cued), and had better recall compared to WT controls. Our results implicate PTP1B in structural plasticity within the hippocampus, likely through modulation of N-cadherin function by ensuring dephosphorylation of ß-catenin on Tyr-654. Disruption of hippocampal PTP1B function or expression leads to elongation of dendritic filopodia and improved learning and memory, demonstrating an exciting novel role for this phosphatase.

The protein tyrosine phosphatase PTP1B is required for efficient delivery of N-cadherin to the cell surface

This work shows a novel role of PTP1B in the regulation of N-cadherin trafficking. PTP1B is required for the association of p120 to the N-cadherin precursor and this event is crucial for trafficking of the complex through the early stages of the secretory pathway.

PTP1B bound to mature N-cadherin promotes the association of β-catenin into the complex, the stable expression of the complex at cell surface, and cadherin-mediated adhesion. Here we show that PTP1B is also required for N-cadherin precursor trafficking through early stages of the secretory pathway. This function does not require association of PTP1B with the precursor. In PTP1B null cells, the N-cadherin precursor showed higher sensitivity to endoglycosidase H than in cells reconstituted with the wild-type enzyme. It also showed slower kinetics of ER-to-Golgi translocation and processing. Trafficking of the viral stomatitis vesicular glycoprotein, VSV-G, however, revealed no differences between PTP1B null and reconstituted cells. N-cadherin precursor complexes contained similar levels of α- and β-catenin regardless of PTP1B expression. In contrast, the associated p120 catenin (p120) was significantly reduced in absence of PTP1B expression. An N-cadherin precursor construct defective in p120 binding, and expressed in PTP1B reconstituted cells, showed higher sensitivity to endoglycosidase H and slower kinetics of processing than the wild-type precursor. Our results suggest that PTP1B promotes the association of p120 to the N-cadherin precursor, facilitating the trafficking of the complex from the ER to the Golgi complex.

Establishment of extracellular signal-regulated kinase 1/2 bistability and sustained activation through Sprouty 2 and its relevance for epithelial function

Our objective was to establish an experimental model of a self-sustained and bistable extracellular signal-regulated kinase 1/2 (ERK1/2) signaling process. A single stimulation of cells with cytokines causes rapid ERK1/2 activation, which returns to baseline in 4 h. Repeated stimulation leads to sustained activation of ERK1/2 but not Jun N-terminal protein kinase (JNK), p38, or STAT6. The ERK1/2 activation lasts for 3 to 7 days and depends upon a positive-feedback mechanism involving Sprouty 2. Overexpression of Sprouty 2 induces, and its genetic deletion abrogates, ERK1/2 bistability. Sprouty 2 directly activates Fyn kinase, which then induces ERK1/2 activation. A genome-wide microarray analysis shows that the bistable phospho-ERK1/2 (pERK1/2) does not induce a high level of gene transcription. This is due to its nuclear exclusion and compartmentalization to Rab5+ endosomes. Cells with sustained endosomal pERK1/2 manifest resistance against growth factor withdrawal-induced cell death. They are primed for heightened cytokine production. Epithelial cells from cases of human asthma and from a mouse model of chronic asthma manifest increased pERK1/2, which is associated with Rab5+ endosomes. The increase in pERK1/2 was associated with a simultaneous increase in Sprouty 2 expression in these tissues. Thus, we have developed a cellular model of sustained ERK1/2 activation, which may provide a mechanistic understanding of self-sustained biological processes in chronic illnesses such as asthma.

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.

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.

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.

Investigation of protein-tyrosine phosphatase 1B function by quantitative proteomics

Because of their antagonistic catalytic functions, protein-tyrosine phosphatases (PTPs) and protein-tyrosine kinases act together to control phosphotyrosine-mediated signaling processes in mammalian cells. However, unlike for protein-tyrosine kinases, little is known about the cellular substrate specificity of many PTPs because of the lack of appropriate methods for the systematic and detailed analysis of cellular PTP function. Even for the most intensely studied, prototypic family member PTP1B many of its physiological functions cannot be explained by its known substrates. To gain better insights into cellular PTP1B function, we used quantitative MS to monitor alterations in the global tyrosine phosphorylation of PTP1B-deficient mouse embryonic fibroblasts in comparison with their wild-type counterparts. In total, we quantified 124 proteins containing 301 phosphotyrosine sites under basal, epidermal growth factor-, or platelet-derived growth factor-stimulated conditions. A subset of 18 proteins was found to harbor hyperphosphorylated phosphotyrosine sites in knock-out cells and was functionally linked to PTP1B. Among these proteins, regulators of cell motility and adhesion are overrepresented, such as cortactin, lipoma-preferred partner, ZO-1, or p120ctn. In addition, regulators of proliferation like p62DOK or p120RasGAP also showed increased cellular tyrosine phosphorylation. Physical interactions of these proteins with PTP1B were further demonstrated by using phosphatase-inactive substrate-trapping mutants in a parallel MS-based analysis. Our results correlate well with the described phenotype of PTP1B-deficient fibroblasts that is characterized by an increase in motility and reduced cell proliferation. The presented study provides a broad overview about phosphotyrosine signaling processes in mouse fibroblasts and, supported by the identification of various new potential substrate proteins, indicates a central role of PTP1B within cellular signaling networks. Importantly the MS-based strategies described here are entirely generic and can be used to address the poorly understood aspects of cellular PTP function.

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.

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.

Electroconvulsive seizures stimulate glial proliferation and reduce expression of Sprouty2 within the prefrontal cortex of rats

BACKGROUND

Reductions in cell number are found within the medial prefrontal cortex (PFC) in major depression and bipolar disorder, conditions for which electroconvulsive therapy (ECT) is a highly effective treatment. We investigated whether electroconvulsive seizure (ECS) in rats stimulates cellular proliferation in the PFC immediately and four weeks after the treatments. In parallel, we examined if ECS also alters the expression of Sprouty2 (SPRY2), an inhibitor of cell proliferation.

METHODS

Sprague-Dawley rats received 10 days of ECS treatments and bromodeoxyuridine (BrdU) injections. After a four week survival period, we estimated the density and number of BrdU-, proliferating cell nuclear antigen (PCNA)-, and SPRY2-immunoreactive cells in the medial (infralimbic) PFC (ILPFC). We also determined the percentage of BrdU-labeled cells that were immunoreactive for markers specific to oligodendrocytes, astrocytes, endothelial cells and neurons.

RESULTS

ECS dramatically enhanced the proliferation of new cells in the infralimbic PFC, and this effect persisted four weeks following the treatments. The percentage of new cells expressing oligodendrocyte precursor cell markers increased slightly following ECS. In contrast, ECS dramatically reduced the number of cells expressing SPRY2.

CONCLUSIONS

ECS stimulates long-lasting increases in glial proliferation within the ILPFC. ECS also decreases SPRY2 expression in the same region, an effect that might contribute to increased glial proliferation.

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.

Functional proteomics identifies protein-tyrosine phosphatase 1B as a target of RhoA signaling

Rho GTPases are signal transduction effectors that control cell motility, cell attachment, and cell shape by the control of actin polymerization and tyrosine phosphorylation. To identify cellular targets regulated by Rho GTPases, we screened global protein responses to Rac1, Cdc42, and RhoA activation by two-dimensional gel electrophoresis and mass spectrometry. A total of 22 targets were identified of which 19 had never been previously linked to Rho GTPase pathways, providing novel insight into pathway function. One novel target of RhoA was protein-tyrosine phosphatase 1B (PTP1B), which catalyzes dephosphorylation of key signaling molecules in response to activation of diverse pathways. Subsequent analysis demonstrated that RhoA enhances post-translational modification of PTP1B, inactivates phosphotyrosine phosphatase activity, and up-regulates tyrosine phosphorylation of p130Cas, a key mediator of focal adhesion turnover and cell migration. Thus, protein profiling reveals a novel role for PTP1B as a mediator of RhoA-dependent phosphorylation of p130Cas.

Sprouty-4 negatively regulates cell spreading by inhibiting the kinase activity of testicular protein kinase

TESK1 (testicular protein kinase 1) is a serine/threonine kinase that phosphorylates cofilin and plays a critical role in integrin-mediated actin cytoskeletal reorganization and cell spreading. We previously showed that TESK1 interacts with Sprouty-4 (referred to as Spry4), an inhibitor of growth factor-induced Ras/MAP (mitogen-activated protein) kinase signalling, but the functional role of this interaction has remained unknown. In the present study, we show that Spry4 inhibits the kinase activity of TESK1 by binding to it through the C-terminal cysteine-rich region. Expression of Spry4 in cultured cells suppressed integrin-mediated cell spreading, and TESK1 reversed the inhibitory effect of Spry4 on cell spreading. Furthermore, Spry4 suppressed integrin- and TESK1-mediated cofilin phosphorylation during the spreading of cells on laminin. These findings suggest that Spry4 suppresses cell spreading by inhibiting the kinase activity of TESK1. Although tyrosine phosphorylation is required for the inhibitory activity of Spry4 on a Ras/MAP kinase pathway, mutation of the corresponding tyrosine residue (Tyr-75 in human Spry4) to an alanine had no apparent effect on its inhibitory actions on TESK1 activity and cell spreading, which suggests a novel cellular function of Spry to regulate the actin cytoskeleton, independent of its inhibitory activity on the Ras/MAP kinase signalling.

The tumor suppressor PTEN is necessary for human Sprouty 2-mediated inhibition of cell proliferation

Sprouty family proteins are novel regulators of growth factor actions. Human Sprouty 2 (hSPRY2) inhibits the proliferation of a number of different cell types. However, the mechanisms involved in the anti-proliferative actions of hSPRY2 remain to be elucidated. Here we have demonstrated that hSPRY2 increases the amount of the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and decreases its phosphorylation. The resultant increase in PTEN activity is reflected in decreased activation of Akt by epidermal growth factor and serum. Consistent with increased PTEN activity, in hSPRY2-expressing cells, the progression of cells from the G1 to S phase is decreased. By using PTEN null primary mouse embryonic fibroblasts and their isogenic controls as well as small interfering RNA against PTEN, we demonstrated that PTEN is necessary for hSPRY2 to inhibit Akt activation by epidermal growth factor as well as cell proliferation. Overall, we concluded that hSPRY2 mediates its anti-proliferative actions by altering PTEN content and activity.

Efficient suppression of FGF-2-induced ERK activation by the cooperative interaction among mammalian Sprouty isoforms

Strict regulation of the receptor tyrosine kinase (RTK)/extracellular signal-regulated kinase (ERK) pathway is essential for maintaining balanced growth in multi-cellular organisms. Several negative regulators of the pathway have been identified which include Sprouty proteins. Mammalian cells express four Sprouty isoforms (Sprouty1-4) in an ERK-dependent manner. In this study, we have examined the molecular mechanisms by which Sprouty proteins elicit their inhibitory effects on the RTK/ERK pathway, with special focus on the co-operation among Sprouty isoforms. The four mammalian Sprouty isoforms interact with each other, most probably to form hetero- as well as homo-oligomers through their C-terminal domains. Sprouty1 specifically interacts with Grb2, whereas Sprouty4 interacts with Sos1. Although any of the Sprouty isoforms by itself inhibits the fibroblast growth factor-2 (FGF-2)-induced activation of the ERK pathway significantly, hetero-oligomers show a more pronounced inhibitory activity. The hetero-oligomer formed between Sprouty1 and Sprouty4 exhibits the most potent inhibitory effect on ERK activation through its highly effective ability to suppress the association of Grb2-Sos1 complex with FRS2. The cooperative interactions observed among Sprouty isoforms could represent an advanced system that functions to regulate strictly the activation state of the RTK/ERK pathway in mammalian cells.

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.

Curcumin inhibits platelet-derived growth factor-stimulated vascular smooth muscle cell function and injury-induced neointima formation

OBJECTIVE

Vascular smooth muscle cell (VSMC) migration, proliferation, and collagen synthesis are key events involved in the pathogenesis of cardiovascular disease. Growth factors, such as platelet-derived growth factor (PDGF) and fibroblast growth factor, released during vascular injury plays a pivotal role in regulating these events. Curcumin (diferuloyl methane), a major component of the spice turmeric (Curcuma longa), has been shown recently to have beneficial effects in chronic conditions, such as inflammation, cancer, cystic fibrosis, and Alzheimer's disease. The objective of this study was to investigate the ability of curcumin to inhibit PDGF-stimulated migration, proliferation, and collagen synthesis in cultured VSMCs and neointima formation after carotid artery injury in rats.

METHODS AND RESULTS

Curcumin (1 to 25 microM) produced a concentration-dependent inhibition of PDGF-elicited VSMC migration, proliferation, and collagen synthesis assessed by chemotaxis, [3H]thymidine incorporation, and [3H]-L-proline incorporation, respectively. Curcumin blocked PDGF-induced VSMC actin-cytoskeleton reorganization, attenuated PDGF signal transduction, and inhibited the binding of PDGF to its receptors. Carotid artery neointima formation was significantly attenuated by perivascular curcumin compared with vehicle controls 14 days after injury, characterized by reduced DNA synthesis, collagen synthesis, and PDGF receptor phosphorylation.

CONCLUSIONS

These data suggest that curcumin is a potent inhibitor of key PDGF-stimulated VSMC functions and may play a critical role in regulating these events after vascular injury.

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.

FGF Signal Interpretation Is Directed by Sprouty and Spred Proteins during Mesoderm Formation

Vertebrate gastrulation requires coordination of mesoderm specification with morphogenetic movements. While both of these processes require FGF signaling, it is not known how mesoderm specification and cell movements are coordinated during gastrulation. The related Sprouty and Spred protein families are recently discovered regulators of receptor tyrosine kinase signaling. We identified two genes for each family in Xenopus tropicalis: Xtsprouty1, Xtsprouty2, Xtspred1, and Xtspred2. In gain- and loss-of-function experiments we show that XtSprouty and XtSpred proteins modulate different signaling pathways downstream of the FGF receptor (FGFR), and consequently different developmental processes. Notably, XtSproutys inhibit morphogenesis and Ca(2+) and PKCdelta signaling, leaving MAPK activation and mesoderm specification intact. In contrast, XtSpreds inhibit MAPK activation and mesoderm specification, with little effect on Ca(2+) or PKCdelta signaling. These differences, combined with the timing of their developmental expression, suggest a mechanism to switch FGFR signal interpretation to coordinate mesoderm formation and cell movements during gastrulation.

Disruption of target cell adhesion structures by the Yersinia effector YopH requires interaction with the substrate domain of p130Cas

The docking protein p130Cas has, together with FAK, been found as a target of the Yersinia virulence effector YopH. YopH is a protein tyrosine phosphatase that is delivered into host cells via the bacterial type III secretion machinery, and the outcome of its activity is inhibition of host cell phagocytosis. In the present study using p130Cas-/- cells, and p130Cas-/- cells expressing variants of GFPp130Cas, we show that this docking protein, via its substrate domain, is responsible for subcellular targeting of YopH in eukaryotic cells. Since YopH inhibits phagocytosis, p130Cas was expected to be critical for signalling mediating bacterial internalization. However, p130Cas-/- cells did not exhibit reduced capacity to internalize Yersinia. On the other hand, when a dominant negative variant of p130Cas was expressed in these cells, the phagocytic capacity was severely impaired. Moreover, the p130Cas-/- cells displayed a marked reduced sensitivity towards YopH-mediated detachment compared to wild-type cells. Transfecting these cells with full-length p130Cas rendered cells hypersensitive to both mechanical and Yersinia-mediated detachment. This hypersensitivity was not seen upon transfection with the dominant negative substrate domain-deleted variant of p130Cas. This implicates p130Cas as a prominent regulator of cell adhesion, where its substrate-binding domain has a significant function.

Regulation of Vascular Smooth Muscle Cell Proliferation and Migration by Human Sprouty 2

OBJECTIVE

To determine whether the human sprouty 2 (hSPRY2) protein, an inhibitor of receptor tyrosine kinase actions, regulates vascular smooth muscle cell (VSMC) proliferation, migration, and neointima formation in injured carotid artery.

METHODS AND RESULTS

The hSPRY2 protein or green fluorescent protein (GFP; control) was transduced into VSMCs by placing an N-terminal TAT epitope on the proteins. The transduction of TAT-tagged hSPRY2 (TAT-hSPRY2) but not TAT-GFP inhibited the ability of serum and different growth factors to stimulate migration of VSMCs. Likewise, TAT-hSPRY2 also inhibited VSMC proliferation in response to serum. The hSPRY2 microtubule association (amino acids 123-177) and membrane translocation (amino acids 178-194) domains were necessary for the biological actions of hSPRY2. In the rat carotid artery injury model, exposure of the injured vessel for 1 hour to TAT-hSPRY2, but not TAT-GFP, markedly inhibited growth of the neointima over the 28-day postangioplasty period as well as VSMC proliferation. The exogenously applied TAT-hSPRY2 was retained in the carotid arteries for at least 3 days after injury, and endogenous SPRY2 expression was maximized around day 14 after injury. The latter is perhaps a compensatory mechanism to regulate neointima formation.

CONCLUSIONS

We conclude that TAT-tagged proteins are efficiently transduced into VSMCs in vitro and in vivo, that hSPRY2 inhibits growth and migration of VSMCs, and that this protein can decrease neointimal growth after blood vessel injury.

Regulation of actin-based cell migration by cAMP/PKA

A wide variety of soluble signaling substances utilize the cyclic AMP-dependent protein kinase (PKA) pathway to regulate cellular behaviors including intermediary metabolism, ion channel conductivity, and transcription. A growing literature suggests that integrin-mediated cell adhesion may also utilize PKA to modulate adhesion-associated events such as actin cytoskeletal dynamics and migration. PKA is dynamically regulated by integrin-mediated cell adhesion to extracellular matrix (ECM). Furthermore, while some hallmarks of cell migration and cytoskeletal organization require PKA activity (e.g. activation of Rac and Cdc42; actin filament assembly), others are inhibited by it (e.g. activation of Rho and PAK; interaction of VASP with the c-Abl tyrosine kinase). Also, cell migration and invasion can be impeded by either inhibition or hyper-activation of PKA. Finally, a number of A-kinase anchoring proteins (AKAPs) serve to associate PKA with various components of the actin cytoskeleton, thereby enhancing and/or specifying cAMP/PKA signaling in those regions. This review discusses the growing literature that supports the hypothesis that PKA plays a central role in cytoskeletal regulation and cell migration.

Sprouty regulates cell migration by inhibiting the activation of Rac1 GTPase

Sprouty (SPRY) protein negatively modulates fibroblast growth factor and epidermal growth factor actions. We showed that human SPRY2 inhibits cell growth and migration in response to serum and several growth factors. Using rat intestinal epithelial (IEC-6) cells, we investigated the involvement of the Rho family of GTPases, RhoA, Rac1, and cdc42 in SPRY2-mediated inhibition of cell migration and proliferation. The ability of TAT-tagged SPRY2 to inhibit proliferation and migration of IEC-6 cells transfected with constitutively active mutants of RhoA(G14V), Rac1(G12V), and cdc42 (F28L) was determined. Constitutively active RhoA(G14V), Rac1(G12V), or cdc42(F28L) did not protect cells from the anti-proliferative actions of TAT-SPRY2. The ability of TAT-hSPRY2 to inhibit migration was not altered by of RhoA(G14V) and cdc42(F28L). However, Rac1(G12V) obliterated the ability of SPRY2 to inhibit cell autonomous or serum-induced migration. Also, the activation of endogenous Rac1 was attenuated by TAT-SPRY2. Thus, SPRY2 mediates its anti-migratory actions by inhibiting Rac1 activation.

Overexpression of sprouty 2 inhibits HGF/SF-mediated cell growth, invasion, migration, and cytokinesis

A strict regulation of hepatocyte growth factor/scatter factor (HGF/SF)-Met signaling is essential for its appropriate function. Several negative regulators of Met signaling have been identified. Here we report that human Spry2 is induced by HGF/SF and negatively regulates HGF/SF-Met signaling. We show that overexpression of Spry2 inhibits cell proliferation, anchorage-independent cell growth, and migration in wound-healing and in vitro invasion assays. Measured in an electric cell-substrate impedance sensing biosensor, cell movement is restricted, because Spry2 dramatically facilitates cell attachment and spreading by enhancing focal adhesions and increasing stress fibers. An analysis of cell cycle distribution shows, unexpectedly, that Spry2-GFP cells are polyploid. Thus, as with FGF and EGF receptors, Spry2-GFP tempers downstream Met signaling in addition to its pronounced effect on cell adhesion, and it has properties suitable to be considered a tumor-suppressor protein.

Tyrosine phosphorylation of Sprouty2 enhances its interaction with c-Cbl and is crucial for its function

Mammalian Sprouty (Spry) proteins are now established as receptor tyrosine kinase-induced modulators of the Ras/mitogen-activated protein kinase pathway. Specifically, hSpry2 inhibits the fibroblast growth factor receptor (FGFR)-induced mitogen-activated protein kinase pathway but conversely prolongs activity of the same pathway following epidermal growth factor (EGF) stimulation, where activated EGF receptors are retained on the cell surface. In this study it is demonstrated that hSpry2 is tyrosine-phosphorylated upon stimulation by either FGFR or EGF and subsequently binds endogenous c-Cbl with high affinity. A conserved motif on hSpry2, together with phosphorylation on tyrosine 55, is required for its enhanced interaction with the SH2-like domain of c-Cbl. A hSpry2 mutant (Y55F) that did not exhibit an enhanced binding with c-Cbl failed to retain EGF receptors on the cell surface. Furthermore, individually mutating hSpry2 residues 52-59 to alanine indicated a tight correlation between their affinity for c-Cbl binding and their inhibition of ERK2 activity in the FGFR pathway. We postulate that tyrosine phosphorylation "activates" hSpry2 by enhancing its interaction with c-Cbl and that this interaction is critical for its physiological function in a signal-specific context.

Sprouty: how does the branch manager work?

Since the discovery of the prototypical Sprouty (Spry) protein in Drosophila, there has been an effort to determine how these novel modulators of the Ras/MAP-kinase pathway function. A clue to their mechanism of action comes from the several highly conserved sequences within all the currently known Spry isoforms: an approximately 110-residue cysteine-rich sequence in the C-terminal half that directs Spry proteins to a concentration of signaling proteins at the plasma membrane; a small motif surrounding a tyrosine residue (Y55 in human Spry2) that is responsible for interaction with other proteins. In cultured mammalian cells, hSpry2 inhibits epidermal growth factor receptor (EGFR) endocytosis and subsequently sustains the activation of MAP kinase but negatively regulates the same pathway following stimulation of fibroblast growth factor receptors (FGFRs). Current evidence indicates that Cbl is a key protein that interacts directly with Spry2 following activation of receptor tyrosine kinases (RTKs). It appears to be the ability of Cbl to interact as an E3 ubiquitin ligase on specific target proteins and as a docking protein in other contexts that dictates the differential effects Spry2 has on the Ras/MAP-kinase pathway following EGFR and FGFR activation.

Mammalian Sprouty4 suppresses Ras-independent ERK activation by binding to Raf1

The signalling cascade including Raf, mitogen-activated protein kinase (MAPK) kinase and extracellular-signal-regulated kinase (ERK) is important in many facets of cellular regulation. Raf is activated through both Ras-dependent and Ras-independent mechanisms, but the regulatory mechanisms of Raf activation remain unclear. Two families of membrane-bound molecules, Sprouty and Sprouty-related EVH1-domain-containing protein (Spred) have been identified and characterized as negative regulators of growth-factor-induced ERK activation. But the molecular functions of mammalian Sproutys have not been clarified. Here we show that mammalian Sprouty4 suppresses vascular epithelial growth factor (VEGF)-induced, Ras-independent activation of Raf1 but does not affect epidermal growth factor (EGF)-induced, Ras-dependent activation of Raf1. Sprouty4 binds to Raf1 through its carboxy-terminal cysteine-rich domain, and this binding is necessary for the inhibitory activity of Sprouty4. In addition, Sprouty4 mutants of the amino-terminal region containing the conserved tyrosine residue, which is necessary for suppressing fibroblast growth factor signalling, still inhibit the VEGF-induced ERK pathway. Our results show that receptor tyrosine kinases use distinct pathways for Raf and ERK activation and that Sprouty4 differentially regulates these pathways.

Molecular cloning of mammalian Spred-3 which suppresses tyrosine kinase-mediated Erk activation

We have reported on Spred-1 and Spred-2, which inhibit MAP kinase activation by interacting with c-kit and ras/raf. Here, we report the cloning of a third member in this family, Spred-3. Spred-3 is expressed exclusively in the brain and its gene locates in chromosome 19q13.13 in human. Like Spred-1 and -2, Spred-3 contains an EVH1 domain in the N-terminus and a Sprouty-related cysteine-rich region (SPR domain) in the C-terminus that is necessary for membrane localization. However, Spred-3 does not possess a functional c-kit binding domain (KBD), since the critical amino acid Arg residue in this region was replaced with Gly in Spred-3. Although Spred-3 suppressed growth factor-induced MAP kinase (Erk) activation, inhibitory activity of Spred-3 was lower than that of Spred-1 or Spred-2. By the analysis of chimeric molecules between Spred-3 and Spred-1, we found that the SPR domain, rather than KBD, is responsible for efficient Erk suppression. The finding of Spred-3 revealed the presence of a novel family of regulators for the Ras/MAP kinase pathway, each member of which may have different specificities for extracellular signals.