Sprouty proteins are targeted to membrane ruffles upon growth factor receptor tyrosine kinase activation. Identification of a novel translocation domain

Paclitaxel Aggravating Radiation-Induced Pulmonary Fibrosis Is Associated with the Down-Regulation of the Negative Regulatory Function of Spry2

Paclitaxel (PTX) is capable of aggravating radiation-induced pulmonary fibrosis (RIPF), but the mechanism is unknown. Spry2 is a negative regulator of receptor tyrosine kinase-related Ras/Raf/extracellular signal regulated kinase (ERK) pathway. This experiment was aimed at exploring whether the aggravation of RIPF by PTX is related to Spry2. The RIPF model was established with C57BL/6 mice by thoracic irradiation, and PTX was administered concurrently. Western blot was used to detect the expression level of ERK signaling molecules and the distribution of Spry2 in the plasma membrane/cytoplasm. Co-immunoprecipitation (co-IP) and immunofluorescence were used to observe the colocalization of Spry2 with the plasma membrane and tubulin. The results showed that PTX-concurrent radiotherapy could aggravate fibrotic lesions in RIPF, downregulate the content of membrane Spry2, and upregulate the levels of p-c-Raf and p-ERK in lung tissue. It was found that knockdown of Spry2 in fibroblast abolished the upregulation of p-c-Raf and p-ERK by PTX. Both co-IP results and immunofluorescence staining showed that PTX increased the binding of Spry2 to tubulin, and microtubule depolymerizing agents could abolish PTX's inhibition of Spry2 membrane distribution and inhibit PTX's upregulation of Raf/ERK signaling. Both nintedanib and ERK inhibitor were effective in relieving PTX-exacerbated RIPF. Taken together, the mechanism of PTX's aggravating RIPF was related to its ability to enhance Spry2's binding to tubulin, thus attenuating Spry2's negative regulation on Raf/ERK pathway. SIGNIFICANCE STATEMENT: This study revealed that paclitaxel (PTX) concurrent radiation therapy exacerbates radiation-induced pulmonary fibrosis during the treatment of thoracic tumors, which is associated with PTX restraining Spry2 and upregulating the Raf/extracellular signal regulated kinase signaling pathway, and provided drug targets for mitigating this complication.

Retinoblastoma gene expression profiling based on bioinformatics analysis

BACKGROUND

Retinoblastoma (RB) is frequently occurring malignant tumors that originate in the retina, and their exact cause and development mechanisms are yet to be fully comprehended. In this study, we identified possible biomarkers for RB and delved into the molecular mechanics linked with such markers.

METHODS

In this study GSE110811 and GSE24673 were analyzed. Weighted gene co-expression network analysis (WGCNA) was applied to screen modules and genes associated with RB. By overlapping RB-related module genes with differentially expressed genes (DEGs) between RB and control samples, differentially expressed retinoblastoma genes (DERBGs) were acquired. A gene ontology (GO) enrichment analysis and a kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis were conducted to explore the functions of these DERBGs. To study the protein interactions of DERBGs, a protein-protein interaction (PPI) network was constructed. Hub DERBGs were screened using the least absolute shrinkage and selection operator (LASSO) regression analysis, as well as the random forest (RF) algorithm. Additionally, the diagnostic performance of RF and LASSO methods was evaluated using receiver operating characteristic (ROC) curves and single-gene gene set enrichment analysis (GSEA) was conducted to explore the potential molecular mechanisms involved with these Hub DERBGs. In addition, the competing endogenous RNA (ceRNA) regulatory network of Hub DERBGs was constructed.

RESULT

About 133 DERBGs were found to be associated with RB. GO and KEGG enrichment analyses revealed that the important pathways of these DERBGs. Furthermore, the PPI network revealed 82 DERBGs interacting with each other. By RF and LASSO methods, PDE8B, ESRRB, and SPRY2 were identified as Hub DERBGs in patients with RB. From the expression assessment of Hub DERBGs, it was found that the levels of expression of PDE8B, ESRRB, and SPRY2 were significantly decreased in the tissues of RB tumors. Secondly, single-gene GSEA revealed a connection between these 3 Hub DERBGs and oocyte meiosis, cell cycle, and spliceosome. Finally, the ceRNA regulatory network revealed that hsa-miR-342-3p, hsa-miR-146b-5p, hsa-miR-665, and hsa-miR-188-5p may play a central role in the disease.

CONCLUSION

Hub DERBGs may provide new insight into RB diagnosis and treatment based on the understanding of disease pathogenesis.

Co-treatment with miR-21-5p inhibitor and Aurora kinase inhibitor reversine suppresses breast cancer progression by targeting sprouty RTK signaling antagonist 2

Numerous studies have reported the regulatory effects of miR-21-5p and reversine in human breast cancer (HBC). However, the mechanism of reversine and miR-21-5p has not been fully investigated in HBC. The aim of the current study was to assess the mechanism of action of reversine, with or without miR-21-5p, in HBC progression. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and Western blot results confirmed the upregulation of miR-21-5p and downregulation of sprouty RTK signaling antagonist 2 (SPRY2) in HBC. Bioinformatics analysis and luciferase assay identified the correlation between miR-21-5p and SPRY2. Cell function experiment results indicated a decrease in migration, proliferation, and invasion of HBC cells treated with miR-21-5p inhibitor and reversine; however, an increase in apoptosis was observed in these cells. Apoptotic ability was more enhanced and migration, proliferation, and invasion were more impaired in HBC cells treated with both miR-21-5p inhibitor and reversine than in those treated individually with either inhibitors. SPRY2, downstream of miR-21-5p, participated in HBC progression with reversine. Overall, our study proved that combining the miR-21-5p inhibitor with reversine produced a synergistic effect by regulating SPRY2, thereby limiting HBC progression. This knowledge might offer insights into the clinical therapy of HBC.

Expression of Spred2 in the urothelial tumorigenesis of the urinary bladder

Aberrant activation of the Ras/Raf/ERK (extracellular-signal-regulated kinase)-MAPK (mitogen-activated protein kinase) pathway is involved in the progression of cancer, including urothelial carcinoma; but the negative regulation remains unclear. In the present study, we investigated pathological expression of Spred2 (Sprouty-related EVH1 domain-containing protein 2), a negative regulator of the Ras/Raf/ERK-MAPK pathway, and the relation to ERK activation and Ki67 index in various categories of 275 urothelial tumors obtained from clinical patients. In situ hybridization demonstrated that Spred2 mRNA was highly expressed in high-grade non-invasive papillary urothelial carcinoma (HGPUC), and the expression was decreased in carcinoma in situ (CIS) and infiltrating urothelial carcinoma (IUC). Immunohistochemically, membranous Spred2 expression, important to interact with Ras/Raf, was preferentially found in HGPUC. Interestingly, membranous Spred2 expression was decreased in CIS and IUC relative to HGPUC, while ERK activation and the expression of the cell proliferation marker Ki67 index were increased. HGPUC with membranous Spred2 expression correlated significantly with lower levels of ERK activation and Ki67 index as compared to those with negative Spred2 expression. Thus, our pathological findings suggest that Spred2 counters cancer progression in non-invasive papillary carcinoma possibly through inhibiting the Ras/Raf/ERK-MAPK pathway, but this regulatory mechanism is lost in cancers with high malignancy. Spred2 appears to be a key regulator in the progression of non-invasive bladder carcinoma.

Identification of key features required for efficient S-acylation and plasma membrane targeting of sprouty-2

Sprouty-2 is an important regulator of growth factor signalling and a tumour suppressor protein. The defining feature of this protein is a cysteine-rich domain (CRD) that contains twenty-six cysteine residues and is modified by S-acylation. In this study, we show that the CRD of sprouty-2 is differentially modified by S-acyltransferase enzymes. The high specificity/low activity zDHHC17 enzyme mediated restricted S-acylation of sprouty-2, and cysteine-265 and -268 were identified as key targets of this enzyme. In contrast, the low specificity/high activity zDHHC3 and zDHHC7 enzymes mediated more expansive modification of the sprouty-2 CRD. Nevertheless, S-acylation by all enzymes enhanced sprouty-2 expression, suggesting that S-acylation stabilises this protein. In addition, we identified two charged residues (aspartate-214 and lysine-223), present on opposite faces of a predicted α-helix in the CRD, which are essential for S-acylation of sprouty-2. Interestingly, mutations that perturbed S-acylation also led to a loss of plasma membrane localisation of sprouty-2 in PC12 cells. This study provides insight into the mechanisms and outcomes of sprouty-2 S-acylation, and highlights distinct patterns of S-acylation mediated by different classes of zDHHC enzymes.

A novel Sprouty4-ERK1/2-Wnt/β-catenin regulatory loop in marrow stromal progenitor cells controls osteogenic and adipogenic differentiation

BACKGROUND

Sprouty (SPRY) proteins play critical roles in controlling cell proliferation, differentiation, and survival by inhibiting receptor tyrosine kinase (RTK)-mediated extracellular signal-regulated kinase (ERK) signaling. Recent studies have demonstrated that SPRY4 negatively regulates angiogenesis and tumor growth. However, whether SPRY4 regulates osteogenic and/or adipogenic differentiation of mesenchymal stem cells remains to be explored.

RESULTS

In this study, we investigated the expression pattern of Spry4 and found that its expression was regulated during the differentiation of mouse marrow stromal progenitor cells and increased in the metaphysis of ovariectomized mice. In vitro loss-of-function and gain-of-function studies demonstrated that SPRY4 inhibited osteogenic differentiation and stimulated adipogenic differentiation of progenitor cells. In vivo experiments showed that silencing of Spry4 in the marrow of C57BL/6 mice blocked fat accumulation and promoted osteoblast differentiation in ovariectomized mice. Mechanistic investigations revealed the inhibitory effect of SPRY4 on canonical wingless-type MMTV integration site (Wnt) signaling and ERK pathway. ERK1/2 was shown to interact with low-density lipoprotein receptor-related protein 6 (LRP6) and activate the canonical Wnt signaling pathway. Inactivation of Wnt signaling attenuated the inhibition of adipogenic differentiation and stimulation of osteogenic differentiation by Spry4 small interfering RNA (siRNA). Finally, promoter study revealed that β-catenin transcriptionally inhibited the expression of Spry4.

CONCLUSIONS

Our study for the first time suggests that a novel SPRY4-ERK1/2-Wnt/β-catenin regulatory loop exists in marrow stromal progenitor cells and plays a key role in cell fate determination. It also highlights the potential of SPRY4 as a novel therapeutic target for the treatment of metabolic bone disorders such as osteoporosis.

Spatial signal repression as an additional role of Sprouty2 protein variants

Sprouty2 (Spry2) is a prominent member of a protein family with crucial functions in the modulation of signal transduction. One of its main actions is the repression of mitogen-activated protein kinase (MAPK) pathway in response to growth factor-induced signalling. A common single nucleotide polymorphism within the Spry2 gene creates two protein variants where a proline adjacent to the serine rich domain is converted to an additional serine. Both protein variants perform similar functions although their efficiency in fulfilling these tasks varies. In this report, we used biochemical fractionation methods as well as confocal microscopy to analyse quantitative and qualitative differences in the distribution of Spry2 variants. We found that Spry2 proteins localize not solely to the plasma membrane, but also to other membrane engulfed compartments like for example the Golgi apparatus. In these less dense organelles, predominantly slower migrating forms reside indicating that posttranslational modification contributes to the distribution profile of Spry2. However there is no significant difference in the distribution of the two variants. Additionally, we found that Spry2 could be found exclusively in membrane fractions irrespective of the mitogen availability and the phosphorylation status. Considering the interference of extracellular signal-regulated kinase (ERK) activation in the cytoplasm, both Spry2 variants inhibited the levels of phosphorylated ERK (pERK) significantly to a similar extent. In contrast, the induction profiles of pERK levels were completely different in the nuclei. Again, both Spry2 variants diminished the levels of pERK. While the proline variant lowered the activation throughout the observation period, the serine variant failed to interfere with immediate accumulation of nuclear pERK levels, but the signal duration was shortened. Since the extent of the pERK inhibition in the nuclei was drastically more pronounced than in the cytoplasm, we conclude that Spry2 - in addition to its known functions as a repressor of general ERK phosphorylation - functions as a spatial repressor of nucleic ERK activation. Accordingly, a dominant negative version of Spry2 was only able to enhance the pERK levels of serum-deprived cells in the cytosol, while in the nucleus the intensity of the pERK signal in response to serum addition was significantly increased.

Subcellular Localization of Sprouty2 in Human Glioma Cells

Sprouty proteins act ubiquitously as signaling integrators and inhibitors of receptor tyrosine kinase (RTK) activated pathways. Among the four Sprouty isoforms, Sprouty2 is a key regulator of growth factor signaling in several neurological disorders. High protein levels correlate with reduced survival of glioma patients. We recently demonstrated that abrogating its function inhibits tumor growth by overstimulation of ERK and induction of DNA replication stress. The important role of Sprouty2 in the proliferation of malignant glioma cells prompted us to investigate its subcellular localization applying super-resolution fluorescence and immunoelectron microscopy. We found that cytoplasmic Sprouty2 is not homogenously distributed but localized to small spots (<100 nm) partly attached to vimentin filaments and co-localized with activated ERK. The protein is associated with early, late and recycling endosomes in response to but also independently of growth factor stimulation. The subcellular localization of Sprouty2 in all areas exhibiting strong RTK activities may reflect a protective response of glioma cells to limit excessive ERK activation and to prevent cellular senescence and apoptosis.

New insights into RAS biology reinvigorate interest in mathematical modeling of RAS signaling

RAS is the most frequently mutated gene across human cancers, but developing inhibitors of mutant RAS has proven to be challenging. Given the difficulties of targeting RAS directly, drugs that impact the other components of pathways where mutant RAS operates may potentially be effective. However, the system-level features, including different localizations of RAS isoforms, competition between downstream effectors, and interlocking feedback and feed-forward loops, must be understood to fully grasp the opportunities and limitations of inhibiting specific targets. Mathematical modeling can help us discern the system-level impacts of these features in normal and cancer cells. New technologies enable the acquisition of experimental data that will facilitate development of realistic models of oncogenic RAS behavior. In light of the wealth of empirical data accumulated over decades of study and the advancement of experimental methods for gathering new data, modelers now have the opportunity to advance progress toward realization of targeted treatment for mutant RAS-driven cancers.

Spred negatively regulates lens growth by modulating epithelial cell proliferation and fiber differentiation

Spred, like Sprouty (Spry) and also Sef proteins, have been identified as important regulators of receptor tyrosine kinase (RTK)-mediated MAPK/ERK-signaling in various developmental systems, controlling cellular processes such as proliferation, migration and differentiation. Spreds are widely expressed during early embryogenesis, and in the eye lens, become more localised in the lens epithelium with later development, overlapping with other antagonists including Spry. Given the synexpression of Spreds and Spry in lens, in order to gain a better understanding of their specific roles in regulating growth factor mediated-signaling and cell behavior, we established and characterised lines of transgenic mice overexpressing Spred1 or Spred2, specifically in the lens. This overexpression of Spreds resulted in a small lens phenotype during ocular morphogenesis, retarding its growth by compromising epithelial cell proliferation and fiber differentiation. These in situ findings were shown to be dependent on the ability of Spreds to suppress MAPK-signaling, in particular FGF-induced ERK1/2-signaling in lens cells. This was validated in vitro using lens epithelial explants, that highlighted the overlapping role of Spreds with Spry2, but not Spry1. This study provides insights into the putative function of Spreds and Spry in situ, some overlapping and some distinct, and their importance in regulating lens cell proliferation and fiber differentiation contributing to lens and eye growth.

Long non-coding RNA GAS5 inhibits ovarian cancer cell proliferation via the control of microRNA-21 and SPRY2 expression

In recent decades, numerous long non-coding (lnc)RNAs, including growth arrest-specific transcript 5 (GAS5), have been demonstrated to exert promoting or suppressive effects in human cancers. Decreased expression of the lncRNA GAS5 was reported to promote cell proliferation, migration and invasion and indicate poor prognosis in ovarian cancer. However, the exact underlying molecular mechanism through which GAS5 is involved in ovarian cancer growth remains unknown. The present study aimed to investigate the regulatory mechanism of GAS5 in ovarian cancer cell proliferation. Quantitative polymerase chain reaction and western blot analysis were used to examine RNA and protein expression, respectively. An MTT assay was used to examine cell proliferation. A luciferase reporter gene assay was conducted to verify the targeting relationship. It was identified that the expression levels of GAS5 and Sprouty homolog 2 (SPRY2) were significantly downregulated, while the expression level of microRNA (miR)-21 was significantly upregulated in ovarian cancer tissues and cell lines compared with adjacent non-tumor tissues and normal ovarian epithelial cells, respectively. Downregulation of GAS5 was significantly associated with advanced clinical stage. Luciferase assay data indicated that miR-21 was a direct target of GAS5 and that SPRY2 was a target gene of miR-21 in ovarian cancer-derived A2780 cells. GAS5 overexpression significantly inhibited the proliferation of ovarian cancer cells, which was accompanied by the downregulation of miR-21 and the upregulation of SPRY2. The overexpression of miR-21 caused a significant decrease in A2780 cell proliferation, which was accompanied by reduced SPRY2 expression. Furthermore, miR-21 overexpression attenuated the suppressive effects of GAS5 on A2780 cell proliferation and rescued the promoting effects of GAS5 on SPRY2 expression. In addition, the knockdown of SPRY2 also rescued the suppressive effects of GAS5 on the proliferation of A2780 cells. In summary, our study demonstrates that GAS5 exerts a suppressive effect on the proliferation of ovarian cancer cells, at least in part via the inhibition of miR-21 expression and subsequent increased SPRY2 expression. These findings suggest that the GAS5/miR-21/SPRY2 signaling pathway may be a potential therapeutic target in ovarian cancer.

microRNA-23a promotes cell growth and metastasis in gastric cancer via targeting SPRY2-mediated ERK signaling

microRNAs (miRs) serve important roles in various human cancer types. Recently, miR-23a has been indicated as an oncogene in gastric cancer, but the underlying mechanism remains unclear. In the present study, reverse transcription-quantitative polymerase chain reaction and western blot analysis was used to explore the effects of miR-23a in gastric cancer. Additionally, cell proliferation, migration and invasion were examined using an MTT assay, wound healing assay and Transwell assay, respectively. Furthermore, a luciferase reporter gene assay was used to confirm the target association. It was determined that miR-23a was significantly upregulated in gastric cancer tissues and cell lines compared with adjacent tissues, and a normal gastric epithelial cell line. Furthermore, its upregulation was significantly associated with cancer progression and poor prognosis of patients. Knockdown of miR-23a caused a notable reduction in the proliferation, migration and invasion of gastric cancer AGS cells. Sprouty homolog 2 (SPRY2) was then predicted to be target gene of miR-23a. A luciferase reporter gene assay data demonstrated that miR-23a has the ability to directly bind to the 3′-untranslational region of SPRY2 mRNA. Further investigation demonstrated that SPRY2 was significantly downregulated in gastric cancer tissues and cell lines, and the protein expression of SPRY2 was negatively regulated by miR-23a in AGS cells. Furthermore, knockdown of SPRY2 reduced the suppressive effects of miR-23a inhibition in AGS cell proliferation, migration and invasion. In addition, the activity of extracellular signal-regulated kinase (ERK) signaling was also inhibited by the miR-23a/SPRY2 knockdown in AGS cells. The present study indicated that miR-23a serves a promoting role in gastric cancer via targeting SPRY2 and downstream ERK signaling.

Sprouty2 Protein Regulates Hypoxia-inducible Factor-α (HIFα) Protein Levels and Transcription of HIFα-responsive Genes

The α-subunits of hypoxia-inducible factors (HIF1α and HIF2α) promote transcription of genes that regulate glycolysis and cell survival and growth. Sprouty2 (Spry2) is a modulator of receptor tyrosine kinase signaling and inhibits cell proliferation by a number of different mechanisms. Because of the seemingly opposite actions of HIFα subunits and Spry2 on cellular processes, we investigated whether Spry2 regulates the levels of HIF1α and HIF2α proteins. In cell lines from different types of tumors in which the decreased protein levels of Spry2 have been associated with poor prognosis, silencing of Spry2 elevated HIF1α protein levels. Increases in HIF1α and HIF2α protein levels due to silencing of Spry2 also up-regulated HIFα target genes. Using HIF1α as a prototype, we show that Spry2 decreases HIF1α stability and enhances the ubiquitylation of HIF1α by a von Hippel-Lindau protein (pVHL)-dependent mechanism. Spry2 also exists in a complex with HIF1α. Because Spry2 can also associate with pVHL, using a mutant form of Spry2 (3P/3A-Spry2) that binds HIF1α, but not pVHL, we show that WT-Spry2, but not the 3P/3A-Spry2 decreases HIF1α protein levels. In accordance, expression of WT-Spry2, but not 3P/3A-Spry2 results in a decrease in HIF1α-sensitive glucose uptake. Together our data suggest that Spry2 acts as a scaffold to bring more pVHL/associated E3 ligase in proximity of HIF1α and increase its ubiquitylation and degradation. This represents a novel action for Spry2 in modulating biological processes regulated by HIFα subunits.

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.

AntagomiR-27a targets FOXO3a in glioblastoma and suppresses U87 cell growth in vitro and in vivo

OBJECTIVE

To study the effect of the antagomiR-27a inhibitor on glioblastoma cells.

METHODS

The miR- 27a expression level in specimens of human glioblastoma and normal human brain tissues excised during decompression for traumatic brain injury was assessed using qRT-PCR; The predicted target gene of miR-27a was screened out through bioinformatics databases, and the predicted gene was verified using genetic report assays; the effect of antagomiR-27a on the invasion and proliferation of glioma cells was analyzed using MTT assays and 5-ethynyl-2'-deoxyuridine (EdU) labeling. A xenograft glioblastoma model in BALB-c nude mice was established to detect the effect of antagomiR-27a on tumour growth.

RESULTS

qRT-PCR results showed that miR-27a significantly increased in specimens from glioblastoma comparing with normal human brain tissues. Th miR-27a inhibitor significantly suppressed invasion and proliferation of glioblastoma cells. FOXO3a was verified as a new target of miR-27a by Western blotting and reporter analyzes. Tumor growth in vivo was suppressed by administration of the miR-27a inhibitor.

CONCLUSION

MiR-27a may be up-regulated in human glioblastoma, and antagomiR-27a could inhibit the proliferation and invasion ability of glioblastoma cells.

Identification of a new Sprouty protein responsible for the inhibition of the Bombyx mori nucleopolyhedrovirus reproduction

The rat sarcoma-extracellular signal regulated kinase mitogen-activated protein kinases pathway, one of the most ancient signaling pathways, is crucial for the defense against Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Sprouty (Spry) proteins can inhibit the activity of this pathway by receptor tyrosine kinases. We cloned and identified a new B. mori gene with a Spry domain similar to the Spry proteins of other organisms, such as fruitfly, mouse, human, chicken, Xenopus and zebrafish, and named it BmSpry. The gene expression analysis showed that BmSpry was transcribed in all of the examined tissues and in all developmental stages from embryo to adult. BmSpry also induced expression of BmNPV in the cells. Our results indicated: (1) the knock-down of BmSpry led to increased BmNPV replication and silkworm larvae mortality; (2) over-expression of BmSpry led to reduced BmNPV replication; and (3) BmSpry regulated the activation of ERK and inhibited BmNPV replication. These results showed that BmSpry plays a crucial role in the antiviral defense of the silkworm both in vitro and in vivo.

Casein kinase 1 regulates Sprouty2 in FGF-ERK signaling

Sprouty2 (SPRY2) is a potent negative regulator of receptor tyrosine kinase signaling, and is implicated as a tumor suppressor. SPRY2 inhibits FGF-RAS-ERK signaling by binding to growth factor receptor bound protein 2 (GRB2) during fibroblast growth factor receptor (FGFR) activation, disrupting the GRB2-SOS (son of sevenless) complex that transduces signals from FGFR to RAS. SPRY2 binding to GRB2 is modulated by phosphorylation but the key regulatory kinase(s) are not known. Prior studies identified the frequent presence of CK1 phosphorylation motifs on SPRY2. We therefore tested if CK1 has a role in SPRY2 phosphorylation and function. Loss of CK1 binding and inhibition of CK1 activity by two structurally distinct small molecules abrogated SPRY2 inhibition of FGF-ERK signaling, leading to decreased SPRY2 interaction with GRB2. Moreover, CK1 activity and binding are necessary for SPRY2 inhibition of FGF-stimulated neurite outgrowth in PC12 cells. Consistent with its proposed role as an inhibitor of FGF signaling, we find that CSNK1E transcript abundance negatively correlates with FGF1/FGF7 message in human gastric cancer samples. Modulation of CK1 activity may be therapeutically useful in the treatment of FGF/SPRY2-related diseases.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

Sprouty2 expression controls endothelial monolayer integrity and quiescence

Vascular integrity is fundamental to the formation of mature blood vessels and depends on a functional, quiescent endothelial monolayer. However, how endothelial cells enter and maintain quiescence in the presence of angiogenic factors is still poorly understood. Here we identify the fibroblast growth factor (FGF) antagonist Sprouty2 (Spry2) as a key player in mediating endothelial quiescence and barrier integrity in mouse aortic endothelial cells (MAECs): Spry2 knockout MAECs show spindle-like shapes and are incapable of forming a functional, impermeable endothelial monolayer in the presence of FGF2. Whereas dense wild type cells exhibit contact inhibition and stop to proliferate, Spry2 knockout MAECs remain responsive to FGF2 and continue to proliferate even at high cell densities. Importantly, the anti-proliferative effect of Spry2 is absent in sparsely plated cells. This cell density-dependent Spry2 function correlates with highly increased Spry2 expression in confluent wild type MAECs. Spry2 protein expression is barely detectable in single cells but steadily increases in cells growing to high cell densities, with hypoxia being one contributing factor. At confluence, Spry2 expression correlates with intact cell-cell contacts, whereas disruption of cell-cell contacts by EGTA, TNFα and thrombin decreases Spry2 protein expression. In confluent cells, high Spry2 levels correlate with decreased extracellular signal-regulated kinase 1/2 (Erk1/2) phosphorylation. In contrast, dense Spry2 knockout MAECs exhibit enhanced signaling by Erk1/2. Moreover, inhibiting Erk1/2 activity in Spry2 knockout cells restores wild type cobblestone monolayer morphology. This study thus reveals a novel Spry2 function, which mediates endothelial contact inhibition and barrier integrity.

G Protein-regulated inducer of neurite outgrowth (GRIN) modulates Sprouty protein repression of mitogen-activated protein kinase (MAPK) activation by growth factor stimulation

Gα(o/i) interacts directly with GRIN (G protein-regulated inducer of neurite outgrowth). Using the yeast two-hybrid system, we identified Sprouty2 as an interacting partner of GRIN. Gα(o) and Sprouty2 bind to overlapping regions of GRIN, thus competing for GRIN binding. Imaging experiments demonstrated that Gα(o) expression promoted GRIN translocation to the plasma membrane, whereas Sprouty2 expression failed to do so. Given the role of Sprouty2 in the regulation of growth factor-mediated MAPK activation, we examined the contribution of the GRIN-Sprouty2 interaction to CB1 cannabinoid receptor regulation of FGF receptor signaling. In Neuro-2A cells, a system that expresses all of the components endogenously, modulation of GRIN levels led to regulation of MAPK activation. Overexpression of GRIN potentiated FGF activation of MAPK and decreased tyrosine phosphorylation of Sprouty2. Pretreatment with G(o/i)-coupled CB1 receptor agonist attenuated subsequent FGF activation of MAPK. Decreased expression of GRIN both diminished FGF activation of MAPK and blocked CB1R attenuation of MAPK activation. These observations indicate that Gα(o) interacts with GRIN and outcompetes GRIN from bound Sprouty. Free Sprouty then in turn inhibits growth factor signaling. Thus, here we present a novel mechanism of how G(o/i)-coupled receptors can inhibit growth factor signaling to MAPK.

Immunolocalization of sprouty-1 and sprouty-2 in developing rat lung

OBJECTIVE

Sprouty, a common antagonist of fibroblast growth factor (FGF) and epidermal growth factor signaling, is a key player regulating tracheal branching and eye development in Drosophila. Four Sprouty homologs have been identified in vertebrates and all share a cysteine-rich region. However, the physiological function(s) of the individual Sprouty homologs is unknown. mRNA of Sprouty homologs is expressed during mouse lung development. In the present study, we investigated the immunolocalization of Sprouty proteins in rat lung at different stages of development.

METHODS

Rabbit antibodies were raised against peptides derived from rat Sprouty-1 and Sprouty-2 and were used in Western blot analysis to determine Sprouty distribution in subcellular fractions (pellets and supernatant centrifuged at 5,000 and 20,000 g) and bronchoalveolar lavage fluid (BAL) from adult rat lungs or used in immunohistochemistry.

RESULTS

Western blot analysis revealed a 30-kDa Sprouty-1 band and a 34-kDa Sprouty-2 band in the supernatant and pellet fractions centrifuged at 20,000 g. BAL contained a band of approximately 16 kDa with Sprouty-1 antibody derived from proteolytic fragmentation of Sprouty-1. In embryonic day (E) 14 and E16 lungs, Sprouty-1 and Sprouty-2 were expressed both in epithelial and peripheral mesenchymal cells. In adult rat lung, bronchiolar and alveolar type II epithelial cells showed staining for both Sprouty-1 and Sprouty-2. Sprouty-1 expression was also seen in alveolar type I epithelial cells.

CONCLUSION

In light of the proximity of the distribution of Sprouty to that of FGF-10 (peripheral mesenchyme) and its receptor FGFR2IIIb (distal tubular epithelium) in lung development, and the finding that FGF-9, which is expressed in mesothelial cells, upregulates FGF-10, it appears that Sprouty expression in epithelial and mesenchymal cells during branching morphogenesis is closely related to signaling by FGF-9 and FGF-10.

Interaction of the receptor FGFRL1 with the negative regulator Spred1

FGFRL1 is a member of the fibroblast growth factor receptor family. It plays an essential role during branching morphogenesis of the metanephric kidneys, as mice with a targeted deletion of the Fgfrl1 gene show severe kidney dysplasia. Here we used the yeast two-hybrid system to demonstrate that FGFRL1 binds with its C-terminal, histidine-rich domain to Spred1 and to other proteins of the Sprouty/Spred family. Members of this family are known to act as negative regulators of the Ras/Raf/Erk signaling pathway. Truncation experiments further showed that FGFRL1 interacts with the SPR domain of Spred1, a domain that is shared by all members of the Sprouty/Spred family. The interaction could be verified by coprecipitation of the interaction partners from solution and by codistribution at the cell membrane of COS1 and HEK293 cells. Interestingly, Spred1 increased the retention time of FGFRL1 at the plasma membrane where the receptor might interact with ligands. FGFRL1 and members of the Sprouty/Spred family belong to the FGF synexpression group, which also includes FGF3, FGF8, Sef and Isthmin. It is conceivable that FGFRL1, Sef and some Sprouty/Spred proteins work in concert to control growth factor signaling during branching morphogenesis of the kidneys and other organs.

Activation of unfolded protein response in transgenic mouse lenses

PURPOSE

Overloading of unfolded or misfolded proteins in the endoplasmic reticulum (ER) can cause ER stress and activate the unfolded protein response (UPR) in the cell. The authors tested whether transgene overexpression in the mouse lens would activate the UPR.

METHODS

Transgenic mice expressing proteins that either enter the ER secretory pathway or are synthesized in cytosol were selected. Activation of the UPR was assessed by determining the expression levels of the ER chaperone protein BiP, the spliced form of X-box binding protein-1 (Xbp-1) mRNA, and the transcription factor CHOP. Changes in the ubiquitin-proteasome system in the mouse lens were detected by ubiquitin immunofluorescence.

RESULTS

BiP expression was upregulated in the fiber cells of transgenic mouse lenses expressing platelet-derived growth factor-A (PDGF-A), dominant-negative fibroblast growth factor receptor (DN-FGFR), or DN-Sprouty2 (DN-Spy2). BiP upregulation occurred around embryonic day 16.5, primarily in the fiber cells adjacent to the organelle free zone. Fiber cell differentiation was disrupted in the PDGF-A and DN-Spry2 lenses, whereas the fiber cells were degenerating in the DN-FGFR lens. High levels of UPR activation and ubiquitin-labeled protein aggregates were found in the DN-FGFR lens, indicating inefficient disposal of unfolded/misfolded proteins in the fiber cells.

CONCLUSIONS

This study implies that overexpression of some transgenes in the lens can induce ER or overall cell stress in fiber cells, resulting in the activation of UPR signaling pathways. Therefore, investigators should assess the levels of UPR activation when they analyze the downstream effects of transgene expression in the lens.

Sprouty2 downregulates angiogenesis during mouse skin wound healing

Angiogenesis is regulated by signals received by receptor tyrosine kinases such as vascular endothelial growth factor receptors. Mammalian Sprouty (Spry) proteins are known to function by specifically antagonizing the activation of the mitogen-activated protein kinase signaling pathway by receptor tyrosine kinases, a pathway known to promote angiogenesis. To examine the role of Spry2 in the regulation of angiogenesis during wound repair, we used a model of murine dermal wound healing. Full-thickness excisional wounds (3 mm) were made on the dorsum of anesthetized adult female FVB mice. Samples were harvested at multiple time points postwounding and analyzed using real-time RT-PCR, Western blot analysis, and immunofluorescent histochemistry. Spry2 mRNA and protein levels in the wound bed increased significantly during the resolving phases of healing, coincident with the onset of vascular regression in this wound model. In another experiment, intracellular levels of Spry2 or its dominant-negative mutant (Y55F) were elevated by a topical application to the wounds of controlled-release gel containing cell permeable, transactivator of transcription-tagged Spry2, Spry2Y55F, or green fluorescent protein (as control). Wound samples were analyzed for vascularity using CD31 immunofluorescent histochemistry as well as for total and phospho-Erk1/2 protein content. The treatment of wounds with Spry2 resulted in a significant decrease in vascularity and a reduced abundance of phospho-Erk1/2 compared with wounds treated with the green fluorescent protein control. In contrast, the wounds treated with the dominant-negative Spry2Y55F exhibited a moderate increase in vascularity and elevated phospho-Erk1/2 content. These results indicate that endogenous Spry2 functions to downregulate angiogenesis in the healing murine skin wound, potentially by inhibiting the mitogen-activated protein kinase signaling pathway.

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.

Sprouty1 inhibits angiogenesis in association with up-regulation of p21 and p27

Sprouty1 (Spry1) is a conserved antagonist of FGF signaling. The goal of this study was to further explore the downstream mechanisms governing Spry1 inhibition of endothelial cell proliferation. Up-regulation of Spry1 in HUVECs inhibited tube formation on Matrigel (n = 6, P < 0.001). This was associated with decreased proliferation as measured by BrdU incorporation (n = 6, P < 0.001) and increased protein expression of the cyclin-dependent kinase inhibitor 1A (CDKN1A), p21 and cyclin-dependent kinase inhibitor 1B (CDKN1B), p27. A transcriptional analysis using a targeted human angiogenesis array following up-regulation of Spry1 demonstrated a >2-fold increase in an anti-angiogenic factor, serpin peptidase inhibitor, clad F (Serpinf1), and a >2-fold decrease in pro-angiogenic factors fms-related tyrosine kinase 1 (FLT1), angiopoietin2 (Ang-2), and placental growth factor (PGF) (n = 2). To define upstream mechanisms that may regulate endogenous Spry1, we performed a search for responsive elements upstream of the promoter region. This search resulted in the identification of multiple degenerate hypoxia responsive elements. Exposure to hypoxia resulted in a significant increase in Spry1 expression (n = 8, P < 0.01). These findings shed new light on downstream signaling pathways associated with Spry1 anti-proliferative responses, and provide new evidence that hypoxia stimulates Spry1 expression.

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.

Sprouty2 interacts with protein kinase C delta and disrupts phosphorylation of protein kinase D1

The Sprouty (Spry) proteins act as inhibitors of the Ras/ERK pathway downstream of receptor tyrosine kinases. In this study, we report a novel interaction between protein kinase C delta (PKCdelta) and Spry2. Endogenous PKCdelta and Spry2 interact in cells upon basic fibroblast growth factor stimulation, indicating a physiological relevance for the interaction. This interaction appeared to require the full-length Spry2 protein and was conformation-dependent. Conformational constraints were released upon FGFR1 activation, allowing the interaction to occur. Although this interaction did not affect the phosphorylation of PKCdelta by another kinase, it reduced the phosphorylation of a PKCdelta substrate, protein kinase D1 (PKD1). Spry2 was found to interact more strongly with PKCdelta with increasing amounts of PKD1, which indicated that instead of competing with PKD1 for binding with PKCdelta, it was more likely to form a trimeric complex with both PKCdelta and PKD1. Formation of the complex was found to be dependent on an existing PKCdelta-PKD1 interaction. By disrupting the interaction between PKCdelta and PKD1, Spry2 was unable to associate with PKCdelta to form the trimeric complex. As a consequence of this trimeric complex, the existing interaction between PKCdelta and PKD1 was increased, and the transfer of phosphate groups from PKCdelta to PKD1 was at least partly blocked by Spry2. The action of Spry2 on PKCdelta resulted in the inhibition of both ERK phosphorylation and invasion of PC-3 cells via PKCdelta signaling. By disrupting the capacity of PKCdelta to phosphorylate its cognate substrates, Spry2 may serve to modulate PKCdelta signaling downstream of receptor tyrosine kinases and to regulate the physiological outcome.

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.

Downregulation of Spry-1, an inhibitor of GDNF/Ret, causes angiotensin II-induced ureteric bud branching

Mutations of genes in the renin-angiotensin system are associated with congenital abnormalities of the kidney and urinary tract. The major signaling pathway for branching morphogenesis during kidney development is the c-Ret receptor tyrosine kinase whose ligand is GDNF and whose downstream target is Wnt11. We determined whether angiotensin II, an inducer of ureteric bud branching in vitro, influences the GDNF/c-Ret/Wnt11 pathway. Mouse metanephroi were grown in the presence or absence of angiotensin II or an angiotensin type 1 receptor (AT1R) antagonist and gene expression was measured by whole mount in situ hybridization. Angiotensin II induced the expression of c-Ret and Wnt11 in ureteric bud tip cells. GDNF, a Wnt11-regulated gene expressed in the mesenchyme, was also upregulated by angiotensin II but this downregulated Spry1, an endogenous inhibitor of Ret tyrosine kinase activity in an AT1R-dependent manner. Angiotensin II also decreased Spry1 mRNA levels in cultured ureteric bud cells. Exogenous angiotensin II preferentially stimulated ureteric bud tip cell proliferation in vivo while AT1R blockade increased cell apoptosis. Our findings suggest AT1R-mediated inhibition of the Spry1 gene increases c-Ret tyrosine kinase activity leading to upregulation of its downstream target Wnt11. Enhanced Wnt11 expression induces GDNF in adjacent mesenchyme causing focal bursts of ureteric bud tip cell proliferation, decreased tip cell apoptosis and branching.

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.

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

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

Sprouty2 downregulation plays a pivotal role in mediating crosstalk between TGF-beta1 signaling and EGF as well as FGF receptor tyrosine kinase-ERK pathways in mesenchymal cells

Mammalian Sprouty2 (Spry2) is a key regulator of the receptor tyrosine kinase/ERK signaling pathway and is involved in many biological processes, including cell growth, differentiation, migration, and embryonic lung branching morphogenesis. Previous studies have shown that Spry2 expression is upregulated by many mitogens, particularly epidermal growth factor (EGF) and fibroblast growth factors (FGFs). In contrast, we report that transforming growth factor-beta1 (TGF-beta1), which stimulates the growth of quiescent Swiss 3T3 cells, induced a dose dependent decrease of mouse Spry2 protein level within 24-h of treatment, and this effect was mediated by a MAP kinase-independent pathway. A concomitant reduction of the level of Spry2 mRNA indicates the involvement of a transcriptional mechanism, which requires histone deacetylase (HDAC) activity and de novo protein synthesis. On the other hand, the turnover rate of Spry2 protein was increased by TGF-beta1 treatment, suggesting enhanced Spry2 degradation. Treatment with lysosomal inhibitors, but not proteasome inhibitors, prevented the degradation of Spry2, thus, indicating that the degradation of Spry2 is mediated through the lysosomal pathway in Swiss 3T3 cells. Furthermore, we demonstrate that TGF-beta1 signaling can modulate EGF and FGF-induced ERK-MAP kinase activation by controlling Spry2 expression and function. Moreover, rescue of the TGF-beta1-induced downregulation of Spry2 by gene over-expression led to inhibition of the mitogenic effect of TGF-beta1 in Swiss 3T3 cells. Together, the combined operation of transcriptional and post-translational mechanisms suggests that regulation of Spry2 is a crucial event and emphasizes the important role that Spry2 plays in controlling cell behaviors.

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.

Regulation of Sprouty2 stability by mammalian Seven-in-Absentia homolog 2

Mammalian Sprouty (Spry) gene expression is rapidly induced upon activation of the FGF receptor signaling pathway in multiple cell types including cells of mesenchymal and epithelial origin. Spry2 inhibits FGF-dependent ERK activation and thus Spry acts as a feedback inhibitor of FGF-mediated proliferation. In addition, Spry2 interacts with the ring-finger-containing E3 ubiquitin ligase, c-Cbl, in a manner that is dependent upon phosphorylation of Tyr55 of Spry2. This interaction results in the poly-ubiquitination and subsequent degradation of Spry2 by the proteasome. Here, we describe the identification of another E3 ubiquitin ligase, human Seven-in-Absentia homolog-2 (SIAH2), as a Spry2 interacting protein. We show by yeast two-hybrid analysis that the N-terminal domain of Spry2 and the ring finger domain of SIAH2 mediated this interaction. Co-expression of SIAH2 resulted in proteasomal degradation of Spry1, 2, and to a lesser extent Spry4. The related E3 ubiquitin-ligase, SIAH1, had little effect on Spry2 protein stability when co-expressed. Unlike c-Cbl-mediated degradation of Spry2, SIAH2-mediated degradation was independent of phosphorylation of Spry2 on Tyr55. Spry2 was also phosphorylated on Tyr227, and phosphorylation of this residue was also dispensable for SIAH2-mediated degradation of Spry2. Finally, co-expression of SIAH2 with Spry2 resulted in a rescue of FGF2-mediated ERK phosphorylation. These data suggest a novel mechanism whereby Spry2 stability is regulated in a manner that is independent of tyrosine phosphorylation, and provides an addition level of control of Spry2 protein levels.

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.

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 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.

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.

Eve-3: a liver enriched suppressor of Ras/MAPK signaling

BACKGROUND/AIMS

The developed liver is able to tightly control cellular proliferation, rapidly switching from quiescence to growth in response to specific stimuli. This suggests that growth inhibitors may be involved in the control of liver growth. We analyzed the role of the Spred-family of growth inhibitors in the liver.

METHODS

We screened human EST databases for Spred-related sequences. Clones were isolated, sequenced, epitope-tagged and expressed. Subcellular localization of clones were determined and their effects on cellular signaling pathways analysed using specific antibodies. Cell cycle progression assays and protein interaction studies were initiated. Organ distribution of transcripts and their expression throughout liver development and in primary hepatocytes were recorded.

RESULTS

We have identified a new, liver-restricted protein, Eve-3, containing a single Ena Vasp homology (EVH1) domain that can potently block activation of the Ras/MAPK pathway. Eve-3 is specific in inhibiting the Ras/MAPK pathway. Eve-3 can block serum-mediated cell cycle progression and its expression is highly regulated during liver development.

CONCLUSIONS

The liver is the only organ that can regulate its growth and mass. Eve-3 may act as an inhibitor of proliferation pathways in the mature liver and be involved in modulating the unique regenerative capacity of this organ.

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.

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.

BNIP-Sα induces cell rounding and apoptosis by displacing p50RhoGAP and facilitating RhoA activation via its unique motifs in the BNIP-2 and Cdc42GAP homology domain

Changes in cell morphology are linked to many cellular events including cytokinesis, differentiation, migration and apoptosis. We recently showed that BNIP-Salpha induced cell rounding that leads to apoptosis via its BNIP-2 and Cdc42GAP Homology (BCH) domain, but the underlying mechanism has not been determined. Here, we have identified a unique region (amino acid 133-177) of the BNIP-Salpha BCH domain that targets RhoA, but not Cdc42 or Rac1 and only the dominant-negative form of RhoA could prevent the resultant cell rounding and apoptotic effect. The RhoA-binding region consists of two parts; one region (residues 133-147) that shows some homology to part of the RhoA switch I region and an adjacent sequence (residues 148-177) that resembles the REM class I RhoA-binding motif. The sequence 133-147 is also necessary for its heterophilic interaction with the BCH domain of the Rho GTPase-activating protein, p50RhoGAP/Cdc42GAP. These overlapping motifs allow tripartite competition such that overexpression of BNIP-Salpha could reduce p50RhoGAP binding to RhoA and restore RhoA activation. Furthermore, BNIP-Salpha mutants lacking the RhoA-binding motif completely failed to induce cell rounding and apoptosis. Therefore, via unique binding motifs within its BCH domain, BNIP-Salpha could interact and activate RhoA while preventing its inhibition by p50RhoGAP. This concerted mechanism could allow effective propagation of the RhoA pathway for cell rounding and apoptosis.

Distinct requirements for the Sprouty domain for functional activity of Spred proteins

Sprouty and Spred {Sprouty-related EVH1 [Ena/VASP (vasodilator-stimulated phosphoprotein) homology 1] domain} proteins have been identified as antagonists of growth factor signalling pathways. We show here that Spred-1 and Spred-2 appear to have distinct mechanisms whereby they induce their effects, as the Sprouty domain of Spred-1 is not required to block MAPK (mitogen-activated protein kinase) activation, while that of Spred-2 is required. Similarly, deletion of the C-terminal Sprouty domain of Spred-1 does not affect cell-cycle progression of G(0)-synchronized cells through to S-phase following growth factor stimulation, while the Sprouty domain is required for Spred-2 function. We also demonstrate that the inhibitory function of Spred proteins is restricted to the Ras/MAPK pathway, that tyrosine phosphorylation is not required for this function, and that the Sprouty domain mediates heterodimer formation of Spred proteins. Growth-factor-mediated activation of the small GTPases, Ras and Rap1, was able to be regulated by Spred-1 and Spred-2, without affecting receptor activation. Taken together, these results highlight the potential for different functional roles of the Sprouty domain within the Spred family of proteins, suggesting that Spred proteins may use different mechanisms to induce inhibition of the MAPK pathway.

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.

A comprehensive pathway map of epidermal growth factor receptor signaling

The epidermal growth factor receptor (EGFR) signaling pathway is one of the most important pathways that regulate growth, survival, proliferation, and differentiation in mammalian cells. Reflecting this importance, it is one of the best-investigated signaling systems, both experimentally and computationally, and several computational models have been developed for dynamic analysis. A map of molecular interactions of the EGFR signaling system is a valuable resource for research in this area. In this paper, we present a comprehensive pathway map of EGFR signaling and other related pathways. The map reveals that the overall architecture of the pathway is a bow-tie (or hourglass) structure with several feedback loops. The map is created using CellDesigner software that enables us to graphically represent interactions using a well-defined and consistent graphical notation, and to store it in Systems Biology Markup Language (SBML).