Alternative splicing generates an isoform of the human Sef gene with altered subcellular localization and specificity

Endothelial IL17RD promotes Western diet-induced aortic myeloid cell infiltration

The Interleukin-17 (IL17) family is a group of cytokines implicated in the etiology of several inflammatory diseases. Interleukin-17 receptor D (IL17RD), also known as Sef (similar expression to fibroblast growth factor) belonging to the family of IL17 receptors, has been shown to modulate IL17A-associated inflammatory phenotypes. The objective of this study was to test the hypothesis that IL17RD promotes endothelial cell activation and consequent leukocyte adhesion. We utilized primary human aortic endothelial cells and demonstrated that RNAi targeting of IL17RD suppressed transcript levels by 83 % compared to non-targeted controls. Further, RNAi knockdown of IL17RD decreased the adhesion of THP-1 monocytic cells onto a monolayer of aortic endothelial cells in response to IL17A. Additionally, we determined that IL17A did not significantly enhance the activation of canonical MAPK and NFκB pathways in endothelial cells, and further did not significantly affect the expression of VCAM-1 and ICAM-1 in aortic endothelial cells, which is contrary to previous findings. We also determined the functional relevance of our findings in vivo by comparing the expression of endothelial VCAM-1 and ICAM-1 and leukocyte infiltration in the aorta in Western diet-fed Il17rd null versus wild-type mice. Our results showed that although Il17rd null mice do not have significant alteration in aortic expression of VCAM-1 and ICAM-1 in endothelial cells, they exhibit decreased accumulation of proinflammatory monocytes and neutrophils, suggesting that endothelial IL17RD induced in vivo myeloid cell accumulation is not dependent on upregulation of VCAM-1 and ICAM-1 expression. We further performed proteomics analysis to identify potential molecular mediators of the IL17A/IL17RD signaling axis. Collectively, our results underscore a critical role for Il17rd in the regulation of aortic myeloid cell infiltration in the context of Western diet feeding.

Physiological roles of human interleukin-17 family

Interleukin-17 s (IL-17s) are well-known proinflammatory cytokines, and their antagonists perform excellently in the treatment of inflammatory skin diseases such as psoriasis. However, their physiological functions have not been given sufficient attention by clinicians. IL-17s can protect the host from extracellular pathogens, maintain epithelial integrity, regulate cognitive processes and modulate adipocyte activity through distinct mechanisms. Here, we present a systematic review concerning the physiological functions of IL-17s. Our goal is not to negate the therapeutic effect of IL-17 antagonists, but to ensure their safe use and reasonably explain the possible adverse events that may occur in their application.

Regulation of stability and inhibitory activity of the tumor suppressor SEF through casein-kinase II-mediated phosphorylation

Inflammation and cancer are intimately linked. A key mediator of inflammation is the transcription-factor NF-κB/RelA:p50. SEF (also known as IL-17RD) is a feedback antagonist of NF-κB/RelA:p50 that is emerging as an important link between inflammation and cancer. SEF acts as a buffer to prevent excessive NF-κB activity by sequestering NF-κB/RelA:p50 in the cytoplasm of unstimulated cells, and consequently attenuating the NF-κB response upon pro-inflammatory cytokine stimulation. SEF contributes to cancer progression also via modulating other signaling pathways, including those triggered by growth-factors. Despite its important role in human physiology and pathology, mechanisms that regulate SEF biochemical properties and inhibitory activity are unknown. Here we show that human SEF is an intrinsically labile protein that is stabilized via CK2-mediated phosphorylation, and identified the residues whom phosphorylation by CK2 stabilizes hSEF. Unlike endogenous SEF, ectopic SEF was rapidly degraded when overexpressed but was stabilized in the presence of excess CK2, suggesting a mechanism for limiting SEF levels depending upon CK2 processivity. Additionally, phosphorylation by CK2 potentiated hSef interaction with NF-κB in cell-free binding assays. Most importantly, we identified a CK2 phosphorylation site that was indispensable for SEF inhibition of pro-inflammatory cytokine signaling but was not required for SEF inhibition of growth-factor signaling. To our knowledge, this is the first demonstration of post-translational modifications that regulate SEF at multiple levels to optimize its inhibitory activity in a specific signaling context. These findings may facilitate the design of SEF variants for treating cytokine-dependent pathologies, including cancer and chronic inflammation.

Interleukin-17 Receptor D in Physiology, Inflammation and Cancer

Interleukin-17 receptor D (IL-17RD) is an evolutionarily conserved member of the IL-17 receptor family. Originally identified as a negative regulator of fibroblast growth factor (FGF) signaling under the name of Sef (Similar expression to FGF genes), IL-17RD was subsequently reported to regulate other receptor tyrosine kinase signaling pathways. In addition, recent studies have shown that IL-17RD also modulates IL-17 and Toll-like receptor (TLR) signaling. Combined genetic and cell biology studies have implicated IL-17RD in the control of cell proliferation and differentiation, cell survival, lineage specification, and inflammation. Accumulating evidence also suggest a role for IL-17RD in tumorigenesis. Expression of IL-17RD is down-regulated in various human cancers and recent work has shown that loss of IL-17RD promotes tumor formation in mice. However, the exact mechanisms underlying the tumor suppressor function of IL-17RD remain unclear and some studies have proposed that IL-17RD may exert pro-tumorigenic effects in certain contexts. Here, we provide an overview of the signaling functions of IL-17RD and review the evidence for its involvement in cancer.

Interleukin-17 receptor D (Sef) is a multi-functional regulator of cell signaling

Interleukin-17 receptor D (IL17RD or IL-17RD) also known as Sef (similar expression to fibroblast growth factor), is a single pass transmembrane protein that is reported to regulate several signaling pathways . IL17RD was initially described as a feedback inhibitor of fibroblast growth factor (FGF) signaling during zebrafish and frog development. It was subsequently determined to regulate other receptor tyrosine kinase signaling cascades as well as several proinflammatory signaling pathways including Interleukin-17A (IL17A), Toll-like receptors (TLR) and Interleukin-1α (IL1α) in several vertebrate species including humans. This review will provide an overview of IL17RD regulation of signaling pathways and functions with emphasis on regulation of development and pathobiological conditions. We will also discuss gaps in our knowledge about IL17RD function to provide insight into opportunities for future investigation.

Growth Inhibitory Signaling of the Raf/MEK/ERK Pathway

In response to extracellular stimuli, the Raf/MEK/extracellular signal-regulated kinase (ERK) pathway regulates diverse cellular processes. While mainly known as a mitogenic signaling pathway, the Raf/MEK/ERK pathway can mediate not only cell proliferation and survival but also cell cycle arrest and death in different cell types. Growing evidence suggests that the cell fate toward these paradoxical physiological outputs may be determined not only at downstream effector levels but also at the pathway level, which involves the magnitude of pathway activity, spatial-temporal regulation, and non-canonical functions of the molecular switches in this pathway. This review discusses recent updates on the molecular mechanisms underlying the pathway-mediated growth inhibitory signaling, with a major focus on the regulation mediated at the pathway level.

Regulators of the RAS-ERK pathway as therapeutic targets in thyroid cancer

Thyroid cancer is mostly an ERK-driven carcinoma, as up to 70% of thyroid carcinomas are caused by mutations that activate the RAS/ERK mitogenic signaling pathway. The incidence of thyroid cancer has been steadily increasing for the last four decades; yet, there is still no effective treatment for advanced thyroid carcinomas. Current research efforts are focused on impairing ERK signaling with small-molecule inhibitors, mainly at the level of BRAF and MEK. However, despite initial promising results in animal models, the clinical success of these inhibitors has been limited by the emergence of tumor resistance and relapse. The RAS/ERK pathway is an extremely complex signaling cascade with multiple points of control, offering many potential therapeutic targets: from the modulatory proteins regulating the activation state of RAS proteins to the scaffolding proteins of the pathway that provide spatial specificity to the signals, and finally, the negative feedbacks and phosphatases responsible for inactivating the pathway. The aim of this review is to give an overview of the biology of RAS/ERK regulators in human cancer highlighting relevant information on thyroid cancer and future areas of research.

Cross-Talk between Fibroblast Growth Factor Receptors and Other Cell Surface Proteins

Fibroblast growth factors (FGFs) and their receptors (FGFRs) constitute signaling circuits that transmit signals across the plasma membrane, regulating pivotal cellular processes like differentiation, migration, proliferation, and apoptosis. The malfunction of FGFs/FGFRs signaling axis is observed in numerous developmental and metabolic disorders, and in various tumors. The large diversity of FGFs/FGFRs functions is attributed to a great complexity in the regulation of FGFs/FGFRs-dependent signaling cascades. The function of FGFRs is modulated at several levels, including gene expression, alternative splicing, posttranslational modifications, and protein trafficking. One of the emerging ways to adjust FGFRs activity is through formation of complexes with other integral proteins of the cell membrane. These proteins may act as coreceptors, modulating binding of FGFs to FGFRs and defining specificity of elicited cellular response. FGFRs may interact with other cell surface receptors, like G-protein-coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs). The cross-talk between various receptors modulates the strength and specificity of intracellular signaling and cell fate. At the cell surface FGFRs can assemble into large complexes involving various cell adhesion molecules (CAMs). The interplay between FGFRs and CAMs affects cell–cell interaction and motility and is especially important for development of the central nervous system. This review summarizes current stage of knowledge about the regulation of FGFRs by the plasma membrane-embedded partner proteins and highlights the importance of FGFRs-containing membrane complexes in pathological conditions, including cancer.

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.

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

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

Delivery of the gene encoding the tumor suppressor Sef into prostate tumors by therapeutic-ultrasound inhibits both tumor angiogenesis and growth

Carcinomas constitute over 80% of all human cancer types with no effective therapy for metastatic disease. Here, we demonstrate, for the first time, the efficacy of therapeutic-ultrasound (TUS) to deliver a human tumor suppressor gene, hSef-b, to prostate tumors in vivo. Sef is downregulated in various human carcinomas, in a manner correlating with tumor aggressiveness. In vitro, hSef-b inhibited proliferation of TRAMP C2 cells and attenuated activation of ERK/MAPK and the master transcription factor NF-κB in response to FGF and IL-1/TNF, respectively. In vivo, transfection efficiency of a plasmid co-expressing hSef-b/eGFP into TRAMP C2 tumors was 14.7 ± 2.5% following a single TUS application. Repeated TUS treatments with hSef-b plasmid, significantly suppressed prostate tumor growth (60%) through inhibition of cell proliferation (60%), and reduction in blood vessel density (56%). In accordance, repeated TUS-treatments with hSef-b significantly inhibited in vivo expression of FGF2 and MMP-9. FGF2 is a known mitogen, and both FGF2/MMP-9 are proangiogenic factors. Taken together our results strongly suggest that hSef-b acts in a cell autonomous as well as non-cell autonomous manner. Moreover, the study demonstrates the efficacy of non-viral TUS-based hSef-b gene delivery approach for the treatment of prostate cancer tumors, and possibly other carcinomas where Sef is downregulated.

Feedback regulation of RTK signaling in development

Precise regulation of the amplitude and duration of receptor tyrosine kinase (RTK) signaling is critical for the execution of cellular programs and behaviors. Understanding these control mechanisms has important implications for the field of developmental biology, and in recent years, the question of how augmentation or attenuation of RTK signaling via feedback loops modulates development has become of increasing interest. RTK feedback regulation is also important for human disease research; for example, germline mutations in genes that encode RTK signaling pathway components cause numerous human congenital syndromes, and somatic alterations contribute to the pathogenesis of diseases such as cancers. In this review, we survey regulators of RTK signaling that tune receptor activity and intracellular transduction cascades, with a focus on the roles of these genes in the developing embryo. We detail the diverse inhibitory mechanisms utilized by negative feedback regulators that, when lost or perturbed, lead to aberrant increases in RTK signaling. We also discuss recent biochemical and genetic insights into positive regulators of RTK signaling and how these proteins function in tandem with negative regulators to guide embryonic development.

The Impact of MicroRNA-223-3p on IL-17 Receptor D Expression in Synovial Cells

OBJECTIVE

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease affecting joints. Elevated plasma levels of microRNA-223-3p (miR-223-3p) in patients with RA are implicated in the pathogenesis of the disease. This study aimed to analyze the functional role of miR-223-3p in the pathogenesis of RA by overexpressing miR-223-3p in synovial cell lines.

METHODS

Arthritis was induced in the RA model of SKG mice by injection of ß-glucan. The histopathologic features of joints were examined using hematoxylin and eosin and immunohistochemical staining. Plasma levels of miRNA were determined by panel real-time PCR analysis. Target genes of the differentially expressed miRNAs in SKG mice were analyzed using miRNA target prediction algorithms. The dual-luciferase reporter system was used to evaluate the relationship between miR-223-3p and IL-17 receptor D (IL-17RD). The activity of miR-223-3p was analyzed by transfection of plasmid vectors overexpressing miR-223-3p into IL-17RD-expressing NIH3T3 and MH7A cell lines. Il6 and Il17rd mRNA expression was analyzed by quantitative real-time PCR. IL-17RD protein expression was analyzed by western blot analysis.

RESULTS

We identified 17 upregulated miRNAs (fold change > 2.0) in plasma of SKG mice injected with ß-glucan relative to untreated SKG mice. Il17rd was identified as the candidate target gene of miR-223-3p using five miRNA target prediction algorithms. The transfection of plasmid vectors overexpressing miR-223-3p into NIH3T3 and MH7A cells resulted in the downregulation of Il17rd expression and upregulation of Il6 expression. Expression of miR-223-3p and Il6 mRNA in MH7A cells was upregulated; however, that of Il17rd mRNA was downregulated following TNF-α stimulation. IL-17RD expression in synovial tissues from SKG mice and RA patients was inversely correlated with the severity of arthritis.

CONCLUSION

This study is the first to demonstrate that miR-223-3p downregulates IL-17RD in both mouse and human cells; miR-223-3p may contribute to the pathogenesis of RA by downregulating the expression of IL-17RD and upregulating that of IL-6 in synovial cells.

Sef Regulates Epithelial-Mesenchymal Transition in Breast Cancer Cells

Sef (similar expression to fgf), also know as IL17RD, is a transmembrane protein shown to inhibit fibroblast growth factor signaling in developmental and cancer contexts; however, its role as a tumor suppressor remains to be fully elucidated. Here, we show that Sef regulates epithelial-mesenchymal transition (EMT) in breast cancer cell lines. Sef expression was highest in the normal breast epithelial cell line MCF10A, intermediate expression in MCF-7 cells and lowest in MDA-MB-231 cells. Knockdown of Sef increased the expression of genes associated with EMT, and promoted cell migration, invasion, and a fibroblastic morphology of MCF-7 cells. Overexpression of Sef inhibited the expression of EMT marker genes and inhibited cell migration and invasion in MCF-7 cells. Induction of EMT in MCF10A cells by TGF-β and TNF-α resulted in downregulation of Sef expression concomitant with upregulation of EMT gene expression and loss of epithelial morphology. Overexpression of Sef in MCF10A cells partially blocked cytokine-induced EMT. Sef was shown to block β-catenin mediated luciferase reporter activity and to cause a decrease in the nuclear localization of active β-catenin. Furthermore, Sef was shown to co-immunoprecipitate with β-catenin. In a mouse orthotopic xenograft model, Sef overexpression in MDA-MB-231 cells slowed tumor growth and reduced expression of EMT marker genes. Together, these data indicate that Sef plays a role in the negative regulation of EMT in a β-catenin dependent manner and that reduced expression of Sef in breast tumor cells may be permissive for EMT and the acquisition of a more metastatic phenotype. J. Cell. Biochem. 117: 2346-2356, 2016. © 2016 Wiley Periodicals, Inc.

Orphan receptor IL-17RD regulates Toll-like receptor signalling via SEFIR/TIR interactions

Receptor families of the innate immune response engage in 'cross-talk' to tailor optimal immune responses against invading pathogens. However, these responses are subject to multiple levels of regulation to keep in check aberrant inflammatory signals. Here, we describe a role for the orphan receptor interleukin-17 receptor D (IL-17RD) in negatively regulating Toll-like receptor (TLR)-induced responses. Deficiency of IL-17RD expression in cells leads to enhanced pro-inflammatory signalling and gene expression in response to TLR stimulation, and Il17rd(-/-) mice are more susceptible to TLR-induced septic shock. We demonstrate that the intracellular Sef/IL-17R (SEFIR) domain of IL-17RD targets TIR adaptor proteins to inhibit TLR downstream signalling thus revealing a paradigm involving cross-regulation of members of the IL-17R and TLR families.

Expression and regulation of the tumor suppressor, SEF, during folliculogenesis in humans and mice

Similar expression to FGF (Sef or IL17-RD), is a tumor suppressor and an inhibitor of growth factors as well as of pro-inflammatory cytokine signaling. In this study, we examined the regulation of Sef expression by gonadotropins during ovarian folliculogenesis. In sexually immature mice, in situ hybridization (ISH) localized Sef gene expression to early developing oocytes and granulosa cells (GC) but not to theca cells. Sef was also expressed in mouse ovarian endothelial cells, in the fallopian tube epithelium as well as in adipose tissue venules. SEF protein expression, determined by immunohistochemistry (IHC), correlated well with Sef mRNA expression in GC, while differential expression was noticed in oocytes. High Sef mRNA but undetectable SEF protein levels were observed in the oocytes of primary/secondary follicles, while an inverse correlation was found in the oocytes of preantral and small antral follicles. Sef mRNA expression dropped after pregnant mare's serum gonadotropin (PMSG) administration, peaked at 6-8 h after human chorionic gonadotropin (hCG) treatment, and declined by 12 h after this treatment. ISH and IHC localized the changes to oocytes and mural GC following PMSG treatment, whereas Sef expression increased in mural GC and declined in granulosa-lutein cells upon hCG treatment. The ovarian expression of SEF was confirmed using human samples. ISH localized SEF transcripts to human GC of antral follicles but not to corpora lutea. Furthermore, SEF mRNA was detected in human GC recovered from preovulatory follicles. These results are the first to demonstrate SEF expression in a healthy ovary during folliculogenesis. Hormonal regulation of its expression suggests that SEF may be an important factor involved in intra-ovarian control mechanisms.

Characterization of Golgi scaffold proteins and their roles in compartmentalizing cell signaling

Subcellular compartmentalization has become an important theme in cell signaling. In particular, the Golgi apparatus (GA) plays a prominent role in compartmentalizing signaling cascades that originate at the plasma membrane or other organelles. To precisely regulate this process, cells have evolved a unique class of organizer proteins, termed "scaffold proteins". Sef, PAQR3, PAQR10 and PAQR11 are scaffold proteins that have recently been identified on the GA and are referred to as Golgi scaffolds. The major cell growth signaling pathways, such as Ras/MAPK, PI3K/AKT, insulin and VEGF (vascular endothelial growth factor), are tightly regulated spatially and temporally by these Golgi scaffolds to ensure a physiologically appropriate outcome. Here, we discuss the subcellular localization and characterization of the topology and functional domains of these Golgi scaffolds and summarize their roles in the compartmentalization of cell signaling. We also highlight the physiological and pathological roles of these Golgi scaffolds in tumorigenesis and developmental disorders.

Activation of TAK1 by Sef-S induces apoptosis in 293T cells

Sef (similar expression to fgf genes, also named IL-17RD) was identified as a negative regulator of fibroblast growth factor signaling. Sef-S, an alternative splice isoform of Sef, inhibits FGF-induced NIH3T3 cell proliferation. Here we report that Sef-S physically interacts with TAK1, induces Lys63-linked TAK1 polyubiquitination on lysine 209 and TAK1-mediated JNK and p38 activation. Co-overexpression of TAK1 WT, K34R, K150R, K158R mutants with Sef-S induces Lys63-linked TAK1 polyubiquitination whereas TAK1 K63R and K209R mutants fail. Furthermore, co-overexpression of Sef-S and TAK1 induce 293T cells apoptosis. These results reveal Sef-S actives Lys63-linked TAK1 polyubiquitination on lysine 209, induces TAK1-mediated JNK and p38 activation and also results apoptosis in 293T cells.

Sef is an inhibitor of proinflammatory cytokine signaling, acting by cytoplasmic sequestration of NF-κB

The NF-κB transcription factor controls diverse biological processes. According to the classical model, NF-κB is retained in the cytoplasm of resting cells via binding to inhibitory, IκB proteins and translocates into the nucleus upon their ligand-induced degradation. Here we reveal that Sef, a known tumor suppressor and inhibitor of growth factor signaling, is a spatial regulator of NF-κB. Sef expression is regulated by the proinflammatory cytokines tumor necrosis factor and interleukin-1, and Sef specifically inhibits "classical" NF-κB (p50:p65) activation by these ligands. Like IκBs, Sef sequesters NF-κB in the cytoplasm of resting cells. However, contrary to IκBs, Sef continues to constrain NF-κB nuclear entry upon ligand stimulation. Accordingly, endogenous Sef knockdown markedly enhances stimulus-induced NF-κB nuclear translocation and consequent activity. This study establishes Sef as a feedback antagonist of proinflammatory cytokines and highlights its potential to regulate the crosstalk between proinflammatory cytokine receptors and receptor tyrosine kinases.

New insights into the control of neurotrophic growth factor receptor signaling: implications for nervous system development and repair

Neurotrophic growth factors control neuronal development by activating specific receptor tyrosine kinase positive signaling pathways, such as Ras-MAPK and PI3K-Akt cascades. Once activated, neurotrophic factor receptors also trigger a cascade of molecular events, named negative receptor signaling, that restricts the intensity of the positive signals and modulates cellular behavior. Thus, to avoid signaling errors that ultimately could lead to aberrant neuronal physiology and disease, negative signaling mechanisms have evolved to ensure that suitable thresholds of neuronal stimulation are achieved and maintained during right periods of time. Recent findings have revealed that neurotrophic factor receptor signaling is tightly modulated through the coordinated action of many different protein regulators that limit or potentiate signal propagation in spatially and temporally controlled manners, acting at specific points after receptor engagement. In this review, we discuss progress in this field, highlighting the importance of these modulators in axonal growth, guidance, neural connectivity, and nervous system regeneration.

ERK1/2 MAP kinases: structure, function, and regulation

ERK1 and ERK2 are related protein-serine/threonine kinases that participate in the Ras-Raf-MEK-ERK signal transduction cascade. This cascade participates in the regulation of a large variety of processes including cell adhesion, cell cycle progression, cell migration, cell survival, differentiation, metabolism, proliferation, and transcription. MEK1/2 catalyze the phosphorylation of human ERK1/2 at Tyr204/187 and then Thr202/185. The phosphorylation of both tyrosine and threonine is required for enzyme activation. Whereas the Raf kinase and MEK families have narrow substrate specificity, ERK1/2 catalyze the phosphorylation of hundreds of cytoplasmic and nuclear substrates including regulatory molecules and transcription factors. ERK1/2 are proline-directed kinases that preferentially catalyze the phosphorylation of substrates containing a Pro-Xxx-Ser/Thr-Pro sequence. Besides this primary structure requirement, many ERK1/2 substrates possess a D-docking site, an F-docking site, or both. A variety of scaffold proteins including KSR1/2, IQGAP1, MP1, β-Arrestin1/2 participate in the regulation of the ERK1/2 MAP kinase cascade. The regulatory dephosphorylation of ERK1/2 is mediated by protein-tyrosine specific phosphatases, protein-serine/threonine phosphatases, and dual specificity phosphatases. The combination of kinases and phosphatases make the overall process reversible. The ERK1/2 catalyzed phosphorylation of nuclear transcription factors including those of Ets, Elk, and c-Fos represents an important function and requires the translocation of ERK1/2 into the nucleus by active and passive processes involving the nuclear pore. These transcription factors participate in the immediate early gene response. The activity of the Ras-Raf-MEK-ERK cascade is increased in about one-third of all human cancers, and inhibition of components of this cascade by targeted inhibitors represents an important anti-tumor strategy. Thus far, however, only inhibition of mutant B-Raf (Val600Glu) has been found to be therapeutically efficacious.

Expression and functional role of negative signalling regulators in tumour development and progression

Alterations in intracellular signalling pathways such as the mitogen-activated protein kinases (MAPKs) are key common mechanisms of tumour development and progression. As such, there has been intense research into developing drugs that can inhibit or attenuate intracellular signalling. In recent years, there has been increasing recognition that the cell already has innate negative regulatory proteins that achieve this in normal homeostasis. These regulators provide a feedback inhibitory mechanism that controls the intensity and duration of activated signalling by exogenous stimuli. Members of this group include Raf kinase inhibitor protein 1, the MAPK phosphatases, the SPROUTY and SPRED families and similar expression to FGF. A number of studies have now demonstrated significant alterations in expression of negative regulators in malignant tissue in different cancer types. In functional studies, manipulated expression of these regulators has been shown to significantly influence tumour cell behaviour and phenotype. Here, we summarise the evidence for the functional expression of negative signalling regulators in tumour growth and progression and discuss their potential role as cancer biomarkers and targets for novel drug therapy.

Sef is a negative regulator of fiber cell differentiation in the ocular lens

Growth factor signaling, mediated via receptor tyrosine kinases (RTKs), needs to be tightly regulated in many developmental systems to ensure a physiologically appropriate biological outcome. At one level this regulation may involve spatially and temporally ordered patterns of expression of specific RTK signaling antagonists, such as Sef (similar expression to fgfs). Growth factors, notably FGFs, play important roles in development of the vertebrate ocular lens. FGF induces lens cell proliferation and differentiation at progressively higher concentrations and there is compelling evidence that a gradient of FGF signaling in the eye determines lens polarity and growth patterns. We have recently identified the presence of Sef in the lens, with strongest expression in the epithelial cells. Given the important role for FGFs in lens developmental biology, we employed transgenic mouse strategies to determine if Sef could be involved in regulating lens cell behaviour. Over-expressing Sef specifically in the lens of transgenic mice led to impaired lens and eye development that resulted in microphthalmia. Sef inhibited primary lens fiber cell elongation and differentiation, as well as increased apoptosis, consistent with a block in FGFR-mediated signaling during lens morphogenesis. These results are consistent with growth factor antagonists, such as Sef, being important negative regulators of growth factor signaling. Moreover, the lens provides a useful paradigm as to how opposing gradients of a growth factor and its antagonist could work together to determine and stabilise tissue patterning during development and growth.

Evidence for distinct alterations in the FGF axis in prostate cancer progression to an aggressive clinical phenotype

Multiple fibroblast growth factor (FGF) axis alterations are known to occur in prostate cancer. Here we simultaneously profiled key components of this axis to determine their relevance in disease progression. An optimized immunohistochemistry protocol was used in expression analysis of FGF2, FGF8, FGFR1, FGFR4, and Sef (similar expression to FGF) in a single TMA of prostate cancer. FGF ligands and receptors were overexpressed in cancers compared to benign samples (p < 0.0001), while Sef expression was reduced (p < 0.0001). There was a positive association between higher grades and increased FGFR4 (p = 0.02), FGF2, and FGF8 (p = 0.002 and p < 0.0001). Sef expression was progressively lower with increasing grade (p = 0.005). Clinical stage was positively associated with FGF2, FGF8, and FGFR4 expression (p = 0.005, 0.03, and 0.012) but not with FGFR1 or Sef expression. Only reduced Sef was associated with bone metastasis (p = 0.02) and was also predictive of subsequent metastasis in initially localized tumours (p = 0.004). Down-regulation of Sef and increased FGFR4 were also the only independent variables associated with disease-specific survival (HR 1.73, p = 0.04 and HR 0.56, p = 0.01). In in vitro studies, silencing Sef enhanced the cell response to FGFs (p < 0.001) and substantially mitigated the effectiveness of an FGFR1 inhibitor. Conversely, increased Sef blocked the response to FGFs and had a comparable suppressive effect to the inhibitor. This study demonstrates that increased FGFR4 and reduced Sef may be critical FGF alterations associated with prostate cancer progression. Sef may also have a role in the tumour response to FGFR inhibition and warrants further investigation in this context.

Similar expression to FGF (Sef) inhibits fibroblast growth factor-induced tumourigenic behaviour in prostate cancer cells and is downregulated in aggressive clinical disease

Background:

The fibroblast growth factor (FGF) axis is an important mitogenic stimulus in prostate carcinogenesis. We have previously reported that transcript level of human similar expression to FGF (hSef), a key regulator of this pathway, is downregulated in clinical prostate cancer. In this study we further analysed the role of hSef in prostate cancer.

Methods:

hSef function was studied in in vitro and in vivo prostate cancer models using stable over-expression clones. Protein expression of hSef was studied in a comprehensive tissue microarray.

Results:

Stable over-expression of hSef resulted in reduced in vitro cancer cell proliferation, migration and invasive potential. In an in vivo xenograft model, the expression of hSef significantly retarded prostate tumour growth as compared with empty vector (P=0.03) and non-transfected (P=0.0001) controls. Histological examination further showed a less invasive tumour phenotype and reduced numbers of proliferating cells (P=0.0002). In signalling studies, hSef inhibited FGF-induced ERK phosphorylation, migration to the nucleus and activation of a reporter gene. Constitutively active Ras, however, was able to reverse these effects, suggesting that hSef exerts an effect either above or at the level of Ras in prostate cancer cells. In a large tissue microarray, we observed a significant loss of hSef protein in high-grade (P<0.0001) and metastatic (P<0.0001) prostate cancer.

Conclusions:

Considered together, the role of hSef in attenuating FGF signalling and evidence of downregulation in advanced tumours argue strongly for a tumour suppressor function in human prostate cancer.

Structure and signalling in the IL-17 receptor family

Interleukin-17A (IL-17A), the hallmark cytokine of the newly defined T helper 17 (T(H)17) cell subset, has important roles in protecting the host against extracellular pathogens, but also promotes inflammatory pathology in autoimmune disease. IL-17A and its receptor (IL-17RA) are the founding members of a newly described family of cytokines and receptors that have unique structural features which distinguish them from other cytokine families. Research defining the signal transduction pathways induced by IL-17R family cytokines has lagged behind that of other cytokine families, but studies in the past 2 years have begun to delineate unusual functional motifs and new proximal signalling mediators used by the IL-17R family to mediate downstream events.

Expression of interleukin-17RC protein in normal human tissues

Background

Interleukin-17 (IL-17) cytokines and receptors play an important role in many autoimmune and inflammatory diseases. IL-17 receptors IL-17RA and IL-17RC have been found to form a heterodimer for mediating the signals of IL-17A and IL-17F cytokines. While the function and signaling pathway of IL-17RA has been revealed, IL-17RC has not been well characterized. The function and signaling pathway of IL-17RC remain largely unknown. The purpose of the present study was to systematically examine IL-17RC protein expression in 53 human tissues.

Results

IL-17RC expression in 51 normal human tissues and two benign tumors (i.e., lymphangioma and parathyroid adenoma) on the tissue microarrays was determined by immunohistochemical staining, using two polyclonal antibodies against IL-17RC. IL-17RC protein was expressed in many cell types including the myocardial cells, vascular and lymphatic endothelial cells, glandular cells (of the adrenal, parathyroid, pituitary, thyroid, pancreas, parotid salivary, and subepidermal glands), epithelial cells (of the esophagus, stomach, intestine, anus, renal tubule, breast, cervix, Fallopian tube, epididymis, seminal vesicle, prostate, gallbladder, bronchus, lung, and skin), oocytes in the ovary, Sertoli cells in the testis, motor neurons in the spinal cord, autonomic ganglia and nerves in the intestine, skeletal muscle cells, adipocytes, articular chondrocytes, and synovial cells. High levels of IL-17RC protein expression were observed in most vascular and lymphatic endothelium and squamous epithelium. The epithelium of the breast, cervix, Fallopian tube, kidney, bladder and bronchus also expressed high levels of IL-17RC, so did the glandular cells in the adrenal cortex, parotid salivary and subepidermal glands. In contrast, IL-17RC protein was not detectable in the smooth muscle cells, fibroblasts, antral mucosa of the stomach, mucosa of the colon, endometrium of the uterus, neurons of the brain, hepatocytes, or lymphocytes. Nevertheless, IL-17RC protein was expressed in the vascular endothelium within the tissues where the IL-17RC-negative cells resided.

Conclusion

IL-17RC protein is expressed in most human tissues, the function of which warrants further investigation.

Developmental gene regulatory networks in the zebrafish embryo

The genomic developmental program operates mainly through the regulated expression of genes encoding transcription factors and signaling pathways. Complex networks of regulatory genetic interactions control developmental cell specification and fates. Development in the zebrafish, Danio rerio, has been studied extensively and large amounts of experimental data, including information on spatial and temporal gene expression patterns, are available. A wide variety of maternal and zygotic regulatory factors and signaling pathways have been discovered in zebrafish, and these provide a useful starting point for reconstructing the gene regulatory networks (GRNs) underlying development. In this review, we describe in detail the genetic regulatory subcircuits responsible for dorsoanterior-ventroposterior patterning and endoderm formation. We describe a number of regulatory motifs, which appear to act as the functional building blocks of the GRNs. Different positive feedback loops drive the ventral and dorsal specification processes. Mutual exclusivity in dorsal-ventral polarity in zebrafish is governed by intra-cellular cross-inhibiting GRN motifs, including vent/dharma and tll1/chordin. The dorsal-ventral axis seems to be determined by competition between two maternally driven positive-feedback loops (one operating on Dharma, the other on Bmp). This is the first systematic approach aimed at developing an integrated model of the GRNs underlying zebrafish development. Comparison of GRNs' organizational motifs between different species will provide insights into developmental specification and its evolution. The online version of the zebrafish GRNs can be found at http://www.zebrafishGRNs.org.

Expression and regulation of Sef, a novel signaling inhibitor of receptor tyrosine kinases-mediated signaling in the nervous system

Fibroblast growth factors (FGFs) signal via four distinct high affinity cell surface tyrosine kinase receptors, termed FGFR1-FGFR4 (FGFR-FGF-receptor). Recently, a new modulator of the FGF signaling pathway, the transmembrane protein 'similar expression to FGF genes' (Sef), has been identified in zebrafish and subsequently in mammals. Sef from mouse and human inhibits FGF mitogenic activity. In the present study, we analyzed the expression of Sef in distinct rat brain areas, in the spinal cord and in peripheral nerves and spinal ganglia using semi-quantitative RT-PCR. Furthermore, we studied the cellular expression pattern of Sef in intact spinal ganglia and sciatic nerves and, in addition, after crush lesion, using in situ hybridization and immunohistochemistry. Sef transcripts were expressed in all brain areas evaluated and in the spinal cord. A neuronal expression was found in both intact and injured spinal ganglia. Intact sciatic nerves, however, showed little or no Sef expression. Seven days after injury, high Sef expression was concentrated to the crush site, and Schwann cells seemed to be the source of Sef. The labeling pattern of up-regulated Sef was complementary to the patterns of FGF-2 and FGFR1-3, which were localized proximal and distal to the crush site. These results suggest an involvement of Sef during the nerve regeneration process, possibly by fine-tuning the effects of FGF signaling.

hSef potentiates EGF-mediated MAPK signaling through affecting EGFR trafficking and degradation

Sef (similar expression to fgf genes) was identified as an effective antagonist of fibroblast growth factor (FGF) in vertebrates. Previous reports have demonstrated that Sef interacts with FGF receptors (FGFRs) and inhibits FGF signaling, however, its role in regulating epidermal growth factor receptor (EGFR) signaling remains unclear. In this report, we found that hSef localizes to the plasma membrane (PM) and is subjected to rapid internalization and well localizes in early/recycling endosomes while poorly in late endosomes/lysosomes. We observed that hSef interacts and functionally colocalizes with EGFR in early endosomes in response to EGF stimulation. Importantly, we demonstrated that overexpression of hSef attenuates EGFR degradation and potentiates EGF-mediated mitogen-activated protein kinase (MAPK) signaling by interfering EGFR trafficking. Finally, our data showed that, with overexpression of hSef, elevated levels of Erk phosphorylation and differentiation of rat pheochromocytoma (PC12) cells occur in response to EGF stimulation. Taken together, these data suggest that hSef plays a positive role in the EGFR-mediated MAPK signaling pathway. This report, for the first time, reveals opposite roles for Sef in EGF and FGF signalings.

Computational and molecular characterization of multiple isoforms of lfe-2 gene in nematode C. elegans

C. elegans C46H11.4 gene encodes a Let-23 fertility effector/regulator protein of the EGF-receptor class of the tyrosine kinase family. Alternative splicing is a major mechanism of generating protein diversity in higher eukaryotes. C. elegans genome sequencing consortium has reported three alternatively spliced transcripts of C46H11.4 gene which encodes for three hypothetical proteins namely, C46H11.4a, C46H11.4b and C46H11.4c. Using a combination of various bioinformatics tools like gene or exon finding programmes, blast searches, alignment tools etc followed by experimental validation, we report the presence of three more alternatively spliced transcripts which encode for novel hypothetical proteins C46H11.4d, C46H11.4e and C46H11.4f. These isoforms arise as a result of alternative splicing in the pre-mRNA encoded by gene C46H11.4. These novel un-reported spliced variants not only point towards the extent of alternative splicing in C. elegans genes but also hint towards the complex nature of alternative splicing.

Changes in Sef levels influence auditory brainstem development and function

During development of the CNS, secreted morphogens of the fibroblast growth factor (FGF) family have multiple effects on cell division, migration, and survival depending on where, when, and how much FGF signal is received. The consequences of misregulating the FGF pathway were studied in a mouse with decreased levels of the FGF antagonist Sef. To uncover effects in the nervous system, we focused on the auditory system, which is accessible to physiological analysis. We found that the mitogen-activated protein kinase pathway is active in the rhombic lip, a germinal zone that generates diverse types of neurons, including the cochlear nucleus complex of the auditory system. Sef is expressed immediately adjacent to the rhombic lip, overlapping with FGF15 and FGFR1, which is also present in the lip itself. This pattern suggests that Sef may normally function in non-rhombic lip cells and prevent them from responding to FGF ligand in the vicinity. Consistent with this idea, overexpression of Sef in chicks decreased the size of the auditory nuclei. Cochlear nucleus defects were also apparent in mice with reduced levels of Sef, with 13% exhibiting grossly dysmorphic cochlear nuclei and 26% showing decreased amounts of GFAP in the cochlear nucleus. Additional evidence for cochlear nucleus defects was obtained by electrophysiological analysis of Sef mutant mice, which have normal auditory thresholds but abnormal auditory brainstem responses. These results show both increases and decreases in Sef levels affect the assembly and function of the auditory brainstem.

Downregulation of Sef, an inhibitor of receptor tyrosine kinase signaling, is common to a variety of human carcinomas

Carcinomas are tumors of epithelial origin accounting for over 80% of all human malignancies. A substantial body of evidence implicates oncogenic signaling by receptor tyrosine kinases (RTKs) in carcinoma development. Here we investigated the expression of Sef, a novel inhibitor of RTK signaling, in normal human epithelial tissues and derived malignancies. Human Sef (hSef) was highly expressed in normal epithelial cells of breast, prostate, thyroid gland and the ovarian surface. By comparison, substantial downregulation of hSef expression was observed in the majority of tumors originating from these epithelia. Among 186 primary carcinomas surveyed by RNA in situ hybridization, hSef expression was undetectable in 116 cases including 72/99 (73%) breast, 11/16 (69%) thyroid, 16/31 (52%) prostate and 17/40 (43%) ovarian carcinomas. Moderate reduction of expression was observed in 17/186, and marked reduction in 40/186 tumors. Only 13/186 cases including 12 low-grade and one intermediate grade tumor retained high hSef expression. The association of hSef downregulation and tumor progression was statistically significant (P<0.001). Functionally, ectopic expression of hSef suppressed proliferation of breast carcinoma cells, whereas inhibition of endogenous hSef expression accelerated fibroblast growth factor and epidermal growth factor-dependent proliferation of cervical carcinoma cells. The inhibitory effect of hSef on cell proliferation combined with consistent downregulation in human carcinoma indicates a tumor suppressor-like role for hSef, and implicates loss of hSef expression as a common mechanism in epithelial neoplasia.

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

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

Tyrosine 330 in hSef is critical for the localization and the inhibitory effect on FGF signaling

Sef (similar expression to fgf genes) was identified as an inhibitor of FGF signaling. The regulation of this inhibitory effect was largely unknown. In this report we demonstrated that tyrosine 330 in hSef protein plays a critical role in the control of the protein localization and thereby in the regulation of Ras/MAPK signaling pathway. We found that the tyrosine 330 is in the form of the YXXcapital EF, Cyrillic signal context and mutation of this residue resulted in preferred plasma membrane localization of hSef. We also observed that both Sef and SefY330F (where tyrosine is substituted by phenylalanine) interacted and co-localized with FGFR in the co-immunoprecipitation assay, and immunostaining assay respectively. We further revealed that the increased amount of Sef localization in the plasma membrane was coupled with the enhanced inhibitory effect on the FGF signaling pathway, indicating that Sef might exert its inhibitory function on the plasma membrane. This paper revealed that tyrosine 330 is critical for the inhibitory function of Sef on FGF signaling.

Sef-S, an alternative splice isoform of sef gene, inhibits NIH3T3 cell proliferation via a mitogen-activated protein kinases p42 and p44 (ERK1/2)-independent mechanism

sef (similar expression to fgf genes) was recently identified as a negative regulator of fibroblast growth factor (FGF) signaling in zebrafish, chicken, mouse and human. By repressing events upstream and/or downstream Ras, Sef inhibits FGF-induced ERK activation and cell proliferation. Here we report that Sef-S, an alternative splice isoform of Sef, lacks a signal peptide and is localized in cytosol. Sef-S inhibits FGF-induced NIH3T3 cell proliferation, a similar function to Sef. However, Sef-S represses neither the intensity nor the duration of ERK activation. Moreover, Sef-S does not inhibit Elk1-dependent transcription. Our study revealed that the signal peptide is critical for the different activities between Sef and Sef-S in FGF-Ras-MAPK signaling cascades. Furthermore, we observed that Sef-S associated with FGFR2 in a co-immunoprecipitated complex. These results indicate that Sef-S inhibits FGF-induced NIH3T3 cell proliferation via an ERK-independent mechanism and therefore suggest that alternative splice licenses sef gene to inhibit cell proliferation via multiple signaling pathways.

A role for extracellular and transmembrane domains of Sef in Sef-mediated inhibition of FGF signaling

Sef (similar expression to fgf genes) is a member of the fibroblast growth factor (FGF) synexpression group that negatively regulates FGF receptor (FGFR) signaling in zebrafish during early embryonic development and in mammalian cell culture systems. The mechanism by which Sef exerts its inhibitory effect remains controversial. It has been reported that Sef functions either through binding to and inhibiting FGFR1 activation or by acting downstream of FGF receptors at the level of MEK/ERK kinases. In both cases, the intracellular domain of Sef was found to play a role in the inhibitory function of Sef. Here we demonstrated that both extracellular and transmembrane domains of Sef contributed to Sef-mediated negative regulation of FGF signaling. In fact, over-expression studies in NIH3T3 cells showed that a truncated mutant of Sef, which lacks the intracellular domain (SefECTM), exerted the inhibitory activity on FGF signaling by inhibiting FGFR1 tyrosine phosphorylation and subsequent activation of the Raf/MEK/ERK signaling cascade. We also showed that SefECTM associated with FGFR1, and inhibited FGF-induced ERK activation in HEK293T cells. Furthermore, we demonstrated that the over-expression of SefECTM was able to inhibit the function of a constitutively activated form of FGFR1, FGFR1-C289R, but not FGFR1-K562E. Finally, we found that SefECTM reduced cell viability when over-expressed in human umbilical vein endothelial cells (HUVEC). These data provide additional insight into the structure-activity relationship in the mechanism of inhibitory action of Sef on FGFR1-mediated signaling.

The human sef-a isoform utilizes different mechanisms to regulate receptor tyrosine kinase signaling pathways and subsequent cell fate

Negative feedback is among the key mechanisms for regulating receptor tyrosine kinase (RTK) signaling. Human Sef, a recently identified inhibitor of RTK signaling, encodes different isoforms, including a membrane spanning (hSef-a) and a cytosolic (hSef-b) isoform. Previously, we reported that hSef-b inhibited fibroblast proliferation and prevented the activation of mitogen-activated protein kinase (MAPK), without affecting protein kinase B/Akt or p38 MAPK. Conflicting results were reported concerning hSef-a inhibition of MAPK activation, and the effect of hSef-a on other RTK-induced signaling pathways is unknown. Here we show that, in fibroblasts, similar to hSef-b, ectopic expression of hSef-a inhibited fibroblast growth factor-induced cell proliferation. Unlike hSef-b, however, the growth arrest was mediated via a MAPK-independent mechanism, and was accompanied by elevated p38 MAPK phosphorylation and inhibition of protein kinase B/Akt. In addition, hSef-a, but not hSef-b, mediated apoptosis in fibroblast growth factor-stimulated cells. Chemical inhibitor of p38 MAPK abrogated the effect of hSef-a on apoptosis. In epithelial cells, ectopic expression of hSef-a inhibited the activation of MAPK, whereas down-regulation of endogenous hSef-a significantly increased MAPK activation and accelerated growth factor-dependent cell proliferation. These results indicate that hSef-a is a multifunctional negative modulator of RTK signaling and clearly demonstrate that hSef-a can inhibit the activation of MAPK, although in a cell type-specific manner. Moreover, the differences between the activities of hSef-a and hSef-b suggest that hSef isoforms can control signal specificity and subsequent cell fate by utilizing different mechanisms to modulate RTK signaling.

Arginyltransferase, its specificity, putative substrates, bidirectional promoter, and splicing-derived isoforms

Substrates of the N-end rule pathway include proteins with destabilizing N-terminal residues. Three of them, Asp, Glu, and (oxidized) Cys, function through their conjugation to Arg, one of destabilizing N-terminal residues that are recognized directly by the pathway's ubiquitin ligases. The conjugation of Arg is mediated by arginyltransferase, encoded by ATE1. Through its regulated degradation of specific proteins, the arginylation branch of the N-end rule pathway mediates, in particular, the cardiovascular development, the fidelity of chromosome segregation, and the control of signaling by nitric oxide. We show that mouse ATE1 specifies at least six mRNA isoforms, which are produced through alternative splicing, encode enzymatically active arginyltransferases, and are expressed at varying levels in mouse tissues. We also show that the ATE1 promoter is bidirectional, mediating the expression of both ATE1 and an oppositely oriented, previously uncharacterized gene. In addition, we identified GRP78 (glucose-regulated protein 78) and protein-disulfide isomerase as putative physiological substrates of arginyltransferase. Purified isoforms of arginyltransferase that contain the alternative first exons differentially arginylate these proteins in extract from ATE1(-/-) embryos, suggesting that specific isoforms may have distinct functions. Although the N-end rule pathway is apparently confined to the cytosol and the nucleus, and although GRP78 and protein-disulfide isomerase are located largely in the endoplasmic reticulum, recent evidence suggests that these proteins are also present in the cytosol and other compartments in vivo, where they may become N-end rule substrates.

hSef co-localizes and interacts with Ras in the inhibition of Ras/MAPK signaling pathway

To study the mechanism of the inhibitory effects of Sef (similar expression to fgf genes) on Ras/mitogen-activated protein kinase (MAPK) signaling pathway, we observed cellular localization of this protein. Immunofluorescent staining results show that Sef locates in the vesicles of the cytoplasm without bFGF treatment but co-localizes with Ras on the plasma membrane (PM) in response to bFGF stimulation. The coimmunoprecipitation assay demonstrates that Sef interacts with Ras or RasG12V, respectively. We observed that Sef inhibited FGF induced, but not RasG12V mediated, signal transduction. We propose that Sef interacted with Ras in the inhibition of Ras/MAPK signaling pathway.

Identification and functional characterization of a novel interleukin 17 receptor: a possible mitogenic activation through ras/mitogen-activated protein kinase signaling pathway

Interleukin-17 receptor (IL-17R) is increasingly emerged as a distinct receptor family functioning in diverse cellular processes including inflammation and cancer. In this study, we uncovered a novel member of IL-17R from mouse tissue that was named mouse IL-17RE (mIL-17R). Mouse IL-17RE cDNA is composed of at least 14 exons and presents at least 6 spliced isoforms (mIL-17RE1-6) with a molecular weight ranging from 34.2 to 70.1 kD. Mouse IL-17RE is expressed in limited tissues such as lung, kidney, stomach, intestine and testis, etc., and is mainly localized in the cytoplasm and on cell membrane. IL-17RE can also be detected in numerous tumor cell lines. Importantly, a mitogenic effect was detected in BaF3 cells stably transfected with the chimeric receptor fused by the ectodomain of erythropoietin receptor (EPOR) with the transmembrane and endomain of IL-17RE in a serum-dependent but EPO-independent manner. Moreover, ERK1/2 phosphorylation was significantly up-regulated as the dose of mIL-17RE increased. Specific RNAi targeting at mIL-17RE dramatically inhibited the activation of ERK1/2, indicating that mIL-17RE could functionally activate RAS/MAPK signaling pathway. Using dominant negative MEK (Dn-MEK) or RAS (Dn-RAS) as a signaling blocker, we were able to show that mIL-17RE probably activated RAS/MAPK signaling at or upstream of RAS. Overall, our results strongly indicate that mIL-17RE may belong to a novel growth-receptor like molecule that has the capability to support cellular mitogenesis through RAS/MAPK pathway.

Targeting perlecan in human keratinocytes reveals novel roles for perlecan in epidermal formation

Heparin-binding growth factors are crucial for the formation of human epidermis, but little is known about the role of heparan sulfate proteoglycans in this process. Here we investigated the role of the heparan sulfate proteoglycan, perlecan, in the formation of human epidermis, by utilizing in vitro engineered human skin. By disrupting perlecan expression either in the dermis or the epidermis, we found that epidermally derived perlecan is essential for epidermal formation. Perlecan-deficient keratinocytes formed a strikingly thin and poorly organized epidermis because of premature apoptosis and failure to complete their stratification program. Exogenous perlecan fully restored epidermal formation. Perlecan deposition in the basement membrane zone correlated with formation of multilayered epidermis. Perlecan deficiency, however, had no effect on the lining and deposition of major basement membrane components as was evident by a continuous linear staining of laminin and collagen IV. Similarly, perlecan deficiency did not affect the distribution of beta1 integrin. Addition of the perlecan ligand, fibroblast growth factor 7, protected perlecan-deficient keratinocytes from cell death and improved the thickness of the epidermis. Taken together, our results revealed novel roles for perlecan in epidermal formation. Perlecan regulates both the survival and terminal differentiation steps of keratinocytes. Our results suggested a model whereby perlecan regulates these processes via controlling the bioavailability of perlecan-binding soluble factors involved in epidermal morphogenesis.

Functions and regulations of fibroblast growth factor signaling during embryonic development

Fibroblast growth factors (FGF) are secreted molecules which function through the activation of specific tyrosine kinases receptors, the FGF receptors that transduce the signal by activating different pathways including the Ras/MAP kinase and the phospholipase-C gamma pathways. FGFs are involved in the regulation of many developmental processes including patterning, morphogenesis, differentiation, cell proliferation or migration. Such a diverse set of activities requires a tight control of the transduction signal which is achieved through the induction of different feedback inhibitors such as the Sproutys, Sef and MAP kinase phosphatase 3 which are responsible for the attenuation of FGF signals, limiting FGF activities in time and space.

Sef is synexpressed with FGFs during chick embryogenesis and its expression is differentially regulated by FGFs in the developing limb

The signaling pathways leading to growth and patterning of various organs are tightly controlled during the development of any organism. These control mechanisms usually involve the utilization of feedback- and pathway-specific antagonists where the pathway induces the expression of its own antagonist. Sef is a feedback antagonist of fibroblast growth factor (FGF) signaling, which has been identified recently in zebrafish and mammals. Here, we report the isolation of chicken Sef (cSef) and demonstrate the conserved nature of the regulatory relationship with FGF signaling. In chick embryos, Sef is expressed in a pattern that coincides with many known sites of FGF signaling. In the developing limb, cSef is expressed in the mesoderm underlying the apical ectodermal ridge (AER) in the region known as the progress zone. cSef message first appeared after limb budding and AER formation. Expression was intense at stages of rapid limb outgrowth, and gradually decreased to almost undetectable levels when differentiation was clearly apparent. Gain- and loss-of-function experiments showed that FGFs differentially regulate the expression of cSef in various tissues. Thus, removal of the AER down-regulated cSef expression, and FGF2 but not FGF4 or FGF8 beads substituted for the AER in maintaining cSef expression. At sites where cSef is not normally expressed, FGF4 and FGF2, but not FGF8 beads, induced cSef expression. Our results demonstrate the complexity of cSef regulation by FGFs and point to FGF2 as a prime candidate in regulating cSef expression during normal limb development. The spatiotemporal pattern of cSef expression during limb development suggests a role for cSef in regulating limb outgrowth but not limb initiation.

Modulation of Fgf8 activity during vertebrate brain development

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