Sef is a spatial regulator for Ras/MAP kinase signaling

Fibroblast growth factor signaling in skeletal development and disease

Fibroblast growth factor (FGF) signaling pathways are essential regulators of vertebrate skeletal development. FGF signaling regulates development of the limb bud and formation of the mesenchymal condensation and has key roles in regulating chondrogenesis, osteogenesis, and bone and mineral homeostasis. This review updates our review on FGFs in skeletal development published in Genes & Development in 2002, examines progress made on understanding the functions of the FGF signaling pathway during critical stages of skeletogenesis, and explores the mechanisms by which mutations in FGF signaling molecules cause skeletal malformations in humans. Links between FGF signaling pathways and other interacting pathways that are critical for skeletal development and could be exploited to treat genetic diseases and repair bone are also explored.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

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.

Quantitative in vivo fluorescence cross-correlation analyses highlight the importance of competitive effects in the regulation of protein-protein interactions

Computer-assisted simulation is a promising approach for clarifying complicated signaling networks. However, this approach is currently limited by a deficiency of kinetic parameters determined in living cells. To overcome this problem, we applied fluorescence cross-correlation spectrometry (FCCS) to measure dissociation constant (Kd) values of signaling molecule complexes in living cells (in vivo Kd). Among the pairs of fluorescent molecules tested, that of monomerized enhanced green fluorescent protein (mEGFP) and HaloTag-tetramethylrhodamine was most suitable for the measurement of in vivo Kd by FCCS. Using this pair, we determined 22 in vivo Kd values of signaling molecule complexes comprising the epidermal growth factor receptor (EGFR)-Ras-extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase pathway. With these parameters, we developed a kinetic simulation model of the EGFR-Ras-ERK MAP kinase pathway and uncovered a potential role played by stoichiometry in Shc binding to EGFR during the peak activations of Ras, MEK, and ERK. Intriguingly, most of the in vivo Kd values determined in this study were higher than the in vitro Kd values reported previously, suggesting the significance of competitive bindings inside cells. These in vivo Kd values will provide a sound basis for the quantitative understanding of signal transduction.

Compensatory regulation of the size of the inner ear in response to excess induction of otic progenitors by fibroblast growth factor signaling

BACKGROUND

The otic placode comprises the progenitors of the inner ear and the neurons that convey hearing and balance information to the brain. Transplantation studies in birds and amphibians demonstrate that when the otic placode is morphologically visible as a thickened patch of ectoderm, it is first committed to an otic fate. Fibroblast growth factor (FGF) signaling initiates induction of the otic placode, and levels of FGF signaling are fine-tuned by the Sprouty family of antagonists of receptor tyrosine kinase signaling.

RESULTS

Here, we examined the size of the otic placode and cup by combinatorial inactivation of the Sprouty1 and Sprouty2 genes. Interestingly, in a Sprouty gene dosage series, early enlargement of the otic placode was progressively restored to normal. Restoration of otic size was preceded by normal levels of FGF signaling, reduced cell proliferation and reduced cell death.

CONCLUSIONS

Our study demonstrates that excess otic placode cells, which form in response to increased FGF signaling, are not maintained in mammals. This suggests that growth plasticity exists in the mammalian otic placode and cup, and that FGF signaling may not be sufficient to induce the genetic program that maintains otic fate.

A human pluripotent stem cell surface N-glycoproteome resource reveals markers, extracellular epitopes, and drug targets

Detailed knowledge of cell-surface proteins for isolating well-defined populations of human pluripotent stem cells (hPSCs) would significantly enhance their characterization and translational potential. Through a chemoproteomic approach, we developed a cell-surface proteome inventory containing 496 N-linked glycoproteins on human embryonic (hESCs) and induced PSCs (hiPSCs). Against a backdrop of human fibroblasts and 50 other cell types, >100 surface proteins of interest for hPSCs were revealed. The >30 positive and negative markers verified here by orthogonal approaches provide experimental justification for the rational selection of pluripotency and lineage markers, epitopes for cell isolation, and reagents for the characterization of putative hiPSC lines. Comparative differences between the chemoproteomic-defined surfaceome and the transcriptome-predicted surfaceome directly led to the discovery that STF-31, a reported GLUT-1 inhibitor, is toxic to hPSCs and efficient for selective elimination of hPSCs from mixed cultures.

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496 cell surface N-glycoproteins on hPSCs

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N-glycosylation site identification dictates accessible epitopes

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>30 positive and negative selection markers for hPSCs are validated

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STF-31 is selectively toxic to hPSCs

Fluoxetine signature on hippocampal MAPK signalling in sex-dependent manner

A growing body of evidence indicates that mitogen-activated protein kinase (MAPK) participates in various stress-induced responses and is considered to be one of the pathophysiological mechanisms in depression. Surprisingly, the effect of antidepressants on MAPKs is almost unexplored, particularly from the perspective of sexes. The present study investigates the cytoplasm-nuclear distribution of MAPK family, c-Jun N-terminal kinases (JNKs) 1, 2 and 3; extracellular signal-regulated kinases (ERKs) 1 and 2; and p38 kinases, as well as their phosphoisoforms in the hippocampus of chronically stressed female and male rats and upon chronic fluoxetine treatment. Additionally, we analysed crosstalk between MAPK signalling and depressive-like behaviour which correlated with brain-derived neurotrophic factor (BDNF) expression. Our results emphasize a gender-specific and compartment-dependent response of MAPKs to stress and fluoxetine. In females, stress decreased pp38 and pJNK and induced cytosolic retention of pERKs which reduced all nuclear pMAPKs. These changes correlated with altered BDNF expression and behaviour. Similarly, in males, stress decreased pp38 but promoted nuclear translocation of pJNKs and pERKs. These stress alterations of pMAPKs in males were not associated with BDNF expression and depressive-like behaviour. Fluoxetine treatment in stressed females upregulated whole pMAPK signalling particularly those in nucleus which was followed with BDNF expression and normalization of behaviour. In stressed males, fluoxetine affected only cytosolic pJNKs, while nuclear pMAPK signalling and BDNF expression were unaffected even though fluoxetine normalized behaviour. Overall, our results suggest existence of gender-specific mechanism of fluoxetine on nuclear pMAPK/BDNF signalling and depressive-like behaviour and reinforce the antidepressant dogma that females and males respond differently to certain antidepressants.

Small GTPase RBJ mediates nuclear entrapment of MEK1/MEK2 in tumor progression

Ras-related small GTPases play important roles in cancer. However, the roles of RBJ, a representative of the sixth subfamily of Ras-related small GTPases, in tumorigenesis and tumor progression remain unknown. Here, we report that RBJ is dysregulated in human gastrointestinal cancers and can promote carcinogenesis and tumor progression via nuclear entrapment of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK)1/MEK2 and activation of ERK1/ERK2. Nucleus-localized RBJ interacts with MEK/ERK and prolongs the duration of MEK/ERK activation. Rbj deficiency abrogates nuclear accumulation of MEK1/MEK2, attenuates ERK1/ERK2 activation, and impairs AOM/DSS-induced colonic carcinogenesis. Moreover, Rbj knockdown inhibits growth of established tumors. Our data suggest that RBJ may be an oncogenic Ras-related small GTPase mediating nuclear accumulation of active MEK1/MEK2 in tumor progression.

Fibroblast growth factor signaling in liver carcinogenesis

Fibroblast growth factors, or FGFs, are a large family of polypeptide cytokines exhibiting a pleiotropy of functions, from cell growth to angiogenesis, wound healing, and tissue repair. This review broadly covers the genetics and protein expression of the FGF family members and the signaling pathways involved in FGF-mediated growth regulation. We emphasize the role of FGFs in the pathogenesis of hepatocellular carcinoma (HCC), including their effects on regulation of the tumor microenvironment and angiogenesis. Finally, we present current views on FGF's potential role as a prognostic marker in clinical practice, as well as its potential as a therapeutic target in HCC.

Deficiency of Sef is associated with increased postnatal cortical bone mass by regulating Runx2 activity

Sef (similar expression to fgf genes) is a feedback inhibitor of fibroblast growth factor (FGF) signaling and functions in part by binding to FGF receptors and inhibiting their activation. Genetic studies in mice and humans indicate an important role for fibroblast growth factor signaling in bone growth and homeostasis. We, therefore, investigated whether Sef had a function role in skeletal acquisition and remodeling. Sef expression is increased during osteoblast differentiation in vitro, and LacZ staining of Sef+/- mice showed high expression of Sef in the periosteum and chondro-osseous junction of neonatal and adult mice. Mice with a global deletion of Sef showed increased cortical bone thickness, bone volume, and increased periosteal perimeter by micro-computed tomography (micro-CT). Histomorphometric analysis of cortical bone revealed a significant increase in osteoblast number. Interestingly, Sef-/- mice showed very little difference in trabecular bone by micro-CT and histomorphometry compared with wild-type mice. Bone marrow cells from Sef-/- mice grown in osteogenic medium showed increased proliferation and increased osteoblast differentiation compared with wild-type bone marrow cells. Bone marrow cells from Sef-/- mice showed enhanced FGF2-induced activation of the ERK pathway, whereas bone marrow cells from Sef transgenic mice showed decreased FGF2-induced signaling. FGF2-induced acetylation and stability of Runx2 was enhanced in Sef-/- bone marrow cells, whereas overexpression of Sef inhibited Runx2-responsive luciferase reporter activity. Bone marrow from Sef-/- mice showed enhanced hematopoietic lineage-dependent and osteoblast-dependent osteoclastogenesis and increased bone resorptive activity relative to wild-type controls in in vitro assays, whereas overexpression of Sef inhibited osteoclast differentiation. Taken together, these studies indicate that Sef has specific roles in osteoblast and osteoclast lineages and that its absence results in increased osteoblast and osteoclast activity with a net increase in cortical bone mass.

Temporal production of the signaling lipid phosphatidic acid by phospholipase D2 determines the output of extracellular signal-regulated kinase signaling in cancer cells

The Ras-extracellular signal-regulated kinase (ERK) cascade is an important signaling module in cells. One regulator of the Ras-ERK cascade is phosphatidic acid (PA) generated by phospholipase D (PLD) and diacylglycerol kinase (DGK). Using a newly developed PA biosensor, PASS (phosphatidic acid biosensor with superior sensitivity), we found that PA was generated sequentially by PLD and DGK in epidermal growth factor (EGF)-stimulated HCC1806 breast cancer cells. Inhibition of PLD2, one of the two PLD members, was sufficient to eliminate most of the PA production, whereas inhibition of DGK decreased PA production only at the later stages of EGF stimulation, suggesting that PLD2 precedes DGK activation. The temporal production of PA by PLD2 is important for the nuclear activation of ERK. While inhibition of both PLD and DGK had no effect on the overall ERK activity, inhibition of PLD2 but not PLD1 or DGK blocked the nuclear ERK activity in several cancer cell lines. The decrease of active ERK in the nucleus inhibited the activation of Elk1, c-fos, and Fra1, the ERK nuclear targets, leading to decreased proliferation of HCC1806 cells. Together, these findings reveal that PA production by PLD2 determines the output of ERK in cancer cell growth factor signaling.

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.

Tay bridge is a negative regulator of EGFR signalling and interacts with Erk and Mkp3 in the Drosophila melanogaster wing

The regulation of Extracellular regulated kinase (Erk) activity is a key aspect of signalling by pathways activated by extracellular ligands acting through tyrosine kinase transmembrane receptors. In this process, participate proteins with kinase activity that phosphorylate and activate Erk, as well as different phosphatases that inactivate Erk by de-phosphorylation. The state of Erk phosphorylation affects not only its activity, but also its subcellular localization, defining the repertoire of Erk target proteins, and consequently, the cellular response to Erk. In this work, we characterise Tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge is a large nuclear protein with a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. We show that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. We suggest that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus.

Extracellular regulated kinases (Erk) mediate signalling by pathways activated by tyrosine kinase transmembrane receptors. The level of activated Erk depends on a highly regulated balance between cytoplasmic kinases and nuclear/cytoplasmic phosphatases, which determine the state of Erk phosphorylation. This affects Erk activity and its subcellular localization, defining the repertoire of Erk targets, and consequently, the cellular response to Erk. In this work, we use a genetic approach to characterise the gene tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge has a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. We show that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. We suggest that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus. These results could provide important insights into the clinical consequences of AUTS2 mutations in humans, which are related to behavioural perturbations including autism, mental retardation, Attention Deficit Hyperactivity Disorder and alcohol drinking behaviour.

Functional roles of fibroblast growth factor receptors (FGFRs) signaling in human cancers

The fibroblast growth factor receptors (FGFRs) regulate important biological processes including cell proliferation and differentiation during development and tissue repair. Over the past decades, numerous pathological conditions and developmental syndromes have emerged as a consequence of deregulation in the FGFRs signaling network. This review aims to provide an overview of FGFR family, their complex signaling pathways in tumorigenesis, and the current development and application of therapeutics targeting the FGFRs signaling for treatment of refractory human cancers.

FGFR signalling in women's cancers

FGFs, in a complex with their receptors (FGFRs) and heparan sulfate (HS), are responsible for a range of cellular functions, from embryogenesis to metabolism. Both germ line and somatic FGFR mutations are known to play a role in a range of diseases, most notably craniosynestosis dysplasias, dwarfism and cancer. Because of the ability of FGFR signalling to induce cell proliferation, migration and survival, FGFRs are readily co-opted by cancer cells. Mutations in, and amplifications of, these receptors are found in a range of cancers with some of the most striking clinical findings relating to their contribution to pathogenesis and progression of female cancers. Here, we outline the molecular mechanisms of FGFR signalling and discuss the role of this pathway in women's cancers, focusing on breast, endometrial, ovarian and cervical carcinomas, and their associated preclinical and clinical data. We also address the rationale for therapeutic intervention and the need for FGFR-targeted therapy to selectively target cancer cells in view of the fundamental roles of FGF signalling in normal physiology.

Pseudophosphatase STYX modulates cell-fate decisions and cell migration by spatiotemporal regulation of ERK1/2

Serine/threonine/tyrosine-interacting protein (STYX) is a catalytically inactive member of the dual-specificity phosphatases (DUSPs) family. Whereas the role of DUSPs in cellular signaling is well explored, the function of STYX is still unknown. Here, we identify STYX as a spatial regulator of ERK signaling. We used predictive-model simulation to test several hypotheses for possible modes of STYX action. We show that STYX localizes to the nucleus, competes with nuclear DUSP4 for binding to ERK, and acts as a nuclear anchor that regulates ERK nuclear export. Depletion of STYX increases ERK activity in both cytosol and nucleus. Importantly, depletion of STYX causes an ERK-dependent fragmentation of the Golgi apparatus and inhibits Golgi polarization and directional cell migration. Finally, we show that overexpression of STYX reduces ERK1/2 activation, thereby blocking PC12 cell differentiation. Overall, our results identify STYX as an important regulator of ERK1/2 signaling critical for cell migration and PC12 cell differentiation.

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.

Mutations in FGF17, IL17RD, DUSP6, SPRY4, and FLRT3 are identified in individuals with congenital hypogonadotropic hypogonadism

Congenital hypogonadotropic hypogonadism (CHH) and its anosmia-associated form (Kallmann syndrome [KS]) are genetically heterogeneous. Among the >15 genes implicated in these conditions, mutations in FGF8 and FGFR1 account for ~12% of cases; notably, KAL1 and HS6ST1 are also involved in FGFR1 signaling and can be mutated in CHH. We therefore hypothesized that mutations in genes encoding a broader range of modulators of the FGFR1 pathway might contribute to the genetics of CHH as causal or modifier mutations. Thus, we aimed to (1) investigate whether CHH individuals harbor mutations in members of the so-called "FGF8 synexpression" group and (2) validate the ability of a bioinformatics algorithm on the basis of protein-protein interactome data (interactome-based affiliation scoring [IBAS]) to identify high-quality candidate genes. On the basis of sequence homology, expression, and structural and functional data, seven genes were selected and sequenced in 386 unrelated CHH individuals and 155 controls. Except for FGF18 and SPRY2, all other genes were found to be mutated in CHH individuals: FGF17 (n = 3 individuals), IL17RD (n = 8), DUSP6 (n = 5), SPRY4 (n = 14), and FLRT3 (n = 3). Independently, IBAS predicted FGF17 and IL17RD as the two top candidates in the entire proteome on the basis of a statistical test of their protein-protein interaction patterns to proteins known to be altered in CHH. Most of the FGF17 and IL17RD mutations altered protein function in vitro. IL17RD mutations were found only in KS individuals and were strongly linked to hearing loss (6/8 individuals). Mutations in genes encoding components of the FGF pathway are associated with complex modes of CHH inheritance and act primarily as contributors to an oligogenic genetic architecture underlying CHH.

Interaction with Shc prevents aberrant Erk activation in the absence of extracellular stimuli

Control mechanisms that prevent aberrant signaling are necessary to maintain cellular homeostasis. We describe a new mechanism by which the adaptor protein Shc directly binds the MAP kinase Erk, thus preventing its activation in the absence of extracellular stimuli. The Shc-Erk complex restricts Erk nuclear translocation, restraining Erk-dependent transcription of genes, including those responsible for oncogenic growth. The complex forms through unique binding sites on both the Shc PTB domain and the N-terminal lobe of Erk. Upon receptor tyrosine kinase stimulation, a conformational change within Shc-induced through interaction with the phosphorylated receptor-releases Erk, allowing it to fulfill its role in signaling. Thus, in addition to its established role in promoting MAP kinase signaling in stimulated cells, Shc negatively regulates Erk activation in the absence of growth factors and thus could be considered a tumor suppressor in human cells.

SiRNA-mediated silencing of paxillin down-regulates ERK1/2 signaling and alters cell ultrastructure in colorectal carcinoma cell line SW480

AIM: To study the effect of silencing of paxillin overexpression on cell signaling and ultrastructure in colorectal carcinoma cell line SW480.

METHODS: Using empty plasmid as a negative control, two siRNA fragments were transfected into a colorectal carcinoma cell line SW480 which overexpresses paxillin. Stably transfected cells were screened and three new cell lines NC, SW545 and SW782 were obtained, which carried the negative control, the siRNA targeting the site 545-565, and the siRNA targeting the site 782-802, respectively. The expression and site-specific phosphorylation of paxillin, FAK, ERK1/2 and AKT1/2/3 were examined in the four cell lines by Western blot. Specimens were prepared with cultured carcinoma cells to observe cell ultrastructure by transmission electron microscopy.

RESULTS: Paxillin overexpression in SW545 cells was not silenced at all, whereas silenced paxillin overexpression and remarkably reduced phosphorylation of paxillin (Tyr118) were observed in SW782 cells. Expression of AKT1/2/3 and FAK as well as their site-specific phosphorylation were substantially the same in the four cell lines. Although expression of ERK1/2 was substantially the same in the four cell lines, significantly reduced phosphorylation of ERK1/2 (Thr202/Tyr204) was observed in SW782 cells. There was no distinct ultrastructural difference between NC cells and SW480 cells, whereas dramatic ultrastructural changes were observed in SW782 cells, such as much more microvilli, microfilament and microtubule bundles, lysosomes and much less mitochondria.

CONCLUSION: Paxillin overexpression may play an important role in the malignant transformation of colorectal carcinoma cells, which is characterized by dramatic ultrastructural changes that can be reversed by silencing paxillin overexpression. Activation of ERK1/2 signaling downstream of paxillin is indispensable for the malignant transformation of colorectal carcinoma cells.

Mitogen-Activated Protein (MAP) Kinase Scaffolding Proteins: A Recount

The mitogen-activated protein kinase (MAPK) pathway is the canonical signaling pathway for many receptor tyrosine kinases, such as the Epidermal Growth Factor Receptor. Downstream of the receptors, this pathway involves the activation of a kinase cascade that culminates in a transcriptional response and affects processes, such as cell migration and adhesion. In addition, the strength and duration of the upstream signal also influence the mode of the cellular response that is switched on. Thus, the same components can in principle coordinate opposite responses, such as proliferation and differentiation. In recent years, it has become evident that MAPK signaling is regulated and fine-tuned by proteins that can bind to several MAPK signaling proteins simultaneously and, thereby, affect their function. These so-called MAPK scaffolding proteins are, thus, important coordinators of the signaling response in cells. In this review, we summarize the recent advances in the research on MAPK/extracellular signal-regulated kinase (ERK) pathway scaffolders. We will not only review the well-known members of the family, such as kinase suppressor of Ras (KSR), but also put a special focus on the function of the recently identified or less studied scaffolders, such as fibroblast growth factor receptor substrate 2, flotillin-1 and mitogen-activated protein kinase organizer 1.

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.

How scaffolds shape MAPK signaling: what we know and opportunities for systems approaches

Scaffolding proteins add a new layer of complexity to the dynamics of cell signaling. Above their basic function to bring several components of a signaling pathway together, recent experimental research has found that scaffolds influence signaling in a much more complex way: scaffolds can exert some catalytic function, influence signaling by allosteric mechanisms, are feedback-regulated, localize signaling activity to distinct regions of the cell or increase pathway fidelity. Here we review experimental and theoretical approaches that address the function of two MAPK scaffolds, Ste5, a scaffold of the yeast mating pathway and KSR1/2, a scaffold of the classical mammalian MAPK signaling pathway. For the yeast scaffold Ste5, detailed mechanistic models have been valuable for the understanding of its function. For scaffolds in mammalian signaling, however, models have been rather generic and sketchy. For example, these models predicted narrow optimal scaffold concentrations, but when revisiting these models by assuming typical concentrations, rather a range of scaffold levels optimally supports signaling. Thus, more realistic models are needed to understand the role of scaffolds in mammalian signal transduction, which opens a big opportunity for systems biology.

Inhibition of CRM1-mediated nucleocytoplasmic transport: triggering human melanoma cell apoptosis by perturbing multiple cellular pathways

Development of multiple drug resistance mechanisms in melanomas necessitates the identification of new drug targets, which when inhibited could impact multiple cellular pathways, thus circumventing potential resistance. By performing complementary DNA microarray analysis, we identified four key components of the nucleocytoplasmic transport machinery-CRM1, RAN (RAN-GTPase), RANGAP1, and RANBP1-to be overexpressed in human melanoma metastases. Chromosome region maintenance 1 (CRM1) inhibition induced a marked depletion of prosurvival/cytoplasmic extracellular signal-regulated kinase 1/2 (Erk1/2) and p90 ribosomal S6 kinase1 and elicited persistent Erk-signaling hyperactivation. Consistently, CRM1 inhibition inflicted extensive apoptosis in melanoma cells while sparing nontransformed melanocytes and primary lung fibroblasts. Apoptosis required both the intrinsic and extrinsic apoptotic pathways and was associated with a nuclear entrapment and downregulation of the antiapoptotic CRM1 target protein, Survivin. Apoptosis was preceded by a G1 cell-cycle arrest, and even though CRM1 inhibition mediated marked p53 and p21 induction in wild-type p53 melanoma cells, the latter's silencing or inactivation failed to alleviate apoptosis. Notably, CRM1 inhibition induced cell line-specific, G1 to S progression-retarding changes in the expression of multiple cell-cycle regulatory proteins, thus potentially explaining p53 dispensability. We propose CRM1 as a potential therapeutic target in human melanoma, whose inhibition induces loss of prosurvival/cytoplasmic Erk1/2, mediates persistent Erk hyperactivation, and initiates a multitude of cell context-dependent molecular events to trigger G1 arrest followed by massive apoptosis.

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.

Rap1-interacting adapter molecule (RIAM) associates with the plasma membrane via a proximity detector

The Ras association and PH domains of RIAM function as a proximity detector for activated Rap1 and PI(4,5)P₂.

Adaptive immunity depends on lymphocyte adhesion that is mediated by the integrin lymphocyte functional antigen 1 (LFA-1). The small guanosine triphosphatase Rap1 regulates LFA-1 adhesiveness through one of its effectors, Rap1-interacting adapter molecule (RIAM). We show that RIAM was recruited to the lymphocyte plasma membrane (PM) through its Ras association (RA) and pleckstrin homology (PH) domains, both of which were required for lymphocyte adhesion. The N terminus of RIAM inhibited membrane translocation. In vitro, the RA domain bound both Rap1 and H-Ras with equal but relatively low affinity, whereas in vivo only Rap1 was required for PM association. The PH domain bound phosphoinositol 4,5-bisphosphate (PI(4,5)P₂) and was responsible for the spatial distribution of RIAM only at the PM of activated T cells. We determined the crystal structure of the RA and PH domains and found that, despite an intervening linker of 50 aa, the two domains were integrated into a single structural unit, which was critical for proper localization to the PM. Thus, the RA-PH domains of RIAM function as a proximity detector for activated Rap1 and PI(4,5)P₂.

Ras and Rap1 govern spatiotemporal dynamic of activated ERK in pituitary living cells

The Ras/Raf/MEK/ERK is a conserved signalling pathway involved in the control of fundamental cellular processes. Despite extensive research, how this pathway can process a myriad of diverse extracellular inputs into substrate specificity to determine biological outcomes is not fully understood. It has been established that the ERK1/2 pathway is an integrative point in the control of the pituitary function exerted by various extracellular signals. In addition we previously established that the GTPases Ras and Rap1 play a key role in the regulation of ERK1/2-dependent prolactin transcription by EGF or the cAMP-dependent neuropeptide VIP. In this report, using the FRET-based biosensor of ERK activity (EKAR) in the pituitary GH4C1 cell line, we show that both EGF and VIP tightly control the spatiotemporal dynamic of activated ERK with different magnitude and duration. Importantly, we provide the first evidence of a differential control of cytoplasmic and nuclear pools of activated ERK by the GTPases Ras and Rap1. Ras is required for nuclear magnitude and duration of EGF-dependent ERK activation, whereas it is required for both VIP-activated cytoplasmic and nuclear ERK pools. Rap1 is exclusively involved in VIP-activated ERK nuclear pool. Moreover, consistent with the control of the nuclear pool of activated ERK by the GTPases, we observe the same differential role of Ras and Rap1 on ERK nuclear translocation triggered by EGF or VIP. Together these findings identify Ras and Rap1 as determinant partners in shaping nuclear and cytoplasmic ERK kinetics in response to EGF and VIP, which in turn should control pituitary secretion.

Modularity and functional plasticity of scaffold proteins as p(l)acemakers in cell signaling

Cells coordinate and integrate various functional modules that control their dynamics, intracellular trafficking, metabolism and gene expression. Such capacity is mediated by specific scaffold proteins that tether multiple components of signaling pathways at plasma membrane, Golgi apparatus, mitochondria, endoplasmic reticulum, nucleus and in more specialized subcellular structures such as focal adhesions, cell-cell junctions, endosomes, vesicles and synapses. Scaffold proteins act as "pacemakers" as well as "placemakers" that regulate the temporal, spatial and kinetic aspects of protein complex assembly by modulating the local concentrations, proximity, subcellular dispositions and biochemical properties of the target proteins through the intricate use of their modular protein domains. These regulatory mechanisms allow them to gate the specificity, integration and crosstalk of different signaling modules. In addition to acting as physical platforms for protein assembly, many professional scaffold proteins can also directly modify the properties of their targets while they themselves can be regulated by post-translational modifications and/or mechanical forces. Furthermore, multiple scaffold proteins can form alliances of higher-order regulatory networks. Here, we highlight the emerging themes of scaffold proteins by analyzing their common and distinctive mechanisms of action and regulation, which underlie their functional plasticity in cell signaling. Understanding these mechanisms in the context of space, time and force should have ramifications for human physiology and for developing new therapeutic approaches to control pathological states and diseases.

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.

Mouse tissues that undergo neoplastic progression after K-Ras activation are distinguished by nuclear translocation of phospho-Erk1/2 and robust tumor suppressor responses

Mutation of K-Ras is a frequent oncogenic event in human cancers, particularly cancers of lungs, pancreas, and colon. It remains unclear why some tissues are more susceptible to Ras-induced transformation than others. Here, we globally activated a mutant oncogenic K-Ras allele (K-Ras(G12D)) in mice and examined the tissue-specific effects of this activation on cancer pathobiology, Ras signaling, tumor suppressor, DNA damage, and inflammatory responses. Within 5 to 6 weeks of oncogenic Ras activation, mice develop oral and gastric papillomas, lung adenomas, and hematopoietic hyperproliferation and turn moribund. The oral, gastric, and lung premalignant lesions display activated extracellular signal-regulated kinases (Erk)1/2 and NF-κB signaling as well as activated tumor suppressor and DNA damage responses. Other organs such as pancreas, liver, and small intestine do not exhibit neoplastic progression within 6 weeks following K-Ras(G12D) activation and do not show a potent tumor suppressor response. Even though robust Erk1/2 signaling is activated in all the tissues examined, the pErk1/2 distribution remains largely cytoplasmic in K-Ras(G12D)-refractory tissues (pancreas, liver, and intestines) as opposed to a predominantly nuclear localization in K-Ras(G12D)-induced neoplasms of lung, oral, and gastric mucosa. The downstream targets of Ras signaling, pElk-1 and c-Myc, are elevated in K-Ras(G12D)-induced neoplastic lesions but not in K-Ras(G12D)-refractory tissues. We propose that oncogenic K-Ras-refractory tissues delay oncogenic progression by spatially limiting the efficacy of Ras/Raf/Erk1/2 signaling, whereas K-Ras-responsive tissues exhibit activated Ras/Raf/Erk1/2 signaling, rapidly form premalignant tumors, and activate potent antitumor responses that effectively prevent further malignant progression.

Monomeric and dimeric models of ERK2 in conjunction with studies on cellular localization, nuclear translocation, and in vitro analysis

Extracellular signal-regulated protein kinase 2 (ERK2) plays many vital roles in cellular signal regulation. Phosphorylation of ERK2 leads to propagation and execution of various extracellular stimuli, which influence cellular responses to stress. The final response of the ERK2 signaling pathway is determined by localization and duration of active ERK2 at specific target cell compartments through protein-protein interactions of ERK2 with various cytoplasmic and nuclear substrates, scaffold proteins, and anchoring counterparts. In this respect, dimerization of phosphorylated ERK2 has been suggested to be a part of crucial regulating mechanism in various protein-protein interactions. After the report of putative dimeric structure of active ERK2 (Canagarajah et al., 1997), dimeric model was employed to explain many in vivo and in vitro experimental results. But more recently, many reports have been presented questioning the validity of dimer hypothesis of active ERK2. In this review, we summarize the various in vitro and in vivo studies concerning the Monomeric or the dimeric forms of ERK2 and the validity of the dimer hypothesis.

Sef downregulation by Ras causes MEK1/2 to become aberrantly nuclear localized leading to polyploidy and neoplastic transformation

Subcellular trafficking of key oncogenic signal pathway components is likely to be crucial for neoplastic transformation, but little is known about how such trafficking processes are spatially controlled. In this study, we show how Ras activation causes aberrant nuclear localization of phosphorylated mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK; MEK) MEK1/2 to drive neoplastic transformation. Phosphorylated MEK1/2 was aberrantly located within the nucleus of primary colorectal tumors and human colon cancer cells, and oncogenic activation of Ras was sufficient to induce nuclear accumulation of phosphorylated MEK1/2 and ERK1/2 in intestinal epithelial cells. Enforced nuclear localization of MEK1 in epithelial cells or fibroblasts was sufficient for hyperactivation of ERK1/2, thereby driving cell proliferation, chromosomal polyploidy, and tumorigenesis. Notably, Ras-induced nuclear accumulation of activated MEK1/2 was reliant on downregulation of the spatial regulator Sef, the reexpression of which was sufficient to restore normal MEK1/2 localization and a reversal of Ras-induced proliferation and tumorigenesis. Taken together, our findings indicate that Ras-induced downregulation of Sef is an early oncogenic event that contributes to genetic instability and tumor progression by sustaining nuclear ERK1/2 signaling.

The ERK Cascade: Distinct Functions within Various Subcellular Organelles

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is a central signaling pathway that regulates a wide variety of stimulated cellular processes, including mainly proliferation, differentiation, and survival, but apoptosis and stress response as well. The ability of this linear cascade to induce so many distinct and even opposing effects after various stimulations raises the question as to how the signaling specificity of the cascade is regulated. Over the past years, several specificity-mediating mechanisms have been elucidated, including temporal regulation, scaffolding interactions, crosstalks with other signaling components, substrate competition, and multiple components in each tier of the cascade. In addition, spatial regulation of various components of the cascade is probably one of the main ways by which signals can be directed to some downstream targets and not to others. In this review, we describe first the components of the ERK1/2 cascade and their mode of regulation by kinases, phosphatases, and scaffold proteins. In the second part, we focus on the role of MEK1/2 and ERK1/2 compartmentalization in the nucleus, mitochondria, endosomes, plasma membrane, cytoskeleton, and Golgi apparatus. We explain that this spatial distribution may direct ERK1/2 signals to regulate the organelles' activities. However, it can also direct the activity of the cascade's components to the outer surface of the organelles in order to bring them to close proximity to specific cytoplasmic targets. We conclude that the dynamic localization of the ERK1/2 cascade components is an important regulatory mechanism in determining the signaling specificity of the cascade, and its understanding should shed a new light on the understanding of many stimulus-dependent processes.

Ras, an actor on many stages: posttranslational modifications, localization, and site-specified events

Among the wealth of information that we have gathered about Ras in the past decade, the introduction of the concept of space in the field has constituted a major revolution that has enabled many pieces of the Ras puzzle to fall into place. In the early days, it was believed that Ras functioned exclusively at the plasma membrane. Today, we know that within the plasma membrane, the 3 Ras isoforms-H-Ras, K-Ras, and N-Ras-occupy different microdomains and that these isoforms are also present and active in endomembranes. We have also discovered that Ras proteins are not statically associated with these localizations; instead, they traffic dynamically between compartments. And we have learned that at these localizations, Ras is under site-specific regulatory mechanisms, distinctively engaging effector pathways and switching on diverse genetic programs to generate different biological responses. All of these processes are possible in great part due to the posttranslational modifications whereby Ras proteins bind to membranes and to regulatory events such as phosphorylation and ubiquitination that Ras is subject to. As such, space and these control mechanisms act in conjunction to endow Ras signals with an enormous signal variability that makes possible its multiple biological roles. These data have established the concept that the Ras signal, instead of being one single, homogeneous entity, results from the integration of multiple, site-specified subsignals, and Ras has become a paradigm of how space can differentially shape signaling.

Ras trafficking, localization and compartmentalized signalling

Ras proteins are proto-oncogenes that are frequently mutated in human cancers. Three closely related isoforms, HRAS, KRAS and NRAS, are expressed in all cells and have overlapping but distinctive functions. Recent work has revealed how differences between the Ras isoforms in their trafficking, localization and protein-membrane orientation enable signalling specificity to be determined. We review the various strategies used to characterize compartmentalized Ras localization and signalling. Localization is an important contextual modifier of signalling networks and insights from the Ras system are of widespread relevance for researchers interested in signalling initiated from membranes.

Ca-dependent Ras/Erk signaling mediates negative selection of autoreactive B cells

Signaling via the Ras/Erk pathway has long been recognized to be critical in lymphocyte development and function, yet the mechanisms that control the distinct functional outputs of this pathway in different cellular contexts remain poorly understood. Our recent results have demonstrated unexpected involvement of Ras/Erk signaling in the sensitization of B cells to apoptosis in order to eliminate autoreactive cells. Increases in cytosolic Ca(2+) are necessary and sufficient to induce activation of this Ras/Erk pathway, and the biochemical events involved in its activation are different from the ones involved in diacylglycerol (DAG)-mediated Ras/Erk activation. Developmental regulation of upstream mediators of these distinct pathways contributes to their predominant activation at different stages of B cell development. These findings have revealed a mechanism by which antigen stimulation can activate distinct Ras/Erk pathways at different developmental stages to mediate appropriate functional outputs that control the selection, development and activation of B cells. Despite these recent findings, however, much remains to be learned about the molecular mechanisms that confer functional specificity to common Ras/Erk signaling modules.

Aconitase regulation of erythropoiesis correlates with a novel licensing function in erythropoietin-induced ERK signaling

BACKGROUND

Erythroid development requires the action of erythropoietin (EPO) on committed progenitors to match red cell output to demand. In this process, iron acts as a critical cofactor, with iron deficiency blunting EPO-responsiveness of erythroid progenitors. Aconitase enzymes have recently been identified as possible signal integration elements that couple erythropoiesis with iron availability. In the current study, a regulatory role for aconitase during erythropoiesis was ascertained using a direct inhibitory strategy.

METHODOLOGY/PRINCIPAL FINDINGS

In C57BL/6 mice, infusion of an aconitase active-site inhibitor caused a hypoplastic anemia and suppressed responsiveness to hemolytic challenge. In a murine model of polycythemia vera, aconitase inhibition rapidly normalized red cell counts, but did not perturb other lineages. In primary erythroid progenitor cultures, aconitase inhibition impaired proliferation and maturation but had no effect on viability or ATP levels. This inhibition correlated with a blockade in EPO signal transmission specifically via ERK, with preservation of JAK2-STAT5 and Akt activation. Correspondingly, a physical interaction between ERK and mitochondrial aconitase was identified and found to be sensitive to aconitase inhibition.

CONCLUSIONS/SIGNIFICANCE

Direct aconitase inhibition interferes with erythropoiesis in vivo and in vitro, confirming a lineage-selective regulatory role involving its enzymatic activity. This inhibition spares metabolic function but impedes EPO-induced ERK signaling and disturbs a newly identified ERK-aconitase physical interaction. We propose a model in which aconitase functions as a licensing factor in ERK-dependent proliferation and differentiation, thereby providing a regulatory input for iron in EPO-dependent erythropoiesis. Directly targeting aconitase may provide an alternative to phlebotomy in the treatment of polycythemia vera.

Raft protein clustering alters N-Ras membrane interactions and activation pattern

The trafficking, membrane localization, and lipid raft association of Ras proteins, which are crucial oncogenic mediators, dictate their isoform-specific biological responses. Accordingly, their spatiotemporal dynamics are tightly regulated. While extensively studied for H- and K-Ras, such information on N-Ras, an etiological oncogenic factor, is limited. Here, we report a novel mechanism regulating the activation-dependent spatiotemporal organization of N-Ras, its modulation by biologically relevant stimuli, and isoform-specific effects on signaling. We combined patching/immobilization of another membrane protein with fluorescence recovery after photobleaching (patch-FRAP) and FRAP beam size analysis to investigate N-Ras membrane interactions. Clustering of raft-associated proteins, either glycosylphosphatidylinositol-anchored influenza virus hemagglutinin (HA-GPI) or fibronectin receptors, selectively enhanced the plasma membrane-cytoplasm exchange of N-Ras-GTP (preferentially associated with raft domains) in a cholesterol-dependent manner. Electron microscopy (EM) analysis showed N-Ras-GTP localization in cholesterol-sensitive clusters, from which it preferentially detached upon HA-GPI cross-linking. HA-GPI clustering enhanced the Golgi compartment (GC) accumulation and signaling of epidermal growth factor (EGF)-stimulated N-Ras-GTP. Notably, the cross-linking-mediated enhancement of N-Ras-GTP exchange and GC accumulation depended strictly on depalmitoylation. We propose that the N-Ras activation pattern (e.g., by EGF) is altered by raft protein clustering, which enhances N-Ras-GTP raft localization and depalmitoylation, entailing its exchange and GC accumulation following repalmitoylation. This mechanism demonstrates a functional signaling role for the activation-dependent differential association of Ras isoforms with raft nanodomains.

Remodeling of the Golgi structure by ERK signaling

Emerging evidence suggests that the Golgi functions as a regulatory node for various signaling cascades. Modules of the MAPK pathway are targeted to the Golgi upon stimulation of cells with mitogens. The target for activated ERK on the Golgi membranes is GRASP65, a peripheral membrane protein required for Golgi cisternal stacking. Phosphorylation of GRASP65 at Serine 277 results in a loss of its oligomerization and causes unstacking of Golgi cisternae. This reorganization of the Golgi structure is required for the polarization of the Golgi and the centrosomes towards the leading edge in migrating cells. Preventing GRASP65 phosphorylation with mutants lacking the phosphorylation site blocks Golgi and centrosome orientation. This demonstrates a mechanism for cell polarization involving dynamic remodeling of the Golgi mediated by local phosphorylation of a Golgi protein induced by mitogen signaling.

PPARgamma and MEK Interactions in Cancer

Peroxisome proliferator-activated receptor-gamma (PPARγ) exerts multiple functions in determination of cell fate, tissue metabolism, and host immunity. Two synthetic PPARγ ligands (rosiglitazone and pioglitazone) were approved for the therapy of type-2 diabetes mellitus and are expected to serve as novel cures for inflammatory diseases and cancer. However, PPARγ and its ligands exhibit a janus-face behaviour as tumor modulators in various systems, resulting in either tumor suppression or tumor promotion. This may be in part due to signaling crosstalk to the mitogen-activated protein kinase (MAPK) cascades. The genomic activity of PPARγ is modulated, in addition to ligand binding, by phosphorylation of a serine residue by MAPKs, such as extracellular signal-regulated protein kinases-1/2 (ERK-1/2), or by nucleocytoplasmic compartmentalization through the ERK activators MAPK kinases-1/2 (MEK-1/2). PPARγ ligands themselves activate the ERK cascade through nongenomic and often PPARγ-independent signaling. In the current review, we discuss the molecular mechanisms and physiological implications of the crosstalk of PPARγ with MEK-ERK signaling and its potential as a novel drug target for cancer therapy in patients.

Involvement of protein kinase Cdelta/extracellular signal-regulated kinase/poly(ADP-ribose) polymerase-1 (PARP-1) signaling pathway in histamine-induced up-regulation of histamine H1 receptor gene expression in HeLa cells

The histamine H(1) receptor (H1R) gene is up-regulated in patients with allergic rhinitis. However, the mechanism and reason underlying this up-regulation are still unknown. Recently, we reported that the H1R expression level is strongly correlated with the severity of allergic symptoms. Therefore, understanding the mechanism of this up-regulation will help to develop new anti-allergic drugs targeted for H1R gene expression. Here we studied the molecular mechanism of H1R up-regulation in HeLa cells that express H1R endogenously in response to histamine and phorbol 12-myristate 13-acetate (PMA). In HeLa cells, histamine stimulation caused up-regulation of H1R gene expression. Rottlerin, a PKCδ-selective inhibitor, inhibited up-regulation of H1R gene expression, but Go6976, an inhibitor of Ca(2+)-dependent PKCs, did not. Histamine or PMA stimulation resulted in PKCδ phosphorylation at Tyr(311) and Thr(505). Activation of PKCδ by H(2)O(2) resulted in H1R mRNA up-regulation. Overexpression of PKCδ enhanced up-regulation of H1R gene expression, and knockdown of the PKCδ gene suppressed this up-regulation. Histamine or PMA caused translocation PKCδ from the cytosol to the Golgi. U0126, an MEK inhibitor, and DPQ, a poly(ADP-ribose) polymerase-1 inhibitor, suppressed PMA-induced up-regulation of H1R gene expression. These results were confirmed by a luciferase assay using the H1R promoter. Phosphorylation of ERK and Raf-1 in response to PMA was also observed. However, real-time PCR analysis showed no inhibition of H1R mRNA up-regulation by a Raf-1 inhibitor. These results suggest the involvement of the PKCδ/ERK/poly(ADP-ribose) polymerase-1 signaling pathway in histamine- or PMA-induced up-regulation of H1R gene expression in HeLa cells.

Similar expression to FGF (Sef) reduces endometrial adenocarcinoma cells proliferation via inhibiting fibroblast growth factor 2-mediated MAPK/ERK signaling pathway

OBJECTIVE

Fibroblast growth factors (FGF) axis, and in particular FGF2 axis, is an important mitogenic stimulus in endometrial carcinogenesis. hSef is a key inhibitory regulator of FGF signaling and aberrant hSef expression is reported to be present in various human carcinomas. The objective of this study was to investigate the role of hSef in the growth and proliferation of endometrial adenocarcinoma cells and to explore the mechanism that may be involved.

METHODS

Using western blot analysis, we determined the expression of hSef in Ishikawa cells under different conditions. Using luciferase reporter assays and western blot analysis, we detected the effect of hSef on MAPK/ERK-mediated FGF2 signaling. Using MTT, cell counting and colony formation assays, we analyzed the growth and proliferation of Ishikawa cells under different conditions.

RESULTS

We found that the hSef expression was positively regulated by FGF2-induced MAPK/ERK signaling and inversely, hSef expression efficiently inhibited the activity of FGF2-induced MAPK/ERK signaling, indicating the presence of hSef-mediated negative feedback mechanism for FGF signaling in endometrial cancer cells. In addition, we found that MAPK/ERK signaling was essential for the growth and proliferation of endometrial cancer cells in vitro, and hSef expression significantly reduced the cell proliferation.

CONCLUSIONS

hSef expression can inhibit the growth and proliferation of endometrial cancer cells via acting on the FGF2/MAPK/ERK signaling.

Pro-apoptotic activity of inhibitory PAS domain protein (IPAS), a negative regulator of HIF-1, through binding to pro-survival Bcl-2 family proteins

Inhibitory PAS (Per/Arnt/Sim) domain protein (IPAS) is a dominant negative transcription factor that represses hypoxia-inducible factor 1 (HIF-1) activity. In this study, we show that IPAS also functions as a pro-apoptotic protein through binding to pro-survival Bcl-2 family members. In a previous paper, we reported that NF-κB-dependent IPAS induction by cobalt chloride repressed the hypoxic response in PC12 cells. We found that prolonged incubation under the same conditions caused apoptosis in PC12 cells. Repression of IPAS induction protected cells from apoptosis. Furthermore, knockdown of IPAS recovered cell viability. EGFP-IPAS protein was localized in both the nucleus and the cytoplasm, with a large fraction associated with mitochondria. Mitochondrial IPAS induced mitochondria depolarization and caspase-3 activation. Immunoprecipitation assays revealed that IPAS is associated with Bcl-x(L), Bcl-w and Mcl-1. The association of IPAS with Bcl-x(L) was also observed in living cells by the FLIM-based FRET analysis, indicating direct binding between the two proteins. IPAS contributed to dysfunction of Bcl-x(L) by inhibiting the interaction of Bcl-x(L) with Bax. These results demonstrate that IPAS functions as a dual function protein involved in transcription repression and apoptosis.

Signalling to and from the secretory pathway

For growth, survival, communication and homeostasis, cells transport a large number of proteins to the plasma membrane and the extracellular medium by using the secretory pathway. Consequently, to adapt to the surrounding environment and the different intracellular contexts, the secretory pathway needs to accommodate and respond to a plethora of endogenous and exogenous stimuli. It is now well established that several kinases, known to be activated by environmental stimuli, signal from the plasma membrane to the secretory pathway in order to remodel its architecture and modulate the cellular secretion capacity. By contrast, membranes of the early secretory pathway, similar to the endosomal system, can also initiate and modulate signalling cascades, thereby spatially organising cellular signalling and eliciting a different cellular outcome than when signalling is localised to the plasma membrane. This Commentary highlights recent contributions to our understanding of the mutual regulation of the secretory pathway and cellular signalling.

Raf family kinases: old dogs have learned new tricks

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.