The CNK1 scaffold binds cytohesins and promotes insulin pathway signaling

Myelination by signaling through Arf guanine nucleotide exchange factor

During myelination, large quantities of proteins are synthesized and transported from the endoplasmic reticulum (ER)-trans-Golgi network (TGN) to their appropriate locations within the intracellular region and/or plasma membrane. It is widely believed that oligodendrocytes uptake neuronal signals from neurons to regulate the endocytosis- and exocytosis-mediated intracellular trafficking of major myelin proteins such as myelin-associated glycoprotein (MAG) and proteolipid protein 1 (PLP1). The small GTPases of the adenosine diphosphate (ADP) ribosylation factor (Arf) family constitute a large group of signal transduction molecules that act as regulators for intracellular signaling, vesicle sorting, or membrane trafficking in cells. Studies on mice deficient in Schwann cell-specific Arfs-related genes have revealed abnormal myelination formation in peripheral nerves, indicating that Arfs-mediated signaling transduction is required for myelination in Schwann cells. However, the complex roles in these events remain poorly understood. This review aims to provide an update on signal transduction, focusing on Arf and its activator ArfGEF (guanine nucleotide exchange factor for Arf) in oligodendrocytes and Schwann cells. Future studies are expected to provide important information regarding the cellular and physiological processes underlying the myelination of oligodendrocytes and Schwann cells and their function in modulating neural activity.

CNK2 promotes cancer cell motility by mediating ARF6 activation downstream of AXL signalling

Cell motility is a critical feature of invasive tumour cells that is governed by complex signal transduction events. Particularly, the underlying mechanisms that bridge extracellular stimuli to the molecular machinery driving motility remain partially understood. Here, we show that the scaffold protein CNK2 promotes cancer cell migration by coupling the pro-metastatic receptor tyrosine kinase AXL to downstream activation of ARF6 GTPase. Mechanistically, AXL signalling induces PI3K-dependent recruitment of CNK2 to the plasma membrane. In turn, CNK2 stimulates ARF6 by associating with cytohesin ARF GEFs and with a novel adaptor protein called SAMD12. ARF6-GTP then controls motile forces by coordinating the respective activation and inhibition of RAC1 and RHOA GTPases. Significantly, genetic ablation of CNK2 or SAMD12 reduces metastasis in a mouse xenograft model. Together, this work identifies CNK2 and its partner SAMD12 as key components of a novel pro-motility pathway in cancer cells, which could be targeted in metastasis.

ARNO is recruited by the neuronal adaptor FE65 to potentiate ARF6-mediated neurite outgrowth

ADP-ribosylation factor 6 (ARF6) is a small GTPase that has a variety of neuronal functions including stimulating neurite outgrowth, a crucial process for the establishment and maintenance of neural connectivity. As impaired and atrophic neurites are often observed in various brain injuries and neurological diseases, understanding the intrinsic pathways that stimulate neurite outgrowth may provide insights into developing strategies to trigger the reconnection of injured neurons. The neuronal adaptor FE65 has been shown to interact with ARF6 and potentiate ARF6-mediated neurite outgrowth. However, the precise mechanism that FE65 activates ARF6 remains unclear, as FE65 does not possess a guanine nucleotide exchange factor (GEF) domain/function. Here, we show that FE65 interacts with the ARF6 GEF, namely the ARF nucleotide-binding site opener (ARNO). Moreover, a complex consisting of ARNO, ARF6 and FE65 is detected. Notably, FE65 potentiates the stimulatory effect of ARNO on ARF6-mediated neurite outgrowth, and the effect of FE65 is abrogated by an FE65 mutation that disrupts FE65–ARNO interaction. Additionally, the intramolecular interaction for mediating the autoinhibited conformation of ARNO is attenuated by FE65. Moreover, FE65 potentiates the effects of wild-type ARNO, but not the monomeric mutant, suggesting an association between FE65 and ARNO dimerization. Collectively, we demonstrate that FE65 binds to and activates ARNO and, consequently, potentiates ARF6-mediated neurite outgrowth.

Physiological and Pathological Roles of the Cytohesin Family in Neurons

The cytohesin proteins, consisting of four closely related members (cytohesins-1, -2, -3, and -4), are a subfamily of the Sec7 domain-containing guanine nucleotide exchange factors for ADP ribosylation factors (Arfs), which are critical regulators of membrane trafficking and actin cytoskeleton remodeling. Recent advances in molecular biological techniques and the development of a specific pharmacological inhibitor for cytohesins, SecinH3, have revealed the functional involvement of the cytohesin-Arf pathway in diverse neuronal functions from the formation of axons and dendrites, axonal pathfinding, and synaptic vesicle recycling, to pathophysiological processes including chronic pain and neurotoxicity induced by proteins related to neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer's disease. Here, we review the physiological and pathological roles of the cytohesin-Arf pathway in neurons and discuss the future directions of this research field.

Akt scaffold proteins: the key to controlling specificity of Akt signaling

The phosphatidylinositol 3-kinase-Akt signaling pathway plays an essential role in regulating cell proliferation and apoptosis. Akt kinase is at the center of this signaling pathway and interacts with a variety of proteins. Akt is overexpressed in almost 80% of tumors. However, inhibiting Akt has serious clinical side effects so is not a suitable treatment for cancer. During recent years, Akt scaffold proteins have received increasing attention for their ability to regulate Akt signaling and have emerged as potential targets for cancer therapy. In this paper, we categorize Akt kinase scaffold proteins into four groups based on their cellular location: membrane-bound activator and inhibitor, cytoplasm, and endosome. We describe how these scaffolds interact with Akt kinase, how they affect Akt activity, and how they regulate the specificity of Akt signaling. We also discuss the clinical application of Akt scaffold proteins as targets for cancer therapy.

Disease-associated synaptic scaffold protein CNK2 modulates PSD size and influences localisation of the regulatory kinase TNIK

Scaffold proteins are responsible for structural organisation within cells; they form complexes with other proteins to facilitate signalling pathways and catalytic reactions. The scaffold protein connector enhancer of kinase suppressor of Ras 2 (CNK2) is predominantly expressed in neural tissues and was recently implicated in X-linked intellectual disability (ID). We have investigated the role of CNK2 in neurons in order to contribute to our understanding of how CNK2 alterations might cause developmental defects, and we have elucidated a functional role for CNK2 in the molecular processes that govern morphology of the postsynaptic density (PSD). We have also identified novel CNK2 interaction partners and explored their functional interdependency with CNK2. We focussed on the novel interaction partner TRAF2- and NCK-interacting kinase TNIK, which is also associated with ID. Both CNK2 and TNIK are expressed in neuronal dendrites and concentrated in dendritic spines, and staining with synaptic markers indicates a clear postsynaptic localisation. Importantly, our data highlight that CNK2 plays a role in directing TNIK subcellular localisation, and in neurons, CNK2 participates in ensuring that this multifunctional kinase is present in the correct place at desirable levels. In summary, our data indicate that CNK2 expression is critical for modulating PSD morphology; moreover, our study highlights that CNK2 functions as a scaffold with the potential to direct the localisation of regulatory proteins within the cell. Importantly, we describe a novel link between CNK2 and the regulatory kinase TNIK, and provide evidence supporting the idea that alterations in CNK2 localisation and expression have the potential to influence the behaviour of TNIK and other important regulatory molecules in neurons.

Arf-GEF localization and function at myosin-rich adherens junctions via coiled-coil heterodimerization with an adaptor protein

Tissue dynamics require regulated interactions between adherens junctions and cytoskeletal networks. For example, myosin-rich adherens junctions recruit the cytohesin Arf-GEF Steppke, which down-regulates junctional tension and facilitates tissue stretching. We dissected this recruitment mechanism with structure–function and other analyses of Steppke and Stepping stone, an implicated adaptor protein. During Drosophila dorsal closure, Steppke’s coiled-coil domain was necessary and sufficient for junctional recruitment. Purified coiled-coil domains of Steppke and Stepping stone heterodimerized through a hydrophobic surface of the Steppke domain. This mapped surface was required for Steppke’s junctional localization and tissue regulation. Stepping stone colocalized with Steppke at junctions, and was required for junctional Steppke localization and proper tissue stretching. A second conserved region of Stepping stone was necessary and largely sufficient for junctional localization. Remarkably, this region could substitute for the Steppke coiled-coil domain for junction localization and regulation, suggesting the main role of the Steppke coiled-coil domain is linkage to the junctional targeting region of Stepping stone. Thus, coiled-coil heterodimerization with Stepping stone normally recruits Step to junctions. Intriguingly, Stepping stone’s junctional localization also seems partly dependent on Steppke.

Insulin/IGF signaling and discoidin domain receptors: An emerging functional connection

The insulin/insulin-like growth factor system (IIGFs) plays a fundamental role in the regulation of prenatal and postnatal growth, metabolism and homeostasis. As a consequence, dysregulation of this axis is associated with growth disturbance, type 2 diabetes, chronic inflammation and tumor progression. A functional crosstalk between IIGFs and discoidin domain receptors (DDRs) has been recently discovered. DDRs are non-integrin collagen receptors that canonically undergo slow and long-lasting autophosphorylation after binding to fibrillar collagen. While both DDR1 and DDR2 functionally interact with IIGFs, the crosstalk with DDR1 is so far better characterized. Notably, the IIGFs-DDR1 crosstalk presents a feed-forward mechanism, which does not require collagen binding, thus identifying novel non-canonical action of DDR1. Further studies are needed to fully explore the role of this IIGFs-DDRs functional loop as potential target in the treatment of inflammatory and neoplastic disorders.

Inhibition of the IGF-1-PI3K-Akt-mTORC2 pathway in lipid rafts increases neuronal vulnerability in a genetic lysosomal glycosphingolipidosis

Glycosphingolipid (GSL) accumulation is implicated in the neuropathology of several lysosomal conditions, such as Krabbe disease, and may also contribute to neuronal and glial dysfunction in adult-onset conditions such as Parkinson's disease, Alzheimer's disease and multiple sclerosis. GSLs accumulate in cellular membranes and disrupt their structure; however, how membrane disruption leads to cellular dysfunction remains unknown. Using authentic cellular and animal models for Krabbe disease, we provide a mechanism explaining the inactivation of lipid raft (LR)-associated IGF-1-PI3K-Akt-mTORC2, a pathway of crucial importance for neuronal function and survival. We show that psychosine, the GSL that accumulates in Krabbe disease, leads to a dose-dependent LR-mediated inhibition of this pathway by uncoupling IGF-1 receptor phosphorylation from downstream Akt activation. This occurs by interfering with the recruitment of PI3K and mTORC2 to LRs. Akt inhibition can be reversed by sustained IGF-1 stimulation, but only during a time window before psychosine accumulation reaches a threshold level. Our study shows a previously unknown connection between LR-dependent regulation of mTORC2 activity at the cell surface and a genetic neurodegenerative disease. Our results show that LR disruption by psychosine desensitizes cells to extracellular growth factors by inhibiting signal transmission from the plasma membrane to intracellular compartments. This mechanism serves also as a mechanistic model to understand how alterations of the membrane architecture by the progressive accumulation of lipids undermines cell function, with potential implications in other genetic sphingolipidoses and adult neurodegenerative conditions. This article has an associated First Person interview with the first author of the paper.

ARF GTPases and their GEFs and GAPs: concepts and challenges

Detailed structural, biochemical, cell biological, and genetic studies of any gene/protein are required to develop models of its actions in cells. Studying a protein family in the aggregate yields additional information, as one can include analyses of their coevolution, acquisition or loss of functionalities, structural pliability, and the emergence of shared or variations in molecular mechanisms. An even richer understanding of cell biology can be achieved through evaluating functionally linked protein families. In this review, we summarize current knowledge of three protein families: the ARF GTPases, the guanine nucleotide exchange factors (ARF GEFs) that activate them, and the GTPase-activating proteins (ARF GAPs) that have the ability to both propagate and terminate signaling. However, despite decades of scrutiny, our understanding of how these essential proteins function in cells remains fragmentary. We believe that the inherent complexity of ARF signaling and its regulation by GEFs and GAPs will require the concerted effort of many laboratories working together, ideally within a consortium to optimally pool information and resources. The collaborative study of these three functionally connected families (≥70 mammalian genes) will yield transformative insights into regulation of cell signaling.

An Inhibitor of the Pleckstrin Homology Domain of CNK1 Selectively Blocks the Growth of Mutant KRAS Cells and Tumors

Cnk1 (connector enhancer of kinase suppressor of Ras 1) is a pleckstrin homology (PH) domain-containing scaffold protein that increases the efficiency of Ras signaling pathways, imparting efficiency and specificity to the response of cell proliferation, survival, and migration. Mutated KRAS (mut-KRAS) is the most common proto-oncogenic event, occurring in approximately 25% of human cancers and has no effective treatment. In this study, we show that selective inhibition of Cnk1 blocks growth and Raf/Mek/Erk, Rho and RalA/B signaling in mut-KRAS lung and colon cancer cells with little effect on wild-type (wt)-KRAS cells. Cnk1 inhibition decreased anchorage-independent mut-KRas cell growth more so than growth on plastic, without the partial "addiction" to mut-KRAS seen on plastic. The PH domain of Cnk1 bound with greater affinity to PtdIns(4,5)P2 than PtdIns(3,4,5)P3, and Cnk1 localized to areas of the plasma membranes rich in PtdIns, suggesting a role for the PH domain in the biological activity of Cnk1. Through molecular modeling and structural modification, we identified a compound PHT-7.3 that bound selectively to the PH domain of Cnk1, preventing plasma membrane colocalization with mut-KRas. PHT-7.3 inhibited mut-KRas, but not wild-type KRas cancer cell and tumor growth and signaling. Thus, the PH domain of Cnk1 is a druggable target whose inhibition selectively blocks mutant KRas activation, making Cnk1 an attractive therapeutic target in patients with mut-KRAS-driven cancer. SIGNIFICANCE: These findings identify a therapeutic strategy to selectively block oncogenic KRas activity through the PH domain of Cnk1, which reduces its cell membrane binding, decreasing the efficiency of Ras signaling and tumor growth.

Cytohesin-3 is required for full insulin receptor signaling and controls body weight via lipid excretion

Insulin plays a central role in regulating metabolic homeostasis and guanine-nucleotide exchange factors of the cytohesin family have been suggested to be involved in insulin signal transduction. The Drosophila homolog of cytohesin-3, steppke, has been shown to be essential for insulin signaling during larval development. However, genetic evidence for the functional importance of cytohesin-3 in mammals is missing. We therefore analyzed the consequences of genetic cytohesin-3-deficiency on insulin signaling and function in young and aged mice, using normal chow or high-fat diet (HFD). Insulin-receptor dependent signaling events are significantly reduced in liver and adipose tissue of young cytohesin-3-deficient mice after insulin-injection, although blood glucose levels and other metabolic parameters remain normal in these animals. Interestingly, however, cytohesin-3-deficient mice showed a reduced age- and HFD-induced weight gain with a significant reduction of body fat compared to wild-type littermates. Furthermore, cytohesin-3-deficient mice on HFD displayed no alterations in energy expenditure, but had an increased lipid excretion instead, as well as a reduced expression of genes essential for bile acid synthesis. Our findings show for the first time that an intact cyth3 locus is required for full insulin signaling in mammals and might constitute a novel therapeutic target for weight reduction.

Pallidin is a novel interacting protein for cytohesin-2 and regulates the early endosomal pathway and dendritic formation in neurons

Cytohesin-2 is a member of the guanine nucleotide exchange factors for ADP ribosylation factor 1 (Arf1) and Arf6, which are small GTPases that regulate membrane traffic and actin dynamics. In this study, we first demonstrated that cytohesin-2 localized to the plasma membrane and vesicles in various subcellular compartment in hippocampal neurons by immunoelectron microscopy. Next, to understand the molecular network of cytohesin-2 in neurons, we conducted yeast two-hybrid screening of brain cDNA libraries using cytohesin-2 as bait and isolated pallidin, a component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1) involved in endosomal trafficking. Pallidin interacted specifically with cytohesin-2 among cytohesin family members. Glutathione S-transferase pull-down and immunoprecipitation assays further confirmed the formation of a protein complex between cytohesin-2 and pallidin. Immunofluorescence demonstrated that cytohesin-2 and pallidin partially colocalized in various subsets of endosomes immunopositive for EEA1, syntaxin 12, and LAMP2 in hippocampal neurons. Knockdown of pallidin or cytohesin-2 reduced cytoplasmic EEA1-positive early endosomes. Furthermore, knockdown of pallidin increased the total dendritic length of cultured hippocampal neurons, which was rescued by co-expression of wild-type pallidin but not a mutant lacking the ability to interact with cytohesin-2. In contrast, knockdown of cytohesin-2 had the opposite effect on total dendritic length. The present results suggested that the interaction between pallidin and cytohesin-2 may participate in various neuronal functions such as endosomal trafficking and dendritic formation in hippocampal neurons. Cover Image for this issue: doi: 10.1111/jnc.14197.

MARK3-mediated phosphorylation of ARHGEF2 couples microtubules to the actin cytoskeleton to establish cell polarity

The PAR-1-MARK pathway controls cell polarity through the phosphorylation of microtubule-associated proteins. Rho-Rac guanine nucleotide exchange factor 2 (ARHGEF2), which activates Ras homolog family member A (RHOA), is anchored to the microtubule network and sequestered in an inhibited state through binding to dynein light chain Tctex-1 type 1 (DYNLT1). We showed in mammalian cells that liver kinase B1 (LKB1) activated the microtubule affinity-regulating kinase 3 (MARK3), which in turn phosphorylated ARHGEF2 at Ser¹⁵¹ This modification disrupted the interaction between ARHGEF2 and DYNLT1 by generating a 14-3-3 binding site in ARHGEF2, thus causing ARHGEF2 to dissociate from microtubules. Phosphorylation of ARHGEF2 by MARK3 stimulated RHOA activation and the formation of stress fibers and focal adhesions, and was required for organized cellular architecture in three-dimensional culture. Protein phosphatase 2A (PP2A) dephosphorylated Ser¹⁵¹ in ARHGEF2 to restore the inhibited state. Thus, we have identified a regulatory switch controlled by MARK3 that couples microtubules to the actin cytoskeleton to establish epithelial cell polarity through ARHGEF2.

Membrane localization of acetylated CNK1 mediates a positive feedback on RAF/ERK signaling

Spatiotemporal control is a common mechanism that modulates activity and function of signal transducers in the signaling network. We identified acetylation of CNK1 (connector enhancer of kinase suppressor of Ras-1) as a late step in the activation of CNK1 signaling, accompanied with prolonged stimulation of extracellular signal-regulated kinase (ERK). We identified the acetyltransferase CREB (cyclic adenosine 3',5'-monophosphate response element-binding protein)-binding protein and the deacetylase SIRT2 (sirtuin type 2) as novel binding partners of CNK1, modulating the acetylation state of CNK1. Acetylation of CNK1 at position Lys414 located in the pleckstrin homology domain drives membrane localization of CNK1 in growth factor-stimulated cells. Inhibition of ERK signaling abolishes CNK1 acetylation. Cosmic database search identified CNK1 mutants at position Arg426 near the acetylation site in several human tumor types. These mutants show constitutive acetylation and membrane localization. CNK1 mutants substituting Arg426, the acetylation mimetic mutant CNK1-K414Q, and membrane-anchored CNK1 mutants all interact with the protein kinase CRAF and stimulate ERK-dependent cell proliferation and cell migration. In RAS-transformed cells, CNK1 is acetylated and membrane-bound and drives cell proliferation. Thus, growth factor-stimulated ERK signaling induces CNK1 acetylation, and acetylated CNK1 promotes ERK signaling, demonstrating a novel function of CNK1 as positive feedback regulator of the RAF/MEK (mitogen-activated protein kinase kinase)/ERK pathway. In addition, acetylation of CNK1 is an important step in oncogenic signaling, promoting cell proliferation and migration.

Absence of IQGAP1 Protein Leads to Insulin Resistance

Insulin binds to the insulin receptor (IR) and induces tyrosine phosphorylation of the receptor and insulin receptor substrate-1 (IRS-1), leading to activation of the PKB/Akt and MAPK/ERK pathways. IQGAP1 is a scaffold protein that interacts with multiple binding partners and integrates diverse signaling cascades. Here we show that IQGAP1 associates with both IR and IRS-1 and influences insulin action. In vitro analysis with pure proteins revealed that the IQ region of IQGAP1 binds directly to the intracellular domain of IR. Similarly, the phosphotyrosine-binding domain of IRS-1 mediates a direct interaction with the C-terminal tail of IQGAP1. Consistent with these observations, both IR and IRS-1 co-immunoprecipitated with IQGAP1 from cells. Investigation of the functional effects of the interactions revealed that in the absence of IQGAP1, insulin-stimulated phosphorylation of Akt and ERK, as well as the association of phosphatidylinositol 3-kinase with IRS-1, were significantly decreased. Importantly, loss of IQGAP1 results in impaired insulin signaling and glucose homeostasis in vivo Collectively, these data reveal that IQGAP1 is a scaffold for IR and IRS-1 and implicate IQGAP1 as a participant in insulin signaling.

Optogenetic clustering of CNK1 reveals mechanistic insights in RAF and AKT signalling controlling cell fate decisions

Scaffold proteins such as the multidomain protein CNK1 orchestrate the signalling network by integrating and controlling the underlying pathways. Using an optogenetic approach to stimulate CNK1 uncoupled from upstream effectors, we identified selective clusters of CNK1 that either stimulate RAF-MEK-ERK or AKT signalling depending on the light intensity applied. OptoCNK1 implemented in MCF7 cells induces differentiation at low light intensity stimulating ERK activity whereas stimulation of AKT signalling by higher light intensity promotes cell proliferation. CNK1 clustering in response to increasing EGF concentrations revealed that CNK1 binds to RAF correlating with ERK activation at low EGF dose. At higher EGF dose active AKT binds to CNK1 and phosphorylates and inhibits RAF. Knockdown of CNK1 protects CNK1 from this AKT/RAF crosstalk. In C2 skeletal muscle cells CNK1 expression is induced with the onset of differentiation. Hence, AKT-bound CNK1 counteracts ERK stimulation in differentiated but not in proliferating cells. Ectopically expressed CNK1 facilitates C2 cell differentiation and knockdown of CNK1 impaired the transcriptional network underlying C2 cell differentiation. Thus, CNK1 expression, CNK1 clustering and the thereto related differential signalling processes decide on proliferation and differentiation in a cell type- and cell stage-dependent manner by orchestrating AKT and RAF signalling.

AKT-dependent phosphorylation of the SAM domain induces oligomerization and activation of the scaffold protein CNK1

Scaffold proteins are hubs for the coordination of intracellular signaling networks. The scaffold protein CNK1 promotes several signal transduction pathway. Here we demonstrate that sterile motif alpha (SAM) domain-dependent oligomerization of CNK1 stimulates CNK1-mediated signaling in growth factor-stimulated cells. We identified Ser22 located within the SAM domain as AKT-dependent phosphorylation site triggering CNK1 oligomerization. Oligomeric CNK1 increased the affinity for active AKT indicating a positive AKT feedback mechanism. A CNK1 mutant lacking the SAM domain and the phosphorylation-defective mutant CNK1_S22A antagonizes oligomerization and prevents CNK1-driven cell proliferation and matrix metalloproteinase 14 promoter activation. The phosphomimetic mutant CNK1_S22D constitutively oligomerizes and stimulates CNK1 downstream signaling. Searching the COSMIC database revealed Ser22 as putative target for oncogenic activation of CNK1. Like the phosphomimetic mutant CNK1_S22D, the oncogenic mutant CNK1_S22F forms clusters in serum-starved cells comparable to clusters of CNK1 in growth factor-stimulated cells. CNK1 clusters induced by activating Ser22 mutants correlate with enhanced cell invasion and binding to and activation of ADP ribosylation factor 1 associated with tumor formation. Mutational analysis indicate that EGF-triggered phosphorylation of Thr8 within the SAM domain prevents AKT binding and antagonizes CNK1-mediated AKT signaling. Our findings reveal SAM domain-dependent oligomerization by AKT as switch for CNK1 activation.

S6 Kinase Reflects and Regulates Ethanol-Induced Sedation

Alcohol use disorders (AUDs) affect people at great individual and societal cost. Individuals at risk for AUDs are sensitive to alcohol's rewarding effects and/or resistant to its aversive and sedating effects. The molecular basis for these traits is poorly understood. Here, we show that p70 S6 kinase (S6k), acting downstream of the insulin receptor (InR) and the small GTPase Arf6, is a key mediator of ethanol-induced sedation in Drosophila. S6k signaling in the adult nervous system determines flies' sensitivity to sedation. Furthermore, S6k activity, measured via levels of phosphorylation (P-S6k), is a molecular marker for sedation and overall neuronal activity: P-S6k levels are decreased when neurons are silenced, as well as after acute ethanol sedation. Conversely, P-S6k levels rebound upon recovery from sedation and are increased when neuronal activity is enhanced. Reducing neural activity increases sensitivity to ethanol-induced sedation, whereas neuronal activation decreases ethanol sensitivity. These data suggest that ethanol has acute silencing effects on adult neuronal activity, which suppresses InR/Arf6/S6k signaling and results in behavioral sedation. In addition, we show that activity of InR/Arf6/S6k signaling determines flies' behavioral sensitivity to ethanol-induced sedation, highlighting this pathway in acute responses to ethanol.

SIGNIFICANCE STATEMENT

Genetic factors play a major role in the development of addiction. Identifying these genes and understanding their molecular mechanisms is a necessary first step in the development of targeted therapeutic intervention. Here, we show that signaling from the insulin receptor in Drosophila neurons determines flies' sensitivity to ethanol-induced sedation. We show that this signaling cascade includes the small GTPase Arf6 and S6 kinase (S6k). In addition, activity of S6k is regulated by acute ethanol exposure and by neuronal activity. S6k activity is therefore both an acute target of ethanol exposure and a regulator of ethanol's effects on behavior.

PH Domain-Arf G Protein Interactions Localize the Arf-GEF Steppke for Cleavage Furrow Regulation in Drosophila

The recruitment of GDP/GTP exchange factors (GEFs) to specific subcellular sites dictates where they activate small G proteins for the regulation of various cellular processes. Cytohesins are a conserved family of plasma membrane GEFs for Arf small G proteins that regulate endocytosis. Analyses of mammalian cytohesins have identified a number of recruitment mechanisms for these multi-domain proteins, but the conservation and developmental roles for these mechanisms are unclear. Here, we report how the pleckstrin homology (PH) domain of the Drosophila cytohesin Steppke affects its localization and activity at cleavage furrows of the early embryo. We found that the PH domain is necessary for Steppke furrow localization, and for it to regulate furrow structure. However, the PH domain was not sufficient for the localization. Next, we examined the role of conserved PH domain amino acid residues that are required for mammalian cytohesins to bind PIP3 or GTP-bound Arf G proteins. We confirmed that the Steppke PH domain preferentially binds PIP3 in vitro through a conserved mechanism. However, disruption of residues for PIP3 binding had no apparent effect on GFP-Steppke localization and effects. Rather, residues for binding to GTP-bound Arf G proteins made major contributions to this Steppke localization and activity. By analyzing GFP-tagged Arf and Arf-like small G proteins, we found that Arf1-GFP, Arf6-GFP and Arl4-GFP, but not Arf4-GFP, localized to furrows. However, analyses of embryos depleted of Arf1, Arf6 or Arl4 revealed either earlier defects than occur in embryos depleted of Steppke, or no detectable furrow defects, possibly because of redundancies, and thus it was difficult to assess how individual Arf small G proteins affect Steppke. Nonetheless, our data show that the Steppke PH domain and its conserved residues for binding to GTP-bound Arf G proteins have substantial effects on Steppke localization and activity in early Drosophila embryos.

Kinome Screen Identifies PFKFB3 and Glucose Metabolism as Important Regulators of the Insulin/Insulin-like Growth Factor (IGF)-1 Signaling Pathway

The insulin/insulin-like growth factor (IGF)-1 signaling pathway (ISP) plays a fundamental role in long term health in a range of organisms. Protein kinases including Akt and ERK are intimately involved in the ISP. To identify other kinases that may participate in this pathway or intersect with it in a regulatory manner, we performed a whole kinome (779 kinases) siRNA screen for positive or negative regulators of the ISP, using GLUT4 translocation to the cell surface as an output for pathway activity. We identified PFKFB3, a positive regulator of glycolysis that is highly expressed in cancer cells and adipocytes, as a positive ISP regulator. Pharmacological inhibition of PFKFB3 suppressed insulin-stimulated glucose uptake, GLUT4 translocation, and Akt signaling in 3T3-L1 adipocytes. In contrast, overexpression of PFKFB3 in HEK293 cells potentiated insulin-dependent phosphorylation of Akt and Akt substrates. Furthermore, pharmacological modulation of glycolysis in 3T3-L1 adipocytes affected Akt phosphorylation. These data add to an emerging body of evidence that metabolism plays a central role in regulating numerous biological processes including the ISP. Our findings have important implications for diseases such as type 2 diabetes and cancer that are characterized by marked disruption of both metabolism and growth factor signaling.

Differential tyrosine phosphorylation controls the function of CNK1 as a molecular switch in signal transduction

Scaffold proteins are multidomain proteins without enzymatic function that play a central role in coordinating signaling processes. The scaffold protein CNK1 interacts with pathway-specific signaling proteins and thereby regulates these respective pathways. Here, we revealed tyrosine phosphorylation as a critical regulation mechanism to control the function of CNK1. We identified Tyr 26 as a PDGF-induced and, additionally, Tyr 519 and Tyr 665 as SRC-induced tyrosine phosphorylation sites. Phosphomimetic mutants indicate that phosphorylation of Tyr 519 recruits CNK1 to the nucleus and additional phosphorylation of Tyr 26 enables CNK1 to promote SRE-dependent gene expression. Contrary, mutants preventing tyrosine phosphorylation promote matrix metalloproteinase MMP14 promoter activity. CNK1-driven cell proliferation partially depends on its tyrosine phosphorylation. Upon PDGF stimulation, CNK1 is recruited to the plasma membrane mediated by SRC. Knock down of CNK1 prevents PDGF-induced SRE-dependent gene expression, MMP14 promoter activity and cell proliferation. Thus, tyrosine phosphorylation is an important mechanism to control the subcellular localization of CNK1 and its distinct biological functions.

The effect of a preparation of minerals, vitamins and trace elements on the cardiac gene expression pattern in male diabetic rats

Background

Diabetic patients have an increased risk of developing cardiovascular diseases, which are the leading cause of death in developed countries. Although multivitamin products are widely used as dietary supplements, the effects of these products have not been investigated in the diabetic heart yet. Therefore, here we investigated if a preparation of different minerals, vitamins, and trace elements (MVT) affects the cardiac gene expression pattern in experimental diabetes.

Methods

Two-day old male Wistar rats were injected with streptozotocin (i.p. 100 mg/kg) or citrate buffer to induce diabetes. From weeks 4 to 12, rats were fed with a vehicle or a MVT preparation. Fasting blood glucose measurement and oral glucose tolerance test were performed at week 12, and then total RNA was isolated from the myocardium and assayed by rat oligonucleotide microarray for 41012 oligonucleotides.

Results

Significantly elevated fasting blood glucose concentration and impaired glucose tolerance were markedly improved by MVT-treatment in diabetic rats at week 12. Genes with significantly altered expression due to diabetes include functional clusters related to cardiac hypertrophy (e.g. caspase recruitment domain family, member 9; cytochrome P450, family 26, subfamily B, polypeptide; FXYD domain containing ion transport regulator 3), stress response (e.g. metallothionein 1a; metallothionein 2a; interleukin-6 receptor; heme oxygenase (decycling) 1; and glutathione S-transferase, theta 3), and hormones associated with insulin resistance (e.g. resistin; FK506 binding protein 5; galanin/GMAP prepropeptide). Moreover the expression of some other genes with no definite cardiac function was also changed such as e.g. similar to apolipoprotein L2; brain expressed X-linked 1; prostaglandin b2 synthase (brain). MVT-treatment in diabetic rats showed opposite gene expression changes in the cases of 19 genes associated with diabetic cardiomyopathy. In healthy hearts, MVT-treatment resulted in cardiac gene expression changes mostly related to immune response (e.g. complement factor B; complement component 4a; interferon regulatory factor 7; hepcidin).

Conclusions

MVT-treatment improved diagnostic markers of diabetes. This is the first demonstration that MVT-treatment significantly alters cardiac gene expression profile in both control and diabetic rats. Our results and further studies exploring the mechanistic role of individual genes may contribute to the prevention or diagnosis of cardiac complications in diabetes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-015-0248-6) contains supplementary material, which is available to authorized users.

Mode of action and pharmacogenomic biomarkers for exceptional responders to didemnin B

Modern cancer treatment employs many effective chemotherapeutic agents originally discovered from natural sources. The cyclic depsipeptide didemnin B has demonstrated impressive anticancer activity in preclinical models. Clinical use has been approved but is limited by sparse patient responses combined with toxicity risk and an unclear mechanism of action. From a broad-scale effort to match antineoplastic natural products to their cellular activities, we found that didemnin B selectively induces rapid and wholesale apoptosis through dual inhibition of PPT1 and EEF1A1. Furthermore, empirical discovery of a small panel of exceptional responders to didemnin B allowed the generation of a regularized regression model to extract a sparse-feature genetic biomarker capable of predicting sensitivity to didemnin B. This may facilitate patient selection in a fashion that could enhance and expand the therapeutic application of didemnin B against neoplastic disease.

General aspects of muscle glucose uptake

Glucose uptake in peripheral tissues is dependent on the translocation of GLUT4 glucose transporters to the plasma membrane. Studies have shown the existence of two major signaling pathways that lead to the translocation of GLUT4. The first, and widely investigated, is the insulin activated signaling pathway through insulin receptor substrate-1 and phosphatidylinositol 3-kinase. The second is the insulin-independent signaling pathway, which is activated by contractions. Individuals with type 2 diabetes mellitus have reduced insulin-stimulated glucose uptake in skeletal muscle due to the phenomenon of insulin resistance. However, those individuals have normal glucose uptake during exercise. In this context, physical exercise is one of the most important interventions that stimulates glucose uptake by insulin-independent pathways, and the main molecules involved are adenosine monophosphate-activated protein kinase, nitric oxide, bradykinin, AKT, reactive oxygen species and calcium. In this review, our main aims were to highlight the different glucose uptake pathways and to report the effects of physical exercise, diet and drugs on their functioning. Lastly, with the better understanding of these pathways, it would be possible to assess, exactly and molecularly, the importance of physical exercise and diet on glucose homeostasis. Furthermore, it would be possible to assess the action of drugs that might optimize glucose uptake and consequently be an important step in controlling the blood glucose levels in diabetic patients, in addition to being important to clarify some pathways that justify the development of drugs capable of mimicking the contraction pathway.

Stepping stone: a cytohesin adaptor for membrane cytoskeleton restraint in the syncytial Drosophila embryo

Cytohesin Arf-GEFs are conserved plasma membrane regulators. The sole Drosophila cytohesin, Steppke, restrains Rho1-dependent membrane cytoskeleton activity at the base of plasma membrane furrows of the syncytial embryo. By mass spectrometry, we identified a single major Steppke-interacting protein from syncytial embryos, which we named Stepping stone (Sstn). By sequence, Sstn seems to be a divergent homologue of the mammalian cytohesin adaptor FRMD4A. Our experiments supported this relationship. Specifically, heterophilic coiled-coil interactions linked Sstn and Steppke in vivo and in vitro, whereas a separate C-terminal region was required for Sstn localization to furrows. Sstn mutant and RNAi embryos displayed abnormal, Rho1-dependent membrane cytoskeleton expansion from the base of pseudocleavage and cellularization furrows, closely mimicking Steppke loss-of-function embryos. Elevating Sstn furrow levels had no effect on the steppke phenotype, but elevating Steppke furrow levels reversed the sstn phenotype, suggesting that Steppke acts downstream of Sstn and that additional mechanisms can recruit Steppke to furrows. Finally, the coiled-coil domain of Steppke was required for Sstn binding and in addition homodimerization, and its removal disrupted Steppke furrow localization and activity in vivo. Overall we propose that Sstn acts as a cytohesin adaptor that promotes Steppke activity for localized membrane cytoskeleton restraint in the syncytial Drosophila embryo.

The insulin/IGF signaling regulators cytohesin/GRP-1 and PIP5K/PPK-1 modulate susceptibility to excitotoxicity in C. elegans

During ischemic stroke, malfunction of excitatory amino acid transporters and reduced synaptic clearance causes accumulation of Glutamate (Glu) and excessive stimulation of postsynaptic neurons, which can lead to their degeneration by excitotoxicity. The balance between cell death-promoting (neurotoxic) and survival-promoting (neuroprotective) signaling cascades determines the fate of neurons exposed to the excitotoxic insult. The evolutionary conserved Insulin/IGF Signaling (IIS) cascade can participate in this balance, as it controls cell stress resistance in nematodes and mammals. Blocking the IIS cascade allows the transcription factor FoxO3/DAF-16 to accumulate in the nucleus and activate a transcriptional program that protects cells from a range of insults. We study the effect of IIS cascade on neurodegeneration in a C. elegans model of excitotoxicity, where a mutation in a central Glu transporter (glt-3) in a sensitizing background causes Glu-Receptor -dependent neuronal necrosis. We expand our studies on the role of the IIS cascade in determining susceptibility to excitotoxic necrosis by either blocking IIS at the level of PI3K/AGE-1 or stimulating it by removing the inhibitory effect of ZFP-1 on the expression of PDK-1. We further show that the components of the Cytohesin/GRP-1, Arf, and PIP5K/PPK-1 complex, known to regulate PIP2 production and the IIS cascade, modulate nematode excitotoxicity: mutations that are expected to reduce the complex's ability to produce PIP2 and inhibit the IIS cascade protect from excitotoxicity, while overstimulation of PIP2 production enhances neurodegeneration. Our observations therefore affirm the importance of the IIS cascade in determining the susceptibility to necrotic neurodegeneration in nematode excitotoxicity, and demonstrate the ability of Cytohesin/GRP-1, Arf, and PIP5K/PPK-1 complex to modulate neuroprotection.

Arf6 guanine nucleotide exchange factor cytohesin-2 binds to CCDC120 and is transported along neurites to mediate neurite growth

The mechanism of neurite growth is complicated, involving continuous cytoskeletal rearrangement and vesicular trafficking. Cytohesin-2 is a guanine nucleotide exchange factor for Arf6, an Arf family molecular switch protein, controlling cell morphological changes such as neuritogenesis. Here, we show that cytohesin-2 binds to a protein with a previously unknown function, CCDC120, which contains three coiled-coil domains, and is transported along neurites in differentiating N1E-115 cells. Transfection of the small interfering RNA (siRNA) specific for CCDC120 into cells inhibits neurite growth and Arf6 activation. When neurites start to extend, vesicles containing CCDC120 and cytohesin-2 are transported in an anterograde manner rather than a retrograde one. As neurites continue extension, anterograde vesicle transport decreases. CCDC120 knockdown inhibits cytohesin-2 localization into vesicles containing CCDC120 and diffuses cytohesin-2 in cytoplasmic regions, illustrating that CCDC120 determines cytohesin-2 localization in growing neurites. Reintroduction of the wild type CCDC120 construct into cells transfected with CCDC120 siRNA reverses blunted neurite growth and Arf6 activity, whereas the cytohesin-2-binding CC1 region-deficient CCDC120 construct does not. Thus, cytohesin-2 is transported along neurites by vesicles containing CCDC120, and it mediates neurite growth. These results suggest a mechanism by which guanine nucleotide exchange factor for Arf6 is transported to mediate neurite growth.

Arfs at a glance

The Arf small G proteins regulate protein and lipid trafficking in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. In their GTP-bound form, Arf proteins recruit a specific set of protein effectors to the membrane surface. These effectors function in vesicle formation and tethering, non-vesicular lipid transport and cytoskeletal regulation. Beyond fundamental membrane trafficking roles, Arf proteins also regulate mitosis, plasma membrane signaling, cilary trafficking and lipid droplet function. Tight spatial and temporal regulation of the relatively small number of Arf proteins is achieved by their guanine nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs), which catalyze GTP binding and hydrolysis, respectively. A unifying function of Arf proteins, performed in conjunction with their regulators and effectors, is sensing, modulating and transporting the lipids that make up cellular membranes. In this Cell Science at a Glance article and the accompanying poster, we discuss the unique features of Arf small G proteins, their functions in vesicular and lipid trafficking in cells, and how these functions are modulated by their regulators, the GEFs and GAPs. We also discuss how these Arf functions are subverted by human pathogens and disease states.

Apical accumulation of the Sevenless receptor tyrosine kinase during Drosophila eye development is promoted by the small GTPase Rap1

The Ras/MAPK-signaling pathway plays pivotal roles during development of metazoans by controlling cell proliferation and cell differentiation elicited, in several instances, by receptor tyrosine kinases (RTKs). While the internal mechanism of RTK-driven Ras/MAPK signaling is well understood, far less is known regarding its interplay with other co-required signaling events involved in developmental decisions. In a genetic screen designed to identify new regulators of RTK/Ras/MAPK signaling during Drosophila eye development, we identified the small GTPase Rap1, PDZ-GEF, and Canoe as components contributing to Ras/MAPK-mediated R7 cell differentiation. Rap1 signaling has recently been found to participate in assembling cadherin-based adherens junctions in various fly epithelial tissues. Here, we show that Rap1 activity is required for the integrity of the apical domains of developing photoreceptor cells and that reduced Rap1 signaling hampers the apical accumulation of the Sevenless RTK in presumptive R7 cells. It thus appears that, in addition to its role in cell-cell adhesion, Rap1 signaling controls the partitioning of the epithelial cell membrane, which in turn influences signaling events that rely on apico-basal cell polarity.

Arf Proteins and Their Regulators: At the Interface Between Membrane Lipids and the Protein Trafficking Machinery

The Arf small GTP-binding (G) proteins regulate membrane traffic and organelle structure in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. The first function identified for Arf proteins was recruitment of cytosolic coat complexes to membranes to mediate vesicle formation. However, subsequent studies have uncovered additional functions, including roles in plasma membrane signalling pathways, cytoskeleton regulation, lipid droplet function, and non-vesicular lipid transport. In contrast to other families of G proteins, there are only a few Arf proteins in each organism, yet they function specifically at many different cellular locations. Part of this specificity is achieved by formation of complexes with their guanine nucleotide-exchange factors (GEFs) and GTPase activating proteins (GAPs) that catalyse GTP binding and hydrolysis, respectively. Because these regulators outnumber their Arf substrates by at least 3-to-1, an important aspect of understanding Arf function is elucidating the mechanisms by which a single Arf protein is incorporated into different GEF, GAP, and effector complexes. New insights into these mechanisms have come from recent studies showing GEF–effector interactions, Arf activation cascades, and positive feedback loops. A unifying theme in the function of Arf proteins, carried out in conjunction with their regulators and effectors, is sensing and modulating the properties of the lipids that make up cellular membranes.

EphrinB1 interacts with CNK1 and promotes cell migration through c-Jun N-terminal kinase (JNK) activation

The Eph receptors and their membrane-bound ligands, ephrins, play important roles in various biological processes such as cell adhesion and movement. The transmembrane ephrinBs transduce reverse signaling in a tyrosine phosphorylation-dependent or -independent, as well as PDZ-dependent manner. Here, we show that ephrinB1 interacts with Connector Enhancer of KSR1 (CNK1) in an EphB receptor-independent manner. In cultured cells, cotransfection of ephrinB1 with CNK1 increases JNK phosphorylation. EphrinB1/CNK1-mediated JNK activation is reduced by overexpression of dominant-negative RhoA. Overexpression of CNK1 alone is sufficient for activation of RhoA; however, both ephrinB1 and CNK1 are required for JNK phosphorylation. Co-immunoprecipitation data showed that ephrinB1 and CNK1 act as scaffold proteins that connect RhoA and JNK signaling components, such as p115RhoGEF and MKK4. Furthermore, adhesion to fibronectin or active Src overexpression increases ephrinB1/CNK1 binding, whereas blocking Src activity by a pharmacological inhibitor decreases not only ephrinB1/CNK1 binding, but also JNK activation. EphrinB1 overexpression increases cell motility, however, CNK1 depletion by siRNA abrogates ephrinB1-mediated cell migration and JNK activation. Moreover, Rho kinase inhibitor or JNK inhibitor treatment suppresses ephrinB1-mediated cell migration. Taken together, our findings suggest that CNK1 is required for ephrinB1-induced JNK activation and cell migration.

The CNK2 scaffold interacts with vilse and modulates Rac cycling during spine morphogenesis in hippocampal neurons

Protein scaffolds play an important role in signal transduction, functioning to facilitate protein interactions and localize key pathway components to specific signaling sites. Connector enhancer of KSR-2 (CNK2) is a neuronally expressed scaffold recently implicated in nonsyndromic, X-linked intellectual disability (NS-XLID) [1-3]. NS-XLID patients have deficits in cognitive function and their neurons often exhibit dendritic spine abnormalities [4], suggesting a role for CNK2 in synaptic signaling and/or spine formation. To gain insight regarding how CNK2 might contribute to these processes, we used mass spectrometry to identify proteins that interact with the endogenous CNK2 scaffold. Here, we report that the major binding partner of CNK2 is Vilse/ARHGAP39 and that CNK2 complexes are enriched for proteins involved in Rac/Cdc42 signaling, including Rac1 itself, α-PIX and β-PIX, GIT1 and GIT2, PAK3 and PAK4, and members of the cytohesin family. Binding between CNK2 and Vilse was found to be constitutive, mediated by the WW domains of Vilse and a proline motif in CNK2. Through mutant analysis, protein depletion and rescue experiments, we identify CNK2 as a spatial modulator of Rac cycling during spine morphogenesis and find that the interaction with Vilse is critical for maintaining RacGDP/GTP levels at a balance required for spine formation.

Design and Synthesis of New (SecinH3) Derivatives as Potential Cytohesin Inhibitors

Cytohesins are small guanine nucleotide exchange factors that stimulate ADP ribosylation factors, Ras-like GTPases, which control various cellular regulatory networks ranging from vesicle trafficking to integrin activation. A small molecule SecinH3 (1,2,4-triazole derivative) in an aptamer displacement assay as a pan-cytohesin Sec7-domain inhibitor was previously identified. Here a series of different SecinH3-analogues was designed and synthesised as potential cytohesin Sec7-domain inhibitors. All final synthesized compounds 6-8, 43-58 and their intermediates were confirmed by (1)H NMR, (13)C NMR and high resolution Mass. Preliminary biological screening of target compounds indictaed that some of the new synthesized secinH3 derivatives showed higher potency and promising activity more than secinH3 itself (unpublished results). Compund 9 and 10 were approximate equal to secinH3 activity as cytohesin antagonist activity. Furthermore compound 52 showed twice inhibition potency if compared to secinH3.

The cytohesin coiled-coil domain interacts with threonine 276 to control membrane association

Cell migration is regulated by a number of small GTPases, including members of the Arf family. Cytohesins, a family of Arf-activating proteins, have been extensively implicated in the regulation of Arfs during migration and cell shape change. Membrane association of both the Arf and its activating protein is a prerequisite for Arf activation. Therefore regulating the extent of cytohesin membrane association is a mechanism for controlling the initiation of cell movement. We have discovered a novel intramolecular interaction that controls the association of cytohesins with membranes. The presence of the coiled-coil domain reduces the association of cytohesin 2 with membranes. We demonstrate that this domain interacts with more C-terminal regions of the protein. This interaction is independent of another previously identified autoinhibitory conformation. A threonine residue (T276) in the cytohesin 2 PH domain is a target for phosphorylation by Akt. Mutation of this threonine to aspartic acid, to mimic phosphorylation, disrupts the binding of the coiled-coil domain to c-terminal regions and promotes membrane association of cytohesin 2. The presence of a second autoinhibitory interaction in the cytohesins suggests that these proteins can act a signal integrators that stimulate migration only after receive multiple pro-migratory signals.

The Drosophila Arf GEF Steppke controls MAPK activation in EGFR signaling

Guanine nucleotide exchange factors (GEFs) of the cytohesin protein family are regulators of GDP/GTP exchange for members of the ADP ribosylation factor (Arf) of small GTPases. They have been identified as modulators of various receptor tyrosine kinase signaling pathways including the insulin, the vascular epidermal growth factor (VEGF) and the epidermal growth factor (EGF) pathways. These pathways control many cellular functions, including cell proliferation and differentiation, and their misregulation is often associated with cancerogenesis. In vivo studies on cytohesins using genetic loss of function alleles are lacking, however, since knockout mouse models are not available yet. We have recently identified mutants for the single cytohesin Steppke (Step) in Drosophila and we could demonstrate an essential role of Step in the insulin signaling cascade. In the present study, we provide in vivo evidence for a role of Step in EGFR signaling during wing and eye development. By analyzing step mutants, transgenic RNA interference (RNAi) and overexpression lines for tissue specific as well as clonal analysis, we found that Step acts downstream of the EGFR and is required for the activation of mitogen-activated protein kinase (MAPK) and the induction of EGFR target genes. We further demonstrate that step transcription is induced by EGFR signaling whereas it is negatively regulated by insulin signaling. Furthermore, genetic studies and biochemical analysis show that Step interacts with the Connector Enhancer of KSR (CNK). We propose that Step may be part of a larger signaling scaffold coordinating receptor tyrosine kinase-dependent MAPK activation.

Cytohesin-2 as a novel prognostic marker for hepatocellular carcinoma

Cytohesin-2 is overexpressed in human lung cancer and it activates cytoplasmic ErbB receptors. Inhibition of cytohesin-2 by SecinH3 reduces growth of EGFR-dependent lung cancer xenografts and improves the treatment of primarily EGFR-TKI-resistant lung cancers. Cytohesin-2 promotes HepG2 proliferation through the IGF pathway, and VEGF-dependent initiation of angiogenesis by regulation of VEGFR-2 internalization in endothelial cells, vessel permeability and ultimately endothelial proliferation. The purpose of this study was to evaluate the effects of cytohesin-2 in hepatocellular carcinoma (HCC). In the current study, we collected 40 HCC tissues and detected cytohesin-2 mRNA expression in the 40 HCC tissues by using quantitative real-time polymerase chain reaction (qRT-PCR), as well as its protein expression by using immunohistochemistry and western blot analysis. We found that cytohesin-2 was more highly expressed in HCC compared to adjacent non-tumorous liver tissues, and cytohesin-2 expression was significantly increased in specimens with high α-fetoprotein and vascular invasion. Both univariate and multivariate analyses indicated that there is an association between cytohesin-2 expression and overall survival (OS) and disease-free survival (DFS). Moreover, stratified analysis showed that patients in tumor-node-metastasis (TNM) stage I with higher cytohesin-2 levels had shorter OS and DFS than those with lower cytohesin-2 levels. In conclusion, cytohesin-2 may identify low-and high-risk individuals with HCC and may be a valuable indicator for stratifying prognosis of TNM stage I patients. Cytohesin-2 may serve as a novel prognostic biomarker for HCC.

Scaffold protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3) coordinates assembly of a multiprotein epithelial sodium channel (ENaC)-regulatory complex

Hormone regulation of ion transport in the kidney tubules is essential for fluid and electrolyte homeostasis in vertebrates. A large body of evidence has suggested that transporters and channels exist in multiprotein regulatory complexes; however, relatively little is known about the composition of these complexes or their assembly. The epithelial sodium channel (ENaC) in particular is tightly regulated by the salt-regulatory hormone aldosterone, which acts at least in part by increasing expression of the serine-threonine kinase SGK1. Here we show that aldosterone induces the formation of a 1.0-1.2-MDa plasma membrane complex, which includes ENaC, SGK1, and the ENaC inhibitor Nedd4-2, a key target of SGK1. We further show that this complex contains the PDZ domain-containing protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3). CNK3 physically interacts with ENaC, Nedd4-2, and SGK1; enhances the interactions among them; and stimulates ENaC function in a PDZ domain-dependent, aldosterone-induced manner. These results strongly suggest that CNK3 is a molecular scaffold, which coordinates the assembly of a multiprotein ENaC-regulatory complex and hence plays a central role in Na(+) homeostasis.

Anti-proliferative effect of cytohesin inhibition in gefitinib-resistant lung cancer cells

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI), such as gefitinib, have been proven to efficiently inhibit the proliferation of a subset of non small-cell lung cancers (NSCLC). Unfortunately, the majority of NSCLC expressing wild type EGFR is primarily resistant to EGFR-TKI treatment. Here, we show that the proliferation of the gefitinib-resistant NSCLC cell lines H460 and A549 is reduced by the small molecule SecinH3 which indirectly attenuates EGFR activation by inhibition of cytohesins, a class of recently discovered cytoplasmic EGFR activators. SecinH3 and gefitinib showed a synergistic antiproliferative effect, which correlated with a profound inhibition of Akt activation and survivin expression. Treating mice bearing H460 xenografts with SecinH3 showed the antiproliferative and pro-apoptotic effect of SecinH3 in vivo. Our data suggest that targeting the EGFR indirectly by inhibiting its cytoplasmic activators, the cytohesins, has the potential to improve the treatment of primarily EGFR-TKI resistant lung cancers.

Organization of the ENaC-regulatory machinery

The control of fluid and electrolyte homeostasis in vertebrates requires the integration of a diverse set of signaling inputs, which control epithelial Na(+) transport, the principal ionic component of extracellular fluid. The key site of regulation is a segment of the kidney tubules, frequently termed the aldosterone-sensitive distal nephron, wherein the epithelial Na(+) channel (or ENaC) mediates apical ion entry. Na(+) transport in this segment is strongly regulated by the salt-retaining hormone, aldosterone, which acts through the mineralocorticoid receptor (MR) to influence the expression of a selected set of target genes, most notably the serine-threonine kinase SGK1, which phosphorylates and inhibits the E3 ubiquitin ligase Nedd4-2. It has long been known that ENaC activity is tightly regulated in vertebrate epithelia. Recent evidence suggests that SGK1 and Nedd4-2, along with other ENaC-regulatory proteins, physically associate with each other and with ENaC in a multi-protein complex. The various components of the complex are regulated by diverse signaling networks, including steroid receptor-, PI3-kinase-, mTOR-, and Raf-MEK-ERK-dependent pathways. In this review, we focus on the organization of the targets of these pathways by multi-domain scaffold proteins and lipid platforms into a unified complex, thereby providing a molecular basis for signal integration in the control of ENaC.

Phosphoinositide specificity determines which cytohesins regulate β1 integrin recycling

Recycling of internalized integrins is a crucial step in adhesion remodeling and cell movement. Recently, we determined that the ADP-ribosylation factor-guanine nucleotide exchange factors (ARF-GEFs) cytohesin 2/ARNO and cytohesin 3/GRP1 have opposing effects on adhesion and stimulated β1 integrin recycling even though they are very closely related proteins (80% sequence identity). We have now determined the sequence differences underlying the differential actions of cytohesin 2/ARNO and cytohesin 3/GRP1. We found that the ability of cytohesins to promote β1 integrin recycling and adhesion depends upon the presence or absence of a key glycine residue in their pleckstrin homology (PH) domains. This glycine residue determines the phosphoinositide specificity and affinity of cytohesin PH domains. Switching the number of glycines in the PH domains of cytohesin 2 and cytohesin 3 is sufficient to reverse their effects on adhesion and spreading and to reverse their subcellular locations. Importantly, we also find that a mutant form of cytohesin 3/GRP1 that has three rather than two glycines in its PH domain rescues β1 integrin recycling in cytohesin 2/ARNO knockdown cells. Conversely, a mutant form of cytohesin 2/ARNO with two glycines in its PH domain fails to rescue β1 integrin recycling. Therefore, we conclude that phosphoinositide specificity is the sole functional difference that determines which cytohesin can promote integrin recycling.

The role of the ENaC-regulatory complex in aldosterone-mediated sodium transport

The mineralocorticoid aldosterone is indispensable for the control of blood pressure and fluid volume in mammals. It acts in large part to increase the abundance and activity of the epithelial Na(+) channel (ENaC), which mediates apical Na(+) entry in the distal parts of the kidney tubules. Aldosterone acts through the mineralocorticoid receptor to alter the transcription of specific genes, including SGK1 and GILZ1. Recent evidence suggests that these key aldosterone-regulated factors function within a unique multi-protein ENaC-regulatory-complex that governs the net cell surface expression and activity of the channel. Another aldosterone-induced protein, CNK3 (connector enhancer of kinase suppressor of Ras 3), also stimulates ENaC and has all of the features of a scaffolding protein. With these observations in mind, we discuss the possibility that CNK3 coordinates the dynamic assembly of the ENaC-regulatory-complex, and promotes context-appropriate aldosterone signal transduction in the regulation of epithelial Na(+) transport.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."