CNK, a RAF-binding multidomain protein required for RAS signaling

CNK1 and other scaffolds for Akt/FoxO signaling

FoxO transcription factors mediate anti-proliferative and pro-apoptotic signals and act as tumor suppressors in cancer. Posttranslational modifications including phosphorylation and acetylation regulate FoxO activity by a cytoplasmic-nuclear shuttle mechanism. Scaffold proteins coordinating signaling pathways in time and space play a critical role in this process. CNK1 acts as a scaffold protein in several signaling pathways controlling the function of FoxO proteins. An understanding of CNK1 and other scaffolds in the FoxO signaling network will provide insights how to release the tumor suppressor function of FoxO as a possibility to block oncogenic pathways. This article is part of a Special Issue entitled: P13K-AKT-FoxO axis in cancer and aging.

Mechanistic principles of RAF kinase signaling

The RAF family of kinases are key components acting downstream of receptor tyrosine kinases and cells employ several distinct mechanisms to strictly control their activity. RAF transitions from an inactive state, where the N-terminal regulatory region binds intramolecularly to the C-terminal kinase domain, to an open state capable of executing the phosphoryl transfer reaction. This transition involves changes both within and between the protein domains in RAF. Many different proteins regulate the transition between inactive and active states of RAF, including RAS and KSR, which are arguably the two most prominent regulators of RAF function. Recent developments have added several new twists to our understanding of RAF regulation. Among others, dimerization of the RAF kinase domain is emerging as a crucial step in the RAF activation process. The multitude of regulatory protein-protein interactions involving RAF remains a largely untapped area for therapeutic applications.

The CNK1 scaffold binds cytohesins and promotes insulin pathway signaling

Protein scaffolds play an important role in signal transduction, regulating the localization of signaling components and mediating key protein interactions. Here, we report that the major binding partners of the Connector Enhancer of KSR 1 (CNK1) scaffold are members of the cytohesin family of Arf guanine nucleotide exchange factors, and that the CNK1/cytohesin interaction is critical for activation of the PI3K/AKT cascade downstream from insulin and insulin-like growth factor 1 (IGF-1) receptors. We identified a domain located in the C-terminal region of CNK1 that interacts constitutively with the coiled-coil domain of the cytohesins, and found that CNK1 facilitates the membrane recruitment of cytohesin-2 following insulin stimulation. Moreover, through protein depletion and rescue experiments, we found that the CNK1/cytohesin interaction promotes signaling from plasma membrane-bound Arf GTPases to the phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) to generate a PIP(2)-rich microenvironment that is critical for the membrane recruitment of insulin receptor substrate 1 (IRS1) and signal transmission to the PI3K/AKT cascade. These findings identify CNK1 as a new positive regulator of insulin signaling.

CNK1 is a novel Akt interaction partner that promotes cell proliferation through the Akt-FoxO signalling axis

The scaffold proteins connector enhancer of KSR (CNK) participate in Raf-, Rho- and NF-kappaB-dependent signalling and promote cell differentiation and invasion. In this study, we demonstrate that CNK1 downregulation inhibits, whereas CNK1 overexpression stimulates the proliferation of breast cancer cells and human embryonic kidney cells, respectively. This stimulatory effect depends on a functional phosphatidylinositol-3 kinase (PI3K) pathway because treatment of cells with the PI3K inhibitor, LY294002, abrogates CNK1-induced proliferation. CNK1 interacts with the PI3K effector Akt and knockdown of CNK1 decreases Akt activity in breast cancer cells. CNK1 controls Akt-dependent phosphorylation and transcriptional activity of FoxO, which is a negative regulator of proliferation. Consistent with this, CNK1-induced cell proliferation is blocked by FoxO overexpression. Moreover, CNK1 regulates anchorage-independent proliferation and focus formation of breast cancer cells. CNK1 is predominantly localized at the plasma membrane of breast cancer cells, whereas in non-transformed mammary epithelial cells, CNK1 is cytoplasmatic. Accordingly, CNK1 is found preferentially at the plasma membrane in carcinoma in situ and invasive breast cancer tumours compared with normal breast tissue sections. Analysis of multiple breast cancer samples reveals that CNK1-negative tumours show less Akt activity. Thus, CNK1 promotes oncogenic signalling through Akt in breast cancer cell lines and tumours.

The ERK signaling cascade--views from different subcellular compartments

The extracellular signal-regulated kinase cascade is a central signaling pathway that is stimulated by various extracellular stimuli. The signals of these stimuli are then transferred by the cascade's components to a large number of targets at distinct subcellular compartments, which in turn induce and regulate a large number of cellular processes. To achieve these functions, the cascade exhibits versatile and dynamic subcellular distribution that allows proper temporal and spatial modulation of the appropriate processes. In this review, we discuss the intracellular localizations of different components of the ERK cascade, and the impact of these localizations on their activation and specificity.

Cnksr3 is a direct mineralocorticoid receptor target gene and plays a key role in the regulation of the epithelial sodium channel

Aldosterone is the principal hormonal regulator of sodium homeostasis in vertebrates. It exerts its actions through the mineralocorticoid receptor (MR) that regulates the transcription of specific target genes. In recent years, a number of MR target genes have been identified that are involved in the regulation of the epithelial sodium channel (ENaC), a key modulator of renal sodium absorption. Here we report the identification of cnksr3 as a direct MR target gene that is up-regulated in response to physiological concentrations of aldosterone. The cnksr3 promoter exhibits two functional aldosterone-responsive regions, which were bound by the MR as assessed by chromatin immunoprecipitation (ChIP). In vivo, CNKSR3 was highly expressed in the renal cortical collecting duct (CCD), the prime target segment of aldosterone-regulated sodium retention in the kidney. CCD cell lines stably overexpressing or silencing CNKSR3 were electrophysiologically analyzed and show that CNKSR3 expression correlated with and is required for ENaC-mediated transepithelial sodium transport. In parallel, CNKSR3 expression led to decreased MEK phosphorylation. We conclude that CNKSR3, a homologue of scaffold proteins involved in MAPK pathway regulation, is a direct target of MR and is required for the maintenance of transepithelial sodium transport in the kidney.

Mechanisms of tumor resistance to EGFR-targeted therapies

BACKGROUND

Much effort has been devoted to development of cancer therapies targeting EGFR, based on its role in regulating cell growth. Small-molecule and antibody EGFR inhibitors have clinical roles based on their efficacy in a subset of cancers, generally as components of combination therapies. Many cancers are either initially resistant to EGFR inhibitors or become resistant during treatment, limiting the efficacy of these reagents.

OBJECTIVE/METHODS

To review cellular resistance mechanisms to EGFR-targeted therapies.

RESULTS/CONCLUSIONS

The best validated of these mechanisms include activation of classic ATP-binding casette (ABC) multidrug transporters; activation or mutation of EGFR; and overexpression or activation of signaling proteins operating in relation to EGFR. We discuss current efforts and potential strategies to override these sources of resistance. We describe emerging systems-biology-based concepts of alternative resistance to EGFR-targeted therapies, and discuss their implications for use of EGFR-targeted and other targeted therapies.

Scaffold proteins and immune-cell signalling

Over the past 20 years great progress has been made in defining most of the key signalling pathways that functionally regulate immune cells. Recently, it has become clear that scaffold proteins have a crucial role in regulating many of these signalling cascades. By binding two or more components of a signalling pathway, scaffold proteins can help to localize signalling molecules to a specific part of the cell or to enhance the efficacy of a signalling pathway. Scaffold proteins can also affect the thresholds and the dynamics of signalling reactions by coordinating positive and negative feedback signals. In this Review, we focus on recent progress in the understanding of the function of scaffold proteins in immune cells.

The PP2C Alphabet is a negative regulator of stress-activated protein kinase signaling in Drosophila

The Jun N-terminal kinase and p38 pathways, also known as stress-activated protein kinase (SAPK) pathways, are signaling conduits reiteratively used throughout the development and adult life of metazoans where they play central roles in the control of apoptosis, immune function, and environmental stress responses. We recently identified a Drosophila Ser/Thr phosphatase of the PP2C family, named Alphabet (Alph), which acts as a negative regulator of the Ras/ERK pathway. Here we show that Alph also plays an inhibitory role with respect to Drosophila SAPK signaling during development as well as under stress conditions such as oxidative or genotoxic stresses. Epistasis experiments suggest that Alph acts at a step upstream of the MAPKKs Hep and Lic. Consistent with this interpretation, biochemical experiments identify the upstream MAPKKKs Slpr, Tak1, and Wnd as putative substrates. Together with previous findings, this work identifies Alph as a general attenuator of MAPK signaling in Drosophila.

Overexpression screen in Drosophila identifies neuronal roles of GSK-3 beta/shaggy as a regulator of AP-1-dependent developmental plasticity

AP-1, an immediate-early transcription factor comprising heterodimers of the Fos and Jun proteins, has been shown in several animal models, including Drosophila, to control neuronal development and plasticity. In spite of this important role, very little is known about additional proteins that regulate, cooperate with, or are downstream targets of AP-1 in neurons. Here, we outline results from an overexpression/misexpression screen in Drosophila to identify potential regulators of AP-1 function at third instar larval neuromuscular junction (NMJ) synapses. First, we utilize >4000 enhancer and promoter (EP) and EPgy2 lines to screen a large subset of Drosophila genes for their ability to modify an AP-1-dependent eye-growth phenotype. Of 303 initially identified genes, we use a set of selection criteria to arrive at 25 prioritized genes from the resulting collection of putative interactors. Of these, perturbations in 13 genes result in synaptic phenotypes. Finally, we show that one candidate, the GSK-3beta-kinase homolog, shaggy, negatively influences AP-1-dependent synaptic growth, by modulating the Jun-N-terminal kinase pathway, and also regulates presynaptic neurotransmitter release at the larval neuromuscular junction. Other candidates identified in this screen provide a useful starting point to investigate genes that interact with AP-1 in vivo to regulate neuronal development and plasticity.

The regulation of extracellular signal-regulated kinase (ERK) in mammalian cells

The mitogen-activated protein (MAP) kinase extracellular-signal-regulated kinases (ERKs) are activated by diverse mechanisms. These include ligation of receptor tyrosine kinases such as epidermal growth factor (EGF) and cell adhesion receptors such as the integrins. In general, ligand binding of these receptors leads to GTP loading and activation of the small GTPase Ras, which recruits Raf to the membrane where it is activated. Raf subsequently phosphorylates the dual specificity MAP/ERK kinase (MEK1/2) which in turn phosphorylates and thereby activates ERK. ERK is a promiscuous kinase and can phosphorylate more than 100 different substrates. Therefore activation of ERK can affect a broad array of cellular functions including proliferation, survival, apoptosis, motility, transcription, metabolism and differentiation. ERK activity is controlled by many distinct mechanisms. Scaffold proteins control when and where ERK is activated while anchoring proteins can restrain ERK localization to specific subcellular compartments. Meanwhile, phosphatases dephosphorylate and inactivate ERK thereby shutting off the pathway. Finally, several feedback mechanisms have been identified downstream of ERK activation. Here we will focus on the diverse mechanisms of ERK regulation in mammalian cells.

CNK and HYP form a discrete dimer by their SAM domains to mediate RAF kinase signaling

RAF kinase functions in the mitogen-activated protein kinase (MAPK) pathway to transmit growth signals to the downstream kinases MEK and ERK. Activation of RAF catalytic activity is facilitated by a regulatory complex comprising the proteins CNK (Connector enhancer of KSR), HYP (Hyphen), and KSR (Kinase Suppressor of Ras). The sterile alpha-motif (SAM) domain found in both CNK and HYP plays an essential role in complex formation. Here, we have determined the x-ray crystal structure of the SAM domain of CNK in complex with the SAM domain of HYP. The structure reveals a single-junction SAM domain dimer of 1:1 stoichiometry in which the binding mode is a variation of polymeric SAM domain interactions. Through in vitro and in vivo mutational analyses, we show that the specific mode of dimerization revealed by the crystal structure is essential for RAF signaling and facilitates the recruitment of KSR to form the CNK/HYP/KSR regulatory complex. We present two docking-site models to account for how SAM domain dimerization might influence the formation of a higher-order CNK/HYP/KSR complex.

Raf kinases: function, regulation and role in human cancer

The Ras-Raf-MAPK pathway regulates diverse physiological processes by transmitting signals from membrane based receptors to various nuclear, cytoplasmic and membrane-bound targets, coordinating a large variety of cellular responses. Function of Raf family kinases has been shown to play a role during organism development, cell cycle regulation, cell proliferation and differentiation, cell survival and apoptosis and many other cellular and physiological processes. Aberrations along the Ras-Raf-MAPK pathway play an integral role in various biological processes concerning human health and disease. Overexpression or activation of the pathway components is a common indicator in proliferative diseases such as cancer and contributes to tumor initiation, progression and metastasis. In this review, we focus on the physiological roles of Raf kinases in normal and disease conditions, specifically cancer, and the current thoughts on Raf regulation.

KSR and CNK: two scaffolds regulating RAS-mediated RAF activation

The RAS-RAF-MEK-extracellular-regulated kinase (RAS/ERK) pathway is a major intracellular route used by metazoan cells to channel to downstream targets a diverse array of signals, including those controlling cell proliferation and survival. Recent findings suggest that the pathway is assembled by specific scaffolding proteins that in turn regulate the efficiency, the location and/or the duration of signal transmission. Here, through the angle of studies conducted in Drosophila and C. elegans, we present two such proteins, the kinase suppressor of RAS (KSR) and connector enhancer of KSR (CNK) scaffolds, and highlight their implication in a novel mechanism regulating RAS-mediated RAF activation. Based on recent findings, we discuss the possibility that KSR, a RAF-like protein, does not solely act as a scaffold, but directly induces RAF catalytic function by a kinase-independent mechanism apparently shared by RAF-like proteins.

The integration of signaling by multiprotein complexes containing Raf kinases

In vivo, eukaryotic cells are subjected simultaneously to a broad array of signals ranging from mitogens and inflammatory inputs to environmental stresses and developmental cues. The combinatorial nature of cellular signaling necessitates that a cell integrate its signal transduction pathways so as to implement rapidly and efficiently an appropriate suite of responses. Emerging evidence indicates that, over the course of evolution, cells have developed multiprotein signaling complexes, or "signalosomes" that mediate the coordinate regulation of different signaling pathways. Such molecular signal integration contrasts with the classical notion of signaling complexes assembled by scaffold proteins-entities that function to segregate specific pathways from one another. This review will focus on two signal integrating multiprotein complexes that involve Raf family kinases: the MLK3-B-Raf-Raf-1 complex and the Raf-1-Mst-2 complex.

Scaffold proteins dictate Rho GTPase-signaling specificity

Given the numerous mechanisms that regulate the activity of Rho GTPases and the multiple effectors for Rho proteins, how is specificity achieved when transducing signals via Rho GTPase-regulated molecular networks? The finding that the scaffold protein hCNK1 links Rho guanine-nucleotide-exchange factors and Rho to JNK (c-Jun N-terminal kinase), while limiting stress-fiber formation and serum-response-factor activation, suggests that scaffold proteins govern the selection of signal outputs, thus helping to solve the Rho GTPase-signaling puzzle.

Alphabet, a Ser/Thr phosphatase of the protein phosphatase 2C family, negatively regulates RAS/MAPK signaling in Drosophila

Signal transduction through the RAS/mitogen-activated protein kinase (MAPK) pathway depends on a diverse collection of proteins regulating positively and negatively signaling flow. We previously conducted a genetic screen in Drosophila to identify novel components of this signaling pathway. Here, we present the identification and characterization of a new gene, alphabet (alph), whose activity negatively regulates RAS/MAPK-dependent developmental processes in Drosophila and this, at a step downstream or in parallel to RAS. alph encodes a protein phosphatase 2C (PP2C) family member closely related to the mammalian PP2C alpha and beta isoforms. Interestingly, although alph gene product does not appear to be essential for viability, its elimination leads to weak but significant developmental defects reminiscent of an overactivated RAS/MAPK pathway. Consistent with this interpretation, strong genetic interactions are observed between alph alleles and mutations in bona fide components of the pathway. Together, this work identifies a PP2C of the alpha/beta subfamily as a novel negative regulator of the RAS/MAPK pathway and suggests that these evolutionarily conserved enzymes play a similar role in other metazoans. Finally, despite the relatively large size of the PP2C gene family in metazoans, this study represents only the second genetic characterization of a PP2C in these organisms.

A KSR/CNK complex mediated by HYP, a novel SAM domain-containing protein, regulates RAS-dependent RAF activation in Drosophila

RAF is a critical effector of the small GTPase RAS in normal and malignant cells. Despite intense scrutiny, the mechanism regulating RAF activation remains partially understood. Here, we show that the scaffold KSR (kinase suppressor of RAS), a RAF homolog known to assemble RAF/MEK/ERK complexes, induces RAF activation in Drosophila by a mechanism mediated by its kinase-like domain, but which is independent of its scaffolding property or putative kinase activity. Interestingly, we found that KSR is recruited to RAF prior to signal activation by the RAF-binding protein CNK (connector enhancer of KSR) in association with a novel SAM (sterile alpha motif) domain-containing protein, named Hyphen (HYP). Moreover, our data suggest that the interaction of KSR to CNK/HYP stimulates the RAS-dependent RAF-activating property of KSR. Together, these findings identify a novel protein complex that controls RAF activation and suggest that KSR does not only act as a scaffold for the MAPK (mitogen-activated protein kinase) module, but may also function as a RAF activator. By analogy to catalytically impaired, but conformationally active B-RAF oncogenic mutants, we discuss the possibility that KSR represents a natural allosteric inducer of RAF catalytic function.

The novel SAM domain protein Aveugle is required for Raf activation in the Drosophila EGF receptor signaling pathway

Activation of the Raf kinase by GTP-bound Ras is a poorly understood step in receptor tyrosine kinase signaling pathways. One such pathway, the epidermal growth factor receptor (EGFR) pathway, is critical for cell differentiation, survival, and cell cycle regulation in many systems, including the Drosophila eye. We have identified a mutation in a novel gene, aveugle, based on its requirement for normal photoreceptor differentiation. The phenotypes of aveugle mutant cells in the eye and wing imaginal discs resemble those caused by reduction of EGFR pathway function. We show that aveugle is required between ras and raf for EGFR signaling in the eye and for mitogen-activated protein kinase phosphorylation in cell culture. aveugle encodes a small protein with a sterile alpha motif (SAM) domain that can physically interact with the scaffold protein connector enhancer of Ksr (Cnk). We propose that Aveugle acts together with Cnk to promote Raf activation, perhaps by recruiting an activating kinase.

Coordinating ERK/MAPK signalling through scaffolds and inhibitors

The pathway from Ras through Raf and MEK (MAPK and ERK kinase) to ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) regulates many fundamental cellular processes. Recently, a number of scaffolding proteins and endogenous inhibitors have been identified, and their important roles in regulating signalling through this pathway are now emerging. Some scaffolds augment the signal flux, but also mediate crosstalk with other pathways; certain adaptors target MEK-ERK/MAPK complexes to subcellular localizations; others provide regulated inhibition. Computational modelling indicates that, together, these modulators can determine the dynamic biological behaviour of the pathway.

The scaffold protein CNK1 interacts with the angiotensin II type 2 receptor

The scaffold protein CNK1 mediates proliferative as well as antiproliferative responses including differentiation and apoptosis. The angiotensin II type 2 (AT2) receptor belongs to the class of G protein-coupled receptors and also promotes antiproliferative effects. Here we report that CNK1 binds through the sterile alpha motif (SAM) and the conserved region in CNK (CRIC) to the AT2 receptor. The exchange of a conserved leucine residue with arginine in the CRIC domain increases the binding affinity of CNK1 to the AT2 receptor. The insertion of a negatively charged amino acid stretch into the linker region between the N- and the C-terminal part of CNK1 strengthens the interaction between CNK1 and the AT2 receptor in a Ras-regulated manner. The biological significance of the interaction was supported by coprecipitation of CNK1 and the AT2 receptor in mouse heart extracts. Thus, CNK1 may play a role in the AT2 receptor-mediated signaling pathways.

SIGNALING NETWORKS IN LIVING CELLS

Recent advances in cell signaling research suggest that multiple sets of signal transducing molecules are preorganized and sequestered in distinct compartments within the cell. These compartments are assembled and maintained by specific cellular machinery. The molecular ecology within a compartment creates an environment that favors the efficient and accurate integration of signaling information arriving from humoral, mechanical, and nutritional sources. The functional organization of these compartments suggests they are the location of signaling networks that naturally organize into hierarchical interconnected sets of molecules through their participation in different classes of interacting units. An important goal is to determine the contribution of the compartment to the function of these networks in living cells.

Caenorhabditis elegans CNK-1 promotes Raf activation but is not essential for Ras/Raf signaling

Connector enhancer of Ksr (CNK) is a conserved multidomain protein essential for Ras signaling in Drosophila melanogaster and thought to be involved in Raf kinase activation. However, the precise role of CNK in Ras signaling is not known, and mammalian CNKs are proposed to have distinct functions. Caenorhabditis elegans has a single CNK homologue, cnk-1. Here, we describe the role of cnk-1 in C. elegans Ras signaling and its requirements for LIN-45 Raf activation. We find that cnk-1 positively regulates multiple Ras signaling events during development, but, unlike Drosophila CNK, cnk-1 does not appear to be essential for signaling. cnk-1 mutants appear to be normal but show cell-type-specific genetic interactions with mutations in two other Ras pathway scaffolds/adaptors ksr-1 and sur-8. Genetic epistasis using various activated LIN-45 Raf transgenes shows that CNK-1 promotes LIN-45 Raf activation at a step between the dephosphorylation of inhibitory sites in the regulatory domain and activating phosphorylation in the kinase domain. Our data are consistent with a model in which CNK promotes Raf phosphorylation/activation through membrane localization, oligomerization, or association with an activating kinase.

A novel isoform of Vinexin, Vinexin gamma, regulates Sox9 gene expression through activation of MAPK cascade in mouse fetal gonad

Recent loss-of-function and gain-of-function studies have revealed that transcription factor Sox9 is required for testis formation by governing Sertoli cell differentiation, and thereafter regulating transcription of Sertoli marker genes. In the present study, we identified a novel isoform of Vinexin, which is expressed in somatic cells but not germ cells of sexually indifferent stages of fetal gonads. After the sex is determined, the expression continues in testicular Sertoli cells. Immunohistochemical analyses with a specific antibody to Vinexin indicated that Vinexin gamma is localized in the cytoplasm. Functional studies with C3H10T1/2 cells showed that Vinexin gamma acted as a scaffold protein to activate MEK and ERK through interaction with c-Raf and ERK. Ultimately, Sox9 transcription was induced by Vinexin gamma. This up-regulation of Sox9 expression disappeared when the cells were treated with a specific MEK inhibitor, U0126. To determine the role of Vinexin gamma during gonad formation, the gene was disrupted by targeted mutagenesis. The phenotype displayed by the mice indicated that ERK activation was decreased in the Vinexin gamma(-/-) XY gonads, and Sox9 expression was down-regulated. Thus, Vinexin gamma seems to be implicated in regulation of Sox9 gene expression by modulating MAPK cascade in mouse fetal gonads.

A Drosophila model of multiple endocrine neoplasia type 2

Dominant mutations in the Ret receptor tyrosine kinase lead to the familial cancer syndrome multiple endocrine neoplasia type 2 (MEN2). Mammalian tissue culture studies suggest that RetMEN2 mutations significantly alter Ret-signaling properties, but the precise mechanisms by which RetMEN2 promotes tumorigenesis remain poorly understood. To determine the signal transduction pathways required for RetMEN2 activity, we analyzed analogous mutations in the Drosophila Ret ortholog dRet. Overexpressed dRetMEN2 isoforms targeted to the developing retina led to aberrant cell proliferation, inappropriate cell fate specification, and excessive Ras pathway activation. Genetic analysis indicated that dRetMEN2 acts through the Ras-ERK, Src, and Jun kinase pathways. A genetic screen for mutations that dominantly suppress or enhance dRetMEN2 phenotypes identified new genes that are required for the phenotypic outcomes of dRetMEN2 activity. Finally, we identified human orthologs for many of these genes and examined their status in human tumors. Two of these loci showed loss of heterozygosity (LOH) within both sporadic and MEN2-associated pheochromocytomas, suggesting that they may contribute to Ret-dependent oncogenesis.

CNK1 is a scaffold protein that regulates Src-mediated Raf-1 activation

Raf-1 is a regulator of cellular proliferation, differentiation, and apoptosis. Activation of the Raf-1 kinase activity is tightly regulated and involves targeting to the membrane by Ras and phosphorylation by various kinases, including the tyrosine kinase Src. Here we demonstrate that the connector enhancer of Ksr1, CNK1, mediates Src-dependent tyrosine phosphorylation and activation of Raf-1. CNK1 binds preactivated Raf-1 and activated Src and forms a trimeric complex. CNK1 regulates the activation of Raf-1 by Src in a concentration-dependent manner typical for a scaffold protein. Down-regulation of endogenously expressed CNK1 by small inhibitory RNA interferes with Src-dependent activation of ERK. Thus, CNK1 allows cross-talk between Src and Raf-1 and is essential for the full activation of Raf-1.

Administration of ricin induces a severe inflammatory response via nonredundant stimulation of ERK, JNK, and P38 MAPK and provides a mouse model of hemolytic uremic syndrome

Recent interest in the health consequences of ricin as a weapon of terrorism has led us to investigate the effects of ricin on cells in vitro and in mice. Our previous studies showed that depurination of the 28S rRNA by ricin results in the inhibition of translation and the coordinate activation of the stress-activated protein kinases JNK and p38 MAPK. In RAW 264.7 macrophages, ricin induced the activation of ERK, JNK, and p38 MAPK, the accumulation of mRNA encoding tumor necrosis factor (TNF)-alpha, interleukin (IL)-1, the transcription factors c-Fos, c-Jun, and EGR1, and the appearance of TNF-alpha protein in the culture medium. Using specific inhibitors of MAPKs, we demonstrated the nonredundant roles of the individual MAPKs in mediating proinflammatory gene activation in response to ricin. Similarly, the intravenous administration of ricin to mice led to the activation of ERK, JNK, and p38 MAPK in the kidneys, and increases in plasma-borne TNF-alpha, IL-1beta, and IL-6. Ricin-injected mice developed the hallmarks of hemolytic uremic syndrome, including thrombotic microangiopathy, hemolytic anemia, thrombocytopenia, and acute renal failure. Microarray analyses demonstrated a massive proinflammatory transcriptional response in the kidneys, coincidental with the symptoms of hemolytic uremic syndrome. Therapeutic management of the inflammatory response may affect the outcome of intoxication by ricin.

Functions and mechanisms of receptor tyrosine kinase Torso signaling: lessons from Drosophila embryonic terminal development

The Torso receptor tyrosine kinase (RTK) is required for cell fate specification in the terminal regions (head and tail) of the early Drosophila embryo. Torso contains a split tyrosine kinase domain and belongs to the type III subgroup of the RTK superfamily that also includes the platelet-derived growth factor receptors, stem cell or steel factor receptor c-Kit proto-oncoprotein, colony-stimulating factor-1 receptor, and vascular endothelial growth factor receptor. The Torso pathway has been a model system for studying RTK signal transduction. Genetic and biochemical studies of Torso signaling have provided valuable insights into the biological functions and mechanisms of RTK signaling during early Drosophila embryogenesis.

Myocardin/MKL family of SRF coactivators: key regulators of immediate early and muscle specific gene expression

Myocardin, megakaryoblastic leukemia-1 (MKL1), and MKL2 belong to a newly defined family of transcriptional coactivators. All three family members bind to serum response factor (SRF) and strongly activate transcription from promoters with SRF binding sites. SRF is required for the serum induction of immediate early genes such as c-fos and for the expression of many muscle specific genes. Consistent with a role in muscle specific gene expression, myocardin is specifically expressed in cardiac and smooth muscle cells while MKL1 and 2 are broadly expressed. Myocardin has particularly been shown to be required for smooth muscle development while MKL1/2 are required for the RhoA signaling pathway for induction of immediate early genes. SRF can be activated by at least two families of coactivators, p62TCF and myocardin/MKL. These factors bind to the same region of SRF such that their binding is mutually exclusive. This provides one mechanism of regulation of SRF target genes by pathways that differentially activate the coactivators. The RhoA pathway appears to activate MKL1 by altering MKL1's binding to actin and causing MKL1's translocation from the cytoplasm to the nucleus. However, this mechanism of activation of the myocardin/MKL family has not been observed in all cell types such that other regulatory mechanism(s) likely exist. In particular, rapid serum inducible phosphorylation of MKL1 was observed. The regulation of this coactivator family is key to understanding how SRF target genes are activated during muscle cell differentiation or growth factor induced cell proliferation.

Hypertonic shock inhibits growth factor receptor signaling, induces caspase-3 activation, and causes reversible fragmentation of the mitochondrial network

Hyperosmotic stress can be encountered by the kidney and the skin, as well as during treatment of acute brain damage. It can lead to cell cycle arrest or apoptosis. Exactly how mammalian cells detect hyperosmolarity and how the cell chooses between cell cycle arrest or death remains to be established. It has been proposed that hyperosmolarity is detected directly by growth factor receptor protein tyrosine kinases. To investigate this, we tested whether growth factors and osmotic stress cooperate in the activation of signaling pathways. Receptors responded normally to the presence of growth factors, and we observed normal levels of GTP-bound Ras under hyperosmotic conditions. In contrast, activation of Raf, Akt, ERK1, ERK2, and c-Jun NH2-terminal kinase was strongly reduced. These observations suggest that hyperosmotic conditions block signaling directly downstream of active Ras. It is thought that apoptotic cell death due to environmental stress is initiated by cytochrome c release from the mitochondria. Visualization of cytochrome c using immunofluorescence showed that hypertonic conditions result in a breakup of the mitochondrial network, which is reestablished within 1 h after hypertonic medium is replaced with isotonic medium. When we carried out live imaging, we observed that the mitochondrial membrane potential disappeared immediately after the onset of hyperosmotic shock. Our observations provide new insights into the hypertonic stress response pathway. In addition, they show that signaling downstream of Ras and mitochondrial dynamics can easily be manipulated by the exposure of cells to hyperosmotic conditions.

Src42 binding activity regulates Drosophila RAF by a novel CNK-dependent derepression mechanism

Connector enhancer of KSR (CNK), an essential component of Drosophila receptor tyrosine kinase/mitogen-activated protein kinase pathways, regulates oppositely RAF function. This bimodal property depends on the N-terminal region of CNK, which integrates RAS activity to stimulate RAF and a bipartite element, called the RAF-inhibitory region (RIR), which binds and inhibits RAF catalytic activity. Here, we show that the repressive effect of the RIR is counteracted by the ability of Src42 to associate, in an RTK-dependent manner, with a conserved region located immediately C-terminal to the RIR. Strikingly, we found that several cnk loss-of-function alleles have mutations clustered in this area and provide evidence that these mutations impair Src42 binding. Surprisingly, the derepressing effect of Src42 does not appear to involve its catalytic function, but critically depends on the ability of its SH3 and SH2 domains to associate with CNK. Together, these findings suggest that the integration of RTK-induced RAS and Src42 signals by CNK as a two-component input is essential for RAF activation in Drosophila.

Bioluminescence resonance energy transfer identify scaffold protein CNK1 interactions in intact cells

Connector enhancer of KSR (CNK) proteins have been proposed to act as scaffolds in the Ras-MAPK pathway. In this work, using in vivo bioluminescence resonance energy transfer (BRET) assays and in vitro co-immunoprecipitation, we show that hCNK1 interacts with the active form of Rho A (G14V) proteins. The domain of hCNK1 that allows binding to Rho proteins involves the C-terminal PH domain. Overexpression of hCNK1 does not affect the actin cytoskeleton and does not modify the appearance of stress fibers in cells overexpressing a constitutively active form of RhoA. In contrast, hCNK1 was able to significantly decrease the RhoA-induced transcriptional activity of the serum response element (SRE) without effect on the Ras-induced SRE activation. These results identify hCNK1 as a specific partner of Rho proteins both in vitro and in vivo and suggest a role of hCNK1 in the signal transduction of Rho proteins.

Crystal structure of the p14/MP1 scaffolding complex: how a twin couple attaches mitogen-activated protein kinase signaling to late endosomes

Signaling pathways in eukaryotic cells are often controlled by the formation of specific signaling complexes, which are coordinated by scaffold and adaptor proteins. Elucidating their molecular architecture is essential to understand the spatial and temporal regulation of cellular signaling. p14 and MP1 form a tight (K(d) = 12.8 nM) endosomal adaptor/scaffold complex, which regulates mitogen-activated protein kinase (MAPK) signaling. Here, we present the 1.9-A crystal structure of a biologically functional p14/MP1 complex. The overall topology of the individual MP1 and p14 proteins is almost identical, having a central five-stranded beta-sheet sandwiched between a two-helix and a one-helix layer. Formation of the p14/MP1 heterodimer proceeds by beta-sheet augmentation and yields a unique, almost symmetrical, complex with several potential protein-binding sites on its surface. Mutational analysis allowed identification of the p14 endosomal adaptor motif, which seems to orient the complex relative to the endosomal membrane. Two highly conserved and hydrophobic protein-binding sites are located on the opposite "cytoplasmic" face of the p14/MP1 heterodimer and might therefore function as docking sites for the target proteins extracellular regulated kinase (ERK) 1 and MAPK/ERK kinase 1. Furthermore, detailed sequence analyses revealed that MP1/p14, together with profilins, define a protein superfamily of small subcellular adaptor proteins, named ProflAP. Taken together, the presented work provides insight into the spatial regulation of MAPK signaling, illustrating how p14 and MP1 collaborate as an endosomal adaptor/scaffold complex.

Kinase Suppressor of Ras Couples Ras to the ERK Cascade during T Cell Development

Ras signaling is critical for many developmental processes and requires the precise coordination of interactions among multiple downstream components. One mechanism by which this regulation is achieved is through the use of scaffolding molecules that coordinate the assembly of multimolecular complexes. Recently, the scaffolding molecule kinase suppressor of Ras (KSR) was isolated in genetic screens as a modifier of Ras signaling, although its contribution to regulating Ras-mediated activation of its different downstream effectors is not well understood. We have analyzed the role of KSR in linking Ras to the ERK cascade during positive selection. Our results demonstrate that KSR overexpression interferes with T cell development, an effect that requires the direct interaction between KSR and MEK. This functional effect correlates with the ability of KSR to uncouple Ras from the ERK cascade when overexpressed.

CNK2 couples NGF signal propagation to multiple regulatory cascades driving cell differentiation

Neuronal precursor cells have the capacity to engage the Raf-MEK-ERK signal module to drive either of two distinctly different regulatory programs, proliferation and differentiation. This is, at least in part, a consequence of stimulus-specific shaping of the kinase cascade response. For example, the mitogen EGF induces a transient ERK activation, whereas the neurotrophin NGF induces prolonged ERK activation. Here we define a novel component of the regulatory machinery contributing to the selective integration of MAP kinase signaling with discrete biological responses. We show that the scaffold/adaptor protein CNK2/MAGUIN-1 is required for NGF- but not EGF-induced ERK activation. In addition, CNK2 makes a separate, essential contribution to the coupling of NGF signaling to membrane/cytoskeletal remodeling. We propose that CNK2 integrates multiple regulatory pathways that must function in concert to drive an appropriate biological response to external stimuli.

The major vault protein is a novel substrate for the tyrosine phosphatase SHP-2 and scaffold protein in epidermal growth factor signaling

The catalytic activity of the Src homology 2 (SH2) domain-containing tyrosine phosphatase, SHP-2, is required for virtually all of its signaling effects. Elucidating the molecular mechanisms of SHP-2 signaling, therefore, rests upon the identification of its target substrates. In this report, we have used SHP-2 substrate-trapping mutants to identify the major vault protein (MVP) as a putative SHP-2 substrate. MVP is the predominant component of vaults that are cytoplasmic ribonucleoprotein complexes of unknown function. We show that MVP is dephosphorylated by SHP-2 in vitro and it forms an enzyme-substrate complex with SHP-2 in vivo. In response to epidermal growth factor (EGF), SHP-2 associates via its SH2 domains with tyrosyl-phosphorylated MVP. MVP also interacts with the activated form of the extracellular-regulated kinases (Erks) in response to EGF and a constitutive complex between tyrosyl-phosphorylated MVP, SHP-2, and the Erks was detected in MCF-7 breast cancer cells. Using MVP-deficient fibroblasts, we demonstrate that MVP cooperates with Ras for optimal EGF-induced Elk-1 activation and is required for cell survival. We propose that MVP functions as a novel scaffold protein for both SHP-2 and Erk. The regulation of MVP tyrosyl phosphorylation by SHP-2 may play an important role in cell survival signaling.

The scaffold protein CNK1 interacts with the tumor suppressor RASSF1A and augments RASSF1A-induced cell death

The connector enhancer of KSR (CNK) is a multidomain scaffold protein discovered in Drosophila, where it is necessary for Ras activation of the Raf kinase. Recent studies have shown that CNK1 also interacts with RalA and Rho and participates in some aspects of signaling by these GTPases. Herein we demonstrate a novel aspect of CNK1 function, i.e. reexpression of CNK1 suppresses tumor cell growth and promotes apoptosis. As shown previously for apoptosis induced by Ki-Ras(G12V), CNK1-induced apoptosis is suppressed by a dominant inhibitor of the mammalian sterile 20 kinases 1 and (MST1/MST2). Immunoprecipitates of MST1 endogenous to LoVo colon cancer cells contain endogenous CNK1; however, no association of these two polypeptides can be detected in a yeast two-hybrid assay. CNK1 does, however, bind directly to the RASSF1A and RASSF1C polypeptides, constitutive binding partners of the MST1/2 kinases. Deletion of the MST1 carboxyl-terminal segment that mediates its binding to RASSF1A/C eliminates the association of MST1 with CNK1. Coexpression of CNK1 with the tumor suppressive isoform, RASSF1A, greatly augments CNK1-induced apoptosis, whereas the nonsuppressive RASSF1C isoform is without effect on CNK1-induced apoptosis. Overexpression of CNK1-(1-282), a fragment that binds RASSF1A but is not proapoptotic, blocks the apoptosis induced by CNK1 and by Ki-Ras(G12V). Thus, in addition to its positive role in the proliferative outputs of active Ras, the CNK1 scaffold protein, through its binding of a RASSF1A.MST complex, also participates in the proapoptotic signaling initiated by active Ras.

Human CNK1 acts as a scaffold protein, linking Rho and Ras signal transduction pathways

Rho family GTPases act as molecular switches to control a variety of cellular responses, including cytoskeletal rearrangements, changes in gene expression, and cell transformation. In the active, GTP-bound state, Rho interacts with an ever-growing number of effector molecules, which promote distinct biochemical pathways. Here, we describe the isolation of hCNK1, the human homologue of Drosophila connector enhancer of ksr, as an effector for Rho. hCNK1 contains several protein-protein interaction domains, and Rho interacts with one of these, the PH domain, in a GTP-dependent manner. A mutant hCNK1, which is unable to bind to Rho, or depletion of endogenous hCNK1 by using RNA interference inhibits Rho-induced gene expression via serum response factor but has no apparent effect on Rho-induced stress fiber formation, suggesting that it acts as a specific effector for transcriptional, but not cytoskeletal, activation pathways. Finally, hCNK1 associates with Rhophilin and RalGDS, Rho and Ras effector molecules, respectively, suggesting that it acts as a scaffold protein to mediate cross talk between the two pathways.

Regulation of MAP kinase signaling modules by scaffold proteins in mammals

The mitogen-activated protein kinase (MAPK) group of serine/threonine protein kinases mediates the response of cells to many extracellular stimuli such as cytokines and growth factors. These protein kinases include the extracellular signal-regulated protein kinases (ERK) and two stress-activated protein kinases (SAPK), the c-Jun N-terminal kinases (JNK), and the p38 MAPK. The enzymes are evolutionarily conserved and are activated by a common mechanism that involves a protein kinase cascade. Scaffold proteins have been proposed to interact with MAPK pathway components to create a functional signaling module and to control the specificity of signal transduction. Here we critically evaluate the evidence that supports a physiologically relevant role of MAPK scaffold proteins in mammals.

Human homologue of Drosophila CNK interacts with Ras effector proteins Raf and Rlf

Connector enhancer of KSR (CNK) is a multidomain protein that participates in Ras signaling in Drosophila eye development. In this report we identify the human homologue of CNK, termed CNK2A, and a truncated alternatively spliced variant, CNK2B. We characterize CNK2 phosphorylation, membrane localization, and interaction with Ras effector molecules. Our results show that MAPK signaling appears to play a role in the phosphorylation of CNK2 in vivo. CNK2 is found in both membrane and cytoplasmic fractions of the cell. In MDCK cells, full-length CNK2 is localized to the lateral plasma membrane. Consistent with previous reports, we show CNK2 interacts with Raf. CNK2 interaction was mapped to the regulatory and kinase domains of Raf, as well as to the carboxyl-terminal half of CNK2. CNK2 also interacts with the Ral signaling components, Ral GTPase, and the RalGDS family member Rlf. CNK2 interaction was mapped to the GEF domain of Rlf. The ability of CNK2 to interact with both Ras effector proteins Raf and Rlf suggests that CNK2 may integrate signals between MAPK and Ral pathways through a complex interplay of components.

Bimodal regulation of RAF by CNK in Drosophila

Connector enhancer of KSR (CNK) is a multidomain-containing protein previously identified as a positive regulator of the RAS/MAPK pathway in Drosophila. Using transfection experiments and an RNAi-based rescue assay in Drosophila S2 cells, we demonstrate that CNK has antagonistic properties with respect to RAF activity. We show that CNK's N-terminal region contains two domains (SAM and CRIC) that are essential for RAF function. Unexpectedly, we also report that the C-terminal region of CNK contains a short bipartite element that strongly inhibits RAF catalytic function. Interestingly, CNK's opposite properties appear to prevent signaling leakage from RAF to MEK in the absence of upstream signals, but then transforms into a potent RAF activator upon signal activation. Together, these findings suggest that CNK not only participates in the elusive RAF activation process, but might also contribute to the switch-like behavior of the MAPK module.

A genetic screen in Drosophila for metastatic behavior

A genetic screen was designed in Drosophila to interrogate its genome for mutations sufficient to cause noninvasive tumors of the eye disc to invade neighboring or distant tissues. We found that cooperation between oncogenic RasV12 expression and inactivation of any one of a number of genes affecting cell polarity leads to metastatic behavior, including basement membrane degradation, loss of E-cadherin expression, migration, invasion, and secondary tumor formation. Inactivation of these cell polarity genes cannot drive metastatic behavior alone or in combination with other tumor-initiating alterations. These findings suggest that the oncogenic background of tissues makes a distinct contribution toward metastatic development.

Mechanisms of regulating the Raf kinase family

The MAP Kinase pathway is a key signalling mechanism that regulates many cellular functions such as cell growth, transformation and apoptosis. One of the essential components of this pathway is the serine/threonine kinase, Raf. Raf (MAPKK kinase, MAPKKK) relays the extracellular signal from the receptor/Ras complex to a cascade of cytosolic kinases by phosphorylating and activating MAPK/ERK kinase (MEK; MAPK kinase, MAPKK) that phosphorylates and activates extracellular signal regulated kinase (ERK; mitogen-activated protein kinase, MAPK), which phosphorylates various cytoplasmic and nuclear proteins. Regulation of both Ras and Raf is crucial in the proper maintenance of cell growth as oncogenic mutations in these genes lead to high transforming activity. Ras is mutated in 30% of all human cancers and B-Raf is mutated in 60% of malignant melanomas. The mechanisms that regulate the small GTPase Ras as well as the downstream kinases MEK and extracellular signal regulated kinase (ERK) are well understood. However, the regulation of Raf is complex and involves the integration of other signalling pathways as well as intramolecular interactions, phosphorylation, dephosphorylation and protein-protein interactions. From studies using mammalian isoforms of Raf, as well as C. elegans lin45-Raf, common patterns and unique differences of regulation have emerged. This review will summarize recent findings on the regulation of Raf kinase.

Expression profiling of osteosarcoma cells transfected with MDR1 and NEO genes: regulation of cell adhesion, apoptosis, and tumor suppression-related genes

The expression patterns of the osteosarcoma cell line U-2 OS, and three derived subclones containing stably transfected MDR1, NEO and MDR1/NEO genes were compared using cDNA microarrays comprising 8976 known genes and expressed sequenced tags. Data provided new insights into three critical issues. First, MDR1 overexpression was associated with altered expression of genes related to several cellular pathways, including (a). drug influx/efflux (eg, dynamin 3), (b). metabolic enzymes (eg, monoamine oxidase A), (c). cell adhesion (eg, EPCAM), (d). apoptotic signaling (eg, I-TRAF), (e). senescence (eg, telomerase RNA binding protein staufen), (f). tumor suppression-related genes (eg, KISS-1 and ephrin B3), and (g). immune system receptors (eg, LENG2). MDR1, EPCAM, and ephrin B3 expression was confirmed by immunohistochemistry. Second, MDR1 transfected cells selected with either doxorubicin or neomycin showed distinct expression profiles that could be related to differential selection. Moreover, hierarchical clustering indicated that cells transfected with MDR1 alone, or cotransfected with NEO, displayed more closely related expression profiles than cells transfected only with NEO. Third, transfection with NEO and selection with neomycin produced a considerable number of expression changes within the cell. This study further elucidates the genetic events associated with MDR1 expression and identifies novel targets associated with multidrug resistance.

Erbin suppresses the MAP kinase pathway

We present evidence here that Erbin is a negative regulator of the Ras-Raf-Erk signaling pathway. Expression of Erbin decreases transcription of the AChR epsilon-subunit gene, an event that is mediated by Erk activation. Although it interacts with the ErbB2 C terminus through the PDZ domain, Erbin has no effect on ErbB2 tyrosine phosphorylation or binding to the adaptor proteins Shc and Grb2. In contrast, expression of Erbin greatly impairs activation of Erk, but not Akt, by ligands that activate receptor tyrosine kinases. Moreover, Erbin inhibits the Erk activation by active Ras, while it fails to do so in the presence of active Raf-1. Erbin associates with active Ras, but not inactive Ras nor Raf. Consistently, Erbin interferes with the interaction between Ras and Raf both in vivo and in vitro. Finally, overexpression of Erbin leads to inhibition of NGF-induced neuronal differentiation of PC12 cells, whereas down-regulation of endogenous Erbin by specific siRNA exhibits an opposite effect. Collectively, our study has identified Erbin as a novel suppressor of the Ras signaling by disrupting the Ras-Raf interaction.

Densin-180, a synaptic protein, links to PSD-95 through its direct interaction with MAGUIN-1

BACKGROUND

Densin-180, a brain-specific protein highly concentrated at the postsynaptic density (PSD), belongs to the LAP [leucine-rich repeats and PSD-95/Dlg-A/ZO-1 (PDZ) domains] family of proteins, some of which play fundamental roles in the establishment of cell polarity.

RESULTS

To identify new Densin-180-interacting proteins, we screened a yeast two-hybrid library using the COOH-terminal fragment of Densin-180 containing the PDZ domain as bait, and we isolated MAGUIN-1 as a Densin-180-binding protein. MAGUIN-1, a mammalian homologue of Drosophila connector enhancer of KSR (CNK), is known to interact with PSD-95 and has a short isoform, MAGUIN-2. The Densin-180 PDZ domain bound to the COOH-terminal PDZ domain-binding motif of MAGUIN-1. Densin-180 co-immunoprecipitated with MAGUIN-1 as well as with PSD-95 from the rat brain. In dissociated hippocampal neurones Densin-180 co-localized with MAGUINs and PSD-95, mainly at neuritic spines. In transfected cells, Densin-180 formed a ternary complex with MAGUIN-1 and PSD-95, whereas no association was detected between Densin-180 and PSD-95 in the absence of MAGUIN-1. MAGUIN-1 formed a dimer or multimer via the COOH-terminal leucine-rich region which is present in MAGUIN-1 but not in -2. Among the PDZ domains of PSD-95, the first was sufficient for interaction with MAGUIN-1.

CONCLUSION

These results suggest that the potential to dimerize or multimerize allows MAGUIN-1 to bind simultaneously to both Densin-180 and PSD-95, leading to the ternary complex assembly of these proteins at the postsynaptic membrane.