Regulation of the Ets-1 transcription factor by sumoylation and ubiquitinylation

Sumoylation and ubiquitinylation reversibly regulate the activity of transcription factors through covalent attachment to lysine residues of target proteins. We examined whether the Ets-1 transcription factor is modified by sumoylation and/or ubiquitinylation. Among four potential SUMO motifs in Ets-1, we identified lysines 15 and 227 within the LK(15)YE and IK(227)QE motifs, as being the sumoylation acceptor sites. Using transfection of Ets-1 wildtype (WT) or its sumoylation deficient version (Ets-1 K15R/K227R), as well as WT or mutant proteins of the SUMO pathway, we further demonstrated that the E2 SUMO-conjugating enzyme Ubc9 and a E3 SUMO ligase, PIASy, can enhance Ets-1 sumoylation, while a SUMO protease, SENP1, can desumoylate Ets-1. We also found that Ets-1 is modified by K48-linked polyubiquitinylation independently of the sumoylation acceptor sites and is degraded through the 26S proteasome pathway, while sumoylation of Ets-1 does not affect its stability. Finally, sumoylation of Ets-1 leads to reduced transactivation and we demonstrated that previously identified critical lysine residues in Synergistic Control motifs are the sumoylation acceptor sites of Ets-1. These data show that Ets-1 can be modified by sumoylation and/or ubiquitinylation, with sumoylation repressing transcriptional activity of Ets-1 and having no clear antagonistic action on the ubiquitin-proteasome degradation pathway.

Proapoptotic function of the MET tyrosine kinase receptor through caspase cleavage

The MET tyrosine kinase, the receptor of hepatocyte growth factor-scatter factor (HGF/SF), is known to be essential for normal development and cell survival. We report that stress stimuli induce the caspase-mediated cleavage of MET in physiological cellular targets, such as epithelial cells, embryonic hepatocytes, and cortical neurons. Cleavage occurs at aspartic residue 1000 within the SVD site of the juxtamembrane region, independently of the crucial docking tyrosine residues Y1001 or Y1347 and Y1354. This cleavage generates an intracellular 40-kDa MET fragment containing the kinase domain. The p40 MET fragment itself causes apoptosis of MDCK epithelial cells and embryonic cortical neurons, whereas its kinase-dead version is impaired in proapoptotic activity. Finally, HGF/SF treatment does not favor MET cleavage and apoptosis, confirming the known survival role of ligand-activated MET. Our results show that stress stimuli convert the MET survival receptor into a proapoptotic factor.

Conservation of epidermal growth factor receptor function in the human parasitic helminth Schistosoma mansoni

The epidermal growth factor receptor (EGF-R) plays an important role in development and cell differentiation, and homologues of EGF-R have been identified in a broad range of vertebrate and invertebrate organisms. This work concerns the functional characterization of SER, the EGF-R-like molecule previously identified in the helminth parasite Schistosoma mansoni. Transactivation assays performed in epithelial Madin-Darby canine kidney cells co-transfected with SER and a Ras-responsive reporter vector indicated that SER was able to trigger a Ras/ERK pathway in response to human epidermal growth factor (EGF). These results were confirmed in Xenopus oocytes showing that human EGF induced meiosis reinitiation characterized by germinal vesicle breakdown in SER-expressing oocytes. Germinal vesicle breakdown induced by EGF was dependent on receptor kinase activity and shown to be associated with phosphorylation of SER and of downstream ERK proteins. (125)I-EGF binding experiments performed on SER-expressing oocytes revealed high affinity (2.9 x 10(-9) M) of the schistosome receptor for human EGF. Phosphorylation of the native SER protein present in S. mansoni membranes was also shown to occur upon binding of human EGF. These data demonstrate the ability of the SER schistosome receptor to be activated by vertebrate EGF ligands as well as to activate the classical ERK pathway downstream, indicating the conservation of EGF-R function in S. mansoni. Moreover, human EGF was shown to increase protein and DNA synthesis as well as protein phosphorylation in parasites, supporting the hypothesis that host EGF could regulate schistosome development. The possible role of SER as a receptor for host EGF peptides and its implication in host-parasite signaling and parasite development are discussed.

Delineation of the region in the glycoprotein VI tail required for association with the Fc receptor gamma-chain

The glycoprotein VI (GPVI).Fc receptor gamma-chain (FcRgamma-chain) complex is the major activation receptor for collagen on platelets. GPVI cross-linking mediates activation through tyrosine phosphorylation of an ITAM (immunoreceptor tyrosine-based activation motif) in the FcR gamma-chain by Src family kinases. It has been previously shown that a transmembrane arginine and the cytoplasmic domain of GPVI are required for association with the FcR gamma-chain in immortalized cell lines. In this study, we have delineated the regions in the GPVI tail that promote binding to FcR gamma-chain and mediate functional responses to the snake venom convulxin by reconstitution of mutant forms of GPVI in RBL-2H3 cells. Sequential truncation of the cytoplasmic tail of GPVI revealed a major role for the basic region and a minor role for the juxtamembrane six amino acids in the association with FcR gamma-chain and functional responses to convulxin. Analysis of selective deletions in the GPVI tail supported this conclusion. In addition, we show that the proline-rich domain is required for optimal Ca2+ release, whereas it is dispensable for FcR gamma-chain association.

Association of Fyn and Lyn with the proline-rich domain of glycoprotein VI regulates intracellular signaling

The glycoprotein VI (GPVI)-Fc receptor (FcR) gamma-chain complex, a key activatory receptor for collagen on platelet surface membranes, is constitutively associated with the Src family kinases Fyn and Lyn. Molecular cloning of GPVI has revealed the presence of a proline-rich domain in the sequence of GPVI cytoplasmic tail which has the consensus for interaction with the Src homology 3 (SH3) domains of Fyn and Lyn. A series of in vitro experiments demonstrated the ability of the SH3 domains of both Src kinases to bind the proline-rich domain of GPVI. Furthermore, depletion of the proline-rich domain in GPVI (Pro(-)-GPVI) prevented binding of Fyn and Lyn and markedly reduced phosphorylation of FcR gamma-chain in transiently transfected COS-7 cells, but did not affect the association of the gamma-chain with GPVI. Jurkat cells stably transfected with wild type GPVI show robust increases in tyrosine phosphorylation and intracellular Ca2+ in response to the snake venom convulxin that targets GPVI. Importantly, convulxin is not able to activate cells transfected with Pro(-)-GPVI, even though the association with the immunoreceptor tyrosine-based activation motif-containing chains is maintained. These findings demonstrate that the proline-rich domain of GPVI mediates the association with Fyn/Lyn via their SH3 domain and that this interaction initiates activation signals through GPVI.

Interaction of calmodulin with the cytoplasmic domain of platelet glycoprotein VI

The platelet collagen receptor, glycoprotein VI (GPVI), and GPIb-IX-V, which binds von Willebrand factor, initiate platelet aggregation at low or high shear stress, respectively. We recently reported that positively charged, membrane-proximal sequences within cytoplasmic domains of GPIbbeta and GPV of GPIb-IX-V bind calmodulin. We now show that GPVI also binds calmodulin as follows-(1) calmodulin coimmunoprecipitated with GPVI from resting platelet lysates using an anti-GPVI IgG, but partially dissociated in platelets activated by collagen or collagen-related peptide; (2) calmodulin coprecipitated from platelet lysates with maltose-binding protein (MBP)-GPVI cytoplasmic domain fusion protein, but not MBP alone; (3) GPVI-related synthetic peptide based on the membrane-proximal sequence, His269-Pro287, induced a shift in calmodulin migration on nondenaturing gels, an assay that identifies calmodulin-binding peptides. His269-Pro287 is analogous to the calmodulin-binding sequence in GPIbbeta. The novel interaction of GPVI and calmodulin may regulate aspects of GPVI function.

The Fc receptor gamma-chain is necessary and sufficient to initiate signalling through glycoprotein VI in transfected cells by the snake C-type lectin, convulxin

There is extensive evidence that FcR gamma-chain couples to the collagen receptor glycoprotein VI (GPVI) and becomes phosphorylated on tyrosines upon receptor cross-linking. However, it is not established whether this receptor complex is sufficient to initiate the signalling cascade. We transfected GPVI and the FcR gamma-chain into the human erythroleukaemia cell line K562, which lacks detectable expression of GPVI and the FcR gamma-chain. The results show that GPVI is unable to signal when expressed alone, despite its surface expression, upon stimulation with the snake C-type lectin, convulxin. Coexpression of the FcR gamma-chain confers signalling properties on the receptor. Furthermore, cotransfection of the FcR gamma-chain and two mutant versions of GPVI shows that the transmembrane arginine and cytoplasmic tail of GPVI are necessary for association with the FcR gamma-chain. These results demonstrate that reconstitution of the GPVI-FcR gamma-chain complex in cells expressing the necessary signalling network is sufficient to initiate signalling events in response to convulxin and collagen-related peptide.

Hepatocyte growth factor/scatter factor activates the ETS1 transcription factor by a RAS-RAF-MEK-ERK signaling pathway

Hepatocyte growth factor/scatter factor (HGF/SF) induces scattering and morphogenesis of epithelial cells through the activation of the MET tyrosine kinase receptor. Although the activated MET receptor recruits a number of signaling proteins, little is known of the downstream signaling pathways activated by HGF/SF. In this study, we wished to examine the signaling pathway leading to activation of the ETS1 transcription factor. Using in vitro and in vivo kinase assays, we found that HGF/SF activates the ERK1 MAP kinase, leading to the phosphorylation of the threonine 38 residue of ETS1 within a putative MAP kinase phosphorylation site (PLLT38P). This threonine residue was neither phosphorylated by JNK1, nor by p38 MAP kinases and was required for the induction of transcriptional activity of ETS1 by HGF/SF. Using kinase and transcription assays, we further demonstrated that phosphorylation and activation of ETS1 occurs downstream of a RAS-RAF-MEK-ERK pathway. The functional involvement of this pathway in HGF/SF action was demonstrated using U0126, a pharmacological inhibitor of MEK, which blocked phosphorylation and activation of ETS1, RAS-dependent transcriptional responses, cell scattering and morphogenesis. These data demonstrated that ETS1 is a downstream target of HGF/SF acting through a RAS-RAF-MEK-ERK pathway and provides a signaling pathway leading to the regulation of gene expression by HGF/SF.

Regulation of RAS in human platelets. Evidence that activation of RAS is not sufficient to lead to ERK1-2 phosphorylation

In this study, we show that the G protein-coupled receptor agonist thrombin, the glycoprotein VI agonist convulxin, and the cytokine receptor Mpl agonist thrombopoietin (TPO) are able to induce activation of RAS in human platelets. Recruitment of GRB2 by tyrosine-phosphorylated proteins in response to TPO and convulxin but not by thrombin occurred with a similar time-course to RAS activation, consistent with a causal relationship. On the other hand, activation of ERK2 by thrombin and convulxin is delayed and also inhibited by the protein kinase C inhibitor Ro-31 8220, whereas RAS activation is unaffected. Further evidence for differential regulation of RAS and ERK is provided by the observations that TPO, which activates RAS but not protein kinase C, does not activate ERK, and that the inhibitor of SRC kinases PP1 inhibits activation of RAS but not ERK2 in response to thrombin. Our results demonstrate that activation of RAS is not necessarily coupled to ERK in human platelets.

C-terminal peptide of thrombospondin-1 induces platelet aggregation through the Fc receptor gamma-chain-associated signaling pathway and by agglutination

A peptide from the C-terminal domain of thrombospondin-1 (Arg-Phe-Tyr-Val-Val-Met-Trp-Lys; known as 4N1-1) has been reported to induce platelet aggregation and to bind to the integrin-associated protein (IAP), which is also known as CD47. In this study, it was discovered that 4N1-1 or its derivative peptide, 4N1K, induces rapid phosphorylation of the Fc receptor (FcR) gamma chain, Syk, SLP-76, and phospholipase C gamma2 in human platelets. A specific inhibitor of Src family kinases, 4-amino-4-(4-methylphenyl)-7-(t-butyl) pyrazola[3,4-d]pyrimidine, prevented phosphorylation of these proteins, abolished platelet secretion, and reduced aggregation by approximately 50%. A similar inhibition of aggregation to 4N1-1 was obtained in the presence of Arg-Gly-Asp-Ser in mouse platelets deficient in FcR gamma chain or SLP-76 and in patients with type I Glanzmann thrombasthenia. These results show that 4N1-1 signals through a pathway similar to that used by the collagen receptor glycoprotein (GP) VI. The alphaIIbbeta3-independent aggregation induced by 4N1-1 was also observed in fixed platelets and platelets from patients with Bernard-Soulier syndrome, which are deficient in GPIbalpha. Surprisingly, the ability of 4N1-1 to stimulate aggregation and tyrosine phosphorylation was not altered in platelets pretreated with anti-IAP antibodies and in IAP-deficient mice. These results show that the C-terminal peptide of thrombospondin induces platelet aggregation through the FcR gamma-chain signaling pathway and through agglutination. The latter pathway is independent of signaling events and does not use GPIbalpha or alphaIIbbeta3. Neither of these pathways is mediated by IAP.

A novel viper venom metalloproteinase, alborhagin, is an agonist at the platelet collagen receptor GPVI

The interaction of platelet membrane glycoprotein VI (GPVI) with collagen can initiate (patho)physiological thrombus formation. The viper venom C-type lectin family proteins convulxin and alboaggregin-A activate platelets by interacting with GPVI. In this study, we isolated from white-lipped tree viper (Trimeresurus albolabris) venom, alborhagin, which is functionally related to convulxin because it activates platelets but is structurally different and related to venom metalloproteinases. Alborhagin-induced platelet aggregation (EC50, <7.5 microg/ml) was inhibitable by an anti-alphaIIbbeta3 antibody, CRC64, and the Src family kinase inhibitor PP1, suggesting that alborhagin activates platelets, leading to alphaIIbbeta3-dependent aggregation. Additional evidence suggested that, like convulxin, alborhagin activated platelets by a mechanism involving GPVI. First, alborhagin- and convulxin-treated platelets showed a similar tyrosine phosphorylation pattern, including a similar level of phospholipase Cgamma2 phosphorylation. Second, alborhagin induced GPVI-dependent responses in GPVI-transfected K562 and Jurkat cells. Third, alborhagin-dependent aggregation of mouse platelets was inhibited by the anti-GPVI monoclonal antibody JAQ1. Alborhagin had minimal effect on convulxin binding to GPVI-expressing cells, indicating that these venom proteins may recognize distinct binding sites. Characterization of alborhagin as a GPVI agonist that is structurally distinct from convulxin demonstrates the versatility of snake venom toxins and provides a novel probe for GPVI-dependent platelet activation.

The role of ITAM- and ITIM-coupled receptors in platelet activation by collagen

The major activation-inducing collagen receptor glycoprotein VI (GPVI) has been cloned within the last two years. It is a member of the Ig superfamily of proteins and is constitutively associated with the ITAM-bearing Fc receptor gamma-chain (FcR gamma-chain). GPVI signals through a pathway that involves several of the proteins used by Fc, B- and T-lymphocyte receptors and which takes place in glycolipid-enriched membrane domains in the plasma membrane known as GEMs. Responses to GPVI are regulated by PECAM-1 (CD31) and possibly other ITIM-bearing receptors. Despite a pivotal role for GPVI, there are important differences between signalling events to collagen and GPVI-specific ligands. This may reflect a role for co-receptors in the response to collagen.

Sequential activation of ERK and repression of JNK by scatter factor/hepatocyte growth factor in madin-darby canine kidney epithelial cells

The scattering of Madin-Darby canine kidney (MDCK) epithelial cells by scatter factor/hepatocyte growth factor (SF/HGF) is associated with transcriptional induction of the urokinase gene, which occurs essentially through activation of an EBS/AP1 response element. We have investigated the signal transduction pathways leading to this transcriptional response. We found that SF/HGF induces rapid and sustained phosphorylation of the extracellular signal-regulated kinase (ERK) MAPK while stimulating weakly and then repressing phosphorylation of the JUN N-terminal kinase (JNK) MAPK for several hours. This delayed repression of JNK was preceded by phosphorylation of the MKP2 phosphatase, and both MKP2 induction and JNK dephosphorylation were under the control of MEK, the upstream kinase of ERK. ERK and MKP2 stimulate the EBS/AP1-dependent transcriptional response to SF/HGF, but not JNK, which inhibits this response. We further demonstrated that depending on cell density, the RAS-ERK-MKP2 pathway controls this transrepressing effect of JNK. Together, these data demonstrate that in a sequential manner SF/HGF activates ERK and MKP2, which in turn dephosphorylates JNK. This sequence of events provides a model for efficient cell scattering by SF/HGF at low cell density.

The multisubstrate docking site of the MET receptor is dispensable for MET-mediated RAS signaling and cell scattering

The scatter factor/hepatocyte growth factor regulates scattering and morphogenesis of epithelial cells through activation of the MET tyrosine kinase receptor. In particular, the noncatalytic C-terminal tail of MET contains two autophosphorylation tyrosine residues, which form a multisubstrate-binding site for several cytoplasmic effectors and are thought to be essential for signal transduction. We show here that a MET receptor mutated on the four C-terminal tyrosine residues, Y1311F, Y1347F, Y1354F, and Y1363F, can induce efficiently a transcriptional response and cell scattering, whereas it cannot induce cell morphogenesis. Although the mutated receptor had lost its ability to recruit and/or activate known signaling molecules, such as GRB2, SHC, GAB1, and PI3K, by using a sensitive association-kinase assay we found that the mutated receptor can still associate and phosphorylate a approximately 250-kDa protein. By further examining signal transduction mediated by the mutated MET receptor, we established that it can transmit efficient RAS signaling and that cell scattering by the mutated MET receptor could be inhibited by a pharmacological inhibitor of the MEK-ERK (MAP kinase kinase-extracellular signal-regulated kinase) pathway. We propose that signal transduction by autophosphorylation of the C-terminal tyrosine residues is not the sole mechanism by which the activated MET receptor can transmit RAS signaling and cell scattering.

The ETS1 transcription factor is expressed during epithelial-mesenchymal transitions in the chick embryo and is activated in scatter factor-stimulated MDCK epithelial cells

In embryos and in human tumors, the expression of the ETS1 transcription factor correlates with the occurrence of invasive processes. Although this was demonstrated in cells of mesodermal origin, the expression of ETS1 was not detected in epithelial cells. In the present study, we show that during early organogenesis in the chick embryo, ETS1 mRNA expression was transiently induced in epithelial structures, during emigration of neural crest cells and dispersion of somites into the mesenchymal sclerotome. In contrast, the expression of ETS1 was not detected in situations where epithelial layers stayed cohesive while forming a new structure, such as the dermomyotome forming the myotome. The involvement of ETS1 in epithelial cell dissociation was examined in MDCK epithelial cells stimulated by scatter factor/hepatocyte growth factor (SF/HGF), a potent inducer of cell dissociation and motility. SF/HGF was found to stimulate ETS1 mRNA and protein expressions, and these increases coincided with the dispersion of cells and the expression of protease mRNAs, such as urokinase-type plasminogen activator and collagenase, but not with the protease inhibitor, plasminogen activator inhibitor type 1. Furthermore, we showed that SF/HGF was able to induce a transcriptional response involving ETS1 by using artificial as well as cellular promoters, such as the urokinase-type plasminogen activator and collagenase 1 promoters, containing RAS-responsive elements with essential ETS-binding sites. These data demonstrate expression of ETS1 during epithelial-mesenchymal transitions in the developing embryo and show that ETS1 can act as a downstream effector of SF/HGF in MDCK epithelial cells. Taken together, these data identify ETS1 as a molecular actor of epithelia cell dissociation.