Véronique Fafeur

The ERK-dependent signalling is stage-specifically modulated by FSH, during primary Sertoli cell maturation

Primary cultures of Sertoli cells provide an interesting model to study how signalling pathways induced by a single hormone in a single cell type evolve, depending on the developmental stage. In vivo, follicle-stimulating hormone (FSH) induces proliferation of Sertoli cells in neonate and controls the subsequent differentiation of the entire population. Molecular mechanisms underlying Sertoli cell pleiotropic responses to FSH have long been investigated. But to date, only cAMP-dependent kinase (PKA) activation has been reported to account for most FSH biological activities in male. Here, we demonstrate that FSH activates the ERK MAP kinase pathway following dual coupling of the FSH-R both to Gs and to Gi heterotrimeric proteins, in a PKA- and also Src-dependent manner. This activation is required for FSH-induced proliferation of Sertoli cells isolated 5 days after birth. Consistently, we show that the ERK-mediated FSH mitogenic effect triggers upregulation of cyclin D1. In sharp contrast, at 19 days after birth, as cells proceed through their differentiation program, the ERK pathway is dramatically inhibited by FSH treatment. Taken together, these results show that FSH can exert opposite effects on the ERK signalling cascade during the maturation process of Sertoli cells. Thus, signalling modules triggered by the FSH-R evolve dynamically throughout development of FSH natural target cells.

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.

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 (Shoemaker, C. B., Ramachandran, H., Landa, A., dos Reis, M. G., and Stein, L. D. (1992) Mol. Biochem. Parasitol. 53, 17–32). 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. 125I-EGF binding experiments performed on SER-expressing oocytes revealed high affinity (2.9 × 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.

A One-Pot Three-Segment Ligation Strategy for Protein Chemical Synthesis†

Protein chemical synthesis by native peptide ligation of unprotected peptide segments is an interesting complement and potential alternative to the use of living systems for producing proteins. Actually, tremendous efforts are focused on the design of one-pot strategies allowing the assembly of three peptide segments. The goal is to get rapid access to small proteins (less than 150 amino acid residues), while saving intermediate purification steps and obtaining the products in good yield. Such methods are gaining increasing significance for the study of protein function and appear as a potential option for producing various protein-based therapeutics currently under development. To date, proteins were mainly assembled by sequential native chemical ligation (NCL) or extended methodologies in the C-to-N direction (for recent achievements, see Refs. [8, 9]). NCL involves the chemoselective ligation of a Cterminal peptide thioester, usually an alkylthioester, with an N-terminal cysteine (Cys) peptide. The one-pot sequential Cto-N ligation of three peptide segments designed by Kent et al. is increasingly used for synthesizing proteins. Methods that enable the assembly of peptide segments in the reverse N-to-C direction are rare. 11] Fundamentally, the combination of N-to-C and C-to-N assembly techniques is at the basis of the convergent total synthesis of proteins. The general principle of the one-pot assembly of three peptide segments in the N-to-C direction is illustrated in Scheme 1. Ligation of peptide segments A-X and H-Cys-B-Y yields segment A-Cys-B-Y (Scheme 1, ligation 1). Group Y must ideally be inert during ligation 1 or at least be significantly less reactive than group X to avoid oligomerization or cyclization of segment B. Activation of group Y into Y* subsequently allows the ligation with the third segment H-Cys-C (Scheme 1, ligation 2). For designing a one-pot process working in the N-to-C direction, this activation must be carried out in situ after ligation 1 by using reagents compatible with ligation 2. Furthermore, the Y* group must enable an efficient ligation with the Cys segment C. To date only few one-pot strategies have been described that work in the N-to-C direction and enable the coupling of three peptide segments. 5, 12] Fundamentally, these methods, such as kinetically controlled ligation, rely on the differential reactivity of X and Y groups for peptide-bond formation. In other words, the purity of the target polypeptide is highly dependent on the C-terminal residues of A and B segments and more generally on the accessibility of the reactive ends. Clearly, a strategy in which Y is inert during the first ligation step would bypass these limitations and constitute a critical advance. Herein we show that the combination of NCL and SEA ligation (Scheme 1) permitted design of a solution to this important problem. Reaction of a peptide featuring a C-terminal bis(2-sulfanylethyl)amido group, called SEA hereafter (Scheme 1), with a Cys peptide results in the formation of a native peptide bond in water at pH 7. This reaction probably proceeds via Scheme 1. Total protein synthesis by one-pot assembly of three peptide segments in the N-to-C direction. The first step is a native chemical ligation between thioester segment A and Cys segment B, during which the cyclic disulfide SEA acts as a blocked thioester group (SEA = bis(2-sulfanylethyl)amido). Activation of SEA into SEA by reduction with a phosphine and addition of the third Cys segment C triggers the second ligation step.

Down-Regulation of the Met Receptor Tyrosine Kinase by Presenilin-dependent Regulated Intramembrane Proteolysis

Hepatocyte growth factor/scatter factor (HGF/SF) acts through the membrane-anchored Met receptor tyrosine kinase to induce invasive growth. Deregulation of this signaling is associated with tumorigenesis and involves, in most cases, overexpression of the receptor. We demonstrate that Met is processed in epithelial cells by presenilin-dependent regulated intramembrane proteolysis (PS-RIP) independently of ligand stimulation. The proteolytic process involves sequential cleavage by metalloproteases and the gamma-secretase complex, leading to generation of labile fragments. In normal epithelial cells, although expression of cleavable Met by PS-RIP is down-regulated, uncleavable Met displayed membrane accumulation and induced ligand-independent motility and morphogenesis. Inversely, in transformed cells, the Met inhibitory antibody DN30 is able to promote Met PS-RIP, resulting in down-regulation of the receptor and inhibition of the Met-dependent invasive growth. This demonstrates the original involvement of a proteolytic process in degradation of the Met receptor implicated in negative regulation of invasive growth.

Proapoptotic Function of the MET Tyrosine Kinase Receptor through Caspase Cleavage

ABSTRACT 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.

Amplification of apoptosis through sequential caspase cleavage of the MET tyrosine kinase receptor.

Activation of the MET tyrosine kinase receptor by hepatocyte growth factor/scatter factor is classically associated with cell survival. Nonetheless, stress stimuli can lead to a caspase-dependent cleavage of MET within its juxtamembrane region, which generate a proapoptotic 40 kDa fragment (p40 MET). We report here that p40 MET is in fact generated through an additional caspase cleavage of MET within its extreme C-terminal region, which removes only few amino acids. We evidenced a hierarchical organization of these cleavages, with the C-terminal cleavage favoring the juxtamembrane one. As a functional consequence, the removal of the last amino acids of p40 MET increases its apoptotic capacity. Finally, cells expressing a MET receptor mutated at the C-terminal caspase site are unable to generate p40 MET and are resistant to apoptosis, indicating that generation of p40 MET amplifies apoptosis. These results revealed a two-step caspase cleavage of MET resulting in the reshaping of this survival receptor to a proapoptotic factor.

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 LK15YE and IK227QE 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.

Autoactivation of the Epstein-Barr virus oncogenic protein LMP1 during type II latency through opposite roles of the NF-kappaB and JNK signaling pathways.

ABSTRACT Epstein-Barr virus (EBV) is associated with several human malignancies where it expresses limited subsets of latent proteins. Of the latent proteins, latent membrane protein 1 (LMP1) is a potent transforming protein that constitutively induces multiple cell signaling pathways and contributes to EBV-associated oncogenesis. Regulation of LMP1 expression has been extensively described during the type III latency of EBV. Nevertheless, in the majority of EBV-associated tumors, the virus is commonly found to display a type II latency program in which it is still unknown which viral or cellular protein is really involved in maintaining LMP1 expression. Here, we demonstrate that LMP1 activates its own promoter pLMP1 through the JNK signaling pathway emerging from the TES2 domain. Our results also reveal that this activation is tightly controlled by LMP1, since pLMP1 is inhibited by LMP1-activated NF-κB signaling pathway. By using our physiological models of EBV-infected cells displaying type II latency as well as lymphoblastoid cell lines expressing a type III latency, we also demonstrate that this balanced autoregulation of LMP1 is shared by both latency programs. Finally, we show that this autoactivation is the most important mechanism to maintain LMP1 expression during the type II latency program of EBV.

The Transcription Factor ETS-1: Its Role in Tumour Development and Strategies for its Inhibition

Transcription factors are an important group of proteins. Changes in expression or activity of transcription factors result in diverse and manifold effects on the whole transcriptome of the cell. Therefore transcription factors are of special interest in physiological as well as pathological processes particularly tumour development and progression. In this review we focus on Ets-1, the prototype of the ETS family of transcription factors. ETS family members play important roles in development, differentiation and proliferation of cells in general and they are involved in apoptosis and tissue remodelling as well. Most of them are downstream nuclear targets of Ras-MAP kinase signalling and the deregulation of ets genes results in malignant transformation of different cells. Several ets genes are rearranged in human leukaemia, Ewing tumours and prostate cancer to produce chimeric oncoproteins. Furthermore, an aberrant expression of several ets genes is often observed in various types of human malignant tumours. With regard to the involvement of some ETS transcription factors, especially Ets-1, in malignant transformation and tumour progression (including invasion, metastasis and neoangiogenesis) through transactivation of cancer related genes, they are potential molecular targets for selective cancer therapy. In this review we focus on the roles of Ets-1 for tumour development and progression with special emphasis on tumour vascularization and invasion. We then discuss specific strategies for Ets-1 inhibition as a potential tool for cancer treatment.