No description available.

The extracellular signal-regulated protein kinase 5 (ERK5), also known as big mitogen-activated protein (MAP) kinase 1 (BMK1), is a non-redundant mitogen-activated protein kinase (MAPK) implicated in mediating the response of cells to mitogens, oxidative and osmotic stresses. The molecular complexity of the ERK5 cascade has been mostly delineated by over-expression studies. For example, like other MAPKs, ERK5 activity increases upon phosphorylation by a MAPK/ERK kinase, namely MEK5. However, the physiological role of ERK5 is not rigorously established by these data. Furthermore, in comparison to the other members of the family, little is known about the downstream targets of ERK5. This constitutes an obstacle for the molecular understanding of the signalling mechanisms that account for the effect of ERK5 activation in vivo. To clarify these issues, I have tested the effect of the conditional loss of ERK5 in primary mouse embryonic fibroblasts (MEFs). My results indicate that ERK5 is required for the proliferation of MEFs, at least in part, by promoting the entry into S phase of the cell cycle. ERK5 suppressed the expression of the cyclin-dependent protein kinase (CDK) inhibitors, p21 and p27. As a result, low-level CDK2 activity detected in ERK5-deficient MEFs correlated with hypo-phosphorylation of the retinoblastoma (Rb) protein and with a defect in G1 to S phase transition of the cell cycle. ERK5 blocks p21 expression by decreasing the stability of the p21 transcript. This process might, at least partially, involve a mechanism implicating c-Myc-induced transcriptional up-regulation of the miR-17-92 cluster. Concerning p27, ERK5 decreases p27 protein stability. The stabilisation of p27 in the absence of ERK5 resulted in the accumulation of the protein in the nucleus. To examine the relevance of my findings in cancer, I tested the effect of pharmacological inhibition of ERK5 in two human breast cancer cell lines, MCF7 and MDA- MB-231, using XMD8-92, a novel potent and selective inhibitor of ERK5. My results show that these cells are dependent on ERK5 to proliferate. Furthermore, I found that incubation of MDA- MB-231 cells with XMD8-92 compromised their ability to invade. In both breast cancer cell lines, ERK5 down-regulates p21 and p27 expression. Together with evidence that cancer patients with poor prognosis display a high-level of expression of components of the ERK5 signalling pathway, these findings support the hypothesis that ERK5 can be a potential target for cancer therapy.

Yes / Brain tumours kill more children and adults under 40 than any other cancer, with approximately half of primary brain tumours being diagnosed as high-grade malignancies known as glioblastomas. Despite de-bulking surgery combined with chemo-/radiotherapy regimens, the mean survival for these patients is only around 15 months, with less than 10% surviving over 5 years. This dismal prognosis highlights the urgent need to develop novel agents to improve the treatment of these tumours. To address this need, we carried out a human kinome siRNA screen to identify potential drug targets that augment the effectiveness of temozolomide (TMZ)-the standard-of-care chemotherapeutic agent used to treat glioblastoma. From this we identified ERK5/MAPK7, which we subsequently validated using a range of siRNA and small molecule inhibitors within a panel of glioma cells. Mechanistically, we find that ERK5 promotes efficient repair of TMZ-induced DNA lesions to confer cell survival and clonogenic capacity. Finally, using several glioblastoma patient cohorts we provide target validation data for ERK5 as a novel drug target, revealing that heightened ERK5 expression at both the mRNA and protein level is associated with increased tumour grade and poorer patient survival. Collectively, these findings provide a foundation to develop clinically effective ERK5 targeting strategies in glioblastomas and establish much-needed enhancement of the therapeutic repertoire used to treat this currently incurable disease.

Les cellules cancéreuses utilisent une glycolyse anaérobie pour générer l'ATP au lieu de la phosphorylation oxydative. Cette spécificité métabolique offre certains avantages aux cellules cancéreuses: une prolifération rapide et une évasion immune qui implique la sous-régulation de l'expression du CMH-I à la surface des cellules, phénomène lié au changement métabolique. Dans nos expériences, nous forçons les cellules leucémiques à produire de l'énergie par phosphorylation oxydative en les incubant avec de la glutamine comme source d'énergie en absence de glucose. La respiration ainsi forcée induit une augmentation de la transcription et de l'expression du CMH-I. Ce changement de métabolisme induit aussi une augmentation de l'expression de MAPK ERK5 et son accumulation dans les mitochondries. ERK5 intervient dans les changements de l'expression du CMH-I et du métabolisme. La sur-régulation du CMH-I induite par la respiration est bloquée dans les cellules leucémiques exprimant le shRNA shERK5. ERK5 régule la transcription de l'histone désacétylase de classe III Sirtuin 1 par l'activation de sa cible MEF2, ayant pour conséquence la liaison de MEF2 au promoteur de SIRT1. La régulation transcriptionnelle de SIRT1 induite par ERK5 intervient dans la réponse antioxydante des cellules leucémiques, et la sous-régulation d'ERK5 affecte cette réponse antioxydante. L'augmentation du métabolisme de la glutamine observée dans les cellules leucémiques est initiée par la glutaminase (GLS), enzyme qui est le facteur limitant de la vitesse du métabolisme de la glutamine. miR-23a cible l'ARN messager de GLS et inhibe l'expression de GLS. Le milieu glutamine induit la translocation de p65 dans le noyau, qui mène à une augmentation de l'activité transcriptionnelle de p65. NF-KB p65 inhibe l'expression de miR-23a en amenant HDAC4 sur le promoteur de miR-23a. Cela permet aux cellules leucémiques d'augmenter l'utilisation de la glutamine en tant que source alternative de carbone. Ainsi, la respiration forcée dans les cellules leucémiques contrôle l'expression du CMH-I, la réponse antioxydante et facilite la prolifération tumorale. / Cancer cells have anaerobic-like glycolysis to generate ATPs instead of oxidative phosphorylation. This specific metabolism provides advantages to cancer cells: rapid growth and immune evasion, which involves downregulation of MHC-I at the cell surface and it is linked to metabolic change. In our experiments, we force leukemic cells to produce energy by oxidative phosphorylation by incubating them with glutamine as an energy source in the absence of glucose. The forced respiration increases MHC-I transcription and protein level. This change of metabolism also leads to increase MAPK ERK5 expression and accumulation in mitochondria. ERK5 mediates changes in both MHC-I and metabolism. The respiration-induced upregulation of MHC-I is blocked in leukemic cells stably expressing short hairpin ERK5 (shERK5). ERK5 transcriptionally regulates the class III histone deacetylase Sirtuin 1 through activation of its target MEF2 and subsequently MEF2 binding to SIRT1 promoter. The ERK5-induced transcriptional regulation of SIRT1 mediates the antioxidant response in leukemic cells and downregulation of ERK5 impairs the antioxidant response. The increased glutamine metabolism found in leukemic cells is initiated by glutaminase (GLS), a rate limiting enzyme for glutamine metabolism. miR-23a targets GLS mRNA and inhibits GLS expression. The glutamine medium induces p65 translocation to the nucleus that leads to increase p65 transcriptional activity. NF-KB p65 inhibits miR-23a expression by bringing HDAC4 to the miR-23a promoter. This allows leukemic cells to increase the use of glutamine as an alternative source of carbon. Thus, forcing respiration in leukemic cells controls MHC-I expression, antioxidant response and facilitate tumor growth.

Yes / Extracellular-signal-regulated kinase 5 (ERK5), also termed big MAPK1 (BMK1), is the most recently discovered member of the mitogen-activated protein kinase (MAPK) family. It is expressed in a variety of tissues and is activated by a range of growth factors, cytokines and cellular stresses. Targeted deletion of Erk5 in mice has revealed that the ERK5 signalling cascade is critical for normal cardiovascular development and vascular integrity. In vitro studies have revealed that, in endothelial cells, ERK5 is required for preventing apoptosis, mediating shear-stress signalling and regulating tumour angiogenesis. The present review focuses on our current understanding of the role of ERK5 in regulating endothelial cell function. / Biotechnology and Biological Sciences Research Council, the Medical Research Council and the Wellcome Trust

Yes / Extracellular-signal-regulated kinase 5 (ERK5) is critical for normal cardiovascular development. Previous studies have defined a canonical pathway for ERK5 activation, showing that ligand stimulation leads to MEK5 activation resulting in dual phosphorylation of ERK5 on Thr218/Tyr220 residues within the activation loop. ERK5 then undergoes a conformational change, facilitating phosphorylation on residues in the C-terminal domain and translocation to the nucleus where it regulates MEF2 transcriptional activity. Our previous research into the importance of ERK5 in endothelial cells highlighted its role in VEGF-mediated tubular morphogenesis and cell survival, suggesting that ERK5 played a unique role in endothelial cells. Our current data show that in contrast to EGF-stimulated HeLa cells, VEGF-mediated ERK5 activation in human dermal microvascular endothelial cells (HDMECs) does not result in C-terminal phosphorylation of ERK5 and translocation to the nucleus, but instead to a more plasma membrane/cytoplasmic localisation. Furthermore, the use of small-molecule inhibitors to MEK5 and ERK5 shows that instead of regulating MEF2 activity, VEGF-mediated ERK5 is important for regulating AKT activity. Our data define a novel pathway for ERK5 activation in endothelial cells leading to cell survival. / This research was funded by grants from: North West Cancer Research (NWCR): M.J.C. and A.K.M.; Medical Research Council (MRC DiMeN PhD): M.J.C. and K.A.; Biotechnology and Biological Sciences Research Council (BBSRC DTG Studentship): M.J.C., C.E.P.G., B.W. and G.N.N.; and Wellcome Trust Institutional Strategic Fund: M.J.C. and A.K.M.

Platelet-derived growth factor (PDGF) is a family of signaling molecules that stimulates cell growth, survival and migration. PDGF is recognized by specific transmembrane proteins, the PDGF receptors, which relay the signals to the cell activating the Mitogen-activated protein (MAP) kinases and other signaling pathways. Aberrant activation of these pathways is frequently detected in cancer. Hence, the study of these processes is essential for identifying potential drug targets or diagnostic markers. In paper I, we identified Receptor Subfamily 4 Group A Member 1 NR4A1 to be regulated by PDGF via MAP kinases, clarifying the role of Extracellular signal–regulated kinases (Erk) 1/2, Erk5 and Nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) in its regulation. NR4A1 was found to be important for the tumorigenic potential, measured as anchorage-independent growth, of glioblastoma cells. Since the cellular responses elicited by PDGF result from the balance between phosphorylation and dephosphorylation events, we investigated the role of the dual specificity phosphatases DUSP4/MKP-2 and DUSP6/MKP-3. In paper II, we describe the crucial role of Erk1/2 and p53 in the expression of DUSP4/MKP2. Moreover, we observed that DUSP4/MKP-2 downregulation decreases Erk5 activation and accelerates PDGFRβ internalization and downregulation resulting in a specific inhibition of Signal transducers and activators of transcription (Stat) 3, Src and protein kinase C (PKC), and partially of p38, Stat1/5 and Phoshoplipase Cγ (PLCγ). In paper III, we report that DUSP6/MKP-3 creates a negative cross-talk between Erk1/2 and Erk5 and an auto-inhibitory feedback loop on the PI3-kinase/Akt pathway. In paper IV, we identify a new regulative mechanism of the PDGF pathway. PDGF induces Erk5 expression and activation that modulates the PDGFRβ activity. After Erk5 downregulation, the receptor undergoes to a faster and stronger activation that results in a faster internalization and degradation. In conclusion, we present a mechanism through which the PDGF/MAP kinases support tumor growth, and elucidate different regulatory pathways involved in PDGF signaling.

Heart failure induced by cardiac hypertrophy is a cause of high mortality in the world and has been the fastest growing cardiovascular disease over the past decade. Cardiac hypertrophy is characterised as a reactive increase in cardiac mass growth with a complex of ventricular remodelling. It occurs initially as a compensatory response to an increased workload but eventually leads to cardiac dysfunction. An in-depth understanding of cardiac hypertrophy and the capacity to regulate it has profound clinical implications. The MAPK pathways provide an important connection between external stimuli and intracellular signals for cardiac hypertrophic response. At least four MAPK subfamilies have been identified: extracellular-regulated protein kinases 1 and 2 (ERK1/2), ERK5, c-Jun NH2-terminal protein kinases (JNKs) and p38 MAPKs. Mitogen-activated protein kinase kinase 4 (MKK4), a vital activator of JNK and p38 is implicated as an important mediator of hypertrophy. ERK5, an atypical MAPK, is also involved in both hypertrophic growth and cardiomyocyte survival. However, conflicting data have been yielded from previously-published studies, since the results are based entirely on experiments conducted in cultured cardiomyocytes or transgenic and conventional knockout mouse models. To elucidate their biological roles and underlying signalling mechanisms in hypertrophy, mice with a cardiomyocyte-specific deletion of MKK4 or ERK5 (MKK4cko and ERK5cko mice) were generated in the present study. In response to pathological hypertrophic stresses including pressure overload or isoprenaline stimulation, MKK4cko mice developed exacerbated pathological hypertrophy with increased cardiomyocyte apoptosis, impaired cardiac function and remarkably upregulated NFAT (nuclear factor of T-cell) transcriptional activity. However, MKK4cko mice exhibited a similar extent of swimming exercise-induced physiological hypertrophy compared with the controls. In response to pathological hypertrophic stimuli, ERK5cko mice were resistant to hypertrophic growth, foetal gene induction and ventricular fibrosis, which is associated with repressed activation of MEF2 (myocyte enhancer factor 2). ERK5 deficiency also caused a profound increase in cardiomyocyte apoptosis which accounted for the impaired cardiac function. In conclusion, the present study provides biological evidence that clarifies in vivo functions of MKK4 and ERK5 in hypertrophy. MKK4 acts a protective role against pathological hypertrophy through inhibiting NFAT signalling, but it is not necessary for the regulation of physiological hypertrophy. ERK5 is essential for pathological hypertrophic remodelling and cardiomyocyte survival and its function in hypertrophic remodelling is mediated through regulation of MEF2 activity. Taken together, these data presented in my thesis advances knowledge about biological functions of MAPK pathways in the heart.

L’ARN polymérase II (ARNPII), l’enzyme responsable de la transcription des ARN messagers, procède au décodage du génome des organismes vivants. Cette fonction requiert l’action concertée de plusieurs protéines, les facteurs généraux de la transcription, par exemple, formant un réseau d’interactions protéine-protéine, plusieurs étant impliquées dans la régulation de l’ARNPII à différents niveaux. La régulation de la transcription a été largement étudiée durant les quatre dernières décennies. Néanmoins, nous en connaissons peu sur les mécanismes qui régulent l’ARNPII avant ou après la transcription. Dans la première partie de cette thèse, nous poursuivons la caractérisation du réseau d’interactions de l’ARNPII dans la fraction soluble de la cellule humaine, travail qui a débuté précédemment dans notre laboratoire. Ce réseau, développé à partir de la méthode de la purification d’affinité en tandem couplée à la spectrométrie de masse (AP-MS) et à des méthodes d’analyses bioinformatiques, nous amène une foule d’informations concernant la régulation de l’ARNPII avant et après son interaction avec la chromatine. Nous y identifions des protéines qui pourraient participer à l’assemblage de l’ARNPII telles des chaperonnes et les protéines du complexe R2TP/prefoldin-like ainsi que des protéines impliquées dans le transport nucléocytoplasmique. Au centre de ce réseau se trouvent RPAP4, une GTPase qui semble se positionner à l’interface entre ces protéines régulatrices et l’ARNPII. Nous avons donc entamé l’étude la fonction de RPAP4, ce qui nous a menés à la conclusion que RPAP4 est essentielle à l’import nucléaire de l’ARNPII au noyau, où elle exerce sa fonction. Nous avons également montré que les motifs G et GPN sont essentiels à la fonction de RPAP4. Le traitement des cellules avec le bénomyl nous montre aussi que la fonction de RPAP4 et l’import nucléaire de l’ARNPII requièrent l’action des microtubules. La deuxième partie de la thèse s’intéresse à une autre protéine positionnée au centre du réseau, RPAP2. Cette dernière partage plusieurs interactions avec RPAP4. Elle est aussi essentielle à la localisation nucléaire de l’ARNPII et interagit directement avec celle-ci. RPAP4 et RPAP2 étant toutes deux des protéines cytoplasmiques qui font la navette entre le noyau et le cytoplasme, nous présentons des évidences que RPAP4 est impliquée dans l’export nucléaire de RPAP2 pour permettre à celle-ci d’être disponible dans le cytoplasme pour l’import de l’ARNPII dans le noyau. Dans la troisième partie de la thèse, nous étudions plus en profondeur les modifications post-traductionnelles de RPAP4, ce qui nous aide à mieux comprendre sa propre régulation et sa fonction auprès de l’ARNPII. RPAP4 est phosphorylée en mitose par la MAP kinase ERK5. Cette phosphorylation favorise l’interaction entre RPAP4 et RPAP2, ce qui empêche RPAP2 d’interagir avec l’ARNPII pendant la mitose, prévenant du même coup, son interaction avec la chromatine pendant cette phase du cycle cellulaire où la transcription est presque inexistante. / RNA polymerase II, the enzyme responsible for transcription of messenger RNA, decodes the genome of living organisms. This function requires the concerted action of several proteins, including transcription factors, which form a protein-protein interaction network. Many of them are implicated in the regulation of RNAPII transcription. Although regulation of transcription has been largely studied during the last four decades, little is known about mechanisms that regulate RNAPII prior and after the transcription reaction. In the first part of this thesis, we continue the characterization of the RNAPII interaction network of RNAPII in the soluble fraction of the human cell. This network, developed using tandem affinity purification method coupled with mass spectrometry (AP-MS) and bioinformatic analysis, provides a wealth of information about RNAPII regulation prior and after its interaction with chromatin for transcription. We identified proteins that can be involved in RNAPII assembly, including chaperones and the cochaperone complex R2TP prefoldin-like, and proteins involved in nucleocytoplasmic shuttling. RPAP4 is a GTPase that occupies a central position in this network being at the interface between these regulatory proteins and RNAPII. We therefore started to study the function of RPAP4, which lead us to conclude that RPAP4 is essential for RNAPII nuclear import. We also report that G domains and the GPN motif are essential for RPAP4 function. Treatment of the cells with benomyl suggests that microtubules are required for RPAP4 function and RNAPII nuclear import. The second part concerns another protein found in the network that is also centrally positioned in the network, called RPAP2. RPAP2 shares many interactions with RPAP4. This protein is also essential for the nuclear import of RNAPII as it interacts directly with it. RPAP4 and RPAP2 being cytoplasmic proteins that shuttle between the cytoplasm and the nucleus, we show evidences that RPAP4 is implicated in RPAP2 nuclear export to make it available for RNAPII nuclear import. In the third part, we study RPAP4 post-translational modifications, which help us to understand its own regulation and its function with RNAPII. RPAP4 is phosphorylated in mitosis by the MAP kinase ERK5. This phosphorylation promotes the interaction between RPAP4 and RPAP2. It prevents RPAP2 and RNAPII interaction and RNAPII chromatin localization in mitosis where transcription is mostly nonexistent.