Role of the protein tyrosine phosphatase DEP-1 in Src activation and the mediation of biological cell functions of endothelial and breast cancer cells

L’implication des protéines tyrosines phosphatases (PTPs) dans la régulation de la signalisation et la médiation des fonctions cellulaires a été bien établie dans les dernières années. Cependant, les mécanismes moléculaires par lesquels les PTPs régulent les processus fondamentaux tels que l’angiogenèse demeurent méconnus. Il a été rapporté que l’expression de la PTP DEP-1 (Density-enhanced phosphatase 1) augmente avec la densité cellulaire et corrèle avec la déphosphorylation du récepteur VEGFR2. Cette déphosphorylation contribue à l’inhibition de contact dans les cellules endothéliales à confluence et diminue l’activité du VEGFR2 en déphosphorylant spécifiquement ses résidus catalytiques Y1054/1059. De plus, la plupart des voies de signalisation en aval du VEGFR2 sont diminuées sauf la voie Src-Gab1-AKT. DEP-1 déphosphoryle la Y529 de Src et contribue à la promotion de la survie dans les cellules endothéliales. L’objectif de cette thèse est de mieux définir le rôle de DEP-1 dans la régulation de l’activité de Src et les réponses biologiques dans les cellules endothéliales. Nous avons identifié les résidus Y1311 et Y1320 dans la queue C-terminale de DEP-1 comme sites majeurs de phosphorylation en réponse au VEGF. La phosphorylation de ces résidus est requise pour l’activation de Src et médie le remodelage des jonctions cellules-cellules dépendantes de Src. Ce remodelage induit la perméabilité, l’invasion et la formation de capillaires en réponse au VEGF. Nos résultats démontrent que la phosphorylation de DEP-1 sur résidu tyrosine est requise pour diriger la spécificité de DEP-1 vers son substrat Src. Les travaux révèlent pour la première fois un rôle positif de DEP-1 sur l’induction du programme angiogénique des cellules endothéliales. En plus de la phosphorylation sur tyrosine, DEP-1 est constitutivement phosphorylé sur la thréonine 1318 situé à proximité de la Y1320 en C-terminal. Cette localisation de la T1318 suggère que ce résidu pourrait être impliqué dans la régulation de la Y1320. En effet, nous avons observé que la T1318 de DEP-1 est phosphorylée potentiellement par CK2, et que cette phosphorylation régule la phosphorylation de DEP-1 sur tyrosine et sa capacité de lier et d’activer Src. En accord avec ces résultats, nos travaux révèlent que la surexpression du mutant DEP-1 T1318A diminue le remodelage des jonctions cellules-cellules et par conséquent la perméabilité. Nos résultats suggèrent donc que la T1318 de DEP-1 constitue un nouveau mécanisme de contrôle de la phosphorylation sur tyrosine et que ceci résulte en l’activation de Src et l’induction des fonctions biologiques des cellules endothéliales en réponse au VEGF. Suite à ces travaux dans les cellules endothéliales qui démontrent un rôle positif de DEP-1 dans la médiation des réponses angiogéniques, nous avons voulu approfondir nos connaissances sur l’implication potentielle de DEP-1 dans les cellules cancéreuses où l’activité de Src est requise pour la progression tumorale. Malgré le rôle connu de DEP-1 comme suppresseur tumoral dans différents types de cancer, nous avons émis l’hypothèse que DEP-1 pourrait promouvoir les fonctions biologiques dépendantes de Src telles que la migration et l’invasion dans les cellules cancéreuses. Ainsi, nous avons observé que l’expression de DEP-1 est plus élevée dans les lignées basales de cancer du sein qui sont plus invasives comparativement aux lignées luminales peu invasives. Dans les lignées basales, DEP-1 active Src, médie la motilité cellulaire dépendante de Src et régule la localisation des protéines impliquées dans l’organisation du cytosquelette. L’analyse d’un micro-étalage de tissu a révélé que l’expression de DEP-1 est associée avec une réduction tendencielle de survie des patients. Nos résultats proposent donc, un rôle de promoteur tumoral pour DEP-1 dans la progression du cancer du sein. Les travaux présentés dans cette thèse démontrent pour la première fois que DEP-1 peut agir comme promoteur des réponses angiogéniques et du phénotype pro-invasif des lignées basales du cancer du sein probablement du à sa capacité d’activer Src. Nos résultats suggèrent ainsi que l’expression de DEP-1 pourrait contribuer à la progression tumorale et la formation de métastases. Ces découvertes laissent donc entrevoir que DEP-1 représente une nouvelle cible thérapeutique potentielle pour contrer l’angiogenèse et le développement du cancer.The implication of protein tyrosine phosphatases (PTPs) in the regulation of cell signalling events and the mediation of cellular functions in response to growth factors such as VEGF has been well-established in the last years. Nonetheless, molecular mechanisms by which PTPs regulate fundamental processes such as angiogenesis are not well-characterized. Expression of the PTP DEP-1 (Density-enhanced phosphatase 1) was reported to increase with cell density and was associated with VEGFR2 dephosphorylation contributing to cell contact inhibition in confluent endothelial cells. We previously demonstrated that DEP-1 attenuates VEGFR2 activity by dephosphorylation of its Y1054/1059 leading to decreased activation of major signalling pathways downstream of VEGFR2 with exception of the Src-Gab1-AKT pathway. Increasing Src activity due to DEP-1-mediated dephosphorylation of its Y529 promotes endothelial cell survival. The objective of this thesis was to gain a better understanding of the role of DEP-1 in the regulation of the Src activity and of biological responses in endothelial cells. We identified tyrosine Y1311 and Y1320 in the C-terminal tail of DEP-1 as major phosphorylation sites in response to VEGF. These residues are required for Src activation and mediate the Src-dependent remodelling of endothelial cell junctions inducing permeability, invasion and capillary formation upon VEGF stimulation. We showed that VEGF-induced DEP-1 tyrosine phosphorylation directs DEP-1 specificity towards its substrate Src. Our results thus highlighted for the first time the promoting role of DEP-1 on the angiogenic program in endothelial cells. In addition to tyrosine phosphorylation, DEP-1 is constitutively phosphorylated on a threonine residue (T1318) proximal to Y1320 in its C-terminal tail suggesting it might be involved in the regulation of Y1320. Indeed, we found that DEP-1 T1318 is phosphorylated, potentially by CK2, and regulates the tyrosine phosphorylation of DEP-1 and its ability to bind to and activate Src. Consistent with this, remodelling of endothelial cell junctions and permeability are impaired in endothelial cells expressing the DEP-1 T1318 mutant. Thus, DEP-1 phosphorylation on T1318 displays a regulatory control over DEP-1 tyrosine phosphorylation and subsequently Src activation and endothelial cell functions in response to VEGF. Our results demonstrating that DEP-1 promotes angiogenic cell responses in endothelial cells, prompted us to consider a possible involvement of DEP-1 in cancer cells, where Src activation has been linked to cancer progression. Thus, although, DEP-1 is believed to act as a tumour suppressor in different cancer types, we hypothesized that it might also promote Src-dependent functions such as migration and invasion in cancer cells. Interestingly, we found that DEP-1 is higher expressed in more invasive basal-like breast cancer cells than in luminal-like cell lines. Moreover, DEP-1 is implicated in the regulation of Src activity, Src-mediated cell motility and appropriate localization of proteins mediating cytoskeletal organization in basal-like breast cancer cell lines. To further support these results, analysis of a breast cancer tissue microarray revealed that DEP-1 expression is associated with a tendency towards reduced overall survival. Thus, our results provide first evidence for a tumour-promoting role of DEP-1 in breast cancer. Altogether, the work performed in the context of this thesis revealed that DEP-1 can similarly behave as a promoter of the angiogenic response and of the pro-invasive phenotype in basal-like breast cancer cell lines, most likely due to its ability to activate Src. This suggests for the first time that DEP-1 expression could contribute to tumour progression and the formation of metastases, and as such, represent a potential new target for anti-angiogenic and anti-cancer therapy

Bioengineered Platforms to Study Carcinoma Cell Response to Drug Treatment

The tumor extracellular matrix (ECM) plays an important role in facilitating tumor growth and mediating tumor cells\u27 resistance to drugs. However, during drug development, potential chemotherapeutics are screened in plastic plates, which lack relevant ECM physicochemical cues. In order to improve drug development process, this dissertation includes the development of relevant 2D and 3D biomaterial systems that can be used to study carcinoma cell response to drug treatment. A novel poly(ethylene glycol)-phosphorylcholine (PEG-PC) high-throughput biomaterial platform was developed to study how the ECM mechanochemical properties affect cancer cells\u27 response to drug. The PEG-PC biomaterial is optically transparent, has a mechanical range from 1 to 10,000 kPa in Young\u27s modulus, and allows easy coupling of cell adhesive proteins. When testing several breast and liver cancer cell lines on PEG-PC gels that had different stiffnesses and integrin-binding sites, there was a significant increase in drug resistance with increasing substrate stiffness. It was found that this stiffness-induced drug resistance was independent of Rho-ROCK and EGFR signaling, but co-administration of a β1 integrin antibody, or an inhibitor to JNK, with sorafenib effectively eliminated the stiffness-mediated sorafenib resistance. Finally, 3D hydrogel systems, poly(N-isopropylacrylamide)-PEG (PNIPAAm-PEG) and PEG-Maleimide, were utilized to create multi-cellular spheroids to study drug resistance in 3D. Both SkBr3s and MDA-MB-231s were tested with sorafenib, lapatinib, temsirolimus, and doxorubicin across varying moduli and geometry (plastic, 2D and 3D hydrogels, spheroid) in different medium conditions. For some drugs, the change in platform or medium was found to have the largest effect on the variation of the IC-50 than the change in modulus. Specifically, the IC-50s varied the most when SkBr3s were treated with sorafenib and temsirolimus and when MDA-MB-231s were treated with sorafenib and lapatinib. However, when treated with doxrorubicin, the IC-50s of both cell types were similar across all platforms. These results demonstrate the utility of tailored biomaterial systems to address basic questions related to tumor microenvironment and drug resistance in cancer, and highlight the importance of incorporating relevant ECM factors into drug testing

Psr1p interacts with SUN/sad1p and EB1/mal3p to establish the bipolar spindle

Regular Abstracts - Sunday Poster Presentations: no. 382During mitosis, interpolar microtubules from two spindle pole bodies (SPBs) interdigitate to create an antiparallel microtubule array for accommodating numerous regulatory proteins. Among these proteins, the kinesin-5 cut7p/Eg5 is the key player responsible for sliding apart antiparallel microtubules and thus helps in establishing the bipolar spindle. At the onset of mitosis, two SPBs are adjacent to one another with most microtubules running nearly parallel toward the nuclear envelope, creating an unfavorable microtubule configuration for the kinesin-5 kinesins. Therefore, how the cell organizes the antiparallel microtubule array in the first place at mitotic onset remains enigmatic. Here, we show that a novel protein psrp1p localizes to the SPB and plays a key role in organizing the antiparallel microtubule array. The absence of psr1+ leads to a transient monopolar spindle and massive chromosome loss. Further functional characterization demonstrates that psr1p is recruited to the SPB through interaction with the conserved SUN protein sad1p and that psr1p physically interacts with the conserved microtubule plus tip protein mal3p/EB1. These results suggest a model that psr1p serves as a linking protein between sad1p/SUN and mal3p/EB1 to allow microtubule plus ends to be coupled to the SPBs for organization of an antiparallel microtubule array. Thus, we conclude that psr1p is involved in organizing the antiparallel microtubule array in the first place at mitosis onset by interaction with SUN/sad1p and EB1/mal3p, thereby establishing the bipolar spindle.postprin

Regulation of blood platelet function by nitric oxide

Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a haemostatic plug. This self-amplifying mechanism requires a tight control to prevent uncontrolled platelet aggregate formation that could occlude the vessel. Endothelial-derived nitric oxide (NO) and prostacyclin (PGI₂) are strong negative regulators that modulate platelet adhesion, activation, aggregation, secretion and shape change. In this study the effects of NO on Ca²+ dependent and independent pathways of activation were investigated. The data produced during the course of this study reveals new insights into the mechanisms by which NO regulates platelet responses via the activation of the AGC family of Ser/Thr protein kinases. NO inhibited platelet shape change in a concentration dependent manner. Platelet shape change is driven by phosphorylation of myosin light chain (MLC) and the experimental data shows that NO blocked this critical phosphorylation event. Phospho-MLC generated in response to platelet agonists occurs through a Ca²+ dependent and RhoA kinase (ROCK)-dependent mechanisms and NO differentially inhibits both pathways. Activation of the ROCK pathway via RhoA leads to the phosphorylation MLC phosphatase Threonine⁶⁹⁶⁄⁸⁵³, which inhibits enzyme activity. Experimental evidence in this thesis indicates that NO, acting through cGMP and protein kinase G, prevents this inhibitory phosphorylation of MLCP by at least two mechanisms, (i) inhibiting the ROCK pathway that phosphorylates MLCP, and (ii) directly phosphorylating MLCP at an independent site, Serine⁶⁹⁵. These original observations hint at a novel mechanism for platelet regulation by the NO-cGMP-signalling pathway

Functional studies on receptor-type protein tyrosine phosphatases of the R3 subgroup

The receptor-type protein tyrosine phosphatases (RPTPs) of the R3subgroup play key roles in the immune, vascular and nervous system. They are characterised by an extracellular domain (ECD), comprised of multiple FNIII-like repeats, a transmembrane domain and a single intracellular phosphatase domain. Although their phosphatase domains have been Fstudied in detail the functional roles of their extracellular regions have not been clearly defined. Potential roles in ligand interaction, dimerisation and cell-cell contacts have been reported. Here I used a bimolecular fluorescence complementation (BiFC) assay in live cells to examine the molecular basis for the interaction of one of the R3 RPTP members, VE-PTP, with VE-cadherin, and explored the potential of others to interact with this protein. The potential of R3 RPTPs to homo-dimerise via extracellular domains in live cells was also addressed. Quantitative BiFC analysis using sialophorin (SPN), an unrelated membrane protein, and a membrane anchored C-terminal Venus-YFP (Myr-VC) fragment as controls revealed a specific interaction between VE-PTP and VE-cadherin using constructs expressing only the extracellular and transmembrane domains. Use of a deletion mutant indicated that, in contrast to previous studies, removal of the 17th FNIII-like domain of VE-PTP is not sufficient to disrupt this interaction. Other members of the R3 RPTP family (DEP-1, GLEPP1 and SAP-1) also exhibited the potential to interact with VE-cadherin suggesting that specificity of this protein-protein interaction is not determined by the ECD alone. The direct interaction of DEP-1 with VE-cadherin is likely to be of physiological relevance since both proteins are expressed in endothelial cells. GLEPP1 and SAP-1 exhibited homo-dimerisation, whereas DEP-1 and VE-PTP did not form dimers via their extracellular and/or transmembrane domains. SPN was identified as a possible bona fide ligand for DEP-1 and their interaction is likely to be of physiological relevance since they were both shown to regulate T cell receptor activation. The interactions identified in the present study suggest a role for both the extracellular domain and transmembrane domain of R3-PTPs in interaction with VE-cadherin. The study also highlights the importance of using multiple controls in BiFC experiments and quantitative analysis of results