Résumé :
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Plasminogen activation system (PA) plays an important role in the degradation of extracellular matrix components. Plasmin (Pli), the activated form of plasminogen (Plg), is implicated in several biological processes such as tissue remodelating and cell migration. In this way, urokinase-plasminogen activator and Pli are required for myogenesis and skeletal muscle regeneration. Furthermore, some components of PA system have been involved in intracellular signaling and several receptors for plasmin(ogen) have been elucidated. Alpha-enolase, a glycolytic enzyme, also constitutes a receptor of Plg enhancing Pli production and focalizing proteolytic activity on the cell surface. Previously, in our laboratory, we have shown that the interaction Plg/alpha-enolase is required for myogenesis in vitro and skeletal muscle regeneration in vivo. However, the implication of alpha-enolase in cell signaling has never been tested in myoblasts. Our recent studies by Western blotting have shown that Plg and Pli induce activation of MAPK-Erk and PI3K-Akt pathways in C2C12 cell line and primary cultures of Muscle Precursor Cells, being Pli-induced signaling more intense than Plg-induced one. Using Pli activity inhibitors, we have proved that Pli activity is required to induce Akt and Erk phosphorylation. Moreover, inhibition of plasmin-induced cell signaling with a lysine analog indicates that Pli binding to cell surface is also necessary. Preliminary results with Mab11G1, a monoclonal antibody synthesized in our laboratory which blocks plasmin(ogen) binding to alpha-enolase, suggest that alpha-enolase is likely to be involved in Pli induced signaling but the participation of other described Plg receptors cannot be discarded. As known, plamin(ogen) receptors have not transmembrane domain so they must be associated with transmembrane adaptors or other possible receptors such as Epidermal Growth Factor Receptor (EGFR), which has been related to other PA system components induced signaling. In the present work, not only we have demonstrated that EGFR is expressed in myoblast, but we have also shown a significant decrease of the Akt phosphorylation in the presence of EGFR tyrosine kinase inhibitors AG1478 and Iressa. Furthermore, we have shown that EGFR becomes phosphorylated in Tyr1068 and Tyr1173 residues in the presence of Pli. These results confirm that Pli activity is able to induce intracellular signaling response in myoblasts and suggest EGFR as possible collaborator.
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