Résumé :
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The epidermal growth factor receptor (EGFR) is a transmembrane protein, of the ErbB family of tyrosine kinase receptors. It is considered a pleitropic signaler, usually associated to survival signal pathways. Overexpression of EGFR has been related to several human tumors. Furthermore, EGFR is expressed in many tissues and it has been described as participating in heart development, skin wound healing, bone formation and adipogenesis. Moreover, it has also been described that EGFR affects negatively osteoblast and chondrocyte differentiation. But a role for EGFR activity in skeletal muscle differentiation has never been addressed.In this work we have analyzed the role of EGFR in myogenesis in vitro and in muscle myodystrophy in vivo, using an inhibitor of EGFR tyrosine kinase activity, AG1478. First, we confirmed that primary cultures of mouse Muscle Precursor Cells (MPCs) expressed EGFR, and it was not modulated during myogenic differentiation. In MPCs under differentiating conditions, AG1478-treatment induced a 20.1+/-5.5% increase of the differentiation ratio and a 23.6+/-6.2% increase of the fusion ratio, as analyzed by immunostaining to myogenic markers, indicating that EGFR inhibition accelerates myogenesis in vitro.EGFR inhibiton was also tested in an in vivo model of myodystrophy, the mdx mice. Mdx mice of 15 days of age, before the onset of the disease, were treated each 2 days by intraperitoneal injections of AG1478, until the mice reached 30, 60 and 90 days of live. Histological analysis of gastrocnemious muscles and diaphragms of AG1478-treated mice showed a marked decrease of fibrotic areas, less extension of degenerating areas and a decrease in mononucleated infiltration areas, when compared to control-treated mice. This difference increases in time, paralleling myopathology progression. Several parameters of muscle dystrophy were also analyzed. Blue Evans Dye staining showed a marked decrease of Blue-positive fibers (damaged fibers) in AG1478-treated mice in gastrocnemious muscles and diaphragms. For instance, at 60 days, AG1478 induced a decrease of 54.15+/-17.02% of Blue-positive fibers. Surprisingly, serum creatin kinase levels were markedly increased in AG1478-treated mice.Altogether, these results clearly indicate that tyrosine kinase inhibition of EGFR stimulates myogenic differentiation in vitro and muscle regeneration in vivo. To our knowledge, this is the first time that a role for EGFR activity in myogenesis is proposed.
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