Résumé :
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Pitx2 is a member of the bicoid family of homeodomain transcription factors that plays a relevant role in morphogenesis. Pitx2 expression has been detected in many tissues during development, including myotomes as well as in migrating myoblasts. Its expression is also maintained in Pax3 positive cells that have completed migration at the proximal limb bud. We have previously documented that overexpression Pitx2c –isoform in undifferentiated myoblasts (Sol8 myogenic cell line) resulted in upregulation of cell cycle genes (c-myc, cyclinD1 and D2) while it arrests differentiation into mature myotubes by upregulating Pax3 and downregulating myogenic transcription factors such as MyoD and myogenin. These observations indicate that c-isoform of Pitx2 plays a pivotal role modulating proliferation vs differentiation during skeletal myogenesis. We report herein that transient tranfections leading to overexpression Pitx2c in Sol8 myoblasts demonstrate that the Pitx2c effects in this cell line are dose-dependent. Therefore, we have determined at which doses of transfection Pitx2c began to induce changes in cell phenotype, inhibiting myocyte differentiation and myoyube formation. Real-time PCR analysis after Pitx2c-transfection reveals that cell cycle genes (Cyclin D1 and Cyclin D2) as well as Pax3 upregulation coincides with induced changes in cell phenotype, whereas myogenic regulatory factors (MyoD, Myogenin) becomes down-regulated at low doses of Pitx2c-transfection before the onset of changes in the phenotype. These data suggests that regulation of genes involved in the maintenance of proliferative stages in myoblast and genes involved in the onset of differentiation (MyoD and Myogenin) require different Pitx2c doses. Interestingly, we found Pax7 down-regulation after low doses of Pitx2c-transfection coinciding with MyoD and Myogenin down-regulation suggesting that Pitx2 could play a role modulating Pax3/7 function in adult satellite cells.
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