Pax3 expression is down-regulated during satellite cell activation. (A) MyoD, Pax3, Pax7, Bcl-2, Bcl-xL, miR-1, and miR-206 gene expression were compared between freshly isolated (day 0) and cultured satellite cells (days 1, 2, and 4) derived from wild-type and MyoD^−/− mice by RT-PCR. 18S rRNA and miR-24 were monitored as loading controls. For miRNAs, all PCR products are ∼90-bp long. (B) Relative Pax3, Bcl-2, and Bcl-xL gene expression levels shown in A normalized by 18S rRNA expression were measured. (C) MyoD and Pax3 gene expression were compared between wild-type satellite cells transfected with siRNA for MyoD and a control siRNA (Cont) or MyoD^−/− satellite cells infected with a lentivirus vector expressing MyoD (Ect-MyoD) and a control empty vector by RT-PCR. (D) Relative Pax3 gene expression shown in C normalized by 18S rRNA expression was measured. (E) The model for apoptotic cascade regulated by MyoD. MyoD is a master regulator for muscle differentiation. Under differentiation or stressful conditions, MyoD transcriptionally activates miR-1 and miR-206 gene expression, which suppresses Pax3 expression. Down-regulation of Pax3 results in down-regulation of Bcl-2 and Bcl-xL, which causes apoptosis. In addition, down-regulation of Pax3 by MyoD also induces muscle differentiation. *, P < 0.05; **, P < 0.01. Error bars indicate SEM.