Regulation of Pax7 protein levels by caspase-3 and proteasome activity in differentiating myoblasts

The transcription factor Pax7 negatively regulates the activity of the muscle regulatory transcription factor MyoD, preventing muscle precursor cells from undergoing terminal differentiation. In this context, the ratio between Pax7 and MyoD protein levels is thought to be critical in allowing myogenesis to proceed or to maintain the undifferentiated muscle precursor state. We have previously shown that Pax7 is subject to rapid down regulation in differentiating myoblasts, via a proteasome-dependent pathway. Here we present evidence indicating that Pax7 is also subject to caspase-3-dependent regulation. Furthermore, simultaneous inhibition of caspase-3 and proteasome activity induced further accumulation of Pax7 protein in differentiating myoblasts. These results suggest that at early stages of muscle differentiation, Pax7 levels are regulated by at least two independent mechanisms involving caspase-3 and proteasome activity.