Regulation of MyoD activity and muscle cell differentiation by MDM2, pRb, and Sp1.

Muscle cell differentiation is controlled by a complex set of interactions between tissue restricted transcription factors, ubiquitously expressed transcription factors, and cell cycle regulatory proteins. We previously found that amplification of MDM2 in rhabdomyosarcoma cells interferes with MyoD activity and consequently inhibits overt muscle cell differentiation (1). Recently, we found that MDM2 interacts with Sp1 and inhibits Sp1-dependent transcription and that pRb can restore Sp1 activity by displacing MDM2 from Sp1 (2). In this report, we show that forced expression of Sp1 can restore MyoD activity and restore overt muscle cell differentiation in cells with amplified MDM2. Furthermore, we show that pRb can also restore MyoD activity and muscle cell differentiation in cells with amplified MDM2. Surprisingly, we found that the MyoD-interacting domain of pRb is dispensable for this activity. We show that the C-terminal, MDM2-interacting domain of pRb is both necessary and sufficient to restore muscle cell differentiation in cells with amplified MDM2. We also show that the C-terminal MDM2-interacting domain of pRb can promote premature differentiation of proliferating myoblast cells. Our data support a model in which the pRb-MDM2 interaction modulates Sp1 activity during normal muscle cell differentiation.