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.We previously found that amplification of MDM2 1 in rhabdomyosarcoma cells inhibits MyoD function and inhibits muscle cell differentiation (1). The oncogenic properties of MDM2 are thought to result from interactions with several cell cycle regulatory proteins. MDM2 interacts directly with the tumor suppressor protein p53 (3) and blocks p53-mediated transactivation (4 -9). In addition, MDM2 has been shown to target p53 for rapid degradation (10, 11). MDM2 also interacts with a second tumor suppressor protein, the retinoblastoma-associated protein, pRb. This MDM2-pRb interaction results in inhibition of pRb growth regulatory function (12, 13). Furthermore, MDM2 interacts with the activation domains of the S-phasepromoting transcription factors E2F1 and DP1, resulting in stimulation of E2F1/DP1 transcriptional activity (14). Taken together, these observations suggest that MDM2 not only relieves the proliferative block mediated by either p53 or pRb but also promotes the G 1 -to-S-phase transition by stimulating E2F1/DP1 activity. The results presented here show that MDM2 can also modulate cellular differentiation through pRb and Sp1.Differentiating muscle cells fuse to form multinucleated myotubes, thereby withdrawing permanently from the cell cycle. This process is controlled by the MyoD family (MyoD, Myf-5, myogenin, and MRF4/Myf-6) of muscle-specific transcription factors (15). The MyoD family of basic helix-loop-helix transcription factors acts at multiple points in the myogenic lineage to establish muscle cell identity and control terminal differentiation. MyoD is found in a multiprotein complex that contains tissue-restricted (SRF or MEF2C) and ubiquitously expressed (E12/E47 and Sp1) transcription facto...

Steel factor (SLF) and its cognate receptor, c-kit, have been implicated in the generation of melanocytes from migrating neural crest (NC) cells during early vertebrate embryogenesis. However, the source of SLF in the early avian embryo and its precise role in melanogenesis are unclear. We report here that NC cells themselves express and release SLF protein, which in turn acts as an autocrine factor to induce melanogenesis in nearby NC cells. These results indicate that NC cell subpopulations play an active role in the determination of their cell fate and suggest a different developmental role for the embryonic microenvironment than what has been previously proposed.