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Inactivation of MyoD-mediated expression of p21 in tumor cell lines
A D Otten1, E J Firpo, A N Gerber
1Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA.
Abstract:
The basic helix-loop-helix protein MyoD induces muscle structural gene expression and cell cycle withdrawal in many nontransformed cell lines. We show that MyoD activation of transcription of the cyclin-dependent kinase inhibitor p21 does not require synthesis of an intermediary protein. In most of the rhabdomyosarcoma and other solid tumor cell lines that we analyzed, p21 levels were abnormally low and correlated with the combined inactivity of MyoD and p53, two known transcriptional activators of p21. Loss of MyoD activation of p21 transcription correlated with the failure to arrest in G1, and expression of p21 caused accumulation of cells in G1, further supporting a role for p21 in MyoD-induced cell cycle arrest. Finally, different tumor types have inactivated distinct factors necessary for p21 expression, because p21 expression was reconstituted in hybrid cell lines. We propose that p21 integrates growth-inhibitory signals from independent p53 and basic helix-loop-helix pathways, and that in the majority of tumor cell lines, both pathways are abrogated.
Insights
MyoD protein activates p21 gene expression, crucial for cell cycle arrest. Many tumors show low p21 due to inactive MyoD and p53 pathways, hindering muscle gene expression and cell growth.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- The basic helix-loop-helix (bHLH) protein MyoD is a key regulator of muscle differentiation.
- MyoD induces muscle structural gene expression and cell cycle withdrawal in various cell types.
- The cyclin-dependent kinase inhibitor p21 plays a critical role in cell cycle regulation.
Purpose of the Study:
- To investigate the role of MyoD in p21 gene activation.
- To determine the correlation between MyoD, p53, and p21 levels in tumor cell lines.
- To elucidate the mechanism of MyoD-induced cell cycle arrest.
Main Methods:
- Analysis of p21 levels in rhabdomyosarcoma and other solid tumor cell lines.
- Assessment of MyoD and p53 activity.
- Generation of hybrid cell lines to study p21 expression reconstitution.
Main Results:
- MyoD activation of p21 transcription does not require intermediary proteins.
- Abnormally low p21 levels in most tumor cells correlated with inactive MyoD and p53.
- Loss of MyoD-mediated p21 activation correlated with failure to arrest in G1 phase.
- p21 expression induced G1 cell cycle arrest.
- Hybrid cell lines demonstrated reconstitution of p21 expression, indicating distinct inactivated factors in different tumor types.
Conclusions:
- p21 integrates growth-inhibitory signals from both p53 and bHLH pathways.
- The majority of tumor cell lines analyzed have abrogated both the p53 and bHLH pathways necessary for p21 expression.
- Dysregulation of the p21 pathway contributes to tumorigenesis.