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Mol. Cell. Biol., Jul 1995, 3823-3829, Vol 15, No. 7
K Guo, J Wang, V Andres, RC Smith and K Walsh
The terminal differentiation of C2C12 skeletal muscle cells involves the
activation of unique sets of genes and an irreversible withdrawal from the
cell cycle. This process is associated with a decrease in cdk2 activity in
cell extracts. The decrease in cdk2 activity correlates with diminished
levels of cdk2 and cyclin A and with a marked induction of the p21
cyclin-dependent kinase (cdk) inhibitor. The upregulation of p21 occurred
at the levels of mRNA and protein, and p21 formed a complex with the cyclin
kinases in myotubes. Further, the immunodepletion of p21 from myotube
extracts neutralized the heat- stable cdk2 inhibitory activity that was
induced upon myogenic differentiation. The levels of p21 mRNA, protein, and
activity remained constant in myotubes when they were reexposed to
mitogen-rich growth medium, indicating that permanent changes in the cell's
genetic program contribute to its sustained expression following terminal
differentiation. Indeed, 10T1/2 fibroblasts transformed with the myogenic
factor MyoD, but not the parental multipotent cells, upregulated p21
transcript levels when induced to differentiate by serum withdrawal,
demonstrating that the upregulation is an integral feature of myogenic
commitment and differentiation. The functional consequences of this
upregulation were indicated by ectopically expressing p21 in myoblasts;
this was sufficient for cell cycle arrest in mitogen-rich growth medium.
The induction and sustained expression of p21 appears to be a contributory
mechanism by which myocytes irreversibly exit the cell cycle upon terminal
differentiation.
Copyright © 1995, American Society for Microbiology
MyoD-induced expression of p21 inhibits cyclin-dependent kinase activity upon myocyte terminal differentiation
Division of Cardiovascular Research, St. Elizabeth's Medical Center, Boston, Massachusetts 02135, USA.
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