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Mol. Cell. Biol., Dec 1996, 7043-7053, Vol 16, No. 12
SX Skapek, J Rhee, PS Kim, BG Novitch and AB Lassar
It was recently demonstrated that ectopic expression of cyclin D1 inhibits
skeletal muscle differentiation and, conversely, that expression of
cyclin-dependent kinase (cdk) inhibitors facilitates activation of this
differentiation program (S. S. Rao, C. Chu, and D. S. Kohtz, Mol. Cell.
Biol. 14:5259-5267, 1994; S. S. Rao and D. S. Kohtz, J. Biol. Chem.
270:4093-4100, 1995; S. X. Skapek, J. Rhee, D. B. Spicer, and A. B. Lassar,
Science 267:1022-1024, 1995). Here we demonstrate that cyclin D1 inhibits
muscle gene expression without affecting MyoD DNA binding activity. Ectopic
expression of cyclin D1 inhibits muscle gene activation by both MyoD and
myogenin, including a mutated form of myogenin in which two potential
inhibitory cdk phosphorylation sites are absent. Because the retinoblastoma
gene product, pRB, is a known target for cyclin D1-cdk phosphorylation, we
determined whether cyclin D1-mediated inhibition of myogenesis was due to
hyperphosphorylation of pRB. In pRB-deficient fibroblasts, the ability of
MyoD to activate the expression of muscle-specific genes requires
coexpression of ectopic pRB (B. G. Novitch, G. J. Mulligan, T. Jacks, and
A. B. Lassar, J. Cell Biol., 135:441-456, 1996). In these cells, the
expression of cyclins A and E can lead to pRB hyperphosphorylation and can
inhibit muscle gene expression. The negative effects of cyclins A or E on
muscle gene expression are, however, reversed by the presence of a mutated
form of pRB which cannot be hyperphosphorylated. In contrast, cyclin D1 can
inhibit muscle gene expression in the presence of the
nonhyperphosphorylatable form of pRB. On the basis of these results we
propose that G1 cyclin-cdk activity blocks the initiation of skeletal
muscle differentiation by two distinct mechanisms: one that is dependent on
pRB hyperphosphorylation and one that is independent of pRB
hyperphosphorylation.
Copyright © 1996, American Society for Microbiology
Cyclin-mediated inhibition of muscle gene expression via a mechanism that is independent of pRB hyperphosphorylation
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA.
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