A pRb-independent mechanism preserves the postmitotic state in terminally differentiated skeletal muscle cells.
In skeletal muscle differentiation, the retinoblastoma protein (pRb) is absolutely necessary to establish definitive mitotic arrest. It is widely assumed that pRb is equally essential to sustain the postmitotic state, but this contention has never been tested. Here, we show that terminal proliferation arrest is maintained in skeletal muscle cells by a pRb-independent mechanism. Acute Rb excision from conditional knockout myotubes caused reexpression of E2F transcriptional activity, cyclin-E and -A kinase activities, PCNA, DNA ligase I, RPA, and MCM2, but did not induce DNA synthesis, showing that pRb is not indispensable to preserve the postmitotic state of these cells. Muscle-specific gene expression was significantly down-regulated, showing that pRb is constantly required for optimal implementation of the muscle differentiation program. Rb-deleted myotubes were efficiently reactivated by forced expression of cyclin D1 and Cdk4, indicating a functionally significant target other than pRb for these molecules. Finally, Rb removal induced no DNA synthesis even in pocket-protein null cells. Thus, the postmitotic state of myotubes is maintained by at least two mechanisms, one of which is pocket-protein independent.
Camarda
Grazia
G
Depatment of Environment and Primary Prevention, Higher Institute of Health, 00161 Roma, Italy.
Siepi
Francesca
F
Pajalunga
Deborah
D
Bernardini
Camilla
C
Rossi
Rossella
R
Montecucco
Alessandra
A
Meccia
Ettore
E
Crescenzi
Marco
M
D000818:Animals; D002453:Cell Cycle; D002454:Cell Differentiation; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000235:genetics / Q000502:physiology; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000502:physiology; D015536:Down-Regulation; D015870:Gene Expression; D051379:Mice; D018345:Mice, Knockout; D008938:Mitosis; D032342:Muscle Cells / Q000166:cytology* / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D018482:Muscle, Skeletal / Q000166:cytology* / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology; D016160:Retinoblastoma Protein / Q000502:physiology*
D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein; D019938:Cyclin D1; C495902:Cdk4 protein, mouse; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases