title: Expression of the positive regulator of cell cycle progression, cyclin D3, is induced during differentiation of myoblasts into quiescent myotubes.
abstract: L6 cells are committed skeletal muscle precursors which can be induced to differentiate into multinucleated, terminally differentiated myotubes. Upon differentiation, these immature skeletal myotubes enter a quiescent state and are unable to reenter the cell cycle. We have examined expression of a series of genes involved in regulation of progression through the G1/S boundary in undifferentiated L6 cells and during terminal differentiation of L6 myoblasts. While no change in the level of cyclin D1 transcript and a transient increase in cyclin D2 transcript were observed, a large increase in cyclin D3 expression was found. Immunohistochemistry demonstrated strong staining for cyclin D3 protein in the nuclei of the multinucleated myotubes from 4 independent myoblast cell lines; L6, L8, G8 and C2C12. Immunoprecipitation confirmed a greater than 20-fold increase in the levels of cyclin D3 protein in the differentiated L6 myotubes as well as its association with a number of proteins. Western assays demonstrated, further, that cyclin D3 was complexed with the cyclin dependent-kinases, cdk2 and cdk4, in differentiated L6 cells. However, while kinase activity specific for a GST-pRB fusion protein was seen for cyclin D3-containing complexes isolated from undifferentiated cells, the high levels of cyclin D3 in the differentiated myotubes had no associated kinase activity. These data demonstrate that cyclin D3 may also have a function in terminally differentiated, quiescent cells. The lack of cyclin D3-associated kinase activity and its association with a number of different proteins suggest that cyclin D3 may regulate the function of other proteins by direct interaction with these factors.
authors: Department of Cellular and Molecular Pathology, University of Toronto, Ontario.
authors: M
authors: M
authors: Kiess
authors: R M
authors: RM
authors: Gill
authors: P A
authors: PA
authors: Hamel
mesh_terms: D000818:Animals; D002453:Cell Cycle; D002454:Cell Differentiation; D002460:Cell Line; D056743:Cyclin D3; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016213:Cyclins / Q000235:genetics*; D009132:Muscles / Q000166:cytology* / Q000201:enzymology / Q000378:metabolism*; D051381:Rats
chemical_list: C534698:Ccnd3 protein, rat; D056743:Cyclin D3; D016213:Cyclins; D018844:Cyclin-Dependent Kinases