Expression of the positive regulator of cell cycle progression, cyclin D3, is induced during differentiation of myoblasts into quiescent myotubes.

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.