Reconstitution of cyclin D1-associated kinase activity drives terminally differentiated cells into the cell cycle

Terminal cell differentiation entails definitive withdrawal from the cell c ycle. Although most of the cells of an adult mammal are terminally differen tiated, the molecular mechanisms preserving the postmitotic state are insuf ficiently understood. Terminally differentiated skeletal muscle cells, or m yotubes, are a prototypic terminally differentiated system. We previously i dentified a mid-G(1) block preventing myotubes from progressing beyond this point in the cell cycle. In this work, we set out to define the molecular basis of such a block. It is shown here that overexpression of highly activ e cyclin E and cdk2 in myotubes induces phosphorylation of pRb but cannot r eactivate DNA synthesis, underscoring the tightness of cell cycle control i n postmitotic cells. In contrast, forced expression of cyclin D1 and wild-t ype or dominant-negative cdk4 in myotubes restores physiological levels of cdk4 kinase activity, allowing progression through the cell cycle. Such rea ctivation occurs in myotubes derived from primary., as well as established, C2C12 myoblasts and is accompanied by impairment of muscle-specific gene e xpression. Other terminally differentiated systems as diverse as adipocytes and nerve cells are similarly reactivated. Thus, the present results indic ate that the suppression of cyclin Dl-associated kinase activity is of cruc ial importance for the maintenance of the postmitotic state in widely diver gent terminally differentiated cell types.