REGULATION OF EXIT FROM QUIESCENCE BY P27 AND CYCLIN D1-CDK4

The synthesis of cyclin D1 and its assembly with cyclin-dependent kina se 4 (CDK4) to form an active complex is a rate-limiting step in progr ession through the G(1) phase of the cell cycle. Using an activated al lele of mitogen-activated protein kinase kinase 1 (MEK1), we show that this kinase plays a significant role in positively regulating the exp ression of cyclin D1. This was found both in quiescent serum-starved c ells and in cells expressing dominant-negative Ras. Despite the observ ation that cyclin D1 is a target of MEK1, in cycling cells, activated MEK1, but not cyclin D1, is capable of overcoming a G(1) arrest induce d by Ras inactivation. Either wild-type or catalytically inactive CDK4 cooperates with cyclin D1 in reversing the G(1) arrest induced by inh ibition of Ras activity. In quiescent NIH 3T3 cells expressing either ectopic cyclin D1 or activated MEK1, cyclin D1 is able to efficiently associate with CDK4; however, the complex is inactive. A significant p ercentage of the cyclin D1-CDK4 complexes are associated with p27 in s erum-starved activated MEK1 or cyclin D1 cell lines. Reduction of p27 levels by expression of antisense p27 allows for S-phase entry from qu iescence in NIH 3T3 cells expressing ectopic cyclin D1, but not in par ental cells.