Inactivation of the cyclin D-dependent kinase in the rat fibroblast cell line, 3Y1, induced by contact inhibition.

Cyclin-dependent kinase (Cdk) inhibitory proteins are involved in cell cycle arrest induced by antiproliferating factors or chemicals. High cell density also induces cell cycle arrest in which the genomic DNA is unreplicated, even in the presence of a mitotic dose of growth factors; this is termed contact inhibition. Although the cell cycle of the rat fibroblast cell line, 3Y1, was arrested in quiescence by contact inhibition, the Cdk4 bound to its regulatory subunit, cyclin D1 or D3. However, these complexes were enzymatically inactive. Phosphorylation of the cyclin D1-bound Cdk4 by the Cdk-activating kinase could convert the inactive cyclin D1-Cdk4 complex into its active form in vitro, suggesting that threonine 172 of the Cdk4, of which phosphorylation is required for its activation, was in part unphosphorylated in contact-inhibited 3Y1 cells. Although MO15 was active in cell extracts prepared from the arrested 3Y1 cells, activation of bacterially produced Cdk4 in the cell extracts was inhibited. Removal of p27(kip1) from the cell extracts allowed the MO15 holoenzyme to phosphorylate the Cdk4 and in turn activate it, indicating that p27(kip1) plays a role in inhibiting the phosphorylation of Cdk4 by MO15 in the contact-inhibited 3Y1 cells.