The Kinase-Dependent and Independent Functions of Cdk4 and Cdk6 Regulate Continuous Proliferation and the Exit From Quiescence Differently

Cdk4 and cdk6 have long been considered functional homologues, despite the fact that tissue-specific differences are detected in the single cdk4 or cdk6 knockout animals. To explore the role of cdk4 and cdk6 in the same model system, we overexpressed variants of cdk4 and cdk6 in Mv1Lu cells to determine their effect on cell cycle progression. We found that cells that overexpressed cdk4 were able to reenter the cell cycle from a contact arrested quiescent state in a kinase-dependent fashion, consistent with the role of this kinase in G0-G1 phase exit. However, we also found that expression of catalytically inactive variants of cdk6 accelerated G0 release, enabled the cell to overcome TGF-β-mediated growth arrest, proliferate in the absence of cyclin D-associated kinase activity and maintain cdk2 activity, while cells expressing catalytically inactive cdk4 were not. This suggested that cdk6 expression was able to affect cell cycle progression in a kinase-independent manner that was distinct from the actions of cdk4. Cdk4 appears unable to associate with cyclin D in the absence of a required assembly factor, such as p27Kip1, but, by gel filtration chromatography, we detected the presence of a previously unidentified, catalytically inactive cyclin D-cdk6 dimer, which might contribute to the kinase-independent, pro-proliferative activity of cdk6.