A cyclin D-CDK6 dimer helps to reshuffle cyclin-dependent kinase inhibitors (CKI) to overcome TGF-beta-mediated arrest and maintain CDK2 activity.

The cyclin D-CDK4/6 complex has two distinct functions. Its kinase-dependent role involves its ability to act as serine/threonine kinase, responsible for phosphorylation of substrates required for cell cycle transitions, while its kinase-independent function involves its ability to act as a reservoir for p27. This association sequesters p27 from cyclin E-CDK2 complexes, allowing them to remain active. The aim of this current study is two-fold: to understand the contribution of the kinase-dependent and kinase-independent functions of CDK4 and CDK6 in epithelial cells and to directly compare CDK4 and CDK6 in a simple model system, TGF-β treatment, where arrest is initiated by the expression of p15. Cells that overexpressed a catalytically inactive, p15-insensitive CDK6 variant (p27 sequestration only mutant) were able to overcome TGF-β-mediated arrest by maintaining CDK2 activity, while cells expressing the identical mutations in CDK4 were not. This result can be partially explained by the presence of a previously unidentified cyclin D-CDK6 dimer, which serves as a sink for free p27 during TGF-β treatment, enabling CDK2 to remain inhibitor free. The use of the TGF-β model system and the characterization of CDK pool dynamics and p27 switching is relevant to the CDK4/6 specific inhibitors, such as palbociclib, whose mechanism of action may resemble that of p15.