Ectopic expression of Cdk6 circumvents transforming growth factor-β mediated growth inhibition

Transforming growth factor-β (TGF-β) induced growth arrest of cells involves regulation of the activities of both D- and E-type cyclin kinase complexes thought to be mediated primarily by the regulation of p15^Ink4b and p27^Kip1 cyclin kinase inhibitors. We show here that TGF-β downregulates Cdk6 and that transient and stable expression of Cdk6 in Mv1Lu mink epithelial cells overrides TGF-β mediated arrest. The main effect of the ectopic Cdk6 expression was to sequester TGF-β induced p15^Ink4b and to maintain more p27^Kip1 in cyclin D-complexes preventing the complete shift of p27^Kip1 to Cdk2 invoked by TGF-β. This led to the presence of an active cyclinD-Cdk6-p27^Kip1 complex and partially active cyclin E-Cdk2 complex and resulted in the failure of TGF-β to fully arrest Mv1Lu cell growth. Though dominant negative Cdk6, expressed similarly in the cells, sequestered both p15^Ink4b and p27^Kip1, it lacks kinase activity and was unable to override the TGF-β arrest. The results demonstrate that downregulation of Cdk6 kinase is required for the enforcement of the G₁-phase arrest by TGF-β and results in changes in association of the p15^Ink4b and p27^Kip1 inhibitors with D- and E-type cyclin kinase complexes.