Inhibition of Cdk4 activity enhances translation of p27kip1 in quiescent Rb-negative cells

Abstract

We show in this work that the inhibition of Cdk4 (6) in Rb-/- 3T3 cells enhances the accumulation of the p27kip1 cyclin-dependent kinase inhibitor when these cells are induced into quiescence. Two different forms of inhibition of Cdk4 (6), namely overexpression of the Cdk4 (6) inhibitor p16 and overexpression of a dominant negative mutant of Cdk4 (Cdk4N158), result in this effect. This suggests that the relevant activity of Cdk4 (6) that has to be inactivated in this setting is its kinase activity. The accumulation of p27kip1 is due to enhanced translation of the protein, mediated by the 3′-untranslated region of the p27 mRNA. Moreover, the cells that overexpress p16ink4a or Cdk4N158 show a delay in G1 when made quiescent and restimulated to proliferate. This delay is overcome by transfection of a plasmid expressing antisense p27kip1, which shows that the accumulation of p27kip1 in these cells is related to their G1 delay. In summary, we report a new functional link between two important cell cycle regulators, Cdk4 and p27kip1, and provide a mechanistic explanation to the previously reported epistatic relations between these two proteins.