Akt-dependent Phosphorylation of p27Kip1Promotes Binding to 14-3-3 and Cytoplasmic Localization*

In many human cancers, the cyclin-dependent kinase inhibitor p27Kip1 is expressed at low or undetectable levels. The decreased p27Kip1 expression allows cyclin-dependent kinase activity to cause cells to enter into S phase and correlates with poor patient survival. Inhibition of serine/threonine kinase Akt signaling by some pharmacological agents or by PTEN induces G1 arrest, in part by up-regulating p27Kip1. However, the role of Akt-dependent phosphorylation in p27Kip1 regulation is not clear. Here, we show that Akt bound directly to and phosphorylated p27Kip1. Screening p27Kip1 phosphorylation sites identified the COOH-terminal Thr198 residue as a novel site. Further analysis revealed that 14-3-3 proteins bound to p27Kip1 through Thr198 only when it was phosphorylated by Akt. Although Akt also phosphorylated p27Kip1 at Ser10 and Thr187, these two sites were not involved in the binding to 14-3-3 proteins. p27Kip1 phosphorylated at Thr198 exists only in the cytoplasm. Therefore, Akt promotes cell-cycle progression through the mechanisms of phosphorylation-dependent 14-3-3 binding to p27Kip1 and cytoplasmic localization.