Phosphorylation of P27 by AKT is required for inhibition of cell cycle progression in cholangiocarcinoma.

BACKGROUND AND OBJECTIVE:P27 is a putative tumor suppressor when located in the nucleus and AKT is an inhibitor of P27 which promotes growth of cholangiocarcinoma. We hypothesized that AKT-dependent phosphorylation at the P27 nuclear localization sequence T157 leads to nuclear export of P27, and thus loss of its tumor suppressive function. This study investigated whether loss of cell cycle regulation in cholangiocarcinoma due to subcellular localization of P27. METHODS:Human cholangiocarcinoma cells were transfected with AKT. P27 was tagged with yellow fluorescence protein. Cell cycle progression was determined by flow cytometry. Migration and invasion of was measured by transwell assay. RESULTS:Overexpression of wildtype P27 or P27-T157A in Mz-ChA-1 cells resulted in G1 arrest; expression of myr-AKT caused translocation of P27-YFP and endogenous P27 from the nucleus to the cytoplasm, leading to inhibition of P27-dependent G1 arrest; the AKT inhibitor and expression of dnAKT increased P27-YFP accumulation in the nucleus and promoted G1 arrest. In contrast, cells expressing YFP-P27-T157A or P27-YFP accumulated only in the nucleus. Co-expression of myr-AKT failed to induce P27-YFP translocation to the cytoplasm or inhibit G1 arrest. Overexpression of P27-T157A significantly increased migration and invasion. CONCLUSIONS:Cholangiocarcinoma growth is associated with nuclear export of P27 that is due to AKT-mediated phosphorylation of P27 at T157.