The peptidyl-prolyl isomerase PIN1 relieves cyclin-dependent kinase 2 (CDK2) inhibition by the CDK inhibitor p27.

PIN1 is a peptidyl-prolyl isomerase that catalyzes the cis/trans isomerization of peptide bonds between proline and phosphorylated serine/threonine residues. By changing the conformation of its protein substrates, PIN1 increases the activities of key proteins that promote cell cycle progression and oncogenesis. Moreover, it has been shown that PIN1 stabilizes and increases the level of the cyclin-dependent kinase (CDK) inhibitor p27, which inhibits cell cycle progression by binding cyclin A- and cyclin E-CDK2. Notwithstanding the associated increase in the p27 level, PIN1 expression promotes rather than retards cell proliferation. To explain the paradoxical effects of PIN1 on p27 levels and cell cycle progression, we hypothesized that PIN1 relieves CDK2 inhibition by suppressing the CDK inhibitory activity of p27. Here, we confirmed that PIN1-expressing cells exhibit higher p27 levels but have increased CDK2 activities and higher proliferation rates in the S-phase compared with Pin1-null fibroblasts or PIN1-depleted hepatoma cells. Using co-immunoprecipitation and CDK kinase activity assays, we found that PIN1 binds the phosphorylated Thr187-Pro motif in p27 and reduces p27's interaction with cyclin A- or cyclin E-CDK2, leading to increased CDK2 kinase activity. In conclusion, our results indicate that although PIN1 increases p27 levels, it also attenuates p27's inhibitory activity on CDK2 and thereby contributes to increased G1-S phase transitions and cell proliferation.