Data from Pim Kinases Promote Cell Cycle Progression by Phosphorylating and Down-regulating p27<sup>Kip1</sup> at the Transcriptional and Posttranscriptional Levels

The serine/threonine kinase Pim is known to promote cell cycle progression and to inhibit apoptosis leading to tumorigenesis. However, the precise mechanisms remain unclear. We show, herein, that all the Pim family members (Pim1, Pim2, and Pim3) bind to and directly phosphorylate the cyclin-dependent kinase inhibitor p27^Kip1 at threonine-157 and threonine-198 residues in cells and in vitro. The Pim-mediated phosphorylation induced p27^Kip1 binding to 14-3-3 protein, resulting in its nuclear export and proteasome-dependent degradation. Ectopic expression of Pim kinases overcome the G₁ arrest mediated by wild-type p27^Kip1 but not by phosphorylation-resistant T157A-p27^Kip1 or T198A-p27^Kip1. In addition to the posttranslational regulations, p27^Kip1 promoter assay revealed that Pim kinases also had the ability to suppress p27^Kip1 transcription. Pim-mediated phosphorylation and inactivation of forkhead transcription factors, FoxO1a and FoxO3a, was involved in the transcriptional repression of the p27^Kip1 gene. In contrast, inhibition of Pim signaling by expressing the dominant-negative form of Pim1 increased nuclear p27^Kip1 level and attenuated cell proliferation. Because the CDK inhibitor p27^Kip1 plays a crucial role in tumor suppression by inhibiting abnormal cell cycle progression, Pim kinases promote cell cycle progression and tumorigenesis by down-regulating p27^Kip1 expression at both transcriptional and posttranslational levels. [Cancer Res 2008;68(13):5076–85]