Pim-2 phosphorylation of p21Cip1/WAF1 enhances its stability and inhibits cell proliferation in HCT116 cells

Pim-2 kinase is one of the three highly conserved Pim family members which are known to be involved in cell survival and cell proliferation. Here we demonstrate that like Pim-1, Pim-2 also phosphorylates the cell cycle inhibitor p21Cip1/WAF1 (p21) on Thr145 in vitro and in vivo. Overexpression of Pim-2 in HCT116 cells leads to the increased stability of p21 and results in enhanced levels of both exogenous and endogenous p21 proteins. Knockdown of Pim-2 expression via siRNA results in reduced level of endogenous p21, indicating that like Pim-1, Pim-2 is another legitimate p21 kinase. However, Pim-2 has no influence on the nuclear localization of p21 in HCT116 cells. In addition, Pim-2 is able to arrest the cell cycle at G1/S phase and inhibit cell proliferation through phosphorylation of p21 in HCT116 cells. These data suggest that Pim-2 phosphorylation of p21 enhances p21's stability and inhibits cell proliferation in HCT116 cells.