PKC-dependent phosphorylation of p27 at T198 contributes to p27 stabilization and cell cycle arrest

In this manuscript, we present experimental evidence that PKCs phosphorylate p27 at T198 in vitro and in vivo, resulting in p27 stabilization and cell cycle arrest in MCF-7 and HeLa cells. Our findings indicate that (1) recombinant PKCα, βII, δ, η and θ isoforms phosphorylate, in in vitro kinase assays, wild-type recombinant p27 protein expressed in E. coli and wild-type p27 protein immunoprecpitated from transfected HEK-293 cells but not the T198A mutant, (2) adoptive expressed PKCα and δ phosphorylate both transfected and endogenous p27 at T198 in HEK-293 cells, (3) T198 phosphorylation of transfected and endogenous p27 is increased by PKC activators [Phorbol 12-myristate 13-acetate (PMA)] and suppressed by PKC inhibitors (Rottlerin A, G06976, Calphostin C), (4) in parallel with increased T198 phosphorylation, PMA induces stabilization of p27 protein in HeLa cells, whereas PKC inhibitors induce a decrease in p27 stability and, finally, (5) PMA-induced p27 upregulation is necessary for growth arrest of HeLa and MCF-7 cells induced by PKC activation by PMA. Overall, these results suggest that PKC-dependent upregulation of p27 induced by its phosphorylation at T198 represents a mechanism that mediates growth arrest promoted by PMA and provide novel insights on the ability of different PKC isoforms to play a role in controlling cell cycle progression.