Akt/PKB Activation-Induced Cytoplasmic Mislocalization of p27Kip1 Protein Is Mediated by Jab1 Shuttling in Human Myeloid Leukemia Cells.

We previously demonstrated the cytoplasmic mislocalization of p27Kip1 protein was highly correlated with constitutive Akt/PKB phosphorylation and poor prognosis in primary acute myeloid leukemia. However, mechanism involved in the Akt/PKB-mediated nuclear export of p27Kip1 in leukemia cells is not fully understood. Jun activation domain-binding protein 1 (Jab1)/CSN5 is the fifth component of the COP9 signalosome complex, which is involved in modulating signal transduction, ubiquitin-proteasome pathway, and nuclear transport. We examined the effects of Akt/PKB activation on the phosphorylation and localization of p27Kip1 protein in relation to Jab1 shuttling in K562 leukemia cells. Akt/PKB activation did not affect the phosphorylation at threonine 187 residue (T187) of p27Kip1 protein in K562 cells. However, the phosphorylation of p27Kip1 at the serine 10 (S10) and threonine T157 (T157) residue was markedly increased in the constitutively active Akt/PKB cDNA-transfected K562 (Myr-Akt/PKB-K562) cells. In the Myr-Akt/PKB-K562 cells, the S10- and T157-phosphorylated form of p27Kip1 localized almost exclusively to the cytoplasm. Leptomycin-B (LMB) pretreatment of Myr-Akt/PKB-K562 cells led to restoration of S10- and T157-phosphorylated p27Kip1 to the nucleus, indicating that Akt/PKB activation-induced cytoplasmic translocation of S10- and T157-phosphorylated p27Kip1 was mediated by CRM1-dependent pathway. We found that Akt/PKB activation did not alter the total cellular levels of Jab1 protein. However, Jab1 protein was exclusively localized to the cytoplasm in the Myr-Akt/PKB-K562 cells, whereas it was preferentially localized to the nucleus in the dominant negative Akt/PKB cDNA-transfected K562 cells. In Myr-Akt/PKB-K562 cells pretreated with LMB, the localization of the Jab1 was essentially nuclear. Jab1 silencing led to an accumulation of S10- and T157-phosphorylated p27Kip1 to the nucleus in the Myr-Akt/PKB-K562 cells, which was accompanied by a prominent decrease in the cytoplasmic level of p27Kip1. Taken together, we demonstrate that Akt/PKB activation-induced cytoplasmic mislocalization of S10- and T157-phosphorylated p27Kip1 protein is mediated by Jab1 shuttling in human myeloid leukemia cells. Apart from our understanding of the mechanism implicated in the regulation of phosphorylation and localization of p27Kip1 protein in leukemia cells, all these data might suggest possibilities for designing leukemia therapy since it should be possible to restore the function of a misplaced p27Kip1 protein.