Cytoplasmic Mislocalization of p27Kip1 Protein Is Associated with Constitutive Phosphorylation of Akt or Protein Kinase B and Poor Prognosis in Acute Myelogenous Leukemia

Abstract

Cyclin-dependent kinase inhibitor p27Kip1 functions at the nuclear level by binding to cyclin E/cyclin-dependent kinase-2. It was shown that Akt or protein kinase B (Akt/PKB)-dependent phosphorylation of p27Kip1 led to the cytoplasmic mislocalization of p27Kip1, suggesting the potential abrogation of its activity. Here, we evaluated the localization of p27Kip1 protein in leukemic blasts in relation to Akt/PKB phosphorylation and clinical outcomes in acute myelogenous leukemia (AML). Western blot analysis of the nuclear and cytoplasmic fractions revealed a heterogenous localization pattern of p27Kip1 in AML. Cytoplasmic mislocalization of p27Kip1 was significantly associated with the constitutive serine 473 Akt/PKB phosphorylation in AML cells (P < 0.05). Transfection of U937 cells with an expression construct encoding the constitutively active form of Akt/PKB resulted in a remarkable increase in the levels of cytoplasmic p27Kip1. Whereas the transfection of U937 cells with a construct encoding dominant-negative Akt/PKB resulted in a recovery of nuclear localization of p27Kip1. Both the disease-free survival and overall survival are significantly shorter in AML cases with high cytoplasmic to nuclear ratio of p27Kip1 localization compared with the cases with low cytoplasmic to nuclear ratio (P ‫؍‬ 0.0353, P ‫؍‬ 0.0023, respectively). Multivariate analysis indicated that the cytoplasmic to nuclear ratio of p27Kip1 localization was an independent prognostic variable for both disease-free survival and overall survival (P ‫؍‬ 0.043, P ‫؍‬ 0.008, respectively). These findings additionally extend our understanding of the role of p27Kip1 in AML, and buttress the case of p27Kip1 mislocalization as a prognostic indicator and Akt/PKB/p27Kip1 pathway as a ready target for antileukemia therapy.

Phosphoinositide 3-kinase/Akt signaling pathway and its therapeutical implications for human acute myeloid leukemia

“…111 Interestingly, cytoplasmic localization of p27Kip1 in AML blasts with upregulated Akt activity was significantly associated with shorter disease-free and overall survival. 123,124 Cyclin D1 levels were also found to be upregulated through PI3K/Akt signaling in HL60 cells. 111 This might depend on Akt-mediated inhibition of glycogen synthase kinase 3b (GSK3b) (see later), because cyclin D1 phosphorylation by GSK3b results in its destabilization.…”

p27 as Jekyll and Hyde: Regulation of cell cycle and cell motility

“…[32][33][34] Cytoplasmic accumulation of T198-phosphorylated p27 also correlated with PKB activation in breast cancers. 72 Cytoplasmic p27 was associated with reduced survival and predicts poor prognosis in breast carcinomas, 32 acute myelogenous leukemia, 73 pancreatic cancer 74 and ovarian carcinomas 75 .…”

Functional characterization of a rare germline mutation in the gene encoding the cyclin-dependent kinase inhibitor p27Kip1 (CDKN1B) in a Spanish patient with multiple endocrine neoplasia-like phenotype

“…Therefore, alternative mechanisms that act at the posttranscriptional or post-translational level have been suggested. These alternative mechanisms include a lower translation rate of CDKN1B mRNA (19), an increase in protein degradation (20,21), and the cytosolic mislocalization of p27 protein (22,23,24).…”