BCR/ABL Regulates Expression of the Cyclin-dependent Kinase Inhibitor p27Kip1 through the Phosphatidylinositol 3-Kinase/AKT Pathway

Abstract

Deregulation of cell cycle checkpoints is an almost universal abnormality in human cancers and is most often due to loss-of-function mutations of tumor suppressor genes such as Rb, p53, or p16INK4a . In this study, we demonstrate that BCR/ABL inhibits the expression of a key cell cycle inhibitor, p27 Kip1 , by signaling through a pathway involving phosphatidylinositol 3-kinase (PI3K). p27Kip1 is a widely expressed inhibitor of cdk2, an essential cell cycle kinase regulating entry into S phase. We demonstrate that the decrease of p27Kip1 is directly due to BCR/ABL in hematopoietic cells by two different approaches. First, induction of BCR/ABL by a tetracycline-regulated promoter is associated with a reversible down-regulation of p27 Kip1 . Second, inhibition of BCR/ABL kinase activity with the Abl tyrosine kinase inhibitor STI571 rapidly increases p27Kip1 levels. The PI3K inhibitor LY-294002 blocks the ability of BCR/ABL to induce p27 Kip1 down-regulation and inhibits BCR/ ABL-induced entry into S phase. The serine/threonine kinase AKT/protein kinase B is a known downstream target of PI3K. Transient expression of an activated mutant of AKT was found to decrease expression of p27 Kip1, even when PI3K was inhibited by LY-294002. The mechanism of p27 Kip1 regulation is primarily related to protein stability, since inhibition of proteasome activity increased p27 Kip1 levels in BCR/ABL-transformed cells, whereas very little change in p27 transcription was found. Overall, these data are consistent with a model in which BCR/ABL suppresses p27Kip1 protein levels through PI3K/AKT, leading to accelerated entry into S phase. This activity is likely to explain in part previous studies showing that activation of PI3K was required for optimum transformation of hematopoietic cells by BCR/ABL in vitro and in vivo.

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“…AKT1 also phosphorylates and inactivates the Forkhead transcription factors, an event that results in the reduced expression of the cell cycle inhibitor, p27 Kip1 , and the Fas ligand (12)(13)(14). Via phosphorylation of IKK, AKT1 also activates NF B, a transcription factor that has been implicated in cell survival (15,16).…”

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“…Thus, although NF B is a transcription factor that has been implicated in cell survival (17,18), there is a possibility that the function of NF B depends on tumor type. However, most reports suggest that paclitaxelinduced NF B activity mediates survival signals that counteract apoptosis (29 -31).…”

“…Akt also phosphorylates and inactivates the Forkhead transcription factors, resulting in reduced expression of the cell cycle inhibitor p27 Kip1 and the Fas ligand (14 -16). Via the phosphorylation of I B kinase (IKK), Akt also activates nuclear factor-B (NF B), a transcription factor that has been implicated in cell survival (17,18).…”

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