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Reciprocal Control of Forkhead box O 3a and c-Myc via the Phosphatidylinositol 3-Kinase Pathway Coordinately Regulates p27^Kip1 Levels¹

Department of Biochemistry, Boston University Medical School, Boston, MA 02118

B cell receptor (BCR) engagement of murine WEHI 231 immature B lymphoma cells leads sequentially to a drop in NF-B and c-Myc, and induction of the p27^Kip1 cyclin-dependent kinase inhibitor, which promotes growth arrest and apoptosis. BCR engagement was recently shown to induce a drop in phosphatidylinositol 3-kinase (PI3K)/Akt signaling, preceding the increase in p27. As induction of p27 is due to an increase in gene transcription, we investigated the role of the Forkhead box O (FOXO) transcription factor family, which has been shown to potently induce p27 promoter activity. We demonstrate that pharmacologic inhibitors of PI3K or BCR engagement lead to decreased inactive cytoplasmic levels and increased active functional nuclear FOXO3a. In contrast, inhibition of PI3K/Akt signaling decreased the levels of NF-B and c-Myc, which has been shown to repress p27 promoter activity. To test the effects of ectopic c-Myc on endogenous p27 levels, WEHI 231 cells stably expressing c-Myc or empty vector DNA were prepared. Ectopic c-Myc blocked the induction of p27 expression upon either inhibition of PI3K or BCR engagement. Thus, p27^Kip1 is coordinately regulated via two arms of a signaling pathway that are inversely controlled upon inhibition of PI3K: induction of the activator FOXO3a and down-regulation of the repressor c-Myc.

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