FOXO transcription factor-dependent p15^INK4b and p19 ^INK4d expression

FOXO (Forkhead box O) transcription factors are involved in cell-cycle arrest or apoptosis induction by transcripting cell-cycle inhibitor p27 ^KIP1 or apoptosis-related genes, respectively. Akt/protein kinase B promotes cell proliferation and suppresses apoptosis, in part, by phosphorylating FOXOs. Phosphorylated FOXOs could not exhibit transcriptional activity because of their nuclear export. Here we show that p15^INK4b and p19^INK4d transcription is associated with FOXO-mediated G ₁ cell-cycle arrest. Inhibition of Akt signaling by PI3K inhibitors, a PDK1 inhibitor, or dominant-negative Akt transfection increased expression of p15^INK4b and p19^INK4d but not p16^INK4a and p18^INK4c. Ectopic expression of wild type or active FOXO but not inactive form also increased p15^INK4b and p19^INK4d levels. FOXOs bound to promoter regions and induced transcription of these genes. No increase in the G₁-arrested cell population, mediated by PI3K inhibitor LY294002, was observed in INK4b^-/- or INK4d^-/- murine embryonic fibroblasts. In summary, FOXOs are involved in G₁ arrest caused by Akt inactivation via p15^INK4b and p19 ^INK4d transcription.