Forkhead transcription factor FKHR-L1 modulates cytokine-dependent transcriptional regulation of p27(KIP1)

Interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating f actor regulate the survival, proliferation, and differentiation of hematopo ietic lineages. Phosphatidylinositol 3-kinase (PI3K) has been implicated in the regulation of these processes, Here we investigate the molecular mecha nism by which PI3K regulates cytokine-mediated proliferation and survival i n the murine pre-B-cell line Ba/F3, IL-3 was found to repress the expressio n of the cyclin-dependent kinase inhibitor p27(KIPI) through activation of PI3K and this occurs at the level of transcription. This transcriptional re gulation occurs through modulation of the forkhead transcription factor FKH R-L1, and IL-3 inhibited FKHR-L1 activity in a PI3K-dependent manner. We ha ve generated Ba/F3 cell lines expressing a tamoxifen-inducible active FKHR- L1 mutant [FKHR-L1(A3):ER*]. Tamoxifen-mediated activation of FKHR-L1(A3):E R* resulted in a striking increase in p27(KIPI) promoter activity and mRNA and protein levels as well as induction of the apoptotic program. The level of p27(KIPI) appears to be critical in the regulation of cell survival sin ce mere ectopic expression of p27(KIPI) was sufficient to induce Ba/F3 apop tosis, Moreover, cell survival was increased in cytokine-starved bone marro w-derived stem cells from p27(KIPI) null-mutant mice compared to that in ce lls from wild-type mice. Taken together, these observations indicate that i nhibition of p27(KIPI) transcription through PI3K-induced FKHR-L1 phosphory lation provides a novel mechanism of regulating cytokine-mediated survival and proliferation.