The Transcription of FOXO Genes Is Stimulated by FOXO3 and Repressed by Growth Factors

Abstract

FOXO (Forkhead box O) transcription factors induce cell growth arrest and apoptosis, which can be prevented by FOXO phosphorylation by AKT in response to growth factors such as platelet-derived growth factors (PDGF) and insulin-like growth factor I (IGF-I). In addition to this well characterized post-translational modification, we showed that FOXO1, FOXO3, and FOXO4 were also regulated at the transcriptional level. PDGF, fibroblast growth factors (FGF), and IGF-I repressed the expression of FOXO genes in human fibroblasts. This process was sensitive to phosphatidylinositol 3-kinase inhibition by LY294002. FOXO1-specific shRNA decreased FOXO1 mRNA expression and enhanced fibroblast proliferation, mimicking the effects of growth factors. Conversely, ectopic FOXO3 activation blocked the proliferation of fibroblasts and induced the expression of FOXO1, FOXO4, and p27-KIP1. Using luciferase reporter assays and chromatin immunoprecipitations, we identified a conserved FOXO-binding site in the promoter of the FOXO1 gene, which was required for regulation by PDGF, and mediated the up-regulation of FOXO1 by itself and by FOXO3. Altogether, our results suggest that the expression of FOXO1 and FOXO4 genes is stimulated by FOXO3 and possibly by other FOXO factors in a positive feedback loop, which is disrupted by growth factors.Forkhead transcription factors, which were initially described in Drosophila melanogaster, constitute a family of transcription factors that share a conserved DNA-binding domain, the so-called forkhead box (1, 2). The FOXO (forkhead box O) group comprises four homologous mammalian proteins, namely FOXO1 (also called FKHR), FOXO3 (FKHR-L1), FOXO4 (AFX), and FOXO6. Activation of these factors induces cell cycle arrest, DNA damage repair, differentiation, and apoptosis. They also increase the resistance to oxidative stress, which was shown to be particularly important in hematopoietic stem cells, and regulate glucose metabolism in various organs. In Caenorhabditis elegans, the FOXO orthologue DAF-16 prolongs life span.The activity of FOXO proteins is tightly controlled by multiple post-translational modifications (3, 4). Growth factors, insulin, and other cell stimuli induce FOXO phosphorylation and inactivation by AKT (also called protein kinase B), a serine/ threonine kinase that is activated via the phosphatidylinositol (PI) 3 3-kinase pathway (1, 3). All FOXO proteins are substrates of AKT, which phosphorylates three conserved sites, resulting in the exclusion of FOXOs from the nucleus and in their subsequent ubiquitination and degradation. Phosphorylation by AKT may also regulate FOXO ability to bind to DNA (3). In addition, the mitogen-activated protein kinases ERK and p38, as well as serum-and glucocorticoid-inducible kinase, DIRK1, and IKK␤, also inactivate FOXO1 and/or FOXO3 by direct phosphorylation (1, 5, 6). By contrast, phosphorylation by c-Jun N-terminal kinase (JNK) kinases upon cell stress activates FOXO4 (7).In the absence of growth factors, FOXOs reside in the nucleus and up-regulate g...

Sirtuin1 (Sirt1) Promotes Cortical Bone Formation by Preventing β-Catenin Sequestration by FoxO Transcription Factors in Osteoblast Progenitors

“…5D). The mRNA level of FoxO3 was also increased by Sirt1 deletion, in line with evidence that FoxO3 can self-amplify (32). The mRNA levels of FoxO1 and FoxO4 were not affected by Sirt1.…”

Effect of growth hormone receptor gene disruption and PMA treatment on the expression of genes involved in primordial follicle activation in mice ovaries

“…1 Expression of Amh, Bmp15, and Gdf9 mRNA in a GHRKO (black bar) and normal mice (white bar), b normal mice treated with PMA (black bar) or DMSO (white bar), and c GHRKO mice treated with PMA (black bar) and DMSO (white bar) Glabowska et al 2012). Studies in other tissues generally indicate that higher FOXO3 mRNA expression predicts also higher level of nuclear FOXO3 (Essaghir et al 2009;Turrel-Davin et al 2010). Despite that, it is known that cellular localization (nuclear vs. cytoplasmic) of FOXO3 rather than the level of expression truly indicates the activation of this protein.…”

The fusion proteins TEL-PDGFR and FIP1L1-PDGFR escape ubiquitination and degradation

“…2 Signal transduction by PDGFR leads to the activation of transcription factors, which ultimately control the proliferation of cells such as fibroblasts and neurons. [3][4][5] However, PDGF-BB, which binds to both PDGFRα and PDGFRβ, is a poor mitogen for hematopoietic cells, although PDGFRα is expressed in platelets and megakaryocytes and PDGFRβ is expressed in megakaryocytes, macrophages and other hematopoietic cells. [6][7][8] Activation of PDGFR also promotes ubiquitination of lysine residues leading to receptor down-regulation.…”