title: The forkhead transcription factor FOXO4 induces the down-regulation of hypoxia-inducible factor 1 alpha by a von Hippel-Lindau protein-independent mechanism.
abstract: Tumors utilize hyperactivation of the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway to cope with deleterious environmental conditions. Activation of the PI3K/AKT pathway has been shown to increase protein expression of the alpha subunit of the hypoxia-inducible factor (HIF) 1, a key regulator of oxygen homeostasis. Elevated levels of HIF-1 alpha induce expression of genes with critical roles in angiogenesis, erythropoiesis, and glucose metabolism, processes that are essential for tumor expansion. Here we examine the involvement of FOXO4 (also known as AFX), a member of the forkhead transcription factor superfamily that is negatively regulated by the PI3K/AKT pathway, in the regulation of HIF-1 alpha protein expression. Nuclear expression of FOXO4 results in the suppression of various responses to hypoxia, including decreased vascular endothelial growth factor, glucose transporter 1, and erythropoietin expression. Interestingly, FOXO4 down-regulates the HIF-1 alpha protein levels, consistent with the lack of hypoxia responsiveness. Previous results have revealed a role for prolyl hydroxylation and resultant von Hippel-Lindau protein (pVHL) interactions in the ubiquitin-proteasome-mediated degradation of HIF-1 alpha. However, neither inhibition of prolyl hydroxylases nor mutation of HIF-1 alpha-hydroxylated prolines involved with pVHL-mediated binding inhibits the observed FOXO4-mediated down-regulation of HIF-1 alpha. These results suggest a novel alternate mechanism for hypoxic regulation that is dependent upon the level of activation of FOXO4-mediated transcription.
authors.affiliation: Department of Molecular Oncology, Genentech, Inc., South San Francisco, California 94080, USA.
authors.forename: Tracy Tzu-Ling
authors.initials: TT
authors.lastname: Tang
authors.forename: Laurence A
authors.initials: LA
authors.lastname: Lasky
mesh_terms: D015153:Blotting, Western; D018797:Cell Cycle Proteins; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D015536:Down-Regulation; D051858:Forkhead Transcription Factors; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D000860:Hypoxia; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D008025:Ligases / Q000378:metabolism*; D008156:Luciferases / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D017434:Protein Structure, Tertiary; D014157:Transcription Factors / Q000737:chemistry* / Q000378:metabolism* / Q000502:physiology*; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins; D025801:Ubiquitin / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases; D051794:Von Hippel-Lindau Tumor Suppressor Protein
chemical_list: D018797:Cell Cycle Proteins; C108803:FOXO4 protein, human; D051858:Forkhead Transcription Factors; C497442:HIF1A protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D009097:Multienzyme Complexes; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D025801:Ubiquitin; D004247:DNA; D008156:Luciferases; D044767:Ubiquitin-Protein Ligases; D051794:Von Hippel-Lindau Tumor Suppressor Protein; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D008025:Ligases; C497446:VHL protein, human; D010100:Oxygen