Functional interference between hypoxia and dioxin signal transduction pathways: competition for recruitment of the Arnt transcription factor.
Hypoxia-inducible factor 1 alpha (HIF-1 alpha) and the intracellular dioxin receptor mediate hypoxia and dioxin signalling, respectively. Both proteins are conditionally regulated basic helix-loop-helix (bHLH) transcription factors that, in addition to the bHLH motif, share a Per-Arnt-Sim (PAS) region of homology and form heterodimeric complexes with the common bHLH/PAS partner ... factor Arnt. Here we demonstrate that HIF-1 alpha required Arnt for DNA binding in vitro and functional activity in vivo. Both the bHLH and PAS motifs of Arnt were critical for dimerization with HIF-1 alpha. Strikingly, HIF-1 alpha exhibited very high affinity for Arnt in coimmunoprecipitation assays in vitro, resulting in competition with the ligand-activated dioxin receptor for recruitment of Arnt. Consistent with these observations, activation of HIF-1 alpha function in vivo or overexpression of HIF-1 alpha inhibited ligand-dependent induction of DNA binding activity by the dioxin receptor and dioxin receptor function on minimal reporter gene constructs. However, HIF-1 alpha- and dioxin receptor-mediated signalling pathways were not mutually exclusive, since activation of dioxin receptor function did not impair HIF-1 alpha-dependent induction of target gene expression. Both HIF-1 alpha and Arnt mRNAs were expressed constitutively in a large number of human tissues and cell lines, and these steady-state expression levels were not affected by exposure to hypoxia. Thus, HIF-1 alpha may be conditionally regulated by a mechanism that is distinct from induced expression levels, the prevalent model of activation of HIF-1 alpha function. Interestingly, we observed that HIF-1 alpha was associated with the molecular chaperone hsp90. Given the critical role of hsp90 for ligand binding activity and activation of the dioxin receptor, it is therefore possible that HIF-1 alpha is regulated by a similar mechanism, possibly by binding an as yet unknown class of ligands.
Mesh Terms:
Aryl Hydrocarbon Receptor Nuclear Translocator, Carcinoma, Hepatocellular, Cell Hypoxia, Cobalt, Cytochrome P-450 CYP1A1, DNA-Binding Proteins, Dioxins, Female, Genes, Reporter, HSP90 Heat-Shock Proteins, HeLa Cells, Helix-Loop-Helix Motifs, Humans, Hypoxia-Inducible Factor 1, Hypoxia-Inducible Factor 1, alpha Subunit, Liver Neoplasms, Luciferases, Male, Mutagenesis, Nuclear Proteins, Organ Specificity, RNA, Messenger, Receptors, Aryl Hydrocarbon, Recombinant Fusion Proteins, Sequence Deletion, Signal Transduction, Transcription Factors, Transfection, Tumor Cells, Cultured
Aryl Hydrocarbon Receptor Nuclear Translocator, Carcinoma, Hepatocellular, Cell Hypoxia, Cobalt, Cytochrome P-450 CYP1A1, DNA-Binding Proteins, Dioxins, Female, Genes, Reporter, HSP90 Heat-Shock Proteins, HeLa Cells, Helix-Loop-Helix Motifs, Humans, Hypoxia-Inducible Factor 1, Hypoxia-Inducible Factor 1, alpha Subunit, Liver Neoplasms, Luciferases, Male, Mutagenesis, Nuclear Proteins, Organ Specificity, RNA, Messenger, Receptors, Aryl Hydrocarbon, Recombinant Fusion Proteins, Sequence Deletion, Signal Transduction, Transcription Factors, Transfection, Tumor Cells, Cultured
Mol. Cell. Biol.
Date: Oct. 01, 1996
PubMed ID: 8816435
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