title: Hypoxia-inducible factor (HIF)-3alpha is subject to extensive alternative splicing in human tissues and cancer cells and is regulated by HIF-1 but not HIF-2.
abstract: The hypoxia-inducible transcription factors (HIFs) play a central role in the response of cells to hypoxia. HIFs are alphabeta dimers, the human alpha subunit having three isoforms. HIF-3alpha is unique among the HIF-alpha isoforms in that its gene is subject to extensive alternative splicing. Database analyses have predicted the generation of six HIF-3alpha splice variants that utilize three alternative transcription initiation sites. None of these variants is likely to act as an efficient transcription factor, but some of them have been reported to inhibit HIF-1 and HIF-2 functions. We analyzed here for the first time in detail whether these six variants are indeed generated in various human tissues and cell lines. We identified four novel variants, named here HIF-3alpha7 to HIF-3alpha10, whereas we obtained no evidence for the predicted HIF-3alpha3 and HIF-3alpha5. Distinct differences in the expression patterns of the variants were found between human tissues, the levels being particularly low in many cancer cell lines. Hypoxia upregulated transcription from all three alternative HIF-3alpha promoters. siRNA experiments showed that this induction is mediated specifically by HIF-1 and not by HIF-2. The tissue-specific differences in the expression patterns and levels of the HIF-3alpha variants can be expected to modulate the hypoxia response of various tissues and cell types to different extents during development and in pathological situations. A further level of regulation is brought about by the fact that the levels of the HIF-3alpha transcripts themselves are regulated by hypoxia and by changes in HIF-1 levels.
authors.affiliation: Oulu Center for Cell-Matrix Research, Biocenter Oulu and Department of Medical Biochemistry and Molecular Biology, University of Oulu, FIN-90014, Finland.
authors.forename: Annika
authors.initials: A
authors.lastname: Pasanen
authors.forename: Minna
authors.initials: M
authors.lastname: Heikkilä
authors.forename: Kati
authors.initials: K
authors.lastname: Rautavuoma
authors.forename: Maija
authors.initials: M
authors.lastname: Hirsilä
authors.forename: Kari I
authors.initials: KI
authors.lastname: Kivirikko
authors.forename: Johanna
authors.initials: J
authors.lastname: Myllyharju
mesh_terms: D017398:Alternative Splicing / Q000235:genetics*; D051017:Apoptosis Regulatory Proteins; D051792:Basic Helix-Loop-Helix Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D015687:Cell Hypoxia / Q000235:genetics; D045744:Cell Line, Tumor; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D051793:Hypoxia-Inducible Factor 1 / Q000235:genetics / Q000378:metabolism*; D008745:Methylation; D050505:Mutant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D009369:Neoplasms / Q000235:genetics* / Q000473:pathology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins; D024363:Transcription Initiation Site; D014158:Transcription, Genetic; D000099162:Endothelial PAS Domain-Containing Protein 1
chemical_list: D051017:Apoptosis Regulatory Proteins; D051792:Basic Helix-Loop-Helix Proteins; D051793:Hypoxia-Inducible Factor 1; D050505:Mutant Proteins; D012333:RNA, Messenger; D012097:Repressor Proteins; D000099162:Endothelial PAS Domain-Containing Protein 1; C116973:HIF3A protein, human