title: Hesx1 homeodomain protein represses transcription as a monomer and antagonises transactivation of specific sites as a homodimer.
abstract: The homeobox repressor Hesx1, expressed throughout Rathke's pouch and required for normal pituitary development, has been implicated in anterior pituitary pathogenesis in man. Prolonged expression of Hesx1 delays the appearance of anterior pituitary terminal differentiation markers in mice, particularly the gonadotroph hormones. We tested if Hesx1 could modulate gonadotrophin gene expression directly, and found that Hesx1 repressed both common alpha subunit (alpha GSU) and luteinising hormone beta-subunit (LH beta) gene promoters. Repression mapped to the Pitx1 homeodomain protein transactivation site in the proximal alpha GSU promoter, but did not map to the equivalent site on LH beta. Hesx1 repression of the alpha GSU Pitx1 site was overridden by co-transfection of Pitx1. In contrast, Hesx1 antagonised Pitx1 transactivation of LH beta in a dose-dependent manner. This was due to monomeric binding of Hesx1 on alpha GSU and homodimerisation on LH beta. The homodimerisation site comprises the Pitx1 DNA binding site and a proximal binding site, and mutation of either inhibited homodimer formation. Conversion of the LH beta Pitx1 DNA binding site to an alpha GSU-type did not promote homodimer formation, arguing that Hesx1 has pronounced site selectivity. Furthermore, mutation of the proximal half of the homodimerisation site blocked Hesx1 antagonisation of Pitx1 transactivation. We conclude that Hesx1 monomers repress gene expression, and homodimers block specific transactivation sites.
authors.affiliation: MRC Human Reproductive Sciences Unit, 37 Chalmers Street, Edinburgh EH3 9ET, UK.
authors.forename: J
authors.initials: J
authors.lastname: Quirk
authors.forename: P
authors.initials: P
authors.lastname: Brown
mesh_terms: D000818:Animals; D001483:Base Sequence; D051792:Basic Helix-Loop-Helix Proteins; D001665:Binding Sites / Q000235:genetics; D004247:DNA / Q000235:genetics / Q000378:metabolism; D019281:Dimerization; D015292:Glycoprotein Hormones, alpha Subunit / Q000235:genetics; D006367:HeLa Cells; D018398:Homeodomain Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007986:Luteinizing Hormone / Q000235:genetics; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation; D051761:Paired Box Transcription Factors; D010903:Pituitary Gland, Anterior / Q000254:growth & development; D011401:Promoter Regions, Genetic; D020836:Protein Structure, Quaternary; D012097:Repressor Proteins / Q000737:chemistry* / Q000378:metabolism*; D012689:Sequence Homology, Nucleic Acid; D000072056:Transcription Factor HES-1; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D015533:Transcriptional Activation; D014162:Transfection
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D004247:DNA; D015292:Glycoprotein Hormones, alpha Subunit; D018398:Homeodomain Proteins; D007986:Luteinizing Hormone; D051761:Paired Box Transcription Factors; D012097:Repressor Proteins; D000072056:Transcription Factor HES-1; D014157:Transcription Factors; C489736:HESX1 protein, human; C489734:Hes1 protein, mouse; C489733:Hesx1 protein, mouse; C101485:homeobox protein PITX1; C078493:HES1 protein, human