title: Ligand-independent activation of estrogen receptor alpha by XBP-1.
abstract: The estrogen receptor (ER) is a member of a large superfamily of nuclear receptors that regulates the transcription of estrogen-responsive genes. Several recent studies have demonstrated that XBP-1 mRNA expression is associated with ERalpha status in breast tumors. However, the role of XBP-1 in ERalpha signaling remains to be elucidated. More recently, two forms of XBP-1 were identified due to its unconventional splicing. We refer to the spliced and unspliced forms of XBP-1 as XBP-1S and XBP-1U, respectively. Here, we report that XBP-1S and XBP-1U enhanced ERalpha-dependent transcriptional activity in a ligand-independent manner. XBP-1S had stronger activity than XBP-1U. The maximal effects of XBP-1S and XBP-1U on ERalpha transactivation were observed when they were co-expressed with full-length ERalpha. SRC-1, the p160 steroid receptor coactivator family member, synergized with XBP-1S or XBP-1U to potentiate ERalpha activity. XBP-1S and XBP-1U bound to the ERalpha both in vitro and in vivo in a ligand-independent fashion. XBP-1S and XBP-1U interacted with the ERalpha region containing the DNA-binding domain. The ERalpha-interacting regions on XBP-1S and XBP-1U have been mapped to two regions, including the N-terminal basic region leucine zipper domain (bZIP) and the C-terminal activation domain. The bZIP-deleted mutants of XBP-1S and XBP-1U completely abolished ERalpha transactivation by XBP-1S and XBP-1U. These findings suggest that XBP-1S and XBP-1U may directly modulate ERalpha signaling in both the absence and presence of estrogen and, therefore, may play important roles in the proliferation of normal and malignant estrogen-regulated tissues.
authors.affiliation: Beijing Institute of Biotechnology, Beijing 100850, Peoples Republic of China.
authors.forename: Lihua
authors.initials: L
authors.lastname: Ding
authors.forename: Jinghua
authors.initials: J
authors.lastname: Yan
authors.forename: Jianhua
authors.initials: J
authors.lastname: Zhu
authors.forename: Hongjun
authors.initials: H
authors.lastname: Zhong
authors.forename: Qiujun
authors.initials: Q
authors.lastname: Lu
authors.forename: Zonghua
authors.initials: Z
authors.lastname: Wang
authors.forename: Cuifen
authors.initials: C
authors.lastname: Huang
authors.forename: Qinong
authors.initials: Q
authors.lastname: Ye
mesh_terms: D015153:Blotting, Western; D002460:Cell Line; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D024202:Electrophoretic Mobility Shift Assay; D047628:Estrogen Receptor alpha; D004967:Estrogens / Q000378:metabolism / Q000494:pharmacology; D051548:Histone Acetyltransferases; D006801:Humans; D008024:Ligands; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D056920:Nuclear Receptor Coactivator 1; D011485:Protein Binding / Q000187:drug effects; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011960:Receptors, Estrogen / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D000071701:Regulatory Factor X Transcription Factors; D020218:Response Elements / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects; D014162:Transfection; D000071717:X-Box Binding Protein 1
chemical_list: D004268:DNA-Binding Proteins; D047628:Estrogen Receptor alpha; D004967:Estrogens; D008024:Ligands; D020033:Protein Isoforms; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; C000605543:XBP1 protein, human; D008156:Luciferases; D051548:Histone Acetyltransferases; C538859:NCOA1 protein, human; D056920:Nuclear Receptor Coactivator 1