title: Potential mechanisms underlying estrogen-induced expression of the molluscan estrogen receptor (ER) gene.
abstract: In vertebrates, estrogens and estrogen mimicking chemicals modulate gene expression mainly through a genomic pathway mediated by the estrogen receptors (ERs). Although the existence of an ER orthologue in the mollusc genome has been known for some time, its role in estrogen signalling has yet to be deciphered. This is largely due to its constitutive (ligand-independent) activation and a limited mechanistic understanding of its regulation. To fill this knowledge gap, we cloned and characterised an ER cDNA (sgER) and the 5'-flanking region of the gene from the Sydney rock oyster Saccostrea glomerata. The sgER cDNA is predicted to encode a 477-amino acid protein that contains a DNA-binding domain (DBD) and a ligand-binding domain (LBD) typically conserved among both vertebrate and invertebrate ERs. A comparison of the sgER LBD sequence with those of other ligand-dependent ERs revealed that the sgER LBD is variable at several conserved residues known to be critical for ligand binding and receptor activation. Ligand binding assays using fluorescent-labelled E2 and purified sgER protein confirmed that sgER is devoid of estrogen binding. In silico analysis of the sgER 5'-flanking sequence indicated the presence of three putative estrogen responsive element (ERE) half-sites and several putative sites for ER-interacting transcription factors, suggesting that the sgER promoter may be autoregulated by its own gene product. sgER mRNA is ubiquitously expressed in adult oyster tissues, with the highest expression found in the ovary. Ovarian expression of sgER mRNA was significantly upregulated following in vitro and in vivo exposure to 17β-estradiol (E2). Notably, the activation of sgER expression by E2 in vitro was abolished by the specific ER antagonist ICI 182, 780. To determine whether sgER expression is epigenetically regulated, the in vivo DNA methylation status of the putative proximal promoter in ovarian tissues was assessed using bisulfite genomic sequencing. The results showed that the promoter is predominantly hypomethylated (with 0-3.3% methylcytosines) regardless of sgER mRNA levels. Overall, our investigations suggest that the estrogen responsiveness of sgER is regulated by a novel ligand-dependent receptor, presumably via a non-genomic pathway(s) of estrogen signalling.
authors: School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia; Department of Agriculture, Forestry and Fisheries, Vinh University, 182 Le Duan St., Vinh City, Nghe An, Vietnam.
authors: Thi Kim Anh
authors: TK
authors: Tran
authors: School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia.
authors: Geoff R
authors: GR
authors: MacFarlane
authors: Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region, China.
authors: Richard Yuen Chong
authors: RY
authors: Kong
authors: New South Wales Department of Primary Industries, Port Stephens Fisheries Institute, Taylors Beach, NSW 2316, Australia.
authors: Wayne A
authors: WA
authors: O'Connor
authors: School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia. Electronic address: Richard.Yu@newcastle.edu.au.
authors: Richard Man Kit
authors: RM
authors: Yu
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019175:DNA Methylation / Q000187:drug effects; D004958:Estradiol / Q000633:toxicity; D004967:Estrogens / Q000633:toxicity*; D005260:Female; D015870:Gene Expression / Q000187:drug effects*; D008969:Molecular Sequence Data; D008974:Mollusca / Q000187:drug effects / Q000378:metabolism; D010125:Ostreidae / Q000187:drug effects / Q000378:metabolism; D010053:Ovary / Q000378:metabolism; D010802:Phylogeny; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D012333:RNA, Messenger / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D011960:Receptors, Estrogen / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D004967:Estrogens; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D014157:Transcription Factors; D014874:Water Pollutants, Chemical; D004958:Estradiol
keywords: Estrogen receptor; Estrogen responsive element; Gene expression; Gene promoter; Molluscs; Sydney rock oyster