title: Fundamentally distinct roles of thyroid hormone receptor isoforms in a thyrotroph cell line are due to differential DNA binding.
abstract: Thyroid hormones have a profound influence on human development and disease. The hypothalamic-pituitary-thyroid axis involves finely tuned feedback mechanisms to maintain thyroid hormone (TH) levels. Despite the important role of TH-negative feedback in regulating this axis, the mechanism by which this occurs is not clearly defined. Previous in vivo studies suggest separate roles for the two thyroid hormone receptor isoforms, THRA and THRB, in this axis. We performed studies using a unique pituitary thyrotroph cell line (TαT1.1) to determine the relative roles of THRA and THRB in the regulation of Tshb. Using chromatin immunoprecipitation assays, we found that THRB, not THRA, bound to the Tshb promoter. By selectively depleting THRB, THRA, or both THRA and THRB in TαT1.1 cells, we found that simultaneous knockdown of both THRB and THRA abolished T(3)-mediated down-regulation of Tshb at concentrations as high as 100 nm T(3). In contrast, THRA knockdown alone had no effect on T(3)-negative regulation, whereas THRB knockdown alone abolished T(3)-mediated down-regulation of Tshb mRNA levels at 10 nm but not 100 nm T(3) concentrations. Interestingly, chromatin immunoprecipitation assays showed that THRA becomes enriched on the Tshb promoter after knockdown of THRB. Thus, a likely mechanism for the differential effects of THR isoforms on Tshb may be based on their differential DNA-binding affinity to the promoter.
authors.affiliation: Universidade Federal de Sao Paulo, 05508-900 Sao Paulo, Brazil.
authors.forename: Maria I
authors.initials: MI
authors.lastname: Chiamolera
authors.forename: Aniket R
authors.initials: AR
authors.lastname: Sidhaye
authors.forename: Shunichi
authors.initials: S
authors.lastname: Matsumoto
authors.forename: Qiyi
authors.initials: Q
authors.lastname: He
authors.forename: Koshi
authors.initials: K
authors.lastname: Hashimoto
authors.forename: Tania M
authors.initials: TM
authors.lastname: Ortiga-Carvalho
authors.forename: Fredric E
authors.initials: FE
authors.lastname: Wondisford
mesh_terms: D000818:Animals; D002460:Cell Line; D004247:DNA / Q000378:metabolism*; D005786:Gene Expression Regulation; D055785:Gene Knockdown Techniques; D015292:Glycoprotein Hormones, alpha Subunit / Q000235:genetics / Q000378:metabolism; D051379:Mice; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D020033:Protein Isoforms / Q000378:metabolism / Q000502:physiology; D034622:RNA Interference; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011988:Receptors, Thyroid Hormone / Q000378:metabolism / Q000502:physiology*; D052684:Thyrotrophs / Q000378:metabolism*; D037322:Thyrotropin, beta Subunit / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D014284:Triiodothyronine / Q000502:physiology
chemical_list: D015292:Glycoprotein Hormones, alpha Subunit; D020033:Protein Isoforms; D012333:RNA, Messenger; D011988:Receptors, Thyroid Hormone; D037322:Thyrotropin, beta Subunit; D014284:Triiodothyronine; D004247:DNA