Molecular and Cellular Endocrinology 246 (2006) 121–127 Thyroid hormone action at the cellular, genomic and target gene levels Paul M. Yen a,b,∗ , Shinichiro Ando b,1 , Xu Feng b,1 , Ying Liu b,1 , Padma Maruvada b,1 , Xianmin Xia a,1 a Department of Medicine, Johns Hopkins Bayview Medical Center, 4940 Eastern Avenue Rm B114, Baltimore, MD, United States b Clinical Endocrinology Branch, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, MD, USA Abstract Thyroid hormone (TH) plays important roles in metabolism, growth and differentiation. Thyroid hormone receptors (TRs) are ligand-regulatable transcription factors that bind both TH and DNA enhancer sequences in the promoter region of target genes where they can interact with co-repressor and co-activator complexes. These interactons, in turn, have consequent effects on transcription. This review describes studies on TH action from our laboratory examining the cellular localization and motility of TRs using green fluorescent fusion proteins, gene expression profiles of TH in WT and TR and TR KO mice, as well as general transcription factor and co-activator recruitment on the promoters of target genes by TH in chromatin immunoprecipitation assays. © 2006 Published by Elsevier Ireland Ltd. Keywords: Thyroid hormone receptor; Transcription; Green fluorescent protein; Microarray; Chromatin immunprecipitation assay 1. Introduction TRs belong to a large superfamily of nuclear hormone recep- tors (NRs) that include the steroid hormone, retinoic acid, Vitamin D and peroxisomal proliferator receptors (PPARs) (Yen, 2001; McKenna and O’Malley, 2002). TRs are cellular homologs of v-erbA, a viral oncogene product that causes ery- throleukemia in chicks. Thyroid hormone receptors (TRs) are ligand-regulatable transcription factors that bind thyroid hor- mone (TH) as well as specific DNA-sequences in enhancer ele- ments (thyroid hormone response elements (TREs)) located in the promoters of target genes. Like other NRs, they have a central DNA-binding domain containing two “zinc finger” motifs and a carboxy-terminal ligand-binding domain (LBD). The hinge region between these two domains contains a nuclear localiza- tion sequence. The carboxy-terminal region also contains mul- tiple contact surfaces that are important for heterodimerization with its partner, retinoid X receptor as well as protein–protein interactions with co-repressors and co-activators. There are two TR two genes, THRA and THRB, located on human chromosomes 17 and 3, respectively (Yen, 2001; Harvey ∗ Corresponding author. Tel.: +1 410 550 4229; fax: +1 410 550 6864. E-mail address: pyen3@jhmi.edu (P.M. Yen). 1 These co-authors contributed significantly to the research described in this manuscript and are listed alphabetically. and Williams, 2002; Zhang and Lazar, 2000). The THRA gene generates two mature mRNAs by alternative splicing that encode two proteins, TR-1 and c-erbA-2, that differ in their carboxy- termini. TR-1 is a bona fide receptor whereas c-erbA-2 is unable to bind TH and blocks TR-mediated transcription. The THRB gene encodes two TR isoforms, TR-1 and TR-2, which most likely are generated by alternative promoter choice (Williams and Brent, 1995) and have differing amino-terminal regions. The major TR isoforms (TR-1, -1, -2) bind T 3 with high affinity and mediate thyroid hormone-regulated tran- scription. They also have highly conserved DNA-binding and ligand-binding domains. TRs bind to distinct thyroid hormone response elements (TREs) typically located in the promoter regions of target genes. TREs usually contain two or more hexamer half-site sequences of AGGT(C/A)A arranged in tandem arrays (Williams and Brent, 1995). TREs vary in the primary nucleotide sequences of half-sites as well as their number, spacing and orientation (Williams and Brent, 1995; Glass, 1994). The ability of TRs to bind to such a wide repertoire of nucleotide sequences and motifs is most likely due to hetereodimerization with RXRs (Glass, 1994). Of note, RXRs also heterodimerize with other members of the NR superfamily such as retinoic acid and Vitamin D recep- tors, and enhance the binding of these receptors to their cognate hormone response elements. Unliganded TRs bind as homod- imers and heterodimers to TREs in vitro whereas liganded TRs bind primarily as heterodimers (Yen, 2001). Thus, TR/RXR 0303-7207/$ – see front matter © 2006 Published by Elsevier Ireland Ltd. doi:10.1016/j.mce.2005.11.030