Effects of ligand and thyroid hormone receptor isoforms on hepatic gene expression profiles of thyroid hormone receptor knockout mice

Abstract

Little is known about the overall patterns of thyroid hormone (Th)-mediated gene regulation by the main Th receptor (Tr) isoforms, Tr-α and Tr-β, in vivo. We used 48 complementary DNA microarrays to examine hepatic gene expression profiles of wildtype and Thra and Thrb knockout mice under different Th conditions: no treatment, treatment with 3,3',5-triiodothyronine (T 3 ), Th-deprivation using propylthiouracil (PTU), and treatment with a combination of PTU and T 3 . Hierarchical clustering analyses showed that positively regulated genes fit into three main expression patterns. In addition, only a subpopulation of target genes repressed basal transcription in the absence of ligand. Interestingly, Thra and Thrb knockout mice showed similar gene expression patterns to wild-type mice, suggesting that these isoforms co-regulate most hepatic target genes. Differences in the gene expression patterns of Thra/Thrb double-knockout mice and Th-deprived wild-type mice show that absence of receptor and of hormone can have different effects. This large-scale study of hormonal regulation reveals the functions of Th and of Tr isoforms in the regulation of gene expression patterns. EMBO reports 4, 581-587 (2003) doi:10.1038/sj.embor.embor862 In the absence of Th, Trs recruit co-repressor complexes (for example, nuclear corepressor (NCoR) and silencing mediator of Tr and retinoic acid receptor (Smrt)) that lead to the histone deacetylation of local chromatin in the promoter region, and decrease the basal transcription of positively regulated target genes (Burke & Baniahmad, 2000). In the presence of Th, Trs recruit co-activator complexes that contain the p160 steroidreceptor co-activator (SRC) family members and other cofactors that have histone acetyltransferase activity, as well as other complexes that can associate with the RNA polymerase II complex (Yen, 2001;Zhang & Lazar, 2000). The overall effect of these ligand-dependent interactions is to promote the transcriptional activation of positively regulated target genes. Trs also negatively regulate target genes, but the molecular mechanism by which they do this is poorly understood (Yen, 2001).Recent studies have shown that Thra and Thrb knockout mice have different phenotypes, and have also suggested that different Tr isoforms may have distinct roles (Flamant & Samarut, 2003;Forrest & Vennstrom, 2000). Two groups have generated Thra knockout mice that have distinct phenotypes (Flamant & Samarut, 2003;Forrest & Vennstrom, 2000). This was due to the targeting of different loci for homologous recombination, which resulted in the loss of different combinations of Tr-α isoforms. Thra knockout mice that lack all Tr-α isoforms (Thra 0/0 ), including short Tr-α isoforms that are generated by internal transcription initiation, are viable and fertile, and have a mild phenotype (Gauthier et al., 2001;Macchia et al., 2001).

The history of a developmental stage: Metamorphosis in chordates

“…This importance of the gene regulatory cascade controlled by a relatively invariant master regulator is reminiscent of the situation observed in other cases such as eye development [with pax6 being the master gene ] or antero-posterior development of arthropods [with Hox genes being the key selector genes (Pearson et al, 2005) and more generally (Carroll et al, 2005)]. Indeed, TH-regulated gene expression pattern is highly variable from one species to another, as shown in X. laevis and the mouse (Das et al, 2006;Yen et al, 2003). It should be interesting to compare gene expression in other vertebrates.…”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…Ultimately, the physiological relevance of such an element would require in vivo evidence based, for example, on analysis of transgenic reporter genes carrying the promoter and/or enhancer elements of the target gene. (127,(390)(391)(392)(393)(394). For the majority of these genes, it remains unknown which respond directly and which indirectly to T 3 , since in many tissues relatively few genes have been defined as direct targets by rigorous criteria that include the identification of the functional TR binding sites in the gene.…”

“…The initial gene targeting studies established that Thrb is primarily responsible for the regulation of the HPT axis (446,447), the development of the auditory and color visual systems (448,449), and the majority of T 3 actions in liver (390,393,450). The Thra gene is primarily responsible for determining thermogenic and cardiac functions (386,451) and the maturation of the intestine (452), bone (453), and certain brain tissues (445,454).…”

Novel Mechanism of Positive versus Negative Regulation by Thyroid Hormone Receptor β1 (TRβ1) Identified by Genome-wide Profiling of Binding Sites in Mouse Liver

“…Previous studies have been based on microarray analysis of mRNA generated from T3-treated samples compared with untreated controls (17)(18)(19)(20). This approach does not address whether the changes seen in gene expression reflect a direct effect of T3 on target genes through the TR or whether some of these changes in expression are secondary effects.…”