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Isoform-specific transcriptional activity of overlapping targets that respond to thyroid hormone receptors a1 and b1

ABSTRACT: Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of physiology and development. TRs are expressed in vertebrates as a series of distinct isoforms that exert distinct biological roles. We wished to determine if the two most widely expressed isoforms, TRa1 and TRb1, exert their different biological effects by regulating different sets of target genes. Using stably transformed HepG2 cells and a microarray analysis, we were able to demonstrate that TRa1 and TRb1 regulate a largely overlapping repertoire of target genes in response to T3 hormone. However, these two isoforms display very different transcriptional properties on each individual target gene, ranging from a much greater T3-mediated regulation by TRa1 than by TRb1, to near equal regulation by both isoforms. We also identified TRa1 and TRb1 target genes that were regulated by these receptors in a hormone-independent fashion. We suggest that it is this gene-specific, isoform-specific amplitude of transcriptional regulation that is the likely basis for the appearance and maintenance of TRa1 and TRb1 over evolutionary time. In essence, TRa1 and TRb1 adjust the magnitude of the transcriptional response at different target genes to different levels; by altering the ratio of these isoforms in different tissues or at different developmental times, the intensity of T3 response can be individually tailored to different physiological and developmental requirements.

ORGANISM(S): Homo sapiens

PROVIDER: GSE17561 | GEO | 2009/08/08

SECONDARY ACCESSION(S): PRJNA118663

REPOSITORIES: GEO

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Project description:Thyroid hormone receptors (TRs) represent important regulators of development, homeostasis, cell proliferation and differentiation. Here we investigated the impact of thyroid hormone (T3) on growth and differentiation of human red blood cells. T3 was found to effectively accelerate differentiation of SCF/Epo dependent red cell progenitors in vitro concomitantly with inducing growth arrest. This T3 activity was further enhanced by 9-cis retinoic acid (9cRA) that activates retinoid X receptor (RXR), the obligate heterodimeric partner of TR. To identify molecular targets for T3 activity in red cells, we employed a genome wide approach with DNA microarrays. We demonstrate that T3 and 9cRA induces specific gene expression patterns of up-or down-regulated genes, including ALAD and GATA-2, respectively. This study also revealed gene clusters, indicating accelerated differentiation in response to treatment. Mining for T3 induced genes identified BTEB1 (KLF9) and GAR22 as TR target genes. BTEB1/ KLF9 (basic transcription element binding protein 1/ KrÃ¼ppel-like factor 9) is a known TR target gene. GAR22 (growth arrest specific 2 (GAS2)-related gene on chromosome 22), initially found as a putative tumor suppressor gene, was induced by T3 also in the presence of cycloheximide, identifying it as a direct target of TR. Thus, our study uncovers novel targets of TR action with a potential function in T3 induced differentiation and growth arrest of red cell progenitors. Experiment Overall Design: 13 hybridizations in two individual experiments. Experiment Overall Design: Experiment 1: Experiment Overall Design: 1_SCF/Epo, 1_Epo/insulin(8h), 1_Epo/insulin(24h), 1_Epo/insulin(48h) Experiment Overall Design: Experiment 2: Experiment Overall Design: 2_SCF/Epo, 2_Epo/insulin(8h), 2_Epo/insulin(24h), 2_Epo/insulin(48h) Experiment Overall Design: 2_Epo/insulin+T3(8h), 2_Epo/insulin+9cRA(8h) Experiment Overall Design: 2_Epo/insulin+T3+9cRA(8h), , 2_Epo/insulin+T3+9cRA(24h), 2_Epo/insulin+T3+9cRA(48h)