Gene Specific Actions of Thyroid Hormone Receptor Subtypes

Abstract

There are two homologous thyroid hormone (TH) receptors (TRs α and β), which are members of the nuclear hormone receptor (NR) family. While TRs regulate different processes in vivo and other highly related NRs regulate distinct gene sets, initial studies of TR action revealed near complete overlaps in their actions at the level of individual genes. Here, we assessed the extent that TRα and TRβ differ in target gene regulation by comparing effects of equal levels of stably expressed exogenous TRs +/− T3 in two cell backgrounds (HepG2 and HeLa). We find that hundreds of genes respond to T3 or to unliganded TRs in both cell types, but were not able to detect verifiable examples of completely TR subtype-specific gene regulation. TR actions are, however, far from identical and we detect TR subtype-specific effects on global T3 response kinetics in HepG2 cells and many examples of TR subtype specificity at the level of individual genes, including effects on magnitude of response to TR +/− T3, TR regulation patterns and T3 dose response. Cycloheximide (CHX) treatment confirms that at least some differential effects involve verifiable direct TR target genes. TR subtype/gene-specific effects emerge in the context of widespread variation in target gene response and we suggest that gene-selective effects on mechanism of TR action highlight differences in TR subtype function that emerge in the environment of specific genes. We propose that differential TR actions could influence physiologic and pharmacologic responses to THs and selective TR modulators (STRMs).

Thyroid hormone receptor regulates most genes independently of fibroblast growth factor 21 in liver

“…6C, full gene names in Table 1). Genes that displayed similar levels of T 3 response in both strains include Cpne2, Rab34, Creld2, and Wfdc6, as identified in our previous studies (Lin et al 2013) (Fig. 6D).…”

“…Furthermore, our studies were performed in euthyroid mice treated with T 3 and not hypothyroid mice. Thus, it is possible that some T 3 -and FGF21-responsive genes with key roles in metabolic response could be fully induced at T 3 concentrations observed in euthyroid mice (data shown in Lin et al (2013)). Arguing against these possibilities, however, is the failure of the Maratos-Flier group also to identify defects in TR-dependent changes in metabolic parameters and gene expression in FGF21 knockouts over time courses of more than 1 week and in hypothyroid mice with varying levels of T 3 (Domouzoglou et al 2014).…”

“…To define the mechanisms of TH-dependent induction of hepatic Fgf21 expression, we examined the response of Fgf21 mRNA to T 3 or GC1 in human HepG2 cells that stably express TRb (Lin et al 2013). We observed induction of FGF21 following short-term treatment with either ligand (Fig.…”

“…HepG2 (ATCC, Bethesda, MD, USA) and HepG2-TRb (Lin et al 2013) cells were grown in DMEM supplemented with 10% fetal bovine serum (FBS), 100 U/ml of penicillin, 0.1 g/l of streptomycin, and 4 mmol/l glutamine, under 95% air and 5% CO 2 at 37 8C. For cycloheximide (CHX) treatment, HepG2-TRb cells were maintained within 10% resin-stripped FBS-DMEM media for 24 h before treatment.…”

“…TH receptors (TRa and TRb) are members of the nuclear hormone receptor family that regulate target genes in response to the main active form of TH, triiodothyronine (T 3 ). TRs usually work by binding DNA-response elements (TREs) in the proximal promoters of target genes as heterodimers with retinoid X receptors (RXRs) but can also regulate target genes via protein-protein interactions with heterologous transcription factors (Baxter & Webb 2009, Lin et al 2013. T 3 induces FGF21 in mouse liver and, in this study, TRb selectively enhances FGF21 expression and promoter activity via interactions with PPARa bound to PPARa-response elements, but does not require PPARa to induce other T 3 -regulated genes (Adams et al 2010).…”