A coregulator shift, rather than the canonical switch, underlies thyroid hormone action in liver

Thyroid hormones (TH) are powerful regulators of metabolism with major effects on body weight, cholesterol, and liver fat that have been exploited pharmacologically for many years. Activation of gene expression by TH action is canonically ascribed to a hormone- dependent “switch” from corepressor to activator binding to thyroid hormone receptors (TR), while the mechanism of TH-dependent repression is controversial. To address this, we generated a mouse line in which endogenous TRß1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of TR, and defined high confidence binding sites where TR functioned at enhancers regulated in the same direction as the nearest gene in a TRß-dependent manner. Remarkably, although positive and negative regulation by TH have been ascribed to different mechanisms, TR binding was highly enriched at canonical DR4 motifs irrespective of the transcriptional direction of the enhancer. The canonical NCoR1/HDAC3 corepressor complex was reduced but not completely dismissed by TH and, surprisingly, similar effects were seen at enhancers associated with negatively as well as positively regulated genes. Conversely, coactivator CBP was found at all TH-regulated enhancers, with transcriptional activity correlating with the ratio of CBP to NCoR rather than their presence or absence. These results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, TH regulates gene expression by orchestrating a shift in the relative binding of corepressors and coactivators. Overall design: mRNA profile, TR beta cistrome and H3K27ac ChIP-seq of hypo- and hyperthyroid state mice livers.