Identical gene regulation patterns of T3 and selective thyroid hormone receptor modulator GC-1.

Synthetic selective thyroid hormone (TH) receptor (TR) modulators (STRM) exhibit beneficial effects on dyslipidemias in animals and humans and reduce obesity, fatty liver, and insulin resistance in preclinical animal models. STRM differ from native TH in preferential binding to the TR beta subtype vs. TR alpha, increased uptake into liver, and reduced uptake into other tissues. However, selective modulators of other nuclear receptors exhibit important gene-selective actions, which are attributed to differential effects on receptor conformation and dynamics and can have profound influences in animals and humans. Although there are suggestions that STRM may exhibit such gene-specific actions, the extent to which they are actually observed in vivo has not been explored. Here, we show that saturating concentrations of the main active form of TH, T-3, and the prototype STRM GC-1 induce identical gene sets in livers of euthyroid and hypothyroid mice and a human cultured hepatoma cell line that only expresses TR beta, HepG2. We find one case in which GC-1 exhibits a modest gene-specific reduction in potency vs. T-3, at angiopoietin-like factor 4 in HepG2. Investigation of the latter effect confirms that GC-1 acts through TR beta to directly induce this gene but this gene-selective activity is not related to unusual T-3-response element sequence, unlike previously documented promoter-selective STRM actions. Our data suggest that T-3 and GC-1 exhibit almost identical gene regulation properties and that gene-selective actions of GC-1 and similar STRM will be subtle and rare. (Endocrinology 153: 501-511, 2012)