A Direct Comparison of Thyroid Hormone Receptor Protein Levels in Mice Provides Unexpected Insights into Thyroid Hormone Action.

Background: Thyroid hormone (TH) action is mediated by three major thyroid hormone receptor (THR) isoforms α1, ß1, and ß2 (THRA1, THRB1, and THRB2). These THRs and a fourth major but non-TH binding isoform, THRA2, are encoded by two genes Thra and Thrb. Reliable antibodies against all THR isoforms are not available, and THR isoform protein levels in mammalian tissues are often inferred from messenger RNA (mRNA) levels. Methods: We generated knock-in mouse models expressing endogenously and identically 2X hemagglutenin epitope (HA)-tagged THRs (THRA1/2, THRB1, and THRB2), which could then be detected by commercially available anti-HA antibodies. Using nuclear enrichment, immunoprecipitation, and Western blotting, we determined relative THR protein expression in 16 mouse organs. Results: In all peripheral organs tested except the liver, the predominant THR isoform was THRA1. Surprisingly, in metabolically active organs such as fat and muscle, THRB1 protein levels were up to 10 times lower than that of THRA1, while their mRNA levels appeared similar. In contrast to peripheral organs, the central nervous system (CNS) had a unique pattern with relatively low levels of both THRB1 and THRA1, and high levels of THRA2 expression. As expected, THRB2 was highly expressed in the pituitary, but a previously unknown sex-specific difference in THRB2 expression was found (female mice having higher pituitary expression than male mice). Higher THRB2 expression appears to make the central axis more sensitive to TH as both serum thyrotropin and Tshb mRNA levels were lower in female mice. Conclusions: Direct comparison of THR protein abundance in different organs using endogenously tagged HA-THR mouse lines shows that expression of THR isoforms is regulated at transcriptional and posttranscriptional levels, and in organ-specific manner. The prevalence of THRA1 and low abundance of THRB1 in majority of peripheral tissues suggest that peripheral actions of these isoforms should be revisited. A unique pattern of high THRA2 in CNS warrants further exploration of this non-TH binding isoform in brain development. Finally, THRB2, in addition to cell-specific control, is also regulated in a sex-specific manner, which may change the hypothalamus-pituitary-thyroid axis set point and perhaps metabolism in males and females.

Thyroid hormone receptor α in breast cancer: prognostic and therapeutic implications.

We determined the expression of two transcriptional variants of thyroid hormone receptor alpha (THRα1 and THRα2) in samples from a cohort of breast cancer patients and correlated expression levels with survival. 130 women who were diagnosed with invasive breast carcinoma between 2007 and 2008 were included. Representative sections of their tumours were analyzed in triplicate on a tissue microarray for expression of THRα1 and THRα2 by immunohistochemistry. The prognostic significance of THRα1 and THRα2 expression was assessed using Kaplan-Meier survival analyses, adjusted for known prognostic factors. Seventy-four percent of tumours had high expression of THRα1 (Allred score ≥6) and 40 % had high expression of THRα2. Expression of THRα2 correlated positively with ER expression (p < 0.001) and with PR expression (p < 0.001), but negatively with HER2 expression (p = 0.018). Patients with low THRα2 expression had inferior 5-year overall survival (75.3 %) compared to those with high expression (91.7 %; p = 0.06). In a multivariate model, high THRα2 expression was a significant and independent prognosticator of improved overall survival (HR = 0.84; 95 % CI 0.71-0.98). Many breast tumours express THRα2 at high levels and these patients experience improved survival. Thyroid hormone signalling may be important in a proportion of breast cancers and THRα2 expression may be a regulator of signalling in this pathway.