SAT-435 A Direct Comparison of Thyroid Hormone Receptor (THR) Protein Levels in Mice Provides Unexpected Insights into TH Action

Thyroid Hormone (TH) action is mediated by three major THR isoforms α1, β1 and β2 (THRA1, THRB1 and THRB2), encoded by two genes Thra and Thrb. A fourth non-T(3) binding isoform is termed THRA2. Previous characterization and comparison of THR isoform expression using QPCR of mRNA levels does not assess translational or post-translational control of THR protein levels. Moreover, reliable antibodies against all THR isoforms are not currently available. To address these concerns, we generated knock-in mouse models expressing endogenously and identically 2X HA tagged THRs (THRA1/2, THRB1 and THRB2), which could then be detected by commercially available anti-HA antibodies. We characterized THR expression in 16 mouse organs. We found that in all peripheral tissues tested except the liver, the dominant THR isoform is THRA1, and THRB1 and THRA2 are expressed at significantly lower levels or are undetectable. Surprising THRB1 levels were very low in metabolically active organs such fat and muscle. In some organs, mRNA transcript levels predicted THR protein amounts, while in others the prediction was inaccurate. For instance, adipose depots have similar levels of Thrb1 and Thra1 transcripts, however THRA1 protein levels are up to 10-fold higher than THRB1. In contrast to peripheral organs, brain tissues express low levels of both THRB1 and THRA1, but have very high levels of THRA2. As expected and confirmed in this study, THRB2 has limited expression and was only detected in pituitary of the organs tested. Interestingly, THRB2 expression in female was much higher than male mice (the only sex-difference in THR expression found); and expression of the THRB2 target gene, Tshb, was lower in female mice. For the first time, these HA-THR mouse lines can be used to accurately compare isoform-specific actions of THRs in organs. The predominant expression of THRA1 in most peripheral organs, for example, suggests that many peripheral actions of THRB1 could be indirectly mediated.