WIDESPREAD DISTRIBUTION OF IMMUNOREACTIVE THYROID-HORMONE BETA-2 RECEPTOR (TR-BETA-2) IN THE NUCLEI OF EXTRAPITUITARY RAT-TISSUES

Messenger RNA for thyroid hormone receptor (TR) isoforms alpha 1 and b eta 1 are widely distributed in rat tissues. Until recently, TR beta 2 mRNA was believed to be limited to the pituitary and the assumption w as made that TR beta 2 protein was similarly restricted. We determined the distribution of TR beta 2 protein in selected adult and fetal rat tissues using three anti-TR beta 2 antisera directed to different ami no acid sequences of the distinctive A/B domain of TR beta 2. The prop ortion of total nuclear binding capacity cleared by each antiserum was determined by saturation analysis. 10-20% of total binding capacity i n adult brain, liver, kidney, and heart was immunoprecipitated by each antiserum. Use of specific antibodies to TR beta 1 and TR alpha 1 sho wed these isoforms accounted for the remainder of total T-3 binding. F etal liver and brain, however, contained only TR alpha 1. Immunohistoc hemical analysis of the adult tissues showed TR beta 2 present in nucl ei. Reverse transcription polymerase chain reaction detected low level s of TR beta 2 mRNA in the adult tissues. We infer that TR beta 2 acco unts for a significant fraction of TR in adult rat tissues despite the low levels of its mRNA.