Nuclear thyroid hormone receptors in cultured bone cells

Thyroid hormones influence bone metabolism, but a direct interaction of triiodothyronine with nuclear T 3 receptors in bone cells has not yet been reported. We investigated 125I-T 3 binding to nuclei isolated from the cloned osteoblastlike rat osteosarcoma cells ROS 17 2.8 . At 37°C, saturable 125I-T 3 binding to isolated nuclei reached equilibrium by 30 minutes and was completely displaced upon the addition of 500 nmol/L unlabeled T 3. Nonsaturable binding represented about 0.5% of the radioactivity added (20% of the total binding). Thyroxine and 3,3′,5′triiodothyronine competed with 125I-T 3 with a 20-fold and 400-fold lower affinity than T 3, respectively. Analysis of equilibrium competition experiments revealed the presence of a single class of homogeneous binding sites with an association constant of 5.0 ± 0.3 × 10 9 mol/L −1 and a maximum nuclear binding capacity of 0.13 ± 0.02 ng/mg DNA. A twofold increase of bone Gla protein (BGP) secretion was observed with T 3 treatment suggesting that these T 3 nuclear receptors are coupled with a biological response.