Thyroid Hormone Stimulates Alkaline Phosphatase Activity in Cultured Rat Osteoblastic Cells (ROS 17/2.8) through 3,5,3′-Triiodo-L-Thyronine Nuclear Receptors*

To investigate the increased alkaline phosphatase activity of bone origin in patients with hyperthyroidism, we studied the thyroid hormone effects on alkaline phosphatase activity in a clonal rat osteoblastic cell line (ROS 17/2.8). T4 and T3 increased alkaline phosphatase activity in ROS 17/ 2.8 cells in a dose-dependent manner. The minimal effective T4 and T3 concentrations in medium containing 10% thyroid hormone-depleted fetal calf serum were 10-8 M (free T4, 8 × 10-11 M) and 10-9M (free T3, 4 × 10-11 M), respectively. ROS 17/2.8 cells possessed high affinity, low capacity nuclear receptors specific for T3 [dissociation constant (Kd) -150 pM; maximal binding capacity, -2000 T3 binding sites per nucleus]. The relative affinity of T3, T4, T3, MIT, and DIT were in good agreement with their biological activity. These findings suggest that rat osteoblast-like cells contain T3 nuclear receptors and that alkaline phosphatase activity is stimulated by thyroid hormone via a nuclear receptor-mediated process at free thyroid hormone concentrations attainable in patients with Graves' disease. (Endocrinology120: 1873–1881, 1987)