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

Abstract

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 (K_d) -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)