Thyroid Hormone Receptors in Human Cultured Fibroblasts: Evidence for Cellular T₄ Transport and Nuclear T₃ Binding

Summary

In cultured normal human skin fibroblasts specific and saturable binding sites for triiodothyronine (T₃) have been revealed. In fact radiolabelled T₃ binds rapidly to intact cells with maximum uptake after 1 hour, while nuclear binding is delayed, the equilibrium being reached after 2 hours. In intact cells it is possible to identify a single binding site for ¹²⁵I-T₃, with a Ka=1.8×10¹⁰ M^-1 and Ro=1.25×10^-11 M, similarly in nuclei it was possible to identify a single binding site of Ka=8.8×10⁹ M^-1 and Ro=2.3×10^-11 M. Intact human fibroblasts take up thyroxine (T₄) even more rapidly than T₃, with maximum after 5 min, showing a lower affinity for T₄ than for T₃ and a negligible specific and saturable binding sites for T₄, the presence of a cellular transport system for T₄ may be hypothesized, considering that iodothyronine cellular binding is increased by preincubation with low doses of T₄.

Key-Words:

Human Fibroblasts - Iodothyronine Binding Sites - Thyroxine - Triiodothyronine