Studies on the Binding of Radiolabeled Thyrotropin to Cultured Human Thyroid Cells*

Abstract

A line of cultured human thyroid adenoma cells was used in a study designed to compare the stimulatory effect of TSH [thyrotropin] on cellular c[cyclic]AMP generation with the binding of radiolabeled TSH to the cells. At 37.degree. C, specific binding of [125I]TSH to suspensions of thyroid cells was maximal at 20 min and was reversed by the addition of excess TSH. Unlike the generation of cellular cAMP in response to TSH stimulation, which was maximal at pH 7.5, the binding of [125I]TSH to the cells was maximal at pH 5.5 and progressively declined up to pH 8.5. Increasing NaCl concentrations progressively inhibited cellular binding of TSH; at physiological salt concentrations, almost no TSH binding was detectable. Competitive inhibition studies of [125I]TSH binding to cells revealed a binding site with a dissociation constant of 5.5 .times. 10-8 M at pH 7.4 GH [growth hormone], PRL [prolactin], hCG [human chorionic gonadotropin], FSH [follitropin], insulin, and glucagon did not compete with [125I]TSH binding. ACTH was a potent inhibitor of [125I]TSH binding. Despite this inhibitory effect on TSH binding, ACTH had little or no effect on cellular cAMP generation. High concentrations of ACTH did not inhibit the biological effect of TSH on cAMP generation. Specific binding of [125I]TSH to empty plastic culture dishes was time dependent, reversible and displayed a hormonal specificity identical to binding to thyroid cells. The effects of pH and NaCl concentrations on TSH binding to dishes were similar but not identical to those on cellular binding. This study raises serious questions as to the biological significance of [125I]TSH binding to cultured human thyroid cells.