Type 1 deiodinase is stimulated by iodothyronines and involved in thyroid hormone metabolism in human somatomammotroph GX cells

Background: Local 5'-deiodination of L-thyroxine (T-4) to the active thyroid hormone, 3,3',5-tri-iodothyronine (T-3) via two deiodinase isoenzymes (D1 and D2) has an important role for various T-3-dependent functions in the anterior pituitary. However. no evidence has been presented yet for thyroid hormone inactivation via the 5-deiodinase (D3) in anterior pituitary models. Methods: Using the human somatomammotroph cell line, GX, we analysed effects of T-3 and its 5'-deiodination product, 3,5-di-iodothyronine (3,5-T-2), on deiodinase activities, measuring release of iodide-125 (I-125(-)) from phenolic-ring- or tyrosyl-ring-labelled substrates respectively. Results: T-3 and 3,5-T-2 rapidly stimulated D1 activity in GX cells in the presence of serum in the culture medium, whereas D2 activity was not detectable under these conditions. However, when the cells were kept under serum-free conditions, specific activity of D2 reached levels similar to those of D1, With tyrosyl-ring labelled 3,5-[I-125]-,3'-T-3 as substrate, a significant release of I-125(-) was observed in GX cell homogenates. This is comparable to the D1 activity of liver membranes, which preferentially catalyses 5'-deiodination, but to some extent also 5-deiodination, at the tyrosyl ring. Conclusions: D1 activity of human GX cells is increased by T-3 and 3,5-T-2, Inactivation of T-3 in the anterior pituitary might occur by deiodination at the tyrosyl ring via D1, thus terminating the stimulatory thyroid hormone signal in human somatomammotroph cells.