3-Iodothyronamine metabolism and functional effects in FRTL5 thyroid cells.

3-Iodothyronamine (T1AM), produced from thyroid hormones (TH) through decarboxylation and deiodination, is a potent agonist of trace amine-associated receptor 1 (TAAR1), a G protein-coupled receptor belonging to the family of TAARs. In vivo T1AM induces functional effects opposite to those produced on a longer time scale by TH and might represent a novel branch of TH signaling. In this study, we investigated the action of T1AM on thyroid and determined its uptake and catabolism using FRTL5 cells. The expression of TAAR1 was determined by PCR and western blot in FRTL5 cells, and cAMP, iodide uptake, and glucose uptake were measured after incubation with increasing concentrations of T1AM for different times. T1AM and its catabolites thyronamine (T0AM), 3-iodothyroacetic acid (TA1), and thyroacetic acid (TA0) were analyzed in FRTL5 cells by HPLC coupled to tandem mass spectrometry. The product of amplification of TAAR1 gene and TAAR1 protein was demonstrated in FRTL5 cells. No persistent and dose-dependent response to T1AM was observed after treatment with increasing doses of this substance for different times in terms of cAMP production and iodide uptake. A slight inhibition of glucose uptake was observed in the presence of 100 mM T1AM after 60 and 120 min (28 and 32% respectively), but the effect disappeared after 18 h. T1AM was taken up by FRTL5 cells and catabolized to T0AM, TA1, and TA0 confirming the presence of deiodinase and amine oxidase activity in thyroid. In conclusion, T1AM determined a slight inhibition of glucose uptake in FRTL5 cells, but it was taken up and catabolized by these cells.