ACTIVATION OF THE THYROTROPIN-RELEASING-HORMONE (TRH) RECEPTOR BY A DIRECT PRECURSOR OF TRH, TRH-GLY

We studied the mechanism by which thyrotropin-releasing hormone (TRH)- Gly stimulated prolactin and thyrotropin (TSH) secretion in pituitary, using a pituitary mammotropic cell line, GH3 cells, and a cell line s tably expressing a human TRH receptor (TRH-R). In GH3 cells expressing endogenous TRH-R, an addition of TRH-Gly evoked an immediate rise of intracellular calcium concentration, indicating that TRH-Gly reacted d irectly without converting from TRH-Gly to TRH. In order to determine whether this reaction might occur through TRH-R, we established a cell line stably expressing a human TRH-R, by transfecting a human TRH-R c DNA into Chinese hamster ovary cells (CHO cells). In this cell line, 1 0 nM TRH elevated intracellular calcium significantly; the K-d for MeT RH was 1.7 nM. One micromolar and 100 nM TRH-Gly also elevated intrace llular concentration of calcium significantly, but not in CHO cells wh ich were not transfected with the TRH-R cDNA. Competition studies furt her revealed that TRH-Gly displaced MeTRH binding (IC50, 12 mu M). The se data indicate that at high concentration, TRH-Gly interacts directl y with TRH-R to activate signal transduction pathway, and that release of prolactin and TSH induced by TRH-Gly in vitro may be due, at least in part, to the direct effect of TRH-Gly on the TRH-R.