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 stably 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 directly 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 cDNA into Chinese hamster ovary cells (CHO cells). In this cell line, 10 nM TRH elevated intracellular calcium significantly; the Kd for MeTRH was 1.7 nM. One micromolar and 100 nM TRH-Gly also elevated intracellular concentration of calcium significantly, but not in CHO cells which were not transfected with the TRH-R cDNA. Competition studies further revealed that TRH-Gly displaced MeTRH binding (IC50, 12 microM). These data indicate that at high concentration, TRH-Gly interacts directly 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.