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Differential Regulation of Pituitary Hormone Secretion and Gene Expression by Thyrotropin-Releasing Hormone. A Role for Mitogen-Activated Protein Kinase Signaling Cascade in Rat Pituitary GH3 Cells¹

^a Department of Obstetrics and Gynecology, Shimane Medical University, Izumo 693-8501, Japan ^b Department of Pharmacology, Kumamoto University School of Medicine, Kumamoto 860-0811, Japan

We examined the possible involvement of mitogen-activated protein (MAP) kinase activation in the secretory process and gene expression of prolactin and growth hormone. Thyrotropin-releasing hormone (TRH) rapidly stimulated the secretion of both prolactin and growth hormone from GH3 cells. Secretion induced by TRH was not inhibited by 50 µM PD098059, but was completely inhibited by 1 µM wortmannin and 10 µM KN93, suggesting that MAP kinase does not mediate the secretory process. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin, whereas expression of growth hormone mRNA was largely attenuated. The increase in prolactin mRNA stimulated by TRH was inhibited by addition of PD098059, and the decrease in growth hormone mRNA was also inhibited by PD098059. Transfection of the cells with a pFC-MEKK vector (a constitutively active MAP kinase kinase kinase), significantly increased the synthesis of prolactin and decreased the synthesis of growth hormone. These data taken together indicate that MAP kinase mediates TRH-induced regulation of prolactin and growth hormone gene expression. Reporter gene assays showed that prolactin promoter activity was increased by TRH and was completely inhibited by addition of PD098059, but that the promoter activity of growth hormone was unchanged by TRH. These results suggest that TRH stimulates both prolactin and growth hormone secretion, but that the gene expressions of prolactin and growth hormone are differentially regulated by TRH and are mediated by different mechanisms.

First decision: 21 December 2001.

¹ This work was supported in part by Grants-in-Aid for Scientific Research form the Ministry of Education, Science, Sports and Culture of Japan; by a research grant from the Human Frontier Science Program (to H.Y., K.F., and E.M.); and by a grant from the Ministry of Health and Welfare (to K.M.).

² Correspondence: Eishichi Miyamoto, Department of Pharmacology, Kumamoto University School of Medicine, 2-2-1 Honjo, Kumamoto 860-0811, Japan. FAX: 81 96 373 5078; emiyamot{at}gpo.kumamoto-u.ac.jp

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