Mitogen-activated protein kinase activation by stimulation with thyrotropin-releasing hormone in rat pituitary GH3 cells

We examined whether mitogen-activated protein (MAP) kinase is activated by thyrotropin-releasing hormone (TRH) in GH3 cells, and whether MAP kinase ac tivation is involved in secretion of prolactin from these cells. Protein ki nase inhibitors-such as PD098059, calphostin C, and genistein-and removal o f extracellular Ca2+ inhibited MAP kinase activation by TRH. A cAMP analogu e activated MAP kinase in these cells. Effects of cAMP on MAP kinase activa tion were inhibited by PD098059. TRH-induced prolactin secretion was not in hibited by levels of PD098059 sufficient to inhibit MAP kinase activation b ut was inhibited by wortmannin (1 mu M) and KN93. Treatment of GH3 cells wi th either TRH or cAMP significantly inhibited DNA synthesis and induced mor phological changes. The effects stimulated by TRH were reversed by PD098059 treatment, but the same effects stimulated by cAMP were not. Treatment of GH3 cells with TRH for 48 h significantly increased the prolactin content i n GH3 cells and decreased growth hormone content. The increase in prolactin was completely abolished by PD098059, but the decrease in growth hormone w as not. These results suggest that TRH-induced MAP kinase activation is involved in prolactin synthesis and differentiation of GH3 cells, but not in prolactin secretion.