ACTIVATION OF MITOGEN-ACTIVATED PROTEIN KINASE FAMILY IN RAT PITUITARY GH3 CELLS

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In GH3 cells, we examined the Mitogen-activated protein kinase (MAP kinase) activation induced by thyrotropin-releasing hormone (TRH) and the role of MAP kinase activation in secretory processes and prolactin (PRL) synthesis. Immunoblotting analysis indicated the presence of only 42 kDa MAP kinase in GH3 cells. TRH induced rapid activation of MAP kinase, which reached a maximal peak of 300 A ィイD4oィエD4i at 5-10 min. TRH-induced MAP kinase activation was inhibited completely by PD098059, and 80 % by calphostin C, respectively. Genistein and removal of extracellular Ca2+ also partially inhibited TRH-induced MAP kinase activation. These results suggest that TRH-induced MAP kinase activation is largely PKC-dependent and partially dependent on the Ca2+-dependent tyrosine kinase. CTPcAMP increased MAP kinase activity significantly, and CPTcAMP effects were inhibited by PD098059. TRH-induced prolactin secretion was not affected by exposure to PD098059, but completely blocked by either wortmannin or KN93. Synthesis of PRL induced by TFH was completely blocked by addition PC098059. These results suggest that TRH-induced MAP kinase activation is correlated with synthesis of PRL but not with PRL secretion. TRH increased the PRL synthesis, while growth hormone synthesis largely decreased by TRH. Treatment of GH3 cells with bromocriptine for 48th increased the p38 MAP kinase activity, and simultaneously increased the number of apoptotic cells. SB202190 and SB203580 completely inhibited the bromocriptine-induced p38 MAP kinase activation, and reduced the number of apoptotic cells. We have shown that TRH induces P44/42 MAP kinase activation in GH3 cells. TRH may be involved in PRL secretion through activation of myosin light chain kinase and CaM kinase II, and in PRL synthesis and differentiation of the cells through activation of P44/42 MAP kinase. The p38 MAP kinase activation induced by bromocriptine was strongly involved in induction of apoptosis.