MAP kinase activity is downregulated by phorbol ester during mouse oocyte maturation and egg activation in vitro

The effects of protein kinase C (PKC) stimulator, phorbol 12-myriatate 13-a cetate (PMA), on meiotic cell cycle regulation and mitogen-activated protei n (MAP) kinase changes have been studied in mouse oocytes and eggs. The res ults showed that MAP kinase activation itself was not necessary for germina l vesicle breakdown (GVBD), but the ability of the ooplasm to phosphorylate MAP kinase was a prerequisite for this event. At concentrations of 1.6 nM, PMA effectively inhibited GVBD and MAP kinase activation, suggesting that PMA inhibits GVBD by inhibiting molecule(s) upstream to MAP kinase. At conc entrations of 16.2 nM, PMA induced metaphase-interphase transition more eff ectively in eggs collected 19 hr after human chorionic gonadotropin (hCG) a dministration than in those collected 15 hr after hCG administration. The d egree of MAP kinase activity decrease was well correlated with the time cou rse and proportion of pronuclear formation. On the other hand, when the eff ect of PMA on cell cycle progression was abolished by protein phosphatase i nhibitor, okadaic acid, MAP kinase was superactivated. The biologically ina ctive 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD) had no evident effect s on either GVBD and interphase transition or on MAP kinase activity. Furth ermore, the effects of PMA on oocyte GVBD, egg activation, and MAP kinase a ctivity could be overcome by the specific PKC inhibitor, calphostin C, sugg esting the possible involvement of this enzyme in the regulation of MAP kin ase activity. The results suggest that activation of PKC by PMA entrains a cascade of events that ultimately inhibits MAP kinase activation and GVBD i n mouse oocytes and induces MAP kinase inactivation and metaphase-interphas e transition in mouse eggs. Mel. Reprod. Dev. 52.310-318, 1999. (C) 1999 Wi ley-Liss, inc.