Mitogen-activated protein kinase kinase inhibitor suppresses cyclin B1 synthesis and reactivation of p34cdc2 kinase, which improves pronuclear formation rate in matured porcine oocytes activated by Ca2+ ionophore

To investigate the role of mitogen-activated protein (MAP) kinase kinase (MEK)/MAP kinase cascade on p34(cdc2) kinase activity and cyclin B1 levels during parthenogenetic activation of porcine oocytes, MEK activity, MAP kinase activity, p34(cdc2) kinase activity, and cyclin B1 levels were assayed in mature porcine oocytes after treatment with different concentrations of Ca2+ ionophore. A high concentration of Ca2+ ionophore (50 muM) rapidly reduced MEK activity in oocytes for up to 8 h of culture. MEK activity in the 10-muM treatment group was significantly higher. The low concentration treatment transiently decreased p34(cdc2) kinase activity but did not affect MAP kinase activity and ultimately induced reactivation of p34cdc2 kinase via the synthesis of cyclin B1. On the other hand, treatments of a high concentration of Ca2+ ionophore or a low concentration of Ca2+ ionophore plus MEK inhibitor, U0126, linearly decreased MAP kinase activity following the decrease of p34(cdc2) kinase activity; most of these oocytes formed pronuclei. These results suggest that decreasing MAP kinase activity is essential to maintaining low p34(cdc2) kinase activity resulting from the degradation of cyclin B via a Ca2+-dependent pathway; lower activities of both MAP kinase and p34(cdc2) kinase induce normal meiotic completion and pronuclear formation of parthenogenetically activated porcine oocytes.