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Mitogen-Activated Protein Kinase Kinase Inhibitor Suppresses Cyclin B1 Synthesis and Reactivation of p34^cdc2 Kinase, Which Improves Pronuclear Formation Rate in Matured Porcine Oocytes Activated by Ca²⁺ Ionophore¹

Laboratory of Animal Reproduction, Graduate School of Biosphere Sciences, Hiroshima University, Higashi-Hiroshima, 739-8528, Japan

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 µM) rapidly reduced MEK activity in oocytes for up to 8 h of culture. MEK activity in the 10-µM 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 p34^cdc2 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.

¹ Supported in part by Grant-in-Aid for Scientific Research (No. 14760179 to M.S.) of the Japan Society for the Promotion of Science (JSPS) and a Research Fellowship (No. 08254 to J.I.) of JSPS for Young Scientists.

² Correspondence: Masayuki Shimada, Laboratory of Animal Reproduction, Graduate School of Biosphere Sciences, Hiroshima University, Higashi-Hiroshima, 739-8528, Japan. FAX: 81 824 24 7988; mashimad{at}hiroshima-u.ac.jp

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