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Effect of Protein Kinase C Activator on Mitogen-Activated Protein Kinase and p34^cdc2 Kinase Activity During Parthenogenetic Activation of Porcine Oocytes by Calcium Ionophore¹

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

The objective of this study was to elucidate the role of a [Ca²⁺]_i rise and protein kinase C (PKC) activation on decreases of p34^cdc2 kinase and mitogen-activated protein (MAP) kinase activity during parthenogenetic activation of porcine oocytes. In oocytes treated with 50 µM Ca²⁺ ionophore, degradations of both p34^cdc2 kinase and MAP kinase activity were observed and half of these oocytes formed pronuclei. However, a supplement of PKC inhibitor, calphostin C, after 50 µM Ca²⁺ ionophore treatment, was sufficient to inhibit the inactivation of MAP kinase and pronuclear formation in the oocytes. These results showed that PKC played an important role in Ca²⁺-induced oocyte activation. On the other hand, 10 µM Ca²⁺ ionophore treatment could not affect the MAP kinase activity but induced a transient decrease of p34^cdc2 kinase activity, which resulted in recovery of p34^cdc2 kinase activity and progression to meiotic metaphase III stage. To investigate the effects of PKC activator on oocytes treated with 10 µM Ca²⁺ ionophore, matured oocytes were cultured with phorbol 12-myriatate 13-acetate (PMA), after 10 µM Ca²⁺ ionophore treatment. The additional treatment suppressed the recovery of p34^cdc2 kinase activity and rapidly induced a decrease of MAP kinase activity, and these low activities were maintained until 12-h cultivation. As a result, a significantly higher percentage of these oocytes (67%) had pronuclei at 12-h cultivation. Moreover, PMA treatment without Ca²⁺ ionophore treatment effectively led to a decrease of MAP kinase activity in a dose-dependent manner but not p34^cdc2 kinase activity in matured porcine oocytes. In conclusion, the parthenogenetic activation of porcine oocytes was mediated by the inactivation of p34^cdc2 kinase via a calcium-dependent pathway and thereafter by the inactivation of MAP kinase via a PKC-dependent pathway.

¹ This work was partly supported by Grant-in-Aid for Scientific Research (M.S., No. 14760179) and Research Fellowship for Young Scientists (J.I., No. 08254) from the Japan Society for the Promotion of Science (JSPS)

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

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