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Activation of Protein Kinase C Induces Mitogen-Activated Protein Kinase Dephosphorylation and Pronucleus Formation in Rat Oocytes¹

^a State Key Laboratory of Reproductive Biology, Institute of Zoology, The Chinese Academy of Sciences, Beijing 100080, People's Republic of China ^b Departments of Obstetrics and Gynecology, Physiology, and Urology, and Reproductive Sciences Program, University of Michigan Medical School, Ann Arbor, Michigan 48105

Mammalian oocytes are arrested at metaphase of the second meiotic division (MII) before fertilization. When oocytes are stimulated by spermatozoa, they exit MII stage and complete meiosis. It has been suggested that an immediate increase in intracellular free calcium concentration and inactivation of maturation promoting factor (MPF) are required for oocyte activation. However, the underlying mechanism is still unclear. In the present study, we investigated the role of protein kinase C (PKC) and mitogen-activated protein (MAP) kinase, and their interplay in rat oocyte activation. We found that MAP kinase became dephosphorylated in correlation with pronucleus formation after fertilization. Protein kinase C activators, phorbol 12-myriatate 13-acetate (PMA) and 1,2-dioctanoyl-rac-glycerol (diC8), triggered dephosphorylation of MAP kinase and pronucleus formation in a dose-dependent and time-dependent manner. Dephosphorylation of MAP kinase was also correlated with pronucleus formation when oocytes were treated with PKC activators. Effects of PKC activators were abolished by the PKC inhibitors, calphostin C and staurosporine, as well as a protein phosphatase blocker, okadaic acid (OA). These results suggest that PKC activation may cause rat oocyte pronucleus formation via MAP kinase dephosphorylation, which is probably mediated by OA-sensitive protein phosphatases. We also provide evidence supporting the involvement of such a process in fertilization.

First decision: 16 August 2001.

¹ This study was supported by The Special Funds for Major State Basic Research Projects (G1999055902) and Knowledge Innovation Program of Chinese Academy of Sciences (KSCX2-SW-303).

² Correspondence. FAX: 86 10 62529248; sunqy1{at}yahoo.com

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