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Phosphorylation of Mitogen-Activated Protein Kinase Is Regulated by Protein Kinase C, Cyclic 3',5'-Adenosine Monophosphate, and Protein Phosphatase Modulators During Meiosis Resumption in Rat Oocytes¹

^a State Key Laboratory of Reproductive Biology, Institute of Zoology, The Chinese Academy of Sciences, Beijing 100080, P.R. China ^b Department of Obstetrics/Gynecology, Physiology, Urology, and Reproductive Sciences Program, University of Michigan Medical School, Ann Arbor, Michigan 48105 ^c Department of Veterinary Pathobiology, University of Missouri, Columbia, Missouri 65201

ABSTRACT

Mitogen-activated protein (MAP) kinase, protein kinase C (PKC), cAMP, and okadaic acid (OA)-sensitive protein phosphatases (PPs) have been suggested to be involved in oocyte meiotic resumption. However, whether these protein kinases and phosphatases act by independent pathways or interact with each other in regulating meiosis resumption is unknown. In the present study, we aimed to determine the regulation of meiosis resumption and MAP kinase phosphorylation by PKC, cAMP, and OA-sensitive PPs in rat oocytes using an in vitro oocyte maturation system and Western blot analysis. We found that ERK1 and ERK2 isoforms of MAP kinases existed in a dephosphorylated (inactive) form in germinal vesicle breakdown (GVBD)-incompetent and GVBD-competent germinal vesicle intact (GVI) oocytes as well as GVBD oocytes at equivalent levels. These results indicate that MAP kinases are not responsible for the initiation of normal meiotic resumption in rat oocytes. However, when GVBD-incompetent and GVBD-competent oocytes were incubated in vitro for 5 h, MAP kinases were phosphorylated (activated) in GVBD-competent oocytes, but not in meiotic-incompetent oocytes, suggesting that oocytes acquire the ability to phosphorylate MAP kinase during acquisition of meiotic competence. We also found that both meiosis resumption and MAP kinase phosphorylation were inhibited by PKC activation or cAMP elevation. Moreover, these inhibitory effects were overcome by OA, which inhibited PP1/PP2A activities. These results suggest that both cAMP elevation and PKC activation inhibit meiosis resumption and MAP kinase phosphorylation at a step prior to OA-sensitive protein phosphatases. In addition, inhibitory effects of cAMP elevation on meiotic resumption and MAP kinase phosphorylation were not reversed by calphostin C-induced PKC inactivation, indicating that cAMP inhibits both meiotic resumption and MAP kinase activation in a PKC-independent manner.

FOOTNOTES

First decision: 10 October 2000.

¹ This study was supported by grants from the Rockefeller Foundation, the Special Funds for Major State Basic Research Projects (G19990559021), Knowledge Innovation Program, and Life Science Special Fund of The Chinese Academy of Sciences.

² Correspondence: Qing-Yuan Sun, Department of Veterinary Pathobiology, RM W 123, College of Veterinary Medicine, University of Missouri-Columbia, 1600 East Rollins, Columbia, MO 65211. FAX: 573 884 5414; sunqy1{at}yahoo.com

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