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 oocytes

Mitogen-activated protein (MAP) kinase, protein kinase C (PKC), cAMP, and o kadaic acid (OA)-sensitive protein phosphatases (PPs) have been suggested t o 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 pho sphorylation by PKC, cAMP and OA-sensitive PPs in rat oocytes using an in v itro 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 ge rminal vesicle intact (GVI) oocytes as well as GVBD oocytes at equivalent l evels. 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 abilit y 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 res ults suggest that both cAMP elevation and PKC activation inhibit meiosis re sumption and MAP kinase phosphorylation at a step prior to OA-sensitive pro tein phosphatases. In addition, inhibitory effects of cAMP elevation on mei otic resumption and MAP kinase phosphorylation were not reversed by calphos tin C-induced PKC inactivation, indicating that cAMP inhibits both meiotic resumption and MAP kinase activation in a PKC-independent manner.