Possible involvement of phosphatidylinositol 3-kinase in the maintenance of metaphase II arrest in porcine oocytes matured in vitro.

It has been reported that phosphatidylinositol 3-kinase (PI3K)-protein kinase B (PKB) pathway plays a crucial role in the meiotic resumption and progression to the metaphase II (MII) stage of oocytes. However, the role of this pathway in meiotic arrest at the MII stage (cytostatic activity) is not well understood. In this study the effect of a PI3K inhibitor, LY294002, on the MAPK and p34(cdc2) kinase activities of matured porcine oocytes was examined. After maturation culture, both the MAPK and p34(cdc2) kinase activities in the oocytes were gradually decreased in a time-dependent manner. Although 25 micromol/L LY294002 did not affect either the MAPK or p34(cdc2) kinase activities, 50 micromol/L LY294002 suppressed the PKB phosphorylation and slightly decreased MAPK activity, but not the p34(cdc2) kinase activity. Therefore the effect of 10 micromol/L Ca(2+) ionophore which was reported as inducing a transient decrease of p34(cdc2) kinase but not MAPK activities, was also examined in LY294002-treated oocytes. By additional treatment with LY294002 after Ca(2+) ionophore, both the MAPK and p34(cdc2) kinase activities were decreased in a time-dependent manner, concomitantly with improvement of pronuclear formation. Therefore, we concluded that PI3K is involved in the maintenance of MAPK activity in matured porcine oocytes.