Effect of dexamethasone on the expression of p34(cdc2) and cyclin B1 in pig oocytes in vitro

The meiotic division in oocytes is arrested in the G2 phase of the cell cycle. Resumption of meiosis, also known as oocyte maturation, entails a G2 to M transition. At the G2-M boundary, maturation promoting factor (MPF) activation is usually induced via several ways, including tyrosine dephosphorylation of p34(cdc2) and synthesis of cyclin B according to cell type and species. Previous studies in our laboratory demonstrated that glucocorticoids directly inhibit the meiotic maturation of pig oocytes in vitro. The aim of this study was therefore to investigate the influence of glucocorticoids on the expression of p34(cdc2) and cyclin B1 in resumption of meiosis of pig oocytes. We detected the relative levels and association of p34(cdc2) and cyclin B1. Isolated cumulus-enclosed oocytes were cultured in Waymouth MB752/1 medium supplemented with sodium pyruvate (50 microgram/ml), LH (0.5 microgram/ml), FSH (0.5 microgram/ml), and estradiol-17beta (1 microgram/ml) in the presence or absence of dexamethasone (DEX) for 24 hr; they then were cultured without hormonal supplements in the presence or absence of DEX for an additional 24 hr. We found that cyclin B1, as well as p34(cdc2), was already present in fully grown G2-arrested pig oocytes when removed from the follicle. In these oocytes, cyclin B1 and p34(cdc2) were already associated in complex. Treatment with DEX at concentrations of 1 microgram/ml or above decreased the level of cyclin B1, but had no effect on the level of p34(cdc2). The exposure of oocytes to DEX also decreased the amount of complexed p34(cdc2)-cyclin B1. These findings suggest that the inhibitory action of DEX on meiotic maturation could be due, at least in part, to the reduced amount of p34(cdc2)-cyclin B1 complex.