Progesterone regulation of AVEN protects bovine oocytes from apoptosis during meiotic maturation

Inhibition of progesterone (P4) synthesis by cumulus cells during bovine in vitro oocyte maturation (IVM) causes a decrease in subsequent embryo development, indicating that P4 intracellular signaling within the cumulus oocyte complex (COC) is important for oocyte developmental competence. The aim of the present study was to further elucidate, on a protein level, the downstream signaling pathway involved in P4 regulation of oocyte developmental competence. COCs were subjected to IVM for 24 h in the presence or absence of trilostane, aglepristone, or promegestone (R5020). These altered IVM conditions resulted in dynamic changes in protein expression of the progesterone receptors and the cell death-regulated proteins AVEN, BCL-xL, and active caspase 3. In addition, AVEN protein localization, caspase 3 activation, and mitochondrial distribution were studied by immunofluorescence. Inhibition of progesterone synthesis (trilostane treatment) resulted in changes in AVEN localization within the COC, corresponding to caspase 3 activation and altered mitochondrial distribution. AVEN was also found to bind BCL-xL in COCs, but this interaction was lost following treatment with trilostane.