204 INADEQUACY OF VASCULAR ENDOTHELIAL GROWTH FACTOR IN CULTURE MEDIUM REDUCES THE VIABILITY OF CUMULUS CELLS AND PREVENTS IN VITRO MATURATION OF PORCINE OOCYTES

Abstract

Recently, vascular endothelial growth factor (VEGF) has been regarded as an important factor associated with not only follicle development but also meiotic competence of oocytes. However, the mechanism of how VEGF works is poorly understood. In this study, we investigated in vitro maturation (IVM) of oocytes from different sizes in the absence or presence of a VEGF receptor inhibitor, Axitinib. Cumulus-oocyte complexes (COC) were obtained from small follicles (SF; l < 3 mm in diameter) and medium follicles (MF; 3–6 mm in diameter). Each group of 30–40 COC with at least 3 layers of clear and compact cumulus cells (CC) was cultured in 500 μL of modified porcine oocyte medium (POM-β-mercaptoethanol) supplemented with 10 IU mL^–1 eCG, 10 IU mL^–1 hCG and 1mM dibutyryl-cyclic-adenosine monophosphate (dbc-AMP) for the first 20 h and then without those supplements for another 24 h at 39°C, 5% CO₂ in air. During the first 20 h of IVM, culture medium was also supplemented with or without 1.25 nM Axitinib. At 20 h and 44 h after the start of IVM, the oocytes were denuded and stained with 4′6-diamidino-2-phenylindole (DAPI) to observe the nuclear stages. At 20 h after the start of IVM, some COC were also stained with PI and SYBR Green I to evaluate the ratio of live/dead cumulus cells. Statistical analyses of data from 5 replications were analysed by ANOVA and Tukey’s multiple comparison test. As compared with controls at 20 h after the start of IVM, the number of dead cumulus cells increased significantly in the groups treated with Axitinib, regardless of COC derived from MF and SF (16.8 v. 43.1% in MF and 25.3 v. 57.7% in SF, P < 0.01, respectively). At that time, a majority of oocytes from MF and SF remained at the germinal vesicle (GV) stage in controls (89.8 and 84.6%, respectively), but the percentage significantly reduced in the presence of Axitinib (57.9 and 48.9% of oocytes from MF and SF, respectively) and proceeded around the metaphase-I stage (37.5 and 44.8% of oocytes from MF and SF, respectively). At 44 h after the start of IVM, lower maturation rates were observed in oocytes treated with Axitinib than controls (35.0 v. 81.2% in MF; 20.1 v. 49.0% in SF; P < 0.01). In conclusions, VEGF plays an important role in maitaining the viability of cumulus cells. The presence of VEGFR inhibitor caused the oocytes to develop uncontrollably, even in the presence of dbc-AMP. Moreover, the deficiency of VEGF prevented oocytes fully competent to resume meiosis and arrest to metaphase II.