SET improved oocyte maturation by serine/threonine protein phosphatase 2A and inhibited oocyte apoptosis in mouse oocytes

SET is a multifunctional protein involved in a variety of molecular processes such as cell apoptosis and cell-cycle regulation. In ovaries SET is predominantly expressed in theca cells and oocytes. In polycystic ovary syndrome (PCOS) patients the expression of SET was increased than healthy people. The current study was designed to determine whether SET plays a role in oocyte maturation and apoptosis, which may provide clues for the underlying pathological mechanism of follicular development in PCOS patients. Oocytes at germinal vesicle (GV) stage were collected from 6-week-old female ICR mice ovaries. The expression of SET was manipulated by AdCMV-SET and AdH1-SiRNA/SET adenoviruses. SET overexpression improved oocyte maturation whereas SET knockdown inhibited oocyte maturation. Moreover, SET negatively regulated serine/threonine protein phosphatase 2A (PP2A) activity in oocytes. Treatment with PP2A inhibitor okadaic acid (OA) promoted oocyte maturation. Furthermore, PP2A knockdown confirmed the role of PP2A in oocyte maturation, and OA was able to block the AdH1-SiRNA/SET-mediated inhibition on oocyte maturation. The central role of PP2A in SET-mediated regulation of oocyte maturation was confirmed by the finding that SET increased the expression of bone morphogenetic protein 15 (BMP15) and growth differentiation factor 9 (GDF9) and PP2A inhibited their expressions. Besides, SET inhibited oocyte apoptosis through decreasing the expression of caspase 3 and caspases 8, while PP2A had no effect on oocyte apoptosis. SET promoted oocyte maturation by inhibiting PP2A activity and inhibited oocyte apoptosis in mouse in-vitro cultured oocytes, which may provide a pathologic pathway leading to impaired oocyte developmental competence in PCOS.