FOXO3a is involved in the apoptosis of naked oocytes and oocytes of primordial follicles from neonatal rat ovaries

Shantou University Medical College, Shantou, Guangdong, PR China.
Biochemical and Biophysical Research Communications (Impact Factor: 2.3). 05/2009; 381(4):722-7. DOI: 10.1016/j.bbrc.2009.02.138
Source: PubMed


Inhibition of the forkhead transcription factor, FOXO3a, can promote the transition from primordial to primary follicle and subsequent follicle development in mammalian ovaries. Stem cell factor (SCF) initiates anti-apoptotic signaling from its membrane receptor, c-kit, to Bcl-2 family members through PI3K/AKT in oocytes of primordial follicles. However, whether FOXO3a mediates the apoptosis of naked oocytes and oocytes of primordial follicles remains unknown. In the present study, oocytes from nests and primordial follicles from neonatal rat ovaries were cultured, and oocyte apoptosis was examined using the TUNEL technique. The pro-apoptotic action of FOXO3a and the potential signal transduction pathways were investigated using RT-PCR, Western blot, and immunocytochemistry. Culturing oocytes in the presence of SCF did not affect the level of total FOXO3a protein, but rapidly elevated the level of phosphorylated FOXO3a (indicating functional suppression). As phosphorylated FOXO3a increased, oocyte apoptosis was inhibited. The specific PI3K/Akt inhibitor, LY 294002, abolished the phosphorylation of FOXO3a and the anti-apoptotic action of SCF. SCF down-regulated the expression of p27KIP1 and pro-apoptotic factors such as Bim, Bad, and Bax, and this activity was reversed by LY 294002. SCF up-regulated the expression of MnSOD, which was also inhibited by LY 294002. However, SCF had no effect on Bcl-2 protein. These results suggest that FOXO3a is involved in oocyte apoptosis in the neonatal rat ovary, and the SCF-PI3K/Akt-FOXO3a signaling pathway mediates oocyte apoptosis and primordial follicle formation.

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    • "Studies have shown that TGF-β1 expression in MCs is increased under high glucose conditions20,21,22,24,25 and that TGF-β1 participates in activation of the PI3K/Akt pathway26,27,28,29. PI3K/Akt pathway activation by TGF-β1 induces the phosphorylation and inactivation of FoxO3a and decreases expression of its downstream target genes, Bim and MnSOD30,31,32,33,34,35,36. Consequently, both the cell proliferation rate and ROS production increase37. "
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    • "The Forkhead transcription factor 3a (FOXO3a) halts the transition from primordial to primary follicle in rodents (Reddy et al., 2005), by inhibiting cyclin-dependent kinases, thus decreasing proliferation (Kops et al., 2002) and stimulating apoptosis of the oocyte and follicular cells (Liu et al., 2009). FOXO3a is phosphorylated by KL (Liu et al., 2009) and testosterone (Yang et al., 2010) inactivating the protein and preventing cell apoptosis allowing the transition from primary to secondary follicle. The anti-Müllerian hormone (AMH), synthesized by granulosa cells (Sadeu et al., 2008), is another member of the TGF-β family known to inhibit the development of primordial follicles (Moniruzzaman and Miyano, 2010): for example, in rat ovarian cultures, the addition of AMH reduces the transition from primordial to primary follicles even in the presence of KL (Nilsson et al., 2007). "
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