Circular RNA circFoxo3 Promotes Granulosa Cell Apoptosis Under Oxidative Stress Through Regulation of FOXO3 Protein

Oxidative stress leads to ovarian functional decline by inducing granulosa cell (GC) apoptosis. Circular RNA circFoxo3 acts as a critical factor in regulating cell cycle and apoptosis, and cellular senescence in tumor cells. However, function of circFoxo3 is little understood in oxidative stress-induced injury of follicular GCs. In this study, we aimed to illustrate the regulation pattern of circFoxo3 in GCs under oxidative stress. CircFoxo3 was confirmed to be expressed in both human and mouse GCs by amplification with divergent primers and sequencing. In vitro and in vivo ovarian oxidative stress model, the expression of circFoxo3, FOXO3 protein, and its downstream targets were examined by quantitative real-time PCR and Western blotting, respectively. Knockdown of circFoxo3 was performed to evaluate the effects of circFoxo3-mediated GC apoptosis in vitro. RNA pull-down was used to discover the protein that interacted with circFoxo3 so as to illustrate the mechanism of circFoxo3 in GCs. Our results demonstrated that circFoxo3 was significantly upregulated in hydrogen peroxide (H2O2)-treated GCs and a 3-nitropropionic acid (3-NP)-induced mouse model of ovarian oxidative stress. Protein level of transcriptional factor FOXO3 was also remarkably increased in both in vitro and in vivo oxidative stress model, but FOXO3 mRNA expression revealed no significant difference. Knockdown of endogenous circFoxo3 downregulated FOXO3 protein level and blocked H2O2-induced cell apoptosis. CircFoxo3 could pull down high levels of MDM2 protein that induced FOXO3 ubiquitination and degradation. Furthermore, knockdown of MDM2 and circFoxo3 showed remarkably higher level of apoptosis when compared with the knockdown of circFoxo3 alone. Our study suggested that circFoxo3 regulated FOXO3 protein level in GCs by reducing interactions between FOXO3 and MDM2. In conclusion, circFoxo3 was positively associated with FOXO3 protein and jointly played crucial roles in mediating GC apoptosis induced by oxidative stress.