Inhibition of integrated stress response promotes primordial follicle activation via the mTOR pathway

Background: The integrated stress response (ISR) is a conserved signaling network that enables cells to cope with stress and maintain homeostasis. Accumulating evidence from both human and animal studies suggests that chronic stress has been implicated as a significant factor in the development of premature ovarian insufficiency (POI). The aim of this study was to investigate the effects of targeted ISR inhibition by ISRIB on primordial follicle activation and explore its underlying mechanisms in a mouse model. Methods: This study used intraperitoneal injection to investigate the effects of ISR activity and its inhibitor ISRIB on primordial follicle activation in vivo. Newborn ICR mice were randomly assigned to the Control, Corticosterone, ISRIB, and Corticosterone + ISRIB groups. Mouse and human ovarian tissues were cultured in vitro and treated with ISRIB and/or a proto-oncogenic receptor tyrosine kinase (KIT) inhibitor. Various methods were used to assess differences in follicle counting, ISR activity, cell proliferation, cell apoptosis, and the mTOR pathway. Results: ISR activity was present in normal newborn mouse ovaries and significantly increased in corticosterone-induced stress mouse ovaries. Excessive ISR activity induced by corticosterone reduced the number of primordial follicles, which could be partially reversed by ISRIB. In vitro treatment with ISRIB inhibited the protein levels of p-eIF2α and ATF4, while promoting granulosa cell proliferation and primordial follicle activation, and increasing the levels of phosphorylated mTOR (p-mTOR), phosphorylated protein kinase B (p-Akt), and KIT ligand (KITL). The KIT inhibitor ISCK03 significantly decreased the phosphorylation levels of Akt and forkhead box O3a (FOXO3a), as well as the number of growing follicles induced by ISRIB. In both in vivo injection and in vitro ovarian tissue culture models, ISRIB promoted primordial follicle activation via the mTOR pathway in pre-granulosa cells. Conclusion: The results suggest that inhibiting ISR can prevent primordial follicle depletion caused by excessive ISR activity. ISRIB has been shown to promote primordial follicle activation in mouse and human ovaries via the mTOR pathway. These results suggest that ISRIB may be a potential therapeutic option for infertility in stress-induced POI patients.