Insulin-like growth factor 2 is produced by antral follicles and promotes preantral follicle development in macaques^†

Insulin-like growth factors (IGFs) are known for their involvement in endocrine and paracrine regulation of ovarian function. Although IGF2 is the predominant circulating and intraovarian form of IGFs in primate species, the stage-specific follicular expression, action, and regulation of IGF2 are not well defined. Therefore, experiments were conducted to investigate the follicular IGF production in response to steroid hormone regulation and the direct IGF actions on follicular development and function in vitro. Preantral follicles were isolated from rhesus macaque ovaries and cultured to the antral stage in media supplemented with follicle-stimulating hormone and insulin. Follicles were randomly assigned to treatment groups: (a) control, (b) trilostane (a steroid synthesis inhibitor), (c) trilostane + estradiol, (d) trilostane + progesterone, and (e) trilostane + dihydrotestosterone. Media was analyzed for IGF concentrations, which were correlated to follicle growth. Follicles produced IGF2, but not IGF1, at the antral stage. Steroid depletion decreased, whereas steroid replacement increased, IGF2 production by antral follicles. Media IGF2 levels correlated positively with antral follicle diameters. Macaque preantral follicles and granulosa cells were subsequently cultured without (control) and with recombinant human IGF2 supplementation. Follicle survival, growth, and paracrine factor production, as well as granulosa cell proliferation and gonadotropin receptor gene expression, were assessed. IGF2 addition increased follicle survival rates, diameters and inhibin B production, as well as granulosa cell proliferation. These data demonstrate that IGF2 produced by antral follicles, in response to steroid hormone regulation, could act as a paracrine factor that positively impacts preantral follicle development and function in primates.