The Role of Insulin-Like Growth Factors in Granulosa Cell Steroidogenesis: Studies with a Neutralizing Antibody to IGF-I

Summary

Evidence that granulosa cells secrete and respond to insulin-like growth factors (IGFs) suggests, but does not prove, the importance of IGFs as intraovarian regulators. To further assess the role of these peptides in ovarian function, we have employed a neutralizing monoclonal antibody to IGF-I to block the actions of IGFs in porcine follicular fluid and in granulosa cell-conditioned medium. Granulosa cells from immature porcine follicles were cultured in medium containing follicular fluid from large (6–10 mm) porcine follicles which had been charcoal stripped to remove steroids. Granulosa cells cultured with follicular fluid produced five- to twentyfold more progesterone than did those cultured with charcoal-treated porcine serum. However, for granulosa cells treated with the blocking antibody, follicular fluid-stimulated progesterone production was inhibited 67%. Granulosa cells were also cultured without exogenous growth factors; the effects of FSH, estradiol, GH, and combinations thereof on progesterone production were inhibited by 45–76% by the antibody. The stimulatory effect of follicular fluid on -thymidine incorporation in granulosa cell cultures was reduced by only 12% by the antibody. These results suggest that the stimulatory effects of follicular fluid and hormones on progesterone production are mediated, at least in part, by IGFs. Under these culture conditions, ovarian IGFs appear to be more important to cytodifferentiation than to replication.