The role of estradiol as a biological amplifier of the actions of follicle-stimulating hormone: In vitro studies in swine granulosa cells

We describe an in vitro system of swine granulosa cells which remain responsive to estradiol (E₂) and FSH. In this system, we examined mechanisms by which E₂ amplifies the stimulatory actions of FSH. E₂ stimulated progesterone production in a dose-dependent manner and enhanced the tropic effects of FSH. The facilitative interaction between E₂ and FSH could not be accounted for by mitogenic effects, by a leftward shift in the dose-response curves for FSH or E₂, or by catabolism of progesterone to 20α-hydroxypregn-4-en-3-one. E₂ also enhanced stimulatory actions of 8-bromo-cAMP, choleratoxin, and the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine. Lipoproteins were required for maximal interactive effects between E₂ and FSH. However, estrogen significantly increased the effects of FSH even in serum- and lipoprotein-free medium. In addition, when de novo cholesterol biosynthesis by granulosa cells was suppressed by Compactin, the actions of E₂ and/or FSH were not impeded. In contrast, E₂ alone and FSH alone significantly augmented progesterone production in response to the oxygenated sterol 5-cholesten-3β, 25-diol, which is an effective substrate for cholesterol side-chain cleavage. In the presence of 5-cholesten-3β, 25-diol, the magnitude of the synergism between E₂ and FSH was also increased markedly. We conclude that E₂ augments the stimulatory actions of FSH or cAMP on steroidogenesis. This facilitative interaction is amplified by exogenous lipoproteins and does not seem to depend upon endogenous cholesterol biosynthesis. Studies with 5-cholesten-3β, 25- diol suggest that estrogen and FSH exert significant stimulatory effects at the level of cholesterol side-chain cleavage. These observations reveal important interactions of E₂ and FSH in preparing granulosa cells for the high rates of steroidogenesis they ultimately express in the luteinized state.