Effect of follicle size on in vitro production of steroids and insulin-like growth factor (IGF)-I, IGF-II, and the IGF-binding proteins by equine ovarian granulosa cells.

This research studies the hormone regulation affecting the steroid production of ovarian granulosa cells in horses, as well as the role of these hormones on the insulin-like growth factor (IGF) system in these cells.

Study Background and Aims

• The study was designed to improve the understanding of the hormonal regulation of ovarian granulosa cells in mares – specifically, their steroidogenesis and the effects on the insulin-like growth factor (IGF) system.
• The first experiment aimed to investigate the effects of hormones like estradiol, insulin and FSH (follicle-stimulating hormone) on the production of steroids by granulosa cells.
• The second experiment was designed to understand if the components of the IGF system are produced by granulosa cells, and whether the aforementioned hormones impact the production of IGF or IGF-binding proteins (IGFBPs).

Methods and Experimental Design

• The test subjects were granulosa cells obtained from the small, medium, and large follicles of the ovary of mares.
• The cells were cultured and treated with or without specific hormones for 2 days.
• The two separate experiments tested the effects of hormones on different types of cells and measured steroid and IGF production.

Results

• Experiment 1 showed that granulosa cells from large follicles produced less progesterone and more estradiol than cells from medium or small follicles.
• FSH and insulin were found to inhibit progesterone production in small and medium follicle cells, but not in the large ones.
• Insulin increased estradiol production in small and medium follicles, but had null effects on the large ones. Estradiol itself had no effect on progesterone production.
• Experiment 2 revealed that insulin inhibited IGF-I production across all follicle sizes, while estradiol or FSH did not make a difference.
• Medium and large follicle cells produced more IGF-II than small follicle cells, and estradiol had no effect here as well. Insulin and FSH inhibited IGF-II production in large and medium follicles, but not in small ones.
• The study also found that only IGFBP-2 and IGFBP-5 were produced by equine granulosa cells. IGFBP-2 production was lower in large follicle cells as compared to small and medium ones. Medium follicle cells produced more IGFBP-5 than small or large follicle cells.
• Considering all follicle sizes, it was found that estradiol increased IGFBP-2 production, FSH mildly increased the production of both IGFBP-2 and -5, and insulin did not have any effect.

Conclusion

• The experimental results showed that IGF-I, IGF-II, IGFBP-2, and IGFBP-5 are all produced by equine granulosa cells and that hormones like insulin, FSH, and estradiol play a regulatory role in steroidogenesis and the IGF system of these cells.