Endocrine Abstracts

Kit ligand (KL)-c-kit interaction is critical for oogenesis and folliculogenesis in the ovary. However, the significance of KL-c-kit loop in ovarian steroidogenesis has yet to be elucidated. We here investigated the impact of KL-c-kit interaction in regulation of steroidogenesis using rat primary granulosa cells co-cultured with oocytes. Treatment with soluble KL suppressed FSH-induced estradiol production and aromatase mRNA expression without changing FSH-induced progesterone production. Blocking the KL-c-kit interaction by its neutralizing antibody increased FSH-induced estrogen production by granulosa cells, suggesting that endogenous KL is functionally involved in suppression of estrogen production by granulosa cells through the interaction between KL and its receptor c-kit on oocytes. The cAMP-PKA pathway activity was not involved in the KL and c-kit actions on estrogen regulation in granulosa cells. To explore the possible oocyte factors induced by KL-c-kit interaction, changes in the expression levels of major oocyte factors including BMP-15, GDF-9 and FGF-8 were investigated. It was of note that KL increased the expression levels of oocyte-derived FGF-8 and GDF-9, while it reduced the expression levels of oocyte-derived BMP-15 in the oocyte-granulosa co-culture. Based on the findings that FGF-8, but not BMP-15 or GDF-9, suppressed FSH-induced estrogen production by granulosa cells, oocyte-derived FGF-8 is possibly involved in suppression of FSH-induced estrogen production through the KL-c-kit interaction. Furthermore, the KL suppression of FSH-induced estrogen production in the co-culture was reversed by the FGFR inhibitor SU5402, which was additionally reversed by the combined treatment with extracellular-domain protein of BMPRII that antagonizes BMP-15 and GDF-9. These data suggest that endogenous oocyte factors including FGF-8 and BMP-15/GDF-9 are likely to be involved in the KL activity that inhibits FSH-induced estradiol production. Thus, KL-c-kit interaction plays a regulatory role in estrogen production through oocyte-granulosa communication.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.