Functional Roles Of Kit Ligand-C-Kit Interaction and Oocyte Factors in Regulation of Steroidogenesis by Rat Granulosa Cells.

The ovarian BMP system plays a key role in regulating follicle development. BMPs have been known to commonly suppress FSH-induced progesterone production. We recently reported roles of oocyte-derived FGF-8 in ovarian steroidogenesis, in which FGF-8 suppressed FSH-induced estrogen production and amplified BMP signaling through oocyte-granulosa cell communication. Regarding this communication, kit ligand (KL)-c-kit interaction is critical for oogenesis and folliculogenesis in the ovary. However, the significance of KL-c-kit in regulation of ovarian steroidogenesis has yet to be elucidated. We here investigated the impact of KL-c-kit interaction in regulating steroidogenesis using rat primary granulosa cells of immature female rat ovaries. It has been reported that oocyte-derived BMP-15 stimulates KL expression in rat granulosa cells leading to stimulation of granulosa cell proliferation, while GDF-9 is reported to suppress KL expression in mouse granulosa cells. In the present study, we revealed that KL treatment suppressed FSH-induced estradiol production and aromatase expression without affecting progesterone production by oocyte-granulosa co-cultures. Interestingly, the KL suppression of FSH-induced estrogen production was reversed in the presence of anti-c-kit neutralizing antibody, suggesting that oocyte factors are functionally involved in suppressing estrogen production by granulosa cells through the interaction between KL and its receptor c-kit on oocytes. 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 that enable to regulate steroidogenesis were examined. It was of note that KL treatment stimulated the expression levels of oocyte-derived FGF-8 and GDF-9, while KL reduced the expression levels of oocyte-derived BMP-15 in oocyte-granulosa co-cultures. Given the findings that GDF-9 and FGF-8, but not BMP-15, suppress FSH-induced estrogen production, oocyte-derived FGF-8 and GDF-9 are possibly involved in reducing estrogen production through the KL-c-kit interaction. In addition, the suppression of BMP-15 expression caused by KL-c-kit signaling may have resulted in maintaining FSH-induced progesterone levels. Thus, KL-c-kit interaction is not only involved in the mitotic regulation of granulosa cells, but also plays a role in estrogenic regulation through oocyte-granulosa communication.

(poster)