Involvement of NR5A1 and NR5A2 in the Regulation of Steroidogenesis by Clock Gene and BMPs by Human Granulosa Cells

We previously reported that the expression levels of Clock gene are linked to the expression levels of steroidogenetic enzymes in human granulosa cells (EJ 2019). However, the downstream molecules of the Clock gene actions in the regulation of ovarian steroidogenesis have yet to be elucidated. In the present study, we investigated the roles of the transcription factors, NR5A1 (also known as SF-1) and NR5A2 (LRH-1), which play key roles in the reproductive function as well as steroidogenesis by focusing on the functional link between Clock gene and bone morphogenetic protein (BMP) signaling using human granulosa KGN cells. First of all, we examined the effects of BMPs/growth differentiation factor (GDF) on forskolin (FSK)-induced steroidogenesis. As a result, FSK-induced mRNA levels of StAR and P450scc, but not P450arom, were potently suppressed by treatments with BMP-6, -9, -15 and GDF-9. The expression levels of NR5A1 and NR5A2 mRNA were also upregulated by FSK treatment, while the BMP-target gene Id-1 mRNA levels were stimulated by the treatment with BMPs. Of interest, treatments with BMPs/GDF increased FSK-induced NR5A1 mRNA levels but suppressed FSK-induced NR5A2 mRNA levels by granulosa cells. The expression levels of NR5A1 mRNA were positively correlated with the changes of P450arom and 3βHSD mRNA, whereas the expression levels of NR5A2 mRNA were correlated with that of StAR and P450scc mRNA. In addition, the expression levels of NR5A1 and NR5A2 mRNAs were positively correlated with the levels of Clock mRNA. In particular, Clock mRNA levels showed highly positive correlation with the levels of NR5A2 mRNA compared with NR5A1 mRNA. Of note, Id-1 mRNA levels were positively correlated with the levels of NR5A1 mRNA, but negatively correlated with that of NR5A2 mRNA. Furthermore, the inhibition of Clock gene expression by siRNA attenuated the expression levels of NR5A1 and NR5A2 mRNA, resulting in decreased mRNA levels of StAR and P450arom in the presence of FSK. Thus, the present results suggested a novel mechanism by which Clock expression is functionally linked to the expression of NR5A1 and NR5A2, the latter of which is further regulated by BMP signaling by granulosa cells. The interaction among Clock, NR5A1/NR5A2 and BMPs may be involved in the fine tuning of steroidogenesis by ovarian follicles.