MON-151 Functional roles of inhibitory Smad6/7 in the regulation of ovarian steroidogenesis by BMP-15

Abstract Disclosure: Y. Soejima: None. K. Yamamoto: None. N. Iwata: None. K. Motohashi: None. A. Suyama: None. Y. Nakano: None. F. Otsuka: None. The transforming growth factor (TGF)-β superfamily, including bone morphogenetic protein (BMP), plays a pivotal role in reproductive function. TGF-β and BMP bind to their receptors and induce expression of target genes through Smad-mediated signaling. Smad6 and Smad7, also known as inhibitory Smads, function as negative regulators of TGF-β and BMP signaling. Although it has been reported that Smad6 and Smad7 are expressed in the ovary, their direct effects on follicular steroidogenesis have not been fully elucidated. In the present study, we investigated the influence of inhibitory Smads on ovarian BMP signaling and follicular steroidogenesis by using human granulosa cell tumor-derived KGN cells and primary cultured rat ovarian granulosa cells. In KGN cells, it was confirmed that knockdown (KD) of Smad6 and Smad7 by siRNA suppressed Smad6 and Smad7 mRNA expression, respectively. Under Smad6 KD or Smad7 KD conditions, BMP receptor signaling, detected by Id-1 mRNA levels, was enhanced in the presence of BMP-15. Under Smad6 KD conditions, BMP-15 stimulation suppressed forskolin-induced StAR and P450scc mRNA levels, enhanced 3βHSD expression, but did not affect P450arom expression. On the other hand, under Smad7 KD conditions, BMP-15 stimulation suppressed the expression of StAR and P450scc, while the expression of 3βHSD and P450arom was not affected. The expression of 20α-HSD, a progesterone degradation enzyme, remained unchanged by BMP-15 stimulation under Smad6 KD conditions but was increased under Smad7 KD conditions. Furthermore, BMP-15 stimulation suppressed the expression of clock genes including Clock and Bmal1b under the KD effects of Smad6 but not that of Smad7. Notably, in the primary culture of rat ovarian granulosa cells, simultaneous KD of Smad6 and Smad7 significantly suppressed progesterone synthesis levels, whereas it did not affect estradiol production. Collectively, it was revealed that the coexistence of inhibitory Smad6 and Smad7 regulates BMP receptor signaling in ovarian granulosa cells, which may lead to the integrated modulation of follicular steroidogenesis by BMP-15. Presentation: Monday, July 14, 2025