Complementary roles of Smad6 and Smad7 in BMP-15-mediated steroidogenic regulation in granulosa cells

Bone morphogenetic proteins (BMPs), members of the transforming growth factor (TGF)-β superfamily, play essential roles in ovarian follicular development and steroidogenesis. Although inhibitory Smads (Smad6 and Smad7) negatively regulate BMP/TGF-β signaling, their roles in ovarian steroidogenesis remain unclear. Here, we investigated the effects of Smad6 and Smad7 on BMP-15-mediated steroidogenic regulation using human granulosa-like KGN cells and primary rat granulosa cells. Of note, siRNA-mediated knockdown of Smad6 enhanced BMP-15-induced Smad1/5/9 phosphorylation and Id-1 expression, whereas Smad7 knockdown increased basal Id-1 expression independently of BMP-15 in KGN cells. BMP-15 reduced progesterogenic StAR expression and this effect was enhanced by Smad6 or Smad7 knockdown. In addition, BMP-15 increased metabolization of progesterone by 20αHSD induction under Smad7-knockdown conditions, whereas aromatase expression was unaffected by BMP-15 regardless of Smad6 or Smad7 knockdown. In primary rat granulosa cells, combined knockdown of Smad6 and Smad7 significantly reduced basal and FSH-induced progesterone production without affecting estradiol production. Collectively, these results suggest that inhibitory Smad6 and Smad7 differentially and complementarily regulate BMP-15 signaling and progesterone synthesis in granulosa cells, contributing to the fine-tuning of ovarian BMP signaling and luteinization.