BAMBI promotes porcine granulosa cell steroidogenesis involving TGF-β signaling.

Transforming growth factor β (TGF-β), acting as the auto/para endocrine factors, has multi-function in mammalian follicle development. Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) is considered as a pseudoreceptor in the TGF-β signal pathway which has the similar extracellular structure of TGF-β receptor but lack of intracellular serine/threonine kinase domains. However, the biological function of BAMBI involved in porcine granulosa cell steroidogenesis remains unknown. This study was thus carried out to explore the effect of BAMBI on the steroidogenesis process in porcine primary granulosa cells. Our results showed overexpression of BAMBI promoted aromatase and StAR, but not P450scc and 3β-HSD mRNA and protein expression levels in porcine primary granulosa cells, and increased the accumulation of estradiol and progesterone in the culture medium. Meanwhile, knockdown endogenous BAMBI decreased the mRNA expression levels of Cyp19a1 and Star and the accumulation levels of estradiol and progesterone. TGF-β1 could decrease Cyp19a1 and Star mRNA expression and estradiol and progesterone production in a dose-dependent manner. Pre-treatment with BAMBI adenovirus reversed TGFβ1-induced downregulation of Cyp19a1 and Star mRNA expression. Moreover, TGF-β1 could induce the phosphorylation of SMAD3 in porcine granulosa cells. Pre-transfected with BAMBI adenovirus also inhibited TGF-β1-induced downregulation of estradiol and progesterone production as well as TGF-β1-induced phosphorylation of SMAD3 in porcine granulosa cells. These findings provided a potential mechanism by which BAMBI could regulate porcine granulosa cell steroidogenesis.