SMAD4 Feedback Activates the Canonical TGF-β Family Signaling Pathways

TGF-beta family signaling pathways, including TGF-beta and BMP pathways, are widely involved in the regulation of health and diseases through downstream SMADs, which are also regulated by multiple validated mechanisms, such as genetic regulation, epigenetic regulation, and feedback regulation. However, it is still unclear whether R-SMADs or Co-SMAD can feedback regulate the TGF-beta family signaling pathways in granulosa cells (GCs). In this study, we report a novel mechanism underlying the feedback regulation of TGF-beta family signaling pathways, i.e., SMAD4, the only Co-SMAD, positive feedback activates the TGF-beta family signaling pathways in GCs with a basal level of TGF-beta ligands by interacting with the core promoters of its upstream receptors. Mechanistically, SMAD4 acts as a transcription factor, and feedback activates the transcription of its upstream receptors, including ACVR1B, BMPR2, and TGFBR2, of the canonical TGF-beta signaling pathways by interacting with three coactivators (c-JUN, CREB1, and SP1), respectively. Notably, three different interaction modes between SMAD4 and coactivators were identified in SMAD4-mediated feedback regulation of upstream receptors through reciprocal ChIP assays. Our findings in the present study indicate for the first time that SMAD4 feedback activates the canonical TGF-beta family signaling pathways in GCs, which improves and expands the regulatory mechanism, especially the feedback regulation modes of TGF-beta family signaling pathways in ovarian GCs.

Automated summary

The study illustrates a novel mechanism in which SMAD4, as the only Co-SMAD, positively regulates the canonical TGF-beta family signaling pathways in granulosa cells (GCs) by interacting with the core promoters of upstream receptors through the recruitment of three coactivators (c-JUN, CREB1, and SP1). Different interaction modes between SMAD4 and coactivators were identified through reciprocal ChIP assays, expanding the understanding of the feedback regulation modes of TGF-beta family signaling pathways in ovarian GCs.