circTGFBR2(3-6) acts as an assembly platform for RNA-binding protein IGF2BP3 and TGFBR1 mRNA to enhance breast cancer cell plasticity

Transforming growth factor (TGF)-β signaling is a key driver to induce epithelial-to-mesenchymal transition (EMT), a process that enhances cancer cell plasticity and metastatic potential. However, the role of circular RNAs (circRNAs) in TGF-β signaling remains largely unexplored. Here, we identify circTGFBR2(3-6), a circRNA derived from TGF-β receptor 2 (TGFBR2) pre-mRNA, as a critical enhancer of TGF-β/SMAD signaling in breast cancer cells. Depletion of circTGFBR2(3-6) inhibits TGF-β-induced EMT, cell migration, and in vivo extravasation of breast cancer cells. Mechanistically, circTGFBR2(3-6) acts as a scaffold that facilitates the interaction between the RNA-binding protein insulin-like growth factor 2 mRNA binding protein 3 (IGF2BP3) and TGF-β receptor 1 (TGFBR1) mRNA in an N⁶-methyladenosine (m⁶A)-dependent manner, and thereby stabilizes TGFBR1 mRNA and promotes its expression. Furthermore, IGF2BP3 knockdown reduces circTGFBR2(3-6)-mediated enhancement of TGF-β/SMAD signaling, as well as TGF-β-induced EMT and cell migration. Our findings identify circTGFBR2(3-6) as a novel potentiator of TGF-β/SMAD signaling at the receptor level and highlight IGF2BP3 as a critical m⁶A reader that mediates circTGFBR2(3-6)-driven breast cancer cell plasticity.