Dynamic SMAD–RUNX2 complexes orchestrate breast cancer bone metastasis

Abstract Bone metastasis of breast cancer is a multistep process involving detachment from the primary tumor, homing and adhesion to bone, clonal survival, and osteolysis, driven by dynamic interactions between tumor cells and the bone microenvironment. Although Runt-related transcription factor 2 (RUNX2) has been recognized as a downstream effector of SMADs in bone metastatic programs, our study has revealed that SMAD–RUNX2 complexes also regulate the intrinsic functions of SMADs, thereby acting as active amplifiers of TGFβ and BMP signaling. We further characterized the distinct roles of the SMAD1–RUNX2 and SMAD2/3–RUNX2 complexes across the bone metastatic cascade using cancer cells overexpressing either wild-type RUNX2 or RUNX2 variants in response to treatment with TGFβ and BMP. SMAD–RUNX2 complexes were found to play microenvironment-dependent roles during bone metastasis. The TGFβ-induced SMAD2/3–RUNX2 complex promoted osteotropic migration and secondary dissemination, whereas the BMP-activated SMAD1–RUNX2 complex increased cancer cell adhesion and survival in the bone microenvironment, with both complexes increasing osteolytic lesion formation. Mechanistically, the competitive binding of SMAD2/3 and SMAD1 to RUNX2 appeared to underlie the crosstalk between the TGFβ and BMP pathways. Together, these findings demonstrate a dynamic interplay among SMAD1, SMAD2/3, and RUNX2 that integrates TGFβ and BMP signaling during bone metastasis, highlighting the potential for developing context-dependent therapeutic strategies.