ACF7 promotes spontaneous metastasis in HER2-driven breast cancer through cytoskeletal coordination

Abstract Background: Metastasis is the primary cause of breast cancer mortality, yet the cellular mechanisms that confer metastatic competence remain incompletely understood. Directed migration requires coordinated regulation of the actin and microtubule cytoskeletons, but how this integration contributes to metastatic progression in vivo remains unclear. Here, we investigated the role of ACF7, a spectraplakin that links actin filaments and microtubules downstream of HER2 signaling, in spontaneous metastasis of HER2-driven breast cancer. Methods: We generated a genetically engineered mouse model combining HER2-driven mammary tumorigenesis with conditional loss of ACF7, enabling its contribution to spontaneous metastasis to be assessed in the context of primary tumor development. Primary tumor-derived cell lines were established and analyzed using live-cell imaging, immunofluorescence microscopy and three-dimensional spheroid assays to define the cellular mechanisms regulated by ACF7. Experimental metastasis models were used to examine its contribution to lung colonization, and its clinical relevance was evaluated in human breast cancer specimens and patient datasets. Results: ACF7 loss markedly reduced spontaneous lung metastasis without affecting primary tumor burden or histopathological features. Mechanistically, ACF7 promoted cancer cell migration and invasion by coordinating actin and microtubule organization, regulating microtubule dynamics and supporting focal adhesion maturation. These functions were conserved across independent HER2-driven tumor cell models. Experimental metastasis assays further demonstrated that ACF7 promotes lung colonization. In human breast tumors, ACF7 expression was elevated in HER2-positive cancers and associated with metastatic features and poorer patient survival. Conclusions: ACF7 promotes spontaneous metastasis in HER2-driven breast cancer by coordinating actin-microtubule organization, microtubule dynamics and focal adhesion maturation, while remaining dispensable for primary tumor growth. These findings establish cytoskeletal coordination as a critical determinant of metastatic competence in vivo and identify ACF7-dependent cytoskeletal programs as potential metastasis-selective therapeutic vulnerabilities.