Transforming Growth Factor-β-Induced Epithelial-Mesenchymal Transition Facilitates Epidermal Growth Factor-Dependent Breast Cancer Progression

TGF-β and EGF play critical roles in regulating the metastasis of aggressive breast cancers, yet the impact of epithelial-mesenchymal transition (EMT) induced by TGF-β in altering the response of breast cancer cells to EGF remains unknown. We show here that murine metastatic 4T1 breast cancer cells formed compact, and dense spheroids when cultured under 3-dimensional (3D) conditions, which contrasted sharply to the branching phenotypes exhibited by their nonmetastatic counterparts. Using the human MCF10A series, we show that epithelial-type and nonmetastatic breast cancer cells were unable to invade to EGF, while their mesenchymal-type and metastatic counterparts readily invaded to EGF. Furthermore, EMT induced by TGF-β was sufficient to manifest spheroid morphologies, a phenotype that increased primary tumor exit and invasion to EGF. Post-EMT invasion to EGF was dependent upon increased activation of EGFR and p38 MAPK, all of which could be abrogated either by pharmacological (PF-271) or genetic (shRNA) targeting of focal adhesion kinase (FAK). Mechanistically, EMT induced by TGF-β increased cell surface levels of EGFR and prevented its physical interaction with E-cadherin, leading instead to the formation of oncogenic signaling complexes with TβR-II. Elevated EGFR expression was sufficient to transform normal mammary epithelial cells, and to progress their 3D morphology from that of hollow acini to branched structures characteristic of nonmetastatic breast cancer cells. Importantly, we show that TGF-β stimulation of EMT enabled this EGFR-driven breast cancer model to abandon their inherent branching architecture and form large, undifferentiated masses that were hyper-invasive to EGF and displayed increased pulmonary tumor growth upon tail vein injection. Finally, chemotherapeutic targeting of FAK was sufficient to revert the aggressive behaviors of these structures. Collectively, this investigation has identified a novel EMT-based approach to neutralize the oncogenic activities of EGF and TGF-β in aggressive and invasive forms of breast cancer.