miR-214 coordinates EMT in a MMTV-PyMT breast cancer mouse model

microRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally silence their mRNA targets and are often abnormally expressed in pathological conditions. The oncogenic miR-214 plays a key role in promoting breast cancer progression, by downregulating the expression of direct targets, such as adhesion molecules and transcription factors, but also by inhibiting the antimetastatic miR-148b. To investigate the role of miR-214 in endogenous breast cancer progression, we crossed our previously established miR-214OVER mice with the Mouse Mammary Tumor Virus-Polyoma Middle Tumor Antigen (MMTV-PyMT) mouse model, characterized by aggressive mammary epithelial tumors. We observed that MMTV-PyMT-miR-214OVER mice developed smaller but more metastatic tumors, compared to MMTV-PyMT animals. Molecular analysis revealed increased Epithelial-to-Mesenchymal Transition (EMT) markers in MMTV-PyMT-miR-214OVER mammary tumors compared to controls. Here, miR-148b levels were diminished. When human breast tumor datasets were analyzed, a correlation between miR-214 and EMT genes was found. Instead, an anticorrelation was detected with miR-148b. Hence, we isolated tumor mammary epithelial cells from MMTV-PyMT mice and engineered them for miR-214 expression, to inquire the molecular mechanism linking miR-214 with EMT. Gene Ontology experiments evidenced that miR-148b-dependent TGFβ signaling was the most relevant EMT-linked pathway. Coherently, in the derived miR-214-overexpressing mammary epithelial cells, SMAD2, one of the main TGFβ signaling players, was found highly phosphorylated and transcriptionally active. Opposite results were found in miR-214-depleted or miR-148-overexpressing cells. Relevantly, TGFβ Receptor I inhibitor in miR-214-overexpressing cells did not restore E-cadherin expression, but impaired miR-214-promoted migration. In conclusion, our findings link the pro-metastatic role of miR-214 to EMT via the activation of the TGFβ pathway in an endogenous mouse model of breast cancer.