miR-9-Mediated Inhibition ofEFEMP1Contributes to the Acquisition of Pro-Tumoral Properties in Normal Fibroblasts

Tumor growth and invasion occurs through a dynamic interaction between cancer and stromal cells, which support an aggressive niche. MicroRNAs are thought to act as tumor messengers to corrupt stromal cells. We previously demonstrated that miR-9, a known metastamiR, is released by triple negative breast cancer (TNBC) cells to enhance the transition of normal fibroblasts (NFs) into cancer-associated fibroblast (CAF)-like cells. EGF containing fibulin extracellular matrix protein 1 (EFEMP1), which encodes for the ECM glycoprotein fibulin-3, emerged as a miR-9 putative target upon miRNA's exogenous upmodulation in NFs. Here we explored the impact ofEFEMP1downmodulation on fibroblast's acquisition of CAF-like features, and how this phenotype influences neoplastic cells to gain chemoresistance. Indeed, upon miR-9 overexpression in NFs,EFEMP1resulted downmodulated, both at RNA and protein levels. The luciferase reporter assay showed that miR-9 directly targetsEFEMP1and its silencing recapitulates miR-9-induced pro-tumoral phenotype in fibroblasts. In particular,EFEMP1siRNA-transfected (si-EFEMP1) fibroblasts have an increased ability to migrate and invade. Moreover, TNBC cells conditioned with the supernatant of NFs transfected with miR-9 or si-EFEMP1became more resistant to cisplatin. Overall, our results demonstrate that miR-9/EFEMP1axis is crucial for the conversion of NFs to CAF-like cells under TNBC signaling.