MiR-135-5p inhibits TGF-β-induced epithelial-mesenchymal transition and metastasis by targeting SMAD3 in breast cancer

Breast cancer (BC) is the most frequently diagnosed malignant tumors and the leading cause of death due to cancer in women around the world. A growing body of studies have documented that microRNA (miR)-135-5p is associated with the development and progression of BC. Considering that sekelsky mothers against dpp3 (SMAD3) plays a crucial role in transforming growth factor (TGF)-β/SMAD pathway and epithelial-mesenchymal transition (EMT) process, it is critical to elucidate the crosstalk and underlying regulatory mechanisms between miR-135-5p and SMAD3 in controlling TGF-β-mediated EMT in BC metastasis. Our results revealed a reciprocal expression pattern between miR-135-5p and SMAD3 mRNA in BC tissues and cell lines. Moreover, miR-135-5p was decreased in BC tissues compared to adjacent breast tissues; more interesting, miR-135-5p mRNA levels (Tumor/Normal, T/N) was further decreased in BC patients with lymph node metastasis, while SMAD3 mRNA levels were increased. Gain- and loss-of-function assays indicated that overexpression of miR-135-5p inhibited TGF-β-mediated EMT and BC metastasis in vitro and in vivo. Furthermore, knockdown of SMAD3 produced a consistent phenotype of miR-135-5p overexpression in breast cancer cells. Mechanistically, SMAD3, a pivotal transcriptional modulator of TGF-β/SMAD pathway, for the first time, was analyzed and identified as a target gene of miR-135-5p by bioinformatic algorithms and dual-luciferase reporter assays. Taken together, we clarified that miR-135-5p suppressed TGF-β-mediated EMT and BC metastasis by negatively regulating SMAD3 and TGF-β/SMAD signaling. Our findings supported that miR-135-5p may serve as a tumor suppressor, and be a valuable diagnostic biomarker for the treatment of BC.