Smad3-related miRNAs regulated oncogenic TRIB2 promoter activity to effectively suppress lung adenocarcinoma growth

MicroRNAs (miRNAs) and Smad3, as key transcription factors in transforming growth factor-β1 (TGF-β1) signaling, help regulate various physiological and pathological processes. We investigated the roles of Smad3-regulated miRNAs with respect to lung adenocarcinoma cell apoptosis, proliferation, and metastasis. We observed that Smad3 and phospho-SMAD3 (p-Smad3) were decreased in miR-206- (or miR-140)-treated cells and there might be a feedback loop between miR-206 (or miR-140) and TGF-β1 expression. Smad3-related miRNAs affected tribbles homolog 2 (TRIB2) expression by regulating trib2 promoter activity through the CAGACA box. MiR-206 and miR-140 inhibited lung adenocarcinoma cell proliferation in vitro and in vivo by suppressing p-Smad3/Smad3 and TRIB2. Moreover, lung adenocarcinoma data supported a suppressive role for miR-206/miR-140 and an oncogenic role for TRIB2—patients with higher TRIB2 levels had poorer survival. In summary, miR-206 and miR-140, as tumor suppressors, induced lung adenocarcinoma cell death and inhibited cell proliferation by modifying oncogenic TRIB2 promoter activity through p-Smad3. MiR-206 and miR-140 also suppressed lung adenocarcinoma cell metastasis in vitro and in vivo by regulating EMT-related factors.