Drosha/Dicer/miR-7/TGF-β2 axis Sustained Acidic Microenvironment-induced Lung Metastasis

Extracellular pH (pHe) is lower in many tumors than in the corresponding normal tissue. Acidic tumor microenvironment has been shown to facilitate epithelial mesenchymal transition (EMT) and tumor metastasis, while the mechanisms underlying tumor acidic microenvironment-induced tumor cell metastasis remain undefined. Here, we aimed to investigate the tumor metastasis and the EMT by acidic microenvironment and to explore their mechanisms and clinical significance in lung cancer. Results showed that acidic pHe remarkably enhanced invasion ability of lung cells accompanying with increased mesenchymal and decreased epithelial markers. Moreover, acidic pHe triggered the inhibition of microRNA-7 (miR-7) expression and activation of TGF-β2/SMAD signaling. Mechanistic studies showed that TGF-β2 is a direct potential target gene of miR-7, and acidity-induced metastasis could be abolished by treatment with a TGFβRI inhibitor, anti-TGF-β2 antibody and miR-7 mimic, respectively. The clinical samples further revealed that miR-7 was decreased in lung tissues and antagonistically correlated with TGF-β2 expression, associating with overall survival and metastasis. In conclusion, our study indicated that acidic pHe showed enhanced invasive potential, and enhanced potential to develop experimental metastases by a novel mechanism involving tumor acidic microenvironment-induced regulation of miR-7/TGF-β2/SMAD axis. Our findings suggest that the possibility that pHe of the primary tumor may be an important prognostic parameter for lung cancer patients merit clinical investigation. Moreover, miR-7 may serve as prognostic molecular marker and a novel therapeutic target for lung cancer.