Impaired miRNA processing by DICER1 downregulation endows thyroid cancer with increased aggressiveness

The global downregulation of miRNAs (miRs) is emerging as a common hallmark of cancer. However, the mechanisms underlying this phenomenon are not well known. We identified that the oncogenic miR-146b-5p attenuates miRNA biosynthesis by targeting DICER1 and reducing its expression. DICER1 overexpression inhibited all the miR-146b-induced aggressive phenotypes in thyroid cells. Systemic injection of an antimiR-146b in mice with orthotopic thyroid tumors suppressed tumor growth and recovered DICER1 levels. Notably, DICER1 downregulation promoted proliferation, migration, invasion and epithelial-mesenchymal transition through miR downregulation. Our analysis of TCGA revealed a general decrease in DICER1 expression in thyroid cancer that was associated with a worse clinical outcome. Administration of the small molecule enoxacin to promote DICER1 complex activity reduced tumor aggressiveness both in vitro and in vivo. Overall, our data establish DICER1 as a tumor suppressor and that the oncogenic miR-146b contributes to its downregulation. Moreover, this study highlights a potential therapeutic application of RNA-based therapies including miR-inhibitors and restoration of the biogenesis machinery, which may provide treatments in for thyroid and other cancers.