miR-216a Acts as a Negative Regulator of Breast Cancer by Modulating Stemness Properties and Tumor Microenvironment.

Breast cancer is the most frequent malignancy in females in terms of both incidence and mortality. Underlying the high mortality rate is the presence of cancer stem cells, which divide indefinitely and are resistant to conventional chemotherapies, so causing tumor relapse. In the present study, we identify miR-216a-5p as a downregulated microRNA in breast cancer stem cells vs. the differentiated counterpart. We demonstrate that overexpression of miR-216a-5p impairs stemness markers, mammosphere formation, ALDH activity, and the level of Toll-like receptor 4 (TLR4), which plays a significant role in breast cancer progression and metastasis by leading to the release of pro-inflammatory molecules, such as interleukin 6 (IL-6). Indeed, miR-216a regulates the crosstalk between cancer cells and the cells of the microenvironment, in particular cancer-associated fibroblasts (CAFs), through regulation of the TLR4/IL6 pathway. Thus, miR-216a has an important role in the regulation of stem phenotype, decreasing stem-like properties and affecting the cross-talk between cancer cells and the tumor microenvironment.

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