Small but mighty: microRNAs as novel signalling molecules in cancer: DOI: 10.14800/rd.627

microRNAs (miRNAs) are short, noncoding RNAs that silence target messenger RNAs by blocking translation or promoting transcript degradation. While the roles of miRNAs within cells have been extensively characterized, emerging evidence suggests that miRNAs are also transported between cells, providing a novel form of intercellular communication. Circulating miRNAs have been identified, packaged in extracellular vesicles or associated with high-density lipoproteins and Argonaute proteins. Specific extracellular miRNAs have been associated with human cancers. They not only serve as measurable disease biomarkers, but recent findings suggest secreted miRNAs may also mediate crosstalk between cancer cells and other cell types, including those that comprise the prometastatic tumor niche. Previous studies, reviewed here, demonstrate that miRNAs released by cancer cells can be internalized by nearby or distant cells, to modify gene expression and alter the tumor microenvironment. As critical drivers of both oncogenesis and metastasis, miRNAs may be attractive therapeutic targets in a wide range of cancers

Tumor-secreted extracellular vesicles promote the activation of cancer-associated fibroblasts via the transfer of microRNA-125b

Tumour cells release large quantities of extracellular vesicles (EVs) to mediate their interactions with other cells in the tumour microenvironment. To identify host cells that naturally take up EVs from tumour cells, we created breast cancer cell lines secreting fluorescent EVs. These fluorescent EVs are taken up most robustly by fibroblasts within the tumour microenvironment. RNA sequencing indicated that miR-125b is one of the most abundant microRNAs secreted by mouse triple-negative breast cancer 4T1 and 4TO7 cells. Treatment with 4T1 EVs leads to an increase in fibroblast activation in isogenic 4TO7 tumours, which is reversed by blocking miR-125b in 4T1 EVs; hence, miR-125b delivery by EVs is responsible for fibroblast activation in mouse tumour models. miR-125b is also secreted by human breast cancer cells and the uptake of EVs from these cells significantly increases cellular levels of miR-125b and expression of multiple cancer-associated fibroblast markers in resident fibroblasts. Overexpression of miR-125b in both mouse and human fibroblasts leads to an activated phenotype similar to the knockdown of established miR-125b target mRNAs. These data indicate that miR-125b is transferred through EVs from breast cancer cells to normal fibroblasts within the tumour microenvironment and contributes to their development into cancer-associated fibroblasts