Activation of cancer-associated fibroblasts via the transfer of miR-125b containing extracellular vesicles from tumor cells

Tumor cells secrete elevated amounts of extracellular vesicles (EVs) as a mean of communication between them and other cells in the tumor microenvironment. Here, we established a series of breast cancer cell lines releasing fluorescent EVs to identify the common cell types that receive tumor EVs within the tumor microenvironment. Among the different cell types, cancer-associated fibroblasts consistently take up the most EVs from cancer cells. Using next generation sequencing, we find that miR-125b is among the top enriched miRNAs in EVs released by mouse isogenic triple-negative breast cancer 4T1 and 4TO7 cell lines. Co-injection of 4TO7 cells with 4T1 EVs results in an upregulation of fibroblast activation in vivo while blockage of miR-125b in 4T1 EVs rescues the effect indicating that EV-delivery of miR-125b is crucial for the fibroblast activation in mouse breast cancer models. Moreover, EVs from human breast cancer cells also contain abundant miR-125b and fibroblasts within the xenografted tumor that take up these EVs gain higher levels of miR-125b and upregulation of cancer-associated fibroblast markers. Both mouse and human fibroblasts that are transfected with miR-125b mimics show an activated phenotype similar to the knockdown of established miR-125b targets. In summary, normal fibroblasts within the tumor microenvironment actively take up the miR-125b-containing EVs from breast cancer cells and develop into cancer-associated fibroblasts.