Fibroblast exosomes induce stromal matrix assembly and breast tumor growth and metastasis

Stromal matrix assembly is a key feature of aggressive tumors and is associated with poor prognosis of breast cancer patients. Fibronectin is a key stromal matrix molecule whose assembly into fibrils is thought to require cells and which serves as a template for stromal matrix assembly. Fibroblasts are the major cell type that secretes and assembles fibronectin and stromal matrix. Here, we identify that small exosome-type small EVs secreted by fibroblasts are critical initiators of fibronectin assembly, revealing a previously unrecognized mechanism of stromal matrix formation. Fibroblasts engineered to be deficient in exosome secretion showed greatly reduced assembly of fibronectin and other stromal matrix molecules in 2D, 3D, and in vivo environments, and led to reduced tumor growth and lung metastasis by triple-negative breast cancer cells. Furthermore, transgenic mice with defects in exosome secretion had greatly reduced lung fibrosis after treatment with bleomycin. In a direct test of exosome function on fibronectin assembly, we find that the addition of purified small EVs to purified soluble fibronectin in a cell-free system is sufficient to induce fibronectin assembly. The EV-induced fibronectin assembly requires the presence of fibronectin-binding integrins and syndecan-1 in the EVs. We propose a new model in which secreted exosomes directly drive stromal matrix assembly and tissue fibrosis.