INTRODUCTION: The majority of cancer fatality is caused by tumour metastasis which is underlined by Epithelial-Mesenchymal-Transition (EMT) at cellular level. Transforming Growth Factor-β (TGF-β) is one of the main molecular driven forces for EMT. Classically intercellular communications are conducted from one molecule to another. However, there are increasing evidences of such communication via extracellular microvesicles or exosomes. How exosomes mediate TGF-β signalling and EMT and cancer metastasis is not known.
METHODS: Exosomes secreted from highly-invasive breast cancer cells MDA-MB-231 and weakly-invasive MCF7 cells as well as fibroblast NIH3T3 were purified by differentially centrifugation. The TGF-β signalling activities of these exosomes were quantified using an adenoviral Smad3 reporter (Ad-CAGA-luc). Exosome secretion inhibitors, DMA (0-100mM) and Wortmannin (0-400nM) as well as Rab27a siRNA were employed to modulate the level of exosome secretion and examine its effect on TGF-β signalling and EMT. Co-culturing fibroblast NIH3T3 cells with MDA-MB-231 or MCF7 cells were used to mimic tumour-stroma interactions mediated by exosomes.
RESULTS: Highly-invasive MDA-MB-231 cells produced much higher amount of exosomes with increased level of TGF-β activity than the weakly-invasive MCF7 cells. Exosomes mediate hyperactivation of TGF-β signalling in both breast cancer cell lines. Exosomes secretion inhibitors or Rab27a knockdown reduced significantly TGF-β responses. Importantly, fibroblast-derived exosomes either directly or in co-culturing transmits enhanced TGF-β signalling in breast cancer cells.
CONCLUSION: Exosomes have been identified as a key mediator of amplified TGF-β signalling in breast cancer cells in an autocrine manner as well as a conductor of intercellular communication between tumour cells and fibroblasts. As such, exosomal TGF-β drives EMT and metastasis. Consequently targeting exosomes may approve to be a new effective therapeutic strategy.