Ral GTPases promote metastasis by controlling biogenesis and organ colonization of exosomes

Cancer extracellular vesicles (EVs) mainly exert pro-tumoral functions by changing the phenotypes of stromal cells to the benefit of tumor growth and metastasis. In particular, they shuttle to distant organs and seed pre-metastatic niches facilitating subsequent colonization by circulating tumor cells. The levels of tumor secreted EVs have been correlated with tumor aggressiveness, however, the link between EV secretion mechanisms and their capacity to form pre-metastatic niches remains obscure. Here, we show that GTPases of the Ral family control, through the phospholipase D1 (PLD1), multi-vesicular bodies homeostasis and thereby tune the biogenesis and secretion of pro-metastatic EVs. Mice experiments revealed that RalA and RalB promote lung metastasis of mammary carcinoma cells without affecting their invasive behaviors. Importantly, we demonstrate that EVs from RalA or RalB depleted cells have limited organotropic capacities in vivo and, as a consequence, are less efficient in seeding lung metastasis. Furthermore, we show that such EVs lack the adhesion molecule MCAM/CD146, which is responsible for EVs organotropism. Finally, we observe that RalA and RalB have increased expression in human breast cancer patients with lung metastasis. Altogether, our study identifies Ral GTPases as central molecules linking the mechanisms of EVs secretion, cargo loading to their capacity to disseminate and induce pre-metastatic niches.