Exosomes: Key mediators of metastasis and pre-metastatic niche formation.

While tumour cells are classically known to communicate via direct cell-to-cell contact and the secretion of soluble protein-based factors such as cytokines and growth factors, alternative novel mechanisms that promote tumour progression have recently emerged. Now, new critical components of the secretome thought to be involved in tumour progression are exosomes, small vesicles of endocytic origin that carry a variety of bioactive molecules, including proteins, lipids, RNA, as well as DNA molecules. Cancer cell-derived exosomes have been shown to participate in crucial steps of metastatic spread of a primary tumour, ranging from oncogenic reprogramming of malignant cells to formation of pre-metastatic niches. These effects are achieved through the mediation of intercellular cross-talk and subsequent modification of both local and distant microenvironments in an autocrine and paracrine fashion. Here, we summarise the recent findings that implicate this non-canonical signalling within the tumour as a critical driver of metastatic disease progression, and discuss how understanding the molecular mechanisms involved in exosome-mediated metastasis is of great value for the development of new therapeutic strategies to prevent cancer progression.