The good and bad sides of exosomes: pre-metastatic niche formation, cancer biomarker and therapy carriers

Exosomes, nanovesicles of endocytic origin, are secreted by most cell types; cancer cells representing no exception. Exosomes facilitate intercellular communication as they deliver diverse proteins, mRNA, miRNA and lipids. In this review, we discuss how exosomes represent one of the main risks associated with cancer but also one of the most promising new tools to fight it. Exosomes appear to function as signalling molecules between the tumour microenvironment, i.e., the complex of both cancer and stromal cells, and the rest of the body. Cancerderived exosomes have been shown to drive the initiation and progression of metastasis, by transporting their cargoes to target tissues. In this respect, exosomes are implicated in cancer progression, dissemination and therapy resistance. However, exosomes are also emerging as a key tool in precision medicine, pivotal for cancer liquid biopsy in early diagnosis and for assessing when there is a recurrence. Profiling exosomal cancer-derived nucleic acids by ultrasensitive next-generation sequencing along with mapping the protein profile utilizing highthroughput proteomics will allow earlier cancer detection, therapeutic stratification and monitoring of response to therapy. Exosomes are also a promising new tool for cancer immunotherapy. Clinically utilizing exosomes for these applications in cancer diagnosis and therapeutics will be the next challenge.