The Role of Extracellular Vesicles in the Control of Vascular Checkpoints for Cancer Metastasis

Systemic cancer progression culminating in metastatic disease is implicitly dependent on tumour cell interactions with the vascular system. Indeed, different facets of the micro- and macro-vasculature can be regarded as rate-limiting 'vascular checkpoints' in the process of cancer dissemination. The underlying complex communication networks drive tumour neovascularization, angiogenesis, immunoregulation, activation of the coagulation system, angiocrine interactions, and non-angiogenic vascular responses across multiple cancer types. Yet, each cancer may represent a unique vascular interaction scenario raising a prospect of targeted modulation of blood and lymphatic vessels for therapeutic purposes, beyond the traditional notion of tumour anti-angiogenesis. While the emphasis of studies aiming to understand this circuitry has traditionally been on soluble, or 'mono-molecular' mediators, the rise of the particulate secretome encompassing heterogeneous subpopulations of extracellular vesicles (EVs; including exosomes) and particles (EPs) brings another dimension into the tumour-vascular communication web during the process of metastasis. EVs and EPs are nanosized cellular fragments, the unique nature of which lies in their ability to encapsulate, protect and deliver to target cells a range of bioactive molecular entities (proteins, RNA, DNA) assembled in ways that enable them to exert a wide spectrum of biological activities. EVs and EPs penetrate through biological barriers and are capable of intracellular uptake. Their emerging vascular functions in metastatic or infiltrative cancers are exemplified by their roles in pre-metastatic niche formation, thrombosis, vasectasia or angiocrine regulation of cancer stem cells. Here, we survey some of the related evidence supporting the biological, diagnostic and interventional significance of EVs/EPs (EVPs) in disseminated neoplastic disease.