Exosomal miRNAs as cancer biomarkers and therapeutic targets

Abstract

Intercommunication between cancer cells and with their surrounding and distant environments is key to the survival, progression and metastasis of the tumour. Exosomes play a role in this communication process. MicroRNA (miRNA) expression is frequently dysregulated in tumour cells and can be reflected by distinct exosomal miRNA (ex-miRNA) profiles isolated from the bodily fluids of cancer patients. Here, the potential of ex-miRNA as a cancer biomarker and therapeutic target is critically analysed. Exosomes are a stable source of miRNA in bodily fluids but, despite a number of methods for exosome extraction and miRNA quantification, their suitability for diagnostics in a clinical setting is questionable. Furthermore, exosomally transferred miRNAs can alter the behaviour of recipient tumour and stromal cells to promote oncogenesis, highlighting a role in cell communication in cancer. However, our incomplete understanding of exosome biogenesis and miRNA loading mechanisms means that strategies to target exosomes or their transferred miRNAs are limited and not specific to tumour cells. Therefore, if ex-miRNA is to be employed in novel non-invasive diagnostic approaches and as a therapeutic target in cancer, two further advances are necessary: in methods to isolate and detect ex-miRNA, and a better understanding of their biogenesis and functions in tumour-cell communication.

Keywords: cell communication; exosome isolation; exosomes; microRNA; oncogenesis; tumour microenvironment.

Fig. 1

miRNA biogenesis and loading into exosomes. miRNA genes are transcribed by RNA polymerase II (Pol II), forming pri-miRNAs in the nucleus. The Drosha complex cleaves pri-RNA to pre-miRNA, which is exported to the cytoplasm via exportin 5. Further cleavage by the Dicer complex generates an intermediary miRNA duplex, of which one strand is incorporated into the RNA-induced silencing complex (RISC) to form mature miRNA, which targets complementary mRNA for translational repression. Inward budding of the early and late endosome forms exosomes. During this process, mature miRNA, some pre-miRNAs and other RNA molecules, proteins, and lipids are loaded into the exosomes. Within exosomes, loaded pre-miRNAs may be processed into mature miRNA. The exosome-loading process involves an endosomal-sorting complex required for transport (ESCRT) or ceramide-dependent mechanisms. The fusion of multivesicular bodies (MVBs) with the plasma membrane releases exosomes. This process is dependent on Rab GTPases (e.g. Rab27). The exosomal fusion with the plasma membrane of the recipient cell, or phagocytosis followed by membrane fusion, leads to the release of miRNA cargo into the cytosol and translational repression.

Fig. 2

Exosome isolation methods. Example procedures and comparison of 4 commonly used methods, which are ultracentrifugation, ExoQuick precipitation, immunoaffinity pulldown/magnetic-activated cell sorting using anti-EpCAM (MACS) and OptiPrep density gradient to isolate exosomes from a plasma sample. These are compared in terms of purity, exosome yield, ease of use, approximate isolation time, approximate hands-on time and approximate cost per sample. *=low, **=moderate, ***=high, ****=very high. The relative assessment of procedures for exosome purity, yield, ease of use, isolation time, hands-on time and cost are adapted from Van Deun et al. (22).

Fig. 3

Role of exosomes in tumour-related pathways. Through the transfer of oncogenic signals, exosomes promote tumorigenesis through interaction with local and distant cells. The intra-tumour transfer of exosomes can facilitate growth and invasiveness, as well as confer chemoresistance. The exosome-mediated activation of fibroblasts facilitates the remodelling of the microenvironment and angiogenesis to enhance growth and invasion of cancer. The direct transfer of tumour-derived exosomes to vascular endothelial cells can promote angiogenesis. Tumour-derived exosomes can also modulate immune function to increase activity of tumour-promoting immune cells and suppress anti-tumour cells. The activation of these and other pathways in distant sites generates a pre-metastatic niche that facilitates metastasis.