Cancer exosomes are MicroRNAs factories

We have unravelled for the first time a cell-independent function of exosomes. Exosomes secreted by cancer cells contain the proteins of the RISC complex that are able to process pre-miRNAS into mature functional microRNAs. In this way cancer exosomes, upon secretion to the extracellular environment, become enriched in mature microRNAs associated with the RISC complex and are much more effective in silencing gene expression in recipient cells. Additionally, we have described that processing of pre-miRNAs also occurs in exosomes extracted from the serum of patients with breast cancer, rendering them potential to alter, by horizontal transmission, the transcriptomic landscape of recipient cells.

HFSP Long-Term Fellow Sonia Melo and colleagues
authored on Mon, 19 January 2015

Exosomes are small vesicles that are secreted from all cells and contain DNA, RNA and proteins enclosed in membranes made up of two lipid layers. They are shed into bodily fluids forming a source of disease-specific nucleic acids and proteins. Increasingly, exosomes have been studied for their potential as both indicators of disease, and as prospective new treatment approaches. All exosomes contain a cellular stew of smaller components including proteins, messenger RNA (mRNA) and microRNAs (miRNAs).

HFSP Long-Term Fellow Sonia Melo and colleagues describe in their recent Cancer Cell paper that breast cancer cell exosomes included proteins that process precursor miRNAs. Exosomes from MDA-MB-231 breast cancer cells incubated in cell- and serum-free culture medium showed increased abundance of mature miRNAs and decreased abundance of pre-miRNAs over time whereas the content of exosomes from MCF10A cells (a normal mammary epithelial cell line) was unchanged.  This suggests active miRNA processing in the cancer cell-derived exosomes. Serum exosomes from breast cancer patients showed a similar switch in the pre-to-mature miRNA ratio over time in culture. Exosomes from MDA-MB-231, but not MCF10A cells, contained proteins involved in miRNA biogenesis (including Dicer, Ago2, TRBP: RISC-loading complex proteins). Expressing Flag-tagged Dicer or green fluorescent protein (GFP)–tagged Ago2 in both cell types showed that Dicer and Ago2 were recruited into exosomes only in the cancer cells. MCF10A cells included GFP-tagged exosomes from MDA-MB-231 cells, resulting in global changes in their transcriptome, including a decrease in PTEN and HOXD10 expression, which are associated with breast cancer tumorigenesis. MCF10A cells cultured with the cancer cell-derived exosomes exhibited increased cell viability, proliferation, and colony formation in culture. When co-injected with exosomes from MDA-MB-231 cells or exosomes from breast cancer patient serum, MCF10A cells formed tumors in the mammary fat pads of mice. The serum from mice bearing these tumors or orthotopic grafts of patient breast, ovarian, or endometrial tumors contained exosomes that carried only human (not mouse) Dicer protein.

The findings suggest that exosomes contain the machinery and molecules that allow miRNA-mediated phenotypic reprogramming of target cells.

Reference

Cancer Exosomes Perform Cell-Independent MicroRNA Biogenesis and Promote Tumorigenesis. Melo SA, Sugimoto H, O'Connell JT, Kato N, Villanueva A, Vidal A, Qiu L, Vitkin E, Perelman LT, Melo CA, Lucci A, Ivan C, Calin GA, Kalluri R, Cancer Cell. 2014 Nov 10;26(5):707-21. doi: 10.1016/j.ccell.2014.09.005.

Link Pubmed