microRNA exchange via extracellular vesicles in cancer

Abstract Cells utilize different means of inter‐cellular communication to function properly. Here, we review the crosstalk between cancer cells and their surrounding environment through microRNA (miRNA)‐containing extracellular vesicles (EVs). The current findings suggest that the export of miRNAs and uptake of miRNA‐containing EVs might be an active process. As post‐transcriptional regulators of gene expression, cancer‐derived miRNAs that are taken up by normal cells can change the translational profile of the recipient cell towards a transformed proteome. Stromal cells can also deliver miRNAs via EVs to cancer cells to support tumour growth and cancer progression. Therefore, gaining a better understanding of EV‐mediated inter‐cellular communication in the tumour microenvironment might lead to the development of novel diagnostic and therapeutic strategies.

by binding (either precisely or imprecisely) to the 3' untranslated region (3'UTR) of their target mRNA, which subsequently leads to translational inhibition and/or target mRNA degradation in the cytosol. In mammals, most protein-coding genes contain at least one miRNA-binding site. 7 Therefore, miRNA-mediated gene regulation is involved in many important processes including embryonic development, the cell cycle and metabolism.
The expression of miRNA is tightly regulated. 6 After being transcribed by RNA polymerase II, miRNA primary transcripts (pri-miR-NAs), which encode one or multiple miRNAs, are further processed via 5'-capping, splicing and polyadenylation. Pri-miRNAs can reach several kilobases in length and they contain stem-loop structures, which are the target excision sites for the nuclear RNase III, Drosha.
After cleavage by Drosha, the hairpins (pre-miRNAs) generated are translocated into the cytosol by exportin-5. The pre-miRNAs then have their terminal loop removed by Dicer, and this results in the formation of double-stranded (ds) RNAs of ~22 bp. The ds-miRNAs are subsequently loaded onto Argonaute (Ago) protein to form a RISC. Ago selects one strand to be a guide (miRNA) and it removes the other (passenger strand or miRNA*) in order to form a functional complex. 8 Although miRNAs are mainly localized in the cytosol, they are also secreted in extracellular complexes such as EVs. Cell-free miR-NAs have been identified in different body fluids such as serum/ plasma, 9,10 breast milk, 11 colostrum, 12 saliva, 13 tears, 14 urine, 15 seminal fluid, 16 amniotic fluid, 17 pleural fluid, 18 bronchoalveolar lavage fluid, 19 gastric juice, 20 peritoneal fluid 21 and cerebrospinal fluid. 22 Moreover, the abnormal expression of miRNAs is indicative of various pathological disorders. 23 For example, aberrant levels of specific circulating miRNAs correlate with diseases such as cancer, 24 diabetes, 25 cardiovascular diseases, 26 muscular disorders 27 and neurodegenerative diseases, 28 as well as toxic and drug-induced organ damage. 29,30 For this reason, many studies have been conducted to determine how circulating miRNAs might be utilized as novel biomarkers for these diseases. Interestingly, extracellular miRNAs can be taken up by and released inside the cytosol of recipient cells, which suggests that they might play an important role in inter-cellular communication. 3 In this review, we will discuss the current knowledge regarding the role of EV-associated miRNAs as mediators of the cell-to-cell communication that occurs in cancer.

| miRNA S ECRE TI ON
Stable circulating miRNAs have been found to complex with lipids, 31 proteins 32 and secreted vesicles, 33 which suggests that there is more than one mechanism of miRNA secretion. In addition to a passive leakage due to cellular injury (ie necrosis), various different pathways have been proposed to explain the secretion of miRNAs into the circulation. 34 For example, high-density lipoprotein (HDL) and other proteins can serve as carriers for different groups of miRNAs. 31,32 However, the mechanism by which miRNAs are selected for HDL secretion is still unknown. It is known that miR-16, miR-92a, miR-122, miR-142-3p and miR-150 in complex with AGO2 can all stably circulate in the blood stream. 32 However, the miRNA secretion pathway where most is known is the export of miRNAs via EVs.
Two processes govern the presence of a specific miRNA in EVs; these are loading and sorting. Loading refers to mechanisms in which EV cargoes are recruited to the site of vesicle formation, whereas sorting is an active selection process in which some specific miRNAs are exclusively destined for secretion. These processes happen simultaneously following vesicle biogenesis.

| miRNA secretion into exosomes
Various attempts have been made to analyse the loading and sorting of miRNA into exosomes. Exosome biogenesis includes either the recruitment of the endosomal sorting complex required for transport system (ESCRT-dependent) or the synthesis of ceramide (ESCRTindependent). 4 miRNA loading is more likely to be involved in the ESCRT-independent pathway. This is because downregulation of neutral sphingomyelinase 2 (nSMase2), a key regulator of ceramide synthesis, leads to decreased miRNA secretion, whereas blocking the ESCRT has little effect on miRNA secretion ( Figure 1). 35 Some miRNAs are known to be selectively sorted into EVs, although the exact mechanisms are mostly unknown. Villarroya-Beltri et al 36  Wild-type and mutant KRAS regulate the secretion of different miR-NAs into colorectal cancer exosomes ( Figure 1). 38 In addition, the RNA-binding protein called Y-box protein 1 has been shown to selectively sort miR-223 into exosomes in HEK-293T cells (Figure 1). 39 Furthermore, Lupus La protein binds to miR-122 at both ends, to target this miRNA for secretion in exosomes of human triple-negative breast cancer MD-MBA231 cells (Figure 1). 40 Together, these findings suggest that there is more than one mechanism for miRNA sorting. However, it is unclear whether the loading of miRNAs is sequence specific although some findings have suggested that the sorting of selective miRNAs is based on motif recognition.  43 These findings reveal that the presence and quantity of EV-derived miRNAs are dynamic and dependent on the response of the mother cell to both internal and external signals.
However, much work is still required to uncover the biological implications of the fluctuation of EV-derived miRNAs.

| miRNA secretion into microvesicles
Not much is known about the loading and sorting of miRNAs into microvesicles. Although the biogenesis of microvesicles and exosomes is regulated by overlapping sets of molecules, the former still utilizes a distinct collection of regulatory proteins. 44 In the biogenesis of microvesicles, annexin A1 and A2, which are calcium ion-(Ca 2+ -) and membrane-binding proteins, play an important role in forming a curved structure on the membrane for blebbing and folding. 45 Indeed, using a combination of high-resolution density gradient fractionation and immunoaffinity methods to purify and classify EVs, Jeppesen et al 46 identified annexin A1 and A2 as exclusive markers of microvesicles. Of note, these members of the annexin family are also RNA-binding proteins.
Annexin A2 is also reported to regulate the levels of miRNAs in EVs ( Figure 1). 47 It both facilitates the loading of miRNAs into EVs and also selectively sorts a number of miRNAs, including miR-16, miR-21, miR-24, miR-29a, miR-100, miR-125, let-7a and let-7b. Recent findings show that a post-translational isoform of annexin A2 partially colocalizes with processing (P-) bodies where the RISC machineries are recruited. 48 This might be a clue for the functional delivery of EV-associated miRNAs, such that annexin A2 is modified and targets miRNAs to the P-bodies in recipient cells.

| UP TAK E OF miRNA-CONTAINING E Vs
Upon secretion, EVs are taken up by their parental cells or by other (either adjacent or distant) cells via autocrine or paracrine signalling. 49 Although much progress has been made to elucidate the mechanisms of miRNA uptake by EVs and other carriers, the EV uptake mechanism is still not fully understood. Since studying exosomes and microvesicles separately is complicated, we will continue to use the general term "EVs" for the following section.

| Uptake of EVs by recipient cells
Since EVs are carriers of proteins and other polynucleotides as well as miRNAs, the endocytic pathway is likely to be common for all EVs, whatever they carry. EVs are taken up into cells by a number of different mechanisms, including endocytosis, phagocytosis, macropinocytosis, and plasma membrane fusion, all of which utilize different sets of proteins for binding and internalization. 50,51 Therefore, it is likely that the uptake of EVs is dependent on surface proteins on the recipient cells as well as ligands on the EVs (Figure 2). Indeed, F I G U R E 1 Incorporation of miRNAs into EVs. Different mechanisms govern the transfer of miRNAs from the cytosol into EVs. Several RNA-binding proteins have been found to direct specific miRNAs to the formation sites of the ILVs on the endosome and the microvesicles on the cell membrane. EVs, extracellular vesicles; ILVs, intraluminal vesicles several studies have demonstrated the differential uptake of EVs by different types of recipient cells. For example, it has been shown that exosomes expressing the Tspan8-alpha4 complex on their surface most likely bind to CD54 on the membrane of endothelial and pancreatic cells (Figure 2). 52 Interestingly, the integrin signatures on the surface of exosomes have been shown to determine metastatic organotropism in different organs ( Figure 2). 53 For example, using labelled exosomes, Hoshino et al 53 discovered a novel homing pattern of tumour EVs with different combinations of integrin molecules.
Specifically, integrin α6β4-and α6β1-positive exosomes were shown to be endocytosed more by lung epithelial cells, whereas αvβ5positive exosomes are more liver-tropic. In another study, activated T cells were reported to capture major histocompatibility complex (MHC) class II-containing exosomes secreted by dendritic cells. 54 This process was shown to be regulated in a dose-dependent manner by leucocyte function-associated antigen-1 on the EV surface. 54 Glycosylation also plays a role in the specific uptake of EVs. For example, the uptake of EVs by recipient cells is, in some cases, regulated by D-mannose, D-glucosamine and β-galactosides, and the underlying mechanism involves sugar-binding proteins. 55,56 It has F I G U R E 2 EV uptake and release of miRNAs into the cytosol of recipient cell. Proteins and sugar chains on EV surface can direct the selective binding of EVs to a specific cell. EV-derived miRNAs are released into the cytosol by either direct fusion of the EVs to the cell membrane or the endosomal-limiting membrane or different mechanisms. EVs, extracellular vesicles been shown that sialyl-lactose-containing EVs, which are secreted by Jurkat cells, bind to (and are captured by) dendritic cells via sialic acid-binding Ig-like lectin 1 (Siglec-1) on the latter (Figure 2). 57 In addition, Saunderson et al 58 reported that B cell-derived EVs are taken up by lymph node macrophages due to the binding of α2,3linked sialic acids on the former to the sugar-binding protein, sialo-adhesin (CD169), on the latter (Figure 2). 59 These findings confirm the hypothesis that the uptake of EVs is neither random nor passive, but rather is regulated by a complex series of pathways. Of note, the specificity of EV uptake leads to another question, which is are different sets of miRNAs selectively secreted into different populations of EVs with specific uptake tropism?

| Release of EV-derived miRNAs into the cytosol
After internalization into the cell, most EVs remain in endosomes. It is therefore challenging for EV-derived miRNAs to get into the cytosol so that they can be loaded onto RISCs and perform their physiological role. Montecalvo et al 58 proposed a functional transfer of miRNAs by direct fusion with the plasma membrane, but also reported that EV cargoes are released to the cytoplasm by back fusion of EVs to the endosomal-limiting membrane (Figure 2). 58 Direct fusion with the plasma membrane is also reported by other groups via a process that is pH dependent. 60 However, the detailed mechanism for EV-plasma membrane fusion is not well understood, although it has been suggested that common intracellular vesicle fusion protein families such as SNAREs, Rab proteins and Sec1/Munc-18 might play a role. 61 Little is known about the fate of EVs after being endocytosed.
However, Heusermann et al 62 traced the uptake of CD63+ exosomes and found that they surf on filopodia to enter recipient cells and shuffle in cytoplasm endosomes. 62 Once these labelled exosomes enter a cell, they follow the usual endosomal trafficking routes that lead to degradation in lysosomes ( Figure 2). Interestingly, around 90% of the labelled vesicles move into close association with the endoplasmic reticulum (ER) before sorted into lysosomes. 62 In human primary fibroblasts, within 48 hours of internalization, 60% of the labelled exosomes are colocalized with lysosomes. 62 The research group proposed the hypothesis that EVs are in contact with ER for efficient release of RNA contents near the local translation machineries. 62 However, when EVs are labelled with a generic lipid dye or the membrane proteins are labelled with a fluorescent tag, the release of EV cargo is rarely observed. 63 Therefore, labelling both the EV contents and membrane, and tracing the end point of EV uptake are all essential for determining the fate of EVs in the cell. Once this mechanism has been revealed, the fate of EVs might be controlled and manipulated for therapeutic purposes.

| Loading of EV-derived miRNAs into RISC of recipient cells
The functionality of EV-derived miRNAs in recipient cells is another key question in the field. In order to perform translational silencing, single-stranded miRNAs must bind to AGO2 and other proteins to form the RISC complex during their maturation. 6 It is unclear whether single-stranded free miRNAs released from EVs are active in recipient cells since they might interact less efficiently with AGO2, when compared with pre-miRNAs. 64 Interestingly, annexin A1 is also capable of binding to both DNA and RNA upon formation of an A1-S100 heterotetramer complex; this suggests an additional possible role for annexin A1 in regulating RNA sorting in EVs. 65 In an attempt to elucidate the underlying mechanism, Melo et al 66 reported that the miRNAs carried in exosomes undergo maturation en route to the recipient cells as pre-miRNAs, Dicer, AGO2 and TRBP are packaged altogether in the EVs. With this cell-independent maturation, miRNAs are thus readily functional upon their release to the cytosol. However, this mechanism has not been confirmed as some other studies could not show the ubiquitous presence of Dicer and AGO2 in EVs when purified using gradient centrifugation. 39,40,46 It has also been suggested that in some cases, mature miRNAs might change the phenotype of their recipient cells without being pre-incorporated onto the RISC complex. 67 Moreover, the selective sorting of miRNAs into exosomes often results in them forming complexes with proteins other than AGO. 39,40 However, the process of switching the miRNA from its carrier protein to AGO in recipient cells is also unknown. Therefore, further investigations are needed to clarify the processing of miRNAs after their release from EVs.

| miRNA E XCHANG E A S INTER-CELLUL AR COMMUNIC ATION MECHANIS M IN C AN CER
With the ability to suppress the expression of multiple target mRNAs, miRNAs transferred via EVs support tumour growth at each stage of cancer progression. This has become a robust means of communication between cancer cells and the tumour microenvironment (TME).
Here, we summarize our current understanding of the mutual communication both among cancer cells and between cancer cells and other cells of the TME.

| The effects of miRNA-containing EVs from cancer cells in the TME
Fibroblast activation is a key event in the growth of primary tumours. 68 Our recent study revealed that breast cancer-derived EVs contain high levels of miR-125b, which is taken up by fibroblasts situated adjacent to the primary tumour (Figure 3). 69 This cellular increase in miR-125b suppresses the expression of TP53 and TP53INP1, and the fibroblasts differentiate into cancer-associated fibroblasts (CAFs). A similar mechanism is observed in pancreatic cancer-derived microvesicles, which contain an elevated level of miR-155 (Figure 3). 70 Similarly, melanoma-derived exosomes (melanosomes) containing miR-211 are able to promote the formation of CAFs by activating the mitogen-activated protein kinase signalling pathway (Figure 3). 71 Moreover, lung adenocarcinoma cancer-derived exosomal miR-142 has been shown to promote the differentiation of WI-38 and IMR90 lung fibroblasts into CAFs via a TGF-β-independent pathway (Figure 3). 72 Evading the immune system is a hallmark of cancer. 73 Cancerderived exosomal miRNAs have been shown to regulate and suppress immune cells in order to support cancer growth. For example, Mutp53 colon cancer cell-derived exosomal miR-1246 reprograms M2 macrophages into tumour-associated macrophages (TAMs) (Figure 3). 74 As a consequence, TAMs produce IL-10, TGF-β and matrix metalloproteinases (MMPs) to enhance tumour growth and progression. Similarly, exosomal miR-940, which is derived from hypoxic epithelial ovarian cancer (EOC) SKOV3 cells, reprogrammes U937 macrophages into the tumour promoting M2 subtype, which in turn promotes the proliferation of EOC cells (Figure 3). 75 EOC-derived exosomal miR-222 has also been shown to promote M2 macrophage polarization, and it achieved this by regulating the SOCS3/ STAT3 signalling pathway (Figure 3). 76 Again, the M2 TAM-like macrophages enhanced EOC growth.
In addition to their canonical function, miRNAs might also exert novel functions in recipient cells. For instance, miR-21 and miR-29a in Lewis lung carcinoma-derived exosomes promote lung tumour multiplicities in B6 mice by binding and activating Toll-like receptor 7 (TLR7) in the surrounding immune cells, especially macrophages ( Figure 3). 77 This results in the activation of nuclear factor κB (NF-κB) and the secretion of pro-inflammatory cytokines TNF-α and IL-6, which together promote tumour growth. Similarly, liposarcoma Lipo246 exosome-derived miR-25 and miR-92a can induce secretion of IL-6 in surrounding TAMs by regulating NF-κB signalling via TLR7/8 (Figure 3). 78 Dendritic cells also play a part in the tumour modulation of the immune system. 79 For example, miR-203-containing exosomes from pancreatic cancer cells inhibit the expression of TLR4 in recipient dendritic cells, and downregulate the production of TNF-α and IL-12, thereby repressing the usual anti-tumour immune response ( Figure 3). 80 Moreover, pancreatic cancer cell PANC-1-derived exosomal miR-212 modulates regulatory factor X-associated protein expression (Figure 3). 81 This leads to a decrease in the expression of MHC-II and thus induces immune tolerance of dendritic cells.
In addition to macrophages and dendritic cells, natural killer (NK) cells are also critical effectors in the innate immune response. 82 Berchem et al 83 showed that tumour-derived exosomal miR-23a F I G U R E 3 The roles of EV-delivered miRNAs in the TME. EV-derived miRNAs from cancer cells mediate cells of the TME into tumour supportive phenotypes. Transformed cells of the TME, in return, secrete EVs containing miRNAs that support tumour growth and progression. EVs, extracellular vesicles; TME, tumour microenvironment modulates the immune response by impairing NK cytotoxicity and function (Figure 3). They demonstrated that when miR-23a and TGF-β were delivered by microvesicles isolated from hypoxic IGR-Heu, K562 and T1 tumour cells into MK-92 cells, then the expression of CD107a/LAMP1 was reduced and the number of NKG2D activator surface receptors was decreased in these cells.

miRNA-containing EVs from tumour cells also modulate and sup-
press T-lymphocyte function. The delivery of miR-214 from Lewis lung cancer cell-derived exosomes to recipient T cells was shown to significantly downregulate PTEN and promote a regulatory T cell (Treg) phenotype (Figure 3). 84 These Tregs in turn secret IL-10, which promotes tumour growth.

reported that
HepG2-derived exosomal miR-23a downregulates SIRT1 in recipient endothelial cells when promoting angiogenesis (Figure 3). In addition, exosomal miR-21 derived from cigarette smoke extract-transformed human bronchial epithelial cells enhances angiogenesis of HUVECs by increasing the levels of VEGF (Figure 3). 98 Furthermore, exosomal miR-142 derived from lung adenocarcinoma, which (as mentioned above) promotes the differentiation of lung fibroblasts into CAFs, also induces tube formation in HMEC-1 endothelial cells by targeting TGFβR1 (Figure 3). 72 In order to undergo metastasis, cancer cells must disrupt the vascular barrier to infiltrate the circulation system. 73 miR-105 targets the mRNA encoding the tight junction protein ZO-1 and in this way disrupts the vascular barrier and facilitates the metastasis of cancer cells (Figure 3). 99 Creating a niche is another premise for metastasis. MCF10A breast cancer cells secrete EVs containing miR-122. This leads to downregulation of the expression of the glycolytic enzyme pyruvate kinase (PKM2) and GLUT1 genes in recipient lung fibroblasts and brain astrocytes in a pre-metastatic niche, resulting in decreased glucose uptake and metabolism in these cells and thereby promoting breast cancer metastasis. 100 In addition, colorectal cancer-derived exosomal miR-21 binds to TLR7 in liver macrophages (Figure 3). 101 Through the TLR7 signalling pathway, liver macrophages polarize to a pro-inflammatory phenotype and produce cytokines such as IL-6, thus generating a pre-metastatic niche for colorectal cancer in the liver. Moreover, miR-203 containing exosomes released from colorectal cancer cells induces the differentiation of monocytes into M2 macrophages in vivo and thus promotes liver metastasis (Figure 3). 102 Furthermore, exosomal miR-25 from SW480 colorectal cancer cells enhances angiogenesis at pre-metastatic niches in endothelial cells, by silencing KLF2 and KLF4, which restricts the expression of tight junction related proteins such as VEGFR2, ZO-1, occludin and claudin-5 (Figure 3). 103 These findings demonstrate that the transfer of miRNAs from cancer cells to cells of the TME is an active and dynamic process.

| Stromal-derived miRNAs
The TME also communicates with cancer cells via miRNAs in EVs. For example, upon activation of ovarian CAFs and cancer-associated adipocytes, miR-21-containing EVs are secreted, and these target apoptotic protease activating factor 1 (APAF1) to promote chemoresistance and suppress apoptosis of recipient cancer cells (Figure 3). 104 Neuroblastoma cells also secrete exosomal miR-21, which upregulates miR-155 levels in TAM and TAM-derived exosomes (Figure 3). 105 The latter then carry miR-155 to neuroblastoma cells where they downregulate telomeric repeat binding factor 1, thereby enhancing neuroblastoma chemoresistance. TAMs also carry miR-223 to breast cancer cells and enhance cancer invasion via the Mef2c-β-catenin pathway (Figure 3). 106 In addition, M2 subtype TAMs secrete exosomal miR-21, which regulates the PTEN/ PI3K/AKT signalling pathway in gastric cancer cells, thereby enhancing cisplatin resistance (Figure 3). 107 Binenbaum et al 108 demonstrated that TAM-derived exosomal miR-365 can induce both in vivo and in vitro chemoresistance in K989 pancreatic adenocarcinoma cells by upregulating triphosphate nucleotide (NTP) (Figure 3). 108 Increased levels of NTP induce the expression of cytidine deaminase, which can metabolize the chemotherapeutic drug, gemcitabine, to its inactive form. Aggressive properties such as the stemness, endothelial-to-mesenchymal transition, anchorage-independent growth and invasive capacity of T47D, BT549 and MDA-MB-231 breast cancer cells are all enhanced by breast CAF-secreted exosomal miR-21, miR-143, and miR-378e (Figure 3). 109 In addition, exosomal miR-21 secreted by hypoxic mesenchymal stem cells is reported to promote NSCLC A549 cell proliferation and mobility by inhibiting the expression of PTEN, PDCD4 and RECK genes. 110 The expression of PTEN is also downregulated when brain astrocyte-derived exosomes transfer miR-19a to melanoma B16BL6 cells, and subsequently recruit myeloid cells to promote brain metastatic tumour cell growth. 111

| miRNAs transfer between cancer cells
As well as modulating the surrounding cells, cancer cells can also exchange EVs to enhance their progression, especially their metastatic capability via miRNAs. 67 For example, when poorly metastatic mouse breast cancer cells take up EVs from highly metastatic isogenic cells, they become more metastatic. The underlying mechanism involves the enrichment of miR-200 family miRNAs exclusively in the highly metastatic breast cancer cells and their secreted EVs (Figure 3). miR-200s target Zeb2 to drive the recipient cells towards a mesenchymal-to-epithelial transition, and this facilitates the colonization of poorly metastatic cells at distant sites. 67 The same phenomenon has also been observed in MB-231 breast cancer cells, which exhibit enhanced lung colonization in mice models after being incubated with EVs from MCF-10CA1a cells.
Together, these findings demonstrate that communication be-

| CON CLUS I ON S AND FUTURE PER S PEC TIVE S
Further investigations are still required to fully understand the crosstalk between cancer cells and their microenvironment via EVs. Among the various cargos (ie proteins and other RNAs), miRNAs play an important role as signalling molecules. However, not enough research has been conducted to determine the synergistic contribution of different molecules on the signal transduction between cancer cells and the microenvironment. As RNA-profiling, proteome analysis and bioinformatics are advancing at a rapid rate, the role of EV-miRNAs can be elucidated further through a comparison of the EV contents and network mapping of the genome and proteome.
Additional problems in the field include both the nomenclature and the purification of EVs. 112 Currently, EVs are classified by their biogenesis. However, experiments often rely on the size and a handful of surface molecules to distinguish EVs. Different publications have shown that the use of such markers cannot clearly distinguish between different types of EVs. 46 Thus, it is necessary to look for novel markers or combinations of molecules that clearly identify the diverse reservoir of EVs released by cancer cells. Identification of such markers will also facilitate the purification of EVs, as currently this also varies widely between publications.
Different mechanisms might govern the export of miRNAs.
However, it is still unclear which pathways contribute the most to the level of circulating miRNAs for each cancer type. Elucidating the export mechanism of miRNAs would help to support their application as biomarkers for cancer. This would be highly advantageous as the detection of miRNAs is both fast and robust, and thus, they would be a valuable tool for cancer diagnosis and prognosis.

ACK N OWLED G EM ENTS
The authors received funding from the National University of

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analysed in this study. Exosome-mediated transfer of mRNAs and microRNAs is a novel