The past decade has witnessed an exponential increase in research on exosomes. For many years considered to be extracellular debris, exosomes are now considered important mediators in intercellular communication. The capability of exosomes to transfer proteins, DNA, mRNA, as well as non-coding RNAs has made them an attractive focus of research into the pathogenesis of different diseases, including cancer. Increasing evidence suggests that tumor cells release a large sum of exosomes, which may not only influence proximal tumor cells and stromal cells in local microenvironment, but also can exert systemic effects when participating in blood circulation. In this study, we review the current understanding on this topic. The literature outlines two broad facets of exosomes in cancer: 1) promotion of tumor growth, tumorigenesis, tumor angiogenesis, tumor immune escape, drug resistance, and metastasis and 2) their role as promising biomarkers for cancer diagnosis and even as potential treatment targets for cancer patients.
In recent years, the function of exosomes has evoked increased interest, particularly in cancer research. Described to harbor and deliver functional molecules to recipient cells, exosomes seemingly add a new layer of complexity to our understanding of molecular biology in multicellular organisms.
The definition of exosomes has evolved over time; the term was coined by Trams et al to describe the release of extracellular vesicles (EVs) with 5′-nucleotidase activity from various normal and neoplastic cell lines (1). In the late 1980s, the usage of the term was adapted to describe small vesicles of endosomal origin that are released during reticulocyte differentiation following the fusion of multivesicular bodies (MVBs) with the plasma membrane (2). Initially ascribed a cell-scavenging function, exosomes are now known to perform a much wider range of biological functions (3–5). However, since exosomes are difficult to discriminate and purify from EVs of other origins, several EV-induced functions reported in the literature may not necessarily be attributable to exosomes alone.
Exosomes are small, lipid bilayer membrane vesicles (30–100 nm) derived from the luminal membrane of multivesicular bodies (MVBs), which are constitutively released by fusion with the cell membrane (6–10). In this review, we highlight the recent advances in our understanding of the biological and functional role of exosomes in cancer, with an emphasis on the potential use of exosomes as biomarkers and therapeutic agents.
Biogenesis, secretion, and uptake of exosomes
Biogenesis of exosomes initiates as an endocytic event at the plasma membrane. The mechanisms by which exosomes sort their cargo into the MVBs are not yet fully understood. The best-described mechanism for exosome biogenesis is driven by the endosomal sorting complex required for transport (ESCRT) and the multimolecular machinery is recruited to the endosomal membrane where the individual steps of the exosome biogenesis are orchestrated (11). The second pathway of MVB formation is independent of the ESCRT machinery and is based on the specific lipid composition of the endosomal membrane. It has been reported that ceramides were involved in exosomes biogenesis (12,13). Moreover, formation of MVBs has also been shown to be controlled by the syndecan heparin sulfate proteoglycans and their cytoplasmic adaptor syntenin (14). Following the formation of MVBs, Rab GTPases govern their degradation as well as their secretion (13,15). The final release of exosomes occurs upon fusion of MVBs with the cellular plasma membrane, a process which is probably mediated, at least in part, by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) (16). Furthermore, there is evidence that enhanced expression of p53, Pyruvate kinase type M2 (PKM2) and tumor suppressor-activated pathway 6 (TSAP6) upregulates the secretion of exosomes in tumor cells (17–19). Of note, the accumulation of intracellular Ca+ and change in microenvironmental pH have been shown to affect the secretion of exosomes (20,21).
Cells appear to take up exosomes by several molecular mechanisms. Most experimental evidence suggested that exosomes are internalized into recipient cells via endocytosis (22,23). Endocytosis is a general statement for a range of molecular pathways, including calthrin-mediated endocytosis, coveolin-mediated endocytosis, phagocytosis and macropinocytosis (24–26). Moreover, the uptake of exosomes by direct cell surface membrane fusion has also been studied. Palolini et al demonstrate that at least a part of exosomes are able to fuse with the recipient cells directly and low pH facilitates this process (21). Additionally, there is also evidence to support the role of lipid rafts in exosome internalization (27,28). Noteworthy, several research groups have shown that the uptake of exosomes by recipient cells is an energy-dependent process. For instance, the internalization of exosomes by ovarian cancer cells via several endocytic pathways were strongly inhibited at 4°C (24). Fig. 1 summarizes the complicated process of biogenesis, release, content and uptake of exosomes.
Contents of exosomes
During the biogenesis of exosomes and prior to their secretion, various molecules are uploaded into the lumen of exosomes. Exosomes contain several types of biomolecules, including lipids, proteins and nucleic acids (6) (Fig. 1). The exosomal content is heterogeneous and in a dynamic state, depending on the cell's origin, its physiological and pathological state, and even on the cellular release site (29).
When comparing exosomes with the total cell membranes, several studies observed that exosomes have a higher expression of sphingomyelin, cholesterol, phosphatidylserine, and generally of saturated fatty acids (6). The protein content in exosomes includes endosomal, plasma, and nuclear proteins (30). Proteins enriched in exosomes include those relevant for individual exosomal biogenesis pathways and for exosome secretion (31). Factors found in exosomes of different cell types include: TSG101, Alix, Rab GTPases, heat shock proteins (HSP70, HSP90), integrins, tetraspanins (CD9, CD63, CD81) and MHC class II proteins (6). A number of reviews have described the protein and lipid composition of exosomes, and various databases, including ExoCarta (http://www.exocarta.org/) and Vesiclepedia (http://microvesicle.org/), have cataloged the protein, lipid, and RNA content of exosomes (32,33). In addition, exosomes can contain genetic material such as mRNA, long noncoding RNA (lncRNA), microRNA (miRNA) and even double-stranded DNA (34–36). Moreover, the composition of exosomes can be different from the cells of their origin due to the selective sorting of cargo into exosomes. However, not much is known about these selective shuttling mechanisms.
Biological function of exosomes in cancer
Emerging evidence suggests that exosomes derived from cancer cells are involved in tumor growth, tumorigenesis, angiogenesis, tumor immune escape, drug resistance and metastasis. Exosomes are also involved in the intercellular communication between cancer cells and stromal cells, especially with cancer-associated fibroblasts (CAFs). In this section, we reviewed recently published articles reporting the roles of exosomes in cancer and their underlying molecular mechanisms (Fig. 2).
Tumorigenesis
Exosomes derived from malignant cells have shown the potential to induce cell transformation. They can also affect other cells in the heterogeneous tumor population, and be involved in the transfer of metastatic capacity. For instance, exosomes released by malignant breast cancer cells are taken up by less malignant tumor cells located within the same tumor, and thereby promote their migratory and metastatic potential (37).
In addition, exosomal miRNA from cancer cells can contribute to tumorigenesis. Both mature miRNA and pre-miRNA transcripts are present in exosomes along with other key components of the machinery for miRNA biogenesis such as DICER, TRBP, and AGO2 (35). Melo et al demonstrated that exosomes derived from breast cancer cells and sera of patients with breast cancer could instigate nontumorigenic epithelial cells to form tumors (35). Although much of the evidence has emanated from in vitro studies, the exchange of exosomes between tumor cells was recently demonstrated in vivo in a study which combined high-resolution imaging with a Cre-LoxP system. Exosomes derived from malignant breast cancer cells are taken up by less malignant tumor cells within the same tumor and distant tumors. The mRNA content in the exosomes promotes migratory behavior and metastatic capacity of the recipient cancer cells (38).
CAFs are major components of the tumor microenvironment. Exosomes derived from CAFs can mediate horizontal transfer of miRNAs and proteins to affect breast cancer progression (39,40). Webber et al demonstrated exosomal TGF-β1 participated in the activation of myofibroblast, which is a rate-limiting step in cancer progression (41). Hoshino et al reported the effect of cancer cell-derived exosomes on multiple steps of invadopodia life cycle, including invadopodia formation and stabilization. Release of exosomal proteinases were also shown to enhance degradation of extracellular matrix (ECM) associated with invadopodia maturation (42). By inducing ECM degradation, cancer cell-derived exosomes promote tumor cell invasiveness and motility. Of note, Lazar et al showed that adipocytes secrete exosomes in abundance, which are then taken up by tumor cells, leading to increased migration and invasion (43). Collectively, these findings point towards a role of exosomes in tumorigenesis.
Tumor growth
The effect of tumor-derived exosomes on tumor growth has been widely reported in the past decade. Exosomes from sera of glioblastoma patients are enriched with EGFRvIII mRNA. The proliferation potential of recipient cells was shown to be greatly enhanced on co-culture with EGFRvIII-containing exosomes (44). Peinado et al demonstrated that melanoma exosomes containing MET oncoprotein can support tumor growth as well (4). In colorectal cancer, tumor-derived exosomes are enriched in cell cycle-related mRNAs, promoting proliferation of endothelial cells and tumor growth (45). Another study by Kogure et al showed that exosomes derived from hepatocellular carcinoma (HCC) cells can modulate TAK1 expression and associated signaling pathways to enhance cell growth in recipient cells (46). In addition, exosomes from stromal cells can promote proliferation through other signaling pathways (47,48). For instance, Au Yeung et al demonstrated that CAF secreted exosomes to regulate survival and proliferation of pancreatic cancer cells, thus may serve as a potential target for overcoming resistance of chemotherapy. Similarly, miRNA-21 enriched in CAF exosomes may profoundly impact ovarian cancer growth by suppressing apoptosis through binding to APAF1 (49). Zhang et al reported that the loss of exosomal miRNA-320a from CAFs contributed to HCC proliferation (50).
While the majority of research evidence pertains to the pro-tumorigenic effect of tumor-derived exosomes, it is important to remember that the function of stromal-derived exosomes may differ from, and, perhaps, be opposite to that of cancer exosomes. The presence of a competitive biological process was well-characterized in multiple myeloma (MM). Bone marrow mesenchymal stromal cells (BM-MSCs) from patients with MM were shown to have a high expression of oncogenic proteins, which facilitated the growth of MM cells in vivo (51). In contrast, the level of miRNA-15a, a known suppressor of MM growth (52), was significantly higher in exosomes derived from BM-MSCs of normal individuals, thus suggesting a tumor suppressive role of MSC-derived miRNA-15a. Thus, exosomes from cancer and stromal cells may modulate tumor growth.
Angiogenesis
Angiogenesis is a fundamental physiological process involved in wound healing and carcinogenesis. Angiogenesis involves a close interaction between endothelial cells and their surrounding microenvironment. Uptake of exosomes by the endothelial cells (ECs) stimulates angiogenesis. Several groups reported the pro-angiogenic effect of tumor-derived exosomes on endothelial cells in various types of cancers such as glioblastoma, leukemia, multiple myeloma, melanoma, ovarian cancer and breast cancer (53–57). In a study by Skog et al, angiogenic-protein rich exosomes released from glioblastoma tumor cells were shown to stimulate tubule formation in ECs (53). Gopal et al demonstrated communication between oncogenic cells undergoing epithelial-mesenchymal transition (EMT) and endothelial cells via exosomes containing Rac1/PAK2 proteins as the angiogenic promoters (58).
Some miRNAs found in exosomes are thought to be specifically involved in tumor angiogenesis (59). For example, in colorectal cancer, miRNA-9 in tumor-derived exosomal vesiculars showed pro-angiogenic effects through inhibiting the expression of suppression of cytokine signaling 5 (SOCS5), promoting the migration of endothelial cells. In addition, miRNA-210 has been observed to suppress the expression of specific genes in endothelial cells, resulting in enhanced pro-angiogenic activity (60–62). Despite the direct pro-angiogenic effect of tumor-derived exosomes on endothelial cells, tumor-derived exosomes also demonstrated indirect effects on other stromal cells, such as CAFs. For example, in leukemia, tumor-derived exosomes induced a CAF phenotype in stromal cells in the surrounding microenvironment, hence leading to increased expression of pro-angiogenic factors in tumor (63). Of note, transfer of miRNA-125a from MSC-derived exosomes to ECs promoted angiogenesis, both in vitro and in vivo (64). The effect of tumor-derived exosomes on vascular remodeling may affect both tumor growth and metastasis. For instance, melanoma-derived exosomes can induce vascular leak at pre-metastatic sites and can also reprogramme bone marrow progenitors towards a pro-vasculogenic phenotype (4).
Exosomes have been shown to contain different kinds of angiogenic factors; it is necessary to understand the exact contributions of these factors to cancer development.
Tumor immune escape
Currently available evidence indicates a dual role of exosomes in mediating crosstalk between immune cells and cancer cells. On the one hand, various immune-stimulatory genes, such as mesothelin, and carcinoembryonic antigen (CEA), are packed in tumor-derived exosomes (65,66). On the other hand, several recent studies suggested that the cancer cells utilized exosomes containing nucleic acids and proteins to enact an immune escape.
An earlier study showed that the dendritic cell (DC)-derived exosomes stimulated an antitumor immune response (67). However, recent research showed tumor-derived exosomes may aid in immune evasion by impairing differentiation and maturation of dendritic cells, which changed their role from effective antigen presenting cells into negative modulators of immune response (68–70).
Exosomes may also play a role in the biology of cytotoxic T cells and regulatory T cells. For instance, in nasopharyngeal carcinoma, tumor-derived exosomes impaired T cell proliferation, differentiation and cytokine secretion in vivo and in vitro. The effect appeared to be mediated via down-regulation of the MAPK1 and JAK/STAT pathways by exosomal miRNAs (71). Clayton et al demonstrated an exosome-mediated mechanism that skewed the IL-2 responsiveness in favor of regulatory T cells and away from cytotoxic cells; this coordinated effect on cellular immunity strongly implicates the role of tumor-derived exosomes in immune escape by tumor cells (72).
Tumor-associated macrophages (TAMs) are currently the most widely studied inflammatory cell component of tumor microenvironment (TME) and may also be activated by tumor-derived exosomes. For instance, Fabbri et al identified a new mechanism of communication between TAMs and cancer cells via exosomal miRNAs. Specifically, exosomal miRNA-21 and miRNA-29a were recruited in the TME by TAMs and bind to their Toll-like receptor 8, triggering the Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway and the secretion of interleukin-6 (IL-6) (73).
Moreover, exosomes derived from human prostate cancer cells were shown to express ligands for NKG2D, which induced downregulation of NKG2D in NK cells and impaired NK cell cytotoxic function (74). Ying et al demonstrated that ovarian tumor-derived exosomes play a crucial role in regulating the polarization of tumor-promoting M2 macrophages (75).
While the above examples demonstrate that tumor-derived exosomes may suppress the immune response, it appears that exosomes derived from immune cells can also influence the cancer cells. For instance, exosomes from activated CD8+ T cells increased tumor immunogenicity by activating ERK and NF-κB signaling, which can promote the metastatic potential of tumor cells (76). Collectively, these studies suggest that exosomes may mediate immunosuppression in tumor-bearing host in different ways.
Drug resistance
Drug resistance has long been a major obstacle in the management of cancers. Recently, exosomes are of great interest in drug resistance studies. Drug-resistant cancer cells may spread resistance to hitherto sensitive ones by releasing exosomes; such effects could be partly attributed to the intercellular transfer of specific proteins, miRNAs, and even long noncoding RNAs (77–80).
Stromal cells were found to initiate cross-talk with cancer cells via exosomes. CAF-derived exosomes were shown to contribute to proliferation and chemoresistance of pancreatic cancer cells (81). Boelens et al demonstrated the transfer of exosomes from stromal cells to breast cancer cells activated antiviral retinoic acid-inducible gene 1 enzyme (RIG-1) signaling to regulate the expansion of therapy-resistant tumor-initiating cells (82). Further study by Au Yeung et al showed that in ovarian cancer, exosomal transportation of CAF-derived miRNA-21 conferred paclitaxel resistance in ovarian cancer cells through targeting APAF1 (49). Moreover, a recent study by Sun et al demonstrated that suppression of miRNA-122 resulted in taxol resistance by upregulating Septin-9 in HCC (29). Qu et al reported that exosome-transmitted LncRNA promoted Sunitinib resistance in renal cancer by acting as a competing endogenous RNA for miRNA-34 and miRNA-449 to promote AXL and c-MET expression (79).
In addition, exosomes may also affect tumor chemotherapy by mediating drug efflux. Shedden et al found that doxorubicin can be encapsulated and exported by tumor-derived exosomes (83). Moreover, exosomes may play a role in lowering the therapeutic effect of antibodies by modulating their binding to cancer cells. For instance, breast cancer cell-derived exosomes were shown to have a high expression of HER2 which interfered with the activity of the monoclonal antibody trastuzumab in vitro (84). Collectively, exosomes derived from both cancer cells and stromal cells can contribute to the development of chemoresistance in tumor cells.
Metastasis
Exosomes not only affect cells in the location where they are produced, but may also influence cells in distant tissues. Exosomes are involved in both the initiation of metastasis and the preparation of a pre-metastatic niche. Exosomes may assist cancer cells in acquiring migratory and invasive properties through EMT. For instance, Aga et al demonstrated that treatment of EBV-negative cells with LMP1-exosomes increases migration and invasiveness of nasopharyngeal cancer cells in functional assays, which correlates with the phenotype associated with EMT (85). Recently, an in vitro imaging experiment also showed the importance of exosome-mediated exchange of molecules for metastasis among tumor cells (38).
The pre-metastatic niche is a prerequisite of tumor metastasis. Exosomes derived from the primary tumor can act as potential mediators for priming the pre-metastatic niche. Pancreatic tumor-derived exosomes enriched in macrophage migration inhibitory factors recruited macrophages to establish pre-metastatic niche in the liver, which resulted in increased hepatic macrometastatic burden (86). Moreover, breast cancer cell-derived exosomal miRNA-122 was shown to suppress glucose uptake by niche cells to promote metastasis, both in vitro and in vivo (3). Liu et al recently reported the Toll-like receptor 3 (TLR3) mediated crosstalk between pulmonary epithelial cells and tumor exosomal RNAs as being critical to the initiation of neutrophil recruitment and lung metastatic niche formation (87). In addition, tumor-derived exosomes were shown to express unique integrins which prepare the pre-metastatic niche by fusing with resident cells and by activating Src phosphorylation and pro-inflammatory S100 expression at the eventual site of metastasis (88).
Recently, Bliss et al reported the nature of regulatory interactions between breast cancer cells and MSC mediated by exosomes and MSC-derived exosomes may stimulate cycling quiescence and early breast cancer dormancy in bone marrow (89). Moreover, tumor-derived exosomes carrying miRNA-181c may trigger the breakdown of blood-brain-barrier (BBB) to promote brain metastasis (90). Zhang et al demonstrated that astrocyte-derived exosomes mediate intercellular transfer of PTEN-targeting miRNAs to metastatic tumor cells, which results in PTEN silencing in these tumor cells. This loss of PTEN expression leads to increased chemokine CCL2 level that recruits myeloid cells to reciprocally promote brain metastasis (91). These findings show that the intercellular crosstalk mediated by exosomes is a crucial mechanism for tumor metastasis.
Exosomes as cancer biomarkers
Exosomes have been detected in nearly all kinds of body fluids, including blood, urine, saliva, amniotic fluid, cerebrospinal fluids, bile, ascites, tears, breast milk and semen (9,92–96). Almost all types of cells can secrete exosomes containing specific proteins, lipids, RNA and even DNA into their microenvironment and circulation (97).
The discovery of elevated level of exosomal miRNAs in the plasma of certain cancer patients was the first indication of the potential use of circulating exosomes as cancer biomarkers. Taylor et al reported that circulating exosomal miRNAs from patients with ovarian cancer were significantly distinct from profiles observed in benign disease and healthy volunteers (98). Microarray analyses of miRNAs derived from circulating exosomes derived from 88 patients with primary colorectal cancer (CRC) and 11 healthy donors, revealed significantly higher levels of 7 miRNAs in the former (99). Recently, Zhu et al demonstrated that the levels of miRNA-19a-3p, miRNA-21-5p and miRNA-425-5p were significantly elevated in exosomes from colorectal cancer patients' serum samples (100).
In addition, Li et al reported that LncRNA can be detected in plasma, and one of the possible mechanisms of its stable existence in blood was its protection by exosomes (101). Li et al recently reported that high levels of exosomal circular RNAs (circRNAs) and serum exosomal circRNAs can distinguish cancer patients from healthy individuals (102).
Proteins associated with tumor-derived exosomes are also seen as potential biomarkers. Exosomes double-positive for CD63 or caveolin-1 and Rab-5b were significantly increased in plasma from melanoma patients as compared to that from healthy donors (103). Noteworthy, Melo et al showed that Glypican-1-positive circulating exosomes was a reliable biomarker for early detection of pancreatic cancer, with superior prognostic value than CA19-9 (104). Similarly, in a study by Lea et al provided proof-of-concept data that supported the high diagnostic power of phosphatidylserine-positive exosomes detected in the blood of women with ovarian malignancies (105).
For prognostic evaluation, circulating exosomes may enhance the stratification of cancer patients with high-risk factors. Manier et al reported that two circulating exosomal miRNAs, namely let-7b and miRNA-18a, improved survival prediction in patients with multiple myeloma (106). Similar results have been found in multiple types of cancers, including non-small cell lung cancer, esophageal squamous cell carcinoma, colorectal cancer, HCC and nasopharyngeal carcinoma (107–112).
For monitoring treatment response, exosomes have been demonstrated to accurately reflect the levels of proteins and mRNAs in parental cells throughout treatment and therefore can serve as potential biomarker of chemotherapy response (113,114). Skog et al identified a protein signature in circulating exosomes linked with clinical stages of melanoma patients. As a predictor of TKI treatment response, exosomal EGFRvIII splice variant was detectable in the serum of glioblastoma multiforme patients, but not the 30 matched controls (53).
Additionally, some newly developed technologies exist to sort and enrich exosomes. Wunsch et al recently demonstrated that nanoscale deterministic lateral displacement pillar arrays, an efficient technology to sort exosomes, may open up the potential for on-chip separation and diagnosis (115).
Collectively, the development of exosome-based novel biomarkers may provide benefits to cancer patients in a wide variety of ways (Table I).
Exosomes as cancer therapeutic targets
Thus far, not much progress has been made in the use of exosomes as therapeutic agents in clinical practice. However, their importance in carcinogenesis envisages their use in individualized cancer therapeutics in the future.
First, complete depletion of circulating exosomes may potentially result in major benefit for cancer patients. Ciravolo et al reported that exosomes purified from HER2-positive early-stage breast cancer patient sera and HER2-overexpressing breast cancer cells can inhibit trastuzumab-induced anti-proliferative activity (84). Removal of such exosomes from advanced breast cancer patients is likely to improve response to trastuzumab therapy (116). Next, dendritic cell-derived exosomes have been used as tumor vaccine to trigger host antitumor immune response and inhibit cell growth and proliferation. Phase I clinical trials of dendritic exosomes in patients with metastatic melanoma and advanced non-small cell lung cancer have shown modest therapeutic effects (117,118).
Exosomes may serve as delivery vehicle. Use of exosomes as nanocarriers for therapeutic agents is being actively explored. For instance, Alcarez et al demonstrated the use of exosomes for targeted delivery of siRNA after systemic administration, which indicates their potential use as vehicle for gene therapies (22). Tian et al reported that exosomes modified by targeting ligands can be used for delivery of doxorubicin to tumor cells (119). Kim et al reported good efficacy of an exosome-based system as a delivery vehicle for PTX to multi-drug resistant cancer cells (120).
More recently, MSC-derived exosomes are being examined for their role in MSC-based cellular therapy. For instance, Katakowski et al demonstrated that exosomes from MSCs expressing miRNA-146b significantly reduced glioma xenograft growth in rat model of primary brain tumor (121). Similarly, a study reported by Ono et al suggested that exosomal transfer of miRNAs from MSCs may promote breast cancer dormancy in a metastatic niche (122). Additionally, use of exosomes derived from BM-MSCs as nanocarriers for targeted delivery of antitumor drugs such as paclitaxel (PTX) has also been investigated (123).
Conclusion
The rapid increase in the number of published studies, especially in oncology, clearly reflects the enthusiasm on research of exosome biology and function. Cancer cells communicate with surrounding and distant cells via exosomes. Exosomes constitute a bi-directional interaction network that mediates the intercellular cross talk between cancer cells and the microcellular environment to promote cancer development, progression, metastasis, and drug resistance. However, most pieces of the aforementioned findings are gathered from cell culture experiments. Therefore, it is vital to substantiate these results in more rigorous in vitro settings. Moreover, exosomes released by cancer cells and TME are highly heterogeneous. But there are few reports explaining the role of each of these subtypes. The difficulties in elucidating the roles of specific exosomes include lack of unique molecules to distinguish each exosome subtype and appropriate exosomes isolation methods. Future technical advances may lead to significant progress in the understanding of the heterogeneity of exosomes.
Exosomes have been proven to be stable carriers of genetic materials, and been nominated as promising tumor biomarkers for cancer diagnosis and prognosis. Moreover, during cancer treatment, exosomes may switch their contents and may therefore exhibit traits for the monitoring of therapeutic efficiency. Furthermore, as vectors for drugs and tumor vaccines, exosome-based delivery is rigorously evaluated as an emerging therapeutic strategy for cancer. Additional studies are much needed to better elucidate their role and mechanism of action in cancer to reduce the risk of off-target effects and therapeutic failures. The several aforementioned results still require further validation in clinical settings, especially in independent prospective cohorts.
Altogether, with the emergence of precision medicine, future studies on exosomes should not only emphasize their roles in cancer biology, but may also open new avenues for cancer diagnostics and therapeutics.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (81372359), the National High Technology Research and Development Program of China (2015AA020106), The National Key Research and Development Program of China (2016YFC1303201, 2016YFC0905400), and the Institutional Fundamental Research Funds (JK2014B14, NCC2016YKY-06).
ReferencesTramsEGLauterCJSalemNJrHeineUExfoliation of membrane ecto-enzymes in the form of micro-vesiclesBiochim Biophys Acta6456370198110.1016/0005-2736(81)90512-56266476HardingCHeuserJStahlPReceptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytesJ Cell Biol97329339198310.1083/jcb.97.2.32963098572112509FongMYZhouWLiuLAlontagaAYChandraMAshbyJChowAO'ConnorSTLiSChinARBreast-cancer-secreted miR-122 reprograms glucose metabolism in premetastatic niche to promote metastasisNat Cell Biol17183194201510.1038/ncb3094256219504380143PeinadoHAleckovicMLavotshkinSMateiICosta-SilvaBMoreno-BuenoGHergueta-RedondoMWilliamsCGarcía-SantosGGhajarCMMelanoma exosomes educate bone marrow progenitor cells toward a pro-metastatic phenotype through METNat Med18883891201210.1038/nm.2753226350053645291LiJLiuKLiuYXuYZhangFYangHLiuJPanTChenJWuMExosomes mediate the cell-to-cell transmission of IFN-[alpha]-induced antiviral activityNat Immunol14793803201310.1038/ni.264723832071ColomboMRaposoGTheryCBiogenesis, secretion, and intercellular interactions of exosomes and other extracellular vesiclesAnnu Rev Cell Dev Biol30255289201410.1146/annurev-cellbio-101512-12232625288114TkachMTheryCCommunication by extracellular vesicles: Where we are and where we need to goCell16412261232201610.1016/j.cell.2016.01.04326967288SchoreyJSHardingCVExtracellular vesicles and infectious diseases: New complexity to an old storyJ Clin Invest12611811189201610.1172/JCI81132270358094811125ZhangXYuanXShiHWuLQianHXuWExosomes in cancer: Small particle, big playerJ Hematol Oncol883201510.1186/s13045-015-0181-x261565174496882KalluriRThe biology and function of exosomes in cancerJ Clin Invest12612081215201610.1172/JCI81135270358124811149WilliamsRLUrbéSThe emerging shape of the ESCRT machineryNat Rev Mol Cell Biol8355368200710.1038/nrm216217450176TrajkovicKHsuCChiantiaSRajendranLWenzelDWielandFSchwillePBruggerBSimonsMCeramide triggers budding of exosome vesicles into multivesicular endosomesScience31912441247200810.1126/science.115312418309083RobbinsPDMorelliAERegulation of immune responses by extracellular vesiclesNat Rev Immunol14195208201410.1038/nri3622245669164350779BaiettiMFZhangZMortierEMelchiorADegeestGGeeraertsAIvarssonYDepoortereFCoomansCVermeirenESyndecan-syntenin-ALIX regulates the biogenesis of exosomesNat Cell Biol14677685201210.1038/ncb250222660413OstrowskiMCarmoNBKrumeichSFangetIRaposoGSavinaAMoitaCFSchauerKHumeANFreitasRPRab27a and Rab27b control different steps of the exosome secretion pathwayNat Cell Biol121930201010.1038/ncb200019966785FaderCMSánchezDGMestreMBColomboMITI-VAMP/VAMP7 and VAMP3/cellubrevin: Two v-SNARE proteins involved in specific steps of the autophagy/multivesicular body pathwaysBiochim Biophys Acta179319011916200910.1016/j.bbamcr.2009.09.01119781582YuXHarrisSLLevineAJThe regulation of exosome secretion: A novel function of the p53 proteinCancer Res6647954801200610.1158/0008-5472.CAN-05-457916651434LespagnolADuflautDBeekmanCBlancLFiucciGMarineJ-CVidalMAmsonRTelermanAExosome secretion, including the DNA damage-induced p53-dependent secretory pathway, is severely compromised in TSAP6/Steap3-null miceCell Death Differ1517231733200810.1038/cdd.2008.10418617898WeiYWangDJinFBianZLiLLiangHLiMShiLPanCZhuDPyruvate kinase type M2 promotes tumour cell exosome release via phosphorylating synaptosome-associated protein 23Nat Commun814041201710.1038/ncomms14041280672305228053SavinaAFurlánMVidalMColomboMIExosome release is regulated by a calcium-dependent mechanism in K562 cellsJ Biol Chem2782008320090200310.1074/jbc.M30164220012639953ParoliniIFedericiCRaggiCLuginiLPalleschiSDe MilitoACosciaCIessiELogozziMMolinariAMicroenvironmental pH is a key factor for exosome traffic in tumor cellsJ Biol Chem2843421134222200910.1074/jbc.M109.041152198016632797191Alvarez-ErvitiLSeowYYinHBettsCLakhalSWoodMJDelivery of siRNA to the mouse brain by systemic injection of targeted exosomesNat Biotechnol29341345201110.1038/nbt.180721423189MorelliAELarreginaATShufeskyWJSullivanMLStolzDBPapworthGDZahorchakAFLogarAJWangZWatkinsSCEndocytosis, intracellular sorting, and processing of exosomes by dendritic cellsBlood10432573266200410.1182/blood-2004-03-082415284116EscreventeCKellerSAltevogtPCostaJInteraction and uptake of exosomes by ovarian cancer cellsBMC Cancer11108201110.1186/1471-2407-11-108214390853072949FengDZhaoWLYeYYBaiX-CLiuR-QChangL-FZhouQSuiS-FCellular internalization of exosomes occurs through phagocytosisTraffic11675687201010.1111/j.1600-0854.2010.01041.x20136776FitznerDSchnaarsMvan RossumDKrishnamoorthyGDibajPBakhtiMRegenTHanischU-KSimonsMSelective transfer of exosomes from oligodendrocytes to microglia by macropinocytosisJ Cell Sci124447458201110.1242/jcs.07408821242314GlebovOOBrightNANicholsBJFlotillin-1 defines a clathrin-independent endocytic pathway in mammalian cellsNat Cell Biol84654200610.1038/ncb134216341206OttoGPNicholsBJThe roles of flotillin microdomains-endocytosis and beyondJ Cell Sci12439333940201110.1242/jcs.09201522194304SunH-LCuiRZhouJTengKHsiaoY-HNakanishiKFassanMLuoZShiGTiliEERK activation globally downregulates miRNAs through phosphorylating exportin-5Cancer Cell30723736201610.1016/j.ccell.2016.10.00127846390KowalJTkachMTheryCBiogenesis and secretion of exosomesCurr Opin Cell Biol29116125201410.1016/j.ceb.2014.05.00424959705Gutiérrez-VázquezCVillarroya-BeltriCMittelbrunnMSánchez-MadridFTransfer of extracellular vesicles during immune cell-cell interactionsImmunol Rev251125142201310.1111/imr.12013232787453740495WoodSLKnowlesMAThompsonDSelbyPJBanksREProteomic studies of urinary biomarkers for prostate, bladder and kidney cancersNat Rev Urol10206218201310.1038/nrurol.2013.2423443013SubraCLaulagnierKPerretBRecordMExosome lipidomics unravels lipid sorting at the level of multivesicular bodiesBiochimie89205212200710.1016/j.biochi.2006.10.01417157973ValadiHEkströmKBossiosASjöstrandMLeeJJLötvallJOExosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cellsNat Cell Biol9654659200710.1038/ncb159617486113MeloSASugimotoHO'ConnellJTKatoNVillanuevaAVidalAQiuLVitkinEPerelmanLTMeloCACancer exosomes perform cell-independent microRNA biogenesis and promote tumorigenesisCancer Cell26707721201410.1016/j.ccell.2014.09.005254468994254633CrescitelliRLässerCSzabóTGKittelAEldhMDianzaniIBuzásEILötvallJDistinct RNA profiles in subpopulations of extracellular vesicles: Apoptotic bodies, microvesicles and exosomesJ Extracell Vesicles 2: eCollection2013doi: 10.3402/jev.v2i0.2067710.3402/jev.v2i0.20677LeMTHamarPGuoCBasarEPerdigão-HenriquesRBalajLLiebermanJmiR-200-containing extracellular vesicles promote breast cancer cell metastasisJ Clin Invest12451095128201410.1172/JCI75695254014714348969ZomerAMaynardCVerweijFJKamermansASchäferRBeerlingESchiffelersRMde WitEBerenguerJEllenbroekSIIn vivo imaging reveals extracellular vesicle-mediated phenocopying of metastatic behaviorCell16110461057201510.1016/j.cell.2015.04.042260004814448148DonnarummaEFioreDNappaMRoscignoGAdamoAIaboniMRussoVAffinitoAPuotiIQuintavalleCCancer-associated fibroblasts release exosomal microRNAs that dictate an aggressive phenotype in breast cancerOncotarget819592196082017281216255386708LugaVZhangLViloria-PetitAMOgunjimiAAInanlouMRChiuEBuchananMHoseinANBasikMWranaJLExosomes mediate stromal mobilization of autocrine Wnt-PCP signaling in breast cancer cell migrationCell15115421556201210.1016/j.cell.2012.11.02423260141WebberJPSparyLKSandersAJChowdhuryRJiangWGSteadmanRWymantJJonesATKynastonHMasonMDDifferentiation of tumour-promoting stromal myofibroblasts by cancer exosomesOncogene34290302201510.1038/onc.2013.56024441045HoshinoDKirkbrideKCCostelloKClarkESSinhaSGrega-LarsonNTyskaMJWeaverAMExosome secretion is enhanced by invadopodia and drives invasive behaviorCell Rep511591168201310.1016/j.celrep.2013.10.05024290760LazarIClementEDauvillierSMilhasDDucoux-PetitMLeGonidecSMoroCSoldanVDalleSBalorSAdipocyte exosomes promote melanoma aggressiveness through fatty acid oxidation: A novel mechanism linking obesity and cancerCancer Res7640514057201610.1158/0008-5472.CAN-16-065127216185Al-NedawiKMeehanBMicallefJLhotakVMayLGuhaARakJIntercellular transfer of the oncogenic receptor EGFRvIII by microvesicles derived from tumour cellsNat Cell Biol10619624200810.1038/ncb172518425114HongBSChoJ-HKimHChoiE-JRhoSKimJKimJChoiD-SKimY-KHwangDColorectal cancer cell-derived microvesicles are enriched in cell cycle-related mRNAs that promote proliferation of endothelial cellsBMC Genomics10556200910.1186/1471-2164-10-556199307202788585KogureTLinWLYanIKBraconiCPatelTIntercellular nanovesicle-mediated microRNA transfer: A mechanism of environmental modulation of hepatocellular cancer cell growthHepatology5412371248201110.1002/hep.24504217210293310362ZhuWHuangLLiYZhangXGuJYanYXuXWangMQianHXuWExosomes derived from human bone marrow mesenchymal stem cells promote tumor growth in vivoCancer Lett3152837201210.1016/j.canlet.2011.10.00222055459XiaoHLässerCShelkeGVWangJRådingerMLunavatTRMalmhällCLinLHLiJLiLMast cell exosomes promote lung adenocarcinoma cell proliferation-role of KIT-stem cell factor signalingCell Commun Signal1264201410.1186/PREACCEPT-1817458803126023253113674206705YeungAu CLCoNNTsurugaTYeungTLKwanSYLeungCSLiYLuESKwanKWongKKExosomal transfer of stroma-derived miR21 confers paclitaxel resistance in ovarian cancer cells through targeting APAF1Nat Commun711150201610.1038/ncomms11150270214364820618ZhangZLiXSunWYueSYangJLiJMaBWangJYangXPuMLoss of exosomal miR-320a from cancer-associated fibroblasts contributes to HCC proliferation and metastasisCancer Lett3973342201710.1016/j.canlet.2017.03.00428288874RoccaroAMSaccoAMaisoPAzabAKTaiY-TReaganMAzabFFloresLMCampigottoFWellerEBM mesenchymal stromal cell-derived exosomes facilitate multiple myeloma progressionJ Clin Invest12315421555201310.1172/JCI66517234547493613927CorthalsSLJongen-LavrencicMde KnegtYPeetersJKBeverlooHBLokhorstHMSonneveldPMicro-RNA-15a and micro-RNA-16 expression and chromosome 13 deletions in multiple myelomaLeuk Res34677681201010.1016/j.leukres.2009.10.02620031211SkogJWürdingerTVan RijnSMeijerDHGaincheLCurryWTCarterBSKrichevskyAMBreakefieldXOGlioblastoma microvesicles transport RNA and proteins that promote tumour growth and provide diagnostic biomarkersNat Cell Biol1014701476200810.1038/ncb1800190116223423894TavernaSFlugyASaievaLKohnECSantoroAMeravigliaSDe LeoGAlessandroRRole of exosomes released by chronic myelogenous leukemia cells in angiogenesisInt J Cancer13020332043201210.1002/ijc.2621721630268HoodJLPanHLanzaGMWicklineSAParacrine induction of endothelium by tumor exosomesLab Invest8913171328200910.1038/labinvest.2009.94197869483316485MillimaggiDMariMD'AscenzoSCarosaEJanniniEAZuckerSCartaGPavanADoloVTumor vesicle-associated CD147 modulates the angiogenic capability of endothelial cellsNeoplasia9349357200710.1593/neo.07133174607791854851WangJDe VeirmanKFaictSFrassanitoMARibattiDVaccaAMenuEMultiple myeloma exosomes establish a favourable bone marrow microenvironment with enhanced angiogenesis and immunosuppressionJ Pathol239162173201610.1002/path.471226956697GopalSKGreeningDWHanssenEGZhuH-JSimpsonRJMathiasRAOncogenic epithelial cell-derived exosomes containing Rac1 and PAK2 induce angiogenesis in recipient endothelial cellsOncotarget719709197222016269190984991413VaderPBreakefieldXOWoodMJExtracellular vesicles: Emerging targets for cancer therapyTrends Mol Med20385393201410.1016/j.molmed.2014.03.002247036194082760KosakaNIguchiHHagiwaraKYoshiokaYTakeshitaFOchiyaTNeutral sphingomyelinase 2 (nSMase2)-dependent exosomal transfer of angiogenic microRNAs regulate cancer cell metastasisJ Biol Chem2881084910859201310.1074/jbc.M112.446831234396453624465FanG-CHypoxic exosomes promote angiogenesisBlood12436693670201410.1182/blood-2014-10-60784625498451TadokoroHUmezuTOhyashikiKHiranoTOhyashikiJHExosomes derived from hypoxic leukemia cells enhance tube formation in endothelial cellsJ Biol Chem2883434334351201310.1074/jbc.M113.480822241332153843049KosakaNDecoding the secret of cancer by means of extracellular vesiclesJ Clin Med522201610.3390/jcm50200224773778LiangXZhangLWangSHanQZhaoRCExosomes secreted by mesenchymal stem cells promote endothelial cell angiogenesis by transferring miR-125aJ Cell Sci12921822189201610.1242/jcs.17037327252357AndreFSchartzNEMovassaghMFlamentCPautierPMoricePPomelCLhommeCEscudierBLe ChevalierTMalignant effusions and immunogenic tumour-derived exosomesLancet360295305200210.1016/S0140-6736(02)09552-112147373DaiSWanTWangBZhouXXiuFChenTWuYCaoXMore efficient induction of HLA-A* 0201-restricted and carcinoembryonic antigen (CEA)-specific CTL response by immunization with exosomes prepared from heat-stressed CEA-positive tumor cellsClin Cancer Res1175547563200510.1158/1078-0432.CCR-05-081016243831ZitvogelLRegnaultALozierAWolfersJFlamentCTenzaDRicciardi-CastagnoliPRaposoGAmigorenaSEradication of established murine tumors using a novel cell-free vaccine: Dendritic cell derived exosomesNat Med4594600199810.1038/nm0598-5949585234CondamineTGabrilovichDIMolecular mechanisms regulating myeloid-derived suppressor cell differentiation and functionTrends Immunol321925201110.1016/j.it.2010.10.00221067974ValentiRHuberVFilipazziPPillaLSovenaGVillaACorbelliAFaisSParmianiGRivoltiniLHuman tumor-released microvesicles promote the differentiation of myeloid cells with transforming growth factor-β-mediated suppressive activity on T lymphocytesCancer Res6692909298200610.1158/0008-5472.CAN-06-181916982774YangCKimS-HBiancoNRRobbinsPDTumor-derived exosomes confer antigen-specific immunosuppression in a murine delayed-type hypersensitivity modelPLoS One6e22517201110.1371/journal.pone.0022517218296293149056YeSLiZ-LLuoDHuangBChenY-SZhangXCuiJZengYLiJTumor-derived exosomes promote tumor progression and T-cell dysfunction through the regulation of enriched exosomal microRNAs in human nasopharyngeal carcinomaOncotarget554395452201410.18632/oncotarget.2118249781374170615ClaytonAMitchellJPCourtJMasonMDTabiZHuman tumor-derived exosomes selectively impair lymphocyte responses to interleukin-2Cancer Res6774587466200710.1158/0008-5472.CAN-06-345617671216FabbriMPaoneACaloreFGalliRGaudioESanthanamRLovatFFaddaPMaoCNuovoGJMicroRNAs bind to Toll-like receptors to induce prometastatic inflammatory responseProc Natl Acad Sci USA109E2110E2116201210.1073/pnas.1209414109227534943412003ClaytonAMitchellJPCourtJLinnaneSMasonMDTabiZHuman tumor-derived exosomes down-modulate NKG2D expressionJ Immunol18072497258200810.4049/jimmunol.180.11.724918490724YingXWuQWuXZhuQWangXJiangLChenXWangXEpithelial ovarian cancer-secreted exosomal miR-222-3p induces polarization of tumor-associated macrophagesOncotarget743076430872016271727985190009CaiZYangFYuLYuZJiangLWangQYangYWangLCaoXWangJActivated T cell exosomes promote tumor invasion via Fas signaling pathwayJ Immunol18859545961201210.4049/jimmunol.110346622573809ChenWLiuXLvMChenLZhaoJZhongSJiMHuQLuoZWuJExosomes from drug-resistant breast cancer cells transmit chemoresistance by a horizontal transfer of microRNAsPLoS One9e95240201410.1371/journal.pone.0095240247404153989268WeiYLaiXYuSChenSMaYZhangYLiHZhuXYaoLZhangJExosomal miR-221/222 enhances tamoxifen resistance in recipient ER-positive breast cancer cellsBreast Cancer Res Treat147423431201410.1007/s10549-014-3037-025007959QuLDingJChenCWuZ-JLiuBGaoYChenWLiuFSunWLiX-FExosome-transmitted lncARSR promotes sunitinib resistance in renal cancer by acting as a competing endogenous RNACancer Cell29653668201610.1016/j.ccell.2016.03.00427117758ChenYWangLZhuYChenZQiXJinLJinJHuaDMaXBreast cancer resistance protein (BCRP)-containing circulating microvesicles contribute to chemoresistance in breast cancerOncol Lett10374237482015267882014665209RichardsKEZeleniakAEFishelMLWuJLittlepageLEHillRCancer-associated fibroblast exosomes regulate survival and proliferation of pancreatic cancer cellsOncogene3617701778201610.1038/onc.2016.353276694415366272BoelensMCWuTJNabetBYXuBQiuYYoonTAzzamDJVictorTwyman-Saint CWiemannBZIshwaranHExosome transfer from stromal to breast cancer cells regulates therapy resistance pathwaysCell159499513201410.1016/j.cell.2014.09.051254171034283810SheddenKXieXTChandaroyPChangYTRosaniaGRExpulsion of small molecules in vesicles shed by cancer cells association with gene expression and chemosensitivity profilesCancer Res6343314337200312907600CiravoloVHuberVGhediniGCVenturelliEBianchiFCampiglioMMorelliDVillaAMinaPDMenardSPotential role of HER2-overexpressing exosomes in countering trastuzumab-based therapyJ Cell Physiol227658667201210.1002/jcp.2277321465472AgaMBentzGLRaffaSTorrisiMRKondoSWakisakaNYoshizakiTPaganoJSShackelfordJExosomal HIF1α supports invasive potential of nasopharyngeal carcinoma-associated LMP1-positive exosomesOncogene3346134622201410.1038/onc.2014.66246628284162459Costa-SilvaBAielloNMOceanAJSinghSZhangHThakurBKBeckerAHoshinoAMarkMTMolinaHPancreatic cancer exosomes initiate pre-metastatic niche formation in the liverNat Cell Biol17816826201510.1038/ncb316925985394LiuYGuYHanYZhangQJiangZZhangXHuangBXuXZhengJCaoXTumor exosomal RNAs promote lung pre-metastatic niche formation by activating alveolar epithelial TLR3 to recruit neutrophilsCancer Cell30243256201610.1016/j.ccell.2016.06.02127505671HoshinoACosta-SilvaBShenT-LRodriguesGHashimotoATesicMark MMolinaHKohsakaSDi GiannataleACederSTumour exosome integrins determine organotropic metastasisNature527329335201510.1038/nature15756265245304788391BlissSASinhaGSandifordOAWilliamsLMEngelberthDJGuiroKIsenalumheLLGrecoSJAyerSBryanMMesenchymal stem cell-derived exosomes stimulate cycling quiescence and early breast cancer dormancy in bone marrowCancer Res7658325844201610.1158/0008-5472.CAN-16-109227569215TominagaNKosakaNOnoMKatsudaTYoshiokaYTamuraKLötvallJNakagamaHOchiyaTBrain metastatic cancer cells release microRNA-181c-containing extracellular vesicles capable of destructing blood-brain barrierNat Commun66716201510.1038/ncomms7716258280994396394ZhangLZhangSYaoJLoweryFJZhangQHuangW-CLiPLiMWangXZhangCMicroenvironment-induced PTEN loss by exosomal microRNA primes brain metastasis outgrowthNature527100104201510.1038/nature15376264790354819404RaposoGStoorvogelWExtracellular vesicles: Exosomes, microvesicles, and friendsJ Cell Biol200373383201310.1083/jcb.201211138234208713575529KellerSRidingerJRuppAKJanssenJWAltevogtPBody fluid derived exosomes as a novel template for clinical diagnosticsJ Transl Med986201110.1186/1479-5876-9-86216517773118335PisitkunTShenR-FKnepperMAIdentification and proteomic profiling of exosomes in human urineProc Natl Acad Sci USA1011336813373200410.1073/pnas.040345310115326289516573GalloATandonMAlevizosIIlleiGGThe majority of microRNAs detectable in serum and saliva is concentrated in exosomesPLoS One7e30679201210.1371/journal.pone.0030679224278003302865LauCKimYChiaDSpielmannNEiblGElashoffDWeiFLinY-LMoroAGroganTRole of pancreatic cancer-derived exosomes in salivary biomarker developmentJ Biol Chem2882688826897201310.1074/jbc.M113.452458238807643772238KosakaNYoshiokaYFujitaYOchiyaTVersatile roles of extracellular vesicles in cancerJ Clin Invest12611631172201610.1172/JCI81130269741614811151TaylorDDGercel-TaylorCMicroRNA signatures of tumor-derived exosomes as diagnostic biomarkers of ovarian cancerGynecol Oncol1101321200810.1016/j.ygyno.2008.04.03318589210Ogata-KawataHIzumiyaMKuriokaDHonmaYYamadaYFurutaKGunjiTOhtaHOkamotoHSonodaHCirculating exosomal microRNAs as biomarkers of colon cancerPLoS One9e92921201410.1371/journal.pone.0092921247052493976275ZhuMHuangZZhuDZhouXShanXQiLWWuLChengWZhuJZhangLA panel of microRNA signature in serum for colorectal cancer diagnosisOncotarget817081170912017281778815370024LiQShaoYZhangXZhengTMiaoMQinLWangBYeGXiaoBGuoJPlasma long noncoding RNA protected by exosomes as a potential stable biomarker for gastric cancerTumour Biol3620072012201510.1007/s13277-014-2807-y25391424LiYZhengQBaoCLiSGuoWZhaoJChenDGuJHeXHuangSCircular RNA is enriched and stable in exosomes: A promising biomarker for cancer diagnosisCell Res25981984201510.1038/cr.2015.82261386774528056LogozziMDe MilitoALuginiLBorghiMCalabròLSpadaMPerdicchioMMarinoMLFedericiCIessiEHigh levels of exosomes expressing CD63 and caveolin-1 in plasma of melanoma patientsPLoS One4e5219200910.1371/journal.pone.0005219193813312667632MeloSALueckeLBKahlertCFernandezAFGammonSTKayeJLeBleuVSMittendorfEAWeitzJRahbariNGlypican-1 identifies cancer exosomes and detects early pancreatic cancerNature523177182201510.1038/nature14581261068584825698LeaJSharmaRYangFZhuHWardESSchroitAJDetection of phosphatidylserine-positive exosomes as a diagnostic marker for ovarian malignancies: A proof of concept studyOncotarget814395144072017281223355362413ManierSLiuCJAvet-LoiseauHParkJShiJCampigottoFSalemKZHuynhDGlaveySVRivottoBPrognostic role of circulating exosomal miRNAs in multiple myelomaBlood12924292436201710.1182/blood-2016-09-74229628213378LiuQYuZYuanSXieWLiCHuZXiangYWuNWuLBaiLCirculating exosomal microRNAs as prognostic biomarkers for non-small-cell lung cancerOncotarget81304813058201728055956MatsumotoYKanoMAkutsuYHanariNHoshinoIMurakamiKUsuiASuitoHTakahashiMOtsukaRQuantification of plasma exosome is a potential prognostic marker for esophageal squamous cell carcinomaOncol Rep36253525432016275997795055211Sandfeld-PaulsenBAggerholm-PedersenNBaekRJakobsenKRMeldgaardPFolkersenBHRasmussenTRVarmingKJørgensenMMSorensenBSExosomal proteins as prognostic biomarkers in non-small cell lung cancerMol Oncol1015951602201610.1016/j.molonc.2016.10.00327856179LiuTZhangXGaoSJingFYangYDuLZhengGLiPLiCWangCExosomal long noncoding RNA CRNDE-h as a novel serum-based biomarker for diagnosis and prognosis of colorectal cancerOncotarget785551855632016278888035356757YeSBZhangHCaiTTLiuY-NNiJ-JHeJPengJ-YChenQ-YMoH-YCuiJExosomal miR-24-3p impedes T-cell function by targeting FGF11 and serves as a potential prognostic biomarker for nasopharyngeal carcinomaJ Pathol240329340201610.1002/path.478127538493WuZZengQCaoKSunYExosomes: Small vesicles with big roles in hepatocellular carcinomaOncotarget760687606972016274630015312412HegiMEDiserensA-CGorliaTHamouM-Fde TriboletNWellerMKrosJMHainfellnerJAMasonWMarianiLMGMT gene silencing and benefit from temozolomide in glioblastomaN Engl J Med3529971003200510.1056/NEJMoa04333115758010ShaoHChungJLeeKBalajLMinCCarterBSHochbergFHBreakefieldXOLeeHWeisslederRChip-based analysis of exosomal mRNA mediating drug resistance in glioblastomaNat Commun66999201510.1038/ncomms7999259595884430127WunschBHSmithJTGiffordSMWangCBrinkMBruceRLAustinRHStolovitzkyGAstierYNanoscale lateral displacement arrays for the separation of exosomes and colloids down to 20 nmNat Nanotechnol11936940201610.1038/nnano.2016.13427479757MarleauAMChenC-SJoyceJATullisRHExosome removal as a therapeutic adjuvant in cancerJ Transl Med10134201210.1186/1479-5876-10-134227381353441244MorseMAGarstJOsadaTKhanSHobeikaAClayTMValenteNShreeniwasRSuttonMDelcayreAA phase I study of dexosome immunotherapy in patients with advanced non-small cell lung cancerJ Transl Med39200510.1186/1479-5876-3-915723705551593EscudierBDorvalTChaputNAndréFCabyM-PNovaultSFlamentCLeboulaireCBorgCAmigorenaSVaccination of metastatic melanoma patients with autologous dendritic cell (DC) derived-exosomes: Results of the first phase I clinical trialJ Transl Med310200510.1186/1479-5876-3-1015740633554765TianYLiSSongJJiTZhuMAndersonGJWeiJNieGA doxorubicin delivery platform using engineered natural membrane vesicle exosomes for targeted tumor therapyBiomaterials3523832390201410.1016/j.biomaterials.2013.11.08324345736KimMSHaneyMJZhaoYMahajanVDeygenIKlyachkoNLInskoeEPiroyanASokolskyMOkolieODevelopment of exosome-encapsulated paclitaxel to overcome MDR in cancer cellsNanomedicine (Lond)12655664201610.1016/j.nano.2015.10.012KatakowskiMBullerBZhengXLuYRogersTOsobamiroOShuWJiangFChoppMExosomes from marrow stromal cells expressing miR-146b inhibit glioma growthCancer Lett335201204201310.1016/j.canlet.2013.02.019234195253665755OnoMKosakaNTominagaNYoshiokaYTakeshitaFTakahashiRYoshidaMTsudaHTamuraKOchiyaTExosomes from bone marrow mesenchymal stem cells contain a microRNA that promotes dormancy in metastatic breast cancer cellsSci Signal7ra63201410.1126/scisignal.200523124985346PascucciLCoccèVBonomiAAmiDCeccarelliPCiusaniEViganòLLocatelliASistoFDogliaSMPaclitaxel is incorporated by mesenchymal stromal cells and released in exosomes that inhibit in vitro tumor growth: A new approach for drug deliveryJ Control Release192262270201410.1016/j.jconrel.2014.07.04225084218WangTNingKLuTXSunXJinLQiXJinJHuaDIncreasing circulating exosomes-carrying TRPC5 predicts chemoresistance in metastatic breast cancer patientsCancer Sci108448454201710.1111/cas.13150280324005378269HannafonBNTrigosoYDCallowayCLZhaoYDLumDHWelmALZhaoZJBlickKEDooleyWCDingWQPlasma exosome microRNAs are indicative of breast cancerBreast Cancer Res1890201610.1186/s13058-016-0753-x276087155016889LiuCEngCShenJLuYTakataYMehdizadehAChangGJRodriguez-BigasMALiYChangPSerum exosomal miR-4772-3p is a predictor of tumor recurrence in stage II and III colon cancerOncotarget776250762602016277884885342811MatsumuraTSugimachiKIinumaHTakahashiYKurashigeJSawadaGUedaMUchiRUeoHTakanoYExosomal microRNA in serum is a novel biomarker of recurrence in human colorectal cancerBr J Cancer113275281201510.1038/bjc.2015.201260574514506387ZhouXWenWShanXZhuWXuJGuoRChengWWangFQiLWChenYA six-microRNA panel in plasma was identified as a potential biomarker for lung adenocarcinoma diagnosisOncotarget865136525201728036284RodríguezMSilvaJLópez-AlfonsoALópez-MuñizMBPeñaCDomínguezGGarcíaJMLópez-GónzalezAMéndezMProvencioMDifferent exosome cargo from plasma/bronchoalveolar lavage in non-small-cell lung cancerGenes Chromosomes Cancer53713724201424764226BrinkmannKCarboneDPEnderleDKoestlerTBentinkSEmeneggerJSpielAMuellerRO'NeillVSkogJExosomal RNA based liquid biopsy detection of EML4-ALK in plasma from NSCLC patientsProc Int Assoc IALSCDenver, CO2016MadhavanBYueSGalliURanaSGrossWMüllerMGieseNAKalthoffHBeckerTBüchlerMWCombined evaluation of a panel of protein and miRNA serum-exosome biomarkers for pancreatic cancer diagnosis increases sensitivity and specificityInt J Cancer13626162627201510.1002/ijc.2932425388097IşınMUysalerEÖzgürEKöseoğluHŞanlıÖYücelÖBGezerUDalayNExosomal lncRNA-p21 levels may help to distinguish prostate cancer from benign diseaseFront Genet61682015259999834422020HuangZZhangLZhuDShanXZhouXQiLWWuLZhuJChengWZhangHA novel serum microRNA signature to screen esophageal squamous cell carcinomaCancer Med6109119201710.1002/cam4.97328035762MirzaeiHFathullahzadehSKhanmohammadiRDarijaniMMomeniFMasoudifarAGoodarziMMardanshahOStanvengJJaafariMRState of the art in microRNA as diagnostic and therapeutic biomarkers in chronic lymphocytic leukemiaJ Cell PhysiolJan132017(Epub ahead of print)10.1002/jcp.25799SugimachiKMatsumuraTHirataHUchiRUedaMUeoHShindenYIguchiTEguchiHShirabeKIdentification of a bona fide microRNA biomarker in serum exosomes that predicts hepatocellular carcinoma recurrence after liver transplantationBr J Cancer112532538201510.1038/bjc.2014.621255844854453648WangHHouLLiADuanYGaoHSongXExpression of serum exosomal microRNA-21 in human hepatocellular carcinomaBiomed Res Int20148648942014249634874052145SohnWKimJKangSHYangSRChoJ-YChoHCShimSGPaikY-HSerum exosomal microRNAs as novel biomarkers for hepatocellular carcinomaExp Mol Med47e184201510.1038/emm.2015.68263809274650928HongCSMullerLBoyiadzisMWhitesideTLIsolation and characterization of CD34 blast-derived exosomes in acute myeloid leukemiaPLoS One9e103310201410.1371/journal.pone.0103310250933294122364
Biogenesis, release, content and uptake of exosomes. Early endosome is formed from the plasma membrane via endocytic pathway. MVB can be formed by the invagination of endosomal membrane. Dependent on the function and content, MVB then can be directed to fuse with plasma membrane and release to the extracellular space as exosomes. During the biogenesis of exosomes and prior to their secretion, proteins (e.g., tetraspanin, cytosolic proteins, receptor), nucleic acids (e.g., mRNA, miRNA, DNA), and lipids (e.g., sphingomyelin, cholesterol) are uploaded to exosomes. Cells appear to take up exosomes via several ways: (a) receptor-/lipidraft medated endocytosis, (b) phagocytosis, (c) macropinocytosis, (d) fusion with the plasma membrane of the target cell. MVB: multivesicular body.
Biological function of exosomes in cancer. Exosomes are involved in tumor growth, tumorigenesis, angiogenesis, tumor immune escape, drug resistance and metastasis. EMT, epithelial-mesenchymal transition.
Exosomes from distinct body fluids of cancer patients as biomarkers.
Exosomal cargos
Cancer types
Clinical value
Biofluids
Refs.
Phosphatidylserine
Ovarian cancer
Elevated level of phosphatidylserine positive exosomes in ovarian cancer patients than healthy controls
Plasma
(108)
TRPC5
Breast cancer
Elevated level of exosomal TRPC5 in cancer patients and higher levels may predict poorer progress
Plasma
(124)
miRNA-1246, 21
Breast cancer
Elevated levels of these two exosomal miRNAs in breast cancer patients than healthy controls
Plasma
(125)
CRNDE-h
Colorectal cancer
Elevated level of exosomal CRNDE-h in cancer patients and higher levels predict poorer progress
Serum
(110)
miRNA-4772-3p
Colon cancer
Lower level of exosomal miRNA-4772-3p predict tumor recurrence
Serum
(126)
miRNA-19a-3p, 21-5p, 425-5p
Colorectal cancer
Elevated levels of the three exosomal miRNAs in colorectal cancer patients than healthy controls
Serum
(100)
miRNA-17-92a
Colorectal cancer
Elevated level in colorectal cancer patients and higher levels may predict poorer progress
Serum
(127)
miRNA-19-3p, 21-5p, 221-3p
Lung adenocarcinoma
Elevated levels of the three miRNAs in lung adenocarcinoma patients than healthy controls
Plasma
(128)
miRNA-302a, 302-c, 126
Non-small cell lung cancer (NSCLC)
Elevated levels of the three miRNAs in NSCLC patients than healthy controls
Plasma/Broncho-alveolar lavage
(129)
EML4-ALK fushion
NSCLC
EML4-ALK fushion transcripts have been identified in the exosomal RNA of NSCLC patients
Plasma
(130)
Glypican-1
Pancreatic cancer
Higher level of Glypican-1 positive exosomes in patients with early- and late-stage pancreatic cancer than healthy controls
Serum
(107)
miRNA-1246, 4644, 3976, 4306
Pancreatic cancer
Elevated levels of these 4 exosomal miRNAs in pancreatic cancer patients compared to healthy controls
Serum
(131)
LncRNA-p21
Prostate cancer
Elevated level of exosomal lncRNA-p21 in patients with prostate cancer than healthy controls
Plasma
(132)
miRNA-21, 375
Prostate cancer
Elevated levels of urinary exosomal miRNA-21 and miRNA-375 in patients with prostate cancer than healthy controls
Urine
(133)
miRNA-375, 1290
Prostate cancer
Castration-resident prostate cancer patients with elevated levels of both exosomal miRNA-375 and miRNA-1290 may predict poorer progress
Serum
(134)
miRNA-718
HCC
Lower level of exosomal miRNA-718 in patients with HCC patients with recurrence after liver transplantation than those without recurrence
Serum
(135)
miRNA-21
HCC
Elevated level of exosomal mircroRNA-21 in HCC cancer patients than healthy controls
Serum
(136)
miRNA-211, 222, 224
HCC
Elevated levels of these three exosomal mircroRNAs in HCC cancer patients than healthy controls
Serum
(137)
CD34
Acute myeloid leukemia (AML)
Higher level of CD34 positive exosomes in AML patients than healthy control
Plasma
(138)
Let-7b, miRNA-18a
MM
Elevated expression in cancer patients and higher levels of these two exosomal miRNAs may predict poorer progress