High cancer mortality is attributed to metastasis to a large extent. However, cancer metastasis remains devoid of dynamic monitoring and early prevention in terms of current advances in diagnostic means and therapeutic modalities. Meanwhile, studies have shown that reciprocal crosstalk among cells via exosomes plays a critical role in maintaining normal physiological state or triggering disease progression, including cancer metastasis. Therefore, in this review, we focus on the latest literature (primarily from 2018) to summarize action mechanisms and experimental studies of exosomes in cancer metastasis and put forward some problems as well as new outlooks of these studies.
Cancer is responsible for approximately 1 out of every 6 deaths and is the second-leading cause of death (following cardiovascular diseases) worldwide [
Despite advances in cancer therapy, including chemoradiotherapy, immunotherapy, and molecular targeted treatment, there has yet to be satisfactory clinical outcome for patients within cancer metastasis [
Cancer metastasis refers to the process of primary tumor cells arriving to other sites of the body, proliferating there and finally forming new tumors. It includes four main stages: intravasation (from primary tumor sites to blood vessels), extravasation (from blood circulation to future metastasis sites), tumor latency, and formation of micrometastasis and macrometastasis. The process of metastasis is modulated by epithelial-mesenchymal transition (EMT) and the reverse (MET), extracellular matrix (ECM) remodeling, activity of immune system, characteristics alteration of tumor cells, reprogramming of microenvironment cells (fibroblasts, macrophages, endothelial cells, etc.), and recruitment of bone marrow-derived cells (BMDC), such as mesenchymal stem cells (MSC) [
Further studies have shown that exosomes play a vital role in cancer metastasis, namely, contributing in forming the premetastatic niche, influencing tumor cells and microenvironment, and determining specific organotropic metastasis [
Therefore, in this review, we will discuss the study of the influence of exosomes in cancer metastasis, which may provide new horizon for monitoring cancer progression, finding new therapeutic targets and realizing early intervention on metastasis.
Exosomes, serving as a cell complement, function mainly via monitoring the specific organotropism of primary tumor cells, and altering the microenvironment of targeted organs and primary tumor organs. They influence the function of tumor cells, and they change the efficacy of chemotherapy, thereby possibly functioning as dynamic monitoring biomarkers and therapeutic targets for cancer metastasis.
The pioneering study from group of Prof. Layden [
Role of exosomes in organ-specific targeting. Pancreatic ductal adenocarcinoma, PDAC.
Tumor cells-derived and microenvironment cells-derived exosomes modify the microenvironment of the primary tumor and make targeted organ suitable for tumor progression (Table
Influence of exosomes in altering the tumor microenvironment.
The role of tumor cells-derived exosomes in influencing the function of tumor microenvironment cells | ||||
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Donor cells | Recipient cells | Mechanisms of action | Effects | Ref. |
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| Promote ECM remodeling, the formation of inflammatory tumor microenvironment and pre-metastatic niche | |||
LLC or B16/F10 melanoma cells | Alveolar epithelial cells | [ | ||
CRC cells | Liver macrophages | [ | ||
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EOC cells | Umbilical vein endothelial cells (HUVECs) | | Contribute to angiogenesis | [ |
CRC cells | [ | |||
Pancreatic cancer cells | [ | |||
CSCC cells | Lymphatic endothelial cells (HLECs) | [ | ||
LAC cells | Lung endothelial cells | [ | ||
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LAC cells | Fibroblasts | | Promote the cancer-associated phenotype transformation of fibroblasts | [ |
Gastric cancer cells | Fibroblasts | [ | ||
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HNSCC cells | neuronal models | Increase the nerve distribution of tumor microenvironment | [ | |
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The role of tumor microenvironment cells-derived exosomes in influencing the function of tumor microenvironment cells | ||||
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EOC-associated macrophages | CD4+ T cells | | Form an immune-suppressive microenvironment | [ |
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MSCs | tumor stromal cells | | Affect angiogenesis, immune response, migration and invasion of tumor | [ |
Note: Lewis lung carcinoma, LLC; Colorectal cancer, CRC; Epithelial ovarian cancer, EOC; Cervical squamous cell carcinoma, CSCC; Lung adenocarcinoma, LAC; Head and neck squamous cell carcinoma, HNSCC; Mesenchymal stem cell, MSC; Human umbilical vein endothelial cell, HUVEC; Human lymphatic endothelial cell, HLEC; ↑, Upregulated or activated; ↓, Downregulated or inhibited;
The tumor cells-derived exosomes transfer some crucial miRNAs, lncRNAs, and proteins to the cancer microenvironment cells, mainly containing epithelial cells, macrophages, endothelial cells, and fibroblasts. This contributes to inflammatory cell infiltration, angiogenesis, obtainment of tumor-associated cell phenotypes, and tumor innervation.
The binding of RNA to toll-like receptor (TLR) of epithelial cells or macrophages can induce tumor microenvironment inflammatory phenotypes. Liu et al. [
The crosstalk between cancer cells and endothelial cells facilitates angiogenesis. Epithelial ovarian cancer (EOC) cells-derived exosomes enhance proangiogenic properties of human umbilical vein endothelial cells (HUVECs) via metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) trafficking which may stimulate the expression of vascular endothelial growth factor (VEGF)-A, VEGF-D, epithelial-derived neutrophil-activating protein 78 (ENA-78), placental growth factor (PlGF), IL-8, angiogenin, basic fibroblast growth factor (bFGF), and leptin in HUVECs [
Exosomes communicating with fibroblasts also trigger reprogramming of recipient cells into cancer-associated phenotypes. These exosomes released from lung adenocarcinoma cells (LAC) transfer miR-142-3p to lung endothelial cells and fibroblasts, which promotes angiogenesis mediated by inhibiting TGF
Exosomes can also increase the nerve distribution of the microenvironment to elevate the malignant degree of tumor cells. Head and neck squamous cell carcinomas (HNSCC) released-exosomal EphrinB1 can induce tumor innervation in the PC12 neuronal model in vitro and the murine model in vivo, and patients with increased tumor innervation are prone to suffer from cancer metastasis [
Meanwhile, surrounding stromal cells-derived exosomes are also involved in preparing microenvironment amenable for tumor colonization.
EOC-associated macrophages transfer miR-29a-3p and miR-21-5p to CD4+T cells via exosomes, which synergistically inhibits the activity of STAT3 and causes the imbalance of regulatory T cells (Treg)/helper T cell 17 (Th17). This contributes to form an immune-suppressive microenvironment [
MSCs play dual roles-stimulative or inhibitory in tumor progression by the interaction of MSC-derived exosomes and tumor microenvironment cells, which affects angiogenesis, immune response, migration, and invasion of tumors [
Tumor cells- and microenvironment cells-derived exosomes commonly act on changing the proliferation activity, migration, invasion, and further distant metastasis of tumor cells (Table
Involvement of exosomes in influencing the function of tumor cells.
The role of tumor cells-derived exosomes in influencing tumor cells | ||||||
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Cancer type | Donor cells | Recipient cells | Study molecule | Signal axis | Effect | Ref. |
| ||||||
Melanoma | Tumor cells | Tumor cells | RAB27A | Migration and invasion↑ | [ | |
Lung cancer | Tumor cells | Tumor cells | lnc-MMP2-2 | lnc-MMP2-2→MMP2↑ | Migration and invasion↑ | [ |
CRC | Hypoxic tumor cells | Normoxic tumor cells | HIF1A | HIF1A→Wnt4-activated | Migration and invasion↑ | [ |
PDAC | Tumor cells | Tumor cells | miR-222 | miR-222→p27↓ | Proliferation, invasion and migration↑ | [ |
Breast cancer | Tumor cells | Tumor cells | CAV1 | Migration and invasion↑ | [ | |
Breast cancer | Exosomes from plasma of healthy donor(the exception of study mode) | Tumor cells | surface proteins | surface proteins→FAK signaling pathway↑ | Adhesive ability and migration↑ | [ |
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The role of microenvironment cells-derived exosomes in influencing tumor cells | ||||||
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CRC | Tumor associated M2 macrophages | Tumor cells | miR-21-5p and miR-155-5p | miR-21-5p and miR-155-5p → BRG1↓ | Migration and invasion↑ | [ |
OSCC | CAFs | Tumor cells | miR-34a-5p | miR-34a-5p→AXL↓→AKT/GSK-3 | Proliferation, EMT and metastasis↑ | [ |
| ||||||
HCC | CSCs | Tumor cells | exosomal molecules | exosomal molecules→Bax and p53↓, Bcl2↑; VEGF↑; P13K, ERK and MMP9↑, TIMP1↓; TGF | Tumor progression↑ | [ |
BM-MSCs | exosomal molecules | contrary to the above expression changes | Tumor progression↓ |
Note: Colorectal cancer, CRC, Pancreatic ductal adenocarcinoma, PDAC; Oral squamous cell carcinoma, OSCC; Cancer-associated fibroblast, CAF; Hepatocellular carcinoma, HCC; Cancer stem cell, CSC; Bone marrow-mesenchymal stem cell, BM-MSC; ↑, Upregulated or activated; ↓, Downregulated or inhibited.
Tumor cells-released exosomes affect activities of tumor cells via autocrine and paracrine processes.
Ras-related protein Rab-27A (RAB27A) is upregulated in melanomas compared with normal skin or nevi and is related to the advanced stage of melanomas for patients. Exosomes enriched with RAB27A can rescue the invasion phenotype of the melanoma cells after the knockdown of RAB27A, which reveals that exosomes promote melanoma metastasis by changing the ability of invasion and motility of surrounding melanoma cells [
In addition, there is a distinct model for studying exosomes function. When most studies focus on tumor-derived exosomes, Shtam et al. pay attention to exosomes from plasma of healthy donor. They have found that these exosomes can increase adhesive ability of breast cancer cells in vitro and migratory activities in Zebrafish model, which is dependent on the interaction of exosomal surface proteins and breast cancer cells, and the activation of focal adhesion kinase (FAK) signaling pathway [
When tumor cells-derived exosomes modify diverse tumor associated microenvironment cells, in turn, these cells release exosomes acting on the functions of tumor cells.
For CRC metastasis, exosomes derived from tumor associated M2 macrophage transfer miR-21-5p and miR-155-5p to CRC cells, which results in downregulated expression of transcription activator BRG1 (BRG1) and enhanced migration and invasion of cancer cells [
Exosomes can transfer resistance to chemotherapy via two different ways (Figure
Influence of exosomes in changing the efficacy of chemotherapy. Tumor associated macrophage, TAM; epithelial ovarian cancer, EOC; ↑, upregulated.
A recent study shows that in hypoxic tumor microenvironment of EOC, tumor associated macrophages- (TAMs-) derived exosomes induce chemotherapy resistance of tumor cells via delivering miR-223 and activating miR-223/ phosphatase and tensin homolog- (PTEN-) PI3K/AKT signaling pathway [
Some studies focus on difference analysis based on different molecular components to select exosomal biomarkers, which sets the stage for in-depth mechanism investigation (Table
Potential exosomal biomarkers of cancer metastasis.
Potential biomarkers | Comparison analysis | Ref. | |
---|---|---|---|
Exosomal RNAs | miR-140-3p, miR-30d-5p, miR-29b-3p, miR-130b-3p, miR-330-5p, miR-296-3p | Exosomes derived from fast- and slow-migrating groups of PDLCs | [ |
miRNA-21 and lncRNA-ATB | Serum exosomes isolated from patients with different HCC stages | [ | |
miR-9 and miR-155 | Exosomes derived from breast cells with different metastatic ability | [ | |
miR-1290 and miR-375 | Plasma exosomes derived from CRPC patients with different prognosis | [ | |
miR-130b and Met | Serum exosomes isolated from prostate cancer patients and healthy donors | [ | |
circPRMT5 | Serum and urine exosomes from normal people and patients with UCB | [ | |
| |||
Exosomal proteins | CD82 | Exosomes derived from tissue, serum, and plasma in breast cancer patients | [ |
Eps8 | Exosomes purified from human pancreatic cancer cell lines with distinct stages | [ | |
CXCR7 and CXCL12 | Exosomes isolated from tissues of primary tumor, lung metastasis, and benign lung disease in CRC patients | [ |
Note: Patient-derived liver cell, PDLC; Hepatocellular carcinoma, HCC; Castration-resistant prostate cancer, CRPC; Urothelial carcinoma of the bladder, UCB; Colorectal cancer, CRC.
Exosomal miR-140-3p, miR-30d-5p, miR-29b-3p, miR-130b-3p, miR-330-5p, and miR-296-3p are associated with the migration ability of hepatocarcinoma cells by the comparison analysis of exosomal miRNAs profile in fast- and slow-migrating groups of patient-derived liver cells (PDLCs). The migration ability is assessed by the wound closure percentage of wound healing assay [
Wang et al. have shown that the level of CD82 antigen (CD82) in exosomes is negatively correlated with that in tissue for breast cancer patients, and the content of serum exosomal CD82 is higher in cancer group than that in the benign group and healthy control group. CD82 expression in serum exosomes is also positively correlated with cancer clinical stage. Therefore, there may be a redistribution of CD82 from tissue to serum exosomes, which reflects tumorigenesis and progression of breast cancer [
The multidirectional communications of tumor cells and tumor associated microenvironment cells via the trafficking of exosomes facilitate the enhancement of malignant phenotypes of tumor cells, promote the formation of premetastatic niche, and finally exhibit clinically detectable metastasis.
In view of the important involvement of exosomes in cancer metastasis, more in-depth studies of exosomes are expected to shed more light on its biogenesis, release, and relevant functions. However, these exosome results may be questionable, due to the lack of standard isolation and characterization methods. Another disturbing factor is the fact that other EV types are likely interfering with the analysis of exosomes [
Moreover, microvesicles as one of the EV types also gave rise to much attention in the cancer field. The prostate cancer cells-derived large oncosomes (a new class of shedded vesicles) are endocytosed by fibroblasts, which activates Myc proto-oncogene protein (MYC) of recipient cells via active AKT1, giving these fibroblasts a protumor phenotype [
A further question arises as to what stimulates these dormant cells into active states and promotes metastasis without a primary tumor. The contributor may be partially remaining exosomes derived from these seemingly stationary tumor cells in predetermined metastasis sites.
To demonstrate this hypothesis, it might be necessary to monitor exosomes alteration in blood of patients without detectable metastasis and then conduct long-term tracking of exosomal biomarkers for patients after tumor resection.
The study reminds us that detection of exosomal biomarkers in blood is dependent on selection of an appropriate specimen. Serum or plasma may give differing diagnostic test values. We need to further investigate the origin of these observed differences for a better prognosis monitoring.
Under the above speculation, exosomes are putative molecular transporters modifying their levels both in tumor cells and in recipient cells. They further alter the state of the two kinds of cells, being either beneficial or obstructive for tumor progression. Deciphering this important question is only in its infancy.
It can be expected that more specific therapeutic targets for cancer metastasis will be developed following these studies. Some research has already demonstrated that tumor cells are inhibited by reducing the production of some exosomes, by interfering with their encapsulated content before or after its packaging, as well as by modifying exosomes as drug carriers [
Bone marrow-derived cell
Bone marrow-mesenchymal stem cell
Cancer-associated fibroblast
Colorectal cancer
Castration-resistant prostate cancer
Cancer stem cell
Cervical squamous cell carcinoma
Extracellular matrix
Epithelial-mesenchymal transition
Epithelial ovarian cancer
Extracellular vesicle
Hepatocellular carcinoma
Human lymphatic endothelial cell
Head and neck squamous cell carcinoma
Human umbilical vein endothelial cell
Lung adenocarcinoma cell
Lewis lung carcinoma
Lymph node
Mesenchymal-epithelial transition
Mesenchymal stem cell
Oral squamous cell carcinoma
Pancreatic ductal adenocarcinoma
Patient-derived liver cell
Tumor associated macrophage
Helper T cell 17
Regulatory T cell
Serine/threonine-protein kinase Akt
Annexin A1
Annexin A6
Tyrosine-protein kinase receptor AXL
Apoptosis regulator BAX
Apoptosis regulator Bcl-2
Basic fibroblast growth factor
Transcription activator BRG1
Caveolin-1
C-C motif chemokine ligand 2
C-C chemokine receptor type 2 positive
CD82 antigen
Protein arginine N-methyltransferase 5 circular RNA
Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9
C-X-C motif chemokine ligand 12
C-X-C chemokine receptor type 7
Diethylnitrosamine
Dual specificity protein phosphatase 14
Epithelial-derived neutrophil-activating protein 78
Epidermal growth factor receptor pathway substrate 8
Extracellular signal-regulated kinase
Focal adhesion kinase
Glycogen synthase kinase-3 beta
Hypoxia-inducible factor 1-alpha
Interleukin
Integrin
Krüppel-like factor
Lnc-matrix metalloproteinase 2-2
LncRNA-activated by tumor growth factor-beta
Lymphocyte antigen 6C positive
Metastasis-associated lung adenocarcinoma transcript 1
Mitogen-activated protein kinase
Tyrosine-protein kinase Met or Hepatocyte growth factor receptor
Migration inhibitory factor
Matrix metalloproteinase
Matrix metalloproteinase 2
Myc proto-oncogene protein
Nuclear factor kappa-light-chain-enhancer of activated B cells
Cyclin-dependent kinase inhibitor 1B (
Cellular tumor antigen p53
Phosphoinositide 3-kinase
Placental growth factor
Serine/threonine-protein phosphatase 2A 55 kDa regulatory subunit B alpha isoform
Phosphatase and tensin homolog
Ras-related protein Rab-27A
A family of genes whose symbols use the S100 prefix; the “S100” symbol prefix is derived from the fact that these proteins are soluble in 100% ammonium sulfate at neutral pH
S100 calcium-binding protein A
Zinc finger transcription factor SNAIL
Small nuclear RNA
Proto-oncogene tyrosine-protein kinase Src
Signal transducer and activator of transcription 3
Transforming growth factor beta
Tissue inhibitor of metalloproteinases
Toll-like receptor
Vasohibin-1
Vascular endothelial growth factor
Protein Wnt-4.
The authors declare no conflicts of interest
The authors thank Quinn Ellner for English editing. This work was supported by grants from the provincial key scientific and technological project (project number: 2014K11-01-01-20).
Table S1: Difference among the three main types of EVs.