Exosomes are nanosized, organelle-like membranous vesicles secreted from various cell types, including normal cells and cancer cells. Exosomes contain abundant bioactive molecules, including nucleic acids, lipids, and proteins and dynamically participate in intercellular communications. By shuttling the functional molecules into the recipient cells, exosomes secreted by cancerous cells can alter the cellular environment to favor tumor growth and metastasis. In this review, we focus on exosomes to promote cancer progression via their various bioactive cargoes through different mechanisms/pathways. By recognizing these pathways, we can design efficient therapeutic strategies to control cancer progression.
Exosome, Cancer progression, microRNA, circRNA, lncRNA
Exosomes are membranous vesicles ranging in size from 30–100 nm in diameter. They are secreted from multiple cell types into the body fluids through exocytosis, a process commonly used for receptor discharge and intercellular communications . The concept of exosomes has evolved since its inception. A more generalized term is extracellular vesicles (EVs), which are classified into exosomes and microvesicles (MVs) . They are both double-layer phospholipid membranous vesicles, but differ in their size (exosomes: 30–100 nm in diameter; MVs: 100–1000 nm in diameter) and origin of cellular compartment . The two types of vesicles have shared biological functions. We follow the term of exosome in this review. Exosomes contain abundant bioactive molecules, including nucleic acids, lipids, and proteins . By shuttling the functional molecules into the recipient cells, exosomes dynamically participate in intercellular communications and are involved in both physiological and pathological processes in the body [4,5]. For example, exosomes secreted by healthy cells can transport homeostatic molecules such as tumor-suppressing proteins, transcriptional regulators, and various necessary genetic information. Exosomes released from immune effector cells are capable of inducing effective immune response to inhibit cancer growth [6,7]. On the other hand, cancer cell-derived exosomes can circulate within the body fluid and execute their biological functions through interorgan communication. Through this “long-distance” control mechanism, cancer cellderived exosomes can prepare sentinel lymph nodes for cancer metastasis . Exosomes released from cancer cells can trigger remote organ-specific pro-metastatic reaction to facilitate cancer metastasis through premetastatic niche formation at targeted organs .
Exosomes are observed at much higher levels in body fluids in pre-cancerous and cancer conditions than those in normal physiological conditions. Besides the change in quantity of the exosomes, the exosome components can also be altered in different biological conditions. One of the interesting finding is that diet can change exosome structure. High-fat diets can alter the lipid composition of exosomes from primarily phosphatidylethanolamine (PE) in exosomes from lean mice to phosphatidylcholine (PC) in exosomes from obese mice. After the intestinal exosomes from obese mice are taken up by macrophages and hepatocytes, they lead to the inhibition of the insulin signaling pathway and decreased insulin sensitivity . However, whether or not diet can affect the cancer-cell derived exosome structure or secretion is not known yet.
RNA Cargoes in Exosomes
miRNAs are the most researched functional cargoes in exosomes. miRNAs are short RNAs (21-23 nucleotides) that bind to the 3’ untranslated regions of target genes, causing translational repression and rapid degradation of the target transcript [11,12]. Different miRNAs are involved in cancer progression through numerous pathways/ mechanisms. Jiang et al. reported that, in breast cancer, the two miRNAs (miR-9 and miR-181a) derived from tumor exosomes can activate the JAK/STAT signaling pathway and promote the expansion of early-stage myeloid-derived suppressor cells (eMDSCs), thus cause immune escape and tumor growth . Merkel cell carcinoma (MCC)- derived exosomes have a high level of miR-375 expression. The cancer cell-derived miR-375 acts as shuttle miRNA and is transferred into the fibroblasts. The fibroblasts go into polarization and change to the phenotype of cancerassociated fibroblasts (CAFs) through p53 pathway. This creates a pro-tumorigenic microenvironment favorable for cancer growth . Exosome miRNAs also act as mediators between primary tumor cells and the distant organs. This communication is crucial for forming the pre-metastatic niche to promote tumor metastasis . A review by Wortzel et al. discusses in detail of the roles of exosome miRNA and other exosome cargoes in the development of pre-metastatic niche and in organotropic metastasis .
With the recent progress of immunotherapy, a substantial amount of research has been focused on the role of tumor-derived exosomes in immune modulation to promote cancer progression . Melanoma cell-derived exosomes contain miR-3187-3p, miR-498, and miR-149. Those miRNAs from exosomes can suppress CD8 T-cell cytotoxicity by regulating T cell receptor (TCR) signaling to promote cancer immune evasion . miR-222–3p secreted from ovarian cancer exosomes can induce macrophage polarization from a tumor-suppressing M1 phenotype to a tumor-promoting M2 phenotype, therefore to facilitate disease progression .
Another emerging field is exosome miRNA-mediated metabolic reprograming to promote cancer progression . Tumors are usually in hypoxia status due to their rapid oxygen consumption. Studies have shown that exosomes produced by hypoxic cancer cells are highly enriched in immunomodulatory proteins and chemokines including CSF-1, CCL2, and TGFβ . Through transferring of let- 7a miRNA, hypoxic cancer exosomes suppress the insulin- Akt-mTOR signaling pathway and evade host immunity to enhance cancer progression . Another study shows that miR-155 and miR-210 secreted from human melanoma cells can remodel stromal cell metabolism and induce the formation of pre-metastatic niche to promote tumor metastasis .
Long non-coding RNAs (LncRNAs)
LncRNAs is a type of RNA transcripts of more than 200 nucleotides with a limited or no protein-coding function. LncRNAs are involved in many diseases including cancer because they can modulate many biological processes including cell proliferation, differentiation, and cell death . Liu et al. has shown that the expression level of exosomal lncRNA 01133 (LINC01133) is high in pancreatic ductal adenocarcinoma (PDAC) patients and is correlated with poor overall survival rate. LINC01133 promotes the proliferation, migration, invasion, and epithelial-to-mesenchymal transition (EMT) of pancreatic cancer cells through Wnt/β-catenin pathway. Exosomal LINC01133 plays an important role in pancreatic cancer progression . Another lncRNA, CRNDE-h, is found to be abundant in colorectal cancer (CRC) exosomes. They can be transmitted to CD4+ T cells and contribute to the differentiation of CD4+ T cells into T helper 17 (Th17) cells to promote cancer progression . Haderk et al. found that chronic lymphocytic leukemia (CLL)-derived exosomal RNA can promote expression of PD-L1 and adopt an immunosuppressive phenotype in CLL patients . Noncoding RNA hY4 is a functional element of CLLderived exosomes acting through TLR7 pathway . In gastric cancer (GC), LINC01559 can be transmitted from mesenchymal stem cells (MSCs) to gastric cancer cells. LINC01559 accelerates GC progression by upregulating PGK1 and downregulating PTEN to activate the phosphatidylinositol 3-kinase/AKT serine/threonine kinase (PI3K/AKT) pathway .
Circular RNAs (circRNAs)
circRNAs is a group of noncoding RNA with a circular structure in eukaryotes. circRNAs have tissue-specific and cell-specific expression patterns. circRNAs act as microRNA or protein inhibitors (‘sponges’). They execute important biological functions by regulating protein function or by being translated themselves [27,28]. Because of the covalently closed structure of these transcripts, it is challenging to detect and quantitate this type of RNA. It is even more difficult to characterize their function and define their roles in diseases. With the recent advances in high-throughput RNA sequencing and computational tools, it makes us possible to illustrate their function in cancer . circRNAs are found to be enriched and stable in exosomes [29,30]. Exosomal circRNA_102481 is shown to be significantly up-regulated in non-small cell lung cancer (NSCLC) with EGFR-TKIs resistance. Expression of exosomal circRNA_102481 is associated with advanced TNM stage, increased odds of brain metastasis, and reduction of overall survival in NSCLC patients. Exosomal circRNA_ 102481 could contribute to EGFR-TKIs resistance via the microRNA-30a-5p/ROR1 axis in NSCLC . Li, et al reported that exosomal circRNA_0044516 is significantly upregulated from prostate cancer patients. circRNA_0044516 plays an oncogenic role in prostate cancer to promote prostate cancer cell survival and metastasis . Exosomal circRNA-100338 is also found to promote the metastasis of hepatocellular carcinoma by enhancing tumor invasiveness and angiogenesis .
Protein Cargoes in Exosomes
ENO1, one of the three major enolases, is a key regulatory enzyme in glycolysis and is widely present in various cells and tissues . Recent studies showed that ENO1 is upregulated in hepatocellular carcinoma (HCC) cells and has even higher expression in highly metastatic HCC cells as well as in exosomes . ENO1 can be transferred between HCC cells via exosomes. Exosome-shuttled ENO1 can upregulate integrin α6β4 expression and activate the FAK/Src-p38MAPK pathway to promote HCC growth, metastasis, and disease progression .
Soluble E-cadherin (sE-cad)
sE-cad is an 80-kDa protein that is highly expressed in the ascites of ovarian cancer patients. It is a potent inducer of angiogenesis . There is evidence to show that plentiful of sE-cad is released in the form of exosomes. Exosomes with positive sE-cad heterodimerize with cadherin on endothelial cells and induce a sequential activation of β-catenin and NFκB signaling. Activation of both pathways activates the angiogenesis process in ovarian cancer, thus promoting cancer progression .
c-Src is a membrane-associated tyrosine kinase with important functions in the signaling transduction to control cell growth and migration . Hikita et al. reported that c-Src is localized in the endosomal membrane. Once c-Src in the endosomal membrane is activated, it can be encapsulated in exosomes and promote exosome secretion. The secretion of exosomes can not only contribute to the maintenance of malignant phenotypes, but also transduce oncogenic signals to promote cancer progression .
One of the most interesting findings is that cancer exosome can cause tumor aggressiveness by inducing cancer innervation through exosome-packaged molecule, EphrinB1 . Recent studies have shown that patients with heavily innervated cancers suffer from increased metastasis and dismal survival when compared to those with fewer innervated cancers [39,40]. EphrinB1 is a single pass transmembrane protein ligand that can bind and activate the Eph receptor, tyrosine kinases . EphrinB1 acts as an axonal guidance molecule in the development of nervous system . Exosome-packaged EphrinB1 works as an axonal guidance molecule to induce neurite outgrowth and to promote cancer innervation .
From the discussions above, we may notice that each type of cancer has its own specific genetic signature. Since exosomes are derived from their original cancer cells, it is rational to recognize that the exosomes from different cancer types have their specific genetic profile that distinguishes them from each other. For example, different types of cancers have unique exosomal RNAs that can differentiate themselves and be used as cancer biomarkers . Proteomic analysis in exosomes also show that protein profiles differ in diverse cancer types and subtypes, as well as stages [44,45].
Table 1 summarizes the mechanisms/pathways involved in cancer progression by selected bioactive cargoes within the exosomes. We want to mention that, there are far more biological molecules and mechanims/pathways within exosomes contribute to this process. The goal of identifying the mechanisms of exosome-mediated cancer progression is to target those specific molecules, control cancer growth and metastasis, and eventually increase overall survival. Instead of targeting specific molecules for control cancer progression, a more general approach is to control exosome release and/or exosome uptake. For example, Rab27 controls exosome release. Rab27 is stabilized by interacting with KIBRA. Knockdown of KIBRA leads to decreased exosome secretion, so it follows that KIBRA can be used to regulate exosome secretion . Other endocytosis inhibitors are used to block the uptake of exosomes as an alternative strategy to inhibit the malignant cell growth .
|Bioactie cargoes||Names of specific
|RNAs||miRNAs||miR-9 and miR-181a||Breast cancer||JAK/STAT signaling pathway||13|
|miR-375||Merkel cell carcinoma||Fibroblasts polarization to cancer-
associated fibroblasts/P53 pathway
|Melanoma||Cancer immune evasion||17|
|miR-222–3p||Ovarian cancer||M2 macrophage polarization||18|
|miR-155 and miR-210||Melanoma||Remodel of stromal cell metabolism||21|
|CRNDE-h||Colorectal cancer||Immune suppression||24|
|LINC01559||Gastric cancer||PI3K/AKT pathway||25,26|
|circRNAs||circRNA_102481||Non-small cell lung cancer||EGFR-TKIs resistance||31|
|circRNA_0044516||Prostate cancer||Oncogenic signaling||32|
|Protein||ENO1||Hepatocellular carcinoma||FAK/Src-p38MAPK pathway||35|
|Soluble E-cadherin||Ovarian cancer||β-catenin and NFkB signaling||36|
|c-Src||Colon cancer||Oncogenic signaling||37|
|EphrinB1||Head and neck squamous cell carcinoma||Cancer innervation||38|
Table 1: Functions of bioactive cargoes in exosomes.
In summary, exosomes have been shown to promote cancer progression via their various bioactive cargoes. Much progress has been made on its mechanisms throughout the years. Accordingly, innovative strategies have been designed to target those molecules/pathways to control malignant progression. Since cancer and the surrounding microenvironment has a complex context and dynamic cross-talk, it is still challenging to design personalized medicine to control cancer progression.
We acknowledge the support from the NIH grant U01HL127518 (Paula Bates, PI), which was funded by NIMHD, NHGRI, NHLBI, NIA, NIAAA, NIBIB, NICHD, NIDA, NINDS, NINR, and NLN.
2. Yang N, Zhao Y, Wu X, Zhang N, Song H, Wei W, et al. Recent Advances in Extracellular Vesicles and their involvements in Vasculitis. Free Radical Biology and Medicine. 2021 May 2;171:203-218.
3. Mathieu M, Martin-Jaular L, Lavieu G, Théry C. Specificities of secretion and uptake of exosomes and other extracellular vesicles for cell-to-cell communication. Nature Cell Biology. 2019 Jan;21(1):9-17.
4. Skog J, Würdinger T, Van Rijn S, Meijer DH, Gainche L, Curry WT, et al. Glioblastoma microvesicles transport RNA and proteins that promote tumour growth and provide diagnostic biomarkers. Nature Cell Biology. 2008 Dec;10(12):1470-6.
5. Valadi H, Ekström K, Bossios A, Sjöstrand M, Lee JJ, Lötvall JO. Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells. Nature Cell Biology. 2007 Jun;9(6):654-9.
6. Barros FM, Carneiro F, Machado JC, Melo SA. Exosomes and immune response in cancer: friends or foes?. Frontiers in Immunology. 2018 Apr 11;9:730.
7. Lindenbergh MF, Stoorvogel W. Antigen presentation by extracellular vesicles from professional antigenpresenting cells. Annual Review of Immunology. 2018 Apr 26;36:435-59.
8. Hood JL, San RS, Wickline SA. Exosomes released by melanoma cells prepare sentinel lymph nodes for tumor metastasis. Cancer Research. 2011 Jun 1;71(11):3792-801.
9. Vidal-Vanaclocha F, Crende O, de Durango CG, Herreros-Pomares A, López-Doménech S, González Á, et al. Liver prometastatic reaction: Stimulating factors and responsive cancer phenotypes. InSeminars in Cancer Biology 2021 Jun 1 (Vol. 71, pp. 122-133).
10. Kumar A, Sundaram K, Mu J, Dryden GW, Sriwastva MK, Lei C, et al. High-fat diet-induced upregulation of exosomal phosphatidylcholine contributes to insulin resistance. Nature Communications. 2021 Jan 11;12(1):213.
11. Lee TH, D’Asti E, Magnus N, Al-Nedawi K, Meehan B, Rak J. Microvesicles as mediators of intercellular communication in cancer—the emerging science of cellular ‘debris’. InSeminars in Immunopathology 2011 Sep (Vol. 33, No. 5, pp. 455-467).
12. Sandhu S, Garzon R. Potential applications of microRNAs in cancer diagnosis, prognosis, and treatment. InSeminars in Oncology 2011 Dec 1 (Vol. 38, No. 6, pp. 781-787).
13. Jiang M, Zhang W, Zhang R, Liu P, Ye Y, Yu W, et al. Cancer exosome-derived miR-9 and miR-181a promote the development of early-stage MDSCs via interfering with SOCS3 and PIAS3 respectively in breast cancer. Oncogene. 2020 Jun;39(24):4681-94.
14. Fan K, Spassova I, Gravemeyer J, Ritter C, Horny K, Lange A, et al. Merkel cell carcinoma-derived exosome-shuttle miR-375 induces fibroblast polarization by inhibition of RBPJ and p53. Oncogene. 2021 Feb;40(5):980-96.
15. Wortzel I, Dror S, Kenific CM, Lyden D. Exosomemediated metastasis: communication from a distance. Developmental Cell. 2019 May 6;49(3):347-60.
16. Lone SN, Bhat AA, Wani NA, Karedath T, Hashem S, Nisar S, et al. miRNAs as novel immunoregulators in cancer. Seminars in Cell & Developmental Biology 2021 Apr 27; S1084-9521(21)00086-0.
17. Vignard V, Labbé M, Marec N, André-Grégoire G, Jouand N, Fonteneau JF, et al. MicroRNAs in tumor exosomes drive immune escape in melanoma. Cancer Immunology Research. 2020 Feb 1;8(2):255-67.
18. Ying X, Wu Q, Wu X, Zhu Q, Wang X, Jiang L, et al. Epithelial ovarian cancer-secreted exosomal miR-222-3p induces polarization of tumor-associated macrophages. Oncotarget. 2016; 7: 43076–43087.
19. Yang E, Wang X, Gong Z, Yu M, Wu H, Zhang D. Exosome-mediated metabolic reprogramming: the emerging role in tumor microenvironment remodeling and its influence on cancer progression. Signal Transduction and Targeted Therapy. 2020 Oct 19;5(1): 242.
20. Park JE, Dutta B, Tse SW, Gupta N, Tan CF, Low JK, et al. Hypoxia-induced tumor exosomes promote M2-like macrophage polarization of infiltrating myeloid cells and microRNA-mediated metabolic shift. Oncogene. 2019 Jun;38(26):5158-73.
21. La Shu S, Yang Y, Allen CL, Maguire O, Minderman H, Sen A, et al. Metabolic reprogramming of stromal fibroblasts by melanoma exosome microRNA favours a pre-metastatic microenvironment. Scientific Reports. 2018 Aug 27;8(1):12905.
22. Ponting CP, Oliver PL, Reik W. Evolution and functions of long noncoding RNAs. Cell. 2009 Feb 20;136(4):629-41.
23. Liu Y, Tang T, Yang X, Qin P, Wang P, Zhang H, et al. Tumor-derived exosomal long noncoding RNA LINC01133, regulated by Periostin, contributes to pancreatic ductal adenocarcinoma epithelial-mesenchymal transition through the Wnt/ß-catenin pathway by silencing AXIN2. Oncogene. 2021 Apr;40(17):3164-79.
24. Sun J, Jia H, Bao X, Wu Y, Zhu T, Li R, et al. Tumor exosome promotes Th17 cell differentiation by transmitting the lncRNA CRNDE-h in colorectal cancer. Cell Death & Disease. 2021 Jan 25;12(1):123.
25. Haderk F, Schulz R, Iskar M, Cid LL, Worst T, Willmund KV, et al. Tumor-derived exosomes modulate PD-L1 expression in monocytes. Science Immunology. 2017 Jul 28;2(13):eaah5509.
26. Wang L, Bo X, Yi X, Xiao X, Zheng Q, Ma L, et al. Exosome-transferred LINC01559 promotes the progression of gastric cancer via PI3K/AKT signaling pathway. Cell Death & Disease. 2020 Sep 7;11(9):723.
27. Zhao X, Cai Y, Xu J. Circular RNAs: biogenesis, mechanism, and function in human cancers. International Journal of Molecular Sciences. 2019 Jan;20(16):3926.
28. Kristensen LS, Andersen MS, Stagsted LV, Ebbesen KK, Hansen TB, Kjems J. The biogenesis, biology and characterization of circular RNAs. Nature Reviews Genetics. 2019 Nov;20(11):675-91.
29. Li Y, Zheng Q, Bao C, Li S, Guo W, Zhao J, et al. Circular RNA is enriched and stable in exosomes: a promising biomarker for cancer diagnosis. Cell Research. 2015 Aug;25(8):981-4.
30. Vo JN, Cieslik M, Zhang Y, Shukla S, Xiao L, Zhang Y, et al. The landscape of circular RNA in cancer. Cell. 2019 Feb 7;176(4):869-81.
31. Yang B, Teng F, Chang L, Wang J, Liu DL, Cui YS, et al. Tumor-derived exosomal circRNA_102481 contributes to EGFR-TKIs resistance via the miR-30a-5p/ROR1 axis in non-small cell lung cancer. Aging (Albany NY). 2021 May 15;13(9):13264.
32. Li T, Sun X, Chen L. Exosome circ_0044516 promotes prostate cancer cell proliferation and metastasis as a potential biomarker. Journal of Cellular Biochemistry. 2020 Mar;121(3):2118-26.
33. Huang XY, Huang ZL, Huang J, Xu B, Huang XY, Xu YH, et al. Exosomal circRNA-100338 promotes hepatocellular carcinoma metastasis via enhancing invasiveness and angiogenesis. Journal of Experimental & Clinical Cancer Research. 2020 Dec 1;39(1):20.
34. Piast M, Kustrzeba-Wójcicka I, Matusiewicz M, Banas T. Molecular evolution of enolase. Acta Biochimica Polonica. 2005 May 15;52(2):507-13.
35. Jiang K, Dong C, Yin Z, Li R, Mao J, Wang C, et al. Exosome-derived ENO1 regulates integrin a6ß4 expression and promotes hepatocellular carcinoma growth and metastasis. Cell Death & Disease. 2020 Nov 12;11(11):972.
36. Tang MK, Yue PY, Ip PP, Huang RL, Lai HC, Cheung AN, et al. Soluble E-cadherin promotes tumor angiogenesis and localizes to exosome surface. Nature Communications. 2018 Jun 11;9(1):2270.
37. Hikita T, Kuwahara A, Watanabe R, Miyata M, Oneyama C. Src in endosomal membranes promotes exosome secretion and tumor progression. Scientific Reports. 2019 Mar 1;9(1):3265.
38. Madeo M, Colbert PL, Vermeer DW, Lucido CT, Cain JT, Vichaya EG, et al. Cancer exosomes induce tumor innervation. Nature Communications 2018; 9 (1): 4284.
39. Fernández EV, Price DK, Figg WD. Prostate cancer progression attributed to autonomic nerve development: potential for therapeutic prevention of localized and metastatic disease. Cancer Biology & Therapy. 2013 Nov 1;14(11):1005-6.
40. Rabben HL, Zhao CM, Hayakawa Y, C Wang T, Chen D. Vagotomy and gastric tumorigenesis. Current Neuropharmacology. 2016 Nov 1;14(8):967-72.
41. Pasquale EB. Eph receptors and ephrins in cancer: bidirectional signalling and beyond. Nature Reviews Cancer. 2010 Mar;10(3):165-80.
42. Thiede-Stan NK, Schwab ME. Attractive and repulsive factors act through multi-subunit receptor complexes to regulate nerve fiber growth. Journal of Cell Science. 2015 Jul 15;128(14):2403-14.
43. Valencia K, Montuenga LM. Exosomes in Liquid Biopsy: The Nanometric World in the Pursuit of Precision Oncology. Cancers. 2021 Jan;13(9):2147.
44. Vinik Y, Ortega FG, Mills GB, Lu Y, Jurkowicz M, Halperin S, et al. Proteomic analysis of circulating extracellular vesicles identifies potential markers of breast cancer progression, recurrence, and response. Science Advances. 2020 Oct 1;6(40):eaba5714.
45. Rontogianni S, Synadaki E, Li B, Liefaard MC, Lips EH, Wesseling J, et al. Proteomic profiling of extracellular vesicles allows for human breast cancer subtyping. Communications Biology. 2019 Sep 3;2(1):325.
46. Song L, Tang S, Han X, Jiang Z, Dong L, Liu C. KIBRA controls exosome secretion via inhibiting the proteasomal degradation of Rab27a. Nature Communications 2019; 10 (1): 1639.
47. Zheng Y, Tu C, Zhang J, Wang J. Inhibition of multiple myeloma derived exosomes uptake suppresses the functional response in bone marrow stromal cell. International Journal of Oncology. 2019 Mar 1;54(3):1061- 70.