Xiao-Dong Wang

1First Affiliated Hospital of Zhejiang University School of Medicine,Hangzhou 310003,China.

Abstract

Tumor invasion and metastasis is the primary cause of death in patients with malignant tumors.Tumor-associated macrophages are macrophages that infiltrate the tumor tissue,and are the most frequent immune cells in the tumor microenvironment.The macrophages could secrete a variety of cytokines and can recognize,identify and clear the tumor cells.As the tumor develops,it plays a vital role in the growth,invasion and metastasis of the cancer.However,the underline mechanisms of how the tumor-associated macrophages promoting tumor metastasis is still not fully understood.Present studies demonstrated that macrophage-related exosomes might play an essential role in tumor metastasis.Exosomes may provide potential targets for the prevention and treatment of macrophage-related tumor metastasis.

Keywords:Exosomes,Cancer metastasis,Macrophages,Treatment targets

Introduction

Tumor-associated macrophages (TAMs) may promote the development of tumors through a variety of pathways,including secretion of various pro-inflammatory factors tumor necrosis factor α,IL-1β,IL-6,vascular endothelial growth factor and basis fibroblast growth factor,matrix metalloproteinases,et al [1-4].At the same time,M2 macrophages may inhibit the proliferation of T cells and weaken the immune system's killing effect on tumors [5-7].Besides,TAMs may also cause the epithelial-mesenchymal transformation of tumor cells around them [4,8-10].Recently,researchers used tumor-derived extracellular vesicles (TEVs)co-cultured with gastric cancer cell lines 44As3 and cancer-associated fibroblasts to treat macrophages,and found that TEVs may cause TAMs to express more inflammatory factors,matrix metalloproteinases And other immunosuppressive molecules.When such TAMs are co-cultured with fibroblasts,TAMs can release and deliver the contained TEVs components to fibroblasts.This process relies on the activation of caspase3 in TAMs,and this phenomenon is alleviated to a certain extent when caspase inhibitors are used.The researchers further found in vitro through matrigel invasion experiments that TEVs-treated TAMs penetrated matrigel more than control macrophages.What's more interesting is that the mixed cells of TEVs treated macrophages and 44As3 cells co-cultured also have stronger invasive ability than the control group.The results of in vivo experiments showed that the distribution of TAMs in the skin of the TEVs-MΦs group was very diffuse and had a tendency to spread to the side of the tumor.In contrast,the macrophages of the control group were regularly distributed with clear edges.Besides,in the in situ gastric cancer model,the mixed cells co-cultured with TEVs treated macrophages and 44As3 cells can penetrate deeper into the stomach wall than the mixed cells of the control group or simple tumor cells.The researchers believe that this study found and proposed the association between TEVs and the progression of macrophage-related tumors,providing new targets for tumor treatment[11].

Exosomes are small membrane vesicles(30-150 nm)containing complex RNA and proteins,for example,miRNAs and mRNAs.In 1983,exosomes were first discovered in sheep reticulocytes,and in 1987 Johnstone named it “exosome” [12,13].A variety of cells can secrete exosomes under normal and pathological conditions.It is mainly derived from the multivesicular body formed by the invagination of lysosomal microparticles in the cell,which is released into the extracellular matrix after fusion of the outer membrane of the multivehicle and the cell membrane.All cultured cell types can secrete exosomes,and exosomes naturally occur in body fluids,including blood,saliva,urine,cerebrospinal fluid,and breast milk [14-18].The precise molecular mechanism of their secretion and ingestion,their composition,“carriers”and corresponding functions have just begun to be studied.Exosomes are currently regarded as specifically secreted membrane vesicles,which are involved in cell-to-cell communication.Research interest in exosomes is growing,whether to study their function or understand how to use them for the development of minimally invasive diagnostics.In recent years,macrophages have become key regulatory cells in the tumor microenvironment.The exosomes produced by macrophages play an important role in the communication between immune cells and tumor cells.At present,the mechanism of TAMs promoting tumor metastasis is still under further study[19].

Effects of TAMs-related exosomes on tumor cells

Yang et al.showed that macrophages derived microvesicles may have the function to promote breast cancer metastasis [19].However,the underline mechanisms were not fully understood.Much research demonstrated that miRNAs and proteins are the main components of exosomes,so most studies focus on the function of miRNA and proteins on the metastasis of tumors [20].The tumor microenvironment recruits macrophages and affects their polarization.Affected by cytokines in the tumor microenvironment,macrophages differentiate into different types of TAMS,which are mainly divided into M1 and M2 types [21-23].With the continuous deepening of related research,there is a lot of evidence that TAMS plays a vital role in the development and progression of malignant tumors,invasion and metastasis,immune escape and the formation of blood vessels and lymphatic vessels [24-26].M1 type macrophages have the effect of killing tumor cells,while M2 type macrophages are shown to promote tumor growth.Zheng et al.showed that in gastric cancer,TAMs are characterized by M2-polarized phenotype and promote migration of gastric cancer cells in vitro and in vivo.M2-derived exosomes determine the TAMs-mediated pro-migratory activity.Using mass spectrometry,they identify that apolipoprotein E (ApoE) is a highly specific and effective protein in M2 macrophages-derived exosomes.Moreover,TAMs are uniquely immune cells population expressed ApoE in the gastric cancer microenvironment.However,exosomes derived from M2 macrophages of ApoE(-/-)mice have no significant effect on the migration of gastric cancer cells in vitro and in vivo[27].Choo et al.showed that M1 macrophage-derived nanovesicles potentiate the anticancer efficacy of immune checkpoint inhibitors [28].However,Xiao et al.showed that when the M1-like TAMs were transferred with THBS1,the macrophage got the function to promote the migration ability of oral squamous cell carcinoma [29].This study demonstrates that M1-like TAMs polarized by exosomes shows excellent potential as a therapeutic target for the control of cancerous migration.At the same time,macrophages not only affect the function of tumor cells,but also affect the function of immune cells,such as T cells.For example,Zhou et al.showed that exosomes released from TAMs transfer miRNAs that induce a Treg/Th17 cell imbalance in epithelial ovarian cancer.This study showed that miR-29a-3p and miR-21-5p were enriched in exosomes derived from TAMs.The two miRNA mimics were transfected into CD4+T cells,they directly suppressed STAT3 and regulated Treg/Th17 cells,inducing an imbalance,and they had a synergistic effect on STAT3 inhibition [30].These studies suggest that macrophages may be a target for treating tumors.Regulating macrophage function through exosomes may be a viable method to break the steady state of the tumor microenvironment and provide a basis for further treatment of tumors.However,there are currently few studies on TAMs exosomes.

Tumor cells affect macrophage function through exosomes

The polarization of TAMS is closely related to the tumor microenvironment.Under the effect of the long-term tumor microenvironment,TAMS mainly displays M2 type,which is closely related to tumor growth,development,invasion and metastasis.The polarization of macrophages to M1 and M2 types is reversible and adjustable [6,7,31-35].At all stages of the tumor,M1 and M2 TAMS exist; M1 is the main type in the early stage,and M2 is the main type in the middle and late stages as the tumor progresses,M1 type is gradually polarized to M2 type and the increase in the number of M2 TAMS also indicates a poor prognosis [9,10,36-39].In many cases,tumor cells regulate the differentiation of macrophages into M2 macrophages through exosomes,thereby playing a role in promoting tumor metastasis [40].Wang et al.showed that encapsulated miRNAs contribute to CXCL12/CXCR4-induced liver metastasis of colorectal cancer by enhancing the M2 polarization of macrophages [41].Lin et al.showed that bladder cancer cell secreted exosomal miR21 activates the PI3K/AKT pathway in macrophages to promote cancer progression [42].Gerloff et al.showed that miR-125b-5p was delivered to macrophages by melanoma exosomes and partially induces the observed tumor-promoting TAM phenotype [43].These studies suggest that miRNA in exosomes may be one of the essential mechanisms to promote tumor metastasis by regulating macrophage function.Therefore,some researchers have observed long-term non-coding RNA (lncRNA) targeting miRNA and observed the possible therapeutic effect.Liang et al.showed that lncRNA RPPH1 promotes colorectal cancer metastasis by interacting with TUBB3 and by promoting exosomes-mediated macrophage M2 polarization [44].This study suggests that lncRNA may become one of the ways to interfere with exosome-related miRNA.However,whether it can be used for cancer treatment by interfering with the action of miRNA requires further research.

At present,curing tumors by drug treatment is still a major challenge.In many cases,tumor resistance and other issues are still problems that need to be solved.But at present,the mechanism of tumor resistance is not completely clear.Many studies suggest that drug resistance of macrophages and tumors may have a certain relationship.TAMS continues to produce pro-angiogenic factors to produce a disordered vascular network,and the permeability of these new blood vessel changes,which can delay the diffusion of chemotherapy drugs into tumor tissues and affect the efficacy of chemotherapy drugs [45-52].Wang et al.showed that tumor-derived exosomes induce PD1+macrophage population in human gastric cancer that promotes disease progression [53].So far,chemotherapy is still an indispensable part of cancer treatment.However,some studies believe that some chemotherapy drugs,when killing cancer cells,will also promote the metastasis of cancer cells.In this process,exosomes play a very critical role.Even apoptotic tumor cells may derive microRNA-375 used CD36 from altering the TAMs phenotype and promoted tumor progress [54].Therefore,it may not be enough to treat tumor cells alone.The simultaneous treatment of multiple drugs,on the one hand against tumor cells and on the other side against some immune cells in the tumor microenvironment,maybe a new approach to tumor therapy.

Discussion and summary

Tumor cells are closely related to some immune cells in the tumor microenvironment,especially macrophages.These cells together form a whole.Exosomes may be a tool for communication between these cells.The interaction between tumor cells and tumor microenvironment is a necessary condition for the metastasis of malignant tumors.Tumor invasion and metastasis are the primary cause of death in patients with malignant tumors [6,24,32,55-58].Tumor metastasis involves multi-step,multi-stage,and multi-gene changes,and its mechanism is far from clear.More and more evidence shows that the interaction between tumor cells and tumor microenvironment is a necessary condition for the metastasis of malignant tumors.

Exosomes secreted by tumor cells can promote angiogenesis at the tumor site [59].During the growth of tumors,tumor cells secrete some carrying exosomes to promote angiogenesis at the tumor site.Studies have shown that exosomes released by hepatoma cells contain miRNAs,which can destroy the growth,integrity and tightness of vascular endothelial cells,promote hepatocellular carcinoma cell transvascular endothelial metastasis,and ultimately promote hepatocellular carcinoma into the bloodstream and other organs metastasis [60].Tumor cell exosomes may also increase drug resistance of the tumor.At present,chemotherapy is one of the most effective means of treating cancer.Still,it is worth noting that tumor cells can excrete chemotherapy drugs by exocrine exosomes,thereby enhancing their resistance to chemotherapy drugs [61-67].For example,biologically active lncARSR can be integrated into exosomes and transmitted to sensitive cells,thereby spreading resistance to sunitinib.And targeting lncARSR can treat the occurrence and development of sunitinib-resistant renal cell carcinoma.Therefore,predicting lncARSR can be used as a potential target for predicting and treating sunitinib resistance [68].Studies have found that the miRNA profiles of exosomes in the blood of patients with cancer and normal people are significantly different,indicating that exosomes are helpful for the diagnosis of cancer[69-72].

The unique biological characteristics of exosomes may be used for cancer diagnosis and treatment.These characteristics are: specific targeting,small size,shuttle signal transduction and biomolecules,and the ability to cross biological barriers.However,the biological limitations of exosomes still need to be further researched to solve,with a view to exosome the technical research of the body is translated into clinical application as soon as possible.