]
Focusing on the exosomal cargo, scientists have
currently been concentrating on exosomal miRNA
as a specific communication message delivered from
cancer cells to other cells after many years of research
on the roles of miRNAs in cancer biology and therapy.
This exosomal miRNA has an important role in the
proliferation and migration of the tumor cell. MicroRNAs
(miRNAs) are small [19-25 nucleotides), noncoding,
single-stranded RNAs that regulate gene expression
by binding imperfectly to the 3' untranslated
region (UTR) of target genes.[,] Over than 60% of all human genes are suggested to be directly regulated by
miRNAs, as a single miRNA can target several hundred
genes, and a single target gene generally contains multiple
miRNA binding sites.[]
In addition to their advantages in terms of quantity,
quality, and stability, several studies have reported
significant variations in the levels of exosomal miRNA
and free circulating miRNA in healthy individuals and
those with pathological conditions including cancer.
However, no significant differences were detected in
their levels among healthy individuals.[-] These
findings suggest that the selective transfer of miRNAs
through exosomes facilitates cancer progression
and dissemination. The above discussed observations
have recently led scientists and researchers to focus on
the significance of investigating the role of exosomal
miRNAs in cancer biogenesis and progression. In the
present review, we will examine the recent studies on
exosomal miRNAs, which are suggested to play an
important role in cancer metastasis in various cancer
types, and explore the potential mechanisms underlying
their involvement in the progression of metastasis.
THE POTENTIAL MECHANISMS OF
EXOSOMAL miRNAs IN METASTASIS
The studies in the last decades have supported the early
ideas regarding metastasis evolution, known as the
"seed and soil" hypothesis, which held that cancer cells
seed metastasis through a series of orderly steps to a
compatible tissue microenvironment.[] These steps
can be summarized as the loss of cellular adhesion, increased
motility and invasiveness, entry and survival in
the circulation, exit into new tissue, and colonization of
a distant site.[] Because the molecular process underlying
tumor metastasis is still complex and not fully understood,
numerous research published since 2007 have
demonstrated the function of miRNA in activating or
preventing metastasis at various stages of the metastatic
pathway.[-] In the same context, various studies
have highlighted the role of cancer secreted miRNAs
by exosomes in controlling many cellular components
of the tumor microenvironment, facilitating metastasis.[] Although all mechanisms are interrelated, they
can be categorized as follows for better clarification:
Promoting Angiogenesis and Vascular
Permeability
Angiogenesis is the process of generating new blood
capillaries from the existing vasculature. The role of this
process is crucial in several physiological activities including embryonic development, female reproductive
processes, and tissue repair. It also plays a significant
function in pathological states such as inflammatory
disorders and cancer.[] The basic steps of sprouting
angiogenesis include enzymatic degradation of capillary
basement membrane, proliferation of endothelial
cells (ECs), directed migration of ECs, tubulogenesis
(ECs tube formation), vessel fusion, vessel pruning,
and pericyte stabilization.[]
To summarize the molecular mechanism of angiogenesis,
hypoxia-inducible factor-α (HIFα) is reported
to have a crucial role as a transcription factor in the
signaling processes associated with angiogenesis. HIFwith angiogenesis
initiates the activation and subsequent release of vascular
endothelial growth factor (VEGF), which then
binds to its receptors VEGFR1 and VEGFR2 on endothelial
cells (ECs), triggering downstream signaling
pathways (ERK, p38 MAPK, and p125FAK, etc.) that
lead to the activation of endothelial cells.
The activation of endothelial cells (ECs) is mediated
by numerous secreted factors, including matrix
metalloproteinases (MMPs), which is important in facilitating
EC migration and promoting vascularization.
In addition to HIFα, other transcription factors such
as Activator Protein-1 (Ap1) and Specificity Protein-1
(Sp1) have been suggested to exert regulatory effects
on VEGF expression by binding to its promoter region.
Conversely, NFκβ is known to facilitate the upregulation
of VEGF, thus enhancing the process of angiogenesis.
Furthermore, the potential contribution of
the SMAD and NOTCH signaling pathways has been
proposed in the stimulation of the migration of endothelial
cells and the promotion of angiogenesis.
Poliseno et al.[] were the first to propose the
possible participation of microRNAs (miRNAs) in
the process of angiogenesis in 2006. Their observation
of the regulatory functions of certain miRNAs in
modulating the expression of receptors for angiogenic
factors supported their hypothesis. Subsequently, various
studies investigated the miRNAs as a component
of exosomes, which are considered an important intercellular
communication mode in cancer progression
to explore their involvement in promoting angiogenesis
and vascular permeability and to explore
their target genes.
Researchers suggested that specific miRNAs in nasopharyngeal
carcinoma may contribute to the suppression
of testis-specific gene antigen (TSGA10) expression
following their transfer from cancer cells to
endothelial cells (ECs) via exosomes. TSGA10 is known
to closely interact with hypoxia-inducible factor-1 (HIF-1α) and exert potent inhibitory effects on tumor
angiogenesis and metastasis.[-] The inhibition
of SMAD4 and STAT6 expression is another possible
target of exosomal miRNAs in which STAT6 depletion
reduces the inhibitory effects of interleukin-13 (IL-13)
on human coronary artery endothelial cell migration
and tube formation.[] In colorectal cancer, exosomal
miR-25-3p was shown to selectively target the transcription
factors Kruppel-like factor KLF2 and KLF4,
leading to the down regulation of ZO-1, Occludin, and
Claudin5, and the up regulation of VEGFR2.[] Similarly,
miR-182-5p employs a similar mechanism to promote
vascular permeability and angiogenesis in glioblastoma.[] Targeting of prolyl-hydroxylase (PHD1
and PHD2) and the consequent accumulation of (HIF-
1α) in endothelial cells by exosomal miR-23a enhances
angiogenesis process in lung cancer.[]
Mediating the Induction of EMT
Exosomal miRNAs are well-recognized as the components
of complex regulatory networks that facilitate
the transition in gene expression from an epithelial
to a mesenchymal phenotype, a process described as
the epithelial-to-mesenchymal transition (EMT). This
transition is pivotal in driving the malignant transformation
of epithelial cancer cells and promoting
metastasis.[,] In this process, epithelial cancer
cells lose the expression of epithelial markers, such
as E-cadherin, occludin, claudins, ZO-1, and connexins,
while they acquire mesenchymal markers, including
N-cadherin, vimentin, and fibronectin. These
morphological and molecular alterations enhance the
metastatic potential of cancer cells.[] Numerous molecular
pathways have been identified to have involved
in exosomal miRNAs in the regulation of epithelial-tomesenchymal
transition (EMT). The activation of the
Wnt/?-catenin signaling pathway, which is a trigger of
the EMT process, is one of the most frequently targeted
pathways by exosomal miRNAs.[] Exosomal miRNAs
may also regulate additional signaling pathways,
including PI3K/AKT and ERK pathways.[,]
As previously discussed, cancer-secreted miRNAs
may promote the formation of a pre-metastatic niche
by downregulating the Krüppel-like factor-2 (KLF2)
and Krüppel-like factor-4 (KLF4) genes, members of
the zinc finger-containing transcription factor family.
These factors regulate the expression of tight junction
(TJ) proteins, including ZO-1, occludin, and Claudin5,
as well as the expression of VEGFR2 in endothelial
cells.[] In the similar context, the suppressing Krev
interaction trapped protein-1 (KRIT1), key regulator of endothelial cell?cell junctions,[] and subsequently
activation of the β-Catenin signaling pathway represent
another proposed mechanism through which
exosomal miRNAs contribute to EMT process.[] Endothelial
junction integrity could also be compromised
by the direct repression of p120 expression through
specific exosomal miRNAs (exomirs).[] Additionally,
the down-regulation of the tumor suppressor gene
CUGBP Elav-Like Family Member 2 (CELF2), via WW
domain-containing oxidoreductase (WWOX) provides
a further molecular mechanism underlying the role of
exosomal miRNAs released from cancer-associated fibroblasts
(CAFs) in colorectal cancer.[]
Promoting Migration, Invasion, and Metastasis
The two distinct patterns of invasive growth are currently
recognized as the collective cell migration and
single-cell migration (also known as individual migration).
In collective cell migration, the entire clusters
of cancer cells infiltrate the surrounding tissues
as cohesive units, either originating from or detaching
from the primary tumor mass. These cells remain
interconnected through adhesion molecules such
as cadherins and intercellular gap junctions. In this
process, tumor cells utilize integrins to form focal
connections with the actin cytoskeleton, facilitating
the proteolytic degradation of the extracellular matrix
(ECM). This degradation creates a permissive
environment for tumor invasion, which is essential
for effective tumor cell migration. This mechanism
is crucial for ensuring the effective migration of tumor
tissue. In contrast, single-cell invasion involves
independent migration of individual tumor cells into
the surrounding tissues, which can occur through
mesenchymal, and amoeboid movement modes. The
transitions between these phenotypes are often driven
by the changes in the activity of specific cellular molecules,
allowing tumor cells to adapt to the unique
characteristics of their microenvironment.[]
A critical component of invasion involves the
enzymatic breakdown of the extracellular matrix
(ECM) and its components, facilitated by enzymes
such as matrix metalloproteinases (MMPs). MMPs
also play a crucial part in the process of invasion in
addition to their significant contribution to cell proliferation,
survival and angiogenesis. Furthermore,
the initiation of distinct signaling pathways, such as
the epidermal growth factor receptor (EGFR) signaling
pathway and phosphatidylinositol 3-kinase
(PI3K) pathway, stimulates the promotion of cancer
cell proliferation and invasion.[,]
Numerous studies suggested that the exosomal
microRNAs (miRNAs) regulate key cellular processes
involved in cancer cell dissemination and metastasis,
including migration and invasion. To exemplify, the
exosomal miRNAs have been shown to target vascular
endothelial growth factor A (VEGFA) in ovarian cancer
cells, thereby influencing these processes.[] Furthermore,
cancer-derived exosomal microRNAs are
implicated in extracellular matrix (ECM) remodeling
and metastasis by activating cancer-associated fibroblasts
(CAFs), which play a critical role in tumor progression.[,] Conversely, CAFs can enhance metastasis
by secreting miRNAs via their own exosomes,
which are subsequently delivered to cancer cells. This
exchange promotes stemness, epithelial-to-mesenchymal
transition (EMT), migration, and invasion in cancer
cells.[-]
Tumor Microenvironment Remodeling
The tumor microenvironment (TME) is a complex
and dynamic structure comprising various cell types
embedded within a modified extracellular matrix
(ECM).[] Tumor-derived exosomes and their cargo
play a pivotal role in mediating intercellular communication
between tumor and non-tumor cells, thereby
contributing to the remodeling of the TME and including
its heterogeneity. The processes facilitate tumor
development, invasion, and metastasis.[] The
effects of exosomal miRNAs on the tumor microenvironment
(TME) can be categorized into two primary
functional classifications within the context of tumor
growth and progression.
The Process of Reshaping the Extracellular Matrix
(ECM)
Tumor-derived exosomal miRNAs have the capacity
to initiate a cascade of signaling pathways that drive
the transformation of normal fibroblasts (NFs), which
are responsible for producing the extracellular matrix
(ECM) into cancer-associated fibroblasts (CAFs). This
transformation alters the physiological properties of
the ECM, creating a microenvironment conducive to
cancer cell proliferation.[] The activation of fibroblasts
by cancer-driven exosomal miRNAs is associated
with the upregulation of key markers including
?-smooth muscle actin (α-SMA), fibroblast growth
factor 2 (FGF2), and fibroblast activating protein
(FAP).[,]
Over the last decade, the increasing evidence has
highlighted the involvement of exosomal miRNAs in
the regulation of ECM remodeling. Wang et al.[] demonstrated that exosomal miR-27a from gastric
cancer (GC) cells is transferred to fibroblasts, leading
to decreased CSRP2 expression, increased α-SMA
expression, and fibroblast differentiation into cancer-
associated fibroblasts (CAFs). Similarly, a 2019
study revealed that exosomal miR-124 plays a role in
ECM remodeling by targeting sphingosine kinase 1
(SPHK1), thereby upregulating α-SMA and FAP expression
and promoting the differentiation of NFs
into CAFs in ovarian cancer.[] Furthermore, the
transfer of exosomal miR-10b from colorectal cancer
cells to fibroblasts results in the downregulation
of PIK3CA expression, reduced activity of the PI3K/
Akt/mTOR signaling pathway, increased TGFβ and
α-SMA expression, and the acquisition of CAF-like
properties by fibroblasts.[]
The Process of Mediating Inflammatory Cell
Invasion, and Immunological Evasion
Tumor cells have the ability to disrupt the maturation
and differentiation of immune cells by releasing
exosomal microRNAs, which activate multiple signal
transduction pathways. This ultimately leads to the
establishment of an immunosuppressive microenvironment
that supports tumor proliferation. Concurrently,
the production of inflammatory mediators by
tumor cells such as prostaglandins and arachidonic
acid, fosters the development of an inflammatory microenvironment.
In this context, exosomal miRNAs
contribute to the stimulation of extracellular receptor
signaling, disruption of cell adhesion, and maintenance
of a chronic low-grade inflammatory state, collectively
facilitating the evasion of tumor cells from
immune surveillance.[,]
Tumor-derived exosomal miRNAs also have the
capacity to influence the maturation and functioning
of dendritic cells (DCs), which serve as antigen-presenting
cells crucial for initiating T cell activation and
sustaining immunological responses. To examplify,
the tumor-derived exosomes containing up-regulated
exosomal miR-let-7i can be internalized by myeloid
dendritic cells (mDCs), modulating intracellular levels
of cytokines and signaling molecules such as IL-
6, IL-17, IL-1b, TGF-β, SOCS1, KLRK1, IFNγ, and
TLR4, thereby suppressing immune response.[]
Similarly, tumor-associated macrophages (TAMs)
represent another subset of immune cells affected by
exosomal miRNAs, as numerous studies have demonstrated
their involvement in modulating TAM phenotypes.[] For instance, in epithelial ovarian cancer
(EOC), the transfer of exosomal miR-222-3p to macrophages leads to the downregulation of SOCS3,
which in turn promotes STAT3 phosphorylation and
subsequent polarization of macrophages toward the
immune-suppressive M2 phenotype.[]
This review aims to analyze the recent studies published
in PubMed that focuses on the role of exosomal
miRNAs in malignancies, with an emphasis on identifying
the most significant exosomal miRNAs implicated
in metastasis across various cancer types.
EXOSOMAL miRNAs ASSOCIATED WITH
METASTASIS
Exosomal miRNAs Associated with Metastasis
in Breast Cancer
Santos et al.[] demonstrated the role of exosomal
miR-155 in the activation of epithelial-mesenchymal
transition (EMT) markers and the downregulation of
E-cadherin in breast cancer cells exposed to exosomes
derived from cells with elevated miR-155 expression.
The process was previously described in a study which
identified the role of miR-155 in depleting C/EBP?,
thereby enhancing the TGF-β response and promoting
EMT.[] The promotion of EMT by miR-155 has been
observed in several other cancer types in addition to
breast cancer cells which will be further discussed.
Researchers in another study reported that exosomal
miR-21, miR-378e, and miR-143 derived from
cancer-associated fibroblasts (CAFs) in breast cancer,
contribute to the enhancement of cancer stemness and
EMT.[] In addition, breast cancer-derived exosomal
miR-146a has been shown to accelerate the differentiation
of normal fibroblasts (NF) into cancer-associated
fibroblasts (CAFs), thereby promoting cell invasion
and migration. miR-146a targets the TXNIP gene -a
well-known metastasis suppressor, by modulating the
Wnt signaling pathway.[] Similarly, exosomal miR-
9 facilitates this transformation by influencing the expression
of MMP1, EFEMP1, and COL1A1.[]
Exosomal miR-105, on the other hand, activates
MYC signal transduction, enabling CAFs to adapt
to various metabolic conditions and thereby enhancing
tumor progression.[] Although the exact
mechanism remains unclear, significantly elevated
levels of exosomal miR-7641 in the plasma of breast
cancer patients with distant metastasis suggest a potential
role in promoting tumor cell progression and
metastasis.[] Additionally, researchers in a recent
study revealed that exosomal miR-19a in estrogen
receptor-positive breast cancer promotes osteolytic bone metastasis by suppressing PTEN expression,
which subsequently activates the NF-κβ and AKT
signaling pathways.[]
Exosomal miRNAs Associated with Metastasis
in Liver Cancer
Elevated serum exosomal miR-1247-3p levels correlate
with lung metastasis in hepatocellular carcinoma
(HCC) patients. This correlation is mediated by the
direct targeting of B4GALT3, which subsequently
activates the β1-integrin-NF-κβ signaling pathway
in fibroblasts.[] In the same context, hepatoma
cell-secreted miR-103 might be transferred into endothelial
cells via exosomes. This transfer leads to
the attenuation of endothelial junction integrity by
inhibiting the expression of VE-Cadherin (VE-Cad),
p120?Catenin (p120), and zonula occludens, ultimately
resulting in increased vascular permeability
and facilitating metastasis.[] Furthermore, the
transfer of exosomal miR-210 from HCC cells to
endothelial cells can promote angiogenesis by targeting
SMAD4 and STAT6 in endothelial cells.[] The
transmission of exosomal miR-21 to cancer-associated
fibroblasts (CAFs) stimulates PDK1/Akt signaling
through direct targeting of PTEN. The activation
leads to the increased expression of factors such as
VEGF, MMP2, MMP9, bFGF, and TGF-β, thus facilitating
the progression of angiogenesis.[]
Exosomal miRNAs Associated with Metastasis
in Lung Cancer
The release of exosomal miR-23a by lung cancer cells
under hypoxic conditions has been shown to selectively
target prolyl-hydroxylase and the tight junction
protein ZO-1, thereby enhancing angiogenesis and increasing
vascular permeability.[] of Mao et al.[]
provided evidence supporting the angiogenic properties
of miR-494 in non-small cell lung cancer, demonstrating
its effect through the activation of the Akt/
eNOS pathway following the targeting of the PTEN
gene. In a different mechanism, exosomal miR-21 and
miR-29 from lung cancer cells contribute to the formation
of an inflammatory microenvironment. These
exosomes act as ligands for Toll-like receptors (TLRs)
on immune cells, thereby activating a TLR-mediated
prometastatic inflammatory response, which can promote
tumor growth and metastasis.[] Additionally,
exosomal miR-1260b is suggested to facilitate cell invasion
in lung cancer cells by regulating the Wnt/-catenin
signaling pathway through the suppression of sFRP1
and Smad4 in lung adenocarcinoma.[]
Exosomal miRNAs Associated with Metastasis
in Colorectal Cancer
Researchers in a recent study demonstrated that
colorectal cancer (CRC) secretes exosomal miR-25-
3p, which plays a role in enhancing vascular permeability
and angiogenesis. This occurs through the silencing
of KLF2 and KLF4, leading to the disruption
of tight junctions in endothelial cells. Consequently,
this mechanism contributes to the formation of premetastatic
niches in distant organs such as lung and
liver.[] Elevated levels of exosomal miR-21 derived
from the plasma of patients with colorectal cancer
(CRC) have been found to correlate with liver metastasis
and TNM stages.[68,69] Moreover, a recent study
elucidated the mechanism by which miR-21 suppresses
Krev interaction trapped protein 1 (KRIT1)
and activates the β-catenin signaling pathway, thereby
inducing angiogenesis and vascular permeability.
[] Exosomal miR-92a-3p derived from CAFs may
promote stemness, invasion, metastasis, and EMT in
colorectal cancer by targeting tumor suppressor genes
FBXW7 and MOAP1.[] These findings align with
a subsequent study which demonstrated the role of
hepatoma-derived exosomal miR92a-3p in promoting
EMT and metastasis by inhibiting PTEN and activating
the Akt/Snail signaling pathway.[] Another
CAF-derived exosomal miRNA, miR-17-5p, increases
CRC metastatic potential by directly targeting RUNX
family transcription factor 3 (RUNX3) in CRC cells.
RUNX3 interacts with the proto-oncogene MYC,
thereby stimulating the TGF-β signaling pathway.
[] The induction of fibroblasts to acquire the characteristics
of cancer-associated fibroblasts (CAFs) in
colorectal cancer may be facilitated by exosomal miR-
10b. This outcome is achieved by inhibiting the PI3K/
Akt/mTOR pathway and promoting the production of
transforming growth factor-beta (TGF-β) and alphasmooth
muscle actin (α-SMA).[]
Exosomal miRNAs Associated with Metastasis
in Other Different Cancers
Li et al.[] demonstrated that the hypoxic microenvironment
can stimulate oral squamous cell carcinoma
cells (OSCC) to produce miR-21-richexosomes,
which are then delivered to normoxic cells, inducing
a pro-metastatic phenotype. The involvement of
miR-21 in EMT and metastasis has been previously
described, where it targets the AKT/ERK1/2 pathway
and PTEN.[]
Angiogenesis is facilitated by the repression of a
recently discovered target gene, testis-specific gene antigen 10 (TSGA10), which functions as a tumor
suppressor in several types of malignancies. In nasopharyngeal
cancer, exosomal miR-23a promotes
metastasis by targeting this gene.[,] The study
of Yang et al.[] provided evidence for the role of
exosomal miR-423-5p in inhibiting the production
of suppressor of fused protein (SUFU), which consequently
affects the proliferation and migration of
gastric cancer. Meanwhile, miR-27a derived from
gastric cancer (GC) is transported to fibroblasts and
promotes their differentiation into CAFs by downregulating
the expression of CSRP2, a protein involved
in regulatory processes essential for cellular differentiation.[] Several studies have suggested that microvesicles
released from human renal cancer stem
cells stimulate angiogenesis and the formation of a
lung pre-metastatic niche.[] Exosomal miR-155-5p
derived from hypoxic tumor-associated macrophages
(TAMs) plays a role in enhancing the stability of the
transcription factor IGF1R mRNA. This, in turn, facilitates
the proliferation and metastasis of renal cell
carcinoma [RCC] cells by upregulating the phosphorylation
of PI3K, p85, and AKT.[] Conversely, the
loss of exosomal miR-148b released from cancer-associated
fibroblasts (CAFs) in endometrial cancer has
the potential to increase the expression of DNMT1.
This process leads to changes in several molecules
associated with epithelial-mesenchymal transition
(EMT), including E-cadherin, N-cadherin, vimentin,
and fibronectin, ultimately promoting metastasis
of cancer cells. Enhancing the transfer of stromal
cell-derived miR-148b may thus represent a potential
strategy for preventing the progression of endometrial
cancer[] (Table 1).
Table 1 Overview of exosomal microRNAs and their potential mechanisms involved in cancer metastasis over the last decades
Table 1 Cont.
Table 1 Cont.
Table 1 Cont.
