1. Introduction
Ovarian cancer, one of the most common malignancies in the female
reproductive system, has the highest mortality rate among gynecological
cancers [1].
Albeit
the
five-year
relative survival rate of patients diagnosed at the early stage (FIGO
stage I or II) can reach 90%, more than 70% of patients are diagnosed
with advanced stage (FIGO stages III-IV) for lack of early symptoms and
sensitive diagnosis [2].
Patients
diagnosed at advanced stages are generally accompanied by poor
prognosis, which possibly
subsumes
extensive peritoneal dissemination, massive ascites, acquired
chemoresistance, and a five-year survival rate of less than 25%[3].
Despite
the
indisputable
fact that novel therapies, such as
targeted
therapy and immunotherapy, have been continuously emerging in recent
years, current treatments for ovarian cancer are still limited[4].
Hence,
there is an urgent need to identify specific diagnostic
biomarkers
and potential therapeutic interventions for ovarian cancer.
Recently,
as promising biomarkers of
ovarian
cancer, exosomes have been widely investigated and have exhibited their
immense potential in the medical field due to their diverse functions in
pathology[5,6].
After the discovery of
membrane
exfoliation
vesicles by Eberhard G. Trams,
the
term ”exosome” was initially used by Johnstone to describe the specific
type of extracellular
vesicles
(EVs) in 1987 [7,8].
Nowadays,
exosomes specifically refer to those
membranous
vesicles with a diameter of ~40 to160 nm (average
~100
nm) that have the same topology as the cells and are enriched in lipids,
nucleic acids, and protein complexes[5,9].
Research
has revealed that exosomes abnormally initiate or suppress various
signaling pathways in cancer cells though transmitting heterogeneous
cargoes, which potentially contribute to the development of cancer[10].
In
ovarian cancer, exosomes play a crucial role in mediating
epithelial-to-mesenchymal transition (EMT),
non-mutational
epigenetic reprogramming, immune
modulation,
thus
involved in the promotion of
tumorigenesis,
peritoneal dissemination,
and
drug resistance [11].
Apart
from this, exosomes are promising biomarkers in biological fluids for
the multicomponent
diagnosis
of
ovarian
cancer [12].
Herein,
this review is aimed to describe the characteristics of exosomes, with a
particular focus on their emerging functions and mechanisms in
ovarian
cancer, we hope to bring new insights into the latest advances in
diagnosis and treatment of
ovarian
cancer.
2.Biological
characteristics ofexosomes
The
biogenesis
of exosomes occurs within
the
endosomal
system,
covering the
involution
of cell membranes to form endosomes, the
generation
of intraluminal vesicles (ILVs)
inside
the multivesicular bodies (MVBs),
and
the
fusion
between the MVBs and the plasma membrane
to
release exosomes (Figure 1)[5].
The
exosomal
membranes mainly consist of lipid layers, and some specific lipids are
enriched
in exosomes compared to their parent cells [13].
Additionally,
exosomes possesses several families of proteins unique to the endosomal
pathway,
such
as tetraspanins
(CD9,
CD63, CD81), tumor-sensitive gene 101 (TSG101), heat shock proteins
(Hsc70), lysosomal proteins (Lamp2b) and fusion proteins (flotillin and
annexin),
which
are
commonly
used to characterize exosomes and distinguish them from
other
vesicles [14].
Moreover,
exosomes
contain multiple types of bioactive molecules such as cargos including
proteins,
nucleic acids, lipids, and metabolites[5].
A
straightforward approach based on analyzing the variation of key
contents is helpful to uncover the biological functions of exosomes
coming from different cell types. For more extensive information about
bio-active contents in exosomes identified in multiple organisms,
ExoCarta
(http://www.exocarta.org)
can
provide further information for
researchers
to conduct in-depth exploration.
3.Roles
of exosomes in ovarian
cancer
Exosomes
presenting in biological fluids are tightly connected with multiple
pathological and physiological
processes.
Generally,
exosomes
participate in
expelling
excess and/or nonfunctional cellular components, recycling materials
between cells for intercellular communication, remodeling the tumor
microenvironment (TME), and many more [15].
Studies
have shown that tumor cells secrete more exosomes than normal cells,
and
exosomes derived from tumor cells have a
potent
capacity to promote tumor
progression
by
modifying
both local and distant microenvironments [16].
Herein,
we summarize the functions of exosomes in the
metastasis,
chemotherapy resistance, and immune regulation of
ovarian
cancer (Figure 2).
3.1 Role inmetastasis
Recent
studies have unraveled the significant contribution of TME
to
ovarian cancer metastasis, of which exosomes are an important
composition[17].
Exosomes
are chiefly involved in
the
formation of pre-metastatic niches and the process of promoting
metastasis.
Primary
ovarian
tumor-derived
exosomes
can prepare the distant tumor microenvironment via regulating
intercellular communication between tumor cells and normal stroma,
cancer-associated fibroblasts, and local immune cells to promote
metastatic invasion[18].
Plasma
cells are capable of mediating phenotypic switch and shaping the
mesenchymal identity of ovarian cancer in
an
exosome-dependent manner [19].
Specifically,
plasma cell-derived exosomes contain miR-330-3p, thereby uplifting the
expression of junctional adhesion molecule B
in
non-mesenchymal ovarian cancer cells in a noncanonical fashion.
Moreover,
exosomes in ascites promote
epithelial-mesenchymal
transition (EMT) of ovarian cancer cells by delivery of miR-6780b-5p and
eventually facilitate ovarian cancer metastasis[20].
Other
exosomal miRNAs can be synthesized and secreted by ovarian cancer cells
as well. It has been found that miR-205 was up-regulated in
ovarian
cancer tissues and promoted ovarian cancer metastasis
in
an exosome-dependent manner by inducing angiogenesis[21].
Furthermore,
proteomic analysis reveals that exosomes from
ovarian
cancer cells contain a specific set of proteins that are representative
of their origin and invasive capacity[22].
Similarly,
Alharbi et al. unveiled that 40 proteins associated with Wnt
canonical pathway (β-catenin) were differentially expressed in tumor
tissues between mice injected with high invasiveness capacity cell line
(exo-SKOV-3) and low invasiveness capacity cell line (exo-OVCAR-3), and
exosomes secreted by a high invasiveness capacity cell line boosted
tumor metastasis in vivo[23].
To
sum up,
exosomes
modulate
TME
via autocrine-paracrine
signaling
that
reinforces the formation of
pre-metastatic
niches and enhances the
invasion
and
migration
of
ovarian
cancer cells.
3.2 Role inchemotherapyresistance
Apart
from metastasis,
chemotherapy
resistance is the major cause of treatment failure in patients with
ovarian
cancer,
especially
in patients with advanced ovarian cancer [24].
And
exosomes
released in TME are prone to bring about chemotherapy
resistance.
For instance, exosomal miRNA miR-1246 assists in establishing paclitaxel
resistance in ovarian cancer via targeting the Cav1/p-gp/M2-type
macrophage axis [25].
Au
Yeunget
al. demonstrated that exosomes transfer stroma-derived miR21 from
cancer-associated adipocytes (CAAs) and fibroblasts (CAFs) to the cancer
cells, therefore helping ovarian cancer cells to develop paclitaxel
resistance through targeting APAF1 [26].
And
Zhu et al . indicated that macrophages-derived
exosomes
deliver miR-223 to epithelial ovarian cancer (EOC) cells to elicit a
chemo-resistant
phenotype [27].
To
put it more specifically, this study emphasized the role of exosomes in
the crosstalk between macrophages and EOC cells for chemotherapy
resistance through the exosomal miR-223/PTEN-PI3K/AKT signaling
pathway.
Lately,
a hypothesis has been proposed that hypoxia-induced changes in exosomes
composition and bioactivity might render carboplatin resistant to target
cells [28].
Conformably,
hypoxia-induced exosomes carry more STAT3 and FAS that are capable of
inducing chemo-resistance[29].
Besides,
plasma gelsolin (pGSN) transported by exosomes upregulates
HIF1α-mediated pGSN expression in
chemo-resistant
ovarian
cancer cells
in
an autocrine manner, coupled with leading to cisplatin resistance in
other chemo-sensitive ovarian cancer cells in a paracrine manner[30].
Notably,
exosomes are also a direct mechanism of intercellular drug transfer.
What’s more, exosomes promote
drug
efflux in a drug concentration-dependent manner and exceed the
p-glycoprotein efflux when it is saturated [31].
3.3 Role inimmune
regulation
The
double-edged role of exosomes originating from TME in immune regulation
has gained continuously increasing
attention.
Most
tumor-associated
exosomes
have
immunosuppressive
effects,
which
collectively inhibit the antitumor response of tumor-specific T
cells
and
induce the functional arrest of adoptively transferred tumor-specific T
cells or chimeric antigen receptor T cells[32].
Consistent
with this, Zhou et al. found that exosomes released from
tumor-associated macrophages deliver miRNAs, including miR-29a-3p and
miR-21-5p, into CD4+ T cells, which could induce a
Treg/Th17
cell imbalance in EOC [33].
Additionally,
exosomes carry PD-L1 on their surface, thus contributing to
immunosuppression by anti-PD-1 response [34]. For
immunosuppressive exosomes, targeting exosomal phosphatidylserine by
ExoBlock,
a
novel PS-binding molecule, represents a promising strategy to enhance
antitumor T-cell responses in TME [35].
However,
some exosomes have antipodal effects. In other words, these exosomes can
activate
dendritic cells (DCs), natural killer cells, and T cells to exert an
antitumorigenic effect through stimulating both the innate and adaptive
immune systems [36].
Since
the immune microenvironment of ovarian cancer is of considerable
complexity,
supererogatory
studies are required to help thoroughly understand the role of exosomes
in immune regulation to provide the possibility for effectively
designing immunotherapeutic regimens.
4.Exosomes
serve as biomarkers of ovarian cancer
To
date, in actual clinical application, the dominating diagnostic and
prognostic biomarkers of ovarian cancer have been carbohydrate antigen
125 (CA125) and human epididymis protein 4 (HE4)[37,38].
Nevertheless,
these markers usually present superior sensitivity and poor specificity
or vice versa and are linked with several other pathological conditions.
Recent
studies have revealed that exosomes
turn
out to be ideally noninvasive or less-invasive
biomarkers
for ovarian cancer
diagnosis
and
monitoring[39].
Because
exosomes represent mobile information
regarding
the
molecular makeup of parental tumors and can be
conveniently
detected in many physical fluids such as blood, urine, saliva, and
malignant effusions of
ascites.
In
this section,
we
mainly summarize
exosomal
proteins and miRNAs as diagnostic and prognostic biomarkers of
ovarian
cancer
(Figure
3).
Remarkably,
in addition to exosomal proteins and miRNAs,
long
noncoding RNAs
(lncRNAs)
and
phosphatidylserine
(PS) carried by exosomes can also be potential and attractive
biomarkers.
For
example,
exosome-delivered lncRNA SOX2-OT contributed to
motility
and proliferation of ovarian cancer cells and was highly
expressed
in
ovarian
cancer.
PS-expressing
exosomes in patient blood and ascites
can
be a surrogate biomarker
for
ovarian cancer, since exosome
membranes
derived from the plasma membrane of parental tumor cells commonly expose
PS [40].
4.1Exosomal
proteins asbiomarkers
Compared
with exosomes derived from normal tissue, exosomes from ovarian cancer
contain
some
types of protein with significantly increased levels, such as CD24 and
Claudin-4, suggesting
that
exosomal protein can serve as biomarkers probably connected with FIGO
classification [41,42].
Other
potential biomarkers, small heat shock proteins
have
remarkably high expression levels in exosomes from
serum
and
peritoneal
fluid of ovarian cancer patients, while more studies are needed to
exploit their diagnostic value[43].
In
addition, the exosomal proteome of the ovarian cancer ascites may
provide monitoring for the therapeutic response to different therapy
strategies.
Meshachet al. have reported that elevated expression of plasma gelsolin
(pGSN) transported by exosomes (Ex-pGSN), correlated with poorer overall
survival and relapse-free survival in patients with ovarian cancer[30].
Consistently,
exosome-mediated pGSN confers cisplatin resistance in chemosensitive
ovarian
cancer cells in an autocrine manner.
Indeed,
in drug-resistant cancer cells derived exosomes, chemoresistance-related
proteins, such as annexin A3, MRP2, ATP7A, and ATP7B, are highly
expressed, indicating the potential value to predict the effectiveness
of chemotherapy methods in patients with ovarian cancer[44].
Surprisingly,
a
microfluidic device enabled the isolation of exosomes and the
establishment of their
protein
profiles
for
the early detection of high-grade serous ovarian cancer (HGSOC)[45].
4.2Exosomal
miRNAs as biomarkers
While
previous studies have reported that the aberrant expression of miRNAs
could represent diagnostic and prognostic
markers
for
ovarian
cancer,
most
of these
studies
regarding miRNAs expression were based on findings from tumor tissue
specimens.
MiRNA
profiling of circulating tumor exosomes in the patient’s plasma has
shown the clinical relevance of
exosomal
miRNAs as circulating biomarkers for cancer[46].
Tayloret
al.have
observed that
8
miRNAs of tumor-derived exosomes had specific expression in
ovarian
cancer[47].
Kanlikilicer et al. have reported that exosomal miR-6126 is
ubiquitously released in high abundance from both chemosensitive and
chemoresistant ovarian cancer cells [48].
Furthermore,
Pan et al. have found that miR-21, miR-100, miR-200b, and miR-320
were significantly enriched in exosomes from plasma of EOC patients
compared with those of healthy women, whereas miR-16, miR-93, miR-126,
and miR-223 were
down-regulated[49].
In particular, the levels of exosomal
miR-200b
and
miR-200c
correlate with the tumor marker CA125 and patient overall survival in
advanced
EOC[50].
Additionally,
ovarian cancer cell-secreted exosomal miR-205 was markedly enriched in
the serum of patients, and further research found that the high level of
exosomal miR-205 promoted ovarian cancer metastasis via inducing
angiogenesis [21].
Conclusively,
despite the existing findings, we are still in sore need of more studies
to elucidate the feasibility of exosomal miRNAs in
ovarian
cancer diagnosis and prognosis, further enabling exosomal miRNAs to be
used in ovarian cancer patients soon.
5.
Therapeutic Potential of Exosomes
In
this section, the therapeutic potential of exosomes will be
elucidated
in the following three aspects, (1)
exosome-mediated
delivery
system,
(2)
exosome-based
immunotherapy, and (3)
exosomes
as therapeutic
targets.
Beyond
that,
we
discuss future perspectives
that
exosomes can serve as therapeutic modalities as well as therapeutic
targets for ovarian cancer treatment (Figure 3).
5.1Exosome-mediated
delivery system
Compared
to non-host vehicles, exosomes are startlingly stable in circulation and
do not elicit
immune
rejection.
Consequently,
their clinical applications as agents delivery nanoplatforms
can
be
leveraged
for treating
ovarian
cancer[51,52].
Specifically,
not only can the advances of in bio-engineering
nanotechnology
enable the encapsulation of
therapeutic
agents such as RNA,
peptides,
and
chemotherapeutic
drugs
into exosomes, but also
modify
the exosomes with diverse
ligands
for
tumor-targeting
strategies[53].
For
instance,
Zhaoet al. found that injection of RGD-modified exosomes loaded with
miR-484 in xenograft model induced vessel normalization and
in
turn sensitized the ovarian cancer cells to chemotherapy[54].
Bioinspired
hybrid nanoparticles, which were formed by CD47-expressing exosomes
and
the target peptide cRGD-modified liposomes with miRNA-497 and
triptolide,
induced
ovarian cancer cells apoptosis by inhibiting the PI3K/AKT/mTOR signaling
pathway and overcame drug resistance in ovarian cancer via regulating
macrophage polarization [55].
Besides,
Pisano et al. proposed that development
immune
derived exosome mimetics (IDEM) as a scalable biomimetic drug-delivery
system can target and treat ovarian cancer[56].
This monocyte-derived exosome
mimetic
can efficiently encapsulate doxorubicin with reduced immunogenicity.
Prominently,
natural products, such as curcumin and
triptolide,
show promising
therapeutic
effects on ovarian cancer, yet the low bioavailability and non-specific
selectivity often hinder their clinical applications[57,58].
Combining
exosomes with these natural anticancer products based on nanotechnology
has exhibited its
high
efficiency for the treatment of
ovarian
cancer [59,60].
Accordingly,
a phase I clinical trial is currently ongoing to investigate the
feasibility and ability of plant exosomes to deliver curcumin
(NCT01294072).
5.2 Exosomes-basedimmunotherapy
Cancer
immunotherapy is
a
novel
type of
treatment
based on manipulating the host
immune
system to reactivate the antitumor immune response
and/or
overcome the pathway leading to tumor escape[61].
Evidence
has revealed
shown
exosomes generally contribute to
immunosuppression
and tumor immunity escapes via a variety of biological mechanisms[62].
Explicitly,
Li et al. reported that ovarian cancer cells secrete exosomes
in
TME
to
suppress
antitumor
immunity based on lymphocyte-cancer cell cross-talk[63].
And
Peng et al. concluded that exosomes existing in ascites of
ovarian
cancer patients
induce
apoptosis of cells in the immune system, such as
dendritic
cells (DCs) and
peripheral
blood mononuclear cells (PBMCs)[64].
Therefore,
eliminating immunosuppressive exosomes in TME of ovarian cancer is
expected to enhance antitumor immune responses for
immunotherapy.
Unexpectedly,
some exosomes with immunogenicity can induce cytotoxic T
lymphocyte-dependent antitumor response[65].
Previous
studies demonstrated that exosomes containing tumor antigens for
APC
activation,
which consequently
manipulate
the immune system
to
recognize and attack cancer cells[66].
In
agreement, according to Li et al ,
ovarian
cancer-derived exosomes with tumor-specific antigens can be presented by
dendritic cells (DCs) derived from unrelated umbilical cord blood to
induce tumor-specific cytotoxicity[67].
These
provided another perspective that
exosomes
may
employ
their immunogenicity to open new avenues for immunization against
ovarian cancer.
5.3 Exosomes astherapeutic
targets
We
considering that exosomes play pivotal roles in
ovarian
cancer progression, and
targeting
exosomes have potential clinical implications for therapy[68].
Removing
exosomes from peripheral circulation and blocking exosome
production
and secretion from cancer cells are the mainstay of exosome-targeting
strategies.
Studiesin vitro have demonstrated the selective removal of exosomes from
patient blood by using Hemopurifier, an extracorporeal hemofiltration
device containing fibers with a high affinity for
exosomes[69].
Apart
from this,
emerging
evidence has suggested that proteins of the Rab family of GTPases play
an important part in the biogenesis and secretion of exosomes[70].
Indeed,
Ostrowski et al. found that
Rab27a
or Rab27b silencing reduces MVE docking to the plasma membrane, hence
indicating
that targeted inhibition of the Rab27 subfamily
led
to reduced
exosome
secretion [71].
Moreover,
the
sphingomyelinase
inhibitor
GW4869
blocks
the
production of exosomes through depleting ceramide, which is a part of
exosomes production [72].
In
ovarian cancer, GW4869 inhibited CD47 overexpressed-exosome secretion in
TME and consequently
promoted
phagocytosis
by
macrophages[73].
In
conclusion, targeted elimination of exosomes as well as
reducing
exosome production and secretion
are
useful for inhibiting the communication between cancer cells and other
cells, thus making them alternative therapeutic strategies.
6.
Conclusions and Perspectives
Over
the past two decades, the spectacularly rapid development of molecular
biological techniques has magnificently contributed to the breakthrough
of discoveries in
exosomes.
As a part of TME, exosomes exert a pivotal role in
tumorigenesis,
metastasis, drug resistance, and immune regulation of ovarian
cancer.
Furthermore,
due
to their
terrific
stability in body fluids and pathological features, exosomes have a
promising value for both ovarian cancer diagnosis and
therapy.
Leveraging
exosomes as a double-edged sword, novel strategies in ovarian cancer
therapy, such as stimulating exosomes secretion or eliminating specific
exosomes, are
attractive.
Nonetheless,
on account of the inestimable complexity of
exosomes,
more
studies regarding the involvement of exosomes in ovarian cancer are
desperately needed to be met prior to large-scale clinical utilization.