Pancreatic cancer and associated exosomes

Abstract

Pancreatic cancers with poor prognosis are highly malignant, readily metastatic and of immune tolerance, mainly due to delayed detection. The metastatic progression and immune tolerance of pancreatic cancer is greatly attributed to the intercellular communication. However, exosomes are deemed to be the most important tool of intercellular communicators. Thus, we present a review of pancreatic cancer and exosomes in this article. We intensively summarize the progress of early pancreatic cancer and the relationship of the proliferation, progression and metastasis of pancreatic cancer and pancreatic cancer-derived exosomes, and propose new ideas of the study of pancreatic cancer.

1. Introduction

Pancreatic cancer (PC), a common malignancy with metastasis, poor prognosis and aggressive disease, is associated with a high mortality rate. The major histological type of pancreatic cancer is pancreatic ductal adenocarcinoma (PDAC) accounting for $>$ 80% of pancreatic neoplasms, and other types are acinar cell carcinomas or neuroendocrine tumors [1]. Pancreatic cancer develops in 3 settings: sporadic pancreatic cancer, accounting for 90% of patients; familial pancreatic cancer, being about 7% of cases; and inherited cancer syndromes, constituting about 3% of patients [2]. The 5 year prevalence proportions of pancreatic cancer is 10.8 per 100,000 [3], and the mortality of pancreatic cancer is currently the sixth of cancer deaths in China [4]. Unfortunately, about 7% of pancreatic cancers are considered localized disease when diagnosed, which is alarmingly low compared to other malignant tumors, such as breast cancer (61%), colon cancer (40%), lung cancer (16%), ovarian cancer (19%) and prostate cancer (91%) [5]. Median survival and 5-year survival of pancreatic cancer are 6 months and less than 5%, respectively, despite 50 years of research and therapeutic development [6]. The 5-year survival of pancreatic cancer is inferior to others recalcitrant cancers, including lung (17%), liver (18%), esophagus (19%), stomach (29%), brain (35%), ovary (44%) and multiple myeloma (45%) [2]. Recent developments in science and technology have not yet resulted in improved survival. At present, for those afflicted with the terrible disease, efficient surgery remains the only hope for cure [7].

Pancreatic cancer often progresses asymptomatically in the early and presents in the late, resulting in only a minority being suitable for resection, because of symptoms of pancreatic cancer being quite non-specific and including nausea, anoxeria, jaundice, weight loss and abdominal pain [8]. Because the pancreas, being a retroperitoneal organ, is buried deep in the torso, there is no external lumps that can be palpated or external skin changes that can be seen during an annual routine physical examination [5]. By the time pancreatic cancer is diagnosed, 80–85% of patients present with advanced unresectable disease, and responds poorly to most chemotherapeutic agents [9]. For patients with advanced pancreatic cancer, the disease usually progresses rapidly, and few patients survive more than 1 year after diagnosis. Encouragingly, prognosis of patients with pancreatic cancer who receive surgery is better, surgical margins are free of disease, and perineural or lymphovascular invasion don’t appear, when the tumor is smaller [5]. Meanwhile, a report of pancreaticoduodenectomies for pancreatic cancer revealed that: for cancers $<$ 3 cm, median survival was 21 months; 1-, 2-, and 5-year survivals were 73%, 45%, and 23%, respectively, and for cancers $>$ 3 cm, median survival was 15 months; 1-, 2-, and 5-year survival were 59%, 31%, and 4%, respectively [10]. So, we suppose that the earlier pancreatic cancer is diagnosed, resected and treated, the longer median survival is obtained by patients, and the higher 5-year survival is improved. However, early detection, resectable stage and effective therapy for pancreatic cancer are faced with great challenges.

2. Pancreatic intraepithelial neoplasia

Precursor lesions of tumor can happen to pancreas, which is a powerful case for screening of early pancreatic cancer. Three precursor lesions, including pancreatic intraepithelial neoplasia (PanINs), intraductal papillary mucinous neoplasms (IPMNs), and mucinous cystic neoplasms (MCNs), have each been identified as distinct precursors to ductal adenocarcinomas of pancreas [5]. PanINs are noninvasive epithelial proliferations within the smaller pancreatic ducts, the progress of which is from PanIN-1 (low-grade) to PanIN-2 (intermediate-grade) to PanIN-3 (high-grade) [11, 12]. We assume the progression model of PanINs in pancreatic cancer, as shown in Fig. 1. PanINs are microscopic (diameter is $<$ 5 mm ) and are not directly visible by pancreatic imaging techniques [9]. PanIN-1 is flat duct lesion without atypia which shall be hyperplastic and benign, PanIN-2 is papillary duct lesion with atypia which may be low-grade dysplasia, and PanIN-3 is high-grade dysplasia or carcinoma in situ [13]. When the cancerous ductal cells move and pass the basement membrane into the adjacent pancreatic parenchyma, PanIN-3 transforms into invasive pancreatic cancer [14].

Figure 1.

Multistep progression model of PanINs in pancreatic cancer.

There are specific molecular alterations in the process from normal epithelium to PanIN-1 to PanIN-2, and then to high-grade PanIN-3 and invasive cancer. For example, alterations of ERBB2, EGFR, 14-3-3 $\sigma$ and telomere shortening, and mutations of $K$ -ras, $p16$ , INK4A and Her-2/neu appear in the whole process from PanIN-1A to invasive cancer, mutation of $p53$ occurs from PanIN-2 to invasive cancer, and mutations of SMAD4/DPC4 and BRCA2 happen from PanIN-3 to invasive cancer [14, 15, 16]. Thus, specific molecular alterations in pancreas can serve as early warning of pancreatic cancer in the early stages.

3. Conventional methods of diagnosing pancreatic cancer

At present, three main conventional methods of diagnosing pancreatic cancer are serum biomarkers, imageological diagnosis and pathological diagnosis. However, currently there are no biomarkers that accurately discriminate between early pancreatic cancer/high-grade PanIN-3 and normal/low-grade (PanIN-1, PanIN-2) lesions [17], as well as no noninvasive screening tools that detect lesions of early pancreatic cancer [2].

3.1 Serum biomarker

Cancer antigen 19-9 (CA 19-9) in serum, the most common biomarker in clinical routine, is the most extensively evaluated with pooled data from 2283 patients with pancreatic cancer, median sensitivity and median specificity of which is 79% (70 $\sim$ 90%) and 82% (68 $\sim$ 91%), respectively [18]. CA 19-9 is prognostic and plays an important role in advanced pancreatic cancer [19], but don’t use to distinguish between low-grade PanIN-1, 2 and pancreatic cancer/high-grade PanIN-3. Although other various representative biomarkers in serum have also been developed, it is insatiable to fulfill the detection of early pancreatic cancer. For example, CEACAM1 was expressed in the serum of 91% of patients with pancreatic cancer, 66% of patients with chronic pancreatitis and 24% of normal patients, the specificity of which is not satisfactory [20]; Mucin4 (MUC4), a transmembrane apomucin, is 91% of sensitivity in immunohistochemical staining, but its low circulatory levels limit its performance in serum [21]. Other cancer antigens, including CA 50, CA 12-5, CA 24-2, are also used for the detection of pancreatic cancer, their sensitivity and specificity are lower in comparison with CA 19-9 [22, 23].

3.2 Imageological diagnosis

Modern non-operative techniques for tumor staging, including transabdominal ultrasound, computed tomography (CT), magnetic resonance imaging (MRI) and endoscopic ultrasound (EUS) are less invasive and less costly than surgery [24]. However, CT is short of sensitivity to detect small pancreatic lesions, such as PanINs, which don’t sensitively detect early pancreatic cancer [2]. EUS with high resolution detects lesions as small as 1 to 2 mm and allows biopsy and/or fine-needle aspiration cytology, which distinguished benign from malignant disease when focal lesions are present [1, 2]. However, pancreatic cancer is more common with increasing age and most frequently diagnosed among people older than 55 years [25]. Because of age [26], smoking [27], alcohol [28] and other factors, the pancreas of a high proportion of individuals older than 50 years appear significant changes, such as pancreatic fibrosis. The changes during EUS imaging could obscure early lesions, especially noninvasive PanIN-3 [2]. Hence, EUS for the detection of early pancreatic cancer has a certain limitation in the clinical.

3.3 Pathological diagnosis

Pathological diagnosis is also an important method for diagnosing pancreatic cancer. At present, needle biopsy is an effective means of obtaining pathologic diagnosis information. Image-guided fine needle aspiration (FNA) is a popular method for evaluating pancreatic lesions, sensitivity, specificity, positive predictive value and negative predictive value of which were 79.4, 99.0, 99.4 and 67.9, respectively [29]. However, for screening of early pancreatic cancer, this approach is in a weak position.

For diagnosing early pancreatic cancer, the three conventional methods of seem to be insufficient and powerless. Therefore, there are currently many challenges for the detection of early pancreatic cancer. With the development of current science and technology, detection of early pancreatic cancer requires to screen asymptomatic subjects among high-risk groups through noninvasive tests. If a marker of early pancreatic cancer is available, some genetically susceptible but asymptomatic individuals could be screened and offered effective therapy in the early stages. Hence, screening early lesion of pancreas through noninvasive tests and molecular imaging will major breakthrough and significant change for the detection of early pancreatic cancer. Noninvasive detecting and screening absolutely depend on specific and sensitive biomarker in blood, saliva and urine, and accurate and distinct imaging technology. Noninvasive screening technology based on the two aspects is more suitable for the detecting and screening of a high proportion of individuals with susceptibility.

Ideal biomarkers of screening early pancreatic cancer should be universally present in curable-stage of pancreatic cancer, while absent in individuals with cancer-free. The biomarkers should be handily detected in readily obtainable biological samples, such as blood, saliva and urine. The detecting process should be rapid, inexpensive, widely distributable to maximize testing results, and the biomarkers must be highly sensitive and specific to accurately detect the critical target. Most recently, the detection of exosomes may offer diagnostic, predictive and prognostic markers of pancreatic cancer.

4. Exosomes

Exosomes are lipid bilayer membrane-enclosed nano-sized (30 $\sim$ 100 nm) vesicles with a density of 1.13 $\sim$ 1.19 g/mL, secreted by virtually all cell types [30, 31]. Formation of exosomes is through the following process: after cell endocytosis, early endosomes form and are integrated into multivesicular bodies (MVB) in which late exosomes are formed into intraluminal vesicles; and then MVB fuses with the plasma membrane to release exosomes with the formation of budding [32]. Exosomes have also been confirmed in all bodily fluids such as blood [33], saliva [34], urine [35], breast milk [36], amniotic fluid [37] and cerebrospinal fluid [38]. In recent years, exosomes have emerged as an important tool for intercellular communication through functional biomole-cules (these are proteins, lipids, RNA and DNA) [39, 40, 41, 42]. Exosomes from different cell types contain a core set of identical proteins, including members of the tetraspanin family (CD9, CD63, CD81 and CD82), members of the endosomal sorting complexes required for transport (TSG101 and Alix), heat shock proteins (Hsp60, Hsp70 and Hsp90) and MHC class II-associated proteins (14-3-3 $\epsilon$ and PKM2) [42, 43, 44]. In addition to these proteins, exosomes also contain some specific proteins which reflect the parental cell. For instance, epithelial tumor cells secrete exosomes that carries the epithelial cell adhesion molecule (EpCAM) [45]; melanoma-derived exosomes contain the tumor associated antigen Mart-1 [46]; exosomes from gastric cancer or breast cancer enrich members of the human epidermal receptor (HER) family [47, 48]; and colon cancer cells harboring mutant K-ras alleles are able to release exosomes with many tumor-promoting proteins, including KRAS, EGFR, SRC family kinases and integrins [49]. At present, emerging evidence suggests that cancer-derived exosomes may contribute to the reprogramming of the tumor microenvironment to form a pro-tumorigenic soil and play a pivotal role in local and systemic cell–cell communication in cancer [50]. For example, Lin et al. demonstrated that tumor cell-derived exosomes endowed bone marrow-derived mesenchymal stromal cells (BM-MSCs) with tumor-promotion capabilities to enhance tumor growth [51]. Because exosomes are very stable under different conditions, they can protect the biological cargo against degradation and denaturation in the extracellular environment [52]. Therefore, exosomes are recognized as promising diagnostic and predictive biomarkers in cancer.

5. Various effects of pancreatic cancer-derived exosomes

Exosomes are deemed to be the most important tool of intercellular communicators, including cancer cells and normal cells [53]. In terms of pancreatic cancer, exosomes deriving from pancreatic malignancies can be loaded with a great deal of substance associated with pancreatic cancer and produce some special physiological functions. We summarized the relative researches of pancreatic cancer and associated exosomes. The relevant principle, mechanism and details were as follows:

5.1 Pancreatic cancer-derived exosomes as vehicles of K-ras and p53 genes

Kahlert et al. identified that mutated K-ras and $p53$ genes could be detected by using genomic DNA in exosomes derived from pancreatic cancer cell lines and serum of patients with PDAC, and demonstrated that serum exosomes from patients with pancreatic cancer contained genomic DNA spanning all chromosomes by using whole genome sequencing, indicating that exosomes have the capacity to carry and transport genomic DNA spanning all chromosomes with mutations of $K$ -ras and $p53$ [54]. $K$ -ras mutations and $p53$ aberration are common in pancreatic cancer [15, 55]. Moreover, $K$ -ras mutations represent an early event in ductal tumorigenesis, and $p53$ alterations represent that an event required for the malignancy progression of PDAC from lower to higher grades [56]. Yang et al. reported that digital PCR analyses of exosomal DNA identified 39.6% of cases with $K$ -ras mutation and 4.2% of cases with $p$ 53 mutation in 48 clinically annotated serum samples from patients with PDAC, and mutations of $K$ -ras and $p$ 53 were also detected in exosomal DNA from patients with IPMN (2 out of 7 with $K$ -ras mutation, one of which also co-presented with $p$ 53 mutation) [57]. They also reported that 2.6% of $K$ -ras mutation and none with $p$ 53 mutation appeared in 114 healthy subject-derived circulating exosomal DNA, respectively. The study highlighted the value of circulating exosomal DNA for a rapid and low-cost identification of cancer driving mutations. Allenson et al. reported that $K$ -ras mutations in exosome-derived DNA were identified in 7.4%, 66.7%, 80% and 85% of age-matched controls, localized, locally advanced and metastatic PDAC patients, respectively [58]. Comparatively, mutant $K$ -ras circulating cell-free DNA was detected in 14.8%, 45.5%, 30.8% and 57.9% of these individuals. In the validation cohort, mutant $K$ -ras exosome-derived DNA was detected in 43.6% of early-stage PDAC patients and 20% of healthy controls. These above results indicated that the additional genetic insights from exosomes would be beneficial to improve the diagnosis for patients with early pancreatic cancer.

5.2 Pancreatic cancer-derived exosomes as biomarkers of noninvasive detection

Melo et al. identified that glypican-1 (GPC1) specifically enriched on cancer-cell-derived exosomes, and verified that GPC1 ${}^{+}$ circulating exosomes (crExos) were monitored and isolated using flow cytometry from the serum of patients and mice with cancer [59]. They confirmed that GPC1 ${}^{+}$ crExos were detected in the serum of patients with pancreatic cancer with absolute specificity and sensitivity, which distinguished healthy subjects and patients with a benign pancreatic disease from patients with early- and late-stage pancreatic cancer, and levels of GPC1 ${}^{+}$ crExos correlated with tumor burden and the survival of pre- and post-surgical patients. GPC1, a cell surface, lipid-raft-associated heparan sulfate proteoglycan, composes of 558 amino acids with a predicted molecular weight of 62 kDa [60]. And Aikawa et al. indicated that down-regulation of GPC1 in the human pancreatic cancer cell line (PANC-1) made cell growth prolong double time and decreased anchorage-independent growth of cell in vitro as well as attenuated tumor growth, angiogenesis, and metastasis when these cells were transplanted into athymic mice [61]. Thus, GPC1 ${}^{+}$ crExos are able to become promising biomarkers of noninvasive detection for early pancreatic cancer.

Lau et al. firstly developed a pancreatic cancer mouse model that yielded discriminatory salivary biomarkers by implanting the mouse pancreatic cancer cell line (Panc02) into the pancreas of the syngeneic host C57BL/6 mouse. The role of pancreatic cancer-derived exosomes in the development of discriminatory salivary biomarkers was then tested by engineering a Panc02 cell line that is suppressed for exosome bio-genesis, implanting into the C56BL/6 mouse, and examining whether the discriminatory salivary biomarker profile was ablated or disrupted. Suppression of exosome biogenesis leads to the ablation of development of discriminatory salivary biomarker [62]. This study supports that tumor-derived exosomes provided a mechanism in the development of discriminatory biomarkers in saliva and distal systemic diseases.

5.3 Pancreatic cancer-derived exosomes as platform of microRNA signature

MicroRNAs (miRNAs) are small non-coding RNAs (17–25 nucleotides) that regulate gene expression at post-transcriptional level, and play important roles in tumorigenesis, tumor-progression and tumor-metast-asis [63, 64]. And previous studies have shown that abnormal expression of miRNAs have the potential to identify various types of cancers, including gastric cancer [65], prostate cancer [66], breast cancer [67], lung cancer [68], pancreatic cancer [69], etc. For pancreatic cancer, miRNA-21, miRNA-155, miRNA-210, miRNA-221 and miRNA-222 were over-expressed in diseased tissues than in the control samples, whereas miRNA-31, miRNA-122, miRNA-145 and miRNA-146a were under-expressed [70]. Lai et al. reported that high exosomal levels of miRNA-10b, miRNA-21, miRNA-30c and miRNA-181a, and low exosomal levels of miRNA-let7a readily differentiated PDAC from normal control and chronic pancreatitis samples, and all 29 cases of PDAC exhibited significantly elevated exosomal miRNA-10b and miRNA-30c levels [71]. Madhavan et al. tested that miRNA-1246, miRNA-4644, miRNA-3976 and 4306 by qPT-PCR and discovered the four miRNAs were significantly upregulated in 83% of serum-exosomes from pancreatic cancer, but rarely in control groups [33]. Takikawa et al. found that pancreatic stellate cell–derived exosomes contained a variety of miRNAs including miRNA-21-5p, and several miRNAs such as miRNA-451a were enriched in exosomes compared to their source pancreatic stellate cells [72]. In addition, Ding et al. reported that pancreatic cancer-derived exosomes transfer miRNAs to dendritic cells and inhibit RFXAP expression via miRNA-212-3p, which decrease MHC II expression and induce immune tolerance of dendritic cells [73]. MiRNAs from pancreatic cancer-derived exosomes are superior to those from serum, and are able to more really reflect the development of pancreatic cancer, which provide a more promising signature for diagnosing pancreatic cancer.

5.4 Pancreatic cancer-derived exosomes as emerging drivers of pre-metastatic niche formation in liver

Primary tumor cells orchestrate pre-metastatic niche formation in secondary organs through secretion of a variety of cytokines and growth factors that promote mobilization and recruitment of bone marrow derived cells to future metastatic sites, and tumor-derived exosomes can serve as drivers of pre-metastatic niche formation [74]. Costa-Silva et al. demonstrated that exosomes from PDAC induced liver pre-metastatic niche formation in naive mice and consequently increase liver metastatic burden, and found that macrophage migration inhibitory factor (MIF) was highly expressed in PDAC-derived exosomes, but its blockade prevented liver pre-metastatic niche formation and metastasis [75]. They found that the PDAC-derived exosomes selectively taken up by liver Kupffer cells caused transforming growth factor $\beta$ secretion and up-regulation of fibronectin by hepatic stellate cells producing. The deposition of fibronectin in the liver leaded to the formation of a fibrotic microenvironment that promoted the recruitment of bone marrow-derived cells such as macrophages and neutrophils. These sequential events established a pre-metastatic niche which permitted the survival and proliferation of disseminated PDAC cells and the formation of metastases in the liver [76].

5.5 Pancreatic cancer-derived exosomes as coordinator of pancreatic tumor immune tolerance

Zhou et al. found that pancreatic cancer-derived exosomes negatively regulated toll-like receptor 4 (TLR4) in dendritic cells (DCs), and also down-regulated the expression of tumor necrosis factor- $\alpha$ (TNF- $\alpha$ ) and interleukin-12 (IL-12) [77]. DCs, antigen-presenting cells (APCs), are the initiator and modulator of the immune response, which express a wide range of TLRs and cytokines [78]. DCs are not only critical for the induction of primary immune responses, but may also be important for the induction of immunological tolerance, as well as for the regulation of the type of T cell-mediated immune response [79]. Toll-like receptors (TLRs), at least 10 members of cellular surface protein receptors, can induce the innate immune defense and recognize specific conserved molecular components of microorganisms in mammals during the inflammatory response that help to eliminate the invading pathogens and coordinate systemic defenses [80, 81, 82, 83]. Different members of TLRs were expressed on DCs subsets, TLR4 of which is expressed on myeloid dendritic cell (MDC) [81]. And TLR4 is stimulated to induce the production of IL-12 [82]. Thus, the immune tolerance of pancreatic tumor for DCs might be related with pancreatic cancer derived exosomes down-regulating TLR4 and downstream cytokines in DCs.

Figure 2.

Various effect of exosomes in pancreatic cancer.

5.6 Pancreatic cancer-derived exosomes as precipitating factor of type 2 diabetes mellitus

Javeed et al. found that pancreatic cancer-derived exosomes contained adrenomedullin and CA19-9, readily entered $\beta$ cells through caveolin-mediated endocytosis or macropinocytosis and inhibited insulin secretion, and confirmed that pancreatic cancer causes paraneoplastic $\beta$ cells dysfunction by shedding adrenomedullin ${}^{+}$ /CA19-9 ${}^{+}$ exosomes into circulation that inhibit insulin secretion [84]. Human adrenomedullin is a 52-amino acid peptide, which was discovered in 1993 [85]. Martínez et al. Clearly demonstrated that adrenomedullin has the inhibitory role for insulin secretion in vitro, and adrenomedullin attenuated the insulin response to oral glucose challenge in vivo, resulting in initial elevated glucose levels [86]. The results explained the causal relationship between type 2 diabetes mellitus and pancreatic cancer, and were consistent with the primary endocrine alteration of $\beta$ -cells in the pancreatic tumor area [87].

5.7 Exosomes of cancer-associated fibroblast as regulator of survival and proliferation of pancreatic cancer cells

Cancer-associated fibroblasts (CAFs) comprise the majority of the tumor bulk of PDAC. Richards et al. found that CAFs exposed to gemcitabine dramatically increased the release of exosomes. These exosomes increased expression of chemoresistance-inducing factor, Snail, in recipient epithelial cells, and promoted survival, proliferation and drug resistance of cancer cells [88]. If gemcitabine-exposed CAFs in vitro was treated with an inhibitor of exosome release, GW4869, survival of co-cultured epithelial cells significantly reduced, suggesting an important role of CAF derived exosomes in chemotherapeutic drug resistance of tumor cells. The findings showed the potential of exosome inhibitors as treatment options alongside chemotherapy for overcoming PDAC chemoresistance.

5.8 Exosomes derived from pancreatic stellate cells as communicator of intercellular communication

Pancreatic stellate cells (PSC), analogous to a resident stellate cell population in the liver or other organs, are initially quiescent, but with appropriate stimuli PSC can convert to an activated state with characteristics of myofibroblasts [89]. Activated PSCs are present in the stromal reaction in pancreatic cancers and are responsible for the production of stromal collagen [90]. Takikawa et al. clarified the microRNA expression profile in exosomes derived from PSC and presented that PSC-derived exosomes might play a pivotal role in the interactions between PSCs and pancreatic cancer cells [72]. And then PSCs dynamically interact with other cell types to constitute the cancer-conditioned microenvironment and promote the progression of pancreatic cancer through interactions with pancreatic cancer cells [91, 92]. So, further elucidation of the roles of PSC-derived exosomes in pancreatic cancer would lead to the identification of novel therapeutic targets in the intractable disease.

Table 1

Comparison between conventional diagnostic methods and exosomes

	Conventional diagnostic methods	Exosomes
Distribution in body	CA 19-9, CA 50, CA 12-5 and CA 24-2 are mainly distributed in blood.	Exosomes is distributed in blood, saliva and urine.
Diagnosis of pancreatic cancer	Pancreatic cancer, which is diagnosed by serum biomarkers and imageological diagnosis, is largely in the middle and late stage. It is unsuitable for diagnosis of early pancreatic cancer.	Exosomes gather mutated K-ras and $p53$ genes, specific GPC1, characteristic miRNAs to provide information of pancreatic cancer at different stages, including different stages of PanINs, especially for diagnosing early pancreatic cancer.
Diagnostic way	Pathological diagnosis is an invasive approach. Image-guided fine needle aspiration is widely applied in clinical, but it’s not easy to be accepted by early screeners.	Exosomes in body fluids are able to act as the best candidate of fluid biopsy. Fluid biopsy is a non-invasive method, which is more suitable to screen the high-risk population of pancreatic cancer.

6. Conclusions and outlook

The conventional diagnostic methods, including serum biomarkers, imageological diagnosis and pathological diagnosis, are clinically used to diagnose pancreatic cancer in, but it seems to be powerless for the screening of early pancreatic cancer. Exosomes, as specific vesicles provide diversified biomarker information about pancreatic cancer, including mutated K-ras and $p53$ genes, specific GPC1, characteristic miRNAs. Exosomes seem like an integrator of biomarkers to gather proteins, DNA and miRNA, and are found in a variety of body fluids, such as blood, saliva and urine. Therefore, exosomes will become a very promising marker for the detection of early pancreatic cancer. In order to better understand the respective characteristic of conventional diagnosing methods and exosomes, comparison between them is summarized, as shown in Table 1.

Moreover, exosomes are related with formation of pre-metastatic niche in liver, immune tolerance of pancreatic tumor, occurrence of type 2 diabetes mellitus, and survival and proliferation of pancreatic cancer cells, which are like an Almighty to exhibit a variety of biological functions. Hence, exosomes provide some new ideas for the study of early metastasis, immune tolerance, proliferation and treatment of pancreatic cancer. In order to comprehensively understand the biological function of exosomes, we summarize a relative figure, as shown in Fig. 2.

Conflict of interest

The authors declare no conflicts of interest.