Recently, the abnormal expression of serum exosomal miRNAs has emerged as a novel potential biomarker of tumor diagnosis and progression, due to their stability in the plasma/serum and their specific expression profiles, which are reflecting the gene information and biological properties of tumor cells [25, 26]. These serum exosomal microRNAs may be used as non-invasive, specific and sensitive biomarkers for the diagnosis and prognosis of tumors . Alterations of circulating microRNAs have been investigated in patients with PC. However, studies on serum exosomal microRNAs in PC are lacking. In the present study, we investigated the levels of free and exosomal miRNA-21 and miRNA-210 in PC patients.
Numerous studies indicate that serum exosomal miRNAs are closely correlated with tumor-derived microRNAs, and that these miRNAs may be of value as diagnostic biomarkers for cancer [28, 29]. In the present study, compared with the CP group, the expression levels of serum exosomal miRNA-21 and miRNA-210 were obviously higher in the PC group. By contrast, the expression levels of serum free miRNA-21 and miRNA-210 did not differ statistically significantly between these two groups. Whole circulating peripheral blood contains other miRNAs in addition to exosomal miRNAs, which are easily degraded, probably because these miRNAs are not enclosed in exosomes [30, 31]. We hypothesized that exosomal rather than free serum miRNAs were preferable and may serve as a new biomarker for PC, as serum free miRNAs could not distinguish between PC and CP patients. In fact, serum exosomal miRNAs originating from tumor-derived miRNAs may be used as novel diagnostic biomarkers . Furthermore, exosomal miRNAs in the serum or plasma and other bodily fluids may be more stable and valuable as biomarkers for PC detection compared with serum free miRNAs.
miRNA-21 is upregulated in different types of tumors and targets tumor-suppressive mRNAs. It was demonstrated that overexpression of serum ex-miRNA-21 was diagnostic of pancreatic ductal adenocarcinoma . By contrast, miRNA-210 acts as an oncogene, and has been shown to be upregulated in various types of tumors compared with adjacent normal tissues, contributing to the progression and development of various cancers, including lung cancer and PC via different signaling pathways [34, 35]. The possible clinical application of miRNA-210 in diagnosing and detecting cancers was previously investigated . Moreover, the serum levels of miRNA-210 were previously found to be induced under hypoxic conditions and linked to adverse prognosis in some cancers ; as hypoxic environment is common in PC [38, 39], it may be worth investigating the potential diagnostic value of serum exosomal miRNAs for patients with PC. The analysis of exosomal miRNA-21 and miRNA-210 levels among circulating serum miRNAs may represent a feasible strategy for PC diagnosis. It may be inferred that exosomal miRNAs in the serum directly reflect the properties of tumor cells, as well as various diseases, as the quantity and content of exosomes may be reflective of the pathophysiological state of the cells .
In the present study, we compared the potential diagnostic value of serum exosomal miRNAs with the serum levels of CA19-9. Exosomal miRNA-21 and miRNA-210 levels may be a new diagnostic or therapeutic target for early PC, while CA19-9 levels remained within the normal range. The sensitivity of the serum test was increased to 90% when combining positive results for exosomal miRNA-21 and miRNA-210 levels with serum CA19-9 levels, while the specificity remained 90%. Serum exosomes are a valuable tool, and different methods may be used to explore their components, which may lead to more accurate, sensitive, cost-effective, and high-throughput diagnoses. The present study demonstrated that the serum exosomal miRNA-21 and miRNA-210 levels were closely related to disease state and other prognostic factors, and they may be used for early diagnosis and prediction of prognosis non-invasively. These investigations reveal that these serum exosomal miRNAs are potential valuable biomarkers for the diagnosis, screening and prognosis of PC.
The limitations of the present study included the insufficient sample size, which may affect the diagnostic value of exosomal miRNAs. In addition, the expression level of serum exosomal miRNAs was not compared between normal controls and PC tissues. The mechanisms regulating transfer of exosomal miRNAs from tumor cells into the blood is rarely reported, while the association between serum and tissue levels of exosomal miRNAs should be a focus of future research. Furthermore, exosomal miRNAs in the plasma, pancreatic juice, bile, urine, saliva, breast milk and other bodily fluids have not yet been investigated as diagnostic biomarkers, and will be examined in future studies. In addition, the level of change of circulating exosomal miRNAs was not examined before and after treatment in PC patients. Finally, the effect of the degree of circulating exosomal miRNA expression on disease progression, tumor cell metastasis and overall survival cannot be concluded from our results due to the small number of PC patients and the short follow-up period, and must be addressed in the future.