Emerging Role of Exosomal-Derived Long Noncoding RNAs in Human PDAC

Abstract

Background

The incidence and mortality of pancreatic ductal adenocarcinoma (PDAC) are increasing recently. Most patients with PDAC are diagnosed at advanced stage because of the high invasiveness of cancer cells and the lack of typical early symptoms. Therefore, early diagnosis of PDAC is very important to improve the prognosis. Exosomes play crucial role in intercellular communication and deliver the contents to recipient cells to regulate their biological behaviors. Recent evidence suggests emerging role of exosomes in the carcinogenesis of a variety of cancers including PDAC. Long noncoding RNAs (LncRNAs) have been reported to be involved in the development of PDAC. It has been proved that LncRNAs have the potential to be biomarkers and therapeutic targets for PDAC. Moreover, increasing number of studies focus on the role of exosomal LncRNAs in PDAC.

Summary

In this review, we summarize the current status on our understanding of the role of exosomal-derived LncRNAs in the progression and metastasis of PDAC.

Key Messages

We focus on challenges in the potential of exosomal-derived LncRNAs as novel diagnostic and prognostic markers and therapeutic targets of PDAC. In addition, we provide an overview about the demonstrated important role of exosomal LncRNAs in the progression of PDAC.

Introduction

Pancreatic cancer is a lethal tumor with high invasiveness and malignancy. According to different tissue sources, pancreatic cancer can be divided into 2 types: pancreatic ductal adenocarcinoma (PDAC) originating from the pancreatic ductal epithelium and other pancreatic tumors of nonpancreatic ductal epithelial origin, including acinar cell carcinoma, serous cystadenocarcinoma, and pancreatic blastoma [1]. PDAC accounts for >85% of all pancreatic tumors, with high malignancy and poor prognosis [2]. Due to the lack of typical early symptoms, most PDAC are diagnosed at advanced stage and are not suitable for surgical treatment [3]. Therefore, early diagnosis of PDAC is essential to improve the prognosis. At present, screening is often recommended for high-risk individuals to achieve early diagnosis of PDAC. A deeper understanding of the risk factors and the pathogenesis of PDAC will help identify high-risk groups and facilitate early diagnosis and timely treatment of PDAC. Currently, there are still no biomarkers with sufficient sensitivity and specificity for the diagnosis of PDAC, although a large number of studies have been carried out to improve the detection rate of different imaging methods and the accuracy of diagnostic methods [4, 5]. For example, recent studies focused on the discovery of new serum biomarkers and the exploration of their combination with CA19-9 for the detection of PDAC [6, 7]. Furthermore, the application of liquid biopsy techniques such as circulating tumor cells, circulating tumor DNA, microRNAs (miRNAs), and exosomes in urine and saliva in the diagnosis of PDAC has received increasing attention [8].

Exosome is a type of extracellular vesicle (EV) which is secreted by all eukaryotic cells and may act as an intercellular messenger for vesicles that recycle and transport body waste. EVs can be divided into 3 categories: exosomes, microvesicles, and pathogenic bodies, which show differences in the biogenesis, size, content, and function. Exosomes have been implicated in carcinogenesis and tumor progression [9]. Exosomes are microvesicles that are produced from the endosomes and have a diameter of 30–150 nm (average 100 nm) [10]. The earliest appearance of exosomes is the formation of multivesicular bodies (MVBs) in early endosomes (Fig. 1). MVBs are endocytosed and enriched in intraluminal vesicles and gradually increase in volume to become late endosomes [11]. The contents of exosomes are from original cells, including molecules such as DNA, mRNA, miRNA, long noncoding RNAs (LncRNAs), proteins, lipids, and metabolites. Because of disparate donor cell types and status, the composition and function of exosomes are different under diverse circumstances [12]. Intercellular communication mediated by exosomes in diverse cells plays a significant role in important processes related to health and disease, such as wound repair, immune response, tissue fibrosis, tumorigenesis, and metastasis [13, 14, 15, 16]. Moreover, exosomes contribute to tumor development, and oncogenic exosomes are an important part of tumor microenvironment to regulate the proliferation, migration, and invasion of tumor cells. In addition, exosomes support angiogenesis, increase vascular permeability, and regulate epithelial-mesenchymal transition [17, 18]. In particular, several studies have suggested clinical significance of exosomes as biomarkers for early and noninvasive diagnosis of PDAC [19]. However, the specificity and sensitivity of a single exosome as a marker are not high enough, and the sensitivity and specificity of diagnostic markers can be increased by combining exosome contents, such as exosomal LncRNA, exosomal miRNA, and exosomal protein, as serum tumor markers for PDAC. Furthermore, exosomes show promise as a therapeutic vehicle of PDAC [20].

Fig. 1.

The process of exosome from a donor cell to a recipient cell. The exosome is first formed in the early endosome, enriched in the intracavitary vesicles, then gradually increases in volume to become a late endosome, and finally forms MVBs, Next, MVBs fuse with the cell membrane to secrete the exosome in exocytosis. Finally, exosome is endocytosed by the recipient cell and delivers all contents into the recipient cell. MVBs, multivesicular bodies.

LncRNAs refer to long nonprotein-coded transcripts with lengths of >200 bp. Increasing evidence suggests that LncRNAs are associated with the development and progression of a variety of cancers [21]. LncRNAs have been reported to be involved in the development of PDAC [22]. As noncoding RNA, LncRNAs regulate gene expression by influencing the processes of DNA transcription and protein translation and play a role in intercellular communication through inclusion in exosomes [23]. Some PDAC-associated LncRNAs are involved in the regulation of cell proliferation, invasion, and migration, as well as epigenetic regulation, thus promoting or inhibiting the progression of PDAC. Therefore, targeted regulation of LncRNA to treat PDAC is a therapeutic strategy being studied [24]. It is valuable to explore the detection of specific LncRNAs in serum and other biofluids as diagnostic and prognostic markers of PDAC [25]. For example, increased expression of HOX transcript antisense RNA (HOTAIR) has been observed in PDAC [26]. Recently, numerous studies have reported the involvement of exosomal LncRNAs in PDAC (Fig. 2). In this review, we summarize progress and prospect of our understanding of the role of exosomal-derived LncRNAs in the occurrence, progression, diagnosis, and treatment of PDAC.

Fig. 2.

Exosomal-derived LncRNAs involved in pancreatic cancer.

Exosomal-Derived LncRNAs Regulate PDAC Cell Proliferation

A study reported that LncRNA PVT1 promoted exosome secretion in PDAC. Mechanically, PVT1 regulated the expression and localization of RAB7, the translocation of YKT6 and VAMP3, and the palmitoylation of YKT6 in PDAC cells [27]. It has been reported that M2 macrophages can profoundly affect tumor development [28]. Yin et al. [29] found that M2 macrophage-derived exosomes promoted the proliferation, migration, invasion, and tumor xenotransplantation while inhibited the apoptosis of PDAC cells. In particular, LncRNA SBF2-AS1 in exosomes could inhibit miR-122-5p as ceRNA and upregulate the expression of X-linked inhibitor of apoptosis protein to promote the progression of PDAC [29]. In PDAC tissues and cell lines, the expression of LncRNA CASC2 was downregulated, while overexpression of CASC2 partially inhibited the growth and progression of PDAC cells by altering intercellular adhesion. The underlying mechanism is that CASC2 acts as a sponge of miR-24, thereby positively regulating target molecule LncRNA MUC6 to modulate the proliferation and apoptosis of PDAC cells [30]. Wang et al. [31] identified micro­RNA-143 from human mesenchymal stem cell-derived exosome and reported that hsa-miR-143-3p may regulate KrasG12D, PI3K, ERK, JNK, p38MAPK, and vimentin to promote the apoptosis and suppress growth, invasion, and migration ability of PDAC cells. Kumar et al. [32] analyzed serum exosomes of healthy subjects, intraductal papillary mucosal neoplasms, and PDAC patients and observed that PDAC-derived cargos contained much more MALAT1 compared to healthy subjects. In addition, LncRNA MALAT1 sponged miR-217 and suppressed its translocation from the nucleus to the cytoplasm, thus downregulating KRAS expression to promote PDAC cell proliferation, migration, and invasion [33]. Furthermore, MALAT1 was reported to interact with HuR to regulate PDAC cell proliferation and migration through hur-tia-1-mediated autophagy activation [34].

Exosomal-Derived LncRNAs Regulate PDAC Invasion and Metastasis

Li et al. [35] found that LncRNA Sox2ot in plasma exosomes promoted EMT and stemness in PDAC cells, and it may be an independent risk factor for the survival of PDAC patients. Sox2ot acts as the ceRNA of the miR-200 family and regulates the migration, invasion, and stemness of PDAC cell through Sox2 [35]. Wang et al. [36] found that melittin can inhibit PDAC cell migration in a dose-dependent manner. They further confirmed that exosome-mediated upregulation of LncRNA NONHSAT105177 could inhibit EMT and cholesterol biosynthesis pathway of PDAC cells to affect the proliferation, migration, and invasion of PDAC cells [37]. Two studies showed that LncRNA UCA1 and CCAT1 derived from PC exosomes could transfer to human umbilical vein endothelial cells and act as a ceRNA of mir-96-5p or miR-138-5p, respectively, to promote angiogenesis and tumor growth [38, 39]. PC cell exosome-derived LINC01133 can be upregulated by periostin via the EGFR pathway. Furthermore, LINC01133 could silence AXIN2 and suppress GSK3 activity via H3K27 trimethylation, leading to β-catenin accumulation and EMT of PDAC cells [40].

Exosomal-Derived LncRNAs Regulate PDAC Angiogenesis

Angiogenesis is a dynamic and complex process regulated by a variety of molecular mechanisms and plays a crucial role in many malignancies. The tumor microenvironment of PDAC is characterized by dense stromal tissue deposition, and there is a positive correlation between vascular density and PDAC progression. The hypoxic microenvironment of PDAC promotes exosome release to enhance tumor angiogenesis [41]. Guo et al. [38] reported that PDAC cell-derived hypoxic exosomes LncRNA UCA1 promoted angiogenesis and tumor growth through the miR-96-5p/AMOTL2/ERK1/2 axis and could serve as a promising target for PDAC treatment. Han et al. [39] showed that exosomal LncRNA CCAT1 derived from PDAC cells could mediate the miR-138-5p/HMGA1 axis to affect angiogenesis of human microvascular endothelial cells in vivo and in vitro. On the other hand, improving the efficiency of drug delivery by normalizing blood vessels is an emerging therapeutic strategy; therefore, we need explore specific biomarkers to screen patients who should either antagonize angiogenesis or normalize vascular [41].

Exosomal-Derived LncRNAs Serve as Diagnostic and Prognostic Markers of PDAC

The occurrence and progression of PDAC depends on the complex tumor microenvironment. Exosomes can exist stably in various circulating biological fluids, facilitate cell-to-cell communication, transport the molecules from donor cells to recipient cells, and regulate cell function and phenotype. Cancer cells secrete more exosomes than normal cells. Therefore, the detection of exosomes in biological fluids such as serum or urine to find biomarkers for PDAC can be used for the diagnosis, prognosis prediction, and monitoring of PDAC [11]. Indeed, a growing body of studies have shown that exosomal LncRNAs have high tissue specificity and can distinguish different cancer subtypes [42, 43, 44]. Li et al. [35] reported that tumor-derived exosomal Lnc-Sox2ot decreased in serum samples from PDAC patients after surgery, suggesting that it may play a role in tumor progression and be a biomarker for the diagnosis of PDAC. In addition, previous RNA sequencing data have demonstrated that EVs in serum as a tumor marker have better diagnostic accuracy than traditional circulating tumor markers [45, 46]. Furthermore, Yu et al. [47] characterized plasma exosome LncRNA profile in PDAC, and the identification of diagnostic features of PDAC based on EV LncRNA (EXLR) spectrum may contribute to the early detection of PDAC and improve the prognosis of patients. Takahashi et al. [48] established a PDAC cell xenografts model in nude mice. Knockout of highly upregulated in liver cancer (HULC) in mouse PDAC cells resulted in the inhibition of tumor growth. In addition, serum samples from 20 PDAC patients, 22 patients with intraductal papillary mucinous neoplasm (IPMN), and 21 healthy individuals were collected to analyze the expression of HULC in serum vesicles. The expression of HULC in PDAC patients was significantly higher than that in healthy individuals or IPMN patients. It is suggested that extracellular vesicula-coated HULC may be a potential circulating biomarker for human PDAC. Currently, in vitro studies on the role of exosome-derived LncRNAs in human PDAC remain to be explored [48]. Kumar et al. [32] examined serum exosomal RNA from healthy individuals, patients with IPMN, and patients diagnosed with PDAC and found that LncRNAs MALAT1 and CRNDE were overexpressed in serum exosomes from patients with PDAC and IPMN. Xie et al. [49] showed that the levels of HOTAIR and PVT1 in the saliva of PDAC patients were significantly higher than those of healthy people and proposed HOTAIR and PVT1 as new noninvasive biomarkers for PDAC. Taken together, these studies suggest that exosome LncRNAs not only have great potential as biomarkers for the diagnosis and prognosis of PDAC but also can distinguish the disease stage of patients, and they show promise for application in precision medicine (Table 1).

Table 1.

Potential role and mechanisms of exosomal-derived LncRNAs in PDAC

Expression of exosomal LncRNA in PDAC	Potential role in PDAC	Mechanism	References
LncRNA PVT1	Promote exosome secretion in PDAC	PVT1 regulated the expression and localization of RAB7	[27]
M2	Inhibit the apoptosis of PDAC cells	Promote the proliferation, migration, invasion, and tumor xenotransplantation	[28]
LncRNA SBF2-AS1	Promote the progression of PDAC	Inhibit miR-122-5p as ceRNA and upregulate the expression of X-linked inhibitor of apoptosis protein	[29]
LncRNA CASC2	Modulate the proliferation and apoptosis of PDAC cells	As a sponge of miR-24, thereby positively regulating target molecule LncRNA MUC6	[30]
LncRNA MALAT1	Promote PDAC cell proliferation, migration, and invasion	Sponged miR-217 and suppressed its translocation	[32, 33, 34]
LncRNA Sox2ot	Promoted EMT and stemness in PDAC cells, and it may be an independent risk factor for the survival of PDAC patients	Acts as the ceRNA of the miR-200 family	[35]
LncRNA NONHSAT105177	Regulate proliferation, migration, and invasion of PDAC cells	Inhibit EMT and cholesterol biosynthesis pathway of PDAC cells	[37]
LncRNA UCA1 and CCAT1	Promote angiogenesis and tumor growth	Act as a ceRNA of mir-96-5p or miR-138-5p	[38, 39]
LINC01133	Affect EMT of PDAC cells	Silence AXIN2 and suppress GSK3 activity via H3K27 trimethylation	[40]
LncRNAs MALAT1 and CRNDE	As diagnostic and prognostic markers of PDAC	Overexpressed in serum exosomes from patients with PDAC and IPMN	[32]
HOTAIR and PVT1	As diagnostic and prognostic markers of PDAC	The levels in the saliva of PDAC patients were significantly higher than those of healthy people	[49]

PDAC, pancreatic ductal adenocarcinoma.

Conclusion

Exosomal-derived LncRNAs have gained more attention as an effective tool for the diagnosis and treatment of PDAC. Abnormal expression of exosomal LncRNAs plays an important role in the occurrence, progression, diagnosis, and treatment of PDAC. One of the main reasons for poor therapeutic effect and poor prognosis of PDAC is chemotherapy resistance. In this review, we summarize recent studies on the role of exosomal LncRNAs in the proliferation, migration, invasion, and angiogenesis of PDAC cells, but there is few report on exosomal LncRNAs and drug resistance of PDAC. Considering that exosomes have a special advantage of protecting their contents from degradation, the combination therapy of exosomal LncRNAs with chemotherapy drugs such as gemcitabine is a new research direction to be developed. Future studies should focus on the delivery of exocrine LncRNAs through nanotechnology. Tang et al. [50] demonstrated the potential of exosomal-derived LncRNAs HOTAIR as a diagnostic and prognostic biomarker in breast cancer, and high expression of exosomal HOTAIR was associated with poor response to neoadjuvant chemotherapy. However, the role of exosomal-derived LncRNAs for monitoring the therapeutic response of PDAC in neoadjuvant chemotherapy remains to be explored.

PDAC has a very low survival rate due to high malignancy and aggression, and the absence of early symptoms leads to poor treatment efficiency. Therefore, we need to develop effective methods for early diagnosis and treatment of PDAC. In recent years, accumulating studies have investigated the role of exosomal LncRNAs in cancer progression and treatment. Because the specificity and sensitivity of LncRNAs are not high enough, experts in the field shift the attention to exosome binding LncRNAs, which show improved sensitivity and specificity. Although most studies are still in the experimental stage, exocrine-related LncRNAs show promise as effective therapeutic targets and specific biomarkers for PDAC. Further studies in the field should identify more exosomal LncRNAs that participate in the initiation, progression, and metastasis of PDAC and reveal the mechanism underlying the role of exosomal LncRNA in PDAC. Furthermore, the role of exosomal-derived LncRNAs in PDAC cells/organoids should be investigated.

Conflict of Interest Statement

The authors declare no conflicts of interest.

Funding Sources

This study was supported by the Natural Science Foundation of China (No. 81871954 and 81672353) and the Beijing Municipal Natural Science Foundation (No. 7212111).

Author Contributions

D.L. designed this review and drafted the manuscript. D.L. searched the related literature. D.L. prepared the figures. X.T. and Y.Y. revised and polished the manuscript and approved to submit the manuscript. All authors contributed to the article and approved the submitted version.

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