Emerging role of exosome-derived long non-coding RNAs in tumor microenvironment

Extracellular vesicles (EVs) are small membranous vesicles that are secreted from numerous cell types, including apoptotic bodies, microvesicles, and exosomes [1]. Exosomes are generated inside multivesicular endosomes or multivesicular bodies (MVBs) and are secreted when these compartments fuse with the plasma membrane [2]. Exosomes facilitate intercellular communication by transporting intracellular components such as protein, RNA and DNA [3]. These components of exosomes are functional in the recipient cells and highly variable depending on the origin cells, and cells can produce different exosomes under different physiological and pathological conditions [4]. In 1981, it was reported that exosomes were 40–150 nm in diameter and could be isolated from various normal and tumor cells [5]. Numerous studies have shown that exosomes are critical mediators of intercellular communication between tumor cells and stromal cells in local and distant microenvironments [6]. The surface molecules of exosomes allow them to target recipient cells [7, 8]. Once attached to a target cell, exosomes induce signaling via receptor-ligand interactions or can be internalized by endocytosis and/or phagocytosis [9]. On the other hand, exosomes even fuse with the membrane of the target cell to deliver their contents into its cytosol. Exosomes can be isolated from diverse biofluids, such as blood [10], urine [11] and saliva [12]. Different techniques have been used to isolate exosomes, such as ultracentrifugation, density gradients, immunoaffinity, and commercial kits [13]. Furthermore, several methods have been applied to identify exosomes, such as electron microscopy, flow cytometry and western blot analysis [14]. Finally, high-throughput sequencing of clinical samples has been performed to analyze the differential expression of exosomal lncRNAs, which may be potential markers for cancer diagnosis.

According to the origin of the exosomes in the tumor microenvironment, exosomes can be divided into tumor cell secretions and stromal cell secretions. Cancer cells aberrantly secrete large amounts of exosomes to reflect the phenotypic state of stromal cells [15]. As tumors progress, the cargo released in exosomes also dynamically changes, thereby promoting cancer progression. Once taken in by recipient cells, tumor-derived exosomes (TEX) contribute to the crosstalk between cancer cells and stromal cells in the tumor microenvironment, such as fibroblasts [16], endothelial cells [17], and immune cells [18]. For instance, cancer cells can inhibit effector functions and induce apoptosis of various immune cells by releasing components of the TEX cargo, such as TGF-β [19], NKG2D [20], miR-203 [21], FasL [22], and TRAIL [23]. Stromal cells in the tumor microenvironment also utilize exosomes to transfer molecular contents that affect cancer initiation and progression. MiR-21 is delivered by exosomes derived from neighboring stromal cells in the omental tumor microenvironment, conferring the malignant phenotype and chemoresistance in metastatic ovarian cancer cells. In breast cancer, exosomes secreted by mesenchymal stem cells (MSCs) might reprogram tumor behavior by transferring their molecular contents. MiR-16 is enriched in MSC-derived exosomes and is partially responsible for the anti-angiogenic effect by targeting vascular endothelial growth factor (VEGF) [24]. As discussed above, exosomes regulate the tumor microenvironment by modifying the physiological state of the recipient cell [25]. Thus, exosomes are emerging as critical messengers in tumor progression and metastasis.

Long noncoding RNAs (lncRNAs) are operationally defined as transcripts of greater than 200 nucleotides that function without evident protein-coding function. LncRNAs are present in either the nucleus or cytoplasm, and they can interact with DNA, RNA or proteins [26]. Notably, lncRNAs were demonstrated to play important roles in multiple biological processes by directly or indirectly interfering with gene expression in various cancers [27]. For instance, H19 promotes tongue squamous cell carcinoma migration and invasion by targeting miR-let-7 [28]. The long noncoding RNA lnc-EGFR promotes hepatocellular carcinoma immune evasion by stimulating T-regulatory cell differentiation [29].

Interestingly, lncRNAs can also be packaged into exosomes and act as messengers in cell-to-cell communication [30]. Notably, some lncRNAs are enriched in exosomes, while others are barely present, indicating that some lncRNAs are selectively sorted into exosomes. Specific proteins might act as fundamental lncRNAs carriers to control lncRNA sorting into exosomes, though the exact regulatory mechanisms are not yet fully elucidated. Recently, exosome-derived long noncoding RNAs were reported to regulate tumor apoptosis [31], proliferation and migration and to induce angiogenesis [32]. These exosome-derived long noncoding RNAs have potential as diagnostic and prognostic biomarkers of various cancers [33]. The dysregulation of exosomal lncRNAs could affect the tumor microenvironment and regulate critical oncological behaviors [17]. For instance, hypoxic bladder cancer cells remodel the tumor microenvironment to facilitate tumor growth and development by secreting the oncogenic lncRNA-UCA1-enriched exosomes, and exosomal lncRNA-UCA1 in human serum has potential as a diagnostic biomarker for bladder cancer [34]. Thus, lncRNAs have gained increasing attention in exosome research.

In this review, we discuss the basic properties of exosomes and the functional roles of exosomes in cancer. In particular, we summarize the current knowledge regarding the contribution of exosome-associated lncRNAs to tumor microenvironments, such as tumor angiogenesis, tumor metastasis, and tumor drug resistance.

Exosomes are a new means of exchanging information between cells, and play a significant role in tumor microenvironment. The rapid development of exosome research has elucidated novel mechanisms underlying the intrinsic intercellular communication networks during cancer initiation and progression [100, 101]. Tumor-derived exosomes promote angiogenesis and coagulation, modulate the immune system, and remodel surrounding parenchymal tissue, which together support tumor progression [102, 103]. Emerging evidence reveals that lncRNAs play significant roles in regulating the tumor microenvironment and tumor progression [104‐106]. However, the physiological and pathological roles of exosomes and exosomal lncRNAs in the tumor microenvironment remain to be further explored. Meanwhile, the quantity and heterogeneity of exosomes in body fluids may be a drawback to their use as biomarkers, for these can lead to false negatives or positives in cancer diagnosis. To overcome these obstacles, we need to learn more about these delivery packets and their precise regulatory mechanisms in cancer progression. We believe that an in-depth understanding of exosomes in the tumor microenvironment may contribute to the design of cancer-diagnostic and cancer-prognostic tools. Effective therapeutic strategies for cancer using exosomes as drug carriers are expected in the near future.

Exosomes are detected in the tumor microenvironment, and exosomal lncRNAs play critical roles in facilitating tumorigenesis by regulating angiogenesis, immunity, and metastasis. In the future, circulating exosomal lncRNAs may be used as liquid biopsies and noninvasive biomarkers for the early detection, diagnosis, and treatment of cancer.