Connection of Cancer Exosomal LncRNAs, Sponging miRNAs, and Exosomal Processing and Their Potential Modulation by Natural Products

Simple Summary Cancer cells generally release special vesicles (exosomes) showing tumor-promoting effects. Some natural products blocking exosome processing (assembly and secretion) can inhibit cancer progression. Long noncoding RNAs (lncRNAs), enclosed in exosomes, can bind and modulate several microRNAs (miRNAs), and, in turn, miRNAs can regulate their targets, such as exosome processing genes. However, there is a gap in the correlation between exosomal lncRNAs and exosomal processing of natural product treatments. After collecting and organizing literature reports, we introduce bioinformatics for retrieving miRNA targets of lncRNAs and exosomal processing gene targets of miRNAs to fill this gap. Consequently, the function of exosomal lncRNAs of cancer cells in regulating miRNA targets that potentially modulate exosomal processing genes is summarized, particularly for the anticancer effects of natural products. Abstract Cancerous exosomes contain diverse biomolecules that regulate cancer progression. Modulating exosome biogenesis with clinical drugs has become an effective strategy for cancer therapy. Suppressing exosomal processing (assembly and secretion) may block exosomal function to reduce the proliferation of cancer cells. However, the information on natural products that modulate cancer exosomes lacks systemic organization, particularly for exosomal long noncoding RNAs (lncRNAs). There is a gap in the connection between exosomal lncRNAs and exosomal processing. This review introduces the database (LncTarD) to explore the potential of exosomal lncRNAs and their sponging miRNAs. The names of sponging miRNAs were transferred to the database (miRDB) for the target prediction of exosomal processing genes. Moreover, the impacts of lncRNAs, sponging miRNAs, and exosomal processing on the tumor microenvironment (TME) and natural-product-modulating anticancer effects were then retrieved and organized. This review sheds light on the functions of exosomal lncRNAs, sponging miRNAs, and exosomal processing in anticancer processes. It also provides future directions for the application of natural products when regulating cancerous exosomal lncRNAs.

Although many exosomal ncRNAs are essential for participating in carcinogenesis [31], the present review focuses on lncRNAs. Noncoding transcripts over 200 nucleotides are called lncRNAs. Mounting literature reports show that exosomal lncRNAs control cancer progression, such as lung [32], gynecological [11], gastric [33], and other cancers. Different lncRNAs exhibit diverse functions. The regulatory mechanisms of lncRNAs were collected in the database LncTarD [34] and include transcriptional regulation, ceRNAing or miRNA sponging, chromatin looping, epigenetic regulation (histone modification and DNA methylation), interacting with mRNA (mRNA splicing and RNA editing), and interacting with protein (protein stability and phosphorylation). LncRNAs have multiple regulatory mechanisms. Each lncRNA, in turn, modulates several miRNAs or proteins transcriptionally and epigenetically, demonstrating a complex network and making it difficult to provide a straightforward direction and clear organization.
For simplification, the present review focuses on the exosomal lncRNAs that may exhibit miRNA-binding sites to sponge miRNAs, causing specific miRNA degradation and preventing them from binding their target mRNAs [35,36] (Figure 1). MiRNAs are another group of ncRNAs with short lengths (21-25 nucleotides). The function of miRNAs is the direct regulation of mRNA degradation and indirect modulation of protein expression by blocking translation [37]. vey, starting from cancer exosomal lncRNAs and continuing to sponging miRNAs and connecting to their target genes, suggests their effects to be similar to a signaling cascade. To converge the connection of exosomal lncRNA-sponging miRNA targets, the targets of the genes involved in exosomal processing were chosen as described above. Moreover, the relationship between miRNAs and exosomal processing genes is poorly known at present. Therefore, in the present review, we focus on the exosomal lncRNA-sponging miRNA-exosomal processing target axis ( Figure 1).

Figure 1.
Overview of the objectives of this review. The lncRNA-miRNAs-exosomal processing targets axis is shown. It starts from exosomal lncRNAs sponging miRNAs that target exosomal processing (including (1) assembly and (2) secretion) genes. For clarification, the detailed components of exosomes are not shown, and only the lncRNAs and miRNAs are indicated.
Exosomal lncRNAs are overexpressed in cancer and responsible for cancer progression. The strategy of downregulating exosomal lncRNAs has potential in anticancer therapy. Some natural products exhibit anticancer effects that may be attributed to suppressing the exosomal lncRNAs of cancer. However, the potential sponging miRNAs and exosomal processing targets of exosomal lncRNAs are rarely summarized. The gaps between exosomal lncRNA and sponging miRNAs and between miRNAs and exosomal processing genes are filled by bioinformatic tools, such as lncTarD [34] and miRDB [38], respectively ( Figure 2).

Figure 2.
Steps in filling the knowledge gap between exosomal lncRNA and sponging miRNAs and between miRNAs and exosomal processing targets, as provided by lncTarD [34] and miRDB [38] databases.
For this review, we collected several cancer exosomal lncRNAs and performed bioinformatic retrieval for their sponging miRNAs and functions and the targeting of exosomal processing genes (Section 2). Next, tumor microenvironment (TME)-associated lncRNAs, sponging miRNAs, and exosomal processing genes were assessed (Section 3). Finally, the lncRNA-modulating effects of natural products, their sponging action on miR-NAs, and the exosomal processing of cancer cells were explored (Section 4). This review examines the relationship between cancer exosomal lncRNAs, miRNAs, and the exosomal Figure 1. Overview of the objectives of this review. The lncRNA-miRNAs-exosomal processing targets axis is shown. It starts from exosomal lncRNAs sponging miRNAs that target exosomal processing (including (1) assembly and (2) secretion) genes. For clarification, the detailed components of exosomes are not shown, and only the lncRNAs and miRNAs are indicated.
Notably, a single miRNA potentially targets several hundred genes. A literature survey, starting from cancer exosomal lncRNAs and continuing to sponging miRNAs and connecting to their target genes, suggests their effects to be similar to a signaling cascade. To converge the connection of exosomal lncRNA-sponging miRNA targets, the targets of the genes involved in exosomal processing were chosen as described above. Moreover, the relationship between miRNAs and exosomal processing genes is poorly known at present. Therefore, in the present review, we focus on the exosomal lncRNA-sponging miRNA-exosomal processing target axis ( Figure 1).
Exosomal lncRNAs are overexpressed in cancer and responsible for cancer progression. The strategy of downregulating exosomal lncRNAs has potential in anticancer therapy. Some natural products exhibit anticancer effects that may be attributed to suppressing the exosomal lncRNAs of cancer. However, the potential sponging miRNAs and exosomal processing targets of exosomal lncRNAs are rarely summarized. The gaps between exosomal lncRNA and sponging miRNAs and between miRNAs and exosomal processing genes are filled by bioinformatic tools, such as lncTarD [34] and miRDB [38], respectively ( Figure 2). is the direct regulation of mRNA degradation and indirect modulation of protein expression by blocking translation [37]. Notably, a single miRNA potentially targets several hundred genes. A literature survey, starting from cancer exosomal lncRNAs and continuing to sponging miRNAs and connecting to their target genes, suggests their effects to be similar to a signaling cascade. To converge the connection of exosomal lncRNA-sponging miRNA targets, the targets of the genes involved in exosomal processing were chosen as described above. Moreover, the relationship between miRNAs and exosomal processing genes is poorly known at present. Therefore, in the present review, we focus on the exosomal lncRNA-sponging miRNA-exosomal processing target axis ( Figure 1).

Figure 1.
Overview of the objectives of this review. The lncRNA-miRNAs-exosomal processing targets axis is shown. It starts from exosomal lncRNAs sponging miRNAs that target exosomal processing (including (1) assembly and (2) secretion) genes. For clarification, the detailed components of exosomes are not shown, and only the lncRNAs and miRNAs are indicated.
Exosomal lncRNAs are overexpressed in cancer and responsible for cancer progression. The strategy of downregulating exosomal lncRNAs has potential in anticancer therapy. Some natural products exhibit anticancer effects that may be attributed to suppressing the exosomal lncRNAs of cancer. However, the potential sponging miRNAs and exosomal processing targets of exosomal lncRNAs are rarely summarized. The gaps between exosomal lncRNA and sponging miRNAs and between miRNAs and exosomal processing genes are filled by bioinformatic tools, such as lncTarD [34] and miRDB [38], respectively ( Figure 2).

Figure 2.
Steps in filling the knowledge gap between exosomal lncRNA and sponging miRNAs and between miRNAs and exosomal processing targets, as provided by lncTarD [34] and miRDB [38] databases.
For this review, we collected several cancer exosomal lncRNAs and performed bioinformatic retrieval for their sponging miRNAs and functions and the targeting of exosomal processing genes (Section 2). Next, tumor microenvironment (TME)-associated lncRNAs, sponging miRNAs, and exosomal processing genes were assessed (Section 3). Finally, the lncRNA-modulating effects of natural products, their sponging action on miR-NAs, and the exosomal processing of cancer cells were explored (Section 4). This review examines the relationship between cancer exosomal lncRNAs, miRNAs, and the exosomal Steps in filling the knowledge gap between exosomal lncRNA and sponging miRNAs and between miRNAs and exosomal processing targets, as provided by lncTarD [34] and miRDB [38] databases.
For this review, we collected several cancer exosomal lncRNAs and performed bioinformatic retrieval for their sponging miRNAs and functions and the targeting of exosomal processing genes (Section 2). Next, tumor microenvironment (TME)-associated lncRNAs, sponging miRNAs, and exosomal processing genes were assessed (Section 3). Finally, the lncRNA-modulating effects of natural products, their sponging action on miRNAs, and the exosomal processing of cancer cells were explored (Section 4). This review examines the relationship between cancer exosomal lncRNAs, miRNAs, and the exosomal processing targets of cancer cells as well as the potential impact of natural products on this axis. Compared to previous literature reports, as mentioned later, this review provides a novel integration for each function of natural-product-modulating exosomal lncRNAs, miRNAs, and the exosomal processing targets by filling the connecting gap with bioinformatic databases.

Cancer Exosomal lncRNAs and Their Sponging miRNAs
Here, we discuss the potential connections of some exosomal lncRNAs of cancer cells to the sponging miRNAs (Table 1). Table 1. The predicted sponging miRNAs of exosomal lncRNAs and predicted exosomal processing targets of these miRNAs.
Detailed information has been provided for these exosomal lncRNAs and their sponging miRNAs (Table 1). For example, hepatocellular carcinoma upregulates the EZH2associated lncRNA (HEIH), binds to miR-939, and then sponges the downstream function of miR-399, thus blocking the interaction between miR-939 and nuclear factor-kB (NF-κB) and improving NF-κB-mediated Bcl-xL expression for anti-apoptosis to promote tumorigenesis of colon cancer cells [50]. LINC02418, overexpressed in lung cancer tissues and cell lines, can sponge miR-4677-3p. In contrast, LINC02418 silencing inhibits the migration and proliferation of lung cancer cells, which is reversed by miR-4677-3p expression [51]. LncRNA POU class 3 homeobox 3 (POU3F3), activated by transcriptional factor SP1, improves the proliferation of cervical cancer cells by sponging miR-127-5p. The cervical cancer growth-promoting function of POU3F3 was validated by POU3F3 knockdown [52].
Furthermore, two sponging miRNAs were shown to be responsible for chemoresistance and migration. The lncRNA regulator of Akt signaling associated with HCC and RCC (LNCARSR; lnc-TALC) overexpression was shown to be accountable for temozolomide (TMZ) resistance and associated with glioblastoma recurrence. LNCARSR can sponge miR-20b-3p to enhance c-Met expression and improve chemoresistance to TMZ [76]. LncRNA-ATB stimulated astrocytes to enhance glioma cell migration by sponging miR-204-3p [16] (Table 1).
In summary, several cancer exosomal lncRNAs sponge miRNAs and promote cancer cell proliferation.

TME and Its Associated lncRNAs
The TME contains several cell types, such as tumor cells, cancer-associated fibroblasts (CAFs), cancer stem cells (CSCs), tumor-associated macrophages (TAMs), natural killer cells, and myeloid-derived suppressor cells [77,78]. This review focuses on the lncRNAs of CAFs, CSCs, and TAMs.
CAFs are activated fibroblasts that secrete several bioactive components to enhance tumor growth, metastasis, and drug stance [79]. CSCs are a minor subpopulation of cancer cells that exhibit the self-renewal ability and can initiate tumor differentiation and development [80,81]. TAMs are macrophages that enhance the establishment of the  [82] by upregulating immunosuppressive M2-like polarization in cancers that play a critical role in the migration and invasion of carcinogenesis [83].
Communication between those TME cells may stimulate cancer exosomes to increase carcinogenesis. Several TME-associated lncRNAs are well-understood [78]. However, the potential impacts on downstream miRNAs are rarely discussed, particularly regarding the sponging effects of TME-associated lncRNAs on miRNAs. Moreover, the putative exosomal processing targets of these predicted miRNAs have not been illustrated to date.

Potential Functions of CAF-Associated lncRNAs That Sponge miRNAs and Modulate miRNA-Targeted Exosomal Processing Genes
In the case of CAF, several TME-associated lncRNAs were reported (Table 2). CASC9, POU3F3, SNHG3, CDKN2B-AS1, and ZEB2-AS1 are predicted to sponge several miRNAs (in cancer cells) to promote cancer proliferation. Furthermore, CCAL is predicted to sponge miRNA to promote cancer metastasis.
Detailed information is presented for these CAF-associated lncRNAs and their sponging miRNAs ( Table 2). For example, CASC9 exhibits oncogenic function, and the CASC9-1 transcript is overexpressed in cervical cancer cells to increase proliferation by sponging miR-383-5p, which is reversed by CASC9 knockdown [86]. The sponging information of POU3F3 is provided in Table 1, while SNHG3, which is enriched in osteosarcoma, promotes proliferation by sponging miR-196a-5p [87]. CDKN2B-AS1, which is highly expressed in oral and ovarian cancer, promotes proliferation or suppressed apoptosis by sponging miR-125a [88] and miR-411-3p [89], respectively. ZEB2-AS1, enriched in bladder cancer cells and tissues, enhances proliferation and suppresses apoptosis by sponging miR-27b-3p, which is reversed by a miR-27b mimic [90]. Furthermore, CCAL is highly expressed in gastric cancer, and its expression levels are correlated with the metastasis stage. CCAL can sponge miR-149 and regulate metastasis of gastric cancer cells [91] ( Table 2). The in vivo functions of CCAL have been validated; CCAL knockdown suppresses tumor growth and metastatic nodules in lungs in a xenograft nude mice model [91].
Consequently, CAF-associated lncRNAs sponging several miRNAs probably cause the proliferation or metastasis of cancer cells (Table 2). By data mining in respective depositories, these CAF-associated lncRNAs show the potential modulation of sponging miRNAs and targeting exosomal processing. This will be helpful in future investigations of exosome biogenesis involving these CAF-associated lncRNAs.

Potential Functions of CSC-Associated lncRNAs That Sponge miRNAs and Modulate miRNA-Targeted Exosomal Processing Genes
In the case of CSCs, several TME-associated lncRNAs were reported ( Table 2). TCF7 is predicted to sponge miRNA (in cancer cells) to promote cancer invasion. Lnc34a, LNCBRM, DLX6-AS1, and LINC01567 are predicted to sponge several miRNAs (in cancer cells) to promote proliferation. Furthermore, HAND2-AS1 and DGCR5 are predicted to sponge miRNA to inhibit the proliferation and radiosensitivity of cancer, respectively.
Detailed information has been provided for these CSC-associated lncRNAs and their sponging miRNAs (Table 2). For example, TCF7 promotes the invasion of cervical cancer cells by sponging miR-155-5p, which is reversed by TCF7 downregulation [92]. Moreover, TCF7 knockdown suppresses cervical tumor growth. Lnc34a shows high levels in colon CSCs, causes an asymmetric division of CSCs, and improves colon cancer proliferation by sponging miR-34a [93]. LNCBRM overexpression enhances colon cancer cell proliferation and invasion by sponging miR-204-3p, which is reversed by LNCBRM knockdown [94]. The sponging information of DLX6-AS1 is provided in Table 1. WLINC01567 was overexpressed in colon CSCs to increase proliferation by sponging miR-93-mediated tumor suppression, which was reversed by LINC01567 knockdown [95] (Table 2).
Consequently, CSC-associated lncRNAs sponge several miRNAs, which may promote or inhibit proliferation and enhance the invasion and radioresistance of cancer cells (Table 2). Using data mining, these CSC-associated lncRNAs modulate sponging miRNA-targeting exosomal processing. This will be helpful for future investigations of exosome biogenesis involving these CSC-associated lncRNAs.

Potential Functions of TAM-Associated lncRNAs That Sponge miRNAs and Modulate the miRNA-Targeted Exosomal Process
In the case of TAMs, several TME-associated lncRNAs have been reported (Table 2). RP11-361F15.2 and RPPH1 are predicted to sponge miRNA (in cancer cells) to promote cancer invasion. FGD5-AS1 and HCG18 are predicted to sponge several miRNAs of cancer cells to promote cancer proliferation. Furthermore, LINC01089 and TP53COR1 are predicted to sponge miRNA to inhibit the proliferation of cancer.
Detailed information is provided for these TAM-associated lncRNAs and their sponging miRNAs ( Table 2). For example, RP11-361F15.2 shows more significant upregulation in osteosarcoma than the normal control, contributing to invasion by promoting the M2-like polarization of TAM, which is reversed by RP11-361F15.2 downregulation. RP11-361F15.2 sponges and downregulates miR-30c-5p expression, contributing to the promotion of invasion in osteosarcoma [83]. Moreover, overexpression of RP11-361F15.2 enhances the growth of xenograft osteosarcoma [83]. RPPH1 shows more unique expression in lung cancer than normal cells, contributing to cisplatin resistance ( Table 2). RPPH1 downregulation suppresses the invasive ability of lung cancer cells, while RPPH1 overexpression shows the opposite effects by sponging miR-326 expression [99]. FGD5-AS1, which shows high expression in glioblastoma cells, is essential for cancer progression. This was reversed by FGD5-AS1 knockdown, leading to binding of miR-129-5p and suppression of miR-129-5p expression [100]. Similarly, FGD5-AS1 shows high expression in colon cancer, enhancing proliferation and inhibiting apoptosis, which is reversed by miR-302e knockdown. MiR-302e is bound to FGD5-AS1. Collectively, FGD5-AS1 upregulated the sponging effects on miR-129-5p [100] and miR-302e [101] and promoted the progression of colon cancer and glioblastoma, respectively. HCG18 was overexpressed in colon cancer for increased proliferation that was reversed by HCG18 knockdown, which induced miR-1271-5p overexpression. HCG18 levels were proportional to the degree of colon cancer malignancy, and this relationship is thought to be due to sponging of miR-1271-5p [102] (Table 2).
Furthermore, LINC01089, showing low levels in cervical cancer, was reversely correlated with tumor growth and lymph node metastasis as a consequence of sponging miR-27a-3p, and this was reversed by LINC01089 knockdown [103] (Table 2). TP53COR1 inhibited liver cancer cell proliferation by sponging and downregulating miR-9 expression [104].
Consequently, TAM-associated lncRNAs sponge several miRNAs and may promote or inhibit proliferation and enhance the invasion of cancer cells (Table 2). Using data mining, these TAM-associated lncRNAs show the potential modulation of the sponging miRNA-targeted exosomal processing. This will be helpful in the future investigation of exosome biogenesis involving these TAM-associated lncRNAs.

The Potential Sponging miRNAs and Exosomal Processing Targets for Natural-Product-Modulated lncRNAs
Several lncRNA-modulating natural products have been reported. However, the potential response of exosomal processing genes in the modulation of the lncRNA-miRNA axis of natural products remains unclear.

The Predicted Sponging miRNAs of Natural-Product-Downregulated lncRNAs
The potential connections between some natural-product-downregulated lncRNAs and sponging miRNAs are shown (Table 3).
Consequently, natural products downregulate some lncRNAs and fail to sponge miRNAs which are responsible for promoting the proliferation, migration, and invasion of cancer cells. To conclude, natural products suppress the miRNA-sponging effects of lncRNAs, thus leading to the inhibition of proliferation, migration, and invasion.

Predicted Sponging miRNAs of lncRNAs Upregulated by Natural Products
The potential connections of some natural-product-upregulated lncRNAs to the sponging of miRNAs are reviewed here (Table 3).
Consequently, natural products upregulate some lncRNAs and sponge miRNAs which are responsible for inhibiting proliferation and chemoresistance. Collectively, natural products promote the miRNA-sponging effects of lncRNAs, thus leading to the inhibition of proliferation and chemoresistance.

Overview of Natural Products That Modulate the Exosomal lncRNA-miRNA Axis to Regulate Exosomal Processing
The relationship between natural products, their exosomal lncRNA-modulating effects, and their sponging miRNAs and potential targets of exosomal processing are summarized (Table 3). To concentrate on the final step of exosomal processing targets, exosomal-processcentric relationships are represented (Table 4). Table 4. Exosome-processing-target-centric view of predicted sponging miRNAs and natural-productmodulated lncRNAs.  ↑ and ↓ indicate the enhancement and inhibition of the lncRNA expression by natural products, respectively. This table provides exosomal-processing-target-centric connections to sponging miRNAs and natural-productmodulated lncRNAs. Exosomal processing targets include exosomal assembly and secretion. All the information is derived from Table 3, but only the information showing exosomal process targets is plotted. Sponging miRNAs and exosomal processing targets were predicted using the lncTarD and miRDB.
Consequently, the relationships between natural products, lncRNAs, sponging miR-NAs, and exosomal processing targets are clearly explored.

Conclusions
Cancerous exosomes contain complicated biomolecule compositions that promote cancer progression. Among other issues, this review focused on exploring the relationship between lncRNAs, miRNAs, and exosomal processing targets. Generally, the lncRNA-miRNA-target axis is straightforward in terms of signaling. However, lncRNAs and miRNAs regulate many downstream effectors, and all complicated signaling regulations are arranged in a cascade. To clarify this rationale, only the sponging miRNAs of exosomal lncRNAs were discussed.
Moreover, several natural products that modulate the exosomal lncRNAs were described. Their potential role in sponging miRNAs and exosomal processes lacks systemic organization in the order of the lncRNA-miRNA-target axis. This gap was filled by the bioinformatic database. The potential exosomal processing targets for the sponging miR-NAs were then retrieved. These sponging miRNAs and exosomal processing targets were summarized using lncTarD and miRDB databases. Consequently, the exosomal lncRNAs and their potential roles in regulating sponging miRNAs and exosomal processing were clarified. Notably, these sponging miRNAs of lncRNAs were bioinformatically predicted, although miRNA-sponging functions were derived from the literature reports with citations. Moreover, these potential candidates may belong to different cancer cell lines and exhibit tissue-specific expression. This warrants a careful investigation by wet experiments in the future. Notably, most studies in this review focus on exploring the lncRNA sponging effects on miRNAs and the targeting effects of miRNA on exosomal processing in cell models, although some of them have evidence from animal studies. A thoughtful in vivo assessment is still required to explore the roles of exosomal lncRNAs, sponging miRNAs, and exosomal processing targets in natural product experiments by animal studies.
Many exosomal lncRNAs control exosomal processing changes. This review provides a future investigation direction, as encouraged by the bioinformatical prediction of several sponging miRNAs. Similarly, TME-associated and natural-product-modulated lncRNAs show similar problems that ignore the potential involvement of exosomal processing in the literature and can be solved using bioinformatic strategies. Accordingly, this review sheds light on explorations of the exosomal lncRNA-sponging miRNA-exosomal process axis and the potential impact of natural products on this axis (Figure 3), providing a possible future direction for cancer therapies using exosomal lncRNA.  . Schematic summary of the natural products acting on the exosomal lncRNA-sponging miRNA-exosomal process axis. Natural products may modulate (inhibit (T) or enhance (arrow)) cancer and TME-associated lnRNAs. Next, the lncRNAs may sponge miRNAs, and, in turn, miR-NAs can modulate target exosome processing genes, providing modulating functions to cancer cells. More detailed information for each step has been mentioned in the Tables 1-3   . Schematic summary of the natural products acting on the exosomal lncRNA-sponging miRNA-exosomal process axis. Natural products may modulate (inhibit (T) or enhance (arrow)) cancer and TME-associated lnRNAs. Next, the lncRNAs may sponge miRNAs, and, in turn, miRNAs can modulate target exosome processing genes, providing modulating functions to cancer cells. More detailed information for each step has been mentioned in the Tables 1-3