The ceRNA network of lncRNA and miRNA in lung cancer

Since lung cancer is a major causative for cancer-related deaths, the investigations for discovering biomarkers to diagnose at an early stage and to apply therapeutic strategies have been continuously conducted. Recently, long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) are being exponentially studied as promising biomarkers of lung cancer. Moreover, supportive evidence provides the competing endogenous RNA (ceRNA) network between lncRNAs and miRNAs participating in lung tumorigenesis. This review introduced the oncogenic or tumor-suppressive roles of lncRNAs and miRNAs in lung cancer cells and summarized the involvement of the lncRNA/miRNA ceRNA networks in carcinogenesis and therapeutic resistance of lung cancer.


Introduction
expressions of genes encoding proteins [25]. LncRNA, which functions as a ceRNA, sequesters miRNA and prevents them regulating the translations of target mRNAs (Fig. 1). In 2014, ln-cRNA AK048451, which is called cardiac hypertrophy related factor, was first identified as a ceRNA of miR-489 and found to inhibit miR-489 expression by direct binding in a sequence-specific manner [20]. Furthermore, abnormal expressions, mutations, and single nucleotide polymorphisms of lncRNA have been associated with tumor formation and metastasis [26], and accumulating evidence indicates networks of lncRNAs, miRNAs, and mRNAs importantly contribute to the epithelial-to-mesenchymal transition (EMT), onset and progression of cancer [27,28]. For example, ln-cRNAs (MEG3, MIAT, and LINC00115) were found to play important roles in carcinogenesis by regulating miRNA-mRNA networks in lung cancer [29].

Oncogenic LncRNAs Acting as ceRNAs in Lung Cancer
Oncogenic lncRNAs are generally upregulated in lung cancer cells and tissues and bind directly to tumor-suppressive miRNAs. Direct lncRNA to miRNA binding upregulates the expressions of oncogenic mRNAs (a target of miRNAs), and thus, promotes cancer cell growth and development. Several lncRNAs that function as oncogenes in lung cancer have been identified (Table 1), for example, lncRNA H19 is highly expressed in the A549, H1299, H23, and SPC-A1 lung cancer cell lines, and inhibits miR-200a, miR-196b, and miR-29b-3p [30][31][32]. The interaction between lncRNA H19 and miR-200a (a tumor-suppressive miRNA downregulated in patients with a high lung cancer stage) regulates the expressions of ZEB1 and ZEB2 [30]. LIN28B is a target of miR-196b and can function as a proto-oncogene, and lncRNA H19 can upregulate LIN28B by 'sponging' miR-196b [31]. MiR-29b-3p is involved in the regulation of apoptosis, the cell cycle, and metastasis, and its targeting by lncRNA H19 transforms STAT3 (signal transducer and activator of transcription 3), and thus, promotes the survival and EMT of lung adenocarcinoma cells [32]. MALAT1 (metastasis associated in lung adenocarcinoma transcript 1) is another representative oncogenic lncRNA and is highly expressed in the A549 and H1299 lung cancer cell lines, in which miR-124 is downregulated. MiR-124 is a direct target of MALAT1 and inhibits the expression of STAT3 [33], and the expression of MALAT1 has also been reported to be correlated with the expressions of miR-200a-3p and programmed death-ligand 1 (PD-L1) [34]. MALAT1 acts as a sponge for miR-200a-3p, and thus, increases the expression of PD-L1 (a direct target of miR-200a-3p), inhibits apoptosis, and promotes the metastasis of NSCLC cells  With IncRNA [34]. PD-L1 is an attractive factor in cancer research, and drugs that target it have been shown to improve patient prognoses [44,45].
Several lncRNAs such as H19, MALAT1, and DANCR act as oncogenes in lung cancer by interacting with miRNAs. Since these lncRNAs are upregulated in lung cancer cells and tissues, and can be used as and are viewed as potential biomarkers for the early diagnosis of lung cancer. Therapies based on the use of ceRNA networks of oncogenic lncRNAs and miRNAs targeting these genes should be useful for the treatment of lung cancer.

Tumor-suppressive lncRNAs acting as ceRNAs in lung cancer
Unlike lncRNAs that are upregulated in lung cancer cells and tissues and function as oncogenes, tumor-suppressive lncRNAs are generally downregulated in lung cancer cells and tissues and bind directly to onco-miRs. Direct binding of tumor-suppressive ln-cRNAs and onco-miRs upregulates the expressions of tumor-suppressive mRNAs, a target of onco-miRs, which inhibit cancer cell growth and development. Several lncRNAs have been identified that function as tumor suppressors in lung cancer (Table 2). For example, lnc ADAMTS9-AS2 is downregulated in lung cancer cells and tissues and inhibits the development of lung cancer cells [46,47]. This function of lnc ADAMTS9-AS2 is due to direct interaction with miR-223-3p, which regulates the expression of TG-FBR3 [46]. Increased lnc ADAMTS9-AS2 expression in lung cancer cells and tissues downregulated mIR-223-3p [46], and as a result, TGFBR3 was upregulated and cancer progression was suppressed [46].
Another tumor-suppressive lncRNA GAS5 (growth arrest-specific transcript 5) inhibits tumor formation in lung cancer by negatively regulating miR-205 expression, and thus, increasing PTEN expression [56]. In NSCLC, GAS5 inhibited the expression of miR-23a, cell proliferation, and invasion and promoted apoptosis [57]. In addition, GAS5 improved the radiosensitivity of NSCLC cells [58]. Radiotherapy kills cancer cells by exposing them to high-energy radiation [59], and greater radiosensitivity of cancer cells is strongly associated with positive treatment results [60].
tumor suppressors such as PTEN and TIMP2. Like oncogenic ln-cRNAs, interactions between tumor-suppressive lncRNAs and onco-miRs may aid early diagnosis and provide gene-targeting therapies for lung cancer.

The ceRNA Roles of LncRNAs in Therapeutic Resistance
Non-surgical methods of treating lung cancer include radiation therapy and drug therapy. Representative drugs for the treatment of lung cancer include gefitinib and cisplatin. Gefitinib inhibits epidermal growth factor receptor (EGFR) tyrosine kinase by binding to the enzyme's ATP-binding site [61]. Gefitinib sensitivity studies in NSCLC have shown that mutations in the tyrosine kinase domain of EGFR activate the anti-apoptotic pathway [61].
On the other hand, cisplatin kills the fastest growing cancer cells by interfering with DNA replication [62]. The developments of radiation and drug resistance are major obstacles to successful non-surgical cancer treatment. Accordingly, studies are being actively conducted on genes involved in signaling pathways that improve sensitivity to radiation or drugs, and evidence is accumulating that lncRNA/miRNA networks are involved. For example, it was reported LINC00483 silences miR-144 in lung adenocarcinoma, and thereby, increases the radiosensitivity of LTEP-A-2 cell lines [63]. Also, FAM201A lncRNA was found to be highly expressed in NSCLC patients resistant to radiation therapy and function as a ceRNA of miR-370 and increase the expressions of EGFR and HIF-1α (hypoxia-inducible factor 1 alpha) [64]. FAM201A knockdown suppressed the expressions of EGFR and HIF-1α and increased the radiosensitivity of NSCLC cells [64]. Furthermore, in NSCLC cells CYTOR (cytoskeleton regulator RNA) lncRNA sponged miR-195 and suppressed radiosensitivity of NSCLC cells in vitro [65]. In a study on drug resistance, overexpression of HOST2 (human ovarian cancer-specific transcript 2) lncRNA inhibited miR-621 and increased gefitinib resistance in NSCLC cells due to the upregulation of SYF2 (a direct target of miR-621) [66]. LIN C00460 was highly expressed in gefitinib-resistant NSCLC cells and tissues and upregulated EGFR through miR-769-5p sponging [67]. Furthermore, EGFR upregulation led to gefitinib resistance [67]. In addition, in cisplatin-resistant NSCLC cells, TATDN1 (Homo sapiens TatD DNase domain containing 1) lncRNA downregulated miR-451, which was overexpressed in these cells, and TATDN1 knockdown improved cisplatin sensitivity [68]. Also in cisplatin-resistant NSCLC cells, TATDN1 and TRIM66 (a target of miR-451) gene expressions were positively correlated and TRIM66 was overexpressed [68]. In SCLC cells, LINC00173 sponged miR-218 and induced cisplatin and etoposide (an SCLC therapeutic) resistance [69].
The lncRNA/miRNA ceRNA network regulates the expressions of several genes that act as oncogenes or tumor suppressors in lung cancer. In several studies, changes in gene expressions by lncRNA/miRNA induced drug and radiation resistance in lung cancer cells, which suggests that the ceRNA network has the potential to contribute to the efficient applications of traditional cancer and gene-targeting therapies.

Conclusion
Studies on the lncRNA/miRNA ceRNA network in lung cancer are being actively conducted. Direct binding between lncRNA and miRNA influences cancer progression by regulating the expressions of various mRNAs that act as oncogenes or tumor suppressors. In the ceRNA network, the expressions of lncRNA and miR-NA are negatively correlated, lncRNA binding to onco-miRs suppresses tumor progression, whereas its binding to tumor-suppressive miRNAs promotes tumor progression. In this review, we summarize the effects of various lncRNAs that function as ceRNAs of miRNAs in lung cancer.
For example, LINC00483, FAM201A, and CYTOR induce radioresistance by directly binding miRNAs, and HOST2, LINC00460, TATDN1, and LINC00173 induce drug resistance to gefitinib and cisplatin. As such, many lncRNAs contribute to the development of lung cancer in various ways by direct binding miRNAs.
In this review, we summarize the lncRNA/miRNA ceRNA networks that impact lung cancer identified to date, and provide insight into the effects of RNAs not translated into proteins and of the various signaling pathways that act on lung cancer through downstream factors. The lncRNA/miRNA ceRNA network offers a means of discovering biomarkers that enable the early diagnosis of lung cancer and provide guidance regarding gene-specific treatments. In addition, the abilities of lncRNA and miRNA interactions to affect radiation and drug resistance suggests they can be targeted in treatment of resistant patients. We believe improved understanding of lncRNA and miRNA interactions is likely to lead to future developments in the lung cancer treatment field.