The molecular mechanisms that contribute to LUSC remain unclear, which hinders early diagnosis, prognosis predictions, and therapeutic target selection in LUSC. Using LUSC samples collected in the TCGA database, we identified potential DEgenes and DElncRNAs. We then constructed a ceRNA network by combining these data with microRNA expression profiles. Additionally, the detailed post-transcriptional regulatory mechanisms of genes involved in the pathological process of LUSC were analyzed, and hub lncRNAs and genes were screened. The present study found that MAGI2-AS3 might be a hub lncRNA involved in LUSC progression. It formed ceRNA networks with miR-374 family members (miR-374a-5p and miR-374b-5p), which are known to participate in cell signal transduction, cell metabolism, and cell migration in LUSC. These processes play an important role in the occurrence, progression, and metastasis of LUSC.
According to the ceRNA hypothesis, lncRNAs can act as molecular sponges of miRNAs and effectively occupy their binding sites, thereby releasing target mRNAs and increasing their post-transcriptional levels. The present study showed that MAGI2-AS3 expression was significantly positively correlated with ZEB2 expression; both of which were downregulated in LUSC samples. Through the construction of the ceRNA network and corresponding analysis, it was shown that MAGI2-AS3 or ZEB2 could act as a molecular sponge of miR-374 family members (miR-374a-5p and miR-374b-5p). The downregulation of MAGI2-AS3 might increase the available binding sites of miR-374 and promote the degradation of downstream mRNAs, thereby leading to the downregulation of ZEB2, and vice versa.
Recently, it was confirmed that MAGI2-AS3 functioned as a sponge of miR-374a/b-5p in vitro(21). The present study verified that MAGI2-AS3 overexpression could inhibit the colony-forming ability of LUSC cells and significantly reduce cell metastasis and invasion. Bioinformatics analysis of prognostic biomarkers for human bladder cancer(22) showed that MAGI2-AS3 was downregulated in bladder cancer samples, and it was identified as the hub lncRNA in the ceRNA network. The tumor-suppressive role of MAGI2-AS3 has also been confirmed in multiple studies related to breast cancer(23-25). In a study on hepatocellular carcinoma (HCC) (26), the tumor-suppressive role of MAGI2-AS3 and targeted regulatory relationship between MAGI2-AS3 and miR-374b-5p were also confirmed. A previous study on epithelial ovarian cancer (27) also confirmed that MAGI2-AS3 played a tumor-suppressive role, which was achieved by targeting miR-374a/b. However, these studies did not further determine the downstream targets of miR-374a/b-5p in the ceRNA network using reliable experiments. For example, He, J, et al. predicted that MAGI2-AS3 might regulate the function of the downstream CADM2 axis via miR-374a/b-5p using only an online database. The study of Yin, Z. et al. showed that MAGI2-AS3, miR-374b-5p, and SMG1 constituted the ceRNA network. This group suggested that SMG1 acted as a molecular sponge of miR-374b-5p and modulated the expression of MAGI2-AS3, thereby participating in the regulation of HCC progression. The results of Du, S. et al. demonstrated that MAGI2-AS3 regulated PTEN, the direct target of miR-374a, by acting as a molecular sponge of miR-374a and thus participating in the regulation of breast cancer cell metastasis and invasion mediated by the PTEN signaling pathway. Although PTEN is an important tumor suppressor and plays an important regulatory role in many tumors, it is inactivated in LUSC(5). Moreover, miR-374a/b-5p does not interact with or regulate PTEN in LUSC cells(21). The results of the present study showed that MAGI2-AS3 expression was significantly reduced in LUSC samples and that MAGI2-AS3 was involved in the ceRNA network together with miR-374a/b-5p, which is consistent with previous studies. However, no differential expression of SMG1, PTEN, or CADM2 was found in the LUSC samples in our study. Therefore, we consider that the MAGI2-AS3-mediated regulation of cell functions via acting as a molecular sponge of miR-374a/b-5p was achieved through other targets.
The miR-374 family has been shown to be involved in many physiological and pathological processes. This family has been well studied in digestive system tumors but not in other tumors. It is speculated that this family may regulate cell differentiation and proliferation and be associated with the occurrence of tumors(28). In the present study, microRNAs targeting ZEB2 were investigated using the TargetScan online database (http://www.targetscan.org/vert_72/), and the results revealed that the miR-374 family targeted and regulated ZEB2 (Figure S8). Based on the results of the present study, MAGI2-AS3, the miR-374 family, and ZEB2 constitute a ceRNA regulatory network and participate in the regulation of LUSC progression.
We suggest that ZEB2 plays very diverse roles in LUSC. ZEB2 expression was low in tumor tissues, suggesting that ZEB2 may play a tumor-suppressive role and that a decrease in ZEB2 expression promotes tumor progression. However, the results of survival analyses demonstrated that low ZEB2 expression was a good prognostic factor for patients with LUSC and contributed to improvements in the 5 year survival rate. This seemingly contradictory result stimulated great interest by our research team. It has been proposed that mRNAs and proteins encoded by the same sequence of a gene may have different biological effects in certain situations. ZEB2 is a practical example that supports this hypothesis(29).
The protein encoded by ZEB2 is a member of the Zfh1 family of two-headed zinc finger/homeodomain proteins that function as DNA-binding transcriptional repressors(15, 30). It has been confirmed that ZEB2 is widely involved in various tumor-related pathological processes, including proliferation induction, differentiation arrest, apoptosis suppression, and angiogenesis(29). ZEB2 promotes tumor progression and invasion by controlling epithelial-to-mesenchymal transition (EMT) in epithelial cancers(31). ZEB2 has been identified as a molecular switch of EMT(29). In prostate cancer (PCa), miR-200c-3p was confirmed to inhibit EMT by targeting ZEB2 and thus reducing the migration and invasion of PCa cells (32). Similarly, miR-145 and ZEB2 form a two-way negative feedback loop that achieves complex functional regulation during PCa invasion and migration and plays a key regulatory role in different pathological processes in the bone metastasis of PCa(31). ZEB2 is also confirmed to be a necessary factor for tumor metastasis in melanoma cells, and the ZEB2-mediated regulation of EMT is one of the most essential mechanisms for the metastasis and ectopic growth of melanoma(33). Furthermore, in a study on triple-negative breast cancers (TNBCs), the researchers confirmed that p53 promoted the transcription of miR-30a-3p and miR-30a-5p, which directly target ZEB2, to inhibit the invasion and distant metastasis of tumor cells. This mechanism is related to the ability of ZEB2 to regulate EMT. Moreover, the inactivation of p53 could affect the biological behavior of TNBCs(34).
Therefore, ZEB2 plays an important role in regulating the invasion and migration of tumor cells. This conclusion may explain the contribution of low ZEB2 expression to the good prognosis of LUSC patients. However, compared with paracancerous normal tissues, we found that ZEB2 mRNA was downregulated in the LUSC samples. According to previous studies, the downregulation of ZEB2 expression promotes the apoptosis of tumor cells but inhibits their invasion and migration. Therefore, we believe that the downregulation of ZEB2 mRNA in LUSC samples achieved the post-transcriptional regulation of other molecules at the mRNA level. TP53 and PTEN, two important tumor-related molecules, are commonly inactivated in LUSC(5). Thus, although TP53 and PTEN are important components of the ZEB2/miR-374 regulatory axis in other tumors(24, 34), they likely do not contribute to the regulatory mechanism of the ZEB2/miR-374 axis in LUSC. The ceRNA regulatory network composed of ZEB2, MAGI2-AS3, and miR-374 family members might play key roles in extensive post-transcriptional regulation. ZEB2 might target the miR-374 family and competitively bind to miR-374, thereby modulating MAGI2-AS3 expression and further regulating the function of downstream molecules. Downregulated ZEB2 mRNA expression may increase the available competitive binding sites on miR-374, thereby enhancing its targeted regulation of other downstream factors. Moreover, MAGI2-AS3 may be directly targeted and regulated by miR-374, resulting in the subsequent downregulation of MAGI2-AS3 expression and induction of tumor progression in LUSC. However, low MAGI2-AS3 expression was also associated with a good prognosis of LUSC patients, indicating that MAGI2-AS3 may have more unknown functions in LUSC. We speculate that the ceRNA network composed of MAGI2-AS3, miR-374, and ZEB2 might have a complex two-way regulatory effect on different biological functions in various stages of tumor development. Additionally, as a transcription factor, ZEB2 might also directly target miR-374/MAGI2-AS3 or other downstream molecules, such as SMADs, to participate in cell signal transduction and more complex regulatory mechanisms.
We believe that the ceRNA regulatory network composed of MAGI2-AS3, miR-374, and ZEB2 has significant research potential. In the future, our team plans to further study the relevant molecular mechanisms involved in the occurrence and progression of LUSC to explore potential therapeutic targets and improve the efficacy of LUSC treatments.