A growing body of evidence has suggested that miR-21 plays a fundamental role in migration, invasion, metastasis, and proliferation of breast cancer, head and neck squamous cell carcinoma, gastric cancer, colorectal cancer, etc.[22–26]. In this study, when exploring the association of miR-21 with clinicopathological parameters, it was related to the tissue type (adjacent noncancerous tissue or CC samples) and the occurrence of distant tumor metastasis (M0 or M1). This also indicate that miR-21 may be involved in the occurrence and development of CC.
Studies have examined that it may be a suitable diagnostic biomarker for colorectal cancer, with moderate sensitivity and specificity[27], and it has good diagnostic value in breast cancer and colorectal cancer[28, 29]. However, the potential diagnostic value of miR-21 in CC and its correlations with clinical features has seldom been studied. The present study focused on investigating the role and latent mechanism of miR-21 in CC by co-applying meta-analysis and bioinformatics approach.
Here, increased expression of miR-21 was found in GSE86100, GSE30656 and TCGA, but in GSE19611, no significant differences were observed. A possible reason for this may be the limited number of tumor samples. Only 4 tumor samples of the 23 samples in GSE19611. Furthermore, a meaningless combined SMD emerged when a sensitivity analysis was conducted (Fig. 2F). We can see from Fig. 2E, although the sample size of GSE30656 was not the largest, its weighting ratio was the highest, reaching 25.7%. Given this, once GSE30656 was removed, the analysis results severely fluctuated. In the future, higher quality and larger sample size studies need to be performed for analysis to obtain more instructive conclusions.
From Fig. 2E, forest plot of the meta-analysis, the combined SMD was 2.50 (95% CI: 0.72 - 4.27, random-effects model). Additionally, the AUC either ROC curves or sROC curve were all > 0.7 (Fig. 4A-E). In a word, our meta-analysis results demonstrated that miR-21 has potential as a highly sensitive and specific biomarker in CC. Subsequently, we performed a series of bioinformatics assay.
On the one hand, PI3K/AKT/mTOR was thought a candidate signaling pathway after the enrichment analysis. Of note, many research suggesting that it could be used as a novel biomarker for the assessment of risk of developing CC[30–32]. Some other miRNAs have been proven to inhibit the activity of human CC cells by inhibiting this pathway, such as miRNA-99b[33] and miRNA-383[34]. What’s more, the effect of miR-21 on PI3K/AKT/mTOR pathway has also been demonstrated in other diseases. Based on the above information, it can be reasonably speculated that miR-21 may affect the invasion and migration of CC cells by regulating the PI3K/AKT/mTOR pathway. This may be a potential mechanism by which miR-21 promotes CC development and progression.
On the other hand, EPAS1 were screened to be the final target of miR-21. EPAS1 is the only one that has a statistically significant correlation with DFS and it has the largest Spearman correlation coefficients with miR-21 among the four candidate target genes. The miR-21 binding sites on it were also identified. Endothelial PAS domain protein 1 (EPAS1), which is also known as hypoxia-inducible factor-2α (HIF-2α), belongs to the family of hypoxia-inducible factors (HIFs)[35]. Hypoxia is a common feature of many solid tumors, including CC. The mTOR pathway, which is essential for cell proliferation, is repressed under hypoxia. As early as 2012, reports showed that activation of the HIF-2α pathway increases mammalian target of rapamycin complex 1 (mTORC1) activity[36]. Moreover, a recent paper reported that HIF-2α could promote the apoptosis of breast cancer cells via the PI3K/AKT/mTOR signaling pathway[37]. Although the function of EPAS1 in the tumorigenesis of CC is yet clear, we have demonstrated that EPAS1 were tightly linked with the PI3K/AKT/mTOR pathway. Thus, we speculated that EPAS1 plays an important role in CC through the PI3K/AKT/mTOR pathway.
It is undeniable that this study has several limitations. First, the composition of the samples differed somewhat due to the differences in data sources. For example, in the TCGA database, there were 309 cases of CC tissues and 3 cases of adjacent nontumor tissues (Table 2 and Fig. 2D), while the corresponding figures in the GEPIA2 database were 306 and 13, respectively (Fig. 6). There is a risk that a mistake will occur, but not to the extent to which it could influence the reliability of the data, as TCGA databases with large data volumes are included. Next, because only four independent datasets were incorporated, we did not assess the risk of publication bias. Finally, due to the HPV infection status in TCGA are unavailable, we failed to identify the relationship of HPV-related CC and miR-21 expression.
Taken together, the results of this study confirm that miR-21 is upregulated and plays a vital role in the occurrence and development of CC. More importantly, miR-21 has the potential to be a novel, highly sensitive, highly specific, noninvasive biomarker for the diagnosis of CC, but additional large-scale studies are required to verify its diagnostic value. The results of bioinformatics research present a new method for exploring the pathogenesis of CC and will guide our experimental thinking.