Background
Colorectal cancer (CRC) is a significant health burden in both men and women, responsible for the third leading factor of cancer-related death [
1]. In spite of the advances in treatment management and early screening, the survival outcome of CRC patients has not improved significantly [
2]. It was found that the gene expression profiles of CRC at different sub-anatomical sites were different and their biological manifestations were also different, which may lead to different therapeutic effects and prognosis of the tumors with same TNM stage [
3]. Thus, exploring a comprehensive biomarker for early predicting clinical outcomes CRC patients is essential for appropriate individual management.
MicroRNAs have been recognized as a subset of small and single-stranded noncoding RNAs which have emerged to be key regulators in various cellular processes containing differentiation, proliferation, apoptosis and stress response [
4]. In human cancers, numerous studies have proved that microRNAs contribute to progression of angiogenesis, proliferation, migration, invasion, and survival of tumor cells [
5]. Moreover, accumulative evidences have demonstrated microRNAs as stable molecular biomarkers in the diagnosis, prognosis and treatment response of some cancers [
6].
Among these microRNAs, microRNA-203 (miR-203) is one of most commonly dramatically studied microRNAs in human cancers, with the location of the chromosome 14q32.33 [
7]. Previous investigations revealed that the up-regulated miR-203 level was related to unsatisfactory clinical outcomes and high probability of carcinogenesis and recurrence in human malignancies including hepatocellular cancer, glioma, gastric cancer, breast cancer and CRC [
8]. In CRC, miR-203 was reported to be associated with poor survival by regulating some genes in the pathogenesis of CRC [
9]. Although miR-203 presented potential huge prognostic values in CRC patients, agreement has not been accomplished for the credibility of miR-203 for clinical application in CRC due to the existence of insignificant and opposing results. In addition, the potential mechanism in the occurrence and development of CRC is poorly evaluated.
Thus, we performed a comprehensive study of all eligible literatures to identify the relevance between miR-203 expression status and clinical outcomes of CRC patients. Moreover, an integrated bioinformatics analysis was also conducted for further understanding of prognostic value of miR-203 and its potential biomarker mechanism in CRC.
Discussion
CRC is still a deadly threat in human health because of tumor metastasis and relapse contributing to locally advanced stages and unsatisfactory survival outcome. In recent decades, the exploration of useful biomarkers for clinical application remains to be the focus of research due to the complexity of CRC. Emerging studies have found that aberrant expression of microRNAs associated with cancers, and some specific members have been identified with important impact during cell growth, migration, invasion and cancer progression and thus could be novel potential tools for predicting cancer initiation and survival outcome. As one of the most frequently mentioned members in CRC, miR-203 has been identified to play a significant part in tumorigenesis and CRC occurrence and development by adjusting the expression level of potential targets. However, it is controversial regarding the prognostic performance of miR-203 for CRC due to the inconsistent results from different studies. For example, recently, Schetter et al. [
24] found higher level of miR-203 might be associated with worse clinical outcome. On the other hand, Deng et al. [
27] analyzed the value of serum miR-203 and yielded a conclusion that circulating higher miR-203 levels had a better overall survival in CRC. Hence, in order to evaluate the prognostic role of miR-203 status in CRC and uncover the potential mechanism of miR-203 involved in CRC, we systematically reviewed the published studies and performed this study for addressing the relevance between miR-203 and survival outcome of CRC patients and explored the potential mechanisms.
The results indicated that miR-203 could be applied to be a useful biomarker for prognosis prediction, as high expression miR-203 was associated with worse overall survival in CRC (HR: 1.55, 95% CI: 1.07–2.24) with a P-value of 0.021. Notably, obvious heterogeneity was found across these studies, with a P-value < 0.001. Stratified analyses indicated that ethnicity, sample size and sample source might be the major factors of heterogeneity. In detail, the predictive value of miR-203 was more significant in non-Asians, large sample size and tissue samples than in Asians, small sample size and serum samples, respectively, which provided new ideas of designs of clinical trials or scientific research for promoting miR-203 into clinical application. Correspondingly, more studies on these points are required for further investigation.
The above clinical data synthesis part has suggested the usefulness of miR-203 expression as a promising biomarker in predicting the survival outcomes for CRC. However, the mechanisms how miR-203 affected the initiation and progression of CRC was not fully understood. The molecular pathogenesis of CRC mainly comes from multistep, multifactor, and polygenic effects and involves changes in various oncogenes and tumor associated microRNAs. Thus we applied several bioinformatics methods for exploring the biological function of miR-203 at the functional level. Since microRNAs play the biological role by regulating downstream genes, we first conducted a functional enrichment analysis including GO and KEGG analysis with the regulated genes of miR-203. The GO analysis demonstrated that the genes regulated by miR-203 were primarily involved in regulation process for the biological process level, mainly enriched in basic cellular component for the cellular component level and significantly clustered in binding function and transcriptional activator activity for the molecular function level. KEGG pathway analysis indicated that some of the signaling pathways regulated by the target genes of miR-203 were involved in some important pathways. The majority of these signaling pathways have been demonstrated significantly related to the occurrence and development for CRC. For example, pathways in cancer may be the most crucial pathway due to the coverage of a series of pivotal signaling, for example the p53, TGF-β, Wnt and MAPK signal pathway, which have dominant impact on almost all the aspects of the tumorigenesis and progression of cancers including CRC. Accumulating evidence suggests that Wnt pathway is important during the pathogenesis of various types of cancers including CRC through influencing cellular proliferation, differentiation, and motility [
31]. The abnormal Wnt signaling may lead to carcinogenesis and targeting this pathway may represent a potential strategy for cancer therapy [
32]. Proteoglycans in cancer, identified as critical constituents of the extracellular matrix, take significant part in the structural organization of the extracellular matrix and cell signaling which may lead to the control of extensive normal and pathological processes [
33]. Moreover, increasing evidence has confirmed that FoxO pathway has a crucial influence on the regulation of cellular homeostasis from development, cell signaling, and tumorigenesis to cell metabolism [
34,
35]. Focal adhesion, a cytoplasmic receptor tyrosine kinase, plays a prominent role in controlling essential cellular activities containing cell growth, proliferation, survival and migration [
36]. In addition, Ras signaling pathway, a well-studied pathway for CRC, the dysregulation of which plays a pivotal part during the progression of CRC and targeting this signaling may provide a potential therapeutic target in the therapy for CRC [
37]. The aforementioned results revealed a possible function of miR-203 and the related pathways during CRC pathogenesis.
As microRNA regulates multiple target genes, and each target gene has close connections with others. So we carried out a PPI analysis of genes regulated by miR-203 to explore more valuable information about miR-203. Next, based on the PPI network construction and analysis, top ten crucial genes were obtained. The crucial genes were demonstrated closely associated with some important pathways, which had pivotal impact on the occurrence and progression according to literature exploration. The most important module of the network map was further identified. The KEGG pathway analysis revealed that proteins of the prominent module were significantly enriched into a series of vital pathways. Most of the pathways have been confirmed by previous literatures. Here, we have to mention some more important pathways. It is worth noting that TNF, as an extraordinarily pleiotropic cytokine, is highly associated with maintenance and homeostasis of the immune system, inflammation as well as host defense [
38]. Adherens junction plays a central role in CRC carcinogenesis and targeting it may promote CRC progression [
39]. Moreover, accumulating preclinical evidence indicates that Toll-like receptor pathway participates in stimulating innate and adaptive immune responses and has a pivotal part during diseases induced by inflammation such as CRC [
40]. Recent reports indicated that targeting this pathway may be beneficial for CRC treatment [
41]. In addition, it is well established that NOD-like receptor plays crucial and complex roles in the homeostasis of the immune system. The abnormal activation of the signaling could cause carcinogenesis and tumor invasion [
42]. The colorectal cancer directly demonstrated that the hub genes were significantly related to the initiation and progression for CRC [
43]. The PPI analysis results are reliably consistent with the present studies. The identified genes, pathways and modules revealed the potential mechanisms of miR-203 involved within the pathological processes of CRC and provided novel insights for the treatment strategy of CRC.
Recently, some studies on the tumor suppressive functions of miR-203 have been investigated. For example, recently, You et al. [
44] reported that miR-203 restrains epithelial-mesenchymal transition (EMT), invasion and migration of papillary thyroid cancer by down-regulating AKT3. On the other hand, Lai et al. [
45] revealed that miR-203 could diminish the stemness of human colon cancer cells through suppressing GATA6 expression. Moreover, growing evidence from recent studies has revealed that miR-203-regulated posttranscriptional deregulation of CPEB4 may promote the progression of CRC and directly targeting CPEB4 by miR-203 might be a novel strategy in CRC treatment [
46]. These studies together with the findings from our bioinformatics analysis may provide help for further understanding the tumor suppressive functions of miR-203 involved in CRC.
To the most of our knowledge, our study may be the most comprehensive study about miR-203 as we not merely quantifies the biomarker performance of miR-203 for predicting the prognosis of CRC by meta-analysis, but also qualitatively explores the potential function and mechanism of miR-203 through integrative bioinformatics analysis. On the whole, our quantitative results strongly agreed with the present mainstream viewpoint that an undesirable impact of miR-203 high expression was related to worse clinical outcome. Moreover, there are still numerous valuable implications from our interesting results for future clinical and scientific research. In addition, although we have not conducted experiment, most of the enriched modules, pathways or networks were successfully confirmed by recent experimental literatures.
However, several limitations exist in the present study. Firstly, the number of studies was relatively small. More clinical studies should be conducted to extend and validate the conclusions in the future. Secondly, we collected and pooled data from public studies rather than individual case data, so subgroup and regression analyses focusing on some important variables containing age, sex and stage of cancer were restricted due to the insufficient data. Thirdly, we only enrolled patients from Asians and Caucasians; so further studies assessed the biomarker roles of miR-203 about more ethnicities are required as the study population are not comprehensive. Besides, language bias may exist as only English studies were enrolled for evidence synthesis which may published more positive results. Due to the fact that all studies that are used for the data pooling have patients of various TNM stage (from stage I to stage IV), there is considerable heterogeneity in survival. Without patient-level data, it would be very difficult to tell whether miR-203 is actually useful as a prognostic factor. Finally, though we have validated the relevance of miR-203 status with the prognosis of CRC and proved the accuracy of our predictive results through an integrated bioinformatics analysis, well-designed biological experiments are still required for future confirmation as the initiation and development of CRC is complex, with multiple cumulative genetic changes.
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