Colorectal cancer (CRC), the third most common cancer in the world, has no specific biomarkers that facilitate its diagnosis and subsequent treatment. The miRNAs, small single-stranded RNAs that repress the mRNA translation and trigger the mRNA degradation, show aberrant levels in the CRC, by which these molecules have been related with the initiation, progression, and drug-resistance of this cancer type. Numerous studies show the microRNAs influence the cellular mechanisms related to the cell cycle, differentiation, apoptosis, and migration of the cancer cells through the post-transcriptionally regulated gene expression. Specific patterns of the upregulated and down-regulated miRNA have been associated with the CRC diagnosis, prognosis, and therapeutic response. Concretely, the downregulated miRNAs represent attractive candidates, not only for the CRC diagnosis, but for the targeted therapies via the tumor-suppressing microRNA replacement. This review shows a general overview of the potential uses of the miRNAs in the CRC diagnosis, prognosis, and treatment with a special focus on the downregulated ones.
The micro-ribonucleic acids (RNAs), or miRNAs, are posttranscriptional regulatory elements consisting of 17–25 short nucleotides and single-strand and noncoding RNAs (1) that are highly conserved between the eukaryotic cells, animals, and plants related to numerous pathophysiological processes (2). The miRNAs are essential for biological processes such as the signal transduction, development, and cell growth and death (3). These processes are regulated by the binding of the miRNAs to the
The miRNAs are generated from the primary transcript (pri-miRNA) in the nucleus, where the RNase III Drosha and the double-strand RNA binding protein, DGCR8 (microprocessor complex), cut its flanked simple strands (5). The result is the 65-nucleotides-hairpin, called the
The miRNAs are usually deregulated in almost all the cancer types. Approximately 50% of the alterations are located in the cancer-associated genomic regions or fragile sites, leading the cells to act abnormally or aberrantly (8), thereby suggesting they play a vital role as the oncogenes or the tumor-suppressor genes. Lu
The colorectal cancer (CRC) is a heterogeneous disease with genetic and localization differences, which are highly related to the patient lifestyle and environment. The risk factors include the hereditary mechanism, polyp formation, large-bowel inflammatory diseases, high-fat diet, physical inactivity, alcoholism, smoking, and obesity (12). Even though the CRC is one of the most common cancers worldwide (13), there are no valid biomarkers that facilitate the diagnosis and subsequent treatment.
The CRC is correlated to the inactivation of the tumor-suppressor genes, and the activation of the oncogenic signaling, amplifications, or mutations of the miRNA loci resulting mostly from the epigenetic alterations (4). The miRNAs may play a role in the physiological and pathological processes by influencing the cancer-stem-cell biology, angiogenesis, epithelial-mesenchymal and mesenchymal-epithelial transitions, or drug resistance (14). Furthermore, the colon-cancer stem cells have been identified as a contributor to the chemotherapy resistance; this is due, among other characteristics, to their plastic nature that lets them switch between the cancer nonstem cells and the cancer stem cells to avoid the chemotherapy effects (15).
The miRNA-sequencing studies regarding the CRC have determined specific expression profiles that could be related to the clinical-pathology and patient prognostics (16). Hamfjord
A remarkable number of miRNAs exhibit the differential expression in the CRC tissues with the downregulated miRNAs (Table 1), and this is especially relevant to the cell proliferation, apoptosis, and metastasis (Fig. 2). Some of these miRNAs has been associated with the CRC risk, patient survival, or treatment outcome.
The miRNA 16-1 is frequently deleted or downregulated in many cancer-cell lines and various tumor tissues. It is usually transcribed with the miRNA 15a (miRNA-15a/16-1) and plays a role in the epithelial-mesenchymal transition, contributing to the metastasis capacity of the CRC cells (18). The p53 activates the miRNA-15a/16-1 that inhibits the expression of the AP4 (activating enhancer binding protein 4), a transcription factor that mediates the epithelial-mesenchymal transition; this leads to the metastasis repression in the lung, which is one of the most frequent produced by CRC (19). The tissue microarray of 90 patients with the CRC correlated with both the lower expressions of the miRNA15A and miRNA16-1 and a greater number of the IgA+ B cells, along with the shorter survival times of the patients. Liu
The ectopic expression of the miRNA-15a/16-1 raised the number of the G2/M phase cells in the CRC cell lines, reducing the colony formation and the tumor-induced mice. The CCNB1 (cyclin B1) protein levels were inversely correlated with the levels of the miRNA-15a/16-1, implying the CCNB1 is the target of these miRNAs (21). The CCNE1 (cyclin E1) comprises two miRNA-16-1 target sites, but a study wherein the small interfering RNA (siRNA) was observed against the CCNE1 shows the lower inhibition of the CCNE1 level compared to when the miRNA-16-1 level is increased, suggesting other miRNA-16-1 targets are involved (1). The MiRNA-16-1 is inversely correlated with the cyclooxygenase-2 (COX-2), whose overexpression is a critical step of the CRC tumorigenesis (22). In the healthy cells, the miRNA-16-1 can bind to both the COX-2 target sequences and mediates its decay; however, in the tumor cells, this mechanism is inhibited by the HuR, which binds to the miRNA-16-1, allowing the COX-2 level to increase (23).
The cluster miRNA-143/miRNA-145 is a tumor suppressor that is usually downregulated in several tumors but is not expressed in the epithelial cells (24–27), while it is noticeably downregulated in the metastatic tumors of the CRC patients (28); its targets include the apoptosis inhibitor 5, K-RAS, ERK5, and insulin-receptor substrate 1. Drebber
The miRNA-365 is often downregulated and involved in the regulation of the cell proliferation, differentiation, and apoptosis in numerous cancers cells, like those of the CRC (36–39). Nie
The miRNA-34 a, b, and c are three similar members of the miRNA family with the same targets (43) and are regulated by the p53 and the DNA hypermethylation (44, 45). Their ectopic expression induces the senescence, apoptosis, and inhibition of the tumor-cell invasion (44), while the loss of the expression produces the resistance against the p53-induced apoptosis. The deregulated miRNA-34 family and the CpG methylation have been associated with the prognosis in the CRC and several other tumors (46). Gao
The miRNA-34 ectopic expression inhibited the invasion, cell growth, and p53 activity in the HCT116 cells, but not in the knockout cells. The same results were obtained with the xenograft knockout, implying the expression is a prognostic marker for the CRC recurrence. A study of the nitric-oxide stress-induced cellular apoptosis, resulting from the p53-dependent miRNA-34 overexpression, increased the resistance of the CRC cells to the apoptosis (49). Roy
The miRNA-137 is embedded in the CpG island and is frequently silenced by the methylation in several tumors. The 5-azacytidine (5-AZA) was used in the CRC cell lines exhibiting a significant demethylation and inducing the miRNA-137 upregulation, suggesting its epigenetically silenced through the promoter methylation, which is not found or has very low levels in the healthy tissues. The restauration of the miRNA-137 levels reduced the cell proliferation, proposing a tumor-suppressor function. Svoboda
An inverse correlation between the miRNA-137 and the lysine (k)-specific demethylase 1A (LSD1), which participates in the maintenance of the global DNA methylation (52), has been found in the CRC cells (53). Also, the LSD1 overexpression has been documented with respect to several cancers (54). The downregulated miRNA-137 in the CRC cells causes the overexpression of the paxillin (PXN) gene, which encodes for a focal adhesion molecule (55), thereby involving a larger tumor size, an adverse differentiation status, a more extensive lymph-node invasion, a higher TNM stage, poor overall survival, a less favorable prognosis, a more proliferative ability, and a higher colony-formation capacity; similarly, this occurs with the Formin-like 2 (FMNL2), a target of the miRNA-137. Furthermore, the FMNL2-promoted proliferation, motility, and invasion of the CRC cell and metastasis
Finally, the miRNA-143, an miRNA with the tumor-suppressor functions, is downregulated in several cancer types. The Ng
The CRC miRNAs are differentially expressed in diverse tissues and are potential biomarkers. Liu
One of the major troubles for the CRC treatment is the acquired chemotherapy resistance. As the miRNAs are involved in the cancer progression, they can be considered as prognostic factors or as therapeutic targets (68). For the miRNA inhibition, different tools may be used, such as the miRNA sponges, miRNA masking, antisense oligonucleotides, or molecule inhibitors.
The miRNA sponge is mRNA that has in its sequence multiple tandem binding sites for the targeting of some specific miRNAs. The union between the mRNA and the miRNA produces the selective blockade of a complete family of the associated miRNAs (69). The sponge was tested first in the breast cancer (70), where the mir-21, -155, and -221 were again involved. Shen
The small molecule inhibitors, such as azobencene, can also be used to modify the miRNA expression (74). Another option is the use of the antisense inhibition of the mature miRNAs (antimiRs) for the blocking of the interaction of the miRNAs with their endogenous mRNA targets. Valeri
A low mir-143 expression is a valuable predictive factor for the effectiveness of the capecitabine treatment in the CRC patient (84), and the same applies regarding the miR-31-3p and the miR-31-5p in response to the Cetuximab in the RAS CRC patients (85). A high miR-320e level is associated with the adverse response to the FOLFOX in the stage-III CRC patients (86). Some of the miR-492 polymorphisms in the CRC patients have been associated with a stronger disease progression (87). The miR-129 and the miR-203 are downregulated in the 5-FU resistant cells, and their restitution induces the chemosensitivity (88). Alternatively, high expressions of the miR-192 and the miR-215 have been detected in the 5-FU resistant cells (89), and a high level of the circulating miR-126 is associated with the Bevacizumab-plus XELOX resistance (90). Finally, some miRNAs are also involved in the radiotherapy sensitivity; in particular, the miR-360 has been identified as a radiosensitivity regulator. Accordingly, the ionic radiation can decrease the mir-360 expression. When this miRNA is ectopically expressed, it can enhance the ionic-radiation-induced cytotoxicity by the negative regulation of the BCL2L2 and TP53RK expressions (91).
Many miRNAs are deregulated in most of the cancer types including the CRC, driving and modulating their progression. The miRNAs may influence the cancer-stem-cell biology, angiogenesis, epithelial-mesenchymal and mesenchymal-epithelial transitions, and/or the drug resistance. Specific patterns of the upregulated and downregulated miRNA have been associated with the CRC diagnosis, prognosis, and therapeutic response. In this review, the miRNA downregulation that occurs in the CRC but not in the normal cells is highlighted, and new strategies of the gene therapy, such as the miRNA sponges, miRNA masking, antisense oligonucleotides, and molecule inhibitors—which are being used to restore their levels, thereby regaining its tumor-suppressor function—are shown. Several miRNAs present the differential expression in the CRC cancer tissues, plasma, or body fluids. These molecules may be used like prognostic and survival biomarkers to activate the tumor-suppression routes or to increase the drug response. In fact, the miRNA panels can be used to quickly identify their circulating concentration, and this has been proposed in relation to the CRC diagnosis and evolution. However, at present the circulating miRNA measurement has rarely been implemented in clinical practice, and the role and function of many miRNAs remain poorly understood. Future research will be necessary to use the miRNAs as a less-invasive technique in the screening for the CRC and to help determine its prognosis.
This review was funded by Consejería de Salud de la Junta de Andalucía through the project PI-0476-2016, Granada University project PP2015-13, and the financial groups 09/112016. We thank Ministerio de Educacion Cultura y Deporte for its research grant (FPU).
The authors have no conflicting interests.
Downregulated miRNAs in CRC Samples
|miRNA||Origin of sample||Biomarker||Reference|
|miRNA-106a||Blood (plasma)||Prognosis and survival biomarker||(92)|
|miRNA-126||Blood (plasma)||Decrease sensitivity to capecitabine and oxaliplatin (XELOX)||(92)|
|miRNA-137||Cancer stem cells and normal colon stem cells||Capacity to suppress the tumorigenicity||(93)|
|miRNA-143||CRC tissue, along with the corresponding normal mucosa specimens||Increased response to 5-fluoracil||(94)|
|miRNA-16.1||Human colon tumors and histologically normal tissue||Stage of CRC and tumorogenesis biomarker||(23)|
|miRNA-29||Normal human colon epithelial cell lines and CRC cell lines||Diagnostic biomarker||(95)|
|miRNA-34||Formalin-fixed paraffin-embedded human CRC tissue and normal colonic mucosa||Prognosis biomarker Resistance to 5-fluorouracil in treatment||(50)|
|miRNA-365||Human CRC tissue and non-neoplastic mucosa tissue||Progression and survival biomarker||(40)|
|miRNA-433-3p||Human CRC and normal human colon epithelial cells||CRC development biomarker||(96)|
|miRNA-497-5p||CRC tissue relative to paired adjacent normal mucosa||Malignancy CRC biomarker||(97)|
|miRNA-675||Primary CRC tissue and paired adjacent non-tumor tissue||Proliferation, invasion and migration related biomarker||(98)|