Current studies have indicated that m
6A modification is involved in the generation process of miRNAs, thus affecting the level of mature miRNAs. Published results in the journal
Nature in 2015 revealed that decreasing the level of m
6A modification on pri-miRNAs by knocking down METTL3 expression could inhibit the binding of DGCR8 to pri-miRNAs, which led to about 70% miRNAs being downregulated by at least 30%
[57][22]. Up to now, the mechanism according to which reduction of m
6A modification on pri-miRNAs inhibits the maturation of miRNAs in a DGCR8-dependent manner has been found in different diseases. For example, catalyzed by over-expressed METTL3, high m
6A modification can promote the maturity of miR-25 and miR-25-3p by strengthening the combination of DGCR8 and pri-miR-25 in pancreatic duct epithelial cells, and this may provoke malignant phenotype of pancreatic cancer cells
[59][24]. The reduction level of m
6A modification mediated by low expression of METTL3 and METTL14 makes the weaker recognition of pri-miR-126 by DGCR8, which hinders the maturation of miR-126, thereby activating the PI3K/AKT/mTOR pathway to promote the proliferation and activation of fibroblasts. Moreover, METTL3-dependent m
6A was involved in the DGCR8-mediated maturation of pri-miR-126 in endometriosis development
[60][25]. In addition, the interaction of METTL3 and DGCR8 positively modulates the biogenesis process of miR-873-5p, miR-365-3p and miR-221/222 in an m
6A-dependent manner in different pathological processes, and as the simplest for specific miRNA, miR-873-5p participated in fighting colistin induced oxidative stress and apoptosis in kidney injury
[61][26], miR-365-3p regulated chronic inflammatory pain induced by Complete Freund’s Adjuvant in the spinal cord
[62][27], and miR-221/222 negatively mediate the PTEN expression, thus leading to the proliferation of bladder cancer cells
[63][28]. In addition, cigarette smoke can stimulate the production of excess mature miRNA-93 in bronchial epithelial cells via enhanced m
6A modification, which was mediated by overexpressed METTL3
[64][29]. METTL3 also plays a major catalytic role in m
6A modification in unilateral ureteral obstruction mice and drove obstructive renal fibrosis development by promoting miR-21-5p maturation
[65][30]. In addition, it has been indicated that silencing of METTL3 expression can elevate the levels of pri-miR-663 and m
6A methylation-modified pri-miR-663, which resulted in suppressing of miR-663 maturation process in A549 and PC9LC cells
[66][31]. In a manner similar to METTL3, the METTL14-mediated m
6A marks also enhanced the recognition of pri-miR-126 by DGCR8, thus subsequently processing to mature miRNA-126, which is involved in hepatocellular carcinoma metastasis
[60][25]. Different from the above mechanisms, METTL3 induced upregulation of miR-143-3p mostly depends on the shear effect of Dicer on pre-miR-143-3p in lung cancer cells
[67][32]. Moreover, as in bone marrow-derived mesenchymal stem cells, METTL3 also methylate pre-miR-320, on which m
6A modification is a key factor that is recognized and decayed by YTHDF2
[68][33]. METTL3 promoted the transition from pri-miR-1246 to mature miR-1246, of which upregulation can significantly enhance the metastasis ability of colorectal cancer cells
[69][34]. METTL3-mediated m
6A modification also promotes the expressions of 9 miRNAs, including miR-106b, miR-18a, miR-18b, miR-3607, miR-432, miR-30a, miR-320b, miR-320d and miR-320e, and bioinformatics analysis has shown that these miRNAs are involved in regulating signaling pathways closely related to malignant transformation induced by arsenite
[70][35]. In addition, four miRNAs (miR-130a-3p, miR-130b-3p, miR-106b-5p and miR-301a-3p) are all related to short overall survival of kidney renal clear cell carcinoma patients and have significantly negative correlation with METTL14 mRNA
[71][36]. Up to now, the importance of methyltransferases-catalyzed m
6A modification on pri-miRNAs has been widely recognized, and a variety of methyltransferase components can affect the generation and function of miRNAs.
Besides methyltransferases, m
6A demethylases and methyl-binding proteins are also involved in miRNA biogenesis. The earliest study showed that knocking down the FTO expression significantly increased levels of 42 miRNAs and decreased levels of 9 miRNAs
[72][37]. A subsequent study reported that FTO regulates cell migration and invasion in breast cancer cells by inhibiting miR-181b-3p
[73][38]. Moreover, FTO has been well evidenced to promoted bladder cancer cell proliferation via the FTO/miR-576/CDK6 pathways
[74][39]. ALKBH5 inhibits tumor growth and metastasis by inhibiting miR-107/LATS2 mediated YAP activity in non-small cell lung cancer
[75][40]. Peng et al. indicated that ALKBH5, the most potent member related to patient outcomes and to suppressing esophageal cancer malignancy in cell and animal models, demethylated pri-miR-194-2 and inhibited miR-194-2 biogenesis through an m
6A/DGCR8-dependent manner
[76][41]. Interestingly, in human non-small cell lung cancer cells, the depletion of ALKBH5 did not change the miR-21-5p level but altered the m
6A abundance on miR-21-5p, thereby changing the miR-21-5p silencing potency towards its target mRNAs, which finally impaired the proliferation and motility of human non-small cell lung cancer cells
[77][42]. In addition, ALKBH5 demethylated pri-miR-320a-3p, thus blocking DGCR8 from interacting with pri-miR-320a-3p and leading to mature process blockage of pri-miR-320a-3p in silica-inhaled mouse lung tissues
[78][43].
In addition to being directly affected by m
6A methyltransferases and demethylases, miRNA generation is also regulated by m
6A methyl binding proteins. YTHDC1, a well-known m
6A reader, facilitated the biogenesis of mature miR-30d via m
6A-mediated regulation of mRNA stability. Furthermore, miR-30d represses pancreatic tumor genesis via suppressing aerobic glycolysis
[79][44]. m
6A reader protein HNRNPA2B1 also binds to a subset of m
6A-modified pri-miRNA transcripts, thus interacting with DGCR8 and promoting primary miRNA processing, and depletion of HNRNPA2B1 caused a reduction in the levels of 61 miRNAs in HEK293 cells. Moreover, transiently overexpressed (5.4-fold) HNRNPA2B1 in MCF-7 cells led to significant alteration of more than 100 miRNAs, which regulate TGFβ and Notch signaling pathways according to MetaCore Enrichment analysis
[80,81,82][45][46][47]. Yi et al. have reported that miR-185 transfer from vascular smooth muscle cells to endothelial cells is controlled by HNRNPA2B1
[83][48], but the role of m
6A modification in this mediate process needs to be further explored. In addition, HNRNPA2B1 reads the m
6A site on pri-miR-106b or pri-Let-7b to facilitate the maturing of miR-106b-5p or Let-7b in the lung cancer cells
[84,85][49][50]. Another m
6A binding protein, IGF2BP1, promotes serum response factor expression in an m
6A-dependent manner by impairing the miRNA-directed downregulation of the
serum response factor (SRF) mRNA in cancer cells
[86][51]. In addition to regulating the generation process of miRNAs, m
6A modification can directly modify mature miRNAs and affect their stability and degradation
[72][37]. Of note, m
6A modification on the
E2F transcription factor 3 (
E2F3) mRNA was required for the interaction between miR-660 and
E2F3 mRNA in gastric cancer, indicating that m
6A also affects the function of miRNA apart from participating in its production process
[87][52].
In a word, emerging studies have identified the roles of m
6A modification during the processing and maturation of miRNAs, which will surely provide good candidate targets for miRNA intervention. Although mechanisms underlying m
6A modification affecting miRNA generation and function are diverse and complex, the general mechanisms are similar.
The Weresearchers drew a schematic diagram, which take miR-30d
[79][44], miR-21-5p
[65][30] and pre-miR-25
[59][24] as examples, to show the specific mechanisms of m
6A regulatory proteins regulating miRNAs (
Figure 2).
Figure 2. Schematic diagram of the mechanisms of m
6A modification and its regulatory proteins regulating miRNA production.
3.2. miRNAs Regulate the m
6
A Modification
Since miRNAs affect the protein level through interacting with mRNAs and m
6A modification is a dynamic reversible methylation
[88][53], it is rationality that miRNAs are involved in the regulation of m
6A modification by affecting the regulatory proteins. At present, several studies have shown that miRNAs regulate m
6A modification via sequence pairing of mRNAs of methyltransferases, demethylases, and methyl-binding proteins in various tissues
[89][54]. In detail, METTL3 was identified as the direct target of miR-1269b
[90][55] and miR-338-5p
[91][56], thus inhibiting gastric cancer development. miR-33a is capable of reducing the METTL3 expression at both mRNA and protein levels, thus affecting proliferation, survival and invasion of non-small cell lung cancer
[92][57]. Moreover, miR-600 can attenuate METTL3 expression and restrain the migration and proliferation of lung cancer cells
[93][58]. Similarly, the down regulation of miR-524-5p also up-regulates the expression of METTL3 in non-small cell lung cancer cells
[94][59]. miR-4429 targeted and repressed METTL3 to inhibit m
6A-mediated stabilization of SEC62, a component belonging to tetrameric Sec62/Sec63-subcomplex of Sec-complex, thus hindering proliferation and encouraging apoptosis in gastric cancer cells
[95][60]. Moreover, Cai et al. concluded that mammalian hepatitis B X-interacting protein (HBXIP) suppresses miRNA let-7g, thus up-regulating METTL3, which in turn promotes the expression of HBXIP through m
6A modification, leading to stimulation or proliferation of breast cancer cells
[96][61]. As an independent prognostic factor in hepatoblastoma patients, METTL3 was identified as a direct target of miR-186, of which low level led to high expression of METTL3, thus significantly inhibiting the proliferation, migration and invasion of hepatoblastoma cells
[97][62]. Moreover, miR-320d has been evidenced to target METTL3, thus affecting KIF3C expression through changing m
6A modification on KIF3C mRNA in prostate cancer cells
[98][63]. Under the treatment of Mono-(2-ethylhexyl)phthalate (MEHP), miRNAs such as miR-16-1-3p, miR-101a-3p, miR-362-5p, miR-501-5p, miR-532-3p and miR-542-3p are dramatically activated in murine macrophage Raw 264.7 cells, and these miRNAs are all predicted to regulate METTL14, thus promoting m
6A modification in
Scavenger Receptor B type 1 (
SR-B1) mRNA
[99][64]. Cui et al. reported that miR-193a-3p directly targets ALKBH5 to inhibit the growth and promote the apoptosis of glioma cells by suppressing the AKT2 pathway both in vitro and in vivo
[100][65]. Interestingly, circGPR137B acted as a sponge for miR-4739 to up-regulate its target FTO, which mediated m
6A demethylation of circGPR137B and promoted its expression, thus finally forming a feedback loop comprising circGPR137B/miR-4739/FTO axis and affecting the hepatocellular carcinoma cells
[101][66]. Results from Yang et al. indicated that imiR-155 directly targets FTO to negatively regulate its expression and increase m
6A level in renal clear cell carcinoma cells. Regarding specific mechanisms, miR-155 is directly bound to the 3′-UTR of FTO mRNA and reduced FTO protein levels
[102][67].
The methyl binding proteins of m
6A modification are also directly targeted by miRNAs. In detail, Zheng et al. reported that miRNA-421-3p targets YTHDF1 to inhibit p65 mRNA translation, thus preventing inflammatory response in cerebral ischemia/reperfusion injury
[103][68]. miR-376c also has been indicated to negatively modulate YTHDF1 expression in non-small cell lung cancer cells
[104][69]. Negative correlations between the miR-145 level and YTHDF2 mRNA expression were observed in hepatocellular carcinoma
[105][70] and epithelial ovarian cancer cells
[106][71], and further detecting results showed that miR-145 decreased the luciferase activities of 3′-UTR of
YTHDF2 mRNA, implicating that YTHDF2 is the direct target gene of miR-145
[105,106][70][71]. In addition,
YTHDF2 mRNA is also regulated by miRNA-495 in prostate cancer cells
[107][72] and miR-6125 in colorectal cancer cells
[108][73]. Bioinformatics analysis from Hao et al. literature revealed IGF2BP1 as the putative target of miR-670, of which mimics and inhibitors were microinjected into parthenogenetic activation embryos, thus confirming these findings
[109][74]. IGF2BP2, another m
6A methyl binding protein, is highly expressed in thyroid cancer cells and identified as a target of miR-204
[110][75]. In addition, inhibition of miR-133b also resulted in the up regulation of IGF2BP2 in colorectal cancer cells
[111][76]. Different from the above-mentioned mechanisms where miRNAs regulated m
6A modification, results from Chen et al. indicated that overexpressing dicer increased the m
6A modification level, and this was not achieved by alternating the quantity of m
6A methyltransferases or demethylases in mouse embryonic fibroblasts. Further experiments showed that miRNAs regulate activity and location of METTL3, which subsequently modulate m
6A modification and impede the reprogramming of mouse embryonic fibroblasts to pluripotent stem cells
[112][77].
In a word, miRNAs can influence m
6A modification by regulating the regulatory proteins of m
6A and ultimately participate in a variety of pathological and physiological processes.
WThe researche
rs drew a schematic diagram, in which
wethe researchers take METTL3
[93][58], FTO
[113][78] and YTHDF2
[108][73] as examples, to show the miRNAs involving in regulation of m
6A modification and its biological effect (
Figure 3).
Figure 3. Schematic diagram of the mechanisms showing that m
6A modification and its biological effects are regulated by miRNAs.
As mentioned above, m
6A modification is regulated by different m
6A regulatory proteins in a variety of diseases by promoting biosynthesis of miRNAs, and miRNA regulates the biological functions of m
6A regulatory proteins. Based on this interplay,
wthe researche
rs summarized the change trends and regulation relationships of m
6A regulatory proteins and miRNAs in tissues or cells during the occurrence and development of different diseases, as shown in
Figure 4.
Figure 4. Mutual regulation between m
6A modifications and miRNAs. The red and green arrows represent the level rise and fall, respectively. The purple arrows point to the regulated object. EC: esophageal cancer; TC: thyroid cancer; BRC: breast cancer; GC: gastric cancer; HCC: hepatocellular carcinoma; CRC: colorectal cancer; RCC: Renal cell carcinoma; BLC: bladder cancer; EOC: epithelial ovarian cancer; NSCLC: non-small cell lung cancer; LUAD: lung adenocarcinoma; PAC: pancreatic cancer; PDAC: Pancreatic ductal adenocarcinoma; PCa: prostate cancer.