N6-methyladenosine (m6A) and m5C methylation are two major types of RNA methylation, but the impact of joint modifications on the same mRNA is unknown. Here, we show that in p21 3′UTR, NSUN2 catalyzes m5C modification and METTL3/METTL14 catalyzes m6A modification. Interestingly, methylation at m6A by METTL3/METTL14 facilitates the methylation of m5C by NSUN2, and vice versa. NSUN2-mediated m5C and METTL3/METTL14-mediated m6A methylation synergistically enhance p21 expression at the translational level, leading to elevated expression of p21 in oxidative stress-induced cellular senescence. Our findings on p21 mRNA methylation and expression reveal that joint m6A and m5C modification of the same RNA may influence each other, coordinately affecting protein expression patterns.

The impact of methylation of the 3′-untranslated region (UTR) of a messenger RNA (mRNA) remains largely unknown. Here we show that NSun2, a transfer RNA methyltransferase, inhibits the turnover of p16INK4 mRNA. Knockdown of NSun2 reduces p16 expression by shortening the half-life of the p16 mRNA, while overexpression of NSun2 stabilizes the p16 mRNA. In vitro methylation assays show that NSun2 methylates the p16 3′UTR at A988. Knockdown of NSun2 reduces the stability of the EGFP-p16 chimeric reporter transcripts bearing wild-type p16 3′UTR, but not p16 3′UTR with a mutant methylation site. Methylation by NSun2 prevents the association of p16 3′UTR with HuR, AUF1 and Ago2/RISC, and prevents the recruitment of EGFP-p16 3′UTR chimeric transcripts to processing bodies. In response to oxidative stress, NSun2 is essential for elevating p16 expression levels. We conclude that NSun2-mediated methylation of the p16 3′UTR is a novel mechanism to stabilize p16 mRNA.

Compelling evidence indicates that epigenetic regulations orchestrate dynamic macrophage polarization. N6-methyladenosine (m6A) methylation is the most abundant epigenetic modification of mammalian mRNA, but its role in macrophage polarization is still completely unknown. Here, we show that the m6A-catalytic enzyme methyltransferase like 3 (METTL3) is specifically upregulated following the M1 polarization of mouse macrophages. Furthermore, METTL3 knockdown through siRNA transfection markedly inhibited M1, but enhanced M2, macrophage polarization. Conversely, its overexpression via plasmid transfection greatly facilitated M1, but attenuated M2, macrophage polarization. Further methylated RNA immunoprecipitation and in vitro m6A methylation assays suggested that METTL3 directly methylates mRNA encoding signal transducer and activator of transcription 1 (STAT1), a master transcription factor controlling M1 macrophage polarization, at its coding sequence and 3′-untranslated regions. In addition, METTL3-mediated STAT1 mRNA methylation significantly increased mRNA stability and subsequently upregulated STAT1 expression. In conclusion, METTL3 drives M1 macrophage polarization by directly methylating STAT1 mRNA, potentially serving as an anti-inflammatory target.

Aims: The N6-methyladenosine (m 6 A) modification plays an important role in various biological processes, but its role in atherosclerosis remains unknown. The aim of this study was to investigate the role and mechanism of m 6 A modification in endothelial cell inflammation and its influence on atherosclerosis development. Methods: We constructed a stable TNF-α-induced endothelial cell inflammation model and assessed the changes in the expression of m 6 A modification-related proteins to identify the major factors involved in this process. The m 6 A-modified mRNAs were identified by methylated RNA immunoprecipitation (RIP) sequencing and forkhead box O1 (FOXO1) was selected as a potential target. Through cytological experiments, we verified whether methyltransferase-like 14 (METTL14) regulates FOXO1 expression by regulating m 6 A-dependent mRNA and protein interaction. The effect of METTL14 on atherosclerosis development in vivo was verified using METTL14 knockout mice. Results: These findings confirmed that METTL14 plays major roles in TNF-α-induced endothelial cell inflammation. During endothelial inflammation, m 6 A modification of FOXO1, an important transcription factor, was remarkably increased. Moreover, METTL14 knockdown significantly decreased TNF-α-induced FOXO1 expression. RIP assay confirmed that METTL14 directly binds to FOXO1 mRNA, increases its m 6 A modification, and enhances its translation through subsequent YTH N6-methyladenosine RNA binding protein 1 recognition. Furthermore, METTL14 was shown to interact with FOXO1 and act directly on the promoter regions of VCAM-1 and ICAM-1 to promote their transcription, thus mediating endothelial cell inflammatory response. In vivo experiments showed that METTL14 gene knockout significantly reduced the development of atherosclerotic plaques. Conclusion: METTL14 promotes FOXO1 expression by enhancing its m 6 A modification and inducing endothelial cell inflammatory response as well as atherosclerotic plaque formation. Decreased expression of METTL14 can inhibit endothelial inflammation and atherosclerosis development. Therefore, METTL14 may serve as a potential target for the clinical treatment of atherosclerosis.

Methylation is a prevalent posttranscriptional modification of RNAs. However, whether mammalian microRNAs are methylated is unknown. Here, we show that the tRNA methyltransferase NSun2 methylates primary (pri-miR-125b), precursor (pre-miR-125b), and mature microRNA 125b (miR-125b) in vitro and in vivo. Methylation by NSun2 inhibits the processing of pri-miR-125b2 into pre-miR-125b2, decreases the cleavage of pre-miR-125b2 into miR-125, and attenuates the recruitment of RISC by miR-125, thereby repressing the function of miR-125b in silencing gene expression. Our results highlight the impact of miR-125b function via methylation by NSun2.

A rise in the levels of the cyclin-dependent kinase (CDK) inhibitor p27KIP1 is important for the growth arrest of senescent cells, but the mechanisms responsible for this increase are poorly understood. Here, we show that the tRNA methyltransferase NSun2 represses the expression of p27 in replicative senescence. NSun2 methylated the 5′-untranslated region (UTR) of p27 mRNA at cytosine C64 in vitro and in cells, thereby repressing the translation of p27. During replicative senescence, increased p27 protein levels were accompanied by decreased NSun2 protein levels. Knockdown of NSun2 in human diploid fibroblasts (HDFs) elevated p27 levels and reduced the expression of CDK1 (encoded by CDK1 mRNA, a previously reported target of NSun2), which in turn further repressed cell proliferation and accelerated replicative senescence, while overexpression of NSun2 exerted the opposite effect. Ectopic overexpression of the p27 5′UTR fragment rescued the effect of NSun2 overexpression in lowering p27, increasing CDK1, promoting cell proliferation, and delaying replicative senescence. Our findings indicate that NSun2-mediated mRNA methylation regulates p27 and CDK1 levels during replicative senescence.

cThe tRNA methytransferase NSun2 promotes cell proliferation, but the molecular mechanism has not been elucidated. Here, we report that NSun2 regulates cyclin-dependent kinase 1 (CDK1) expression in a cell cycle-dependent manner. Knockdown of NSun2 decreased the CDK1 protein level, while overexpression of NSun2 elevated it without altering CDK1 mRNA levels. Further studies revealed that NSun2 methylated CDK1 mRNA in vitro and in cells and that methylation by NSun2 enhanced CDK1 translation. Importantly, NSun2-mediated regulation of CDK1 expression had an impact on the cell division cycle. These results provide new insight into the regulation of CDK1 during the cell division cycle.

Basigin, which has four isoforms, plays an important role in invasion of hepatocellular carcinoma (HCC).Detailed transcriptional regulation and functions of the basigin isoforms have not been reported except in the case of the predominant isoform basigin-2, which act as inducer of matrix metalloproteinases (MMPs). Here we determined that basigin-2, basigin-3, and basigin-4 were the most abundant transcript variants in human cell lines. GeneRacer PCR and luciferase reporter assays showed that basigin-3 and basigin-4 were initiated from an alternative promoter. Basigin-3 and basigin-4 were widely expressed in various normal human tissues at the mRNA level and were upregulated in HCC tissues compared to in normal tissues. Western blotting and confocal imaging showed that glycosylated basigin-3 and basigin-4 were expressed and localized to the plasma membrane. However, in cultured cell lines, only native basigin-3, and not basigin-4, was detected at protein level. Overexpression of basigin-3 inhibited HCC cell proliferation, MMP induction, and cell invasion in vitro and in vivo. Bimolecular fluorescence complementation assays and nuclear magnetic resonance (NMR) analysis indicated that basigin-3 interacted with basigin-2 to form hetero-oligomers. In conclusion, we systematically investigated the alternative splicing of basigin and found that basigin-3 could inhibit HCC proliferation and invasion, probably through interaction with basigin-2 as an endogenous inhibitor via hetero-oligomerization.