m6A mRNA methylation-directed myeloid cell activation controls progression of NAFLD and obesity

RNA m6A modification in liver biology and its implication in hepatic diseases and carcinogenesis

N6-methyladenosine (m6A) is the most prevalent internal modification in eukaryotic RNAs. This modification is regulated by three different factors (writers, erasers, and readers) and affects multiple aspects of RNA metabolism, including RNA splicing, nuclear export, translation, stability and decay. The m6A-mediated modification plays important roles in posttranscriptional regulation of gene expression and mediates a variety of cellular and biological processes. Accordingly, deregulation in m6A modification is closely related to the occurrence and development of human diseases. The liver is the largest digestive and metabolic organ in human and recent studies have shown that m6A modification is importantly implicated in liver cellular and physiological functions and in the pathogenesis of hepatic diseases and cancers. In the current review, we summarize the functions of m6A in RNA metabolism and its roles in liver cell biology and discuss its implication in hepatic diseases and carcinogenesis.

The regulation and potential roles of m6A modifications in early embryonic development and immune tolerance at the maternal-fetal interface

The immune microenvironment at the maternal-fetal interface was determined by the crosstalk between the trophoblast and maternal-derived cells, which dynamically changed during the whole gestation. Trophoblasts act as innate immune cells and dialogue with maternal-derived cells to ensure early embryonic development, depending on the local immune microenvironment. Therefore, dysfunctions in trophoblasts and maternal decidual cells contribute to pregnancy complications, especially recurrent pregnancy loss in early pregnancy. Since many unknown regulatory factors still affect the complex immune status, exploring new potential aspects that could influence early pregnancy is essential. RNA methylation plays an important role in contributing to the transcriptional regulation of various cells. Sufficient studies have shown the crucial roles of N6-methyladenosine (m6A)- and m6A-associated- regulators in embryogenesis during implantation. They are also essential in regulating innate and adaptive immune cells and the immune response and shaping the local and systemic immune microenvironment. However, the function of m6A modifications at the maternal-fetal interface still lacks wide research. This review highlights the critical functions of m6A in early embryonic development, summarizes the reported research on m6A in regulating immune cells and tumor immune microenvironment, and identifies the potential value of m6A modifications in shaping trophoblasts, decidual immune cells, and the microenvironment at the maternal-fetal interface. The m6A modifications are more likely to contribute to embryogenesis, placentation and shape the immune microenvironment at the maternal-fetal interface. Uncovering these crucial regulatory mechanisms could provide novel therapeutic targets for RNA methylation in early pregnancy.

Phytic Acid Improves Hepatic Steatosis, Inflammation, and Oxidative Stress in High-Fat Diet (HFD)-Fed Mice by Modulating the Gut-Liver Axis

Nonalcoholic fatty liver disease (NAFLD) induced by obesity is a grave threat to human health. Phytic acid (PA) is a natural compound found in high-fiber diets, such as soybeans. This study investigated the effects and mechanisms of PA on obesity, hepatic lipid metabolism, and gut-liver axis homeostasis in high-fat diet (HFD)-fed mice. PA was observed to significantly inhibit obesity and alleviate liver steatosis in mice. PA improved HFD-induced liver inflammation, oxidative stress and fibrosis. Moreover, PA improved HFD-induced colonic inflammation, gut barrier damage and systemic inflammation in mice. Furthermore, PA effectively ameliorated the decreased diversity and gut microbiota composition in HFD-fed mice. Additionally, PA decreased the abundance of harmful bacteria Proteobacteria and Desulfovibrionaceae and increased the abundance of probiotic bacteria Muribaculaceae and Lachnospiraceae. Thus, PA is effective in restoring the homeostasis of the gut-liver axis. It further provides a theoretical basis for the prevention and treatment of NAFLD in patients with obesity by the rational intake of foods containing PA.

New roles of N6-methyladenosine methylation system regulating the occurrence of non-alcoholic fatty liver disease with N6-methyladenosine-modified MYC

Non-alcoholic fatty liver disease (NAFLD) has become a major chronic disease in contemporary society, affected by N6-methyladenosine (m6A) RNA methylation, one of the most common RNA modifications. Compared with healthy control, m6A RNA methyltransferase 3 (METTL3) and METTL14 increased, while Wilms tumor 1-associated protein (WTAP) and RNA-binding motif protein 15 (RBM15) decreased significantly in NAFLD, and the m6A demethylases fat mass and obesity-associated protein (FTO) elevated. Meanwhile, the m6A binding proteins, YT521-B homology (YTH) domain-containing 1 (YTHDC1), YTHDC2, insulin-like growth factor 2 mRNA binding protein 1 (IGF2BP1), heterogeneous nuclear ribonucleoprotein C (HNRNPC), and HNRNPA2B1 were decreased, while eukaryotic translation initiation factor 3 subunit H (EIF3H) was increased significantly. All these changes of m6A regulators had significant differences between healthy control and NAFLD, but no differences between the NAFL and NASH group. The expression level of RBM15, HNRNPC, and HNRNPA2B1 were related to body fat index. RBM15, YTHDC2, HNRNPC, HNRNPA2B1, and EIF3H were related to steatosis. Also, KIAA1429 and YTH domain family 1 (YTHDF1) were related to lobular inflammation. Taken together, m6A regulators were involved in the occurrence of NAFLD. More importantly, abnormal MYC was determined as a key link to m6A regulation of NAFLD. The higher MYC mRNA level was accompanied by higher HDL cholesterol and unsaturated fatty acid proportions, as well as lower fat mass, glucose, and transaminase. Taken together, dysregulation of m6A methylation caused steatosis and fibrosis, affecting the occurrence of NAFLD, and MYC might be its potential target.

METTL3 inhibits inflammation of retinal pigment epithelium cells by regulating NR2F1 in an m6A-dependent manner

N6-metyladenosine (m6A) RNA methylation has been proven to be involved in diverse biological processes, but its potential roles in the development of lipopolysaccharide (LPS) induced retinal pigment epithelium (RPE) inflammation have not been revealed. In this study, we explored the effects and underlying mechanisms of methyltransferase-like 3 (METTL3) in LPS stimulated RPE cells. Proliferation of METTL3-silenced RPE cells was examined by Cell counting kit-8 (CCK8) and 5-Ethynyl-2´-Deoxyuridine (Edu). Expression of tight junction proteins ZO-1 and Occludin, and secretion of inflammatory factors interleukins (IL)-1, 6 and 8 were detected by Western blotting or Enzyme-linked immunosorbent assay (ELISA). RNA sequencing and methylated RNA immunoprecipitation (MeRIP) sequencing were used to analyze the target gene nuclear receptor subfamily 2 group F member 1 (NR2F1) of METTL3. Our results showed that both human RPE (hRPE) cells and ARPE19 cells exhibited inhibited proliferation, tight junction protein expression, and increased inflammatory factor secretion after METTL3 silencing. Mechanistically, we found that NR2F1, as a METTL3-methylated target gene, inhibits Occludin level and promotes IL-6 secretion of RPE cells in an m6A-dependent manner. Interestingly, NR2F1 deficiency reversed the decreased Occludin expression and increased IL-6 secretion in METTL3-defective RPE cells. In conclusion, our study revealed that METTL3 attenuates RPE cell inflammation by methylating NR2F1, suggesting the critical role of METTL3 in RPE cells.

One genome, many cell states: epigenetic control of innate immunity

A hallmark of the innate immune system is its ability to rapidly initiate short-lived or sustained transcriptional programs in a cell-specific and pathogen-specific manner that is dependent on dynamic chromatin states. Much of the epigenetic landscape is set during cellular differentiation; however, pathogens and other environmental cues also induce changes in chromatin that can either promote tolerance or 'train' innate immune cells for amplified secondary responses. We review chromatin processes that enable innate immune cell differentiation and functional transcriptional responses in naive or experienced cells, in concert with signal transduction and cellular metabolic shifts. We discuss how immune chromatin mechanisms are maladapted in disease and novel therapeutic approaches for cellular reprogramming.

The role of RNA m6A methylation in lipid metabolism

The m6A methylation is the most numerous modification of mRNA in mammals, coordinated by RNA m6A methyltransferases, RNA m6A demethylases, and RNA m6A binding proteins. They change the RNA m6A methylation level in their specific manner. RNA m6A modification has a significant impact on lipid metabolic regulation. The "writer" METTL3/METTL14 and the "eraser" FTO can promote the accumulation of lipids in various cells by affecting the decomposition and synthesis of lipids. The "reader" YTHDF recognizes m6A methylation sites of RNA and regulates the target genes' translation. Due to this function that regulates lipid metabolism, RNA m6A methylation plays a pivotal role in metabolic diseases and makes it a great potential target for therapy.