1. m⁶A RNA regulators are closely associated with metabolic syndrome development and present novel prospects for clinical applications.

Metabolic sydromes are a group of disorders caused by glucolipid metabolism dysregulation, with symptoms such as obesity, hyperglycemia, hyperlipidemia, and nonalcoholic fatty liver severely affecting physical health. Furthermore, these symptoms are interrelated metabolic risk factors directly contributing to atherosclerotic cardiovascular disease development and increasing the risk of type 2 diabetes mellitus (T2DM) and its complications (Wang et al., 2015; Shi et al., 2017). Recently, research into the pathogenic mechanisms of metabolic sydromes has increasingly focused on excessive lipid accumulation, inflammatory responses, autophagy, apoptosis and epigenetic modifications (Shi et al., 2017; Huang et al., 2018; Wu et al., 2020).

Several epigenetic modifications exist, including deoxyribonucleic acid (DNA) methylation, histone modifications, messenger RNA (mRNA), and noncoding RNA chemical modifications. Methylated RNA is reportedly functional in pathophysiological processes at all stages of life. Adenylate methylation modifications account for over half of the total methylated ribonucleotides in cellular RNA and 0.1%–0.4% of all adenosines (Goldberg et al., 2007). N6-methyladenosine (m⁶A), a widespread trans-epigenetic adenylate modification, has become relevant to the specific mechanisms of metabolic sydrome pathogenesis (Li et al., 2017; Wu et al., 2020). m⁶A modification is a dynamic and reversible biological progress owing to the interaction between “writers” and “erasers.” The “writers” mean methyltransferase, such as methyltransferase-like 3 (METTL3), methyltransferase-like 14 (METTL14). The “erasers” mean demethylase, such as fat mass and obesity-associated protein (FTO) (Jia et al., 2011) and ALKB homolog 5 (ALKBH5). Also, “readers” are the third critical factor which recognizes m⁶A modifications and ultimately paly function.

m⁶A primarily controls post-transcriptional gene expression and is involved in DNA repair, cellular reprogramming, cell differentiation, cellular stress responses, and programmed cell death (PCD) (Wu et al., 2020). In addition, m⁶A is highly conserved in the 3′untranslated region (UTR) and the consensus motif RRACH in the coding region (Desrosiers et al., 1974; Wu et al., 2020). m⁶A methylation regulates almost all aspects of mRNA metabolism through “readers,” including nuclear export (Roundtree et al., 2017a; Roundtree et al., 2017b), stability (Wang et al., 2015), translation (Huang et al., 2018), and pre-mRNA processing to mRNA decay (Li et al., 2017; Shi et al., 2017). As reported, m⁶A methylation has been a hot research field in metabolic sydromes. In high-fat diet-induced fatty liver in mice, m⁶A hyper-methylated genes were significantly enriched in processes and pathways associated with lipid metabolism, such as fatty acid synthesis, triglyceride metabolism, and the PPAR signaling pathways (Zhong et al., 2020). m⁶A methylation controls the insulin IGF1-AKT-PDX1 pathway and T2DM is exacerbated by targeting METTL3 or METTL14 to reduce m⁶A levels and decrease AKT phosphorylation and PDX1 protein levels (De Jesus et al., 2019). YTHDF2 accelerates JAK2 mRNA degradation and attenuates JAK2-STAT3-C/EBPβ signaling. In our previous studies, we found that m⁶A ultimately contributes to the development of NAFLD and insulin resistance by regulating the expression of CYP2B6 (Li et al., 2023). Existing literature indicated the potential value of m⁶A methylation as novel targets in metabolic sydromes treatment.

In the present study, we review the relationship between m⁶A and metabolic syndromes and summarize the role of RNA m⁶A modification in metabolic syndrome pathogenesis and progression. We first introduce the role of m⁶A methylation “writers,” “erasers,” and “readers,” then the association of m⁶A methylation modification with inflammatory response, PCD, and other essential biological functions in metabolic syndromes. For the literature review, we searched the PubMed database of the NCBI Homepage using the keywords “m⁶A and inflammation”, “m⁶A and autophagy”, “m⁶A and programmed cell death,” and “m⁶A and metabolic syndrome” to filter the key publications related to metabolic syndromes and categorize and extract the content of interest. This study can provide evidence for the clinical application of m⁶A methylation in preventing and treating metabolic syndromes.

We summarized the components of m⁶A methylation process in Figure 1. The primary molecules involved in m⁶A are methyltransferases that methylate RNA, called the “writers”; demethylases that remove m⁶A, called the “erasers”; and “readers” which recognize m⁶A modifications and ultimately play functions. Thus, m⁶A modification is dynamically and reversibly regulated at various cellular stages (Roundtree et al., 2017b; Zaccara et al., 2019). Methyltransferase is a multi-component protein primarily comprising methyltransferase-like 3 (METTL3), −14 (METTL14), and Wilms tumor 1-associated protein (WTAP). KIAA1429, RBM15, RBM15B, ZC3H13, and CBLL1. m⁶A demethylation is primarily achieved by two enzymes: fat mass and obesity-associated protein (FTO) (Jia et al., 2011) and ALKB homolog 5 (ALKBH5) (Zhang et al., 2017). Generally, m⁶A “readers” are divided into three classes: the YTH family proteins (YTHDF1/2/3, YTHDC1/2), m⁶A conversion proteins (hnRNPC, hnRNPG, and hnRNPA2B1), and RNA-binding proteins (IGF2BP 1/2/3 and hnRNPA2B1) (Zaccara et al., 2019). Additionally, eIF3 and ELAVL1 are reported as “readers.” Several other m⁶A RNA binding proteins have been identified, such as LRPPRC, FMR1, FMRP, and SND1 (Chen et al., 2011; Darnell et al., 2011; Arguello et al., 2017).

Metabolic syndromes are primarily associated with inflammatory responses during their development. For example, Day and James proposed the second-strike theory of nonalcoholic fatty liver disease (NAFLD). The “first strike” triggers a metabolic derailment of the mitochondria, endoplasmic reticulum stress, and hepatocyte peroxisomes. Reactive oxygen species (ROS) and further lipid peroxidation, cytokine production, and lipid accumulation promote inflammation and fibrosis. In atherosclerosis pathogenesis, oxidized low-density lipoprotein (ox-LDL) is absorbed by endothelial macrophages to form foam cells, causing inflammatory lesions in the endothelium and leading to atherosclerosis. Diabetes mellitus pathogenesis also involves the interleukin (IL)-1β inflammatory factor that damages islet cells and causes insufficient insulin secretion. Therefore, we focus on the impact of m⁶A methylation on inflammatory-related responses in metabolic syndromes.

PCD, in addition to typical cell death modalities such as autophagy, ferroptosis, necrosis, and thermoproteolysis, is a programmed mechanism that eliminates abnormal cells to maintain the balance of the internal environment. Autophagy is an evolutionarily conserved degradation pathway that is primarily free of unnecessary and senescent organelles and proteins and is tightly regulated by autophagy-associated proteins and transcription factors. Autophagic dysregulation is associated with many diseases, such as neurodegenerative syndromes, cardiovascular diseases, and cancer, owing to its several cytoplasmic targets. Furthermore, autophagy is a normal cell function in metabolic syndrome development and progression. Therefore, we focus on the impact of m⁶A methylation in PCD in metabolic syndromes.

Inflammatory vesicle-mediated pyroptosis leads to cell death characterized by activating multiple caspases, including caspase-1, in an immortalized mouse podocyte cell-5 line (Liu et al., 2021). m⁶A is essential in regulating intracellular pyroptosis based on the human RPE cell line ARPE-19 (Zha et al., 2020). Nod-like receptor protein 3 (NLRP3) is a crucial inflammatory vesicle component that causes pyroptosis and increases pro-inflammatory cytokine levels (Yu et al., 2019). High sugar levels induce the increased expression of heat degradation-related proteins (caspase-1, gasdermin D, NLRP3, IL-1β, and IL-18), leading to pyroptosis (Mulay, 2019). These proteins are less expressed when METTL3 is overexpressed and cellular damage is reduced. In contrast, the damage is exacerbated in human peripheral blood mononuclear cell-derived macrophages when METTL3 is knocked down (Guo et al., 2020). A previous study investigated how interferon regulatory factor-1 (IRF-1) facilitates macrophage scorching among patients with acute coronary syndrome, and elevated m⁶A and METTL3 levels were observed in macrophages (Kunjathoor et al., 2002). IRF-1 overexpression increases m⁶A and METTL3 levels and promotes acute coronary syndrome (Liu et al., 2019c). According to a study on disc degeneration, METTL14 specifically induces the NLRP3 mRNA m⁶A modification and increases NLRP3 protein expression in the human osteosarcoma cell line U2OS (Yuan et al., 2021). Therefore, we hypothesized that METTL3/14 plays a joint role in pyroptosis death by regulating NLRP3, thus influencing metabolic syndrome development.

Ferroptosis, a new type of programmed cell death, involves iron-dependent lipid peroxidation and glutathione peroxidase 4, and mitochondrial membrane loss. m6A is vital in ferroptosis as a novel post-transcriptional regulatory mechanism (Shen et al., 2021). YTHDF1 enhanced the BECN1 stability by binding to the BECN1 m6A site and activating hepatic autophagic vesicle formation in primary hepatic stellate cells from ICR mice. Autophagic vesicles trigger ferroptosis in stellate cells and attenuate liver inflammation (Shen et al., 2021). METTL3/ASK1-p38 pathway-induced ferroptosis might be one of the leading causes of diabetic osteoporosis induced by high sugar and a high-fat diet (Lin et al., 2022). m6A modification induces diabetic erectile dysfunction by regulating ferroptosis in penile cells in a diabetes mellitus with erectile dysfunction model in SD rats (Wang et al., 2023b). ALKBH5-mediated m6 A demethylation was reported to lead to the posttranscriptional inhibition of NFE2L2/NRF2, which is crucial for the regulation of antioxidant molecules in cells. Knocking down ALKBH5 subsequently increased the expression of NFE2L2/NRF2 and increased the resistance of HPSCC cells to ferroptosis (Ye et al., 2022). In a diabetic cataract (DC) research, RBM15, WTAP, ALKBH5, FTO, and YTHDF1-were upregulated in DC samples, the mechanism of which is related with the ferroptosis pathway (Cai et al., 2023).

Glucolipotoxicity is toxicity caused by elevated glucose and fatty acid levels, frequently occurring during diabetes development due to hyperglycemia and hyperlipidemia. Autophagy is activated as an adaptive response and provides a possible protective mechanism for β-cells to eliminate damaged mitochondria, unwanted proteins, or both to avoid dysfunction and apoptosis (Meijer and Codogno, 2007). However, glucolipotoxicity induces excessive autophagy in β-cells, leading to cell death. RNA demethylation of critical autophagy pathway genes is an essential factor of β-cells dysfunction and the pathophysiology of diabetes. FTO-m6A demethylation is responsible for NR3C1-stimulated ATG gene expression under glucolipotoxic conditions, triggering excessive autophagy and β-cell death (Wu et al., 2023).

m⁶A methylation exacerbates metabolic syndrome progression by affecting programmed cell death. In diabetes and its complications, m⁶A-mediated apoptosis, ferroptosis, and excessive autophagy-induced-cell and osteoblast damage are particularly important. Similarly, markers related to programmed cell death indicate that m⁶A is a potential emerging target for detecting and treating metabolic syndromes.

To summarize, we included Table 1 to present the m⁶A-related proteins that play essential roles in modulating metabolic syndrome development. Table 1 also summarizes the target genes related to these proteins. These essential proteins will likely become vital targets for m⁶A in clinical applications.

RNA methylation is one of the most common post-transcriptional modifications. m⁶A regulates transcription, translocation, splicing, and translation. m⁶A modifications are commonly detected in biological processes using RNA-seq and methylation RIP-seq (or miCLIP-seq) to explore the role of m⁶A modifications in metabolic syndromes. m⁶A modification is reversible using m⁶A regulators (methylesterases, demethylases, and m⁶A-RNA binding proteins).

Comparative observations across disease conditions have revealed that the m⁶A modification levels and their associated regulators vary in T2DM, NAFLD, and atherosclerosis. However, further studies are needed to identify the exact association between m⁶A modification and the pathogenesis of metabolic syndrome-mediated regulatory factors. In addition, m⁶A RNA modification is tissue-specific, and whether it is specific to the disease stage remains unclear. Currently, treating metabolic syndromes involves exercise and controlling a high-calorie diet. Although weight-loss surgery effectively slows the progression of metabolic syndromes, it is not widely used because it harms patients. Studying the m⁶A levels in peripheral blood samples from patients with metabolic syndromes can help identify early biomarkers. Although only confirmed in cellular experiments and animal tests in many studies, these m⁶A-related markers can become therapeutic alternatives to weight loss surgery in the future. This potential therapeutic alternative can treat patients with metabolic syndromes without the adverse events of surgery.

We summarized the Abbreviation list in Table 2 to better understand the present review. m⁶A-related proteins, especially METTL3, are critical in regulating m⁶A in inflammation, autophagy, and cell death. We believe that targeting m⁶A-related proteins may be ideal for developing metabolic syndrome treatments (Yankova et al., 2021). For example, in cellular and animal studies, STM2457, a small molecule active inhibitor of METTL3, inhibits acute leukemia development caused by elevated m6A levels. Drugs such as these inhibitors may shed new light on treating metabolic syndromes at the epigenetic level. In recent years, some m⁶A-related clinical trials have been progressively reported (Luo et al., 2020; Cai et al., 2021; Yang et al., 2021). We concluded the information in Table 3. Such as, it has been shown that SNP rs34269950 in the RRACH genome, located in the 3′UTR of RUNX1T1, is significantly regulated by FTO in rural populations in the Caucasus region of Australia, providing important clinical evidence for the association of m6A with metabolic syndrome (Zhou et al., 2021b).

In conclusion, m⁶A RNA modification regulates many factors involved in the pathogenesis of metabolic syndromes and may play a vital role in metabolic syndrome development and progression. However, many studies are still needed to answer multiple outstanding questions, such as whether specific inhibitors possess clinical functions in metabolic disorders, how to achieve tissue-specific drug delivery, and whether side effects exist during m⁶A RNA modification regulation. Clarifying these research questions will provide a new frontier in epigenetic modification and metabolic syndrome research.

In the future, the detection of m⁶A and its regulatory molecules will also be developed to be more convenient, with a wider range of samples and higher sensitivity, and closer and closer to bedside detection. As basic research on m⁶A and metabolic sydromes intensifies, more and more basic findings will be confirmed in clinical trials. More convenient detection Methods will facilitate these clinical trials, and clinical trials will provide evidence for drug development targeting m⁶A regulators. With the maturation of screening technology and the development of artificial intelligence (AI), more and more drugs targeting m⁶A regulatory molecules will be discovered. The study of Chinese medicinal decoction for m⁶A regulatory factors will also become a direction for future drug research on m⁶A-related metabolic sydromes.