Hepatic FTO expression is increased in NASH and its silencing attenuates palmitic acid-induced lipotoxicity

Nucleic acid demethylase MpAlkB1 regulates the growth, development, and secondary metabolite biosynthesis in Monascus purpureus

Nucleic acid demethylases of α-ketoglutarate-dependent dioxygenase (AlkB) family can reversibly erase methyl adducts from nucleobases, thus dynamically regulating the methylation status of DNA/RNA and playing critical roles in multiple cellular processes. But little is known about AlkB demethylases in filamentous fungi so far. The present study reports that Monascus purpureus genomes contain a total of five MpAlkB genes. The MpAlkB1 gene was disrupted and complemented through homologous recombination strategy to analyze its biological functions in M. purpureus. MpAlkB1 knockout significantly accelerated the growth of strain, increased biomass, promoted sporulation and cleistothecia development, reduced the content of Monascus pigments (Mps), and strongly inhibited citrinin biosynthesis. The downregulated expression of the global regulator gene LaeA, and genes of Mps biosynthesis gene cluster (BGC) or citrinin BGC in MpAlkB1 disruption strain supported the pleiotropic trait changes caused by MpAlkB1 deletion. These results indicate that MpAlkB1-mediated demethylation of nucleic acid plays important roles in regulating the growth and development, and secondary metabolism in Monascus spp.

SUMOylation modification of FTO facilitates oxidative damage response of arsenic by IGF2BP3 in an m6A-dependent manner

N6-methyladenosine (m6A) is the most common form of internal post-transcriptional methylation observed in eukaryotic mRNAs. The abnormally increased level of m6A within the cells can be catalyzed by specific demethylase fat mass and obesity-associated protein (FTO) and stay in a dynamic and reversible state. However, whether and how FTO regulates oxidative damage via m6A modification remain largely unclear. Herein, by using both in vitro and in vivo models of oxidative damage induced by arsenic, we demonstrated for the first time that exposure to arsenic caused a significant increase in SUMOylation of FTO protein, and FTO SUMOylation at lysine (K)- 216 site promoted the down-regulation of FTO expression in arsenic target organ lung, and therefore, remarkably elevating the oxidative damage via an m6A-dependent pathway by its specific m6A reader insulin-like growth factor-2 mRNA-binding protein-3 (IGF2BP3). Consequently, these findings not only reveal a novel mechanism underlying FTO-mediated oxidative damage from the perspective of m6A, but also imply that regulation of FTO SUMOylation may serve as potential approach for treatment of oxidative damage.

FTO Deficiency Alleviates LPS-induced Acute Lung Injury by TXNIP/NLRP3-mediated Alveolar Epithelial Cell Pyroptosis

N6-methyladenosine (m6A) plays a role in various diseases, but it has rarely been reported in acute lung injury (ALI). The FTO (fat mass and obesity-associated) protein can regulate mRNA metabolism by removing m6A residues. The aim of this study was to examine the role and mechanism of the m6A demethylase FTO in LPS-induced ALI. Lung epithelial FTO-knockout mice and FTO-knockdown/overexpression human alveolar epithelial (A549) cell lines were constructed to evaluate the effects of FTO on ALI. Bioinformatics analysis and a series of in vivo and in vitro assays were used to examine the mechanism of FTO regulation. Rescue assays were conducted to examine whether the impact of FTO on ALI depended on the TXNIP/NLRP3 pathway. In LPS-induced ALI, RNA m6A modification amounts were upregulated, and FTO expression was downregulated. In vivo, lung epithelial FTO knockout alleviated alveolar structure disorder, tissue edema, and pulmonary inflammation and improved the survival of ALI mice. In vitro, FTO knockdown reduced A549 cell damage and death induced by LPS, whereas FTO overexpression exacerbated cell damage and death. Mechanistically, bioinformatics analysis revealed that TXNIP was a downstream target of FTO. FTO deficiency mitigated pyroptosis in LPS-induced ALI via the TXNIP/NLRP3 pathway. Rescue assays confirmed that the impact of FTO on the TXNIP/NLRP3 pathway was significantly reversed by the TXNIP inhibitor SRI-37330. Deficiency of FTO alleviates LPS-induced ALI via TXNIP/NLRP3 pathway-mediated alveolar epithelial cell pyroptosis, which might be a novel therapeutic strategy for combating ALI.

Emerging roles of RNA-binding proteins in fatty liver disease

A rampant and urgent global health issue of the 21st century is the emergence and progression of fatty liver disease (FLD), including alcoholic fatty liver disease and the more heterogenous metabolism-associated (or non-alcoholic) fatty liver disease (MAFLD/NAFLD) phenotypes. These conditions manifest as disease spectra, progressing from benign hepatic steatosis to symptomatic steatohepatitis, cirrhosis, and, ultimately, hepatocellular carcinoma. With numerous intricately regulated molecular pathways implicated in its pathophysiology, recent data have emphasized the critical roles of RNA-binding proteins (RBPs) in the onset and development of FLD. They regulate gene transcription and post-transcriptional processes, including pre-mRNA splicing, capping, and polyadenylation, as well as mature mRNA transport, stability, and translation. RBP dysfunction at every point along the mRNA life cycle has been associated with altered lipid metabolism and cellular stress response, resulting in hepatic inflammation and fibrosis. Here, we discuss the current understanding of the role of RBPs in the post-transcriptional processes associated with FLD and highlight the possible and emerging therapeutic strategies leveraging RBP function for FLD treatment. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Fat mass and obesity-associated (FTO) gene is essential for insulin secretion and β-cell function: In vitro studies using INS-1 cells and human pancreatic islets

AIMS

In this study, we investigated the role of the FTO gene in pancreatic β-cell biology and its association with type 2 diabetes (T2D). To address this issue, human pancreatic islets and rat INS-1 (832/13) cells were used to perform gene silencing, overexpression, and functional analysis of FTO expression; levels of FTO were also measured in serum samples obtained from diabetic and obese individuals.

RESULTS

The findings revealed that FTO expression was reduced in islets from hyperglycemic/diabetic donors compared to normal donors. This reduction correlated with decreased INS and GLUT1 expression and increased PDX1, GCK, and SNAP25 expression. Silencing of Fto in INS-1 cells impaired insulin release and mitochondrial ATP production and increased apoptosis in pro-apoptotic cytokine-treated cells. However, glucose uptake and reactive oxygen species production rates remained unaffected. Downregulation of key β-cell genes was observed following Fto-silencing, while Glut2 and Gck were unaffected. RNA-seq analysis identified several dysregulated genes involved in metal ion binding, calcium ion binding, and protein serine/threonine kinase activity. Furthermore, our findings showed that Pdx1 or Mafa-silencing did not influence FTO protein expression. Overexpression of FTO in human islets promoted insulin secretion and upregulated INS, PDX1, MAFA, and GLUT1 expression. Serum FTO levels did not significantly differ between individuals with diabetes or obesity and their healthy counterparts.

CONCLUSION

These findings suggest that FTO plays a crucial role in β-cell survival, metabolism, and function and point to a potential therapeutic utility of FTO in T2D patients.

m6 A mRNA methylation: Biological features, mechanisms, and therapeutic potentials in type 2 diabetes mellitus

As the most common internal post-transcriptional RNA modification in eukaryotic cells, N6-methyladenosine (m6 A) performs a dynamic and reversible role in a variety of biological processes mediated by methyltransferases (writers), demethylases (erasers), and m6 A binding proteins (readers). M6 A methylation enables transcriptome conversion in different signals that regulate various physiological activities and organ development. Over the past few years, emerging studies have identified that mRNA m6 A regulators defect in β-cell leads to abnormal regulation of the target mRNAs, thereby resulting in β-cell dysfunction and loss of β-cell identity and mass, which are strongly associated with type 2 diabetes mellitus (T2DM) pathogenesis. Also, mRNA m6 A modification has been implicated with insulin resistance in muscles, fat, and liver cells/tissues. In this review, we elaborate on the biological features of m6 A methylation; provide a comprehensive overview of the underlying mechanisms that how it controls β-cell function, identity, and mass as well as insulin resistance; highlight its connections to glucose metabolism and lipid metabolism linking to T2DM; and further discuss its role in diabetes complications and its therapeutic potentials for T2DM diagnosis and treatment.

Studies on the fat mass and obesity-associated (FTO) gene and its impact on obesity-associated diseases

Obesity has become a major health crisis in the past ∼50 years. The fat mass and obesity-associated (FTO) gene, identified by genome-wide association studies (GWAS), was first reported to be positively associated with obesity in humans. Mice with more copies of the FTO gene were observed to be obese, while loss of the gene in mice was found to protect from obesity. Later, FTO was found to encode an m6A RNA demethylase and has a profound effect on many biological and metabolic processes. In this review, we first summarize recent studies that demonstrate the critical roles and regulatory mechanisms of FTO in obesity and metabolic disease. Second, we discuss the ongoing debates concerning the association between FTO polymorphisms and obesity. Third, since several small molecule drugs and micronutrients have been found to regulate metabolic homeostasis through controlling the expression or activity of FTO, we highlight the broad potential of targeting FTO for obesity treatment. Improving our understanding of FTO and the underlying mechanisms may provide new approaches for treating obesity and metabolic diseases.

Changes in m6A in Steatotic Liver Disease

Fatty liver disease is one of the major causes of morbidity and mortality worldwide. Fatty liver includes non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH), now replaced by a consensus group as metabolic dysfunction-associated steatotic liver disease (MASLD). While excess nutrition and obesity are major contributors to fatty liver, the underlying mechanisms remain largely unknown and therapeutic interventions are limited. Reversible chemical modifications in RNA are newly recognized critical regulators controlling post-transcriptional gene expression. Among these modifications, N6-methyladenosine (m6A) is the most abundant and regulates transcript abundance in fatty liver disease. Modulation of m6A by readers, writers, and erasers (RWE) impacts mRNA processing, translation, nuclear export, localization, and degradation. While many studies focus on m6A RWE expression in human liver pathologies, limitations of technology and bioinformatic methods to detect m6A present challenges in understanding the epitranscriptomic mechanisms driving fatty liver disease progression. In this review, we summarize the RWE of m6A and current methods of detecting m6A in specific genes associated with fatty liver disease.

N6-methyladenosine RNA modification: an emerging molecule in type 2 diabetes metabolism

Type 2 diabetes (T2D) is a metabolic disease with an increasing rate of incidence worldwide. Despite the considerable progress in the prevention and intervention, T2D and its complications cannot be reversed easily after diagnosis, thereby necessitating an in-depth investigation of the pathophysiology. In recent years, the role of epigenetics has been increasingly demonstrated in the disease, of which N6-methyladenosine (m6A) is one of the most common post-transcriptional modifications. Interestingly, patients with T2D show a low m6A abundance. Thus, a comprehensive analysis and understanding of this phenomenon would improve our understanding of the pathophysiology, as well as the search for new biomarkers and therapeutic approaches for T2D. In this review, we systematically introduced the metabolic roles of m6A modification in organs, the metabolic signaling pathways involved, and the effects of clinical drugs on T2D.

RNA-binding proteins in metabolic-associated fatty liver disease (MAFLD): From mechanism to therapy

Metabolic-associated fatty liver disease (MAFLD) is the most common chronic liver disease globally and seriously increases the public health burden, affecting approximately one quarter of the world population. Recently, RNA binding proteins (RBPs)-related pathogenesis of MAFLD has received increasing attention. RBPs, vividly called the gate keepers of MAFLD, play an important role in the development of MAFLD through transcription regulation, alternative splicing, alternative polyadenylation, stability and subcellular localization. In this review, we describe the mechanisms of different RBPs in the occurrence and development of MAFLD, as well as list some drugs that can improve MAFLD by targeting RBPs. Considering the important role of RBPs in the development of MAFLD, elucidating the RNA regulatory networks involved in RBPs will facilitate the design of new drugs and biomarkers discovery.

Roles of RNA m6A modification in nonalcoholic fatty liver disease

NAFLD is a series of liver disorders, and it has become the most prevalent hepatic disease to date. However, there are no approved and effective pharmaceuticals for NAFLD owing to a poor understanding of its pathological mechanisms. While emerging studies have demonstrated that m6A modification is highly associated with NAFLD. In this review, we summarize the general profile of NAFLD and m6A modification, and the role of m6A regulators including erasers, writers, and readers in NAFLD. Finally, we also highlight the clinical significance of m6A in NAFLD.

In silico profiling of nonsynonymous SNPs of fat mass and obesity-associated gene: possible impacts on the treatment of non-alcoholic fatty liver disease

BACKGROUND

Nonalcoholic fatty liver, or NAFLD, is the most common chronic liver ailment. It is characterized by excessive fat deposition in hepatocytes of individuals who consume little or no alcohol and are unaffected by specific liver damaging factors. It is also associated with extrahepatic manifestations such as chronic kidney disease, cardiovascular disease, and sleep apnea. The global burden of NAFLD is increasing at an alarming rate. However, no pharmacologically approved drugs against NAFLD are available owing to their complex pathophysiology. Genome-wide association studies have uncovered SNPs in the fat mass and obesity-associated gene (FTO) that are robustly associated with obesity and higher BMI. The prevalence of NAFLD increases in parallel with the increasing prevalence of obesity. Since FTO might play a crucial role in NAFLD development, the current study identified five potentially deleterious mutations from 383 ns-SNPs in the human FTO gene using various in silico tools.

METHODS

This study aims to identify potentially deleterious nonsynonymous SNPs (ns-SNPs) employing various in silico tools. Additionally, molecular modeling approaches further studied the structural changes caused by identified SNPs. Moreover, molecular dynamics studies finally investigated the binding potentials of the phytochemicals resveratrol, rosmarinic acid, and capsaicin with different mutant forms of FTO.

RESULTS

The current investigation has five potentially deleterious mutations from 383 ns-SNPs in the human FTO gene using various in silico tools. The present study identified five nsSNPs of the human gene FTO, Gly103Asp, Arg96Pro, Tyr295Cys, and Arg322Gln, with an apparent connection to the disease condition. Modulation of demethylation activity by phytomolecule scanning explains the hepatoprotective action of molecules. The current investigation also suggested that predicted mutations did not affect the binding ability of three polyphenols: rosamarinic acid, resveratrol, and capsaicin.

CONCLUSION

This study showed that the predicted mutations in FTO did not affect the binding of three polyphenols. Thus, these three molecules can significantly aid drug development against FTO and NAFLD.

METTL16-mediated translation of CIDEA promotes non-alcoholic fatty liver disease progression via m6A-dependent manner

Background

As the most prevalent chemical modifications on eukaryotic mRNAs, N6-methyladenosine (m6A) methylation was reported to participate in the regulation of various metabolic diseases. This study aimed to investigate the roles of m6A methylation and methyltransferase-like16 (METTL16) in non-alcoholic fatty liver disease (NAFLD).

Methods

In this study, we used a model of diet-induced NAFLD, maintaining six male C57BL/6J mice on high-fat diet (HFD) to generate hepatic steatosis. The high-throughput sequencing and RNA sequencing were performed to identify the m6A methylation patterns and differentially expressed mRNAs in HFD mice livers. Furthermore, we detected the expression levels of m6A modify enzymes by qRT-PCR in liver tissues, and further investigated the potential role of METTL16 in NAFLD through constructing overexpression and a knockdown model of METTL16 in HepG2 cells.

Results

In total, we confirmed 15,999 m6A recurrent peaks in HFD mice and 12,322 in the control. Genes with differentially methylated m6A peaks were significantly associated with the dysregulated glucolipid metabolism and aggravated hepatic inflammatory response. In addition, we identified five genes (CIDEA, THRSP, OSBPL3, GDF15 and LGALS1) that played important roles in NAFLD progression after analyzing the differentially expressed genes containing differentially methylated m6A peaks. Intriguingly, we found that the expression levels of METTL16 were substantially increased in the NAFLD model in vivo and in vitro, and further confirmed that METTL16 upregulated the expression level of lipogenic genes CIDEA in HepG2 cells.

Conclusions

These results indicate the critical roles of m6A methylation and METTL16 in HFD-induced mice and cell NAFLD models, which broaden people's perspectives on potential m6A-related treatments and biomarkers for NAFLD.

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.

FTO promotes liver inflammation by suppressing m6A mRNA methylation of IL-17RA

Background

Previous studies have demonstrated that inflammation-related interleukin-17 (IL-17) signaling plays a pivotal role in the pathogenesis of non-alcoholic steatohepatitis (NASH)- and alcoholic liver disease (ALD)-induced hepatocellular carcinoma (HCC). However, rare efforts have been intended at implementing the analysis of N6-methyladenosine (m6A) mRNA methylation to elucidate the underpinning function of the IL-17 receptor A (IL-17RA) during the inflammation-carcinogenesis transformation of HCC.

Methods

We performed methylated RNA immunoprecipitation sequencing (MeRIP-seq) using normal, HCC tumor and paired tumor adjacent tissues from patients to investigate the dynamic changes of m6A mRNA methylation in the process of HCC. Additionally, murine non-alcoholic fatty liver disease (NAFLD) model and murine chronic liver injury model were utilized to investigate the role of IL-17RA regulated by m6A mRNA modulator fat mass and obesity-associated (FTO) in chronic hepatic inflammation.

Results

MeRIP-seq revealed the reduction of m6A mRNA methylation of IL-17RA in tumor adjacent tissues with chronic inflammation, suggesting the potential role of IL-17RA in the inflammation-carcinogenesis transformation of HCC. Besides, we demonstrated that FTO, rather than methyltransferase-like 3 (METTL3), methyltransferase-like 14 (METTL14), and alkB homolog 5 (ALKBH5) functions as a main modulator for the decrease of m6A mRNA methylation of IL-17RA via knockdown and overexpression of FTO in vitro and in vivo.

Conclusions

Overall, we elaborated the underlying mechanisms of the increase of IL-17RA resulting in chronic inflammation via the demethylation of FTO in tumor adjacent tissues and demonstrated that targeting the specific m6A modulator FTO may provide an effective treatment for hepatitis patients to prevent the development of HCC.

Spermidine-mediated hypusination of translation factor EIF5A improves mitochondrial fatty acid oxidation and prevents non-alcoholic steatohepatitis progression

Spermidine is a natural polyamine that has health benefits and extends life span in several species. Deoxyhypusine synthase (DHPS) and deoxyhypusine hydroxylase (DOHH) are key enzymes that utilize spermidine to catalyze the post-translational hypusination of the translation factor EIF5A (EIF5A^H). Here, we have found that hepatic DOHH mRNA expression is decreased in patients and mice with non-alcoholic steatohepatitis (NASH), and hepatic cells treated with fatty acids. The mouse and cell culture models of NASH have concomitant decreases in Eif5a^H and mitochondrial protein synthesis which leads to lower mitochondrial activity and fatty acid β-oxidation. Spermidine treatment restores EIF5A^H, partially restores protein synthesis and mitochondrial function in NASH, and prevents NASH progression in vivo. Thus, the disrupted DHPS-DOHH-EIF5A^H pathway during NASH represents a therapeutic target to increase hepatic protein synthesis and mitochondrial fatty acid oxidation (FAO) and prevent NASH progression.

Thyroid Hormone Decreases Hepatic Steatosis, Inflammation, and Fibrosis in a Dietary Mouse Model of Nonalcoholic Steatohepatitis

Background: Nonalcoholic steatohepatitis (NASH) is characterized by hepatic steatosis, lobular inflammation, and fibrosis. Thyroid hormone (TH) reduces steatosis; however, the therapeutic effect of TH on NASH-associated inflammation and fibrosis is not known. This study examined the therapeutic effect of TH on hepatic inflammation and fibrosis during NASH and investigated THs molecular actions on autophagy and mitochondrial biogenesis. Methods: HepG2-TRβ cells were treated with bovine serum albumin-conjugated palmitic acid (PA) to mimic lipotoxic conditions in vitro. Mice with NASH were established by feeding C57BL/6J mice Western diet with 15% fructose in drinking water for 16 weeks. These mice were administered triiodothyronine (T3)/thyroxine (T4) supplemented in drinking water for the next eight weeks. Results: In cultured HepG2-TRβ cells, TH treatment increased mitochondrial respiration and fatty acid oxidation under basal and PA-treated conditions, as well as decreased lipopolysaccharides and PA-stimulated inflammatory and fibrotic responses. In a dietary mouse model of NASH, TH administration decreased hepatic triglyceride content (3.19 ± 0.68 vs. 8.04 ± 0.42 mM/g liver) and hydroxyproline (1.44 ± 0.07 vs. 2.58 ± 0.30 mg/g liver) when compared with mice with untreated NASH. Metabolomics profiling of lipid metabolites showed that mice with NASH had increased triacylglycerol, diacylglycerol, monoacylglycerol, and hepatic cholesterol esters species, and these lipid species were decreased by TH treatment. Mice with NASH also showed decreased autophagic degradation as evidenced by decreased transcription Factor EB and lysosomal protease expression, and accumulation of LC3B-II and p62. TH treatment restored the level of lysosomal proteins and resolved the accumulation of LC3B-II and p62. Impaired mitochondrial biogenesis was also restored by TH. The simultaneous restoration of autophagy and mitochondrial biogenesis by TH increased β-oxidation of fatty acids. Additionally, the elevated oxidative stress and inflammasome activation in NASH liver were also decreased by TH. Conclusions: In a mouse model of NASH, TH restored autophagy and mitochondrial biogenesis to increase β-oxidation of fatty acids and to reduce lipotoxicity, oxidative stress, hepatic inflammation, and fibrosis. Activating thyroid hormone receptor in the liver may represent an effective strategy for NASH treatment.

Dysregulated m6A modification promotes lipogenesis and development of non-alcoholic fatty liver disease and hepatocellular carcinoma

Type 2 diabetes mellitus (DM2) is associated closely with non-alcoholic fatty liver disease (NAFLD) by affecting lipid metabolism, which may lead to non-alcoholic steatohepatitis (NASH), fibrosis, and hepatocellular carcinoma (HCC). N6-methyladenosine (m6A) RNA methylation is an important epigenetic regulation for gene expression and is related to HCC development. We developed a new NAFLD model oriented from DM2 mouse, which spontaneously progressed to histological features of NASH, fibrosis, and HCC with high incidence. By RNA sequencing, protein expression and methylated RNA immunoprecipitation (MeRIP)-qPCR analysis, we found that enhanced expression of ACLY and SCD1 in this NAFLD model and human HCC samples was due to excessive m6A modification, but not elevation of mature SREBP1. Moreover, targeting METTL3/14 in vitro increases protein level of ACLY and SCD1 as well as triglyceride and cholesterol production and accumulation of lipid droplets. m6A sequencing analysis revealed that overexpressed METTL14 binds to mRNA of ACLY and SCD1 and alters their expression pattern. Our findings demonstrate a new NAFLD mouse model that provides a study platform for DM2-related NAFLD and reveals a unique epitranscriptional regulating mechanism for lipid metabolism via m6A-modified protein expression of ACLY and SCD1.

Fat mass and obesity–associated protein promotes liver steatosis by targeting PPARα

Background

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease worldwide. The fat mass and obesity–associated protein (FTO) has been shown to be involved in obesity; however, its role in NAFLD and the underlying molecular mechanisms remain largely unknown.

Methods

FTO expression was first examined in the livers of patients with NAFLD and animal and cellular models of NAFLD by real-time PCR and Western blotting. Next, its role in lipid accumulation in hepatocytes was assessed both in vitro and in vivo via gene overexpression and knockdown studies.

Results

FTO expression was obviously elevated in the livers of mice and humans with hepatic steatosis, probably due to its decreased ubiquitination. FTO overexpression in HepG2 cells induced triglyceride accumulation, whereas FTO knockdown exerted an opposing effect. Consistent with the findings of in vitro studies, adeno-associated viruses 8 (AAV8)-mediated FTO overexpression in the liver promoted hepatic steatosis in C57BL/6J mice. Mechanistically, FTO inhibited the mRNA of peroxisome proliferator-activated receptor α (PPARα) in hepatocytes. Activation of PPARα by its agonist GW7647 reversed lipid accumulation in hepatocytes induced by FTO overexpression.

Conclusions

Overall, FTO expression is increased in NAFLD, and it promotes hepatic steatosis by targeting PPARα.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-022-01640-y.

Critical roles of FTO-mediated mRNA m6A demethylation in regulating adipogenesis and lipid metabolism: Implications in lipid metabolic disorders

The goal this review is to clarify the effects of the fat mass and obesity-associated protein (FTO) in lipid metabolism regulation and related underlying mechanisms through the FTO-mediated demethylation of m⁶A modification. FTO catalyzes the demethylation of m⁶A to alter the processing, maturation and translation of the mRNAs of lipid-related genes. FTO overexpression in the liver promotes lipogenesis and lipid droplet (LD) enlargement and suppresses CPT-1–mediated fatty acid oxidation via the SREBP1c pathway, promoting excessive lipid storage and nonalcoholic fatty liver diseases (NAFLD). FTO enhances preadipocyte differentiation through the C/EBPβ pathway, and facilitates adipogenesis and fat deposition by altering the alternative splicing of RUNX1T1, the expression of PPARγ and ANGPTL4, and the phosphorylation of PLIN1, whereas it inhibits lipolysis by inhibiting IRX3 expression and the leptin pathway, causing the occurrence and development of obesity. Suppression of the PPARβ/δ and AMPK pathways by FTO-mediated m⁶A demethylation damages lipid utilization in skeletal muscles, leading to the occurrence of diabetic hyperlipidemia. m⁶A demethylation by FTO inhibits macrophage lipid influx by downregulating PPARγ protein expression and accelerates cholesterol efflux by phosphorylating AMPK, thereby impeding foam cell formation and atherosclerosis development. In summary, FTO-mediated m⁶A demethylation modulates the expression of lipid-related genes to regulate lipid metabolism and lipid disorder diseases.

Inhibition of AlkB Nucleic Acid Demethylases: Promising New Epigenetic Targets

The AlkB family of nucleic acid demethylases is currently of intense chemical, biological, and medical interest because of its critical roles in several key cellular processes, including epigenetic gene regulation, RNA metabolism, and DNA repair. Emerging evidence suggests that dysregulation of AlkB demethylases may underlie the pathogenesis of several human diseases, particularly obesity, diabetes, and cancer. Hence there is strong interest in developing selective inhibitors for these enzymes to facilitate their mechanistic and functional studies and to validate their therapeutic potential. Herein we review the remarkable advances made over the past 20 years in AlkB demethylase inhibition research. We discuss the rational design of reported inhibitors, their mode-of-binding, selectivity, cellular activity, and therapeutic opportunities. We further discuss unexplored structural elements of the AlkB subfamilies and propose potential strategies to enable subfamily selectivity. It is hoped that this perspective will inspire novel inhibitor design and advance drug discovery research in this field.

Structural characteristics of small-molecule inhibitors targeting FTO demethylase

Studies have shown that the FTO gene is closely related to obesity and weight gain in humans. FTO is an N⁶-methyladenosine demethylase and is linked to an increased risk of obesity and a variety of diseases, such as acute myeloid leukemia, type 2 diabetes, breast cancer, glioblastoma and cervical squamous cell carcinoma. In light of the significant role of FTO, the development of small-molecule inhibitors targeting the FTO protein provides not only a powerful tool for grasping the active site of FTO but also a theoretical basis for the design and synthesis of drugs targeting the FTO protein. This review focuses on the structural characteristics of FTO inhibitors and discusses the occurrence of obesity and cancer caused by FTO gene overexpression.

FTO inhibits UPRmt-induced apoptosis by activating JAK2/STAT3 pathway and reducing m6A level in adipocytes

As a nucleic acid demethylase, Fat and obesity associated gene (FTO) plays a vital role in modulating adipose metabolism. However, it is still unknown how FTO affects apoptosis in adipocytes. In this study, we found that overexpression of FTO inhibited the expression of pro-apoptosis factors Caspase-3, Caspase-9 and Bax and mitochondrial unfolded protein response (UPR^mt) markers HSP60 and ClpP in vivo and in vitro. Particularly, overexpression of FTO inhibited mitochondria-dependent apoptosis in adipocytes. Further studies revealed that FTO suppressed UPR^mt by reducing HSP60 mRNA N6-methyladenosine (m6A) modification. Moreover, FTO inhibited the activation of Caspase-3 via JAK2/STAT3 signaling pathway in adipocytes. Further experiments showed that pro-apoptosis gene Bax was upregulated by UPR^mt-activated PKR/eIF2α/ATF5 axis in adipocytes. In summary, this study confirms that FTO reduces adipocytes apoptosis by activiting JAK2/STAT3 signaling pathway and inhibiting UPR^mt, revealing a novel mechanism of FTO on adipocytes apoptosis, which provides some new potential therapy for treating obesity and related metabolic syndromes.

N6-Methyladenosine RNA Modification in Inflammation: Roles, Mechanisms, and Applications

N6-methyladenosine (m6A) is the most prevalent internal mRNA modification. m6A can be installed by the methyltransferase complex and removed by demethylases, which are involved in regulating post-transcriptional expression of target genes. RNA methylation is linked to various inflammatory states, including autoimmunity, infection, metabolic disease, cancer, neurodegenerative diseases, heart diseases, and bone diseases. However, systematic knowledge of the relationship between m6A modification and inflammation in human diseases remains unclear. In this review, we will discuss the association between m6A modification and inflammatory response in diseases, especially the role, mechanisms, and potential clinical application of m6A as a biomarker and therapeutic target for inflammatory diseases.

Triclosan-induced abnormal expression of miR-30b regulates fto-mediated m6A methylation level to cause lipid metabolism disorder in zebrafish

Chronic exposure of triclosan (TCS) to zebrafish triggers high incidence of fatty liver and hepatitis; however, the underlying molecular mechanisms remain unclear. Herein, we identified miR-30b as a sensitive biomarker to TCS stress, reflecting in that its decreased expression caused metabolic toxicity, abnormal development and behavior, and lipid-metabolism disorder. By microinjecting the inhibitor and mimic experiments, miR-30b was proved to regulate lipid metabolism by its main target gene fto. Over-expression of FTO resulted in fat accumulation, elevation of the TG and TC levels and up-regulation of the PPARγ and CEBPα, as well as decrease of the global m⁶A level in larvae. On the contrary, the knock-down of FTO using MO caused the anti-lipogenic effect, decrease of the TG and T-CHO levels, and abnormal changes of cebpɑ, acsl5, fasn, ppap2c and pparγ etc. Further fortification tests of cycloleucine and betaine evidenced that the toxic effect was strongly dependent on regulation of the m⁶A level. The toxicity effects in the treatments of methylated donors and receptors were consistent with the changes in physiological functions of FTO knockdown and overexpression. The effects of cycloleucine on m⁶A level and lipid metabolism generally consisted with those of FTO, but this was not the case for betaine, reflecting in increased m⁶A level and lipid accumulation in larval liver. Consequently, we posit that TCS exposure caused zebrafish lipid-metabolism disorder by decreasing miR-30b expression to regulate fto-mediated m⁶A methylation level. These findings contribute to our deep understanding of the underlying molecular mechanisms regarding contaminant-originating fatty liver and hepatocellular carcinoma, and also have practical significance in pollution warning and target therapy for related diseases.

Targeted Inhibition of FTO Demethylase Protects Mice Against LPS-Induced Septic Shock by Suppressing NLRP3 Inflammasome

Sepsis refers to the systemic inflammatory response syndrome caused by infection. It is a major clinical problem and cause of death for patients in intensive care units worldwide. The Fat mass and obesity-related protein (FTO) is the primary N⁶-methyladenosine demethylase. However, the role of FTO in the pathogenesis of inflammatory diseases remains unclear. We herein show that nanoparticle-mediated Fto-siRNA delivery or FTO inhibitor entacapone administration dramatically inhibited macrophage activation, reduced the tissue damage and improved survival in a mouse model of LPS-induced endotoxic shock. Importantly, ablation of FTO could inhibit NLRP3 inflammasome through FoxO1/NF-κB signaling in macrophages. In conclusion, FTO is involved in inflammatory response of LPS-induced septic shock and inhibition of FTO is promising for the treatment of septic shock.

Fat mass and obesity-associated protein regulates lipogenesis via m6 A modification in fatty acid synthase mRNA

As the first identified N⁶ -methyladenosine (m⁶ A) demethylase, fat mass and obesity-associated (FTO) protein is associated with fatty acid synthase (FASN) and lipid accumulation. However, little is known about the regulatory role of FTO in the expression of FASN and de novo lipogenesis through m⁶ A modification. In this study, we used FTO small interfering RNA to explore the effects of FTO knockdown on hepatic lipogenesis and its underlying epigenetic mechanism in HepG2 cells. We found that knockdown of FTO increased m⁶ A levels in total RNA and enhanced the expression of YTH domain family member 2 which serves as the m⁶ A-binding protein. The de novo lipogenic enzymes and intracellular lipid content were significantly decreased under FTO knockdown. Mechanistically, knockdown of FTO dramatically enhanced m⁶ A levels in FASN messenger RNA (mRNA), leading to the reduced expression of FASN mRNA through m⁶ A-mediated mRNA decay. The protein expressions of FASN along with acetyl CoA carboxylase and ATP-citrate lyase were further decreased, which inhibited de novo lipogenesis, thereby resulting in the deficiency of lipid accumulation in HepG2 cells and the induction of cellular apoptosis. The results reveal that FTO regulates hepatic lipogenesis via FTO-dependent m⁶ A demethylation in FASN mRNA and indicate the critical role of FTO-mediated lipid metabolism in the survival of HepG2 cells. This study provides novel insights into a unique RNA epigenetic mechanism by which FTO mediates hepatic lipid accumulation through m⁶ A modification and indicates that FTO could be a potential target for obesity-related diseases and cancer.

Hepatic FTO is dispensable for the regulation of metabolism but counteracts HCC development in vivo

Objective

Single-nucleotide polymorphisms in the FTO gene encoding an m⁶Am and an m⁶A demethylase are associated with obesity. Moreover, recent studies have linked a dysregulation of m⁶A modifications and its machinery, including FTO, to the development of several forms of cancers. However, the functional role of hepatic FTO in metabolism and the development and progression of hepatocellular carcinoma (HCC), a proteotypic obesity-associated cancer, remains unclear. Thus, we aimed to reveal the role of hepatic FTO in metabolism and in the initiation and progression of HCC in vivo.

Methods

We generated mice with hepatic FTO deficiency (FTO^L−KO). The effect of hepatic FTO on metabolism was investigated by extensive metabolic phenotyping. To determine the impact of hepatic FTO on HCC development, FTO^L−KO and Ctrl mice were subjected to long-term diethylnitrosamine (DEN)-induced HCC-development and the tumor initiation phase was examined via a short-term DEN protocol.

Results

In long-term DEN experiments, FTO^L−KO mice exhibit increased HCC burden compared to Ctrl mice. In the tumor initiation phase, Ctrl mice display a dynamic regulation of FTO upon induction of liver damage, while this response is abrogated in FTO-deficient mice. Proteomic analyses revealed that liver damage-induced increases in FTO expression reduce CUL4A protein abundance. Functionally, simultaneous knockdown of Cul4a reverses the increased hepatocyte proliferation observed upon loss of FTO.

Conclusion

Collectively, our study demonstrates that hepatic FTO is dispensable for the control of energy homeostasis and glucose metabolism. However, we show a protective function of FTO in liver carcinogenesis and suggest the FTO-dependent dynamic mRNA demethylation of Cul4a in the initiation of HCC development contributes to this effect.

•

Hepatic FTO is dispensable for whole body metabolism.

•

FTO is dynamically regulated upon acute liver damage and controls proliferation.

•

Hepatic FTO function protects against the development of hepatocellular carcinoma (HCC).

•

Cul4a is a downstream target of FTO, and Cul4a knockdown reduces damage-induced proliferation in FTO^L−KO livers.

Epitranscriptomics in liver disease: Basic concepts and therapeutic potential

The development of next-generation sequencing technology and the discovery of specific antibodies targeting chemically modified nucleotides have paved the way for a new era of epitranscriptomics. Cellular RNA is known to dynamically and reversibly undergo different chemical modifications after transcription, such as N⁶-methyladenosine (m⁶A), N¹-methyladenosine, N⁶,2'-O-dimethyladenosine, 5-methylcytosine, and 5-hydroxymethylcytidine, whose identity and location comprise the field of epitranscriptomics. Dynamic post-transcriptional modifications determine the fate of target RNAs by regulating various aspects of their processing, including RNA export, transcript processing, splicing, and degradation. The most abundant internal mRNA modification in eukaryotic cells is m⁶A, which exhibits essential roles in physiological processes, such as embryogenesis, carcinogenesis, and neurogenesis. m⁶A is deposited by the m⁶A methyltransferase complex (composed of METTL3/14/16, WTAP, KIAA1429, and RBM15/15B), erased by demethylases (FTO and ALKBH5), and recognised by binding proteins (e.g., YTHDF1/2/3, YTHDC1/2, IGF2BP1/2/3). The liver is the largest digestive and metabolic organ, and m⁶A modifications play unique roles in critical physiological hepatic functions and various liver diseases. This review focuses on the biological roles of m⁶A RNA methylation in lipid metabolism, viral hepatitis, non-alcoholic fatty liver disease, liver cancer, and tumour metastasis. In addition, we summarise the existing inhibitors targeting m⁶A regulators and discuss the potential of modulating m⁶A modifications as a therapeutic strategy.

GR-mediated FTO transactivation induces lipid accumulation in hepatocytes via demethylation of m6A on lipogenic mRNAs

Chronic stress or excessive exposure to glucocorticoids (GC) contributes to the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Glucocorticoid receptor (GR) mediates the action of GC, but its downstream signalling is not fully understood. Fat mass and obesity associated (FTO) protein and its demethylation substrate N6-methyladenosine (m⁶A) are both reported to participate in the regulation of lipid metabolism, yet it remains unknown whether they are involved in GC-induced hepatic lipid accumulation as new components of GR signalling. In this study, we use both in vivo and in vitro models of GC-induced hepatic lipid accumulation and demonstrate that the activation of lipogenic genes and accumulation of lipid in liver cells are mediated by GR-dependent FTO transactivation and m⁶A demethylation on mRNA of lipogenic genes. Targeted mutation of m⁶A methylation sites and FTO knockdown further validated the role of m⁶A on 3'UTR of sterol regulatory element-binding transcription factor 1 and stearoyl-CoA desaturase mRNAs. Finally, FTO knockdown significantly alleviated dexamethasone-induced fatty liver in mice. These results demonstrate a role of GR-mediated FTO transactivation and m⁶A demethylation in the pathogenesis of NAFLD and provide new insight into GR signalling in the regulation of fat metablism in the liver.

Fat Mass and Obesity-Associated (FTO) Stimulates Osteogenic Differentiation of C3H10T1/2 Cells by Inducing Mild Endoplasmic Reticulum Stress via a Positive Feedback Loop with p-AMPK

Fat mass and obesity-associated (FTO) gene helps to regulate energy homeostasis in mammals by controlling energy expenditure. In addition, FTO functions in the regulation of obesity and adipogenic differentiation; however, a role in osteogenic differentiation is unknown. This study investigated the effects of FTO on osteogenic differentiation of C3H10T1/2 cells and the underlying mechanism. Expression of osteogenic and endoplasmic reticulum (ER) stress markers were characterized by reverse-transcriptase polymerase chain reaction and western blotting. Alkaline phosphatase (ALP) staining was performed to assess ALP activity. BMP2 treatment increased mRNA expression of osteogenic genes and FTO. Overexpression of FTO increased expression of the osteogenic genes distal-less homeobox5 (Dlx5) and runt-related transcription factor 2 (Runx2). Activation of adenosine monophosphate-activated protein kinase (AMPK) increased FTO expression, and there was a positive feedback loop between FTO and p-AMPK. p-AMPK and FTO induced mild ER stress; however, tunicamycin-induced severe ER stress suppressed FTO expression and AMPK activation. In summary, FTO induces osteogenic differentiation of C3H10T1/2 cells upon BMP2 treatment by inducing mild ER stress via a positive feedback loop with p-AMPK. FTO expression and AMPK activation induce mild ER stress. By contrast, severe ER stress inhibits osteogenic differentiation by suppressing FTO expression and AMPK activation.

RNA N6-methyladenosine modification in solid tumors: new therapeutic frontiers

Epigenetic mRNA modification is an evolving field. N⁶-methyladenosine (m⁶A) is the most frequent internal transcriptional modification in eukaryotic messenger RNAs (mRNAs). This review will discuss the functions of the m⁶A mRNA machinery, including its "writers" that are components of the methyltransferase complex, its "readers" and its "erasers" (specifically FTO and ALKBH5) in cancer. The writers deposit the m⁶A and include METTL3, METTL14, WTAP, VIRMA, and RBM15. M⁶A methylation is removed by the m6A demethylases (FTO and ALKBH5). Lastly, the most diverse members are the readers that can contribute to mRNA splicing, stability, translation, and nuclear export. Many of these functions continue to be elucidated. The dysregulation of this machinery in various malignancies and the associated impact on tumorigenesis and drug response will be discussed herein with a focus on solid tumors. It is clear that, by contributing to either mRNA stability or translation, there are downstream targets that are impacted, contributing to cancer progression and the self-renewal ability of cancer stem cells.

Correlation of insulin-resistance with blood fat and glucose in elder patients after surgery for hepatic carcinoma

The present study was designed to analyze variations in blood fat, blood glucose and insulin-resistance in elder patients following surgery for hepatic carcinoma. It also investigated the correlation of insulin with the level of serum leptin and blood fat. A total of 80 patients with primary hepatic cancer who were admitted to The First Hospital of Lanzhou University for treatment between October 2014 and June 2016 were enrolled in the study. At the 1-year follow-up, the patients were divided into two groups based on their recurrence of hepatic cancer after surgery. The levels of serum leptin were detected prior to, one month and one year after surgery; the changes in blood fat, body mass index (BMI), waistline and hipline were measured at one year after surgery; alterations in the fasting blood glucose and blood glucose were measured at 2 h after meal. The fasting insulin (FINS) level and homeostasis model assessment-insulin resistance (HOMA-IR) index were also measured. Correlations between serum leptin and total cholesterol, FINS and fasting blood glucose were analyzed. In the recurrence group, the levels of serum leptin and FINS level were significantly reduced, while waistline and hipline were increased, compared with the non-recurrence group (P<0.05). The BMI and fasting blood glucose in the recurrence group was significantly elevated in comparison with the non-recurrence group (P<0.05). The HOMA-IR index was significantly increased in the recurrence group compared with the non-recurrence group (P<0.05). These results indicated that following surgery for hepatic cancer, the level of serum leptin in patients with recurrence was decreased with an increase in susceptibility to abnormal metabolism of blood fat and glucose. In addition, the serum leptin was negatively correlated with the total cholesterol level and fasting blood glucose and positively correlated with the FINS level in patients. It was concluded that leptin levels decreased in patients with postoperative recurrence, as well as the accumulation of visceral adipose tissue and the development of abnormal blood glucose metabolism was observed.

Fat Mass and Obesity Associated (FTO) Gene and Hepatic Glucose and Lipid Metabolism

Common genetic variants of the fat mass and obesity associated (FTO) gene are strongly associated with obesity and type 2 diabetes. FTO is ubiquitously expressed. Earlier studies have focused on the role of hypothalamic FTO in the regulation of metabolism. However, recent studies suggest that expression of hepatic FTO is regulated by metabolic signals, such as nutrients and hormones, and altered FTO levels in the liver affect glucose and lipid metabolism. This review outlines recent findings on hepatic FTO in the regulation of metabolism, with particular focus on hepatic glucose and lipid metabolism. It is proposed that abnormal activity of hepatic signaling pathways involving FTO links metabolic impairments such as obesity, type 2 diabetes and nonalcoholic fatty liver disease (NAFLD). Therefore, a better understanding of these pathways may lead to therapeutic approaches to treat these metabolic diseases by targeting hepatic FTO. The overall goal of this review is to place FTO within the context of hepatic regulation of metabolism.

A fluorescent methylation-switchable probe for highly sensitive analysis of FTO N 6-methyladenosine demethylase activity in cells

N 6-Methyladenosine (m6A) is one of the most abundant epigenetic modifications on mRNA. It is dynamically regulated by the m6A demethylases FTO and ALKBH5, which are currently attracting intense medical interest because of their strong association with several human diseases. Despite their clinical significance, the molecular mechanisms of m6A demethylases remain unclear, hence there is tremendous interest in developing analytical tools to facilitate their functional studies, with a longer term view of validating their therapeutic potentials. To date, no method exists which permits the analysis of m6A-demethylase activity in cells. To overcome this challenge, herein, we describe the first example of a fluorescent m6A-switchable oligonucleotide probe, which enables the direct detection of FTO demethylase activity both in vitro and in living cells. The m6A probe provides a simple, yet powerful visual tool for highly sensitive detection of demethylase activity. Through the use of m6A-probe, we were able to achieve real-time imaging and single-cell flow cytometry analyses of FTO activity in HepG2 cells. We also successfully applied the probe to monitor dynamic changes in FTO activity and m6A methylation levels during 3T3-L1 pre-adipocyte differentiation. The strategy outlined here is highly versatile and may, in principle, be adapted to the study of a range of RNA demethylases and, more widely, other RNA modifying enzymes. To the best of our knowledge, the present study represents not only the first assay for monitoring FTO activity in living cells, but also a new strategy for sensing m6A methylation dynamics.

Multiprotein Dynamic Combinatorial Chemistry: A Strategy for the Simultaneous Discovery of Subfamily-Selective Inhibitors for Nucleic Acid Demethylases FTO and ALKBH3

Dynamic combinatorial chemistry (DCC) is a powerful supramolecular approach for discovering ligands for biomolecules. To date, most, if not all, biologically templated DCC systems employ only a single biomolecule to direct the self-assembly process. To expand the scope of DCC, herein, a novel multiprotein DCC strategy has been developed that combines the discriminatory power of a zwitterionic "thermal tag" with the sensitivity of differential scanning fluorimetry. This strategy is highly sensitive and could differentiate the binding of ligands to structurally similar subfamily members. Through this strategy, it was possible to simultaneously identify subfamily-selective probes against two clinically important epigenetic enzymes: FTO (7; IC₅₀ =2.6 μm) and ALKBH3 (8; IC₅₀ =3.7 μm). To date, this is the first report of a subfamily-selective ALKBH3 inhibitor. The developed strategy could, in principle, be adapted to a broad range of proteins; thus it is of broad scientific interest.