m6A mRNA Methylation in Brown Fat Regulates Systemic Insulin Sensitivity via an Inter-Organ Prostaglandin Signaling Axis Independent of UCP1

Endocrinology Biochemistry Life Sciences Medicine Cell Biology
brown fat m6A methylation insulin sensitivity prostaglandin lipid metabolism METTL14 signaling

The Role of m6A mRNA Methylation in Adipose Tissue: A Breakthrough Discovery of Cross-Organ Prostaglandin Signaling Axis in Insulin Sensitivity Regulation

Research Background and Motivation

In recent years, the potential role of brown adipose tissue (BAT) in human metabolic regulation has attracted widespread attention. Known for its thermogenic properties mediated by uncoupling protein 1 (UCP1), BAT can consume energy and reduce body fat under cold stimulation, making it an essential target for treating obesity and metabolic syndrome. However, besides its thermogenic function, BAT also influences systemic metabolism by secreting factors that regulate the utilization of glucose, fatty acids, and branched-chain amino acids. Though BAT activation is generally associated with high UCP1 expression, increasing evidence suggests that UCP1-independent mechanisms can also exert metabolic regulatory functions in adipose tissue.

The authors of this study propose that N6-methyladenosine (m6A), an epigenetic modification widely present in mRNA, might play a crucial role in the secretory functions of BAT and the regulation of systemic insulin sensitivity. Specifically, the study focuses on an m6A writer enzyme, methyltransferase-like protein 14 (METTL14), and explores its effect on insulin sensitivity in BAT, thereby uncovering how m6A influences systemic metabolism through the regulation of BAT secretions.

Research Origin

This research was conducted by Ling Xiao and her team, with members from Joslin Diabetes Center, Harvard Medical School, University of Chicago, University of Eastern Finland, and Leipzig University in Germany. The findings were published in the October 2024 edition of the journal “Cell Metabolism.” Through multi-institutional collaboration and interdisciplinary experimental design, the team provided in-depth insights into molecular biology and metabolism.

Research Process

The research team generated brown adipose tissue-specific METTL14 knockout mice (M14KO) to analyze their insulin sensitivity, glucose tolerance, and metabolic responses under different dietary conditions (high-fat vs. low-fat diet). Experiments included:

  1. Gene Expression Analysis: Initially, the authors detected METTL14 expression under obesity conditions in both human and mouse samples, finding it significantly upregulated in the BAT of obese individuals and high-fat diet mice.

  2. Mouse Model Generation and Detection: By crossing METTL14-floxed mice with UCP1-Cre mice, the researchers obtained a mouse model (M14KO) with BAT-specific METTL14 knockout. The study confirmed the loss of METTL14 in the BAT of M14KO mice, validating that this knockout did not affect other metabolic tissues.

  3. Insulin Tolerance Testing: Under controlled diet and high-fat diet conditions, it was found that M14KO mice showed noticeably improved insulin sensitivity and glucose tolerance. This phenomenon was consistent across different sexes of mice and was independent of weight changes.

  4. Cold Exposure Experiments: To explore UCP1’s role in this process, the study further tested M14KO mice under cold (5°C) and thermoneutral (30°C) conditions. The results indicated that regardless of UCP1 activity changes, M14KO mice maintained improved insulin sensitivity, suggesting that the enhancement was independent of UCP1.

  5. Lipidomics Analysis: Using liquid chromatography-mass spectrometry, the study identified significant increases in prostaglandin E2 (PGE2) and prostaglandin F2α (PGF2α) in M14KO-BAT and human brown adipocytes (HBAT), confirming their release in BAT and further exploring their role in regulating insulin sensitivity.

  6. Cellular and Co-culture Experiments: In vitro experiments were conducted to examine the effects of PGE2 and PGF2α on hepatocytes, skeletal muscle cells, and white adipocytes, finding that they significantly enhance Akt phosphorylation in the insulin signaling pathway at the cellular level.

  7. Neutralization Experiments: To verify PGE2’s effects, neutralizing antibodies were used to block PGE2 in M14KO mice, showing partial inhibition of insulin sensitivity improvement, further confirming PGE2’s contribution to insulin sensitivity in M14KO mice.

Research Results

  1. Improved Insulin Sensitivity: M14KO mice demonstrated significant enhancements in insulin sensitivity, and this improvement remained unaffected by cold exposure or thermoneutral conditions, proving that METTL14 deficiency impacts metabolism through non-UCP1-dependent mechanisms.

  2. Changes in BAT Secretions: Lipidomics and mass spectrometry analyses identified increased levels of PGE2 and PGF2α in M14KO-BAT. Further experiments revealed that these prostaglandins enhance insulin signaling in the liver, skeletal muscle, and white fat, promoting glucose uptake.

  3. Mechanism of Prostaglandins: In vivo and in vitro experiments showed that PGE2 and PGF2α promoted insulin sensitivity through activation of the Akt signaling pathway, with receptor blockade significantly reducing their effects. The study indicates that PGE2 enhances the Akt signal by inhibiting protein phosphatases such as PHLPP and SHIP1/2.

  4. Role of m6A Modification in Gene Regulation: RNA sequencing and m6A immunoprecipitation analyses showed that METTL14 deficiency resulted in lower methylation of mRNAs encoding PGE2 and PGF2α synthases (e.g., PTGES2 and CBR1), delaying their degradation and increasing protein levels. METTL14 regulates the stability of these mRNAs through YTHDF2/3 proteins, ultimately facilitating prostaglandin secretion.

  5. Human Correlation Analysis: In multiple human cohorts, the study found a negative correlation between PGE2 and PGF2α levels and BMI and insulin sensitivity, further supporting the role of these prostaglandins in human metabolic regulation.

Research Conclusion and Significance

This study reveals, for the first time, the mechanism by which METTL14 regulates BAT secretory functions through m6A modification, identifying its significant role in improving systemic insulin sensitivity by regulating prostaglandin synthesis. This process is independent of the UCP1-mediated thermogenic mechanism, offering a novel perspective and potential targets for metabolic regulation. Specifically, the study confirmed the role of BAT-secreted PGE2 and PGF2α as modulators of insulin sensitivity, expanding our understanding of how BAT affects metabolism under non-thermogenic conditions.

Additionally, the study points out that PGE2 and PGF2α exhibit similar metabolic regulatory properties in human cohorts, providing a theoretical basis for targeted metabolic disease interventions using BAT secretory factors. Overall, the study presents a new biological mechanism for enhancing systemic insulin sensitivity through prostaglandin signaling in BAT secretions, suggesting potential new directions for treating obesity and related metabolic disorders.