Short-term Cold Exposure Induces Persistent Epigenomic Memory in Brown Adipose Tissue

Background

Brown Adipose Tissue (BAT) is the primary non-shivering thermogenic organ in mammals that dissipates chemical energy as heat under cold stimuli. BAT is characterized by a high density of mitochondria containing Uncoupling Protein 1 (UCP1), which generates heat by releasing fatty acids under the proton gradient established by the electron transport chain. Activation of BAT, through cold exposure, localized heat, or β-adrenergic agonists, may benefit human metabolism by burning stored calories, thus combating type 2 diabetes, insulin resistance, obesity, and cardiovascular diseases.

However, there is significant individual variability in BAT quantity and activity levels, influenced by factors such as photoperiod, ambient temperature, age, sex, body mass index (BMI), plasma glucose levels, and diabetic status. These individual differences are likely driven in part by genetic and epigenetic variations. Thus, understanding the molecular mechanisms affecting BAT thermogenesis and metabolism is of considerable significance for potential metabolic disease therapies.

Study Source

This study was published by Inoue et al. on August 6, 2024, in the journal “Cell Metabolism”. The research team, including Shin-ichi Inoue, Matthew J. Emmett, Hee-Woong Lim, among others, is affiliated with the Institute for Diabetes, Obesity, and Metabolism at the Perelman School of Medicine, University of Pennsylvania, as well as researchers from MIT and Harvard University.

Research Process

Research Process

1. Experimental Procedure and Subjects

   • Cold Exposure Experiment: The research team divided UCP1-Cre/HDAC3flox/flox mice into different groups, conducting experiments at room temperature (22°C), under moderate cold exposure (15°C, 24 hours), and acute cold exposure (4°C). The team also used mice with specific gene knockouts, such as HDAC3/PGC-1α double knockout mice, and adeno-associated virus (AAV) mediated targeted mice to knock down specific gene expressions in BAT.
   • Molecular Analysis: Thorough analyses of the mice’s BAT were performed using RNA sequencing, chip sequencing, chromatin immunoprecipitation sequencing (ChIP-seq), and global run-on sequencing (GRO-seq) technologies. The experiments also included assessing the mice’s body temperature and survival to evaluate physiological responses post cold exposure.

2. Data Analysis

   • Gene Expression Analysis: Using RNA-seq (RNA sequencing) and ChIP-seq (chromatin immunoprecipitation sequencing) data, the research team analyzed the impact of cold exposure on gene expression and transcriptional regulation. Expression of some genes was further validated through quantitative PCR.

3. Epigenomic Memory

   • Memory Effect: The study reports that a short-term moderate cold exposure (STEMCT, 15°C, 24 hours) can restore the impaired UCP1 expression in HDAC3-deficient mouse BAT, allowing the mice to survive cold stress when subjected to acute cold exposure. Furthermore, the protective effect of STEMCT can last up to 7 days. By inducing transcription factor C/EBPβ, STEMCT significantly enhances epigenomic memory effects.

Study Results

1. Cold Exposure Restores UCP1 Expression

   • In the absence of HDAC3, the RNA and protein expression of UCP1 are nearly completely inactive but restored under STEMCT. RNA-seq analysis revealed that many genes regulating mitochondrial function (e.g., electron transport, fatty acid oxidation) are re-expressed post cold exposure, particularly with significant upregulation observed in UCP1 and oxidative phosphorylation (OXPHOS) related genes.

2. Epigenomic Memory

   • STEMCT triggered a persistent high expression of the transcription factor C/EBPβ, which remained elevated even 7 days post STEMCT. Reducing C/EBPβ abolished the long-term protective effect of STEMCT, indicating the existence of a C/EBPβ-dependent epigenomic memory effect for cold adaptation.

3. Molecular Mechanism Exploration

   • Data analysis revealed a cascade of new transcriptional regulation mechanisms under cold exposure, particularly involving the function of ERRα (estrogen-related receptor α) and its coactivator PGC-1α. While PGC-1α is crucial in the short term post cold exposure, C/EBPβ is critical for determining the long-term memory effect. C/EBPβ binds extensively to enhancer sites of UCP1 and oxidase-related genes, significantly enhancing their expression.

Conclusion

1. Scientific and Applied Value

   • This study reveals that even in the absence of HDAC3, short-term moderate cold exposure (STEMCT) can provide persistent cold resistance effects in brown fat through epigenomic memory mechanisms. This mechanism shows value in basic scientific research and offers new perspectives for future innovative therapies for obesity and related metabolic disorders.

2. Innovation and Highlights of the Study

   • The study reveals for the first time the complex cold adaptation mechanisms and persistent epigenomic memory effects in brown adipose tissue, emphasizing the indispensable role of C/EBPβ in this process. The study identified new transcription regulation networks and widespread gene expression changes, significantly advancing our understanding of the impact of environmental and epigenetic regulation on metabolism.

3. Directions for Further Research

   • Future research can explore further the interactions between C/EBPβ and other transcription factors and cofactors, delving into the complex network of epigenomic memory induced by cold exposure. Investigating these mechanisms under other environmental stress conditions and their potential clinical applications will also be an important direction for future research.

References

The study references numerous relevant literatures and supports its conclusions with rich experimental data, providing a solid foundation for our future research advancements.