Obesity is a global health crisis with severe complications. While lifestyle changes and surgery help manage weight, a deeper understanding of the underlying mechanisms is needed for better therapeutic options. Epigenetics, particularly DNA methylation, plays a key role in the connection between environmental factors and obesity. DNA methylation is regulated by DNA methyltransferases (DNMTs) and ten-eleven translocation (TET) enzymes, which catalyze active DNA demethylation. Studies have shown a link between adipose tissue DNA methylation and obesity, but the causal relationship remains unclear. TET enzymes, particularly TET2, are implicated in energy homeostasis; their loss can protect against diet-induced obesity by increasing β3-AR expression. TET2 is unique among the TET family, lacking the CXXC domain and interacting with transcription factors to regulate gene expression. Previous in vitro studies suggest a metabolic role for TET2 in adipocytes, influencing PPARγ expression and lipolysis. However, a clear mechanism linking TET2-mediated epigenetic changes to obesity in vivo is lacking. Leptin, an adipokine, signals to reduce food intake and increase energy expenditure. While effective in congenital leptin deficiency, leptin treatment fails to impact diet-induced obesity due to leptin resistance, which involves decreased hypothalamic responsiveness to leptin. Hyperleptinemia exacerbates leptin resistance and contributes to obesity. Reducing circulating leptin levels improves leptin sensitivity and reduces weight gain. This study investigates the regulation and function of TET2 in adipocytes during obesity, focusing on its relationship with leptin.

The literature extensively demonstrates the association between DNA methylation and obesity. Epigenome-wide association studies (EWAS) have identified methylation loci linked to body mass index (BMI) and waist circumference in various populations. Studies analyzing adipose tissue have shown associations between DNA methylation patterns of numerous genes and BMI. Weight loss itself alters DNA methylation profiles in adipose tissue, further highlighting the dynamic interplay between epigenetics and obesity. However, the causal role of adipose tissue DNA methylation alteration in obesity remains elusive. Recent research suggests a crucial role for TET enzymes in energy homeostasis, particularly TET2 in adipocytes. In vitro studies indicate that TET2 impacts adipogenesis by enhancing PPARγ expression and increasing lipolysis by upregulating Adrb3 gene expression. Leptin, a key adipokine, is also central to obesity research. Although effective in treating congenital leptin deficiency, its efficacy in diet-induced obesity is limited by leptin resistance, a complex phenomenon involving impaired leptin signaling in the hypothalamus and hyperleptinemia. Studies have shown that reducing circulating leptin levels can improve leptin sensitivity and lead to weight loss, providing further impetus to explore the interplay between leptin and TET2.

The study employed various in vivo and in vitro approaches. In vivo experiments used C57BL/6J mice, leptin-deficient (ob/ob) mice, and adipocyte-specific Tet2 knockout (AKO) mice, fed either normal or high-fat diets (HFD) for varying durations. Metabolic parameters like body weight, body composition, glucose tolerance, insulin sensitivity, energy expenditure, and food intake were assessed. Adipocytes and stromal vascular fractions (SVF) were isolated from adipose tissue for analyses. In vitro studies utilized differentiated 3T3-L1 adipocytes and primary adipocytes. The effects of leptin, JAK2 inhibitors, and siRNAs targeting JAK2 and STAT3 on Tet2 expression were evaluated. Genomic DNA and RNA were extracted from adipose tissue and adipocytes for analyses of 5-hmC, 5-mC levels, and gene expression using dot blot assays, qRT-PCR, and western blotting. Immunohistochemistry was used to assess TET2 protein levels in adipose tissue. ChIP-seq and ChIP-qPCR were performed to identify TET2 binding sites in the genome and the leptin gene promoter. Co-immunoprecipitation (Co-IP) and sequential ChIP (reChIP) were used to investigate the interaction between TET2 and C/EBPa. Human subcutaneous adipose tissue (SAT) samples from obese and non-obese individuals were analyzed for 5-hmC, 5-mC levels, and TET2 and LEPTIN gene expression. Statistical analyses were performed using appropriate methods.

Obesity decreased 5-hmC and TET2 levels in adipocytes, primarily attributed to hyperleptinemia. Leptin inhibited adipocyte Tet2 expression via the JAK2-STAT3 signaling pathway. Tet2 deficiency attenuated HFD-induced obesity and insulin resistance, increasing energy expenditure and decreasing food intake. Adipocyte-specific Tet2 knockout mice exhibited similar protective effects against HFD-induced obesity. Tet2 deficiency improved HFD-induced obesity and insulin resistance by partially reducing leptin levels and enhancing leptin sensitivity. TET2 upregulated leptin gene expression by interacting with C/EBPa, increasing hydroxymethylcytosine levels at the leptin promoter. In humans, TET2 levels were negatively correlated with LEPTIN levels and BMI. Inhibition of TET2 suppressed leptin production in human adipocytes. Experiments involving double knockout (Tet2 ob/ob) mice and leptin supplementation in AKO mice confirmed the critical role of leptin in mediating the effects of Tet2 deficiency on metabolism.

This study identifies a novel negative feedback loop between TET2 and leptin in adipocytes, providing a mechanistic explanation for the interplay between epigenetics and metabolic regulation in obesity. The findings highlight the crucial role of adipocyte TET2 in modulating leptin levels and sensitivity. The reduction in TET2 in obesity, driven by hyperleptinemia, appears to be a compensatory mechanism to counteract the effects of excessive leptin. While TET2 deficiency does not entirely prevent leptin elevation in obesity, it significantly improves leptin sensitivity and leads to weight loss. The interaction between TET2 and C/EBPa in regulating leptin gene expression provides a specific epigenetic target for potential therapeutic interventions. The consistent findings in both mouse models and human data strengthen the translational relevance of this discovery. This research opens avenues for developing targeted therapies focused on the TET2-C/EBPa interaction to control leptin levels and manage obesity.

This study reveals a crucial negative feedback loop between TET2 and leptin in adipocytes that regulates body weight. Leptin suppresses TET2 levels via JAK2-STAT3 signaling, and TET2 deficiency improves leptin sensitivity and reduces weight gain. TET2 regulates leptin expression through interaction with C/EBPa. These findings provide a novel epigenetic mechanism underlying obesity and suggest the potential for therapeutic strategies targeting the TET2-C/EBPa interaction.

The study primarily used male mice, limiting the generalizability to females. While the human data supports the mouse model findings, the sample size is relatively small. Further research is needed to fully elucidate the complex interplay between TET2, leptin, and other factors in different adipose depots and sexes. The long-term effects of Tet2 deficiency on metabolic health require further investigation.