Type I interferon sensing unlocks dormant adipocyte inflammatory potential

Obesity is a significant public health concern, characterized by the expansion of adipocytes in white adipose tissue (WAT) and the creation of a highly inflammatory environment. This chronic inflammation contributes to obesity-related diseases like type 2 diabetes mellitus (T2D) and non-alcoholic fatty liver disease (NAFLD). While the role of myeloid cells in WAT inflammation is well-understood, the mechanisms regulating adipocyte inflammatory potential and its contribution to obesity-associated inflammation remain poorly defined. Adipocytes, similar to myeloid cells, produce pro-inflammatory cytokines, express innate immune receptors (e.g., Toll-like receptors or TLRs), and major histocompatibility complex (MHC) molecules, suggesting a potential for immune-like behavior. However, the extent of this potential and the regulatory mechanisms involved are unclear. Type I interferons (IFNs), primarily IFNα and IFNβ, are crucial immune mediators that regulate innate and adaptive immune responses, including myeloid cell inflammation. IFNβ is widely expressed, while IFNα is mainly produced by hematopoietic cells. Obesity-associated metabolic endotoxemia (e.g., lipopolysaccharide or LPS) activates TLR signaling, a potent inducer of type I IFN production. Type I IFNs bind to the ubiquitously expressed IFNAR, initiating inflammatory signaling pathways. The role of the IFNAR axis in obesity-associated diseases is debated, with some studies suggesting detrimental effects and others indicating beneficial effects. However, the impact of the type I IFN/IFNAR axis on adipocyte inflammatory potential has not been thoroughly investigated. Given the IFNAR axis's influence on myeloid cell inflammation and the shared inflammatory functions between myeloid cells and adipocytes, the researchers hypothesized that activating the type I IFN/IFNAR axis in adipocytes would reveal immune-like inflammatory signatures and enhance adipocyte inflammation.

The literature review section summarizes existing knowledge on the role of type I interferons and their receptor (IFNAR) in obesity and related metabolic disorders. There is conflicting evidence regarding the beneficial or detrimental roles of the IFNAR axis in the development of obesity-associated sequelae. Some studies suggest detrimental effects of type I interferon signaling in hepatic and metabolic dysfunction, linking it to the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Others suggest that interferon-beta overexpression might have protective effects on weight gain and glucose homeostasis. However, there is a lack of research specifically focusing on the contribution of the type I IFN/IFNAR axis in regulating the inflammatory potential of adipocytes themselves. This gap highlights the need for further investigation into the specific role of adipocyte-intrinsic IFNAR signaling in the context of obesity.

The study utilized several key methodologies to investigate the role of the type I interferon (IFN)/interferon-alpha receptor (IFNAR) axis in adipocyte inflammation. The researchers employed both in vitro and in vivo models. **In vitro studies:** Primary adipocytes were isolated from mice fed either a chow diet (CD) or a high-fat diet (HFD). These adipocytes were then treated with various stimuli, including lipopolysaccharide (LPS), interferon-beta (IFNβ), and inhibitors of glycolysis (2-Deoxy-D-Glucose or 2-DG). The effects of these treatments on cytokine production (e.g., IL-6, TNF), gene expression (via qPCR and RNA sequencing), and metabolic activity (using a Seahorse XF96 analyzer to measure extracellular acidification rate (ECAR) and oxygen consumption rate (OCR)) were assessed. Human primary adipocytes were also isolated and tested in a similar way with the addition of ELISA to measure IFNβ, IL-6 and other relevant cytokines. **In vivo studies:** The researchers utilized mouse models with genetic modifications to investigate the effects of IFNAR signaling on obesity-associated metabolic phenotypes. This included using IFNAR knockout mice and mice with adipocyte-specific IFNAR deletion (*Adipoq*Cre-*IFNAR*fl/fl). High-fat diet (HFD)-fed mice were compared to chow diet (CD)-fed controls. Obesity phenotypes (body weight, adipose tissue distribution, glucose tolerance, insulin sensitivity), inflammation markers, and immune cell infiltration in adipose tissue and liver were evaluated. Reciprocal bone marrow transfer experiments were performed to delineate the contribution of hematopoietic versus non-hematopoietic IFNAR expression. **Molecular techniques:** qPCR was used to measure the expression of specific genes. RNA sequencing was performed to study global gene expression changes in adipocytes and macrophages following IFNβ treatment. Bioinformatics analysis was conducted to identify enriched pathways and transcription factors. The study also employed flow cytometry for immune cell analysis (adipose tissue and liver) in mouse models, ELISA for cytokine quantification, and a Type I IFN activity assay. Statistical analyses were conducted to compare experimental groups.

The study's key findings demonstrate a significant role for the type I IFN/IFNAR axis in regulating adipocyte inflammatory responses and contributing to obesity-associated metabolic derangements. The researchers found that: 1. **Activation of the IFNAR axis amplifies adipocyte inflammation:** LPS treatment of primary adipocytes induced IFNβ production and the expression of type I IFN signature genes. IFNβ treatment further enhanced LPS-driven pro-inflammatory cytokine production in an IFNAR-dependent manner, mirroring observations in myeloid cells. 2. **Type I IFN reveals dormant adipocyte inflammatory networks:** RNA sequencing revealed that IFNβ + LPS treatment of adipocytes drove their gene expression patterns toward a resemblance with inflammatory macrophages. A significant overlap was observed between the genes upregulated in adipocytes and macrophages after this combined treatment, including genes involved in inflammation and antigen presentation. These findings suggest adipocytes possess a dormant immunological capacity that is unmasked by type I IFN signaling. 3. **Type I IFN promotes adipocyte glycolysis:** IFNβ treatment enhanced the expression of key glycolytic enzymes, increased basal extracellular acidification rate (ECAR), and increased cellular respiration. Inhibition of glycolysis (using 2-DG) reversed the IFNβ-mediated increase in cellular respiration and dampened the expression of type I IFN signature genes and inflammation. This suggests that metabolic alterations, particularly in glycolysis, are linked to IFNβ-driven adipocyte inflammation. 4. **Obesity induces the type I IFN axis:** HFD-fed mice exhibited increased expression of type I IFN signature genes in adipose tissue and other organs. Adipocytes from HFD-fed mice showed enhanced inflammatory responses after IFNβ priming. 5. **IFNAR signaling exacerbates obesity-associated sequelae:** Total body IFNAR knockout mice and mice with adipocyte-specific IFNAR deletion had altered adipose tissue distribution, but total body weight was similar to their controls. However, these genetic modifications resulted in reduced immune cell infiltration in epididymal WAT (eWAT) and decreased pro-inflammatory cytokine production by macrophages, while improving glucose tolerance and insulin sensitivity. These findings suggest that IFNAR signaling in adipocytes contributes to metabolic dysfunction rather than influencing weight gain directly. 6. **Type I IFN axis effects are conserved in humans:** Human primary adipocytes exhibited similar responses to IFNβ and LPS treatment as those in mice. An association was seen between systemic IFNβ levels and markers of liver damage, highlighting the translational relevance of these findings.

The findings of this study offer a novel perspective on the role of adipocytes in obesity-associated inflammation and metabolic dysfunction. The data demonstrate that the type I IFN/IFNAR axis plays a significant role in amplifying adipocyte inflammatory responses and activating dormant immune-like functions. The interplay between IFN signaling and glycolysis in adipocytes highlights a previously underappreciated aspect of adipocyte biology. This emphasizes the need to consider adipocytes not merely as passive energy storage units but as active players in the inflammatory landscape of WAT. The findings from the in vivo experiments further support the importance of IFNAR signaling in the pathogenesis of obesity-associated diseases. The selective impact of adipocyte-specific IFNAR deletion on glucose metabolism but not on total body weight or liver damage is noteworthy, suggesting the complex interactions between different cell types in this metabolic milieu. These findings are partly corroborated with similar studies using other mouse models for IFNAR-deletion. The conservation of type I IFN axis effects in human adipocytes enhances the translational relevance of the study's findings. The correlation between systemic IFNβ levels and markers of liver disease in humans points to a potential clinical significance. These findings contribute substantially to our understanding of obesity pathogenesis and suggest new potential therapeutic targets.

This study provides compelling evidence for a previously unrecognized role of the type I IFN/IFNAR axis in regulating adipocyte inflammatory responses and contributing to obesity-associated metabolic derangements. The findings highlight the importance of considering adipocytes as active participants in the inflammatory processes of WAT and suggest that targeting the type I IFN/IFNAR axis could offer new therapeutic strategies for obesity-related metabolic diseases. Future research should focus on elucidating the precise mechanisms underlying the crosstalk between type I IFN signaling, adipocyte metabolism, and immune cell activation. Furthermore, investigating the potential therapeutic implications of modulating the type I IFN/IFNAR axis in both adipocytes and immune cells deserves further attention.

One limitation of the study is the use of a relatively small cohort of human subjects for the in vivo experiments, which might limit the generalizability of the findings. Furthermore, the focus on severe obesity in the human cohort might not fully represent the spectrum of obesity-related metabolic phenotypes. Additional studies with larger and more diverse cohorts are needed to confirm and extend these observations. The use of certain cre-driver mice might also introduce off-target effects which might confound the interpretation of the data. Future studies might benefit from using multiple different approaches to delete the IFNAR gene in adipocytes to reduce the uncertainty introduced by potential off-target effects. Further, the use of different types of high fat diets might also influence the final results.