Identification of an adipogenic niche for adipose tissue remodeling and restoration

Perivascular Adipose Tissue Niches for Modulating Immune Cell Function

Perivascular adipose tissue is a unique fat depot surrounding most blood vessels with a significant role in vascular function. While adipocytes compose the vast majority of the perivascular adipose tissue by area, they only account for around 20% of the total cell number. Most of the cellular component belongs to resident immune cells, with macrophages and lymphoid cells representing ≈30% and 15% of total cells, respectively. Recently, new evidence has shown that aside from their well-known role in modulating the inflammatory tone, immune cells in perivascular adipose tissue can control adipogenesis, vessel integrity, and vascular contractility through complex cellular interactions. These interactions are spatially coordinated and influenced by the environmental state. Here, we review the mechanism by which immune cells regulate perivascular adipose tissue function with a special focus on the spatial organization of immune cells and their heterotypic interactions, supporting tissue function in health and disease.

The Landscape of SPP1+ Macrophages Across Tissues and Diseases: A Comprehensive Review

Macrophages play a critical role in shaping the immune landscape of various diseases, with secreted phosphoprotein 1 (SPP1)-expressing macrophages emerging as a distinct subset implicated in both cancerous and non-cancerous conditions. Leveraging recent advances in single-cell RNA sequencing, numerous studies have identified SPP1+ macrophages across diverse pathological contexts, shedding light on their functional heterogeneity. In cancer, SPP1+ tumour-associated macrophages contribute to tumour growth, angiogenesis, and immune evasion, often interacting with T cells and stromal components to sustain an immunosuppressive microenvironment. Conversely, in non-cancerous diseases, these macrophages exhibit both profibrotic and disease-promoting properties, depending on the tissue context. This review provides a comprehensive synthesis of the latest findings on SPP1+ macrophages, highlighting their roles in tumour progression, immune suppression, tissue remodelling, and fibrosis. By comparing their shared traits and tissue-specific differences, we explore how SPP1+ macrophages adapt to distinct microenvironments and influence disease progression. Understanding their conserved and context-dependent functions may open new avenues for therapeutic targeting.

Adipocyte ZAG improves obesity-triggered insulin resistance by reshaping macrophages populations in adipose tissue

Adipose tissues macrophages (ATMs) serve as a critical effector in the mediating occurrence of metabolic inflammation to impact whole-body insulin sensitivity in obesity. Discovering the key adipokines mediating crosstalk of adipocytes-macrophages and understanding the molecular mechanism of ATMs polarization and function have become hot topic issues in the immunometabolism fields. Zinc-α2-glycoprotein (ZAG) as a anti-inflammatory adipokines plays important roles in obesity-related metabolic diseases. We attempt to explore the precise role of adipose ZAG in metabolic inflammation and obesity-associated insulin resistance. Here we showed that Omental ZAG was positively associated with insulin sensitivity and M2 macrophages markers. ZAG-specific ablation in adipocyte aggravated insulin resistance and adipose tissues inflammation as evidenced by enhanced M1 macrophages proportion and inhibited AKT signaling pathway in mice fed with a high-fat diet. Exogenous ZAG inhibits PA-induced M1 macrophage polarization via β3-AR/PKA/STAT3 signaling in RAW264.7 macrophages.These findings suggest that adipocyte ZAG maintain insulin sensitivity via the cross talk with adipose-resident macrophages.

The Advanced Glycation End-Products (AGE)-Receptor for AGE System (RAGE): An Inflammatory Pathway Linking Obesity and Cardiovascular Diseases

The AGE (advanced glycation end-products)-RAGE (receptor for AGE) system is a pro-inflammatory pathway that contributes to the pathogenesis of obesity and obesity-related cardiovascular disorders (CVD). Circulating AGE and the soluble form of RAGE (sRAGE) has been suggested as a potential biomarker of CVD related to obesity. In this study, we aim to (1) summarize the current knowledge about the role of obesity in the onset and progression of CVD, (2) discuss the role of the AGE-RAGE system as a pathway promoting obesity and linking obesity to CVD, and (3) highlight available strategies for reducing AGE-RAGE system activation and the associated beneficial effects.

SPP1 macrophages across diseases: A call for reclassification?

SPP1+ macrophages, characterized by elevated expression of the osteopontin gene (secreted phosphoprotein 1, SPP1), have emerged as key players in various pathological contexts, including aging, chronic inflammatory diseases, and cancer. While frequently classified as a subclass of tumor-associated macrophages in oncological settings, their presence in noncancer conditions, such as aging-related disorders and muscular diseases, suggests a broader role beyond tumors. These macrophages share conserved traits, including fibrosis promotion, extracellular matrix remodeling, and immune modulation, often linked to poor clinical outcomes. This perspective explores the multifaceted roles of SPP1+ macrophages across diseases and advocates for their reclassification as a distinct macrophage subtype associated with chronic or prolonged inflammation. Recognizing their cross-disease relevance could reshape macrophage biology and inform targeted therapeutic strategies.

Ablation of CD44 Attenuates Adipogenesis in White Adipocytes via the Tryptophan 5-Hydroxylase 2/5-Hydroxytryptamine Axis to Protect Mice from High-Fat Diet-Induced Obesity

CD44 is a transmembrane protein that plays an essential role in transducing extracellular stimuli into intracellular signaling cascades, especially in cancer cells. Recent studies have shown that CD44 contributes to metabolic regulation. However, the effect of CD44 on adipogenesis in white adipose tissue (WAT) remains unclear. Herein, the expression of CD44 was largely increased in the inguinal and epididymal WAT of obese mice. Ablation or neutralization of CD44 inhibited adipogenesis in cultured adipocytes. CD44-deficient mice were resistant to high-fat diet-induced obesity and metabolic dysfunction. RNA-sequencing, together with functional studies, revealed that reduced expression of tryptophan 5-hydroxylase 2 (Tph2) in WAT was responsible for the repressed adipogenesis in the absence of CD44. The application of 5-hydroxytryptamine, a product of TPH2, rescued the repressed adipogenesis induced by CD44 neutralization. Moreover, the inhibition of TPH2 by p-chlorophenylalanine recapitulated the beneficial phenotypes observed in CD44-deficient mice. Taken together, these data indicate that CD44 plays a pivotal role in adipogenesis in WAT. In this regard, CD44 and its downstream target TPH2 may hold great therapeutic potential for treating excessive adiposity-related metabolic disorders, such as obesity, insulin resistance, and type 2 diabetes.

Immune Regulatory Crosstalk in Adipose Tissue Thermogenesis

Brown adipose tissue (BAT) and thermogenic beige fat within white adipose tissue (WAT), collectively known as adaptive thermogenic fat, dissipate energy as heat, offering promising therapeutic potential to combat obesity and metabolic disorders. The specific biological functions of these fat depots are determined by their unique interaction with the microenvironments, composed of immune cells, endothelial cells, pericytes, and nerve fibers. Immune cells residing in these depots play a key role in regulating energy expenditure and systemic energy homeostasis. The dynamic microenvironment of thermogenic fat depots is essential for maintaining tissue health and function. Immune cells infiltrate both BAT and beige WAT, contributing to their homeostasis and activation through intricate cellular communications. Emerging evidence underscores the importance of various immune cell populations in regulating thermogenic adipose tissue, though many remain undercharacterized. This review provides a comprehensive overview of the immune cells that regulate adaptive thermogenesis and their complex interactions within the adipose niche, highlighting their potential to influence metabolic health and contribute to therapeutic interventions for obesity and metabolic syndrome.

How obesity affects adipocyte turnover

Cellular turnover is fundamental for tissue homeostasis and integrity. Adipocyte turnover, accounting for 4% of the total cellular mass turnover in humans, is essential for adipose tissue homeostasis during metabolic stress. In obesity, an altered adipose tissue microenvironment promotes adipocyte death. To clear dead adipocytes, macrophages are recruited and form a distinctive structure known as crown-like structure; subsequently, new adipocytes are generated from adipose stem and progenitor cells in the adipogenic niche to replace dead adipocytes. Accumulating evidence indicates that adipocyte death, clearance, and adipogenesis are sophisticatedly orchestrated during adipocyte turnover. In this Review, we summarize our current understandings of each step in adipocyte turnover, discussing its key players and regulatory mechanisms.

5-Aminolevulinic Acid (5-ALA) Plays an Important Role in the Function of Innate Immune Cells

5-aminolevulinic acid (5-ALA) is an amino acid essential for the synthesis of heme, which is important for various cellular functions, including the mitochondrial electron transport chain. We previously established heterozygous knockout mice (Alas1+/-) for 5-ALA synthase 1 (ALAS1), the rate-limiting enzyme for 5-ALA synthesis, and reported that the mice developed non-obese insulin-resistant diabetes. In the present study, we used these mice to analyze the role of 5-ALA in the immune system. Using a lipopolysaccharide (LPS)-induced septic shock model, Alas1+/- mice showed reduced mortality compared to wild-type (WT) mice. In this model experiment, the plasma concentration of inflammatory cytokines such as tumor necrosis factor α (TNFα) and interleukin-6 (IL-6), and the chemokine monocyte chemoattractant protein-1 (MCP1) decreased in Alas1+/- mice compared that in WT mice, and inflammatory cell infiltration into the peritoneal cavity was also decreased. In ex vivo experiments, exogenous 5-ALA pretreatment enhanced LPS-induced TNFα and IL-6 production from peripheral blood leukocytes of Alas1+/- mice. Additionally, 5-ALA pretreatment enhanced LPS-induced activation of inflammatory cytokine genes in innate immune cells. Interestingly, the phagocytosis and reactive oxygen species (ROS) producing abilities of neutrophils were clearly hampered in Alas1+/- mice compared to WT mice, but after 2 weeks of 5-ALA administration to Alas1+/- mice, both abilities were significantly recovered up to the level in WT mice. These results reveal that 5-ALA is essential for the function of innate immune cells. Because 5-ALA can be supplemented orally, it has the potential to be used as a drug to restore innate immune function.

Interplay between the brain and adipose tissue: a metabolic conversation

The central nervous system and adipose tissue interact through complex communication. This bidirectional signaling regulates metabolic functions. The hypothalamus, a key homeostatic brain region, integrates exteroceptive and interoceptive signals to control appetite, energy expenditure, glucose, and lipid metabolism. This regulation is partly achieved via the nervous modulation of white (WAT) and brown (BAT) adipose tissue. In this review, we highlight the roles of sympathetic and parasympathetic innervation in regulating WAT and BAT activities, such as lipolysis and thermogenesis. Adipose tissue, in turn, plays a dual role as an energy reservoir and an endocrine organ, secreting hormones that influence brain function and metabolic health. In addition, this review focuses on recently uncovered communication pathways, including extracellular vesicles and neuro-mesenchymal units, which add new layers of regulation and complexity to the brain-adipose tissue interaction. Finally, we also examine the consequences of disrupted communication between the brain and adipose tissue in metabolic disorders like obesity and type-2 diabetes, emphasizing the potential for new therapeutic strategies targeting these pathways to improve metabolic health.

Dynamic Roles and Expanding Diversity of Adipose Tissue Macrophages in Obesity

Adipose tissue macrophages (ATMs) are key regulators of adipose tissue (AT) inflammation and insulin resistance in obesity, and the traditional M1/M2 characterization of ATMs is inadequate for capturing their diversity in obese conditions. Single-cell transcriptomic profiling has revealed heterogeneity among ATMs that goes beyond the old paradigm and identified new subsets with unique functions. Furthermore, explorations of their developmental origins suggest that multiple differentiation pathways contribute to ATM variety. These advances raise concerns about how to define ATM functions, how they are regulated, and how they orchestrate changes in AT. This review provides an overview of the current understanding of ATMs and their updated categorization in both mice and humans during obesity. Additionally, diverse ATM functions and contributions in the context of obesity are discussed. Finally, potential strategies for targeting ATM functions as therapeutic interventions for obesity-induced metabolic diseases are addressed.

Dissecting human adipose tissue heterogeneity using single-cell omics technologies

Single-cell omics technologies that profile genes (genomic and epigenomic) and determine the abundance of mRNA (transcriptomic), protein (proteomic and secretomic), lipids (lipidomic), and extracellular matrix (matrisomic) support the dissection of adipose tissue heterogeneity at unprecedented resolution in a temporally and spatially defined manner. In particular, cell omics technologies may provide innovative biomarkers for the identification of rare specific progenitor cell subpopulations, assess transcriptional and proteomic changes affecting cell proliferation and immunomodulatory potential, and accurately define the lineage hierarchy and differentiation status of progenitor cells. Unraveling adipose tissue complexity may also provide for the precise assessment of a dysfunctional state, which has been associated with cancer, as cancer-associated adipocytes play an important role in shaping the tumor microenvironment supporting tumor progression and metastasis, obesity, metabolic syndrome, and type 2 diabetes mellitus. The information collected by single-cell omics has relevant implications for regenerative medicine because adipose tissue is an accessible source of multipotent cells; alternative cell-free approaches, including the use of adipose tissue stromal cell-conditioned medium, extracellular vesicles, or decellularized extracellular matrix, are clinically valid options. Subcutaneous white adipose tissue, which is generally harvested via liposuction, is highly heterogeneous because of intrinsic biological variability and extrinsic inconsistencies in the harvesting and processing procedures. The current limited understanding of adipose tissue heterogeneity impinges on the definition of quality standards appropriate for clinical translation, which requires consistency and uniformity of the administered product. We review the methods used for dissecting adipose tissue heterogeneity and provide an overview of advances in omics technology that may contribute to the exploration of heterogeneity and dynamics of adipose tissue at the single-cell level.

IL-10 Enhances the Inhibitory Effect of Adipose-Derived Stromal Cells on Insulin Resistance/Liver Gluconeogenesis by Treg Cell Induction

The modulation of cellular phenotypes within adipose tissue provides a potential means for therapeutic intervention for diabetes. Endogenous interleukin-10 (IL-10) protects against diet-induced insulin resistance. We examined the effects and mechanisms of action of IL-10-treated adipose-derived stromal cells on diabetes-induced insulin resistance and liver gluconeogenesis. We harvested stromal vascular fractions (SVFs) from the adipose tissue of diabetic (Leprdb/db) mice and treated them with IL-10 in vitro. SVFs treated with 10 or 100 ng of IL-10 were injected into the inguinal adipose tissue of Leprdb/db mice. IL-10 treatment suppressed the mRNA expression of IL-6, IL-33, CCL2, TNF-α, and IL-1β. Additionally, it suppressed the protein expression of IL-6, pmTOR, pJNK, and pNF-κB but enhanced Foxp3 mRNA expression in SVFs from diabetic mice. Meanwhile, IL-10 treatment repressed CCL2 and PDGFRα expression in adipose tissue macrophages (ATMs) and IL-6 expression in non-ATMs but increased the Foxp3 and IL-10 mRNA expression of ATMs from diabetic mice. Injection of IL-10-treated SVFs decreased the IL-6, IL-33, CCL2, IL-1β, and CCL2 but enhanced the Foxp3 and IL-10 mRNA expression of adipose tissue from Leprdb/db mice. Furthermore, injection of IL-10-treated SVFs increased CD4+ regulatory T cells (Tregs) in SVFs and adipose IL-10 levels and suppressed plasma adiponectin levels and DPP4 activity in diabetic mice. Injection of IL-10-treated SVFs decreased hepatic G6PC and PCK1 mRNA expression and increased Akt activation, STAT3 phosphorylation in the liver, and glucose tolerance in diabetic mice. Our data suggest that IL-10 treatment decreases inflammation in adipose SVFs of diabetic mice. Injection of IL-10-treated SVFs into the adipose tissue decreased diabetes-induced gluconeogenesis gene expression, DPP4 activity, and insulin resistance by enhancing Treg cells in diabetic mice. These data suggest that IL-10-treated adipose stromal vascular cells could be a promising therapeutic strategy for diabetes mellitus.

Anatomical location, sex, and age modulate adipocyte progenitor populations in perivascular adipose tissues

Perivascular adipose tissue (PVAT) regulates vascular function due to its capacity to synthesize vasoactive products and its mechanical properties. PVATs most abundant cells are adipocytes, and their populations are maintained by the maturation of adipocyte progenitor cells (APC), which may play a pivotal role in the pathogenesis of cardiovascular diseases. However, the distribution of APC within PVAT depots, their potential variation in spatial location, and the influence of sex and age on their abundance remain unknown. We hypothesize that APC abundance in PVAT is affected by location, age, sex and that APC subtypes have specific spatial distributions. PVAT from thoracic and abdominal aorta, and mesenteric arteries, and AT from interscapular, gonadal, and subcutaneous depots from 13-week and 30-week-old females and males Pdgfrα-CreERT2 x LSL-tdTomato mice (n = 28) were analyzed. Abdominal aorta PVAT had fewer progenitors than mesenteric PVAT and gonadal AT. Aging reduced the abundance of APC in the thoracic aorta but increased their numbers in mesenteric PVAT. Females had more APC than males in mesenteric PVAT and gonadal AT depots. APC exhibited unique spatial distribution in the aorta and mesenteric PVAT where they localized neighboring vasa vasorum and arteries. APC subtypes (APC1, APC2, APC3, diff APC) were identified in all PVAT depots. Thoracic aorta PVAT APC3 were located in the adventitia while diff APC were in the parenchyma. This study identified variability in APC populations based on depot, age, and sex. The distinctive spatial distribution and the presence of diverse APC subtypes suggest that they may contribute differently to cardiovascular diseases-induced PVAT remodeling.

Adipocyte Mitochondria: Deciphering Energetic Functions across Fat Depots in Obesity and Type 2 Diabetes

Adipose tissue, a central player in energy balance, exhibits significant metabolic flexibility that is often compromised in obesity and type 2 diabetes (T2D). Mitochondrial dysfunction within adipocytes leads to inefficient lipid handling and increased oxidative stress, which together promote systemic metabolic disruptions central to obesity and its complications. This review explores the pivotal role that mitochondria play in altering the metabolic functions of the primary adipocyte types, white, brown, and beige, within the context of obesity and T2D. Specifically, in white adipocytes, these dysfunctions contribute to impaired lipid processing and an increased burden of oxidative stress, worsening metabolic disturbances. Conversely, compromised mitochondrial function undermines their thermogenic capabilities, reducing the capacity for optimal energy expenditure in brown adipocytes. Beige adipocytes uniquely combine the functional properties of white and brown adipocytes, maintaining morphological similarities to white adipocytes while possessing the capability to transform into mitochondria-rich, energy-burning cells under appropriate stimuli. Each type of adipocyte displays unique metabolic characteristics, governed by the mitochondrial dynamics specific to each cell type. These distinct mitochondrial metabolic phenotypes are regulated by specialized networks comprising transcription factors, co-activators, and enzymes, which together ensure the precise control of cellular energy processes. Strong evidence has shown impaired adipocyte mitochondrial metabolism and faulty upstream regulators in a causal relationship with obesity-induced T2D. Targeted interventions aimed at improving mitochondrial function in adipocytes offer a promising therapeutic avenue for enhancing systemic macronutrient oxidation, thereby potentially mitigating obesity. Advances in understanding mitochondrial function within adipocytes underscore a pivotal shift in approach to combating obesity and associated comorbidities. Reigniting the burning of calories in adipose tissues, and other important metabolic organs such as the muscle and liver, is crucial given the extensive role of adipose tissue in energy storage and release.

Adipocyte Progenitor Pools Composition and Cellular Niches Affect Adipogenesis Divergence in Porcine Subcutaneous and Intramuscular Fat

Intramuscular fat (IMF) contributed positively to pork quality, whereas subcutaneous fat (SCF) was often considered to be a detrimental factor impacting growth and carcass traits. Reducing SCF while maintaining optimal IMF levels requires a thorough understanding of the adipogenic differences between these two adipose depots. Our study explored the differences in adipogenesis between porcine IMF and SCF, and the results showed that subcutaneous adipocytes (SCAs) demonstrate a greater potential for adipogenic differentiation, both in vivo and in vitro. Lipidomic and transcriptomic analyses suggested that intramuscular adipocytes (IMAs) are more inclined to biosynthesize unsaturated fatty acids. Furthermore, single-cell RNA sequencing (scRNA-seq) was employed to dissect the intrinsic and microenvironmental discrepancies in adipogenesis between porcine IMF and SCF. Comparative analysis indicated that SCF was enriched with preadipocytes, exhibiting an enhanced adipogenic potential, while IMF was characterized by a higher abundance of stem cells. Furthermore, coculture analyses of porcine intramuscular adipogenic cells and myogenetic cells indicated that the niche of IMAs inhibited its adipogenic differentiation. Cell communication analysis identified 160 ligand-receptor pairs and channels between adipogenic and myogenetic cells in IMF. Collectively, our study elucidated two intrinsic and microenvironmental novel mechanisms underpinning the divergence in adipogenesis between porcine SCF and IMF.

Effects of COL1A1 and SYTL2 on inflammatory cell infiltration and poor extracellular matrix remodeling of the vascular wall in thoracic aortic aneurysm

BACKGROUND

Thoracic aortic aneurysm (TAA) is a fatal cardiovascular disease, the pathogenesis of which has not yet been clarified. This study aimed to identify and validate the diagnostic markers of TAA to provide a strong theoretical basis for developing new methods to prevent and treat this disease.

METHODS

Gene expression profiles of the GSE9106, GSE26155, and GSE155468 datasets were acquired from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were identified using the "limma" package in R. Least absolute shrinkage and selection operator (LASSO), support vector machine-recursive feature elimination (SVM-RFE), random forest, and binary logistic regression analyses were used to screen the diagnostic marker genes. Single-sample gene set enrichment analysis (ssGSEA) was used to estimate immune cell infiltration in TAA.

RESULTS

A total of 16 DEGs were identified. The enrichment and functional correlation analyses showed that DEGs were mainly associated with inflammatory response pathways and collagen-related diseases. Collagen type I alpha 1 chain ( COL1A1 ) and synaptotagmin like 2 ( SYTL2 ) were identified as diagnostic marker genes with a high diagnostic value for TAA. The expression of COL1A1 and SYTL2 was considerably higher in TAA vascular wall tissues than in the corresponding normal tissues, and there were significant differences in the infiltration of immune cells between TAA and normal vascular wall tissues. Additionally, COL1A1 and SYTL2 expression were associated with the infiltration of immune cells in the vascular wall tissue. Single-cell analysis showed that COL1A1 in TAA was mainly derived from fibroblasts and SYTL2 mainly from cluster of differentiation (CD)8 + T cells. In addition, single-cell analysis indicated that fibroblasts and CD8 + T cells in TAA were significantly higher than those in normal arterial wall tissue.

CONCLUSIONS

COL1A1 and SYTL2 may serve as diagnostic marker genes for TAA. The upregulation of SYTL2 and COL1A1 may be involved in the inflammatory infiltration of the vessel wall and poor extracellular matrix remodeling, promoting the progression of TAA.

Extracellular matrix hyaluronan modulates fat cell differentiation and primary cilia dynamics

Hyaluronan is an important extracellular matrix component, with poorly documented physiological role in the context of lipid-rich adipose tissue. We have investigated the global impact of hyaluronan removal from adipose tissue environment by in vitro exposure to exogenous hyaluronidase (or heat inactivated enzyme). Gene set expression analysis from RNA sequencing revealed downregulated adipogenesis as a main response to hyaluronan removal from human adipose tissue samples, which was confirmed by hyaluronidase-mediated inhibition of adipocyte differentiation in the 3T3L1 adipose cell line. Hyaluronidase exposure starting from the time of induction with the differentiation cocktail reduced lipid accumulation in mature adipocytes, limited the expression of terminal differentiation marker genes, and impaired the early induction of co-regulated Cebpa and Pparg mRNA. Reduction of Cebpa and Pparg expression by exogenous hyaluronidase was also observed in cultured primary preadipocytes from subcutaneous, visceral or brown adipose tissue of mice. Mechanistically, inhibition of adipogenesis by hyaluronan removal was not caused by changes in osmotic pressure or cell inflammatory status, could not be mimicked by exposure to threose, a metabolite generated by hyaluronan degradation, and was not linked to alteration in endogenous Wnt ligands expression. Rather, we observed that hyaluronan removal associated with disrupted primary cilia dynamics, with elongated cilium and higher proportions of preadipocytes that remained ciliated in hyaluronidase-treated conditions. Thus, our study points to a new link between ciliogenesis and hyaluronan impacting adipose tissue development.

TM4SF19-mediated control of lysosomal activity in macrophages contributes to obesity-induced inflammation and metabolic dysfunction

Adipose tissue (AT) adapts to overnutrition in a complex process, wherein specialized immune cells remove and replace dysfunctional and stressed adipocytes with new fat cells. Among immune cells recruited to AT, lipid-associated macrophages (LAMs) have emerged as key players in obesity and in diseases involving lipid stress and inflammation. Here, we show that LAMs selectively express transmembrane 4 L six family member 19 (TM4SF19), a lysosomal protein that represses acidification through its interaction with Vacuolar-ATPase. Inactivation of TM4SF19 elevates lysosomal acidification and accelerates the clearance of dying/dead adipocytes in vitro and in vivo. TM4SF19 deletion reduces the LAM accumulation and increases the proportion of restorative macrophages in AT of male mice fed a high-fat diet. Importantly, male mice lacking TM4SF19 adapt to high-fat feeding through adipocyte hyperplasia, rather than hypertrophy. This adaptation significantly improves local and systemic insulin sensitivity, and energy expenditure, offering a potential avenue to combat obesity-related metabolic dysfunction.

Caloric restriction for the immunometabolic control of human health

Nutrition affects all physiological processes occurring in our body, including those related to the function of the immune system; indeed, metabolism has been closely associated with the differentiation and activity of both innate and adaptive immune cells. While excessive energy intake and adiposity have been demonstrated to cause systemic inflammation, several clinical and experimental evidence show that calorie restriction (CR), not leading to malnutrition, is able to delay aging and exert potent anti-inflammatory effects in different pathological conditions. This review provides an overview of the ability of different CR-related nutritional strategies to control autoimmune, cardiovascular, and infectious diseases, as tested by preclinical studies and human clinical trials, with a specific focus on the immunological aspects of these interventions. In particular, we recapitulate the state of the art on the cellular and molecular mechanisms pertaining to immune cell metabolic rewiring, regulatory T cell expansion, and gut microbiota composition, which possibly underline the beneficial effects of CR. Although studies are still needed to fully evaluate the feasibility and efficacy of the nutritional intervention in clinical practice, the experimental observations discussed here suggest a relevant role of CR in lowering the inflammatory state in a plethora of different pathologies, thus representing a promising therapeutic strategy for the control of human health.

Glucocorticoids increase adiposity by stimulating Krüppel-like factor 9 expression in macrophages

The mechanisms underlying glucocorticoid (GC)-induced obesity are poorly understood. Macrophages are the primary targets by which GCs exert pharmacological effects and perform critical functions in adipose tissue homeostasis. Here, we show that macrophages are essential for GC-induced obesity. Dexamethasone (Dex) strongly induced Krüppel-like factor 9 (Klf9) expression in macrophages. Similar to Dex, lentivirus-mediated Klf9 overexpression inhibits M1 and M2a markers expression, causing macrophage deactivation. Furthermore, the myeloid-specific Klf9 transgene promotes obesity. Conversely, myeloid-specific Klf9-knockout (mKlf9KO) mice are lean. Moreover, myeloid Klf9 knockout largely blocks obesity induced by chronic GC treatment. Mechanistically, GC-inducible KLF9 recruits the SIN3A/HDAC complex to the promoter regions of Il6, Ptgs2, Il10, Arg1, and Chil3 to inhibit their expression, subsequently reducing thermogenesis and increasing lipid accumulation by inhibiting STAT3 signaling in adipocytes. Thus, KLF9 in macrophages integrates the beneficial anti-inflammatory and adverse metabolic effects of GCs and represents a potential target for therapeutic interventions.

Macrophage and T cell networks in adipose tissue

The signals and structure of the tissues in which leukocytes reside critically mould leukocyte function and development and have challenged our fundamental understanding of how to define and categorize tissue-resident immune cells. One specialized tissue niche that has a powerful effect on immune cell function is adipose tissue. The field of adipose tissue leukocyte biology has expanded dramatically and has revealed how tissue niches can shape immune cell function and reshape them in a setting of metabolic stress, such as obesity. Most notably, adipose tissue macrophages and T cells are under intense investigation due to their contributions to adipose tissue in the lean and obese states. Both adipose tissue macrophages and T cells have features associated with the metabolic function of adipose tissue that are distinct from features of macrophages and T cells that are classically characterized in other tissues. This Review provides state-of-the-art understanding of adipose tissue macrophages and T cells and discusses how their unique niche can help us to better understand diversity in leukocyte responses.

Unique adipose tissue invariant natural killer T cell subpopulations control adipocyte turnover in mice

Adipose tissue invariant natural killer T (iNKT) cells are a crucial cell type for adipose tissue homeostasis in obese animals. However, heterogeneity of adipose iNKT cells and their function in adipocyte turnover are not thoroughly understood. Here, we investigate transcriptional heterogeneity in adipose iNKT cells and their hierarchy using single-cell RNA sequencing in lean and obese mice. We report that distinct subpopulations of adipose iNKT cells modulate adipose tissue homeostasis through adipocyte death and birth. We identify KLRG1+ iNKT cells as a unique iNKT cell subpopulation in adipose tissue. Adoptive transfer experiments showed that KLRG1+ iNKT cells are selectively generated within adipose tissue microenvironment and differentiate into a CX3CR1+ cytotoxic subpopulation in obese mice. In addition, CX3CR1+ iNKT cells specifically kill enlarged and inflamed adipocytes and recruit macrophages through CCL5. Furthermore, adipose iNKT17 cells have the potential to secrete AREG, and AREG is involved in stimulating adipose stem cell proliferation. Collectively, our data suggest that each adipose iNKT cell subpopulation plays key roles in the control of adipocyte turnover via interaction with adipocytes, adipose stem cells, and macrophages in adipose tissue.

Adipocytes in the Uterine Wall during Experimental Healing and in Cesarean Scars during Pregnancy

We have suggested that adipocytes in uterine scars may affect the development of the placenta accrete spectrum (PAS). In the experimental part, we explored adipocytes in the uterine wall by the twelfth sexual cycle after surgery. In the clinical part, we investigated adipocyte clusters in the cesarean scar of pregnant women with and without PAS. The uterine wall was evaluated in gross and histological sections using morphometry, histochemistry (hematoxylin and eosin stain, Mallory stain), and immunohistochemistry for FABP4 (adipocyte markers), CD68, CD163, CD206 (macrophages), CD 34 (endothelium), cytokeratin 8 (epithelium), aSMA (smooth muscle cells). The design included an experimental study on Sprague-Dawley rats (n = 18) after a full-thickness surgical incision on the seventh (n = 6), 30th (n = 6), and 60th day (n = 6). The clinical groups include pregnant women without uterine scars (n = 10), pregnant women with a uterine scar after previous cesarean sections (n = 10), and women with PAS (n = 11). Statistical processing was carried out using nonparametric methods. Comparisons were conducted using the Mann-Whitney U-test and Kruskal-Wallis test. Statistical significance was considered at p < 0.05. On the seventh day, the rat uterine horn was enveloped by adipose tissue, which contained crown-like structures with FABP4+, CD68+, CD206+, and CD163+ cells. FABP4+ cells in the uterine wall were absent by the 30th day. The number of CD206+ and CD163+ cells in the adipose tissue decreased by the 30th day. On the 60th day, the attachment of fat tissue was revealed in the form of single strands. The serous layer around the damaged area totally recovered on the 60th day. FABP4+ cells were not detected in the uterine wall samples from pregnant women without a previous cesarean section. Adipocytes were found in the scar during non-complicated pregnancy and with PAS. Reducing the number of CD68+ cells in adipocyte clusters, there were in myometrium with PAS. Increased CD206+ and CD163+ cells were revealed in uterine adipocyte clusters of the group. According to the experimental finding, adipocytes should be absent in the uterine wall by the 12th sexual cycle after a full-thickness surgical incision. The presence of adipocyte clusters in cesarean scar indicated the disturbance of cell interaction. Differences in the numbers of CD206 and CD163 cells in adipocyte clusters between groups with and without PAS may be indirect evidence that uterine adipocytes affect the development of PAS.

Fat and inflammation: adipocyte-myeloid cell crosstalk in atherosclerosis

Adipose tissue inflammation has been implicated in various chronic inflammatory diseases and cancer. Perivascular adipose tissue (PVAT) surrounds the aorta as an extra layer and was suggested to contribute to atherosclerosis development. PVAT regulates the function of endothelial and vascular smooth muscle cells in the aorta and represent a reservoir for various immune cells which may participate in aortic inflammation. Recent studies demonstrate that adipocytes also express various cytokine receptors and, therefore, may directly respond to inflammatory stimuli. Here we will summarize current knowledge on immune mechanisms regulating adipocyte activation and the crosstalk between myeloid cells and adipocytes in pathogenesis of atherosclerosis.

Obesity and Fibrosis: Setting the Stage for Breast Cancer

Obesity is a rising health concern and is linked to a worsened breast cancer prognosis. Tumor desmoplasia, which is characterized by elevated numbers of cancer-associated fibroblasts and the deposition of fibrillar collagens within the stroma, may contribute to the aggressive clinical behavior of breast cancer in obesity. A major component of the breast is adipose tissue, and fibrotic changes in adipose tissue due to obesity may contribute to breast cancer development and the biology of the resulting tumors. Adipose tissue fibrosis is a consequence of obesity that has multiple sources. Adipocytes and adipose-derived stromal cells secrete extracellular matrix composed of collagen family members and matricellular proteins that are altered by obesity. Adipose tissue also becomes a site of chronic, macrophage-driven inflammation. Macrophages exist as a diverse population within obese adipose tissue and mediate the development of fibrosis through the secretion of growth factors and matricellular proteins and interactions with other stromal cells. While weight loss is recommended to resolve obesity, the long-term effects of weight loss on adipose tissue fibrosis and inflammation within breast tissue are less clear. Increased fibrosis within breast tissue may increase the risk for tumor development as well as promote characteristics associated with tumor aggressiveness.

Oleic acid abomasal infusion limits lipolysis and improves insulin sensitivity in adipose tissue from periparturient dairy cows

Excessive adipose tissue (AT) lipolysis around parturition in dairy cows is associated with impaired AT insulin sensitivity and increased incidence of metabolic diseases. Supplementing cows with oleic acid (OA) reduces circulating biomarkers of lipolysis and improves energy balance. Nevertheless, it is unclear if OA alters lipid trafficking in AT. In the liver and skeletal muscle, OA improves mitochondrial function and promotes lipid droplet formation by activating perilipin 5 (PLIN5) and peroxisome proliferator-activated receptor α (PPARα). However, it is unknown if this mechanism occurs in AT. The objective of this study was to determine the effect of OA on AT lipolysis, systemic and AT insulin sensitivity, and AT mitochondrial function in periparturient dairy cows. Twelve rumen-cannulated Holstein cows were infused abomasally following parturition with ethanol (CON) or OA (60 g/d) for 14 d. Subcutaneous AT samples were collected at 11 ± 3.6 d before calving (-12 d), and 6 ± 1.0 d (7 d) and 13 ± 1.4 d (14 d) after parturition. An intravenous glucose tolerance test was performed on d 14. Adipocyte morphometry was performed on hematoxylin and eosin-stained AT sections. The antilipolytic effect of insulin (1 μg/L) was evaluated using an ex vivo explant culture following lipolysis stimulation. PLIN5 and PPARα transcription and translation were determined by real-time quantitative PCR and capillary electrophoresis, respectively. RNA sequencing was used to evaluate the transcriptomic profile of mitochondrial gene networks. In CON cows, postpartum lipolysis increased the percentage of smaller (<3,000 µm²) adipocytes at 14 d compared with -12 d. However, OA limited adipocyte size reduction at 14 d. Likewise, OA decreased lipolysis plasma markers nonesterified free fatty acids and β-hydroxybutyrate at 5 and 7 d. Over the 14-d period, compared with CON, OA increased the concentration of plasma insulin and decreased plasma glucose. During the glucose tolerance test, OA decreased circulating glucose concentration (at 10, 20, 30, 40 min) and the glucose clearance rate. Moreover, OA increased insulin at 10 and 20 min and tended to increase it at 30 min. Following lipolysis stimulation, OA improved the antilipolytic effect of insulin in the AT at 14 d. PLIN5 and PPARA gene expression decreased postpartum regardless of treatment. However, OA increased PLIN5 protein expression at 14 d and increased PPARA at 7 and 14 d. Immunohistochemical analysis of AT and RNA sequencing data showed that OA increased the number of mitochondria and improved mitochondrial function. However, OA had no effect on production and digestibility. Our results demonstrate that OA limits AT lipolysis, improves systemic and AT insulin sensitivity, and is associated with markers of mitochondrial function supporting a shift to lipogenesis in AT of periparturient dairy cows.

Adipose tissue macrophages as potential targets for obesity and metabolic diseases

Macrophage infiltration into adipose tissue is a key pathological factor inducing adipose tissue dysfunction and contributing to obesity-induced inflammation and metabolic disorders. In this review, we aim to present the most recent research on macrophage heterogeneity in adipose tissue, with a focus on the molecular targets applied to macrophages as potential therapeutics for metabolic diseases. We begin by discussing the recruitment of macrophages and their roles in adipose tissue. While resident adipose tissue macrophages display an anti-inflammatory phenotype and promote the development of metabolically favorable beige adipose tissue, an increase in pro-inflammatory macrophages in adipose tissue has negative effects on adipose tissue function, including inhibition of adipogenesis, promotion of inflammation, insulin resistance, and fibrosis. Then, we presented the identities of the newly discovered adipose tissue macrophage subtypes (e.g. metabolically activated macrophages, CD9+ macrophages, lipid-associated macrophages, DARC+ macrophages, and MFehi macrophages), the majority of which are located in crown-like structures within adipose tissue during obesity. Finally, we discussed macrophage-targeting strategies to ameliorate obesity-related inflammation and metabolic abnormalities, with a focus on transcriptional factors such as PPARγ, KLF4, NFATc3, and HoxA5, which promote macrophage anti-inflammatory M2 polarization, as well as TLR4/NF-κB-mediated inflammatory pathways that activate pro-inflammatory M1 macrophages. In addition, a number of intracellular metabolic pathways closely associated with glucose metabolism, oxidative stress, nutrient sensing, and circadian clock regulation were examined. Understanding the complexities of macrophage plasticity and functionality may open up new avenues for the development of macrophage-based treatments for obesity and other metabolic diseases.

Metabolic Factors at the Crossroads of Periparturient Immunity and Inflammation

Periparturient cows have the highest risk for disease and culling in the adult dairy herd. This risk is compounded by the multiple physiological changes of metabolism and immune function occurring around calving that alter the cow's inflammatory response. In this article, the authors summarize the current knowledge on immunometabolism in the periparturient cow, discussing major changes in immune and metabolic function around parturition that will facilitate the assessment of periparturient cow management programs.

Adipose tissue macrophages and their role in obesity-associated insulin resistance: an overview of the complex dynamics at play

Obesity, a major global health concern, is characterized by serious imbalance between energy intake and expenditure leading to excess accumulation of fat in adipose tissue (AT). A state of chronic low-grade AT inflammation is prevalent during obesity. The adipose tissue macrophages (ATM) with astounding heterogeneity and complex regulation play a decisive role in mediating obesity-induced insulin resistance. Adipose-derived macrophages were broadly classified as proinflammatory M1 and anti-inflammatory M2 subtypes but recent reports have proclaimed several novel and intermediate profiles, which are crucial in understanding the dynamics of macrophage phenotypes during development of obesity. Lipid-laden hypertrophic adipocytes release various chemotactic signals that aggravate macrophage infiltration into AT skewing toward mostly proinflammatory status. The ratio of M1-like to M2-like macrophages is increased substantially resulting in copious secretion of proinflammatory mediators such as TNFα, IL-6, IL-1β, MCP-1, fetuin-A (FetA), etc. further worsening insulin resistance. Several AT-derived factors could influence ATM content and activation. Apart from being detrimental, ATM exerts beneficial effects during obesity. Recent studies have highlighted the prime role of AT-resident macrophage subpopulations in not only effective clearance of excess fat and dying adipocytes but also in controlling vascular integrity, adipocyte secretions, and fibrosis within obese AT. The role of ATM subpopulations as friend or foe is determined by an intricate interplay of such factors arising within hyperlipidemic microenvironment of obese AT. The present review article highlights some of the key research advances in ATM function and regulation, and appreciates the complex dynamics of ATM in the pathophysiologic scenario of obesity-associated insulin resistance.

Capturing the multifaceted function of adipose tissue macrophages

Adipose tissue macrophages (ATMs) bolster obesity-induced metabolic dysfunction and represent a targetable population to lessen obesity-associated health risks. However, ATMs also facilitate adipose tissue function through multiple actions, including adipocyte clearance, lipid scavenging and metabolism, extracellular remodeling, and supporting angiogenesis and adipogenesis. Thus, high-resolution methods are needed to capture macrophages' dynamic and multifaceted functions in adipose tissue. Herein, we review current knowledge on regulatory networks critical to macrophage plasticity and their multifaceted response in the complex adipose tissue microenvironment.

Differential effects of temperature and mTOR and Wnt-planar cell polarity pathways on syndecan-4 and CD44 expression in growth-selected turkey satellite cell populations

Satellite cells (SCs) comprise a heterogeneous population of muscle stem cells. Thermal stress during the first week after hatch alters proliferation, myogenesis, and adipogenesis of SCs of turkey pectoralis major (p. major) muscle via mechanistic target of rapamycin (mTOR) and wingless-type mouse mammary tumor virus integration site family/planar cell polarity (Wnt/PCP) pathways. Pivotal genes in mTOR and Wnt/PCP pathways are mTOR and frizzled-7 (Fzd7), respectively. The objective of this study was to determine the differential effects of thermal stress on SDC4 and CD44 expression in turkey p. major muscle SCs and how the expression of SDC4 and CD44 is modulated by the mTOR and Wnt/PCP pathways. Satellite cells were isolated from the p. major muscle of 1-week-old faster-growing modern-commercial (NC) turkeys and slower-growing historic Randombred Control Line 2 (RBC2) turkeys, and were challenged with hot (43°C) and cold (33°C) thermal stress for 72 h of proliferation followed by 48 h of differentiation. The NC line SCs were found to contain a lower proportion of SDC4 positive and CD44 negative (SDC4+CD44-) cells and a greater proportion of SDC4 negative and CD44 positive (SDC4-CD44+) cells compared to the RBC2 line at the control temperature (38°C) at both 72 h of proliferation and 48 h of differentiation. In general, at 72 h of proliferation, the proportion of SDC4+CD44- cells decreased with heat stress (43°C) and increased with cold stress (33°C) relative to the control temperature (38°C) in both lines, whereas the proportion of SDC4-CD44+ cells increased with heat stress and decreased with cold stress. In general, the expression of SDC4 and CD44 in the NC SCs showed greater response to both hot and cold thermal stress compared to the RBC2 cells. Knockdown of mTOR or Fzd7 expression increased the proportion of SDC4+CD44- cells while the proportion of SDC4-CD44+ cells decreased during differentiation with line differences being specific to treatment temperatures. Thus, differential composition of p. major muscle SCs in growth-selected commercial turkey may be resulted, in part, from the alteration in SDC4 and CD44 expression. Results indicate differential temperature sensitivity and mTOR and Wnt/PCP pathway responses of growth-selected SC populations and this may have long-lasting effect on muscle development and growth.

Hyaluronan in Adipose Tissue, Metabolic Inflammation, and Diabetes: Innocent Bystander or Guilty Party?

Hyaluronic acid, or hyaluronan (HA), is a nonsulfated glucosaminoglycan that has long been recognized for its hydrophilic properties and is widely used as a dermal filler. Despite much attention given to the study of other extracellular matrix (ECM) components, in the field of ECM properties and their contribution to tissue fibroinflammation, little is known of HA's potential role in the extracellular milieu. However, recent studies suggest that it is involved in inflammatory response, diet-induced insulin resistance, adipogenesis, and autoimmunity in type 1 diabetes. Based on its unique physical property as a regulator of osmotic pressure, we emphasize underestimated implications in adipose tissue function, adipogenesis, and obesity-related dysfunction.

Extracellular Matrix (ECM) and Fibrosis in Adipose Tissue: Overview and Perspectives

Fibrosis in adipose tissue is a major driver of obesity-related metabolic dysregulation. It is characterized by an overaccumulation of extracellular matrix (ECM) during unhealthy expansion of adipose tissue in response to over nutrition. In obese adipose-depots, hypoxia stimulates multiple pro-fibrotic signaling pathways in different cell populations, thereby inducing the overproduction of the ECM components, including collagens, noncollagenous proteins, and additional enzymatic components of ECM synthesis. As a consequence, local fibrosis develops. The result of fibrosis-induced mechanical stress not only triggers cell necrosis and inflammation locally in adipose tissue but also leads to system-wide lipotoxicity and insulin resistance. A better understanding of the mechanisms underlying the obesity-induced fibrosis will help design therapeutic approaches to reduce or reverse the pathological changes associated with obese adipose tissue. Here, we aim to summarize the major advances in the field, which include newly identified fibrotic factors, cell populations that contribute to the fibrosis in adipose tissue, as well as novel mechanisms underlying the development of fibrosis. We further discuss the potential therapeutic strategies to target fibrosis in adipose tissue for the treatment of obesity-linked metabolic diseases and cancer. © 2023 American Physiological Society. Compr Physiol 13:4387-4407, 2023.

The Effect of TGFβ1 in Adipocyte on Inflammatory and Fibrotic Markers at Different Stages of Adipocyte Differentiation

Transforming growth factor beta (TGFβ) is a versatile cytokine. Although a profibrotic role of TGFβ is well established, its effect on tissue inhibitor of metalloproteinase (TIMPs) and inflammatory mediators are incompletely described. This study investigates the profibrotic and pro-inflammatory role of TGFβ1 during adipocyte differentiation. NIH3T3L1 cells were used for the in vitro study and were differentiated by adding a standard differentiation mix either with rosiglitazone (R-Diff) or without (S-Diff). Recombinant TGFβ1 (2 ng/mL) was added to the undifferentiated preadipocyte during the commitment stage and at the terminal differentiation stage. TGFβ1 treatment significantly decreased adiponectin mRNA at both early commitment (>300 fold) and terminal differentiated cells [S-Diff (~33%) or R-Diff (~20%)]. TGFβ1 upregulated collagen VI mRNA and its regulators connective tissue growth factor (CCN2/CTGF), TIMP1 and TIMP3 mRNA levels in undifferentiated preadipocytes and adipocytes at commitment stage. But in the terminal differentiated adipocytes, changes in mRNA and protein of collagen VI and TIMP3 mRNA were not observed despite an increase in CCN2/CTGF, TIMP1 mRNA. Although TGFβ1 upregulated interleukin-6 (IL6) and monocyte chemoattractant protein-1 (MCP1) mRNA at all stages of differentiation, decreased tumor necrosis factor-α (TNFα) mRNA was observed early in adipocyte differentiation. This study highlights the complex role of TGFβ1 on extracellular matrix (ECM) remodeling and inflammatory markers in stimulating both synthetic and inhibitory markers of fibrosis at different stages of adipocyte differentiation.

The sternum reconstruction: Present and future perspectives

Sternectomy is a procedure mainly used for removing tumor masses infiltrating the sternum or treating infections. Moreover, the removal of the sternum involves the additional challenge of performing a functional reconstruction. Fortunately, various approaches have been proposed for improving the operation and outcome of reconstruction, including allograft transplantation, using novel materials, and developing innovative surgical approaches, which promise to enhance the quality of life for the patient. This review will highlight the surgical approaches to sternum reconstruction and the new perspectives in the current literature.

Adipose tissue macrophage in obesity-associated metabolic diseases

Adipose tissue macrophage (ATM) has been appreciated for its critical contribution to obesity-associated metabolic diseases in recent years. Here, we discuss the regulation of ATM on both metabolic homeostatsis and dysfunction. In particular, the macrophage polarization and recruitment as well as the crosstalk between ATM and adipocyte in thermogenesis, obesity, insulin resistance and adipose tissue fibrosis have been reviewed. A better understanding of how ATM regulates adipose tissue remodeling may provide novel therapeutic strategies against obesity and associated metabolic diseases.

Deconstructing cold-induced brown adipocyte neogenesis in mice

Cold exposure triggers neogenesis in classic interscapular brown adipose tissue (iBAT) that involves activation of β1-adrenergic receptors, proliferation of PDGFRA+ adipose tissue stromal cells (ASCs), and recruitment of immune cells whose phenotypes are presently unknown. Single-cell RNA-sequencing (scRNA-seq) in mice identified three ASC subpopulations that occupied distinct tissue locations. Of these, interstitial ASC1 were found to be direct precursors of new brown adipocytes (BAs). Surprisingly, knockout of β1-adrenergic receptors in ASCs did not prevent cold-induced neogenesis, whereas pharmacological activation of the β3-adrenergic receptor on BAs was sufficient, suggesting that signals derived from mature BAs indirectly trigger ASC proliferation and differentiation. In this regard, cold exposure induced the delayed appearance of multiple macrophage and dendritic cell populations whose recruitment strongly correlated with the onset and magnitude of neogenesis across diverse experimental conditions. High-resolution immunofluorescence and single-molecule fluorescence in situ hybridization demonstrated that cold-induced neogenesis involves dynamic interactions between ASC1 and recruited immune cells that occur on the micrometer scale in distinct tissue regions. Our results indicate that neogenesis is not a reflexive response of progenitors to β-adrenergic signaling, but rather is a complex adaptive response to elevated metabolic demand within brown adipocytes.

Role of adipose tissue macrophages in obesity-related disorders

The obesity epidemic has led researchers and clinicians to reconsider the etiology of this disease and precisely decipher its molecular mechanisms. The excessive accumulation of fat by cells, most notably adipocytes, which play a key role in this process, has many repercussions in tissue physiology. Herein, we focus on how macrophages, immune cells well known for their tissue gatekeeping functions, assume fundamental, yet ill-defined, roles in the genesis and development of obesity-related metabolic disorders. We first discuss the determinants of the biology of these cells before introducing the specifics of the adipose tissue environment, while highlighting its heterogeneity. Finally, we detail how obesity transforms both adipose tissue and local macrophage populations. Understanding macrophage diversity and their cross talk with the diverse cell types constituting the adipose tissue environment will allow us to frame the therapeutic potential of adipose tissue macrophages in obesity.

Immune and non-immune functions of adipose tissue leukocytes

Adipose tissue is a complex dynamic organ with whole-body immunometabolic influence. Much of the work into understanding the role of immune cells in adipose tissue has been in the context of obesity. These investigations have also uncovered a range of typical (immune) and non-typical functions exerted by adipose tissue leukocytes. Here we provide an overview of the adipose tissue immune system, including its role as an immune reservoir in the whole-body response to infection and as a site of parasitic and viral infections. We also describe the functional roles of specialized immunological structures found within adipose tissue. However, our main focus is on the recently discovered 'non-immune' functions of adipose tissue immune cells, which include the regulation of adipocyte homeostasis, as well as responses to changing nutrient status and body temperature. In doing so, we outline the therapeutic potential of the adipose tissue immune system in health and disease.

Tissue Transglutaminase Knock-Out Preadipocytes and Beige Cells of Epididymal Fat Origin Possess Decreased Mitochondrial Functions Required for Thermogenesis

Beige adipocytes with thermogenic function are activated during cold exposure in white adipose tissue through the process of browning. These cells, similar to brown adipocytes, dissipate stored chemical energy in the form of heat with the help of uncoupling protein 1 (UCP1). Recently, we have shown that tissue transglutaminase (TG2) knock-out mice have decreased cold tolerance in parallel with lower utilization of their epididymal adipose tissue and reduced browning. To learn more about the thermogenic function of this fat depot, we isolated preadipocytes from the epididymal adipose tissue of wild-type and TG2 knock-out mice and differentiated them in the beige direction. Although differentiation of TG2 knock-out preadipocytes is phenotypically similar to the wild-type cells, the mitochondria of the knock-out beige cells have multiple impairments including an altered electron transport system generating lower electrochemical potential difference, reduced oxygen consumption, lower UCP1 protein content, and a higher portion of fragmented mitochondria. Most of these differences are present in preadipocytes as well, and the differentiation process cannot overcome the functional disadvantages completely. TG2 knock-out beige adipocytes produce more iodothyronine deiodinase 3 (DIO3) which may inactivate thyroid hormones required for the establishment of optimal mitochondrial function. The TG2 knock-out preadipocytes and beige cells are both hypometabolic as compared with the wild-type controls which may also be explained by the lower expression of solute carrier proteins SLC25A45, SLC25A47, and SLC25A42 which transport acylcarnitine, Co-A, and amino acids into the mitochondrial matrix. As a consequence, the mitochondria in TG2 knock-out beige adipocytes probably cannot reach the energy-producing threshold required for normal thermogenic functions, which may contribute to the decreased cold tolerance of TG2 knock-out mice.

The membrane receptor CD44: novel insights into metabolism

CD44, a cell-surface glycoprotein, has long been studied as a cancer molecule due to its essential role in physiological activities in normal cells and pathological activities in cancer cells, such as cell proliferation, adhesion, and migration; angiogenesis; inflammation; and cytoskeleton rearrangement. Yet, recent evidence suggests a role of CD44 in metabolism, especially insulin resistance in obesity and diabetes. In line with the current concept of fibroinflammation in obesity and insulin resistance, CD44 as the main receptor of the extracellular matrix component, hyaluronan (HA), has been shown to regulate diet-induced insulin resistance in muscle and other insulin-sensitive tissues. In this review, we integrate current evidence for a role of CD44 in regulating glucose and lipid homeostasis and speculate about its involvement in the pathogenesis of chronic metabolic diseases, including obesity and diabetes. We summarize the current development of CD44-targeted therapies and discuss its potential for the use in treating metabolic diseases.

Spontaneous Browning of White Adipose Tissue Improves Angiogenesis and Reduces Macrophage Infiltration After Fat Grafting in Mice

Background: Fat grafting is a frequently used technique; however, its survival/ regeneration mechanism is not fully understood. The browning of white adipocytes, a process initiated in response to external stimuli, is the conversion of white to beige adipocytes. The physiologic significance of the browning of adipocytes following transplantation is unclear.

Methods: C57BL/6 mice received 150 mg grafts of inguinal adipose tissue, and then the transplanted fat was harvested and analyzed at different time points to assess the browning process. To verify the role of browning of adipocytes in fat grafting, the recipient mice were allocated to three groups, which were administered CL316243 or SR59230A to stimulate or suppress browning, respectively, or a control group after transplantation.

Results: Browning of the grafts was present in the center of each as early as 7 days post-transplantation. The number of beige cells peaked at day 14 and then decreased gradually until they were almost absent at day 90. The activation of browning resulted in superior angiogenesis, higher expression of the pro-angiogenic molecules vascular endothelial growth factor A (VEGF-A) and fibroblast growth factor 21 (FGF21), fewer macrophages, and ultimately better graft survival (Upregulation, 59.17% ± 6.64% vs. Control, 40.33% ± 4.03%, *p < 0.05), whereas the inhibition of browning led to poor angiogenesis, lower expression of VEGF-A, increased inflammatory macrophages, and poor transplant retention at week 10 (Downregulation, 20.67% ± 3.69% vs. Control, 40.33% ± 4.03%, *p < 0.05).

Conclusion: The browning of WAT following transplantation improves the survival of fat grafts by the promotion of angiogenesis and reducing macrophage.

Impact of microRNA Regulated Macrophage Actions on Adipose Tissue Function in Obesity

Obesity-induced adipose tissue dysfunction is bolstered by chronic, low-grade inflammation and impairs systemic metabolic health. Adipose tissue macrophages (ATMs) perpetuate local inflammation but are crucial to adipose tissue homeostasis, exerting heterogeneous, niche-specific functions. Diversified macrophage actions are shaped through finely regulated factors, including microRNAs, which post-transcriptionally alter macrophage activation. Numerous studies have highlighted microRNAs' importance to immune function and potential as inflammation-modulatory. This review summarizes current knowledge of regulatory networks governed by microRNAs in ATMs in white adipose tissue under obesity stress.

Deconstructing Adipose Tissue Heterogeneity One Cell at a Time

As a central coordinator of physiologic metabolism, adipose tissue has long been appreciated as a highly plastic organ that dynamically responds to environmental cues. Once thought of as a homogenous storage depot, recent advances have enabled deep characterizations of the underlying structure and composition of adipose tissue depots. As the obesity and metabolic disease epidemics continue to accelerate due to modern lifestyles and an aging population, elucidation of the underlying mechanisms that control adipose and systemic homeostasis are of critical importance. Within the past decade, the emergence of deep cell profiling at tissue- and, recently, single-cell level has furthered our understanding of the complex dynamics that contribute to tissue function and their implications in disease development. Although many paradigm-shifting findings may lie ahead, profound advances have been made to forward our understanding of the adipose tissue niche in both health and disease. Now widely accepted as a highly heterogenous organ with major roles in metabolic homeostasis, endocrine signaling, and immune function, the study of adipose tissue dynamics has reached a new frontier. In this review, we will provide a synthesis of the latest advances in adipose tissue biology made possible by the use of single-cell technologies, the impact of epigenetic mechanisms on adipose function, and suggest what next steps will further our understanding of the role that adipose tissue plays in systemic physiology.

Quercetin May Improve Fat Graft Survival by Promoting Fat Browning Peripherally

BACKGROUND

Adipose browning occurs after white fat transfer. But its location and effects on fat graft survival remains controversial. This study was performed to locate the browning of fat grafts, and to explore the effects of quercetin on fat graft browning and fat graft survival.

METHODS

Human fat granules were injected into the subcutaneous layer of 12 nude mice. Control group was injected with fat granules and 10% of normal saline, while quercetin group was injected with fat granules and 10% of quercetin. The graft samples (n = 6 for each group) were obtained in weeks 2, 4, 8 and 12. Weight retention rate of the grafts was calculated. Gene and protein expression of mitochondrial markers (silent information regulator 1, SIRT1; heat shock protein 60, HSP60), browning marker (uncoupling protein 1, UCP1), peroxisome proliferator-activated receptor-γ (PPAR-γ), vascular endothelial growth factor A (VEGF-A) were evaluated. Hematoxylin and eosin staining and anti-UCP1 staining were performed.

RESULTS

Clusters of small multilocular beige adipocytes were observed in the periphery of fat grafts. Compared with control group, quercetin group had a higher weight retention rate, a higher gene/protein expression of SIRT1, HSP60, UCP1, PPAR-γ and VEGF-A, and a higher occurrence of peripheral adipose browning.

CONCLUSIONS

Peripherally located adipose browning occurred after white fat transfer. It can be enhanced by the addition of quercetin through promoting mitochondrial function of fat cells, and may be one of the mechanisms that quercetin improves fat graft survival.

NO LEVEL ASSIGNED

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Expression of the preadipocyte marker ZFP423 is dysregulated between well-differentiated and dedifferentiated liposarcoma

BACKGROUND

Well-differentiated and dedifferentiated liposarcomas are rare soft tissue tumors originating in adipose tissue that share genetic abnormalities but have significantly different metastatic potential. Dedifferentiated liposarcoma (DDLPS) is highly aggressive and has an overall 5-year survival rate of 30% as compared to 90% for well-differentiated liposarcoma (WDLPS). This discrepancy may be connected to their potential to form adipocytes, where WDLPS is adipogenic but DDLPS is adipogenic-impaired. Normal adipogenesis requires Zinc Finger Protein 423 (ZFP423), a transcriptional coregulator of Perixosome Proliferator Activated Receptor gamma (PPARG2) mRNA expression that defines committed preadipocytes. Expression of ZFP423 in preadipocytes is promoted by Seven-In-Absentia Homolog 2 (SIAH2)-mediated degradation of Zinc Finger Protein 521 (ZFP521). This study investigated the potential role of ZFP423, SIAH2 and ZFP521 in the adipogenic potential of WDLPS and DDLPS.

METHODS

Human WDLPS and DDLPS fresh and paraffin-embedded tissues were used to assess the gene and protein expression of proadipogenic regulators. In parallel, normal adipose tissue stromal cells along with WDLPS and DDLPS cell lines were cultured, genetically modified, and induced to undergo adipogenesis in vitro.

RESULTS

Impaired adipogenic potential in DDLPS was associated with reduced ZFP423 protein levels in parallel with reduced PPARG2 expression, potentially involving regulation of ZFP521. SIAH2 protein levels did not define a clear distinction related to adipogenesis in these liposarcomas. However, in primary tumor specimens, SIAH2 mRNA was consistently upregulated in DDLPS compared to WDLPS when assayed by fluorescence in situ hybridization or real-time PCR.

CONCLUSIONS

These data provide novel insights into ZFP423 expression in adipogenic regulation between WDLPS and DDLPS adipocytic tumor development. The data also introduces SIAH2 mRNA levels as a possible molecular marker to distinguish between WDLPS and DDLPS.

A Cycle of Inflammatory Adipocyte Death and Regeneration in Murine Adipose Tissue

Adipose tissue (AT) expands by a combination of two fundamental cellular mechanisms: hypertrophic growth of existing adipocytes or through generation of new adipocytes, also known as hyperplastic growth. Multiple lines of evidence suggest a limited capacity for hyperplastic growth of AT in adulthood and that adipocyte number is relatively stable, even with fluctuations in AT mass. If the adipocyte number is stable in adulthood, despite well-documented birth and death of adipocytes, then this would suggest that birth may be coupled to death in a regenerative cycle. To test this hypothesis, we examined the dynamics of birth of new fat cells in relationship to adipocyte death by using high-fidelity stable isotope tracer methods in C57Bl6 mice. We discovered birth of new adipocytes at higher frequency in histological proximity to dead adipocytes. In diet-induced obesity, adipogenesis surged after an adipocyte death peak beyond 8 weeks of high-fat feeding. Through transcriptional analyses of AT and fractionated adipocytes, we found that the dominant cell death signals were inflammasome related. Proinflammatory signals were particularly evident in hypertrophied adipocytes or with deletion of a constitutive oxygen sensor and inhibitor of hypoxia-inducible factor, Egln1. We leveraged the potential role for the inflammasome in adipocyte death to test the adipocyte death-birth hypothesis, finding that caspase 1 loss of function attenuated adipocyte death and birth in murine visceral AT. These data collectively point to a regenerative cycle of adipocyte death and birth as a driver of adipogenesis in adult murine AT.

The composition, function, and regulation of adipose stem and progenitor cells

White adipose tissue (WAT) is a highly plastic organ that plays a central role in regulating whole-body energy metabolism. Adipose stem and progenitor cells (ASPCs) are essential components of the stromal vascular fraction (SVF) of adipose tissue. They give rise to mature adipocytes and play a critical role in maintaining adipose tissue function. However, the molecular heterogeneity and functional diversity of ASPCs are still poorly understood. Recently, single-cell RNA sequencing (scRNA-seq) analysis has identified distinct subtypes of ASPCs in murine and human adipose tissues, providing new insights into the cellular complexity of ASPCs among multiple fat depots. This review summarizes the current knowledge on ASPC populations, including their markers, functions, and regulatory mechanisms. Targeting one or several of these cell populations may ameliorate metabolic disorders by promoting adaptive hyperplastic adipose growth.