Adipocyte Turnover: Relevance to Human Adipose Tissue Morphology

OBJECTIVE

Adipose tissue may contain few large adipocytes (hypertrophy) or many small adipocytes (hyperplasia). We investigated factors of putative importance for adipose tissue morphology.

RESEARCH DESIGN AND METHODS

Subcutaneous adipocyte size and total fat mass were compared in 764 subjects with BMI 18–60 kg/m². A morphology value was defined as the difference between the measured adipocyte volume and the expected volume given by a curved-line fit for a given body fat mass and was related to insulin values. In 35 subjects, in vivo adipocyte turnover was measured by exploiting incorporation of atmospheric ¹⁴C into DNA.

RESULTS

Occurrence of hyperplasia (negative morphology value) or hypertrophy (positive morphology value) was independent of sex and body weight but correlated with fasting plasma insulin levels and insulin sensitivity, independent of adipocyte volume (β-coefficient = 0.3, P < 0.0001). Total adipocyte number and morphology were negatively related (r = −0.66); i.e., the total adipocyte number was greatest in pronounced hyperplasia and smallest in pronounced hypertrophy. The absolute number of new adipocytes generated each year was 70% lower (P < 0.001) in hypertrophy than in hyperplasia, and individual values for adipocyte generation and morphology were strongly related (r = 0.7, P < 0.001). The relative death rate (∼10% per year) or mean age of adipocytes (∼10 years) was not correlated with morphology.

CONCLUSIONS

Adipose tissue morphology correlates with insulin measures and is linked to the total adipocyte number independently of sex and body fat level. Low generation rates of adipocytes associate with adipose tissue hypertrophy, whereas high generation rates associate with adipose hyperplasia.Adipose tissue expands by increasing the volume of preexisting adipocytes (adipose hypertrophy), by generating new small adipocytes (hyperplasia), or by both. Although the amount and distribution of adipose tissue associate independently with insulin resistance, type 2 diabetes, and other metabolic disorders (1), the size of adipocytes within the adipose tissue is also important (2). Increased adipocyte size correlates with serum insulin concentrations, insulin resistance, and increased risk of developing type 2 diabetes (3–10). Obese subjects with few large adipocytes are more glucose intolerant and hyperinsulinemic than those having the same degree of obesity and many small fat cells (5,7,9–14). Furthermore, adipocyte hypertrophy may impair adipose tissue function by inducing local inflammation, mechanical stress, and altered metabolism (15–17). There is, however, a large interindividual variation in adipocyte size among lean and obese individuals (10,18,19). Lean individuals can have larger adipocytes than obese individuals and the other way around. Hitherto there is no straightforward method to assess adipose morphology. It is not valid to merely adjust fat cell size for BMI by linear regression because the relationship between BMI or fat mass and adipocyte size is curve-linear (10,18,19).The mechanisms responsible for the development of different forms of adipose morphology are unknown; however, adipocyte turnover may be involved. The turnover rate of adipocytes is high at all adult ages and body fat levels (18). Approximately one-tenth of the total fat cell pool is renewed every year by ongoing adipogenesis and adipocyte death.We presently investigated whether adipocyte turnover was involved in the different morphologies of subcutaneous adipose tissue (the body''s dominant fat depot). A method to quantitatively assess adipose morphology was developed. Based on the relationship between adipocyte size and total body fat, the subjects were categorized as having different degrees of either adipose hypertrophy or hyperplasia. Thereafter, we set the different forms of adipose morphology in relation to adipocyte turnover in vivo using previously generated data on the incorporation of atmospheric ¹⁴C into adipocyte DNA (18). Finally, we correlated adipose morphology with fasting plasma insulin and insulin sensitivity in vivo.     

Effect of Halothane on Lipolysis and Lipogenesis in Human Adipose Tissue

Es wurde die Wirkung von Halothan auf den Fettgewebsstoffwechsel untersucht. Die Patienten erhielten 0,5% Halothan während einer Lachgas-Sauerstoff-Suxamethonium-Narkose. Vor und nach der Verarbreichung von Halothan wurden Biopsien von Fettgewebe entnommen.
Die Untersuchung zeigte, daß die Einbauraten von Glukose durch Halothan nicht beeinflußt wurden. Die basale Freisetzung von Glyzerol war signifikant vermindert und der antilipolytische Effekt von Insulin scheint nach Halothan-Verabreichung weniger deutlich zu sein. Der lipolytische Effekt von Noradrenalin unterschied sich nicht signifikant zwischen den beiden Gruppen, aber in Prozenten schien sich die durch Noradrenalin stimulierte Lipolyse nach Halothan zu verändern.     

Microvascular Transplantation of Adipose Tissue and Serum Level of Adipocyte Products

Microvascular transplantation of subcutaneous adipose tissue is an essential step in reconstructive surgery after breast carcinoma. Serum levels of adipose tissue products may serve as indicators for transplant function. This study aimed to determine serum leptin and tumor necrosis factor (TNF)-alpha plasma levels pre-, intra-, and postoperatively in 20 patients undergoing reconstructive breast surgery and in 7 women undergoing abdominoplasty operation. In the patients undergoing reconstructive breast surgery, the serum leptin levels decreased intraoperatively from 14.5 +/- 13.1 to 9.1 +/- 7.3 ng/ml, a decrease of 63%. An increase in serum leptin levels to 13.5 +/- 12.7 ng/ml (93% of the initial value) was found on postoperative day 1. This was paralleled by similar changes in the plasma levels of TNF-alpha (preoperatively, 20 +/- 7.3 pg/ml; intraoperatively, 17 +/- 11.4 pg/ml; postoperatively, 21 +/- 10.8 pg/ml). In the patients undergoing abdominoplasty, plasma leptin and TNF-alpha levels decreased intraoperatively (20% and 27%, respectively) and postoperatively (44% and 27%, respectively). The results of our pilot study indicate that a postoperative increase in the level of serum leptin after reconstructive breast surgery may be related to successful transplant function.     

Noncanonical Wnt Signaling Promotes Obesity-Induced Adipose Tissue Inflammation and Metabolic Dysfunction Independent of Adipose Tissue Expansion

Adipose tissue dysfunction plays a pivotal role in the development of insulin resistance in obese individuals. Cell culture studies and gain-of-function mouse models suggest that canonical Wnt proteins modulate adipose tissue expansion. However, no genetic evidence supports a role for endogenous Wnt proteins in adipose tissue dysfunction, and the role of noncanonical Wnt signaling remains largely unexplored. Here we provide evidence from human, mouse, and cell culture studies showing that Wnt5a-mediated, noncanonical Wnt signaling contributes to obesity-associated metabolic dysfunction by increasing adipose tissue inflammation. Wnt5a expression is significantly upregulated in human visceral fat compared with subcutaneous fat in obese individuals. In obese mice, Wnt5a ablation ameliorates insulin resistance, in parallel with reductions in adipose tissue inflammation. Conversely, Wnt5a overexpression in myeloid cells augments adipose tissue inflammation and leads to greater impairments in glucose homeostasis. Wnt5a ablation or overexpression did not affect fat mass or adipocyte size. Mechanistically, Wnt5a promotes the expression of proinflammatory cytokines by macrophages in a Jun NH₂-terminal kinase–dependent manner, leading to defective insulin signaling in adipocytes. Exogenous interleukin-6 administration restores insulin resistance in obese Wnt5a-deficient mice, suggesting a central role for this cytokine in Wnt5a-mediated metabolic dysfunction. Taken together, these results demonstrate that noncanonical Wnt signaling contributes to obesity-induced insulin resistance independent of adipose tissue expansion.     

Leucine Deprivation Decreases Fat Mass by Stimulation of Lipolysis in White Adipose Tissue and Upregulation of Uncoupling Protein 1 (UCP1) in Brown Adipose Tissue

OBJECTIVE

White adipose tissue (WAT) and brown adipose tissue (BAT) play distinct roles in adaptation to changes in nutrient availability, with WAT serving as an energy store and BAT regulating thermogenesis. We previously showed that mice maintained on a leucine-deficient diet unexpectedly experienced a dramatic reduction in abdominal fat mass. The cellular mechanisms responsible for this loss, however, are unclear. The goal of current study is to investigate possible mechanisms.

RESEARCH DESIGN AND METHODS

Male C57BL/6J mice were fed either control, leucine-deficient, or pair-fed diets for 7 days. Changes in metabolic parameters and expression of genes and proteins related to lipid metabolism were analyzed in WAT and BAT.

RESULTS

We found that leucine deprivation for 7 days increases oxygen consumption, suggesting increased energy expenditure. We also observed increases in lipolysis and expression of β-oxidation genes and decreases in expression of lipogenic genes and activity of fatty acid synthase in WAT, consistent with increased use and decreased synthesis of fatty acids, respectively. Furthermore, we observed that leucine deprivation increases expression of uncoupling protein (UCP)-1 in BAT, suggesting increased thermogenesis.

CONCLUSIONS

We show for the first time that elimination of dietary leucine produces significant metabolic changes in WAT and BAT. The effect of leucine deprivation on UCP1 expression is a novel and unexpected observation and suggests that the observed increase in energy expenditure may reflect an increase in thermogenesis in BAT. Further investigation will be required to determine the relative contribution of UCP1 upregulation and thermogenesis in BAT to leucine deprivation-stimulated fat loss.Obesity develops from an imbalance between calorie intake and energy expenditure (1). Excess calories are stored in the white adipose tissue (WAT) as triglyceride (TG), which are mobilized in response to increased energy demands (2). Various strategies have been proposed to treat obesity by promoting fat mobilization and/or increasing energy expenditure (3–5).Recently, there has been a growing interest in controlling body weight by manipulating macronutrients (6–8). Recent studies have shown that dietary manipulation of essential amino acids, including leucine, arginine, and glutamine, have significant effects on lipid metabolism and glucose utilization (9–14). Most of these studies, however, have focused on the effects of increased levels of essential amino acids in the diet (4,14–18). For example, Zhang et al. (15) recently demonstrated that doubling intake of dietary leucine decreases body weight and improves glucose metabolism in mice maintained on a high-fat diet. The effect of increasing dietary leucine, however, is controversial. Additional studies have shown that dietary supplementation of leucine has no effect on lipid metabolism (16).By contrast, our research has focused on the effect of eliminating leucine from the diet on lipid metabolism. As we recently reported, mice maintained on a leucine-deficient diet for 7 days experienced a dramatic reduction in abdominal fat mass (9). The cellular mechanisms responsible for this loss, however, are unclear. The goal of our current research is to elucidate the molecular and cellular mechanisms underlying the rapid abdominal fat loss induced by leucine deprivation.In our current study, we observed increases in lipolysis and expression of β-oxidation genes and decreases in expression of lipogenic genes and activity of fatty acid synthase (FAS) in WAT, consistent with increased use and decreased synthesis of fatty acids, respectively. In addition, we observed for the first time that leucine deprivation increases expression of uncoupling protein (UCP)-1 in brown adipose tissue (BAT), suggesting increased thermogenesis. We hypothesize that these changes in WAT and BAT account for the significant loss of abdominal fat mass under leucine deprivation.     

Adipose Tissue Free Fatty Acid Storage In Vivo: Effects of Insulin Versus Niacin as a Control for Suppression of Lipolysis

Insulin stimulates the translocation fatty acid transport protein 1 (FATP1) to plasma membrane, and thus greater free fatty acid (FFA) uptake, in adipocyte cell models. Whether insulin stimulates greater FFA clearance into adipose tissue in vivo is unknown. We tested this hypothesis by comparing direct FFA storage in subcutaneous adipose tissue during insulin versus niacin-medicated suppression of lipolysis. We measured direct FFA storage in abdominal and femoral subcutaneous fat in 10 and 11 adults, respectively, during euglycemic hyperinsulinemia or after oral niacin to suppress FFA compared with 11 saline control experiments. Direct palmitate storage was assessed using a [U-¹³C]palmitate infusion to measure palmitate kinetics and an intravenous palmitate radiotracer bolus/timed biopsy. Plasma palmitate concentrations and flux were suppressed to 23 ± 3 and 26 ± 5 µmol ⋅ L⁻¹ (P = 0.91) and 44 ± 4 and 39 ± 5 µmol ⋅ min⁻¹ (P = 0.41) in the insulin and niacin groups, respectively, much less (P < 0.001) than the saline control group (102 ± 8 and 104 ± 12 µmol ⋅ min⁻¹, respectively). In the insulin, niacin, and saline groups, abdominal palmitate storage rates were 0.25 ± 0.05 vs. 0.25 ± 0.07 vs. 0.32 ± 0.05 µmol ⋅ kg adipose lipid⁻¹ ⋅ min⁻¹, respectively (P = NS), and femoral adipose storage rates were 0.19 ± 0.06 vs. 0.20 ± 0.05 vs. 0.31 ± 0.05 µmol ⋅ kg adipose lipid⁻¹ ⋅ min⁻¹, respectively (P = NS). In conclusion, insulin does not increase FFA storage in adipose tissue compared with niacin, which suppresses lipolysis via a different pathway.     

Bone Marrow Adipose Tissue and Skeletal Health

Purpose of Review

To summarize and discuss recent progress and novel signaling mechanisms relevant to bone marrow adipocyte formation and its physiological/pathophysiological implications for bone remodeling.

Recent Findings

Skeletal remodeling is a coordinated process entailing removal of old bone and formation of new bone. Several bone loss disorders such as osteoporosis are commonly associated with increased bone marrow adipose tissue. Experimental and clinical evidence supports that a reduction in osteoblastogenesis from mesenchymal stem cells at the expense of adipogenesis, as well as the deleterious effects of adipocyte-derived signaling, contributes to the etiology of osteoporosis as well as bone loss associated with aging, diabetes mellitus, post-menopause, and chronic drug therapy. However, this view is challenged by findings indicating that, in some contexts, bone marrow adipose tissue may have a beneficial impact on skeletal health.

Summary

Further research is needed to better define the role of marrow adipocytes in bone physiology/pathophysiology and to determine the therapeutic potential of manipulating mesenchymal stem cell differentiation.

     

Obesity Alters Adipose Tissue Macrophage Iron Content and Tissue Iron Distribution

Adipose tissue (AT) expansion is accompanied by the infiltration and accumulation of AT macrophages (ATMs), as well as a shift in ATM polarization. Several studies have implicated recruited M1 ATMs in the metabolic consequences of obesity; however, little is known regarding the role of alternatively activated resident M2 ATMs in AT homeostasis or how their function is altered in obesity. Herein, we report the discovery of a population of alternatively activated ATMs with elevated cellular iron content and an iron-recycling gene expression profile. These iron-rich ATMs are referred to as MFe^hi, and the remaining ATMs are referred to as MFe^lo. In lean mice, ~25% of the ATMs are MFe^hi; this percentage decreases in obesity owing to the recruitment of MFe^lo macrophages. Similar to MFe^lo cells, MFe^hi ATMs undergo an inflammatory shift in obesity. In vivo, obesity reduces the iron content of MFe^hi ATMs and the gene expression of iron importers as well as the iron exporter, ferroportin, suggesting an impaired ability to handle iron. In vitro, exposure of primary peritoneal macrophages to saturated fatty acids also alters iron metabolism gene expression. Finally, the impaired MFe^hi iron handling coincides with adipocyte iron overload in obese mice. In conclusion, in obesity, iron distribution is altered both at the cellular and tissue levels, with AT playing a predominant role in this change. An increased availability of fatty acids during obesity may contribute to the observed changes in MFe^hi ATM phenotype and their reduced capacity to handle iron.     

Adipose Tissue Macrophages Function As Antigen-Presenting Cells and Regulate Adipose Tissue CD4+ T Cells in Mice

The proinflammatory activation of leukocytes in adipose tissue contributes to metabolic disease. How crosstalk between immune cells initiates and sustains adipose tissue inflammation remains an unresolved question. We have examined the hypothesis that adipose tissue macrophages (ATMs) interact with and regulate the function of T cells. Dietary obesity was shown to activate the proliferation of effector memory CD4⁺ T cells in adipose tissue. Our studies further demonstrate that ATMs are functional antigen-presenting cells that promote the proliferation of interferon-γ–producing CD4⁺ T cells in adipose tissue. ATMs from lean and obese visceral fat process and present major histocompatibility complex (MHC) class II–restricted antigens. ATMs were sufficient to promote proliferation and interferon-γ production from antigen-specific CD4⁺ T cells in vitro and in vivo. Diet-induced obesity increased the expression of MHC II and T-cell costimulatory molecules on ATMs in visceral fat, which correlated with an induction of T-cell proliferation in that depot. Collectively, these data indicate that ATMs provide a functional link between the innate and adaptive immune systems within visceral fat in mice.Obesity-induced inflammation contributes to the development of type 2 diabetes, metabolic syndrome, and cardiovascular disease (1–3). Accumulation of activated leukocytes in metabolic tissues is a driving force for obesity-associated metabolic inflammation (metainflammation) and insulin resistance (3,4). In adipose tissue, a vast array of leukocytes have been identified and reported to contribute to obesity-induced metainflammation. How adipose tissue leukocytes interact to shape the inflammatory environment within fat is an important unresolved gap in our current understanding of metabolic disease.In humans and rodent models, F4/80⁺ adipose tissue macrophages (ATMs) are the predominant leukocyte found in metabolically healthy and insulin-resistant fat (5). Resident (type 2) ATMs are distributed between adipocytes in healthy adipose tissue throughout development, express anti-inflammatory markers typical of “alternatively activated” or M2 polarized macrophages, and promote tissue homeostasis (6,7). Disruption of macrophage M2 polarization increases the susceptibility to insulin resistance induced by a high-fat diet (HFD) (8–10). Obesity triggers the accumulation of F4/80⁺ ATMs that coexpress the dendritic cell (DC) marker CD11c as well as genes typically expressed by “classically activated” or proinflammatory M1 polarized macrophages (11–13). M1 ATMs form multicellular lipid-laden clusters, known as crown-like structures (CLS), around dead adipocytes in obese fat (6,14,15) and produce inflammatory cytokines (e.g., interleukin [IL]-1β, IL-6, and tumor necrosis factor-α [TNF-α]) that can impair insulin action in adipocytes (16,17). Current models suggest that obesity promotes metainflammation in part by altering the balance between type 2 and type 1 ATMs in visceral fat (13,18).In addition to ATMs, adipose tissue contains lymphocytes (e.g., natural killer T cells [NKTs], conventional CD4⁺ T cells [Tconvs], regulatory CD4⁺ T cells [Tregs], cytotoxic CD8⁺ T cells, and B cells) that are also regulated by metabolic status (19–24). Treg content in visceral fat is inversely correlated with measures of insulin resistance and inflammation (19,25,26), suggesting that Tregs are anti-inflammatory. In contrast, T helper 1 (Th1) CD4⁺ T cells and CD8⁺ adipose tissue T cells (ATTs) accumulate in fat during obesity, promoting IFN-γ and TNF-α production and insulin resistance (20,21,27). Thus, analogous to ATMs, the imbalance between anti-inflammatory Tregs and proinflammatory CD4⁺/CD8⁺ ATTs contributes to metainflammation.The mechanisms that regulate ATTs in adipose tissue are largely unknown. Spectratyping experiments suggest that CD4⁺ ATTs (but not CD8⁺ ATTs) undergo monoclonal expansion within fat and have an effector-memory (CD44^High CD62L^Low) phenotype (19,21,28). This implies that ATT activation and expansion may be an adaptive immune response to an obesity-induced antigen. T-cell activation depends on an intricate relationship between T cells and antigen-presenting cells (APCs) (29). Classically, APCs (specifically, macrophages and DCs) shape CD4⁺ T-cell activation by three signals: 1) presentation of peptide antigens on major histocompatibility complex (MHC) class II (MHC II) molecules (signal 1), 2) expression of T-cell costimulatory molecules (e.g., CD40, CD80, and CD86) (signal 2), and 3) production of cytokines (e.g., transforming growth factor-β, IL-10, or IL-12) (signal 3). These three signals shape the differentiation of naïve CD4⁺ T cells into effector T-cell subsets (e.g., Th1, Th2, Th17, Treg).The APCs that interact with ATTs in fat have not been well characterized but could include ATMs, adipose tissue DCs, adipose tissue B cells, mast cells, and neutrophils (24,30–34). Quantitative changes in ATTs can precede the accumulation of type 1 CD11c⁺ ATMs in visceral fat in obese mice, suggesting that APCs present in lean and obese fat could trigger an adaptive immune response. Because ATMs are the predominant leukocyte population in lean and obese fat and ATMs from obese mice and humans express MHC II molecules (35–37), we tested the hypothesis that ATMs (CD11b⁺ F4/80⁺) are capable of functioning as APCs to regulate CD4⁺ ATT activation and proliferation. We report that ATMs within visceral fat from mice phagocytose and process antigens for presentation, express costimulatory molecules, and induce antigen-specific CD4⁺ T-cell proliferation in vitro and in situ. Furthermore, we found proliferating ATTs localized with ATMs in fat-associated lymphoid clusters (FALCs) where antigen-specific T-cell activation and proliferation may be initiated. Our data indicate that ATMs meet the functional definition of APCs and suggest that MHC II-restricted antigens presented by ATMs in visceral fat regulate Tregs and Tconvs CD4⁺ ATTs in mice.     

Brown Adipose Tissue and Seasonal Variation in Humans

OBJECTIVE

Brown adipose tissue (BAT) is present in adult humans where it may be important in the prevention of obesity, although the main factors regulating its abundance are not well established. BAT demonstrates seasonal variation relating to ambient temperature and photoperiod in mammals. The objective of our study was therefore to determine whether seasonal variation in BAT activity in humans was more closely related to the prevailing photoperiod or temperature.

RESEARCH DESIGN AND METHODS

We studied 3,614 consecutive patients who underwent positron emission tomography followed by computed tomography scans. The presence and location of BAT depots were documented and correlated with monthly changes in photoperiod and ambient temperature.

RESULTS

BAT activity was demonstrated in 167 (4.6%) scans. BAT was demonstrated in 52/724 scans (7.2%) in winter compared with 27/1,067 (2.5%) in summer months (P < 0.00001, χ² test). Monthly changes in the occurrence of BAT were more closely related to differences in photoperiod (r² = 0.876) rather than ambient temperature (r² = 0.696). Individuals with serial scans also demonstrated strong seasonal variation in BAT activity (average standardized uptake value [SUV_max] 1.5 in July and 9.4 in January). BAT was also more common in female patients (female: n = 107, 7.2%; male: n = 60, 2.8%; P < 0.00001, χ² test).

CONCLUSIONS

Our study demonstrates a very strong seasonal variation in the presence of BAT. This effect is more closely associated with photoperiod than ambient temperature, suggesting a previously undescribed mechanism for mediating BAT function in humans that could now potentially be recruited for the prevention or reversal of obesity.Brown adipose tissue (BAT) was first discovered several hundreds of years ago (1). Extensive research has been conducted in animals to elucidate its function. BAT has been proven to be important in small mammals for nonshivering thermogenesis, particularly following hibernation (2). BAT often coexists with white adipose tissue (WAT) and is structurally very different (3). WAT is unilocular containing a large single vacuole, whereas BAT is multilocular containing large complex mitochondria with a rich vascular supply and is extensively innervated by the sympathetic nervous system. The functions of WAT and BAT differ. WAT acts as a chemical store and an insulator. BAT, however, enables rapid heat production directly from the metabolism of triglycerides, suitable for nonshivering thermogenesis. This function is important in mammals and also helps to counteract the cold stress of birth in newborn mammals including humans (4). BAT is activated by the cold, a function mediated by the sympathetic nervous system (2), and is capable of producing up to 300 times more heat per unit mass compared with all other tissues (5). Uncoupling protein (UCP)-1 is exclusively expressed in brown adipocytes and uncouples oxidative phosphorylation from respiration and the production of ATP, resulting in the production of large amounts of heat (2).BAT is thought to have a protective role against obesity because genetic knockout mice lacking BAT become obese (6). Strong recent evidence for the existence of active BAT in adults has come from positron emission tomography (PET)/computed tomography (CT) imaging, a technique enabling the visualization and anatomical localization of sites of glucose metabolism. Sites of high activity are seen within adipose tissue, particularly in adipose located in the supraclavicular regions but also in paraspinal and suprarenal regions. Three recent studies have demonstrated the presence of UCP-1 in areas corresponding to these areas of activity on PET/CT. Biopsy of the supraclavicular areas of activity confirmed the presence of BAT at this site, providing conclusive evidence for the presence of active BAT in adult humans (7,8,9). Although very likely to represent BAT, the other sites of presumed BAT activity have not been definitively confirmed on biopsy samples.The purpose of our study was to further examine the characteristics of BAT expression in a large cohort of adults undergoing PET/CT at our institution and to determine how this related to time of year. We specifically focused on the impact of photoperiod and ambient temperature because these are two key factors determining BAT function in small mammals (10,11).     

Matrix-Assisted Transplantation of Functional Beige Adipose Tissue

Novel, clinically relevant, approaches to shift energy balance are urgently needed to combat metabolic disorders such as obesity and diabetes. One promising approach has been the expansion of brown adipose tissues that express uncoupling protein (UCP) 1 and thus can uncouple mitochondrial respiration from ATP synthesis. While expansion of UCP1-expressing adipose depots may be achieved in rodents via genetic and pharmacological manipulations or the transplantation of brown fat depots, these methods are difficult to use for human clinical intervention. We present a novel cell scaffold technology optimized to establish functional brown fat–like depots in vivo. We adapted the biophysical properties of hyaluronic acid–based hydrogels to support the differentiation of white adipose tissue–derived multipotent stem cells (ADMSCs) into lipid-accumulating, UCP1-expressing beige adipose tissue. Subcutaneous implantation of ADMSCs within optimized hydrogels resulted in the establishment of distinct UCP1-expressing implants that successfully attracted host vasculature and persisted for several weeks. Importantly, implant recipients demonstrated elevated core body temperature during cold challenges, enhanced respiration rates, improved glucose homeostasis, and reduced weight gain, demonstrating the therapeutic merit of this highly translatable approach. This novel approach is the first truly clinically translatable system to unlock the therapeutic potential of brown fat–like tissue expansion.     

Insulin and Metformin Regulate Circulating and Adipose Tissue Chemerin

OBJECTIVE

To assess chemerin levels and regulation in sera and adipose tissue from women with polycystic ovary syndrome (PCOS) and matched control subjects.

RESEARCH DESIGN AND METHODS

Real-time RT-PCR and Western blotting were used to assess mRNA and protein expression of chemerin. Serum chemerin was measured by enzyme-linked immunosorbent assay. We investigated the in vivo effects of insulin on serum chemerin levels via a prolonged insulin-glucose infusion. Ex vivo effects of insulin, metformin, and steroid hormones on adipose tissue chemerin protein production and secretion into conditioned media were assessed by Western blotting and enzyme-linked immunosorbent assay, respectively.

RESULTS

Serum chemerin, subcutaneous, and omental adipose tissue chemerin were significantly higher in women with PCOS (n = 14; P < 0.05, P < 0.01). Hyperinsulinemic induction in human subjects significantly increased serum chemerin levels (n = 6; P < 0.05, P < 0.01). In adipose tissue explants, insulin significantly increased (n = 6; P < 0.05, P < 0.01) whereas metformin significantly decreased (n = 6; P < 0.05, P < 0.01) chemerin protein production and secretion into conditioned media, respectively. After 6 months of metformin treatment, there was a significant decrease in serum chemerin (n = 21; P < 0.01). Importantly, changes in homeostasis model assessment–insulin resistance were predictive of changes in serum chemerin (P = 0.046).

CONCLUSIONS

Serum and adipose tissue chemerin levels are increased in women with PCOS and are upregulated by insulin. Metformin treatment decreases serum chemerin in these women.Polycystic ovary syndrome (PCOS), a common endocrinopathy affecting 5–10% of women in the reproductive age, is characterized by menstrual dysfunction and hyperandrogenism and is associated with insulin resistance and pancreatic β-cell dysfunction, impaired glucose tolerance (IGT), type 2 diabetes, dyslipidemia, and visceral obesity (1,2). The consequent hyperinsulinemia is more prevalent in lean and obese women with PCOS when compared with age- and weight-matched normal women (3).The metabolic syndrome is associated with excessive accumulation of central body fat. As well as its role in energy storage, adipose tissue produces several hormones and cytokines termed ‘adipokines’ that have widespread effects on carbohydrate and lipid metabolism. They appear to play an important role in the pathogenesis of insulin resistance, diabetes, and atherosclerosis (4). Furthermore, it is apparent that accumulation of visceral adipose tissue poses a greater cardiometabolic risk than subcutaneous adipose tissue (5) as removal of visceral rather than subcutaneous adipose tissue has been shown to improve insulin sensitivity (6). Moreover, differences in gene expression of adipocyte-secreted molecules (adipokines) suggest that there are inherent adipose tissue depot–specific differences in the endocrine function of adipose tissue. In relation to this, we have published data on the increased levels of vaspin in women with PCOS (7); vaspin is a recently described adipokine mainly formed in human visceral adipose tissue that has insulin-sensitizing effects (8).Recently, Bozaoglu et al. (9) reported chemerin as a novel adipokine, circulating levels of which significantly correlated with BMI, circulating triglycerides, and blood pressure, features of the metabolic syndrome. In addition, chemerin or chemerin receptor knockdown impaired differentiation of 3T3-L1 cells and attenuated the expression of adipocyte genes involved in glucose and lipid homeostasis (10).With the aforementioned in mind and the fact that there is no literature with regards to chemerin in human adipose tissue and its regulation, in study 1, we assessed circulating chemerin as well as mRNA expression and protein levels of chemerin in subcutaneous and omental adipose tissue depots in women with PCOS against age, BMI, and waist-to-hip ratio (WHR) in matched control subjects. Furthermore, we studied the in vivo (study 2) and ex vivo effects of insulin on circulating chemerin levels via a prolonged insulin-glucose infusion in humans and primary adipose tissue explant cultures, respectively. In study 3 we studied the effects of metformin therapy, widely used in the treatment of PCOS in women, on circulating chemerin levels in tandem with associated changes to clinical, hormonal, and metabolic parameters in the same cohort of PCOS in women. Additionally, we studied the ex vivo effects of metformin and steroid hormones in human primary adipose tissue explants.     

Synthetic Adipose Tissue Models for Studying Mammary Gland Development and Breast Tissue Engineering

The mammary gland is a dynamic organ that continually changes its architecture and function. Reciprocal interactions between epithelium and adipocyte-containing stroma exert profound effects on all stages of its development, even though the details of these events are not fully understood. To address this issue, enormous potential exists in the utilization of synthetic adipose tissue model systems to uncover the properties and functions of adipocytes in the mammary gland. The first part of this review focuses on mammary adipose tissue (or adipocyte)-related model systems developed in recent years and their utility in investigating adipose-epithelial interactions, mammary gland morphogenesis, development and tumorigenesis. The second part shifts to the field of adipose-based breast tissue engineering, focusing on how these synthetic adipose tissue models are being constructed in vitro or in vivo for regeneration of the mammary gland, and their potentials in adipose tissue engineering also are discussed.