The Adipose Tissue Microenvironment Regulates Depot-Specific Adipogenesis in Obesity

Betaine Supplementation Enhances Lipid Metabolism and Improves Insulin Resistance in Mice Fed a High-Fat Diet

Obesity is a major driver of metabolic diseases such as nonalcoholic fatty liver disease, certain cancers, and insulin resistance. However, there are no effective drugs to treat obesity. Betaine is a nontoxic, chemically stable and naturally occurring molecule. This study shows that dietary betaine supplementation significantly inhibits the white fat production in a high-fat diet (HFD)-induced obese mice. This might be due to betaine preventing the formation of new white fat (WAT), and guiding the original WAT to burn through stimulated mitochondrial biogenesis and promoting browning of WAT. Furthermore, dietary betaine supplementation decreases intramyocellular lipid accumulation in HFD-induced obese mice. Further analysis shows that betaine supplementation reduced intramyocellular lipid accumulation might be associated with increasing polyunsaturated fatty acids (PUFA), fatty acid oxidation, and the inhibition of fatty acid synthesis in muscle. Notably, by performing insulin-tolerance tests (ITTs) and glucose-tolerance tests (GTTs), dietary betaine supplementation could be observed for improvement of obesity and non-obesity induced insulin resistance. Together, these findings could suggest that inhibiting WAT production, intramyocellular lipid accumulation and inflammation, betaine supplementation limits HFD-induced obesity and improves insulin resistance.

An obesity-associated gut microbiome reprograms the intestinal epigenome and leads to altered colonic gene expression

Background

The gut microbiome, a key constituent of the colonic environment, has been implicated as an important modulator of human health. The eukaryotic epigenome is postulated to respond to environmental stimuli through alterations in chromatin features and, ultimately, gene expression. How the host mediates epigenomic responses to gut microbiota is an emerging area of interest. Here, we profile the gut microbiome and chromatin characteristics in colon epithelium from mice fed either an obesogenic or control diet, followed by an analysis of the resultant changes in gene expression.

Results

The obesogenic diet shapes the microbiome prior to the development of obesity, leading to altered bacterial metabolite production which predisposes the host to obesity. This microbiota–diet interaction leads to changes in histone modification at active enhancers that are enriched for binding sites for signal responsive transcription factors. These alterations of histone methylation and acetylation are associated with signaling pathways integral to the development of colon cancer. The transplantation of obesogenic diet-conditioned microbiota into germ free mice, combined with an obesogenic diet, recapitulates the features of the long-term diet regimen. The diet/microbiome-dependent changes are reflected in both the composition of the recipient animals’ microbiome as well as in the set of transcription factor motifs identified at diet-influenced enhancers.

Conclusions

These findings suggest that the gut microbiome, under specific dietary exposures, stimulates a reprogramming of the enhancer landscape in the colon, with downstream effects on transcription factors. These chromatin changes may be associated with those seen during colon cancer development.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1389-1) contains supplementary material, which is available to authorized users.

SnapShot: Niche Determines Adipocyte Character II

The intrinsic cellular and metabolic properties of an adipocyte are shaped by the specific niche in which it resides. The diverse and discrete locations of major and minor rodent adipose depots are depicted in Part I. In Part II, the molecular and functional characteristics of four major types of adipocytes are described. Identified functions of relatively understudied but undoubtedly important depots are also highlighted.

Deciphering adipose tissue heterogeneity

Obesity is an excess accumulation of adipose tissue mass, and, together with its sequelae, in particular type II diabetes and metabolic syndrome, obesity presents a major health crisis. Although obesity is simply caused by increased adipose mass, the heterogeneity of adipose tissue in humans means that the response to increased energy balance is highly complex. Individual subjects with similar phenotypes may respond very differently to the same treatments; therefore, obesity may benefit from a personalized precision medicine approach. The variability in the development of obesity is indeed driven by differences in sex, genetics, and environment, but also by the various types of adipose tissue as well as the different cell types that compose it. By describing the distinct cell populations that reside in different fat depots, we can interpret the complex effect of these various players in the maintenance of whole-body energy homeostasis. To further understand adipose tissue, adipogenic differentiation and the transcriptional program of lipid accumulation must be investigated. As the cell- and depot-specific functions are described, they can be placed in the context of energy excess to understand how the heterogeneity of adipose tissue shapes individual metabolic status and condition.

Immunologic and endocrine functions of adipose tissue: implications for kidney disease

Excess adiposity can induce adverse sequelae in multiple cell types and organ systems. The transition from the lean to the obese state is characterized by fundamental cellular changes at the level of the adipocyte. These changes affect the local microenvironment within the respective adipose tissue but can also affect nonadipose systems. Adipocytes within fat pads respond to chronic nutrient excess through hyperplasia or hypertrophy, which can differentially affect interorgan crosstalk between various adipose depots and other organs. This crosstalk is dependent on the unique ability of the adipocyte to coordinate metabolic adjustments throughout the body and to integrate responses to maintain metabolic homeostasis. These actions occur through the release of free fatty acids and metabolites during times of energy need - a process that is altered in the obese state. In addition, adipocytes release a wide array of signalling molecules, such as sphingolipids, as well as inflammatory and hormonal factors (adipokines) that are critical for interorgan crosstalk. The interactions of adipose tissue with the kidney - referred to as the adipo-renal axis - are important for normal kidney function as well as the response of the kidney to injury. Here, we discuss the mechanistic basis of this interorgan crosstalk, which clearly has great therapeutic potential given the increasing rates of chronic kidney disease secondary to obesity and type 2 diabetes mellitus.

Pu'erh tea extract-mediated protection against hepatosteatosis and insulin resistance in mice with diet-induced obesity is associated with the induction of de novo lipogenesis in visceral adipose tissue

BACKGROUND

White adipose tissue (WAT) is important for the maintenance of metabolic homeostasis, and metabolic syndrome is sometimes associated with WAT dysfunction in humans and animals. WAT reportedly plays a key, beneficial role in the maintenance of glucose and lipid homeostasis during de novo lipogenesis (DNL). Pu'erh tea extract (PTE) can inhibit harmful, ectopic DNL in the liver, thus protecting against hepatosteatosis, in mice with diet-induced obesity. We examined whether PTE could induce DNL in WAT and consequently protect against hepatosteatosis.

METHODS

C57BL/6 male mice were fed a high-fat diet (HFD) with/without PTE for 16 weeks. Systemic insulin sensitivity was determined using HOMA-IR, insulin- and glucose-tolerance tests, and WAT adipogenesis was evaluated by histological analysis. Adipogenesis-, inflammation-, and DNL-related gene expression in visceral AT (VAT) and subcutaneous AT (SAT) was measured using quantitative reverse transcription-PCR. Regression analysis was used to investigate the association between DNL in WAT and systemic insulin resistance or hepatosteatosis.

RESULTS

Pu'erh tea extract significantly reduced the gain of body weight and SAT, but not VAT adiposity, in mice fed the high-fat diet and induced adipogenesis in VAT. The expression of DNL-related genes, including Glut4, encoding an important insulin-regulated glucose transporter (GLUT4), were highly elevated in VAT. Moreover, PTE inhibited VAT inflammation by simultaneously downregulating inflammatory molecules and inducing expression of Gpr120 that encodes an anti-inflammatory and pro-adipogenesis receptor (GPR-120) that recognizes unsaturated long-chain fatty acids, including DNL products. The expression of DNL-related genes in VAT was inversely correlated with hepatosteatosis and systemic insulin resistance.

CONCLUSIONS

Activation of DNL in VAT may explain PTE-mediated alleviation of hepatosteatosis symptoms and systemic insulin resistance.

Gene expression profiles indicate tissue-specific obesity regulation changes and strong obesity relevant tissues

BACKGROUND

With the growing evidence that other tissues, apart from adipose, could have strong relevance to obesity, it is necessary to comprehensively understand the relationship between obesity and other tissues, and to point out the most relevant tissues.

METHODS

There were 549 participants with 20 different tissue types involved in this study. We firstly employed both Spearman's correlation test and WGCNA (weighted correlation network analysis) to identify body mass index (BMI)-related genes. Subsequently, we performed enrichment analyses with obesity genes and pathways to see the different regulation patterns among tissues. In addition, we compared obesity genes identified by genome-wide association studies (GWAS) with BMI-related genes to find the overlapping proportion in each tissue. Finally, we integrated preceding results to identify six strong obesity relevant tissues and indicate three categories to represent different obesity relevant tissues.

RESULTS

Statistical analyses revealed diverse BMI-related genes and tissue-specific enrichment patterns among tissues. Comparison between BMI-related genes and GWAS findings showed tissue-specific expression changes of GWAS genes. Ultimately, six tissues that showed predominant performance in enrichment analyses and significantly embraced GWAS genes were referred to as strong obesity relevant tissues, including adipose, esophagus, nerve, pancreas, pituitary and skin. We also proposed three categories to represent different obesity relevant tissues.

CONCLUSIONS

We performed the first study to investigate the BMI-related gene expression changes across 20 tissues at the same time. With valid data analyses and comparison with GWAS findings, our study provides a holistic view of how different tissues correlate with obesity, and proposes target tissues for obesity pathogenesis investigation.

The genetic underpinnings of body fat distribution

Obesity, defined as a body mass index (BMI) ≥ 30 kg/m², has reached epidemic proportions; people who are overweight (BMI > 25 kg/m²) or obese now comprise more than 25% of the world's population. Obese individuals have a higher risk of comorbidity development including type 2 diabetes, cardiovascular disease, cancer, and fertility complications. Areas covered: The study of monogenic and syndromic forms of obesity have revealed a small number of genes key to metabolic perturbations. Further, obesity and body shape in the general population are highly heritable phenotypes. Study of obesity at the population level, through genome-wide association studies of BMI and waist-to-hip ratio (WHR), have revealed > 150 genomic loci that associate with these traits, and highlight the role of adipose tissue and the central nervous system in obesity-related traits. Studies in animal models and cell lines have helped further elucidate the potential biological mechanisms underlying obesity. In particular, these studies implicate adipogenesis and expansion of adipose tissue as key biological pathways in obesity and weight gain. Expert commentary: Further work, including a focus on integrating genetic and additional genomic data types, as well as modeling obesity-like features in vitro, will be crucial in translating genome-wide association signals to the causal mechanisms driving disease.

Depot-Specific Response of Adipose Tissue to Diet-Induced Inflammation: The Retinoid-Related Orphan Receptor α (RORα) Involved?

OBJECTIVE

Epididymal adipose tissue (EAT), a visceral fat depot, is more closely associated with metabolic dysfunction than inguinal adipose tissue (IAT), a subcutaneous depot. This study evaluated whether the nuclear receptor RORα, which controls inflammatory processes, could be implicated.

METHODS

EAT and IAT were compared in a RORα loss-of-function mouse (sg/sg) and in wild-type (WT) littermates, fed a standard diet (SD) or a Western diet (WD), to evaluate the impact of RORα expression on inflammatory status and on insulin sensitivity (IS) of each fat depot according to the diet.

RESULTS

Sg/sg mice fed the SD exhibited a decreased inflammatory status and a higher IS in their fat depots than WT mice. WD-induced obesity had distinct effects on the two fat depots. In WT mice, EAT exhibited increased inflammation and insulin resistance while IAT showed reduced inflammation and improved IS, together with a depot-specific increase of RORα, and its target gene IκBα, in the stroma vascular fraction (SVF). Conversely, in sg/sg mice, WD increased inflammation and lowered IS of IAT but not of EAT.

CONCLUSIONS

These findings suggest an anti-inflammatory role for RORα in response to WD, which occurs at the level of SVF of IAT, thus possibly contributing to the "healthy" expansion of IAT.

Wnt5a regulates the cell proliferation and adipogenesis via MAPK-independent pathway in early stage of obesity

The early stage of obesity is an important stage in the development of obesity. However, there are few studies which explored the property or changes in obesity at early stage especially involving Wnt5a. The associated gene expression of Wnt5a on cell regeneration and the effect of Wnt5a on rat adipose-derived stem cell (rASC) proliferation and adipogenesis need additional study. Here, we investigated the changes in obesity at early stage and how Wnt5a regulates rASC regeneration, proliferation, and adipogenesis. Our data revealed that obesity at early stage measured by Lee index presented a state with impaired adipogenesis and more infiltrated inflammatory cells but without significant changes in adipocyte sizes and inflammatory factors. The process might be associated with anti-canonical Wnt pathway and a reciprocal Wnt5a/JNK pathway. Besides the gene expression of Wnt5a decreased from cell passage 1 to passage 3. The cell proliferation was regulated by increasing dose of Wnt5a with the maximal effect at 50 ng/mL and 50 ng/mL Wnt5a suppressed adipogenic differentiation at middle-late stage of adipogenesis via anti-β-catenin and a mitogen-activated protein kinase (MAPK) signaling-independent manner. Accordingly, the research helps to gain further insights into the early stage of obesity and its associated changes on a cellular and molecular level.

Sex-dependent Depot Differences in Adipose Tissue Development and Function; Role of Sex Steroids

Men and women are different in their fat mass and distribution pattern. The gynoid-type fat distribution, accumulation in lower-body, is considered to be protective while the android-type accumulation in upper-body, both in abdominal subcutaneous and visceral depots, is detrimental. Sex-dependent depot differences in adipose metabolic and endocrine functions are thought to contribute to the sexual disparity in fat distribution as well as its association with cardiometabolic risks. Although molecular details have not been completely elucidated, available evidence shows that sex steroid hormones are important factors governing sexual dimorphism in adipose tissue distribution and hence, risks for metabolic diseases. We will review sex-dependent heterogeneities in adipose tissue properties that can link their depot-specific biology to metabolic complications in men and women. In addition, we will also review how sex steroids regulate adipose tissue biology, both development and functional characteristics, with emphasis on their depot-dependent actions.

Genetic Basis for Sex Differences in Obesity and Lipid Metabolism

Men and women exhibit significant differences in obesity, cardiovascular disease, and diabetes. To provide better diagnosis and treatment for both sexes, it is important to identify factors that underlie the observed sex differences. Traditionally, sex differences have been attributed to the differential effects of male and female gonadal secretions (commonly referred to as sex hormones), which substantially influence many aspects of metabolism and related diseases. Less appreciated as a contributor to sex differences are the fundamental genetic differences between males and females, which are ultimately determined by the presence of an XX or XY sex chromosome complement. Here, we review the mechanisms by which gonadal hormones and sex chromosome complement each contribute to lipid metabolism and associated diseases, and the current approaches that are used to study them. We focus particularly on genetic approaches including genome-wide association studies in humans and mice, -omics and systems genetics approaches, and unique experimental mouse models that allow distinction between gonadal and sex chromosome effects.

Directing visceral white adipocyte precursors to a thermogenic adipocyte fate improves insulin sensitivity in obese mice

Visceral adiposity confers significant risk for developing metabolic disease in obesity whereas preferential expansion of subcutaneous white adipose tissue (WAT) appears protective. Unlike subcutaneous WAT, visceral WAT is resistant to adopting a protective thermogenic phenotype characterized by the accumulation of Ucp1⁺ beige/BRITE adipocytes (termed ‘browning’). In this study, we investigated the physiological consequences of browning murine visceral WAT by selective genetic ablation of Zfp423, a transcriptional suppressor of the adipocyte thermogenic program. Zfp423 deletion in fetal visceral adipose precursors (Zfp423^loxP/loxP; Wt1-Cre), or adult visceral white adipose precursors (Pdgfrb^rtTA; TRE-Cre; Zfp423^loxP/loxP), results in the accumulation of beige-like thermogenic adipocytes within multiple visceral adipose depots. Thermogenic visceral WAT improves cold tolerance and prevents and reverses insulin resistance in obesity. These data indicate that beneficial visceral WAT browning can be engineered by directing visceral white adipocyte precursors to a thermogenic adipocyte fate, and suggest a novel strategy to combat insulin resistance in obesity.

DOI:http://dx.doi.org/10.7554/eLife.27669.001

Mammals have different types of fat cells in their bodies. White fat cells store energy for later use, and brown and beige fat cells burn energy to help keep the body warm. Individuals who are obese typically have too many white fat cells in and around their belly. This belly fat, also called visceral fat, accumulates around the organs and is believed to contribute to metabolic diseases, such as diabetes and heart disease. Individuals who are obese also have relatively few brown and beige energy-burning fat cells.

Boosting the amount of brown and beige fat in individuals who are obese has been proposed as a potential way to reduce their risk of metabolic disease. One way to do this would be to encourage white visceral fat cells to become more like energy-burning beige or brown fat cells.

Recent research has shown that white fat cells contain higher amounts of a protein called Zfp423 than brown or beige fat cells. This protein turns off the genes that fat cells use to burn energy and so keeps white fat cells in an energy-storing state. Now, Hepler et al. show that genetically modifying mice to turn off the gene that produces Zfp423 specifically in the precursor cells that become white fat cells causes more energy-burning beige cells to appear in their visceral fat.

The genetically modified mice were better able to tolerate cold than normal mice. When placed on a high-fat diet, the modified mice were also less likely to become resistant to the effects of the hormone insulin – a process that can lead to the development of type 2 diabetes and may be linked to heart disease. This suggests that treatments that prevent Zfp423 from working in fat cells could help to treat or prevent diabetes and heart disease in people who are obese. Before such treatments can be developed, further work is needed to investigate how Zfp423 works in more detail, and to confirm that it has the same effects in human fat cells as it does in mice.

DOI:http://dx.doi.org/10.7554/eLife.27669.002

Fifty shades of white: Understanding heterogeneity in white adipose stem cells

The excessive expansion of white adipose tissue underlies the global obesity epidemic. However, not all fat is equal, and the impact of heterogeneity on the development and expansion of different adipose depots is becoming increasingly apparent. Two mechanisms are responsible for the growth of adipose tissue: hyperplasia (increasing adipocyte number) and hypertrophy (increasing adipocyte size). The former relies on the differentiation of adipocyte stem cells, which reside within the adipose stromal vascular fraction. Many differences in gene expression, adipogenesis, and the response to obesogenic stimuli have been described when comparing adipose stem cells from different depots. Considering that there is disparity in the pathogenicity of the depots, understanding this heterogeneity has clinically relevant implications. Here we review the current knowledge surrounding such differences, in the context of development, expansion and therapeutics. Moreover, given the importance of these differences, we suggest that careful consideration for the precise methodologies used, is essential if we are to truly understand the physiologically relevant consequences of this heterogeneity.

Puberty is an important developmental period for the establishment of adipose tissue mass and metabolic homeostasis

Over the past 2 decades, the incidence of childhood obesity has risen dramatically. This recent rise in childhood obesity is particularly concerning as adults who were obese during childhood develop type II diabetes that is intractable to current forms of treatment compared with individuals who develop obesity in adulthood. While the mechanisms responsible for the exacerbated diabetic phenotype associated with childhood obesity is not clear, it is well known that childhood is an important time period for the establishment of normal white adipose tissue in humans. This association suggests that exposure to obesogenic stimuli during adipose development may have detrimental effects on adipose function and metabolic homeostasis. In this study, we identify the period of development associated with puberty, postnatal days 18-34, as critical for the establishment of normal adipose mass in mice. Exposure of mice to high fat diet only during this time period results in metabolic dysfunction, increased leptin expression, and increased adipocyte size in adulthood in the absence of sustained increased fat mass or body weight. These findings indicate that exposure to obesogenic stimuli during critical developmental periods have prolonged effects on adipose tissue function that may contribute to the exacerbated metabolic dysfunctions associated with childhood obesity.

Impact of Growth Hormone on Regulation of Adipose Tissue

Increasing prevalence of obesity and obesity-related conditions worldwide has necessitated a more thorough understanding of adipose tissue (AT) and expanded the scope of research in this field. AT is now understood to be far more complex and dynamic than previously thought, which has also fueled research to reevaluate how hormones, such as growth hormone (GH), alter the tissue. In this review, we will introduce properties of AT important for understanding how GH alters the tissue, such as anatomical location of depots and adipokine output. We will provide an overview of GH structure and function and define several human conditions and cognate mouse lines with extremes in GH action that have helped shape our understanding of GH and AT. A detailed discussion of the GH/AT relationship will be included that addresses adipokine production, immune cell populations, lipid metabolism, senescence, differentiation, and fibrosis, as well as brown AT and beiging of white AT. A brief overview of how GH levels are altered in an obese state, and the efficacy of GH as a therapeutic option to manage obesity will be given. As we will reveal, the effects of GH on AT are numerous, dynamic and depot-dependent. © 2017 American Physiological Society. Compr Physiol 7:819-840, 2017.

Adipose tissue: between the extremes

Adipose tissue represents a critical component in healthy energy homeostasis. It fulfills important roles in whole-body lipid handling, serves as the body's major energy storage compartment and insulation barrier, and secretes numerous endocrine mediators such as adipokines or lipokines. As a consequence, dysfunction of these processes in adipose tissue compartments is tightly linked to severe metabolic disorders, including obesity, metabolic syndrome, lipodystrophy, and cachexia. While numerous studies have addressed causes and consequences of obesity-related adipose tissue hypertrophy and hyperplasia for health, critical pathways and mechanisms in (involuntary) adipose tissue loss as well as its systemic metabolic consequences are far less understood. In this review, we discuss the current understanding of conditions of adipose tissue wasting and review microenvironmental determinants of adipocyte (dys)function in related pathophysiologies.

Maternal obesity programs increased leptin gene expression in rat male offspring via epigenetic modifications in a depot-specific manner

Objective

According to the Developmental Origin of Health and Disease (DOHaD) concept, maternal obesity and accelerated growth in neonates predispose offspring to white adipose tissue (WAT) accumulation. In rodents, adipogenesis mainly develops during lactation. The mechanisms underlying the phenomenon known as developmental programming remain elusive. We previously reported that adult rat offspring from high-fat diet-fed dams (called HF) exhibited hypertrophic adipocyte, hyperleptinemia and increased leptin mRNA levels in a depot-specific manner. We hypothesized that leptin upregulation occurs via epigenetic malprogramming, which takes place early during development of WAT.

Methods

As a first step, we identified in silico two potential enhancers located upstream and downstream of the leptin transcription start site that exhibit strong dynamic epigenomic remodeling during adipocyte differentiation. We then focused on epigenetic modifications (methylation, hydroxymethylation, and histone modifications) of the promoter and the two potential enhancers regulating leptin gene expression in perirenal (pWAT) and inguinal (iWAT) fat pads of HF offspring during lactation (postnatal days 12 (PND12) and 21 (PND21)) and in adulthood.

Results

PND12 is an active period for epigenomic remodeling in both deposits especially in the upstream enhancer, consistent with leptin gene induction during adipogenesis. Unlike iWAT, some of these epigenetic marks were still observable in pWAT of weaned HF offspring. Retained marks were only visible in pWAT of 9-month-old HF rats that showed a persistent “expandable” phenotype.

Conclusions

Consistent with the DOHaD hypothesis, persistent epigenetic remodeling occurs at regulatory regions especially within intergenic sequences, linked to higher leptin gene expression in adult HF offspring in a depot-specific manner.

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The white adipose tissue is an important target of developmental programming.

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Higher leptin gene expression occurs in offspring from obese dams in a depot-specific manner.

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Leptin upregulation occurs via epigenetic malprogramming during development of adipose tissue.

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Persistent genomic epigenetic remodeling occurs in adipose tissue of offspring from obese dams.

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Intergenic regions were more affected than the leptin promoter region in offspring of obese dams.

CD36 Is a Marker of Human Adipocyte Progenitors with Pronounced Adipogenic and Triglyceride Accumulation Potential

White adipose tissue (WAT) expands in part through adipogenesis, a process involving fat cell generation and fatty acid (FA) storage into triglycerides (TGs). Several findings suggest that inter-individual and regional variations in adipogenesis are linked to metabolic complications. We aimed to identify cellular markers that define human adipocyte progenitors (APs) with pronounced adipogenic/TG storage ability. Using an unbiased single cell screen of passaged human adipose-derived stromal cells (hADSCs), we identified cell clones with similar proliferation rates but discordant capabilities to undergo adipogenic differentiation. Transcriptomic analyses prior to induction of differentiation showed that adipogenic clones displayed a significantly higher expression of CD36, encoding the scavenger receptor CD36. CD36+ hADSCs, in comparison with CD36-cells, displayed almost complete adipogenic differentiation while CD36 RNAi attenuated lipid accumulation. Similar findings were observed in primary CD45-/CD34+/CD31-APs isolated from human WAT where the subpopulation of MSCA1+/CD36+ cells displayed a significantly higher differentiation degree/TG storage capacity than MSCA1+/CD36-cells. Functional analyses in vitro and ex vivo confirmed that CD36 conferred APs an increased capacity to take up FAs thereby facilitating terminal differentiation. Among primary APs from subcutaneous femoral, abdominal and visceral human WAT, the fraction of CD36+ cells was significantly higher in depots associated with higher adipogenesis and reduced metabolic risk (i.e., femoral WAT). We conclude that CD36 marks APs with pronounced adipogenic potential, most probably by facilitating lipid uptake. This may be of value in developing human adipocyte cell clones and possibly in linking regional variations in adipogenesis to metabolic phenotype. Stem Cells 2017;35:1799-1814.

The Complex Roles of Mechanistic Target of Rapamycin in Adipocytes and Beyond

Having healthy adipose tissue is essential for metabolic fitness. This is clear from the obesity epidemic, which is unveiling a myriad of comorbidities associated with excess adipose tissue including type 2 diabetes, cardiovascular disease, and cancer. Lipodystrophy also causes insulin resistance, emphasizing the importance of having a balanced amount of fat. In cells, the mechanistic target of rapamycin (mTOR) complexes 1 and 2 (mTORC1 and mTORC2, respectively) link nutrient and hormonal signaling with metabolism, and recent studies are shedding new light on their in vivo roles in adipocytes. In this review, we discuss how recent advances in adipose tissue and mTOR biology are converging to reveal new mechanisms that maintain healthy adipose tissue, and discuss ongoing mysteries of mTOR signaling, particularly for the less understood complex mTORC2.

The expanding problem of adipose depot remodeling and postnatal adipocyte progenitor recruitment

The rising incidence of obesity and associated metabolic diseases has increased the urgency in understanding all aspects of adipose tissue biology. This includes the function of adipocytes, how adipose tissue expands in obesity, and how expanded adipose tissues in adults can impact physiology. Here, we highlight the growing appreciation for the importance of de novo adipocyte differentiation to adipose tissue expansion in adult humans and animals. We detail recent efforts to identify adipose precursor populations that contribute to the physiological postnatal recruitment of white, brown, and beige adipocytes in mice, and summarize new data that reveal the complexity of adipose tissue development in vivo.

Transcriptional Regulation of Adipogenesis

Adipocytes are the defining cell type of adipose tissue. Once considered a passive participant in energy storage, adipose tissue is now recognized as a dynamic organ that contributes to several important physiological processes, such as lipid metabolism, systemic energy homeostasis, and whole-body insulin sensitivity. Therefore, understanding the mechanisms involved in its development and function is of great importance. Adipocyte differentiation is a highly orchestrated process which can vary between different fat depots as well as between the sexes. While hormones, miRNAs, cytoskeletal proteins, and many other effectors can modulate adipocyte development, the best understood regulators of adipogenesis are the transcription factors that inhibit or promote this process. Ectopic expression and knockdown approaches in cultured cells have been widely used to understand the contribution of transcription factors to adipocyte development, providing a basis for more sophisticated in vivo strategies to examine adipogenesis. To date, over two dozen transcription factors have been shown to play important roles in adipocyte development. These transcription factors belong to several families with many different DNA-binding domains. While peroxisome proliferator-activated receptor gamma (PPARγ) is undoubtedly the most important transcriptional modulator of adipocyte development in all types of adipose tissue, members of the CCAAT/enhancer-binding protein, Krüppel-like transcription factor, signal transducer and activator of transcription, GATA, early B cell factor, and interferon-regulatory factor families also regulate adipogenesis. The importance of PPARγ activity is underscored by several covalent modifications that modulate its activity and its ability to modulate adipocyte development. This review will primarily focus on the transcriptional control of adipogenesis in white fat cells and on the mechanisms involved in this fine-tuned developmental process. © 2017 American Physiological Society. Compr Physiol 7:635-674, 2017.

A PDGFRα-Mediated Switch toward CD9high Adipocyte Progenitors Controls Obesity-Induced Adipose Tissue Fibrosis

Obesity-induced white adipose tissue (WAT) fibrosis is believed to accelerate WAT dysfunction. However, the cellular origin of WAT fibrosis remains unclear. Here, we show that adipocyte platelet-derived growth factor receptor-α-positive (PDGFRα⁺) progenitors adopt a fibrogenic phenotype in obese mice prone to visceral WAT fibrosis. More specifically, a subset of PDGFRα⁺ cells with high CD9 expression (CD9^high) originates pro-fibrotic cells whereas their CD9^low counterparts, committed to adipogenesis, are almost completely lost in the fibrotic WAT. PDGFRα pathway activation promotes a phenotypic shift toward PDGFRα⁺CD9^high fibrogenic cells, driving pathological remodeling and altering WAT function in obesity. These findings translated to human obesity as the frequency of CD9^high progenitors in omental WAT (oWAT) correlates with oWAT fibrosis level, insulin-resistance severity, and type 2 diabetes. Collectively, our data demonstrate that in addition to representing a WAT adipogenic niche, different PDGFRα⁺ cell subsets modulate obesity-induced WAT fibrogenesis and are associated with loss of metabolic fitness.

A causal role for hyperinsulinemia in obesity

Insulin modulates the biochemical pathways controlling lipid uptake, lipolysis and lipogenesis at multiple levels. Elevated insulin levels are associated with obesity, and conversely, dietary and pharmacological manipulations that reduce insulin have occasionally been reported to cause weight loss. However, the causal role of insulin hypersecretion in the development of mammalian obesity remained controversial in the absence of direct loss-of-function experiments. Here, we discuss theoretical considerations around the causal role of excess insulin for obesity, as well as recent studies employing mice that are genetically incapable of the rapid and sustained hyperinsulinemia that normally accompanies a high-fat diet. We also discuss new evidence demonstrating that modest reductions in circulating insulin prevent weight gain, with sustained effects that can persist after insulin levels normalize. Importantly, evidence from long-term studies reveals that a modest reduction in circulating insulin is not associated with impaired glucose homeostasis, meaning that body weight and lipid homeostasis are actually more sensitive to small changes in circulating insulin than glucose homeostasis in these models. Collectively, the evidence from new studies on genetic loss-of-function models forces a re-evaluation of current paradigms related to obesity, insulin resistance and diabetes. The potential for translation of these findings to humans is briefly discussed.

Receptor for hyaluronan mediated motility (RHAMM/HMMR) is a novel target for promoting subcutaneous adipogenesis

Hyaluronan, CD44 and the Receptor for Hyaluronan-Mediated Motility (RHAMM, gene name HMMR) regulate stem cell differentiation including mesenchymal progenitor differentiation. Here, we show that CD44 expression is required for subcutaneous adipogenesis, whereas RHAMM expression suppresses this process. We designed RHAMM function blocking peptides to promote subcutaneous adipogenesis as a clinical and tissue engineering tool. Adipogenic RHAMM peptides were identified by screening for their ability to promote adipogenesis in culture assays using rat bone marrow mesenchymal stem cells, mouse pre-adipocyte cell lines and primary human subcutaneous pre-adipocytes. Oil red O uptake into fat droplets and adiponectin production were used as biomarkers of adipogenesis. Positive peptides were formulated in either collagen I or hyaluronan (Orthovisc) gels then assessed for their adipogenic potential in vivo following injection into dorsal rat skin and mammary fat pads. Fat content was quantified and characterized using micro CT imaging, morphometry, histology, RT-PCR and ELISA analyses of adipogenic gene expression. Injection of screened peptides increased dorsal back subcutaneous fat pad area (208.3 ± 10.4 mm²versus control 84.11 ± 4.2 mm²; p < 0.05) and mammary fat pad size (45 ± 11 mg above control background, p = 0.002) in female rats. This effect lasted >5 weeks as detected by micro CT imaging and perilipin 1 mRNA expression. RHAMM expression suppresses while blocking peptides promote expression of PPARγ, C/EBP and their target genes. Blocking RHAMM function by peptide injection or topical application is a novel and minimally invasive method for potentially promoting subcutaneous adipogenesis in lipodystrophic diseases and a complementary tool to subcutaneous fat augmentation techniques.

Long Term High Fat Diet Treatment: An Appropriate Approach to Study the Sex-Specificity of the Autonomic and Cardiovascular Responses to Obesity in Mice

Obesity-related cardiovascular disease (CVD) involves increased sympathetic activity in men and male animals. Although women exhibit increased visceral fat, metabolic disorders, inflammation and CVD with obesity, whether body weight gain affects autonomic control of cardiovascular function in females remain unknown. Due to the lack of adequate model to mimic the human pathology, this study aimed to develop a murine model, which would allow studying the sex-specificity of the response of the autonomic nervous system to obesity and identifying the origin of potential sex-differences. We tested the hypothesis that sexual dimorphisms in the autonomic response to obesity disappear in mice matched for changes in body weight, metabolic and inflammatory disorders. Male and female C57Bl/6 mice were submitted to control (CD) or high fat diet (HFD) for 24 weeks. Female mice gained more adipose mass and lost more lean mass than males but reached similar visceral adipose mass and body weight, as males, at the end of the diet. 24 weeks of HFD matched male and female mice for visceral adiposity, glycaemia, plasma insulin, lipids, and inflammatory cytokines levels, demonstrating the suitability of the model to study human pathology. HFD did not elevate BP, but similarly increased heart rate (HR) in males (CD: 571 ± 9 vs. HFD: 631 ± 14 bpm, P < 0.05) and females (CD: 589 ± 19 vs. HFD: 642 ± 6 bpm, P < 0.05). Indices of autonomic control of BP and HR were obtained by measuring BP and HR response to ganglionic blockade, β-adrenergic, and muscarinic receptors antagonists. HFD increased vascular but reduced cardiac sympathetic drive in males (CD: -43 ± 4 and HFD: -60 ± 7% drop in BP, P < 0.05). HFD did not alter females' vascular or cardiac sympathetic drive. HFD specifically reduced aortic α-adrenergic constriction in males and lowered HR response to muscarinic receptor antagonism in females. These data suggest that obesity-associated increases in HR could be caused by a reduced cardiac vagal tone in females, while HR increases in males may compensate for the reduced vascular adrenergic contractility to preserve baseline BP. These data suggest that obesity impairs autonomic control of cardiovascular function in males and females, via sex-specific mechanisms and independent of fat distribution, metabolic disorder or inflammation.

HO-1 inhibits preadipocyte proliferation and differentiation at the onset of obesity via ROS dependent activation of Akt2

Excessive accumulation of white adipose tissue (WAT) is a hallmark of obesity. The expansion of WAT in obesity involves proliferation and differentiation of adipose precursors, however, the underlying molecular mechanisms remain unclear. Here, we used an unbiased transcriptomics approach to identify the earliest molecular underpinnings occuring in adipose precursors following a brief HFD in mice. Our analysis identifies Heme Oxygenase-1 (HO-1) as strongly and selectively being upregulated in the adipose precursor fraction of WAT, upon high-fat diet (HFD) feeding. Specific deletion of HO-1 in adipose precursors of Hmox1^fl/flPdgfra^Cre mice enhanced HFD-dependent visceral adipose precursor proliferation and differentiation. Mechanistically, HO-1 reduces HFD-induced AKT2 phosphorylation via ROS thresholding in mitochondria to reduce visceral adipose precursor proliferation. HO-1 influences adipogenesis in a cell-autonomous way by regulating events early in adipogenesis, during the process of mitotic clonal expansion, upstream of Cebpα and PPARγ. Similar effects on human preadipocyte proliferation and differentiation in vitro were observed upon modulation of HO-1 expression. This collectively renders HO-1 as an essential factor linking extrinsic factors (HFD) with inhibition of specific downstream molecular mediators (ROS & AKT2), resulting in diminished adipogenesis that may contribute to hyperplastic adipose tissue expansion.

An estrogen receptor β-selective agonist inhibits non-alcoholic steatohepatitis in preclinical models by regulating bile acid and xenobiotic receptors

Non-alcoholic steatohepatitis (NASH) affects 8-10 million people in the US and up to 75% of obese individuals. Despite this, there are no approved oral therapeutics to treat NASH and therefore the need for novel approaches exists. The estrogen receptor β (ER-β)-selective agonist, β-LGND2, inhibits body weight and white adipose tissue, and increases metabolism, resulting in higher energy expenditure and thermogenesis. Due to favorable effects of β-LGND2 on obesity, we hypothesized that β-LGND2 will prevent NASH directly by reducing lipid accumulation in the liver or indirectly by favorably changing body composition. Male C57BL/6 mice fed with high fat diet (HFD) for 10 weeks or methionine choline-deficient diet for four weeks and treated with vehicle exhibited altered liver weights by twofold and increased serum transaminases by 2-6-folds. These changes were not observed in β-LGND2-treated animals. Infiltration of inflammatory cells and collagen deposits, an indication of fibrosis, were observed in the liver of mice fed with HFD for 10 weeks, which were effectively blocked by β-LGND2. Gene expression studies in the liver indicate that pregnane X receptor target genes were significantly increased by HFD, and the increase was inhibited by β-LGND2. On the other hand, metabolomics indicate that bile acid metabolites were significantly increased by β-LGND2. These studies demonstrate that an ER-β agonist might provide therapeutic benefits in NASH by directly modulating the function of xenobiotic and bile acid receptors in the liver, which have important functions in the liver, and indirectly, as demonstrated before, by inhibiting adiposity. Impact statement Over 75-90% of those classified as clinically obese suffer from co-morbidities, the most common of which is non-alcoholic steatohepatitis (NASH). While there are currently no effective treatment approaches for NASH, data presented here provide preliminary evidence that an estrogen receptor β-selective ligand could have the potential to reduce lipid accumulation and inflammation, and protect liver from NASH.

Sorting out adipocyte precursors and their role in physiology and disease

The ability to maintain and expand the pool of adipocytes in adults is integral to the regulation of energy balance, tissue/stem cell homeostasis, and disease pathogenesis. For decades, our knowledge of adipocyte precursors has relied on cellular models. The identity of native adipocyte precursors has remained unclear. Recent studies have identified distinct adipocyte precursor populations that are physiologically regulated and contribute to the development, maintenance, and expansion of adipocyte pools in mice. With new tools available, the properties of adipocyte precursors can now be defined, and the regulation and function of adipose plasticity in development and physiology can be explored.

Cellular Mechanisms Driving Sex Differences in Adipose Tissue Biology and Body Shape in Humans and Mouse Models

Sex differences in adipose tissue distribution and the metabolic, endocrine, and immune functions of different anatomical fat depots have been described, but they are incompletely documented in the literature. It is becoming increasingly clear that adipose depots serve distinct functions in males and females and have specific physiological roles. However, the mechanisms that regulate the size and function of specific adipose tissues in men and women remain poorly understood. New insights from mouse models have advanced our understanding of depot differences in adipose growth and remodeling via the proliferation and differentiation of adipose progenitors that can expand adipocyte number in the tissue or simply replace dysfunctional older and larger adipocytes. A limited ability of a depot to expand or remodel can lead to excessive adipocyte hypertrophy, which is often correlated with metabolic dysfunction. However, the relationship of adipocyte size and function varies by depot and sex. For example, femoral adipose tissues of premenopausal women appear to have a greater capacity for adipose expansion via hyperplasia and hypertrophy; although larger, these gluteal-femoral adipocytes remain insulin sensitive. The microenvironment of specific depots, including the composition of the extracellular matrix and cellular composition, as well as cell-autonomous genetic differences, influences sex- and depot-dependent metabolic and growth properties. Although there are some species differences, studies of the molecular and physiological determinants of sex differences in adipocyte growth and function in humans and rodents are both needed for understanding sex differences in health and disease.

High-fat diet-induced obesity regulates MMP3 to modulate depot- and sex-dependent adipose expansion in C57BL/6J mice

Increased adipocyte size is hypothesized to signal the recruitment of adipose progenitor cells (APCs) to expand tissue storage capacity. To investigate depot and sex differences in adipose growth, male and female C57BL/6J mice (10 wk-old) were challenged with high-fat (HF) or low-fat (LF) diets (D) for 14 wk. The HFD increased gonadal (GON) depot weight by adipocyte hypertrophy and hyperplasia in females but hypertrophy alone in males. In both sexes, inguinal (ING) adipocytes were smaller than GON, and depot expansion was due to hypertrophy. Matrix metalloproteinase 3 (Mmp3), an antiadipogenic factor, and its inhibitor Timps modulate the extracellular matrix remodeling needed for depot expansion. Mmp3 mRNA was depot different (ING > GON), higher in females than males and mainly expressed in APCs. In males, HFD-induced obesity increased tissue and APC Mmp3 mRNA levels and MMP3 protein and enzymatic activity. In females however, HFD significantly decreased MMP3 protein without affecting its mRNA levels. MMP3 activity also decreased (significant in ING). Timp4 mRNA was expressed mainly in adipocytes, and HFD-induced obesity tended to increase the ratio of TIMP4 to MMP3 protein in females, whereas it decreased it in males. Overexpression of Mmp3 in 3T3-L1 preadipocytes or rhMMP3 protein added to primary human preadipocytes inhibited differentiation, whereas rhTIMP4 improved adipogenesis and attenuated the inhibitory effect of rhMMP3. These data suggest that HFD-induced obesity downregulates APC MMP3 expression to trigger adipogenesis, and adipocyte TIMP4 may modulate this process to regulate hyperplastic vs. hypertrophic adipose tissue expansion, fat distribution, and metabolic health in a sex- and depot-dependent manner.

The Role of Sex and Sex Hormones in Regulating Obesity-Induced Inflammation

Metabolic and non-metabolic complications due to obesity are becoming more prevalent, yet our understanding of the mechanisms driving these is not. This is due to individual risk factor variability making it difficult to predict disease outcomes such as diabetes and insulin resistance. Gender is a critical factor in obesity outcomes with women having more adiposity but reduced metabolic complications compared to men. The role of immune system activation during obesity is an emerging field that links adiposity to metabolic syndrome. Furthermore, evidence from animal models suggests that sex differences exist in immune responses and, therefore, could be a possible mechanism leading to sex differences in metabolic disease. While there is still much to learn in the area of sex-differences research, this chapter will review the current knowledge and literature detailing the role of sex and sex hormones on adiposity and metabolically induced inflammation in obesity.

Adipose tissue development and the molecular regulation of lipid metabolism

The production of new adipocytes is required to maintain adipose tissue mass and involves the proliferation and differentiation of adipocyte precursor cells (APCs). In this review, we outline new developments in understanding the phenotype of APCs and provide evidence suggesting that APCs differ between distinct adipose tissue depots and are affected by obesity. Post-mitotic mature adipocytes regulate systemic lipid homeostasis by storing and releasing free fatty acids, and also modulate energy balance via the secretion of adipokines. The review highlights recent advances in understanding the cellular and molecular mechanisms regulating adipocyte metabolism, with a particular focus on lipolysis regulation and the involvement of microribonucleic acids (miRNAs).

Skin Adipocyte Stem Cell Self-Renewal Is Regulated by a PDGFA/AKT-Signaling Axis

Tissue growth and maintenance requires stem cell populations that self-renew, proliferate, and differentiate. Maintenance of white adipose tissue (WAT) requires the proliferation and differentiation of adipocyte stem cells (ASCs) to form postmitotic, lipid-filled mature adipocytes. Here we use the dynamic adipogenic program that occurs during hair growth to uncover an unrecognized regulator of ASC self-renewal and proliferation, PDGFA, which activates AKT signaling to drive and maintain the adipogenic program in the skin. Pdgfa expression is reduced in aged ASCs and is required for ASC proliferation and maintenance in the dermis, but not in other WATs. Our molecular and genetic studies uncover PI3K/AKT2 as a direct PDGFA target that is activated in ASCs during WAT hyperplasia and is functionally required for dermal ASC proliferation. Our data therefore reveal active mechanisms that regulate ASC self-renewal in the skin and show that distinct regulatory mechanisms operate in different WAT depots.

Transgenic Adipose-specific Expression of the Nuclear Receptor RORα Drives a Striking Shift in Fat Distribution and Impairs Glycemic Control

RORα is a member of the nuclear receptor (NR) superfamily and analysis of the (global) RORα-deficient mouse model revealed this NR has a role in glycemic control and fat deposition. Therefore, we generated an adipose-specific RORα ‘gain of function’ mouse model under the control of the fatty acid binding protein 4 (FABP4) promoter to elucidate the function of RORα in adipose tissue. The Tg-FABP4-RORα4 mice demonstrated a shift in fat distribution to non-adipose tissues when challenged with a high fat diet (HFD). Specifically, we observed a subcutaneous lipodystrophy, accompanied by hepatomegaly (fatty liver/mild portal fibrosis) and splenomegaly; in a background of decreased weight gain and total body fat after HFD. Moreover, we observed significantly higher fasting blood glucose and impaired clearance of glucose in Tg-FABP4-RORα4 mice. Genome wide expression and qPCR profiling analysis identified: (i) subcutaneous adipose specific decreases in the expression of genes involved in fatty acid biosynthesis, lipid droplet expansion and glycemic control, and (ii) the fibrosis pathway as the most significant pathway [including dysregulation of the collagen/extracellular matrix (ECM) pathways] in subcutaneous adipose and liver. The pathology presented in the Tg-FABP4-RORα4 mice is reminiscent of human metabolic disease (associated with aberrant ECM expression) highlighting the therapeutic potential of this NR.

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Adipose-specific expression of RORα is associated with subcutaneous lipodystrophy and hepatomegaly with fibrosis.

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The phenotype is associated with impaired glycemic control and decreased weight gain on a high fat diet.

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Gene expression profiling reveals significant dysregulation of extra cellular matrix signaling.

We have generated a ‘gain of function’ animal model with the nuclear hormone receptor RORα4 to understand the function of this protein in fat. Over expression of the RORα4 gene, was associated with fat deposition in non-adipose tissues on a high fat diet. Moreover, we observed a decrease in fat tissue (located under the skin) accompanied by enlargement of the liver and spleen. In addition, over expression of this receptor was associated with impaired glycemic control. The pathology in this animal model is reminiscent of metabolic disease in humans, highlighting the therapeutic potential of pharmacologically manipulating this nuclear receptor.