Explore the vast range of titles available.

In mammals, the white adipocyte is a cell type that is specialized for storage of energy (in the form of triacylglycerols) and for energy mobilization (as fatty acids). White adipocyte metabolism confers an essential role to adipose tissue in whole-body homeostasis. Dysfunction in white adipocyte metabolism is a cardinal event in the development of insulin resistance and associated disorders. This Review focuses on our current understanding of lipid and glucose metabolic pathways in the white adipocyte. We survey recent advances in humans on the importance of adipocyte hypertrophy and on the in vivo turnover of adipocytes and stored lipids. At the molecular level, the identification of novel regulators and of the interplay between metabolic pathways explains the fine-tuning between the anabolic and catabolic fates of fatty acids and glucose in different physiological states. We also examine the metabolic alterations involved in the genesis of obesity-associated metabolic disorders, lipodystrophic states, cancers and cancer-associated cachexia. New challenges include defining the heterogeneity of white adipocytes in different anatomical locations throughout the lifespan and investigating the importance of rhythmic processes. Targeting white fat metabolism offers opportunities for improved patient stratification and a wide, yet unexploited, range of therapeutic opportunities.White adipocyte metabolism is important for the regulation of systemic metabolism and is often dysregulated in various conditions, such as cancer and type 2 diabetes mellitus. In this Review, Langin and colleagues provide an overview of lipid metabolism in white adipocytes and the related metabolism of glucose and discuss how these pathways provide therapeutic targets in metabolic disorders.

Key Points MicroRNAs (miRNAs) are important for fat cell formation (adipogenesis) and for regulating the metabolic and endocrine functions of these cells Obesity influences the expression of miRNAs in adipose tissue, but altered expression of only a few of these miRNAs has been experimentally verified in humans Regional variations in expression of miRNAs in human adipose tissues have been demonstrated miRNAs signal through complex networks involving transcription factors, which has been demonstrated in the context of regulation of inflammation in human adipose tissue Extracellular miRNAs have specific expression profiles in obesity MicroRNAs (miRNAs) regulate the differentiation of adipoctyes, as well as endocrine and inflammatory processes in adipose tissues. In this Review, Arner and Kulyté addressed the characterization and functions of miRNA regulatory networks in human adipose tissue, in particular the networks that contribute to chronic low-level inflammation. The potential for targeting these networks, as well as individual miRNAs, in obesity and other metabolic disorders is also discussed. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression and, therefore, biological processes in different tissues. A major function of miRNAs in adipose tissue is to stimulate or inhibit the differentiation of adipocytes, and to regulate specific metabolic and endocrine functions. Numerous miRNAs are present in human adipose tissue; however, the expression of only a few is altered in individuals with obesity and type 2 diabetes mellitus or are differentially expressed in various adipose depots. In humans, obesity is associated with chronic low-grade inflammation that is regulated by signal transduction networks, in which miRNAs, either directly or indirectly (through regulatory elements such as transcription factors), influence the expression and secretion of inflammatory proteins. In addition to their diverse effects on signalling, miRNAs and transcription factors can interact to amplify the inflammatory effect. Although additional miRNA signal networks in human adipose tissue are not yet known, similar regulatory circuits have been described in brown adipose tissue in mice. miRNAs can also be secreted from fat cells into the circulation and serve as markers of disturbed adipose tissue function. Given their role in regulating transcriptional networks, miRNAs in adipose tissue might offer tangible targets for treating metabolic disorders.

Methylation patterns of circulating cell-free DNA (cfDNA) contain rich information about recent cell death events in the body. Here, we present an approach for unbiased determination of the tissue origins of cfDNA, using a reference methylation atlas of 25 human tissues and cell types. The method is validated using in silico simulations as well as in vitro mixes of DNA from different tissue sources at known proportions. We show that plasma cfDNA of healthy donors originates from white blood cells (55%), erythrocyte progenitors (30%), vascular endothelial cells (10%) and hepatocytes (1%). Deconvolution of cfDNA from patients reveals tissue contributions that agree with clinical findings in sepsis, islet transplantation, cancer of the colon, lung, breast and prostate, and cancer of unknown primary. We propose a procedure which can be easily adapted to study the cellular contributors to cfDNA in many settings, opening a broad window into healthy and pathologic human tissue dynamics. The methylation status of circulating cell-free DNA (cfDNA) can be informative about recent cell death events. Here the authors present an approach to determine the tissue origins of cfDNA, using a reference methylation atlas of 25 human tissues and cell types, and find that cfDNA from patients reveals tissue contributions that agree with clinical findings.

Development of Type 2 diabetes, like obesity, is promoted by a genetic predisposition. Although several genetic variants have been identified they only account for a small proportion of risk. We have asked if genetic risk is associated with abnormalities in storing excess lipids in the abdominal subcutaneous adipose tissue. We recruited 164 lean and 500 overweight/obese individuals with or without a genetic predisposition for Type 2 diabetes or obesity. Adipose cell size was measured in biopsies from the abdominal adipose tissue as well as insulin sensitivity (HOMA index), HDL-cholesterol and Apo AI and Apo B. 166 additional non-obese individuals with a genetic predisposition for Type 2 diabetes underwent a euglycemic hyperinsulinemic clamp to measure insulin sensitivity. Genetic predisposition for Type 2 diabetes, but not for overweight/obesity, was associated with inappropriate expansion of the adipose cells, reduced insulin sensitivity and a more proatherogenic lipid profile in non-obese individuals. However, obesity per se induced a similar expansion of adipose cells and dysmetabolic state irrespective of genetic predisposition. Genetic predisposition for Type 2 diabetes, but not obesity, is associated with an impaired ability to recruit new adipose cells to store excess lipids in the subcutaneous adipose tissue, thereby promoting ectopic lipid deposition. This becomes particularly evident in non-obese individuals since obesity per se promotes a dysmetabolic state irrespective of genetic predisposition. These results identify a novel susceptibility factor making individuals with a genetic predisposition for Type 2 diabetes particularly sensitive to the environment and caloric excess.

DNA methylation is a fundamental epigenetic mark that governs gene expression and chromatin organization, thus providing a window into cellular identity and developmental processes 1 . Current datasets typically include only a fraction of methylation sites and are often based either on cell lines that underwent massive changes in culture or on tissues containing unspecified mixtures of cells 2 – 5 . Here we describe a human methylome atlas, based on deep whole-genome bisulfite sequencing, allowing fragment-level analysis across thousands of unique markers for 39 cell types sorted from 205 healthy tissue samples. Replicates of the same cell type are more than 99.5% identical, demonstrating the robustness of cell identity programmes to environmental perturbation. Unsupervised clustering of the atlas recapitulates key elements of tissue ontogeny and identifies methylation patterns retained since embryonic development. Loci uniquely unmethylated in an individual cell type often reside in transcriptional enhancers and contain DNA binding sites for tissue-specific transcriptional regulators. Uniquely hypermethylated loci are rare and are enriched for CpG islands, Polycomb targets and CTCF binding sites, suggesting a new role in shaping cell-type-specific chromatin looping. The atlas provides an essential resource for study of gene regulation and disease-associated genetic variants, and a wealth of potential tissue-specific biomarkers for use in liquid biopsies. We describe a human DNA methylome atlas based on deep whole-genome bisulfite sequencing, allowing fragment-level analysis of cell-type-specific markers and providing an essential resource for studies of gene regulation and for deconvolution of cell mixtures and liquid biopsies.

Long-term lipid turnover in adipose tissue Lipid turnover is an important factor in determining how adipose tissue stores and releases energy in humans. By measuring carbon-14, derived from nuclear bomb tests, in the fat-cell lipids of a large cohort of individuals, Arner et al . estimate lipid turnover over a prolonged period in sickness and in health. The data show that during the ten-year lifespan of a typical human adipocyte, its triglyceride content is renewed six times. Unusual lipid turnover patterns correlate to conditions with disturbed lipid metabolism, suggesting it as a potential target for the treatment of metabolic disease. Adipose tissue mass is determined by the storage and removal of triglycerides in adipocytes 1 . Little is known, however, about adipose lipid turnover in humans in health and pathology. To study this in vivo , here we determined lipid age by measuring 14 C derived from above ground nuclear bomb tests in adipocyte lipids. We report that during the average ten-year lifespan of human adipocytes, triglycerides are renewed six times. Lipid age is independent of adipocyte size, is very stable across a wide range of adult ages and does not differ between genders. Adipocyte lipid turnover, however, is strongly related to conditions with disturbed lipid metabolism. In obesity, triglyceride removal rate (lipolysis followed by oxidation) is decreased and the amount of triglycerides stored each year is increased. In contrast, both lipid removal and storage rates are decreased in non-obese patients diagnosed with the most common hereditary form of dyslipidaemia, familial combined hyperlipidaemia. Lipid removal rate is positively correlated with the capacity of adipocytes to break down triglycerides, as assessed through lipolysis, and is inversely related to insulin resistance. Our data support a mechanism in which adipocyte lipid storage and removal have different roles in health and pathology. High storage but low triglyceride removal promotes fat tissue accumulation and obesity. Reduction of both triglyceride storage and removal decreases lipid shunting through adipose tissue and thus promotes dyslipidaemia. We identify adipocyte lipid turnover as a novel target for prevention and treatment of metabolic disease.

Adipocyte Turnover: Relevance to Human Adipose Tissue Morphology Erik Arner 1 , Pål O. Westermark 2 , Kirsty L. Spalding 3 , Tom Britton 4 , Mikael Rydén 1 , Jonas Frisén 3 , Samuel Bernard 5 and Peter Arner 1 1 Department of Medicine, Karolinska Institutet at Karolinska University Hospital, Stockholm, Sweden; 2 Institute for Theoretical Biology, Humboldt University Berlin and Charité, Berlin, Germany; 3 Department of Cell and Molecular Biology, Karolinska Institutet, Stockholm, Sweden; 4 Department of Mathematics, Stockholm University, Stockholm, Sweden; 5 Institut Camille Jordan, University of Lyon, Villeurbanne, France. Corresponding author: Peter Arner, peter.arner{at}ki.se . Abstract 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/m 2 . 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 14 C 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. Footnotes The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Received June 26, 2009. Accepted October 8, 2009. © 2010 American Diabetes Association

In obesity, white adipose tissue (WAT) inflammation is linked to insulin resistance. Increased adipocyte chemokine (C-C motif) ligand 2 (CCL2) secretion may initiate adipose inflammation by attracting the migration of inflammatory cells into the tissue. Using an unbiased approach, we identified adipose microRNAs (miRNAs) that are dysregulated in human obesity and assessed their possible role in controlling CCL2 production. In subcutaneous WAT obtained from 56 subjects, 11 miRNAs were present in all subjects and downregulated in obesity. Of these, 10 affected adipocyte CCL2 secretion in vitro and for 2 miRNAs (miR-126 and miR-193b), regulatory circuits were defined. While miR-126 bound directly to the 3'-untranslated region of CCL2 mRNA, miR-193b regulated CCL2 production indirectly through a network of transcription factors, many of which have been identified in other inflammatory conditions. In addition, overexpression of miR-193b and miR-126 in a human monocyte/macrophage cell line attenuated CCL2 production. The levels of the two miRNAs in subcutaneous WAT were significantly associated with CCL2 secretion (miR-193b) and expression of integrin, α-X, an inflammatory macrophage marker (miR-193b and miR-126). Taken together, our data suggest that miRNAs may be important regulators of adipose inflammation through their effects on CCL2 release from human adipocytes and macrophages.

Nutrition is a major variable factor in human environments. The composition of nutrients has changed markedly in recent decades which may contribute to the increased prevalence of metabolic diseases, such as obesity and type 2 diabetes. Fat is an important component of the diet which comes in various forms with fatty acids (FA) of different carbon chain lengths and saturation degrees. In addition to being an energy supply, FA function as potent signalling molecules and influence transcriptional activity. Among other tissues, dietary FA target white adipose tissue function, which is central in maintaining metabolic health. This review focuses on the possible role of dietary FA composition and its effect on human white adipose tissue expandability and transcriptional response. Altogether, the existing literature suggests that unsaturated fat has more benign effects on adipose tissue distribution when compared to long-chain saturated fat. However, the mechanisms of action remain poorly characterised.

Abstract Objective Although IL-10 is generally considered as an anti-inflammatory cytokine, it was recently shown to have detrimental effects on insulin sensitivity and fat cell metabolism in rodents. Whether this also pertains to human white adipose tissue (hWAT) is unclear. We therefore determined the main cellular source and effects of IL-10 on human adipocytes and hWAT-resident immune cells and its link to insulin resistance. Methods Associations between hWAT IL-10 production and metabolic parameters were investigated in 216 participants with large interindividual variations in body mass index and insulin sensitivity. Adipose cells expressing or secreting IL-10 and the cognate IL-10 receptor α (IL10RA) were identified by flow cytometry sorting. Effects on adipogenesis, lipolysis, and inflammatory/metabolic gene expression were measured in two human primary adipocyte models. Secretion of inflammatory cytokines was investigated in cultures of IL-10–treated hWAT macrophages and leukocytes by Luminex analysis (Luminex Corp.). Results IL-10 gene expression and protein secretion in hWAT correlated positively with body mass index (BMI) and homeostasis model assessment-insulin resistance (HOMA-IR). Gene expression analyses in mature fat cells and flow cytometry–sorted hWAT-resident adipocyte progenitors, macrophages, and leukocytes demonstrated that the expression of IL-10 and the IL10RA were significantly enriched in proinflammatory M1 macrophages. In contrast to murine data, functional studies showed that recombinant IL-10 had no effect on adipocyte phenotype. In hWAT-derived macrophages and leukocytes, it induced an anti-inflammatory profile. Conclusion In hWAT, IL-10 is upregulated in proinflammatory macrophages of obese and insulin-resistant persons. However, in contrast to findings in mice, IL-10 does not directly affect human adipocyte function. In human white adipose tissue, IL-10 is secreted by proinflammatory macrophages and correlates with adverse metabolic phenotype but unlike in rodents, it has no effect on adipocyte function.