OBJECTIVETo investigate how insulin sensitivity and glucose metabolism differ in adipocytes between different fat depots of male and female mice and how sex steroids contribute to these differences.RESEARCH DESIGN AND METHODSAdipocytes from intra-abdominal/perigonadal (PG) and subcutaneous (SC) adipose tissue from normal, castrated, or steroid-implanted animals were isolated and analyzed for differences in insulin sensitivity and glucose metabolism.RESULTSAdipocytes from both PG and SC depots of females have increased lipogenic rates compared with those from males. In females, intra-abdominal PG adipocytes are more insulin-sensitive than SC adipocytes and more insulin-sensitive than male adipocytes from either depot. When stimulated by low physiological concentrations of insulin, female PG adipocytes show a robust increase in Akt and extracellular signal–related kinase (ERK) phosphorylation and lipogenesis, whereas male adipocytes show activation only at higher insulin concentrations. Adipocytes from females have higher mRNA/protein levels of several genes involved in glucose and lipid metabolism. After castration, adipocytes of male mice showed increased insulin sensitivity and increased lipogenic rates, whereas adipocytes of females demonstrate decreased lipid production. Increasing estrogen above physiological levels, however, also reduced lipid synthesis in females, whereas increasing dihydrotestosterone in males had no effect.CONCLUSIONSThere are major sex differences in insulin sensitivity in adipose tissue, particularly in the intra-abdominal depot, that are regulated by physiological levels of sex steroids. The increased sensitivity to insulin and lipogenesis observed in adipocytes from females may account for their lower level of insulin resistance and diabetes risk despite similar or higher fat content than in males.

SUMMARY Excess adipose tissue is associated with metabolic disease and reduced lifespan, whereas caloric restriction decreases these risks. Here we show that as mice age, there is down-regulation of Dicer and miRNA processing in adipose tissue resulting in decreases of multiple miRNAs. A similar decline of Dicer with age is observed in C. elegans. This is prevented in both species by caloric restriction. Decreased Dicer expression also occurs in preadipocytes from elderly humans and can be produced in cells by exposure to oxidative stress and UV radiation. Knockdown of Dicer in cells results in premature senescence, and fat-specific Dicer knockout renders mice hypersensitive to oxidative stress. Finally, Dicer loss-of-function mutations in worms reduce lifespan and stress tolerance, while overexpression of Dicer confers stress resistance. Thus, regulation of miRNA processing in adipose-related tissues plays an important role in longevity and the ability of an organism to respond to environmental stress and age-related disease.

Obesity and body fat distribution are important risk factors for the development of type 2 diabetes and metabolic syndrome. Evidence has accumulated that this risk is related to intrinsic differences in behavior of adipocytes in different fat depots. In the current study, we demonstrate that adipocyte precursor cells (APCs) isolated from visceral and subcutaneous white adipose depots of mice have distinct patterns of gene expression, differentiation potential, and response to environmental and genetic influences. APCs derived from subcutaneous fat differentiate well in the presence of classical induction cocktail, whereas those from visceral fat differentiate poorly but can be induced to differentiate by addition of bone morphogenetic protein (BMP)-2 or BMP-4. This difference correlates with major differences in gene expression signature between subcutaneous and visceral APCs. The number of APCs is higher in obesity-prone C57BL/6 mice than obesity-resistant 129 mice, and the number in both depots is increased by up to 270% by exposure of mice to high-fat diet. Thus, APCs from visceral and subcutaneous depots are dynamic populations, which have intrinsic differences in gene expression, differentiation properties, and responses to environmental/genetic factors. Regulation of these populations may provide a new target for the treatment and prevention of obesity and its metabolic complications.

Environmental factors, such as the macronutrient composition of the diet, can have a profound impact on risk of diabetes and metabolic syndrome. In the present study we demonstrate how a single, simple dietary factor—leucine—can modify insulin resistance by acting on multiple tissues and at multiple levels of metabolism. Mice were placed on a normal or high fat diet (HFD). Dietary leucine was doubled by addition to the drinking water. mRNA, protein and complete metabolomic profiles were assessed in the major insulin sensitive tissues and serum, and correlated with changes in glucose homeostasis and insulin signaling. After 8 weeks on HFD, mice developed obesity, fatty liver, inflammatory changes in adipose tissue and insulin resistance at the level of IRS-1 phosphorylation, as well as alterations in metabolomic profile of amino acid metabolites, TCA cycle intermediates, glucose and cholesterol metabolites, and fatty acids in liver, muscle, fat and serum. Doubling dietary leucine reversed many of the metabolite abnormalities and caused a marked improvement in glucose tolerance and insulin signaling without altering food intake or weight gain. Increased dietary leucine was also associated with a decrease in hepatic steatosis and a decrease in inflammation in adipose tissue. These changes occurred despite an increase in insulin-stimulated phosphorylation of p70S6 kinase indicating enhanced activation of mTOR, a phenomenon normally associated with insulin resistance. These data indicate that modest changes in a single environmental/nutrient factor can modify multiple metabolic and signaling pathways and modify HFD induced metabolic syndrome by acting at a systemic level on multiple tissues. These data also suggest that increasing dietary leucine may provide an adjunct in the management of obesity-related insulin resistance.

White, beige and brown adipocytes are developmentally and functionally distinct but often occur mixed together within individual depots. To target white, beige and brown adipocytes for diagnostic or therapeutic purposes, a better understanding of the cell surface properties of these cell types is essential. Using a combination of in silico, in vitro and in vivo methods, we have identified three new cell surface markers of adipose cell types. The amino acid transporter Asc-1 is a white adipocyte-specific cell surface protein, with little or no expression in brown adipocytes, whereas the amino acid transporter PAT2 and the purinergic receptor P2RX5 are cell surface markers expressed in classical brown and beige adipocytes in mice. These markers also selectively mark brown/beige and white adipocytes in human tissue. Thus, Asc-1, PAT2 and P2RX5 are membrane surface proteins that may serve as tools to identify and target white and brown/beige adipocytes for therapeutic purposes.

Obesity and type 2 diabetes are associated with mitochondrial dysfunction in adipose tissue, but the role for adipose tissues mitochondria in the development of these disorders is currently unknown. To understand the impact of adipose tissue mitochondria on whole body metabolism, we have generated a mouse model with disruption of the mitochondrial transcription factor A (TFAM) specifically in fat. F-TFKO adipose tissue exhibit decreased mtDNA copy number, altered levels of proteins of the electron transport chain, and perturbed mitochondrial function with decreased Complex I activity and greater oxygen consumption and uncoupling. As a result, F-TFKO mice exhibit higher energy expenditure and are protected from age- and diet-induced obesity, insulin resistance and hepatosteatosis, despite a greater food intake. Thus, TFAM deletion in the adipose tissue increases mitochondrial oxidation that has positive metabolic effects suggesting that regulation of adipose tissue mitochondria may be a potential therapeutic target for the treatment of obesity.

Insulin and insulin-like growth factor 1 (IGF-1) play important roles in adipocyte differentiation, glucose tolerance and insulin sensitivity. Here, to assess how these pathways can compensate for each other, we created mice with a double tissue-specific knockout of insulin and IGF-1 receptors to eliminate all insulin/IGF-1 signaling in fat. These FIGIRKO mice had markedly decreased white and brown fat mass and were completely resistant to high fat diet (HFD) induced obesity and age- and HFD-induced glucose intolerance. Energy expenditure was increased in FIGIRKO mice despite a >85% reduction in brown fat mass. However, FIGIRKO mice were unable to maintain body temperature when placed at 4°C. Brown fat activity was markedly decreased in FIGIRKO mice but was responsive to β3-receptor stimulation. Thus, insulin/IGF-1 signaling has a crucial role in the control of brown and white fat development, and, when disrupted, leads to defective thermogenesis and a paradoxical increase in basal metabolic rate.

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