Lipolysis in Brown Adipocytes Is Not Essential for Cold-Induced Thermogenesis in Mice.

Lipid droplet (LD) lipolysis in brown adipose tissue (BAT) is generally considered to be required for cold-induced nonshivering thermogenesis. Here, we show that mice lacking BAT Comparative Gene Identification-58 (CGI-58), a lipolytic activator essential for the stimulated LD lipolysis, have normal thermogenic capacity and are not cold sensitive. Relative to littermate controls, these animals had higher body temperatures when they were provided food during cold exposure. The increase in body temperature in the fed, cold-exposed knockout mice was associated with increased energy expenditure and with increased sympathetic innervation and browning of white adipose tissue (WAT). Mice lacking CGI-58 in both BAT and WAT were cold sensitive, but only in the fasted state. Thus, LD lipolysis in BAT is not essential for cold-induced nonshivering thermogenesis in vivo. Rather, CGI-58-dependent LD lipolysis in BAT regulates WAT thermogenesis, and our data uncover an essential role of WAT lipolysis in fueling thermogenesis during fasting.

Gsα deficiency in adipose tissue improves glucose metabolism and insulin sensitivity without an effect on body weight.

Gsα, the G protein that transduces receptor-stimulated cAMP generation, mediates sympathetic nervous system stimulation of brown adipose tissue (BAT) thermogenesis and browning of white adipose tissue (WAT), which are both potential targets for treating obesity, as well as lipolysis. We generated a mouse line with Gsα deficiency in mature BAT and WAT adipocytes (Ad-GsKO). Ad-GsKO mice had impaired BAT function, absent browning of WAT, and reduced lipolysis, and were therefore cold-intolerant. Despite the presence of these abnormalities, Ad-GsKO mice maintained normal energy balance on both standard and high-fat diets, associated with decreases in both lipolysis and lipid synthesis. In addition, Ad-GsKO mice maintained at thermoneutrality on a standard diet also had normal energy balance. Ad-GsKO mice had improved insulin sensitivity and glucose metabolism, possibly secondary to the effects of reduced lipolysis and lower circulating fatty acid binding protein 4 levels. Gsα signaling in adipose tissues may therefore affect whole-body glucose metabolism in the absence of an effect on body weight.

Adipocyte-specific disruption of fat-specific protein 27 causes hepatosteatosis and insulin resistance in high-fat diet-fed mice.

White adipose tissue (WAT) functions as an energy reservoir where excess circulating fatty acids are transported to WAT, converted to triglycerides, and stored as unilocular lipid droplets. Fat-specific protein 27 (FSP27, CIDEC in humans) is a lipid-coating protein highly expressed in mature white adipocytes that contributes to unilocular lipid droplet formation. However, the influence of FSP27 in adipose tissue on whole-body energy homeostasis remains unclear. Mice with adipocyte-specific disruption of the Fsp27 gene (Fsp27(ΔAd)) were generated using an aP2-Cre transgene with the Cre/LoxP system. Upon high-fat diet feeding, Fsp27(ΔAd) mice were resistant to weight gain. In the small WAT of these mice, small adipocytes containing multilocular lipid droplets were dispersed. The expression levels of the genes associated with mitochondrial abundance and brown adipocyte identity were increased, and basal lipolytic activities were significantly augmented in adipocytes isolated from Fsp27(ΔAd) mice compared with the Fsp27(F/F) counterparts. The impaired fat-storing function in Fsp27(ΔAd) adipocytes and the resultant lipid overflow from WAT led to marked hepatosteatosis, dyslipidemia, and systemic insulin resistance in high-fat diet-treated Fsp27(ΔAd) mice. These results demonstrate a critical role for FSP27 in the storage of excess fat in WAT with minimizing ectopic fat accumulation that causes insulin-resistant diabetes and non-alcoholic fatty liver disease. This mouse model may be useful for understanding the significance of fat-storing properties of white adipocytes and the role of local FSP27 in whole-body metabolism and estimating the pathogenesis of human partial lipodystrophy caused by CIDEC mutations.

Differentially regulated protein kinase A (PKA) activity in adipose tissue and liver is associated with resistance to diet-induced obesity and glucose intolerance in mice that lack PKA regulatory subunit type IIα.

The cAMP-dependent protein kinase A (PKA) signaling system is widely expressed and has a central role in regulating cellular metabolism in all organ systems affected by obesity. PKA has four regulatory (RIα, RIIα, RIß, RIIß) and four catalytic (Cα, Cß, Cγ, Prkx) subunit isoforms that have tissue-specific expression profiles. In mice, knockout (KO) of RIIß, the primary PKA regulatory subunit in adipose tissue or knockout of the catalytic subunit Cß resulted in a lean phenotype that resists diet-induced obesity and associated metabolic complications. Here we report that the disruption of the ubiquitously expressed PKA RIIα subunit in mice (RIIαKO) confers resistance to diet-induced obesity, glucose intolerance, and hepatic steatosis. After 2-week high-fat diet exposure, RIIαKO mice weighed less than wild-type littermates. Over time this effect was more pronounced in female mice that were also leaner than their wild-type counterparts, regardless of the diet. Decreased intake of a high-fat diet contributed to the attenuated weight gain in RIIαKO mice. Additionally, RIIα deficiency caused differential regulation of PKA in key metabolic organs: cAMP-stimulated PKA activity was decreased in liver and increased in gonadal adipose tissue. We conclude that RIIα represents a potential target for therapeutic interventions in obesity, glucose intolerance, and nonalcoholic fatty liver disease.

The chemical uncoupler 2,4-dinitrophenol (DNP) protects against diet-induced obesity and improves energy homeostasis in mice at thermoneutrality.

The chemical uncoupler 2,4-dinitrophenol (DNP) was an effective and widely used weight loss drug in the early 1930s. However, the physiology of DNP has not been studied in detail because toxicity, including hyperthermia and death, reduced interest in the clinical use of chemical uncouplers. To investigate DNP action, mice fed a high fat diet and housed at 30 °C (to minimize facultative thermogenesis) were treated with 800 mg/liter DNP in drinking water. DNP treatment increased energy expenditure by ∼ 17%, but did not change food intake. DNP-treated mice weighed 26% less than controls after 2 months of treatment due to decreased fat mass, without a change in lean mass. DNP improved glucose tolerance and reduced hepatic steatosis without observed toxicity. DNP treatment also reduced circulating T3 and T4 levels, Ucp1 expression, and brown adipose tissue activity, demonstrating that DNP-mediated heat generation substituted for brown adipose tissue thermogenesis. At 22 °C, a typical vivarium temperature that is below thermoneutrality, DNP treatment had no effect on body weight, adiposity, or glucose homeostasis. Thus, environmental temperature should be considered when assessing an anti-obesity drug in mice, particularly agents acting on energy expenditure. Furthermore, the beneficial effects of DNP suggest that chemical uncouplers deserve further investigation for the treatment of obesity and its comorbidities.

Erythropoietin signaling: a novel regulator of white adipose tissue inflammation during diet-induced obesity.

Obesity-induced white adipose tissue (WAT) inflammation and insulin resistance are associated with macrophage (Mф) infiltration and phenotypic shift from "anti-inflammatory" M2-like to predominantly "proinflammatory" M1-like cells. Erythropoietin (EPO), a glycoprotein hormone indispensable for erythropoiesis, has biological activities that extend to nonerythroid tissues, including antiapoptotic and anti-inflammatory effects. Using comprehensive in vivo and in vitro analyses in mice, EPO treatment inhibited WAT inflammation, normalized insulin sensitivity, and reduced glucose intolerance. We investigated EPO receptor (EPO-R) expression in WAT and characterized the role of its signaling during obesity-induced inflammation. Remarkably, and prior to any detectable changes in body weight or composition, EPO treatment reduced M1-like Mф and increased M2-like Mф in WAT, while decreasing inflammatory monocytes. These anti-inflammatory effects were found to be driven, at least in part, by direct EPO-R response in Mф via Stat3 activation, where EPO effects on M2 but not M1 Mф required interleukin-4 receptor/Stat6. Using obese ∆EpoR mice with EPO-R restricted to erythroid cells, we demonstrated an anti-inflammatory role for endogenous EPO. Collectively, our findings identify EPO-R signaling as a novel regulator of WAT inflammation, extending its nonerythroid activity to encompass effects on both Mф infiltration and subset composition in WAT.

Disrupted erythropoietin signalling promotes obesity and alters hypothalamus proopiomelanocortin production.

Although erythropoietin (Epo) is the cytokine known to regulate erythropoiesis, erythropoietin receptor (EpoR) expression and associated activity beyond haematopoietic tissue remain uncertain. Here we show that mice with EpoR expression restricted to haematopoietic tissues (Tg) develop obesity and insulin resistance. Tg-mice exhibit a decrease in energy expenditure and an increase in white fat mass and adipocyte number. Conversely, Epo treatment of wild-type (WT)-mice increases energy expenditure and reduces food intake and fat mass accumulation but shows no effect in body weight of Tg-mice. EpoR is expressed at a high level in white adipose tissue and in the proopiomelanocortin (POMC) neurons of the hypothalamus. Although Epo treatment in WT-mice induces the expression of the polypeptide hormone precursor, POMC, mice lacking EpoR show reduced levels of POMC in the hypothalamus. This study provides the first evidence that mice lacking EpoR in non-haematopoietic tissue become obese and insulin resistant with loss of Epo regulation of energy homeostasis.

Disruption of hypoxia-inducible factor 1 in adipocytes improves insulin sensitivity and decreases adiposity in high-fat diet-fed mice.

OBJECTIVE: Obesity, insulin resistance, and type 2 diabetes form a tightly correlated cluster of metabolic disorders in which adipose is one of the first affected tissues. The role of hypoxia and hypoxia-inducible factor 1 (HIF1) in the development of high-fat diet (HFD)-induced obesity and insulin resistance was investigated using animal models. RESEARCH DESIGN AND METHODS: Mice with adipocyte-specific targeted disruption of the genes encoding the HIF1 obligatory subunits Hif1α or Arnt (Hif1ß) were generated using an aP2-Cre transgene with the Cre/LoxP system. The mice were fed an HFD for 12 weeks and their metabolic phenotypes were determined. Gene expression patterns in adipose tissues were also determined by microarray and quantitative PCR. RESULTS: On an HFD, adipocyte-specific ARNT knockout mice and adipocyte-specific HIF1α knockout mice exhibit similar metabolic phenotypes, including reduced fat formation, protection from HFD-induced obesity, and insulin resistance compared with similarly fed wild-type controls. The cumulative food intake remained similar; however, the metabolic efficiency was lower in adipocyte-specific HIF1α knockout mice. Moreover, indirect calorimetry revealed respiratory exchange ratios were reduced in adipocyte-specific HIF1α knockout mice. Hyperinsulinemic-euglycemic clamp studies demonstrated that targeted disruption of HIF1α in adipocytes enhanced whole-body insulin sensitivity. The improvement of insulin resistance is associated with decreased expression of Socs3 and induction of adiponectin. CONCLUSIONS: Inhibition of HIF1 in adipose tissue ameliorates obesity and insulin resistance. This study reveals that HIF1 could provide a novel potential therapeutic target for obesity and type 2 diabetes.

Targeted deletion of one or two copies of the G protein ß subunit Gß5 gene has distinct effects on body weight and behavior in mice.

We investigated the physiological role of Gß5, a unique G protein ß subunit that dimerizes with regulators of G protein signaling (RGS) proteins of the R7 family instead of Gγ. Gß5 is essential for stability of these complexes, so that its knockout (KO)causes degradation of the entire Gß5-R7 family. We report that the Gß5-KO mice remain leaner than the wild type (WT) throughout their lifetime and are resistant to a high-fat diet. They have a 5-fold increase in locomotor activity, increased thermogenesis, and lower serum insulin, all of which correlate with a higher level of secreted epinephrine. Heterozygous (HET) mice are 2-fold more active than WT mice. Surprisingly, with respect to body weight, the HET mice display a phenotype opposite to that of the KO mice: by the age of 6 mo, they are ≥ 15% heavier than the WT and have increased adiposity, insulin resistance, and liver steatosis. These changes occur in HET mice fed a normal diet and without apparent hyperphagia, mimicking basic characteristics of human metabolic syndrome. We conclude that even a partial reduction in Gß5-R7 level can perturb normal animal metabolism and behavior. Our data on Gß5 haploinsufficient mice may explain earlier observations of genetic linkage between R7 family mutations and obesity in humans.

Myostatin inhibition in muscle, but not adipose tissue, decreases fat mass and improves insulin sensitivity.

Myostatin (Mstn) is a secreted growth factor expressed in skeletal muscle and adipose tissue that negatively regulates skeletal muscle mass. Mstn(-/-) mice have a dramatic increase in muscle mass, reduction in fat mass, and resistance to diet-induced and genetic obesity. To determine how Mstn deletion causes reduced adiposity and resistance to obesity, we analyzed substrate utilization and insulin sensitivity in Mstn(-/-) mice fed a standard chow. Despite reduced lipid oxidation in skeletal muscle, Mstn(-/-) mice had no change in the rate of whole body lipid oxidation. In contrast, Mstn(-/-) mice had increased glucose utilization and insulin sensitivity as measured by indirect calorimetry, glucose and insulin tolerance tests, and hyperinsulinemic-euglycemic clamp. To determine whether these metabolic effects were due primarily to the loss of myostatin signaling in muscle or adipose tissue, we compared two transgenic mouse lines carrying a dominant negative activin IIB receptor expressed specifically in adipocytes or skeletal muscle. We found that inhibition of myostatin signaling in adipose tissue had no effect on body composition, weight gain, or glucose and insulin tolerance in mice fed a standard diet or a high-fat diet. In contrast, inhibition of myostatin signaling in skeletal muscle, like Mstn deletion, resulted in increased lean mass, decreased fat mass, improved glucose metabolism on standard and high-fat diets, and resistance to diet-induced obesity. Our results demonstrate that Mstn(-/-) mice have an increase in insulin sensitivity and glucose uptake, and that the reduction in adipose tissue mass in Mstn(-/-) mice is an indirect result of metabolic changes in skeletal muscle. These data suggest that increasing muscle mass by administration of myostatin antagonists may be a promising therapeutic target for treating patients with obesity or diabetes.