Featured ArticlesVolume 5 | No. 12 | December 2016
|Hypoxia-inducible gene 2 promotes fat deposition and insulin resistanceHypoxia-inducible gene 2 (Hig2) has been identified as a protein that localizes to lipid droplets (LDs) in hepatocytes and promotes hepatic lipid deposition by inhibiting lipolysis. In this present study, DiStefano and colleagues observed that Hig2 is highly expressed in the adipocyte fraction of adipose tissue samples. They demonstrate that Hig2 also localizes to LDs in adipocytes, and its expression is increased with both adipogenic differentiation and fat deposition in vitro. Observations in mice harboring an adipocyte-specific deletion of Hig2 suggest that adipocyte-specific Hig2 promotes adipocyte lipid deposition and glucose intolerance.
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Objective: Adipose tissue relies on lipid droplet (LD) proteins in its role as a lipid-storing endocrine organ that controls whole body metabolism. Hypoxia-inducible Gene 2 (Hig2) is a recently identified LD-associated protein in hepatocytes that promotes hepatic lipid storage, but its role in the adipocyte had not been investigated. Here we tested the hypothesis that Hig2 localization to LDs in adipocytes promotes adipose tissue lipid deposition and systemic glucose homeostasis.
Methods: White and brown adipocyte-deficient (Hig2fl/fl × Adiponection cre+) and selective brown/beige adipocyte-deficient (Hig2fl/fl × Ucp1 cre+) mice were generated to investigate the role of Hig2 in adipose depots. Additionally, we used multiple housing temperatures to investigate the role of active brown/beige adipocytes in this process.
Results: Hig2 localized to LDs in SGBS cells, a human adipocyte cell strain. Mice with adipocyte-specific Hig2 deficiency in all adipose depots demonstrated reduced visceral adipose tissue weight and increased glucose tolerance. This metabolic effect could be attributed to brown/beige adipocyte-specific Hig2 deficiency since Hig2fl/fl × Ucp1 cre+ mice displayed the same phenotype. Furthermore, when adipocyte-deficient Hig2 mice were moved to thermoneutral conditions in which non-shivering thermogenesis is deactivated, these improvements were abrogated and glucose intolerance ensued. Adipocyte-specific Hig2 deficient animals displayed no detectable changes in adipocyte lipolysis or energy expenditure, suggesting that Hig2 may not mediate these metabolic effects by restraining lipolysis in adipocytes.
Conclusions: We conclude that Hig2 localizes to LDs in adipocytes, promoting adipose tissue lipid deposition and that its selective deficiency in active brown/beige adipose tissue mediates improved glucose tolerance at 23 °C. Reversal of this phenotype at thermoneutrality in the absence of detectable changes in energy expenditure, adipose mass, or liver triglyceride suggests that Hig2 deficiency triggers a deleterious endocrine or neuroendocrine pathway emanating from brown/beige fat cells.[Hide abstract]
|Fat and gut microbiota interactions determine obesity in miceThe gut microbial ecosystem, positioned at the interface between diet and host energy metabolism, can affect energy balance. In the present study, Kübeck et al. assessed the impact of dietary fat source and the gut microbiota on diet-induced obesity (DIO) by performing comprehensive phenotyping of the host combined with metabolite profiling. This was achieved by feeding high-fat diets to germfree and specific pathogen free mice. This work assessed host energy balance in response to different dietary fat sources in combination with metabolite profiling, qPCR, and high-throughput sequencing approaches, providing novel insights into the physiological relevance of gut microbiota and cholesterol-derived metabolites interactions.|
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Objective: Gut microbiota may promote positive energy balance; however, germfree mice can be either resistant or susceptible to diet-induced obesity (DIO) depending on the type of dietary intervention. We here sought to identify the dietary constituents that determine the susceptibility to body fat accretion in germfree (GF) mice.
Methods: GF and specific pathogen free (SPF) male C57BL/6N mice were fed high-fat diets either based on lard or palm oil for 4 wks. Mice were metabolically characterized at the end of the feeding trial. FT-ICR-MS and UPLC-TOF-MS were used for cecal as well as hepatic metabolite profiling and cecal bile acids quantification, respectively. Hepatic gene expression was examined by qRT-PCR and cecal gut microbiota of SPF mice was analyzed by high-throughput 16S rRNA gene sequencing.
Results: GF mice, but not SPF mice, were completely DIO resistant when fed a cholesterol-rich lard-based high-fat diet, whereas on a cholesterol-free palm oil-based high-fat diet, DIO was independent of gut microbiota. In GF lard-fed mice, DIO resistance was conveyed by increased energy expenditure, preferential carbohydrate oxidation, and increased fecal fat and energy excretion. Cecal metabolite profiling revealed a shift in bile acid and steroid metabolites in these lean mice, with a significant rise in 17β-estradiol, which is known to stimulate energy expenditure and interfere with bile acid metabolism. Decreased cecal bile acid levels were associated with decreased hepatic expression of genes involved in bile acid synthesis. These metabolic adaptations were largely attenuated in GF mice fed the palm-oil based high-fat diet. We propose that an interaction of gut microbiota and cholesterol metabolism is essential for fat accretion in normal SPF mice fed cholesterol-rich lard as the main dietary fat source. This is supported by a positive correlation between bile acid levels and specific bacteria of the order Clostridiales (phylum Firmicutes) as a characteristic feature of normal SPF mice fed lard.
Conclusions: In conclusion, our study identified dietary cholesterol as a candidate ingredient affecting the crosstalk between gut microbiota and host metabolism.[Hide abstract]
|Gut Microbiota and Glucometabolic Alterations in Response to Sleep Deprivation The last two decades have revealed an increasingly important role of the gut microbiome for human health. Benedict et al. conducted a study in which the impact of sleep loss on the human gut microbiota was assessed in healthy, young normal-weight individuals. This study is the first to provide evidence for sleep deprivation-induced changes in microbial families of bacterial gut species, which have previously been linked to metabolic pathologies.|
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Objective: Changes to the microbial community in the human gut have been proposed to promote metabolic disturbances that also occur after short periods of sleep loss (including insulin resistance). However, whether sleep loss affects the gut microbiota remains unknown.
Methods: In a randomized within-subject crossover study utilizing a standardized in-lab protocol (with fixed meal times and exercise schedules), we studied nine normal-weight men at two occasions: after two nights of partial sleep deprivation (PSD; sleep opportunity 02:45–07:00 h), and after two nights of normal sleep (NS; sleep opportunity 22:30–07:00 h). Fecal samples were collected within 24 h before, and after two in-lab nights, of either NS or PSD. In addition, participants underwent an oral glucose tolerance test following each sleep intervention.
Results: Microbiota composition analysis (V4 16S rRNA gene sequencing) revealed that after two days of PSD vs. after two days of NS, individuals exhibited an increased Firmicutes:Bacteroidetes ratio, higher abundances of the families Coriobacteriaceae and Erysipelotrichaceae, and lower abundance of Tenericutes (all P < 0.05) – previously all associated with metabolic perturbations in animal or human models. However, no PSD vs. NS effect on beta diversity or on fecal short-chain fatty acid concentrations was found. Fasting and postprandial insulin sensitivity decreased after PSD vs. NS (all P < 0.05).
Conclusions: Our findings demonstrate that short-term sleep loss induces subtle effects on human microbiota. To what extent the observed changes to the microbial community contribute to metabolic consequences of sleep loss warrants further investigations in larger and more prolonged sleep studies, to also assess how sleep loss impacts the microbiota in individuals who already are metabolically compromised.[Hide abstract]
|Targeting the endocannabinoid/CB1 receptor system for treating obesity Prader-Willi syndrome (PWS) is a complex neurogenetic disorder characterized by childhood-onset hyperphagia-associated obesity and intellectual disability, among other symptoms. The study by Knani and colleagues provides the first evidence that the endocannabinoid (eCB) system may contribute to severe obesity both in PWS children and adults and in an established mouse model for this syndrome. In subjects with PWS, increased circulating eCBs were associated with their metabolic abnormalities. Specifically targeting the peripheral eCB system in obese Magel2-null mice was found to be as efficacious as in diet-induced obese animals. Therefore, it may represent a novel approach to treating obesity and its complications in PWS. |
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Objective: Extreme obesity is a core phenotypic feature of Prader–Willi syndrome (PWS). Among numerous metabolic regulators, the endocannabinoid (eCB) system is critically involved in controlling feeding, body weight, and energy metabolism, and a globally acting cannabinoid-1 receptor (CB1R) blockade reverses obesity both in animals and humans. The first-in-class CB1R antagonist rimonabant proved effective in inducing weight loss in adults with PWS. However, it is no longer available for clinical use because of its centrally mediated, neuropsychiatric, adverse effects.
Methods: We studied eCB ‘tone’ in individuals with PWS and in the Magel2-null mouse model that recapitulates the major metabolic phenotypes of PWS and determined the efficacy of a peripherally restricted CB1R antagonist, JD5037 in treating obesity in these mice.
Results: Individuals with PWS had elevated circulating levels of 2-arachidonoylglycerol and its endogenous precursor and breakdown ligand, arachidonic acid. Increased hypothalamic eCB ‘tone’, manifested by increased eCBs and upregulated CB1R, was associated with increased fat mass, reduced energy expenditure, and decreased voluntary activity in Magel2-null mice. Daily chronic treatment of obese Magel2-null mice and their littermate wild-type controls with JD5037 (3 mg/kg/d for 28 days) reduced body weight, reversed hyperphagia, and improved metabolic parameters related to their obese phenotype.
Conclusions: Dysregulation of the eCB/CB1R system may contribute to hyperphagia and obesity in Magel2-null mice and in individuals with PWS. Our results demonstrate that treatment with peripherally restricted CB1R antagonists may be an effective strategy for the management of severe obesity in PWS.[Hide abstract]
|Impaired histone deacetylases expression mimics the effects of obesity and hypoxia In both human and mice obesity, hypoxia is thought to contribute to impaired adipokine production. Bricambert, Favre, Brajkovic and colleagues have identified altered adipocyte histone deacetylase (Hdac) 5 and Hdac6 expression in obesity and associated hypoxia. Modeling the defect of the two Hdacs in adipocytes by genetic silencing and selective inhibitors mimicked the effect of hypoxia and obesity on the expression of key adipokines. These findings provide a better understanding of the possible mechanism responsible of adipose dysfunction, obesity related insulin resistance, and metabolic complications.|
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Objective: The goal of the study was to investigate the role of histone deacetylases (HDACs) in adipocyte function associated with obesity and hypoxia.
Methods: Total proteins and RNA were prepared from human visceral adipose tissues (VAT) of human obese and normal weight subjects and from white adipose tissue (WAT) of C57Bl6-Rj mice fed a normal or high fat diet (HFD) for 16 weeks. HDAC activity was measured by colorimetric assay whereas the gene and protein expression were monitored by real-time PCR and by western blotting, respectively. RNA interference (RNAi) was used to silence the expression of genes in 3T3-L1 adipocytes.
Results: Total HDAC activity was decreased in VAT and WAT from obese individuals and from mice fed a HFD, respectively. The HDAC activity reduction was associated with decreased HDAC5/Hdac5 and HDAC6/Hdac6 expression in human and mice adipocyte fraction. Similarly, hypoxia hampered total Hdac activity and reduced the expression of Hdac5 and Hdac6 in 3T3-L1 adipocytes. The decrease of both Hdac5 and Hdac6 by hypoxia was associated with altered expression of adipokines and of the inducible cAMP early repressor (Icer), a key repressor that is defective in human and mice obesity. Silencing of Icer in adipocytes reproduced the changes in adipokine levels under hypoxia and obesity, suggesting a causative effect. Finally, modeling the defect of the two Hdacs in adipocytes by RNAi or selective inhibitors mimicked the effects of hypoxia on the expression of Icer, leading to impairment of insulin-induced glucose uptake.
Conclusions: Hdac5 and Hdac6 expression are required for the adequate expression of Icer and adipocyte function. Altered adipose expression of the two Hdacs in obesity by hypoxia may contribute to the development of metabolic abnormalities.[Hide abstract]
|Islet ChREBP-β is increased in diabetes and controls ChREBP-α and glucose-induced gene expression Carbohydrate-response element-binding protein (ChREBP) is the major transcription factor conferring glucose-induced expression of glycolytic, gluconeogenic, and lipogenic genes. Jing et al. have found that a novel ChREBP-β isoform is increased in diabetic islets, whereas ChREBP-α is decreased. These results suggest that ChREBP-β provides a compensatory response, limiting excessive ChREBP-α-mediated gene expression in the context of diabetes or high glucose conditions. The study sheds new light on the physiological role of this novel β-isoform of ChREBP in pancreatic islets and on the regulation of glucose-induced gene expression in general.|
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Objective: Carbohydrate-response element-binding protein (ChREBP) is the major transcription factor conferring glucose-induced gene expression in pancreatic islets, liver and adipose tissue. Recently, a novel ChREBP isoform, ChREBP-β, was identified in adipose tissue and found to be also expressed in islets and involved in glucose-induced beta cell proliferation. However, the physiological function of this less abundant β-isoform in the islet, and in diabetes, is largely unknown. The aims of the present study, therefore, were to determine how diabetes affects ChREBP-β and elucidate its physiological role in pancreatic beta cells.
Methods: Non-obese diabetic and obese, diabetic ob/ob mice were used as models of T1D and T2D and human islets and the rat INS-1 beta cell line were exposed to low/high glucose and used for ChREBP isoform-specific gain-and-loss-of-function experiments. Changes in ChREBP-β and ChREBP-a were assessed by qRT-PCR, immunoblotting, promoter luciferase, and chromatin immunoprecipitation studies.
Results: Expression of the ChREBP-β isoform was highly induced in diabetes and by glucose, whereas ChREBP-a was downregulated. Interestingly, ChREBP-β gain-of-function experiments further revealed that it was ChREBP-β that downregulated ChREBP-a through a negative feedback loop. On the other hand, ChREBP-β knockdown led to unabated ChREBP-a activity and glucose-induced expression of target genes, suggesting that one of the physiological roles of this novel β-isoform is to help keep glucose-induced and ChREBP-a-mediated gene expression under control.
Conclusions: We have identified a previously unappreciated negative feedback loop by which glucose-induced ChREBP-β downregulates ChREBP-a-signaling providing new insight into the physiological role of islet ChREBP-β and into the regulation of glucose-induced gene expression.[Hide abstract]