Featured ArticlesVolume 25 | July 2019
|Acute thyroxine-induced thermogenesis is independent of UCP1Hyperthyroidism is associated with an increased body temperature. The molecular nature of this has not been clarified although it is thought that the reason is activation of brown adipose tissue. In the brown fat cells, thermogenesis would occur via uncoupling-protein-1 (UCP1). To test this hypothesis, Dittner et al. have examined thyroid thermogenesis induced by peripherally administered thyroxine in wild-type mice and UCP1 knockout mice. The authors found that thyroid thermogenesis was independent of the presence of UCP1.|
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Objective: Hyperthyroidism is associated with increased metabolism (“thyroid thermogenesis”) and elevated body temperature, often referred to as hyperthermia. Uncoupling protein-1 (UCP1) is the protein responsible for nonshivering thermogenesis in brown adipose tissue. We here examine whether UCP1 is essential for thyroid thermogenesis.
Methods: We investigated the significance of UCP1 for thyroid thermogenesis by using UCP1-ablated (UCP1 KO) mice. To avoid confounding factors from cold-induced thermogenesis and to approach human conditions, the experiments were conducted at thermoneutrality, and to resemble conditions of endogenous release, thyroid hormone (thyroxine, T4) was injected peripherally.
Results: Both short-term and chronic thyroxine treatment led to a marked increase in metabolism that was largely UCP1-independent. Chronic thyroxine treatment led to a 1–2 °C increase in body temperature. This increase was also UCP1-independent and was maintained even at lower ambient temperatures. Thus, it was pyrexia, i.e. a defended increase in body temperature, not hyperthermia. In wildtype mice, chronic thyroxine treatment induced a large relative increase in the total amounts of UCP1 in the brown adipose tissue (practically no UCP1 in brite/beige adipose tissue), corresponding to an enhanced thermogenic response to norepinephrine injection. The increased UCP1 amount had minimal effects on thyroxine-induced thermogenesis and pyrexia.
Conclusions: These results establish that thyroid thermogenesis is a UCP1-independent process. The fact that the increased metabolism coincides with elevated body temperature and thus with accelerated kinetics accentuates the unsolved issue of the molecular background for thyroid thermogenesis.[Hide abstract]
|SIRT3 controls brown fat thermogenesis by regulation upstream of UCP1Brown adipose tissue (BAT) is capable of thermogenesis via uncoupling protein 1 (UCP1) and has high expression of sirtuin 3 (SIRT3), a deacetylase in the mitochondria. Much of the acetylation that occurs in mitochondria is thought to occur nonenzymatically. SIRT3 appears to play a critical role in reversing these acetylations, which are generally thought to have an inhibitory effect on enzymatic activities. The findings of Sebaa et al. suggest that SIRT3 indirectly controls BAT thermogenesis by deacetylating and promoting the function of pathways upstream of UCP1.|
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Objective: Brown adipose tissue (BAT) is important for thermoregulation in many mammals. Uncoupling protein 1 (UCP1) is the critical regulator of thermogenesis in BAT. Here we aimed to investigate the deacetylation control of BAT and to investigate a possible functional connection between UCP1 and sirtuin 3 (SIRT3), the master mitochondrial lysine deacetylase.
Methods: We carried out physiological, molecular, and proteomic analyses of BAT from wild-type and Sirt3KO mice when BAT is activated. Mice were either cold exposed for 2 days or were injected with the β3-adrenergic agonist, CL316,243 (1 mg/kg; i.p.). Mutagenesis studies were conducted in a cellular model to assess the impact of acetylation lysine sites on UCP1 function. Cardiac punctures were collected for proteomic analysis of blood acylcarnitines. Isolated mitochondria were used for functional analysis of OXPHOS proteins.
Results: Our findings showed that SIRT3 absence in mice resulted in impaired BAT lipid use, whole body thermoregulation, and respiration in BAT mitochondria, without affecting UCP1 expression. Acetylome profiling of BAT mitochondria revealed that SIRT3 regulates acetylation status of many BAT mitochondrial proteins including UCP1 and crucial upstream proteins. Mutagenesis work in cells suggested that UCP1 activity was independent of direct SIRT3-regulated lysine acetylation. However, SIRT3 impacted BAT mitochondrial proteins activities of acylcarnitine metabolism and specific electron transport chain complexes, CI and CII.
Conclusions: Our data highlight that SIRT3 likely controls BAT thermogenesis indirectly by targeting pathways upstream of UCP1.[Hide abstract]
|Noggin depletion in adipocytes promotes obesityNoggin is an extracellular inhibitor of bone morphogenetic proteins (BMPs), growth factors that regulate development and tissue differentiation through their influences on cell proliferation, lineage and migration. Blázquez-Medela and colleagues investigated whether Noggin has the ability to modulate adipose tissue and obesity. They deleted the Noggin gene in mouse adipocytes in vivo and found that Noggin deficiency promotes age-related whitening of brown adipose tissue and white adipose hypertrophy associated with obesity.|
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Objective: Obesity has increased to pandemic levels and enhanced understanding of adipose regulation is required for new treatment strategies. Although bone morphogenetic proteins (BMPs) influence adipogenesis, the effect of BMP antagonists such as Noggin is largely unknown. The aim of the study was to define the role of Noggin, an extracellular BMP inhibitor, in adipogenesis.
Methods: We generated adipose-derived progenitor cells and a mouse model with adipocyte-specific Noggin deletion using the Adiponectin Cre transgenic mouse, and determined the adipose phenotype of Noggin-deficiency.
Results: Our studies showed that Noggin is expressed in progenitor cells but declines in adipocytes, possibly allowing for lipid accumulation. Correspondingly, adipocyte-specific Noggin deletion in vivo promoted age-related obesity in both genders with no change in food intake. Although the loss of Noggin caused white adipose tissue hypertrophy, and whitening and impaired function in brown adipose tissue in both genders, there were clear gender differences with the females being most affected. The females had suppressed expression of brown adipose markers and thermogenic genes including peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC1alpha) and uncoupling protein 1 (UCP1) as well as genes associated with adipogenesis and lipid metabolism. The males, on the other hand, had early changes in a few BAT markers and thermogenic genes, but the main changes were in the genes associated with adipogenesis and lipid metabolism. Further characterization revealed that both genders had reductions in VO2, VCO2, and RER, whereas females also had reduced heat production. Noggin was also reduced in diet-induced obesity in inbred mice consistent with the obesity phenotype of the Noggin-deficient mice.
Conclusions: BMP signaling regulates female and male adipogenesis through different metabolic pathways. Modulation of adipose tissue metabolism by select BMP antagonists may be a strategy for long-term regulation of age-related weight gain and obesity.[Hide abstract]
|Loss of GLP-1R and Y2R does not alter progression of obesity or response to RYGB surgeryRoux-en-Y gastric bypass (RYGB) results in large and sustained weight loss. It is thought that the beneficial effects of RYGB are mediated, at least in part, by the gut hormones GLP-1 and PYY. Boland, Mumphrey, et al. hypothesized that deleting both signaling pathways simultaneously in GLP1R/Y2R double knockout mice would significantly attenuate the effects of RYGB. Unexpectedly, however, combined loss of GLP-1R and Y2R signaling had only minor effects on body weight, fat mass, and glucose homeostasis. Furthermore, RYGB was just as effective in lowering body weight and adiposity as well as improving glucose tolerance and insulin sensitivity.|
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Objective: Understanding the mechanisms underlying the remarkable beneficial effects of gastric bypass surgery is important for the development of non-surgical therapies or less invasive surgeries in the fight against obesity and metabolic disease. Although the intestinal L-cell hormones glucagon-like peptide-1 (GLP-1) and peptide tyrosine–tyrosine (PYY) have attracted the most attention, direct tests in humans and rodents with pharmacological blockade or genetic deletion of either the GLP1-receptor (GLP1R) or the Y2-receptor (Y2R) were unable to confirm their critical roles in the beneficial effects gastric bypass surgery on body weight and glucose homeostasis. However, new awareness of the power of combinatorial therapies in the treatment of metabolic disease would suggest that combined blockade of more than one signaling pathway may be necessary to reverse the beneficial effects of bariatric surgery.
Methods: The metabolic effects of high-fat diet and the ability of Roux-en-Y gastric bypass surgery to lower food intake and body weight, as well as improve glucose handling, was tested in GLP1R and Y2R-double knockout (GLP1RKO/Y2RKO) and C57BL6J wildtype (WT) mice.
Results: GLP1RKO/Y2RKO and WT mice responded similarly for up to 20 weeks on high-fat diet and 16 weeks after RYGB. There were no significant differences in loss of body and liver weight, fat mass, reduced food intake, relative increase in energy expenditure, improved fasting insulin, glucose tolerance, and insulin tolerance between WT and GLP1RKO/Y2RKO mice after RYGB.
Conclusions: Combined loss of GLP1R and Y2R-signaling was not able to negate or attenuate the beneficial effects of RYGB on body weight and glucose homeostasis in mice, suggesting that a larger number of signaling pathways is involved or that the critical pathway has not yet been identified.[Hide abstract]
|Maternal Western-style diet affects offspring isletsThe Developmental Origins of Health and Disease (DOHaD) hypothesis states that the gestational and immediate postnatal environments influence long-term offspring health and play a role in adult disease etiology. Elsakr and colleagues used a well-established, non-human primate (NHP) model, the Japanese Macaque, to determine the consequences of exposure to Western-Style Diet (WSD) in utero and during lactation on islet cell mass and function in the offspring. Islets from offspring whose mothers had received WSD secreted more insulin in response to a glucose challenge ex vivo. Reduction of α-cell mass as a result of maternal WSD persists to three years of age even when offspring are weaned onto control diet.|
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Objective: In humans, offspring of women who are overweight or obese are more likely to develop metabolic disease later in life. Studies in lower animal species reveal that a calorically-dense maternal diet is associated with alterations in islet cell mass and function. The long-term effects of maternal diet on the structure and function of offspring islets with characteristics similar to humans are unknown. We used a well-established non-human primate (NHP) model to determine the consequences of exposure to Western-Style Diet (WSD) in utero and during lactation on islet cell mass and function in the offspring.
Methods: Female Japanese Macaques (Macaca fuscata) were fed either control (CTR) or WSD before and throughout pregnancy and lactation. Offspring were weaned onto CTR or WSD to generate four different groups based on maternal/offspring diets: CTR/CTR, WSD/CTR, CTR/WSD, and WSD/WSD. Offspring were analyzed at three years of age. Pancreatic tissue sections were immunolabelled to measure α- and β-cell mass and proliferation as well as islet vascularization. Live islets were also isolated to test the effects of WSD-exposure on islet function ex vivo. Offspring glucose tolerance was correlated with various maternal characteristics.
Results: α-cell mass was reduced as a result of maternal WSD exposure. α-cell proliferation was reduced in response to offspring WSD. Islet vasculature did not differ among the diet groups. Islets from WSD/CTR offspring secreted a greater amount of insulin in response to glucose ex vivo. We also found that maternal glucose tolerance and parity correlated with offspring glucose tolerance.
Conclusions: Maternal WSD exposure results in persistently decreased α-cell mass in the three-year old offspring. WSD/CTR islets secreted greater amounts of insulin ex vivo, suggesting that these islets are primed to hyper-secrete insulin under certain metabolic stressors. Although WSD did not induce overt impaired glucose tolerance in dams or offspring, offspring born to mothers with higher glucose excursions during a glucose tolerance test were more likely to also show higher glucose excursions.[Hide abstract]
|ABHD15 regulates adipose tissue lipolysis and hepatic lipid accumulationInsulin suppresses lipolysis in adipocytes. However, the mechanism for this is not well understood. ABHD15 displays several properties implicating it as having an important role in lipolysis. It is a member of the α/β-hydrolase family of proteins that includes several other lipolytic regulatory proteins. Stöckli, Zadoorian, et al. show that insulin-stimulated suppression of lipolysis is blocked in the absence of ABHD15 in adipocytes. ABHD15 knockout mice displayed elevated circulating free fatty acids upon fasting and in response to insulin. Furthermore, ABHD15 knockout mice showed increased liver triglycerides upon β-adrenergic receptor activation.|
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Objective: Insulin suppresses adipose tissue lipolysis after a meal, playing a key role in metabolic homeostasis. This is mediated via the kinase Akt and its substrate phosphodiesterase 3B (PDE3B). Once phosphorylated and activated, PDE3B hydrolyses cAMP leading to the inactivation of cAMP-dependent protein kinase (PKA) and suppression of lipolysis. However, several gaps have emerged in this model. Here we investigated the role of the PDE3B-interacting protein, α/β-hydrolase ABHD15 in this process.
Methods: Lipolysis, glucose uptake, and signaling were assessed in ABHD15 knock down and knock out adipocytes and fat explants in response to insulin and/or β-adrenergic receptor agonist. Glucose and fatty acid metabolism were determined in wild type and ABHD15−/− littermate mice.
Results: Deletion of ABHD15 in adipocytes resulted in a significant defect in insulin-mediated suppression of lipolysis with no effect on insulin-mediated glucose uptake. ABHD15 played a role in suppressing PKA signaling as phosphorylation of the PKA substrate Perilipin-1 remained elevated in response to insulin upon ABHD15 deletion. ABHD15−/− mice had normal glucose metabolism but defective fatty acid metabolism: plasma fatty acids were elevated upon fasting and in response to insulin, and this was accompanied by elevated liver triglycerides upon β-adrenergic receptor activation. This is likely due to hyperactive lipolysis as evident by the larger triglyceride depletion in brown adipose tissue in these mice. Finally, ABHD15 protein levels were reduced in adipocytes from mice fed a Western diet, further implicating this protein in metabolic homeostasis.
Conclusions: Collectively, ABHD15 regulates adipocyte lipolysis and liver lipid accumulation, providing novel therapeutic opportunities for modulating lipid homeostasis in disease.[Hide abstract]
|JUND regulates pancreatic β cell survival JUND is a member of the Jun family of transcription factors. It has been previously linked to regulating oxidative stress in other cell types, but its role in β cells is unknown. Good et al. uncover JUND as a novel stress-responsive factor that is translationally upregulated in β cells during metabolic stress. Assessments of oxidative stress and apoptosis in primary islets as well as transcriptome analyses indicate that JUND promotes a maladaptive response in β cells during conditions associated with type 2 diabetes.|
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Objective: In type 2 diabetes (T2D), oxidative stress contributes to the dysfunction and loss of pancreatic β cells. A highly conserved feature of the cellular response to stress is the regulation of mRNA translation; however, the genes regulated at the level of translation are often overlooked due to the convenience of RNA sequencing technologies. Our goal is to investigate translational regulation in β cells as a means to uncover novel factors and pathways pertinent to cellular adaptation and survival during T2D-associated conditions.
Methods: Translating ribosome affinity purification (TRAP) followed by RNA-seq or RT-qPCR was used to identify changes in the ribosome occupancy of mRNAs in Min6 cells. Gene depletion studies used lentiviral delivery of shRNAs to primary mouse islets or CRISPR-Cas9 to Min6 cells. Oxidative stress and apoptosis were measured in primary islets using cell-permeable dyes with fluorescence readouts of oxidation and activated cleaved caspase-3 and-7, respectively. Gene expression was assessed by RNA-seq, RT-qPCR, and western blot. ChIP-qPCR was used to determine chromatin enrichment.
Results: TRAP-seq in a PDX1-deficiency model of β cell dysfunction uncovered a cohort of genes regulated at the level of mRNA translation, including the transcription factor JUND. Using a panel of diabetes-associated stressors, JUND was found to be upregulated in mouse islets cultured with high concentrations of glucose and free fatty acid, but not after treatment with hydrogen peroxide or thapsigargin. This induction of JUND could be attributed to increased mRNA translation. JUND was also upregulated in islets from diabetic db/db mice and in human islets treated with high glucose and free fatty acid. Depletion of JUND in primary islets reduced oxidative stress and apoptosis in β cells during metabolic stress. Transcriptome assessment identified a cohort of genes, including pro-oxidant and pro-inflammatory genes, regulated by JUND that are commonly dysregulated in models of β cell dysfunction, consistent with a maladaptive role for JUND in islets.
Conclusions: A translation-centric approach uncovered JUND as a stress-responsive factor in β cells that contributes to redox imbalance and apoptosis during pathophysiologically relevant stress.[Hide abstract]
|Reduced insulin action in muscle not associated with defective insulin signalingEnergy metabolism in mammals is regulated by mechanisms that follow a clear diurnal rhythm. Assessing glucose metabolism and insulin action for any dysregulation relevant to the risk of developing type 2 diabetes at a single point in time therefore provides only a limited picture of the 24-hour physiology of glucose homeostasis. Small and colleagues investigated the relationship between circulating insulin, insulin signaling, and glucose metabolism in vivo over multiple points of the normal diurnal cycle. The results show that reduced glucose uptake in the muscle of high fat, high sugar-fed rats over the diurnal cycle was not associated with reduced Akt signaling.|
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Objective: Energy metabolism and insulin action follow a diurnal rhythm. It is therefore important that investigations into dysregulation of these pathways are relevant to the physiology of this diurnal rhythm.
Methods: We examined glucose uptake, markers of insulin action, and the phosphorylation of insulin signaling intermediates in muscle of chow and high fat, high sucrose (HFHS) diet-fed rats over the normal diurnal cycle.
Results: HFHS animals displayed hyperinsulinemia but had reduced systemic glucose disposal and lower muscle glucose uptake during the feeding period. Analysis of gene expression, enzyme activity, protein abundance and phosphorylation revealed a clear diurnal regulation of substrate oxidation pathways with no difference in Akt signaling in muscle. Transfection of a constitutively active Akt2 into the muscle of HFHS rats did not rescue diet-induced reductions in insulin-stimulated glucose uptake.
Conclusions: These studies suggest that reduced glucose uptake in muscle during the diurnal cycle induced by short-term HFHS-feeding is not the result of reduced insulin signaling.[Hide abstract]
|A sterol-regulated feedback loop modulates statin sensitivity in prostate cancerProstate cancer (PCa) is the most commonly diagnosed malignancy in men. Statins are clinically approved agents that are commonly prescribed for the management of high cholesterol, but more recently have been shown to possess anti-cancer properties. However, heterogeneous responses to statin exposure have been reported. Longo et al. investigated the conditions which may make PCa cells sensitive to statins. They report that inhibition of the sterol regulatory element-binding protein 2 potentiated statin-induced apoptosis in PCa cells that were relatively insensitive to statin as a single agent.|
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Objective: The statin family of cholesterol-lowering drugs has been shown to induce tumor-specific apoptosis by inhibiting the rate-limiting enzyme of the mevalonate (MVA) pathway, HMG-CoA reductase (HMGCR). Accumulating evidence suggests that statin use may delay prostate cancer (PCa) progression in a subset of patients; however, the determinants of statin drug sensitivity in PCa remain unclear. Our goal was to identify molecular features of statin-sensitive PCa and opportunities to potentiate statin-induced PCa cell death.
Methods: Deregulation of HMGCR expression in PCa was evaluated by immunohistochemistry. The response of PCa cell lines to fluvastatin-mediated HMGCR inhibition was assessed using cell viability and apoptosis assays. Activation of the sterol-regulated feedback loop of the MVA pathway, which was hypothesized to modulate statin sensitivity in PCa, was also evaluated. Inhibition of this statin-induced feedback loop was performed using RNA interference or small molecule inhibitors. The achievable levels of fluvastatin in mouse prostate tissue were measured using liquid chromatography–mass spectrometry.
Results: High HMGCR expression in PCa was associated with poor prognosis; however, not all PCa cell lines underwent apoptosis in response to treatment with physiologically-achievable concentrations of fluvastatin. Rather, most cell lines initiated a feedback response mediated by sterol regulatory element-binding protein 2 (SREBP2), which led to the further upregulation of HMGCR and other lipid metabolism genes. Overcoming this feedback mechanism by knocking down or inhibiting SREBP2 potentiated fluvastatin-induced PCa cell death. Notably, we demonstrated that this feedback loop is pharmacologically-actionable, as the drug dipyridamole can be used to block fluvastatin-induced SREBP activation and augment apoptosis in statin-insensitive PCa cells.
Conclusions: Our study implicates statin-induced SREBP2 activation as a PCa vulnerability that can be exploited for therapeutic purposes using clinically-approved agents.[Hide abstract]
|A leptin receptor point mutation resulting in metabolic dysfunctions distinct from db/db miceLeptin plays a crucial role in controlling food intake and energy expenditure. Leptin receptor deficient (db/db) mice suffer from fat accumulation, massive obesity, and the development of diabetes due to excessive food intake. Piattini, Le Foll, and colleagues describe a novel spontaneous point mutation of the leptin receptor gene in mice, leading to a truncated protein. Partial responsiveness to leptin in homozygous mutants resulted in differences in the control of energy expenditure and the severity of diabetes compared to db/db mice, although the development of obesity was comparable to full KO mice.|
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Objective: Leptin (Lep) plays a crucial role in controlling food intake and energy expenditure. Defective Lep/LepRb-signaling leads to fat accumulation, massive obesity, and the development of diabetes. We serendipitously noticed spontaneous development of obesity similar to LepR-deficient (db/db) mice in offspring from a C57BL/6J breeding and transmittance of the phenotype in a Mendelian manner. Candidate gene sequencing revealed a spontaneous point mutation in the LepRb gene. We investigated leptin responsiveness, leptin receptor signaling and metabolic phenotype of this novel LepRb mutant mouse variant.
Methods: Overexpression and functional tests of the mutant LepRb in 3T3 cells. Measurement of leptin responsiveness in hypothalamic nuclei, glucose tolerance, food uptake and energy expenditure in the mutant mice.
Results: The mutation results in the exchange of a glycine for serine (G506S) and introduces an alternative splice acceptor which, when used, encodes for a protein with a 40aa deletion that is retained in the cytoplasm. LepRb signaling was abrogated in the hypothalamic ventromedial nucleus (VMN) and dorsomedial nucleus (DMN), but only partially reduced in the hypothalamic arcuate nucleus (ARC) of LepRbG506S/G506S mice, most likely due to differential splicing in neurons located in the respective regions of the hypothalamus. Extensive metabolic characterization of these mice revealed interesting differences in the control of food intake, glucose tolerance, energy expenditure, and fat accumulation in LepRbG506S/G506S compared with LepRb-deficient db/db mice.
Conclusions: This study provides further insight into differences of the leptin responsiveness in VMN, DMN, and ARC and its metabolic consequences.[Hide abstract]
|Gsα deficiency in the dorsomedial hypothalamus leads to obesity
Gsα couples multiple receptors to intracellular cAMP generation. Germline inactivating Gsα mutations lead to obesity in humans and mice. Mice with brain-specific Gsα deficiency also develop obesity with reduced energy expenditure and locomotor activity, and impaired adaptive thermogenesis, but the underlying mechanisms remain unclear. Chen et al. created mice with complete Gsα deficiency in the dorsomedial hypothalamus. These mice develop severe early-onset obesity associated with hyperphagia, along with decreases in energy expenditure. This is associated with impaired leptin signaling.|
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Objective: Gsα couples multiple receptors, including the melanocortin 4 receptor (MC4R), to intracellular cAMP generation. Germline inactivating Gsα mutations lead to obesity in humans and mice. Mice with brain-specific Gsα deficiency also develop obesity with reduced energy expenditure and locomotor activity, and impaired adaptive thermogenesis, but the underlying mechanisms remain unclear.
Methods: We created mice (DMHGsKO) with Gsα deficiency limited to the dorsomedial hypothalamus (DMH) and examined the effects on energy balance and thermogenesis.
Results: DMHGsKO mice developed severe, early-onset obesity associated with hyperphagia and reduced energy expenditure and locomotor activity, along with impaired brown adipose tissue thermogenesis. Studies in mice with loss of MC4R in the DMH suggest that defective DMH MC4R/Gsα signaling contributes to abnormal energy balance but not to abnormal locomotor activity or cold-induced thermogenesis. Instead, DMHGsKO mice had impaired leptin signaling along with increased expression of the leptin signaling inhibitor protein tyrosine phosphatase 1B in the DMH, which likely contributes to the observed hyperphagia and reductions in energy expenditure, locomotor activity, and cold-induced thermogenesis.
Conclusions: DMH Gsα signaling is critical for energy balance, thermogenesis, and leptin signaling. This study provides insight into how distinct signaling pathways can interact to regulate energy homeostasis and temperature regulation.[Hide abstract]
|Imaging adipose tissue browning using the TSPO-18kDa tracer [18F]FEPPAChemical stimulation with β3-adrenergic receptor agonists has been shown to induce browning in white adipose tissue (WAT), but determination of the quantity and location of this new beige fat mass is a challenge. The difference in mitochondrial abundance between WAT and beige fat suggests that an imaging agent able to quantify mitochondrial expression may be ideal for identifying beige fat mass. TSPO-18kDa (TSPO) is a five transmembrane domain protein located on the outer membrane of mitochondria, which may be detected using positron emission tomography (PET). Hartimath et al. tested the TSPO tracer [18F]FEPPAand found that it may be useful for detecting unstimulated beige adipocytes.|
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Objective: The browning of white adipose tissue (WAT) into beige has been proposed as a strategy to enhance energy expenditure to combat the growing epidemic of obesity. Research into browning strategies are hampered by the lack of sensitive, translatable, imaging tools capable of detecting beige fat mass non-invasively. [18F]FDG is able to detect activated beige fat but provides little information on unstimulated beige fat mass. We have assessed the use of [18F]FEPPA, a tracer for the TSPO-18KDa found on the outer mitochondrial membrane, as an alternative imaging agent capable of detecting unstimulated brown fat (BAT) and beige fat.
Methods: Female Balb/c mice (n = 5) were treated for 7 days with the β3 adrenergic agonist CL-316,243 to induce the browning of inguinal WAT (beige fat). Animals were imaged longitudinally with [18F]FDG and [18F]FEPPA and uptake in interscapular BAT and inguinal WAT assessed. The browning of inguinal WAT was confirmed using H&E and immunohistochemical detection of UCP-1 and TSPO.
Results: Repeated dosing with β3-adrenergic agonist CL-316,243 caused a significant increase in [18F]FDG uptake in both interscapular BAT and inguinal WAT associated with the increased metabolic activity of brown and beige adipocytes respectively. [18F]FEPPA uptake was likewise increased in inguinal WAT but showed no increase in BAT uptake due to stimulation over the same time course. Furthermore, inguinal WAT uptake was unaffected by pharmacological blockade, indicating that [18F]FEPPA uptake is associated with the expression of mitochondria in BAT and beige adipocytes and independent of activation.
Conclusions: These data show that [18F]FEPPA can detect BAT and newly formed beige fat under non-stimulated, thermoneutral conditions and that uptake after stimulation is linked to mitochondrial expression as opposed to activation.[Hide abstract]
|Epigenetic regulation of diabetogenic adipose morphologyWhite adipose tissue (WAT) expands by increasing adipocyte number (hyperplasia) and size (hypertrophy). Their relative importance differs between individuals resulting in alternate adipose morphologies. Hypertrophic WAT is the pernicious morphology, which is associated with dyslipidemia, insulin resistance, and T2D. Kerr and colleagues tested whether WAT morphology is epigenetically regulated and performed CpG-methylome profiling on abdominal subcutaneous adipocytes from a large cohort of women. Their results support the notion that differential CpG-methylation in adipocytes is linked to hypertrophic WAT morphology predisposing to T2D.|
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Objective: Hypertrophic white adipose tissue (WAT) morphology is associated with insulin resistance and type 2 diabetes. The mechanisms governing hyperplastic versus hypertrophic WAT expansion are poorly understood. We assessed if epigenetic modifications in adipocytes are associated with hypertrophic adipose morphology. A subset of genes with differentially methylated CpG-sites (DMS) in the promoters was taken forward for functional evaluation.
Methods: The study included 126 women who underwent abdominal subcutaneous biopsy to determine adipose morphology. Global transcriptome profiling was performed on WAT from 113 of the women, and CpG methylome profiling on isolated adipocytes from 78 women. Small interfering RNAs (siRNA) knockdown in human mesenchymal stem cells (hMSCs) was used to assess influence of specific genes on lipid storage.
Results: A higher proportion of CpG-sites were methylated in hypertrophic compared to hyperplastic WAT. Methylation at 35,138 CpG-sites was found to correlate to adipose morphology. 2,102 of these CpG-sites were also differentially methylated in T2D; 98% showed directionally consistent change in methylation in WAT hypertrophy and T2D. We identified 2,508 DMS in 638 adipose morphology-associated genes where methylation correlated with gene expression. These genes were over-represented in gene sets relevant to WAT hypertrophy, such as insulin resistance, lipolysis, extracellular matrix organization, and innate immunity. siRNA knockdown of ADH1B, AZGP1, C14orf180, GYG2, HADH, PRKAR2B, PFKFB3, and AQP7 influenced lipid storage and metabolism.
Conclusions: CpG methylation could be influential in determining adipose morphology and thereby constitute a novel antidiabetic target. We identified C14orf180 as a novel regulator of adipocyte lipid storage and possibly differentiation.[Hide abstract]
|The best housing temperature to translate mouse experiments to humansTemperature is a key environmental variable that has various impacts on the physiology and health of all animals, including humans. Mice are currently the most widely used animal model for human disease and fundamental biology. Therefore, it is important to choose a housing temperature for mice that best mimics everyday human conditions; however, which temperature to choose is not trivial. Keijer and colleagues studied the oxygen consumption of C57BL/6 mice measured across the range of temperatures from 21.4 to 30.2 °C in order to find the best housing temperature. From their results, they conclude that 25.5 °C to 27.6 °C may be optimal for solitary housed mice and discuss this in the light of suggestions and results from other groups.|
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Objective: Ambient temperature impinges on energy metabolism in a body size dependent manner. This has implications for the housing temperature at which mice are best compared to humans. In 2013, we suggested that, for comparative studies, solitary mice are best housed at 23–25 °C, because this is 3–5 °C below the mouse thermoneutral zone and humans routinely live 3–5 °C below thermoneutrality, and because this generates a ratio of DEE to BMR of 1.6–1.9, mimicking the ratio found in free-living humans.
Methods: Recently, Fischer et al. (2017) challenged this estimate. By studying mice at 21 °C and at 30 °C (but notably not at 23–25 °C) they concluded that 30 °C is the optimal housing temperature. Here, we measured energy metabolism of C57BL/6 mice over a range of temperatures, between 21.4 °C and 30.2 °C.
Results: We observed a ratio of DEE to BMR of 1.7 at 27.6 °C and of 1.8 at 25.5 °C, suggesting that this is the best temperature range for housing C57BL/6 mice to mimic human thermal relations. We used a 24 min average to calculate the ratio, similar to that used in human studies, while the ratio calculated by Fisher et al. dependent on short, transient metabolic declines.
Conclusions: We concur with Fisher et al. and others that 21 °C is too cool, but we continue to suggest that 30 °C is too warm. We support this with other data. Finally, to mimic living environments of all humans, and not just those in controlled Western environments, mouse experimentation at various temperatures is likely required.[Hide abstract]