Exendin-4 regulates lipid metabolism and fibroblast growth factor 21 in hepatic steatosis

Objective

Hepatokine fibroblast growth factor (FGF) 21 takes part in the regulation of lipid metabolism in the liver and adipose tissue. We investigated whether exendin-4 regulates the expression of FGF21 in the liver, and whether the effects of exendin-4 on the regulation of FGF21 expression are mediated via silent mating type information regulation 2 homolog (SIRT) 1 or SIRT6.

Materials/methods

The C57BL/6J mice were fed a low fat diet, high fat diet, or high fat diet with 1 nmol/kg/day exendin-4 intraperitoneal injection for 10 weeks. HepG2 used in vitro study was treated with palmitic aicd (0.4 mM) with or without exendin-4 (100 nM) and FGF21 (50 nM) for 24 hours. The change of FGF21 and its receptors expression by exendin-4 were measured using quantitative real-time RT-PCR and Western blot. The intracellular lipid content in HepG2 cells was evaluated by Oil Red O staining. Inhibition of FGF21, SIRT1 and SIRT6, by 10 nM siRNA was performed to establish the signaling pathway of exendin-4 action in hepatic lipid metabolism.

Results

Exendin-4 increased the expression of FGF21 and its receptors in high fat diet-induced obese mice. In addition, recombinant FGF21 treatment reduced lipid content in palmitic acid-treated HepG2 cells. We also observed significantly decreased expression of peroxisomal proliferator-activated receptor (PPAR) α and medium-chain acyl-coenzyme A dehydrogenase (MCAD) in hepatocytes transfected with FGF21 siRNA. In cells treated with exendin-4, inhibition of SIRT1, but not SIRT6, by siRNA significantly repressed the expression of FGF21 mRNA, whereas decreased SIRT1 expression by inhibition of FGF21 was not observed.

Conclusions

These data suggest that exendin-4 could improve fatty liver by increasing SIRT1-mediated FGF21.     

Brown fat like gene expression in the epicardial fat depot correlates with circulating HDL-cholesterol and triglycerides in patients with coronary artery disease

Background

Recent evidence indicates that epicardial adipose tissue (EAT) expresses uncoupling protein-1 (UCP1), a marker of brown adipocytes. However, the putative effects of the presence of brown adipocytes in EAT remain unknown.

Methods

The mRNA expression of genes related to brown adipocyte-mediated thermogenesis was measured in the fat samples collected from the epicardial-, mediastinal- and subcutaneous-depots of patients undergoing coronary artery bypass grafting. Both univariate and multivariate analyses were then utilized to determine any association between gene expression and the anthropometrics and fasting blood chemistries of these patients.

Results

EAT exhibited significantly higher expression of UCP1 and cytochrome c oxidase subunit-IV (COX-IV) compared to mediastinal- and subcutaneous-fat depots (P ≤ 0.05). EAT expression of UCP1 (r = 0.50), COX-IV (r = 0.37) and lipoprotein lipase (LPL) (r = 0.58) positively associated with circulating levels of HDL-cholesterol (P ≤ 0.05). In addition, EAT expression of LPL, acyl coA dehydrogenase-short, -medium and -long chain genes associated negatively with circulating TG levels (P ≤ 0.05).

Conclusions

Abundance of UCP-1 in the EAT relative to other fat depots confirms the presence of brown adipocytes in human EAT. Furthermore, the correlations among the EAT expression of thermogenesis-related genes with the circulating HDL and TG levels indicate that presence of active brown adipocytes shares a functional association with the circulating plasma lipids in humans.     

Measurement of brown adipose tissue mass using a novel dual-echo magnetic resonance imaging approach: A validation study

Objective

The aim of this study was to evaluate and validate magnetic resonance imaging (MRI) for the visualization and quantification of brown adipose tissue (BAT) in vivo in a rat model. We hypothesized that, based on differences in tissue water and lipid content, MRI could reliably differentiate between BAT and white adipose tissue (WAT) and could therefore be a possible alternative for ¹⁸ F-Fluorodeoxyglucose Positron Emission Tomography (¹⁸FDG-PET), the current gold standard for non-invasive BAT quantification.

Materials/Methods

Eleven rats were studied using both ¹⁸FDG-PET/CT and MRI (1.5 T). A dual echo (in-and-out-of-phase) sequence was used, both with and without spectral presaturation inversion recovery (SPIR) fat suppression (DUAL-SPIR) to visualize BAT, after which all BAT was surgically excised. The BAT volume measurements obtained via ¹⁸FDG-PET/CT and DUAL-SPIR MR were quantitatively compared with the histological findings. All study protocols were reviewed and approved by the local ethics committee.

Results

The BAT mass measurements that were obtained using DUAL-SPIR MR subtraction images correlated better with the histological findings (P = 0.017, R = 0.89) than did the measurements obtained using ¹⁸FDG-PET/CT (P = 0.78, R = 0.15), regardless of the BAT metabolic activation state. Additionally, the basic feasibility of the DUAL-SPIR method was demonstrated in three human pilot subjects.

Conclusions

This study demonstrates the potential for MRI to reliably detect and quantify BAT in vivo. MRI can provide information beyond that provided by ¹⁸FDG-PET imaging, and its ability to detect BAT is independent of its metabolic activation state. Additionally, MRI is a low-cost alternative that does not require radiation.     

Visceral/epicardial adiposity in nonobese and apparently healthy young adults: Association with the cardiometabolic profile

Objective

We investigate associations of regional adipose tissues with cardiometabolic profile of nonobese and apparently healthy young adults.

Methods

Four hundred twenty-five nonobese and apparently healthy individuals were assessed for blood pressure and fasting lipid profile, blood glucose and adiponectin. Subcutaneous abdominal adipose tissue (SAT) and ectopic fat depots (visceral abdominal adipose tissue [VAT], epicardial adipose tissue [EAT] and hepatic fat fraction [HFF]) were quantified by magnetic resonance imaging.

Results

According to anthropometric measurements, blood pressure and blood markers, the population (18–35 years, 54% women) had a low cardiometabolic risk. Compared to women, men had more VAT, EAT and HFF, but less SAT. Regional adipose tissues were positively correlated with each other. VAT and EAT carried significant correlations with all markers of cardiometabolic risk, while SAT and HFF correlated variably with these markers. While taking into account age and gender, SAT, VAT and EAT were associated with most cardiometabolic markers, while HFF was only associated with total cholesterol/high-density lipoprotein ratio (TC/HDL-C) and triglycerides (TG). When comparing SAT, VAT and EAT head-to-head, VAT was the only adipose tissue location maintaining significant association with most markers of cardiometabolic risk. Greater VAT (≥50th percentile) was associated with a worse cardiometabolic profile, whether individuals were overweight or normal weight.

Conclusion

Even in nonobese and apparently healthy young women and men, accumulation of ectopic visceral adiposity in general, and of VAT in particular, is associated with a worse cardiometabolic profile whether individuals were overweight or normal weight.     

Liver X receptor β controls thyroid hormone feedback in the brain and regulates browning of subcutaneous white adipose tissue

The recent discovery of browning of white adipose tissue (WAT) has raised great research interest because of its significant potential in counteracting obesity and type 2 diabetes. Browning is the result of the induction in WAT of a newly discovered type of adipocyte, the beige cell. When mice are exposed to cold or several kinds of hormones or treatments with chemicals, specific depots of WAT undergo a browning process, characterized by highly activated mitochondria and increased heat production and energy expenditure. However, the mechanisms underlying browning are still poorly understood. Liver X receptors (LXRs) are one class of nuclear receptors, which play a vital role in regulating cholesterol, triglyceride, and glucose metabolism. Following our previous finding that LXRs serve as repressors of uncoupling protein-1 (UCP1) in classic brown adipose tissue in female mice, we found that LXRs, especially LXRβ, also repress the browning process of subcutaneous adipose tissue (SAT) in male rodents fed a normal diet. Depletion of LXRs activated thyroid-stimulating hormone (TSH)-releasing hormone (TRH)-positive neurons in the paraventricular nucleus area of the hypothalamus and thus stimulated secretion of TSH from the pituitary. Consequently, production of thyroid hormones in the thyroid gland and circulating thyroid hormone level were increased. Moreover, the activity of thyroid signaling in SAT was markedly increased. Together, our findings have uncovered the basis of increased energy expenditure in male LXR knockout mice and provided support for targeting LXRs in treatment of obesity.The metabolic syndrome is a constellation of related disorders (obesity, insulin resistance, dyslipidemia, fatty liver, hypertension, and atherosclerosis) (1–3). Of note, obesity, which is attributable to the chronic imbalance between energy intake and energy expenditure, is an epidemic, for which there is no effective therapy (4). A major challenge in battling this epidemic is to identify a target that can either decrease energy intake or increase energy expenditure. There is great research interest in brown adipose tissue (BAT), which is specialized for the dissipation of chemical energy in the form of heat (4, 5). BAT defends mammals against hypothermia, obesity, and type 2 diabetes; however, adult humans lack this thermogenic interscapular organ (6). Recently, studies have demonstrated that adult humans harbor a distinct cold-inducible depot of brown adipocytes that are expressed in WAT in the supraclavicular, paraaortic, and suprarenal regions (7–9). These cells, called beige or brite fat cells because of their beige color, undergo a browning process following cold stimulus and share some molecular, histologic and functional characteristics with beige adipocytes found in the subcutaneous white adipose tissue (SAT) of mice (7, 10, 11). The discovery of beige cells has raised clinical interest in the potential of these cells in the treatment of obesity.Uncoupling protein-1 (UCP1), which dissipates the mitochondrial electrochemical gradient which is the key for ATP formation, mediates the thermogenic activity of brown and beige adipocytes (12). Cell death-inducing DNA fragmentation factor α-like effector A (CIDEA), a member of a novel family of proapoptotic proteins, is expressed abundantly in both BAT and beige cells (10). Despite the similarity in thermogenic function, multiple lines of evidence indicate that they have unique expression profiles and distinct characters that likely contribute to their tissue-specific functions (4). Several genes such as Prdm16 (PR domain containing 16), Tbx1 (T-box 1), Tmem26 (transmembrane 26), pRb (protein retinoblastoma), Foxc2 (forkhead box protein C2), and CD137 [also know as TNFRSF9 (tumor necrosis factor receptor superfamily, member 9)] are preferentially expressed in beige adipocytes and ablation of some of these genes or of beige cells make mice more prone to develop obesity and metabolic dysfunction (10, 13, 14). Although recent evidence suggested that beige and brown adipocytes are likely to function in the maintenance of energy balance and thermogenesis, the safety of therapeutic stimulation of the browning process in treatment of obesity has not been established partly because the mechanisms underlying this process are not understood.Liver X receptors (LXRs) α and β are two members of the nuclear receptor family involved in multiple metabolic pathways, including insulin sensitivity; metabolism of glucose, lipid, and cholesterol; and energy expenditure (15). Our team has shown that LXR participates in regulation of key genes of energy pathways in the BAT in female rodents (16, 17). Genetic knock out of LXRs in both male and female mice provided them protection from diet-induced obesity, which was consistent with findings observed by other research groups using different LXR knockout mice (18, 19). These phenomena were explained by an ectopic expression of UCP1 in visceral white adipose and skeletal muscles or increased fat oxidation (18, 20). However, we have speculated that alteration of the browning process in LXR knockout mice could contribute to the metabolic protection against obesity and type 2 diabetes. We now present the evidence that this is the case.We found that depletion of LXRs in male mice reduced fat content and body weight. This finding was associated with an increased browning of SAT and consequently increased energy expenditure. Meanwhile, activated TSH-releasing hormone (TRH) signaling in the paraventricular nucleus (PVN) area of the hypothalamus in LXRαβ^−/− mice increased the activity of the hypothalamic–pituitary–thyroid (HPT) axis, which ultimately led to the enhanced browning of SAT.     

Artemisia scoparia extract attenuates non-alcoholic fatty liver disease in diet-induced obesity mice by enhancing hepatic insulin and AMPK signaling independently of FGF21 pathway

Objective

Nonalcoholic fatty liver disease (NAFLD) is a common liver disease which has no standard treatment. In this regard, we sought to evaluate the effects of extracts of Artemisia santolinaefolia (SANT) and Artemisia scoparia (SCO) on hepatic lipid deposition and cellular signaling in a diet-induced obesity (DIO) animal model.

Materials/Methods

DIO C57/B6J mice were randomly divided into three groups, i.e. HFD, SANT and SCO. Both extracts were incorporated into HFD at a concentration of 0.5% (w/w). Fasting plasma glucose, insulin, adiponectin, and FGF21 concentrations were measured.

Results

At the end of the 4-week intervention, liver tissues were collected for analysis of insulin, AMPK, and FGF21 signaling. SANT and SCO supplementation significantly increased plasma adiponectin levels when compared with the HFD mice (P < 0.001). Fasting insulin levels were significantly lower in the SCO than HFD mice, but not in SANT group. Hepatic H&E staining showed fewer lipid droplets in the SCO group than in the other two groups. Cellular signaling data demonstrated that SCO significantly increased liver IRS-2 content, phosphorylation of IRS-1, IR β, Akt1 and Akt2, AMPK α1 and AMPK activity and significantly reduced PTP 1B abundance when compared with the HFD group. SCO also significantly decreased fatty acid synthase (FAS), HMG-CoA Reductase (HMGR), and Sterol regulatory element-binding protein 1c (SREBP1c), but not Carnitine palmitoyltransferase I (CPT-1) when compared with HFD group. Neither SANT nor SCO significantly altered plasma FGF21 concentrations and liver FGF21 signaling.

Conclusion

This study suggests that SCO may attenuate liver lipid accumulation in DIO mice. Contributing mechanisms were postulated to include promotion of adiponectin expression, inhibition of hepatic lipogenesis, and/or enhanced insulin and AMPK signaling independent of FGF21 pathway.     

Association between low SIRT1 expression in visceral and subcutaneous adipose tissues and metabolic abnormalities in women with obesity and type 2 diabetes

Aims

To assess the importance of adipose tissue sirtuin 1 (SIRT1) in the regulation of whole-body metabolism in humans with obesity and type 2 diabetes.

Methods

In total, 19 non-diabetic obese women, 19 type 2 diabetic women undergoing gastric bypass surgery, and 27 normal-weight women undergoing gynecological surgery (total 65 women) were enrolled. Their anthropometric variables, abdominal fat distribution and metabolic parameters, serum adiponectin concentrations, and SIRT1 mRNA and protein and adiponectin mRNA expressions in visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) were measured.

Results

SIRT1 mRNA levels in VAT and SAT were similar and these levels were suppressed in obese and type 2 diabetic women compared to normal-weight subjects. These decreases in SIRT1 expression were observed in both adipocytes and non-fat cells. There was a strong association between adipose tissue SIRT1 mRNA and protein levels. Adipose SIRT1 expression correlated inversely with HOMA-IR and other insulin resistance-related parameters. Adipose SIRT1 and adiponectin mRNA expression correlated very strongly and positively. SIRT1 mRNA level in VAT correlated inversely with visceral obesity whereas its expression in SAT correlated negatively with body mass index.

Conclusions

Adipose tissue SIRT1 may play a key role in the regulation of whole body metabolic homeostasis in humans. Downregulation of SIRT1 in VAT may contribute to the metabolic abnormalities that are associated with visceral obesity.     

Correlative studies on the effects of obesity,diabetes and hypertension on gene expression in omental adipose tissue of obese women

Objective:

A major consequence of obesity is the enormous expansion of and enhanced inflammatory response seen in visceral adipose tissue. I hypothesized that the expression of inflammatory markers in visceral omental fat would correlate with the extent of visceral adiposity as measured by waist circumference or body mass index and that diabetes and hypertension, defined as subjects taking anti-hypertensive drugs, would be associated with changes in mRNA expression in visceral fat.

Design and methods:

The expression of 106 mRNAs by RT-PCR was examined in observational studies using extracts of omental fat of obese women undergoing bariatric surgery as well as the circulating levels of some adipokines. We also compared the mRNA levels of 65 proteins in omental fat removed during gastric bypass surgery of women with and without hypertension and those with type 2 diabetes.

Results:

Out of 106 mRNAs the expression of 10 mRNAs in omental fat of women not taking anti-hypertensive drugs correlated with waist circumference while 7 different mRNAs had significant correlations with circulating glucose. The correlations of waist circumference with mRNA expression were abolished, except for interleukin (IL)-1 receptor antagonist (IL-1RA), in women taking anti-hypertensive drugs. The correlations of blood glucose with omental fat mRNA expression were abolished, except for that of Akt1 and Akt2, in women taking anti-hypertensive drugs. However, the expression of 4 different mRNAs in omental fat was affected by circulating glucose in subjects taking anti-hypertensive drugs. The circulating levels of IL-1 RA, but not fatty acid binding protein 4, adipsin and phospholipase A2, correlated with both waist circumference and mRNA expression in omental fat.

Conclusion:

In female bariatric surgery patients, the mRNA expression of some proteins in omental fat was affected by the degree of obesity, whereas hypertension and diabetes affected a separate set of mRNAs.     

Detection of brown adipose tissue and thermogenic activity in mice by hyperpolarized xenon MRI

The study of brown adipose tissue (BAT) in human weight regulation has been constrained by the lack of a noninvasive tool for measuring this tissue and its function in vivo. Existing imaging modalities are nonspecific and intrinsically insensitive to the less active, lipid-rich BAT of obese subjects, the target population for BAT studies. We demonstrate noninvasive imaging of BAT in mice by hyperpolarized xenon gas MRI. We detect a greater than 15-fold increase in xenon uptake by BAT during stimulation of BAT thermogenesis, which enables us to acquire background-free maps of the tissue in both lean and obese mouse phenotypes. We also demonstrate in vivo MR thermometry of BAT by hyperpolarized xenon gas. Finally, we use the linear temperature dependence of the chemical shift of xenon dissolved in adipose tissue to directly measure BAT temperature and to track thermogenic activity in vivo.Obesity is the result of an imbalance between energy intake and energy expenditure. The latter seems to be modulated, at least in part, by the activity of brown adipose tissue (BAT). BAT is a fatty tissue specialized in cold-induced and diet-induced thermogenesis, a metabolic activity during which this tissue burns fat to produce heat (1). Because of its high “fat-burning” capacity and its ability to regulate glucose homeostasis and insulin sensitivity (2, 3), this tissue is now considered to be the next target for antiobesity drugs (4). However, as interventions that aim to decrease body weight by increasing energy expenditure through BAT volume and/or activity modulations are being investigated (5, 6), the ability to detect this tissue in humans represents an unmet need. Current imaging techniques fail to detect BAT in the target population, obese and overweight subjects, and it is not clear whether this is due to a lack of BAT mass or BAT activity (7). For example, ¹⁸FluoroDeoxyGlucose Positron Emission Tomography (¹⁸FDG-PET), considered to be the gold standard for the detection of BAT activity, can detect BAT activity in morbidly obese and obese subjects only after bariatric surgery or substantial weight loss (8, 9). This is because ¹⁸FDG-PET can only be used to detect active BAT, and in obese subjects, activity is substantially reduced. In addition, the detection of BAT by ¹⁸FDG-PET is performed indirectly, through measurements of BAT uptake of glucose, which is not the primary fuel for BAT thermogenesis. Heat production in BAT is primarily fueled by oxidation of fatty acids released from triglycerides stored in the intracellular fat droplets (10), and, as such, measurements of exogenous glucose or exogenous fatty acid uptake are clearly intrinsically insensitive to BAT thermogenic activity (11, 12).In computed tomography (CT) and ¹H MRI studies, fat fraction measurements are used to differentiate the highly hydrated brown adipose tissue from the less hydrated normal white adipose tissue (WAT) (13–15). Unfortunately, human BAT is a heterogeneous mixture of white and brown adipocytes (16), and partial volume effects that arise from the limited spatial resolution of these imaging modalities make these types of measurements unreliable and nonspecific. Indeed, in one study in which ¹⁸FDG-PET BAT maps and MRI BAT maps were compared, areas that were BAT-positive for ¹⁸FDG-PET had the same fat fraction of nearby areas that were BAT-negative (17). Nonlinear MRI techniques based on intermolecular zero-quantum coherences between water and fat spins can be used to overcome partial volume effects and obtain background-free maps of BAT in lean phenotypes (18). However, in overweight and obese subjects, the strong reduction in tissue hydration and increase in the average distance between water and fat spins would cause a reduction of the already small nonlinear MR signal, making the entire detection unfeasible (18). More importantly, BAT hydration is not a good marker for BAT, as it is highly modulated by tissue thermogenic activity and therefore exhibits very high intersubject and intrasubject variability (19). For example, whereas in lean subjects BAT may have a fat fraction close to 50%, in obese subjects the BAT fat fraction is close to 80%, making it impossible to distinguish this tissue from the normal WAT with conventional fat fraction methods or more sophisticated nonlinear MR techniques.Other less expensive imaging modalities such as infrared thermography and contrast ultrasound have similar problems. BAT thermogenesis is still detected indirectly, either through surface temperature measurements (infrared thermography) that are inherently affected by local changes in tissue blood flow (20) or by measurements of tissue blood flow (ultrasound), which is only partially coupled to BAT thermogenic activity (11) and which exhibits the same insensitivity to the less vascularized lipid-rich BAT of obese phenotypes (21).Our primary goal with this study is to evaluate the possibility to detect BAT tissue and thermogenic activity by using hyperpolarized (HP) xenon gas MRI. In hyperpolarized xenon MRI studies, prior hyperpolarization of the gas through a process called spin-exchange optical pumping (SEOP) is the necessary step to make the gas MR-visible (22). After hyperpolarization, the gas can be inhaled by the subject or by the animal to visualize lung ventilation function (23), xenon gas exchange (23, 24), or other distal organs of interest (25). After inhalation the gas diffuses from the lung airspaces to the lung parenchyma and to the blood. The dissolved gas is then transported to distal organs, where it accumulates proportionally to tissue perfusion rate and to tissue–blood partition coefficient. More interestingly, as xenon diffuses in different tissue compartments, its chemical shift changes, making it possible to differentiate between xenon dissolved in blood and xenon dissolved in tissue or lipids. However, as the amount of xenon that is transferred at any given point in time from the lung airspaces to tissue–blood is relatively small, and as the T1 relaxation time of xenon in blood is relatively short, imaging of distal organs by hyperpolarized xenon gas remains a major challenge.Here we capitalize on the lipophilic nature of xenon and on the strong increase in blood flow to brown fat that occurs during stimulation of thermogenic activity to detect the highly vascularized BAT (26). In addition, we measure and use the chemical shift temperature coefficient of xenon dissolved in adipose tissue to directly detect its thermogenic activity in vivo.     

Effects of a eucaloric reduced-carbohydrate diet on body composition and fat distribution in women with PCOS

Objective

To determine if consumption of a reduced-carbohydrate (CHO) diet would result in preferential loss of adipose tissue under eucaloric conditions, and whether changes in adiposity were associated with changes in postprandial insulin concentration.

Methods

In a crossover-diet intervention, 30 women with PCOS consumed a reduced-CHO diet (41:19:40% energy from CHO:protein:fat) for 8 weeks and a standard diet (55:18:27) for 8 weeks. Body composition by DXA and fat distribution by CT were assessed at baseline and following each diet phase. Insulin AUC was obtained from a solid meal test (SMT) during each diet phase.

Results

Participants lost 3.7% and 2.2% total fat following the reduced-CHO diet and STD diet, resp. (p < 0.05 for difference between diets). The reduced-CHO diet induced a decrease in subcutaneous-abdominal, intra-abdominal, and thigh-intermuscular adipose tissue (− 7.1%, − 4.6%, and − 11.5%, resp.), and the STD diet induced a decrease in total lean mass. Loss of fat mass following the reduced CHO diet arm was associated with lower insulin AUC (p < 0.05) during the SMT.

Conclusions

In women with PCOS, consumption of a diet lower in CHO resulted in preferential loss of fat mass from metabolically harmful adipose depots, whereas a diet high in CHO appeared to promote repartitioning of lean mass to fat mass.     

Effects of a nonnutritive sweetener on body adiposity and energy metabolism in mice with diet-induced obesity

Objective

Nonnutritive sweeteners (NNSs) have been studied in terms of their potential roles in type 2 diabetes, obesity, and related metabolic disorders. Several studies have suggested that NNSs have several specific effects on metabolism such as reduced postprandial hyperglycemia and insulin resistance. However, the detailed effects of NNSs on body adiposity and energy metabolism have not been fully elucidated. We investigated the effects of an NNS on energy metabolism in mice with diet-induced obesity (DIO).

Methods

DIO mice were divided into NNS-administered (4% NNS in drinking water), sucrose-administered (33% sucrose in drinking water), and control (normal water) groups. After supplementation for 4 weeks, metabolic parameters, including uncoupling protein (UCP) levels and energy expenditure, were assessed.

Results

Sucrose supplementation increased hyperglycemia, body adiposity, and body weight compared to the NNS-administered and control groups (P < 0.05 for each). In addition, NNS supplementation decreased hyperglycemia compared to the sucrose-administered group (P < 0.05). Interestingly, NNS supplementation increased body adiposity, which was accompanied by hyperinsulinemia, compared to controls (P < 0.05 for each). NNS also increased leptin levels in white adipose tissue and triglyceride levels in tissues compared to controls (P < 0.05 for each). Notably, compared to controls, NNS supplementation decreased the UCP1 level in brown adipose tissue and decreased O₂ consumption in the dark phase.

Conclusions

NNSs may be good sugar substitutes for people with hyperglycemia, but appear to influence energy metabolism in DIO mice.     

Adipose Tissue and Modulation of Hypertension

Purpose of Review

Obesity is a major risk factor for the development of hypertension (HTN), a leading cause of cardiovascular morbidity and mortality. Growing body of research suggests that adipose tissue function is directly associated with the pathogenesis of obesity-related HTN. In this review, we will discuss recent research on the role of adipose tissue in blood pressure (BP) regulation and activation of brown adipose tissue (BAT) as a potentially new therapeutic means for obesity-related HTN.

Recent Findings

Adipose tissue provides mechanical protection of the blood vessels and plays a role in regulation of vascular tone. Exercise and fasting activate BAT and induce browning of white adipose tissue (WAT). BAT-secreted FGF21 lowers BP and protects against HTN. Browning of perivascular WAT improves HTN.

Summary

New insights on WAT browning and BAT activation can open new avenues of potential therapeutic interventions to treat obesity-related HTN.

     

Markers of endothelial cell dysfunction are increased in human omental adipose tissue from women with pre-existing maternal obesity and gestational diabetes

Objective

To determine the effect of maternal obesity and gestational diabetes mellitus (GDM) on the expression and release of genes involved in endothelial cell dysfunction in human placenta and omental adipose tissue.

Materials/Methods

Human placenta and omental adipose tissue were obtained from non-obese and obese normal glucose tolerant (NGT) women and women with GDM at the time of Caesarean section. Quantitative RT-PCR was performed to determine the level of expression. Tissue explants were performed to determine the release of proteins of interest.

Results

There was no effect of pre-existing maternal obesity or GDM on placental gene expression or secretion of members of the VEGF family members (PLGF and VEGF-A expression and secretion; sFlt-1 release; VEGFR1 and VEGFR2 mRNA expression); FGFR1 mRNA expression, FGF2 mRNA expression and secretion; endoglin mRNA expression and secretion (sEng); and the adhesion molecules ICAM-1 and VCAM-1. On the other hand, in omental adipose tissue, pre-existing maternal obesity and GDM were associated with increased gene expression of PLGF, endoglin and ICAM-1 and increased secretion of PLGF, sFlt-1, FGF2, sEng and sICAM-1. There was, however, no effect of maternal pre-existing obesity and GDM on VEGF-A, VEGFR1, VEGFR2, FGFR1 and VCAM-1 expression or secretion.

Conclusions

This study demonstrated the presence of abnormal expression and secretion of angiogenic proteins and adhesion molecules in omental adipose tissue, but not placenta, from pregnant women with GDM and pre-existing maternal obesity. Increased angiogenic and adhesion molecules released from adipose tissue may affect angiogenesis, inflammation and or lipid and glucose metabolism in both mum and her offspring.     

Inappropriate heat dissipation ignites brown fat thermogenesis in mice with a mutant thyroid hormone receptor α1

Thyroid hormone is a major regulator of thermogenesis, acting both in peripheral organs and on central autonomic pathways. Mice heterozygous for a point mutation in thyroid hormone receptor α1 display increased thermogenesis as a consequence of high sympathetic brown fat stimulation. Surprisingly, despite the hypermetabolism, their body temperature is not elevated. Here we show, using isolated tail arteries, that defective thyroid hormone receptor α1 signaling impairs acetylcholine-mediated vascular relaxation as well as phenylephrine-induced vasoconstriction. Using infrared thermography on conscious animals, we demonstrate that these defects severely interfere with appropriate peripheral heat conservation and dissipation, which in turn leads to compensatory alterations in brown fat activity. Consequently, when the vasoconstrictive defect in mice heterozygous for a point mutation in thyroid hormone receptor α1 was reversed with the selective α1-adrenergic agonist midodrine, the inappropriate heat loss over their tail surface was reduced, normalizing brown fat activity and energy expenditure. Our analyses demonstrate that thyroid hormone plays a key role in vascular heat conservation and dissipation processes, adding a unique aspect to its well-documented functions in thermoregulation. The data thus facilitate understanding of temperature hypersensitivity in patients with thyroid disorders. Moreover, the previously unrecognized connection between cardiovascular regulation and metabolic activity revealed in this study challenges the interpretation of several experimental paradigms and questions some of the currently derived hypotheses on the role of thyroid hormone in thermogenesis.Thyroid hormone affects energy metabolism, body temperature, and cardiovascular function (1, 2). This is evident in hypo- and hyperthyroid patients, who display metabolic and cardiovascular problems as well as inadequacies in heat and cold tolerance (3, 4). The latter effects are thought to be a consequence of a shift in obligatory thermogenesis, because thyroid hormone affects basal metabolic rate through the regulation of genes controlling cellular metabolism and mitochondrial function. In addition, facultative thermogenesis, brought about by shivering and brown adipose tissue (BAT) activation (5, 6), is also modulated by thyroid hormone. The recent discovery of BAT in adult humans, where previously thought to exist only in rodents and neonates (7), suggests that the role of BAT in thermoregulation and energy expenditure may be underestimated. Recently, a new role of thyroid hormone in facultative thermogenesis has emerged, controlling BAT via the central nervous system (8, 9). The importance of central thyroid hormone receptor α1 (TRα1) signaling for BAT activity was further supported by findings in mice heterozygous for a R384C mutation in TRα1 (TRα1+m), which display a strong hypermetabolism as the result of a central overactivation of BAT (10). However, the finding was unexpected, as the specific mutation causes a 10-fold reduced affinity to 3,3′,5-triiodothyronine (T3), and lower brown fat thermogenesis would be expected from impaired TRα1 signaling (11). Even more surprisingly, when TRα1+m mice were treated with supraphysiological doses of T3 to reactivate the mutant TRα1 (12, 13), their BAT activity returned to normal levels, although T3 is known to stimulate thermogenesis. This puzzling phenotype of TRα1+m animals therefore prompted us to investigate their thermoregulation in greater detail.Here we demonstrate that TRα1+m mice display lower nocturnal body temperature despite BAT hyperactivity. Studying the tail as an important thermoregulatory effector organ (14), we reveal a defective vascular control of heat dissipation and conservation in vivo and identify possible vascular mechanisms affected by the impaired TRα1 signaling in ex vivo studies. Our findings show that the BAT hyperactivity observed in TRα1+m mice is a consequence of impaired vasoconstriction causing increased heat loss. The results describe a previously unknown role for TRα1 in vascular control of thermoregulation, thus constituting a unique link between thyroid hormone signaling and facultative thermogenesis. Moreover, they provide a possible explanation for the exaggerated thermosensitivity observed in patients with defects in thyroid hormone metabolism.     

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

G_sα, 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 G_sα 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. G_sα signaling in adipose tissues may therefore affect whole-body glucose metabolism in the absence of an effect on body weight.The sympathetic nervous system (SNS) regulates energy homeostasis and adiposity through several mechanisms, including activation of nonshivering thermogenesis in brown adipose tissue (BAT), browning (formation of BAT-like “beige” cells) of white adipose tissue (WAT), and stimulation of lipolysis. Although these processes have been shown to be potential targets in treating obesity and diabetes (1–3), ablation of sympathetic nerves (4–6) or their main effectors (norepinephrine and epinephrine) (7) does not result in obesity or insulin resistance. Although mice lacking β adrenergic receptors (β-less mice) do develop obesity (8), it is likely that this effect is not due only to loss of β-adrenergic signaling in adipose tissue.The main mediator of SNS function in adipose tissues is G_sα (9, 10), a ubiquitously expressed G protein α-subunit that in adipose tissue couples adrenergic and other receptors, such as the adenosine A2A receptor (11), to the generation of intracellular cAMP. We have previously generated adipose-specific G_sα knockout mice (FGsKO) using fatty acid binding protein 4 (FABP4) (aP2)-cre and showed these mice to have significant early mortality and a severely lean phenotype (12). However, the usefulness of this model to examine the role of G_sα in mature adipocytes is limited due to both the lack of specificity of FABP4-cre expression in adipose tissue and the presence of a severe defect in adipogenesis due to expression of FABP4, and therefore loss of G_sα, during an early step in adipocyte differentiation.Adiponectin is a mature adipocyte marker expressed late in adipocyte differentiation (13). The more recent availability of adiponectin-cre mouse lines (14, 15) has enabled us to generate adipose-specific G_sα knockout mice (Ad-GsKO) in which G_sα deletion is restricted to mature adipocytes. Despite having loss of BAT function or browning of WAT, Ad-GsKO mice failed to develop obesity on either standard chow or a high-fat diet (HFD). Moreover, Ad-GsKO mice had improved glucose tolerance and insulin sensitivity associated with a significant reduction of circulating FABP4. Our results show that thermogenesis in BAT and in beige adipocytes is not required for normal weight maintenance and that G_sα signaling in adipose tissue has an effect on whole-body glucose metabolism independent of adiposity.     

Stearic acid content of abdominal adipose tissues in obese women

Objective:

Subcutaneous (SC) adipose tissue stearic acid (18:0) content and stearoyl-CoA desaturase-1 (SCD1)-mediated production of oleic acid (18:1) have been suggested to be altered in obesity. The objective of our study was to examine abdominal adipose tissue fatty acid content and SCD1 mRNA/protein level in women.

Subjects and methods:

Fatty acid content was determined by capillary gas chromatography in SC and omental (OM) fat tissues from two subgroups of 10 women with either small or large OM adipocytes. Samples from 10 additional women were used to measure SCD1 mRNA and protein expression, total extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphorylated ERK1/2 protein as well as insulin receptor (IR) expression levels.

Results:

OM fat 18:0 content was significantly lower in women with large OM adipocytes compared with women who had similar adiposity, but small OM adipocytes (2.37±0.45 vs 2.75±0.30 mg per 100 g adipose tissue, respectively, P⩽0.05). OM fat 18:0 content was negatively related to the visceral adipose tissue area (r=−0.44, P=0.05) and serum triglyceride levels (r=−0.56, P<0.05), while SC fat 18:0 content was negatively correlated with total body fat mass (BFM) (r=−0.48, P<0.05) and fasting insulin concentration (r=−0.73, P<0.005). SC adipose tissue desaturation index (18:1/18:0), SCD1 expression and protein levels were positively correlated with BFM. Moreover, obese women were characterized by a reduced OM/SC ratio of SCD1 mRNA and protein levels. A similar pattern was observed for ERK1/2 and IR expression.

Conclusion:

The presence of large adipocytes and increased adipose mass in a given fat compartment is related to reduced 18:0 content and increased desaturation index in women, independently of dietary fat intake. The depot-specific difference in ERK1/2 expression and activation, as well as in SCD1 and IR expression in obese women is consistent with the hypothesis that they may predominantly develop SC fat, which could in turn help protect from metabolic disorders.