Adipose Tissue Oxygen Research Articles

Moderate Hypoxia Exposure: A Novel Strategy to Improve Glucose Metabolism in Humans?

The obesity epidemic calls for novel strategies to prevent and treat obesity and its comorbidities. Several studies have indicated that the amount of oxygen to which tissues are exposed may substantially impact cardiometabolic health. Interestingly, living at high altitude (hypobaric hypoxia) seems to be associated with improved glucose homeostasis and a decreased prevalence of Type 2 diabetes. Furthermore, normobaric hypoxia exposure has been shown to exert beneficial effects on glucose homeostasis and insulin sensitivity in rodents and humans. This may, at least in part, be explained by altered adipose tissue and skeletal muscle oxygen tension. In contrast, patients with obstructive sleep apnoea syndrome, which is characterised by episodes of severe intermittent hypoxia due to periodic collapse of the upper airway during sleep, show impairments in glucose homeostasis and are at increased cardiovascular risk. These discrepancies may be explained by the severity, duration, and pattern (number of cycles) of hypoxic episodes, but underlying mechanisms have not yet been studied in detail. The purpose of this review is to provide an overview of available studies on the link between oxygen tension, inflammation, and glucose homeostasis. Detailed studies to elucidate the effects of moderate hypoxia exposure on wholebody and tissue-specific insulin sensitivity in humans are clearly warranted.

Adipose tissue dysfunction and impaired metabolic health in human obesity: a matter of oxygen?

The number of studies in the field of adipose tissue biology has increased exponentially over the last decade. This shift in research focus is primarily driven by the tremendous increase in the prevalence of obesity and related chronic diseases, including cardiovascular disease and type 2 diabetes mellitus. Adipose tissue is a fascinating and complex organ, with marked effects on whole-body physiology. Intriguingly, expansion of adipose tissue does not necessarily translate into increased metabolic and cardiovascular disease risk. A proportion of obese individuals seems to be relatively protected against worsening of metabolic health (1), suggesting that adipose tissue dysfunction, rather than the amount of fat mass, may be a key factor in the pathophysiology of obesity-related metabolic and cardiovascular diseases (2–4). It is widely accepted that impairments in adipose tissue lipid metabolism, a decreased adipose tissue blood flow (ATBF) and an increased production of pro-inflammatory cytokines by hypertrophic adipocytes and infiltrating adaptive and innate immune cells are characteristics of dysfunctional adipose tissue in obesity (2, 5). These impairments not only induce insulin resistance locally in the adipose tissue but also have detrimental effects at the whole-body level, thereby affecting metabolic health. The reason for this is that adipose tissue dysfunction in obesity is accompanied by lipid spillover in the circulation and subsequent lipid accumulation in non-adipose tissues (ectopic fat storage), and may contribute to systemic low-grade inflammation, thereby accelerating the development and progression of obesity-related insulin resistance and chronic metabolic diseases (Figure ​(Figure1)1) (2). Figure 1 Adipose tissue dysfunction in obesity is related to impaired metabolic health. A long-term positive energy balance, leading to body weight gain, will increase adipocyte size. Adipocyte hypertrophy in obesity is accompanied by disturbances in lipid metabolism ... Adipose Tissue Oxygen Tension in Human Obesity Since adipose tissue dysfunction has been recognized as a key process in the pathophysiology of obesity-related disorders (2, 5), the number of studies aimed at identifying the trigger of adipose tissue dysfunction in obesity has increased substantially. A prevailing concept is that an insufficient amount of oxygen within adipose tissue, commonly referred to as “adipose tissue hypoxia,” may underlie adipose tissue dysfunction in obesity (6, 7). Adipose tissue hypoxia in obesity: The concept It has been postulated that adipose tissue angiogenesis is insufficient to maintain normoxia in the entire fat depot during the progressive development of obesity (8). In other words, a reduced supply of oxygen to the tissue has been proposed to instigate adipose tissue dysfunction. Indeed, a lower expression of angiogenic genes (e.g., VEGF) and lower capillary density have been found in abdominal subcutaneous adipose tissue of obese as compared to lean individuals (9, 10). The net result of structural and functional properties of the adipose tissue vasculature determines tissue blood flow. A consistent observation made by our lab and others is that fasting and postprandial ATBF is decreased in obese insulin resistant versus lean insulin sensitive subjects (9, 11–13), indicating that oxygen delivery to adipose tissue is indeed impaired in obesity. We have recently demonstrated, for the first time, that both pharmacological (local administration of vasoactive agents into adipose tissue) and physiological (oral glucose drink) manipulation of ATBF induce concomitant alterations in adipose tissue oxygen partial pressure (AT PO2) in humans (9), indicating that adipose tissue oxygen supply indeed affects AT PO2. A second argument that has been put forward to develop the concept of adipose tissue hypoxia in obesity is that the diameter of hypertrophic adipocytes in obesity exceeds the normal diffusion distance of oxygen across tissues (100–200 μm) (14). However, in human adipose tissue, there seems to be only a very small proportion of adipocytes with a diameter >100 μm (9, 15, 16). Therefore, the significance of reduced oxygen diffusion from the capillaries to hypertrophic fat cells in obese humans can be questioned.

Chronic Adrenergic Stress Causes Adrenergic β‐3 Receptor Up‐regulation in White Adipose Tissue of Burn Patients

Chronic adrenergic stress results in the browning of subcutaneous white adipose tissue (WAT), i.e. increased oxidative capacity and uncoupling protein 1 (UCP1) expression in rodents and possibly in humans. There are multiple adrenergic receptors that could mediate this response. The aim of this study was to determine if severe burn injury increases β3‐adrenergic receptors (β3AR) in human WAT.
 Subcutaneous WAT was collected from ten burned (11±3 yrs; 59±21% total body surface area burned) on two separate occasions (10±8 (early) and 34±24 (late) days post injury) and five non‐burn children (10 ± 5 years). Whole body metabolic rate and WAT thermogenesis were measured. Histology, immunohistochemistry, ELISA and q‐PCR were performed in WAT.
 Resting energy expenditure rate (35±5.9 vs 45±7 Kcal/kg/day), p<0.01) and WAT mitochondrial respiration (1.5±0.3 vs. 0.56±0.1 pmol/sec/mg, p<0.05) were higher in late after burn compared to early after burn, respectively, suggesting increased whole body and adipose tissue oxygen consumption. β3AR mRNA and protein expression in WAT progressively increased over time post burn. Further, UCP1 (p<0.01), but not UCP2, UCP3 and UCP4, mRNA expression was higher in WAT at later, compared to early, times post‐burn. Similarly, UCP1 protein was significantly increased in the late burn group compared to healthy controls (22.9±4.5ng/mg vs 2.7±0.3; P<0.001). In summary, WAT β3AR up‐regulation in WAT is associated with increased UCP1 and oxygen consumption in WAT and increased whole body oxygen consumption. The possible contribution of β3ARs to browning of WAT in humans with severe burns is being investigated.

Adipose tissue fibrosis.

The increasing prevalence of obesity causes a major interest in white adipose tissue biology. Adipose tissue cells are surrounded by extracellular matrix proteins whose composition and remodeling is of crucial importance for cell function. The expansion of adipose tissue in obesity is linked to an inappropriate supply with oxygen and hypoxia development. Subsequent activation of hypoxia inducible factor 1 (HIF-1) inhibits preadipocyte differentiation and initiates adipose tissue fibrosis. Thereby adipose tissue growth is limited and excess triglycerides are stored in ectopic tissues. Stressed adipocytes and hypoxia contribute to immune cell immigration and activation which further aggravates adipose tissue fibrosis. There is substantial evidence that adipose tissue fibrosis is linked to metabolic dysfunction, both in rodent models and in the clinical setting. Peroxisome proliferator activated receptor gamma agonists and adiponectin both reduce adipose tissue fibrosis, inflammation and insulin resistance. Current knowledge suggests that antifibrotic drugs, increasing adipose tissue oxygen supply or HIF-1 antagonists will improve adipose tissue function and thereby ameliorate metabolic diseases.

Augmented expression and secretion of adipose-derived pigment epithelium-derived factor does not alter local angiogenesis or contribute to the development of systemic metabolic derangements.

Impaired coupling of adipose tissue expansion and vascularization is proposed to lead to adipocyte hypoxia and inflammation, which in turn contributes to systemic metabolic derangements. Pigment epithelium-derived factor (PEDF) is a powerful antiangiogenic factor that is secreted by adipocytes, elevated in obesity, and implicated in the development of insulin resistance. We explored the angiogenic and metabolic role of adipose-derived PEDF through in vivo studies of mice with overexpression of PEDF in adipocytes (PEDF-aP2). PEDF expression in white adipocytes and PEDF secretion from adipose tissue was increased in transgenic mice, but circulating levels of PEDF were not increased. Overexpression of PEDF did not alter vascularization, the partial pressure of O2, cellular hypoxia, or gene expression of inflammatory markers in adipose tissue. Energy expenditure and metabolic substrate utilization, body mass, and adiposity were not altered in PEDF-aP2 mice. Whole body glycemic control was normal as assessed by glucose and insulin tolerance tests, and adipocyte-specific glucose uptake was unaffected by PEDF overexpression. Adipocyte lipolysis was increased in PEDF-aP2 mice and associated with increased adipose triglyceride lipase and decreased perilipin 1 expression. Experiments conducted in mice rendered obese by high-fat feeding showed no differences between PEDF-aP2 and wild-type mice for body mass, adiposity, whole body energy expenditure, glucose tolerance, or adipose tissue oxygenation. Together, these data indicate that adipocyte-generated PEDF enhances lipolysis but question the role of PEDF as a major antiangiogenic or proinflammatory mediator in adipose tissue in vivo.

High-salt intake induced visceral adipose tissue hypoxia and its association with circulating monocyte subsets in humans

To investigate the feasibility of blood oxygen level dependent magnetic resonance imaging (BOLD-MRI) in evaluating human visceral adipose tissue (AT) oxygenation induced by salt loading/depletion and its association with changes in circulating monocyte subsets. A dietary intervention study was performed in 23 healthy volunteers beginning with a 3-day usual diet followed by a 7-day high-salt diet (≥15 g NaCl/day) and a 7-day low-salt diet (≤5 g NaCl/day). BOLD-MRI was used to evaluate oxygenation in perirenal AT. Salt loading led to a consistent AT hypoxia (increase in the R2* signal, 25.2 ± 0.90 s(-1) vs. baseline 21.5 ± 0.71 s(-1) , P < 0.001) and suppression of circulating renin-angiotensin-aldosterone system (RAAS), as well as an expansion of the CD14++CD16+ monocytes and monocyte pro-inflammatory activation. In salt depletion phase, the hypoxic state of AT and the expanded CD14++CD16+ monocyte pool were regressed to baseline levels, accompanied by a rebound activation of RAAS. Moreover, AT oxygenation level was positively correlated with the CD14++CD16+ monocytes (r = 0.419, P < 0.001). This work provides proof-of-principle evidence supporting the feasibility of BOLD-MRI in monitoring visceral AT oxygenation in humans induced by dietary salt loading/depletion. In addition, the CD14++CD16+ monocytes may participate in the pathogenesis of high-salt intake induced AT hypoxia.

Effects of Exercise Training on Chronic Inflammation in Obesity

Chronic, systemic inflammation is an independent risk factor for several major clinical diseases. In obesity, circulating levels of inflammatory markers are elevated, possibly due to increased production of pro-inflammatory cytokines from several tissues/cells, including macrophages within adipose tissue, vascular endothelial cells and peripheral blood mononuclear cells. Recent evidence supports that adipose tissue hypoxia may be an important mechanism through which enlarged adipose tissue elicits local tissue inflammation and further contributes to systemic inflammation. Current evidence supports that exercise training, such as aerobic and resistance exercise, reduces chronic inflammation, especially in obese individuals with high levels of inflammatory biomarkers undergoing a longer-term intervention. Several studies have reported that this effect is independent of the exercise-induced weight loss. There are several mechanisms through which exercise training reduces chronic inflammation, including its effect on muscle tissue to generate muscle-derived, anti-inflammatory 'myokine', its effect on adipose tissue to improve hypoxia and reduce local adipose tissue inflammation, its effect on endothelial cells to reduce leukocyte adhesion and cytokine production systemically, and its effect on the immune system to lower the number of pro-inflammatory cells and reduce pro-inflammatory cytokine production per cell. Of these potential mechanisms, the effect of exercise training on adipose tissue oxygenation is worth further investigation, as it is very likely that exercise training stimulates adipose tissue angiogenesis and increases blood flow, thereby reducing hypoxia and the associated chronic inflammation in adipose tissue of obese individuals.

Hepatocyte growth factor and interferon‐γ inducible protein‐10 are related to visceral adiposity

Increased production of chemokines by adipose tissue and defective adipose tissue oxygenation as a result of obesity may induce leucocyte infiltration and subsequent systemic inflammation. 1-To determine the relation between the amount of visceral and subcutaneous adipose tissue and the chemokine interferon-γ-inducible protein 10 (IP-10) and angiogenic factor hepatocyte growth factor (HGF). 2-To determine the relation between the metabolic syndrome and IP-10 as well as HGF. Patients originated from the Secondary Manifestations of ARTerial disease (SMART) cohort. In this study, a cohort of 1251 patients with manifest vascular disease was included. Subcutaneous and visceral adipose tissue thickness (SAT and VAT respectively) were measured ultrasonographically. IP-10 and HGF concentrations were measured with Luminex multiplex immuno assay in addition to fasting metabolic parameters. Linear regression analyses with adjustments for age, gender, smoking, estimated glomerular filtration rate, type 2 diabetes mellitus and medication use were applied to quantify the relations between adiposity or metabolic syndrome and IP-10 and HGF concentrations. VAT was significantly associated with (log)IP-10 and (log)HGF, reflected by significant higher β-values in VAT quartile 4 compared with VAT quartile 1 (reference): β0.155 (95%CI:0.073-0.237) for IP-10 and β0.147 (95%CI:0.076-0.218) for HGF. Per standard deviation increase in VAT, (log)IP-10 levels increased with 0.057 pg/mL (95%CI:0.027-0.087) and (log)HGF increased with 0.051 pg/mL (95%CI:0.025-0.077). Effect estimates were not affected by including body mass index(BMI) in the model. In contrast, SAT was not associated with IP-10 and HGF. Furthermore, the presence of the metabolic syndrome was associated with IP-10 and HGF. Visceral adipose tissue but not subcutaneous adipose tissue is significantly associated with circulating levels of IP-10 and HGF, irrespective of BMI.

Adipose tissue oxygen tension

The present review aims to address the role of adipose tissue oxygen partial pressure (PO2) in the metabolic and endocrine derangements in conditions characterized by insulin resistance. The balance between adipose tissue oxygen supply and its metabolic rate seems to determine adipose tissue PO2. Studies in ob/ob and dietary-induced obese mice have provided evidence for adipose tissue hypoxia in obesity, which has been explained by insufficient adipose tissue angiogenesis during the massive and rapid weight gain in these animals. However, conflicting data have been reported in humans, showing both increased and decreased adipose tissue PO2 in obese compared with lean individuals. Both low and high adipose tissue PO2 may induce a proinflammatory phenotype in (pre)adipocytes, but most studies have been performed under rather extreme PO2 levels, not reflecting human adipose tissue physiology. Furthermore, adipose tissue PO2 may affect glucose and lipid metabolism as well as adipogenic differentiation, but many issues still need to be addressed. Adipose tissue hypoxia has been demonstrated in animal models of obesity, but findings in humans are controversial and require further investigation. Although adipose tissue PO2 seems to be involved in metabolic and endocrine derangements in human adipose tissue, future studies should investigate how low and high adipose tissue PO2 within the human physiological range (3-11% O2) relates to adipose tissue blood flow and oxygen consumption, cellular metabolic responses, and the inflammatory phenotype.

Roles for Peroxisome Proliferator-activated Receptor γ (PPARγ) and PPARγ Coactivators 1α and 1β in Regulating Response of White and Brown Adipocytes to Hypoxia

Obese white adipose tissue is hypoxic but is incapable of inducing compensatory angiogenesis. Brown adipose tissue is highly vascularized, facilitating delivery of nutrients to brown adipocytes for heat production. In this study, we investigated the mechanisms by which white and brown adipocytes respond to hypoxia. Brown adipocytes produced lower amounts of hypoxia-inducible factor 1α (HIF-1α) than white adipocytes in response to low O(2) but induced higher levels of hypoxia-associated genes. The response of white adipocytes to hypoxia required HIF-1α, but its presence alone was incapable of inducing target gene expression under normoxic conditions. In addition to the HIF-1α targets, hypoxia also induced many inflammatory genes. Exposure of white adipocytes to a peroxisome proliferator-activated receptor γ (PPARγ) ligand (troglitazone) attenuated induction of these genes but enhanced expression of the HIF-1α targets. Knockdown of PPARγ in mature white adipocytes prevented the usual robust induction of HIF-1α targets in response to hypoxia. Similarly, knockdown of PPARγ coactivator (PGC) 1β in PGC-1α-deficient brown adipocytes eliminated their response to hypoxia. These data demonstrate that the response of white adipocytes requires HIF-1α but also depends on PPARγ in white cells and the PPARγ cofactors PGC-1α and PGC-1β in brown cells.

Response to Letter Regarding Article, “Increased Adipose Tissue Oxygen Tension in Obese Compared With Lean Men Is Accompanied by Insulin Resistance, Impaired Adipose Tissue Capillarization, and Inflammation”

We thank Drs Lecoultre and Tam for their thoughtful comments and interest in our recent publication.1 They question whether the higher adipose tissue (AT) oxygen partial pressure (Po2) in obese insulin resistant men compared with lean insulin sensitive men should be considered as hyperoxia. Abdominal subcutaneous AT Po2 showed substantial interindividual variation both in our study (range 24–85 mm Hg)1 and in another human study that assessed AT Po2 in subcutaneous AT of the abdomen.2 We also demonstrated that pharmacological and physiological manipulation of AT blood flow induced concomitant alterations in AT Po2 in lean subjects. In our opinion, these findings indicate substantial flexibility of AT …

Letter by Lecoultre and Tam Regarding Article, “Increased Adipose Tissue Oxygen Tension in Obese Compared With Lean Men Is Accompanied by Insulin Resistance, Impaired Adipose Tissue Capillarization, and Inflammation”

To the Editor: At odds with recent reports and reviews implicating adipose tissue (AT) hypoxia as a factor inducing AT dysfunction,1–4 Goossens et al5 published in Circulation a very elegant study suggesting that the impaired responsiveness of AT blood flow in obese patients is accompanied by reduced oxygen extraction and mitochondrial dysfunction leading to impaired tissue oxygen consumption. As a result, AT oxygen partial pressure is paradoxically higher in obese (≈67 mm Hg) versus lean (45 mm Hg) subjects, a condition referred as hyperoxic5 as opposed to normoxic. Interestingly, in both groups, Goossens et al reported wide ranges of Po2 (20–90 mm …

PS15 - 78. Increased adipose tissue oxygen tension in obesity is accompanied by insulin resistance, impaired adipose tissue capillarisation and inflammation

Adipose tissue (AT) dysfunction in obesity contributes to chronic, low-grade inflammation that predisposes to type 2 diabetes and cardiovascular disease. Recent in vitro studies suggest that AT hypoxia may induce inflammation. We hypothesized that AT blood flow (ATBF) regulates AT oxygen partial pressure (AT Po2), thereby affecting AT inflammation and insulin sensitivity.

Increased Adipose Tissue Oxygen Tension in Obese Compared With Lean Men Is Accompanied by Insulin Resistance, Impaired Adipose Tissue Capillarization, and Inflammation

Adipose tissue (AT) dysfunction in obesity contributes to chronic, low-grade inflammation that predisposes to type 2 diabetes mellitus and cardiovascular disease. Recent in vitro studies suggest that AT hypoxia may induce inflammation. We hypothesized that adipose tissue blood flow (ATBF) regulates AT oxygen partial pressure (AT P(O2)), thereby affecting AT inflammation and insulin sensitivity. We developed an optochemical measurement system for continuous monitoring of AT P(O2) using microdialysis. The effect of alterations in ATBF on AT P(O2) was investigated in lean and obese subjects with both pharmacological and physiological approaches to manipulate ATBF. Local administration of angiotensin II (vasoconstrictor) in abdominal subcutaneous AT decreased ATBF and AT P(O2), whereas infusion of isoprenaline (vasodilator) evoked opposite effects. Ingestion of a glucose drink increased ATBF and AT P(O2) in lean subjects, but these responses were blunted in obese individuals. However, AT P(O2) was higher (hyperoxia) in obese subjects despite lower ATBF, which appears to be explained by lower AT oxygen consumption. This was accompanied by insulin resistance, lower AT capillarization, lower AT expression of genes encoding proteins involved in mitochondrial biogenesis and function, and higher AT gene expression of macrophage infiltration and inflammatory markers. Our findings establish ATBF as an important regulator of AT P(O2). Nevertheless, obese individuals exhibit AT hyperoxia despite lower ATBF, which seems to be explained by lower AT oxygen consumption. This is accompanied by insulin resistance, impaired AT capillarization, and higher AT gene expression of inflammatory cell markers. http://www.trialregister.nl. Unique identifier: NTR2451.

Effects of the Oral Administration of a β3-Adrenergic Agonist on Lipid Metabolism in Alloxan-Diabetic Rats

Previous studies have reported that beta3-adrenergic agonists regulate plasma glucose, triglycerides and free fatty acids in situations of hyperglycaemia and dyslipidaemia in rodents. In this study Trecadrine, a novel compound with affinity for beta3-adrenergic receptors, has been tested in an alloxan-induced model of hyperglycaemia in rats. Alloxan-induced hyperglycaemic rats were orally treated with Trecadrine (1 mg/kg/day for 4 days), resulting in an improvement of hyperglycaemia (from 16.6 to 8.3 mmol L(-1), P < 0.001). This effect was not associated with statistical differences in plasma insulin levels, which may be explained by changes in insulin resistance and carbohydrate oxidation in peripheral tissues. Furthermore, a reduction in internal white fat weight (-39%), which was not statistically significant, as well as in plasma triglycerides (from 1.89 to 0.33 mmol L(-1), P < 0.001) and free fatty acids (from 0.70 to 0.39 mmol L(-1), P < 0.001), was found after Trecadrine administration. Trecadrine apparently induced lipolytic activity in adipocytes, as suggested by the increase of oxygen consumption in white adipose tissue (+282%, P < 0.001), while free fatty acids decreased apparently through their utilisation in other tissues. Furthermore, the increase in brown adipose tissue oxygen consumption (+50%, P < 0.01) and in rectal temperature (P < 0.05) suggests that both glucose and fatty acid oxidation may be enhanced in this tissue. These results give support to the possible therapeutic use of beta3-adrenergic compounds in situations of hyperglycaemia, particularly when this is accompanied by hypertriglyceridaemia.

Reduced Adipose Tissue Oxygenation in Human Obesity

OBJECTIVE— Based on rodent studies, we examined the hypothesis that increased adipose tissue (AT) mass in obesity without an adequate support of vascularization might lead to hypoxia, macrophage infiltration, and inflammation.RESEARCH DESIGN AND METHODS— Oxygen partial pressure (AT pO2) and AT temperature in abdominal AT (9 lean and 12 overweight/obese men and women) was measured by direct insertion of a polarographic Clark electrode. Body composition was measured by dual-energy X-ray absorptiometry, and insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp. Abdominal subcutaneous tissue was used for staining, quantitative RT-PCR, and chemokine secretion assay.RESULTS— AT pO2 was lower in overweight/obese subjects than lean subjects (47 ± 10.6 vs. 55 ± 9.1 mmHg); however, this level of pO2 did not activate the classic hypoxia targets (pyruvate dehydrogenase kinase and vascular endothelial growth factor [VEGF]). AT pO2 was negatively correlated with percent body fat (R = −0.50, P < 0.05). Compared with lean subjects, overweight/obese subjects had 44% lower capillary density and 58% lower VEGF, suggesting AT rarefaction (capillary drop out). This might be due to lower peroxisome proliferator–activated receptor γ1 and higher collagen VI mRNA expression, which correlated with AT pO2 (P < 0.05). Of clinical importance, AT pO2 negatively correlated with CD68 mRNA and macrophage inflammatory protein 1α secretion (R = −0.58, R = −0.79, P < 0.05), suggesting that lower AT pO2 could drive AT inflammation in obesity.CONCLUSIONS— Adipose tissue rarefaction might lie upstream of both low AT pO2 and inflammation in obesity. These results suggest novel approaches to treat the dysfunctional AT found in obesity.

Olive oil feeding up-regulates uncoupling protein genes in rat brown adipose tissue and skeletal muscle

Some nutrients, such as carotenoids, retinoic acid, and certain types of fatty acids, increase thermogenic capacity. The influence of 4 dietary lipid sources (olive oil, sunflower oil, palm oil, and beef tallow) on the content of uncoupling proteins 1, 2, and 3 (UCP1, UCP2, and UCP3) and their messenger RNA (mRNA) expression in several tissues of rats was compared. Wistar rats were randomly divided into 4 groups and fed ad libitum diets containing 40% of energy as fat. UCP1, UCP2, and UCP3 mRNA and protein were assessed by Northern blot and Western blot, respectively. Oxygen consumption in tissues was measured by polarography. Total-body oxygen consumption was assessed in an open-circuit chamber system. Circulating fuels (fatty acids and glucose) and hormones (triiodothyronine, thyroxine, corticosterone, and insulin) were measured. Olive oil feeding induced the highest UCP1, UCP2, and UCP3 mRNA expression in interscapular brown adipose tissue. An analogous effect was observed in gastrocnemius muscle UCP3 mRNA. No significant differences were observed in perirenal white adipose tissue UCP2 mRNA. Changes in mRNAs were not accompanied by close changes in the protein content of UCPs and were not associated with changes in adipose tissue oxygen consumption. Nevertheless, total-body oxygen consumption was higher in rats fed olive oil than in those fed the other 3 diets. No significant differences were found in body and tissue weights or in serum indexes. Olive oil induced an up-regulating effect on UCP mRNA that was probably not mediated by systemic metabolic changes, but rather related to a local effect on interscapular brown adipose tissue and skeletal muscle.

Divergent effects of an α2-adrenergic antagonist on lipolysis and thermogenesis: Interactions with a β3-adrenergic agonist in rats

This study was undertaken in order to test the hypothesis that selective beta3-AR stimulation and simultaneous blockade of alpha2-AR would result in an increase of lipolysis and thermogenesis in rats. Incubation of isolated white adipocytes with the alpha2-AR antagonist yohimbine produced a concentration-dependent increase in glycerol release (P<0.001) for all assayed concentrations (10-12-10-6 M) and potentiated the lipolytic effect of the beta3-AR agonist Trecadrine. However, in vivo administration of yohimbine produced a marked decrease in body temperature (1.3-1.5 degrees C, P<0.001) and blocked the thermogenic effect of Trecadrine when simultaneously administered. A similar response was observed for whole body oxygen consumption. Furthermore, yohimbine did not modify brown adipose tissue oxygen consumption, but blocked the beta3-AR-mediated increase triggered by Trecadrine. Brown adipose tissue UCP-2 and -3 mRNA expression was not changed by yohimbine. In conclusion, the present work indicates that in vitro alpha2-AR blockade by yohimbine potentiates the beta3-AR-mediated stimulation of lipolysis. On the other hand, in vivo alpha2-AR antagonism blocks the thermogenic effects mediated by beta3-AR stimulation, suggesting a possible interplay between the receptors.

Administration of muscimol into the posterior hypothalamus reduces hyperthermia induced by hippocampal neostigmine injection

The firing rate of the sympathetic nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures ( T IBAT and T C) and oxygen (O 2) consumption were monitored in urethane-anesthetized male Sprague–Dawley rats. These variables were measured for 40 min before (baseline values) and 40 min after an injection of neostigmine (5×10 −7 mol in 1 μl of saline) into the hippocampus and a bilateral administration of a GABA a-agonist, muscimol (28 ng in 0.5 μl of saline, per side) into the posterior hypothalamus. The same variables were recorded in other rats, but the muscimol was replaced by saline. Control animals were used with muscimol or saline alone. The results show an increase of sympathetic firing rate, T IBAT, T C and O 2 consumption after neostigmine injection. Muscimol significantly reduces this enhancement. The findings suggest that hippocampus controls the sympathetic and thermogenic activation induced by neostigmine through an influence on GABAergic tone of the posterior hypothalamus.

Up-regulation of muscle UCP2 gene expression by a new beta3-adrenoceptor agonist, trecadrine, in obese (cafeteria) rodents, but down-regulation in lean animals.

The anti-obesity properties of a new beta3-adrenergic agonist (Trecadrine) were examined in a diet-induced obesity model, including the effects on OB and uncoupling protein (UCP-1 and -2) gene expression. Control rats and cafeteria-fed rats were treated with placebo or Trecadrine for 35 days. Leptin and UCP (1 and 2) mRNA levels were determined by reverse transcription-polymerase chain reaction (RT-PCR) methodology in adipose tissue and gastrocnemius muscle. Animals fed a cafeteria diet increased body weight, fat content, white adipose tissue (WAT), brown adipose tissue (BAT) weights and oxygen consumption in relation to lean controls. A rise in plasma leptin, WAT OB gene expression as well as circulating free fatty acids levels was found in obese rats as compared with lean controls. Trecadrine administration to cafeteria-fed animals decreased fat content, WAT weight, circulating leptin and fatty acids concentrations, and WAT OB gene expression, reaching comparable values to lean controls, while WAT O2 consumption was increased in these animals. Also, an increase in BAT UCP1 mRNA levels was found through a two-way analysis of variance in control and obese animals after Trecadrine administration. Gastrocnemius muscle UCP2 gene expression was reduced in lean Trecadrine-treated and diet-induced obese animals as compared to controls, while an increase was found in cafeteria-fed animals after Trecadrine administration. A negative correlation between WAT O2 consumption and UCP2 expression was found in control animals, but not in the cafeteria-fed groups, suggesting a differential response to the beta3-adrenergic compound in lean and obese animals, which is in agreement with the reported statistical interactions between obesity and Trecadrine administration found for WAT O2 consumption and muscle UCP2 expression, as well as for plasma leptin and WAT leptin expression. The new beta3-adrenergic agonist, Trecadrine, decreases fat content and increases gastrocnemius muscle UCP2 gene expression in a diet-induced obesity model. This sheds additional light on the action mechanism of compounds with affinity for beta3-adrenoceptors and other potential anti-obesity agents.