Increased lipolysis of subcutaneous abdominal adipose tissue and altered noradrenergic activity in patients with Cushing's syndrome: an in-vivo microdialysis study

Cushing's syndrome is associated with typical central redistribution of adipose tissue. The aim of the study was to assess lipolysis and catecholamines and their metabolites in subcutaneous abdominal adipose tissue using an in-vivo microdialysis technique. Nine patients with Cushing's syndrome and nine age-, gender- and body mass index (BMI)-matched control subjects were included in the study. Local glycerol concentrations were significantly increased in subcutaneous adipose tissue of patients with Cushing's syndrome (p<0.001). Plasma noradrenaline, dihydroxyphenylglycol and dihydroxyphenylalanine were decreased in patients with Cushing's syndrome (p<0.02, p<0.05, and p<0.02, respectively). Adrenaline, noradrenaline, dihydroxyphenylglycol and dihydroxyphenylalanine concentrations in subcutaneous abdominal adipose were non-significantly higher in patients with Cushing's syndrome. In conclusion, we showed that lipolysis in subcutaneous adipose tissue of patients with Cushing's syndrome is significantly increased as compared to healthy subjects. This finding together with non-significantly increased local catecholamine concentrations in these patients suggests a possible link between increased lipolysis and catecholaminergic activity in subcutaneous adipose tissue.     

TNF-alpha, but not IL-6, stimulates plasminogen activator inhibitor-1 expression in human subcutaneous adipose tissue.

Plasminogen activator inhibitor-1 (PAI-1) is produced by adipose tissue, and elevated PAI-1 levels in plasma are a risk factor in the metabolic syndrome. We investigated the regulatory effects of TNF-alpha and IL-6 on PAI-1 gene induction in human adipose tissue. Twenty healthy men underwent a 3-h infusion of either recombinant human TNF-alpha (n = 8), recombinant human IL-6 (n = 6), or vehicle (n = 6). Biopsies were obtained from the subcutaneous abdominal adipose tissue at preinfusion, at 1, 2, and 3 h during the infusion, and at 2 h after the infusion. The mRNA expression of PAI-1 in the adipose tissue was measured using real-time PCR. The plasma levels of TNF-alpha and IL-6 reached 18 and 99 pg/ml, respectively, during the infusions. During the TNF-alpha infusion, adipose PAI-1 mRNA expression increased 2.5-fold at 1 h, 6-fold at 2 h, 9-fold at 3 h, and declined to 2-fold 2 h after the infusion stopped but did not change during IL-6 infusion and vehicle. These data demonstrate that TNF-alpha rather than IL-6 stimulates an increase in PAI-1 mRNA in the subcutaneous adipose tissue, suggesting that TNF-alpha may be involved in the pathogenesis of related metabolic disorders.     

Glyceroneogenesis is inhibited through HIV protease inhibitor-induced inflammation in human subcutaneous but not visceral adipose tissue

Glyceroneogenesis, a metabolic pathway that participates during lipolysis in the recycling of free fatty acids to triglycerides into adipocytes, contributes to the lipid-buffering function of adipose tissue. We investigated whether glyceroneogenesis could be affected by human immunodeficiency virus (HIV) protease inhibitors (PIs) responsible or not for dyslipidemia in HIV-infected patients. We treated explants obtained from subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) depots from lean individuals. We observed that the dyslipidemic PIs nelfinavir, lopinavir and ritonavir, but not the lipid-neutral PI atazanavir, increased lipolysis and decreased glyceroneogenesis, leading to an increased release of fatty acids from SAT but not from VAT. At the same time, dyslipidemic PIs decreased the amount of perilipin and increased interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) secretion in SAT but not in VAT. Parthenolide, an inhibitor of the NFκB pathway, counteracted PI-induced increased inflammation and decreased glyceroneogenesis. IL-6 (100 ng) inhibited the activity of phosphoenolpyruvate carboxykinase, the key enzyme of glyceroneogenesis, in SAT but not in VAT. Our data show that dyslipidemic but not lipid-neutral PIs decreased glyceroneogenesis as a consequence of PI-induced increased inflammation in SAT that could have an affect on adipocytes and/or macrophages. These results add a new link between fat inflammation and increased fatty acids release and suggest a greater sensitivity of SAT than VAT to PI-induced inflammation.     

Increased lipolysis but diminished gene expression of lipases in subcutaneous adipose tissue of healthy young males with intrauterine growth retardation

Intrauterine growth retardation (IUGR) is associated with a central fat distribution and risk of developing type 2 diabetes in adults when exposed to a sedentary Western lifestyle. Increased lipolysis is an early defect of metabolism in IUGR subjects, but the sites and molecular mechanisms involved are unknown. Twenty IUGR and 20 control (CON) subjects, aged 20-30 years, were studied before and after 10 days of bed rest using the glucose clamp technique combined with measurements of in vivo metabolism by microdialysis technique and blood flow by (133)Xe washout technique in subcutaneous abdominal (SCAAT) and femoral (SCFAT) adipose tissue. Additionally, mRNA expression of lipases was evaluated in biopsies from SCAAT. Lipolysis in SCAAT was substantially higher in IUGR than in CON subjects despite markedly lower mRNA expression of lipases. Blood flow was higher in IUGR compared with CON in both SCAAT and SCFAT. Whole body insulin sensitivity did not differ between groups and decreased after bed rest. After bed rest, SCAAT lipolysis remained higher in IUGR compared with CON, and SCFAT lipolysis decreased in CON but not in IUGR. Prior to the development of whole body insulin resistance, young men with IUGR are characterized by increased in vivo adipose tissue lipolysis and blood flow with a paradoxically decreased expression of lipases compared with CON, and 10 days of physical inactivity underlined the baseline findings. Subjects with IUGR exhibit primary defects in adipose tissue metabolism.     

Metallothioneins 1 and 2, but not 3, are regulated by nutritional status in rat white adipose tissue

Background

Cumulating evidence underlines the role of adipose tissue metallothionein (MT) in the development of obesity and type 2 diabetes. Fasting/refeeding was shown to affect MT gene expression in the rodent liver. The influence of nutritional status on MT gene expression in white adipose tissue (WAT) is inconclusive. The aim of this study was to verify if fasting and fasting/refeeding may influence expression of MT genes in WAT of rats.

Results

Fasting resulted in a significant increase in MT1 and MT2 gene expressions in retroperitoneal, epididymal, and inguinal WAT of rats, and this effect was reversed by refeeding. Altered expressions of MT1 and MT2 genes in all main fat depots were reflected by changes in serum MT1 and MT2 levels. MT1 and MT2 messenger RNA (mRNA) levels in WAT correlated inversely with serum insulin concentration. Changes in MT1 and MT2 mRNA levels were apparently not related to total zinc concentrations and MTF1 and Zn transporter mRNA levels in WAT. Fasting or fasting/refeeding exerted no effect on the expression of MT3 gene in WAT. Addition of insulin to isolated adipocytes resulted in a significant decrease in MT1 and MT2 gene expressions. In contrast, forskolin or dibutyryl-cAMP (dB-cAMP) enhanced the expressions of MT1 and MT2 genes in isolated adipocytes. Insulin partially reversed the effect of dB-cAMP on MT1 and MT2 gene expressions.

Conclusions

This study showed that the expressions of MT1 and MT2 genes in WAT are regulated by nutritional status, and the regulation may be independent of total zinc concentration.

     

Increased postprandial fatty acid trapping in subcutaneous adipose tissue in obese women

The objective of this study was to test the hypothesis that increased fatty acid trapping by subcutaneous adipose tissue might contribute to the development and/or maintenance of obesity. To do so, venoarterial (V-A) gradients across subcutaneous adipose tissue for triglycerides, glycerol, nonesterified fatty acid (NEFA), and acylation-stimulating protein (ASP) were determined in eight lean females [body mass index (BMI), 22.2 +/- 0.6] and eight obese females (BMI, 34.4 +/- 3.4). Plasma insulin was also measured at intervals throughout this period. Fasting plasma triglyceride was significantly higher in the obese group and postprandial triglyceride was also significantly delayed. In contrast, both triglyceride clearance and fatty acid uptake by subcutaneous adipose tissue were significantly greater in the obese group compared with the lean group. Fasting insulin did not differ between the groups, but postprandial insulin values were significantly higher in the obese group. The pattern of ASP release from subcutaneous adipose tissue also appeared to differ in that it was significantly greater in the early postprandial period (0;-90 min) in the obese group versus the lean group and this correlated with greater triglyceride clearance during this period. Moreover, there were strong, positive correlations between BMI and the V-A gradient for fasting ASP, the 0- to 90-min area under the curve (AUC) for ASP V-A gradient fasting insulin, and the 0- to 90-min AUC for fatty acid incorporation into adipose tissue. Taken together, these data demonstrate that fatty acid trapping by adipose tissue can be increased even when overall plasma triglyceride clearance is delayed. The postprandial pattern of insulin, in particular, was altered in the obese, although it is certainly possible that differences in ASP release or response could also contribute to increased fatty acid trapping in the obese.The data, therefore, suggest that increased fatty acid trapping by adipose tissue may be a feature of some forms of obesity.     

Increased glycolysis in skeletal muscle coordinates with adipose tissue in systemic metabolic homeostasis

Insulin-independent glucose metabolism, including anaerobic glycolysis that is promoted in resistance training, plays critical roles in glucose disposal and systemic metabolic regulation. However, the underlying mechanisms are not completely understood. In this study, through genetically manipulating the glycolytic process by overexpressing human glucose transporter 1 (GLUT1), hexokinase 2 (HK2) and 6-phosphofructo-2-kinase-fructose-2,6-biphosphatase 3 (PFKFB3) in mouse skeletal muscle, we examined the impact of enhanced glycolysis in metabolic homeostasis. Enhanced glycolysis in skeletal muscle promoted accelerated glucose disposal, a lean phenotype and a high metabolic rate in mice despite attenuated lipid metabolism in muscle, even under High-Fat diet (HFD). Further study revealed that the glucose metabolite sensor carbohydrate-response element-binding protein (ChREBP) was activated in the highly glycolytic muscle and stimulated the elevation of plasma fibroblast growth factor 21 (FGF21), possibly mediating enhanced lipid oxidation in adipose tissue and contributing to a systemic effect. PFKFB3 was critically involved in promoting the glucose-sensing mechanism in myocytes. Thus, a high level of glycolysis in skeletal muscle may be intrinsically coupled to distal lipid metabolism through intracellular glucose sensing. This study provides novel insights for the benefit of resistance training and for manipulating insulin-independent glucose metabolism.     

Cardiotrophin-1 is expressed in adipose tissue and upregulated in the metabolic syndrome

Adipose tissue is a target for cardiotrophin-1 (CT-1), a cytokine member of the IL-6 family of cytokines that is involved in cardiac growth and dysfunction. However, it is unknown whether adipocytes are a source of CT-1 and whether CT-1 is overexpressed in diseases characterized by increased fat depots [i.e., the metabolic syndrome (MS)]. Thus this work aimed 1) to test whether adipose tissue expresses CT-1 and whether CT-1 expression can be modulated and 2) to compare serum CT-1 levels in subjects with and without MS diagnosed by National Cholesterol Education Program Adult Treatment Panel III criteria. Gene and protein expression of CT-1 was determined by real-time RT-PCR, ELISA, and Western blotting. CT-1 expression progressively increased, along with differentiation time from preadipocyte to mature adipocyte in 3T3-L1 cells. CT-1 expression was enhanced by glucose in a dose-dependent manner in these cells. mRNA and protein CT-1 expression was also demonstrated in human adipose biopsies. Immunostaining showed positive staining in adipocytes. Finally, increased CT-1 serum levels were observed in patients with MS compared with control subjects (127 +/- 9 vs. 106 +/- 4 ng/ml, P < 0.05). Circulating levels of CT-1 were associated with glucose levels (r = 0.2, P < 0.05). Taken together, our data suggest that adipose tissue can be recognized as a source of CT-1, which could account for the high circulating levels of CT-1 in patients with MS.     

Dietary fat modifies lipid metabolism in the adipose tissue of metabolic syndrome patients

Adipose tissue (AT) is a key organ in the regulation of total body lipid homeostasis, which is responsible for the storage and release of fatty acids according to metabolic needs. We aimed to investigate the effect of the quantity and quality of dietary fat on the lipogenesis and lipolysis processes in the AT of metabolic syndrome (MetS) patients. A randomized, controlled trial conducted within the LIPGENE study assigned MetS patients to one of four diets: (a) high-saturated fatty acid (HSFA) (b) high-monounsaturated fatty acid, and (c, d) two low-fat, high-complex carbohydrate diets supplemented with long chain (LC) n-3 (LFHCC n-3) polyunsaturated fatty acids (PUFA) or placebo (LFHCC), for 12 weeks each. A fat challenge reflecting the same fatty acid composition as the original diets was conducted post-intervention. Long-term consumption of the LFHCC diet induced an increase in the fasting expression levels of the sterol regulatory element binding protein-1 and stearoyl-CoA desaturase D9-desaturase genes, whereas the supplementation of this diet with n-3 PUFA reversed this effect (p = 0.007). In contrast, long-term consumption of the HSFA diet increased the expression of the adipose triglyceride lipase (ATGL) gene, at both fasting and postprandial states (both, p < 0.001). Our results showed the anti-lipogenic effect exerted by LC n-3 PUFA when administered together with a LFHCC diet. Conversely, a diet high in saturated fat increased the expression of the lipolytic gene ATGL relative to the other diets.

Electronic supplementary material

The online version of this article (doi:10.1007/s12263-014-0409-3) contains supplementary material, which is available to authorized users.     

Depot-specific hormonal characteristics of subcutaneous and visceral adipose tissue and their relation to the metabolic syndrome.

Visceral adipose tissue (VAT) imaged by computed tomography (CT) or magnetic resonance imaging (MRI) is associated with the metabolic syndrome features, being morphologically and functionally different from subcutaneous adipose tissue (SAT). Insulin effect is lower and catecholamine effect higher in visceral adipose tissue, with its metabolites and its secretions draining through portal system, partially at least, to the liver. Thus, visceral cells transfer and release fatty acids more extensively, have increased glucocorticoid and reduced thiazolidinedione responses, produce more angiotensinogen, interleukin-6 and plasminogen activator inhibitor-1, and secrete less leptin and adiponectin than SAT. Furthermore, there are regional differences in the intrinsic characteristics of the preadipocytes, with those of SAT presenting greater differentiation and fat cell gene expression but less apoptosis than that of VAT. All features contribute to the morbidity associated with increased VAT. To evaluate the relationship between VAT and components of the metabolic syndrome, 55 non-diabetic women, 11 lean (VAT < 68 cm 2) and 44 obese were studied. The obese with VAT within the normal range (VAT < or = 68 cm 2) had higher BMI, WHR, BP and resistance to FFA suppression during oGTT in comparison to the lean controls. The obese with VAT > 68 cm 2 compared to those with VAT < or = 68 cm 2 had similar body mass index (BMI) but significantly higher in vivo homeostasis model assessment for insulin resistance (HOMA IR ) results and triglycerides. By pooling all data, correlation analysis indicated that VAT contributes more to insulin resistance (HOMA IR ) than SAT does, but not when insulin-suppressed plasma free fatty acids during oral glucose tolerance test as an index of insulin resistance are taken into consideration.     

Increased systemic and adipose tissue cytokines in patients with HIV-associated lipodystrophy

The lipodystrophy syndrome (adipose tissue redistribution and metabolic abnormalities) observed with highly active antiretroviral therapy (HAART) during human immunodeficiency virus (HIV) infection may be related to increased proinflammatory cytokine activity. We measured acute cytokine (TNF-alpha, IL-6, leptin), glycerol, and lactate secretion from abdominal subcutaneous adipose tissue (SAT), and systemic cytokine levels, in HIV-infected subjects with and without lipodystrophy (HIVL+ and HIVL-, respectively) and healthy non-HIV controls. Lipodystrophy was confirmed and characterized as adipose tissue redistribution in HIVL+ compared with HIVL- and controls, by dual-energy X-ray absorptiometry and by whole body MRI. TNF-alpha secretion from abdominal SAT and circulating levels of IL-6, soluble TNF receptors I and II, and insulin were elevated in HIVL+ relative to HIVL- and/or controls, particularly in HIVL+ undergoing HAART. In the HIV-infected group as a whole, IL-6 secretion from abdominal SAT and serum IL-6 were positively associated with visceral fat and were negatively associated with the relative amount of lower limb adipose tissue (P < 0.01). Decreased leptin and increased lactate secretion from abdominal SAT were specifically associated with HAART. In conclusion, increased cytokine secretion from adipose tissue and increased systemic proinflammatory cytokine activity may play a significant role in the adipose tissue remodeling and/or the metabolic abnormalities associated with the HIV-lipodystrophy syndrome in patients undergoing HAART.     

SCD1 promotes lipid mobilization in subcutaneous white adipose tissue

Beiging of white adipose tissue (WAT) has beneficial effects on metabolism. Although it is known that beige adipocytes are active in lipid catabolism and thermogenesis, how they are regulated deserves more explorations. In this study, we demonstrate that stearoyl-CoA desaturase 1 (SCD1) in subcutaneous WAT (scWAT) responded to cold stimulation and was able to promote mobilization of triacylglycerol [TAG (triglyceride)]. In vitro studies showed that SCD1 promoted lipolysis in C3H10T1/2 white adipocytes. The lipolytic effect was contributed by one of SCD1’s products, oleic acid (OA). OA upregulated adipose TAG lipase and hormone-sensitive lipase expression. When SCD1 was overexpressed in the scWAT of mice, lipolysis was enhanced, and oxygen consumption and heat generation were increased. These effects were also demonstrated by the SCD1 knockdown experiments in mice. In conclusion, our study suggests that SCD1, known as an enzyme for lipid synthesis, plays a role in upregulating lipid mobilization through its desaturation product, OA.     

Metformin, but not thiazolidinediones, inhibits plasminogen activator inhibitor-1 production in human adipose tissue in vitro.

Biguanides and thiazolidinediones (TZDs), which are primarily used as anti-diabetic drugs, are also associated with other beneficial effects on cardiovascular risk factors such as reduced plasma plasminogen activator inhibitor-1 (PAI-1) concentration in both diabetic and non-diabetic obese subjects. Since human adipose tissue is of importance for the production of PAI-1, the aim of the present study was to investigate the possible direct effects of these anti-diabetic agents on PAI-1 mRNA and secretion by human adipose tissue. Adipose tissue was obtained from biopsies taken from the subcutaneous abdominal depot. Adipose tissue fragments, isolated mature adipocytes, and preadipocytes were incubated in vitro with metformin and various TZDs. Metformin (0.1 - 10 mM) dose-dependently decreased PAI-1 production (and PAI-1 mRNA) under both basal (43 % inhibition at 10 mM, p < 0.05) and interleukin-1beta (IL-1beta)-stimulated conditions where the levels were inhibited by 47.8 % at 1 mM metformin (p < 0.05) and by 100 % at 10 mM (p < 0.01). None of the TZDs tested (PPAR-gamma agonists: troglitazone, pioglitazone, or ciglitazone) had any effects on PAI-1 production. Moreover, no effects on PAI-1 production were observed using various PPAR-alpha agonists such as 5, 8, 11, 14-eicosatetraynoic acid (ETYA), Wy14643 and fenofibrate. Our findings indicate no direct effects of TZDs on PAI-1 secretion, whereas metformin was able to directly inhibit PAI-1 production in human adipose tissue.     

Adaptive changes in enzyme activity and metabolic pathways in adipose tissue from meal-fed rats

A number of metabolic factors and the activity of a number of enzymes were determined in meal-fed (animals fed a single daily 2 hr meal) and nibbling (ad libitum-fed) rats. The dependency of the observed adaptive changes on the ingestion of carbohydrate was studied by feeding diets high in carbohydrate or fat. Glucose-6-phosphate dehydrogenase and NADP-malic dehydrogenase were more active in adipose tissue from high carbohydrate meal-fed rats than in tissue from ad libitum-fed rats. The activity in adipose tissue of isocitric dehydrogenase, 6-phosphogluconate dehydrogenase, and NAD-malic dehydrogenase did not increase significantly in response to meal-feeding the high carbohydrate diet. No increase in lipogenesis or enzyme activity could be demonstrated in adipose tissue from rats meal-fed a high fat diet. Lipase activity of adipose tissue was increased by high carbohydrate meal-feeding and decreased by feeding a high fat diet. The in vitro uptake of palmitate-1-(14)C by adipose tissue was depressed by a high fat diet and enhanced in rats meal-fed a high carbohydrate diet. Diaphragm or slices of liver from high fat-fed rats oxidized palmitate-1-(14)C more rapidly than did tissue from ad libitum-fed animals. Evidence is presented for the quantitative importance of citrate as a source of extramitochondrial acetyl CoA in adipose tissue of meal-eating and ad libitum-fed rats. The relationship of extramitochondrially formed citrate to the NAD-malic dehydrogenase-malic enzyme system in adipose tissue is discussed.