Expression of human hormone-sensitive lipase in white adipose tissue of transgenic mice increases lipase activity but does not enhance in vitro lipolysis

Hormone-sensitive lipase (HSL) catalyzes the hydrolysis of acylglycerols and cholesteryl esters (CEs). The enzyme is highly expressed in adipose tissues (ATs), where it is thought to play an important role in fat mobilization. The purpose of the present work was to study the effect of a physiological increase of HSL expression in vivo. Transgenic mice were produced with a 21 kb human genomic fragment encompassing the exons encoding the adipocyte form of HSL. hHSL mRNA was expressed at 3-fold higher levels than murine HSL mRNA in white adipocytes. Transgene expression was also observed in brown adipose tissue (BAT) and skeletal muscle. The human protein was detected in ATs of transgenic (Tg) mice. The hydrolytic activities against triacylglycerol (TG), diacylglycerol (DG) analog, and CE were increased in transgenic mouse AT. However, cAMP-inducible adipocyte lipolysis was lower in transgenic animals. In the B6CBA genetic background, transgenic mice up to 14 weeks of age showed lower body weight and fat mass. The phenotype was not observed in older animals and in mice fed a high-fat diet (HFD). In the OF1 genetic background, there was no difference in fat mass of mice fed ad libitum. However, transgenic mice became leaner than their wild-type (WT) littermates after a 4 day calorie restriction. The data show that overexpression of HSL, despite increased lipase activity, does not lead to enhanced lipolysis.     

Neutralization of tumor necrosis factor-alpha reverses insulin resistance in skeletal muscle but not adipose tissue

We examined the possible role of tumor necrosis factor-alpha (TNF-alpha) as a mediator of insulin resistance in maturing male Sprague-Dawley rats. Rats were treated either with goat anti-murine TNF-alpha IgG (anti-TNF-alpha) or goat nonimmune IgG (NI) for 7 days. Vascular catheters were implanted, and rats were fasted overnight before hyperinsulinemic euglycemic clamp (HUC) studies were performed. TNF-alpha neutralization increased the rate of glucose infusion required to maintain euglycemia by 68%. Insulin-stimulated glucose transport into individual tissues was measured after bolus administration of 2-deoxy-[(14)C]glucose during HUC. Anti-TNF-alpha administration increased glucose transport in muscles composed predominantly of fast-twitch fibers: white gastrocnemius muscle (68% increase) and tibialis anterior muscle (64% increase). There were nonsignificant trends for increased glucose transport in the slow-twitch soleus muscle and in the mixed-fiber red gastrocnemius muscle. Glucose transport was unchanged in visceral and subcutaneous fat. Anti-TNF treatment did not alter body weight, muscle mass, or fat mass. Anti-TNF-alpha did not alter the distribution of the 17-kDa and 26-kDa forms of TNF-alpha in either muscle or fat. However, anti-TNF-alpha treatment caused an approximately 50% reduction in the secretion of TNF-alpha bioactivity in vitro by explants of visceral and subcutaneous fat. We conclude that TNF-alpha neutralization reversed insulin resistance substantially in fast-twitch muscle and may have done so in other muscles, while having little effect in fat. TNF-alpha neutralization was accompanied by reduced TNF-alpha bioactivity without tissue depletion of TNF-alpha protein.     

Upregulation of uncoupling protein homologues in skeletal muscle but not adipose tissue in posttraumatic insulin resistance

Metabolic alterations after surgical stress include peripheral insulin resistance and increased utilization of fat as a fuel substrate. An up-regulation of skeletal muscle uncoupling proteins (UCPs) has been associated with physiologic states of insulin resistance and enhanced fat metabolism in rodents. We examined whether posttraumatic insulin resistance induced the UCPs in gastrocnemius and soleus muscle and white adipose tissue in an experimental model of surgical trauma. Insulin sensitivity was significantly reduced in isolated soleus muscles but unchanged in adipocytes after trauma. In traumatized rats, mRNA and protein contents of UCP2 and UCP3 and were significantly increased in both muscle types. UCP2 protein content in adipose tissue was unaltered by surgical stress. Circulating NEFAs and glycerol were reduced after surgical trauma. We hypothesize that the changes in UCP2 and UCP3 gene and protein expression are involved in the regulation of substrate utilization in posttraumatic insulin resistance.     

Hyperglycemia contributes insulin resistance in hepatic and adipose tissue but not skeletal muscle of ZDF rats

To determine the contribution of hyperglycemia to the insulin resistance in various insulin-sensitive tissues of Zucker diabetic fatty (ZDF) rats, T-1095, an oral sodium-dependent glucose transporter (SGLT) inhibitor, was administered by being mixed into food. Long-term treatment with T-1095 lowered both fed and fasting blood glucose levels to near normal ranges. A hyperinsulinemic euglycemic clamp study that was performed after 4 wk of T-1095 treatment demonstrated partial recovery of the reduced glucose infusion rate (GIR) in the T-1095-treated group. In the livers of T-1095-treated ZDF rats, hepatic glucose production rate (HGP) and glucose utilization rate (GUR) showed marked recovery, with almost complete normalization of reduced glucokinase/glucose-6-phosphatase (G-6-Pase) activities ratio. In adipose tissues, decreased GUR was also shown to be significantly improved with a normalization of insulin-induced GLUT-4 translocation. In contrast, in skeletal muscles, the reduced GUR was not significantly improved in response to amelioration of hyperglycemia by T-1095 treatment. These results suggest that the contribution of hyperglycemia to insulin resistance in ZDF rats is very high in the liver and considerably elevated in adipose tissues, although it is very low in skeletal muscle.     

Decreased sensitivity to insulin in white adipose tissue from adrenalectomised rats

The ability of insulin to increase both [14C]-glucose incorporation into fatty acids and pyruvate dehydrogenase activity in incubated rat epididymal adipose tissues was considerably lessened after adrenalectomy. Insulin antagonism of adrenaline-stimulated lipolysis in isolated fat cells was abolished after adrenalectomy. Percentage stimulation of lipolysis above basal by adrenaline was not appreciably altered by adrenalectomy.     

Glucocorticoid-regulated adipose tissue secretion of PAI-1, but not IL-6, TNFalpha or leptin in vivo.

OBJECTIVE: Glucocorticoids are well-known regulators of adipose tissue metabolism and endocrine function. The aim of this study was to examine glucocorticoid effect on plasminogen activator inhibitor 1 (PAI-1), tumour necrosis factor alpha (TNFalpha), leptin and interleukin-6 (IL-6) adipose tissue secretion. MATERIAL AND METHODS: Twelve healthy postmenopausal women with mean BMI of 28.9 kg/m(2) (+/- 0.8 SEM) received 25 mg prednisolone daily for 7 days. Before and after glucocorticoid treatment adipose tissue secretion of PAI-1, leptin, IL-6 and TNFalpha were measured, and adipocyte PAI-1 mRNA as well as anthropometrical and bio-chemical data were obtained. RESULTS: Anthropometric measurements remained unaffected. Analyses of venous blood-samples showed a borderline increase of insulin levels (p = 0.062). PAI-1 secretion from adipose tissue increased (1.9 +/- 0.2 vs. 3.5 +/- 0.5 ng/g triglycerides, p = 0.012), but PAI-1 mRNA levels did not (0.19 +/- 0.02 vs. 0.21 +/- 0.04 arbitrary units after normalised to beta-actin, p = 0.51). There were no apparent differences in IL-6, TNFalpha or leptin secretion after glucocorticoid exposure. CONCLUSION: This study shows an increased secretion of PAI-1, but not IL-6, TNFalpha or leptin, from abdominal adipose tissue after in vivo glucocorticoid treatment, which may be a finding of pathophysiological importance given the well-known effect of glucocorticoid excess on metabolic aberrations and cardiovascular morbidity.     

Mild-cold water swimming does not exacerbate white adipose tissue browning and brown adipose tissue activation in mice

Journal of Physiology and Biochemistry - The present study investigated the effects of swimming physical training either thermoneutral or below thermoneutral water temperature on white (WAT) and...     

Lipoprotein lipase mRNA in white adipose tissue but not in skeletal muscle is increased by pioglitazone through PPAR-gamma

Lipoprotein lipase (LPL), a key enzyme for triglyceride hydrolysis, is an insulin-dependent enzyme and mainly synthesized in white adipose tissue (WAT) and skeletal muscles (SM). To explore how pioglitazone, an enhancer of insulin action, affects LPL synthesis, we examined the effect of pioglitazone on LPL mRNA levels in WAT or SM of brown adipose tissue (BAT)-deficient mice, which develop insulin resistance and hypertriglyceridemia. Both LPL mRNA of WAT and SM were halved in BAT-deficient mice. Pioglitazone increased LPL mRNA in WAT by 8-fold, which was substantially associated with a 4-fold increase of peroxisome proliferator activated receptor (PPAR)-gamma mRNA (r=0.97, p<0.0001), whereas pioglitazone did not affect LPL mRNA in SM. These results suggest that pioglitazone exclusively increases LPL production in WAT via stimulation of PPAR-gamma synthesis. Since pioglitazone does not affect LPL production in SM, this would contribute to prevent the development of insulin resistance due to lipotoxicity.     

Reconstitution of insulin action in muscle, white adipose tissue, and brain of insulin receptor knock-out mice fails to rescue diabetes

Type 2 diabetes results from an impairment of insulin action. The first demonstrable abnormality of insulin signaling is a decrease of insulin-dependent glucose disposal followed by an increase in hepatic glucose production. In an attempt to dissect the relative importance of these two changes in disease progression, we have employed genetic knock-outs/knock-ins of the insulin receptor. Previously, we demonstrated that insulin receptor knock-out mice (Insr(-/-)) could be rescued from diabetes by reconstitution of insulin signaling in liver, brain, and pancreatic β cells (L1 mice). In this study, we used a similar approach to reconstitute insulin signaling in tissues that display insulin-dependent glucose uptake. Using GLUT4-Cre mice, we restored InsR expression in muscle, fat, and brain of Insr(-/-) mice (GIRKI (Glut4-insulin receptor knock-in line 1) mice). Unlike L1 mice, GIRKI mice failed to thrive and developed diabetes, although their survival was modestly extended when compared with Insr(-/-). The data underscore the role of developmental factors in the presentation of murine diabetes. The broader implication of our findings is that diabetes treatment should not necessarily target the same tissues that are responsible for disease pathogenesis.     

Effects of noradrenaline and insulin on electrical activity in white adipose tissue of rat.

An active change in membrane voltage responses to hyperpolarizing pulses has been identified by intracellular recording on an in vitro preparation of white adipose tissue. This change was characterized by a slow return to baseline at the offset of the pulses. Amplitude and duration of the slow return to baseline were dependent on extracellular K+ concentration, and were diminished by external application of Ba2+. Such properties suggest that this electrical response can be mainly due to activation of transient K+ conductances. The effects that noradrenaline and insulin have over the slow return to baseline have been also studied. While external addition of noradrenaline decreased amplitude and duration of this electrical response, insulin produced the opposite effect. These results suggest that noradrenaline and insulin could modulate K+ conductances in white adipocytes.     

Catecholamine activation of pyruvate dehydrogenase in white adipose tissue of the rat in vivo.

Intraperitoneal injections of noradrenaline or adrenaline into rats increased the proportion of pyruvate dehydrogenase in the active state in white adipose tissue; this effect of catecholamines was also apparent in streptozotocin-diabetic rats, showing that it was not due to an increase in serum insulin concentration. The catecholamine-induced increase in pyruvate dehydrogenase of white adipose tissue in vivo was completely blocked by prior injection of either the beta-antagonist propranolol or the alpha 1-antagonist prazosin. Cervical dislocation of conscious rats increased pyruvate dehydrogenase activity of white adipose tissue, which was prevented by prior injection of propranolol. Adrenaline (30 nM) activated pyruvate dehydrogenase in white adipocytes in vitro; the maximum effect of adrenaline required activation of both alpha 1- and beta-receptors. The results show that catecholamines activate pyruvate dehydrogenase of white adipose tissue both in vivo and in vitro and that this effect is mediated by a combination of alpha 1- and beta-adrenergic receptors.     

Autologous fat transplants influence compensatory white adipose tissue mass increases after lipectomy

Direct tests of the hypothesized total body fat regulatory system have been accomplished by partial surgical lipectomy. This usually results in the restoration of the lipid deficit through compensatory increases in nonexcised white adipose tissue (WAT) masses of ground squirrels, laboratory rats, and mice, as well as Siberian and Syrian hamsters. We challenged this hypothesized total body fat regulatory system by testing the response of Siberian hamsters to 1) lipid deficits [lipectomy; primarily bilateral epididymal WAT (EWAT) removal], 2) lipid surfeits (addition of donor EWAT with no lipectomy), 3) no net change in lipid [EWAT or inguinal WAT (IWAT) lipectomy with the excised fat replaced to a new location (autologous)], 4) lipectomy with the same pad (EWAT lipectomy only) added from a sibling (nonautologous), and 5) sham surgeries for each treatment. Food intake generally was not affected. Body mass was not affected across all treatments. Grafts approximately 3 mo later had normal appearance both macro- and microscopically and were revascularized. The normal lipectomy-induced compensatory increases in nonexcised WAT masses surprisingly were exaggerated with autologous EWAT transplants, but not for autologous IWAT or nonautologous EWAT transplants. There was no compensatory decrease in native WAT masses with nonautologous EWAT additions. Collectively, only lipectomy triggered reparation of the lipid deficit, but the other manipulations did not, suggesting a system biased toward rectifying decreases in lipid or an inability of the hypothesized total body fat regulatory system to recognize WAT transplants.     

Racial differences in visceral adipose tissue but not anthropometric markers of health-related variables.

This study sought to determine whether visceral adipose tissue (VAT) and/or its anthropometric surrogates could significantly predict health-related variables (HRV) in overweight Caucasian (CC) (n = 36) and African-American (AA) (n = 30) women. With the use of magnetic resonance imaging, findings showed significantly higher volume and area of VAT (P < 0.0001 for both) as well as higher triacylglycerol (P = 0.009) in CC compared with AA women. Furthermore, VAT volume, race, and VAT volume x race interaction could significantly predict triacylglycerol (P = 0.0094), high-density lipoprotein cholesterol (P = 0.0057), insulin (P = 0.0002), and insulin resistance (P < 0. 0001). Additionally, the VAT volume x race interaction for insulin (P = 0.040) and insulin resistance (P = 0.003) was significant. In a separate analysis, waist circumference and race predicted the identical variables. Our results support the use of volume or area of VAT in predicting HRV in CC women; however, its use in AA women appears limited. In contrast, waist circumference can provide a suitable VAT alternative for both CC and AA women; however, VAT clearly represents the more powerful predictor.     

Diabetogenic diet-induced insulin resistance associates with lipid droplet proteins and adipose tissue secretome,but not with sexual dimorphic adipose tissue fat accumulation in Wistar rats

The role of sexual dimorphic adipose tissue fat accumulation in the development of insulin resistance is well known. However, whether vitamin A status and/or its metabolic pathway display any sex- or depot (visceral/subcutaneous)-specific pattern and have a role in sexual dimorphic adipose tissue development and insulin resistance are not completely understood. Therefore, to assess this, 5 weeks old Wistar male and female rats of eight from each sex were provided either control or diabetogenic (high fat, high sucrose) diet for 26 weeks. At the end, consumption of diabetogenic diet increased the visceral fat depots (p < 0.001) in the males and subcutaneous depot (p < 0.05) in the female rats, compared to their sex-matched controls. On the other hand, it caused adipocyte hypertrophy (p < 0.05) of visceral depot (retroperitoneal) in the females and subcutaneous depot of the male rats. Although vitamin A levels displayed sex- and depot-specific increase due to the consumption of diabetogenic diet, the expression of most of its metabolic pathway genes in adipose depots remained unaltered. However, the mRNA levels of some of lipid droplet proteins (perilipins) and adipose tissue secretory proteins (interleukins, lipocalin-2) did display sexual dimorphism. Nonetheless, the long-term feeding of diabetogenic diet impaired the insulin sensitivity, thus affected glucose clearance rate and muscle glucose-uptake in both the sexes of rats. In conclusion, the chronic consumption of diabetogenic diet caused insulin resistance in the male and female rats, but did not corroborate with sexual dimorphic adipose tissue fat accumulation or its vitamin A status.