Effects of a beta3-adrenergic agonist on glucose uptake and leptin expression and secretion in cultured adipocytes from lean and overweight (cafeteria) rats

Eicosapentaenoic acid actions on adiposity and insulin resistance in control and high-fat-fed rats: role of apoptosis, adiponectin and tumour necrosis factor-alpha

n-3 PUFA have shown potential anti-obesity and insulin-sensitising properties. However, the mechanisms involved are not clearly established. The aim of the present study was to assess the effects of EPA administration, one of the n-3 PUFA, on body-weight gain and adiposity in rats fed on a standard or a high-fat (cafeteria) diet. The actions on white adipose tissue lipolysis, apoptosis and on several genes related to obesity and insulin resistance were also studied. Control and cafeteria-induced overweight male Wistar rats were assigned into two subgroups, one of them daily received EPA ethyl ester (1 g/kg) for 5 weeks by oral administration. The high-fat diet induced a very significant increase in both body weight and fat mass. Rats fed with the cafeteria diet and orally treated with EPA showed a marginally lower body-weight gain (P = 0.09), a decrease in food intake (P < 0.01) and an increase in leptin production (P < 0.05). EPA administration reduced retroperitoneal adipose tissue weight (P < 0.05) which could be secondary to the inhibition of the adipogenic transcription factor PPARgamma gene expression (P < 0.001), and also to the increase in apoptosis (P < 0.05) found in rats fed with a control diet. TNFalpha gene expression was significantly increased (P < 0.05) by the cafeteria diet, while EPA treatment was able to prevent (P < 0.01) the rise in this inflammatory cytokine. Adiposity-corrected adiponectin plasma levels were increased by EPA. These actions on both TNFalpha and adiponectin could explain the beneficial effects of EPA on insulin resistance induced by the cafeteria diet.

Indispensable role of mitochondrial UCP1 for antiobesity effect of beta3-adrenergic stimulation

Mitochondrial uncoupling protein-1 (UCP1) has been thought to be a key molecule for thermogenesis during cold exposure and spontaneous hyperphagia and thereby in the autonomic regulation of energy expenditure and adiposity. However, UCP1 knockout (KO) mice were reported to be cold intolerant but unexpectedly did not get obese even after hyperphagia, implying that UCP1 may not be involved in the regulation of adiposity. Treatment of obese animals with beta3-adrenergic agonists is known to increase lipid mobilization, induce UCP1, and, finally, reduce body fat content. To obtain direct evidence for the role of UCP1 in the anti-obesity effect of beta3-adrenergic stimulation, in the present study, UCP1-KO and wild-type (WT) mice were fed on cafeteria diets for 8 wk and then given a beta3-adrenergic agonist, CL-316,243 (CL), or saline for 2 wk. A single injection of CL increased whole body oxygen consumption and brown fat temperature in WT mice but not in KO mice, and it elicited almost the same plasma free fatty acid response in WT and KO mice. WT and KO mice increased similarly their body and white fat pad weights on cafeteria diets compared with those on laboratory chow. Daily treatment with CL resulted in a marked reduction of white fat pad weight and the size of adipocytes in WT mice, but not in KO mice. Compared with WT mice, KO mice expressed increased levels of UCP2 in brown fat but decreased levels in white fat and comparable levels of UCP3. It was concluded that the anti-obesity effect of beta3-adrenergic stimulation is largely attributable to UCP1, but less to UCP2 and UCP3, and thereby to UCP1-dependent degradation of fatty acids released from white adipose tissue.

Serum and gene expression levels of leptin and adiponectin in rats susceptible or resistant to diet-induced obesity

The aim of the present study was to identify the role of leptin and adiponectin in the development of resistance or susceptibility to diet-induced obesity in rats. For this purpose, male Wistar rats were fed with standard laboratory diet (control group) or cafeteria diet. After 15 days, two groups of rats with different response respect to the cafeteria diet were identified, and were assigned as diet-induced obesity (DIO) and diet resistant (DR) rats. The high-fat diet induced a very significant increase in both body and fat mass weight in DIO group. However, DR rats, gained even less weight than control-fed animals. Food intake was increased in cafeteria-fed rats (both DIO and DR) in comparison to control group; but hyperphagia was higher in DIO rats. In addition, feed efficiency (the ratio of weight gained to calories consumed) was significantly decreased in DR as compared to DIO rats. Regarding leptin, a significant increase in both adipose tissue gene expression and serum levels was observed in DIO rats in comparison with other groups (control and DR). A significant increase in both adiponectin circulating levels and adipose tissue mRNA expression was also observed in DIO animals as compared with the other groups. These data suggest that the susceptibility to obesity of DIO rats might be secondary, at least in part, to an earlier development of leptin resistance, which could lead to alterations in food intake (hyperphagia) and energetic metabolism. However, neither changes in leptin or adiponectin seem to be involved in the adaptive mechanisms that confer resistance to high fat intake.

Eicosapentaenoic fatty acid increases leptin secretion from primary cultured rat adipocytes: role of glucose metabolism

Eicosapentaenoic acid (EPA), one of the n-3 polyunsaturated fatty acids, has been shown to stimulate leptin mRNA expression and secretion in 3T3-L1 cells. However, other studies have reported inhibitory effects of EPA on leptin expression and secretion in vivo and in vitro. To determine the direct effects of EPA on basal and insulin-stimulated leptin secretion, isolated rat adipocytes were incubated with EPA in the absence and presence of insulin. EPA (10, 100, and 200 μM) increased basal leptin gene expression and secretion (+43.8%, P < 0.05; +71.1%, P < 0.01; and +73.7%, P < 0.01, respectively). EPA also increased leptin secretion in the presence of 1.6 nM insulin; however, the effect was less pronounced than in the absence of it. Because adipocyte glucose and lipid metabolism are involved in the regulation of leptin production, the metabolic effects of this fatty acid were also examined. EPA (200 μM) increased basal glucose uptake in isolated adipocytes (+50%, P < 0.05). Anaerobic metabolism of glucose, as assessed by lactate production and proportion of glucose metabolized to lactate, has been shown to be inversely correlated to leptin secretion and was decreased by EPA in both the absence and presence of insulin. EPA increased basal glucose oxidation as determined by the proportion of14C-labeled glucose metabolized to CO2. Lipogenesis (14C-labeled glucose incorporation into triglyceride) was decreased by EPA in the absence of insulin, whereas lipolysis (glycerol release) was unaffected. The EPA-induced increase of basal leptin secretion was highly correlated with increased glucose utilization ( r = +0.89, P < 0.01) and inversely related to the anaerobic glucose metabolism to lactate. EPA’s effect on insulin-stimulated leptin secretion was not related to increased glucose utilization but was inversely correlated with anaerobic glucose metabolism to lactate ( r = −0.84, P < 0.01). Together, the results suggest that EPA, like insulin, stimulates leptin production by increasing the nonanaerobic/oxidative metabolism of glucose.

Does weight loss prognosis depend on genetic make-up?

The prevalence of obesity is rising throughout the world. Indeed, obesity has reached epidemic proportions in many developed and transition countries. Obesity is a complex disease with multifactorial origin, which in many cases appears as a polygenic condition affected by environmental factors. Treatment or prevention of obesity is necessary to reverse or avoid the onset of type 2 diabetes and other obesity-related diseases. Weight loss is a complex trait that depends on many environmental, behavioural and genetic influences. An effective programme for the management of overweight and obesity must take into account all of these factors. Individual responses to weight loss interventions vary widely and reliable predictors of successful slimming are poorly understood. The individual genetic make-up participating in energy expenditure regulation, appetite control, lipid metabolism and adipogenesis, have been reported to affect the risk of treatment failure in some subjects. In addition, the genotype could also help to predict the changes in lipid profile, cardiovascular risk factors and insulin sensitivity in response to weight loss. Herein, the current evidence from human studies that support the existence of a genetic component and the participation of different polymorphisms in the prognosis of weight loss induced by interventions leading to a negative energy balance are reviewed.

Energy restriction restores the impaired immune response in overweight (cafeteria) rats

Impaired immune function linked to obesity has been shown in both human and animal studies. The purpose of this work was to evaluate the effects of a 4-week energy restriction (50% of total energy intake) on immune function in previously diet-induced (cafeteria) overweight rats. Flow cytometric analysis revealed that the number of spleen T helper cells were significantly (P < 0.05) elevated in control and overweight energy-restricted rats as compared with groups fed ad libitum groups. The proliferative response of splenocytes to phytohaemaglutinin and concanavalin A from overweight rats after energy restriction was significantly (P < 0.05) higher compared to overweight nonrestricted rats. The cytotoxic activity of natural killer cells tended to be lower in overweight rats compared to controls. Finally, control rats under the dietary deprivation period presented higher levels of uncoupling protein 2 mRNA and lower levels of leptin receptor mRNA compared with the reference control group. These results suggest that energy restriction is able to restore, at least in part, the impaired immune response commonly observed in overweight animals.

beta3-adrenoceptor agonist prevents alterations of muscle diacylglycerol and adipose tissue phospholipids induced by a cafeteria diet

Background

Insulin resistance induced by a high fat diet has been associated with alterations in lipid content and composition in skeletal muscle and adipose tissue. Administration of β3-adrenoceptor (β3-AR) agonists was recently reported to prevent insulin resistance induced by a high fat diet, such as the cafeteria diet. The objective of the present study was to determine whether a selective β3-AR agonist (ZD7114) could prevent alterations of the lipid profile of skeletal muscle and adipose tissue lipids induced by a cafeteria diet.

Methods

Male Sprague-Dawley rats fed a cafeteria diet were treated orally with either the β3-AR agonist ZD7114 (1 mg/kg per day) or the vehicle for 60 days. Rats fed a chow diet were used as a reference group. In addition to the determination of body weight and insulin plasma level, lipid content and fatty acid composition in gastronemius and in epididymal adipose tissue were measured by gas-liquid chromatography, at the end of the study.

Results

In addition to higher body weights and plasma insulin concentrations, rats fed a cafeteria diet had greater triacylglycerol (TAG) and diacylglycerol (DAG) accumulation in skeletal muscle, contrary to animals fed a chow diet. As expected, ZD7114 treatment prevented the excessive weight gain and hyperinsulinemia induced by the cafeteria diet. Furthermore, in ZD7114 treated rats, intramyocellular DAG levels were lower and the proportion of polyunsaturated fatty acids, particularly arachidonic acid, in adipose tissue phospholipids was higher than in animals fed a cafeteria diet.

Conclusions

These results show that activation of the β3-AR was able to prevent lipid alterations in muscle and adipose tissue associated with insulin resistance induced by the cafeteria diet. These changes in intramyocellular DAG levels and adipose tissue PL composition may contribute to the improved insulin sensitivity associated with β3-AR activation.

Gene expression changes in rat white adipose tissue after a high-fat diet determined by differential display

The differences in gene expression pattern of visceral white adipose tissue between control and high-fat-fed rats were compared using the mRNA differential display methodology. The results, confirmed by Northern blot, showed eight genes upregulated: adiponectin, fibrillin-1, transferrin, Y-box binding protein-1, IgE receptor beta chain (FcRIbeta), alpha-1 haemoglobin, and ribosomal proteins S10 and L7 and four genes downregulated: caveolin-2, lactate dehydrogenase-A, mitochondrial 16S rRNA, and mitochondrial cytochrome oxidase subunit I/serine tRNA. Two of these genes have been already related to obesity (adiponectin and caveolin-2) while the others are known to participate in metabolic, signalling or transcription regulation pathways that can be relevant in energy (lipid and/or carbohydrate) metabolism.

Cold exposure induces tissue-specific modulation of the insulin-signalling pathway in Rattus norvegicus

Cold exposure provides a reproducible model of improved glucose turnover accompanied by reduced steady state and glucose-induced insulin levels. In the present report we performed immunoprecipitation and immunoblot studies to evaluate the initial and intermediate steps of the insulin-signalling pathway in white and brown adipose tissues, liver and skeletal muscle of rats exposed to cold. Basal and glucose-induced insulin secretion were significantly impaired, while glucose clearance rates during a glucose tolerance test and the constant for glucose decay during a 15 min insulin tolerance test were increased, indicating a significantly improved glucose turnover and insulin sensitivity in rats exposed to cold. Evaluation of protein levels and insulin-induced tyrosine (insulin receptor, insulin receptor substrates (IRS)-1 and -2, ERK (extracellular signal-related kinase)) or serine (Akt; protein kinase B) phosphorylation of proteins of the insulin signalling cascade revealed a tissue-specific pattern of regulation of the molecular events triggered by insulin such that in white adipose tissue and skeletal muscle an impaired molecular response to insulin was detected, while in brown adipose tissue an enhanced response to insulin was evident. In muscle and white and brown adipose tissues, increased 2-deoxy-D-glucose (2-DG) uptake was detected. Thus, during cold exposure there is a tissue-specific regulation of the insulin-signalling pathway, which seems to favour heat-producing brown adipose tissue. Nevertheless, muscle and white adipose tissue are able to take up large amounts of glucose, even in the face of an apparent molecular resistance to insulin.

Effects of arachidonic acid on leptin secretion and expression in primary cultured rat adipocytes

Leptin, a hormone produced in adipocytes, is a key signal in the regulation of food intake and energy expenditure. Several studies have suggested that leptin can be regulated by macronutrients intake. Arachidonic acid is a dietary fatty acid known to affect cell metabolism. Controversial effects of this fatty acid on leptin have been reported. The aim of this experimental trial was to evaluate the effect of the arachidonic acid on basal and insulin-stimulated leptin secretion and expression in isolated rat adipocytes. Because insulin-stimulated glucose metabolism is an important regulator of leptin expression and secretion by the adipocytes, the effects of the arachidonic acid on indices of adipocyte metabolism were also examined. Isolated adipocytes were incubated with arachidonic acid (1-200 microM) in the absence and presence of insulin (1.6 nM). Leptin secretion and expression, glucose utilization and lactate production were determined at 96 h. The arachidonic acid (200 microM) inhibited both the basal and insulin stimulated leptin secretion and expression. Glucose utilization was not affected by the acid. Basal lactate production was increased by the fatty acid at the highest concentration used (200 microM), however lactate production in presence of insulin was not modified. Finally, the percentage of glucose carbon released as lactate was significantly increased (200 microM). These results suggest that the inhibitory effect of the arachidonic acid on leptin secretion and expression may be due, al least in part, to the increase in the anaerobic utilization of glucose.