The thrifty 'catch-up fat' phenotype: its impact on insulin sensitivity during growth trajectories to obesity and metabolic syndrome

The analyses of large epidemiological databases have suggested that infants and children who show catch-up growth, or adiposity rebound at a younger age, are predisposed to the development of obesity, type 2 diabetes and cardiovascular diseases later in life. The pathophysiological mechanisms by which these growth trajectories confer increased risks for these diseases are obscure, but there is compelling evidence that the dynamic process of catch-up growth per se, which often overlaps with adiposity rebound at a younger age, is characterized by hyperinsulinemia and by a disproportionately higher rate in the recovery of body fat than lean tissue (i.e. preferential 'catch-up fat'). This paper first focuses upon the almost ubiquitous nature of this preferential 'catch-up fat' phenotype across the life cycle as a risk factor for obesity and insulin-related complications - not only in infants and children who experienced catch-up growth after earlier fetal or neonatal growth retardation, or after preterm birth, but also in adults who show weight recovery after substantial weight loss owing to famine, disease-cachexia or periodic dieting. It subsequently reviews the evidence indicating that such preferential catch-up fat is primarily driven by energy conservation (thrifty) mechanisms operating via suppressed thermogenesis, with glucose thus spared from oxidation in skeletal muscle being directed towards de novo lipogenesis and storage in white adipose tissue. A molecular-physiological framework is presented which integrates emerging insights into the mechanisms by which this thrifty 'catch-up fat' phenotype crosslinks with early development of insulin and leptin resistance. In the complex interactions between genetic constitution of the individual, programming earlier in life, and a subsequent lifestyle of energy dense foods and low physical activity, this thrifty 'catch-up fat' phenotype--which probably evolved to increase survival capacity in a hunter-gatherer lifestyle of periodic food shortages--is a central event in growth trajectories to obesity and to diseases that cluster into the insulin resistance (metabolic) syndrome.

[Warthin tumor with carcinoma: malignant transformation or metastasis?]

We present the unusual case of a cytologically diagnosed Warthin tumor (WT) of long standing with sudden enlargement und subsequent resection. Histologically, the diagnosis of WT was confirmed, but the tumor additionally showed diffuse infiltrates of an adenocarcinoma undergoing unrestrained growth. Warthin tumor with malignant transformation was suspected and radiological staging examinations were conducted. PET scans detected a metastasizing carcinoma of the breast, morphologically identical to the WT infiltrates. Care should always be taken when the diagnosis of malignant WT is made to exclude metastatic disease.

[Laparoscopy in the management of endometrial cancer]

Recent advance in laparoscopy have changed the surgical approach of endometrial cancer patients. The Swissendos Center, Fribourg, in collaboration with AGO (Groupe de travail pour la gynécologie oncologique) and AGE (groupe de travail pour la gynécologie endoscopique) have established a consensus based on the available evidence for the use of laparoscopy in the management of patients with endometrial cancer The main objective was to define Swiss clinical practice guidelines appropriate to the country and consistent with the needs of the physicians.

Thrifty metabolism that favors fat storage after caloric restriction: a role for skeletal muscle phosphatidylinositol-3-kinase activity and AMP-activated protein kinase

Energy conservation directed at accelerating body fat recovery (or catch-up fat) contributes to obesity relapse after slimming and to excess fat gain during catch-up growth after malnutrition. To investigate the mechanisms underlying such thrifty metabolism for catch-up fat, we tested whether during refeeding after caloric restriction rats exhibiting catch-up fat driven by suppressed thermogenesis have diminished skeletal muscle phosphatidylinositol-3-kinase (PI3K) activity or AMP-activated protein kinase (AMPK) signaling-two pathways required for hormone-induced thermogenesis in ex vivo muscle preparations. The results show that during isocaloric refeeding with a low-fat diet, at time points when body fat, circulating free fatty acids, and intramyocellular lipids in refed animals do not exceed those of controls, muscle insulin receptor substrate 1-associated PI3K activity (basal and in vivo insulin-stimulated) is lower than that in controls. Isocaloric refeeding with a high-fat diet, which exacerbates the suppression of thermogenesis, results in further reductions in muscle PI3K activity and in impaired AMPK phosphorylation (basal and in vivo leptin-stimulated). It is proposed that reduced skeletal muscle PI3K/AMPK signaling and suppressed thermogenesis are interdependent. Defective PI3K or AMPK signaling will reduce the rate of substrate cycling between de novo lipogenesis and lipid oxidation, leading to suppressed thermogenesis, which accelerates body fat recovery and furthermore sensitizes skeletal muscle to dietary fat-induced impairments in PI3K/AMPK signaling.

β-Adrenergic control of stearoyl-CoA desaturase 1 repression in relation to sympathoadrenal regulation of thermogenesis

Mice lacking beta-adrenoceptors, which mediate the thermogenic effects of norepinephrine and epinephrine, show diminished thermogenesis and high susceptibility to obesity, whereas mice lacking stearoyl-CoA desaturase 1 (SCD1), which catalyzes the synthesis of monounsaturated fatty acids, show enhanced thermogenesis and high resistance to obesity. In testing whether beta-adrenergic control of thermogenesis might be mediated via repression of the SCD1 gene, we found that in mice lacking beta-adrenoceptors, the gene expression of SCD1 is elevated in liver, skeletal muscle and white adipose tissue. In none of these tissues/organs, however, could a link be found between increased sympathetic nervous system activity and diminished SCD1 gene expression when thermogenesis is increased in response to diet or cold, nor is the SCD1 transcript repressed by the administration of epinephrine. Taken together, these studies suggest that the elevated SCD1 transcript in tissues of mice lacking beta-adrenoceptors is not a direct effect of blunted beta-adrenergic signalling, and that beta-adrenergic control of SCD1 repression is unlikely to be a primary effector mechanism in sympathoadrenal regulation of thermogenesis. Whether approaches that target both SCD1 and molecular effectors of thermogenesis under beta-adrenergic control might be more effective than targeting SCD1 alone are potential avenues for future research in obesity management.

Corticotropin-releasing hormone directly stimulates thermogenesis in skeletal muscle possibly through substrate cycling between de novo lipogenesis and lipid oxidation

The mechanisms by which CRH and related peptides (i.e. the CRH/urocortin system) exert their control over thermogenesis and weight regulation have until now focused only upon their effects on brain centers controlling sympathetic outflow. Using a method that involves repeated oxygen uptake determinations in intact mouse skeletal muscle, we report here that CRH can act directly on skeletal muscle to stimulate thermogenesis, an effect that is more pronounced in oxidative than in glycolytic muscles and that can be inhibited by a selective CRH-R2 antagonist or blunted by a nonselective CRH receptor antagonist. This thermogenic effect of CRH can also be blocked by interference along pathways of de novo lipogenesis and lipid oxidation, as well as by inhibitors of phosphatidylinositol 3-kinase or AMP-activated protein kinase. Taken together, these studies demonstrate that CRH can directly stimulate thermogenesis in skeletal muscle, and in addition raise the possibility that this thermogenic effect, which requires both phosphatidylinositol 3-kinase and AMP-activated protein kinase signaling, might occur via substrate cycling between de novo lipogenesis and lipid oxidation. The effect of CRH in directly stimulating thermogenesis in skeletal muscle underscores a potentially important peripheral role for the CRH/urocortin system in the control of thermogenesis in this tissue, in its protection against excessive intramyocellular lipid storage, and hence against skeletal muscle lipotoxicity and insulin resistance.

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

Life is a combustion, but how the major fuel substrates that sustain human life compete and interact with each other for combustion has been at the epicenter of research into the pathogenesis of insulin resistance ever since Randle proposed a 'glucose-fatty acid cycle' in 1963. Since then, several features of a mutual interaction that is characterized by both reciprocality and dependency between glucose and lipid metabolism have been unravelled, namely: the inhibitory effects of elevated concentrations of fatty acids on glucose oxidation (via inactivation of mitochondrial pyruvate dehydrogenase or via desensitization of insulin-mediated glucose transport),the inhibitory effects of elevated concentrations of glucose on fatty acid oxidation (via malonyl-CoA regulation of fatty acid entry into the mitochondria), and more recentlythe stimulatory effects of elevated concentrations of glucose on de novo lipogenesis, that is, synthesis of lipids from glucose (via SREBP1c regulation of glycolytic and lipogenic enzymes). This paper first revisits the physiological significance of these mutual interactions between glucose and lipids in skeletal muscle pertaining to both blood glucose and intramyocellular lipid homeostasis. It then concentrates upon emerging evidence, from calorimetric studies investigating the direct effect of leptin on thermogenesis in intact skeletal muscle, of yet another feature of the mutual interaction between glucose and lipid oxidation: that of substrate cycling between de novo lipogenesis and lipid oxidation. It is proposed that this energy-dissipating substrate cycling that links glucose and lipid metabolism to thermogenesis could function as a 'fine-tuning' mechanism that regulates intramyocellular lipid homeostasis, and hence contributes to the protection of skeletal muscle against lipotoxicity.

Skeletal muscle mitochondrial oxidative capacity and uncoupling protein 3 are differently influenced by semistarvation and refeeding

We investigated, in skeletal muscle mitochondria isolated from semistarved and refed rats, the relation between the protein expression of uncoupling protein 3 (UCP3) and mitochondrial oxidative capacity, assessed as state 4 and state 3 respiration rates in presence of substrates that are either non-lipids (glutamate, succinate) or lipids (palmitoyl CoA, palmitoylcarnitine). During semistarvation, when whole-body thermogenesis is diminished, state 3 respiration was lower than in fed controls by about 30% independently of substrate types, while state 4 respiration was lower by 20% only during succinate oxidation, but UCP3 was unaltered. After 5 days of refeeding, when thermogenesis is still diminished, neither state 4, state 3 nor UCP3 were lower than in controls. Refeeding on a high-fat diet, which exacerbates the suppression of thermogenesis, resulted in a two-fold elevation in UCP3 but no change in state 4 or state 3 respiration. These results during semistarvation and refeeding, in line with those previously reported for fasting, are not in support of the hypothesis that UCP3 is a mediator of adaptive thermogenesis pertaining to weight regulation, and underscore the need for caution in interpreting parallel changes in UCP3 and mitochondrial oxidative capacity as the reflection of mitochondrial uncoupling by UCP3.

Skeletal muscle heterogeneity in fasting-induced upregulation of genes encoding UCP2, UCP3, PPARgamma and key enzymes of lipid oxidation

The uncoupling protein homologs UCP2 and UCP3 have been proposed as candidate genes for the regulation of lipid metabolism. Within the context of this hypothesis, we have compared, from fed and fasted rats, changes in gene expression of skeletal muscle UCP2 and UCP3 with those of carnitine palmitoyltransferase I and medium-chain acyl-CoA dehydrogenase, two key enzymes regulating lipid flux across the mitochondrial beta-oxidation pathway. In addition, changes in gene expression of peroxisome proliferator-activated receptor gamma, a nuclear transcription factor implicated in lipid metabolism, were also investigated. The results indicate that in response to fasting, the mRNA levels of UCP2, UCP3, carnitine palmitoyltransferase I and medium-chain acyl-CoA dehydrogenase are markedly increased, by three- to sevenfold, in the gastrocnemius and tibialis anterior (fast-twitch muscles, predominantly glycolytic or oxidative-glycolytic), but only mildly increased, by less than twofold, in the soleus (slow-twitch muscle, predominantly oxidative). Furthermore, such muscle-type dependency in fasting-induced transcriptional changes in UCP2, UCP3, carnitine palmitoyltransferase and medium-chain acyl-CoA dehydrogenase persists when the increase in circulating levels of free fatty acids during fasting is abolished by the anti-lipolytic agent nicotinic acid - with blunted responses only in the slow-twitch muscle contrasting with unabated increases in fast-twitch muscles. Independently of muscle type, however, the mRNA levels of peroxisome proliferator-activated receptor gamma are not altered during fasting. Taken together, these studies indicate a close association between fasting-induced changes in UCP2 and UCP3 gene expression with those of key regulators of lipid oxidation, and are hence consistent with the hypothesis that these UCP homologs may be involved in the regulation of lipid metabolism. Furthermore, they suggest that in response to fasting, neither the surge of free fatty acids in the circulation nor induction of the peroxisome proliferator-activated receptor gamma gene may be required for the marked upregulation of genes encoding the UCP homologs and key enzymes regulating lipid oxidation in fast-twitch muscles.

Angiotensinogen-deficient mice exhibit impairment of diet-induced weight gain with alteration in adipose tissue development and increased locomotor activity

White adipose tissue is known to contain the components of the renin-angiotensin system, which gives rise to angiotensin II from angiotensinogen (AGT). Recent evidence obtained in vitro and ex vivo is in favor of angiotensin II acting as a trophic factor of adipose tissue development. To determine whether AGT plays a role in vivo in this process, comparative studies were performed in AGT-deficient (agt(-/-)) mice and control wild-type mice. The results showed that agt(-/-) mice gain less weight than wild-type mice in response to a chow or high fat diet. Adipose tissue mass from weaning to adulthood appeared altered rather specifically, as both the size and the weight of other organs were almost unchanged. Food intake was similar for both genotypes, suggesting a decreased metabolic efficiency in agt(-/-) mice. Consistent with this hypothesis, cellularity measurement indicated hypotrophy of adipocytes in agt(-/-) mice with a parallel decrease in the fatty acid synthase activity. Moreover, AGT-deficient mice exhibited a significantly increased locomotor activity, whereas metabolic rate and mRNA levels of uncoupling proteins remained similar in both genotypes. Thus, AGT appears to be involved in the regulation of fat mass through a combination of decreased lipogenesis and increased locomotor activity that may be centrally mediated.

Adipose angiotensinogen is involved in adipose tissue growth and blood pressure regulation

White adipose tissue and liver are important angiotensinogen (AGT) production sites. Until now, plasma AGT was considered to be a reflection of hepatic production. Because plasma AGT concentration has been reported to correlate with blood pressure, and to be associated with body mass index, we investigated whether adipose AGT is released locally and into the blood stream. For this purpose, we have generated transgenic mice either in which adipose AGT is overexpressed or in which AGT expression is restricted to adipose tissue. This was achieved by the use of the aP2 adipocyte-specific promoter driving the expression of rat agt cDNA in both wild-type and hypotensive AGT-deficient mice. Our results show that in both genotypes, targeted expression of AGT in adipose tissue increases fat mass. Mice whose AGT expression is restricted to adipose tissue have AGT circulating in the blood stream, are normotensive, and exhibit restored renal function compared with AGT-deficient mice. Moreover, mice that overexpress adipose AGT have increased levels of circulating AGT, compared with wild-type mice, and are hypertensive. These animal models demonstrate that AGT produced by adipose tissue plays a role in both local adipose tissue development and in the endocrine system, which supports a role of adipose AGT in hypertensive obese patients.

Uncoupling protein 3 and fatty acid metabolism

A role for uncoupling protein (UCP) 3 in fatty acid metabolism is reviewed within the context of our proposal, first put forward in 1998, that this homologue of UCP1 may be involved in the regulation of lipids as fuel substrate rather than in the mediation of thermogenesis. Since then, the demonstrations of muscle-type differences in UCP3 gene regulation in response to dietary manipulations (starvation, high-fat feeding) or to pharmacological interferences with the flux of lipid substrates between adipose-tissue stores and skeletal-muscle mitochondrial oxidation are all in accord with this proposed role for UCP3 in regulating lipids as fuel substrate. However, given the current limitations of gene-knockout technology for evaluating/interpreting the functional importance of genes encoding mitochondrial membrane proteins, the transition from 'associative' to 'cause-and-effect' evidence for a physiological role of UCP3 in regulating fatty acid metabolism will have to await the development of assays that are sensitive to changes in UCP3 activity. Furthermore, in evaluating the physiological regulators of UCP3, the available evidence points to the existence of adipose-derived factor(s) which, independently of circulating levels of free fatty acids, initiates events leading to the transcription of genes encoding UCP3 and key enzymes of lipid oxidation in the fast glycolytic or fast oxidative-glycolytic muscles, i.e. in the bulk of the skeletal-muscle mass. It is proposed that in tissues where UCP3 co-exists with UCP2 (skeletal muscle, brown adipose tissue, heart) they may act in concert in the overall regulation of lipid oxidation, concomitant to the prevention of lipid-induced oxidative damage.

Fasting-induced changes in the expression of genes controlling substrate metabolism in the rat heart

During fasting, when overall metabolism changes, the contribution of glucose and fatty acids (FA) to cardiac energy production alters as well. Here, we examined if the heart is able to adapt to such fasting-induced changes by modulation of its gene expression. Rats were fed ad libitum or fasted for 46 h, resulting in reduced circulating glucose levels and a 3-fold rise in FA. Besides changes in the cardiac activity or content of proteins involved in glucose or FA metabolism, mRNA levels also altered. The cardiac expression of genes coding for glucose-handling proteins (glucose transporter GLUT4, hexokinase I and II) was up to 70% lower in fasted than in fed rats. In contrast, the mRNA levels of various genes involved in FA transport and metabolism (FA translocase/CD36, muscle-type carnitine palmitoyl transferase 1, long-chain acyl-CoA dehydrogenase) and of the uncoupling protein UCP-3 increased over 50% in hearts of fasted rats. Surprisingly, mRNA levels of the fatty acid- activated transcription factors PPARalpha and PPARbeta/delta were reduced in hearts of fasted rats, whereas in livers, fasting led to a marked rise in PPARalpha mRNA. Reducing FA levels by nicotinic acid administration during the final 8 h of fasting did not affect the expression of the majority of metabolic genes, but totally abolished the induction of UCP-3. In conclusion, the adult rat heart responds to changes in nutritional status, as provoked by 46 h fasting, through adjustment of glucose as well as FA metabolism at the level of gene expression.

Increased insulin concentrations and glucose storage in neuropeptide Y Y1 receptor-deficient mice

Mice lacking NPY Y1 receptors develop obesity without hyperphagia indicating increased energy storage and/or decreased energy expenditure. Then, we investigated glucose utilization in these animals at the onset of obesity. Fasted NPY Y1 knockouts showed hyperinsulinemia associated with increased whole body and adipose tissue glucose utilization and glycogen synthesis but normal glycolysis. Since leptin modulates NPY actions, we studied whether the lack of NPY Y1 receptor affected leptin-mediated regulation of glucose metabolism. Leptin infusion normalized hyperinsulinemia and glucose turnover. These results suggest a possible mechanism for the development of obesity without hyperphagia via dysfunction in regulatory loops involving NPY, leptin and insulin.

Differential roles of tumor necrosis factor-alpha and interferon-gamma in mouse hypermetabolic and anorectic responses induced by LPS

Lipopolysaccharide (LPS)-induced effects on energy balance are characterized by alterations in energy expenditure (hypermetabolism) and food intake (anorexia). To study the role of tumour necrosis factor alpha (TNF-alpha) on some of these metabolic responses to endotoxin, we have used transgenic mice expressing soluble tumour necrosis factor receptor-1 IgG fusion protein (TNFR1-IgG) as well as TNF-alpha knockout (KO), lymphotoxin-alpha (LT-alpha) KO, and interferon-gamma receptor (IFN-gamma R) KO mice. The results from TNFR1-IgG transgenic mice suggest that the hypermetabolic and anorectic responses induced by LPS are independently regulated since, in the absence of TNF-alpha or LT-alpha, the LPS-induced hypermetabolism is almost prevented but not the anorexia. The anorectic response shows the strongest association with IFN-gamma since both IFN-gamma R KO mice and mice treated with anti-IFN-gamma antibody showed marked reduction in the LPS-induced anorexia compared to other mice. IFN-gamma R KO mice also have an attenuated thermogenic response to endotoxin. Anti-Asialo GM1 antibody treatment attenuated both the hypermetabolic and anorectic responses to LPS, to an extent comparable to that observed in IFN-gamma R KO mice. This finding suggests that natural killer cells (lymphocytic subsets) may be involved in IFN-gamma production and play an important role in the metabolic alterations induced by LPS. We also showed that the hypermetabolic response of control mice is associated with an upregulation of cytokine expression within the brain and an increase in permeability of the blood brain barrier. LPS-induced anorexia appears to involve peripheral cytokine expression.

Downregulation of skeletal muscle UCP-3 gene expression during refeeding is prevented by cold exposure

We wished to gain insights into the role of skeletal muscle uncoupling protein-3 (UCP-3) in the elevated efficiency of fat recovery during refeeding after starvation. Previous observations have revealed that muscle UCP-3 expression is downregulated in rats during refeeding at 22 degrees C. Therefore, we investigated whether this also occurs during refeeding at thermoneutrality (29 C) or in the cold (6 C), since at these environmental temperatures the refed animals also show diminished thermogenesis and a higher rate of fat deposition than controls. The UCP-3 mRNA level in the skeletal muscles studied (soleus, gastrocnemius and tibialis anterior) was significantly lower in the refed group than in controls at thermoneutrality, but there were no such differences between these two groups in the cold. This effect of cold, namely abolishing refeeding-induced downregulation of skeletal muscle UCP, is specific to UCP-3 since the gene expression of skeletal muscle UCP-2 remained significantly lower in the refed than in the controls both at thermoneutrality and in the cold. These findings during refeeding in the cold therefore dissociate UCP-3 gene regulation from the adaptive reduction in thermogenesis that accelerates fat deposition during weight recovery. They also reveal differential responses of UCP-3 and UCP-2, whose significance is discussed in the light of our previously proposed hypothesis, which centers upon a role for these UCP homologues in the regulation of lipids as a fuel substrate.

Green tea and thermogenesis: interactions between catechin-polyphenols, caffeine and sympathetic activity

The thermogenic effect of tea is generally attributed to its caffeine content. We report here that a green tea extract stimulates brown adipose tissue thermogenesis to an extent which is much greater than can be attributed to its caffeine content per se, and that its thermogenic properties could reside primarily in an interaction between its high content in catechin-polyphenols and caffeine with sympathetically released noradrenaline (NA). Since catechin-polyphenols are known to be capable of inhibiting catechol-O-methyl-transferase (the enzyme that degrades NA), and caffeine to inhibit trancellular phosphodiesterases (enzymes that break down NA-induced cAMP), it is proposed that the green tea extract, via its catechin-polyphenols and caffeine, is effective in stimulating thermogenesis by relieving inhibition at different control points along the NA-cAMP axis. Such a synergistic interaction between catechin-polyphenols and caffeine to augment and prolong sympathetic stimulation of thermogenesis could be of value in assisting the management of obesity. International Journal of Obesity (2000) 24, 252-258

UCP2 and UCP3 rise in starved rat skeletal muscle but mitochondrial proton conductance is unchanged

The relationship between UCP2 and UCP3 expression and mitochondrial proton conductance of rat skeletal muscle was examined. Rats were starved for 24 h and the levels of UCP2 and UCP3 mRNA and UCP3 protein were determined by Northern and Western blots. Proton conductance was measured by titrating mitochondrial respiration rate and membrane potential with malonate. Starvation increased UCP2 and UCP3 mRNA levels more than 5-fold and 4-fold, respectively, and UCP3 protein levels by 2-fold. However, proton conductance remained unchanged. These results suggest either that Northern and Western blots do not reflect the levels of active protein or that these UCPs do not catalyse the basal proton conductance in skeletal muscle mitochondria.

Skeletal muscle UCP3 and UCP2 gene expression in response to inhibition of free fatty acid flux through mitochondrial beta-oxidation

Studies of starvation and refeeding have implicated the genes coding for uncoupling protein-3 and -2 (UCP3, UCP2) as candidate genes in the regulation of lipids as metabolic fuels in skeletal muscle. To gain insight into the role of free fatty acid (FFA) flux in regulating the expression of these muscle UCP homologues, we recently reported that, in response to the anti-lipolytic agent nicotinic acid, utilized to reduce FFA flux at the input supply (i.e. circulating) level in fed and fasted rats, expression of the UCP3 and UCP2 genes was reduced in the soleus (predominantly slow-oxidative fibres), but not in the gastrocnemius (predominantly fast-glycolytic fibres) or tibials anterior (predominantly fast-oxidative-glycolytic fibres) muscles. In the present study, we examined UCP2 and UCP3 gene expression in these muscles from fed or fasted rats treated with etomoxir, an inhibitor of FFA flux at the output (i.e. mitochondrial oxidation) level. Fasting per se resulted in a threefold increase in serum FFA (P < 0.001) and in marked increases in the messenger ribonucleic acid (mRNA) expression of both UCP2 and UCP3 in all three muscles (P < 0.001). Treatment with etomoxir had no significant effect on serum FFA in the fed rats, but further elevated serum FFA in the fasted rats (P < 0.001). The mRNA levels of both UCP3 and UCP2 in response to etomoxir were significantly reduced in the tibialis anterior muscle in both fed and fasted states (P < 0.01), unaltered in the gastrocnemius muscle in both fed and fasted states and unaltered in the soleus muscle in the fed state, but increased in the fasted state, in parallel with the etomoxir-induced changes in serum FFA levels. Taken together, these results suggest the existence of positive feedback loops between FFA flux and muscle UCPs only in oxidative muscles--with that loop operating at the input FFA supply level for muscles with predominantly slow-oxidative fibres, and at the output FFA oxidation level for muscles with predominantly fast-oxidative-glycolytic fibres.

Peroxisome proliferator-activated receptor alpha mediates the adaptive response to fasting

Prolonged deprivation of food induces dramatic changes in mammalian metabolism, including the release of large amounts of fatty acids from the adipose tissue, followed by their oxidation in the liver. The nuclear receptor known as peroxisome proliferator-activated receptor alpha (PPARalpha) was found to play a role in regulating mitochondrial and peroxisomal fatty acid oxidation, suggesting that PPARalpha may be involved in the transcriptional response to fasting. To investigate this possibility, PPARalpha-null mice were subjected to a high fat diet or to fasting, and their responses were compared with those of wild-type mice. PPARalpha-null mice chronically fed a high fat diet showed a massive accumulation of lipid in their livers. A similar phenotype was noted in PPARalpha-null mice fasted for 24 hours, who also displayed severe hypoglycemia, hypoketonemia, hypothermia, and elevated plasma free fatty acid levels, indicating a dramatic inhibition of fatty acid uptake and oxidation. It is shown that to accommodate the increased requirement for hepatic fatty acid oxidation, PPARalpha mRNA is induced during fasting in wild-type mice. The data indicate that PPARalpha plays a pivotal role in the management of energy stores during fasting. By modulating gene expression, PPARalpha stimulates hepatic fatty acid oxidation to supply substrates that can be metabolized by other tissues.

Post-starvation gene expression of skeletal muscle uncoupling protein 2 and uncoupling protein 3 in response to dietary fat levels and fatty acid composition: a link with insulin resistance

UCP2 and UCP3 are two recently cloned genes with high sequence homology to the gene for uncoupling protein (UCP)-1, which regulates thermogenesis in brown adipose tissue. In the context of the current debate about whether UCP2 and UCP3 in the skeletal muscle may also function as mediators of thermogenesis or as regulators of lipids as fuel substrate, we have examined their mRNA expressions in rat gastrocnemius muscle in response to dietary manipulations known to differentially affect thermogenesis during the phase of weight recovery after starvation. Compared with ad libitum-fed control rats, the refeeding of isocaloric amounts of a low-fat (high-carbohydrate) diet resulted in lower energy expenditure and lower mRNA levels of muscle UCP2 and UCP3. This downregulation of UCP homologs was abolished by the refeeding of a high-fat diet, even though energy expenditure was significantly lower during refeeding on the high-fat than on the low-fat diet. Furthermore, major alterations in the fatty acid composition of the refeeding diet in favor of n-6 polyunsaturated or medium-chain fatty acids resulted in significant increases in energy expenditure, but with no significant changes in the expression of skeletal muscle UCP homologs. Regression analysis of gastrocnemius UCP mRNA levels against parameters that included body composition, energy expenditure, and plasma levels of free fatty acids (FFAs), insulin, and glucose as well as the increase in plasma glucose after a glucose load, revealed that only the latter (an index of insulin resistance) could explain the variability in muscle UCP2 and UCP3 mRNA expressions (r = 0.41, P < 0.02; r = 0.45, P < 0.01, respectively). Taken together, these data are at variance with a role for skeletal muscle UCP2 and UCP3 in dietary regulation (or modulation) of thermogenesis. However, they are consistent with the notion that these UCP homologs may function as regulators of lipids as fuel substrate and raise the possibility that high-fat induced upregulation of muscle UCP2 and UCP3 may be more closely linked to insulin resistance than to changes in circulating FFAs.

Interorgan signaling between adipose tissue metabolism and skeletal muscle uncoupling protein homologs: is there a role for circulating free fatty acids?

Uncoupling proteins 3 and 2 (UCP3 and UCP2) are two newly cloned genes that have been implicated in the regulation of lipids as fuel substrate in skeletal muscle on the basis that their mRNA expressions are upregulated during starvation (when fat stores are being rapidly mobilized) and downregulated during the early phase of refeeding (when fat stores are being rapidly replenished). To test the hypothesis that circulating free fatty acids (FFAs) may have a physiological role as an interorgan signal linking these dynamic changes in the fat stores to skeletal muscle expression of UCP3 and UCP2, the mRNA levels of these UCP homologs were examined in fed and fasted rats treated with the antilipolytic agent nicotinic acid. In 46-h fasted rats, we observed a threefold increase in serum FFA levels and increases in UCP3 and UCP2 mRNA levels that were more marked in the gastrocnemius and tibialis anterior muscles (predominantly fast-twitch fibers) than in the soleus muscle (predominantly slow-twitch fibers). Treatment with nicotinic acid blunted the fasting-induced increase in serum FFA levels and prevented the increase in mRNA levels of UCP3 and UCP2 in the soleus muscle, but had little or no effect on the elevated mRNA levels of these UCP homologs in the gastrocnemius and tibialis anterior muscles. Furthermore, treatment of ad libitum-fed animals with nicotinic acid resulted in a twofold reduction in serum FFA levels (i.e., by a magnitude similar to that observed during early refeeding) and significant reductions in UCP3 and UCP2 mRNA levels in the soleus muscle, but not in the gastrocnemius or tibialis anterior muscles. These results revealed a muscle-type dependency in the way UCP2 and UCP3 gene expression in skeletal muscle is regulated, and suggest that the hypothesis that circulating FFAs function as an interorgan signal between fat stores and skeletal muscle UCP3 and UCP2 gene expression is adequate only for slow-twitch (oxidative) muscles. Consequently, a signal(s) other than circulating FFAs must be implicated in the link between dynamic changes in body fat stores and UCP expression in predominantly fast-twitch (glycolytic/oxidative-glycolytic) muscles, which constitute the major fiber type of the total skeletal muscle mass and which have high susceptibility to developing insulin resistance and impairment in substrate utilization in metabolic diseases.

Metabolic response to various beta-adrenoceptor agonists in beta3-adrenoceptor knockout mice: evidence for a new beta-adrenergic receptor in brown adipose tissue

The beta3-adrenoceptor plays an important role in the adrenergic response of brown and white adipose tissues (BAT and WAT). In this study, in vitro metabolic responses to beta-adrenoceptor stimulation were compared in adipose tissues of beta3-adrenoceptor knockout and wild type mice. The measured parameters were BAT fragment oxygen uptake (MO2) and isolated white adipocyte lipolysis. In BAT of wild type mice (-)-norepinephrine maximally stimulated MO2 4.1+/-0.8 fold. Similar maximal stimulations were obtained with beta1-, beta2- or beta3-adrenoceptor selective agonists (dobutamine 5.1+/-0.3, terbutaline 5.3+/-0.3 and CL 316,243 4.8+/-0.9 fold, respectively); in BAT of beta3-adrenoceptor knockout mice, the beta1- and beta2-responses were fully conserved. In BAT of wild type mice, the beta1/beta2-antagonist and beta3-partial agonist CGP 12177 elicited a maximal MO2 response (4.7+/-0.4 fold). In beta3-adrenoceptor knockout BAT, this response was fully conserved despite an absence of response to CL 316,243. This unexpected result suggests that an atypical beta-adrenoceptor, distinct from the beta1-, beta2- and beta3-subtypes and referred to as a putative beta4-adrenoceptor is present in BAT and that it can mediate in vitro a maximal MO2 stimulation. In isolated white adipocytes of wild type mice, (-)-epinephrine maximally stimulated lipolysis 12.1+/-2.6 fold. Similar maximal stimulations were obtained with beta1-, beta2- or beta3-adrenoceptor selective agonists (TO509 12+/-2, procaterol 11+/-3, CL 316,243 11+/-3 fold, respectively) or with CGP 12177 (7.1+/-1.5 fold). In isolated white adipocytes of beta3-adrenoceptor knockout mice, the lipolytic responses to (-)epinephrine, to the beta1-, beta2-, beta3-adrenoceptor selective agonists and to CGP 12177 were almost or totally depressed, whereas those to ACTH, forskolin and dibutyryl cyclic AMP were conserved.

Cardiovascular response, feeding behavior and locomotor activity in mice lacking the NPY Y1 receptor

Neuropeptide Y (NPY) is a 36-amino-acid neurotransmitter which is widely distributed throughout the central and peripheral nervous system. NPY involvement has been suggested in various physiological responses including cardiovascular homeostasis and the hypothalamic control of food intake. At least six subtypes of NPY receptors have been described. Because of the lack of selective antagonists, the specific role of each receptor subtype has been difficult to establish. Here we describe mice deficient for the expression of the Y1 receptor subtype. Homozygous mutant mice demonstrate a complete absence of blood pressure response to NPY, whereas they retain normal response to other vasoconstrictors. Daily food intake, as well as NPY-stimulated feeding, are only slightly diminished, whereas fast-induced refeeding is markedly reduced. Adult mice lacking the NPY Y1 receptor are characterized by increased body fat with no change in protein content. The higher energetic efficiency of mutant mice might result, in part, from the lower metabolic rate measured during the active period, associated with reduced locomotor activity. These results demonstrate the importance of NPY Y1 receptors in NPY-mediated cardiovascular response and in the regulation of body weight through central control of energy expenditure. In addition, these data are also indicative of a role for the Y1 receptor in the control of food intake.

Role of UCP homologues in skeletal muscles and brown adipose tissue: mediators of thermogenesis or regulators of lipids as fuel substrate?

The mRNA expressions of UCP2 and UCP3, two newly described genes with high sequence homology to the uncoupling protein UCP1 in brown adipose tissue (BAT), were examined in two skeletal muscles (gastrocnemius and soleus) as well as in interscapular BAT (IBAT) of the rat in response to food deprivation and controlled refeeding. In IBAT (a tissue highly dependent on lipids for thermogenesis), the pattern of mRNA expression of UCP2 and UCP3 closely follows that of UCP1: it was markedly down-regulated during food deprivation (when this tissue's thermogenesis and lipid fuel requirements are decreased) and restored to control levels by day 5 of refeeding. By contrast, in the gastrocnemius muscle (a mixed fiber type muscle with a high capacity to shift between glucose and lipids as fuel substrate), mRNA expression of both UCP2 and UCP3 mRNA was found to be markedly up-regulated during food deprivation (when this tissue's thermogenesis is also decreased but its lipid fuel utilization is increased). The expressions were subsequently found to be markedly down-regulated upon transition to refeeding, with mRNA levels remaining below control levels on days 3, 5, and 10 of refeeding (period of enhanced efficiency of body fat deposition). In the soleus muscle (an oxidative type muscle with higher dependency on lipids than the gastrocnemius, and hence with a lower capacity to shift between lipids and glucose as fuel substrate), UCP homologues were also found to be up-regulated during food deprivation, but changes in their mRNA expression contrast with those in the gastrocnemius muscle both in their much lower magnitude of response to food deprivation and in their more rapid restoration to control levels during refeeding. Up-regulation of UCP2 and UCP3 gene expressions in skeletal muscle during food deprivation was found to persist at thermoneutrality (i.e., under conditions of reduced thermoregulatory thermogenesis). Together, these tissue-dependent differential mRNA expressions of the UCP homologues in IBAT, gastrocnemius, and soleus muscles during food deprivation and refeeding are much more consistent with a role for UCP2 and UCP3 in the regulation of lipids as fuel substrate rather than as mediators of regulatory thermogenesis.

Metabolic-cytokine responses to a second immunological challenge with LPS in mice with T. gondii infection

Injection of 10 cysts of Toxoplasma gondii (Me49 strain) into Swiss Webster mice results in 1) an acute phase of infection lasting for 2-3 wk, characterized by weight loss, and 2) a chronic phase in which surviving mice show either partial weight recovery (Gainers) or persistent, although stable, cachexia (Nongainers). In response to a second immunological stimulation with lipopolysaccharide (LPS) in the chronic phase of the infection, it is shown that 1) the increase in energy expenditure was more prolonged in both groups of infected mice than in controls, 2) the intensity and duration of hypophagia were also differently affected with Nongainers > Gainers > controls, and 3) the infected mice had higher serum levels of tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-10 and a lower ratio of IL-10 to TNF-alpha than controls. In contrast, serum IL-4 increased to the same level in all three groups. Evaluation of the permeability of the blood-brain barrier by intravenous injection of Evans blue revealed a marked staining in the brain of only the infected Nongainers. Taken together, these results indicate that, in mice with chronic toxoplasmosis, a second nonspecific challenge (with LPS) exacerbates the hypophagic and hypermetabolic states, the latter being associated with hyperresponsiveness in TNF-alpha and IL-10 production. Furthermore, the greater exacerbation of the hypophagic state in mice showing persistent cachexia may be due to a preexisting higher permeability of the blood-brain barrier, which would allow a greater access of plasma-borne cytokines and/or other neuroimmunologically active substances to the central nervous system.

Effect of endurance training on mRNA expression of uncoupling proteins 1, 2, and 3 in the rat

Endurance exercise training has been shown to decrease diet-induced thermogenesis (DIT) in rats and humans. In rodents, most thermogenesis is thought to occur in brown adipose tissue via activation of the uncoupling protein-1 (UCP1) and in skeletal muscle. Since the level of UCP1 mRNA in rat BAT was reported to be unmodified by exercise training, the newly described uncoupling proteins UCP2 and UCP3 could be responsible for the decreased DIT in trained rats. UCP3 mRNA levels in endurance-trained rats were found to be reduced by 76% and 59% in tibialis anterior and soleus muscles, respectively. UCP2 mRNA levels were also decreased in tibialis anterior and in heart by 54% and 41%, respectively. Neither white adipose tissue UCP2 nor brown adipose tissue UCP1, UCP2, and UCP3 mRNA levels were modified. The results of this study show that a need for a higher metabolic efficiency is associated with decreased mRNA expression of the uncoupling proteins in skeletal and heart muscles, which would decrease energy dissipation in these tissues. The down-regulation of UCP3 and UCP2 expressions might also contribute to the rapid weight gain known to occur when exercise training ceased.

Uncoupling protein-3 expression in rodent skeletal muscle is modulated by food intake but not by changes in environmental temperature

A new member of the uncoupling protein (UCP) family called UCP3 has recently been cloned and shown to be highly expressed in skeletal muscle of rodents and humans. In the present study, UCP3 was overexpressed in C2C12 myoblasts where it acts as an uncoupling protein. Changes in UCP3 mRNA expression were examined in rodent muscles under conditions known to modulate thermogenesis in brown adipose tissue. In skeletal muscle, UCP3 expression did not change in response to 48 h of cold exposure (6 degrees C), whereas it was decreased by 81% or increased 5.6-fold by 1 week of 50% food restriction or fasting, respectively. It was also decreased by 36% in soleus muscle of obese (fa/fa) as compared with lean Zucker rats. The unexpected rise of UCP3 mRNA level induced by fasting did not change in vitro muscle basal heat production rate but decreased by 31% the capacity to produce heat in response to the uncoupler carbonylcyanide p-trifluoromethoxyphenylhydrazone. This decrease may reflect underlying uncoupling by UCP3. Up-regulation of UCP3 mRNA after a 24-h fast was still observed in mice exposed at thermoneutrality. These results show that the increase in UCP3 expression induced by fasting is associated with the maintenance of thermogenesis measured in muscle in vitro and is not modulated by environmental temperature. The notion that UCP3 expression is modulated by food intake is of importance to better understand the pathophysiology of obesity in humans.

Tissue-dependent upregulation of rat uncoupling protein-2 expression in response to fasting or cold

The control of uncoupling protein-2 (UCP2) mRNA expression in rat brown adipose tissue (BAT), heart and skeletal muscles was examined. Cold exposure (48 h) increased UCP2 mRNA in BAT, heart and soleus muscle by 2.4-, 4.3- and 2.6-fold, respectively. Fasting (48 h) had no effect on UCP2 mRNA expression neither in BAT nor in heart, but markedly increased it in skeletal muscles. While the upregulation of UCP2 mRNA in response to cold exposure is in line with a putative uncoupling role for this protein in thermoregulatory thermogenesis, the unexpected upregulation of UCP2 in skeletal muscles in response to fasting seems inconsistent with its role as an uncoupling protein involved in dietary regulation of thermogenesis.

Targeted gene disruption reveals a leptin-independent role for the mouse beta3-adrenoceptor in the regulation of body composition

Targeted disruption of mouse beta3-adrenoceptor was generated by homologous recombination, and validated by an acute in vivo study showing a complete lack of effect of the beta3-adrenoceptor agonist CL 316,243 on the metabolic rate of homozygous null (-/-) mice. In brown adipose tissue, beta3-adrenoceptor disruption induced a 66% decrease (P < 0.005) in beta1-adrenoceptor mRNA level, whereas leptin mRNA remained unchanged. Chronic energy balance studies in chow-fed mice showed that in -/- mice, body fat accumulation was favored (+41%, P < 0.01), with a slight increase in food intake (+6%, NS). These effects were accentuated by high fat feeding: -/- mice showed increased total body fat (+56%, P < 0.025) and food intake (+12%, P < 0.01), and a decrease in the fat-free dry mass (-10%, P < 0.05), which reflects a reduction in body protein content. Circulating leptin levels were not different in -/- and control mice regardless of diet. The significant shift to the right in the positive correlation between circulating leptin and percentage of body fat in high fat-fed -/- mice suggests that the threshold of body fat content inducing leptin secretion is higher in -/- than in control mice. Taken together, these studies demonstrate that beta3-adrenoceptor disruption creates conditions which predispose to the development of obesity.

Tissue-dependent upregulation of rat uncoupling protein-2 expression in response to fasting or cold

The control of uncoupling protein-2 (UCP2) mRNA expression in rat brown adipose tissue (BAT), heart and skeletal muscles was examined. Cold exposure (48 h) increased UCP2 mRNA in BAT, heart and soleus muscle by 2.4-, 4.3- and 2.6-fold, respectively. Fasting (48 h) had no effect on UCP2 mRNA expression neither in BAT nor in heart, but markedly increased it in skeletal muscles. While the upregulation of UCP2 mRNA in response to cold exposure is in line with a putative uncoupling role for this protein in thermoregulatory thermogenesis, the unexpected upregulation of UCP2 in skeletal muscles in response to fasting seems inconsistent with its role as an uncoupling protein involved in dietary regulation of thermogenesis.

Uncoupling protein-3: a new member of the mitochondrial carrier family with tissue-specific expression

Brown adipose tissue (BAT) and skeletal muscle are important sites of nonshivering thermogenesis. The uncoupling protein-1 (UCP1) is the main effector of nonshivering thermogenesis in BAT and the recently described ubiquitous UCP2 [1] has been implicated in energy balance. In an attempt to better understand the biochemical events underlying nonshivering thermogenesis in muscle, we screened a human skeletal muscle cDNA library and isolated three clones: UCP2, UCP3L and UCP3S. The novel UCP3 was 57% and 73% identical to human UCP1 and UCP2, respectively, highly skeletal muscle-specific and its expression was unaffected by cold acclimation. This new member of the UCP family is a candidate protein for the modulation of the respiratory control in skeletal muscle.

Altered energy balance and cytokine gene expression in a murine model of chronic infection with Toxoplasma gondii

The temporal pattern of changes in energy balance and cytokine mRNA expression in spleen and brain were examined in a mouse model of infection with Toxoplasma gondii. During days 1-7 postinfection, food intake was unaltered, but energy expenditure was significantly increased, and this was associated with elevated tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1, IL-5, and interferon (IFN)-gamma. The hypermetabolic state persisted during subsequent anorexia, whose onset coincided with elevated IL-2, and at the end of the acute phase of cachexia, the dual anorexic and hypermetabolic states were associated with the cytokines examined: TNF-alpha, IL-1 beta, IL-2, IL-4, IL-5, IL-6, IL-10, and IFN-gamma. In the chronic phase of the infection, the mice showed either partial weight recovery (gainers) or no weight regain (nongainers). The infected gainers, though still hypophagic, were no longer hypermetabolic, and their cytokine mRNA was no longer elevated, except for TNF-alpha and IL-10. In contrast, the infected nongainers continued to show both anoroxia and hypermetabolism, which were associated with elevations in all cytokines examined and particularly those of the TH2 profile (IL-4 and IL-5) and IL-6. Taken together, these studies reveal a distinct pattern of cytokine mRNA expression underlying 1) hypermetabolism vs. anorexia, 2) acute vs. chronic cachexia, and 3) stable weight loss vs. partial weight recovery.

Effects of beta-adrenoceptor subtype stimulation on obese gene messenger ribonucleic acid and on leptin secretion in mouse brown adipocytes differentiated in culture

The ob gene product is known to control food intake and energy expenditure. To determine whether thermogenic agents directly control ob gene expression, the effects of beta-adrenoceptor agonists on the level of the ob gene messenger RNA (mRNA) and on leptin secretion have been studied in mouse brown adipocytes differentiated in culture. These cells highly expressed the beta 3-adrenoceptor, the uncoupling protein, and the ob gene mRNAs. The ob gene was expressed in mouse brown adipocytes earlier than in mouse white adipocytes under the same culture conditions and to a similar level. The beta 3-, beta L-, and beta 2-adrenoceptor agonists BRL 37344, dobutamine, and terbutaline inhibited ob gene expression in mouse brown adipocytes differentiated in culture with EC50 values of 0.3, 1.0, and 85 nM, respectively. Leptin secretion by the cells under basal conditions was 78 +/- 10 pg/microgram DNA-4 h and was decreased by exposure to the beta-adrenoceptor agonists. The ob gene mRNA half-life was 9.4 h and was decreased to 2.4 h by 1 nM BRL 37344, indicating that the inhibitory effect of the beta 3-agonist might be due to destabilization of ob gene mRNA. (Bu)2cAMP (10-100 microM) and forskolin (20 microM) mimicked the effect of the beta-adrenoceptor agonists. FFA (150-800 microM) had only a small inhibitory effect on ob gene mRNA expression. The results suggest the existence in brown adipose tissue of a retroregulatory pathway by which leptin production in inhibited when the sympathetic nervous system is stimulated.

Adrenoceptor heterogeneity in human white adipocytes differentiated in culture as assessed by cytosolic free calcium measurements

Changes in intracellular calcium concentration [Ca2+]i in response to norepinephrine (NE) and to various adrenergic agonists were monitored by dual excitation microfluorimetry in single human adipocytes differentiated in culture and loaded with fura-2 acetoxymethyl ester (fura-2 AM). The addition of NE elicited increases in [Ca2+]i that were depending on the cell, (1) either rapid (time to peak: 9 +/- 3 s), large, and transient; or (2) slow (time to peak: 125 +/- 8 s), small, and sustained. The rapid and large [Ca+]i response, which was inhibited by 90% by the alpha 1-antagonist prazosin and only by 20% by the non-specific beta antagonist (-)-propranolol, was considered to be mediated by the alpha 1-adrenoceptor. In fact, an alpha 1A-adrenoceptor was found to be expressed in human white adipose tissue. Consecutive additions of beta-agonists specific for each subtype of alpha-adrenoceptor enabled the characterization of four cell populations with different response patterns: 47% of the cells had alpha 1- and beta 1-, beta 2- and beta 3-induced [Ca2+]i responses; 29% had only beta 1-, beta 2-, beta 3-responses; 14% had alpha 1- and beta 3-responses, and 10% had only an alpha 1-response. Taken together, these results show that in differentiated human adipocytes: (1) alpha 1- and beta-adrenergic stimulations induce [Ca2+]i increases with different kinetics and amplitudes; (2) there is a beta 3-adrenergic response similar to the beta 1- or beta 2-adrenergic responses; and (3) there is a marked adrenoceptor heterogeneity.

Thermogenesis associated with spontaneous activity: an important component of thermoregulatory needs in rats

1. Quantification of the energy cost of spontaneous activity in freely moving lean and obese Zucker rats was performed at 28 degrees C and during acute cold exposure (from 28 to 5 degrees C). 2. An open-circuit metabolic chamber was supplemented with an ultrasensitive ergometric platform equipped with six undirectional accelerometers and with an opto-electronic device for location of the rat's centre of mass. 3. Resting and mean metabolic rates during control and cold-exposure periods were similar in both groups of rats. The 'extra thermogenesis' (ET), i.e. the difference between mean and resting metabolic rate, amounted to 11.7 +/- 1.1 and 8.6 +/- 0.7% of resting metabolic rate at 28 degrees C, and 39.7 +/- 2.9 and 34.1 +/- 2.9% of resting metabolic rate during cold exposure for lean and obese rats, respectively. 4. During the control period obese rats moved 3.71 +/- 0.61 m h-1 and lean rats 8.69 +/- 0.57 m h-1, but during cold exposure the distance moved by obese rats increased 3.58 +/- 0.33-fold whereas that moved by lean rats only increased 1.40 +/- 0.06-fold. The external work performed during spontaneous activity seldom reached 1.0% of the increase in metabolic rate. 5. In obese rats, weight was a good predictor of the distance covered, and cold exposure induced the same percentage increase in both distance and ET. Activity-associated thermogenesis of obese rats was the predominant thermogenic source that substituted for their atrophied brown fat thermogenesis whereas in lean rats with active brown fat these correlations were not found.

Modulation of obese gene expression in rat brown and white adipose tissues

The ob gene mRNA expression in rat brown adipose tissue (BAT) and epididymal white adipose tissue (WAT) was measured on Northern blots hybridized with a rat ob gene probe. The level of ob gene mRNA in BAT was about 40% of that in WAT. Fasting (36 h) or semi-starvation (10 days) decreased the ob gene mRNA level in both tissues by 62-68%, and cold exposure at 6 degrees C (24 h) decreased it in BAT (-84%) but not in WAT. Acute administration of the beta 3-adrenergic agonist Ro 16-8714 decreased the ob gene mRNA level in BAT (-51%) and WAT (-28%) of lean Zucker rats and only in BAT (-74%) of obese falfa rats. This study demonstrates that, in the rat, the ob gene is not only expressed in WAT but also in BAT, and suggests that in these two tissues, the modulation of the ob gene expression might be more closely associated with known alterations in cell lipid content than with changes in sympathetic activity.

Dissociation of enhanced efficiency of fat deposition during weight recovery from sympathetic control of thermogenesis

Studies reported here examined the extent to which conditions known to suppress or markedly increase the sympathetic control of thermogenesis influence enhanced efficiency of fat deposition during weight recovery after caloric restriction. To this end, measurements of energy balance and changes in body energy compartments during refeeding of rats pair fed with weight-matched controls were conducted over a 2-wk period at 22 degrees C, at thermoneutrality (29 degrees C), or in the cold (6 degrees C). The results indicate that, despite identical (or slightly lower) energy intake relative to the respective controls, the refed animals showed greater gain in body fat (by 2- to 2.5-fold), 10-12% lower energy expenditure, and higher energetic efficiency (60-80%) than the controls at all three environmental temperatures. In contrast, protein gain was not different between the refed and control groups. Thus the energy-conserving mechanism specific to acceleration of fat deposition during weight recovery persists when sympathetically driven thermogenesis is shifted from very low to very high intensity. These findings raise the possibility that this energy-conserving mechanism during refeeding may be distinct from sympathetic-dependent mechanisms underlying adaptive reduction in thermogenesis during severe energy deficit and weight loss.

Differential effects of high-fat diets varying in fatty acid composition on the efficiency of lean and fat tissue deposition during weight recovery after low food intake

The energetics of body weight recovery after low food intake was examined in the rat during refeeding for 2 weeks with isocaloric amounts of high-fat (HF) diets providing 50% of energy as either lard, coconut oil, olive oil, safflower oil, menhaden fish oil, or a mixture of all these fat types. The results indicate that for both body fat and protein, the efficiency of deposition was dependent on the dietary fat type. The most striking differences were found (1) between diets rich in n-3 and n-6 polyunsaturated fatty acids (PUFA), with the diet high in fish oil resulting in a greater body fat deposition and lower protein gain than the diet high in safflower oil; and (2) between diets rich in long-chain (LCT) and medium-chain triglycerides (MCT), with the diet high in lard resulting in a greater gain in both body fat and protein than the diet high in coconut oil. Furthermore, the diet high in olive oil (a monounsaturated fat) and the mixed-fat diet (containing all fat types) were found to be similar to the fish oil diet in that the efficiency of fat deposition was greater (and that of protein gain lower) than with the diet high in safflower oil. Neither the efficiency of fat gain nor that of protein gain were found to correlate with fasting plasma insulin, the insulin to glucose ratio, or plasma lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Paraxanthine (metabolite of caffeine) mimics caffeine's interaction with sympathetic control of thermogenesis

The notion that paraxanthine (the major dimethylated by-product of caffeine) may be a biologically active metabolite that could mediate some of the effects of caffeine was tested in relation to the well-established property of caffeine as a thermogenic stimulant. From studies measuring the in vitro respiration rates of rat brown adipose tissue in the basal state and in response to ephedrine (an enhancer of norepinephrine release from sympathetic nerve endings), it is shown that paraxanthine has the same potency as its parent compound, caffeine, in interacting with the adrenergic system to potentiate thermogenesis. These data provide the first direct demonstration of a physiological effect of the main metabolite of caffeine and raise the possibility that paraxanthine may contribute importantly to the ability of caffeine to potentiate the thermogenic effects of well-known stimuli of the sympathetic nervous system such as cold exposure, moderate exercise, and sympathomimetic drugs.

Real-time measurement of the contribution of the muscular activity to the metabolic rate in freely moving rats

A new real-time ergometric system, ERGORAT, for measuring the energy expenditure due to muscular activity of small mammals is described. The method is based on measuring the vibrations induced by a freely moving rat to a platform on which it is living. Six accelerometers placed on the platform detect all the vibrations produced by the rat. The co-ordinates of the centre of mass of the animal are located by an optoelectronic device. This location and the six accelerations are fed to a microprocessor-based data acquisition and processing system. Using a Lagrangian dynamic model, the values of the mechanical energy transferred to the platform are computed every second. ERGORAT is used as part of an experimental setup allowing the measurement of the energetic balance of lean and obese rats. The results clearly show that the increase in metabolic rate of obese rats during cold exposure can be entirely explained by the cost of their increase in locomotor activity, whereas for lean rats, this cost accounts for only 41.5 per cent of their metabolic increase, the remaining being the contribution of their active brown adipose tissue.

Potentiation of the thermogenic antiobesity effects of ephedrine by dietary methylxanthines: adenosine antagonism or phosphodiesterase inhibition?

Current concepts about the mechanisms underlying the therapeutic effects of dietary methylxanthines (caffeine, theophylline, and theobromine) favor their actions as antagonists of adenosine receptors, and attribute their other possible modes of action, namely those associated with translocation of intracellular calcium, inhibition of phosphodiesterase enzyme (PDE) activity, or the release of catecholamines, to high (near-toxic) doses. From studies measuring the respiration rate of brown adipose tissue (BAT), evidence is provided here that at concentrations compatible with therapeutic doses, the ability of methylxanthines (25 to 50 mumol/L) to potentiate the thermogenic effect of the sympathomimetic drug, ephedrine (0.25 mumol/L), particularly under conditions of caloric restriction, involves a minor contribution of adenosine antagonism, but could mainly be explained by the inhibition of PDE activity. In view of current interest in the pharmacological stimulation of metabolic rate to assist the management of obesity with low-calorie regimens, the targeting of PDE activity is therefore a rational approach in the search for drugs that could potentiate sympathomimetic stimulation of metabolic rate.